Fast and highly reversible Na⁺ intercalation/extraction in Zn/Mg dual-doped P2-Na_0.67MnO₂ cathode material for high-performance Na-ion batteries

P2-type layered Na_0.67MnO₂ has been considered to be a promising candidate cathode material for sodium ion batteries. Nevertheless, the undesired phase transitions during operation and the large Na⁺ radius induced sluggish ion diffusion remain the stumbling blocks to realize its high performance. Herein, we propose a Zn/Mg co-doping strategy, which is proved to have bifunctional effects. First, relative to the pristine P2-Na_0.67MnO₂ and the single-ion (Zn/Mg) doped samples, the Zn/Mg dual-doped P2-Na_0.67MnO₂ demonstrates a lower Mn³⁺/Mn⁴⁺ ratio and a higher lattice O content, which facilitate the structural stability of the cathode material. More intriguingly, the Zn/Mg co-doping gives rise to enlarged interplanar spacing, which provides spacious ion diffusion channels for fast Na⁺ intercalation/extraction. As a result, the Zn/Mg dual-doped sample exhibits a high Na⁺ diffusion coefficient and a solid-solution reaction during charge/discharge, with a cell volume change determined to be only 0.55%. Taking advantages of the above favorable features, the Zn/Mg dual-doped P2-Na_0.67MnO₂ demonstrates a high rate performance with 67.2 mAhdg^-1 delivered at 10 C and a decent cycling stability with a capacity retention of 93.8% achieved at 1 C after 100 cycles. This work introduces the Zn/Mg co-doping strategy to simultaneously improve the cycling stability and rate capability of P2-Na_0.67MnO₂, which may offer a promising avenue for further performance enhancement of the layered Na-ion batteries cathode materials.