Understanding the lithiation/delithiation mechanism

of Si1-xGex Alloys

Supporting Information

Laura C. Loaizaa; Elodie Salagerb,c;Nicolas Louvainc,d; Athmane Boulaouedd,e,f; Antonella

Iadecolac; Patrik Johanssone,f; Lorenzo Stievanoc,d,e; Vincent Sezneca,c and Laure

Monconduitc,d,e

a. Laboratoire de Réactivité et Chimie des Solides (CNRS UMR 7314), Université de

Picardie Jules Verne, 33 Rue Saint Leu, 80039 Amiens Cedex, France.

b. CNRS, CEMHTI UPR3079, Université d’Orléans, 1D Avenue de la Recherche

Scientifique, 45071 Orléans Cedex 2, France.

c. Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 Rue Saint

Leu, 80039 Amiens, Cedex, France.

d. Institut Charles Gerhardt -AIME (CNRS UMR 5253), Université de Montpellier CC 15-02,

Pl. E. Bataillon, 34095 Montpellier Cedex 5, France.

e. ALISTORE European Research Institute, Université de Picardie Jules Verne, 33 rue Saint

Leu, 80039 Amiens Cedex, France

f. Department of Physics Chalmers University of Technology, SE-412 96 Göteborg,

Sweden

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2017

1. Experimental Details

(i) Raman Spectroscopy.

In situ Measurements. For the in situ analysis a Si0.5Ge0.5 self-supported film was placed in a Raman

operando cell with quartz window (Figure S1).1 The half-cell was assembled in a glove box filled with

argon, with a lithium foil as a negative electrode and a glass fiber separator (WhatmanTM filter paper)

wet with ~60 µL of LP30 electrolyte (1 M LiPF6 in EC: DMC (1:1)). A drilled copper grid was used as

the current collector to enable the laser light to reach the Si0.5Ge0.5 electrode. The focal plane was

adjusted at the surface of the Si0.5Ge0.5 electrode, slightly separated from the quartz window of the

cell by the Cu foil. The pinhole and the optical slits were adjusted to probe a large volume of the

electrode, increasing the laser cross section. This, unfortunately, led to the presence of a small signal

(peak at 490 cm-1) of the quartz window in the spectra.

In order to prevent the electrode from heating, the laser power was appropriately limited (power

measured on the sample < 10mW). Additionally, some Raman spectra present a high fluorescence

effects, thus to compare them at different stages of lithiation the baseline has been subtracted.

Figure S1. Scheme of the cell configuration used in operando confocal Raman spectroscopy.

(ii) X-ray Absorption Spectroscopy

For the XAS measurements, the intensity of the monochromatic X-ray beam was measured by three

consecutive ionisation detectors. The in situ electrochemical cell2 was placed between the first and

the second ionisation chambers, and the homogeneity of the sample was checked before running the

electrochemical test. The latter was carried on continuously for the first lithiation. Successive spectra

were collected at a rate of 2 Hz, and averaged out over periods of 20 minutes. The electrochemical

signature of the in situ cell and the correspondence with the operando average spectra is reported

Figure S3.

PCA and MCR-ALS analysis details. PCA is a factor analysis, i.e., a multivariate technique for

reducing matrices of data to their lowest dimensionality by the use of an orthogonal factor space: in

such methods, the analysed matrix corresponds to the experimental spectra, and each spectrum is

considered as an n-dimensions vector with n corresponding to the number of points within each XAS

spectrum. The first principal component has the largest initial variance, i.e., alone it takes into

account as much as possible the whole set of data. The following principal components are built

orthogonal to the first one, and allow the reproduction of the experimental spectra with the highest

possible variance via adapted linear combinations. This supposes that the group of analysed spectra

is intrinsically bilinear, which means that all spectra can be expressed as linear combinations of an

orthogonal basis set of uncorrelated spectra. This basis set, has a lower dimensionality than the

number of original spectra and is obtained from the calculation. In the different linear combinations

that are built to reconstruct each experimental spectrum, the multiplication factors of the principal

components are usually called scores.

Even though the so-obtained principal components are only orthogonal mathematical functions

and not real XAS spectra, their determination reveals the number of independent spectral

components that can be used to describe the whole set of experimental data. Several methodologies

are then available to reconstruct the corresponding real XAS components as well as their evolution

such as the MCR-ALS (Multivariate Curve Resolution - Alternating Least Squares) analysis tool.3,4 A

detailed description of this method from a theoretical point of view is given by Tauler et al.5,6 who

were the first to propose it for the analysis of in situ spectroscopic data. The intrinsic limits of this

method and of its application are discussed in ref.7

In the present study, the MCR-ALS analysis was performed with the following constraints: (i) non-

negativity of the intensity of the components, (ii) unimodality for all components and (iii) closure

(sum of the components always equal to 100 of the intensity).

(iii) Scanning electron microscopy – Energy dispersive X-ray analysis (SEM/EDX):

Morphology and composition of the synthetized samples as well as of the electrodes were checked

by SEM (Hitachi S4800) operating at 20 kV and EDX (Hitachi S4500). Cells containing Si0.5Ge0.5 self-

supported films were cycled vs Li+/Li0 (C/10) at different depths of charge/discharge in Swagelok

assembly. The electrolyte was 1 M LiPF6 in EC:PC:DMC with 1% VC and 5% FEC and the separator a

Whatman glass fiber. The cells were disassembled in an Ar filled glove box; the electrodes were

washed with DMC, dried under vacuum and analyzed.

1. Results and Discussion

Figure S2. The refined cell Si1-xGex parameters reported as a function of the Ge content. The values

follow a linear increase with increasing Ge content, in agreement with Vegard’s Law. Based on this

results it can be assumed that the 50-50 composition was obtained.

0 5 10 15 200.0

0.5

1.0

1.5

2.0

2.5

595040302010

E we (V

vs.

Li+ /L

i)

Time (h)

1

Figure S3. Electrochemical signature of a Si0.5Ge0.5 self-supported electrode cyled at C/5 in the in

situ XAS cell.

Figure S4. EXAFS spectrum of pristine Si0.5Ge0.5 (top) and relative Fourier transform (bottom). The

experimental spectrum is drawn in blue, and the fitting result in red. The spectrum was fitted by

imposing a total number of nearest and second neighbors equal to those expected from the crystal

structure determined by XRD (4 and 6, respectively), and by leaving variable their nature (Si or Ge)

and their bond distance.

Figure S5. XRD patterns for Li15Si4, Li15Ge4 and Li15(Si0.5Ge0.5)4.

Figure S6. 7L i NMR spectra of Li15Si4 (top) and Li15Ge4 (bottom)

Figure S7. SEM images of a Si0.5Ge0.5 electrode taken at different stages of charge/discharge.

When discharged below 250 mV, a flower-like morphology (1 to 6 μm) can be observed in the

Si0.5Ge0.5 electrode. These formations are more abundant once the electrode is fully discharged and

tend to aggregate in the shape of islands. Although, some segments of the electrode do not show

any volume expansion, probably due to disconnection from the current collector. Upon de-lithiation

a porous-like morphology can be appreciated. In the second discharge, the flower-like morphology

reappears. Subsequent cycles resemble the first one. The same morphology was identified in all the

mechanically lithiated phases (Li15Si4, Li15Ge4, Li15(Si0.5Ge0.5)4).

Figure S8. EDX mapping of a Si0.5Ge0.5 electrode at different stages of charge/discharge.

EDX mapping for silicon and germanium was done on a Si0.5Ge0.5 electrode. It can be observed that

over the course of (de)lithiation; both silicon and germanium remain homogeneously mixed. This is

an indicator of the presence of a solid solution.

Figure S9. Zoomed area for an in situ XRD experiment for a self-suported Si0.5Ge0.5 electrode

performed at a C/14 rate.

An in situ XRD experiment for Si0.5Ge0.5 performed at C/14 allowed to observe the crystallization of

a Li15(Si0.5Ge0.5)4 phase at the end of discharge. Particularly, it was noticed that by the time the

Li15(Si0.5Ge0.5)4 starts to crystallize there is a shifting of its Bragg reflections towards lower angles

(corresponding to a higher value of the refined cell parameter). Following, the peaks start to shift

towards higher angles (lower value of the refined cell parameter) until the end of the discharge

(Green pattern). Upon charging the Li15(Si0.5Ge0.5)4 peaks continue to shift towards higher angles until

the amorphization of the phase. During the second discharge, the same phenomenon was observed.

Figure S10. In situ XRD experiment for a self-suported Si0.5Ge0.5 electrode performed at a C/5 rate.

An in situ XRD experiment was performed on a self-supported Si0.5Ge0.5 film at C/5. It can be

evidenced that upon lithiation the c-Si0.5Ge0.5 gets amorphous. The system remains amorphous until

the crystallization of Li15(Si0.5Ge0.5)4 at the end of the discharge. Upon charging, the previously

crystalline phase gets amorphous and the system remains in this state until the end of charge, where

a-Si0.5Ge0.5 is recovered. In the second discharge, the system stays amorphous until crystallization of

Li15(Si0.5Ge0.5)4 at the end of the discharge. In analogy with the experiment at C/14 there is an

evolution of the refined cell parameter. And contrary, there is a delay in the crystallization of

Li15(Si0.5Ge0.5)4, here the pattern with the highest intensity is collected after the end of discharge.

7Li shift (ppm)-80-40040

-17 ppm

1st discharge

Tim

e (h

ours

)

0

10

20

Voltage (V)00.5

0.201V0.172V0.145V0.124V0.094V0.067V0.028V0.028V0.001V0.001V

Figure S11. Extracted slices in the first discharge and first potentiostatic step at 0 V.

Figure S12. Extracted slices during the first open circuit (OCV) step.

Figure S13. Extracted slices during the first charge

Figure S14. Extracted slices during the second discharge.

Figure S15. Extracted slices during the second potentiostatic step at 0V.

Figure S16. Extracted slices during the second open circuit (OCV) step.

Figure S17. Fitting results of the second reconstructed MCR-ALS component in the k (top) and in

the R (bottom) space. The experimental spectrum is drawn in blue, and the fitting result in red. R-

factor = 0.0085

Figure S18. Fitting results of the third reconstructed MCR-ALS component in the k (top) and in the

R (bottom) space. The experimental spectrum is drawn in blue, and the fitting result in red. R-factor

= 0.0207

Figure S19. Fitting results of the fourth reconstructed MCR-ALS component in the k (top) and in the

R (bottom) space. The experimental spectrum is drawn in blue, and the fitting result in red. R-factor

= 0.0220

Figure S20. Scheme of the different structural modifications of Si0.5Ge0.5 during

lithiation/delithiation

Table S1. Fitting parameter for pristine Si0.5Ge0.5

Shell S02 N Effective N

(S02 x N)

R (theory)

R (measured) 2

Ge-Ge 0.52(3) 4 2.5 2.398 2.452(5)

Ge-Si 0.30((3) 4 1.5 2.398 2.390(9)0.0043(3)

Ge-Ge 0.40(5) 6 2.9 3.916 3.92(2)

Ge-Si 0.42(5) 6 3.1 3.916 3.91(3)0.016(1)

Table S2: Fitting parameter for four reconstructed MCR components

Component Shell S02 N Effective N

(S02 x N)

R (theory)

R (measured) 2

Ge-Ge 0.53(3) 4 2.6 2.398 2.451(6)

Ge-Si 0.29(3) 4 1.4 2.398 2.39(1)0.0043(3)

Ge-Ge 0.40(5) 6 2.9 3.916 3.92(2)Comp. 1

Ge-Si 0.42(5) 6 3.1 3.916 3.91(3)0.016(1)

Ge-Ge 0.36(4) 4 2.3 2.398 2.457(7)

Ge-Si 0.26(3) 4 1.7 2.398 2.37(1)0.0041(4)

Ge-Ge 0.28(6) 6 2.7 3.916 3.90(2)

Ge-Si 0.34(6) 6 3.3 3.916 3.89(3)0.011(2)

Comp. 2

Ge-Li 0.19(5) 12 - 2.576 2.6(3) 0.02(1)

Ge-Ge 0.08(6) 4 1.2 2.398 2.40(1)

Ge-Si 0.21(4) 4 2.8 2.398 2.37(3)0.004(1)

Ge-Ge 0.11(6) 6 2.3 3.916 3.85(4)

Ge-Si 0.18(5) 6 3.7 3.916 3.85(4)0.01(1)

Comp. 3

Ge-Li 0.53(4) 12 - 2.576 2.6(1) 0.015(8)

Comp. 4 Ge-Li 6 - 2.576 2.452(5) 0.016(4)

Ge-Li0.82(3)

6 - 2.856 2.75(3) 0.018(6)

Table S3. Refined cell parameters for the different crystalline phases identified in an in situ XRD experiment performed on a self-supported Si0.5Ge0.5 film at C/14.

First Discharge First ChargePotential (V) x Li Cell Parameter (Å) Potential (V) x Li Cell Parameter (Å)

Si0.5Ge0.5 Li15(Si0.5Ge0.5)4

2.25 0.286 5.539(5) 0.186 4.145 10.732(8)2.32 0.354 5.540(5) 0.293 4.073 10.730(5)0.934 0.426 5.539(5) 0.356 4.00 10.727(2)0.499 0.499 5.539(5) 0.393 3.937 10.724(2)0.415 0.57 5.540(5) 0.416 3.862 10.721(4)0.383 0.638 5.539(5) 0.427 3.790 10.719(4)0.356 0.713 5.530(4) 0.434 3.730 10.717(2)0.334 0.782 5.530(4) 0.442 3.649 10.714(5)0.318 0.847 5.530(4) 0.444 3.611 10.711(4)0.306 0.848 5.529(4) 0.451 3.501 10.709(4)0.297 0.995 5.529(4) 0.454 3.461 10.707(2)0.289 1.062 5.526(5) 0.456 3.437 10.704(3)0.282 1.141 5.529(4) 0.458 3.348 10.702(5)0.276 1.205 5.524(4) 0.462 3.243 10.670(5)0.27 1.286 5.524(3) 0.468 3.156 10.699(4)0.264 1.354 5.520(4) 0.471 3.100 10.697(5)0.259 1.429 5.518(3) 0.478 3.001 10.696(6)0.253 1.489 5.516(4) 0.483 2.92 10.693(5)0.248 1.555 5.515(6) 0.484 2.864 10.694(8)0.242 1.64 5.514(3) 0.493 2.796 10.695(8)0.237 1.711 5.510(3)0.232 1.783 5.509(6) Second Discharge0.228 1.856 5.508(6) Li15(Si0.5Ge0.5)4

0.224 1.912 5.507(4) 0.127 3.530 10.757(4)0.219 1.995 5.540(6) 0.121 3.609 10.756(5)0.215 2.056 5.536(6) 0.114 3.685 10.754(6)0.211 2.131 5.535(6) 0.108 3.752 10.754(2)0.207 2.2 5.530(6) 0.102 3.820 10.754(5)0.204 2.275 5.53(1) 0.094 3.897 10.754(5)0.2004 2.276 5.532(9) 0.085 3.967 10.754(2)0.196 2.416 5.525(9) 0.072 4.042 10.740(2)0.192 0.286 5.529(6) 0.054 4.112 10.733(2)0.188 0.354 5.523(9) 0.029 4.183 10.727(2)0.184 0.426 5.52(2) 0.0025 4.256 10.724(2)0.18 0.499 5.519(9)0.176 0.57 5.51(1) Second Charge

Li15(Si0.5Ge0.5)4 Li15(Si0.5Ge0.5)4

0.119 3.419 10.764(5) 0.196 4.201 10.722(2)0.115 3.489 10.760(4) 0.301 4.130 10.720(2)0.11 3.490 10.756(4) 0.363 4.058 10.717(2)0.105 3.638 10.758(3) 0.402 3.988 10.715(2)0.099 3.704 10.763(6) 0.424 3.915 10.712(2)0.091 3.782 10.764(8) 0.439 3.845 10.709(2)0.081 3.854 10.767(3) 0.453 3.772 10.707(2)0.069 3.923 10.768(2) 0.48 3.702 10.704(2)0.053 4.000 10.769(5) 0.616 3.629 10.705(8)

0.035 4.071 10.772(5) 0.864 3.564 10.704(4)0.017 4.139 10.757(2) 0.989 3.515 10.705(4)

3.48E-04 4.214 10.745(2) 1.5 3.319 10.704(4)

Table S4. Values for the refined cell parameter for Li15(Si0.5Ge0.5)4, Li15Ge4 and Li15Si4.

PhaseCell

Parameter (Å)

PhaseCell

Parameter (Å)

PhaseCell

Parameter (Å)

Li15(Si0.5Ge0.5)4

Mechanically synthesized

10.753(1)Li15Ge4

mechanically synthesized

10.768(1)Li15Si4

Mechanically synthesized

10.65(8)

Li15(Si0.5Ge0.5)4

From in situ XRD(C/14)

10.745(2)Li14Ge4

ICSD 43235 10.72(5)Li15Si4

ICSD 237945 10.6546

Table S5. Fit parameters for the 7Li ex situ NMR spectrum of sample A.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 35 50 0 54

2 0 23 0 30

3 9 15 1 7

4 -9 9 1 5

5 3 5 0 4

Table S6. Fit parameters for the 7Li ex situ NMR spectrum of sample B.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 -2 45 0 66

2 -10 14 0 27

3 9 8 0 4

4 3 4 0 2

Table S7. Fit parameters for the 7Li ex situ NMR spectrum of sample C.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 -11 17 0 72

2 -12 6 1 14

3 10 13 0 9

4 1 5 0 3

5 -1 1 1 2

Table S8. Fit parameters for the 7Li ex situ NMR spectrum of sample D.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 -4 22 0 54

2 -10 9 0 45

3 0 2 1 1

Table S9. Fit parameters for the 7Li ex situ NMR spectrum of Li15Si4.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 10 7 0 42

2 16 34 0 36

3 3 6 0 22

Table S10. Fit parameters for the 7Li ex situ NMR spectrum of Li15Ge4.

Component Position (ppm) Width (ppm) Gaussio-lorentzian ratio

Contribution (%)

1 3 12 0 40

2 5 33 0 33

3 -7 14 0 22

4 7 5 0 4

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