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nanocarbon families as lithium-ion battery anodes
Solid State Sciences 14 (2012) 111e116
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Structural and electrochemical properties of SnO2/nanocarbon families as lithium-ion battery anodes Li-Li Xing, Chun-Hua Ma, Chun-Xiao Cui, Xin-Yu Xue* College of Sciences, Northeastern University, Shenyang 110004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2011 Received in revised form 1 September 2011 Accepted 5 November 2011 Available online 13 November 2011
Hybrid SnO2/nanocarbon families (graphene nanosheets (GNSs), single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) and carbon nanospheres (CNSs)) have been synthesized by a similar wet chemical method. SnO2 nanoparticles are uniformly loaded on the surface of the nanocarbon families. As lithium battery anodes, their electrochemical properties of the reaction of lithium are investigated under the same conditions. To compare between them, SnO2/GNSs have the largest capacity; SnO2/GNSs and SnO2/SWCNTs have high cyclability; and SnO2/MWCNTs can maintain the capacity at high current density. Such behaviors are ascribed to their surface-to-volume ratio, structure flexibility, ion mobility and electron conductivity. The present results are the bases for their practical applications in lithium-ion battery anodes. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nanomaterials Composite Lithium battery Tin oxide Nanocarbon families
1. Introduction Lithium-ion batteries have attracted special attention in the scientific and industrial fields due to their high electromotive force and high energy density [1,2]. For anode materials, graphite have been commonly employed because of their high cyclability although their capacity is relatively low (372 mAh g1) [3]. To meet the increasing demand for higher energy density of lithium-ion batteries, much research effort has been taken to explore new anode materials [4e8]. Tin oxide is an attractive anode material with high theoretical capacity (782 mAh g1) [9e11]. Unfortunately, the volume of SnO2 anodes changes significantly upon insertion and extraction of lithium, which usually result in pulverization, capacity fading, poor cyclability, and limited practical applications [12,13]. To enhance the cyclability of the SnO2 anode, hybridizing SnO2 with carbon at nano-scale is an effective method to accommodate the strain of volume change and circumvent the aggregation of SnO2 nanoparticles during charge-discharge process [14,15]. Graphene nanosheets (GNSs), single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) and carbon nanospheres (CNSs) are promising candidates for carrying SnO2 nanoparticles because these nanocarbon families have excellent electrical
* Corresponding authors. Tel./fax: þ86 024 83687658. E-mail address: [email protected] (X.-Y. Xue). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.11.004
conductivities, high surface-to-volume ratio and broad electrochemical activity [16e23]. However, the reported performances of SnO2/nanocarbon families as lithium-ion battery anodes are in dispute [24e28], and the direct comparison between them has not been carried out. Such a comparison is not only a fundamental scientific issue, but also can facilitate the practical applications of the technologies. Furthermore, it has been reported that the nanostructures and binder effects can greatly affect the cyclability [29], and CMC binder shows enhanced performance than PVDF binder [30]. Thus CMC is chosen as binder in this work. In this paper, SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are all synthesized by a similar wet chemical method, and their electrochemical properties of the reaction of lithium as lithium battery anodes are investigated under the same conditions. SnO2/GNSs have the largest capacity; SnO2/GNSs and SnO2/SWCNTs have high cyclability; and SnO2/MWCNTs can maintain the capacity at high current density. Such behaviors are related to the difference of effective surface-to-volume ratio, structure flexibility, ion mobility and electron conductivity. 2. Experiments GNSs and CNSs were synthesized by hydrothermal methods as reported elsewhere previously [31,32]. MWCNTs and SWCNTs were purchased from Shenzhen Seasunnano Co. Ltd.. SnCl2$2H2O, sulfuric acid, nitric acid and hydrochloric acid (Sinopharm Chemical Reagent Co. Ltd.) were analytical-grade reagents and used as
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purchased without further purification. SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs were all synthesized by a similar wet chemical method at room temperature. Firstly, nanocarbon families were treated in a mixture of sulfuric acid and nitric acid, respectively. Then, 20 mg nanocarbon was ultrasonically dispersed into 160 ml distilled water for 10 min. 2.8 ml hydrochloric acid and 4.8 g SnCl2$2H2O were added into the solution. After ultrasonically treatment for 10 min and vigorously stirring for 4 h, the resulting
precipitates were collected and washed with distilled water. The morphologies and crystal phases of SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are investigated by scanning electron microscope (SEM; JEOL JSM-6700F), transmission electron microscopy (TEM; JEOL JEM-2010) and X-ray powder diffraction (XRD; D/max 2550 V, Cu Ka radiation). The test cells were assembled in an argon filled glove box at room temperature. An aqueous suspension was prepared by mixing
Fig. 1. (a) SEM image of SnO2/GNSs. (b) TEM image of one single SnO2/GNS (inset is high resolution TEM image of SnO2/GNSs). (c) TEM image of SnO2/MWCNTs. (d) High resolution TEM image of one single SnO2/MWCNT. (e) TEM image of SnO2/SWCNTs. (f) TEM image of one single SnO2/SWCNT (inset is high resolution TEM image of SnO2 nanoparticles). (g) TEM image of SnO2/CNSs (inset is high resolution TEM image of SnO2/CNSs). (h) XRD patterns of SnO2/nanocarbon families and pure SnO2 nanoparticles. Pure SnO2 nanoparticles are obtained by completely removing nanocarbon by annealing at 600 C in air for 1 h.
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SnO2/nanocarbon families, acetylene black and (carboxymethyl cellulose) CMC with the mass ratio of 8:1:1. Then the slurry was coated onto a copper foil substrate. The electrode sheets were dried at 100 C for 12 h under vacuum. Li foil was used as both counter electrode and reference electrode, and the electrolyte was 1 M LiPF6 in a 1:1 w/w mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC). The cells were galvanostatically charged and discharged under different current densities.
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3. Results and discussion Fig. 1a is a typical SEM image of SnO2/GNSs, showing a general view of their morphologies. SnO2/GNSs are mainly dominated by the nanosheets with the size of several micrometers. The aggregation of SnO2 nanoparticles is not observed and SnO2 nanoparticles are uniformly distributed on the surface of GNSs. Fig. 1b is a TEM image of one single SnO2/GNS, showing that all the surface of
Fig. 2. Charge/discharge profile at C/10 rate for (a) SnO2/GNSs, (b) SnO2/MWCNTs, (c) SnO2/SWCNTs, (d) SnO2/CNSs, (e) GNSs, (f) MWCNTs, (g) SWCNTs and (h) CNSs.
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GNS is uniformly coated with SnO2 nanoparticles. As shown in the inset of Fig. 1b, the average diameter of SnO2 nanoparticles is about 4 nm, and lattice fringe spacing is 0.33 nm, which corresponds to the (110) plane of SnO2. Fig. 1c is a TEM image of SnO2/MWCNTs, showing that SnO2 nanoparticles are uniformly coated on the surface of MWCNTs. The average diameter of SnO2 nanoparticles is about 4 nm. Fig. 1d is a high-resolution TEM image of SnO2 nanoparticles enveloping a single MWCNT. The lattice fringe spacing of 0.33 nm corresponds to the (110) plane of SnO2. Fig. 1e is a lowresolution TEM image of SnO2/SWCNTs. The SWCNTs usually appear as bundles with some appearing as individuals. Almost all nanotubes or nanotube bundles in the sample have been uniformly coated with SnO2 nanoparticles. Fig. 1f is a high-resolution TEM image of a single SnO2/SWCNT. The average diameter of SnO2 nanoparticles is about 4 nm. As shown in the inset of Fig. 1f, the lattice fringe spacing is 0.27 nm, which corresponds to the (101) plane of SnO2. Fig. 1g is a TEM image of SnO2/CNSs, which clearly shows that all CNSs were uniformly decorated with nanoparticles and the diameter of CNSs is about 300 nm SnO2 nanoparticles have the diameters of about 4 nm and the lattice fringe spacing is observed to be 0.27 nm. Fig. 1h is XRD patterns of SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs, SnO2/CNSs and pure SnO2 nanoparticles. All the peaks can be indexed to SnO2 (JCPDS NO.41e1445) and nanocarbon families [24e28]. No peaks can be indexed to other impurities. The XRD peaks are broad, which indicate that SnO2 nanoparticles are relatively small [33,34]. From the data above, SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are all uniformly coated with single-crystalline SnO2 nanoparticles, and the sizes of nanoparticles are almost the same. The electrochemical properties of the reaction of lithium among SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs as lithium battery anodes are investigated under the same conditions. Their charge/discharge properties of 1st, 2nd, 10th and 20th cycle between 0.001 and 2.5 V at C/10 rate (10 h per half cycle) are shown in Fig. 2aed. The first cycle discharge capacities are 1430, 985, 867
and 914 mAh g1 for SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs, respectively. And the first cycle charge capacities are 1010, 755, 652 and 375 mAh g1, respectively. The first cycle Coulombic efficiencies are calculated to be 71%, 77%, 75% and 41%, respectively. The initial Coulombic efficiencies are relatively low, which can be attributed to the formation of SEI layer at large surface in the first discharge process. After 20 cycles, the discharge capacities are 922, 713, 632 and 330 mAh g1, and the coulombic efficiencies are over 98%, 99%, 99% and 98%, respectively. High capacity, cyclability and coulumbic efficiency of SnO2/GNSs, SnO2/MWCNTs and SnO2/SWCNTs support their potential applications as lithium battery anodes. In comparison among them, SnO2/GNSs have the largest capacity. Such a behavior can be attributed to that both surfaces of SnO2/GNSs can be the host of lithium ions, which is confirmed by the measured electrochemical properties of pure GNSs, MWCNTs, SWCNTs and CNSs as shown in Fig. 2eeh. The reversible capacity of GNSs, MWCNTs, SWCNTs and CNSs are 523, 482, 250 and 265 mAh g1, respectively. The capacity of GNSs is the highest because GNSs are proposed to form Li2C6 since both of GNS surfaces can host lithium ions [16,35,36]. And many groups have confirmed that GNSs have extremely high surface-to-volume ratio [37e43]. Also, the masking effect of GNSs leads to an additional reversible capacity as discussed below. Differential capacity curves (dC/dV vs. potential) of SnO2/nanocarbon families in the potential range of 0.1e2.5 V are used to obtain the information about their structural transformations during lithiation and delithation, as shown in Fig. 3. During the first discharge process, a broad peak around 0.8e1.3 V has been observed in all SnO2/nanocarbon families. This broad peak can be ascribed to two processes: formation of solid electrolyte interfaces (SEI) at the SnO2/electrolyte and nanocarbon/electrolyte interfaces, respectively [24,44e46]. For SnO2/CNSs, this peak is very weak, much weaker than those of other nanocarbon families. For both SnO2/ MWCNTs and SnO2/CNSs, this broad peak is observed to disappear in the 2nd and following discharge cycles. These indicate that SEI at
Fig. 3. Differential capacity curves of (a) SnO2/GNSs, (b) SnO2/MWCNTs, (c) SnO2/SWCNTs and (d) SnO2/CNSs at C/10 rate.
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Fig. 4. Cycling performances at C/10 and 1C rate for (a) SnO2/nanocarbon families and (b) Pure nanocarbon families.
SnO2/electrolyte interface has been well formed and carbon/electrolyte interfaces may not exist because MWCNTs and CNSs with relatively small surface-to-volume ratio have been thoroughly coated by SnO2 nanoparticles [22,24,28]. Especially for CNSs, their surface-to-volume ratio is very small, thus their peak is very weak. In contrast, for SnO2/GNSs and SnO2/SWCNTs, this broad peak does not disappear after the first discharge process, but shifts to about 0.8 V. The SEI is formed not only at SnO2/electrolyte interfaces, but also at carbon/electrolyte interfaces due to their large surface-tovolume ratio and masking effect [46e51]. Thus, the reversible capacity of SnO2/GNSs is higher than that of the others. During the first discharge process, another two broad peaks around about 0.5 and 0.2 V have been observed in all SnO2/nanocarbon families. These two peaks are related to the transformation of SnO2 to Sn (reaction (1)) and the formation of series of LieSn alloy (reaction (2)), respectively [24,44,51,52]. The chemical reactions are as follows:
discharge capacity of SnO2/MWCNTs at 1C rate is close to (slightly higher than) that at C/10 rate, this phenomenon is not valuable because initial discharge process are contributed by irreversible reactions. The following discharge processes reflect their real capacity. The high retention of SnO2/MWCNTs at high current rate may be attributed to the high electrical conductance of MWCNTs. Such behaviors also arises from the difference in the electrochemical properties among them. As shown in Fig. 4b, pure MWCNTs can also maintain the capacity at high current density. One-dimensional nanostructures of MWCNTs can facilitate lithium ions and electrons passing through the system [19,46,53e55]. The sheet-like nanostructures of GNSs may block the ions transporting through the system. SWCNTs have relatively low electrical conductivity because almost half of SWCNTs are semiconductors [16e18,56e59].
4Liþ þ 4e þ SnO2 /2Li2 O þ Sn
(1)
4. Conclusion
xLiþ þ xe þ Sn4Lix Snð0 x 4:4Þ
(2)
The peak around 0.5 V of all SnO2/nanocarbon families decrease drastically after the first cycles, suggesting capacity loss after the first cycle, which is a well-known and unavoidable phenomenon due to the decomposition of SnO2 into Sn [24,44,51,52]. The peak around 0.2 V of SnO2/GNSs and SnO2/SWCNTs decreases slightly after the first cycle, but that of SnO2/MWCNTs and SnO2/CNSs decreases significantly. GNSs and SWCNTs have higher surface-tovolume ratio and better structure flexibility, which can accommodate the strain of volume change of SnO2 and circumvent the aggregation of SnO2 nanoparticles during charge-discharge process [16e18,46]. The cycling performances of SnO2/nanocarbon families and pure nanocarbon families at C/10 and 1C rate are shown in Fig. 4a and b, respectively. By comparing between 30th and 2nd cycles, the capacity retentions of SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs at C/10 rate are 82%, 76%, 89% and 73%, respectively. And the capacity retentions at 1C rate are 90%, 76%, 82% and 60%, respectively. These results further confirm that SnO2/GNSs and SnO2/SWCNTs have higher surface-to-volume ratio and structure flexibility. The cyclability of pure GNSs, MWCNTs, SWCNTs and CNSs at C/10 and 1C rate is shown in Fig. 4b, and the capacity retentions are 78%, 73%, 90% and 75% at C/10 rate and are 80%, 66%, 82% and 46% at 1C rate, respectively. From Fig. 4a, it can be observed that the capacities at 1C rate of SnO2/GNSs, SnO2/MWCNTs, SnO2/ SWCNTs and SnO2/CNSs fall down to 74%, 98%, 55% and 50% of the capacity at C/10 rate, respectively. Surprisingly, SnO2/MWCNTs can maintain the capacity at high current density. Although the initial
SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs were all synthesized by a similar wet chemical method, and their electrochemical properties of the reaction of lithium were investigated under the same conditions. SnO2/GNSs had the largest capacity, SnO2/GNSs and SnO2/SWCNTs had high cyclability, and SnO2/ MWCNTs could maintain the capacity at high current density. Such behaviors were related to the difference of effective surface-tovolume ratio, structure flexibility, ion mobility and electron conductivity. Our results could facilitate their practical applications in lithium-ion battery anodes. Acknowledgments This work was partly supported from the Fundamental Research Funds for the Central Universities (N090405017 and N100405109), and the National Natural Science Foundation of China (51102041 and 11104025). References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] Y. Wang, K. Takahashi, K.H. Lee, G.Z. Cao, Adv. Funct. Mater. 16 (2006) 1133e1144. [3] M.G. Kim, J. Cho, Adv. Funct. Mater. 19 (2009) 1497e1514. [4] X. Wang, X.L. Wu, Y.G. Guo, Y.T. Zhong, X.Q. Cao, Y. Ma, J.N. Yao, Adv. Funct. Mater. 20 (2010) 1e7. [5] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Nano Lett. 9 (2009) 1002e1006. [6] C.M. Ban, Z.C. Wu, D.T. Gillaspie, L. Chen, Y.F. Yan, J.L. Blackburn, A.C. Dillon, Adv. Mater. 22 (2010) E1eE5.
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Structural and electrochemical properties of SnO2/nanocarbon families as lithium-ion battery anodes Li-Li Xing, Chun-Hua Ma, Chun-Xiao Cui, Xin-Yu Xue* College of Sciences, Northeastern University, Shenyang 110004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 May 2011 Received in revised form 1 September 2011 Accepted 5 November 2011 Available online 13 November 2011
Hybrid SnO2/nanocarbon families (graphene nanosheets (GNSs), single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) and carbon nanospheres (CNSs)) have been synthesized by a similar wet chemical method. SnO2 nanoparticles are uniformly loaded on the surface of the nanocarbon families. As lithium battery anodes, their electrochemical properties of the reaction of lithium are investigated under the same conditions. To compare between them, SnO2/GNSs have the largest capacity; SnO2/GNSs and SnO2/SWCNTs have high cyclability; and SnO2/MWCNTs can maintain the capacity at high current density. Such behaviors are ascribed to their surface-to-volume ratio, structure flexibility, ion mobility and electron conductivity. The present results are the bases for their practical applications in lithium-ion battery anodes. Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords: Nanomaterials Composite Lithium battery Tin oxide Nanocarbon families
1. Introduction Lithium-ion batteries have attracted special attention in the scientific and industrial fields due to their high electromotive force and high energy density [1,2]. For anode materials, graphite have been commonly employed because of their high cyclability although their capacity is relatively low (372 mAh g1) [3]. To meet the increasing demand for higher energy density of lithium-ion batteries, much research effort has been taken to explore new anode materials [4e8]. Tin oxide is an attractive anode material with high theoretical capacity (782 mAh g1) [9e11]. Unfortunately, the volume of SnO2 anodes changes significantly upon insertion and extraction of lithium, which usually result in pulverization, capacity fading, poor cyclability, and limited practical applications [12,13]. To enhance the cyclability of the SnO2 anode, hybridizing SnO2 with carbon at nano-scale is an effective method to accommodate the strain of volume change and circumvent the aggregation of SnO2 nanoparticles during charge-discharge process [14,15]. Graphene nanosheets (GNSs), single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs) and carbon nanospheres (CNSs) are promising candidates for carrying SnO2 nanoparticles because these nanocarbon families have excellent electrical
* Corresponding authors. Tel./fax: þ86 024 83687658. E-mail address: [email protected] (X.-Y. Xue). 1293-2558/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2011.11.004
conductivities, high surface-to-volume ratio and broad electrochemical activity [16e23]. However, the reported performances of SnO2/nanocarbon families as lithium-ion battery anodes are in dispute [24e28], and the direct comparison between them has not been carried out. Such a comparison is not only a fundamental scientific issue, but also can facilitate the practical applications of the technologies. Furthermore, it has been reported that the nanostructures and binder effects can greatly affect the cyclability [29], and CMC binder shows enhanced performance than PVDF binder [30]. Thus CMC is chosen as binder in this work. In this paper, SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are all synthesized by a similar wet chemical method, and their electrochemical properties of the reaction of lithium as lithium battery anodes are investigated under the same conditions. SnO2/GNSs have the largest capacity; SnO2/GNSs and SnO2/SWCNTs have high cyclability; and SnO2/MWCNTs can maintain the capacity at high current density. Such behaviors are related to the difference of effective surface-to-volume ratio, structure flexibility, ion mobility and electron conductivity. 2. Experiments GNSs and CNSs were synthesized by hydrothermal methods as reported elsewhere previously [31,32]. MWCNTs and SWCNTs were purchased from Shenzhen Seasunnano Co. Ltd.. SnCl2$2H2O, sulfuric acid, nitric acid and hydrochloric acid (Sinopharm Chemical Reagent Co. Ltd.) were analytical-grade reagents and used as
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purchased without further purification. SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs were all synthesized by a similar wet chemical method at room temperature. Firstly, nanocarbon families were treated in a mixture of sulfuric acid and nitric acid, respectively. Then, 20 mg nanocarbon was ultrasonically dispersed into 160 ml distilled water for 10 min. 2.8 ml hydrochloric acid and 4.8 g SnCl2$2H2O were added into the solution. After ultrasonically treatment for 10 min and vigorously stirring for 4 h, the resulting
precipitates were collected and washed with distilled water. The morphologies and crystal phases of SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are investigated by scanning electron microscope (SEM; JEOL JSM-6700F), transmission electron microscopy (TEM; JEOL JEM-2010) and X-ray powder diffraction (XRD; D/max 2550 V, Cu Ka radiation). The test cells were assembled in an argon filled glove box at room temperature. An aqueous suspension was prepared by mixing
Fig. 1. (a) SEM image of SnO2/GNSs. (b) TEM image of one single SnO2/GNS (inset is high resolution TEM image of SnO2/GNSs). (c) TEM image of SnO2/MWCNTs. (d) High resolution TEM image of one single SnO2/MWCNT. (e) TEM image of SnO2/SWCNTs. (f) TEM image of one single SnO2/SWCNT (inset is high resolution TEM image of SnO2 nanoparticles). (g) TEM image of SnO2/CNSs (inset is high resolution TEM image of SnO2/CNSs). (h) XRD patterns of SnO2/nanocarbon families and pure SnO2 nanoparticles. Pure SnO2 nanoparticles are obtained by completely removing nanocarbon by annealing at 600 C in air for 1 h.
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SnO2/nanocarbon families, acetylene black and (carboxymethyl cellulose) CMC with the mass ratio of 8:1:1. Then the slurry was coated onto a copper foil substrate. The electrode sheets were dried at 100 C for 12 h under vacuum. Li foil was used as both counter electrode and reference electrode, and the electrolyte was 1 M LiPF6 in a 1:1 w/w mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC). The cells were galvanostatically charged and discharged under different current densities.
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3. Results and discussion Fig. 1a is a typical SEM image of SnO2/GNSs, showing a general view of their morphologies. SnO2/GNSs are mainly dominated by the nanosheets with the size of several micrometers. The aggregation of SnO2 nanoparticles is not observed and SnO2 nanoparticles are uniformly distributed on the surface of GNSs. Fig. 1b is a TEM image of one single SnO2/GNS, showing that all the surface of
Fig. 2. Charge/discharge profile at C/10 rate for (a) SnO2/GNSs, (b) SnO2/MWCNTs, (c) SnO2/SWCNTs, (d) SnO2/CNSs, (e) GNSs, (f) MWCNTs, (g) SWCNTs and (h) CNSs.
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GNS is uniformly coated with SnO2 nanoparticles. As shown in the inset of Fig. 1b, the average diameter of SnO2 nanoparticles is about 4 nm, and lattice fringe spacing is 0.33 nm, which corresponds to the (110) plane of SnO2. Fig. 1c is a TEM image of SnO2/MWCNTs, showing that SnO2 nanoparticles are uniformly coated on the surface of MWCNTs. The average diameter of SnO2 nanoparticles is about 4 nm. Fig. 1d is a high-resolution TEM image of SnO2 nanoparticles enveloping a single MWCNT. The lattice fringe spacing of 0.33 nm corresponds to the (110) plane of SnO2. Fig. 1e is a lowresolution TEM image of SnO2/SWCNTs. The SWCNTs usually appear as bundles with some appearing as individuals. Almost all nanotubes or nanotube bundles in the sample have been uniformly coated with SnO2 nanoparticles. Fig. 1f is a high-resolution TEM image of a single SnO2/SWCNT. The average diameter of SnO2 nanoparticles is about 4 nm. As shown in the inset of Fig. 1f, the lattice fringe spacing is 0.27 nm, which corresponds to the (101) plane of SnO2. Fig. 1g is a TEM image of SnO2/CNSs, which clearly shows that all CNSs were uniformly decorated with nanoparticles and the diameter of CNSs is about 300 nm SnO2 nanoparticles have the diameters of about 4 nm and the lattice fringe spacing is observed to be 0.27 nm. Fig. 1h is XRD patterns of SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs, SnO2/CNSs and pure SnO2 nanoparticles. All the peaks can be indexed to SnO2 (JCPDS NO.41e1445) and nanocarbon families [24e28]. No peaks can be indexed to other impurities. The XRD peaks are broad, which indicate that SnO2 nanoparticles are relatively small [33,34]. From the data above, SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs are all uniformly coated with single-crystalline SnO2 nanoparticles, and the sizes of nanoparticles are almost the same. The electrochemical properties of the reaction of lithium among SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs as lithium battery anodes are investigated under the same conditions. Their charge/discharge properties of 1st, 2nd, 10th and 20th cycle between 0.001 and 2.5 V at C/10 rate (10 h per half cycle) are shown in Fig. 2aed. The first cycle discharge capacities are 1430, 985, 867
and 914 mAh g1 for SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs, respectively. And the first cycle charge capacities are 1010, 755, 652 and 375 mAh g1, respectively. The first cycle Coulombic efficiencies are calculated to be 71%, 77%, 75% and 41%, respectively. The initial Coulombic efficiencies are relatively low, which can be attributed to the formation of SEI layer at large surface in the first discharge process. After 20 cycles, the discharge capacities are 922, 713, 632 and 330 mAh g1, and the coulombic efficiencies are over 98%, 99%, 99% and 98%, respectively. High capacity, cyclability and coulumbic efficiency of SnO2/GNSs, SnO2/MWCNTs and SnO2/SWCNTs support their potential applications as lithium battery anodes. In comparison among them, SnO2/GNSs have the largest capacity. Such a behavior can be attributed to that both surfaces of SnO2/GNSs can be the host of lithium ions, which is confirmed by the measured electrochemical properties of pure GNSs, MWCNTs, SWCNTs and CNSs as shown in Fig. 2eeh. The reversible capacity of GNSs, MWCNTs, SWCNTs and CNSs are 523, 482, 250 and 265 mAh g1, respectively. The capacity of GNSs is the highest because GNSs are proposed to form Li2C6 since both of GNS surfaces can host lithium ions [16,35,36]. And many groups have confirmed that GNSs have extremely high surface-to-volume ratio [37e43]. Also, the masking effect of GNSs leads to an additional reversible capacity as discussed below. Differential capacity curves (dC/dV vs. potential) of SnO2/nanocarbon families in the potential range of 0.1e2.5 V are used to obtain the information about their structural transformations during lithiation and delithation, as shown in Fig. 3. During the first discharge process, a broad peak around 0.8e1.3 V has been observed in all SnO2/nanocarbon families. This broad peak can be ascribed to two processes: formation of solid electrolyte interfaces (SEI) at the SnO2/electrolyte and nanocarbon/electrolyte interfaces, respectively [24,44e46]. For SnO2/CNSs, this peak is very weak, much weaker than those of other nanocarbon families. For both SnO2/ MWCNTs and SnO2/CNSs, this broad peak is observed to disappear in the 2nd and following discharge cycles. These indicate that SEI at
Fig. 3. Differential capacity curves of (a) SnO2/GNSs, (b) SnO2/MWCNTs, (c) SnO2/SWCNTs and (d) SnO2/CNSs at C/10 rate.
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Fig. 4. Cycling performances at C/10 and 1C rate for (a) SnO2/nanocarbon families and (b) Pure nanocarbon families.
SnO2/electrolyte interface has been well formed and carbon/electrolyte interfaces may not exist because MWCNTs and CNSs with relatively small surface-to-volume ratio have been thoroughly coated by SnO2 nanoparticles [22,24,28]. Especially for CNSs, their surface-to-volume ratio is very small, thus their peak is very weak. In contrast, for SnO2/GNSs and SnO2/SWCNTs, this broad peak does not disappear after the first discharge process, but shifts to about 0.8 V. The SEI is formed not only at SnO2/electrolyte interfaces, but also at carbon/electrolyte interfaces due to their large surface-tovolume ratio and masking effect [46e51]. Thus, the reversible capacity of SnO2/GNSs is higher than that of the others. During the first discharge process, another two broad peaks around about 0.5 and 0.2 V have been observed in all SnO2/nanocarbon families. These two peaks are related to the transformation of SnO2 to Sn (reaction (1)) and the formation of series of LieSn alloy (reaction (2)), respectively [24,44,51,52]. The chemical reactions are as follows:
discharge capacity of SnO2/MWCNTs at 1C rate is close to (slightly higher than) that at C/10 rate, this phenomenon is not valuable because initial discharge process are contributed by irreversible reactions. The following discharge processes reflect their real capacity. The high retention of SnO2/MWCNTs at high current rate may be attributed to the high electrical conductance of MWCNTs. Such behaviors also arises from the difference in the electrochemical properties among them. As shown in Fig. 4b, pure MWCNTs can also maintain the capacity at high current density. One-dimensional nanostructures of MWCNTs can facilitate lithium ions and electrons passing through the system [19,46,53e55]. The sheet-like nanostructures of GNSs may block the ions transporting through the system. SWCNTs have relatively low electrical conductivity because almost half of SWCNTs are semiconductors [16e18,56e59].
4Liþ þ 4e þ SnO2 /2Li2 O þ Sn
(1)
4. Conclusion
xLiþ þ xe þ Sn4Lix Snð0 x 4:4Þ
(2)
The peak around 0.5 V of all SnO2/nanocarbon families decrease drastically after the first cycles, suggesting capacity loss after the first cycle, which is a well-known and unavoidable phenomenon due to the decomposition of SnO2 into Sn [24,44,51,52]. The peak around 0.2 V of SnO2/GNSs and SnO2/SWCNTs decreases slightly after the first cycle, but that of SnO2/MWCNTs and SnO2/CNSs decreases significantly. GNSs and SWCNTs have higher surface-tovolume ratio and better structure flexibility, which can accommodate the strain of volume change of SnO2 and circumvent the aggregation of SnO2 nanoparticles during charge-discharge process [16e18,46]. The cycling performances of SnO2/nanocarbon families and pure nanocarbon families at C/10 and 1C rate are shown in Fig. 4a and b, respectively. By comparing between 30th and 2nd cycles, the capacity retentions of SnO2/GNSs, SnO2/MWCNTs, SnO2/SWCNTs and SnO2/CNSs at C/10 rate are 82%, 76%, 89% and 73%, respectively. And the capacity retentions at 1C rate are 90%, 76%, 82% and 60%, respectively. These results further confirm that SnO2/GNSs and SnO2/SWCNTs have higher surface-to-volume ratio and structure flexibility. The cyclability of pure GNSs, MWCNTs, SWCNTs and CNSs at C/10 and 1C rate is shown in Fig. 4b, and the capacity retentions are 78%, 73%, 90% and 75% at C/10 rate and are 80%, 66%, 82% and 46% at 1C rate, respectively. From Fig. 4a, it can be observed that the capacities at 1C rate of SnO2/GNSs, SnO2/MWCNTs, SnO2/ SWCNTs and SnO2/CNSs fall down to 74%, 98%, 55% and 50% of the capacity at C/10 rate, respectively. Surprisingly, SnO2/MWCNTs can maintain the capacity at high current density. Although the initial
SnO2/GNSs, SnO2/SWCNTs, SnO2/MWCNTs and SnO2/CNSs were all synthesized by a similar wet chemical method, and their electrochemical properties of the reaction of lithium were investigated under the same conditions. SnO2/GNSs had the largest capacity, SnO2/GNSs and SnO2/SWCNTs had high cyclability, and SnO2/ MWCNTs could maintain the capacity at high current density. Such behaviors were related to the difference of effective surface-tovolume ratio, structure flexibility, ion mobility and electron conductivity. Our results could facilitate their practical applications in lithium-ion battery anodes. Acknowledgments This work was partly supported from the Fundamental Research Funds for the Central Universities (N090405017 and N100405109), and the National Natural Science Foundation of China (51102041 and 11104025). References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] Y. Wang, K. Takahashi, K.H. Lee, G.Z. Cao, Adv. Funct. Mater. 16 (2006) 1133e1144. [3] M.G. Kim, J. Cho, Adv. Funct. Mater. 19 (2009) 1497e1514. [4] X. Wang, X.L. Wu, Y.G. Guo, Y.T. Zhong, X.Q. Cao, Y. Ma, J.N. Yao, Adv. Funct. Mater. 20 (2010) 1e7. [5] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Nano Lett. 9 (2009) 1002e1006. [6] C.M. Ban, Z.C. Wu, D.T. Gillaspie, L. Chen, Y.F. Yan, J.L. Blackburn, A.C. Dillon, Adv. Mater. 22 (2010) E1eE5.
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