Correlating hydrogen evolution and zinc deposition/dissolution kinetics to the cyclability of metallic zinc electrodes

The reversibility of metal plating/stripping critically constrains the cyclability of batteries. Prior reports have unexpectedly noted that zinc (Zn) plating/stripping gains higher reversibility when subjected to elevated current densities, yet the underlying reasons remains unclear. Our study has led to the finding that at high current densities (high-C), Zn metal is less prone to suffer effects of the hydrogen evolution reaction (HER), thereby mitigating the accumulation of insulating byproducts. This discrepancy can be correlated with the concentration of a solvated Zn(H2O)62+ species at the interface, which is proved to be the main substance responsible for the HER. High working current densities combined with a high electric field creates a sparse Zn(H2O)62+ concentration gradient near the interface and thus mitigates the hydrogen evolution during the Zn plating period. Moreover, we also observed an inherent asymmetrical Zn plating/stripping mechanism that tends towards a more symmetrical pattern at high-C, contrasting starkly with the highly asymmetrical plating/stripping kinetics at low current density (e.g. 0.2 mA cm-2). Our study establishes a novel conceptual framework for understanding the Zn metal plating/stripping process by elucidating the kinetics of hydrogen evolution and Zn electrodeposition. We correlate the Zn plating/stripping cyclability with the kinetics of hydrogen evolution. The pronounced hydrogen evolution kinetics and asymmetric Zn plating/stripping kinetics explain the poor Zn reversibility at low current density.