Si composite nanofibers as high-capacity battery electrodes

Electrochemistry Communications 11 (2009) 1146–1149

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Fabrication of porous carbon/Si composite nanofibers as high-capacity battery electrodes Liwen Ji, Xiangwu Zhang * Fiber and Polymer Science Program, Department of Textile Engineering, Chemistry and Science, North Carolina State University, 2401 Research Drive/Box 8301, Raleigh, NC 27695-8301, USA

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Article history: Received 2 March 2009 Received in revised form 21 March 2009 Accepted 24 March 2009 Available online 5 April 2009 Keywords: Electrospinning Porous carbon nanofibers Si nanoparticles Lithium-ion batteries

a b s t r a c t Carbon/Si composite nanofibers with porous structures are prepared by electrospinning and subsequent carbonization processes. It is found that these porous composite nanofibers can be used as anode materials for rechargeable lithium-ion batteries (LIBs) without adding any binding or conducting additive. The resultant anodes exhibit good electrochemical performance; for example, a large discharge capacity of 1100 mAh g 1 at a high current density of 200 mA g 1. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Silicon (Si) has long been considered as an ideal anode material for next-generation high-capacity rechargeable lithium-ion batteries (LIBs) because it has the highest known theoretical capacity of about 4200 mAh g 1, which is significantly greater than those of graphite and metal oxides [1,2]. However, this material suffers from severe pulverization triggered by more than 300% volume changes upon lithium insertion and extraction, which inevitably causes serious capacity fading during charge/discharge cycling, especially at high current densities [3]. Several strategies were proposed to accommodate the large volume change of Si in order to improve its cyclability. One of the most promising ways is to disperse nano-sized Si into a carbon matrix, in which the carbon phase acts as both a structural buffer and an electroactive material [4]. The nanostructured composite has the potential to accommodate the large volume change of Si particles by keeping the integrity of the electrode during cycling [5]. However, so far, these nano-Si embedded carbons still have limited electrochemical performance, e.g., low capacities at high current densities, and hence they cannot meet the need of practical battery applications. Dispersing Si nanoparticles into carbon nanofibers (CNFs) with porous structures may be an effective means to prevent severe pul-

* Corresponding author. Tel.: +1 919 515 6547; fax: +1 919 515 6532. E-mail address: [email protected] (X. Zhang). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.03.042

verization of Si and finally achieve satisfactory electrochemical performance for LIBs. The one-dimensional (1D) porous carbon matrix can hinder Si particle aggregation, provide continuous long-distance electron transport pathway, support numerous active sites for charge-transfer reactions, and eliminate the need for binding or conducting additive [6,7]. In addition, these porous composite materials have large surface area combined with extra pore volume, which permits a high contact area between electrode and electrolyte, reduces the distance for lithium-ion transport, and increases the rate of lithium insertion/extraction [6]. As a result, these materials can have improved reversible capacity, enhanced cycling performance, and elevated rate capability. In this work, Si nanoparticle-loaded porous carbon (C/Si) nanofibers are fabricated simply by the stabilization (in air) and carbonization (in argon) of electrospun polyacrylonitrile (PAN)/ poly-L-lactic acid (PLLA)/Si composite nanofibers. These 1D porous C/Si composite nanofibers are directly used as anodes for LIBs without adding any binder or conducting additive, and exhibit large reversible capacity and relatively good cycling performance at a high current density of 200 mA g 1.

2. Experimental PAN, PLLA and DMF were purchased from Aldrich (USA). Si nanoparticles were obtained from Nanostructured & Amorphous Materials, Inc. DMF solution of 8 wt% PAN/PLLA blend containing 30 wt% Si nanoparticles (PAN:PLLA:Si = 17:3:6) was prepared at 60 °C. Electrospinning was carried out with 0.75 mL h 1 flow rate,

L. Ji, X. Zhang / Electrochemistry Communications 11 (2009) 1146–1149

15 cm needle-to-collector distance, and 21 kV voltage. Electrospun PAN/PLLA/Si nanofibers were firstly stabilized in air at 280 °C for 8 h (heating rate was 5 °C min 1) and then carbonized at 700 °C for 1 h in argon (heating rate was 2 °C min 1). The morphology of electrospun PAN/PLLA/Si and carbonized C/ Si nanofibers was evaluated using Field Emission-Scanning Electron Microscope (JEOL 6400F FESEM) at 20 kV. The structure variations of nanofibers were identified using wide angle X-ray diffraction (WAXD, Philips X’Pert PRO MRD HR X-Ray Diffraction System, Cu ka, k = 1.5405 Å). Surface area analysis was carried out using the Brunauer–Emmett–Teller (BET) nitrogen adsorption method (Micromeritics Gemini 2360). Electrochemical performance measurements were performed using 2032 coin-type cells. Porous C/Si composite nanofibers were attached onto copper foil (Lyon industries) to be used directly as the working electrode. Lithium ribbon (Aldrich) and Separion S240 P25 (Degussa) were directly used as the counter electrode and separator, respectively. Electrolyte used was 1 M lithium hexafluorophosphate (LiPF6), dissolved in 1/ 1 (V/V) ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (Ferro Corp.). Charge and discharge were conducted using an Arbin automatic battery cycler at a constant current density of 200 mA g 1 between cut-off potentials of 2.80 and 0.01 V. The morphology of C/Si nanofibers after 30 charge/dis-

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charge cycles was also examined with analytical UHR FESEM (SU-70) at 5 kV.

3. Results and discussion SEM images of electrospun PAN/PLLA/Si composite nanofibers (Fig. 1A–C) display irregular, highly uneven surface morphology and heterogeneous diameter distribution. A large number of Si agglomerates and beads or coarse nanofibers with so-called ‘beads on a string’ morphology are also clearly shown. After carbonization, all fibers exhibit convolute fibrous structure and wrinkled ‘‘molecular sponge” surface morphology (Fig. 1D–F). This structure change after carbonization is closely related to the different behaviors of PAN, PLLA, and Si during the stabilization and carbonization processes. During these processes, the PAN component in nanofibers transfers into carbon, while the PLLA phase is decomposed without producing carbon residues and acts as the pore generator [8]. At the same time, Si still preserves its particle structure and remains dispersed in carbon matrix. As a result, porous C/Si nanofibers are formed. However, the pores cannot be seen directly from the SEM images in Fig. 1 D–F due to their small sizes, and hence other techniques are needed to evaluate the pore structure in these nanofibers.

Fig. 1. SEM images of PAN/PLLA/Si (17/3/6) precursor nanofibers (A–C) and their corresponding porous C/Si composite nanofibers (D–F).

L. Ji, X. Zhang / Electrochemistry Communications 11 (2009) 1146–1149

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CuKα (2θ/θ) Fig. 2. WAXD patterns of porous C/Si composite nanofibers prepared from PAN/ PLLA/Si (17/3/6) precursor.

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Nitrogen adsorption studies were carried out in order to investigate the surface area and pore volume of as-prepared porous C/Si nanofibers. BET results show that porous C/Si nanofibers have a surface area of about 150 m2 g 1, which is 400% larger than that (33.9 m2 g 1) of nonporous pure PAN-based CNFs with similar diameters. In addition, porous C/Si nanofibers also have a relatively large pore volume of about 0.208 cm3 g 1. The BET results demonstrate the formation of porous structure in C/Si nanofibers. The structural variations of porous C/Si nanofibers were further explored using WAXD. From WAXD patterns shown in Fig. 2, it is seen that C/Si nanofibers exhibit clear diffusion peaks at scattering angels (2h) of about 28.4°, 47.4°, 56.2°, 69.2°, 76.5° and 88.1°, which are ascribed to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1) and (4 2 2) planes of Si crystallites, respectively [4]. However, the peak near 2h = 25.0°, which can be attributed to the (0 0 2) layer of the graphite [9], is very small, revealing that the carbon matrix in porous C/Si nanofibers is largely un-graphitized. In order to understand the electrochemical performance of these materials at high current densities, porous C/Si composite nanofibers were directly used as LIB anodes for electrochemical evaluation. Fig. 3 shows their typical voltage profiles and cycling performance at a relatively high current density of 200 mA g 1 between a cut-off potential range between 2.8 and 0.01 V. From Fig. 3A, it can be seen that porous C/Si nanofibers deliver charge and discharge capacities of about 1340 and 1100 mAh g 1, respectively, at the first cycle, corresponding to a coulombic efficiency of 82.1%. The irreversible capacity is 240 mAh g 1, which can be mainly ascribed to the reductive decomposition of the electrolyte solution and the subsequent formation of the solid electrolyte interface (SEI) film on the anode surface [10]. When Li inserts into porous C/Si nanofibers during the first cycle, the voltage initially drops quickly to about 0.55 V, and then experiences a slow decrease as shown in the potential plateau in Fig. 3A. The plateau below 0.50 V may be due to the formation of Li–Si alloys, which coexist with Si as two-phase regions [11]. During the first discharge process, a relatively slow increase in voltage is observed. At the second and sequential cycles, the irreversible capacity decreases significantly, leading to high coulombic efficiencies (>98.0%). This means that after the first cycle, the microstructure of porous C/Si composite nanofibers become stable. Fig. 3B shows the cycling performance of porous C/Si composite nanofibers. For comparison, the theoretical capacity of graphite is also given. It is seen that, at the second cycle, the discharge capacity of porous C/Si anodes decreases from 1100 to 1050 mAh g 1, corresponding to a 95.8% capacity retention. At the 30th cycle, this reversible capacity decreases to 630 mAh g 1. Although this capacity is just 56.9% of the initial value, it is still much larger than the 372 mAh g 1 theoretical value of graphite [3,11]. The relatively good electrochemical performance of porous C/Si composite nanofibers can be ascribed to the buffering effect of

Fig. 4. SEM images of porous C/Si composite nanofibers after 30 charge/discharge cycles at a constant current density of 200 mA g PLLA/Si (17/3/6) precursor.

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. Nanofibers were prepared from PAN/

L. Ji, X. Zhang / Electrochemistry Communications 11 (2009) 1146–1149

nanostructured porous carbon matrix, which provides facile strain relaxation during electrode structure changes, accommodates the large Si volume expansion and shrinkage during lithium insertion/extraction, ensures good electronic contact between Si and carbon, and preserves the integrity of the electrode structure [4– 7]. As a result, LIBs can benefit from both Si phases (large lithium-storage capacity) and carbon matrix (long stable cycle life). In addition, the porous structure of these nanofibers provides more active sites for Li-ion diffusion [1,4–7,12,13]. These combined effects finally lead to acceptable electrochemical performance at high current density. SEM images of porous C/Si composite nanofibers were taken after 30 charge/discharge cycles and are shown in Fig. 4. Compared with the images of the materials before cycling (Fig. 1), some particulate-like and mushroom-shaped structures appear in cycled nanofibers. It may be caused by the large volume expansion and shrinkage upon repeated lithium insertion/extraction. However, the nanofiber structure still remains in the cycled nanofibers. 4. Conclusions Si nanoparticle-loaded porous CNFs are prepared by using relatively convenient and low cost electrospinning and thermal treatment methods. The resultant porous C/Si composite nanofiber anodes exhibit large accessible surface area, high reversible capacity, and relatively good cycling performance at high current densities. It is envisaged that by judiciously optimizing the parameters, such as Si grain size, carbon crystallite structure, and pore size and

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porosity, both the capacity and reversibility of these materials can be further improved. Acknowledgements This work was supported by the US National Science Foundation (Grant Nos. 0555959 and 0833837), the ERC Program of the National Science Foundation under Award Number EEC08212121, and ACS Petroleum Research Fund 47863-G10. The authors thank Mr. Andrew J. Medford for his help with sample characterizations. References [1] C.K. Chan, H. Peng, G. Liu, K. Mcilwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31. [2] H.J. Kim, B. Han, J. Choo, J. Cho, Angew. Chem. Int. Ed. 47 (2008) 10151. [3] X.W. Zhang, P.K. Patil, C.S. Wang, A.J. Appleby, F.E. Little, D.L. Cocke, J. Power Sources 125 (2004) 206. [4] Y.S. Hu, R. Demir-Cakan, M.M. Titirici, J.O. Müller, R. Schögl, M. Antonietti, J. Maier, Angew. Chem. Int. Ed. 47 (2008) 1645. [5] C.H. Jiang, E. Hosono, H.S. Zhou, Nanotoday 1 (2006) 28. [6] L.W. Ji, X.W. Zhang, Electrochem. Commun. 11 (2009) 684. [7] F.Y. Cheng, Z.L. Tao, J. Liang, J. Chen, Chem. Mater. 20 (2008) 667. [8] L.W. Ji, A.J. Medford, X.W. Zhang, J. Polym. Sci. Part B: Polym. Phys. 47 (2009) 493. [9] C. Kim, K.S. Yang, M. Kojima, K. Yoshida, Y.J. Kim, Y.A. Kim, M. Endo, Adv. Funct. Mater. 16 (2006) 2393. [10] T. Zhang, J. Gao, L.J. Fu, L.C. Yang, Y.P. Wu, H.Q. Wu, J. Mater. Chem. 17 (2007) 1321. [11] U. Kasabajjula, C.S. Wang, A.J. Appleby, J. Power Sources 163 (2007) 1003. [12] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Ed. 47 (2008) 2930. [13] L.F. Cui, R. Ruffo, C.K. Chan, H.L. Peng, Y. Cui, Nano Lett. 9 (2009) 491.