High-Voltage Na_0.76Ni_{0.25–<i>x</i>/2}Mg_<i>x</i>/2Mn_0.75O_2–<i>x</i>F_<i>x</i> Cathode Improved by One-Step In Situ MgF₂ Doping with Superior Low-Temperature Performance and Extra-Stable Air Stability

Abstract

P2-Na_<i>x</i>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-Na_0.76Ni_0.225Mg_0.025Mn_0.75O_1.95F_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 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 Na_0.76Ni_0.25Mn_0.75O₂. 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.