Functional‐Group Engineering for Durable Solid Electrolyte Interphase Toward Aqueous Copper Batteries

ABSTRACT Aqueous copper‐based batteries intrinsically suppress hydrogen evolution, offering a safer alternative to zinc systems. However, their practical deployment remains limited by poor Cu 2+ plating/stripping reversibility. Here we report a functional‐group‐guided strategy to in situ construct an organic‐inorganic hybrid solid electrolyte interphase (SEI) on Cu electrodes. By comparing electrolytes with and without additives containing fluorine and sulfur, a clear structure‐function relationship is established. Fluorine‐free molecules fail to form a stable SEI, whereas fluorinated additives generate an organic‐rich SEI with moderate benefits. In contrast, the group with both fluorine and sulfur produces a robust organic‐inorganic hybrid SEI with high mechanical modulus (54.7 GPa) and rapid Cu 2+ transport. This hybrid interphase homogenizes interfacial ion flux and markedly improves Cu 2+ reversibility, enabling Cu||Cu cells to cycle more than 2000 h at 5 mA cm −2 and Cu–MnO 2 full cells to retain stable operation over 2200 cycles. The excellent dimensional stability of the pouch cells further underscores their practical promise. This work establishes functional‐group engineering of the SEI for copper‐based aqueous batteries.