Vanadium is a silvery white soft metal from Group 5 of the periodic table. It is used in alloys with steel and iron to produce high-speed tool steel, high-strength low-alloy steel, and wear-resistant cast iron. It is also used in making batteries, catalysts, and chemical reagents.
Pure vanadium is difficult to obtain because it is prone to oxidation at elevated temperatures. Commercial vanadium is obtained from ores as vanadium pentoxide (V2O5) through a series of smelting, leaching, and roasting processes. It is then reduced to ferrovanadium, a powder, or to high-strength low-alloy vanadium alloys by sintering in reducing atmospheres.
Several vanadium sulfide compounds have been reported to be good cathode materials for aqueous zinc-ion batteries (AZIBs). They feature large layer spacings, different oxidation states, and are soluble in sulfuric acid. However, the performance of these materials in AZIBs has yet to be satisfactorily demonstrated.
Youqi Zhu and Chuanbao Cao from the Beijing Institute of Technology, China, have developed ultrathin VS 4 nanosheets that can be self-assembled into different hierarchical structures. By varying the reaction temperature and time, they were able to form hollow nanospheres, microspheres, and nanoflowers. They also measured the sodium storage performance of each morphology and found that the hollow VS 4 nanospheres had the highest capacity with the best long-term cycling stability.
To make the VS4 nanosheets, the team started with ammonium metavanadate dissolved in water. When exposed to oxygen in air, the solution turns green and is oxidized back to vanadium(III) – producing blue VO2+ ions. Adding nitric acid, which is a reasonably powerful oxidizing agent, will further reduce the ions to their vanadium(IV) state, producing the blue dioxovanadium(V) ion.