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Publication Date (Web): October 11, 2012 ... Sn nanoparticles inside few-walled carbon nanotubes and its effect on lithium ion storage property have b...
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Enhanced Lithium Ion Storage Property of Sn Nanoparticles: The Confinement Effect of Few-Walled Carbon Nanotubes Hongkun Zhang,† Huaihe Song,*,† Xiaohong Chen,† and Jisheng Zhou† †

State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: The confinement of Sn nanoparticles inside few-walled carbon nanotubes and its effect on lithium ion storage property have been investigated in detail. It was found that the charge transfer and electronic interaction facilitated Sn to remain in a more reduced state and to link strongly with interior surface of carbon nanotubes by Sn−C bonds, leading to a high reversible capacity of 732 mAh g−1 and capacity retention of 639.7 mAh g−1 after 170 cycles at 50 mA g−1. We also found that the volume restriction inside CNTs protected Sn−C bonds against breakage during lithium insertion/extraction, contributing to the excellent cycling performance with the fade rate of only 0.074% per cycle.

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INTRODUCTION Owing to their high thermal stability, great mechanical properties, and excellent electrical properties, carbon nanotubes (CNTs) have triggered interest as one-dimensional conductor, catalyst support, hydrogen storage, electron field emission, electronic devices, and electrochemical devices.1 CNTs are composed of rolled-up graphene layer with unique tubular structure, and this high curvature of rolled graphene wall leads to the shift of π-electron density from concave to convex surface, leaving the interior surface electron-deficient and exterior surface electron-rich.2 Therefore, the inner tubular channels of CNTs, like a nanoreactor,3 can create a wellconfined environment ranging from less than 1 nm up to 100 nm, which is supposed to be different from the outside. For example, water molecules form a layered cylindrical structure inside CNTs, and each layer is composed of seven water molecules hydrogen-bonded into a heptagonal ring;4 H2 molecules inside CNTs favor being aligned along the long axis in deuterium−hydrogen exchange reaction;5 the reduction of Fe2O3 nanoparticles and Fe3O4 nanowire inside CNTs is facilitated;6 the activation energy and reaction endothermicity are considerably reduced for the Menshutkin SN2 reaction inside CNTs.7 In addition, the catalytic activity is also significantly affected by the confinement inside CNTs:8 a higher ethanol production in syngas conversion being obtained © 2012 American Chemical Society

from Rh−Mn inside CNTs for Mn remains in a more reduced state and attracts O of CO that is absorbed on adjacent Rh species to form a titled absorption of CO, facilitating the dissociation of CO and increasing the C 2 oxygenate formation;8a the activity and selectivity toward C5+ hydrocarbon of Fe inside CNTs being enhanced in Fischer−Tropsch synthesis (FTS), because the iron species are prone to form more iron carbides with high FTS activity, and the spatial restriction of well-confined channel traps the interaction intermediates to prolong their contact time with Fe and prevent it from diffusion and aggregation;8b high activity in NH3 decomposition being produced from Fe−Co inside CNTs for the thermal stability of Fe−Co particles is enhanced.8c As one of the promising anode materials in lithium ion batteries (LIBs), Sn possesses high specific capacity (991 mAh g−1), no solvent intercalation, and low cost,9 but its major drawback is the large volume expansion (about 300%) in Li−Sn alloying and dealloying process, leading to pulverization and electrical disconnection of the electrode as well as rapid capacity fading.10 However, CNT-support Sn-based composites exhibit an enhanced reversible capacity and stable cyclability. Received: August 29, 2012 Revised: October 8, 2012 Published: October 11, 2012 22774

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ethanol solution and dried at 323.15 K in vacuum, and Sn-inCNT was obtained. Sn-out-CNT was prepared through the same process of Sn-in-CNT, except reducing the filling time from 96 to 4 h with only 3 g of SnCl2·2H2O. The contents of Sn were obtained by thermogravimetric analysis (TGA; Netzsch STA 449C) to be 39.3% and 39.8% in Sn-in-CNT and Sn-out-CNT, respectively. The CNTs−Sn composites were characterized by X-ray diffraction (XRD; Rigaku D/max-2500B2+/PCX system with Cu Kα radiation, λ = 1.5406 Å, 2θ = 5−90°), high-resolution transmission electron microscopy (HRTEM; JEOL JEM-3010F operating at 300 kV), Raman spectra (JY HR800 with an excitation line of λ = 532 nm), and X-ray photoelectron spectroscopy (XPS; ESCALAB 250 with monochromatic Al K X-ray sources of 30 eV pass energy in 0.5 eV step over an area of 650 mm × 650 mm). The working electrode was prepared by mixing the CNT−Sn powder with poly(vinylidene difluoride) (PVDF) in a mass ratio of 9:1, and N-methylpyrrolidinone (NMP) was added to form a slurry for spreading the composites. Then the working electrodes were dried at 353 K for 4 h and 393 K for 12 h and assembled with the lithium foil used as the negative electrode and 1 mol/L LiPF6 solution in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte in a recirculating argon glovebox where both the moisture and oxygen contents are below 1 ppm. The cell was galvanostatically charged and discharged over the potential range from 0.01 to 2.50 V versus Li/Li+ at various current densities (50 to 2000 mA g−1). Over the range from 2.50 to 0.01 V at a scanning rate of 0.1 mV/s, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical workstation (CHI 660B). The ac impedance spectra were obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range from 100 to 0.01 kHz.

SnSb-out-CNT with 47.6% of SnSb content showed the capacity retention of 480 mAh g−1 after 50 cycles at 100 mA g−1, covering 81.5% of the theoretical capacity,11 while SnSb-inCNT with 92.4% of SnSb content exhibited a higher reversible capacity of 672 mAh g−1 after 80 cycles at 0.2 C and more excellent cyclability of the average fade rate to be only 0.064% than SnSb-out-CNT (0.59%).12 The robust tube wall of CNTs was ascribed to be the key factor to this excellent cycling performance which kept Sn−Sb in the good electrical and mechanical contact inside CNTs. Wang et al.13 have produced Sn-in-CNT composites with 37.6 wt % of Sn and found it exhibited good lithium storage property with the capacity retention of 470 mAh g−1 after 80 cycles which equaled 82.8% of its theoretical capacity. The large void space inside CNTs (200 nm) was supposed to be beneficial to buffering the volume expansion of Sn. Besides, they believed the encapsulation structure of Sn inside CNTs was very responsible for the cycle improvement due to their good electrical contact and the reconnection again of dislocated Sn to other interior part of CNTs. On the contrary, if Sn was coated on exterior surface, its detachment would result in electrical disconnection and permanent loss of use. Up to now, the advantages of CNTs incorporation for most Sn/SnO2-in-CNT composites are simply ascribed to their excellent electrical conductivity, ensuring the good contact of Sn with CNTs, as well as to the interior space and the tube wall accommodating the volume expansion of Sn,13,14 but there are no systematic study or convincing evidence to confirm how the confined spaces of CNTs affect the lithium ion storage properties. Until recently, Chen and co-workers15 have found that the confinement of MnO2 inside CNTs leads to partially formation of Mn2O3, which is very helpful to obtain superior pseudocapacitance in electrochemical capacitors. In the present work, we synthesized few-walled carbon nanotubes (FWCNTs) encapsulated Sn composites (Sn-inCNT) and investigated the interaction and confinement of Sn inside the interior nanospace. We found the charge transfer and electronic interaction facilitated Sn to remain in a more reduced state and to link strongly with interior surface of CNTs by Sn− C bonds, which greatly improved its electrochemical performance: with 39.3% Sn content, it exhibited the reversible capacity of 732 mAh g−1 and maintained 639.7 mAh g−1 after 170 cycles at 50 mA g−1, while the reversible capacity of Sn-out-CNT (39.8 wt % of Sn) was 493 mAh g−1 and its retention was only 351.1 mAh g−1. To the best of our knowledge, it was the first experimental observation and confirmation that the confinement of Sn inside CNTs significantly affected its electrochemical performances for LIBs.

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RESULTS AND DISCUSSION XRD results confirm the presence of Sn in composites with good crystallinity (JCPDS card No. 04-0673). As shown in Figures 1a and 1d, both Sn-in-CNT and Sn-out-CNT show the characteristic peaks of metallic Sn at diffraction angles of 2θ = 30.64°, 32.00°, 43.84°, and 44.06°, corresponding to the (200), (101), (220), and (211) planes, respectively. There is no SnO2 diffraction peak, which indicates high purity of Sn. From HRTEM images in Figures 1b and 1c, it can be seen that Sn

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EXPERIMENTAL SECTION Carbon nanotubes (Shenzhen Nanotech Port Co. Ltd.), comprising 2−5 tube walls with average diameter of 2−5 nm and length of 5−15 μm, were activated to open the end by KOH (mass ratio of 1:7 for CNTs to KOH) at 973 K for 2 h under a N2 atmosphere and were reheated at 1073 K for 2 h after eliminating KOH by deionized water. Then 50 mg of activated CNTs and 3 g of SnCl2·2H2O were put into 10 mL of ethanol to be sonicated for 30 min and mechanically stirred for 96 h, during which another 6 g of SnCl2·2H2O was put into the suspension within 48 h. After that, the mixture was washed by ethanol solution several times to remove the outer SnCl2 around CNTs and reduced by NaBH4 ethanol solution under a N2 atmosphere. Finally, the sample was washed again by

Figure 1. XRD patterns and HRTEM images of Sn-in-CNT (a−c) and Sn-out-CNT (d−f). 22775

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breathing mode (RBM, 100−300 cm−1) is indicative of smaller diameters of CNTs. The frequency ωRBM (cm−1) is inversely related to tube diameter of CNTs,18which can be calculated from the equation ωRBM = 234/dt +10, in which dt is the tube diameter (nm). The D-band corresponds to A1g mode that arises from the disordered carbon, such as defects of the graphitic lattice and the vibration of carbon atoms at the edge of carbon nanotubes.18a,19 The G-band is related to the graphite E2g mode, resulting from the vibration of sp2-bonding carbon atoms in hexagonal graphene plane. The shift of G peak’s location is always used to reveal the charge transfer between metal and carbon nanotubes.20 In our cases, the G peak of CNTs locates at 1592.0 cm−1, and there exists a blue-shift by 2.4 cm−1 for Sn-in-CNT and red-shift by 3.6 cm−1 for Sn-outCNT (Supporting Information SI-1). Because of deviation from planarity, π-electron density shifts from concave inner to convex outer surface of CNTs, leading to electron deficiency of interior surface and electron enrichment of exterior surface.2 Therefore, for Sn-out-CNT, the electrons transfer from the electron-enriched exterior surface to Sn corresponding to the red-shift of G peak; on the contrary, for Sn-in-CNT, the charge transfer from electron-donor Sn to the electron-deficient interior surface to compensate for the electron density loss resulting in the blue-shift. And the blue-shift leaves Sn in a more reduced state, which is helpful to attract the absorbate like lithium ions. The ID/IG ratio, reflecting the degree of disordered structure in carbon materials, is 0.215 for CNTs, and it increases to 0.325 (Sn-out-CNT) and 0.557 (Sn-in-CNT). The increase of ID/IG ratio suggests not only the breaking of sp2 bonds with the formation of shorter carbon chains because of metallic Sn incorporation,21 but also the chemical interaction between Sn and CNTs, rather than physical adsorption.11,22 Cyclic voltammogram (CV) is a useful technique to investigate the lithium insertion/extraction behavior of the electrode. For CNTs electrode (Supporting Information SI-2), there is a sharp and intense reduction peak at around 0.6 V in the first cathodic sweep, and it disappears in the next cycles. This peak corresponds to the formation of solid electrolyte interface (SEI) film.23 In the first anodic sweep, there are two oxidation peaks at 0.2 and 1.23 V: one at 0.2 V is related to the lithium extraction out from the graphite layer,23 and the other at 1.23 V is a reversible process being ascribed to the reaction between lithium ions and the surface functional groups on CNTs.24 For Sn-out-CNT (Figure 4a), there are four intense reduction peaks at 1.5, 1.0, 0.7, and 0.12 V in the first cathodic sweep. The peak at 0.7 V can be attributed to the SEI film, and the other three peaks are related to the characteristic alloying process between lithium ions and Sn, described in eq 1.10,25

nanoparticles (NP) is encapsulated into interior hollow cavity of CNTs, and the exterior surface of CNTs is clear, which is very indicative of the ideal encapsulation structure. Owing to tiny confined interior space, the particle size of Sn is restrained to be 4 nm (Figure 1c). As to Sn-out-CNT (Figures 1e and 1f), Sn is dispersed on exterior surface of CNTs with the average particles size of 6 nm. The contact between metal/metal oxide and carbon can be classified into physical adsorption and chemical links, and the chemical links greatly affect the oxidation reactivity of carbon. Neeft et al.16 reported the tight contact between Fe2O3/MoO3 and carbon could lead to the accelerated oxidation of carbon. Zhou et al.17 also found that the oxidation temperature of graphene sheets decreased a lot on account of the tight contact between Fe3O4 and graphene. DSC analysis in air atmosphere is carried out to explore the oxidation behaviors of CNT−Sn composites (Figure 2). For CNTs, there is an exothermic peak

Figure 2. DSC curves of CNTs and CNT−Sn composites.

at 490 °C, which can be attributed to the oxidation of CNTs and emission of CO2/CO gas, while for Sn-out-CNT and Snin-CNT, the combustion temperatures decrease to 396 and 360 °C, respectively, and this decrease indicates that Sn incorporation leads to the higher oxidation activity of CNTs. The lower CNTs combustion temperature of Sn-in-CNT suggests the tighter contact of Sn with interior concave surface than exterior surface.16 Raman spectroscopy is employed to further investigate the contact and interaction between Sn and CNTs. In Figure 3a, we can see that CNTs display three typical features at 100−300, 1340, and 1592 cm−1, which correspond to the radial breathing mode, D-band, and the G-band, respectively. The radial

Sn + x Li+ + x e− ↔ LixSn

(0 ≤ x ≤ 4.4)

(1)

Figure 4. Cyclic voltammograms of Sn-out-CNT (a) and Sn-in-CNT (b).

Figure 3. Raman spectra of CNTs and CNTs−Sn composites. 22776

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And in the first three anodic sweeps, there are four intense oxidation peaks between 0.4 and 0.8 V. It is at the potential plateau from 0.8 to 0.4 V that a subsequence of intermetallic phases LixSn (x = 0.4−4.4) can be formed in the electrochemical reaction of Sn and lithium ions.9 And the reduction peaks in the second and third cathodic sweeps can also be ascribed to the typical LixSn alloying process.10 Comparing to Sn-out-CNT, Sn-in-CNT shows significantly different electrochemical behavior (Figure 4b). There are two reduction peaks at 0.65 and 0.1 V in the first cathodic sweep, and the absence of 1.5 and 1.02 V confirms that Sn surface in Sn-in-CNT is not exposed to electrolyte, but encapsulated into interior cavity.25,26 In all three anodic sweeps, we can only find one typical LixSn dealloying oxidation peak at 0.56 V. On the basis of Raman and DSC results that the electronic interaction facilitates Sn to bind stronger to CNT interior surface and to remain in a more reduced state, we suppose that Sn inside CNTs is more energetically favorable for rapid Li−Sn alloying and dealloying process. Figure 5 shows the electrochemical performance of CNTs and CNT−Sn composites. At the current density of 50 mA g−1

cycles) of Sn-in-CNT is 64.8%, much higher than 44.6% of Snout-CNT and 34.3% of CNTs. In addition, Sn-in-CNT exhibits a good rate capacity at discharge rate of 50−2000 mA g−1 (Figure 5c). By returning to the initial rate, it resumes the capacity of 630 mAh g−1 and continues to operate with stable cycling response. It can be seen that, although containing the same contents of Sn (40%), the reversible capacity and cycling performance of Sn-in-CNT is much higher than that of Sn-out-CNT, and the difference of capacity between these two CNT-Sn composites reaches to 288 mAh g−1 after 170 cycles at 50 mA g−1 and 230 mAh g−1 after 100 cycles at 1 A g−1. The reason for this different electrochemical performance should be further investigated. Lithium ions transport speed is one of important factors affecting the electrochemical performance, and lower chargetransfer resistance is helpful for rapid immigration of lithium ions in the electrode. The typical Randles equivalent circuit is used to stimulate the AC impendence spectra, which is shown in Figure 6a,29 and the kinetic parameters are illustrated in

Figure 6. The AC impedance spectra (a), Randles equivalent circuit for composites electrode/electrolyte interfaces (b), and kinetic parameters (c) of the CNTs, Sn, and CNT−Sn composites electrodes after 100 cycles at a current density of 50 mA g−1.

Figure 5. Cycling performances of CNTs and CNT−Sn composites at 50 mA g−1 (a), 1 A g−1 (b), and rate capacity (c).

Figure 6c. It can be seen that pure Sn electrodes exhibit both high contact resistance (Rf = 4.98 Ω) and charge-transfer resistance (Rct = 13.68 Ω), and these resistances of CNT−Sn composites electrodes decrease for its excellent conductivity of CNTs. It is noticeable that the charge-transfer resistance of Snin-CNT is 2.17 Ω, which is lower than 4.94 Ω of Sn-out-CNT and even lower than 2.58 Ω of CNTs. This result is consistent with density functional theory calculations that CNTencapsulated Sn nanowire composites possess higher conductivity than standalone CNTs.30 Previous reports showed the diffusion and molecular transport of gas or liquid were enhanced inside CNTs,31 as n-heptane molecules preferred orienting themselves parallel to the nanotube axis inside CNTs, which made translational motion along the nanotube axis easier, and the diffusion of N2 was enhanced for the formation of ordered structure inside CNTs. The exchange current i0 can be calculated according to eq 2:29

(Figure 5a), CNTs electrode delivers a first discharge (Liinsertion) and charge capacities of 1889.8 and 345.0 mAh g−1, respectively, and the initial columbic efficiency is only 12.3%. The low efficiency and large initial irreversible capacity would be related to SEI film and lithium ions storage inside CNTs, which cannot be extracted.26,27 Sn-out-CNT exhibits first discharge capacity of 1245.2 mAh g−1 and first reversible capacity of 492.7 mAh g−1, but Sn-in-CNT shows 1340.9 and 731.6 mAh g−1, respectively. After 170 cycles, the capacity retention of Sn-out-CNT is 351.1 mAh g−1, which equals 56.8% of the theoretical capacity (theoretical capacity = 991·0.398 + 372·0.602), while it is very inspiriting to find Sn-in-CNT remains at 639.7 mAh g−1, much higher than the capacity of Snout-CNT and even higher than its theoretical capacity (615 mAh g−1). What is more, the fade rate is only 0.074% per cycle, and the contribution from Sn to the overall capacity is more than 1300 mAh g−1 at the first 40 cycles and 1200 mAh g−1 during the next 130 cycles. The reversible capacity and stable cycling performance of Sn-in-CNT are much better than the most reported results (Support Information SI-3),11−13,25,27,28 and its utilization efficiency of Sn in composites is also superior to the excellent reports.26 These cells are also discharged and charged at high current density of 1A g−1, as shown in Figure 5b. The reversible capacity of Sn-in-CNT after 100 cycles reaches to higher 335.2 mAh g−1, compared to 70.8 mAh g−1 of CNTs and 122.9 mAh g−1 of Sn-out-CNT. The C100/C2 ratio (Cn = capacity after n

i0 = RT /nFR ct

(2)

We can also find that Sn-in-CNT exhibit a higher value of i0 (7.44 mA cm−2) than Sn-out-CNT (3.27 mA cm−2) and CNTs (6.25 mA cm−2), indicating its enhanced electrochemical activity. So we speculate that the molecular ordering inside CNTs results in the enhanced diffusion and transport of the confined lithium ions, which ensures the good electrical contact and rapid immigration. The interfacial interaction between metal oxide and carbon is attributed to covalent, noncovalent, π-stacking, and electronic 22777

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interaction,32 and this interaction is of importance to catalytic growth of carbon.33 Our previous work had found that the covalent Fe−O−C bonds between Fe3O4 nanoparticles and graphene played a leading role in homogeneous locating Fe3O4 nanoparticles on graphene, which could result in high specific capacity and long-period life cycling performance for lithium ion storage.17 Therefore, we employed XPS to explore the bonding state of Sn in composites and its changes after cycling, and the curve fitting of Sn 3d was carried out by using Gaussian−Lorentzian peak shape after a Shirley background correlation. In Figure 7a, before cycling the peak of Sn 3d5/2 for Figure 8. HRTEM images of Sn-in-CNT (a) and Sn-out-CNT (c) after 100 cycles at 1 A g−1; modes for the changes of Sn nanoparticles and Sn−C bonds after lithium insertion/extraction for Sn-in-CNT (b) and Sn-out-CNT (d).

trapped in confined interior hollow core of CNTs (inside the dotted rectangle), and the spatial restriction inside CNTs is helpful to hinder Sn from aggregation and prolong their reaction time.8c However, Sn on exterior CNT aggregates into large clusters with diameter of more than 10 nm (Figure 8c). We build a model to describe the changes of Sn and Sn−C bonds after cycling (Figures 8b and 8d). In brief summary, the excellent lithium ion storage property of Sn-in-CNT and its huge capacity difference with respect to Sn-out-CNT are closely related to the confinement effect of Sn inside the confined interior space of CNTs, and it can be ascribed to the following specific four parts: (1) higher electrical conductivity, ensuring the fast immigration of Li+ during cycling and excellent electron contact between Li+ and Sn; (2) the charge transfer from Sn to electron-deficient interior surface of CNTs, enabling Sn to stay in a more reduced state and be favorable for lithium ions absorption; (3) the electronic interaction, forming strong covalent Sn−C bonds to facilitate Sn to embed into interior surface of CNTs and to make Sn flexible for cushioning the intern stress by volume expansion of Sn; (4) the spatial restriction, keeping Sn−C bonds alive and preventing Sn from exfoliation out of CNT interior space.

Figure 7. Sn 3d narrow scanning spectra before and after cycling for Sn-in-CNT a (c) and Sn-out-CNT b (d).

Sn-in-CNT is composed of two peaks at 486.7 and 485.3 eV, which corresponds to Sn−C bonds and Sn−Sn bonds, respectively,11,34 and these two bonds can also be found in Sn-out-CNT (Figure 7b). After cycling at 1 A g−1, it is very surprising that the Sn 3d5/2 spectrum of CNTs−Sn composites displays significant difference (Figures 7c and 7d). For Sn-in-CNT, the peak at 486.5 eV is still very intense, indicating Sn still strongly linked by Sn−C bonds, while for Sn-out-CNT, the intensity of the peak at 486.5 eV is very low, suggesting the broken of Sn−C bonds during the charge and discharge processes. However, the intensity of Sn−Sn bonds shows a different behavior: for Sn-in-CNT, the peak at 484.4 eV is weak, while for Sn-out-CNT the peak is very intense. A little decrease of binding energy (0.9 eV) for Sn−Sn bonds (484.4 eV) can be ascribed to the volume expansion of Sn. XPS results clearly validate the change of bonding states of Sn in CNTs−Sn composites after cycling. When Sn nanoparticles are encapsulated into interior hollow core of CNTs, the compact wall and confined inner space of CNTs can effectively cushion its intern stress and restrain the volume expansion. Therefore, Sn is still trapped in interior CNTs, and Sn−C bonds are preserved. But if Sn nanoparticles are coated on exterior surface of CNTs, there will be no other barrier to restrain the volume change, and so the large intern stress of Sn during volume expansion will inevitably result in the broken of Sn−C bonds and further exfoliation of Sn out from CNTs. On the basis of above XPS results, we employ HRTEM measurement to explore the morphology change of CNTs−Sn composites after cycling. For Sn-in-CNT (Figure 8a), Sn is still

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CONCLUSION

In this work, the confinement of Sn inside carbon nanotubes and its effect on lithium ions storage property have been studied in detail. The charge transfer and electronic interaction facilitate Sn to remain in a more reduced state and to link strongly with CNT interior surface by Sn−C bonds, leading to excellent electrochemical performance of Sn-in-CNT with the first reversible capacity being 731.6 mAh g−1 and superior retention of 639.7 mAh g−1 after 170 cycles at 50 mA g−1 rate, much higher than that of Sn-out-CNT (492.7 and 351.1 mAh g−1). Besides, Sn-in-CNT shows a reversible capacity of 335 mAh g−1 after 100 cycles at 1 A g−1 and delivers a good rate capacity. Our results indicate that the confinement inside CNTs and the interaction between Sn with CNTs enhance their electrochemical performance a lot. We believe our work is not only very practically useful for anode materials design of confined nanospace to encapsulate high specific capacity elements for higher energy density LIBs but also meaningful to promote the catalytic activity and electrochemical property of metal/carbon composites by exploring their interactions. 22778

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ASSOCIATED CONTENT

S Supporting Information *

Variations of Raman factors, cyclic voltammograms of CNTs, and electrochemical performances of reported Sn−carbon composites. 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 This work was supported by the National Natural Science Foundation of China (50572003 and 50972004) and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20081001001).

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ABBREVIATIONS CNTs, carbon nanotubes; Sn-in-CNT, Sn encapsulated into CNTs; Sn-out-CNT, Sn coated on exterior surface of CNTs. REFERENCES

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dx.doi.org/10.1021/jp308571p | J. Phys. Chem. C 2012, 116, 22774−22779