# Fileset

[260204 SiOx Supporting Information.docx](https://mdr.nims.go.jp/filesets/d37a8e65-5312-4112-acca-cbcb5d2b0c59/download)

## Creator

[Tsukasa Iwama](https://orcid.org/0000-0002-1453-6936), Ryosuke Sugimoto, Raimu Endo, [Tsuyoshi Ohnishi](https://orcid.org/0000-0002-2333-7752), Masakazu Haruta, [Takayuki Doi](https://orcid.org/0000-0003-1081-9223), [Takuya Masuda](https://orcid.org/0000-0001-7462-2177)

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[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Electrochemical Lithiation and Delithiation of Amorphous Nonstoichiometric Silicon Oxide Thin-Film Electrode Studied by                    <i>Operando</i>                    X-ray Photoelectron Spectroscopy](https://mdr.nims.go.jp/datasets/e95da878-0aff-4d1b-8450-74e98a54c48d)

## Fulltext

Template for Electronic Submission to ACS JournalsSupporting Information of“Electrochemical Lithiation and Delithiation of Amorphous Nonstoichiometric Silicon OxideThin-Film Electrode Studied by Operando X-ray Photoelectron Spectroscopy”Tsukasa Iwama1,2, Ryosuke Sugimoto3, Raimu Endo1,2, Tsuyoshi Ohnishi1, Masakazu Haruta4, Takayuki Doi3, and Takuya Masuda1,2,*1. Research Center for Energy and Environmental Materials (GREEN), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan2.  Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan3. Department of Molecular Chemistry and Biochemistry, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan4. Department of Electric and Electronic Engineering, Kindai University, Iizuka, Fukuoka 820-8555, Japan*Corresponding Author: Takuya Masuda MASUDA.Takuya@nims.go.jpExperimental procedures. An a-SiOx thin film with a diameter of 10 mm and thickness of around 100 nm was deposited on a LLZT sheet (10 mm × 10 mm × 500 μm; Toshima Manufacturing Co., Ltd.) by radio frequency magnetron sputtering using Ar/O2 gas mixtures.1, 2 Then, a Cu layer serving as a current collector was deposited on the sputter-deposited SiOx layer by direct current sputtering. During the Cu coating, the center area with dimensions of 10 mm × 4 mm was masked by a stainless-steel stencil plate to produce an uncoated SiOx region for XPS measurements. Finally, a Li metal layer with a thickness of around 1.5 μm was formed on the other surface of the LLZT sheet by thermal evaporation to yield a Cu/SiOx/LLZT/Li all-solid-state (ASS) half-cell, as shown in Figure 1.The electrochemical lithiation/delithiation and XPS measurements were simultaneously carried out using an operando XPS system.3-5 The ASS half-cell was mounted onto a sample holder in an Ar-filled glovebox and then transferred into the XPS apparatus (Kratos AXIS Nova, Shimadzu Corporation) without exposure to open air. The Cu layer on the a-SiOx thin-film electrode and the Li metal layer were electrically connected to terminal A and B, respectively, with being insulated from each other by a polyimide film, as shown in Figure 1. After transferring the cell into the analysis chamber, terminal A and B were connected to a potentiostat (VSP-300, BioLogic Science instruments) at the outside by coaxial cables via a vacuum feedthrough, while terminal A was grounded with a hemispherical electron analyzer. Electrochemical lithiation and delithiation of the ASS half-cell were carried out in constant-current (CC)–constant-voltage (CV) and constant-current (CC) mode, respectively. In the first lithiation with a CC–CV mode, the cell was first lithiated with a constant current of 4.9 μA cm-2 (~0.134 C, 1 C = 3579 mA g-1 for Li3.75Si) until the cell voltage reached 0.02 V (109 min), and then the cell voltage was held constant at 0.02 V until the current density decreased to 0.49 μA cm-2 (546 min). In the delithiation with a CC mode, constant current of 2.45 μA cm-2 (~0.067 C) was applied until the cell voltage reached 1.5 V (285 min). The same procedure was applied in the subsequent lithiation/delithiation cycles, except that the current densities for CC and CV for lithiation were set to half of those for the first lithiation. In the present study, a low current density was used in order to obtain a sufficient number of photoelectron spectra during the lithiation/delithiation processes. In addition, it should be noted that, at higher current densities, the voltage immediately reaches the cutoff voltage due to a lower electric and ionic conductivity of SiOx.XPS measurements were performed in the analysis chamber kept under a pressure of 4 × 10-9 Torr. X-rays from a monochromatic Al Kα (1486.7 eV) source at a power of 300 W were incident to the exposed region of a-SiOx thin-film electrode. The analysis area, takeoff angle, and pass energy of photoelectrons were fixed at 700 × 300 μm2, 90°, and 80 eV, respectively. XPS measurements composed of Si 2p, C 1s, O 1s, and Li 1s regions were repeatedly applied to the a-SiOx thin-film electrode throughout the lithiation and delithiation. The obtained spectra were calibrated by the hydrocarbon peak assignable to surface contamination at 285.0 eV in C 1s region as previously reported.5 After the background subtraction by using the Shirley method, the spectra were fitted using the Voight function.6 As for the Si 2p region, all the peaks were deconvoluted into Si 2p3/2 and 2p1/2 peaks and only the Si 2p3/2 peaks were used as the subject of discussion unless specified. In our previous study on an a-Si thin-film electrode sputter-deposited on a LLZT, a linear correlation between the peak position of LiySi in the Si 2p region and its Li content was obtained based on an assumption that all the electrochemical charge was consumed for the lithiation of Si.5 In the present study, Li content in LiySi was determined based on the refined linear correlation of our previous study on a-Si thin-film electrode5 as shown in Table S2 and Figure S2 for the first lithiation, and Table S3 and Figure S3 for the first delithiation.Hard X-ray photoelectron spectroscopy (HAXPES) measurements were performed in the analysis chamber equipped with a hemispherical analyzer (EW4000, Scienta Omicron, Inc.) kept under a pressure of 2 × 10-8 mbar.7 Focused X-rays from a monochromatic Cr-Kα (5414.9 eV) source (ULVAC-PHI, Inc.) at a power of 50 W with a spot size of 200 μm were incident to the exposed area of a-SiOx thin-film electrode. The takeoff angle, and pass energy of photoelectrons were fixed at 75° and 200 eV, respectively. HAXPES measurements composed of Si 2p and C 1s regions were applied to the pristine SiOx thin-film electrode. The obtained spectra were calibrated and fitted in the same manner as those in the XPS measurements.Elemental analysis of the a-SiOx thin-film electrode in the pristine state was conducted using a SEM (SU8220, Hitachi) equipped with an energy dispersive X-ray spectroscopy (EDS, XFlash5060FQ, Bruker).Estimation of oxygen content x in a-SiOx The oxygen content x in the a-SiOx thin film was estimated from SEM-EDS, XPS and HAXPES. According to elemental analysis using SEM-EDS, the oxygen content x in the SiOx thin film in the pristine state was estimated to be 0.53.Figure S1 (a) and (b) show the Si 2p photoelectron spectra of the SiOx thin-film electrode in the pristine state obtained using Al Kα (XPS) and Cr Kα rays (HAXPES), respectively. The curve fitting analysis was performed using five symmetric Voigt functions6 with each energy difference of 1 eV, corresponding to five species with the oxidation states of bulk Si (Si0), Si2O (Si1+), SiO (Si2+), Si2O3 (Si3+), and SiO2 (Si4+).8-11 The binding energy and the full width half maximum (FWHM) of Si 2p3/2 peak due to Si0 were set as variable parameters. In contrast, the FWHMs of peaks due to Si oxides such as Si1+, Si2+, Si3+, and Si4+ species were fixed at the same value to reduce the number of refinement parameters. In addition, the intensity ratio and the difference of binding energy of the Si 2p1/2 to 2p3/2 peaks were fixed at 0.5 and 0.6 eV, respectively.8-11 As a result, the best fit was obtained with Si0, Si3+ and Si4+ components. The formation of silicon suboxides is commonly observed in nonstoichiometric silicon oxide and at the SiO2/Si interface, and their chemical structures have been studied in the field of semiconductor technologies.12-16The oxygen content x in SiOx was estimated from the following equation ;8 where , , and  are Si 2p3/2 peak intensities of Si0, Si3+, and Si4+, respectively. The intensity ratio of Si0, Si3+, Si4+, and estimated oxygen content x are summarized in Table S1. The estimated value of x = 0.50 from the photoelectron spectra using Cr Kα rays (HAXPES) was in good agreement with that of x = 0.53 estimated from SEM-EDS although value of x = 0.86 from the photoelectron spectra using Al Kα rays (XPS) was overestimated because Si surface is covered by a native oxide layer. Figure S1. Si 2p photoelectron spectra of SiOx thin film electrode in the pristine state obtained using (a) Al Kα and (b) Cr Kα rays.Table S1. The oxygen content x in SiOx estimated from the results of curve fitting of SiOx thin-film electrode in the pristine state using equation . Intensity ratioTechniques     x in SiOx Al Kα (XPS) 0.52 0.19 0.29 0.87 Cr Kα (HAXPES) 0.72 0.12 0.16 0.50 EDS    0.53The correlation between the position of Si 2p peaks due to LiySi and the Li content y during the first lithiation and delithiation of a-Si.5In our previous study, electrochemical lithiation/delithiation reaction of an a-Si thin film electrode on a LLZT was dynamically analyzed by operando XPS.5 The correlation between the position of Si 2p and deconvoluted Si 2p3/2 peaks due to LiySi and the Li content y obtained from electrochemical charge density during the first lithiation and delithiation are shown in Table S2 and S3, respectively. Figure S2 and S3 show the Si 2p and Si 2p3/2 peak positions as a function of Li content y in LiySi and linear approximations generated from those plots during the first lithiation and delithiation, respectively.Table S2. Curve fitting results of Si 2p and Si 2p3/2 peaks due to Si0/LiySi as a function of Li content y during the first lithiation.5 1st Lithiation Si0/LiySi  Si0/LiySi y in LiySi Si 2p Si 2p3/2 y in LiySi Si 2p Si 2p3/2 0 99.07 98.95 1.77 96.68 96.52 0.018 98.27 98.27 1.88 96.61 96.45 0.128 97.71 97.58 1.99 96.55 96.38 0.237 97.68 97.53 2.10 96.49 96.30 0.347 97.57 97.42 2.20 96.40 96.23 0.456 97.48 97.33 2.31 96.35 96.19 0.566 97.39 97.21 2.42 96.25 96.11 0.675 97.33 97.15 2.53 96.25 96.06 0.785 97.26 97.09 2.64 96.15 95.97 0.894 97.24 97.04 2.75 96.13 95.94 1.00 97.16 97.01 2.86 96.02 95.87 1.11 97.06 96.92 2.97 95.98 95.83 1.22 97.02 96.83 3.08 95.93 95.79 1.33 96.90 96.75 3.19 95.95 95.78 1.44 96.85 96.71 3.30 95.87 95.72 1.55 96.79 96.63 3.41 95.74 95.59 1.66 96.77 96.58 3.50 95.36 95.22Figure S2. Positions of (a) Si 2p and (b) Si 2p3/2 peaks due to Si0/LiySi as a function of Li content y during the first lithiation. Red lines are linear fits to the data in a range of 0.25 < y < 3.3 [y : Li content in LiySi, z : Peak position / eV].Table S3. Curve fitting results of Si 2p and Si2p3/2 peaks due to Si0/LiySi as a function of Li content y during the first delithiation. 1st Delithiation Si0/LiySi  Si0/LiySi y in LiySi Si 2p Si 2p3/2 y in LiySi Si 2p Si 2p3/2 3.502 95.36 95.22 1.844 96.08 95.91 3.484 95.41 95.24 1.735 96.72 96.56 3.374 95.48 95.33 1.626 97.08 96.91 3.265 95.48 95.33 1.516 97.12 96.96 3.155 95.51 95.37 1.352 97.19 97.04 3.045 95.50 95.37 1.214 97.17 97.00 2.935 95.59 95.41 1.101 97.23 97.05 2.825 95.61 95.43 0.991 97.28 97.10 2.717 95.60 95.44 0.882 97.31 97.14 2.608 95.63 95.44 0.773 97.34 97.20 2.499 95.61 95.47 0.664 97.45 97.26 2.390 95.63 95.49 0.555 97.47 97.29 2.281 95.69 95.52 0.446 97.44 97.32 2.172 95.75 95.57 0.337 97.53 97.36 2.063 95.75 95.60 0.228 97.55 97.41 1.954 95.85 95.69 0.189 97.59 97.45Figure S3. Positions of (a) Si 2p and (b) Si 2p3/2 peaks due to Si0/LiySi as a function of Li content y during the first delithiation. Bule and green lines are linear fits to the data in ranges of 0.2 < y < 1.6 and 2.0 < y < 3.5, respectively [y : Li content in LiySi, z : Peak position / eV].Si 2p3/2 peak positions corresponding to LiySi at the end of the first lithiation.Figure S4 shows the Si 2p3/2 peak positions of each chemical species obtained by the curve fitting of XPS spectra of the a-SiOx thin film (Figure 4) during the first lithiation, and Si 2p3/2 peak positions corresponding to LiySi at the end of the first lithiation. The LiySi peak gradually shifted from 96.4 eV at the capacity of 2093 mAh gSi-1 to 96.1 eV at the capacity of 2591 mAh gSi-1.Figure S4. (a) Si 2p3/2 peak positions of each component with respect to capacity during the first lithiation. (b) Magnified graph of dashed box of (a). The green area shown in (a) and (b) represent the regions where Si0, Si3+, Si4+, Li4SiO4, and LiySi coexisted.In the first lithiation, the Li content y in LiySi was estimated by the following equation  (y: Li content y in LiySi, z: peak position / eV), as shown in Figure S2  (b);Here, the LiySi peak position, z was 96.40 eV when the capacity density reached 1427 mAh gSi-1 where LiySi and Li silicates were formedThe LiySi peak position, z was 96.37 eV when the capacity density reached 2093 mAh gSi-1 where almost all of the Si0, Si3+, and Si4+ were lithiatedIn addition, the LiySi peak position, z was 96.07 eV when the capacity density reached 2591 mAh gSi-1In the first delithiation, the Li content y in LiySi was estimated by the following equation , as shown in Figure S3 (b);Here, the LiySi peak position, z was 97.05 eV after the first delithiationPhotoelectron spectra during the second lithiation and delithiation.Figure S5. Li 1s and Si 2p photoelectron spectra of the a-SiOx thin film electrode in a Cu/SiOx/LLZT/Li ASS half-cell during the second (a) lithiation and (b) delithiation.Figure S6. Potential profiles and Li 1s and Si 2p3/2 peak positions of each chemical species as a function of capacity density during the second lithiation and delithiation.References(1) Sugimoto, R.; Marumoto, K.; Haruta, M.; Inaba, M.; Doi, T. Quantitative Evaluation and Improvement of Interfacial Li+ Transfer Between SiOx Electrode and Garnet‐Type Ta‐Doped Li7La3Zr2O12 Electrolyte. ChemElectroChem 2022, 9 (17), e202200491. DOI: 10.1002/celc.202200491.(2) Haruta, M.; Doi, T.; Inaba, M. Oxygen-Content Dependence of Cycle Performance and Morphology Changes in Amorphous-SiOx Thin-Film Negative Electrodes for Lithium-Ion Batteries. J. Electrochem. Soc. 2019, 166 (2), A258-A263. DOI: 10.1149/2.0861902jes.(3) Endo, R.; Ohnishi, T.; Takada, K.; Masuda, T. 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F.; Zamini, N.; Maserjian, J.; Madhukar, A. XPS STUDIES OF STRUCTURE-INDUCED RADIATION EFFECTS AT THE Si/SiO2 INTERFACE. IEEE Trans. Nucl. Sci. 1980, 27 (6), 1640-1646. DOI: 10.1109/TNS.1980.4331082.(16) Barranco, A.; Yubero, F.; Holgado, J. P.; Caballero, A.; Gonzalez-Elipe, A. R.; Mejias, J. A. Structure and chemistry of SiOx (x<2) systems. Vacuum 2002, 67 (3-4), 491-499. DOI: 10.1016/S0042-207X(02)00218-X.1S2image2.pngimage3.pngimage4.pngimage5.pngimage6.pngimage1.png