Unexpected discovery of magnesium-vanadium spinel oxide containing extractable Mg2+ as a high-capacity cathode material for magnesium ion batteries

Magnesium ion batteries (MIBs) have attracted significant attention as a promising candidate for the next generation energy storage system owing to their large volumetric capacity and abundant resource. Currently, several researchers have focused on Mg-free materials to be used as cathode in MIBs; But the exploration of Mg-rich electrode materials will promote the development of magnesium batteries towards obtaining more flexible MIBs. Herein, a Mg(Mg0.5V1.5)O4 is investigated as a potential cathode material, in which the Mg2+ can be extracted and the fast Mg2+ reaction kinetics. Benefited from the exceptional cathode, the Mg battery shows a high reversible specific capacity of 250 mA h g−1 at 100 mA g−1 and capacity retention of 100 mA h g−1 after 500 cycles at high rate of 1 A g−1. The excellent rate capability and desirable cycling performance obtained herein outperformed those of previously reported magnesium spinel oxides. It is also demonstrated that Mg2+ extraction/insertion mechanism of Mg(Mg0.5V1.5)O4 is related to the coexistence of two-phase process and single-phase solid solution reaction through a series of systematic in situ/ex situ characterizations. X-ray absorption near edge structure (XANES) demonstrates that the valence state of vanadium changes and the octahedral symmetry of vanadium site varies due to the extraction of Mg2+ from spinel Mg(Mg0.5V1.5)O4. Significantly, the cathode containing extractable Mg2+ can be coupled with Mg-free anode materials (Na2Ti3O7) to assemble a full cell in Mg(TFSI)2/acetonitrile electrolyte, displaying a discharge capacity of 102 mA h g−1 after 100 cycles at 50 mA g−1. The encouraging results show that the Mg(Mg0.5V1.5)O4 is a promising electrode material, which paves ways for the development and further improvements of MIBs.