Ultrathin CuF2‐Rich Solid‐Electrolyte Interphase Induced by Cation‐Tailored Double Electrical Layer toward Durable Sodium Storage

Abstract Solid‐electrolyte interphase (SEI) seriously affects battery's cycling life, especially for high‐capacity anode due to excessive electrolyte decomposition from particle fracture. Herein, we report an ultrathin SEI (3–4 nm) induced by Cu + ‐tailored double electrical layer (EDL) to suppress electrolyte consumption and enhance cycling stability of CuS anode in sodium‐ion batteries. Unique EDL with SO 3 CF 3 ‐Cu complex absorbing on CuS in NaSO 3 CF 3 /diglyme electrolyte is demonstrated by in situ surface‐enhanced Raman, Cyro‐ TEM and theoretical calculation, in which SO 3 CF 3 ‐Cu could be reduced to CuF 2 ‐rich SEI. Dispersed CuF 2 and F‐containing compound can provide good interfacial contact for formation of ultrathin and stable SEI film to minimize electrolyte consumption and reduce activation energy of Na + transport. As a result, the modified CuS delivers high capacity of 402.8 mAh g −1 after 7000 cycles without capacity decay. The insights of SEI construction pave a way for high‐stability electrode.