Capacity Fade Studies of LiCoO 2 Based Li-ion Cells Cycled at Different Temperatures Bala S. Haran, P.Ramadass, Ralph E. White and Branko N. Popov Center for Electrochemical Engineering Department of Chemical Engineering, University of South Carolina Columbia, SC 29208

Objectives  Study the change in capacity of commercially available Sony 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.

Characteristics of a Sony Li-ion cell  Cathode (positive electrode) - LiCoO 2.  Anode (negative electrode) - MCMB.  Cell capacity – 1.8 Ah

Characteristics of a Sony Li-ion cell Characteristics Positive LiCoO 2 Negative Carbon Mass of the electrode material (g) Geometric area (both sides) (cm 2 ) Loading on one side (mg/cm 2 ) Total Thickness of the Electrode (  m) Specific Capacity (mAh/g)

Experimental – Cycling Studies  Cells cycled using Constant Current-Constant Potential (CC-CV) protocol.  Cells were discharged at a constant current of 1 A.  Batteries were cycled at 3 different temperatures – 25 o C, 45 o C and 55 o C.  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.

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 kHz ~ 1 mHz ±5 mV.  Material characterization - XRD studies and SEM, EPMA analysis.

Experimental - Characterization

Discharge Curve Comparison of Sony Cells after 800 Cycles

Capacity Fade as a Function of Cycle Life

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

Change in Charging Times with Cycling Constant Current Constant Voltage

Rate Capability after 150 and 800 Cycles

Nyquist Plots of Sony Cell at RT and 55 o C

Nyquist Plots of Sony Cell at RT and 45 o C

Negative Electrode Resistance (Fully Lithiated)

Positive Electrode Resistance (Fully Lithiated)

Comparison of Electrode Resistances 150 Cycles 300 Cycles

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 (55 o C).  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.

Nyquist Plot of Fresh LiCoO 2 as a function of SOC at RT

Nyquist Plot of Fully Delithiated LiCoO 2 as a function of Storage Time at RT

Nyquist Plot of Fully Lithiated LiCoO 2 as a function of Storage Time at RT

Specific Capacity of Positive and Negative Electrodes at Various Cycles and Temperature Cell (Cycle No. – Temperature) Specific capacity (mAh/g) LiCoO 2 Carbon Fresh RT % % % % % % 300-RT % % % % % %

Comparison of Capacity Fade of Individual Electrodes with Full Cell Loss Cell (Cycle No. – Temperature) Capacity Lost (mAh) Full Cell Capacity Loss LiCoO 2 Carbon(mAh) 150-RT RT

CV’s of Sony Cell Room Temperature

CV’s of Sony Cell

XRD Patterns of LiCoO 2 after Different Charge-Discharge Cycles Cellc/a Fresh RT RT

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

Capacity Fade Loss of Li (Primary Active Material) Degradation of C, LiCoO 2 (Secondary Active Material) SEI Formation Overcharge Salt Reduction Solvent Reduction Electrolyte Oxidation Structural Degradation

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 (LiCoO 2, 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.

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.