Effects of High-Rate Cycling on the Bulk Internal Pressure Rise and Capacity Degradation of Commercial LiCoO₂Cells

Commercial lithium-ion cells with the same LiCoO 2 /graphite electrodes were cycled at high-rate discharge, high-rate charge, and both high-rate charge and discharge until the capacity reached 60%. Periodic baseline characterization tests were performed including internal pressure, discharge capacity, and electrochemical impedance spectroscopy (EIS). A paralinear behavior with an early parabolic dependence on the square root of the cycle number, followed by a transition into a linear dependence on cycle number was seen in both the pressure rise and capacity fade in all three conditions. A direct correlation indicating a very strong, statistically significant relationship between the two variables was identified in all the cells tested. CO, CO 2 , CH 4 , C 2 H 6 , and C 3 H 8 were identified with post cycling gas-chromatography analysis indicating reactions with trace impurities and a reduction of the electrolyte. SEM analysis identified an excessive passivation layer on the surface of the anode presumably due to the reduction of electrolyte at the anode surface, while the cathode demonstrated no significant change with cycling. EIS analysis indicated an minimal change in R CT in the early stages, however the low frequency semicircle underwent considerable change due to a stronger contribution of the charge-transfer kinetics and Li + transport through the passivation layer in later stages.