Dynamic Fluorine Compensation Strategy Purifies Na₃V₂(PO₄)₂F₃ Phase Toward High-Energy and Stable Sodium Storage

Na₃V₂(PO₄)₂F₃ (NVPF) is a promising sodium-ion battery cathode due to its high voltage and structural stability, but suffers from fluorine loss, suboptimal capacity from limited Na⁺ extraction, and insufficient stability at deeper Na⁺ (de)intercalation. Therefore, this study used in situ thermogravimetry-infrared-mass spectrometry for the first time to clarify the fluorine loss mechanism during the synthesis of NVPF. Based on this, a targeted dynamic fluorine compensation strategy is proposed, using NH₄F as a dual-functional additive that combines the functions of fluorine source and pH buffer. The optimal NVPF-12.5NHF shows the characteristics of completely suppressing the impurity phase. In addition, by expanding the voltage window to 1.0-4.4 V to activate the redox reaction of V²⁺/V³⁺, the ultrahigh specific capacity of NVPF is achieved via the electrochemical insertion of additional Na⁺. The NVPF-12.5NHF cathode exhibits an astonishing specific capacity of 173.6 mAh g^-1 at a rate of 0.3C (64 mA g^-1), and maintains a specific capacity of 96.1 mAh g^-1 at 33C. Furthermore, the capacity retention rate is more than 70% after 2000 cycles at 13C. These findings establish a scalable strategy for designing electrode materials with volatile components, offering significant potential to accelerate the practical deployment of sodium-ion batteries.