Capacity Fade Studies of LiCoO 2 Based Li-ion Cells Cycled at Different Temperatures

1. Capacity Fade Studies of LiCoO2 Based Li-ion Cells Cycled at Different Temperatures Bala S. Haran, P.Ramadass, Ralph E. Whiteand Branko N. Popov Center for Electrochemical Engineering Department of Chemical Engineering, University of South Carolina Columbia, SC 29208

2. Objectives • Study the change in capacity of commercially available Sony 18650 Cells cycled at different temperatures. • Perform rate capability studies on cells cycled to different charge-discharge cycles. • Perform half-cell studies to analyze causes for capacity fade. • Use impedance spectroscopy to analyze the change in cathode and anode resistance with SOC. • Study structural and phase changes at both electrodes using XRD.

3. Cathode (positive electrode) - LiCoO2. • Anode (negative electrode) - MCMB. • Cell capacity – 1.8 Ah Characteristics of a Sony 18650 Li-ion cell

4. Characteristics Positive LiCoO2 Negative Carbon Mass of the electrode material (g) 15.1 7.1 Geometric area (both sides) (cm2) 531 603 Loading on one side (mg/cm2) 28.4 11.9 Total Thickness of the Electrode (m) 183 193 Specific Capacity (mAh/g) 148 306 Characteristics of a Sony 18650 Li-ion cell

5. Experimental – Cycling Studies • Cells were discharged at a constant current of 1 A. • Batteries were cycled at 3 different temperatures – 25oC, 45oC and 55oC. • Experiments done on three cells for each temperature. • Rate capability studies done after 150, 300 and 800 cycles - Cells charged at 1 A and discharged at currents of 0.2, 0.4, 0.6, 0.8 and 1.0 A. • Cells cycled using Constant Current-Constant Potential (CC-CV) protocol.

6. Experimental - Characterization • Batteries were cut open in a glove box after 150, 300 and 800 cycles. • Cylindrical disk electrodes (1.2 cm dia) were punched from both the electrodes. • Electrochemical characterization studies were done using a three electrode setup. • Impedance analysis - 100 kHz ~ 1 mHz ±5 mV. • Material characterization - XRD studies and SEM, EPMA analysis.

7. Experimental - Characterization

8. Discharge Curve Comparison of Sony 18650 Cells after 800 Cycles

9. Capacity Fade as a Function of Cycle Life

10. Capacity Fade as a Function of Cycle Life

11. 45 deg C Room Temperature 55 deg C Charge Curves at Various Cycles

12. Constant Current Constant Voltage Change in Charging Times with Cycling

13. Rate Capability after 150 and 800 Cycles

14. Nyquist Plots of Sony Cell at RT and 55oC

15. Nyquist Plots of Sony Cell at RT and 45oC

16. Negative Electrode Resistance (Fully Lithiated)

17. Positive Electrode Resistance (Fully Lithiated)

18. Comparison of Electrode Resistances 150 Cycles 300 Cycles

19. Possible Reasons for Rapid Capacity Fade at Elevated Temperatures • The SEI layer formed on a graphite electrode changes in both morphology and chemical composition during cycling at elevated temperature. • The R-OCO2Li phase is not stable on the surface and decomposes readily when cycled at elevated temperatures (55oC). • This creates a more porous SEI layer and also partially exposes the graphite surface, causing loss of charge on continued cycling. • The LiF content on the surface increases with increasing storage temperature mainly due to decomposition of the electrolyte salt. • SEI and electrolyte (both solvents and salt)decomposition have a more significant influence than redox reactions on the electrochemical performance of graphite electrodes at elevated temperatures.

20. Nyquist Plot of Fresh LiCoO2 as a function of SOC at RT

21. Nyquist Plot of Fully Delithiated LiCoO2 as a function of Storage Time at RT

22. Nyquist Plot of Fully Lithiated LiCoO2 as a function of Storage Time at RT

23. Specific Capacity of Positive and Negative Electrodes at Various Cycles and Temperature

24. Comparison of Capacity Fade of Individual Electrodes with Full Cell Loss

25. Room Temperature CV’s of Sony Cell

26. CV’s of Sony Cell

27. XRD Patterns of LiCoO2 after Different Charge-Discharge Cycles

28. Variation of Lattice Constants with Cycling and Temperature Decrease in c/a ratio leads to decrease in Li stoichiometry* *G. Ting-Kuo Fey et al., Electrochemistry Comm. 3 (2001) 234

29. Capacity Fade Loss of Li (Primary Active Material) Degradation of C, LiCoO2 (Secondary Active Material) SEI Formation Electrolyte Oxidation Salt Reduction Overcharge Structural Degradation Solvent Reduction

30. Conclusions • Capacity fade increases with increase in temperature. • For all cells decrease in rate capability with cycling is associated with increased resistance at both electrodes. • Both primary (Li+) and secondary active material (LiCoO2, C) are lost during cycling. • The fade in anode capacity with cycling could be due to repeated film formation. • XRD reveals a decrease in Li stoichiometry at the positive electrode with cycling.

31. Acknowledgements This work was carried out under a contract with Mr. Joe Stockel, National Reconnaissance Office for Hybrid Advanced Power Sources # NRO-00-C-1034.