While α-V2O5 has traditionally been considered as a promising oxide to reversibly intercalate high levels of Mg2+ at high potential, recent reports indicate that previously observed electrochemical activity is dominated by intercalation of H+ rather than Mg2+, even in moderately dry nonaqueous electrolytes. Consequently, the inherent functionality of oxides to intercalate Mg2+ remains in question. By conducting electrochemistry in a chemically and anodically stable ionic liquid electrolyte, we report that, at 110 °C, layered α-V2O5 is indeed capable of reversibly intercalating 1 mol Mg2+ per unit formula, to accumulate capacities above 280 mAh g–1. Multimodal characterization confirmed intercalation of Mg2+ by probing the elemental, redox, and morphological changes undergone by the oxide. After cycling at 110 °C, the electrochemical activity at room temperature was significantly enhanced. The results renew prospects for functional Mg rechargeable batteries surpassing the levels of energy density of current Li-ion batteries.

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The presence of H2O has been linked to enhancements in the reactivity of cathodes for Mg2+ electrochemistry. If the enhancements were mimicked by nonaqueous solvents, they could enable Mg batteries with transformational energy density. However, the extent to which H2O may boost actual intercalation of Mg2+, as opposed to competing reactions, has not been elucidated. Here, we evaluate its role as additive in the electrochemistry of a tunnel polymorph of V2O5 in a nonaqueous Mg2+ electrolyte. The electrochemical response and V reduction in the cathodes positively correlated with H2O concentration, but it was not concurrent with commensurate changes in cell volume and Mg content. These observations indicate that H2O does not enhance Mg2+ intercalation, but rather, it promotes competing pathways. This work shows the importance of accurately probing reactions in multivalent electrolytes. Importantly, it indicates that H2O is not a universal solution to the challenge of Mg2+ intercalation in oxides.