Copper‐substituted P3‐type Na 0.54 Mn 0.64 Fe 0.16 Mg 0.1 Cu 0.1 O 2 cathode material for sodium‐ion batteries with enhanced anionic redox reversibility

Abstract P3‐type manganese‐iron‐based cathodes with high specific capacity and abundant resource have attracted considerable attention for sodium‐ion batteries. However, the long‐term cycle stability of P3‐type cathodes is still not satisfactory. In this work, we design a new quaternary manganese‐iron‐based cathode material (P3‐Na 0.54 Mn 0.64 Fe 0.16 Mg 0.1 Cu 0.1 O 2 ) by Cu substitution. The strong covalent Cu–O bonds improve the structural stability and the reversibility of O redox during charge and discharge processes. Cu substitution also mitigates the structure change with less unit cell volume variation, and improves the Na‐ion transport kinetics effectively. As a result, NMFMC delivers much improved cycling stability and rate capability compared with NMFM. It reveals that the charge compensation of NMFMC is mainly contributed by Mn 3+/4+ , Fe 3+/3.5+ and O 2−/− during the charge and discharge processes, and Cu substitution can also enhance the activity and reversibility of Fe redox. This strategy provides a new pathway toward improving the stability and O redox reversibility of P3‐type cathode materials for sodium‐ion batteries.