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Article Cite This: J. Phys. Chem. C 2019, 123, 347−355

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Insights into the Li-Metal/Organic Carbonate Interfacial Chemistry by Combined First-Principles Theory and X‑ray Photoelectron Spectroscopy Mahsa Ebadi,† Antoine Nasser,‡,§ Marco Carboni,† Reza Younesi,† Cleber F. N. Marchiori,‡ Daniel Brandell,*,† and C. Moyses Araujo*,‡ †

Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden § ENSTA ParisTech, 828, boulevard des Maréchaux, 91120 Palaiseau, France

J. Phys. Chem. C 2019.123:347-355. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/11/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: X-ray photoelectron spectroscopy (XPS) is a widely used technique to study surfaces and interfaces. In complex chemical systems, however, interpretation of the XPS results and peak assignments is not straightforward. This is not least true for Li-batteries, where XPS yet remains a standard technique for interface characterization. In this work, a combined density functional theory (DFT) and experimental XPS study is carried out to obtain the C 1s and O 1s core-level binding energies of organic carbonate molecules on the surface of Li metal. Decomposition of organic carbonates is frequently encountered in electrochemical cells employing this electrode, contributing to the build up of a complex solid electrolyte interphase (SEI). The goal in this current study is to identify the XPS fingerprints of the formed compounds, degradation pathways, and thereby the early formation stages of the SEI. The contribution of partial atomic charges on the core-ionized atoms and the electrostatic potential due to the surrounding atoms on the core-level binding energies, which is decisive for interpretation of the XPS spectra, are addressed based on the DFT calculations. The results display strong correlations between these two terms and the binding energies, whereas electrostatic potential is found to be the dominating factor. The organic carbonate molecules, decomposed at the surface of the Li metal, are considered based on two different decomposition pathways. The trends of calculated binding energies for products from ethereal carbon−ethereal oxygen bond cleavage in the organic carbonates are better supported when compared to the experimental XPS results.

1. INTRODUCTION The high reactivity of Li metal with electrolytes is a wellknown issue in the application of Li-metal batteries (LMBs) or when employing “half-cells” for the study of Li-ion battery electrodes.1 The electrolyte decomposition products formed on the surface of the Li metal contributes to the formation of a solid electrolyte interphase (SEI),2 which can be observed also on other types of anode materials such as graphite and silicon. This complex film, highly dependent on battery chemistry and operating conditions, is decisive for the electrochemical performance of the battery, and has therefore been the subject of vast research.3 Indeed, a deeper knowledge about the surface chemistry of Li metal when in contact with electrolytes is required for the development of LMB. X-ray photoelectron spectroscopy (XPS) is arguably the most widely used characterization techniques for exploring the surface chemistry of Li-battery electrodes. For Li metal, it has been used in the presence of different electrolyte systems, and thereby provided vital information about the components and structural composition of the SEI layer.3,4 © 2018 American Chemical Society

XPS has been used to study different organic carbonate solvents and different salts at the surface of Li-metal electrode.5 The results from XPS and Fourier transform infrared (FTIR) spectroscopy were in agreement. It has been reported that the film on the Li surface has a multilayer structure in which the organic salts exist in higher concentrations farther from the surface. In another study, Aurbach et al. applied XPS together with FTIR to study the SEI on the Li-metal surface in contact with propylene carbonate (PC) solvents and different salts.6 They identified Li2O, LiOH, LiF, Li2CO3, Li alkylcarbonate (RCOOLi), and hydrocarbons as the major species on the Li surface. XPS, FTIR, NMR spectroscopy, evolved gas analysis, and quantitative chemical analysis have been applied by Ota et al. to study the surface of Li electrode in Li imide/ cyclic ether based electrolytes. The authors found species such Received: August 7, 2018 Revised: November 15, 2018 Published: December 11, 2018 347

DOI: 10.1021/acs.jpcc.8b07679 J. Phys. Chem. C 2019, 123, 347−355

Article

The Journal of Physical Chemistry C

the exchange−correlation functional. The plane-wave cutoff for all the systems is set to 550 eV. The Brillouin zone integration for organic carbonates adsorbed at the Li metal surface (100) and for molecules before adsorption on the surface were performed with 3 × 3 × 1 Monkhorst and Pack k-point mesh31,32 and at gamma point, respectively. Gaussian smearing with a width of 0.1 and 0.2 eV was used for molecules in the gas phase and adsorbed + decomposed molecules, respectively. The DFT-D3 van der Waals (vdW) correction method is used in this work.33 Bader charge analyses34 are performed on the total charge density. More details on the construction of slab geometries and gas phase calculations can be found in our previous publication.26 The core-level binding energies (BEs) have been calculated using the Slater−Janak transition state method.21−23 Provided that the Kohn−Sham eigenvalue ϵ is a linear function of the occupation number η, the binding energies can be calculated through the following equation

as ROCO2Li, Li2CO3, polymer constituents, and LiF in the top layer and Li2O and carbide species in the inner layers.7 Watanabe et al. performed an XPS study of Li metal electrodes in contact with polymer electrolytes containing different lithium salts.8 They could identify LiF formed on the surface of the Li metal after contact with the polymer electrolytes, and suggested a relation between the type of salt and the thickness and porosity of the film formed. XPS has also been used for the Li metal electrode in more novel systems such as dual salts electrolyte.9 Besides the powerfulness of the XPS technique, it is also associated with certain limitations. XPS spectra can reveal the type of chemical bonds present on the surface; however, the results may not be strictly conclusive to reveal the exact composition of species formed on the surface. The X-ray may, due to radiation damage, be destructive to the surface of the Li electrode, rendering the information obtained unreliable.1,4,10 The ultra-high-vacuum conditions used in XPS are also significantly different from the conditions in the working battery cell.1,3 Existence of unwanted contaminations originating from glovebox atmosphere are frequently observed, which can significantly affect the XPS spectra and therefore render the interpretations complicated.10,11 The peak assignment is often less straightforward and generally relies on comparisons with model systems. Moreover, unexpected peak shifts often occur, whose origins are still under debate.11−14 Hence, computational modeling can be very helpful for better guidance of these interpretations.15,16 In this context, density functional theory (DFT) has been shown to be a promising approach to assess the core-level binding energies (BEs) for various types of systems such as metallic surfaces,17 organic materials,18 catalysts,19 and energy storage systems.20 In previous work,20 we have studied the electronic structure and the core-level BEs of LiNO3 and a number of likely decomposition species (N2, N2O, LiNO2, Li3N, and Li2N2O2) obtained from the reduction of this electrolyte additive on the surface of Li metal electrode. The Janak−Slater (JS) transition state method21 was used to calculate the core-level BEs. This method has shown to describe the BEs well when compared to the experimental data.22−25 In the current study, the core-level BEs of organic carbonate molecules, conventional liquid electrolyte solvents for Li-based batteries, are studied at the surface of the Li metal by a combination of DFT calculations and XPS measurements. The electronic structure and thermodynamic (in)stability of these organic carbonates interacting with Li-metal surface have been explored in a previous DFT study,26 and the most stable structures are therefore used as the employed model systems. The main goal is to describe the central features of the XPS spectra of the adsorbed and decomposed molecules on the reactive surface of Li metal using the first principle calculations, and the results are compared with the experimentally measured data to achieve XPS peak assignments and decomposition routes of higher quality than when using either DFT or XPS separately.

BEi =

∫0

1

i1y ϵi(ηi)dηi ≈ ϵijjj zzz k2{

(1)

(1)

where ϵi 2 is the energy of the core-level state with half occupation. This method formally takes into account the change in the total energy of the system when removing one electron from the core state. Then, the C 1s and O 1s core-level binding energies for the adsorbed molecules have been obtained as i1y BEO 1s/C 1s = ϵFermi − ϵO 1s/C 1sjjj zzz k2{

(2)

where ϵO 1s/C 1s is the Kohn−Sham core state energy of the 1s orbital of the carbon/oxygen atom with half a core electron and ϵFermi is the Fermi energy of the system. In the case of the gas phase molecules, the highest occupied level is considered instead. It should be pointed out that the excited electrons have been placed on the bottom of the conduction band to avoid periodically charged systems. This approach has been applied and discussed in previous studies to calculate the corelevel BEs and core-level shift (CLS) in different materials.22−24 We have also considered the core-level BEs referred to the vacuum level as applied in other previous studies.25,35 It is achieved by calculating the electrostatic potential, Evac, in the vacuum region of the supercell containing the free molecules and the ones with the adsorbed molecules on the Li surface. In this case, the binding energies are instead calculated as i1y BEO 1s/C 1s = Evac − ϵO 1s/C 1sjjj zzz k2{

(3)

2.2. Experimental Section. 2.2.1. Sample Preparation. A lithium foil was scratched with a scalpel to remove any trace of surface oxidations and was cut into disks of ø = 6 mm. One of the as-prepared disk was measured as pristine material, whereas the other disks were soaked for 1 h into four different vials containing (a) propylene carbonate (PC, Sigma-Aldrich), (b) ethylene carbonate (EC, BASF), (c) dimethyl carbonate (DMC, Sigma-Aldrich), and (d) diethyl carbonate (DEC, Novolyte Tech.). The lithium disks were then dried under vacuum for 15 min and introduced into the XPS analysis chamber using a special built air-tight argon-filled transfer cup. EC was warmed at 55 °C overnight before use. The water content in the solvents was measured by means of an 831KF

2. COMPUTATIONAL AND EXPERIMENTAL SECTION 2.1. Computational Methods. All calculations in this work have been carried out within the framework of DFT and using the projector-augmented wave (PAW) method27,28 as implemented in the Vienna ab initio Simulation Package (VASP).29 The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE)30 have been applied as 348


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Below, we compare the calculated BEs and their trends in these systems with the results reported in literature. The calculated C 1s BEs for the free EC molecules (Figure 2a) show one large peak and a small shoulder that belong to the C2−C3 and C1 atoms, respectively (Figure 2b). Upon the adsorption of the EC molecule on the Li surface, these peaks shift slightly to lower values, which could be due to the observed electron transfer from the Li surface to the EC molecule by 0.32 e. A greater split in the peak can also be seen for the adsorbed molecule. The reason for the position of the C1 peak at higher values than that of the C2 and C3 atoms, both before and after the adsorption, is related to the changes in the hybridization of the C atoms in the molecule. Besides, the C1 atom is connected to more oxygens than the C2 and C3 atoms. Figure 2c shows two peaks at 543 and 545.5 eV for O1 and O2−O3, respectively. This trend and the difference between the BE values are well in accordance with the reported experimental results for carbonyl O (CO) at 532 eV and ethereal O (C−O−C) around 533 eV.37−39 For the adsorbed EC, the gap between these peaks becomes narrower because of the small shift in the BE of the O1 atom (O2−O3 atoms) to a higher (lower) value. The electron transfer from the Li surface to the π* molecular orbital of the carbonyl group and consequently the elongation of the C1−O1 bond are likely to be responsible for the O 1s BE shift. For the PC molecule (Figure 2d), two major C 1s peaks are calculated: one overlapping from the carbons in the ring (C1, C2, and C3) and another from the C4 atom in the methyl group, which is lower by ca. 2 eV (Figure 2e). This is in good agreement with the experimental values for the C 1s BEs for the C−H and C−O bonds for which the difference is reported to be around 2 eV.5 As for the EC molecule, a shift to lower BEs occurs for the PC molecule upon adsorption on the Li surface. The reason for this shift and split of the C 1s peak is likely similar as for the EC molecule. The calculated charge transfer from Li to the PC molecule is 0.33 e, which should be responsible for the negative BE shift. The same pattern as that for EC is observed for O 1s spectra of PC in Figure 2f. For DMC (Figure 2g), the C 1s BEs are shifted to lower values upon adsorption, similar to the cyclic carbonates (Figure 2h). The calculated charge transfer from the Li surface to DMC is 0.29 e. The same pattern as for the cyclic carbonates is observed for the O 1s BEs in DMC (Figure 2i). For DEC (Figure 2j), the obtained results (Figure 2k,l) are, in turn, similar to DMC. The calculated charge transfer to the molecule is in this case around 0.27 e. In contrast to the cyclic carbonates, the BE values for C1 of the linear carbonates are slightly higher. Moreover, the gap between the core-level BE peaks for the C1 atom and the C2 and C3 atoms in the linear carbonates is larger than their corresponding values in the cyclic carbonates. This could be due to either different hybridization of the C atoms in the linear carbonates, or the difference in adsorption sites and molecular orientation on the surface. The calculated BEs referred to the vacuum level are shown in Figure S1 (Supporting Information file). The main effect of changing the reference energy is the inversion of some trends when comparing a given peak (for instance, the C1 of EC) before and after the adsorption on the metallic surface. However, the relative position of the peaks (for instance, C1, C2, and C3 of EC) on both free and adsorbed molecules are basically the same. On the basis of the above discussions, one can notice that the physical chemistry of the systems under

Karl Fischer coulometer (Metrohm) and found to be lower than 5 ppm. The whole procedure for the sample preparations was carried out in an Argon-filled glovebox with moisture and O2 concentrations below 1 and 5 ppm, respectively, to avoid surface contamination of lithium metal by moisture, CO2, etc. The exposure of lithium metal to ambient atmosphere can lead to the formation of Li2CO3, which not only changes the surface composition but also influences the partial charging of the lithium metal surface.11,36 However, the survey of Li 1s, O 1s, and C 1s spectra of pristine Li metal shown in Figures S3−S6 displays that some surface contaminations still exist on the surface, although the samples were prepared in the glovebox. 2.2.2. XPS Measurement. XPS measurements were performed on an in-house spectrometer (PHI 5500) using monochromatized Al Kα radiation (hν = 1486.7 eV), an emission angle of 45°, and a base pressure about 2 × 10−9 mbar in the analyzer chamber during the spectra detection. The C 1s binding energy (BE) of the adventitious carbon component at 284.8 eV was adopted as the internal standard reference for the BE scale with an accuracy of ±0.1 eV.

3. RESULTS AND DISCUSSION 3.1. Core-Level Binding Energies of Organic Carbonates at Li-Metal Surface. The organic carbonates studied are ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The optimized structures are illustrated in Figure 1.

Figure 1. Structure of the investigated organic carbonate molecules: ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

Following our previous study,26 the most stable configurations for EC and PC have been found to be a tilted orientation with the head of carbonyl O on the “bridge” sites of the Li-metal surface. DMC and DEC also had the most stable adsorption at the bridge site, but with a perpendicular orientation of the molecule on the surface. These configurations have been considered for the molecules on the surface in this study. The calculated BEs of C 1s and O 1s for organic carbonates before and after adsorption on the Li-metal surface along with the optimized structures for their corresponding adsorbed molecules on the surface are shown in Figure 2. The calculated BEs with respect to the vacuum level are shown in Figure S1. 349


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Figure 2. Optimized structure (a), C 1s (b), and O 1s (c) XPS spectra of EC at the Li-metal surface; optimized structure (d), C 1s (e), and O 1s (f) XPS spectra of PC at the Li-metal surface; optimized structure (g), C 1s (h), and O 1s (i) XPS spectra of DMC at the Li-metal surface; optimized structure (j), C 1s (k), and O 1s (l) XPS spectra of DEC at the Li-metal surface; red, brown, green, and pink denote O, C, Li, and H atoms, respectively.

study here can be better represented by the BEs trend obtained with the Fermi level reference. Furthermore, such reference choice will not affect the analysis of our experimental results because the theoretical spectra are aligned with the experimental peak, as shall be discussed below. 3.2. Computed C 1s and O 1s Core-Level Shifts. The PAW method27 as implemented in VASP code,29 in general, cannot generate accurate absolute BE values. Therefore, a better interpretation of the results is achieved by instead focusing on the BE shifts and the relative position of the spectral features.16 The C 1s and O 1s CLS can be calculated from the changes in the binding energy of the organic carbonates upon adsorption on the Li(100) surface such as CLS = BEmolecule@Li − BEmolecule

BEA = β0qA + β1

The C1 atom in the organic carbonate molecules is denoted by Ccarbonyl, whereas C2 and C3 atoms, bonded to one oxygen, are denoted Cethereal. Similarly, O1 and O2−O3 are denoted by Ocarbonyl and Oethereal, respectively. The calculated CLS of C and O of the carbonyl group and that of Oethereal in the studied organic carbonates are shown in Table 1. Different factors control the CLS and the location of the peaks.15,40 Here, we discuss a simple potential model that expresses the BEs in terms of intra-atomic and extra-atomic potentials41 Table 1. C 1s and O 1s Core-Level Shifts (CLS) of EC, PC, DMC, and DEC (Unit: eV) C 1s (Ccarbonyl)

O 1s (Ocarbonyl)

O 1s (Oethereal)

EC PC DMC DEC

−0.43 −0.29 −0.18 −0.11

0.50 0.39 0.46 0.30

−0.23 −0.50 −0.61 −0.60

qi

r (i ≠ A) iA

+ β2 (5)

where qA is the charge on core-ionized atom A and the second term contains information of the electrostatic potential originating from the other atoms in the system. Here, qi is the partial atomic charge on the neighboring atoms i of A (only i atoms bonded to A within the molecule are considered) and riA is the distance between atom A and the neighboring atom i. β0, β1, and β2 are coefficients to be determined through ordinary least-square (OLS) multiregression. We start our analysis by investigating the correlation between the BE and either the local charge or the nearestneighbors’ potential separately. In Figure 3a−d, the correlation between the computed Bader charge on the C and O atoms of the adsorbed molecules are plotted with respect to their corresponding C 1s and O 1s BEs. As shown in Figure 3a, there is a fairly good positive linear relation between BE and the charge. This shows that as the charge on the C atom is more positive, the BE of the C atom shifts to higher values. The correlation between the C 1s BE and the potential is stronger with a negative slope. Although the same behavior can be observed for O 1s BEs in Figure 3c,d, the correlations are much better than those of C 1s BEs. However, to obtain the coefficients in eq 5, multiple regression is performed for the results considering BEs, partial atomic charges, and electrostatic potentials. The R2, which suggests the percentage variation in BE by the two variables, charge and electrostatic potential, is calculated as 0.89 and 0.99 for C 1s and O 1s BEs, respectively. The calculated R2 here shows a strong correlation between the variables and the BEs. The calculated coefficients are β0 = −1.31 V, β1 = −1.63 V Å, and β2 = 294.84 eV from the OLS multiregression of C 1s BEs and β0 = −1.4 V, β1 = −1.69 V Å, and β2 = 544.51 eV from the OLS multiregression of O 1s

(4)

molecule

∑

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Figure 3. (a) Computed C 1s binding energy (eV) vs calculated Bader charge on the C atoms; (b) electrostatic potential/Ke calculated on the adjacent atoms to C; (c) computed O 1s binding energy (eV) vs calculated Bader charge on the O atoms; and (d) electrostatic potential/Ke calculated on the adjacent atoms to O, in EC, PC, DMC, and DEC molecules at the Li-metal surface (Ke is the Coulomb constant).

BEs. The negative coefficients β0 and β1 indicate a downward slope of the fitted lines for O 1s and C 1s BEs. The sign of the slope of BEs changes with respect to the charge from the simple (Figure 3a,c) to multiple regression, which is due to the dominating effect of the nearest-neighbors’ potential. Although the correlation between C 1s and O 1s BEs with the charge and electrostatic potential are promising, there can be other factors and mechanisms involved in changing the BEs,15 i.e., interpretations of the origin of BE shift is likely not as straightforward as Figure 3 might suggest. 3.3. Core-Level Binding Energies of Decomposed Organic Carbonates at Li-Metal Surface. Next, we have studied the core-level BEs of the products from the decomposed organic carbonates on the surface of Li metal and compared them with the experimentally measured XPS data for these solvents. The decomposition products studied likely to result at the very early stage of SEI formationare based on previous DFT studies of the decomposition reactions of these organic carbonates on the surface of Li metal.26 Two pathways have then been proposed: via Ccarbonyl−Oethereal and Cethereal−Oethereal bond cleavages. Details regarding these reaction pathways a and b are available in the Supporting Information (Figure S2), but are also in short described in Table 2.

For comparison, the BE values computed by DFT are shifted to align them with the experimental peaks. In each case for the calculated C 1s BEs, the peaks for C−H/C−C are shifted to their corresponding assigned experimental peaks, which is 284.8 eV (i.e., a shift of 6.5 eV for decomposed EC, 7.1 eV for decomposed PC, 6.6 eV for DMC, and 6.7 eV for decomposed DEC). For the calculated O 1s BEs, the peaks for the O atoms in the carbonate group are shifted to 532 eV. The experimental O 1s peaks for each solvent are shifted with the same value as their C 1s BEs counterparts. The experimentally measured XPS spectra, the calculated C 1s and O 1s spectra for these organic carbonates, and the final geometries for their corresponding decomposition pathways are shown in Figure 4. The experimental spectra of pristine Li metal (Figures S3− S6) also display some surface contaminations. These are common to observe due to ppm-level contamination in the gloveboxes used for handling the reactive Li-metal samples. These contributions are, however, not significant. The adventitious hydrocarbon peak at 284.8 eV was selected as an internal reference for the calibrations. For a better understanding of the type of C−O groups in the decomposed fragments, the DFT-calculated C−O bond distances in species before and after adsorption and decomposition at the surface are reported in Tables S1−S4. The EC molecule is decomposed to CO32− and ethylene species through pathway a on the Li-metal surface (Table 2 and Figure S2a). The calculated C−O bond distance in the CO32− fragment is 1.3 Å. The C 1s BE for C atoms in the ethylene species shows a peak lower than that of the C atom in CO32− fragment by 5 eV. This is in good agreement with the experimental results in Figure 4b. Other experimental studies have also reported the C 1s peak for carbonate and aliphatic carbons typically around 290 and 285 eV, respectively.5,38 The products from pathway b are CO and C2H4O22− (Figure S2a). The peak for C2H4O22− appears between the peaks for

Table 2. Products from Decomposition of Organic Carbonate Molecules at the Li-Metal Surface via Pathways a and b solvent molecules EC PC DMC DEC

products for pathway a 2−

CO3 , C2H4 CO32−, C3H6 CH3, O3C2H3 C2H5, O3C3H5

products for pathway b C2H4O22−, CO C3H6O22−, CO OCH3, O2C2H3 OC2H5, O2C3H5 351


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Figure 4. Pathway a and b and C 1s and O 1s XPS spectra calculated by DFT and measured experimentally; (a), (b), and (c) for EC; (d), (e), and (f) for PC; (g), (h), and (i) for DMC; and (j), (k), and (l) for DEC, respectively; the calculated binding energies of C 1s and O 1s are shifted 6−7 and 11 eV to lower values in the plot, respectively. The intensity of the calculated peaks is also adjusted in comparison to the experimental results (the intensity is multiplied by 2 × 103 and 3.5 × 103 for C 1s and O 1s, respectively).

carbonate and ethylene. The difference between the calculated BE of C2H4O22− from the values for CO32− and ethylene is around 2 and 3 eV, respectively. Our experimental results show a small shoulder around 286 eV, which can be assigned to C− O carbons. A peak assigned to C−O has been reported also in other studies to occur around 287 eV.5 Therefore, the order of the computed XPS peaks and their differences are in very good agreement with the experimental results. The CO molecule produced in pathway b diffused to the surface, thereby having interactions with the inner Li atoms of the surface. Its calculated bond distance is 1.41 Å. The calculated BEs of the CO molecule are however not shown in the spectra because this molecule likely does not stay in this interfacial region under normal experimental conditions, and it can therefore not

be measured by XPS. In the O 1s XPS spectrum (Figure 4c) of EC, only one peak for the carbonate ion is produced from pathway a. This is expected because three oxygens in this ion have quite similar chemical environment. For pathway b, there is a peak corresponding to the C2H4O22− oxygens around 1 eV lower than the peak of carbonates. The measured O 1s XPS data show only one broad peak. The interpretation of this peak is difficult because all Ocontaining species with similar binding energies in the electrolyte have contributions to this peak. The trend of the calculated BEs is in good agreement with the reported experimental peaks, O (carbonate) 531−532 eV > O (alkoxides) 528 eV, although the difference between the BE values are somewhat lower than the reported experimental 352


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The Journal of Physical Chemistry C results.5 It seems that products from both pathways may have contributions to the experimental results. In previous DFT calculations,26 the formation of products from pathway b has been shown to be more thermodynamically favorable. The energy differences are, however, small, indicating that both products may have formed during the SEI formation. The PC molecule is decomposed to propylene and carbonate species via pathway a (Figures 4d and S2b). The C 1s and O 1s spectra produced from the DFT and experiment are shown in Figure 4e,f, respectively. The relative order of the peaks for these species from pathway a is very similar to the EC decomposition. The propylene produced from PC has two different types of C, sp2 and sp3, with the former at lower BE values than the latter. For the species produced from pathway b, the position of carbon from the alkoxide group is in the expected region; however, there is quite a small difference between this peak and the peak for sp3 C. The calculated BEs fit with the measured XPS, which is similar to the decomposed EC. The relative order of the O 1s peaks is also the same as for the EC molecule. Therefore, for PC, similar as for EC, both decomposition pathways can be expected to occur based on the agreement with the experimental XPS data. It is somewhat more complicated to interpret the results from the linear carbonates as compared to the cyclic ones. For the C 1s spectra of decomposed DMC (Figures 4g,h and S2c), the relative order of the peaks from pathway a are very well reproduced. In pathway b, the C atom connected to two O atoms in the C2O2H3 ion has the C−O bond distance around 1.32 Å, and the BE value (after the alignments of the peaks) should therefore be around 290 eV; however, the BE calculated for this carbon is close to the BE of the aliphatic C. This can be related to other factors affecting the BE shifts. One such possibility is the distance of the molecule from the surface and the interaction of the neighboring Li atoms with the adsorbed molecule. In the O 1s plot (Figure 4i), the peak corresponding to the O atoms of C2O3CH3 in contact with Li surface (in pathway a) are located around 532 eV (the common BE value for O of carbonates groups). There exists another peak around 534 eV corresponding to the O atom further from the surface (−OCH3 part of the C2O3CH3 fragment). The trend and the difference between the BE values for O atoms in this fragment are well in agreement with the BEs of carbonyl and ethereal O atoms in the reported experimental results.37−39 The BEs of the products from pathway b (Figure 4i) are lower than expected. The O BE of the −COC− group is very close to that of the carbonate groups. The measured O 1s XPS spectra exhibit a very similar peak to the measured O 1s plots of the cyclic carbonates. For decomposed DEC (Figures 4j and S2d), the C 1s BEs for the products from pathway a are shown in Figure 4k. A splitting of the peaks depending on their distance from the Limetal surface can also be observed in the calculated BEs. The interpretation of the results for the species in pathway b are more complex. Although the C−O peak is both in an expected position and in order with the rest of the peaks, the peaks for C of the aliphatic parts are unexpectedly higher in BE, whereas that of C in the −CO2 part is unexpectedly lower. The O 1s spectra (Figure 4l), on the other hand, follow the trend of the one for decomposed DMC.

agreement with the experimental results. Analyzing the factors contributing to the BE shifts is a challenging task that could be greatly aided by computational studies. In this study, we have established a consistent set of linear relations between the core-level BEs (C 1s and O 1s) with the partial atomic charges on the core-ionized atoms and the electrostatic potential for different organic carbonates adsorbed on the surface of Li metal. The results indicate a strong relation between these two terms and the BEs, where the dominating factor is the electrostatic potential. For cyclic carbonates, the results through decomposition pathway a (i.e., decomposition through Cethereal−Oethereal bond cleavage) exhibit excellent agreement with the experimental spectrum in terms of shapes and order of BE peaks, particularly for C 1s. Pathway b (decomposition through Ccarbonyl− Oethereal), on the other hand, involves a CO molecule, which most likely cannot be detected experimentally. The BEs of this pathway are in fairly good agreement with the experimental results. Therefore, products from both pathways can have contributions to the experimental peaks. For the linear carbonates, the appearance of BE peaks in pathway a match better with the experimental XPS. Finally, it should be noted that this DFT approach can be successful in identifying the XPS fingerprints of the compounds formed through different possible degradation pathways in the early stage of SEI formation. However, the complexity of applied battery system as compared to models will certainly mean that more method development is necessary.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b07679.

■

(a) C 1s and (b) O 1s XPS spectra of EC; (c) C 1s and (d) O 1s XPS spectra of PC; (e) C 1s and (f) O 1s XPS spectra of DMC; (g) C 1s and (h) O 1s XPS spectra of DEC; before and after adsorption on the Li-metal surface; binding energies are calculated from the difference between the core-level energies and the vacuum-level energy (Figure S1); decomposition products from pathway a and b for EC, PC, DMC, and DEC on the surface of Li metal/calculated bond distances (Å) of C−O of the gas phase molecules, adsorbed at the Limetal surface, decomposed via pathway a and b (Figure S2 and Tables S1−S4); XP survey spectrum of the pristine Li-metal surface (Figure S3) and XP spectra of the Li 1s for the pristine Li metal and the Li metals soaked in PC, DMC, DEC, and EC solvents (Figure S4), XPS spectra of the O 1s and C 1s for the pristine Li metal (Figures S5 and S6, respectively) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.B.). *E-mail: [email protected] (C.M.A.). ORCID

Mahsa Ebadi: 0000-0001-8525-7339 Reza Younesi: 0000-0003-2538-8104 Cleber F. N. Marchiori: 0000-0003-0377-3669 Daniel Brandell: 0000-0002-8019-2801

4. CONCLUSIONS In conclusion, the JS method implemented in VASP can reproduce the CL BE shift, and the order of BE peaks is in fair 353


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(17) Olovsson, W.; Marten, T.; Holmström, E.; Johansson, B.; Abrikosov, I. A. First Principle Calculations of Core-Level Binding Energy and Auger Kinetic Energy Shifts in Metallic Solids. J. Electron Spectrosc. Relat. Phenom. 2010, 178−179, 88−99. (18) Taucher, T. C.; Hehn, I.; Hofmann, O. T.; Zharnikov, M.; Zojer, E. Understanding Chemical versus Electrostatic Shifts in X-Ray Photoelectron Spectra of Organic Self-Assembled Monolayers. J. Phys. Chem. C 2016, 120, 3428−3437. (19) Kabir, S.; Artyushkova, K.; Serov, A.; Kiefer, B.; Atanassov, P. Binding Energy Shifts for Nitrogen-Containing Graphene-Based Electrocatalysts - Experiments and DFT Calculations. Surf. Interface Anal. 2016, 48, 293−300. (20) Ebadi, M.; Lacey, M. J.; Brandell, D.; Araujo, C. M. Density Functional Theory Modeling the Interfacial Chemistry of the LiNO3 Additive for Lithium−Sulfur Batteries by Means of Simulated Photoelectron Spectroscopy. J. Phys. Chem. C 2017, 121, 23324− 23332. (21) Janak, J. F. Proof That dE/dn=ε in Density-Functional Theory. Phys. Rev. B 1978, 18, 7165−7168. (22) Olovsson, W.; Göransson, C.; Pourovskii, L. V.; Johansson, B.; Abrikosov, I. A. Core-Level Shifts in Fcc Random Alloys: A FirstPrinciples Approach. Phys. Rev. B 2005, 72, No. 064203. (23) Olovsson, W.; Göransson, C.; Marten, T.; Abrikosov, I. A. Core-Level Shifts in Complex Metallic Systems from First Principle. Phys. Status Solidi B 2006, 243, 2447−2464. (24) Salvarezza, R. C.; Carro, P. Exploring the Core Level Shift Origin of Sulfur and Thiolates on Pd(111) Surfaces. Phys. Chem. Chem. Phys. 2015, 17, 24349−24355. (25) Pueyo Bellafont, N.; Viñes, F.; Hieringer, W.; Illas, F. Predicting Core Level Binding Energies Shifts: Suitability of the Projector Augmented Wave Approach as Implemented in VASP. J. Comput. Chem. 2017, 38, 518−522. (26) Ebadi, M.; Brandell, D.; Araujo, C. M. Electrolyte Decomposition on Li-Metal Surfaces from First-Principles Theory. J. Chem. Phys. 2016, 145, No. 204701. (27) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17978. (28) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (29) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (30) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett 1996, 77, 3865−3868. (31) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188. (32) Pack, J. D.; Monkhorst, H. J. “Special Points for Brillouin-Zone Integrations”a Reply. Phys. Rev. B 1977, 16, 1748. (33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, S. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (Dft-d) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, No. 154104. (34) Henkelman, G.; Arnaldsson, A.; Jonsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (35) Torelli, P.; Giordano, L.; Benedetti, S.; Luches, P.; Annese, E.; Valeri, S.; Pacchioni, G. X-Ray Photoemission Study of the Charge State of Au Nanoparticles on Thin MgO/Fe(001) Films. J. Phys. Chem. C 2009, 113, 19957−19965. (36) Oswald, S.; Hoffmann, M.; Zier, M. Peak Position Differences Observed during XPS Sputter Depth Profiling of the SEI on Lithiated and Delithiated Carbon-Based Anode Material for Li-Ion Batteries. Appl. Surf. Sci. 2017, 401, 408−413. (37) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992. (38) Dedryvère, R.; Laruelle, S.; Grugeon, S.; Gireaud, L.; Tarascon, J.-M.; Gonbeau, D. XPS Identification of the Organic and Inorganic

C. Moyses Araujo: 0000-0001-5192-0016 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Swedish Energy Agency grant number 39036-1, STandUP for Energy, the Carl Tryggers Foundation and the Swedish Research Council (VR). The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC and NSC Centre for High Performance Computing.

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REFERENCES

(1) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Wei, F.; Zhang, J.-G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. Adv. Sci. 2015, 3, No. 1500213. (2) Peled, E. The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery SystemsThe Solid Electrolyte Interphase Model. J. Electrochem. Soc. 1979, 126, 2047−2051. (3) Yu, X.; Manthiram, A. Electrode-Electrolyte Interfaces in Lithium-Based Batteries. Energy Environ. Sci. 2018, 11, 527−543. (4) Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J.-G. Lithium Metal Anodes for Rechargeable Batteries. Energy Environ. Sci. 2014, 7, 513−537. (5) Aurbach, D.; Weissman, I.; Schechter, A.; Cohen, H. X-Ray Photoelectron Spectroscopy Studies of Lithium Surfaces Prepared in Several Important Electrolyte Solutions. A Comparison with Previous Studies by Fourier Transform Infrared Spectroscopy. Langmuir 1996, 12, 3991−4007. (6) Aurbach, D.; Daroux, M. L.; Faguy, P. W.; Yeager, E. Identification of Surface Films Formed on Lithium in Propylene Carbonate Solutions. J. Electrochem. Soc. 1987, 134, 1611. (7) Ota, H.; Sakata, Y.; Wang, X.; Sasahara, J.; Yasukawa, E. Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate and Cyclic Ether Electrolytes. J. Electrochem. Soc. 2004, 151, A437. (8) Ismail, I.; Noda, A.; Nishimoto, A.; Watanabe, M. XPS Study of Lithium Surface after Contact with Lithium-Salt Doped Polymer Electrolytes. Electrochim. Acta 2001, 46, 1595−1603. (9) Miao, R.; Yang, J.; Feng, X.; Jia, H.; Wang, J.; Nuli, Y. Novel Dual-Salts Electrolyte Solution for Dendrite-Free Lithium-Metal Based Rechargeable Batteries with High Cycle Reversibility. J. Power Sources 2014, 271, 291−297. (10) Philippe, B.; Hahlin, M.; Edström, K.; Gustafsson, T.; Siegbahn, H.; Rensmo, H. Photoelectron Spectroscopy for Lithium Battery Interface Studies. J. Electrochem. Soc. 2016, 163, A178−A191. (11) Oswald, S. Binding Energy Referencing for XPS in Alkali MetalBased Battery Materials Research (I): Basic Model Investigations. Appl. Surf. Sci. 2015, 351, 492−503. (12) Kelly, M. A. Historical Perspectives on Charging Issues in XPS. J. Electron Spectrosc. Relat. Phenom. 2010, 176, 5−7. (13) Maibach, J.; Lindgren, F.; Eriksson, H.; Edström, K.; Hahlin, M. Electric Potential Gradient at the Buried Interface between LithiumIon Battery Electrodes and the SEI Observed Using Photoelectron Spectroscopy. J. Phys. Chem. Lett. 2016, 7, 1775−1780. (14) Oswald, S.; Thoss, F.; Zier, M.; Hoffmann, M.; Jaumann, T.; Herklotz, M.; Nikolowski, K.; Scheiba, F.; Kohl, M.; Giebeler, L.; et al. Binding Energy Referencing for XPS in Alkali Metal-Based Battery Materials Research (II): Application to Complex Composite Electrodes. Batteries 2018, 4, 36. (15) Bagus, P. S.; Illas, F.; Pacchioni, G.; Parmigiani, F. Mechanisms Responsible for Chemical Shifts of Core-Level Binding Energies and Their Relationship to Chemical Bonding. J. Electron Spectrosc. Relat. Phenom. 1999, 100, 215−236. (16) Köhler, L.; Kresse, G. Density Functional Study of CO on Rh(111). Phys. Rev. B 2004, 70, No. 165405. 354


Article

The Journal of Physical Chemistry C Components of the Electrode/Electrolyte Interface Formed on a Metallic Cathode. J. Electrochem. Soc. 2005, 152, A689. (39) Ota, H.; Sakata, Y.; Inoue, A.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode. J. Electrochem. Soc. 2004, 151, A1659. (40) Bagus, P. S.; Ilton, E. S.; Nelin, C. J. The Interpretation of XPS Spectra: Insights into Materials Properties. Surf. Sci. Rep. 2013, 68, 273−304. (41) Brundle, C. R.; Baker, A. D. Electron Spectroscopy: Theory, Techniques, and Applications. Phys. Today 1978, 14, 487−488.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on December 20, 2018. Table 2 has been updated. The revised version re-posted on December 21, 2018.

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