One-pot synthesis of carbon-coated VO₂(B) nanobelts for high-rate lithium storage

Abstract

Uniform carbon-coated single crystalline vanadium dioxide (VO₂(B)@C) nanobelts were successfully prepared by using a facile one-pot hydrothermal approach. Sucrose plays a dual role in this hydrothermal process, namely as a carbon precursor for the carbon shell and, as a reductant to reduce V₂O₅ to VO₂(B). The thickness of the carbon coating layer is tunable from 3.0 to 6.9 nm by changing the ratio of the precursors. Although a high carbon content can improve the electrical conductivity of VO₂(B)@C nanobelts, a thick carbon coating layer would block the lithium ion diffusion. The optimal thickness is found to be 4.3 nm (carbon content: 6.6 wt%), where the cathode displays superior performance with highly reversible specific capacities, good cycling stabilities and excellent rate capabilities (e.g. 100 mA h g⁻¹ at 12.4 C).

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Introduction

Vanadium oxides with multiple valence states from +2 in VO to +5 in V₂O₅ have been extensively studied as well-known transition-metal oxides for the potential applications in catalysis,¹ sensors,² optical switching devices³ and high energy density lithium ion batteries (LIBs).^4–10 For LIB applications, vanadium oxides (e.g.V₂O₅, V₆O₁₃ and VO₂(B)etc.) offer the advantages of high capacity, simple synthesis procedure, low cost and being abundant with respect to the commercial cathode LiCoO₂.^7,11–13V₂O₅ is known to undergo a series of phase transformation during Li⁺ intercalation/deintercalation processes, giving theoretical capacities up to 294 (x = 2 in Li_xV₂O₅) and 442 mA h g⁻¹ (x = 3 in Li_xV₂O₅). However, V₂O₅ suffers from a large irreversible capacity at a deep discharge/charge depth (x > 1 in Li_xV₂O₅), and thus affects the battery cycling performance.⁷ Beside V₂O₅, vanadium oxides with the nominal stoichiometry of VO₂₊x (0 ≤ x ≤ 0.33), e.g.VO₂(B), V₆O₁₃, V₄O₉ and V₃O₇, can also serve as promising cathode materials for LIBs.^7,14 Among them, VO₂(B) is found to show a better cycling stability than V₂O₅. For example, VO₂(B) nanofiber aerogels prepared through a sol–gel method exhibited a capacity retention of ∼80% between 4.0 and 1.5 V at C/8 after 20 cycles, while V₂O₅ nanorods obtained by annealing the VO₂(B) aerogels under air only attained a capacity retention of ∼55% under the same testing conditions.⁷ This is mainly due to the higher structural stability of VO₂(B), which arises from the increased edge sharing and the consequent resistance to lattice shearing during cycling,⁴ and emerging of undesired agglomerate irregular V₂O₅ particles during the annealing process. The capacity retention of V₂O₅ based electrodes can be improved by a sol–gel route,^15,16e.g., showing a value of ∼75% at C/20 after 10 cycles¹⁵.

To develop high-performance VO₂(B) based cathodes, especially at high current densities, one-dimensional (1D) nanowires or nanobelts are the most favourable as they can offer a range of unique advantages over their traditional counterparts including large interfacial contact area between the electrode and electrolyte, short Li⁺ transport distance, efficient 1D electron transport pathways and facile strain relaxation upon electrochemical cycling.^6,17,18 For example, an electrode platform¹⁸ was developed to enable silicon nanowires to grow directly onto the current collector to accommodate the large volume change and to avoid capacity fading. Similar enhanced Li storage properties could also be obtained in 1D VO₂(B) nanofibers,⁷nanorods,¹⁹nanowires,²⁰ and nanobelts.^21,22 Although the VO₂(B) nanostructures were reported to show improved cyclability at relatively low current densities (e.g. capacity retention: 75% during the first 100 cycles at 1 C)²¹, their rate capabilities are still not satisfactory. For example, a VO₂(B) nanobelts/multiwall carbon nanotube composite²¹ exhibited capacity retentions of 77% (0.3 C), 65% (1 C), 49% (3 C) and 39% (5 C), respectively, on the basis of the 2nd discharge capacity (327 mA h g⁻¹) at 0.1 C. Such performance is significantly lower than other reported high-performance cathodes such as flower-shaped LiFePO₄/C/polypyrrole microspheres,²³ with capacity retentions of 92% (2 C), 81% (5 C) and 63% (10 C), respectively, based on the 2nd discharge capacity of 136 mA h g⁻¹ at 0.5 C. Hence, further performance improvement on VO₂(B) based cathode electrodes is highly desired.

Coating active materials with a conductive and elastic carbon layer is a simple and effective strategy to improve the rate capability and cycling stability of electrodes, where the carbon coating can effectively (1) enhance the kinetics of charge transfer, (2) buffer the strain caused by structural or volume changes and (3) prevent the aggregation of active materials during cycling.^24–32 In this regard, cathodes with superior performance for LIBs have been successfully developed by coating a thin carbon layer onto the surface of LiCoO₂,²⁷LiMn₂O₄,²⁸LiFePO₄^29,30 and Li₃V₂(PO₄)₃.^31,32 However, a thick carbon coating layer would hinder the Li⁺ diffusion.^32,33 Therefore, the thickness of the carbon layer needs to be adjusted to achieve optimized Li storage performance.

Herein, we report a facile one-pot hydrothermal approach to synthesize uniform carbon-coated VO₂(B) (denoted as VO₂(B)@C) nanobelts. The thickness of carbon coating layer is controlled by changing the concentration of the precursors, which can affect the Li storage performance of the VO₂(B)@C samples. The VO₂(B)@C samples with different thicknesses of the carbon layer show high specific capacities and stable cyclability at a current density of 100 mA g⁻¹ (0.6 C). It is found that a thick carbon layer may decrease the apparent diffusion coefficient of lithium ions although it can increase the electrical conductivity and buffer the structural strain during the lithiation process. The VO₂(B)@C cathode with a carbon content of 6.6 wt% (e.g. the thickness of carbon shell is ∼4.3 nm) shows an optimized Li ion storage performance at high current densities, achieving a highly reversible discharge capacity of 100 mA h g⁻¹ at a current density of 2000 mA g⁻¹ (12.4 C).

Experimental

Synthesis of VO₂(B)@C nanobelts

In a typical procedure, 50 mg V₂O₅ (Alfa Aesar) and 10 mg sucrose (C₁₂H₂₂O₁₁, Sigma-Aldrich) were put in 40 mL distilled water under ultrasonication to give a yellow suspension. Successively, the suspension was transferred to a 50 mL autoclave and kept in an oven at 180 °C for 24 h. After cooling to room temperature, a black product was collected and washed with distilled water and ethanol solution several times, and then dried in a vacuum oven at 70 °C for 12 h. During the hydrothermal process, sucrose serves as a carbon precursor for the carbon shell as well as a reductant to reduce V₂O₅ to VO₂(B), since the sucrose possesses the same hydroxyl group as the alcohol.³⁴ To obtain a thicker carbon coating layer, the quantity of sucrose was sequentially increased to 20, 30 and 40 mg.

Materials characterization

X-Ray powder diffraction (XRD) patterns were recorded on a Bruker AXS D8 advance X-ray diffractometer at the 2θ range of 10 to 60° using Cu Kα radiation. The morphology of the VO₂(B)@C samples was investigated by using a field-emission scanning electron microscopy (FESEM) system (JEOL, Model JSM-7600F), and the nanostructure was characterized by using a transmission electron microscopy (TEM) system (JEOL, Model JEM-2010) operating at 200 kV. Thermogravimetry analysis was carried out from room temperature to 550 °C at a heating rate of 10 K min⁻¹ in air using TGA Model Q500. Raman spectra were obtained by WITec CRM200 confocal Raman microscopy system at a laser wavelength of 488 nm and a spot size of 0.5 mm. X-Ray photoelectron spectroscopy (XPS) measurement was performed on VG Escalab 250 spectrometer using an Al Kα 1846.6 eV anode.

Electrochemical measurements

The coin-type cells were assembled in an argon-filled glove-box, where both moisture and oxygen levels were less than 1 ppm. The cathodes were fabricated by mixing VO₂(B)@C nanobelts, carbon black and poly(vinyldifluoride) (PVDF) at a weight ratio of 70 : 20 : 10 in N-methylpyrrolidone (NMP) solvent. The resulting mixture was then pasted onto the aluminum foil and punched into small disks (Ø = 14 mm). The mass loading was around 2.0 mg. Lithium foils were used as anodes and the electrolyte solution was made of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, w/w). The cells were tested on a NEWARE multi-channel battery test system with galvanostatic charge and discharge in the voltage range 4.0–2.0 V. For current density, 1 C equals 161 mA g⁻¹ based on the theoretical capacity (161 mA h g⁻¹) of VO₂(B) between 4.0 and 2.0 V. The capacity values in this paper were estimated based on the bare VO₂(B) without the amorphous coating carbon and carbon black. The electrochemical impedance spectra (frequency range: 0.001∼10⁵ Hz) were performed with an electrochemical workstation (CHI 660C). Cyclic voltammetry of VO₂(B)@C electrodes in coin-type cells were performed with a Solartron analytical equipment (model 1470E) at different scan rates.

Results and discussion

The crystal structure of the as-prepared samples obtained at different weight ratio of V₂O₅ to sucrose (herein known as I_V2O5:sucrose) in the hydrothermal process was examined by X-ray diffraction (XRD). Fig. 1 reveals that all the reflection peaks of the four samples can be indexed to the monoclinic VO₂(B) phase (space group: C2/m, JCPDS 31-1438). No detectable crystalline carbon peaks can be observed. It has been reported that the carbon derived from sucrose (or glucose) under hydrothermal conditions is amorphous.^25,35,36 The nature of the carbon coating of the VO₂(B) products was studied using Raman spectroscopy (see ESI,† Fig. S1a). Two characteristic bands of carbonaceous materials were detected at 1370 cm⁻¹ (D-band) and at 1600 cm⁻¹ (G-band), which agree well with those of amorphous carbon.^23,37 The carbon structure was further analyzed by X-ray photoelectron spectroscopy (XPS) C 1s spectrum (see ESI,† Fig. S1b). The spectrum was fitted with two components, centered at 284.5 and 285.7 eV, which correspond to sp² and sp³ bonded carbon, respectively. The ratio of sp² to sp³ was then determined to be as high as ∼2.5, which is comparable to that of amorphous carbon thin films prepared by magnetron sputtering and cathodic arc deposition,³⁸ implying a relatively high electronic conductivity.

Fig. 1 XRD patterns of the as-prepared samples obtained at different I_V2O5:sucrose in the hydrothermal process. (a) I_V2O5:sucrose = 5.00, (b) I_V2O5:sucrose = 2.50, (c) I_V2O5:sucrose = 1.67 and (d) I_V2O5:sucrose = 1.25.

To determine the carbon content in the VO₂(B)@C composites, thermogravimetric analysis (TGA) was carried out in air (see Fig. 2). The weight loss of 1.7% below 100 °C is attributed to the evaporation of free water and physically adsorbed water. The weight changes in the temperature range 100–450 °C are ascribed to the burning-out of amorphous carbon (weight loss) and oxidization of VO₂ to V₂O₅ (weight gain). Based on the total weight changes, the carbon contents are estimated to be 4.2 wt%, 6.6 wt%, 8.4 wt% and 9.6 wt% for VO₂(B)@C composites prepared with I_V2O5:sucrose = 5.00–1.25, respectively. Thus, these four samples are named as VO₂(B)@C (4.2 wt%), VO₂(B)@C (6.6 wt%), VO₂(B)@C (8.4 wt%) and VO₂(B)@C (9.6 wt%).

Fig. 2 TGA curves of VO₂(B)@C composites prepared at different I_V2O5:sucrose values, (a) = 5.00, (b) = 2.50, (c) = 1.67 and (d) = 1.25.

The field emission scanning electron microscopy (FESEM) images show that the sample prepared with I_V2O5:sucrose = 5.00 is made up of nanobelts (see Fig. 3a–c) of 1–2 μm in length and 120–160 nm in width with a thickness of ∼25 nm. Decreasing the I_V2O5:sucrose values during the hydrothermal process from 5.00 to 2.50 and 1.67 does not lead to obvious changes in the nanobelt shape except for the slight decrease in the length (see Fig. 3d–i). Further decreasing I_V2O5:sucrose to 1.25 results in a significant decrease in the length of the sample to 200∼500 nm, making the sample nanosheets (see Fig. 3j–l). This is mainly attributed to the binding of the sucrose molecules onto the surface of VO₂(B) nanocrystals and hence confines their growth under high sucrose concentration. Thus, the sucrose does not only act as the carbon source but also as an effective surfactant similar to the synthesis of other reported nanocrystals.^35,39 Here, the formation of the VO₂(B)@C nanobelts is mainly ascribed to the anisotropic bonding of its layered structure, resulting in the faster growth along the [010] direction and the slower growth along [001] direction.⁴⁰ Meanwhile, the 1D soft templates formed by the stacking of the sucrose molecules may also contribute to the growth of 1D VO₂(B) nanocrystals.³⁹

Fig. 3 Representative FESEM images of VO₂(B)@C nanobelts. (a–c) VO₂(B)@C (4.2 wt%), (d–f) VO₂(B)@C (6.6 wt%), (g–i) VO₂(B)@C (8.4 wt%) and (j–l) VO₂(B)@C (9.6 wt%). Representative high-magnification FESEM images of (b, e, h and k) and (c, f, i and l) are the magnified view of VO₂(B)@C nanobelts to show the width and thickness of the nanobelts, respectively.

The nanostructure of VO₂(B)@C nanobelts was further examined by using the high resolution transmission electron microscopy (HRTEM). Fig. 4 reveals that the VO₂(B) nanocrystals are coated with a layer of amorphous carbon, which forms the VO₂(B)@carbon core–shell structure. Here, the carbon shell forms through the carbonization of the sucrose under hydrothermal conditions. The thickness of the carbon shell can be simply tuned by changing the values of I_V2O5:sucrose during the hydrothermal process. The thickness of the carbon layer increases from ∼3.0 to ∼6.9 nm when I_V2O5:sucrose is decreased from 5.00 to 1.25 (see Fig. 4). The lattice fringes observed from the HRTEM images indicate the single crystalline nature of the VO₂(B) nanobelts. The observed interplanar spacing of 0.34 nm in the HRTEM images of these four samples agrees well with the d value of the (110) plane of the monoclinic VO₂(B) (JCPDS 31-1438). The inset in Fig. 4d is the indexed fast Fourier transform (FFT) pattern of Fig. 4d.

Fig. 4 Representative HRTEM images of VO₂(B)@C nanobelts. (a) VO₂(B)@C (4.2 wt%), (b) VO₂(B)@C (6.6 wt%), (c) VO₂(B)@C (8.4 wt%) and (d) VO₂(B)@C (9.6 wt%). The inset in Fig. 4d is the FFT pattern of Fig. 4d.

To study the Li-ion storage properties of VO₂(B)@C nanobelts, a series of electrochemical measurements were carried out based on the half cell configuration.^41–43Fig. 5a shows the initial galvanostatic discharge (Li⁺ insertion) and charge (Li⁺ extraction) voltage profiles of the VO₂(B)@C electrodes at a current density of 100 mA g⁻¹ (0.6 C) between 4.0 and 2.0 V vs.Li⁺/Li. A typical long plateau at around 2.5 V is clearly observed in the discharge curves, which corresponds to a phase transition of crystalline VO₂(B) to Li_0.5VO₂(B) and the corresponding electrochemical reaction is written as:⁵

VO₂ + 0.5Li⁺ + 0.5e⁻ → Li_0.5VO₂ (1)

Half lithium insertion into VO₂ yields a theoretical capacity of 161 mA h g⁻¹. This reaction is highly reversible as indicated by the long plateau at ∼2.6 V in the subsequent charge process. As shown in Fig. 5a, the VO₂(B)@C (4.2 wt%), VO₂(B)@C (6.6 wt%), VO₂(B)@C (8.4 wt%) and VO₂(B)@C (9.6 wt%) electrodes depict initial discharge capacities of 160, 161, 159 and 158 mA h g⁻¹ with the subsequent charge capacities of 160, 161, 159 and 158 mA h g⁻¹, respectively, indicating no irreversible capacity loss (e.g. the coulombic efficiency is ∼100%). During the 50th cycle, the discharge capacities are 125, 128, 120 and 116 mA h g⁻¹ for VO₂(B)@C (4.2 wt%), VO₂(B)@C (6.6 wt%), VO₂(B)@C (8.4 wt%) and VO₂(B)@C (9.6 wt%) electrodes (see Fig. 5b), which are 78%, 80%, 76% and 73% of the initial capacity, respectively. Although initial capacities are relatively lower as compared to flexible free-standing VO₂(B) nanobelt films (218.7 mA h g⁻¹ at current density of 10 mA g⁻¹ in the voltage range 4.0–1.5 V),²² these cycling performances are much better than that electrode (53% of capacity retention at 50 mA g⁻¹ during the first 50 cycles) as well as other reported VO₂(B) and V₂O₅ cathodes, e.g., 50% of capacity retention for VO₂(B) nanorods between 3.44 and 1.5 V at 0.1 C during the 17 cycles,⁴⁴ 68% of capacity retention for VO₂(B) nanowires at 50 mA g⁻¹ (0.3 C) between 3.2 and 2.0 V during the first 50 cycles²⁰ and 61% of capacity retention for V₂O₅ submicron spheres at 0.125 C between 3.8 and 1.8 V during the first 35 cycles.⁴⁵

Good rate capability is one of the keys in developing high power/fast charging lithium ion batteries. The cycling responses of the VO₂(B)@C electrodes at different C rates are evaluated and are shown in Fig. 5c. Among the four types of cathodes, the VO₂(B)@C (6.6 wt%) electrode shows the best rate capability, delivering discharge capacities of 157, 155, 146 and 136 mA h g⁻¹ during the 2nd cycle at current densities of 200 (1.2 C), 500 (3.1 C), 800 (5.0 C) and 1000 mA g⁻¹ (6.2 C), respectively. Even at a very high current density of 2000 mA g⁻¹ (12.4 C), the VO₂(B)@C (6.6 wt%) electrode can still achieve a high specific capacity of 100 mA h g⁻¹ during the 2nd cycle. The retention of the rate capacity is calculated to be 99% (3.1 C), 93% (5.0 C), 87% (6.2 C) and 64% (12.4 C) on the basis of the capacity value of 157 mA h g⁻¹ at 1.2 C. The other three electrodes exhibit relatively low specific capacities, especially at high C rates, e.g. showing discharge capacities of 90, 69 and 41 mA h g⁻¹ at 12.4 C for VO₂(B)@C (4.2 wt%), VO₂(B)@C (8.4 wt%) and VO₂(B)@C (9.6 wt%), respectively. The difference of the rate capabilities of these four cathodes is verified by the electrochemical impedance spectra (see Fig. 5d). The VO₂(B)@C (6.6 wt%) cathode shows the smallest radius of semi-circle in the Nyquist plots as compared to the other three, indicating the smallest charge-transfer resistance and the correspondence fastest kinetics of the Faradic reaction.⁴⁶

Fig. 5 Cathode performance of VO₂(B)@C nanobelts in the voltage range 4.0–2.0 V. (a) Initial galvanostatic charge–discharge voltage profiles at a current density of 100 mA g⁻¹. (b) Cycling performance at a current density of 100 mA g⁻¹. (c) Discharge capacities of VO₂(B)@C electrodes at various current densities of 200 to 2000 mA g⁻¹. (d) Electrochemical impedance spectra of VO₂(B)@C electrodes measured at the 4th fully discharged state. The high-middle frequency semicircle represents the charge-transfer process and a straight slopping line at low frequencies corresponds to Li⁺ diffusion in VO₂(B)@C electrodes known as Warburg impedance.

It is interesting to find out that the VO₂(B)@C (6.6 wt%) electrode shows the best rate capabilities among these four samples. Generally, a high carbon content should improve the electrical conductivity of the VO₂(B)@C composites. However, a thick layer of carbon coating may also block the lithium ion diffusion.^32,33 To verify this, cyclic voltammetry (CV) was used to estimate the apparent diffusion coefficient of lithium ions (D_Li⁺) in VO₂(B)@C nanobelt cathodes. Fig. S2 (ESI†) shows the CV plots of VO₂(B)@C nanobelts with different scan rates: 0.1, 0.2 0.3 and 0.5 mV s⁻¹. A pair of redox peaks is clearly seen in the CV curves, corresponding to the phase transition between VO₂ and Li_0.5VO₂ as indicated in eqn (1), which is consistent with the potential plateaus observed in the charge–discharge curves (see Fig. 5a). As shown in Fig. 6, it is found that the current of the oxidation peaks (i_p) has a linear relationship with the square root of scan rate (ν^1/2), which is typical of the equilibrium behaviour of an intercalation type electrode.⁴⁶ If the charge transfer at the interface is fast enough and the limiting step of the Li transfer is the lithium diffusion in the electrode, the relationship of the peak current and the CV sweep rate is given as:

Fig. 6 The relationship of the oxidation peak current (i_p) and the square root of scan rate (v^1/2).

where i_p is the peak current (A), n is the charge-transfer number, S is the contact area between electrode and electrolyte (here the geometric area of electrode, 1.54 cm², is used for simplicity), ν is the potential scan rate (V s⁻¹), and C_Li⁺ is the concentration of lithium ions in the cathode (mol cm⁻³), which is calculated as:

C _Li⁺= m/(N_A/V) (3)

Here, m = 0.5 is the lithium number in the unit cell of Li_0.5VO₂, N_A is Avogadro's constant (6.02 × 10²³), and V is the volume of unit cell of VO₂ ignoring slight volume changing during the lithium insertion and extraction processes. In addition, owing to comparable capacities (161–156 mA h g⁻¹) for four cathodes during the 3rd cycle (see Fig. 5b), it is reasonable to assume that the concentrations of Li⁺ within these cathodes are the same. Thus, based on the slopes in Fig. 6, the D_Li⁺ is calculated to be 8.6 × 10⁻¹⁰, 4.7 × 10⁻¹⁰, 3.6 × 10⁻¹⁰ and 2.6 × 10⁻¹⁰ cm² s⁻¹ for VO₂(B)@C(4.2 wt%), VO₂(B)@C(6.6 wt%), VO₂(B)@C(8.4 wt%) and VO₂(B)@C(9.6 wt%) cathodes, respectively. With increasing of carbon content, the value of D_Li⁺ decreases slightly, which results from the following two aspects: (1) it is more difficult for lithium ions to transport through thicker carbon coating layer; (2) a thicker carbon layer hinders the electrolyte penetration into electrode and thus reduces the interfacial contact area for Li⁺ insertion/extraction. Thus, in order to achieve high electrical conductivity, buffering of the structural strain and high value of D_Li⁺, a thickness of ∼4.3 nm for the carbon layer is ideal to achieve high Li storage properties, especially at high current densities, for the VO₂(B)@C cathodes.

Conclusions

We have developed a facile one-pot hydrothermal approach to synthesize uniform carbon-coated VO₂(B) nanobelts. The thickness of carbon coating layer is tunable from 3.0 to 6.9 nm by changing the ratio of the precursors. The electrochemical test shows that the VO₂(B)@C is promising as a high-power cathode material for LIBs. For example, the VO₂(B)@C electrode with a 4.3 nm carbon layer (e.g.carbon content: 6.6 wt%) depicts high specific capacities, good cycling stabilities and excellent rate capabilities (e.g. 100 mA h g⁻¹ at 12.4 C).

Acknowledgements

The authors gratefully acknowledge AcRF Tier 1 RG 31/08 of MOE (Singapore), NRF2009EWT-CERP001-026 (Singapore), Singapore Ministry of Education (MOE2010-T2-1-017), A*STAR SERC grant 1021700144 and Singapore MPA 23/04.15.03 RDP 009/10/102 and MPA 23/04.15.03 RDP 020/10/113 grant.

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Footnote

† Electronic Supplementary Information (ESI) available: Raman spectra and XPS of VO₂(B)@C composites and CV plots of VO₂(B)@C electrodes at various scan rates. See DOI: 10.1039/c1ra00698c/

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