Revisiting of Tetragonal NaVPO₄F: A High Energy Density Cathode for Sodium-Ion Batteries

Tetragonal NaVPO₄F has been regarded as an ideal cathode for sodium-ion batteries because of its high average plateau (3.8 V) and theoretical specific capacity (143 mA h g^-1). However, the Na-storage performance is still hindered by unsatisfying thermal stability and poor conductivity. Herein, a stable tetragonal NaVPO₄F has been synthesized by a novel solvent thermal method using a carbon coating precursor. The as-prepared NaVPO₄F@C nanocomposite delivers a capacity of 133 mA h g^-1 at 0.2 C, corresponding to an excellent energy density of 509 W h kg^-1; when coupled with an HC anode, the full cell still displays an outstanding performance of 124 mA h g^-1 at 0.05 C. Fast Na⁺ diffusion kinetics (D^Na = 10^-12 to 10^-10 cm² s^-1) and small volume change (4.4%) are exploited, which ensures good rate trait and cycling stability of tetragonal NaVPO₄F. Further, the Na⁺ extraction-insertion mechanism has been explored by analyzing the crystal structure change during in situ X-ray powder diffraction cycles.