A dilute kosmotropic eutectic electrolyte for practical four-electron aqueous Zn−Iodine batteries

Abstract

Hydrolysis of I⁺ and instability of zinc anode in dilute aqueous electrolytes are two main obstacles for constructing high-rate, long-cycle-life and cost-effective aqueous Zn−iodine batteries with I⁺/I₂/I⁻ conversion (4eZIBs). Here, a kosmotropic eutectic electrolyte (KEE) was obtained using ZnSO₄·7H₂O and 1-Butyl-3-methylimidazolium chloride structure makers. The combined kosmotropic and eutectic features enable highly effective regulation of water activity while simultaneously facilitating reversible and stable chemistries of both the iodine cathode and zinc anode. Specifically, appropriate amount of water is crucial to reversible, fast and stable four-electron transfer of the iodine cathode. Meanwhile, the restricted water activity and water-deficient solvation structure of ZnCl(SO₄)(H₂O)₄⁻ grant stable zinc anodes. Consequently, the 4eZIB with dilute KEE (3.2 M) exhibits stable cycle performance for over 27,000 cycles, along with high Coulombic efficiency of 99.6 %. Moreover, the 4eZIB also demonstrates overall performance enhancement, as manifested in a practical pouch cell. This work provides mechanistic understanding on electrolyte design for solving complex issues in 4eZIBs.