Direct Synthesis of Alloyed Si1–xGex Nanowires for Performance

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Direct Synthesis of Alloyed Si1−xGex Nanowires for Performance-Tunable Lithium Ion Battery Anodes Killian Stokes, Hugh Geaney, Grace Flynn, Martin Sheehan, Tadhg Kennedy, and Kevin M. Ryan* Bernal Institute and Department of Chemical Sciences, University of Limerick, Limerick V94 T9PX, Ireland S Supporting Information *

ABSTRACT: Here we report the formation of high capacity Li-ion battery anodes from Si1−xGex alloy nanowire arrays that are grown directly on stainless steel current collectors, in a single-step synthesis. The direct formation of these Si1−xGex nanowires (ranging from Si0.20Ge0.80 to Si0.67Ge0.33) represents a simple and efficient processing route for the production of Li-ion battery anodes possessing the benefits of both Si (high capacity) and Ge (improved rate performance and capacity retention). The nanowires were characterized through SEM, TEM, XRD and ex situ HRSEM/HRTEM. Electrochemical analysis was conducted on these nanowires, in half-cell configurations, with capacities of up to 1360 mAh/g (Si0.67Ge0.33) sustained after 250 cycles and in full cells, against a commercial cathode, where capacities up to 1364 mAh/g (Si0.67Ge0.33) were retained after 100 cycles. KEYWORDS: Si1−xGex nanowires, alloy, lithium-ion battery, ex situ, full-cell growth of NWs serves to inhibit the formation of “parasitic” microscale Si islands.26 Despite the benefits of these approaches, Si NWs still perform poorly at faster cycling rates, and the requirements for additional nonactive materials in multiple step processes may limit their viability. Directly grown Ge NWs exhibit better capacity retention and rate capability compared to Si, due to their higher rate of Li diffusivity at room temperature (400×) and greater electrical conductivity (1000×).27−31 Ge NWs have been shown to maintain capacities of 900 mAh/g over 1000 cycles with discharge capacities greater than 400 mAh/g at rates of 100C.32 Despite their promise, the commercial use of anodes consisting entirely of Ge is unlikely due to its high material cost. The ability to combine the high capacity and lower cost of Si with the greater stability and rate capability offered by Ge is attractive as a potentially viable Li-ion battery technology. Recently, we have demonstrated a process whereby Si NWs were grown as branches from Ge NW stems.33 When cycled, these materials exhibited capacities greater than the theoretical figure for Ge and showed improved capacity retention at high C rates compared to pure Si NWs. This two-stage seed mediated growth process demonstrated the capability to harness the advantages of both elements although a one-stage growth process is more desirable.

H

igher capacity Li-ion battery anodes are required to meet the ever-increasing stored energy demands in consumer electronics and electric vehicles.1−3 Anodes of lithium alloying materials Si (3579 mAh/g) and Ge (1384 mAh/g) have been the subject of significant research interest, with their formation as nanostructures allowing them to withstand the volume changes (∼300%) during cycling which has proven detrimental to the use of their bulk forms in Li-ion batteries.4−17 The catalytic seeding mechanisms for Si and Ge nanowires (NWs) allows their growth directly from current collectors, resulting in improved conductivity and shorter Li diffusion distances due to good electronic contact.18 Furthermore, this direct anchoring means these NW arrays offer a more electrode efficient fabrication route which does not require the preparation and casting of slurries while also eliminating nonactive material weight (in the form of conductive additives and binders) from the overall battery. The viability of Si NWs as a commercial Li-ion anode material to date is inhibited by an appreciable capacity fade during longterm cycling and poor capacity retention in rate capability testing.19 In attempts to improve the electrochemical performance of Si NWs, many different material architectures have been investigated, most notably core/shell structures with crystalline Si NW coated with amorphous Si, carbon, TiO2, or Al2O3 to improve the mechanical stability and/or conductivity of the NWs.20−24 Deposition of an interfacial layer of graphene onto the current collector prior to NW growth has also been shown to improve the anode’s interfacial stability,25 while AAO-templated © 2017 American Chemical Society

Received: June 28, 2017 Accepted: September 13, 2017 Published: September 13, 2017 10088

DOI: 10.1021/acsnano.7b04523 ACS Nano 2017, 11, 10088−10096

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Cite This: ACS Nano 2017, 11, 10088-10096

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ACS Nano Alloyed Si1−xGex NWs with controllable compositions have been the subject of interest for semiconductor technologies.34 There has been particular attention given to their use as thermoelectric materials for high temperature energy conversion,35 transistors with enhanced transport properties,36,37 and as photonic materials for optical communications and IR sensors.37 These NWs have been synthesized using laser-assisted catalyzed growth (LCG),38 molecular beam epitaxy (MBE),39 and chemical vapor deposition (CVD) where gaseous precursors such as germane gas and either silane or disilane gas are combined to grow NWs from catalyst decorated substrates.40,41 Si1−xGex active materials have also received interest as Li-ion battery anodes in a range of different morphologies to date,42 which have included: crystalline SiGe nanotubes,43,44 nanostructured Si1−xGex thin films,45 SiGe templated nanostructures,46 and Si1−xGex that was cosputtered onto Cu or NixSiy NWs.47,48 Au seeded growth of SiGe alloy NWs were grown by Kim et al. using CVD, however, the most Si-rich composition obtained was Si0.15Ge0.85.49 While these materials displayed higher gravimetric capacities compared to Ge and better rate capability than Si, long-term cycling performance was not demonstrated. Here, we report the direct synthesis of Sn seeded Si1−xGex alloy NWs, in high densities from stainless steel current collectors, in a single step wet chemical reaction, using a high boiling point solvent system.50−59 Different Si to Ge atomic ratios (Si0.20Ge0.80 to Si0.67Ge0.33) can be obtained by balancing the ratios and reactivities of the precursors that are simultaneously introduced into the flask in a single injection. The electrochemical performance of the Si1−xGex NWs were studied in both half-cell and full-cell configurations with the most Si rich anodes displaying the highest capacities and the more Gerich anodes showing the best capacity retention at faster charge and discharge rates.

Figure 1. Schematic illustrating the synthetic method used for the synthesis of a range of Si1−xGex alloy NWs. A SS current collector coated with a 20 nm evaporated layer of Sn was placed into a high boiling point solvent via a simple glassware based system. The temperature was ramped to 460 °C prior to the injection of a Si and Ge precursor mixture. Growth proceeds via the SLS mechanism where Si and Ge monomers were simultaneously delivered to the Sn catalyst seed.

RESULTS AND DISCUSSION A schematic of the wet chemical synthetic setup for Si1−xGex alloy NW formation is outlined in Figure 1. The process involves the simultaneous injection of liquid precursors, phenylsilane (Si) and triphenylgermane (Ge), into a reaction flask containing Sncoated SS substrates submerged in squalane at 460 °C, where NW growth occurs through the solution-liquid−solid (SLS) mechanism. Figure 2a shows a low magnification SEM image of a typical Si1−xGex NW array demonstrating the dense coverage across the substrate. A higher resolution SEM image, Figure 2b, of the as-synthesized material (Si0.67Ge0.33) confirms the majority of the NWs are 8−10 μm in length with diameters typically between 80 and 100 nm. While the quantity of the Si precursor (PS) was kept constant for each reaction, it was possible to control the Si to Ge ratios of the electrodes through varying the quantity of the Ge precursor (TPG) within the mixed precursor injection. Overall the NW mass loadings ranged from 0.246 mg/ cm2 to 0.308 mg/cm2. EDX analysis, performed on SEM, was used to quantify the atomic % of Si and Ge within each alloy composition by analyzing 5−6 different locations of approximately 100 μm2 across each substrate. Additional details are available in the Supporting Information (Figures S1−S3). Figure 2c shows XRD data for each alloy composition scanned between 25° and 30° in order to highlight the (111) reflection. As the concentration of Si in the alloy NWs increases, a clear shift is observed in the (111) going from the pure Ge peak (26.9°) to pure the Si peak (28.2°). It should be noted that reflections corresponding to pure Si and Ge were not observed in any of the

Figure 2. (a) SEM image of the resulting high density NW growth of Sn seeded Si0.67Ge0.33 alloy NWs. Scale bar = 5 μm. (b) Higher magnification SEM image of an alloy NW. (c) X-ray diffraction of 4 different alloy compositions as well as cubic Si and cubic Ge with normalized intensities. A slow scan was performed on each composition between 25° and 30° in order to clearly measure the shift in the (111) diffraction reflection. 10089

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Figure 3. TEM and STEM analysis performed on the Si0.67Ge0.33 NWs. (a) TEM image of a Sn-seeded Si0.67Ge0.33 NW. (b) HRTEM of the highlighted area in (a) where defects are observed across the diameter of the wire. (c) HRTEM image of the area highlighted in (b) giving a clearer image of the defects with the inset showing the diffraction pattern for this region. (d) DF-STEM image of an alloy NW. (e) EDX line profile with Si in green, Ge in blue, and Sn in red with the direction of the scan indicated by the blue line in (d).

alloy samples. This shift in the (111) reflection matches closely with the lattice constants for each composition.60 Full XRD scans of Si NWs, Ge NWs, and Si0.50Ge0.50 NWs are available in the Supporting Information (Figure S4). Defects were found in each of the four Si1−xGex NWs similar to what has been reported for pure Si and Ge NWs.61 Parts a and b of Figure 3 show low- and high-magnification TEM images of a Sn-seeded Si0.67Ge0.33 NW with longitudinal defects along its length. A higher magnification TEM image, Figure 3c, shows the defects in clearer detail. These defects were found to propagate in the ⟨111⟩ direction, seen by the presence of twinning in the inset diffraction pattern in Figure 3c. These [111] orientated twin

defects have previously been reported for NWs and are likely due to the low surface tension of the Sn catalyst seed which results in an unstable growth front.62,63 Using the distance in reciprocal space between the (002) and (002̅) planes (7.22 1/nm), the lattice parameter for the NW was determined to be 5.539 Å, which is intermediate between the theoretical values of pure Si (5.4305 Å) and pure Ge (5.6400 Å). This intermediate lattice parameter again suggests alloying of Si and Ge in the NWs and supports the XRD data shown in Figure 2c. The DF-STEM image (Figure 3d) shows no obvious contrast difference across the length of the NW suggesting a homogeneous composition. EDX line scan analysis confirms this (Figure 3e) where the 10090

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Figure 4. (a) Discharge capacities and Coulombic efficiencies over 250 cycles of the four different compositions of Sn seeded Si1−xGex alloy NWs. The active material was charged and discharged at a C/5 rate in the potential range of 0.01−1.0 V. (b) First charge and discharge voltage profiles of each alloy composition. (c) Rate capability testing for each of the alloy compositions at C/10, C/5, C/2, C, 2C, 5C, 10C, and C/10. (d) Differential capacity plots for the first charge and discharge for each of the alloy NW compositions and reference plots for Si and Ge.

respectively (in line with the increasing Si to Ge ratio), with the corresponding first charge and discharge voltage profiles seen in Figure 4b. While the exhibited capacities are slightly below the maximum theoretical specific capacities calculated for each composition, this has been found to be a consequence of a native oxide layer that forms on Si (or Ge) as the substrates have been exposed to air during different steps in the fabrication process.45 These oxide layers have been found to irreversibly alloy with lithium resulting in a loss in capacity.65 Over the 250 cycles, the four different compositions of Si1−xGex NWs all exhibited enhanced capacity retention compared to pure Si NWs.66 After 250 cycles, the pure Si NWs maintained just 70% of their initial capacities. In comparison, the alloy compositions demonstrated an improvement in capacity retention of up to 25% over the pure Si NW anode. Despite a slightly larger percentage capacity fade compared to pure Snseeded Ge NW anodes,32 the incorporation of Si atoms into the NW structures has a major capacity boosting effect with the Si0.67Ge0.33 NWs sustaining a capacity of 1360 mAh/g after 250 cycles. Each of the Si1−xGex alloy ratios were analyzed at different rates by charging and discharging the material for five cycles at rates of C/10, C/5, C/2, C, 2C. 5C, 10C, and back to C/10 (Figure 4c). Electrodes with higher compositions of Si exhibited the highest capacities at the slowest rates (C/10 and C/5). At these rates, there was little strain on the active material during the lithiation and delithiation processes and the capacities increased with increasing Si content. Once faster cycling rates were applied, the more Ge-rich compositions began outperforming those with higher Si content. The most Ge-rich composition (Si0.20Ge0.80) exhibited the best capacity value of 532 mAh/g at the 10C rate,

signals for Si and Ge remain constant throughout the measured area of the NW before decreasing to near zero as the scan entered the Sn seed. Interestingly, there is a measurable Sn signal through the length of the NW which could potentially be attributed to incorporation of Sn metal at the twin defects. This correlates with previous reports where defects act as sites for metal incorporation.53,64 Defects were observed for each of the four alloy ratios presented in this work with further TEM, STEM and EDX data available in the Supporting Information (Figures S5 and S6). The electrochemical performance of the alloy NWs was evaluated through galvanostatic cycling of the material in a two electrode Swagelok type cell cycled in a potential range of 0.01−1 V versus Li/Li+ with a 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) electrolyte +3% vinylene carbonate additive present. The electrochemical properties of the four different Si1−xGex alloy atomic ratios (Si0.20Ge0.80, Si0.33Ge0.67, Si0.50Ge0.50, and Si0.67Ge0.33) were investigated. The maximum theoretical capacity for each ratio was determined to be 1516, 1698, 1868, and 2047 mAh/g, respectively, which were calculated using the atomic ratios and the maximum theoretical capacities for Si, Ge, and Sn. The masses of the entire Li-active anode material (i.e., NW masses including Si, Ge and Sn catalyst) were taken into account when calculating the charge/discharge rates and capacity figures for the plots shown in Figure 4a. The as-grown NWs on SS were used directly as the working electrode, and the material was cycled at a rate of C/5 for 250 cycles. The discharge capacities and Coulombic efficiencies for each of the four ratios over the first 250 cycles are shown in Figure 4a where the initial discharge capacities were 1242, 1460, 1600, and 1709 mAh/g, 10091

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Figure 5. TEM and STEM images showing the progression of deformation as a result of lithiation and delithiation on the Si0.67Ge0.33 morphology after one cycle: (a, b), 10 cycles (c, d), 20 cycles (e, f) and 100 cycles (g, h) where the NW morphology has been replaced by a network consisting of interwoven ligaments of active material. (i) and (ii) represent Si and Ge, respectively.

materials (Figure 4d). The DCP for the first cycle of each of the four Si1−xGex alloy NW compositions and for pure Si and pure Ge NWs were normalized and plotted against voltage. A prominent lithiation peak for the Si NWs was seen at 0.13 V, and a peak for the Ge NWs can be observed at 0.38 V. The alloy NWs show distinct peaks within this voltage range based on their composition. The differential capacity data is another strong indicator of the alloyed nature of the NWs and confirms the interplay between Ge and Si occurring during lithiation/ delithiation. Additional analysis on the DCPs, as well as cyclic voltammetry (CV) plots and voltage profiles of the 1st, 10th, 50th, 100th, and 250th charge/discharge cycles have been included in the Supporting Information (Figures S8 and S9).

an effect also witnessed by Abel et al. in thin-film alloys.45 This is likely due to the higher rate of Li+ diffusivity in Ge and its greater conductivity versus Si.27,28 The Si0.67Ge0.33 alloy NWs went from the best performing anode at the slowest rates to the worst performing at the fastest C rates. However, this anode still maintained a capacity of 320 mAh/g and is a substantial improvement over the pure Si NW anodes which displayed capacities of 180 mAh/g at 1 °C (Figure S7). After cycling at 10C, the charge/discharge rate was returned to the initial rate of C/10, and all of the different alloy compositions showed good capacity recovery, regaining over 96% of their initial capacities. Differential capacity plots (DCP) were used to gain insight into the lithiation and delithiation behaviors of the active 10092

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Figure 6. Full-cell testing of alloy NW anodes. Specific capacity retention (a) and Coulombic efficiencies (b) over 100 charge/discharge cycles. First charge and discharge voltage profiles (c) and associated differential capacity plots (d).

Full-cell testing of the Si1−xGex NWs was conducted using a LCO cathode within a two-electrode configuration at a C/5 rate. The initial specific capacities (based on the anode masses) are, in order of increasing Si content, 1272, 1426, 1551, and 1654 mAh/ g (Figure 6a). The initial capacity for the Si0.67Ge0.33 NW anode was the highest for the full-cells tested; however, it suffered the quickest rate of capacity fade, further illustrating the stabilizing effect from the presence of Ge in the alloy NWs. For comparison, full-cells consisting of pure elemental Si and Ge NWs were also prepared (Figure S12). The Coulombic efficiency values for the full-cells are provided in Figure 6b. All of the initial CE values were ≤60%, indicating the need for anode preconditioning or pretreatment in practical full-cells.71,72 For the various compositions, the CE values were 1−2% lower in the full-cell compared to the half-cell measurements. This, along with the increased rate of capacity fade, is to be expected given the limited Li inventory in the LCO cathode (full-cell) compared to the Li metal electrode (half-cell). Parts c and d, respectively, of Figure 6 show that the anode lithiation peaks are unique for each alloy composition, and the voltage profiles in Figure 6c show that the reversible capacity for the first discharge closely follows the composition of the alloy NWs (decreasing capacity from high Si content to low Si content). More detailed analysis on the voltage profiles and the differential capacity (including plots for anodes of pure Si and pure Ge), as well as the CVs for each composition investigated, is included in the Supporting Information (Figures S13 and S14). The full-cell capacities followed similar trends to those seen for the analogous half-cells, with a clear trade-off between enhanced capacity stability for the Ge rich alloys and

The structural changes occurring to the Si1−xGex NW active material due to the effects of Li-ion cycling were investigated by ex situ SEM, TEM, and DF-STEM analysis. After one cycle, the NW morphology remained distinguishable from TEM analysis (Figure 5a); however, nanopore formation throughout the NW due to the insertion and extraction of lithium during cycling was clearly visible.67,68 Examination of the material after 10 (Figure 5b) and 20 (Figure 5c) cycles revealed an increasing amount of deformation to the NWs. Although the outlines of some wires could still be identified using SEM (Figure S10) at this stage, extensive texturing and material restructuring had occurred due to fusion with surrounding NWs via lithium assisted electrochemical welding.69,70 After 100 cycles (Figure 5d), the original NW morphology has been completely lost and is now replaced by a continuous porous network structure. EDX elemental mapping shows that the restructured material is comprised of interlinked ligaments of Si, Ge, and small quantities of Sn (Figure S11). The EDX elemental mapping (Figure 5 insets) reveals that the dispersion of Si and Ge atoms remains constant throughout the active material. This shows that the initial alloy homogeneity (in terms of the uniform distribution of Si and Ge atoms within the NWs) is not disrupted even in the most Si rich composition. Despite this extensive restructuring, the active material remains well contacted to the current collector, which is clear from the stable capacities exhibited in Figure 4a. SEM images showing the changes in the morphology after 1, 10, 20, and 100 cycles for the Si0.67Ge0.33 alloy composition are available in the Supporting Information (Figure S10). 10093

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cool. Once the setup cooled to room temperature, the substrates were extracted and cleaned in toluene (Aldrich, 99%). Pure Si NWs and pure Ge NWs can be produced in the same system by injecting solely PS or diphenylgermane (DPG). Analysis. Scanning electron microscopy (SEM) analysis was performed on a Hitachi SU-70 system operating between 5 and 20 kV. The substrates required no prior treatment before SEM analysis. Alloy ratios were investigated using SEM Oxford instruments EDS detector where the atomic and weight % of multiple areas of the substrate were determined and an average Si/Ge ratio was determined per substrate. For transmission electron microscopy (TEM) analysis, the alloy NWs were removed from the growth substrate through sonication before being drop cast onto a lacey carbon TEM grid. TEM analysis was conducted at 200 kV on a JEOL JEM-2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS detector. X-ray diffraction was conducted on a PANalytical X’Pert PRO MRD instrument with a Cu Kα radiation source (λ = 1.5418 Å) and an X’celerator detector. The masses of Sn and Ge/Si were determined using a Sartorius Ultra-Microbalance (SE2) (repeatability ±0.25 μg). Electrochemical Measurements. The electrochemical performance was evaluated by assembling two-electrode Swagelok-type cells in an Ar-filled glovebox. The cells consisted of Si1−xGex alloy NWs on an SS current collector as the working electrode, Li foil as the counter and reference electrode, a Celgard separator, and an electrolyte solution of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v) + 3% vinylene carbonate. Galvanostatic measurements were carried out using a Biologic MPG-2 in a potential range of 0.01−1.0 V versus Li/Li+. Fullcell experiments were conducted by pairing NW-based substrates as the counter/reference electrode with commercial (LiCoO2) LCO on Al foil (NEI corporation) as the working electrode in a two-electrode configuration. 0.66 cm2 cathodes were punched from the foil, leading to a fixed cathode capacity of 555.52 μAh in each test. As a result, there was a sufficient cathode excess to account for irreversible losses due to SEI formation and material restructuring. This level of cathode excess is not practical for a real-world application; however, it allows the NWs to be examined in full-cell configuration to gauge the impact of composition on the fundamental full-cell responses. Further studies are underway to investigate the role of anode/cathode balancing in NW based anodes. The currents applied for galvanostatic testing at a C/5 rate were calculated based on the masses of the NW anodes. Cycling was performed between 3.9 and 2.8 V to minimize Li plating on the anode at higher potentials.

higher specific capacities for the Si rich alloys. All full-cell capacities, after 100 cycles, were lower than their half-cell counterparts at this point, a likely consequence of the limited lithium resources in the commercial cathode. Nevertheless, encouraging cycling performance for the directly grown alloy NWs in full-cell configurations was observed for all ratios where the anodes with the Si0.67Ge0.33 NWs retaining the highest capacities of 1364 mAh/g after 1oo cycles. Rate capability analysis of the Si1−xGex NW compositions in a full cell is available in the Supporting Information (Figure S15).

CONCLUSION Sn-seeded Si1−xGex alloy NWs were successfully grown on SS substrates using a low-cost, glassware-based solvent vapor growth system. The NWs, of varying atomic ratios, were grown via the SLS mechanism through the simultaneous delivery of Si and Ge precursors to a Sn seed. The alloyed nature of the NWs was confirmed through XRD, TEM, and EDX analysis. The electrochemical performance of the Si1−xGex NWs showed that the most Si-rich compositions exhibited the highest specific capacities, while the most Ge-rich compositions were found to perform the best at faster charge/discharge rates. This synthetic method represents a versatile, tunable platform for the formation of Si1−xGex alloy NWs of varying ratios, combining the high capacities of Si with the higher rate performance of Ge. This allows for synthetic design of advanced LIB anode materials where capacity, rate performance, and stability can be compositionally tuned. EXPERIMENTAL SECTION Substrate Preparation. Stainless steel (SS, 316) foil was purchased from Pi-Kem, Ltd. with a thickness of 0.1 mm. The foil was roughened using P600 grit sandpaper to increase the surface area and improve the contact between the current collector and the active material. From this, substrates of approximately 8 mm × 8 mm were cut and weighed. A 20 nm layer of Sn (99.999%, Kurt J. Lesker) was then thermally evaporated onto these pieces in a glovebox-based evaporation unit. The substrates were stored in the Ar-filled glovebox prior to reactions to minimize oxidation. Chemicals Used. Phenylsilane (PS, 97%) was supplied by Fluorochem and was stored and dispensed from an Ar-filled glovebox. Triphenylgermane (TPG, 97%) was supplied by Fluorochem and also stored in an Ar-filled glovebox. The TPG was dissolved in squalane (1:4 wt %) and sonicated to ensure thorough mixing prior to being dispensed for reactions. The TPG/squalane solution was stored and dispensed from an Ar-filled glovebox. Reaction Set-up. Reactions were carried out in a 100 mL Pyrex long-neck round-bottom flask. The substrates with a 20 nm evaporated layer of Sn were slotted into a custom holder and placed horizontally into the bulb of the flask. The flask was connected to a Schlenk line setup via a water condenser. A volume of 8 mL squalane (99%, Aldrich) was added and the system was ramped up to a temperature of 125 °C using a three zone furnace. A vacuum of at least 100 mTorr was applied for 1 h to remove any moisture from the system. Following this the system was purged with Ar. The flask was allowed to ramp up to a reaction temperature of 460 °C under a constant flow of Ar. A water condenser controlled the solvent reflux and kept the reaction under control. Once the desired reaction temperature was reached, a mixture of phenylsilane and the TPG/squalane was injected into the system via the septum cap sealing the condenser. By varying the amount of TPG in the precursor mixture, different ratios of Si−Ge alloy NWs were achieved. A constant amount of 700 μL of PS was used, while 200, 100, 50, and 30 μL quantities of TPG/squalane were added to this to synthesize the desired alloy compositions of Si0.20Ge0.80, Si0.33Ge0.67, Si0.50Ge0.50, and Si0.67Ge0.33, respectively. The reaction was allowed to proceed for 1 h and was then terminated by opening the furnace to allow the system to

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04523. Additional SEM images of each Si1−xGex composition; additional TEM showing the presence of defects for each Si1−xGex composition and DF-STEM elemental mapping of a NW showing signals for Si, Ge and Sn; cycling data (at C/5) and rate capability data for Si and Ge NWs as well as SEM images showing the changes in NW morphology as a result of Li cycling after 1, 10, 20, and 100 cycles and an additional DF-STEM image with elemental maps of the alloy NW morphology after 100 cycles (showing signals for Si, Ge and Sn); full-cell specific capacities for Si and Ge NWs (100 cycles) and voltage profiles along with differential capacity plots of the first charge/discharge cycle for four different Si1−xGex compositions, Si and Ge NWs and cyclic voltammetry for Si NWs, Ge NWs and each of the Si1−xGex compositions analyzed carried out in half cell and full cell configurations (PDF) 10094

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Kevin M. Ryan: 0000-0003-3670-8505 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Science Foundation Ireland (SFI) under the Principal Investigator Program under Contract No. 11PI-1148. K.S. thanks the Irish Research Council for funding through the Government of Ireland Postgraduate Scheme, and H.G. acknowledges Enterprise Ireland under Contract No. CF20144014. M.S., G.F. and T.K. acknowledge Intel Ireland and the Irish Research Council for funding through the Enterprise Partnership Scheme. REFERENCES (1) Kennedy, T.; Brandon, M.; Ryan, K. M. Advances in the Application of Silicon and Germanium Nanowires for High-Performance Lithium-Ion Batteries. Adv. Mater. 2016, 28, 5696−5704. (2) Osiak, M.; Geaney, H.; Armstrong, E.; O’Dwyer, C. Structuring Materials for Lithium-Ion Batteries: Advancements in Nanomaterial Structure, Composition, and Defined Assembly on Cell Performance. J. Mater. Chem. A 2014, 2, 9433−9460. (3) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366−377. (4) Bogart, T. D.; Chockla, A. M.; Korgel, B. A. High Capacity Lithium Ion Battery Anodes of Silicon and Germanium. Curr. Opin. Chem. Eng. 2013, 2, 286−293. (5) 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. (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) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Control of Thickness and Orientation of Solution-Grown Silicon Nanowires. Science 2000, 287, 1471−1473. (8) Zhang, W.-J. A Review of the Electrochemical Performance of Alloy Anodes for Lithium-Ion Batteries. J. Power Sources 2011, 196, 13−24. (9) Liu, Y.; Zhang, S.; Zhu, T. Germanium-Based Electrode Materials for Lithium-Ion Batteries. ChemElectroChem 2014, 1, 706−713. (10) Chockla, A. M.; Klavetter, K. C.; Mullins, C. B.; Korgel, B. A. Solution-Grown Germanium Nanowire Anodes for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 4658−4664. (11) Chan, C. K.; Ruffo, R.; Hong, S. S.; Huggins, R. A.; Cui, Y. Structural and Electrochemical Study of the Reaction of Lithium with Silicon Nanowires. J. Power Sources 2009, 189, 34−39. (12) Obrovac, M. N.; Christensen, L. Structural Changes in Silicon Anodes during Lithium Insertion/Extraction. Electrochem. Solid-State Lett. 2004, 7, A93−A96. (13) Winter, M.; Besenhard, J. O. Electrochemical Lithiation of Tin and Tin-Based Intermetallics and Composites. Electrochim. Acta 1999, 45, 31−50. (14) McDowell, M. T.; Cui, Y. Single Nanostructure Electrochemical Devices for Studying Electronic Properties and Structural Changes in Lithiated Si Nanowires. Adv. Energy Mater. 2011, 1, 894−900. (15) Mullane, E.; Kennedy, T.; Geaney, H.; Ryan, K. M. A Rapid, Solvent-Free Protocol for the Synthesis of Germanium Nanowire Lithium-Ion Anodes with a Long Cycle Life and High Rate Capability. ACS Appl. Mater. Interfaces 2014, 6, 18800−18807. 10095

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