High-Voltage Na_0.76Ni_0.25–x/2Mg_x/2Mn_0.75O_2–xF_x Cathode Improved by One-Step In Situ MgF₂ Doping with Superior Low-Temperature Performance and Extra-Stable Air Stability

P2-Na x MnO 2 has garnered significant attention due to its favorable Na + conductivity and structural stability for large-scale energy storage fields. However, achieving a balance between high energy density and extended cycling stability remains a challenge due to the Jahn–Teller distortion of Mn 3+ and anionic activity above 4.1 V. Herein, we propose a one-step in situ MgF 2 strategy to synthesize a P2-Na 0.76 Ni 0.225 Mg 0.025 Mn 0.75 O 1.95 F 0.05 cathode with improved Na-storage performance and decent water/air stability. By partially substituting cost-effective Mg for Ni and incorporating extra F for O, the optimized material demonstrates both enhanced capacity and structure stability via promoting Ni 2+ /Ni 4+ and oxygen redox activity. It delivers a high capacity of 132.9 mA h g –1 with an elevated working potential of ≈3.48 V and maintains ≈83.0% capacity retention after 150 cycles at 100 mA g –1 within 2–4.3 V, compared to the 114.9 mA h g –1 capacity and 3.32 V discharging potential of the undoped Na 0.76 Ni 0.25 Mn 0.75 O 2 . While increasing the charging voltage to 4.5 V, 133.1 mA h g –1 capacity and 3.55 V discharging potential (vs Na/Na + ) were achieved with 72.8% capacity retention after 100 cycles, far beyond that of the pristine sample (123.7 mA h g –1, 3.45 V, and 43.8%@100 cycles). Moreover, exceptional low-temperature cycling stability is achieved, with 95.0% after 150 cycles. Finally, the Na-storage mechanism of samples employing various doping strategies was investigated using in situ EIS, in situ XRD, and ex situ XPS techniques.