Abstract
Structural changes in VO2F, which allow two-electron transfer during electrochemical Li cycling, were investigated. This compound adopts a rhombohedral structure, space group R3c, with O and F sharing one site, and was synthesized by high-energy ball-milling. The thermal stability of VO2F, which is related to the battery safety, is studied by in situ XRD upon heating and by thermal gravimetric analysis. VO2F is found to be stable up to 160 °C under inert atmosphere; above this temperature, it converts into vanadium oxide with fluorine loss. The structure evolution upon lithium cycling was studied by ex situ X-ray diffraction and absorption techniques. The results show that lithiation of VO2F goes through a solid-solution reaction, and the rhombohedral structure is preserved if no more than one lithium ion is intercalated. Upon a second Li insertion, an irreversible transition to a rock-salt structure occurs. We show using first-principles calculations that this irreversible transformation can be explaine...
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