Wide-Temperature Operational P2-Type Cathode via Protective Coating: Synergistic Air Stability Improvement and Mn Dissolution Mitigation

Enhancing the performance of cathode materials for sodium-ion batteries through surface modification has proven to be an effective approach. In this study, P2-Na_0.67Mn_0.95Mg_0.05O₂ coated with different masses of Al₂O₃ (NMMO@Ax; x = 0.5, 1, 1.5, and 2) is synthesized by the sol-gel method in a facile manner. By regulating the interfacial configuration and the quality of a surface modification layer, the air stability of a P2-type cathode is improved by reducing the residual alkali. Meanwhile, the Al₂O₃ interface can inhibit the side reactions of the Mn dissolution and achieve a long-duration service life. Al₂O₃-coated Na_0.67Mn_0.95Mg_0.05O₂ (NMMO@A1) (1 wt %) has the highest pseudocapacitance contribution and lowest impedance in the NMMO@Ax series. The initial specific capacity of NMMO@A1 is 184.9 mAh g^-1 at 100 mA g^-1, with 85.9% capacity retention after 100 cycles in the SIBs. After 1000 cycles, the capacity retention of NMMO@A1 is 67.7% at 1A g^-1. Notably, NMMO@A1 exhibits acceptable specific capacity and cycling stability from -20 to 60 °C with favorable thermal adaptability. Furthermore, the full battery consisting of the NMMO@A1 cathode and hard carbon anode demonstrates a high energy density of 326.5 Wh kg^-1.