A1604
Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
0013-4651/2013/160(9)/A1604/7/$31.00 © The Electrochemical Society
Physicochemical and Electrochemical Properties of Ionic Liquids
Containing Aprotic Heterocyclic Anions Doped With Lithium Salts
Chaojun Shi,∗ Mauricio Quiroz-Guzman, Aruni DeSilva, and Joan F. Brenneckez
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA
A series of novel halogen-free ionic liquids (ILs) based on N, N’-dialkylimidazolium, tetra-alkylphosphonium, and dialkylpyrrolidinium cations paired with two aprotic heterocyclic anions (AHAs) were synthesized and tested as potential electrolytes.
In addition, the corresponding lithium salts were prepared to investigate the impacts of the Li salts on the physical properties
of the IL-Li mixtures. Physicochemical and electrochemical properties (density (ρ), viscosity (η), electrical conductivity (σ) and
electrochemical window (EW) of the neat ILs and their mixtures with the lithium salts at various concentrations were measured at
temperatures between 283.15 K and 343.15 K and at atmospheric pressure. The results indicate that ILs with both planar anions and
cations exhibit lower viscosities, and better conductivities than those with more spherical geometries, while maintaining competitive
EWs. The Walden plot behavior of the ILs provides qualitative insight into the suitability of these ILs as electrolytes. Based on
systematic studies shown in this paper, it can be concluded that these new ILs are promising halogen-free electrolytes that deserve
further investigation
© 2013 The Electrochemical Society. [DOI: 10.1149/2.116309jes] All rights reserved.
Manuscript submitted April 2, 2013; revised manuscript received June 25, 2013. Published July 20, 2013. This was Paper 3626
presented at the Honolulu, Hawaii, Meeting of the Society, October 7–12, 2012.
Ionic liquids (ILs) are organic salts comprised entirely of ions,
which have relatively low melting points.1–3 ILs have attracted considerable attention due to their negligibly small vapor pressure, high
thermal stability, relatively high conductivity, and wide electrochemical windows.4–7 These unique properties make ILs suitable for many
applications.8,9 In the field of electrochemical devices (e.g., lithium ion
batteries) ILs now appear to be one of the best alternative electrolytes
since they could significantly reduce safety concerns.10–13
ILs are structurally tunable liquids; thus, the possible combination of anions, cations and substituents are endless. A tremendous
amount of experimental data on the physical and electrochemical
properties of various pure ILs is available.14–18 Imidazolium-based ILs
are the most widely investigated class of ILs, based on their moderately low viscosity and acceptable conductivity, but their applications
are limited by their electrochemical stability.17,19–21 Phosphoniumbased ILs have also been studied by several researchers.22–24 For
example, Tsunashima et al.25 have shown that phosphonium-based
ILs provide better chemical and electrochemical stability than
their imidazolium-based counterparts, but these ILs have relatively
higher viscosities. More recently, pyrrolidinium-based ILs have attracted interest because of their wide electrochemical windows.26–30
Studies of ILs doped with lithium salts have been limited to
ones with perfluorinated-based anions,31–33 such as hexafluorophosphate ([PF6 ]− ), tetrafluoroborate ([BF4 ]− ), bis(fluorosulfonyl)-amide
([FSA]− ) and bis(trifluoromethanesulfonyl)imide ([Tf2 N]− ). [BF4 ]−
and [PF6 ]− anions yield hydrophilic ILs, but ILs based on the [Tf2 N]−
anion are by far the most popular because the electron withdrawing
-CF3 group can delocalize the negative charge on the anion. This
weakens interactions with the cation, which reduces viscosity and increases conductivity. Even though ILs based on [FSA]− have attractive
physical properties and battery cycling behavior that surpasses that of
[Tf2 N]− based ILs, its thermal stability remains a major concern.34 In
addition, halogen-based ILs are likely to illicit concerns about corrosion, toxicity and environmental persistence.
To our knowledge, there are relatively few publications that have
investigated non-fluorinated ionic liquids for electrochemical applications. In order to expand the range of ILs considered as electrolytes for
battery applications, and to avoid potential concerns with corrosion,
toxicity and persistence, this paper focuses on the design and testing
of new halogen-free ILs and their corresponding lithium salts. Eight
ILs based on N, N’-dialkylimidazolium, tetra-alkylphosphonium, and
dialkylpyrrolidinium cations paired with aprotic heterocyclic anions
(AHAs) are investigated. Furthermore, we investigate how the structure of the IL and temperature influence the physicochemical and
∗
Electrochemical Society Student Member.
z
E-mail: [email protected]
electrochemical properties of both neat ILs and lithium salt doped IL
mixtures.
Experimental
Ionic Liquid Preparation.— All ILs and lithium salts studied in
this work were synthesized in our laboratory and are listed in Table I
with structures and abbreviations. Details of the synthesis and characterization of both the ILs and the corresponding lithium salts can be
found in Supplementary Material. In order to reduce impurities and
residual water in the ILs, they were dried under vacuum at ∼50◦ C
for a minimum of 48 hours before use. Water content was determined
before and after measurements using a Brinkman 831 Karl Fischer
Coulometer, which has an uncertainty of +/−3% of the measured
value. The water content of all ILs for all measurements was less than
300 ppm. Mixing of samples and loading of samples into syringes
(for density and viscosity measurements) and the conductivity cells
was carried out in a nitrogen atmosphere inside a M. Braun dry glove
box, which has a water content of less than 0.1 ppm. The syringes and
conductivity cells were sealed with parafilm before removing them
from the glove box. Electrochemical window measurements were
performed in the glove box. The lithium doped ILs were prepared by
adding a known mass of the lithium salt with the same anion as the
IL, followed by heating and stirring to ensure formation of a homogeneous binary mixture. All properties measurements were recorded
immediately after sample preparation to avoid absorption of water.
Density Measurement.— A DMA 4500 Anton Paar oscillating
U-tube densitometer was used to measure the densities of all
ILs and IL/lithium salt mixtures at atmospheric pressure with ±5
× 10−5 g/cm3 uncertainty.35 Considering sample impurity, the actual uncertainty of the density measurements is estimated as ±1
× 10−4 g/cm3 . Besides the built-in automatic viscosity correction
in this type of densitometer, there are two integrated Pt 100 platinum
thermometers to control temperature within ±0.01 K.
Viscosity Measurement.— The viscosity of each IL and IL/lithium
salt mixture was measured with an ATS Rheosystems Viscoanalyzer
containing a ETC-3 Joule Thomson effect temperature cell (−10 to
400◦ C). It was operated in the cone and plate (10 mm) geometry.35
The shear stress range of this equipment was from 7.1 × 10−6 to 7.1
× 103 Pa. Measured viscosities have an uncertainty of ±5% above
100 cP and ±10% for viscosities below 100 cP.
Conductivity Measurement.— The temperature dependence of the
conductivities of the pure ILs and lithium doped ILs were determined with an electrochemical impedance spectroscopy (EIS) system,
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Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
A1605
Table I. Name, abbreviation and structure of the ILs and lithium salts studied in this report.
Structure
Name
N
N
N
N
N
N
N
N
1-ethyl-3-methylimidazolium
2-cyanopyrrolide
[emim][CNPyr]
1-butyl-3-methylimidazolium
2-cyanopyrrolide
[bmim][CNPyr]
N
1-ethyl-3-methylimidazolium
1,2,4-triazolide
[emim][4triz]
N
1-butyl-3-methylimidazolium
1,2,4-triazolide
[bmim][4triz]
trihexyl(tetradecyl)phosphonium
2-cyanopyrrolide
[P66614][CNPyr]
trihexyl(tetradecyl)phosphonium
1,2,4-triazolide
[P66614][4triz]
N-methyl-N’-butyl pyrrolidinium
2-cyanopyrrolide
[C4mpyr][CNPyr]
N-methyl-N’-butyl pyrrolidinium
1,2,4-triazolide
[C4mpyr][4triz]
Lithium 2-cyanopyrrolide
Li[CNPyr]
Lithium 1,2,4-triazolide
Li[4triz]
C
N
C
N
N
N
N
N
N
Abbreviation
N
C6H13
C6H13
N
P+ C14H29
C
N
C6H13
C6H13
C6H13
P+ C14H29
C6H13
N
N
N
N
C
N
N
N
N
N
N
N
C
N
Li+
N
N
N
Electrochemical Window (EW) Measurement.— The determination of the EWs of the ILs was carried out with a VoltaLab50 potentiostat (Radiometer Analytical) and a conventional three electrode cell system under a nitrogen atmosphere at ambient temperature using a 100 mV/s scan rate. Glassy carbon (BAS Inc.,
d = 3 mm) and platinum wire were used as working and counter electrodes, respectively. The reference electrode was composed of a silver
wire immersed in 0.1 M silver trifluoromethanesulfonate (AgOTf)
in a 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
([hmim][Tf2 N]) solution with a Vycor glass separator.
Material. In general, IL density decreases linearly with temperature
and can be described by a straight line with the parameters shown in
Table II:
ρ = a + bT
[1]
The density of ILs with the same anion decreases as follows:
[emim]+ > [bmim]+ > [C4 mpyr]+ > [P66614 ]+ . Imidazolium-based
ILs exhibit the highest densities and phosphonium-based ILs have the
lowest densities. In the imidazolium-based IL group, the most dense
1.15
1.10
3
comprised of a Solartron SI 1260 Impedance / Gain-phase analyzer
connected to a Solartron 1287 electrochemical interface, The uncertainty of the conductivities is estimated as ±3%.36,37 The conductivity
sample cell (Materials Mates, Italia) constant was calibrated with diluted KCl standard solutions and all ILs samples were packed and
sealed in a M. Braun dry glove box with gas purification platform
labmaster with O2 < 30 ppm and H2 O < 0.1 ppm, as described
above. All samples were thermally equilibrated at each temperature
for at least two hours before measurement to avoid any temperature
fluctuations.
Density (g/cm )
Li+
1.05
1.00
0.95
0.90
Results and Discussion
Density.— Figure 1 presents the measured densities of the pure
dried ILs at temperatures from 283.15 K to 343.15 K at atmospheric
pressure. The uncertainties of all of the measurements are given in
the Experimental Section. In all of the figures, the size of the symbols is chosen to be consistent with the experimental uncertainties.
Tables of all of the experimental data can be found in Supplementary
0.85
280
290
300
310
320
330
340
350
T (K)
[emim][4triz]
[C4mpyr][4triz]
[emim][CNPyr]
[C4mpyr][CNPyr]
[bmim][4triz]
[P66614][4triz]
[bmim][CNPyr]
[P66614][CNPyr]
Figure 1. Densities of selected ILs as a function of temperature.
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Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
Table II. Molecular weights and density equation parameters of
the neat ILs.
a
g · cm−3
b/10−4
g · cm−3 · K−1
[emim][4triz]
[emim][CNPyr]
[bmim][4triz]
[bmim][CNPyr]
[C4 mpyr][4triz]
[C4 mpyr][CNPyr]
[P66614 ][4triz]
[P66614 ][CNPyr]
179.2
202.2
207.3
230.3
210.3
233.4
551.9
575.0
1.3213 ± 0.0005
1.2680 ± 0.0003
1.2638 ± 0.0007
1.2317 ± 0.0006
1.2114 ± 0.0005
1.1731 ± 0.0005
1.0736 ± 0.001
1.0692 ± 0.0005
−6.11 ± 0.02
−6.08 ± 0.01
−5.97 ± 0.02
−6.01 ± 0.02
−5.71 ± 0.02
−5.52 ± 0.02
−5.84 ± 0.03
−5.76 ± 0.02
1.06
3
MW
g · mol−1
Density (g/cm )
ILs
1.05
1.04
1.03
neat
0.02
0.04
1.15
neat
0.025
0.05
0.10
0.15
1.07
1.02
280
290
300
310
320
330
340
350
T (K)
Figure 3. Densities of neat [bmim][CNPyr] and mixtures of [bmim][CNPyr]
doped with 0.025, 0.05, 0.10, and 0.15 mole fraction Li[CNPyr].
3
Density (g/cm )
1.14
1.13
[P66614][4triz]
1.12
[C4mpyr][CNPyr]
[bmim][CNPyr]
[emim][4triz]
[emim][CNPyr]
[P66614][CNPyr]
1000
[C4mpyr][4triz]
[bmim][4triz]
280
290
300
310
320
330
340
350
T (K)
Figure 2. Densities of neat [emim][4triz] and mixtures of [emim][4triz] doped
with 0.02 and 0.04 mole fraction Li[4triz].
IL is [emim][4triz], followed by [emim][CNPyr] and [bmim][4triz].
ILs with longer alkyl chains have lower densities due to more weak
dispersion interactions, which is consistent with previous studies.21,38
Densities also vary with the choice of anion. As shown in
Figure 1, the densities of the ILs with the [4triz]− anion are always
higher than the ones with the [CNPyr]− anion, as expected, since
[4triz]− is smaller than [CNPyr]− . Figures 2 and 3 show that the addition of Li[4triz] and Li[CNPyr] to [emim][4triz] and [bmim][CNPyr]
increases the densities. Equation 1 was used to fit the density dependence on temperature for these mixtures, as well, and the parameters
are listed in Table III.
Viscosity.— The temperature dependence of the viscosities of the
neat ILs and ILs doped with lithium salts was measured from 283.15 K
to 343.15 K at atmospheric pressure and the results are shown in
Figures 4–6. As with molecular liquids, the viscosity of ILs decreases with increasing temperature because the species are further
apart (lower density) and they have higher kinetic energy, which reduces interaction times between species. The neat [emim][CNPyr]
has the lowest viscosity of the ILs studied, as shown in Figure 4.
Viscosity (cP)
1.11
100
10
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
Figure 4. Arrhenius plot (viscosity vs. inverse temperature) of neat ILs.
Longer alkyl chains on the cation increase van der Waals interactions
and, subsequently, the friction between the chains. This results in
more viscous ILs regardless of the anion, which is in agreement with
previous reports.39 Pyrrolidinium-based ILs are generally more viscous than the corresponding imidazolium-based ILs. Moreover, ILs
containing the [4triz]− anion always exhibit higher viscosity than the
ones containing the [CNPyr]− anion, perhaps due to differences in
charge delocalization attributable to the strong electron-withdrawing
cyano group, as well as the potential for π-π stacking. The addition of lithium salts increases the viscosity of both [emim][4triz] and
[bmim][CNPyr].
Table III. Density equation parameters of [emim][4triz] and [bmim][CNPyr] doped with lithium salts.
ILs + Li-salts
[emim][4triz] + Li[4triz]
[bmim][CNPyr] + Li[CNPyr]
Lithium salts
mole fraction
a
g · cm−3
b/10−4 g · cm−3 · K−1
0.02
0.04
0.025
0.05
0.10
0.15
1.3207 ± 0.0008
1.3214 ± 0.001
1.2351 ± 0.001
1.2390 ± 0.001
1.2394 ± 0.0009
1.2393 ± 0.0006
−6.04 ± 0.02
−6.00 ± 0.03
−6.08 ± 0.04
−6.18 ± 0.03
−6.17 ± 0.03
−6.05 ± 0.02
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Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
neat
0.02
0.04
0.01
Conductivity (S/cm)
Viscosity (cP)
1000
A1607
100
1E-3
1E-4
1E-5
2.9
10
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
Figure 5. Arrhenius plot (viscosity vs. inverse temperature) of [emim][4triz]
and [emim][4triz] doped with 0.02 and 0.04 mole fraction Li[4triz].
neat
0.025
0.05
0.10
0.15
1000
Viscosity (cP)
3.0
2.9
3.0
[emim][4triz]
[C4mpyr][CNPyr]
[bmim][CNPyr]
[P66614][CNPyr]
[bmim][4triz]
[P66614][4triz]
Figure 7. Arrhenius plot (ionic conductivity vs. inverse temperature) of neat
ILs.
The Vogel-Fulcher-Tamman (VFT) equation was used to fit the
experimental data:
η = η0 exp [B/ (T − T0 )]
[2]
where η0 , B, and T0 are the adjustable parameters. The best-fit
parameters are shown in Tables IV and V and the solid lines in
Figures 4–6 are the VFT fits. Theoretically, the Vogel temperature
(T0 ) 19 is the ideal glass transition temperature. While many of the
T0 values are close to typical experimental glass transition values for
ILs (e.g., −70◦ C), here T0 should simply be regarded as a fitting
parameter. The addition of lithium salts results in an increase in the
viscosity of the IL mixtures, which reduces the mobility of ions, which
is consistent with previous reports for other ILs.19
100
10
[emim][CNPyr]
[C4mpyr][4triz]
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
Figure 6. Arrhenius plot (viscosity vs. inverse temperature) of [bmim]
[CNPyr] and [bmim] [CNPyr] doped with 0.025, 0.05, 0.10, and 0.15 mole
fraction Li[CNPyr].
Table IV. VFT equation parameters of experimental viscosity data
for neat ILs.
ILs
η0 (cP)
B (K)
T0 (K)
[P66614 ][4triz]
[P66614 ][CNpyr]
[C4 mpyr][4triz]
[C4 mpyr][CNPyr]
[bmim][4triz]
[bmim][CNPyr]
[emim][4triz]
[emim][CNPyr]
0.05 ± 0.02
0.09 ± 0.02
0.16 ± 0.04
0.21 ± 0.05
0.23 ± 0.04
0.18 ± 0.04
0.78 ± 0.08
0.72 ± 0.08
1375 ± 88
1116 ± 64
764 ± 41
602 ± 38
680 ± 27
608 ± 34
381 ± 18
349 ±16
158 ± 4
165 ± 4
197 ± 2
208 ± 2
202 ± 2
209 ± 2
215 ± 2
222 ± 1
Conductivity.— Conductivity has been regarded as one of the
most important properties of ILs if they are to be used for electrochemical applications. Conductivities of all neat ILs studied here
were measured over a temperature range of 283.15–343.15 K.
The very low temperature region, which is below or near the
melting points of the ILs, is not included because of the experimental limitations of the environmental chamber. Arrhenius
plots of all ILs investigated are shown in Figure 7. The conductivity trend of the neat ILs is: [emim][CNPyr] > [emim][4triz]
> [bmim][CNPyr] > [C4 mpyr][CNPyr] > [C4 mpyr][4triz]
≈ [bmim] [4triz] > [P66614 ][CNPyr] > [P66614 ][4triz]. The long chain
phosphonium-based ILs exhibit the lowest conductivity, in the range
of 1.63 × 10−2 – 4.2 × 10−1 mS/cm. The other ILs exhibit relatively
high conductivities, up to 31 mS/cm, which is two orders of magnitude
higher than the phosphonium-based ILs. In particular, [emim][CNPyr]
is the most conductive among the imidazolium-based ILs tested here.
As shown by comparing Figures 6 and 7, IL conductivity is mainly
controlled by viscosity; higher viscosity generally leads to lower conductivity. However, structure also plays an important role in determining conductivity. This study demonstrates that ILs with planar
anions and cations give rise to higher ionic conductivity compared to
Table V. VFT equation parameters of experimental viscosity data for ILs doped with lithium salts.
ILs + Li-salts
[emim][4triz] + Li[4triz]
[bmim][CNPyr] + Li[CNPyr]
Lithium salt
mole fraction
η0 (cP)
B (K)
T0 (K)
0.02
0.04
0.025
0.050
0.100
0.150
0.62 ± 0.05
0.52 ± 0.09
0.53 ± 0.04
0.44 ± 0.04
0.22 ± 0.03
0.32 ± 0.15
427 ± 13
474 ± 26
449 ± 9
511 ± 13
644 ± 24
610 ± 71
214 ± 1
211 ± 2
221 ± 1
216 ± 1
207 ± 1
210 ± 4
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Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
Table VI. VFT equation parameters of conductivity data for
selected neat ILs.
Conductivity (S/cm)
neat
0.02
0.04
0.01
1E-3
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
Figure 8. Arrhenius plot (ionic conductivity vs. inverse temperature) of
[emim][4triz] and [emim][4triz] doped with 0.02, 0.04 mole fraction Li[4triz].
ILs with more spherically shaped ions. All four imidazolium-AHA
ILs investigated here have planar anions and planar cations. From the
conductivity measurements, it is clear that they provide the fastest ion
diffusivity. Pyrrolidinium-AHA ILs can be regarded as having pseudoplanar cations and planar anions. They have higher conductivity than
ILs with the more spherical tetra-alkyl phosphonium cation.
In addition, conductivities of [emim][4triz] and [bmim][CNPyr]
doped with Li[4triz] and Li[CNPyr] were measured. Similarly, Arrhenius plots of ionic conductivities for these two ILs and their corresponding IL-Li salt mixtures are shown in Figures 8 and 9. The neat
ILs exhibit the highest ionic conductivity, which is never surpassed by
their corresponding mixtures with the lithium salts. Therefore, lithium
salts reduce the overall ionic conductivity of the mixture.
The temperature dependence of the conductivities is welldescribed by the Vogel-Fulcher-Tamman (VFT) equation:
σ = σ0 exp [k/ (T − T0 )]
[3]
where σ0 , k, and T0 are fitting parameters. σ0 is the infinite temperature conductivity, which is a combination constant that depends on
the model and relates to the fragility. The fit parameters, with their
uncertainties, are shown in Tables VI and VII.
σ0 (S/cm)
k (K)
T0 (K)
[P66614 ][4triz]
[P66614 ][CNpyr]
[C4 mpyr][4triz]
[C4 mpyr][CNPyr]
[bmim][4triz]
[bmim][CNPyr]
[emim][4triz]
[emim][CNPyr]
0.49 ± 0.11
2.92 ± 1.83
1.28 ± 0.21
0.68 ± 0.02
1.34 ± 0.07
0.84 ± 0.07
0.86 ± 0.24
0.85 ± 0.10
−1548 ± 83
−2057 ± 270
−751 ± 42
−642 ± 9
−733 ± 14
−595 ± 21
−487 ± 67
−460 ± 27
144 ± 5
112 ± 14
192 ± 3
196 ± 1
194 ± 1
205 ± 2
202 ± 8
206 ± 3
Generally, the conductivity decreases with increasing lithium salt
concentration because Li+ can pair with anions to form ion aggregates.
Literature suggests that the Li+ ion itself displays low conductivity,
which may be only 10−3 S/cm at ambient temperature.4 According to
molecular dynamics (MD) simulations, Liu et al.40 found that one Li+
can interact with several anions to form a Li[anion]n complex, where
n depends on the anion type. Figures 8 and 9 show how the conductivity of [emim][4triz]/lithium salt and [bmim][CNPyr]/lithium salt
mixtures decrease with increasing concentration of their corresponding lithium salts. Note that the concentration of added lithium salt is
limited by the solubility of that salt in the IL. For [bmim][CNPyr], the
conductivity just decreases slightly when the Li[CNPyr] mole fraction is 0.025, but the conductivity drops further as more Li[CNPyr]
is added. The overall decreasing tendency agrees well with previous
reports.28
Ionicity.— A straight-forward way to evaluate the suitability of
neat ILs and ILs doped with lithium salts for electrochemical applications is to investigate their ionicity in terms of the empirical Walden
Rule41 presented by Angell et al.,42,43 which is expressed as follows:
η = C
log(Λ) (Scm2mol-1)
1
1E-3
1E-4
[4]
This rule connects the equivalent conductivity ( = σM/ρ, M
is the molecular weight, σ is the ionic conductivity, and ρ is the density
of the respective IL) and the fluidity ϕ = 1/η (where η is viscosity).44
Figure 10 displays the ionicities of the ILs in a logarithmic plot of
equivalent conductivity as a function of fluidity. The solid line that
runs from corner to corner is known as the ideal Walden line, which
is determined by data for a 1 M KCl solution at ambient temperature.
This ideal line stands for the behavior of an ideal electrolyte with unity
slope.45 An ideal ILs is supposed to run through the ideal line, and the
deviation from the ideal line can be interpreted as a measure of the
neat
0.025
0.05
0.10
0.15
0.01
Conductivity (S/cm)
ILs
0
[emim][CNPyr]
[emim][4triz]
[bmim][CNPyr]
[bmim][4triz]
[C4mpyr][4triz]
[C4mpyr][CNPyr]
[P66614][CNPyr]
[P66614][4triz]
KCl ideal line
-1
-2
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
-1
1000/T (K )
Figure 9. Arrhenius plot (ionic conductivity vs. inverse temperature) of
[bmim][CNPyr] and [bmim][CNPyr] doped with 0.025, 0.05, 0.10, and
0.15 mole fraction Li[CNPyr].
-2
-1
0
1
log(η-1) (P-1)
Figure 10. Walden plot of neat ILs at different temperatures.
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Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
A1609
Table VII. VFT equation parameters of conductivity data for lithium salt doped ILs.
ILs + Li-salts
[emim][4triz] + Li[4triz]
[bmim][CNPyr] + Li[CNPyr]
Lithium salt
mole fraction
σ0 (S/cm)
k (K)
T0 (K)
0.02
0.04
0.025
0.05
0.10
0.15
0.97 ± 0.03
0.81 ± 0.15
1.07 ± 0.06
1.23 ± 0.06
0.98 ± 0.25
0.60 ± 0.07
−548 ± 8
−528 ± 46
−647 ± 14
−689 ± 8
−638 ± 62
−560 ± 27
197 ± 1
201± 5
201± 1
200 ± 1
207 ± 6
213 ± 3
degree of ion association. As shown in Figure 10, all of the ILs fall
below the ideal line, which means that they exhibit non-ideal behavior
because of the strong interactions between the ions (note: an expanded
version of this figure can be found in the Supplementary Material). In
general, imidazolium-based and pyrrolidinium-based ILs present better ionicity compared to the phosphonium-based ILs investigated here.
One possible reason that could explain this is the differences in structure of the various cations. Imidazolium-based and pyrrolidiniumbased ILs paired with AHAs have relatively planar structure for both
the cation and the anion, which we believe promotes fast diffusion.
However, the more bulky spherical cations of phosphonium-based ILs
(especially those with long alkyl chains) may hinder ion movement to
some extent. It is noticeable that [emim][CNPyr] and [emim][4triz]
lie closer to the ideal line than the other ILs. The dashed line in
Figure 10 is used to indicate the situation where the IL exhibits just
10% of the expected conductivity.45 Even though [P66614 ][CNPyr] and
[P66614 ][4triz] have lower ionicities than the other ILs, they still fall in
1
[P66614][4triz]
0
-1
[P66614][CNPyr]
1
0
2
Current density (mA/cm )
-1
2
[C4mpyr][4triz]
0
-2
2
[C4mpyr][CNPyr]
0
-2
1
Electrochemical Window (EW).— The electrochemical behavior
of all of the pure ILs has been evaluated by measuring their EWs.
These values, as determined by cyclic voltammetry, are shown in
Figure 11. The EWs are reported as the cathodic reduction potential
(Epc ) and anodic oxidation potential (Epa ) when 1.0 mA/cm2 current
density is reached. Normally, ILs containing the same cations or
anions would be expected to have the same Epc or Epa . However, the
combination of cations from different families with the AHAs shows
variation in both the cathodic and anodic potentials, indicating that
the counterions do affected the oxidation and reduction potentials.
The intrinsic EWs of the ILs studied in this paper decreased in the
order of: [P66614 ][4triz] > [C4 mpyr][4triz] > [P66614 ][CNPyr]
> [bmim][4triz] > [C4 mpyr][CNPyr] > [bmim][CNPyr]
≈ [emim][4triz] > [emim][CNPyr]. [P66614 ][4triz] has the widest
EW of 4.2 V, and [emim][CNPyr] exhibits the smallest EW of just
2.4 V. It is worth noticing that [bmim]+ and [P66614 ]+ combined with
AHAs do have similar Epa values. However, the Epa of [emim]+
and [C4 mpyr]+ shifted in the negative direction, which means they
have less oxidation stability. Imidazolium-based ILs have smaller
EWs, and phosphonium-based and pyrrolidinium-based ILs have
wider EWs. It appears that the EWs expand with increasing alkyl
chain length for imidazolium-based ILs. For instance, the EWs of
[bmim][AHAs] are ∼0.5 V higher than the corresponding ILs with the
[emim]+ cation. There is significant cathodic stability improvement
when replacing imidazolium-based cations with phosphonium-based
or pyrrolidinium-based ones. For a particular cation the EW of ILs
containing the [4triz]− anions are about 0.5 V larger than the ones
with the [CNPyr] anion, which indicates that [CNPyr]− is less stable
than [4triz]− .
[bmim][4triz]
0
Conclusions
-1
1
0
-1
-2
1
the region between the ideal line and the 10% dashed line. The slopes
of all the neat ILs are almost one, except for [C4 mpyr][CNPyr] (0.85),
in the temperature range of 283.15–343.15 K, which demonstrates
that the degree of dissociation of those ILs is essentially independent
of temperature.
[emim][4triz]
[bmim][CNPyr]
0
-1
4
2
0
-2
[emim][CNPyr]
-4
-3
-2
-1
0
1
+
Potential (V) vs. (Fc/Fc )
Figure 11. Electrochemical windows of investigated ILs with glassy carbon
working electrode at room temperature and 100 mV/s scan rate.
A group of novel halogen-free ILs composed of imidazoliumbased, pyrrolidinium-based, and phosphonium-based cations paired
with two aprotic heterocyclic anions (AHAs), as well as these ILs
doped with their corresponding lithium salts, were investigated for
the first time. Basic physical and electrochemical properties were
measured for both neat ILs and ILs doped with lithium salts. As
expected, the IL density decreases with increasing temperature, and
IL-Li salt mixtures exhibit increasing density with increasing lithium
salt concentration. All ILs have relatively low viscosities except for the
phosphonium-based ILs. As for the cation, viscosity increases in the
following order: [emim]+ < [bmim]+ < [C4 mpyr]+ < [P66614 ]+ . ILs
containing the [4triz]− anion always exhibit higher viscosities compared to those with the [CNPyr]− anion. The viscosities of ILs doped
with lithium salts are always higher than the neat ILs, and increase
with increasing lithium salt concentration. There is a strong connection between viscosity and conductivity. The temperature dependence
of the conductivity follows VFT behavior for both neat ILs and ILs
doped with lithium salts. Overall, ILs with both planar cations and
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A1610
Journal of The Electrochemical Society, 160 (9) A1604-A1610 (2013)
anions exhibit higher conductivity than ILs with more spherical shaped
ions. This can be explained by a better delocalized charge distribution
in the planar ions and weaker interaction between the cation and anion.
Li[4triz] and Li[CNPyr] have varying degrees of decreasing mixture
conductivity. Although addition of lithium salts to neat ILs leads to
conductivity reduction, [emim][4triz] and [bmim][CNPyr] doped with
lithium salts retain a relatively high conductivity, which is of interesting for Li-ion battery applications. According to the Walden Rule,
all ILs in this study have relatively good ionicity and some lie very
close to the ideal line. The temperature dependence of the neat ILs on
the Walden plots indicates that the degrees of dissociation of all pure
ILs are almost independent of temperature. Among the cations investigated in this paper, phosphonium-based ILs have the widest EWs,
followed by pyrrolidinium-based ones. These favorable electrochemical stability combined with outstanding physicochemical properties
suggest that AHAs-based ILs may be of interest in electrochemical
fields.
Acknowledgments
This paper is based upon work supported by the U.S. Army
TARDEC under Contract No. W56HZV-08-C-0236, through a subcontract with Mississippi State University. Any opinions, results, conclusions, and recommendations expressed in this paper are those of
the authors and do not reflect the views of the U.S. Army TARDEC.
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