Mechanochemical Reduction of V2O5: Alkali Metals vs. Alkali Metal Hydrides—Which Are the More Suitable Reducing Agents?

Vanadium oxides and alkali metal vanadates are promising electrode materials for electrochemical energy storage owing to their ability to access multiple oxidation states. In this study, mechanochemical reduction of V2O5 with alkali metal hydrides is explored as a facile route to obtain reduced vanadium oxide phases. Sodium hydride enables the rapid formation of mixed sodium vanadium oxide phases at room temperature, proceeding in a self-propagating manner after only a short milling period. To assess the generality of this approach, lithium hydride was evaluated as an alternative reducing agent and exhibited comparable reaction behavior and product distributions. Complementary theoretical calculations indicate that the corresponding reductions with elemental sodium and lithium are thermodynamically more favorable, displaying lower reaction enthalpies. This prediction is experimentally corroborated by the instantaneous ignition observed when elemental metals are used. Rietveld refinement reveals that all investigated reducing agents follow similar reaction pathways, yielding comparable mixtures of alkali metal vanadium oxides with consistent main phases. However, the use of highly ductile sodium metal results in significant mixing limitations, which can be mitigated only through cryogenic milling. These findings highlight mechanochemical reduction with alkali metal hydrides as a robust and practical strategy, particularly for systems in which metal ductility limits process efficiency, despite the associated risk of hydrogen evolution.