A curved relationship in the Arrhenius plots was observed
for all the mixtures used in the present study. In general, an ion
conductive process is expressed by the Vogel–Fulcher–
Tamman (VFT) equation,22 which empirically explains the
temperature dependence of viscosity in amorphous materials.
The best fitting results are also depicted as the solid lines in
Fig. 3. Thus, the temperature dependence of the ionic
conductivity for all the mixtures obeyed the VFT equation
quite well.
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The OE1imps/LiN(Tf)2 mixture showed the ionic
conductivity of 9.2 Â 10À5 S cmÀ1 at 80 1C, which was
almost identical to that of Bimps/LiN(Tf)2 mixture. The
ionic conductivity of OE2imps/LiN(Tf)2 mixture was
3.8 Â 10À4 S cmÀ1 at 80 1C, which was 4 times higher than
that of OE1imps/LiN(Tf)2 mixture. It is known that there is a
good correlation between ionic conductivity values and Tg
values for ILs.23 The higher conductivity of the OE2imps/
LiN(Tf)2 mixture should be mainly attributed to the Tg of
À27 1C, which is the lowest among the mixtures. In addition,
OE units attached to the imidazolium cations will be effective
not only in lowering the crystallinity of zwitterions but also in
enhancing the dissociation of lithium salts and the mobility
of lithium ions in the matrix. The lithium transference
number (tLi+) and the electrochemical stability are important
properties for applying zwitterions as electrolyte materials.
Recently, the tLi+ values were determined by an electro-
chemical technique to be over 0.5 for the equimolar mixtures
of LiN(Tf)2 and the zwitterions containing dissociable anion
structures such as sulfonate, amide, and borate anions.13,14
It has been suggested that zwitterions act as excellent ion
conductive matrices, in which the target ions such as lithium
cations preferentially migrate. It is, however, well known
that poly(ethylene oxide) solubilizes lithium salts, which is
attributable to a large polarity on the ether oxygen. OE units
would trap lithium cations through ion–dipole interactions.
Those investigations are now in progress and will be reported
elsewhere.
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In conclusion, the zwitterion containing two OE units on the
imidazolium cation was obtained as a viscous liquid at room
temperature. In addition, the equimolar mixture of the liquid
zwitterion and LiN(Tf)2 maintained a liquid state even after a
few months and showed higher ionic conductivity than those
of other mixtures based on the solid zwitterions. Ether groups
were effective not only in lowering the crystallinity of
zwitterions but also in improving the ionic conductivity.
Low-melting and low-viscosity zwitterions will be an interesting
matrix for electrochemical applications.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2345–2347 2347