Interplays between TM migration, cation mixing and oxygen defects and their impacts on degradation of layer-structured oxide cathode materials

Abstract

The Li-rich and Ni-rich layer-structured transition metal (TM) oxide cathode materials have attracted extensive attention due to their high energy densities. However, they suffer from rapid structural and performance degradations due to TM migration and oxygen loss during cycling. Herein, the $R\overline{3}m$ structured LiNiO₂ was investigated as a model of the layered oxides to reveal their structural features and structural degradation mechanisms. On the basis of comprehensive characterizations, we attributed the lowering of the TM oxidation potential to the deteriorated Li/TM cation mixing and the resultant change of the phase transition pathways. Notably, the cation mixing was found deteriorated even at a very low dosage of Li extraction and is enhanced with generation of extra oxygen vacancies at high potentials. Density functional theory (DFT) calculations explain the self-restriction of the Ni migration as well as these observations. Our findings clarify the mutual reinforcement between the TM migration, cation mixing and oxygen redox: the intrinsic and extrinsic Li/TM cation mixings result in the formation of Li-O-Li and/or Li-O-Vac_TM configuration(s) (Vac_TM for TM vacancy) and induce the oxygen oxidation, while the formation of oxygen vacancies further facilitate Ni migration by lowering its oxygen coordination. Clarification the relationship of TM migration, oxygen redox and structural degradation of LiNiO₂ and the Ni-rich materials provides a new insight for the development of other layer-structured oxide cathode materials.