Cornlike Ordered Mesoporous Silicon Particles ... - ACS Publications

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Cornlike Ordered Mesoporous Silicon Particles Modified by Nitrogen-Doped Carbon Layer for the Application of Li-Ion Battery Bing Lu, Bingjie Ma, Xinglan Deng, Wangwu Li, Zhenyu Wu, Hongbo Shu, 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 and Conversion, School of Chemistry, Xiangtan University, Xiangtan 411105, China ABSTRACT: The cornlike ordered mesoporous silicon (OMSi) particles modified by the nitrogen-doped carbon layer (OM-Si@NC) are successfully fabricated and used as the anode of lithium-ion battery (LIBs). The influences of the Ndoped carbon layer on the structure and electrochemical properties of the OM-Si@NC composite are detailedly investigated by transmission electron microscopy (TEM), Xray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS), and charge/discharge tests. The results reveal that the amorphous N-doped carbon layer can offer the abundant conductive pathways for fast lithium ion transportation and electron transfer, which not only leads to a high specific capacity under high ampere density but also serves as a structural barrier maintaining the whole integrity and settling the mechanical breaking due to the huge volume changes of Si host. Therefore, the as-synthesized OM-Si@NC composite exhibits a high original discharge capacity of 2548 mA h g−1 under 0.2 A g−1 as well as a large reversible capacity of 1336 mA h g−1 under 1 A g−1 after 200 circles. The OM-Si@NC composite prepared by a relatively simple and feasible synthesis method shows excellent electrochemical performances and turns out to be promising for the application of high power LIBs. KEYWORDS: cornlike morphology, nitrogen-doped carbon, OM-Si@NC composite, lithium-ion battery, anode

1. INTRODUCTION The LIBs with relatively high energy density, long life span, and low self-discharge rate turn out to be a key technology that has various adhibitions, for example in electric vehicles (EVs), plugin hybrid electric vehicles (PHEVs), and massive grid energy storage applications.1−5 Since the extensive applications in the fields of the transportations and consumer electronics, the Battery 500 Consortium proposes an ambitious goal to achieve 500 Wh kg−1, more than 10-year life, and total mileage of ∼150000 miles. However, the commercial graphite anodes which only have a capacity of 372 mA h g−1, theoretically for LIBs are impossible to meet the above requirements.6 In this regard, silicon with numerous advantages, including the high theoretical capacity (3572 mA h g−1, Li15Si4), the relatively low discharge potential versus Li/Li+, the rich resources, nontoxicity, as well as eco-friendliness, has been considered to be a promising alternative anode material.7 Unfortunately, the largescale practical applications of Si anode material are still hindered by structure instability because of the huge volume expansion (≥300%) during the (de)alloying with lithium,8 which will lead to the fracture and pulverization of the silicon active material, major damage of electric contact, and continuous generation of unsteady solid electrolyte interphase (SEI) on newly exposed silicon surface.9 All of the above issues result in large irreversible capacity loss, increasingly poor electrical conductivity, and inferior cycling life. © 2017 American Chemical Society

In recent years, two main strategies are considered for accommodating the volume changes and enhancing the structure stability as well as improving electrochemical performances of Si materials. One is to tailor the dimension of the silicon material to nanograde,10,11 which can effectively mitigate the fracture and afford a large capacity with minimal fading, such as the silicon nanospheres,12 silicon nanotubes,13 silicon nanowires,14 etc. Besides, the etch of Si-based alloys15 and the synthesis of porous structures from the magnesiothermic reduction procedure16 are also effective pathways for the preparation of nanoscale Si-based anode material. Among these nanosized designs, preparing Si-based porosint is considered as an extremely effective method,17 which avoids the common serious aggregation problem of less than 100 nm ultrafine powders.18 Furthermore, this porosity conformation can afford abundant buffer space to assimilate the big volume changes and therefore can improve the cycle stability. Simultaneously, the sufficient pore pathways offered by the exoteric porosity conformation are conducive to abridge the route of electrolyte and Li+, thereby enhancing the rate capability. For example, the microscaled porous Si particles prepared by acid etching demonstrated a high capacity of 1459 mA h g−1 and remained Received: July 26, 2017 Accepted: September 8, 2017 Published: September 8, 2017 32829

DOI: 10.1021/acsami.7b10922 ACS Appl. Mater. Interfaces 2017, 9, 32829−32839

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ACS Applied Materials & Interfaces

Figure 1. Flowchart of the preparation of the cornlike OM-Si@NC composite.

97.8% of the initial capacity after 200 cycles.7 Du et al.19 fabricated a three-dimensional Si/C composite by uniting the calcining and acid soaking process that delivered a reversible capacity of 1700 mA h g−1 over 70 cycles at 0.2 C. Another strategy is to combine electronically conductive carbon with nano-Si particles to generate silicon/carbon (Si/C) composites which own yolk−shell structure, watermelon, pomegranate, and so on.20,21 The conductive carbon not only isolates the Si particles from contacting the electrolyte directly and is conductive to generate relatively stable SEI film but also serves as a vital elastic intermediary to absorb the stress of volume changes and enhances the electroconductibility of the nano-Si particles. For example, Tian et al.22 synthesized the microscaled nanoporous Si/C composite by a simple, low-cost, and scaleable preparation route, which obtained a capacity of 1182 mA h g−1 at 50 mA g−1 and maintained 86.8% of the first discharge capacity after 300 cycles. In addition, a graphenewrapped silver-porous Si/C was synthesized by Du et al.23 using magnesiothermic reduction and surface-modified via Ag nanoparticles, which revealed an original capacity of 3531 mA h g−1 under 0.1 A g−1. Obviously, the porous structure of the silicon with a carbon coating layer can stabilize the SEI film, and the inside pore offers adequate space for Si expansion to relax the stress/strain. Nevertheless, the most advanced techniques still cannot satisfy the demand of industrial application, which could be ascribed to the unsatisfied synthetical property of the prepared composites, security issues, and/or high expenses for manufacture on a large scale. Herein, we designed a scalable approach for the preparation of N-doped carbon coating layer on the ordered mesoporous silicon (OM-Si@NC) composite via the magnesiothermic reduction of mesoporous silica (SBA-15), chemical selfassembly of polypyrrole on the OM-Si surface, and subsequent carbonization treatment. The as-synthesized OM-Si@NC composite shows homogeneous mesoporous structure, which consists of ca. 45 nm primary particles. Besides, the robust Ndoped carbon-coating layer with a thickness of ∼6 nm cannot merely offer sufficient conductive pathway to quick Li+ transportation and electronic transfer, and promote a high specific capacity at high current density, but also function as a structural barrier which can keep the integrity to deal with the mechanical breaking because of the huge volume expansions of silicon host. Benefiting from the advantages of high porosity property and high conductivity, the as-synthesized OM-Si@NC composite delivers the outstanding electrochemical perform-

ances as anode material for LIBs, such as high capacity retention, long-term cycle stability, and excellent rate property.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. 2.1.1. Preparation of Ordered Mesoporous SBA-15 Silica. All chemicals were used without further purification. Pluronic P123 (3.0 g, EO20PO70EO20) was completely dissolved in deionized water (23 mL) and HCl (88 mL, 2 M) solution first. Afterward, tetraethoxysilane (8 mL, TEOS) was dropwise added into the above solution. The commixture solution was stirred in an oil bath under 35 °C for 24 h and followed with hydrothermal reaction at 105 °C for 24 h with standing methods. Then, the resultant sample was filtered, desiccated, and ultimately calcinated under 550 °C for 2 h. 2.1.2. Preparation of Highly Ordered Mesoporous Silicon (OM-Si). Magnesium powder and the SBA-15 were laid in an adamantine spar boat (weight ratio 1:1) and retained at 700 °C for 6 h with a rising speed of 5 °C min−1 in Ar atmosphere. The as-synthesized sample was first soaked in hydrochloric acid solution (100 mL, 2 M) for 12 h, after that filtered by absolute ethyl alcohol and deionized water, and finally desiccated under 80 °C for 12 h. Afterward, magnesium oxide was wiped off, and the OM-Si sample was obtained. 2.1.3. Preparation of Nitrogen-Doped Carbon Coated on Ordered Mesoporous Silicon (OM-Si@NC). The obtained OM-Si sample (0.1 g) was decentralized in 250 mL deionized water, including sodium dodecylbenzenesulfonate (SDBS, 5 mg) under ultrasonication for 15 min, and the commixture was ulteriorly churned for 1 h at room temperature. Afterward, a pipet was used to drop pyrrole monomer (0.2 mL) into the above solution successively. Later, (NH4)2S2O8 solution (0.34 g in 40 mL H2O) was poured into the mixed solution which was used as an oxidizing agent to trigger the polyreaction. The OM-Si@PPy composite was collected by filtration after rabbling in a low temperature (below 5 °C) for 24 h, followed by rinsing with deionized water, and drying via vacuum oven under 50 °C for 12 h. The PPy nanoparticles were synthesized under the uniform condition for contrast. Afterward, the OM-Si@PPy sample was retained at 700 °C for 3 h with a rising speed of 5 °C min−1 in Ar atmosphere to prepare the OM-Si@NC composite. In addition, the pure PPy nanoparticles was also carbonized to prepare the N-doped carbon composite under the same condition for comparison. 2.2. Material Characterization. The Rigaku D/MAX-2500 powder diffractometer equipped with Cu Ka radiation (λ = 0.154178 nm) were used to record the X-ray diffraction (XRD) patterns to investigate the crystalline phase of the samples. Raman spectrum was performed on a LabRAM HR800 Raman spectrometer by a 532 nm laser wavelength, which was first calibrated with a Si wafer (520 cm−1). The morphology of samples was conducted on a field emission scanning electron microscopy (FE-SEM, Helios, nanolab, 600i). The JEOL-2100F microscope was used to present the transmission electron microscopy (TEM) and element mapping images. TG analysis was performed using a thermogravimetric analyzer 32830


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Figure 2. (a) XRD patterns of SBA-15 silica, OM-Si particles, PPy nanoparticles, OM-Si@PPy, and OM-Si@NC composites, (b) Raman spectra of OM-Si particles, PPy nanoparticles, OM-Si@PPy, and OM-Si@NC composites; the inset is the expanded view of PPy nanoparticles. (c) Thermogravimetric curves of the OM-Si particles and OM-Si@NC composite. (d and e) Nitrogen adsorption/desorption isotherms and the related pore diameter scattergrams of SBA-15 silica, OM-Si particles, OM-Si@PPy, and OM-Si@NC composites. (f) XPS survey scan spectrum of OM-Si@ NC composite and the corresponding high-resolution XPS spectra with Gaussian fitting of (g) Si 2p, (h) C 1s, and (i) N 1s. (SHIMADZU, DTG-60) in air atmosphere at 10 °C min−1 from room temperature to 900 °C. 2.3. Electrochemical Measurement. The anode electrodes were obtained by casting a commixture of OM-Si@NC composite, acetylene black and sodium alginate (weight ratio: 7:2:1). Next, the commixture was dissolved in deionized water and homogenized for 4 h at 600 rpm; afterward, the slurry was coated onto Cu foil (99.8%, Goodfellow). The working electrodes were desiccated vacuum condition at 80 °C for 12 h. Later, the dried electrodes were pressurized and tailored into 1 cm disks. Finally, the area mass of the active materials is typically 1.0−1.5 mg cm−2. CR2025 coin cells were fitted together in a glovebox with the OM-Si@NC electrodes, the lithium wafer as the cathode pole piece, the Celgard 2400 membranes as the segregator, and 1 mol L−1 lithium hexafluorophate in solvent of ethylene carbonate: dimethyl carbonate = 1:1 serving for the electrolyte under Ar atmosphere. The galvanostatic charge−discharge measurements were conducted from 0.01 to 1.5 V (vs Li+/Li) using CT-3008 battery tester (Neware Co., China). Cyclic voltammetry was performed on CHI 660e at 0.1 mV s−1. Electrochemical impedance spectroscopy tests were conducted from 100 kHz to 100 mHz on the Versa STAT 4. The specific capacity was measured on the basis of the total weight of OM-Si@NC composite.

3. RESULTS AND DISCUSSION The synthesis process of the cornlike ordered mesoporous silicon particles modified by the nitrogen-doped carbon (OMSi@NC, for short) is schematically depicted in Figure 1. It has been reported that SBA-15 was served as a silicon source and hard template to obtain mesoporous silicon nanorods or nanowires due to its high superficial area, proper pore size distribution, as well as continuous nanorods.24,25 Therefore, first, the ordered SBA-15 silica is utilized straight for the preparation of ordered mesoporous Si particles (OM-Si, for short) via the magnesiothermic reduction method, which acts as not only a template but also a silicon source. Second, as the mixture of the mesoporous SBA-15 and magnesium powder is under calefaction at 700 °C, the liquated magnesium soaks the well-aligned mesoporous of silica and deoxidates the silica to generate the MgO-Si composite. Then the HCl solution was used to remove MgO from the obtained mixture. Whereafter, the OM-Si particles are obtained, which can well duplicate the initial morphology of SBA-15 silica. Third, the OM-Si particles 32831


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Figure 3. FE-SEM images of (a−c) the SBA-15 silica, (d−f) the OM-Si particles, (g−i) the OM-Si@PPy composite, and (j−l) OM-Si@NC composite.

composite, one of 1561 cm−1, the other two of 1404 and 1349 cm−1, are attributed to the stretching of carbon−carbon double bonds and the ring of PPy severally. The peak at 1044 cm−1 pertains to the C−H in-plane bending of PPy, meanwhile the peaks at 966 and 925 cm−1 are assigned to in-plane deformation of the ring.29 In contrast to those of pure PPy nanoparticles and the OM-Si particles, the bands of the OMSi@PPy composite are moved mildly, owing to the synergistic effect of the OM-Si and PPy. Another two peaks of OM-Si@ NC composite at 1318 and 1595 cm−1 accorded with the disorder-induced band and the graphitic band severely. Generally speaking, the proportion of integrated area of the disorder-induced band and graphitic band displays the graphitization degree.28 The ID/IG value (0.9725) of OM-Si@ NC composite indicates a low degree of graphitization, which is in conformity with the XRD analysis of OM-Si@NC composite. The thermogravimetric (TG) curves of OM-Si particles and OM-Si@NC composite are shown in Figure 2c, which were measured in the air at 900 °C to burn up the nitrogen-doped carbon material. The OM-Si content in the OM-Si@NC composite could be calculated based on the equation: (W2‑R (W1/W1‑R))/W2= XSi,30 in which W1 and W2 are the original weight of the OM-Si particles and OM-Si@NC composite at the room temperature. W2‑R and W1‑R are the residual weight of OM-Si@NC composite and OM-Si particles at 900 °C, respectively, and XSi is the content of Si in OM-Si@NC composite. In accordance with the mass loss percentage data, it is estimated that the nitrogen-doped carbon content in the OM-Si@NC composite is about 31 wt %.

are decentralized in deionized water and further modified by a supple layer of sodium dodecylbenzenesulfonate (SDBS), and then the chemical polymerization of pyrrole monomers are triggered by the (NH4)2S2O8 oxidant, which occurs between the region of the supple SDBS layer and the OM-Si particles to generate the OM-Si@PPy composite. Finally, the resultant OM-Si@PPy composite is carbonized to obtain the OM-Si@ NC composite. The X-ray diffraction (XRD) patterns of the PPy nanoparticles, OM-Si particles, OM-Si@PPy, and OM-Si@NC composites are shown in Figure 2a. All the diffractions of the OM-Si particles after the etching-disposed process correspond to a cubic silicon phase (Card no. 27-1402),26 demonstrating that SBA-15 silica is successfully reduced to Si by the magnesiothermic reduction procedure. A broadened diffraction peak at ca. 22° for OM-Si@PPy composite should be credited with the rough PPy layer deposited on the OM-Si particles surface. When the OM-Si@PPy composite is carbonized at 700 °C, another broadened diffraction peak around 24° is observed, indicating that the nitrogen-doped carbon matrix is amorphous.27 Figure 2b depicts the Raman spectra of the OM-Si particles, the OM-Si@PPy, and the OM-Si@NC composites. The strong peak around 512 cm−1 for OM-Si particles is attributed to the specific diffraction of the Si first-order optical phonon. In addition, the peaks ca. 303.8 and 946.9 cm−1 are ascribed to the scatterings of two transverse acoustic and optical phonons, respectively.28 In contrast to the OM-Si particles, the Si peaks of the OM-Si@PPy composite becomes subdued apparently, which is attributed to the coating layer of PPy nanoparticles. In addition, the peaks of OM-Si@PPy 32832


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Figure 4. TEM images of (a−c) the OM-Si particles and (d−f) the OM-Si@NC composite; (g) EDS mapping images of the OM-Si@NC composite.

For researching the pore structure characteristics of the samples systematically, the nitrogen absorption/desorption isotherms and related pore diameter scattergrams are depicted in Figure 2 (panels d and e). All the samples display symbolic type-IV curves, manifesting the characteristic of the mesopore material. SBA-15 silica possesses a specific superficial area of 655.03 m2 g−1 and an average pore size of 5.5 nm. After the magnesiothermic reduction procedure, the OM-Si particles show a shrinking specific superficial area of 220.52 m2 g−1, while the average pore diameter increases to 9.4 nm. Simultaneously, the OM-Si@PPy composite still owns a specific superficial area of 142.4 m2 g−1, which increases to 156.7 m2 g−1 after the carbonization process. In addition, the average pore size decreases slightly from 14.46 to 13.63 nm. The decrease of specific surface area from the OM-Si particles to the OM-Si@PPy composite could be credited with the PPy nanoparticles coating layer on the OM-Si surface. Besides, the increase of the average pore size could be owing to the stacking of the nanoscaled PPy on the OM-Si particles. When the H elements runoff from the PPy nanoparticles, the micropores are formed simultaneously during the carbonization process. This leads to the decrease of the average BJH pore size of OM-Si@ NC composite and the slight increase of the specific surface area.

The chemical state of each element in the OM-Si@NC composite is ulteriorly notarized through the XPS survey scan spectrum (Figure 2f). As shown in the Si 2p high-resolution XPS spectrum (Figure 2g), the peaks at 98.88 and 99.48 eV are verified to the Si−Si bond, while the other peaks at 102.26 and 103.1 eV are confirmed to SiOx due to the slight oxidation of the OM-Si particles.28 In addition, the high-resolution XPS spectrum of nitrogen−carbon bond (286.28 eV in C 1s spectrum, Figure 2h) manifests that the nitrogen is doped into the carbon framework apparently,31 and it has been found that the nitrogen concentration in the OM-Si@NC composite is 3.49%. Moreover, the N 1s high-resolution XPS spectrum can be divided and fitted into three single-peaks centralizing at 397.78, 399.61, and 400.59 eV, respectively, which are in accord with the pyridinic, pyrrolic, and graphitic types of nitrogen atoms doped in the carbon framework.28 Therefore, the Ndoped carbon framework by adopting PPy as both carbon and nitrogen source is conducive to acquire better electronic conductivity between N-doped carbon layer and adjacent OMSi particles because N atoms can act as electron donors and offer electron carriers. The FE-SEM images in Figure 3 display the surface morphologies of the as-synthesized SBA-15 silica, OM-Si particles, OM-Si@PPy composite, and OM-Si@NC composite. As observed in Figure 3 (panels a−c), the short rodlike SBA-15 32833


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Figure 5. (a) CV curves of the OM-Si@NC electrode at 0.1 mV s−1 from 0.01 to 1.5 V (vs Li/Li+). (b) Charge/discharge profiles of OM-Si@NC electrode. The current densities were 0.2 A g−1 for the first and second cycle, and then 1 A g−1 for the latter circles. (c) Cycling performance of OMSi (red curve) and OM-Si@NC electrodes (magenta curve) at different rates. (d) Rate properties of OM-Si and OM-Si@NC electrodes at different rates from 0.01 to 1.5 V. (e) EIS curves of OM-Si and OM-Si@NC electrodes before cycling and after 200 cycles. (f) Equivalent circuits for OM-Si and OM-Si@NC electrodes used to produce fitting results before and after 200 cycles.

silica links lengthways to generate the cornlike nanorods. After the magnesiothermic reduction procedure, the as-prepared OM-Si with the length of 500−600 nm and the diameter of ∼400 nm retains the cornlike nanorod morphology and delivers a rough surface (Figure 3, panels d−f). The porous characteristic of the OM-Si particles is further notarized in Figure 3f, in which 35 nm primary silicon crystallites are uniformly distributed. These OM-Si chains bind into bundles and form a cornlike morphology, which can substantially increase the packing density and the tap density of OM-Si reaching to 0.85 g cm−3. The poor-conductive OM-Si active materials are modified by the conducting surface carbon coating layer, which often exert a significant influence on the cycling stability and fast charge−discharge property. Therefore, the surface modification by nitrogen-doped carbon coating, which derives from the carbonization of PPy, can further enhance the electrochemical properties of OM-Si particles. Figure 3 (panels g−i) shows the FE-SEM images of OM-Si@PPy composite. Apparently, the OM-Si particles are coated by the PPy nanoparticles homogeneously with a diameter of ∼65 nm. Even though the surface of OM-Si is coated by the nitrogendoped carbon, the porous structure of OM-Si@NC composite is still visible in Figure 3 (panels j−l). Moreover, the homogeneous and gauzy nitrogen-doped carbon layer is coated on every OM-Si primary particle with diameter of ∼45 nm. The TEM and energy-dispersive spectroscopy (EDS) mapping analyses were used to ulteriorly display the structure information on the materials. Figure 4 (panels a and b) presents the OM-Si particles TEM images and reveal that the porous structure and typical morphology of the SBA-15 are well reserved during the reduction procedure. Figure 4c depicts the HR-TEM image of the OM-Si particles. The SAED pattern

reveals the (111), (220), and (311) diffraction rings of Si planes. As observed in Figure 4 (panels d−f), the cornlike OMSi particles are entirely encapsulated by the homogeneously amorphous N-doped carbon layer which is about 6 nm in thickness. An interplanar spacing of 0.31 nm can be clearly observed and verified to be the (111) plane of Si.28 The EDS analyses show that the OM-Si@NC is constituted of silicon, carbon, and nitrogen elements. On the basis of the observation of bright/dark field scanning FE-SEM images and elemental mappings, the silicon, carbon, and nitrogen elements of the OM-Si@NC composite apparently indicate a uniform distribution. To evaluate the potential application of the OM-Si@NC composite as the anode of LIBs, the CV curves of the OM-Si@ NC electrode at 0.1 mV s−1 in different cycle numbers are shown in Figure 5a. During the first return-scan process, the wide cathodal peak appears around 0.72 V on account of the generation of the SEI film, which leads to the initial irreversible capacity loss.32 In addition, a peak around 0.38 V is observed and accorded with the insertion of Li+ into OM-Si particles. The peak around 0.17 V in the next CV curves is ascribed to the formation of LixSi, meanwhile the peaks at 0.37 and 0.53 V at the anodal sweep could be the Li extraction process from LixSi. In the following cycles, the reinforced peaks with high repetition indicate the improved (de)lithiation kinetics. Moreover, the amorphous nitrogen-doped carbon layer contains disordered structures and defects which can promote Li+ insertion. The OM-Si@NC composite has a long and flat discharge terrace around 0.09 V in the first circle (Figure 5b), which is in accordance with the characteristic terrace of the Li insertions of crystalline Si. The well-crystalline silicon turns to amorphic shape and shows the representative charge/discharge 32834


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A g−1 (Figure 5d), respectively. Furthermore, the specific capacity of the OM-Si@NC composite turns back to 1557 mA h g−1, while the ampere density becomes 2 A g−1, which reveals a high reversibility. Such a satisfactory electrochemical performance is probably because of the elegant structural design of the active material. On the one hand, the N-doped carbon layer can offer abundant conductive pathways for fast lithium ion transportation and electron transfer, enabling a high specific capacity at high rate; on the other hand, the robust Ndoped carbon framework can function as a structural barrier which can maintain the whole integrity to settle the mechanical fracture during the huge volume expansions of silicon host. EIS analyses were further performed to test the conductivity and ion diffusion in the electrode/electrolyte interface. Before cycling (Figure 5e), both the OM-Si and OM-Si@NC electrodes show one semicircle representing the charge transfer resistance between the electrode and electrolyte interface (Rct)40,41 as well as a sloping low-frequency line, known as Warburg impedance,42,43 indicating the resistance corresponding to ion diffusion into the active material.44 The Rct of the OM-Si electrode (116.9 Ω) is apparently higher than that of the OM-Si@NC electrode (48.23 Ω), revealing that the OM-Si@ NC electrode owns much higher electrochemical reaction activity because the N-doped carbon layer derived from PPy can offer more than enough charge transfer channels on the electrode/electrolyte interface (Table 2). It can also be

profiles of amorphic silicon in the latter cycles. Nevertheless, the initial discharge specific capacity of OM-Si@NC composite reaches up to 2548 mA h g−1 with a high Coulombic efficiency (CE) of 71.4%, which demonstrates that the nitrogen-doped carbon coated on the OM-Si particles surface could significantly decrease the undesired surface reactions and increase discharge capacity. As observed in Figure 5c, the OM-Si@NC composite could still deliver a specific capacity over 1336 mA h g−1 with a capacity retention of 73.5% and an average CE of 99.36% after 200 circles under 1 A g−1, which is nearly triple higher than the capacity of graphite theoretically. The excellent CE and cycling performance indicate that the OM-Si@NC composite owns excellent reversibility during the repeated (de)lithiation process. After 200 cycles, the N-doped carbon (NC) electrode delivers an average capacity of 134.5 mA h g−1, while the OMSi@NC electrode affords an average capacity of 1406.4 mA h g−1 under the same test condition. In accordance with the C (31 wt %) and Si (69 wt %) contents of the OM-Si@NC composite, the capacity contributions derived from carbon components are 4.1%. In addition, the charge/discharge tests are further executed at higher current density of 4.0 A g−1 and 6.0 A g−1. After 200 circles, the OM-Si@NC composite still maintains large reversible capacities of 984 and 758 mA h g−1, respectively, indicating an outstanding cycling stability. Nevertheless, the specific capacity of OM-Si without coated N-doped carbon layer decreases slightly to 750 mA h g−1 under 1 A g−1 after only 100 circles. Table 1 generalizes the latest works on Si-based materials as anodes for LIBs. For example, Jaumann et al.33 provided a

Table 2. Resistance Parameters Fitted from the Equivalent Circuits cycle number

Table 1. Contrast of the Latest Work on Si-Based Materials As Anodes for LIBs samples nc-Si@HCS P-doped Si/ graphite HSi@C Si@NPC h-SiNT-15/C pSS/CNTs Si-PBI OM-Si@NC

current density (A g−1)

cycle number

capacity (mA h g−1 after cycles)

initial CE (%)

0.25 0.2

250 200

810 883.4

69 64

[33] [34]

0.5 1.0 1.0 0.5 1.0 1.0

200 200 120 200 200 200

886.2 1372 975 1200 1128 1336

52.4 57.1 − − 60.27 71.4

[35] [36] [37] [38] [39] in this work

4.0

200

984

72.3

before cycle after 200 cycles

ref

resistance

OM-Si

OM-Si@NC

Re (Ω) Rct (Ω) Re (Ω) Rf (Ω) Rct (Ω)

5.807 116.9 10.93 114.7 108.5

2.165 48.23 5.706 56.03 46.58

observed that two semicircles show up in the Nyquist plots of OM-Si and OM-Si@NC electrodes after cycling. The high frequency one corresponds to the resistance caused by SEI films (Rf),44 and the adjacent mid-frequency one is assigned to the Rct. The slight change of the Rct after cycling could be owing to the activation of materials and the full infiltration of electrolyte into electrode materials depending on the sufficiently ordered mesoporous and conductive pathways. Simultaneously, Figure 6 (panels a−d) depicts the FE-SEM images of the OM-Si and OM-Si@NC electrodes after 200 circles. Apparently, the OM-Si electrode generates large cracks and presents an agglomeration phenomenon (Figure 6a). Meanwhile, in Figure 6b the OM-Si electrode is pulverized, and the structure is collapsed after 200 cycles. On the contrary, the OM-Si@NC composite with cornlike morphology is remained intact, and the electrode is flat without any fracture or pulverization after 200 cycles, indicating that the OM-Si@NC composite exhibits low volume expansion, and the shape of each nanoparticle is well-preserved with the help of the void spaces. In addition, the TEM and HRTEM images (Figure 6, panels e, f, and g) of the OM-Si@NC composite after 200 cycles are used to further testify to the structural integrity. It can be found that the cornlike structure is roughly observable, and the deepened gray levels could be ascribed to the homogeneous lithiation during cycling.36 And the amorphous layer after 200 cycles is thicker than the original coating layer

viable preparation method to gain nc-Si@HCS structures, which obtained an original capacity of 1560 mA h g−1 with a CE of 69% and a capacity retention of 51.9% under 0.25 A g−1. Zhu et al.36 prepared Si@NPC composites by a scalable procedure, which exhibited an original capacity of 2473 mA h g−1 under 0.5 A g−1 with an CE of 57.1%. It can be found that the initial CE and the cycling performance of the OM-Si@NC composite in this work are better than most of the previously reported results. Moreover, the enhanced cyclability of the OMSi@NC electrode can be attributed to the below reasons: (1) the ordered mesopore structure can provide enough buffer zone to accommodate the huge volume expansion of Si and (2) the N-doped carbon layer can form an effective conducting network on the OM-Si particles surface in the process of (dis)charge tests. Besides, the OM-Si@NC electrode obtains specific capacities of 2455 and 781.2 mA h g−1 under 0.5 and 8 32835


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Figure 6. (a and b) FESEM images of the OM-Si particles and (c and d) OM-Si@NC composite after 200 cycles; (e and f) the TEM images and (g) HRTEM image of OM-Si@NC composite after 200 cycles.

on account of the stable formation of SEI film. Obviously, the N-doped carbon layer of the OM-Si@NC composite could efficiently boost the structural stability and electrical conductivity of the electrode material during the (dis)charge processes. Furthermore, in order to identify the changes in thickness of the OM-Si and OM-Si@NC electrodes before and after 200 cycles, the cross-sectional FE-SEM images are revealed in Figure 7 (panels a−d). It can be observed in Figure 7 (panels a and c) that the pristine thickness of the OM-Si and OM-Si@ NC electrodes are both 20 μm. The thickness of the OM-Si electrode after 200 cycles is increased to 40.9 μm with a swelling rate of 104.5%. In addition, the electrode shows that the active material is shed from the current collector partly caused by the huge volume expansions of Si during the (de)lithiation process, thus generating the large electrical contact loss and sharp capacity fading.45 However, the OMSi@NC electrode exhibits a small swelling rate of 53.5% without apparent crack, disintegration, and separation from the current collector. To better distinguish the differences of electrochemical behaviors between the conventional Si@C composite and the OM-Si@NC composite, the possible structure evolutions

during the cycling processes are illustrated in Figure 8 (panels a and b). As being seen from the schematic in Figure 8, compared with the conventional Si@C composite, the OM-Si@ NC composite possesses two prominent advantages that can be clarify for its improved electrochemical properties: (1) the porosity of the surface could adsorb the volume expansion of silicon during the (de)lithiation process and (2) the N-doped carbon layer is able to efficiently improve the electrical conductivity of the material during the charge/discharge processes, and meanwhile the carbon layer combined with active particles can act as a physical support to protect Si from fracture and pulverization, thus enhancing the structural stability.

4. CONCLUSIONS The highly ordered mesoporous silicon particles with cornlike morphology was successfully synthesized by the magnesiothermic reduction procedure. The homogeneous mesoporous structure could function as a buffer layer to efficiently mitigate the volume variations of silicon during (de)lithiation. A thickness of 6 nm N-doped carbon layer derived from carbonization of PPy nanoparticles is uniformly coated on the OM-Si surface to generate the OM-Si@NC, and the resultant 32836


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Figure 7. Cross-sectional FESEM images of OM-Si and OM-Si@NC electrodes. (a and c) Original electrodes and (b and d) the electrodes of OM-Si particles and OM-Si@NC composite after 200 cycles.

Figure 8. Flowchart of the possible structure transformation of (a) conventional Si@C composite and (b) OM-Si@NC composite during (dis)charge process.

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ACKNOWLEDGMENTS The work is supported financially by the National Natural Science Foundation of China under project 51272221, Key Project of Strategic New Industry of Hunan Province under projects 2016GK4005 and 2016GK4030, Scientific Research Fund of Xiangtan University under project 2015SEP03.

OM-Si@NC composite obtains an initial discharge specific capacity of 2548 mA h g−1 under 0.2 A g−1 and a stable reversible capacity of 1336 mA h g−1 with an average CE of 99.36% after 200 circles under 1 A g−1. Especially, at a high rate up to 8 A g−1, the discharge capacity still retained at 781.2 mA h g−1. The great improvement could be attributed to the wellorganized nanoparticles linked to the homogeneous distribution of ordered mesoporous, optimized primary particles of ∼45 nm, and high-conductive N-doped carbon layer. The outstanding electrochemical performances and the facile preparation route make the N-doped carbon-coated layer on the highly ordered mesoporous silicon surface promising for the applications of the high-property LIBs.

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AUTHOR INFORMATION

Corresponding Author

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

Xianyou Wang: 0000-0001-8888-6405 Notes

The authors declare no competing financial interest. 32837


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Cornlike Ordered Mesoporous Silicon Particles ... - ACS Publications

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