Lithium insertion or extraction from/into tavorite-type LiVPO4F : an in situ x-ray diffraction study

The Tavorite-type composition LiVPO4F undergoes a complex series of phase transformations upon Li+ electrochemical extraction or insertion at ∼4.25 V (V4+/V3+ redox couple) and 1.8 V vs. Li (V3+/V2+ redox couple), respectively. In all the reactions involved, followed by in-house in situ X-Ray diffraction, two-phase topotactic mechanisms are systematically encountered, without noticeable modification of the [VPO4F] framework besides significant unit-cell contractions/expansions and overall shifts in symmetry from triclinic P-1 to monoclinic C2/c. Two electrochemical oxidation reactions occur at 4.24 V and 4.28 V vs. Li due to the formation of the intermediate phase Li0.67VPO4F, isolated for the first time. Noticeably, this peculiar composition is not encountered upon reduction from VPO4F to LiVPO4F