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Graphene supported Sn–Sb@carbon core-shell particles as a superior anode for lithium ion batteries
Electrochemistry Communications 12 (2010) 1302–1306
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Graphene supported Sn–Sb@carbon core-shell particles as a superior anode for lithium ion batteries Shuangqiang Chen a, Peng Chen a, Minghong Wu b, Dengyu Pan b,⁎, Yong Wang a,⁎ a b
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99,Shanghai, 200444, PR China Institute of Nanochemistry and Nanobiology, Shanghai University, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 30 June 2010 Accepted 6 July 2010 Available online 13 July 2010 Keywords: Core-shell nanostructure Graphene nanosheets Lithium-ion battery Sn-Sb
a b s t r a c t This paper reports the preparation and Li-storage properties of graphene nanosheets(GNS), GNS supported Sn–Sb@carbon (50–150 nm) and Sn–Sb nanoparticles (5–10 nm). The best cycling performance and excellent high rate capabilities were observed for GNS-supported Sn–Sb@carbon core-shell particles, which exhibited initial capacities of 978, 850 and 668 mAh/g respectively at 0.1C, 2C and 5C (1C = 800 mA/g) with good cyclability. Besides the GNS support, the carbon skin around Sn–Sb particles is believed to be a key factor to improve electrochemical properties of Sn–Sb. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Graphite is the commercial anode material for Li-ion batteries with a limited theoretical capacity of 372 mAh/g, which fails to meet the fast-growing requirements of high-power Li-ion batteries[1–3]. Tinbased anodes have long been considered as an attractive replacement of graphite anodes because they can deliver as high as 2–3 times the capacity of graphite. However, large volume changes are often associated with lithium alloying and de-alloying process, which eventually lead to the electrode pulverization and capacity fading [4–7]. Among various tin-based anodes, Sn–Sb alloys have been considered promising, because a stepwise lithium insertion mechanism can relieve volume changes and improve the mechanical stability of the electrode, ensuring better electrochemical properties than a single Sn phase [8–14]. Graphene, a flat one-atom-thick monolayer, owns outstanding electronic behavior, large surface area and amazing mechanical properties, which has attracted a great deal of research interest for many applications [15]. In particular, graphene nanosheets(GNS) [16–21] and GNS-based composites [22–28] have been explored as anode materials, which generally showed good Li-storage properties. This communication reports the preparation and Li-storage properties of two new GNS-based composites, namely GNS-supported Sn–Sb nanoparticles or Sn–Sb@carbon core-shell particles.
Preparation of graphite oxide(GO) and GNS: natural graphite powders (1 g, Shanghai Colloid Chemical) were oxidized by a mixed aqueous solution (40 ml 65 wt.% HNO3 and 60 ml 98 wt.% H2SO4)in an ice-water bath. After oxidation for 20 min, 5 g KMnO4 was gradually added and reacted for 2 h at 35 °C. Then 200 ml deionized water and 5 ml 30 wt.% H2O2 aqueous solution were added into the system, followed by washing with 15 ml 10 wt.% HCl aqueous solution. GO sheets were obtained after centrifugation, copious washing with deionized water and drying. GNS was obtained by thermal reduction of GO at 300 °C in N2 for 2 h. Preparation of GNS-supported Sn–Sb composites: 20 ml 0.03 M SnCl4·5H2O ethanol solution was mixed with 20 ml 0.01 M SbCl3 ethanol solution to form a transparent solution. GNS(0.04 g) or GO(0.06 g) were ultrasonically dispersed in 10 ml deionized water for 30 min, and then mixed with the solution containing SnCl4 and SbCl3. The suspension was reduced by a 20 ml 0.3 M NaBH4 aqueous solution in an ice-water bath. After strong magnetic stirring for 2 h, the precipitates (GNS- or GOsupported Sn–Sb nanoparticles) were collected by centrifugation, copious washing with deionized water, and drying. The GO-supported Sn–Sb nanoparticles were heated in a tube furnace in a flow of 80 sccm C2H2 at 500 °C for 2 h to obtain GNS-supported Sn–Sb@carbon. The products were characterized by various instruments and assembled into a half battery for electrochemical properties. More details on the instruments for material characterizations and cell assembly can be found elsewhere[10]. The cells were discharged (lithium insertion) and charged (lithium extraction) at a constant current (80 mA/g, 0.1C) in 0.005–3.0 V. Higher hourly rates (0.5, 1, 2, or 5C) were also used and the first cycle discharging was kept at 0.1C.
⁎ Corresponding authors. E-mail addresses: [email protected] (D. Pan), [email protected] (Y. Wang). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.07.005
S. Chen et al. / Electrochemistry Communications 12 (2010) 1302–1306
3. Results and discussion The XRD patterns of GO, GNS and GNS-supported Sn–Sb nanoparticles or Sn–Sb@carbon particles are shown in Fig. 1a. There is a characteristic peak of GO at 11.5°, which shifts to 23.5° for GNS. The corresponding interplanar spacing of GNS is 0.38 nm, which is slightly larger than that of standard graphite (0.335 nm). For the GNS-supported Sn–Sb or Sn–Sb@carbon, a few peaks ascribed to a mixed phase of Sn (JCPDS 04-0673) and SnSb (JCPDS 33-0018) [8–10] can be observed. The SEM images in Fig. 1b, c, d, e–f show GNS, Sn–Sb nanoparticles, GNSsupported Sn–Sb, and GNS-supported Sn–Sb@C, respectively. A number of Sn–Sb nanoparticles (∼5–10 nm) are shown as white dots on GNS (Fig. 1d). In comparison, a few larger Sn–Sb particles (∼50–150 nm) were found in Fig. 1e–f. These particles were distributed uniformly on GNS and a thin carbon layer was coated around each Sn–Sb particle, as indicated as arrows in Fig. 1f. Fig. 2a–b shows the TEM and HRTEM images of GNS. GNS are almost transparent with a rippled structure and ∼4–8 nm in thickness, corresponding to ∼10–20 layers stacking of single graphene nanosheet. Fig. 2c shows the TEM image of GNS-supported Sn–Sb nanoparticles, which are uniformly dispersed without detectable agglomeration on GNS surface. GNS-supported Sn–Sb@carbon particles are shown in
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Fig. 2d–e. Because chemical vapor deposition and reduction was performed at 500 °C and the melting point of Sn–Sb is ∼230–240 °C, Sn–Sb droplets adopt a nearly sphere-like morphology due to surface tension. Sn–Sb droplets were used as self-catalysts to promote the C2H2 decomposition and formation of carbon shell [9,10]. Compared to GNSsupported Sn–Sb nanoparticles, there was a significant particle size growth from 5–10 nm to ∼50–150 nm for Sn–Sb@carbon particles, which is attributed to the movement and fusion of small Sn–Sb droplets at a high temperature. It is believed that once a carbon shell is completely formed around Sn–Sb material, there should be no further droplet coalescence and particle size growth. The carbon shells are clearly observed around Sn–Sb particles (Fig. 2e) with the thickness ∼5– 10 nm. CHN elemental analysis indicated there was ∼48.2 wt.% of carbon in GNS–SnSb@carbon composite, which was substantially higher than carbon content (∼29.9 wt.%) in GNS–SnSb composite. The molar ratio of Sn:Sb was estimated to be ∼3:1 in two composites by the EDS analysis. A schematic sketch of the growth process is illustrated in the Fig. 2f. Fig. 3a shows the initial discharge and charge curves of various anodes. Initial charge capacities were 755 and 905 mAh/g for Sn–Sb nanoparticles and GNS respectively. GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon particles showed higher initial charge capacities
Fig. 1. (a)XRD patterns of GO, GNS, GNS-supported Sn–Sb or Sn–Sb@carbon particles. SEM images of the products: (b) GNS. (c) Sn–Sb nanoparticles. (d) GNS-supported Sn–Sb nanoparticles. (e–f) GNS-supported Sn–Sb@carbon particles.
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S. Chen et al. / Electrochemistry Communications 12 (2010) 1302–1306
Fig. 2. (a) TEM image of GNS. (b) HRTEM image of GNS. (c) TEM image of GNS-supported Sn–Sb nanoparticles. (d–e) TEM image of GNS-supported Sn–Sb@carbon particles. (f) schematic illustration of the formation process of nanocomposites.
of 1120 and 978 mAh/g. These values were substantial larger than Sn– Sb/C composites reported previously [9–14] and the theoretical value of GNS (744 mAh/g), which may be attributed to additional storage of Liions in the defects or stacking of nanoscale composites. The initial coulombic efficiencies were 58.7% and 65.3% for GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon particles respectively. The initial large capacity loss seems to be inevitable for most GNS-based anodes [16–28], which is mainly ascribed to the formation of solid electrolyte interface (SEI) on GNS and Sn–Sb particles. Fig. 3b shows cycling performances of various anode materials at 0.1C. Unsupported Sn–Sb nanoparticles displayed a fast capacity fading and the charge capacity was reduced to below 10 mAh/g after 20 cycles. This poor cycling performance has long been understood as a sequence of particle aggregation and electrode pulverization[8–14]. The charge capacity of GNS-supported Sn–Sb nanoparticles (5–10 nm) was 714 mAh/g after 30 cycles, which corresponded to 63.8% of initial charge capacity. In spite of a larger particle size, Sn–Sb@carbon particles (50–150 nm) supported on GNS showed a larger charge capacity of 896 mAh/g after 30 cycles and the capacity retention rate was a high 91.6% compared to initial value. It is believed particle size is not the determining factor to cycling performance. The excellent cycling performance of GNS-supported Sn–Sb@carbon, which was superior to
GNS-supported Sn–Sb nanoparticles, is largely ascribed to a carbon shell around each Sn–Sb particle. The carbon shell functions as a stabilizer to immobilize Sn–Sb particles, a matrix to buffer local volume changes during cycling, and a conducting bridge between GNS and Sn–Sb particles. Therefore mechanical stability and electrical contact were increased for GNS-supported Sn–Sb@carbon. Fig. 3c shows cyclic voltammetry of the composite, a few characteristic cathodic peaks (0.75, 0.58, 0.32 V) and anodic peaks (0.71, 0.81, 0.86 V) could be observed, which correspond to the stepwise lithium ion insertion and extraction reactions in Sn–Sb alloy[8–10]. It should be noted that there are two small anodic peaks above 2.5 V, which may be attributed to nanoscale Sn–Sb[29]. The rate performance of GNS-supported Sn– Sb@carbon was also explored (Fig. 3d–f). Initial charge capacities were 916, 890, 850 and 668 mAh/g at large currents of 0.5C, 1C, 2C and 5C respectively (1C = 800 mA/g). Highly stable capacities of ∼660–700 and 400–450 mAh/g could be maintained for the nanocomposite in the prolonged cycles at 2C and 5C respectively. 4. Conclusion GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon core-shell particles were prepared and showed improved electrochemical
S. Chen et al. / Electrochemistry Communications 12 (2010) 1302–1306
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Fig. 3. Electrochemical properties of GNS-supported Sn–Sb@carbon particles: (a) the first cycle discharge and charge curves at 0.1C. (b) cycling performances at 0.1C. (c) cyclic voltammograms. (d) the first cycle discharge and charge curves at large currents of 0.5C, 1C, 2C and 5C. (e) cycling performances at stepwise current rates. (f) cycling performances at large current rates.
properties in this work. Despite the larger particle size, GNSsupported Sn–Sb@C (50–150 nm) showed much better lithium ion storage properties than GNS-supported Sn–Sb nanoparticles (5– 10 nm) and also an excellent high-rate cycling performance, which was largely attributed to the presence of a carbon skin around Sn–Sb particle and GNS support. Acknowledgements The authors gratefully acknowledge the Program for Professor of Special Appointment (Eastern Scholar), National Natural Science Foundation of China (50971085, 50701029, 10774118), Shanghai Science & Technology Committee (09JC1406100, 09530501200, 09ZR1411800) and Shanghai Municipal Education Commission (09zz96, 10YZ03, S30109) for the financial support.
References [1] N.A. Kaskhedikar, J. Maier, Adv. Mater. 21 (2009) 2664. [2] D.R. Rolison, R.W. Long, J.C. Lytle, A.E. Fischer, C.P. Rhodes, T.M. McEvoy, M.E. Bourga, A.M. Lubers, Chem. Soc. Rev. 38 (2009) 226. [3] F. Cheng, Z. Tao, J. Liang, J. Chen, Chem. Mater. 20 (2008) 667. [4] S.C. Chao, Y.C. Yen, Y.F. Song, Y.M. Chen, H.C. Wu, N.L. Wu, Electrochem. Commun. 12 (2010) 234. [5] I.T. Lucas, E. Pollak, R. Kostecki, Electrochem. Commun. 11 (2009) 2157. [6] C.M. Park, W.S. Chang, H. Jung, J.H. Kim, H.J. Sohn, Electrochem. Commun. 11 (2009) 2165. [7] J. Park, J. Eom, H. Kwon, Electrochem. Commun. 11 (2009) 596. [8] J. Yang, M. Winter, J.O. Besenhard, Solid State Ionics 90 (1996) 281. [9] S.H. Lee, M. Mathews, H. Toghiani, D.O. Wipf, C.U. Pittman, Chem. Mater. 21 (2009) 2306. [10] Y. Wang, J.Y. Lee, Angew. Chem. Int. Ed. 45 (2006) 7039. [11] J. Hassoun, G. Derrien, S. Panero, B. Scrosati, Electrochim. Acta 54 (2009) 4441. [12] C.M. Park, H.J. Sohn, Electrochim. Acta 54 (2009) 6367. [13] J.S. Zhao, L. Wang, X.M. He, C.R. Wan, C.Y. Jiang, Electrochim. Acta 53 (2008) 7048.
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[14] H. Li, G. Zhu, X. Huang, L. Chen, J. Mater. Chem. 10 (2000) 693. [15] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, Science 306 (2004) 666. [16] P.C. Lian, X.F. Zhu, S.Z. Liang, Z. Li, W.S. Yang, H.H. Wang, Electrochim. Acta 55 (2010) 3909. [17] Y. Shao, J. Wang, M. Engelhard, C. Wang, Y. Lin, J. Mater. Chem. 20 (2010) 743. [18] E.J. Yoo, J. Kim, E. Hosono, H.S. Zhou, T. Kudo, I. Honma, Nano Lett. 8 (2008) 2277. [19] P. Guo, H. Song, X. Chen, Electrochem. Commun. 11 (2009) 1320. [20] G.X. Wang, X.P. Shen, J. Yao, J. Park, Carbon 47 (2009) 2049. [21] D.Y. Pan, S. Wang, B. Zhao, M.H. Wu, H.J. Zhang, Y. Wang, Z. Jiao, Chem. Mater. 21 (2009) 3136. [22] Y.S. He, D.W. Bai, X.W. Yang, J. Chen, X.Z. Liao, Z.F. Ma, Electrochem. Commun. 12 (2010) 570.
[23] S.B. Yang, G.L. Cui, S.P. Pang, Q. Cao, U. Kolb, X.L. Feng, J. Maier, ChemSusChem 2 (2009) 236. [24] S.M. Paek, E.J. Yoo, I. Honma, Nano Lett. 9 (2009) 72. [25] F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska, L. Niu, Nanotech 20 (2009) 455602. [26] J. Yao, X.P. Shen, B. Wang, H.K. Liu, G.X. Wang, Electrochem. Commun. 11 (2009) 1849. [27] A.V. Murugan, T. Muraliganth, A. Manthiram, Chem. Mater. 21 (2009) 5004. [28] S.L. Chou, J.Z. Wang, M. Choucair, H.K. Liu, J.A. Stride, S.X. Dou, Electrochem. Commun. 12 (2010) 234. [29] B. Guo, J. Shu, K. Tang, Y. Bai, Z. Wang, L. Chen, J. Power Sources 177 (2008) 205.
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Graphene supported Sn–Sb@carbon core-shell particles as a superior anode for lithium ion batteries Shuangqiang Chen a, Peng Chen a, Minghong Wu b, Dengyu Pan b,⁎, Yong Wang a,⁎ a b
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shangda Road 99,Shanghai, 200444, PR China Institute of Nanochemistry and Nanobiology, Shanghai University, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2010 Received in revised form 30 June 2010 Accepted 6 July 2010 Available online 13 July 2010 Keywords: Core-shell nanostructure Graphene nanosheets Lithium-ion battery Sn-Sb
a b s t r a c t This paper reports the preparation and Li-storage properties of graphene nanosheets(GNS), GNS supported Sn–Sb@carbon (50–150 nm) and Sn–Sb nanoparticles (5–10 nm). The best cycling performance and excellent high rate capabilities were observed for GNS-supported Sn–Sb@carbon core-shell particles, which exhibited initial capacities of 978, 850 and 668 mAh/g respectively at 0.1C, 2C and 5C (1C = 800 mA/g) with good cyclability. Besides the GNS support, the carbon skin around Sn–Sb particles is believed to be a key factor to improve electrochemical properties of Sn–Sb. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Graphite is the commercial anode material for Li-ion batteries with a limited theoretical capacity of 372 mAh/g, which fails to meet the fast-growing requirements of high-power Li-ion batteries[1–3]. Tinbased anodes have long been considered as an attractive replacement of graphite anodes because they can deliver as high as 2–3 times the capacity of graphite. However, large volume changes are often associated with lithium alloying and de-alloying process, which eventually lead to the electrode pulverization and capacity fading [4–7]. Among various tin-based anodes, Sn–Sb alloys have been considered promising, because a stepwise lithium insertion mechanism can relieve volume changes and improve the mechanical stability of the electrode, ensuring better electrochemical properties than a single Sn phase [8–14]. Graphene, a flat one-atom-thick monolayer, owns outstanding electronic behavior, large surface area and amazing mechanical properties, which has attracted a great deal of research interest for many applications [15]. In particular, graphene nanosheets(GNS) [16–21] and GNS-based composites [22–28] have been explored as anode materials, which generally showed good Li-storage properties. This communication reports the preparation and Li-storage properties of two new GNS-based composites, namely GNS-supported Sn–Sb nanoparticles or Sn–Sb@carbon core-shell particles.
Preparation of graphite oxide(GO) and GNS: natural graphite powders (1 g, Shanghai Colloid Chemical) were oxidized by a mixed aqueous solution (40 ml 65 wt.% HNO3 and 60 ml 98 wt.% H2SO4)in an ice-water bath. After oxidation for 20 min, 5 g KMnO4 was gradually added and reacted for 2 h at 35 °C. Then 200 ml deionized water and 5 ml 30 wt.% H2O2 aqueous solution were added into the system, followed by washing with 15 ml 10 wt.% HCl aqueous solution. GO sheets were obtained after centrifugation, copious washing with deionized water and drying. GNS was obtained by thermal reduction of GO at 300 °C in N2 for 2 h. Preparation of GNS-supported Sn–Sb composites: 20 ml 0.03 M SnCl4·5H2O ethanol solution was mixed with 20 ml 0.01 M SbCl3 ethanol solution to form a transparent solution. GNS(0.04 g) or GO(0.06 g) were ultrasonically dispersed in 10 ml deionized water for 30 min, and then mixed with the solution containing SnCl4 and SbCl3. The suspension was reduced by a 20 ml 0.3 M NaBH4 aqueous solution in an ice-water bath. After strong magnetic stirring for 2 h, the precipitates (GNS- or GOsupported Sn–Sb nanoparticles) were collected by centrifugation, copious washing with deionized water, and drying. The GO-supported Sn–Sb nanoparticles were heated in a tube furnace in a flow of 80 sccm C2H2 at 500 °C for 2 h to obtain GNS-supported Sn–Sb@carbon. The products were characterized by various instruments and assembled into a half battery for electrochemical properties. More details on the instruments for material characterizations and cell assembly can be found elsewhere[10]. The cells were discharged (lithium insertion) and charged (lithium extraction) at a constant current (80 mA/g, 0.1C) in 0.005–3.0 V. Higher hourly rates (0.5, 1, 2, or 5C) were also used and the first cycle discharging was kept at 0.1C.
⁎ Corresponding authors. E-mail addresses: [email protected] (D. Pan), [email protected] (Y. Wang). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.07.005
S. Chen et al. / Electrochemistry Communications 12 (2010) 1302–1306
3. Results and discussion The XRD patterns of GO, GNS and GNS-supported Sn–Sb nanoparticles or Sn–Sb@carbon particles are shown in Fig. 1a. There is a characteristic peak of GO at 11.5°, which shifts to 23.5° for GNS. The corresponding interplanar spacing of GNS is 0.38 nm, which is slightly larger than that of standard graphite (0.335 nm). For the GNS-supported Sn–Sb or Sn–Sb@carbon, a few peaks ascribed to a mixed phase of Sn (JCPDS 04-0673) and SnSb (JCPDS 33-0018) [8–10] can be observed. The SEM images in Fig. 1b, c, d, e–f show GNS, Sn–Sb nanoparticles, GNSsupported Sn–Sb, and GNS-supported Sn–Sb@C, respectively. A number of Sn–Sb nanoparticles (∼5–10 nm) are shown as white dots on GNS (Fig. 1d). In comparison, a few larger Sn–Sb particles (∼50–150 nm) were found in Fig. 1e–f. These particles were distributed uniformly on GNS and a thin carbon layer was coated around each Sn–Sb particle, as indicated as arrows in Fig. 1f. Fig. 2a–b shows the TEM and HRTEM images of GNS. GNS are almost transparent with a rippled structure and ∼4–8 nm in thickness, corresponding to ∼10–20 layers stacking of single graphene nanosheet. Fig. 2c shows the TEM image of GNS-supported Sn–Sb nanoparticles, which are uniformly dispersed without detectable agglomeration on GNS surface. GNS-supported Sn–Sb@carbon particles are shown in
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Fig. 2d–e. Because chemical vapor deposition and reduction was performed at 500 °C and the melting point of Sn–Sb is ∼230–240 °C, Sn–Sb droplets adopt a nearly sphere-like morphology due to surface tension. Sn–Sb droplets were used as self-catalysts to promote the C2H2 decomposition and formation of carbon shell [9,10]. Compared to GNSsupported Sn–Sb nanoparticles, there was a significant particle size growth from 5–10 nm to ∼50–150 nm for Sn–Sb@carbon particles, which is attributed to the movement and fusion of small Sn–Sb droplets at a high temperature. It is believed that once a carbon shell is completely formed around Sn–Sb material, there should be no further droplet coalescence and particle size growth. The carbon shells are clearly observed around Sn–Sb particles (Fig. 2e) with the thickness ∼5– 10 nm. CHN elemental analysis indicated there was ∼48.2 wt.% of carbon in GNS–SnSb@carbon composite, which was substantially higher than carbon content (∼29.9 wt.%) in GNS–SnSb composite. The molar ratio of Sn:Sb was estimated to be ∼3:1 in two composites by the EDS analysis. A schematic sketch of the growth process is illustrated in the Fig. 2f. Fig. 3a shows the initial discharge and charge curves of various anodes. Initial charge capacities were 755 and 905 mAh/g for Sn–Sb nanoparticles and GNS respectively. GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon particles showed higher initial charge capacities
Fig. 1. (a)XRD patterns of GO, GNS, GNS-supported Sn–Sb or Sn–Sb@carbon particles. SEM images of the products: (b) GNS. (c) Sn–Sb nanoparticles. (d) GNS-supported Sn–Sb nanoparticles. (e–f) GNS-supported Sn–Sb@carbon particles.
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Fig. 2. (a) TEM image of GNS. (b) HRTEM image of GNS. (c) TEM image of GNS-supported Sn–Sb nanoparticles. (d–e) TEM image of GNS-supported Sn–Sb@carbon particles. (f) schematic illustration of the formation process of nanocomposites.
of 1120 and 978 mAh/g. These values were substantial larger than Sn– Sb/C composites reported previously [9–14] and the theoretical value of GNS (744 mAh/g), which may be attributed to additional storage of Liions in the defects or stacking of nanoscale composites. The initial coulombic efficiencies were 58.7% and 65.3% for GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon particles respectively. The initial large capacity loss seems to be inevitable for most GNS-based anodes [16–28], which is mainly ascribed to the formation of solid electrolyte interface (SEI) on GNS and Sn–Sb particles. Fig. 3b shows cycling performances of various anode materials at 0.1C. Unsupported Sn–Sb nanoparticles displayed a fast capacity fading and the charge capacity was reduced to below 10 mAh/g after 20 cycles. This poor cycling performance has long been understood as a sequence of particle aggregation and electrode pulverization[8–14]. The charge capacity of GNS-supported Sn–Sb nanoparticles (5–10 nm) was 714 mAh/g after 30 cycles, which corresponded to 63.8% of initial charge capacity. In spite of a larger particle size, Sn–Sb@carbon particles (50–150 nm) supported on GNS showed a larger charge capacity of 896 mAh/g after 30 cycles and the capacity retention rate was a high 91.6% compared to initial value. It is believed particle size is not the determining factor to cycling performance. The excellent cycling performance of GNS-supported Sn–Sb@carbon, which was superior to
GNS-supported Sn–Sb nanoparticles, is largely ascribed to a carbon shell around each Sn–Sb particle. The carbon shell functions as a stabilizer to immobilize Sn–Sb particles, a matrix to buffer local volume changes during cycling, and a conducting bridge between GNS and Sn–Sb particles. Therefore mechanical stability and electrical contact were increased for GNS-supported Sn–Sb@carbon. Fig. 3c shows cyclic voltammetry of the composite, a few characteristic cathodic peaks (0.75, 0.58, 0.32 V) and anodic peaks (0.71, 0.81, 0.86 V) could be observed, which correspond to the stepwise lithium ion insertion and extraction reactions in Sn–Sb alloy[8–10]. It should be noted that there are two small anodic peaks above 2.5 V, which may be attributed to nanoscale Sn–Sb[29]. The rate performance of GNS-supported Sn– Sb@carbon was also explored (Fig. 3d–f). Initial charge capacities were 916, 890, 850 and 668 mAh/g at large currents of 0.5C, 1C, 2C and 5C respectively (1C = 800 mA/g). Highly stable capacities of ∼660–700 and 400–450 mAh/g could be maintained for the nanocomposite in the prolonged cycles at 2C and 5C respectively. 4. Conclusion GNS-supported Sn–Sb nanoparticles and Sn–Sb@carbon core-shell particles were prepared and showed improved electrochemical
S. Chen et al. / Electrochemistry Communications 12 (2010) 1302–1306
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Fig. 3. Electrochemical properties of GNS-supported Sn–Sb@carbon particles: (a) the first cycle discharge and charge curves at 0.1C. (b) cycling performances at 0.1C. (c) cyclic voltammograms. (d) the first cycle discharge and charge curves at large currents of 0.5C, 1C, 2C and 5C. (e) cycling performances at stepwise current rates. (f) cycling performances at large current rates.
properties in this work. Despite the larger particle size, GNSsupported Sn–Sb@C (50–150 nm) showed much better lithium ion storage properties than GNS-supported Sn–Sb nanoparticles (5– 10 nm) and also an excellent high-rate cycling performance, which was largely attributed to the presence of a carbon skin around Sn–Sb particle and GNS support. Acknowledgements The authors gratefully acknowledge the Program for Professor of Special Appointment (Eastern Scholar), National Natural Science Foundation of China (50971085, 50701029, 10774118), Shanghai Science & Technology Committee (09JC1406100, 09530501200, 09ZR1411800) and Shanghai Municipal Education Commission (09zz96, 10YZ03, S30109) for the financial support.
References [1] N.A. Kaskhedikar, J. Maier, Adv. Mater. 21 (2009) 2664. [2] D.R. Rolison, R.W. Long, J.C. Lytle, A.E. Fischer, C.P. Rhodes, T.M. McEvoy, M.E. Bourga, A.M. Lubers, Chem. Soc. Rev. 38 (2009) 226. [3] F. Cheng, Z. Tao, J. Liang, J. Chen, Chem. Mater. 20 (2008) 667. [4] S.C. Chao, Y.C. Yen, Y.F. Song, Y.M. Chen, H.C. Wu, N.L. Wu, Electrochem. Commun. 12 (2010) 234. [5] I.T. Lucas, E. Pollak, R. Kostecki, Electrochem. Commun. 11 (2009) 2157. [6] C.M. Park, W.S. Chang, H. Jung, J.H. Kim, H.J. Sohn, Electrochem. Commun. 11 (2009) 2165. [7] J. Park, J. Eom, H. Kwon, Electrochem. Commun. 11 (2009) 596. [8] J. Yang, M. Winter, J.O. Besenhard, Solid State Ionics 90 (1996) 281. [9] S.H. Lee, M. Mathews, H. Toghiani, D.O. Wipf, C.U. Pittman, Chem. Mater. 21 (2009) 2306. [10] Y. Wang, J.Y. Lee, Angew. Chem. Int. Ed. 45 (2006) 7039. [11] J. Hassoun, G. Derrien, S. Panero, B. Scrosati, Electrochim. Acta 54 (2009) 4441. [12] C.M. Park, H.J. Sohn, Electrochim. Acta 54 (2009) 6367. [13] J.S. Zhao, L. Wang, X.M. He, C.R. Wan, C.Y. Jiang, Electrochim. Acta 53 (2008) 7048.
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