Elucidating the Reaction Mechanism of Mn 2+ 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 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.