Solvothermal synthesis of electroactive lithium iron tavorites and structure of Li2FePO4F

Lithium transition metal fluorophosphates with a tavorite structure have been recognized as promising electrode materials for lithium-ion batteries because of their good energy storage capacity combined with electrochemical and thermal stability. We report here a new low-cost and environmentally friendly solvothermal process to prepare LiFePO4F, which exhibits a complex single phase regime followed by a two-phase plateau at 2.75 V on electrochemical lithium insertion. The structure of the pure single phase end member Li2FePO4F was synthesized by lithiation of LiFePO4F, and solved via Rietveld refinement of the combined X-ray and neutron diffraction patterns, showing that Li+ occupies multiple sites in the tavorite lattice. LiFePO4OH was prepared by a new synthetic route and the electrochemical capacity for this material is the highest reported to date. LiFePO4OH was found to intercalate lithium at 2.40 V and the reduced Li2FePO4OH phase was found to be amorphous.