Reviving reversible anion redox in 3d-transition-metal Li rich oxides by introducing surface defects

The reversible anion redox of O²⁻/(O₂)ⁿ⁻ in 3d-transition-metal based Li layered oxides (LLO) has received renewed attention due to its capability of hosting additional redox centers, which can further improve the energy density of Li-ion batteries. However, over-oxidized (O₂)ⁿ⁻ was found to be unstable upon cycling, resulting in an irreversible crystal structure transformation that lowers long-term cycling stability. Herein, we demonstrated that the anion redox can be tuned through surface defect engineering, which consequently improves the cycling instability. By reconstructing the atomic configuration of the surface, a highly defective surface layer with oxygen vacancies is achieved, substantially enhancing the reversibility of anion redox as well as the stability of bulk crystal structure. The modified LLO expresses a high performances of discharge capacity (94.5%), redox potential (>3.0 V during discharge) and energy density (90.2%) after 100 cycles. Ex-situ XPS measurements confirm a high reversibility of the O²⁻/(O₂)ⁿ⁻ redox couple, which is supported by DFT calculations showing that the oxygen vacancies formed at the fully lithiated state of LLO mitigate the over-oxidization of oxygen and the formation of unstable superoxides ((O₂)^-) through a reductive coupling mechanism.