Li-ion batteries are the top choice in electrochemical energy storage for portable devices and mobile vehicles. However, the limited storage capacities do not quite yet meet the emerging energy demands in transportation and grid markets. Multivalent batteries based on divalent cations have attracted wide attention with high energy density/capacity in different framework of oxide cathodes. For instance, spinel structure M₂O₄, has been theoretically predicted as one of the most suitable oxide cathode for the reversible intercalation with Mg²⁺ [1]. Further, reversible, yet sluggish Mg intercalation into Mn₂O₄ host, prepared from de-lithiated LiMn₂O₄ electrode, was reported in aqueous electrolyte environments [2]. In order to enhance kinetics, reduction of particles to the smallest possible sizes is desired. Further, spinel-type Mn­₂O₄ can only be prepared by delithiation of LiMn₂O₄. Therefore, the direct synthesis of the parent spinel oxide, MgMn₂O₄, in the form nanocrystals would be desirable to investigate the fundamental limits of Mg intercalation within the spinel oxide framework, preferably in non-aqueous electrolytes.

Here, we report a series of Mg-based spinel nanocrystals with different phase (Mg_xMn_3-xO₄ and Mg_xCr_3-xO₄), size (5nm~50nm), and shape (cube and sheet) via a low temperature hydrothermal process, followed by calcination (Figure 1). All samples were characterized with X-ray diffraction and spectroscopy, as well as electron microscopy in order to define the products of electrochemical reactions in non-aqueous cells. Our goal was to explore spinel oxide nanocrystals with optimal size, phase, and shape and establish size-structure-property relationship for the experimental realization of Mg reversible intercalation in spinel oxide hosts.

1. Liu, M., et al., Energy Environ. Sci. 2015, 8, 964.

2. Kim, C. et al., Adv. Mater. 2015, 27, 3377.

Figure 1 (a) Representative STEM image of 50nm MgMn_­2O₄ nanocubes (b) Representative STEM image of 2~5nm thickness MgMn_­2O₄ nanosheets