CNTs nanocomposite as a superior high-rate anode material for lithium-ion batteries

CNTs nanocomposite as a superior high-rate anode material for lithium-ion batteries

Journal of Alloys and Compounds 603 (2014) 144–148 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 603 (2014) 144–148

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Anatase-TiO2/CNTs nanocomposite as a superior high-rate anode material for lithium-ion batteries Jinlong Liu a,b, Haibo Feng a, Jianbo Jiang a, Dong Qian a,b,⇑, Junhua Li a, Sanjun Peng a, Youcai Liu a,⇑ a b

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China

a r t i c l e

i n f o

Article history: Received 8 February 2014 Received in revised form 8 March 2014 Accepted 10 March 2014 Available online 25 March 2014 Keywords: Oxide materials Composite materials Nanostructured materials Electrode materials Chemical synthesis

a b s t r a c t Anatase-TiO2/carbon nanotubes (CNTs) with robust nanostructure is fabricated via a facile two-step synthesis by ammonia water assisted hydrolysis and subsequent calcination. The as-synthesized nanocomposite was characterized employing X-ray powder diffraction, Fourier transform infrared spectrophotometry, Raman spectrophotometry, thermal gravimetric analysis, transmission electron microscopy, high-resolution transmission electron microscopy and selected area electronic diffraction, and its electrochemical properties as an anode material for lithium-ion batteries (LIBs) were investigated by cyclic voltammetry, galvanostatic discharge/charge test and electrochemical impendence spectroscopy. The results show that the pure anatase TiO2 nanoparticles with diameters of about 10 nm are uniformly distributed on/among the CNTs conducting network. The as-synthesized nanocomposite exhibits remarkably improved performances in LIBs, especially super-high rate capability and excellent cycling stability. Specifically, a reversible capacity as high as 92 mA h g 1 is achieved even at a current density of 10 A g 1 (60 C). After 100 cycles at 0.1 A g 1, it shows good capacity retention of 185 mA h g 1 with an outstanding coulombic efficiency up to 99%. Such superior Li+ storage properties demonstrate the reinforced synergistic effects between the nano-sized TiO2 and the interweaved CNTs network, endowing the nanocomposite with great application potential in high-power LIBs. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Confronted with the climate change and foreseeable depletion of fossil fuels, it is ever-increasingly urgent to develop alternative clean energy, especially in the realm of electrify transportation [1–3]. During the past decades, extensive efforts have been devoted to power sources for electric and/or hybrid-electric vehicles [4–7]. As one of the most promising candidates, rechargeable lithium-ion batteries (LIBs) are qualified in terms of energy densities and power densities [8]. However, graphite anodes in the conventional LIBs have poor rate capabilities, which cannot satisfy the need of high-rate devices. Another thorny issue is the safety hazard of carbonaceous anodes because of the generation of Li dendrites at low voltage [9,10]. Accordingly, there are great interests in exploring advanced materials with superior Li+ storage properties, particularly at high current densities [11].

⇑ Corresponding authors at: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China (D. Qian). Tel./fax: +86 731 88879616. E-mail addresses: [email protected] (D. Qian), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jallcom.2014.03.089 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

TiO2 is one of most investigated semiconductors because of its low cost, easy availability, environmental friendliness, and extremely broad applications [12–14]. There exist different phases of TiO2, such as anatase, rutile, brookite and TiO2 (B) (bronze). Most strikingly, anatase TiO2, due to its unique crystal structure and more established synthetic conditions [15], has been widely studied in photocatalysis [16,17], bio-sensing [18], energy storage and conversion [19–22], and so on [23,24]. When used as anode materials for LIBs, anatase TiO2 has shown excellent electrochemical performances and improved safety [20–22]. It has been demonstrated that Li+ can insert/extract from anatase TiO2 with volume variation less than 4% [25], which is favourable to achieving high-rate performance. More importantly, the charge/discharge voltage plateaus locate at around 1.7 V vs. Li+/Li, thereby circumventing the safety problem by avoiding the evolution of Li dendrites. However, the low electrical conductivity (ca. 10 12– 10 7 S cm 1) and Li+ diffusivity (ca. 10 15–10 9 cm2 s 1) hinder its practical application as the anode materials for high-power LIBs [26,27]. To overcome such a deficiency, various methods have been developed, including reducing particle sizes to nanoscale [28–30], doping with other elements [31–34], and compositing with

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conductive materials [35–44]. Among these methods, hybridizing with carbon materials is considered to be the most facile and effective approach. A good example is that Isamu Moriguch et al. have reported a mesoporous nanocomposite of TiO2 and single-walled carbon nanotubes (SWCNTs) as a high-rate anode material for LIBs, which delivered about 200–75 mA h g 1 at the charging rate of 1– 40 C (1 C = 0.168 A g 1) [37]. It is well known that the synthetic strategy usually plays a decisive role in material structures, and that the electrochemical performances depend heavily on the structures of electrode materials. Albeit the mesoporous TiO2/ SWCNTs exhibited improved rate performance, the bicontinuous microemulsion-acided process involved in the uses of surfactant, oil and SWCNTs, which was complicated, environmentally harmful and expensive. Moreover, mesoporous materials have low volumetric energy density. Therefore, it is still a great challenge to develop facile, eco-friendly, economical and large-scale synthetic methods, with aim to fabricate superior high-rate electrode materials for real applications. To this end, herein we propose a simple and scalable synthesis strategy to fabricate a nanocomposite with anatase TiO2 nanoparticles anchored on/among carbon nanotubes (CNTs) (denoted as TCs) via ammonia water assisted hydrolysis and in situ crystal transformation under calcination. In the nanostructured TCs, interweaved CNTs serve as highway for electrons to facilitate the charge transfer. The intimate connection between CNTs and TiO2 is able to promote the shift of electrons toward the active TiO2 nanoparticles. Furthermore, nano-sized TiO2 is beneficial to increase its contact area with electrolyte and reduce the Li+ diffusion path, which can remarkably accelerate the Li+ transport. In virtue of the advantageous synergistic effects as well as the nano-sized effects, the assynthesized TCs nanocomposite exhibits tremendously enhanced high-rate capability and cycling stability compared with commercial P25 (Degussa TiO2 with 80% anatase and 20% rutile) and previous TiO2/CNTs hybrids. Specifically, a high reversible capacity of 92 mA h g 1 has been remained at a current density of 10 A g 1 (ca. 60 C, 1 C = 0.168 A g 1). As far as we know, this fascinating high-rate performance is superior to those of most of TiO2/CNTs composites ever reported, manifesting profound prospect in high-power LIBs. 2. Experimental 2.1. Synthesis All the chemical reagents were analytically pure and used without any further purification. Commercial P25 and CNTs were purchased from Degussa Corporation and Shenzhen Nanotech Port C. Ltd., respectively. Before the synthesis of TCs, 1 g of CNTs were first sonicated in mixed acids of 20 mL of HNO3 (65 wt%) and 60 mL of H2SO4 (98 wt%) for 1 h, and further refluxed at 80 °C for another 2 h in order to introduce oxygen-containing groups on their surfaces. The TCs nanocomposite was synthesized by a facile ammonia water assisted hydrolysis method. In a typical procedure, 100 mg of acid-treated CNTs were dispersed in 200 mL of absolute ethanol under sonication for 30 min. Then 0.6 mL of ammonia water (28 wt%) was added dropwise into the mixture, followed by the addition of 1 mL of tetra-n-butyl titanate (TBT). After the hydrolysis reaction under magnetic agitation for 24 h, the

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precipitates were collected by vacuum filtration, washed by deionized water and absolute ethanol three times, respectively, and dried at 60 °C overnight. The collected amorphous TiO2 (aTiO2)/CNTs were finally calcined at 500 °C for 2 h in Ar to transform aTiO2 in situ into anatase TiO2, thus obtaining TCs.

2.2. Characterization Field emission transmission electron microscopy (TEM) (JEOL-2010), high-resolution TEM (HRTEM) (JEOL-2010) and selected area electron diffraction (SAED) were employed to observe the morphology and structure of the as-obtained TCs. X-ray diffraction (XRD) patterns, Fourier transform infrared spectra (FT-IR) and Raman spectra of samples were recorded on a Rigaka D/max-2500 X-ray diffractometer with Cu Ka radiation, an IRPrestige-21 FT-IR spectrometer with a KBr disk and a LabRAM HR 800 Raman microscope with an excitation laser beam wavelength of 633 nm, respectively. Thermal gravimetric analysis (TGA) was measured with a Netzsch-Gerätebau GmbH-STA 449 C Jupiter thermo-microbalance.

2.3. Electrochemical measurements The working electrodes were made by mixing the electroactive materials, acetylence black and polyvinylidene (PVDF) with a mass ratio of 80:10:10 in N-methyl2-pyrrodidone (NMP). Electrode film was fabricated by pasting the resulting slurry on a Cu foil, which was dried at 60 °C for 6 h and then at 120 °C for another 10 h. CR2032 type coin half cells were assembled in an Ar-filled glovebox (MBRAUN) using a pure Li foil as the counter electrode, Celgard 2400 as the separator, and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1, by volume) as the electrolyte. The cyclic voltammogram (CV) analysis was investigated on a CHI660 electrochemical workstation within the voltage range of 1–3 V at a scan rate of 0.1 mV s 1. The galvanostatic discharge/charge tests were performed on a Land battery tester (China) at 0.1–10 A g 1 within the same voltage window at room temperature. The electrochemical impedance spectra (EIS) were evaluated on a PARSTAT 2273 advanced electrochemical system in a frequency range from 100 kHz to 10 mHz with an amplitude of 5 mV.

3. Results and discussion Our strategy to fabricate the TCs nanocomposite principally includes ammonia water assisted hydrolysis and calcination of aTiO2/CNTs. As illustrated in Fig. 1, CNTs were firstly treated in HNO3/H2SO4 to introduce oxygen-containing groups, such as – COOH and –OH, on their surfaces, which provide numerous active molecules for the deposition of aTiO2 by forming Ti–O bonds. The acid-treated CNTs were dispersed into ethanol to construct a three-dimensional (3D) conducting network. Under the assistance of ammonia water, TBT hydrolyzed in ethanol at a very slow rate, thus producing uniform aTiO2 nanoparticles (blue balls) on/among the CNTs network without evident agglomeration. After the hydrolysis, the resulting aTiO2/CNTs were calcined at 500 °C in Ar for 2 h to convert aTiO2 into anatase TiO2 (red balls). In this 3D architecture, the porous CNTs framework can not only facilitate fast electron transfer, but also provide more Li+ diffusion paths. The uniformly scattered TiO2 nanoparticles shorten the diffusion distances for quick Li+ supply, and greatly increase the contact between the electrolyte and the electrode materials. Thanks to these features, such a 3D porous nanostructure will contribute to the overall electronic and ionic conductivity of the hybrid material, making it exceptionally attractive for high-rate anodes in LIBs.

Fig. 1. Schematic illustration of the fabrication mechanism for TCs nanocomposite.

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Fig. 2. (a and b) TEM images (inset of b: a representative HRTEM image showing the anchoring of TiO2 on CNT), (c) HRTEM image and (d) SAED pattern of TCs.

Fig. 2a–c gives the typical TEM and HRTEM images of TCs. Apparently, CNTs interconnect with each other, on/among which plenty of uniform nanoparticles can be clearly observed. There is no obvious agglomeration amongst these nanoparticles, demonstrating the fabrication mechanism illustrated in Fig. 1. Close observations of the inset of Fig. 2b and c indicate that TiO2 nanoparticles with diameters of about 10 nm bond tightly with CNTs. The lattice fringes with d-spacings of 0.34 and 0.35 nm correspond to the (0 0 2) planes of CNTs and the (1 0 1) planes of anatase TiO2, respectively. SAED (Fig. 2d) was also carried out for TCs to deter-

mine series of d-distances. The vicinity of the (0 0 2) planes of CNTs and the (1 0 1) planes of anatase TiO2 results in a bright broadened diffraction ring. As can be seen, a set of concentric rings in the SAED image suggest the presence of both CNTs and polycrystalline anatase TiO2, further verifying the formation of TCs composite. The crystal structure and phase purity of TCs were confirmed by XRD, FT-IR, Raman spectra and TGA. In Fig. 3a, the XRD pattern of TCs can be readily indexed to pure anatase phase (JCPDS 84– 1285) without any observable other peaks from a secondary phase. No peak of CNTs is identified in TCs because the (1 0 1) peak of anatase overlaps the (0 0 2) peak of CNTs [45], in accordance with the result in SAED image. By means of the Scherrer equation, the particle size of anatase is calculated to be 11 nm from the broadening of the (1 0 1) peak, completely tallying with the value attained from the HRTEM observation. Fig. 3b shows the FT-IR spectra of acid-treated CNTs and TCs. For the acid treated CNTs, the peaks at 3433 and 1077 cm 1 can be ascribed to the stretching vibrations of O–H and C–O, respectively, which were introduced through the acid treatment [46]. Whereas in TCs, the peaks located at 660, 594 and 548 cm 1 are attributed to the Ti–O stretching and Ti–O–Ti bridging stretching modes [47], and the O–H and C–O stretching vibrations move to 3417 and 1039 cm 1, respectively. The observed red shifts of oxygen-containing groups give evidence of strong bonding interactions between CNTs and TiO2. Meanwhile, Raman spectra are adopted to further certify the purity of TCs. The peak at 145 cm 1 is assigned to the Eg mode for typical anatase [47], and the peaks at 1347 and 1582 cm 1 belong to the disordered (D) band and graphitic (G) band of CNTs, respectively [46]. It is clear that TCs display the characteristic peaks of anatase and CNTs without impurity peaks, agreeing well with the high purity in XRD analysis. Note that the ID/IG ratio of TCs increases to approximate 1, higher than that of acid treated CNTs (0.85), implying the strong chemical valence between CNTs and TiO2 on their contact surface. In addition, TGA was used to determine the content of CNTs in the final TCs. According to the TGA curve in Fig. 3d, TCs nanocomposite is consisted of 27 wt% CNTs and 73 wt% TiO2.

Fig. 3. (a) XRD patterns, (b) FTIR spectra and (c) Raman spectra of acid-treated CNTs and TCs, and (d) TGA curve of TCs.

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Fig. 4. CV curves of TCs at a scan rate of 0.1 mV s

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To highlight the as-synthesized TCs as a superior anode for high-power LIBs, its electrochemical behaviours were carefully investigated and compared with those of nano-sized TiO2 P25 (as shown in Figs. 4–8). Fig. 4 depicts the first three CV curves of TCs in the voltage range of 1–3 V at a scan rate of 0.1 mV s 1. In accord with previous literature, a pair of redox peaks at 1.7 and 2.0 V are observed during the cathodic and anodic processes, matching with the Li+ insertion and extraction reactions of anatase, respectively [29,40,43]. Interestingly, the first three CV curves almost overlap each other, suggesting the highly stability and excellent reversibility of the TCs nanocomposite. Fig. 5 shows the typical discharge/charge profiles of P25 and TCs at 0.1 A g 1. It can be seen that there is a discharging plateau at 1.75 V and a charging plateau at 1.90 V, which are consistent with the result in CV curves. The initial discharge and charge capacities are 302 and 281 mA h g 1, respectively, exceeding those of P25 (269 and 173 mA h g 1). Only 4.6% capacity loss for TCs is observed in the second cycle. Even at the 100th cycle, the discharge/charge profiles of TCs are very stable. On the contrary, P25 has no discharging or charging plateaus, indicating its inactivation with prolonging the cycle number. The cycling performance and corresponding coulombic efficiency (CE) of P25 and TCs at 0.1 A g 1 are listed in Fig. 6a and b, respectively. Clearly, TCs show much better cyclic retention and reversible capacity than those for commercial P25. After the slow capacity fading in the first 35 cycles, the capacities of TCs keep constant around 185 mA h g 1 in the following cycles. At the same time, its CE gradually increases from 92% to 99%. Although P25 also has an improved CE during cycling, its capacities decline rapidly. These results prove that the 3D porous CNTs conducting network

Fig. 5. Galvanostatic discharge/charge profiles of P25 and TCs at 0.1 A g

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Fig. 6. (a) Cycling performance and (b) corresponding coulombic efficiency of P25 and TCs at 0.1 A g 1.

is helpful to improve the overall structural stability of the nanocomposite, leading to excellent cycling stability with high CE. As the most important performance in high-power LIBs, rate capability of TCs is further demonstrated in Fig. 7. Fresh cells are successively tested at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 0.1 A g 1 (5 cycles under each current density). As expected, TCs nanocomposite exhibits overwhelmingly superior high-rate capability. Specifically, it delivers a reversible capacity of 162 mA h g 1 at 0.5 A g 1 (3 C) after activation under low current densities, 140 mA h g 1 at 1 A g 1 (6 C), and 117 mA h g 1 at 2 A g 1 (12 C). Even at 5 A g 1 (30 C) and 10 A g 1 (60 C), the reversible capacities still maintain at 97 and 92 mA h g 1, respectively. Most notably, when the current density returns to

Fig. 7. Rate capability of P25 and TCs at various current densities from 0.1 to 10 A g 1.

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References

Fig. 8. Nyquist plots of P25 and TCs.

0.1 A g 1, the capacities of TCs can recover from 189 to 200 mA h g 1. By contrast, P25 displays evidently poor capacity and stability at high current densities. Indeed, to the best of our knowledge, the extraordinary ultra-high rate capability for the as-synthesized TCs remarkably outperforms the previously reported work on TiO2/CNTs hybrids [37–44]. For instance, the mesoporous TiO2/SWCNTs fabricated by Isamu Moriguch et al. has a capacity of about 75 mA h g 1 at the maximum rate of 40 C [37]. To get insight into the different electrochemical behaviours of TCs and P25, EIS were conducted in the frequency range of 100 kHz to 10 mHz, and their Nyguist plots are depicted in Fig. 8. Each plot is composed of a semicircle in the high frequency region and a straight line in the low frequency region, reflecting the charge transfer and Li+ diffusion kinetics [43]. The smaller loop and steep line in the plot of TCs indicate its much outstanding electronic and ionic conductivity. Based on the above characterizations, it is reasonable to conclude that the excellent electronic conductivity benefits from the integrated CNTs network and its strong bonding interaction with TiO2, and that the significantly enhanced Li+ diffusion originates from the porous 3D architecture along with the nano-sized channels in TiO2 nanoparticles. 4. Conclusions In summary, a facile strategy has been developed to synthesize anatase-TiO2/CNTs nanocomposite by an ammonia water assisted hydrolysis method and subsequent calciantion in Ar at 500 °C for 2 h. The characterizations demonstrated that uniform TiO2 nanoparticles with about 10 nm in diameter tightly anchored on/among CNTs conducting network, which contribute to the enhancement of overall electronic conductivity as well as Li+ diffusivity. Using the nanocomposite as an anode for LIBs, it exhibits vastly superior rate capability and cycling stability, compared to commercial nanosized TiO2 (P25) and previous reported works. Additionally, the synthetic strategy is eco-friendly, economical and highly scalable. Therefore, we believe that the fascinating anatase-TiO2/CNTs nanocomposite holds great potential as a superior anode material for high-power LIBs. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 21171174), Provincial Natural Science Foundation of Hunan (No. 09JJ3024), Provincial Environmental Science and Technology Foundation of Hunan, the opening subject of State Key Laboratory of Powder Metallurgy and the Open-end Fund for the Valuable and Precision Instruments of Central South University.

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