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The natural abundance of magnesium together with its high volumetric energy capacity and less-dendritic anodes makes Mg-ion batteries an appealing alternative to the widely used Li-ion batteries. However, Mg cathode materials under current investigation suffer from various shortcomings such as low operation voltage and high energy barrier for ion migration, resulting in poor battery performance. Here, we propose a garnet-type intercalation cathode active material, Mg3Si3(MoO6)2, for high-performance Mg-ion batteries. Through first-principles density functional theory calculations, it is demonstrated that Mg3Si3(MoO6)2 possesses a high average discharge voltage (2.35 V vs Mg/Mg2+), a low ion migration barrier (∼0.2 eV), and a minimal volume change (∼4%) concurrently, which comprises excellent intercalation cathode chemistry. The small energy barrier for ion migration is shown to arise from the favorable change in the Mg coordination along the migration route within the garnet host. These findings present an additional direction to develop competent Mg-ion batteries for future energy storage applications.

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