Fluorophosphates from Solid‐State Synthesis and Electrochemical Ion Exchange: NaVPO₄F or Na₃V₂(PO₄)₂F₃?

Abstract Vanadium‐based fluorophosphates are promising sodium‐ion battery cathode materials. Different phases of NaVPO₄F and Na₃V₂(PO₄)₂F₃ are reported in the literature. However, experiments in this work suggest that there could be confusions about the single‐phase NaVPO₄F in solid‐state synthesis. Here, systematic investigation of the mechanism underlying structural and compositional evolution of solid‐state synthesis (NaF:VPO₄ = 1:1) is determined by in situ and ex situ X‐ray diffraction and electrochemical measurements. Three reactions—3NaF + 3VPO₄ → Na₃V₂(PO₄)₂F₃ + VPO₄ (up to 500 °C), Na₃V₂(PO₄)₂F₃ + VPO₄ → Na₃V₂(PO₄)₃ + VF₃↑ (600–800 °C), and 2Na₃V₂(PO₄)₃ → 2(VO)₂P₂O₇ + Na₄P₂O₇ + amorphous products (above 800 °C)—are validated by in situ XRD and thermogravimetric analysis/differential scanning calorimetry. None of the products reported in this work is consistent with single‐phase NaVPO₄F at any temperature. It is speculated that the assignments of I4/mmm and C₂/c NaVPO₄F from solid‐state synthesis are incorrect, which are instead multiphase mixtures of Le Meins' Na₃V₂(PO₄)₂F₃, unreacted VPO₄, and hexagonal Na₃V₂(PO₄)₃. Liquid‐electrolyte‐based electrochemical ion exchange of LiVPO₄F produces a tavorite NaVPO₄F structure, which is very different from Le Meins' family of Na₃Al₂(PO₄)₂F₃ polymorphs.