A Comparison among Viscosity, Density, Conductivity, and Electrochemical Windows of N-n-Butyl-N-methylpyrrolidinium and Triethyl-n-pentylphosphonium Bis(fluorosulfonyl imide) Ionic Liquids and Their Analogues Containing Bis(trifluoromethylsulfonyl) Imide Anion

Article abstract of DOI:10.1021/acs.jced.7b00458

Ionic liquid consists of an organic cation and generally an inorganic anion. The properties of liquids can be modulated by changing the anion or the cation or both. Two ionic liquids derived from the anion [FSI] were synthesized and characterized: (N-n-butyl-N-methylpyrrolidinium bis(fluorosulfonyl imide), [BMPYR][FSI]) and triethyl-n-pentylphosphonium bis(fluorosulfonyl)imide), [P₂₂₂₅][FSI]). We also report the comparison with [TFSI] based ionic liquids, namely: (N-n-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide), [BMPYR][TFSI]), (N-n-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide), [BMP][TFSI]), triethyl-n-pentylphosphonium bis(trifluoromethylsulfonyl)imide), [P₂₂₂₅][TFSI]) and (triethyl(2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl imide), [P₂₂₂₂₀₁][TFSI]. Thermophysical properties, such as viscosity (298.15-373.15 K), density (298.15-373.15 K), conductivity (298.15-361.15 K), electrochemical (cutoff density current within 150 μA·cm^-2), and thermal stability (303.15-973.15 K) were experimentally determined. The results showed that [FSI] derivatives exhibit better transport properties and lower thermal and electrochemical stability when compared with their [TFSI] counterparts.

Full text of DOI:10.1021/acs.jced.7b00458

Article
pubs.acs.org/jced
A Comparison among Viscosity, Density, Conductivity, and
Electrochemical Windows of N‑n‑Butyl‑N‑methylpyrrolidinium and
Triethyl‑n‑pentylphosphonium Bis(fluorosulfonyl imide) Ionic
Liquids and Their Analogues Containing Bis(trifluoromethylsulfonyl)
Imide Anion
Nedher Sanchez-Ramírez,†,‡ Birhanu Desalegn Assresahegn,‡ Daniel Belanger,‡
́
́
́
and Roberto M. Torresi*,†
†Departamento de Química Fundamental, Instituto de Química, Universidade de Sao Paulo, Av. Prof. Lineu Prestes 748, 05508-000,
̃
Sao Paulo, Brazil
̃
‡Dep
́ ́ ́ ̀ ́ ́ ́
artement de Chimie, Universite du Quebec a Montreal, Case Postale 8888 succursale Centre-Ville, Montreal, Quebec H3C 3P8,
Canada
S
* Supporting Information
ABSTRACT: Ionic liquid consists of an organic cation and generally an
inorganic anion. The properties of liquids can be modulated by changing the
anion or the cation or both. Two ionic liquids derived from the anion [FSI]
were synthesized and characterized: (N-n-butyl-N-methylpyrrolidinium bis-
(fluorosulfonyl imide), [BMPYR][FSI]) and triethyl-n-pentylphosphonium
bis(fluorosulfonyl)imide), [P2225][FSI]). We also report the comparison with
[TFSI] based ionic liquids, namely: (N-n-butyl-N-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide), [BMPYR][TFSI]), (N-n-butyl-N-methyl-
piperidinium bis(trifluoromethylsulfonyl)imide), [BMP][TFSI]), triethyl-n-
pentylphosphonium bis(trifluoromethylsulfonyl)imide), [P2225][TFSI]) and
(triethyl(2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl imide),
[P222201][TFSI]. Thermophysical properties, such as viscosity (298.15−373.15
K), density (298.15−373.15 K), conductivity (298.15−361.15 K), electro-
chemical (cutoff density current within 150 μA·cm−2), and thermal stability (303.15−973.15 K) were experimentally determined.
The results showed that [FSI] derivatives exhibit better transport properties and lower thermal and electrochemical stability
when compared with their [TFSI] counterparts.
characteristics, which include: (1) high stability, (2) good
transport properties, (3) nonvolatility, and (4) modulation
properties.9,10 These unique properties of liquids as electrolytes
have the possibility of attenuating the above-mentioned
problems concerning lithium batteries. Here, we report a
complete characterization of a variety of ILs derived from
[TFSI] (bis(trifluoromethylsulfonyl)imide) and [FSI] (bis-
(fluorosulfonyl) imide) along with various cations derived from
phosphorus and nitrogen. The results reveal how these
properties are affected when the anion or cation are changed.
An adequate selection of the IL made up will allow their
application for the lithium-ion technology requirement. Figure
1 displays the cations and anions used in this work.
1. INTRODUCTION
Ionic liquids (ILs) are liquids consisting exclusively or almost
exclusively of ions. They therefore exhibit ionic conductivity;
this definition includes liquids that are traditionally known as
molten salts or fused salts, which have high melting points.1
Nevertheless, in the last few years, a new definition has
appeared: “The term IL refers to compounds consisting entirely
of ions and existing in the liquid state below 100 °C”. In many
cases the melting point is even below room temperature.2 The
scientific and technological importance of ILs today spans a
wide range of applications.3 In particular, Li-ion technology has
captured the portable electronic market, invaded the power tool
equipment market, and is now penetrating the electric vehicles
(EVs) segment.4−7 However, lithium-ion batteries must operate
within a safe and reliable operating area, which is delimited by
temperature and voltage window; both are related to the
organic electrolyte typically used as an electrolyte in this
technology. Exceeding these restrictions leads to the rapid
attenuation of battery performance and even results in safety
problems.8 Nowadays, ILs are of great interest for their unique
Received: May 19, 2017
Accepted: September 11, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jced.7b00458
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3.44 (4H; m); δC (500 MHz; CD3CN; ppm): 13.8; 20.4; 22.3;
26.3; 49.3; 65.1; 65.4. (Figure S1, SI)
[P2225][FSI] Data. Elemental analysis data: Found: C; 35.6;
H; 7.23; N; 4.08. Calcd for: PC11H26F2NO4S2: C; 35.8; H;
7.09; N; 3.79; δH (500 MHz; CD3CN; ppm): 0.90−0.93 (3H;
t; J = 7,5 Hz); 1.15−1.21 (9H; m); 1.33−1.56 (6H; m); 2.03−
2.15 (8H; m); δ13 (500 MHz; CD3CN; ppm): 5.80 (3C);
C
12.0−12.4 (3C; d; 1JCP = 50 Hz); 14.1; 17.8−18.2 (1C; d; 1JCP
= 49 Hz); 21.5; 22.7; 33.4. (Figures S2, SI)
[BMPYR][TFSI] Data. Elemental analysis data: Found: C;
31.3; H; 4.74; N; 6.65. Calcd for C11H20N2S2O4F6: C; 31.3; H;
4.77; N; 6.63; δH (500 MHz; CD3CN; ppm): 0.95−0.98 (3H;
t; J = 7.5 Hz); 1.33−1.40 (2H; sh; J = 7 Hz); 1.68−1.75 (2H;
m); 2.11−2.18 (4H; m); 2.93 (3H; s); 3.20−3.23 (2H; m);
3.35−3.44 (4H; m); δC (500 MHz; CD3CN; ppm): 13.8; 20.4;
1
22.4; 26.3; 49.3; 65.1; 65.4; 117.2−124.9 (2C; q; JCF = 319
Figure 1. Schematic diagram of the constituent ions in the ILs studied.
Hz) (Figure S3, SI). All of the characterization of [P2225][TFSI]
and [P222201][TFSI] can be found elsewhere.10 The NMR
spectra and elemental analysis demonstrate the purity of the all
compounds.
2. EXPERIMENTAL SECTION
2.3. Thermal Properties, Density, and Transport
Properties. Thermogravimetric analysis was carried out with
a thermogravimetric analyzer (TA Instruments TGA (Q500)/
Discovery MS). Samples (typically 2 mg) were placed in a Pt
pan and heated from 303.15 to 973.15 K with a temperature
ramp of 5 K·min−1, under flowing helium atmosphere.
Density and viscosity were measured with a viscometer-
densimeter SVM 3000 (Anton Paar) from 298.15 to 373.15 K
at atmospheric pressure. The standard uncertainties for the
temperature, viscosity, and density are u(T) = 0.01 K, ur(η) =
0.002, and u(ρ) = 0.0003 g·cm−3, respectively. The viscometer/
densimeter was calibrated with mineral oils, namely, S3, N7.5,
N26, and N415 according to the manufacturer’s specification.
2.4. Electrochemical Analysis. The electrochemical
stabilities of the electrolytes were determined by linear
voltammetry using glassy carbon as a working electrode at a
scan rate of 0.05 V·s−1 starting from an open circuit potential
until the density current reached was within 150 μA·cm−2. The
stability was determined by a linear sweep starting at the OCP
and moving to the positive or negative potentials. Platinum and
silver were used as the counter and pseudoreference electrodes,
respectively. Experiments were performed with an Autolab
MAC80132 (Metrohm). The standard uncertainty of the
applied potential is [0.001 + 0.001] V.
2.1. Materials. N-n-Butyl-N-methylpyrrolidinium bromide,
lithium bis(fluorosulfonyl)imide, lithium bis(trifluorosulfonyl)-
imide, triethylphosphine, N-n-butyl-N-methylpiperidinium bis-
(trifluoromethylsulfonyl)imide, 1-bromopentane, and 2-bromo-
ethylmethyl ether were used as received from the supplier
(Table 1). The ILs were dried under vacuum at optimal
temperatures for several days until the amount of water was
under 0.0001 weight fraction of water; the moisture content of
all the electrolytes studied here were determined via Karl Fisher
titration (Metrohm).
2.2. Ionic Liquid Synthesis. The ILs, [BMPYR][TFSI],
[P2225][TFSI], [P222201][TFSI], [BMPYR][FSI], and [P2225]-
[FSI], were synthesized as described elsewhere.10−12 Briefly, 26
mmol of cations (BMPYRBr or P2225Br or P222201Br) were
dissolved in 50 mL of water and mixed with 26 mmol of anion
(LiTFSI or LiFSI) which were also dissolved in 50 mL of water.
The mixture was stirred for 2 h. The resultant IL is immiscible
in water and was separated using dichloromethane. The phase
containing the IL was washed five times with water, stirred with
activated carbon for 2 h, and subjected to column
chromatography (alumina, dichloromethane).
[BMPYR][FSI] Data. Elemental analysis data: Found: C; 33.3;
H; 6.29; N; 8.71. Calcd for C9H20N2S2O4F2: C; 33.5; H; 6.25;
N; 8.69; δH (500 MHz; CD3CN; ppm): 0.95−0.98 (3H; t; J =
7.5 Hz); 1.33−1.41 (2H; sh; J = 8 Hz); 1.68−1.75 (2H; m);
2.13−2.17 (4H; m); 2.93 (3H; s); 3.20−3.23 (2H; m); 3.35−
The ionic conductivity was determined by electrochemical
impedance spectroscopy (EIS) of two parallel Pt electrodes
Table 1. Sample Information
water content (weight
chemical name
abbreviation
CAS No.
source
fraction)
N-n-butyl-N-methylpyrrolidinium bromide (≥99.0%)
triethylphosphine (≥95%)
1-bromopentane (98%)
BMPYRBr
P222
93457-69-3
554-70-1
SIGMA-Aldrich
SIGMA-Aldrich
SIGMA-Aldrich
SIGMA-Aldrich
Solvay
5Br
110-53-2
2-bromoethyl methyl ether (90%)
201Br
6482-24-2
90076-65-6
lithium bis(trifluoromethylsulfonyl)imide (99.9%)
N-n-butyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide (99%)
N-n-butyl-N-methylpyrrolidinium bis(fluorosulfonyl imide (99%)
N-n-butyl-N-methylpyrrolidinium bis(trifluorosulfonyl imide (99%)
triethyl-n-pentylphosphonium bis(fluorosulfonyl)imide (99%)
triethyl-n-pentylphosphonium bis(trifluorosulfonyl)imide (99%)
LiTFSI
[BMP][TFSI]
[BMPYR][FSI]
[BMPYR][TFSI]
[P2225][FSI]
[P2225][TFSI]
[P222201][TFSI]
623580-02-9 Iolitec
this work
≤0.0001
≤0.0001
≤0.0001
≤0.0001
≤0.0001
≤0.0001
this work
this work
this work
this work
triethyl (2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl)imide
(99%)
B
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a
Table 2. Selected Properties of the ILs at 298.15 K at p = 0.1 MPa
LI
η (mPa·s)
ρ (g·cm−3
)
σ (mS·cm−·1
)
MM (g·mol−1
)
EW (V)
[BMPYR][FSI]
[P2225][FSI]
53.24
69.38
77.76
48.14
85.30
188.9
1.307
1.219
1.395
1.378
1.304
1.382
8.66
4.63
3.93
3.84
2.10
1.39
322.4
369.4
422.4
457.4
469.4
436.5
5.07
4.30
5.67
4.11
4.93
4.44
[BMPYR][TFSI]
[P222201][TFSI]10
[P2225][TFSI]10
[BMP[TFSI]
a
Viscosity, η, density, ρ, electrical conductivity, σ, and electrochemical windows, EW. Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3,
u(EW) = [0.001 + 0.001] V, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density, electrochemical window, temperature, and pressure,
respectively. The uncertainties for conductivity are listed in Tables 3−8.
with an Autolab MAC80132 in the frequency range of 0.1−
100 000 Hz in the range of temperature = 298.15−361.15 K.
The cell constant (A/l) (a = area and l = distance between the
electrodes) was determined with a standard KCl solution. The
ionic resistance (R) value was taken from the extrapolation at
high frequencies (impedance imaginary part = 0) on the real
part of the impedance. So, the conductivity is σ = RxA/l. The
temperature was controlled by a thermostat with an uncertainty
within 0.02 K.
caused by various factors, such as the percentage weight loss in
which the Td is taken, the heating rate, the gas atmosphere, and
the pan material used the TGA experiment.14
3.2. Viscosity, Conductivity, and Density Measure-
ment. The viscosity of a fluid arises from the internal friction
of the fluid, and it manifests itself externally as the resistance of
the fluid to flow.13 This is a very important property since high
viscosities limit some electrochemical applications and slow
down the rate of diffusion-controlled chemical reactions.20 In
general, ILs are 1−2 orders of magnitude more viscous than
molecular solvents, and their viscosities at room temperatures
typically lie in the range of 10 to over 500 mPa·s.1
3. RESULTS AND DISCUSSION
3.1. Thermal Stability of the Electrolytes. ILs are
thermally stable but certainly decompose at high temperatures.
The decomposition temperature (Td) of ILs can be seen from
Figure S4, using the maximum of the derivate of loss weight as
a reference; the Td values are about 730, 730, 740, 731, 615,
and 644 K for [BMPYR][TFSI], [BMP][TFSI], [P222201]-
[TFSI], [P2225][TFSI], [BMPYR][FSI], and [P2225][FSI],
respectively.
Table 2 presents the viscosity at 298.15 K and Figure 2 and
Tables 3−8 at different temperatures.
The ILs under study have no distinguishable vapor pressure;
consequently, the first event upon heating these ILs is their
thermal decomposition.12 The Td of ILs depends on the
component ion structure, similarly to other thermal properties.2
The nature of the ILs containing organic cations generally
restricts upper stability temperature, up to 623 K, where
pyrolysis occurs if no other lower temperature decomposition
pathways are accessible.13 This implies that, in general, Td is
more affected by anion structure, being less stable for ILs with
strong nucleophilic anions.2,13,14 The decomposition temper-
atures (Td) for all ILs used in the project were higher than 573
K. The [TFSI]-based ILs are more thermally stable than the
[FSI] derivatives presumably due to the higher nucleophilicity
of the [FSI] anion. The [TFSI] has two more fluorine atoms in
the structure than [FSI]. This provokes a better negative charge
delocalization and stabilization by the electron-withdrawing
effect of halogen atoms.2 Consequently, [FSI] has a stronger
nucleophile effect compared with the [TFSI] anion. It should
also be emphasized that the thermal decomposition of the ILs
was found to be a time-dependent process. Indeed, the long-
term thermal stability of ILs can be more than 50 K lower than
the decomposition temperature obtained by step-tangent
analysis of rising temperature scanning differential thermal
analysis (TGA-onset) experiments13 that are commonly
performed at a heating rate of 10 K·min−1.
Figure 2. Arrhenius plots of the viscosity of neat ILs. [BMP][TFSI]
(red *), [P2225][TFSI] (light green ), [BMPYR][TFSI] (light blue
□
■
○
●
), [P2225][FSI] (dark green ), [BMPYR][FSI] (dark blue ),
△
[P222201][TFSI] ( ). The lines, with the same color nomenclature,
represent the best fits of the VTF equation.
The full lines in Figure 1 represent the best fits of the VTF
(Vogel−Tammann−Fulcher) equation:21
⎛
⎜
⎝
⎞
⎟
⎠
B
η = η exp
o
T − T
(1)
0
where η0, B, and T0 are adjustable parameters whose values can
be seen in Table S1. T0 is the temperature at which the viscosity
(η) goes to zero. The relationship B/T0 can be related to the
fragility of the liquid or, in other words, how the transport
properties vary with the temperature change.21,22 The transport
properties of strong liquids (high B/T0) suffer less change with
the temperature than do weak liquids (low B/T0). Table S1
show the VTF parameters for viscosity. For both FSI-based ILs,
B/T0 is higher compared with TFSI derivative, indicating that
The Td reported in the literature are 698,15 685,16 677,17
653,17 563,18 and 58219 for [BMPYR][TFSI], [BMP][TFSI],
[P222201][TFSI], [P2225][TFSI], [BMPYR][FSI], and [P2225]-
[FSI], respectively. These values show the same trend of our
data in the sense that [FSI]-based ILs present a lower thermal
stability. The discrepancies between measured values could be
C
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the latter are more fragile, bringing about the “funnel shape”
curve observed in Figure 1. It is worth noting, however, that
fragility analyses are usually made near the glass transition
temperature, and in this work, we are considering the strong/
fragile classification only in the temperature range studied here.
A different η(T) profile must be considered at a different range
of temperatures.9
Table 3. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [BMPYR][FSI] at Several
Temperatures and p = 0.1 MPa
a
T (K)
density (g·cm−3
)
viscosity (mPa·s) conductivity (mS·cm−1
)
298.15
300.15
306.15
312.15
318.15
324.15
330.15
336.15
341.15
348.15
355.15
361.15
373.15
1.307
1.306
1.301
1.296
1.292
1.287
1.283
1.278
1.274
1.269
1.264
1.259
1.251
53.24
49.74
40.74
33.83
28.42
24.15
20.73
17.94
16.01
13.77
11.95
10.65
8.610
8.66
9.17
10.9
The viscosity of RT ILs is, generally, affected by the nature of
both the cations and anions. It is noticeable from Table 2 that
all ILs containing [FSI] ([BMPYR][FSI] and [P2225][FSI]) are
19 and 31% less viscous than their [TFSI] analogues, which is
ascribable to the smaller size of the [FSI] anion (van der Waals
force) and the weaker electrostatic interactions caused by the
smaller interaction energies for the [FSI] based ILs.23−25
Furthermore, the same result was found when [FSI]-Li+ and
[TFSI]-Li+ interactions were compared.24,26 The lower
viscosity observed in [P222201][TFSI] compared with [P2225]-
[TFSI] is attributed to a weaker electrostatic cation−anion
interaction provoked by the alkyl ether chain. It is also
suggested that the methoxy group can decrease the positive
charge in the central atom. Additionally, the alkyl ether group
increases the chain flexibility, which increases the ionic
mobility.10,17 The high viscosity of of [BMP][TFSI] was
explained by DFT calculations which clearly showed the higher
number of hydrogen bonding present in piperidinium based ILs
compared to imidazolium.27 This tendency of piperidium
cation to form viscous ILs was also observed using a
tetracyanoborate anion and a pyrrolodinium cation as a
comparison.9 The viscosity values for these ILs at 298 K
agree with those reported in the literature: 53.2,28 70,19 76,16
44,17 88,17 and 18216 mPa·s for [BMPYR][FSI], [P2225][ FSI],
[BMPYR] [TFSI], [P222201][TFSI], [P2225][TFSI], and [BMP]-
[FSI] respectively. The divergences between measured values
can be ascribing to the issues with the instrument or differences
in purity of the ILs.
12.7
14.7
16.8
19.1
21.6
26.9
33.1
a
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
= 0.05σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
conductivity, temperature, and pressure, respectively.
Table 4. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [BMPYR][TFSI] at Several
a
Temperatures and p = 0.1 MPa
T (K)
density (g·cm−3
)
viscosity (mPa·s) conductivity (mS·cm−1
)
298.15
300.15
306.15
312.15
318.15
324.15
330.15
336.15
341.15
348.15
355.15
361.15
373.15
1.395
1.393
1.388
1.382
1.377
1.372
1.367
1.361
1.357
1.351
1.345
1.340
1.330
77.76
71.11
54.97
43.41
34.92
28.56
23.69
19.91
17.37
14.53
12.31
10.78
8.272
3.93
4.23
5.20
6.38
7.72
9.24
10.8
12.6
16.4
21.1
The ionic conductivity as an electrochemical window (EW)
of a IL are of critical importance in its selection for an
electrochemical application.13 Unlike aqueous electrolytes, ILs
display intrinsic ionic conductivity because; by definition they
consist of ions. Table 2 show the ionic conductivity at 298.15 K
and Figure 3 and Tables 3−8 at different temperatures.
a
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
= 0.05σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
conductivity, temperature, and pressure, respectively.
As well as viscosity, the conductivity is often best described
using the empirical Vogel−Tammann−Fulcher (VTF) equa-
tion:
⎛
⎜
⎝
⎞
⎟
⎠
−B′
σ = σ exp
o
′
T − T
(2)
0
The parameters can be seen in Table S2. The B′/T′0
quotients for all of the systems are consistent with those
obtained in the viscosity analyses.
The ionic conductivities measured for the [FSI] based ILs
were found (at RT) to be approximately 50% greater than their
[TFSI] analogues. This inverse trend with respect to viscosity is
expected taking into account that, in general, the mobility of
charge carriers correlates with their rate of diffusion in the
electrolyte. The diffusion coefficient is also inversely propor-
tional to the viscosity of the electrolyte (Stokes−Einstein
equation).1
Figure 3. Arrhenius plot of ILs’ temperature-dependent conductivity
data. [BMP][TFSI] (red *), [P2225][TFSI] (light green ),
□
■
○
[BMPYR][TFSI] (light blue ), [P2225][FSI] (dark green ),
●
△
[BMPYR][FSI] (dark blue ), [P222201][TFSI] ( ). The lines, with
the same color nomenclature, represent the best fits of the VTF
equation.
Nevertheless, these facts indicate that the decrease of the
ionic conductivity in ILs containing [TFSI] cannot be
D
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Table 5. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [P2225][FSI] at Several
Temperatures and p = 0.1 MPa
Table 7. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [P2225][TFSI] at Several
Temperatures at p = 0.1 MPa
a
a 10
,
T (K)
density (g·cm−3
)
viscosity (mPa·s) conductivity (mS·cm−1
)
T (K)
density (g·cm−3
)
viscosity (mPa·s)
conductivity (mS·cm−1
)
298.15
300.15
306.15
312.15
318.15
324.15
330.15
336.15
341.15
348.15
355.15
361.15
373.15
1.219
1.217
1.213
1.208
1.204
1.199
1.195
1.191
1.187
1.182
1.177
1.173
1.165
69.38
64.02
50.72
40.86
33.40
27.65
23.17
19.66
17.25
14.52
12.37
10.86
8.543
4.63
4.95
6.02
7.23
8.61
25
27
33
39
45
51
57
63
68
75
82
88
101
1.304
1.302
1.297
1.292
1.287
1.282
1.277
1.272
1.268
1.262
1.256
1.251
1.241
85.30
77.43
58.68
45.42
35.82
28.79
23.50
19.46
16.79
13.86
11.58
10.02
7.531
2.10
2.23
2.86
3.58
4.35
5.21
6.12
7.11
10.2
11.9
13.7
17.6
22.3
9.17
11.7
a
a
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
= 0.06σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
= 0.06σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
conductivity, temperature, and pressure, respectively.
conductivity, temperature, and pressure, respectively.
Table 6. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [BMP][TFSI] at Several
Temperatures and p = 0.1 MPa
Table 8. Experimental Values of Density, ρ, Viscosity, η, and
Electrical Conductivity, σ, of [P222201][TFSI] at Several
Temperatures and p = 0.1 MPa
a
a10
,
T (K)
density (g·cm−3
)
viscosity (mPa·s)
conductivity (mS·cm−1
)
T (K)
density (g·cm−3
)
viscosity (mPa·s)
conductivity (mS·cm−1
)
25
27
33
39
45
51
57
63
68
75
82
88
100
1.382
1.380
1.375
1.370
1.365
1.360
1.355
1.349
1.345
1.339
1.333
1.329
1.319
188.9
168.0
120.2
88.67
67.13
52.01
41.12
33.12
27.98
22.51
18.36
15.62
11.62
1.39
1.53
2.02
2.64
3.37
4.24
5.19
6.26
25
27
33
39
45
51
57
63
68
75
82
88
101
1.378
1.376
1.370
1.365
1.360
1.354
1.349
1.344
1.339
1.333
1.327
1.322
1.311
48.14
44.28
34.95
28.10
22.97
19.04
16.01
13.62
11.98
10.15
8.692
7.672
5.990
3.84
4.11
4.98
6.08
7.19
8.48
9.87
11.3
8.76
11.9
14.5
18.4
a
a
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
Standard uncertainties are ur(η) = 0.002, u(ρ) = 0.0003 g·cm−3, uc(σ)
= 0.06σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
= 0.06σ, u(T) = 0.01 K, and u(p) = 0.005 MPa for viscosity, density,
conductivity, temperature, and pressure, respectively.
conductivity, temperature, and pressure, respectively.
completely attributed to the detrimental effect of the viscosity
because the viscosity of ILs based on [FSI] is 19−30% lower
than their corresponding analogues containing [TFSI]. In other
words, the increase of the ionic conductivity of the ILs
containing [FSI] cannot be attributed merely to the resistance
effect of the ions that is associated with the viscosity. Therefore,
to explain this result, one must consider the actual
concentration of the charge carriers in both types of ILs.
To evaluate the ionicity of the reported electrolytes, that is,
state of ionization of each IL, a classical Walden rule diagram
was used. This approach allows evaluating the state of
ionization of ILs and classifying them according to the position
in the Walden plot.29,30 The Walden rule correlates the
logarithm of molar conductivity (Λ) and the logarithm of
fluidity (1/η) through a temperature-dependent constant (k),
as reported in Figure 4a and b.
Figure 4. Walden plot of neat ILs: (a) log−log, (b) linear scale.
□
[BMP][TFSI] (red *), [P2225][TFSI] (light green ), [BMPYR]-
■
○
[TFSI] (light blue ), [P2225][FSI] (dark green ), [BMPYR][FSI]
●
△
(dark blue ), [P222201][TFSI] ( ).
Plotting the molar conductivity (Λ) instead of the absolute
conductivity (κ), to an extent, normalizes the effects of molar
concentration and density on the conductivity and, thus, gives a
better indication of the number of mobile charge carriers in an
IL.13
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Figure 4a presents the Walden plot, the logarithm of molar
ionic conductivity as a function of the logarithm of fluidity
(inverse of viscosity). The dotted line indicates the ideal
Walden line (dilute aqueous KCl solution). Deviations from
this line is related to the IL ionicity. As it can be observed from
Figure 4a, the [FSI]-based ILs, namely, [BMPYR][FSI] and
[P2225][FSI], are closer to the ideal line; the rest of the
substances lie even below, indicating that the former have
higher number of charge carriers lessening the aggregate
formation between cations and anions. The distance from the
line and the position of the set of point on the graph suggests
that both ILs, according to the Walden representation, are
closer to the “good” IL zone.29,30
Another way to present the Walden plot is shown in Figure
4b, with a larger slope signifying a larger ionicity change with
respect to change in viscosity. The slopes of these lines are 85,
73, 66, 60, 47, and 41 S·cm2·mol−1·(mPa·s)−1 for [BMPYR]-
[FSI], [P2225][FSI], [BMPYR][TFSI], [BMP][FSI][P222201]-
[TFSI], and [P2225][TFSI], respectively. This indicates that
both ILs have a higher increase in molar ionic conductivity
related to an equal decrease in viscosity or increase in fluidity
compared to [TFSI] derivates.
Figure 5. Density at different temperatures for neat ILs. [BMP][TFSI]
(red *), [P2225][TFSI] (light green ), [BMPYR][TFSI] (light blue
□
■
○
●
), [P2225][FSI] (dark green ), [BMPYR][FSI] (dark blue ),
△
[P222201][TFSI] ( ).
The conductivities reported for these ILs at 298 K are 6.9,31
3.0,19 3.0,32 3.58,17 1.73,17 and 1.116 for [BMPYR][FSI],
[P2225][FSI], [BMPYR][TFSI], [P222201][TFSI], [P2225][TFSI],
and [BMP][FSI], respectively. Unlike the viscosity the
comparison of the obtained conductivities with the literature
seems to have higher differences which are assigned to the
more sensitivity of EIS to systematic error. For instance, for
[BMPYR][TFSI], the following conductivity values can be
found: 4.8,18 6.2,28 6.9,31 and so on.
In Figure 4 and Tables 3−8, the densities of all of the ILs are
shown as a function of temperature. All ILs are denser than
water. Unlike the viscosity and conductivity, it is observed that
the densities decrease slightly with an increase in temperature.
In addition, all of the ILs studied exhibited a similar degree of
volume expansion with temperature. From Figure 4 it is
possible to infer that the density of the ILs increases with the
increasing molecular weight of the anion. [BMPYR][TFSI] and
[P2225][TFSI] have a higher density with their [FSI] counter-
parts. The molecular weights of [TFSI] and [FSI] are 280.1
and 180.1 g·mol−1, respectively. This trend is most likely due to
the nature of the anions studied. Since none of the anions have
a particularly long or steric chain, it is possible that they all can
occupy favorable, relatively close-approach positions around the
cation, resulting in high densities.33
The effect of having different substituents on the cation is
also shown in Figure 5. It is noticeable that the pentyl chain
[P2225] present a more irregular and less flexible structure
compared to [P222201] that provokes less dense ILs. In general,
the density decreases as the alkyl-substituted chain length on
the cations or anions increases. Adding CH2 groups to the alkyl
chain on the cyclic cation decreases the density since CH2 is
less dense than the cyclic cation ring.20,33 In addition, the long
chain could enable a more irregular shape in the ion hindering
and packing. At 298 K, the densities in Table 2 matches
literature values, namely, 1.307,28 1.24,19 1.393,16 1.39,17 1.32,17
and 1.37916 for [BMPYR][FSI], [P2225][FSI], [BMPYR]-
[TFSI], [P222201][TFSI], [P2225][TFSI], and [BMP][FSI],
respectively. Unsurprisingly, the impact of impurities are less
for density than in viscosity.13
Figure 6. Combination of two linear voltammograms recorded by
starting from open circuit potential and cycling to either more positive
or negative potentials. Scan rate of 50 mV·s−1 and 150 μA·cm−2 of
current cutoff. Working electrode: glassy carbon; counter electrode:
Pt; pseudoreference electrode: Ag; [BMP][TFSI] (red line), [P2225]-
[TFSI] (light green line), [BMPYR][TFSI] (light blue line),
[P2225][FSI] (dark green line), [BMPYR][FSI] (dark blue line),
[P222201][TFSI] (black line).
sweep voltammograms measured in the neat ILs on GC
electrode. The overall values of the EW are listed in Table 2. By
definition, the EW of the IL is the voltage range in which the
substance is neither oxidized or reduced at an electrode.1,34
Therefore, for a specific IL, the oxidation potential is
determined by the voltage of the anion oxidation (HOMO
energy), and the reduction potential is determined by the
voltage of the cation reduction (LUMO energy).34
Nevertheless, Ong et al.35 has demonstrated that both
cations and anions can play a role in the overall electrochemical
stability of the ILs. The values of the potential window are at
least 4.0 V for all ILs investigated in this work. ILs derivative
from [TFSI]−[P2225][TFSI] and [BMPYR][TFSI] registered
an electrochemical window of 4.9 and 5.7 V, respectively. These
values are slightly higher than those presented by the [FSI]-
based ILs, whose values were 4.3 and 5.1 V for [P2225][FSI] and
[BMPYR][FSI], respectively. The side chain that contains ether
in [P222201][TFSI] causes a decrease of the electrochemical
window, as previously reported.36 It was also found that the
pyrrolidynium cation seems to increase the electrochemical
3.3. Electrochemical Analysis. Working Potential
Window of the Ionic Liquids. Figure 6 illustrates the linear
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Journal of Chemical & Engineering Data
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stability compared with the other cations. However, more
research needs to be carried out to know what happens in each
application, for instance, in the lithium ion battery, during
lithium intercalation taking into account that the EW was
measured using glassy carbon as the working electrode.
We have avoided comparing the EW with those reported in
the literature. Since the electrode material, sweep rate, and the
cutoff current are arbitrary, the comparison is almost
impossible. The reader is referred to corresponding bibliog-
raphy, for BMPYRTFSI37 BMPTFSI,38 P2225FSI,19 P2225TFSI,17
P222201TFSI,17 and BMPYRFSI.18
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.7b00458.
■
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +54 1130912350. E-mail: rtorresi@iq.usp.br.
ORCID
Roberto M. Torresi: 0000-0003-4414-5431
Funding
The authors acknowledge FAPESP (15/26308-7) for funding.
N.S.R. thanks FAPESP (2014/01987-6 and 2015/11164-0) for
scholarship support. This work was financially supported by the
Natural Sciences and Engineering Research Council of Canada
(NSERC) by a Discovery Grant to D.B. The research
infrastructure of NanoQAM was used during this work.
■
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Notes
The authors declare no competing financial interest.
Author e-mail: Daniel Belanger, belanger.daniel@uqam.ca;
Nedher Sanchez-Ramirez, nedher2002@gmail.com; Birhanu
Desalegn Assresahegn, mignot22008@gmail.com.
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