In this work, Li-rich Li1.2Mn0.43+Mnx4+Ti0.4-xO2 (LMMxTO, 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, LMM0.2TO delivers a high specific capacity of 322 mAh g(-1) at room temperature (30 degrees C, 30 mA g(-1)) and even 352 mAh g(-1) (45 degrees C, 30 mA g(-1)) with an energy density of 1041 Wh kg(-1). The reason for such a high capacity of LMM0.2TO 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 LMM0.2TO 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.