Atomic-Scale Study of Spinel MgV₂O₄ Nanocrystals for Multivalent Ion Battery Cathodes

Mg-ions based batteries have emerged as a promising candidate in the search for post lithium-ion energy storage. Since Mg ions are divalent, batteries based on their transfer inherently offer higher energy densities than those using Li. Additionally, batteries with Mg metal anodes offer a route towards higher gravimetric energy densities needed for applications in electric vehicles, aircraft, and energy grid storage. While Mg ions are comparable in ionic radii to Li ions, their increased reactivity has complicated the search for compatible cathode materials. To date many systems have been explored but few materials for real-world applications have emerged [1]. To overcome this, a fundamental understanding of Mg electrochemistry and solid-state cathode materials are needed. Here we present a study of a spinel vanadium oxides, a cathode material that has been shown to cycle Mg at capacities greater than 170mAh/g [2], using an aberration corrected cold field scanning transmission electron microscopy (STEM) to develop atomic scale structure-property relationships in the material.<br/>In addition to atomic scale imaging, we will utilize electron energy loss spectroscopy (EELS) to map local changes in the density of states (DOS) of the material and conduct a quantitative chemical analysis of the material using energy dispersive spectroscopy (XEDS).<br/>We will show that MgV₂O₄ crystals tend to form well-defined polyhedra with [111], [001] and [110] surface types. The prevalence of each surface type will be examined through a statistical analysis of imaged crystals and the results will be compared to <i>in-silico</i> calculations of the associated energies of each surface type. Additionally, we will present atomic-resolution images of the surface reconstructions observed within the first two-three atomic layers on a variety of surface types. Our analysis will include the atomic-scale effects of electrochemical cycling on the crystals. We will show that upon charging a ~5 nm amorphous layer forms on the crystal surface and notably is the region with most pronounced electrochemical activity. The relationship between the crystal surface type and the depth of the surface layer formation will be explored. Finally, by comparing MgV₂O₄ material at different number of cycles (1,20,50) we will present insights into the failure mechanism associated with this material [3].<br/><br/>References<br/>[1] Ponrouch, Alexandre, et al. "Multivalent rechargeable batteries." <i>Energy Storage Materials</i> 20 (2019): 253-262.<br/>[2] Hu, Linhua, et al. "High capacity for Mg2+ deintercalation in spinel vanadium oxide nanocrystals." <i>ACS Energy Letters</i> 5.8 (2020): 2721-2727.<br/>[3] This work is solely supported by the Joint Center for Energy Storage Research (JCESR) and Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences.