Reaction mechanism for the α-MnO₂ cathode in aqueous Zn ion batteries revisited: elucidating the irreversible transformation of α-MnO₂ into Zn-vernadite

Rechargeable aqueous Zn/α-MnO₂ batteries have attracted much attention due to the advantages of safety and high capacity. However, the reaction process of α-MnO₂ electrodes in long cycles is still unclear. Here, a reaction mechanism for explaining the complete processes in long cycles was developed and the irreversible transformation of α-MnO₂ into Zn-vernadite was elucidated by employing electrochemical measurements, comprehensive characterization including operando XRD, SEM, TEM, and XPS, and DFT calculations. The electrochemical reactions involved in the discharge/charge process include intercalation/extraction of H⁺ in α-MnO₂, electrochemical dissolution of α-MnO₂ and subsequent electrochemical dissolution-deposition of Zn-vernadite. α-MnO₂ irreversibly transformed into Zn-vernadite after long cycles owing to the gradual dissolution of α-MnO₂ and accumulation of Zn-vernadite. These findings shed new light on the reaction mechanism of the MnO₂ electrode in aqueous zinc ion batteries, and will benefit the development of strategies for improving the performance of aqueous rechargeable batteries.