Oxygen-Tuned Na₃V₂(PO₄)₂F_{3-2 y}O_{2 y}(0 ≤ y < 1) as High-Rate Cathode Materials for Rechargeable Sodium Batteries

The abundant reserves of sodium resources and its low price make the desire to build energy storage systems on room-temperature rechargeable sodium batteries. The current challenge exists in exploring high-performance key materials, especially cathode materials. Vanadium-based phosphates have attracted extensive attention as a class of promising cathode materials for sodium batteries due to their stable structure, large capacity, and high voltage advantages. In this paper, sodium vanadium oxyfluorophosphate, Na₃V₂(PO₄)₂F_3-2yO_2y(0 ≤ y < 1), is exploited to improve its electrochemical performance through oxygen tuning. The effect of oxygen amount on the structure, morphology, and performance is comprehensively and comparatively investigated. It is found that the optimal electrochemical performance is achieved in Na₃V₂(PO₄)₂F₂O when y = 0.5. Its specific discharge capacity at 0.1, 0.5, 1, 8, and 20 C is 125, 111, 101, 71, and 58 mA h g^-1, respectively. The superior rate performance and good cyclability are ascribed to the low impurity, mixed V³⁺/V⁴⁺valence state, small particle size, suitable residual carbon coating, low charge transfer resistance, fast Na⁺diffusion, and particularly the regulation of charge/discharge potentials as well as polarizations due to proper oxygen tuning.