Published on the web June 12, 2010
753
Physicochemical Properties of Glyme-Li Salt Complexes
as a New Family of Room-temperature Ionic Liquids
Takashi Tamura, Kazuki Yoshida, Takeshi Hachida, Mizuho Tsuchiya, Megumi Nakamura,
Yuichi Kazue, Naoki Tachikawa, Kaoru Dokko, and Masayoshi Watanabe*
Department of Chemistry and Biotechnology, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501
(Received April 30, 2010; CL-100417; E-mail: mwatanab@ynu.ac.jp)
Certain glyme-Li salt complexes, which are composed of
equimolar mixtures of a glyme and a Li salt, are liquid under
ambient conditions with physicochemical properties such as
high thermal stability, wide potential window, high ionic
conductivity, and high Li+ transference number and can be
regarded as a new family of room-temperature ionic liquids.
that particular molar ratio mixtures of Li salts and oligoethers
such as crown ethers, triglyme (G3), and tetraglyme (G4) form
complexes. Henderson et al. have conducted a systematic study
of glyme-Li salt complexes and reported the crystallographic
structures, thermal properties, and electrochemical properties of
the complexes.7 Among the series of glyme-Li salt complexes,
they found that [Li(G4)][TFSA] remains in the liquid state at
¹3
room temperature and exhibits an ionic conductivity of 10
Room-temperature ionic liquids (RTILs), which are liquid at
room temperature and composed entirely of ions, have attracted
much attention because of their unique properties such as non-
flammability, low-volatility, high chemical stability, and high
ionic conductivity.1 RTILs are expected to be applied to
electrochemical devices, including electric double-layer capaci-
tors,2 fuel cells,3 dye-sensitized solar cells,4 and lithium ion
batteries (LIBs).5 Most of the RTILs reported to date can be
classified as combinations of weakly Lewis-acidic cations and
weakly Lewis-basic anions, which leads to ionic dissociation
without strong coordination of solvent molecules around each
ion. Thus, the most common compositions of RTILs are
combinations of onium cations such as imidazolium cations,
quaternary ammonium cations, and quaternary phosphonium
cations and soft anions such as bis(trifluoromethylsulfonyl)-
S cm¹1. Our group also reported thermal, transport, and electro-
chemical properties of equimolar complexes of glyme (G3 or
G4)-Li salt complexes (LiTFSA or LiN(C2F4S2O4)).8 We found
that the equimolar complexes of glyme (G3 or G4) and Li salts
show unique physicochemical properties similar to those of
RTILs, for example, high thermal stability, low volatility, low
flammability, and high ionic conductivity. Significantly, a 4 V-
class LiCoO2 cathode can be successfully operated in the molten
glyme-Li salt complexes;8 therefore, these complexes are now
promising electrolytes for LIBs.
Through the donation of lone pairs of a glyme molecule
(Lewis base) to a Li+ cation (Lewis acid), the weakly Lewis-
acidic complex cation [Li(glyme)]+ can be obtained. Due to the
formation of the complex, the Lewis acidity of Li+ is greatly
weakened, which can be regarded as an analogous concept to the
¹
¹
¹
¹
amide (TFSA ), tetrafluoroborate (BF4 ), and hexafluorophos-
formation of weakly Lewis-basic anion such as BF4 and PF6
¹
phate (PF6 ). There are few reports of RTILs consisting of
by the reaction between a Lewis acid and a Lewis base. The
combination of a weakly Lewis-acidic complex cation [Li-
(glyme)]+ and a weakly Lewis-basic anion leads to ionic
dissociation in the fused complex similar to conventional RTILs.
Here, we propose glyme-Li salt complexes as a new family of
room-temperature ionic liquids. It is anticipated that modifica-
tion of the chemical structure of the ligand, i.e., glyme in this
study, brings about dramatic changes in the physicochemical
properties of the glyme-Li salt complexes. So far, the phys-
icochemical properties of conventional RTILs have been tailored
by the modification of the chemical structures of cations and/or
anions. For example, it has been reported that introducing
asymmetric structure into the cation and/or anion of an RTIL is
effective in decreasing melting point, glass transition temper-
ature, and viscosity.9 In this work, asymmetric structure is
introduced into the glyme molecules by changing the terminal
alkyl group, as shown in Figure 1, and the dependences of
physicochemical properties of glyme-Li salt complexes on the
chemical structure are investigated.
strongly Lewis-acidic cations such as Li+ and Na+ and strongly
¹
¹
Lewis-basic anions such as F and Cl . Melting points of salts
consisting of strongly Lewis-acidic cations and strongly Lewis-
basic anions are generally much higher than room temperature,
resulting in the formation of ionic crystals at room temperature.
So far, we have reported the preparation of lithium ionic liquids
consisting of lithium salts of borates having electron-withdraw-
ing groups, to reduce the anionic basicity, and lithium
coordinating ether-ligands, to dissociate the lithium cations
from the anionic centers.6 However, possibly due to the strong
Lewis acidity of Li+, the viscosity and ionicity (dissociativity)
of the lithium ionic liquids at room temperature are as high as
500 mPa s and as low as 0.1-0.2, respectively, resulting in a low
ionic conductivity of 10¹5 S cm at its maximum.
¹1
¹
¹
Weakly Lewis-basic anions such as BF4 and PF6 are
prepared by the reactions between Lewis acids (BF3 and PF5)
¹
and a Lewis base (F ) by forming coordination bonds. However,
the preparation of weakly Lewis-acidic cations for RTILs by the
reaction between a Lewis acid and a Lewis base has not been
proposed. It is anticipated that weakly Lewis-acidic cations can
be prepared by the combination of alkali metal cations (Lewis
acid) and suitable ligands (Lewis base).
DSC thermograms (Figure 2) of different glyme (G3 or
G4)-LiTFSA complexes show their melting points to be lower
than room temperature. In all the DSC curves, no peak due to the
melting of pure glymes is observed (see Supporting Information
(SI) Table S1),10 suggesting that no free glyme molecules exist
in the equimolar mixtures. It has been reported that the
Ethers are relatively strong Lewis bases, and alkali metal
cations are strongly coordinated with ethers. It is well-known
Chem. Lett. 2010, 39, 753-755
© 2010 The Chemical Society of Japan