Synthesis and characterization of vanadium diboride and its use in air battery applications

Battery technology, for which there is a serious need in various fields such as EV (electric vehicle), energy production, mobile devices, has long been studied. True potential of state-of-the-art Lithium-ion technology (600 Wh kg-1), offering a 400 Wh kg-1 energy density today, is estimated to be achieved within 5 years. However, even estimated 500-600 Wh kg-1 energy density barely provides a feasible operating range for EVs. On top of that, many researchers are concerned by safety and toxicity. As a result, seeking distinctive types of battery technology is of crucial interest especially in the current decade; metal-air battery technology is a strong candidate to fill this need. Zinc-air batteries have been commercially available mostly in hearing aid devices for more than 40 years. Concurrently, lithium-air technology is being intensely developed. Metal borides class is another potential player in the air battery league. Among metal borides, vanadium diboride (VB2) performs superior to others, combining a flat discharge potential at 1.1 V with an exceptional high theoretical charge density of 4060 mAh g-1. In this study, VB2 nanopowders are synthesized employing the gel method. The process is carried out using ammonium metavanadate (NH4VO3) and boric acid (H3BO3) as vanadium and boron sources respectively, and citric acid (C6H8O7) and tartaric acid (C4H6O6) as chelating agents. The resulting material is characterized to be at least 98% VB2 within the possibility of excessive amorphous B2O3 and ß-rhombohedral boron remaining. Thermodynamic considerations in regards to boro/carbothermal synthesis of VB2 and chelating mechanism throughout the major reaction steps are described. Finally, the routes to assemble a VB2-air battery using as-synthesized anode material are presented along with final remarks towards achieving VB2-air batteries with Coulombic efficiency up to 56% and rechargability.