Mg-ions based technologies have emerged as a promising candidate in the search for beyond-lithium-ion batteries. To make them a feasible alternative, compatible cathode materials are needed. Spinel vanadium oxides are a cathode material of active interest because of their reported impressive cycling capacities. Typically, vanadium oxides are synthesized as nanocrystals on the order of a few nm in radius and thus are difficult to study using high resolution electron microscopy techniques. However, recent work has shown that post synthetic thermal annealing can be used to produce crystals on the order of 50 nm with well-defined crystal surface types. Taking advantage of the larger crystal size, here we will use scanning transmission electron microscopy [STEM] to study atomic scale changes undergone by the MgV₂O₄ crystals upon electrochemical cycling.
As part of this contribution, we will show that charged MgV₂O₄ crystals form a surface layer with distinct structural and chemical properties. Atomic-scale STEM images show this layer is roughly 5 nm in thickness and is characterized by a loss of crystallinity. Additionally, electron probe spectroscopy measurements reveal this surface layer contains low Mg concentration and vanadium in an elevated valence state. In the absence of significant changes to the bulk of the crystals, we believe the changes in the surface layer gives rise to most of the activity observed in charge and discharge curves. As part of our study, we will demonstrate that the nature of the boundary between the crystal bulk and surface layer is dependent on the exposed crystal surface type. Figure 1 show an example where the smoothness of this boundary changes along two different exposed surfaces. This study will help guide the synthesis of multivalent ion cathodes, particularly in thins films, where the exposed surface play an important role in the perfomance of the material.

Figure 1a. A z-contrast image showing a charged MgV<sub>2</sub>O<sub>4</sub> crystal oriented along the [1-1-2] direction. A surface layer ~5 nm in thickness is observed along the edge of the crystal. 1b. Magnified view of the [-11-1] and 1c. the [110] crystal surface. The dotted line in each highlights the interface between the bulk of the crystal and the surface layer.

Figure 1a. A z-contrast image showing a charged MgV₂O₄ crystal oriented along the [1-1-2] direction. A surface layer ~5 nm in thickness is observed along the edge of the crystal. 1b. Magnified view of the [-11-1] and 1c. the [110] crystal surface. The dotted line in each highlights the interface between the bulk of the crystal and the surface layer.

Grant C.B. Alexander

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