Sodium-ion batteries (NIBs) have been selected as a promising candidate for large-scale energy storage systems due to their abundance. Among several NIB cathode materials, P2-type transition metal layered oxides (Na_xTMO2, TM = Ti, V, Cr, Mn, Fe, Co, Ni) featuring high theoretical capacity and better rate performance have attracted much attention. However, the practical applications have to endure the low energy density of NIB cathode materials compared to lithium-ion batteries. In tradition, the capacity is constrained by transition metal ions and is closed to their limits. Hence,in order to obtain extraordinarily high capacity in cathode materials, both anionic and cationic redox chemistry are utilized. Nevertheless, their performance is impeded by irreversible structure evolution and lattice oxygen emission. Therefore, it is highly urgent to develop stable anionic redox chemistry for high energy density and long-cycle-life layered oxide cathode materials.

In this study, cation-doped Na_xMg_yCu_zMn_(1−y−z)O2 cathode material featuring synergistic effects of cationic and anionic redox was reported. By cations doping, the inhibited structure evolution and lattice oxygen stabilization were achieved. Moreover, the effects of cation-doped Na_xMg_yCu_zMn_(1−y−z)O2 were also studied by electrochemical measurements. Also, the mechanism of cation-doped NaxMgyCu_zMn_(1−y−z)O2 was confirmed by operando synchrotron X-ray absorption spectrum, operando X-ray diffraction, and density functional theory computations. Cation-doped Na_xMg_yCu_zMn_(1−y−z)O2 was synthesized through a facile sol-gel method followed by heat treatment. The cation-doped Na_xMg_yCu_zMn_(1−y−z)O2 showed high specific capacity (203 mAh g⁻¹ cycled at 0.1C) as well as better cycling stability, providing sodium layered oxides a new developing stage toward high-performance cathode materials in NIBs for large scale energy storage systems.

Keywords: Na-ion batteries, layered oxides, anionic redox, cathode