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Supplementary Information for

In situ chemical synthesis of lithium fluoride/metal

nanocomposite for high capacity prelithiation of cathodes

Yongming Sun1, Hyun-Wook Lee

1, Guangyuan Zheng

1, Zhi Wei Seh

1, Jie Sun

1, Yanbin Li

1

and Yi Cui1,2*

1Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, United States

2Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,

2575 Sand Hill Road, Menlo Park, California 94025, United States

* To whom correspondence should be addressed. Email: yicui@stanford.edu (Y. C.)

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Methods

Materials Synthesis. The preparation of LiF/Co and LiF/Fe nanocomposites was carried out in

an Argon-filled glove box with moisture level below 0.1 ppm and oxygen level below 3.0 ppm.

To synthesize LiF/Co nanocomposite, lithium metal foil (12 mmol, 99.9%, Alfa Aesar) was

firstly melted at 185 ºC, followed by the addition of CoF3 powder (4 mmol, 99.98%,

Sigma-Aldrich). After the mixture was mechanical stirred for 20 minutes at 185 ºC and 2 hours

at 240 ºC, a black LiF/Co product was obtained. LiF/Fe nanocomposite was synthesized with the

same procedure using FeF3 powder (4 mmol, Sigma-Aldrich) and lithium metal foil (12 mmol)

as the starting materials. Samples were stored in dry air condition to eliminate the residual

lithium metal.

Characterization. Power X-ray diffraction (XRD) patterns were recorded on a panalytical

X’pert diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5406 Å). During the XRD

measurement, the samples were encapsulated with a Kapton tape. The protection from the

Kapton tape helped to confirm whether there was residual lithium metal in the products.

Scanning electron microscopy (SEM) characterizations were performed using an FEI XL30

Sirion SEM. Transmission electron microscopy (TEM), high resolution TEM (HRTEM) and

scanning TEM (STEM) images were conducted on an FEI Titan 80-300 environmental TEM. A

PHI Versa Probe 5000 system (Physical Electronics, MN) was used for X-ray photoelectron

spectroscopy (XPS) analyses.

Electrochemical measurements. The LiF/Co and LiF/Fe working electrodes were prepared by

mixing 70% active materials, 20% carbon black and 10% polyvinylidene fluoride (PVDF) binder

in N-methyl-2-pyrrolidone (NMP) solvent, casting the slurry on an Al foil and drying it at 80 ºC

in vacuum. Their typical mass loading is ~1 mg/cm2. The same slurry method was also used to

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construct LiFePO4 electrodes with cathode prelithiation materials. The as-made cathodes consist

of 75.2% commercial LiFePO4 powder (MTI Corporation), 4.8% prelithiation materials (LiF/Co

or LiF/Fe nanocomposite), 10% carbon black and 10% PVDF binder with a typical mass loading

is ~4.5 mg/cm2. 2032-type coin cells (MTI Corporation) were assembled in an Argon-filled

glove box. A lithium metal foil worked as both the counter and reference electrode. The

electrolyte was 1 M LiPF6 in 1:1 v/v ethylene carbonate (EC) and diethyl carbonate (DEC). A

Celgard 2300 membrane was used as the separator. The electrochemical performances were

carried out on an Arbin Battery Cycler instrument. The galvanostatic charge/discharge

measurements for cells with LiF/Co and LiF/Fe cathodes and lithium metal anodes were

performed at a current density of 50 mA g−1 in the cut-off potential range of 4.2–2.5 V and

4.3–2.5 V, respectively. The LiFePO4/lithium metal half cells were cycled in the cut-off potential

range of 2.5–4.2 V at 0.1 C for the first cycle and 0.2 C upon the following cycles.

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Figure S1. SEM images of (a) the starting CoF3 and (b) LiF/Co product.

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Figure S2. TEM images of the starting CoF3.

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0 100 200 300 400 500 600

0

1

2

3

4

Chemically

synthesized LiF/Co

Potential (V)

Capacity (mAh g−1)

Electrochemically

produced LiF/Co

Figure S3. The charge potential profiles of the electrochemically produced LiF/Co composite

and the chemically synthesized LiF/Co composite.

0 50 100 150 200

2.0

2.5

3.0

3.5

4.0

4.5

1 st

2 nd

5 th

10 th

Potential (V)

Capacity (mAh g−1)

Figure S4. Charge/discharge curves for the LiFePO4 electrode with 4.8% LiF/Co additive.

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Intensity (a.u.)

10 20 30 40 50 60 70 80 90

2 Theta (degree)

FeF3

Figure S5. XRD pattern of the FeF3 powder for the synthesis of LiF/Fe composite.

Fe

LiF

FeF3

FeF2

Intensity (a.u.)

10 20 30 40 50 60 70 80 90

2 Theta (degree)

Figure S6. XRD pattern of the as-obtained LiF/Fe nanocomposite.

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The main XRD peaks in Figure S6 can be readily indexed to LiF (JCPDS No. 04-0857) and Fe

(JCPDS No. 06-0696), suggesting the formation of LiF/Fe composite. The the significant

broadening of these diffraction peaks indicates their small crystallite size. Additionally,

characteristic XRD peaks with low intensity for FeF3 (JCPDS No. 33-0647) and FeF2 (JCPDS

No. 45-1062) are observed, arising from the uncompleted reaction between FeF3 and lithium

metal.

Figure S7. Digital image of the starting FeF3 and the as-obtained LiF/Fe nanocomposite. After

the chemical transformation reaction, the color turns from pale to black.