Probing Kinetics Beyond Tafel Unlocks Highly Accurate Exchange Currents for Aqueous Zinc Metal Batteries

Understanding the kinetics of zinc electrodeposition on current collectors is crucial for improving aqueous zinc-metal battery (AZMB) performance, yet it remains largely unexplored. A major challenge within the field is the inconsistent reporting of kinetic parameters, particularly the exchange current density (j₀), which is essential for accurately modeling and simulating zinc electrodeposition reactions. In this study, fast scan voltammetry on tungsten ultramicroelectrodes (UMEs) is employed to decouple mass transfer effects and isolate charge transfer kinetics. These results show that while zinc electrodeposition is a two-electron process, the rate-limiting step involves a one-electron transfer, validated through Butler-Volmer and Marcus-Hush models. This work also identifies significant limitations of Tafel analysis for certain zinc electrolyte systems, as their kinetically quasi-reversible/irreversible nature prevents the existence of a true Tafel regime (±118 mV kinetic regime). For such systems, the Allen-Hickling approach is proposed as a more accurate method for probing zinc electrodeposition kinetics. We report j₀ values for ZnSO₄ (0.20 A/cm²), Zn(OTf)₂ (0.42 A/cm²), and ZnCl₂ (0.46 A/cm²) and provide clear guidelines for precise kinetic analysis based on the width of the kinetic regime. Finally, the impact of accurate kinetic parameter measurements on coin cell performance is demonstrated, revealing that lower accurately determined j₀ values correlate with improved long-term cycling stability, following the trend ZnSO₄ > Zn(OTf)₂ > ZnCl₂, aligning with predictions from Sands' model. This work provides the first systematic kinetic investigation of counter anions in aqueous zinc-metal batteries, offering critical insights into how electrolyte composition influences charge transfer kinetics. These findings advance our fundamental understanding of AZMB kinetics and offer a framework for optimizing electrolyte compositions and electrode designs to enhance battery performance and durability.