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 $H_2{\mathrm{PO}}_4^{-}$ / ${\mathrm{HPO}}_4^{2-}$ 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.