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

Mg batteries have the potential to deliver rechargeableenergystorage devices with high energy densities from low-cost materials.Cathode materials are arguably the largest roadblock to develop thesetechnologies since there is a severe dearth of phases known to de/insertMg(2+) with reasonable kinetics and material stability. Despitepromising predictions based on materials modeling, all systems thathave been explored experimentally remain essentially uncompetitiveagainst available Li-ion technologies. This limited progress highlightsthe need for fundamental studies to provide a more complete understandingof the Mg electrochemistry in solids. Here, we show that atomic-resolutionscanning transmission electron microscopy combined with electron energy-lossspectroscopy offers unique insights into the structural and chemicalevolution during electrochemical cycling of MgV2O4, a candidate oxide cathode potentially enabling high energy density.We discover that the mechanism underpinning electrochemical activitydemands that the MgV2O4 crystals undergo a structuraltransformation, resulting in a reduction of the local crystallineorder. Importantly, we observe that this transformation primes thematerial for a high degree of Mg2+ de/insertion.