Insights into the Structural and Thermodynamic Instability of Ni-Rich NMC Cathode

The Ni-rich LiNi1-x-yMnxCoyO2 (NMC) layered oxides have been promising materials to replace the LiCoO2 commercial cathode due to their high operation voltages, low cost, and enhanced capacity. However, structural instability is the main drawback of the Ni-rich NMC system, leading to capacity fading, which prevents its commerce application. While numerous efforts have been made to comprehend and overcome this issue, there remains a lack of systematic phase stability analysis which is an essential factor to be addressed. Herein, systematic first-principles calculations were performed in this study to reveal the structural, oxidation, and phase stability of Ni-rich NMC during delithiation. This is the first report on competing phases of Ni-rich NMC at various states of charge (SoCs). The compounds with the spinel and rock salt structures formed during delithiation and oxygen evolution takes place at highly delithiation cases, which are likely the origins of capacity fading and crack formation at the surface. This work also outlined the thermodynamic foundation of doping and oxidation state gradient-based core-shell strategies for resolving the instability and enhancing the capacity retention of Ni-rich NMC layered oxides as cathode materials in Li-ion batteries.