physical and electrochemical properties of binary ionic liquid mixtures: (1−x) pyr14tfsi–(x)...

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Electrochimica Acta 60 (2012) 163– 169

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Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

Physical and electrochemical properties of binary ionic liquid mixtures: (1 − x)PYR14TFSI–(x) PYR14IM14

M. Montaninoa,∗, M. Morenoa, F. Alessandrinia, G.B. Appetecchia,∗, S. Passerinib, Q. Zhouc, W.A. Hendersonc

a ENEA, Agency for New Technologies, Energy and Sustainable Economic Development, UTRINN-IFC, Via Anguillarese 301, Rome 00123, Italyb Westfälische Wilhelm Universität, Institut fur Physikalische Chemie, Corrensstr. 28/30, D48149 Münster, Germanyc ILEET (Ionic Liquids & Electrolytes for Energy Technologies Laboratory), Department of Chemical & Biomolecular Engineering,North Carolina State University, Raleigh, NC 27695, USA

a r t i c l e i n f o

Article history:Received 20 July 2011Received in revised form 7 November 2011Accepted 7 November 2011Available online 15 November 2011

Keywords:Ionic liquid mixturesN-Butyl-N-methylpyrrolidiniumbis(trifluoromethanesulfonyl)imideN-Butyl-N-methylpyrrolidinium(trifluoromethanesul-fonyl)(nonafluorobutanesulfonyl)imide

a b s t r a c t

The synergistic effect on the physical and electrochemical properties derived from mixingbis(trifluoromethanesulfonyl)imide-based, TFSI−, and (trifluoromethanesulfonyl)(nonafluorobutane-sulfonyl)imide-based, IM14

−, ionic liquids (ILs) with a common N-butyl-N-methylpyrrolidinium, PYR14+,

cation has been examined. The incorporation of even small mole fractions (x ≤ 0.3) of PYR14IM14 intoPYR14TFSI is capable of strongly hindering the ability of the mixtures to crystallize. The lowering ofthe melting point caused by PYR14IM14 addition, in conjunction with the relatively high conductivity ofPYR14TFSI, results in an ionic conductivity for all of the mixtures approaching 10−4 and 10−3 S cm−1 at−20 ◦C and 20 ◦C, respectively. The PYR14TFSI/PYR14IM14 binary mixtures may therefore be appealing forelectrolyte applications in electrochemical devices operating at low temperatures.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Ionic liquids (ILs) are widely considered as possible replace-ments for volatile and hazardous organic solvents to realize saferelectrochemical devices such as supercapacitors [1–4] and lithiumbatteries [5–8]. Some ILs are characterized by desirable propertiessuch as non-flammability, negligible vapor pressure, high chemical,thermal and electrochemical stability, and a high ionic conductiv-ity at ambient temperature and above. However, they also tendto have a high viscosity, especially at low temperatures, and theionic conductivity typically drops off rapidly with the rapid increasein viscosity. Conversely, electrochemical devices for sub-ambienttemperature applications require fast ion transport. Unfortunately,no single IL reported thus far matches the combination of suitableelectrochemical characteristics with a reasonable conductivity (andviscosity) at low temperatures.

Previous work has demonstrated that ILs can be favorably com-bined to give mixtures with enhanced ionic transport propertiesand electrochemical stability [9–11], while also providing excellentcompatibility with carbonaceous electrodes [7,8,12]. Generally, ILs

∗ Corresponding authors.E-mail addresses: [email protected] (M. Montanino),

[email protected], [email protected] (G.B. Appetecchi).

based on relatively small anions (i.e., lower molecular weight werefound to have a higher ionic conductivity (e.g., resulting from theirlower viscosity) than those with larger anions [13] whereas ILswith asymmetric anions often exhibit a lower melting point thanthose with symmetric anions resulting from unfavorable ion pack-ing within the crystal lattice [13].

In this work we have combined two different ILs sharing thecation N-butyl-N-methylpyrrolidinium (PYR14

+), but with two dif-ferent structurally similar anions, bis(trifluoromethanesulfonyl)imide (TFSI−) and (trifluoromethanesulfonyl)(nonafluorobutane-sulfonyl)imide (IM14

−) (Scheme 1). PYR14TFSI was selected becauseof its high ionic conductivity in the molten state (1.8 × 10−3 S cm−1

at 20 ◦C) in conjunction with a wide electrochemical stability win-dow [13], whereas PYR14IM14 displays the interesting propertyof not being readily crystallizable [13]. The physical and electro-chemical properties of PYR14TFSI/PYR14IM14 mixtures over a widetemperature range are presented and discussed here.

2. Experimental

2.1. Sample preparation

The PYR14TFSI and PYR14IM14 ILs were synthesized througha procedure developed at ENEA and described in detailselsewhere [13,14]. The chemicals N-methylpyrrolidine (97 wt.%),

0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2011.11.030


164 M. Montanino et al. / Electrochimica Acta 60 (2012) 163– 169

Scheme 1. Chemical structure of PYR14TFSI and PYR14IM14 ILs.

1-bromobutane (99 wt.%) and ethyl acetate (ACS grade, >99.5 wt.%)were purchased from Aldrich and purified (with the exceptionof ethyl acetate) by passing them through activated carbon(Aldrich, Darco-G60) and alumina (acidic, Aldrich Brock-mann I). Lithium bis(trifluoromethanesulfonyl)imide, LiTFSI(99.9 wt.%, battery grade), and acidic (trifluoromethanesul-fonyl)(nonafluorobutanesulfonyl)imide, HIM14 (59 wt.% solutionin water), were purchased and received as a gift, respectively, from3 M and used as-received. Deionized H2O was obtained with a Mil-lipore ion-exchange resin deionizer. The binary mixtures (1 − x)PYR14TFSI–(x) PYR14IM14 (where x represents the PYR14IM14mole fraction) were prepared by blending the starting ILs in theappropriate proportions.

A screening of impurities in the IL samples was run by X-rayfluorescence spectroscopy using a Philips PW 2404 spectrometer.In addition, the concentration of specific cations (Li and Fe) waschecked by atomic absorption analysis (SpetcrAA Mod. 220 atomicabsorption spectrometer). The water content was measured usingthe standard Karl Fischer method. The titrations were performedby an automatic Karl Fischer coulometer titrator (Mettler ToledoDL32) in a dry room (R.H. < 0.1%) at 20 ◦C. The Karl Fischer titrantwas a one-component reagent (Hydranal 34836 Coulomat AG) pur-chased from Aldrich.

The synthetic route utilized resulted in clear, colorless and odor-less PYR14TFSI and PYR14IM14 ILs with an overall yield higher than85 mol% and a water content below 2 ppmw. Contents lower than100 ppmw were found for Cl, Si, Mo, Al, Na and Ni, whereas Fe andLi were found to be lower than 40 and 2 ppmw, respectively.

2.2. Thermal measurements

DSC measurements were performed using a TA InstrumentsQ1000 differential scanning calorimeter with liquid N2 cooling.The instrument was calibrated with cyclohexane (solid–solid phasetransition at −87.06 ◦C, melt transition at 6.54 ◦C) and indium (melttransition at 156.60 ◦C). Hermetically sealed Al sample pans wereprepared in a N2 glove box. The samples were cooled from ambi-ent temperature down to −150 ◦C and then heated (5 ◦C min−1) tohigher temperature (i.e., up to −15 ◦C), held for 10 min, then cooledand the cycle repeated with the high temperature being slightlylower than for the preceding cycle. This has been found to be themost effective means of crystallizing ILs with slow crystallizationkinetics in the DSC pans.

2.3. Ionic conductivity measurements

All electrochemical tests were performed in the dry-room. Thesample mixtures were loaded in sealed glass conductivity cells(AMEL 192/K1, assembled in the dry room) equipped with twoporous platinum electrodes (cell constant equal to 1.0 ± 0.1 cm−1).In order to fully crystallize the samples, the cells were immersedin liquid N2 for a few seconds and then transferred into a cli-matic test chamber (Binder GmbH MK53) at −40 ◦C. After a few

minutes of storage at this temperature, the amorphous solidsturned back into liquids. The procedure was repeated several timesto favor, if possible, the crystallization of the mixtures. In previouswork, it was demonstrated that incomplete crystallization of ILsresults in non-equilibrium behavior which dramatically influencesthe conductivity measurements, as well as the thermal properties[15]. However, any of the samples did not solidify (at −40 ◦C) evenafter repeated quenching steps. Following the quenching proceduredescribed above, the sample mixtures were stored at −40 ◦C for atleast 18 h before starting the measurements. The ionic conductivitywas measured with an AMEL 160 conductivitymeter by running aheating scan at 1 ◦C h−1 from −40 ◦C to 100 ◦C. The entire setup wascontrolled by software developed at ENEA.

2.4. Viscosity measurements

Viscosity measurements were conducted using a HAAKERheoStress 600 rheometer located in the dry room. The tests wereperformed from −20 ◦C to 80 ◦C (1 ◦C min−1 heating rate) in the100–2000 s−1 rotation speed range. Measurements were takenafter 10 ◦C steps.

2.5. Density measurements

The density measurements were performed from 90 ◦C to 20 ◦Cin 10 ◦C step using a densimeter (Mettler Toledo DE40) located inthe dry room. The samples were previously degassed under vacuumat 70 ◦C overnight to avoid bubble formation during the cooling scantests.

2.6. Electrochemical stability

The electrochemical stability window (ESW) of the mixtureswas evaluated by linear sweep voltammetry (LSV) at 5 mV s−1. Asealed, three-electrode, glass micro-cell, described in details else-where [16], was used for the LSV tests. The cell was loaded with asmall amount of sample (about 0.5 ml). A glass-sealed, platinumworking microelectrode (active area equal to 0.78 mm2) and aplatinum foil counter electrode (about 0.5 cm2) were used. The ref-erence electrode was a silver wire immersed in a 0.01 M solutionof AgCF3SO3 in PYR14TFSI, separated from the cell compartmentwith a fine glass frit. This reference electrode was found to be sta-ble for at least three weeks. High purity argon (3 ppmv water and2 ppmv oxygen) was flown over the samples for 30 min before thestart of the tests. The gas flow was continued during the exper-iment. Separate LSV tests were carried out on each IL sample todetermine the cathodic and anodic electrochemical stability limits.The measurements were run scanning the cell potential from theopen circuit potential (OCP) towards more negative (cathodic limit)or positive (anodic limit) potential, respectively. Clean electrodesand fresh samples were used for each test. To confirm the resultsobtained, the LSV tests were performed at least twice on differentfresh samples of each investigated composition. The measurementswere performed at 20 ◦C using a Schlumberger (Solartron) Electro-chemical Interface (model 1287) controlled by software developedat ENEA.

3. Results and discussion

The DSC heating traces for the (1 − x) PYR14TFSI–(x) PYR14IM14mixtures are shown in Fig. 1 (the thermal cycling features forcrystallization are not reported). The pure PYR14TFSI (x = 0) sam-ple melts near −7 ◦C (Table 1) with no other transitions detected[13,15]. In contrast, pure PYR14IM14 (x = 1) could not be crystallizedin the DSC pans, remaining liquid even at very low tempera-tures. This is likely ascribed to the largely unfavorable packing of


M. Montanino et al. / Electrochimica Acta 60 (2012) 163– 169 165

Table 1Physical properties of (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures. The density (d) and the viscosity (�) values are referred to 20 ◦C, whereas the conductivity (�) is reportedalso at −20 ◦C.

PYR14IM14 mole fraction (x) d (g cm−3) � (mPa s) � (S cm−1)

20 ◦C −20 ◦C

0 1.399 ± 0.002 100 ± 10 (1.8 ± 0.2) × 10−3 (4.0 ± 0.4) × 10−8

0.1 1.413 ± 0.002 130 ± 20 (1.5 ± 0.2) × 10−3 (2.1 ± 0.2) × 10−6

0.2 1.427 ± 0.002 150 ± 20 (1.3 ± 0.1) × 10−3 (7.5 ± 0.8) × 10−5

0.3 1.440 ± 0.002 160 ± 20 (1.0 ± 0.1) × 10−3 (5.4 ± 0.5) × 10−5

0.4 1.449 ± 0.002 n.a. (0.80 ± 0.08) × 10−3 (4.1 ± 0.4) × 10−5

0.5 1.462 ± 0.002 260 ± 30 (0.71 ± 0.07) × 10−3 (3.6 ± 0.4) × 10−5

0.6 1.473 ± 0.002 n.a. (0.60 ± 0.06) × 10−3 (2.1 ± 0.2) × 10−5

0.7 1.483 ± 0.002 350 ± 40 (0.50 ± 0.05) × 10−3 (1.6 ± 0.2) × 10−5

0.8 1.494 ± 0.002 n.a. (0.40 ± 0.04) × 10−3 (1.1 ± 0.1) × 10−5

1.0 1.512 ± 0.002 550 ± 50 (0.28 ± 0.03) × 10−3 (7.6 ± 0.5) × 10−6

the highly asymmetric IM14− anion which remarkably lowers the

lattice energy of the IL material [13]. As evidenced from Fig. 1,the progressive addition of PYR14IM14 hinders (in lower concen-tration) or inhibits (in higher concentration) the crystallizationof the binary mixtures. Such behavior is highlighted by a melt-ing peak shift from −7 ◦C to −10 ◦C on passing from x = 0 (purePYR14TFSI) to x = 0.1. In addition, the phase transition peak is seento broaden upon PYR14IM14 incorporation. For a PYR14IM14 molefraction equal to or larger than 0.2, the binary mixtures could notbe crystallized in the DSC pans despite repeated cycling at lowtemperatures, e.g., the melting peak disappears, and no featurewith the exception of the glass transition was observed. An anal-ogous behavior was previously observed in similar mixed anion(i.e., TFSI− and FSI−, where the latter is the bis(fluorosulfonyl)imideanion whereas PYR14

+ is the common cation) IL binary systemswhich were not able to be crystallized [17]. The absence of a crys-talline phase for x ≥ 0.2 (Fig. 1) indicates that the IL mixtures remainamorphous, even at very low temperatures. This suggests that thehighly asymmetric anion IM14

− prevents the crystallization of the

Fig. 1. DSC heating trace of (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures. Scan rate:5 ◦C min−1.

mixtures, instead resulting in the solidification of the mixtures ina glassy state, at very low temperatures. The glass transition tem-perature (Tg) does increase somewhat with increasing PYR14IM14mole fraction (Fig. 1), e.g., from −83 ◦C (x = 0.2) to −75 ◦C (x = 1),thus progressively approaching the behavior of the pure PYR14IM14material.

The ionic conductivity vs. temperature behavior of the IL mix-tures is illustrated in Fig. 2. Error bars from repeated measurementsfall within the data markers. The pure PYR14TFSI (x = 0) materialdisplays an onset for increasing conductivity near −6 ◦C followed

Fig. 2. Ionic conductivity vs. temperature behavior of (1 − x) PYR14TFSI–(x)PYR14IM14 mixtures (panel A). In panel B the plot of selected (1 − x) PYR14TFSI–(x)PYR14IM14 sample mixtures (see legend) is magnified. The error bar (when notshown) falls within the data markers.



by a steep rise of more than four orders of magnitude. This increaseis due to the melting of the IL in good agreement with the DSC trace(Fig. 1). From −40 ◦C to the melting, the PYR14TFSI sample exhibitsa stable but low conductivity (e.g., around 4 × 10−8 S cm−1), typi-cal of a solid crystalline material. The incorporation of PYR14IM14up to a mole fraction equal to 0.1 results in a progressive increasein conductivity at a temperature well below the DSC meltingpoint. This behavior is better highlighted in panel B which mag-nifies the conductivity vs. temperature plot of selected (1 − x)PYR14TFSI–(x) PYR14IM14 mixtures (0 ≤ x ≤ 0.2). However, no glasstransition is evident in Fig. 1 for the x = 0.1 mixture, this indicatingthat a large fraction of the sample is still crystalline. Our hypoth-esis is that PYR14IM14 remains liquid and may even interact withsome of the PYR14TFSI (solid) preventing it also from crystallizing.Thus, the progressive increase in conductivity is due to this liquidphase (PYR14IM14) dispersed in the crystalline solid (PYR14TFSI).An analogous nanostructural organization was previously postu-lated in similar binary IL mixtures composed of a common cation(3-methyl-1-pentylimidazolium, MPI) and anions which differedgreatly in size, i.e., Br− and TFSI− [18]. Optical Kerr Effect Spec-troscopy measurements revealed that the MPIBr/MPITFSI mixturesare nanostructurally organized into ionic networks characterizedby block co-networks (e.g., Br− · · ·· · · MPI+ · · ·· · · Br− or TFSI− · · ·· · ·MPI+ · · ·· · · TFSI−) instead of “random co-networks” (Br− · · ·· · · MPI+

· · ·· · · TFSI−) as found for anions that are nearly the same in size.The conductivity data for the x = 0.2 sample (Fig. 2) indicate thatsome of the sample crystallized, in contrast with the DSC data(Fig. 1). This is likely due to the difference in the amount of mate-rial used for the two measurements (<15 mg for DSC vs. <2 g forconductivity). Crystallization relies on nucleation of the PYR14TFSIfollowed by growth of the crystals. Given the slow crystallizationkinetics, the probability of forming a nucleus of critical size is muchhigher in larger samples. In addition, the crystallized samples wereheld at −40 ◦C for 18 h prior to the conductivity measurementswhereas the DSC samples were only cycled for several hours undera variety of low temperature cycling conditions. The conductiv-ity of the sample mixtures with x > 0.2 follows the expected trendfor liquid/amorphous electrolytes which is often modeled by VTFbehavior [19–21]. In the molten state, the ionic conductivity of the(1 − x) PYR14TFSI–(x) PYR14IM14 mixtures is seen to progressively,even if slightly, decrease with the increase in the average molecularweight of the mixtures.

The effect on the low temperature ion mobility is more evidentin Fig. 3 which reports the ionic conductivity vs. PYR14IM14 molefraction dependence in the temperature range from −40 ◦C to 0 ◦C.When the samples are fully molten, the conductivity of the binarymixtures decreases linearly with increasing the PYR14IM14 molefraction (see also Fig. 4). In particular, for PYR14IM14 mole frac-tions ranging from 0.1 to 0.3 the conductivity values are at least oneorder of magnitude higher than that of PYR14IM14 and more thantwo orders of magnitude higher than that of PYR14TFSI (the lattersample crystallizes). This interesting effect is due to ionic confusionpresent in the binary mixtures (e.g., resulting from mixing two dif-ferent ILs) that shifts the crystallization and, therefore, the meltingpoint to a much lower temperature [11]. For instance, a conductiv-ity approaching 10−4 S cm−1 is achieved at −20 ◦C (x = 0.2) whereasappreciable conduction values (>10−5 S cm−1) are detected even ata very low temperature (−30 ◦C) for x ranging from 0.3 to 0.4. Table 1reports the conductivity values, obtained at 20 ◦C and −20 ◦C, of theIL mixtures investigated.

Viscosity measurements were performed to better understandthe conduction phenomena. Fig. 4 shows the dependence of theviscosity (panel A) and resistivity (panel B, for comparison pur-pose) of the mixtures with respect to the PYR14IM14 mole fraction.The measurements were carried out in an interval ranging froma temperature slightly above the melting point to 80 ◦C. The pure

Table 2Activation energy (Ea) values obtained from the conductivity (*) and inverse of vis-cosity (**) VTF plots depicted in Fig. 5 for (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures.The slope of the Ea vs. PYR14IM14 mole fraction plots of Fig. 6 are also reported.

PYR14IM14 mole fraction (x) Ea(*) (kJ mol−1) Ea(**) (kJ mol−1)

0 22.4 ± 0.9 28.4 ± 0.40.1 24 ± 2 29.9 ± 0.90.2 24.9 ± 0.9 30.8 ± 0.90.3 27.4 ± 0.9 32.4 ± 0.90.5 30 ± 2 34.1 ± 0.90.7 32.4 ± 0.9 36.6 ± 0.91 34.9 ± 0.9 41 ± 2

Slope (Ea vs. PYR14IM14 fraction) 12.7 ± 0.8 12.3 ± 0.4

components, as well as the (1 − x) PYR14TFSI–(x) PYR14IM14 mix-tures, exhibited a Newtonian behavior (not shown) above 20 ◦C.However, at lower temperatures, slight viscosity variations withthe rotational speed were observed for all the investigated materi-als. The logarithm of viscosity, as well as the logarithm of resistivity,was found to linearly increase with the PYR14IM14 mole fraction(Fig. 4 and Table 1) through the whole temperature range inves-tigated. The slope of both the resistivity and the viscosity trend isseen to decrease with the temperature increase, due to the pro-gressively decreasing differences among the conductivity valuesexhibited from the mixture samples on passing from low to hightemperatures.

Fig. 5 illustrates the temperature dependence of the ionic con-ductivity (panel A) and the inverse of viscosity (panel B) reportedas VTF plots. Only the data obtained at ≥20 ◦C, i.e., where the ILmixtures showed Newtonian behavior, are reported. The Tg val-ues of the x = 0 and x = 0.1 samples (not detectable in Fig. 1) wereobtained from thermal scans run on non-crystallized materials.The VTF diagrams exhibit a linear behavior with a slope progres-sively increasing with increasing content of the less conductive ILcomponent, i.e., PYR14IM14. The activation energies (Ea), calculatedfrom the slope of the plots of Fig. 5, are reported in Table 2 anddepicted as a function of the PYR14IM14 mole fraction in Fig. 6.It is interesting to note that the Ea values, obtained from bothconductivity and viscosity data, depict linearly increasing trends(with the increase of the more viscous PYR14IM14 material) withvery similar slopes, e.g., 12.7 ± 0.8 kJ mol−1 vs. 12.3 ± 0.4 kJ mol−1

(Table 2). These results highlight that even in binary IL mixtures

Fig. 3. Ionic conductivity vs. PYR14IM14 mole fraction of (1 − x) PYR14TFSI–(x)PYR14IM14 mixtures at different temperatures (see legend). The error bar (whennot shown) falls within the data markers.



Fig. 4. Viscosity (panel A) and resistivity (panel B) dependence of (1 − x)PYR14TFSI–(x) PYR14IM14 mixtures as a function of the PYR14IM14 mole fractionat different temperatures. The error bar (when not shown) falls within the datamarkers.

the conductivity is directly correlated with the viscosity, i.e., theion movement is mostly affected by viscous drag. The progressivedecrease in conductivity with increasing fraction of IM14

− is likelyattributed to an analogous linear increase in viscosity, affected byion steric hindrance, resulting in a higher ionic resistance of the ILblends [22,23].

The relationship between conductivity and viscosity,as previously proposed by Angell and co-workers for ILs[24,25], may be also described on the basis of the Waldenrule:

�� = k (1)

where � is the molar conductivity, � the viscosity and k is atemperature dependent constant. In particular, the Walden rulerepresents a qualitative approach for investigating the “ionicity” ofILs [26]. Fig. 7 plots the log � vs. log �−1 for the (1 − x) PYR14TFSI–(x)PYR14IM14 mixtures. The solid straight line (through the origin) of

Fig. 5. VTF plot of the ionic conductivity (panel A) and the inverse of viscosity (panelB) for (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures. The error bar (when not shown) fallswithin the data markers.

Fig. 7 is for a 0.01 N KCl aqueous solution; this system, known tobe fully dissociated and to have ions of equal mobility [24], wasused as a calibration point (e.g., ideal Walden line). The (1 − x)PYR14TFSI–(x) PYR14IM14 mixtures are seen to fall close below theideal line, suggesting that the mixtures mostly consist of indepen-dently mobile ions [25]. For instance, it has been suggested that aquasi-ideal lattice for an IL system is characterized by an uniform(although non-periodic) distribution of positive charges aroundthe negative ones [27–29]. The (1 − x) PYR14TFSI–(x) PYR14IM14mixtures are located near the top right-hand corner of the dia-gram, corresponding to the most favorable conditions for ILs[24] since high conductivity is combined with low viscosity.Finally, no relevant difference is observed with the progressiveincrease of the content of the asymmetric IM14

− anion, indicat-ing a similar “ionicity” for the (1 − x) PYR14TFSI–(x) PYR14IM14mixtures.



The physico-chemical properties of the mixtures in the moltenstate were also investigated in terms of density at differenttemperatures (Fig. 8). The density of pure PYR14IM14 is found tobe 7% higher than that of PYR14TFSI. Fig. 8 shows that a slight, butprogressive density decrease of about 5% is observed from 20 ◦Cto 90 ◦C. The increase of the PYR14IM14 mole fraction results inlinearly increasing density vs. composition trends with a knee atx = 0.3. This is better seen in Table 3 which reports the slope valuescalculated for the two different PYR14IM14 mole fraction ranges(e.g., 0 ≤ x ≤ 0.3 and 0.3 ≤ x ≤ 1, respectively) at various tempera-tures. Such behavior suggests an organizational rearrangement ofthe cations and anions within the mixtures, leading to differention packing at a PYR14IM14 mole fraction equal to 0.3 which corre-sponds to a IM14

−/TFSI− mole ratio of about 1:2.Exceptional electrochemical stability is a crucial property of IL

electrolytes for their application in practical devices. The electro-chemical stability window (ESW) of selected (1 − x) PYR14TFSI–(x)PYR14IM14 mixtures (at 20 ◦C) is depicted in Fig. 9. All potentialsare given vs. the Ag/Ag+ reference electrode. The potential vs. Li/Li+

Fig. 6. Activation energy (Ea) vs. PYR14IM14 mole fraction dependence for (1 − x)PYR14TFSI–(x) PYR14IM14 mixtures. The Ea values were obtained from the VTF plotsof Fig. 5. The error bar (when not shown) falls within the data markers.

Fig. 7. Walden plot for (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures. The solid straightline, due to a 0.01 N KCl solution, fixes the position of the ideal Walden line.

Fig. 8. Density vs. PYR14IM14 mole fraction dependence of (1 − x) PYR14TFSI–(x)PYR14IM14 mixtures at different temperatures. The error bar (when not shown) fallswithin the data markers.

Table 3Slope values of the density vs. composition plots reported in Fig. 8.

T (◦C) PYR14IM14 mole fraction range (x)

0–0.3 0.3–1

20 0.135 ± 0.003 0.104 ± 0.00330 0.134 ± 0.003 0.103 ± 0.00340 0.132 ± 0.003 0.102 ± 0.00350 0.129 ± 0.003 0.101 ± 0.00360 0.129 ± 0.003 0.101 ± 0.00370 0.131 ± 0.001 0.098 ± 0.00380 0.127 ± 0.004 0.097 ± 0.00390 0.124 ± 0.003 0.096 ± 0.003

Fig. 9. Linear sweep voltammetries of selected (1 − x) PYR14TFSI–(x) PYR14IM14 mix-tures at 20 ◦C. Platinum as working and counter electrode. The reference electrodeis a silver wire immersed in a 0.01 M solution of AgCF3SO3 in PYR14TFSI. Scan rate:5 mV s−1.



Table 4Cathodic (EC1 and EC2) and anodic (EA1 and EA2) potential values determined from LSV tests for selected (1 − x) PYR14TFSI–(x) PYR14IM14 mixtures at 20 ◦C. The potentials weretaken when the current density through the cell reached 0.05 mA cm−2 (EC1 and EA1) and 0.1 mA cm−2 (EC2 and EA2), respectively. The cathodic (ECLimit) and anodic (EALimit)potential limit values were determined by the intercept of the step current raises with the x-axis. The potentials are given versus the Ag/0.01 M AgCF3SO3 in PYR14TFSIreference electrode.

PYR14IM14 mole fraction Cathodic potentiala (V) Anodic potentiala (V)

EC1 EC2 ECLimit EA1 EA2 EALimit

0 −3.61 ± 0.01 −3.71 ± 0.01 −3.76 ± 0.01 2.00 ± 0.01 2.05 ± 0.01 2.10 ± 0.010.1 −2.92 ± 0.01 −3.08 ± 0.01 −3.88 ± 0.01 2.03 ± 0.01 2.14 ± 0.01 2.16 ± 0.010.3 −2.85 ± 0.01 −3.20 ± 0.01 −3.49 ± 0.01 2.04 ± 0.01 2.14 ± 0.01 2.12 ± 0.011.0 −2.66 ± 0.01 −2.87 ± 0.01 −3.08 ± 0.01 2.29 ± 0.01 2.54 ± 0.01 2.00 ± 0.01

a Potential vs. Ag/0.01 M AgCF3SO3 in PYR14TFSI.

is also indicated. The cathodic (EClimit) and anodic (EAlimit) stabilitylimit potential values, determined by the intercept of the step cur-rent rise from the x-axis, are listed in Table 4. The potentials takenwhen the current density through the cell reached 0.05 mA cm−2

(EC1 and EA1) and 0.1 mA cm−2 (EC2 and EA2) are also reported inTable 4 for comparison purpose. From Fig. 9 it is evident that the(1 − x) PYR14TFSI–(x) PYR14IM14 mixtures exhibit an ESW close to5 V. All ILs and IL mixtures show nearly equivalent anodic stabil-ity (from 2.00 V to 2.16 V vs. Ag/Ag+), suggesting that the anodicdecomposition of the IM14

− anion is close to that of TFSI− [11]. Thepure PYR14TFSI material shows a steep rise in the current densitywhen the anodic decomposition processes occur, but this currentdensity is lowered with increasing PYR14IM14 mole fraction (Fig. 9).This behavior is likely attributable to both the lower conductiv-ity of the IM14

− anion and, perhaps, the growth of an insulatingpassivation layer on the working electrode, due to IM14

− anodicdecomposition [11], which restricts further oxidation of the mix-tures. No other feature is observed at lower voltages during theanodic scan, thus excluding the presence of impurities that couldbe oxidized prior to the anodic breakdown potential of the mix-tures.

On the cathodic side, however, the electrochemical stabilityis found to be progressively reduced with increasing PYR14IM14mole fraction, e.g., a decrease of EClimit from −3.76 V to −3.08 Vis noted (Table 3) on changing from PYR14TFSI (x = 0) to PYR14IM14(x = 1). Such behavior is not easily explainable. We hypothesize thatthe IM14

− anion and/or particular impurities, even if in very lowamounts (<100 ppmw), in the PYR14IM14 material may affect thereduction processes of the PYR14

+ cation or may be reduced instead.The reasons for this effect are not yet fully understood and furtherwork is in progress to clarify this issue.

4. Conclusions

The physical and electrochemical properties of (1 − x)PYR14TFSI–(x) PYR14IM14 IL mixtures have been investigated.The results show that mixing the different ILs generates new mate-rials with improved characteristics with respect to the startingcomponents. The addition of an IL (PYR14IM14) which cannot becrystallized (but which has a relatively low ionic conductivity)to a highly conductive IL (PYR14TFSI) (which is easily crystal-lized) was found to largely enhance the low temperature ionicconductivity (10−4 S cm−1 at −20 ◦C), while still retaining highelectrochemical (4.5–5 V) stability. Even moderate mole fractions(x ≤ 0.3) of PYR14IM14 are able to prevent the crystallization ofthe IL mixtures, thus retaining them in the molten state below−40 ◦C. Mixing of different ILs, therefore, may represent a practicalapproach to obtain new electrolytic materials with enhancedproperties as required in electrochemical devices for particularapplications—something which is generally not possible throughthe use of a single IL material.

Acknowledgements

The authors gratefully acknowledge the financial support of(QZ and WAH) the Army Research Office (ARO) under Con-tract W911NF-07-1-0556 and (GBA, MM and SP) EuropeanCommission under Contracts TST4-CT-2005-518307 and NMP3-CT-2006-033181.

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