Mg Secondary Batteries Using Nano-Crystalline V2O5

Abstract

Vanadium oxide (V2O5) is available as an active material for secondary batteries. Intercalation of cations to the layer of V2O5 crystal occurs with a reduction of vanadium receiving electrons up to 3 (theoretical capacity: 442 mAh g−1). Fabrications of V2O5 crystal directly affect the battery property. Nano-sizing, thinly supporting on carbon materials with large specific surface area, and solvent-insertion were studied to shorten a diffusion resistance of ions in layers. When carrier cation is Mg2+, V2O5 can be applied for a positive material of Mg battery. In this study, nano-crystalline V2O5 was synthesized by ball-mill and calcination. Ball-mill in toluene as a solvent with φ0.2 mm zirconia balls, removal of toluene, and burning in air at 400°C during 4 h were conducted. SEM and particle size distribution measurement show V2O5 was spherical in shape with an average diameter of 200 nm. XPS analysis shows this ball-mill process partially caused a chemical reaction including reduction of V2O5 and oxidation of toluene. Chemical etching and physical ball-milling gave the uniform nano-particles. Burning in air removed carbon species derived from toluene, re-oxidizes vanadium species to V2O5, and raises crystallinity. Obtained V2O5 was mixed with AB and PVdF (80 : 10 : 10, wt%) in NMP and the slurry was pasted/dried on Al foil to make a working electrode. The effect of nano-sizing on the electrochemical performance as an active material was tested in a system of Li secondary battery. Compared to unprocessed V2O5, nano-crystalline V2O5 has better electrochemical properties of cyclability and capacity. As to Mg secondary battery test, different electrolytes were utilized: 5 mol% Mg[TFSA]2 / 95 mol% acetonitrile (AN), diglyme (G2), triglyme (G3), tetraglyme (G4), 12-crown-4ether (12C4), and diethylmethylmethoxyethylammonium TFSA (DEMETFSA); 5 mol% Mg[TFSA]2 / 5 mol% 18C6 and 90 mol% THF; 3 mol% Mg[TFSA]2 and MgCl2 / 94 mol% G3; 5 mol% Mg[TFSA]2 and MgBr2 / 90 mol% G3. Battery tests were performed in a three electrode cell: counter electrode, magnesium metal; reference electrode, Ag wire immersing in 0.05 M AgBF4/EMImBF4. In many cases of these electrolytes, only Mg[TFSA]2/G3 shows electrochemically active against Mg intercalation to V2O5. The discharge and charge capacities at the 10th cycle were 180.3 and 176.5 mAh g−1, and cycle efficiency is over 95 %. The electrolytes with only different carbon number to G3, Mg[TFSA]2/G2 or G4, gave little capacity. In the case of 12C4 with the same carbon number to G3, lower capacity and overpotential were shown. In the case of ionic liquid, only 6 mAh g−1 appeared at room temperature, whereas 100 mAh g−1 appeared at 100°C. These results indicate that intercalation and decalation of Mg2+ to V2O5 are mainly influenced by solvents which are desolvated and solvated to Mg2+. The SEM images of charged and discharged electrodes show little morphology change of V2O5 particle. The XPS analyses show that the peaks of V2p1/2 and V2p3/2 in the discharged electrode appeared at a lower energy compared to the charged electrode, indicating reduction of vanadium from +5 to a lower valence. The XRD pattern of discharged electrode has a small difference to charged one, where the peaks derived from (020) and (040) shift to a lower angle (b axis means the vertical direction to V2O5 layer). This result shows an extension of interlayer distance from 5.76 Å to 6.15 Å, indicating intercalation of Mg2+, not co-intercalation of Mg2+ and organic solvent.