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Elucidating the surface reactions of an amorphous Si thin film as a model electrode for Li-ion batteries Giulio Ferraresi, Lukas Czornomaz, Claire Villevieille, Petr Novák, and Mario El Kazzi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10929 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Elucidating the surface reactions of an amorphous Si thin film as a model electrode for Li-ion batteries Giulio Ferraresi†, Lukas Czornomaz‡, Claire Villevieille†, Petr Novák†, Mario El Kazzi†*

† Paul Scherrer Institute, Electrochemical Energy Storage Section, CH-5232 Villigen PSI, Switzerland ‡ IBM Research-Zürich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland

KEYWORDS: Thin film, amorphous silicon, solid electrolyte interphase, X-ray photoemission spectroscopy, Li-ion batteries

ABSTRACT We investigated during the first lithiation/delithiation process the electrochemical reaction mechanisms at the surface of 30 nm n-doped amorphous silicon (a-Si) thin film used as negative model electrode for Li-ion batteries. Usage of thin film allowed us to accurately discern the different reaction mechanisms occurring at the surface by avoiding interference from carbon and binder components. The potential-dependency of the evolution of the solid electrolyte interphase (SEI) and the reactions on the a-Si and on the copper current collector were elucidated by

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coupling galvanostatic cycling with post-mortem X-ray photoemission spectroscopy (XPS) and Scanning Electron Microscopy (SEM) analyses. Our approach revealed the clear reversibility of lithiation/delithiation in the a-Si and native SiO2 layers; such a reaction for SiO2 has not been previously detected and was considered to be an irreversible process. Quantitative and qualitative analyses of the potential-dependent surface evolution revealed the decomposition products of both the salt (LiPF6) and solvent (DMC/EC), giving insight into the complex SEI formation mechanism on the a-Si film but also underline the strong influence of “inert” materials such as the role of the current collector in the irreversible charge loss. A model mechanism describing the evolutionary complexity of the a-Si surface during the first galvanostatic cycle is proposed and discussed.

1. INTRODUCTION Silicon is considered to be the most promising negative electrode material for replacing graphite in Li-ion batteries because of its abundance and its high specific charge, which exceeds that of graphite (370 mAh/g) by nearly a factor of ten.1 Unfortunately, its commercialization in Li-ion batteries has been delayed for two main reasons. First, Si undergoes substantial volume expansion during lithiation/delithiation (ca. 300%), which disconnects Si particles from the conductive carbon/current collector and leads to the rapid fading of the specific charge.2 Second, the continuous solid electrolyte interphase (SEI) formation caused by the volume expansion/contraction and thus a continuous creation of fresh surface that occurs during cycling results in the steady consumption of Li ions.3 Several approaches have been considered to buffer the fading, such as the implementation of nano-sized silicon electrodes4-11 or the use of an alternative electrolyte or electrolyte additives that help to form a passivation layer able to prevent further electrolyte reduction.


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Various studies were conducted to understand the bulk reactions and the surface electrolyte/electrode interaction of Si nanoparticles in conventional electrodes (Si particles mixed with a conductive carbon and a binder) or as thin films free of additives.12 Lee et al. investigated thin films of a-Si (thickness of ca. 430 nm) by X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS), concluding that the native SiO2 layer covering the a-Si surface is destroyed at the beginning of the lithiation process.13 However, the formation of a thick SEI that attenuated the signals due to SiO2 and Si0 prevented the observation of the LixSi alloying reaction. Philippe et al. overcame this limitation by performing XPS using hard Xrays to probe the Si particles through the thick SEI.14 They found that the native SiO2 was involved in two parallel reactions at ~ 0.2 V vs. Li+/Li during lithiation: a reduction to Si (SiO2 + 4 Li 2 Li2O + Si) and lithiation (2 SiO2 + 4 Li Li4SiO4 + Si). They also observed that the SiO2 layer was only partially involved in the above reactions and considered the processes irreversible. With respect to the SEI, an obvious increase in thickness was observed during lithiation but, during delithiation, the SEI layer was rather stable, exhibiting only slight dissolution. Similarly, Young et al. investigated the SEI composition on binder-free Si nanoparticles, which was mainly composed of LiF and lithium ethylene dicarbonate species after the first cycle.15 In agreement with previous results, Guo et al. proposed that nano-SiO2 particles irreversibly form Li-silicate compounds at 0.25 V vs. Li+/Li during lithiation.16 An excellent understanding of the surface reaction mechanisms that occur at the electrolyte/electrode interface during the cycling of a-Si is crucial for its further development.17 In this context, we revisited this topic using a 30 nm thin film of a-Si as a model electrode, free of any conductive carbon and polymeric binder. Due to the small amount of active material present in such film, the charge consumption carried by electrolyte reduction on the current



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collector during first charge is also highlighted and discussed. The final goal of this study was to perform post-mortem XPS and SEM by monitoring the Si 2p, O 1s, and Li 1s core levels to elucidate the reaction mechanisms of both the LixSi alloy and native SiO2 layer during the early stages of cycling. Simultaneously, by probing the F 1s, C 1s, and P 2p core levels, we anticipated being able to track the decomposition of the salt and solvent mixture in the electrolyte, and qualitatively and quantitatively investigated the SEI composition.

2. EXPERIMENTAL 2.1 a-Si thin film deposition. Copper metal foils (10 × 10 cm) were mounted on a Si carrier wafer and loaded in a plasma-enhanced chemical vapor deposition (PECVD) tool with a base temperature of 350 °C. Silane (SiH4, 2% in Ar) and phosphine (PH3, 3% in Ar) were introduced in the deposition chamber at flow rates of 1250 and 20 sccm, respectively, at a pressure of 1.5 Torr. The deposition was carried out with an RF power of 10 W at a rate of 11 nm/min. The resulting deposited layer comprised a 30 nm thick film of in situ n-doped a-Si with a resistivity of 70 Ω·cm, calculated from the current-voltage (I-V) characteristics measured by the 4-point probes method on a reference sample of a-Si on SiO2. The deposition conditions were selected to minimize the film resistivity. It is noteworthy that no surface cleaning of the metal foils was carried out prior to a-Si deposition. SEM images of the as-deposited Si film are shown in Figure S1 (Supporting Information) to confirm the thickness and homogeneity of the film. . 2.2 Electrochemical investigation. Different electrodes were obtained from the same deposited batch in order to avoid any reproducibility problems (change in the growth conditions, film thickness and doping concentration). Electrodes were inserted inside an argon-filled glovebox after being dried at 120 °C overnight under dynamic vacuum. The active mass was indirectly


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calculated by considering the full density of the amorphous film (2.33 g/cm3). Half-cells were assembled using Celgard 2400 as a separator soaked with 500 µL LP30 electrolyte (1 M LiPF6 salt in ethylene carbonate/ dimethyl carbonate, EC:DMC 1:1 w/w, BASF SE) and lithium metal as the counter electrode. Galvanostatic cycling was performed at the 1 C rate (one hour for charge, one hour for discharge) in the 5 mV–1.5 V vs. Li+/Li potential range. All the potentials given in the manuscript are referred to the Li+/Li potential. Regarding the samples measured at the open-circuit potential (soaked electrodes), the electrodes were placed inside the assembled testing cell and rested for 72 h. The cycling of each cell was stopped at specific potentials, the cell then disassembled inside an argon-filled glovebox, and the Si electrode rinsed with DMC (BASF SE) before being dried and transferred into the XPS chamber using an argon-filled transfer vessel. 2.3 Post-mortem XPS. The XPS measurements were conducted with a VG ESCALAB 220iXL spectrometer (Thermo Fisher Scientific) using focused monochromatized Al Kα radiation (1486.6 eV) with a beam size of ∼ 500 µm2 (power, 150 W). The pressure in the analysis chamber was approximately 2 × 10−9 mbar. The spectrometer was calibrated on a clean silver surface by measuring the Ag 3d5/2 peak at a binding energy (BE) of 368.25 eV with a full-widthat-half-maximum (FWHM) of 0.78 eV. All the spectra were recorded using pass energy of 30 eV in steps of 50 meV and a dwell time of 50 ms. Peak deconvolution was performed by applying the sum of Gaussian (70%) and Lorentzian (30%) line shapes after Shirley-type background subtraction. Deconvolutions of the Si 2p peaks were performed by fixing their spin-orbit splitting and branching ratio to 0.59 eV and 0.5, respectively.18 No calibration of the peak positions was applied in this study since all the core levels evolve during lithiation/delithiation and have the tendency to shift, even the hydrocarbon (CH2) peaks in the C 1s spectra which are commonly



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selected as a reference.11, 14 Inhomogeneous shifts are always observed and reported when XPS measurements are performed on cycled electrodes.19 The disparate shifts for the signals of the different compounds have been explained by differential local charging effects between the surface layer (composed of Li-rich organic/inorganic species) and the electrode components (active material, conductive carbon, and organic binder) due to poor conductivity. Here, by using a thin film of n-doped a-Si, there was low risk of any charging effect when measuring the Si 2p core levels because the substrate is highly conductive and adequately grounded. Thus, any shift of the Si 2p peak would always be related to a change in the electronic structure. Spectra recorded on different samples were normalized by the intensities of the respective Si0 or LixSi components of the Si 2p core level. The reproducibility of the XPS measurement is confirmed by measuring different spots on the surface of each electrode.

3. RESULTS AND DISCUSSION 3.1 Electrochemical investigation. Figures 1a and b show the galvanostatic profile of the first cycle and the derivative curve, respectively, for the 30 nm a-Si thin film at the 1 C rate between 5 mV and 1.5 V. The lithiation of a-Si is known to take place in two distinct processes negative to 0.4 V, which correspond to the two sharp peaks observed at 0.24 V and 0.09 V.20 Both processes are reversible, and delithiation shows approximately three times lower charge consumption compared to the lithiation, resulting in the irreversible consumption of 60% of the charge during the first cycle. Almost 50% of the charge is consumed positive to 0.4 V during lithiation. This activity is thought to be related to electrolyte reduction on Si and is often reported as the initial formation of the SEI.12, 14, 21-22 However, for a thin film a significant contribution could arise from electrolyte reduction at the current collector, as we observed comparable


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activity in the 1.5–0.6 V region when we cycled a bare Cu foil under the same conditions (Figure 1b). The second galvanostatic cycle does not show any further electrochemical activity above 0.5 V. Instead, two reversible peaks at 0.24 V and 0.09 V are present with reduced intensity compared to the first cycle. The delithiation curve in the second galvanostatic cycle is comparable to the first cycle leading to an estimated specific charge of 4200mAh/g with respect to the calculated mass of 9 ± 1 µg from geometrical parameters.

Figure 1. (a) First galvanostatic cycle of the 30 nm a-Si thin film at the 1 C rate. (b) Derivative curve of the first galvanostatic cycle compared to the first cycle of a bare Cu substrate.



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In terms of electrochemical stability in the cathodic potential window, metallic Cu is a stable current collector for negative electrode in Li-ion batteries. However, on the surface a thin layer of Cu2O/CuO species is always present upon air exposure. These metal oxides are known to be reactive with lithium in the 2.5 - 0.6 V region forming Li2O and Cu° as end products23. Cu oxides reduction is generally neglected when the loading of the active material is greater than the mass of such a thin layer (i.e. conventional thick electrodes). In our specific case, due to the low active mass in the 30 nm thin film, the irreversible charge consumption during the first cycle is possibly carried by the reduction of copper oxide and electrolyte reduction on the current collector. To sustain this hypothesis, post-mortem XPS investigation was performed on the surface of the Cu current collector in parallel to the investigation on the surface of the a-Si thin film, and analysis of various potential steps during first discharge was conducted. As observed in Figure 2, the Cu-side of the pristine electrode shows the presence of two clear components of Cu0/+1 and Cu2+ between 933.1 eV and 934.5 eV in the Cu 2p spectra, and a collection of satellites peaks (in the range 941-944 eV) associated to Cu2+. We clearly observe the disappearance of the Cu2+ component and a net increase in intensity of Cu0/+1 at 1.5 V, corresponding to the reduction of the copper oxides layer at the surface of the current collector. As also clarified in Figure S2 and S3 (see Supporting Information), by decreasing the potential from 1.5 V to 1 V and 0.2 V there is a subsequent decrease of the Cu0 component intensity which is a clear indication of the formation of a surface layer on top of the current collector. Regarding the C 1s core level, the surface of the current collector starts to evolve at 1.5 V with the increased intensity of the component at 291.4 eV associated to carbonate-species. The simultaneous intensity decrease of Cu0 and increase of carbonate components are attributed to the signature of


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a surface layer formation on the current collector mainly composed of carbonate-based species and responsible of relevant charge consumption occurred at this potential.

Figure 2. XPS spectra on Cu 2p and C 1s core levels acquired on the Cu-side of the a-Si thin film for pristine and cycled electrodes at various potentials during first charge.

Based on the galvanostatic cycle profile, specific potential steps were chosen to investigate the evolution of the surface reaction mechanisms on a-Si film for the lithiation/delithiation as well as SEI formation and subsequent decomposition processes. 3.2 Unreacted a-Si and adsorbed species on the surface [OCP–0.6 V]. The Si 2p XPS spectrum obtained with the pristine sample is displayed in Figure 3. It shows the two major components of Si 2p3/2 at 99.63 eV and 103.75 eV, associated with the core levels of Si0 (Si–Si bonds) and Si in the +4 oxidation state, respectively. The latter, attributed to SiO2, originates from the native silicon oxide formed after air exposure of the as-prepared samples. The C 1s core



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level peak exhibits very low intensity compared to the Si 2p and can be de-convoluted into three main components: a major one at 285.8 eV associated with hydrocarbon (CH2) species and two minor components at 289.8 eV and 287.2 eV which can be attributed to the adsorbed carbon species CO3 and O-C=O, respectively, as already found on the surfaces of air-exposed samples.24

Figure 3. Si 2p, C 1s, and F 1s XPS core levels acquired on a-Si thin films for a pristine sample, soaked (OCP), and during lithiation and delithiation process.

The O 1s core level peak in Figure 4 shows a single broad peak at 533.6 eV attributed to the native SiO2 layer; any oxygenated carbon species that are present would be obscured due to the energy overlap with the SiO2 component. At the OCP and at 0.6 V slight changes are observed in the Si 2p peaks compared to the pristine, in which a shift of 0.2 eV to higher BE of the Si0 component is observed (see Table 1 for values). 10 Environment ACS Paragon Plus

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Figure 4. O 1s, Li 1s, and P 2p XPS core levels acquired on a-Si thin films for a pristine sample, soaked (OCP), and during lithiation and delithiation process.

The measured |∆E| values (BE Si0 − Si4+) also show increases, from 4.1 eV for the pristine to 4.3 eV and 4.7 eV at OCP and 0.6 V, respectively. The increase in the |∆E| can only be explained by the reaction of the SiO2 with the fluorine present in the LiPF6 salt, leading to the formation of SiOxFy species.25-26 To confirm the formation of SiOxFy species, a comparative test was performed by soaking an electrode in a cell with LC30 electrolyte solution free of any fluorine (LiClO4 salt in EC:DMC solvent 1:1). As shown in Figure S4 (Supporting Information), no Si4+ peak shift to higher BE was observed with LC30, supporting the diffusion hypothesis of the fluorine in the oxide layer. Regarding the carbonaceous and fluorinated species, an obvious difference is visible between the sample pristine and at OCP in Figure 3. The C 1s peak exhibits



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an intensity increase for the hydrocarbon species and the appearance of additional components at 287.8 eV and 290.2 eV, belonging to O–C=O and CO3 species, respectively. The evolution of the surface layer at OCP can only be related to adsorbed and/or reacted species that originate from electrolyte/a-Si interactions, since no external current flows inside the cell and electrolyte reduction is not expected. Fluorinated species are also detected in the F 1s core level at ~688 and ~690 eV, which are associated with SiOxFy and LiPFxOy (salt remaining on the surface), respectively. Post mortem SEM performed on soaked a-Si confirms that the surface is covered by remaining salt despite the washing with DMC (Figure S3, Supporting Information). At 0.6 V, the absolute intensity of the Si 2p peak remains roughly the same as at OCP, and only a slight decrease of the C 1s signal intensity is observed (Figure 5). Such observations allow us to presume no change in the surface layer thickness. This suggests that no SEI surface coverage related to EC/DMC reduction occurs at 0.6 V, and especially, that no changes are observed in the carbon species composition or intensity; thus, the detected surface products are still related to the originally adsorbed species. However, the shift to higher BE observed for the C 1s components at 0.6 V compared to OCP is believed to be related to a ″local charging effect″. On the other hand, the slight increase in the F 1s peak absolute intensity at ~ 686.5 eV is associated with the formation of LiF organic species. The little amount of LiF detected can be explained by the LiPF6 decomposition either electrochemically or chemically e.g. under the beam exposure27. The SEM images at 0.6 V measured on a-Si in Figure S3 show that the density of the remaining salt increases respect to the soaked electrode. Regarding the O 1s core level, no additional components are detected between the pristine, OCP and 0.6 V samples; the only noticeable features are the slight increases in the peak width and position, related to the overlap of the


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SiOxFy component with the oxygenated species28 and to the local charging effect already observed in the C 1s spectra, respectively.

Table 1. Binding energy (BE) position and full-width-at-half-maximum (FWHM) of the Si 2p3/2 peak for the Si0 and Si4+ components.

Si0

Si4+

|∆E| (Si0 − Si4+)

Lithiation

Sample

Delithiation

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BEmax (eV)

FWHM

BEmax (eV)

FWHM

BEmax (eV)

Pristine

99.6

0.81

103.8

1.41

4.2

OCP

99.8

0.82

104.1

1.59

4.3

0.6 V

99.8

0.86

104.5

1.71

4.7

0.2 V

98.9

1.04

103.4

2.06

4.5

0.07 V

98.4

1.29

103.7

2.22

5.3

5 mV

98.3

1.28

103.6

2.09

5.3

0.28 V

98.7

1.36

103.8

2.27

5.1

0.47 V

99.1

1.02

103.5

2.16

4.4

1.5 V

99.4

1.05

104.5

1.70

5.1

3.3 Lithiation reaction mechanism and SEI formation [0.2 V–5 mV]. Once the voltage reaches 0.2 V, an important intensity drop is observed for the Si 2p peaks and both the Si0 and Si4+ components are shifted to lower BE at 0.8 eV and 1.1 eV, respectively. The shift of the SiOxFy component is related to the formation of a Li-silicate compound, whereas the shift of the Si0 is associated with formation of the LixSi alloy. Both peaks exhibit a broadening of the FWHM (Table 1). The shift of the Si4+ component proves that the SiOxFy is electrochemically



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active. The previous reaction of SiO2 with fluorine makes the identification of the Li-silicate composition more complex, since it can influence the peak position. At the same time, the lithiation of the a-Si is accompanied by a shift of ~ 1.6 eV to lower BE for the P 2p component relative to the inactive P0 detected on the surface. This shift is believed to be caused by the lithiation of P0, leading to the formation of LixP alloy. On the other hand, the intensity drop of the Si 2p signal is accompanied by an important increase in the LiF component intensity at 688 eV. An accurate quantitative analysis of LiF species is trivial due to the sensitivity of this compound under the X-ray beam, however the observed increase can only be explained by a strong salt reduction at this potential. Simultaneously, the C 1s peak shows slight intensity increase and remarkable increase in the BE position with a shift of 1.5 eV to higher BE. This behavior confirms the growth of a surface layer on top of the a-Si which is rich in LiF species. In the C 1s peak, the components located at 291.5 and 289.3 eV associated with Li2CO3 and O– C=O, respectively, can still be identified. The silicon/silicate lithiation and SEI formation can also be monitored by following the evolution of the Li 1s and O 1s core levels presented in Figure 4. The Li 1s spectrum shows two defined peaks: that at 58.1 eV is related to electrolyte decomposition with Li-rich species (LiF, Li2CO3, ROLi, oligomers, etc.),24 and eventually the Li-silicate species, whereas the peak at 55.6 eV is associated with the LixSi alloy component. Regarding the O 1s core level, three main resolved peaks can be distinguished: the one at 532.6 eV belongs to the Li-silicate species, whereas the other two peaks at 534.6 eV and 536 eV belong to lithiated oxygen species generated by electrolyte reduction. Focusing on the lithiation at 0.07 V and 5 mV, noticeable shifts to lower BE are observed at 0.07 V for the Si 2p and Li 1s components related to the LixSi alloy reaction. However, the shifts


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are less pronounced at 5 mV (see Table 1 for detailed values). The same trend is also noticed for the FWHM, which increases from 1.04 eV at 0.2 V to 1.29 eV at 0.07 V and remains constant at 5 mV. Interestingly, the BE position and FWHM related to the Li-silicate do not follow the trend for the LixSi component but rather shift to higher BE for samples D and E. However, the FWHM increases at 0.07 V and then decreases at 5 mV, proving that silicate formation evolves during the lithiation, with the possible presence of two different silicate phases at 0.07 V. Based on the evolution of the Si 2p relative intensities between 0.2 V and 0.07 V, we can assert that the SEI thickness increases at 0.07 V. This increase is accompanied by a drop in the LiF component intensity and an increase in that of the CH2 component. This can be explained by the strong reduction of the solvent with respect to the salt at 0.07 V, leading to the formation of a more pronounced oligomeric layer covering the LiF species already present at the surface and attenuating its intensity. Between 0.07 V and 5 mV, no further reduction is observed for DMC/EC, since the C 1s signal does not evolve whereas the LiF intensity increases by 4 times in the F 1s spectra. Surprisingly, also the Si 2p intensity remains nearly invariant, as if the surface layer thickness does not increase. The only reasonable model which can explain this behavior is that the LiF grows as spherical clusters on the previously formed oligomeric layer instead of covering the surface as a 2D layer. 3.4. Delithiation reaction mechanism and SEI decomposition [0.28–1.5 V]. During delithiation, the reaction to form the LixSi alloy is observed to be reversible, as confirmed by the constant increase in the Si 2p BE from 98.3 eV to 99.4 eV between 5 mV and 1.5 V. We notice that the Si0 BE position at 1.5 V is not fully restored if compared to the OCP, since there is a |∆E| of 0.4 eV between the two samples. Despite this ∆E, we consider that the a-Si is fully delithiated (in the depth range of 8 nm), as attested by the complete disappearance of the



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LixSi component in the Li 1s spectra. Two plausible explanations may account for this divergence: (i) the n-dopant concentration drops within the a-Si, which can lead to a shift in the Fermi-level from the conductive band (highly n-doped) more toward the mid-gap; and/or (ii) volume expansion occurs which results in significant structural damage leading to Fermi-level pinning. A drop in the n-dopant concentration is the more plausible hypothesis, especially because the phosphorus dopant takes part in the lithiation process either as LixP or LixSiP29 and is observed to be completely reversible at 1.5 V. Concerning the Li-silicate component, it is observed to be fully delithiated at 1.5 V, according to the shift of the BE back to 104.8 eV, as on sample B. No obvious shift is detected before 1.5 V, which is evidence that the delithiation process of the Li-silicate is different than that of the LixSi alloy. Moreover, the slight increase in the intensity ratio between Si0 and Si4+ (ISi0/Si4+) at the end of delithiation suggests the partial decomposition of the SiO2 layer (supporting information, Figure S5).

Figure 5. Evolution of the absolute intensities of the Si 2p (Si0/LixSi), C 1s (CH2), and F 1s (LiF) core level peaks acquired on the different samples.


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As expected, the delithiation of the LixSi alloy and the Li-silicate leads to the disappearance of the two Li 1s and O 1s components at 55.5 eV and 532.5 eV at 1.5 V. In parallel, the continuous increase of the absolute Si 2p intensity until the end of the delithiation can only be explained by SEI decomposition during discharge. In fact, based on the absolute intensity values for the electrode at 0.28 V (Figure 5), we notice the drop of C 1s intensity respect to 5 mV followed by the increase of F 1s. This means that, at 0.28 V the surface layer composed of CH2, O–C=O, and Li2CO3 species is already mostly dissolved. At higher potentials and until the end of the delithiation, the C 1s intensity remains in the same range; however the F 1s intensity continuously decreases. Based on these results, at the end of the delithiation process, the surface of the a-Si is mainly composed of (i) remaining inorganic LiF compound and (ii) adsorbed species originating from the electrolyte/electrode interaction.

3.5 Proposed reaction mechanisms. By coupling the results obtained by galvanostatic cycling and post-mortem XPS, we are able to schematically illustrate the voltage-dependency of the reaction mechanisms taking place during the first cycle (Figure 6). Once the electrode is inserted into the cell, an interaction occurs between the electrolyte and the a-Si. Fluorine diffuses within the thin film reacting with the SiO2 native layer to form SiOxFy. On the current collector side of the electrode, XPS analysis reveals the presence of Cu2O/CuO species on the pristine sample. These species are reduced already at 1.5 V, leading to initial charge consumption. Subsequently, when the potential drops to 1 V and 0.6 V, the continuous decrease of the Cu0 component and the simultaneous appearance in the C 1s core level of a component assigned to CO3 reveals the deposition of a surface layer mainly composed of



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carbonates originating from the electrolyte decomposition, which justifies the relevant charge consumption during the first charge. Regarding the a-Si thin film surface, up to 0.6 V the lithiation of the a-Si and SiOxFy are not noticeable, and EC/DMC reduction has not started yet. The absence of alloy reactions at this voltage is in good agreement with the galvanostatic curve where the onset of lithiation is expected at 0.2 V. However, the absence of solvent decomposition is surprising since SEI formation on a conventional Si nanoparticle electrode was reported to start at this stage.14 This difference can be explained by the surface complexity of the conventional Li-ion battery electrode, where we also cannot exclude some preferential electrolyte reduction and SEI growth on the carbon nanoparticles.12 Possible LiPF6 reduction at 0.6 V leading to a slight increase of LiF species is also observed but its contribution to the charge consumption is considered negligible. When the first potential plateau at 0.2 V vs. Li+/Li is completed different phenomena take place on the a-Si surface: (1) Both the a-Si and SiOxFy are lithiated, leading to the formation of the LixSi alloy and Li silicate. The lithiation of the a-Si reaches its maximum at 0.07 V, and no further change is observed at lower potentials. Compared to results found in the literature, at this stage, the Si nanoparticles are just partially lithiated,14 and the difference can be explained by the faster diffusion of Li+ in thin films compared to nanoparticles. Regarding the Li-silicate component, if we apply the same adjustment of the BE positions according to the CH2 component located at 285 eV,14 we find that our silicate signal is located at 101.3 eV, which is only 0.4 eV higher than that of Li4SiO4


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(100.9 eV), in good agreement with the shift of our fluorinated phase. Thus, the lithiation of the silicate can be described by the following reactions: 4 Li + 2 SiOxFy Si + Li4Si(OxFy)2

(1)

4 Li + SiO2 Si + 2 Li2O (at ~ 0.19 V)30

(2)

Although the variation in the ISi0/Si4+ intensity in sample at 1.5 V compared to sample at 0.6 V suggests a partial loss of the native oxide layer (Supporting information, Figure S5), the irreversible reaction (2) involving the formation of the Si0 and Li2O species is not observed in this study, as the Li2O species is not detected, neither in the O 1s nor in the Li 1s spectra. (2) SEI formation begins simultaneously with a-Si lithiation, and the surface layer is composed of LiF species with organic/inorganic reduction products consisting mainly of CH2, O‒C=O, and CO3 bonds. The LiF species is the decomposition product due to LiPF6 reduction, whereas the organic/inorganic surface layer is related to the DMC/EC reduction. At 0.2 V, the electrolyte decomposition is mostly dominated by LiPF6 reduction, whereas at 0.07 V, the DMC/EC reduction is dominant. At lower potentials, no further DMC/EC reduction is observed, unlike the LiPF6 which is continuously reduced until the end of the lithiation, forming clusters of LiF. The percentages of the different carbon species are estimated to be in the range of 80% CH2, 14% O–C=O, and 6% CO3, and are presented in supporting information (Table S1 in Supporting Information) for all the analyzed samples. During the delithiation process, both the LixSi alloy and Li-silicate are observed to be completely reversible. The onset of LixSi delithiation is detected at 0.24 V and the reaction continues until the silicate is reversibly formed at 1.5 V. Regarding the SEI, the organic/inorganic layer is mostly decomposed at 0.24 V, whereas part of the LiF species are continuously detached until



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the end of the delithiation, leading to a significant decrease in the SEI thickness. As a result of using the model a-Si thin film electrode, the delithiation of the Li silicate and the decomposition of the SEI could be clearly revealed, and thus, the continuous consumption of the electrolyte observed on Si electrodes is not caused just by possible film cracks, but also by SEI detachment during delithiation.

Figure 6. Scheme of the proposed reaction mechanisms occurring during the early cycling of an amorphous Si thin film in LP30 liquid electrolyte.

4. CONCLUSIONS We investigated a 30 nm n-doped amorphous silicon thin-film electrode during different stages of the first lithiation/delithiation cycle in a half-cell vs. lithium. To the best of our knowledge for the first time, we were able to probe the lithiation/delithiation reversibility of the SiO2 native layer as well as to discriminate between the voltage dependency of the C-rich and F-rich SEI formation/detachment processes by post-mortem XPS on a-Si thin films. By monitoring the Si 2p, O 1s, P 2p, and Li 1s core levels, the lithiation/delithiation mechanism of the a-Si and native SiO2 layer was explored. Simultaneously, by monitoring the C 1s and F 1s core levels, the SEI thickness and compositional evolution were elucidated. The onset of LiPF6 reduction occurs first


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at 0.2 V, followed by the reduction of the DMC/EC at 0.07 V and finally further salt decomposition at 5 mV, leading to the formation of LiF inorganic clusters embedded in an oligomeric layer, respectively. The removal of the SEI was also revealed by the complete and partial detachment of the oligomeric layer and LiF, respectively. A detailed model mechanism was proposed to describe the reactions taking place during the first cycle.

SUPPORTING INFORMATION SEM images of pristine and cycled electrodes; Extensive analysis of XPS data; XPS Si 2p region analysis of soaked electrode in different electrolyte conditions AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is part of Project 911 from the Competence Center Energy and Mobility (CCEM) and the SwissElectric Research initiative (SER).



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