Solid‐Electrolyte Interphase for Ultra‐Stable Aqueous Dual‐Ion Storage

Abstract The solid‐electrolyte interphase (SEI) is the key component of the electrochemical electrode as a passivation layer, which enables a long calendar life for commercial applications. However, owing to the poor understanding of aqueous SEI in conventional aqueous electrolytes, the adoption of aqueous Li‐ion batteries (ALIBs) has been drastically limited. Herein, the construction of a robust aqueous SEI is successfully demonstrated by introducing a ladderized heterocyclic polymer electrode into a water‐in‐salt electrolyte, leading to ultra‐stable dual‐ion storage with long‐term calendar life. The sophisticated SEI composed of fluorinated/oxygenated species is revealed. Electrochemical and structural analysis confirm that such SEI formation is characterized by a proton co‐intercalation, which consumes the water in the Li⁺'s solvation shell to trigger the subsequent decomposition of organic anions. This new SEI formation path is expected to broaden the horizons of aqueous SEI and open new avenues for the design of versatile electrode materials for further advances in ALIBs.