Breaking the Thermodynamic Ceiling of Aqueous Selenium Chemistry via Intrinsic Mn Cation Modulation

Aqueous conversion-type batteries hold great promise for safe, low-cost, and high-energy storage, yet selenium (Se) cathodes are limited by the mismatch between their high redox potential and the narrow electrochemical stability window of aqueous electrolytes. Here, we report the first aqueous manganese-selenium battery that overcomes this limitation via the intrinsic hydrolysis acidity of Mn-based cations, eliminating the need for organic additives. This enables a highly reversible dual-phase conversion between Se, MnSe, and MnSe₂, effectively unlocking multi-electron reactions. Electrolyte anions critically regulate the reaction pathway: Cl^- induces soluble species and shuttle effects, while OTf^- stabilizes the interface. Consequently, The Mn-Se battery delivers a high initial capacity of 621 mAh g^-1, retaining 501 mAh g^-1 after 1200 cycles and achieving a record-high energy density of 414 Wh kg^-1, surpassing all previously reported aqueous manganese-metal batteries. This work provides valuable insights into the rational design of high-reversibility conversion-type cathode materials.