Nickel and Cobalt Oxidation State Evolution at Ni-Rich NMC Cathode Surfaces during Treatment

Abstract

Continued improvement of Ni-rich cathode materials is necessary to achieve higher energy density and cycle life targets for lithium-ion batteries. Despite having a significant influence on these parameters, Ni and Co surface states and their evolution have not been described in detail for realistic cathode particles, making it difficult to identify optimal states and tune treatments. Here, we analyze the chemical oxidation states of LiNi0.6Mn0.2Co0.2O2 (NMC 622) and LiNi0.8Mn0.1Co0.1O2 (NMC 811). Surfaces are found to be nonstoichometric compared to the bulk and evolve significantly in response to low-temperature heating and coating treatments. Some of the observed phenomena include Mn enrichment of the surface, varying ratios of Ni2+/Ni3+ and Co3+/Co4+, and depletion of lattice Li+, suggesting new paths for optimization when connected with electrochemical impacts.