Study of the Effects of Thermal Annealing Temperature to Electrochemical Cycling of Mg2+ and Structure of Spinel MgV2O4 Crystals
Authors: Lagunas Francisco Javier , Hu Linhua , Alexander Grant C. B , Cabana Jordi , Klie Robert F. 
Published: July 2021

Spinel vanadium oxides are a promising candidate material for multivalent battery cathodes due to the various possible valence states of V within the spinel framework, which allows for pathways in Mg2+ ion deintercalation [1]. In recent work, Hu et al. experimentally demonstrated that spinel vanadium oxides nanocrystals have a high capacity for Mg2+ electrochemical cycling [2]. While they reported capacities above 200mAh/g at 110 °C, thus outperforming counterparts such as Cr and Mn oxide spinels, it was noted that these capacities are limited by complex defect structures, as suggested by peak pair distances that cannot be explained in the spinel framework in X-ray diffraction measurements.
Here, we will utilize atomic-resolution scanning transmission electron microscopy to characterize the structural defects in spinel V2O4 nanocrystals and their possible effect on Mg2+ intercalation. Additionally, we will report the effects of thermal annealing on the structure as a function of temperature and explore the stability of the MgV2O4 nanocrystals. We find that the effects of electron beam exposure can be used as a proxy for damage incurred on the crystals during electrochemical cycling, on the structure of the crystals. The atomic-resolution characterization will be conducted with an aberration-corrected cold field emission JEOL ARM200CF operated at 200kV primary electron energy and emission current of 15μA. The electron probe will be operated at 24mrad convergence semi-angle. High angle annular dark field, low angle annular dark field and annular bright field detectors will be set to the following inner angles: 75 mrad, 30 mrad, 11mrad. X-ray energy dispersive spectroscopy measurements will be conducted using a large solid-angle Oxford XMAX100TLE detector. Additionally, the ARM200CF is equipped with a post-column Gatan Continuum GIF spectrometer providing capabilities for electron energy loss spectroscopy measurements.
In this contribution, we will demonstrate that thermal annealing at higher temperatures increases the crystal size of the MgV2O4 crystals while simultaneously decreasing the presence of defect structures. Additionally, we will characterize a variety of defect structures, similar to those shown in Figure 1, present in crystals annealed at lower temperatures using atomic resolved images and electron spectroscopy techniques. [3]
Figure 1. LAADF Image showing a grain boundary defect in a spinel MgV2O4 nanocrystal thermally anneal at 400 degrees Celsius.
[1] Computer Modeling Investigation of MgV2O4 for Mg-ion Batteries. Navaratnarajah Kuganathan, Konstantinos Davazolglou, Alexander Chroneos; Journal of Applied Physics 2020 127, 035106.
[2] High Capacity for Mg2+ Deintercalation in Spinel Vanadium Oxide Nanocrystals. Linhua Hu, Jacob R. Jokisaari, Bob Jin Kwon, Liang Yin, Soojeong Kim, Haesun Park, Saul H. Lapidus, Robert F. Klie, Baris Key,Peter Zapol, Brian J. Ingram, John T. Vaughey, and Jordi Cabana; ACS Energy Letters 2020 5 (8), 2721-2727.
[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.

Figure 1