Origin of Structural Degradation During Cycling and Low Thermal Stability of Ni-Rich Layered Transition Metal-Based Electrode Materials
Ni-rich lithiated layered oxides composed of Ni, Co, and Mn (NCMs) have shown tremendous promise as cathode materials in lithium-ion batteries (LIB) for electromobility applications. The capacity of these materials increases with nickel content, but there is a concomitant decrease in stability and stable operating voltage during cycling. Hence, it is of great importance to probe ways to increase the nickel content without sacrificing other important aspects. In this study, we performed a detailed comparative theoretical study of Ni-rich NCMs to advance our understanding of the cycling and thermal stability. On the basis of extensive analysis of density of states, magnetic structure, bond covalency, molecular orbital diagrams, Bader atomic charges, and oxygen binding energies, we draw several crucial conclusions: as the NCM materials become increasingly rich in Ni, (1) the amount of high-valence Ni-ions increases (i.e., N3+, Ni4+), (2) Ni4+ ions are readily reduced due to a low-lying LUMO, and hence can easily react with electrolyte species, (3) Ni4+-O bonds become increasingly covalent, and (4) molecular oxygen release becomes more feasible and, hence, may result in cathode degradation. Importantly, these conclusions are found to be appropriate also for the deintercalation process for all NCM materials and therefore also explain cycling behavior. On the basis of the current results, we suggest that a strategy of doping NCMs with high-valent cations, which suppresses Ni-ions in high oxidation states via charge compensation, should be adopted. These results will be beneficial for understanding and designing high capacity LIB cathodes for electric vehicles.