Regular Papers

Ionic Liquids for Lithium Secondary
Batteries
Katsuya Hayashi†, Yasue Nemoto, Keiji Akuto,
and Yoji Sakurai
Abstract
We focused on the nonflammability of imidazolium salts and investigated these salts as electrolyte solvents for lithium secondary cells. An imidazolium salt, 1-ethyl-3-methylimidazolium tetrafluoroborate(EMI-BF4), has been reported to exhibit a high reduction potential of about 1 V vs. Li/Li+, which is insufficient for the electrolyte of a lithium secondary cell. Therefore, we attempted to lower its reduction
potential by modifying its molecular structure. We synthesized two types of alkylated EMI-BF4: 1-ethyl2,3,4,5-tetramethylimidazolium tetrafluoroborate (ETMI-BF4) and 1,2-diethyl-3,4(5)-dimethylimidazolium tetrafluoroborate (DEDMI-BF4). Both salts exhibited very little decomposition at 0 V vs. Li/Li+
and a wide electrochemical window up to 5 V vs. Li/Li+. Moreover, DEDMI-BF4 had a relatively low
melting point of about 20°C and a good specific conductivity of 1.44 mS cm–1 at 20°C.

1. Introduction
High-energy-density lithium ion batteries are
expected to be employed as energy storage devices
for uninterruptible power supplies for telecommunications, load leveling equipment, and electric vehicles. This application requires a large lithium ion battery with a high energy. However, it is generally difficult to ensure the safety of such batteries, whichmust not catch fire or burst when the battery experiences problems such as a short circuit, overcharging,a high temperature, or a crushing force.
Various organic solvent-based solutions have been
widely marketed for use as lithium ion battery electrolytes, including propylene carbonate, ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate,
diethyl carbonate, and γ-butyrolactone as the solvent.
Electrolytes with these organic solvents provide thefollowing features: a high lithium salt solubility of† NTT Microsystem Integration Laboratories
Present address: NTT Energy and Environment Systems Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: hayashik@aecl.ntt.co.jp

48

1.0 to 3.0 mol dm–3, high specific conductivity over awide temperature range such as –20 to 60°C, a wide
electrochemical window of 0 to over 5.0 V vs. Li/Li+,and good chemical stability against both the cathodeand anode.
However, these organic solvents are flammable andvolatile, which makes the batteries that employ thema safety risk. Therefore, various measures have beentaken to make lithium ion batteries safe. However,year by year the energy density has increased, andlarger batteries are being developed. This raises serious concerns about the future safety of this type ofbattery.
One promising solution is to use a nonflammable,
nonvolatile electrolyte since this would eliminate
most of the flammable material from the battery. This
nonflammability is effective in preventing batteries
from catching fire, and a nonvolatile electrolyte prevents batteries from bursting. One candidate for anonflammable electrolyte solvent is a room-temperature molten salt (ionic liquid).
Ionic liquids consist of a cation and an anion, andthe total charge of the compound is neutral. They generally have a number of features including nonflamNTT Technical Review

Regular Papers(PF 6–, CF 3SO 3–, and BF 4–) shows good stability
against air and moisture [3]. However, the reduction
potential of these EMI salts is about 1 V vs. Li/Li+,
which is insufficient for lithium ion battery applications. It is thought that the reduction potential of thesesalts depends mainly on EMI+ [4]. There have beensome reports of lithium cell applications using imidazolium salts with high potential anodes [5]-[8] orelectrolyte additives [9], [10]. If we can obtain an
imidazolium salt with no decomposition within thepotential range of a lithium ion cell, we believe that
an electrolyte with this salt could enhance the safetyof high-energy lithium ion cells.
In this study, we modified the cation structure of 1ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) to lower its reduction potential for use in lithium
ion batteries.

mability, nonvolatility, a relatively wide electrochemical window, a high specific conductivity, andgood chemical stability. Moreover, they are liquid at
about room temperature. Therefore, ionic liquids arepromising materials for both battery electrolytes andother chemical devices.
Figure 1 shows examples of cations and anions forionic liquids. Butyl pyridinium (BP) cations are
suitable cations for molten salts, but they are not very
stable and have a narrow electrochemical window.
Trimethylpropylammonium (TMPA) cations and Nmethylpiperidinium (PP13) cations are stable and canbe used as the electrolyte of lithium ion batteries [1].
However, their specific conductivity is not very highand only a limited number of counter anions can beused with them to form ionic liquids. 1-ethyl-3methylimidazolium cations (EMI+) are compact
and very stable and have a structure that can be modified. Cl – with AlCl 3 , BF 4 – , PF 6 – , CF 3 SO 3 – andN(CF3SO2)2– are examples of anions for ionic liquids.
Initially, ionic liquids with imidazolium and pyridinium cations were widely studied. For example, 1ethyl-3-methylimidazolium chloride (EMI-Cl) with
AlCl3 provides high specific conductivity, relatively
low viscosity, and a relatively wide electrochemical
window [2]. However, this salt is sensitive to air andmoisture, so it must be handled carefully.
Then it was reported that EMI+ with other anions

2. Experimental
EMI-BF4 (Aldrich) was used as received. EMI-BF4
and other imidazolium salts were dried in vacuo at
80°C for 24 hrs. All salt preparation was performed in
an argon-filled glove box. 1-ethyl-2,3,4,5-tetramethylimidazolium tetrafluoroborate (ETMI-BF4)
and 1,2-diethyl-3,4(5)-dimethylimidazolium tetrafluoroborate (DEDMI-BF4), which were synthesized inthis study for the first time, were identified by H1NMR (JEOL JNM-ECX400), mass spectroscopy

Cations

AnionsCH2CH2CH2CH3N+

Cl–,AlCl3
CH3CH2

N+

N

CH3
BF4–

BP

EMI

CH2CH2CH3

PF6–
CH3CH2

CH3
+

N
CH3

N+

CF3SO3–

CH3

CH3
TMPA

PP13

N(CF3SO2)2–

BP: butyl pyridinium, EMI: 1-ethyl-3-methylimidazolium,
TMPA: trimethylpropylammonium, PP13: N-methylpiperidinium

Fig. 1. Cations and anions for ionic liquids.

Vol. 2 No. 9 Sep. 200449

Regular Papers

Potentiostat

W.E.
(glassy carbon)

LCR bridge

C.E.
(Li metal)
Electrodes(platinum black)

R.E.
(Li metal)
W.E.: working electrode, C.E.: counter-electrode,
R.E.: reference electrodeFig. 3. Electrochemical cell for specific conductivity
measurement.

Fig. 2. Electrochemical cell for electrochemical window
evaluation.

0.0

Current (mA)

(JEOL JMS-700), high-throughput liquid chromatography, HPLC (Shimadzu LC-2010), and elemental
analysis. The melting points of these salts were determined by observation.
The cell we used for the potential measurement(Fig. 2) had three electrodes: a glassy carbon working
electrode, a lithium metal reference electrode, and alithium metal counter electrode. Measurements werecarried out using the linear potential sweep methodwith a potentiostat (BioLogic, Macpile II) at 80°C.
The voltage range with respect to Li/Li+ was from 0
to 5 V, and the sweep speed was 0.1 mV s–1.
We measured the specific conductivity of eachelectrolyte at 1 kHz by using an LCR bridge (GenRad, 1658 RLC digibridge) with a cell (Yanako, Itype cell) in a thermostat (ESPEC, SU-240) (Fig. 3).

0.5

–0.5

C2H5

N

+

CH3

N

–1.0
BF4–

–1.5

0

1

2

3

4

5

6

+

Potential vs. Li/Li (V)

3. Results and discussion

Fig. 4. Potential-current curve of EMI-BF4.

3.1 Drawback of EMI-BF4 and improvement
strategy
The potential-current profile of EMI-BF4 with athree-electrode cell is shown in Fig. 4. The reduction
current of EMI-BF 4 appeared at around 1 V vs.
Li/Li+, which is the same as that in a previous report[4]. This reduction decomposition of EMI-BF4 wasdue to imidazolium cations (EMI+) [4], and it implies

that we cannot use conventional graphite anode material with a redox potential close to 0 V vs. Li/Li+ inthis salt. We believe this high reduction potential iscaused by the high charge density of the cations andthat reducing this charge density will lead to a low
reduction potential.
The imidazolium cation itself has one positive
charge, and the position of the highest charge density

50

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Regular Papers

Imidazolium cation for alkylation
R2

1-ethyl-3-methylimidazolium cation (EMI cation)
H

C

C

R1

R3
N

CH3

N

+

C

C2H5
N

C

C

R5

R4

H

2,4,5-substituted EMI cation
R2

C

C

C2H5

C2H5

CH3

H

CH3

N

+

C

C

H

2-substituted EMI cation
R2

N

N

+

N

C

C

H

N

+

C

R5

R4

Fig. 5. Structure of modified EMI cations.
CH3

CH3

CH3

Br–
N

CH3

CH3

N

C2H5
C2H5Br

CH3

N

BF4–
+

CH3

C2H5

CH3

N

HBF4

CH3

N

CH3

+

CH3

N

CH3
ETMI-BF4

ETMI-BF4: 1-ethyl-2, 3, 4, 5-tetramethylimidazolium tetrafluoroborate

Fig. 6. Synthesis scheme of ETMI-BF4.

is believed to be between two nitrogens. Figure 5
shows the structures of modified imidazolium cations
designed by using the above hypothesis. Here EMI+is at R1=C2H5, R3=CH3, R2=R4=R5=H. We substituted the electron donor R2 at the 2nd carbon. We
also substituted the electron donors R4 and R5 at the
4th and 5th carbons. These substitutions were expected to reduce the peak positive charge density and also
to lessen the reactivity of EMI+ with electrons due tosteric hindrance. We selected an alkyl group as anelectron donor substituent.
3.2 Modified imidazolium salt I, ETMI-BF4
We examined the possibility of synthesizing alkyVol. 2 No. 9 Sep. 2004

lated EMI+ and selected a methyl group, in this casethe smallest alkyl group, for the electron donors R2,
R4, and R5. We then synthesized 1-ethyl-2,3,4,5tetramethylimidazolium tetrafluoroborate (ETMIBF 4 ). Figure 6 shows the ETMI-BF 4 synthesis
scheme. Ethyl bromide was reacted with 1,2,4,5tetramethyl imidazole and the anion was changed
from Br – to BF4–. ETMI-BF4 was identified by H1NMR spectrometry, mass spectrometry, elemental
analysis, and HPLC.
Table 1 shows the physical properties of various
imidazolium salts. The melting point of ETMI-BF4
was observed to be 78–80°C. This is higher than thatof EMI-BF4 and similar to that of EMI-Cl. ETMI-BF4
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and EMI-BF4 show almost the same symmetry, so
this trend is mainly caused by the volume of ETMIBF4 being larger than that of EMI-BF4.
Figure 7 shows the electrochemical profile of thecurrent and potential for ETMI-BF4 at 80°C. The current around 1 V vs. Li/Li+, which was observed in
EMI-BF4 (Fig. 4), was not present in ETMI-BF4. In
addition, there was no decomposition of ETMI-BF4
from 0 to 5 V vs. Li/Li+; that is, ETMI-BF4 is stablewithin the electrochemical potential range of lithium
ion batteries. Consequently, as we expected, alkylating EMI + gave it a much lower electrochemical
reduction potential than EMI+.
The specific conductivities of ETMI-BF4 and EMIBF4 are shown in Fig. 8. The specific conductivity of
ETMI-BF4 at 20°C was 0.12 mS cm–1 and this value
is much lower than that of EMI-BF 4. Although
ETMI-BF4 has a high melting point of 78–80°C,
ETMI-BF4 is liquid down to 10°C as a result of supercooling. A similar phenomenon was observed for
EMI-BF4: the melting point is 15°C and the liquidstate was maintained down to –30°C as a result ofTable 1. Physical properties of imidazolium salts.
Anion

Formula weight

Melting point (°C)

ETMI-BF4

BF4–240.0578-80

DEDMI-BF4

BF4–

240.05

19-20

EMI-BF4

BF4–

197.97

15

EMI-Cl–Cl

146.62

77-79

EMI-PF6

PF6–

256.1358-62

Abbreviation

supercooling.
Thus, our investigation of the alkylation of imidazolium salts as a way of boosting the performance ofEMI-BF 4 showed that alkylation of the cations
improves the stability when the electrochemical
potential is low and provides a wide enough electrochemical window for use in Li-ion batteries. However, ETMI-BF4 exhibits a high melting point and a low
specific conductivity, so both these characteristics
must be improved before it can be used as a lithium
ion battery electrolyte solvent.
3.3 Modified imidazolium salt II, DEDMI-BF4
We redesigned the structure of the imidazolium
cations and investigated 1,2-diethyl-3,4(5)dimethylimidazolium tetrafluoroborate (DEDMIBF4). DEDMI-BF4 is less symmetrical than ETMIBF4 and is a mixture of 4- and 5-methyl isomers.
Asymmetric molecules tend to have a lower melting
point than symmetric compounds with the same formula weight. Moreover, the solidification point of the
mixture is lower than that of any of the original components. Therefore, we expected the melting point tobe lower than that of ETMI-BF4.
Figure 9 shows the DEDMI-BF4 synthesis scheme.
2-ethyl-4(5)-dimethyl imidazole was reacted with
ethyl bromide and then reacted with methyl iodide.
Finally, I– was replaced by BF4–. DEDMI-BF4 was
identified by H1-NMR spectrometry, mass spectrometry, elemental analysis, and HPLC.
50

Specific conductivity (mS cm–1)

0.5

Current (mA)

0.0

–0.5

CH3
–

BF4
C2H5

N

N

+

CH3

40
(1)
30

20
m.p.
m.p.
10

–1.0

(2)
CH3
–1.5

0

1

2

3

CH3
4

0
–50
5

Potential vs. Li/Li+ (V)Fig. 7. Potential-current curve of ETMI-BF4 at 80°C.

52

6

0

100
50
Temperature (°C)
m.p.: melting point

Fig. 8. Temperature dependence of specific conductivityof each salt. (1) EMI-BF4 and (2) ETMI-BF4.

NTT Technical Review

Regular Papers

C2H5

H

N

C2H5

C2H5Br

N

C2H5

N

N

CH3

CH3
C2H5

CH3I

C2H5

N

+

C2H5

HBF4

CH3

N

C2H5

N

+

CH3

N

I–

BF4–

CH3

CH3
DEDMI-BF4

DEDMI-BF4: 1,2-diethyl-3,4(5)-dimethylimidazolium tetrafluoroborate

Fig. 9. Synthesis scheme for DEDMI-BF4.
0.05

8Specific conductivity (mS cm–1)

Current (mA)

0.00

C2H5
BF4–

–0.10
C2H5

N

+

H
–0.20

0

1

2
3
4
Potential vs. Li/Li+ (V)

N

CH3

(2)
4
m.p.
m.p.
(1)
2

CH3

5

6

Fig. 10. Potential-current curve of DEDMI-BF4 at 80°C.

The melting point of DEDMI-BF4 was observed tobe 19–20°C, as shown in Table 1. As we expected,
this is much lower than that of ETMI-BF4 and close
to that of EMI-BF 4. Therefore, we succeeded in
obtaining an imidazolium salt with a room-temperature melting point.
The electrochemical profile of the current andpotential for DEDMI-BF4 at 80°C is shown in Fig.
10. The profile is almost flat, similar to that for
ETMI-BF4 (Fig. 7). There was no decomposition
when the potential was between 0 and 5 V. Therefore,
Vol. 2 No. 9 Sep. 2004

6

0
–20

0

20
40
Temperature (°C)

6080Fig. 11. Temperature dependence of specific conductivityof each salt. (1) ETMI-BF4 and (2) DEDMI-BF4.

DEDMI-BF4 provides a sufficiently wide electrochemical window for use in Li-ion batteries.
Figure 11 shows how the specific conductivity varied with temperature. DEDMI-BF4 had a higher specific conductivity of 1.44 mS cm–1 at 20°C thanETMI-BF4. DEDMI-BF4 was in the liquid state, evenbelow the melting point. It also exhibited supercooling, with the limit being –10°C.
DEDMI-BF4 still does not meet the requirements
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Regular Papers

for a lithium ion cell electrolyte solvent. We should
expand the temperature range of its liquid state to atleast below –20°C, select a suitable lithium salt andelectrolyte composition, and then evaluate the lithium cell performance and cell safety with this type ofelectrolyte.
4. Conclusion
We found that alkylating EMI+ is an effective way
to improve its stability when the electrochemical
potential is low. It also provides a sufficiently wide
electrochemical window for use in Li-ion batteries.
DEDMI-BF4 retained its liquid state down to –10°C
and has a good specific conductivity at room temperature. However, we must further improve the performance of DEDMI-BF4 for use in lithium ion cells.
We firmly believe that ionic liquid electrolytes will
be used as nonflammable electrolytes for lithium ion
cells, which will greatly improve their safety.

Katsuya Hayashi
Senior Research Engineer, Energy System
Laboratory, NTT Energy and Environment Systems Laboratories.
He received the B.S. and M.S. degrees inchemistry from Waseda University, Tokyo in
1990 and 1992, respectively. He joined NTT
Interdisciplinary Research Laboratories in 1992
and moved to NTT Integrated Information andEnergy Laboratories in 1996, NTT Telecommunication Energy Laboratories in 1999, NTT
Microsystem Integration Laboratories in 2002,
and NTT Energy and Environment Systems Laboratories in 2004. He has mainly been engaged inresearch on electrolyte materials for lithium secondary batteries. He is a member of the Chemical Society of Japan (CSJ) and the Electrochemical Society of Japan. He has received awards
from CSJ and the Electronics Division of the
Ceramics Society of Japan.

Yasue Nemoto
Engineer, Smart Devices Laboratory, NTT
Microsystem Integration Laboratories.
She joined NTT Electrical Communication
Laboratories in 1986 and moved to NTT Applied
Electronics Laboratories in 1987, NTT Interdisciplinary Research Laboratories in 1991, NTT
Integrated Information and Energy Laboratories
in 1996, NTT Telecommunication Energy Laboratories in 1999, and NTT Microsystem Integration Laboratories in 2002. She has mainly been
engaged in the analysis of materials for lithium
secondary batteries.

References[1]

H. Matsumoto, M. Yanagida, K. Tanimoto, T. Kojima, Y. Tamiya,
and Y. Miyazaki, “Improvement of Ionic Conductivity of Room Temperature Molten Salt Based on Quaternary Ammonium Cation and
Imide Anion ,” Molten Salts XII, edited by P. C. Trulove et al., Electrochem. Soc., Pennington, NJ, p. 186, 2000.
[2] J. S. Wilkes, T. J. Melton, J. Joyce, J. T. Maloy, and J. A. Boon “Electrochemical Studies of Sodium Chloride as a Lewis Buffer for Room
Temperature Chloroaluminate Molten Salts,” J. Electrochem. Soc.,
Vol. 137, No. 12, p. 3865, 1990.
[3] J. Fuller, R. T. Carlin, H. C. De Long, and D. Haworth, “Structure of
1-ethyl-3-methylimidazolium hexafluorophosphate: model for room
temperature molten salts,” J. Chem. Soc., Chemical Communications,p.299 1994.
[4] H. Matsumoto and Y. Miyazaki, “Highly Conductive Room Temperature Molten Salts Based on Small Trimethylalkylammonium Cations
and Bis (trifluoromethylsulfonyl) imide,” Chem. Lett., 922, 2000.
[5] J. Dvynck, R. Messinam, J. Pingarron, B. Tremillon, and L. Trichet,“Electrochemical Intercalation of Lithium into Transition Metal
Compounds in Low Temperature Chloroaluminate Melts,” J. Electrochem. Soc., Vol. 131, No.10, p.2274, 1984.
[6] Y. S. Fung and R. Q. Zhou, “Room temperature molten salt as medium for lithium battery,” J. Power Sources, Vol.81-82, p.891, 1999.
[7] K. Ui, N. Koura, Y. Idemoto, and K. Iizuka, “Positive Electrode
Active Material for the Rocking Chair Type Lithium Secondary Battery with an Ambient Temperature Molten Salt as a Nonflammable
Electrolyte,” Denki Kagaku, Vol.65, p.161, 1997.
[8] Y. S. Fung and D. R. Zhou, “Electrodeposited Tin Coating as Negative Electrode Material for Lithium-Ion Battery in Room Temperature
Molten Salt,” J. Electrochem. Soc., Vol.149, p. A319, 2002.
[9] N. Koura, K. Etoh, Y. Idemoto, and F. Matsumoto, “Electrochemical
Behavior of Graphite-Lithium Intercalation Electrode in AlCl3EMIC-LiCl-SOCl2 Room-Temperature Molten Salt,” Chem. Lett.,
1320, 2001.
[10] J. Fuller, R. T. Carlin, and R. A. Osteryoung, “The Room Temperature Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate:
Electrochemical Couples and Physical Properties,” J. Electrochem.
Soc., Vol. 144, No. 11 p. 3881, 1997.

54

Keiji Akuto
Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E. degree from Gunma University, Kiryu, Gunma in 1975 and the M.E. and
Ph.D. degrees from Keio University, Tokyo in
1977 and 2001, respectively. Since joining theElectrical Communication Laboratories, Nippon
Telegraph and Telephone Public Corporation(now NTT), in 1977, he has mainly been engaged
in research on catalytic oxidation of hydrogen,photorechargeable air batteries, and lithium-ion
batteries. He is a member of the Electrochemical
Society, CSJ, the Electrochemical Society ofJapan, the Surface Science Society of Japan, andthe Institute of Electronics, Information andCommunication Engineers.

Yoji Sakurai
Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.E. and M.E. degrees inapplied electronic engineering from the University of Electro-Communications, Tokyo in 1978
and 1980, respectively. He received the Ph.D.
degree in industrial chemistry from Kyoto University, Kyoto in 1989. In 1980, he joined theElectrical Communication Laboratories, Nippon
Telegraph and Telephone Public Corporation(now NTT), Ibaraki, where he engaged inresearch on cathode active materials for lithium
secondary batteries. He has been researching
several high-energy density batteries, especiallylithium-ion batteries. He is a member of the Electrochemical Society of Japan and the Electrochemical Society.

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