Elucidating the Reaction Mechanism of Mn²⁺ 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 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 Co₃ O₄ 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/Co₃ O₄ 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⁺ /Zn₄ SO₄ (OH)₆ ∙5H₂ O deposition/dissolution is involved during part of the charge/discharge cycle due to the change in the electrolyte environment. The reversible Zn²⁺ /Zn₄ SO₄ (OH)₆ ∙5H₂ O reaction contributes no capacity and lowers the diffusion kinetics of the Mn²⁺ /MnO₂ reaction, which prevents the operation of ZIBs at high current densities.