- Email: [email protected]
C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode
Nano Energy (]]]]) ], ]]]–]]]
1
Available online at www.sciencedirect.com
3 5 journal homepage: www.elsevier.com/locate/nanoenergy
7 9
RAPID COMMUNICATION
11
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode
13 15 17 19 21 23 25
Q1
Q2
27 29
Leigang Xue, Kun Fu, Ying Li, Guanjie Xu, Yao Lu, Shu Zhang, Ozan Toprakci, Xiangwu Zhangn Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA Received 8 October 2012; received in revised form 27 October 2012; accepted 4 November 2012
31 33 35 37 39
KEYWORDS
Abstract
Lithium-ion battery anode; Volume change; Electric conductive network; Cycling stability; Carbon coating; Nanofiber
High-energy anode materials have attracted significant attention because of their potential applications in large-scale energy storage devices. However, they often suffer from rapid capacity fading due to the pulverization of the electrode and the breakdown of electric conductive network caused by the large volume changes of active material upon repeated lithium insertion and extraction. In this work, a new electrode composed of Si/C composite nanofibers was prepared, aiming at the improvement of cycling performance of Si anodes through the establishment of a stable electric conductive network for Si during cycling. By electrospinning, a three-dimensional network of carbon nanofibers, which possesses good elasticity to maintain the structure integrity and stable electric conductive network, is formed; by carbon coating, all Si nanoparticles are tightly bonded with carbon fibers to form a stable electric conductive pathway for electrode reactions. The nanofiber structure and the carbon coating on Si, combined with the binder, lead to a stable network structure that can accommodate the huge volume change of Si during the repeated volume expansion and contraction, thus resulting in excellent cycling performance. & 2012 Elsevier Ltd. All rights reserved.
41 43 45 47 49
63
51
Introduction 53 Lithium-ion batteries (LIBs) have been wildly used in various wireless electronic devices and the application targets are
55 57
n
Q3
59 61
Corresponding author. Tel.: +1 919 515 6547. E-mail address: [email protected] (X. Zhang).
currently moving from small-sized mobile devices to largescale electric vehicles and grid storage. Therefore, LIBs with higher energy densities are in urgent need. For high-energy anode materials, Si has attracted much attention because of its highest known lithium storage capacity (4200 mAh g1), good safety, and abundant reserves on earth [1,2]. The limited cycling life, however, resulted from the significant volume changes upon lithium insertion and extraction, greatly
2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2012.11.001 Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
65 67 69 71 73
2 1
L. Xue et al.
35
restricts its practical use in LIBs. Studies have shown that each silicon atom can theoretically accommodate up to 4.4 lithium atoms to form Li22Si5 alloy, accompanied by a volume expansion of about 400% [3,4]. This huge volume change gives rise to the pulverization of electrodes, which in turn causes breakdown of electric conductive network and insulation of active material, eventually resulting in rapid capacity fading [5–7]. One wildly used method to address the abovementioned problems is to control the morphology of Si materials. Many Si-based materials with different morphologies such as nanoparticles [8–10], nanotubes [11,12] and nanofibers [13–15] have been investigated. Among these materials, Si or Si-containing nanotubes and nanofibers can not only provide higher electric conductivity through the one-dimensional structure, but also have the ability to form three-dimensional interconnected webs to provide additional structural buffering effect to absorb the mechanical stress induced by the huge volume changes of active Si, thus preventing the deterioration of the electrode integrity and the breakdown of electric conductive network. Recently, our team reported the Si/C composite nanofibers prepared by using electrospinning to disperse Si nanoparticles into carbon nanofibers (CNFs) [13–15]. The results show that the cycling performance of these Si/C nanofiber anodes are significantly greater than that of conventional Si nanoparticlebased electrodes, but the cycling stability is still not sufficient for practical battery application due to the poor distribution of Si nanoparticles and the insufficient bonding between the Si nanoparticles and CNFs. From SEM and TEM characterizations, a significant amount of Si nanoparticles were found on the fiber surface, which tends to loss contact with CNFs during the volume changes. As a result, the expected cycling stability has not been achieved. In this paper, carbon coating method was introduced to avoid the instability of the Si nanoparticles on carbon nanofibers, so as to achieve improved cycling stability.
37
Experimental
3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33
39 41 43 45 47 49 51 53 55 57
Preparation of carbon-coated Si (Si@C) Si nanoparticles (0.40 g, 30–50 nm in diameter, Nanostructured and Amorphous Materials Inc.) were first ultrasonically dispersed in a mixed solvent of tetrahydrofuran (THF) and ethanol (600 ml, 1:1 by volume). Phosphonitrilic chloride trimer (0.40 g, Aldrich), 4,4-sulfonyldiphenol (0.86 g, Aldrich) and triethylamine (40 ml, Aldrich) were then added to the solution and kept in the ultrasonic bath for 10 h. During this process, poly(cyclotriphosphazene-4,40 -sulfonyldiphenol) was formed on the surface of Si nanoparticles. The mixture was then washed with THF/ethanol solution (1:1 by volume) for 3 times, followed by solvent evaporation at 80 1C. Finally, the resultant mixture was calcined at 900 1C in argon for 2 h and the polymer on the Si nanoparticle surface was transformed to a carbon coating.
Preparation of Si/C composite nanofibers (Si@C/CNFs)
59 61
Dimethylformamide solution of 8 wt% PAN and Si@C nanoparticles (PAN:Si@C= 5:2 by weight) was prepared at 60 1C. Strong mechanical stirring was applied for at least 24 h to
obtain a homogeneous dispersion. Electrospinning was carried out to form Si@C/PAN nanofibers with a 0.75 ml h1 flow rate, 15 cm needle-to-collector distance, and 15 kV voltage. Electrospun Si@C/PAN nanofibers were first stabilized in air at 280 1C for 5 h (heating rate was 5 1C min1) and then carbonized at 700 1C for 2 h in argon (heating rate was 2 1C min1) to form Si@C/CNFs. For comparison, CNF and Si/CNFs without the carbon coating were also prepared.
63 65 67 69 71
Characterizations and electrochemical evaluation
73
X-ray diffraction pattern was collected on a Rigaku SmartLab X-Ray Diffractometer with Cu Ka radiation between 101 and 901 at a scan rate of 51/min. Field-emission scanning electron microscopy (SEM, JEOL 6400) and field-emission transmission electron microscopy (TEM, Hitachi HF2000) were used to observe the morphology. Elemental Analyzer (Perkin Elmer, CHN 2400) was used to determine the composition of the as-prepared materials. Electrochemical performance evaluation was performed using lithium-ion half cells. The working electrodes were prepared by using two different methods. In the first method, the freestanding nanofiber mat was punched into small disks (diameter: 0.5 in.) to be used directly as the working electrode without any binder or current collector. In the second method, the nanofiber mat was ground into a powder consisting of short fibers to form the working electrode by traditional slurry coating manufacturing process. Slurry was made by mixing ground nanofibers (80% weight) and polyamideimide (PAI) binder (20% weight) in N-methylpyrrolidinone. No additional conductive carbon additive was used since Si@C/CNFs are electronically conductive. The typical mass load of the active material was about 2 mg cm2. Coin-type 2032 cells (20 mm diameter, 3.2 mm thickness) were assembled in an argon glove box with lithium as the counter electrode, Celgard 2400 membrane as the separator, and 1 M LiPF6/EC+DMC+DEC (1:1:1 by volume, MTI Corporation) as the electrolyte. Charge/ discharge test was performed using a LAND CT2001A Battery Testing System.
75 77 79 81 83 85 87 89 91 93 95 97 99 101 103
Results and discussion
105
Fig. 1a shows the preparation process of the new Si/C composite nanofibers. Before the formation of composite nanofibers, Si nanoparticles were firstly coated with a thin carbon layer. The color change from yellow (Si, Fig. 1b) to black (Si@C, Fig. 1c) indicates that the Si nanoparticles have been successfully coated by carbon. The obtained Si@C nanoparticles were then dispersed in PAN to form Si@C/PAN nanofibers by electrospinning. After thermal treatment at 700 1C in argon atmosphere, black Si@C/CNF mat was obtained (Fig. 1d). The mat has a uniform appearance and is free-standing because it is composed of continuous nanofibers which form an interconnected network. The free-standing mat can be directly used as the working electrode without using any polymer binder or current collector. At the same time, the mat can also be ground into fine powder of short fibers, and then made into electrodes by using a PAI binder. Both methods will be discussed in this paper.
107
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
109 111 113 115 117 119 121 123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode 1 3 5 7 9 11 13 15 17
Fig. 1 (a) Illustration of the preparation process of the Si@C/ CNF mat, and photographs of (b) Si nanoparticles, (c) Si@C nanoparticles and (d) Si@C/CNF mat.
19 21 23 25 27 29 31 33 35 37
Fig. 2 X-ray diffraction patterns of (a) Si nanoparticles, (b) Si@C nanoparticles, and (c) Si@C/CNF mat.
39 41 43 45 47 49 51 53 55 57 59 61
Fig. 2 shows the XRD patterns of the Si nanoparticles, Si@C nanoparticles, and Si@C/CNFs. The Si nanoparticles (Fig. 2a) present well-defined peaks at 2y of 28.41, 47.41, 56.21, 69.21, 76.51, and 88.11, which are assigned to the (111), (220), (311), (400), (331) and (422) planes of crystalized Si, respectively (JCPDS No. 27-1402). After heat treatment at 900 1C in argon, the Si nanoparticles coated by carbon are still crystalline since no apparent changes can be observed for all Si peaks (Fig. 2b). One broad peak appears at around 241, indicating that the carbon coating is amorphous. A similar XRD pattern is also obtained for the Si@C/CNFs (Fig. 4c). The carbon peak shifts to 261, which can be assigned to the (002) layer of carbon. This suggests that the PAN-derived CNFs possess a low degree of graphitization (amorphous), which is quite reasonable for the case with PAN as the carbon source [16]. Fig. 3a and b show the plane-view and cross-sectional SEM images of the Si@C/CNF mat. From the surface direction, the mat exhibits an interconnected network structure composed of nanofibers with diameters of 200–300 nm, and the fiber surface appears uneven due to the presence of Si@C nanoparticles. This network structure is good for the cycling stability of Si because it has excellent structural
3
buffering effect to absorb the mechanical stress induced by volume changes of Si. From cross-sectional SEM image of this mat, however, it shows a layered structure. Theoretically, the layered structure is not ideal for the cycling stability because it tends to delaminate upon repeated volume expansion and contraction. Fig. 3c and d show the plane-view and cross-sectional SEM images of Si@C/CNF electrode prepared by the traditional slurry coating method. Both the plane-view and cross-section directions present the desirable network structure that is composed of short fibers and does not have separated layers. TEM was employed to investigate the detailed microscopic structure of the fibers. As shown in Fig. 4a, some Si@C particles are embedded in the nanofiber, and each individual particle is coated by a homogeneous carbon layer with a thickness of about 10 nm (Fig. 4b). In addition, the carbon layer does not present any ordered structure, which is in agreement with the amorphous carbon structure revealed by the XRD analysis (Fig. 2). From Fig. 4c and d, it is seen that some Si@C particles are dispersed on the fiber surface and form agglomerates. In general, the aggregation of Si particles reduces the structural stability of nanofiber anodes, so does the exposure of Si particles on the fiber surface since they tend to fall off from the fibers when the volume change occurs. However, in this work, all Si particles are pre-coated with a carbon layer, which provides a buffering zone between adjacent Si nanoparticles even in the aggregates. More importantly, the carbon coating may strengthen the bonding and electric connections between Si nanoparticles and the carbon fiber matrix because of the better compatibility between the carbon coating and CNFs (see inset in Fig. 4c). In order to avoid the exposure of Si, core–shell Si/C nanofibers were reported by using a spinneret consisting of two coaxial capillaries [17,18]. In these nanofibers, Si nanoparticles are dispersed in the core to avoid the exposure of Si, and the cycling performance was improved. However, the coaxial electrospinning process is complicated and has a narrower processing window than regular electrospinning because of the mismatches in the viscosity, conductivity, and/or surface tension of the ‘‘core’’ and ‘‘shell’’ solutions. In contrast to coaxial electrospinning, the introduction of carbon coating is a simple, low-cost approach to address the Si distribution problem and is easy to scale up. Fig. 5a shows the charge/discharge curves of the PANderived CNF mat. The initial discharge (lithiation) and charge (delithiation) capacities of the PAN-derived CNF mat are 879 and 515 mAh g1, respectively, corresponding to an initial Coulombic efficiency (ICE) of 58.6%. The relatively low ICE or large irreversible capacity (364 mAh g1) is mainly related to the electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) at around 500 mV. The potential plateau of SEI formation disappears at the second cycle. After the introduction of uncoated Si nanoparticles (Fig. 5b), the discharge and charge capacities of the resultant Si/CNF mat increase to 1182 and 822 mAh g1, respectively, and the ICE increases to 69.5% due to the higher ICE of Si than PANderived CNFs [19]. The long flat potential plateau at 100 mV can be attributed to the lithium alloying of the crystalline Si [20]. After the first cycle, the charging and discharging profile shows the typical behavior of Li intercalating with amorphous Si [20–23]. For the Si@C/CNF mat (Fig. 5c), the discharge and
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
4
L. Xue et al.
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25 27
87 Fig. 3 (a) Plane view and (b) cross-sectional SEM images of the Si@C/CNF mat electrode, and (c) plane view and (d) cross-sectional SEM images of the Si@C/CNF powder electrode prepared by the slurry coating manufacturing process.
89
29
91
31
93
33
95
35
97
37
99
39
101
41
103
43
105
45
107
47
109
49
111
51
113
53
115
55
117
57
119
59
121
61
Fig. 4 TEM images showing that Si@C nanoparticles are located in different places along the CNFs. The inset in (c) indicates that the Si nanoparticle on the CNF surface is fastened to the carbon matrix by a carbon coating layer. Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode
5
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
87 Fig. 5 Charge/discharge curves of (a) PAN-derived CNF mat, (b) Si/CNF mat, (c) Si@C/CNF mat, and (d) ground Si@C/CNF powder at 50 mA g1 and within a voltage window of 0.02–2.0 V.
charge capacities are 1171 and 745 mAh g1, respectively, and the ICE decreases to 63.6% due to the introduction of amorphous carbon coating. Its charge/discharge behavior is similar to that of Si/CNF mat. Fig. 5d shows the charge/ discharge curves of the polymer-bound Si@C/CNF electrode. It exhibits discharge and charge capacities of 1418 and 728 mAh g1, respectively. The ICE decreases to 51.3% because of the higher irreversible capacity caused by the increased surface area. The initial irreversible capacity loss of these four electrodes mainly originates from the reduction of the electrolyte on the surface of carbon and the formation of SEI film. This phenomenon wildly exists in reported Si/C composite materials because of the low graphitization degree of carbon, and it might be overcome by prelithiation, graphitization of carbon, and/or employing new electrolyte in future studies [24–26]. The cycling performances of these four different electrodes are shown in Fig. 6. For comparison, the theoretical capacity of graphite (372 mAh g1) is also shown. Here, all capacities were calculated based on the total weight of composite material including carbon coating, CNFs, and Si. The Si contents in Si/CNFs and Si@C/CNFs are 33% and 31%, respectively. We limited the anode capacity to be below 800 mAh g1 so that this material can work with current cathode materials (o250 mAh g1) in LIBs. However, the capacity can be improved simply by increasing the Si content. As shown in Fig. 6, the PAN-derived CNF mat (curve a) shows fast capacity fading in the first several cycles and the capacity gradually stabilizes after 20 cycles. After the incorporation of uncoated Si, the initial reversible specific capacity of the resultant Si/CNF mat (curve b) increases from 515 mAh g1 to 822 mAh g1, and the electrode reaction of Si is reversible due to the presence of the CNF network structure. When Si@C nanoparticles are used
89 91 93 95 97 99 101 103 105 107
Fig. 6 Cycling performance of (a) PAN-derived CNF mat, (b) Si/CNF mat, (c) Si@C/CNF mat, and (d) ground Si@C/CNF powder at 50 mA g1 and within a voltage window of 0.02–2.0 V.
109 111
to replace uncoated Si, the cycling stability of the Si@C/CNF mat (curve c) is improved and the capacity retention increases to 92% in the first 15 cycles. This is because the pre-coating of Si nanoparticles with a carbon layer enhances the electric connection and bonding between Si particles and the fiber matrix. However, the capacity fading of the Si@C/CNF mat starts to accelerate after 15 cycles. This problem was resolved by grinding the Si@C/CNF mat into short fibers and using the traditional slurry coating manufacturing process to form the polymer-bound electrode, as shown in curve d. The polymer-bound Si@C/CNF electrode shows the best cycling stability among all four electrodes.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
113 115 117 119 121 123
6
L. Xue et al.
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
Fig. 7 Schematic of Si-based anodes consisting of (a) nanoparticles, (b) nanofiber mat, and (c) short nanofibers before and after cycling. The nanofiber mat is free-standing and can be directly used as an electrode. For nanoparticles and short nanofibers, they need to be bound together by a polymer binder.
The capacity retention increases to 92% at the 40th cycle. The possible mechanism for the excellent cycling performance of polymer-bound Si@C/CNF electrode is proposed below. To make Si nanoparticle or carbon-coated Si nanoparticle electrodes, the particles need to be held together and adhered to the current collector by a polymer binder (Fig. 7a). However, most polymer materials cannot accommodate the huge volume changes of Si during lithium insertion and extraction. Hence, the electrodes tend to crack and pulverize after repeated volume expansion and contraction, resulting in the breakdown of electric conductive network and the insulation of active Si. On the other hand, the Si-containing nanofiber mat is freestanding and can be directly used as an electrode without binder or current collector. From the cross-sectional direction, however, it shows a layered structure, which tends to delaminate and is not ideal for maintaining the integrity of the electric conductive network (Fig. 7b). For grounded Si@C/ CNFs, they can be bound together with a polymer binder to form a durable three-dimensional network, which provides additional structural buffering effect to accommodate the huge volume changes of active Si (Fig. 7c). In addition, the carbon coating helps connect all Si nanoparticles tightly with the carbon fiber matrix to form a stable electric conductive pathway for electrode reactions. As a result, among all electrodes studied, the polymer-bound electrode made from ground Si@C/CNF powder has the best cycling performance due to the synergic effect of nanofiber structure, carbon coating, and binder.
Conclusions
55 57 59 61
By the pre-coating of a thin carbon layer onto Si nanoparticles and the formation of a three-dimensional network structure using nanofibers, a new Si/C composite nanofiber electrode with high capacity and excellent cycling performance was prepared. The excellent cycling stability of the new electrode was mainly attributed to three aspects: (1) the network structure composed of nanofibers helped
the entire electrode to possess good structural integrity for preventing the crack and pulverization; (2) the carbon coating made the Si nanoparticles more stable on/in the carbon nanofibers, which provided a stable electric pathway for the electrode reactions of the active Si; (3) a nanofiber electrode with the more stable three-dimensional network structure was obtained by using the traditional slurry coating manufacturing process instead of the direct use of nanofiber mat which shows layered structure in the crosssectional direction. The synergic effect of nanofiber structure, carbon coating, and binder led to a stable electric conductive network that can accommodate the huge volume change of Si during the repeated volume expansion and contraction, which in turn resulted in the greatly improved cycling stability.
81 83 85 87 89 91 93 95 97 99
Acknowledgments
101
This research was supported by the U.S. Department of Energy under Grant no. DE-EE0001177, Advanced Transportation Energy Center, and ERC Program of the National Science Foundation under Award no. EEC-08212121.
103
References
109
[1] S. Cahen, R. Janot, L. Laffont-Dantras, J.M. Tarascon, Journal of the Electrochemical Society 155 (2008) A512. [2] H. Li, X.J. Huang, L.Q. Chen, Z.G. Wu, Y. Liang, Electrochemical and Solid-State Letters 2 (1999) 547. [3] R.A. Sharma, R.N. Seefurth, Journal of the Electrochemical Society 123 (1976) 1763. [4] B.A. Boukamp, G.C. Lesh, R.A. Huggins, Journal of the Electrochemical Society 128 (1981) 725. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Journal of Power Sources 97 (2001) 235. [6] R.A. Huggins, Journal of Power Sources 81 (1999) 1319. [7] I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov, G. Yushin, Science 334 (2011) 75. [8] J.H. Ryu, J.W. Kim, Y.E. Sung, S.M. Oh, Electrochemical and Solid-State Letters 7 (2004) A306.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
105 107
111 113 115 117 119 121 123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode 1 3 5 7 9 11 13 15 17
[9] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Electrochemical and Solid-State Letters 6 (2003) A194. [10] B. Gao, S. Sinha, L. Fleming, O. Zhou, Advanced Materials 13 (2001) 816. [11] M.H. Park, M.G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui, J. Cho, Nano Letters 9 (2009) 3844. [12] T. Song, J.L. Xia, J.H. Lee, D.H. Lee, M.S. Kwon, J.M. Choi, J. Wu, S.K. Doo, H. Chang, W. Il Park, D.S. Zang, H. Kim, Y.G. Huang, K.C. Hwang, J.A. Rogers, U. Paik, Nano Letters 10 (2010) 1710. [13] L. Ji, X. Zhang, Energy & Environmental Science 3 (2010) 124. [14] L. Ji, K.H. Jung, A.J. Medford, X. Zhang, Journal of Materials Chemistry 19 (2009) 4992. [15] L. Ji, X. Zhang, Carbon 47 (2009) 3219. [16] C. Kim, K.S. Yang, M. Kojima, K. Yoshida, Y.J. Kim, Y.A. Kim, M. Endo, Advanced Functional Materials 16 (2006) 2393. [17] B.S. Lee, S.B. Son, K.M. Park, J.H. Seo, S.H. Lee, I.S. Choi, K.H. Oh, W.R. Yu, Journal of Power Sources 206 (2012) 267.
7
[18] T.H. Hwang, Y.M. Lee, B.S. Kong, J.S. Seo, J.W. Choi, Nano Letters 12 (2012) 802. [19] C.K. Chan, H.L. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nature Nanotechnology 3 (2008) 31. [20] M.N. Obrovac, L.J. Krause, Journal of the Electrochemical Society 154 (2007) A103. Q4 [21] A. Netz, R.A. Huggins, W. Weppner, Journal of Power Sources 119–121 (2003) 95. [22] J. Li, J.R. Dahn, Journal of the Electrochemical Society 154 (2007) A156. [23] T.D. Hatchard, J.R. Dahn, Journal of the Electrochemical Society 151 (2004) A838. [24] N. Liu, L. Hu, M.T. McDowell, A. Jackson, Y. Cui, ACS Nano 5 (2011) 6487. [25] Q.M. Pan, H.B. Wang, Y.H. Jiang, Electrochemistry Communications 9 (2007) 754. [26] Y.T. Lee, C.S. Yoon, Y.K. Sun, Journal of Power Sources 139 (2005) 230.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
19 21 23 25 27 29 31 33 35
1
Available online at www.sciencedirect.com
3 5 journal homepage: www.elsevier.com/locate/nanoenergy
7 9
RAPID COMMUNICATION
11
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode
13 15 17 19 21 23 25
Q1
Q2
27 29
Leigang Xue, Kun Fu, Ying Li, Guanjie Xu, Yao Lu, Shu Zhang, Ozan Toprakci, Xiangwu Zhangn Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC 27695-8301, USA Received 8 October 2012; received in revised form 27 October 2012; accepted 4 November 2012
31 33 35 37 39
KEYWORDS
Abstract
Lithium-ion battery anode; Volume change; Electric conductive network; Cycling stability; Carbon coating; Nanofiber
High-energy anode materials have attracted significant attention because of their potential applications in large-scale energy storage devices. However, they often suffer from rapid capacity fading due to the pulverization of the electrode and the breakdown of electric conductive network caused by the large volume changes of active material upon repeated lithium insertion and extraction. In this work, a new electrode composed of Si/C composite nanofibers was prepared, aiming at the improvement of cycling performance of Si anodes through the establishment of a stable electric conductive network for Si during cycling. By electrospinning, a three-dimensional network of carbon nanofibers, which possesses good elasticity to maintain the structure integrity and stable electric conductive network, is formed; by carbon coating, all Si nanoparticles are tightly bonded with carbon fibers to form a stable electric conductive pathway for electrode reactions. The nanofiber structure and the carbon coating on Si, combined with the binder, lead to a stable network structure that can accommodate the huge volume change of Si during the repeated volume expansion and contraction, thus resulting in excellent cycling performance. & 2012 Elsevier Ltd. All rights reserved.
41 43 45 47 49
63
51
Introduction 53 Lithium-ion batteries (LIBs) have been wildly used in various wireless electronic devices and the application targets are
55 57
n
Q3
59 61
Corresponding author. Tel.: +1 919 515 6547. E-mail address: [email protected] (X. Zhang).
currently moving from small-sized mobile devices to largescale electric vehicles and grid storage. Therefore, LIBs with higher energy densities are in urgent need. For high-energy anode materials, Si has attracted much attention because of its highest known lithium storage capacity (4200 mAh g1), good safety, and abundant reserves on earth [1,2]. The limited cycling life, however, resulted from the significant volume changes upon lithium insertion and extraction, greatly
2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2012.11.001 Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
65 67 69 71 73
2 1
L. Xue et al.
35
restricts its practical use in LIBs. Studies have shown that each silicon atom can theoretically accommodate up to 4.4 lithium atoms to form Li22Si5 alloy, accompanied by a volume expansion of about 400% [3,4]. This huge volume change gives rise to the pulverization of electrodes, which in turn causes breakdown of electric conductive network and insulation of active material, eventually resulting in rapid capacity fading [5–7]. One wildly used method to address the abovementioned problems is to control the morphology of Si materials. Many Si-based materials with different morphologies such as nanoparticles [8–10], nanotubes [11,12] and nanofibers [13–15] have been investigated. Among these materials, Si or Si-containing nanotubes and nanofibers can not only provide higher electric conductivity through the one-dimensional structure, but also have the ability to form three-dimensional interconnected webs to provide additional structural buffering effect to absorb the mechanical stress induced by the huge volume changes of active Si, thus preventing the deterioration of the electrode integrity and the breakdown of electric conductive network. Recently, our team reported the Si/C composite nanofibers prepared by using electrospinning to disperse Si nanoparticles into carbon nanofibers (CNFs) [13–15]. The results show that the cycling performance of these Si/C nanofiber anodes are significantly greater than that of conventional Si nanoparticlebased electrodes, but the cycling stability is still not sufficient for practical battery application due to the poor distribution of Si nanoparticles and the insufficient bonding between the Si nanoparticles and CNFs. From SEM and TEM characterizations, a significant amount of Si nanoparticles were found on the fiber surface, which tends to loss contact with CNFs during the volume changes. As a result, the expected cycling stability has not been achieved. In this paper, carbon coating method was introduced to avoid the instability of the Si nanoparticles on carbon nanofibers, so as to achieve improved cycling stability.
37
Experimental
3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33
39 41 43 45 47 49 51 53 55 57
Preparation of carbon-coated Si (Si@C) Si nanoparticles (0.40 g, 30–50 nm in diameter, Nanostructured and Amorphous Materials Inc.) were first ultrasonically dispersed in a mixed solvent of tetrahydrofuran (THF) and ethanol (600 ml, 1:1 by volume). Phosphonitrilic chloride trimer (0.40 g, Aldrich), 4,4-sulfonyldiphenol (0.86 g, Aldrich) and triethylamine (40 ml, Aldrich) were then added to the solution and kept in the ultrasonic bath for 10 h. During this process, poly(cyclotriphosphazene-4,40 -sulfonyldiphenol) was formed on the surface of Si nanoparticles. The mixture was then washed with THF/ethanol solution (1:1 by volume) for 3 times, followed by solvent evaporation at 80 1C. Finally, the resultant mixture was calcined at 900 1C in argon for 2 h and the polymer on the Si nanoparticle surface was transformed to a carbon coating.
Preparation of Si/C composite nanofibers (Si@C/CNFs)
59 61
Dimethylformamide solution of 8 wt% PAN and Si@C nanoparticles (PAN:Si@C= 5:2 by weight) was prepared at 60 1C. Strong mechanical stirring was applied for at least 24 h to
obtain a homogeneous dispersion. Electrospinning was carried out to form Si@C/PAN nanofibers with a 0.75 ml h1 flow rate, 15 cm needle-to-collector distance, and 15 kV voltage. Electrospun Si@C/PAN nanofibers were first stabilized in air at 280 1C for 5 h (heating rate was 5 1C min1) and then carbonized at 700 1C for 2 h in argon (heating rate was 2 1C min1) to form Si@C/CNFs. For comparison, CNF and Si/CNFs without the carbon coating were also prepared.
63 65 67 69 71
Characterizations and electrochemical evaluation
73
X-ray diffraction pattern was collected on a Rigaku SmartLab X-Ray Diffractometer with Cu Ka radiation between 101 and 901 at a scan rate of 51/min. Field-emission scanning electron microscopy (SEM, JEOL 6400) and field-emission transmission electron microscopy (TEM, Hitachi HF2000) were used to observe the morphology. Elemental Analyzer (Perkin Elmer, CHN 2400) was used to determine the composition of the as-prepared materials. Electrochemical performance evaluation was performed using lithium-ion half cells. The working electrodes were prepared by using two different methods. In the first method, the freestanding nanofiber mat was punched into small disks (diameter: 0.5 in.) to be used directly as the working electrode without any binder or current collector. In the second method, the nanofiber mat was ground into a powder consisting of short fibers to form the working electrode by traditional slurry coating manufacturing process. Slurry was made by mixing ground nanofibers (80% weight) and polyamideimide (PAI) binder (20% weight) in N-methylpyrrolidinone. No additional conductive carbon additive was used since Si@C/CNFs are electronically conductive. The typical mass load of the active material was about 2 mg cm2. Coin-type 2032 cells (20 mm diameter, 3.2 mm thickness) were assembled in an argon glove box with lithium as the counter electrode, Celgard 2400 membrane as the separator, and 1 M LiPF6/EC+DMC+DEC (1:1:1 by volume, MTI Corporation) as the electrolyte. Charge/ discharge test was performed using a LAND CT2001A Battery Testing System.
75 77 79 81 83 85 87 89 91 93 95 97 99 101 103
Results and discussion
105
Fig. 1a shows the preparation process of the new Si/C composite nanofibers. Before the formation of composite nanofibers, Si nanoparticles were firstly coated with a thin carbon layer. The color change from yellow (Si, Fig. 1b) to black (Si@C, Fig. 1c) indicates that the Si nanoparticles have been successfully coated by carbon. The obtained Si@C nanoparticles were then dispersed in PAN to form Si@C/PAN nanofibers by electrospinning. After thermal treatment at 700 1C in argon atmosphere, black Si@C/CNF mat was obtained (Fig. 1d). The mat has a uniform appearance and is free-standing because it is composed of continuous nanofibers which form an interconnected network. The free-standing mat can be directly used as the working electrode without using any polymer binder or current collector. At the same time, the mat can also be ground into fine powder of short fibers, and then made into electrodes by using a PAI binder. Both methods will be discussed in this paper.
107
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
109 111 113 115 117 119 121 123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode 1 3 5 7 9 11 13 15 17
Fig. 1 (a) Illustration of the preparation process of the Si@C/ CNF mat, and photographs of (b) Si nanoparticles, (c) Si@C nanoparticles and (d) Si@C/CNF mat.
19 21 23 25 27 29 31 33 35 37
Fig. 2 X-ray diffraction patterns of (a) Si nanoparticles, (b) Si@C nanoparticles, and (c) Si@C/CNF mat.
39 41 43 45 47 49 51 53 55 57 59 61
Fig. 2 shows the XRD patterns of the Si nanoparticles, Si@C nanoparticles, and Si@C/CNFs. The Si nanoparticles (Fig. 2a) present well-defined peaks at 2y of 28.41, 47.41, 56.21, 69.21, 76.51, and 88.11, which are assigned to the (111), (220), (311), (400), (331) and (422) planes of crystalized Si, respectively (JCPDS No. 27-1402). After heat treatment at 900 1C in argon, the Si nanoparticles coated by carbon are still crystalline since no apparent changes can be observed for all Si peaks (Fig. 2b). One broad peak appears at around 241, indicating that the carbon coating is amorphous. A similar XRD pattern is also obtained for the Si@C/CNFs (Fig. 4c). The carbon peak shifts to 261, which can be assigned to the (002) layer of carbon. This suggests that the PAN-derived CNFs possess a low degree of graphitization (amorphous), which is quite reasonable for the case with PAN as the carbon source [16]. Fig. 3a and b show the plane-view and cross-sectional SEM images of the Si@C/CNF mat. From the surface direction, the mat exhibits an interconnected network structure composed of nanofibers with diameters of 200–300 nm, and the fiber surface appears uneven due to the presence of Si@C nanoparticles. This network structure is good for the cycling stability of Si because it has excellent structural
3
buffering effect to absorb the mechanical stress induced by volume changes of Si. From cross-sectional SEM image of this mat, however, it shows a layered structure. Theoretically, the layered structure is not ideal for the cycling stability because it tends to delaminate upon repeated volume expansion and contraction. Fig. 3c and d show the plane-view and cross-sectional SEM images of Si@C/CNF electrode prepared by the traditional slurry coating method. Both the plane-view and cross-section directions present the desirable network structure that is composed of short fibers and does not have separated layers. TEM was employed to investigate the detailed microscopic structure of the fibers. As shown in Fig. 4a, some Si@C particles are embedded in the nanofiber, and each individual particle is coated by a homogeneous carbon layer with a thickness of about 10 nm (Fig. 4b). In addition, the carbon layer does not present any ordered structure, which is in agreement with the amorphous carbon structure revealed by the XRD analysis (Fig. 2). From Fig. 4c and d, it is seen that some Si@C particles are dispersed on the fiber surface and form agglomerates. In general, the aggregation of Si particles reduces the structural stability of nanofiber anodes, so does the exposure of Si particles on the fiber surface since they tend to fall off from the fibers when the volume change occurs. However, in this work, all Si particles are pre-coated with a carbon layer, which provides a buffering zone between adjacent Si nanoparticles even in the aggregates. More importantly, the carbon coating may strengthen the bonding and electric connections between Si nanoparticles and the carbon fiber matrix because of the better compatibility between the carbon coating and CNFs (see inset in Fig. 4c). In order to avoid the exposure of Si, core–shell Si/C nanofibers were reported by using a spinneret consisting of two coaxial capillaries [17,18]. In these nanofibers, Si nanoparticles are dispersed in the core to avoid the exposure of Si, and the cycling performance was improved. However, the coaxial electrospinning process is complicated and has a narrower processing window than regular electrospinning because of the mismatches in the viscosity, conductivity, and/or surface tension of the ‘‘core’’ and ‘‘shell’’ solutions. In contrast to coaxial electrospinning, the introduction of carbon coating is a simple, low-cost approach to address the Si distribution problem and is easy to scale up. Fig. 5a shows the charge/discharge curves of the PANderived CNF mat. The initial discharge (lithiation) and charge (delithiation) capacities of the PAN-derived CNF mat are 879 and 515 mAh g1, respectively, corresponding to an initial Coulombic efficiency (ICE) of 58.6%. The relatively low ICE or large irreversible capacity (364 mAh g1) is mainly related to the electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) at around 500 mV. The potential plateau of SEI formation disappears at the second cycle. After the introduction of uncoated Si nanoparticles (Fig. 5b), the discharge and charge capacities of the resultant Si/CNF mat increase to 1182 and 822 mAh g1, respectively, and the ICE increases to 69.5% due to the higher ICE of Si than PANderived CNFs [19]. The long flat potential plateau at 100 mV can be attributed to the lithium alloying of the crystalline Si [20]. After the first cycle, the charging and discharging profile shows the typical behavior of Li intercalating with amorphous Si [20–23]. For the Si@C/CNF mat (Fig. 5c), the discharge and
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 101 103 105 107 109 111 113 115 117 119 121 123
4
L. Xue et al.
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25 27
87 Fig. 3 (a) Plane view and (b) cross-sectional SEM images of the Si@C/CNF mat electrode, and (c) plane view and (d) cross-sectional SEM images of the Si@C/CNF powder electrode prepared by the slurry coating manufacturing process.
89
29
91
31
93
33
95
35
97
37
99
39
101
41
103
43
105
45
107
47
109
49
111
51
113
53
115
55
117
57
119
59
121
61
Fig. 4 TEM images showing that Si@C nanoparticles are located in different places along the CNFs. The inset in (c) indicates that the Si nanoparticle on the CNF surface is fastened to the carbon matrix by a carbon coating layer. Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode
5
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19
81
21
83
23
85
25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61
87 Fig. 5 Charge/discharge curves of (a) PAN-derived CNF mat, (b) Si/CNF mat, (c) Si@C/CNF mat, and (d) ground Si@C/CNF powder at 50 mA g1 and within a voltage window of 0.02–2.0 V.
charge capacities are 1171 and 745 mAh g1, respectively, and the ICE decreases to 63.6% due to the introduction of amorphous carbon coating. Its charge/discharge behavior is similar to that of Si/CNF mat. Fig. 5d shows the charge/ discharge curves of the polymer-bound Si@C/CNF electrode. It exhibits discharge and charge capacities of 1418 and 728 mAh g1, respectively. The ICE decreases to 51.3% because of the higher irreversible capacity caused by the increased surface area. The initial irreversible capacity loss of these four electrodes mainly originates from the reduction of the electrolyte on the surface of carbon and the formation of SEI film. This phenomenon wildly exists in reported Si/C composite materials because of the low graphitization degree of carbon, and it might be overcome by prelithiation, graphitization of carbon, and/or employing new electrolyte in future studies [24–26]. The cycling performances of these four different electrodes are shown in Fig. 6. For comparison, the theoretical capacity of graphite (372 mAh g1) is also shown. Here, all capacities were calculated based on the total weight of composite material including carbon coating, CNFs, and Si. The Si contents in Si/CNFs and Si@C/CNFs are 33% and 31%, respectively. We limited the anode capacity to be below 800 mAh g1 so that this material can work with current cathode materials (o250 mAh g1) in LIBs. However, the capacity can be improved simply by increasing the Si content. As shown in Fig. 6, the PAN-derived CNF mat (curve a) shows fast capacity fading in the first several cycles and the capacity gradually stabilizes after 20 cycles. After the incorporation of uncoated Si, the initial reversible specific capacity of the resultant Si/CNF mat (curve b) increases from 515 mAh g1 to 822 mAh g1, and the electrode reaction of Si is reversible due to the presence of the CNF network structure. When Si@C nanoparticles are used
89 91 93 95 97 99 101 103 105 107
Fig. 6 Cycling performance of (a) PAN-derived CNF mat, (b) Si/CNF mat, (c) Si@C/CNF mat, and (d) ground Si@C/CNF powder at 50 mA g1 and within a voltage window of 0.02–2.0 V.
109 111
to replace uncoated Si, the cycling stability of the Si@C/CNF mat (curve c) is improved and the capacity retention increases to 92% in the first 15 cycles. This is because the pre-coating of Si nanoparticles with a carbon layer enhances the electric connection and bonding between Si particles and the fiber matrix. However, the capacity fading of the Si@C/CNF mat starts to accelerate after 15 cycles. This problem was resolved by grinding the Si@C/CNF mat into short fibers and using the traditional slurry coating manufacturing process to form the polymer-bound electrode, as shown in curve d. The polymer-bound Si@C/CNF electrode shows the best cycling stability among all four electrodes.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
113 115 117 119 121 123
6
L. Xue et al.
1
63
3
65
5
67
7
69
9
71
11
73
13
75
15
77
17
79
19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
Fig. 7 Schematic of Si-based anodes consisting of (a) nanoparticles, (b) nanofiber mat, and (c) short nanofibers before and after cycling. The nanofiber mat is free-standing and can be directly used as an electrode. For nanoparticles and short nanofibers, they need to be bound together by a polymer binder.
The capacity retention increases to 92% at the 40th cycle. The possible mechanism for the excellent cycling performance of polymer-bound Si@C/CNF electrode is proposed below. To make Si nanoparticle or carbon-coated Si nanoparticle electrodes, the particles need to be held together and adhered to the current collector by a polymer binder (Fig. 7a). However, most polymer materials cannot accommodate the huge volume changes of Si during lithium insertion and extraction. Hence, the electrodes tend to crack and pulverize after repeated volume expansion and contraction, resulting in the breakdown of electric conductive network and the insulation of active Si. On the other hand, the Si-containing nanofiber mat is freestanding and can be directly used as an electrode without binder or current collector. From the cross-sectional direction, however, it shows a layered structure, which tends to delaminate and is not ideal for maintaining the integrity of the electric conductive network (Fig. 7b). For grounded Si@C/ CNFs, they can be bound together with a polymer binder to form a durable three-dimensional network, which provides additional structural buffering effect to accommodate the huge volume changes of active Si (Fig. 7c). In addition, the carbon coating helps connect all Si nanoparticles tightly with the carbon fiber matrix to form a stable electric conductive pathway for electrode reactions. As a result, among all electrodes studied, the polymer-bound electrode made from ground Si@C/CNF powder has the best cycling performance due to the synergic effect of nanofiber structure, carbon coating, and binder.
Conclusions
55 57 59 61
By the pre-coating of a thin carbon layer onto Si nanoparticles and the formation of a three-dimensional network structure using nanofibers, a new Si/C composite nanofiber electrode with high capacity and excellent cycling performance was prepared. The excellent cycling stability of the new electrode was mainly attributed to three aspects: (1) the network structure composed of nanofibers helped
the entire electrode to possess good structural integrity for preventing the crack and pulverization; (2) the carbon coating made the Si nanoparticles more stable on/in the carbon nanofibers, which provided a stable electric pathway for the electrode reactions of the active Si; (3) a nanofiber electrode with the more stable three-dimensional network structure was obtained by using the traditional slurry coating manufacturing process instead of the direct use of nanofiber mat which shows layered structure in the crosssectional direction. The synergic effect of nanofiber structure, carbon coating, and binder led to a stable electric conductive network that can accommodate the huge volume change of Si during the repeated volume expansion and contraction, which in turn resulted in the greatly improved cycling stability.
81 83 85 87 89 91 93 95 97 99
Acknowledgments
101
This research was supported by the U.S. Department of Energy under Grant no. DE-EE0001177, Advanced Transportation Energy Center, and ERC Program of the National Science Foundation under Award no. EEC-08212121.
103
References
109
[1] S. Cahen, R. Janot, L. Laffont-Dantras, J.M. Tarascon, Journal of the Electrochemical Society 155 (2008) A512. [2] H. Li, X.J. Huang, L.Q. Chen, Z.G. Wu, Y. Liang, Electrochemical and Solid-State Letters 2 (1999) 547. [3] R.A. Sharma, R.N. Seefurth, Journal of the Electrochemical Society 123 (1976) 1763. [4] B.A. Boukamp, G.C. Lesh, R.A. Huggins, Journal of the Electrochemical Society 128 (1981) 725. [5] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Journal of Power Sources 97 (2001) 235. [6] R.A. Huggins, Journal of Power Sources 81 (1999) 1319. [7] I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov, G. Yushin, Science 334 (2011) 75. [8] J.H. Ryu, J.W. Kim, Y.E. Sung, S.M. Oh, Electrochemical and Solid-State Letters 7 (2004) A306.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
105 107
111 113 115 117 119 121 123
Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode 1 3 5 7 9 11 13 15 17
[9] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Electrochemical and Solid-State Letters 6 (2003) A194. [10] B. Gao, S. Sinha, L. Fleming, O. Zhou, Advanced Materials 13 (2001) 816. [11] M.H. Park, M.G. Kim, J. Joo, K. Kim, J. Kim, S. Ahn, Y. Cui, J. Cho, Nano Letters 9 (2009) 3844. [12] T. Song, J.L. Xia, J.H. Lee, D.H. Lee, M.S. Kwon, J.M. Choi, J. Wu, S.K. Doo, H. Chang, W. Il Park, D.S. Zang, H. Kim, Y.G. Huang, K.C. Hwang, J.A. Rogers, U. Paik, Nano Letters 10 (2010) 1710. [13] L. Ji, X. Zhang, Energy & Environmental Science 3 (2010) 124. [14] L. Ji, K.H. Jung, A.J. Medford, X. Zhang, Journal of Materials Chemistry 19 (2009) 4992. [15] L. Ji, X. Zhang, Carbon 47 (2009) 3219. [16] C. Kim, K.S. Yang, M. Kojima, K. Yoshida, Y.J. Kim, Y.A. Kim, M. Endo, Advanced Functional Materials 16 (2006) 2393. [17] B.S. Lee, S.B. Son, K.M. Park, J.H. Seo, S.H. Lee, I.S. Choi, K.H. Oh, W.R. Yu, Journal of Power Sources 206 (2012) 267.
7
[18] T.H. Hwang, Y.M. Lee, B.S. Kong, J.S. Seo, J.W. Choi, Nano Letters 12 (2012) 802. [19] C.K. Chan, H.L. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nature Nanotechnology 3 (2008) 31. [20] M.N. Obrovac, L.J. Krause, Journal of the Electrochemical Society 154 (2007) A103. Q4 [21] A. Netz, R.A. Huggins, W. Weppner, Journal of Power Sources 119–121 (2003) 95. [22] J. Li, J.R. Dahn, Journal of the Electrochemical Society 154 (2007) A156. [23] T.D. Hatchard, J.R. Dahn, Journal of the Electrochemical Society 151 (2004) A838. [24] N. Liu, L. Hu, M.T. McDowell, A. Jackson, Y. Cui, ACS Nano 5 (2011) 6487. [25] Q.M. Pan, H.B. Wang, Y.H. Jiang, Electrochemistry Communications 9 (2007) 754. [26] Y.T. Lee, C.S. Yoon, Y.K. Sun, Journal of Power Sources 139 (2005) 230.
Please cite this article as: L. Xue, et al., Si/C composite nanofibers with stable electric conductive network for use as durable lithium-ion battery anode, Nano Energy (2012), http://dx.doi.org/10.1016/j.nanoen.2012.11.001
19 21 23 25 27 29 31 33 35