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

Abstract 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(BF 4 ) 2 ·xH 2 O and sulfolane (SL) is developed to prevent the side reactions and achieve the outstanding cyclability of ZMAs. The strong coordination between Zn 2+ and SL triggers the eutectic feature, enabling the low‐temperature availability of HEEs. The restriction of BF 4 − hydrolysis in the eutectic system can realize favorable compatibility between Zn(BF 4 ) 2 ‐based electrolyte and ZMAs. Besides, the newly‐established solvation structure with the participation of SL, H 2 O, and BF 4 − , can induce in situ formation of desirable SEI with gradient structure consisting of B,O‐rich species, ZnS, and ZnF 2 , 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||NH 4 V 4 O 10 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.