The electrochemical insertion properties of sodium vanadium fluorophosphate, Na3V2(PO4)2F3

The structural and alkali ion insertion characteristics of sodium vanadium fluorophosphate, Na3V2(PO4)2F3 are presented. The material was prepared using a solid-state carbothermal reduction approach involving the precursors VPO4 and NaF. Electrochemical characterization of the Na3V2(PO4)2F3 phase in the potential range 3.0–4.6 V vs. Li, revealed a structured voltage response corresponding to the reversible cycling of two alkali ions per formula unit. The associated specific capacity was around 120 mAh/g, at an average discharge voltage of around 4.1 V vs. Li. The stability of the alkali ion insertion reactions was confirmed by long term cycling experiments, which demonstrated low capacity fade over the initial 220 cycles. Voltage excursions to 5 V vs. Li suggest that all three Na ions may be successfully extracted from the fluorophosphate phase, although this process is likely accompanied by some concurrent structural degradation.