Gradational anionic redox enabling high-energy P2-type Na-layered oxide cathode

Anionic-redox-based layered oxide materials are considered promising cathodes for Na-ion batteries because of their high energy densities. However, the anionic redox reaction at high voltage results in structural instability of the layered oxides, leading to not only poor electrochemical properties but also structural degradation after prolonged cycling. Herein, through combined studies using first-principles calculation and various experimental techniques, we investigate the role of the combination of earth-abundant Mn, Fe, and Mg in enabling a stable and gradational anionic redox reaction in a P2-type Na-layered oxide cathode during charge/discharge, resulting in outstanding electrochemical performance. At 10 mA g⁻¹, P2-type Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ delivers a large specific capacity of ∼207 mAh g⁻¹, corresponding to ∼0.8 mol Na⁺ de/intercalation via both cationic and anionic redox reactions. The outstanding cycle performance, well-retained crystal structure, and morphology after prolonged cycling indicate that the anionic redox reaction of O²⁻/O⁻ stably occurred in the P2-type Na_0.67[Mg_0.22Mn_0.55Fe_0.23]O₂ structure despite the charging process in the high-voltage region. Furthermore, the use of earth-abundant Mn, Fe, and Mg is beneficial in terms of the economic feasibility for low-cost and high-energy Na-ion batteries. These intensive investigations provide key knowledge for understanding anionic-redox-based cathode materials with high structural stability for Na-ion batteries.