Unveiling Capacity Degradation Mechanism of Li-ion Battery in Fast-charging Process

In this work, capacity degradation in fast-charging process is studied by using a power-optimized graphite-LiNi0.80Co0.15Al0.05O2 (NCA) electrode couple and a three-electrode coin cell as the testing vehicle. It is shown that capacity degradation consists of two distinct stages, (1) rapid degradation in early period and (2) progressive degradation over lifetime, which are associated with the electrochemical reduction of electrolyte solvents on the graphite anode and the structural deterioration of the NCA cathode, and are well correlated to a decrease in Coulomb efficiency. Analyses on the change in differential capacity profile and the recoverability of the lost capacity reveal that capacity degradation mainly originates from structural deterioration of the NCA cathode over lifetime, and it aggravates with the state-of-charge and temperature. Based on this guideline, we show that capacity degradation of the Li-ion batteries in the fast-charging applications can be greatly mitigated by limiting the cell's operation to a relatively low state-of-charge (i. e., a relatively low charging cut-off voltage).