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

The structural and alkali ion insertion characteristics of sodium vanadium fluorophosphate, Na3V2(PO4)(2)F-3 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)(2)F-3 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. (c) 2006 Elsevier B.V. All rights reserved.