Re‐evaluating the Magnesium‐ion Storage Capability of Vanadium Dioxide, VO2(B): Uncovering the Influence of Water Content on the Previously Overestimated High Capacity

Abstract Magnesium batteries have emerged as a promising alternative to lithium‐ion batteries due to their theoretical high energy density and abundant magnesium resources. Vanadium dioxide, VO 2 (B), has been reported as a high‐capacity cathode material for magnesium batteries. However, the electrochemical intercalation mechanism requires further elucidation due to a limited understanding of the structure‐property relationship. In this study, we re‐evaluated the magnesium storage capability of the material, with a particular focus on the influence of water content in nonaqueous electrolytes. The higher discharge capacity of 250 mAh g −1 is achieved exclusively in the wet electrolyte with 650 ppm water content. A significantly lower capacity of 51 mAh g −1 was observed in the dry electrolyte solution containing 40 ppm water content. Through X‐ray structural and elemental analyses, as well as magnesium‐ion diffusion pathway analysis using bond‐valence‐energy‐landscape calculations, the restricted capacity was clarified by examining the reaction mechanism. According to this study, the impressive capacity of magnesium‐ion battery cathodes may be exaggerated due to the involvement of non‐magnesium‐ion insertion unless the electrolytes′ water content is appropriately regulated.