Toward Better Stability and Reversibility of the Mn4+/Mn2+ Double Redox Activity in Disordered Rocksalt Oxyfluoride Cathode Materials

Cation-disordered rocksalt (DRS) materials have shown good initial reversibility and facile Li+ insertion and extraction in the structure at high rates. However, all of the Lirich oxyfluorides introduced so far suffer from short cycle lifetimes and severe capacity fading. In the current study, we combine the strategy of using high-valent cations with partial substitution of oxygen anions by fluorine ions to achieve the optimal Mn4+/Mn2+ double redox reaction in the composition system Li(2)Mn(1-x)TixO(2)F (0 <= x <= 2/3). While Ti-rich compositions correlate to an Ooxidation plateau and a partial Mn3+-Mn4+ redox process at high voltages, owing to the presence of Ti3+ in the structure, a new composition Li2Mn2/3Ti1/3O2F with a lower amount of Ti shows better electrochemical performance with an initial high discharge capacity of 227 mAh g(-1) (1.5-4.3 V window) and a Coulombic efficiency of 82% after 200 cycles with a capacity of 136 mAh g(-1) (>462 Wh kg(-1)). The structural characteristics, oxidation states, and charge-transfer mechanism have been examined as a function of composition and state of charge. The results indicate a double redox mechanism of Mn4+/Mn2+ in agreement with Mn-Ti structural charge compensation. The findings point to a way for designing high-capacity DRS materials with multi-electron redox reactions.