Mn⁴⁺-Substituted Li-Rich Li_1.2Mn_0.4³⁺Mn⁴⁺Ti_0.4-O₂ Materials with High Energy Density

In this work, Li-rich Li_1.2Mn_0.4³⁺Mn⁴⁺Ti_0.4-O₂ (LMMTO, 0 ≤ x ≤ 0.4) oxides have been studied for the first time. X-ray diffraction (XRD) patterns show a cation-disordered rocksalt structure when x ranges from 0 to 0.2. After Mn⁴⁺ substitution, LMM_0.2TO delivers a high specific capacity of 322 mAh g^-1 at room temperature (30 °C, 30 mA g^-1) and even 352 mAh g^-1 (45 °C, 30 mA g^-1) with an energy density of 1041 Wh kg^-1. The reason for such a high capacity of LMM_0.2TO is ascribed to the increase of both cationic (Mn) and anionic (O) redox after Mn⁴⁺ substitution, which is proved by dQ/dV curves, X-ray absorption near edge structure, DFT calculations, and in situ XRD results. In addition, the roles of Mn³⁺ and Ti⁴⁺ in LMM_0.2TO are also discussed in detail. A ternary phase diagram is established to comprehend and further optimize the earth-abundant Mn³⁺-Mn⁴⁺-Ti⁴⁺ system. This work gives an innovative strategy to improve the energy density, broadening the ideas of designing Li-rich materials with better performance.