Structure and Electrochemistry of Two-Electron Redox Couples in Lithium Metal Fluorophosphates Based on the Tavorite Structure

An electrochemical and structural study of the two-electron redox couple comprising the tavorite-type series of fluorophosphates Li1±xVPO4F (x = 0, 1) shows that both intercalation of LiVPO4F with Li (to give Li2VPO4F) and deintercalation (to give VPO4F) 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. LiVPO4F crystallizes in the triclinic space group P1- and is isostructural to many known fluorophosphates whereas both Li2VPO4F and VPO4F crystallize in the monoclinic space group C2/c, although they have very closely related structures to the parent. Solid-state 6,7Li NMR studies of Li2VPO4F 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.