Silica-Derived Hydrophobic Colloidal Nano-Si for ... - ACS Publications

Jun 1, 2017 - BTR New Energy Materials, Inc., High Tech Industrial Park, Xitian, Gongming Town, Guangming New District, Shenzhen 518106,. China. ∥...
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Silica-Derived Hydrophobic Colloidal Nano-Si for Lithium-Ion Batteries Zhiliang Liu,† Xinghua Chang,†,‡ Teng Wang,† Wei Li,§ Haidong Ju,∥ Xinyao Zheng,† Xiuqi Wu,† Cong Wang,† Jie Zheng,*,† and Xingguo Li*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering and ‡Academy for Advanced Interdisciplinary Studies, Peking University, 5 Yiheyuan Road, Beijing 100871, China § BTR New Energy Materials, Inc., High Tech Industrial Park, Xitian, Gongming Town, Guangming New District, Shenzhen 518106, China ∥ Department of Chemistry, Kunming University, Kunming 650214, China S Supporting Information *

ABSTRACT: Silica can be converted to silicon by magnesium reduction. Here, this classical reaction is renovated for more efficient preparation of silicon nanoparticles (nano-Si). By reducing the particle size of the starting materials, the reaction can be completed within 10 min by mechanical milling at ambient temperature. The obtained nano-Si with high surface reactivity are directly reacted with 1-pentanol to form an alkoxyl-functionalized hydrophobic colloid, which significantly simplifies the separation process and minimizes the loss of small Si particles. Nano-Si in 5 g scale can be obtained in one single batch with laboratory scale setups with very high yield of 89%. Utilizing the excellent dispersion in ethanol of the alkoxyl-functionalized nano-Si, surface carbon coating can be readily achieved by using ethanol soluble oligomeric phenolic resin as the precursor. The nano-Si after carbon coating exhibit excellent lithium storage performance comparable to the state of the art Si-based anode materials, featured for the high reversible capacity of 1756 mAh·g−1 after 500 cycles at a current density of 2.1 A·g−1. The preparation approach will effectively promote the development of nano-Si-based anode materials for lithium-ion batteries. KEYWORDS: silicon nanoparticles, magnesium, surface functionalization, colloidal, lithium-ion batteries

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nano-Si using earth abundant reactants. This preparation method is especially suited for nano-Si used in LIBs, where very high purity of Si is not required. Bao et al. pioneered in using Mg vapor to reduce diatoms into porous silicon at 650 °C.12 Later, this method was successfully adopted by many researchers to prepare Si nanostructures using various silica nanostructures.13−16 One major drawback of this method is the high reaction temperature, which is typically above 600 °C in order to generate sufficient Mg vapor. In addition to the poor energy efficiency, high temperature also causes formation of side products such as Mg2Si.13−18 There have been a few efforts to reduce the reaction temperature.19−22 A major progress was made by Lin et al., who reduced the reaction temperature to 250 °C in molten anhydrous AlCl3.19 However, a large amount of anhydrous molten salt is required in this method, which is a heavy burden for the subsequent separation.

ilicon nanostructures (nano-Si) are highly attractive for applications in photovoltaics, optoelectronics, energy storage, and sensing.1−6 One very promising application of nano-Si is the anode material in lithium-ion batteries (LIB) due to the very high theoretical lithium storage capacity up to 3579 mAh·g−1. Although Si-based anode materials suffer from poor cyclic stability due to the huge volume change during repeated charge/discharges, recent studies demonstrate that nano-Si after proper modification can effectively improve the cyclic stability.7−11 Currently, developing high-performance anode materials based on nano-Si has attracted considerable academic and industrial interest. However, the application of nano-Si is severely limited by their high cost. The commercially available Si nanoparticles are sold at unreasonably high price considering that Si is the second abundant element in the earth’s crust. Thus, low-cost and scalable preparation methods for nano-Si are highly desirable to support the development of Si-based anode for LIBs. Thermal reduction of silica by magnesium, or known as the magnesiothermal reduction, offers the possibility to prepare © 2017 American Chemical Society

Received: March 23, 2017 Accepted: June 1, 2017 Published: June 1, 2017 6065

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Figure 1. Formation process of hydrophobic colloidal nano-Si from mechanical milling of nanosilica (shown by the TEM image in a) and nano-Mg (shown by the SEM in b), followed by surface functionalization with 1-pentanol. During the acid wash separation step, the hydrophobic nano-Si upper layer is clearly separated with the lower transparent aqueous solution containing MgCl2 (c), which significantly simplifies the separation process and results in near complete recovery of nano-Si. Dry powder of nano-Si can be obtained in ∼5 g scale in one single batch (d). For comparison, conventional acid wash and centrifugation result in considerable loss of small Si particles. Even after 10,000 rpm centrifugation for more than 10 min, the supernant remains highly turbid (e).

Figure 2. (a) XRD patterns of the reactants and products at different stage of the reaction. (b−g) Structural characterization of nano-Si: (b) SEM image. (c) TEM image. (d) A lattice image obtained by HRTEM. (e) Particle size distribution obtained by DLS. (f) Raman spectrum. (g) Si 2p XPS spectrum.

Mg reduction yields a mixture of MgO and Si. MgO has to be removed by acid wash before pure nano-Si can be obtained. In this process, small Si nanoparticles tend to form stable suspensions and are difficult to recover with sufficient efficiency by conventional centrifugation or filtration.13−18 This issue was often overlooked in the literature. Even in the most recent works, the conventional acid washing approach is still exclusively used.19−23 However, the loss of small Si particle in the separation step is a critical issue limiting the preparation efficiency of this method. In this work, we renovate this classical reaction for more efficient preparation of nano-Si. Using nanoparticles of Mg as the reducing agent, the high-temperature thermal reduction can be replaced by mechanical milling for only 10 min at ambient temperature. Moreover, nano-Si are separated in the form of hydrophobic colloidal by surface functionalization, which

minimizes the loss of small nano-Si particles. Nano-Si of 5 g scale with an average diameter of 40 nm can be obtained in one single batch using lab-scale setups with high yield up to 89%. The nano-Si obtained by this method exhibit excellent lithium storage performance comparable to that of the state of the art nano-Si anode materials.

RESULTS AND DISCUSSION Figure 1 shows our renovated preparation process of nano-Si. To promote the preparation efficiency, starting materials with smaller particle size are used. Silica nanoparticles with an average diameter of 40 nm (Figure 1a) are prepared by emulsion-assisted hydrolysis of tetraethyloxysilane (TEOS).24 Mg nanoparticles with an average diameter of 300 nm (Figure 1b) are prepared by thermal plasma evaporation and condensation, as reported in our previous study.25,26 As 6066

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Figure 3. (a) A photograph of the colloidal suspension of the nano-Si in ethanol, obtained by redispersing of the dry nano-Si powder. (b) The FT-IR spectrum of the nano-Si, showing vibrational bands corresponding to surface alkoxyl groups. (c) A schematic illustration of an alkoxylfunctionalized nano-Si particle. (d) The reaction mechanism between 1-pentanol and the surface dangling bonds. (e) The H2 evolution profile when 1-pentanol is injected into the mechanical milled product.

The nano-Si sample is composed of crystalline Si with a facecentered cubic structure without other crystalline impurities (Figure 2a). As shown by the SEM (Figure 2b) and TEM (Figure 2c) images, the average diameter of the nano-Si sample is around 40 nm, which is in excellent agreement with the size distribution obtained by dynamic light scattering (DLS) measurement (Figure 2e). The tap density of the nano-Si sample is 0.33 g·cm−3, which is higher than that of the nano-Si samples with similar size prepared by laser ablation (∼0.15 g· cm−3).19,27 The higher tap density is attributed to the aggregation of particles, as observed in the SEM and TEM images (Figure 2b,c). The aggregation is mainly driven by the capillary force during the evaporation of the solvent, which leads to more compact packing of the particles.28 The higher tap density is favorable for enhancing the volumetric energy density of the electrode. The HRTEM image shows welldefined lattice fringes with an interplanar space of 0.31 nm, corresponding to the (111) crystal planes of Si (Figure 2d). The Raman spectrum shows a sharp peak at 513 cm−1, which is originated from the optical phonons in crystalline Si nanoparticles (Figure 2g).29,30 The Si 2p X-ray photoelectron spectroscopy (XPS) spectrum (Figure 2f) shows a dominant peak at 99 eV, corresponding to elemental Si. The silicon oxide related peaks (100.5 eV for surface oxide and 103−104 eV for dioxide) are very weak.31−33 The above results show that the obtained nano-Si are of very high crystallinity and purity. The hydrophobic nature of the nano-Si can be understood from the FT-IR spectrum (Figure 3b). Vibrational bands corresponding to C−H stretching (2800−3000 cm−1) are clearly observed. The absorption band at 1100 cm−1 can be assigned to the Si−O stretching.34 The above results suggest that the surface of nano-Si is functionalized by the hydrophobic alkoxyl groups, as schematically illustrated in Figure 3c. Therefore, the nano-Si can be enriched in the 1-pentanol layer for efficient separation. Due to the surface alkoxyl groups, the dry powder of nano-Si can be easily redispersed to form a colloidal suspension in ethanol (Figure 3a).

shown by the X-ray diffraction (XRD) patterns (Figure 2a), the Mg nanoparticles are highly crystallized, while the nanosilica is amorphous. After mechanical milling in an Ar atmosphere for only 10 min, a black powder (Figure S1) is obtained. XRD suggests that the product is composed of crystalline Si (JCPDS no. 27-1402) and MgO (JCPDS no. 45-0946) without unconverted reactants or side products (Figure 2a). The FTIR spectrum shows that the Si−O vibration bands at around 1100 cm−1 almost completely disappear after milling (Figure S2). The above results indicate that the reaction is completed within 10 min. To the best of our knowledge, this is the most efficient conversion from silica to silicon reported so far. To separate nano-Si from the mixture, the conventional method is to dissolve MgO using acid and recover nano-Si by centrifugation.13−18 However, this method is rather inefficient to recover small Si particles. As shown in Figure 1e, even after centrifugation at 10,000 rpm for more than 10 min, the supernatant remains highly turbid, indicating that a considerable amount of Si particles are not recovered even after highspeed centrifugation for a long time. Small Si particles tend to form very stable colloidal suspension in water. Thus, the conventional separation method will result in a significant loss of small Si particles, while small Si particles are the most desired for LIB applications. In our improved separation procedure, we inject 1-pentanol into the mechanical milling vessel to passivate the nano-Si before it is exposed to air. Then HCl (1 M) is added to dissolve MgO. As 1-pentanol is immiscible with water, a clear phase boundary develops (Figure 1c). All of the Si particles are enriched in the upper dark-brown organic phase, which can be easily separated from the aqueous phase using a standard liquid separation method. The organic phase can be thoroughly washed with deionized water to completely remove the inorganic salts. The final wash step uses a 5 wt % HF solution to etch away some surface oxides. After removing 1-pentanol by vacuum evaporation, a dry powder of nano-Si of about 5 g is obtained (Figure 1d), corresponding to a very high final yield of 89% based on nanosilica. 6067

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Figure 4. Lithium storage performance of the pristine nano-Si: (a) The voltage−capacity profiles tested at 0.42 A·g−1. (b) Cyclic voltammograms at a scan rate of 0.1 mV·s−1. (c) Cycling performance tested at 0.42 A·g−1 for the first 3 cycles and 2.1 A·g−1 for the later cycles. The capacity is based on the mass of nano-Si.

the reduction is actually achieved by the Mg vapor.13−18 Mechanical milling provides sufficient mechanical energy to initiate the reaction. The following reaction can be sustained by the reaction heat. Clearly, this is a much more efficient energy source for this exothermic reaction. Furthermore, mechanical milling effectively eliminates the side products. Here we achieve complete conversion using near stoichiometric ratio of reactants without any side products. For comparison, the vapor reduction method has an intrinsic drawback of inhomogeneous contact of the reactants. The silica closer to the Mg evaporation source tend to over-react into Mg2Si, while the silica on the far end to the Mg source remain incompletely converted.13,14,17 In the ball milling process, the reactants are constantly agitated. Side products due to local inhomogeneity can be effectively eliminated. Mechanical milling has been utilized to reduce the particle size of Si for LIB application by direct physical pulverization of Si powder in micrometer size.38,39 However, the particle size obtained by conventional high-energy mechanical milling is limited to submicrometer size, usually with severe aggregation and wide size distribution. Different from conventional mechanical milling which is based on physical pulverization, the method in this work is better termed as reactive milling. Here mechanical milling provides energy to initiate the reaction and sustains the reaction by continuously generating a fresh surface of the solid-state reactants. Moreover, it constantly agitates the reactants to ensure homogeneous reaction. In the reactive milling, formation of Si is limited to the location where Mg and SiO2 are in contact. The side product MgO can also inhibit the growth of the obtained Si particles. In addition, here the particle size of both reactants is quite small, which leads to complete reaction within only 10 min. The short milling time is

The nano-Si prepared by mechanical milling contain many highly reactive dangling bonds (DBs) on the surface, providing reactive sites for surface functionalization. It has been reported that DBs in amorphous silicon and on the Si/SiO2 interface can directly react with H2 by breaking the H−H bond.35,36 The O− H bond in 1-pentanol is more reactive than the H−H bond in H2. Therefore, it is expected that 1-pentanol can react with the surface DBs through a similar mechanism, as shown in Figure 3d. The DBs can be passivated by the alkoxyl group in 1pentanol, accompanied by H2 generation. Si−H bonds can also be formed, while the Si−H bonds will undergo further reaction with 1-pentanol (alcoholysis), yielding the same products. Indeed, some bubbles are generated when injecting 1-pentanol into the ball milled product, which are identified to be H2 using mass spectroscopy (Figure 3e). According to the XRD pattern (Figure 2a), Mg has been completely consumed after milling. Thus, the H2 generation must be from the reaction of 1pentanol with the reactive Si surface. Clearly, other alcohols can also be used for surface functionalization through the same mechanism. Here 1-pentanol is deliberately chosen because it has the lowest boiling point among the water immiscible alcohols. Therefore, 1-pentanol can be easily removed by evaporation to obtain the nano-Si in the dry powder form. One major improvement in this work is using ball milling instead of heating to initiate the reaction, which leads to more efficient conversion at ambient temperature. As reduction of silica by Mg is exothermic at room temperature (ΔH = −69.9 kJ/(mol Si)),37 external heating is quite unnecessary. Due to the exothermic nature, direct heating of thoroughly mixed Mg and silica powder will cause a highly vigorous reaction that is very difficult to control. That is why this approach is rarely used in literature. Instead, Mg and silica are located separately, and 6068

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Figure 5. (a) Schematically illustration of the surface carbon coating process of the alkoxyl-functionalized hydrophobic nano-Si using oligomeric PR as the carbon precursor. (b−g) Structural characterization of the nano-Si@C sample with 21 wt % carbon: (b) the XRD pattern, (c, d) SEM images, (e) particle size distribution obtained by DLS, and (f, g) TEM and HRTEM images.

also favorable to inhibit particle growth. As a result, the reactive milling method in this work is able to produce much smaller Si nanoparticles with a more homogeneous size distribution compared to direct pulverization. By combining reactive milling and surface functionalizationbased separation, we substantially improve the preparation efficiency of nano-Si. Using laboratory scale setups, we can easily obtain nano-Si in 5 g scale in one single batch (Figure 1d) with very high yield up to 89% (based on nanosilica). Although there are some requirements for the particle size of the starting materials, preparation of the nanoparticles of both silica and Mg is not difficult. The preparation method for nanosilica is already well established. Moreover, nanosilica with particle size smaller than 50 nm can also be derived from lowcost natural products such as husk and reed plants.40,41 Mg nanoparticles can be easily prepared by the thermal plasma evaporation and condensation technique, which is commonly used for metal powder production. Due to the high vapor pressure of Mg, the production rate of Mg nanoparticles is much faster compared to most metals. Even with our laboratory scale setup, a high production rate of 1 kg·h−1 can be easily achieved. Thus, the current form of the preparation method reported here is at least highly promising for laboratory scale preparation of nano-Si. It also has the potential for further scaling up, as mechanical milling and solvent extraction

technologies are readily available in both laboratories and industries. However, the strong exothermic effect of the Mgsilica reaction should be seriously taken into account to ensure safe and controllable reaction when scaling up the reaction. As a promising anode material for LIBs, the lithium storage performance of the pristine nano-Si sample is tested. As shown in Figure 4a, the nano-Si exhibit well-defined lithiation and delithiation plateaus at 0.1 and 0.4 V (vs Li+/Li), respectively. The peaks in cyclic voltammetry (CV) curves are also characteristic of the stepwise reactions with Li+ (Figure 4b).41−44 The initial Coulombic efficiency reaches 81.4%, which is very high for pristine nano-Si samples. At a large current density of 2.1 A·g−1, the electrode still displays a capacity over 1590 mAh·g−1 after 200 cycles (Figure 4c). As compared in Table S1, this performance, particularly the cyclic stability, is already very impressive for pristine Si. The lithium storage performance of the nano-Si sample can be further improved by carbon coating. To achieve homogeneous carbon coating, the carbon precursor needs to access the particles in all directions. Thanks to the alkoxyl groups on the surface, our nano-Si can form a very stable colloidal suspension in ethanol (Figure 3a), which significantly faciliates the surface carbon coating process. We use oligomeric phenolic resin (PR) as the carbon precursor as it is also soluble in enthanol. As schematically shown in Figure 5a, nano-Si and 6069

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Figure 6. Lithium storage performance of the nano-Si@C sample (21 wt % carbon). (a) The CV curves of the first three cycles. (b) The charge/discharge profile at a current density of 0.1 A·g−1. (c) Cyclic performance at a current density of 0.1 A·g−1. (d) The capacity at a current density ranging from 0.1 to 21 A·g−1. The current density (in A·g−1) is given by the numbers in the graph. (e) Cycling performance at 2.1 A·g−1. In (c−e), the first three cycles are tested at 0.1 A·g−1. (f) The full battery charge/discharge profiles at 2.1 A·g−1. (g) The anode capacity of the full battery in 100 cycles at 0.42 A·g−1 for the first three cycles and 2.1 A·g−1 for the following cycles. The capacity is based on the mass of the nano-Si@C sample.

filled with the PR-derived amorphous carbon, as suggested from the TEM image (Figure 5f). Figure 5g shows a crystalline Si nanoparticle covered with an amorphous carbon layer around 5 nm. The aggregation leads to lower specific surface area (41.3 m2·g−1 for nano-Si@C and 56.7 m2·g−1 for nano-Si) and higher tap density (0.50 g·cm−3 for nano-Si@C and 0.33 g· cm−3 for nano-Si) compared to the pristine nano-Si sample, respectively. The lithium storage performance of the nano-Si@C sample is shown in Figure 6. The characteristic CV peaks (Figure 6a) and potential plateaus (Figure 6b) of the pristine nano-Si during reversible lithation/delithiation are retained. Carbon coating significantly improves the cyclic stability. The nano-Si@C sample shows a capacity of 2273 mAh·g−1 after 100 cycles at 0.42 A·g−1 (Figure 6c) and very stable capacity at gradual increasing current density up to 21 A·g−1(Figure 6d). When the current density is switched back to 0.42 A·g−1, a high capacity of 2140 mAh·g−1 is restored. The reversible capacity remains 1750 mAh·g−1 after 1000 cycles (Figure 6e) at 2.1 A·g−1, showing excellent long-term stability.

oligomeric PR form a homogeneous suspension, in which the PR chains can approach the nano-Si in all directions. After spray drying and curing, a solid product with nano-Si homogeneously distributed in cross-linked PR is obtained, which can be converted to the carbon-coated nano-Si (nanoSi@C) by a single carbonization step. Elemental analysis shows that the obtained nano-Si@C contains 21 wt % carbon. The XRD pattern (Figure 5b) suggests that nano-Si remain highly crystalline after carbon coating, while the carbon obtained from pyrolysis of PR is amorphous. According to the SEM image and the DLS particle size distribution, the nano-Si@C sample contains particles in submicrometer size with relatively wide size distribution (Figure 5c,e). As implied by the high-magnification SEM image (Figure 5d), the submicrometer particles are formed by aggregation of the nano-Si particles (∼40 nm) interconnected by the PR-derived carbon. The structure is also schematically illustrated in Figure 5a. During the drying process of the nanoSi/oligomeric PR suspension, the oligomeric PR connects the nano-Si particles into submicrometer size aggregation. As a result, the inter particle voids in the nano-Si sample is largely 6070

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EXPERIMENTAL DETAILS

We further assemble a full battery by pairing the nano-Si@C anode and a LiNi0.8Co0.15Al0.05O2 cathode and evaluate its performance at current density of 2.1 A·g−1. As shown in Figure 6f, discharge and charge capacities during the first cycle reach 3181 and 2298 mAh·g−1 respectively. After 100 cycles, the anode capacity remains 1595 mAh·g−1 (Figure 6g). The slightly faster decay compared to that in the half-cell measurement is mainly from the cathode. The capacity decay of our cathode material is about 10% after 100 cycles, slightly improved compared to the same material reported in recent literature.45,46 When taking the decay of the cathode into account, the cyclic stability of the nano-Si@C anode is similar to that in the halfcell measurement, indicating the promising application of the nano-Si@C sample for LIB application. Carbon coating has been extensively used to promote the cyclic stability of Si and other alloy-type anode materials in LIBs. Carbon can serve as a buffer to accommodate the volume change in the lithiation/delithiation process and also improve the electrical conductivity.28,47−49 The benefit of the carbon coating can be implied by comparing the Nyquist plots of the nano-Si@C and nano-Si electrodes (Figure S3). The diameter of the quasi-semicircle in the high-frequency region corresponds to the charge-transfer impedance (Rct), which is indicative of the electrochemical reaction kinetics.50,51 The nano-Si@C sample exhibits a much lower Rct value compared to that of the pure nano-Si sample, indicating more favorable electrochemical kinetics after carbon coating. It should be noted that homogeneous distribution of nano-Si in carbon is essential to maximize the stabilization effect of carbon. One key advantage of our nano-Si sample is the excellent dispersion in ethanol due to the surface alkoxyl groups. Thus, homogeneous carbon coating can be readily achieved using very simple approaches as described above. The performance of Si-based anode depends on many electrode fabrication and measurement details.52 In Tables S1 and S2, we provide a comprehensive performance comparison of the Si-based anodes reported in recent literature. The lithium storage performance of our nano-Si, both in the pristine form and after simple carbon coating, is highly competitive compared to the state of the art Si-based anode materials. Particularly, both the nano-Si and nano-Si@C samples can be prepared in large quantities using simple processing techniques suitable for scale-up production. This is very encouraging for the long pursuing target of developing high-performance Si-based anode for LIBs.

Materials Preparation and Characterization. Mg nanoparticles are prepared by thermal plasma evaporation and condensation in pure argon according to our previous work.25,26 A typical reaction runs for 15 min, producing more than 200 g Mg nanoparticles. Silica nanoparticles are prepared by hydrolysis of TEOS in an alkaline aqueous emulsion containing surfactants following the procedure reported by Qiao et al.,24 while the preparation is scaled proportionally to obtain over 50 g nano-SiO2 per batch. To prepare nano-Si, 12 g of silica nanoparticles, 9.8 g of Mg nanoparticles, and agate beads (3.5 g per bead) are loaded into a home designed airtight stainless steel ball milling vessel. The ratio of the reactants is near stoichiometry with a slight Mg excess (Silica:Mg = 1:2.04 in molar ratio). A low ball-to-material ratio 8:1 is used to maximize the processing amount of each batch. The mixture is milled in a planetary ball miller (Fritsch Pulverizer 5) at 250 rpm for 10 min under pure Ar atmosphere. A 100 mL of 1-pentanol is injected into the ball milling vessel to passivate the powder before it is exposed to air. The mixture is transferred into a glass flask and sonicated for 30 min. HCl (1 M) is added into the mixture under stirring until the pH of the aqueous solution is lower than 1. The mixture is allowed to settle to give a clear phase boundary. The lower aqueous phase containing MgCl2 and excessive HCl is discarded. The organic phase is thoroughly washed by deionized water to completely remove the inorganic salt. The final wash step uses 5 wt % hydrofluoric acid to remove the surface oxide layer in a Teflon beaker. Solid nano-Si in the form of a dark-brown powder is obtained by evaporating the 1pentanol in dynamic vacuum at 80 °C. For surface carbon coating of nano-Si, oligomeric PR is used as the carbon precursor. 6.1 g of phenol is dissolved in 1.3 g of 20 wt % NaOH solution. 1.05 g of formalin solution (40 wt %) is added dropwise. The solution is maintained at 70 °C for 1 h for oligomerization. After cooling to room temperature, the pH is adjusted to 7.0 by 0.5 M HCl. The oligomeric PR is extracted by 20 mL of ethyl acetate and dried at 40 °C to obtain ethanol soluble PR. The nano-Si powder and oligomeric PR are dispersed in ethanol by sonication, yielding a homogeneous suspension in which the concentration of nano-Si is 5 g·L−1. The suspension is sprayed onto a hot plate using a syringe with a spray nozzle to rapidly evaporate the ethanol to yield a homogeneous solid mixture, which is cured at 100 °C for 1 h to further cross-link the oligomeric PR. The nano-Si coated with cross-linked PR is carbonized at 1000 °C for 2 h in high-purity Ar to yield the carbon-coated nano-Si (nano-Si@C). Characterization. The samples are characterized by XRD (PANalytical X’Pert3 Powder, Cu Kα), Raman spectroscopy (Jobin Yvon LabRam ARAMIS), infrared spectroscopy (Spolight 200), SEM (Hitachi S4800), TEM (Tecnai F20 and HRTEM, Tecnai F20), and XPS (AXIS-Ultra spectrometer, Kratos Analytical). The carbon content of nano-Si@C is determined by element analysis (Elementar Vario MICRO CUBE). Electrochemistry Measurements. The working electrode is composed of nano-Si or nano-Si@C, acetylene black, and sodium alginate in a mass ratio of 60:20:20 pasted onto a Cu foil. The mass loading of the active material is about 1.0 mg·cm−2. Coin-type halfcells are assembled in an argon-filled glovebox, using a lithium foil counter electrode, 1 M LiPF6 in ethylene carbonate/dimethyl carbonate = 1/1 in volume with 5 wt % fluoroethylene carbonate (FEC) as the electrolyte, and polypropylene film (Celgard 2400) as the separator. The cells are tested in the voltage range of 0.005−3.0 V (versus Li/Li+) at ambient temperature. CV is carried out at a scan rate of 0.1 mV·s−1 from 0.005−3.0 V (versus Li/Li+). In the full battery test, the cathode active material is LiNi0.8Co0.15Al0.05O2, which is mixed with carbon black and polyvinylidene fluoride in 90:5:5 mass ratio and coated on an aluminum foil. The mass loading of the cathode is 16 mg·cm−2. The LiNi0.8Co0.15Al0.05O2 sample is prepared following the method reported by Kim et al.,53 which has a high capacity of 200 mAh·g−1 with ∼10% decay in 100 cycles. The anode preparation and cell assembly are the same as described above.

CONCLUSIONS In conclusion, we present two major improvements to achieve more efficient preparation of nano-Si by Mg reduction of silica. By reducing the particle size of both Mg and silica, nano-Si can be efficiently prepared by ball milling at ambient temperature for only 10 min. The nano-Si are separated in the form of hydrophobic colloidal by reacting with 1-petanol, which minimizes the loss of small Si particles during the separation. The results here demonstrate that nano-Si with good lithium storage performance can be prepared in large quantities using widely available starting materials with simple processing methods, which is very promising for the development of a low-cost and high-performance Si-based anode for the next generation of LIBs. 6071

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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/acsnano.7b02021. Additional figures and tables (PDF)

AUTHOR INFORMATION Corresponding Authors

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

Jie Zheng: 0000-0003-1817-6357 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are indebted to Prof. Henghui Zhou in Peking University for providing the cathode material for the full battery test. The authors acknowledge the financial support from National Science Foundation of China (NSFC, nos. U1607126, 51431001, 11375020, and 21321001). REFERENCES (1) Becker, C. R.; Apperson, S.; Morris, C. J.; Gangopadhyay, S.; Currano, L. J.; Churaman, W. A.; Stoldt, C. R. Galvanic Porous Silicon Composites for High-Velocity Nanoenergetics. Nano Lett. 2011, 11, 803−807. (2) Cullis, A. G.; Canham, L. T. Visible Light Emission due to Quantum Size Effects in Highly Porous Crystalline Silicon. Nature 1991, 353, 335−338. (3) Qu, Y.; Liao, L.; Li, Y.; Zhang, H.; Huang, Y.; Duan, X. Electrically Conductive and Optically Active Porous Silicon Nanowires. Nano Lett. 2009, 9, 4539−4543. (4) Zywietz, U.; Evlyukhin, A. B.; Reinhardt, C.; Chichkov, B. N. Laser Printing of Silicon Nanoparticles with Resonant Optical Electric and Magnetic Responses. Nat. Commun. 2014, 5, 4402. (5) Wang, Y. L.; Wang, T. Y.; Da, P. M.; Xu, M.; Wu, H.; Zheng, G. F. Silicon Nanowires for Biosensing, Energy Storage, and Conversion. Adv. Mater. 2013, 25, 5177−5195. (6) 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. (7) Xue, L.; Fu, K.; Li, Y.; Xu, G.; Lu, Y.; Zhang, S.; Toprakci, O.; Zhang, X. Si/C Composite Nanofibers with Stable Electric Conductive Network for Use as Durable Lithium-ion Battery Anode. Nano Energy 2013, 2, 361−367. (8) Zhong, L.; Kwok, T.; Mangolini, L. Spray Pyrolysis of Yolk−Shell Particles and Their Use for Anodes in Lithium-ion Batteries. Electrochem. Commun. 2015, 53, 1−5. (9) Nguyen, B. P. N.; Kumar, N. A.; Gaubicher, J.; Duclairoir, F.; Brousse, T.; Crosnier, O.; Dubois, L.; Bidan, G.; Guyomard, D.; Lestriez, B. Nanosilicon-Based Thick Negative Composite Electrodes for Lithium Batteries with Graphene as Conductive Additive. Adv. Energy Mater. 2013, 3, 1351−1357. (10) Cui, L.-F.; Yang, Y.; Hsu, C.-M.; Cui, Y. Carbon−Silicon Core− Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries. Nano Lett. 2009, 9, 3370−3374. (11) Fu, K.; Yildiz, O.; Bhanushali, H.; Wang, X.; Stano, K.; Xue, L.; Zhang, X.; Bradford, P. D. Aligned Carbon Nanotube-Silicon sheets: a Novel Nano-Architecture for Flexible Lithium Ion Battery Electrodes. Adv. Mater. 2013, 25, 5109−5114. (12) Bao, Z.; Weatherspoon, M. R.; Shian, S.; Cai, Y.; Graham, P. D.; Allan, S. M.; Ahmad, G.; Dickerson, M. B.; Church, B. C.; Kang, Z.; Abernathy, H. W., 3rd; Summers, C. J.; Liu, M.; Sandhage, K. H. 6072

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DOI: 10.1021/acsnano.7b02021 ACS Nano 2017, 11, 6065−6073