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High Capacity Graphite Anodes for Li-Ion battery applications using Tin microencapsulation Basker Veeraraghavan, Anand Durairajan, Bala Haran Ralph White and Branko Popov University of South Carolina, Columbia, SC 29208 and Ronald Guidotti Sandia National Laboratories Albuquerque, NM 87185-0614

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High Capacity Graphite Anodes for Li-Ion battery applications

using Tin microencapsulation

Basker Veeraraghavan, Anand Durairajan, Bala Haran

Ralph White and Branko Popov

University of South Carolina, Columbia, SC 29208

and

Ronald Guidotti

Sandia National Laboratories

Albuquerque, NM 87185-0614

Introduction

Graphite has good cycle life but low theoretical capacity (372 mAh/g)

Tin has high theoretical capacity (991 mAh/g)

Tin based anodes have poor cycling characteristics due to density changes of Tin

Reducing the Sn particle size may mitigate the problem

Objectives

To obtain an anode material with high specific capacity, better rate capability and good cycle life

To use electroless deposition for preparing Sn-C composites and to optimize the deposition conditions

To optimize the Sn loading on graphite based on discharge characteristics

To study the effect of Sn loading on the electrochemical performance of the composite

Preparation of Sn/Graphite compositesElectroless deposition of Sn using

hypophosphite bathpH-10 (using NaOH) and T-50C

Cell Preparation for testing1/2” T-cells used for electrochemical testingElectrodes prepared by cold rolling using

PTFE binder (10wt%)Whatman fiber used as separator and Li-foil

used as counter and reference electrode1M LiPF6 in EC/DMC (1:1 v/v) used as

electrolyte

Experimental

Experimental (Cont’d.)

Electrochemical characterizationCharge-discharge and cycling behavior

Cycling was performed between 2V and 5 mV at C/15 rate (0.1mA/cm2)

Electrochemical Impedance Spectroscopy (EIS)100kHz to 1mHz with 5mV sinusoidal signal

Cyclic VoltammetryCVs were performed in the potential range 1.6V to

0.01V at 0.05 mV/s

Physical characterizationSEM, EDAX and XRD

SEM images of bare and 15% sn-coated SFG10 samples

15% SnBare

10 m

EDAX studies of bare and 15% sn-coated SFG10 samples

Bare

15% Sn

XRD analysis of 15% sn-coated SFG10 samples as a function of heat treatment temperature

40 41 42 43 44 45 46 47 48 49 50

2

0

250

500

750

1000In

tens

ity

Bare100oC200oC300oC400oC

XRD patterns of SFG10 with 15% Sn heat treated at different temperatures.

The XRD analysis of Bare SFG10 is shown for comparison.

Charge discharge studies of 15% sn-coated SFG10 samples as a function of heat treatment temperature

-100 100 300 500 700

Specific Capacity (mAh/g)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Pote

ntia

l (V

vs

Li/

Li+

)

100oC (332 mAh/g)200oC (400 mAh/g)300oC (343 mAh/g)400oC (337 mAh/g)

Specific Capacities of SFG10 coated with 15% Sn as a function of temperature of drying

Comparison of charge-discharge curves of bare and 15 wt% sn-coated graphite.

0 1000 2000 3000 4000 5000

Specific capacity (mAh/g)

0 .0

1.0

2.0

3.0

4.0P

ote

ntia

l ( V

vs

Li/L

i+)

Bare15% Sn

Charge-Discharge curves of bare and sn-coated SFG10 samples

100 300 500 700

Specific Capacity (mAh/g)

-0.2

0.3

0.8

1.3

1.8

Pote

ntia

l (V

vs

Li/

Li+

)

Bare5% Sn

10% 20%

15%

Specific Capacity as a function of Sn loading at C/15 rate

Percentage increase in reversible capacity as a function of composition of sn

0 5 10 15 20 25

Composition of Sn (%)

0

10

20

30

40

50

60

70

Perc

enta

ge in

crea

se in

reve

rsib

le c

apac

ity (%

)

Percentage increase in reversible capacity as function of Sn compositionfor two different rates

Utilization of sn in the coated samples as a function of the composition of tin

Sample Reversible

Capacity (mAh/g)

Capacity due to Sn (mAh/g)

Utilization of Sn1 (%)

Specific Surface

area (m2/g)

Volumetric Surface

area (m2/cm3)

Volumetric Capacity

(mAh/cm3)

Bare

 

5% Sn

 10% Sn

 

15% Sn

 

20% Sn

284.6

 

327.8

374.6

 

433.2

 

381.7

-

 

57.4

118.5

191.3

154.0

-

 

55.0

53.8

54.7

31.1

9.84

8.52

8.30

7.61

7.12

21.65

20.49

21.66

21.42

21.50

626.1

788.4

977.7

1219.5

1152.7

1Utilization of tin = (Capacity due to tin/weight of tin in the composite)/Theoretical capacity of tin (991 mAh/g)*100

Impedance plots for the bare and sn-coated SFG10 samples at fully discharged state

0 1 2 3 4

Real Z (g)

0.0

0.5

1.0

1.5

2.0

Imag

inar

y Z

(g

Impedance plot of SFG10 samplesImpedance plot of SFG10 samples

Bare

5% Sn

10% Sn15% Sn

20% Sn

Cyclic Voltammograms of bare and 15% sn coated SFG10 samples for the reversible cycle

0.0 0.4 0.8 1.2 1.6 2.0

Potential (V vs Li/Li+)

-300

-100

100

300

Spec

ific

Cur

rent

(mA

/g)

15% SnBare

Cycle life studies of bare and 15% sn coated SFG10 samples at C/15 rate

5 10 15 20 25 30 35

Cycle number

0

100

200

300

400

500

Spec

ific

Cap

acit

y (m

Ah/

g)

Bare

15% Sn

Rate Capability studies of bare and 15% sn coated SFG10 samples

0.0 0.5 1.0 1.5 2.0

Discharge current density in mA/cm2

0

100

200

300

400

500Sp

ecifi

c C

apac

ity (m

Ah/

g)

C/15

C/6

C/3

C

2C

Bare15% Sn

Conclusions Tin encapsulation on SFG10 graphite results in high

performance anodes for use in Li-ion batteriesReversible capacities are improved upto 15% Sn, relative to

bare graphite Cycle life of the bare graphite is improved on Sn-encapsulation

The optimum heat treatment temperature was found to be 200 CCrystallinity increases with temperature

Sn-C based anodes show better conductivity and lower polarization resistance compared to virgin carbon

Addition of Polypyrrole reduces irreversible capacity and further studies need to be done to optimize the amount of polypyrrole

Acknowledgements

This work was funded by the Dept. of Energy division of Chemical Science, Office of Basic Energy Sciences and, in part, by Sandia National Laboratories

(Sandia National Laboratories is a multi-program laboratory operated by Sandia corp., a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.)