Synthesis of Multiwalled Carbon Nanotubes That Are Both Filled and

Publication Date (Web): November 2, 2009 .... The SnO2/MWCNT hybrids showed a good electrochemical performance in Li ion batteries at current density ...
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J. Phys. Chem. C 2009, 113, 20509–20513

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Synthesis of Multiwalled Carbon Nanotubes That Are Both Filled and Coated by SnO2 Nanoparticles and Their High Performance in Lithium-Ion Batteries Chaohe Xu, Jing Sun,* and Lian Gao* The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: October 14, 2009

Multiwalled carbon nanotubes (MWCNTs) filled and coated with SnO2 nanoparticles (NPs) have been prepared using a simple one-step chemical solution method at 50 °C. The SnO2/MWCNT hybrids were characterized by transmission electron microscopy and X-ray diffraction, and the formation mechanism has been discussed. The discharge capacities showed that MWCNTs coated and filled with SnO2 had superior electrochemical performance. The first discharge capacities are 2127.4 and 1880.2 mAh/g at 70 and 200 mA/g and remain at 469 and 362 mAh/g after 40 cycles. Our results demonstrated that they had a better cycling performance at large discharge/charge current densities. The main reason is the huge volume expansion usually occurred in SnO2 NPs during the Li+ insertion/extraction process has been prevented when they were filled into the cavities of MWNTs who have a superior mechanical properties. Introduction Rechargeable Li-ion batteries (LIBs) have long been considered as an attractive power source for lots of mobile devices such as cellular phones, lap-top computers, and electric vehicles.1-4 However, the next generations of LIBs, which are expected to have superior performance such as energy capacity, cycling stability and rate capability, can only be achieved by making breakthroughs in electrode materials.5 Therefore, there has been great interest in developing new anode materials to replace the graphite anode materials with a theoretical specific capacity of only 372 mAh/g.6 SnO2, an n-type semiconductor, has been regarded as a potential anode material for LIBs owing to its high theoretical specific capacity (∼781 mAh/g).7 The main disadvantage of the SnO2 anode materials is the large volume effect (>300%) during the discharge/charge process. Such volume variation results in pulverization and a loss of electrical contact with the current collectors thus greatly limits the cycling capability of those metal oxide electrodes.8-12 Recently, nanorods, nanotubes, and nanowires of SnO2 have been synthesized as anode materials for LIBs to solve the above problems.13-16 Kim et al. investigated the electrochemical Li intercalation performances with different particle sizes SnO2 NPs and demonstrated that the smaller ones had superior electrochemical performance than the larger ones.17 Wang et al. showed the polycrystalline SnO2 nanotubes had a better Li storage performance.18 Recently, composites with SnO2, such as SnO2/ polypyrrole,19 SnO2/Fe2O3,20 SnO2/In2O3,21 SnO2/graphite,22-25 SnO2/carbon nanotubes (CNTs)26-29 have been synthesized as the anode materials of LIBs. The SnO2/CNT hybrid materials have drawn much attention because their superior performance in LIBs. The incorporation of tin oxide nanoparticles into CNTs is expected to result in a high capacity and good cycle ability. For SnO2/CNT hybrids as an anode material, the CNTs can form a three-dimensional electric network and have high conductivity and large surface * To whom correspondence should be addressed. Tel: +86 12 52412718. Fax: +86 21 52413122. E-mail address: (J.S.)[email protected]; (L.G.) [email protected].

area. These properties are important for LIBs because they can strongly restrain the volume expansion or pulverization of electrode materials. To date, several methods, such as chemical solution method,30 sol-gel method,31 and anodic aluminum oxide template method32 have been developed to synthesize the composites. For example, Zhao and co-workers33,34 coated and filled MWCNTs with SnO2 NPs by a one-step wet chemical method in which a long time of acid and heat treatment are needed. An et al. synthesized SnO2/CNTs composites in supercritical fluids and the as-synthesized composites exhibited good chemical sensor and lithium storage performance, but the supercritical fluid method is complex and cost expensive.35 Han and co-workers36 synthesized the SnO2/single-walled carbon nanotube (SWCNT) hybrids by a simple chemical solution method at room temperature; however, all of those SnO2 NPs are coated on the outside walls of SWCNTs. To our knowledge, there is less work on SnO2 NPs filled into the cavities of CNTs done and no results about the hybrid materials applied in LIBs. In this study, we adopted the one-step chemical solution method developed by Han and co-workers36 in which MWCNTs both filled and coated by SnO2 NPs have been successfully synthesized at 50 °C. The well-crystallized SnO2 NPs have particle size of only 3-5 nm. The SnO2/MWCNT hybrids showed a good electrochemical performance in Li ion batteries at current density of 70 and 200 mA/g. Experimental Section All chemicals were analytical grade and used as received. MWCNTs were kindly provided by Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences. MWCNTs prepared by the catalytic decomposition of CH4 were purified in the following process. First, 1.0 g of pristine MWCNTs was dried in a vacuum oven at 60 °C for 24 h. The dried samples were added into 45 mL concentrated nitric acid and sonicated for 30 min. Then, the suspensions were transferred into a 250 mL three-necked round-bottomed flask and refluxed at 140 °C for 6 h with magnetic stirring. The mixtures were separated through 0.22 µm member filter and washed with

10.1021/jp909740h CCC: $40.75  2009 American Chemical Society Published on Web 11/02/2009

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Figure 1. XRD patterns of SnO2/MWCNT hybrids synthesized at (a) 20, (b) 50, and (c) 90 °C.

distilled water and absolute ethanol until the pH value reached neutral. Finally, the acid-MWCNTs were dried at 60 °C for 24 h in a vacuum oven. MWCNTs coated and filled with nanocrystalline SnO2 particles were synthesized using a simple one-step chemical solution method.36 Briefly, 1.2 g of SnCl2 · H2O was dissolved in 40 mL of distilled H2O in a glass flask, and 0.7 mL of HCl (38%) was added. Fifty milligrams of acid-treated MWCNTs were added into the above solution and sonicated for 5 min,

Xu et al. then the mixture was stirred for 60 min at 20, 50, and 90 °C, respectively. The reactants were separated and rinsed with distilled H2O and absolute ethanol for several times. Finally, they were dried at 60 °C for 10 h. Phase identification was performed on the powder X-ray diffraction pattern (XRD), using D/max 2550 V X-ray diffraction-meter with Cu KR irradiation at λ ) 1.5406 Å. The specific surface area was investigated by Brunauer-Emmett-Teller (BET) method at 77 K in N2 atmosphere carried out by Micromeritics ASAP 2010 surface area analyzer. Thermogravimetric analysis was performed on the STA 449C system (Netzsch, Germany). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were performed on JEM-2100F Electron Microscope with an accelerating voltage of 200 kV. The electrochemical properties of the SnO2/MWCNT hybrids as the negative electrode used in Li-ion batteries were characterized at room temperature. The working electrode was made from the mixture of the active materials, acetylene black, and polyvinylidene fluoride (PVDF) binder in weight ratio of 80: 10:10. Li foil was used as the counter electrode. The electrolyte was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Cell assembly was carried out in a glovebox with the concentrations of moisture and oxygen below 1 ppm. The electrode activities were measured using a CT2001 battery tester. The cell was charged and

Figure 2. TEM images of SnO2/MWCNT hybrids synthesized at (a) 20, (b, c) 50, and (d) 90 °C.

MWCNTs Filled and Coated by SnO2 Nanoparticles

Figure 3. Thermogravimetric analysis showing the weight losses of sample synthesized at (a) 20 (marked as S1), (b) 50 (marked as S2), and (c) 90 °C (marked as S3).

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Figure 4. HRTEM images of SnO2/MWCNT hybrids synthesized at (a) 20 and (b) 50 °C. The inset of (b) is the high magnification of the square area of Figure 2b.

discharged at different current density and the fixed voltage limits between 2.0 and 0.0 V. Results and Discussion 1. Sample Characterization. The XRD pattern of SnO2/ MWCNT hybrids synthesized at 20, 50, and 90 °C are shown in Figure 1. The XRD curves indexed both the tetragonal SnO2 (t-SnO2) and the main peak of carbon nanotubes (002). The (110) peak of t-SnO2 almost overlaps with the main peak of h-C (002). The broad peaks in the XRD pattern indicate that the crystalline SnO2 particles are very small. The mean particle size of SnO2 calculated by Scherrer equation is about 4 nm, and the temperature has less influence on the particle size from the Scherrer equation. Besides, the intensity of the XRD peaks increases with reacted temperature, indicating the better crystallization of SnO2 NPs. Figure 2 shows the TEM images of the SnO2/MWCNT hybrids synthesized at 20, 50, and 90 °C. The SnO2 NPs are uniformly coated on the side walls of the MWCNTs at 20 and 50 °C as shown in Figure 2a,b, no significant difference could be found at the low magnifications of TEM images. Figure 2c showed the high magnification of the sample synthesized at 50 °C, lots of small SnO2 NPs were filled into the cavities of MWCNTs compared with the inset figure in Figure 2a. This is because the capillary force decreased in high temperature compared with that in low temperature. Thus, more reactants filled into the cavities of the MWCNTs. When the temperature increased to 90 °C (Figure 2d), the SnO2/ MWCNT hybrids with more SnO2 NPs coated on the side walls of MWCNTs were observed, and SnO2 NPs aggregated seriously and formed a thicker coating layer compared to Figure 2a,c. In the meantime, a large amount of reactants filled into the cavity of MWCNTs, thus the quantities of SnO2 NPs also increased intensively in the cavity of MWCNTs. The increased temperature accelerated the nucleation and the crystal-growth velocity of the SnO2 NPs. Therefore, the hydrolysis reaction occurred speedily both on the surface and in the cavity of MWCNTs and produced the high ratio of SnO2 to MWCNTs in the hybrids. Thermogravimetric analysis results are shown in Figure 3. The percentage of weight loss is 74.96, 65.08, and 48.32% for samples that synthesized at 20, 50, and 90 °C respectively (marked as S1, S2, and S3 in Figure 3). The contents of SnO2 NPs are 25.04, 34.92, and 51.68% for sample S1, S2 and S3, increased significantly with the temperature and this coincided well with the TEM images in Figure 2. Figure 4 shows the typical HRTEM images of SnO2 NPs that focused on the surface of MWCNTs synthesized at 20 and 50 °C (Figure 4a focused on the square area of the inset figure of

Figure 5. Schematic illustration of the SnO2 NPs coated and filled the MWCNTs.

Figure 6. Discharge curves of SnO2/MWCNT hybrids at various current densities.

Figure 2a; panel b is focused on the square area of the inset figure of itself). The particle size of SnO2 NPs in the two samples is 3-5 nm, which coincided well with 4 nm calculated by the Scherrer equation from Figure 1. The d-spacing shown in Figure 4 are 0.33 nm for two specimens, corresponding to the (110) face of t-SnO2. Figure 5 shows the schematic illustration of SnO2 NPs coated on and filled into the MWCNTs. First, MWCNTs were dispersed in the tin bichloride solution. Then, stannous cations, O2, and H2O molecules diffused into the cavities of MWCNTs when the solution temperature increased. Finally, the hydrolysis reaction took place on the outside walls and in the cavities of MWCNTs and the SnO2/MWCNT hybrids formed. The chemical reactions are as follows36

2Sn2+ + 2H2O + O2 f 2SnO2 + 4H+

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Figure 7. Schematic illustration of the volume effect of the SnO2/MWCNT hybrids in the insertion/extraction process (a) SnO2 NPs coated on outside walls of MWCNTs, and (b) SnO2 NPs filled in the MWCNTs.

2. Galvanostatic Charge/Discharge Tests. The discharge capacities curves of SnO2/MWCNT hybrids synthesized at 20, 50, and 90 °C (marked as S1, S2 and S3, respectively) are shown in Figure 6, which was measured at current densities of 70, 100, 140, and 200 mA/g. As shown in Figure 6, the initial discharge capacities are 2377.2, 2127.4, and 1209.2 mAh/g for sample S1, S2, and S3 at 70 mA/g, respectively, and sample S1 has the highest initial lithium storage capacity than S2 and S3. It is because the SnO2 NPs are mainly coated on the outside walls of MWCNTs, resulting in the ease of Li+ inserting into the SnO2 nanocrystals to form Li-Sn alloy LixSn, and the Li+ intercalate into the cavities of MWCNTs at sidewall defects and opened tips. Therefore, the distance of Li+ inserting into the SnO2 NPs filled in the MWCNTs is longer than that of the SnO2 NPs coated on the outside walls of MWCNTs. Except the attaching states of NPs with MWNTs, the crystallinity of SnO2 NPs also plays an important role on their lithium storage performance. The well-crystallized S2 sample has higher discharge capacities than those of S1 from Figure 1 and 6. The discharge capacities retained around 500 mAh/g for sample S1, 650 mAh/g for S2 and 425 mAh/g for S3 when the current density increased to 200 mA/g, and these values are higher than the theoretic capacity of graphite (372 mAh/g). S2 sample with SnO2 NPs coated on and filled into MWCNTs has an improved rate capability than S1 and S3. This is because that the quantities of SnO2 NPs coated on and loaded into the cavities of MWCNTs larger than S1 from the TEM images (Figure 2a) and TG curves (Figure 3). Although the S3 sample has the most SnO2 NPs combined with the MWCNTs, much more free particles are formed from the TEM images (Figure 2d) and TG curves (Figure 3); thus, the SnO2 NPs may lose contact with the MWCNTs and each other because of the large volume expansion in the insertion/extraction process and result in poor initial lithium storage properties and rate capabilities compared to S2 and S1. Figure 7 is the schematic illustration of the volume effect of SnO2/MWCNT hybrids in the insertion/extraction process. When Li+ insert into the SnO2 NPs coated on the outside walls of MWCNTs, huge volume expansion occurs in the three directions as the red arrows marked. MWCNTs were against the volume expansion at the interface with SnO2 NPs by absorbing the deformation energy because of their superior mechanical properties. In the directions of red arrows, the stress relaxation occurs when Li+ extract from the SnO2 NPs and result pulverization and aggregation in the charge/discharge process. For the NPs filled into the cavities of MWCNTs, the volume expansion occurs when Li+ inserts into the SnO2 NPs. It can be prevented by the inner walls of carbon nanotubes and the neighbor SnO2 NPs because of the mechanical performance of MWCNTs and the steric effect of the NPs as shown in Figure 7b. The pulverization and volume expansion prevented effect

Figure 8. Discharge curves of SnO2/MWCNT hybrids synthesized at 50 °C at different current densities (a) 70 and (b) 200 mA/g; the inset figure in (b) is the cycle performance result of S2.

was similar to that of the nanoparticles embedded in a carbon matrix, such as Sn-C composite,37,38 and Si-C composite.39 Figure 8 presents the first ten cycles discharge curves at different current densities (70 and 200 mA/g) of sample S2. The discharge capacities are 2127.4 and 1880.2 mAh/g at 70 and 200 mA/g. The capacity loss is 497 mAh/g. The second discharge capacities are 1317.7 and 1078 mAh/g. The capacity retention is only 55% compared with the first cycle. After 40 cycles, the discharge capacities are 469 and 362 mAh/g with loss of 1.2 and 1.5% per cycle under the current densities of 70 and 200 mA/g (the inset curve of Figure 8b) and higher than that of SnO2 NPs coated only on the surface of MWCNTs.27 The discharge capacity is only 20% of the first cycle but is still larger than the theoretical capacity of graphite (372 mAh/g). The main reason of this capacity loss in the charge/discharge process is that the large BET surface area (111.3 m2 for sample S2 and 82.87 m2 for S1) result in the formation of the solid electrolyte interface films between the active materials and electrolytes interface, which can reduce the electric and ionic conductivities and the capacity loss at the charge/discharge cycles. Conclusions SnO2 NPs coated and filled the MWCNT hybrids were synthesized by a simple one-step chemical solution method at 50 °C. XRD patterns and HRTEM images validated that the

MWCNTs Filled and Coated by SnO2 Nanoparticles SnO2 NPs had an average size of 3-5 nm. The SnO2 NPs homogenously coated and filled MWCNT hybrids exhibited enhanced discharge capacity and cycling performance because MWCNTs prevented the volume expansion and increased the electric conductivity. The sample shows initial discharge capacities of 2127.4 and 1880.2 mAh/g at 70 and 200 mA/g and remains at 469 and 362 mAh/g after 40 cycles with a loss of 1.2 and 1.5% per cycle under these current densities. Though the capacity retained about only 20% of the first cycle, it is still larger than the theoretical capacity of graphite (372 mAh/ g). In conclusion, SnO2/MWCNT hybrids with well-crystallized nanoparticles, the suitable chemical composition, and the homogeneous distribution of NPs on MWNTs show a better lithium storage performance. Therefore, further optimizing the above parameters will be the main topic for SnO2/MWCNT anode electrode materials for lithium ion batteries. Acknowledgment. This work is supported by the 973 Project (2005CB623605), the Shanghai Nanotechnology Promotion Center (0852 nm01900) and Shanghai Talents Program Foundation. The authors thank Professor Z. Y. Wen and Dr. X. Y. Wang for the help in the cell assembly. References and Notes (1) Kang, K. S.; Meng, Y. S.; Breger, J.; Grey, C. P.; Ceder, G. Science 2006, 311, 977. (2) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem., Int. Ed. 2008, 47, 2930. (3) Wang, Y.; Takahashi, K.; Lee, K. H.; Cao, G. Z. AdV. Funct. Mater. 2006, 16, 1133. (4) Larcher, D.; Beattie, S.; Morcrette, M.; Edstro¨m, K.; Jumas, J. C.; Tarascon, J. M. J. Mater. Chem. 2007, 17, 3759. (5) Liu, J. P.; Li, Y. Y.; Huang, X. T.; Ding, R. M.; Hu, Y. Y.; Jiang, J.; Liao, L. J. Mater. Chem. 2009, 19, 1859. (6) Kim, M. G.; Cho, J. AdV. Funct. Mater. 2009, 19, 1497. (7) Xie, J.; Varadan, V. K. Mater. Chem. Phys. 2005, 91, 274. (8) Brousse, T.; Retoux, R.; Herterich, U.; Schleich, D. M. J. Electrochem. Soc. 1998, 145, 1. (9) Choi, S. H.; Kim, J. S.; Yoon, Y. S. Electrochim. Acta 2004, 50, 547. (10) Demir, C. R.; Hu, Y. S.; Antonietti, M.; Maier, J.; Titirici, M. M. Chem. Mater. 2008, 20, 1227. (11) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2943.

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