VO₂ (A) nanostructures with controllable feature sizes and giant aspect ratios: one-step hydrothermal synthesis and lithium-ion battery performance

Abstract

In this paper, uniform 1D VO₂ (A) nanostructures with controllable morphologies were prepared via an easy one-step hydrothermal method for the first time. Aspect ratios are tunable from 10 : 1 to 1000 : 1 by changing the synthesis parameters such as the concentration of vanadium source and the reaction time. The mechanism of morphology evolution was investigated and discussed based on the nucleation and growth process of VO₂ (A) particles. Electrochemical analyses of VO₂ (A) nanostructures were performed, and the results showed a capacity of 277 mAh g⁻¹ for a Li-ion battery using these nanostructures as cathode materials. This shows a significant improvement compared with other vanadium oxides such as VO₂ (B) (approximately 180 mAh g⁻¹) and V₂O₅ (approximately 230 mAh g⁻¹). Unfortunately, the VO₂ (A) nanostructures exhibit high initial irreversible loss due to the formation of an intermediate layer after electrochemical reactions.

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Introduction

Due to the worldwide growing energy crisis, energy storage is more important today than ever before. Rechargeable lithium-ion batteries (LIBs) are a good choice for energy storage.^1–3 Currently, the LIBs have powered hybrid electric vehicles and most modern portable electronic devices such as computers, cell phones, and cameras.⁴ However, in order to develop financially marketable, high-quality LIBs for a potentially massive market, further developments in safety, capacity, charge/discharge rates, and service life are still required.^5,6 Among the components required of a battery containing a cathode, anode and electrolyte, the quality of the cathode material which concurrently stores electrons and Li⁺ is the bottleneck of LIB's performances. The exploration of novel materials and technological solutions are two major research focal points.^7,8

Most cathode materials used in LIBs have a layered structure. Vanadium oxide,^9–17 especially V₂O₅,^11–16 has been studied as a cathode material for decades,¹⁸ namely due to its high energy-density (the theoretical capacity can reach 440 mAh g⁻¹ by inserting 3 Li⁺ in one molecular V₂O₅) and low cost.¹⁹ The layered structure of V₂O₅ is comprised of corner sharing VO₅ square pyramids which form a sheet (Fig. 1a). This structure provides a variety of sites that are available for lithium intercalation. However, the low diffusion coefficient of lithium ions (D∼10⁻¹² cm² s⁻¹),²⁰ poor electrical conductivity (10⁻² to 10⁻³ S cm⁻¹)²¹ and limited cycling stability²² of vanadium pentoxide electrodes hinder the practical application of this material.

Fig. 1 (a) The crystal structure of V₂O₅ projected along the b-axis; (b) the crystal structure of VO₂ (B) projected along the b-axis; (c) the crystal structure of VO₂ (A) projected along the c-axis.

Vanadium dioxide (VO₂) exhibits a number of polymorphs.²³ The thermodynamically stable phase of VO₂ is a rutile type structure that undergoes a sharp, first order metal-to-insulator phase transition accompanied by a structural change from a high temperature, metallic rutile structure VO₂ (R) (P4₂/mnm, 136) to a low temperature, insulating monoclinic form VO₂ (M) (P2₁/c, 14),²⁴ and has been studied abundantly for applications to smart windows.^25–28 Another two polymorphs, VO₂ (A) (P4₂/ncm, 138) and VO₂ (B) (C2/m, 12) are thermodynamically metastable phases that also have layered structures similar to V₂O₅ (Fig. 1) and are assessed as the cathode material of LIBs. As shown in Fig. 1b, two edge-sharing octahedra in the VO₂ (B) structure form an octahedral layer. These adjacent octahedral layers are linked to each other by corner sharing along the c-axis to construct a three-dimensional framework.^23,29 The tunnels in VO₂ (B) function as pathways for the Li⁺ diffusion during insertion and extraction. VO₂ (A) shares a similar structure as VO₂ (B) except with two differences (Fig. 1b–c). First, VO₆ octahedra in VO₂ (B) are slightly distorted. Second, the superposition of VO₆ sheets is different; two VO₆ sheets form a layered unit that stacks along the direction of the [110] plane in VO₂ (A) and the [010] plane in VO₂ (B).^30,31 Previous research shows that VO₂ (B) is a promising cathode material for LIBs.^{10,29,32–37} However, VO₂ (A) can also be considered as an alternative, but has not been reported until now.

The existence of the VO₂ (A) polymorph was first reported as an intermediate phase of the transformation from VO₂ (B) to VO₂ (R) by Théobald in a study on the hydrothermal reaction of the V₂O₃–V₂O₅–H₂O system.³⁸ Decades later, Y. Oka thoroughly studied the crystal structures of VO₂ (A) phases.^30,39,40 Since then, there were almost no reports on VO₂ (A) for a long time. Recently, Jin et al. synthesized pure phase VO₂ (A) micrometer-scale rod-like powders with tens of micrometers in length and several micrometers in width via the reduction of V₂O₅ by oxalic acid through hydrothermal treatment.⁴¹ However, to date, there is still no study showing a tunable preparation for low-dimensional nanopowders and/or the investigation of the properties and applications of VO₂ (A), which implies that the controllable preparation of VO₂ (A) nanostructures is still a challenge.

The crystalline structure and morphology of the cathode materials have a dominant influence on the storage capacity and Li⁺/e⁻ transport in LIBs. Nanostructured cathode materials have short Li-ion insertion/extraction distances and large surface to volume ratios, which can effectively improve the performance of LIBs.^42–47 Specifically, one-dimensional (1D) nanostructured materials that possess excellent surface activities have attracted much attention for cathodes in LIBs.^48–56 Recently, a review article summarizing the synthesis and the battery properties of some typical vanadium oxide nanowires, such as VO₂ (B), V₂O₅, and hydrated vanadium oxides, has been published.⁵⁷ However, no battery information about nanostructured VO₂ (A) is discussed.

This current study reports the synthesis and the property studies of VO₂ (A) nanopowders as a cathode material for LIBs. A hydrothermal method using a novel precursor for the vanadium source was developed to controllably synthesize a series of VO₂ (A) nanostructures including nanorods, nanowires and nanobelts with tunable aspect ratios from 10 to 1000. The morphology control could easily be achieved by varying the synthesis parameters. The electrochemical performance of the nanostructured VO₂ (A) was investigated to determine its potential use as a cathode material in LIBs. The capacity of VO₂ (A) nanowires was 277.1 mAh g⁻¹, which is an encouraging result compared to other vanadium oxide materials. This finding suggests that VO₂ (A) is a promising new cathode material for LIBs. As the first article to report on the battery property of VO₂ (A), the result is very interesting and should attract continuous attention with regard to researching the applications of VO₂ (A) nanostructures.

2. Experimental section

2.1 Chemical reagents

All chemical reagents used, including vanadyl sulfate (VOSO₄), ammonia water (NH₃·H₂O) and hydrazine hydrate (N₂H₄·H₂O), were of analytical grade without further purification.

2.2 Sample preparation

Three types of VO₂ (A) nanostructures were synthesised via a one-step hydrothermal process using VOSO₄ as the vanadium source. In a typical procedure, VOSO₄ powders were dissolved in 100 mL deionised water under magnetic stirring to produce a clear, blue solution. An appropriate amount of hydrazine hydrate was added to the solution under continuous stirring. The pH of the resultant solution was adjusted to pH7 using a NH₃·H₂O (28 wt %) solution. A brown precursor formed during the addition of NH₃·H₂O solution. The precursor was collected and then dispersed in 30 mL of deionised water. The total volume of the precursor solution was transferred into a 50-mL teflon-lined stainless-steel autoclave followed by hydrothermal treatment under auto-generated pressure at 260 °C for different periods of time, ranging from 2 to 16 h. The final black product was collected by filtration or centrifugation, washed with deionised water and ethanol several times, and then dried in air at 80 °C for 12 h.

2.3 Sample characterization

The morphologies and sizes of the resulted products were observed by field-emission scanning electron microscopy (FESEM, JSM 6700F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM2010, JEOL, Tokyo, Japan). The crystal structures of the resultant products were characterized with a Japan Rigaku D/max 2550 V X-ray powder diffractometer (Cu-Kα, λ = 0.15406 nm). N₂ adsorption-desorption isotherms at 77 K were measured on a Micrometitics Tristar 3000 system. Specific surface area was calculated by using Langmuir methods.

2.4 Electrochemical measurements

The charge/discharge tests were performed at ambient temperature. The working electrode was fabricated by compressing a mixture of the active materials (the as-prepared VO₂ (A) powders), conductive materials (acetylene black, AB), and binder (poly (vinylidene) fluoride, PVDF) with a weight ratio of VO₂/AB/PVDF of 8 : 1 : 1 onto an aluminium grid and dried at 80 °C for 4 h. The cathode was typically punched in the form of disk with a diameter of 10 mm. Then the electrode was dried again at 120 °C for 5 h in a vacuum before assembly. Finally, the prepared cathode and Celgard2400 separator (diameter of 16 mm) were placed into an argon atmosphere filled glove box (H₂O and O₂ < 1 ppm) and assembled into a coin cell (CR2032) with a lithium anode, electrolyte of 1 M LiPF₆ in EC-DEC-DMC (1 : 1 : 1, v/v/v) and all other components of a typical coin type cell. The cells were charged and discharged at the current density of 90 mA g⁻¹ in a voltage range of 1.25–4.5 V on a LAND CT2001A cell test instrument at room temperature.

3. Results and discussion

The crystal structure and phase composition of the obtained products were revealed by XRD, as shown in Fig. 2a. All the diffraction peaks of the as-prepared product match well with the standard XRD pattern of a VO₂ (A) phase (space group: P4₂/ncm, JCPDS No. 42-0876) and no peaks of any other vanadium oxide phases or impurities were detected. The strong and sharp diffraction peaks suggest high crystallinity of the products. The relative intensities of the (nn0) peaks are stronger than those of the JCPDS card, indicating crystalline orientation along the [110] direction, which is in agreement with high-resolution TEM analysis (Fig. 2d). A panoramic view demonstrates that the entire VO₂ (A) sample consists of nanowires with a uniform size, as shown in the overview of SEM and TEM images (Fig. 2b, c). The average diameter and length of the nanowires is estimated to be about 70 nm and tens of micrometres, respectively. The aspect ratio of as-prepared VO₂ (A) nanowires can reach 1000. The growth direction of the VO₂ (A) crystal may be determined by the relative stacking rate of the VO₆ octahedral at various crystal faces. As for the crystal structure of VO₂ (A), the V–O distance along the [110] direction is 0.1784 nm which is shorter than that along the other directions. This indicates that the (110) plane has a relatively high stacking rate, which favours growth along the [110] axis.

Fig. 2 (a) XRD pattern; (b) Overview SEM image; (c) TEM image of VO₂ (A) nanowires that were obtained by hydrothermal treatment at 260 °C for 8 h in the presence of 0.025 mol L⁻¹ VOSO₄, (d) High-resolution TEM image of the circular area in (c), inset is the corresponding SAED pattern.

The aspect ratio of 1D VO₂ (A) nanostructures can be tuned by varying the concentrations of the vanadium source. The VO₂ (A) nanowires with a diameter of 70 nm and a length of over 20 μm that represents an aspect ratio of 300 were obtained by using 0.025 M of V⁴⁺ (Fig. 3a). However, doubling the V⁴⁺ concentration produced VO₂ (A) nanobelts that were 100 nm in width and up to about 10 μm in length with an aspect ratio of 100 (Fig. 3b). Further increasing the vanadium source concentration to 0.1 M resulted in the formation of a large quantity of VO₂ (A) nanorods that were several micrometres in length and 200–500 nm in width with an aspect ratio of 10–50 (Fig. 3c). All three nanostructures described were of a pure VO₂ (A) phase and no other phase peaks or impurities were detected (Fig. 3d). The relative intensity of the (nn0) diffraction peaks in the XRD patterns of the three nanostructures were obviously stronger than others, especially for nanowires, suggesting an orientation growth along [110], which was in accord with the TEM results.

Fig. 3 (a,b,c) TEM images and (d) XRD patterns of the products obtained by hydrothermal treatment at 260 °C for 8 h in solutions with different concentrations of VOSO₄: (a) The VO₂ (A) nanowires obtained in the presence of 0.025 mol L⁻¹ VOSO₄; (b) The VO₂ (A) nanobelts obtained in the presence of 0.05 mol L⁻¹ VOSO₄; (c) The VO₂ (A) nanorods obtained in the presence of 0.1 mol L⁻¹ VOSO₄.

In order to understand the formation and growth processes of the 1D VO₂ (A) nanostructures, the samples prepared at the vanadium concentration of 0.1 M were collected at different reaction times, and the corresponding time-resolved TEM and SEM images of the products are shown in Fig. 4a–g. During the initial stages (less than 2 h), the VO₂ (A) mainly grows along the [110] axis by orientation attachment of VO₂ (A) nanosilks to form nanowires, which can be directly observed from the TEM image (Fig. 4a) and can also be confirmed by the selected area electron diffraction (SAED). The {110} plane shows the highest surface energy and is eliminated. After growth for another 1 h, VO₂ (A) nanowires begin to attach themselves to each other by a two-step process (Fig. 4b–c): contact–combination. When the reaction time is prolonged to 4 h, the VO₂ (A) nanowires develop into a nanobelt, and then further grow to a nanorod through continued attachment of other nanowires and nanobelts (Fig. 4d). The inserted high resolution-TEM shows that there is a clear interface (approximately 6 nm) between nanobelts and nanowires. By further increasing the reaction time to 8 h, VO₂ (A) nanobelts begin to attach and stack along the thickness orientation, resulting in the formation of VO₂ (A) nanorods (Fig. 4e–f). Further increasing the reaction time only yields gradual increases in size, but no further changes in morphology occurred (Fig. 4g). These findings suggest that 1D VO₂ (A) nanostructures grow through the attachment and combination model. In this formation process, the concentration of vanadium has a similar impact with reaction time. When the concentration of the vanadium source is too low, the VO₂ (A) grows along [110] axis by consuming large amounts of the vanadium source. This inhibits the development of other crystal planes, thereby only forming nanowires even after a reaction time of 16 h (Figure S1, ESI†), which corresponds to the lowest energy principle.

Fig. 4 (a–g) TEM images and SEM image of the products by hydrothermal treatment in the presence of 0.1 mol L⁻¹ VOSO₄ at 260 °C for different reaction stages. (a) the reaction time is 2 h, the insets are the corresponding SAED patterns; (b–c) the reaction time is 3 h; (d) the reaction time is 4 h, the inset is the corresponding HRTEM images and the scale bar is 5 nm; (e–f) the reaction time is 8 h; (g) the reaction time is 12 h.

Based on the above results, the self-assembly oriented attachment growth mechanism of 1D nanostructures VO₂ (A) was schematically described in Scheme 1. The oriented attachment growth mechanism was first observed in the hydrolytic synthesis of titania by Penn and Banfield.⁵⁸ This mechanism is quite distinct from Ostwald ripening, which involves the dissolution of fine particles and growth of larger particles. Nanoparticles generated in solution have a large surface-to-volume ratio, i.e. a large surface energy, which can be reduced by the oriented attachment growth and is the basis of the mechanism. Among the crystal planes of VO₂ (A), {110} has the highest surface energy, and is thermodynamically favourable to be removed. At low concentrations of the vanadium precursor, the {110} crystal plane adsorbs and consumes most of the vanadium source; the nanocrystals are grown mainly along the [110] direction to form nanowires. When the concentration of vanadium source is high enough, some of the vanadium source can be consumed to make the crystal planes of low surface energies begin to grow. By prolonging the reaction time, the appearance of different orientations increases significantly. These results show that the vanadium concentration is the main requirement for morphology evolution, while the reaction time mainly influences the size of 1D VO₂ (A) nanostructures. The combination of vanadium concentration and reaction time can lead to different VO₂ (A) nanostructures such as nanowires, nanobelts and nanorods.

Scheme 1 Formation mechanism of 1D VO₂ (A) nanostructures.

The electrochemical properties of the LIBs using the as-prepared VO₂ (A) nanoparticles as cathode materials were investigated. The discharge curves and the corresponding cycle behaviour of the cell, which was cycled at a current density of 90 mA g⁻¹ in a potential window of 1.5–4.0 V are shown in Fig. 5. The nanowire and nanobelt cathodes show a remarkably large initial discharge capacity of 277.1 mAh g⁻¹ and 254.9 mAh g⁻¹ (Fig. 5a), which is almost twice that of the nanorod structure (162.7 mAh g⁻¹). Similarly, VO₂ (A) nanowires exhibit the best cycling performance among three nanostructures: 116 mAh g⁻¹ (nanowire) vs. 66.3 mAh g⁻¹ (nanobelt) vs. 53.1 mAh g⁻¹ (nanorod) at 50C. The VO₂ (A) nanowires offer improved energy storage capacity and better cycling stabilities, namely due to their large surface areas; the BET specific surface area of nanowires is 100.22 m² g⁻¹, much higher than 56.95 m² g⁻¹ and 10.42 m² g⁻¹ for those of nanobelts and nanorods, respectively; a large specific area permits a high contact area with the electrolyte and short distance for lithium-ion and electron diffusion. The diffusion time varies as the square of the length, so the greater available surface area and reduction of particle sizes result in orders of magnitude increases in lithium insertion/extraction kinetics and lithium-ion flux across the electrode/electrolyte interface.

Fig. 5 (a) First discharge profiles of as-obtained VO₂ (A) nanostructures cycled between 1.5 and 4.0 V at a current density of 90 mA g⁻¹. (b) Cycling performance of three VO₂ (A) nanostructures at a rate of 90 mA g⁻¹.

However, both VO₂ (A) nanowires and nanobelts exhibit a higher initial irreversible capacity loss of 16% and 17%, which are much larger than VO₂ (A) nanorods (3.5%). These losses are probably due to the irreversible phase transition and the undesirable reactions with liquid electrolytes during change–recharge cycling, as reported for other vanadium oxides in the references.^59,60 Actually, the formation of an intermediate layer on VO₂ (A) nanowires has been experimentally observed by comparing the TEM images (Fig. 6) of VO₂ (A) nanowires and nanorods before and after charge–discharge treatments. A clear amorphous film was formed on the superficial layer of the nanowires after charge–discharge treatments, but this phenomenon was not observed for the nanorods. Although the XRD patterns (Fig. 7) of VO₂ (A) nanowires and nanorods after charge–discharge treatments don't reveal any impure phases, it is obvious that the nanowires are instable during charge–discharge treatments. The nanowires possessing a large surface area and small dimensions lead to increased chemical sensitivity to impurities, contaminants, and side reactions with the electrolyte. Also, the thickness of the solid electrolyte interface (SEI) layer may be larger than in the case of bulk material impeding electron and Lithium-ion transport in and out of the structure. These results serve to illustrate the importance of controlling the dimensions of nanostructure materials to optimize performance, e.g., to form an insulating layer on VO₂ (A) nanowires to prohibit the contact with electrolytes.

Fig. 6 TEM images of VO₂ (A) nanowires (a, b) and nanorods (c, d) before (a, c) and after (b, d) charge–discharge treatments at 90 mA g⁻¹ for one cycle. The insets in (b) and (d) are the corresponding HRTEM images with scale bars of 5 nm.

Fig. 7 XRD patterns of VO₂ (A) nanowires and nanorods after one-cycle charge–discharge treatments at 90 mA g⁻¹.

Conclusions

Different morphologies of VO₂ (A) nanostructure are controllably prepared by the hydrothermal treatment of a vanadium precursor. The aspect ratios of 1D VO₂ (A) nanostructures can be effectively tuned from 10 to 1000 by changing the vanadium source concentration and reaction time. When evaluated as potential cathode materials for LIBs, VO₂ (A) nanowires exhibit the highest capacity (the first discharging capacity was 277.1 mAh g⁻¹) and excellent cycling performance (the 50th discharging capacity was 116 mAh g⁻¹), which could be ascribed to the huge surface area and short Li⁺ transport distance. This finding suggests that VO₂ (A) is a promising new cathode material for LIBs.

Acknowledgements

This study was supported in part by the One-Hundred-Talent Program of CAS, National Key Basic Research Project (NKBRP, 2009CB939904), the NSFC (50772126, 51172265, 51032008)), Shanghai Basic Research Project (09DJ1400200, 08JC1420300), the key project of CAS (4912009YC006), Shanghai Key Basic Research Program (09DJ1400200), and high-tech project of MOST (2012AA030605, 2012BAA10B03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra20587d

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