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

Abstract

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. The need for better performing energy storage units increases as the development of new portable electronic and power generation technologies continues. Lithium-ion batteries (LIBs) are becoming a dominant player in the market for long and short term energy storage. However, new applications such as renewable energy systems and hybrid electric vehicles require more in depth knowledge of the degradation and safety features of LIBs. This lack of knowledge hinders the performance and development of these technologies which are increasingly playing a larger role in today's energy generation and usage market. Thus, a better understanding of the degradation mechanisms as well as a more sophisticated approach for battery life prediction is required. While research into these mechanisms has been ongoing for years, most of them have trouble relating the results back to full scale systems and battery modules used in these applications. Safety is usually the chief concern when utilizing LIBs in high power systems. Therefore it would be more prevalent to conduct a study that focuses on the hazardous reactions that can cause battery failure and also contribute to long term capacity fade. The cause of power and capacity fade of LIBs can generally be grouped into three categories: structural degradation (e.g., volume change, phase transition, and binder decomposition), chemical changes to the electrodes (e.g., chemical decomposition, dissolution reaction), and surface layer formation at the electrode-electrolyte interface. Current collector corrosion and metallic lithium plating also may contribute to power and capacity fade. The dominant deg...

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