Fully Reversible Conversion between SnO2 and Sn in SWNTs@SnO2

Aug 9, 2012 - Honeycomb-inspired design of ultrafine SnO 2 @C nanospheres embedded in carbon film as anode materials for high performance lithium- and...
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Fully Reversible Conversion between SnO2 and Sn in SWNTs@SnO2@ PPy Coaxial Nanocable As High Performance Anode Material for Lithium Ion Batteries Yi Zhao,†,‡ Jiaxin Li,† Ning Wang,†,‡ Chuxin Wu,† Guofa Dong,† and Lunhui Guan*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing, P. R. China S Supporting Information *

ABSTRACT: In this report, we designed a novel SWNTs@SnO2@PPy coaxial nanocable as superior anode material for the first time. The nanosized SnO2 particles (2−3 nm) were uniformly distributed between one dimension SWNTs core and PPy shell, as confirmed by XRD, SEM, and TEM characterizations. As an anode material for lithium ion batteries, this composite delivered a high capacity of 823 mAh g−1 at 150 mA g−1 after 100 cycles. Even at a high rate of 3000 mA g−1, a reversible capacity of 480 mAh g−1 still remained. Furthermore, the SnO2 in this composite exhibited a large capacity of 1486 mAh g−1 as well as good capacity retention of 95% over 100 cycles. This result indicated the completely reversible reaction between Li4.4Sn and SnO2, greatly improving the theoretical capacity of SnO2 from 782 to 1493 mAh g−1.

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INTRODUCTION In the past decade, transition metal oxides with high capacities from 700 to 1000 mAh g−1 have been widely investigated as alternative anode materials to replace the commercial graphite anode for lithium ion batteries (LIBs).1,2 Among them, SnO2 attracts particular attention due to its high theoretical capacity and safe working potential.3 Unfortunately, the huge volume change of SnO2 during discharge/charge process eventually causes the aggregation and pulverization of SnO2 with quick capacity fading. Meanwhile, the electron conductivity of SnO2 is poor. These two flaws severely hampered the practical usage of the SnO2 anode for LIBs. To mitigate these problems, nanosized SnO2 with porous structures, such as hollow spheres,4 nanotubes,5 nanosheets,6 and nanoboxes,7 have been synthesized to improve the cycling life. The other method is to design composites with SnO2 and carbon nanomaterials, which not only improve the conductivity but also accommodate the volume change of SnO2 during cycles.8−12 However, most of the reported composites delivered capacities below 600 mAh g−1 after cycles, because the phenomenon of aggregation and pulverization was inevitable if the loading ratio of SnO2 was too high. Thus, it is still a challenge to synthesize SnO2 based anode materials with high capacity and good cycling life. When SnO2 is used as an anode material for LIBs, both conversion and alloy mechanisms are involved. The reaction process could be described as SnO2 + 4Li+ + 4e− → Sn + 2Li 2O +

−

x Li + Sn + x e ↔ LixSn

(0 ≤ x ≤ 4.4)

© 2012 American Chemical Society

Normally, the alloy reaction 2 is thought to be highly reversible, providing a theoretical specific capacity of 782 mAh g−1 for SnO2. The conversion reaction in eq 1 is usually considered as electrochemically irreversible with no capacity contribution. Recently, several reports indicated the partially reversible of reaction 1.13−15 However, the extra capacity of 711 mAh g−1 from eq 1 was not manifested in these reports with capacities only around 600 mAh g−1. If the conversion reaction is highly reversible, the theoretical capacity of SnO2 could be improved from 782 to 1493 mAh g−1, which greatly meets the high capacity demand for next generation LIBs. In this work, SnO 2 nanoparticles (NPs) were first monodispersed on the surface of single-walled carbon nanotubes (SWNTs). The obtained SWNTs@SnO2 composite showed a high capacity of 1080 mAh g−1 after 20 cycles. To further improve the cycling performance, a conductive polypyrrole (PPy) coating was applied. The obtained SWNTs@SnO2@PPy composite exhibited high capacity, good cycling performance, and excellent rate performance as an anode material for LIBs. It delivered a high capacity of 823 at 150 mA g−1 after 100 cycles. Even at a high rate of 3000 mA g−1, the reversible capacity could be still retained at 480 mAh g−1. Furthermore, the SnO2 in the SWNTs@SnO2@PPy composite showed a large initial capacity of 1486 mAh g−1, which was almost equal to the expected theoretical capacity of 1493 mAh g−1, as well as good capacity retention of 95% over

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Received: April 27, 2012 Revised: July 16, 2012 Published: August 9, 2012

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100 cycles. To the best of our knowledge, our composite is among the first to demonstrate completely reversible conversion and alloy reactions of SnO2 with good capacity retention up to 100 cycles.

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EXPERIMENTAL SECTION Preparation of SWNTs@SnO2 Composite. The SWNTs with high purity were produced by an arc-discharge method.16 The SWNTs@SnO2 composite was synthesized via a noncovalent method based on our previous reports.10,17,18 Briefly, 20 mg of SWNTs was sonicated in 50 mL of deionized water containing 0.66 mmol phthalic acid and SnCl4·5H2O to form a homogeneous solution. Then, 500 mg urea was added into the above black dispersion and the mixture was stirred at 80 °C for 20 h. The obtained product was filtered, washed several times with deionized water, and dried at 80 °C for 12 h. Preparation of SWNTs@SnO2@PPy Composite. In a typical experiment, 40 mg of SWNTs@SnO2 composite was first sonicated in 20 mL of deionized water containing 2 mg of sodium lauryl sulfate (SDS) for 0.5 h and stirred magnetically for 3 h. Then, 10.3 μL of pyrrole was added and stirring continued for 1 h. Finally, 2 mL of 0.1 M ammonium persulfate aqueous solution was slowly dropped into the above solution. The polymerization process was kept under stirring for 4 h at room temperature. The final composite was filtered, washed with deionized water and ethanol several times, and then dried at 80 °C overnight. Materials Characterization. The structure and morphology of the composites were characterized by X-ray diffraction (XRD, RIGAKU SCXmini), X-ray photoelectron spectroscopy (XPS, ESCALAB 250), scanning electron microscope (SEM, JSM-6700F), and transmission electron microscope (TEM, JEM-2010). Thermogravimetry analyses (TGA, NETZSCH STA449C) were measured from 30 to 1000 °C at a heating rate of 10 K min−1 in air, to evaluate the weight content of SnO2 in these composites. Electrochemical Measurements. The electrochemical tests were performed via CR2025 coin-type test cells. To fabricate the working electrodes, 80 wt % active material (SWNTs@SnO2@PPy, or SWNTs@SnO2), 10 wt % conductivity agent (ketjen black, KB), and 10 wt % polymer binder (Carboxymethyl cellulose, Na-CMC) were mixed with deionized water and then pasted on Ni foam. The electrodes were dried at 80 °C for 12 h in a vacuum. Cells were assembled in an Ar-filled golve box with the concentration of moisture and oxygen below 1 ppm. Pure lithium foil was used as both counter and reference electrode. The electrolyte was 1 M LiPF6 in EC/EMC/DMC (1:1:1 in volume). A Celgard 2300 membrane was used as the separator. The galvanostatic discharge/charge cycles were carried out on a LAND 2001A system over a voltage range of 0.05 to 3.00 V at room temperature. Cyclic voltammetry (CV) tests were performed on a CHI660C Electrochemical Workstation. The specific capacities in this article were calculated based on the overall mass of the composite.

Figure 1. Schematic illustration (not to scale) of the preparation process of the SWNTs@SnO2@PPy coaxial nanocable.

coated with a conductive PPy layer, to obtain the final SWNTs@SnO2@PPy coaxial nanocable. This unique architecture could not only improve the electron conductivity but also effectively alleviate the volume expansion and aggregation of SnO2 NPs during discharge/charge cycles, which is benefit for the superior electrochemical performance of SWNTs@SnO2@ PPy composite. The crystalline structures of these samples were characterized by X-ray diffraction (XRD). As shown in Figure 2, the main

Figure 2. X-ray power diffraction (XRD) patterns of the SWNTs, SWNTs@SnO2, and SWNTs@SnO2@PPy composites.

diffraction peaks of SWNTs@SnO2 and SWNTs@SnO2@PPy composites at 26.2°, 33.5°, 51.6°, and 65.2° were well assigned to (110), (101), (211), and (112) planes of the tetragonal structure of SnO2 (JCPDS Card No. 41-1445), respectively. The diffraction peak of SWNTs near 26.5° was overlapped with the (110) plane of the SnO2 in the composites. The broad peaks with low intensities in these two composites indicated the small size of the SnO2 particles. The morphologies of the samples were carried out using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Figure 3a−c shows the SEM and TEM images of the SWNTs. The pristine SWNTs had an ultrahigh purity of 99.5 wt % with no impurities could be found.16 The average diameter of a SWNT was about 1.4 nm, as clearly observed from the SWNTs bundles in Figure 3c. Figure 3d−f disclosed that the SnO2 NPs were uniformly monodispersed on the surface of the SWNTs. Isolated SnO2 outside SWNTs were barely found, indicating that nearly all SnO2 NPs with a weight content of 73 wt % (Figure S1, Supporting Information) were located on SWNTs. The average diameter of the SnO2 NPs was determined to be 2−3 nm (Figure 3f), which was consistent with the XRD results. It should be noted that, even after strong ultrasonic treatment during the preparation of TEM sample, SnO2 NPs were still closely anchored on the SWNTs, showing the strong interaction between SnO2 and SWNTs. To further enhance

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RESULTS AND DISCUSSION Figure 1 illustrates the schematic process for the synthesis of SWNTs@SnO2@PPy coaxial nanocable. First, SnO2 NPs were monodispersed on the surface of SWNTs via a noncovalent method, which effectively preserved the good conductivity of the SWNTs. Then, the SWNTs@SnO2 composite was further 18613

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Figure 3. SEM and TEM images of (a−c) SWNTs, (d−f) SWNTs@SnO2 composite, and (g−i) SWNTs@SnO2@PPy composite.

the electrochemical performance of SWNTs@SnO2, a conductive PPy coating was applied.19 Compared with the carbon coating, the synthesis of the PPy layer is more facile without the hydrothermal conditions and high temperature calcination process.20,21 The obtained SWNTs@SnO2@PPy composite exhibited one-dimensional morphology with diameters around 50−100 nm and lengths of several micrometers, as shown in the SEM images in Figure 3g and Figure S2. Figure 3h clearly exhibited the coaxial nanocable structure of the SWNTs@ SnO2@PPy composite with SnO2 homogeneously distributed between SWNTs core and PPy shell. The thickness of the PPy layer was about 8−20 nm. The clear interlayer spacing of 0.33 nm, corresponding to the (110) planes of SnO2, could be found in the HRTEM images (Figure S3). The loading ratio of SnO2 in the SWNTs@SnO2@PPy composite was about 55 wt % based on the TGA result in Figure S1. To investigate the electrochemical behavior of the SWNTs@ SnO2@PPy composite, a cyclic voltammetry (CV) test was first carried out. Figure 4a shows the initial five CV curves of the SWNTs@SnO2@PPy composite at a scan of 0.2 mV s−1 from 0.05 to 3.00 V. There was a broad reduction peak near 0.9 V in the first cathodic scan, which was attributed to the formation of a solid electrolyte interface (SEI) layer and the conversion of SnO2 to metallic Sn, as depicted in eq 1. This peak with a significant drop in current after the first cycle corresponded to the conversion reaction only. Two characteristic pairs of redox peaks were clearly observed in the CV curves, located at 0.08 and 0.57 V and 0.9 and 1.3 V, respectively. The first pair was due to the highly reversible alloying and dealloying process of LixSn in eq 2. The second redox couple could be attributed to the conversion reaction between SnO2 and Sn in eq 1. Meanwhile, the CV curves after the first cycle were almost overlapped together, indicating the reversible reactions in eqs 1

Figure 4. (a) Cyclic voltammograms of the SWNTs@SnO2@PPy composite at a scan rate of 0.2 mV s−1 between 3 and 0.05 V. (b) High resolution XPS spectra of Sn3d in the SWNTs@SnO2@PPy composite before and after cycles.

and 2 as well as the good cycling performance of the SWNTs@ SnO2@PPy composite. 18614

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Figure 5. Discharge/charge curves for selected cycles of (a) the SWNTs@SnO2 composite and (b) the SWNTs@SnO2@PPy composite. (c) Cycling performance of the SWNTs@SnO2 and SWNTs@SnO2@PPy composites. (d) Charge capacity of the SnO2 in the SWNTs@SnO2@PPy composite (capacity calculated based on the mass of SnO2) at a rate of 150 mA g−1 from 0.05 to 3.00 V.

eqs 2 and 1, respectively. The electrochemical behavior of SWNTs@SnO2 was similar to that of the SWNTs@SnO2@PPy composite. The initial discharge/charge curves of the SWNTs@SnO2@PPy composite in Figure 5b delivered capacities of 1314 and 847 mAh g−1, corresponding to a high Coulombic efficiency of 64.4%, which was higher than ∼50% of the SnO2/graphene composite.23,24 The initial capacity loss was attributed to the irreversible formation of the SEI layer and electrolyte decomposition. It is worth noting that the charge curves in the following cycles almost overlapped with the first one, showing a good cycling performance of the SWNTs@ SnO2@PPy composite. Figure 5c compares the cyclic performance of the SWNTs@ SnO2 and SWNTs@SnO2@PPy composites between 0.05 and 3.00 V at a rate of 150 mA g−1. In our previous work, the pure SnO2 NPs suffered from drastic capacity fading with only a low capacity of 65 mAh g−1 remaining after 100 cycles.10 By using SWNTs as a carbon matrix, the cycling performance of the SWNTs@SnO2 composite was slightly improved. As shown in Figure 5c, the SWNTs@SnO2 composite delivered a high charge capacity of 1080 mAh g−1 after 20 cycles with a small capacity fading of 4.7 mAh g−1 per cycle. Such high capacity was barely observed in the SnO2 based composite. However, the capacity of the SWNTs@SnO2 composite gradually decreased after 20 cycles and only remained at 196 mAh g−1 in 85 cycles, which was due to the inevitable aggregation and pulverization of SnO2 NPs on the surface of SWNTs without coating.21 In comparison, the SWNTs@SnO2@PPy coaxial nanocable delivered excellent cycling performance. After 100 successive cycles, the SWNTs@SnO2@PPy composite was able to deliver a high capacity of 823 mAh g−1 with minimal capacity

Since eq 1 is commonly thought to be irreversible, we further prove the possibility of the reversible conversion reaction via the X-ray photoelectron spectroscopic (XPS) experiment. Figure 4b shows the high resolution XPS spectra of Sn3d in the SWNTs@SnO2@PPy composite before and after cycles. As can be seen, there were two peaks at 486.7 and 495.1 eV, corresponding to Sn3d2/5 and Sn3d2/3 of the SnO2, in the XPS spectrum of the pristine composite.22 Even after 100 discharge/ charge cycles, these two representative peaks of SnO2 were still observed, which provided further evidence for the reversible reaction of Li2O and Sn to yield SnO2 during cycles. The full XPS spectra of these two samples are shown in Figure S4. Compared with the pristine sample, some additional peaks from the SEI layer (F 1s) and Ni current collector (Ni 2P) were also observed in the XPS spectrum after cycles. According to the CV and XPS experiments, we could deduce that the reaction in eq 1 is reversible. Thus the overall electrochemical reaction of SnO2 in this composite could be described as SnO2 + 8.4Li+ + 8.4e− ↔ Li4.4Sn + 2Li 2O

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Figure 5a,b exhibits the typical discharge/charge profiles of the SWNTs@SnO2 and SWNTs@SnO2@PPy composites at a current density of 150 mA g−1 from 0.05 to 3.00 V. As shown in Figure 5a, the first discharge and charge capacities of the SWNTs@SnO2 composite were 1733 and 1175 mAh g−1, respectively. In the first discharge profile, the sloped region from 1.2 to 0.7 V was ascribed to the formation of the SEI layer and reduction of SnO2 to Sn. The region below 0.7 V corresponded to the formation of a series of Sn−Li alloys. The voltage regions at 0.05−1.0 V and above 1.0 V in the first charge curve could be due to the reverse reaction described in 18615

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fading, which was better than most of the SnO2 based composites. Furthermore, these two anodes showed different Coulombic efficiency, as shown in Figure S5. The Coulombic efficiency of the SWNTs@SnO2@PPy anode quickly increased to 98% since the fifth cycle, whereas the SWNTs@SnO2 anode delivered lower Coulombic efficiency. Especially from the 20th to 60th cycle, only Coulombic efficiencies of ∼95% remained. This low value of the SWNTs@SnO2 anode was due to the formation of fresh surface area, resulting from the large volume expansion and extraction of SnO2 NPs during cycles, increasing the side reaction between the electrolyte and the electrode.24 Thus, the SWNTs@SnO2@PPy composite delivered a superior cycling performance as well as higher Coulombic efficiency than the SWNTs@SnO2 anode during cycles. To evaluate the contribution of SnO2 in the SWNTs@ SnO2@PPy composite, the specific capacity calculated based on the mass of SnO2 is given in Figure 5d. The weight content of SWNTs in the SWNTs@SnO2@PPy composite could be calculated to be 11 wt % based on the feed ratio of SWNTs and SnO2. Since the lithium storage of pure PPy was negligible,25 the contribution of SnO2 in this composite was calculated as follows: CSnO2 = [Ctotal − CSWNTs0.11]/0.55. The SWNTs delivered a specific capacity of ∼270 mAh g−1 based on our previous experiments.26 Thus, the first charge capacity of the SnO2 was about 1486 mAh g−1, which was almost equal with the expected theoretical capacity of 1493 mAh g−1 for the SnO2 anode, according to eq 3 with 8.4 Li+ ion per formula. As shown in Figure 5d, the SnO2 in this composite showed stable cycling performance with a high capacity retention of 95% after 100 cycles. Thus we could demonstrate the completely reversible electrochemical reaction between SnO2 and Li4.4Sn in our composite with a high theoretical capacity of 1493 mAh g−1 based on eq 3. To our best knowledge, the SWNTs@SnO2@ PPy composite is among the first to prove the highly reversible conversion and alloy reactions of the SnO2 anode with a long cycling life up to 100 cycles. Figure 6 illustrates the rate performance of the SWNTs@ SnO2@PPy composite at various current densities from 150 to 3000 mA g−1. The typical discharge/charge profiles are shown in Figure 6a. As can be seen, these curves kept similar shapes with only a small overpotential observed and exhibited high capacities even at large current densities. As shown in Figure 6b, the specific capacities of this composite were 880, 752, 703, and 603 mAh g−1 when cycled at 150, 500, 1000, and 2000 mA g−1, respectively. Even at an ultrahigh current density of 3000 mA g−1, this composite still delivered a capacity of 480 mAh g−1, which was higher than the theoretical capacity (372 mAh g−1) of the commercial graphite anode. Such excellent rate performance of our composite is much better than that of the recently reported SnO2/graphene composite.22,23,27 When back to 150 mA g−1, a capacity of 860 mAh g−1 can be restored and kept stable up to 60 cycles, indicating the good stability of this composite. Figure 6c shows the good cycling performance of the SWNTs@SnO2@PPy composite at 1000 mA g−1 after being activated at 150 mA g−1 in the initial two cycles. As can be seen, a capacity of 553 mAh g−1 was still remained after 100 cycles even at 1000 mA g−1 and the Coulombic efficiency kept above 98% during cycles. Compared with the recently reported SiC@SnO2@graphene composite with full use of the lithium storage of SnO 2 , this SWNTs@SnO 2 @PPy composite exhibited better cycling performance and rate capabilities.28 To the best of our knowledge, such outstanding electro-

Figure 6. (a) Typical discharge/charge curves of the SWNTs@SnO2@ PPy composite at various rates from 150 to 3000 mA g−1. (b) Rate capabilities of the SWNTs@SnO2@PPy composite. (c) Cycling performance of the SWNTs@SnO2@PPy composite at a high rate of 1000 mA g−1.

chemical performance of the SWNTs@SnO2@PPy composite, including high capacity, good cycling retention (823 at 150 mA g−1 after 100 cycles), and excellent rate performance (480 at 3000 mA g−1), was one of the best results for SnO2 based composite. To further understand the excellent electrochemical performance of the SWNTs@SnO2@PPy composite, we decomposed a cell after 100 cycles and characterized the morphology by TEM. Seen in Figure 7, the one-dimensional morphology was still preserved, showing the good stability of this composite during cycles. The SAED pattern in Figure S6 indicated the amorphous property of the SnO2 particles after cycles. Thus,

Figure 7. TEM images of the SWNTs@SnO2@PPy composites after 100 cycles at 150 mA g−1. 18616

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(7) Wang, Z. Y.; Luan, D. Y.; Boey, F. Y. C.; Lou, X. W. J. Am. Chem. Soc. 2011, 133, 4738. (8) Lou, X. W.; Li, C. M.; Archer, L. A. Adv. Mater. 2009, 21, 2536. (9) Paek, S. M.; Yoo, E.; Honma, I. Nano Lett. 2009, 9, 72. (10) Zhao, Y.; Li, J. X.; Ding, Y. H.; Guan, L. H. R. Soc. Chem. Adv. 2011, 1, 852. (11) Noerochim, L.; Wang, J. Z.; Chou, S. L.; Wexler, D.; Liu, H. K. Carbon 2012, 50, 1289. (12) Ren, J. G.; Yang, J. B.; Abouimrane, A.; Wang, D. P.; Amine, K. J. Power Sources 2011, 196, 8701. (13) Demir-Cakan, R.; Hu, Y. S.; Antonietti, M.; Maier, J.; Titirici, M. M. Chem. Mater. 2008, 20, 1227. (14) Chen, Y.; Huang, Q. Z.; Wang, J.; Wang, Q.; Xue, J. M. J. Mater. Chem. 2011, 21, 17448. (15) Liu, H. P.; Long, D. H.; Liu, X. J.; Qiao, W. M.; Zhan, L.; Ling, L. C. Electrochim. Acta 2009, 54, 5782. (16) Wu, C. X.; Li, J. X.; Dong, G. F.; Guan, L. H. J. Phys. Chem. C 2009, 113, 3612. (17) Zhao, Y.; Li, J. X.; Wu, C. X.; Guan, L. H. Nanoscale Res. Lett. 2011, 6, 71. (18) Zhao, Y.; Li, J. X.; Ding, Y. H.; Guan, L. H. J. Mater. Chem. 2011, 21, 19101. (19) Shao, Q. G.; Chen, W. M.; Wang, Z. H.; Qie, L.; Yuan, L. X.; Zhang, W. X.; Hu, X. L.; Huang, Y. H. Electrochem. Commun. 2011, 13, 1431. (20) Wu, P.; Du, N.; Zhang, H.; Yu, J. X.; Yang, D. R. J. Phys. Chem. C 2010, 114, 22535. (21) Ding, S. J.; Chen, J. S.; Lou, X. W. Chem.-Asian J. 2011, 6, 2278. (22) Liang, R. L.; Cao, H. Q.; Qian, D.; Zhang, J. X.; Qu, M. Z. J. Mater. Chem. 2011, 21, 17654. (23) Zhong, C.; Wang, J. Z.; Chen, Z. X.; Liu, H. K. J. Phys. Chem. C 2011, 115, 25115. (24) Li, X.; Meng, X.; Liu, J.; Geng, D.; Zhang, Y.; Banis, M. N.; Li, Y.; Yang, J.; Li, R.; Sun, X. Adv. Funct. Mater. 2012, DOI: 10.1002/ adfm.201101068. (25) Cui, L. F.; Shen, J. A.; Cheng, F. Y.; Tao, Z. L.; Chen, J. J. Power Sources 2011, 196, 2195. (26) Li, J. X.; Wu, C. X.; Guan, L. H. J. Phys. Chem. C 2009, 113, 18431. (27) Zhang, B. A.; Zheng, Q. B.; Huang, Z. D.; Oh, S. W.; Kim, J. K. Carbon 2011, 49, 4524. (28) Chen, Z. X.; Zhou, M.; Cao, Y. L.; Ai, X. P.; Yang, H. X.; Liu, J. Adv. Energy Mater. 2012, 2, 95. (29) Wu, H.; Zheng, G. Y.; Liu, N. A.; Carney, T. J.; Yang, Y.; Cui, Y. Nano Lett. 2012, 12, 904.

this unique coaxial nanocable structure had several advantages for improving the electrochemical performance. First, the one dimension structure of the SWNTs core and PPy coating greatly improved the electron and ion conductivity of this composite. Second, the PPy coating hampered the direct contact between electrolyte and SnO2 and formed a stable thin SEI layer on the surface, resulting in the high Coulombic efficiency of this composite during cycles.29 Third, this unique coaxial nanocable architecture, with SnO2 NPs uniformly monodispersed between SWNTs and PPy, could effectively accommodate the volume expansion of SnO2, prevent the aggregation of Sn, and maintain the good contact between Li2O and Sn under discharge/charge process, to realize the highly reversible conversion and alloy reactions of SnO2 during cycles with good cycling performance.

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CONCLUSIONS In summary, a novel SWNTs@SnO2@PPy composite was developed to realize the fully reversible conversion between SnO2 and Sn. Due to the unique coaxial nanocable architecture, the SnO2 in this composite delivered a high capacity of 1487 mAh g−1 with good capacity retention, which was among the first to demonstrate the completely reversible conversion and alloy reactions of SnO2 as well as long cycling life up to 100 cycles. Meanwhile, as an anode material for LIBs, the SWNTs@ SnO2@PPy composite showed outstanding electrochemical performance with good cycling retention (high capacity of 823 mAh g−1 after 100 cycles) and excellent rate capacity (480 at 3000 mA g−1).

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ASSOCIATED CONTENT

S Supporting Information *

TGA, low magnification SEM, HRTEM, TEM after cycles, and Coulombic efficiency of the SWNTs@SnO2@PPy composite. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support provided by the National Key Project on Basic Research (Grant No. 2011CB935904) and the National Natural Science Foundation of China (Grant Nos. 21171163 and 91127020).

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REFERENCES

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