Size and Surface Effects of Silicon Nanocrystals in Graphene Aerogel

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Size and Surface Effects of Silicon Nanocrystals in Graphene Aerogel Composite Anodes for Lithium Ion Batteries Maryam Aghajamali,† Hezhen Xie,†,‡ Morteza Javadi,† W. Peter Kalisvaart,†,‡ Jillian M. Buriak,*,†,‡ and Jonathan G. C. Veinot*,†,‡ †

Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada National Institute for Nanotechnology, National Research Council Canada, 11421 Saskatchewan Drive, Edmonton, AB T6G 2M9, Canada

Chem. Mater. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/29/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Silicon is recognized as a promising anode material for high-performance lithium ion batteries due to its high theoretical specific capacity and elemental abundance. Challenges related to the low electrical conductivity of Si and large volume changes during the lithiation/delithiation cycles, as well as the low rate of lithium diffusion in silicon anodes, hinder practical applications. To provide fundamental insights into these issues, silicon nanocrystal/graphene aerogel nanocomposites were synthesized by combining undecanoic acid-functionalized silicon nanocrystals of various sizes (SiX-COOH, where X represents the nanocrystal diameter of 3, 5, 8, and 15 nm) with conductive mesoporous graphene aerogels (GAs). The silicon nanocrystals are evenly dispersed throughout the graphene aerogel as shown by energy-dispersive X-ray (EDX) mapping. In terms of electrochemical performance, SiX-COOH/GA nanocomposites demonstrated a clear dependence on the size of the embedded silicon nanocrystals, with the composites comprising the larger silicon nanocrystals showing a higher initial capacity but accompanied by rapid decay of capacity retention over 100 cycles. To study the effect of thermal processing on the electrochemical performance, SiX-COOH/GA nanocomposites were annealed at 600 °C to yield annealed SiX/GA nanocomposites. The annealed nanocomposite composed of the smallest silicon nanocrystals, Si3/GA, exhibits a stable specific capacity of ∼1100 mAh/g and capacity retention of over 90% after 500 cycles when tested at a current density of 400 mA/g.

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INTRODUCTION The development of high-performance lithium ion batteries (LIBs) with higher energy and power densities is of great importance for applications in portable electronics, stationary energy storage, and electric vehicles, to name a few.1−3 Silicon is a promising anode material for high-performance LIBs because of its elemental abundance, low discharge potential (∼0.4 V vs Li/Li+), and high theoretical specific capacity (4200 mAh/g based on Li 22 Si 5).2−8 This remarkable theoretical capacity is ∼10 times higher than that of a graphite anode (372 mAh/g) used in commercial LIBs.5−8 Practical applications of Si anodes are held back due to several reasons, including the low electrical conductivity of Si, the low rate of lithium diffusion, and large volume changes (>300%) observed during the lithiation/delithiation cycles.2,7,8 The latter results in the pulverization of the silicon into isolated particles and © XXXX American Chemical Society

unstable solid−electrolyte interphase (SEI) formation, leading to loss of electrical contact and poor cyclability.5,7,9 To address these issues, one approach is to use nanoscale silicon of various morphologies (e.g., nanoparticles,10 nanowires,11,12 nanotubes,13,14 nanorods,15 hollow nanospheres,16 and nanoporous silicon17,18). Nanoscale materials offer short Li diffusion distances within the electrode and higher stress/strain tolerance, leading to improved rates and cycling performance.3,7,19 A fruitful extension of this approach is to integrate nanoscale silicon with conductive carbon materials to improve the electrical conductivity of the silicon anodes, which results in better rates and cycling performance.20−30 Received: July 27, 2018 Revised: October 18, 2018 Published: October 19, 2018 A

DOI: 10.1021/acs.chemmater.8b03198 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Scheme 1. (a) Different Sizes of Hydride-Terminated Silicon Nanocrystals (H-SiNCs); (b) Hydrosilylation of Silicon Nanocrystals with 10-Undecenoic Acid; (c) Preparation of SiX-COOH/GA Nanocomposites; TEM Micrograph of Si3COOH/GA and Optical Image of Si3-COOH/GA under UV Light Exposure, Showing Typical Red Photoluminescence of Embedded 3 nm Diameter Silicon Nanocrystals

“initial” interphase layer at their interface during LIB cycling. The GA chosen was prepared by CO2 supercritical drying of a rGO hydrogel, which has been shown to exhibit higher electrical conductivity (∼100 S/m), higher specific surface area (∼512 m2/g), and increased mesoporosity compared to freezedried GAs.34 The electrochemical properties for LIBs of nanocomposites of 3, 5, 8, and 15 nm silicon nanocrystals in a GA matrix were screened and showed a strong size dependence, particularly with respect to much improved Coulombic efficiency and cycling stability; smaller is better, as will be described.

Graphene aerogels (GAs) are nanoporous carbon materials exhibiting tunable porosity and high electrical conductivity.31,32 They are commonly prepared by reduction of a graphene oxide (GO) precursor to reduced graphene oxide (rGO), followed by various drying procedures (e.g., freezedrying and CO2 supercritical drying) to form the aerogel structure.33−35 Nanocomposites comprising a graphene aerogel host and nanoscale silicon guests are of interest for LIB applications as they take advantage of the conductivity and porosity of the GA and the very high theoretical specific capacity of the silicon. Recently, interesting works showing the synthesis and characterization of nanoscale silicon/GA composites as anode materials for LIBs have been published;36,37 these composites were prepared by incorporating commercial silicon nanoparticles (diameter ∼100−120 nm) into graphene aerogels by freeze-drying. Some of these composites exhibited a high initial capacity of >2500 mAh/g, but the capacity retention was less than 75% over 40 cycles. While the GA in these composites clearly facilitated lithium ion diffusion and charge transport, it did not stabilize these relatively large silicon nanoparticles over repeated cycling, leading to the observed decrease in charge/discharge capacities.36,37 Building on these encouraging results,36,37 we chose to systematically investigate a series of much smaller and sizecontrolled silicon nanocrystals (SiNCs) and functionalize them with a layer of covalently bound ligands linked to their surface by silicon−carbon bonds. Sub-15 nm diameter silicon nanocrystals could improve the cycling performance of silicon anodes since smaller particles would have a greater stress/ strain tolerance. The ligands also facilitate their processing by improving their water solubility and provide a well-defined

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RESULTS AND DISCUSSION The overall approach to prepare the silicon nanocrystal/ graphene aerogel composites is shown in Scheme 1. Hydrideterminated SiNCs (H-SiNCs) were synthesized following wellestablished procedures developed in the Veinot laboratory.38,39 SiNC/SiO2 composites obtained from thermal processing of hydrogen silsesquioxane (HSQ) at 1100, 1200, 1300, and 1400 °C were etched using a hydrofluoric acid solution to remove the oxide and liberate 3, 5, 8, and 15 nm silicon nanocrystals, respectively. H-SiNCs are susceptible to oxidation, and their surfaces must be passivated to prevent oxidation reactions and render them hydrophobic/hydrophilic.40−43 Because the GO precursor used in graphene aerogel synthesis is dispersible in aqueous media, a radical44 hydrosilylation with 10-undecenoic acid was employed to hydrosilylate the H-SiNCs and render them hydrophilic, compatible with the aerogel synthetic method. The resulting hydrophilic SiNCs (d ∼ 3, 5, 8, and 15 nm) were introduced to an aqueous solution of GO containing L-ascorbic acid (L-AA) as a reducing agent with a mass ratio of SiX-COOH:GO of 8:1. Following chemical B


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Chemistry of Materials reduction of GO to rGO, hydrogels were formed and dried using a CO2 supercritical dryer to yield monolithic aerogels. The hydrosilylation step of the silicon nanocrystals with 10undecenoic acid to form the functionalized SiX-COOH nanomaterials was characterized by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Representative FTIR spectra of SiX-COOH (X = 3, 5, 8, and 15 nm) show the expected features corresponding to the expected methylene chains and carboxylic acid group (Figure 1); intense absorptions corresponding to νas(CH2) and

Figure 1. FTIR spectra of undecanoic acid-functionalized silicon nanocrystals, SiX-COOH (where X represents the diameter of the nanocrystal, ∼3, 5, 8, and 15 nm).

νs(CH2) and ν(CO) appear at 2850−3000 and 1700− 1725 cm−1, respectively.45 Broad features at ∼1000−1130 cm−1 related to ν(Si−O), as well as weak absorptions of ν(SiHx) at ∼2100 cm−1, are noted in the spectra of all functionalized silicon nanocrystals. High-resolution XP spectra of the Si 2p region of the functionalized SiX-COOH nanocrystals and their corresponding SiX-COOH/GA nanocomposites show emission characteristic of the Si(0) core at 99.3 eV (Figure 2). Higher binding energy features in this region are attributed to low quantities of Si suboxides. Only a slight increase in oxidation of the silicon nanocrystals is observed in the SiX-COOH/GA nanocomposites (Figure 2b) after the aqueous processing, suggesting that the undecanoic acid termination offers some degree of protection of the nanocrystal cores. High-resolution XP spectra of the C 1s region of graphene oxide and SiX-COOH/GA nanocomposites are shown in the Supporting Information (Figure S1). Transmission electron microscopy (TEM) images of the functionalized SiX-COOH nanocrystals and their size distributions are presented in Figures 3a and 3b. The Si3-COOH, Si5-COOH, and Si8-COOH nanocrystals are well separated, consistent with their dispersibility in polar solvents. The larger Si15-COOH nanocrystals, however, appear somewhat more aggregated on the TEM grids. The corresponding TEM images of the SiX-COOH/GA nanocomposites are shown in Figure 3c, revealing that the silicon nanocrystals appear to adhere to the surface of the rGO within the aerogels. X-ray diffraction (XRD) data support the observation by TEM of intact silicon nanocrystals embedded within the graphene aerogel (Figure 4). All XRD patterns show broad reflections at 2θ of 28° and

Figure 2. High-resolution XP spectra of the Si 2p region of (a) SiXCOOH nanocrystals and (b) SiX-COOH/GA nanocomposites (X = 3, 5, 8, and 15 nm). Only Si 2p3/2 components are shown; Si 2p1/2 components are omitted for clarity.

47°, characteristic of Si (111) and (220) lattice planes, respectively.46 As the size of silicon nanocrystals increases, these reflections become narrower and of higher intensity, as expected. A broad peak at a 2θ of 20° noted in XRD patterns of SiX-COOH/GA (X = 3, 5, and 8 nm) corresponds to the graphene aerogel (indicating the poor ordering of graphene sheets along their stacking direction). To probe the distribution of SiX-COOH within the GA, elemental mapping was acquired using a scanning electron microscope equipped with an energy-dispersive X-ray (EDX) spectroscopy system as shown in Figure 5. These aerogel nanocomposites point to a uniform distribution of silicon throughout the graphene aerogel, for all four sizes of silicon nanocrystals. Quantification of the silicon content of the SiXCOOH nanocrystals and SiX-COOH/GA nanocomposites was determined using thermal gravimetric analysis (TGA in an argon atmosphere, Figure S2) and elemental analysis (Table S1). The weight loss of SiX-COOH nanocrystals at 600 °C was 38% (3 nm), 31% (5 nm), 13% (8 nm), and 8% (15 nm) due to loss of the undecanoic acid ligands; the silicon content (remaining mass) was calculated to be 62, 69, 87, and 92%, respectively (Figure S2a). The corresponding silicon content of the SiX-COOH/GA nanocomposites, as determined in the C


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Figure 3. (a) Bright-field TEM images of the SiX-COOH nanocrystals, (b) their size distributions represented as a histogram, and (c) the brightfield TEM images of SiX-COOH/GA nanocomposites (X = 3, 5, 8, and 15 nm).

nanocomposites at a current density of 200 mA/g. The specific capacity values were calculated based on the silicon content obtained from CHNS analysis (Table S1). The 5, 8, and 15 nm nanocrystals (i.e., Si5-COOH, Si8-COOH, and Si15-COOH) show poor cycling stability and low specific capacities (Figure 6a). The 3 nm nanocrystal size, Si3-COOH, is the worst of the series, with a specific capacity of essentially zero. Given that the conductive carbon black used for electrode preparation has a much larger average particle size (diameter of ∼42 nm) compared to the silicon nanocrystals, the SiXCOOH nanocrystals are likely to aggregate with themselves instead of distributing uniformly in the carbon black, leading to poor electrical contact.47 In comparison with the SiX-COOH nanocrystals in carbon black, the SiX-COOH/GA (X = 5, 8, and 15 nm) nanocomposites exhibit improved cycling stability and increased specific capacities, with initial specific capacities of >2500 mAh/g for Si8-COOH/GA and Si15-COOH/GA. The specific capacity decreases significantly, however, over 100 cycles. The initial specific capacity of Si5-COOH/GA is lower (∼1750 mAh/g) but more stable over the same number of cycles. It is noteworthy that the contribution of the GA host in the SiX-COOH/GA nanocomposites (X = 5, 8, and 15 nm) is negligible given that the specific capacity of the NC-free graphene aerogel is less than 50 mAh/g between 0 and 2 V (Figure S6). Furthermore, the composite electrodes contain only 10 wt % GA, making its anticipated contribution to the electrode performance essentially zero. Also, the Si3-COOH/ GA shows almost no electrochemical performance, similar to what was observed with the silicon nanocrystals in carbon black. The very poor electrochemical performance of the smaller silicon nanocrystals, in particular Si3-COOH and Si3-COOH/

Figure 4. XRD patterns of graphene aerogels containing undecanoic acid-functionalized SiNCs, SiX-COOH/GA (X = 3, 5, 8, and 15 nm).

same manner by TGA, was revealed to be 58% (3 nm), 67%, (5 nm) 83% (8 nm), and 86% (15 nm), as shown in Figure S2b. Additional characterization of the SiX-COOH/GA nanocomposites includes Raman spectroscopy (Figure S3) and nitrogen adsorption−desorption isotherms (BET/BJH, Figure S4). The electrochemical performance of the SiX-COOH nanocrystals and SiX-COOH/GA nanocomposites as anode materials was tested using CR2032 coin cells, where Li foils were used as the counter electrode. Figure 6 shows the cycling performance of the SiX-COOH nanocrystals (without GA, on carbon black) and their corresponding SiX-COOH/GA D


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Figure 5. (a) Secondary electron SEM images, (b) EDX spectra, and (c) EDX mapping of the SiX-COOH/GA nanocomposites (X = 3, 5, 8, and 15 nm).

Figure 6. Cycling performance and associated Coulombic efficiency of (a) undecanoic acid-functionalized silicon nanocrystals, SiX-COOH, and (b) their corresponding aerogel nanocomposites, SiX-COOH/GA (X = 3, 5, 8, and 15 nm).

GA, was surprising since the expectation was that smaller silicon nanocrystals would be less sensitive to the large volume changes upon cycling. The higher ratio of surface ligands to silicon could be the cause; these surface ligands could reasonably be expected to electrically isolate the SiNCs from the conductive graphene aerogel matrix while simultaneously physically hindering Li diffusion. Because TGA experiments indicated that the undecanoic acid functional groups decompose at 600 °C under an argon atmosphere (Figure S2), a series of the four SiX-COOH/GA (X = 3, 5, 8, and 15 nm) nanocomposites were annealed at 600 °C to decompose the surface ligands and produce annealed SiX/GA nanocomposites. SEM imaging of the SiX-COOH/GA and annealed SiX/GA nanocomposites indicates negligible morphological changes following annealing (Figures S7 and S8). The XRD patterns, however, reveal some changes to both the Si and GA structure that are particularly evident for the

aerogels containing smaller silicon nanocrystals (i.e., Si3/GA and Si5/GA; compare Figure 4 and Figure S9). The broad GA reflection observed at 2θ of 20° (Figure 4) shifts to higher angles, meaning that the graphene sheets now have a lower average spacing, and now overlaps with the broad Si (111) reflection at 2θ of 28°. A reduction in the interplanar spacing is often observed when annealing (reduced) graphene oxide, possibly due to further thermal reduction.48 In addition, the Si 2p region of the high-resolution XP spectra of annealed SiX/ GA nanocomposites shows that these nanocomposites are oxidized compared to the SiX-COOH/GA nanocomposites, with marked oxidation observed for the annealed Si3/GA nanocomposite (Figure S10). As a result, only very broad, overlapping peaks are visible in the XRD pattern of Si3/GA in the 2θ region between 15° and 40°, and the previously observed distinct Si (220) is no longer visible. The electrochemical performance of the annealed Si3/GA nanoE


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Figure 7. (a) Cycling performance and associated Coulombic efficiency of the annealed SiX/GA nanocomposites at the current density of 200 mA/ g, (b, c) galvanostatic charge−discharge curves of the annealed SiX/GA nanocomposites at the first and second cycles, (d) rate tests of the annealed SiX/GA nanocomposites at various rates ranging from 200 to 8000 mA/g, and (e) cycling performance and associated Coulombic efficiency of the annealed Si3/GA nanocomposite at the current density of 400 mA/g.

capacities of ∼1900 and ∼1500 mAh/g, respectively (Figure 7a). As the size of the silicon nanocrystals decreases from 15 to 3 nm, their initial specific capacity decreases, perhaps due to the surface oxidation observed in XPS data of Si3(−COOH)/ GA and Si5(−COOH)/GA (Figure 2 and Figure S10), which not only lowers the actual amount of active Si in the electrode but also limits the diffusion of lithium into the silicon cores.49,50 The cycling stability of SiX/GA nanocomposites improved significantly when the size of the nanocrystals decreased from 15 to 3 nm. This observation is consistent with the original hypothesis that decreasing the size of the silicon

composite was completely different from that of Si3-COOH/ GAthis material exhibits a stable specific capacity of ∼1500 mAh/g, with no detectable capacity loss after 100 cycles (Figure 7a). Improvement in cycling stability was observed for the Si5/GA and Si8/GA nanocomposites over 100 cycles, but the performance of Si15/GA deteriorated after heat treatment, possibly because the Si particles were more susceptible to aggregation (see Figure 3a). The initial specific capacity of the Si15/GA and Si8/GA nanocomposites is ∼2600 mAh/g at a current density of 200 mA/g, while Si5/GA and Si3/GA exhibit initial specific F


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composed of smaller SiNCs; a higher fraction of conductive GA can also be expected to improve the cycling stability. The combined XRD and XPS results (Figures S9 and S10) warrant a closer examination for the annealed Si3/GA nanocomposite, as it is obviously oxidized. Figure S13 shows differential capacity plots (dQ/dV) for the Si3/GA nanocomposite, essentially the reciprocal of the derivative of the galvanostatic charge−discharge curves in Figures 7b and 7c, of the selected cycles at 200 mA/g. The characteristic response of amorphous Si showing two broad delithiation peaks centered at ∼0.30 and ∼0.47 V vs Li/Li+ is more readily recognizable in Figures 7b and 7c and takes many cycles to fully develop.55 Lithium silicate phases have been shown to reversibly store Li ions when particle size is very small.56 It appears that the SiOx phase formed after annealing only decomposes into Li2O and Si after repeated lithiation/delithiation cycling. The influence of thermal processing on the composition of the SEI was investigated using XPS. Si8-COOH/GA and Si8/ GA nanocomposites were chosen for this study because both show high reversible capacity. Because the influence of the Si surface on SEI formation is expected to be most dramatic during the first few cycles, the XP spectra were collected after the first and third cycles. The survey XP spectra and the corresponding calculated surface compositions of the Si8(−COOH)/GA nanocomposites are shown in Figures S14 and S15. While Si was detectable using EDX (Figures S16 and S17), it was present below the detection limit of XPS. These observations are consistent with the Si being covered with an SEI. These analyses also suggest that the impact of thermal processing on the overall SEI composition is negligible. We note a slightly higher combined carbon and oxygen content for the electrodes prepared with Si8/GA compared to Si8COOH/GA, particularly after the third cycle. The highresolution C 1s spectra (Figure S18a) show a relative increase of the signal at higher binding energies, ∼290 eV, after annealing, consistent with the presence of a larger proportion of lithium carbonate (Li2CO3), which is a two-electron reduction product of ethylene carbonate; these data are consistent with the lower CE of Si8/GA compared to Si8COOH/GA (Figures 6b and 7a). However, it is important to recall that the contribution of the GA component is expected to be disproportionately large because of its high surface area; the same will also be true for its contribution to the XPS signal as it will be encapsulated with SEI even if the reversible capacity is low.

nanocrystals could improve resistance toward mechanical fracture and pulverization induced by large volume change.19 The initial Coulombic efficiency (CE) of the functionalized silicon nanocrystals and their corresponding aerogel nanocomposites (both unannealed and annealed) is between 40% and 60% (Figures 6 and 7a and Figure S11b) but could be improved by prelithiation.49 The low initial CE likely arises as a result of initial SEI formation, reduction of SiOx on the surface of the silicon nanocrystals, and reactions between the remaining oxygen-containing groups in the graphene aerogel and lithium ions.49 The final CE of all nanocomposites reaches above 99% (Figures 6 and 7a). Figures 7b and 7c show the galvanostatic charge−discharge curves of the SiX/GA nanocomposites at the first and second cycles. The first discharge (lithiation) curves of Si5/GA, Si8/ GA, and Si15/GA show flat plateaus at around 0.11 V, characteristic of crystalline Si.51 After the first cycle, the charge−discharge curves of these anodes show sloping curves, indicative of Li alloying with amorphous Si.52,53 The result is consistent with previous studies on Si anodes, which indicate that crystalline silicon becomes amorphous after the first cycle. The rate capability of the annealed SiX/GA anodes were evaluated at current densities ranging from 200 to 8000 mA/g. Figure 7d and Figure S11a show the specific capacity and capacity retention at various rates, ranging from 200 to 8000 mA/g. The specific capacities of Si3/GA, Si5/GA, Si8/GA, and Si15/GA are ∼500, ∼1000, ∼1200, and ∼500 at 4000 mA/g, respectively (Figure 7d). Si15/GA retained only ∼20% of its initial specific capacity at 4000 mA/g, while Si8/GA, Si5/GA, and Si3/GA show higher capacity retention and retained ∼45%, ∼50%, and ∼33% of their initial specific capacity at 4000 mA/g, respectively (Figure S11a). In addition, they recovered up to 90% of their initial specific capacity when the current density was returned to 200 mA/g (Figure S11a). Upon further cycling at 200 mA/g, Si15/GA degrades rapidly whereas the capacity of Si3/GA, Si5/GA, and Si8/GA seems to be stable (Figure 7d). The prolonged cycling test of Si3/GA at a higher current density (400 mA/g) is shown in Figure 7e, which indicates a stable specific capacity of ∼1100 mAh/g, a capacity retention of over 90% after 500 cycles, and final CE of above 99%; this result highlights the favorable effect of reduced particle size on the pulverization resistance of the active material. This material still shows good cycling stability at higher rates of 1 and 2 A/g, with a maximum capacity of 1231 mAh/g after 100 cycles at 1 A/g, and the capacity retention of 79% after an additional 300 cycles at 2 A/g (Figure S12). We observe a continuous improvement in the cycling stability with decreasing SiNC diameter, even though our largest particles are already much smaller than the “critical diameter” (∼150 nm), established through in situ TEM experiments performed by others, below which SiNCs become resistant to fracture.10 While these observations may seem counterintuitive, bare surfaces of lithiated silicon are known to “weld” together when they come into contact.54 This process could lead to the formation of Si agglomerates that exceed the critical size. It is reasonable that the denser ligand shells of the smaller SiNCs would make them more resistant to this effect, leading to further improvements in the cycling stability. The improvement of cycling stability with decreasing SiNC size was observed before and after thermal processing, showing that the original ligand shell and its decomposition products after annealing play a similar role. In addition, TGA and CHNS data (Figure S2 and Table S1) show lower Si to C ratios for GAs

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CONCLUSIONS Nanocomposite LIB anode materials exhibiting high stable specific capacity were synthesized by integrating the size- and surface-dependent properties of silicon nanocrystals with conductive mesoporous graphene aerogels. The improved cycling performance of the SiX/GA nanocomposites in this work can be ascribed to the following reasons: (1) smaller sizes of silicon nanocrystals minimize pulverization of Si particles due to high stress/strain tolerance, (2) effective surface functionalization prevents their surface oxidation and facilitates uniform distribution of the silicon nanocrystals in the GA matrix, and (3) a conductive mesoporous graphene aerogel improves electrical conductivity and provides a nanoporous structure to accommodate the large volume changes of Si particles during the lithiation/delithiation cycling. We believe that our approach could be further extended to other highG


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supercritical drying. Briefly, SiX-COOH nanocrystals (60 mg) were dispersed in an aqueous solution of GO (6 mg/mL, 1.25 mL), diluted to 4 mL with water, and stirred vigorously with a magnetic stirrer for 5 min to obtain a uniform dispersion (the initial mass ratio of SiXCOOH:GO was 8:1). L-Ascorbic acid (72 mg) was added into the dispersion, which was stirred for another 5 min. The resulting dispersion was transferred to a plastic syringe (12 × 60 mm2) and heated at 90 °C for 3 h to reduce the GO and form the nanocomposite hydrogel. The resulting hydrogel was placed in a capped glass vial and rinsed with DI water three times to remove any impurities. Subsequently, DI water was exchanged with acetone, refreshing the acetone bath every 6 h, for 1 day. The acetoneexchanged gel was placed in a home-built CO2 supercritical dryer (shown in Figure S19) and dried, in the same manner as described in ref 59, to yield a monolithic nanocomposite aerogel labeled SiXCOOH/GA. When studying the effect of thermal processing on the electrochemical performance of the SiX-COOH/GA nanocomposites, these materials were annealed at 600 °C under argon flow for 3 h to yield annealed SiX/GA nanocomposites. Materials Characterization. Powder samples of nanocrystals and nanocomposites were used for characterization, unless otherwise indicated. Fourier transform infrared (FTIR) spectra of SiX-COOH nanocrystals were recorded using a Nicolet Magna 750 IR spectrometer. X-ray photoelectron spectroscopy (XPS) data were acquired using a Kratos Axis Ultra instrument as described previously.59 Transmission electron microscopy (TEM) images were obtained using a JEOL-2010 electron microscope equipped with a LaB6 filament and operated at an accelerating voltage of 200 kV. The TEM samples were prepared by drop-coating dilute suspensions of the SiX-COOH nanocrystals and SiX-COOH/GA nanocomposites onto a holey carbon-coated copper grid. Particle size distributions were measured manually by counting at least 300 particles using ImageJ software (1.48v). X-ray diffraction (XRD) analysis was performed on an AXS diffractometer (Discover 8, Bruker, Madison, WI) with Cu Kα radiation (λ = 1.5406 Å). The diffractometer was equipped with a Histar general-area two-dimensional detection system (GADDs) with a sample−detector distance of 22.25 cm. Raman spectra were acquired using a Renishaw inVia Raman microscope equipped with a 514 nm excitation laser and a power of 3.98 mW on the sample. Powder samples were measured on a gold-coated glass substrate. Scanning electron microscopy (SEM) images were obtained using a Zeiss Sigma 300 VP-FESEM equipped with a secondary electron detector and a Bruker energy-dispersive X-ray (EDX) spectroscopy system operated at 10 kV. A conductive carbon coating was applied on all samples using a Leica EM SCD005 evaporative carbon coater prior to characterization. Nitrogen adsorption−desorption isotherms were recorded using a Quantachrome ASiQwin surface area and porosimetry analyzer at 77 K. Fine powders of aerogel nanocomposites were degassed under vacuum at 130 °C for 6 h prior to the measurements, and the isotherm data were fitted as described previously.59 Thermal gravimetric analysis (TGA) was performed on a Mettler Toledo TGA/DSC 1 Star system under an argon atmosphere (25− 650 °C, 10 °C/min). Carbon, hydrogen, and nitrogen contents were measured using a Thermo Scientific Flash 2000 organic elemental analyzer equipped with Eager Xperience software. The electrochemical performance of the materials tested was evaluated using CR2032 coin cells. The working electrodes were prepared using a slurry method; for silicon nanocrystal samples (no GA), the nanocrystals were mixed with conductive carbon black and PVDF dissolved in NMP with a mass ratio of 8:1:1 to form homogeneous slurries. For the SiX-COOH/GA and SiX/GA nanocomposites, they were mixed with PVDF dissolved in NMP with a mass ratio of 9:1. Next, the slurries were spread onto stainless steel spacers and dried overnight at 65 °C in a vacuum oven. The average mass loading of all the electrode materials, including binder, was ∼0.42 mg/cm2. The electrolyte was a mixture of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) and fluoroethylene

capacity active materials (e.g., Sn, Sb, and SnSb) for various battery applications (e.g., LIBs, SIBs, etc.).

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EXPERIMENTAL SECTION

Materials. All reagents were used as received, unless otherwise indicated. A methyl isobutyl ketone solution of hydrogen silsesquioxane (HSQ, trade name Fox-17) was obtained from Dow Corning; the solvent was removed under vacuum, and the resulting white solid was used without further purification. Electronic grade hydrofluoric acid (HF, 49% aqueous solution) and L-ascorbic acid (L-AA) were purchased from J.T. Baker. 10-Undecenoic acid (98%), azobis(isobutyronitrile) (AIBN, 98%), lithium foil (99.9%), 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/diethyl carbonate (1:1 v/v), fluoroethylene carbonate (99%), N-methyl-2-pyrrolidone (NMP), methanol (reagent grade), ethanol (reagent grade), toluene (reagent grade), acetone (reagent grade), and hydrogen peroxide (H2O2, 30%) were obtained from Sigma-Aldrich. Natural graphite flake (99.9%), carbon black (50% compressed, 99.9+%), and poly(vinylidene fluoride) (PVDF) were purchased from Alfa Aesar. Potassium permanganate (KMnO4, reagent grade), sulfuric acid (H2SO4, reagent grade), phosphoric acid (H3PO4, reagent grade), and hydrochloric acid (HCl, 30%) were obtained from Caledon. CR2032 coin cells and stainless steel spacers were purchased from MTI Corporation. Trilayer polypropylene−polyethylene−polypropylene separators (porosity of 39%) were obtained from Celgard. Toluene was dried using an Innovative Technologies, Inc., solvent purification system. SiNC/SiO2 Composite Synthesis and SiNC Liberation. A detailed procedure of silicon nanocrystal synthesis can be found elsewhere.38,39 Briefly, a sample of HSQ (i.e., 10 g) was annealed under 5% H2/95% Ar for 1 h at 1100, 1200, 1300, and 1400 °C to produce SiO2-like composites containing silicon nanocrystals of sizes ∼3, 5, 8, and 15 nm, respectively. Hydride-terminated silicon nanocrystals (H-SiNCs) were liberated from the silica matrix by etching SiNC/SiO2 composites (0.9 g) in a 1:1:1 ethanol/H2O/HF solution (30 mL) for 1 h, followed by extraction into toluene. (Caution! HF is extremely dangerous and must be handled with extreme care.) Synthesis of Undecanoic Acid-Functionalized SiNCs (Referred to as SiX-COOH, Where X Is the Diameter, ∼3, 5, 8, and 15 nm). The procedure used for radical hydrosilylation has been previously described.44 Briefly, after etching of SiNC/SiO2 composites (0.9 g), H-SiNCs (d ∼ 3, 5, 8, or 15 nm) suspended in toluene were centrifuged twice (3000 rpm, 5 min) and redispersed in dry toluene (16 mL) in a Schlenk flask equipped with a magnetic stir bar. 10Undecenoic acid (4 g) and AIBN (200 mg) were added to the flask, and the mixture was subjected to three freeze/pump/thaw cycles using an argon-charged Schlenk line. After stirring at 75 °C for 15 h, the resulting particles were collected by centrifugation (3000 rpm, 5 min) and purified by three successive cycles of dispersion/ precipitation using methanol/toluene as the solvent/antisolvent mixture. The functionalized SiNCs were dispersed in benzene and freeze-dried. Synthesis of Graphene Oxide (GO). GO was prepared by oxidation of natural graphite following the improved Hummers’ method.57,58 Briefly, graphite flakes (3.0 g) and KMnO4 (18.0 g) were dispersed in a 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL). After stirring for 15 h at 50 °C, the green suspension turned dark purple. The mixture was cooled to room temperature and slowly poured into an ice cold solution of water (400 mL) and 30% H2O2 (3 mL). The mixture was centrifuged (12000 rpm, 1 h), and the filtrate was washed successively with deionized (DI) water (200 mL × 2), 30% HCl (200 mL × 1), and ethanol (200 mL × 3). After each washing, the supernatant was discarded, and the unreacted graphite was removed. Finally, the brown GO precipitate was dispersed in DI water (6 mg/mL). Synthesis of Silicon Nanocrystal/Graphene Aerogel Composites. SiNC/graphene aerogel composites were synthesized by incorporating SiX-COOH (X = 3, 5, 8, or 15 nm) into the GO solution, followed by chemical reduction (to rGO) and CO2 H


Article

Chemistry of Materials carbonate, with a 9:1 volume ratio. A Celgard 2325 polypropylene− polyethylene−polypropylene membrane with a porosity of 39% was used as the separator. Coin cells were assembled using the working electrode and a Li foil, as the counter electrode, in an argon-filled glovebox with oxygen and moisture contents below 1 and 0.1 ppm, respectively. Galvanostatic cycling measurements were performed using an Arbin BT2000 battery testing system at the voltage window of 0.01−2 V at 25 °C. The current densities of 200 or 400 mA/g were applied for cycle life measurements and up to 8000 mA/g for rate capability tests.

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also thank Dr. Erik J. Luber and Brian C. Olson for discussions about battery performance and Dr. Md Hosnay Mobarok and all Veinot and Buriak team members for their assistance and useful suggestions.

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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/acs.chemmater.8b03198. Additional data related to the characterization of SiXCOOH nanocrystals (i.e., TGA, CHNS, N2 adsorption− desorption isotherms, electrochemical), SiX-COOH/GA nanocomposites (i.e., TGA, CHNS, Raman, N 2 adsorption−desorption isotherms, SEM/EDX, XPS, electrochemical), and annealed SiX/GA nanocomposites (i.e., SEM/EDX, XRD, XPS, electrochemical); photos of the CO2 supercritical dryer (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(J.G.C.V.) E-mail: [email protected]. *(J.M.B.) E-mail: [email protected]. ORCID

Maryam Aghajamali: 0000-0003-2802-9721 Hezhen Xie: 0000-0001-9275-9169 Morteza Javadi: 0000-0002-2249-326X W. Peter Kalisvaart: 0000-0003-1228-906X Jillian M. Buriak: 0000-0002-9567-4328 Jonathan G. C. Veinot: 0000-0001-7511-510X Author Contributions

M.A. and H.X. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge continued generous funding from the NSERC Discovery Grant Program (RGPIN-2014-05195, RPGIN-2015-03893), the NSERC CREATE supporting the Alberta/Technical University of Munich International Graduate School for Hybrid Functional Materials (ATUMS, CREATE-463990-2015), the Department of Chemistry at the University of Alberta (UofA), Future Energy Systems of the University of Alberta (Grants T12-P04 and T06-Z01), Alberta Innovates Technology Futures (Grant AITF iCORE IC50-T1 G2013000198), and the Canada Research Chairs program (CRC 207142). Support from the Faculty of Science and FGSR of the University of Alberta for ATUMS is acknowledged. We thank the staff at the UofA Chemistry Design and Manufacturing Centre for building the CO2 supercritical dryer and the staff at the UofA Chemistry Analytical and Instrumentation Laboratory for their assistance with FTIR, TGA, and CHNS analyses. The authors thank Prof. Dr. Tom Nilges and Claudia Ott at the Department of Chemistry, Technical University of Munich, for the initial electrochemical characterization and useful discussions. We I


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