Elucidating the Reaction Mechanism of Mn Electrolyte Additives in Aqueous Zinc Batteries.

Aqueous 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 Mn salts to ZnSO electrolytes enhanced overall energy densities and extended the cycling life of Zn/MnO batteries. It is commonly believed that Mn additives in the electrolyte inhibit the dissolution of MnO cathode. To better understand the role of Mn electrolyte additives, the ZIB using a Co O cathode instead of MnO in 0.3 m MnSO + 3 m ZnSO electrolyte is built to avoid interference from MnO cathode. As expected, the Zn/Co O battery exhibits electrochemical characteristics nearly identical to those of Zn/MnO 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 Mn /MnO deposition/dissolution process, while a chemical reaction of Zn /Zn SO (OH) ∙5H O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn /Zn SO (OH) ∙5H O reaction contributes no capacity and lowers the diffusion kinetics of the Mn /MnO reaction, which prevents the operation of ZIBs at high current densities.