Oxygen vacancies boosted fast Mg2+ migration in solids at room temperature

Magnesium (Mg) batteries are promising candidates for lithium-ion batteries owing to their high theoretical capacity and the rich natural abundance of Mg. However, the development of Mg metal batteries, especially in an attractive solid-state configuration, is hampered by sluggish Mg2+ migration. Here, we discover that by incorporating oxygen vacancies on the surface of metal oxide nanopowders, the Mg2+ ion conductivity (σi) at room temperature in Mg(BH4)2·1.5NH3 is dramatically improved on the order of 10−4 S cm−1, e.g., 2.96 × 10−4 S cm−1, by the addition of 60 wt.% TiO2. Theoretical simulations indicate that the high σi is due to the “coordination-unlock” phenomenon at the interface, where the coordination sheath of Mg(BH4)2·1.5NH3 is unlocked by oxygen vacancies and coordination-deficient Mg transfers on the surface through interactions with oxygen sites. The strategy involving the assistance of oxygen vacancies could create a new path for designing new divalent cation conductors.