Medium‐entropy configuration enabling reversible P2‐OP4 phase transition in layered oxides for high‐rate sodium‐ion batteries

Layered transition metal oxides have emerged as promising cathode materials for sodium ion batteries. However, irreversible phase transitions cause structural distortion and cation rearrangement, leading to sluggish Na$^+$ dynamics and rapid capacity decay. In this study, we propose a medium-entropy cathode by simultaneously introducing Fe, Mg, and Li dopants into a typical P2-type Na$_{0.75}$Ni$_{0.25}$Mn$_{0.75}$O$_2$ cathode. The modified Na$_{0.75}$Ni$_{0.2125}$Mn$_{0.6375}$Fe$_{0.05}$Mg$_{0.05}$Li$_{0.05}$O$_2$ cathode predominantly exhibits a main P2 phase (93.5%) with a minor O3 phase (6.5%). Through spectroscopy techniques and electrochemical investigations, we elucidate the redox mechanisms of Ni$^{2+/3+/4+}$, Mn$^{3+/4+}$, Fe$^{3+/4+}$, and O$^{2−}$/O$_2$$^{n−}$ during charging/discharging. The medium-entropy doping mitigates the detrimental P2-O2 phase transition at high-voltage, replacing it with a moderate and reversible structural evolution (P2-OP4), thereby enhancing structural stability. Consequently, the modified cathode exhibits a remarkable rate capacity of 108.4 mAh·g$^{−1}$ at 10C, with a capacity retention of 99.0% after 200 cycles at 1C, 82.5% after 500 cycles at 5C, and 76.7% after 600 cycles at 10C. ... mehrFurthermore, it also demonstrates superior electrochemical performance at high cutoff voltage of 4.5 V and extreme temperature (55 and 0 °C). This work offers solutions to critical challenges in sodium ion batteries cathode materials.