<p>Aqueous Zn-MnO₂ batteries are promising candidates for large-scale energy storage owing to their low cost and intrinsic safety. However, their long-term stability is severely compromised by Mn²⁺ dissolution and structural degradation, driven by pronounced Jahn-Teller distortion and a complex charge storage mechanism. Here, we develop an efficient modulation of interfacial chemical potential for the MnO₂ cathode with remarkably enhanced stability via introducing a solid Mn₃(PO₄)₂ interphase with ultralow solubility product (<i>K</i>_sp ≈ 10⁻³¹). The Mn₃(PO₄)₂ interphase establishes a locally Mn²⁺-rich microenvironment at the MnO₂ surface, which elevates the chemical potential of Mn²⁺ and increases the Gibbs free energy change (Δ<i>G</i>) of the dissolution reaction, thereby thermodynamically suppressing further Mn loss. We appropriately integrate this interfacial chemical potential strategy into a core-shell MnO₂ structure, which accommodates volume changes during cycling and further enhances structural and interfacial stability. Therefore, the aqueous Zn-MnO₂ battery achieves a remarkable cycling lifespan of 10000 cycles at 3 C. This work demonstrates a synergistic strategy combining interfacial chemical potential modulation and structural engineering to enable high-performance, long-life Zn-MnO₂ batteries.</p>