Synthesis, structure and electrochemical performance of V₂O₅ nanostructures as cathode materials for advanced lithium-ion batteries

Vanadium oxide nanotubes (VONTs) were synthesised by hydrothermal treatment of a vanadium oxide xerogel mixed with primary amines. A systematic investigation into the factors affecting the structure of VONTs is presented. Amine molecules act as structure-maintaining templates preserving the layered structure of vanadium oxide lamina during scrolling to form VONTs. By varying the molar ratio of xerogel to amine additive, we show using electron microscopy, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy, that a ratio of 2 amine molecules per V2O5 structural unit optimizes the quality of the VONTs. This observation is consistent for a range of primary amines of different molecular chain lengths (from hexylamine to octadecylamine). The interlayer spacings for VONTs synthesised within this range of amine lengths, and amine-xerogel mixture ratios were measured by XRD and TEM. Correlation of these measurements with the vibrational characteristics of intercalated amines, shows conclusively that interdigitation of amines occurs between V2O5 lamina. This molecular interdigitation maintains and defines the structural consistency and quality of the scrolled layers in the VONT.

We show that as-synthesised VONTs suffer from severe capacity fading issues when tested as a cathode material for Li-ion batteries. Our results suggest that organic amne molecules embedded within the vanadium oxide interlayers impede the reversible intercalation of Li+ ions. We detail two methods for removing amine molecules and compare the treated products structurally and electrochemically. The first route involves the thermal treatment of as-synthesised VONTs to form poly-NRs. The second route involves an ion exchange treatment to partially substitute amine molecules with Na+ions to form Na-VONTs. We also anneal Na-VONTs to 600 o C to examine the effects of a combination of the two methods on electrochemical performance. The structural transitions which occur during both routes are characterised by TGA, XRD, electron diffraction, TEM, SEM, FTIR and X-ray photoelectron spectroscopy (XPS).

Thermal treatment of the nanotubes to 600 o C is shown to remove the organic template and cause a specific structural conversion from nanotubes to polycrystalline nanorods (poly-NRs). Annealing VONTs to this temperature is shown to significantly improve electrochemical performance when tested as a cathode material in Li-ion battery half-cells. Capacity fading issues associated with blocked intercalation sites on the (010) faces of layered vanadium oxide that form the nanotubes are reduced. The electrochemical response for Na-VONTs before and after thermal treatment is also discussed and compared to as-synthesised VONTs and poly-NRs vanadium oxide through galvanostatic cycling and high rate capability tests. Removing amine molecules via ion exchange is also shown to improve capacity values compared to the as-synthesised material. 

We detail how poly-NRs are the most promising cathode material out of all of the vanadium oxide nanostructure samples we investigated. Poly-NRs and Na-VONTs demonstrate significantly larger capacity values than as-synthesised VONTs, and Na-VONTs before and after thermal treatment. In a potential window of 4.0 – 1.2 V drawing 30 µA (C/20), poly-NRs exhibited enhanced initial specific capacities values of ~260 mAh g-1, compared to capacities obtained for VONTs (~6 mAh g-1), Na-VONTs (~100 mAh g-1) and Na-VONTs heated to after annealing (~40 mAh g-1). High rate capability tests demonstrate the stability of poly-NRs at different charging rates and long term cycle life testing indicates a high level of capacity retention over 500 cycles.