Polymer graphite composite anodes for Li-ion batteries

Basker Veeraraghavan, Bala Haran, Ralph White and Branko Popov University of South Carolina, Columbia, SC 29208 Plamen Atanassov University of New Mexico, Albuquerque, NM 87131

Problem Definition

 Electrolyte decomposition  Solvated lithium intercalation and reduction  Irreversible reactions lead to  Losses in capacity / active lithium material  Lowers cell energy densities, increases cell cost

Previous approaches



Modification to the electrolyte

 Addition of SO 2 , CO 2  Other solvents like DMPC 

Modification to the electrode

 Mild oxidation  Coating with Ni, Pd

Objectives

 To prepare PPy/C composite which will reduce the initial irreversible capacity  To improve the conductivity and the coulombic efficiency of the electrode  To obtain material with better rate capability and good cycle life

Approach

 Produce a matrix of PPy which forms a conducting backbone for the graphite particles by polymerization

in-situ

Experimental

 Preparation of PPy/Graphite composites  Dropwise addition of pyrrole into aqueous slurry of graphite at 0  C with nitric acid acting as an oxidizer for 40 h  Wash repeatedly with water and methanol and vacuum dried at 200  C for 24h  Cell Preparation for testing  Electrodes prepared by cold rolling using PTFE binder (10wt%)  Whatman fiber used as separator and Li-foil used as counter and reference electrode  1M LiPF 6 in EC/DMC (1:1 v/v) used as electrolyte

Experimental (Cont’d.)

 Electrochemical characterizations  Charge-discharge and cycling behaviors  Arbin Battery test system used for the testing  Cycling was performed between 2V and 5 mV at C/15 rate (0.25 mA/cm 2 )  Cyclic Voltammetry  CVs were performed from 1.6V to 0.01V at 0.05 mV/s  Electrochemical Impedance Spectroscopy (EIS)  100kHz to 1mHz with 5mV PP signal  Physical characterizations  SEM micrographs  TGA and BET analysis

120

TGA analysis of polymer composite SFG10 samples

100 80 60 40 20 0 Bare 5% PPy 6% PPy 7.8% PPy 8.4% PPy PPy -0.0

150.0

300.0

450.0

Temperature 600.0

750.0

900.0

Charge-discharge curves of polymer composite SFG10 samples

4.0

3.0

Bare 5% polymer 6% polymer 7.8% polymer 8.4%polymer 2.0

1.0

0.0

0 200 400 Specific Capacity (mAh/g) 600 800

Change in irreversible capacity loss with PPy loading at C/15 rate

Amount of PPy loading (wt%) Initial lithiation capacity (mAh/g) Initial de lithiation capacity (mAh/g) Overall irreversible Capacity (%) Initial coulombic efficiency (%) 0 5 6 7.8

8.4

485.9

483.7

471.7

456.6

432.5

232.7 309.3

313.6

310.1

290.3

52.1

36.1

33.5

32.1

32.9

47.9

63.9

66.5

67.9

67.1

Comparison of surface area and capacity for polymer composite electrodes

Amount of PPy loading (wt%) Reversible Capacity (mAh/g) Specific Surface area (m 2 /g) Volumetric Surface area (m 2 /cm 3 ) Volumetric Capacity (mAh/cm 3 ) 0 5 6 7.8

8.4

284.6

338.8

359.8 362.3 359.0

9.84

8.98 8.55

7.78

7.69

21.65

19.76

18.81

17.12

16.92

626.1

745.4

791.6

797.1

789.8

Cyclic voltammograms of polymer composite SFG10 samples

400 300 200 100 0 -100 -200 -300 -400 -500 -600 0.0

0.4

0.8

Potential ( V vs Li/Li + ) 1.2

Bare 5% PPy 6% PPy 7.8% PPy 8.4% PPy 1.6

SEM pictures of polymer composite SFG10 samples

10  m Bare 10  m PPy/C

Impedance studies of polymer composite SFG10 samples

0.5

0.4

0.3

0.2

Bare 5% polymer 6% Polymer 7.8% Polymer 8.4% Polymer 0.1

0.0

0.0

0.1

0.2

0.3

0.4

0.5

Real Z (  -g) 0.6

0.7

0.8

0.9

1.0

Impedance comparison of Bare and Polymer composites of SFG10.

Impedance was done at unlithiated state for all the samples.

Equivalent circuit used to fit the experimental data C

1

C 2 DPE 1 DPE 2 R

 R 1 R 2 – SEI layer resistance – Polarization resistance

R

1 R  – ohmic resistance

R 2

C 1 C 2 – SEI layer capacitance – Double layer capacitance

Equivalent circuit parameters for polymer composite electrode

Sample Bare 5% PPy 6% PPy 7.8% PPy 8.4% PPy R  (ohm) 7.9

7.6

7.9

7.8

8.3

R 1 (ohm) C 1 (Farad) 197.4

2.3x10

-7 R 2 (ohm) 17.7

C 2 (Farad) 4.3x10

-6 27.1

21.7

13.9

8.7

4.7x10

-6 7.0x10

-6 7.2x10

-6 8.2x10

-6 14.8

12.1

10.4

9.1

4.3x10

4.5x10

7.0x10

9.9x10

-6 -6 -6 -6

Comparison of coulombic efficiencies for SFG10 samples

100 99 98 97 96 95 94 93 92 91 90 1.0

2.0

7.8% PPy 3.0

4.0

5.0

6.0

7.0

Cycle number Bare 8.0

9.0

10.0

11.0

Rate capability studies of composite SFG10 samples

400 7.8% polymer 300 200 Bare 100 0 0

C/15 rate

5

C/6 rate C/3 rate

10 15 Cycle number

C rate

20

C/15 rate

25

Cycle life studies of composite SFG10 samples

400 7.8% PPy 300 200 100 0 0 Bare 6 12 18 Cycle number 24 30 36

Charge-Discharge curves of polymer composite SFG10-15% sn samples

4.0

SFG10-15%Sn 15% Sn-PPy 3.0

2.0

1.0

0.0

200 600 Specific Capacity (mAh/g) 1000

Comparison of irreversible capacities for bare and polymer composite SFG10 samples

Sample Bare Initial lithiation capacity (mAh/g) 485.9

Initial de lithiation capacity (mAh/g) Irreversible capacity (%) 232.7

52.1

Initial coulombic efficiency (%) 47.9

Bare-PPy 15% Sn 15% Sn-PPy 456.6

719.9

606.2

310.1

350.8

370.4

32.1

51.3

38.9

67.9

48.7

61.1

Conclusions

 Polypyrrole on SFG10 graphite results in high performance anodes for use in Li-ion batteries  Irreversible capacity is reduced up to 7.8% PPy composite  Charge discharge studies are supported by CV data  Reduction in irreversible capacity seen during cathodic scan  Polymer composite anodes show better conductivity and lower polarization resistance compared to virgin carbon  Polymer composite anode show better rate capability and longer cycle life

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.)