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

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 SO₃ CF₃ -Cu complex absorbing on CuS in NaSO₃ CF₃ /diglyme electrolyte is demonstrated by in situ surface-enhanced Raman, Cyro-TEM and theoretical calculation, in which SO₃ CF₃ -Cu could be reduced to CuF₂ -rich SEI. Dispersed CuF₂ 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.