Functioning Mechanism of the Secondary Aqueous Zn-β-MnO₂ Battery

Rechargeable aqueous Zn-MnO₂ batteries are a promising candidate for large-scale energy storage systems due to their outstanding advantages, such as high energy density, high safety, low cost, and environmental friendliness. Considering the controversies surrounding the mechanism of this battery containing a mildly acidic electrolyte, the electrochemical behavior of this type of battery using β-MnO₂ as the cathode is systematically investigated. The results indicate that the reversible intercalation of Zn²⁺ ions into MnO₂ is not likely to take place in the aqueous system. We conclude that it is the existence of the water molecule and its participation in the electrochemical reactions, for instance, the reversible insertion of proton into MnO₂ and the electrolysis of water, that makes the mechanism of aqueous Zn-MnO₂ batteries complicated. Besides, the capacity fading of this mildly acidic Zn-MnO₂ battery is assigned to the generation of the inert layer of Zn₄SO₄(OH)₆·nH₂O and the ZnMn₂O₄ on the cathode via electrochemical conversion reactions, the dissolution of the active material during discharging, and the release of gases. When Mn²⁺ ions are available in the electrolyte, they will be electrodeposited on the cathode during charging, and the kinetics of the electrochemical reactions of the electrode is improved, leading to the higher electrochemical performance of the battery.