Degradation modes and mechanisms analysis of lithium-ion batteries with knee points

The capacity of lithium-ion batteries decreases during usage (cycling) and storage (rest). After some initial charge-discharge cycling, the capacity fade rate has been observed to increase, and the capacity fade curve visibly bends, the onset of which is described as a knee point. The occurrence of the knee point above the end-of-life capacity threshold leads to a shorter life than expected based on the initial capacity fade rate. Although various degradation mechanisms and their effects on lithium-ion batteries are generally known, the degradation mechanisms for the knee point phenomenon have been in contention in the literature. In this paper, aging tests are conducted on custom three-electrode lithium-ion pouch cells to distinguish the contribution of all degradation modes to the cell's capacity fade and determine the one that causes the knee point, which is the acceleration of cathode resistance increase due to the increased degradation rate in cathode interfacial kinetics. Incremental capacity analysis and destructive analysis were further conducted to determine the degradation mechanisms that lead to the acceleration of cathode resistance increase.