Buffer reactions can prevent changes induced by external causes. Here, we demonstrate the significant buffer role of a very small amount of Mn in a self-optimized cathode for an aqueous Zn-ion battery. Our operando X-ray characterization studies reveal that the dissolution of most of the Mn in MnV₂O₄ during the first charging cycle induces atomic re-arrangement to form a disordered vanadium oxide phase with 0.88 at% Mn. Interestingly, the residual Mn ions exhibit voluntary migration between tetrahedral and octahedral sites during Zn²⁺ de/intercalation. This Mn migration not only stabilizes the main structure of the vanadium-based electrode, but also modulates the Fermi surface of V 3d against excessive drift. As result, the optimized cathode delivers an excellent capacity of 610.2 mA h g⁻¹ at 0.2 A g⁻¹ and long-term cycling stability over 4000 cycles. This buffer contribution via tunable metal ions exhibits high potential for applications in long-life metal-ion batteries and related fields.