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C composites with tunable structure and high reversible lithium storage capacity
Nano Energy (2013) 2, 1314–1321
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journal homepage: www.elsevier.com/locate/nanoenergy
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Tandem plasma reactions for Sn/C composites with tunable structure and high reversible lithium storage capacity Wei Li, Rong Yang, Jie Zhengn, Xingguo Lin Beijing National Laboratory for Molecular Sciences (BNLMS), (The State Key Laboratory of Rare Earth Materials Chemistry and Applications), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received 14 May 2013; received in revised form 20 June 2013; accepted 20 June 2013 Available online 3 July 2013
KEYWORDS
Abstract
Tandem plasma reaction; Sn/C binary composites; Controlled preparation; Lithium ion batteries; Anode materials
The Sn/C nanocomposites are of great interest as high capacity anode materials for lithium ion batteries (LIBs). In this paper, we employ a tandem plasma reaction method for controlled preparation of Sn/C binary composites. The Sn and C components are generated by magnetron sputtering and plasma decomposition of CH4 in two tandem plasma zones, respectively. The obtained Sn/C composites are composed of ultrafine Sn particles homogeneously embedded in carbon matrix, which exhibit very high reversible lithium storage capacity. The tandem plasma reaction method offers great versatility in controlling the Sn/C ratio and the Sn particle size, allowing a systematic study on the relationship between the structural parameters and the electrode performance. The reversible anode capacity is found to be strongly affected by the Sn particle size while it shows a much weaker correlation with the carbon coating layer. & 2013 Elsevier Ltd. All rights reserved.
Introduction Binary composites of metal and carbon are extensively investigated as electrode materials, which have been extensively used in LIBs [1,2], fuel cells [3,4] and metal-air batteries [5,6]. An essential point to improve the electrode performance of metal/carbon binary composites is the accurate control of the nanoscale structure during the preparation process. Among the numerous efforts devoted to the controlled preparation of n
Corresponding authors. Tel.: +86 6276 5930. E-mail addresses: [email protected] (J. Zheng), [email protected] (X. Li). 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.06.012
metal/carbon binary composites, the preparation strategies can be classified into two categories depending on the origin of carbon. A straightforward strategy is using readily available carbon materials and introducing the metal component by impregnation, vapor phase deposition or direct loading of metal nanostructures. This approach is advantageous for the broad choice of the carbon materials, including amorphous carbon black, carbon nanotubes, graphene nanosheets and porous carbon [7–9]. However, this approach tends to yield metal-oncarbon structures, i.e. the metal is simply loaded onto the surface of the carbon materials (or the interior surface of pores in case of porous carbon). In many application circumstances such as the electrodes of LIBs, however, the metalencapsulated-in-carbon structure is preferred [10–12]. A more
Tandem plasma reactions for Sn/C composites sophisticated approach employs an intermediate composite in which both the metal and carbon components are in the precursor states. The intermediate composites can be obtained by various soft chemistry methods, which is more favorable for nanoscale structure control. The carbon precursor is usually obtained by polymerization or condensation of small organic molecules [13–16]. Therefore, high temperature treatment is inevitable for carbonization, which is energy inefficient and may also cause side reactions. The intermediate guided method, though is favorable for nanoscale structure control, is limited by the difficulty to obtain carbon from organic or polymer precursors without high temperature treatment. This obstacle, however, can be easily overcome in plasma reactions. By decomposition of hydrocarbon in low pressure gas discharge, various carbon materials, including diamond and diamond like carbon films [17], carbon nanotubes [18], fullerenes and graphene [19,20] can be prepared in mild conditions. Plasma techniques are also very effective in preparation of metal nanostructures. Magnetron sputtering, for instance, is a well-established method to prepare various metal thin films. Inspired by the success of plasma techniques in the single component preparation, we developed a tandem plasma reaction approach for controlled preparation of metal/carbon binary composites, in which the generation of metal and carbon are separately controlled by two tandem plasma reactions. As will show in this paper, this approach enables preparing metal/carbon composites in one single step at room temperature and provides large freedom in composition and structure control. In this paper, we will focus on the Sn/C binary nanocomposites, which are promising high capacity anode materials for LIBs. Compared to the graphite materials used in commercial LIBs, Sn has a much higher theoretical capacity (990 mAh g1 of Sn compared to 372 mAh g1 of graphite), offering the opportunity to significantly enhance the cell capacity [21,22]. However, Sn will experience severe volume change during charge/discharge processes, resulting in rapid capacity decay [23]. To alleviate this volume change, carbon is introduced as the buffer and a conductive matrix [24,25]. There have been several successful examples demonstrating that enhanced reversible capacity can be obtained in Sn/C binary composites with well-tailored nanostructures [26–28]. For example, by hydrothermal reaction or electrostatic spray deposition, intermediates composed of SnO2 nanostructures with polymeric organic coating are first prepared, which can be converted into Sn/C composites by subsequent high temperature carbonization and hydrogen reduction [29,30]. By pyrolysis of organic tin compounds with polymeric organic coating, Sn particles can be encapsulated in carbon hollow spheres or tubular fibers [11,31,32]. Most recent advances are based on Sn-encapsulated-in-carbon structures, which employ the intermediate composite guided approaches and require the high temperature carbonization step [33].
1315 inductively coil (13.56 MHz, 50–150 W), which is shown in Figure 1. The distance between the target and the substrate is 15 cm. The inductively coupled plasma (ICP) coil is located 2 cm above the substrate. The chamber is first evacuated to 8 104 Pa by a diffusion pump. Ar (99.995%) and CH4 (99.95%) are inlet separately for Sn sputtering and carbon coating, respectively. The working pressure of the chamber is maintained at 2.0 Pa. Sn particles are generated by magnetron sputtering of a high purity Sn target (99.99%), and are coated with carbon in the ICP zone by decomposition of CH4. Such configuration enables independent control of the generation of the two components, offering great versatility in compositional and structural control of binary composites. The products are directly deposited on copper foils or silicon substrates at room temperature. And the obtained products are uniform films with sufficient adhesion to the substrates, which can be directly used as the electrodes without further treatment. Plasma reactors based on similar concept have been previously used for grafting alkyl groups on to the surface of silicon nanoparticles [34].
Characterization The products deposited on substrates can be directly characterized by X-ray diffraction (XRD, Rigaku D/max 2000 diffractometer, Cu Kα) and scanning electron microscopy (SEM, Hitachi S4800, 10 kV) with an energy dispersive X-ray spectroscopy (EDS) analyzer. For transmission electron microscopy (TEM, JEOL JEM-2100, 200 kV) measurement, the samples are scratched from the SiO2/Si substrate to copper grids. The electrodes used in electrochemical performance measurements are fabricated by direct deposition of Sn/C composite on copper foils. Coin type half-cells were assembled in an argon-filled glove-box. A lithium foil was utilized as the counter electrode. 1 M LiPF6 in ethylene carbonate/dimethyl carbonate =1/1 in volume was used as the electrolyte. Polypropylene films (Celgard 2400) were used as the separators. All the cells were tested at a current density of 2000 mA g1 for both charge and discharge at room temperature in the voltage range of 0.005–1.5 V
Experimental Preparation of Sn/C composites The Sn/C binary composite samples are deposited in a home-designed tandem plasma reactor which consists of a magnetron sputtering source (AC, 50–150 W) and an
Figure 1 Schematic illustration of the tandem plasma reactor and the preparation process of the Sn/C composites.
1316 (versus Li/Li+). Cyclic voltammetric measurements were performed in the voltage range of 2.5–0 V (versus Li/Li+) at a scan rate of 0.1 mV s1.
Results and discussion Figure 2a is a typical X-ray diffraction (XRD) pattern of the obtained products collected on a copper foil. All the diffraction peaks are originated from the metallic Sn and the copper substrate. The energy dispersed spectroscopy (EDS) measurement suggests that the composite contains both Sn and C (Figure 2b). The above information implies that the composite is composed of crystalline Sn and amorphous carbon. Figure 2c–f are SEM images of the Sn/C composites with different carbon fraction (fC). The pure Sn sample is composed of well-defined nanoparticles around 80 nm with large inter-particle voids. Carbon incorporation shows strong effects on the particle size and surface morphology. Higher fC leads to smaller particle size and more uniform and smooth surface. A prominent advantage of the tandem plasma reactions is the versatility in composition control. The Sn/C ratio can be easily tuned by controlling the component generating rate in the two plasma zones. The key parameters are the CH4 fraction and the sputtering power. As shown in Figure 3a, the fC in the composite monotonically increases with the CH4 fraction in the plasma, which is straightforward since carbon is generated from the decomposition of CH4. Higher power of the sputtering source, on the other hand, leads to higher Sn fraction (namely, lower fC) in the composite (Figure 3b). This is explained by the fact that higher sputtering power increases the Sn generation rate. Proper combination of these parameters results in tunable Sn/C ratio in a wide range.
W. Li et al. Figure 4a–f are TEM images of the Sn/C composites with different fC. The darker spots are the Sn particles. A clear decrease trend of the Sn particle size with the increase of fC is observed. At low fC (10 wt%), the sample is composed of overlapped particles around 60 nm with relatively wide size distribution. The Sn particles become smaller and more uniform in size as fC increases, which is in agreement with the SEM results (Figure 2c–f). The Sn particles size approaches a lower limit of 3 nm when fC is higher than 39 wt%. Further increase of fC only leads to slightly more sparse distribution of the Sn particles. Higher fC also leads to more homogeneous distribution of the Sn particles (Figure 4d–f). The HRTEM image shows a crystalline Sn particle with well-defined lattice fringes surrounded by an amorphous carbon layer around 4 nm (Figure 4g). In the selected area electron diffraction pattern (Figure 4h), clear diffraction rings attributed to tetragonal metallic Sn can be resolved, which is in agreement with the XRD result (Figure 2a). Therefore, the nanoscale structure of the Sn/C composite can be described as crystalline Sn nanoparticles homogeneously embedded in amorphous carbon matrix. Both the SEM and TEM results suggest that carbon incorporation can effectively inhibit the growth of the Sn particles. In conventional magnetron sputtering processes, the initial products sputtered from the target are small clusters of atoms. During the flight to the substrate, the clusters collide with each other and agglomerate into larger particles. In the tandem plasma reaction, however, the Sn clusters can be coated by carbon when they pass the ICP zone. Carbon coating is essential to inhibit the agglomeration of the Sn clusters. The effectiveness of carbon coating depends on the relative generation rate of Sn and C. If the Sn clusters are generated too fast, some Sn clusters may not be completely coated with carbon, which will aggregate into larger particles. This explains the relatively wide size
Figure 2 (a) XRD patterns of the Sn/C composite on a copper foil. (b) EDS spectrum of the Sn/C composite on a SiO2/Si substrate. (c–f) Planar view SEM images of the Sn/C composites with different fC.
Tandem plasma reactions for Sn/C composites
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Figure 3 The fC in the composites versus preparation conditions. (a) The influence of the CH4 fraction in the plasma, the sputtering power is 80 W; (b) The influence of the sputtering power, the CH4 fraction is 28.6%. The ICP power is fixed at 100 W in all experiments.
distribution of the samples with lower fC (Figure 4a). Higher carbon generation rate, on the other hand, will result in more homogeneous coating on the Sn particles. As a result, the Sn particles are well separated by carbon layers and inhibited from agglomeration (Figure 4c–f). Similar effects have been observed in the preparation of Mg/C nanocomposites by arc thermal plasma, in which the growth of Mg particle is inhibited by carbon generated from decomposition of acetylene [35]. Sn/C binary composites are promising high capacity anode materials for LIBs. The Sn/C composites deposited on copper foils can be directly used as the electrodes. Figure 5a is a typical cyclic voltammogram (CV) graph for the Sn/C samples. The pair peaks A–A′ in the voltage range of 0.17–0.51 V vs. Li/Li+ are associated with the electrochemical reaction between Li and C (Eq. (1)) [36,37]. And the pair peaks in the higher voltage range B–B′, C–C′ and D–D ′ are corresponding to the stepwise reactions between Li–Sn (Eqs. 2–4) [23,38]. Li+6C-LiC6
(1)
5Sn+2Li-Li2Sn5
(2)
Li2Sn5+3Li-5LiSn
(3)
5LiSn+17Li-Li22Sn5
(4)
More sophisticated study reveals that the formation of Li22Sn5 from LiSn also experiences several intermediate phases [23]. The first cycle shows some irreversible capacity which is attributed to the formation of the solid electrolyte interface (SEI). The CV curves of the following cycles are almost overlapped, indicating that the Sn/C composites have good cycling stability during charge/discharge. A key parameter measuring the electrode performance of the Sn/C binary composites is the reversible capacity. Since the carbon component contributes a low but stable capacity, the strategy to achieve high reversible capacity is to stabilize the capacity of the high capacity Sn component. The reversible capacity of the Sn component in the Sn/C composites with different fC is compared in Figure 5b. The amorphous carbon obtained from ICP plasma decomposition of CH4 contributes a stable capacity of 310 mAh g1, which is subtracted to calculate the capacity of the Sn component. Not surprisingly, the capacity of the pure Sn sample decays
very rapidly in the first 10 cycles, which is attributed to the severe volume change during the charge/discharge processes. Carbon incorporation significantly improves the capacity stability. The sample with 31 wt% carbon already exhibits decent cycling stability. The capacity of Sn remains 660 mAh g1 after 100 cycles. An even higher reversible capacity of 850 mAh g1 is obtained for the sample with 51 wt% carbon, which is more than 85% of the theoretical value. The excellent capacity retention property is attributed to the well-tailored nanoscale structure of the Sn/C composites. The main reason for the capacity decay of the Sn component is the severe volume change during the charge/discharge processes. Smaller Sn particle size is beneficial for capacity stabilization since it reduces the absolute volume change. The carbon coating surrounded the Sn particles provides further buffering effect. The Sn/C composites prepared by tandem plasma reaction are composed of ultrafine Sn particles homogeneously embedded in amorphous carbon matrix. This structure combines the two advantages, which represents one of the ideal structures to achieve high stable capacity. The stabilization effect of the composite structure is further confirmed by the surface morphology of the electrodes after repeated charge/discharge cycles. As shown in Figure A1 (Supporting information), after 100 cycles, the electrode made of pure Sn is pulverized while the Sn/C composite electrode remains intact. Understanding how the composition and nanoscale structure will affect the electrode performance is of critical importance for a binary composite electrode system. Although it is generally accepted that smaller Sn particle size and more homogeneous carbon coating are beneficial for maintaining high reversible capacity, systematic studies on the correlation between the electrode performance and the compositional and structural parameters are still rare. Such systematic studies require samples with tunable structural parameters and thus are highly demanding for the sample preparation techniques. As shown above, the tandem plasma reactions are very versatile in controlling the composition and Sn particle size of the Sn/C composites, which allows a systematic study on the structureperformance correlation. The variation of the reversible capacity (defined as the capacity after 100 cycles) with the composition of the Sn/C composites is summarized in Figure 6a, showing both the capacity of the Sn/C composites and the contribution of the
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Figure 5 (a) A typical cyclic voltammograph of a Sn/C composite electrode and (b) the cycling performance of Sn/C composites with different content of carbon, hollow symbols: discharge, solid symbols: charge.
Figure 4 (a–f) TEM images of the Sn/C composites with 10 wt%, 21 wt%, 26 wt%, 35 wt%, 45 wt%, 51 wt% carbon, respectively. (g) An HRTEM image of a single carbon coated Sn particle, the observed lattice fringe corresponds to the (200) crystalline planes of tetragonal Sn. (h) Selected area electron diffraction pattern of the Sn/C sample with 26 wt% carbon.
Sn component. The reversible capacity of the Sn component increases steadily with fC and reaches the higher limit of 850 mAh g1 when fC = 35 wt% or higher. The carbon fraction of 35 wt% is the critical value at which the capacity of the Sn component starts to level off. Further incorporation of carbon shows no improvement of the Sn capacity and will lower the overall capacity. It is also at this fC that maximum overall reversible capacity of the Sn/C composites is obtained (660 mAh g1). The dashed line is the theoretical capacity of Sn/C binary composites with different composition, plotted using the theoretical capacity of Sn (990 mAh g1) and graphite carbon (372 mAh g1). The Sn/C composites with fC higher than 35 wt% approach the theoretical limit very closely. The electrode performance of the Sn/C
composites is apparently superior to the commercial graphite materials. It is also comparable to the state-of-the-art Sn/C binary composite structures [25,28,39]. The structural parameters also exhibit strong correlation with fC. The mean diameter of the Sn particle dSn is obtained from the TEM results. As shown in Figure 6b, the Sn particle size shows a clear decrease trend with the increase of fC in the low region (0–25 wt%). When fC increases from 10 wt% to 20 wt%, the Sn particle size decreases from 60 nm to 20 nm. The rapid decrease in particle size occurs in the exact same region that the reversible capacity of the Sn component rapidly increases. When fC = 35 wt% or higher, the Sn particle size approaches the lower limit. Correspondingly, the reversible capacity of Sn also levels off in this region. Such strong correlation provides unambiguous evidence that smaller Sn particle size is beneficial for stabilizing the capacity of the Sn component. The improvement due to the Sn size effect reaches the maximum when the Sn particle size is reduced to 5 nm and levels off when the Sn particle size is further reduced. Another important structural parameter is the thickness of the carbon layer separating two neighboring Sn particles δC. Assuming that the Sn particles are identical spheres and distribute in the carbon matrix in a homogeneous by random
Tandem plasma reactions for Sn/C composites
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Figure 6 (a) The capacity after 100 cycles of the Sn component (solid squares) and the Sn/C composites (hollow circles) versus fC in the composite. The theoretical capacity of Sn/C composites is shown by the dashed line, plotted using the theoretical capacity of Sn (990 mAh g1) and graphite (372 mAh g1). (b) The mean particle diameter of Sn dSn and (c) the thickness of the carbon separation layer of two neighboring Sn particles δC for Sn/C composites with different fC.
pattern, the value of δC can be estimated from fC and dSn (Eq. (5), see supporting information for detailed derivation). ( 1=3 ) πk 1 1 þ 1 1 d Sn ð5Þ δC ¼ DSn d Sn ¼ 6β 1 f C k where DSn and dSn are the average distance between two neighboring Sn particles and average diameter of the Sn particles, respectively. k = 3.22 is the ratio of the mass density of Sn and C. β is a constant depending on the packing density of the Sn particles, defined by V T ¼ βDSn 3
ð6Þ
where VT is the average volume containing one Sn particle, including the Sn particle itself and the carbon surround the particle. Smaller β value corresponds to higher packing density of the Sn particles: β = 1 for simple cubic packing and β =0.71 for closest packing. The model predicts that the minimum fC to obtain positive δC value is fC = 10 wt%, calculated using b= 0.71 corresponding to the closest packing structure. Carbon fraction lower than this value will result in inhomogeneous size and direct contact of the Sn particles, which is indeed observed in the fC = 10 wt% sample (Figure 4a). To estimate the δC value for the samples with higher fC, we use an intermediate value β = 0.77, corresponding to the packing density of the body centered cubic structure. For the fC = 20 wt% sample, δC is calculated to be 5 nm, i.e. the Sn particles are already separated by a carbon layer. Carbon separation also contributes to the
improvement of the reversible capacity in the fC region 10–20 wt%. For higher fC region, the model predicts that the carbon layer thickness only shows a rather weak change in the range of 2–5 nm (Figure 6c). This is a rather weak change compared to that of the Sn particle size. Therefore, the change in the reversible capacity of Sn in the region of fC420 wt% should be mainly attributed to the Sn particle size effect rather than the change of the carbon layer thickness. Finally, it is worthwhile to compare the tandem plasma reaction method to conventional chemical approaches in preparation of Sn/C nanocomposites. In conventional chemical approaches, an intermediate composite, in which carbon is usually in the form of polymeric organics, is often employed. Therefore, a high temperature treatment step is inevitable for carbonization. Moreover, electrode preparation using powder samples requires addition of conductive additives (e.g. acetylene black) and binders (e.g. poly (vinylidene fluoride)). The real capacity of the electrodes is much lower than that of the active materials. The tandem plasma reaction method, on the other hand, enables direct preparation of the Sn/C composites in one single step at room temperature [40]. The Sn/C composites deposited on the current collectors (such as copper foils in this work) have uniform thickness and good adhesion to the current collector, which can be directly used as the electrodes without the need of any additives or binders. Besides, the tandem plasma reaction method enables systematic controlling of the Sn/C structures. Moreover, it is also promising to prepare nanotubes or nanosheets with organic compounds as precursor [41]. Therefore, the tandem plasma reaction method has apparent advantage over conventional preparation methods for Sn/C binary composite electrode materials. Moreover, this preparation method is not limited to the Sn/C binary system. By simply using different metal targets, the preparation method and the principle for composition control can be readily extended to other metal/carbon binary composites. We expect that this new preparation technique will assist the controlled preparation and nanoscale structure design of a large variety of metal/ carbon binary nanocomposites, which may be very promising for LIBs and other electric energy storage devices such as super-capacitors.
Conclusion In conclusion, Sn/C composites with tunable composition and Sn particle size are prepared by a tandem plasma reaction method. In this method, the generations of the Sn and C components are independently controlled in separated plasma zones, enabling facile control of the composition and the Sn particle size and distribution of the Sn particles. The obtained Sn/C composites are composed of uniform Sn particles homogeneously embedded in amorphous carbon matrix. This well-tailored structure results in high reversible capacity. Composite samples with different composition and Sn particle size are prepared, allowing a systematic study on the correlation between the reversible capacity and the structural parameters. The results suggest that reducing the Sn particle size is beneficial for attaining high reversible capacity. However, no further improvement
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is obtained when the Sn particle size is smaller than 5 nm. The carbon layer thickness, on the other hand, shows a much weaker correlation with the reversible capacity. This performance-structure correlation will provide useful insights into the design of carbon based composite anode materials for LIBs. The tandem plasma reaction method and the principals for composition control can also be readily extended to a large variety of metal/carbon binary composites.
Acknowledgment This work was supported by MOST of China (Nos. 2009CB939902 and 2010CB631301) and NSFC (Nos. 20821091, U1201241 and 51071003).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.06.012.
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Rong Yang received his Ph.D. degree in inorganic chemistry from Peking University in 2011. He is now working at General Research Institute of Nonferrous Metals in Beijing, China. His research is focused on high lithium storage materials.
Tandem plasma reactions for Sn/C composites Jie Zheng received his Ph.D. degree from Peking University, China in 2009. He is now an assistant professor in College of Chemistry and Molecular Engineering, Peking University. His research interest is plasma processing of inorganic nanostructures and their applications in batteries and hydrogen storage.
1321 Xingguo Li received his Ph.D. degree from Tohoku University, Japan in 1990. He is currently a professor in College of Chemistry and Molecular Engineering, Peking University. His recent research is focused mainly on lithium-ion batteries, hydrogen storage, fuel cells and solar cells.
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journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Tandem plasma reactions for Sn/C composites with tunable structure and high reversible lithium storage capacity Wei Li, Rong Yang, Jie Zhengn, Xingguo Lin Beijing National Laboratory for Molecular Sciences (BNLMS), (The State Key Laboratory of Rare Earth Materials Chemistry and Applications), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received 14 May 2013; received in revised form 20 June 2013; accepted 20 June 2013 Available online 3 July 2013
KEYWORDS
Abstract
Tandem plasma reaction; Sn/C binary composites; Controlled preparation; Lithium ion batteries; Anode materials
The Sn/C nanocomposites are of great interest as high capacity anode materials for lithium ion batteries (LIBs). In this paper, we employ a tandem plasma reaction method for controlled preparation of Sn/C binary composites. The Sn and C components are generated by magnetron sputtering and plasma decomposition of CH4 in two tandem plasma zones, respectively. The obtained Sn/C composites are composed of ultrafine Sn particles homogeneously embedded in carbon matrix, which exhibit very high reversible lithium storage capacity. The tandem plasma reaction method offers great versatility in controlling the Sn/C ratio and the Sn particle size, allowing a systematic study on the relationship between the structural parameters and the electrode performance. The reversible anode capacity is found to be strongly affected by the Sn particle size while it shows a much weaker correlation with the carbon coating layer. & 2013 Elsevier Ltd. All rights reserved.
Introduction Binary composites of metal and carbon are extensively investigated as electrode materials, which have been extensively used in LIBs [1,2], fuel cells [3,4] and metal-air batteries [5,6]. An essential point to improve the electrode performance of metal/carbon binary composites is the accurate control of the nanoscale structure during the preparation process. Among the numerous efforts devoted to the controlled preparation of n
Corresponding authors. Tel.: +86 6276 5930. E-mail addresses: [email protected] (J. Zheng), [email protected] (X. Li). 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.06.012
metal/carbon binary composites, the preparation strategies can be classified into two categories depending on the origin of carbon. A straightforward strategy is using readily available carbon materials and introducing the metal component by impregnation, vapor phase deposition or direct loading of metal nanostructures. This approach is advantageous for the broad choice of the carbon materials, including amorphous carbon black, carbon nanotubes, graphene nanosheets and porous carbon [7–9]. However, this approach tends to yield metal-oncarbon structures, i.e. the metal is simply loaded onto the surface of the carbon materials (or the interior surface of pores in case of porous carbon). In many application circumstances such as the electrodes of LIBs, however, the metalencapsulated-in-carbon structure is preferred [10–12]. A more
Tandem plasma reactions for Sn/C composites sophisticated approach employs an intermediate composite in which both the metal and carbon components are in the precursor states. The intermediate composites can be obtained by various soft chemistry methods, which is more favorable for nanoscale structure control. The carbon precursor is usually obtained by polymerization or condensation of small organic molecules [13–16]. Therefore, high temperature treatment is inevitable for carbonization, which is energy inefficient and may also cause side reactions. The intermediate guided method, though is favorable for nanoscale structure control, is limited by the difficulty to obtain carbon from organic or polymer precursors without high temperature treatment. This obstacle, however, can be easily overcome in plasma reactions. By decomposition of hydrocarbon in low pressure gas discharge, various carbon materials, including diamond and diamond like carbon films [17], carbon nanotubes [18], fullerenes and graphene [19,20] can be prepared in mild conditions. Plasma techniques are also very effective in preparation of metal nanostructures. Magnetron sputtering, for instance, is a well-established method to prepare various metal thin films. Inspired by the success of plasma techniques in the single component preparation, we developed a tandem plasma reaction approach for controlled preparation of metal/carbon binary composites, in which the generation of metal and carbon are separately controlled by two tandem plasma reactions. As will show in this paper, this approach enables preparing metal/carbon composites in one single step at room temperature and provides large freedom in composition and structure control. In this paper, we will focus on the Sn/C binary nanocomposites, which are promising high capacity anode materials for LIBs. Compared to the graphite materials used in commercial LIBs, Sn has a much higher theoretical capacity (990 mAh g1 of Sn compared to 372 mAh g1 of graphite), offering the opportunity to significantly enhance the cell capacity [21,22]. However, Sn will experience severe volume change during charge/discharge processes, resulting in rapid capacity decay [23]. To alleviate this volume change, carbon is introduced as the buffer and a conductive matrix [24,25]. There have been several successful examples demonstrating that enhanced reversible capacity can be obtained in Sn/C binary composites with well-tailored nanostructures [26–28]. For example, by hydrothermal reaction or electrostatic spray deposition, intermediates composed of SnO2 nanostructures with polymeric organic coating are first prepared, which can be converted into Sn/C composites by subsequent high temperature carbonization and hydrogen reduction [29,30]. By pyrolysis of organic tin compounds with polymeric organic coating, Sn particles can be encapsulated in carbon hollow spheres or tubular fibers [11,31,32]. Most recent advances are based on Sn-encapsulated-in-carbon structures, which employ the intermediate composite guided approaches and require the high temperature carbonization step [33].
1315 inductively coil (13.56 MHz, 50–150 W), which is shown in Figure 1. The distance between the target and the substrate is 15 cm. The inductively coupled plasma (ICP) coil is located 2 cm above the substrate. The chamber is first evacuated to 8 104 Pa by a diffusion pump. Ar (99.995%) and CH4 (99.95%) are inlet separately for Sn sputtering and carbon coating, respectively. The working pressure of the chamber is maintained at 2.0 Pa. Sn particles are generated by magnetron sputtering of a high purity Sn target (99.99%), and are coated with carbon in the ICP zone by decomposition of CH4. Such configuration enables independent control of the generation of the two components, offering great versatility in compositional and structural control of binary composites. The products are directly deposited on copper foils or silicon substrates at room temperature. And the obtained products are uniform films with sufficient adhesion to the substrates, which can be directly used as the electrodes without further treatment. Plasma reactors based on similar concept have been previously used for grafting alkyl groups on to the surface of silicon nanoparticles [34].
Characterization The products deposited on substrates can be directly characterized by X-ray diffraction (XRD, Rigaku D/max 2000 diffractometer, Cu Kα) and scanning electron microscopy (SEM, Hitachi S4800, 10 kV) with an energy dispersive X-ray spectroscopy (EDS) analyzer. For transmission electron microscopy (TEM, JEOL JEM-2100, 200 kV) measurement, the samples are scratched from the SiO2/Si substrate to copper grids. The electrodes used in electrochemical performance measurements are fabricated by direct deposition of Sn/C composite on copper foils. Coin type half-cells were assembled in an argon-filled glove-box. A lithium foil was utilized as the counter electrode. 1 M LiPF6 in ethylene carbonate/dimethyl carbonate =1/1 in volume was used as the electrolyte. Polypropylene films (Celgard 2400) were used as the separators. All the cells were tested at a current density of 2000 mA g1 for both charge and discharge at room temperature in the voltage range of 0.005–1.5 V
Experimental Preparation of Sn/C composites The Sn/C binary composite samples are deposited in a home-designed tandem plasma reactor which consists of a magnetron sputtering source (AC, 50–150 W) and an
Figure 1 Schematic illustration of the tandem plasma reactor and the preparation process of the Sn/C composites.
1316 (versus Li/Li+). Cyclic voltammetric measurements were performed in the voltage range of 2.5–0 V (versus Li/Li+) at a scan rate of 0.1 mV s1.
Results and discussion Figure 2a is a typical X-ray diffraction (XRD) pattern of the obtained products collected on a copper foil. All the diffraction peaks are originated from the metallic Sn and the copper substrate. The energy dispersed spectroscopy (EDS) measurement suggests that the composite contains both Sn and C (Figure 2b). The above information implies that the composite is composed of crystalline Sn and amorphous carbon. Figure 2c–f are SEM images of the Sn/C composites with different carbon fraction (fC). The pure Sn sample is composed of well-defined nanoparticles around 80 nm with large inter-particle voids. Carbon incorporation shows strong effects on the particle size and surface morphology. Higher fC leads to smaller particle size and more uniform and smooth surface. A prominent advantage of the tandem plasma reactions is the versatility in composition control. The Sn/C ratio can be easily tuned by controlling the component generating rate in the two plasma zones. The key parameters are the CH4 fraction and the sputtering power. As shown in Figure 3a, the fC in the composite monotonically increases with the CH4 fraction in the plasma, which is straightforward since carbon is generated from the decomposition of CH4. Higher power of the sputtering source, on the other hand, leads to higher Sn fraction (namely, lower fC) in the composite (Figure 3b). This is explained by the fact that higher sputtering power increases the Sn generation rate. Proper combination of these parameters results in tunable Sn/C ratio in a wide range.
W. Li et al. Figure 4a–f are TEM images of the Sn/C composites with different fC. The darker spots are the Sn particles. A clear decrease trend of the Sn particle size with the increase of fC is observed. At low fC (10 wt%), the sample is composed of overlapped particles around 60 nm with relatively wide size distribution. The Sn particles become smaller and more uniform in size as fC increases, which is in agreement with the SEM results (Figure 2c–f). The Sn particles size approaches a lower limit of 3 nm when fC is higher than 39 wt%. Further increase of fC only leads to slightly more sparse distribution of the Sn particles. Higher fC also leads to more homogeneous distribution of the Sn particles (Figure 4d–f). The HRTEM image shows a crystalline Sn particle with well-defined lattice fringes surrounded by an amorphous carbon layer around 4 nm (Figure 4g). In the selected area electron diffraction pattern (Figure 4h), clear diffraction rings attributed to tetragonal metallic Sn can be resolved, which is in agreement with the XRD result (Figure 2a). Therefore, the nanoscale structure of the Sn/C composite can be described as crystalline Sn nanoparticles homogeneously embedded in amorphous carbon matrix. Both the SEM and TEM results suggest that carbon incorporation can effectively inhibit the growth of the Sn particles. In conventional magnetron sputtering processes, the initial products sputtered from the target are small clusters of atoms. During the flight to the substrate, the clusters collide with each other and agglomerate into larger particles. In the tandem plasma reaction, however, the Sn clusters can be coated by carbon when they pass the ICP zone. Carbon coating is essential to inhibit the agglomeration of the Sn clusters. The effectiveness of carbon coating depends on the relative generation rate of Sn and C. If the Sn clusters are generated too fast, some Sn clusters may not be completely coated with carbon, which will aggregate into larger particles. This explains the relatively wide size
Figure 2 (a) XRD patterns of the Sn/C composite on a copper foil. (b) EDS spectrum of the Sn/C composite on a SiO2/Si substrate. (c–f) Planar view SEM images of the Sn/C composites with different fC.
Tandem plasma reactions for Sn/C composites
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Figure 3 The fC in the composites versus preparation conditions. (a) The influence of the CH4 fraction in the plasma, the sputtering power is 80 W; (b) The influence of the sputtering power, the CH4 fraction is 28.6%. The ICP power is fixed at 100 W in all experiments.
distribution of the samples with lower fC (Figure 4a). Higher carbon generation rate, on the other hand, will result in more homogeneous coating on the Sn particles. As a result, the Sn particles are well separated by carbon layers and inhibited from agglomeration (Figure 4c–f). Similar effects have been observed in the preparation of Mg/C nanocomposites by arc thermal plasma, in which the growth of Mg particle is inhibited by carbon generated from decomposition of acetylene [35]. Sn/C binary composites are promising high capacity anode materials for LIBs. The Sn/C composites deposited on copper foils can be directly used as the electrodes. Figure 5a is a typical cyclic voltammogram (CV) graph for the Sn/C samples. The pair peaks A–A′ in the voltage range of 0.17–0.51 V vs. Li/Li+ are associated with the electrochemical reaction between Li and C (Eq. (1)) [36,37]. And the pair peaks in the higher voltage range B–B′, C–C′ and D–D ′ are corresponding to the stepwise reactions between Li–Sn (Eqs. 2–4) [23,38]. Li+6C-LiC6
(1)
5Sn+2Li-Li2Sn5
(2)
Li2Sn5+3Li-5LiSn
(3)
5LiSn+17Li-Li22Sn5
(4)
More sophisticated study reveals that the formation of Li22Sn5 from LiSn also experiences several intermediate phases [23]. The first cycle shows some irreversible capacity which is attributed to the formation of the solid electrolyte interface (SEI). The CV curves of the following cycles are almost overlapped, indicating that the Sn/C composites have good cycling stability during charge/discharge. A key parameter measuring the electrode performance of the Sn/C binary composites is the reversible capacity. Since the carbon component contributes a low but stable capacity, the strategy to achieve high reversible capacity is to stabilize the capacity of the high capacity Sn component. The reversible capacity of the Sn component in the Sn/C composites with different fC is compared in Figure 5b. The amorphous carbon obtained from ICP plasma decomposition of CH4 contributes a stable capacity of 310 mAh g1, which is subtracted to calculate the capacity of the Sn component. Not surprisingly, the capacity of the pure Sn sample decays
very rapidly in the first 10 cycles, which is attributed to the severe volume change during the charge/discharge processes. Carbon incorporation significantly improves the capacity stability. The sample with 31 wt% carbon already exhibits decent cycling stability. The capacity of Sn remains 660 mAh g1 after 100 cycles. An even higher reversible capacity of 850 mAh g1 is obtained for the sample with 51 wt% carbon, which is more than 85% of the theoretical value. The excellent capacity retention property is attributed to the well-tailored nanoscale structure of the Sn/C composites. The main reason for the capacity decay of the Sn component is the severe volume change during the charge/discharge processes. Smaller Sn particle size is beneficial for capacity stabilization since it reduces the absolute volume change. The carbon coating surrounded the Sn particles provides further buffering effect. The Sn/C composites prepared by tandem plasma reaction are composed of ultrafine Sn particles homogeneously embedded in amorphous carbon matrix. This structure combines the two advantages, which represents one of the ideal structures to achieve high stable capacity. The stabilization effect of the composite structure is further confirmed by the surface morphology of the electrodes after repeated charge/discharge cycles. As shown in Figure A1 (Supporting information), after 100 cycles, the electrode made of pure Sn is pulverized while the Sn/C composite electrode remains intact. Understanding how the composition and nanoscale structure will affect the electrode performance is of critical importance for a binary composite electrode system. Although it is generally accepted that smaller Sn particle size and more homogeneous carbon coating are beneficial for maintaining high reversible capacity, systematic studies on the correlation between the electrode performance and the compositional and structural parameters are still rare. Such systematic studies require samples with tunable structural parameters and thus are highly demanding for the sample preparation techniques. As shown above, the tandem plasma reactions are very versatile in controlling the composition and Sn particle size of the Sn/C composites, which allows a systematic study on the structureperformance correlation. The variation of the reversible capacity (defined as the capacity after 100 cycles) with the composition of the Sn/C composites is summarized in Figure 6a, showing both the capacity of the Sn/C composites and the contribution of the
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Figure 5 (a) A typical cyclic voltammograph of a Sn/C composite electrode and (b) the cycling performance of Sn/C composites with different content of carbon, hollow symbols: discharge, solid symbols: charge.
Figure 4 (a–f) TEM images of the Sn/C composites with 10 wt%, 21 wt%, 26 wt%, 35 wt%, 45 wt%, 51 wt% carbon, respectively. (g) An HRTEM image of a single carbon coated Sn particle, the observed lattice fringe corresponds to the (200) crystalline planes of tetragonal Sn. (h) Selected area electron diffraction pattern of the Sn/C sample with 26 wt% carbon.
Sn component. The reversible capacity of the Sn component increases steadily with fC and reaches the higher limit of 850 mAh g1 when fC = 35 wt% or higher. The carbon fraction of 35 wt% is the critical value at which the capacity of the Sn component starts to level off. Further incorporation of carbon shows no improvement of the Sn capacity and will lower the overall capacity. It is also at this fC that maximum overall reversible capacity of the Sn/C composites is obtained (660 mAh g1). The dashed line is the theoretical capacity of Sn/C binary composites with different composition, plotted using the theoretical capacity of Sn (990 mAh g1) and graphite carbon (372 mAh g1). The Sn/C composites with fC higher than 35 wt% approach the theoretical limit very closely. The electrode performance of the Sn/C
composites is apparently superior to the commercial graphite materials. It is also comparable to the state-of-the-art Sn/C binary composite structures [25,28,39]. The structural parameters also exhibit strong correlation with fC. The mean diameter of the Sn particle dSn is obtained from the TEM results. As shown in Figure 6b, the Sn particle size shows a clear decrease trend with the increase of fC in the low region (0–25 wt%). When fC increases from 10 wt% to 20 wt%, the Sn particle size decreases from 60 nm to 20 nm. The rapid decrease in particle size occurs in the exact same region that the reversible capacity of the Sn component rapidly increases. When fC = 35 wt% or higher, the Sn particle size approaches the lower limit. Correspondingly, the reversible capacity of Sn also levels off in this region. Such strong correlation provides unambiguous evidence that smaller Sn particle size is beneficial for stabilizing the capacity of the Sn component. The improvement due to the Sn size effect reaches the maximum when the Sn particle size is reduced to 5 nm and levels off when the Sn particle size is further reduced. Another important structural parameter is the thickness of the carbon layer separating two neighboring Sn particles δC. Assuming that the Sn particles are identical spheres and distribute in the carbon matrix in a homogeneous by random
Tandem plasma reactions for Sn/C composites
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Figure 6 (a) The capacity after 100 cycles of the Sn component (solid squares) and the Sn/C composites (hollow circles) versus fC in the composite. The theoretical capacity of Sn/C composites is shown by the dashed line, plotted using the theoretical capacity of Sn (990 mAh g1) and graphite (372 mAh g1). (b) The mean particle diameter of Sn dSn and (c) the thickness of the carbon separation layer of two neighboring Sn particles δC for Sn/C composites with different fC.
pattern, the value of δC can be estimated from fC and dSn (Eq. (5), see supporting information for detailed derivation). ( 1=3 ) πk 1 1 þ 1 1 d Sn ð5Þ δC ¼ DSn d Sn ¼ 6β 1 f C k where DSn and dSn are the average distance between two neighboring Sn particles and average diameter of the Sn particles, respectively. k = 3.22 is the ratio of the mass density of Sn and C. β is a constant depending on the packing density of the Sn particles, defined by V T ¼ βDSn 3
ð6Þ
where VT is the average volume containing one Sn particle, including the Sn particle itself and the carbon surround the particle. Smaller β value corresponds to higher packing density of the Sn particles: β = 1 for simple cubic packing and β =0.71 for closest packing. The model predicts that the minimum fC to obtain positive δC value is fC = 10 wt%, calculated using b= 0.71 corresponding to the closest packing structure. Carbon fraction lower than this value will result in inhomogeneous size and direct contact of the Sn particles, which is indeed observed in the fC = 10 wt% sample (Figure 4a). To estimate the δC value for the samples with higher fC, we use an intermediate value β = 0.77, corresponding to the packing density of the body centered cubic structure. For the fC = 20 wt% sample, δC is calculated to be 5 nm, i.e. the Sn particles are already separated by a carbon layer. Carbon separation also contributes to the
improvement of the reversible capacity in the fC region 10–20 wt%. For higher fC region, the model predicts that the carbon layer thickness only shows a rather weak change in the range of 2–5 nm (Figure 6c). This is a rather weak change compared to that of the Sn particle size. Therefore, the change in the reversible capacity of Sn in the region of fC420 wt% should be mainly attributed to the Sn particle size effect rather than the change of the carbon layer thickness. Finally, it is worthwhile to compare the tandem plasma reaction method to conventional chemical approaches in preparation of Sn/C nanocomposites. In conventional chemical approaches, an intermediate composite, in which carbon is usually in the form of polymeric organics, is often employed. Therefore, a high temperature treatment step is inevitable for carbonization. Moreover, electrode preparation using powder samples requires addition of conductive additives (e.g. acetylene black) and binders (e.g. poly (vinylidene fluoride)). The real capacity of the electrodes is much lower than that of the active materials. The tandem plasma reaction method, on the other hand, enables direct preparation of the Sn/C composites in one single step at room temperature [40]. The Sn/C composites deposited on the current collectors (such as copper foils in this work) have uniform thickness and good adhesion to the current collector, which can be directly used as the electrodes without the need of any additives or binders. Besides, the tandem plasma reaction method enables systematic controlling of the Sn/C structures. Moreover, it is also promising to prepare nanotubes or nanosheets with organic compounds as precursor [41]. Therefore, the tandem plasma reaction method has apparent advantage over conventional preparation methods for Sn/C binary composite electrode materials. Moreover, this preparation method is not limited to the Sn/C binary system. By simply using different metal targets, the preparation method and the principle for composition control can be readily extended to other metal/carbon binary composites. We expect that this new preparation technique will assist the controlled preparation and nanoscale structure design of a large variety of metal/ carbon binary nanocomposites, which may be very promising for LIBs and other electric energy storage devices such as super-capacitors.
Conclusion In conclusion, Sn/C composites with tunable composition and Sn particle size are prepared by a tandem plasma reaction method. In this method, the generations of the Sn and C components are independently controlled in separated plasma zones, enabling facile control of the composition and the Sn particle size and distribution of the Sn particles. The obtained Sn/C composites are composed of uniform Sn particles homogeneously embedded in amorphous carbon matrix. This well-tailored structure results in high reversible capacity. Composite samples with different composition and Sn particle size are prepared, allowing a systematic study on the correlation between the reversible capacity and the structural parameters. The results suggest that reducing the Sn particle size is beneficial for attaining high reversible capacity. However, no further improvement
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is obtained when the Sn particle size is smaller than 5 nm. The carbon layer thickness, on the other hand, shows a much weaker correlation with the reversible capacity. This performance-structure correlation will provide useful insights into the design of carbon based composite anode materials for LIBs. The tandem plasma reaction method and the principals for composition control can also be readily extended to a large variety of metal/carbon binary composites.
Acknowledgment This work was supported by MOST of China (Nos. 2009CB939902 and 2010CB631301) and NSFC (Nos. 20821091, U1201241 and 51071003).
Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2013.06.012.
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Rong Yang received his Ph.D. degree in inorganic chemistry from Peking University in 2011. He is now working at General Research Institute of Nonferrous Metals in Beijing, China. His research is focused on high lithium storage materials.
Tandem plasma reactions for Sn/C composites Jie Zheng received his Ph.D. degree from Peking University, China in 2009. He is now an assistant professor in College of Chemistry and Molecular Engineering, Peking University. His research interest is plasma processing of inorganic nanostructures and their applications in batteries and hydrogen storage.
1321 Xingguo Li received his Ph.D. degree from Tohoku University, Japan in 1990. He is currently a professor in College of Chemistry and Molecular Engineering, Peking University. His recent research is focused mainly on lithium-ion batteries, hydrogen storage, fuel cells and solar cells.