Suppressing Dissolution of Vanadium from Cation-Disordered Li2-xVO2F via a Concentrated Electrolyte Approach

Li2VO2F with the cation-disordered rock-salt structure is an attractive high-energy-density positive electrode material but suffers from severe capacity fading upon cycling. The underlying reasons are yet unclear. In this study, we unveil the overlooked role of vanadium dissolution and electrode-electrolyte interactions and provide insight into the failure mechanism. Interfacial reactions, in general, can be tuned by either surface coatings or the modification of the electrolyte chemistry. Here we modify the interfacial reactions through the use of a concentrated electrolyte 5.5 M LiFSI in dimethyl carbonate, effectively reducing vanadium dissolution. Moreover, it results in a lower interfacial resistance build-up as compared to conventional 1.0 M LiPF6 electrolyte, thus increasing the cycling stability. The solubility of vanadium enhances significantly with higher oxidation states. Furthermore, a chemical prelithiation strategy has been presented, which allows the full lithiation of VO2F to Li2VO2F, with an outlook on the intermediate phases. We argue that the optimization of cathode-electrolyte interactions is of significant importance to improve the cycling performance of disordered rock-salts, where a thorough understanding of the limiting factors is still missing.