Elucidating a dissolution-deposition reaction mechanism by multimodal synchrotron X-ray characterization in aqueous Zn/MnO2 batteries

Aqueous Zn/MnO2 batteries with their environmental sustainability and competitive cost, are becoming a promising, safe alternative for grid-scale electrochemical energy storage. Presented as a promising design principle to deliver a higher theoretical capacity, this work offers fundamental understanding of the dissolution-deposition mechanism of Zn/beta-MnO2. A multimodal synchrotron characterization approach including three operando X-ray techniques (powder diffraction, absorption spectroscopy, and fluorescence microscopy) is coupled with elementally resolved synchrotron X-ray nano-tomography. Together they provide a direct correlation between structural evolution, reaction chemistry, and 3D morphological changes. Operando synchrotron X-ray diffraction and spectroscopy show a crystalline-to-amorphous phase transition. Quantitative modeling of the operando data by Rietveld refinement for X-ray diffraction and multivariate curve resolution (MCR) for X-ray absorption spectroscopy are used in a complementary fashion to track the structural and chemical transitions of both the long-range (crystalline phases) and short-range (including amorphous phases) ordering upon cycling. Scanning X-ray microscopy and full-field nano-tomography visualizes the morphology of electrodes at different electrochemical states with elemental sensitivity to spatially resolve the formation of the Zn- and Mn-containing phases. Overall, this work critically indicates that for Zn/MnO2 aqueous batteries, the reaction pathways involving Zn-Mn complex formation upon cycling become independent of the polymorphs of the initial electrode and sheds light on the interplay among structural, chemical, and morphological evolution for electrochemically driven phase transitions.

Automated summary

This study uses multiple synchrotron X-ray techniques and elementally resolved synchrotron X-ray nano-tomography to directly reveal the correlation between structural evolution, reaction chemistry, and 3D morphological changes in Zn/MnO2 aqueous batteries. The research finds that the reaction pathways of Zn/MnO2 aqueous batteries are independent of the polymorphs of the initial electrode, and sheds light on the interplay among structural, chemical, and morphological evolution in electrochemically driven phase transitions.