Tavorite Lithium Iron Fluorophosphate Cathode Materials: Phase Transition and Electrochemistry of LiFePOsub 4F–Lisub 2FePOsub 4F

We have synthesized LiFePO4 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, Li2FePO4 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.