Tavorite Lithium Iron Fluorophosphate Cathode Materials: Phase Transition and Electrochemistry of LiFePO4F – Li2FePO4F

We have synthesized LiFePO 4 F by a simple solid-state route as a pure single phase, which we show is isostructural with that of the minerals tavorite and amblygonite, and we report the first isolation of its fully lithium-inserted crystalline analog, Li 2 FePO 4 F. We show that the latter adopts a triclinic P1 tavorite-type framework that is very closely related to the parent phase. The redox activity between these two compositions is very facile and occurs with an 8% change in volume to result in a reversible and stable capacity of about 145 mAh/g. The electrochemical cycling at both room temperature and 55 °C is very stable.