V2O5-SiO2 and V2O5-B2O3 As Cathode Materials for Rechargeable Mg-Ion Batteries

Abstract

Rechargeable Mg batteries have gained considerable interest, as it possesses considerably higher volumetric capacity (3833 mAh/cm3) vs. Li metal (2046 mAh/cm3) due to the bivalency of Mg1. Other advantages of Mg is its abundancy in the earth's crust, low cost, environmentally friendly, low equilibrium potential (-2.31 V vs NHE2) and the benefit of using Mg directly as anode without dendrite formation thus alleviating the safety concerns as opposed to using Li metal as anode. Nevertheless, the development of rechargeable Mg batteries has yet not received strong commercial interest due major issues such as lack of high-voltage Mg intercalation cathodes with faster Mg insertion/extraction kinetics and passivation of metallic magnesium at the anode when exposed to oxygen, water and other protic solvents (including organics). These limitations result in restricted choice of electrolytes with poor anodic stability and potential window thus hindering the research on high energy density cathodes. The need for high energy density batteries as mentioned earlier would require cathode materials with high voltages (> 3 V). Much of the promising cathode materials for rechargeable Mg-ion batteries are metal oxides and sulfides. Of them, V2O5 is particularly interesting as it provides comparatively higher open circuit potential (OCV, about 2.66 V vs. Mg2+ in 1 M Mg(ClO4)2 in THF))2. Last year, we successfully demonstrated high specific capacity (245 mAh/g) for nanostructured V2O5 synthesized by flame spray pyrolysis (FSP)3. However, strong electrostatic interaction between Mg2+ ions and inorganic host materials hamper the kinetics of these oxides for reversible Mg2+ intercalation. Nevertheless, recently, amorphous structures are found to show better kinetics owing to their favourable interlayer spacing4. To this end, a series of amorphous V2O5-based cathode materials such as V2O5-SiO2 and V2O5-B2O3 are synthesized by flame spray pyrolysis (FSP). The effect of employing different solvents and flow rates during synthesis on the structure and morphology of the materials is studied. While XRD, SEM and BET are used to characterize the physical properties, 3-electrode cell assembly and coin cells study the electrochemical properties of the materials. The cathodes materials are coated on graphite sheet current collector with Mg plate and Mg(BH4)2in tetraglyme acting as anode and electrolyte, respectively. This work, as a whole will discuss the effect of synthesis parameters, heat treatment, composition and morphology of V2O5-based cathode materials (with varying degree of amorphicity) with respect to their electrochemical performance. H.D. Yoo, I. Shterenberg, Y. Gofer, G. Gershinsky, N. Pour and D. Aurbach, Energy Environ. Sci., 2013, 6, 2265M.M. Huie, D.C. Bock, E.S: Takeuchi, A.C. Marschilok and K.J. Takeuchi, Coordination Chemistry Reviews, 2015, 287, 15S. M. Hanetho, K. Jayasayee, P.I. Dahl, J. Kvello, J. R. Tolchard, A. Fossdal, T. Mokkelbost, F. Vullum-Bruer, "Mg Ion Batteries: V2O5 Cathode Materials by Flame Spray Pyrolysis", 227th ECS Meeting, 2015, Chicago, USAT.S Arthur, K. Kato, F. Mizuno and J. Germain, 20th International Conference on Solid State Ionics, June 14-19, 2015