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CHAPTER - I1

CHAPTER-I1

EXPERIMENTAL TECHNIQUES

A number of techniques are a~ai lable for the synthesis and

characterisation of cathode and polymer electrolyte materials for lithium ion

batteries. Some of the synthesis and charaterisation techniques employed in this

work are briefly explained here.

2.1. SYNTHESIS TECHNIQUES

Synthesizing techniques that Mere used for the preparation of cathode

and f'erroclectric flller materials \ \err solid-state. ball milling. solution co-

precipitation and citrate gel technique. Solvent cast technique mas used for the

synthesis of composite polymer electrolyte films.

2.1.1. Solid-state and Ball-milling technique

Grinding by hand in an agate mortar remains a useful method in modern

synthesis. For many small-scale studies, hand mortaring is perfectly adequate

for reducing the sizes of powder particles. Grinding of materials b) hand ~ i t h a

mortar and pestle was used in this study for mixing and particle-size reduction

of small amounts of material (-50 g). Once the homogenous precursor powder

Chapter - I1

was obtained, it was placed into a furnace and heated to a suitable temperature.

Gases were generated and released followed by chemical interaction to form

final product(s1. Generally. this material may be ground finel) again and then

put into a furnace a second time (for a ..2"d sinter". or to allow cysta ls to form

and grow in size). Sometimes powder materials are hydraulically pressed into a

pellet shape to increase the interactivity between the powder grains and fbrm a

higher quality andlor h~gher density final product

Ball mills. also known as centrifugal or planetary mills, are devices used

to rapidly grind materials to colloidal fineness (approximately I micron and

below) by developing high grinding energy via centrifugal andlor planetary

action. To grind a sample in a ball mill. the particle size should alread) have

been reduced to less than 10 mm. uslng a mortar and pestle if necessar). The

samples are placed in one of the b o ~ l s and several balls arc added. Samples

can be run wet or dry

Fig. 2.1. Grinding mechanism within the bowl of a ball mill.

D. Shanrnukaraj Ph.D. Thesis (2007) 68

Each bowl sits on an independent rotatable platform. and the entire

assembly of four bowls is also rotated in a direction opposite to the direction of

the bowl platform rotation. In planetary action, centrifugal forces alternately

add and subtract. The grinding balls roll halfway around the bowls and then are

thrown across the bowls. impacting on the opposite walls at high speed as

shown in Fig. 2.1. Grinding is further intensiiied by interaction of the balls and

sample. Planetary action gives up to 20g acceleration and reduces the grinding

time to about 213 of a simple centrifugal mill (one that simply spins around).

Grinding media is available in agate, sintered comndum, tungsten

carbide. tempered chrome steel. stainless steel. zirconium oxide, and polyamide

plastic. The exact type of bowl and balls that are used depend on the type of

material being ground. For example. very hard samples might requlre tungsten

carbide balls in steel ho\+ls. For Qpical use. agate is a good choice. As with any

riiethod of grinding. contaminat~on of the sample with the grinding unit

material can be a complication.

2.1.2. Co-precipitation

Chemical co-precipitation can provide uniform nucleation growth and

aging of the particles in the solution. The size and morphology of the panicles

can be manipulated by controlling the different reaction parameters: a

technique generally inexpensive to perform that relies on simple coordination


Chapter - I1

chemistry allowing the synthesis of the required solid "compound" w ~ t h the

desired composition and at high uniformity. On the other hand, the process of

establishing and controlling the precipitation conditions are quite complex.

Materials for co-precipitation are usually dissolved in a suitable solvent

most often water. For example Lu and Dahn [ l ] synthesized

I.i[Cr,Li,l~i.,,3,Mn(2,3.~,3,]02 with lithium acetate. chromium nitrate and

tnangnnese nitrate. all of wh~ch are easil) water soluble. After adding the

correct ratio of materials into continuously stirred water. they added

ammonium hydroxide solution and metal hydroxide products which started

forming a cloud of precipitation in the solution. Heating was done to evaporate

the water and the mix had a slightly jelly like consistency. The mix was then

d r ~ e d thoroughly by purtlng In an oven at 130 "C overn~ght and then ground up

In d mechan~cal mill to make sure no segregat~on of ~ngred~ents had occurred

du r~ng the preblous process The resultant powder ivas then heated at 900 C in

a turnace to ) ~ e l d LI[C~,LI I ,.3,Mn,2 3 > , I 10:

2.1.3. Sol-gel

The sol-gel technique can be described as chemical technique of

simplicity and effectiveness to synthesize different type of inorganic and

organic-inorganic hybrid materials. which can be used in solid state d e ~ i c e s .

Wide ranges of new and known materials containing oxide components have

D. Shanmukaraj Ph.D. Thesis (2007) 70

Chapter - I1

been successfully prepared in recent times 12-41, This process allows des~gning

rhc morphologc of electrochemical materials by which the propenics of the

surface and interface can be modified. Fig. 2.2 shows the schematic d~agram of

the sol-gel process. Sol-gel technique follows the route line of hydrolysis and

pol\.merisation to form amorphous or crystalline material at low temperature

processing in solut~on state [3 . 51.

The sol-gel process takes up two different routes to prepare various

tqpes ot amorphous nlater~al. (a) collo~dal process and (b) alkox~de routes

Colloidal process in\~olves the dispersion of particle of colloidal size in

aqueous s o l ~ e n t medlum to form a sol and thus formed sol is destabilised in a

controlled manner. by allowing the particles to approach each other to

overcome the stabilising barrier. This could be achieved by heating, freezing,

adjusting the pH of the sol to obtain a gel. The solvent from the gel can be

removed by maintaining it at a particular temperature and then sintered to give

a cps~a l l i ne~dense amorphous so l~d niaterial

Sol-gel process through alkoxide roure involves the following steps;

mixing, casting. gelation. aging, drying. chemical stabilisation and

densilication. Metal alkoxide is the main precursor chemical, which requires

the use of an organic solvent. normally an alcohol, In order to act as mutual

solvent for the alkoxide and water for hydrolysis. The citrate gel process is a

modified sol-gel technique in which the precursor ingredients include metal

nitrates dissolved in c i t r~c acid and water.


Fast fire k' I Controlled slow

heating rate _1

Fig. 2.2. Schematic diagram of the sol-gel process.

D. Shlmmukamj ph.D. Thesis (2007) 72

Chapter - I1

2.1.4. Solvent casting technologv for the production of films

Nouadays. solvent cast technolog\, is becoming increasingl? attractive

for the production of films with extremel) high quality requirements. The

advantages of this technology include uniform thickness distribution.

maximum optical purity and extremely low haze. The optical orientation is

virtually isotropic and the films have excellent flatness and dimensional

stability. Solvent casting is an important commercial technique utilized to

fabricate thin layered films for diverse applications. Most familiar is the solvent

casting o f cellulose acetate (CA) for photographic films having good

dimensional stabilit!. clarit>, flexibility and fiacturr resistance. Some of the

advantages of solvent casting method (as compared to the melt process) are as

t'ollo\hs.

1. Higher quality (uniformity) and thinner film

2. Freedom from pinholes and gel marks

3. Purity and clarity

4. Lack of residual stresses

5. Possible to produce patterns or dull finishes

Thc solvent-cast process consists of dissolution of the film ingredicnts in

a suitable carrier that conveys the solution through a drier &here the solcent is

ecaporated. The resulting film is removed from the substrate and wound into

rolls or cut into desired shape as required


Chapter - I1 2.2. TEST CELL CONSTRUCTION

2.2.1. Cathode paste mixing

Materials to be evaluated as cathode materials for storing and releasing

Li should be in the form o f ' a powder uith particle size < -10 pm. Cathode

substrates should be prepared first. For CK2032 coin cells, Al disks ofdiameter

15.5 mm and thickness between 0.50 and 1.10 rnm are suitable. These

substrates are cleaned and de-burred by abrading with glass paper or SIC

abrasive paper and then degreased in alcohol in an ultrasonic bath. Care should

to be taken not to let the discs become re-oxidised by exposure to atmosphere

once abraded and degreased Discs are then weighed carefully and individually

so that their masses are known exactly.

If the cathode material has a rebistance of <-lo0 MR then it is mixed

\s ith -1 5 wt04 of acetylene carbon-black as a conductive matrix and - I ? \vt O h

of poly vinylidene difluoride (PVdf) as an adhesive. Cathode material of

resistance >I00 MR (eg: 5GR) are mixed with -20wt% of acetylene carbon-

black (to improve conductivity).

Powder ingredients are dry-ground in an agate mortar for 10 minutes.

then a fefi drops of ethanol is added to make a thin paste and the mix is wet-

ground in the agate mortar for I0 more minutes. Drops of dimethyl phthalate

(UMP) or normal methyl pyrolidinone (NMP) are then added and ground in the

mortar for 5 minutes to make a smooth. thin paste. The paste is applied to the


Chapter - I1

entire upper surface of Al cathode substrates via a nylon or camel-hair artist's

brush and then allowed to dry in a vacuum oven between 50 and YO "C' for at

least one hour.

After drqing. cathode disks are compacted using a hydraulic press at

- 40 MPa between smooth dies to aboid voids. to homogenise and optimize the

density and thickness of the cathode coating. Then the disks are re-weighed to

determine (by subtraction) how much "active mass" has been added by the

cathode-pasting process. The active mass is vital for determining the exact

charge and discharge capacity of the material. Disks are then baked at high

temperature (120 - 170 "C) to make the paste adhere strongly to the Al

subitrates.

2.2.2. Cell construction and assembly

1-he fabricated electrodes are placed into an argon filled glove box

(MBraun Unilab) ready for assembly into CR2032 test cells. A schematic

diagram of a true electrode CR2032 test cell is presented in Fig. 2.3. Standard

'.half cell" test cells are assembled by placing the electrode to be tested at the

bottom of the cell followed by a microporous polqpropylene separator

(Celgunrd 250 '~ ' ) . Pure 1.i foil is used as an anode on a clean aluminium

backing disc. Electrolyte is then added to the cell which is Merck LP30-IM

LiPF6 in a 1:l mixture by volume of ethqlene carbonate (EC) and dimethyl


c h a w - n

carbonate (DMC). A "crinkle washer" flat profile stainless steel spring is used

to provide the constant compaction function.

I C n s i n g (Hermetic)

Anode Subsink

Anode Maierpl

Electrolyte

P o l ~ s mepantor mernbnn

I

Fig. 2.3. Basic design of test cells for charge-discharge and similar tests.

2.3. CHARACTERISATION

2.3.1. Structural characterisation

2.3.1.1. X - Ray Dtflraction (XRD)

X-Ray Diffraction (XRD) measurements were carried out using x-ray

diflixctometers (XPERT Philips analytical and Philips PW 1730) with CU-K,

radiation. Scans were typically acquired at an accelerating voltage of 40 kV

and current 20 mA with a scan rate of one degree per minute. XRD

measurement was carried out by smearing the powder samples on slides using

ethanol. The x-ray diffiactometer (Philips PW 1730) is shown in Fig. 2.4.

D. Shanmubraj P~.D. Thesis (2007) 76

cham-n

Based on the interaction of x-rays with periodic arrangements of atoms in

crystalline materials, details on the structure of the material can be gathered.

The interactions which make this analysis possible are found to satisfy Braggs

Law as shown in Eqn. 2.1 (61.

nA = 2dhk, Sin8 2.1

Where

n is the order of reflection and is any integer such that sin0 i 1

h is the wavelength of x-ray

dhkl is the distance between two adjacent parallel planes

0 is the incident angle of the x-ray beam to the planes of atoms

Fig. 2.4. X-ray diffractometer (Philips PW173O).

By subjecting materials to incident x-rays at a variety of angles an x-ray

diffraction pattern can be produced. An X-Ray Diffraction (XRD) pattern

typically consists of a number of peaks of various intensities over a range of

angles. Analysis of the angular positions, peak intensities and shapes can then

he used to give information on the crystal structure and physical state of the

material being investigated [7].

Information that can he gained from application of XRD includes

identification and quantitative analysis of crystalline compounds. crystal

structural determination and analysis of residual stress and crystalline size [7]

Accurate determination of the interplanar distance (dhll) and relative intensities

is necessary for phase identification [7]. Routine identification can however be

carried out by comparison to the data puhlished in the x-ray pouder diffraction

file b> the Joint Committee on Powder Diffraction Standards (JCPDS).

Kesidual stress can affect peak width as a result of microstress while their

position can be influenced by marcostresses [7]. The peak width is however

also influenced by crystallite size with finer crystalline size materials exhibiting

broader peaks. Crystallite size can he estimated from XRD patterns using the

Scherrer equation as given in Eqn. 2.2 [8].

Where K is a constant which is frequently taken as 0.9


h is the wavelength of the source used

t is the particle dimension

p is the peak width in radians

2.3.1.2. Fourier Transform Infrared (FTIR) spectroscopy

Molecular vibrational information can be obtained from the absorption

of infrared radiations and also from the inelastic scattering of light. The study

of molecular structure by spectrometry depends primarily on the existence of

the vibrating motion of atoms within the molecule. These motions in turn

depend on the nature and arrangement of constituent atoms. Radiant energy,

particularly infrared, incident upon matter, is affected by the presence of such

motions. A study of this behaviour of infrared radiation is thus capable of

giving indirect but very valuable information on molecular structure.

Fig. 2.5. Fourier Transform Infrared (FTIR) spectrometer.


Chapter - I1

'l'he Fourier Transform Infrared (FTIR) measurements for the

synthesized cathode materials were taken using a FTIR spectrometer (ABB

ROMEM MB104) shown in Fig. 2.5. The samples were pelletised with KBr.

FI IK spectrometer is based on a Michelson interferometer that proxides a

spectrum in the time domain, which is Fourier-transformed by a computer to a

spectrum in the frequency domain. The sample can be scanned repeatedly and

the accumulated spectra can be averaged. thu5 producing a representative

~nfrared spectrum of a r e v high signal-to-noise ratio. This enables the

measurement of samples containing a x e p low concentration of the active

materials

Its main appl~cations are: to study the intramolecular forces,

intermolecular forces or degree of association in condensed phases and in the

determination of molecular symmetries. Other applications include. the

identification of functional groups or compound identification. determination

of' the strength of chemical bond. structural elucidation and the calculation of

therniodynamical properties.

2.3.2. Microstructural characterisation

The microstructure of the electrode materials is also important to the

electrochemical properties of materials and a variety of techniques can be used

to investigate it.


Chapter - I1

2.3.2.1. Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy (SEM) investigation was carried out

using an electron microscope (JEOL JSM-6460A) shown in Fig. 2.6. SEM is a

powerful technique for investigating the microstructure of a variety of

materials. The SEM is similar in many ways to a reflected light microscope but

it makes use of a beam of electrons rather than light. The working distance in

the SEM is the distance between the sample and the final objective lens and is

an important parameter for the SEM. A short working distance provides higher

Fig. 2.6. Scanning Electron Microscope (JEOL J S M M A ) .

Chapter - I1

resolutions hut at the expense of depth of field while a long working distance

probides a high depth of field at the expense of resolution. 'The \rorking

distance is not the only parameter that influences the image but also depends on

the size of the electron spot which in turn depends on the magnetic elecrron-

optical system which produces the scanning beam. The resolution is also

lim~ted by the size of the interaction volume. or the extent to which the material

interacts with the electron beam.

The SEM has compensating advantages. though, including the ability to

image a comparatively large area of the specimen: the ability to image hulk

~iiaterials (not just thin films or foils); and the variety of' analytical modes

abailable for measuring the composition and nature of the specirncn.

Depending on the instrument. the resolution can fall somewhere between less

than 1 nm and 20 nm. In general. SEM images are much easier to interpret than

Iransmission Electron Microscope (TEM) images. Prior to examination

samples were mounted on aluminium stubs with carbon conductive tape.

2.3.2.2. Transmission Electron Microscopy fTE.M)

The panicle size of the carbon coated LiFel'O, particles prepared in this

work u a s studied using a Transmission Electron Microscope (HRTEM. 300 kV

JEOL JEM-3000F) as shown in Fig. 2.7. The material to be studied was

suspended in a droplet of ethanol and some of the material adsorbed onto a Cu

b. Shanmukaraj Ph.D. Thesis (2007) 82

Fig. 2.7. Transmission Electron Microscope (HRTEM, 300 kV JEOL JEM-3000F).

TEM specimen grid by carefully immersing the Cu specimen grid in the

ethanol suspension. The Transmission Electron Microscope (TEM) operates on

the same basic principles as the light microscope but uses electrons instead of

light. TEMs use electrons as "light source" and their much lower wavelength

make it possible to get a resolution a thousand times better than with a light

microscope. Particles to the order of a few angstroms (10.'' m) can be viewed

with TEM. The possibility for high magnifications has made the TEM a

valuable tool in materials research. A "light source" at the top of the

microscope emits the electrons that travel through vacuum in the column of the

Chapter - I1

microscope. Instead of glass lenses focusing the light in the light microscope,

the TEM uses electromagnetic lenses to focus the electrons into a v e q thin

beam. The electron beam then travels through the specimen under study

Ilcpending on the density of the material present. some of the electrons are

scattered and disappear from the beam. At the bottom of the microscope the

unscattered electrons hit a fluorescent screen, which gives rise to a "shadow

image" of the specimen with its different parts displayed in varied darkness

according to their density. The image can be studied directly by the operator or

photographed with a camera.

2.3.3. Thermal analysis technique

2.3.3.1. Diffrrenfial Thermal .Ana!r.sis (DTA)

IlitTerential Thermal Anal!.sis (DTA) for the prepared pol?mer

elccti.ol~tes b a s taken using a d~tt'erential thcr~i~al anal)ser (Linseis) as shown

in Fig. 7.8. It IS one of the simplest and most Lvidely used thermal analysis

technique. The difference in temperature. AT. betiveen the sample and the

reference material is recorded while both are subjected to heating programme.

The classical DTA instrument is shown in Fig. 2.9. The DTA curves should be

marked either endo or cxo directions. The negative peak is called as an

cndotherrn (AH is positive, e.g. melting) and for an exotherm (AH is negat i~e .

e.y. oxidation) the peak will he in the posit~ve d~rection.


Fig. 2.8. Differential thermal analyser (Linseis).

Fig. 2.9. Classical apparatus of differential thermal analysis (S = samplc; R = reference).

Chapter - I1

The reference material should have the following characteristics:

8 It should undergo no thermal events over the operating temperature

range.

It should not react with the sample holder or thermocouple

*:* Both thermal conductivity and heat capacity of the reference should he

similar to those of the sample.

AIZO; and SiC have been extensi~el) used as reference substance for

inurganic samples. while for organic compounds. especiall!, for polbmers. octyl

phthalate and silicone oil were used as reference substance.

2.3.4. Electrochemical characterisation

Techniques such as constant current charge'discharge and cyclic

\oltammetr> are useful for observing a \ a r i e ~ of electrochemical properties.

2.3.4.1. Constant current (galvanostatic) charge - discharge

In this study constant current charge-dischargt. testing was carried out

on 5 V-5 mA B a n e 9 Test System (BTS) (Neware) utilising a current ranging

from 20. 40. 80 and I00 PA. The experimental setup for testing cells is shown

schematically in Fig. 2.10. Charging and discharging of cells *ere carried out

hetween the voltage end points of 2.75 V to 4.25 V for LiFePOJ/C

materials and 2.75 V to 4.95 V for LiFe,.,Co, PO, cathode materials. The


N e W u e BMey T e e System

( 8 Ch-el. I I Can el CsU Y2 CeU U3

Fig. 2.10. Contemporary equipment for charge-discharge testing.

"Neware" rack-mounted batter) testers can manage tests for upto 8 cells

simultaneously. The controlling sotiware used was either Nesare Celltest 3 1.

3.2 or t i w a r e RTS.

\'hen carrying out constant current charging to determine the capacity.

two different criteria are frequently used to determine the appropriate current to

be used. These are hascd on either the 'electrode surface area' or its 'mass'.

The current denslt) criterion is based on the electrode surface area. where the

cument density is simply the current divided by the electrodes surface

area.


Chapter - I1

Fig. 2.11. Variation of current density with electrode diameter for a fixed current

of 50 uA.

Fig.

1:; I .'- I,. "

2.12. Variation of C-rate with electrode mass for a fixed current of 50 PA.


Chapter - I1

'The C-rate is the other method. for e.g. LiFePO, has a theoretical

capacity of 170 mAhig over a period of time. A discharge current of 34 mAig

ocer five hours is referred to as the C'J5 rate ( 5 x 34 = 170) and 17 mAtg over

ten hours is referred to as CI10 rate. When the charging current is tixed, the

variation of the current density to the surface area of the electrode and C-rate to

the mass of the electrode is as shown in Fig. 2 . I 1 and Fig. 2.12. Such variation

should be taken lnto account as the measilred capacity \.aries with the current

densit) and C-rate employed

3 3.4 1 2. C'apuciry

A given discharge capacity can be represented as a reversible and

irreversible capacity as shown in Fig. 2.13.

Fig. 2.13. Schematic diagram illustrating reversible and irreversible capacih.


Chapter - I1

The rebersiblc capacitj represents the portion of the capacltg that was

re~ers ible on charge and is given by the charge capacity. The irreversible

capacitc on the other hand is the portion of the capacity that cannot be

recovered on charge and is given bc the difference between the discharge and

charge capacities. Capacity is often expressed in mAh or .4h on commercial

cells.

'l'he specific capacity (niAh1g) is the capacity (mAh) divided b! the

mass (g) of the electrode material whereas the \olurnetric capacit! ( m n h em')

is the capacit) dl\ ided b) the \olume occupied b) the electrode material.

[Inless and otherwise stated the capacit? being referred to in t h ~ s thzsis is the

specific capacit?. The theoretical capaclt! can he calcul~ted using a

relationship between the number of moles of lithium in the reaction product

and the molar mass of the lithium host as shown in Eqn. 2.3.

Fn Specific Capacity =

3.hhl

Where t is Farada>'s conslant 96187 (C mole)

n is the number of moles of lithium in the reaction product

M is the molar mass in grams of the lithium host

0. Shanmukaraj Ph.D. Thesis (2007) 90

Chapter - I1

2.3.4.1.3. Cvcle l ~ f e

Cycle life is typically considered as the number of cycles it takes for the

d~scharge capacity to tall to 80% of the lnitial discharge capaci). C'!cle life

and variation of capacity with cycle number can he obtained from

charge'discharye data.

2 3.1 1.l . Chulomhic eflicfenoy

The coulombic efEciency can he calculated from the discharge and

iuhsequent charge capacity as shown in Eqn. 2.4. This can be considered as a

measure o f a nuniber of propenies including the ease ui th \\hich lithium ions

cdn be extracted fiom the structure (occurs during charging). .4 h~gher

coulomhic e f f i c~mcy indicates that lithiuln ions are more difficult to evtract

from the struclure and hcnce the re\erslbilir\ c~t'the reaction Ir reducsd

,, - Discharge capacity CoulombicEFficiency ( , o ) - xl00 2.4

Charge capacit)

2.3 -1 1 5. L)(ffcrcn/iul c,~~pacir>'

The differenliat~on of charge and discharge protiles (cnpacir? ~ i t h

respect to voltage) can be used to further assess the reaction mechanism and the

changes that occur in the reaction mechanism as shown in Fig. 2.14. The lower

portion of the differential capacity plot corresponds to the discharge bhile the

D. Shanmukaraj Ph.D. Thesis (2007) 9 1

Chapter - I1

upper to the charge. I f the corresponding charge-discharge peaks can be clearly

identified then the reversible potential E, and overpotential q can be calculated

using Eqn. 2.5 and 2.6. respectivel) 191.

Fig 2.14. Example ofa differential capacih plot.

D. Shanmukaraj Ph.D. Theois (2007) 92

Sharp peaks in differential capacity plots are typical of large crystalline

materials while broad peaks are typical of nanocrystalline materials where

reaction plateaus are less defined.

2.3.4.2. Cyclic L'o1tamrnelrq. (CV)

Cyclic Voltammetry (CV) for the synthesized polymer electrolytes was

carried out on an electrochemical work station (CH660B) over the \.ullage

range 0.01 V to 3.0 V at a scan rate of 1 mV per second. In Cyclic

Voltammetry (CV), the potential of a small, stationary working electrode is

changed linearly with time starting from a potentiai where no electrode reaction

occurs and moving to potentials where reduction or oxidation of a solute (the

material being studied) occurs. After traversing the potentiai region In which

one or more electrode reactions takes place. the direction of the linear sneep is

reversed and the electrode reactions of intermediate and products. formed

during the forward scan can be detected. The time scale of the experiment.

controlled by the scan (or sweep) rate and the total potential traversed. can be

varied over the range of 10'-10.' s through quantitative experiments.

The position (potential) of peaks in CV can be used to detcrm~ne the

reaction occurring at the given potential while the current describes the

intensity of the reaction. The separation of respecti\e oxidation and reduction

peaks for a given reaction also provides an indication of the stability and

reversibility of the reaction.


- -

2.3.5. Ac impedance spectroscopy

Ac impedance spectroscopic measurements were carried out for the

synthesized cathode materials using a 1.CR meter (1-IP 4284A) and the

experimental setup is shown in Fig. 2.15. The impedance measurements for the

polymer electrolytes were taken using an electrochemical workstation

(CH 660R).

Fig. 2.15. Experimental setup for ac impedance analysis.

Ac impedance spectroscopy involves the application of a small potential

perturbation (E) at various frequencies (f) at a given dc potential (E&) as shown

in Fig. 2.16. The current response is monitored and as a result the variation of

resistance with frequency for the material can be examined.

/

Time

Fig 2.16. Basics of ac impedance technique.

Impedance spectroscopy is a powerful technique for the stud! of fast

ionic conductor materials. as it enables the bulk ionic conductl\it\ to be

resol\ ed tiom other resistl\,e or capacl t i~e elements \\lthin these conductors.

it single crystal ionic sample would show a sen~icircle in an impedance

plot as shown i l l Fig. 2. I 7 [ lo] . rrsult~ng from the bulk resistance and the hulk

capacitance of the crystal. This is modelled by an equibalent circuit of a

resistor in parallel with a capacitor and is shown in Fig. 2.18 [lo].

Measurement of the electrical conductivity for polqcr~stalline materials [ I I ]

using impedance spectroscopy provides information relating to the electrical

behavior of both the grain interiors and to the grain boundar? regions.

D. Shanmukaraj Ph.0. Thesis (2007) 95

Chapter - I1

Fig. 2.17 A simulation o f the semicircle obtained for an impedance plot of a 1 MQ resistor i n parallel w i th a 10 pF capacitor (an ideallsed ionic conductor). The crosses represent simulated ionic conductivity measurements from 0.1 Hz to 10 MHz at regular intervals in log (frequency) (101.

Fig. 2.18 The equivalent circuit for an idealised ionic conductor, consisting of a single sample capacitance i n parallel ~ i t h the bulk crystal resistance

1101.


Chapter - I1

In Fig. 2.19 (a), the equivalent circuit for the electrical response of a

polycrystalline material is shown. The circuit has a direct relationship to the

frequency

2''

2'

'ha R e R,

Fig. 2.19. a) Equibalent circuit for the electrical response of a polycrystalline sample showing contributions from the grain interiors (gi), grain boundaries (gb) and electrolytelelectrode interface (e); b) Complex impedance plot corresponding to the circuit in a).

complex impedance plot (Fig. 2.19 b), in which 2". the imaginary part of the

complex impedance, is ploned against Z'. the real part. for a ~ i d e range of

frequencies (typically 10-'-10~ Hz).


Chapter - I1

l 'he frequency increases as shown in the arrou in Fig. 2.19h. The

highest frequency is located at the origin. Comparing with the other two. the R,

could normally be ignored due to the use of very conductive metal except for

the imperfect contact. which causes interfhcial contact resistance [ I 1 j.

The capacitance C,,. Cgbr and C, corresponding to the grain interior,

grain boundary and electrode, respectivelq, can be obtained from the dielectric

relaxation peaks. uhich have their specific capacitance reference value 1121.

The ac theop is concerned uith sinusoidnl varqing currents and toitages

of the Ibrm

E = E,, (cos wt * .i sin on) 1 7

and the real part of the expression E,,cos wt and locos wt represents the

obsenable quant~ties. These equations all have the form I = E/Z uhere Z = R.

1,' ,jtr,C and joL for resistance, capacitance and induction respectively.

Modulus Z is known as electrical impedance and is exprrssed in unlts of ohms.

-1'11~ impedance behaves mathematicall) in sim~lar manner to resistance.

Z = ( I / R L 1 , ' - ' JWC

2.9

1~ 1 - (1 IR' + 11 t o 2 ~ 2 ) ' . 2.10

This is the same expression as found for the impedance by graphical means.

The frame of the complex impedance may be generalized to

z* = Z' - iZ" 2 I 1


Chapter - XI

'The other related complex quantities similarly defined

Complex admittance Y' = Y' - jY" =(z')-' 2.12

Complex permitivity E' = Y' i ' oCc = E'- je" 2.13

Complex modulus M' = (c').' 2.14

These relationships form the basis of complex plane analqtical technique.

The impedance spectra convey information about the microscopic

dynamics o f the mobile carriers, because o(o) is the Fourier transform of the

autocorrelation function of the current density i. Its autocorrelation function is

I here i ( t )=--Zqjvi( t )

I

real function of time t, V is the volume of the sample. and the summation is

over the charge carriers. The charge on the mobile species and their yelocities

has been denoted by q, and v, respectlvel>. o. ki, T are angular frequency.

Boltzmams constant and temperature respectively

Thus in recent )ears t h ~ s techn~que has become a \<ell accepted

fundanietital tool for characterizing ionic conductors dut. to its ad\antagcs like

rapid acquisition of data (otien ~ i t h i n minutes). high accurac!. rcpeatahility

and high adaptability to a wide variety of different application.

0. Shanmukaraj Ph.0. Thesis (2007) 99

-

2.3.6. Temperature dependant ionic conductivity studies

Temperature dependant ionic conductivity studies for the polymer

samples were taken by casting the polymer samples on teflon moulds.

Preparing the sample in a teflon mould ensure the intactness of the sample

during conductivity measurement. The teflon mould was then placed in a

specially designed vacuum chamber coupled with a heating setup. Silver

electrodes were used for the conductivity measurements. The temperature

dependant conductivity setup used for the study is as shown in Fig. 2.20.

Fig. 220. Temperature dependant conductivity setup.

Chapter - I1

2.4. VARIABLES AFFECTING ELECTROCHEMICAL PROPERTIES

Variables affecting the electrochemical properties of materials include

the makeup and fabrication of the electrode. the electrochemical testing

procedure and those associated with the material themselves. Properties of the

materials that can influence the electrochemical properties ~nclude its

composition. particle size and its distribution as well as its morphology. A

number of variables from the fabrication of the electrode influence the

properties as well. such as the use and amount of conductive additives. hinder

choice and proportion as well as the film thickness and denslty. The

electrochemical properties are also influenced by the electrochemical test

procedure such as the testing current and the potential \\indo% in \\hich the

c>cles are cycled.

When nanostructured materials (crystallite size less than 10 nm) are

utilised a number of important material propert). changes occur [ I 31 which will

ultimately affect the electrochemical performance of such materials.

Nanostructured electrodes [14-171 and those composed of nanocrystalline

panicles [I81 have demonstrated better rate capahilih than those with larger

crystal sizes.

It is not just the crystalline size that can ~ntlucnce the properties but also

the distribution o i the particles [19]. Mass transport often limits the

electrochemical performance of lithium ion batteries and as a result the


- -- ~ -- -p

fabrication of electrodes as thin films (60-90 pm) consisting of small particles

(5-30 pm) on substrates of 20 Fm thick is common [20]. Shim el. al.

investigated the effect of electrode thickness and density on the electrochemical

properties of natural graphite electrodes [21 j . Dlft'erent densities bere achieced

by pressing the electrodes with a variety of forces includ~ng an unpressed

sample Pressing resulted in a decrease in both the reversible capacit). and

~rreversible capacity loss during the formation of cycles. The cycl~ng

performance of pressed electrodes was also more stable than those of unpressed

samples.

Takamura el . 01. examined the influence of conductive additive loading

on rlectrochemical properlies and found that homogeneity of the electrode was

more important 122). The addition of conductive a d d ~ t i ~ e s did improve

electrochemical performance however homogenous s lurp was found to be

n e c e s s q tbr high performance regardless of the presence of conductive

additives.

Dominko e!. al. made a similar observation and concluded the

distribi~tion of carbon black around active particles is critical. birh eben an

electrode consisting of on]) 2 wi% carbon black uniforml) distributed offering

better kinetics than one with 10 W% carbon black distributed non-unifbrmly

1231. The homogeneit) of the electrodes \\as also recognised as a ke! factor h!

Nanjundaswamy er. 01, in their examination of the coating techniques [24].

D. Shanmukaraj Ph.D. Thais (2007) 102

Franson el. al. examined the effect of carbon black and binder on

electrochemical properties [ 2 5 ] . In the voltage range 0.01-1.5V no difference in

cycling performance was observed between the binders polyvinyledene

fluoride (PVdf) and ethylene propqlene diene terpolynier (EPDM) though

irrcvcrsihle capacit) increased with increasing amounts of carbon black Cyclic

voltammetry in the same voltage range however showed an additional peak

during the first discharge at 0.35 V that \\,as associated with PVdF binder. In

many of these cases however the differences on a percentage basis are very

small and the results observed might in fact be within the normal error limits of

the experiment.


Chapter - I1

REFERENCES

I. Z. Lu. J. R. Dahn. J. Electrochem. Soc., 149 (2002) 1454.

2. R. Zollen. 'Physics of amorphous materials'. John Wiley 6t Sons. Nem

York (1983).

3. L. C. Kleen. 'Sol-gel technology fbr thin films. fihres. perfirrns,

electronics. and speciality shapes'. Noyes Publications, Park Ridge. N.J

( 1988).

1. C. J. Br~nker. G. W. Scherrer. 'Sol-gel science: The physics and

chemistry of sol-gel processing'. Academic Press Inc.. London (1990).

5. L. L. Hinch. J. K. West, Chem. Rev.. 90 (1 990) 33.

6. W. L). Callister Jr.. 'Materials science and engineering - An

introduction'. 31d ed.. John U'ileq and Sons lnc.. he \ \ York (1994).

7. Smith Hells. .Metal reference book (Ed: C j R. Brook)'. 7"; ed..

Butterworth Heinemann ( 1997).

8. A. L. Patterson. Phys. Rev.. 56 (1939) 978

9. A. lilus. Y. Rosenberg. L. Burstein. E. Peled. J. Electrochem. Soc.. 149

(2002) 635.

10. N. J . G. Gardher. S. Hull. D. A. Keen. P. Berastegui. 'User manual for

impedance spectroscopy measurements at ISIS'. ISlS facility.

Rutherford Appleton Laboratory. Chilton. Didcot. Oxon.

I I . H. Ye. 0. A. Williams. R. B. Jackman. R. Rudkin. A. Arkinson. Ph!

Stat. Sol., 193 (2002) 462.


Chapter - I1

12. H. Ye, 'Dielectric behaviour and thermal stability of synthetic diamond'.

M. Eng. Thesis, Nanyang Technological Universit), Singapore (2001 ).

13. Z . S. Wronski, International Materials Reviews. 46 (2001) 1

14. N. Li. C. R. Martin, B. Scrosati. Electrochem. Solid-State Lett.. 3 (2000)

3 16.

I . N. Li. C. J . Parrissi. Ci. Che. C. R. Martin. J. Llectrochem. Soc.. I47

(2000) 2044.

16. C'. R. Sides. N. Li. C. J . Patrissi. B. Scrosati. C' R Martin. Matcr. Kes.

Hull. 27 (2002) 004

17. N. Li, C. R. Martin, J. Electrochem Soc.. 148 (2001) A164.

18. Y . Shao-Horn, S, Osmialowski, Q. C. Horn. J. Electrochem. Soc.. 149

(2002) 1499.

19. Y . Sato. J . Nakamo. K Kobayaka\ra. T. Ka~ la i . A. Yoko>ania. 1 . P o ~ \ e r

Sources. 75 ( 1998) 27 1

20, hl. U'in~er. J 0. Bcsenhard. 1.1. E. Spahr. P. N o ~ a h . !Id\ . h1att.r. 10

(1998) 725.

2 1 J. Shim. K. A. Strieble. J. Power Sources. 119 (2003) 934.

22. T. Takamura. M. Saito. A. Shimokawa. C. Nakahara. K. Sekine.

S. Meano. N . Kibayashi. J. Power Sources. 90 (2000) 45.

23. R. Dominko. M. Gaberscek, J. Drofenik, M. Bele. S. Prjo\,nil\.

J. Janinik. J . Power Sources. 119 (2003) 770.


Chapter - If

24. K. S. Nanjundaswamy, H. D. Friend. C. 0. Kelly. D. J. Standiee. R. L .

Higgins. Proceedings of the 32" Intersociety Energ? Conversion

Engineering Conference, IEEE, (1997) 42.

25. L. Franson. T. Eriksson. K . Edstrom. T. Ciustafsson. J. 0. Thomas. J .

Power Sources. 101 (2001) 1


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