Design, synthesis, and electrochemistry of room-temperature ionic liquids functionalized with propylene carbonate

Article abstract of DOI:10.1002/anie.201005208

A PC organic salt: A series of piperidinium salts with a covalently attached propylene carbonate (PC) moiety provides a novel room-temperature ionic liquid (RTIL). This uniquely functionalized RTIL exhibits favorable electrochemical stability, leading to lithium metal deposition/stripping.

Full text of DOI:10.1002/anie.201005208

Communications
DOI: 10.1002/anie.201005208
Ionic Liquids
Design, Synthesis, and Electrochemistry of Room-Temperature Ionic
Liquids Functionalized with Propylene Carbonate**
Tetsuya Tsuda,* Koshiro Kondo, Takashi Tomioka,* Yusuke Takahashi, Hajime Matsumoto,
Susumu Kuwabata, and Charles L. Hussey
Alkyl carbonates are often employed as solvents for the study
of energy-storage devices (ESD), such as lithium secondary
batteries (LSB) and electric double-layer capacitors. Some of
these solvents, including propylene carbonate (PC) and
diethyl carbonate, have already been put to practical use in
modern electronics technology, such as in mobile phones and
laptop computers. However, all of these organic solvents have
potential safety drawbacks related to their flammable and
volatile nature that can lead to explosions and/or fire
accidents. Furthermore, lithium anodes with a high theoret-
ical discharge capacity (3860 mAhgÀ1) cannot be utilized in
such solvents owing to dendritic lithium deposition during the
charging cycle. However, room-temperature ionic liquids
(RTILs)[1–3] and RTIL-like solvents[3–6] are expected to be a
new class of solvents for next-generation rechargeable high-
energy-density batteries because RTILs possess unique salt-
like properties. Some of these properties, such as high
electrochemical stability, negligible vapor pressure, and
resistance to combustion, are highly advantageous in electro-
chemical applications.[1–3,7,8] Thus, we anticipated that chemi-
cally combining an appropriate carbonate and organic salt
may remove some of the undesirable properties of alkyl
carbonates and provide uniquely functionalized RTILs for
ESDs, and particularly LSB systems (Figure 1).
Figure 1. An organic salt (room-temperature ionic liquid; RTIL)
attached to propylene carbonate (PC).
Many of common cations of organic salts (for example,
imidazolium, pyridinium, ammonium, and phosphonium
species) could be used for this ESD-oriented study; however,
full-scale electrochemical and physicochemical measurement
analysis often requires ten- to hundred-gram quantities of
impurity-free samples, and a large-scale preparation of
structurally complex organic salts is still synthetically chal-
lenging. Therefore, a readily accessible and generally inex-
pensive tetraalkylammonium salt (R4N+XÀ), more specifi-
cally piperidinium salt 1, seems to be a reasonable substruc-
ture to combine with a carbonate functionality (Figure 2).[9]
[*] Prof. T. Tsuda
Frontier Research Base for Global Young Researchers
Graduate School of Engineering, Osaka University
2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7374
E-mail: ttsuda@chem.eng.osaka-u.ac.jp
Prof. T. Tsuda, K. Kondo, Prof. S. Kuwabata
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)
Prof. T. Tomioka, Y. Takahashi, Prof. C. L. Hussey
Department of Chemistry and Biochemistry
University of Mississippi
University, MS 38677-1848 (USA)
Fax: (+1)662-915-7300
E-mail: tomioka@olemiss.edu
Dr. H. Matsumoto
Research Institute for Ubiquitous Energy Devices
National Institute of Advanced Industrial Science and Technology
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577 (Japan)
Figure 2. Common organic salts and piperidinium salt attached to
PC (1).
Prof. S. Kuwabata
Japan Science and Technology Agency, CREST
Kawaguchi, Saitama 332-0012 (Japan)
Herein, we describe a novel RTIL containing a piperidinium
cation with a PC moiety, which can form a solid electrolyte
interphase (SEI) layer on the lithium deposit.[10] A facile
synthetic approach to 1 and the electrochemical properties of
1 as a solvent in a LSB system are also discussed.
The synthesis (Scheme 1) started from inexpensive, com-
mercially available reagents, piperidine and epichlorohydrin,
which afforded epoxypropylpiperidine 2 by using a modified
Heywood–Phillips procedure.[11] The TBAI-catalyzed cyclic
[**] This research was supported by The University of Mississippi and
Core Research for Evolution Science and Technology (CREST) from
the Japan Science and Technology Agency (JST). C.L.H. acknowl-
edges support for this work by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences of
the U.S. Department of Energy through a subcontract to Grant DE-
AC02-98CH10886.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005208.
1310
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1310 –1313

stoichiometry (1.2 versus 4.0 equiv) were also assessed.
Methyl methanesulfonate (MeOMs) reacted smoothly at
room temperature (entry 8). The resulting mesylate salt 1h
(m.p. 134–1368C) showed a lower melting point than the
iodide salt 1a (m.p. 171–1728C). Delocalized charge on the
mesylate strongly affected the decrease in melting point of the
salt.[13] The near-stoichiometric conditions (entry 9) still
worked highly efficiently and gave a 91% yield of 1h.
Interestingly, among these examples, the salt 1c (entry 3)
displayed the lowest melting point (100–1028C) and, com-
pared to the other alkyl groups (entries 1–5), a three-carbon
propyl chain is obviously a critical length to yield a low-
temperature melt in this salt system. This is a common
behavior of organic salts; for example, 1-alkyl-3-methylimi-
dazolium tetrafluoroborate;[1] that is, the propyl group creates
a sterically hindered environment around the cation leading
to the appropriate ion–ion separation, which eventually
lowers the melting point of 1c.[14] Although anion exchange
of 1c may help to lower the melting point further, owing to
the lack of reactivity of the propyl halide, a more reactive
three-carbon-chain reactant, allyl halide, was then employed
(Scheme 2). Treatment of 3 with allyl bromide instantly
Scheme 1. Large-scale synthesis of salt 1a. TBAI=tetrabutylammo-
nium iodide.
carbonation of 2 then provided the desired carbonate 3 in
86% yield.[12] Both reactions were successfully operated
under solvent-free, large-scale conditions (ca. 250 mmol).
Subsequently, exposing carbonate 3 (ca. 175 mmol) with four
equivalents of methyl iodide in acetone gave the analytically
pure salt 1a in quantitative yield. This simple three-step
approach did not require any chromatographic purification
and enabled the large-scale synthesis of the functionalized salt
1a. Indeed, more than 400 g of the salt was easily prepared;
however, subsequent anion exchange of 1a did not give the
desired RTIL (see the Supporting Information, Figure S1).
To test the generality of this route and to explore a low-
melting/room-temperature ionic liquid, the other piperidi-
nium salt analogues of 1 were prepared from carbonate 3
(Table 1). For practical reasons, all syntheses were carried out
Table 1: Synthesis of a series of piperidinium salts 1.
Scheme 2. Preparation of room-temperature ionic liquids 1i and
1i-Tf2N.
afforded salt 1i in excellent yield, even under large-scale neat
conditions (ca. 120 mmol). Furthermore, 1i has a glass
transition temperature of only À21.48C without a true
melting point. It should be noted that allyl iodide also
worked well (658C, 2 hours, near quantitative yield), but
minor organic impurities (< 10%) could not be separated
from the salt, at least by standard trituration techniques,
owing to the high viscosity of the product. Anion exchange of
1i with bis(trifluoromethanesulfonyl)amine (HTf2N) finally
provided the RTIL 1i-Tf2N.
Entry
RX
1
T [8C]
t [h]
2.5
2 days
3 days
3 days
3 days
1
Yield [%]
m.p. [8C]
1
2
3
4
5
6
7
8
9
MeI
EtI
n-PrI
nBuI
nHexI
BnBr
BrCH2CN
MeOMs
MeOMs
1a
1b
1c
1d
1e
1 f
1g
1h
1h
RT
70
65
85
85
65
RT
RT
RT
96
171–172
187–188
100–102
129–131
132–133
140–142
n.a.[b]
84[a]
82
84
92
98
98
3.5
3.5
3.5
99
134–136
134–136
91[c]
Although we have successfully prepared several ILs, 1a-
Tf2N (m.p. 92.08C), 1c, 1i, and 1i-Tf2N with melting points of
below 1008C, only 1i-Tf2N was a true room-temperature ionic
liquid. Physical properties of 1i-Tf2N are summarized in
Table S1 in the Supporting Information. The viscosity
exceeded 30000 cP at 358C and the conductivity was
considerably low. The thermal stability and electrochemical
stability were also assessed in this investigation. Figure 3
shows thermoanalytical data of 1i and 1i-Tf2N estimated by
means of thermogravimetric analysis (TGA) and differential
thermal analysis (DTA). The decomposition temperature Td
was found at 76.48C for 1i and 286.98C for 1i-Tf2N, with the
assumption that the decomposition begins at the temperature
at which 1 wt% loss was detected. The higher decomposition
temperature, that is, higher thermal stability, of 1i-Tf2N can
[a] Used 8 equiv of EtI. [b] Not measured owing to product impurities
(1g/BrCH2CN=5:1). [c] Used 1.2 equiv of MeOMs.
under neat conditions. As expected, methyl iodide (entry 1)
rapidly formed the desired salt 1a in 96% yield. Other
primary alkyl halides (entries 2–5) also provided the corre-
sponding products 1b–e in good yields, although higher
reaction temperatures (65-858C) and longer reaction times
(2–3 days) were required. The use of activated halides,
namely benzyl bromide (entry 6) and bromoacetonitrile
(entry 7), afforded 1 f and 1g, respectively in nearly quanti-
tative yields and with short reaction times. The impacts of
different counteranions (iodide versus mesylate) and reagent
Angew. Chem. Int. Ed. 2011, 50, 1310 –1313
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1311

Communications
reactions at limiting potentials, suggesting a high-purity grade
of this salt.
Figure 5a shows a cyclic voltammogram recorded at a
copper wire electrode in 91.0–9.0 mol% [1i-Tf2N][LiTf2N]
binary RTIL system (thermal stability data are given in
Figure 3. Results of thermogravimetric analysis (TGA) and differential
thermal analysis (DTA). a) 1i and b) 1i-Tf2N. Rate of temperature
increase: 108C minÀ1
.
be reasonably explained by the lower nucleophilicity of the
Tf2NÀ anion. It should be noted that 1a and 1a-Tf2N showed
the same tendency (see Figure S1 in the Supporting Informa-
tion). Similar observations were also reported by several
research groups.[15–17] Differential scanning calorimetry
(DSC) confirmed that these salts do not have melting
points, but rather have glass transition temperatures at
À21.48C (1i) and À24.38C (1i-Tf2N; see Figure S2 in the
Supporting Information). The former salt had almost no
fluidity at 258C owing to the strong cation–anion interactions
compared to 1i-Tf2N, but the latter was a barely fluidic salt.
Thus, we concluded that 1i-Tf2N has a wide liquid temper-
ature range exceeding 3008C.
The electrochemical stability of 1i-Tf2N was studied by
using linear sweep voltammetry, which was carried out at
458C to avoid unfavorable voltammograms resulting from the
large IR drops that appeared below this temperature. The
potential scan was initiated from the rest potential, which is
indicated by the origin of the directional arrows in Figure 4.
The electrochemical window at a glassy carbon (GC) and a Pt
electrode were about 4.5 V and 5.0 V, respectively. The
window was estimated by measuring the cathodic and
anodic limiting potentials at which a current density of
Æ 0.05 mAcmÀ2 was observed. The electrochemical stability is
comparable to the other piperidinium-based RTILs that have
been reported.[18] There is no obvious redox peaks except the
Figure 5. a) A cyclic voltammogram recorded at a copper wire elec-
trode in 91.0–9.0 mol% [1i-Tf2N][LiTf2N]. T=858C. Sweep rate:
10 mV sÀ1. b) A photograph of the copper wire electrode after the
electrodeposition experiment at À100 mA. T=858C. Electrodeposition
time: 600 s.
Figure S3 in the Supporting Information). A very sharp
deposition wave with an associated stripping wave appeared
at about À3.0 V. Further to this, a very small redox couple,
which may be related to the formation of SEI layer, also
appeared at about À2.0 V. As shown in Figure 5b, controlled-
current electrolysis at À100 mA for 600 s yielded a light gray
electrodeposit of lithium metal deposited on the copper
substrate; no additives were used. This result implies that the
[1i-Tf2N][LiTf2N] binary RTIL system is a promising solvent
for next-generation LSB systems.
In summary, a highly practical and scalable approach for a
series of piperidinium salts 1 has been established. All three-
step reactions were carried out under solvent-free/neat
conditions and no chromatographic purification was required.
Controlling the length of alkyl chain as well as the type of
counteranion successfully afforded a room-temperature ionic
liquid (1i-Tf2N). This novel RTIL possesses wide thermal
stability and lithium metal deposition ability. Thus, the
positive aspects of the melt in both synthesis and electro-
chemistry hold great promise as a candidate solvent for next-
generation ESD.
Experimental Section
Piperidinium salts 1a and 1i were purified by precipitation from dry
ethanol with dry ethyl acetate. Bis(trifluoromethylsulfonyl)amine
(HTf2N, Morita Chemical Industries Co.) was used as received. The
ionic liquids were prepared by mixing exactly equal molar amounts of
1a or 1i and HTf2N in ultrapure water. This solution was stirred at
room temperature for 24 h. The resulting IL was then extracted with
anhydrous dichloromethane (Wako Pure Chemical Industries, dehy-
drated). The solution was washed repeatedly with reverse-osmosed
(RO) water until the aqueous phase was found to contain no chloride
as determined by the addition of a few drops of a silver nitrate
Figure 4. Linear sweep voltammograms recorded at (c) a glassy
carbon and (b) a platinum disk electrode in 1i-Tf2N at 458C. Sweep
rate: 10 mV secÀ1
.
1312
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1310 –1313

solution. Finally the dichloromethane solvent was removed by stirring
the extraction solution under vacuum (1 ꢀ 10À4 torr) at 1008C for 24 h.
The composition of the final products was confirmed by quantitative
elemental analysis and FAB-MS (for data, see the Supporting
Information). A binary 91.0–9.0 mol% [1i-Tf2N][LiTf2N] IL system
was prepared by adding lithium bis(trifluoromethylsulfonyl)amide
(LiTf2N, Aldrich, > 99.95%) to 1i-Tf2N.
[2] H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley-
Interscience, New Jersey, 2005.
[3] T. Tsuda, C. L. Hussey in Modern Aspects of Electrochemistry,
Vol. 45 (Ed.: R. E. White), Springer, New York, 2009, pp. 63 –
174.
[4] T. Tsuda, T. Tomioka, C. L. Hussey, Chem. Commun. 2008, 2908.
[5] T. Tamura, K. Yoshida, T. Hachida, M. Tsuchiya, M. Nakamura,
Y. Kazue, N. Tachikawa, K. Dokko, M. Watanabe, Chem. Lett.
2010, 39, 753.
[6] T. Tsuda, L. E. Boyd, S. Kuwabata, C. L. Hussey, J. Electrochem.
Soc. 2010, 157, F96.
[7] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M.
Kono, J. Power Sources 2006, 160, 1308.
[8] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono,
J. Power Sources 2006, 162, 658.
[9] A conceptually similar approach towards the development of
hydrogen-storage materials employing imidazolium ILs has
been recently reported: M. P. Stracke, G. Ebeling, R. Cataluꢁa,
J. Dupont, Energy Fuels 2007, 21, 1695.
Thermoanalytical investigations were carried out by the use of a
Bruker TG-DTA2000SA and DSC3100SA. This instrument was
controlled with a Bruker MTC1000SA workstation utilizing Bruker
WS003 software. The analytical data were obtained in aluminum
pans; the standard sample was a-alumina. If the sample was
hygroscopic, the aluminum pan was sealed in an argon-gas-filled
glove box. The analysis was conducted at 108CminÀ1 under dry air.
Electrochemical experiments were conducted with a computer-
controlled Hokuto Denko HZ-5000 potentiostat/galvanostat. All
electrochemical experiments were carried out in three-electrode cells.
The working electrodes were a Pt disk electrode (0.020 cm2), a glassy
carbon (GC) disk electrode (0.00785 cm2), and
a copper-wire
electrode. A 0.05 cm diameter platinum wire and a large surface-
area coiled platinum wire were used as a quasi reference electrode
and a counter electrode, respectively. The platinum quasi reference
and counter electrodes were immersed in the RTIL. All electro-
chemical experiments were carried out in an argon gas-filled glove
box (VAC Atmospheres NEXUS system) with O2 and H2O < 1 ppm.
[10] E. Peled, J. Electrochem. Soc. 1979, 126, 2047.
[11] D. L. Heywood, B. Phillips, J. Am. Chem. Soc. 1958, 80, 1257.
[12] V. Calꢂ, A. Nacci, A. Monopoli, A. Fanizzi, Org. Lett. 2002, 4,
2561.
[13] E. I. Izgorodina, M. Forsyth, D. R. MacFarlane, Aust. J. Chem.
2007, 60, 15.
[14] S. T. Handy, Curr. Org. Chem. 2005, 9, 959.
[15] H. L. Ngo, K. LeCompte, L. Hargens, A. B. McEwen, Thermo-
chim. Acta 2000, 357–358, 97.
Received: August 20, 2010
Revised: October 7, 2010
[16] W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, Jr., T. E.
Sutto, J. Callahan, P. C. Trulove, H. C. DeLong, D. M. Fox,
Thermochim. Acta 2004, 409, 3.
[17] C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, J. F.
Brennecke, J. Chem. Eng. Data 2004, 49, 954.
[18] H. Matsumoto, H. Sakaebe, K. Tatsumi, J. Power Sources 2005,
146, 45.
Published online: January 5, 2011
Keywords: electrochemistry · energy storage · ionic liquids ·
.
lithium · propylene carbonate
[1] Ionic Liquids in Synthesis (Eds.: P. Wasserscheid, T. Welton),
Wiley-VCH, Weinheim, 2003.
Angew. Chem. Int. Ed. 2011, 50, 1310 –1313
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1313