Electrochemical Impedance Spectroscopy and X-ray Photoelectron

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Surfaces, Interfaces, and Applications

EIS and XPS Study of Lithium Metal Surface Aging in Imidazolium-Based Ionic Liquids Electrolytes Performed at OCV Jorge Eduardo Morales-Ugarte, Anass Benayad, Catherine C Santini, and Renaud Bouchet ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00753 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

EIS and XPS Study of Lithium Metal Surface Aging in Imidazolium-Based Ionic Liquids Electrolytes Performed at OCV J. E. Morales-Ugarte 1,2, A. Benayad*,1, C. C. Santini 3, and R. Bouchet**,2,4 1. Université Grenoble Alpes, CEA–LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France 2. Université Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 1130 rue de La Piscine, 38402 St. Martin d’Hères, France 3. Université Lyon, CNRS-UMR 5265, 43 Bd du 11 Novembre 1918, 69616, Villeurbanne, France 4. Réseau sur le Stockage Électrochimique de l’Énergie (RS2E), CNRS, 80039 Amiens, France

ABSTRACT Lithium reactivity towards electrolytic media and dendrite growth phenomenon constitute the main drawback for its use as anode material for lithium battery technology. Ionic liquids were pointed out as promising electrolyte solvent candidates to prevent thermal runaway in lithium battery system. However, the reactivity of lithium towards such kind of electrolyte is still under debate. In this study, the interaction between lithium metal and imidazolium based ionic liquids, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C1C6ImTFSI) and 1-hexyl-3methylimidazolium bis(fluorosulfonyl)imide (C1C6ImFSI), has been investigated based on nondestructive methodology coupling electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) in coin cells aged several days in open circuit voltage (OCV). 1 ACS Paragon Plus Environment

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The main components detected by XPS in the bulk separator and at the surface of the lithium metal are the byproducts of cation and anion degradation. Similarities and differences were noticed depending on the anion nature TFSI vs. FSI. The role of lithium salt addition (LiTFSI) was also pointed giving rise to stability improvement of the electrolytic solution towards lithium anode. A direct correlation between the resistance of the bulk electrolyte and of the interface electrolyte/lithium, and chemical composition changes was established based on a detailed EIS and XPS combined study. Keywords: Lithium metal batteries, ionic liquids, electrochemical stability, SEI, EIS, XPS

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1. INTRODUCTION Future challenges for current energy storage systems reside in the improvement of their safety and the increase of energy density.1,2 Among the different approaches to tackle these issues,3,4 the use of ionic liquids (ILs)5,6 and ionic liquid-based polymers7 constitutes an alternative to integrate a new generation of electrolytes for batteries because of their retarded flammability in comparison with conventional electrolyte solvents based on carbonate solvents. Most of the works using ILs for batteries include cations based on imidazolium (Im) and pyrrolidinium (Pyrr) in combination with the anions bis(trifluoromethylsulfonyl)imide (TFSI) or bis(fluorosulfonyl)imide (FSI),8–10 because of their good ionic conductivity (> 1 mS.cm-1), high thermal stability and wide electrochemical window (up to 5 V).5,6 Imidazolium based ILs performance have been evaluated with relative high success in several systems including graphite/LiFePO4

(Cgr/LFP),

Li4Ti5O12/LiFePO4

(LTO/LFP)

and

Li4Ti5O12/LiNi1/3Mn1/3Co1/3O2 (LTO/NMC).11 However, detailed studies of the stability of these ILs face to lithium metal and their impact on the performance of lithium metal batteries (LMB) systems remain relatively limited. This point is the key factor to use such electrolytes in a system that targets more energy density. Lithium metal has been regarded for longtime as one of the most promising alternatives for negative electrodes because of its low reduction potential (-3.04 V vs. standard hydrogen electrode) and large specific capacity (3.86 Ah.g-1). Nevertheless, battery systems using lithium metal as anode suffer from a poor cycling performance as a result of a low coulombic efficiency, and the growth of dendrites when lithium is cycled with most of the organic electrolytes.12,13 Furthermore, because of its high reactivity, passive layers are formed to the surface of lithium at the contact with the electrolyte. These layers are termed as the Solid Electrolyte Interphase (SEI).14 It is widely accepted that the different properties of the SEI such as morphology, chemical composition and mechanical strength have a great impact on the cycling life of lithium 3 ACS Paragon Plus Environment


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based batteries, as well as on the dendrite growth phenomenon.15,16 SEI on lithium metal has been studied by several techniques such as scanning electron microscopy (SEM) for morphology, Fourier Transform Infrared spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) for surface chemical composition, and Electrochemical Impedance Spectroscopy (EIS) for electrochemical properties.17-20 Howlett et al.17 have characterized the SEI with lithium electrodes in contact with the N-methylN-alkylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyrr1,xTFSI). By coupling EIS and XPS techniques, they proposed a bilayer model: an inner layer of about 180 nm composed mainly of LiF resulting from the degradation of the anion TFSI and of some Li2O from the initial native layer on lithium, and an outer layer of about 40 nm composed of degradation products from TFSI (LiF, Li2S2O4, LiSO2CF3, lithium sulfides and lithium nitrides) and of some Li2CO3 from initial native layer. Similarly, with N-methyl-N-propyl-pyrrolidinium bis(fluorosulfonyl)imide (Pyrr13FSI), Budi et al.,18 through the ab initio molecular dynamic simulations and XPS studies, reported that the nature of SEI is resulting from the FSI anion decomposition. Their XPS results have been corroborated by the study performed by Basile et al.19 All these authors agreed that breakdown products of cation degradation are also present in the SEI. Olchewski et al.20 performed a XPS comparative study between three different ILs, (1butyl-1-methylpyrrolidinium methylpyrrolidinium

bis(fluorosulfonyl)imide

bis(trifluoromethylsulfonyl)imide

(Pyrr14FSI), (Pyrr14TFSI),

and

1-butyl-11-octyl-3-

methylimidazolium bis(trifluoromethylsulfonyl)imide (C1C8ImTFSI)) deposed by physical vapor deposition on a lithium layer. They demonstrated that TFSI anion is more stable than FSI anion when the cation is Pyrr14+. Besides, they proved the decomposition of both cations Pyrr14+ and C1C8Im+, although they reported that the decomposition of C1C8Im+ was much more significant.


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In complement to these reports, it is known that imidazolium based cations show a poor cathodic stability due to the activity of the hydrogen atom at the imidazolium ring in the C2 position.21 For example, it has been reported that neat ILs 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

(C1C2ImTFSI)

and

1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide (C1C4ImTFSI) are not chemically stable versus metallic lithium in comparison with neat Pyrr14TFSI.22 In contrast with these results, it has been demonstrated that the cathodic limit of these ILs can be enlarged by the addition of a lithium salt such as LiTFSI.23 In any case, to the best of our knowledge, the relation between the stability of these ILs and the electrical and chemical evolution of the formed interfaces at room temperature and in OCV conditions is still not clear. Herein, in order to extend this discussion, we propose in this paper a comparison of the interactions between lithium metal and two different imidazolium-based ionic liquids doped with LiTFSI in symmetric Li/Li coin cell systems. Since lithium metal electrode is a reactive material, a careful characterization must be undertaken in order to study the evolution of the interfaces between lithium metal and ILs based electrolyte with good enough accuracy. Hence, the in-situ evolution of the electrolyte resistance and interface resistance measured by EIS was correlated with the final chemical environment of electrolytes and lithium surfaces probed post mortem by XPS on the electrodes and separators. This study, based on the coupling of two nondestructive and well adapted techniques to study interfaces and surfaces, paves the way to further understanding the reactivity between lithium and imidazolium IL based electrolyte and its impact on the formation of passive layers. 2. EXPERIMENTAL SECTION 2.1 Ionic Liquids Synthesis



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Ionic liquids 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C1C6ImTFSI) and 1-hexyl-3-methylimidazolium bis(fluorosulfonyl)imide (C1C6ImFSI) were synthesized following procedures described elsewhere.24 ILs were dried 12 h under vacuum using Schlenk line techniques at room temperature. This method assures a content of water impurities below 20 ppm, as it was confirmed by Karl-Fischer titration.24 Electrolytes were prepared at room temperature in a glove box (O2, H2O < 5 ppm) by dissolving 1 mol.L-1 of lithium bis(trifluoromethylsulfonyl)imide salt (LiTFSI, 99.9 %, Solvionic) in the ILs. Mixtures were stirred and dried one night under vacuum at room temperature prior to utilization. 2.2 Conductivity Measurements The ionic conductivity of ILs based electrolytes (σ0) was determined from electrochemical impedance measurements carried out by a VMP-300 multichannel potentiostat (Bio- Logic Science Instruments) in the frequency range from 7 MHz to 100 Hz. The IL based electrolytes were filled in a tightened conductivity cell constructed with platinized platinum electrode cells and thermostated by a Temperature Controlled System between 273.15 ± 0.10 K and 343.15 ± 0.10 K. The cell constants were determined using a standard KCl aqueous solution. 2.3 Coin Cells Assembly CR2032-type cells were assembled in an argon filled glove box using two lithium disks as electrodes (Rockwood type, thickness: 135 µm, diameter: 16 mm). A separator (Freudenberg, Viledon FS 2207-25, thickness: 0.25 mm, diameter: 16.5 mm) soaked with 150 µL of the electrolyte solution, was inserted between the two lithium electrodes. Stainless steel disks (thickness: 0.5 mm) were used as current collectors. 2.4 Impedance Measurements


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The EIS was performed on the freshly assembled coin cells at OCV at a temperature of 295.0 ± 0.3 K using a VMP-2 multichannel potentiostat (Bio- Logic Science Instruments). Spectra were recorded each hour during 168 hours by applying an electric signal of 20 mV of amplitude (RMS) in the frequency range from 1 MHz to 10 mHz. All spectra were fitted with ZView software (Scribner Associates Inc.). 2.5 XPS Measurements Chemical characterization by XPS was carried out using a Versaprobe II ULVAC-PHI spectrometer. A monochromatic beam (X-ray source Al-Kα 1486.6 eV) of 100 µm of diameter and 97 W of power was focused at the surface of the samples. Survey spectra were measured over a spectral range of 0-1200 eV to identify the elements present in the material using pass energy of 117 eV which corresponds to a resolution of 1.6 eV. High-resolution spectral analyses were performed using a pass energy of 23 eV which corresponds to a resolution of 0.5 eV. All XPS measurements were carried out in an ultra-high vacuum chamber (7.10-8 mbar). All XPS spectra binding energies were corrected using the C 1s line of alkyl groups in C1C6Im+ cation at 285.0 eV. Curve fitting and background subtraction were accomplished using Casa XPS software. The spectra curve fitting was performed using Voigt function, convolution product of Gaussian (80%) and Lorentzian (20%) distributions. XPS measurements were fulfilled at first on neat ionic liquid droplets over silicon substrates and on a neat lithium foil in order to establish proper references for the coin cells post-mortem studies. After 168 hours of aging at OCV, the cells were disassembled in a glove box, and the Viledon separators were extracted to study the degradation of the bulk electrolyte. At the same time, lithium electrodes were extracted and washed with dimethyl carbonate (DMC) during 30



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seconds, dried under vacuum at room temperature in order to eliminate the remaining DMC and analyze the composition of the formed interphases. 3. RESULTS AND DISCUSSIONS 3.1 Evolution of the Electrolyte and Interphase Resistances Figures 1 show the evolution, at the OCV, of the impedances measurements on symmetrical Li/Li coin cells configuration filled with neat IL and electrolyte based IL as a function of time, at a temperature of 295.0 ± 0.3 K. The plots correspond to a Nyquist representation of the spectra which can be separated in three frequency domains: At frequencies higher than 0.1 MHz, the impedance is dominated by the resistance of the bulk of the electrolyte Rel and contributions associated to the electric connections represented by a resistance Rc (typically 0.1 Ω for our setting) in series with an inductance element Lc (typically 10-6 H for our setting). Rel can be written as in the equation 1: 𝑑

𝑅𝑒𝑙 = 𝜎𝑒𝑓𝑓 ∗ 𝑆

(Eq. 1)

where σeff is the effective electrolyte conductivity, d is the distance between the electrodes, i. e. the thickness of the separator, and S is the surface area of the electrode. At middle frequencies (MF, 0.1 MHz – 1 Hz), Nyquist plot exhibits a dissymmetrical loop generally associated to the formation of surface layers and to the electrochemical charge transfer. Finally, the tail at frequencies below 1 mHz is associated to diffusional processes. The MF contribution increases as a function of aging time for all the ILs used. Due to its large dissymmetry, at least two circuits in series composed of a resistance in parallel with constant phase element (Rint//CPE) might be used to fit it. However, this equivalent electrical circuit


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(EEC) is non-univocal and always subject to debate.25–27 In addition, using an EEC corresponds to a parameterization a priori of the experimental data with two main drawbacks: i) the physical meaning of each electrical elements is not always obvious and ii) the non-unicity of the decomposition, especially when different phenomena occur in the same range of characteristic frequencies. Therefore, as a first approach to probe the main changes of spectra during aging, we calculated a time difference impedance spectra, Zt(), by subtracting for each pulsation the impedance obtained at a given aging time t, Zt(), to the initial impedance, Z0(), according to the following equation 2: Zt(ω) = Zt(ω) – Z0(ω)

(Eq. 2)

The figure 2 shows the time difference impedance spectra resulting from the equation 2. The spectra exhibit only one semi-circle that roughly increases by a factor of two to three in 60 h for all the systems. For both neat ILs, we also observe a positive shift on the real part of the impedance at high frequency that shows an evolution of the bulk properties of the ILs. Interestingly, the characteristic frequency fdiff of the semi-circle depends on the electrolyte composition, especially on the nature of the anion: with C1C6ImTFSI, fdiff is lower (59 Hz for neat IL or 73 Hz when the LiTFSI salt is added) than the one with C1C6ImFSI (226 Hz for neat IL or 162 Hz when LiTFSI salt is added). Therefore, this loop may be associated to the formation of the SEI in contact with neat ILs and electrolytes. It is possible to fit the resulting difference spectra using the simple circuit Rel in series with Rint/CPEint, (Fig. 3a). Note, that the parallel circuit R/CPE has a characteristic angular frequency (ω) represented by the equation 3a, where Y and n are the pseudo-capacitance (Fs1n)

and the exponent of the CPE (ZCPE =1/Y(j)n), respectively:28

𝑅 ∗ 𝑌 ∗ 𝜔𝑛 = 1

(Eq. 3a)



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The equivalent capacitance that would give in the R/C circuit the same characteristic angular frequency obeys the same law with n=1:

𝑅∗𝐶∗𝜔=1

(Eq. 3b)

Therefore, one can calculate the equivalent capacitance of the circuit by combining both equations 3a and 3b (equation 4):

∆𝐶𝑖𝑛𝑡 = ∆𝑅𝑖𝑛𝑡(1 ― 𝑛)/𝑛 ∗ ∆𝑌𝑖𝑛𝑡1/𝑛

(Eq. 4)

The evolutions of the time difference of the electrolyte interphase resistances, Rel and Rint , and of the inverse of the equivalent capacitance, 1/Cint, as a function of the square root of aging time are depicted in the figures 3b, 3c and 3d, respectively. Surprisingly, the Rel of neat C1C6ImTFSI shows a continuous linear evolution as a function of the square root of time with a pronounced slope (Fig. 3c). The Rel of neat C1C6ImFSI evolves in a similar way with less pronounced slope. In the presence of LiTFSI lithium salt, for both IL, the slope of Rel is reduced, and is fairly steady after 24 h of aging time. From the equation 1, it is clear that the significant increase of Rel values for neat C1C6ImTFSI must be attributed to a modification of its effective conductivity σeff. This evolution could result either from a poor wettability of the separator or a chemical change of the ILs during the aging process. To exclude the hypothesis of poor wettability, the effective conductivity (σeff) was compared with the experimental conductivity (σ0) (see experimental part). The ratio σeff / σ0 is related to the effective tortuosity (τeff) and the porosity ( )of the separator following the equation 5:29 𝜎0

𝜏𝑒𝑓𝑓 = 𝜎𝑒𝑓𝑓𝜀

(Eq. 5)


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A theoretical value of the tortuosity (τ0) can be calculated knowing the porosity ε of the separator from the equation 6:30

τ0 = 1 ― 0.5 ln (ε)

(Eq. 6)

The similarities of τeff and τ0 values (Table 1) exclude the poor wettability of the separators. Consequently, the Rel increase would result from a chemical transformation of the bulk properties of ILs related to their decomposition on lithium metal surface. Thus, with neat ILs, we suggest that some degradation compounds are partially soluble in ILs which may modify the IL viscosity, for example,31 leading to a reduction of the effective conductivity. Note that for C1C6ImFSI, the increase of Rel is less pronounced suggesting a higher stability compared to C1C6ImTFSI (Fig. 3b) or less soluble byproducts. Interestingly, the addition of LiTFSI salt protects both neat ILs from progressive bulk decomposition. This could be related to the increase of the electrochemical window range upon addition of lithium salt e.g. the cathodic limit of C1C6ImTFSI changes from + 0.64 V vs. Li+/Li to - 0.28 V vs. Li+/Li when LiTFSI (1 mol.L-1) is added.23 Concerning the interface at medium frequency (fig. 3c and 3d), for both neat ILs and electrolytes, in this representation after a linear increase of Rint and 1/Cint for the first 15 hours, the slope angle diminishes suggesting a lowering of the interface resistance evolution. This linearity is not trivial since it verifies that the growth of the fresh passive layer follows a diffusion law i.e. the interphase resistance is proportional to

𝐷𝑡, where D is a diffusion

coefficient. Interestingly, one can notice that the initial slope of the curves depends on the nature of the anion. In the case of C1C6ImTFSI and related electrolyte these slopes are similar, whereas in C1C6ImFSI it is rather lower. For C1C6ImFSI doped with LiTFSI an intermediate behavior is observed. This variance in the slopes of the curves suggest a different mechanism of 11 ACS Paragon Plus Environment


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degradation of the TFSI and FSI anions on lithium surface, leading to a different chemical composition of the fresh SEI. This is confirmed by the two sets of values of the characteristic frequencies, fdiff, of the fresh SEI (see fig. 2) depending on the nature of the anions for both neat ILs. From the initial slopes of the resistance and inverse capacitance as function of

𝑡, rough

estimates of the diffusion coefficient (D) and of the conductivity (σ) of the fresh SEI can be calculated28 using the equations 7 and 8, assuming a diffusion limited growth of its thickness, lint:

∆𝑅𝑖𝑛𝑡 =

𝑙𝑖𝑛𝑡 𝜎𝑆

=

𝐷𝑡

(Eq. 7)

𝜎𝑆

𝑙𝑖𝑛𝑡 𝐷𝑡 1 = = ∆𝐶𝑖𝑛𝑡 𝜀𝑟𝜀0𝑆 𝜀𝑟𝜀0𝑆

(Eq. 8)

Where εr, is the relative dielectric permittivity of the passive layer and ε0 the vacuum dielectric permittivity. The calculated values for the ionic conductivities and diffusion coefficients of the fresh SEI for all electrolyte combinations are listed in Table 2. Although these calculations constitute a rough estimation, the obtained values are consistent with the literature. For instance Guan et al., based on computational simulations,32 found a lithium-ion diffusion coefficient in LiF at 298 K of 3.5 x 10-16 cm2.s-1 and a conductivity of 3.2 x 10-7 S.cm-1. Note that the fresh SEI ionic conductivity is larger with FSI anion, and the diffusion coefficients are larger when ILs are doped with LiTFSI. Finally, this approach allows us to propose a bilayer SEI model for the interfacial response corresponding to one contribution at higher frequencies (1.2 ± 0.2 kHz) due to the native layers 12 ACS Paragon Plus Environment

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that do not evolve over time, and a second contribution at medium frequencies (200 ± 100 Hz) which increases continuously during the cell aging, constituting the fresh SEI generated by the electrolyte decomposition on the lithium surface. Therefore, we can propose an electric equivalent circuit based on two (Rint/CPE) in series in order to describe the interfacial loop. The first contribution (Rnative/CPEnative) has been fitted on the initial spectrum and all the parameters have been fixed for the following registered spectra assuming that all the evolution are due to the lower frequency contribution (Rfresh/CPEfresh). With this approach, we have been able to obtain excellent fitting of the data without any ambiguity on the determined parameters. The evolution of the set of parameters as a function of time is given in the Supplementary Information Figures S1-S2. Obviously, the main results are similar to what have been obtained in the analysis of the time difference spectra and will not be discussed further. However, one can notice that the interfacial resistance due to the fresh SEI formed in the case of the neat C1C6ImFSI and C1C6ImFSI/LiTFSI is systematically lower by a factor 2 to 3 compared to the TFSI based IL and electrolyte, confirming the strong impact of the anion nature on the SEI. 3.2 Chemical Environment Evolution of Bulk Electrolytes and Interfaces after Aging At first, the XPS spectra of lithium foil reference used in this study is examined. The XPS spectra of C 1s, O 1s and Li 1s core levels recorded at the surface of lithium foil are reported in the figure 4. The surface chemical composition of pristine lithium foils is mainly dominated by the presence of carbonates Li2CO3 (289.8 eV, C 1s and 531.6 eV, O 1s), of carbon from hydrocarbon contamination (C-C) (285.0 eV, C 1s) and of carbon atoms bonded to oxygen (ROLi, 286.8 eV).



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The evolution of the chemical structure of the bulk electrolyte was probed by analyzing by XPS the separator after aging and without washing with DMC. The evolution of the passivation layer at the interface electrolyte/lithium was probed by XPS performed at the surface of lithium after aging. Both bulk and interface XPS evolution was studied relatively to neat IL and IL based electrolyte. The chemical structures of C1C6Im+ cation and TFSI- and FSI- anions are shown in the figure 5. The signatures of the different chemical groups of the neat ILs and ILs based electrolytes are depicted in the XPS spectra of the figures 6a to 10a and Tables SI-1 to SI-4. The reference spectra are recorded from a drop of ILs and ILs based electrolyte deposited of silicon wafer substrate. The XPS peak assignment has been done according to data reported in the literature.33,34 Recently, we reported the XPS spectra of neat C1C6ImTFSI and C1C6ImTFSI doped with LiTFSI (1 mol.L- 1), registered at the surface of pristine graphite (Cgr) electrode with similar results.35 After aging, the coin cell is disassembled in a glove box and both Viledon separators (Figures 6b to 10b) and lithium electrodes (Figures 6c to 10c) are analyzed by XPS to probe the ILs and ILs based electrolyte bulk and interface chemical structure change. For neat ILs and ILs doped with LiTFSI (Fig. 6a), the XPS spectra of carbon C 1s core level exhibit a peak at 286.8 eV named Chetero (for hetero-structure) and corresponding to the double bonds C=N and C=C from the imidazolium ring. The C-C bonds of the (C1C6) alkyl chain correspond to the peak centered at 285.0 eV. The peak related to -CF3 from the anion TFSI appears at 293.0 eV. After aging, the spectra recorded at the Viledon separators do not show significant changes (Fig. 6b) for C1C6ImFSI, C1C6ImTFSI/LiTFSI and C1C6ImFSI/LiTFSI. However, for 14 ACS Paragon Plus Environment

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C1C6ImTFSI the relative intensities of –CF3 and Chetero related peaks in comparison with Calkyl are reduced. Moreover, the position of the peak Chetero is shifted to lower binding energies (286.0 eV), and its full width at half maximum (FWHM) become broader compared to neat reference. These observations suggest that chemical modifications or degradations of C1C6Im+ cation occurred during aging. Similar behavior was observed in the C 1s peak recorded in the bulk of C1C6ImFSI. In this case the shift toward lower binding energies and the broadness of FWHM is less pronounced (Table S1 and S3). On lithium electrodes (Fig. 6c), the spectra of C 1s are different and mainly dominated by a peak at 289.8 eV identified as Li2CO3. The presence of Li2CO3 has as origin the initial native layer on the surface of the lithium electrode (Fig. 4). Surprisingly, the intensity of the peak corresponding to –CF3 group is negligible in every case, and disappears in the case of C1C6ImFSI and C1C6ImFSI/LiTFSI, whereas the peak at 285.0 eV is clearly still visible. By comparison with the C 1s spectra of the lithium foil, the peak corresponds to the native carbon layer and not to a residuary Calkyl from ILs. This explains the fading of the –CF3 peak and the shifts of the Chetero peak that correspond probably to the formation of precipitate organic complexes from the C1C6Im+ degradation. On the other hand, since the Li2CO3 related peak is still visible, the deposited SEI layer coming from decomposition products of IL based electrolytes must be thinner than ~5 nm (depth probed by XPS). In the case of the C1C6ImFSI IL, the Li2CO3 contribution is less pronounced which is a signature of thicker and/or more uniform SEI. For neat ILs on silicon substrate (Fig. 7a), the nitrogen spectrum is characterized by two peaks corresponding to the (C1C6Im+) cation at 402.1 eV and to the anion TFSI or FSI at 399.5 and 399.8 eV, respectively, with an atomic concentration ratio Ncation/Nanion equal to 1.9 for both ILs (Tables S1 and S3). 15 ACS Paragon Plus Environment


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On Viledon separators (Fig. 7b), this atomic concentration ratio Ncation/Nanion decreases to 0.3 for C1C6ImTFSI and 0.8 for C1C6ImFSI. For both ILs doped with LiTFSI the ratio Ncation/Nanion on silicon substrate changes from 1.4 to 1.1 on Viledon separators (Tables S2 and S4). These observations demonstrate an improved stability of the cation when neat ILs are doped with LiTFSI. On lithium surface electrodes (Fig. 7c), the peaks of Ncation and Nanion still remain visible with an enlargement of its FWHM and a new peak labelled Nd around 398.6 – 398.9 eV is formed. This peak can be attributed to decomposition products from C1C6Im+ cation e.g. N- heterocyclic carbenes formation.36 From the C 1s and N 1s core level analyses, in the case of neat C1C6ImTFSI, C1C6Im+ cation reacts with the lithium affording byproducts partially soluble in the electrolytic solution, which induces a continuous change in the IL composition that strongly affects its conductivity as observed by EIS. The cation byproducts are almost absent in the bulk electrolyte with neat C1C6ImFSI. That effect may be due to a better passivation layer induced by the FSI anion degradation on the lithium surface than with TFSI anion. In the figure 8a, the spectra of fluorine F 1s core level show two peaks corresponding to the – CF3 group for TFSI anion at 688.9 eV and the SO2-F bonds in FSI anion at 687.8 eV. No significant changes of both peaks recorded at the surface of the separators (Fig. 8b), although they are accompanied by a new peak positioned at 685 eV associated to LiF salt traces mainly for FSI based IL and electrolyte. On the lithium electrodes (Fig. 8c), the LiF related peak is more intense especially in the case of FSI based IL and electrolyte. Besides, the broad peak between 687 – 689 eV associated to the formation of fluorine derivatives from anion such as C2FxLiy,37 since there is not more initial –CF3 group, as it was demonstrated in the C 1s spectra (vide supra). 16 ACS Paragon Plus Environment

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Considering that LiF is scarcely soluble in IL,38 and that it has been proposed as an agent that stabilizes the SEI layer preventing any side reaction,39 we can suggest that FSI decomposition byproducts leads to the formation of a more passivating interface than in the case of TFSI decomposition. This leads to the formation of a SEI layer mainly constituted of LiF which could prevents further decomposition of the C1C6Im+ cation. This result was corroborated by Auger and SEM analyses on the surface of 7 days aged lithium in both studied ILs in figure S3. Regarding the S 2p spectra (Fig.9), it contains the initial doublets (S 2p3/2-1/2) assigned to the SO2-CF3 bonds at 169.0 eV and SO2-F bonds at 169.9 eV. For the IL and electrolyte in the separator, one can notice that at lower binding energies, Li2SxOy species are also present at 167.0 eV. The amount of degraded anion into Li2SxOy is more important on the lithium electrode surfaces. We noticed a small contribution of lithium sulfide Li2S (159.9 eV) at the surface of lithium aged in C1C6ImTFSI doped with LiTFSI, which was also observed in previous works.17,20 This contribution remains minor in our case. The reference spectra of oxygen O 1s core levels of neat ILs (Fig. 10a) exhibit only one peak centered around 532.7 – 533.1 eV related to -N-SO2- bonds. On the Viledon separators, these peaks are present but are no more symmetric, and a new peak of reduced intensity is present at 531.3 – 531.5 eV (Fig. 10b). This new peak could be related to traces of Li2SxOy products (167 eV, S 2p spectra). As H2O contamination cannot be excluded, these peaks can also be assigned to LiOH located in the same binding energy range. The lithium electrodes exhibit the same two sets of peaks, the first one between 533.2 – 533.4 eV representing the -N-SO2- bonds with a lower intensity and the second one between 531.7 – 531.9 eV more intense associated to Li2CO3 (289.8 eV, C 1s spectra), but also related to the formation of Li2SxOy products.



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4. CONCLUSIONS The interaction between lithium metal and two different imidazolium-based ionic liquids C1C6ImTFSI and C1C6ImFSI, neat and laden with 1 mol.L-1 of LiTFSI in symmetric Li/Li coin cell systems has been studied at the OCV by EIS and XPS coupled study. The bulk electrolyte resistance of both neat ILs was found to be not stable even at the OCV condition, mainly in the case of neat C1C6ImTFSI. The changes in the IL resistivity upon aging followed by EIS, could be related to the presence of soluble byproduct of the IL decomposition onto the lithium surface. This has been confirmed by the XPS analyses performed at the surface of the separators after cell disassembling which evidenced the presence of C1C6Im+ degradation products, especially in the case of neat C1C6ImTFSI. In both ILs, the addition of LiTFSI induces stabilization of the bulk electrolyte resistance and the disappearance of the cation degradation byproduct. The increase of TFSI/ C1C6Im+ ratio seems to play the role of charge screening around the cation constituting the IL. Indeed, it has been shown upon LiTFSI addition, local reorganizations of the IL leading to the formation of negative Li(TFSI)2C1C6Im complexes40, protecting the cation. In all combinations, XPS analyses on the lithium surface show the deposition of thin layers of inorganic and organic byproducts resulting from anion and cation decomposition giving rise to the deposition of mainly LiF, Li2SxOy and N-heterocyclic carbenes. In parallel, the slope of the interfacial resistance evolution as function of time at OCV seems to be characteristic of the anion type with a stronger slope with combinations containing TFSI rather than FSI. Especially, in case of FSI the presence of LiF byproducts is observed and it seems to favor the formation of a more homogeneous and less resistive passive layer. In conclusion, it appears that the role and the nature of the anion and the salt concentration in the electrolytic solution is the key factor to design robust electrolytes that produce stable and protecting SEI which prevents the continuous decomposition of the bulk electrolyte.


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This work paves the way to understand the reactivity of the ionic liquids C1C6ImTFSI and C1C6ImFSI combined to LiTFSI salt towards lithium metal electrodes upon electrochemical cycling, that we are currently studying. ASSOCIATED CONTENT Supporting Information The evolution of the interface resistances and capacitances of the native and fresh SEI layers as a function of aging time, obtained from an equivalent circuit of a bilayer model, are depicted in figures S1 and S2. Surface characterizations by scanning electron microscope (SEM) and Auger electron spectroscopy (AES) of the lithium electrodes are shown in figure S3. The binding energy positions, atomic concentrations and full width at high maximum of the peaks observed in XPS spectra are detailed in tables S1, S2, S3 and S4. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] ** E-mail: [email protected] Funding Sources J.E.M.-U. received funding for his scholarship from FONDECYT - CONCYTEC (Grant 2322015-FONDECYT).



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Figure caption Figure 1. Evolution of the impedance response as a function of time of Li/Li coin cells containing a) C1C6ImTFSI, b) C1C6ImFSI, c) C1C6ImTFSI/LiTFSI, and d) C1C6ImFSI/LiTFSI measured at OCV at 295 K at initial time (■), after 5 hours (●), after 36 hours (▲) and after 60 hours (▼). Figure 2. Differences of impedance spectra upon aging for Li/Li coin cells containing a) C1C6ImTFSI, b) C1C6ImFSI, c) C1C6ImTFSI/LiTFSI, and d) C1C6ImFSI/LiTFSI measured at OCV at 295 K after 5 hours (●), after 36 hours (▲) and after 60 hours (▼). Figure 3. a) The Randles circuit used to fit the resulting difference impedance spectra, b) Evolution of the electrolyte resistance ∆Rel as a function of the square root of aging time, c) Evolution of the interface resistance ∆Rint as a function of the square root of aging time, and d) Evolution of the inverse of the capacitance 1/∆Cint as a function of the square root of aging time for the ionic liquids C1C6ImTFSI (■), C1C6ImFSI (▲) and electrolytes C1C6ImTFSI/LiTFSI (●) and C1C6ImFSI/LiTFSI (▼) at OCV at 295 K. Figure 4. XPS high-resolution spectra of C 1s, O 1s, F 1s and Li 1s core levels registered at the surface of pristine lithium foil. Figure 5. Chemical structure of 1-hexyl-3-methylimidazolium [C1C6Im]+ cation and bis(trifluoromethane)sulfonamide [TFSI]- and bis(fluorosulfonyl)imide [FSI]- anions. Figure 6. XPS spectra of carbon C 1s core level registered at the surface of a) neat IL based electrolytes, b) on separator and c) on lithium electrode surfaces. Figure 7. XPS spectra of nitrogen N 1s core level registered at the surface of a) neat IL based electrolytes, b) on separator and c) on lithium electrode surfaces. Figure 8. XPS spectra of fluorine F 1s core level registered at the surface of a) neat IL based electrolytes, b) on separator and c) on lithium electrode surfaces.



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Figure 9. XPS spectra of sulfur S 2p core level registered at the surface of a) neat IL based electrolytes, b) on separator and c) on lithium electrode surfaces. Figure 10. XPS spectra of oxygen O 1s core level registered at the surface of a) neat IL based electrolytes, b) on separator and c) on lithium electrode surfaces.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Figure 7



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Figure 8


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Figure 9



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Figure 10


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Table caption Table 1. Effective and theoretical conductivities of ionic liquids based electrolytes and effective and theoretical tortuosities of the separator (Thickness of separator: 250 µm, active area of electrode: 2 cm2, porosity: 60%). Table 2. Conductivities and diffusion coefficients of the interphase layer formed under the contact, at the OCV, between the IL based electrolytes and lithium electrodes



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Table 1 Electrolyte C1C6ImTFSI C1C6ImTFSI/LiTFSI C1C6ImFSI C1C6ImFSI/LiTFSI

Effective conductivity σeff (mS.cm-1) 0.9 0.3 2.1 1.0

Experimental conductivity σ0 (mS.cm-1) 1.91 0.70 3.78 1.88

Effective tortuosity τeff 1.3 1.4 1.1 1.1

Theoretical tortuosity τ0 1.2 1.2 1.2 1.2


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Table 2

IL based electrolyte

Conductivity σSEI (10-8 S.cm-1)

C1C6ImTFSI C1C6ImTFSI/LiTFSI C1C6ImFSI C1C6ImFSI/LiTFSI

4 6 19 12

Diffusion coefficient DSEI -15 (10 cm2.s) 5 13 3 25



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TOC Graphic


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