Influence of the Vinylene Carbonate on the Wetting and Interface

Jul 7, 2017 - Laboratoire de Physico-Chimie des Matériaux et des Electrolytes pour l,Energie (EA 6299), Université François Rabelais, Parc de. Grandmo...
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Influence of the Vinylene Carbonate on the Wetting and Interface Chemical Structure of Doped Ionic Liquid Electrolyte at Porous Graphite Surface E. Bolimowska,†,§ J. E. Morales-Ugarte,‡,§ H. Rouault,‡,§ J. Santos-Peña,∥,⊥ C. Santini,† and A. Benayad*,‡,§ †

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

ABSTRACT: Adding organic carbonates (e.g., vinylene carbonate, VC) into ionic liquid (IL)-based electrolytes generally improves the electrochemical performance of graphite electrode (Cgr)-based Li-ion batteries. The impact of VC on the wettability at open-circuit voltage and the interface chemical structure change between pristine Cgr and the ionic liquid-based electrolyte have been investigated by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The XPS results show a change in the surface dipole and a better impregnation of the IL-based electrolyte when the VC is added to the electrolytic media. The Cgr dipole change and the better impregnation of IL-based electrolyte induce a change in the electrical series resistance and double-layer capacitance as deduced by EIS measurements.

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INTRODUCTION

Recently, we tested the harmless and low-cost full cell configuration based on the electrochemical couple Cgr// LiFePO4 (LFP) using an electrolyte composed of a mixture of (1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) ([C1C6Im][NTf2]) with LiNTf2 (1 mol·L−1) and vinylene carbonate (VC) (5% vol.), referred to as IL* in the text. VC was required to perform the lithium intercalation and deintercalation in a Cgr-based electrochemical cell.8 The role of VC is mainly associated with the creation of interfacial compatibility between the Cgr electrode and IL-based electrolyte and formation of a solid electrolyte interphase (SEI).5,6 However, VC additive impacts the viscosity, beneficial to the [Li+] diffusion, and the electrolyte impregnation on porous Cgr electrode.10−13 The full wetting of the porous electrode and separator, before cycling, is a key factor and was highlighted in the literature with conventional organic and IL-based electrolytes.10,11 For instance, the LiFePO4 (LFP) electrode is fully wetted with PYR14NTf2-LiNTf2 + 50% propylene carbonate (PC) at 293 K after 15 h.12 To our knowledge, there are no

Lithium-ion battery technology has reached a mature level as the most used power source for consumer electronics, such as mobile phones and laptops, because of its high voltage and high-energy density.1 Electricity storage required for versatile application needs strict safety regulations from a battery pack to the individual electrochemical cell level. To achieve such requirement, the choice of electrolyte, usually based on a carbonate mixture, has a significant impact on the safety, thermal stability, and abuse tolerance of the electrochemical cell.2,3 Hence, there is active research for alternative approaches to eliminate or mitigate these problems. One of the approaches is the replacement of conventional organic electrolytes with ionic liquids (IL) due to their thermal and electrochemical stability, flame retardant performance, and high ionic conductivity.4 Nevertheless, the poor electrochemical cyclability widely observed with neat IL−electrolytes in a graphite (Cgr) electrode-based electrochemical cell is a significant drawback for industrial implementation. Previous studies have shown that the addition of carbonates in IL−electrolytes is crucial to improve the performance of the graphite-based batteries,5−8 except for a very few cases.9 © 2017 American Chemical Society

Received: May 19, 2017 Revised: July 3, 2017 Published: July 7, 2017 16166

DOI: 10.1021/acs.jpcc.7b04867 J. Phys. Chem. C 2017, 121, 16166−16173

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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 0.4 eV. We investigated in parallel the interface between neat IL and IL-based electrolyte and pristine Cgr electrode and glass fiber separator in order to distinguish between the self-evolution of media and the surface impact on the media. As the electrode required a certain time to be fully wetted with IL and its constituent, Cgr electrode and glass fiber were kept at 333 K during at least 12h before XPS measurement. It goes without saying that the low pressure vapor of IL allowed the XPS measurement performed in an ultrahigh vacuum chamber (6 × 10−8 Pa).

reported data on the Cgr electrode wetting with the IL-based electrolytes in the literature contrary to the wetting of the separator. Hence, in this paper we will point out the role of VC on the electrolyte wettability, interfacial resistance, and chemical structure change through a coupled study combining electrochemical impedance spectroscopy (EIS) and X-ray photoemission spectroscopy (XPS).

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EXPERIMENTAL SECTION Ionic Liquid Synthesis and Electrolyte Preparation. The experiments were performed in the absence of oxygen and water under a purified argon atmosphere using glovebox (Jacomex or MBraun) or vacuum-line techniques. 1-Hexyl-3-methylimidazolium chloride [C 1 C 6 Im][Cl] (>99%, Solvionic), bis(trifluoromethanesulphonyl)imide lithium salt LiNTf2 (>99%, Solvionic), and vinylene carbonate (VC) (Sigma-Aldrich) were kept in the glovebox and used as received. 1-Hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide, ([C1C6Im][NTf2]), was synthesized as already reported.8 After drying IL under vacuum (10−5 mbar) for 24 h at 40 °C, the electrolytes were prepared, at room temperature, in an argon-filled glovebox by dissolving anhydrous LiNTf2 (1 mol· L−1) in the ionic liquid. The mixture was stirred for 1 h. Then the required volume of VC was added under stirring at RT, and the resulting electrolyte was kept in a refrigerator of the argonfilled glovebox. The electrolyte was a mixture of 63 mol % of C1C6ImNTf2, 21 mol % of LiNTf2, and 17 mol % of VC. Electrolytes. Electrolytes were prepared by adding LiNTf2 (1 mol·L−1) in a well-stirred IL at room temperature, followed by storage in an argon-filled glovebox. A graphite composite electrode, supplied by the CEA, was prepared from graphite (SLP30 Timcal, 16 μm D50), carboxymethylcellulose (CMC), and styrene butadiene rubber (SBR) in a weight ratio 96:2:2, suspended in water solution (slurry preparation), deposited onto a copper collector (12 μm, Oak Mitsui) using a doctor blade (ink deposition and coating), dried for 24 h at 333 K, and calendared (thickness 48 μm, porosity of 37%). The electrodes have a loading of 2.6 mAh/ cm2. Before assembling in cells, they were disk-shape cut (diameter Ø = 14 or 16 mm) and then dried for 48 h at 353 K under vacuum. Impedance Spectroscopy. Three-electrode Swagelok cells were assembled in the glovebox using Cgr as working electrode (WE), lithium ribbon as counter electrode (RE), and lithium foil or stainless steel as reference (CE) electrode (ss). The electrodes were separated with Whatman glass fiber membrane soaked with 100 μL of IL*. The cells were placed in an oven at 333 K just before starting the electrochemical testing. Impedance spectra were measured using a Bio-Logic VMP3 multichannel potentiostat by applying a potentiostatic signal with 1 mV amplitude within the frequency range from 240 kHz to 100 mHz. Measurements were repeated each 2 h to monitor the electrode wetting. Nyquist plots were fitted using ZView software (Scribner Associates Inc.). The fitting was considered suitable when goodness of fit was lower than 10−4. X-ray Photoemission 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. The 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

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RESULTS AND DISCUSSION Electrochemical Impedance Spectroscopy Measurements. EIS in its potentiostatic variant (PEIS) was performed with three-electrode Swagelok cells in a CE-Li or CE-ss, WECgr, RE-Li configuration, at different impregnation times at 333 K. The results are presented in Nyquist plots (Figure 1), i.e., imaginary part of the impedance versus real part. With the (CE-Li/WE-Cgr/RE-Li) system (Figure 1a), all measurements, taken at the beginning of cell assemblage (0 h) and after 2 h wetting periods, show similar shape of Nyquist diagrams, reflecting a small dependence of the features describing the Cgr/IL* interface evolution with time. With CE-ss, the Nyquist diagrams are also similar at various wetting times; however, their shapes appear slightly different (Figure 1b). Thus, at 0 h, the Nyquist plots obtained using CE-Li (Figure 1a) shows a half-circle developed at high frequencies followed by a 45° slope at middle frequencies and a linear slope at low frequencies. After 2 h of wetting, a shift of the Nyquist plot toward smaller real impedance values is noticed. The Nyquist plots remain unchanged with the time, evidencing a stabilization of the interface Cgr/IL*. Considering that no charge transfer is involved in the process, the Nyquist plot modeled according to the equivalent electrical circuit described in Scheme 1, containing two elements, labeled ESR and Cdl, associated with a capacitive behavior of graphite occurring instantly after cell assembly at 333 K and after cell evolution with time.14 This hypothesis is plausible since impedance measurements are performed close to the open cell voltage (OCV), 3.0 V vs Li/Li+, above the lithium insertion voltage (0.2 V). Thus, the capacitor behavior is due to equivalent series resistance (ESR) and the graphite double-layer capacitance (Cdl). For better fitting of the Nyquist plot, the equivalent circuit contains two additional elements placed in series Rel and Rf//Cf, which might be associated with electrolyte resistance and charge stored at the graphite electrode grains/copper collector boundaries, respectively.14 The equivalent circuit Rf//Cf could also be related to the presence of an electronically insulating solid film onto the lithium counter electrode,15−19 which can be solved in part, migrate, and precipitate onto the working electrode (graphite).20 To shed light on the (Rel + (Rf//Cf)) part, we performed an EIS using stainless steel counter electrode (CE-ss/WE-Cgr/RELi). In this case, the Nyquist plots collected and reported in Figure 1b show different features compared to the previous system. As for CE-Li/WE-Cgr/RE-Li system, we noticed a shift toward smaller real impedance values with time until reaching stabilization after ∼6 h, with negligible differences between the profiles at 2 and 6 h. No evidence of a half-circle in these plots 16167

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seems evident that the first hypothesis about the presence of (Rel + (Rf//Cf)) elements due to the grain/collector charge storage must be discarded as it should create a half-circle independently of the counter electrode nature. The (Rel + (Rf// Cf)) contribution is only present when the counter electrode is lithium, in agreement with the assignation of the Rf//Cf element to a solid film. Using lithium as counter electrode, a thin layer is deposited on the surface of graphite surface, resulting from the migration of the electrolyte byproduct formed at the surface of lithium electrode.16−20 Note that this solid interface layer cannot be considered as a conventional SEI formed onto the graphite electrode particles during the discharge process as the measurements are carried out at OCV.21 For both electrode configurations, the evolution of ESR values vs wetting time is presented, Figure 2. The measured

Figure 1. Evolution of the Nyquist plot with time at 333 K of CE/WECgr/RE-Li cell using C1C6ImNTf2/VC (5% vol.)/LiNTf2 (1 mol·L−1) electrolyte and lithium (a) or stainless steel as counter electrode (b). (c) CE-Li/WE-Cgr/RE-Li using C1C6ImNTf2/LiNTf2 (1 mol·L−1) electrolyte with lithium as counter electrode.

Scheme 1. Schemes I and II Showing the Equivalent Electrical Circuit Employed for Modeling the Nyquist Plots Presented in Figure 1a

a

Figure 2. Evolution of interfacial resistances with wetting time obtained by simulation of Nyquist plots (a) from Figure 1a and 1b corresponding to C1C6ImNTf2/VC (5% vol.)/LiNTf2 (1 mol·L−1) electrolyte with lithium or stainless steel as counter electrode and (b) from Figure 1c corresponding to C1C6ImNTf2/LiNTf2 (1 mol·L−1) electrolyte with lithium as counter electrode.

ESR values are similar, lower than 1 Ω, and slightly decrease as the wetting time increases. The ESR value evolution indicates that the electrolyte fully wets the graphite electrode in 2−4 h, independent of the nature of the counter electrode. The Whatman separator (glass fiber separator), rather hydrophilic, does not limit the wetting, since it is rapidly impregnated.10−12 The increased contact area between the electrolyte and the electrodes is proved by the decrease of Rel and ESR values and the increase of Cdl value from 300 to 360 μF·cm−2 with wetting

Elements are described in the text.

was demonstrated with time, and the diagrams are thus modeled by using scheme II as shown in Scheme 1, containing the above-described ESR + Cdl elements, associated with the capacitor behavior of the graphite at the OCV. From the comparison between schemes I and II as shown in Scheme 1, it 16168

DOI: 10.1021/acs.jpcc.7b04867 J. Phys. Chem. C 2017, 121, 16166−16173

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Figure 3. C 1s core peak signature of (a) C1C6ImNTf2, (b) C1C6ImNTf2/LiNTf2 (1 mol·L−1), and (c) IL* = C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) registered at separator and pristine Cgr surfaces.

Figure 4. (A) Evolution of cation and anion N 1s related peaks in (a) C1C6ImNTf2, (b) C1C6ImNTf2/LiNTf2 (1 mol·L−1), and (c) IL* = C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) registered at separator and pristine Cgr surfaces. (B) Change in the ratio intensity between cationic and anionic N 1s related peaks of C1C6ImNTf2, C1C6ImNTf2/LiNTf2 (1 mol·L−1), and IL* = C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) registered at separator and pristine Cgr surfaces. (C) Cationic and anionic N 1s binding energy variation of C1C6ImNTf2, C1C6ImNTf2/LiNTf2 (1 mol·L−1), and IL* = C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) registered at separator and pristine Cgr surfaces.

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The Journal of Physical Chemistry C time. Note that with CE-Li the decrease of the film resistance, Rf, could be due to an immediate film formation at 333 K and its partial dissolution during the wetting, although further studies should be devoted to clarify this point. This phenomenon does not affect the others electrode properties since Rel and ESR values do not increase over time. In order to determine the influence of VC on the wetting of graphite electrode, we performed EIS in the same conditions with a VC free electrolyte using Li as counter electrode. The Nyquist plots and the corresponding electrical parameters are shown in Figures 1c and 2b, respectively. The diagrams can be fitted with scheme I as shown in Scheme 1, indicating a similar behavior to that observed in the case of VC free electrolyte. However, the absence of VC enhances the total system impedance. For instance, electrolyte resistance values are slightly higher due to a decreased electrolyte conductivity. Furthermore, the film resistance is two times higher in agreement with the higher reactivity of the lithium electrode with the electrolyte in the absence of a well-known stabilizer SEI-builder as VC. Nevertheless, the ESR values are similar and the interface is stabilized after 2−4 h as for VC-containing electrolyte. X-ray Photoelectron Spectroscopy Measurements. In parallel, the XPS spectra of core levels C 1s, N 1s, O 1s, F 1s, S 2p, and Li 1s of C1C6ImNTf2, C1C6ImNTf2/LiNTf2 (1 mol· L−1), and C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) on Whatman separator and a pristine Cgr electrode were performed. 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.22 The C 1s core peaks registered at the separator and Cgr surface after dipping until complete wetting in the three different media are shown in Figure 3. For all media, the spectral envelopes display the same shape with the main peak at 285 eV assigned to carbon in the aliphatic group (Caliph), a broad peak at higher binding energies centered at ∼286.6 eV associated with different C−N-related bonds in C1C6Im (this peak is labeled heterocarbon (Chetero)), and a peak at 292.9 eV assigned to carbon surrounded by three fluorine atoms (CCF3) in NTf2 anion. The intensity of the CCF3 peak is higher with the solutions containing LiNTf2 as the solution contains additional CF3 groups constituting the salt. The C 1s peak registered at the surface of Cgr impregnated IL* shows a sharp peak at 284.3 eV assigned at sp2 graphitic carbon. Since in-depth analysis in XPS is ∼5 nm, the observation of a Csp2-related peak corroborates the good IL* impregnation in Cgr as reported by EIS measurements. In this case we do not observe clear evidence of the VC signature at the surface of the graphite, because the C−O-related peak at 286.5 eV overlaps with the peak labeled Cheter, and the intensity of the OC−O related peak at 290.5 eV (labeled with asterisks (*) in Figure 3c) is at noise level.23 In a macroscopic point of view, adding VC to C1C6ImNTf2/LiNTf2 induces a change of the viscosity of the electrolyte from 221.1 to 131.4 mPa·s at room temperature.13 Hence, the better impregnation of electrolyte containing VC in porous graphite electrode could explain the low intensity of the peak at 290.5 eV. The Ncation (∼402.1 eV) and Nanion (∼399.4 eV) 1s core peaks taken at the separator and Cgr electrode surfaces are reported in Figure 4A. In the case of neat IL impregnated in separator and Cgr, the Ncation/Nanion peak intensity ratio is ∼1.8, lower than the theoretical stoichiometric ratio (2 nitrogen

atoms in C1C6Im cation vs one nitrogen atom in NTf2 anion).24 When LiNTf2 is present in the solution, the ratio Ncation/Nanion decreases from 1.75 to 0.97 for separator and 1.84 to 0.94 for Cgr, Tables 1 and 2. Table 1. Binding Energies (eV) and Relative Atomic Concentration of Different Species of the Ionic Liquids Mixtures on Glass Fiber Separator chemical nature C 1s

N 1s O 1s F 1s S 2p Li 1s

Calkyl CF3 Chetero cation anion anion anion LiF 3/2 1/2 LiNTf2

IL 285.0 292.9 286.7 402.0 399.4 532.7 688.8 685.7 168.9 170.1

(24.8) (6.4) (19.2) (6.4) (3.6) (12.1) (15.8) (0.8) (7.3) (3.6)

IL + LiNTf2 285.0 293.1 286.8 402.1 399.7 533.0 689.0 685.2 169.3 170.4 56.1

(10.9) (7.8) (11.9) (3.8) (4.0) (14.1) (18.8) (2.2) (8.6) (4.3) (13.6)

IL* 284.9 293.1 286.8 402.1 399.7 533.1 689.0 685.2 169.3 170.5 56.2

(12.4) (7.9) (11.0) (3.7) (3.9) (14.2) (19.9) (1.4) (8.1) (4.1) (13.4)

Table 2. Binding Energies (eV) and Relative Atomic Concentration of Different Species of the Ionic Liquids Mixtures on Pristine Graphite chemical nature C 1s

N 1s O 1s F 1s S 2p Li 1s

Calkyl CF3 Chetero cation anion anion anion LiF 3/2 1/2 LiNTf2

IL 285.1 293.0 286.9 402.2 399.5 532.8 689.0

(17.7) (6.0) (16.6) (6.8) (3.7) (14.5) (25.7)

169.0 (6.0) 170.1 (3.0)

IL + LiNTf2 285.0 293.0 286.7 402.1 399.6 533.0 689.0

(11.1) (7.2) (11.9) (3.9) (4.2) (14.3) (21.6)

169.2 (7.8) 170.4 (3.9) 55.7 (14.1)

IL* 284.6 293.0 286.5 402.1 399.6 532.8 688.9

(25.0) (8.7) (25.1) (5.1) (3.8) (11.5) (11.2)

169.1 (6.4) 170.3 (3.2)

Note that in IL* this ratio remains constant at 0.97 for separator (Figure 4B) but increases to 1.4 for Cgr electrode. On the other hand, the Ncation and Nanion peak position difference shifts from 2.6 to 2.4 eV for separator and from 2.7 to 2.5 for Cgr (Figure 4C), indicating a change in the cation/anion charge balance in the presence of (LiNTf2). Simulation and NMR spectrometry-based results have shown that [Li]+ cations were linked to two and four [NTf2]− anions through strong oxygen−lithium bonds.25 These SO−Li bonds make a decrease in the basicity of Nanion causing a shift to higher binding energies in the Nanion core level. In the case of IL* the cation/ anion charge balance seems to be similar at least from an XPS point of view.25 Due to the mild and neutral experimental conditions, the intensity ratio Ncation/Nanion increase could not be explained by a decomposition/insertion of LiNTf2 on Cgr electrode. However, based on molecular dynamics simulations it has been proposed that [Li+] remains partially solvated by the IL anions even at the contact with the negatively charged surface. Consequently, it will not be the case at the direct contact with the graphite surface.26 The IL anions in the solvation shell of the alkali metal ions favorably interact with the excessive positive charge accumulated in the first layer due to the effect of overscreening in ILs.27 This result can be supported by the increase of the relative atomic concentration 16170

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Figure 5. F 1s core peak signature of (a) C1C6ImNTf2, (b) C1C6ImNTf2/LiNTf2 (1 mol·L−1), and (c) IL* = C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.) registered at separator and pristine Cgr surfaces.

Figure 6. S 2p core peak signature of (a) C1C6ImNTf2, (b) C1C6ImNTf2/VC (5% vol.), and (c) C1C6ImNTf2/LiNTf2 (1 mol·L−1) registered at separator and pristine Cgr surfaces.

stable N−SO2−CF3 chemical structure in the IL-based electrolyte. However, in the media with LiNTf2, the S 2p3/2−1/2 doublet position shifts to higher binding energies resulting from the [Li]+ cations coordination with [NTf2]− anions through strong oxygen−lithium bonds.24 In the case of Cgr, the S 2p doublet shift to higher binding energies reflects a change in the surface dipole charge as already suggested from the C 1s and N 1s spectral analyses. The EIS analyses indicated an interfacial time-dependent behavior of the Cgr/IL* interface in the first hours of wetting. Both XPS and EIS analyses suggested the good insertion process using the VC as additive in C1C6ImNTf2/LiNTf2 electrolyte.8

of cation/anion (Chetero/F) from 0.5 to 2.2 in the presence of VC, Table 2. The main fluorine F 1s core peak positioned at 689.0 eV (Figure 5), assigned to CF3-like bonds, does not change at a simple contact with separator or Cgr. However, a low-intensity peak at 685.7 eV signature of LiF is observed at the separator surface with C1C6ImNTf2:LiNTf2. The LiF formation could result from an interaction between residual surface silanol on the separator and the anion [NTf2]28 and not from the IL degradation under the X-ray beam,28,29 since no peak is observed when Cgr is wetted by any media. Regarding the S 2p3/2−1/2 doublet (Figure 6), assigned to SO2 chemical state, we do not observe any peak modification upon contact with the separator or the Cgr electrode, indicating a 16171

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(4) Tarascon, J.-M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (5) Holzapfel, M.; Novak, P.; Jost, C. Stable Cycling of Graphite in an Ionic Liquid Based Electrolyte. Chem. Commun. 2004, 2098−2099. (6) Holzapfel, M.; Jost, C.; Prodi-Schwab, A.; Krumeich, F.; Wursig, A.; Buqa, H. Stabilisation of Lithiated Graphite in an Electrolyte Based on Ionic Liquids: an Electrochemical and Scanning Electron Microscopy Study. Carbon 2005, 43, 1488−1498. (7) Yan, C.; Zaijun, L.; Hailang, Z.; Yinjun, F.; Xu, F.; Junkang, L. 1Alkyl-2,3-dimethyl Imidazolium Bis(trifluoromethanesulfonyl)Imide Ionic Liquids as Highly Safe Electrolyte for Li/LiFePO4 Battery. Electrochim. Acta 2010, 55, 4728−4733. (8) Srour, H.; Rouault, H.; Santini, C. C. Imidazolium-Based Ionic Liquid Electrolytes for Li-ion Secondary Batteries Based on Graphite and LiFePO4. J. Electrochem. Soc. 2013, 160, A66−A69. (9) Sugimoto, T.; Atsumi, Y.; Kikuta, M.; Ishiko, E.; Kono, M.; Ishikawa, M. Ionic Liquid Electrolyte Systems Based on Bis(fluorosulfonyl)imide for Lithium-Ion Batteries. J. Power Sources 2009, 189, 802−805. (10) Stefan, C. S.; Lemordant, D.; Claude-Montigny, B.; Violleau, D. Are Ionic Liquids Based on Pyrrolidinium Imide Able to Wet Separators and Electrodes used for Li-ion Batteries? J. Power Sources 2009, 189 (2), 1174−1178. (11) Dahbi, M.; Violleau, D.; Ghamouss, F.; Jacquemin, J.; Tran-Van, F.; Lemordant, D.; Anouti, M. Interfacial Properties of LiTFSI and LiPF6-Based Electrolytes in Binary and Ternary Mixtures of Alkylcarbonates on Graphite Electrodes and Celgard Separator. Ind. Eng. Chem. Res. 2012, 51, 5240−5245. (12) Kühnel, R.-S.; Obeidi, S.; Lübke, M.; Lex-Balducci, A.; Balducci, A. Evaluation of the Wetting Time of Porous Electrodes in Electrolytic Solutions Containing Ionic Liquids. J. Appl. Electrochem. 2013, 43 (7), 697−704. (13) Bolimowska, E. Investigation of the Interphase Between Ionic Liquid/Graphite Electrodes, Ph.D. Dissertation, Lyon 1, 2016. (14) Portet, C.; Taberna, P. L.; Simon, P.; Laberty-Robert, C. Modification of Al Current Collector Surface by Sol−Gel Deposit for Carbon−Carbon Supercapacitor Applications. Electrochim. Acta 2004, 49, 905−912. (15) Thomas, M. G. S. R.; Bruce, P. G.; Goodenough, J. B. AC Impedance Analysis of Polycrystalline Insertion Electrodes: Application to Li1−xCoO2. J. Electrochem. Soc. 1985, 132, 1521−1528. (16) 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, 1611−1620. (17) Aurbach, D.; Zaban, A.; Schetcher, A.; Ein-Eli, Y.; Zinigrad, E.; Markovsky, B. The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries. I. Li Metal Anodes. J. Electrochem. Soc. 1995, 142 (9), 2873−2882. (18) Trócoli, R.; Franger, S.; Morales, J.; Santos-Peña, J. Insights on the Electrode/Electrolyte Interfaces in LiFePO4 Based Cells with LiAl(Al) and Li(Mg) Anodes. J. Electroanal. Chem. 2014, 732, 53−60. (19) Thevenin, J. G.; Muller, H. Impedance of Lithium Electrodes in a Propylene Carbonate Electrolyte. J. Electrochem. Soc. 1987, 134 (2), 273−280. (20) Xu, K. Electrolytes and Interphases in Li-ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (21) Aurbach, D.; Ein-Eli, Y.; Markovsky, B.; Zaban, A.; Luski, S.; Carmeli, Y.; Yamin, H. The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries. II. Graphite Electrodes. J. Electrochem. Soc. 1995, 142 (9), 2882−2890. (22) Smith, E. F.; Rutten, F. J. M.; Villar-Garcia, I. J.; Briggs, D.; Licence, P. Langmuir, Ionic liquids in vacuo: Analysis of Liquid Surfaces using Ultra-High-Vacuum Techniques. Langmuir 2006, 22, 9386−9392. (23) El Ouatani, L.; Dedryvère, R.; Siret, C.; Biensan, P.; Reynaud, S.; Iratçabal, P.; Gonbeau, D. The Effect of Vinylene Carbonate Additive on Surface Film Formation on Both Electrodes in Li-Ion Batteries. J. Electrochem. Soc. 2009, 156 (2), A103−A113.

If we consider that the bulk electrolyte structure remained homogeneous after 2 h of impregnation, the change in the surface dipole could be due to a change of the LiNTf2 concentration at the vicinity of the Cgr surface, inducing modifications of the ESR and the double layer as deduced from EIS results. Adding VC to the electrolytic solution induced better impregnation, facilitating the LiNTf2 infiltration in the Cgr porous electrode structure.

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CONCLUSIONS Impedance spectroscopy experiments were carried out to follow up the wetting of the Cgr electrode by the IL* (C1C6ImNTf2/LiNTf2 (1 mol·L−1)/VC (5% vol.)) electrolyte. After assembly and every 2 h we conducted PEIS on electrodes at the OCV state at 333 K. When stainless steel was used as counter electrode, impedance measurements agree with a capacitor behavior of the graphite, which is highly likely as no charge transfer occurs at OCV. However, when lithium was used as counter electrode, we detected a half circle in the Nyquist plots associated with a solid film immediately forming onto the graphite surface containing products formed at the lithium electrode. The wetting was followed up by analyzing the ESR resistance evolution with time. We found that a period of 2−4 h is long enough at 333 K to stabilize the graphite/IL* interface by an efficient impregnation. A similar impregnation mechanism is found for the VC-free electrolyte, although the system total impedance is higher due to a resistive film formed onto the graphite. On the basis of a systematic XPS study, the stability of IL* on glass fiber support and graphite electrode after 12 h of wetting at 333 K was investigated. The C 1s core level analyses evidenced the better impregnation of IL* on Cgr electrode and an increase of cation concentration at the surface. The qualitative and quantitative analyses of C 1s, N 1s, and S 2p core peaks have shown that the surface dipole is modified when the VC was added to the electrolytic media. This underwent Cgr dipole change, and the better impregnation of IL* in the Cgr electrode pores accounts for ESR and double-layer capacitance value modification as deduced from EIS results. This study has paved the way for better understanding of the role of VC in IL-based electrolyte on the electrochemical stability of Li-ion battery under specific electrochemical working conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Benayad: 0000-0002-8854-1105 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b04867 J. Phys. Chem. C 2017, 121, 16166−16173

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DOI: 10.1021/acs.jpcc.7b04867 J. Phys. Chem. C 2017, 121, 16166−16173