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

ABSTRACT 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 KH ₂ PO ₄ , guided by theoretical calculations, to construct a functionality‐graded hierarchical CEI on a carbon‐coated Cu‐MnO ₂ 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 Zn ₃ (PO ₄ ) ₂ /ZnHPO ₄ 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 / 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.