Structural Changes in a High-Energy Density VO2F Cathode upon Heating and Li Cycling

Structural changes in VO2F, which allow twoelectron transfer during electrochemical Li cycling, were investigated. This compound adopts a rhombohedral structure, space group R-3c, 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 {$^\circ$}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 explained by an asymmetric energetic preference between the rhombohedral and rock-salt forms of LixVO2F, which result in large thermodynamic driving forces to convert to the rock-salt structure at x \textbackslash textgreater 1 and relatively small thermodynamic driving forces to convert back to the rhombohedral structure when delithiating to x \textbackslash textless 1.