Supporting Information

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1 Supporting Information A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes Nian Liu,, Hui Wu,, Matthew T. McDowell, Yan Yao, Chongmin Wang, and Yi Cui *,, Department of Chemistry, Department of Materials Science and Engineering, Stanford University, CA 94305, USA Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA These authors contributed equally. Correspondence and requests for materials should be addressed to Y.C. ( yicui@stanford.edu) EXPERIMENTAL PROCEDURES Synthesis of Si@void@C. For a typical synthesis of Si@void@C, 150 mg Si nanoparticles (<100 nm, Aldrich) were first dispersed in a mixture of 240 ml ethanol and 60 ml water by ultrasonication, followed by the addition of concentrated ammonium hydroxide (3.0 ml). Under vigorous stirring, 2.4 g tetraethoxysilane (TEOS, Aldrich) was added dropwise into the dispersion and the reaction was left at room temperature under stirring for 12 hr. SiNPs coated by SiO 2 were collected by centrifugation, and washed three times using water. The obtained Si@SiO 2 were mixed with dopamine hydrochloride (150 S1

2 mg, Aldrich) in Tris-buffer (75 ml, 10 mm; ph 8.5) and stirred for 24 hr. The obtained particles were collected by centrifugation and washed three times using water. To carbonize the polydopamine coating, the dried powders were placed in a tube furnace and heated under N 2 at 400 o C for 2 hr with a heating rate of 1 o C/min, and then at 800 o C for 3 hr with a heating rate of 5 o C/min. Finally, to remove the SiO 2 sacrificial layer, the Si@SiO powders were immersed in 10 wt% HF aqueous solution for 30 minutes, followed by centrifugation and ethanol washing three times. Si@void@C powders (~100 mg) were obtained. Characterization. The weight percentage of Si and C in Si@void@C was determined from the weight loss curves measured under simulated air atmosphere (20% O % Ar, both are ultra purity grade gases from Airgas) on a TG/DTA instrument (Netzsch STA 449) with a heating rate of 5 o C/min. Other characterization was carried out using scanning electron microscopy (FEI Sirion), transmission electron microscopy (FEI Tecnai G2 F20 X-twin), X-ray photoelectron spectroscopy (XPS, SSI S-Probe Monochromatized, Al Kα radiation at 1486 ev), and X-ray diffraction (PANalytical X Pert, Ni-filtered Cu Kα radiation). In situ TEM characterization. The in situ TEM experiments were carried out using a dual-probe STM- TEM holder manufactured by Nanofactory Instruments AB. This holder allows for nanoscale positioning of each probe along with the capability of applying an electrical bias. Using this holder, a nanoscale electrochemical cell was fabricated in a similar manner to a recent report. 1 Figure 3A shows a schematic of the experimental setup. The working electrode consists of a silicon nanowire (NW) attached to the Au probe at the bottom of the schematic. This Si NW has a thin (~20 nm) Cu layer S2

3 thermally evaporated onto one side to increase the electrical conductivity. The yolk-shell particles are physically attached to the sidewalls of the NW. The counter electrode consists of LiCoO 2 particles attached to the other probe. A drop of ionic liquid electrolyte, which has a very low vapor pressure and does not evaporate inside the TEM column, was placed on this electrode. The ionic liquid electrolyte was 10 wt% lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) in a solvent of 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (P 14 TFSI). Inside the TEM column (FEI Titan), the NW/particle electrode was positioned so that the tip of the NW was immersed in the electrolyte while the remaining section of the NW and the particles remained out of the electrolyte in the field of view. A bias of -4V was applied to the NW electrode with respect to the LiCoO 2 electrode, which caused lithium ions to be reduced at the NW electrode and to diffuse into the NW from the electrolyte as the NW became lithiated. In a typical experiment, the entire NW length can become lithiated within minutes. As soon as the NW section in contact with the Si@void@C particles became lithiated, the particles themselves were also lithiated due to diffusion of Li atoms between the structures. In this way, it was possible to observe the lithiation of the particles. The Si NWs for the experiment were grown on silicon substrates using the vapor-liquid-solid (VLS) method with 50 nm gold nanoparticles as the catalyst, and copper was deposited on the NWs with thermal evaporation. Electrochemistry. To make the working electrode, an aqueous slurry method was used. Si@void@C powders, carbon black (Super P, TIMCAL, Switzerland), and sodium alginate binder (MP Biomedicals LLC, USA) or polyvinylidene difluoride binder (PVDF, Kynar HSV 900) with a ratio of 65:20:15 were added to water, and stirred for over 8 hr to ensure thorough mixing. The slurry was cast onto a thin copper foil and dried. Prior to cell fabrication, the electrodes were degassed in vacuum oven at 100 o C S3

4 for at least 4 hr. Coin-type cells (2032) were fabricated inside an Ar-filled glovebox using the Si@void@C working electrode, Li metal foil counter/reference electrode, and a Celgard 2250 separator soaked in the electrolyte. The electrolyte employed was 1.0 M LiPF 6 in 1:1 w/w ethylene carbonate/diethyl carbonate (EMD Chemicals) with 3 vol % vinylene carbonate (Novolyte Technologies) added to improve the cycling stability. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on a BioLogic VMP3 system. Galvanostatic cycling was performed using a BioLogic VMP3 system or an MTI 8 Channels battery tester ( ma). If not mentioned otherwise, the galvanostatic voltage cutoffs were 0.01 and 1 V vs Li/Li +. The specific capacity was calculated based on the total mass of Si@void@C composite. The charge/discharge rate was calculated with respect to the theoretical capacity of Si (4200 mah/g). The Coulombic efficiency was calculated as C delithiation /C lithiation, where C delithiation and C lithiation are the capacity during Li extraction and insertion, respectively. To characterize the electrode after cycling, cells were discharged to 1 V and opened. The Si@void@C electrodes were washed in acetonitrile to remove the electrolyte while keeping the SEI. S4

5 Figure S1. Optical images of (A) bare Si nanoparticles, (B) and (C) The synthesis of is scalable. Figure S2. SEM images of (A) bare Si nanoparticles, (B) and (C) Figure S3. SEM images of the samples with various void space sizes prepared by controlling the amount of TEOS in the SiO 2 coating step to be (A) 0.6, (B) 1.2, and (C) 2.4g (in a batch of 150 mg SiNPs as described in the experimental procedure). The average SiO 2 coating thickness and therefore the gap size is measured to be ~20, ~30 and ~50 nm, respectively. A small amount of ~50 nm hollow carbon spheres without internal Si particles are present in (C) because the concentrated TEOS synthesis will cause SiO 2 spheres to nucleate directly in the solution as well as coat SiO 2 on the SiNPs. S5

6 Figure S4. Thermogravimetric Analysis (TGA) results of the sample. The sample was heated at a constant rate of 5 o C/min under 20% oxygen and 80% argon. Figure S5. (A) Cyclic voltammogram (CV) for Si@void@C from 1.0 V to 0.01 V versus Li/Li + at a 0.05 mv/s scan rate. The first ten cycles are shown. (B) EIS results for the same cell showing the evolution of the real and imaginary impedance after each cycle of CV scan. 0 in legend means before CV. (C) Magnified view of (B). S6

7 Figure S6. Constant capacity cycling of with alginate binder. The delithiation capacity and the Coulombic efficiency are shown. The lithiation capacity is limited to 1000 mah/g. The current density is 4 A/g. Figure S7. SEM image of a bare SiNP electrode after electrochemical cycling. S7

8 Figure S8. Voltage profiles of under different cycling rates. REFERENCES (1) Huang, J. Y.; Zhong, L.; Wang, C. M.; Sullivan, J. P.; Xu, W.; Zhang, L. Q.; Mao, S. X.; Hudak, N. S.; Liu, X. H.; Subramanian, A.; Fan, H.; Qi, L.; Kushima, A.; Li, J. Science 2010, 330, S8