Toward Better Stability and Reversibility of the Mn$^{4+}$/Mn$^{2+}$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 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. The 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