Structural Transformations at the Atomic Scale in Spinel Vanadium Oxides upon Mg²⁺ Extraction

Mg batteries have the potential to deliver rechargeable energy storage devices with high energy densities from low-cost materials. Cathode materials are arguably the largest roadblock to develop these technologies since there is a severe dearth of phases known to de/insert Mg²⁺ with reasonable kinetics and material stability. Despite promising predictions based on materials modeling, all systems that have been explored experimentally remain essentially uncompetitive against available Li-ion technologies. This limited progress highlights the need for fundamental studies to provide a more complete understanding of the Mg electrochemistry in solids. Here, we show that atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy offers unique insights into the structural and chemical evolution during electrochemical cycling of MgV₂O₄, a candidate oxide cathode potentially enabling high energy density. We discover that the mechanism underpinning electrochemical activity demands that the MgV₂O₄ crystals undergo a structural transformation, resulting in a reduction of the local crystalline order. Importantly, we observe that this transformation primes the material for a high degree of Mg²⁺ de/insertion.