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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 39371-39379

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Two-Dimensional Porous Sandwich-Like C/Si−Graphene−Si/C Nanosheets for Superior Lithium Storage Weiqi Yao,† Jie Chen,† Liang Zhan,*,†,‡ Yanli Wang,† and Shubin Yang*,§ †

State Key Laboratory of Chemical Engineering, Key Laboratory for Specially Functional Polymers and Related Technology of Ministry of Education, Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China § Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing 100191, China S Supporting Information *

ABSTRACT: A novel two-dimensional porous sandwich-like Si/ carbon nanosheet is designed and successfully fabricated as an anode for superior lithium storage, where a porous Si nanofilm grows on the two sides of reduced graphene oxide (rGO) and is then coated with a carbon layer (denoted as C/Si−rGO−Si/C). The coexistence of micropores and mesopores in C/Si−rGO−Si/ C nanosheets offers a rapid Li+ diffusion rate, and the porous Si provides a short pathway for electric transportation. Meanwhile, the coated carbon layer not only can promote to form a stable SEI layer, but also can improve the electric conductivity of nanoscale Si coupled with rGO. Thus, the unique nanostructures offer the resultant C/Si−rGO−Si/C electrode with high reversible capacity (1187 mA h g−1 after 200 cycles at 0.2 A g−1), excellent cycle stability (894 mA h g−1 after 1000 cycles at 1 A g−1), and high rate capability (694 mA h g−1 at 5 A g−1, 447 mA h g−1 at 10 A g−1). KEYWORDS: silicon, graphene, anode material, lithium-ion batteries, sandwich-like structure cm−1), and then, an inferior rate performance is inevitably achieved.21 For a solution to the volume expansion of bulk Si, various nanostructures have been designed for nanoscale Si, such as Si nanowires,22 Si nanotubes,23 Si nanosheets,24 and hollow Si nanospheres.25 The nanostructured Si ensures a short pathway for Li ion diffusion and electron transportation. For example, Cui et al. synthesized Si-nanolayer-embedded graphite/carbon hybrids (SGCs); the electrode achieved a stable reversible capacity of 517 mA h g−1 with a high initial Coulombic efficiency (ICE) of 92%.26 Wang et al. has designed a kind of Si nanowire coated with graphene nanosheets and reduced graphene oxide (denoted as SiNW@G@rGO); the electrode shows a high reversible specific capacity of 1600 mA h g−1 at 2.1 A g−1 and excellent rate capability (500 mA h g−1 at 8.4 A g−1).27 Cui et al. has reported double-walled silicon nanotubes, and the electrode exhibits high reversible capacity of 600 mA h g−1 at 12 C and ultralong cycle stability (88% capacity retention after 6000 cycles).28

1. INTRODUCTION Lithium-ion batteries (LIBs) have been extensively utilized in portable electronics because of their relatively high energy density, long life cycle, and environmental friendliness.1 Because of the low theoretical specific capacity of traditional graphite anodes (372 mA h g−1), commercial LIBs still cannot satisfy the urgent requirements of renewable energy storage for electric vehicles (EVs), hybrid electric vehicles (HEVs), and other energy conversion systems.2 Therefore, many endeavors have been concentrated on exploring novel anode materials, such as transition metal oxides (Co3O4,3 Fe3O4,4 MnO2,5 and SnO26), molybdenum disulfide (MoS2),7 tin (Sn),8 and silicon (Si).9−18 Among the novel candidates, Si has been considered as the most attractive one for lithium storage, because of the high theoretical capacity (Li4.4Si ≈ 4200 mA h g−1) and relatively low discharge potential (about 0.1 V versus Li/Li+).19 However, practical implementation of bulk Si suffers from dramatic volume expansion (>300%) during the delithiation/ lithiation process, which results in serious pulverization and repeated formation of an unstable solid−electrolyte interphase (SEI) film, leading to rapid capacity decay.20 Meanwhile, bulk Si also endures a low intrinsic electric conductivity (10−3 S © 2017 American Chemical Society

Received: August 7, 2017 Accepted: September 22, 2017 Published: September 22, 2017 39371

DOI: 10.1021/acsami.7b11721 ACS Appl. Mater. Interfaces 2017, 9, 39371−39379

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustrations of the fabrication process for the 2D sandwich-like C/Si−rGO−Si/C nanosheets.

(1187 mA h g−1 at 0.2 A g−1 after 200 cycles), good cycle stability (894 mA h g−1 at 1 A g−1 after 1000 cycles), and high rate capability (694 mA h g−1 at 5 A g−1, 447 mA h g−1 at 10 A g−1). For exploration of the effects of the coated-carbon layer toward the electrochemical performance of nano-Si, another type of carbon/rGO electric conductive network was designed for comparison, where porous carbon and mesoporous Si simultaneously and directly grew on the two sides of rGO (denoted as Si@C−rGO−Si@C).

For enhancement of the low electrical conductivity of nanostructured silicon, various conductive agents (such as Ag,29 Cu,30 carbon,31 and conductive polymer32) were further coated on the surface of nanoscale Si33 or embedded in nanostructured Si.34 Among the tremendous efforts, carbonaceous materials have been extensively adopted as the conductive agents, such as carbon nanotubes,35 graphene,36 porous carbon and hollow carbon spheres,37 because of their high electrical conductivity and special physicochemical properties. For example, Zhao et al. has reported a kind of material of silicon nanoparticles (NPs) embedded into mesoporous carbon, and the ordered mesoporous Si/C electrode shows high reversible specific capacity and good cycle stability (1480 mA h g−1 after 1000 cycles at 2 A g−1).38 Hu et al. has developed silicon NPs simultaneously coated by a thin SiOx and carbon layer; the Si@SiOx/C electrode showed a stable reversible capacity of 1100 mA h g−1 at 150 mA g−1 after 60 cycles.39 Kang et al. synthesized novel carbon-coated Si NPs on reduced graphene oxide (rGO) nanocomposites via layer-bylayer assembly; the Si@C-rGO electrode exhibited a stable reversible capacity of 930 mA h g−1under a current density of 0.3 A g−1 after 400 cycles.40 Yang et al. prepared amorphousTiO2-coated commercial Si NP core−shell nanostructures; the Si@a−TiO2 electrode delivered a high ICE of 86.1%, high reversible capacity (1720 mA h g−1 after 200 cycles at 420 mA g−1), and excellent rate capability (812 mA h g−1 at 8.4 A g−1).41 Herein, a novel two-dimensional (2D) mesoporous silicon nanofilm embedded into three-dimensional (3D) conductive networks of carbon−graphene−carbon was designed and successfully fabricated, which was denoted as C/Si−rGO−Si/ C. In detail, a 2D sandwich-like mesoporous Si−rGO−Si was initially fabricated, and then coated with a porous carbon layer. The porous carbon layer grew on the two sides of graphene forming a conductive network, which not only can make up the shortage of nanoscale Si with a high electric conductivity, but also can provide enough void spaces for the volume expansion of nano-Si during the Li+ delithiation/lithiation process. Meanwhile, the 2D porous Si nanofilm can offer a short pathway for Li ion diffusion and electron transportation. As a consequence, its unique nanostructures offer the resultant C/ Si−rGO−Si/C electrode a high reversible specific capacity

2. EXPERIMENTAL SECTION 2.1. Preparation of the C/Si−rGO−Si/C Nanosheets. The sandwich-like SiO2−rGO−SiO2 nanosheets were prepared as in our previous works,42 and the Si−rGO−Si nanosheets were synthesized via a magnesiothermic reduction reaction (MRR). In a typical experiment, the SiO2−rGO−SiO2 nanosheets, magnesium (Mg) powder, and NaCl (Mg/SiO2−rGO−SiO2/NaCl = 1:1:10, weight ratio) were mixed and ground. The mixture was loaded and sealed into a stainless Swagelok-type apparatus under an argon-filled glovebox, then quickly transferred and sealed in a tube furnace and heated to 650 °C for 2 h in argon, and the powder was next immersed in HCl (1 M) and HF (10 wt %) solution. Next, the suspension was further centrifuged, then washed by DI (deionized) water, and subsequently dispersed in ethanol solution for further use without drying. The C/Si−rGO−Si/C nanosheets were fabricated through in situ polymerization of cyclotriphosphazene-4,4′-sulfonyldiphenol (PZS) in the Si−rGO−Si suspension. In detail, 0.05 g of hexachlorocyclotriphosphazene (HCCP), 0.215 g of 4-4′-sulfonyldiphenol (BPS), and 10 mL of triethylamine (TEA) were slowly added into the above mixture, stirred at 40 °C for 10 h, washed three times, and then vacuum-dried at 80 °C overnight to achieve PZS/Si−rGO−Si/PZS intermediate. Finally, the dried products were carbonized at 900 °C for 3 h under nitrogen, resulting in the 2D C/Si−rGO−Si/C nanosheets. For comparison, Si@C−rGO−Si@C was also synthesized, and the detailed synthesis process is illustrated in the Supporting Information. 2.2. Characterization. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were performed on JEOL 7100F and JEOL 2100F devices, respectively. Thermogravimetric analysis (TGA) was carried out on the TA Instrument Q600 analyzer under air atmosphere. Nitrogen adsorption−desorption isotherms at 77 K were achieved by Quadrasorb SI analysis, and the pore size distribution was analyzed by the Barrett−Joyner−Halenda (BJH) method. X-ray diffraction (XRD) measurements were carried out on a Rigaku D/Max2550 instrument utilizing Cu Kα radiation (λ = 0.154 06 nm) and 2θ ranging from 10° to 80°. Raman measurements 39372


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ACS Applied Materials & Interfaces were performed on the Renishaw system by using an argon-ion laser of 514 nm. The X-ray photoelectron spectroscopy (XPS) measurements determined the surface chemistry compositions of the as-prepared sample on the Axis Ultra DLD instrument. The tap density of the sample was measured by the tap density meter (GeoPyc 1360, Micromeritics). 2.3. Cell Assembly and Electrochemical Measurements of the Anode Materials. The anode electrode was fabricated from mixing active materials (C/Si−rGO−Si/C or Si@C−rGO−Si@C), Super C, and PVDF (binder) in a weight ratio of 8:1:1 and dissolving them in the N-methyl-2-pyrrolidone (NMP) solvent. The homogeneous slurries were then coated on a pure Cu foil and dried at 80 °C overnight. The working electrode was punched in a diameter of 12 mm disks and dried in vacuum at 80 °C. The loading mass of the active materials is traditionally about 0.8−1.0 mg cm−2. For comparison, electrochemical performance with different mass loadings of 0.50, 0.98, 1.53, and 2.03 mg cm−2 were also measured. The 2016 standard coin-type cell assembly was operated in a glovebox filled with argon. Li metal foil and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) with a volume ratio of 1:1 were used as the reference electrode and electrolyte, respectively. The galvanostatic discharge−charge measurements were performed on a LAND CT2001A system within a voltage window 0.01−2.0 V (versus Li/ Li+). The Arbin electrochemical workstation (BT2000) was used for the cyclic voltammetry (CV) tests with a scan rate of 0.1 mV s−1. The Gamry Instrument was used to examine the electrochemical impedance spectroscopy (EIS) tests with the frequency ranging from 105 to 0.01 Hz. 2.4. Electrochemical Measurements of the Full Cell. The 2025 coin-type full-cell LIB was assembled in a glovebox, where the C/Si− rGO−Si/C nanosheets and commercial lithium cobalt oxide (LiCoO2) were utilized as anode and cathode, respectively. The cathode electrode was fabricated by mixing LiCoO2, Super C, and PVDF in a weight ratio of 8:1:1 in NMP to form a homogeneous slurry, and the slurries were coated on aluminum foil. Before the full-cell assembly, the C/Si−rGO−Si/C anode electrode was preactivated in a half cell. The weight ratio between cathode and anode materials was about 10:1 to match the specific capacity of anode materials. The galvanostatic discharge−charge full cells were tested within a voltage window 2.5− 4.2 V, and the specific capacity of the electrode was calculated according to weight of the cathode active materials.

Figure 2. SEM (a), TEM (b), and HRTEM (c) images and EDS spectrum (d) of the 2D sandwich-like C/Si−rGO−Si/C nanosheets. (e) Bright field STEM images of the C/Si−rGO−Si/C nanosheets and the corresponding elemental mappings of C (f) and Si (g) elements.

observed from the inset TEM image in Figure 2b. The above results indicate that the structures of C/Si−rGO−Si/C inherit those of both SiO2−rGO−SiO2 (Figure S3) and Si−rGO−Si (Figure S4) nanosheets with a two-dimensional and sandwichlike structure. Importantly, a carbon layer was clearly observed and coated on the surface of Si−rGO−Si nanosheets (Figure 2b) with a thickness of about 10 nm (Figure 2c). Then, a continuous conductive network is formed by the coated carbon layer grown on the two sides of rGO. Figure 2c also reveals the wormlike mesoporous structure for the coated carbon layer and nanoscale Si, where the typical lattice fringe with the interplanar distance of 0.31 nm corresponds well to the (111) planes of the diamond-structured silicon. The inset image in Figure 2c displays the selected-area electron diffraction (SAED) patterns of crystalline Si. The spotted diffraction rings from inside to outside could be indexed to the (111), (220), and (311) planes of silicon, respectively, identifying the good crystalline nature of silicon.46 The EDS (Figure 2d) spectrum analysis displays that the composite consists of Si, C, and O elements. It is noteworthy that a weak oxygen signal emerges in the sample area, suggesting that a SiOx (1 < x < 2) film is produced on the surface of nanoscale Si.47 The bright field (BF) STEM image (Figure 2e) combined with its elemental mapping images (Figure 2f,g) further reveal the C and Si elements homogeneously distributing in C/Si−rGO−Si/C nanosheets. Interestingly, the SiO2@C−rGO−SiO2@C nanosheets possess the similar 2D and independent dispersed morphology (Figure S5); however, the resultant MRR product Si@C−rGO−Si@C hybrids were observed with a twist or fragment structure, unlike

3. RESULTS AND DISCUSSION Figure 1 schematically illustrates the typical synthetic procedure of the 2D sandwich-like mesoporous C/Si−rGO−Si/C nanosheets. Two-dimensional mesoporous SiO2−GO−SiO2 nanosheets were initially prepared through hydrolysis of tetraethyl orthosilicate (TEOS) on both sides of GO via the assistance of the cationic surfactant (cetyltrimethylammonium bromide, CTAB). Subsequently, SiO2−GO−SiO2 was heat-treated at 800 °C under nitrogen atmosphere, during which graphene oxide was reduced into graphene, giving 2D sandwich-like SiO2−rGO−SiO2 nanosheets.43 Afterward, in situ magnesiothermic reduction was carried out at 650 °C utilizing NaCl as the heat scavenger under argon protection,44 resulting in 2D sandwich-like mesoporous Si−rGO−Si nanosheets. Finally, cyclotriphosphazene-4,4′- sulfonyldiphenol (PZS,45 see Figure S1 in the Supporting Information) was coated onto the surface of Si−rGO−Si and then carbonized at 900 °C for 3 h, giving the final product C/Si−rGO−Si/C nanosheets. For comparison, other nanostructured 2D SiO2@C−rGO−SiO2@C nanosheets and Si@C−rGO−Si@C hybrids were synthesized as illustrated in Figure S2. As shown in Figure 2a, a number of monodispersed, 2D nanosheets can be observed with a lateral size ranging from 100 nm to several micrometers. Meanwhile, the sandwich-like structure of the resultant C/Si−rGO−Si/C was also clearly 39373


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Figure 3. Nitrogen adsorption−desorption isotherm (a) and its corresponding pore size distribution (b) of the samples. TGA curves (c) and Raman spectrum (d) of C/Si−rGO−Si/C nanosheets and C@Si−rGO−Si@C hybrids.

Figure 4. (a) XRD patterns of the as-synthesized C/Si−rGO−Si/C nanosheets and C@Si−rGO−Si@C hybrids. XPS (b), C 1s (c), and Si 2p (d) spectra of the C/Si−rGO−Si/C nanosheets.

the 2D nanostructure (Figure S6), which can be attributed to the reaction heat unable to transfer immediately during the rapid exothermic MRR process. The nitrogen adsorption−desorption isotherm of the C/Si− rGO−Si/C nanosheets deliver the typical type-IV curve with a hysteresis loop (Figure 3a), predicting the coexistence of micropores and mesopores (Figure 3b). The pore size of the C/Si−rGO−Si/C nanosheets distribute in a range 0.7−25 nm with an average pore size of 3.79 nm. Although the resultant

C@Si−rGO−Si@C was also derived from the same 2D sandwich-like SiO2−rGO−SiO2, its nitrogen adsorption− desorption isotherms show a typical type-I curve (Figure 3a), and its pore size distribution ranges from 0.7 to 2 nm with an average pore size of 1.56 nm (Figure 3b). During the polymerization process of PZS, the formed PZS would exhaust some space of mesopores in the 2D SiO2−rGO−SiO2, and thus the final C@Si−rGO−Si@C sample only has micropores. The C/Si−rGO−Si/C nanosheets possess a specific surface area of 39374


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Figure 5. (a) CV curves of the initial five cycles of the C/Si−rGO−Si/C electrode with a scan rate of 0.1 mV s−1. (b) Galvanostatic discharge− charge profiles of the C/Si−rGO−Si/C electrode under a current density of 200 mA g−1. (c) Cycling performance of the C/Si−rGO−Si/C and Si@ C−rGO−Si@C electrodes at 200 mA g−1. (d) Rate capacities of the C/Si−rGO−Si/C and Si@C−rGO−Si@C electrodes at various rates. (e) Longterm cycle performance of the C/Si−rGO−Si/C electrode in 1000 cycles at 1 A g−1.

532.5 m2 g−1, which is higher than those of both Si−rGO−Si (458.9 m2 g−1, Figure S7) and Si@C−rGO−Si@C (388.4 m2 g−1). For determination of the silicon and carbon contents of the C/Si−rGO−Si/C nanosheets and the Si@C−rGO−Si@C hybrids, thermal gravimetric analysis (TGA) was carried out in air flow (Figure 3c). The weight loss lower than 300 °C should be ascribed to the removal of adsorbed water in the samples,48,49 and the distinct weight loss in the range 500− 600 °C is ascribed to the combustion of graphene and coated carbon into CO2. Interestingly, a distinct weight increase was detected after 650 °C, which should be ascribed to the oxidation reaction between silicon and O2, forming certain SiOx (1 < x < 2) or SiO2.50 The silicon content and total mass fractions of the graphene and coated carbon in the C/Si− rGO−Si/C nanosheets are 76.6 and 21.3 wt %, respectively (Figure 3c and Figure S8). Additionally, the carbon content of SiO2@C−rGO−SiO2@C nanosheets was calculated to be about 28 wt % (Figure S9). The Raman spectra of the C/ Si−rGO−Si/C nanosheets and Si@C−rGO−Si@C hybrids are shown in Figure 3d; three characteristic peaks at 515, 1360 (D band), and 1590 (G band) cm−1 are observed, corresponding to crystalline Si, and sp3 and sp2 carbon atoms, respectively.51 The ratio of relative intensity ID/IG manifests the degree of graphitization, defects, as well as the domain size of

graphitization.52 The ID/IG value of the C/Si−rGO−Si/C nanosheets is about 1.01, which is lower than those of the Si@ C−rGO−Si@C hybrids (1.10) and Si−rGO−Si nanosheets (1.06, Figure S10), indicating that the excellent graphitic crystallinity of the coated carbon layer results from pyrolysis of PZS and fewer defects in the C/Si−rGO−Si/C nanosheets. Figure 4a exhibits the X-ray diffraction (XRD) patterns of asprepared C/Si−rGO−Si/C nanosheets and the Si@C−rGO− Si@C hybrids. In the C/Si−rGO−Si/C sample, the major five diffraction peaks occurred at 28.5°, 47.3°, 56.1°, 69.5°, and 76.6° corresponding to (111), (220), (311), (400), and (331) planes of the crystallization silicon (JCPDS 27-1402),53 which are the same as those of its counterpart Si−rGO−Si (Figure S11). An additional broad peak around 23° is also observed, corresponding to the amorphous carbon-coated layer originating from PZS during the pyrolysis process. However, after the MRR process, an additional crystalline diffraction peak at 35.1° for the Si@C−rGO−Si@C hybrid can be indexed to the (111) plane of silicon carbide (β-SiC).54 The micropore pore size is too small to transport aggregated heat produced during the rapid exothermic reaction (MRR), so the local reaction temperature will be extremely high among the synthesized Si@C−rGO−Si@C hybrids; forming a β-SiC interface between porous carbon and nanoscale Si is inevitable. The chemical 39375


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Figure 6. (a) Nyquist plots of the C/Si−rGO−Si/C and Si@C−rGO−Si@C electrode. (b) Cycle performance of the C/Si−rGO−Si/C−LiCoO2 full cell under a current density of 0.5C (1C = 140 mA h g−1 of LiCoO2).

chemical mechanisms as discussed in the CV curves. Figure 5c shows that the C/Si−rGO−Si/C electrode exhibits better cycling performance and a higher stability capacity than the Si@ C−rGO−Si@C electrode at 200 mA g−1. The C/Si−rGO−Si/ C electrode exhibited the first discharge and charge capacity of 2281 and 1553 mA h g−1, respectively, corresponding to an ICE of 68%. The irreversible capacity loss of the first cycle should be related to the formation of the SEI layer and irreversible phase. Because the SiOx species and coated amorphous carbon would interact with Li+, an irreversible phase forms (e.g., Li2O or Li4SiO4).60−62 The reversible capacity can be obtained as high as 1187 mA h g−1 after 200 cycles, and this value is higher than those of both Si−rGO−Si (711 mA h g−1, Figure S13) and Si@ C−rGO−Si@C (514 mA h g−1). The result reveals that the coated carbon layer of the C/Si−rGO−Si/C electrode will improve the cycle stability of Si−rGO−Si electrodes, because it is beneficial to form the stable SEI film, and its thickness (10 nm) is high enough to effectively alleviate mechanical stress caused from the volume expansion during the lithiation/ delithiation process. For the Si@C−rGO−Si@C electrode, the unavoidably produced β-SiC will increase the irreversible capacity loss because of the electrochemically inactive nature,63 and its micropores are too small to resolve the huge expansion of Si during the delithiation/lithiation process. Because of the space-efficient packing, the tap density of C/Si−rGO−Si nanosheets (0.69 g cm−3) is about four times higher than that of commercial Si NPs (0.15 g cm−3). The rate capabilities of C/Si−rGO−Si/C and Si@C−rGO−Si@C electrodes were then examined under different current densities. As exhibited in Figure 5d and Figure S14, the reversible capability of C/Si− rGO−Si/C electrodes is up to 1189, 1051, 848, and 694 mA h g−1 at 0.5, 1, 2, and 5 A g−1, respectively. Even under a high current density of 10 A g−1, the high reversible capacity of 447 mA h g−1 can be attained. When the rate finally recovered to 0.2 A g−1, the capacity of 1165 mA h g−1 can be recovered with a little capacity loss. Notably, the C/Si−rGO−Si/C electrode obviously delivers a better rate performance than the Si@C− rGO−Si@C electrode, suggesting that employed graphene sheets and the high-conductive carbon layer formed the extraordinary hierarchical sandwiched conductive network, guaranteeing the overall electrode integrity and continuity; meanwhile, this facilitated the fast ion diffusion and transportation. Additionally, the coexistence of micropores and mesopores not only can be beneficial to the Li+ diffusion especially at the high current density, but also facilitate a decrease of the polarization of the electrode. As depicted in Figure 5e, the C/Si−rGO−Si/C anode also shows superior cycle stability under a high current density of 1 A g−1. After an

compositions of the C/Si−rGO−Si/C nanosheets were probed by X-ray photoelectron spectroscopy (XPS), as depicted in Figure 4b, and the four peaks at 99.7, 151.7, 284.6, and 532.5 eV are detected in the XPS survey spectrum of C/Si−rGO−Si/ C nanosheets, corresponding to Si 2p, Si 2s, C 1s, and O 1s, respectively.55 According to the C 1s spectrum (Figure 4c), the peaks at 284.6, 286.2, and 289.2 eV are related to CC (73.08%), CO (18.09%), and CO (8.83%), respectively. The low intensity of CO and CO bonds indicates that most of the oxygen-containing functional groups in the GO and amorphous carbon have been pyrolyzed during the carbonization process. The fitted Si 2p peaks show two evident peaks at 99.7 and 102.7 eV (Figure 4d), attributed to the SiSi band (85.55%) indexed to metallic Si and SiO band (14.45%), respectively. The SiO band reveals the presence of SiOx (1 < x < 2) residues, producing a thin silica film on the fresh reduced Si surface in the presence of the chemical surroundings with a high surface activity.56 The typical peaks of SiO2 at 110 and 105 eV were not detected, and the EDS spectrum illustrated the same results (Figure 2d), which further reveals that the residual SiO2 after the MRR reaction has been completely etched by HF. Notably, the intensity of the SiO peak at 102.7 eV for C/Si−rGO−Si/C (Figure 4d, 14.45%) is weaker than that of Si−rGO−Si (Figure S12, 31.60%), suggesting that being carbon-coated is beneficial to inhibit the exposed Si from oxidizing into SiOx. For exploration of the electrochemical behaviors of the assynthesized C/Si−rGO−Si/C electrode, the cyclic voltammetry (CV) curves and discharge−charge voltage profiles are initially characterized. As exhibited in Figure 5a, the C/Si−rGO−Si/C electrode delivered a cathodic peak at about 0.70 V in the first scan and disappears from subsequent cycles, indicating formation of the SEI layer on the electrode and the electrolyte decomposition, which is the major reason for the irreversible capacity loss in the first cycle.57 An obvious reduction peak around 0.18 V emerged from the second cycles, which is associated with the lithiation reaction of Si to form the LixSi phase. Two anodic peaks located at about 0.32 and 0.53 V were detected in the first charge process and become more distinct in the following cycles, which connect with the phase transition from the Li−Si alloy to the amorphous Si.58 Importantly, CV curves in the subsequent cycles are almost overlapping, indicating the good reversibility of the following alloying/ dealloying process. In the charge and discharge voltage profiles of the C/Si−rGO−Si/C electrode (Figure 5b), the first discharge curve exhibits a long plateau at about 0.1 V, which is related to the discharge potential of crystalline Si to form the amorphous LixSi phase;59 this is consistent with the electro39376


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potential candidate as a novel anode material for practical applications of LIBs.

ultra-long-term test of 1000 cycles, a stable reversible capacity can be achieved with 894 mA h g−1, a high capacity retention of 70.3%, as well as a small capacity decay of 0.030% per cycle. Additionally, the CE can reach up to approximately 100% after several cycles, indicating the excellent reversibility during the electrochemical processes. For further investigation of the practical application of LIBs, volumetric and areal capacity parameters of the C/Si−rGO−Si/C electrodes are also represented in Table S1. Importantly, no obvious morphology change or serious fracture was detected after the C/Si−rGO− Si/C electrode cycled 1000 times under a high current density of 1 A g−1 (Figure S15), contributing to its peculiar nanostructures. Additionally, although the discharge capacity of the as-synthesized C/Si−rGO−Si/C electrode decreased gradually with the increasing mass loading of active materials (Figure S16a), the specific capacity can still be maintained at 961 mA h g−1 under a current density of 500 mA g−1 after 100 cycles even under a high mass loading of 2.03 mg cm−2 (Figure S16b). For insight into the remarkable electrochemical behaviors of the C/Si−rGO−Si/C electrode, EIS measurements were further investigated after the cycling and rate performance test. The Nyquist plots (Figure 6a) exhibit a straight line in the low-frequency region and a depressed semicircle in the highfrequency region, which are related to the resistance of the surface film formed on electrodes (Rf) and charge-transfer resistance (Rct), respectively.64 It is obviously that diameter for the semicircle of the C/Si−rGO−Si/C electrode in highmedium frequency region is much smaller than that of the Si@ C−rGO−Si@C electrode, which indicates that charge transfer resistance and SEI impedance of the former are much lower.65 The exact kinetic differences between the C/Si−rGO−Si/C and Si@C−rGO−Si@C electrodes were simulated using the modeling results and based on equivalent circuit diagram (Figure S17). Obviously, Li+ transport and diffusion in the coexistence of micropores and mesopores is quicker than that of in pure micropores, resulting the value of Rct for C/Si− rGO−Si/C electrode (23.6 Ohm) is lower than that of the Si@ C−rGO−Si@C electrode (99.2 Ohm). Additionally, the value of Rf for C/Si−rGO−Si/C electrode (8.7 Ohm) is lower than that of the Si@C−rGO−Si@C electrodes (31.2 Ohm), because there exists a β-SiC interface between nanoscale Si and carbon for Si@C−rGO−Si@C hybrids contributed to formation of a passivated SEI film during the delithiation/lithiation process.66 To evaluate practical application of the C/Si−rGO−Si/C electrode as anode material for LIBs, a full cell was assembled with the synthesized C/Si−rGO−Si/C nanosheets and commercial lithium cobalt oxide (LiCoO2) as anode and cathode, respectively. The corresponding redox reaction of the full cell is based on the following chemical equation.

4. CONCLUSION We have designed and successfully fabricated novel 2D sandwich-like C/Si−rGO−Si/C nanosheets for superior lithium storage, where a mesoporous silicon nanofilm grows on the two sides of graphene which was then coated by a porous carbon layer. For comparison, the C/Si−rGO−Si/C nanosheets exhibit a better electrochemical performance than another nanostructured Si@C−rGO−Si@C hybrid. The coexistence of micropores and mesopores in C/Si−rGO−Si/ C nanosheets offer a rapid Li+ diffusion rate, and the porous Si provides a short pathway for electric transportation. Meanwhile, the coated carbon layer not only can promote to form a stable SEI layer, but also can enhance the electric conductivity of nano-Si coupled with graphene. Thus, the unique nanostructures provide the resultant C/Si−rGO−Si/C electrode with a high reversible capacity (1187 mA h g−1 after 200 cycles at 0.2 A g−1), excellent cycle stability (894 mA h g−1 over 1000 cycles at 1 A g−1), and high rate capability (694 mA h g−1 at 5 A g−1, 447 mA h g−1 at 10 A g−1).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11721. Additional experimental details, schematic illustration, SEM and TEM images, TG curves, Raman spectra, XRD and XPS results, and electrochemical performances (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liang Zhan: 0000-0002-5640-7198 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (51472086, 51572007, 51622203, and 51002051), Recruitment Program of Global Experts and the Fundamental Research Funds for the Central Universities (YWF-16-BJ-Y-12), and CAS Key Laboratory of Carbon Materials (KLCMKFJJ1703).

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[C/Si−rGO−Si/C] + LiCoO2 discharge

HooooooooI Lix[C/Si−rGO−Si/C] + Li1 − xCoO2

REFERENCES

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charge

The galvanostatic charge and discharge profiles of the full cell under a current density of 0.5C (1C = 140 mA h g−1 of LiCoO2) with a voltage window 2.5−4.2 V are exhibited in Figure S18. The C/Si−rGO−Si/C−LiCoO2 full cell delivers a high reversible capacity of 132 mA h g−1 with an ICE of 86.3% (Figure 6b), and exhibits a high capacity retention of 87.6% after 50 cycles. The cycling performance of the full cell predicts that the C/Si−rGO−Si/C nanosheets will be an attractive and 39377


Research Article

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