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

We have synthesized LiFePO4F 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, Li2FePO4F. We show that the latter adopts a triclinic P (1) over bar 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 degrees C is very stable. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3298353] All rights reserved.