Designing Anion-Type Water-Free Zn²⁺ Solvation Structure for Robust Zn Metal Anode

Rechargeable aqueous Zn batteries are potential for large-scale electrochemical energy storage due to their low cost and high security. However, Zn metal anode suffers from the dendritic growth and interfacial hydrogen evolution reaction (HER), resulting in the deterioration of electrode/battery performance. Here we propose that both dendrites and HER are related to the water participated Zn²⁺ solvation structure-Zn(H₂ O)₆ ²⁺ and thus can be resolved by transforming Zn(H₂ O)₆ ²⁺ to an anion-type water-free solvation structure-ZnCl₄ ^2- , which is achieved in traditional ZnSO₄ aqueous electrolyte after adding chloride salt with a bulky cation (1-ethyl-3-methylimidazolium chloride). The elimination of cation-water interaction suppresses HER, while the electrostatic repulsion between Zn tips and the anion solvation structure inhibits dendrite formation. As a result, the electrolyte shows uniform Zn deposition with an average Zn plating/stripping Coulombic efficiency of ≈99.9 %, enabling a capacity retention of 78.8 % after 300 cycles in anode-free Zn batteries with pre-zincificated polyaniline as the cathode. This work provides a novel electrolyte design strategy to prevent HER and realize long-lifespan metal anode.