Deep Eutectic Solvents Based on N-Methylacetamide and a Lithium

Article pubs.acs.org/JPCC

Deep Eutectic Solvents Based on N‑Methylacetamide and a Lithium Salt as Electrolytes at Elevated Temperature for Activated CarbonBased Supercapacitors Warda Zaidi,† Aurélien Boisset,† Johan Jacquemin,‡ Laure Timperman,† and Mérièm Anouti*,† †

Laboratoire PCM2E, Université François Rabelais, Parc de grandmont, 37200 Tours, France The QUILL Research Centre, School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom

‡

ABSTRACT: This study describes the utilization of deep eutectic solvents (DESs) based on the mixture of the N-methylacetamide (MAc) with a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF6; or nitrate, NO3) as electrolytes for carbon-based supercapacitors at 80 °C. The investigated DESs were formulated by mixing a LiX with the MAc (at xLi = 0.25). All DESs show the typical eutectic characteristic with eutectic points localized in the temperature range from −85 to −52 °C. Using thermal properties measured by differential scanning calorimetry (DSC), solid−liquid equilibrium phase diagrams of investigated LiX−MAc mixtures were then depicted and also compared with those predicted by using the COSMOThermX software. However, the transport properties of selected DESs (such as the conductivity (σ) and the fluidity (η−1)) are not very interesting at ambient temperature, while by increasing the temperature up to 80 °C, these properties become more favorable for electrochemical applications, as shown by the calculated Walden products: w = ση−1 (mS cm−1 Pa−1 s−1) (7 < w < 16 at 25 °C and 513 < w < 649 at 80 °C). This “superionicity” behavior of selected DESs used as electrolytes explains their good cycling ability, which was determined herein by cyclic voltammetry and galvanostic charge−discharge methods, with high capacities up to 380 F g−1 at elevated voltage and temperature, i.e., ΔE = 2.8 V and 80 °C for the LiTFSI−MAc mixture at xLi = 0.25, for example. The electrochemical resistances ESR (equivalent series resistance) and EDR (equivalent diffusion resistance) evaluated using electrochemical impedance spectroscopy (EIS) measurements clearly demonstrate that according to the nature of anion, the mechanism of ions adsorption can be described by pure double-layer adsorption at the specific surface or by the insertion of desolvated ions into the ultramicropores of the activated carbon material. The insertion of lithium ions is observed by the presence of two reversible peaks in the CVs when the operating voltage exceeds 2 V. Finally, the efficiency and capacitance of symmetric AC/AC systems were then evaluated to show the imbalance carbon electrodes caused by important lithium insertion at the negative and by the saturation of the positive by anions, both mechanisms prevent in fact the system to be operational. Considering the promising properties, especially their cost, hazard, and risks of these DESs series, their introduction as safer electrolytes could represent an important challenge for the realization of environmentally friendly EDLCs operating at high temperature.

1. INTRODUCTION Supercapacitors have been receiving great interest because of their high power storage capability, which is highly desirable for applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs).1−3 Carbon-based capacitors have attracted great attention since these materials possess diversified morphologies with high stability and conductivity,4−9 especially activated carbons (ACs) which are the most commonly used electrode materials for EDLC (electrical double-layer capacitor) applications. This is due to their relatively low cost and high surface area in comparison with other carbon-based materials. Another factor that influences the properties of supercapacitors is the selected electrolyte. Currently, aqueous solutions are the most utilized.10−13 However, the main drawbacks of aqueous electrolyte-based supercapacitors are a narrow cell voltage and low energy.14 Other electrolytes © 2014 American Chemical Society

described into the literature are based on organic solvent, or ionic liquid, based mixtures, even if their large-scale applications are still limited mainly caused by their environmental impact and/or cost. For ACs, only part of the “theoretical” capacitance was observed due to the presence of micropores that are inaccessible by the solvated anion in the electrolyte, wetting deficiencies of electrolytes on the electrode surface, and/or the inability to successfully form a double layer in the pores. In fact, the nature of the electrolyte is essentially responsible of this. The attainable cell voltage of supercapacitors depends largely on the electrolyte breakdown voltage, while ESR depends on Received: December 22, 2013 Revised: February 5, 2014 Published: February 7, 2014 4033

dx.doi.org/10.1021/jp412552v | J. Phys. Chem. C 2014, 118, 4033−4042

The Journal of Physical Chemistry C

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mg, with an active mass of 5.0 mg) made of aluminum foil coated with AC with poly(vinylidene fluoride) (PVDF) binder (80 wt % AC, 10 wt % PVDF, 10 wt % carbon black) was kindly supplied by Batscap. The temperature was controlled at 60−80 °C by a thermostatic oven within an accuracy close to 0.1 °C. Gaussian software (Gaussian version 03-D1, DFTB3LYP, DGTZVP as basic set) and then used to generate from each resultant optimized structure its COSMO file (TURBOMOLE, BP-DFT-Ahlrichs-TZVP).19 The COSMO volume and the sigma profile of each structure were then directly obtained by using the COSMOthermX software (version 2.1, release 01.08)20 based on the COSMO-RS (conductor-like screening model for real solvents) model.19,20

the electrode and electrolyte conductivity. The choice of the electrolyte is therefore very influential. Deep eutectic solvents (DESs) thus have been paid great attention to replace current organic solvents and have been applied to many chemical processes. DESs are liquid media obtained by mixing in certain ratios a metal salt, with hydrogen bond donor (HBD) such as alcohol, amide, and carboxylic acid15,16 as a complexing agent. This results in the formation of a eutectic mixture with a melting point temperature that is considerably lower than its original precursors mainly due to the generation of intermolecular hydrogen bonds. We demonstrate in two recent papers17,18 that the N-methylacetamide (MAc) can be considered as a good and green solvent for the formulation of new DES-based electrolytes. Furthermore, our group demonstrates also that the MAc is able to decrease the melting point of a complex system containing a molten salt due to its ‘‘water-like’’ physical properties, e.g., very high dielectric constant and dipolar moment.17 We also demonstrated in these works that each formulated DES-based electrolyte containing the MAc and a LiX salt (X = NO3, TFSI, or PF6) allows the realization of LIBs devices showing promising performances even at 80 °C. The present work reports on the use of DES as electrolytes for symmetrical supercapacitors with activated carbon as the electrode at elevated temperatures (60 and 80 °C) and a discussion centered on the mechanism of pore accessibility for nonsolvated lithium ions.

3. RESULTS AND DISCUSSION 3.1. Physical Properties of DESs. The MAc has very good physical properties such as dissociating solvent (e.g., very high dielectric constant (ε = 178.9), a high dipolar moment μ = 6.8 D, and a very low vapor pressure pvap = 0.050 kPa at 40 °C). When it is mixed in a specific compositions range with a lithium salt, a transparent homogeneous solution can be formed at room temperature after 10 min of stirring. This behavior is similar to that of an ionic liquid in which the presence of large ions hinders the crystallization of the liquid at room temperature. In fact, the polarizing power due to its two polar groups (CO and NH2) can separate the two ions by weakening and even breaking the bonds between Li+ cations and X anions of a lithium salt (LiX, with X = bis[(trifluoromethyl)sulfonyl]imide, TFSI; hexafluorophosphate, PF6; or nitrate, NO3). The ability of a solvent to solvate and separate the ions of a salt are linked to its ability to establish bonds around ions especially when ions are very structuring such as lithium. In the case of water or organic solvents, the lithium ions form swarms ion surrounded by a sphere of solvation of several molecules. In the case of MAc, the presence of two donors group promotes coordination of both the anion and the cation. This case is different from the solvation sphere model (see Figure 1). We can then consider the Li+ ions as free

2. EXPERIMENTAL SECTION Lithium bis[(trifluoromethyl)sulfonyl]imide (LiTFSI, >99.0%) was obtained from Solvionic; lithium hexafluorophosphate (LiPF6, >99.0%) and lithium nitrate (LiNO3, >99.0%) were obtained from Sigma-Aldrich. Anhydrous N-methylacetamide (MAc, 99.8%) was purchased from Sigma-Aldrich and redistilled before any utilization. Each DES solution was prepared in glovebox by mixing known quantities by mass of the dried lithium salt and a freshly distillated and degassed Nmethylacetamide (MAc) in mole fraction composition xLi+ = 1/ (1 + n) (e.g., 1 mol of LiTFSI mixed into n mol of MAc), giving an uncolored viscous liquid. The DESs were stored in argonfilled glovebox Mbraun with oxygen and water contents lower than 1 ppm. The water content in each investigated electrolyte, determined by Coulometric Karl Fischer titration, was controlled to investigate the water content effect before and after any measurement during this work; the water content in electrolytes is found to be close to 300 ppm (LiTFSI), 400 ppm (LiPF6), and 800 ppm (LiNO3). This result is linked to the hydrophobic and hydrophilic nature of the TFSI and NO3 anions, respectively. Differential scanning calorimetry (DSC) was carried out on a PerkinElmer DSC 4000 under nitrogen atmosphere, coupled with an Intracooler SP VLT 100. Samples for DSC measurements were sealed in Al pans. Electrochemical measurements were carried out on a versatile multichannel potentiostat (Biologic S.A). The cyclic voltammograms (CVs) was measured using a using a Teflon Swagelok-type system with three-electrode cell, with activated carbon as the working and counter electrode and a silver wire as the reference electrode. Galvanostatic charge−discharge experiments were conducted using same system with a two-electrode cell with activated carbon as the working and counter electrodes. Whatman glass microfiber filter papers were utilized as the separator. The activated carbon electrode material (12 mm in diameter, 7.2

Figure 1. Li+−MAc−NO3− system.

ions that easily separate from the MAc. In other words, the interactions of the acetamide (−CO−NH−CH3) groups with ions (cations and anions) can lead to a formation of coordinated complexes producing microdomains in solution as supported by several works.21−23 Abdulnur and Laki have calculated optimal distance of approach for lithium cations to the oxygen atom on the MAc structure, which is close to d = 2.16 Å,22 and the resultant electronic charge on MAc, obtained from the Mulliken population analysis, close to δ = +0.27. 4034



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Additionally, they evaluated also the interaction energy of the Li+ with MAc as ΔE = 346.9 kJ/mol.22 These considerations can be used to explain the behavior of adsorption in the activated carbon as electrochemical double-layer capacitor material according to the pore size and shape as discussed below. The ionic bond in selected DESs based on LiX−MAc mixtures is a weakening process between LiX and MAc. Considering the size and the charge delocalization of selected anions, it is then easy to imagine that the formation of DESs by mixing MAc with LiX is easier by following this order: TFSI > PF6 > NO3. The thermal behavior of the studied DESs was recorded by using a DSC from −150 to 150 °C during two heating−cooling cycles. Figure 2a presents a typical thermogram obtained for the investigated DESs, as shown here in the case of the LiTFSI− MAc, as example. As shown in this figure, the crystallization by cooling requires long-time annealing in the supercooled liquid regions from 150 to −150 °C. No phase change is visible except a vitrification Tg observed at −82.9 °C. During the heating scan, we observe first a cold crystallization followed by the melting temperature at 18 and 45 °C, respectively. These two processes require only 7 J g−1, indicating a low energy of crystal organization. This result reveals that at this composition (xLi+ = 0.25) this DES is liquid from −82 to 150 °C. Figure 2b shows the differential scanning calorimeter (DSC) curves of LiNO3−MAc binary system as a function of the lithium salt mole fraction, xLi+, from 0.1 to 0.5 and of the temperature from −150 to 150 °C. Only one endothermic peak in each DSC curve is obtained when the salt mole fraction xLi+ is higher than 0.3. However, two endothermic peaks can be observed in the samples beyond a mole fraction composition of 0.3, which clearly indicate the existence of liquid−solid coexistence regime between the eutectic temperature (−82 °C) and the liquidus curve. The solid−liquid equilibrium phase diagram of the LiX−MAc mixtures can be also calculated by using the SLE option within the COSMOThermX software (Figure 2c). Calculations of salts solubility in MAc as well as the solid− liquid equilibrium (SLE) of binary (solvent + salt) mixtures as a function of the composition using the salt solubility and SLE options can be obtained within COSMOthermX software (version 2.1, release 01.08). With these options, COSMOthermX can compute a range of mixtures and search for possible concentrations of solidification. The solid−liquid equilibrium properties (Figure 2c) are calculated from μisolid = μi liquid + RT ln(xi)

(1)

Herein, it should be important to notify that the COSMOthermX SLE calculation assumes that there is a simple eutectic point in the binary mixture. Since COSMOthermX can only calculate compounds in a liquid state, the Gibbs free energy of fusion of the compound, ΔGfus, is taken into account for the solid−liquid equilibrium of a solid compound with a solvent. In other words, COSMOthermX calculates a temperature-dependent free energy of fusion if the compounds enthalpy or entropy of fusion (ΔHfus or ΔSfus, respectively) and melting temperature (Tm) are known. ΔHfus and Tm values were taken from DSC experimental results, as well as from the Aspen 2006 plus database in the case of the selected solvent, and from the literature in the case of the investigated salts.17 Like in a VLE/ LLE binary calculation, it is possible to compute solid−liquid

Figure 2. Thermal analysis of DES. (a) Thermogram of LiTFSI−MAc xLi+ = 0.25. (b) DSC thermograms of LiNO3−MAc mixtures from −150 to 150 °C. (c) Solid−liquid equilibrium phase diagram of the LiX−MAc mixtures calculated by SLE COSMOThermX.

phase equilibria (SLE) for pseudobinary solutions with an ionic liquid or salt phase. Solid−liquid equilibrium, SLE, of investigated DES MAcbased binary mixtures (e.g., MAc + LiTFSI, LiPF6, or LiNO3) was then calculated by using COSMOthermX (SLE/LLE 4035



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Table 1. Conductivity, Viscosity, and Walden Product of DES at xLi+ = 0.25 at Various Temperatures conductivity (mS cm−1)

DES system temp (°C) LiTFSI−MAc LiNO3−MAc LiPF6−MAc a

25 1.35 0.76 1.41

60 5.59 3.25 5.86

Walden product (mS cm−1 Pa−1 s−1)

viscosity (mPa s) 80 9.22 6.32 10.13

25 78.38 107.19 nda

60 22.15 23.31 34.22

80 14.20 12.14 19.73

25 17 7.1 nd

60 252 139 171

80 649 520 513

nd: not determined.

Figure 3. Three-electrode cyclic voltammograms of activated carbon in (a) LiTFSI−MAc (xLi+ = 0.25) amd (b) LiPF6−MAc (xLi+ = 0.25); scan rate 1 mV s−1.

option using melting temperatures and ΔHfus data reported in the literature17) and then compared with experimental data reported by our group previously.18 As shown in Figure 2c, a very low LiX solubility in the MAc is predicted by COSMOthermX at room temperature. As expected from experimental results, the lithium salt solubility in the MAc is strongly affected by the temperature and classical eutectic curve is then obtained for each investigated DES. Interestingly, COSMOthermX predicts, as expected also from experimental results, a strong difference on the Tx diagram of DES-based on LiNO3 salt in comparison with those obtained using LiTFSI or LiPF6 salt. Additionally, COSMOthermX predicts an eutectic point in the MAc-rich region in each case at TE = 229.5 K and xMAc = 0.8334, TE = 241.4 K and xMAc = 0.8352, and TE = 152.8 K and xMAc = 0.5552 in the case of MAc-based DES containing the LiTFSI, LiPF6, and LiNO3 salt, respectively. This solubility difference can be explained by structure and charge distribution differences between selected anion TFSI asymmetric structures and charges highly delocalized, PF6 and NO3 symmetric structures and charges highly localized; the main difference between the two latter anions are their volume and structure differences as well as by their hydrogen-bond abilities to interact with each MAc molecule. However, in comparison with experimental results already reported by our group previously,18 we can conclude that COSMOthermX provides a qualitative prediction of the LiX salt solubility in MAc as a function of the lithium structure and temperature, as these eutectic points are localized experimentally in the MAc-rich region at TE = 201.2 K and xMAc = 0.80, TE = 219.4 K and xMAc = 0.84, and TE = 198.2 K and xMAc = 0.70 in the case of MAc-based DES containing the LiNO3, LiPF6, and LiTFSI salt, respectively. A comparison between experimental data18 with COSMOthermX calculations shows that this methodology can be used to predict qualitatively not only the salt solubility in a solvent but also its general solid−liquid equilibrium trend, which can be very useful to formulate prior any experimental measurement novel and safer electrolytes.

The conductivity of these DES measured in previously work18 ranges from 0.76 mS cm−1 for LiNO3−MAc at 25 °C and reaches 10.13 mS cm−1 at 80 °C for the LiPF6−MAc mixtures. At 25 °C, the conductivity of the LiTFSI−MAc system at xLi+ = 0.25 (e.g., 1.35 mS cm−1) is close to that reported by Chen et al.24 for the LiTFSI−acetamide mixture (e.g., 1.26 mS cm−1), showing that the presence of an extra methyl group on the structure of the solvent does not seem to affect this transport property. This value is seven time higher than for the LiPF6−acetamide mixture at xLi+ = 0.20 (σ = 0.2 mS cm−1)25 and 2 times higher than that of the LiNO3−MAc mixture at same temperature. However, the more expressive parameter in the transport properties is, in fact, the Walden product, denoted herein w, calculated from the knowledge of the conductivity σ and the fluidity η−1 of the medium through which the ions move (w = ση−1) which is relates the ionic mobility. The larger is the value of w, the better is the performance of the electrolyte under electric field, especially at high temperature when both η−1 and σ increase. However, as reported in Table 1, the transport properties of selected DESs (such as the conductivity (σ) and the fluidity (η−1)) are not very high at ambient temperature, while by increasing the temperature up to 80 °C, these properties become more favorable for electrochemical applications, as shown by the calculated Walden products: w = ση−1 (mS cm−1 Pa−1 s−1) (7 < w < 16 at 25 °C and 513 < w < 649 at 80 °C). This “superionicity”17 behavior observed in the case of the investigated DESs explains their favorable role as electrolytes in the supercapacitors systems as depicted below. 3.2. Electrochemical Study. 3.2.2. Three-Electrode Cell Configuration. The cyclic voltammetry (CV) was employed to examine the electrochemical performance of the cells with the selected DESs. All electrochemical measurements were carried out at high temperature (at 60 or 80 °C) due to the relative high viscosity of DESs at room temperature (e.g., 78−100 mPa s). Cyclic voltammetry was carried out using a three-electrode system with Ag wire as pseudo reference electrode at 2, 5, 10, 4036



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Figure 4. (a) Three-electrode cyclic voltammograms of activated carbon in LiTFSI−MAc (xLi+ = 0.25): scan rate 5 mV s−1. (b) Specific capacitance calculated from voltammograms according to eq 2 for ΔE = 2.6 V.

Figure 5. Three-electrode cyclic voltammograms of positive electrode of activated carbon in LiNO3−MAc (xLi+ = 0.25): (a) scan rate v = 2 mV s−1 the rectangular dashed surface represents the double-layer part of capacity; (b) 1st and 100th cycles at scan rate v = 5 mV s−1.

and 20 mV s−1 scan rates. From cyclic voltammetry, the specific gravimetric capacitance (Csp) was derived using

Csp =

Figures 3a and 3b show the CVs recorded with threeelectrode cell assembled with LiTFSI−MAc, and LiPF6−MAc, respectively. Voltammograms reported in Figure 3a (scan rate of 1 mV s−1) visualize the tension variation of each electrode separately in order to decouple the mutual influence between them. From these figures, no peak attributed to the activity of surface groups can be observed. This would seem to indicate that, at this scan rate and voltage tension, the lithium ions cannot be inserted in the microporosity of carbon. The probable pseudocapacitive reactions of MAc with functions groups of carbon are not involved in theses condition probably because each MAc molecule close to the carbon surface is not probably free to interact with the electrode material. Figure 4a shows CVs recorded at 80 °C, with the LiTFSI−MAc system, in this case, two reversible peaks of lithium-ion insertion (1)/ (1′) and (2)/(2′) are observed beyond a cell voltage of 2.2 V, while at operating voltage ΔE = 2 V (blue curve), no peak is observed. Specific capacitance Csp calculated according to eq 2 is shown in Figure 4b for a tension voltage ΔE = 2.8 V. Based on which, very high Csp values are obtained in comparison with similar system using aqueous or organic electrolytes presenting Csp data generally up to 380 F g−1. By examining the CVs curves obtained in the case of LiNO3based DES (Figure 5), a fast lithium ion diffusion into the porosity is demonstrated (Figure 5a shows the CV curves of the samples at a scan rate of 2 mV s−1, for example). The microporous structure still facilitates an easy and smooth movement of the electrolyte ions, presenting no obvious

2 ∫ i dt ΔEm

(2)

where ΔE is the operating voltage, i is the discharge current, and m is the mass of active material (carbon) of working electrode.26 By assuming that the charge is evenly distributed between two capacitors in series (1/C = 1/C1 + 1/C2) with C1 = C2 = Csp, then we can multiply by a factor of 2 to account for the two electrodes’ setup. All values for the current or capacitance were normalized by the weight of the carbon material to enable a direct comparison with conventional organic or aqueous electrolytes. Recently, several experimental and theoretical works refer an universal increase of the EDLCs capacitance using small pore sizes-based materials (microporous carbons) even by using aqueous, organic, or ionic liquid-based electrolytes.27 However, the experimental study done by Centeno et al. reports a regular (i.e., pore-sizeindependent) normalized capacitance of carbon pores with a size of 0.7−15 nm, using the same electrolyte of [TEA][BF4]ACN.28 This challenging experimental observation raised answer concerning the existence of the enhancement of micropore capacitance and how the capacitance changes over a wide range of pore size, including the “anomalous increase” regime. The answer may be linked to the nature of the ions in the electrolyte and their interaction with the solvent in solution. 4037



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Figure 6. Typical impedance plots of positive AC electrode in the three electrode cells recorded at 80 °C in the frequency ranging from 100 kHz to 10 mHz with LiNO3−MAc (xLi+ = 0.25) and LiTFSI−MAc (xLi+ = 0.25) as electrolyte before cycling (a) and after cycling (b).

frequency region or Warburg region, represented by a 45° line, is rather due to the frequency dependent resistance R(ω) associated with the ionic resistance in the porosity;30 (iii) the low-frequency region corresponds to purely capacitive phenomena and is represented by a quasi-vertical line;31 the associated resistance, named equivalent diffusion resistance (EDR), is obtained from the intercept of the line with the Z′axis. Figure 6 compares the Nyquist plots for positive AC electrode in a supercapacitor assembled with three-electrode cells recorded at 80 °C in the frequency ranging from 100 kHz to 10 mHz with LiNO3−MAc (xLi+ = 0.25) or with LiTFSI− MAc (xLi+ = 0.25) as electrolyte before cycling (Figure 6a) and after cycling (Figure 6b). The relative small Warburg region obtained in both cells at the positive electrode (Figure 6a,b) indicates a low ion diffusion impedance, driving in fact a lower obstruction of the ions movement and a better charge propagation in the mesoporous AC electrodes for the formation of double layers. Before cycling, the ESR in the case of LiTFSI−MAc electrolyte (ESR = 6 Ω) is lower than that of LiNO3−MAc (ESR = 9 Ω) as expected by the relative low conductivity of LiNO3-based DES, while the EDR data are quite similar for both electrolytes. After cycling, a significant enhancement in the resistance on ionic resistance in the carbon porosity is showed by an increase of ESR and EDR values, which indicates the insertion of anions into the porosity of the activated carbon electrode. This increase is more pronounced in the case of the TFSI-based electrolyte; interestingly, this can be linked to the fact of this anion is 4 times larger than that of NO3 anion (e.g., VCOSMO(TFSI) = 219.4 Å3 ; VCOSMO(NO3) = 56.7 Å3).18 A significant enhancement in the EDR resistance when LiTFSI−MAc electrolyte can be explained by the saturation of carbon microporosity (see Scheme 1) and can prevent the cyclability of the system by increasing its impedance. This aspect will be further discussed in the Cycling Tests section. Furthermore, the ESR values obtained here in all DES systems are lower than those reported using pure aprotic ionic liquids (AILs) containing the TFSI anion, such as [MPPip][TFSI], [MPPyr][TFSI], and [EMIm][TFSI], which display ESR values of 58, 30.2, and 15.4 Ω, respectively, but much higher than those reported in electrolytes containing AILs mixed with

polarization (distortion from the standard rectangle) in the CV curve. The importance of the electrosorption of lithium ions and its reversibility to the specific capacitance can be illustrated clearly by Figure 5a. The area of the CV curve of the electrochemical active MAc (black curve in Figure 5a) is 25% (no-shaded area of Figure 5) higher than that of the inactive Li+ (dashed rectangle in Figure 5a). In the case of the AC/LiNO3/ MAc/AC supercapacitor system recorded at for the positive electrode at 0.8 V, the observed changes in the intensity of the adsorbed ions corresponds, in fact, to changes in the populations of adsorbed NO3 species. The dissymmetric form of CV suggests that when the tension voltage increases, the anions population in the positive electrode is approximately constant, while the cations population in the negative electrode increases of indicating that the charge process is not dominated by the adsorption of counterions, instead rearrangements and the ejection of cations achieved through short scale from the micropores. This phenomenon is determined by the difference of ions size. At high cell voltages (greater than 0.5 V), the abrupt decrease in the adsorbed anion intensity in the positive electrode, which can be combined with the continued loss of anions from the electrode, suggests that both counterion adsorption and ions ejection are important factors. This behavior is essentially observed at the beginning of cycling as shown in Figure 5b (red curve) during the first cycle. However, after several cycles, the saturation of the microporosity of carbon explains the more rectangular form of CVs (blue curve in Figure 5b). Similar observations were studied by Richey et al. and explain by ion dynamics in porous carbon electrodes for porous nanosized carbide-derived carbons (CDCs) and nonporous onion-like carbons (OLCs) using in-situ infrared spectroelectrochemistry.29 3.2.1. Electrochemical Impedance Spectroscopy (EIS) on Two-Electrode Cells. Electrochemical impedance spectroscopy (EIS) measurements were conducted at open circuit voltage (OCV) with a sinusoidal signal of 5 mV over the frequency range from 1 mHz to 100 kHz. In general, the Z″ = f(Z′) Nyquist plot of an EDLC comprises three domains: (i) the semicircle at high frequencies is representative of the contact resistance between the active material and the current collector; the equivalent resistance (sum of resistances of active material, current collectors, electrolyte in the separator and contact resistance) is the value obtained at ω → ∞; (ii) the middle4038



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resulting from the fluidity of the electrolyte during voltage reversal. From Figures 7b and 7d, it is clear that the temperature effect on the cyclability of the selected DES is not a predominant factor (with the respect of temperature range investigated). In fact, the CV curves at 60 and 80 °C are quite similar, while the more rectangular aspect of the CV at higher temperature indicates a better mobility of ions in the material. This result can be linked to the Walden product (Table 1) change from 60 and 80 °C, which shows that the transport properties of the DES-based electrolytes are improved by increasing the temperature from 60 to 80 °C. This indicates that the transport properties in the bulk electrolyte and within the microporosity of the carbon are mainly different. Galvanostatic charge−discharge measurements were performed on the samples to obtain more detailed information on the electrochemical capacitance. Figure 8a shows typical charge− discharge curves of two-electrode cells with active carbon on LiNO3−MAc (xLi+ = 0.25). The discharge cycle was used to determine the specific capacitance (Csc) that was calculated using Csp = 2i/[m(dV/dt)], where m is the mass of one electrode and dV/dt is the slope of the discharge curve starting from the bottom of the IR drop to half of the high potential, Vmax.26 The charge−discharge measurements were performed at different current densities to assess the rate performance that is of particular importance for practical applications. As can be seen from Figure 8b, the LiNO3 retained 87% of their capacitances as the current density increased from 0.1 to 0.4 A g−1. The good rate performance suggests rapid ions transport characteristics in these system, which is associated with the good adequacy of ions size with porous structure in the AC in the case of NO3-based DES. Figures 8c and 8d present electrochemical stability (capacitance, Figure 8c, and

Scheme 1. Illustration of Ions in Negative Electrodes Consisting of Porous Activated Carbon Particles before and after Polarization

propylene carbonate (PC) or acetonitrile (ACN) with values close to 4−5 Ω.32 3.2.3. Cycling Tests, Temperature, and Current Density Effect. The symmetric AC/AC two-electrode cells were applied to evaluate the performance of the DES electrolytes within the voltage range from 0 to 1.8, 2.0, and 2.8 V for LiNO3−MAc, LiPF6−MAc, and LiTFSI−MAc, respectively. Figures 7a and 7c show the cyclic voltammograms (CVs) recorded at a scan rate from 2 to 20 mV s−1 at 80 °C for the AC electrodes with LiNO3−MAc and LiTFSI−MAc, respectively. The nearly rectangular shape of the CVs in the voltage range from 0 to 2 V characterizes typical capacitive properties. The less rectangular shape in the case of LiTFSI−MAc reveals a worse charge propagation although its conductivity is higher than that of LiNO3−MAc, which reflects obstruction of the TFSI ions movement and better charge propagation for LiNO3 based electrolyte in the pores of AC electrodes.29 The effect of the scan rate on this rectangular shape is not clearly visible by comparing the curves from V = 2 to 20 mV s−1 (Figure 7a). In fact, only the shape of the voltammograms aspect is changed

Figure 7. Cyclic voltammograms of two-electrode cells with active carbon on LiNO3−MAc (xLi+ = 0.25) and LiTFSI−MAc (xLi+ = 0.25) as a function of scan rates (a, c) and temperature (b, d). 4039



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Figure 8. Galvanostatic charge−discharge curves (a) and capacitances calculated from eq 2 (b) at current density of 100, 200, and 400 mA g−1 and (c) at current density of 200 mA g−1 for 3000 cycles. (d) Efficiency for 3000 cycles for activated carbon electrode with LiNO3−MAc (xLi+ = 0.25) as electrolyte at 80 °C.

Figure 9. Capacitances calculated according to eq 2 from charge−discharge curves (a) and efficiency (b) at current density of 200 mA g−1 for activated carbon electrode with LiNO3−MAc, LiPF6−MAc, and LiTFSI−MAc (xLi+ = 0.25) as electrolyte at 80 °C.

efficiency, Figure 8d) of the LiNO3−MAc (xLi+ = 0.25) during 3000 charge discharge cycles of two-cell system by using the galvanostatic method at current density of 200 mA g−1 at 80 °C. The comparative electrochemical stability of the DES-based electrolytes with lithium salt has been studied during 600 charge−discharge cycles of two-cell system by using the galvanostatic method at current density of 200 mA g−1. Figure 9a clearly shows the collapse capacity after 100 cycles, especially for systems running with DES electrolytes based on LiTFSI;

however, the efficiency of systems remains correct, close to 90%. These observations suggest that there is no decomposition of the electrolyte or of materials but only an increase of the saturation of the material porosity inducing in fact a steady decrease of the specific surface of activated carbon. The imbalance carbon electrodes versus the size of the ions with important lithium insertion at the negative and the degree of the saturation by anions at the positive, in fact, prevent the system to be operational. Moreover, as shown Figure 9b, during the cycling the efficiency in each electrolyte slowly increases. 4040



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Most likely, since the three electrolytes are quite viscous, the observed behavior is due to an improvement in the wetting over the cycles. This kind behavior has been already observed in many IL-based electrolytes at room temperature, and it is therefore not a surprising result.18 Herein, after 100 cycles, the efficiency reaches values close to 99.9%. The encouraging capacitance values obtained during the first cycles (up to 380 F g−1), as shown in Figure 4b, allow to consider performance improvement of the system by balancing the masses of the electrodes in order to avoid the negative becoming a limiting electrode.33 Work is in progress to decrease the resistance by means of balancing masses of carbon electrode materials in order to improve the energy density delivered by these systems. This should render DES even more attractive as environmental friendly electrolytes for energy storage systems at high temperature.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax (33)247367360; Tel (33)247366951 (M.A.). Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS In this study three deep eutectic solvents based on a lithium salt (LiNO3, LiPF6, or LiTFSI) and the N-methylacetamide were formulated and their physicochemical properties were characterized. Each DES was found to be liquid at ambient temperature in the case of mole fraction xLi+ = 0.25, electrochemically and thermally stable up to 80 °C toward the activated carbon. At 80 °C, as these DES have a good conductivity up to 10 mS cm−1 (LiTFSI−MAc) and a relatively low viscosity close to 12 mPa s (LiNO3−MAc), they can be investigated as potential electrolytes in electrochemical devices. From electrochemical studies, it appears that DES can be operated up to 2.8 V on the activated carbon in a two-electrode cells. Herein, we demonstrated by characterizing separately positive and negative electrodes that there is an insertion of lithium ions in the AC electrode material. The access of ions to the microporosity of electrode material is manifested by a higher initial capacity up to 240 F g−1. However, the obstruction of the carbon pores by the electrolyte ions prevents the correct cycling symmetrical systems AC//AC based on which the negative becomes the limiting electrode. This study shows that these electrolytes are highly promising as a substitute for organic electrolytes to increase the operating voltage up to 2.8 V and for temperature up to 80 °C. Moreover, selected DESs constitute the models electrolytes composed by nonsolvated ions that can be adsorbed in the well microporosity of the porous material. A more comprehensive study on other electrode materials such as onion-like carbons (OLCs), carbon nanotubes (CNTs), or carbide-derived carbons (CDCs) is actually underway and will be presented in the near future.

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ACKNOWLEDGMENTS

This research was supported by ADEME-CEA and Region Centre through the financial support for postdoctoral position in academic initiative project. Many thanks are expressed to Batscap for providing the electrode material. 4041



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Deep Eutectic Solvents Based on N-Methylacetamide and a Lithium

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