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Supporting Information

Unraveling Nanoscale Electrochemical Dynamics of Graphite Fluoride by in-situ Electron Microscopy: Key Difference between Lithiation and SodiationJianlin Wang,‡ab Muhua Sun,‡a Yu Liu,a Jinfang Lin,d Lifen Wang,a Zhi Xu,ac Wenlong Wang,*abc Zhongzhi Yuan,*d Jincheng Liu,e and Xuedong Bai*abc

a State Key Laboratory for Surface Physics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. E-mail: wwl@iphy.ac.cn; xdbai@iphy.ac.cnb School of Physical Sciences, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, Chinac Songshan Lake Materials Laboratory, Dongguan 523808, Chinad School of Chemistry, South China Normal University, Guangzhou 510006, China. E-mail: yuanzz@scnu.edu.cne EVE Energy Co. Ltd., Huizhou 516006, China‡ These authors contributed equally to the work.

1. Description of the Supplementary Movies

Supplementary Movie S1. In-situ TEM movie showing the full electrochemical lithiation

process of CFx nanosheet. The video was recorded at 1 frame/s and played at a speed of ~12.5×.

Supplementary Movie S2. In-situ TEM movie showing the electrochemical lithiation process

of CFx nanosheet until the lithiation process was enforced to stop. The video was recorded at 1

frame/s and played at a speed of ~5×.

Supplementary Movie S3. In-situ TEM movie showing the electrochemical sodiation process

of CFx nanosheet and was enforced to stop at 216s. The video was recorded at 1 frame/s and

played at a speed of ~12.5×.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020

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2. Experimental section

2.1 Structural Characterization

The morphology of CFx nanosheet was characterized by employing field-emission scanning

electron microscopy (FESEM, JSM-6700F, JEOL). SEM images were obtained with FESEM

operated at an acceleration voltage of 10 kV.

2.2 In-situ TEM Experiment

In-situ characterization of a nanoscale lithium (sodium) battery with metallic Li (Na), Li2O

(Na2O), and CFx nanosheet as the counter electrode, electrolyte, and working electrode,

respectively, was carried out using a homemade single-tilt specimen holder in a JEOL-2010F

TEM working at 200 kV. CFx nanosheets were adhered on a gold wire by electrostatic

adsorption. Metallic Li (Na) was scratched by an electrochemically etched tungsten tip in a

glovebox. After that, the gold wire with CFx nanosheets and the tungsten tip with metal Li (Na)

were loaded onto the homemade single-tilt TEM specimen holder and sealed in a full argon-

filled bag inside the glovebox. Next, the holder was transferred into TEM fast. The naturally

grown thin Li2O (Na2O) layer on the surface of metallic Li (Na) served as the solid electrolyte.

A homemade piezo-driven nanomanipulator was used to manipulate the tungsten tip with

Li2O/Li (Na2O/Na) to contact the free end of a selected CFx nanosheet. Once they were

contacted, a constant voltage was applied on the Li (Na) electrode with respect to CFx nanosheet

electrode. In order to prevent the CFx nanosheet from being burned out, we set a limit current

of 10 μA. During the lithiation process, a series of TEM images were recorded at a frequency

of one frame per second.

2.3 STEM – ADF & EELS Characterization

The STEM-ADF image and EELS spectrum imaging (EELS-SI) were performed using

aberration-corrected (S)TEM (JEM-ARM300F, JEOL Ltd.) operated at 300 kV with a cold

field-emission gun and double dodeca-poles Cs correctors, which also are equipped with a GIF

Quantum electron energy loss spectrometer with Dual-EELS functionality. The convergence

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angle is 22 mrad and the angular range of collected electrons for ADF imaging is about 54-220

mrad. Typically, the dual-EELS-SI was operated at 300 kV with a collection semi-angle of 78.5

mrad, a dispersion of 0.5 eV/channel and a scanning step size of 0.93 nm. The Li, C, F and Na

mappings are obtained from the low-loss, high-loss EELS-SI, respectively.

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3. Supplementary Figures

Fig. S1 SEM image of CFx nanosheet, showing a typical layered stacking structure.

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Fig. S2. HRTEM image of folded edges for CFx nanosheet, clearly showing the interlayer lattice fringes. The interlayer spacing of CFx nanosheet is measured to be ~ 5.9 Å, which is consistent with the results (~ 6 Å) reported in previous literatures.1,2

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Fig. S3 I-t curve showing the enlargement of the first I-t curve of Fig. 1f within 0-5s, where the current is not equal to zero but is several nano-amperes.

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Fig. S4 The rotationally averaged normalized intensity profiles obtained from the pristine and lithiated SAED patterns, i.e. Fig. 1e and Fig. 2b. It is obviously discovered that the peaks of LiF/graphite phase emerge.

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Fig. S5 Fast Fourier space filtered image and the FFT pattern (inset) of Fig. 2c. The higher contrast marked by the red dotted ellipses corresponds to LiF nanoparticles. And the FFT pattern shows three Bragg circles, corresponding to {200}, {220} and {222} crystal planes of LiF.

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Fig. S6 Li-K edge EELS. Nothing about the signal of lithium was detected in the pristine CFx nanosheet, but there is obvious Li-K edge signal in lithiated nanosheet.

Fig. S7 F-K edge EELS of the pristine and lithiated CFx nanosheet. After lithiation, the peaks move towards lower energy, which indicates that the covalent C-F bond changes into the ionic Li-F bond.

Fig. S6 and Fig. S7 show the Li-K and F-K edge EELS, which vary considerably between the pristine and lithiated CFx nanosheet. After lithiation, the obvious Li-K peaks emerge at ~60.6 eV and ~66.2 eV (Fig. S6), indicating the lithiated CFx nanosheet contains Li. And the characteristics of Li-K edge are in good agreement with previous data.3, 4 As shown in Fig. S7, the F-K edge shift toward lower energy losses, indicating that the electronic environment of F must be very different from that in pristine CFx nanosheet, namely that the covalent C-F bond changes into the ionic Li-F bond.5 In addition, the F-K edge EELS of lithiated CFx nanosheet consists of two intense peaks with a small post peak at 699 eV marked by a red arrow. Such a post peak is the characteristic of LiF.3, 6 Both EELS spectrums of Li-K edge and F-K edge indicate the presence of LiF.

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Fig. S8 Normalized rotationally averaged intensity profiles obtained from the SAED patterns of Fig. 2b and Fig. 3c. The peaks marked by “#” and “+” can be indexed as LiF and graphite, respectively.

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Fig. S9 HRTEM image and corresponding FFT pattern (inset) of intermediate lithiated CFx nanosheet, where the crystal lattice points or fringes of LiF are very few and the sizes of LiF are very small, corresponding to the weak and unsharp Bragg circles in FFT pattern.

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Fig. S10 (left) Fourier space filtered HRTEM image of the sodiation front, showing the C/NaF-CFx phase boundary, and (right) FFT patterns from the areas marked by red dash squares with “1” and “2” in HRTEM image, respectively. The yellow dash line indicates roughly the sodiation front (phase boundary).

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