Effects of long-term fast charging on a layered cathode for lithium-ion batteries

Fast charging, which aims to shorten recharge times to 10–15 min, is crucial for electric vehicles (EVs), but battery capacity usually decays rapidly if batteries are charged under such severe conditions. Revealing the failure mechanism is a prerequisite to improving the charging performance of lithium (Li)-ion batteries. Previous studies have focused less on cathode materials while also mostly focusing on their early changes. Thus, the cumulative effect of long-term fast charging on cathode materials has not been fully studied. Here, we study the changes in a layered cathode material during 1000 cycles of 6C charging based on 1.6 Ah LiCoO2/graphite pouch cells. Postmortem analysis reveals that the surface structure, charge transfer resistance and Li-ion diffusion coefficient of the cathode degenerate during repeated fast charging, causing a large increase in polarization. This polarization-induced poor utilization of the Li inventory is an important reason for the rapid capacity fading of batteries. These findings deepen the understanding of the aging mechanism for cells undergoing fast charging and can be used as benchmarks for the future development of high-capacity, fast-charging layered cathode materials.