Physical and electrochemical properties of 1-alkyl-3-methylimidazolium tetrafluoroborate for electrolyte

Article abstract of DOI:10.1016/S0022-1139(02)00322-6

Three kinds of ionic liquids, 1-alkyl-3-methylimidazolium tetrafluoroborate (n = 2-4), were prepared and fundamental properties of ionic liquids and those mixed with industrially used organic solvents (PC, GBL and AN) were investigated compared to solid salts, TEMABF₄. It was found that degree of ionization of the ionic liquids were almost same as that of TEMABF₄ from the conductivity measurement in diluted system of PC. The ionic liquids and the organic solvents intermingle with each other. Some enhancement in conductivity was observed compared to TEMABF₄.

Full text of DOI:10.1016/S0022-1139(02)00322-6

Journal of Fluorine Chemistry 120 (2003) 135–141
Physical and electrochemical properties of
1-alkyl-3-methylimidazolium tetrafluoroborate for electrolyte
Tetsuo Nishida*, Yasutaka Tashiro, Masashi Yamamoto
Research Division, Stella Chemifa Corporation 227, 7-cho, Kaizan-cho, Sakai, Osaka 590-0982, Japan
Received 1 August 2002; received in revised form 9 September 2002; accepted 20 September 2002
Abstract
Three kinds of ionic liquids, 1-alkyl-3-methylimidazolium tetrafluoroborate (n ¼ 2À4), were prepared and fundamental properties of ionic
liquids and those mixed with industrially used organic solvents (PC, GBL and AN) were investigated compared to solid salts, TEMABF4. It
was found that degree of ionization of the ionic liquids were almost same as that of TEMABF4 from the conductivity measurement in diluted
system of PC. The ionic liquids and the organic solvents intermingle with each other. Some enhancement in conductivity was observed
compared to TEMABF4.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Ionic liquid; 1-Alkyl-3-methylimidazolium tetrafluoroborate; Organic electrolyte
1. Introduction
The characteristics of ionic liquids, for example, high
ionic conductivity, physical stability, non-volatility,
and, etc. are attracted as solutions of those problems.
Several studies have been done so far and many researchers
tried to apply the liquid to devices but the results pointed
out high viscosity and lack of electrochemical stability
[11–16].
Electrochemical double layer capacitor (EDLC) is one
of the candidates for auxiliary power supply of next-
generation automobile. Propylene carbonate (PC) solutions
of quaternary ammonium tetrafluoroborate are often used
for electrolytes of EDLC. The numerous investigations of
quaternary ammonium cation structures and those solutions
conclude that triethylmethylammonium tetrafluoroborates
(TEMABF4) and tetraethylammonium tetrafluoroborates
(TEABF4) are the best electrolytes at the present time
[21–23]. However, these solid salts have limitations of
ionic concentration and conductivity, which are caused by
the solubility to solvents especially at low-temperature
range.
Various kinds of ionic liquids have been taken as a subject
for studies since a room temperature molten salt which is
inert against water was reported by Wilkes and Zaworotko
in 1992 [1]. Typical ionic liquids consist of organic cation
with delocalized charges and organic or inorganic fluor-
oanions [2–10]. For example, 1-alkyl-3-methylimidazo-
lium tetrafluorobrates is one of the most popular ionic
liquids. Some of them preserve the state of liquid in wide
temperature range. They are difficult to be volatile and
expected to have high conductivity because they are essen-
tially formed by ions only. These features are not seen on
usual materials and receive much attention for applications
[11–20].
Performances of secondary battery represented by lithium
ion battery have been elevated remarkably for last decade
due to earnest study on electrode materials, electrolyte and
cell arrangement. Now farther improvement is required
for energy storage systems of hybrid electric vehicle
(HEV) and fuel cell electric vehicle (FCEV) because those
devices have several drawbacks to be gotten over such as
power decline at low-temperature and degradation at high
temperature.
Ionic liquids exist as liquids at room temperature and they
are soluble to organic solvents. Several properties of some of
1-alkyl-3-methylimidazolium tetrafluoroborates have been
described in a previous report [24]. In this work, we prepared
three kinds of 1-alkyl-3-methylimidazolium tetrafluorobo-
rates, 1-ethyl-(EMIBF4), 1-propyl-(MPIBF4) and 1-butyl-
(BMIBF4), and investigated the physical and electrochemical
properties as electrolyte.
* Corresponding author. Tel.: þ81-72-229-3104; fax: þ81-72-229-5916.
E-mail address: nishida@stella-chemifa.co.jp (T. Nishida).
0022-1139/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 3 2 2 - 6

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T. Nishida et al. / Journal of Fluorine Chemistry 120 (2003) 135–141
Table 1
Physical properties of 1-alkyl-3-methylimidazolium tetrafluoroborate
R
Tg (8C)
Tc (8C)
Tm (8C)
Td (8C)
Z (mPa s)
d (g cmÀ3
)
s (m S cmÀ1
)
Ethyl
Propyl
Butyl
À92
À88
À85
À58
À38
None
13
À17
None
447
435
435
37
103
180
1.28
1.24
1.21
14
5.9
3.5
Viscosities, densities and conductivites were measured at 25 8C. Tg, glass transition temperature; Tc, crystallization point; Tm, melting point; Td,
decomposition temperature.
2. Results and discussion
BF4 anions to crystallize due to steric problem arising from
the length of alkyl chain.
2.1. Synthesis of ionic liquids
Decomposition temperature was decided by thermo-grav-
imetrical reduction point. The decomposition of these ionic
liquids occurred at almost the same temperature regardless
the length of alkyl chain.
The conductivity of EMIBF4 was the highest among the
ionic liquids in this study. It exists as a super-cooled liquid
below the melting point during this measurement. The
temperature dependence of the viscosity and conductivity
of the ionic liquids are shown in Figs. 2 and 3, respectively.
The length of the alkyl group influences those properties.
Liquids become more viscous with the elongation of alkyl
chain. Entwining of the alkyl chains is considered to be one
of the reasons for the increase of viscosity.
Generally, 1-alkyl-3-methylimidazolium tetrafluorobo-
rates are synthesized by ion-exchange reaction of 1-alkyl-
3-methylimidazolium halide and tetrafluoroborate of metals
such as silver, sodium and potassium. Here, 1-alkyl-3-
methylimidazolium halides are prepared by the reaction
of 1-methylimidazole and alkyl halide. However, prepara-
tion of high-purity ionic liquid is difficult because AgX
(X ¼ Cl, Br, I), which is by-product, is soluble to ionic
liquids. Moreover, use of Ag salts takes much cost. Another
way to synthesize them is the reaction of fluoroboric acid
directly with 1-alkyl-3-methylimidazolium halide. This
reaction also forms by-product HX instead of AgX but
gaseous by-products is removed easily from ionic liquids
and high-purity ionic liquids are synthesized with high yield.
This reaction reduces the cost because fluoroboric acid is
cheaper than tetrafluoroborate salts. The latter reaction was
taken in order to synthesize 1-alkyl-3-methylimidazolium
tetrafluoroborate to be examined in this report.
Conductivity decreased with the increase of viscosity. In
the middle temperature range, the conductivity exhibits the
Arrhenius-type relation, however, not at lower temperatures.
There is no relationship between potential window and the
length of alkyl chain among these ionic liquids. Linear
2.2. Properties of ionic liquids
Physical properties obtained in the present study (glass
transition point, crystallization point, melting point, decom-
position point, viscosity and conductivity) are summarized
in Table 1. DSC curves of the ionic liquids are shown in
Fig. 1. Samples are cooled down to À150 8C with liquid
nitrogen and successively heated at the rate of 5 8C minÀ1
.
The curve of EMIBF4 shows glass transition, crystallization
and melting. There is a small endothermic peak at À18 8C on
the curve of EMIBF4, which was observed repeatedly for the
other samples of EMIBF4. The question of whether this
endothermic peak is ascribed to essential characteristics of
EMIBF4 or the existence of contaminants needs further
research. A higher glass transition point is observed as
the cation has a longer alkyl side chain. EMIBF4 and
MPIBF4 have crystallization and melting points. The former
has a lower crystallization point and higher melting point
than the latter. BMIBF4 does not exhibit crystallization. This
might be explained by the difficulties of BMI cations and
Fig. 1. DSC curves for EMIBF4, MPIBF4 and BMIBF4.

T. Nishida et al. / Journal of Fluorine Chemistry 120 (2003) 135–141
137
Fig. 3. Arrhenius plots for viscosity of ionic liquids. (*) EMIBF4, (~)
MPIBF4, (&) BMIBF4.
Fig. 2. Temperature dependences of the conductivity of ionic liquids. (*)
EMIBF4, (&) MPIBF4, (~) BMIBF4.
Table 2
Physical properties of selected solvents at 25 8C
sweep voltammograms on glassy carbon electrode are
shown in Fig. 4. Difference of the cations does not effect
on both the oxidation and reduction potentials. The potential
windows were approximately 4 V as for all these ionic
liquids.
Solvent
Viscosity (mPa s)
Dielectric (e)
Liquid range (8C)
PC
2.5
1.7
0.3
65
42
36
À49 to 242
À44 to 204
À49 to 82
GBL
AN
2.3. Properties of ionic liquid mixed with solvents
(AN) were chosen in this work from the industrial point
of view. Their physical properties were listed in Table 2.
Propylene carbonate (PC) is often used to EDLC’s electro-
lyte for its high dielectric, wide liquid range and electro-
chemical stability. It was chosen as a main solvent for this
examination.
As to the neat ionic liquids, the viscosity increases and the
conductivity decreases with the increase of the length of
alkyl chain, especially at lower temperature as explained in
the previous section. We examined the properties of the
mixed system of ionic liquid and organic solution to
decrease the viscosity. Three kinds of solvents, propylene
carbonate (PC), g-butyrolactone (GBL) and acetonitorile
Firstly, fundamental properties of electrolyte such as the
association constant and limiting molar conductivity are
Fig. 4. Linear sweep voltammograms of EMIBF4, MPIBF4 and BMIBF4. Scan rate was 50 mV sÀ1
.

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T. Nishida et al. / Journal of Fluorine Chemistry 120 (2003) 135–141
Fig. 6. Concentration dependences of conductivity of ionic liquids and
TEMABF4 dissolved in PC. (*) EMIBF4, (&) MPIBF4, (~) BMIBF4,
(Â) TEMABF4.
water in the solution might cause the difference. The salts
which have larger molecular weight show smaller limiting
molar conductivity. The ionic liquids have almost the same
association constants as TEMABF4. These results suggest
that the ionic liquids do not consist of only ions but some ion
pairs, only dissociated ions being able to contribute to the
conductance in ionic liquids.
TEMABF4/PC solution has limit concentration around
2 M though ionic liquid and PC intermingle easily with each
other. The maximum solubility of EMIBF4 in PC is 6.5 M.
Conductivities of the mixed solutions have the maximums as
shown in Fig. 6. The highest conductivity of 15 m S cmÀ1 is
observed for 2 M TEMABF4/PC. On the other hand, the
value of EMIBF4 is 19 m S cmÀ1 at 2.8 M at 25 8C, that is
40% increase in molar concentration and 25% enhancement
in conductivity. The comparison among three kinds of ionic
liquids/PC solutions indicates that the difference of con-
ductivity appears over 1 M of each solution and the longer
alkyl chain causes the decrease of conductivity.
Fig. 5. Molar conductivity of ionic liquids and TEMABF4 diluted with
PC. (*) EMIBF4, (&) MPIBF4, (~) BMIBF4, (Â) TEMABF4.
examined in diluted systems. A series of different concen-
trations of PC solutions of EMIBF4, MPIBF4, BMIBF4 and
TEMABF4 were prepared in the range from 7:5 Â 10À4 to
1:0 Â 10À2 M. The relationship of the molar conductivity
and concentration are shown in Fig. 5. Molar conductivities
of all the samples show the same tendency. They decrease
with increase of the concentration. Fuoss–Kraus Eq. (1) was
used for calculations of approximate value of the limiting
molar conductivities and association constants. Procedures
of calculations were followed by the previous reports
[25,26].
1
KA cf2L
FðzÞ
L
¼
þ
(1)
L02
L0
FðzÞ
where L is the molar conductivity, L0, limiting molar
conductivity, KA, association constant, f, mean molar activ-
ity coefficient and F(z) is the Fuoss extrapolation function.
Table 3 shows the values of limiting molar conductivities
(L0), association constants (KA), and cations’ single ion
limiting molar conductivities (l0þ) which were calculated
from the anion’s single ion limiting molar conductivity
(20.3 m S cmÀ1) [21]. The value of TEMABF4 is slightly
different from the result reported by Ue [21]. Contaminated
The temperature dependences of conductivities of the
solutions are shown in Fig. 7. Concentration of each sample
Table 3
Calculated results of limiting molar conductivities (L0), single ion molar
conductivities (l0þ) and association constants (KA)
Salts
L0
l0þ
KA
(dmÀ3 molÀ1
)
(S cm2 molÀ1
)
(S cm2 molÀ1
)
EMIBF4
MPIBF4
BMIBF4
TEMABF4
32.40
31.84
30.97
32.05
12.10
11.54
10.67
11.75
12.6
12.4
12.1
12.5
Fig. 7. Temperature dependences of conductivities of the solutions: 2.8 M
EMIBF4/PC (*), 2.2 M MPIBF4/PC (&), 1.6 M BMIBF4/PC (~) and
2.2 M TEMABF4/PC (Â).

T. Nishida et al. / Journal of Fluorine Chemistry 120 (2003) 135–141
139
3. Experimental
3.1. General experimental procedures
Synthesis of ionic liquids is described below in detail.
Thermal properties were analyzed by TG-DTA (RIGAKU,
TG-8110) and DSC (RIGAKU, DSC8230B). Decomposi-
tion temperature was decided by gravimetrical reduction
point of TG-DTA curve. Heating rate and terminal tempera-
ture was set at 10 8C minÀ1 and 800 8C, respectively. The
glass transition point, crystallization point and melting point
were decided by DSC analysis. Initial temperature was
À150 8C, which is cooled with liquid nitrogen, and heated
at the rate of 5 8C minÀ1. These measurements were carried
out under nitrogen atmosphere. The viscosity of solutions
was measured with oscillating type viscosity meter
(YAMAICHI ELECTRONICS, VM-1G). The density of
solutions was measured with density meter by resonant
frequency method (KYOTO ELECTRONICS, DA-130).
Conductivity was measured by conductivity meter
(RADIOMETER ANALYTICAL, CDM210) with a glass
cell with a cell constant 0.85 cmÀ1 (RADIOMETER ANA-
LYTICAL, CDC641T). The calibration of the instrument
was carried out with 0.1N KCl or 0.001N KCl solution
according to conductivity range of measured sample. A glass
vessel was equipped to the conductivity cell so that sample
liquid is sealed up. The cell temperature was controlled to
ꢀ0.1 8C by low-temperature controlled bath into which the
vessel was immersed.
Electrochemical measurement was carried out with three-
electrode cell. Glassy carbon electrode (7:85 Â 10À3 cm2,
BAS 11-2411) was used as working electrode. The surface
of the electrode was polished before every measurement. A
platinum wire was used as counter electrode. A silver wire
was used as a quasi-reference electrode. Potential between a
silver wire electrode and sample liquid was referred to Liþ
(0.5 M)/Li electrode in advance. For example, silver wire
immersed in EMIBF4 showed 3.0 V referred to lithium foil
immersed in 0.5 M LiBF4/EMIBF4 solution separated by
glass cell with a bicole glass (BAS, 11-2051). This measure-
ment was carried out in a vacuum glove box (TAKASUGI
SEISAKUSHO, G-65-MV-AV) under argon atmosphere.
Electrochemical measurements were performed with an
electrochemical measurement system (HOKUTO DENKO,
HZ-3000).
Fig. 8. Concentration dependences of conductivity of TEMABF4 and
EMIBF4 dissolved in GBL. (*) EMIBF4, (Â) TEMABF4.
on this figure is taken from the value which exhibits the
highest conductivity of each solution in Fig. 6. Each sample
of the ionic liquids shows similar dependence to that of
TEMABF4. The conductivities decrease as the temperature
goes down. The solutions of the ionic liquids which have
melting points such as EMIBF4 and MPIBF4 do not exhibit
step-wise conductivity-drop resulting from phase separation
which leads to sudden decrease in ion concentration at the
lower temperatures than their melting points. DSC analysis
of 2.8 M EMIBF4/PC solution was carried out in order to
confirm whether there was phase separation. The DSC curve
did not show endothermic peak ascribed to phase separation
in the temperature range of À150 to 25 8C.
Solutions of EMIBF4 and TEMABF4 mixed with GBL or
AN exhibited higher conductivity as shown in Figs. 8 and 9.
EMIBF4 mingled with GBL and AN as well as PC with each
other. Solubilities of TEMABF4 in GBL and AN are 2.3 and
3.3 M, respectively. The maximum conductivities of
EMIBF4/GBL and EMIBF4/AN are 26 m S cmÀ1 at 2.8 M
and 68 m S cmÀ1 at 2.3 M, respectively.
3.2. Synthetic procedures
3.2.1. Synthesis of 1-ethyl-3-methylimidazolium bromide
The equipments are set up as follows. Reflux condenser,
300 ml dropping funnel and nitrogen gas line (20À30 ml
minÀ1) were connected to three-neck flask to keep the
reactors dry. Stirring was done by magnetic stirrer. Ethyl-
bromide (1.1 mol, TCI) was dropped into 300 ml dehydrated
toluene solution of 1-methylimidazole (1 mol, TCI) for 3 h
at 30 8C. After stirring for 5 h, the reaction solution was
Fig. 9. Concentration dependences of conductivities of TEMABF4 and
EMIBF4 dissolved in AN. (*) EMIBF4, (Â) TEMABF4.

140
T. Nishida et al. / Journal of Fluorine Chemistry 120 (2003) 135–141
Scheme 1. Synthetic methods of 1-alkyl-3-methylimidazolium tetrafluoroborate.
cooled to 0 8C. The precipitated crystal was filtered. Each
starting material was distilled in advance under reduced
pressure with molecular sieve 4A (WAKO). Filtered crystal
was dissolved into 100 ml dehydrated acetonitrile and re-
crystallized by addition of dehydrated ethylacetate (300 ml,
TCI). Whole operations were carried out under nitrogen
atmosphere.
Melting point: 74.5 8C. 1H-NMR (Me2SO2-d6, s (ppm)
relative to Me4Si): 9.27 (s, 1H), 7.83 (t, 1H), 7.74 (t, 1H)
4.20 (q, 2H), 3.85 (s, 1H), 1.40 (t, 3H).
4. Conclusions
From the conductivity measurement in diluted system,
the association constants and limiting molar conductivities
of 1-alkyl-3-methylimidazolium tetrafluoroborates and
TEMABF4 in PC were examined. The limiting molar con-
ductivities of the salts became smaller as their molecular
weights increased. Association constants of the ionic
liquids were almost same as that of the solid solute such
as TEMABF4. This suggests that the ionic liquids do not
consist of only ions but some ion pairs. The ionic liquids
mingled with PC, GBL and AN with each other. These
solutions showed some enhancement in conductivity com-
pared to TEMABF4 solutions.
3.2.2. Synthesis of 1-methyl-3-propylimidazolium bromide
Same equipments for synthesis of 1-ethyl-3-methylimi-
dazolium bromide were applied. Reaction temperature was
70 8C. Melting point was 35.3 8C. 1H-NMR (Me2SO2-d6, s
(ppm) relative to Me4Si): 9.32 (s, 1H), 7.86 (t, 1H), 7.79
(t, 1H) 4.17 (t, 2H), 3.89 (s, 1H), 1.82 (m, 2H), 0.86 (t, 3H).
Acknowledgements
3.2.3. Synthesis of 1-butyl-3-methylimidazolium bromide
Same equipments for synthesis of 1-ethyl-3-methylimi-
dazolium bromide were applied. Reaction temperature was
70 8C. Melting point was 77.6 8C. 1H-NMR (Me2SO2-d6, s
(ppm) relative to Me4Si): 9.11 (s, 1H), 7.77 (t, 1H), 7.70 (t,
1H) 4.16 (t, 2H), 3.84 (s, 3H), 1.76 (m, 2H), 1.26 (m, 2H),
0.90 (s, 3H).
We are thankful to the JSPS 155th Committee on Fluorine
Chemistry for providing us such a valuable opportunity to
present our activity.
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