Improved cycling stability in high-capacity Li-rich vanadium containing disordered rock salt oxyfluoride cathodes

Lithium-rich transition metal disordered rock salt (DRS) oxyfluorides have the potential to lessen one large bottleneck for lithium ion batteries by improving the cathode capacity. However, irreversible reactions at the electrode/electrolyte interface have so far led to fast capacity fading during electrochemical cycling. Here, we report the synthesis of two new Li-rich transition metal oxyfluorides Li₂V_0.5Ti_0.5O₂F and Li₂V_0.5Fe_0.5O₂F using the mechanochemical ball milling procedure. Both materials show substantially improved cycling stability compared to Li₂VO₂F. Rietveld refinements of synchrotron X-ray diffraction patterns reveal the DRS structure of the materials. Based on density functional theory (DFT) calculations, we demonstrate that substitution of V³⁺ with Ti³⁺ and Fe³⁺ favors disordering of the mixed metastable DRS oxyfluoride phase. Hard X-ray photoelectron spectroscopy shows that the substitution stabilizes the active material electrode particle surface and increases the reversibility of the V³⁺/V⁵⁺ redox couple. This work presents a strategy for stabilization of the DRS structure leading to improved electrochemical cyclability of the materials.