Improving Aqueous Zinc Ion Batteries with Alkali Metal Ions

Aqueous zinc (Zn) ion batteries have received broad attention recently. However, their practical application is limited by severe Zn dendrite growth and the hydrogen evolution reaction. In this study, three alkali metal ions (Li⁺, Na⁺, and K⁺) are added in ZnSO₄ electrolytes, which are subjected to electrochemical measurements and molecular dynamics simulations. The studies show that since K⁺ has the highest mobility and self-diffusion coefficient among the four ions (Li⁺, Na⁺, K⁺, and Zn²⁺), it enables K⁺ to preferentially approach a zinc dendrite at an earlier time, driven by a negative electric field during a cathodic process. The electric double layer, with K⁺ around the negatively charged Zn dendrite, inhibits dendrite growth and mitigates the hydrogen evolution reaction on the Zn anode. Under this kinetic effect, the Zn-Zn symmetric cell with K⁺ exhibits a long cycling stability of 1000 h at 1 mA·cm^-2 of 1 mAh·cm^-2 and 190 h at 30 mA·cm^-2 of 2 mAh·cm^-2. Such a kinetic effect is also observed with additives Na⁺ and Li⁺, though less profound than that of K⁺.