VO2F: a new transition metal oxyfluoride with high specific capacity for Li ion batteries

Abstract

Hitherto unreported vanadium oxyfluoride VO₂F has been synthesized using a solid state reaction at a pressure of 4 GPa and 800 °C. This long awaited vanadium oxyfluoride fills the existing gap of ReO₃-type MO₂F compounds of Group 5 elements, from which only NbO₂F and TaO₂F have been known to exist to date. VO₂F crystallizes with the VF₃-type structure, space group Rc, with a = 5.1226(1) Å and c = 13.0686(3) Å as determined by powder X-ray diffraction. Highly structured diffuse streaking observed in electron diffraction patterns evidences local O/F ordering. VO₂F exhibits two regions upon discharge in a lithium cell, an upper sloped region in the range of 3.9–2.2 V and a lower plateau at 2.15 V. Discharge of VO₂F to 1 V provides a gravimetric capacity of 450 mA h g⁻¹. VO₂F can reversibly insert up to 1 Li⁺ per vanadium above 2.15 V without destruction of the host structure, delivering a gravimetric capacity as high as 250 mA h g⁻¹ and pointing to VO₂F as a promising intercalation electrode.