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

Abstract

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 Li-rich 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 Li2Mn1-xTixO2F (0 ≤ x ≤ 2/3). While Ti-rich compositions correlate to an O-oxidation 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.