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DOI: 10.1039/C5RA02457A (Paper) RSC Adv., 2015, 5, 20692-20698

Copper vanadates/polyaniline composites as anode materials for lithium-ion batteries†

Shaoyan Zhang*a, Shengjie Pengb, Ruisheng Hua and Seeram Ramakrishnab
aCollege of Chemical Engineering, Shijiazhuang University, Shijiazhuang, Hebei Province 050035, China. E-mail: zsyedu@hotmail.com; Tel: +86-311-66617326
bMechanical Engineering, National University of Singapore, Singapore, 117574, Singapore

Received 8th February 2015 , Accepted 13th February 2015

First published on 13th February 2015

Abstract

In this study, a simple hydrothermal process has been carried out for the synthesis of Cu5(VO4)2(OH)4·H2O nanorods and Cu5V2O10 nanoparticles. The copper vanadates/polyaniline (CVO/PANI) core–shell composites have also been synthesized via a simple dip-and-dry method on the surface of Cu5(VO4)2(OH)4·H2O nanorods and Cu5V2O10 nanoparticles. When tested as anode materials in lithium-ion batteries, the CVO/PANI composites exhibit improved electrochemical properties of high discharge capacity, excellent cycling reversibility and rate capability, compared with the pure CVO materials. The results suggest that the conductive PANI nanolayer coating helps to preserve high capacity, maintain high electrochemical stability, and reduce charge transfer resistance during cycling performance.

Introduction

As important functional inorganic materials, transition metal vanadates have long been studied as potential battery materials for primary or secondary lithium-ion batteries (LIBs) applications owing to their layered nature and excellent kinetics.1–4 Among them, copper vanadates (CVO) have drawn significant attention as owing to their high theoretical capacity, safety, easy preparation, and low cost.4–8 For example, Tarascon and co-workers showed that Cu2.33V4O11 can operate through a reversible process involving lithium insertion and metal extrusion. The electrochemical capacity of the Cu2.33V4O11 cathode is almost double that of existing LiCoO2 electrodes (250–270 mA h g−1 compared with 140–150 mA h g−1) owing to the insertion/extraction of five or more Li+ per formula unit.9 Hu et al. reported that ε-Cu0.95V2O5 nanoribbons can react with 2.64 Li (292 mA h g−1) through a reversible process.5 However, the main challenge for employing copper vanadates as active materials for rechargeable lithium-ion batteries is the poor electrical conductivity and large volume change during discharge–charge process.10–12

In recent years, conducting polymers coating have been investigated as an effective way to improve the electrode performance, because it improves the surface electronic conductivity and the electric contact between particles and conducting agents.13–16 In addition, the soft polymer matrix also shows the function of accommodating the internal stress of electrodes that suffer from severe volume change. Among the conducting polymers, polyaniline (PANI) is particularly interesting due to its ease of doping, moderate conductivity, simple preparation, environmental stability, and low cost.17–21 The combination of PANI with electrode materials to prepare composites that combine the properties of both materials is a very promising approach to improve the electrochemical performance of the electrodes for applications in lithium-ion batteries and capacitors. For example, Chapal et al. reported a supercapacitor based on PANI-coated CoMoO4·0.75H2O nanorods with a specific capacitance of 380 F g−1 at 1 A g−1 current density.22 Karthikeyan et al. developed a Li/Li(Mn1/3Ni1/3Fe1/3)O2/PANI composite exhibited the reversible capacity of ∼127, ∼114 and ∼110 mA h g−1 at ultra-high current rate of 5, 30 and 40 C, respectively with exceptional cycleability between 2 and 4.5 V vs. Li.23 Ahn et al. produced a PANI-coated Li[Li0.2Co0.1Mn0.7]O2 nanodisks and the discharge capacity was improved by about 15%.24

Herein, for the first time, we successfully synthesized Cu5(VO4)2(OH)4·H2O nanorods via a template-free hydrothermal route and then converted the Cu5(VO4)2(OH)4·H2O precursors into Cu5V2O10 nanoparticles by calcination treatment. The PANI-coated CVO composites were prepared by a simple dip-and-dry method. Moreover, the electrochemical performances of the pure CVO and CVO/PANI nanocomposites were evaluated as an anode material for LIBs. The results showed that the as-prepared CVO/PANI nanocomposites show high discharge capacity, excellent cycling reversibility and rate capability, indicating that they are promising anode candidates for LIBs. The present strategy could provide an effective and facile technique for improving the cycling performance and rate capability of transition metal vanadate electrodes for applications in lithium-ion batteries.

Experimental

Synthesis of Cu5(VO4)2(OH)4·H2O nanorods and Cu5V2O10 nanoparticles

The Cu5(VO4)2(OH)4·H2O nanorods were synthesized by a simple hydrothermal procedure. In a typical synthesis, a solution containing 2 mmol CuSO4·5H2O and 2 mmol CH3COONa dissolved in 8.0 mL deionized water was added to a 25 mL beaker under constant electromagnetic stirring. Then 8 mL 0.25 M Na3VO4 solution was added drop wise into the solution under stirring. After the mixture had been stirred for about 10 min, the mixture was sealed in a Teflon-lined autoclave (20 mL), heated to 180 °C and maintained at that temperature for 20 h. After the reaction, the autoclave was cooled down naturally to room temperature. The final products were collected through centrifugation, washed several times with deionized water and ethanol and finally dried in a vacuum oven at 60 °C for 12 h. To get the Cu5V2O10 nanoparticles, the as-prepared Cu5(VO4)2(OH)4·H2O nanorods was calcined at 600 °C for 2 h in air without any other treatment.

Synthesis of CVO/PANI composites

The strategy used to obtain CVO/PANI composites can be described as a simple dip-and-dry process. Conducting polyaniline was supplied by Shijiazhuang Jianyada New Material Technology Co., LTD, P. R. China. Conducting polyaniline (0.5 g) was dispersed in 50 mL N-methylpyrrolidinone with magnetic stirring for 2 h at 80 °C and filtrated to obtain a homogeneous solution. To attain the CVO/PANI composites, 0.15 g the as-prepared CVO powders were added into the PANI solution, then treated by 10 min ultrasonic treatment and 2 h agitation. The resulting sample was collected by centrifugation, and then vacuum dried at 110 °C for 8 h. The dip-and-dry process was repeated for two times to get CVO/PANI composites.

Material characterization

The crystalline structures of the products were analyzed with an X-ray diffractometer (XRD, Bruker, D8 ADVANCE) with CuKα radiation (λ = 1.5418 Å) at a scan rate of 2° min−1 over the range of 10–60°. The microstructure properties were observed by field emission scanning electron microscopy (FESEM, JEOL JSM-6700), transmission electron microscopy (TEM, JEOL 2100F) and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX, FEI Tecnai, F20). The thermal properties of the samples were investigated using a thermogravimeter (NETZSCH STA 449F3). Fourier transform infrared (FTIR) spectra were recorded with a Perkin-Elmer IR spectrophotometer.

Electrochemical measurements

The electrochemical behaviors of the as-prepared CVO and CVO/PANI products were measured using CR2032 coin-type cells at room temperature. The working electrode was fabricated by compressing a mixture of the active material, carbon black, and polyvinylidenefluoride (PVDF) in a weight ratio of active material[thin space (1/6-em)]:[thin space (1/6-em)]carbon black[thin space (1/6-em)]:[thin space (1/6-em)]PVDF = 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1. Lithium metal was used as both the counter and reference electrode, and the polypropylene membrane (Celgard 2400) was served as separator. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1). The galvanostatic discharge–charge tests were carried out on a LAND-CT2001A instrument (Land, China) at various current densities with a cutoff voltage window of 0.01–3.3 V. The cyclic voltammograms (CV) and electrochemical impedance spectrometry (EIS) was tested with an electrochemical workstation (Gamry Reference 600).

Results and discussions

The phase and structure of the products were examined using X-ray diffractometer (XRD), and the results are shown in Fig. 1. Fig. 1a gives the XRD pattern of Cu5(VO4)2(OH)4·H2O nanorods. All diffraction peaks could be readily indexed to the pure phase of Cu5(VO4)2(OH)4·H2O with the turanite structure (JCPDS card no. 12-522). No other phase of copper and vanadium is detected in the XRD spectrum, indicating the high purity of the product. For the Cu5(VO4)2(OH)4·H2O/PANI composite (Fig. 1b), the XRD pattern shows the diffraction peaks of Cu5(VO4)2(OH)4·H2O became weaker after PANI coating. All the characteristic peaks in Fig. 1b are consistent with the Cu5(VO4)2(OH)4·H2O characteristic peaks with no shift occurring, indicating the PANI does not intercalate into the interlayers of Cu5(VO4)2(OH)4·H2O but just coats on the surface of the nanorods. The infrared diffuse reflectance spectra of the obtained products well support the presence of PANI and its surface interaction with Cu5(VO4)2(OH)4·H2O (Fig. S1†). To estimate the amount of PANI in the Cu5(VO4)2(OH)4·H2O/PANI composite, thermogravimetric analysis (TGA) was carried out (Fig. S2†). The result shows a typical TGA curve of the Cu5(VO4)2(OH)4·H2O/PANI composite sample along with that of pure Cu5(VO4)2(OH)4·H2O. The difference in weight between Cu5(VO4)2(OH)4·H2O and Cu5(VO4)2(OH)4·H2O/PANI could be directly translated into the amount of PANI in the Cu5(VO4)2(OH)4·H2O/PANI composite. With this method, the amount of PANI in the composite nanostructure was estimated approximately 14.1 wt%.
image file: c5ra02457a-f1.tif
Fig. 1 XRD patterns of (a) pure Cu5(VO4)2(OH)4·H2O and (b) Cu5(VO4)2(OH)4·H2O/PANI composite.

The morphologies of the products were investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Fig. 2a shows a typical FESEM image of the as-prepared Cu5(VO4)2(OH)4·H2O nanorods. From this image, we can see that the diameter of the nanorods is in the range of 50–100 nm and the length is several hundred nanometers. TEM examination reveals that the surface of the Cu5(VO4)2(OH)4·H2O nanorods is smooth (Fig. 2b and c). High-resolution TEM (HRTEM) was used to study the detailed crystalline structures of nanorods. The clear lattice fringes of the nanorods with a lattice of 5.39 Å correspond to the spacing for the (100) planes of Cu5(VO4)2(OH)4·H2O (Fig. 2d). Fig. 2e shows the FESEM images of the Cu5(VO4)2(OH)4·H2O/PANI nanorods. Compared with the bare Cu5(VO4)2(OH)4·H2O nanorods, the external surface of the Cu5(VO4)2(OH)4·H2O/PANI composite nanorods are much rougher and many particles covered on the surface of the nanorods, suggesting the PANI layer had coated on the Cu5(VO4)2(OH)4·H2O surface. TEM image provides further insight into the structure details of Cu5(VO4)2(OH)4·H2O/PANI nanorods. Compared with the clean edge of pure Cu5(VO4)2(OH)4·H2O nanorod, the nanorods with dark color in Fig. 2f should be Cu5(VO4)2(OH)4·H2O nanorods, and thin coating layer around the nanorods should be PANI.


image file: c5ra02457a-f2.tif
Fig. 2 FESEM and TEM images of (a–d) pure Cu5(VO4)2(OH)4·H2O and (e and f) Cu5(VO4)2(OH)4·H2O/PANI composite. (g) STEM-EDX mapping profiles of Cu5(VO4)2(OH)4·H2O/PANI composite.

To further investigate element distribution in the Cu5(VO4)2(OH)4·H2O/PANI composite, scanning transmission electron microscope-energy-dispersive X-ray spectroscopy (STEM-EDX) mapping was carried out, as shown in Fig. 2g. It can be seen that the images were overlapped with each other, which indicates that V and O for Cu5(VO4)2(OH)4·H2O as well as N for PANI distributed uniformly through the Cu5(VO4)2(OH)4·H2O/PANI composite. The STEM-EDX mapping analysis confirms the successful formation of the Cu5(VO4)2(OH)4·H2O/PANI composite in which the nanorods are buried under the PANI, this result is consistent with the assumption from XRD and FTIR results.

Fig. 3 shows the XRD patterns of the as-prepared Cu5V2O10 and Cu5V2O10/PANI nanoparticles. Fig. 3a gives the XRD pattern of Cu5V2O10 particles that calcined at 600 °C for 2 h in air. All the diffraction peaks can be indexed to monoclinic phase Cu5V2O10 (JCPDS card no. 33-504), indicating the successful synthesis of pure Cu5V2O10 by calcining the Cu5(VO4)2(OH)4·H2O precursor. For the Cu5V2O10/PANI nanocomposites (curve b), the XRD pattern shows the diffraction peaks of Cu5V2O10 became weaker after PANI coating. All the characteristic peaks of Cu5V2O10/PANI nanocomposites are consistent with the Cu5V2O10 characteristic peaks with no shift occurring, indicating PANI did not intercalate into the interlayers of Cu5V2O10. The amount of PANI in the Cu5V2O10/PANI composite was about 10.3 wt% based on the TGA result in Fig. S3.†


image file: c5ra02457a-f3.tif
Fig. 3 XRD patterns of (a) pure Cu5V2O10 and (b) Cu5V2O10/PANI composite.

Fig. 4a gives the FESEM image of the as-prepared Cu5V2O10 sample, which consists of nearly spherical particles with a diameter of approximately 100 nm. TEM examination reveals that the surface of pure Cu5V2O10 is very smooth (Fig. 4b and c). In Fig. 4d, the lattice spacing measured by HRTEM is 2.75 Å, which is corresponding to the (−511) lattice plane. Fig. 4e shows the FESEM image of the Cu5V2O10/PANI nanocomposites. Compared with the pure Cu5V2O10, the external surface of the Cu5V2O10/PANI nanoparticles became much rougher, and some particles stick together. TEM image shows that Cu5V2O10 nanoparticles is coated with thin PANI layer (Fig. 4f), displaying a typical core–shell nanostructure, which is beneficial for enhancing electronic conductivity and facilitating electrochemical performance of the composite electrode. STEM-EDX mapping (Fig. 4g) results of V, O and N elements further proves the formation of the PANI layer on the surface of Cu5V2O10 particles.


image file: c5ra02457a-f4.tif
Fig. 4 FESEM and TEM images of (a–d) pure Cu5V2O10 and (e and f) Cu5V2O10/PANI composite. (g) STEM-EDX mapping profiles of Cu5V2O10/PANI composite.

To evaluate their electrochemical activity, cyclic voltammetry (CV) of Cu5(VO4)2(OH)4·H2O/PANI nanocomposites was investigated at a scan rate of 0.2 mV s−1 within a voltage window of 0–3.3 V (vs. Li+/Li), as shown in Fig. 5a. In the first cathodic process, three reduction peaks at 1.86, 0.89 and 0.45 V can be related to the multi-step lithium intercalation of Cu5(VO4)2(OH)4·H2O. The broad peak centred at 1.86 V could be associated with the reduction of Cu2+ to Cu0. The peaks at 0.89 and 0.45 V can be attributed to the conversion of V5+ to V4+ and V3+ and the formation of the solid electrolyte interface (SEI) layer. With the formation of Cu0, most of Cu releases from the framework, which induces large strains in the Cu5(VO4)2(OH)4·H2O host material and leads to volume change and fracture. Due to the structure transformation, most of Cu0 can not return to the host in the subsequent charging process. Therefore, the weak peak (2.83 V) observed in the following anodic polarization may result from the partial oxidation of Cu0 into Cu2+. The broad peak at 1.37 V can be attributed to the oxidation of V3+ and V4+ to V5+. In the subsequent cycles, the pair of redox peaks at 1.86/2.83 V completely disappeared, indicating irreversible capacity losses due to the irreversible electrochemical process of Cu2+ to Cu0. The characteristic pair of redox peaks at 0.89/1.37 V could be attributed to the reversible conversion of vanadium. Fig. 5b shows the 1st, 2nd, 5th and 50th galvanostatic discharge–charge voltage profiles of Cu5(VO4)2(OH)4·H2O/PANI nanocomposites at a current density of 150 mA g−1 with a voltage window of 0.01–3.3 V. It can be seen that the as-prepared Cu5(VO4)2(OH)4·H2O/PANI nanocomposite display a initial discharge curve with two distinct plateaus (2.1–1.1, 1.0–0.3 V) for the lithium intercalation reaction, which are in good agreement with the cathodic peaks in the above CVs. The initial discharge and charge capacities of Cu5(VO4)2(OH)4·H2O/PANI composite are 1237.6 and 757.5 mA h g−1, respectively, with the Coulombic efficiency of 61.2%. From the second cycle, only one discharge slope was observed in the range of 1.1–0.4 V, the discharge and charge capacities reached 703.3 and 649.2 mA h g−1 with a Coulombic efficiency of about 92.3%. As for pure Cu5(VO4)2(OH)4·H2O nanorods (Fig. S4†), the initial discharge and charge capacities are 869.4 and 517.1 mA h g−1, respectively, with a low Coulombic efficiency of 59.4%. In the second cycle, the discharge and charge capacities decrease to 504.2 and 447.3 mA h g−1, respectively, and the Coulombic efficiency reaches to 88.7%. The remarkable irreversible capacities in the first cycle can be partially attributed to the possible irreversible processes, such as electrolyte decomposition, irreversible conversion of transition metal ions and formation the solid electrolyte interphase film (SEI), which are common to the transition metal vanadate systems.25–27 A comparison of the cycling performance between Cu5(VO4)2(OH)4·H2O nanorods and Cu5(VO4)2(OH)4·H2O/PANI composite was carried out. As shown in Fig. 5c, the Cu5(VO4)2(OH)4·H2O/PANI nanocomposites shows decreased capacity in the first five cycles and then the capacity become stable for the rest of the cycles. The Coulombic efficiency for Cu5(VO4)2(OH)4·H2O/PANI is 61.2% in the initial cycle, which rapidly reaches over 98.6% after 5 cycles, and remains at ∼99% in the following cycles. The reversible capacity is 443.0 mA h g−1 after 50 cycles, compared to 457.2 mA h g−1 for the 5th cycle, indicating close to 97% capacity retention. In contrast, the pure Cu5(VO4)2(OH)4·H2O nanorod electrode shows poor cycling performance and the discharge capacity decreases to 385.4 mA h g−1 after 50 cycles. To better illustrate the advantage of the Cu5(VO4)2(OH)4·H2O/PANI composites in lithium storage, the rate capability was evaluated at different current densities. In Fig. 5d, the rate capabilities detected for both the pure Cu5(VO4)2(OH)4·H2O nanorods and Cu5(VO4)2(OH)4·H2O/PANI composite from 50 to 200 mA g−1. It is obvious that the Cu5(VO4)2(OH)4·H2O/PANI displays a much better rate capability than pure Cu5(VO4)2(OH)4·H2O nanorods. When the current density reaches 200 mA g−1, the Cu5(VO4)2(OH)4·H2O/PANI composite can still retain a discharge capacity of about 400 mA h g−1. When the current density was brought down to 50 mA g−1 after 50 cycles, a stable highly reversible capacity of 611.3 mA h g−1 could be recovered. By contrast, the charge capacities of pure Cu5(VO4)2(OH)4·H2O nanorods drop dramatically with increasing the current density.


image file: c5ra02457a-f5.tif
Fig. 5 (a) Cyclic voltammetry (CV) curves of Cu5(VO4)2(OH)4·H2O/PANI at the first three cycles at a scan rate of 0.2 mV s−1, (b) discharge–charge profiles of Cu5(VO4)2(OH)4·H2O/PANI at a current density of 150 mA g−1 between 0.01 V and 3.3 V, (c) cycling performances of Cu5(VO4)2(OH)4·H2O and Cu5(VO4)2(OH)4·H2O/PANI at the current density of 150 mA g−1, (d) rate capability of Cu5(VO4)2(OH)4·H2O and Cu5(VO4)2(OH)4·H2O/PANI at various current density from 50 to 200 mA g−1.

Fig. 6a shows the CV profiles of the Cu5V2O10/PANI nanocomposite for the first three cycles in the voltage range of 0–3.3 V versus Li/Li+ at a scan rate of 0.2 mV s−1. It can be seen that the Cu5(VO4)2(OH)4·H2O/PANI and Cu5V2O10/PANI have a similar electrochemical process during the intercalation/deintercalation of lithium. The broad peak at 1.76 V in the first cathodic process could be associated with the reduction of Cu2+ to Cu0. The peaks at 0.81 and 0.47 V can be attributed to the conversion of V5+ to V4+ and V3+and the formation of the solid electrolyte interface (SEI) layer. During the anodic process, two peaks are present at 1.48 V and ∼2.8 V, which are associated with the oxidation of V4+ and Cu0 to V5+ and Cu2+, respectively. In the subsequent cycles, a pair of redox peaks at 1.88/2.77 V might be attributed to the incomplete conversion reaction between Cu2+ and Cu0. Fig. 6b shows the 1st, 2nd, 5th and 50th galvanostatic discharge–charge voltage profiles of Cu5V2O10/PANI nanocomposite at a current density of 150 mA g−1 within the voltage range of 0.01–3.3 V. The plateaux on the voltage profiles are consistent with the CV curves shown in Fig. 6a. The initial discharge and charge capacities of the Cu5V2O10/PANI nanocomposite are 1209.2 and 781.1 mA h g−1, respectively, with a Coulombic efficiency of 64.6%. In the second cycle, the discharge and charge capacities reach 704.0 and 644.9 mA h g−1 with a Coulombic efficiency of about 91.6%. As for pure Cu5V2O10 nanoparticle, the initial discharge and charge capacities are 950.4 and 565.5 mA h g−1, respectively, and the irreversible capacity loss is about 40.5% (Fig. S5†). Fig. 6c shows the cycling performance and the Coulombic efficiency of the Cu5V2O10 nanoparticles and Cu5V2O10/PANI nanocomposite cycled in the voltage range of 0.01–3.3 V. The Cu5V2O10/PANI nanocomposite clearly demonstrates its superior cycling performance and much better long-term stability than that of Cu5V2O10 nanoparticles. The Coulombic efficiency for Cu5V2O10/PANI nanocomposite is 64.6% in the initial cycle, which increases to 98.7% after 5 cycles, and remains at ∼99% in the following cycles. Even after 50 cycles, a reversible capacity as high as 494.1 mA h g−1 can still be retained, compared to 510.2 mA h g−1 for the 5th cycle, indicating close to 97% capacity retention. For pure Cu5V2O10 nanoparticles, the discharge capacity decreases to 440.1 mA h g−1 after 50 cycles. The rate performance of Cu5V2O10/PANI and Cu5V2O10 were also evaluated at different current densities, as shown in Fig. 6d. It can be seen that even at a high current density of 200 mA g−1, Cu5V2O10/PANI can deliver a capacity of 453.7 mA h g−1. When the current density changed back from 200 to 50 mA g−1, the capacity also increased to 651.8 mA h g−1, which is near to the capacity at a current density of 50 mA g−1 for the first ten cycles. But, in the case of pure Cu5V2O10, the specific capacity reduces quickly upon the current density increased from 50 to 200 mA g−1 and with a specific capacity of 375.8 mA h g−1 at 200 mA g−1. Based on the above results, the Cu5(VO4)2(OH)4·H2O/PANI and Cu5V2O10/PANI composites both exhibit enhanced capacity. This is mainly because of the enhanced electronic conductivity offered by the conducting polymer PANI, which also directly promotes the redox reaction of transition metals.28,29 Similar results have also been observed in Li/Li(Mn1/3Ni1/3Fe1/3)O2/PANI composites,30 LiFePO4/PANI,24 and Li[Li0.2Co0.1Mn0.7]O2/PANI nanodisks.23


image file: c5ra02457a-f6.tif
Fig. 6 (a) Cyclic voltammetry (CV) curves of Cu5V2O10/PANI at the first three cycles at a scan rate of 0.2 mV s−1, (b) discharge–charge profiles of Cu5V2O10/PANI at a current density of 150 mA g−1 between 0.01 V and 3.3 V, (c) cycling performances of Cu5V2O10 and Cu5V2O10/PANI at the current density of 150 mA g−1, (d) rate capability of Cu5V2O10 and Cu5V2O10/PANI at various current density from 50 to 200 mA g−1.

To gain an insight view for the reason of such excellent performance of CVO/PANI composites, electrochemical impedance spectroscopy (EIS) measurements of the CVO/PANI and CVO electrodes were carried out. Fig. 7 shows the Nyquist plots of CVO/PANI and CVO electrodes in the frequency ranging from 100 kHz to 0.01 Hz under open circuit potential. As shown in Fig. 7, all Nyquist plots consist of two distinct parts including a distorted semicircle in the high frequency region and a sloped line in the low frequency region. In general, the semicircle can be assigned to the combination of Li+ migration resistance through the SEI film and the charge-transfer resistance at the electrode surface, while the slope of the line is closely related to the lithium-diffusion process within the electrodes.31–33 Apparently, the semicircle diameter of the CVO/PANI composite is much smaller than that of the pure CVO electrodes, indicating that CVO/PANI composites have a much lower SEI resistance and charge-transfer resistance. Furthermore, the CVO/PANI electrodes have a more vertical shape at low frequencies than the CVO electrodes, indicating their better capacitive behavior with lower diffusion resistance. All the EIS datas therein demonstrated that the CVO/PANI composite electrodes possess a higher electrical conductivity and a faster charge-transfer reaction for lithium ion intercalation and deintercalation.


image file: c5ra02457a-f7.tif
Fig. 7 Nyquist plots of CVO and CVO/PANI composites.

To further understand the excellent electrochemical performance of the CVO/PANI composites, the morphology of the Cu5(VO4)2(OH)4·H2O/PANI nanorod electrode was characterized after 50 cycles of discharge–charge at 150 mA g−1. Seen in Fig. S6,† the one-dimensional morphology was still preserved, showing the good stability of this composite during cycles. This structural stability is likely to be responsible for the excellent electrochemical performance of the Cu5(VO4)2(OH)4·H2O/PANI nanorod electrode. It should be noted that, to the best of our knowledge, this is the first report on Cu5(VO4)2(OH)4·H2O/PANI and Cu5V2O10/PANI composites as anode materials in lithium-ion batteries. According to the results presented above, the as-prepared CVO/PANI electrodes exhibited superior electrochemical performances in terms of discharge capacity, cycling reversibility, Coulombic efficiency and rate capability, which is of great significance for LIBs. We believe the significantly improved electrochemical properties of the CVO/PANI composites may result from the following factors. First, the PANI coating can provide a continuous conductive network for the whole electrode and greatly improves the electron and ion conductivity of this composite. Second, the PANI coating can suffer the structural strain and volume variations created by the core CVO nanostructure during repeated electrochemical reactions. Third, the PANI coating hamper the direct contact between electrolyte and CVO and formed a stable thin SEI layer on the surface, resulting in the high Coulombic efficiency of this composite during cycles.

Conclusions

In summary, the Cu5(VO4)2(OH)4·H2O/PANI and Cu5V2O10/PANI nanocomposites have been successfully synthesized using a hydrothermal process followed by a simple dip-and-dry method. FESEM and TEM analyses confirm the core–shell nanoarchitecture of the CVO/PANI composites. The PANI coating provides a continuous electronic conducting network that can effectively improve electronic conductivity of the materials. Consequently, the resultant CVO/PANI composite anodes exhibit improved electrochemical performance, including improved discharge capacity (443.0 and 494.1 mA h g−1 after 50 cycles at a current density of 150 mA g−1 for Cu5(VO4)2(OH)4·H2O/PANI and Cu5V2O10/PANI), excellent cycling stability, high Coulombic efficiency, and good rate capability. The simple, synthetic approach and the core–shell nanoarchitecture of the CVO/PANI composites may offer a promising approach for practical applications, including LIBs, supercapacitors, and catalysis.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21303107), Natural Science Foundation of Hebei Province (no. B2014106056, B2010001946), Program of One Hundred Innovative Talents of Higher Education Institutions of Hebei Province (no. BR2-264).

Notes and references

  1. H. Ma, S. Y. Zhang, W. Q. Ji, Z. L. Tao and J. Chen, J. Am. Chem. Soc., 2008, 130, 5361 CrossRef CAS PubMed.
  2. Y. Wang and G. Z. Cao, J. Mater. Chem., 2007, 17, 894 RSC.
  3. H. J. Zhang, J. Shu, X. Wei, K. X. Wang and J. S. Chen, RSC Adv., 2013, 3, 7413 Search PubMed.
  4. L. Zhou, W. J. Cui, J. M. Wu, Q. F. Zhao, H. X. Li, Y. Y. Xia, Y. H. Wang and C. Z. Yu, Nanoscale, 2011, 3, 999 RSC.
  5. W. Hu, X. B. Zhang, Y. L. Cheng, Y. M. Wu and L. M. Wang, Chem. Commun., 2011, 47, 5250 RSC.
  6. S. Y. Zhang, Y. Sun, C. S. Li and L. J. Ci, Solid State Sci., 2013, 25, 15 CrossRef CAS PubMed.
  7. S. Y. Zhang, L. J. Ci and H. R. Liu, J. Phys. Chem. C, 2009, 113, 8624 CAS.
  8. W. Hu, X. C. Du, Y. M. Wu and L. M. Wang, J. Power Sources, 2013, 237, 112 CrossRef CAS PubMed.
  9. M. Morcrette, P. Rozier, L. Dupont, E. Mugnier, L. Sannier, J. Galy and J. M. Tarascon, Nat. Mater., 2003, 2, 755 CrossRef CAS PubMed.
  10. M. Kamiya, M. Eguchi, T. Miura and T. Kishi, Solid State Ionics, 1998, 109, 321 CrossRef CAS.
  11. Y. J. Wei, C. E. Ryu, G. Chen and K. B. Kim, Electrochem. Solid-State Lett., 2006, 9, A487 CrossRef CAS PubMed.
  12. X. Y. Cao, J. G. Xie, H. Zhan and Y. H. Zhou, Mater. Chem. Phys., 2006, 98, 71 CrossRef CAS PubMed.
  13. J. H. Huang and Z. H. Yang, RSC Adv., 2014, 4, 19205 RSC.
  14. Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya and L. C. Qin, J. Phys. Chem. C, 2011, 115, 23584 CAS.
  15. W. Xiao, J. S. Chen, Q. Lu and X. W. Lou, J. Phys. Chem. C, 2010, 114, 12048 CAS.
  16. Y. Zhao, J. X. Li, N. Wang, C. X. Wu, G. F. Dong and L. H. Guan, J. Phys. Chem. C, 2012, 116, 18612 CAS.
  17. Z. J. Huang, W. B. Ni, H. Pang, Q. Y. Lu, D. C. Wang and J. W. Zhao, Chem. Lett., 2010, 39, 192 CrossRef CAS.
  18. L. J. Pan, L. Pu, Y. Shi, T. Sun, R. Zhang and Y. D. Zheng, Adv. Funct. Mater., 2006, 16, 1279 CrossRef CAS.
  19. Z. L. Wang, R. Guo, G. R. Li, H. L. Lu, Z. Q. Liu, F. M. Xiao, M. Q. Zhang and Y. X. Tong, J. Mater. Chem., 2012, 22, 2401 RSC.
  20. D. Lepage, C. Michot, G. X. Liang, M. Gauthier and S. B. Schougaard, Angew. Chem., Int. Ed., 2011, 50, 6884 CrossRef CAS PubMed.
  21. F. Y. Cheng, W. Tang, C. S. Li, J. Chen, H. K. Liu, P. W. Shen and S. X. Dou, Chem.–Eur. J., 2006, 12, 3082 CrossRef CAS PubMed.
  22. M. Manas, G. Debasis, G. Soumen, S. Imran and K. D. Chapal, RSC Adv., 2014, 4, 30832 RSC.
  23. K. Karthikeyan, S. Amaresh, V. Aravindan, W. S. Kim, K. W. Nam, X. Q. Yang and Y. S. Lee, J. Power Sources, 2013, 232, 240 CrossRef CAS PubMed.
  24. D. Ahn, Y. M. Koo, M. G. Kim, N. Shin, J. Park, J. Eom, J. Cho and T. J. Shin, J. Phys. Chem. C, 2010, 114, 3675 CAS.
  25. D. Aurbach, B. Markovsky, M. D. Levi, E. Levi, A. Schechter, M. Moshkovich and Y. Cohen, J. Power Sources, 1999, 81–82, 95 CrossRef CAS.
  26. A. Rong, X. P. Gao, G. R. Li, T. Y. Yan, H. Y. Zhu, J. Q. Qu and D. Y. Song, J. Phys. Chem. B, 2006, 110, 14754 CrossRef CAS PubMed.
  27. S. Y. Zhang, L. Ren and S. J. Peng, CrystEngComm, 2014, 16, 6195 RSC.
  28. E. Perez-Cappe, Y. Mosqueda, R. Martinez, C. R. Milian, O. Sanchez, J. A. Varela, A. Hortencia, E. Souza, P. Aranda and E. Ruiz-Hitzky, J. Mater. Chem., 2008, 18, 3965 RSC.
  29. K. S. Park, S. B. Schougaard and J. B. Goodenough, Adv. Mater., 2007, 19, 848 CrossRef CAS.
  30. Y. H. Huang and J. B. Goodenough, Chem. Mater., 2008, 20, 7237 CrossRef CAS.
  31. L. F. Cui, J. Shen, F. Y. Cheng, Z. L. Tao and J. Chen, J. Power Sources, 2011, 196, 2195 CrossRef CAS PubMed.
  32. L. Q. Mai, X. Xu, C. H. Han, Y. Z. Luo, L. Xu, Y. M. Wu and Y. L. Zhao, Nano Lett., 2011, 11, 4992 CrossRef CAS PubMed.
  33. C. L. Zhang, H. S. Li, N. Ping, G. Pang, G. Y. Xu and X. G. Zhang, RSC Adv., 2014, 4, 38791 RSC.

Footnote

† Electronic supplementary information (ESI) available: Infrared diffuse reflectance spectra of Cu5(VO4)2(OH)4·H2O and Cu5(VO4)2(OH)4·H2O/PANI nanorods, thermal gravimetric analysis (TGA) curves of the Cu5(VO4)2(OH)4·H2O and Cu5(VO4)2(OH)4·H2O/PANI composite obtained from 40 to 700 °C at a heating rate of 20 °C min−1, TGA curves of the Cu5V2O10 and Cu5V2O10/PANI composite obtained from 40 to 700 °C at a heating rate of 20 °C min−1, the discharge–charge profiles of pure Cu5(VO4)2(OH)4·H2O at a current density of 150 mA g−1 between 0.01 V and 3.3 V, The discharge–charge profiles of pure Cu5V2O10 at a current density of 150 mA g−1 between 0.01 V and 3.3 V, FESEM and TEM images of the electrode mixture removed from the Cu5(VO4)2(OH)4·H2O/PANI electrode after 50 discharge–charge cycles. See DOI: 10.1039/c5ra02457a
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