synthesis and characterization of carbon coated...

SYNTHESIS AND CHARACTERIZATION OF CARBON COATED

LITHIUM TRANSITION METAL PHOSPHATES FOR LITHIUM ION

BATTERY APPLICATIONS

Thesis submitted to the Pondicherry University

for the award of the degree of

Doctor of Philosophy

in

PHYSICS

by

P. RAMESH KUMAR

Supervisor: Prof. N. SATYANARAYANA

Department of Physics

Pondicherry University

Puducherry – 605014

India

July 2012

To My Friends & Parents

Prof. N. Satyanarayana

Department of Physics Pondicherry University Puducherry 605014 India E- mail: [email protected] Phone: 91-413-2654404 (Off.) 91-413-2251281 (Res.)

CERTIFICATE

This is to certify that the thesis entitled “Synthesis and Characterization of

Carbon Coated Lithium Transition Metal Phosphates for Lithium Ion Battery

Applications” is an authentic record of the work carried out by Mr. P. Ramesh

Kumar, in the department of physics, Pondicherry University for the award of the

Degree of Doctor of Philosophy, under my guidance and supervision. This thesis or

any part thereof has not formed the basis for the award of any Degree, Diploma,

Associateship, Fellowship or other similar title.

I further state that the entire thesis represents the independent work of

Mr. P. Ramesh Kumar is actually undertaken by the candidate under my

supervision.

Place : Puducherry (N. Satyanarayana)

Date : Supervisor

DECLARATION

I hereby declare that the work presented in this Ph.D. thesis entitled “Synthesis

and Characterization of Carbon Coated Lithium Transition Metal Phosphates

for Lithium Ion Battery Applications”, is original and has been done by me

under the supervision of Prof. N. Satyanarayana, department of physics,

Pondicherry University. I further declare that this thesis or any part thereof has not

formed the basis for the Degree, Diploma, Associateship, Fellowship or other

similar title.

Place : Puducherry (P. Ramesh Kumar)

Date : Research Scholar Department Physics Pondicherry University Puducherry 605014

ACKNOWLEDGEMENTS

I express my deep sense of gratitude to Prof. N. Satyanarayana, Department of

Physics, Pondicherry University for his inspiring and valuable guidance, fruitful

discussions, motivation and persistent as well as continuous encouragement

throughout my research work.

I thank Prof. G. Chandrasekharan, Head, Department of Physics, Pondicherry

University for his encouragement and support during my research tenure.

I also thank Dr. S. Subramanian, Professor, Department of Chemistry,

Pondicherry Engineering College, Puducherry, for his valuable encouragement and

suggestion during my research work.

I am profoundly thankful to Dr. V. V. Ravi Kanth kumar, Department of Physics,

Pondicherry University for being a constant source of encouragement, and inspiring

discussion during the present work.

I thank Prof. G. Govindaraj, Coordinator, Central Instrumentation facilities

Pondicherry University for his encouragement and support during my research

tenure.

I am very much thankful to Dr. T. Premkumar, Advanced battery division,

CECRI, Karaikudi, for providing the facilities for the fabrication and

characterization of lithium batteries and also his valuable discussions.

I wish to express my sincere thanks to Dr. M. Venkateswarlu, Manager, Research

and Development, Amara Raja Batteries, Tirupati, A.P., for his encouragement,

suggestions and timely help during my research work.

I express my heartiest thanks to Prof. Amar K. Mohanty, and Prof. Manjusri

Misra, Bio-products discovery & development centre (BDDC), Department of

Plant Agriculture, School of engineering, University of Guelph, Canada and Mr.

David Carnahan, NanoLab Inc., Bostan, Massachusetts, USA for their constant

encouragement as well as collaboration during my research work.

I express my thankful and gratitude to Dr. P. Muralidharan, Dr. S.

Vivekanandhan, Dr. M. Vijayakumar, Dr. I. Prakash, Mr. N. Nallamuthu ,

Mr. O. Padmaraj, Mr. B. Nageswara rao, Mr. Paramananda Jana, Mr. K.

Hari Prasad and Mr. D. Narsimulu for their help in my research work.

I wish to express my sincere thanks to all the faculty members, staffs and research

scholars, Department of Physics, Pondicherry University for their timely help.

I am very much thankful to my friends from various other departments in

Pondicherry University, for their constant encouragements and valuable discussion

during my research work.

I am happy to express my sincere thanks to all the staff members in CIF (Central

Instrumentation Facilities) and various divisions of Pondicherry University for

their timely help.

I express my deep sense of gratitude to my wife Mrs. P. Anuradha as well as my

family members and also my best friend Mr. M. Rajesh Babu for their invaluable

care, affection, cooperation and also encouragement during the period of my

research work.

Finally, I thank CSIR for the financial support in the form Junior Research Fellow

(JRF). I am also grateful to DST, UGC, ACITE, DRDO and Government of India,

for utilizing the research facilities, developed in our laboratory using the grants

received in the form of major research projects from the above funding agencies.

P. Ramesh Kumar

CONTENTS

Chapter Title Page

No

Preface i

Chapter-I Introduction 1

Chapter - II Experimental techniques 41

Chapter - III Synthesis of pure and carbon coated

LiMPO4 (M= Mn, Co & Ni)

nanoparticles using PVP assisted polyol

and resin coating processes

88

Chapter - IV Characterization of pure and carbon

coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles.

108

Chapter - V Impedance and ac conductivity studies of

pure and carbon coated LiMPO4 (M=

Mn, Co & Ni) nanoparticles

139

Chapter - VI Fabrication and electrochemical

characterization of the CR2032 coin cells

using the synthesized pure and carbon

coated LiMPO4 (M= Mn, Co & Ni)

nanoparticle samples.

181

Chapter - VII Summary & future work 199

Publications 204

List of Figures

Figure Page

1.1Various applications of batteries like electronic devices, battery bikes,

digital camera and large scale storage 6

1.2 Schematic diagram of zinc carbon battery 15

1.3 Volumetric and gravimetric energy densities comparison graph for

different secondary batteries 17

1.4 Schematic diagram of the lithium ion battery operation 21

1.5 Schematic diagrams of a) spinel structure and b) layered structures 27

1.6 Schematic diagram of olivine structure 29

2.1 Schematic diagram of various stages involved in sol-gel process 47

2.2 Photograph of experimental setup of polyol process 56

2.3 Photograph of the PANalytical X’ Pert PRO MPD X-ray diffractometer 64

2.4 Schematic diagram of custom built Raman spectrometer 67

2.5 Schematic diagram of SEM 69

2.6 Schematic diagram of TEM 70

2.7 Photograph of impedance analyzer (Nova control, Germany) 73

2.8 Schematic diagrams of impedance plots for the a) series and

b) parallel combination of R and C circuits 77

2.9 Photograph of the VAC atmosphere glow box 80

2.10 Flowchart of the total coin cell fabrication process 81

2.11 Battery cycle tester, ARBIN Instrument, USA. 82

3.1a) Jahn-Teller distortion in Mn3+ ion and b) changes in MnO6 octahedral

symmetry 93

3.2 Photograph of experimental setup of polyol synthesis process 96

3.3 Flow chart for the PVP assisted polyol process 98

3.4 Final prepared olivine structured LiMPO4 (M= Mn, Co & Ni)

nanoparticle samples 99

3.5 Flowchart for the total resin coating process 102

3.6 Schematic synthesis scheme of resin coating process 103

4.1 a) XRD patterns of the pure and carbon coated LiMnPO4 nanorods

along with JCPDS data and b) Observed XRD pattern fitted with the

calculated one, using Rietveld analysis 112

4.2 XRD patterns of the pure and carbon coated LiCoPO4 nanoparticles

along with JCPDS data 113

4.3 XRD patterns of the pure and carbon coated LiNiPO4 nanoparticles

along with JCPDS data 114

4.4 FTIR spectra of the pure and carbon coated LiMnPO4 nanoparticles 116

4.5 FTIR spectra of the pure and carbon coated LiCoPO4 nanoparticles 117

4.6 FTIR spectra of the pure and carbon coated LiNiPO4 nanoparticles 118

4.7 Raman spectra of the pure and carbon coated LiMnPO4 nanorods 121

4.8 Raman spectra of the pure and carbon coated LiCoPO4 nanoparticles 122

4.9 Raman spectra of the carbon coated LiNiPO4 nanoparticles 123

4.10 Vibrational modes of single crystalline graphite 124

4.11 SEM images of the pure LiMnPO4 nanorods in low magnification 126

4.12 a & b) SEM images of the carbon coated LiMnPO4 nanorods at

different magnification 127

4.13 a & b) SEM images of the pure LiCoPO4 nanoparticles at different

magnification 128

4.14 a & b) SEM images of the carbon coated LiCoPO4 nanoparticles at


4.15 a & b) SEM images of the Pure LiNiPO4 nanoparticles at different

magnification 130

4.16 a & b) SEM images of the carbon coated LiNiPO4 nanoparticles at


4.17 a) HRTEM image and b) EDS spectrum of the carbon coated

LiMnPO4 nanorods 133

4.18 a) HRTEM image and b) EDS spectrum of the carbon coated

LiCoPO4 nanoparticles 134

4.19 EDS spectrum of the prepared carbon coated LiNiPO4 nanoparticles 135

5.1 Impedance plots obtained at different temperatures of pure


5.2 Impedance plots obtained at different temperatures of carbon

coated LiMnPO4 nanorods 144


LiCoPO4 nanoparticles 146


coated LiCoPO4 nanoparticles 147


LiNiPO4 nanoparticles 148


coated LiNiPO4 nanoparticles 149

5.7 a) log (σT) vs. 1000/T plots of the pure and carbon coated LiMnPO4

nanorods b) log (σT) vs. 1000/T plots of the pure and carbon

coated LiMnPO4 nanorods region 3 (high temperature) 151

5.8 log (σT) vs. 1000/T plots of the pure and carbon coated LiCoPO4

nanoparticles 152

5.9 log (σT) vs. 1000/T plots of the pure and carbon coated LiNiPO4

nanoparticles 153

5.10 log σ vs log ω plots obtained at different temperatures of a) pure

and b) carbon coated LiMnPO4 nanorods 155


and b) carbon coated LiCoPO4 nanoparticles 156


and b) carbon coated LiNiPO4 nanoparticles 157

5.13 log (σ/σ0) vs log (ω/ωp) plots obtained at different temperatures of pure

and carbon coated LiMnPO4 nanorods 158


and carbon coated LiCoPO4 nanoparticles 159


and carbon coated LiNiPO4 nanoparticles 160

5.16 Real part of dielectric permittivity ε' vs. log ω obtained at different

temperatures of pure and carbon coated LiMnPO4 nanorods 163


temperatures of pure and carbon coated LiCoPO4 nanoparticles 164


temperatures of pure and carbon coated LiNiPO4 nanoparticles 165

5.19 Imaginary part of electric modulus M'' vs log ω plots obtained at different

temperatures of the pure and carbon coated LiMnPO4 nanorods 167


temperatures of the pure and carbon coated LiCoPO4 nanoparticles 168


temperatures of the pure and carbon coated LiNiPO4 nanoparticles 169

5.22 log τ vs. 1000/T of the pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticle samples 170

5.23 Wagner’s polarization method for measuring transport numbers 173

5.24 Current vs time graphs of pure and carbon coated LiMnPO4 nanorods 174

5.25 Current vs time graphs of pure and carbon coated LiCoPO4 nanoparticles 175

5.26 Current vs time graphs of pure and carbon coated LiNiPO4 nanoparticles 176

6.1 Flowchart for the general electrode processing for the lithium ion batteries 185

6.2 Schematic diagram of CR2032 coin cell construction 186

6.3 Photographs of fabricated CR 2032 coin cells using the prepared


6.4 Discharge profile of the lithium batteries fabricated using pure and carbon

coated LiMnPO4 nanorods 188

6.5 Capacity retain plot of the lithium batteries fabricated using pure

and carbon coated LiMnPO4 nanorods 189

6.6 Discharge profile of the lithium batteries fabricated using a) pure

and b) carbon coated LiCoPO4 nanoparticles 191

6.7 Capacity retain plot of the lithium batteries fabricated using pure and carbon

coated LiCoPO4 nanoparticles 192

6.8 Discharge profile of lithium batteries fabricated using a) pure and b) carbon


6.9 Capacity retain plot of lithium batteries fabricated using pure and carbon


List of Tables

Table Page

5.1 The activation energies of carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles obtained from conductivity and relaxation plots 171

5.2 Electronic and ionic transport numbers of pure and carbon coated

LiMPO4 (M= Mn, Co &Ni) nanoparticles 177

6.1 Discharge capacities of all pure and carbon coated LiMPO4

(M= Mn, Co & Ni) nanoparticles along with the reported work 195

i

PREFACE

The enormous growth in portable electronic devices such as cellular phones, laptop

computers, etc., has motivated interest in compact, light weight batteries with high

energy densities. Lithium ion batteries are used in the above applications, since

they provide higher energy density compared to other available rechargeable

batteries such as lead acid, nickel- cadmium, nickel- metal hydride batteries, etc. In

recent years, the rechargeable battery market has further expanded and tends to

increase exponentially. Hence, the lithium battery technology is receiving most

attention. Ever growing demand for batteries leads the industry and government to

make huge investment in battery research and development. Lithium battery

research not only involves enhancement of cyclability, high energy density but also

seriously considers the safety, environmental friendliness, cost issues, etc. Hence, a

wide range of materials (anode, cathode, electrolytes, etc) have been developed and

investigated to improve the lithium battery technology.

In lithium ion batteries, chemical energy stored in the positive electrode is released

and converted into electrical energy through an intercalation process. Lithium

transition metal oxides have been used as promising positive electrode (cathode)

materials, since they show large stability region with respect to lithium content than

other class of intercalation compounds. The intercalation process of lithium ions are

accompanied by redox of transition metal ions. Therefore, much research has been

focused on the understanding of lithium intercalation / de-intercalation mechanism,

and modifying materials to suppress the structural change of lithium transition

metal oxides during the intercalation / de-intercalation process to achieve better

battery performance. Layered LiCoO2, LiNiO2 and LiMnO2, spinel LiMn2O4,

ii

inverse spinel LiNiVO4, LiCoVO4 and olivine LiFePO4 are the most used and

studied cathode materials in the last two decades.

Recently, it has been found that the nanocrystalline cathode materials exhibit high

storage capacity, voltage and charging / discharging rates due to much shorter

diffusion paths for Li+ diffusion and the smaller dimensional change during

intercalation and de-intercalation process. Synthesis process plays major role in the

development of nanocrystalline cathode materials with desired physiochemical

properties. Wet chemical processes such as polyol, sol-gel, combustion etc., are

used for this purpose.

Due to the less structural stability, high capacity fading, while charging/

discharging, less energy density in the layered and spinal structured cathode

materials and so, there is a need for further development, in order to achieve an

improved electrochemical performance of materials. In 1997, J.B. Goodenough et

al., first discovered the electrochemically active lithium iron phosphate (LiFePO4)

olivine compound. The lithium iron phosphate is found to have good

electrochemical properties compared to layer and spinel structure materials with

potentially low cost, high natural abundance of raw materials and environmentally

friendliness. However, lithium iron phosphate cathode material has several

disadvantages, which impede its industrial applications. Recently, researchers have

developed other olivine materials [LiMPO4: M= Mn, Co &Ni] using various

preparative methods. But, they obtained an acceptable electrochemical properties

due to the some intensive drawbacks like (i) very low electronic conductivity in its

pure form, of the order of 10-9 S cm-1, (ii) slow lithium ion diffusion in the solid

phase due to high defects formation in lithium ion diffusion path, and (iii) Jahn-

Teller distortion. These drawbacks can be overcome by various strategies,

iii

including carbon coating, conductive additives, partial substitution, alien cation

doping, and synthesizing various shapes of nanosize crystallites (like spheres, rods

.etc.), using various synthesis methods. The advantages and properties of olivine

structured materials motivated me to develop the shape controlled pure and carbon

coated [LiMPO4: M= Mn, Co & Ni] nanocrystalline cathode materials for

improving the storage capacity and cycleability of lithium ion batteries.

In the present work, pure and carbon coated lithium transition metal phosphate

[LiMPO4: M= Mn, Co & Ni] systems were chosen to develop in different shapes

such as nanorods, nanospheres using polyol and resin coating processes. Further, all

prepared materials are characterized by powder X – ray diffraction (XRD), Fourier

transform infrared radiation (FTIR), Raman spectroscopy (RS), scanning electron

microscopy (SEM), transmission electron microscopy and energy dispersive X-ray

spectroscopy (TEM-EDS). The best property compounds were chosen as a cathode

material for the fabrication of CR2032 coin cell type of lithium ion batteries.

The thesis entitled “Synthesis and characterization of pure and carbon coated

LiMPO4 (M=Mn, Co & Ni) nanomaterials for lithium ion battery applications”

contains seven chapters.

First chapter of the thesis briefly discusses the importance of lithium ion batteries

among the available batteries, based on their energy density and also briefly

classifies the types of batteries with examples, based on the reversibility of the

electrode reaction. It also briefly discusses the major components of battery and

highlights the existing problems involved in the commercial cathode materials with

respect to safety, environmental concern and fulfillment of growing energy

demands for the recent portable electronic device applications. Finally, it proposes

the possible remedies to overcome the above mentioned problems by preparing the

iv

nanostructured materials and introducing the surface modified olivine structured

lithium based transition metal phosphates.

Second chapter briefly discusses the available wet chemical methods like polyol,

sol-gel, combustion, thermo-mechanical, hydro-thermal, co-precipitation, etc.,

along with their respective merits and demerits for the preparation of

nanocrystalline materials. It also briefly discusses the characterization techniques

used in the present investigation like powder X-ray diffraction (XRD), Fourier

transform infrared radiation (FTIR), Raman spectrometer, scanning electron

microscope- Energy dispersion spectroscopy (SEM-EDS), High-resolution

transmission electron microscopy (HRTEM), Raman Spectroscopy. Also, briefly

explains the transport studies like dc & ac conductivity, dielectric and modulus and

transport number studies respectively, through impedance and Wagner polarization

measurements. Finally, describing the fabrication and electrochemical

characterization of CR2032 coin cells using newly developed cathode materials.

Third chapter discusses the preparation of pure and carbon coated LiMPO4 (M=

Mn, Co & Ni) nanoparticles using PVP assisted polyol and resin coating processes.

Pure LiMPO4 (M= Mn, Co & Ni) nanoparticles were prepared by

polyvinylpyrrolidone (PVP) assisted polyol process. All precursor materials

are mixed with the ethylene glycol (polyol) and heated under distillation

condition up to 465 K for 3 h. The precipitation in the bottom of the flask

was washed with acetone and heated up to 873 K to get the pure crystalline

nanomaterials. At this temperature, ethylene glycol will convert in to the

glycolic acid, which has high chelating agent property due its hydroxyl and

carboxyl functional groups. In this reaction, PVP acts as a stabilizer to

obtain the different shaped nanoparticles.

v

The novel resin coating process was used for the thin and uniform carbon

coating upon the nanoparticle surface to increase the electrical conductivity

of the materials. In this process, synthesized pure LiMPO4 (M= Mn, Co &

Ni) nanoparticles were added in to the solution of EG (ethylene glycol)+

PAA (Poly acrylic acid) and heated up to 353 K under constant stirring for

1h. After formation of high density resin, nanoparticles were again heated

up to 623 K for 1h to get the thin carbon coating over the surface of the

LiMnPO4 nanorods. At 623 K, hydrogen and oxygen compounds will

evaporate, and the remaining carbon will form a thin layer upon the

LiMPO4 (M= Mn, Co & Ni) nanoparticles.

Fourth chapter discusses the characterization of pure and carbon coated LiMPO4

(M= Mn, Co & Ni) nanoparticles.

Phase purity and structure of both pure and carbon coated LiMPO4 (M=

Mn, Co & Ni) nanoparticles were examined by the XRD and FTIR. FTIR

confirmed the presence of the carbon in the LiMPO4 (M= Mn, Co & Ni)

nanoparticle samples by showing the C=C symmetrical stretching band.

Further, Raman spectral results show the amorphous nature of the carbon

coating in the LiMPO4 (M= Mn, Co & Ni) nanoparticle samples. SEM

images of LiMPO4 (M= Mn, Co & Ni) nanoparticles gives the shape and

size of the nanoparticles. Finally, HRTEM confirmed the thickness and

uniformity of the carbon coating upon LiMPO4 (M= Mn, Co & Ni)

nanoparticles.

Fifth chapter discusses the impedance and ac conductivity studies of pure and

carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles.

vi

The Impedance plots were plotted for the both pure and carbon coated

LiMPO4 (M= Mn, Co & Ni) nanoparticles using the measured impedance

data by the Alpha frequency analyzer (NOVOCONTROL, Germany) from

room temperature to 773 K. The resistance of samples was obtained by

analyzing measured impedance data using winfit software. Also, this

chapter discuss the frequency dependent and independent conductivity,

dielectric constant and modulus of pure and carbon coated LiMPO4 (M=

Mn, Co & Ni) nanoparticles with respect to temperature. Finally, this

chapter includes the transport number studies of both pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) nanoparticles using Wagner

polarization method to estimate the electronic conductivity enhancement.

Sixth chapter discusses the fabrication and electrochemical characterization of the

CR2032 coin cells using the synthesized pure and carbon coated LiMPO4 (M= Mn,

Co & Ni) nanoparticle samples.

CR 2032 coin cells are fabricated by using synthesized both pure ad carbon

coated LiMPO4 (M= Mn, Co & Ni) nanoparticles. For fabricating coin cells,

the cathode is made with composition of 80% of active material + 15%

Super P carbon + 5% PVDF and anode is the lithium metal. The electrolytes

used for pure and carbon coated LiMnPO4/Li coin cells is 1M LiPF6 in EC:

DMC: EMC (1:1:1) but, for LiCoPO4/Li and LiNiPO4/Li cells, the

electrolyte used 1M LiPF6 in EC: DMC (LP30, Merck chemicals) due to

high reduction potentials.

The discharge capacities and cycleability of the fabricated batteries are

examined by measuring the cycleability, discharge characteristics using the

battery cycle tester (BCT).

vii

Seventh chapter discusses the summary of the present investigation and the future

work

Pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were

synthesized using PVP assisted polyol and resin coating processes and

characterized by XRD, FTIR, SEM, TEM-EDS, Raman & Impedance

spectroscopy. Finally, fabrication of CR2032 coin cells were made using the

newly developed samples and characterized using battery cycle tester.

It is concluded that the carbon coated lithium transition metal phosphates

[LiMPO4: M= Mn, Co & Ni] are the important materials as cathodes for high

voltage and high energy density Li-ion batteries.

1

INTRODUCTION

CHAPTER – I

2

CHAPTER I

INTRODUCTION

1.1 General Introduction

1.2 Battery

1.2.1 Components of battery

1.2.1.1 Anode

1.2.1.2 Cathode

1.2.1.3 Electrolyte

1.3 Parameters of batteries

1.3.1 Voltage (V)

1.3.2 Theoretical capacity (mAh g-1)

1.3.3 Energy density (Wh L-1 or Wh kg-1)

1.3.4 Specific power (W kg-1 or W L-1)

1.3.5 Cycle life

1.4 Types of batteries

3

1.4.1 Primary batteries

1.4.1.1 Silver battery

1.4.1.2 Zinc carbon battery

1.4.1.3 Alkaline manganese battery

1.4.2 Secondary batteries

1.4.2.1 Lead acid battery

1.4.2.2 Nickel cadmium battery

1.4.2.3 Nickel metal hydride battery

1.4.2.4 Lithium battery

1.4.2.5 Lithium ion battery

1.5 Review of lithium ion batteries

1.5.1 Advantages of lithium ion batteries

1.5.1.1 High cell voltage

1.5.1.2 High energy density by weight and volume

1.5.1.3 Discharge characteristics

4

1.5.1.4 Low temperature capability

1.5.1.5 Shelf life

1.5.1.6 Environmental compatibility

1.6 Challenges in developing of cathode materials for the lithium ion

batteries

1.7 LiMPO4 (M= Mn, Co & Ni) cathode materials for better performance of

the batteries

1.8 Role of surface modification and doping effect on nanosize cathode

materials

1.9 Present work

References

5

CHAPTER I

INTRODUCTION

1.1 General Introduction

Energy is the primary source of all socio-economic activities of the human community.

The enhancement of economic growth would result in demand of energy at an

incremental rate of 7 to 8 % annually. It is very essential to preserve the conventional

sources of energy and explore possible alternatives. Such alternatives are environment

friendly and easily refillable. Lithium ion battery is one of the alternatives that offer high

energy density, high capacity and long cycle life, in the most common light weight, sizes

with no memory effect [1-5]. These cells, which operate over a wide temperature range,

are ideal for many portable electronic devices such as cell phones, mobile computers, life

saving contribution by powering implantable devices, strong military equipments, electric

vehicles, etc [6-7]. The research and development in the lithium ion battery technology

has been increasing to improve the capacity and energy that includes the improvement of

electrolyte and electrode materials with better properties and low cost to suite various

applications. Various types of applications of batteries are mentioned in figure 1.1.

Lithium metal oxides like layered LiCoO2, LiNiO2 and LiMnO2, spinel LiMn2O4 and

inverse spinel LiNiVO4 materials are used as cathode materials in the Li ion batteries. On

the other hand, due to low conductivity, thermal and structural stability, these materials

deliver less capacity. In order to improve number of charge and discharge cycles,

6

capacity and structural stability, some modifications such as substitution, coating, etc.,

are implemented to the existing cathode materials.

Figure 1.1: Various applications of batteries like electronic devices, battery bikes,

digital camera and large scale storage

7

The capacity fading during charge-discharge cycling process in the lithiated oxide

cathodes are mainly due to an anisotropic volume change, which results in a structural

deformation of the electrode material during repeated cycling [8-15]. The capacity fading

can be reduced by coating the surface of the cathode materials with a thin layer of

materials such as C, La2O3, SiO2. Doping of super ions ( like Mg2+, Al3+, Ti4+, Zr4+, Nb5+

or W6+), which also significantly improves the cycling stability and cell performance [R].

The conductive carbon has been used in various applications as they possess excellent

electronic conductivity, increase the maximum current carrier capacity and support high

voltages [16]. The substitution of carbon into cathodes will increase the strength of the

nanocrystalline materials that have more surface area compared to the bulk material.

Hence, the electrochemical reactions will be more in the nanocrystalline cathodes and it

reduces the battery charging time. Therefore, it is proposed to prepare nanocrystalline

cathode materials in the form of pure and carbon coated lithium transition metal

phosphates (LiMPO4: M= Mn, Co & Ni), in order to study and compare the

electrochemical properties of the pure and coated nanocrystalline cathode materials.

Synthesis process plays a vital role in the development of nanocrystalline and

nanocomposite cathode materials with desired physical and chemical properties. Hence,

the famed wet chemical route, polyol process was used to synthesize lithium metal

phosphates and the novel resin coating process was used effectively for conductive

coating. Present chapter starts with an introduction and classification of batteries,

components of battery. Also describes the use and challenges in development of lithium

based cathode materials and finally, conclusion with the present work.

8

1.2 Battery

A battery is one electrochemical cell or more electrically (series or parallel) connected

electrochemical cells. The electrochemical cell, that converts chemical energy into

electrical energy by means of an electrochemical reaction called oxidation and reduction.

In batteries, the energy of chemical compounds acts as storage medium, during discharge,

a chemical process occurs, which generates energy in the form of voltage.

Each oxidation and reduction reactions are associated with the standard cell potential Eo,

which can be calculated from the thermodynamic, as follows,

Eo = - ΔGo/ZF -----------------(1.1)

Where ΔGo - standard Gibbs free energy

Z = Number of electrons exchanged during oxidation or reduction reactions

F = Faraday constant (96,487 C mol-1)

The thermodynamic parameters are

1. Enthalpy (∆H) represents the amount of energy released or absorbed by

electrochemical reaction.

2. Free energy (∆G) also called change of Gibb’s free energy, describes the maximum

amount of chemical energy that can be converted into electrical energy and vice versa.

3. Entropy (∆S) characterizes the reversible energy loss or gain connected with the

chemical or electrochemical process.

9

Important relations between the three parameters are

∆G = ∆H – T ∆S or ∆H - ∆G = T ∆S -----------------(1.2)

Where T is temperature in Kelvin (K)

The overall theoretical cell voltage ΔEo is obtained by subtracting the negative electrode

potential Eo(-), from the positive electrode potential Eo(+).

ΔEo = Eo(+) – Eo(-) -----------------(1.3)

1.2.1 Components of battery

Battery consists of three components and they are cathode (positive electrode), anode

(negative electrode) and electrolyte.

1.2.1.1 Cathode

The cathode is a positive electrode and it accepts electrons from the external circuit

through reduction reaction. The oxidizing efficiency as well as high structural stability is

considered for selecting cathode materials. LiMn2O4, LiNiO2, LiCoO2, LiFePO4 and

LiNiVO4, etc., are used as cathode materials in lithium battery. The brief description of

various types of cathode materials is given in section 1.6.

10

1.2.1.2 Anode

The anode is a negative electrode and it releases electrons to the external circuit through

oxidation reaction. Anode materials are selected based on their high reducing efficiency,

high columbic output (Ah g-1) and good conductivity. Different metals, alloys and metal

oxides are used as anode materials. Li, C, SnO2, Si, Ge, CoO, NiO, CuO, CoFe2O4,

NiFe2O4, CuFe2O4, etc., are used as anode materials in lithium battery [17-33]. The

problems like low discharge rate capability, safety, life spans, structural change during

charge and discharge processes, researchers have focused their attention on conversion

electrodes. Conversion reactions are those in which an active electrode material, MXy, is

consumed by Li and reduced to the metal M and a corresponding lithium compound,

according to the general reaction:

where M represents the cation and X the anion [34-36]

1.2.1.3 Electrolyte

The electrolyte is an ionic conductor with negligibly small electronic conduction, which

provides the medium for transfer of ions from one electrode to another electrode and does

not participate in cell reaction to prevent short circuit. The important properties of the

electrolyte are non reactivity with electrode materials, small property change with respect

11

to temperature variation, safety in handling, etc. In certain cases, the interaction between

the electrolyte and the active material cannot be prevented and often influences aging of

the battery. Most of the battery systems uses aqueous electrolytes, lithium as active

material would readily react with water. Batteries with lithium electrodes therefore have

to use nonaqueous inorganic electrolytes, like thionyl chloride or organic electrolytes. A

general disadvantage of organic electrolytes is the conductivity that at least is one order

of magnitude below that of aqueous electrolytes. It must be compensated by narrow

spacing of thin electrodes. Furthermore, interaction between the electrolyte and the active

material is unavoidable at the high cell voltage. Solid state electrolytes are also used

mainly in special long lasting batteries for extremely low loads, like lithium/LiI/iodine

batteries that are applied in pacemakers [37, 38].

1.3 Battery parameters

1.3.1 Voltage (V)

The voltage of the cell was determined by the Gibbs free energy of the electrochemical

reactions. The electrochemical potential of one component is the sum of electrical

potential and chemical potential.

-----------------(1.4)

Where μi is the chemical potential of species i, zi is the effective charge on lithium, F is

Faraday's constant (96,487 C mol-1) and ɸ is the electrical potential.

12

The electrochemical potential should make equilibrium at the cathode and anode, if there

is no external current. Under the equilibrium condition, the cell voltage is obtained by the

Nernst equation.

-----------------(1.5)

The cell voltage is driven by the difference of Li chemical potential in both electrodes.

During charging and discharging of a cell, the measuring voltage is equal to measuring

chemical potential of lithium. For example, potentiostatic intermittent titration technique

(PITT) measurement controls the Li chemical potential to measure current relaxation

with time. The voltage is primarily determined by the chemistry of the active material in

the electrode and measured with respect to the metallic lithium (Vs. Li+/Li).

1.3.2 Theoretical capacity (mAh g-1)

Theoretical capacity of a cell is the amount of charge that can be generated, typically

defined in terms of ampere-hours. Theoretical capacity of a compound is calculated by

using its molecular weight and total electrical charge. The magnitude of electric charge

per mole of electrons is 26.8 Ah or 96,487 C mol-1 , which is called as Faraday constant.

The capacity in lithium ion cell is based on cathode active materials participating in

electrochemical reaction. This value also depends on the chemistry of the cathode active

material. The calculation of theoretical capacity of LiMnPO4 cathode material is shown

below.

Calculation of theoretical capacity of a LiMnPO4 compound:

Theoretical capacity (CT) = 26800 mAh/ Molecular weight of compound

13

= 26800 mAh/ 156.85 g (Molecular weight of LiMnPO4)

= 170.86 mAh g-1

1.3.3 Energy density (Wh L-1 or Wh kg-1)

The energy obtained from the cell is the voltage times the capacity of the cell, typically

defined in terms of watt-hour (Wh). Gravimetric and volumetric energy densities are

defined as energy per unit weight and energy per unit volume respectively.

1.3.4 Specific power (W kg-1 or W L-1)

Specific power is defined as the power delivered from the cell per unit weight (W kg-1).

Power is a product of the voltage and the current.

1.3.5 Cycle life

It is defined as possible number of charge/discharge cycles before the capacity falls

below a certain percentage (often 80%) of its initial capacity. The cycle life depends on

many factors such as the formation of a stable solid electrolyte interface (SEI) layer, the

stability of electrolyte, and the structural stability of electrodes. Most of all, the cycle life

depends primarily on the stability of the electrode material during operation.

1.4 Types of batteries

Based on the reversibility of the electrochemical reactions at the anode and cathode,

batteries are divided into primary (non rechargeable) and secondary (rechargeable).

1.4.1 Primary batteries

14

In primary batteries, the electrode reactions are not reversible and hence, the cells are not

rechargeable. Once they get discharged completely, they should be discarded. Some

important primary batteries are briefly discussed below.


Lithium primary batteries are using nonaqueous solvents for the electrolyte. Organic

solvents such as acetonitrile, propylene carbonate, and dimethoxyethane and inorganic

solvents such as thionyl chloride are typically employed. A compatible solute is added to

provide the necessary electrolyte conductivity (solid-state and molten-salt electrolytes are

also used in some other primary and secondary lithium cells). Many different materials

were considered for the active cathode. Sulfur dioxide, manganese dioxide, iron disulfide,

and carbon monofluoride are now in common use [39]. The possible cell reactions for

lithium primary battery with lithium anode and manganese dioxide cathode are

At anode: Li → Li+ + e–

At cathode: MnO2 + e– → MnO2– (Li+)

Total reaction: MnO2– + Li+ → MnO2

-(Li+)

1.4.1.2 Zinc carbon battery

Zinc carbon battery is also known as Leclanche cell, where zinc anode, manganese

dioxide cathode and ammonium chloride or zinc chloride dissolved in water used as

electrolyte. This is commonly used in flash lights and toys. In an ordinary Leclanche cell,

the electrolyte consists of 26% NH4Cl (ammonium chloride), 8.8% ZnCl2 (zinc chloride),

15

and 65.2% water [40]. The schematic diagram of zinc carbon battery is shown in figure

1.2. The overall cell reaction can be expressed as,

Zn + 2MnO2 +2NH4Cl —> 2MnOOH + Zn(NH3)2Cl2 ; Eo=1.26

1.4.1.3 Alkaline manganese battery

Alkaline manganese batteries are commonly used in radios, toys, photo-flash

applications, watches, high-drain applications. In alkaline manganese battery, zinc is used

as anode, manganese dioxide as cathode and potassium hydroxide as electrolyte.

Figure 1.2: Schematic diagram of zinc carbon battery

Cardboard disk Carbon pin

Hot bitumen

Top washer

Manganese dioxide

Zinc can

Insulating sleeve With top cap

16

than the zinc-carbon batteries. But its improved performance makes it more cost

effective, especially in high drain situations, where the alkaline cell's energy density is

much higher [41].

The half-reactions at both the electrodes are:

Zn + 2 OH– → ZnO + H2O + 2 e–

2 MnO2 + H2O + 2 e– →Mn2O3 + 2 OH–

The overall reaction is

Zn + 2MnO2 → ZnO + Mn2O3 ; E=1.5 V

1.4.2 Secondary batteries

In secondary batteries, the electrode reactions are reversible. Hence, the secondary

batteries act as storage batteries. Figure 1.3 shows the volumetric and gravimetric energy

density comparison graph for different secondary batteries. Some important secondary

batteries are discussed below.

17

1.4.2.1 Lead acid battery

Lead acid batteries are still highly demandable batteries due to their automobile

applications. Lead (Pb) is used as negative electrode material. In the negative electrode,

lead is oxidized into Pb2+ ion and forms lead sulfate PbSO4 after interaction with

electrolyte (sulfuric acid) [42]. At the positive electrode, the charged active material

(PbO2) is reduced to Pb2+. The ejected two electrons were passing through external

circuit, while charging and discharging. The discharging/ charging reactions can be

written as:

Figure1.3: Volumetric and gravimetric energy densities comparison graph for

different secondary batteries

18

Charging ↔ Discharging

Negative electrode Pb + H2SO4 ↔ PbSO4 + 2H++ 2 e–

Positive electrode PbO2 + H2SO4 + 2H+ + 2 e– ↔ PbSO4 + 2H2O

----------------------------------------------------------------------------------------

Cell reaction Pb + PbO2 + 2H2SO4 ↔ 2 PbSO4 + 2H2O

The nominal equilibrium voltage amounts to Eo = 2.0 V as the difference between the

equilibrium values of the electrode reactions Eo of PbO2/PbSO4 = 1.7 V and Eo of

Pb/PbSO4 = - 0.3 V (referred to standard hydrogen electrode). These values depend on

acid concentration. Lead acid battery finds application in cars, trucks, forklifts,

construction equipment, recreational water craft and standby/backup systems, etc.

1.4.2.2 Nickel cadmium battery

Nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries are the most

important in secondary battery group. In nickel cadmium batteries, nickel-hydroxide is

used as the positive electrode. At the positive electrode during discharge state, In nickel-

hydroxide, Ni3+ ion are reduced to Ni2+ as shown in the below equation

NiOOH + H2O + e– → Ni(OH)2 + OH–

It is combined with the cadmium electrode that reacts via the solution similar to the lead

electrode and is discharged according to

Cd + 2(OH–) → Cd(OH)2 + 2 e–

19

The total cell reaction during discharge can be written as,

2 Ni(OOH) + 2H2O + Cd → 2 Ni(OH)2 + Cd(OH)2

When the battery is recharged, the reaction is reversed. Nickel cadmium battery finds

application in calculators, digital cameras, pagers, lap tops, tape recorders, flashlights,

electric vehicles, space applications, etc. The main disadvantages of these nickel-

cadmium batteries are high hydrogen evolution rate and comparatively low power

efficiency [43].

1.4.2.3 Nickel metal hydride battery

In nickel/metal hydride batteries, hydrogen is not stored as a gas, but during charging, it

is absorbed by the negative electrode material from cathode and desorbed during

discharging, both occur at a low hydrogen pressure. This is achieved by special alloys

that act as the catalytic electrode surface and simultaneously absorb the hydrogen (H2) by

forming metal hydrides [44]. Thus, the negative electrode material catalyzes the reaction

and simultaneously stores the hydrogen that is formed during charging. Due to the low

internal pressure, nickel/metal hydride batteries do not require a cell container that can

withstand high pressures, but they can be encapsulated like sealed nickel/cadmium

batteries. Nickel metal hydride batteries find applications in cellular phones, camcorders,

emergency backup lighting, power tools, laptops, portable, electric vehicles, etc.


20

Secondary lithium battery uses lithium metal as an anode material and LiCoO2, LiMn2O4,

LiNiO2 etc. as cathode. It exhibits highest specific energy and energy density [45]. Even

though lithium battery is having highest specific energy, the safety issues related with the

use of lithium, due to dendrite growth and consecutive short circuit, forces the

researchers to look for other metals and metal oxides instead of lithium as anode. The non

lithium based anode materials come under lithium ion battery category and it is discussed

below.

1.4.2.5 Lithium ion Battery

In a lithium ion battery, the Li ions are moving between cathode and anodes during the

discharge and charging process. The principle of lithium-ion battery operation is shown

in Figure 1.4. In charging state, lithium ions are extracted from the cathode, go through

the electrolyte and separator and are inserted into the anode structure. The reverse process

happens during discharging. In order to achieve high cycling efficiency and long cycle

life, the movement of Li ions in anode and cathode hosts should not change or damage

the host crystal structure. The design of a lithium-ion battery system requires careful

selection of electrode pairs to obtain a high operating voltage (Vc). The operation of

lithium-ion batteries obey the thermodynamic laws. Therefore, the electrochemical

reaction in lithium-ion batteries follows the Gibbs-Helmholtz equation (1.1).

A high Vc can be realized with an anode and cathode that have, respectively, smaller and

larger work functions Фa and Фc. The open-circuit voltage Voc of the cell can be

calculated from the following formula:

21

Voc = (Фc -Фa) / e -----------------(1.6)

Where “e” is the electronic

charge. The Fermi energies (EF) of anode and cathode must lie within the band gap (Eg)

of the electrolyte. Therefore, the anode and cathode materials are thermodynamically

stable in contact with the electrolyte, and there will be no side reaction or oxidation of the

electrolyte.

Figure 1.4: Schematic diagram of the lithium ion battery operation

22

Example, in LiCoO2/C cell, at the carbon anode contains the lithium in the battery

charged state and delivers it to the cathode during discharge [46]. Because of lithium ions

migration during cycling back and forth between the anode and cathode, lithium ion

batteries are named ‘rocking chair’ or ‘swing’ batteries.

The following reactions at both anode and cathode are showed in below

Anode: LiC6 → C6 + Li+ +e–

Cathode: CoO2 + Li+ e– → LiCoO2

Cell: LiC6 + CoO2 → C6 + LiCoO2 ; Eo = 3.9V

1.5 Review of lithium ion batteries

Lithium batteries were first proposed by M.S. Whittingham, while he was working for

Exxon in the 1970s and he used titanium sulfide as cathode and lithium metal as anode

[47]. In 1979, John Goodenough demonstrated a rechargeable battery with high cell

voltage using lithium cobalt oxide (LiCoO2) as the positive electrode and lithium metal as

the negative electrode. In 1980, Rachid Yazami also demonstrated the reversible

electrochemical intercalation of lithium in graphite, which is useful for the good anode

for the future lithium ion batteries. Akira Yoshino assembled a prototype cell using

carbonaceous material as anode and lithium cobalt oxide (LiCoO2) in 1985. The safety

was dramatically improved over batteries by using this carbonaceous material [48]. In

1996, Goodenough, Padhi and coworkers discovered phospho-olivines (lithium metal

phosphates) as cathode materials. In 2002, Chiang again demonstrated high capacity and

23

performance Li-ion battery by utilizing high surface iron phosphate nanoparticles [49,

50].

From the above literature review of lithium ion batteries, one can make two generic

observations for improving the performance of the Li-ion batteries: first, open structured

cathode materials; second, nanosize particles with surface modification.

1.5.1 Advantages of lithium Ion batteries

Lithium batteries show remarkable advantages when compared with the traditional

primary and secondary batteries. Advantages of lithium ion batteries are clearly explained

in this chapter.

1.5.1.1 High cell voltage

Most important advantage of lithium ion batteries is the high voltage, the lithium batteries

show a cell voltage in the upper range of 1.5 to 4.0 V or even higher. This high voltage is

related to the energy density and specific energy of these cells. So, in many cases, only

one lithium cell is sufficient, where two or three conventional Leclanche or alkaline cells

are necessary [51].

1.5.1.2 High specific energy and energy density

The gravimetric energy content or the specific energy (SE) of lithium batteries is 100 to

500Wh per kg or per liter depending on active materials and cell type. Lead acid batteries

show a specific energy between 35 and 55 Wh kg-1 and NiCd batteries, a bit more

powerful, from 50 to 70 Wh kg-1. The volumetric energy or the energy density goes from

24

300 to 1300 Wh L-1 for lithium batteries and therefore, require less space than

conventional batteries. Leclanche´ cells can deliver 165 Wh L-1 and alkaline cells deliver

330 Wh L-1 [52].

1.5.1.3 Discharge characteristics

Lithium batteries show a flat and stable discharge curve (voltage against time) for the

whole capacity. This supports electronic devices, which are designed for little tolerances

for voltage fluctuations [53].

1.5.1.4 Thermal stability

These batteries can be stored and operated for wide range of temperatures. It can be

operated even in low temperature range of -10 to -40 and even -55 oC without any

additional support such as heaters or special insulation [54].

1.5.1.5 Self life

Most of the lithium primary batteries can be stored for over 10 up to 20 years with

negligible self-discharge and deliver most of their nominal capacity. At normal

temperature storage, for nearly 10 years, the self-discharge is only 5 to 10% [55].

1.5.1.6 Environmental compatibility

Lithium metal is not poisonous to biological systems when compared to other metals

used in common batteries, such as lead or nickel, cadmium, etc. Hence, disposal of used

lithium batteries is not a severe problem to environment [56].

25

1.6 Challenges in development of cathode materials for the lithium ion batteries

Cathode materials play an important role in the operation of lithium-ion batteries. They

provide lithium ion sources (Li+) for the Li-ion "shuttle" between the cathode and the

anode. Since carbon materials are widely used as anodes in lithium-ion batteries, which

have a potential close to that of Li/Li+ reference electrode, the voltage of lithium-ion

batteries is mainly determined by the potential of cathode materials [57]. Therefore,

cathode materials must meet some strict requirements: (i) the cathode host must be

structurally stable for repeated lithium ion intercalation and de-intercalation. The variation

of the lattice structure should be as small as possible during the battery operation process

to maintain long cycle life. (ii) The cathode materials must have a high potential relative

to Li/Li+ reference electrode to enable high operating voltage (high energy density). (iii)

The cathode materials should contain maximum amount of lithium as possible, and those

lithium atoms must be electrochemically extractable from the host crystal structure (high

capacity), (iv) The cathode materials should have high electronic conductivity and high

lithium chemical diffusion coefficient to keep high rate capacity (high power density). (v)

The cathode materials should be low cost, non-toxic, and easy to prepare. A large number

of materials were investigated as cathode materials for lithium-ion batteries. However,

the different compounds, suitable as cathode hosts for lithium-ion batteries are quite

limited, due to the critical requirements such as high energy density, good cyclability, and

safety.

Although, LiCoO2 cathode materials are popularly used in commercial lithium-ion

battery production, currently, layered and spinel structure cathode materials are under

26

extensive investigation worldwide. Their selection is mainly driven by reasons such as

reducing the cost, increasing environmental friendliness and developing a new

generation of lithium-ion batteries for power storage and electric vehicle (EV)

applications. The layered and spinel structure cathode materials for lithium-ion batteries

mainly include LiCoO2, LiNiO2, LiMnO2 [58-68], LiMn2O4 and their doped derivatives.

The major drawback is the high cost for LiCoO2 due to the high price and limited natural

abundance of Co. Another problem associated with Co is it’s toxicity, which is known to

cause a serious environmental impact when batteries are disposed. These two factors

forbid the application of LiCoO2 as the cathode material in any large batteries for electric

vehicle and power storage applications.

In spinel LiMn2O4, when the discharge voltage is close to 3.0 V, the average Mn

oxidation state changes to 3.5. A Jahn-Teller distortion might occur at the particle

surface, inducing the growth of tetragonal phase. The incompatibility between the

oxygen arrays of the cubic and tetragonal phases could then cause a highly crystalline

electrode particle to be degraded at the crystal surface, which would damage the

structural integrity and particle-to-particle contacts. Therefore, the electronic

conductivity and lithium diffusivity will deteriorate. Fortunately, this damaging effect

can be reduced by suppressing the onset of the Jahn-Teller distortion by modifying the

composition of the spinel electrode to keep the average Mn oxidation state slightly

above 3.5 at the end of the 4. Recently, lithium iron phosphate compounds have also

emerged as new cathode materials for lithium-ion batteries [69-76].

27

Figure 1.5: Schematic diagrams of a) spinel structure and b) layered structures

a

b

28

1.7 LiMPO4 (M= Fe, Mn, Co & Ni) cathode materials for better performance of

the battery

In 1997, J.B. Goodenough et al. discovered the electrochemically active lithium transition

metal phosphate, which is one of the best compounds of olivine structured materials. This

is a new cathode material with potentially low cost due to natural abundance of raw

materials. It is expected that lithium transition metal phosphate will have a significant

impact in electrochemical energy storage. Phospho-olivine compounds belong to

orthorhombic system (space group Pmnb). The oxygen atoms are in slightly distorted

hexagonal close packed arrangement, the phosphorous atoms are located in tetrahedral

sites (PO4) and both the lithium and the metal cations occupy octahedral sites (LiO6 and

MO6). The MO6 units lie in bc plane connected through corners, the LiO6 groups grow

along b axis having common edges and the PO4 tetrahedral share one and two edges with

MO6 and LiO6 groups, respectively. The presence of PO4 tetrahedral plays an important

role in the olivine phase stability during lithium deintercalation. The partially covalent P-

O-M interactions stabilize the MPO4 framework.

Among the Olivine compounds, LiFePO4 has attracted a lot of interest for friendly

environmental reason. This compound, also exists as mineral (triphylite) and can be

prepared using both solid state reaction and soft chemistry strategy. The latter was

mainly developed to decrease the particle size for avoiding reversible capacity tack, at

higher current density [77-88].

29

Figure 1.6: Schematic diagram of olivine structure

Olivine structure cathode materials characteristics:

A high theoretical capacity (170 mAh g-1)

A high and relatively stable open-circuit voltage of ~4V relative to lithium metal

Easy to synthesize and inexpensive to fabricate

Little hygroscope, which makes it easy to handle

Excellent stability during cycling when used with common organic electrolyte

systems

Good overcharge tolerance compared to other cathode materials, so that on

overcharge, there is less heat released and thus less risk of fire or explosion

Good capacity retention during cycling

30

Non-toxic and safe in operation

Environmentally benign

Low cost

However, lithium transition metal phosphate cathode material also has several

disadvantages, which impede its industrial applications. These include: (i) very low

electronic conductivity in its pure form, on the order of 10-9 S cm-1; (ii) slow lithium

diffusion in the solid phase; and (iii) low rate capacity. Those drawbacks can be

overcome by various strategies, including carbon coating, conductive additives, partial

substitution, alien cation doping, and synthesis of nanosize crystallites [89-93].

1.8 Role of surface modification and doping effect on nanosize cathode materials

Size reduction in nanocrystals leads to a variety of exciting phenomena due to enhanced

surface-to-volume ratio and reduced scale of transport lengths for both mass and charge

transport. Unfortunately, for high voltage cathode materials, increasing the electrode

surface area of course allows for significantly more unfavorable side reactions (e.g.,

dissolution of species within the active material). Coating nanoparticles with a stabilizing

surface layer is one way to alleviate such problems, as long as the coating is “ultrathin”

and therefore does not reduce the overall capacity and also allows for a high rate of Li-

ion diffusion [94, 95]. Methods applied to micrometer-sized particles including metal

oxides, phosphates and fluorides generally employ “sol-gel” wet-chemical methods.

Despite the numerous reports dedicated to sol-gel coatings, it is apparent that the lack of

control over surface coverage, thickness, and uniformity prohibits sol-gel methods for

nanoscale materials.

31

Cathode materials should have the ionic conductivity as well as the electronic

conductivity but most of the existing cathodes are not having good electronic

conductivity. Hence, researchers have attempted to prepare electronically active coating

upon the cathode materials by solid state reaction, hydrothermal process and also doping

by supervalent (Mg2+, Al3+, Ti4+, Zr4+, Nb5+ and W6+) cations to Li+ in M1 sites [96-99].

1.9 Present work

In the present work, pure and carbon coated LiMPO4 (M=Mn, Co & Ni) nanoparticles

are developed by using PVP assisted Polyol synthesis followed by the resin coating

process. Polyvinylpyrrolidone (PVP) was used as stabilizer in to this polyol process to

reduce the nanoparticles agglomeration as well as to obtain shape controlled

nanoparticles. All the prepared pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles are characterized by the XRD, FTIR, SEM, Raman and TEM-EDX.

Impedance, ac conductivity and transport studies of the prepared materials are obtained

from frequency analyzer. Finally, CR2032 coin cell are fabricated by using prepared pure

and carbon coated LiMPO4 (M=Mn, Co & Ni) nanoparticle cathode materials. The

discharge characteristics of fabricated CR2032 coin cell are recorded using battery cycle

tester.

32

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41

EXPERIMENTAL TECHNIQUES

CHAPTER – II

42

CHAPTER – II

EXPERIMENTAL TECHNIQUES

2.1 Introduction

2.2 Synthesis processes for the preparation of nanocrystalline and

nanocomposite materials

2.2.1 Sol-gel process

2.2.2 Combustion process

2.2.3 Hydrothermal process

2.2.4 Polyol process

2.2.5 Co-precipitation process

2.2.6 Sonochemical process

2.2.7 Other wet chemical processes

2.3 Characterization techniques

2.3.1 Thermogravimetric and differential thermal analysis (TG/DTA)

2.3.2 Differential scanning calorimetry (DSC)

43

2.3.3 X- ray powder diffraction (XRD)

2.3.4 Raman spectroscopy

2.3.5 Scanning electron microscope (SEM)

2.3.6 Transmission electron microscope (TEM)

2.3.7 UV-VIS-NIR absorption

2.4 Impedance measurements

2.5 Fabrication of CR2032 Coin cell

2.6 Electrochemical characterization

References

44

2.1 Introduction

Nanocrystalline materials have superior physical and chemical properties over the bulk

materials, which are due to the high surface to volume ratio when the particles are in

nano size. Number of atoms located on the surface are more in nanocrystalline materials

than the bulk and hence, exhibits the enhanced physical and chemical properties [1-5]. In

the case of lithium ion battery electrodes, nanocrystalline material minimizes the lithium

ion diffusion length and the lithium ion accommodation, as nanocrystalline materials

have more surface area.

The electrochemical properties and cycle life of lithium ion batteries mainly depends on

the structural stability of cathodes. Structural deformation occurs during charge and

discharge cycles, which reduces the life time of the battery [6]. In order to avoid these

problems, researchers have tried to coat the electrode material with the oxide materials

like Al2O3, La2O3 and SiO2 etc. as well as doping trivalent and divalent cations (Mg2+,

Ca2+, La3+, etc) to stabilize the structure under different physical and chemical conditions

[7, 8]. Synthesis of nanoparticles mainly depends upon the synthesis temperature and

hence, controlling the growth of the crystals to nano level with particular morphology is

difficult even in a well controlled temperature. This is because the nanoparticles are

highly reactive due to the occupancy of majority of atoms at the surfaces, consequently

the nanoparticles agglomerate when the temperature is changed [9, 10]. In order to obtain

the low agglomeration and also nanosized crystals, it is grown in the polymeric or glassy

or ceramic matrices, which forms nanocomposite [11-13]. The supporting matrix is used

to stabilize the nanocrystals with a controlled growth/loading level, where the

45

nanocrystals were embedded in the supporting matrix to form the nanocomposite

structure [14]. Thus, series of efforts have been made to develop the synthesis processes

to prepare single and multi-component nanocrystalline metal oxides dispersed in the

matrix in various forms [15, 16]. Hence, the preparation of nanocrystalline pure and

carbon coated LiMPO4 (M = Mn, Co & Ni) materials have become important, since they

are used in the lithium batteries as cathode materials. The wet chemical methods like

polyol and novel resin coating processes are used for the preparation of nanocrystalline

materials [17–20]. This chapter describes the processes used for the preparation of

nanocrystalline and nanocomposite materials as well as characterization techniques.

2.2 Synthesis processes for the preparation of nanocrystalline and nanocomposite

materials

2.2.1 Sol-gel process

Sol–gel is the wet chemical technique to prepare polycrystalline glass and glass ceramic

materials in bulk as well as nano size, relatively at low temperature. Through this

process, homogeneous inorganic oxide materials with desirable properties of hardness,

optical transparence, chemical durability, tailored porosity, and thermal resistance, can be

produced at room temperature. The sol- gel process, as the name implies, involves the

evolution of inorganic networks through the formation of a colloidal suspension (sol) and

gelation of the sol to form a network in a continuous liquid phase (gel). According to the

chemical mechanism, sol-gel process can be divided into two distinct routes. [21, 22]

46

1. Colloidal route

2. Chemical polymerization route

2.2.1.1 Colloidal route

A colloid is a suspension in which the dispersed phase is very small ~ up to 100 Å. Here,

the gravitational forces are negligible and interactions are due to short-range forces. The

inertia of the dispersed phase is so small that it exhibits Brownian motion. This route

involves dispersion of colloidal particles in a liquid to form a sol. Then this sol is

destabilized to form a gel.

2.2.1.2 Chemical polymerization route

In chemical polymerization route, an appropriate chemical is added to the solution of

organometalic compounds or metallic salts to obtain sol. Further, the sol transforms into

gel at low temperature. Aging and drying of gels at low temperature produce

polycrystalline material. The seven steps involved in this process of material synthesis

are mixing, casting, gelation, aging, drying, dehydration and densification. Different

stages involved in the sol-gel process are represented pictorially and it is shown in fig.

2.1.

The gel is obtained by adjusting a particular pH. In chemical polymerization route,

mixing involves hydrolysis and condensation. The liquid alkoxide precursor (Si(OR)4,

(R= CH3, C2H5, C3H7, etc)) is hydrolyzed by mixing with water. Metal alkoxide are

popular precursors, because they react readily with water and it is called hydrolysis with

the following reactions.

47

Figure 2.1: Schematic diagram of various stages involved in sol-gel process

Wet gel

Xerogel

Dense ceramics

Sol Alkoxide Solution

Acid catalyst growth Base catalyst growth

48

Depending upon the amount of water and catalyst, the hydrolysis may go up to

completion or stop up to a particular length of required network. The hydrolyzed

molecule can link together through condensation reaction to form the 1D, 2D and 3D

networks. During condensation reaction, alcohol or water is liberated.

Si (OR)4 + H2O → HO – Si (OR)3 + ROH

(or)

OR OH

| |

RO –Si – OR + 4H2O → HO – Si – OH + 4ROH

| |

OR OH

49

OR OR OR OR

| | | |

RO – Si - OR + HO – Si – OR → RO – Si – O – Si - OR + ROH

| | | |

OR OR OR OR

OH OH OH OH

| | | |

HO – Si – OH + HO – Si – OH → HO – Si – O – Si – OH + H2O

| | | |

OH OH OH OH

Casting

The prepared condensed sol can be cast into a mould. The mould must be selected to

avoid adhesion of the sol.

50

Gelation

The condensation reaction builds up larger and larger by the process of polymerization to

form three-dimensional network of macroparticle. When sol becomes a gel, it can support

stress elastically and this is defined as the gelation point or gelation time. The gelation

time will depend upon the temperature, solvent, pH condition and also removal of the

solvent.

Aging

During aging, the casted object is maintained for a period of time. The polycondensation

continues and the liquids are expelled from the pores. This increases the thickness of the

particle necks and decreases the porosity. By aging, strength of the gel increases.

Drying

The existing liquid in the interconnected pore network is removed during drying. Thus,

there is a decrease in the volume of the gel, which is equal to the volume of the liquid lost

by evaporation. After drying, the liquid in the pores is emptied.

Dehydration

During dehydration, chemically stable solid is obtained. This is due to the removal of

unwanted surface elements like H and R from Si-OH and Si-OR bonds.

Densification

Densification is the last step in the sol gel process. This porous gel is heated to a higher

temperature. The pores can be eliminated and densified glasses can be obtained like fused

51

quartz or fused silica. The densification temperature depends on the dimensions of the

pore network, the connectivity of the pores, surface area, etc.

2.2.1.3 Advantages of sol-gel process

The chemicals are in solution form. The reaction is in the atomic level. Better

homogeneity and better purity are possible.

It is a low temperature preparation process. This leads to saving of energy,

minimum evaporation losses, minimum air pollution, no reaction with the

container, no phase separation.

New non-crystalline solids outside the range of normal glass formation can be

prepared. New crystalline phases are formed from new non-crystalline solids.

Better glass products can be formed from the special properties of gels.

Special products like films and fibers can be formed. Mixing of organic and

inorganic compounds is possible.

2.2.1.4 Disadvantages of sol-gel process

High cost of raw materials and health hazards of organic solutions.

Long processing time, shrinkage and difficulty in producing large pieces.

Micro porosity, residual hydroxyl group and carbon change the properties.

2.2.2 Combustion routes

52

2.2.2.1 Combustion of nitrates solutions

Metal nitrates generally decompose on heating to yield the oxides with evolution of

gaseous nitrogen oxides. If the nitrates are rapidly heated the evolved gas would be

“violent” resulting in the formation of fine oxide powders. The rapid evolution of gas

would not allow agglomeration of the oxide particle [23].

2.2.2.2 Combustion of fuel – oxidant

This combustion technique involves an exothermic decomposition of a fuel – oxidant

precursors such as urea- nitrate, glycine-nitrate, DHF- nitrate, etc, relatively at lower

temperatures [24]. This method explores highly fast and self sustaining exothermic

reaction between the metal salts and organic fuels. The heat required for the phase

formation is supplied from the reaction and not from an external source. During the

ignition process, large volume of gases will evolve, which prevent the agglomeration and

lead to the formation of fine powders with nano structures. The release of heat during the

combustion reaction depends on the fuel–oxidant stoichiometry in the precursor

composition. The fuel–oxidant stoichiometry is calculated based on the thermodynamical

concepts used in the field of propellants and explosives, for the required nature of the

combustion process [25].

2.2.2.3 Polymeric precursor’s route

The polymeric precursor route is known to be simple cost effective and versatile low

temperature route for the synthesis of multicomponent metal oxides relatively at low

temperatures. The general idea of this process is to distribute the metal ions atomistically

53

through the polymeric structure and to inhibit their segregation and precipitation from the

solution [26]. Further heating these polymeric intermediates yields ultra fine nano

crystalline metal oxides. Generally, hydroxyl carboxylic acids such as citric acid tartaric

acid, etc, are used as a polymerizing as well as chelating agents in this process. The

physiochemical properties of the synthesized powders are critically depend on the

properties of polymeric intermediates, which influence on the combustion parameters

such as ignition temperature, heat evolution, combustion duration, etc. Hence, wide

ranges of polymeric precursors have been investigated in order to control the structural

properties of final products [27].

2.2.2.4 Advantages of combustion methods

Gel combustion methods show advantages over the earlier mentioned processes mainly

due to the following reasons,

Low cost and low temperature process.

Better control over stoichiometry of the compound.

Size of the powders produced by these methods is invariably in the nanometer

range.

Exothermic reaction gives product almost instantaneously.

Possibility of multi component oxides with high surface area.

2.2.2.5 Disadvantages of combustion methods

54

Contamination due to carbonaceous residue, particle agglomeration, no control on

particle morphology.

Reliable data on fuel characteristics are needed to achieve the required properties.

Violent reaction needs special production.

2.2.3 Hydrothermal process

Hydrothermal synthesis has attracted great interest because, it is a promising route to

produce highly crystallized, weakly agglomerated powders having a narrow size

distribution. Water has been effectively used for this process as pressure transmitting

medium and solvent for the precursor chemicals. An autoclave is used to achieve the

required hydrothermal condition. The pressure is in the range of 10 to 150 Kilo bar.

Several compositions were prepared by crystallization under hydrothermal conditions.

Simple evidence is available to show that this process has some advantages in controlling

particle size, morphology and other characteristics by adjusting reaction temperature,

duration of the process, additives and other factors [28].

2.2.3.1 Advantages of hydrothermal process

• Possibility of controlling particle size and shape by using different precursor

materials and hydrothermal conditions.

• The prepared materials are highly reactive and low temperature sintering is

enough.

55

2.2.3.2 Disadvantages of hydrothermal process

Prior knowledge of solubility of starting material is essential.

Accidental explosion of high pressure vessel is possible.

2.2.4 Polyol process

In polyol process, the precursor material is suspended in a liquid polyol. The mixture is

stirred and heated to a boiling point of the polyol to get the metals [29, 30]. The reduction

of the starting precursor quantitatively yields the metal as a fine powder. The main

advantage of this reaction mechanism is that the reduction reaction proceeds in the

solution rather than in the solid state. Hence, the metal particles are formed by nucleation

and growth from the solution. According to the reaction mechanism, the polyol acts first

as a solvent for the starting inorganic precursors due to its high dielectric constant of

these organic media. In ethylene glycol, for instance, salts such as cobalt, nickel or

copper acetate hydrate are soluble to such an extent that a complete dissolution is

observed as the first step of the reaction. Nevertheless, in most cases, the starting

compound is only slightly soluble but this solubility reaction mechanism is high enough

to allow the reaction like crystallized hydroxide → intermediate phase and intermediate

phase → metal. The metal is generated in the liquid phase and the nucleation and growth

occur when the supersaturation is high enough. Figure 2.2 shows the photograph of

polyol process experimental set up.

2.2.4.1 Advantages of polyol process

• Low temperature process

56

• Possibility of controlling the particle size, shape and distribution

Figure 2.2: Photograph of experimental setup of polyol process

2.2.4.2 Disadvantages of polyol process

Large amount of polyhydroxy alcohol is required

Phase separation in multicomponent metal oxide preparation

57

Purification of intermediate is complicated and tedious

2.2.5 Co-precipitation process

Co-precipitation is an attractive method of producing metal oxides because of increased

homogeneity, purity, and reactivity over standard ceramic processing. The chemical

species most frequently used are the hydroxides, oxalates, and carbonates [31]. The

hydroxides are gelatinous, leading to filtration difficulty and, in addition, cause loss of

some ions by complexing, particularly when ammonia is the precipitant. Oxalates are

costly to produce and the oxalic acid cannot be recycled. Carbonates do not form from

aqueous ferric solutions and when formed from ferrous salts, give poor ceramic reactivity

when decomposed. Control over pH, concentration of solution and stirring speed is

essential to obtain the final product with required properties.

2.2.5.1 Advantages of co-precipitation process

Simple and direct process for preparing fine metal oxide powders

Homogeneous mixing and reactant precipitates reduces the reaction temperature

2.2.5.2 Disadvantages of co-precipitation process

o Accurate stoichiometry cannot be obtained

o Precursor materials with different solubility and precipitate rate cannot be used.

58

2.2.6 Sonochemical process

Sonochemical process uses the application of ultrasound for chemical reactions and

processes. Ultrasound is the part of the sonic spectrum which ranges from about 20 kHz

to 10 MHz and can be roughly subdivided in three main regions: low frequency, high

power ultrasound (20-100 kHz), high frequency, medium power ultrasound (100 kHz-1

MHz), and high frequency, low power ultrasound (1-10 MHz). The range from 20 kHz to

around 1 MHz is used in sonochemical synthesis [32]. In this process, cavitation of

microbubbles, which are created during the rarefaction (or negative pressure) period of

sound waves. The cavitational collapse creates drastic conditions such as temperatures of

2000-5000 K and pressures up to 1800 atm. inside the collapsing cavity for an extremely

short time. The collapse causes a couple of strong physical effects outside the bubble are

shear forces, jets and shock waves. Thus, there are basically two groups of effects, radical

and mechanical effects. These cavitation-induced effects can cause physical, chemical,

and biological effects also this effect is enough for the material synthesis.

2.2.6.1 Advantages of sonochemical process

Decrease of reaction time and increase of yield

Lower processing temperature

Possibility to use different precursors

2.2.6.2 Disadvantage of sonochemical process

59

High cost instrument is required

2.2.7 Other wet chemical processes

Other than the above discussed processess, there are many other wet chemical processes

available for the synthesis of nanocrystalline and nanocomposite metal oxides such as

micro-emulsion, microwave synthesis, etc. Some of the processes are discussed below.

2.2.7.1 Microemulsion

Microemulsion methods provide a synthetic approach that allows for high-quality

nanoparticles with a narrow size distribution to be made. Furthermore, by minor

adjustments to the synthesis conditions, this approach easily allows the size of

nanoparticles to be controlled by means other than thermal annealing of the nanoparticles

at various temperatures [33]. In other methods, the crystalline phases are frequently

developed during calcinations from the amorphous room temperature phases, whereas in

the microemulsion technique, crystalline phases may be formed during synthesis. It is

also easier to produce well-dispersed nanosized particles with a narrow size distribution

in microemulsions, and also processing costs are less compared with other production

techniques of nanosized powders.

2.2.7.2 Microwave synthesis

Microwaves are a part of the electromagnetic spectrum with frequencies ranging from

300 MHz to 300 GHz and the corresponding wavelengths are 1 m to 1 mm. The most

commonly used frequency is 2.45 GHz. The degree of interaction of microwaves with a

dielectric medium is related to the material dielectric constant and dielectric loss [34].

60

When microwaves penetrate and propagate through a dielectric solution or suspension,

the internal electric fields generated within the affected volume induce translation

motions of free or bound charges such as electrons or ions and rotate charge complexes

such as dipoles. The resistance of these induced motions due to inertial, elastic, and

frictional force, which is frequency dependent, causes losses and attenuates the electric

field. Compared to ultrasonic irradiation, there is no similar bubble formation during

microwave heating but superheating occurs in localized spots.

2.3 Characterization techniques

2.3.1 Thermogravimetric and differential thermal analysis (TG/DTA)

TG/DTA involves measuring thermal changes while measuring change in weight using a

thermo balance. This is a combination of both thermogravimetric and differential thermal

analysis techniques.

2.3.1.1 Thermogravimetric analysis (TGA)

Thermogravimetry (TG) measures the change in mass of a material as a function

temperature over a temperature range using a predetermined heating rate. Essentially, a

TG consists of a microbalance surrounded by a furnace. A computer records any mass

gains or losses. Weight is plotted against a function of time for isothermal studies and as

61

a function of temperature for experiments at constant heating rate. Thus, this technique is

very useful in monitoring heat stability and loss of components [35].

2.3.1.2 Differential thermal analysis (DTA)

DTA involves heating or cooling a test sample and an inert reference under identical

conditions, while recording any temperature difference between the sample and

reference. This differential temperature is then plotted against time, or against

temperature. Changes in the sample absorption or evolution of heat can be detected

relative to the inert reference. DTA can therefore be used to study thermal properties and

phase changes, which do not lead to a change in enthalpy. A DTA curve can be used as a

finger print for identification purposes. The area under a DTA peak can be the enthalpy

change and is not affected by the heat capacity of the sample. DTA may be defined

formally as a technique for recording the difference in temperature between a substance

and a reference material against either time or temperature as the two specimens are

subjected to identical temperature regimes in an environment heated or cooled at a

controlled rate [36].

2.3.2 Differential scanning calorimetry (DSC)

Differential scanning calorimetry or DSC is a thermal technique in which the difference

in the amount of heat required to increase the temperature of a sample and reference are

measured as a function of temperature. In this experiment, the sample and reference are

maintained at nearly the same temperature. DSC is used to identify phase transitions,

62

melting, glass transitions, or exothermic decompositions. The above transitions involved

in energy changes that can be detected by DSC with great sensitivity [37]. There are two

types of DSC systems in commercial use and they are power compensation DSC where,

the temperatures of the sample and reference are controlled independently using separate,

identical furnaces. The temperatures of the sample and reference are made identical by

varying the power input to the two furnaces and the energy required to do this is a

measure of the enthalpy or heat capacity changes in the sample relative to the reference.

In heat flux DSC, the sample and reference are connected by a low resistance heat flow

path. The assembly is enclosed in a single furnace. Enthalpy or heat capacity changes in

the sample cause a difference in the temperature relative to the reference and the resulting

heat flow is small compared with that in differential thermal analysis (DTA) because the

sample and reference are in good thermal contact. The temperature difference is recorded

and related to enthalpy change in the sample using calibration experiments. DSC plot is a

curve of heat flux versus temperature. The exothermic reactions in the sample are shown

with a positive peak. This curve can be used to calculate enthalpies of transitions. This is

done by integrating the peak corresponding to a given transition. The enthalpy of

transition can be expressed using the following equation:

ΔH = KA

Where, ΔH is the enthalpy of transition, K is the calorimetric constant, and A is the area

under the curve. The calorimetric constant will vary from instrument to instrument, and

can be determined by analyzing a well-characterized sample with known enthalpies of

transitions.

63

The thermal behavior of the polymeric intermediate is analyzed using differential

scanning calorimetry as well as thermogravimetric and differential thermal analysis.

Approximately, 7 mg of PI sample was placed in an alumina crucible and heated at a rate

of 10 K per minute from 333 K to 773 K under nitrogen atmosphere and the differential

thermal analysis and thermal gravimetric curves were recorded using SETARAM Labsys,

France, TG-DTA instrument from ambient to 873 K. Similarly, DSC measurement was

carried out for the polymeric intermediate between 303 and 773 K at a heating rate of 10

K per minute under nitrogen atmosphere using Mettler Toledo Star e system module DSC

821e/500/575/414183/578.

2.3.3 X- ray powder diffraction (XRD)

X-ray diffraction is based on constructive interference of monochromatic X-rays and a

crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce

monochromatic radiation, collimated and directed towards the sample. The interaction of

the incident rays with the sample produces constructive interference when it satisfies the

Bragg condition nλ=2d sin θ. This law relates the wavelength of electromagnetic

radiation to the diffraction angle and the lattice spacing in a crystalline sample [38]. The

prepared sample grounded well and filled in the sample holder and mounted on the X-ray

diffractometer. The powder X ray diffraction patterns were recorded between 10 and 80o

with the scan rate of 1.5o per minute using PANalytical X’ Pert PRO MPD X-ray

diffractometer using Cu Kα radiation. Figure 2.3 shows the photograph of the

PANalytical X’ Pert PRO MPD X-ray Diffractometer. The obtained patterns were

compared with JCPDS data for confirming the formation of the crystalline phase.

64

Crystallite size calculation:

The broadening of crystalline peaks is due to instrument broadening as well as due to

sample crystallite size reduction. The peak broadened with crystallite size decrement. The

crystallite size of the sample is calculated using Scherrer’s formula.

D = 0.9 λ/β1/2 cosθ

Where, λ is the wavelength of X rays, θ Bragg angle in degree and β1/2 full width at half

maximum. β1/2 is calculated using the following relation,

β1/2 = (βM – βS)1/2

Where, βM is the FWHM of the sample, βS is the FWHM of the Si standard. The peak

corresponds to (111) plane of Si standard. NBS silicon standard was used for estimating

the instrumental broadening.

Figure 2.3: Photograph of the PANalytical X’ Pert PRO MPD X-ray diffractometer

65

2.3.4 Fourier transform infrared spectroscopy (FTIR)

Infrared spectroscopy is a technique based on the vibrations of the atoms of a molecule.

An infrared spectrum is commonly obtained by passing infrared radiation through a

sample and determining the fraction of the incident radiation is absorbed by the sample at

a particular energy. Vibrational spectra are the characteristic of the material and are

specific to the chemical bonds. Changes in the chemical characteristics of matter are

reflected in the vibrational spectra [39]. Vibrational energies are much smaller as

compared to the chemical bond energies and even minute changes in the local

atmosphere of a sample are reflected in the spectra. For a molecule to show infrared

absorptions, it must possess a specific feature, i.e. an electric dipole moment of the

molecule must change during the vibration. The dipole moment of such a molecule

changes as the bond expands and contracts. The equation relating the force constant, the

reduced mass and the frequency of absorption is:

ν = (1/2π)√(k/μ)

A molecule can only absorb radiation when the incoming infrared radiation is of the same

frequency as one of the fundamental modes of vibration of the molecule. Vibrations can

involve either a change in bond length (stretching) or bond angle (bending). Some bonds

can stretch in-phase (symmetrical stretching) or out-of-phase (asymmetric stretching).

Fourier-transform infrared (FTIR) spectroscopy is based on the idea of the interference of

radiation between two beams to yield an interferogram. The latter is a signal produced as

a function of the change of path length between the two beams. The two domains of

distance and frequency are inter-convertible by the mathematical method of fourier-

66

transformation. The fine powdered polymeric intermediate as well as calcined polymeric

intermediate at different temperatures mixed with spectral pure KBr powder in 1:20 ratio,

thin transparent pellets were made using KBr press and used for the FTIR spectral

measurements using Schimadzu FTIR/8300/8700 spectrophotometer in the range of 4000

– 400 cm–1 with 2 cm–1 resolution for 20 scans.


Raman spectroscopy is a spectroscopic technique based on inelastic scattering of

monochromatic light, usually from a laser source. Inelastic scattering means that the

frequency of photons in monochromatic light changes upon interaction with a sample.

Photons of the laser light are absorbed by the sample and then reemitted. Frequency of

the reemitted photons is shifted up or down in comparison with original monochromatic

frequency, which is called the Raman effect. This shift provides information about

vibrational, rotational and other low frequency transitions in molecules. Raman

spectroscopy can be used to study solid, liquid and gaseous samples.

A sample is normally illuminated with a laser beam in the ultraviolet (UV), visible (Vis)

or near infrared (NIR) range. Scattered light is collected with a lens and is sent through

interference filter or spectrophotometer to obtain Raman spectrum of a sample. A

molecule with no Raman-active modes absorbs a photon with the frequency υ0. The

excited molecule returns back to the same basic vibrational state and emits light with the

same frequency υ0 as an excitation source. This type of interaction is called an elastic

67

Rayleigh scattering. A photon with frequency υ0 is absorbed by Raman-active molecule

which at the time of interaction is in the basic vibrational state. Part of the photon’s

energy is transferred to the Raman-active mode with frequency υm and the resulting

frequency of scattered light is reduced to υ0 - υm. This Raman frequency is called Stokes

frequency, or just “Stokes”. And photon with frequency υ0 is absorbed by a Raman-active

molecule, which, at the time of interaction, is already in the excited vibrational state.

Excessive energy of excited Raman active mode is released, molecule returns to the

basic vibrational state and the resulting frequency of scattered light goes up to υ0 + υm.

This Raman frequency is called Anti- Stokes frequency, or just “Anti-Stokes”. The

schematic diagram of custom built Raman spectrometer is shown in figure 2.4.

Figure 2.4: Schematic diagram of custom built Raman spectrometer

Synthesized carbon coated LiMPO4 (M= Mn, Co & Ni) samples were characterized by

the Renishaw InVia Laser Raman Microscope with 633 nm He-Ne laser source and it’s

confirmed that the presence of the carbon in the carbon coated LiMPO4 samples.

68

2.3.6 Scanning electron microscope (SEM)

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons

to generate a variety of signals at the surface of solid sample. The schematic diagram of

SEM instrument is shown in figure 2.5. The signals that derive from electron-sample

interactions reveal information about the sample including external morphology,

chemical composition and orientation of materials. Accelerated electrons in a SEM have

significant amounts of kinetic energy, and this energy is dissipated in different forms of

signals produced by electron-sample interactions when the incident electrons are

decelerated in the solid sample. These signals include secondary electrons (SE),

backscattered electrons (BSE), diffracted backscattered electrons, photons, visible light,

and heat. Secondary electrons and backscattered electrons are commonly used for

imaging samples. Secondary electrons are most valuable for showing morphology and

topography on samples and backscattered electrons are most valuable for illustrating

contrasts in composition in multiphase samples [40]. Small piece of polymeric

intermediate is sprayed over a conducting carbon tape, which is pasted over an

aluminium stub. Carbon is coated over the polymeric intermediate for good conduction.

Similarly, small amount of final product is sprayed over carbon tape and coated with

gold. The SEM images and SEM-EDS elemental mappings were taken using Hitachi,

SN-3400 N SEM instrument.

Elemental analysis:

The electron beam causes various excitations in the sample, which are characteristic of

the elements present in the material. Characteristic X-rays emitted by the sample can be

69

used for elemental identification. The intensity of the signal can be used for quantitative

analysis. When a fast moving electron approaches the atom, it gets decelerated due to the

coulombic field. This results in a loss of energy for the electron and that energy appears

as photon, referred to as breaking radiation. The characteristic X rays emitted by the

atoms will appear as spikes over this smoothly varying photon intensity. There are

several characteristic X-ray lines with which an atom can be identified. The intensities of

these lines can be related to the concentrations of the emitting species in the sample.

Figure 2.5: Schematic diagram of SEM

70

2.3.7 Transmission electron microscope (TEM)

In TEM, the transmitted electrons are used to create an image of the sample. The

schematic diagram of TEM instrument is shown in figure 2.6. The transmission electron

microscope uses a high energy electron beam transmitted through a very thin sample to

image and analyze the microstructure of materials with atomic scale resolution. The

electrons are focused with electromagnetic lenses and the image is observed on a

fluorescent screen, or recorded on film or digital camera. The electrons are accelerated at

several hundred kV, giving wavelengths much smaller than that of light: 200 kV

electrons have a wavelength of 0.025 Å. However,

Figure 2.6: Schematic diagram of TEM

71

whereas the resolution of the optical microscope is limited by the wavelength of light,

that of the electron microscope is limited by aberrations inherent in electromagnetic

lenses, to about 1-2 Å. High resolution imaging mode of the microscope images the

crystal lattice of a material as an interference pattern between the transmitted and

diffracted beams. This allows one to observe planar and line defects, grain boundaries,

interfaces, etc. with atomic scale resolution [41]. The bright field/dark field imaging

modes of the microscope, which operate at intermediate magnification, combined with

electron diffraction, are also invaluable for giving information about the morphology,

crystal phases, and defects in a material. The samples are dispersed in absolute ethanol

under sonication and were deposited over the dipped carbon coated copper grid. This is

used for taking TEM images and selected area diffraction pattern. The particles sizes

were examined by JEOL-2010F TEM instrument with an accelerating voltage of 200 kV.

2.3.9 UV-VIS-NIR optical absorption

The UV-visible EM radiation causes electronic transitions within a molecule, promoting

bonding and non-bonding electrons to higher, less stable antibonding orbitals. When a

sample of an unknown compound is exposed to light, certain functional groups within the

molecule absorb light of different wavelengths in the UV or visible or NIR region. UV-

VIS-NIR spectroscopy is used for qualitative and quantitative analysis of materials. It

measures the intensity of light passing through a sample (I), and compares it to the

intensity of light before it passes through the sample (Io) [43]. The ratio I / Io is called the

72

transmittance, and is usually expressed as a percentage (%T). The absorbance, A, is

based on the transmittance:

A = − log (T% / 100%)

The sample is placed in the sample holder and the UV–VIS–NIR absorption spectra were

recorded using a Carry 5000 spectrophotometer (Varian, Palo Alto, CA) in the spectral

range from 200 to 1500 nm, the spectrometer resolution being 2 nm, for the prepared fine

powder in the diffuse reflectance mode.

2.4 Impedance measurements

Impedance spectroscopy is superior over other laboratory techniques by using very small

amplitude voltage signals without significantly disturbing the properties being measured.

Such small AC voltage perturbations cause variations in impedance in a way that is

related to the properties of the material under investigation. This may be due to the

physical structure of the material, chemical processes within it or to a combination of

both. Consequently, impedance spectroscopy is constantly used as a nondestructive

technique for providing accurate and repeatable measurements regarding surface

conditions such as adsorption and desorption processes at electrode surfaces. Impedance

is the opposition to flow of current, which is given by the ratio of the applied voltage to

the resultant current. Impedance spectroscopy is a powerful technique used for electrical

characterization of an electrolyte material. It is a perturbation technique, which involves

73

the measurement of the current through a solid electrolyte when a sinusoidal voltage of

low amplitude is applied [44, 45]. The photograph of Impedance analyzer (Alpha-A high

frequency analyser, Nova control, Germany) is shows in figure 2.7.

Figure 2.7: Photograph of Impedance analyzer (Nova control, Germany)

74

2.4.1 Basic theory

A small amplitude A.C signal is used in impedance measurement to perturb the system

and the applied potential is given by,

E = Eo exp (jωt)

The output current of the system is also a sinusoidal and represented by,

I = Io exp (jωt +φ)

According to the Ohm’s law, impedance of the circuit (Z) at any angular frequency ω can

be represented by,

Z = E/I = ( Eo / Io )exp (-jφ)

= Zo exp (-jφ)

= Z cosφ - jZ sinφ

Z = Zr - jZi

Where j is the complex number having the value of √ -1, Zr and ZI are real and imaginary

parts of the impedance. The phase difference is represented by

φ = tan-1(jZi /Zr)

For pure resistor (R), capacitor (C) and inductor (L) impedance is given by the following

representations:

Z = R + j 0

75

Z = 0 - j/ωC

Z = 0 + jωL

From above equations it is seen that the impedance due to capacitor and an inductor

depend on the frequency of the input signal. The plot of real (in phase) and imaginary

(out of phase) parts is known as impedance spectrum and it appears as semi circles or

straight lines depending on the combination. Apart from impedance, three other

important representations are used in complex plane analysis, and they are given below.

Admittance

Y=Z-1= Yr + j Zi

Permittivity

ε = A/jωCo=εr-jεi

Modulus

M = ε-1=Mr+jMi

Where Co = εo (A/L), εo = 8.854 x 10-12 F m-1 and (A/L) is cell constant.

The complex permittivity and modulus representations are used for analyzing the

dielectric response of the system.

76

2.4.2 Equivalent circuit representation

The electrical behavior of any sample can be represented in the form of equivalent

circuit, which represents the various current conducting elements in the sample [45].

2.4.2.1. Series combination of R and C

A circuit containing a resistance and a capacitance in series is shown in fig. 2.8. The

voltage drop across the circuit, E, is given by E=E1+E2 and therefore, the total impedance

of the circuit is given by

Z = R + (1/jωC) = R- (j/ωC)

The impedance contains real and imaginary terms as indicated below:

Z' = R, Z" = 1/ωC

The complex impedance plot gives a vertical spike as shown in fig 2.8a, because Z’ is of

fixed value and Z” decrease with increasing ω.

2.4.2.2. Parallel combination of R and C

A circuit containing a resistance and a capacitance in parallel is shown in fig 2.8b. Since

resistance is in parallel reciprocal of complex impedance called complex admittance,

A is given by, A = 1/Z = 1 / R + jωC

The complex impedance,

77

Figure 2.8: Schematic diagrams of impedance plots for the a) series and

b) parallel combination of R and C

Z = (A)-1 = (1 / R + jωC)-1

= R/ (1 + jωC)

= {R / (1 + (ωRC)2)} – {RjωRC / (1+(ωRC)2)}

Therefore,

Z' = R / (1 + (ωRC)2) and Z" = RωRC / (1+(ωRC)2)

a b

78

The complex impedance plot for the circuit containing a resistance and a capacitance in

parallel is shown in fig.2.8.

The semi circle has intercepts on the Z' axis at zero and R; the maximum of the semi-

circle equals 0.5R and occurs at a frequency when ωRC =1. For more complex circuits,

each parallel RC element gives rise to a semi circle in the complex Z-plane. In the present

work, Novacontrol high performance frequency analyzer is used for impedance

measurements. The measured impedance data were analyzed using winfit software and

evaluated the bulk/sample resistance and its electrical behavior in the form of equivalent

circuits. The conductivity of the pellet sample is calculated from the dimension of the

pellet as well as the bulk resistance obtained through win fit software using the relation,

σ = t/RA

Where, “t” is the thickness of the pellet, “A” = Area of the pellet

“R” is the bulk resistance of the sample.

The activation energy (Ea) is calculated from the slope obtained from Log σT vs. 1000/T

plot

79

2.5 Fabrication and Characterization of CR2032 Coin cell

2.5.1 Preparation of electrodes for coin cell & Fabrication of CR2032 Coin cell

The electrode materials were hand mixed with 15 wt% carbon black (C65, Timcal

cooperation, USA) and 5 wt% PVDF (Sigma Aldrich) binder in N-Methyl-2-pyrrolidone

(NMP) solvent by mortar and pestle to form viscous slurry. Electrodes were prepared by

coating slurries either of cathode disc or on aluminum foil and dried in vaccum oven at

80 0C for 4 hr to remove the moisture from the cathode materials. Before going for the

fabrication, cathode discs should be kept in the desiccator to remove the remaining water

molecules from material to avoid the oxidation of the lithium metal which is using as

anode. CR2032 coin cells were assembled for all electrochemical testing which are

purchased from Hohsen Crop., USA. . A microporous plastic film (Cellgard 2400,

Cellgard Co., USA) was used as separator and the electrolyte solution was comprised of

1.5 M LiPF6 in a 1:1:1 mixture of ethylene carbonate (EC), eythyl methyl carbonate

(EMC), and dimethyl carbonate (DMC) in weight percent procured from Sigma Aldrich.

We used the DMC: DEC: EMC (1:1:1) ratio electrolyte for the LiCoPO4 and LiNiPO4

cathode materials cells.

Coin cell were fabricated in argon filled glow box (OMINI, VAC Atmosphere, USA)

with both moisture and oxygen concentrations less than 0.1 ppm, as shown in figure 2.9

and the total coin cell fabrication process flowchart is shown in figure 2.10.

80

Figure 2.9: Photograph of the VAC atmosphere glow box

81

Figure 2.10: Flowchart of the total coin cell fabrication process

Binder PVDF NMP

Positive Active Material LiMnPO4 (M=Mn, Co &Ni)

Conductive agent Carbon black

Mixing and coated on the Aluminum foil

Drying in vacuum oven at 80 oC for 8 h

Sealing all cathode/separator/anode using CR2032 Coin cell parts

24 h aging at RT before testing coin cells

82

2.5.2 Charge/discharge tests:

Charge/discharge tests were carried out by using a battery cycle tester (BT2000 with

voltage range as + 6V and current as + 100 mA to +10 μA, ARBIN Instruments, USA)

with a computer (PC) and software, which is shown in figure 2.11. The system is

capable of switching between charge and discharge automatically, according to the cut

off potentials set. All the coin cells, which were prepared by the pure and carbon coated

LiMnPO4 nanorods tested between 2.9 and 4.5 V at room temperature for 30 cycles. But,

LiCoPO4/Li and LiNiPO4/Li cells were tested between 4 and 5.1 V for 20 cycles.

Figure 2.11: Battery cycle tester, ARBIN Instrument, USA.

83

We constrained the number of cycles because, In the high voltage the electrolyte form a

poly(ethylene carbonate) (PEC) derived from the oxidative polymerization of EC and it

act as a lithium ion barrier. Hence, for more than 4.5 V researchers are using DMC:

DEC: EMC ratio as 1:1:1 electrolyte up to 5 V.

84

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88

SYNTHESIS OF PURE AND CARBON

COATED LiMnPO4 (M= Mn, Co & Ni)

NANOPARTICLES BY POLYOL AND

RESIN COATING PROCESSES

CHAPTER – III

89

CHAPTER – III

SYNTHESIS OF PURE AND CARBON COATED LiMPO4 (M= Mn,

Co &Ni) NANOPARTICLES BY POLYOL AND RESIN COATING

PROCESSES.

3.1 General introduction

3.2 Synthesis of pure LiMPO4 (M= Mn, Co &Ni) nanoparticles using PVP

assisted polyol process

3.2.1 Introduction

3.2.2 Synthesis of pure LiMPO4 (M= Mn, Co &Ni) nanoparticles

3.3 Preparation of carbon coated LiMPO4 (M= Mn, Co &Ni) nanoparticles

by using resin coating Process

3.3.1 Novelty of resin coating process

3.3.2 Preparation of carbon coated LiMPO4 (M= Mn, Co &Ni)

nanoparticles

3.4 Conclusions

References

90

3.1 General Introduction Olivine structured lithium transition metal phosphates are interesting cathode materials

for rechargeable lithium ion batteries due to their excellent thermal and structural stability

[1-6]. But these olivines are having some drawbacks like, defect formation, poor

electronic conductivity and distortion of the Jahn-Teller active M2+ ion, which were

obstacle for real applications. Recently, these problems have been solved by carbon

coating, metallic coating and doping techniques [7-9].

Basically, olivine structured type lithium metal phosphates are having one dimensional

diffusion path for lithium ions. These compounds have an open phosphate structure,

which provides an easy motion of lithium ions and the strong bonding of oxygen atoms

with phosphorus helps to avoid oxidation with the electrolyte. Hence, in this work

nanosize lithium metal phosphates were prepared to get the high power capability

through improving defect free lithium ion transport characteristics. There are several

techniques available to prepare nanoparticles. Among all, wet chemical process like, co-

precipitation, hydrothermal and polyol processes are effective to prepare nanoparticles

[10-13]. Particularly, the polyol process is very efficient to achieve nanoparticles with

high purity. In polyol process, we can control the nucleation growth in polyol medium

and also achieve less structural imperfections like anti-site defects, which act as obstacle

for lithium ion motion in local structure. The high pure olivine lithium metal phosphate

nanoparticles prepared by polyol process have high capacity and good cycleability due to

the defect free short diffusion path [14,15].

Another drawback in olivine lithium metal phosphate nanoparticles is its poor electronic

conductivity [16,17]. To overcome this problem, researchers have made composite

91

cathodes with conductive carbon by ball milling process. In this ball milling process,

acetylene black, sucrose…etc., are used as carbon source, but coating is not uniform. In

addition, it will give more carbon agglomeration in composite, which will cause blocking

for lithium ion migration [18]. The novel resin coating process gives very thin and

uniform coating upon olivine lithium metal phosphate nanoparticles to enhance the

electrochemical properties of materials. Most important drawback in olivine lithium

metal phosphate nanoparticles is Jahn-Teller distortion in M2+ ions [19].

Jahn-Teller distortion:

In an electronically degenerate state, a nonlinear molecule undergoes distortion to remove

the degeneracy by lowering the symmetry and thus by lowering the energy. In charging

state of LiMPO4 (M= Mn, Co & Ni), material will turn into MPO4, here the oxidation

state of M is 3+, which are Jahn-Teller effective ions in octahedral complex due to their

high or low spin configuration. The d-orbital in octahedral complex are dxy, dyz, dxz, dz2

and dx2

-y2

and in octahedral complex dz2 and dx

2-y

2 are having high energy due to the direct

interaction with negative legends of octahedral complex. The energy difference between

lower and higher energy orbital is called “crystal field splitting energy (∆) ”. In high spin

configuration, the crystal field splitting energy is less compared to paring energy and

hence the last electron will occupy the high energy orbital and the total spin is more than

1. If the crystal field splitting energy is higher than the paring energy then last electron

will occupy the lower energy orbital, hence it will give low spin configuration [20].

LiMnPO4: At discharging state of LiMnPO4/Li battery, LiMnPO4 cathode material

becomes MnPO4, here, the manganese becomes oxidized and form Mn3+. This Mn3+ ion

has high spin configuration due to the outer most electrons arrangement in d-orbital as

92

shown in Figure 3.1. The crystal field splitting energy (∆ ) is very less compared to

paring energy. Hence, four valence electrons in Mn3+ are occupying three lower energy

states dxy, dyz, dxz and one higher state dz2 respectively, the total spin is 2, which is called

as high spin configuration. Due to the high spin configuration [Mn3+: d4 (t2g3 eg

1) high

spin], the fourth electron occupies dz2 higher energy orbital, which is having directly

interaction with negative ligands of octahedral complex. Hence, the electron density is

more in Z-direction compared to the XY- direction. This asymmetrical distribution of

electron density leads increment in energy of the system. Finally, the octahedral complex

lowering energy through elongated the bonds between metal ion and ligands in z-

direction. Due to the deformation of octahedral complex in LiMnPO4 materials, one

dimensional lithium ion diffusion path will be shrinkage, so the capacity of battery will

decrease cycle by cycle, which is known as capacity fading. Figure 3.1 shows the Jahn-

Teller distortion in MnO6 octahedral complex [21].

LiCoPO4: At LiCoPO4/Li battery discharge state, the LiCoPO4 cathode material

becomes CoPO4. The Co3+ has six valence electrons, out of which, two electrons occupy

dxy state and remaining four electrons will occupy four dyz, dxz, dz2 and dx

2-y2 states. So,

the Co3+ has high spin configuration [Co3+: d6 (t2g4 eg

2)] with total spin value of 2. Here,

the distribution of electron density in all directions is equal. As a result, the total

octahedral complex will equally expand in X, Y & Z-directions. Because of the

expansion of octahedral complex, there is no impact on one dimensional lithium ion

diffusion path [22].

LiNiPO4: At LiNiPO4/Li battery discharge state, LiNiPO4 cathode material becomes

NiPO4. The oxidation state of Ni is 3+ with seven valence electrons. The crystal field

93

splitting energy (∆) is very high compared to paring energy. In seven valence electrons of

Ni3+, the first six electrons are occupying three lower energy states dxy, dyz, dxz and

remaining one electron will occupy higher state dx2

-y2

. The total spin is 1/2, which is

called as low spin configuration [Ni3+: d7 (t2g6 eg

1) Low Spin]. Due to the low spin

configuration the electron density is more in XY-direction compared to the Z- direction.

This asymmetrical distribution of electron density leads increment in energy of the

system. Finally, the octahedral complex lowering energy through elongates the bonds

between metal ion and ligands in XY-direction. The effect of octahedral complex

deformation in LiNiPO4 materials is negligible [23].

Figure 3.1: a) Jahn-Teller distortion in Mn3+ ion and b) changes in MnO6 octahedral

symmetry

a

b

94

To overcome this Jahn-Teller distortion in these olivine cathode materials, researchers

have made doping with the equal size divalent metal ions. Doping replaces Jhan -Teller

active metal ion in olivine structured cathode materials by divalent metal ions like Mg2+,

Ca2+, Zn4+ …etc.

In this chapter, PVP assisted polyol process was used for the synthesis of LiMPO4 (M=

Mn, Co & Ni) nanomaterial. Ethylene glycol was used as a solvent and PVP acted as a

stabilizer to prepare the nanoparticles in different shape and size [24, 25]. Acetates of

metals were used as precursors for the synthesis and the total solutions were heated up to

465 K for 2 h under continuous stirring. Finally, the formed precipitate at bottom of the

flask was washed with water and acetone followed by heated up to 873 K to remove all

organic and inorganic impurities from the sample. PVP assisted polyol process would

give the shape controlled nanoparticles to avoid the defects formation as well as Jhan-

Teller effect and the resin coating process was used for obtaining very thin and uniform

carbon coating over the nanoparticles. This conductive carbon coating was assumed to

improve electronic conductivity of the material [26].

3.2 Synthesis of pure LiMPO4 (M= Mn, Co &Ni) using PVP assisted polyol process

3.2.1 Introduction

Polyol is an alcohol having high hydroxyl groups, example: Polyethylene glycol (PEG),

Tetraethylene glycol (TEG), Ethylene glycol (EG)…etc. This polyol process is one of the

well known wet chemical processes to prepare the metal nanoparticles in different

shapes. While preparing metal nanostructures, polyol acts as a solvent as well as a

reducing agent. On the other hand, in metal oxides the same polyol acts as a solvent and a

95

chelating agent at higher temperature. Further, PVP is used as a stabilizer to obtain shape

controlled nanoparticles.

Olivine structured LiMPO4 (M= Mn, Co &Ni) nanoparticles were synthesized by PVP

assisted polyol process. Ethylene glycol was used as solvent in this process and metal

acetates were used as precursors for LiMPO4 (M= Mn, Co &Ni) nanoparticles. The total

solution was heated up to 465 K (just below the boiling point of ethylene glycol) under

distillation conditions to increase the solution pressure. At higher temperature, the rate of

dissolution of the intermediate phase, the rate of reduction in solution and rate of

spontaneous nucleation increase. Thus, the number of nuclei formed during the

nucleation burst increases. At the spontaneous nucleation, the added PVP acts as a

stabilizing agent to form shape controlled nanoparticles.

The dehydration reaction will occur in the polyol process at higher temperature, so

ethylene glycol will turn in to acetaldehyde and form glycolic acid. Glycolic acid has

high chelating property due to hydrophilic functional groups like hydroxyl and carboxyl

groups [27].

Hence, we used PVP assisted polyol process for synthesizing shape controlled LiMPO4

(M= Mn, Co &Ni) nanoparticles.

2 HO-CH2-CH2-OH 2 CH3CHO+2 H2O

2HO-CH2-COOH Glycolic Acid

96

Figure 3.2: Photograph of experimental setup of polyol synthesis process

97

3.2.2 Synthesis of pure LiMPO4 (M= Mn, Co &Ni) nanoparticles:

LiMPO4 (M= Mn, Co & Ni) nanoparticles were synthesized by PVP assisted polyol

process using lithium acetate [Li(CHOO), Fisher scientific company], metal acetates

[Mn(CHOO)2 for LiMnPO4, Co(CHOO)2 for LiCoPO4 & Ni(CHOO)2 for LiNiPO4 ,

Merck chemicals] and ammonium dihydrogen phosphate [NH4H2PO4, Merck chemical].

Ethylene glycol (EG), which was used as solvent, obtained from Fisher scientific

company and the stabilizer (PVP: Mw=106) procured from Sigma Aldrich. The required

stoichiometric quantities of metal acetates were mixed with ethylene glycol+ PVP

solution, by keeping end product to PVP ratio of 1:4 wt%, under constant stirring and

distillation condition as shown in figure 3.2. The resulting white colored solution was

stirred at 465 K for 2 h and it turned into a light brown colour solution with precipitation.

The precipitate was washed with acetone and water. Further, washed samples were

heated up to 873 K for 3 h to remove the organic and inorganic impurities, the flowchart

of the total polyol process was shown in figure 3.3. Figure 3.4 showing the photographs

of end product of a) LiMnPO4, b) LiCoPO4 & c) LiNiPO4 samples. All the samples were

cooled down to room temperature after the heat treatment, grounded and manually

pelletized with 3 ton pressure using 10 mm disc shaped mold. The final samples were

characterized by XRD, FTIR SEM, HRTEM, Raman and impedance spectroscopy.

98

Figure 3.3: Flowchart of the PVP assisted polyol process

Li acetate +

Metal acetate+ Ethylene Glycol (A)

NH4H2PO4 +

PVP +Ethylene Glycol (B)

Mixing of Solution A &

Solution B

Precipitation in bottom of the flask

Pure LiMPO4 (M=Mn, Co & Ni) Nanoparticles

Heated up to 465 K for 2 h

Washing and Heated up to 873 K for 3 h

99

Figure 3.4: Final prepared olivine structured LiMPO4 (M= Mn, Co & Ni) nanoparticle

samples

LiMnPO4 LiCoPO4

LiNiPO4

100

3.3 Preparation of carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles by

using resin coating Process

3.3.1 Novelty of resin coating process for the surface modification

Initially, different coating (surface modification) techniques were used with various

metals and metal oxide to avoid the instability of structure and also to enhance the

different properties of the materials. In case of olivine structure cathode materials, the

structural stability is very high compared to other exiting cathode materials for Li-ion

batteries. But, these olivine structure cathode materials are having poor electrical

conductivity. Hence, researchers have made surface modification to increase the

electrical conductivity of cathode materials using different methods such as sol-gel,

polymeric precursor, spray coating, etc. However, these processes exhibit disadvantages

such as choosing and the availability of the precursor chemicals (metal alkoxides), long

duration of preparation, non uniform coating, etc. Thus, there is need to develop a simple

and cost effective common process to overcome the above mentioned disadvantages.

Recently, we have investigated a novel polymeric resin process for uniform and effective

coating of metal oxides and carbon over nanocrystalline cathode powders for lithium

battery application using polyacrylic acid and ethylene glycol. Using the resin coating

process, very thin nanosize coating with high uniformity and high purity can be obtained

[28]. The tendency of ethylene glycol to form polyester (making the bond with the

surface) was effectively exploited for the fulfillment of efficient nanosize thin coating.

Thermal decomposition of hydrogen and oxygen in polymeric matrix of polyacrylic acid

and ethylene glycol leads to the formation of carbon thin layer over the cathode materials.

Hence, in the present work, the polymeric resin process was exploited for the uniform

coating of carbon layer over LiMPO4 (M= Mn, Co & Ni) nanomaterials.

101

3.3.2 Preparation of carbon coated LiMPO4 (M= Mn, Co &Ni) nanoparticles

Following chemicals were used for the coating of carbon over LiMPO4 (M= Mn, Co

&Ni) nanomaterials using polymeric resin coating process. Polyacrylic acid (chelating

and resin forming agent) AR grade, Merck, India and ethylene glycol (polymerizing

agent) SQ grade Qualigens,India). LiMPO4 (M= Mn, Co &Ni) nanomaterials samples

(pure samples) were prepared by PVP assisted polyol process. The obtained LiMPO4

(M= Mn, Co &Ni) nanomaterials were first dispersed in acetone and sonicated for 30 min

to remove the agglomerations and then acetone was removed by drying the dispersion in

hot air oven. The dried LiMPO4 (M= Mn, Co & Ni) nanomaterials were further dispersed

in ethanol, which is called as colloidal solution A. Solutions of polyacrylic acid and

ethylene glycol were mixed under constant stirring with their respective molar ratio of

10:2, which is labeled as solution B. Solutions A and B were mixed together under

vigorous stirring and the evaporation was continued at 353 K for 1 h to remove the

remaining water. Finally, thick brown colored solid mass was formed and it was calcined

at 623 K for 1 h to obtain carbon coated LiMPO4 (M= Mn, Co &Ni) nanoparticles. While

heating the samples up to 623 K, hydrogen and oxygen elements will leave from sample,

only carbon will stick on surface of nanoparticles. The flowchart of resin coating process

and schematic representation for coating of carbon layer over the LiMPO4 (M= Mn, Co

&Ni) nanoparticles are shown in figure 3.5 and figure 3.6 respectively.

102

Figure 3.5: Flowchart for the total resin coating process

Prepared LiMPO4 (M=Mn, Co & Ni) +

Ethanol (A)

Poly acrylic acid +

Ethylene Glycol (B)

Mixing of Solution A &

Solution B

Dark colored solid material

Carbon coated LiMPO4 (M=Mn, Co & Ni)

Nanoparticles



103

Figure 3.6: Schematic synthesis scheme of resin coating process

104

3.4 Conclusions

In this chapter, detailed results of pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles synthesis using PVP assisted polyol process followed by the resin coating

process were explained. PVP assisted polyol process gives the shape controlled

nanoparticles due to the presence of high chelating agents and PVP stabilizer.

Furthermore, novel resin coating process was adopted for obtaining very thin nanosize

carbon coating upon the LiMPO4 (M= Mn, Co & Ni) nanoparticles.

105

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108

CHARACTERIZATION OF PURE AND

CARBON COATED LiMPO4 (M= Mn,

Co & Ni) NANOPARTICLES

CHAPTER – IV

109

CHAPTER - IV

CHARACTERIZATION OF PURE AND CARBON COATED LiMPO4

(M= Mn, Co & Ni) NANOPARTICLES


4.2 Characterization of pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles

4.2.1 X-ray Diffraction (XRD)

4.2.2 Fourier transform infrared spectroscopy (FTIR)


4.2.4 Scanning electron microscopy (SEM)

4.2.5 High resolution transmission electron microscope – energy

dispersion X- ray spectroscope (HRTEM-EDS)

4.3 Conclusions

References

110

4.1 General Introduction In this chapter, detailed results obtained from XRD, FTIR, SEM, RS and HRTEM-EDS

for all the prepared pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles are

presented and discussed. The pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticles were successfully prepared by PVP assisted polyol and novel resin coating

processes. The structural characterizations like phase purity and structure of all the

prepared materials were respectively obtained from the X-ray diffraction (XRD) and

Fourier transform infrared spectroscopy (FTIR). The lattice parameters and the weight

percentage of the prepared material were determined by Rietveld refinement analysis

using the X'pert high score plus software. Microstructure of prepared pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) nanoparticles was observed under the scanning

electron microscopy (SEM). The amorphous nature and thickness of carbon coating over

the carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were observed by Raman

spectra and high resolution transmission electron microscope (HRTEM) images. Finally,

the chemical compositions of all synthesized materials were obtained by energy

dispersive spectroscopy (EDS).

4.2 Characterization of pure and carbon coated LiMPO4 (M= Mn, Co

&Ni) nanoparticles

4.2.1 X-ray Diffraction (XRD)

Figures 4.1, 4.2 & 4.3 show the XRD patterns of the pure and carbon coated LiMPO4

(M= Mn, Co & Ni) nanoparticle samples along with their respective JCPDS data. From

figure 4.1a, XRD patterns of pure and carbon coated LiMnPO4 nanorods, prepared at 873

111

K and the formation of the crystalline phase of pure and carbon coated LiMnPO4 nanorod

samples were confirmed by comparing with JCPDS (# 740375) data. XRD patterns of

both pure and carbon coated LiMnPO4 nanorods showed two extra peaks at 29 and 30.5,

due to the crystalline Mn2P2O7 phase. This peak was further analyzed by Rietveld

refinement, which confirmed the presence of 98 wt.% LiMnPO4 and 2 wt.% Mn2P2O7 in

the LiMnPO4 nanorod sample. Figure 4.1b shows the Rietveld refinement results for the

pure LiMnPO4 nanorods. The crystallite size of the LiMnPO4 nanorods was calculated

using Scherrer’s formula (D = 0.9 × λ/β Cos θ) and was found to be ~63 nm. The XRD

pattern of the carbon coated LiMnPO4 nanorods did not show any extra peak for carbon

phase, suggesting that the coated carbon over the LiMnPO4 nanorods may be in an

amorphous phase [1-3]. From figure 4.2, XRD patters of pure and carbon coated lithium

cobalt phosphate (LiCoPO4) nanoparticles along with the carbon and LiCoPO4 JCPDS

data. JCPDS data confirmed the formation of structural phase. The crystallite size of the

LiCoPO4 nanoparticles was calculated using Scherrer’s formula and was found to be ~45

nm. The XRD patterns of carbon coated LiCoPO4 nanoparticles were showing two extra

peaks at 28.5 and 31.5 other than LiCoPO4 peaks, which is indicated that presence of

crystalline nature of coated carbon on LiCoPO4 nanoparticles.

112

Figure 4.1: a) XRD patterns of the pure and carbon coated LiMnPO4 nanorods along

with JCPDS data and b) Observed XRD pattern fitted with the calculated one, using

Rietveld analysis

10 20 30 40 50 60 70

Inte

nsity

2θ

JCPDS# 740375

** LiMnPO4

LiMnPO4/C

15 20 25 30 35 40

-40

0

40

80

120

LiMnPO4 nanorods Observerd Calculated Differnce Peak Position

Inte

nsity

(a.u

)

2θ (deg.)

a

b

113

Figure 4.2: XRD patterns of the pure and carbon coated LiCoPO4 nanoparticles along

with JCPDS data

114

Figure 4.3: XRD patterns of the pure and carbon coated LiNiPO4 nanoparticles along

with JCPDS data

From figure 4.3, XRD patterns of pure and carbon coated lithium nickel phosphate

(LiNiPO4) nanoparticles along with the carbon and LiNiPO4 JCPDS data. JCPDS data

confirmed the formation of structural phase. The crystallite size of the LiNiPO4 nanorods

was calculated using Scherrer’s formula and was found to be ~50 nm. Furthermore, the

XRD patterns of carbon coated LiNiPO4 nanoparticles were showing two extra peaks,

which are due to the crystalline carbon (JCPDS#46-0943). It confirmed that the coated

carbon on LiNiPO4 nanoparticles was showing partially crystalline nature, which was

further confirmed by the Raman Spectroscopy.

115

4.2.2 Fourier transforms infrared spectroscopy (FTIR)

Figures 4.4, 4.5 & 4.6 show the FTIR spectra of all pure and carbon coated LiMnPO4,

LiCoPO4 and LiNiPO4 nanoparticles respectively. From figure 4.4, the pure and carbon

coated LiMnPO4 nanorods FTIR spectra show bands at 900 cm-1 and 1055 cm-1, which

are due to symmetrical and asymmetrical stretching of PO4-3. The IR bands between 630

and 460 cm−1 are due to the asymmetric bending O–P–O mode. Thus, FTIR results

confirmed the presence of PO4-3 structure in the pure and carbon coated LiMnPO4

nanoparticles. The observed common peaks at 3417-3444 cm-1 is assigned to the

stretching frequency of OH (due to the adsorbed moisture). Furthermore, carbon coated

LiMnPO4 nanoparticle samples showed extra bands at 1712 and 1637 cm−1, which are

attributed to the symmetrical stretching of C-O and C-C respectively. IR band at 744

cm−1 is due to P–O–C symmetric stretching and confirmed the presence of carbon over

LiMnPO4 nanorods in carbon coated samples [4, 5].

From figure 4.5, the FTIR spectra of the pure and carbon coated LiCoPO4 nanorods show

bands at 950 cm-1 and 1070 cm-1, which are due to symmetrical and asymmetrical

stretching of PO4-3 respectively. The IR bands between 641 and 480 cm−1 are due to the

asymmetric bending O–P–O mode. Thus, FTIR results confirmed the presence of PO4-3

structure in the pure and carbon coated LiCoPO4 nanoparticles. The observed broad peak

from 3582-3236 cm-1 in pure LiCoPO4 sample is assigned to the stretching frequency of

O-H (due to the adsorbed moisture). Furthermore, carbon coated LiCoPO4 nanoparticle

samples showed extra bands at 1728 and 1621cm−1, which are attributed to the

symmetrical stretching of C-O and C-C respectively. IR band at 745 cm−1 was due to P–

116

4000 3500 3000 2500 2000 1500 1000 500

LiMnPO4/C

Tran

smis

sion

(%)

Wavenumber ( cm-1)

LiMnPO4

O–C symmetric stretching and confirmed the presence of carbon over LiCoPO4 nanorods

in carbon coated samples [6].

Figure 4.4: FTIR spectra of the pure and carbon coated LiMnPO4 nanorods

117

3500 3000 2500 2000 1500 1000

LiCoPO4

Tran

smis

sion

(%)

Wavenumber (cm-1)

LiCoPO4/C

Figure 4.5: FTIR spectra of the pure and carbon coated LiCoPO4 nanoparticles

118

Figure 4.6: FTIR spectra of pure and carbon coated LiNiPO4 nanoparticles

From figure 4.6, the pure and carbon coated LiNiPO4 nanoparticles FTIR spectra show

bands at 920 cm-1 and 1045 cm-1, which are respectively due to symmetrical and

asymmetrical stretching of PO4-3. The IR bands between 660 and 480 cm−1 are due to the

asymmetric bending O–P–O mode. Thus, FTIR results confirmed the presence of PO4-3

structure in the pure and carbon coated LiCoPO4 nanoparticles. Furthermore, carbon

coated LNiPO4 nanoparticle samples showed extra bands at 1705 and 1642 cm−1,

attributed to the symmetrical stretching of C-O and C-C respectively, broad peak from

3740-2780 cm-1 is assigned to the stretching frequency of O-H. The IR band at 748 cm−1

119

is due to P–O–C symmetric stretching and confirmed the presence of carbon over

LiNiPO4 nanoparticles in carbon coated samples [7].

4.2.3 Raman Spectroscopy

Figures 4.7, 4.8 and 4.9 show the Raman spectra of the pure and carbon coated LiMPO4

(M=Mn, Co & Ni) nanoparticle samples. From figure 4.7, the observed bands at 953

cm−1 corresponded to the symmetric Ag mode, and the two low intensity bands at 997 and

1090 cm−1 are due to the asymmetric stretching modes of the PO4−3 polygon. Raman

spectra of the carbon coated samples showed two bands at 1350 [D (Disordered) band of

sp3] and 1592 cm−1 [G (Graphene) band of sp2-type] along with PO4−3 anion bands, which

are due to the residual carbon [8].

Raman spectroscopy of carbon materials:

Single crystal graphite belongs to the D6h symmetry group, and vibrational modes (2E2g,

2B2g, E1u, and A2u) are as shown in Figure 4.10. The two E2g modes are Raman active and

have been identified with the Raman band which obtained at 1582 cm-1 and a low-

frequency neutron scattering feature at 47 cm-l. The E1u (1588 cm-1) and A2u (868 cm-1)

are IR active and observable with IR reflectance. The B2g modes are optically inactive,

but one has been observed by neutron scattering at 127 cm-1. From figures 4.7, 4.8 and

4.9, the Raman spectra of carbon coated LiMPO4 (M= Mn, Co & Ni) shows the presence

of one new band at around 1360 cm-1 other than 1582 cm-1 band. The reasons for the

appearance of the 1360 cm-1 band is quite controversial. Several workers have attributed

it to a decrease in symmetry near microcrystallite edges, reducing the symmetry from D6h

to C3v (Vibrational modes: 2A1 and 2E) or Cs. New vibration modes of the lattice may then

become active, such as an Al mode. A related mechanism is breakdown of the k = 0

120

selection rule for optical phonons near crystallite edges. Such breakdown permits

phonons other than 1582 and 47 cm-1 to become active and the spectra reflect the density

of phonon states in the lattice. A very different explanation for the 1360 cm-1 mode is the

existence of specific vibrations at the edges. All reports concur that the 1360 cm-1 mode

is related to structural disorder and it will be referred to as the D band hereafter [9-13].

From figure 4.8, the observed bands at 947 cm−1 corresponded to the symmetric Ag mode,

and the two low intensity bands at 1006 and 10410 cm−1 are due to the asymmetric

stretching modes of the PO4 −3 polygon. Raman spectra of the carbon coated samples

showed two bands at 1311 and 1598 cm−1 (D and G bands) along with PO4−3 anion bands,

which are due to the residual carbon. From figure 4.9, Raman spectra of the carbon

coated LiNiPO4 sample showed band at 944 cm−1 corresponded to the symmetric Ag

mode of the PO4−3 polygon. Also, observed two bands at 1313 and 1593 cm−1 (D and G

bands) along with PO4−3 anion bands, which were due to the residual carbon. The relative

peak heights and widths of carbon bands change substantially with the reaction

temperature and the nature of the precursor materials. The variation of the width and

intensity of the D and G bands are related to the growth and size of different carbon

phases, the presence of functional groups and impurities. Hence, the Raman spectral

results of carbon coated LiMPO4 (M= Mn, Co & Ni) samples confirmed the presence of

carbon. From figures 4.7, 4.8 &4.9, the high sp3/sp2 value is correlating to a high

amorphous nature which may help to enhance the electrochemical performance of

LiMPO4 (M= Mn, Co & Ni) materials.

121

Figure 4.7: Raman spectra for the pure and carbon coated LiMnPO4 nanorods

122

800 1000 1200 1400 1600

C=C

LiCoPO4/C

Inte

nsity

(a.u

)

Raman Shift (Cm-1)

PO4 C-C

LiCoPO4

Figure 4.8: Raman spectra of the pure and carbon coated LiCoPO4 nanoparticles

123

Figure 4.9: Raman spectra of the carbon coated LiNiPO4 nanoparticles

124

Figure 4.10: Vibrational modes of Single crystalline graphite

(Reprinted with permission from [6], Y. Wang et.al, Chem. mater. 1990, 2, 557-563.

Copyright (1990) American Chemical Society)

125

4.2.4 Scanning Electron Microscopy (SEM)

Figures 4.11, 4.12, 4.13, 4.14, 4.15 & 4.16 show the SEM images obtained at different

magnifications of the pure and carbon coated LiMPO4 (M=Mn, Co & Ni) nanoparticle

samples. From figure 4.11, the SEM images of pure LiMnPO4 nanorods with high

agglomeration. Figure 4.12 shows SEM images the carbon coated LiMnPO4 nanorods

taken at different magnifications. From figure 4.12a, the width of the carbon coated

LiMnPO4 nanorods was found to be 250 nm. From figure 4.13, the SEM images of the

pure LiCoPO4 nanoparticles at different magnifications are observed. The agglomerated

spherical shape LiCoPO4 particle size was found to be ~120 nm. Figure 4.14 shows the

SEM images of the carbon coated LiCoPO4 nanoparticles. From figures 4.14, the SEM

images of the carbon coated LiCoPO4 nanoparticles show highly agglomeration and the

average size of the particles is found to be ~ 90 nm.

From figure 4.15, the SEM images of pure LiNiPO4 nanoparticles with high

agglomeration were observed. The particle size of pure LiNiPO4 nanoparticles is found

to be ~ 110 nm. Figure 4.16 shows the SEM images of carbon coated LiNiPO4 nanorods

taken at different magnifications. From figure 4.16, the particle size of the carbon coated

LiNiPO4 nanoparticles was found to be ~ 80 nm. XRD, FTIR, Raman and SEM results

confirmed the formation of pure and carbon coated nanocrystalline lithium transition

metal phosphates prepared by polyol process followed by novel resin coating process.

126

Figure 4.11: SEM image of the pure LiMnPO4 nanorods in low magnification

127

Figure 4.12: a & b) SEM images of the carbon coated LiMnPO4 nanorods

a

b

128

Figure 4.13: a & b) SEM images of the pure LiCoPO4 nanoparticles at different

magnifications

a

b

129

Figure 4.14: a & b) SEM images of the carbon coated LiCoPO4 nanoparticles at different

magnifications

a

b

130

Figure 4.15: a & b) SEM images of the pure LiNiPO4 nanoparticles at different

magnifications

a

b

131

Figure 4.16: a & b) SEM images of the carbon coated LiNiPO4 nanoparticles at different

magnifications

a

b

132

4.2.5 High resolution transmission electron microscope – energy dispersion X-ray

spectroscope (HRTEM-EDS)

Figures 4.17 & 4.18 show the HRTEM images and EDS spectra of the carbon coated

LiMPO4 (M=Mn & Co) nanoparticle samples. From figure 4.17a, the TEM micrograph

provided the direct evidence for the uniform coating of thin carbon layer about 3 nm over

the nanocrystalline LiMnPO4 nanorods. Figure 4.17b shows the EDS spectra of carbon

coated nanocrystalline LiMnPO4 nanorods and it shows the presence of carbon peak.

Figure 4.18 shows the TEM image of the carbon coated LiCoPO4 nanoparticles with EDS

spectra. From figure 4.18a, an uniform thickness of carbon coating was found to be

around 5 nm. The EDS spectra confirmed the presence of carbon in carbon coated

LiNiPO4 sample, showing carbon peak in figure 4.18b. Figure 4.19 shows the EDS

spectra of the carbon coated LiNiPO4 nanoparticles and it confirmed the presence of

carbon in coated sample. Hence, from the TEM micrographs, FTIR results and Raman

spectra, it is concluded that an uniform coating of the thin layer of carbon over the

nanocrystalline LiMPO4 (M=Mn, Co & Ni) nanoparticles was achieved.

133

Figure 4.17: a) HRTEM images and b) EDS spectrum of the carbon coated LiMnPO4

nanorods

134

Figure 4.18: a) HRTEM images and b) EDS spectrum of the carbon coated LiCoPO4

nanoparticles

Carbon coating

a

b

135

Figure 4.19: EDS spectrum of the prepared carbon coated LiNiPO4 nanoparticles

4.3 Conclusions

Effect of PVP (stabilizer) and experimental conditions for polyol as well as resin coating

processes were investigated for the synthesis of nanocrystalline pure and carbon coated

LiMPO4 (M=Mn, Co & Ni) samples. A systematic study was carried out through XRD,

FTIR, Raman, SEM and TEM-EDS analysis of prepared pure and carbon coated LiMPO4

(M=Mn, Co & Ni) nanoparticle samples.

136

The crystalline phases and structure of pure and carbon coated LiMPO4 (M=Mn, Co &

Ni) nanoparticle were confirmed from the analysis of XRD and FTIR results. In FTIR

spectra of all carbon coated LiMPO4 (M=Mn, Co & Ni) nanoparticle samples showed

evidence for the presence of carbon. SEM images gave information of the microstructure

of both pure and carbon coated samples. FTIR and Raman spectra of all carbon coated

samples confirmed the presence of carbon and dominated amorphous nature of the coated

carbon. Microstructure, shape, size, carbon coating and presence of transition metals, P,

O, and C in the carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were confirmed

from the SEM images, TEM –EDS and Raman spectral results. HRTEM images of

carbon coated LiMPO4 (M=Mn, Co & Ni) nanoparticle samples give the evidence for

uniformity of thickness of carbon coating upon the nanoparticles. Using a polyol and

resin coating processes, we have shown that the pure and carbon coating LiMPO4 (M=

Mn, Co & Ni) nano materials could be synthesized.

137

Reference

[1] S. Geller and J. L. Durand, Acta Cryst., 13(1960) 325-331.

[2] A. K. Padhi, K. Nanjundaswamy and J. B. Goodenough, J. Electrochem. Soc., 144

(1997) 1188-1194.

[3] B. D. Cullity, Elements Of X Ray Diffraction, Addison-Wesley Publishing Company,

Inc. USA, 1956.

[4] N. S. Norberg and R. Kostecki, Electrochim. Acta, 56 (2011) 9168–9171.

[5]G. Socrates, Infrared and Raman Characteristic Group Frequencies, John Wiley

&Sons, New York, 2001.

[6] C. M. Julien1, P. Jozwiak and J. Garbarczyk, Proceedings of the International

Workshop, Advanced Techniques for Energy Sources Investigation and Testing,4 –

9, Sofia, Bulgaria, 2004.

[7] C. M. Julien, A. Mauger, K. Zaghib, R. Veillette and H. Groult, Ionics, 18(2012) 625-

633.

[8] Y. Mizuno, M. Kotobuki, H. Munakata and K. Kanamura, J. Ceram. Soc. Jpn.,

117(2009)1225-1228.

[9] Yan Wang, Daniel C. Alsmeyer, and R. L. McCreery, Chem. Mater., 2 (1990) 557-

563.

[10] J. P. Tessonnier, D. Rosenthal, T. W. Hansen, C. Hess, M. E. Schuster, R. Blume, F.

Girgsdies, N. Pfander, O. Timpe, D. S. Su and R. Schlogl, Carbon, 47 (2009) 1779-

1798.

[11] C. Li, H.P. Zhang, L.J. Fu, H. Liu, Y.P. Wu, E. Rahm, R. Holze and H. Q. Wu,

Electrochim. Acta, 51 (2006) 3872–3883.

138

[12] F. Pan, X. Chen, H. Li, X. Xin, Q. Chang, K. Jiang and W. Wang, Electrochem.

Comm., 13(2011) 726–729.

[13] J. D. Wilcox, M. M. Doeff, M. Marcinek and R. Kostecki, J. Electrochem. Soc.,

154(2007) A389-A395.

139

IMPEDANCE, AC CONDUCTIVITY

AND TRANSPORT STUDIES OF PURE

AND CARBON COATED LiMPO4 (M=

Mn, Co & Ni) NANOPARTICLES

CHAPTER – V

140

CHAPTER - V

IMPEDANCE, AC CONDUCTIVITY AND TRANSPORT STUDIES

OF PURE AND CARBON COATED LiMPO4 (M= Mn, Co & Ni)

NANOPARTICLES


5.2 Electrical conductivity studies

5.2.1 Complex impedance

5.2.2 Frequency dependent conductivity

5.2.3 Dielectric constant

5.2.4 Electrical modulus

5.3 Wagner polarization

5.4 Conclusions

References

141

5.1 General introduction The synthesized samples of pure and carbon coated LiMPO4 (M=Mn, Co & Ni) were

ground in to very fine powder and then made into 10 mm diameter pellets and their

thickness various from 1.5 mm to 3 mm. All the pellets were dried in the oven for one

hour at 333 K, coated with silver paste on either faces of the pellets, and dried in oven for

four hours. Followed by drying, coated pellets were stored in a dessicator and used for

the impedance measurements. The real and imaginary parts of complex impedance

measurements were made on the pressed pellets of pure and carbon coated LiMPO4

(M=Mn, Co & Ni) samples as a function of frequency (1MHz – 10 mHz) and temperature

(305 – 773 K). The obtained impedance data were analyzed using Winfit software. In this

software, all the circuit parameters are adjusted simultaneously, in order to fit the

measured data and to obtain bulk resistance and circuit description code of the physical

model (equivalent circuit) of the materials. The ionic transport number for the pure and

carbon coated LiMPO4 (M=Mn, Co & Ni) samples was obtained by analyzing the

measured current Vs time data using nanoammeter (Keithley, 2182A) and DC power

supply (Keithley, 2308).

5.2 Result & Discussion

5.2.1 Complex impedance

Figures 5.1, 5.2, 5.3, 5.4, 5.5 and 5.6 show the complex impedance plots (imaginary -Z"

vs real Z') obtained at different temperatures up to 773 K of both pure and carbon coated

LiMPO4 (M= Mn, Co & Ni) nanoparticles. From figures 5.1a, b, c & d, the typical

complex impedance plots (imaginary -Z" vs real Z') obtained at various temperatures

142

(303 to 773 K) of pure LiMnPO4 nanorods. The semicircle and one inclined spike were

observed in the impedance spectra, in which, the semicircle at the higher frequency

region is related to the bulk conductivity of the pure LiMnPO4 nanorods sample and the

appearance of inclined spike is due to the effect of interfacial polarization of the

LiMnPO4 sample [1]. In figures 5.1, the scatter symbols and continuous line represent

the experimentally observed impedance data and their fitted data value. Using the winfit

software, the impedance plot is deconvolated into a semicircle and an inclined spike,

represented as continuous line. The bulk resistance (Rb) of LiMnPO4 material was

obtained from the respective intercept of the depressed semicircles with the real axis. The

semicircle showed in the impedance plot is due to the parallel combination of RC

elements for Debye behaviour. For non ideal case, the semicircles were slightly

depressed and it replaced the capacitor by constant phase element Q (CPE) in the

equivalent circuit with the impedance function [ZCPE = (1/Y(jω)n), n=1]. As the

temperature increases; the size of the low frequency spike is reducing, which indicating

the decreasing of interfacial polarization effect.

Figures 5.3 a, b, c & d show the analyzed real (Z') and imaginary (Z") parts of the

impedance plots obtained at various temperatures (303 to 733 K) for carbon coated

LiMnPO4 samples. Figure 5.2a shows the impedance plot of the carbon coated LiMnPO4

nanorods sample, two depressed semicircles were represented by two parallel

combinations of RC elements in series and first semicircle represents the grain interior

and another semicircle represents the carbon coating responses. From the impedance

plot, R values were obtained from the intercept of the semicircles on Z' real axis and the

capacitances C were obtained using the relation ω max RC =ωmax(RQ)1/n =1 [2]. In figure

143

5.2a, the middle frequency semicircle was reduced with increase of temperature; it might

be due to the decomposition of carbon at higher temperature. From these figures, it is

observed that the intercept of the semicircle with real axis shifts towards the origin with

temperature and also frequency of the intersection with real axis, increases with

temperature.

Figure 5.1: Impedance plots obtained at different temperatures of pure LiMnPO4

nanorods

a b

c d

144

Figure 5.2: Impedance plots obtained at different temperatures of carbon coated

LiMnPO4 nanorods

a c

b

d

145

Figures 5.3 a, b, c & d show the analyzed real (Z') and imaginary (Z") parts of the

impedance plots obtained at various temperatures (313 to 733 K) for pure LiCoPO4

samples. From these figures, it was observed that the intersection of the semicircle with

the real axis gave the best bulk resistance (Rb) of the crystalline sample and it shifted

towards the origin with an increase in temperature. Also, it was observed that the

frequency of the intersection with real axis increased with temperature. From figures

5.4a-d, the typical complex impedance plot (imaginary -Z" vs real Z') for carbon coated

LiCoPO4 nanoparticles were obtained at various temperatures (333 to 743 K). From the

impedance plot, R values were obtained from the intercept of the semicircles on Z’ real

axis and the capacitances C were obtained using the relation ω max RC =ωmax(RQ)1/n =1

[2]. Figures 5.5 a, b, c & d show the analyzed real (Z') and imaginary (Z") parts of the

impedance plots obtained at various temperatures (293 to 733 K) for pure LiNiPO4

samples.. From these figures, it was observed that the intersection of the semicircle with

the real axis gave the best bulk resistance (Rb) of the crystalline sample and it shifts

towards the origin with an increase in temperature. Also, it was observed that the

frequency of the intersection with real axis increases with temperature. Figures 5.6 a, b, c

& d show the analyzed real (Z') and imaginary (Z") parts of the impedance plots obtained

at various temperatures (353 to 733 K) for carbon coated LiNiPO4 samples. By analyzing

the depressed semicircles shown in figures 5.6a-d, the equivalent circuits are obtained

and are represented as parallel combination of resistance and (CPE) for all the samples

shown in figure 2.8. CPE is the representation of the distributed elements.

146

Figure 5.3: Impedance plots obtained at different temperatures of pure LiCoPO4

nanoparticles

a b

c d

147

Figure 5.4: Impedance plots obtained at different temperatures of carbon coated LiCoPO4

nanoparticles.

a

b

c d

148

Figure 5.5: Impedance plots obtained at different temperatures of pure LiNiPO4

nanoparticles

0 2 4 6 8 100123456789

10

503 K 523 K 543 K 563 K Fit

-Z'' X

106 (O

hm)

Z' x106 (Ohm) 0 200 400 600 800 1000

0100200300400500600700800900

1000 683 K 713 K 733 K 743 K 763 K 783 K Fit

-Z'' X

103 (O

hm)

Z' X103 (Ohm)

a

b

c d

149

Figure 5.6: Impedance plots obtained at different temperatures of carbon coated LiNiPO4

nanoparticles

0 10 20 30 40 50 60 700

10

20

30

40

50

60

70

433 K 453 K 473 K 493 K 513 K Fit

-Z'' X

107 (O

hm)

Z' X107 (Ohm)

0 5 10 15 20 25 300

5

10

15

20

25

30 633 K 653 K 673 K 693 K 713 K Fit

-Z'' X

104 (O

hm)

Z' X104 (Ohm)

a b

c d

150

Electrical conductivity

Figure 5.7a &b show the log (σT) vs. 1000/T plots for the pure and carbon coated

LiMnPO4 nanorods. Bulk conductivities of pure and carbon coated LiMPO4 (M= Mn, Co

& Ni) nanoparticles were calculated using sample dimensions and bulk resistance,

evaluated by analyzing the impedance data measured at different temperatures using the

winfit software. From figure 5.7a, conductivity increases from 313 K to 373 K (region1)

and then decreases up to 533 K (region2). The increase of conductivity in region 1 may

be due to both Li+ and H+ of adsorbed water molecules on thermal activation. The

decrease of conductivity in region 2 may be due to the loss of adsorbed water molecules.

The linear increase of conductivity in region 3 may be due to pure Li+ ions. Hence,

activation energy (Ea) for Li+ ion migration was evaluated by fitting the temperature

dependence conductivity region 3 data with the Arrhenius equation using linear least

square fit [3-5 ]. Figure 5.7b shows the log σT vs 1000 /T plots region II of pure and

carbon coated LiMnPO4 samples. From figure 5.7b, the carbon coated LiMnPO4

nanorods sample shows the higher conductivity than the pure, which may be due to the

conductive carbon coating.

Figure 5.8 shows the log σT vs 1000 /T plots of pure and carbon coated LiCoPO4

nanoparticles samples at higher temperature (413 K to 733 K). From figure 5.8, the

activation energy of the pure and carbon coated LiCoPO4 nanoparticles obtained from

slop of the curves. Figure 5.9 show that the log σT vs 1000 /T plots of pure and carbon

coated LiNiPO4 nanoparticles at higher temperature (413 K to 783 K). From figure 5.9,

the temperature dependence of conductivity is fitted to Arrhenius equation for the pure

and carbon coated LINiPO4 samples & the activation energy Ea was obtained from the

151

slopes of each plot. All calculated activation energies for ions migration in pure and

carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles are listed in table 1.

Figure 5.7: a) log (σT) vs. 1000/T plots for the pure and carbon coated LiMnPO4

nanorods b) log (σT) vs. 1000/T plots for the pure and carbon coated LiMnPO4 nanorods

region 3 (high temperature)

region 3

region 2 region 1

152

1.4 1.6 1.8 2.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0 LiCoPO4/C LiCoPO4 Linear Fit

Log(

σT) (

S Cm

-1 K

)

1000/T (K-1)

Figure 5.8: log (σT) vs. 1000/T plots of the pure and carbon coated LiCoPO4

nanoparticles

153

1.2 1.4 1.6 1.8 2.0 2.2 2.4-10-9-8-7-6-5-4-3-2-1 LiNiPO4

LiNiPO4/C Linear Fit

Log

(σT)

S C

m-1 K

1000/T K-1

Figure 5.9: log (σT) vs. 1000/T plots of the pure and carbon coated LiNiPO4

nanoparticles

154

5.2.2 A.C. conductivity

Figures 5.10, 5.11 & 5.12 show the log σ vs log ω plots, obtained at different

temperatures, for pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles. AC

conductivity was calculated using the measured impedance data of pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) nanoparticles. Figure 5.10a and b show the log σ vs

log ω plots, obtained at different temperatures, for pure and carbon coated LiMnPO4

nanoparticles respectively. From figures 5.10a & b, frequency dependence of

conductivity shows (1) at low frequency plateau region and (2) at high frequency

dispersion region. The plateau region corresponds to frequency independent conductivity

(σ0) or (σdc) and evaluated by extrapolating the conductivity value to zero frequency. The

frequency dependent conductivity was fitted using Jonscher’s universal power law

(JUPL) of equation (σω= σ0 + Aωs) [6,7]. Figures 5.11 and 5.12 show the log σ vs log ω

plots for pure and carbon coated LiCoPO4 and LiNiPO4 samples respectively. From

figures 5.10, 5.11 and 5.12, it was observed that the frequency at the dispersion region

deviated from the dc conductivity plateau is defined as characteristic frequency (ωp) and

also known as the hopping rate, at which σω = 2σ0. The relation between the dc

conductivity and the hopping rate is given by σ0 = kωp where k is the empirical constant,

which depends on the concentration of mobile ions and the conduction mechanism. Also,

it was observed that the hoping frequency moved towards the higher frequency with

increase of temperature, which was thermally activated with same activation energy of

σ0T. The value of σ0 (σdc) obtained from log σ vs log ω plots was in good agreement with

the σdc obtained from impedance measurements of LiMnPO4 nanoparticles. Hence, the

observed dispersion region of conductivity may follow the diffusion controlled relaxation

155

0 1 2 3 4 5 6 7 8-8

-7

-6

-5

-4LiMnPO4/C

553 K 593 K 633 K 673 K 723 K 773 K

Log

σ (S

Cm

-1)

Log ω

(DCR) model [8-10]. In conclusion, from the observed impedance & power law, the

frequency at which the intersection occurs with real axis and the frequency at which

relaxation effects ωp began to shift towards the higher frequencies with increase in

temperature.

Figure 5.10: log σ vs log ω plots obtained at different temperatures of a) pure and b)

carbon coated LiMnPO4 nanorods

156

Figure 5.11: log σ vs log ω plots obtained at different temperatures of a) pure and b)

carbon coated LiCoPO4 nanoparticles

0 1 2 3 4 5 6 7 8 9-7

-6

-5

-4

-3

743 K 723 K 703 K 683 K 663 K 643 K 623 K

Log

σ

Log ω

LiCoPO4/C

0 1 2 3 4 5 6 7 8 9-8

-7

-6

-5

-4

-3

533 K 553 K 573 K 593 K 613 K 653 K 693 K 733 K

Log

σ

Log ω

LiCoPO4

a

b

157

Figure 5.12: log σ vs log ω plots obtained at different temperatures of a) pure and b) carbon coated LiNiPO4 nanoparticles

a

b

158

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6-0.50.00.51.01.52.02.53.03.54.0

LiMnPO4/C 553 K 593 K 633 K 673 K 723 K 773 K

log

(σ/σ

0)

log (ω/ωp)

0 1 2 3 4 5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

LiMnPO4

653 K 673 K 693 K 713 K 733 K 753 K 773 Klo

g (σ

/σ0)

log (ω/ωp)

Figure 5.13: log (σ/σ0) vs log (ω/ωp) plots obtained at different temperatures of pure and carbon coated LiMnPO4 nanorods

159

Figure 5.14: log (σ/σ0) vs log (ω/ωp) plots obtained at different temperatures of pure

and carbon coated LiCoPO4 nanoparticles

-7 -6 -5 -4 -3 -2 -1 0 1 2 3-1.0

-0.5

0.0

0.5

1.0

1.5

2.0LiCoPO4/C

623 K 643 K 663 K 683 K 703 K 723 K 743 K

Log

(σ/σ

0)

Log (ω/ω0)

-2 -1 0 1 2 3 4 5 6 7-0.50.00.51.01.52.02.53.03.54.0

LiCoPO4

533 K 553 k 573 K 593 K 613 K 653 K 693 K 733 KLo

g (σ

/σ0)

Log(ω/ω0)

a

b

160

Figure 5.15: log (σ/σ0) vs log (ω/ωp) plots obtained at different temperatures of pure

and carbon coated LiNiPO4 nanoparticles

-6 -3 0 3

0.0

0.7

1.4 653 K 673 K 693 K 713 K 733 K 753 K 773 K

Log

(σ/σ

0)

Log (ω/ω0)

LiNiPO4

-3 0 3

0

1

2

3 653 K 673 K 693 K 713 K 733 K 753 K 773 K

log

(σ/σ

0)

Log (ω/ω0)

LiNiPO4/C

a

b

161

Figures 5.13, 5.14 & 5.15 show the log (σ/σ0) vs log (ω/ωp) plots obtained at various

temperatures of pure and carbon coated LiMnPO4, LiCoPO4 & LiNiPO4 nanoparticles.

From figure 5.13 a & b, it 1s observed for pure and carbon coated LiMnPO4 nanorods

samples that the superimposed curves of log (σ/σ0) vs log (ω/ωp) at various temperatures,

suggest the conductivity relaxation mechanism is temperature independent and the slight

deviation in higher temperature may be due to the ionic transport in materials is affected

by structural aspects and ionic concentrations with temperature [36]. The perfect overlap

of the curves at different temperatures into a single master curve indicates that the

dynamical processes occurring at different frequencies are independent of temperature

and have the same thermal activation energy.

From figure 5.14a & b, the superimposed curves of log (σ/σ0) vs. log (ω/ωp) plots

obtained at various temperatures of the pure and carbon coated LiCoPO4 nanoparticles,

suggest the conductivity relaxation mechanism is temperature independent and slight

deviation in a particular sample may be due to the ionic transport in sample is affected by

structural aspects and ionic concentrations with temperature. From figure 5.15a & b, the

superimposed curves of log (σ/σ0) vs. log (ω/ωp) plots for the pure and carbon coated

LiNiPO4 nanoparticles at various temperatures. Also, the conductivity relaxation

mechanism is temperature independent and slight deviation in higher temperature may be

due to the ionic transport in materials is affected by structural aspects. Olivine structured

LiMPO4 (M= Mn, Co & Ni) nanoparticles had constant ionic concentration, but at higher

temperature, the volume of the unit cell was increasing with temperature. Due to this

superimposed curves of log (σ/σo) vs, log (ω/ωp) curves of LiMPO4 (M= Mn, Co & Ni),

nanoparticles showed some deviation from master curve [11].

162

5.2.3 Electrical permittivity

Figures 5.16, 5.17& 5.18 represent the real part of dielectric permittivity ε' vs. log ω of

the pure and carbon coated LiMnPO4, LiCoPO4 & LiNiPO4 nanoparticle samples,

respectively. The real ε' and imaginary ε" of the dielectric permittivity ε* were calculated

using the measured impedance data Z' & -Z" and pellet dimensions according to the

equations described in section 2.4. From the figures 5.16a & b, it is observed that with

increase in the frequency, ε' decreased and attained a constant value for pure and carbon

coated LiMnPO4 nanorods samples at high frequencies. From figures 5.17a &b, it is

observed the increase of dielectric constant ε' with decrease of frequency, which can be

attributed to the contribution of charge accumulation at the interface and leads to a net

polarization of the ionic medium result in the formation of space charge region at

electrode-electrolyte interface in pure and carbon coated LiCoPO4 nanoparticles. From

figures 5.18a & b, that with increase in the frequency, ε' decreased and attained a constant

value for pure and carbon coated LiNiPO4 nano particles sample at high frequencies. At

high frequencies, periodic reversal of the ac field resulted in no charge accumulation at

the interface and hence ε' remained constant. The motion of the charge without

accumulation can be explained in terms of the ion diffusion mechanism. The increase of

ε' towards lower frequencies can be attributed to the contribution from polarization of

charges. In the low frequency region, the ions jump in the field direction and pile up at

sites with high free energy barrier in the field direction after successfully hopping the

sites with low free energy barrier. The pile up of charges leads to a net polarization of the

ionic medium. At high frequencies, the periodic reversal of the field takes place so

rapidly that there are no excess ion jumping in the field direction, the polarization due to

163

charge pile up at high free energy barrier sites disappears and the observed value of ε'

decreases. When the temperature of the sample is increased, the values of dielectric

constant ε' increased and dispersion at ε' was moving towards the higher frequencies [12-

16].

Figure 5.16: Real part of dielectric permittivity ε' vs. log ω obtained at different

temperatures of pure and carbon coated LiMnPO4 nanorods

0 2 4 6 8-50

0

50

100

150

200

250 773 K 753 K 733 K 713 K 693 K 673 K 653 K 633 Kε'

Log ω

LiMnPO4

a

b

164


temperatures of pure and carbon coated LiCoPO4 nanoparticles

-1 0 1 2 3 4 5 6 7 8

0

200

400

600

800

1000 743 K 723 K 703 K 683 K 663 K 643 K 623 K 603 Kε'

Log ω

LiCoPO4/C

a

b

165


temperatures of pure and carbon coated LiNiPO4 nanoparticles

-1 0 1 2 3 4 5 6 7 8 9

0

800

1600

2400

653 K 673 K 693 K 713 K 733 K 753 K 773 Kε'

Log ω

LiNiPO4/C

a

b

166

5.2.4 Electrical modulus

Figures 5.19, 5.20 & 5.21 show the imaginary part of electric modulus M'' vs log ω plots

obtained at different temperatures of the pure and carbon coated LiMnPO4, LiCoPO4 and

LiNiPO4 nanoparticle samples respectively. The electric modulus data are calculated

using the real & imaginary parts of the measured impedance data and the pellet

dimensions using the equations described in section 2.4 for all the pure and carbon coated

LiMPO4 (M= Mn, Co & Ni) nanoparticles.

From figures 5.19a &b, the continuous line represents KWW fit and the symbols

correspond to the experimental data of pure and carbon coated LiMnPO4 nanorods

sample. From Fig. 11, it is observed that the shape of each curve is asymmetric of non-

Lorentzian type exhibiting a peak at the relaxation frequency, fmax, with a long tail

extending in the region of shorter relaxation time. Similarly, from figure 5.20 & 5.21, the

M''max value is remains constant and it shifts towards the higher frequency while

increasing the temperature. At lower frequency, M'' value was nearly zero and it raised

while increasing the frequency and saturated (M∞) at higher frequencies. The dispersion

region of M'' moved towards the higher frequencies as the temperature increased. The

shape of observed two overlapped M'' peak curve exhibited as an asymmetric and the

maximum of the M'' (M''max) were not centered at their dispersion regions of M'' in the

measured frequency window, which were indicated as non Debye behavior. The M''max

value shifted towards the higher frequency while increasing the temperature [17]. it is

observed that the shape of each curve is asymmetric of non-Lorentzian type exhibiting a

peak at the relaxation frequency, fmax, with a long tail extending in the region of shorter

167

relaxation time τ =1/fmax, (fmax is the relaxation frequency at M''max and of M'' vs. log ω

curves).

Figure 5.19: Imaginary part of electric modulus M'' vs log ω plots obtained at different

temperatures of the pure and carbon coated LiMnPO4 nanorods

LiMnPO4

168

Figure 5.20: Imaginary part of electric modulus M'' vs log ω plots measured at different

temperatures of the pure and carbon coated LiCoPO4 nanoparticles

169

Figure 5.21: Imaginary part of electric modulus M'' vs log ω plots obtained at different

temperatures of the pure and carbon coated LiNiPO4 nanoparticles

170

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0-6.8-6.4-6.0-5.6-5.2-4.8-4.4-4.0-3.6-3.2

LiMnPO4 LiMnPO4/C LiCoPO4 LiCoPO4/C LiNiPO4 LiNiPO4/C Linear Fit

log

τ

1000/T K-1

The shift in the fmax with the temperature could be explained based on the distribution of

attempt frequencies for the barrier crossover or a distribution of jump or flight distances

following the crossover. There is slight variation in M'' peak height with temperature may

be due to the variation of the ionic relaxation in the samples. The broadness of the M'' vs.

log ω curves is interpreted in terms of the distribution of relaxation times for

distinguishable physical processes.

Figure 5.22: log τ vs. 1000/T of the pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanoparticle samples

171

Figure 5.22 shows the log τ vs. 1000/T for pure and carbon coated LiMPO4 (M= Mn, Co

& Ni) nanoparticle samples. From figure 5.22, Arrhenius linear least square fitted the log

τ vs. 1000/T curve slope gives the relaxation activation energy Eτ. The relaxation time τ

is obtained from 1/fmax of the M'' vs. log f spectra. The relaxation time follows Arrhenius

relation (check the exponential form for minus sign)

From table 5.1, it is observed that the activation energies for the relaxation Eτ are slightly

different than thermal activation energy, Ea.

Table 5.1: The activation energies of carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticle obtained from conductivity and relaxation plots

S. No

Compound Conductivity (σ in S cm-1) at RT

Activation Energy (Ea in eV)

Activation Energy from relaxation plot (Ea in eV)

1 LiMnPO4 3.95X10-7 0.6±0.02 0.57±0.01 2 LiMnPO4/C 4.7X10-5 0.57±0.01 0.53±0.02 3 LiCoPO4 2.46X10-9 0.58±0.02 0.61±0.05 4 LiCoPO4/C 4.51X10-6 0.42±0.02 0.48±0.04 5 LiNiPO4 8.91X10-9 0.72±0.03 0.65±0.01 6 LiNiPO4/C 2.17X10-7 0.68±0.01 0.72±0.03

172

5.3 Wagner polarization method

Figure 5.23 shows the Wagner’s Polarization method for measuring transport numbers of

materials. Olivine structured LiMPO4 (M= Mn, Co & Ni) nanomaterials pellet was

sandwich between ion blocking electrode (graphite) and non-ion blocking electrode (Ag)

as in figure 5.22

[ion blocking electrode(gaphite)] /( LiMPO4 (M= Mn, Co & Ni)) / [ion non-blocking

electrode

(Ag)].

For all three systems, each sample pellet was subjected to dc potential and measured the

resultant current as a function of time using a nanoammeter. In Wagner's polarization

method, the solid electrolyte was placed between two electrodes - one blocking electrode

and the other non-blocking electrode. The current vs. time was monitored for a fixed

applied dc potential. The initial total current ‘i', decreased with time, which was the

depletion of ionic species in the electrolyte and became constant in the fully depleted

situation. At this stage, the residual current is the only electronic current (ie) [18].

The total current (it) flowing in the system is the sum of the currents due to ions (ii) and

the current due to electrons (ie).

it = ii + ie -------- (1)

By measuring the it and ie, ii can be calculated.

Ionic and electronic transport numbers were calculated using the measured currents

(it and ie) and from the following equations (2 and 3)

Ionic transport number ti = ie /it -------(2)

and Electronic transport number te = ie/it -------(3)

173

Figure 5.23: Wagner’s Polarization method for measuring transport numbers

174

Figure 5.24: Current vs time graphs of pure and carbon coated LiMnPO4 nanorods

0 1 2 3 4 5

0.00

0.05

0.10

0.15

0.20

0.25

LiMnPO4

Curre

nt (n

A)

Time ( h)

0 1 2 3 4 5 6 7 8

0.20.40.60.81.01.21.41.61.8

LiMnPO4/C

Curre

nt (n

A)

Time (h)

a

b

175

Figure 5.25: Current vs time graphs of pure and carbon coated LiCoPO4 nanoparticles

0 1 2 3 4 5

0.0

0.5

1.0

1.5

2.0

2.5

LiCoPO4/C

Curre

nt (n

A)

Time (hr)

0 1 2 3 4 5

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

LiCoPO4

Curre

nt (n

A)

Time (hr)

a

b

176

Figure 5.26: Current vs time graphs of pure and carbon coated LiNiPO4 nanoparticles

0 1 2 3 4 5

0.0

0.1

0.2

0.3

0.4

0.5

LiNiPO4

Curre

nt (n

A)

Time (hr)

b

a

177

Figures 5.24 a & b show the current vs time graphs of pure and carbon coated LiMnPO4,

LiCoPO4 and LiNiPO4 nanomaterials respectively. From figures 5.23a & b, total it ionic

ii, and electronic ie currents for pure and carbon coated LiMnPO4 nanorods were

obtained. The transport number values indicated that the carbon coated LiMnPO4

nanorods showed a few order enhancement of the electronic transport number compared

to the pure ones.

From figure 5.25 a & b, total it, ionic ii, and electronic ie currents for pure and carbon

coated LiCoPO4 nanoparticles were obtained and the electronic transport number of

carbon coated LiCoPO4 nanoparticles is higher than pure materials. Similarly, the total it,

ionic ii, and electronic ie currents for pure and carbon coated LiNiPO4 nanoparticles were

obtained from figure 5.26. The electronic transport number of carbon coated LiNiPO4

nanoparticles is more than pure materials, which may be due to the carbon coating. The

transport number values complement the conductivity values of pure and carbon coated

LiMPO4 (M=Mn, Co & Ni) nanomaterials. Hence, an enhanced electronic conductivity

might be due to the presence of carbon over the LiMPO4 (M=Mn, Co & Ni)

nanomaterials. The evaluated transport numbers due to ions and electrons for pure and

carbon coated LiMPO4 (M= Mn, Co &Ni) nanoparticles are listed in table 5.2.

Table 5.2: Electronic and ionic transport number of pure and carbon coated LiMPO4 (M= Mn, Co &Ni) nanoparticles S. No Compound Electronic transport

Number (te) Ionic transport

Number (ti)

1 LiMnPO4 0.004 0.995 2 LiMnPO4/C 0.143 0.856 3 LiCoPO4 0.0397 0.9602 4 LiCoPO4/C 0.1078 0.8921 5 LiNiPO4 0.0178 0.9821 6 LiNiPO4/C 0.1358 0.8641

178

5.3 Conclusion

The impedance measurements were made for all the pure and carbon coated Olivine

structured lithium transition metal phosphate cathode materials in the frequency range of

10 MHz to 1 kHz at various temperatures. The conductivity (σ), ac conductivity,

dielectric permittivity (ε') and electric modulus (M*) were calculated using the measured

real & imaginary parts of the impedance data and pellet dimensions, using the

corresponding inter relation formalisms. From the Arrhenius least square fit of the log

(σT) vs. 1000/T plot, the activation energy of the mobile charge carriers were calculated.

The activation energy was found to be low for the high conducting sample in each

system. The conductivity variation with frequency was fitted to the Jonscher's power law

expression and fit parameters σ0 & ωp were obtained. The conductivity spectral results

were explained with the existing theoretical diffusion controlled relaxation (DCR) model.

The real dielectric permittivity and the conductivity vs. frequency increased with increase

in temperature. The relaxation behavior of the grain interior and grain boundary of the

pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were also obtained

from the analyzed electrical permittivity and electrical modulus data fitted to Havriliak

and Negami function using winFIT software. The electronic and ionic transport numbers

for pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were obtained by

using Wagner’s polarization method.

179

Reference

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Irvine and P. Nunez, Solid State Ionics, 176(2005) 1807-1958.

[3] J. Ross Macdonald, (ed.1, Impedance Spectroscopy, John Wiley & Sons, New York

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836.

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180

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(1974) 639-648.

[17] C.T. Moynrhan, L.P. Boesch and N.L. Laberge, Phys. Chem. Glasses, 14 (1973)

122-125.

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Publishing Company, (1981)

181

FABRICATION AND

ELECTROCHEMICAL

CHARECTARIZATION OF THE

CR2032 COIN CELLS USING THE

DEVELOPED PURE AND CARBON

COATED LiMPO4 (M= Mn, Co & Ni)

NANOPARTICLES

CHAPTER – VI

182

CHAPTER - VI

FABRICATION AND ELECTROCHEMICAL

CHARACTERIZATION OF THE CR2032 COIN CELLS USING THE

DEVELOPED PURE AND CARBON COATED LiMPO4 (M= Mn, Co

& Ni) NANOPARTICLES


6.2 Fabrication of CR2032 coin cells

6.3 Electrochemical characterization

6.3.1 LiMnPO4/Li and carbon coated LiMnPO4/ Li cells

6.3.2 LiCoPO4/Li and carbon coated LiCoPO4/ Li cells

6.3.3 LiNiPO4/Li and carbon coated LiNiPO4/ Li cells

6.4 Conclusions

References

183

6.1 General Introduction The international electrotechemical commission (IEC) is developing standards for the

designation, marking, electrical testing, and safety testing of Li-ion cells and batteries. A

proposed designation and marking system for Li-ion cells utilize five figures in the case

of cylindrical cells and six figures in the case of prismatic cells. For example, the

common round cell that uses the C/LiCoO2 cell chemistry is designated ICR2032. The

first letter “I” designates an intercalation negative electrode. The second letter designates

the type of positive electrode employed, such as C for a cobalt type, N for Nickel type, M

for Manganese type or V for a Vanadium type, etc... The third letter will designate the

shape of the cell, R for round. The next two figures will designate the diameter in

millimeters and then the next three figures the height of the cell, in tenths of millimeters,

as they are 20 mm in diameter and 3.2 mm in height. Battery researchers will use to call

ICR2032 cell as CR2032.

In this chapter, The CR2032 coin cells were assembled with a metallic Li anode to

evaluate the pure and carbon coated LiMPO4 (M=Mn, Co & Ni) and evaluate their

electrochemical properties. There are two types of processes to evaluate the

electrochemical properties of material. The first is rate capability test, here the cell is

charged at slow rate with a constant voltage and then the fully charged cell is discharged

at various current rates to measure the capacity obtained from each discharge rate.

Another one is capacity retention test, in this method; the cell is charged at specific rate

and is discharged at the same rate and measure how long the cell sustains the initial

capacity without significant degradation of the capacity. The effect of carbon coating

184

over LiMPO4 (M= Mn, Co & Ni) on the electrochemical properties of carbon coated

LiMPO4/ Li batteries have been investigated and presented in this chapter.

6.2 Fabrication of CR2032 coin cells

The cathode materials are coated on aluminum foil and metallic lithium is used as anode.

The aluminum foil acts as a current collector for conducting the current in and out of the

cell. Both of cathode and anode materials are delivered to the factory in the form of black

powder and to the untrained eye and these are almost indistinguishable from each other.

Since contamination between the anode and cathode materials will ruin the battery, great

care must be taken to prevent these materials from coming into contact with each other.

For this reason, the anodes and cathodes are usually processed in different rooms. The

flowchart for the general electrode processing is shown in figure 6.1.

The electrochemical performance of CR2032 coin-type cells made up of pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) nanoparticles were evaluated. The cathodes were

prepared by the following process. First, the synthesized samples of pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) were ground to slurry with carbon black (C65,

Timcal cooperation, USA) and PVDF (Polyvinylidene fluoride; Sigma Aldrich) binder in

N-Methyl-2-pyrrolidone (NMP) solvent. The formulation of electrode was 80 (active

material): 15(carbon black): 5(binder) in weight percentage. Cathodes were prepared by

coating of cathode materials slurries on aluminum foil and then dried at 80 oC for 5 h

under vacuum. Lithium metal (Sigma Aldrich) was used as an anode, and a micro porous

plastic film (Cellgard 2400, Cellgard Co., USA) was used as separator and the electrolyte

solution used comprised of 1.5 M LiPF6 in a 1:1:1 mixture of ethylene carbonate (EC),

185

eythyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in weight percent

procured from Sigma Aldrich. We have used DMC: DEC: EMC ratio as 1:1:1 for

electrolytes of LiCoPO4 and LiNiPO4 cathode materials cells to avoid the oxidation

polymerization of EC in to PEC. CR2032 coin cells were assembled for all

electrochemical testing, purchased from Hohsen Crop., USA. Figure 6.2 shows the

schematic diagram of CR2032 coin cell construction. Figure 6.3 shows the photographs

of fabricated CR 2032 coin cells using the prepared LiMnPO4 nanorods.

Figure 6.1: Flowchart for the general electrode processing for the lithium ion batteries

186

Figure 6.2: Schematic diagram of CR2032 coin cell construction

Figure 6.3: Photographs of fabricated CR 2032 coin cells using the prepared LiMnPO4

nanorods

Coin cell

187

6.3 Electrochemical performance

6.3.1 LiMnPO4/Li and carbon coated LiMnPO4/ Li cells

The voltage profiles obtained using the measured voltage between 2.9 to 4.5 V at 1C rate

for the 1st and 30th cycles of cells made up of the pure and carbon coated LiMnPO4

nanorods are shown in Figure 6.4. The discharge curves of LiMnPO4/Li cells showed

continuous sloping because of the electronic structure and nanosized properties of the

material. Kim et. al, suggested that nano-size particles resulted in continuous sloping in

discharge curves of the Li/LiMnPO4 cell, because electronic structure and property of

nano-size particle lies in between bulk crystalline material and individual molecular state

and Jahn–Teller distortions leading to capacity fading [1]. However, the carbon coated

LiMnPO4 nanorods showed better discharge capacity at 1C than the pure ones. The

discharge capacity for the 1st cycle of pure LiMnPO4 nanorods was almost same as the

30th cycle of the carbon coated LiMnPO4 nanorods, indicating an improvement in the rate

capability by coating of carbon over LiMnPO4. From figure 6.4, the LiMnPO4/Li cells are

delivered discharge capacity of 98 mAh g-1 during the 1st cycle and 64 mAh g-1 during

30th cycle at 1 C rate. Carbon coated LiMnPO4/Li cells are delivered 122 mAh g-1 during

the 1st cycle and 30th cycle it was 97 mAh g-1 at 1C rate. Figure 6.5 shows the capacity

retention of the pure and carbon coated LiMnPO4 nanorods at 1C rate. From figure 6.5,

the cells made up of pure and carbon coated materials showed good capacity retention

with cycling. The carbon coated LiMnPO4 achieved 122 mAh g-1 at 1C, which is higher

than the capacity of the pure LiMnPO4 (about 98 mAh g-1). The improved capacity was

attributed to a one dimensional electron transport pathway in LiMnPO4 nanorods [2,3].

The significance of our pure LiMnPO4 nanorods is its increased capacity at 1C compared

188

to recently reported nanocomposite LiMnPO4/C, [4] off-stoichiometric LiMnPO4,[5]

carbon coated LiMnPO4 particles,[6] and also, multiwalled carbon nanotube coated

LiMnPO4 thumb shaped nanorods [7].

Figure 6.4: Discharge profile of the lithium batteries fabricated using pure and carbon

coated LiMnPO4 nanorods

189

Figure 6.5: Capacity retain plot of the lithium batteries fabricated using pure and carbon

coated LiMnPO4 nanorods

190

6.3.2 LiCoPO4/Li and carbon coated LiCoPO4/ Li cells

Figure 6.6 shows the charge-discharge curves of the cells made up of pure and carbon

coated LiCoPO4 samples at 0.1C rate. These cells were characterized between the

voltages of 4.1- 5 V up to 20 cycles. Both pure and carbon coated LiCoPO4 samples

exhibited flat voltage plateau at 4.8 V, which is the characteristics of the lithium

intercalation at 4.8 V (Vs. Li) of LiCoPO4/Li cell. Similarly, cells made up of carbon

coated LiCoPO4 sample delivered high capacity than pure LiCoPO4/Li cell. From figure

6.6a, the pure LiCoPO4/Li cells show the continuous discharge curve, which may be due

to the capacity fading. The discharge capacity of LiCoPO4/Li cells at 0.1C rate during the

1st cycle was observed as 144 mAh g-1, while at the 20th cycle it was 133 mAh g-1. From

figure 6.6b, The LiCoPO4/Li cell shows the flat lithium intercalation voltage curve at 4.8

V (Vs. Li). The discharge capacity of lithium-ion coin cells at 0.1C rate during the 1st

cycle was observed as 180 mAh g-1, while at the 20th cycle it was 162 mAh g-1. The

discharge capacity loss in the Li/carbon coated LiCoPO4 cell was around 18% between

the 1st and the 20th cycles.

Figure 6.7 shows the cycling performance of cells made up of pure and carbon coated

LiCoPO4 samples at 0.1C. The capacities of LiCoPO4/Li, carbon coated LiCoPO4/Li cells

were found to be 144 mAh g-1 and 180 mAh g-1 respectively. The enhancement in

electrochemical performance of carbon coated LiCoPO4/Li cells compared to pure

LiCoPO4/Li may be due to the increased electrical conductivity through carbon coating.

The capacity retentions of both pure and carbon coated LiCoPO4 were almost 80% even

after 20 cycles, which indicate that the pure and carbon coated LiCoPO4 nanoparticle

191

cathode materials prepared by PVP assisted polyol process had the better cycling

performance [8].

Figure 6.6: Discharge profile of the lithium batteries fabricated using a) pure and b)

carbon coated LiCoPO4 nanoparticles

a

b

192

Figure 6.7: Capacity retain plot of the lithium batteries fabricated using pure and carbon

coated LiCoPO4 nanoparticles

193

6.3.3 LiNiPO4, carbon coated LiNiPO4/ Li cells

The discharge curves of the cells made up of pure and carbon coated LiNiPO4 samples,

characterized between 5.3- 4.3 V at 0.1 C shown in figure 6.8. All the cells exhibited

voltage plateau at 5.1 V as a characteristic of the oxidation/reduction reaction of LiNiPO4

with respect to lithium anode. From figure 6.8a, the discharge capacity of pure

LiNiPO4/Li cell is 74 mAh g-1 for 1st cycle and 56 mAh g-1 for 20th cycles at 0.1C rate.

From figure 6.8b, the carbon coated LiNiPO4/Li cell is showing enhanced discharge

capacities like 98 mAh g-1 for 1st cycle and 66 mAh g-1 for 20th cycles at 0.1C rate. Figure

6.9 shows the cycling performance of the cells made up of pure and carbon coated

LiNiPO4 samples at 0.1C. The capacities of pure and carbon coated LiNiPO4 samples

were found to be 74 mAh g-1 and 98 mAh g-1 respectively. The improvement in

discharge capacity for carbon coated sample might be due to the enhancement in

electrical conductivity of material. The capacity retentions of both pure and carbon

coated LiNiPO4 were almost 80% even after 20 cycles, which indicated that LiNiPO4

prepared by PVP assisted polyol process also has the good cycling performance and it

can be used as a high voltage cathode in lithium batteries [9].

It is reported that the electrochemical performance of cathodes could be significantly

improved by adding amount of conductive materials [10-15]. In this work, we have

successfully synthesized pure and carbon coated Lithium transition metal phosphate

nanoparticles with high electrochemical properties compared to earlier reported results.

The discharge capacities of all prepared compounds along with previous work are listed

in table 6.1.

194

Figure 6 8: Discharge profile of lithium batteries fabricated using a) pure and b) carbon

coated LiNiPO4 nanoparticles

a

b

195

Figure 6.9: Capacity retain plot of lithium batteries fabricated using pure and carbon

coated LiNiPO4 nanoparticles

Table 6.1: Discharge capacities of all pure and carbon coated LiMPO4 (M= Mn, Co & Ni) nanoparticles along with reported work

Compound Present obtained capacity (mAh g-1)

Reported capacity (mAh g-1)

References

LiMnPO4 98 99 6, 7 LiMnPO4/C 122 108 6,

LiCoPO4 144 58 7 LiCoPO4/C 178 89 7

LiNiPO4 72 54 9 LiNiPO4/C 97 - -

196

6.4 Conclusion

In this chapter, CR2032 coin cells fabricated by using synthesized pure and carbon coated

LiMPO4 (M= Mn, Co & Ni) cathode materials and their electrochemical properties were

discussed. The discharge capacity of the carbon coated LiMPO4 (M= Mn, Co & Ni)

nanomaterial increased 20% compared to the pure ones. Furthermore, the pure and

carbon coated LiMPO4 (M= Mn, Co & Ni) material shows the good electrochemical

performance, especially rate capability. This improvement is partly ascribed to small

particle size, which acts as an inhibitor for grain growth because small particles improve

bulk lithium transport. Carbon coating upon the LiMPO4 (M= Mn, Co & Ni) materials

will increase the electronic conductivity of the cathodes. Therefore, these electrode

materials show the extremely high discharge capacity and good cycleability. Hence,

newly developed carbon coated LiMnPO4 nanorods, using modified polyol and resin

coating processes will improve the lithium battery performance.

197

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Chem. Solids, 68 (2007)1203-1206.

[2] D. K. Kim, P. Muralidharan, H. W. Lee, R. Ruffo, Y. Yang, C. K. Chan, H. Peng,R.

A. Huggins, and Y. Cui, Nano Lett., 8 (2008) 3948-3952.

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T. Drezen, D. Wang and G. Deghenghi, J. Electrochem. Soc., 156 (2009) A541-

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[7] A. V. Murugan, T. Muraliganth, and A. Manthiram, J. Inorg. Chem., 48(2009) 946-

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[8] P. Ramesh Kumar, M. Venkateswarlu, Manjusri Misra, Amar K. Mohanty, N.

Satyanayana, J. Nanosci. Nanotech., 11(2011) 3314–3322.

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(2011) 4356–4360

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Electrochimica Acta, 51 (2006) 3872–3883.

[11] G. X. Wang, L. Yang, S. L. Bewlay, Y. Chen, H. K. Liu, J. H. Ahn, J. Power

Sources, 146 (2005) 521–524.

[12] J. Cho, J. Power Sources, 126 (2004) 186–189.

198

[13] V. Aravindan, K. Karthikeyan, K. S. Kang, W. S. Yoon, W. S. Kim, Y. S. Lee, J.

Mater. Chem., 21 (2011) 2470-2475.

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Comm., 13 (2011) 726–729.

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199

SUMMARY

CHAPTER – VII

200

CHAPTER-VII

SUMMARY 7.1 Summary and Conclusion The objective of this thesis is to improve the rate capability of LiMPO4 (M= Mn, Co &

Ni) cathode materials by optimizing lithium transport and increasing the electronic

conductivity. Present chapter consolidate all the experimental results observed in the

present investigation of synthesis and characterization of nanocrystalline LiMPO4 (M=

Mn, Co & Ni) powders by PVP assisted polyol process and their surface modification by

conductive carbon for secondary lithium batteries.

The olivine structured LiMPO4 (M= Mn, Co & Ni) are considered as the most

encouraging cathode materials because they are cheaper, less toxic, safety, good energy

and power density , but some problems hind the practical application. For example, low

electronic conductivity and lithium ion diffusion coefficient leads to its lower discharge

capacity and poor rate capability. In order to overcome these problems, I have

systematically investigated the effect of stabilizer on microstructure, electrochemical

characteristics of olivine’s and have optimized syntheses conditions. Furthermore,

lithium transport in particle interior is improved by reducing the particle size without

blocking 1D lithium diffusion as well as electron transport by coating with conducting

carbon.

Polyvinylpyrrolidone (PVP) assisted polyol method was used for the preparation of

LiMnPO4 (M= Mn, Co & Ni) nanomaterials with improved physical properties and

electrochemical properties. Further, carbon coating upon LiMnPO4 (M= Mn, Co & Ni)

nanomaterials is achieved by novel resin coating process for enhancing the electrical

201

conductivity of nanomaterials. XRD and FTIR results of synthesized pure and carbon

coated LiMPO4 (M= Mn, Co & Ni) cathodes show the formation of phase and structure.

SEM images give the information about the microstructures of the synthesized materials.

The LiMnPO4 sample is composed of uniform and dispersed rod like structure and also

both LiNiPO4, LiCoPO4 samples showed sphere like structures under the SEM. HRTEM

images of the prepared carbon coated LiMPO4 (M= Mn, Co & Ni) materials show the

uniformity and nanosize thickness of the carbon coating over the LiMPO4 materials.

Furthermore, Raman Spectra of all carbon coated LiMPO4 (M= Mn, Co & Ni) materials

confirm the nature of the carbon coating in the samples. The transport studies were

carried out by using impedance spectroscopy. The bulk conductivity of all prepared

materials is increasing with temperature. Carbon coated samples delivers high bulk

conductivity than pure samples which might be due to the enhancement in electronic

conductivity of materials. The activation energies of pure and carbon coated LiMPO4

(M= Mn, Co & Ni) materials were calculated from log (σT) versus 1000/T plots. The

super-impossibility of log (σ/σ0) vs, log (ω/ωp) at various temperatures, suggest the

conductivity relaxation mechanism is temperature independent and slight deviation in a

high temperature may be due to the volume of the total unit cell increasing linearly with

temperature for all olivine samples. The increase in ε’ with decreasing frequency can be

attributed to the contribution of charge accumulation at the interface. This leads to a net

polarization of the ionic medium. Whereas at high frequencies, the periodic reversal of

the field takes place so rapidly such a way that there is no charge accumulation at the

interface, resulting in constant ε’ value in olivine samples. In the modulus curves, the

continuous line represents the simulated M" curve, whereas the symbols correspond to

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the experimental data. From these figures, it is observed that the shape of each curve is

asymmetric of Non-Lorentzian type exhibiting a peak at the relaxation frequency, fmax,

with a long tail extending in the region of shorter relaxation time τ =1/fmax, (fmax is the

relaxation frequency at M''max and of M'' vs. log ω curves) and it is also observed that

there is a shift in peak frequency & variation in peak height with temperature. Finally,

enhancement of electrical conductivity in carbon coated LiMPO4 (M= Mn, Co & Ni)

materials confirmed by the Wagner Polarization method through finding the transport

number of electrons and ions. The LiMnPO4/Li CR2032 coin cells fabricated by using

pure LiMnPO4 nanorods prepared by PVP assisted polyol method delivers higher

discharge capacity of 98 mAh g-1 at 1 C. The remaining two cells which are made up of

pure LiCoPO4 and LiNiPO4 also delivered 144 mAh g-1 and 72 mAh g-1 of the discharge

capacity at the rate of 0.1C, respectively. Furthermore, it is confirmed that the

combination of carbon coating through resin coating technology brings further

improvement in electrochemical properties of LiMPO4 (M= Mn, Co & Ni) cathodes.

Hence, newly developed pure and carbon coated LiMPO4 (M= Mn, Co & Ni)

nanomaterials by PVP assisted polyol and novel resin coating processes are suitable

cathodes for the high voltage Lithium ion batteries.

7.2 Scope of the further work on surface modified cathode materials

There has been a significant research work over a decade in the area of surface modified

cathode materials in order to enhance the performance of rechargeable lithium batteries

due to the huge requirement of portable energy devices. Synthetic approaches (physical

and chemical) towards the fabrication of surface modified nanocrystalline cathode

materials received more attention to meet the requirements such as uniform and

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homogeneous coating. In order to investigate the effect of surface modified cathode

materials in lithium battery application, various techniques such as electrochemical

impedance spectroscopy, accelerating colorimetry, atomic force microscopy,

electrochemical force microscopy, etc., are can be used.

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PUBLICATIONS

Research Publications in reputed international journals

1. Carbon Coated LiMnPO4 Nanorods for Lithium Batteries, P. Ramesh Kumar, M.

Venkateswarlu, Manjusri Misra, Amar K. Mohanty, N. Satyanarayana, Journal of

Electrochemical Society, 158 , A227-A230, 2011

(Selected in Vir. J. Nan. Sci. & Tech. / Volume 23 / Issue 2 / surface and interface

properties)

2. Synthesis, Characterization and Electrical Properties of Carbon Coated LiCoPO4

Nanoparticles, P. Ramesh Kumar, M. Venkateswarlu, Manjusri Misra,

Amar K. Mohanty, and N. Satyanarayana, Journal of Nanoscience and

Nanotechnology, 11, 3314–3322, 2011

3. Nanofibers: Effective Generation by Electrospinning and Their Applications ,P.

Ramesh Kumar, N. Khan, S. Vivekanandhan, N. Satyanarayana, A. K.

Mohanthy, and M. Misra, Journal of Nanoscience and Nanotechnology, 12, 1-25,

2012

4. Soy Bean (Glycine Max) Leaf Extract Based Green Synthesis of Palladium

Nanoparticles. P. Ramesh Kumar, S. Vivekanandhan, M. Misra, A. K. Mohanty,

N. Satyanarayana, Journal of Biomaterials and Nanobiotechnology, 3, 14-19, 2012

5. Three-dimensional lithium manganese phosphate microflowers for lithium-ion

battery applications, P. Ramesh Kumar, M. Venkateswarlu, N. Satyanarayana,

Journal of Applied Electrochemistry, 42, 163–167, 2012

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6. Enhanced conductivity and electrical relaxation studies of carbon coated LiMnPO4

nanorods, P. Ramesh Kumar, M. Venkateswarlu, Manjusri Misra, Amar K.

Mohanty, N. Satyanarayana, Ionics 2012 (Accepted)

Research Publications in International Conference Proceedings

1. Preparation of carbon coated lithium manganese phosphate nanorods by modified

polyol process, Ramesh Kumar, P., Venkateswarlu, M., Satyanarayana, N;

Technical Proceedings of the 2009 NSTI Nanotechnology Conference and Expo,

NSTI-Nanotech 2009 1, pp. 121-124.

2. Synthesis of Carbon Coated Lithium manganese phosphate Nano particles.

Ramesh Kumar, P., Venkateswarlu, M., Satyanarayana, N; Materials research

Symposium 2009, IITB.

3. Synthesis of Lanthanum Oxide Nanoparticles By sol gel combustion Process, P.

Ramesh Kumar, O. Padmaraj, M. Venkateswarlu , N. Satyanarayana, International

conference on Sol-Gel Process for Advanced Ceramics (SGPAC -2009) IGCAR,

2009.

4. Synthesis and ac conductivity studies of PEO + LiClO4 + La2O3 + MoO3

nanocomposite polymer solid electrolyte, Ramesh Kumar, P., Venkateswarlu, M.,

Satyanarayana, N; Technical Proceedings of the 2011 NSTI Nanotechnology

Conference and Expo, NSTI-Nanotech 2011, pp.546-549 .

5. Synthesis and Transport studies of Nanocomposite (PEO + LiClO4 + MoO3):

Al2O3 Polymer solid Electrolyte, P. Ramesh Kumar, M. Venkateswarlu , N.

Satyanarayana, ISAEST 09, Chennai.

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6. Synthesis and electrical conductivity studies of PEO + LiClO4 + TiO2 + MoO3

nanocomposite solid polymer electrolyte, P. Ramesh Kumar, M. Venkateswerlu,

N. Satyanarayana, Technical Proceedings of the 2012 NSTI Nanotechnology

Conference and Expo, NSTI-Nanotech 2012, USA.

7. Carbon Coated Nano Cellulose for Lightweight Energy Storage Devices, P.

Ramesh Kumar, N. Satyanarayana, Amar K. Mohanthy, Manjusri Misra, 2010

BioEnvironmental Polymer Society (BEPS) Annual Meeting, Toronto, Canada.

synthesis and characterization of carbon coated...

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