Copper-substituted P3-type Na0.54Mn0.64Fe0.16Mg0.1Cu0.1O2 cathode material for sodium-ion batteries with enhanced anionic redox reversibility

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_0.54Mn_0.64Fe_0.16Mg_0.1Cu_0.1O₂) 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.