Measuring Zn Transference with Precision: Insights for Dendrite-Free Zinc Metal Anodes

Electrolyte engineering in Zn-metal batteries frequently explores the use of alkaline metal supporting salts to enhance conductivity and reduce overpotential for Zn plating and stripping. While these additives improve conductivity, the presence of more mobile alkali cations can negatively affect the Zn2+ transference number. Optimizing the Zn2+ transference number is crucial for high-rate performance, efficiency, and safety, as a high Zn2+ transference number minimizes concentration polarization and dendrite formation during high-current cycling. However, reliably measuring the transference number of Zn ions in non-binary electrolytes presents significant experimental challenges due to the dynamic nature of Zn metal interfaces, rendering traditional methods ineffective. Here, we introduce a modified Hittorf-type method to reliably measure Zn2+ transference numbers in complex electrolytes. Supported by MD simulations, we apply this method to accurately obtain transport numbers of Zn2+, K+ and acetate ions in Zn-K acetate electrolytes. By varying the Zn2+ fraction, we study the impact of co-electrolytes on transport properties and correlate these with the Zn solvation environment using X-ray absorption spectroscopy. We reveal that while overall ionic conductivity notably increases with the addition of KOAc co-salt, the Zn2+ transference number dramatically decreases. Electrolytes with higher Zn2+ transference numbers enable longer high-rate cycling, underscoring the importance of optimizing Zn2+ transference for improved performance and stability of Zn-metal anodes.