Operando Tracking the Dissolution and Deposition Dynamics in Aqueous Zn-MnO2 Batteries

Abstract

Aqueous Zn–MnO2 batteries have received extensive attention for next-generation large-scale energy storage because of their low cost and outstanding safety. Despite efforts to achieve better performances, the charge-storage chemistry of MnO2 cathodes remains controversial. Zn-ion insertion, Zn-ion/proton co-insertion, proton insertion, and electro-dissolution have been proposed as the mechanism of the MnO2 cathodes. More specifically, the pathway of Mn dissolution is debatable: while intercalation chemistry leads to dissolution caused by the Jahn–Teller effect of Mn(III), electro-dissolution relies on a conversion from Mn(IV) to Mn(II). Furthermore, it is recently considered that Mn dissolution/redeposition efficiency is crucial to the reversibility of MnO2 cathodes, depending on pre-added Mn in the electrolyte. The role of trace Mn additive is also confusing. Last but not least, Mn dissolution and deposition is conjectured to be linked to the dissolution/redeposition of Zn‐based species-that is, Zn4SO4·(OH)6·xH2O (ZSH). To understand these key scientific questions that enlighten the further design of materials and electrolytes, it is imperative to develop operando measurements of the MnO2 cathodes. In our study, we utilized synchrotron X-ray fluorescence microscopy with high spatial resolution to probe the evolution of Zn and Mn species during electrochemical cycling. We can quantify the relationship between Zn and Mn concentrations from the electrode level to the single-particle level. We conclude that during the first discharge, MnO2 delivers the capacity by electro-dissolution, accompanied by the rapid precipitation of ZSH due to change of pH. ZSH surrounds the MnO2, which has high reversibility with the pre-added Mn additive in electrolytes. Our work provides in-depth insights into the complex Zn/Mn deposition and deposition near the electrode surface which will guide the further development of aqueous Zn-MnO2 batteries.