Does Water Enhance Mg Intercalation in Oxides? The Case of a Tunnel Framework

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.