carbon nanotube nanocomposites

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To solve the problem, a highly porous layer of MnO2 coating on the CNTs ..... In addition to the formation of the SEI layer, the irreversible capacity for the first.

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www.rsc.org/materials | Journal of Materials Chemistry

Nanoflaky MnO2/carbon nanotube nanocomposites as anode materials for lithium-ion batteries† Hui Xia, ManOn Lai and Li Lu*

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Received 19th March 2010, Accepted 24th May 2010 DOI: 10.1039/c0jm00759e Caterpillar-like nanoflaky MnO2/carbon nanotube (CNT) nanocomposites are synthesized via a facile solution method. To build a three-dimensional hierarchy architecture, highly porous interconnected MnO2 nanoflakes are uniformly coated on the surface of the CNTs. As a promising anode material for lithium-ion batteries, the nanoflaky MnO2/CNT nanocomposite electrode exhibits a large reversible capacity of 801 mA h g1 (1000 mA h g1 can be attributed to the MnO2 porous layer alone) for the first cycle without capacity fade for the first 20 cycles and with a good rate capability. The superior electrochemical performance of the MnO2/CNT nanocomposite electrode compared to the pure MnO2 electrode can be attributed to its unique hierarchy architecture, which is able to provide fast lithium ion and electron transport and to accommodate a large volume change during the conversion reactions.

Introduction To develop next-generation lithium-ion batteries with high power and energy densities, nanotechnology has become one of the key approaches to realize novel electrode materials with high electrochemical performance.1–3 Recently, various transition metal oxides have been extensively studied as anode materials for lithium-ion batteries due to their high theoretical capacity based on the conversion mechanism.4–7 However, most of transition metal oxides suffer from limited reversible capacity and fast capacity fade due to their poor electronic conductivity and large volume changes during the conversion reaction. Fabrication of nanostructures and mixing the active material with electronically conductive phases have been demonstrated to be effective ways to alleviate these problems.8,9 Manganese dioxides (MnO2) with various nanostructures, such as nanowires, nanorods, nanoflakes, and nanotubes, have attracted great attention due to their wide applications in catalysis and energy storage.10–12 MnO2 has traditionally been used as cathode material but rarely been reported as an anode material for lithium-ion batteries.13 Until recently, the nanostructured MnO2 has been reported to achieve high reversible capacities for anode materials.9,14–17 MnO2/CNT composite structures have been proven to be an effective way to improve electron transfer for MnO2 electrodes for supercapacitors.18–20 As reported by Reddy et al.,9 although coaxial MnO2/CNT array electrodes can deliver a reversible capacity of about 500 mA h g1 after 15 cycles, fast capacity decay within 15 cycles for such a composite is observed. To solve the problem, a highly porous layer of MnO2 coating on the CNTs could be an effective way to suppress the fast capacity fade as the porous structure can accommodate

Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576. E-mail: [email protected]; Fax: +65 6779 1459; Tel: +65 6516 2236 † Electronic supplementary information (ESI) available: Raman sepctra of CNT, MnO2 and MnO2/CNT samples, FESEM images of CNT, MnO2 and MnO2/CNT samples, TEM images of nanocrystalline MnO2 formation on the CNT surface after the mixing of KMnO4 and CNT. See DOI: 10.1039/c0jm00759e

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a large volume change during charge/discharge cycling. However, a facile and fast synthesis of uniformly distributed MnO2 porous layers on the CNTs is still a challenge. It could be a beneficial design if some of nanostructures (nanowire, nanorod, nanoflake, etc.) of MnO2 can be transferred onto the CNTs as this hierarchy architecture will be able to accommodate large volume change and will provide fast electron transport. In this study, we design a unique hierarchy architecture, where highly porous interconnected MnO2 nanoflakes are coated on the CNTs. The caterpillar-like nanoflaky MnO2/CNT nanocomposite has been found to have high reversible capacity, good cycling stability and good rate capability as anode material for lithium-ion batteries.

Experimental Preparation of pure MnO2 powder and MnO2/CNT nanocomposite Commercial multiwalled CNTs (20–50 nm in diameter, Shenzhen Nanotech Port Co., Ltd.) were purified by refluxing the asreceived sample in 10 wt% nitric acid for 12 h. The acid-treated CNTs were then collected by filtration and dried in vacuum at 120  C for 12 h. Typical synthesis process of the MnO2/CNT nanocomposite can be described as follows. Firstly, 0.1 g CNTs were dispersed in 25 ml deionized (DI) water by ultrasonic vibration for 2 h. 0.3 g KMnO4 was then added into the above suspension and the mixed solution was stirred by magnetic bar for 6 h. After that, the mixed solution was transferred to a 30 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and put in an electric oven at 150  C for 3–12 h and then naturally cooled to room temperature. After the hydrothermal treatment, the produced samples were collected by filtration and washed with DI water. The MnO2/CNT samples were finally dried in an oven at 100  C for 12 h for further characterization. To prepare MnO2 powders, 0.3 g KMnO4 and 0.2 mL H2SO4 (95 wt%) were mixed with 25 mL DI water to form the precursor. The precursor solution was then treated with hydrothermal reaction at 150  C for 4 h. This journal is ª The Royal Society of Chemistry 2010

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Structural and compositional characterization The crystallographic information of the products was investigated using powder X-ray diffraction (XRD, Shimadzu X-ray diffractormeter 6000, Cu-Ka radiation) with a scan rate of 2 min1. Morphologies of acid-treated CNTs, MnO2 powders and MnO2/CNT nanocomposites were characterized by field emission scanning electron microscopy (FESEM, Hitachi S4300). The morphologies and structures of the MnO2/CNT nanocomposites were further investigated by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM, JEOL, JEM-2010). Compositional investigation of samples was carried out with X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM Scanning X-ray Microprobe) and energy-dispersive X-ray (EDX, Noran System SIX). The contents of interlayer water and CNTs of the nanocomposites were determined using thermogravimetric analysis (TGA, Shimadzu DTG-60H). Electrochemical characterization To prepare the working electrode, 80 wt% of the active material (CNT, MnO2 powder or MnO2/CNT nanocomposite), 15 wt% carbon black and 5 wt% polyvinylidene difluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) were mixed to form a slurry. The slurry was pasted on the Ti foil and dried in a vacuum oven for 12 h. The loading of working electrode is typically in the range of 0.3–0.4 mg (2 mg cm2). To evaluate the potential practical application of the MnO2/CNT nanocomposite electrode, high loading electrodes (10 mg cm2) were also prepared. Half cells using Li foil as both counter and reference electrodes were constructed using Swagelok cells for electrochemical measurements. 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC/DEC, v/v ¼ 1 : 1) solution was used as the electrolyte. Galvanostatic charge and discharge measurements were carried out in the voltage range between 3.0 and 0.01 V at different current densities using a Neware battery system.

Results and discussion

and then the solution was treated in a hydrothermal reaction to produce the final product. When the KMnO4 solution is mixed with the CNTs at room temperature before the hydrothermal processing, the nanocrystalline MnO2 will be formed on the surface of the CNTs due to the slow redox process according to eqn (1).18 When the solution is further treated in the hydrothermal reaction, the nanoflaky MnO2 grows from the preformed nanocrystalline due to the decomposition of KMnO4 in water according to eqn (2),21 where a highly porous layer of nanostructured MnO2 can be uniformly coated on the surfaces of the CNTs to form nanocomposite architecture. 4MnO4 + 3C + H2O / 4MnO2 + CO32 + 2HCO3

(1)

4MnO4 + 2H2O / 4MnO2 + 4OH + 3O2

(2)

XRD patterns of the pure CNTs, pure MnO2 powder and different MnO2/CNT nanocomposites are shown in Fig. 2. The XRD pattern of the pure MnO2 powder synthesized by hydrothermal reaction can be indexed to monoclinic potassium birnessite (JCPDS no. 80-1098), which consists of 2D edge-shared MnO6 octahedra layers with K+ cations and water molecules in the interlayer space. The two stronger diffraction peaks correspond to (001) and (002) basal reflections, while another two weaker diffraction peaks can be indexed as the (20l/11l) and (02l/ 31l) diffraction bands, respectively.22 Diffraction peaks from both birnessite-type MnO2 phase and CNT can be observed from the XRD pattern for the MnO2/CNT product after 3 h hydrothermal reaction. All the diffraction peaks from MnO2 are very weak and broad, indicating the nature of nanocrystals. As the reaction time increases, the diffraction peaks from MnO2 become stronger and the diffraction peaks from the CNTs become almost invisible, indicating the growth of thicker layers of MnO2. The birnessite-type structure of MnO2 is further confirmed by Raman measurements (Fig. S1, ESI†). The three major Raman bands located at 501, 575, and 645 cm1 for both MnO2 and MnO2/ CNT samples are in good agreement with the three major vibrational features of birnessite-type MnO2 compounds previously reported at 500–510, 575–585, and 625–650 cm1.23

The mechanisms of growth of the caterpillar-like nanoflaky MnO2/CNT nanocomposite architecture is illustrated in Fig. 1. Briefly, the CNTs were first dispersed well in a KMnO4 solution

Fig. 1 Schematic illustration of preparation of caterpillar-like nanoflaky MnO2/CNT nanocomposite.

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Fig. 2 XRD patterns of the pure CNTs, pure MnO2, and MnO2/CNT nanocomposites with different synthesis times.

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Morphologies of the CNTs and the birnessite-type MnO2 powder are characterized by FESEM (Fig. S2a and 2b, ESI†). The diameter of the CNT is about 20–50 nm. MnO2 synthesized by hydrothermal reaction consists of monodisperse microspheres of 2–3 mm in diameter. The MnO2 microspheres exhibit a flowerlike structure composed of many nanoflakes radiating from the centre. The morphologies of the MnO2/CNT nanocomposites with different synthesis times are characterized by both FESEM (Fig. S2c–2f, ESI†) and TEM (Fig. 3). TEM images as shown in Fig. 3a–3c reveal a porous MnO2 nanostructure coated on the CNT surface. With a highly porous MnO2 layer coated on the CNT, the morphology resembles that of a caterpillar, the spiny larva of butterflies. As shown in the high magnification FESEM images of the MnO2/CNT nanocomposites (Fig. S2c–2f, ESI†), it can be seen that the MnO2 coating on the CNT is quite uniform. For a short synthesis time such as 3 h (Fig. 3a), only a very thin layer of porous MnO2 forms on the surface of CNT. As the synthesis time increases, the MnO2 layer gets thicker. MnO2 nanowires start to develop among the nanocomposite as the synthesis time is prolonged to 12 h as shown in Fig. 3c (also see in Fig. S2e, ESI†). Fig. 3d and 3e show the TEM images of one single CNT coated with porous MnO2 (6 h sample) at different magnifications. It can be seen that the porous MnO2 layer is composed of numerous tiny nanoflakes. These MnO2 nanoflakes are interconnected and uniformly distributed on the surface of the CNTs. The thickness of the porous MnO2 layer is estimated

to be about 20 nm from Fig. 3e. By controlling the synthesis time, the thickness of the MnO2 layer can be easily varied. Fig. 3f shows the HRTEM of the interface between the CNT and the MnO2 layer. It can be seen that MnO2 nanoflakes directly grow from the CNT wall. There are a number of methods to prepare the MnO2/CNT composite such as mixing KMnO4 solution and the CNTs in a suspension,18,24 leading to a redox reaction between the CNTs and KMnO4 based on the ‘‘microelectrochemical cell’’ mechanism.18 The as-synthesized MnO2 layers are, however, not uniform due to the non-uniform consumption of the CNTs during the reaction. Other methods like decomposition of manganese nitrates,25 electrochemical deposition26 or polymer-assisted synthesis27 may be able to produce relatively uniform MnO2 layer coatings, but it is difficult to achieve MnO2/CNT nano-architecture with uniform and highly porous MnO2 nanoflakes on the CNTs. The present nanoarchitecture can be attributed to the hydrothermal process. Porous MnO2 films composed of nanoflakes have been reported to be easily produced in a hydrothermal reaction with KMnO4 solution.21,22 In the present study, small nanocrystallines of MnO2 will form at the surface of the CNTs during the mixing of the CNTs and KMnO4 (Fig. S3, ESI†). When the mixed solution is further undergone through the hydrothermal reaction, the preformed nanocrystalline MnO2 may serve as the nucleation sites for MnO2 crystals to continuously grow. The flaky morphology forms due to preferred growth along the ab plane of

Fig. 3 TEM images of caterpillar-like MnO2/CNT nanocomposites synthesized by hydrothermal reaction for: (a) 3 h, (b) 6 h, and (c) 12 h. d) TEM and (e) magnified TEM images of one single CNT coated with porous MnO2 layer. (f) HRTEM image of the interface between the MnO2 and CNT. (g) HRTEM image of the MnO2 nanoflake, (h) ED pattern for MnO2 nanoflake. (Images d–h are obtained from the 6 h sample.)

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the layered birnessite-type MnO2.22,28 As shown in Fig. 3g, the interplanar spacing of MnO2 nanoflake is measured to be 0.67 nm, which is in good agreement with literature report of 0.7 nm for birnessite-type MnO2.22,28 These MnO2 nanoflakes are typically less than 5 nm in thickness. Fig. 3h shows the diffraction pattern (ED) of MnO2 nanoflakes. The multiple diffraction rings indicate that either the MnO2 nanoflake is polycrystalline or multiple single crystals presented in the incident electron beam. As the sample hydrothermally synthesized for 6 h exhibits the best morphology with thick layer of MnO2 coating without formation of separate MnO2 nanowires, further characterizations were performed on those samples only. To investigate the oxidation state of Mn and the composition of manganese oxide of the MnO2/CNT nanocomposite, the sample was further characterized by XPS. Fig. 4a shows the Mn 2p spectrum, where two peaks located at 642.1 and 653.7 eV can be attributed to Mn 2p3/2 and Mn 2p1/2, respectively. The peak values well agree with those reported for MnO2, indicating 4+ oxidation state for Mn.9,17 Fig. 4b shows the deconvoluted O 1s spectrum, where one sharp peak located at 529.5 eV and two broad peaks located at 530.9 and 532.4 eV can be observed. This spectrum is in good agreement with literature reports of 529.3– 530.3 eV for oxide, 530.5–531.5 eV for hydroxide, and 531.8– 532.8 eV for water.21 The K/Mn ratio obtained from XPS result is about 0.2, which is in good agreement with EDX spectroscopy result shown in Fig. 4c. The interlayer water content and the CNTs content can be evaluated from TGA measurement as shown in Fig. 4d. According to 6% weight loss below 250  C, the calculated interlayer water is about 0.33 H2O per chemical

formula (K0.2MnO2$0.33H2O). Based on this formula, the average oxidation state of Mn is 3.8+, which is in good agreement with the XPS result. 28% weight loss at about 400  C is due to the oxidation of the CNTs in air.19 Therefore, the composition of MnO2/CNT nanocompsite can be expressed as 0.72(K0.2MnO2$0.33H2O)/0.28CNT. For convenience, MnO2/ CNT is still used in the following text. The hierarchy architecture of the caterpillar-like MnO2/CNT nanocomposite makes it promising for application in catalysis and energy storage. Therefore the electrochemical performance of the MnO2/CNT nanocomposite electrode as anode material for lithium-ion batteries was investigated. The voltage profiles of the 1st, 5th, 10th and 20th cycles for the MnO2/CNT nanocomposite electrode cycled between 0.01 and 3 V at a current density of 200 mA g1 are shown in Fig. 5c. The first discharge and charge capacities are 1402 and 801 mA h g1, respectively, with a coulumbic efficiency of 57%, while multiwall CNTs alone can only deliver a reversible capacity of about 280 mA h g1 for the first cycle (Fig. 5a). Subtracting the contribution from the CNTs in MnO2/CNT nanocomposite, a reversible capacity about 1003 mA h g1 can be attributed to the 72 wt% MnO2. Compared to the flower-like MnO2 composite electrode which respectively has discharge and charge capacities of 1471 and 455 mA h g1 with a columbic efficiency of 31% in the first cycle (Fig. 5b), it is clear that the MnO2/CNT nanocomposite electrode can deliver a much higher reversible capacity than that of the pure MnO2 electrode. The reaction mechanism of transition metal oxide with lithium has been proposed as conversion mechanism.29 It was found that electrochemical

Fig. 4 XPS narrow scan spectra of (a) Mn 2p and (b) O 1s for the MnO2/CNT nanocomposite. (c) EDX spectrum of the MnO2/CNT nanocomposite synthesized by 6 h hydrothermal reaction.(d) TGA curve of the MnO2/CNT nanocomposite synthesized by 6 h hydrothermal reaction.

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Fig. 5 (a) The first 5 cycles’ voltage profiles of a CNT electrode cycled between 0.01 and 3 V at a current density of 200 mA h g1. (b) The first 5 cycles’ voltage profiles of a MnO2 electrode cycled between 0.01 and 3 V at a current density of 200 mA h g1. (c) Voltage profiles of a MnO2/CNT nanocomposite electrode cycled between 0.01 and 3 V at a current density of 200 mA g1. (d) Differential charge/discharge capacity plots for the first two cycles of (c).

lithiation typically leads to formation of nanometre-scale metal clusters embedded in a Li2O matrix, accompanying a large volume expansion. If this mechanism also works for MnO2 system, the reaction between MnO2 and Li can be expressed as eqn (3).17 MnO2 + 4Li+ + 4e 4 2Li2O + Mn

(3)

Formation of Mn/Li2O leads to a theoretical capacity of 1230 mA h g1 for MnO2.29 The discharge capacity for the first cycle of MnO2/CNT nanocomposite electrode is larger than the theoretical capacity, which may be attributed to the decomposition of electrolyte and the formation of solid electrolyte interface (SEI) layer on the surface of the electrode.30 In addition to the formation of the SEI layer, the irreversible capacity for the first cycle is highly dependent on the mobility of Li+ and e during Li extraction process. As discussed by Li et al.,29 Li extraction reaction stops at a depth where the carriers cannot be sufficiently transported through the metal oxide phase in the time scale of the experiment. Li ion transport could be enhanced as a nanoporous structure is usually formed at the end of the first discharge,31 which facilitates the penetration of electrolyte into nanopores in the electrode. However, electron transport in the electrode may not be affected since electrolyte cannot supply the free electron to the electrode. Therefore, electron mobility is highly dependent on the original particle size. It can be seen that the MnO2/CNT nanocomposite electrode exhibits a much smaller irreversible 6900 | J. Mater. Chem., 2010, 20, 6896–6902

capacity for the first cycle compared to the pure MnO2 electrode, which suggests that the small nanoflakes with CNT core provide fast electron transport through the MnO2/CNT nanocomposite electrode. For the following cycles, charge and discharge are highly reversible due to stable microstructure after the first cycle.29 One voltage plateau for the discharge process and two voltage plateaus for the charge process can be observed, corresponding to different redox reactions during Li insertion/ extraction. To better illustrate the redox reactions, dQ/dV curves for the first two cycles are plotted in Fig. 5d. The two reduction peaks at 0.4 V and 0.8 V in the first cycle can be assigned to the electrochemical reduction of MnO2 with Li and formation of SEI layer. Two oxidation peaks at 1.2 and 2.2 V in the first cycle can be observed, indicating that the electrochemical oxidation reaction may proceed by two steps. In the second cycle, the 0.8 V reduction peak disappear, indicating that formation of the SEI layer only takes place during the first cycle. All reduction and oxidation peaks in the second cycle overlap well with those in the first cycle, indicating good electrochemical reversibility and structural stability. Fig. 6a compares the cyclic performance of the MnO2/CNT nanocomposite electrode with the flower-like MnO2 electrode. Both electrodes are cycled between 0.01 and 3 V at a current density of 200 mA g1 for 50 cycles. It can be seen that the MnO2/ CNT nanocmoposite electrode exhibits much better cycling stability than that of the MnO2 electrode. There is no capacity decay for the first 20 cycle for the MnO2/CNT nanocomposite electrode. However, the capacity decay for the MnO2 electrode is This journal is ª The Royal Society of Chemistry 2010

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Fig. 6 (a) Capacity vs. cycle number curves of the MnO2/CNT nanocomposite electrode and the MnO2 composite electrode at a current density of 200 mA g1. (b) Voltage profiles of a MnO2/CNT nanocomposite electrode at different current densities from 200 to 4000 mA g1.

drastic for the first 10 cycles. After 50 cycles, a specific capacity of 620 mA h g1 can be remained for the MnO2/CNT nanocomposite electrode, corresponding to 77% capacity retention. For the MnO2 electrode, a specific capacity of 115 mA h g1 can be retained, corresponding to 25% capacity retention. The cell using the MnO2/CNT nanocomposite electrode was further cycled at a high current density of 2000 mA g1 for another 50 cycles. The capacity decay after the initial 20th cycle slows down and the capacity is finally stabilized after 100 cycles (Fig. S4, ESI†). Similar cyclic performance is observed from the high loading electrode (Fig. S5, ESI†). The capacity decay for the MnO2/CNT nanocomposite electrode could be explained by two possible mechanisms. Firstly, there are probably some separated MnO2 nanoparticles formed without connection with CNT among the MnO2/CNT nanocomposite during the hydrothermal reaction. This phenomenon is observed and also obvious for the sample hydrothermally synthesized for 12 h. Due to the big volume change of MnO2 during repeat charge/discharge processes, these separated MnO2 nanoparticles may lose their electrical contact, leading to the capacity decay. Secondly, some MnO2 nanoflakes could peel off from the CNTs during the This journal is ª The Royal Society of Chemistry 2010

charge/discharge processes. The interfaces between the MnO2 nanoflakes and the CNTs are critical for a good capacity retention as poor interfaces may lead to poor adhesion between the MnO2 nanoflakes and the CNTs, finally resulting in debonding of the MnO2 nanoflakes from the CNTs. The acid-treated CNTs may have some defects on the surfaces, and hence the deposited MnO2 nanoflakes may have relatively poor adhesion to the CNTs’ surfaces. Nevertheless, most of the MnO2 nanoflakes have good adhesion with the CNTs, which is able to withstand big volume changes during charge/discharge, as can be demonstrated by the stabilized capacity after 100 cycles (Fig. S4, S5, ESI†). In addition to the good capacity retention ability, the MnO2/CNT nanocomposite electrode also exhibits a good rate capability as shown in Fig. 6b. With increase in current density, the MnO2/CNT electrode can deliver a reversible capacity of 820, 800, 650, 420, and 250 mA h g1 at a current density of 200, 400, 800, 2000, and 4000 mA g1, respectively. The high specific capacity, improved cycle performance and good rate capability of the MnO2/CNT nanocomposite electrode can be attributed to its unique hierarchical architecture. First, the highly porous structure of the MnO2 layer composing uniformly distributed nanoflakes enables large accommodation of volume change during Li insertion/extraction processes. It has been assumed that high strain induced electrical contact lose between electrode and current collector is the main reason for the capacity fade during the cycling for transition metal oxide. The extra space between the MnO2 nanoflakes can easily accommodate the volume change during Li insertion/extraction and reduce the associated stain. This advantage of present MnO2/CNT nanocomposite can be confirmed by its improved cycle performance compared to that of the coaxial MnO2/CNT array,9 which has a relatively denser MnO2 layer. Secondly, the deposition of MnO2 nanoflakes on CNT produce tiny flakes with much smaller dimension compared to self-assembled MnO2 nanoflakes synthesized by direct hydrothermal reaction or other methods.21,22 The small size of the MnO2 nanoflakes grown on CNTs enable fast ion and electron transport during Li insertion/ extraction. Last but not least, this nanocomposite using CNT as core enables improved electron transport compared to conventional nanostructured MnO2.

Conclusions In summary, a simple method has been developed to fabricate nanoflaky MnO2/CNT nanocomposite architecture with caterpillar-like morphology. The birnessite-type MnO2 synthesized by hydrothermal reaction for 6 h contains 0.2 K+ and 0.3 H2O per formula. When evaluated as anode material for lithium-ion batteries, the nanocomposite with a highly porous MnO2 layer deposited on CNT exhibits high reversible capacity, good cycle performance and good rate capability. The superior electrochemical performance of the nanocomposite electrode is due to its unique hierarchy architecture, which is able to accommodate large volume change and enable fast ion and electron transport. This caterpillar-like nanoflaky MnO2/CNT nanocomposite could be promising electrode material for lithium-ion batteries and also for other applications such as catalysis and supercapacitors. J. Mater. Chem., 2010, 20, 6896–6902 | 6901

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Acknowledgements This work was supported by the National University of Singapore, and Agency for Science, Technology and Research through the research grant R-265-000-292-305.

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References 1 Y. Wang and J. Y. Lee, Angew. Chem., Int. Ed., 2006, 45, 7039. 2 J. Liu, F. Liu, K. Gao, J. S. Wu and D. F. Xue, J. Mater. Chem., 2009, 19, 1051. 3 F. Chen, Z. Tao, J. Liang and J. Chen, Chem. Mater., 2008, 20, 667. 4 M. V. Reddy, T. Yu, C. H. Sow, Z. X. Shen, C. T. Lim, G. V. S. Rao and B. V. R. Chowdari, Adv. Funct. Mater., 2007, 17, 2792. 5 J. Liu, H. Xia, D. F. Xue and L. Lu, J. Am. Chem. Soc., 2009, 131, 12086. 6 N. Du, H. Zhang, B. Chen, J. B. Wu, X. Y. Ma, Z. H. Liu, Y. Q. Zhang, D. Yang, X. H. Huang and J. P. Tu, Adv. Mater., 2007, 19, 4505. 7 J. Liu, H. Xia, L. Lu and D. F. Xue, J. Mater. Chem., 2010, 20, 1506. 8 K. Saravanan, M. V. Reddy, P. Balaya, H. Gong, B. V. R. Chowdari and J. J. Vittal, J. Mater. Chem., 2009, 19, 605. 9 A. L. M. Reddy, M. M. Shaijumon, S. R. Gowda and P. M. Ajayan, Nano Lett., 2009, 9, 1002. 10 D. H. Park, S. H. Lee, T. W. Kim, S. T. Lim, S. J. Hwang, Y. S. Yoon, Y. H. Lee and J. H. Choy, Adv. Funct. Mater., 2007, 17, 2949. 11 R. Liu and S. B. Lee, J. Am. Chem. Soc., 2008, 130, 2942. 12 J. Luo, H. T. Zhu, H. M. Fan, J. K. Liang, H. L. Shi, G. H. Rao, J. B. Li, Z. M. Du and Z. X. Shen, J. Phys. Chem. C, 2008, 112, 12594. 13 J. X. Dai, S. F. Y. Li, K. S. Siow and Z. Q. Gao, Electrochim. Acta, 2000, 45, 2211.

6902 | J. Mater. Chem., 2010, 20, 6896–6902

14 J. Z. Zhao, Z. L. Tao, J. Liang and J. Chen, Cryst. Growth Des., 2008, 8, 2799. 15 M. S. Wu and P. C. Chiang, Electrochem. Commun., 2006, 8, 383. 16 B. X. Li, G. X. Rong, Y. Xie, L. F. Huang and C. Q. Feng, Inorg. Chem., 2006, 45, 6404. 17 M. S. Wu and P. C. Chiang, J. Phys. Chem. B, 2005, 109, 23279. 18 X. B. Jin, W. Z. Zhou, S. W. Zhang and G. Z. Chen, Small, 2007, 3, 1513. 19 X. F. Xie and L. Gao, Carbon, 2007, 45, 2365. 20 S. R. Sivakkumar, J. M. Ko, D. Y. Kim, B. C. Kim and G. G. Wallace, Electrochim. Acta, 2007, 52, 7377. 21 D. Yan, P. X. Yan, S. Cheng, J. T. Chen, R. F. Zhuo, J. J. Feng and G. A. Zhang, Cryst. Growth Des., 2009, 9, 218. 22 H. T. Zhu, J. Luo, H. X. Yang, J. K. Liang, G. H. Rao, J. B. Li and Z. M. Du, J. Phys. Chem. C, 2008, 112, 17089. 23 A. Ogata, S. Komaba, R. Baddour-Hadjean, J. P. Pereira-Ramos and N. Kumagai, Electrochim. Acta, 2008, 53, 3084. 24 S. B. Ma, K. W. Nam, W. S. Yoon, X. Q. Yang, K. Y. Ahn, K. H. Oh and K. B. Kim, J. Power Sources, 2008, 178, 483. 25 Z. Fan, J. H. Chen, M. Y. Wang, K. Z. Cui, H. H. Zhou and Y. F. Kuang, Diamond Relat. Mater., 2006, 15, 1478. 26 Z. Fan, J. H. Chen, B. Zhang, F. Sun, B. Liu and Y. F. Kuang, Mater. Res. Bull., 2008, 43, 2085. 27 K. P. Gong, P. Yu, L. Su, S. X. Xiong and L. Q. Mao, J. Phys. Chem. C, 2007, 111, 1882. 28 Z. P. Liu, R. Z. Ma, Y. Ebina, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2007, 19, 6504. 29 H. Li, P. Balaya and J. Maier, J. Electrochem. Soc., 2004, 151, A1878. 30 J. P. Liu, Y. Y. Li, X. T. Huang, R. Ding, Y. Y. Hu, J. Jiang and L. Liao, J. Mater. Chem., 2009, 19, 1859. 31 Y. S. Hu, Y. G. Guo, W. Sigle, S. Hore, P. Balaya and J. Maier, Nat. Mater., 2006, 5, 713.

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