Li+ Storage Sites in Amorphous V2O5 Prepared by Precipitation Method

Abstract

Amorphous vanadium oxide (a-V2O5) powders were prepared by acidifying aqueous NH4VO3 solution with concentrated nitric acid. Samples having a different degree of layer stacking and surface area were obtained either by changing the NH4VO3 concentration or by employing additional solvent exchange process. The pentane-exchanged precipitate gave the largest surface area (60 m2 g-1) after vacuum drying at 100°C for 24 h. This electrode material delivered an initial discharge capacity of 426 mAh g21 in the voltage range of 1.5-4.0 V ~vs. Li/Li1), which amounts to 2.9 equiv Li1 ions per mol of V2O5 . X-ray absorption near-edge spectra ~XANES! clearly showed a vanadium reduction down to V~III! when Li1 ions were inserted at Li1/V2O5 . 2.0. The Li1 storage sites were analyzed by correlating the peak intensity in differential capacity plots to the surface area and degree of layer stacking, from which two different Li1 storage sites were identified. The discharging capacity at 1.7 V was strongly correlated with the surface area of electrode material, suggesting that Li1 ions are inserted into the amorphous region at this potential. The intensity of 2.5 V peak was, however, proportional to the peak intensity of ~001! diffraction, illustrating that Li1 ions are inserted into the quasi-ordered layer stacking region at this potential. The latter feature was further confirmed by X-ray diffraction analysis, whereby it was found that the interlayer spacing decreases most significantly near 2.5 V along with a sharp decrease in the ~001! diffraction intensity.