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Flexible free-standing graphene-silicon composite film for lithium-ion batteries
Electrochemistry Communications 12 (2010) 1467–1470
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Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Flexible free-standing graphene-silicon composite film for lithium-ion batteries Jia-Zhao Wang ⁎, Chao Zhong, Shu-Lei Chou, Hua-Kun Liu Institute for Superconducting and Electronic Materials, and ARC Center of Excellence for Electromaterials Science, University of Wollongong, NSW 2519, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2010 Received in revised form 29 July 2010 Accepted 9 August 2010 Available online 14 August 2010 Keywords: Graphene-silicon composite Free-standing electrode In-situ filtration method Lithium-ion battery
a b s t r a c t Flexible, free-standing, paper-like, graphene-silicon composite materials have been synthesized by a simple, one-step, in-situ filtration method. The Si nanoparticles are highly encapsulated in a graphene nanosheet matrix. The electrochemical results show that graphene-Si composite film has much higher discharge capacity beyond 100 cycles (708 mAh g− 1) than that of the cell with pure graphene (304 mAh g− 1). The graphene functions as a flexible mechanical support for strain release, offering an efficient electrically conducting channel, while the nanosized silicon provides the high capacity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With recent advances in the technology of various types of soft portable electronic equipment, such as roll-up displays, wearable devices, and implanted medical devices, there has been a strong market demand for ultra-thin and flexible batteries to power them. Active radio-frequency identification tags and integrated-circuit smart cards also require flexible or bendable batteries for durability in everyday use [1,2]. Making such ultra-thin and flexible batteries requires the development of active materials for soft and robust thin film electrode. Carbon-based materials have shown favourable flexibility and hence, are promising for producing flexible and bendable freestanding electrodes [3,4]. Carbon-based paper-like flexible materials for batteries and supercapacitors have been extensively studied [5,6]. The free-standing paper-like carbon-based materials, such as carbon nanotube and graphene electrode materials, are lightweight, flexible, and have good cycling stability, however, their capacities are only around 100–200 mAh g− 1 [7,8], although conventional carbon nanotube and graphene electrodes have shown good electrochemical performance in lithium-ion batteries [9,10]. To improve the practical capacity of the free-standing electrode materials, an electrochemically active second phase with higher capacity can be incorporated into the carbon-based paper-like films [11]. Silicon is the most attractive anode material for lithium-ion batteries because it has a low discharge potential and the highest known theoretical charge capacity of about 4200 mAh g − 1 , corresponding to the fully lithiated composition of Li4.4Si [12], which is more than ten times higher than for commercial graphite
⁎ Corresponding author. Tel: + 61 2 4298 1478; fax: + 61 2 4221 5731. E-mail address: [email protected] (J.-Z. Wang). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.008
anode (372 mAh g− 1). Therefore, Si is a promising candidate as a second electrochemically active phase to incorporate into carbonbased free-standing electrode for higher specific capacity. Recently, Kung et al. reported that Si-graphene paper had been prepared using a two-step method [13]. This method is quite complicated, since flammable H2 has to be involved in the second stage of preparation. Here, we have developed a simple, one-step, in-situ filtration method to fabricate free-standing graphene-Si composite thin films. The methodology demonstrated here can be extended to preparation of other paper-like flexible inorganic-carbon materials for various applications, such as solar cells [14], fuel cells [15], and supercapacitors [16]. 2. Experimental procedures The free-standing graphene-Si nanocomposite film was prepared by an in-situ chemical method using graphite oxide (GO) as the precursor. GO was synthesized from natural graphite powder by a modified Hummers method, as reported elsewhere [17,18]. To prepare the composite material, 5 mg nano-silicon powder (Hefei Kaier NanoTech & Development Co., Ltd (China), 3–80 nm in diameter) was simply added into 200 mL (0.1 mg mL− 1) of GO dispersion together with 200 mg NaOH and 290 mg pyrenebutyric acid, followed by 2 h ultrasonication. The mixed aqueous suspension of the GO and the nanoparticles of Si was then reduced with 1 mL hydrazine monohydrate at 80 °C, with continuous stirring for 24 h, followed by cooling down to room temperature naturally. A poly (vinylidene) fluoride (PVDF) membrane was wetted in a 50:50 v/v deionized water to ethanol solution for 30 min. The graphene-Si composite suspension was passed through the wetted PVDF filter in a filtration cell under a positive pressure to produce a free-standing mat of the graphene-Si composite. Subsequently, the resultant composite
Si (220)
(a)
Graphene (100)
mat was washed with deionized water and then peeled off from the PVDF filter after drying overnight in a vacuum oven at 60 °C. Finally, the as-prepared sample was heat-treated at 500 °C for 10 h in argon atmosphere to remove − H and −OH groups. For comparison, pure free-standing graphene paper was also prepared using the same method. Graphene, graphene-Si film, and Si powder were characterised by X-ray diffraction (XRD, GBC-MMA), Raman spectroscopy (JOBIN YVON HR800 confocal with 632.8 nm diode laser excitation), fieldemission scanning electron microscopy (FESEM, JEOL 7500) with energy dispersive spectroscopy (EDS) mapping, transmission electron microscopy (TEM, JEOL 2011), and thermogravimetric analysis (TGA, Mettler Toledo). The as-prepared free-standing graphene and graphene-Si composite films were used as working electrode in CR 2032 coin-type cells with a lithium foil counter and reference electrode. The electrolyte used was 1 M LiPF6 in a 50:50 (v/v) mixture of ethylene carbonate and dimethyl carbonate. The cells were galvanostatically discharged and charged within a voltage window of 0.02–1.2 V (vs. Li/Li+) at a current density of 50 mA g− 1 and a temperature of 20 °C. The discharge capacities are based on the total amount of active material in the electrodes.
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J.-Z. Wang et al. / Electrochemistry Communications 12 (2010) 1467–1470 Graphene (002)
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The X-ray diffraction patterns obtained from the free-standing graphene and graphene-Si composite are shown in Fig. 1(a), together with those of the pristine graphite, GO, and Si. The diffraction peak around 26.4º for graphite has completely disappeared in the graphite oxide pattern, and a broad peak has arisen at 2θ = 10.9°, indicating the successful oxidation of raw graphite to graphite oxide [19]. For the free-standing graphene, the diffraction peaks at 2θ = 26.0° and 42.8° can be attributed to graphite-like (002) and (100) structure, respectively [20]. Fig. 1(a) shows that the peak of GO at 2θ = 10.9° is absent in the graphene-Si sample, which indicates that GO was reduced to graphene during the in-situ chemical processing, while the nanoparticles of Si were still retained in the crystalline structure, as indicated by two major peaks (28.43° and 47.60°) [21]. Fig. 1(b) presents the Raman spectrum of graphene-Si composite film, along with the Raman spectra of Si nanopowder and pure graphene film, which are also plotted for comparison. The Raman spectrum of the graphene-Si composite displays a main peak at around 512 cm− 1, which is due to the Si particles [22], and another two peaks at around 1355 cm− 1 and 1597 cm− 1, which are identified respectively as the D band and G band of graphene [23]. This indicates that the graphene-Si composite was successfully synthesized via the in-situ chemical method from graphite oxide and Si powder. Our previous results showed that the conventional electrode of carbon coated Si nanocomposite with 45 wt.% Si–55 wt.% C had the highest capacity and best cycle life comparing with other electrodes [21]. The conductivity of free-standing electrode must be lower than that of the traditional electrode with metal substrate. Therefore, a lower Si content was selected for the free-standing composite film electrode in this study. For quantifying the amount of Si in the composite materials, TGA was carried out in air from 50 to 800 °C (Fig. 1(c)). It was estimated that the amount of graphene in the composite was 73.6 wt.%. Therefore, the Si content in the composite was 26.4 wt.%. Fig. 2(a) presents a FESEM image of the graphene film from the top view, showing well packed layered platelets composed of cured nanosheets. Such sheets are at times folded or continuous, and it is possible to distinguish the edges of individual sheets, including kinked and wrinkled areas. Fig. 2(b) shows a planar-view FESEM image of the free-standing graphene-Si film. Both the edges of the graphene and the nanosize of the Si are clearly observed. Si nanoparticles with a diameter of 20–80 nm are spread over the graphene sheet surface. A cross-sectional image of a free-standing
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Temperature (oC) Fig. 1. (a) X-ray diffraction patterns for graphite, GO, Si, and free-standing graphene and graphene-Si films. (b) Raman spectra for free-standing graphene-Si composite film, graphene film, and Si nanopowder. (c) TGA curves of bare Si, graphene-Si composite, and pure graphene.
graphene-Si film is shown in Fig. 2(c); the thickness is about 10 μm, and pockets of void spaces are clearly visible. The Si particles are embedded uniformly into the layers of graphene sheets (Fig. 2(d)). The pure free-standing graphene electrode also presents pockets of void spaces (see Fig. 2(a) inset). The TEM image shows that the Si nanoparticles are wrapped up in the multiple overlapping layers of
J.-Z. Wang et al. / Electrochemistry Communications 12 (2010) 1467–1470
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Fig. 2. FESEM images: top view of free-standing graphene film (a) with a cross-sectional view (inset), free-standing graphene-Si composite film (b), and cross-sectional views of the composite film at low (c) and high (d) magnification; TEM image of graphene-Si composite (e); FESEM image of graphene-Si (f), with corresponding EDS mapping of carbon and silicon.
suffer from poor cycling stability due to silicon's volume changes by 400% upon insertion and extraction of lithium, leading to cracking and crumbling of the electrode, which results in the failure of the anode within a few cycle [25,26]. In the case of the free-standing graphene-Si
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graphene nanosheets (Fig. 2(e)). Results of FESEM examination combined with EDS mapping for the elements C and Si are shown in Fig. 2(f). The bright regions correspond to the presence of the elements C and Si, respectively, and indicate that C and Si are distributed uniformly throughout the whole area. The uniform distribution of Si nanoparticles among the graphene is due to the in-situ synthesis method used for preparation of the free-standing electrode, as mentioned in the experimental part. Fig. 3 shows discharge capacity versus cycle number for cells made from free-standing graphene and graphene-Si composite film electrodes. It can be seen that the capacity of the pure graphene film electrode is about 304 mAh g− 1. The Li storage capacity of the freestanding graphene is lower than that of the conventional graphene electrode reported by Yoo's group. This is attributed to the lower conductivity of the free-standing electrode compared to the conventional electrode [8]. By adding a second electrochemically active phase, Si nanoparticles, the capacity of the composite material is significantly improved to about 708 mAh g− 1, even after 100 cycles. On the other hand, the cycling performance of Si in the graphene composite is much better than that of pure Si electrode. Our previous results showed that pristine nanosize Si electrode has an initial discharge capacity of 3026 mAh g− 1, with a coulombic efficiency of 58%. The capacity drops rapidly to 346 mAh g− 1 after 30 cycles [24]. Interestingly, the free-standing graphene-Si electrode gives a reversible capacity of 708 mAh g− 1 without obvious capacity loss over 100 cycles. It is well known that conventional silicon-based electrodes
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composite electrode, nanosize Si particles are homogeneously distributed between the graphene sheets. The graphene serves not only for dilution, to prevent Si particles from aggregating, but also as an efficient elastic matrix to accommodate the mechanical stresses/ strains experienced by the Si phase, thus maintaining the structural integrity of the composite electrode during the alloying/de-alloying processes [27,28]. Even though the volume expansion still occurs, the electrode is not pulverized, since graphene-Si has enough void spaces to buffer the volume change. The excellent electron and ion transfer kinetics associated with graphene also contributes to the superior electrochemical performance of the graphene-Si composite by lowering the internal resistance for both electrons and lithium ions to facilitate the de-alloying reaction over the small surface area of the silicon particles [12]. On the other hand, the specially designed freestanding film materials, integrating the active materials and the current collector into the one flexible film, can prevent loss of the electrical contact between active materials and substrate, which normally occurs for the conventional electrode [29]. The charge and discharge curves of the first and second cycles are shown in Fig. 3 (inset). The large irreversible capacity observed in the first cycle may be caused by the formation of the solid electrolyte interphase (SEI) [21]. It may also occur because of the large amount of graphene used in the composite. It has been reported that graphene anode showed large irreversible capacity for the first cycle, which is associated with the formation of an SEI layer [30]. 4. Conclusion A free-standing graphene-Si composite film was synthesized, based on the idea of adding high capacity nanoparticle Si into graphene electrode to improve the capacity of the free-standing, film-like, graphene electrode. A reversible discharge capacity of 708 mAh g− 1 for the as-obtained composite material was achieved, even after 100 cycles. The preliminary results demonstrate good prospects for the combination of graphene with a second high capacity, electrochemically active phase to improve the total capacity of the electrode. On the other hand, the methodology for the preparation of this film-like material can be extended to other flexible, free-standing, inorganic-carbon materials for various applications. Acknowledgements Financial support provided by the Australian Research Council (ARC) through a Discovery Project (DP 0987805) is gratefully
acknowledged. Many thanks also go to Dr. Tania Silver for critical reading of the manuscript.
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Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Flexible free-standing graphene-silicon composite film for lithium-ion batteries Jia-Zhao Wang ⁎, Chao Zhong, Shu-Lei Chou, Hua-Kun Liu Institute for Superconducting and Electronic Materials, and ARC Center of Excellence for Electromaterials Science, University of Wollongong, NSW 2519, Australia
a r t i c l e
i n f o
Article history: Received 21 June 2010 Received in revised form 29 July 2010 Accepted 9 August 2010 Available online 14 August 2010 Keywords: Graphene-silicon composite Free-standing electrode In-situ filtration method Lithium-ion battery
a b s t r a c t Flexible, free-standing, paper-like, graphene-silicon composite materials have been synthesized by a simple, one-step, in-situ filtration method. The Si nanoparticles are highly encapsulated in a graphene nanosheet matrix. The electrochemical results show that graphene-Si composite film has much higher discharge capacity beyond 100 cycles (708 mAh g− 1) than that of the cell with pure graphene (304 mAh g− 1). The graphene functions as a flexible mechanical support for strain release, offering an efficient electrically conducting channel, while the nanosized silicon provides the high capacity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction With recent advances in the technology of various types of soft portable electronic equipment, such as roll-up displays, wearable devices, and implanted medical devices, there has been a strong market demand for ultra-thin and flexible batteries to power them. Active radio-frequency identification tags and integrated-circuit smart cards also require flexible or bendable batteries for durability in everyday use [1,2]. Making such ultra-thin and flexible batteries requires the development of active materials for soft and robust thin film electrode. Carbon-based materials have shown favourable flexibility and hence, are promising for producing flexible and bendable freestanding electrodes [3,4]. Carbon-based paper-like flexible materials for batteries and supercapacitors have been extensively studied [5,6]. The free-standing paper-like carbon-based materials, such as carbon nanotube and graphene electrode materials, are lightweight, flexible, and have good cycling stability, however, their capacities are only around 100–200 mAh g− 1 [7,8], although conventional carbon nanotube and graphene electrodes have shown good electrochemical performance in lithium-ion batteries [9,10]. To improve the practical capacity of the free-standing electrode materials, an electrochemically active second phase with higher capacity can be incorporated into the carbon-based paper-like films [11]. Silicon is the most attractive anode material for lithium-ion batteries because it has a low discharge potential and the highest known theoretical charge capacity of about 4200 mAh g − 1 , corresponding to the fully lithiated composition of Li4.4Si [12], which is more than ten times higher than for commercial graphite
⁎ Corresponding author. Tel: + 61 2 4298 1478; fax: + 61 2 4221 5731. E-mail address: [email protected] (J.-Z. Wang). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.08.008
anode (372 mAh g− 1). Therefore, Si is a promising candidate as a second electrochemically active phase to incorporate into carbonbased free-standing electrode for higher specific capacity. Recently, Kung et al. reported that Si-graphene paper had been prepared using a two-step method [13]. This method is quite complicated, since flammable H2 has to be involved in the second stage of preparation. Here, we have developed a simple, one-step, in-situ filtration method to fabricate free-standing graphene-Si composite thin films. The methodology demonstrated here can be extended to preparation of other paper-like flexible inorganic-carbon materials for various applications, such as solar cells [14], fuel cells [15], and supercapacitors [16]. 2. Experimental procedures The free-standing graphene-Si nanocomposite film was prepared by an in-situ chemical method using graphite oxide (GO) as the precursor. GO was synthesized from natural graphite powder by a modified Hummers method, as reported elsewhere [17,18]. To prepare the composite material, 5 mg nano-silicon powder (Hefei Kaier NanoTech & Development Co., Ltd (China), 3–80 nm in diameter) was simply added into 200 mL (0.1 mg mL− 1) of GO dispersion together with 200 mg NaOH and 290 mg pyrenebutyric acid, followed by 2 h ultrasonication. The mixed aqueous suspension of the GO and the nanoparticles of Si was then reduced with 1 mL hydrazine monohydrate at 80 °C, with continuous stirring for 24 h, followed by cooling down to room temperature naturally. A poly (vinylidene) fluoride (PVDF) membrane was wetted in a 50:50 v/v deionized water to ethanol solution for 30 min. The graphene-Si composite suspension was passed through the wetted PVDF filter in a filtration cell under a positive pressure to produce a free-standing mat of the graphene-Si composite. Subsequently, the resultant composite
Si (220)
(a)
Graphene (100)
mat was washed with deionized water and then peeled off from the PVDF filter after drying overnight in a vacuum oven at 60 °C. Finally, the as-prepared sample was heat-treated at 500 °C for 10 h in argon atmosphere to remove − H and −OH groups. For comparison, pure free-standing graphene paper was also prepared using the same method. Graphene, graphene-Si film, and Si powder were characterised by X-ray diffraction (XRD, GBC-MMA), Raman spectroscopy (JOBIN YVON HR800 confocal with 632.8 nm diode laser excitation), fieldemission scanning electron microscopy (FESEM, JEOL 7500) with energy dispersive spectroscopy (EDS) mapping, transmission electron microscopy (TEM, JEOL 2011), and thermogravimetric analysis (TGA, Mettler Toledo). The as-prepared free-standing graphene and graphene-Si composite films were used as working electrode in CR 2032 coin-type cells with a lithium foil counter and reference electrode. The electrolyte used was 1 M LiPF6 in a 50:50 (v/v) mixture of ethylene carbonate and dimethyl carbonate. The cells were galvanostatically discharged and charged within a voltage window of 0.02–1.2 V (vs. Li/Li+) at a current density of 50 mA g− 1 and a temperature of 20 °C. The discharge capacities are based on the total amount of active material in the electrodes.
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J.-Z. Wang et al. / Electrochemistry Communications 12 (2010) 1467–1470 Graphene (002)
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The X-ray diffraction patterns obtained from the free-standing graphene and graphene-Si composite are shown in Fig. 1(a), together with those of the pristine graphite, GO, and Si. The diffraction peak around 26.4º for graphite has completely disappeared in the graphite oxide pattern, and a broad peak has arisen at 2θ = 10.9°, indicating the successful oxidation of raw graphite to graphite oxide [19]. For the free-standing graphene, the diffraction peaks at 2θ = 26.0° and 42.8° can be attributed to graphite-like (002) and (100) structure, respectively [20]. Fig. 1(a) shows that the peak of GO at 2θ = 10.9° is absent in the graphene-Si sample, which indicates that GO was reduced to graphene during the in-situ chemical processing, while the nanoparticles of Si were still retained in the crystalline structure, as indicated by two major peaks (28.43° and 47.60°) [21]. Fig. 1(b) presents the Raman spectrum of graphene-Si composite film, along with the Raman spectra of Si nanopowder and pure graphene film, which are also plotted for comparison. The Raman spectrum of the graphene-Si composite displays a main peak at around 512 cm− 1, which is due to the Si particles [22], and another two peaks at around 1355 cm− 1 and 1597 cm− 1, which are identified respectively as the D band and G band of graphene [23]. This indicates that the graphene-Si composite was successfully synthesized via the in-situ chemical method from graphite oxide and Si powder. Our previous results showed that the conventional electrode of carbon coated Si nanocomposite with 45 wt.% Si–55 wt.% C had the highest capacity and best cycle life comparing with other electrodes [21]. The conductivity of free-standing electrode must be lower than that of the traditional electrode with metal substrate. Therefore, a lower Si content was selected for the free-standing composite film electrode in this study. For quantifying the amount of Si in the composite materials, TGA was carried out in air from 50 to 800 °C (Fig. 1(c)). It was estimated that the amount of graphene in the composite was 73.6 wt.%. Therefore, the Si content in the composite was 26.4 wt.%. Fig. 2(a) presents a FESEM image of the graphene film from the top view, showing well packed layered platelets composed of cured nanosheets. Such sheets are at times folded or continuous, and it is possible to distinguish the edges of individual sheets, including kinked and wrinkled areas. Fig. 2(b) shows a planar-view FESEM image of the free-standing graphene-Si film. Both the edges of the graphene and the nanosize of the Si are clearly observed. Si nanoparticles with a diameter of 20–80 nm are spread over the graphene sheet surface. A cross-sectional image of a free-standing
Intensity (a.u.)
3. Results and discussion
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Temperature (oC) Fig. 1. (a) X-ray diffraction patterns for graphite, GO, Si, and free-standing graphene and graphene-Si films. (b) Raman spectra for free-standing graphene-Si composite film, graphene film, and Si nanopowder. (c) TGA curves of bare Si, graphene-Si composite, and pure graphene.
graphene-Si film is shown in Fig. 2(c); the thickness is about 10 μm, and pockets of void spaces are clearly visible. The Si particles are embedded uniformly into the layers of graphene sheets (Fig. 2(d)). The pure free-standing graphene electrode also presents pockets of void spaces (see Fig. 2(a) inset). The TEM image shows that the Si nanoparticles are wrapped up in the multiple overlapping layers of
J.-Z. Wang et al. / Electrochemistry Communications 12 (2010) 1467–1470
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Fig. 2. FESEM images: top view of free-standing graphene film (a) with a cross-sectional view (inset), free-standing graphene-Si composite film (b), and cross-sectional views of the composite film at low (c) and high (d) magnification; TEM image of graphene-Si composite (e); FESEM image of graphene-Si (f), with corresponding EDS mapping of carbon and silicon.
suffer from poor cycling stability due to silicon's volume changes by 400% upon insertion and extraction of lithium, leading to cracking and crumbling of the electrode, which results in the failure of the anode within a few cycle [25,26]. In the case of the free-standing graphene-Si
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graphene nanosheets (Fig. 2(e)). Results of FESEM examination combined with EDS mapping for the elements C and Si are shown in Fig. 2(f). The bright regions correspond to the presence of the elements C and Si, respectively, and indicate that C and Si are distributed uniformly throughout the whole area. The uniform distribution of Si nanoparticles among the graphene is due to the in-situ synthesis method used for preparation of the free-standing electrode, as mentioned in the experimental part. Fig. 3 shows discharge capacity versus cycle number for cells made from free-standing graphene and graphene-Si composite film electrodes. It can be seen that the capacity of the pure graphene film electrode is about 304 mAh g− 1. The Li storage capacity of the freestanding graphene is lower than that of the conventional graphene electrode reported by Yoo's group. This is attributed to the lower conductivity of the free-standing electrode compared to the conventional electrode [8]. By adding a second electrochemically active phase, Si nanoparticles, the capacity of the composite material is significantly improved to about 708 mAh g− 1, even after 100 cycles. On the other hand, the cycling performance of Si in the graphene composite is much better than that of pure Si electrode. Our previous results showed that pristine nanosize Si electrode has an initial discharge capacity of 3026 mAh g− 1, with a coulombic efficiency of 58%. The capacity drops rapidly to 346 mAh g− 1 after 30 cycles [24]. Interestingly, the free-standing graphene-Si electrode gives a reversible capacity of 708 mAh g− 1 without obvious capacity loss over 100 cycles. It is well known that conventional silicon-based electrodes
1st discharge 2nd discharge 1st charge 2nd charge
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Cycle number Fig. 3. Discharge capacity vs. cycle number for the electrodes made from graphene film and graphene-Si composite film, and charge and discharge curves for the first and second cycles (inset).
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composite electrode, nanosize Si particles are homogeneously distributed between the graphene sheets. The graphene serves not only for dilution, to prevent Si particles from aggregating, but also as an efficient elastic matrix to accommodate the mechanical stresses/ strains experienced by the Si phase, thus maintaining the structural integrity of the composite electrode during the alloying/de-alloying processes [27,28]. Even though the volume expansion still occurs, the electrode is not pulverized, since graphene-Si has enough void spaces to buffer the volume change. The excellent electron and ion transfer kinetics associated with graphene also contributes to the superior electrochemical performance of the graphene-Si composite by lowering the internal resistance for both electrons and lithium ions to facilitate the de-alloying reaction over the small surface area of the silicon particles [12]. On the other hand, the specially designed freestanding film materials, integrating the active materials and the current collector into the one flexible film, can prevent loss of the electrical contact between active materials and substrate, which normally occurs for the conventional electrode [29]. The charge and discharge curves of the first and second cycles are shown in Fig. 3 (inset). The large irreversible capacity observed in the first cycle may be caused by the formation of the solid electrolyte interphase (SEI) [21]. It may also occur because of the large amount of graphene used in the composite. It has been reported that graphene anode showed large irreversible capacity for the first cycle, which is associated with the formation of an SEI layer [30]. 4. Conclusion A free-standing graphene-Si composite film was synthesized, based on the idea of adding high capacity nanoparticle Si into graphene electrode to improve the capacity of the free-standing, film-like, graphene electrode. A reversible discharge capacity of 708 mAh g− 1 for the as-obtained composite material was achieved, even after 100 cycles. The preliminary results demonstrate good prospects for the combination of graphene with a second high capacity, electrochemically active phase to improve the total capacity of the electrode. On the other hand, the methodology for the preparation of this film-like material can be extended to other flexible, free-standing, inorganic-carbon materials for various applications. Acknowledgements Financial support provided by the Australian Research Council (ARC) through a Discovery Project (DP 0987805) is gratefully
acknowledged. Many thanks also go to Dr. Tania Silver for critical reading of the manuscript.
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