Li[sup +] Storage Sites in Amorphous V[sub 2]O[sub 5] Prepared by Precipitation Method

Amorphous vanadium oxide ( a ­ V 2 O 5 ) powders were prepared by acidifying aqueous NH 4 VO 3 solution with concentrated nitric acid. Samples having a different degree of layer stacking and surface area were obtained either by changing the NH 4 VO 3 concentration or by employing additional solvent exchange process. The pentane-exchanged precipitate gave the largest surface area ( 60 m 2 g ­ 1 ) after vacuum drying at 100°C for 24 h. This electrode material delivered an initial discharge capacity of 426 mAh g − 1 in the voltage range of 1.5-4.0 V ( vs. Li / Li + ) , which amounts to 2.9 equiv Li + ions per mol of V 2 O 5 . X-ray absorption near-edge spectra (XANES) clearly showed a vanadium reduction down to V(III) when Li + ions were inserted at Li + / V 2 O 5 > 2.0 . The Li + 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 Li + storage sites were identified. The discharging capacity at 1.7 V was strongly correlated with the surface area of electrode material, suggesting that Li + 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 Li + 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.