Ion Transport Properties of Lithium Salts
J. Phys. Chem. B, Vol. 108, No. 50, 2004 19521
TABLE 2: Ionic Conductivities (mS cm-1) of Electrolyte
TABLE 3: Ionic Conductivity, Viscosity, and the Ratios of
Carrier Ion Density Calculated by Ionic Conductivities and
Viscosities in the Presence and Absence of the Boric Acid
Ester Monomers for CF3COOLi and LiTFSI Solutions in
GBL at 25 °C
-
1
Solutions (0.2 mol kg ) at 25 °C in the Absence and
Presence of Boric Acid Ester Monomers (0.2 mol kg-1)
salt
LiCl
added monomer
PC
GBL
DME
DMC
insoluble 0.45 insoluble insoluble
boron
monomer
CT monomer
BP monomer
NP monomer
1.9
0.83
1.6
0.34
1.3
0.82
1.1
1.8
1.5
1.4
1.6
2.9
2.7
2.2
2.0
3.3
2.6
2.4
2.5
2.8
1.6
2.6
0.20
0.17
0.33
insoluble
insoluble
insoluble
σ (mScm-
1)
η (cP)
np/n
a
a
a
GBL/CF COOLi
3
0.51
1.2
1.7
1.8
4.3
3.3
3.5
3.3
1.8
2.7
2.1
2.2
1.9
2.9
2.2
2.6
1.0
3.3
3.9
4.4
1.0
1.1
-
4
CF3COOLi
CF3SO3Li
LiBF4
0.51 0.024
<5 × 10
0.0033
<5 × 10
0.014
0.0028
0.0056
0.0041
0.010
0.0014
0,013
0.027
0.029
0.24
BP
CT
NP
CT monomer
1.7
1.2
1.7
2.9
2.3
2.2
2.3
4.3
3.4
3.1
3.0
4.3
3.7
3.3
3.3
0.15
0.19
0.38
0.18
0.17
0.20
0.25
0.28
0.27
0.24
0.39
2.8
a
-4
BP monomer
a
NP monomer
GBL/LiTFSI
BP
CT
NP
CT monomer
0.94
1.0
a
BP monomer
a
NP monomer
the ionic conductivity of the electrolyte solutions does not
exhibit remarkable structural dependence on the boric acid ester
monomers.
To determine the factors controlling ionic conductivity in such
systems, we examined the relationship between ionic conductiv-
ity and viscosity, which can roughly be correlated to the number
of carrier ions by the equation
CT monomer
a
BP monomer
a
NP monomer
LiTFSI
CT monomer
2.7
2.5
2.5
0.45
0.42
0.34
a
BP monomer
a
NP monomer
a
Data from ref 13.
np σp ηp
electron density of the boron atoms in each species. The boron
atoms in CT and NP are involved in the five-membered rings,
whereas that in BP is involved in the seven-membered ring.
Such structural difference around the boron atoms might result
in the difference in the chemical shifts. The number of peaks
for the monomers interestingly differs in DME, which possesses
higher electron donor property compared to toluene. Although
a single peak is also observed for the BP monomer in DME,
two different peaks of boron are observed both for CT and NP
monomer systems. The peaks at the lower magnetic fields can
be assigned to the free monomer species, whereas that of the
higher magnetic fields corresponds to the boron atoms interacted
with DME molecules. The single peak for the BP monomer in
DME at ca. 0 ppm is expected to correspond to the BP
monomers interacted with DME and no free BP monomers
might exist in the system. This necessitates further investigation
≈
(2)
na
σa η
a
where na and np are the number of carrier ions, σa and σp are
the conductivities, ηa and ηp are the viscosities in the absence
a) and presence (p) of the anion receptors, respectively. The
(
ionic conductivity, viscosity, and the ratio, np/na, for LiTFSI
and CF3COOLi solutions in GBL with or without the addition
of boric acid ester monomers, are summarized in Table 3.
Regardless of the type of the boric acid ester monomers added
to the CF3COOLi solutions, the number of carrier ions
significantly increases, compared to the initial values, despite
increase in the viscosity. It is noteworthy that, despite the low
dissociability of CF3COOLi in an apolar solvent due to strong
Lewis basicity of the anion, the addition of boric acid ester
monomers facilitates ionic dissociation in GBL, resulting in
enhancement of the ionic conductivity. However, as we reported
earlier, with the excess addition of boric acid monomers, the
ionic mobility decreases, resulting in a decrease in the
conductivity. When the boric acid ester monomers are added
to the LiTFSI solutions (Table 3), the number of carrier ions
does not increase compared to the initial values, possibly due
to the high dissociation of LiTFSI in GBL, even in the absence
of the boric ester monomers. It is the increase in solution
viscosity due to the addition of boric acid ester monomers, which
causes a decrease in ionic conductivity in the highly dissociable
LiTFSI solution systems.
The molar conductivity of four electrolyte solutions (GBL/
LiTFSI, GBL/LiTFSI/BP, GBL/CF3COOLi, and GBL/CF3-
COOLi/BP) is plotted against the square root of the respective
salt concentrations in Figure 2. For dilute solutions, the molar
conductivity of electrolyte solutions of highly dissociable salts
usually tends to proportionally decrease with the square root of
their respective salt concentrations. The molar conductivity of
electrolyte solutions of low dissociable salts, on the other hand,
is inclined to be inversely proportional. When BP is added to
GBL/LiTFSI solutions, the molar conductivity is found to
decrease in the concentration range studied, because of the
increase in solution viscosity at each concentration. In contrast,
11
to correlate the structure of boron compounds, B NMR spectra,
and Lewis acidity of the boron atoms.
13b
Ionic Conductivities of the Electrolyte Solutions. The ionic
conductivities of various electrolyte solutions with and without
addition of the boric acid ester monomers are shown in Table
2
6
, where aprotic solvents having different permittivities (ꢀPC )
5, ꢀGBL ) 42, ꢀDME ) 7.2, ꢀDMC ) 3.1) and lithium salts of
anions with a wide variety of structures with different Lewis
basicity were used. LiF was insoluble in all of the solvents used,
irrespective of the absence or presence of the boric acid ester
monomers. As shown in Table 2, LiCl is insoluble in the
solvents except for GBL. The solubility behaviors are, however,
changed upon addition of the boric acid monomers. LiCl can
be solubilized in PC and DME with added boric acid monomers,
although it still remains insoluble in DMC. The ionic conductiv-
ity of the electrolyte solutions containing poorly dissociable salts
(LiCl, and CF3COOLi) considerably increases upon addition
of the boric acid monomers in the system. For salts with high
dissociability (CF3SO3Li, LiBF4, and LiTFSI), a similar increase
in conductivity by the addition of boric acid ester monomers is
observed only in the solutions of low permittivity solvents (DME
and DMC). In contrast, the solutions of high permittivity
solvents (PC and GBL) show an apparent decrease in conduc-
tivity with the addition of boric acid ester monomers. However,
the addition of the BP monomer to the GBL/CF COOLi solution
appreciably enhances the molar conductivity, showing that the
ionic dissociation more appreciably facilitates than an increase
3