High-Voltage Na 0.76 Ni 0.25- x /2 Mg x /2 Mn 0.75 O 2- x F x Cathode Improved by One-Step In Situ MgF 2 Doping with Superior Low-Temperature Performance and Extra-Stable Air Stability.

P2-Na x MnO2 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 Mn3+ and anionic activity above 4.1 V. Herein, we propose a one-step in situ MgF2 strategy to synthesize a P2-Na0.76 Ni0.225 Mg0.025 Mn0.75 O1.95 F0.05 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 Ni2+ /Ni4+ 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 Na0.76 Ni0.25 Mn0.75 O2 . 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.