Decreasing Water Activity Using the Tetrahydrofuran Electrolyte Additive for Highly Reversible Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries show great promise in large-scale energy storage. However, the decomposition of water molecules leads to severe side reactions, resulting in the limited lifespan of Zn batteries. Here, the tetrahydrofuran (THF) additive was introduced into the zinc sulfate (ZnSO4) electrolyte to reduce water activity by modulating the solvation structure of the Zn hydration layer. The THF molecule can play as a proton acceptor to form hydrogen bonds with water molecules, which can prevent water-induced undesired reactions. Thus, in an optimal 2 M ZnSO4/THF (5% by volume) electrolyte, the hydrogen evolution reaction and byproduct precipitation can be suppressed, which greatly improves the cycling stability and Coulombic efficiency of reversible Zn plating/stripping. The Zn symmetrical cells exhibit ultralong working cycles with a wide range of current density and capacity. The THF additive also enables a high Coulombic efficiency in the Zn||Cu cell with an average value of 99.59% over 400 cycles and a high reversible capacity with a capacity retention of 97.56% after 250 cycles in the Zn||MnO2 full cells. This work offers an effective strategy with high scalability and low cost for the protection of the Zn metal electrodes in aqueous rechargeable batteries.