Toward Better Stability and Reversibility of the Mn${}^{4+}$/Mn${}^{2+}$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 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 Mn${}^{4+}$/Mn${}^{2+}$ double redox reaction in the composition system Li${}_2$Mn${}_{1-x}$Ti${}_x$O${}_2$F (0 ≤ x ≤ 2/3). While Ti-rich compositions correlate to an O-oxidation plateau and a partial Mn${}^{3+}$–Mn${}^{4+}$ redox process at high voltages, owing to the presence of Ti3+ in the structure, a new composition Li${}_2$Mn${}_{2/3}$Ti${}_{1/3}$O${}_2$F 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. ... mehrThe results indicate a double redox mechanism of Mn${}^{4+}$/Mn${}^{2+}$ 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.