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

• Single-crystal LiNixCoyMnzO2 (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 LiNi0.65Co0.15Mn0.20O2 (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.

Subject headings

NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

Keyword

Single-Crystal Cathodes

Transfer Kinetics

Structural Stability

Rock-Salt Formation

Multiphysics Analysis

Publication and Content Type

art (subject category)

ref (subject category)