Solvothermal synthesis of electroactive lithium iron tavorites and structure of Li2FePO4F

Abstract

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.