Tailoring the Solvation of Aqueous Zinc Electrolytes by Balancing Kosmotropic and Chaotropic Ions

Aqueous zinc (Zn) batteries (AZBs) have emerged as a highly promising concept for grid-scale electrochemical energy storage due to the prospects of high safety, low cost, and competitive energy density. However, the commonly employed electrolytes, at ca. 0.5–2 M salt concentration, significantly limit the cycling stability due to the uncontrolled hydrogen evolution reaction (HER). This originates from the plentiful access of free water molecules that become hydrolyzed. As a remedy, highly concentrated electrolytes, ca. 10 m and higher, have been suggested by means of altering the local solvation, promoting Zn$^{2+}$–anion rather than Zn$^{2+}$–H$_2$O coordination, but this renders high viscosity electrolytes with reduced ion transport. Here, by balancing a combination of kosmotropic and chaotropic ions, specifically acetate (Ac) and guanidinium (Gua), it is possible to tailor their strong and weak coordination with water, respectively. This strategy results in a weakly solvated electrolyte with improved ion transport properties alongside stabilization of the Zn metal anode. Furthermore, our electrolyte also enhances the cathode stability, rendering an overall increase in the battery lifetime and performance. Hence, this electrolyte design strategy can be applied to the development of a new generation of AZBs.