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

Abstract 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 2+ salts to ZnSO 4 electrolytes enhanced overall energy densities and extended the cycling life of Zn/MnO 2 batteries. It is commonly believed that Mn 2+ additives in the electrolyte inhibit the dissolution of MnO 2 cathode. To better understand the role of Mn 2+ electrolyte additives, the ZIB using a Co 3 O 4 cathode instead of MnO 2 in 0.3 m MnSO 4 + 3 m ZnSO 4 electrolyte is built to avoid interference from MnO 2 cathode. As expected, the Zn/Co 3 O 4 battery exhibits electrochemical characteristics nearly identical to those of Zn/MnO 2 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 2+ /MnO 2 deposition/dissolution process, while a chemical reaction of Zn 2+ /Zn 4 SO 4 (OH) 6 ∙5H 2 O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn 2+ /Zn 4 SO 4 (OH) 6 ∙5H 2 O reaction contributes no capacity and lowers the diffusion kinetics of the Mn 2+ /MnO 2 reaction, which prevents the operation of ZIBs at high current densities.