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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.

Rechargeable lithium-ion cells are considered the most advanced energy storage systems. Commercial cells utilize layered metal-based oxides as the positive electrode, [1] but their high cost and safety could limit use in large-scale applications. In an intensive search for alternative materials, phosphates such as LiFePO 4 , [2][3][4][5] and other polyanion ("XO 4 ") structures have also been explored, including those based on silicates.[6] In addition to possessing high redox potentials and promising Li + transport, polyanion frameworks can exhibit remarkable electrochemical and thermal stability. These make them particularly suitable where safety and longevity are a concern. Fluorinated polyanion moieties have also received much attention owing to their similar properties. The addition of fluoride engenders a charge difference and modification to the dimensionality of the lattice along with an increase in redox potential, thus offering the tantalizing promise of new architectures and electrochemical behavior. [7]

An electrochemical and structural study of the two-electron redox couple comprising the tavorite-type series of fluorophosphates Li 1±x VPO 4 F (x = 0, 1) shows that both intercalation of LiVPO 4 F with Li (to give Li 2 VPO 4 F) and deintercalation (to give VPO 4 F) proceed by a two-phase mechanism. Structural models for each of the three phases were determined by Rietveld refinements of combined neutron and X-ray diffraction data of the isolated pure phase materials. LiVPO 4 F crystallizes in the triclinic space group P1̅ and is isostructural to many known fluorophosphates whereas both Li 2 VPO 4 F and VPO 4 F crystallize in the monoclinic space group C2/c , although they have very closely related structures to the parent. Solid-state 6,7 Li NMR studies of Li 2 VPO 4 F reveal the two lithium sites are clearly distinguishable, with more than 100 ppm separation between the resonances. 2D exchange NMR is used to demonstrate the time scale of ion dynamics between the two sites.

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