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SUPPORTING INFORMATION FOR

Artificial Solid Electrolyte Interphase-Protected LixSi Nanoparticles:

An Efficient and Stable Prelithiation Reagent for Lithium-Ion

Batteries Jie Zhao,† Zhenda Lu,† Haotian Wang,‡ Wei Liu,† Hyun-Wook Lee,† Kai Yan,† Denys Zhuo,† Dingchang Lin,† Nian Liu,§ and Yi Cui*,†,|| †Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ‡Department of Applied Physics, Stanford University, Stanford, California 94305, United States §Department of Chemistry, Stanford University, Stanford, California 94305, United States ||Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States

Materials and Synthesis.

1.1 Synthesis of LixSi NPs

Si NPs (50 nm, MTI, Inc.) were dried under vacuum for 48h and then heated to 120 ºC in the argon glove box for 12h

to remove trapped water and oxygen. 500 mg of Si NPs were heated at 200 ºC in a tantalum crucible with a cap, and

then 550 mg of Li metal foil (99.9%, Alfa Aesar) was added inside. Crystalline LixSi NPs were synthesized by heating

the mixture of Si NPs and Li foil at 200 ºC under mechanical stirring at 200 rpm for 3 days in a glove box (Ar

atmosphere, H2O level <0.1 ppm and O2 level <1.2 ppm). For a small amount of Si NPs, 6 h is enough to transform Si

to LixSi. For gram-synthesis, the heating time can increase to several days to ensure complete reaction. In the process,

the color of the powder changes from brown to black, indicating the formation of the LixSi alloy. After synthesis of the

crystalline LixSi NPs, they were cooled down to room temperature, and stored in the argon glove box.

1.2 Synthesis of artificial-SEI coated LixSi NPs

8 µL 1-fluorodecane (Sigma Aldrich) was dissolved in 5 ml anhydrous cyclohexane (Sigma Aldrich) and then stirred at

60 ºC for 2 h. After cooling the solution, 50 mg of LixSi NPs was then added into the solution and stirred at 200 rpm for

2 h at room temperature. After the reaction, the particles were washed with cyclohexane three times to remove

non-reacted 1-fluorodecane and then dried under vacuum. No change in color of the powder is observed in the

subsequent coating process. The packing density of coated LixSi powder is low as inherited from original Si NPs. SEM

images under different magnifications are shown in Figure S2, revealing uniformly distributed nanopowder without

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strong aggregations. The dimension of the LixSi NPs ranges from 200 to 300 nm, larger than the pristine Si NPs (50 nm)

due to the volume expansion. As synthesized LixSi NPs exhibit clean and smooth surface texture (Figure S2c). The

surface of NPs is roughened (Figure S2f) after the coating process, indicating the formation of protective coating on the

surface.

1.3 Waste disposal

LixSi should be kept away from any possible contact with water and any solvent with active protons, because of violent

reaction and possible flash fire. Any waste containing LixSi should be exposed in fume hood for more than 2 days, and

then react with ethanol to remove remaining active materials.

Characterizations.

Characterizations were carried out using TEM (FEI Tecnai G2 F20 X-TWIN), Raman spectroscopy (WITEC Raman

spectrometer), X-ray Diffraction (PANalyticalX’Pert, Ni-filtered Cu Kα radiation) and X-ray photoelectron

spectroscopy (SSI SProbe XPS spectrometerwith Al Kα source). A FEI Titan 80–300 environmental TEM was

employed for Electron energy loss (EELS) collection at an acceleration voltage of 300 kV. The energy window of the

EELS was 40–145 eV for Li (Li K-edge, 54.7 eV). LixSi NPs are sensitive to ambient moisture, so the samples were

sealed within a quartz cell in the glove box before Raman characterization. The wavelength of excitation laser for

Raman spectroscopy is 532 nm. The XRD samples were sealed with Kapton tape (DuPont) to eliminate possible side

reactions with moisture and oxygen in the air. The broad background of the XRD patterns in Figure 2a and Figure S13

comes from the Kapton tape.

Electrochemical Measurements.

Si NPs (MTI, Inc.), Sn NPs (Sigma Aldrich), graphite, carbon black (Super P, TIMCAL, Switzerland), and

polyvinylidenefluoride binder (PVDF, Kynar HSV 900) were dried under vacuum for 24h to remove trapped water. To

prepare the working electrodes, various materials were dispersed uniformly in tetrahydrofuran (THF, Sigma Aldrich) to

form a slurry. (Anode materials and mass ratio are based on specific cells.) The slurry was then cast onto a thin copper

foil and dried under vacuum. Coin-type cells (2032) were assembled in an Ar-filled glovebox using a Li metal foil as

counter/reference electrode. The electrolyte is 1.0 M LiPF6 in 1:1 w/w ethylene carbonate/diethyl carbonate (BASF).

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Cyclic voltammetry measurements were carried out on a BioLogic VMP3 system. Galvanostatic cycling was carried

out using an MTI 8 Channel battery tester. The total mass loading of the Si or Sn based anode was 0.7-1.0 mg cm-2 and

a typical total mass loading of the graphite based anode was 2.0-2.5mg cm-2.

Supplementary Figures

 Figure S1. Digital photographs of 1 g as-prepared LixSi NPs in (a) the tantalum crucible and (b) 20 ml vial. (c) Digital photograph of artificial-SEI-coated LixSi powder on the weighing paper (3×3 in).

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Figure S2. SEM images of LixSi NPs (a-c) before and (d-f) after coating.

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Figure S3.TEM images of LixSi NPs with (a) thin and (b) thick coating.

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Figure S4. TEM-EDS analysis of artificial-SEI-coated LixSi NPs. The inset is the magnified EDS spectrum.

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Figure S5. Electron energy loss spectrum (EELS) of Li K-edge.

e- （LixSi）

LiF

F

+CH2•

CH2• e- （LixSi）

Li

LiO2

OLiCO2

O

O

OLi  

Figure S6. The detailed reaction mechanism we proposed.

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Figure S7. High-resolution XPS spectra of (a) Li 1s and (b) O 1s.

Figure S8. (a) Extraction capacities of LixSi NPs before and after coating. (b) Cyclicvoltammetry (CV) measurements of artificial-SEI-coated LixSi NPs at a scan rate of 0.1  mV/s over the potential window of 0.01 to 1.5 V versus Li/Li+.

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Figure S9. First cycle voltage profiles of tin NPs/coated LixSi composite (60:5 w/w) and tin NPs control cells (The capacity is based on the mass of both tin and Si in coated LixSi NPs.).

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Figure S10. Cycling performance of coated LixSi NPs (red), LixSi NPs (blue) and Si control cells (orange) at C/20 for the first three cycles and C/2 for the following cycles (1C = 4.2 A/g, the capacity is based on the mass of Si). The purple line is the Coulombic efficiency of coated LixSi NPs.

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Figure S11. The Coulombic efficiency of graphite/coated LixSi composite (85:5 w/w) and graphite control cells.

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Figure S12. First cyclevoltage profile of graphite added with coated LixSi, exposed to humid air with 10% RH for 6h (graphite/coated LixSi=85:5 w/w).

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Figure S13. XRD patterns of uncoated (top) and coated (bottom) LixSi NPs in humid air for 2 h with 10% RH.