Three-electron reversible redox for a high-energy fluorophosphate cathode: Na3V2O2(PO4)2F.

A three-electron structural reaction for Na3V2O2(PO4)2F (Na3VOPF) in space group I4/mmm shows a priori stabilisation in terms of long-life in the voltage range of 2.0-4.5 V, with embedding of more than one sodium ion to generate Na5VOPF upon discharge to 1.0 V in the first cycle, thereby increasing the specific capacity from ∼170 mA h g-1. The capacity of the crystals was 180 mA h g-1 after 20 cycles. The results provide a probable route to improve fluorophosphate cathode performance.