A Functionality‐Graded Cathode Electrolyte Interphase Enables Ultra‐Long Cycling Stability in Aqueous Zn–Mn Batteries

The rational design of cathode electrolyte interphases (CEI) is pivotal for enhancing reaction kinetics and stability in zinc-manganese batteries, yet their design principle and formation mechanisms remain unclear. In this work, we introduce an electrolyte additive-driven in situ strategy using trace KH₂PO₄, guided by theoretical calculations, to construct a functionality-graded hierarchical CEI on a carbon-coated Cu-MnO₂ cathode. This precisely engineered structure effectively regulates the interfacial water environment, mitigates volume stress, and promotes efficient charge carrier transport. Specifically, the inner amorphous inorganic Zn₃(PO₄)₂/ZnHPO₄ layer enhances ion transport, the intermediate organic phosphate ester layer with C─O─P bonds provides mechanical flexibility, and the hydrated outer layer with adsorbed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:msubsup><mml:mi>PO</mml:mi> <mml:mn>4</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> <mml:annotation>${{{\mathrm{H}}}_{\mathrm{2}}}{\mathrm{PO}}_{\mathrm{4}}^{\mathrm{ - }}$</mml:annotation></mml:semantics> </mml:math> / <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>HPO</mml:mi> <mml:mn>4</mml:mn> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> <mml:annotation>${\mathrm{HPO}}_{\mathrm{4}}^{{\mathrm{2 - }}}$</mml:annotation></mml:semantics> </mml:math> traps water molecules via hydrogen bonding, suppressing corrosion. As a result, the battery achieves exceptional cycling stability of 100 000 cycles at 5.0 A g^-1, nearly a 10-fold improvement over conventional systems. This work presents a universal approach for interfacial engineering in aqueous batteries, offering new insights into regulating CEI formation and reaction kinetics via electrolyte engineering to achieve durable energy storage performance.