Operando investigation of aqueous zinc manganese oxide batteries: multi-stage reaction mechanism revealed

Aqueous Zn/MnO₂ batteries with mildly acidic electrolytes are promising candidates for low cost, high safety electrochemical energy storage for grid-scale applications. However, the complexity of the chemistry results in conflicting reports of operation principles, making rational improvements challenging. In this work, operando synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) as well as ex situ Raman spectroscopy, XRD, and XAS characterization were used to probe the mechanism of aqueous Zn/α-MnO₂ batteries with ZnSO₄ electrolyte. A multi-stage Mn dissolution-conversion charge storage mechanism was revealed, which consists of reversible solid-aqueous phase transformation via Mn dissolution-deposition reactions and a solid redox mechanism via Zn-ion insertion. This mechanism is supported by thermodynamic calculations paired with in situ electrolyte pH measurements to provide further mechanistic insights. The findings establish a detailed charge storage mechanism for aqueous Zn/α-MnO₂ batteries with a well resolved reversible layered charge product structure, that can serve as a reference for future studies on advancing the reversibility and stability of aqueous Zn/α-MnO₂ batteries.