Highly reduced VOx nanotube cathode materials with ultra-high capacity for magnesium ion batteries

Abstract

Here, we describe novel VO_x nanotubes with vanadium at various oxidation states (V³⁺/V⁴⁺/V⁵⁺) as cathode materials for magnesium ion batteries. The VO_x nanotubes synthesized by a microwave-assisted hydrothermal process using an amine as an organic template show a high initial discharge capacity (∼218 mA h g⁻¹) of more than 200 mA h g⁻¹ and an outstanding cycling performance, which have not been previously reported for magnesium ion batteries. These improvements in the electrochemical performance of our VO_x nanotubes originate from the trivalent vanadium ions generated in the highly reduced VO_x nanotubes. The VO_x nanotubes with trivalent vanadium ions exhibit a lower charge transfer resistance at the electrode/electrolyte interfaces and superior cycling performance than the VO_x nanotubes containing vanadium ions of a higher oxidation state. We first suggest that the pristine oxidation state of the vanadium ions and the maintenance of a bonding structure on the surface of the VO_x nanotubes are the most important factors determining the magnesium insertion/extraction kinetics into/out of the VO_x nanotubes. Our findings offer a breakthrough strategy for achieving high-energy-density magnesium rechargeable batteries using VO_x nanotube cathode materials in combination with nanoarchitecture tailoring.