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Jingxugroup.netNano Res. 2011, 4(3): 290–296 Nano Res. 2011, 4(3): 290–296 DOI 10.1007/s12274-010-0081-x CN Research Article Hybrid Silicon-Carbon Nanostructured Composites as Superior
Anodes for Lithium Ion Batteries
Po-Chiang Chen1, Jing Xu1, Haitian Chen2, and Chongwu Zhou2 () 1 Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089, USA2 Ming-Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA Received: 6 October 2010 / Revised: 13 November 2010 / Accepted: 13 November 2010 Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2010 ABSTRACT
We have successfully fabricated a hybrid silicon–carbon nanostructured composite with large area (about
25.5 in2) in a simple fashion using a conventional sputtering system. When used as the anode in lithium ion
batteries, the uniformly deposited amorphous silicon (a-Si) works as the active material to store electrical energy,
and the pre-coated carbon nanofibers (CNFs) serve as both the electron conducting pathway and a strain/stress
relaxation layer for the sputtered a-Si layers during the intercalation process of lithium ions. As a result, the
as-fabricated lithium ion batteries, with deposited a-Si thicknesses of 200 nm or 300 nm, not only exhibit a high
specific capacity of >2000 mA·h/g, but also show a good capacity retention of over 80% and Coulombic efficiency
of >98% after a large number of charge/discharge experiments. Our approach offers an efficient and scalable
method to obtain silicon–carbon nanostructured composites for application in lithium ion batteries.
Amorphous silicon, carbon nanofibers, lithium ion batteries, hybrid nanostructured composite
(1-D) nanostructures, have the advantages of high surface area (such as single-walled carbon nanotubes In recent years, due to the depletion of fossil fuels with a surface area of 300–600 m2/g) and short ion and the increasing concern about environmental issues, diffusion length, and have been viewed as components interest in developing alternative energy sources and for next-generation electrochemical energy conversion devices, including solar cells, fuel cells, supercapacitors, and storage devices [9–11]. For instance, TiO2–B nano- and lithium ion batteries, has increased rapidly [1–6]. wires have been shown to have a higher specific Among the above-mentioned energy conversion and capacity than TiO2–B nanoparticle-based materials storage devices, lithium ion batteries have been widely when used as anodes in lithium ion batteries . applied in both portable electronics and consumer More recently, silicon (Si) nanostructures have been electronics because of their high energy density, high suggested to be one of the most promising anode power density, and low fabrication cost. Most research materials for lithium ion batteries and have attracted efforts have been devoted to the synthesis of electrode considerable attention because of their large theoretical materials and the design of electrode structures [7, 8] by gravimetric specific capacity of 4200 mA·h/g and adapting nanostructured materials. One-dimensional relatively low working potential at around 0.5 V vs. Address correspondence to firstname.lastname@example.org
Nano Res. 2011, 4(3): 290–296 Li/Li+ [12–20]. For instance, Chan et al. reported Si nanowires as anode materials and successfully de- monstrated an excellent specific capacity of 3124 mA·h/g during the first discharge process . Further develop- ments include using homogeneous Si nanowires and hybrid Si core–shell nanowires, which have been demonstrated to deliver superior device performance as battery electrodes [21, 22]. For example, TiSi2–Si core–shell nanowires have been reported as one example of such novel Si nanostructure-based anodes . The highly conductive core TiSi2 nanowires provide the electron conducting pathway and improve the battery power density by a factor of two. The TiSi2 nanowires also serve as a mechanical support and release the structural stress/strain associated with lithium ion insertion/extraction processes. In addition to core–shell nanowires, the preparation of nano- structured carbon–silicon composites seems to be another promising approach to solve the capacity retention problem [23–26]. However, synthesis of the above-mentioned materials usually requires a high temperature chemical vapor deposition process, and therefore alternative methods that can avoid a high- temperature process would be very interesting and attractive. In this paper, we present a rather simple but efficient approach to obtain hybrid Si-based nanostructured Figure 1 (a) Schematic diagram of sputtered silicon on CNF/copper
composites. By simply using a conventional sputtering foil. (b) Photographs of CNFs on a copper foil before and after system to deposit amorphous silicon (a-Si) on car- 200 nm silicon deposition. (c) Top-view SEM and EDX images bonaceous materials pre-deposited on copper foil, we of sputtered Si on CNFs. (d) Side-view of CNFs before and after are able to obtain a hybrid Si-based nanostructured 500 nm silicon deposition composite with a large area of 25.5 in2 and employ it as an anode in lithium ion batteries. In our design, 2. Experimental
carbon nanofibers (CNFs) are used as both mechanical support and electron conducting pathway. Different The fabrication of Si hybrid nanostructured thin film thicknesses of sputtered silicon were deposited on electrodes began with the preparation of a CNF film pre-deposited carbonaceous materials as the active on copper foil. CNFs (Sigma-Aldrich) were deposited materials to store electrical energy, as illustrated in on copper (Cu) foil via a so-called slurry spreading Fig. 1(a). These hybrid Si-based nanostructured method, which has been widely adopted in the pre- electrodes (Si/CNF) exhibit a high charge storage paration of lithium ion battery electrodes. In brief, capacity of 2528 mA·h/g and good cycling life up to a CNFs were mixed with polyvinylidene fluoride (PVDF, large number of charge/discharge cycles (>105 cycles). 10 wt%) in N-methylpyrrolidone (NMP) to form a In addition, the as-fabricated batteries also show a uniform slurry, and then spread onto a copper foil Coulombic efficiency of 88% in the first cycle and over using a stainless steel blade. The samples were installed 98% in the following cycles, which are comparable in a conventional sputtering system (Denton Discovery with results in the literature [25–28]. Sputtering System) for Si deposition. The loading Nano Res. 2011, 4(3): 290–296 density of as-prepared CNF films was about 0.4 mg/cm2 to distinguish between carbon nanofibers (green) and sputtered silicon (yellow). It can be seen that sputtered Silicon deposition on CNFs was carried out in an Si forms a uniform coating on CNFs, which is consistent argon (Ar)-filled environment at room temperature, with the image in Fig. 1(c). with a deposition rate of 6 nm/min and varied Electrochemical measurements of sputtered Si/CNF deposition thicknesses of 100 nm, 200 nm, 300 nm, or anodes were carried out with a battery testing system 500 nm. In order to compare the sputtered Si/CNF (MSTAT, Arbin) and a potentiostat (Gamry Reference anodes and Si electrodes, we sputtered 200 nm Si 600). The cyclic voltammetry (CV) profiles of sputtered directly on to a Cu foil as the reference sample. The Si/CNF anodes were obtained using the Si/CNF as-prepared samples were then characterized by field- electrode as the working electrode and Li foil as the emission scanning electron microscopy (FE-SEM, reference electrode. Figure 2(a) shows the first three CV Hitachi S-4800) and energy-dispersive X-ray spectros- curves of a sputtered Si/CNF anode with a deposited copy (EDS, Jeol, JSM-7001F). CR2032 coin cells were Si thickness of 200 nm, in a potential window between assembled in an Ar-filled glove box by using the 0.01 V and 3.0 V, with a scan rate of 0.05 mV/s. Two as-prepared Si/CNF anode as the working electrode pairs of signature redox peaks of amorphous Si and lithium metal foil as the counter electrode. The can be found at around 0.18/0.03 V (reduction) and electrolyte was 1 mol/L LiClO4 dissolved in a 1:1 0.50/0.30 V (oxidation) in the first cycling curve (blue (weight ratio) mixture of ethylene carbonate (EC) and line), which indicates the occurrence of Si–Li reactions diethyl carbonate (DEC). in the sputtered Si/CNF anode . The shape of the first cycling curve differs from that of the second and third curves, where the sharp peak at 0.18 V disappears 3. Results and discussion
and the peaks at 0.50/0.30 V shift towards a lower Figure 1(b) displays a photograph of a typical CNF potential. The irreversible reaction in the first cycle film on a Cu foil before (upper image) and after can be attributed to the formation of a solid electrolyte (lower image) the deposition of 500 nm Si. The Si interface (SEI) layer or a phase transformation . sputtered area is about 8.5 in × 3 in, making this method However, the second and the third cycles match almost a more efficient way to prepare large area Si-based exactly, which implies the system has reached a steady electrodes than the chemical vapor deposition (CVD) state. No significant peaks related to CNFs  can processes previously employed to prepare CNF- be observed in Fig. 2(a). Overall, our sputtered Si/CNF amorphous Si core–shell nanowire or Si nanowire anodes display CV characteristics similar to amorphous anodes [12, 22]. The zoomed-in SEM images in Fig. 1(b) silicon , but different from carbon nanofibers , are the top-view SEM images before and after the Si suggesting the redox behavior of our anodes is deposition. A uniform pre-deposited CNF thin film dominated by the sputtered Si layer rather than the on a the Cu foil can be clearly observed before Si pre-coated CNF thin films. deposition. After Si deposition, the CNFs were fully Galvanostatic (GV) charge/discharge measurements covered by a continuous Si film forming a hybrid were used to determine the specific capacity (Csp), Si/CNF nanostructured thin film electrode. In order to and the Coulombic efficiency of the devices in a two- study the interface between the CNFs and the sputtered electrode configuration. Figure 2(b) shows the cycling Si layer, we intentionally stretched a Si/CNF hybrid performance of a 200 nm sputtered Si/CNF anode up film electrode. In the top-view SEM image, shown in to 95 cycles, with a constant charge/discharge current Fig. 1(c), a uniform layered structure of a sputtered Si of 0.05 A/g. In the initial state, the potential dropped layer and CNFs can be clearly identified by the to 0.22 V and maintained a flat plateau at around contrast in the image. The thickness of the sputtered 0.25 V, before gradually decreasing to 0.01 V. The Si layer shown in Fig. 1(c) is about 500 nm. Fig. 1(d) stepped behavior of the discharge potential may shows side-view SEM images of the electrodes before result from the inactive nature of the Si film in the and after silicon sputtering with artificial colors used electrochemical reactions . The capacity in the
Nano Res. 2011, 4(3): 290–296 that of commercial graphite anodes ( 90%) and better than that of lithium ion batteries made of single- crystalline Si nanowires ( 73%) . The second discharge capacity decreased to 1608 mA·h/g, which can be attributed to the irreversible nature of Li–Si insertion and the formation of an SEI layer. After the first cycle, the reversible charge/discharge reactions were maintained over the subsequent 80 cycles. To further understand the GV behavior of sputtered Si/CNF anodes, we prepared Si/CNF anodes with different sputtering Si thicknesses of 100 nm, 300 nm, and 500 nm, and then performed GV measurements with the resulting Si/CNF anodes using a constant charge/discharge current of 0.05 A/g. The Si weight loadings were 5.8%, 15.5%, and 23.6% for 100 nm, 300 nm, and 500 nm sputtered Si, respectively. The first cycle for the Si/CNF anodes can be found in Fig. 2(c). As one can see, the voltage profile of 100 nm sputtered Si/CNF anode is quite different from that of 200 nm, 300 nm, and 500 nm sputtered Si/CNF anodes. The plateau at 0.22 V is short and not easy to observe, which indicates the discharge capacity might be contributed by the CNFs underneath the 100 nm sputtered Si, and the discharge capacity is only 786 mA·h/g. On the other hand, with more Si deposition on the CNFs, both 300 nm and 500 nm sputtered Si/CNF anodes exhibit prolonged plateaus at around 0.22 V. The 300 nm sputtered Si/CNF anode shows the highest discharge capacity of 2528 mA·h/g, which is more than three-times that of the 100 nm sputtered Si/CNF anode. However, the discharge capacity of 500 nm sputtered Si/CNF anode is only 648 mA·h/g. In con- clusion, our observations indicate that 200 nm and Figure 2 (a) Cyclic voltammograms for 200 nm Si on CNFs from
300 nm films offer better performance than 500 nm 0.01 to 3.0 V. The first three cycles are shown. (b) Voltage profile for films. More experiments are required to understand first 90 cycles of the 200 nm sputtered Si/CNF anode at a constant charge/discharge current of 0.05 A/g. (c) Voltage profile for the these phenomena. first cycle of the 100 nm, 200 nm, 300 nm, and 500 nm Si/CNF Figure 3(a) shows plots of the capacity retention anodes at a constant charge/discharge current of 0.05 A/g versus cycling number for electrodes with different thicknesses of sputtered Si (100 nm, 200 nm, and first discharge process of sputtered Si/CNF anodes 300 nm). The weight densities for 100 nm, 200 nm and was about 2320 mA·h/g. Our results are better than 300 nm sputtered Si are 0.02 mg/cm2, 0.049 mg/cm2 those reported for an amorphous Si microbattery and 0.071 mg/cm2, respectively. When calculating the ( 650 mA·h/g), and close to those reported for CNF– capacity, we have included the weight of the CNFs. amorphous Si core–shell nanostructured electrodes. As one can see, all Si/CNF anodes displayed good In addition, the Coulombic efficiency of our devices capacity retention, especially the 100 nm Si/CNF anode in the first cycle is about 88%, which is comparable to ( 84% after 95 cycles). This is probably because there
Nano Res. 2011, 4(3): 290–296 is more space for volume expansion than in the thicker sputtered Si samples. For 200 nm Si/CNF anodes, the capacity retention up to the 50th cycle ( 80%) is close to values reported in the literature. Good capacity retention was also observed even at a higher charge/ discharge rate (C/4). In this case, as shown in Fig. 3(b), the 300 nm Si/CNF anode exhibited a high specific capacity up to 1200 mA·h/g after 105 cycles, corres- ponding to over 90% capacity retention. In the initial stage of the cycle tests, a reduced capacity was obser- ved, and after about 10 cycles, the capacity recovered to about 1200 mA·h/g. Similar observations have reported by Takamura et al. [31, 32] and can be understood in terms of the Si film initially being very compact and unable to accept a large amount of Li, but after repeated Li insertion/extraction, the film structure underwent changes enabling it to accept more Li. To highlight the advantage of our Si/CNF anodes, we directly sputtered 200 nm Si on Cu foil (Si/Cu) and carried out GV measurements as a comparison. As expected, the Si/Cu anode showed extremely poor capacity retention, which can be attributed to the huge volume expansion after Li ion insertion/extraction. After 20 cycles, there was almost no measurable capacity from the anode, indicating a total loss of active material. We also investigated the Coulombic efficiency of our devices and, as an example, the results for the 200 nm sputtered Si/CNF anodes are shown in Fig. 3(c). The cell exhibited a good first cycle of Coulombic efficiency of 88%. The Coulombic efficiency in the second cycle increased to 93% and a very high Coulombic efficiency between 98%–99% was Figure 3 (a) Discharge capacity of 100 nm, 200 nm, and 300 nm
sputtered Si/CNF anodes, and 200 nm Si/Cu anodes at a current
maintained over the next 80 cycles. Subsequently, the rate C/10. (b) Discharge capacity of 300 nm sputtered Si/CNF discharge capacities faded a little but remained at anodes at a current rate C/4. (c) Coulombic efficiency of a 200 nm 70% up to 90 cycles (relative to the second cycle). sputtered Si/CNF anode Such good capacity retention was not observed for the 200 nm electrode, but also for 100 nm and 300 nm the Si surface after 95 cycles appears rough, as a sputtered Si/CNF anodes. consequence of the lithiation/delithiation processes. In order to study any variation in chemical com- Figures 4(b) and 4(c) show EDS images of the sputtered position of our sputtered Si/CNF anodes after the Si/CNF electrodes. The red color represents carbon cycling experiments, EDS was carried out and the atoms and the green color indicates the presence of results are shown in Fig. 4. The SEM image of a 200 nm silicon atoms. The EDS mapping strongly suggests sputtered Si/CNF anode after 95 charge/discharge that the sputtered Si layer is still well coated on CNFs cycles (Fig. 4(a)) clearly shows that there is still good and has a strong adhesion with them, even after a adhesion between Si and the substrate. In addition, large number of cycling experiments. Although the compared with the pristine surface shown in Fig. 1(b), sputtered Si film was broken into small grain domains
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