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

Rechargeable aqueous Zn/alpha-MnO2 batteries have attracted much attention due to the advantages of safety and high capacity. However, the reaction process of alpha-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 alpha-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 alpha-MnO2, electrochemical dissolution of alpha-MnO2 and subsequent electrochemical dissolution-deposition of Zn-vernadite. alpha-MnO2 irreversibly transformed into Zn-vernadite after long cycles owing to the gradual dissolution of alpha-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.