Optimizing zinc-ion battery performance by regulating hydrogel electrolyte water and salt concentrations

Abstract

Zinc-ion batteries (ZIBs) are promising for energy storage due to their safe, non-flammable electrolytes and low-cost materials. However, practical use is limited by water-related issues such as hydrogen evolution reaction (HER), zinc corrosion, and cathode decay. Polymer hydrogel electrolytes can mitigate these reactions, but the effects of varying salt and water concentrations on their performance are not well-studied. This thesis explores polyacrylamide (PAM) hydrogels synthesized via in-situ polymerization with different concentrations of Zn(ClO4)2 (0.5 to 2.0 mol kg−1) and water (40 to 90 wt.%). These hydrogels were tested in Zn||Ti half cells, Zn||Zn symmetrical cells, and Zn||V2O5 full cells to evaluate their electrochemical performance. Results show that while higher salt concentrations increase ionic conductivity, a concentration of 2.0 mol kg−1 causes severe HER and zinc corrosion. An optimal water concentration of 70–80 wt.% balances high ionic conductivity and minimizes undesirable reactions, with 64.1–73.1 wt.% of the water being chemically active. A PAM hydrogel with 1.0 mol kg−1 Zn(ClO4)2 and 80 wt.% water enabled stable cycling for 1200 hours in a Zn||Zn cell and achieved 99.24% Coulombic efficiency in a Zn||Ti cell. In Zn||V2O5 cells, an electrolyte with 70 wt.% water retained 73.7% capacity after 400 cycles. The thesis concludes that precise control of salt and water concentrations in hydrogel electrolytes is crucial to reducing active water fractions while maintaining high ionic conductivity, essential for high-performance ZIBs.