Enabling Gradient‐Structured Solid Electrolyte Interphase by a Hydrated Eutectic Electrolyte for High‐Performance Zn Metal Batteries

Aqueous Zn metal batteries are attracting tremendous interest as promising energy storage systems due to their intrinsic safety and cost-effectiveness. Nevertheless, the reversibility of Zn metal anodes (ZMAs) is hindered by water-induced parasitic reactions and dendrite growth. Herein, a novel hydrated eutectic electrolyte (HEE) consisting of Zn(BF₄)₂·xH₂O and sulfolane (SL) is developed to prevent the side reactions and achieve the outstanding cyclability of ZMAs. The strong coordination between Zn²⁺ and SL triggers the eutectic feature, enabling the low-temperature availability of HEEs. The restriction of BF₄ ^- hydrolysis in the eutectic system can realize favorable compatibility between Zn(BF₄)₂-based electrolyte and ZMAs. Besides, the newly-established solvation structure with the participation of SL, H₂O, and BF₄ ^-, can induce in situ formation of desirable SEI with gradient structure consisting of B,O-rich species, ZnS, and ZnF₂, to offer satisfactory protection toward ZMAs. Consequently, the HEE allows the Zn||Zn symmetric cell to cycle over 1650 h at 2 mA cm^-2 and 1 mA h cm^-2. Moreover, the Zn||NH₄V₄O₁₀ full batteries can deliver a prolonged lifespan for 1000 cycles with a high capacity retention of 83.4%. This work represents a feasible approach toward the elaborate design of advanced electrolyte systems for next-generation batteries.