Toward Long‐Life High‐Voltage Aqueous Li‐Ion Batteries: from Solvation Chemistry to Solid‐Electrolyte‐Interphase Layer Optimization Against Electron Tunneling Effect

Water is pursued as an electrolyte solvent for its non-flammable nature compared to traditional organic solvents, yet its narrow electrochemical stability window (ESW) limits its performance. Solvation chemistry design is widely adopted as the key to suppress the reactivity of water, thereby expanding the ESW. In this study, an acetamide-based ternary eutectic electrolyte achieved an ESW ranging from 1.4 to 5.1 V. The electrolyte confines water molecules within the primary solvation sheath of Li-ions, reducing the free water and breaking the hydrogen bond network. Despite this, initial capacity retention is suboptimal due to inadequate formation of solid-electrolyte-interphase (SEI) layers. To address this, additional hydrogen evolution reaction is induced by widening the operation voltage range, thereby optimizing the SEI layer to mitigate the electron tunneling effect. This approach resulted in a denser LiF-rich SEI layer, effectively preventing water decomposition and improving long-term cycle stability. The optimized SEI layer reduced the electron tunneling barrier, achieving a discharge capacity of 152 mAh g^-1 at 1 C and maintaining 76% of its capacity (116 mAh g^-1) after 1000 cycles. This study highlights the critical role of both solvation structure and SEI layer optimization in enhancing the performance of high-voltage aqueous Li-ion batteries.