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 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)P) 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. LiVPO4F crystallizes in the triclinic space group P (1) over bar 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.