Huge active Li⁺ loss in the initial cycle owing to the formation of solid electrolyte interface (SEI) layer of anode restrict the enhancement of high-energy–density lithium-ion batteries (LIBs). Here, a method by reducing surface oxygen vacancies of lithium-rich manganese-based oxide (LRMO) to obtain huge number of irreversibly active Li⁺ for the application of cathode pre-lithiation, which is highly adaptive to electrode preparation process with excellent air stability, is proposed. Through modulating different lithium ratios, Li_2-x(Mn_2/3Ni_1/3)_xO₂ (x = 0.67, LR-2) with fewer oxygen vacancies is synthesized. By simply adjusting cell voltage, it exhibits different electrochemical properties to match the requirements of different LIB cathode. When used as an agent to a LiFePO₄ cathode, LR-2 can further enhance the capacity of cells, due to its “donor” lithium-ion capacity (244.1 mAh/g) and exceptional cyclic performance. By coupling with LiNi_0.5Mn_1.5O₄, LR-2 provides high lithium compensation efficiency (320.0 mAh/g), while also releasing reversible capacities of 98.5 mAh/g, achieving a balance between pre-lithiation capacities and reversible discharge capacities. As a consequence, the LiNi_0.5Mn_1.5O₄|graphite full cell containing LR-2 exhibits a reversible capacity of 123.3 mAh/g, 47% greater than the counterpart without LR-2, and significantly better cyclic performance. This study introduces a novel approach to design cathode pre-lithiation agent with Li-rich oxides for high-energy–density LIBs.