Lithium Insertion or Extraction from/into Tavorite-Type LiVPO₄F: An In Situ X-ray Diffraction Study

The Tavorite-type composition LiVPO 4 F undergoes a complex series of phase transformations upon Li + electrochemical extraction or insertion at ∼4.25 V (V 4+ /V 3+ redox couple) and 1.8 V vs. Li (V 3+ /V 2+ 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 [VPO 4 F] 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 Li 0.67 VPO 4 F, isolated for the first time. Noticeably, this peculiar composition is not encountered upon reduction from VPO 4 F to LiVPO 4 F.