Enhanced Electrochemical Performance of Sodium Manganese Ferrocyanide by Na₃(VOPO₄)₂F Coating for Sodium-Ion Batteries

Sodium manganese ferrocyanide Na_xMn[Fe(CN)₆]_y is an attractive cathode material for sodium-ion batteries. However, Na_xMn[Fe(CN)₆]_y prepared by simple coprecipitation of Mn²⁺ and [Fe(CN)₆]^4- usually shows poor cycling performance, which hinders its practical application. In this work, electrochemical performance of a Na_1.6Mn[Fe(CN)₆]_0.9 (PBM) sample prepared by the simple precipitation method was greatly improved by coating with Na₃(VOPO₄)₂F (NVOPF) via a solution precipitation method. The as-prepared PBM@NVOPF with a coating quantity of 2.0% molar ratio showed enhanced rate capability and superior cyclic stability. The discharge capacities of PBM@NVOPF were 101.5 mA h g^-1 (1 C) and 91.4 mA h g^-1 (10 C), with a capacity retention of 84.3% after 500 cycles at 1 C, 20 °C. It also exhibited excellent cyclic stability at elevated temperature with an initial capacity of 109.5 mA h g^-1 and a capacity retention of 78.8% after 200 cycles at 1 C, 55 °C. In comparison, uncoated PBM showed a discharge capacity of 105.7 mA h g^-1 (1 C) and 76.7 mA h g^-1 (10 C), with a capacity retention of only 42.0% after 500 cycles at 1 C, 20 °C. The high-temperature performance of bare PBM was very poor, and the capacity retention was only 35.7% after 40 cycles because of serious Mn/Fe dissolution which caused structural deterioration of PBM. NVOPF coating protected the PBM from suffering corrosion in the electrolyte, thus ensured the framework stability of PBM during long-term cycling and contributed to the excellent electrochemical performance.