Fluorophosphates from Solid‐State Synthesis and Electrochemical Ion Exchange: NaVPO 4 F or Na 3 V 2 (PO 4 ) 2 F 3 ?

Vanadium‐based fluorophosphates are promising sodium‐ion battery cathode materials. Different phases of NaVPO 4 F and Na 3 V 2 (PO 4 ) 2 F 3 are reported in the literature. However, experiments in this work suggest that there could be confusions about the single‐phase NaVPO 4 F in solid‐state synthesis. Here, systematic investigation of the mechanism underlying structural and compositional evolution of solid‐state synthesis (NaF:VPO 4 = 1:1) is determined by in situ and ex situ X‐ray diffraction and electrochemical measurements. Three reactions—3NaF + 3VPO 4 → Na 3 V 2 (PO 4 ) 2 F 3 + VPO 4 (up to 500 °C), Na 3 V 2 (PO 4 ) 2 F 3 + VPO 4 → Na 3 V 2 (PO 4 ) 3 + VF 3 ↑ (600–800 °C), and 2Na 3 V 2 (PO 4 ) 3 → 2(VO) 2 P 2 O 7 + Na 4 P 2 O 7 + 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 NaVPO 4 F at any temperature. It is speculated that the assignments of I4/mmm and C 2 /c NaVPO 4 F from solid‐state synthesis are incorrect, which are instead multiphase mixtures of Le Meins' Na 3 V 2 (PO 4 ) 2 F 3 , unreacted VPO 4 , and hexagonal Na 3 V 2 (PO 4 ) 3 . Liquid‐electrolyte‐based electrochemical ion exchange of LiVPO 4 F produces a tavorite NaVPO 4 F structure, which is very different from Le Meins' family of Na 3 Al 2 (PO 4 ) 2 F 3 polymorphs.