Soft-TXM study of electrolyte-driven MnO2 redox in aqueous Zn-MnO2 batteries

Aqueous zinc metal batteries, featuring high-capacity zinc anodes and nonflammable electrolytes, are promising for electric mobility and grid storage. However, cathode optimization remains challenging. MnO₂ is an attractive cathode material due to its low cost and strong electrochemical performance, but its mechanism deviates from  conventional Zn-ion insertion. Instead, MnO₂ undergoes a two-electron dissolution-precipitation pathway controlled by local pH, buffer, and electrolyte volume, leading to various Mn compounds with distinct morphologies. Incorporation of FeSO₄ alters the reaction, potentially via Fe intercalation into MnO₂, improving coulombic efficiency and cycle life. These findings underscore the complexity of MnO₂ electrochemistry in aqueous Zn batteries. We propose an in-depth TXM study of Zn-MnO 2  cathode material at MISTRAL, leveraging high energy and spatial resolution absorption spectra at the Zn L-edge, Fe L-edge, and Mn L-edge to gain insight into elemental distributions and oxidation states. Our primary objective is to determine how additives such as acetate drive MnO 2 morphology changes and Zn incorporation, and to examine whether Fe integration stabilizes intermediate Mn valence states. By systematically analysing samples harvested at different charged state and electrolyte, we aim to map the morphological and chemical state of Mn, Zn, and Fe species. This information will complement data from the surface (XPS), with higher spatial resolution (STXM-EELS) and operando (XAS), for a more detailed understanding of the deposited structures and linked to observed differences in capacity, reversibility, and overall cell efficiency.