The effect of Mn2+additives on the capacity of aqueous Zn/8-MnO2 batteries: Elucidating the Mn2+concentration dependence of the irreversible transformation of 8-MnO2

Rechargeable aqueous Zn/MnO2 batteries receive extensive attention owing to the high capacity and safety. Pre-adding Mn2+ in electrolyte is widely adopted as an effective strategy to improve the electrochemical performance of Zn/MnO2 batteries. However, the reaction mechanism of MnO2 electrode during long cycles is still unclear with Mn2+ pre-added in electrolyte because of the complex reactions involved Mn2+. In this work, the reaction process of 8-MnO2 electrode with different concentration of Mn2+ pre-added in electrolyte is elucidated. The pre-addition of 0.1 M Mn2+ promotes the formation of vernadite on 8-MnO2 electrode and delivers extra capacity. Nevertheless, with the increasing Mn2+ concentration in electrolyte, electrochemical inactive ZnMn2O4 forms and the coverage of ZnMn2O4 nanoparticles on 8-MnO2 electrode hinders the electrodissolution of 8-MnO2 thus leading to the severe capacity fading. The different electrodeposition reactions with different concentration of Mn2+ pre-added in electrolyte cause the distinct irreversible transformation of 8-MnO2 into vernadite or ZnMn2O4 after long cycles. These findings shed new light on the reaction mechanism for 8-MnO2 electrode in aqueous zinc ion batteries.