Unveiling the intricate intercalation mechanism in manganese sesquioxide as positive electrode in aqueous Zn‐metal battery

Abstract

This work presents a comprehensive and systematic study of the charge storage mechanism of α‐Mn2O3 as a positive electrode in rechargeable aqueous Zn‐metal batteries. Upon the first and second cathodic steps a sequence of electrochemical and chemical reactions occurs, including the dissolution of Mn2+ ions, leading to the formation of a layered‐type L‐ZnxMnO2, which enables the reversible Zn2+ de‐/intercalation. In the family of Zn/manganese oxide batteries with mild aqueous electrolytes, cubic α‐Mn2O3 with bixbyite structure is rarely considered, because of the lack of the tunnel and/or layered structure that are usually believed to be indispensable for the incorporation of Zn ions. In this work, the charge storage mechanism of α‐Mn2O3 is systematically and comprehensively investigated. It is demonstrated that the electrochemically induced irreversible phase transition from α‐Mn2O3 to layered‐typed L‐ZnxMnO2, coupled with the dissolution of Mn2+ and OH− into the electrolyte, allows for the subsequent reversible de‐/intercalation of Zn2+. Moreover, it is proven that α‐Mn2O3 is not a host for H+. Instead, the MnO2 formed from L‐ZnxMnO2 and the Mn2+ in the electrolyte upon the initial charge is the host for H+. Based on this electrode mechanism, combined with fabricating hierarchically structured mesoporous α‐Mn2O3 microrod array material, an unprecedented rate capability with 103 mAh g−1 at 5.0 A g−1 as well as an appealing stability of 2000 cycles (at 2.0 A g−1) with a capacity decay of only ≈0.009% per‐cycle are obtained.