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C-C (MWCNTs) composite anode materials for lithium ion battery by carbothermal reduction
Diamond & Related Materials 20 (2011) 413–417
Contents lists available at ScienceDirect
Diamond & Related Materials 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 / d i a m o n d
Synthesis of confinement structure of Sn/C-C (MWCNTs) composite anode materials for lithium ion battery by carbothermal reduction☆ Yi-Ruei Jhan a, Jenq-Gong Duh a,⁎, Su-Yueh Tsai b a b
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan Precision Instrument Center, National Tsing Hua University, Hsinchu, Taiwan
a r t i c l e
i n f o
Available online 12 January 2011 Keywords: Sn Carbonaceous mixture Composite anode Carbothermal reduction Lithium ion batteries
a b s t r a c t A composite anode material was prepared with confined tin into multiwall carbon nanotube by carbothermal reduction. The morphology and structure of Sn/C (nature graphite) and Sn/C-C (nature graphite + multiwall carbon nanotube) were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was revealed that the additive of MWCNT was a crucial factor to improve Sn /C composite anodes for cyclability and reversible capacity. Volume changes and morphological changes in Sn can be reduced by encasing MWCNT in a carbonaceous material that has sufficient flexibility to act as a buffer. Electrochemical performance test shows that the charge capacity of the Sn/C-C (NG + MWCNT) electrode in the fiftieth cycle was 400 mAh/g, which was higher than that of the Sn/C (NG) electrode. After 50 cycles, the retention of the Sn/C-C electrode and the Sn/C electrode was 80% and 63%, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium (Li) alloys containing tin (Sn) have higher energy density than pure Li and are, hence, preferred as negative electrode materials in Li batteries [1–5]. Metallic tin has a theoretical capacity of 994 mAh/g, which is approximately two times higher than that of graphite. However, the electrodes made of these alloy systems showed poor capacity retention during cycle life [6–8]. This behavior is attributed to the pulverization of particles, which is in turn caused by a change in volume (N300%) due to compositional changes that occurred during cycling. The cycling performance of the electrodes can be enhanced by reducing the change in volume. Many approaches have been focused on reducing such volume change through the use of composites of carbon (C) and Sn nanoparticles [9,10]. Kim and co-workers synthesized Sn/C composites by infiltrating tetraethyltin into mechanically milled polystyrene resin powder [11]. The electrode made of these synthesized Sn/C composites shows better cycling performance than conventional Sn anodes. They suggested that the volume changes and morphological changes in Sn can be reduced by encasing it in a carbonaceous material that has sufficient flexibility to act as a buffer. However, this process was virulent and involved expensive starting materials, such as tetraethyltin and hydrophobized Sn nanoparticles. Recently, it was demonstrated that combining ductile carbonaceous materials with Sn-based compounds can effectively enhance the cycling stability [12]. ☆ Presented at NDNC 2010, the 4th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. Tel./fax: +886 3 5712686. E-mail address: [email protected] (J.-G. Duh). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.01.012
In this study, a multiwall carbon nanotube (MWCNT) was selected as a separator for inhibiting the aggregation of Sn particles and thus increasing their structural stability during cycling. The MWCNT also acted as a buffering matrix to reduce the expansion occurred within the electrode because of the Li+ ion intercalation/deintercalation process. The matrix structure of Sn/C-C (MWCNT) composite anode material was prepared by a simple and inexpensive method of carbothermal reduction.
2. Experimental 2.1. Preparation of Sn/C, C-C composite powders The carbothermal reduction was carried out in accordance with the following reaction. SnO2 þ 2C→Sn þ 2CO
ð1Þ
To reduce the size of reduced Sn particles, nanosized SnO2 particles were firstly coated on carbonaceous matrix via a wet chemical process. Nature graphite (NG) and multiwall carbon nanotube (MWCNT) were adopted as the carbonaceous matrix. Tin (II) chloride (SnCl2) was the source of Sn. Firstly, appropriate amount of SnCl2 was dissolved in methanol to obtain the metal-precursor. The NG and NG+ MWCNT powders were immediately added into the precursor under strong stirring for 1 h. Afterwards, the Sn2+-coated NG and NG+ MWCNT powders were dried at room temperature in vacuum and heat-treated to 600 °C at a rate of 1 °C /min to derive tin oxides, and then heated to 850 °C at a rate of 2 °C/min to obtain metallic Sn/C (NG) and Sn/C (NG + MWCNT).
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Y.-R. Jhan et al. / Diamond & Related Materials 20 (2011) 413–417
2.2. Materials characterizations The phase identification of derived anode materials was carried out by X-ray diffraction (XRD) operated at 40 kV and 30 mA from 20° to 80° with a wavelength of λ = 1.5405 Å for CuKα. The tin and carbon content in the composites were determined by inductively coupled plasma (ICP) spectroscopy using filtered digestions of the composites in a mixture of HCl and HNO3. Surface morphology of Sn-containing composite anode materials was observed using a field-emission scanning electron microscope (FE-SEM, JSM-7600F) with a controlled accelerating voltage at 15 kV. 2.3. Electrochemical characterization
Table 1 Composition of Sn/C and Sn/C-C electrodes. Sample
C (wt.%)
Sn (wt.%)
Sn/C Sn/C-C
62.2 63.9
37.8 36.1
Sn was inserted between NG and NG. The particle size of Sn was 0.8– 1.2 μm. These features of the surface morphology and Sn particles easily result in aggregation of Sn particles during cycling. As shown in Fig. 2(b), the morphology of the Sn/C-C powder indicates that the addition of MWCNTs to the powder results in the segregation of Sn particles. The particle size of Sn in this case was 0.4–0.5 μm. Fig. 2(c)
The electrochemical performance of the Sn-coating composite negative electrode was examined by two-electrode test cells consisted of the composite electrode, metallic lithium electrode, polypropylene separator and an electrolyte composed of 1 M LiPF6 in EC/EMC (1:2 vol.%) for 2032 coin cells. The composite electrode was prepared by coating the ball-milled slurry comprised of Sn-containing composite materials (95 wt.%) and PVDF binder (5 wt.%) on an cooper foil, followed by drying the coated electrode at 100 °C for 12 h in vacuum and then by roll-pressing the derived electrode. The assembled cells were galvanostatically cycled at the rate of 0.1 C between 0.001 and 1.5 V. 3. Results and discussion 3.1. Phase identification and surface morphology of Sn/C and Sn/C-C composite materials As shown in Fig. 1, the X-ray diffraction (XRD) patterns of Sn/C and Sn/C-C composite materials revealed that the crystalline phases of these materials were not affected by the addition of MWCNTs for identical precursor solution and carbothermal reduction condition. The phases included pure Sn and C, indicating that tin oxide was completely reduced to pure Sn at the reduction temperature of 850 °C. Table 1 represents the composition of Sn/C and Sn/C-C electrode, which indicated that the containment of tin and carbon in both electrode was nearly the same under identical concentration of precursor. The carbon composition of Sn/C was 100 wt.% of nature graphite, as compared to Sn/C-C, which was 95 wt.% of nature graphite and 5 wt.% of multiwall carbon nanotube. The morphologies of the Sn/C (NG) and Sn/C-C (NG + MWCNT) composite anode material powders were observed using scanning electron microscopy (SEM). These SEM images are shown in Fig. 2. As shown in Fig. 2(a), the Sn/C powder contained numerous round particles of Sn dispersed on the flake-shaped surface of NG and other
Fig. 1. XRD patterns of the as-prepared Sn/C and Sn/C-C powders.
Fig. 2. SEM images of the as-prepared powders of (a) Sn/C and (b) Sn/C-C. (c) Magnification of (b).
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shows a highly magnified image of the Sn/C-C powder morphology. The Sn particle was tightly entangled by MWCNTs. The cross-link space of MWCNTs was 0.1–0.3 μm, indicating that Sn particles in the Sn/C-C powder cannot escape from this space. Therefore, MWCNTs provided a reliable matrix space that separated Sn particle in the Sn/C-C powder. 3.2. Electrochemical characterization of Sn/C and Sn/C-C materials Cyclic voltammogram (CV) curves of Sn/C (NG) and Sn/C-C (NG + MWCNTS) electrodes in the first cycle are shown in Fig. 3(a). The measurement was carried out at a scanning rate of 0.05 mV/s. The potential was swept from 1.5 to 0.0 V versus Li/Li± reference electrode and back to 1.5 V. According to the literature [13], the intense peaks which were present below 0.2 V and near 0.0 V were observed both in the reduction and oxidation curves. It was attributed to the reversible intercalation of Li± into graphite of Sn/C and Sn/C-C electrodes. Four other oxidation peaks can be clearly observed at 0.47, 0.62, 0.72, and 0.81 V vs. Li/Li+. These peaks were related to the Li extraction from Sn in Sn/C and Sn/C-C electrodes. Moreover, four corresponding reduction peaks at 0.18, 0.41, 0.54 and 0.68 V can be observed in the reduction CV scans. The CV curve of the Sn/C electrode showed higher reduction and oxidation peak currents than those of the Sn/C-C electrode at near 0.0 and 0.2 V, respectively. This implies that C in the Sn/C electrode can provide more capacity than C in the Sn/C-C electrode at the first cycle. The difference in the peak currents between Sn/C and Sn/C-C electrodes at near 0.0 and 0.2 V was attributed to the fact that the Sn/C-C electrode has a higher surface area than that of the Sn/C electrode. Therefore, the electrolyte does not immediately wet the Sn/C-C electrode completely. This observation was verified by the second cycle of CVs, as shown in Fig. 3(b).
Fig. 3. Cyclic voltammogram curves of Sn/C and Sn/C-C electrodes in (a) the first cycle and (b) the second cycle.
415
Reduction and oxidation peak currents of Sn/C-C electrodes at near 0.0 and 0.2 V, respectively, are close to those of the Sn/C electrode. Sn, like C, affects the reversible capacity of anode materials. Therefore, the stability of the oxidation peak current corresponding to Sn at 0.47, 0.62, 0.72, and 0.81 V vs. Li/Li+ influences the overall reversible capacity of the Sn/C and Sn/C-C electrodes. The CV curves of Sn/C and Sn/C-C electrodes for the first eight cycles are shown in Fig. 4. Fig. 4(a) shows that the oxidation peaks of Sn current for the Sn/C electrode rapidly faded with each cycle. However, as shown in Fig. 4(b), the Sn/C-C electrode with the structure for confined Sn particles exhibited good stability in cycling. This observation indicates that the cyclability of Sn/C electrodes can be improved by adding MWCNTs. Fig. 5(a) shows the cycling performance of Sn/C and Sn/C-C electrodes cycled between 0.001 and 1.5 V at a 0.1 C rate (approximately 0.15 mA/cm2). After 50 cycles, the Sn/C and Sn/C-C electrodes, respectively, can retain 63% (305 mAh/g) and 80% (400 mAh/g) reversible capacity of the second cycle. Hence, the Sn/C-C electrode exhibited superior electrochemical performance than the Sn/C electrode. Fig. 5(b) shows the coulombic efficiency of Sn/C and Sn/C-C electrodes. The Sn/C electrode delivered lower coulombic efficiency than the Sn/C-C electrode, which was attributed to the change in the Sn volume of the Sn/C electrode during the charge/ discharge process, resulting in the formation of cracks in the electrode. The formation of a crack usually accompanied the formation of a solid electrolyte interface (SEI) film. The formation of SEI films would consume Li+, leading to irreversible components. Therefore, the coulombic efficiency of Sn/C was lower than that of Sn/C-C in each cycle. Sn/C-C with the confinement structure can entangle Sn particles tightly. Consequently, the stress cause of volume change during charge/discharge can be suppressed by the
Fig. 4. Cyclic voltammogram curves of (a) Sn/C and (b) Sn/C-C electrodes in the first eight cycles.
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Fig. 6. The fresh surface morphology of the as-prepared (a) Sn/C electrode and (b) the Sn/C-C electrode. Fig. 5. (a) Cycling performance of Sn/C and Sn/C-C cycles between 0.001 and 1.5 V at a 0.1 C rate. (b) Comparison between coulombic efficiencies of Sn/C and Sn/C-C.
confinement structure. Fig. 6(a) and (b) show the fresh surface morphologies of the as-prepared Sn/C and Sn/C-C electrodes, which were similar to those of the as-prepared powders. After 50 cycles, several cracks can be observed on the surface of the Sn/C electrode, as shown in Fig. 7(a). These cracks result in a decreased electronic conductivity between the particle aggregates. However, as shown in Fig. 7(b), the surface morphology of the Sn/C-C electrode after 50 cycles did not exhibit any cracks. The added MWCNTs not only serve as the separator, but also mitigate the conductivity loss and reduce the electrode impedance. In addition, the effect of the addition of MWCNTs on the electrode impedance was verified by measuring the impedance of the Sn/C and Sn/C-C electrodes. Fig. 8 shows the impedance spectra for the electrodes in the tenth cycle. The shapes of these two spectra are rather similar. The high-frequency arc in the spectra is attributed to the charge-transfer reaction at the interface of the electrolyte and the electrode. The subsequent inclined line is attributed to the diffusion of Li+ ions in the Sn/C and Sn/C-C electrodes. The resistance of the Sn/C-C electrode is lower than that of the Sn/C electrode. 4. Conclusions Dispersion of MWCNT in the Sn/C composite anode material during the process of carbothermal reduction has been proved to be a key factor for obtaining a reliable composite anode material. Improvement of the anode performance due to the MWCNT loading includes (1) remarkable increase of the cyclability; (2) the active material utilization was improved; (3) effective suppression of the stress of volume change during charge/discharge; (4) the electroconductivity was enhanced. Therefore, it is concluded that the
Fig. 7. Surface morphology of (a) Sn/C and (b) Sn/C-C electrodes after 50 cycles.
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417
Acknowledgements This work was mainly supported by the National Science Council, Taiwan, under Contract No. NSC 96-2221-E-007-093-MY3.
References
Fig. 8. Comparison between impedance spectra for Sn/C and Sn/C-C electrodes in the tenth cycle.
electrochemical performance of electrodes can be enhanced by controlling the morphology of the electrode materials. The confined structure of Sn/C-C (NG-MWCNTs) composite material can, therefore, be considered as a candidate for the anodes of lithium ion batteries.
[1] R.A. Huggins, in: J.O. Besenhard (Ed.), Handbook of Battery Materials, Wiley-VCH, Weinhelm, 1999, p. 359. [2] K.T. Lee, Y.S. Jung, S.M. Oh, J. Am. Chem. Soc. 125 (2003) 5652. [3] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novak, Adv. Mater. 10 (1998) 725. [4] G.R. Goward, F. Leroux, W.P. Power, G. Ouvrard, W. Dmowski, T. Egami, L.F. Nazar, Electrochem. Solid State Lett. 2 (1999) 367. [5] O. Crosnier, T. Brousse, X. Devaux, P. Fragnaud, D.M. Schleich, J. Power Sources 94 (2001) 169. [6] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [7] J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87. [8] M.M. Thackeray, J.T. Vaughey, A.J. Kahaian, K.D. Kepler, R. Benedek, Electrochem. Commun. 1 (1999) 111. [9] Y.J. Chen, C.L. Zhu, X.Y. Xue, X.L. Shi, M.S. Cao, Appl. Phys. Lett. 92 (2008) 223101. [10] G.X. Wang, J.H. Ahn, M.J. Lindsay, L. Sun, D.H. Bradhurst, S.X. Dou, J. Power Sources 97–98 (2001) 211. [11] I.S. Kim, G.E. Blomgren, P.N. Kumta, Electrochem. Solid State Lett. 7 (2004) A44. [12] H. Li, Q. Wang, L. Shi, L. Chen, X. Huang, Chem. Mater. 14 (2002) 103. [13] D. Billaud, F.X. Henry, P. William, Mater. Res. Bull. 28 (1999) 477.
Contents lists available at ScienceDirect
Diamond & Related Materials 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 / d i a m o n d
Synthesis of confinement structure of Sn/C-C (MWCNTs) composite anode materials for lithium ion battery by carbothermal reduction☆ Yi-Ruei Jhan a, Jenq-Gong Duh a,⁎, Su-Yueh Tsai b a b
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan Precision Instrument Center, National Tsing Hua University, Hsinchu, Taiwan
a r t i c l e
i n f o
Available online 12 January 2011 Keywords: Sn Carbonaceous mixture Composite anode Carbothermal reduction Lithium ion batteries
a b s t r a c t A composite anode material was prepared with confined tin into multiwall carbon nanotube by carbothermal reduction. The morphology and structure of Sn/C (nature graphite) and Sn/C-C (nature graphite + multiwall carbon nanotube) were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was revealed that the additive of MWCNT was a crucial factor to improve Sn /C composite anodes for cyclability and reversible capacity. Volume changes and morphological changes in Sn can be reduced by encasing MWCNT in a carbonaceous material that has sufficient flexibility to act as a buffer. Electrochemical performance test shows that the charge capacity of the Sn/C-C (NG + MWCNT) electrode in the fiftieth cycle was 400 mAh/g, which was higher than that of the Sn/C (NG) electrode. After 50 cycles, the retention of the Sn/C-C electrode and the Sn/C electrode was 80% and 63%, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Lithium (Li) alloys containing tin (Sn) have higher energy density than pure Li and are, hence, preferred as negative electrode materials in Li batteries [1–5]. Metallic tin has a theoretical capacity of 994 mAh/g, which is approximately two times higher than that of graphite. However, the electrodes made of these alloy systems showed poor capacity retention during cycle life [6–8]. This behavior is attributed to the pulverization of particles, which is in turn caused by a change in volume (N300%) due to compositional changes that occurred during cycling. The cycling performance of the electrodes can be enhanced by reducing the change in volume. Many approaches have been focused on reducing such volume change through the use of composites of carbon (C) and Sn nanoparticles [9,10]. Kim and co-workers synthesized Sn/C composites by infiltrating tetraethyltin into mechanically milled polystyrene resin powder [11]. The electrode made of these synthesized Sn/C composites shows better cycling performance than conventional Sn anodes. They suggested that the volume changes and morphological changes in Sn can be reduced by encasing it in a carbonaceous material that has sufficient flexibility to act as a buffer. However, this process was virulent and involved expensive starting materials, such as tetraethyltin and hydrophobized Sn nanoparticles. Recently, it was demonstrated that combining ductile carbonaceous materials with Sn-based compounds can effectively enhance the cycling stability [12]. ☆ Presented at NDNC 2010, the 4th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author. Tel./fax: +886 3 5712686. E-mail address: [email protected] (J.-G. Duh). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.01.012
In this study, a multiwall carbon nanotube (MWCNT) was selected as a separator for inhibiting the aggregation of Sn particles and thus increasing their structural stability during cycling. The MWCNT also acted as a buffering matrix to reduce the expansion occurred within the electrode because of the Li+ ion intercalation/deintercalation process. The matrix structure of Sn/C-C (MWCNT) composite anode material was prepared by a simple and inexpensive method of carbothermal reduction.
2. Experimental 2.1. Preparation of Sn/C, C-C composite powders The carbothermal reduction was carried out in accordance with the following reaction. SnO2 þ 2C→Sn þ 2CO
ð1Þ
To reduce the size of reduced Sn particles, nanosized SnO2 particles were firstly coated on carbonaceous matrix via a wet chemical process. Nature graphite (NG) and multiwall carbon nanotube (MWCNT) were adopted as the carbonaceous matrix. Tin (II) chloride (SnCl2) was the source of Sn. Firstly, appropriate amount of SnCl2 was dissolved in methanol to obtain the metal-precursor. The NG and NG+ MWCNT powders were immediately added into the precursor under strong stirring for 1 h. Afterwards, the Sn2+-coated NG and NG+ MWCNT powders were dried at room temperature in vacuum and heat-treated to 600 °C at a rate of 1 °C /min to derive tin oxides, and then heated to 850 °C at a rate of 2 °C/min to obtain metallic Sn/C (NG) and Sn/C (NG + MWCNT).
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Y.-R. Jhan et al. / Diamond & Related Materials 20 (2011) 413–417
2.2. Materials characterizations The phase identification of derived anode materials was carried out by X-ray diffraction (XRD) operated at 40 kV and 30 mA from 20° to 80° with a wavelength of λ = 1.5405 Å for CuKα. The tin and carbon content in the composites were determined by inductively coupled plasma (ICP) spectroscopy using filtered digestions of the composites in a mixture of HCl and HNO3. Surface morphology of Sn-containing composite anode materials was observed using a field-emission scanning electron microscope (FE-SEM, JSM-7600F) with a controlled accelerating voltage at 15 kV. 2.3. Electrochemical characterization
Table 1 Composition of Sn/C and Sn/C-C electrodes. Sample
C (wt.%)
Sn (wt.%)
Sn/C Sn/C-C
62.2 63.9
37.8 36.1
Sn was inserted between NG and NG. The particle size of Sn was 0.8– 1.2 μm. These features of the surface morphology and Sn particles easily result in aggregation of Sn particles during cycling. As shown in Fig. 2(b), the morphology of the Sn/C-C powder indicates that the addition of MWCNTs to the powder results in the segregation of Sn particles. The particle size of Sn in this case was 0.4–0.5 μm. Fig. 2(c)
The electrochemical performance of the Sn-coating composite negative electrode was examined by two-electrode test cells consisted of the composite electrode, metallic lithium electrode, polypropylene separator and an electrolyte composed of 1 M LiPF6 in EC/EMC (1:2 vol.%) for 2032 coin cells. The composite electrode was prepared by coating the ball-milled slurry comprised of Sn-containing composite materials (95 wt.%) and PVDF binder (5 wt.%) on an cooper foil, followed by drying the coated electrode at 100 °C for 12 h in vacuum and then by roll-pressing the derived electrode. The assembled cells were galvanostatically cycled at the rate of 0.1 C between 0.001 and 1.5 V. 3. Results and discussion 3.1. Phase identification and surface morphology of Sn/C and Sn/C-C composite materials As shown in Fig. 1, the X-ray diffraction (XRD) patterns of Sn/C and Sn/C-C composite materials revealed that the crystalline phases of these materials were not affected by the addition of MWCNTs for identical precursor solution and carbothermal reduction condition. The phases included pure Sn and C, indicating that tin oxide was completely reduced to pure Sn at the reduction temperature of 850 °C. Table 1 represents the composition of Sn/C and Sn/C-C electrode, which indicated that the containment of tin and carbon in both electrode was nearly the same under identical concentration of precursor. The carbon composition of Sn/C was 100 wt.% of nature graphite, as compared to Sn/C-C, which was 95 wt.% of nature graphite and 5 wt.% of multiwall carbon nanotube. The morphologies of the Sn/C (NG) and Sn/C-C (NG + MWCNT) composite anode material powders were observed using scanning electron microscopy (SEM). These SEM images are shown in Fig. 2. As shown in Fig. 2(a), the Sn/C powder contained numerous round particles of Sn dispersed on the flake-shaped surface of NG and other
Fig. 1. XRD patterns of the as-prepared Sn/C and Sn/C-C powders.
Fig. 2. SEM images of the as-prepared powders of (a) Sn/C and (b) Sn/C-C. (c) Magnification of (b).
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shows a highly magnified image of the Sn/C-C powder morphology. The Sn particle was tightly entangled by MWCNTs. The cross-link space of MWCNTs was 0.1–0.3 μm, indicating that Sn particles in the Sn/C-C powder cannot escape from this space. Therefore, MWCNTs provided a reliable matrix space that separated Sn particle in the Sn/C-C powder. 3.2. Electrochemical characterization of Sn/C and Sn/C-C materials Cyclic voltammogram (CV) curves of Sn/C (NG) and Sn/C-C (NG + MWCNTS) electrodes in the first cycle are shown in Fig. 3(a). The measurement was carried out at a scanning rate of 0.05 mV/s. The potential was swept from 1.5 to 0.0 V versus Li/Li± reference electrode and back to 1.5 V. According to the literature [13], the intense peaks which were present below 0.2 V and near 0.0 V were observed both in the reduction and oxidation curves. It was attributed to the reversible intercalation of Li± into graphite of Sn/C and Sn/C-C electrodes. Four other oxidation peaks can be clearly observed at 0.47, 0.62, 0.72, and 0.81 V vs. Li/Li+. These peaks were related to the Li extraction from Sn in Sn/C and Sn/C-C electrodes. Moreover, four corresponding reduction peaks at 0.18, 0.41, 0.54 and 0.68 V can be observed in the reduction CV scans. The CV curve of the Sn/C electrode showed higher reduction and oxidation peak currents than those of the Sn/C-C electrode at near 0.0 and 0.2 V, respectively. This implies that C in the Sn/C electrode can provide more capacity than C in the Sn/C-C electrode at the first cycle. The difference in the peak currents between Sn/C and Sn/C-C electrodes at near 0.0 and 0.2 V was attributed to the fact that the Sn/C-C electrode has a higher surface area than that of the Sn/C electrode. Therefore, the electrolyte does not immediately wet the Sn/C-C electrode completely. This observation was verified by the second cycle of CVs, as shown in Fig. 3(b).
Fig. 3. Cyclic voltammogram curves of Sn/C and Sn/C-C electrodes in (a) the first cycle and (b) the second cycle.
415
Reduction and oxidation peak currents of Sn/C-C electrodes at near 0.0 and 0.2 V, respectively, are close to those of the Sn/C electrode. Sn, like C, affects the reversible capacity of anode materials. Therefore, the stability of the oxidation peak current corresponding to Sn at 0.47, 0.62, 0.72, and 0.81 V vs. Li/Li+ influences the overall reversible capacity of the Sn/C and Sn/C-C electrodes. The CV curves of Sn/C and Sn/C-C electrodes for the first eight cycles are shown in Fig. 4. Fig. 4(a) shows that the oxidation peaks of Sn current for the Sn/C electrode rapidly faded with each cycle. However, as shown in Fig. 4(b), the Sn/C-C electrode with the structure for confined Sn particles exhibited good stability in cycling. This observation indicates that the cyclability of Sn/C electrodes can be improved by adding MWCNTs. Fig. 5(a) shows the cycling performance of Sn/C and Sn/C-C electrodes cycled between 0.001 and 1.5 V at a 0.1 C rate (approximately 0.15 mA/cm2). After 50 cycles, the Sn/C and Sn/C-C electrodes, respectively, can retain 63% (305 mAh/g) and 80% (400 mAh/g) reversible capacity of the second cycle. Hence, the Sn/C-C electrode exhibited superior electrochemical performance than the Sn/C electrode. Fig. 5(b) shows the coulombic efficiency of Sn/C and Sn/C-C electrodes. The Sn/C electrode delivered lower coulombic efficiency than the Sn/C-C electrode, which was attributed to the change in the Sn volume of the Sn/C electrode during the charge/ discharge process, resulting in the formation of cracks in the electrode. The formation of a crack usually accompanied the formation of a solid electrolyte interface (SEI) film. The formation of SEI films would consume Li+, leading to irreversible components. Therefore, the coulombic efficiency of Sn/C was lower than that of Sn/C-C in each cycle. Sn/C-C with the confinement structure can entangle Sn particles tightly. Consequently, the stress cause of volume change during charge/discharge can be suppressed by the
Fig. 4. Cyclic voltammogram curves of (a) Sn/C and (b) Sn/C-C electrodes in the first eight cycles.
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Fig. 6. The fresh surface morphology of the as-prepared (a) Sn/C electrode and (b) the Sn/C-C electrode. Fig. 5. (a) Cycling performance of Sn/C and Sn/C-C cycles between 0.001 and 1.5 V at a 0.1 C rate. (b) Comparison between coulombic efficiencies of Sn/C and Sn/C-C.
confinement structure. Fig. 6(a) and (b) show the fresh surface morphologies of the as-prepared Sn/C and Sn/C-C electrodes, which were similar to those of the as-prepared powders. After 50 cycles, several cracks can be observed on the surface of the Sn/C electrode, as shown in Fig. 7(a). These cracks result in a decreased electronic conductivity between the particle aggregates. However, as shown in Fig. 7(b), the surface morphology of the Sn/C-C electrode after 50 cycles did not exhibit any cracks. The added MWCNTs not only serve as the separator, but also mitigate the conductivity loss and reduce the electrode impedance. In addition, the effect of the addition of MWCNTs on the electrode impedance was verified by measuring the impedance of the Sn/C and Sn/C-C electrodes. Fig. 8 shows the impedance spectra for the electrodes in the tenth cycle. The shapes of these two spectra are rather similar. The high-frequency arc in the spectra is attributed to the charge-transfer reaction at the interface of the electrolyte and the electrode. The subsequent inclined line is attributed to the diffusion of Li+ ions in the Sn/C and Sn/C-C electrodes. The resistance of the Sn/C-C electrode is lower than that of the Sn/C electrode. 4. Conclusions Dispersion of MWCNT in the Sn/C composite anode material during the process of carbothermal reduction has been proved to be a key factor for obtaining a reliable composite anode material. Improvement of the anode performance due to the MWCNT loading includes (1) remarkable increase of the cyclability; (2) the active material utilization was improved; (3) effective suppression of the stress of volume change during charge/discharge; (4) the electroconductivity was enhanced. Therefore, it is concluded that the
Fig. 7. Surface morphology of (a) Sn/C and (b) Sn/C-C electrodes after 50 cycles.
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417
Acknowledgements This work was mainly supported by the National Science Council, Taiwan, under Contract No. NSC 96-2221-E-007-093-MY3.
References
Fig. 8. Comparison between impedance spectra for Sn/C and Sn/C-C electrodes in the tenth cycle.
electrochemical performance of electrodes can be enhanced by controlling the morphology of the electrode materials. The confined structure of Sn/C-C (NG-MWCNTs) composite material can, therefore, be considered as a candidate for the anodes of lithium ion batteries.
[1] R.A. Huggins, in: J.O. Besenhard (Ed.), Handbook of Battery Materials, Wiley-VCH, Weinhelm, 1999, p. 359. [2] K.T. Lee, Y.S. Jung, S.M. Oh, J. Am. Chem. Soc. 125 (2003) 5652. [3] M. Winter, J.O. Besenhard, M.E. Spahr, P. Novak, Adv. Mater. 10 (1998) 725. [4] G.R. Goward, F. Leroux, W.P. Power, G. Ouvrard, W. Dmowski, T. Egami, L.F. Nazar, Electrochem. Solid State Lett. 2 (1999) 367. [5] O. Crosnier, T. Brousse, X. Devaux, P. Fragnaud, D.M. Schleich, J. Power Sources 94 (2001) 169. [6] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276 (1997) 1395. [7] J.O. Besenhard, J. Yang, M. Winter, J. Power Sources 68 (1997) 87. [8] M.M. Thackeray, J.T. Vaughey, A.J. Kahaian, K.D. Kepler, R. Benedek, Electrochem. Commun. 1 (1999) 111. [9] Y.J. Chen, C.L. Zhu, X.Y. Xue, X.L. Shi, M.S. Cao, Appl. Phys. Lett. 92 (2008) 223101. [10] G.X. Wang, J.H. Ahn, M.J. Lindsay, L. Sun, D.H. Bradhurst, S.X. Dou, J. Power Sources 97–98 (2001) 211. [11] I.S. Kim, G.E. Blomgren, P.N. Kumta, Electrochem. Solid State Lett. 7 (2004) A44. [12] H. Li, Q. Wang, L. Shi, L. Chen, X. Huang, Chem. Mater. 14 (2002) 103. [13] D. Billaud, F.X. Henry, P. William, Mater. Res. Bull. 28 (1999) 477.