Stabilizing fluorine to achieve high-voltage and ultra-stable Na₃V₂(PO₄)₂F₃ cathode for sodium ion batteries

Na₃V₂(PO₄)₂F₃ is a promising cathode candidate for sodium ion batteries due to its high working voltage. However, an “unknown” low-voltage plateau around ~3.3 V lowers its operating voltage and puzzles researchers. Herein, the reasons for its appearance are explored. It is found that the formation of the low-voltage plateau originates from the extra-generated [VO₆] induced by the fluorine loss during the synthesis. To enable better performance, a feasible strategy of stabilizing fluorine is proposed to eliminate the low-voltage plateau. It is demonstrated that this strategy can effectively guarantee the existence of fluorine and modulate the local electronic structure of V to maintain [VO₄F₂] in Na₃V₂(PO₄)₂F₃ phase, thus eliminating the undesirable plateau and achieving an operating-voltage increase of ~100 mV. Moreover, the enhanced structural stability and kinetics of the optimized Na₃V₂(PO₄)₂F₃ cathode are confirmed by in-situ XRD. As expected, the Na₃V₂(PO₄)₂F₃ cathode delivers higher energy density (446.4 Wh kg⁻¹), better rate capability and longer cycling performance (89.2%@30 C after 1000 cycles). Furthermore, its excellent adaptability to wide temperature range (−25 °C–55 °C) and strong competence in Na₃V₂(PO₄)₂F₃||hard carbon full-cell are verified. These excellent properties further qualify Na₃V₂(PO₄)₂F₃ as a competitive cathode for sodium ion batteries.