Activity of metal-fluorine states upon delithiation of disordered rocksalt oxyfluorides.

The capacity of transition metal oxides as Li-ion battery cathodes is limited by instabilities that arise when high states of charge are achieved. Oxyfluorides with a disordered rock-salt structure have emerged as attractive alternatives, but the role of F in their electrochemical function, particularly when metals reach high formal oxidation states through cationic redox, remains to be ascertained. Using XAS measurements of Mn, O and F, we reveal the existence of Mn-F covalent interactions in Li2MnO2F and Li2Mn2/3Nb1/3O2F. New unoccupied states evolve from both Mn-F and Mn-O interactions when the phases are delithiated. The results challenge the assumption of F as largely a spectator ion, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggest the existence of unique knobs of design of battery cathodes in these chemical spaces, by manipulating the covalent interactions between transition metals and two different anions.