Activity of Metal-Fluorine States upon Delithiation of Disordered Rocksalt Oxyfluorides

Abstract

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 rocksalt structure have emerged as attractive alternatives, but the role of F in their electrochemical function, particularly when cationic redox produces high formal oxidation states, remains to be ascertained. Using X-ray absorption spectroscopy, we confirm the existence of Mn–F covalent interactions in Li2MnO2F and Li2Mn2/3Nb1/3O2F. New unoccupied states evolve from hybrid 3d-2p states of both Mn–F and Mn–O bonds when the phases are delithiated, particularly in the presence of Mn­(IV). The results challenge the assumption of F as anion whose covalent states with the metal are tapped at very high potentials, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggest the existence of unique knobs of design of battery cathodes by manipulating the covalent interactions between transition metals and two different anions.

Structural and Chemical Evolution of Highly Fluorinated Li‐Rich Disordered Rocksalt Oxyfluorides as a Function of Temperature

“…The existence of signals between 685 and 687 eV in the F K‐edge XAS of pristine Li 2 CoO 2 F and Li 2 MnO 2 F (Figure 4c,f) confirms the existence of Co/Mn 3d‐F 2p covalent bonding. [ 40 ] The lower intensity of all pre‐edge signals compared to the O counterparts was ascribed to a weaker overlap of metal‐anion states, and it hampered any analysis of their fine structure. However, a decrease in pre‐edge intensity and a sharpening of the edge signals at 692 eV was observed upon annealing in all materials (Figure 4c,f,i), which suggested a reorganization of the structure toward less prevalent M─F bonds, implying that temperature favored the formation of more Li─F and M─O bonds until phase separation into the ternary oxide and LiF.…”

“…Its shape and intensity reflect the expected distribution of unoccupied states, which can, in many cases, be derived from simple crystal field theory (e.g., we observe directly the T 2g and E g states as two peaks in the case of an octahedral field). [37,40] Above the pre-edge, a signal corresponding to the absorption edge results from the promotion to hybridized states with TM 4sp, followed by photoionization and multiple scattering.…”