Quantitative Characterization of the Surface Evolution for LiNi_0.5Co_0.2Mn_0.3O₂/Graphite Cell during Long-Term Cycling

Many factors have been brought forward to explain the capacity degradation mechanisms of LiNi_xCo_yMn_zO₂ (NCM)/graphite cells at extreme conditions such as under high temperature or with high cutoff voltage. However, the main factors dominating the long-term cycling performance under normal operations remain elusive. Quantitative analyses of the electrode surface evolution for a commercial 18650 LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523)/graphite cell during ca. 3000 cycles under normal operation are presented. Electrochemical analyses and inductively coupled plasma-optical emission spectroscopy (ICP-OES) confirm lithium inventory loss makes up for ca. 60% of the cell's capacity loss. Electrochemical deterioration of the NCM523 cathode is identified to be another important factor, which accounts for more than 30% of the capacity decay. Irregular primary particle cracking due to the mechanical stress and the phase change aroused from Li-Ni mixing during repetitive cycles are identified to be the main contributors for the NCM cathode deterioration. The amount of transition metal dissolved into electrolyte is determined to be quite low, and the resulting impedance rise after about 3000 cycles is obtained to be twice that of the reference cell, which are not very significant affecting the long-term cycling performance under normal operations.