Multimodal Synchrotron X-Ray Tools for Studying Spinel Oxide Nanocrystals-Based Mg Cathodes

Abstract

Magnesium battery has huge potential to meet the future needs of large-scale energy storage due to the high abundance of Mg element, high volumetric and gravimetric capacities, and improved safety compared to Li metal battery. The complex solid-electrolyte interface (SEI) formed on Mg anode surface and slow diffusion rate of magnesium ion in cathode bulk are two critical challenges to achieve high capacity and energy density of Mg battery. Nanostructured spinel oxides with well-defined morphology and crystalline structures are promising candidates for Mg battery cathodes, but their dynamic evolutions of chemical composition and electronic/geometric structure during charging and discharging are still on the early stage, which hinders understanding of the interaction and degradation mechanisms on spinel oxide-based cathodes. Herein, in-situ/or ex-situ soft X-ray absorption spectroscopy (sXAS) and small/wide-angle X-ray scattering (SAXS/WAXS) to track the changes of structure and surface/bulk chemistry of spinel oxide nanocrystals during Mg intercalation/deintercalation processes at both local and macroscopical levels. The Mg0.5Fe2.5O4 (MFO) nanocrystals were tested in a Swagelok cell with an electrolyte of 0.3 M Mg(TFSI)2 and 0.15 M MgCl2 at room temperature. An initial discharge capacity of 187 mAh g-1 was achieved and 83% capacity was retained after 20 cycles. sXAS results reveal that Fe3+/Fe3+ redox is highly reversible during charge/discharge process, and MgF2 slowly formed on the surface as well. In-situ WAXS shows that rocksalt phases of FeO and MgO were both observed during discharge process, and the former one quickly disappeared and the latter one slowly converted to pristine phase during charge process. Moreover, MFO with small size (~5 nm) delivered double specific capacity but poorer cyclability compared to large size (~18 nm), which is ascribed to the nanocrystal aggregation and decrease of active surface area and formation of interfacial crystal boundary based on in-situ SAXS and ex-situ STEM. More structural features of shape and crystallinity degree of MFO were also investigated and correlated to their battery performance. This work highlight the powerful role of multimodal in-situ synchrotron X-ray techniques in understanding the structure-performance of spinel oxide nanocrystals-based Mg cathodes. Figure 1