Abstract
Electrolyte engineering in Zn-metal batteries frequently employs alkaline metal salts to enhance conductivity and reduce overpotential for Zn plating. While these additives improve conductivity, the presence of more mobile alkali cations can negatively affect the Zn(2+) transference number. Optimizing this property is crucial for high-rate performance, efficiency, and safety, as high Zn(2+) transference number minimizes concentration polarization and dendrite formation during high-current cycling. However, reliably measuring the transference number in non-binary electrolytes presents significant experimental challenges due to dynamic Zn metal interfaces, rendering traditional methods ineffective. Here, we use a modified Hittorf-type method to measure Zn(2+) transference numbers in complex electrolytes. Supported by molecular dynamics simulations, this method is applied to obtain transference numbers of Zn(2+), K(+,) and acetate ions in Zn-K acetate electrolytes. By varying the Zn(2+) fraction, the impact of co-salts on transport properties is studied and correlated with the Zn solvation environment using X-ray absorption spectroscopy. It is revealed that while ionic conductivity increases with the addition of KOAc co-salt, the Zn(2+) transference number dramatically decreases. Electrolytes with higher Zn(2+) transference numbers enable longer high-rate cycling, underscoring the importance of optimizing Zn(2+) transference for improved performance of Zn-metal anodes.