Magnesium Ion Storage Properties in a Layered (NH₄)₂V₆O₁₆·1.5H₂O Nanobelt Cathode Material Activated by Lattice Water

Magnesium ion batteries have attracted increasing attention as a promising energy storage device due to the high safety, high volumetric capacity, and low cost of Mg. However, the strong Coulombic interactions between Mg2+ ions and cathode materials seriously hinder the electrochemical performance of the batteries. To seek a promising cathode material for magnesium ion batteries, in this work, (NH4)2V6O16·1.5H2O and water-free (NH4)2V6O16 materials are synthesized by a one-step hydrothermal method. The effects of NH4+ and lattice water on the Mg2+ storage properties in these kinds of layered cathode materials are investigated by experiments and first-principles calculations. Lattice water is demonstrated to be of vital importance for Mg2+ storage, which not only stabilizes the layered structure of (NH4)2V6O16·1.5H2O but also promotes the transport kinetics of Mg2+. Electrochemical experiments of (NH4)2V6O16·1.5H2O show a specific capacity of 100 mA·h·g-1 with an average discharge voltage of 2.16 V vs Mg2+/Mg, highlighting the potential of (NH4)2V6O16·1.5H2O as a high-voltage cathode material for magnesium ion batteries.