Structural Transformations at the Atomic Scale in Vanadium oxides upon Mg²⁺ intercalation

The advancements in mobile energy storage systems rely upon the evolution of battery technologies, to keep up with the increasing demands and deliver higher energy densities. Despite the widespread use of lithium-ion batteries in portable devices, there is growing research into alternative multivalent battery chemistries with more abundant elements that may also offer higher energy densities. Mg-ion batteries are one such candidate, showing advantages such as greater material abundance, enhanced safety, and reduced cost compared to Li-ion batteries. A previous study demonstrated morphological changes in Mg-ion battery cathodes during chemical cycling at elevated temperatures. It was found that the charge-discharge cycle induces structural transformation and the formation of an amorphous layer with a distinct bond structure compared to the crystalline region. However, the intercalation mechanisms and their impact on the parent cathode during electrochemical cycling for Mg-ion batteries are not well understood, which is crucial for device optimization.

In this contribution, we investigate MgV₂O₄ and -V₂O₅ as two possible Mg²⁺ intercalation cathodes using scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), and energy dispersive spectroscopy (EDS). The intercalation and de-intercalation of Mg²⁺ during discharging and charging, respectively, are expected to cause significant changes in the atomic, chemical, and electronic structure of the cathodes. We use atomic-resolution imaging and spectroscopy to analyze the transformation of the crystal structure and changes in the electronic structures, including the oxidation state and coordination environment, of the parent cathode during charge/discharge cycles at elevated temperatures. Here, we will focus on delineating the influence of Mg (de)intercalation during electrochemical cycling on the morphology and local crystalline order at the cathodes, and further elucidate the Mg (de)intercalation pathways.