Correlation of Bulk Internal Pressure Rise and Capacity Degradation of Commercial LiCoO2 Cells

Abstract

A commercial lithium-ion cell with LiCoO2/graphite electrodes was cycled at high-rates (2C) at room temperature. Periodic measurements were performed including internal pressure measurement, discharge capacity, and electrochemical impedance spectroscopy (EIS). Poor cell cyclability was demonstrated with 39% capacity fade after 250 cycles. Both the pressure rise and capacity fade demonstrated a paralinear behavior that is primarily parabolic in the early stage with a dependency on the square-root of the cycle number, followed by a transition to a linear dependency on cycle number in later cycles. An examination of the pressure and capacity evolution presented a direct correlation indicating a very strong, statistically significant relationship between the two (rs = 0.9455). Post cycling gas-chromatography analysis of the gases detected CO, CO2, CH4, C2H6, and C3H8 indicating reactions with trace impurities and a reduction of the electrolyte. Scanning electron microscope (SEM) analysis revealed minimal changes to the surface morphology of the cathode, while demonstrating an ostensible passivation layer buildup as well as crack formations inducing continued electrolyte reduction. EIS analysis indicated an apparent increase in RCT in the early stages, followed by a stronger contribution of the charge-transfer kinetics and Li+ transport through the solid-electrolyte interphase (SEI) in later stages.