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

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 explained by an asymmetric energetic preference between the rhombohedral and rock-salt forms of Li_xVO₂F, which result in large thermodynamic driving forces to convert to the rock-salt structure at x > 1 and relatively small thermodynamic driving forces to convert back to the rhombohedral structure when delithiating to x < 1.