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Aug 25, 2017 - Silicon has been considered to be an attractive high-capacity anode material for next-generation lithium-ion batteries (LIBs). Currentl...
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Constructing Three-Dimensional Honeycombed Graphene/Silicon Skeletons for High-Performance Li-Ion Batteries Peng Chang, Xiaoxiao Liu, Qianjin Zhao, Yaqun Huang, Yunhui Huang, and Xianluo Hu* State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: Silicon has been considered to be an attractive highcapacity anode material for next-generation lithium-ion batteries (LIBs). Currently, the commercial application of Si-based anodes is still restricted by its limited cycle life and rate capacity, which could be ascribed to the colossal volumetric change during the cycling process and poor electronic conductivity. We report the design of a unique Si-based nanocomposite of three-dimensional (3D) honeycombed graphene aerogel and the reduced graphene oxide sheets preprotected silicon secondary particles (SiNPs@rGO1). Through simple electrostatic self-assembly and hydrothermal processes, SiNPs are able to be wrapped with rGO1 to form reunited SiNPs@rGO1, and embedded into the backbone of 3D graphene honeycomb (rGO2). Such an intriguing design (namely, SiNPs@rGO1/rGO2) not only provides a conductive skeleton to improve the electrical conductivity, but also possesses abundant void spaces to accommodate the dramatic volume changes of SiNPs. Meanwhile, the outer rGO1 coats protect the inner SiNPs away from the electrolyte and prevent the destruction of the solid electrolyte interphase (SEI) film. As a result, the 3D honeycombed architecture achieves a high cyclability and excellent rate capability. KEYWORDS: silicon, anode, Li-ion batteries, honeycombed architecture, 3D graphene

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INTRODUCTION The increasing demand for high energy density of Li-ion batteries (LIBs) has aroused much interest in various fields of emerging technologies, especially for intelligent electronic devices, hybrid electric vehicles (HEVs), and electric vehicles (EVs). It is urgent to find other feasible alternative electrode materials with high specific energy and power density to substitute the graphite anode (the limited theoretical capacity: ∼372 mAh g−1).1−5 Among a variety of new anode materials, Si is the most attractive candidate because of its superiorities, including high earth’s crust abundance, low working voltage potential and a much higher theoretical capacity (approximately 4200 mA h g−1, Li4.4Si).6−9 If the graphite anode can be replaced by Si-based materials, the energy density of LIBs will be increased about 40%. Nevertheless, the large-scale commercialization of Si-based negative electrodes is still hampered by several challenges. Some large hurdles associated with the huge volume expansion (∼410%) upon lithiation and delithiation cycling, consisting of severe pulverization of electrodes, delamination of active materials from the cooper current collector, and the repeated destruction and reconstruction of the solid electrolyte interphase film, would lead to rapid capacity deterioration.10−12 In the past several decades, researchers have been trying to devise appropriate strategies in an attempt to optimize the electrochemical performance of Si anodes. One of the most © 2017 American Chemical Society

promising strategy is taking advantages of nanoengineering technologies, such as fabricating difform nanosized silicon precursors that include nanoparticles,13,14 nanowires,15−17 nanotubes,18 nanoporous structures,17,19,20 and their derivatives with conductive additives.21−26 Among them, the preparation of new Si/C composites is a widely accepted method on account of the excellent conductivity, good chemical durability and special mechanical strength of carbonaceous materials. Considerable recent studies are focused on hybridizing silicon species with conductive carbonaceous matrixes including carbon back,27 carbon layer,28 carbon nanotube,29 graphene,24,30−33 and build charming core−shell and yolk−shell nanostructures.34,35 For example, Zhao et al. have reported a kind of ordered mesoporous Si/C nanocomposites that Si nanoparticles were embedded in a mesoporous carbon via a magnesiothermic reduction method.36 The unique mesoporous Si/C nanocomposites showed excellent rate performance and brilliant cycle stability. Cui et al. introduced a nonfilling carboncoated porous Si microparticle (nC-pSiMP), ascribing to the appealing construction, and the as-prepared nC-pSiMP exhibited superior cyclability (∼1500 mAh g−1 at 1.05 A g−1 over 1000 cycles) and high areal capacity (2.01 mg cm−2, 3 Received: June 26, 2017 Accepted: August 25, 2017 Published: August 25, 2017 31879

DOI: 10.1021/acsami.7b09169 ACS Appl. Mater. Interfaces 2017, 9, 31879−31886

Research Article

ACS Applied Materials & Interfaces mAh cm−2).37 Despite those advances, it is still a great challenge to construct a novel and effective architecture that could behave excellent electrochemical activities via a simple method. In general, the current effective strategies have some common features, such as introducing conductive additives to overcome the low intrinsic electronic conductivity, constructing suitable void spaces to buffer the dramatic volume change upon cycling, and building stable SEI films forming on the surface to avoid sustained lithium consumption. In this work, we design a novel three-dimensional (3D) honeycombed framework of reduced graphene oxide, enabling protected Si nanoparticles as high-performance anodes for LIBs. It is well-established that 3D graphene networks are much different from 2D graphene sheets, since the 3D architecture shows an interconnected porous macrostructure with some intriguing properties.38,39 For instance, Shi et al. first reported a kind of 3D graphene hydrogel prepared by a convenient hydrothermal method.39 The hydrogel exhibited excellent mechanical strength, electrical conductivity, and thermal stability, which were attractive in many applications including electrochemical energy storage devices, sensing, and so forth.40 Moreover, various 3D graphene networks and 3D graphene-related hybrids were constructed for high-performance energy-storage devices.41−45 Herein, we report a neoteric 3D SiNPs@rGO1/rGO2 composite composed of a 3D conductive graphene backbone and flexible prewrapping graphene coats. They play a vital role in preprotecting the SiNPs by adapting the stress generated during volume expansion and maintaining the integrity of the anode. Simultaneously, the dual graphene coats can act as separating layers to isolate the electrolyte and silicon, thus allowing a thin and stable SEI film to grow on graphene sheets rather than SiNPs, and avoiding continuous consumption of the lithium source for the regeneration of the SEI film. Benefiting from its unique and stable construction, the as-prepared 3D SiNPs@rGO1/rGO2 composite displays significantly enhanced cyclability and excellent rate capability.

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mixture was kept quietly to make it layered. The supernatant was decanted and the product was subsequently collected by centrifugation. Fabrication of the 3D Honeycombed SiNPs@rGO1/rGO2 Aerogel Composite. The SiNPs@GO1 secondary particles were embedded into the skeleton of 3D graphene hydrogel through a facile hydrothermal assembly. The obtained SiNPs@GO1 particles without drying were well redispersed in 15 mL of UP water, and 18.3 mL of GO2 aqueous dispersion containing 100 mg of GO was added. They were homogenized by means of ultrasonication and stirring. The mixture was then transferred into a Teflon-lined autoclave and held for 5 h at 180 °C to generate a 3D hydrogel composite. The resultant hydrogel was rinsed with UP water, and then completely frozen by liquid nitrogen. After that, it was lyophilized for 2 days until all of the water was removed and the aerogel was generated. Finally, the asprepared 3D SiNPs@GO1/GO2 aerogel was reduced at 500 °C (5 °C min−1) in H2 (5%)/Ar atmosphere for 2 h to gain the final 3D SiNPs@rGO1/rGO2 aerogel composite. For comparison, the samples denoted as SiNPs@rGO1, SiNPs@rGO2 were prepared by the electrostatic self-assembly method and the hydrothermal reaction, respectively. The contents of Si remained the same and the thermal treatment was carried under the identical conditions. Material Characterizations. The shape and microstructure were observed by scanning electron microscopy (SEM, FEI, SIRION200), and transmission electron microscopy (TEM, JEOL 2100F). The phase was detected by powder X-ray diffraction (XRD, PANalytical B.V., Holland). The content of carbon in the composites was studied by thermogravimetric (TG) analysis using Pyrisl TGA (PerkinElmer Instruments) in air from room temperature to 800 °C at a heating rate of 10 °C min−1. Raman spectra were obtained through a LabRAM HR800 spectrometer. X-ray photoelectron spectroscopy (XPS) was conducted on a VG Multilab 2000 system using a monochromatic Al Kα radiation (Thermo VG Scientific). Electrochemical Measurements. Electrochemical properties were investigated on typical CR2032 coin-type half cells. The working electrode was prepared through a conventional slurry coating method, while the mass ratio of active material, super P, and sodium alginate was 6:2:2. The slurry was then pasted on a Cu foil, and dried in a vacuum oven at 80 °C overnight. The average mass loading of the active material is about 0.5−0.8 mg cm−2. The cells were assembled in a glovebox filled with argon atmosphere. The lithium metal foil and Celgard 2325 membrane were used as the counter electrode and the separator, respectively. The electrolyte was 1 M LiPF6 in a mixed solvent of ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume), with 5 wt % fluoroethylene carbonate (FEC) additive to facilitate the formation of a more stable SEI layer.46 The galvanostatic charge−discharge tests were carried out to examine cyclability and rate performance on a LAND battery testing system over a potential range of 2−0.05 V (vs Li/Li+) at room temperature. An electrochemical workstation (CHI, 660E) was used to record cyclic voltammetry (CV, 0.1 mV s−1) plots and electrochemical impedance spectra (EIS, 100 kHz to 0.1 Hz).

EXPERIMENTAL SECTION

Materials Synthesis. Chemicals. All the chemicals were used as received. Commercial silicon nanoparticles (∼70 nm) were purchased from Nanostructured & Amorphous Materials, Inc. Twenty wt % Poly(diallyldimethylammonium chloride) (PDDA) solution was purchased from Aladdin. Ammonium hydroxide and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co. Ltd. Graphene oxide was obtained from Sinocarbon Materials Technology Co. Ltd. Ultrapure (UP) water was used in the experiment. Synthesis of SiNPs@GO1 Secondary Particles. The silicon nanoparticles were first positive-charged by PDDA. 100 mg of Si nanoparticles were evenly dispersed in 100 mL of UP water under ultrasonication for 1 h. Then, 400 uL of PDDA solution was dissolved into the above suspension. After vigorously stirring for 30 min, the PDDA-modified silicon nanoparticles (PDDA-SiNPs) were retrieved and washed by centrifugation for three times using UP water and ethanol. The precipitate was dried in a vacuum oven at 60 °C overnight to get the PDDA-SiNPs powder. The SiNPs@GO1 composites were obtained by an electrostatic self-assembly process. 50 mg of PDDA-SiNPs were evenly dispersed in 500 mL of UP water under ultrasonication for 1 h, and the as-received GO suspension was diluted with UP water to a concentration of 0.1 mg mL−1 (GO1). The pH value of the aqueous PDDA-SiNPs and GO1 dispersions was adjusted to pH 2 by ammonium hydroxide and hydrochloric acid. Subsequently, the aqueous PDDA-SiNPs dispersion was dropwise added into the aqueous GO1 dispersion using a separatory funnel under magnetic stirring. Followed by another 2 h of stirring, the

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RESULTS AND DISCUSSION Typically, the fabrication of the 3D holey-SiNPs@rGO1/rGO2 composite includes two main steps: self-assembly of preprotected SiNPs@GO1 and construction of the 3D honeycomb-like architecture (Scheme 1). First, the secondary particles made of the preprotected SiNPs@GO1 are prepared by electrostatic self-assembly (i−iii). Electrostatic self-assembly is an effective way for engineering the nanocomposites,47 whereby two oppositely charged substances are combined with each other by electrostatic attraction, and therefore strong Coulombic forces may occur between the two substances. Normally, graphene oxide has a negatively charged character due to the presence of many oxygen-containing functional groups on the surface.47 In this work, we used PDDA, a watersoluble long-chain cationic polyelectrolyte to positively charge 31880

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Fabrication Process of the SiNPs@rGO1/rGO2 Composite

the Si nanoparticles.48 The electrostatic self-assembly process is realized by mixing PDDA-modified SiNPs (PDDA-SiNPs) suspension with GO1 dispersion dropwise under magnetic stirring, as shown in ii, iii of Scheme 1. The dispersed GO1 sheets could be adsorbed on the surface of silicon nanoparticles by the Coulombic force, leading to the assembly of SiNPs@ GO1 secondary particles. It is worth noting that the pH value of the suspension is crucial to the successful self-assembly process.44 As shown in Figure S1, when the pH value is controlled at about 2, the silicon suspension will show the strongest positively charged characteristic (Figure S1a), thus prompting the effective combination of silicon nanoparticles and GO sheets (Figure S1b). Subsequently, a 3D holey graphene network (named as GO2) is built up through a hydrothermal process actuated by π−π stacking and hydrophilic oxygen-containing groups,39 and the as-formed SiNPs@GO1 secondary particles are used as the building blocks within the 3D backbone of GO2 hydrogel, as shown in iv−vi of Scheme 1. The 3D honeycombed holes could then be formed by the ice crystallization and the isotropic growth in liquid nitrogen during the lyophilization treatment.49 After the subsequent thermal reduction treatment, the final product of 3D honeycombed graphene/silicon skeletons could be successfully achieved. Figure 1a shows the TEM image for the product of the SiNPs@GO1 secondary particles, indicating that they are well wrapped within GO1 sheets. Further TEM observations at a higher magnification (Figure 1b) and high-resolution TEM (HRTEM) images (Figure 1c and 1d) reveal that a large number of voids exist in the SiNPs@GO1 composite. It is observed that the SiNPs@GO1 composite has a three-layered architecture. The innermost part is silicon nanocrystals, and a layer of silicon oxide is formed on the surface because of the natural oxidation of silicon in air. The outermost layer is the GO1 sheets attracted by the Coulombic force. The GO1 sheets on the surface could not only improve the conductivity of the composite but also prevent serious agglomeration of SiNPs. Furthermore, it is clear to see that numerous void spaces exist in the composite at the junctions of Si−Si, Si−GO1, and GO1−GO1. These abundant void spaces are beneficial to the electrochemical performance of the silicon-based anode, as they can buffer the large volume effect upon cycling. After hydrothermal, freeze-drying and annealing processes, the 3D honeycombed SiNPs@rGO1/rGO2 aerogel composite, and the architecture is shown in Figure 2. The SEM results (Figure 2a, b) demonstrate that it possesses a 3D honeycomb-

Figure 1. (a, b) TEM images of SiNPs@GO1 secondary particles. The red circles in the inset of b are two representative regions for highresolution TEM analysis. (c, d) HRTEM images of SiNPs@GO1 secondary particles.

Figure 2. Characterization of the SiNPs@rGO1/rGO2 composite. (a, b) SEM images. (c, d) TEM images. The red circles in c correspond to pores in the 3D skeleton.

like texture. The cross-linked rGO2 skeletons act as the honeycomb backbone while the large number of pores act as the hives. Together with the TEM results (Figure 2c, d), we observe that the preformed SiNPs@GO1 particles are wellembedded in the backbone of the honeycomb. The grapheneprotected silicon structure of SiNPs@rGO1/rGO2 is further confirmed by dark-field scanning TEM (STEM) image and EDX mapping (Figure S2). The results verify that the silicon nanoparticles are well encapsulated by graphene sheets. For comparison, the SEM images for the control samples of SiNPs@rGO1 and SiNPs@rGO2 are also provided in Figure S3. It is observed that the SiNPs in both of the control samples 31881

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Figure 3. (a) XRD patterns and (b) Raman spectra of pure SiNPs and the SiNPs@rGO1/rGO2 composite.

show serious agglomeration. Nitrogen isothermal adsorption− desorption measurements are conducted for the surface and pore size analysis to further demonstrate the porous nature of the 3D honeycombed SiNPs@rGO1/rGO2 aerogel composite (Figure S4). The Brunauer−Emmett−Teller (BET) specific surface area of the composite is calculated to be 244.4 m2 g−1, and the pores mainly distribute in the range from 2 to 100 nm with an average diameter of 14.6 nm, based on the Barrett− Joyner−Halenda (BJH) model. Benefiting from the carefully designed structure, it is anticipated that the SiNPs@rGO1/ rGO2 composite would distinctly solve the dilemma of Si-based anodes. On the one hand, the continuous 3D carbon network could greatly increase the electronic conductivity, while the rich void spaces act as cushion spaces for volume expansion of active SiNPs during the discharge/charge cycles. On the other hand, the dual graphene coats could prevent direct contact between the electrolyte and inner SiNPs, which is beneficial to the formation of a stable SEI film and conducive to structural integrity of the composite.26 Figure 3a shows the XRD patterns of pure SiNPs and the SiNPs@rGO1/rGO2 composite. Both of them have the characteristic peaks at around 28.4, 47.3, 56.1, 69.1, and 76.4°, which can be well-indexed to the (111), (220), (311), (400), and (331) lattice planes of crystalline silicon (JCPDS no. 27−1402), respectively.11 A distinct hump at around 25.7° confirms the existence of disordered rGO sheets.50 Since SiNPs are well protected in the graphene framework, the intensity of the typical five peaks for the SiNPs@rGO1/rGO2 composite is weakened in comparison to pure Si. The components in the SiNPs@rGO1/rGO2 composite are probed by Raman spectroscopy (Figure 3b). Three peaks centered at 292, 510, and 930 cm−1 are signals of silicon,24 but their intensity in the SiNPs@rGO1/rGO2 is much lower than that pure SiNPs due to the well protected character of inner SiNPs. Besides, the peaks representing the emblematic D and G bands of graphene locate at ∼1347 and 1600 cm−1, respectively. The ratio of ID/IG is figured out to be 1.08, indicating the presence of defects as they could not be completely removed even after thermal treatment.51 According to previous report, it is believed that an adequate amount of defects are good for the LIBs as they can offer additional channels for lithium ion diffusion.52 The surface electronic states and the chemical composition of the SiNPs@rGO1/rGO2 composites were further explored by XPS. The survey XPS spectra (Figure 4a) indicate the coexistence of Si, C and O elements arising from the silicon nanoparticles and rGO in the product. In the high-resolution Si 2p spectrum (Figure 4b), three evident peaks at 99.9, 101.4, and 103.9 eV could be attributed to the monatomic silicon (Si− Si band), silicon oxide (Si−O band) and silicon carbide (Si−C

Figure 4. XPS spectra: (a) survey scan of pure SiNPs and SiNPs@ rGO1/rGO2, and high-resolution spectra of (b) Si 2p, (c) C 1s for SiNPs@rGO1/rGO2.

band) components, respectively.53 The C 1s narrow scanning spectrum could be resolved into four peaks (Figure 4c) of 284.4, 285.0, 286.0, and 289.4 eV. They can be assigned to the C−Si, C−C, C−O, and CO bands, respectively. The sharp C−C peak and weak C−O and CO peaks testifies that there are still some oxygen-containing functional groups on the surface of rGO sheets,54 which is consistent with the results of the Raman spectra. The content of Si in the SiNPs@rGO1/ rGO2 composite is calculated to be 42.5% based on the TG analysis (Figure S5). The cyclic voltammetry (CV) plots of the SiNPs@rGO1/ rGO2 composite anode are demonstrated in Figure 5a for the initial 5 cycles in a potential range of 2.00−0.05 V, and the scanning rate is set as 0.1 mV s−1. It is clearly observed that two irreversible cathodic peaks appear during the first discharge process. The first one at 1.62 V mainly results from the irreversible Li+ storage behavior in the defects such as oxygencontaining groups and edges in rGO.51 The broad one ranging from 1.10 to 0.30 V can be ascribed to the formation of a surface passivation layer of the SEI film.55 The alloying reaction between crystalline silicon and lithium is revealed by the sharp peak at around 0.05 V, which will evolve into another fuzzy peak centered at 0.15 V in the subsequent cycles due to the reaction between amorphous silicon and lithium.15 During the charge process, two prominent humps at about 0.36 and 0.53 V are observed, arising from the dealloying transition from Li15Si4 31882

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Figure 5. Electrochemical characteristics: (a) CV plots of SiNPs@rGO1/rGO2 at 0.1 mV s−1. (b) Charge−discharge profiles of SiNPs@rGO1/ rGO2 at 500 mA g−1. (c) Cycling performances of SiNPs@rGO1/rGO2, SiNPs@rGO1, SiNPs@rGO2 and pure SiNPs at 500 mA g−1 for 50 cycles. (d) Long-term cycling performance of SiNPs@rGO1/rGO2 at 500 mA g−1 for the first 5 cycles and then at 1000 mA g−1 for 200 cycles. (e) Rate performance of SiNPs@rGO1/rGO2, SiNPs@rGO1, and SiNPs@rGO2 at current densities of 0.2−10 A g−1. (f) Comparison of the rate capacity retention of SiNPs@rGO1/rGO2 with some recent reports.

reversible specific capacity as high as 1280 mA h g−1 without decay in capacity over initial 50 cycles. In contrast, the specific discharge capacities for the electrodes made of pure SiNPs, SiNPs@rGO1 and SiNPs@rGO2 decay rapidly. The cyclic performance at a higher current density of 1000 mA g−1 is further investigated after an activating process at 500 mA g−1 for the first five cycles (Figure 5d). The cell maintains a high reversible discharge capacity of 880 mA h g−1 (more than twice as high compared with the theoretical capacity of the commercial graphite anode) even upon cycling up to 200 cycles. For better understanding of the superior property of the SiNPs@rGO1/rGO2 composite compared with the pure SiNPs, SiNPs@rGO1 and SiNPs@rGO2, the surface characteristics of the electrodes before and after Li-cycling are investigated by SEM (Figure S6). As shown in Figure S6, SEM observations reveal that the SiNPs@rGO1/rGO2 electrode upon cycling over 50 cycles shows a smooth surface, suggesting the stability and integrity of the electrode. This benefits from the advantage of the unique 3D honeycombed SiNPs@rGO1/rGO2 skeleton that effectively accommodates the large volume expansion of Si upon cycling. In contrast, the electrodes made of pure SiNPs, SiNPs@rGO1, or SiNPs@ rGO2 have the huge cracks and humps after Li-cycling, indicating the poor cyclability.

to amorphous Si.15 The subsequent cycles show higher peaks related to reversible Li+ storage/release processes, which can be attributed to an activation effect from the charge/discharge cycles.15 Figure 5b illustrates the charge and discharge profiles of SiNPs@rGO1/rGO2 for the 1, 10, 20, 50, and 100 cycles at a current density of 500 mA g−1. It should be pointed out that all the specific capacities in this work are calculated based on the total weight of the SiNPs@rGO1/rGO2 composites. In the first charge/discharge cycle, we can see a short plateau at about 1.60 V and a long slope plateau from 1.10 to 0.30 V, which are consistent with the cathodic peaks in Figure 5a. This special plateau will disappear in the subsequent cycles, resulting from the irreversible consumption of lithium in defects and the formation of the SEI film. The first reversible charge capacity is about 1335 mA h g−1 with an initial Coulombic efficiency of 52.3%. The discharge curves overlap each other well in the subsequent cycles, suggesting that the SEI film is very stable and only formed during the first cycle, benefiting from the double cladding of rGO sheets.24 The cyclability has been analyzed using galvanostatic charge−discharge measurements at a current density of 500 mA g−1 within the potential window of 2.00 to 0.05 V (Figure 5c). Surprisingly, the SiNPs@rGO1/rGO2 anode delivers a 31883

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Figure 6. (a) Electrochemical impedance spectra after different cycles. (b) Corresponding equivalent circuits and (c) fitted values.

the skeleton of the 3D conductive graphene aerogel. The resulting 3D honeycombed SiNPs@rGO1/rGO2 composite delivers a high cyclability (1205 mA h g−1 at 500 mA g−1 over 100 cycles) and excellent rate capability (800 mA h g−1 at 4 A g−1; 460 mA h g−1 at 8 A g−1). The high lithium-storage performance of SiNPs@rGO1/rGO2 composite could be attributed to the intriguing 3D honeycombed nanoarchitecture. The high porosity facilitates the rapid Li-ion transport, and the highly interconnected graphene skeleton in the 3D honeycombed architecture offers excellent electron transport properties. Meanwhile, both the protecting layer of rGO sheets and numerous void spaces involved in the backbone are beneficial to maintaining the integrity of the electrode and buffer the volumetric effect upon Li-cycling.

Apart from the superior cyclability, the SiNPs@rGO1/rGO2 anode displays an outstanding rate capability. As shown in Figure 5e, when the cells are tested at a series of current densities from 0.2 to 1, 2, 4, 6, 8, and 10 A g−1, the SiNPs@ rGO1/rGO2 anode exhibits the highest reversible capacities from 1490 to 1200, 1010, 800, 615, 460, and 360 mA h g−1 respectively, compared with the control samples. Even after such a violent cycle journey, as the current density is switched back to 1 A g−1, the recovered reversible capacity is still 1300 mA h g−1, a little higher than the previously because of the activation. Meanwhile, the excellent rate capacity retention of our product is also proved by comparing with some recent reports on Si-based composite anodes,24,56−60 as depicted in Figure 5f. It can be confirmed that the favorable cyclability and rate capability are ascribed to the rational structure engineering of the SiNPs@rGO1/rGO2 composite, which has a 3D conductive network to encapsulate the well-protected SiNPs@rGO1 particles into the skeleton and isolates internal SiNPs from the electrolyte. The synergistic effect of the graphene network and the active material can not only improve the electrical conductivity and the ionic diffusion rate, but also buffer the remarkable volume variation and avoid the constant destruction-reconstruction of SEI films, maintaining the integrity of the electrode upon cycling. Finally the SiNPs@ rGO1/rGO2 composite shows superior the electrochemical performance.26 The EIS measurements have been carried out to further understand the enhanced performance of our product in contrast to pure Si. The electrochemical impedance spectra are recorded using the electrodes before and after cycles. Notably, the Nyquist plots in Figure 6a exhibit a conventional semicircle and straight line. Figure 6b, c show the corresponding equivalent circuits and the fitted values, respectively. It is clear that our product has lower ohmic resistance (R0) and charge transfer resistance (Rct) than the pure Si electrode. This can be explained by the fact that the graphene network improves the electrical conductivity of the composite and an adequate amount of defects also increase the diffusion rate of lithium ions.56 After 100 cycles, the Rct for our product decreases because of the activation effect. Moreover, a new resistor element ascribed to the SEI film resistance (Rs) could be observed. Its low value suggests that the SEI film formed on the outer graphene coats is thin and stable even after the repeated charge and discharge processes.

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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.7b09169. Photograph of the electrostatic self-assembly process, nitrogen adsorption−desorption isotherm, and TG curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yunhui Huang: 0000-0003-3852-7038 Xianluo Hu: 0000-0002-5769-167X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology of the People’s Republic of China (2015AA034601), National Natural Science Foundation of China (51772116, 51472098, and 51522205), and the fund for Academic Frontier Youth Team of HUST. The authors thank the Analytical and Testing Center of HUST for XRD, SEM, and other measurements.

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REFERENCES

(1) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (2) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (3) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935.

CONCLUSIONS We have successfully designed and fabricated a 3D honeycombed SiNPs@rGO1/rGO2 composite. The SiNPs are preprotected by the graphene coats and then embedded into 31884

DOI: 10.1021/acsami.7b09169 ACS Appl. Mater. Interfaces 2017, 9, 31879−31886

Research Article

ACS Applied Materials & Interfaces (4) Guo, Y. G.; Hu, J. S.; Wan, L. J. Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices. Adv. Mater. 2008, 20, 2878−2887. (5) Zhou, G.; Li, F.; Cheng, H. M. Progress in Flexible Lithium Batteries and Future Prospects. Energy Environ. Sci. 2014, 7, 1307− 1338. (6) Wu, H.; Cui, Y. Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nano Today 2012, 7, 414−429. (7) Boukamp, B. A.; Lesh, G. C.; Huggins, R. A. All-Solid Lithium Electrodes with Mixed-Conductor Matrix. J. Electrochem. Soc. 1981, 128, 725−729. (8) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for LiIon Batteries. Chem. Rev. 2014, 114, 11444−11502. (9) Zhang, L.; Liu, X. X.; Zhao, Q. J.; Dou, S. X.; Liu, H. K.; Huang, Y. H.; Hu, X. L. Si-Containing Precursors for Si-Based Anode Materials of Li-Ion Batteries: A Review. Energy Storage Materials 2016, 4, 92−102. (10) Key, B.; Morcrette, M.; Tarascon, J. M.; Grey, C. P. Pair Distribution Function Analysis and Solid State NMR Studies of Silicon Electrodes for Lithium Ion Batteries: Understanding the (De) lithiation Mechanisms. J. Am. Chem. Soc. 2011, 133, 503−512. (11) Hu, Y. S.; Demir-Cakan, R.; Titirici, M. M.; Müller, J. O.; Schlögl, R.; Antonietti, M.; Maier, J. Superior Storage Performance of a Si@SiOx/C Nanocomposite as Anode Material for Lithium-Ion Batteries. Angew. Chem., Int. Ed. 2008, 47, 1645−1649. (12) Xu, Q.; Li, J. Y.; Sun, J. K.; Yin, Y. X.; Wan, L. J.; Guo, Y. G. Watermelon-Inspired Si/C Microspheres with Hierarchical Buffer Structures for Densely Compacted Lithium-Ion Battery Anodes. Adv. Energy Mater. 2017, 7, 1601481. (13) Kasavajjula, U.; Wang, C.; Appleby, A. J. Nano-and Bulk-SiliconBased Insertion Anodes for Lithium-Ion Secondary Cells. J. Power Sources 2007, 163, 1003−1039. (14) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6, 1522−1531. (15) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-Performance Lithium Battery Anodes Using Silicon Nanowires. Nat. Nanotechnol. 2008, 3, 31−35. (16) Peled, E.; Patolsky, F.; Golodnitsky, D.; Freedman, K.; Davidi, G.; Schneier, D. Tissue-Like Silicon Nanowires-Based Three-Dimensional Anodes for High-Capacity Lithium Ion Batteries. Nano Lett. 2015, 15, 3907−3916. (17) Chen, Y.; Liu, L.; Xiong, J.; Yang, T.; Qin, Y.; Yan, C. Porous Si Nanowires from Cheap Metallurgical Silicon Stabilized by a Surface Oxide Layer for Lithium Ion Batteries. Adv. Funct. Mater. 2015, 25, 6701−6709. (18) Park, M. H.; Kim, M. G.; Joo, J.; Kim, K.; Kim, J.; Ahn, S.; Cui, Y.; Cho, J. Silicon Nanotube Battery Anodes. Nano Lett. 2009, 9, 3844−3847. (19) Jung, D. S.; Ryou, M. H.; Sung, Y. J.; Park, S. B.; Choi, J. W. Recycling Rice Husks for High-Capacity Lithium Battery Anodes. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 12229−12234. (20) Li, X.; Gu, M.; Hu, S.; Kennard, R.; Yan, P.; Chen, X.; Wang, C.; Sailor, M. J.; Zhang, J.; Liu, J. Mesoporous Silicon Sponge as an AntiPulverization Structure for High-Performance Lithium-Ion Battery Anodes. Nat. Commun. 2014, 5, 4105. (21) Chae, S.; Ko, M.; Park, S.; Kim, N.; Ma, J.; Cho, J. Micron-Sized Fe−Cu−Si Ternary Composite Anodes for High Energy Li-Ion Batteries. Energy Environ. Sci. 2016, 9, 1251−1257. (22) Yi, R.; Dai, F.; Gordin, M. L.; Chen, S.; Wang, D. Micro-Sized Si-C Composite with Interconnected Nanoscale Building Blocks as High-Performance Anodes for Practical Application in Lithium-Ion Batteries. Adv. Energy Mater. 2013, 3, 295−300. (23) Liu, N.; Lu, Z.; Zhao, J.; McDowell, M. T.; Lee, H. W.; Zhao, W.; Cui, Y. A Pomegranate-Inspired Nanoscale Design for LargeVolume-Change Lithium Battery Anodes. Nat. Nanotechnol. 2014, 9, 187−192. (24) Ding, X.; Liu, X.; Huang, Y.; Zhang, X.; Zhao, Q.; Xiang, X.; Li, G.; He, P.; Wen, Z.; Li, J.; Huang, Y. Enhanced Electrochemical

Performance Promoted by Monolayer Graphene and Void Space in Silicon Composite Anode Materials. Nano Energy 2016, 27, 647−657. (25) Chen, Y.; Qian, J.; Cao, Y.; Yang, H.; Ai, X. Green Synthesis and Stable Li-Storage Performance of FeSi2/Si@C Nanocomposite for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 3753− 3758. (26) Wang, B.; Li, X.; Zhang, X.; Luo, B.; Jin, M.; Liang, M.; Dayeh, S. A.; Picraux, S. T.; Zhi, L. Adaptable Silicon-Carbon Nanocables Sandwiched Between Reduced Graphene Oxide Sheets as Lithium Ion Battery Anodes. ACS Nano 2013, 7, 1437−1445. (27) Magasinski, A.; Dixon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin, G. High-Performance Lithium-Ion Anodes Using a Hierarchical Bottom-up Approach. Nat. Mater. 2010, 9, 353−358. (28) Xu, Z. L.; Gang, Y.; Garakani, M. A.; Abouali, S.; Huang, J. Q.; Kim, J. K. Carbon-Coated Mesoporous Silicon Microsphere Anodes with Greatly Reduced Volume Expansion. J. Mater. Chem. A 2016, 4, 6098−6106. (29) Wang, W.; Kumta, P. N. Nanostructured Hybrid Silicon/ Carbon Nanotube Heterostructures: Reversible High-Capacity Lithium-Ion Anodes. ACS Nano 2010, 4, 2233−2241. (30) Chang, J.; Huang, X.; Zhou, G.; Cui, S.; Hallac, P. B.; Jiang, J.; Hurley, P. T.; Chen, J. Multilayered Si Nanoparticle/Reduced Graphene Oxide Hybrid as a High-Performance Lithium-Ion Battery Anode. Adv. Mater. 2014, 26, 758−764. (31) Zhou, X.; Yin, Y. X.; Cao, A. M.; Wan, L. J.; Guo, Y. G. Efficient 3D conducting networks built by graphene sheets and carbon nanoparticles for high-performance silicon anode. ACS Appl. Mater. Interfaces 2012, 4, 2824−2828. (32) Zhou, X.; Guo, Y. G. A PEO-assisted electrospun silicon− graphene composite as an anode material for lithium-ion batteries. J. Mater. Chem. A 2013, 1, 9019−9023. (33) Liu, X.; Du, Y.; Hu, L.; Zhou, X.; Li, Y.; Dai, Z.; Bao, J. Understanding the effect of different polymeric surfactants on enhancing the silicon/reduced graphene oxide anode performance. J. Phys. Chem. C 2015, 119, 5848−5854. (34) Su, L.; Zhou, Z.; Ren, M. Core Double-Shell Si@SiO2@C Nanocomposites as Anode Materials for Li-Ion Batteries. Chem. Commun. 2010, 46, 2590−2592. (35) Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y. A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett. 2012, 12, 3315−3321. (36) Zhang, R.; Du, Y.; Li, D.; Shen, D.; Yang, J.; Guo, Z.; Liu, H.; Elzatahry, A.; Zhao, D. Highly Reversible and Large Lithium Storage in Mesoporous Si/C Nanocomposite Anodes with Silicon Nanoparticles Embedded in a Carbon Framework. Adv. Mater. 2014, 26, 6749−6755. (37) Lu, Z.; Liu, N.; Lee, H. W.; Zhao, J.; Li, W.; Li, Y.; Cui, Y. Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS Nano 2015, 9, 2540−2547. (38) Chen, Z.; Ren, W.; Gao, L.; Liu, B.; Pei, S.; Cheng, H. M. ThreeDimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 424−428. (39) Xu, Y.; Sheng, K.; Li, C.; Shi, G. Self-Assembled Graphene Hydrogel via a One-Step Hydrothermal Process. ACS Nano 2010, 4, 4324−4330. (40) Hong, J. Y.; Wie, J. J.; Xu, Y.; Park, H. S. Chemical modification of graphene aerogels for electrochemical capacitor applications. Phys. Chem. Chem. Phys. 2015, 17, 30946−30962. (41) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films. ACS Nano 2013, 7, 4042−4049. (42) Chen, W.; Li, S.; Chen, C.; Yan, L. Self-Assembly and Embedding of Nanoparticles by In Situ Reduced Graphene for Preparation of a 3D Graphene/Nanoparticle Aerogel. Adv. Mater. 2011, 23, 5679−5683. (43) Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Müllen, K. 3D Graphene Foams Cross-Linked with Pre-encapsulated Fe3O4 31885

DOI: 10.1021/acsami.7b09169 ACS Appl. Mater. Interfaces 2017, 9, 31879−31886

Research Article

ACS Applied Materials & Interfaces Nanospheres for Enhanced Lithium Storage. Adv. Mater. 2013, 25, 2909−2914. (44) Zhang, Y.; Chen, P.; Gao, X.; Wang, B.; Liu, H.; Wu, H.; Liu, H.; Dou, S. Nitrogen-Doped Graphene Ribbon Assembled CoreSheath MnO@Graphene Scrolls as Hierarchically Ordered 3D Porous Electrodes for Fast and Durable Lithium Storage. Adv. Funct. Mater. 2016, 26, 7754−7765. (45) Mahmood, Q.; Kim, M. G.; Yun, S.; Bak, S. M.; Yang, X. Q.; Shin, H. S.; Kim, W. S.; Braun, P. V.; Park, H. S. Unveiling surface redox charge storage of interacting two-dimensional heteronanosheets in hierarchical architectures. Nano Lett. 2015, 15, 2269−2277. (46) Xu, C.; Lindgren, F.; Philippe, B.; Gorgoi, M.; Björefors, F.; Edström, K.; Gustafsson, T. Improved performance of the silicon anode for Li-ion batteries: understanding the surface modification mechanism of fluoroethylene carbonate as an effective electrolyte additive. Chem. Mater. 2015, 27, 2591−2599. (47) Xia, F.; Hu, X.; Sun, Y.; Luo, W.; Huang, Y. Layer-By-Layer Assembled MoO2-Graphene Thin Film as a High-Capacity and Binder-Free Anode for Lithium-Ion Batteries. Nanoscale 2012, 4, 4707−4711. (48) Zhou, X.; Yin, Y. X.; Wan, L. J.; Guo, Y. G. Self-Assembled Nanocomposite of Silicon Nanoparticles Encapsulated in Graphene through Electrostatic Attraction for Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 1086−1090. (49) Xie, X.; Zhou, Y.; Bi, H.; Yin, K.; Wan, S.; Sun, L. Large-range control of the microstructures and properties of three-dimensional porous graphene. Sci. Rep. 2013, 3, 2117. (50) Li, Q.; Chen, D.; Li, K.; Wang, J.; Zhao, J. Electrostatic SelfAssembly bmSi@C/rGO Composite as Anode Material for Lithium Ion Battery. Electrochim. Acta 2016, 202, 140−146. (51) Raccichini, R.; Varzi, A.; Passerini, S.; Scrosati, B. The Role of Graphene for Electrochemical Energy Storage. Nat. Mater. 2015, 14, 271−279. (52) Erickson, K.; Erni, R.; Lee, Z.; Alem, N.; Gannett, W.; Zettl, A. Determination of the Local Chemical Structure of Graphene Oxide and Reduced Graphene Oxide. Adv. Mater. 2010, 22, 4467−4472. (53) Zhao, Q.; Huang, Y.; Hu, X. A Si/C Nanocomposite Anode by Ball Milling for Highly Reversible Sodium Storage. Electrochem. Commun. 2016, 70, 8−12. (54) Tang, H.; Zhang, J.; Zhang, Y. J.; Xiong, Q. Q.; Tong, Y. Y.; Li, Y.; Wang, X. L.; Gu, C. D.; Tu, J. P. Porous Reduced Graphene Oxide Sheet Wrapped Silicon Composite Fabricated by Steam Etching for Lithium-Ion Battery Application. J. Power Sources 2015, 286, 431−437. (55) Lee, Y. M.; Lee, J. Y.; Shim, H. T.; Lee, J. K.; Park, J. K. SEI Layer Formation on Amorphous Si Thin Electrode During Precycling. J. Electrochem. Soc. 2007, 154, A515−A519. (56) Wu, J.; Qin, X.; Zhang, H.; He, Y. B.; Li, B.; Ke, L.; Lv, W.; Du, H.; Yang, Q.; Kang, F. Multilayered Silicon Embedded Porous Carbon/Graphene Hybrid Film as a High Performance Anode. Carbon 2015, 84, 434−443. (57) Li, B.; Jiang, Y.; Jiang, F.; Cao, D.; Wang, H.; Niu, C. Bird’s Nest-Like Nanographene Shell Encapsulated Si Nanoparticles-Their Structural and Li Anode Properties. J. Power Sources 2017, 341, 46−52. (58) Li, B.; Yang, S.; Li, S.; Wang, B.; Liu, J. From Commercial Sponge Toward 3D Graphene-Silicon Networks for Superior Lithium Storage. Adv. Energy Mater. 2015, 5, 1500289. (59) Niu, J.; Zhang, S.; Niu, Y.; Song, H.; Chen, X.; Zhou, J.; Cao, B. Direct Amination of Si Nanoparticles for the Preparation of Si@ Ultrathin SiOx@Graphene Nanosheets as High Performance LithiumIon Battery Anodes. J. Mater. Chem. A 2015, 3, 19892−19900. (60) Gao, X.; Li, J.; Xie, Y.; Guan, D.; Yuan, C. A Multilayered Silicon-Reduced Graphene Oxide Electrode for High Performance Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 7855− 7862.

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DOI: 10.1021/acsami.7b09169 ACS Appl. Mater. Interfaces 2017, 9, 31879−31886