Article pubs.acs.org/Langmuir
Directing Silicon−Graphene Self-Assembly as a Core/Shell Anode for High-Performance Lithium-Ion Batteries Yuanhua Zhu,‡,§ Wen Liu,∥,§ Xinyue Zhang,‡ Jinchao He,‡ Jitao Chen,*,∥ Yapei Wang,*,‡ and Tingbing Cao† ‡
Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
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ABSTRACT: There is great interest in utilization of siliconcontaining nanostructures as anode materials for lithium-ion batteries but usually limited by manufacturing cost, their intrinsic low electric conductivity, and large volume changes during cycling. Here we present a facile process to fabricate graphene-wrapped silicon nanowires (GNS@Si NWs) directed by electrostatic self-assembly. The highly conductive and mechanical flexible graphene could partially accommodate the large volume change associated with the conversion reaction and also contributed to the enhanced electronic conductivity. The as-prepared GNS@Si NWs delivered a reversible capacity of 1648 mAh·g−1 with an initial Coulombic efficiency as high as 80%. Moreover, capacity remained 1335 mAh·g−1 after 80 cycles at a current of 200 mA·g−1, showing significantly improved electrochemical performance in terms of rate capability and cycling performance.
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INTRODUCTION
hybrid structures encourage us to develop analogous structures for anode materials while using an easy process. From a brief survey, graphene may serve as an excellent carbon source for preparing carbon/Si hybrid anodes with respect to its high mechanical strength, high chemical and thermal stability, and high electrical conductivity.32−37 Direct blending of graphene with Si cannot form a distinct association between them. As such, the giant interstitial volume between carbon and Si still leaves too much space to confine fatal expansion of anodes. The promise that graphene can tailor Si expansion cannot be fulfilled without their intimate association. In this contribution, we present a versatile strategy for decorating graphite layers on Si nanowires. Graphene oxide (GO), having plenty of hydroxyl, epoxy, and carboxyl moieties,38 was uniformly wrapped on positively charged Si nanowires by electrostatic self-assembling. The preassembled GO−Si NWs composite can be straightforward converted to reduced graphene nanosheets (GNS) @Si NWs composite by subsequent heat treatment.35,39 The as-prepared GNS@Si NWs hybrid material was characterized and demonstrated improving electrochemical performance as anode for high-performance LIBs. In addition, this system integrates nanoscale in diameter and microscale in length. It renders a possibility to bridge the research among nano, micro, and macro, fundamentally leading to a roadmap for balancing high performance and high throughput of anode materials, Scheme 1.
A critical demand for portable electronic devices has been rising sharply in recent years, in part because of the increasing global dependence on mobile information, entertainment, and communication. All of these devices are critically dependent on portable power sources. Lithium-ion batteries (LIBs) are one of the most popular types of rechargeable power sources in electronic devices. Efforts have been devoted to improving energy density, prolonging cycle life, stabilizing working voltage, and impairing memory effect for high-performance LIBs.1−7 New anode materials, such as silicon (Si), sulfur (S8), titanium dioxide (TiO2), and tin oxide (SnO),8−11 have been suggested. In terms of abundant amount in earth and high association ability with lithium ions, Si is attracting enormous attention for high-capacity anode materials. Its theoretical capacity of 4200 mAh·g−1 (ca. Li4.4Si) is 10 times higher than the graphite (372 mAh·g−1) which is currently used as an anode material.12−14 However, the increase of lithium insertion leads a significant volume expansion of Si, which causes pulverization of anodes and a rapid decrease in cycling stability.15,16 Creating nanovoids or hollow structures to Si routinely alleviates the anode pulverization resulting from volume expansion, while they are technically challenging to be scaled up.17−25 Another effective strategy is integrating carbon materials with nanostructural Si that addition of carbon materials can resist the volume expansion to some extent.26−28 There are examples of hybrid Si/carbon composites, such as physical blends of Si nanowires (Si NWs) with graphene nanosheets (GNS),29 chemical vapor deposition (CVD) of Si onto carbon surface,30 calcination of carbon-rich polymers on Si nanowires.31 The improvements of anode stability and energy density using © 2012 American Chemical Society
Received: November 4, 2012 Revised: December 12, 2012 Published: December 26, 2012 744
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h at 120 °C, the electrode was punched into disks with a diameter of 10 mm and pressed at 10 MPa thereafter. The electrolyte was comprised of 1.0 M LiPF6 in a mixture of ethylene carbonate (EC), dimethyl carbonate (DEC), and ethyl methyl carbonate (1:1:1, v/v) (LB315, Huarong Co. Ltd.). Celgard 2400 was used as a separator. Cells were assembled in an Ar-filled glovebox (Master 100 Lab, Braun, Germany) with less than 1 ppm of both oxygen and moisture. The cyclic voltammogram for GNS@Si NWs was measured from 2.0 to 0 V versus Li/Li+ at a 1 mV s−1 scan rate at an electrochemistry workstation (CHI660D, CH Instruments).
Scheme 1. Schematic Illustration of the Direct Self-Assembly Between GO Nanosheets and Si Nanowires
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RESULTS AND DISCUSSION Generation of Si−graphene hybrid with a core/shell structure was separated into three steps. First, Si-based nanowires were prepared and their surfaces modified with primary amines that endowed nanowires with positive charge. Second, negativecharged GO with carboxyl and hydroxyl groups was obtained via oxidation of natural graphite. Third, direct self-assembly based on electrostatic interaction was undergone between Si nanowires and GO nanosheets. There are several ways to produce Si nanowires: top-down approach and bottom-up approach, such as CVD, RIE, and solution process, which need very expensive instruments and very strict conditions.43,44 A simple top-down method of metalassisted chemical etching (MACE) was utilized to produce Si NWs.40−42 An array of Si NWs was created via an electroless process as described below. Typically, a p-type doped Si (100 0.3−10 Ω cm) wafer was soaked in a HF−AgNO3 mixture, causing the adjacent Ag ions to be quickly reduced on the Si surface
EXPERIMENTAL SECTION
Materials. Natural graphite powder (325 mesh) was purchased from Qingdao HuaTai Lubricant Sealling S&T Co. Ltd. Silicon wafer was purchased from Zhejiang Kaihua Lijing Ltd. (3-Aminopropyl)triethoxysilane (APTES) was bought from Alfa Aesar. Other reagents were purchased from TCI. All reagents described above were used as received without further purification. Preparation of Silicon Nanowires. Si nanowires were obtained via an electroless etching process.40−42 Commercial Si wafers (p-type, ⟨100⟩ oriented, 0.3−10 Ω cm) were cut to 4 × 4 cm pieces. Si wafer was ultrasonicated in acetone and ethanol at room temperature for 10 min to remove contamination from organic grease. Degreased Si substrate was heated in boiling Piranha solution (H2SO4/H2O2 3:1 v/ v) for 1 h. Then, the wafer was dipped into HF (5%) solution to remove the native oxide layer. Subsequently, the Si substrate was rinsed several times with deionized water. The treated Si wafers were immediately dipped into an Ag coating solution containing 10% HF and 0.04 M AgNO3. The solution was slowly stirred for 1 min under ambient air. After a thin uniform film of Ag was deposited, wafers were washed with deionized water to remove extra Ag+ and then immersed in an etchant composed of 4.6 M HF and 0.5 M H2O2. In this manner, Si NWs were formed on the Si wafer. After 240 min of etching in the dark at room temperature, the wafer was washed by a HNO3 solution (30%) to remove excess Ag. Finally, the black wafer was washed 3 times with deionized water and dried in the N2 atmosphere. Preparation of NH2-Terminated Silicon Nanowires. Pretreated Si wafers were cleaned in boiling Piranha solution (H2SO4/H2O2 3:1 v/v) for 1 h, rinsed thoroughly with deionized water, and dried. Then the OH-terminated Si wafers were immersed in a 1 wt % toluene solution of APTES for 24 h at room temperature, subsequently washed by THF and acetone three times, and dried in vacuum overnight. To remove the Si nanowires from the substrate, functionalized Si wafers were ultrasonicated in deionized water and free Si nanowires were collected from the solution. Preparation of Silicon Nanowires Core/Graphene Shell Structure. GO nanosheets were prepared from natural graphite (325 mesh, Qingdao) by a modified Hummers’ method.46,47 The solution of APTES-functionalized Si NWs (1 mg mL−1) was stepwise dropped into a diluted (GO) dispersion (1 mg mL−1) with slight ultrasonication for 0.5 h. The precipitate was freeze dried, followed by a thermal reduction process at 950 °C in a furnace for 10 h with a reductive atmosphere (95% N2 and 5% H2).35 Afterward, the graphene-wrapped Si NWs were soaked in 20% HF solution for 0.5 h and dried in vacuum for 24 h, which was thereafter called GNS@Si NWs. Characterization. The morphology and nanostructure were examined on a JEOL JSM-7401F field-emission scanning electron microscope (FESEM) and Hitachi T20 transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) was carried out by a Kratos Axis Ultra spectrometer with Al Kα monochromatized X-ray source. Galvanostatic charge and discharge cycling (LAND CT2001A, Wuhan Kinguo Electronics Co., Ltd.) were performed in the potential window from 0.0005 to 2.0 V vs Li/Li + with a two-electrode 2032 coin-type half cell, where Li metal foil was used as the counter electrode. The working electrode was prepared by mixing the GNS@Si NWs (80 wt %), carbon black (10 wt %), and PVDF (10 wt %) with solvent (NMP). After coating the slurry onto Cu foils and drying for 1
4Ag + + Si + 6F− → 4Ag + SiF6 2 −
The resulting Ag clusters uniformly distributing on Si surface started to be reoxidized into the ionic state in the presence of hydrogen peroxide (H2O2) 2Ag + H 2O2 + 2H+ → 2Ag + + 2H 2O
In case those Ag ions could be reduced by Si again, the Ag clusters acted as catalysts that accelerated the etching of Si in the HF solution. As such, Si underneath the Ag nanoparticles was indeed dissolved much faster than that without the coverage of Ag nanoparticles. The etching process of Si followed a total reaction as given below Si + n/2H 2O2 + 6HF → nH 2O + H 2SiF6 + (4 − n)/2H 2↑
The Ag-covering regions gradually went down after the etching reaction occurred for a while. The Si without Ag covering apparently suffered little from chemical etching; in turn, they stood as distinct nanowires. Ag-assisted etching could be rapidly quenched by removing Ag clusters from the Si surface. In this regard, desirable nanowires lengths are excellently controlled by the etching time, e.g., the average length of Si nanowires in this contribution is 10 μm with a diameter of about 200 nm after the etching reaction performs for 4 h, as shown in Figure 1a and 1b. The high degree of uniformity affords relative ease of surface functionalization for Si nanowires on the wafer. The interstitial areas among the Si nanowires could be easily fully filled with liquid chemical agents due to capillary forces. In an effort to functionalize nanowire surfaces with primary amines, the assynthesized Si NWs on a wafer were treated by a harsh Piranha 745
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Well-dispersed GO nanosheets were prepared by the modified Hummer’s method.46,47 A typical GO solution was presented in Figure 2a. Since GO has lots of functional groups, mainly hydroxyl and carboxylic acids (−OH, −COOH), GO exhibit unique affinity to water that the dispersed GO did not precipitate over months. Individual GO nanosheets were conceivably visualized by an atomic force microscope (AFM) and transmission electron microscope (TEM), as shown in Figure 2b and 2c, respectively. The thickness of a GO nanosheet with a single layer was evaluated as around 1 nm, which is entirely consistent with previous results.48,49 Self-assembly between Si nanowires and GO nanosheets occurs based on electrostatic interaction. As shown in Figure 3a, insoluble solids separated from the GO aqueous solution
Figure 1. (a) Top view and (b) side view of the as-prepared Si NWs (inset shows a high-magnification image). (c) Etched Si wafer after ultrasonication for 0.5 h. (d) Free Si NWs functionalized with primary amines (inset shows a digital photograph of the solution having dispersed Si NWs). (e) Molecular structure of APTES. (f) XPS spectra of blank Si NWs and Si NWs functionalized with APTES.
solution (H 2 SO 4 /H 2 O 2 3:1 v/v), (3-aminopropyl)triethoxysilane (APTES) dissolved in anhydrous toluene entirely wetted Si NWs, leading to a hydrolysis reaction where silane covalently bonds to hydroxyl groups.45 Free Si nanowires functionalized with primary amines (NH2−Si NWs) were collected via a vigorous sonication for 0.5 h. As shown in Figure 1d, APTES-functionalized Si nanowires could be well dispersed in water as the surface hydrophilicity had been extremely enhanced. As a control, nonfunctionalized Si nanowires precipitate from aqueous solution after a few minutes. It should be noted that successful surface functionalization was verified by X-ray photoelectron spectroscopy (XPS) as indicated by the appearance of the N 1s peak at 398 eV (Figure 1f).
Figure 3. (a) Photographic images of rapid self-assembly between Si nanowires and GO nanosheets in aqueous solution. (b) FESEM image. (c) Low- and (d) high-magnification TEM images of the core/ shell Si−graphene complex. (e) HRTEM image of NH2−Si NWs core/graphene shell structure (inset shows electron diffraction of single-crystal Si).
upon dropwise addition of NH2−Si NWs solution. This precipitation was due to an electrostatic attraction that depressed the solubility of both GO nanosheets and Si NWs.
Figure 2. (a) Photograph image of dispersed GO solution; the concentration of GO nanosheets is 0.5 mg mL−1. (b) AFM and (c) TEM images of GO nanosheets. 746
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Figure 4. (a) Galvanostatic discharge−charge profiles of the first three cycles for graphene-wrapped Si NWs. (b) Cyclic voltammetry (CV) differential capacity curves of GNS@Si NWs electrode. (c) Rate capability of the GNS@Si NWs composite compared with Si NWs. (d) Cycling performance of the GNS@Si composite compared with bare Si NWs.
Coulombic efficiency at 80%. The irreversible capacity observed in the first cycle can be attributed to formation of a solid electrolyte interface (SEI). It is noteworthy that the GNS@Si NWs still exhibit a flat charging voltage plateau after 80 cycles, showing excellent cyclability. A typical cyclic voltammetry (CV) characterization of the GNS@Si NWs composite is shown in Figure 4b. A discharge potential plateau around 0.79 V is clearly observed in the first discharge curves, corresponding mainly to formation of SEI film and subsequent possible irreversible accumulation of Li+ in the active Si particles. Below 0.3 V, a sharp reduction peak for insertion of Li+ into Si could be observed, and subsequently, the extraction process occurred at 0.39 and 0.53 V with a broad peak; these redox peaks were attributed the alloying/dealloying of Li with active Si NWs. In the following cycles, the cathodic peak of 0.6 V disappears, indicating a stable SEI layer has maintained. The cathodic peak at 0.17 V gradually evolved, corresponding to generation of Li−Si alloy phases. The anodic part shows two peaks at 0.39 and 0.53 V, which could be assigned to dealloying of Li−Si alloys, and become more distinct after the following cycles. Figure 4c compared the rate performance of GNS@Si NWs and bare Si NWs at various current densities. The cell was first cycled at 200 mA·g−1 for 10 cycles, in which a stable reversible capacity of about 1649 mAh·g−1 was observed. At the high current densities of 500, 1000, 5000, and 200 mA·g−1, the GNS@Si NWs can still deliver high reversible specific capacities of 1090, 701, 441, and 1251 mAh·g−1, respectively. In contrast, the bare Si NWs showed sharply deteriorating capacity with the increasing charging current. When the charging current reached 1000 mA·g−1, the bare Si NWs cannot deliver lithiation capacity at
As a consequence, the self-assembled complex was solidified by a powerful means of lyophilization which can not only remove water but also enhance the electrostatic association. The dry solid was calcinated at a high temperature under a flow of N2/ H2 gas mixture (95:5), enabling conversion of GO to graphene. Noting that a native SiO2 layer may exist on the Si nanowire, the calcinated hybrid material was etched by a diluted HF solution to remove SiO2. Subtitle structure of graphene on Si NWs was identified by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). As shown in Figure 3b, an intriguing thin layer apparently exists on a Si nanowire. Such a core/shell structure could be also observed by TEM (Figure 3c and 3d). In addition, the HRTEM image indicates that there is a clear boundary between Si NWs and the graphene layer. The thickness of the graphene layer is approximately 9−10 nm (Figure 3e). Both the rate of electron diffusion and the conductivity of Si nanowires coated with graphene shells are assumed to be improved. In addition, the core/shell structure is also envisioned to accommodate the Si expansion during lithiation and delithiation cycling. To validate these hypotheses, the core/ shell complex as a function of anode electrode was evaluated in a sealed 2032 coin cell with Li foil as the counter electrode. Figure 4a shows the 1st, 2nd, 3rd, 10th, 25th, 50th, and 80th charge−discharge curves of GNS@Si NWs electrode at a current density of 200 mA·g−1 in a voltage range of 0.005−2.0 V vs Li/Li+. The initial small plateaus at 0.8 V and long slope profiles of electrode are similar to bare Si electrode that were reported previously. The GNS@Si NWs discharged a capacity of 2142 mAh·g−1 and a charge capacity of 1649 mAh·g−1 with 747
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all. Figure 4d shows the cycling performance of various anodes at a current density of 200 mA·g−1. The bare Si NWs electrode shows dramatic capacity fading, with charge capacity at 250 mAh·g−1 after 80 cycles. With the graphene wrapping, GNS@Si NWs exhibit great enhanced stability, maintaining a capacity of 1335 mAh·g−1 after 80 cycles. We assume that the enhanced reversibility and rate capability of GNS@Si NWs is attributed mainly to the good mechanical flexibility of graphene that it can readily accommodate the large volume change associated with the conversion reaction and preventing cracking of the Si NWs. Use of graphene can also contribute to the enhanced electronic conductivity and act as a three-dimensional conductive network for the GNS@Si NWs electrode, which could promote electron transfer during the lithiation and delithiation processes.
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CONCLUSION In summary, we demonstrate a facile, low-cost, and scalable approach to prepare GNS@Si NWs for high-performance LIBs. The approach has enabled preparation of a well-defined selfassembly structure of Si nanowires with spatially defined wrap of graphene. This unique structure enhances electron diffusion and conductivity of Si nanowires. More importantly, the core/ shell self-assembly structure can accommodate Si expansion during lithiation and delithiation cycling, thus significantly improving the discharge capacity and prolonging the anode life in contrast to bare Si anodes materials. These Si nanowires/ graphene-based structures and the self-assembly method used to produce them represent a new paradigm for integrating nanoscale materials properties into microscale structures, opening opportunities in generating functional nanomaterials for the next generation of LIBs. It is envisioned that the capacity retention can be further improved by decreasing the diameter scale of the Si nanowires and introducing extra space to reversibly tailor the volume expansion.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest. † In memory of Prof. Cao. Deceased on March, 16, 2012
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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (20334010, 20473045, and 20574040).
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