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carbon nanospheres composite as high performance anode material for lithium-ion batteries
Journal of Energy Chemistry 23(2014)315–323
Interconnected sandwich structure carbon/Si-SiO2/carbon nanospheres composite as high performance anode material for lithium-ion batteries Yuanjin Du, Mengyan Hou, Dandan Zhou, Yonggang Wang, Congxiao Wang∗ , Yongyao Xia∗ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, China [ Manuscript received December 9, 2013; revised April 2, 2014 ]
Abstract In the present work, an interconnected sandwich carbon/Si-SiO2 /carbon nanospheres composite was prepared by template method and carbon thermal vapor deposition (TVD). The carbon conductive layer can not only efficiently improve the electronic conductivity of Si-based anode, but also play a key role in alleviating the negative effect from huge volume expansion over discharge/charge of Si-based anode. The resulting material delivered a reversible capacity of 1094 mAh/g, and exhibited excellent cycling stability. It kept a reversible capacity of 1050 mAh/g over 200 cycles with a capacity retention of 96%. Key words silicon; carbon; anode materials; lithium-ion batteries; template method; carbon thermal vapor deposition
1. Introduction Silicon has been demonstrated as a promising anode material for lithium-ion batteries (LIB), due to its high theoretical capacity of 4200 mAh/g, which is more than ten times higher than that of the graphitic carbon (372 mAh/g) [1,2]. However, one major problem preventing them from the commercial application is that they undergo large volume changes during cycling, which results in disintegration of the electrodes and subsequent rapid capacity fading [3]. Many efforts have been devoted to solving these problems, such as reducing particle size [4,5], controlling the morphology Si [6−10] and preparing variety of Si-C composites [11−15]. The fabrication of Si-C composite is a viable solution to circumvent the limitations of pure Si power, because carbon can not only play a buffering role, but also act as an electrical connecting media between the nanoparticles. Moreover, carbon/silicon composites with various structures have been developed to improve their cycling stability, such as microporous carbon coated silicon core/shell nanocomposite [13], common carbon-nano-silicon composite [16], Si@SiOx/C nanocomposite and hollow core-shell structured porous Si-
C nanocomposite [17,18]. Among these structures, the coreshell structure normally shows better electrochemical performance enhancement. Gao et al. [13] reported a microporous carbon coated core/shell Si@C nanocomposite with 87% retention of the first cycle after 40 cycles. Li et al. [18] reported a hollow core-shell structured porous Si-C nanocomposite with a capacity retention of 86% after 100 cycles. Recently, Chen et al. [19] prepared silver-treated nanoscale hollow porous silicon particles by a template method and the composite presented good cycling behavior and rate performance. Yao et al. [20] has reported the interconnected silicon hollow nanospheres prepared by chemical vapor deposition (CVD) of Si on the surface of hollow spherical silica. The obtained interconnected silicon hollow nanospheres showed high capacity over 700 cycles, which is the longest cycle life ever reported for silicon anode. In the present work, we designed and prepared an interconnected sandwich carbon/SiSiO2 /carbon nanospheres composite (hereafter abbreviated as IS-C/Si-SiO2 /C) by a template method combining the magnesiothermic reduction and carbon CVD process. The asprepared material was evaluated as anode material for Li-ion batteries.
Corresponding authors. Tel/Fax: +86-21-51630318; E-mail: [email protected]; [email protected] This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903), the National Natural Science Foundation of China (No.20925312) and Shanghai Science Technology Committee (13JC1407900). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60153-4
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2. Experimental 2.1. Materials Tetraethylorthosilicate (TEOS, 98 wt%), tolueneand magnesium powder were purchased from Sinopharm Chemical Reagent Co. Ltd. Aqueous ammonia (25 wt%), styrene, sodium dodecyl sulfate, potassium persulfate, cetyltrimethylammonium bromide (CTAB), hydrochloric acid and hydrofluoric acid were analytical reagents and purchased from Shanghai Chemical Corp. Deionized water was used in all the experiments. 2.2. Synthesis of polystyrene sphere (PS) The template polystyrene spheres (PS) about 225 nm were synthesized according to the literature [21]: 150 mL deionized water and 0.03 g sodium dodecyl sulfate were added into a 500 mL three-necked round-bottomed flask, and then it was heated to 70 ◦ C, followed by the addition of 18.3 mL styrene. An inert gas was bubbled through the roundbottomed flask to deaerate the mixture. After 30 min, 0.083 g potassium persulfate was added into the solution and the mixture was heated to 80 ◦ C. The emulsion was then refluxed for 20 h. The resulting polystyrene spheres (PS) were produced until the solvent was completely evaporated at 70 ◦ C in air. 2.3. Synthesis of interconnected hollow spherical SiO2 The interconnected hollow spherical SiO2 was prepared as previously described elsewhere [22]. A homogeneous solution containing 0.7 g cetyltrimethylammonium bromide (CTAB), 60 mL deionized water, 60 mL ethanol and 3.0 mL ammonia aqueous solution (25 wt%) was obtained after vigorous stirring. Then 1.5 g the as-prepared PS was dispersed in the above solution via 60 min sonication. 3.0 g silicon source tetraethoxysilane (TEOS) was then added into the above homogeneous solution. The mixture was stirred at 600 rpm at room temperature for 12 h. The resulting precipitate was filtered and washed thoroughly with ethanol, and dried at 80 ◦ C. Polystyrene and CTAB were removed by calcination in air at 600 ◦ C for 6 h and the interconnected hollow spherical SiO2 nanoparticles were collected. 2.4. Synthesis of interconnected hollow spherical Si-SiO2 0.15 g interconnected hollow spherical SiO2 nanoparticles and 0.165 g magnesium powder were put into a corundum boat and then heated in a tube furnace at 650 ◦ C for 6 h under an argon atmosphere (with 5 vol% H2 ). The collected brown powder was washed with HCl solution (14 mL, 2 mol/L) for 12 h and then washed with ethanol and distilled water by centrifugation until pH value was 7. Finally, the interconnected
hollow spherical Si-SiO2 was vacuum dried at 80 ◦ C for 12 h and stored in a desiccator. 2.5. Synthesis of interconnected sandwich carbon/SiSiO2 /carbon nanospheres composite (IS-C/Si-SiO2/C) and SiC composite The obtained interconnected hollow spherical Si-SiO2 was further treated through thermal vapour deposition (TVD) synthesis process, which was carried out at 700 ◦ C for 2 h under a nitrogen atmosphere using toluene as the carbon source. Carbon was deposited in the surface crack and inner hollow part during TVD process. The interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite (IS-C/SiSiO2 /C) was obtained. In order to compare the advantages of this structure, nanosize Si, carbon coated Si-C composite were obtained by the following process: the interconnected hollow spherical SiSiO2 was treated with HF (5%). In this process, SiO2 was dissolved, and the interconnected hollow spherical structure was finally destroyed and pure Si nanoparticles were collected. The carbon coated Si-C composite was obtained by further treating the nano-sized Si through carbon TVD process. The obtained pure Si, IS-C/Si-SiO2 /C nanospheres composite and Si-C composite were all stored in a desiccator. 2.6. Characterization X-ray diffraction (XRD) patterns were recorded on a BrukerD8 powder X-ray diffractometer using Ni-filtered CuKα radiation (40 kV, 40 mA). Scanning electron microscopy (SEM) was conducted on a JSM-6390 microscope from JEOL. Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2011 microscope (Japan) operated at 200 kV. The high-resolution transmission electron microscopy (HRTEM) images were acquired to identify the wellresolved lattice planes. The samples for TEM and HRTEM measurements were suspended in ethanol and supported onto a holey carbon film on a Cu grid. The thermogravimetric analysis (TGA) was obtained by NETZSCH TG 209 F1 Libra. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). 2.7. Electrochemical measurements Electrochemical measurements were carried out in CR2016-type coin cells at room temperature consisting of a working electrode, a separator and a negative electrode (lithium metal in half-cells). The working electrode was prepared by mixing 85 wt% active material, 5 wt% carbon black and 10 wt% binder (styrene butadiene rubber/sodium carboxymethyl cellulose, 1 : 1 by weight). Then the slurry was cast onto a copper foil current collector by doctor-blade tech-
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nique. The electrode films were vacuum dried at 80 ◦ C for 12 h to remove the solvent before pressing. The electrode film was punched in the form of disks, typically with a diameter of 12 mm and then dried at 80 ◦ C for 12 h under vacuum. The typical mass loading was about 2.5 mg/cm2. The cells were assembled with the working electrode as cathode, lithium metal as anode, and Celgard 2300 film as separator in a glove-box filled with pure argon. The electrolyte solution was 1 mol/L LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 by volume), plus 2 wt% vinylene carbonate (VC). And then the cells were tested in the potential range of 0.005−3 V at a current density of 150 mA/g (except rate capability tests). The cell capacity was calculated based on the weight of pure active material. The cyclic voltammograms (CV) were conducted on CHI 660D in the potential window of 0.005−3 V at a scan rate of 0.1 mV/s.
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3. Results and discussion The preparation procedure of IS-C/Si-SiO2 /C nanospheres composite is shown in Scheme 1, which includes four steps: first, PS@SiO2 was prepared using 225 nm polystyrene spheres (PS) as the template and through TEOS hydrolyzing. Second, the interconnected hollow spherical SiO2 was obtained by removing the PS template with calcination treatment. And then, the as-prepared interconnected hollow spherical SiO2 was employed to prepare the interconnected hollow spherical Si-SiO2 by magnesiothermic reduction. Finally, a carbon conductive framework along with the connected surface was built through carbon TVD and IS-C/Si-SiO2 /C nanospheres composite was obtained.
Scheme 1. Preparation of the interconnected sandwich carbon-silicon-carbon (IS-C/Si-SiO2 /C) nanospheres composite
Figure 1(1) gives the XRD pattern of IS-C/Si-SiO2 /C nanospheres composite. All the main diffraction peaks can be indexed to Si (JCPDS file No. 27-1402). The broad peak at about 20o –28o should be ascribed to carbon (002) and the residual SiO2 . The broad peak of the residual SiO2 was also observed in the hollow spherical Si-SiO2 (Figure 1(2)). Furthermore, as the XPS spectrum detected, there was some SiOx in ISC-Si/SiO2 -C composite (Figure 2). In the case of SiO2 , two dominant peaks at 110 and 105 eV were observed, thus, the broad peak should be ascribed to SiO2 and SiOx with x<2 [4].
Figure 2. XPS spectrum (Mg Kα , hν = 1253.6 eV) of ISC-Si/SiO2 -C composite
Figure 1. XRD patterns of (1) IS-C/Si-SiO2 /C nanospheres composite, (2) interconnected hollow spherical Si-SiO2 and (3) interconnected hollow spherical SiO2
Figure 3 gives the SEM images of the 225 nm template polystyrene spheres (PS), the interconnected hollow spherical SiO2 synthesized through the template method, and the ISC/Si-SiO2 /C nanospheres composite. The SiO2 spheres had a smooth surface (Figure 3b), which was converted into the interconnected hollow spherical Si-SiO2 after the magnesiothermic reduction process, keeping the morphology of SiO2 precursor (Figure 3c). However, different from the smooth surface of SiO2 spheres, the resulting interconnected hollow spherical Si-SiO2 exhibited a rough surface, owing to SiO2 /Si conversion and the consequent HCl treatment for removing the remnant MgO particles. IS-C/Si-SiO2 /C nanospheres composite shown in Figure 3(d) kept the same morphology of the interconnected hollow spherical Si-SiO2 .
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Figure 3. SEM images of (a) 225 nm polystyrene spheres (PS), (b) interconnected hollow spherical SiO2 , (c) interconnected hollow spherical Si-SiO2 and (d) IS-C/Si-SiO2 /C nanospheres composite
Figure 4. TEM images of (a, b) interconnected hollow spherical SiO2 , (c, d) interconnected hollow spherical Si-SiO2 and (e) IS-C/Si-SiO2 /C nanospheres composite; HRTEM image of (f) IS-C/Si-SiO2 /C nanospheres composite
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In order to clarify the structure change during the preparation process, TEM investigation was conducted. TEM image of the as-prepared interconnected hollow spherical SiO2 is shown in Figure 4(a). It displayed a typical hollow structure with ∼300 nm in diameter and ∼ 35 nm thickness wall, which consists with the observation of SEM. The area outlined indicated a shared shell between two interconnected spheres. Figure 4(b) presents the TEM image with higher magnification of the spherical SiO2 . There were many channels in the shell of the hollow spherical SiO2 . The porous SiO2 precursor provided a favorable structural framework for the subsequent reactions. Next, the hollow spherical SiO2 was converted into interconnected hollow spherical Si-SiO2 with magnesium thermal reduction. TEM image shown in Figure 4(c) indicates that the resulting spherical Si-SiO2 kept the morphology of its precursor (i.e. interconnected hollow
spherical SiO2 ), but the diameter of the resulting spherical SiSiO2 shrank about 40 nm compared with its precursor (in Figure 4d). Furthermore, the shells of the spherical Si-SiO2 became looser and more porous than that of the interconnected hollow spherical SiO2 , which could be ascribed to SiO2 /Si conversion and the subsequent acid treatment. Finally, the as-prepared spherical Si-SiO2 was treated with carbon TVD process, which was converted into interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite. As shown in Figure 4(e), the spherical shells became tight again after carbon TVD process, in which carbon filled the loose surface and the inner hollow part. HRTEM image for IS-C/Si-SiO2 /C nanospheres composite was shown in Figure 4(f). The amorphous SiO2 , crystalline Si and graphitized carbon layer were all detected in IS-C/Si-SiO2 /C nanospheres composite. The crystalline Si was surrounded by SiO2 and carbon.
Figure 5. SEM images of (a) silicon nanoparticles obtained from the interconnected hollow spherical Si-SiO2 after HF (5%) treatment and (b) Si-C composite
ure 5b). All above results indicate that SiO2 supports the interconnected hollow spherical structure, and then the structure avoids the particle aggregation. The carbon content in the composite was determined by thermogravimetric analysis (TGA), and the results are given in Figure 6. The carbon content in IS-C/Si-SiO2 /C nanospheres composite (31%) was less than that in Si-C composite (43%). However, as presented in Table 1, the electronic conductivity of IS-C/Si-SiO2 /C nanospheres composite was higher than that of Si-C composite, indicating that the structure of ISC/Si-SiO2 /C nanospheres composite can provide more active site for electrochemical reactions. Table 1. Electronic conductivities of ISC-Si/SiO2 -C, Si-SiO2 and Si-C measured at a pressure of 4 MPa Figure 6. Thermogravimetry analysis (TGA) results of interconnected hollow spherical Si-SiO2 , IS-C/Si-SiO2 /C nanospheres composite and Si-C composite
Figure 5 shows the SEM images of the sample after SiO2 was removed from the interconnected hollow spherical SiSiO2 with HF (5%). It is evident that the typical hollow spheres structure was broken down and became into the aggregated nanoparticles (Figure 5a). The carbon coated Si also showed similar morphologies with Si nanoparticles (Fig-
Materials Conductivity (S/cm)
IS-C/Si-SiO2 /C 0.196
Si-SiO2 2.95×10−6
Si-C 1.50×10−3
The first three cyclic voltammograms (CV) of IS-C/SiSiO2 /C nanospheres composite in the potential window of 0.005−3 V at a scan rate of 0.1 mV/s are shown in Figure 7. In the first cycle two broad cathodic peaks appeared around 0.75 V and 1.1 V, resulting from the formation of solid electrolyte interphase (SEI) and the reductive decomposition of
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VC respectively, which is in good agreement with previous reports [12,23,24]. The cathodic sweep of the second CV curve displayed one peak at 0.231 V, corresponding to the formation of Li-Si alloy phases, while the two peaks at 0.33 and 0.463 V at the anodic sweep could be ascribed to dealloying of Li-Si alloys, which are well consistent with the data reported in the literature [20].
and Si-C composite. Figure 9(a) gives the discharge capacity of all above three samples as a function of the cycling number at a current density of 150 mA/g. ISC-Si/SiO2 /C showed slightly increased capacitance at the first 50 cycles, which may be related to the gradual activation of Si host [13], and then kept the constant capacity. It still delivered a reversible capacity of 1050 mAh/g over 200 cycles, corresponding to the capacity retention of 96%. Obviously, IS-C/Si-SiO2 /C nanospheres composite showed much better cycling performance than thoes of Si-SiO2 and Si-C composites, which demonstrates that both the interconnected hollow spherical structure and carbon coating greatly improve the electrochemical performance of the interconnected hollow spherical SiSiO2 . The rate performance of IS-C/Si-SiO2 /C nanospheres composite was also evaluated under different current densities, varying from 0.1 A/g to 0.8 A/g. As shown in Figure 9(b), IS-C/Si-SiO2 /C nanospheres composite displayed about 800 mAh/g even at high current rate of 1 A/g.
Figure 7. Cyclic voltammograms of IS-C/Si-SiO2 /C nanospheres composite in the first three cycles at a scan rate of 0.1 mV/s
The first discharge/charge profiles of IS-C/Si-SiO2 /C nanospheres composite, the interconnected hollow spherical Si-SiO2 and Si-C composite are shown in Figure 8. The first discharge and charge capacities for IS-C/Si-SiO2 /C nanospheres composite were 1693 and 1094 mAh/g, respectively, with an initial coulomb efficiency of 64% (Figure 8a). The first discharge and charge capacities of Si-SiO2 were 2643 and 1046 mAh/g, respectively, with an initial coulomb efficiency of 40% (Figure 8b). The irreversible capacity loss of the two samples could mainly originate from the reduction of electrolyte, the formation of solid electrolyte interphase (SEI) film and the deposition of SiOx . SiOx can leads to irreversible capacity due to the irreversible reaction of decomposition of SiOx to Si and xLi2 O [25]. Therefore, the poor initial coulomb efficiency of Si-SiO2 and IS-C/SiSiO2 /C nanospheres composite should be mainly attributed to SiOx . The first discharge and charge capacities of Si-C composite without SiO2 and SiOx were 2233 and 1756 mAh/g, respectively, with an initial coulomb efficiency of 79% (Figure 8c). Careful inspection of the first discharge curves for all the samples reveals a discharge plateau at ∼1.1 V, illustrating the reductive decomposition of electrolyte vinylene carbonate (VC), which is consistent with the results of cyclic voltammograms (CV) in Figure 7. The cycling stability of the as-prepared IS-C/Si-SiO2 /C nanospheres composite was investigated in comparision with the interconnected hollow spherical Si-SiO2 without carbon
Figure 8. The first discharge/charge profiles of (a) IS-C/Si-SiO2 /C nanospheres composite, (b) interconnected hollow spherical Si-SiO2 and (c) Si-C composite
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Figure 9. (a) Cycling performance of interconnected hollow spherical Si-SiO2 , IS-C/Si-SiO2 /C nanospheres composite and Si-C composite electrodes; (b) Reversible capacities of IS-C/Si-SiO2 /C nanospheres composite cycled at different current rates
Figure 10. SEM images of the electrodes before cycling and after 100 cycles: (a) and (b) IS-C/Si-SiO2 /C nanospheres composite, (c) and (d) Si-C composite, (e) and (f) interconnected hollow spherical Si-SiO2 . The insets of (b), (d) and (f) are the magnification of local areas
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In order to further study the mechanism of the improved electrochemical performance of IS-C/Si-SiO2 /C nanospheres composite, SEM investigation was also employed to detect the surface of IS-C/Si-SiO2 /C nanospheres composite electrode, Si-C composite electrode and the interconnected hollow spherical Si-SiO2 electrode after charge/discharge cycling in Figure 10. Obviously, IS-C/Si-SiO2 /C nanospheres composite electrode did not crack and pulverize after 100 charge/discharge cycles, although volume expansion can be obviously observed (Figure 10b), compared with that of before cycling (Figure 10a). However, the surface of Si-C composite and the interconnected hollow spherical Si-SiO2 electrodes suffered serious crack and pulverization (Figure 10d and 10f), compared with the surface before cycling (Figure 10c and 10e). The investigation indicated that the typical structure of IS-C/Si-SiO2 /C nanospheres composite owns higher elasticity to accommodate huge volume changes of Sibased anode. Thus, IS-C/Si-SiO2 /C nanospheres composite showed much better cycling life compared with Si-C composite and the interconnected hollow spherical Si-SiO2 . To identify the relationship between the electrochemical performance and electrode kinetics, electrochemical impedance spectroscopy (EIS) was carried out. The experimental was employed at 1.2 V vs. Li/Li+ of both Si-C composite and IS-C/Si-SiO2 /C nanospheres composite after the second charge. The Nyquist plots consisted of a depressed semicircle in the high frequency region between 105 and 100 Hz and a straight line in the low frequency between 100 Hz and 0.1 Hz, as shown in Figure 11. The diameter of the depressed semicircle mainly represents the charge transfer resistance and the straight line is related to a diffusion controlled process [26]. IS-C/Si-SiO2 /C nanospheres composite provided smaller diameter of high frequency semicircle, indicating that the composite has a decreased ionic resistance. The building of carbon conductive framework maybe the main reason for the reduction of the charge transfer resistance.
Figure 11. Electrochemical impedance spectra of IS-C/Si-SiO2 /C nanospheres composite and Si-C composite electrodes at 1.2 V vs. Li/Li+ after the second charge
4. Conclusions An interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite has been successfully obtained by combining the template method, magnesium thermal reduction and carbon chemical thermal decomposition process. IS-C/Si-SiO2 /C nanospheres composite greatly improves the electrochemical properties of the Si-based anode. As a result, the interconnected sandwich C/Si-SiO2 /C nanospheres composite exhibits excellent cycle stability with a reversible capacity of 1050 mAh/g over 200 cycles, corresponding to 96% capacity retention. The carbon coating layer plays a key role in both buffering the volume expansion and increasing the electronic conductivity. These findings described in the present work indicate that building interconnected sandwich C/Si-SiO2 /C structure is an efficient method for preparing nanostructured Si-based electrode material. Acknowledgements This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903) the National Natural Science Foundation of China (No.20925312) and Shanghai Science Technology Committee (13JC1407900).
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[18] Li X L, Meduri P, Chen X L, Qi W, Engelhard M H, Xu W, Ding F, Xiao J, Wang W, Wang C M, Zhang J G, Liu J. J Mater Chem, 2012, 22(22): 11014 [19] Chen D Y, Mei X, Ji G, Lu M H, Xie J P, Lu J M, Lee J Y. Angew Chem Int Ed, 2012, 51(10): 2409 [20] Yao Y, McDowell M T, Ryu I, Wu H, Liu N, Hu L B, Nix W D, Cui Y. Nano Lett, 2011, 11(7): 2949 [21] Holland B T, Blanford C F, Do T, Stein A. Chem Mater, 1999, 11(3): 795
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Interconnected sandwich structure carbon/Si-SiO2/carbon nanospheres composite as high performance anode material for lithium-ion batteries Yuanjin Du, Mengyan Hou, Dandan Zhou, Yonggang Wang, Congxiao Wang∗ , Yongyao Xia∗ Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200433, China [ Manuscript received December 9, 2013; revised April 2, 2014 ]
Abstract In the present work, an interconnected sandwich carbon/Si-SiO2 /carbon nanospheres composite was prepared by template method and carbon thermal vapor deposition (TVD). The carbon conductive layer can not only efficiently improve the electronic conductivity of Si-based anode, but also play a key role in alleviating the negative effect from huge volume expansion over discharge/charge of Si-based anode. The resulting material delivered a reversible capacity of 1094 mAh/g, and exhibited excellent cycling stability. It kept a reversible capacity of 1050 mAh/g over 200 cycles with a capacity retention of 96%. Key words silicon; carbon; anode materials; lithium-ion batteries; template method; carbon thermal vapor deposition
1. Introduction Silicon has been demonstrated as a promising anode material for lithium-ion batteries (LIB), due to its high theoretical capacity of 4200 mAh/g, which is more than ten times higher than that of the graphitic carbon (372 mAh/g) [1,2]. However, one major problem preventing them from the commercial application is that they undergo large volume changes during cycling, which results in disintegration of the electrodes and subsequent rapid capacity fading [3]. Many efforts have been devoted to solving these problems, such as reducing particle size [4,5], controlling the morphology Si [6−10] and preparing variety of Si-C composites [11−15]. The fabrication of Si-C composite is a viable solution to circumvent the limitations of pure Si power, because carbon can not only play a buffering role, but also act as an electrical connecting media between the nanoparticles. Moreover, carbon/silicon composites with various structures have been developed to improve their cycling stability, such as microporous carbon coated silicon core/shell nanocomposite [13], common carbon-nano-silicon composite [16], Si@SiOx/C nanocomposite and hollow core-shell structured porous Si-
C nanocomposite [17,18]. Among these structures, the coreshell structure normally shows better electrochemical performance enhancement. Gao et al. [13] reported a microporous carbon coated core/shell Si@C nanocomposite with 87% retention of the first cycle after 40 cycles. Li et al. [18] reported a hollow core-shell structured porous Si-C nanocomposite with a capacity retention of 86% after 100 cycles. Recently, Chen et al. [19] prepared silver-treated nanoscale hollow porous silicon particles by a template method and the composite presented good cycling behavior and rate performance. Yao et al. [20] has reported the interconnected silicon hollow nanospheres prepared by chemical vapor deposition (CVD) of Si on the surface of hollow spherical silica. The obtained interconnected silicon hollow nanospheres showed high capacity over 700 cycles, which is the longest cycle life ever reported for silicon anode. In the present work, we designed and prepared an interconnected sandwich carbon/SiSiO2 /carbon nanospheres composite (hereafter abbreviated as IS-C/Si-SiO2 /C) by a template method combining the magnesiothermic reduction and carbon CVD process. The asprepared material was evaluated as anode material for Li-ion batteries.
Corresponding authors. Tel/Fax: +86-21-51630318; E-mail: [email protected]; [email protected] This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903), the National Natural Science Foundation of China (No.20925312) and Shanghai Science Technology Committee (13JC1407900). ∗
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60153-4
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2. Experimental 2.1. Materials Tetraethylorthosilicate (TEOS, 98 wt%), tolueneand magnesium powder were purchased from Sinopharm Chemical Reagent Co. Ltd. Aqueous ammonia (25 wt%), styrene, sodium dodecyl sulfate, potassium persulfate, cetyltrimethylammonium bromide (CTAB), hydrochloric acid and hydrofluoric acid were analytical reagents and purchased from Shanghai Chemical Corp. Deionized water was used in all the experiments. 2.2. Synthesis of polystyrene sphere (PS) The template polystyrene spheres (PS) about 225 nm were synthesized according to the literature [21]: 150 mL deionized water and 0.03 g sodium dodecyl sulfate were added into a 500 mL three-necked round-bottomed flask, and then it was heated to 70 ◦ C, followed by the addition of 18.3 mL styrene. An inert gas was bubbled through the roundbottomed flask to deaerate the mixture. After 30 min, 0.083 g potassium persulfate was added into the solution and the mixture was heated to 80 ◦ C. The emulsion was then refluxed for 20 h. The resulting polystyrene spheres (PS) were produced until the solvent was completely evaporated at 70 ◦ C in air. 2.3. Synthesis of interconnected hollow spherical SiO2 The interconnected hollow spherical SiO2 was prepared as previously described elsewhere [22]. A homogeneous solution containing 0.7 g cetyltrimethylammonium bromide (CTAB), 60 mL deionized water, 60 mL ethanol and 3.0 mL ammonia aqueous solution (25 wt%) was obtained after vigorous stirring. Then 1.5 g the as-prepared PS was dispersed in the above solution via 60 min sonication. 3.0 g silicon source tetraethoxysilane (TEOS) was then added into the above homogeneous solution. The mixture was stirred at 600 rpm at room temperature for 12 h. The resulting precipitate was filtered and washed thoroughly with ethanol, and dried at 80 ◦ C. Polystyrene and CTAB were removed by calcination in air at 600 ◦ C for 6 h and the interconnected hollow spherical SiO2 nanoparticles were collected. 2.4. Synthesis of interconnected hollow spherical Si-SiO2 0.15 g interconnected hollow spherical SiO2 nanoparticles and 0.165 g magnesium powder were put into a corundum boat and then heated in a tube furnace at 650 ◦ C for 6 h under an argon atmosphere (with 5 vol% H2 ). The collected brown powder was washed with HCl solution (14 mL, 2 mol/L) for 12 h and then washed with ethanol and distilled water by centrifugation until pH value was 7. Finally, the interconnected
hollow spherical Si-SiO2 was vacuum dried at 80 ◦ C for 12 h and stored in a desiccator. 2.5. Synthesis of interconnected sandwich carbon/SiSiO2 /carbon nanospheres composite (IS-C/Si-SiO2/C) and SiC composite The obtained interconnected hollow spherical Si-SiO2 was further treated through thermal vapour deposition (TVD) synthesis process, which was carried out at 700 ◦ C for 2 h under a nitrogen atmosphere using toluene as the carbon source. Carbon was deposited in the surface crack and inner hollow part during TVD process. The interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite (IS-C/SiSiO2 /C) was obtained. In order to compare the advantages of this structure, nanosize Si, carbon coated Si-C composite were obtained by the following process: the interconnected hollow spherical SiSiO2 was treated with HF (5%). In this process, SiO2 was dissolved, and the interconnected hollow spherical structure was finally destroyed and pure Si nanoparticles were collected. The carbon coated Si-C composite was obtained by further treating the nano-sized Si through carbon TVD process. The obtained pure Si, IS-C/Si-SiO2 /C nanospheres composite and Si-C composite were all stored in a desiccator. 2.6. Characterization X-ray diffraction (XRD) patterns were recorded on a BrukerD8 powder X-ray diffractometer using Ni-filtered CuKα radiation (40 kV, 40 mA). Scanning electron microscopy (SEM) was conducted on a JSM-6390 microscope from JEOL. Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2011 microscope (Japan) operated at 200 kV. The high-resolution transmission electron microscopy (HRTEM) images were acquired to identify the wellresolved lattice planes. The samples for TEM and HRTEM measurements were suspended in ethanol and supported onto a holey carbon film on a Cu grid. The thermogravimetric analysis (TGA) was obtained by NETZSCH TG 209 F1 Libra. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). 2.7. Electrochemical measurements Electrochemical measurements were carried out in CR2016-type coin cells at room temperature consisting of a working electrode, a separator and a negative electrode (lithium metal in half-cells). The working electrode was prepared by mixing 85 wt% active material, 5 wt% carbon black and 10 wt% binder (styrene butadiene rubber/sodium carboxymethyl cellulose, 1 : 1 by weight). Then the slurry was cast onto a copper foil current collector by doctor-blade tech-
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nique. The electrode films were vacuum dried at 80 ◦ C for 12 h to remove the solvent before pressing. The electrode film was punched in the form of disks, typically with a diameter of 12 mm and then dried at 80 ◦ C for 12 h under vacuum. The typical mass loading was about 2.5 mg/cm2. The cells were assembled with the working electrode as cathode, lithium metal as anode, and Celgard 2300 film as separator in a glove-box filled with pure argon. The electrolyte solution was 1 mol/L LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 by volume), plus 2 wt% vinylene carbonate (VC). And then the cells were tested in the potential range of 0.005−3 V at a current density of 150 mA/g (except rate capability tests). The cell capacity was calculated based on the weight of pure active material. The cyclic voltammograms (CV) were conducted on CHI 660D in the potential window of 0.005−3 V at a scan rate of 0.1 mV/s.
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3. Results and discussion The preparation procedure of IS-C/Si-SiO2 /C nanospheres composite is shown in Scheme 1, which includes four steps: first, PS@SiO2 was prepared using 225 nm polystyrene spheres (PS) as the template and through TEOS hydrolyzing. Second, the interconnected hollow spherical SiO2 was obtained by removing the PS template with calcination treatment. And then, the as-prepared interconnected hollow spherical SiO2 was employed to prepare the interconnected hollow spherical Si-SiO2 by magnesiothermic reduction. Finally, a carbon conductive framework along with the connected surface was built through carbon TVD and IS-C/Si-SiO2 /C nanospheres composite was obtained.
Scheme 1. Preparation of the interconnected sandwich carbon-silicon-carbon (IS-C/Si-SiO2 /C) nanospheres composite
Figure 1(1) gives the XRD pattern of IS-C/Si-SiO2 /C nanospheres composite. All the main diffraction peaks can be indexed to Si (JCPDS file No. 27-1402). The broad peak at about 20o –28o should be ascribed to carbon (002) and the residual SiO2 . The broad peak of the residual SiO2 was also observed in the hollow spherical Si-SiO2 (Figure 1(2)). Furthermore, as the XPS spectrum detected, there was some SiOx in ISC-Si/SiO2 -C composite (Figure 2). In the case of SiO2 , two dominant peaks at 110 and 105 eV were observed, thus, the broad peak should be ascribed to SiO2 and SiOx with x<2 [4].
Figure 2. XPS spectrum (Mg Kα , hν = 1253.6 eV) of ISC-Si/SiO2 -C composite
Figure 1. XRD patterns of (1) IS-C/Si-SiO2 /C nanospheres composite, (2) interconnected hollow spherical Si-SiO2 and (3) interconnected hollow spherical SiO2
Figure 3 gives the SEM images of the 225 nm template polystyrene spheres (PS), the interconnected hollow spherical SiO2 synthesized through the template method, and the ISC/Si-SiO2 /C nanospheres composite. The SiO2 spheres had a smooth surface (Figure 3b), which was converted into the interconnected hollow spherical Si-SiO2 after the magnesiothermic reduction process, keeping the morphology of SiO2 precursor (Figure 3c). However, different from the smooth surface of SiO2 spheres, the resulting interconnected hollow spherical Si-SiO2 exhibited a rough surface, owing to SiO2 /Si conversion and the consequent HCl treatment for removing the remnant MgO particles. IS-C/Si-SiO2 /C nanospheres composite shown in Figure 3(d) kept the same morphology of the interconnected hollow spherical Si-SiO2 .
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Figure 3. SEM images of (a) 225 nm polystyrene spheres (PS), (b) interconnected hollow spherical SiO2 , (c) interconnected hollow spherical Si-SiO2 and (d) IS-C/Si-SiO2 /C nanospheres composite
Figure 4. TEM images of (a, b) interconnected hollow spherical SiO2 , (c, d) interconnected hollow spherical Si-SiO2 and (e) IS-C/Si-SiO2 /C nanospheres composite; HRTEM image of (f) IS-C/Si-SiO2 /C nanospheres composite
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In order to clarify the structure change during the preparation process, TEM investigation was conducted. TEM image of the as-prepared interconnected hollow spherical SiO2 is shown in Figure 4(a). It displayed a typical hollow structure with ∼300 nm in diameter and ∼ 35 nm thickness wall, which consists with the observation of SEM. The area outlined indicated a shared shell between two interconnected spheres. Figure 4(b) presents the TEM image with higher magnification of the spherical SiO2 . There were many channels in the shell of the hollow spherical SiO2 . The porous SiO2 precursor provided a favorable structural framework for the subsequent reactions. Next, the hollow spherical SiO2 was converted into interconnected hollow spherical Si-SiO2 with magnesium thermal reduction. TEM image shown in Figure 4(c) indicates that the resulting spherical Si-SiO2 kept the morphology of its precursor (i.e. interconnected hollow
spherical SiO2 ), but the diameter of the resulting spherical SiSiO2 shrank about 40 nm compared with its precursor (in Figure 4d). Furthermore, the shells of the spherical Si-SiO2 became looser and more porous than that of the interconnected hollow spherical SiO2 , which could be ascribed to SiO2 /Si conversion and the subsequent acid treatment. Finally, the as-prepared spherical Si-SiO2 was treated with carbon TVD process, which was converted into interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite. As shown in Figure 4(e), the spherical shells became tight again after carbon TVD process, in which carbon filled the loose surface and the inner hollow part. HRTEM image for IS-C/Si-SiO2 /C nanospheres composite was shown in Figure 4(f). The amorphous SiO2 , crystalline Si and graphitized carbon layer were all detected in IS-C/Si-SiO2 /C nanospheres composite. The crystalline Si was surrounded by SiO2 and carbon.
Figure 5. SEM images of (a) silicon nanoparticles obtained from the interconnected hollow spherical Si-SiO2 after HF (5%) treatment and (b) Si-C composite
ure 5b). All above results indicate that SiO2 supports the interconnected hollow spherical structure, and then the structure avoids the particle aggregation. The carbon content in the composite was determined by thermogravimetric analysis (TGA), and the results are given in Figure 6. The carbon content in IS-C/Si-SiO2 /C nanospheres composite (31%) was less than that in Si-C composite (43%). However, as presented in Table 1, the electronic conductivity of IS-C/Si-SiO2 /C nanospheres composite was higher than that of Si-C composite, indicating that the structure of ISC/Si-SiO2 /C nanospheres composite can provide more active site for electrochemical reactions. Table 1. Electronic conductivities of ISC-Si/SiO2 -C, Si-SiO2 and Si-C measured at a pressure of 4 MPa Figure 6. Thermogravimetry analysis (TGA) results of interconnected hollow spherical Si-SiO2 , IS-C/Si-SiO2 /C nanospheres composite and Si-C composite
Figure 5 shows the SEM images of the sample after SiO2 was removed from the interconnected hollow spherical SiSiO2 with HF (5%). It is evident that the typical hollow spheres structure was broken down and became into the aggregated nanoparticles (Figure 5a). The carbon coated Si also showed similar morphologies with Si nanoparticles (Fig-
Materials Conductivity (S/cm)
IS-C/Si-SiO2 /C 0.196
Si-SiO2 2.95×10−6
Si-C 1.50×10−3
The first three cyclic voltammograms (CV) of IS-C/SiSiO2 /C nanospheres composite in the potential window of 0.005−3 V at a scan rate of 0.1 mV/s are shown in Figure 7. In the first cycle two broad cathodic peaks appeared around 0.75 V and 1.1 V, resulting from the formation of solid electrolyte interphase (SEI) and the reductive decomposition of
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VC respectively, which is in good agreement with previous reports [12,23,24]. The cathodic sweep of the second CV curve displayed one peak at 0.231 V, corresponding to the formation of Li-Si alloy phases, while the two peaks at 0.33 and 0.463 V at the anodic sweep could be ascribed to dealloying of Li-Si alloys, which are well consistent with the data reported in the literature [20].
and Si-C composite. Figure 9(a) gives the discharge capacity of all above three samples as a function of the cycling number at a current density of 150 mA/g. ISC-Si/SiO2 /C showed slightly increased capacitance at the first 50 cycles, which may be related to the gradual activation of Si host [13], and then kept the constant capacity. It still delivered a reversible capacity of 1050 mAh/g over 200 cycles, corresponding to the capacity retention of 96%. Obviously, IS-C/Si-SiO2 /C nanospheres composite showed much better cycling performance than thoes of Si-SiO2 and Si-C composites, which demonstrates that both the interconnected hollow spherical structure and carbon coating greatly improve the electrochemical performance of the interconnected hollow spherical SiSiO2 . The rate performance of IS-C/Si-SiO2 /C nanospheres composite was also evaluated under different current densities, varying from 0.1 A/g to 0.8 A/g. As shown in Figure 9(b), IS-C/Si-SiO2 /C nanospheres composite displayed about 800 mAh/g even at high current rate of 1 A/g.
Figure 7. Cyclic voltammograms of IS-C/Si-SiO2 /C nanospheres composite in the first three cycles at a scan rate of 0.1 mV/s
The first discharge/charge profiles of IS-C/Si-SiO2 /C nanospheres composite, the interconnected hollow spherical Si-SiO2 and Si-C composite are shown in Figure 8. The first discharge and charge capacities for IS-C/Si-SiO2 /C nanospheres composite were 1693 and 1094 mAh/g, respectively, with an initial coulomb efficiency of 64% (Figure 8a). The first discharge and charge capacities of Si-SiO2 were 2643 and 1046 mAh/g, respectively, with an initial coulomb efficiency of 40% (Figure 8b). The irreversible capacity loss of the two samples could mainly originate from the reduction of electrolyte, the formation of solid electrolyte interphase (SEI) film and the deposition of SiOx . SiOx can leads to irreversible capacity due to the irreversible reaction of decomposition of SiOx to Si and xLi2 O [25]. Therefore, the poor initial coulomb efficiency of Si-SiO2 and IS-C/SiSiO2 /C nanospheres composite should be mainly attributed to SiOx . The first discharge and charge capacities of Si-C composite without SiO2 and SiOx were 2233 and 1756 mAh/g, respectively, with an initial coulomb efficiency of 79% (Figure 8c). Careful inspection of the first discharge curves for all the samples reveals a discharge plateau at ∼1.1 V, illustrating the reductive decomposition of electrolyte vinylene carbonate (VC), which is consistent with the results of cyclic voltammograms (CV) in Figure 7. The cycling stability of the as-prepared IS-C/Si-SiO2 /C nanospheres composite was investigated in comparision with the interconnected hollow spherical Si-SiO2 without carbon
Figure 8. The first discharge/charge profiles of (a) IS-C/Si-SiO2 /C nanospheres composite, (b) interconnected hollow spherical Si-SiO2 and (c) Si-C composite
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Figure 9. (a) Cycling performance of interconnected hollow spherical Si-SiO2 , IS-C/Si-SiO2 /C nanospheres composite and Si-C composite electrodes; (b) Reversible capacities of IS-C/Si-SiO2 /C nanospheres composite cycled at different current rates
Figure 10. SEM images of the electrodes before cycling and after 100 cycles: (a) and (b) IS-C/Si-SiO2 /C nanospheres composite, (c) and (d) Si-C composite, (e) and (f) interconnected hollow spherical Si-SiO2 . The insets of (b), (d) and (f) are the magnification of local areas
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In order to further study the mechanism of the improved electrochemical performance of IS-C/Si-SiO2 /C nanospheres composite, SEM investigation was also employed to detect the surface of IS-C/Si-SiO2 /C nanospheres composite electrode, Si-C composite electrode and the interconnected hollow spherical Si-SiO2 electrode after charge/discharge cycling in Figure 10. Obviously, IS-C/Si-SiO2 /C nanospheres composite electrode did not crack and pulverize after 100 charge/discharge cycles, although volume expansion can be obviously observed (Figure 10b), compared with that of before cycling (Figure 10a). However, the surface of Si-C composite and the interconnected hollow spherical Si-SiO2 electrodes suffered serious crack and pulverization (Figure 10d and 10f), compared with the surface before cycling (Figure 10c and 10e). The investigation indicated that the typical structure of IS-C/Si-SiO2 /C nanospheres composite owns higher elasticity to accommodate huge volume changes of Sibased anode. Thus, IS-C/Si-SiO2 /C nanospheres composite showed much better cycling life compared with Si-C composite and the interconnected hollow spherical Si-SiO2 . To identify the relationship between the electrochemical performance and electrode kinetics, electrochemical impedance spectroscopy (EIS) was carried out. The experimental was employed at 1.2 V vs. Li/Li+ of both Si-C composite and IS-C/Si-SiO2 /C nanospheres composite after the second charge. The Nyquist plots consisted of a depressed semicircle in the high frequency region between 105 and 100 Hz and a straight line in the low frequency between 100 Hz and 0.1 Hz, as shown in Figure 11. The diameter of the depressed semicircle mainly represents the charge transfer resistance and the straight line is related to a diffusion controlled process [26]. IS-C/Si-SiO2 /C nanospheres composite provided smaller diameter of high frequency semicircle, indicating that the composite has a decreased ionic resistance. The building of carbon conductive framework maybe the main reason for the reduction of the charge transfer resistance.
Figure 11. Electrochemical impedance spectra of IS-C/Si-SiO2 /C nanospheres composite and Si-C composite electrodes at 1.2 V vs. Li/Li+ after the second charge
4. Conclusions An interconnected sandwich carbon/Si-SiO2/carbon nanospheres composite has been successfully obtained by combining the template method, magnesium thermal reduction and carbon chemical thermal decomposition process. IS-C/Si-SiO2 /C nanospheres composite greatly improves the electrochemical properties of the Si-based anode. As a result, the interconnected sandwich C/Si-SiO2 /C nanospheres composite exhibits excellent cycle stability with a reversible capacity of 1050 mAh/g over 200 cycles, corresponding to 96% capacity retention. The carbon coating layer plays a key role in both buffering the volume expansion and increasing the electronic conductivity. These findings described in the present work indicate that building interconnected sandwich C/Si-SiO2 /C structure is an efficient method for preparing nanostructured Si-based electrode material. Acknowledgements This work was partially supported by the State Key Basic Research Program of PRC (2011CB935903) the National Natural Science Foundation of China (No.20925312) and Shanghai Science Technology Committee (13JC1407900).
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