Effects of partial isovalent substitution of V with (Ti,Fe) on redox reactivity in Li2VO2F battery cathodes

Abstract

We applied photon-only X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study the bulk properties of Li-rich disordered rock salt oxyfluoride cathodes Li₂VO₂F, Li₂V_0.5Ti_0.5O₂F and Li₂V_0.5Fe_0.5O₂F. We have systematically investigated V 3d- and O 2p-states, both of which are crucial for charge compensation during battery cycling. A combined analysis of both spectroscopies reveals that the vanadium ions deviate from the expected V³⁺/V⁵⁺ redox couple for all systems. Moreover, our results suggest that the oxygen ions partake in charge compensation to a considerable degree. Large O 2p-band width changes as a function of doping material are observed. While the diverging trend makes it less plausible that these states have a significant influence on degradation, we find evidence that the presence of O 2p–V 3d hybridized states may constitute a common factor for the previously found improvement in long term cycling of the Ti/Fe-substituted materials. This study demonstrates the power of using a combined spectroscopy approach to understand the tuning properties of Li₂VO₂F to find ways of designing better cathode materials based on this parent compound.