Modified Chestnut-Like Structure Silicon Carbon Composite as Anode

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Modified Chestnut-Like Structure Silicon Carbon Composite as Anode Material for Lithium-Ion Batteries Jing Luo, Bingjie Ma, Jiao Peng, Zhenyu Wu, Zhigao Luo, and Xianyou Wang* National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Yuhu District, Xiangtan 411105, Hunan, China

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ABSTRACT: The chestnut-like structure mesporous silicon sphere@C@void@ nitrogen-doped carbon (MSN@C@ void@N-C) composite is designed and prepared successfully by introducing an internal carbon layer as a shell layer on the surface of a mesporous silicon core and then using nickel oxide as template to obtain a cavity between a carbon-coated mesporous silicon core and an external nitrogen-doped carbon layer. The influences of the double carbon layer and cavity on the morphology and electrochemical properties of the composite are systematically investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and galvanostatic charge−discharge tests. The results show that the coordination of the double carbon layer and the middle cavity can not only protect the silicon core from electrolyte corrosion but also improve the electron transmission rate of silicon-based materials as well as provide the space to accommodate the volume expansion of silicon without destruction of the electrode structure. It has been found that the MSN@ C@void@N-C composite exhibits excellent electrochemical performance. The first discharge specific capacity is 2499 mAh g−1 and still maintains a discharge specific capacity of 1372 mAh g−1 with a capacity retention rate of 54.9% after 150 cycles. Therefore, the reasonable designs of the structure and morphology for Si/C composites are of great significance for improving the electrochemical performance of silicon-based materials, and this work provides a helpful exploration for development of the next-generation high-energy density lithium-ion batteries. KEYWORDS: Chestnut-like structure, Nitrogen-doped carbon, MSN@C@void@N-C composites, Anode, Lithium-ion batteries

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INTRODUCTION With the rapid development of economy and society, simultaneously, a train of serious consequences began to emerge, such as environmental pollution, resource depletion, and other issues.1−3 Therefore, there is an urgent need to seek an environmentally friendly and sustainable green new energy source.4−6 Undoubtedly, the emergence of secondary batteries is conducive to solving the above problems.7,8 Lithium-ion batteries (LIBs) are one of the most widely used secondary battery systems.9 Compared with other rechargeable batteries, such as nickel cadmium10 and nickel hydrogen batteries,11 LIBs have higher energy density, higher operating voltage, limited self-discharge, and lower maintenance costs.12 However, in the commercial LIBs, graphite is usually used as the negative electrode, which possesses a lower theoretical specific capacity of 372 mAh g−1, and cannot well meet the everincreasing demands of energy density and operation reliability for portable electronic equipment, electric vehicles, and largescale energy storage applications.13 Therefore, it is imperative to exploit a high-performance anode material to improve the performance of LIBs. Among series of potential alternatives, as a new generation of negative electrode materials, silicon has attracted wide attention in academic and business circles.14 In © XXXX American Chemical Society

contrast to the traditional graphite materials, silicon has an ultrahigh theoretical specific capacity (3752 mAh g−1), and the voltage platform of silicon is slightly higher than graphite.15 Furthermore, it can avoid the generation of lithium precipitation on the surface during charging, which has a better safety performance.16 In addition, Si is the second richest element in the earth’s crust. However, as a semiconductor material, the conductivity of silicon is poor. During the lithium-ions extraction/insertion process, the silicon electrodes will lead to huge volume expansion and shrinking (>300%), which causes pulverization of electrode materials and collapsing of structure with destruction of ion and electron conduction channels so that the cycle performance of the battery will be seriously deteriorated.17 Therefore, a series of modification studies is proposed to relieve pulverization of silicon-based materials and improve the electrical conductivity of silicon-based materials as well as enhance the electrochemical properties of silicon-based materials.18 At present, preparation of the nanocrystalline Received: January 30, 2019 Revised: March 26, 2019

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DOI: 10.1021/acssuschemeng.9b00616 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

conductivity of the material and buffer the volume change of silicon, which is formed by using silicon as the core and mesoporous carbon acting as the conducting layer. However, during the cycling process the electrolyte can still pass through the mesoporous carbon layer to the surface of the silicon core, so that the silicon core is corroded to form an unstable SEI film. Furthermore, previous works have been reported to use hydrofluoric acid (HF) to etch SiO2 template to obtain the cavity, yet HF not only reacts with SiO2 but also continually corrodes with silicon core, thus resulting in the uncontrollable etching process.34 In order to solve the above problems, some researchers adopt Si@C as the core and then introduce the cavity and carbon layer to form the structure of Si@C@void@ C.35 Addition of an intermediate carbon layer can not only increase the electron conductivity between the silicon core and the outer carbon shell but also guard the Si core. In summary, this is an important strategy to further improve the electrochemical properties of Si/C composites by introducing an inner cavity and a carbon layer by means of the synergistic effect of each component. In this paper we propose a method for preparing chestnutlike structure MSN@C@void@N-C nanocomposites by introducing an internal carbon layer as a shell layer on the surface of mesporous silicon core and then using nickel oxide as template to obtain a cavity between a carbon-coated mesporous silicon core and an external nitrogen-doped carbon layer. The inner carbon layer was prepared by the sol−gel method with phenolic resin as the carbon source. The interspace was obtained by chemical etching of NiO nanocrystals, and the outer carbon layer was a nitrogendoped carbon layer. On the basis of the reasonable designs of the structure and morphology, the physicochemical and electrochemical performances of the as-prepared chestnut-like MSN@C@void@N-C composite are in detail studied.

materials and Si/C composites or alloying is usually applied to increase the structural stability and cyclic performance of silicon-based anode materials. The nanocrystallized silicon materials are mainly studied based on different morphology design, e.g., silicon nanospheres,19 silicon nanowires,20 silicon nanosheets,21 and silicon nanotubes.22 In particular, nanocrystallized silicon materials can shorten the diffusion distance of lithium ions due to their small particle size and large contact area between particles, while the voids between particles can alleviate the volume expansion of silicon to a certain extent. Huang23 and colleagues pointed out that the critical particle size can maintain the structural stability of the silicon-based anode materials and found that the particle size of ∼150 nm was the critical particle size. In other words, the particle size below this value can avoid rupture and will cause cracking and crumbling of the electrode material over this value. In addition to the above nanocrystallization, the Si/C composites are usually used to enhance the electrochemical performance of batteries. For this reason, a lot of works on the modified Si/C materials have been conducted. For example, putting silicon nanoparticles into carbon buffer layers can effectively improve the cyclic properties of Si−C composite and accommodate the stress of volume expansion/contraction. The commonly used carbon materials are graphite,24 carbon nanotube,25 carbon nanowires,26 graphene,27 and so on. In addition, the conductivity of the material can be improved by encapsulating silicon into a continuous conductive carbon matrix. Therefore, the diffusion rate of lithium ion in composites will be increased. Guo28 and collaborator dispersed micrometer-sized silicon, carbon, and binder in alcohol solution and obtained nanoscale silicon particles by ball milling and then mixed with super P and graphite to prepare Si/C material by spray drying. It has a reversible capacity of 450−750 mAh g−1 under highquality load (8.5 mg cm−2). Except for carbon materials, the electrochemical properties of silicon-based negative electrodes can also be effectively increased by adding metals and metal oxides to form alloys with silicon since metals can not only limit the volume expansion but also increase the electronic conductivity, e.g., Sn−Si,29 Cu−Si,30 and other silicon alloy materials. Besides, Wang et al. reported that the porous silicon can first be prepared by acid etching of Al−Si alloy;31 then a P−Si−Ag/C composite with double conductive core−shell structure was obtained by a silver mirror reaction and CVD method. Because of the double-layer conductive effect of silver particles and carbon layer, the as-prepared material has remarkable electrochemical performance. At the current density of 0.2 A g−1, the specific capacity was 1000 mAh g−1 after 200 cycles. In order to further promote the properties of silicon materials, it is necessary to develop in the direction of the design of the structure and morphology in addition to modification and recombination.32 The cavity is formed inside the material to accommodate the volume expansion effect of silicon during the cycling process, which can be produced by etching templates, such as SiO2, CaCO3, etc. Moreover, the outer carbon shell can not only act as an electrolyte barrier to protect the silicon core from the formation of repeated SEI but also serve as a conductive framework to reduce the resistance of electrons and Li+. Li et al.33 prepared Si@void@C materials by Al2O3 coating and magnetocaloric reduction using silica waste as raw material. At a current density of 400 mA g−1, the reversible capacity is over 1450 mAh g−1 after 100 cycles. Generally, Si@void@C materials can greatly optimize the

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EXPERIMENTAL SECTION

Material Synthesis. Synthesis of Mesoporous Core−Shell MSN@C Nanospheres. The chemical reagents used for synthetic materials do not undergo further purification treatment. Mesoporous silica (M-SiO2) was prepared by a modified Stober method. First, 0.6 g of hexadecyltrimethylammonium bromide (CTAB) was dissolved in a mixed solution of water and ethanol, the solution was stirred for 30 min, and then ultrasonication was carried out for 30 min. Next, 9 mL of concentrated ammonia (28 wt %) was added as catalyst to catalyze hydrolysis of the silicon source. Then 4.5 mL of tetraethyl orthosilicate (TEOS) was added to the mixed solution, and the mixed solution was stirred for 8 h at 35 °C. Finally, the product was collected by centrifugation and washed alternately with water and alcohol five times. Mesoporous silica was prepared by calcining the final product in a muffle furnace at 550 °C for 6 h. M−SiO2 was mixed with magnesium powder and sodium chloride at a mass ratio of 1:0.85:5. Magnesium thermal reduction was carried out in Ar atmosphere at a heating rate of 5 °C min−1 and calcined at 670 °C for 5 h. Then the calcined mixture was treated at 2 M hydrochloric acid (HCl) for 4 h, filtered, and dried to obtain mesoporous silicon spheres (MSN). Then the MSN is used as the core, and an amorphous carbon layer is coated on the surface. Typically, at room temperature, 0.01 g of CTAB is dissolved in 180 mL of deionized water. After stirring and ultrasonic treatment, 0.1 g of MSN is added under stirring to evenly disperse MSN. Finally, 0.1 g of 3aminophenol (3-AP) and 0.1 mL of formaldehyde (37 wt %) were added to the uniformly dispersed solution, and concentrated ammonia solution was used as initiator to initiate the polymerization of phenolic resin (RF). The mixture solution was stirred for 4 h at room temperature. Finally, the products were collected by centrifugation, and RF-coated mesoporous Si nanoparticles (MSN@RF) were B


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Figure 1. (a) Flowchart of the preparation of the yolk−shell chestnut-like MSN@C@void@N-C composite. (b) Structural schematic diagram of MSN@void@N-C. (c and d) SEM images of the microstructure of the SiO2. material to neutral with deionized water. Finally, the final product, yolk−shell chestnut-like structure MSN@C@void@N-C nanoparticles were obtained by drying at 60 °C for 12 h in a vacuum. Synthesis of Yolk−Shell Chestnut-Like Structure MSN@void@NC Nanoparticles. Similarly, the MSN@void@N-C was prepared by using nickel oxide as template and PPy as carbon source. Material Characterization. The JSM-5600LV scanning electron microscope (SEM) of JEOL Co. of Japan was used to characterize the surface morphology of the material with an electron acceleration voltage of 30 kV. The internal structure and crystal plane structure of the materials were further analyzed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM), respectively. In order to express the crystal structure of the material, the instrument we used was the LabX-6000 type X-ray powder diffractometer (XRD) produced by Shimadzu Co. of Japan. The test conditions are as follows: continuous scanning in the scanning range 2θ = 10−90°, Cu Kα radiation, scanning rate 5° min−1. The thermal stability of the material was investigated by using a Series Q500 synchronous thermogravimetric analyzer (TGA) of TA Instruments in the United States from room temperature to 800 °C with a heating rate of 20 °C min−1. Infrared spectroscopy (FTIR7600) was used to determine the composition of the composites. The specific surface and pore size distribution of the material were analyzed by the N2 adsorption−desorption method. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250 Xi) was used to study the element compositions of material surface, in which qualitative analysis and valence analysis were based on the energy value of the peak position in the photoelectron spectra. Electrochemical Measurement. The as-prepared MSN@C@ void@N-C nanoparticles, Super P, and sodium alginate were mixed to form a homogenate at a mass ratio of 7:2:1, stirred with distilled water at 600 rpm for 4 h, coated on copper foil (99.8%, Good fellow), and dried for 12 h under 80 °C in vacuum. Finally, the working electrode was cut into a disc with a diameter of 1 cm to obtain a negative electrode. The loading of active materials was usually 1.4−1.5 mg cm−2. A button cell (CR-2025) was assembled in a high-purity argon glovebox (Mikrouna, Co, Ltd.) using the silicon matrix composite as the anode electrode, lithium metal as the positive and auxiliary

obtained after cleaning and drying. Then mesoporous core−shell MSN@C composites were prepared by calcining MSN@RF nanoparticles at a heating rate of 3 °C min−1 at 800 °C in argon atmosphere for 3 h. Synthesis of Core−Shell Chestnut-Like Structure MSN@C@NiO Nanoparticles. NiO layer was deposited on the surface of MSN@C by chemical bath deposition (CBD). In a typical synthesis process, 0.1 g of MSN@C was added to 50 mL of deionized water dissolved with 0.01 g of CTAB, stirred, and went through ultrasonic treatment for 30 min, so that the MSN@C was well distributed and reserved. A 0.1 g amount of nickel acetate and 0.02 g of potassium persulfate were dissolved in another 50 mL of distilled water. After being fully dissolved, they were slowly added to the MSN@C dispersion. After 30 min of stirring, 20 mL of low-concentration ammonia−water was added. They were stirred for another 4 h at 25 °C in an oil bath. Finally, the product was collected by centrifugation and dried at 60 °C for 6 h in vacuum to obtain MSN@C@NiOOH. Then the core−shell chestnut-like structure MSN@C@NiO nanoparticles were prepared by calcining MSN@C@NiOOH nanoparticles at a heating rate of 2 °C min−1 in nitrogen atmosphere at 450 °C for 2 h. Synthesis of Yolk−Shell Chestnut-Like Structure MSN@C@void@ N-C Nanoparticles. A layer of the amorphous nitrogen-doped carbon layer was coated on the surface of MSN@C@NiO nanoparticles by the sol−gel method. In common synthesis processes, 5 mg of sodium dodecylbenzenesulfonate (SDBS) was dissolved in 250 mL of deionized water. A 0.1 g amount of MSN@C@NiO nanoparticles was added and stirred for another 30 min; 0.2 mL of pyrrole (Py) was added to an ice bath at 0 −5 °C and stirred for an extra 30 min. At the same time, 0.34 g of ammonium persulfate (APS) was dissolved in 40 mL of distilled water. After being completely dissolved, it was added into Py mixed solution and stirred for 24 h at a reaction temperature of below 5 °C. At the end of the reaction, the products were collected by centrifugation, washed successively with distilled water and ethanol to remove impurities, and dried for 12 h at 60 °C in vacuum. Finally, the product MSN@C@NiO@PPy was calcined at 700 °C in an argon atmosphere for 3 h to obtain MSN@C@NiO@N-C coated with a nitrogen-doped carbon layer. The chemical etching method was used to remove the NiO layer in MSN@C@NiO@N-C nanoparticles with 5 M HCl. Recently, we used the centrifugal method to wash the C


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ACS Sustainable Chemistry & Engineering electrode, and Celgard 2400 as the diaphragm; 1 mol L−1 LiPF6 was dissolved in a volume ratio of 1:1 ethylenecarbonate (EC), dimethyl carbonate (DMC) solvent with 5.0% fFluoroethylene carbonate (FEC) as electrolyte. The voltage range of electrochemical performance was 0.01−2.0 V, and the test temperature was 25 °C. The constant voltage charge−discharge test of the battery was carried out using a XinWei tester. Cyclic voltammetry was tested by chi660e at a scanning rate of 0.1 mV s−1. The electrochemical workstation VersaSTAT3 was used to test the electrochemical impedance spectroscopy (EIS) of the sample battery. The frequency ranged from 1 × 10−2 to 1 × 105 Hz.

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RESULTS AND DISCUSSION Figure 1a shows a schematic procedure for synthesis of the chestnut-like MSN@C@void@N-C structure composite. First, m-SiO2 is prepared by a modified Stober method.36 Then MSN is prepared by magnesium thermal reduction of molten salt in an argon atmosphere following with acid treatment. Also, the amorphous carbon layer is uniformly encapsulated on the surface of silicon material by high-temperature carbonization using phenolic resin as the carbon source. The surface of MSN@C is coated with a chestnut-like NiO layer through catalyzing hydrolysis of nickel acetate with ammonia. A uniform layer of polypyrrole is formed on the outside of MSN@C@NiO by in situ polymerization and subsequent carbonization in Ar atmosphere. Finally, the MSN@C@void@ N-C composite is obtained by removing NiO with hydrochloric acid. SEM graphs of silica at different magnification (Figure 1c and 1d) show that the surface of the as-prepared silica has a smooth surface and uniform particle size with a diameter of about 400 nm. As shown in Figure 2, the surface morphology and microstructure of MSN, MSN@C, and MSN@C@NiO are further characterized by SEM and TEM. After magnesium thermal reduction, the silicon nanospheres preferably inherit the spherical morphology of silica, and the surface roughness proves the existence of mesoporous structure (Figure 2a−c), in which 15 nm primary silicon crystal is uniformly distributed. The insertion diagram of Figure 2c is a high-resolution electron diffraction pattern in a specific region. The diffraction rings of Si (111), (220), and (311) planes can be clearly seen, indicating the high crystallinity of MSN.37 The MSN@C obtained by coating the amorphous carbon layer shows that the surface tends to be smooth and the silicon core is evenly embedded in the carbon layer (Figure 2d and 2e), which can be further confirmed by the high-resolution image in Figure 2f. The plane spacings of 0.31 and 0.16 nm can be clearly observed, which can be ascribed to the (111) and (311) planes of Si, respectively.38 The chestnut-like MSN@C@NiO is prepared by hydrolyzing nickel acetate and depositing an ∼20 nm nickel oxide nanosheet layer on the MSN@C surface, which can be clearly seen from the high-resolution image of Figure 2i. The lattice fringe spacing is 0.24 and 0.20 nm, which correspond to the (111) and (200) planes of NiO, respectively,39 confirming the existence of nickel oxide nanoparticles on the MSN@C surface, Figure 2j. X-ray spectroscopy (EDS) further confirms that MSN@C@NiO is composed of silicon, carbon, nitrogen, and nickel.40 As shown in Figure 3, physical characterization is used to study the chestnut-like structure of MSN@C@void@N-C. The outermost nitrogen-doped carbon layer is formed by carbonization of polypyrrole. It can be seen from Figure 3a and 3b that MSN@C@void@N-C composite maintains the morphology of NiO nanocrystalline. In addition, as seen

Figure 2. SEM images of (a) MSN, (d) MSN@C, and (g) MSN@ C@NiO. TEM images of (b) MSN, (e) MSN@C, and (h) MSN@ C@NiO composite. HRTEM images of (c) MSN, (f) MSN@C, and (i) MSN@C@NiO composite. EDS images of (j) MSN@C@NiO composite.

from the TEM image of the MSN@C@NiO, the thickness of the NiO template is about 30 nm; thus, the 10−30 nm bright field is a cavity inside the MSN@C@void@N-C. In order to describe further the pore/hole structure of MSN@C@void@ N-C composite, a more distinct TEM image is given in Figure S1. It can be seen from Figure S1 that the material has a cavity with a diameter of 30−50 nm. After the acid etches the NiO layer, a cavity structure is obtained from the middle of the material, which is beneficial to volume expansion during the (dis)charge process, and introduction of nitrogen atoms helps to improve the conductivity of the material. The surface elemental composition and valence state distribution of MSN@C@void@N-C have been analyzed by full-spectrum X-ray photoelectron spectroscopy (XPS) (Figure 3c). The binding energy peaks of several elements near 103.6, 153.7, 283.0 and 531.eV are clearly visible, which correspond to Si 2p, Si 2s, C 1s, N 1s, and O 1s, respectively, indicating that Si, N, and C elements exist in nanocomposites.41 In Si 2p spectra (Figure 3d), the fitting peaks near 103.3 and 103.8 eV are attributed to the Si2+ and Si4+ of SiOx species, while the peak at 99.7 eV is confirmed as Si−Si bonds.42 High-resolution C 1s spectra (Figure 3e) show that the strong peak at 284.9 eV is attributed to the C−C bond, while the weak peaks with 286.3 and 289.3 eV as the center correspond to the N−C and CO double bonds, which further confirms that nitrogen atoms have been doped into the carbon layer.43 In addition, the N 1s spectra of MSN@C@void@N-C (Figure 3f) show three distinct peaks at 398.6, 401.3, and 403.4 eV, which match to D


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Figure 3. SEM images of (a) MSN@C@void@N-C composite. TEM images of (b) MSN@C@void@N-C composite. (c) XPS survey spectra of the MSN@C@void@N-C composite. (d) Si 2p, (e) C 1s, and (f) N 1s high-resolution spectra of the MSN@C@void@N-C composite.

Figure 4. (a) XRD patterns of SiO2, MSN, MSN@C, MSN@void@C, MSN@C@NiO, MSN@C@NiO@N-C, and MSN@C@void@N-C composites. (b) IR spectra of MSN, RF, MSN@RF, MSN@C, MSN@C@NiO, and MSN@C@NiO@PPy, and MSN@C@void@N-C. (c) TGA curves of MSN@C and MSN@C@void@N-C in air. (d−f) Nitrogen adsorption−desorption isotherms and corresponding pore size distribution curves of SiO2, MSN, MSN@C, MSN@C@NiO, and MSN@C@void@N-C composites.

charge transfer but also strengthen the electrochemical properties of the material.45 In Figure 4a the crystal structures of SiO2, MSN, MSN@C, MSN@void@C, MSN@C@NiO, MSN@C@NiO@N-C, and

pyridinic nitrogen, pyrrolic nitrogen, and nitrogen doped in a graphitized carbon layer.44 Apparently, doping nitrogen atoms into the carbon layer is beneficial to not only improve the conductivity of the carbon layer and accelerate the rate of E


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carbon content of ∼13.1% and 33.4%. The decrease of weight at ∼100 °C corresponds to loss of water in the material and removal of water in the molecule. The weight loss between 300 and 600 °C matches the decomposition of carbon in the material. The faster the mass loss rate of MSN@C@void@NC, the greater the mass loss, which deduces to the higher the carbon content in the nanocomposites material. The specific surface area and texture of the material are studied in Figure 4d−f by nitrogen adsorption−desorption isotherm and pore size distribution. It can be seen that all samples show typical IV curves, which are characteristic of mesoporous materials. The specific surface of silica is 1121.7 m2 g−1, and the average pore size is 2.88 nm. However, after magnesium thermal reduction, the specific surface of MSN decreased to 343.1 m2 g−1, and the average pore size increased to 9.66 nm. This is attributed to the process of oxygen flowing out of silica and removal of impurities such as magnesium oxide during reduction, making the pore size larger. After being coated with a carbon layer, some voids are covered on the surface of pure silicon, which further reduces the specific surface to 179.63 m2 g−1. However, accumulation of NiO nanoparticles on the surface of MSN@C can produce a new pore, thus increasing the specific surface area (∼189.543 m2 g−1) and average pore size (∼19.348 nm) of MSN@C@NiO. It is worth noting that the specific surface area of the final obtained MSN@C@void@N-C composite is larger than that of MSN@C@NiO, which can be ascribed to volatilization of O and H elements during carbonization of PPy. The existence of the mesoporous structure can shorten the diffusion distance of the lithium ion and further promote the performance of the material. Moreover, the porosity property of composites is given in Table 1.

MSN@C@void@N-C are studied by X-ray diffraction. The cubic Si phase (JCPDS No. 27-1402) belongs to the diffraction peaks of the Si that is prepared by magnesium thermal reduction and acid treatment. The diffraction patterns at 28°, 47°, 56°, 69°, and 76° can be indexed as the cubic Si phase (111), (220), (311), (400), and (331), respectively.46 As a result, SiO2 is successfully reduced to Si. The MSN@C composite prepared by sol−gel and carbonization shows that the diffraction peak of Si can be still seen, and a broad peak appeared around 22° corresponding to amorphous carbon after carbonization of phenolic resin.47 Due to the existence of the NiO layer coated on the surface of MSN@C by chemical deposition, the diffraction peaks at 37°, 43°, 62°, 75°, and 79° belong to the (111), (200), (311), and (222) planes of NiO (JCPDS No. 47-1049).48 The diffraction peaks of Si and NiO in MSN@C@NiO@N-C composite prepared by using NiO as template are well maintained. In the MSN@void@C and MSN@C@NiO@N-C prepared by acid treatment, the diffraction peaks of NiO disappear and the Si peaks are well retained. The broad peak appearing at around 22° is ascribed to the nitrogen-doped carbon prepared by polypyrrole carbonization. Introduction of a nitrogen-doped carbon layer is beneficial to heighten the conductivity of the materials. The absence of other impurity peaks indicates that the as-prepared materials have high purity. In order to further explore the encapsulation and structural details of the material, FTIR is shown in Figure 4b. It is used to characterize the specific active groups of phenolic resin and polypyrrole. It can be seen from the spectrum of MSN that MSN has three strong vibration peaks at 1090, 1643, and 3450 cm−1. As monocrystalline silicon is a noninfrared active material, it can be inferred that these three peaks may be related to the natural oxide layer on the surface of silicon material.49 Besides, it can be seen from the FTIR spectrum of phenolic resin that the vibration peak near 3400 cm−1 is the main stretching vibration peak of hydrogen bonding. The vibrational peak of methylene (−CH2−) at 2852 cm−1 and the stretching peak of the carbon−oxygen bond (C−O−C) at 1121 cm−1 indicate a phenol−formaldehyde-type polymerization reaction, which can be further extended to hydroxymethylation of 3-aminophenol and ether bridge condensation.50 The vibration peak of Ar−NH2 appears at 1621 cm−1. Obviously, the phenolic resin is successfully synthesized. The characteristic peak of meta-phenolic resin (m-RF) appears at 835 cm−1, so it is the m-RF. Compared with RF, the peak intensity of RF in MSN@RF decreases owing to the larger vibration peak intensity of MSN, and the peaks at 3400, 1621, and 1121 cm−1 have been covered. The spectra of MSN@C and MSN@C@NiO obtained by high-temperature carbonization and chemical deposition are basically the same as those of MSN, which can be attributed to the fact that C and NiO are inactive groups, which have no obvious vibration peaks in infrared spectra. However, after coating with polypyrrole on the surface of the material, the intensity of the silicon peak decreases and a new vibration peak appears in the material. The peaks located at 1560, 1472, and 1394 cm−1 correspond to the stretching vibrations of CC, C−C, and C−N of the pyrrole ring, respectively.51 As shown in Figure 4c, the content of carbon in MSN@C and MSN@C@void@N-C was determined by thermogravimetric analysis (TGA) in which the materials were sintered at 800 °C in air. The decrease of weight percentage of MSN@C and MSN@C@void@N-C can be attributed to the increase of

Table 1. Porosity Property from the N2 Adsorption− Desorption Isotherm of SiO2, MSN, MSN@C, MSN@C@ NiO, and MSN@C@void@N-C sample

BET surface area (m2 g−1)

total pore volume (cm3 g−1)

average pore size (nm)

SiO2 MSN MSN@C MSN@C@NiO MSN@C@void@N-C

1121.7 343.1 179.63 189.54 239.87

0.809 0.828 0.502 0.916 0.490

2.88 9.66 11.17 19.34 8.18

In order to study the electrochemical performance of MSN@C@void@N-C, the first five cycles of the cyclic voltammetry curves (CV) are obtained at a sweep rate of 0.1 mV s−1 in the voltage range from 0.01 to 2.0 V. As shown in Figure 5a, during the first cycle of lithiation, a broad reduction peak appears at ∼1.0 V, corresponding to the reduction and decomposition of FEC in electrolyte, and an obvious cathodic peak appears at 0.72 V due to decomposition of electrolytes to form SEI films on the surface of the material, which is irreversible and will not occur again in subsequent cycles.52 The peak in the voltage range from 0.1 to 0.3 V corresponds to the process of Li+ embedded to nitrogen-doped carbon layer and Li−Si alloying. In the oxidation process, two different anodic peaks appear at 0.35 and 0.52 V, which are the two characteristic peaks of lithium removal from LixSi alloying material to silicon.53 During the subsequent cycle, it can be F


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Figure 5. (a) CV curves of the MSN@C@void@N-C electrode at 0.1 mV s−1 from 0.01 to 2.0 V (vs Li/Li+). (b) Charge−discharge profiles of MSN@C@void@N-C electrode with a current density of 0.3 A g−1. (c) Cycling performance of MSN, MSN@C, MSN@void@C, and MSN@C@ void@N-C electrodes at a current density of 300 mA g−1. (d) Rate properties of MSN, MSN@C, MSN@void@C, and MSN@C@void@N-C electrodes at differentrates from 0.01 to 2.0 V. (e) EIS curves of MSN and MSN@C@void@N-C electrodes before cycling and after 150 cycles. (f) Equivalent circuits for MSN and MSN@C@void@N-C electrodes used to produce fitting results before and after 150 cycles.

void@N-C electrode reached 1372 mAh g−1, which is four times higher than that of commercial graphite. The Si/C composite with chestnut-like structure formed by introducing amorphous carbon layer to MSN@C core on silicon surface shows good cycling performance. It is strongly indicated that the material has good structural stability. It can not only buffer the volume change but also provide high conductivity and prevent corrosion of the electrolyte to the silicon core. Besides, as shown in Figure 5d, the rate performances of MSN, MSN@C, MSN@void@C, and MSN@C@void@N-C at different current densities are further studied. It can be seen that the capacity of pure silicon electrode decreases faster than other electrodes during cycling. Meanwhile, all of MSN, MSN@C, and MSN@void@C display poor rate capability at high current density. Comparatively, the MSN@C@void@NC electrode shows good rate capability. At current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 4.0 A g−1 the discharge capacities of MSN@C@void@N-C nanocomposite are 1694, 1483, 1290, 976, and 816 mAh g−1, respectively. When the current density is turned back to 0.2 A g−1, the specific discharge capacity is also 1405 mAh g−1. This can be due to the reasonable structural design of the active materials. The external nitrogendoped carbon layer can provide rapid lithium-ion transport, and the inner carbon layer can protect the silicon core. At the same time, the central cavity and the outer carbon frame can also act as a cushion for large volume expansion of silicon, so that the structure of the material will not collapse and break. Table 2 generalizes recent work on Si/C anode materials in LIBs. For example, Sun et al.54 proposed a “self-sacrificing template strategy”, that is, using polyethylenimine (PEI) as carbon source to form voids in the carbonization process,

seen that the peak value of the characteristic peak increases continuously, which is due to complete activation of Si. Figure 5b shows the galvanostatic charge−discharge curves of MSN@C@void@N-C in the 0.01−2.0 V voltage window for different periods. In the first discharge curve, there is a long flat discharge platform when the potential is less than 0.1 V, which is consistent with formation of amorphous LixSi by lithium alloying of crystalline silicon. Afterward, silicon is converted to the amorphous state, which can show the characteristics of amorphous silicon in subsequent charge− discharge curves. The initial discharge−charge capacity of MSN@C@void@N-C composite is 2600 and 1810 mAh g−1, in which the original coulomb efficiency is 70% and the second cycle is increased to 92%. Therefore, introduction of the chestnut-like structure MSN@C@void@N-C with double carbon layer and cavity structure can significantly reduce the excess surface reaction and enhance the discharge specific capacity of the material. As shown in Figure 5c, MSN, MSN@C, MSN@void@C, and MSN@C@void@N-C electrodes are tested for long cycle stability at a current density of 0.3 A g−1. The pure silicon has a discharge capacity of up to 2819 mAh g−1 for the first time. However, in the first 50 cycles the capacity begins to decay rapidly, which is attributed to repeated growth of surface SEI and constant consumption of electrolyte. By contrast, with the introduction of a carbon layer and cavity, the initial discharge capacity of MSN@C and MSN@void@C decreases to 2652 and 2580 mAh g−1, respectively, but the cycle stability is slightly improved. In sharp contrast, the MSN@C@void@N-C electrode has no obvious capacity decrease after 30 cycles. After 150 cycles, the discharge specific capacity of MSN@C@ G


Research Article


MSN@C@void@N-C after cycling. The new semicircle of the high-frequency region corresponds to the impedance of SEI films (Rf) formed on the surface of material during cycling, while the adjacent semicircle corresponds to Rct, and the diffusion impedance of Li+ ions can be attributed to the sloping straight line in the impedance spectrum.62

Table 2. Comparison of Recent Work on Si/C Anode Materials in LIBs sample

current density (A g−1)

cycle no.

capcity (mAh g−1 after cycle)

Si@void@C Si/C/void/SiO2/C P-SiNPs@HC [email protected] SiVC-2 Si@void@C@TiO2 MSN@C@void@N-C

0.2 0.1 0.2 0.5 1 0.2 0.3

200 100 100 200 200 300 150

854 900 1400 710 657 774 1372

initial CE (%) 62 64

62.5 66.5 70

ref

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54 55 56 57 58 59 this work

CONCLUSION In this work, we successfully prepared a chestnut-like structure Si/C composite, which is used as anode material for lithiumion batteries. On the basis of the structure design and reasonable composite scheme, the thickness of the double carbon layer and inner cavity of MSN@C@void@N-C composite can be achieved by adjusting reaction conditions. The electrochemical performance of the composite has been greatly improved, which can be attributed to the following advantages: (1) introduction of an amorphous carbon layer on the surface of the inner silicon core can not only prevent the electrolyte from corroding the silicon core but also solve the blocking problem of the electron transport between the MSN@C core and the nitrogen-doped carbon shell, (2) the cavity with a certain thickness of the middle layer can provide space for the volume expansion of silicon, and (3) in the external amorphous carbon layer introduction of nitrogen can effectively increase the conductivity of the material. Therefore, the MSN@C@void@N-C composite exhibits excellent electrochemical properties. At a current density of 300 mAh g−1 the initial discharge capacity is as high as 2499 mAh g−1; after 150 cycles, the capacity of MSN@C@void@N-C nanoparticles is 1372 mAh g−1. Therefore, the design strategy of the structure and morphology are of great significance to the performance improvement of the silicon-based anode material.

instead of acid etching or base etching to obtain the yolk−shell structure Si@void@C. The composite material has excellent cycling performance. After 200 cycles, the capacity of Si@ void@C is 854.1 mAh g−1 at a current density of 200 mA g−1. Besides, Chen et al.55 designed a complex silicon−carbon negative material with two carbon layers, one cavity, and one silica layer to solve the problems of expansion and poor conductivity of silicon-based negative material. Introduction of a cavity, double carbon layer, and silica can provide a more stable guarantee for the electrode structure. At a current density of 0.1 A g−1, the nanomaterials possess 900 mAh g−1 after 200 cycles. Relatively speaking, the MSN@C@void@N-C has better cyclic stability, which can be attributed to the protection and conductivity of double-layer carbon and the buffer effect of the cavity on the volume expansion of silicon. In Figure 5e electrochemical impedance spectroscopy (EIS) in the frequency range of 10−2−10−5 is used to further study the electronic conductivity of the electrode materials and the ion transport between the electrode and the electrolyte interface. The equivalent circuit model and the simulated electrochemical parameters are shown in Figure 5f and Table 3, respectively. The Nyquist diagrams of both MSN and

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00616.

Table 3. Simulated Equivalent Resistance Parameters cycle no.

resistance

MSN

MSN@C@void@C

before cycle

Rs (Ω)a Rct (Ω)b Rs (Ω) Rf (Ω)c Rct (Ω)

10.97 141.81 29.40 108.52 172.45

2.17 47.95 3.74 82.59 65.91

after 150 cycles

ASSOCIATED CONTENT

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TEM image of the microstructure of the MSN@C@ void@N-C (PDF)

AUTHOR INFORMATION

Corresponding Author

a Impedance of electrolyte. bCharge transfer impedance. cImpedance of SEI films.

*Tel.: +86 731 58293377. Fax: +86 731 58292052. E-mail: [email protected]. ORCID

MSN@C@void@N-C before cycling are composed of a semicircle in the high-frequency region and a straight line in the low-frequency region. The high-frequency semicircle matches the charge transfer impedance (Rct) between the electrode and the electrolyte,60 while the inclined line is adapted to the rate of ion transport impedance (Warburg impedance) to the active material at the interface.61 Compared with MSN@C@void@N-C (47.95 Ω), the Rct of the MSN electrode is about 141.81 Ω, suggesting that MSN@C@void@ N-C has faster reaction kinetics. Introduction of double-layer carbon can stabilize the SEI film of the material and significantly improve the conductivity of the material, ensuring higher charge and lithium-ion transfer efficiency at the interface between the electrode and the electrolyte. However, there are two semicircles in the Nyquist plots of MSN and

Xianyou Wang: 0000-0001-8888-6405 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (No. 51272221) and Key Project of Strategic New Industry of Hunan Province (2016GK4005 and 2016GK4030).

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Modified Chestnut-Like Structure Silicon Carbon Composite as Anode

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