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Preparation of polypyrrole-coated silicon nanoparticles
Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
Preparation of polypyrrole-coated silicon nanoparticles Hyun-Shil La a , Ki-Soo Park a , Kee-Suk Nahm a , Kyu-Kwan Jeong b , Youn-Sik Lee a,∗ a
Division of Environmental and Chemical Engineering, Nanomaterials Research Center, Chonbuk National University, Chonju 561-756, Republic of Korea b Division of Chemical Education, Chonbuk National University, Chonju 561-756, Republic of Korea Received 3 March 2005; received in revised form 28 June 2005; accepted 7 July 2005 Available online 18 August 2005
Abstract Silicon (Si) nanoparticles were stabilized by sodium dodecyl sulfate and poly(N-vinylpyrrolidone) in water, and coated with polypyrrole (PPy) via in situ polymerization of pyrrole with FeCl3 . TEM images revealed that the Si nanoparticles were successfully coated with PPy (average thickness, ∼2 nm). The Li/PPy-coated Si electrode exhibited improved discharge capacities, compared to that of a reported Li/pure Si electrode, even though the capacity fading problem caused by large-scaled crumbling of the electrode was not overcome. © 2005 Elsevier B.V. All rights reserved. Keywords: Anode material; Nanoparticle; Silicon; Polypyrrole; Coating
1. Introduction Recently, there has been considerable interest in new electrode materials for improved performances of secondary lithium batteries [1]. For example, binary lithium alloys, such as Li–Al and Li–Sn were tested as anode materials. However, a common problem in all these cases was the large volume increase (∼100–300%), due to Li+ insertion into the alloy anodes, resulting in the internal crack of the materials [2]. The internal crack can cause dramatic morphological changes in the anode, and then the contact between the active material and current collector becomes difficult, resulting in poor cycleabilty of the discharge capacity. The Si electrode system, fabricated with carbon as a conducting material, usually exhibits a very rapid fade of discharge capacity after the first discharge cycle. The discharge capacity decreases successively as the cycling number increases. For example, Hong et al. reported that the Li/nano-sized Si electrode delivered the discharge capacity of 2900 mAh/g in the first cycle. However, the discharge capacity decreased rapidly with increasing the cycling number, and
∗
Corresponding author. Tel.: +82 63 270 2312; fax: +82 63 270 2306. E-mail address: [email protected] (Y.-S. Lee).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.07.003
became 600 mAh/g after 10 cycles. Many researchers also reported similar results [3–5]. Extensive efforts have been made to improve the cyclic performance of the Si-based electrode system [3–12]. However, the cyclic performance is still unsatisfactory. During insertion/extraction of Li+ ion, the Si particles are cracked to form nano-sized Si particles of electrode. After some cycles, the newly formed nanoparticles can merge together, due to their high surface energy, to form dense blocks, which cannot participate in the electrochemical reaction. Therefore, the nano-sized Si particle–particle interaction should be avoided. One of the ways to overcome the problem may be this is to coat the Si particles with an electronand Li+ -conducting material [8,9]. Polypyrroles (PPy) can be easily synthesized using chemical oxidants, such as FeCl3 and exhibits good electrical conductivities and high air stabilities [13–16]. Recently, PPycoated polystyrene latex nanoparticles were successfully prepared by polymerization of pyrrole in the presence of polystyrene latex nanoparticles in our laboratory [17]. The PPy-coated latex nanoparticles exhibited largely improved electrical conductivities, as compared to the micron-sized PPy-coated polystyrene latex particles. PPy-coated Si nanoparticles are very interesting, since the loose contact between the active material and current collec-
H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
23
tor, and/or the extraction of the cracked Si fragments into electrolyte may be prohibited by the PPy coatings. In this research, we attempted to coat Si nanoparticles with PPy via in situ polymerization of pyrrole with FeCl3 . This paper describes the preparation, morphology and discharge capacity behavior of the PPy-coated Si nanoparticles.
2. Experimental 2.1. Measurements The electrochemical characterization was carried out using CR2032 coin-type cells. A mixture of PPy-coated Si nanoparticles (20 mg) and polytetrafluoroethylene (13 mg) were pressed onto a stainless steel grid (25 mm2 mesh) under a pressure of 300 kg/cm2 , and dried at 100 ◦ C for 5 h in an oven. The test cell was composed of a working electrode and a lithium counter electrode (Cyprus Foote Mineral Co.), separated by a porous polypropylene film (Celgard 3401), where the electrolyte was a solution of 1.0 M LiPF6 -EC/DMC (1/2, v/v). The cell was assembled in an argon-filled dry box and tested at room temperature, which was then charged and discharged at a current density of 80 mA/g over a voltage range of 0.05–0.8 V (versus Li/Li+ ). Cyclic voltammetry
Fig. 1. Schematic representation for the preparation of PPy-coated Si particles.
Fig. 2. SEM images: Si powders (a) before and (b) after Au coating and PPy-coated Si powders (c) before and (d) after Au coating.
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H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
(CV) experiment was performed in the range of 0.0–1.5 V at 10 mV/min. 2.2. Preparation of PPy-coated Si nanoparticles The Si nanoparticles (diameter ≤50 nm) were treated with a solution of 1% HF, 60% HNO3 and deionized water (1/5/10, v/v/v) for 10 min, and washed with deionized water by centrifugation thrice. The etched Si powders (0.20 g) and sodium dodecyl sulfate (0.56 g) were dispersed in deionized water (200 mL) in a three-neck round-bottomed flask fitted with a mechanical stirrer, followed by the addition of poly(Nvinylpyrrolidone) (Mw 55,000; 0.10 g) and ferric chloride (FeCl3 ·6H2 O) (0.91 g). Pyrrole (0.10 mL) was added to the above mixture via a syringe, and the in situ polymerization was allowed to proceed for 24 h at room temperature [18,19]. The pH of the reaction mixture was adjusted to be 7.0 to prevent aggregation of the particles. The resulting PPy-coated Si nanoparticles were purified by repeated centrifugation–dispersion cycles (Fig. 1).
3. Results and discussion Fig. 2a and b show FE-SEM images of the pure Si nanoparticles, before and after Au coating, indicating that the Aucoated Si particles are more clearly seen than the uncoated Si particles, due to a more efficient electron flow at the particle surfaces. On the other hand, FE-SEM images of PPy-coated Si particles, before and after Au coating (Fig. 2c and d), indicate that the PPy-coated Si particles are very clearly seen even before the Au coating. This result suggests that the Si particles were successfully coated with PPy. According to TEM images of the PPy-coated Si nanoparticles, the thickness of the PPy coating increased almost linearly as the relative amount of pyrrole added increased. For example, when the amount of pyrrole added was increased from 0.10 to 0.20 mL for 0.20 g of Si nanoparticles, the thickness of the resulting PPy coating on the Si nanoparticles was increased from around 2 and 4 nm, respectively. However, the CV data of the PPy-coated Si nanoparticles-based devices were very similar to each other. Thus, in this research, the PPy-coated Si nanoparticles with the average thickness value of ∼2 nm were employed for further characterization. Fig. 3 shows TEM images for the PPy-coated Si nanoparticles along with a part of the magnified Si particle. It can be seen that the Si particles were pretty uniformly coated with PPy (thickness, about 2 nm). The inset in Fig. 3b shows the selected area diffraction (SAD) of the PPy-coated Si particle, which clearly shows the direction (0 2 2) of Si crystal growth as indicated by the arrow. This result indicates that the core material underneath the amorphous PPy coating is the typical Si crystal. Fig. 4 is a plot of charge/discharge capacity versus voltage for the Li/PPy-coated Si powder electrode. The open-circuitvoltage (OCV) was 2.98 V. Overall, the Li/PPy-coated Si
Fig. 3. TEM micrographs: (a) PPy-coated Si particles and (b) a magnified part in the circle along with the inset for the selected area diffraction (SAD) pattern.
electrode exhibited the typical charge/discharge behavior of a Si anode, but with much higher discharge capacity than the reported Si electrode system in each cycle. For example, during the first discharge, the voltage profile exhibited an initial rapid drop and slow increase, which then reached a plateau region, followed by the final drop down to the discharge capacity of 2590 mAh/g. The initial fast potential drop is because Li began to insert into Si at potential below 0.35 V. The high intial overpotential is due to the slow nucleation for Li–Si alloy and subsequent decreases of overpotential is due to the slight fast growth of Li–Si alloy. The second discharge profile was very similar to the first one, but with a slightly decreased discharge capacity of 2480 mAh/g. However, the discharge capacity of the electrode system was significantly reduced in the third cycle and remained relatively unaltered thereafter up to tenth cycle. The discharge capacity in the tenth cycle was around 1000 mAh/g, which is still much
H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
25
Fig. 5. Cyclic voltammograms of the PPy-coated Si in 1.0 M LiPF6 EC/DMC with scanning rate of 0.6 mV/s.
kinetics. However, a broad reduction peak observed between 0 and 0.1 V suggests that the alloying process was faster than the de-alloying process. The intensities of peaks related to the SEI layer formation were weak, but not changed. On the other hand, the peaks related to the de-alloying kinetics increased in intensity and shifted to higher potentials up to the fifth cycle, and then decreased thereafter. This result indicates that PPy coating on the Si nanoparticles was not sustained during multiple cycles, but may contribute in preventing the binding among the particles from loosing and/or preventing cracked Si fragments from extracting into the electrolyte.
Fig. 4. Electrochemical performance: (a) charge–discharge voltage profile and (b) discharge capacity vs. cycle number of the electrode prepared from the PPy-coated Si nanoparticles along with the reported cyclic performance of the electrode prepared from the pure Si nanoparticles [4].
greater than that of the reported pure Si electrode system measured in the eigth cycle (∼130 mAh/g) [4]. As mentioned in the Section 1, the conventional Si electrode system usually exhibits a very rapid fade of discharge capacity after the first cycle. Dimov et al. observed that the capacity fading increased as the cycling number increased, and proposed the following five reasons for the capacity fading: (1) solid electrolyte interface (SEI) layer formation, (2) influence of the particle size and particle morphology, (3) internal cracks and insertion-generated default formations, (4) accumulation of Li+ in the bulk of the silicon particles and (5) reaction of the fluorine-containing electrolyte with silicon [8]. Our PPy-coated Si electrode system exhibited only a slightly reduced discharge capacity in the second cycle. However, the discharge capacity of the PPy-coated Si electrode was significantly dropped in the third cycle. In order to clearly understand this result, we obtained cyclovoltammogram of the PPy-coated Si electrode, as shown in Fig. 5. The two oxidation peaks observed at 0.31 and 0.6 V are due to de-alloying
4. Conclusions According to the experimental data of FE-SEM and TEM, the PPy-coated Si nanoparticles were successfully prepared. The Li/PPy-coated Si electrodes exhibited much higher discharge capacities than the reported Li/pure Si electrodes in each cycle. Even though the discharge capacity of the PPycoated Si electrode was significantly dropped in the third cycle. Based on the current result, the limit improvement can be attributed to improved conductive contacts among particles caused by the PPy coating, even though the capacity fading problem caused by large-scaled crumbling of the electrode has not been solved.
Acknowledgement This research was financially supported by the Brain Korea-21 Project in 2003.
References [1] H.-Y. Lee, S.-M. Lee, Electrochem. Commun. 6 (2004) 465. [2] H. Li, X. Huang, L. Chen, Z. Wu, Y. Liang, Electrochem. Solid State Lett. 2 (1999) 547.
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H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
[3] H. Li, X. Huang, L. Chen, G. Zhou, Z. Zhang, D. Yu, Y.J. Mo, N. Pei, Solid State Ionics 135 (2000) 181. [4] Z.S. Wen, J. Yang, S.F. Wang, K. Wang, Y. Liu, Electrochem. Commun. 5 (2003) 165. [5] I.S. Kim, P.N. Kumta, G.E. Blomgren, Electrochem. Solid State Lett. 3 (2000) 493. [6] S.M. Hwang, H.Y. Lee, S.W. Jang, S.M. Lee, S.J. Lee, H.K. Baik, J.Y. Lee, Electrochem. Solid State Lett. 4 (2001) 97. [7] M. Yoshio, H. Wang, K. Fukuda, T. Umeno, N. Dimov, Z. Ogumi, J. Electrochem. Soc. 149 (2002) 1598. [8] N. Dimov, S. Kugino, M. Yoshio, Electrochim. Acta 48 (2003) 1579. [9] W.R. Liu, Z.Z. Guo, W.S. Young, D.T. Shieh, H.C. Wu, M.H. Yang, N.L. Wu, J. Power Sources 140 (2005) 139. [10] J. Yang, B.F. Wang, K. Wang, Y. Liu, J.Y. Xie, Z.S. Wen, Electrochem. Solid State Lett. 6 (2003) A154.
[11] H. Dong, X.P. Ai, H.X. Yang, Electrochem. Commun. 5 (2003) 952. [12] X.W. Zhang, P.K. Patil, C. Wang, A.J. Appleby, F.E. Little, D.L. Cocke, J. Power Sources 125 (2004) 206. [13] E.P. Kluwer, in: M. Aldissi (Ed.), Intrinsically Conducting Polymers: An Emerging Technology, vol. 246, NATO Advanced Research, Boston, 1993, p. 165. [14] T.A. Skotheim, R.L. Elsenbaumer, Handbook of Conducting Polymers, second ed., Marcel Dekker, J.R. Reynolds, New York, 1998. [15] J.C. Thieblemont, A. Brun, J. Marty, M.F. Planche, P. Calo, Polymer 36 (1995) 1605. [16] J. Jang, H. Yoon, Chem. Commun. (2003) 720. [17] S.H. Cho, W.Y. Kim, G.K. Jeong, Y.S. Lee, Colloid Surface A: Physicochem. Eng. Aspects 255 (2005) 79. [18] S.F. Lascelles, S.P. Armes, J. Mater. Chem. 7 (1997) 1339. [19] D.B. Cairns, M.A. Khan, C. Perruchot, A. Riede, S.P. Armes, Chem. Mater. 15 (2003) 233.
Preparation of polypyrrole-coated silicon nanoparticles Hyun-Shil La a , Ki-Soo Park a , Kee-Suk Nahm a , Kyu-Kwan Jeong b , Youn-Sik Lee a,∗ a
Division of Environmental and Chemical Engineering, Nanomaterials Research Center, Chonbuk National University, Chonju 561-756, Republic of Korea b Division of Chemical Education, Chonbuk National University, Chonju 561-756, Republic of Korea Received 3 March 2005; received in revised form 28 June 2005; accepted 7 July 2005 Available online 18 August 2005
Abstract Silicon (Si) nanoparticles were stabilized by sodium dodecyl sulfate and poly(N-vinylpyrrolidone) in water, and coated with polypyrrole (PPy) via in situ polymerization of pyrrole with FeCl3 . TEM images revealed that the Si nanoparticles were successfully coated with PPy (average thickness, ∼2 nm). The Li/PPy-coated Si electrode exhibited improved discharge capacities, compared to that of a reported Li/pure Si electrode, even though the capacity fading problem caused by large-scaled crumbling of the electrode was not overcome. © 2005 Elsevier B.V. All rights reserved. Keywords: Anode material; Nanoparticle; Silicon; Polypyrrole; Coating
1. Introduction Recently, there has been considerable interest in new electrode materials for improved performances of secondary lithium batteries [1]. For example, binary lithium alloys, such as Li–Al and Li–Sn were tested as anode materials. However, a common problem in all these cases was the large volume increase (∼100–300%), due to Li+ insertion into the alloy anodes, resulting in the internal crack of the materials [2]. The internal crack can cause dramatic morphological changes in the anode, and then the contact between the active material and current collector becomes difficult, resulting in poor cycleabilty of the discharge capacity. The Si electrode system, fabricated with carbon as a conducting material, usually exhibits a very rapid fade of discharge capacity after the first discharge cycle. The discharge capacity decreases successively as the cycling number increases. For example, Hong et al. reported that the Li/nano-sized Si electrode delivered the discharge capacity of 2900 mAh/g in the first cycle. However, the discharge capacity decreased rapidly with increasing the cycling number, and
∗
Corresponding author. Tel.: +82 63 270 2312; fax: +82 63 270 2306. E-mail address: [email protected] (Y.-S. Lee).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.07.003
became 600 mAh/g after 10 cycles. Many researchers also reported similar results [3–5]. Extensive efforts have been made to improve the cyclic performance of the Si-based electrode system [3–12]. However, the cyclic performance is still unsatisfactory. During insertion/extraction of Li+ ion, the Si particles are cracked to form nano-sized Si particles of electrode. After some cycles, the newly formed nanoparticles can merge together, due to their high surface energy, to form dense blocks, which cannot participate in the electrochemical reaction. Therefore, the nano-sized Si particle–particle interaction should be avoided. One of the ways to overcome the problem may be this is to coat the Si particles with an electronand Li+ -conducting material [8,9]. Polypyrroles (PPy) can be easily synthesized using chemical oxidants, such as FeCl3 and exhibits good electrical conductivities and high air stabilities [13–16]. Recently, PPycoated polystyrene latex nanoparticles were successfully prepared by polymerization of pyrrole in the presence of polystyrene latex nanoparticles in our laboratory [17]. The PPy-coated latex nanoparticles exhibited largely improved electrical conductivities, as compared to the micron-sized PPy-coated polystyrene latex particles. PPy-coated Si nanoparticles are very interesting, since the loose contact between the active material and current collec-
H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
23
tor, and/or the extraction of the cracked Si fragments into electrolyte may be prohibited by the PPy coatings. In this research, we attempted to coat Si nanoparticles with PPy via in situ polymerization of pyrrole with FeCl3 . This paper describes the preparation, morphology and discharge capacity behavior of the PPy-coated Si nanoparticles.
2. Experimental 2.1. Measurements The electrochemical characterization was carried out using CR2032 coin-type cells. A mixture of PPy-coated Si nanoparticles (20 mg) and polytetrafluoroethylene (13 mg) were pressed onto a stainless steel grid (25 mm2 mesh) under a pressure of 300 kg/cm2 , and dried at 100 ◦ C for 5 h in an oven. The test cell was composed of a working electrode and a lithium counter electrode (Cyprus Foote Mineral Co.), separated by a porous polypropylene film (Celgard 3401), where the electrolyte was a solution of 1.0 M LiPF6 -EC/DMC (1/2, v/v). The cell was assembled in an argon-filled dry box and tested at room temperature, which was then charged and discharged at a current density of 80 mA/g over a voltage range of 0.05–0.8 V (versus Li/Li+ ). Cyclic voltammetry
Fig. 1. Schematic representation for the preparation of PPy-coated Si particles.
Fig. 2. SEM images: Si powders (a) before and (b) after Au coating and PPy-coated Si powders (c) before and (d) after Au coating.
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H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
(CV) experiment was performed in the range of 0.0–1.5 V at 10 mV/min. 2.2. Preparation of PPy-coated Si nanoparticles The Si nanoparticles (diameter ≤50 nm) were treated with a solution of 1% HF, 60% HNO3 and deionized water (1/5/10, v/v/v) for 10 min, and washed with deionized water by centrifugation thrice. The etched Si powders (0.20 g) and sodium dodecyl sulfate (0.56 g) were dispersed in deionized water (200 mL) in a three-neck round-bottomed flask fitted with a mechanical stirrer, followed by the addition of poly(Nvinylpyrrolidone) (Mw 55,000; 0.10 g) and ferric chloride (FeCl3 ·6H2 O) (0.91 g). Pyrrole (0.10 mL) was added to the above mixture via a syringe, and the in situ polymerization was allowed to proceed for 24 h at room temperature [18,19]. The pH of the reaction mixture was adjusted to be 7.0 to prevent aggregation of the particles. The resulting PPy-coated Si nanoparticles were purified by repeated centrifugation–dispersion cycles (Fig. 1).
3. Results and discussion Fig. 2a and b show FE-SEM images of the pure Si nanoparticles, before and after Au coating, indicating that the Aucoated Si particles are more clearly seen than the uncoated Si particles, due to a more efficient electron flow at the particle surfaces. On the other hand, FE-SEM images of PPy-coated Si particles, before and after Au coating (Fig. 2c and d), indicate that the PPy-coated Si particles are very clearly seen even before the Au coating. This result suggests that the Si particles were successfully coated with PPy. According to TEM images of the PPy-coated Si nanoparticles, the thickness of the PPy coating increased almost linearly as the relative amount of pyrrole added increased. For example, when the amount of pyrrole added was increased from 0.10 to 0.20 mL for 0.20 g of Si nanoparticles, the thickness of the resulting PPy coating on the Si nanoparticles was increased from around 2 and 4 nm, respectively. However, the CV data of the PPy-coated Si nanoparticles-based devices were very similar to each other. Thus, in this research, the PPy-coated Si nanoparticles with the average thickness value of ∼2 nm were employed for further characterization. Fig. 3 shows TEM images for the PPy-coated Si nanoparticles along with a part of the magnified Si particle. It can be seen that the Si particles were pretty uniformly coated with PPy (thickness, about 2 nm). The inset in Fig. 3b shows the selected area diffraction (SAD) of the PPy-coated Si particle, which clearly shows the direction (0 2 2) of Si crystal growth as indicated by the arrow. This result indicates that the core material underneath the amorphous PPy coating is the typical Si crystal. Fig. 4 is a plot of charge/discharge capacity versus voltage for the Li/PPy-coated Si powder electrode. The open-circuitvoltage (OCV) was 2.98 V. Overall, the Li/PPy-coated Si
Fig. 3. TEM micrographs: (a) PPy-coated Si particles and (b) a magnified part in the circle along with the inset for the selected area diffraction (SAD) pattern.
electrode exhibited the typical charge/discharge behavior of a Si anode, but with much higher discharge capacity than the reported Si electrode system in each cycle. For example, during the first discharge, the voltage profile exhibited an initial rapid drop and slow increase, which then reached a plateau region, followed by the final drop down to the discharge capacity of 2590 mAh/g. The initial fast potential drop is because Li began to insert into Si at potential below 0.35 V. The high intial overpotential is due to the slow nucleation for Li–Si alloy and subsequent decreases of overpotential is due to the slight fast growth of Li–Si alloy. The second discharge profile was very similar to the first one, but with a slightly decreased discharge capacity of 2480 mAh/g. However, the discharge capacity of the electrode system was significantly reduced in the third cycle and remained relatively unaltered thereafter up to tenth cycle. The discharge capacity in the tenth cycle was around 1000 mAh/g, which is still much
H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
25
Fig. 5. Cyclic voltammograms of the PPy-coated Si in 1.0 M LiPF6 EC/DMC with scanning rate of 0.6 mV/s.
kinetics. However, a broad reduction peak observed between 0 and 0.1 V suggests that the alloying process was faster than the de-alloying process. The intensities of peaks related to the SEI layer formation were weak, but not changed. On the other hand, the peaks related to the de-alloying kinetics increased in intensity and shifted to higher potentials up to the fifth cycle, and then decreased thereafter. This result indicates that PPy coating on the Si nanoparticles was not sustained during multiple cycles, but may contribute in preventing the binding among the particles from loosing and/or preventing cracked Si fragments from extracting into the electrolyte.
Fig. 4. Electrochemical performance: (a) charge–discharge voltage profile and (b) discharge capacity vs. cycle number of the electrode prepared from the PPy-coated Si nanoparticles along with the reported cyclic performance of the electrode prepared from the pure Si nanoparticles [4].
greater than that of the reported pure Si electrode system measured in the eigth cycle (∼130 mAh/g) [4]. As mentioned in the Section 1, the conventional Si electrode system usually exhibits a very rapid fade of discharge capacity after the first cycle. Dimov et al. observed that the capacity fading increased as the cycling number increased, and proposed the following five reasons for the capacity fading: (1) solid electrolyte interface (SEI) layer formation, (2) influence of the particle size and particle morphology, (3) internal cracks and insertion-generated default formations, (4) accumulation of Li+ in the bulk of the silicon particles and (5) reaction of the fluorine-containing electrolyte with silicon [8]. Our PPy-coated Si electrode system exhibited only a slightly reduced discharge capacity in the second cycle. However, the discharge capacity of the PPy-coated Si electrode was significantly dropped in the third cycle. In order to clearly understand this result, we obtained cyclovoltammogram of the PPy-coated Si electrode, as shown in Fig. 5. The two oxidation peaks observed at 0.31 and 0.6 V are due to de-alloying
4. Conclusions According to the experimental data of FE-SEM and TEM, the PPy-coated Si nanoparticles were successfully prepared. The Li/PPy-coated Si electrodes exhibited much higher discharge capacities than the reported Li/pure Si electrodes in each cycle. Even though the discharge capacity of the PPycoated Si electrode was significantly dropped in the third cycle. Based on the current result, the limit improvement can be attributed to improved conductive contacts among particles caused by the PPy coating, even though the capacity fading problem caused by large-scaled crumbling of the electrode has not been solved.
Acknowledgement This research was financially supported by the Brain Korea-21 Project in 2003.
References [1] H.-Y. Lee, S.-M. Lee, Electrochem. Commun. 6 (2004) 465. [2] H. Li, X. Huang, L. Chen, Z. Wu, Y. Liang, Electrochem. Solid State Lett. 2 (1999) 547.
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H.-S. La et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 272 (2006) 22–26
[3] H. Li, X. Huang, L. Chen, G. Zhou, Z. Zhang, D. Yu, Y.J. Mo, N. Pei, Solid State Ionics 135 (2000) 181. [4] Z.S. Wen, J. Yang, S.F. Wang, K. Wang, Y. Liu, Electrochem. Commun. 5 (2003) 165. [5] I.S. Kim, P.N. Kumta, G.E. Blomgren, Electrochem. Solid State Lett. 3 (2000) 493. [6] S.M. Hwang, H.Y. Lee, S.W. Jang, S.M. Lee, S.J. Lee, H.K. Baik, J.Y. Lee, Electrochem. Solid State Lett. 4 (2001) 97. [7] M. Yoshio, H. Wang, K. Fukuda, T. Umeno, N. Dimov, Z. Ogumi, J. Electrochem. Soc. 149 (2002) 1598. [8] N. Dimov, S. Kugino, M. Yoshio, Electrochim. Acta 48 (2003) 1579. [9] W.R. Liu, Z.Z. Guo, W.S. Young, D.T. Shieh, H.C. Wu, M.H. Yang, N.L. Wu, J. Power Sources 140 (2005) 139. [10] J. Yang, B.F. Wang, K. Wang, Y. Liu, J.Y. Xie, Z.S. Wen, Electrochem. Solid State Lett. 6 (2003) A154.
[11] H. Dong, X.P. Ai, H.X. Yang, Electrochem. Commun. 5 (2003) 952. [12] X.W. Zhang, P.K. Patil, C. Wang, A.J. Appleby, F.E. Little, D.L. Cocke, J. Power Sources 125 (2004) 206. [13] E.P. Kluwer, in: M. Aldissi (Ed.), Intrinsically Conducting Polymers: An Emerging Technology, vol. 246, NATO Advanced Research, Boston, 1993, p. 165. [14] T.A. Skotheim, R.L. Elsenbaumer, Handbook of Conducting Polymers, second ed., Marcel Dekker, J.R. Reynolds, New York, 1998. [15] J.C. Thieblemont, A. Brun, J. Marty, M.F. Planche, P. Calo, Polymer 36 (1995) 1605. [16] J. Jang, H. Yoon, Chem. Commun. (2003) 720. [17] S.H. Cho, W.Y. Kim, G.K. Jeong, Y.S. Lee, Colloid Surface A: Physicochem. Eng. Aspects 255 (2005) 79. [18] S.F. Lascelles, S.P. Armes, J. Mater. Chem. 7 (1997) 1339. [19] D.B. Cairns, M.A. Khan, C. Perruchot, A. Riede, S.P. Armes, Chem. Mater. 15 (2003) 233.