Simultaneously tuning cationic and anionic redox in a P2-Na0.67Mn0.75Ni0.25O2 cathode material through synergic Cu/Mg co-doping

Abstract

Anionic redox chemistry has attracted increasing attention in research on P2-type layered oxide cathodes for sodium-ion batteries. But how to modulate the crystal or electronic structure to achieve high activity and reversibility of oxygen redox is extremely challenging. Herein, we propose to enhance the activity and reversibility of both cationic and anionic redox in Na_0.67Mn_0.75Ni_0.25O₂ by a synergy of Cu and Mg co-doping. The cycling stability and the rate capability were found to be largely enhanced, and the synergy mechanism of Cu and Mg co-doping was unveiled. Firstly, Cu/Mg co-doping shortens TM–O and enhances TM–O bonding energy, which improve the oxygen redox reversibility and structure stability. Secondly, Cu/Mg co-doping improves the thermal decomposition temperature and the reversible transformation from P2 to O2 phase, indicating the enhancement of anionic redox reversibility. Thirdly, Cu/Mg co-doping reduces the ratio of Mn³⁺/Mn⁴⁺, which alleviates the Jahn–Teller effect and enhances the layered structure stability. Moreover, the d-spacing was also enlarged by Cu/Mg co-doping, which facilitates the Na⁺ diffusion. This study presents some insights into simultaneously tuning the activity and reversibility of cationic and anionic redox in layered oxide cathode materials.