Unveiling the Intricate Intercalation Mechanism in Manganese Sesquioxide as Positive Electrode in Aqueous Zn-Metal Battery

In the family of Zn/manganese oxide batteries with mild aqueous electrolytes, cubic α-Mn$_{2}$O$_{3}$ 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-Mn$_{2}$O$_{3}$ is systematically and comprehensively investigated. It is demonstrated that the electrochemically induced irreversible phase transition from α-Mn$_{2}$O$_{3}$ to layered-typed L-Zn$_{x}$MnO$_{2}$, coupled with the dissolution of Mn$^{2+}$ and OH$^{-}$ into the electrolyte, allows for the subsequent reversible de-/intercalation of Zn$^{2+}$. Moreover, it is proven that α-Mn$_{2}$O$_{3}$ is not a host for H$^{+}$. Instead, the MnO$_{2}$ formed from L-Zn$_{x}$MnO$_{2}$ and the Mn$^{2+$ in the electrolyte upon the initial charge is the host for H$^{+}$. Based on this electrode mechanism, combined with fabricating hierarchically structured mesoporous α-Mn$_{2}$O$_{3}$ 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.