www.sciencemag.org/content/358/6362/506/suppl/DC1

Supplementary Materials for

Atomic structure of sensitive battery materials and interfaces revealed by cryo-electron microscopy

Yuzhang Li,* Yanbin Li,* Allen Pei, Kai Yan, Yongming Sun, Chun-Lan Wu,

Lydia-Marie Joubert, Richard Chin, Ai Leen Koh, Yi Yu, John Perrino, Benjamin Butz, Steven Chu, Yi Cui†

*These authors contributed equally to this work.

†Corresponding author. Email: yicui@stanford.edu

Published 27 October 2017, Science 358, 506 (2017) DOI: 10.1126/aam6014

This PDF file includes:

Materials and Methods Supplementary Text Figs. S1 to S16 References

Materials and Methods

Electrochemistry

In an inert argon (Ar) atmosphere, type-2032 coin cells were assembled with Cu transmission

electron microscopy (TEM) grid as the working electrode and Li metal (Alfa Aesar) as the

counter/reference electrode. ~20 μL of 1.0 M LiPF6 in 1:1 w/w ethylene carbonate/diethyl

carbonate (BASF Selectilyte LP40) was added as the electrolyte, using a polymer separator

(Celgard 2250) to divide the two electrodes. For SEI formed in fluorine-functionalized

electrolyte additive, ~20 μL of 1.0 M LiPF6 in 90 vol% 1:1 v/v ethylene carbonate/diethyl

carbonate with 10 vol% fluoroethylene carbonate was added as the electrolyte. Li metal with an

areal capacity of 1 mAh cm-2 was deposited onto the working electrode by applying a current of

2 mA cm-2 for 30 min (BioLogic VMP3).

Cryo-transfer procedure

After Li metal was deposited, coin cells were immediately disassembled in an inert Ar

atmosphere (glovebox) and working electrodes were washed briefly with 1,3-dioxolane to

remove Li salts. Once dry, the TEM grid with plated Li was placed in a Teflon-sealed Eppendorf

tube and transferred out into the ambient air. The pressure inside the Ar-filled glovebox (and thus

the Eppendorf tube) is greater than the ambient pressure, which prevents potential air from

leaking into the tube. The sealed Eppendorf tube (with Li metal still immersed in an Ar

environment inside) was plunged directly into a bath of liquid nitrogen (LN2). We then quickly

crushed this airtight container with a bolt cutter while still immersed in LN2 to quickly expose

the Li metal to the cryogen. The TEM grid was then carefully mounted onto a TEM cryo-holder

(Gatan) using a cryo-transfer station to ensure this entire process occurred under LN2. During

insertion into the TEM column (~1 second), a built-in shutter on the holder was closed to prevent

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contact of Li metal with the air. In this way, the reactive battery material can be safely

transferred from the coin cell to the TEM without any chance of ambient air exposure. Once

inside the TEM column, the sample is kept cold at –178 °C.

Standard TEM procedure (control)

As with the cryo-transfer method, coin cells were disassembled in an Ar-filled glovebox and

working electrodes were washed with 1,3-dioxolane. Once dry, the TEM grid was loaded onto a

single-tilt TEM holder and sealed in an airtight container. This container was then transported to

the TEM facility in ~1 minute. The TEM holder was inserted into the TEM column by opening

the airtight container and exposing the sample briefly to air for ~1 second. Imaging took place at

room temperature.

Electron Microscopy

All TEM characterizations were carried out using a FEI Titan 80-300 environmental (scanning)

transmission electron microscope (E(S)TEM) operated at 300 kV. The microscope was equipped

with an aberration corrector in the image-forming (objective) lens, which was tuned before each

sample analysis. During the TEM image acquisition, the corresponding electron dose flux

(measured in units of number of electrons per square angström per second, e− Å-2·s-1) was also

recorded. This parameter had been calibrated for the instrument using an analytical TEM holder

with a Faraday cup. Lattice spacings of Li metal and its salts were analyzed using

DigitalMicrograph (Gatan) software. SEM characterizations were carried out using a FEI Helios

600i Nanolab dual-beam FIB/SEM. Samples immersed in LN2 were transferred under vacuum

using a Quorum PP3000T cryo-SEM system.

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Cryo-Transferred Samples Imaged at Elevated Temperature

We performed a control experiment in which a cryo-transferred sample was allowed to gradually

heat up in vacuum within the TEM column. The sample was allowed to reach thermal

equilibrium at each temperature step, during which the electron beam was shut off. We then

exposed the dendrite tip region for ~1 second at an electron dose rate of ~500 e-/A2/s. The low

magnification images show the area in which the sample was exposed to electron beam

irradiation (fig. S7).

Although the pristine Li metal dendrite is stable at cryo conditions (–178 °C), damage begins to

occur at the dendrite tip due to beam heating effects at –100 °C. At increasing temperatures, this

damage becomes more pronounced until the dendrite becomes completely unstable under high

electron dose rate imaging conditions at room temperature. The accumulated dose for the entirety

of the temperature ramping experiment (~5 seconds; ~500 e-/A2/s) is much less than that for

samples imaged at cryo conditions (~30 seconds; ~1000 e-/A2/s). This experiment shows the

importance of imaging sensitive battery materials at cryo conditions.

TEM Diffraction Pattern Rotation Calibration

We show the rotation calibration between the diffraction pattern and its image in the

supplementary fig. S8. The standard calibration sample used was a piece of silicon with a gap.

The diffraction pattern was taken after adjusting the focus such that the sample features can be

seen as a bright field or dark field image in the transmitted beam or the diffracted beams,

respectively. The vertical gap is clearly visible in the diffracted beam of the diffraction pattern

and is pointing in the same direction as the gap in the high-resolution TEM image. When

comparing the silicon [110] direction (horizontal line) in the high-resolution TEM image with

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the diffraction pattern, it is clear that the two directions are well aligned. This calibration is done

at each magnification for which our TEM images are taken.

As an example, we have provided a properly indexed SAED pattern of the image taken from Fig.

2E. The growth direction can be determined by drawing a line from the transmitted spot to a

fundamental reflection (fig. S9).

5

Supplementary Text

TEM Radiation Damage Mechanisms

Sample damage in a TEM can occur by elastic (electron-nucleus) or inelastic (electron-electron)

scattering between the electron beam and the specimen (18). Li metal has a low melting point

and weak bonding interactions, making it particularly susceptible to damage from specimen

heating, radiolysis, and sputtering (35). In principle, cooling the sample can reduce beam damage

from specimen heating and radiolysis. Damage from sputtering is primarily in the form of knock-

on displacement from high-angle elastic scattering. According to (35), optimizing the electron

beam accelerating voltage can minimize sputter damage. Conventional wisdom suggests that a

low TEM operating voltage (<100 kV) should reduce damage. However, when calculating the

sputtering cross-sections for Li metal atoms in the elemental state, it is more suitable to image Li

compounds at higher voltages in order to reduce the sputtering cross-section and minimize the

interaction damage from the electron beam (35). Thus, we have imaged our Li metal dendrites at

300 kV under cryogenic temperatures to minimize damage from specimen heating, radiolysis,

and sputtering.

Image Simulation

High-resolution images were taken under negative Cs imaging (NCSI) conditions. In principle,

when the spherical aberration (Cs) value is –15 μm and the defocus value is ~5-8 nm, the areas of

bright contrast normally correspond to atoms (36–38). To confirm this, we performed image

simulations with varying focus values and crystal thickness, assuming an accelerating voltage of

300 kV, Cs value of –15 μm, beam convergence angle of 0.2 mrad, and spread of defocus of 1

nm. Other aberrations (e.g. two-fold, three-fold, coma) are not considered (set to zero). Image

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simulations were performed in the STEM_CELL Version 2.5.2.1 (39) and MacTempasX Version

2.4.33 (40) software, using the multislice method. Image simulations for both the atomic

resolution images along the [111] and [001] zone axis are presented in fig. S10 and fig. S11,

respectively. Although HRTEM images typically require sample thicknesses of less than 50 nm,

Li metal has the third lowest atomic number and allows us to obtain high quality HRTEM

images with thicker samples. The changes of HRTEM patterns can be explained by dynamical

channeling effect. In many cases, the bright spots represent the position of Li atom columns,

indicating that the bright spots indeed correspond to atomic columns. Furthermore, the simulated

HRTEM patterns quite robust to defocus and thickness variation. Under our experimental

conditions, the contrast reverse problem in HRTEM is not as a big issue for Li as it is a light

element.

7

Fig. S1. Li deposition voltage profile. In coin cells, Li metal is deposited electrochemically

onto Cu TEM grids, exhibiting typical voltage behavior as shown in the plot above. Thus, direct

Li deposition onto Cu TEM grids is similar to deposition onto typical Cu current collectors.

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Fig. S2. Measuring Li metal lattice spacing. (A) Atomic-resolution TEM image of Li metal

lattice viewed from [111] zone axis. Colored boxes represent regions where pixel intensities are

summed along each line parallel to the analyzed facet. (B) The summed pixel intensities are then

plotted vs. the distances along the perpendicular direction. For the <110> facets, each peak

intensity represents one plane of {110} atoms. For the <211> facets, each peak intensity

represents two planes of {211} atoms.

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Fig. S3. Faceting behavior of Li metal dendrite. (A) Cryo-EM image showing hexagonal

outline in Li dendrite growing along <111> when tilted towards a grazing angle. (B)

Corresponding SAED pattern showing the <111> growth direction.

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Fig. S4. Li dendrite stability after TEM-EDS characterization. (A) Pristine Li metal dendrite

before TEM-EDS. (B) The same Li metal dendrite after TEM-EDS. The red circle indicates the

region where the EDS spectra were collected. The Li metal morphology remains unchanged after

5 minutes of exposure to an electron dose rate of ~100 e-/Å2/s.

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Fig. S5. Energy-dispersive X-ray spectroscopy of Li metal dendrite. (A) Spectra are collected

along a single dendrite. The chemical compositions of the SEI formed in standard EC/DEC and

that formed in 10 vol% FEC electrolyte are similar. The copper signal is from the copper TEM

grid. (B) Li metal dendrite formed in 10 vol% FEC electrolyte. Blue circle indicates size of the

electron beam when focused to collect EDS spectra. (C) Li metal dendrite formed in standard

EC/DEC electrolyte. Red circle indicates size of the electron beam when focused to collect EDS

spectra.

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Fig. S6. TEM radiation damage mechanisms. Elastic interactions from electron-nucleus

scattering can cause sputtering damage. This can be reduced by optimizing the accelerating

voltage such that the sputtering cross section is minimized. Inelastic interactions from electron-

electron scattering can cause damage from specimen heating and radiolysis. By cooling the

sample, this damage can be reduced.

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Fig. S7. Li metal beam damage at elevated temperatures. Samples were allowed to gradually

heat up and exposed for ~1 second at ~500 e-/A2/s for each temperature step. Low magnification

images were taken at lower doses (<10 e-/A2/s). Beam heating effects begin to damage the

dendrite at –100 °C.

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Fig. S8. Electron diffraction alignment calibration. (A), (B) The vertical gap feature of the

silicon sample (A) is visible in the diffracted beams (B) of the diffraction pattern. Furthermore,

the [110] direction along the horizontal matches that of the diffraction pattern, indicating that the

instrument is calibrated and aligned.

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Fig. S9. Indexing SAED patterns. Images are taken from Fig. 2 in the main text. Proper miller

indices are assigned to each observed reflection. The exact direction of this Li metal dendrite can

be determined by drawing a line from the transmitted spot (000) to the fundamental reflection

(1�12�). The (110) lattice distance can be determined from SAED pattern: We measure the length

between (110) and (1�1�0), which is 8.1 nm-1. This is twice the (110) lattice spacing in reciprocal

space. We divide 8.1 nm-1 by 2 and convert to real space to arrive at a calculated (110) lattice

spacing of 2.47 nm.

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Fig. S10. Image simulation of atomic positions along [111] zone axis. (A) Simulated HRTEM

images under NCSI condition, with varying focus values and crystal thicknesses. (B) Simulated

HRTEM images overlaid with atomic models. The green circles indicate the position of Li atom

columns. Overlaying images shows that, in most of cases, the bright spots represent the positions

of Li atom columns, indicating that the bright spots indeed correspond to atomic columns and

that the HRTEM patterns are quite robust to defocus and thickness.

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Fig. S11. Image simulation of atomic positions along [001] zone axis. (A) Simulated HRTEM

images under NCSI condition, with varying focus values and crystal thicknesses. (B) Simulated

HRTEM images overlaid with atomic models. The green circles indicate the position of Li atom

columns. Overlaying images shows that, in most of cases, the bright spots represent the positions

of Li atom columns, indicating that the bright spots indeed correspond to atomic columns and

that the HRTEM patterns are quite robust to defocus and thickness.

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Fig. S12. Electron energy loss spectroscopy of Li metal dendrites. Combined EELS spectrum

of lithium dendrite. Lithium, carbon, oxygen spectra were obtained simultaneously using Dual-

EELS. Exposure time for lithium K-edge was 0.1 sec with 130x vertical binning. Exposure time

for carbon and oxygen K-edge was 2.0 sec with 130x vertical binning. The carbon K-edge

intensity is scaled by 37x and the oxygen K-edge intensity is scaled by 54x in order to fit on the

plot. Quantitative analysis estimates lithium atomic percent to be greater than 80%, with that of

carbon and oxygen less than 5% and 15%, respectively.

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Fig. S13. Low magnification SEM and TEM image of Li metal dendrites. (A) SEM image of

Li metal dendrites deposited onto a Cu TEM grid. (B) TEM image of Li metal dendrites at the

Cu edge of the TEM grid. The grid is too thick for high resolution imaging at the nucleation

interface.

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Fig. S14. High-resolution TEM image of SEI formed in EC/DEC electrolyte. Raw TEM

image of the Li metal SEI formed in EC/DEC electrolyte from Fig. 4E in the main text.

5 nm

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Fig. S15. High-resolution TEM image of SEI formed in 10 vol% FEC electrolyte. Raw TEM

image of the Li metal SEI formed in 10 vol% FEC electrolyte from Fig. 4H in the main text.

5 nm

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Fig. S16. EFTEM chemical mapping of lithium metal dendrites. (A) TEM image of typical

lithium dendrite (B) Lithium map (white areas are rich in lithium) obtained using slit width of 10

eV. 40, 50 eV for pre-edge 1 and 2; 60 eV for post-edge. Exposure time of 10 sec. (C) Oxygen

map (white areas are rich in oxygen) obtained using slit width of 50 eV. 455, 505 eV for pre-

edge 1 and 2; 555 eV for post-edge. Exposure time of 20 sec. (D) Carbon map (white areas are

rich in carbon) obtained using slit width of 30 eV. 240, 270 eV for pre-edge 1 and 2; 300 eV for

post-edge. Exposure time of 20 sec. (E) Fluorine map (white areas are rich in fluorine) obtained

using slit width of 50 eV. 605, 655 eV for pre-edge 1 and 2; 710 eV for post-edge. Exposure time

of 20 sec. (F) Line profiles of lithium (red), oxygen (green), carbon (orange), and fluorine (blue)

EFTEM averaged intensities across the dendrite. It appears that carbon and oxygen

concentrations increase at the edge of the dendrite, whereas lithium concentrations are focused in

the middle region.

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