Cation–anion co-redox induced high-capacity cathode for high energy density sodium-ion batteries†

Iron/manganese-based layered transition metal oxides have emerged as competitive cathode candidates for sodium-ion batteries (SIBs) due to their high theoretical capacity and naturally abundant constituent elements. Nevertheless, implementation challenges persist due to irreversible phase transformations and substantial capacity fading during cycling. Herein, we present a novel P2-type Na_0.73Fe_0.2Mn_0.52Co_0.2Mg_0.05Li_0.03O₂ (designated as P2-NFMO-CoMgLi) cathode material characterized by coupled cationic and anionic redox. The rational doping of Li⁺ into transition metal (TM) sites activates the reversible oxygen redox chemistry and suppresses the Jahn–Teller distortions. In addition, the doping of Mg²⁺ effectively inhibits Na⁺ vacancy ordering in low-voltage regimes (<2.5 V), and Co²⁺ incorporation concurrently improves specific capacity and stabilizes the TM–O bonding networks. Consequently, the P2-NFMO-CoMgLi cathode exhibits a high capacity of 178 mA h g⁻¹ at 20 mA g⁻¹ and a 68% capacity retention over 150 cycles at 200 mA g⁻¹. Ex situ XPS characterization reveals that oxygen redox processes occur and provide extra capacity at high voltage. These findings provide a fundamental understanding of voltage-induced cation–anion redox in layered oxide cathodes, advancing the rational design of high-energy-density SIB systems.