Elucidating the Reaction Mechanism of Mn2+ Electrolyte Additives in Aqueous Zinc Batteries

Abstract

AbstractAqueous zinc batteries (ZIBs) have attracted considerable attention in recent years because of their high safety and eco‐friendly features. Numerous studies have shown that adding Mn2+ salts to ZnSO4 electrolytes enhanced overall energy densities and extended the cycling life of Zn/MnO2 batteries. It is commonly believed that Mn2+ additives in the electrolyte inhibit the dissolution of MnO2 cathode. To better understand the role of Mn2+ electrolyte additives, the ZIB using a Co3O4 cathode instead of MnO2 in 0.3 m MnSO4 + 3 m ZnSO4 electrolyte is built to avoid interference from MnO2 cathode. As expected, the Zn/Co3O4 battery exhibits electrochemical characteristics nearly identical to those of Zn/MnO2 batteries. Operando synchrotron X‐ray diffraction (XRD), ex situ X‐ray absorption spectroscopy (XAS), and electrochemical analyses are carried out to determine the reaction mechanism and pathway. This work demonstrates that the electrochemical reaction occurring at cathode involves a reversible Mn2+/MnO2 deposition/dissolution process, while a chemical reaction of Zn2+/Zn4SO4(OH)6∙5H2O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn2+/Zn4SO4(OH)6∙5H2O reaction contributes no capacity and lowers the diffusion kinetics of the Mn2+/MnO2 reaction, which prevents the operation of ZIBs at high current densities.