Unraveling the Dissolution‐Mediated Reaction Mechanism of α‐MnO2 Cathodes for Aqueous Zn‐Ion Batteries

Abstract Aqueous Zn/α‐MnO 2 batteries have attracted immense interest owing to their high energy density, low cost, and safety, making them desirable for future large‐scale energy application. Despite these merits, the comprehensive understanding of their reaction mechanism has been elusive due to the limitations of standard bulk characterization. Here, via transmission electron microscopy, the dissolution‐mediated reaction mechanism of a Zn/α‐MnO 2 system is discovered and explored in full scope to involve reversible formation of Zn 4 SO 4 (OH) 6 · x H 2 O and “birnessite‐like” Zn‐MnO x phase upon cycling. Overall, α‐MnO 2 acts primarily as a source for cell activation through dissolution and thus is not directly involved in the Zn redox chemistry. This microscopic study offers a unique knowledge on the unconventional reaction chemistry of Zn/α‐MnO 2 batteries.