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Silicon Nanowires-Reduced Graphene Oxide Composite as a High-Performance Lithium Ion Battery Anode Material Jian-Guo Ren,a ChundongWang,a Qi-Hui Wu,*b Xiang Liu,c Yang Yang,a Lifang He,a and Wenjun Zhang*a Toward the increasing demands of portable energy storage and electric vehicle applications, silicon has been emerging as a promising anode material for lithium-ion batteries (LIBs) owing to its high specific capacity. However, serious pulverization of bulk silicon during cycling limits its cycle life. Herein, we report a novel hierarchical Si nanowires (Si NWs)reduced graphene oxide (rGO) composite fabricated using a solvothermal method followed by chemical vapor deposition process. In the composite, the uniform-sized [111]-oriented Si NWs are well dispersed on rGO surface and in between rGO sheets. The flexible rGO enables maintaining the structural integrity and providing continuous conductive network of the electrode, which results in over 100 cycles serving as anode in half cells at a high lithium storage capacity of 2300 mAh g -1. Due to its [111] growth direction and the large contact area with rGO, the Si NWs in the composite show a substantially enhanced reaction kinetics compared with other Si NWs or Si particles.
1 Introduction Lithium-ion battery (LIB) has currently dominated the market of rechargeable batteries for consumer electronics devices (e.g., smart phones and laptops) and is playing an increasingly important role in industrial, transportation, and grid-scale energy storage system applications. Commercial LIBs consisting of LiMn2O4 and graphite as the cathode and anode, respectively, generally deliver a capacity of ~120 Wh kg-1 based on the mass of battery pack, which, however, can hardly satisfy the electric vehicles and energy storage system requirements.1,2 Developing new cathode and anode materials with increased energy density and extended cycle life is of critical importance to address the ever-increasing energy storage demands.3,4 Graphite has a theoretical capacity of 372 mAh g-1 and is lithiated only by Li+ intercalation process. In contrast, Si, the second most abundant element in the Earth’s crust, has a maximum capacity of 3579 mAh g-1 because a Si atom can accommodate 3.75 Li atoms at room temperature.5 The high capacity of Si, however, is associated with massive volume (~300%) and crystal structural changes during the charging and discharging processes, which cause mechanical fracture of the electrode, and consequently detachment between the Si particles and irreversible capacity loss of the electrode.6 A variety of Si nanostructures and their composites have been reported for diminishing the volume expansion-induced failure and thus improving the stability of Si-based anodes, for example, Si nanoparticles (NPs),7 Si nanowires (NWs),8,9 Si nanotubes (NTs),10 3D nanoporous Si frameworks,11,12 yolk-
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shell Si nanoparticle-amorphous carbon structure,13 Si-carbon nanotube (CNT) and Si-graphene composites.14-19 Among all the reported nanostructures, an electrode architecture consisting of Si NWs grown vertically on a metallic current collector using chemical vapor deposition (CVD) method allows efficient volume change without pulverization and maintains good electrical contact between NWs and the current collector. Therefore, the electron transport along the length of each NW is provided, which leads to both increased specific capacity and improved cycling performance.8,20 However, the CVD method for growing Si NWs directly on the metallic current collector is so far limited by the difficulty to synthesize large quantities of NWs on the substrates (only ~200–250 µg cm-2 or ~0.75 mg h-1) which is insufficient for the practical applications.20 A recent work has reported a new CVD method to significantly enhance the Si NWs quantity on the stainless steel current collectors by growing Si NWs at a catalytic Au–CNT hybrid interface, and demonstrated high areal energy storage capacities (~1.0–1.6 mAhcm-2) and good electrochemical performance in half cells.17 In that case, however, the capacity decay of the Si NW– CNT electrode was clearly observed after 30 cycles, which could be ascribed to the small connection area between a single Si NW and the CNT network. The strong shear stress during LiSi alloying and de-alloying processes resulted in detachment of Si NWs from CNT network. Moreover, the formation of α−FeSi2 purity phase on the surface of metallic current collector during the high temperature NW growth process was also found.8,17 Hence, further refinement of the carbon–Si NWs
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hybrid material interfacial architecture and new approach are still required for the improvement of cyclic stability. Herein, we report a new method to improve the cycling performance and energy storage capacities of Si NWs electrode by growing Si NWs directly on the reduced graphene oxide (rGO) sheets. In our approach, Au nanoparticles decorated rGO (Au–rGO) hybrid materials were synthesized in ethylene glycol (EG) based on solvothermal reduction reaction, and then [111]oriented single crystalline and uniform Si NWs were grown directly on rGO surface by CVD method through vapor–liquid– solid mechanism. In the composite, Si NWs were wrapped in the rGO matrix, leading to enhanced multiple connections between Si NWs and rGO. The Si NWs–rGO composite anodes exhibited a highly stable cycling retention over 100 cycles with a high Li storage capacity of 2300 mAh g-1 at a C/3 rate in half cells. The salient cyclability and high reversible capacity are suggested to be credited to the enhanced contacts between Si NWs and the rGO overcoats, which as a mechanically robust and flexible matrix, accommodate the volume change of Si NWs, and maintain the structural and electrical integrity of the electrode.
2 Experimental GO was prepared from graphite following the modified Hummers’ method,21 and then decorated with Au nanoparticles using an EG–mediated solvothermal reduction process. In a typical synthesis, 20 mg GO powders were dispersed in 10 mL EG and 2 mL deionized water and then treated with sonication for 1 hour to get a brown suspension. Subsequently, 400 µL of HAuCl4 solution (50 mM) was added into the above GO solution. The mixed solution was transferred into a Teflon-lined stainless steel autoclave and kept at 220 °C for 12 hours inside a furnace. The black precipitate was separated by centrifugation and washed with acetone three times before drying at 120 °C for 2 hours to obtain the Au–rGO nanocomposites. Growth of Si NWs on rGO was carried out in a horizontal low-pressure CVD tube reactor. The as-synthesized Au–rGO powders were placed in the middle of the reactor and then heated up to 1000 °C under H2 flowing at a rate of 300 sccm. The Si NWs growth was performed by the introduction of SiCl4 and H2 at flow rates of 30 and 300 sccm, respectively, for 30 mins. The growth pressure was kept at ~180 mbar. In the case of control samples, Au thin films (2–5 nm in thickness) were deposited onto the single crystalline Si (100) wafers by electron beam evaporation. The prepared Au modified Si wafers were loaded into the horizontal CVD tube reactor. The control samples were synthesized in the same growth conditions as the Si NWs–rGO samples, resulting in the epitaxial growth of single crystalline Si NWs. Because epitaxial growth of single crystalline Si NWs was achieved on Si wafers, the control samples are denoted as Ep–Si NWs in this paper. The morphology and crystal structure of the samples were characterized by scanning electron microscopy (SEM, Philips XL 30 FEG) and transmission electron microscopy (TEM, Philips CM 20 FEG TEM operated at 200 kV). The X–ray diffraction (XRD) patterns were recorded using a Philips X'Pert MRD X–ray diffractometer with Cu Kα radiation. Thermal gravimetric analysis (TGA) was conducted on a TA instrument (TGAQ50, TA USA) under a static air atmosphere from room temperature to 1000 °C at a heating rate of 10 ºC min-1. Raman scattering spectra were measured with a Renishaw 2000 microRaman spectrometer using a laser wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) was performed to study the
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Journal Name DOI: 10.1039/C3NR05093A chemical composition of samples using a VG ESCALAB 220iXL surface analysis system equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. Electrochemical measurements were performed using CR2032 coin–type cells assembled in an argon-filled glove box. For the preparation of the working electrode, Si NWs–rGO or Ep–Si NWs (scraped off from Si wafers), Super–P carbon black and polyimide binder (dissolved in N–methylpyrrolidone) were mixed in a mass ratio of 80:10:10. The resultant slurry was then uniformly coated on a Cu foil current collector and dried overnight under vacuum. The total mass loading of the electrode in this study for half cell tests was 1.5 mg cm-2. The electrode thickness was pressed to be 20 µm by calendaring. Electrochemical cells were assembled with Si NWs–rGO or Ep–Si NWs as the working electrode, metallic lithium foil as the counter electrode, and Celgard 2325 porous film as a separator. The electrolyte used in this work was a solution of 1.2 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) (3:7 by weight). In addition, 10 wt% of fluoro–ethylene carbonate (FEC) was added into the above electrolyte as an additive. Galvanostatic electrochemical experiments were carried out in a Maccor Series4000 battery testing system. The electrochemical tests were performed between 0.01–1.5 V vs. Li at room temperature.
Figure 1. Schematic illustration for the synthetic procedure of Si NWs– rGO composite.
3 Results and discussion The synthesis procedure for Si NWs–rGO composite is schematically illustrated in Figure 1. For anchoring Au nanoparticles uniformly on the surface of carbon matrix, GO rather than graphene was taken as the starting material due to its high solubility in water and the abundance of functional groups (such as carboxyl, carbonyl, hydroxyl, and epoxide) on its surface.22 GO with an ultrahigh surface area and an open porous texture can serve as an excellent building block to effectively support Au nanoparticles, affording high catalyst loading and thus enhancing VLS synthesis of Si NWs. Figure 2 displays representative SEM images of as-prepared Au–rGO composites, which were fabricated by a typical polyolsolvothermal method using EG as the solvent and reducing agent. The formation of Au nanoparticles on the planes and edges of rGO sheets is demonstrated in Figure 2a. The presence of oxygen functionalities at GO surface are believed to be responsible for the nucleation and growth of Au nanoparticles, as reported previously.23 Basically, the oxygen functional groups led to previous attachment of the free Au(III) ions in solution because of the electrostatic interactions. Afterward, EG promoted the subsequent reductions of Au(III) ions and GO simultaneously under the solvothermal condition in the sealed
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Teflon reactor, enabling the efficient growth of Au nanoparticles at the rGO surface. As a result, a homogeneous distribution of discrete Au nanoparticles with diameters of around 60 nm was achieved at the rGO surface with negligible nanoparticle agglomeration (Figure 2b).
Figure 2. SEM images of the as-prepared Au–rGO nanocomposites by polyolsolvothermal reaction. The diameter of Au nanoparticle is measured to be around 60 nm in Figure 2(b).
Growth of Si NWs on the Au–rGO matrix was conducted by means of low-pressure (ca. 180 mbar) SiCl4 decomposition in an H2 atmosphere at 1000 ºC. It has been demonstrated that the SiCl4 precursor was highly effective for the epitaxial growth of vertically aligned, single-crystalline Si NWs for photovoltaic application.24-26 Compared with SiH4 (the most frequently used precursor for CVD synthesis of Si NWs for LIBs),8,27,28 the use of SiCl4 precursor in the presence of hydrogen leads to the formation of hydrochloric acid (HCl), which may cause some desirable etching of the oxide layer on Si surface and result in the production of high-purity and oxide-free Si NWs. 25,26 Furthermore, for SiCl4, the growth temperatures typically range from about 800 ºC to well beyond 1000 ºC, compared to temperatures of about 400–600 ºC, typically for Si NWs growth in the presence of SiH 4. Growth at such elevated processing temperatures, the corresponding NWs are expected to be free of crystallographic defects,29 which benefits the application of the NWs in LIBs since the defects may lead to the irreversible lithium trapping in the initial lithiation process. 30 Figure 3 depicts the morphology and crystal structure of the Si NWs– rGO composite. The Si NWs are densely and randomly aligned
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Figure 3. (a) SEM image of the CVD–synthesized Si NWs–rGO composite. (b) TEM image of a single Si NW in the Si NWs–rGO composite. (c) SEM image of the as-prepared Si NWs array on Si (100) wafer. The inset in Figure 3(a) shows the diameter of a single Si NW in the composite. The inset in Figure 3(b) is its corresponding Fourier transform image. The inset in Figure 3(c) is the top-view image.
on the rGO surface and in-between rGO sheets. The diameter of most NWs is around 60 nm, as shown in the inset of Figure 3a, which is consistent with the original diameter of the Au nanoparticles in the Au–rGO hybrids. These observations suggest that the aggregation of Au nanoparticles on rGO is negligible during the high temperature synthesis process,
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probably due to the strong interaction between Au and rGO. Although the agglomeration of rGO might result in partial overlapping and coalescing of the rGO sheets, the crumpled rGO sheets were assembled in a porous structure, as shown in Figure 2, and the adsorbed solvent species might be removed from rGO surfaces during the initial CVD heating process in H2 atmosphere, leaving space for the gaseous Si precursors to access the inner rGO sheets' surfaces. As a result, the NWs grew up from discrete Au particles on rGO surfaces were well dispersed and in turn sandwiched between rGO sheets, forming rGO-sandwiched Si NWs nanocomposites (Figure S1). With this architecture, the multiple contacts between each NW and rGO sheets were provided and a three-dimensional porous network was formed. The thus-constructed Si NWs–rGO composite, as a representative model shown in Figure 1, can freely switch between a contracted state and an expanded state upon lithiation and delithiation. The void space between NWs and rGO can effectively accommodate the volume expansion of NWs and the flexible rGO can be adaptable to maintain the structural integrity and continuous conductive network of the electrode. The high-resolution TEM image of a single Si NW is shown in Figure 3b accompanying with a fast Fourier transform (FFT) pattern. The ordered crystalline structure with the interlayer distance of 3.14 Å demonstrates that the Si NWs are single crystal with the growth direction along [111] axis. The FFT pattern can be indexed as the diffraction along the [110] zone axis of crystalline Si and further verifies the growth of Si NWs along [111] direction.31 Recent studies have clearly shown that the lithiation of Si is highly dependent on the crystallographic orientations, resembling the anisotropic swelling of Si.32,33 The lithiation of Si has exhibited the largest swelling with the fastest lithiation speed along the [110] direction because the crystalline Si structure presents much larger interstitial spaces between atoms along the [110] direction than [111] and [100] directions.32 In this work, Si NWs were grown along [111] direction. Thus, there are six [110] directions perpendicular to the NW's axis arranged hexagonally, which can definitely promote fast diffusion of Li ions into NWs along its radial direction. For comparison, the single crystalline Si NWs used as the control samples (Ep-Si NWs) were prepared using the same CVD conditions with Au thin films directly deposited on single crystalline Si wafers as the growth catalyst. Figure 3c shows the self-oriented Si NWs array grown on Si (100) wafers. A rectangular networks is clearly formed, in which the Si NWs orient along four directions that make an angle of about 35º with the substrate. Taking into account that the Si (100) substrate has four [111] directions at an angle of 35.3º to the surface of the substrate on which orthographic projections of these directions form rectangular networks,34 these Si NWs are thus believed to grow along the [111] directions. This is the same with that observed on Au-rGO matrix and indicates that the Si NW growth direction is independent of the substrate type. After growth, the Ep–Si NWs were scraped off from Si wafers for the electrode preparation by the slurry method for further electrochemical performance characterization. The formation of Si NWs on the rGO matrix was further confirmed by XRD and Raman spectroscopy (Figure 4). XRD characterization shows that the obtained product is a mixture of Si and graphitic carbon (Figure 4a). A weak peak at 26.8º corresponds to the (002) plane reflections of graphitic carbon. The peaks at 28.6º, 47.5º, 56.3º, 68.5º, and 75.7º can be assigned to the (111), (220), (311), (400), and (331) reflections of Si (ICDD number 01-0791), respectively. Further structural
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Figure 4. (a) XRD pattern, (b) Raman spectroscopy, and (c) TGA–DSC curves of the Si NWs–rGO composite.
information is provided by the Raman spectroscopy by examining the intensity ratios among the D, G, and 2D peaks together with the full-width-at-half-maximum (FWHM) of the 2D peak (Figure 4b). The appearance of both G and 2D peaks signifies the readily formation of graphene after the reduction of GO under solvothermal condition. As the peak intensity ratio I2D/IG is less than 1 and the FWHM of the 2D band is ~80 cm-1, the obtained rGO scaffold should be the stacking of multilayer graphene sheets.35,36 The existence of D band suggests that the as-synthesized rGO scaffold is defective. The presence of such
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Journal Name defects helps the efficient access for Li ions to diffuse in and out of the Si NWs.37The strong Raman peak located at 520 cm-1 confirms the formation of crystalline Si. In this work, the small D band and high 2D band in Raman spectrum could be ascribed to the further reduction of rGO during CVD growth of Si NWs at 1000 ºC in H2 atmosphere, in which the oxygen moieties were removed and the sp2 network was restored due to structural relaxation. The further reduction of rGO during CVD process could be verified by XPS studies of rGO before and after Si NW growth, as shown in Figure S2. The atomic ratio of carbon to oxygen (C/O) increased from 4.05 to 25.04 after Si NW growth, suggesting that most of the attached epoxide and hydroxyl functional groups on rGO were removed. TGA (Figure 4c), carried out in air at a heating rate of 10 ºC min-1, was used to determine the chemical composition of the Si NWs–rGO composite. The result shows that the composite contains approximately 90.9 wt% Si and 9.1 wt% graphene based on the assumptions that the weight loss is due to the complete combustion of rGO in air and the partial oxidation of Si NWs is ignored in this temperature region. An exothermic peak centred at 700 ºC was observed in differential scanning calorimetry (DSC) plot, along with a pronounced weight loss, which corresponds to the graphene combustion reaction. After that, a gradual increase in weight and an endothermic peak (at ~900ºC) in DSC curve may be ascribed to the partial oxidation of Si to SiOx (x < 2) in air. Electrochemical properties of the Si NWs–rGO and Ep-Si NWs electrodes were characterized in the potential range from 10 mV to 1.5 V in CR2032-type coin cells, respectively. Since Si is a semiconducting material, Super P carbon is needed for the preparation of pure Si NWs electrode. In order to make a fair comparison, Super P carbon is also used to make Si NWsrGO electrode with the same electrode's composition. In this work, the specific capacities and current densities of the Si NWs-rGO electrode were calculated on the basis of the total weight of Si NWs and rGO. The cells were galvanostatically tested at a low current density of 300 mA g-1 (C/12, 1C = 3600 mA g-1) in the first cycle and at 600 mA g-1 (C/6) for the second cycle, followed by at 1.2 A g-1 (C/3) for the subsequent cycles. The first cycle of the Si NWs–rGO electrode shows the discharge (lithiation) and charge (delithiation) capacities of 3111 mAh g-1 and 2428 mAh g-1, respectively, giving an initial Coulombic efficiency (ICE) of 78% (Figure 5a). The irreversible capacity loss can be mainly attributed to the formation of a solid electrolyte interface (SEI) layers on the electrode surface and the consumption of lithium by the defects on the rGO.38,39 The reversible capacity of the Si NWs–rGO electrode is 1900 mAh g-1 for the third cycle at a rate of C/3, and gradually increases to 2300 mAh g-1 in the tenth cycle followed by a stable cyclability for the subsequent cycles. Upon cycling, a 2230 mAh g-1 capacity is retained after 100 full cycles. Thus the capacity retention after 100 cycles relative to the capacity value at the first cycle is 91.8%. The Ep–Si NWs control electrode shows the discharge and charge capacities of 3272 mAh g-1 and 2618 mAh g-1 for the first cycle at the same discharge/charge current rate as the Si NWs–rGO electrode. An ICE of 80% is achieved which is greater than that observed for the Si NWs–rGO electrode. However, upon cycling, noticeable and continuous capacity degradation is observed for the Ep–Si NWs electrode, which exhibited a specific capacity of 1246 mAh g-1 at the 82nd cycle, almost half of the corresponding value for the Si NWs–rGO electrode at the same rate. This highlights the great effect of rGO on improving the cyclic stability of Si NWs anode. The rate capabilities were also characterized in Si NWs-rGO composite electrode (Figure S3).
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Figure 5. (a) Cycling performance of the Si NWs–rGO and Ep–Si NWs electrodes, (b) voltage profiles and (c) differential capacity curves of the Si NWs–rGO electrode.
At current rates of C/12, C/6, C/3, 1C, and 3C (1 C = 3.600 mA g-1), the electrode exhibited reversible capacities of 2424, 2226, 1812, 1371 and 825 mAh g-1, respectively. The capacity could be recovered to 2407 mAh g-1 when the rate is returned to C/12. Moreover, after 100 cycles at C/3 rate, the Si NW in the composite was transformed to a porous structure, and its diameter increased to about 120 nm, as shown in Figure S4. However，the integrity of Si NW was kept and NW did not turn into broken species.
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In addition, the electrochemical properties of the Si NWs– rGO composite prepared by vacuum filtration of an aqueous Si NWs-GO dispersion followed by thermal reduction have been reported recently.39 In that case, the composite presented a relatively low (∼50%) Coulombic efficiency in the first cycle and a gradual capacity fading after 50 cycles, and the specific capacity retained 80% of its initial capacity after 100 cycles. The enhanced electrochemical properties of the Si NWs–rGO composite in this work could be attributed to the relatively low density of defects on the rGO as demonstrated in Raman results (Figure 4b) and the well dispersion of Si NWs since the NWs were directly grown from the discrete Au nanoparticle on the rGO surface. The restoration of the sp2 network of rGO during Si NW growth at 1000 ºC in H2 atmosphere could lead to higher conductivity than that of the conventional rGO prepared by chemical reduction or thermal reduction at 600–700 ºC in argon,18,39which is believed to benefit the electron transport in lithiation and delithiation reactions. Furthermore, the Si NWs– rGO composite electrode in this work was made using a traditional slurry coating method and more compatible to the current battery technology. The areal and volumetric energy storage capacities of the Si NWs–rGO composite electrode were 3.45 mAh cm-2 and 1725 mAh cm-3, respectively, which was calculated based on the electrode density of 1.5 mg cm-2, the calendared electrode thickness of 20 µm and the specific capacity of 2300 mAh g-1. These values are much greater than those for earlier-reported Si NWs electrodes8,17,20,27,30 and also very well lying in the range of capacity densities commonly found in the electrodes of commercial LIBs (between 2 and 5 mAh cm-2).40 The substantially enhanced cycling stability and reversibility of the Si NWs–rGO composite in comparison with the Si NWs grown directly on stainless steel current collector8 or the Si NWs grown on CNTs matrix17 may be ascribed to the unique composite architecture fabricated. First, the rGO overcoats in the composite function as a flexible and adaptable matrix to accommodate the contracting and expanding deformation of the sandwiched Si NWs, playing a critically important role in guaranteeing the structural integrity of the electrode upon cycling. A large connection area between NWs and rGO sheets could be maintained in the repetitive swelling/shrinkage processes (as illustrated in Figure 1). Second, the flexible rGO sheets which are tightly attached onto the NW surfaces, enables the formation of a three-dimensional porous network, thus significantly facilitating the diffusion and transport of lithium ions. Third, the lateral contact between NW and rGO sheet greatly shortens the electron transport distance from/to Si NW, and is highly favourable for improving the reaction kinetics, which is clearly better than the axial transport in the Si NWs deposited directly on metal current collector or CNTs matrix.41,42 Figure 5b exhibits voltage profiles of the Si NWs–rGO composite electrode in the first cycle for electrode activation together with the subsequent cycles. The first discharge curve displays a slope from 1.3 V to 0.1 V and a long flat plateau below 0.1 V, which corresponds to the irreversible reaction on the interface or host materials, and the lithium-alloying process of crystalline Si to form an amorphous LixSi phase, respectively.16,40 Afterwards, the discharge and charge curves show the characteristics of amorphous Si.43 The lithium insertion profiles at the 50th cycle and the 100th cycle almost overlap each other, which suggests a stable property in the discharge kinetics.44 Figure 5c displays differential capacity curves of the first, second, and third cycles for the Si NWs–
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Journal Name DOI: 10.1039/C3NR05093A rGO electrode, from which information about the structural transformations during lithiation and delithiation can be obtained. The first cycle shows a large lithiation peak at 70 mV and a delithiation peak at 0.43 V, which correspond to a twophase region where crystalline Si reacts with Li to form an amorphous Li silicide and a two-phase region between two different amorphous lithiated phases, respectively.45 In the second cycle, lithiation into the amorphous Si shows three small peaks at 0.25 V, 90 mV and 40 mV, respectively, indicating the successively formation of three amorphous lithium silicide phases. Delithiation of the ultimate Li silicide phase occurs in a two-phase region to show a similar delithiation peak as in the first cycle. In the third cycle, with increased current density (from 600 mA g-1 to 1.2 A g-1), only two peaks at 0.25 V and 90 mV are observed in the lithiation process, in which 40 mV peak disappears mainly due to the reaction kinetics in a higher current rate. Consequently, two small peaks at 0.26 V and 0.44 V appear in the delithiation curve which can be ascribed to a two-phase region between two different amorphous Li silicide phases and a two-phase region between amorphous Li silicide and amorphous Si phases. It should be pointed out that in this case the delithiation peaks arise at relatively low voltage and the lithiation peaks at relatively high voltage compared with that observed in the [100]-oriented Si NWs prepared by metal-assisted chemical etching30 and Si submicron particles prepared by CVD approach.16 As discussed before, [111]-oriented Si NWs obtained in this work present six (110) facets parallel to the NW's axis arranged hexagonally, promoting fast lithium diffusion into/out Si NWs. At the same time, the rGO sheets cover large areas of the NWs, i.e., affording great areas of contact for charge transfer and, thus facilitating the electrode reaction. As a result, the Si NWs in this work show a substantially enhanced reaction kinetics.
4 Conclusions We described a facile approach to significantly enhance the cyclability and energy storage density of Si NWs as anode materials for high-performance LIBs by growing Si NWs directly on the rGO matrix. The Au nanoparticles decorated rGO hybrids were successfully prepared using solvothermal method and then used as an integrated growth catalyst to support the VLS growth of uniformly dispersed Si NWs on rGO surface and in between rGO sheets. The flexible and conductive rGO sheets accommodate the volume change of embedded Si NWs, maintaining the structural and electrical integrity of the electrode. As a result, the Si NWs–rGO composite electrode exhibited a stable cycling retention over 100 cycles at a high specific capacity of 2300 mAh g-1, simultaneously providing greatly improved cycling performance and areal/volumetric energy storage capacities compared with the Si NWs grown directly on metallic current collector or the Si NWs on the CNTs matrix. The strategy to encapsulate Si with adaptable matrices demonstrated here opens up a new avenue for developing high-performance Sibased anodes, and can be also extended to other fascinating anode and cathode materials that undergo large volume expansion.
Acknowledgements This work was financially supported by CityU Applied Research Grant, Hong Kong Special Administrative Region, China (ARG 9667075). Wu would like to thank Quanzhou
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“Tong-Jiang Scholar” program, Fujian "Min-Jiang Scholar" program, program for New Century Excellent Talents in University (NCET), and the Education and Scientific Research Foundation (Class A) for Young Teachers of Education Bureau of Fujian Province, China (Grant No. JA13263) for financial support.
Notes and references a
Centre of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China. E-mail: [email protected]; Fax: 852-3442-0538; Tel: 852-3442-7433. b Department of Chemistry, College of Chemistry and Life Science, Quanzhou Normal University, Quanzhou 362000, P. R. China. E-mail: [email protected]. c Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, P. R. China. 1 2 3 4 5 6 7 8 9 10
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ARTICLE DOI: 10.1039/C3NR05093A 21 W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 22 C. Wang, F. Ye, H. Wu, Y. Qian, Int. J. Electrochem.Sci., 2013, 8, 2440. 23 G. Goncalves, P. A. A. P. Marques, C. M. Granadeiro, H. I. S. Nogueira, M. K. Singh, J. Gracio, Chem. Mater., 2009, 21, 4796. 24 S. W. Boettcher, J. M. Spurgeon, M. C. Putnam, E. L. Warren, D. B. Turner-Evans, M. D. Kelzenberg, J. R. Maiolo, H. A. Atwater, N. S. Lewis, Science, 2010, 327, 185. 25 J. Goldberger, A. I. Hochbaum, R. Fan, P. Yang, NanoLett., 2006, 6, 973. 26 V. Schmidt, J. V. Wittemann, U. Gosele, Chem. Rev., 2010, 110, 361. 27 S. Jeong, J. P. Lee, M. Ko, G. Kim, S. Park, J. Cho, NanoLett., 2013, 13, 3403. 28 A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nat. Mater., 2010, 9, 353. 29 B. M. Kayes, M. A. Filler, M. C. Putnam, M. D. Kelzenberg, N. S. Lewis, H. A. Atwater, Appl. Phys. Lett., 2007, 91, 103110. 30 Y. Yang, J. G. Ren, X. Wang, Y. S. Chui, Q. H. Wu, X. Chen, W. J. Zhang, Nanoscale, 2013, 5, 8689. 31 Y. Wu, R. Fan, P. Yang, NanoLett., 2002, 2, 83. 32 S. W. Lee, M. T. McDowell, J. W. Choi, Y. Cui, NanoLett., 2011, 11, 3034. 33 X. H. Liu, Y. Liu, A. Kushima, S. Zhang, T. Zhu, J. Li, J. Y. Huang, Adv. Energy Mater., 2012, 2, 722. 34 S. Ge, K. Jiang, X. Lu, Y. Chen, R. Wang, S. Fan, Adv. Mater., 2005, 17, 56. 35 A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97, 187401. 36 Y. Hao, Y. Wang, L. Wang, Z. Ni, Z. Wang, R. Wang, C. K. Koo, Z. Shen and J. T. L. Thong, Small, 2006, 2, 195. 37 H. Kim, Y. Son, C. Park, J. Cho and H. C. Choi, Angew. Chem. Int. Ed., 2013, 52, 5997. 38 Y. C. Yen, S. C. Chao, H. C. Wu, N. L. Wu, J. Electrochem. Soc., 2009, 156, A95. 39 B. Wang, X. Li, X. Zhang, B. Luo, M. Jin, M. Liang, S. A. Dayeh, S. T. Picraux, L. Zhi, ACS Nano, 2013, 7, 1437. 40 J. Li, R. B. Lewis, J. R. Dahn, Electrochem. Solid-State Lett., 2007, 10, A17. 41 B. Liu, X. Wang, H. Chen, Z. Wang, D. Chen, Y. B. Cheng, C. Zhou, G. Shen, Sci. Rep., 2013, 3, 1622. 42 X. L. Wang, W. Q. Han, ACS Appl. Mat. Interfaces, 2010, 2, 3709. 43 C. D. Wang, Y. S. Chui, R. Ma, T. Wong, J. G. Ren, Q. H. Wu, X. Chen, W. J. Zhang, J. Mater. Chem.A, 2013, 1, 10092. 44 K. Evanoff, A. Magasinski, J. Yang and G. Yushin, Adv. Energy Mater., 2011, 1, 495. 45 C. K. Chan, R. Ruffo, S. S. Hong, R. A. Huggins, Y. Cui, J Power Sources, 2009, 189, 34.
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DOI: 10.1039/C3NR05093A
Direct growth of silicon nanowires on reduced graphene oxide provides an anode material for lithium ion battery with enhanced cyclability.
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Silicon Nanowires-Reduced Graphene Oxide Composite as a High-Performance Lithium Ion Battery Anode Material Jian-Guo Ren,a ChundongWang,a Qi-Hui Wu,*b Xiang Liu,c Yang Yang,a Lifang He,a and Wenjun Zhang*a Toward the increasing demands of portable energy storage and electric vehicle applications, silicon has been emerging as a promising anode material for lithium-ion batteries (LIBs) owing to its high specific capacity. However, serious pulverization of bulk silicon during cycling limits its cycle life. Herein, we report a novel hierarchical Si nanowires (Si NWs)reduced graphene oxide (rGO) composite fabricated using a solvothermal method followed by chemical vapor deposition process. In the composite, the uniform-sized [111]-oriented Si NWs are well dispersed on rGO surface and in between rGO sheets. The flexible rGO enables maintaining the structural integrity and providing continuous conductive network of the electrode, which results in over 100 cycles serving as anode in half cells at a high lithium storage capacity of 2300 mAh g -1. Due to its [111] growth direction and the large contact area with rGO, the Si NWs in the composite show a substantially enhanced reaction kinetics compared with other Si NWs or Si particles.
1 Introduction Lithium-ion battery (LIB) has currently dominated the market of rechargeable batteries for consumer electronics devices (e.g., smart phones and laptops) and is playing an increasingly important role in industrial, transportation, and grid-scale energy storage system applications. Commercial LIBs consisting of LiMn2O4 and graphite as the cathode and anode, respectively, generally deliver a capacity of ~120 Wh kg-1 based on the mass of battery pack, which, however, can hardly satisfy the electric vehicles and energy storage system requirements.1,2 Developing new cathode and anode materials with increased energy density and extended cycle life is of critical importance to address the ever-increasing energy storage demands.3,4 Graphite has a theoretical capacity of 372 mAh g-1 and is lithiated only by Li+ intercalation process. In contrast, Si, the second most abundant element in the Earth’s crust, has a maximum capacity of 3579 mAh g-1 because a Si atom can accommodate 3.75 Li atoms at room temperature.5 The high capacity of Si, however, is associated with massive volume (~300%) and crystal structural changes during the charging and discharging processes, which cause mechanical fracture of the electrode, and consequently detachment between the Si particles and irreversible capacity loss of the electrode.6 A variety of Si nanostructures and their composites have been reported for diminishing the volume expansion-induced failure and thus improving the stability of Si-based anodes, for example, Si nanoparticles (NPs),7 Si nanowires (NWs),8,9 Si nanotubes (NTs),10 3D nanoporous Si frameworks,11,12 yolk-
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shell Si nanoparticle-amorphous carbon structure,13 Si-carbon nanotube (CNT) and Si-graphene composites.14-19 Among all the reported nanostructures, an electrode architecture consisting of Si NWs grown vertically on a metallic current collector using chemical vapor deposition (CVD) method allows efficient volume change without pulverization and maintains good electrical contact between NWs and the current collector. Therefore, the electron transport along the length of each NW is provided, which leads to both increased specific capacity and improved cycling performance.8,20 However, the CVD method for growing Si NWs directly on the metallic current collector is so far limited by the difficulty to synthesize large quantities of NWs on the substrates (only ~200–250 µg cm-2 or ~0.75 mg h-1) which is insufficient for the practical applications.20 A recent work has reported a new CVD method to significantly enhance the Si NWs quantity on the stainless steel current collectors by growing Si NWs at a catalytic Au–CNT hybrid interface, and demonstrated high areal energy storage capacities (~1.0–1.6 mAhcm-2) and good electrochemical performance in half cells.17 In that case, however, the capacity decay of the Si NW– CNT electrode was clearly observed after 30 cycles, which could be ascribed to the small connection area between a single Si NW and the CNT network. The strong shear stress during LiSi alloying and de-alloying processes resulted in detachment of Si NWs from CNT network. Moreover, the formation of α−FeSi2 purity phase on the surface of metallic current collector during the high temperature NW growth process was also found.8,17 Hence, further refinement of the carbon–Si NWs
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hybrid material interfacial architecture and new approach are still required for the improvement of cyclic stability. Herein, we report a new method to improve the cycling performance and energy storage capacities of Si NWs electrode by growing Si NWs directly on the reduced graphene oxide (rGO) sheets. In our approach, Au nanoparticles decorated rGO (Au–rGO) hybrid materials were synthesized in ethylene glycol (EG) based on solvothermal reduction reaction, and then [111]oriented single crystalline and uniform Si NWs were grown directly on rGO surface by CVD method through vapor–liquid– solid mechanism. In the composite, Si NWs were wrapped in the rGO matrix, leading to enhanced multiple connections between Si NWs and rGO. The Si NWs–rGO composite anodes exhibited a highly stable cycling retention over 100 cycles with a high Li storage capacity of 2300 mAh g-1 at a C/3 rate in half cells. The salient cyclability and high reversible capacity are suggested to be credited to the enhanced contacts between Si NWs and the rGO overcoats, which as a mechanically robust and flexible matrix, accommodate the volume change of Si NWs, and maintain the structural and electrical integrity of the electrode.
2 Experimental GO was prepared from graphite following the modified Hummers’ method,21 and then decorated with Au nanoparticles using an EG–mediated solvothermal reduction process. In a typical synthesis, 20 mg GO powders were dispersed in 10 mL EG and 2 mL deionized water and then treated with sonication for 1 hour to get a brown suspension. Subsequently, 400 µL of HAuCl4 solution (50 mM) was added into the above GO solution. The mixed solution was transferred into a Teflon-lined stainless steel autoclave and kept at 220 °C for 12 hours inside a furnace. The black precipitate was separated by centrifugation and washed with acetone three times before drying at 120 °C for 2 hours to obtain the Au–rGO nanocomposites. Growth of Si NWs on rGO was carried out in a horizontal low-pressure CVD tube reactor. The as-synthesized Au–rGO powders were placed in the middle of the reactor and then heated up to 1000 °C under H2 flowing at a rate of 300 sccm. The Si NWs growth was performed by the introduction of SiCl4 and H2 at flow rates of 30 and 300 sccm, respectively, for 30 mins. The growth pressure was kept at ~180 mbar. In the case of control samples, Au thin films (2–5 nm in thickness) were deposited onto the single crystalline Si (100) wafers by electron beam evaporation. The prepared Au modified Si wafers were loaded into the horizontal CVD tube reactor. The control samples were synthesized in the same growth conditions as the Si NWs–rGO samples, resulting in the epitaxial growth of single crystalline Si NWs. Because epitaxial growth of single crystalline Si NWs was achieved on Si wafers, the control samples are denoted as Ep–Si NWs in this paper. The morphology and crystal structure of the samples were characterized by scanning electron microscopy (SEM, Philips XL 30 FEG) and transmission electron microscopy (TEM, Philips CM 20 FEG TEM operated at 200 kV). The X–ray diffraction (XRD) patterns were recorded using a Philips X'Pert MRD X–ray diffractometer with Cu Kα radiation. Thermal gravimetric analysis (TGA) was conducted on a TA instrument (TGAQ50, TA USA) under a static air atmosphere from room temperature to 1000 °C at a heating rate of 10 ºC min-1. Raman scattering spectra were measured with a Renishaw 2000 microRaman spectrometer using a laser wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) was performed to study the
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Journal Name DOI: 10.1039/C3NR05093A chemical composition of samples using a VG ESCALAB 220iXL surface analysis system equipped with a monochromatic Al Kα (1486.6 eV) X-ray source. Electrochemical measurements were performed using CR2032 coin–type cells assembled in an argon-filled glove box. For the preparation of the working electrode, Si NWs–rGO or Ep–Si NWs (scraped off from Si wafers), Super–P carbon black and polyimide binder (dissolved in N–methylpyrrolidone) were mixed in a mass ratio of 80:10:10. The resultant slurry was then uniformly coated on a Cu foil current collector and dried overnight under vacuum. The total mass loading of the electrode in this study for half cell tests was 1.5 mg cm-2. The electrode thickness was pressed to be 20 µm by calendaring. Electrochemical cells were assembled with Si NWs–rGO or Ep–Si NWs as the working electrode, metallic lithium foil as the counter electrode, and Celgard 2325 porous film as a separator. The electrolyte used in this work was a solution of 1.2 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethylene methyl carbonate (EMC) (3:7 by weight). In addition, 10 wt% of fluoro–ethylene carbonate (FEC) was added into the above electrolyte as an additive. Galvanostatic electrochemical experiments were carried out in a Maccor Series4000 battery testing system. The electrochemical tests were performed between 0.01–1.5 V vs. Li at room temperature.
Figure 1. Schematic illustration for the synthetic procedure of Si NWs– rGO composite.
3 Results and discussion The synthesis procedure for Si NWs–rGO composite is schematically illustrated in Figure 1. For anchoring Au nanoparticles uniformly on the surface of carbon matrix, GO rather than graphene was taken as the starting material due to its high solubility in water and the abundance of functional groups (such as carboxyl, carbonyl, hydroxyl, and epoxide) on its surface.22 GO with an ultrahigh surface area and an open porous texture can serve as an excellent building block to effectively support Au nanoparticles, affording high catalyst loading and thus enhancing VLS synthesis of Si NWs. Figure 2 displays representative SEM images of as-prepared Au–rGO composites, which were fabricated by a typical polyolsolvothermal method using EG as the solvent and reducing agent. The formation of Au nanoparticles on the planes and edges of rGO sheets is demonstrated in Figure 2a. The presence of oxygen functionalities at GO surface are believed to be responsible for the nucleation and growth of Au nanoparticles, as reported previously.23 Basically, the oxygen functional groups led to previous attachment of the free Au(III) ions in solution because of the electrostatic interactions. Afterward, EG promoted the subsequent reductions of Au(III) ions and GO simultaneously under the solvothermal condition in the sealed
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Teflon reactor, enabling the efficient growth of Au nanoparticles at the rGO surface. As a result, a homogeneous distribution of discrete Au nanoparticles with diameters of around 60 nm was achieved at the rGO surface with negligible nanoparticle agglomeration (Figure 2b).
Figure 2. SEM images of the as-prepared Au–rGO nanocomposites by polyolsolvothermal reaction. The diameter of Au nanoparticle is measured to be around 60 nm in Figure 2(b).
Growth of Si NWs on the Au–rGO matrix was conducted by means of low-pressure (ca. 180 mbar) SiCl4 decomposition in an H2 atmosphere at 1000 ºC. It has been demonstrated that the SiCl4 precursor was highly effective for the epitaxial growth of vertically aligned, single-crystalline Si NWs for photovoltaic application.24-26 Compared with SiH4 (the most frequently used precursor for CVD synthesis of Si NWs for LIBs),8,27,28 the use of SiCl4 precursor in the presence of hydrogen leads to the formation of hydrochloric acid (HCl), which may cause some desirable etching of the oxide layer on Si surface and result in the production of high-purity and oxide-free Si NWs. 25,26 Furthermore, for SiCl4, the growth temperatures typically range from about 800 ºC to well beyond 1000 ºC, compared to temperatures of about 400–600 ºC, typically for Si NWs growth in the presence of SiH 4. Growth at such elevated processing temperatures, the corresponding NWs are expected to be free of crystallographic defects,29 which benefits the application of the NWs in LIBs since the defects may lead to the irreversible lithium trapping in the initial lithiation process. 30 Figure 3 depicts the morphology and crystal structure of the Si NWs– rGO composite. The Si NWs are densely and randomly aligned
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Figure 3. (a) SEM image of the CVD–synthesized Si NWs–rGO composite. (b) TEM image of a single Si NW in the Si NWs–rGO composite. (c) SEM image of the as-prepared Si NWs array on Si (100) wafer. The inset in Figure 3(a) shows the diameter of a single Si NW in the composite. The inset in Figure 3(b) is its corresponding Fourier transform image. The inset in Figure 3(c) is the top-view image.
on the rGO surface and in-between rGO sheets. The diameter of most NWs is around 60 nm, as shown in the inset of Figure 3a, which is consistent with the original diameter of the Au nanoparticles in the Au–rGO hybrids. These observations suggest that the aggregation of Au nanoparticles on rGO is negligible during the high temperature synthesis process,
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probably due to the strong interaction between Au and rGO. Although the agglomeration of rGO might result in partial overlapping and coalescing of the rGO sheets, the crumpled rGO sheets were assembled in a porous structure, as shown in Figure 2, and the adsorbed solvent species might be removed from rGO surfaces during the initial CVD heating process in H2 atmosphere, leaving space for the gaseous Si precursors to access the inner rGO sheets' surfaces. As a result, the NWs grew up from discrete Au particles on rGO surfaces were well dispersed and in turn sandwiched between rGO sheets, forming rGO-sandwiched Si NWs nanocomposites (Figure S1). With this architecture, the multiple contacts between each NW and rGO sheets were provided and a three-dimensional porous network was formed. The thus-constructed Si NWs–rGO composite, as a representative model shown in Figure 1, can freely switch between a contracted state and an expanded state upon lithiation and delithiation. The void space between NWs and rGO can effectively accommodate the volume expansion of NWs and the flexible rGO can be adaptable to maintain the structural integrity and continuous conductive network of the electrode. The high-resolution TEM image of a single Si NW is shown in Figure 3b accompanying with a fast Fourier transform (FFT) pattern. The ordered crystalline structure with the interlayer distance of 3.14 Å demonstrates that the Si NWs are single crystal with the growth direction along [111] axis. The FFT pattern can be indexed as the diffraction along the [110] zone axis of crystalline Si and further verifies the growth of Si NWs along [111] direction.31 Recent studies have clearly shown that the lithiation of Si is highly dependent on the crystallographic orientations, resembling the anisotropic swelling of Si.32,33 The lithiation of Si has exhibited the largest swelling with the fastest lithiation speed along the [110] direction because the crystalline Si structure presents much larger interstitial spaces between atoms along the [110] direction than [111] and [100] directions.32 In this work, Si NWs were grown along [111] direction. Thus, there are six [110] directions perpendicular to the NW's axis arranged hexagonally, which can definitely promote fast diffusion of Li ions into NWs along its radial direction. For comparison, the single crystalline Si NWs used as the control samples (Ep-Si NWs) were prepared using the same CVD conditions with Au thin films directly deposited on single crystalline Si wafers as the growth catalyst. Figure 3c shows the self-oriented Si NWs array grown on Si (100) wafers. A rectangular networks is clearly formed, in which the Si NWs orient along four directions that make an angle of about 35º with the substrate. Taking into account that the Si (100) substrate has four [111] directions at an angle of 35.3º to the surface of the substrate on which orthographic projections of these directions form rectangular networks,34 these Si NWs are thus believed to grow along the [111] directions. This is the same with that observed on Au-rGO matrix and indicates that the Si NW growth direction is independent of the substrate type. After growth, the Ep–Si NWs were scraped off from Si wafers for the electrode preparation by the slurry method for further electrochemical performance characterization. The formation of Si NWs on the rGO matrix was further confirmed by XRD and Raman spectroscopy (Figure 4). XRD characterization shows that the obtained product is a mixture of Si and graphitic carbon (Figure 4a). A weak peak at 26.8º corresponds to the (002) plane reflections of graphitic carbon. The peaks at 28.6º, 47.5º, 56.3º, 68.5º, and 75.7º can be assigned to the (111), (220), (311), (400), and (331) reflections of Si (ICDD number 01-0791), respectively. Further structural
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Figure 4. (a) XRD pattern, (b) Raman spectroscopy, and (c) TGA–DSC curves of the Si NWs–rGO composite.
information is provided by the Raman spectroscopy by examining the intensity ratios among the D, G, and 2D peaks together with the full-width-at-half-maximum (FWHM) of the 2D peak (Figure 4b). The appearance of both G and 2D peaks signifies the readily formation of graphene after the reduction of GO under solvothermal condition. As the peak intensity ratio I2D/IG is less than 1 and the FWHM of the 2D band is ~80 cm-1, the obtained rGO scaffold should be the stacking of multilayer graphene sheets.35,36 The existence of D band suggests that the as-synthesized rGO scaffold is defective. The presence of such
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Journal Name defects helps the efficient access for Li ions to diffuse in and out of the Si NWs.37The strong Raman peak located at 520 cm-1 confirms the formation of crystalline Si. In this work, the small D band and high 2D band in Raman spectrum could be ascribed to the further reduction of rGO during CVD growth of Si NWs at 1000 ºC in H2 atmosphere, in which the oxygen moieties were removed and the sp2 network was restored due to structural relaxation. The further reduction of rGO during CVD process could be verified by XPS studies of rGO before and after Si NW growth, as shown in Figure S2. The atomic ratio of carbon to oxygen (C/O) increased from 4.05 to 25.04 after Si NW growth, suggesting that most of the attached epoxide and hydroxyl functional groups on rGO were removed. TGA (Figure 4c), carried out in air at a heating rate of 10 ºC min-1, was used to determine the chemical composition of the Si NWs–rGO composite. The result shows that the composite contains approximately 90.9 wt% Si and 9.1 wt% graphene based on the assumptions that the weight loss is due to the complete combustion of rGO in air and the partial oxidation of Si NWs is ignored in this temperature region. An exothermic peak centred at 700 ºC was observed in differential scanning calorimetry (DSC) plot, along with a pronounced weight loss, which corresponds to the graphene combustion reaction. After that, a gradual increase in weight and an endothermic peak (at ~900ºC) in DSC curve may be ascribed to the partial oxidation of Si to SiOx (x < 2) in air. Electrochemical properties of the Si NWs–rGO and Ep-Si NWs electrodes were characterized in the potential range from 10 mV to 1.5 V in CR2032-type coin cells, respectively. Since Si is a semiconducting material, Super P carbon is needed for the preparation of pure Si NWs electrode. In order to make a fair comparison, Super P carbon is also used to make Si NWsrGO electrode with the same electrode's composition. In this work, the specific capacities and current densities of the Si NWs-rGO electrode were calculated on the basis of the total weight of Si NWs and rGO. The cells were galvanostatically tested at a low current density of 300 mA g-1 (C/12, 1C = 3600 mA g-1) in the first cycle and at 600 mA g-1 (C/6) for the second cycle, followed by at 1.2 A g-1 (C/3) for the subsequent cycles. The first cycle of the Si NWs–rGO electrode shows the discharge (lithiation) and charge (delithiation) capacities of 3111 mAh g-1 and 2428 mAh g-1, respectively, giving an initial Coulombic efficiency (ICE) of 78% (Figure 5a). The irreversible capacity loss can be mainly attributed to the formation of a solid electrolyte interface (SEI) layers on the electrode surface and the consumption of lithium by the defects on the rGO.38,39 The reversible capacity of the Si NWs–rGO electrode is 1900 mAh g-1 for the third cycle at a rate of C/3, and gradually increases to 2300 mAh g-1 in the tenth cycle followed by a stable cyclability for the subsequent cycles. Upon cycling, a 2230 mAh g-1 capacity is retained after 100 full cycles. Thus the capacity retention after 100 cycles relative to the capacity value at the first cycle is 91.8%. The Ep–Si NWs control electrode shows the discharge and charge capacities of 3272 mAh g-1 and 2618 mAh g-1 for the first cycle at the same discharge/charge current rate as the Si NWs–rGO electrode. An ICE of 80% is achieved which is greater than that observed for the Si NWs–rGO electrode. However, upon cycling, noticeable and continuous capacity degradation is observed for the Ep–Si NWs electrode, which exhibited a specific capacity of 1246 mAh g-1 at the 82nd cycle, almost half of the corresponding value for the Si NWs–rGO electrode at the same rate. This highlights the great effect of rGO on improving the cyclic stability of Si NWs anode. The rate capabilities were also characterized in Si NWs-rGO composite electrode (Figure S3).
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Figure 5. (a) Cycling performance of the Si NWs–rGO and Ep–Si NWs electrodes, (b) voltage profiles and (c) differential capacity curves of the Si NWs–rGO electrode.
At current rates of C/12, C/6, C/3, 1C, and 3C (1 C = 3.600 mA g-1), the electrode exhibited reversible capacities of 2424, 2226, 1812, 1371 and 825 mAh g-1, respectively. The capacity could be recovered to 2407 mAh g-1 when the rate is returned to C/12. Moreover, after 100 cycles at C/3 rate, the Si NW in the composite was transformed to a porous structure, and its diameter increased to about 120 nm, as shown in Figure S4. However，the integrity of Si NW was kept and NW did not turn into broken species.
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In addition, the electrochemical properties of the Si NWs– rGO composite prepared by vacuum filtration of an aqueous Si NWs-GO dispersion followed by thermal reduction have been reported recently.39 In that case, the composite presented a relatively low (∼50%) Coulombic efficiency in the first cycle and a gradual capacity fading after 50 cycles, and the specific capacity retained 80% of its initial capacity after 100 cycles. The enhanced electrochemical properties of the Si NWs–rGO composite in this work could be attributed to the relatively low density of defects on the rGO as demonstrated in Raman results (Figure 4b) and the well dispersion of Si NWs since the NWs were directly grown from the discrete Au nanoparticle on the rGO surface. The restoration of the sp2 network of rGO during Si NW growth at 1000 ºC in H2 atmosphere could lead to higher conductivity than that of the conventional rGO prepared by chemical reduction or thermal reduction at 600–700 ºC in argon,18,39which is believed to benefit the electron transport in lithiation and delithiation reactions. Furthermore, the Si NWs– rGO composite electrode in this work was made using a traditional slurry coating method and more compatible to the current battery technology. The areal and volumetric energy storage capacities of the Si NWs–rGO composite electrode were 3.45 mAh cm-2 and 1725 mAh cm-3, respectively, which was calculated based on the electrode density of 1.5 mg cm-2, the calendared electrode thickness of 20 µm and the specific capacity of 2300 mAh g-1. These values are much greater than those for earlier-reported Si NWs electrodes8,17,20,27,30 and also very well lying in the range of capacity densities commonly found in the electrodes of commercial LIBs (between 2 and 5 mAh cm-2).40 The substantially enhanced cycling stability and reversibility of the Si NWs–rGO composite in comparison with the Si NWs grown directly on stainless steel current collector8 or the Si NWs grown on CNTs matrix17 may be ascribed to the unique composite architecture fabricated. First, the rGO overcoats in the composite function as a flexible and adaptable matrix to accommodate the contracting and expanding deformation of the sandwiched Si NWs, playing a critically important role in guaranteeing the structural integrity of the electrode upon cycling. A large connection area between NWs and rGO sheets could be maintained in the repetitive swelling/shrinkage processes (as illustrated in Figure 1). Second, the flexible rGO sheets which are tightly attached onto the NW surfaces, enables the formation of a three-dimensional porous network, thus significantly facilitating the diffusion and transport of lithium ions. Third, the lateral contact between NW and rGO sheet greatly shortens the electron transport distance from/to Si NW, and is highly favourable for improving the reaction kinetics, which is clearly better than the axial transport in the Si NWs deposited directly on metal current collector or CNTs matrix.41,42 Figure 5b exhibits voltage profiles of the Si NWs–rGO composite electrode in the first cycle for electrode activation together with the subsequent cycles. The first discharge curve displays a slope from 1.3 V to 0.1 V and a long flat plateau below 0.1 V, which corresponds to the irreversible reaction on the interface or host materials, and the lithium-alloying process of crystalline Si to form an amorphous LixSi phase, respectively.16,40 Afterwards, the discharge and charge curves show the characteristics of amorphous Si.43 The lithium insertion profiles at the 50th cycle and the 100th cycle almost overlap each other, which suggests a stable property in the discharge kinetics.44 Figure 5c displays differential capacity curves of the first, second, and third cycles for the Si NWs–
6 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C3NR05093A rGO electrode, from which information about the structural transformations during lithiation and delithiation can be obtained. The first cycle shows a large lithiation peak at 70 mV and a delithiation peak at 0.43 V, which correspond to a twophase region where crystalline Si reacts with Li to form an amorphous Li silicide and a two-phase region between two different amorphous lithiated phases, respectively.45 In the second cycle, lithiation into the amorphous Si shows three small peaks at 0.25 V, 90 mV and 40 mV, respectively, indicating the successively formation of three amorphous lithium silicide phases. Delithiation of the ultimate Li silicide phase occurs in a two-phase region to show a similar delithiation peak as in the first cycle. In the third cycle, with increased current density (from 600 mA g-1 to 1.2 A g-1), only two peaks at 0.25 V and 90 mV are observed in the lithiation process, in which 40 mV peak disappears mainly due to the reaction kinetics in a higher current rate. Consequently, two small peaks at 0.26 V and 0.44 V appear in the delithiation curve which can be ascribed to a two-phase region between two different amorphous Li silicide phases and a two-phase region between amorphous Li silicide and amorphous Si phases. It should be pointed out that in this case the delithiation peaks arise at relatively low voltage and the lithiation peaks at relatively high voltage compared with that observed in the [100]-oriented Si NWs prepared by metal-assisted chemical etching30 and Si submicron particles prepared by CVD approach.16 As discussed before, [111]-oriented Si NWs obtained in this work present six (110) facets parallel to the NW's axis arranged hexagonally, promoting fast lithium diffusion into/out Si NWs. At the same time, the rGO sheets cover large areas of the NWs, i.e., affording great areas of contact for charge transfer and, thus facilitating the electrode reaction. As a result, the Si NWs in this work show a substantially enhanced reaction kinetics.
4 Conclusions We described a facile approach to significantly enhance the cyclability and energy storage density of Si NWs as anode materials for high-performance LIBs by growing Si NWs directly on the rGO matrix. The Au nanoparticles decorated rGO hybrids were successfully prepared using solvothermal method and then used as an integrated growth catalyst to support the VLS growth of uniformly dispersed Si NWs on rGO surface and in between rGO sheets. The flexible and conductive rGO sheets accommodate the volume change of embedded Si NWs, maintaining the structural and electrical integrity of the electrode. As a result, the Si NWs–rGO composite electrode exhibited a stable cycling retention over 100 cycles at a high specific capacity of 2300 mAh g-1, simultaneously providing greatly improved cycling performance and areal/volumetric energy storage capacities compared with the Si NWs grown directly on metallic current collector or the Si NWs on the CNTs matrix. The strategy to encapsulate Si with adaptable matrices demonstrated here opens up a new avenue for developing high-performance Sibased anodes, and can be also extended to other fascinating anode and cathode materials that undergo large volume expansion.
Acknowledgements This work was financially supported by CityU Applied Research Grant, Hong Kong Special Administrative Region, China (ARG 9667075). Wu would like to thank Quanzhou
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ARTICLE
Nanoscale
Journal Name
Published on 07 January 2014. Downloaded by Heinrich Heine University of Duesseldorf on 07/01/2014 21:45:23.
“Tong-Jiang Scholar” program, Fujian "Min-Jiang Scholar" program, program for New Century Excellent Talents in University (NCET), and the Education and Scientific Research Foundation (Class A) for Young Teachers of Education Bureau of Fujian Province, China (Grant No. JA13263) for financial support.
Notes and references a
Centre of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P. R. China. E-mail: [email protected]; Fax: 852-3442-0538; Tel: 852-3442-7433. b Department of Chemistry, College of Chemistry and Life Science, Quanzhou Normal University, Quanzhou 362000, P. R. China. E-mail: [email protected]. c Institute of Advanced Materials, Nanjing University of Technology, Nanjing 210009, P. R. China. 1 2 3 4 5 6 7 8 9 10
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DOI: 10.1039/C3NR05093A
Direct growth of silicon nanowires on reduced graphene oxide provides an anode material for lithium ion battery with enhanced cyclability.
Nanoscale Accepted Manuscript
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