Structural factors to improve physico-chemical properties of zwitterions as ion conductive matrices

Article abstract of DOI:10.1039/b515287a

Thermal properties and ion conductive characteristics of equimolar mixtures of zwitterion and lithium salts (Z/L mixtures) have been investigated. Through a systematic study of their structure-property relationship, we describe a zwitterion structure that produces high ionic conductivity when the zwitterion is mixed with lithium salt. We found that a pure zwitterion containing imide anion melted at 87 °C and the melting point of pure zwitterions was lowered in proportion to the alkyl spacer chain length between the anion and cation sites. In Z/L mixtures, ionic liquid-like domain formed between the cation sites of zwitterions and the anions of lithium salts regardless of the anion structure of the zwitterions. However, the anion structure affected the transference number of the lithium cation in the Z/L mixtures. To improve the thermal stability and the ionic conductivity of Z/L mixtures, suitable alkyl chain length between the cation and anion sites was determined to be 5 to 7 CH₂ units. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515287a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Structural factors to improve physico-chemical properties of zwitterions as
ion conductive matrices
Asako Narita, Wataru Shibayama and Hiroyuki Ohno*
Received 27th October 2005, Accepted 20th January 2006
First published as an Advance Article on the web 1st February 2006
DOI: 10.1039/b515287a
Thermal properties and ion conductive characteristics of equimolar mixtures of zwitterion and
lithium salts (Z/L mixtures) have been investigated. Through a systematic study of their structure–
property relationship, we describe a zwitterion structure that produces high ionic conductivity
when the zwitterion is mixed with lithium salt. We found that a pure zwitterion containing imide
anion melted at 87 uC and the melting point of pure zwitterions was lowered in proportion to the
alkyl spacer chain length between the anion and cation sites. In Z/L mixtures, ionic liquid-like
domain formed between the cation sites of zwitterions and the anions of lithium salts regardless of
the anion structure of the zwitterions. However, the anion structure affected the transference
number of the lithium cation in the Z/L mixtures. To improve the thermal stability and the ionic
conductivity of Z/L mixtures, suitable alkyl chain length between the cation and anion sites was
determined to be 5 to 7 CH2 units.
zwitterion and the added TFSI anion.5 Furthermore, the
Introduction
transference number of the lithium cation in this mixture
was 0.567 due to the coupling of the cation site of zwitterion
Ionic liquids (ILs) are expected to function as non-volatile ion-
conductive liquids for electrochemical devices such as batteries
and the added TFSI anion which contributed to fix the
and capacitors.1 ILs may be useful for replacing the volatile
anion and to provide the domain with lower glass transition
electrolyte solutions because ILs are thermally stable, have
temperature (Tg).
negligibly small vapour pressures, have a wide potential
window, and have a high ionic conductivity.2,3 However, for
electrochemical applications, their carrier-ion mobility is too
low. Carrier ions, especially cations, generated from added
salts or acids, do not migrate dominantly because all of the
component ions of ILs are majority in number and have
high mobility. For example, the transference numbers of
lithium cation in polymer films containing ILs and lithium
salts are only 0.1–0.3.4
There is no satisfactory strategy to design zwitterions with
high ionic conductivity and large transference number for
the carrier ions. To obtain zwitterions having low Tm, many
zwitterions have been tested.6 Certain strategy should be
proposed to efficiently improve their properties. We focus
here on the three parts, namely, cation-site, spacer chain, and
anion-site of zwitterions (Scheme 1, left).
The purpose of this study is to improve the properties of
both pure zwitterions and Z/L mixtures. We analyzed the
properties of pure zwitterions and Z/L mixtures and the effect
of the structural characteristics of every part. We focused
on the effect of spacer-chain length, cation- and anion-site
structure on the thermal property and ion conductive
characteristics of both pure zwitterions and Z/L mixtures.
To reduce the mobility of the component ions, both cation
and anion have been tethered.5 Unfortunately, the tethering
elevated the melting point (Tm) of the salt for more than
120 uC, and accordingly the synthesized zwitterions showed
melting points much higher than 100 uC. For example, a
zwitterion having an N-ethylimidazolium cation-site and
a sulfonate anion site melts at 176 uC,5,6 and the ionic
conductivity at 50 uC was lower than 1028 S cm21 due to both
a solid state and a lack of mobile ions. In contrast, the Tm of
N-ethylimidazolium methyl sulfonate is 29 uC. In spite of the
high melting point of the zwitterion, we found that a mixture
of the above zwitterion and equimolar amount of lithium
bis(trifluoromethylsulfonyl)-imide (LiTFSI) turned liquid at
room temperature5 (see Scheme 1, right). The mixture showed
an ionic conductivity of about 1025 S cm21 at 50 uC. This
relatively high ionic conductivity is attributed to the ionic
liquid-like domain formed by the coupling of the cation site of
Department of Biotechnology, Tokyo University of Agriculture and
Technology, 2-24-16 Naka-cho, Koganei, Tokyo, 184-8588, Japan.
E-mail: ohnoh@cc.tuat.ac.jp; Fax: +81 42 388 7024; Tel: +81 42 388 7024
Scheme 1 Three parts of zwitterions affect the characteristics (left).
Zwitterion was mixed with LiTFSI to form liquid salts (right).
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dissolved in 20 ml of acetonitrile. The mixture was stirred
for 7 days at room temperature, and then it was poured into
1000 ml diethylether. The precipitate formed in diethylether
was collected, washed, dried, and dissolved in milli-Q water.
After the solution was passed through a column filled with
anion exchange resin (Amberlite IRA-400 (Cl), Aldrich),
which was treated with 1 M of NaOH aqueous solution in
advance, the water was removed by evaporation. The resulting
product was obtained as a white solid. Yield: 81%. NMR:
dH(DMSO-d6, d/ppm Me4Si) 1.12–1.20 (m, 2H 6 3, 3–5-H),
1.32 (m, 2H, 2H), 1.36 (t, 3H, N-CH2CH3), 1.72 (m, 2H, 6-H),
1.74 (t, 2H, 7-H), 4.11 (t, 2H, 1-H), 4.15 (q, 2H, N-CH2CH3),
7.75 (s, 1H, Im), 7.78 (d, 1H, Im), and 9.72 (s, 1H, Im). Tm:
140 uC. Td: 226 uC.
1-(1-Ethyl-3-imidazolio)undecane-11-carboxylate (EIm-10C).
N-Ethylimidazole (3.6 g, 4 mmol, TCI) was reacted with
methyl 11-bromoundecanoate (1.0 g, 4.0 mmol, Aldrich)
for 6 days by the same procedure as that for EIm-8C. The
resulting product was a white solid. Yield: 88%. NMR:
dH(D2O, d/ppm Me4Si) 1.23–1.34 (m, 2H 6 6, 4H-9H), 1.50
(t, 3H, N-CH2CH3), 1.54 (m, 2H, 3-H), 1.86 (m, 2H, 2-H), 2.15
(t, 2H, 10-H), 4.19 (t, 2H, N-CH2CH3), 4.23 (q, 2H, 1-H), 7.49
(d, 1H, Im), 7.51 (d, 1H, Im), and 8.79 (s, 1H, Im). Tm: 103 uC.
Td: 224 uC.
Scheme 2 Synthesis of imide-anion-type zwitterions.
Experimental
Materials
We used previously reported methods for synthesizing
zwitterions having sulfonate5,6 and carboxylate6 groups.
Zwitterions having an imide group were synthesized by the
method as shown in Scheme 2. The base was used in three
such ways; the cation unit to be introduced in the zwitterion,
the trapping site for the generated hydrochloride, and the
neutralizer for the generated imide acids. Therefore, trimolar
N-ethylimidazole to alkanoyl chloride was used. This proce-
dure allows us to generate pure products. The halide ions
are exchanged to hydroxyl ions by a column filled with
anion exchange resin (Amberlite IRA-400(Cl), Aldrich). This
procedure provides non-ionic amines except for the zwitterion.
These non-ionic amines were eliminated by aprotic solvent
to isolate the zwitterions. The structure of zwitterions was
1-(1-Ethyl-3-imidazolio)hexane-6[(6-carbonyl)(trifluoro-
methylsulfonyl)]imide (EIm-5csI). Trifluoromethanesulfon-
amide (2.0 g, 14 mmol, TCI) was dissolved in 40 ml of
˚
acetonitrile and dried using 3 A molecular sieves under dry
argon gas atmosphere. After 6-bromo-n-hexanoylchloride
(3.1 g, 15 mmol, Wako Chemical Industry) was dissolved in
40 ml dry acetonitrile, the solution was added to trifluoro-
methanesulfonamide solution. And then, we added a solution
of N-ethylimidazole (3.9 g, 41 mmol, TCI) dissolved in 40 ml
of dry acetonitrile to it slowly about 1 h. The solution was
then stirred for 72 h at room temperature. After the solvent
was removed by evaporation, the residue was washed with
tetrahydrofuran. The washed residue was dried and then
dissolved in milli-Q water. This aqueous solution was passed
through an anion-exchange column. After the water was
removed by evaporation, the product was washed by tetra-
hydrofuran and then by diethylether. It was then dried in
vacuum at 100 uC for 2 days. The product was obtained as
a colorless, viscous liquid that slowly crystallized at room
temperature. The yield was 8%. NMR: dH(D2O, d/ppm Me4Si)
1.22 (m, 2H, 3-H), 1.42 (t, 3H, N-CH2CH3), 1.48 (m, 2H, 4-H),
1.78 (m, 2H, 2-H), 2.06 (t, 2H, 5-H), 4.15(q, 2H, N-CH2CH3),
4.20 (t, 2H, 1-H), 7.79 (d, 1H 6 2, Im), and 9.18 (s, 1H, Im).
TOF MASS: (ESI+, m/z) 342 (H+), 364 (Na+), 380 (K+), and
705 (dimer, Na+). Tm: 87 uC. Td: 295 uC.
1
then determined by H-NMR (a-500, JEOL). The molecular
weight was determined by TOF-mass spectrometer (JMS-
T100X, JEOL).
1-(1-Methylpyrolidinio)butane-4-sulfonate (MPrldn-4S).
N-Methylpyrrolidine 4.2
g (49 mmol, Tokyo Chemical
Industry (TCI)) was dissolved in 40 ml of acetonitrile. 1,4-
Butanesultone (6.4 g, 47 mmol, TCI) was dissolved in 40 ml
acetonitrile. Then the mixture was added dropwise into the
N-methylpyrrolidine for 30 min under nitrogen atmosphere
at 60 uC. After the solution was stirred for 24 h at room
temperature, formed precipitate was then removed and washed
with acetone. The resulting product was obtained as a white
powder. Yield: 78%. dH(D2O, d/ppm Me4Si) 1.81 (m, 2H, 3-H),
1.97 (m, 2H, 2-H), 2.20 (m, 2H+2H, pyrrolidine-3H and -4H ),
3.97 (t, 2H, 4-H), 3.04 (s, 3H, NMe), 3.37 (t, 2H, 1-H), and 3.51
(m, 2H+2H, pyrrolidine-2H and -5H). Tm: 320 uC. Td: 321 uC.
1-(N-Butyl-N,N-dimethylammonio)propane-3-[(3-sulfonyl)
(trifluoromethylsulfonyl)]imide (BDMAm-3ssI). Trifluoro-
methanesulfonamide (4.5 g, 30 mmol, TCI) dissolved in 30 ml
dry acetonitrile, 3-chloro-n-propane-sulfonylchloride (4.9 g,
30 mmol, Wako) dissolved in 30 ml dry acetonitrile, and
dimethylbutylamine (10.0 g, 41 mmol, TCI) dissolved in 60 ml
of dry acetonitrile were mixed and reacted according to the
1-(1-Ethyl-3-imidozolio)octane-8-carboxylate (EIm-8C).
N-Ethylimidazole 1.4 g (15 mmol, TCI) was added to a solu-
tion conaining 8-bromooctanoate (1.6 g, 18 mmol, TCI)
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same procedure as that for EIm-5csI. The product was
obtained as a white powder. Yield: 5%. NMR: dH(DMSO-
d6, d/ppm Me4Si) 0.95 (t, 2H, N(CH2)3CH3), 1.30 (m, 2H,
N(CH2)2CH2CH3), 1.64 (m, 2H, NCH2CH2CH2CH3), 2.09
(m, 2H, 2-H), 3.01 (s, 3H 6 2, NMe), 3.07 (t, 2H, 3-H), 3.26
(m, 2H, NCH2(CH2)2CH3), and 3.38 (m, 2H, 1-H). TOF
MASS: (ESI+, m/z) 377 (Na+), and 731 (Na+, dimer). Tm:
104 uC. Td: 267 uC.
below Td with a sweep rate of 5 uC min21. The second heating
curve was used to determine Tm (peak top) and Tg. When Tm
and Td were very close to each other, the Tm was determined
from the DTA curve during TG measurement. In these cases,
the first heating profiles were used.
LiTFSI (99.97%, Sumitomo 3 M) was used without further
purification. Equimolar amount of zwitterions and LiTFSI
were dissolved in methanol. After the solution was well stirred,
methanol was removed under reduced pressure. The mixture
was dried in vacuum at 80 uC for 2 days.
1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)
imide ([EMIm][TFSI]). N-Methylimidazole (15.0 g, 183 mmol,
Wako) was dissolved in 30 ml acetonitrile. Bromoethane
(30.0 g, 275 mmol, Wako) was added to the solution at 0 uC
under stirring for 24 h under nitrogen gas while being kept in
the dark. Then the solvent and excess bromoethane were
removed by evaporation. The residue was stirred in 60 ml
diethylether at 0 uC for 3 h, and the diethylether was removed
by decantation. This diethylether washing was repeated twice.
The resulting white solid (14 g) was dissolved in 10 ml milli-Q
water. Separately, LiTFSI (21 g, 73 mmol) was dissolved in
10 ml milli-Q water. These two aqueous solutions were then
mixed and stirred for 12 h. Dichloromethane (200 ml) was
added to the aqueous solution. After swiftly stirring, the
dichloromethane fraction was collected using a separating
funnel, and the washing procedure with dichloromethane was
repeated. Milli-Q water 200 ml was mixed with the solution
and stirred swiftly. The water phase was then removed using a
separating funnel and the dichloromethane was removed by
evaporation. After the remaining liquid was dried under
reduced pressure at 60 uC for 12 h, the liquid was passed
Ionic conductivity was measured by complex impedance
method using Impedance/Gain-phase analyzer (Solartron 1260,
Schlumberger) under dry nitrogen gas. The tLi+ was determined
by combination of AC impedance and DC polarization
methods (potential-static field: 10 mV, lithium metal electrode,
under argon atmosphere at room temperature).
FT-IR spectra were obtained at room temperature (FT/IR-
420, JASCO International Co. Ltd.). Raman shifts were
measured at room temperature using LASER Raman spectro-
photometer (NRS-1000, JASCO International Co. Ltd.) with
Kr LASER (647.1 nm, Innova, COHERENT Inc.).
Results and discussion
Effect of cation site on the properties of zwitterions
Pure zwitterion MPrldn-4S (Scheme 3) melted at 320 uC, which
was very close to the decomposition temperature (Td) of
321 uC. In our previous report, we described some candidates
of suitable cation structure for the design of zwitterions
with low Tm.6 These preliminary results helped us to select
some cation structures. Imidazolium cation sites are relatively
effective for lowering the melting points in a series of sulfonate
anion type zwitterions. The structures of zwitterions having
relatively low Tm in a series of sulfonate anion type (EIm-3S,
EIm-4S5–7) are also shown in Scheme 3. However, their
melting points are all above 150 uC. The Tm of a zwitterion
through
a column filled with alumina (activated acid,
Brockmann 1 standard grade, 150 mesh, Aldrich). The product
was obtained as colourless liquid. NMR: dH(DMSO-d6, d/ppm
Me4Si) 1.36 (t, 3H, N-CH2CH3), 3.30 (s, 3H, NMe), 4.14 (q,
2H, N-CH2CH3), 7.62 (s, 1H), 7.76 (s, 1H), and 9.15(s, 1H).
1-Ethyl-3-methylimidazolium tetrafluorosulfonate
([EMIm][Tf]). Methyl trifluoromethanesulfonate (10 g,
61 mmol, Aldrich) was dissolved in 150 ml toluene at 0 uC.
N-Ethylimidazole 7.6 g was added to the solution under
stirring for 15 min, and the mixture was further stirred for
30 min. Then, precipitated phase in the solution was collected
using a separating funnel. The liquid phase was washed with
150 ml toluene, and then it was dried in vacuum at 100 uC for
2 days. The product was obtained as colorless liquid with
62% yield. NMR: dH(DMSO-d6, d/ppm Me4Si) 1.38 (t, 3H,
N-CH2CH3), 3.81 (s, 3H, NMe), 4.17 (q, 2H, N-CH2CH3),
7.62 (s, 1H), 7.75 (s, 1H), and 9.05(s, 1H).
with
a
pyridinium structure was reported as 273 uC,6
even though the pyridinium ring is one of excellent cationic
component to prepare ILs.6 These results imply that the cation
structures for typical ILs are not always useful for designing
excellent zwitterions. The report also shows that zwitterions
with ammonium cation sites have relatively low melting
points.6 Zwitterions that have a highly asymmetric ammonium
cation structure with a long alkyl-chain length have melting
points below 100 uC. However, a long alkyl-chain also results
in low ionic density and low ionic conductivity, not desirable
characteristics for ionic devices.
The mixture of MPrldn-4S and an equimolar amount of
LiTFSI, hereafter MPrldn-4S/LiTFSI (and similar notation is
used for other mixtures) was obtained as a transparent liquid
Methods
The decomposition temperature (Td) of the zwitterions
was determined by thermogravimetric analysis (TG/DTA220,
Seiko Instruments). The Td was assumed to be the temperature
at the onset of weight loss on the TG chart. The phase transi-
tion temperatures (Tg, Tm) were measured using differential
scanning calorimetry (DSC, EXSTAR6000, Seiko Instruments)
or differential thermal analysis (DTA, TG/DTA220). They
were measured over a range from 2120 uC to the point 20 uC
Scheme 3 Sulfonate-anion-type zwitterions.
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Fig. 1 Arrhenius plot of the ionic conductivity of zwitterion–LiTFSI
mixtures. #: EIm-4S/LiTFSI,7 &: MPrldn-4S/LiTFSI.
Fig. 2 Melting points of carboxylate-type zwitterions with various
spacer chain lengths.
at room temperature. Both EIm-3S/LiTFSI and EIm-4S/
LiTFSI were also obtained as liquids. However, the Tg of
MPrldn-4S/LiTFSI is 16 uC and the ionic conductivity is
lower than that of EIm-4S/LiTFSI (Fig. 1). This suggests
that the ionic liquid-like domain formed by the coupling of
N-methylpyrrolidinio-sites and TFSI anions could not provide
effective ion conductive pathway as N-ethylimidazolio-sites
did. These results indicate again that cation structures like
typical ILs are not always effective for the improvement
of ionic conductivity of Z/L mixtures. At this stage,
N-ethylimidazolio group was selected as one of the most
suitable cation structure for controlling the Tm of pure
zwitterions. Altough it is possible to prepare zwitterions with
low Tm by introducing long alkyl-chains to cation-sites.6 Such
alkyl-chains however results low ionic conductivity of Z/L
mixtures. Considering also the ion density of Z/L mixture,
N-ethylimidazolio group is concluded to be most suitable
structure. Further improvement of the remaining two parts
(spacer and anion-site) is needed both to lower the Tm of
pure zwitterions and to increase the ionic conductivity of
Z/L mixtures.
Scheme 4 Carboxylate-anion-type zwitterions.
with spacer-chain lengths of more than six carbon units
have lower melting points than that of EIm-6C. This finding
suggests that inter- and intra-electrostatic interactions for
zwitterions were weaken as the alkyl-chain became longer.
All of the mixtures composed of carboxylate-type zwitter-
ions and LiTFSI were viscous liquids at room temperature,
like as EIm-3S/LiTFSI5 and EIm-4S/LiTFSI.7 Against these,
EIm-1C/LiTFSI was prepared as a white solid at room
temperature. The fact that EIm-1C/LiTFSI is a solid at room
temperature may be due to at least the following two reasons.
One is a micro-phase separation of zwitterions and LiTFSI
due to strong electrostatic interaction among zwitterions. The
other is a rigid structure of the zwitterion. This rigidity should
be maintained even after mixing with LiTFSI. To investigate
the state of the EIm-1C/LiTFSI mixture, we applied FT-IR
spectroscopy to EIm-1C and EIm-1C/LiTFSI mixtures. As
seen in Fig. 3, the spectrum for EIm-1C/LiTFSI (1) does not
have any common features with that of pure EIm-1C (2),
suggesting that the zwitterion–LiTFSI complex was formed
without micro-phase separation.
Effect of spacer chain length
Fig. 2 shows the melting points of a series of carboxylate anion
type zwitterions shown in Scheme 4. The carboxylate anion
type zwitterions are useful to analyze the effect of spacer-chain
length on the thermal properties because a series of starting
materials for the spacer units are commercially available. The
Tm of these pure zwitterions depends on the length of spacer-
chains as seen in Fig. 2. Some Tm values of carboxylate-type
zwitterions are slightly different from those in our previous
report6 because we used here a different method to determine
them. In the present study, Tm was determined from a peak top
for the second heating profile. In our previous report,6 Tm was
determined by onset of the peak for the first heating chart.
Zwitterion EIm-1C and EIm-10C melt at 250 uC6 and 103 uC,
respectively. Zwitterion EIm-7C melts at 140 uC, which is
slightly lower than that for EIm-6C.6 Also, the temperature
range for the liquid phase was in proportion to the n value of
spacers. The results in the present study show that zwitterions
We also examined the FT-IR spectra of all of carboxylate-
type zwitterion–LiTFSI mixtures. In Fig. 4, spectrum (1) is
that of a typical ionic liquid (1-ethyl-3-methylimidazolium
TFSI ([EMIm][TFSI])), which is shown as a reference. The
lower most spectrum (10) for LiTFSI powder was also used
as a reference. The spectra in this wavenumber range mainly
show stretching (n) and bending (d) modes of the TFSI anions.
Signals in the range from about 1000 to 1400 cm21 are not
useful because peaks from the zwitterions overlap with those
from LiTFSI (these peaks have been characterized in refs 8
and 9). Spectra (2)–(8) in Fig. 4 are similar to that of
[EMIm][TFSI]. This implies that the LiTFSI are fully dis-
sociated in EIm-nC–LiTFSI mixtures (n > 2). Also this strongly
suggests that the TFSI anions interact with imidazolium
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Fig. 5 Decomposition temperature of pure zwitterions having
various spacer lengths (%) and the corresponding zwitterion/LiTFSI
mixtures (&).
Fig. 3 FT-IR spectra for pure EIm-1C (1), EIm-1C/LiTFSI mixture
(2), and LiTFSI powder (3).
temperature of the zwitterions increased after mixing with
LiTFSI, a stabilization that is known as one of advantages
of the mixture.5 The stabilization was particularly large for
the EIm-nC/LiTFSI with n ¢ 4. Among these, EIm-6C/LiTFSI
decomposed at 401 uC, showing the most thermally stable
mixture.
cation sites rather than lithium cations. The only peak that
does not appear in the spectra of both [EMIm][TFSI] and
LiTFSI is (dS–N–S) at around 650 cm21. This peak seemed to
be unique to EIm-nC/LiTFSI mixtures (n > 2). On the other
hand, the spectrum for EIm-1C/LiTFSI is similar to that for
LiTFSI, especially the peaks of nC–S, nS–N–S, dS–N–S, and
daSO2 are almost the same. This indicates that the LiTFSI
are not fully dissociated in this mixture (EIm-1C/LiTFSI);
however, the cation sites of EIm-1C interacted with TFSI
anions. Accordingly, the spatial separation provided by
spacers with n > 2 is required for the design of excellent
mixtures for electrochemical applications.
We also measured the ionic conductivity for a series of
EIm-nC/LiTFSI mixtures (Fig. 6). Except for the case with
n = 1, all mixtures showed the ionic conductivity of 1 6 1025
–
1 6 1024 S cm21. EIm-6C/LiTFSI showed the highest
ionic conductivity in this series. However the reason for this
tendency has not been clarified yet, this might be concerned
with the Tg, viscosity, ion density, etc. For electrochemical
applications, we conclude that the most suitable spacer chain
length (n) of zwitterions is 6 or should be in between 5 and 7,
considering both the thermal stability (Fig. 5) and the ionic
conductivity (Fig. 6) of a series of the mixtures of zwitterion
and LiTFSI.
Fig. 5 shows the Td for a series of pure zwitterions and
their mixtures with LiTFSI. In all cases, the decomposition
Fig. 4 FT-IR spectra of the mixtures of LiTFSI and zwitterions
having various spacer chain lengths. (1) [EMIm][TFSI], (2) EIm-10C/
LiTFSI, (3) EIm-7C/LiTFSI, (4) EIm-6C/LiTFSI, (5) EIm-5C/LiTFSI,
(6) EIm-4C/LiTFSI, (7) EIm-3C/LiTFSI, (8) EIm-2C/LiTFSI, (9) EIm-
1C/LiTFSI, and (10) LiTFSI powder. Subscript ‘‘s’’ means symmetric
mode, and subscript ‘‘a’’ means asymmetric mode.
Fig. 6 Effect of temperature on the ionic conductivity of zwitterion–
LiTFSI mixtures. N: EIm-1C/LiTFSI (no Tg), %: EIm-3C/LiTFSI (Tg =
230 uC), .: EIm-4C/LiTFSI, (Tg = 229 uC) #: EIm-5C/LiTFSI, (Tg =
229 uC) m: EIm-6C/LiTFSI (Tg = 235 uC), &: EIm-7C/LiTFSI.
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Scheme 5 Structure of asymmetric-imide-type zwitterion.
Effect of anion site
According to our results, a mixture of LiTFSI and zwitterion
having N-ethylimidazolio-cation site with suitable alky spacer
is expected to be a better ion conductor than other mixtures.
The use of asymmetric (carbonyl)(sulfonyl)imide-type anion
(CS-imide) in the preparation of ionic liquids have been
reported to give lower Tm or Tg values than those of symmetric
systems.10, 11 Thus, an asymmetric structure should be examined
for lowering the Tm of the zwitterions. We introduced an
asymmetric (carbonyl)(trifluoromethyl-sulfonyl)imide anion
site into zwitterions with favourable cation and spacer struc-
ture (EIm-5csI as seen in Scheme 5). Pure EIm-5csI melts at
87 uC and decomposes at 295 uC. The Tm is significantly
lower than those of EIm-5C (148 uC) and EIm-4S (175 uC).6
This relatively low Tm for EIm-5csI shows that the systematic
design of zwitterions is helpful for the improvement of
thermal property.
Fig. 8 Arrhenius plots of ionic conductivity for EIm-4S/LiTf (&) and
EIm-5csI/LiTf (#).
A comparison of the ionic conductivity of these two mixtures
is shown in Fig. 8.
However, EIm-5csI/LiTFSI has a small transference number
of 0.2 for the lithium cation (tLi+), which is only slightly larger
than that of EIm-4C/LiTFSI (0.14). Further charge delocaliza-
tion must be effective to improve the tLi+. Bis(sulfonyl)imide
(SS-imide) anion is a good candidate for an anion that can
delocalize the negative charge. There was little effect of the
substitution of a symmetric anion to an asymmetric one. An
SS-imide-type zwitterion (BDMAm-3ssI, see Scheme 6) was
prepared according to the method similar to that for EIm-5csI.
The Tm of this zwitterion was 104 uC. The mixture, BDMAm-
The mixture EIm-5csI/LiTFSI is liquid at room temperature
and shows a Tg at 235 uC. This value is the same as that for
the EIm-6C/LiTFSI mixture but lower than that of EIm-4S/
LiTFSI (216 uC). The ionic conductivity of EIm-5csI/LiTFSI
is a little lower than that of EIm-6C/LiTFSI but higher than
that of EIm-4S/LiTFSI, which is expected from a comparison
of their Tg values. Fig. 7 shows the Arrhenius plots of the ionic
conductivity of the EIm-5csI/LiTFSI and the EIm-4S/LiTFSI
mixtures. However LiTFSI is considered to be only salt to
improve the ionic conductivity of the Z/L mixture, there
thould be some other partner salts for EIm-5csI to improve the
conductivity. Among a series of organic lithium salts, we found
that EIm-5csI/lithium tetrafluoromethanesulfonate (LiTf)
mixture has higher ionic conductivity than EIm-4S/LiTf.
+
3ssI/LiTFSI, had a tLi value of 0.55, which was almost the
same as that of EIm-4S/LiTFSI. This result suggests that the
structure of the anion site affects the tLi+ of zwitterion–LiTFSI
mixtures. Considering relatively high tLi+ of the mixtures
containing sulfonate-type or SS-imide-type zwitterions, the
degree of dissociation of the anion site seems to have a
significant effect on the mobility of carrier cations.
Spectroscopic measurements were carried out for Z/L
mixtures composed of various anion-type zwitterions to
analyze the effect of these anion structures on the state of
the ionic liquid-like phase. Fig. 9 shows FT-IR and Raman
spectra for zwitterion–LiTFSI mixtures having various anion
sites. Spectra for all of these mixtures are quite similar to that
of [EMIm][TFSI] ionic liquid. The marker peaks coincide
with the peaks of free TFSI anions9 (dsCF3) in the spectrum
of [EMIm][TFSI] as seen in Fig. 9b. The specific peaks
in zwitterion/LiTFSI mixtures (dS–N–S) were found in all
mixtures (Fig. 9a). These spectra suggest that the anion sites of
the zwitterions do not affect the ionic liquid-like domain
formed after mixing with LiTFSI. This tendency has also been
Fig. 7 Arrhenius plots of ionic conductivity of EIm-4S/LiTFSI (&)
and EIm-5csI/LiTFSI (#).
Scheme 6 Structure of a symmetric-imide-type zwitterion.
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Fig. 9 FT-IR spectra (a) and Raman shift (b) for various anion-type
zwitterion mixtures with LiTFSI. (1) [EMIm][TFSI], (2) EIm-5csI/
LiTFSI, (3) EIm-4C/LiTFSI, (4) EIm-4S/LiTFSI, and (5) LiTFSI
powder.
Fig. 10 FT-IR spectra (a) and Raman shift (b) for various anion-type
zwitterion mixtures with LiTf. (1) [EMIm][Tf], (2) EIm-5csI/LiTf, (3)
EIm-4C/LiTf, (4) EIm-4S/LiTf, and (5) LiTf powder.
Zwitterions having a relatively low Tm value became liquid
after mixing with salt, and these Z/L mixtures had high ionic
conductivity. To obtain a relatively high ionic conductivity, the
Tm value of the pure zwitterion should be relatively low. In
additions, it is important to find a suitable cation structure
besides imidazolium cation. (2) Formation of an ionic liquid-
like domain. This domain is necessary for preparation of
effective ion conductive pathway. (3) A high degree of
dissociation to generate sufficient amount of carrier cations.
This factor improves transference number of carrier cations.
The cation structure affects factors (1) and (2), as does the
spacer-chain length, whereas the anion structure affects factors
(1) and (3). By considering these factors, one can improve the
performance of zwitterions as ion conductive matrices or
materials for other purposes.
found in the case of zwitterion/LiTf mixed systems. Although
some peaks have not been characterized, the results in Fig. 10
indicate sufficient dissociation of triflate anions after mixing
with zwitterions. Spectra for all Z/L mixtures are quite similar
to that of [EMIm][Tf] ionic liquid. These results strongly
support that the N-ethylimidazolium cation of the zwitterions
interacted with the added anions but received little influence
from the tethered anions.
Important points for designing zwitterions
For simple ionic liquids, there is a known relation between the
melting point and ionic conductivity. In contrast, it is difficult
to estimate the ionic conductivity of a salt–zwitterion mixture
from the Tm of the pure zwitterion. For example, there is
no correlation between the Tm value of pure zwitterions and
the Tg (or ionic conductivity) of zwitterion–salt mixtures. As
shown in Figs. 2 and 6, no correlation was found in a series
of carboxylate-type zwitterions. Nevertheless, we found the
following three points for designing excellent zwitterions for
electrochemical applications: (1) Pure zwitterions with a low
Tm. Although the Tm value of pure zwitterions does not
correlate with the properties of Z/L mixtures, there is a rough
relation between Tm and the ionic conductivity of the mixture.
Conclusions
Strategies to obtain zwitterions having good properties have
been investigated. We discussed three parts of zwitterions,
i.e., cation-site, spacer-chain, and anion-site to clarify the
structural effects on the properties of the zwitterions and their
mixture with salts. Regarding the cation-sites, it is difficult
to improve the properties of pure zwitterions through the
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modification of the cation sites. However, the Tm of a pure
zwitterion can be lowered by increasing the spacer chain
length. The optimum spacer was found to be the hydrocarbons
with about 5 to 7 CH2 units. With such a chain length, the Z/L
mixtures had high thermal stability and high ionic conduc-
tivity. Modifications to the anion structure, particularly imide
anion sites, affected both the Tm value of pure zwitterions
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Acknowledgements
The financial support for this study by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology (No. 17205020 and 17073005)
is gratefully acknowledged. LiTFSI was a gift from Sumitomo
3M. The present study was carried out under the 21st COE
program of Future Nano-Materials.
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