EIS and XPS Investigation on SEI Layer Formation during First

Article Cite This: J. Phys. Chem. C 2018, 122, 18223−18230

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EIS and XPS Investigation on SEI Layer Formation during First Discharge on Graphite Electrode with a Vinylene Carbonate Doped Imidazolium Based Ionic Liquid Electrolyte J. E. Morales-Ugarte,†,‡ E. Bolimowska,‡,§ H. Rouault,†,‡ J. Santos-Peña,*,∥,⊥ C. C. Santini,§ and A. Benayad*,†,‡ †

Université Grenoble Alpes, 38000 Grenoble, France CEA-LITEN, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France § Université de Lyon, CNRS-UMR 5265, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne, France ∥ Laboratoire de Physico-Chimie des Matériaux et des Electrolytes pour l’Energie (EA 6299), Université de Tours, Parc de Grandmont, 37200 Tours, France ⊥ Laboratoire de Recherche Correspondant, LRC CEA/PCM2E, CEA/DAM No. 1, Le Ripault, 37260 Monts, France

J. Phys. Chem. C 2018.122:18223-18230. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 08/16/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Adding organic carbonates (e.g., vinylene carbonates, VC) into ionic liquid based electrolyte generally improves the electrochemical performances of graphite electrode (Cgr) in lithium ion batteries. In this study, step-by-step electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) were performed during the first reduction cycle to follow the formation of the solid electrolyte interphase (SEI) in lithium/graphite (Li/Cgr) cell using (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) [C1C6Im][NTf2] doped with vinylene carbonate (VC). The EIS spectra evolution, recorded at different cutoff voltages, indicated a two-step SEI thin film formation. These results were supported by XPS measurement at the same cutoff voltage. Hence, we pointed out that the first film results from the VC decomposition at 0.8 V to form an organic layer constituted of lithium alkylcarbonates. This film was developed up to 0.2 V. From 0.8 to 0.2 V, we detected a slight decomposition of IL solvent. This process was driven by a progressive sulfone decomposition reaction through the formation of polyoxysulfone, Li2S, Li3N, and LiF. This process was at the origin of the formation of the second film of inorganic nature beyond 0.2 V. This SEI film was stable up to 0.01 V; the composition probed by XPS remained unchanged.

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INTRODUCTION Lithium-ion batteries (LIB) are nowadays the most widespread energy storage system for use in portable electronic devices because of their relatively high power and energy densities. However, concerning the fast demand of automotive industry for nomad energy, LIB performance is far from satisfactory for large scale commercialization of the next generation of electric vehicles that need more energy density with strict safety regulations.1−3 The conventional electrolytes used in LIB contain organic solvents such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). Their volatility and flammability are at the origin of safety problems, such as fire and explosion of LIB, even if the accident may be initiated by nonadapted use conditions or battery pack engineering failure.4,5 In order to eliminate or mitigate these problems, ionic liquids (IL) represent a safer suitable alternative because of their negligible vapor pressure and very low flammability.6−9 A challenge of their application is the limited lithium ion transport properties related to their high viscosity implying low cycling and power delivery for IL-based batteries.10−13 © 2018 American Chemical Society

Previous studies have shown that the addition of carbonates in IL-electrolytes improves the electrochemical cell performance.14−18 Hence, it has been reported that addition of the organic compound VC (vinylene carbonate) to the IL C1C6ImNTf2 (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) containing the lithium salt LiNTf2 (lithium bis(trifluoromethanesulfonyl)imide), in the LFP/graphite system, is crucial for improving the performance of the carbon based batteries.18 The addition of VC influences both the mobility and the coordination shell of [Li+].19−23 Moreover, it contributes to the creation of interfacial compatibility between the graphite (Cgr) electrode and IL through the formation of a solid electrolyte interphase (SEI).11−15 The composition of the SEI is a highly debated subject in the literature, and the proposed composition of SEI varies from Received: April 17, 2018 Revised: July 23, 2018 Published: July 23, 2018 18223

DOI: 10.1021/acs.jpcc.8b03636 J. Phys. Chem. C 2018, 122, 18223−18230

Article

The Journal of Physical Chemistry C

Figure 1. First galvanostatic discharge of Cgr/Li/Li cell with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte at C/50 rate.

performed in diverse experimental conditions and with different ILs, lithium salts, and electrodes. With the electrolyte C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC, the performance of graphite based batteries was improved.18 We already depicted, on the basis of EIS experiments and XPS coupled study, the impregnation mechanism (wetting) on graphite electrode with this electrolyte.23 To extend this study, we propose in this paper to determine the impact of VC on the structure of the graphite/ILelectrolyte interface during the first galvanostatic discharge in Li/Cgr cell using similar electrolyte, through a EIS and high resolution XPS coupled study.

electrochemical system to another as the electrolyte composition and stability are electrochemical potential dependent. Because carbon-based electrodes are thermodynamically unstable in all known electrolytes during lithium ion intercalation, the SEI layer plays an important role in protecting the graphite surfaces. Several SEI models on graphite were proposed by Peled,24 Aurbach,25,26 and Edström.27,28 All of authors suggest that the SEI is a dense layer of inorganic components close to the carbon, followed by a porous organic or polymeric layer close to the electrolyte phase. Peled’s model (called also mosaic model) assumes multiple simultaneous reductive decompositions between the negatively charged anode surface and the various electrolyte components. According to this model, SEI contains microregions with the inorganic species located onto the anode surface and organic species (corresponding to less reduced compounds) close to the electrolyte. For carbonate electrolytes containing VC, the SEI consists of polymer species such as polyvinylene carbonate and polyacetylene associated with lithium vinylene dicarbonate, (CHOCO2Li)2, lithium divinylene dicarbonate, (CHCHOCO2Li)2, lithium divinylene dialkoxide, (CHCHOLi)2, and lithium carboxylate, RCOOLi.29 Note that the cyclic voltammograms performed on Li/Cgr cells using the methylpropylpyrrolidinium (MPPY) or the methylpropylpiperidinium (MPPI) cations associated with NTf2 anion mixed with 0.5 M LiNTf2 and 10 wt % VC have shown that the major cathodic peak at 0.8 V results from VC reduction, well before the reduction of pure IL-electrolytes at 0.4 V.30,31 As a result, graphite electrodes were successfully passivated, and reversible lithium intercalation/deintercalation peaks were observed for both IL-electrolytes.31 Hence, the addition of VC improves the performance of the cell with lithium and graphite electrodes. However, contrary to the organic based electrolyte, the information concerning the impact of VC on the nature of SEI with IL based electrolytes with VC as additive is scarce and

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EXPERIMENTAL SECTION Electrolyte Preparation. Experiments were performed under a purified argon atmosphere using glovebox (Jacomex or MBraun, H2O < 1 ppm, O2 < 1 ppm) or vacuum-line techniques. Ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C1C6Im][NTf2] was synthesized as already reported.18 After IL purification, the halide content was found between 100 and 200 ppm (AgNO3 tests, elementary analysis, ESI mass), and water content was ∼20 ppm estimated by Karl−Fischer titration. The electrolyte, referred in the text as IL*, was prepared at room temperature in an argon-filled glovebox by dissolving LiNTf2 (1 mol·L−1) in [C1C6Im][NTf2] and adding VC (5% vol.). Electrodes Preparation. Graphite based electrodes were prepared using natural graphite (SLP30 Timcal, 16 μm D50), carboxymethylcellulose (CMC), and styrene butadiene rubber (SBR) in a weight ratio of 96:2:2, suspended in a water solution (slurry preparation). It was deposited onto a copper collector (Oak Mitsui, 12 μm) using a doctor blade (ink deposition and coating) and dried for 24 h at 333 K. Electrodes have a loading of 2.6 mAh·cm−2. The electrode first was pressed, then dried at 353 K under vacuum for 48 h and 18224


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Figure 2 shows the different Nyquist plots obtained at the cutoff voltages of 0.6, 0.4, 0.2, 0.1, 0.035, and 0.01 V. The

stored in an anhydrous argon-filled glovebox. Metallic lithium electrode was provided by Rockwood Lithium GmBH, with the thickness of 135 μm, and was used as counter electrode. Characterization of the Graphite/Electrolyte Interface during First Discharge by EIS. Potentiostatic electrochemical impedance spectroscopy (PEIS) was applied in three electrode Swagelock cells with Cgr as working electrode (WE), Li foil as counter electrode (CE), and Li ribbon as reference electrode (RE). PEIS was conducted after different galvanostatic pulses in order to obtain interfaces at selected voltage values. For this purpose, galvanostatic discharges were conducted under C/20 or C/50 after a rest time of 4 h, for improving the electrolyte wettability on the electrode. PEIS was applied with ac voltage signal of ΔE = 10 mV in the frequency range between 240 kHz and 2.5 mHz. Prior to the PEIS application, a relaxation time of 2−4 h was imposed to the cell in order to reach the equilibrium before the EIS measurement. Nyquist plots were fitted using ZView software (Scribner Associates Inc.). Fitting was considered suitable when goodness of fit was lower than 10−4 . All the electrochemical experiments were performed with a BioLogic VMP3 multichannel potentiostat. Evolution of the Graphite/Electrolyte Interface during First Discharge by Surface Analysis. Six different lithium/graphite coin cells with IL* as electrolyte were discharged at C/50 rate at 333 K and stopped during the first reduction step at different potentials from 0.8, 0.6, 0.4, 0.2, 0.035, and 0.010 V vs Li+/Li. To be sure that the potential of each coin cell remains at the defined potential, a floating step has been applied before opening the cell. The cells were dismantled in a glovebox, and graphite electrodes were washed with dimethyl carbonate (DMC) before XPS analysis. X-ray photoelectron spectroscopy (XPS) analysis was performed with Versaprobe II PHI 5000 (ULVAC-PHI) spectrometer using a 100 μm focused monochromatic Al Kα X-ray source (1486.6 eV) beam. Survey spectra were performed over a spectral range of 0−1200 eV to identify the elements present in the material within the pass energy of 117 eV, corresponding to an energy resolution of 1.6 eV and an acquisition time of 0.5 s/eV. The high-resolution spectral analysis was performed using a pass energy of 23 eV allowing an energy resolution of 0.5 eV. It goes without saying that the low pressure vapor of IL allowed the XPS measurement in ultrahigh vacuum chamber (6 × 10−8 Pa).

Figure 2. Nyquist plot (Zim vs Zreal) for Cgr/Li/Li cell with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC.

Nyquist plots registered at 0.6 and 0.4 V discharged states are similar to those obtained during the wettability tests.23 Nevertheless, there is a significant change in the Nyquist diagrams beyond 0.2 V, associated with the presence of halfcircles in the middle frequencies region, commonly attributed to surface films formation.25,26 As the Nyquist plot shape is very different depending on the voltage range, we focused initially our study on the first discharge steps between 0.6 and 0.4 V. We demonstrated that at the OCV, the Cgr/electrolyte interface is similar to that of a capacitor with a contribution of R//C element associated with a solid film onto the Cgr electrode.23 This film is due to the electrolyte degradation at the negative lithium metal electrode yielding products that subsequently precipitate on graphite. The corresponding equivalent electrical circuit is shown on “equivalent circuit I” in Figure 3, where Rel and Rf//Cf are, respectively, associated with the electrolyte resistance and the electronically insulating solid film onto graphite.32 The equivalent series resistance (ESR) and the graphite double layer capacitance (Cdl) provide information on the capacitive behavior of graphite. In a first approach, the same equivalent circuit for the spectra registered at 0.6 and 0.4 V has been proposed to fit the Nyquist plots despite the fact that these voltages are clearly far from the OCV, where the graphite electrode is not negatively polarized (or slightly polarized during the EIS measurements). As expected, the fitting with this model was unsatisfactory. In order to confirm that the graphite does not behave as a capacitor at these voltages, Figure 4 shows the galvanostatic cycling in the voltage limits of 2 V (developed in the inset), 0.6 V, and 0.4 V. Clearly, the capacitive behavior defined by straight lines is not observed at 0.6 and 0.4 V. However, the presence of plateaus corresponding to the faradaic process (during reduction step) indicates clearly that the equivalent circuit I cannot be applied to describe the Nyquist plots beyond 2 V.

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RESULTS AND DISCUSSION Evolution of the SEI over First Discharge. Figure 1 shows the first galvanostatic discharge of a Cgr/Li/Li cell containing IL* (C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC) electrolyte under a regime of C/50. One can detect several features in the profile: a polarization before 1 V followed by two sloping regions before 0.2 V, and the wellknown pseudo-plateau corresponding to lithium intercalation in the graphite and developing in the 0.2−0.01 V voltage range. In order to investigate the possible VC reduction and SEI formation, the electrode/electrolyte interface has been monitored by EIS. The predefined limit potentials for EIS measurements were 0.6, 0.4, 0.2, 0.1, 0.035, and 0.01 V as reported in Figure 1. The choice of the cutoff voltage of 0.6 and 0.4 V allows monitoring of the reduction of VC and IL prior to lithium intercalation in the graphite electrode. Lithium staging (from stage IV to stage I) and further electrolyte reduction were followed at 0.2, 0.1, 0.035, and 0.01 V. 18225


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Figure 3. Equivalent electrical circuits employed to fit the Nyquist plots in Figure 2. The equivalent circuits I and II correspond to EIS spectra at 0.6 and 0.4 V, whereas equivalent circuit III is used to fit the Nyquist plot beyond 0.4 V.

The suitable fit of Nyquist plots between 0.6 and 0.4 V is the equivalent circuit II (Figure 3), corresponding to an association of two R//C elements in series with a Warburg element. The first Rf//Cf element is associated with the deposition of electrolyte degradation byproducts on graphite electrode. The second R//C (Rf1//Cf1) may correspond to a solid film formed on the graphite electrode during the reduction step and containing organic and/or inorganic products. This equivalent circuit II is similar to a classic Randles circuit containing an additional dipole associated with the SEI.33 In fact, modeling with both schemes (equivalent circuit II or a SEI-modified Randles classic) provides similar goodness of fit with 10% more accuracy if the Warburg element is excluded from the last element (equivalent circuit II). Furthermore, the SEI-modified Randles classic model is hardly suitable at these voltage values since graphite is far from the voltage region where it inserts lithium. Physical meaning of the Warburg element included in equivalent circuit II is complicated although its presence has also been stated prior in graphite negative electrodes.

Figure 4. First galvanostatic cycle at different voltage limits ((black dot) 2 V, (red dot) 0.6 V, (blue dot) 0.4 V) for Cgr/Li/Li cells with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC.

Figure 5. Evolution of electrical parameters for Cgr/Li/Li cells discharged at different voltage limits (0.6 V, 0.4 V, 0.2 V, 0.1 V, 0.035 V, 0.01 V) with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte. 18226


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Figure 6. Evolution of the C 1s and O 1s peaks registered at the surface of the graphite electrode during the first discharge with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte. The inset figure shows the C 1s peak in the energy range of 288−290 eV registered at 0.8, 0.6, 0.4, and 0.2 V.

In conclusion, there is proof that at 0.6 and 0.2 V a film is developed onto the graphite electrode due to the reduction of electrolyte components at negatively polarized surface. The presence of this film has also been confirmed by XPS measurements discussed in the next section of this paper. At 0.2 V and beyond, the Nyquist plots (Figure 2) contain new features, specifically two new half-circles. The occurrence of such half-circles indicates the inclusion of a new R//C element corresponding to a second surface film produced by the reduction of the electrolyte onto the graphite electrode (Rf2// Cf2). It goes with a classic Randles circuit, containing a charge transfer resistance Rct in series with a Warburg element (Wo), associated with the lithium ion diffusion into the graphite network, and both in parallel with the double layer capacitance, Cdl. Consequently, this model represented by equivalent circuit III considers a second step for the electrolyte degradation at voltages lower than 0.4 V. The bilayer nature of the SEI when IL is the electrolyte solvent has recently been observed on silicon-based negative electrodes.34 The evolution of the resistive parameters extracted from the modeled equivalent circuits (electrolyte, films, and charge transfer) as a function of potential is reported in Figure 5. First, the electrolyte resistance, Rel, increases upon discharge, indicating that electrolyte becomes less conductive due to its instability beyond 0.6 V. Regarding the film constituted of species produced on the lithium electrode and precipitated on graphite, its resistance, Rf1, increases beyond 0.2 V, reaching at 0.01 V twice the value at 0.6 V. This evolution agrees with the long exposure time of the graphite electrode to the electrolyte during the discharge.23 Indeed, we have performed galvanostatic intermittent titration measurement at every selected voltage and we found that about 6 h for higher voltage (OCV to 0.2 V) and

12 h for lower voltage (0.2 to 0.01 V) are necessary for relaxation time before EIS measurements. Nevertheless, the evolution of both film resistances during the discharge is different. The film 1 resistance Rf1 (and consequently, thickness) decreases from 0.4 to 0.2 V, then increases slightly up to 0.1 V and finally decreases. Unlike the film 2 resistance, Rf2 grows up to 0.01 V. The particular composition of both films is clarified in the XPS section. The changes of the Rf1 and Rf2 values indicate that the bilayered SEI characteristics hardly change when the lithium insertion in the graphite occurs, beyond 0.2 V. These observations clearly reveal the effect of a protection of the graphite electrode with the products issued from the degradation of VC doped IL electrolyte before 0.2 V. Composition of the SEI formed at such low voltages will be defined in the next section. Evolution of the XPS Spectra over First Discharge. After a floating step, the cell was unlocked in a glovebox and graphite electrodes were washed with DMC before XPS analyses. The survey spectra recorded at different voltage are reported in the Supporting Information, showing that only the species related to electrode and electrolyte are present at the surface of the electrodes. The XPS spectra of core levels C 1s, N 1s, O 1s, F 1s, and S 2p of IL* recorded at the surface of pristine Cgr electrode after 12 h of wetting at 333 K were considered as reference.23 Core level binding energy changes were used to probe the magnitude of cation−anion based interactions and the effect of the anion on the electronic environment of the cation. Figure 6 shows the C 1s and O 1s core levels spectra evolution of noncycled graphite electrode wetting in IL* electrolyte (labeled neat) and graphite electrodes at different voltage stages during first discharge. For noncycled graphite, the C 1s spectrum presents four main peaks centered at 284.6, 285.0, 286.7, and 292.9 eV 18227


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Figure 7. Evolution of the F 1s, N 1s, and S 2p core peaks registered at the surface of the graphite electrode during the first discharge with C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte.

associated with the carbons in sp2 configuration related to graphite electrode, the aliphatic group of C1C6Im (Calkyl), carbon in a nitrogen environment (C−N (Chetero)), and a carbon surrounded by three fluorine atoms (CF3) from the anion NTf2. At 0.8 V, the C 1s spectrum features a new peak of lower intensity at 289 eV (inset spectrum in Figure 6) associated with carbon bonded to three oxygens atoms. This peak can be assigned to Li−OCOOC2H5 and/or Li2CO3, resulting from VC degradation. This peak is visible up to 0.2 V and disappears upon discharge up to 0.035 V. This result is corroborated by the presence of ephemeral shoulder at 287.8 eV (inset Figure 6) which disappeared at 0.035 V, associated with CO species. The CO related bonds suggest that the VC degradation byproduct is mainly the Li−OCOOC2H5. At 0.6 V, one can observe a slight decrease of the intensity of the peak associated with CN bindings (Chetero), associated with the presence of organic thin layer at the surface of the electrode between 0.6 and 0.2 V. The C 1s spectra between 0.4 and 0.01 V remain unchanged.35 In the case of O 1s spectra evolution, Figure 6, the main peak at 532.8 eV associated with the −SO2 bindings of NTf2 anion is slightly shifted at lower binding energies during galvanostatic cycling at all potentials. Since the amount of shift matches across all core level peaks, we assumed that the shift is related to surface charging effect due to the formation of surface dipole related to the SEI formed film. At 0.8 V, one can notice the presence of a new peak centered at 531.3 eV increasing until 0.4 V, then decreasing at 0.2 V with the concomitant formation of a third peak at 528.0 eV assigned to Li2O. The peak at 531.3 eV is assigned to polyoxysulfone groups (such as Li2S2O4 and Li2SO3). The S 2p spectrum recorded at 0.8 V and reported in Figure 7 presents two new doublets 2p3/2−1/2, at 167.2−168.4 eV and 160.6−161.8 eV, resulting from the evolution of SO2−CF3 into polyoxysulfone and Li2S species, respectively.36

Regarding the F 1s spectra (Figure 7), the main peak around 688.8 eV corresponding to −CF3 group of NTf2 anion suffers a slight shift to lower binding energies from 0.8 to 0.6 V. For all reduction potential values the presence of LiF (685.0 eV) is observed. The N 1s peak registered at the surface of neat electrode shows two peaks at 402.1 and 399.5 eV assigned to nitrogen in the cation C1C6Im and anion NTf2, labeled Ncation and Nanion, respectively.23 At the cutoff voltage of 0.8 V, the Ncation 1s and the anion Nanion 1s peaks are shifted to 401.5 and 399 eV, respectively. The N 1s binding energy peak difference between Ncation and Nanion (ΔNcation − Nanion) changes from 2.4 to 2.5 and the ratio of peak intensities Ncation/Nanion lowers from 1 to 0.8. In the experimental error range, these values do not change from 0.8 to 0.01 V. During the discharge, the ratio Ncation/Nanion decreases while the Nanion related peak full width at halfmaximum (fwhm) increases. This observation suggests an anion degradation and the presence of different related anion byproducts for which the binding peak energies are within the resolution limit of the spectrometer (0.5 eV), Table 1. This result can be corroborated with the appearance at 0.8 eV of a Table 1. Evolution of Atomic Concentrations of Ncation, Nanion, and NLi3N and Intensity Ratio Ncation/Nanion at Each Potential

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potential, V

Ncation, %

Nanion, %

NLi3N, %

Ncation/Nanion

neat IL* 0.8 0.6 0.4 0.2 0.035 0.01

51 40 40 34 32 36 33

38 47 43 48 45 42 44

0 13 17 18 23 23 23

1.4 0.84 0.91 0.70 0.72 0.85 0.75


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LiF, and Li2O slightly increase and remain quite stable during the reduction.

third peak centered at 396.5 eV associated with Li3N whose the intensity increases during the discharge. The relative atomic concentration changes of the Ncation, Nanion, and NLi3N are reported in Table 1. From the XPS study, a partial decomposition of the electrolyte was detected incorporating into the SEI mainly inorganic products (Li3N, LiF, Li2O, and Li2S) coming from the partial decomposition of the ionic liquid C1C6ImNTf2 and lithium salt LiNTf2.. The organic compounds (lithium alkylcarbonates) resulting from VC degradation are observed beyond the cutoff voltage of 0.8 V. Interestingly, the formation of Li2CO3 is not detected in our case. The formation of salts LiF, Li2O, and Li2S results from the degradation of the anion NTf2. According to XPS results, there is a concomitant formation of these three species and their amount increases with the cycle number. According to these results, we propose that stepwise decomposition mechanism of the anion, proposed by Aurbach et al.,37 eq 1a, is more accurate than the one step proposed by Xu et al.,38 eq 1b.

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CONCLUSIONS In a previous paper, we pointed out, based on an EIS/XPS coupled study, the role of VC on the wettability of C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte on the graphite electrode. We evidenced a better impregnation of the IL-based electrolyte when the VC is added to the electrolytic media. Hence in this paper, we demonstrated the role of VC in IL-based electrolyte on the electrochemical performance of graphite based electrode. EIS and XPS experiments were carried out to follow the evolution of the SEI at the surface of the graphite electrode upon the first discharge cycled in C1C6ImNTf2//1 mol·L−1 LiNTf2//5 vol % VC electrolyte. The EIS spectra evolution, registered at different cutoff voltage, indicated a two-step SEI thin film formation. Equivalent circuit models were used to estimate the dynamic evolution of both films through the film resistance changes. These results were supported by step-bystep XPS measurements at the same cutoff voltage. The impedance results revealed the presence of two films onto the graphite electrode. The first film is formed between 0.8 and 0.4 V and the second one beyond 0.2 V. Both films were modeled using two equivalents circuits represented in equivalent circuit II and III. Hence, we pointed out from the XPS measurement that the first film results from the VC decomposition from 0.8 V to form an organic layer constituted of lithium alkylcarbonates and some amount of LiF; this film was developed up to 0.2 V. The second film, whose thickness is slightly increasing during the discharge, was associated with polyoxysulfone, Li3N, Li2S, and LiF products resulting from a slight decomposition of IL solvent. This process was driven by a progressive sulfone decomposition reaction beyond 0.2 V. Since the depth analyses in XPS is 5 nm, the disappearance of the alkylcarbonate related peak indicates that the second film is thicker than 5 nm and mainly composed of inorganic species. This SEI film was stable up to 0.01 V; the composition probed by XPS remained unchanged.

(SO2 CF3)2 + ne− + nLi+ → Li3N + Li 2S2O4 + LiF + C2Fx Li y (SO2 CF3)2 + 2e− + 2Li+ → Li 2NSO2 CF3 + Li 2SO2 CF3 Li 2S2 O4 + 6e− + 6Li+ → 2Li 2S + 4Li 2O Li 2S2 O4 + 4e− + 4Li+ → Li 2SO3 + Li 2S + Li 2O (1a) −

+

(SO2 CF3)2 + ne + n Li

→ a Li3N + b LiF + c Li 2S + d Li 2SO3 + eSz + f C2Fx Li y (1b)

According to the stepwise decomposition mechanism, the presence of low amount of polysulfides cannot be excluded as shown in S 2p XPS peaks recorded at 0.2 and 0.035 V (spectral region labeled with ∗ in Figure 7). Figure 8 summarizes the evolution of the relative concentration of the byproducts detected by XPS at the

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ASSOCIATED CONTENT

S Supporting Information *

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b03636. Survey spectra registered at the surface of the graphite electrode during the first discharge with C1C6ImNTf2// 1 mol·L−1 LiNTf2//5 vol % VC electrolyte (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 8. Graphic representation of the atomic concentration of identified components at 0.8, 0.6, 0.4, 0.2, 0.035, and 0.010 V vs Li+/ Li.

ORCID

J. E. Morales-Ugarte: 0000-0002-2483-6537 A. Benayad: 0000-0002-8854-1105 Notes

surface of the graphite electrode upon discharge. The intensities of the peaks corresponding to LiSxOy as well as some low reduced electrolyte organic components are appearing, then fading, during the discharge. This might be related to the presence of a second thicker film (above the depth detection limit of XPS, 5 nm). The amounts of Li3N,

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.E.M.-U. received funding for his scholarship from FONDECYT-CONCYTEC (Grant 232-2015-FONDECYT). 18229


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