Redox Chemistry and Reversible Structural Changes in Rhombohedral VO2F Cathode during Li Intercalation

Abstract

Metal oxyfluorides are currently attracting much attention for next-generation rechargeable batteries because of their high theoretical capacity and resulting high energy density. Rhombohedral VO2F is promising because it allows two-electron transfer during electrochemical lithium cycling, with a theoretical capacity of 526 mAh g-1. However, the chemical changes it undergoe during operation are not clearly understood. In this work, a combination of synchrotron X-ray and neutron diffraction was employed to accurately describe the crystal structure of both pristine and lithiated VO2F, using samples with high crystallinity to overcome challenges in previous studies. The mechanism and reversibility of the lithium insertion was monitored in real time by high angular synchrotron diffraction measurements, performed in operando on a lithium battery in the high-voltage range: 3.9-2.3 V vs Li+/Li. Insertion of up to one lithium ion proceeds through a solid-solution reaction, while Rietveld refinements of neutron powder diffraction data revealed that the lithiated states adopt the noncentrosymmetric R3c framework, uncovering an octahedral Li-(O/F)6 coordination with reasonable Li-O/F bond lengths. This work further evaluates the redox changes of VO2F upon Li intercalation. By a comparison of changes in electronic states of all the elements in the compound, it clarifies the critical role of both anions, O and F, in the charge compensation through their covalent interactions with the 3d states of V. The clear evidence of participation of F challenges existing assumptions that its high electronegativity renders this anion largely a spectator in the redox reaction.