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 (DNa+ = 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.