Electrochemical Lithiation of Ge: New Insights by Operando

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Electrochemical Lithiation of Ge: New Insights by Operando Spectroscopy and Diffraction Laura C. Loaiza,† Nicolas Louvain,‡,§,∥ Bernard Fraisse,‡ Athmane Boulaoued,‡,∥,⊥ Antonella Iadecola,§ Patrik Johansson,∥,⊥ Lorenzo Stievano,‡,§,∥ Vincent Seznec,*,†,§,∥ and Laure Monconduit‡,§,∥ †

Laboratoire de Réactivité et Chimie des Solides (CNRS UMR 7314), and ∥ALISTORE European Research Institute, Université de Picardie Jules Verne, 33 Rue Saint Leu, 80039 Amiens Cedex, France ‡ Institut Charles Gerhardt-AIME (CNRS UMR 5253), Université de Montpellier CC 15-02, Pl. E. Bataillon, 34095 Montpellier, Cedex 5, France § Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 Rue Saint Leu, 80039 Amiens, Cedex, France ⊥ Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden S Supporting Information *

ABSTRACT: The relatively high cost of metallic germanium (Ge) as a lithiumion battery negative electrode material is more than counterbalanced by its high capacity, high lithium diffusivity, and electronic conductivity. Using a unique and highly complementary set of operando characterization techniques, we propose a complete mechanism for the reversible lithiation of Ge. The electrochemical mechanism is found to be determined by the process of discharge/charge: (i) independent of the charge/discharge rate amorphous a-LiGe is proposed as the first intermediate during the lithiation of c-Ge, followed by Li7Ge3, and (ii) at low potential Li15Ge4 is observed, but only for moderate rates and never at low rates, where indeed an “overlithiated” phase is preferred. The complementarity of the data obtained from XAS, Raman spectroscopies, and XRD, all in operando mode, was crucial in order to understand the complex mechanism based on reversible formation of the various crystalline and amorphous phases.

1. INTRODUCTION

network, permitting a facile stress relaxation, whereas Si electrodes often are heavily fractured.6 Most reports in the scientific literature are devoted to the improving of the Ge electrode performance,5 especially regarding the optimization of the volume expansion by using nanostructuring2,3,7−9 or composites,10−13 while lesser information is focused on the basic lithiation mechanism.4,14−18 Other studies demonstrate that the Ge particle size and morphology both have a strong impact on lithiation mechanism and phases created.4,14−21 Indeed, many different amorphous and crystalline phases can be obtained upon lithiation (Table 1). The results are, however, difficult to rationalize; even if it seems clear that crystalline Li15Ge4 and Li15+δGe4 (or Li22Ge5) are the end-products under electrochemical conditions, there is still no consensus on the intermediate phases, neither the crystalline nor the amorphous. Moreover, the presence of such intermediate phases depends strongly on the current density, revealing the importance of kinetics in the electrochemical mechanism.

Silicon (Si) and germanium (Ge) have both been at the very center of electronics development since the 1940s and are now becoming of huge interest as part of the next generation negative electrode materials for Li-ion batteries (LIB). Both elements can alloy with lithium, either chemically or electrochemically, and give rise to maximum contents of Li4.4Si and Li4.4Ge (Li22Si5 and Li22Ge5), respectively, rendering specific capacities up to 11 times larger than conventional graphitebased electrodes.1 Germanium is less abundant and therefore more expensive than silicon. Nevertheless, it presents many advantages over silicon as an electrode: (i) Its theoretical specific capacity is higher than those of graphite (1623 vs 372 mAh g−1) and the positive electrode material lithium cobalt oxide (LiCoO2) (137 mAh g−1). As a comparison, Si has a 30 times higher theoretical specific capacity than LiCoO2 (4199 mAh g−1), and in fact, such a huge difference can represent a problem in manufacturing well-balanced full cells. (ii) Ge allows very fast Li+ diffusion (ca. 400 times faster than in silicon)2,3 and exhibits an electronic conductivity 2 orders of magnitude larger.4 (iii) Lithiated Ge alloys have slightly lower volumes than the corresponding Si ones, the reason why the volumetric expansion by lithiation is smaller and isotropic, while for Si it is highly anisotropic.5 (iv) Ge electrodes react with Li through a reversible formation of a nanoporous © XXXX American Chemical Society

Received: November 14, 2017 Revised: February 6, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.jpcc.7b11249 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Literature Review of the Chemical Species Formed during Lithiation of Germanium Electrodes author (ref)

C/rate

type of electrode

proposed species during lithiation

identification technique

Yoon et al.19

micrometricsized c-Ge and carbon-coated composite prepared by mechanical ball milling and pyrolysis

100 mA/g

c-Li9Ge4 c-Li7Ge2 c-Li15Ge4 c-Li22Ge5

ex situ XRD

Baggetto et al.14,15

evaporated/sputtered a-Ge thin film (50 nm thickness)

C/20

a-LiGe a-Li7Ge2 c-Li15Ge4

in situ XRD in situ XAS

Graetz et al.3

evaporated/ballistically deposited a-Ge thin film

C/4

c-LiGe c-Li7Ge2 c-Li15Ge4 c-Li11Ge6 c-Li9Ge4 c-Li22Ge5 a-LixGe c-Ge upon delithiation

ex situ XRD

Lim et al.17

micrometric-sized c-Ge

C/15

a-Ge a-Li9Ge4 a-LiGe a-Li7Ge2 a-LixGe c-Li15Ge4

operando XRD and XAS

Lim et al.16,18


>C/5 C/5 < C < C/10

a-LixGe a-Ge a-LixGe a-Li9Ge4 c-Li15Ge5 a-LixGe c-Li15Ge4

operando XRD and XAS

c-Li7Ge3 a-Li7Ge2 a-Li13Ge5 c-Li15Ge4 c-Li15+δGe4

ex situ XRD in situ/ex situ solid state NMR

C/20

Jung et al.4

C/50


pair distribution function

Tang et al.20

Ge nanorods embedded in multiwall carbon nanotubes

C/5

a-Li2.26Ge a-Li3.5Ge c-Li15Ge4 c-Li15+δGe4

in situ XRD in situ solid state NMR

Morris et al.21

−

−

LiGe Li7Ge3 Li13Ge5 Li8Ge3 Li15Ge4 Li17Ge4

density functional theory calculations

2. METHODOLOGY

Two types of electrodes were prepared: (a) for the electrochemical measurements the composition was 70 wt % of active material, 18 wt % of carbon black (SN2A, Y50A), and 12 wt % of CMC (carboxyl methyl cellulose) (Aldrich, MW = 250 000, DS = 0.7); (b) for the in situ measurements, a selfsupported electrode with 70 wt % of active material, 12 wt % of carbon black, and 18 wt % of CMC. The two slurries were casted on a copper foil (15 μm thickness) and Mylar foil, respectively, using a Doctor Blade (50 μm thickness) and

Ge powder (100-Mesh, 99.99%, Alfa Aesar) was ball milled, and a reference sample of Li15Ge4 was mechanically synthesized by mixing stoichiometric amounts of Li and Ge. Both processes were performed under argon atmosphere for 20 h (planetary ball mill, RETSCH PM 100) at 600 rpm, using a 50 mL tungsten carbide jar and a 10:2 balls:active material ratio. B


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The Journal of Physical Chemistry C

Figure 1. Galvanostatic discharge−charge curves at (a) C/5 and (b) C/14 rate cycling of the Ge electrode (second discharge in dashed lines). The corresponding derivative curves are shown in the inset.

were globally analyzed using a chemometric approach including principal component analysis (PCA) and multivariate curve resolution−alternating least squares (MCR−ALS),27−29 using a methodology described in full detail elsewhere29−31 and here applied to the full XAS signal (absorption edge plus extended fine structure). The reconstructed spectral components as well as the spectrum of pristine Ge were fitted using the Demeter software package.26 Scattering factors and phase-shifts were calculated by FEFF7 starting from the calculated lattice parameters of Ge, LiGe, Li7Ge2, Li15Ge4, and Li22Ge5. Interatomic distances and the Debye−Waller factors (σ2) were calculated for all paths included in the fits.

subsequently dried in air at room temperature. Following, disklike electrodes were cut out and dried in vacuo at 100 °C for 2 h. Coin cells using Cu-foil casted electrodes were assembled in a glovebox, using lithium foil as the counter electrode, a 1 M LiPF6 in EC:PC:3DMC (ethylene carbonate/ polycarbonate/dimethyl carbonate) (1:1:3, w/w/w) electrolyte containing 1% VC (vinyl carbonate) and 5% FEC (fluoroethylene carbonate), and a Whatman glass fiber separator (GF/ D, 675 μm). The galvanostatic tests were performed at room temperature between 0 and 2 V versus Li/Li+ at the rates of C/ 5 and C/14 (C/n = 1Li/n h), using a MPG (Biologic) galvanostat. Operando XRD measurements were carried out on the Ge self-supported electrode casted on Mylar foil, with abovementioned cycling conditions using a Panalytical Empyrean diffractometer equipped with Cu−Kα1/Kα2 radiation in theta− theta configuration. The in situ cell, assembled in an argon glovebox, is described in detail in ref 22. Operando Raman spectra were recorded using a confocal Dilor LabRAM spectrometer, equipped with a HeNe laser, a 600 grooves/mm grating, and a Peltier-cooled CCD detector. A backscattering geometry with a 50× optical microscope objective under an excitation laser line of 647 nm was used. Each spectrum was recorded during 10 min with a resolution of 1 cm−1. The in situ cell described in ref 23 was assembled in a glovebox and cycled vs Li/Li+ at C/5 rate using the Ge selfsupported electrode, a Whatman glass separator, and an LP30 electrolyte (1 M LiPF6 in EC:DMC (1:1)). Operando X-ray absorption spectroscopy (XAS) was made at the Ge K-edge in transmission mode at the ROCK beamline24 of synchrotron SOLEIL (France). A Si (111) channel-cut quick-XAS monochromator with an energy resolution of 2 eV at 12 keV was used. The intensity of the monochromatic X-ray beam was measured by three consecutive ionization detectors. The in situ cell25 was assembled using the exact same specifications as for the operando XRD measurements, cycled at C/8, and placed between the first and the second ionization chambers. The energy calibration was established with simultaneous absorption measurements on a Ge metal foil placed between the second and the third ionization chamber. The absolute energy reproducibility of the measured spectra was ±0.05 eV. One hundred four successive spectra were collected using the oscillation frequency of the Quick-XAS monochromator set at 2 Hz and averaged out over periods of 10 min during the Li insertion at a C/8 rate. The corresponding electrochemical curve is shown in the Supporting Information. The energy calibration and normalization on the XAS spectra were done using the Athena software.26 The normalized spectra

3. RESULTS AND DISCUSSION 3.1. Electrochemical Studies. The electrochemical response for the Ge electrode at different rates shows the practical capacity to be 1370 and 1351 mAhg−1 at C/5 and C/ 14, respectively (Figure 1.). For both C/rates, the galvanostatic curve presents four main processes during the lithiation. From 1.0 to 0.7 V the continuous drop in potential is attributed to partial reduction of the electrolyte leading to the formation of the solid electrolyte interphase (SEI) at the surface of the electrode.3,15,32 From ≈380−250 and ≈250−150 mV, two pseudoplateaus indicate the coexistence of two phases and are followed by a drop of the potential (after ca. 200 mV), corresponding to a one-phase region. Upon delithiation, the slope increases abruptly until approximately 350 mV (independently of the rate used), followed by the presence of two pseudoplateaus at 420−450 and 590 mV for C/14 and C/5, respectively. The second lithiation is slightly different; the first pseudoplateau (at 410 mV) presents a steeper slope, whereas the second (at 250 mV) shows a more marked plateau. Both processes, however, occur at higher potentials, suggesting the electrochemical path followed during the second discharge to possibly be different. All the processes in the galvanostatic curves are in line with those displayed in the derivative curves, but there is a slight difference in the intensities of the peak at 590 mV between the C/5 and C/14 rate experiments during charge and additionally a broadening of the peak at 380 mV during the second discharge indicating a change in the nature of the Ge network compared to the pristine sample. 3.2. Operando XRD. In order to observe in depth the cycling processes, a series of operando XRD experiments were performed at the rates of C/5 and C/14. The electrochemical signatures were fully consistent with the coin cell experiments above. C


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Figure 2. Operando XRD and associated galvanostatic discharge−charge curves at (a) C/5 and (c) C/14 cycling rate of the Ge electrodes; (b, d) magnifications of the [18−28°] and [38−44°] 2θ ranges.

For both C-rates, the intensity of the diffraction patterns of cGe initially gradually decrease as the potential drops until complete amorphization (x ∼ 2) without any growth of a crystalline phase (Figure 2). When the potential reaches 250 mV, however, the results of the two C-rates differ, even though the shapes of both galvanostatic curves are similar. For the C/5 rate broad humps grow at 2θ = 24.5° and 41°, which both correspond to poorly crystallized/amorphous lithiated states, possibly Li7Ge2 (or Li7Ge3). At lower potentials betterdefined diffraction patterns appear, which can be clearly attributed to Li15Ge4.4,21,33 Indeed crystalline Li15Ge4 coexists with Li7Ge2 or Li7Ge3. In contrast for the C/14 rate, after the full amorphization of c-Ge, there is no growth of any new peaks during the insertion of about 2.7 Li; only three broad peaks centered at about 23.3°, 24.5°, and 40.5° appear during the end of the discharge, and no crystalline Li15Ge4 is observed. Again it is difficult to unambiguously associate these features to Li7Ge3, Li7Ge2, or Li17Ge4, whereas the finally achieved insertion of 4 Li is more in agreement with either Li7Ge2 or Li17Ge4.4,21,34 Figure S2 shows the diffraction pattern for the discharged product of this operando XRD. The observed Bragg reflections are likely attributable to Li17Ge4 (main peak at 40.5°), in analogy with the reports of Iwamura et al.35 and Zeilinger.36 Upon delithiation at the C/5 rate, the Li15Ge4 phase rapidly disappears, and only the humps centered at 24.5° and 41° are still visible (Li7Ge2 or Li7Ge3) with a shift to higher angles, corresponding to the volume contraction upon Li extraction. For the C/14 rate the broad peaks disappear and are replaced

by the hump at 24.5°, which continuously shifts to 26° at the end of charge. In both cases the humps eventually disappear and give place to a completely flat diffraction pattern, confirming the formation of a-Ge and/or low lithiated a-LixGe.4 Most substantial differences in the XRD evolution between the two C-rates are observed for the second discharge/charge cycle. For the C/5 rate, the second discharge path resembles the first one, but with a better crystallization of Li15Ge4 at the expense of the humps centered at 2θ = 24.5° and 41°. Indeed, the presence of a-Ge or low-lithiated a-LixGe rather than pristine bulk c-Ge at the end of the discharge may contribute to better kinetics of lithiation, facilitating the formation of Li15Ge4. In stark contrast, for the C/14 rate, no Li15Ge4 is formed at the end of the second discharge, the reason why the two broad peaks centered at 23.3° and 24.5°, together with the reflection at 40.5° (with an increased intensity), are more likely to be a highly lithiated phase such as Li17Ge4 (in agreement with x ∼ 4.1) rather than Li7Ge2.21,34 The possible formation of such highly lithiated phases has previously been observed by Yoon et al.,19 Iwamura et al.,35 Zeilinger,36 and Goward et al.34 and predicted by Morris et al.21 for very low potentials, close to 0.10 V. It is possible that with the presence of a-Ge cycled and thus with optimized electrochemical interfaces and by cycling at low rates, the overpotentials are sufficiently low to make such very highly lithiated phases accessible. For both rates the Bragg peaks of the crystalline phases that grow during the discharge process initially slightly shift to lower angles (Figure 2b) during the discharge and then toward higher angles during the beginning of charge, in agreement with an D


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Figure 3. (a) Galvanostatic discharge−charge curves at C/5 rate for the Ge electrode; (b) the corresponding Raman spectra and (c) Raman spectra: (A, B) from the operando experiment at E = 0.20 and 0.06 V, respectively, (C) in situ of the Ge electrode after discharging and 40 h potentiostatic step at 0.01 V, and (D) ex situ of the Li15Ge4 reference electrode prepared by ball milling.

toward lower frequencies suggests the evolution of the freshly formed Li15Ge4 to a new amorphous LixGe-like state which could possibly correspond to Li7Ge3 (or Li7Ge2), proposed to compete with Li15Ge4 at the end of discharge, based on the XRD analysis. The feature might, however, also correspond to the evolution of Li15Ge4 to Li15+δGe4, as observed by Jung et al.4 and Tang et al.20 by in situ solid state NMR and already discussed above (vide supra). In order to complement the latter propositions, a Ge electrode discharged down to 0.01 V and held there for 40 h (Figure 3c, C) to allow a better crystallization of the lithiated Ge phase(s) was measured. The resulting spectrum shows several features, matching those of the Li15Ge4 reference (Figure 3c, D) and also resembles the spectra of the analogous LixSi phases.38 After the crystallization process, however, the main peak is well centered at 222 cm−1, corresponding to the presence of “normal” Li15Ge4. Finally, during delithiation, the presence of a final peak at ∼300 cm−1 indicates the reformation of Ge−Ge bonds, while its broad shape and slightly shifted position compared to pristine c-Ge agrees well with the formation of a-Ge during the charge process. 3.4. Operando X-ray Absorption Spectroscopy. Further complementary information about the electrochemical mechanism of Ge during lithiation was gathered by operando XAS, a site selective and local probe technique ideal for the study of amorphous phases, as already demonstrated in our previous work on Si1−xGex alloys.37 In total, 104 spectra were collected during the first discharge at C/8 rate (a rate chosen as optimal in terms of low rate cycling without exceeding the beam time allocated) and are shown in Figure 4. Previous XAS studies have mainly centered on interpreting the signal corresponding to the first peak of the Fourier transform (FT) of their extended fine structure (EXAFS) in terms of contributions of Ge−Ge and Ge−Li shells.14,16−18 Here, we have proposed a different analysis approach based on

increasing/decreasing of the crystallographic cell parameters during lithiation/delithiation.4,37 This structural evolution might be related to the existence of solubility domains. Such domains have already been explained as an overlithiation process transferring c-Li15Ge4 into Li15+δGe4, as observed by in situ solid state NMR by both Jung et al.4 and Tang et al.20at low voltages. c-Li15Ge4 and Li15+δGe4 are in fact structurally related and have until now only been differentiated by solid state NMR, with the defects in the lattice playing an important role in accommodating the extra charges. The existence of a solubility domain affects also the hypothetic Li7Ge3 (or Li7Ge2) (C/5) and Li17Ge4 (C/14). 3.3. Operando Raman Spectroscopy. The different hypothesis brought by XRD was further targeted by operando confocal Raman spectroscopy, performed at C/5 rate to gain complementary information about the phase formed at the end of the discharge and charge processes. The evolution of the Raman spectra during the first lithiation/delithiation cycle is shown in Figure 3, together with the galvanostatic curve. The mode of c-Ge at 298 cm−1 is clearly visible for the pristine electrode measured under open circuit voltage (OCV) conditions, in agreement with the literature.7 As the lithiation progresses, this peak decreases continuously until its complete extinction at 240 mV, in line with the depletion of c-Ge (also observed by XRD), through the first biphasic process leading to an amorphous a-Li xGe intermediate. Shortly after (at ∼200 mV), as the potential keeps decreasing, a new weak peak that appears at around 220 cm−1 grows and shifts to lower frequencies, until reaching 208 cm−1 at the end of discharge (∼20 mV). During delithiation, another peak at 278 cm−1 (arrow in Figure 3b) emerges at ∼750 mV and reaches a maximum intensity at ∼1 V. This peak is gradually replaced by a broad one centered at ∼300 cm−1. The peak at 220 cm−1 that appears toward the end of the discharge matches with the spectrum of the Li15Ge4 reference (Figure 3c, D). Its shift E


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Figure 4. XAS spectra collected during the first discharge at C/8 for a Ge electrode and reconstructed components from the analysis of the whole series: (a) XANES and (b) EXAFS regions of the experimental spectra (the evolution of the spectra from the beginning to the end of the discharge is represented by spectra going from dark blue to green); (c) XANES and (d) EXAFS regions of the reconstructed components.

chemometric tools that take into account the whole evolution of the XAS spectra during the lithiation. This approach was used in order to extract the maximum information from this new whole series of operando XAS spectra, especially on the intermediate phases. First, the number of orthogonal “pure” components was obtained by principal component analysis (PCA); then, by multivariate curve resolution-alternating least squares (MC-ALS) method, we were able to reconstruct these “pure” components. In fact, this statistical method allows not only to reduce the dimension’s problem but also to reveal the hidden intermediated species. The PCA (Figure S4) strongly indicates the existence of four dominant components expressing more than 99.9% of the variance of the whole series of spectra. The inspection of their shape and of the evolution of their scores strengthens this assumption. These four components were then used in the MCR−ALS analysis, which provides “pure” spectral components expressing, by their linear combination, the main modifications of the XAS spectra. The evolution of their respective relative concentrations along the series of spectra is shown in Figure 5. Clearly, components 1 and 4 represent the pristine c-Ge and the final product of the first discharge. Components 2 and 3 show maxima after 30 and 65 measured spectra from the beginning of the discharge, respectively, corresponding to the reaction of about 1 and 2 mol of Li per mol of Ge.

Figure 5. Evolution of the concentration of the MCR−ALS components during the lithiation of Ge.

By observing the shape of the absorption edge of the four components, we find that the evolution of the XANES spectra through the first discharge cannot simply be expressed as a linear combination of the XANES spectra of the pristine and the fully lithiated material, but at least two new XANES spectra are needed. Since the shape of the XANES spectra is directly F


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Figure 6. Fit of the EXAFS portion of the MCR−ALS components in (a) k and (b) R space. The black line corresponds to the data and the red one to the fit.

Table 2. Fitting Parameters of the EXAFS Portion of the MCR−ALS Components in (a) k and (b) R Space comp.

phase

1

Ge

2

Ge

LiGe 3

Li7Ge3 (Li9Ge4)

4

Li15Ge4

shell Ge−Ge Ge−Ge Ge−Ge Ge−Ge Ge−Ge Ge−Ge Ge−Ge Ge−Li Ge−Ge Ge−Li Ge−Li Ge−Li Ge−Ge

S02 0.93

0.32(5)

0.61(5) 0.93 0.93

related to band structure, this means that at least two additional phases are formed during the first discharge. The analysis of the EXAFS portions of the MCR−ALS components shows the fit in both k and R spaces (Figure 6, Table 2). The fitting of the first component, representing pristine c-Ge, is rather trivial and is performed up to R = 5 using the first three distance shells of the diamond structure of Ge.39 While the bond distances coincide perfectly with those expected from the structure, rather high values of mean-square relative fluctuations (σ2) are observed for the second and the third shells, indicating a somewhat disordered structure or a partial amorphization of the pristine c-Ge, in line with the ball-milling activation pretreatment before preparation of the electrodes. The FT of the second component shows the same three peaks as for the first component. However, the peaks are now

N

Rtheory (Å)

Rmeasured (Å)

σ2

4 12 12 4 12 12 3 5 1 7 3 9 5

2.449 4.000 4.690 2.449 4.000 4.690 2.554 2.64−2.99 2.444 2.55−2.98 2.576 2.78−2.90 4.56−4.64

2.461(3) 4.03(1) 4.73(2) 2.462(5) 4.00(1) 4.69(2) 2.57(3) 2.70(6) 2.57(2) 2.8(1) 2.54(2) 2.81(2) 4.54(5)

0.0052(2) 0.019(1) 0.022(3) 0.0045(6) 0.0091(8) 0.012(2) 0.015(8) 0.015(8) 0.008(1) 0.020(9) 0.007(2) 0.015(2) 0.030(3)

globally less intense and have different intensity ratios, suggesting this component to not represent a pure compound but rather also some remaining c-Ge as well as at least one lithiated phase. A fit was attempted using a two-phase approach, dividing the total intensity of the spectrum between the threeshell contribution of c-Ge and the two-shell contribution of LiGe, whereby the different Ge−Ge and Li−Ge bonds distances were averaged out. While the LiGe fraction dominates, more than 2/3 of the total component intensity corresponds to the three-shell of c-Ge, similar to the pristine material, but with clearly lower σ2, indicating an increase in the crystallinity. Taking into account the general amorphization during this part of the discharge, an increase in crystallinity is rather unexpected, but this apparent incoherence can be explained by the fraction of more a-Ge, contained in the pristine sample, reacting first and thereby increasing the relative G


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4. CONCLUSION In summary, we have investigated the electrochemical mechanism of a Ge electrode during lithiation/delithiation by operando XRD, Raman spectroscopy, and XAS. The results indicate that the lithiated phases/states formed during lithiation/delithiation are kinetically dependent on the C/rate. This is, relatively higher C/rates promote the growing of cLi15Ge4 at the end of discharge after the successive formation of amorphous/poorly crystallized low-lithiated LixGe (x = 7/3 or 7/2) states. Low C/rates, however, are characterized by the absence of c-Li15Ge4 and the presence Li17Ge4. In both cases during delithiation, the mechanism is reversed; however, the crystalline Ge network is not rebuilt, and an amorphous product is obtained instead (as confirmed by the Raman and XAS results). This work illustrates the high interest of performing longrange and short-range sensitive experiments in operando conditions and brings new essential information to understand the puzzling mechanism of alloys-based electrode material like Ge. It was demonstrated that the cycling rate and the morphology of the pristine electrodes play an important role in the kinetics and the participating phases in the electrochemical mechanism, underlining that the differences in the literature arise from the material and the conditions used for cycling. Nonetheless, a deep understanding of the electrochemical mechanism is crucial for improving the performances of such electrode materials.

concentration of better crystallized c-Ge reacting at a later stage. The FT of component 3, which replaces component 2 about halfway through the discharge, shows only a main contribution corresponding to the first coordination shell of Ge, which can be shared between a Ge−Ge and a Ge−Li shell. It is interesting to note the shifting of the main peak of FT toward higher R distances compared to the first two components, associated with an important change of the unit cell volume. These data could not be fitted by using the typical Ge−Ge and Ge−Li shells intensity of Li7Ge2, whereas a better fit was obtained by increasing the Ge−Ge contribution, indicating Li7Ge3 as an alternative21,40 Therefore, we used coordination shells simulated for Li9Ge4, a phase with a similar composition and numbers of Ge and Li neighbors. Both phases contain the same type of Ge−Ge dumbbell-like structures, and taking into account plausible longer Ge−Ge distances in Li7Ge3, the fit confirms this component to be due to Li7Ge3. Finally, component 4 could be easily fitted with three shells: two Ge−Li shells describing the first coordination shell of Ge and a Ge−Ge shell at longer distances indicating a somewhat better midrange order. These shells correspond perfectly with the bond distances observed in Li15Ge4, in line with the formation of this phase with a relatively good crystallinity at the end of the discharge. 3.5. Proposed Mechanism. By correlating all the information provided by operando XRD, Raman spectroscopy, XAS, and the electrochemical results, we can follow the relation of the electrochemical mechanism of Ge upon lithiation at different C/rates: At relatively higher C/rates (C/5 and C/8), the lithiation of c-Ge leads to a phase amorphization, followed by the formation of amorphous/poorly crystallized intermediates: LiGe as first lower lithiated state and Li7Ge3 as second one. Subsequently, at the end of discharge there is a competitive growth between these amorphous lithiated states and c-Li15Ge4. The mechanism is reversed during delithiation with the eventual formation of aGe. Hence, the lithiation of Ge occurs by the successive biphasic processes and can be summarized as follows:

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11249. SEM pictures of pristine Ge and pristine self-supported Ge film; XRD pattern of the discharged product obtained during the operando XRD at C/14; galvanostatic curve for the operando XAS performed at C/8 and the corresponding spectra number; PCA analysis of the components and the evolution of their scores (PDF)

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Discharge: c‐Ge → LiGe → Li 7Ge3 → c‐Li15Ge4

AUTHOR INFORMATION

Corresponding Author

*Address: Laboratoire de Réactivité et Chimie des Solides, CNRS, UMR 7314, Université de Picardie Jules Verne, 33 Rue Saint Leu, 80039 Amiens Cedex, France. E-mail: vincent. [email protected].

Charge: c‐Li15Ge4 → LiGe → Li 7Ge3 → a‐Ge

At lower C/rates (C/14), the lithiation of c-Ge also leads to amorphous intermediates, but with the formation of Li17Ge4 at the end of the first discharge and the absence of c-Li15Ge4. Upon delithiation, a-Ge is present. The mechanism can be described as

ORCID

Nicolas Louvain: 0000-0001-8727-6832 Vincent Seznec: 0000-0001-5233-5943 Laure Monconduit: 0000-0003-3698-856X

Discharge: c‐Ge → a − LixGe → c‐Li17Ge4

Notes

The authors declare no competing financial interest.

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Charge: → c‐Li17Ge4 → a‐LixGe → a‐Ge

ACKNOWLEDGMENTS We warmly thank L. Daenens (ICGM) and E. Zanghellini (Chalmers) for the technical help with the Raman measurements, M. Khokhlova for the assistance in the preparation of the XAS cells, S. Belin and V. Briois for their expert advice on the beamline operation, Synchrotron Soleil (France) for providing beamtime at the ROCK beamline (financed by the French National Research Agency (ANR), reference: ANR-10EQPX-45), and Alistore-European Research Institute for its

The second discharge/charge cycles in both cases are characterized by the enhanced crystallinity of the different phases/states due to the presence of a-Ge and optimized electrochemical interphases. On the other hand, the end of the discharge/beginning of charge for both C/rates is characterized by a solid solution process with cell parameters modification that is related to the existence of solubility domains and to a process of accommodation of extra charges in the defects of the structure (overlithiation). H


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The Journal of Physical Chemistry C financial support through the postdoctoral grant to A.B. The travel grants from Chalmers Area of Advance Energy to P.J. for the visit by L. Stievano and A. Boulaoued to Chalmers are also gratefully acknowledged. The research group “Catalyse, Réactivité de Surface et Rayonnement Synchrotron” (GDR CNRS 3590) is gratefully acknowledged for the organization of the workshop on chemometric approach for XAS data analysis, and the Ministry of Higher Education & Research of France for the financial support of L.L. via a “Contrat Doctoral” at UPJV Amiens.

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