Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Chemistry of Materials, 30, 21, pp. 7782-7792, 2018-10-19
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=090f4abe-5259-4cb1-a611-0e8c9e1fab9d https://publications-cnrc.canada.ca/fra/voir/objet/?id=090f4abe-5259-4cb1-a611-0e8c9e1fab9d
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.1021/acs.chemmater.8b03198
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Size and surface effects of silicon nanocrystals in graphene aerogel
composite anodes for lithium ion batteries
Aghajamali, Maryam; Xie, Hezhen; Javadi, Morteza; Kalisvaart, W. Peter;
Buriak, Jillian M.; Veinot, Jonathan G. C.
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
*
S Supporting InformationABSTRACT: 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.
■
INTRODUCTIONThe 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.1−3Silicon 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 Li22Si5).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
unstable solid−electrolyte interphase (SEI) formation, leading to loss of electrical contact and poor cyclability.5,7,9To address these issues, one approach is to use nanoscale silicon of various morphologies (e.g., nanoparticles,10 nanowires,11,12 nano-tubes,13,14nanorods,15hollow nanospheres,16and nanoporous silicon17,18). Nanoscale materials offer short Li diffusion distances within the electrode and higher stress/strain tolerance, leading to improved rates and cycling perform-ance.3,7,19A 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
pubs.acs.org/cm Cite This:Chem. Mater. 2018, 30, 7782−7792
Downloaded via NATL RESEARCH COUNCIL CANADA on April 30, 2019 at 18:26:38 (UTC).
Graphene aerogels (GAs) are nanoporous carbon materials exhibiting tunable porosity and high electrical conductiv-ity.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., freeze-drying and CO2 supercritical drying) to form the aerogel
structure.33−35Nanocomposites 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;36,37these composites were prepared by incorporat-ing 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 size-controlled 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
“initial” interphase layer at their interface during LIB cycling. The GA chosen was prepared by CO2supercritical 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
freeze-dried 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.
■
RESULTS AND DISCUSSIONThe overall approach to prepare the silicon nanocrystal/ graphene aerogel composites is shown inScheme 1. Hydride-terminated SiNCs (H-SiNCs) were synthesized following well-established 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 radical44hydrosilylation 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 containingL-ascorbic acid (L-AA) as a reducing agent with a
mass ratio of SiX-COOH:GO of 8:1. Following chemical 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 Si3-COOH/GA and Optical Image of Si3-Si3-COOH/GA under UV Light Exposure, Showing Typical Red Photoluminescence of Embedded 3 nm Diameter Silicon Nanocrystals
reduction of GO to rGO, hydrogels were formed and dried using a CO2supercritical dryer to yield monolithic aerogels.
The hydrosilylation step of the silicon nanocrystals with 10-undecenoic 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
ν
s(CH2) and ν(CO) 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(SiH
x) 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 nano-composites 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 theSupporting Information(Figure S1).
Transmission electron microscopy (TEM) images of the functionalized SiX-COOH nanocrystals and their size dis-tributions are presented inFigures 3a and3b. 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
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 SiX-COOH nanocrystals and SiX-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 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).
Figure 2.High-resolution XP spectra of the Si 2p region of (a) SiX-COOH nanocrystals and (b) SiX-SiX-COOH/GA nanocomposites (X = 3, 5, 8, and 15 nm). Only Si 2p3/2components are shown; Si 2p1/2 components are omitted for clarity.
same manner by TGA, was revealed to be 58% (3 nm), 67%, (5 nm) 83% (8 nm), and 86% (15 nm), as shown inFigure 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 6shows the cycling performance of the SiX-COOH nanocrystals (without GA, on carbon black) and their corresponding SiX-COOH/GA
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 SiX-COOH nanocrystals are likely to aggregate with themselves instead of distributing uniformly in the carbon black, leading to poor electrical contact.47In 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 3.(a) Bright-field TEM images of the SiX-COOH nanocrystals, (b) their size distributions represented as a histogram, and (c) the bright-field TEM images of SiX-COOH/GA nanocomposites (X = 3, 5, 8, and 15 nm).
Figure 4.XRD patterns of graphene aerogels containing undecanoic acid-functionalized SiNCs, 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 nano-composites. SEM imaging of the SiX-COOH/GA and annealed SiX/GA nanocomposites indicates negligible mor-phological 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; compareFigure 4andFigure 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.48In 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 nano-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).
composite was completely different from that of Si3-COOH/ GAthis 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 (seeFigure 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
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 2andFigure 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 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.
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 nano-composites (both unannealed and annealed) is between 40% and 60% (Figures 6 and 7a and Figure S11b) but could be improved by prelithiation.49The 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.49The final CE of all nanocomposites reaches above 99% (Figures 6and7a).
Figures 7b and7c 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.52,53The 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.54This 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
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 S13shows differential capacity plots (dQ/dV) for the Si3/GA nano-composite, essentially the reciprocal of the derivative of the galvanostatic charge−discharge curves inFigures 7b and7c, 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.56It 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 inFigures 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 Si8-COOH/GA, particularly after the third cycle. The high-resolution 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 Si8-COOH/GA (Figures 6b and7a). 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.
■
CONCLUSIONSNanocomposite 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
high-capacity active materials (e.g., Sn, Sb, and SnSb) for various battery applications (e.g., LIBs, SIBs, etc.).
■
EXPERIMENTAL SECTIONMaterials. All reagents were used as received, unless otherwise indicated. A methyl isobutyl ketone solution of hydrogen silsesquiox-ane (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/SiO2composites (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 (Re-ferred 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.44Briefly, after etching of SiNC/SiO
2composites (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. 10-Undecenoic 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.57,58Briefly, graphite flakes (3.0 g) and KMnO
4(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 Com-posites. 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
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 SiX-COOH: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 acetone-exchanged gel was placed in a home-built CO2 supercritical dryer (shown inFigure S19) and dried, in the same manner as described in ref 59, to yield a monolithic nanocomposite aerogel labeled SiX-COOH/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 LiPF
6in ethylene carbonate/diethyl carbonate (1:1 v/v) and fluoroethylene
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.
■
ASSOCIATED CONTENT*
S Supporting InformationThe Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-ter.8b03198.
Additional data related to the characterization of SiX-COOH nanocrystals (i.e., TGA, CHNS, N2adsorption−
desorption isotherms, electrochemical), SiX-COOH/GA nanocomposites (i.e., TGA, CHNS, Raman, N2
adsorption−desorption isotherms, SEM/EDX, XPS, electrochemical), and annealed SiX/GA nanocomposites (i.e., SEM/EDX, XRD, XPS, electrochemical); photos of the CO2supercritical dryer (PDF)
■
AUTHOR INFORMATION Corresponding Authors *(J.G.C.V.) E-mail: jveinot@ualberta.ca. *(J.M.B.) E-mail:jburiak@ualberta.ca. 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 ContributionsM.A. and H.X. contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTSThe 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 Gradu-ate School for Hybrid Functional MGradu-aterials (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
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.
■
REFERENCES(1) Goodenough, J. B.; Park, K. S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135 (4), 1167−1176.
(2) Choi, J. W.; Aurbach, D. Promise and Reality of Post-Lithium-Ion Batteries with High Energy Densities. Nat. Rev. Mater. 2016, 1, 16013.
(3) Liu, Y.; Zhou, G.; Liu, K.; Cui, Y. Design of Complex Nanomaterials for Energy Storage: Past Success and Future Opportunity. Acc. Chem. Res. 2017, 50 (12), 2895−2905.
(4) Jin, Y.; Zhu, B.; Lu, Z.; Liu, N.; Zhu, J. Challenges and Recent Progress in the Development of Si Anodes for Lithium-Ion Battery. Adv. Energy Mater. 2017, 7, 1700715.
(5) Wu, H.; Cui, Y. Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nano Today 2012, 7 (5), 414−429.
(6) Zamfir, M. R.; Nguyen, H. T.; Moyen, E.; Lee, Y. H.; Pribat, D. Silicon Nanowires for Li-Based Battery Anodes: A Review. J. Mater. Chem. A 2013, 1, 9566−9586.
(7) Su, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review. Adv. Energy Mater. 2014, 4, 1300882.
(8) Zuo, X.; Zhu, J.; Müller-Buschbaum, P.; Cheng, Y.-J. Silicon Based Lithium-Ion Battery Anodes: A Chronicle Perspective Review. Nano Energy 2017, 31, 113−143.
(9) Lawes, S.; Sun, Q.; Lushington, A.; Xiao, B.; Liu, Y.; Sun, X. Inkjet-Printed Silicon as High Performance Anodes for Li-Ion Batteries. Nano Energy 2017, 36, 313−321.
(10) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles during Lithiation. ACS Nano 2012, 6 (2), 1522−1531.
(11) Chockla, A. M.; Harris, J. T.; Akhavan, V. A.; Bogart, T. D.; Holmberg, V. C.; Steinhagen, C.; Mullins, C. B.; Stevenson, K. J.; Korgel, B. A. Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material. J. Am. Chem. Soc. 2011, 133 (51), 20914−20921. (12) Nguyen, H. T.; Yao, F.; Zamfir, M. R.; Biswas, C.; So, K. P.; Lee, Y. H.; Kim, S. M.; Cha, S. N.; Kim, J. M.; Pribat, D. Highly Interconnected Si Nanowires for Improved Stability Li-Ion Battery Anodes. Adv. Energy Mater. 2011, 1 (6), 1154−1161.
(13) Park, M.; Kim, M. G.; Joo, J.; Kim, K.; Kim, J.; Ahn, S.; Cui, Y.; Cho, J. Silicon Nanotube Battery Anodes. Nano Lett. 2009, 9 (11), 3844−3847.
(14) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; Mcdowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; et al. Stable Cycling of Double-Walled Silicon Nanotube Battery Anodes through Solid-Electrolyte Interphase Control. Nat. Nanotechnol. 2012, 7 (5), 310− 315.
(15) Ghassemi, H.; Au, M.; Chen, N.; Heiden, P. A.; Yassar, R. S. In Situ Electrochemical Lithiation/Delithiation Observation of Individ-ual Amorphous Si Nanorods. ACS Nano 2011, 5 (10), 7805−7811.
(16) Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N.; Hu, L.; Nix, W. D.; Cui, Y. Interconnected Silicon Hollow Nanospheres for Lithium-Ion Battery Anodes with Long Cycle Life. Nano Lett. 2011, 11 (7), 2949−2954.
(17) Zhu, J.; Gladden, C.; Liu, N.; Cui, Y.; Zhang, X. Nanoporous Silicon Networks as Anodes for Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2013, 15 (2), 440−443.
(18) Li, X.; Gu, M.; Hu, S.; Kennard, R.; Yan, P.; Chen, X.; Wang, C.; Sailor, M. J.; Zhang, J. G.; Liu, J. Mesoporous Silicon Sponge as an Anti-Pulverization Structure for High-Performance Lithium-Ion Battery Anodes. Nat. Commun. 2014, 5, 5105.
(19) Szczech, J. R.; Jin, S. Nanostructured Silicon for High Capacity Lithium Battery Anodes. Energy Environ. Sci. 2011, 4 (1), 56−72.
(20) Lee, J. K.; Smith, K. B.; Hayner, C. M.; Kung, H. H. Silicon Nanoparticles-Graphene Paper Composites for Li Ion Battery Anodes. Chem. Commun. 2010, 46, 2025−2027.
(21) Luo, J.; Zhao, X.; Wu, J.; Jang, H. D.; Kung, H. H.; Huang, J. Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes. J. Phys. Chem. Lett. 2012, 3, 1824−1829.
(22) Wu, H.; Yu, G.; Pan, L.; Liu, N.; McDowell, M. T.; Bao, Z.; Cui, Y. Stable Li-Ion Battery Anodes by in-Situ Polymerization of Conducting Hydrogel to Conformally Coat Silicon Nanoparticles. Nat. Commun. 2013, 4, 1943.
(23) Liu, B.; Soares, P.; Checkles, C.; Zhao, Y.; Yu, G. Three-Dimensional Hierarchical Ternary Nanostructures for High-Perform-ance Li-Ion Battery Anodes. Nano Lett. 2013, 13 (7), 3414−3419.
(24) Liu, N.; Lu, Z.; Zhao, J.; Mcdowell, M. T.; Lee, H. W.; Zhao, W.; Cui, Y. A Pomegranate-Inspired Nanoscale Design for Large-Volume-Change Lithium Battery Anodes. Nat. Nanotechnol. 2014, 9 (3), 187−192.
(25) Chang, J.; Huang, X.; Zhou, G.; Cui, S.; Hallac, P. B.; Jiang, J.; Hurley, P. T.; Chen, J. Multilayered Si Nanoparticle/reduced Graphene Oxide Hybrid as a High-Performance Lithium-Ion Battery Anode. Adv. Mater. 2014, 26 (5), 758−764.
(26) Lu, Z.; Liu, N.; Lee, H. W.; Zhao, J.; Li, W.; Li, Y.; Cui, Y. Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS Nano 2015, 9 (3), 2540−2547.
(27) Zong, L.; Jin, Y.; Liu, C.; Zhu, B.; Hu, X.; Lu, Z.; Zhu, J. Precise Perforation and Scalable Production of Si Particles from Low-Grade Sources for High-Performance Lithium Ion Battery Anodes. Nano Lett. 2016, 16 (11), 7210−7215.
(28) Jin, Y.; Tan, Y.; Hu, X.; Zhu, B.; Zheng, Q.; Zhang, Z.; Zhu, G.; Yu, Q.; Jin, Z.; Zhu, J. Scalable Production of the Silicon-Tin Yin-Yang Hybrid Structure with Graphene Coating for High Performance Lithium-Ion Battery Anodes. ACS Appl. Mater. Interfaces 2017, 9 (18), 15388−15393.
(29) Xu, Q.; Li, J. Y.; Sun, J. K.; Yin, Y. X.; Wan, L. J.; Guo, Y. G. Watermelon-Inspired Si/C Microspheres with Hierarchical Buffer Structures for Densely Compacted Lithium-Ion Battery Anodes. Adv. Energy Mater. 2017, 7 (3), 1601481.
(30) Chen, S.; Shen, L.; van Aken, P. A.; Maier, J.; Yu, Y. Dual-Functionalized Double Carbon Shells Coated Silicon Nanoparticles for High Performance Lithium-Ion Batteries. Adv. Mater. 2017, 29, 1605650.
(31) Gorgolis, G.; Galiotis, C. Graphene Aerogels: A Review. 2D Mater. 2017, 4, 032001.
(32) Ma, Y.; Chen, Y. Three-Dimensional Graphene Networks: Synthesis, Properties and Applications. Natl. Sci. Rev. 2015, 2 (1), 40−53.
(33) Chen, W.; Yan, L. In Situ Self-Assembly of Mild Chemical Reduction Graphene for Three-Dimensional Architectures. Nanoscale 2011, 3, 3132−3137.
(34) Zhang, X.; Sui, Z.; Xu, B.; Yue, S.; Luo, Y.; Zhan, W.; Liu, B. Mechanically Strong and Highly Conductive Graphene Aerogel and Its Use as Electrodes for Electrochemical Power Sources. J. Mater. Chem. 2011, 21, 6494−6497.
(35) Shan, H.; Xiong, D.; Li, X.; Sun, Y.; Yan, B.; Li, D.; Lawes, S.; Cui, Y.; Sun, X. Tailored Lithium Storage Performance of Graphene Aerogel Anodes with Controlled Surface Defects for Lithium-Ion Batteries. Appl. Surf. Sci. 2016, 364, 651−659.
(36) Xu, B.; Wu, H.; Lin, C. X.; Wang, B.; Zhang, Z.; Zhao, X. S. Stabilization of Silicon Nanoparticles in Graphene Aerogel Frame-work for Lithium Ion Storage. RSC Adv. 2015, 5 (39), 30624−30630. (37) Hu, X.; Jin, Y.; Zhu, B.; Tan, Y.; Zhang, S.; Zong, L.; Lu, Z.; Zhu, J. Free-Standing Graphene-Encapsulated Silicon Nanoparticle Aerogel as an Anode for Lithium Ion Batteries. ChemNanoMat 2016, 2 (7), 671−674.
(38) Hessel, C. M.; Henderson, E. J.; Veinot, J. G. C. Hydrogen Silsesquioxane: A Molecular Precursor for Nanocrystalline Si-SiO2 Composites and Freestanding Hydride-Surface-Terminated Silicon Nanoparticles. Chem. Mater. 2006, 18 (26), 6139−6146.
(39) Clark, R. J.; Aghajamali, M.; Gonzalez, C. M.; Hadidi, L.; Islam, M. A.; Javadi, M.; Mobarok, M. H.; Purkait, T. K.; Robidillo, C. J. T.; Sinelnikov, R.; et al. From Hydrogen Silsesquioxane to Functionalized Silicon Nanocrystals. Chem. Mater. 2017, 29 (1), 80−89.
(40) Hua, F.; Swihart, M. T.; Ruckenstein, E. Efficient Surface Grafting of Luminescent Silicon Quantum Dots by Photoinitiated Hydrosilylation. Langmuir 2005, 21 (13), 6054−6062.
(41) Rosso-Vasic, M.; Spruijt, E.; Van Lagen, B.; De Cola, L.; Zuilhof, H. Alkyl-Functionalized Oxide-Free Silicon Nanoparticles: Synthesis and Optical Properties. Small 2008, 4 (10), 1835−1841.
(42) Dasog, M.; De Los Reyes, G. B.; Titova, L. V.; Hegmann, F. A.; Veinot, J. G. C. Size vs Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum via Surface Groups. ACS Nano 2014, 8 (9), 9636−9648.
(43) McVey, B. F. P.; Tilley, R. D. Solution Synthesis, Optical Properties, and Bioimaging Applications of Silicon Nanocrystals. Acc. Chem. Res. 2014, 47 (10), 3045−3051.
(44) Yang, Z.; Gonzalez, C. M.; Purkait, T. K.; Iqbal, M.; Meldrum, A.; Veinot, J. G. C. Radical Initiated Hydrosilylation on Silicon Nanocrystal Surfaces: An Evaluation of Functional Group Tolerance and Mechanistic Study. Langmuir 2015, 31 (38), 10540−10548.
(45) Clark, R. J.; Dang, M. K. M.; Veinot, J. G. C. Exploration of Organic Acid Chain Length on Water-Soluble Silicon Quantum Dot Surfaces. Langmuir 2010, 26 (19), 15657−15664.
(46) Henderson, E. J.; Kelly, J. A.; Veinot, J. G. C. Influence of HSiO 1.5 Sol−Gel Polymer Structure and Composition on the Size and Luminescent Properties of Silicon Nanocrystals. Chem. Mater. 2009, 21 (22), 5426−5434.
(47) Wetjen, M.; Pritzl, D.; Jung, R.; Solchenbach, S.; Ghadimi, R.; Gasteiger, H. A. Differentiating the Degradation Phenomena in Silicon-Graphite Electrodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2017, 164 (12), A2840−A2852.
(48) Wen, Y.; He, K.; Zhu, Y.; Han, F.; Xu, Y.; Matsuda, I.; Ishii, Y.; Cumings, J.; Wang, C. Expanded Graphite as Superior Anode for Sodium-Ion Batteries. Nat. Commun. 2014, 5, 4033.
(49) Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y. A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett. 2012, 12 (6), 3315−3321.
(50) Xun, S.; Song, X.; Wang, L.; Grass, M. E.; Liu, Z.; Battaglia, V. S.; Liu, G. The Effects of Native Oxide Surface Layer on the Electrochemical Performance of Si Nanoparticle-Based Electrodes. J. Electrochem. Soc. 2011, 158 (12), A1260−A1266.
(51) Obrovac, M. N.; Krause, L. J. Reversible Cycling of Crystalline Silicon Powder. J. Electrochem. Soc. 2007, 154 (2), A103−A108.
(52) Chen, L. B.; Xie, J. Y.; Yu, H. C.; Wang, T. H. An Amorphous Si Thin Film Anode with High Capacity and Long Cycling Life for Lithium Ion Batteries. J. Appl. Electrochem. 2009, 39 (8), 1157−1162. (53) Li, J.; Dahn, J. R. An In Situ X-Ray Diffraction Study of the Reaction of Li with Crystalline Si. J. Electrochem. Soc. 2007, 154 (3), A156−A161.
(54) Karki, K.; Epstein, E.; Cho, J. H.; Jia, Z.; Li, T.; Picraux, S. T.; Wang, C.; Cumings, J. Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes. Nano Lett. 2012, 12 (3), 1392− 1397.
(55) Du, Z.; Hatchard, T. D.; Dunlap, R. A.; Obrovac, M. N. Combinatorial Investigations of Ni-Si Negative Electrode Materials for Li-Ion Batteries. J. Electrochem. Soc. 2015, 162 (9), A1858−A1863. (56) Kim, T.; Park, S.; Oh, S. M. Solid-State NMR and Electrochemical Dilatometry Study on Li+ Uptake/Extraction Mechanism in SiO Electrode. J. Electrochem. Soc. 2007, 154 (12), A1112−A1117.
(57) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z. Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4 (8), 4806−4814.
(58) Thomas, H. R.; Day, S. P.; Woodruff, W. E.; Vallés, C.; Young, R. J.; Kinloch, I. A.; Morley, G. W.; Hanna, J. V.; Wilson, N. R.; Rourke, J. P. Deoxygenation of Graphene Oxide: Reduction or Cleaning? Chem. Mater. 2013, 25 (18), 3580−3588.
(59) Aghajamali, M.; Iqbal, M.; Purkait, T. K.; Hadidi, L.; Sinelnikov, R.; Veinot, J. G. C. Synthesis and Properties of Luminescent Silicon Nanocrystal/Silica Aerogel Hybrid Materials. Chem. Mater. 2016, 28 (11), 3877−3886.
