Cobalt-free layered lithium-rich nickel manganese oxides, Li[Li_xNi_yMn_1−x−y]O₂, are promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. Utilization of the oxygen anionic redox in this series of materials enables realization of a high capacity beyond that achieved via the conventional transition metal cationic redox when charging above 4.5 V vs. Li/Li⁺. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. The undesirable voltage decay has been proposed to be linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. Herein, we demonstrate that the voltage decay of the electrode is correlated to the activation of Mn⁴⁺/Mn³⁺ redox and subsequent cation disordering, which is able to be remarkably suppressed via simple compositional tuning to induce the formation of Ni³⁺ in the pristine material. By implementing our new strategy, an alternative redox reaction involving the use of this pristine Ni³⁺ as a redox buffer, which has been designed to be widened from Ni³⁺/Ni⁴⁺ to Ni²⁺/Ni⁴⁺, subdued the Mn⁴⁺/Mn³⁺ reduction without compensation for the capacity in principle. Negligible change in the voltage profile of the modified lithium-rich nickel manganese oxide electrode is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. Based on these findings, we propose a general strategy to suppress the voltage decay of Mn-containing lithium-rich oxides to achieve long-lasting high energy density from this class of materials.