Quantifying Degradation Parameters of Single-Crystalline Ni-Rich Cathodes in Lithium-Ion Batteries.

Single‐crystal LiNi x Co y Mn z O 2 (SC‐NCM, x + y + z =1) cathodes are renowned for their high structural stability and reduced accumulation of adverse side products during long‐term cycling. While advances have been made using SC‐NCM cathode materials, careful studies of cathode degradation mechanisms are scarce. Herein, we employed quasi single‐crystalline LiNi 0.65 Co 0.15 Mn 0.20 O 2 (SC‐NCM65) to test the relationship between cycling performance and material degradation for different charge cutoff potentials. The Li/SC‐NCM65 cells showed >77 % capacity retention below 4.6 V vs. Li + /Li after 400 cycles and revealed a significant decay to 56 % for 4.7 V cutoff. We demonstrate that the SC‐NCM65 degradation is due to accumulation of rock‐salt (NiO) species at the particle surface rather than intragranular cracking or side reactions with the electrolyte. The NiO‐type layer formation is also responsible for the strongly increased impedance and transition‐metal dissolution. Notably, the capacity loss is found to have a linear relationship with the thickness of the rock‐salt surface layer. Density functional theory and COMSOL Multiphysics modeling analysis further indicate that the charge‐transfer kinetics is decisive, as the lower lithium diffusivity of the NiO phase hinders charge transport from the surface to the bulk.