Physical and electrolytic properties of difluorinated dimethyl carbonate

Article abstract of DOI:10.1016/j.jfluchem.2004.05.008

The physical and electrolytic properties of difluorinated dimethyl carbonate (DFDMC) synthesized using F₂ gas (direct fluorination) were examined. The dielectric constant and viscosity of DFDMC are higher than those of monofluorinated dimethyl carbonate (MFDMC) and dimethyl carbonate (DMC). The oxidative decomposition voltage of DFDMC is higher than those of DMC and MFDMC. The specific conductivity in DFDMC solution is considerably lower than those in MFDMC and DMC solutions. The ethylene carbonate (EC)-DFDMC equimolar binary solution containing 1 mol dm^-3 LiPF₆ shows a moderate conductivity of 6.91 mS cm^-1 at 25 °C. The lithium electrode cycling efficiency (charge-discharge coulombic cycling efficiency of lithium electrode) in EC-DFDMC equimolar binary solution containing 1 mol dm^-3 LiPF₆ is higher than 80%. The EC-DFDMC solution is a good electrolyte for rechargeable lithium batteries.

Full text of DOI:10.1016/j.jfluchem.2004.05.008

Journal of Fluorine Chemistry 125 (2004) 1205–1209
Physical and electrolytic properties of difluorinated
dimethyl carbonate
Yukio Sasakia,*, Masahiro Takeharaa, Susumu Watanabea,
Noritoshi Nanbua, Makoto Ueb
aDepartment of Applied Chemistry, Faculty of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan
bMitsubishi Chemical Group Science and Technology Research Center, Inc., 8-3-1 Chuo, Ami, Inashiki, Ibaraki 300-0332, Japan
Abstract
The physical and electrolytic properties of difluorinated dimethyl carbonate (DFDMC) synthesized using F2 gas (direct fluorination) were
examined. The dielectric constant and viscosity of DFDMC are higher than those of monofluorinated dimethyl carbonate (MFDMC) and
dimethyl carbonate (DMC). The oxidative decomposition voltage of DFDMC is higher than those of DMC and MFDMC. The specific
conductivity in DFDMC solution is considerably lower than those in MFDMC and DMC solutions. The ethylene carbonate (EC)-DFDMC
equimolar binary solution containing 1 mol dmꢀ3 LiPF6 shows a moderate conductivity of 6.91 mS cmꢀ1 at 25 8C. The lithium electrode
cycling efficiency (charge–discharge coulombic cycling efficiency of lithium electrode) in EC-DFDMC equimolar binary solution containing
1 mol dmꢀ3 LiPF6 is higher than 80%. The EC-DFDMC solution is a good electrolyte for rechargeable lithium batteries.
# 2004 Elsevier B.V. All rights reserved.
1. Introduction
(DFDMC) in Fig. 1 newly synthesized by direct fluorination,
compared with those of DMC and MFDMC.
There has been increasing interest in development of new
solvents on rechargeable lithium batteries with high energy
density, oxidation durability, high liquidus range and non-
flammability as power sources of mobile equipments and
electric vehicles. One of the appropriate methods to find a
solvent with good cell performance is the introduction of
fluorine atoms into the solvent molecules. Among the
important solvents for practical lithium batteries, fluorina-
tion using F2 gas (direct fluorination) has already been
applied to 1,2-dimethoxyethane (DME) [1], propylene car-
bonate (PC) [2] and ethylene carbonate (EC) [2]. In recent
years, we reported the formation of three monofluorinated
g-butyrolactones [3]. It is possible for direct fluorination to
synthesize various fluorinated organic solvents because of
high reactivity of F2. In addition, physical and electrolytic
properties, and application to lithium batteries of mono-
fluorinated dimethyl carbonate (MFDMC) obtained by
direct fluorination of dimethyl carbonate (DMC) were first
examined by us [4].
2. Results and discussion
Fig. 2 shows GC–MS analysis of the fluorinated DMC
derivatives after fluorination for 24 h [4]. RT means the
retention time. It is found that four fluorinated DMC deri-
vatives are formed by direct fluorination of DMC, which
are monofluorinated dimethyl carbonate (MFDMC), two
difluorinated dimethyl carbonates (DFDMC and gem-
DFDMC), and trifluorinated dimethyl carbonate (TFDMC)
as shown in Fig. 2. In Fig. 2, the boiling point of DFDMC is
very different from that of gem-DFDMC, which has two
fluorine atoms similar to DFDMC. This means that physical
properties of the fluorinated DMC with same molecular
weight such as DFDMC and gem-DFDMC are highly
dependent on the positions of fluorine atoms introduced
to DMC molecule. Variation in yields with time of the
fluorinated DMC derivatives is shown in Fig. 3 [4]. In the
present study, pure DFDMC was obtained by distillation of
DMC sample after fluorination for more than 20 h.
The purpose of the present paper is to elucidate the
physical and electrolytic properties, and the charge–dis-
charge characteristics of difluorinated dimethyl carbonate
Fig. 4 shows i–E curves in DFDMC, MFDMC and DMC
solutions containing 1.0 mol dmꢀ3 LiPF6. In Fig. 4, oxida-
tive decomposition voltage for these three solvents decreases
in the order, DFDMC > FDMC > DMC, as expected from
* Corresponding author. Tel.: þ81 462 42 95 30; fax: þ81 462 42 30 00.
E-mail address: sasaki@chem.t-kougei.ac.jp (Y. Sasaki).
0022-1139/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2004.05.008

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Y. Sasaki et al. / Journal of Fluorine Chemistry 125 (2004) 1205–1209
Fig. 1. Structure of difluorinated dimethyl carbonate (DFDMC).
HOMO energies calculated by B3LYP [4]. In other words,
oxidation durability of the fluorinated DMC derivatives
becomes higher with increasing the number of fluorine atom
in the fluorinated DMC.
Fig. 5 shows dielectric constants of DFDMC, MFDMC
and DMC with temperature. The dielectric constant of
DFDMC is very high compared with those of MFDMC
and DMC at whole temperature range. It seems that the high
dielectric constant of DFDMC is dependent on the high
polarity of DFDMC due to the introduction of two fluorine
atoms with high electron withdrawing into DMC molecule.
In addition, the dielectric constant of DFDMC decreases
linearly with increasing temperature, in a similar manner to
MFDMC. This may mean that the orientation of the dipoles
in DFDMC and MFDMC reduces with the rise of tempera-
ture. On the other hand, dielectric constant of DMC with low
one is almost constant at whole temperature range, as shown
in Fig. 5. Variation of viscosities (Z) with temperature
for DFDMC, MFDMC and DMC is shown in Fig. 6. The
viscosities of these three solvents decrease in the order,
DFDMC > MFDMC > DMC, similarly to those of the
dielectric constants. Fig. 7 shows plot of log Z versus 1/T
Fig. 3. Variation in yields with time of the fluorinated DMC derivatives.
for DFDMC, MFDMC and DMC. Activation energies of
viscous flow estimated from the straight lines in Fig. 7 are
16.1, 12.9 and 10.3 kJ molꢀ1 for DFDMC, MFDMC and
DMC, respectively. The activation energy of DFDMC with
the highest viscosity shows a maximum value.
Table 1 shows specific conductivities with various con-
centrations of LiPF6 in DFDMC, MFDMC and DMC at
25 8C. In Table 1, it was difficult to measure the conductiv-
ities for DFDMC solutions of more than 1.50 mol dmꢀ3
because of low solubility of LiPF6. Specific conductivities in
DFDMC solutions (except 0.25 mol dmꢀ3) with higher
dielectric constant in Fig. 5 are considerably lower than
those in MFDMC and DMC solutions. Accordingly, the
Fig. 2. GC–MS analysis of the fluorinated DMC derivatives after fluorination for 24 h.

Y. Sasaki et al. / Journal of Fluorine Chemistry 125 (2004) 1205–1209
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Fig. 7. Plot of log (Z/cP) vs. 1/T for DFDMC, MFDMC and DMC.
Fig. 4. i–E curves in 1.0 mol dmꢀ3 LiPF6 solutions using a Pt electrode at
a scan rate of 5.0 mV sꢀ1 at 25 8C.
Table 1
Specific conductivities with various LiPF6 concentrations in DFDMC,
MFDMC and DMC at 25 8C
Concentration (mol dmꢀ3
)
Specific conductivities (mS cmꢀ1
)
DFDMC
MFDMC
DMC
0.25
0.50
1.00
1.25
1.50
1.75
2.00
0.70
1.36
2.39
2.80
3.10
–
0.76
2.00
4.21
4.93
5.30
5.24
4.99
0.57
2.33
5.99
7.33
7.90
8.15
7.71
–
Fig. 5. Temperature dependence on dielectric constants of DFDMC,
MFDMC and DMC measured with 1 MHz.
Fig. 9 shows lithium electrode cycling efficiencies in
DFDMC, MFDMC, EC-DFDMC, EC-MFDMC and EC-
DMC equimolar binary solutions containing 1 mol dmꢀ3
LiPF6 at 25 8C. It is found that DFDMC and EC-DFDMC
solutions show higher efficiencies than those of another
solutions. In particular, EC-DFDMC solution shows the
highest efficiency of more than 80% in a higher range of
cycle number. This is a good electrolyte for rechargeable
lithium batteries. In addition, it is very useful for fluorina-
tion of DMC to obtain a high lithium electrode cycling
efficiency. It seems that the high cycling efficiency is
conductivities seem to be affected much by the viscosities of
these three solvents as shown in Fig. 6. The conductivities
with various LiPF6 concentrations in EC-DFDMC, EC-
MFDMC and EC-DMC equimolar binary solutions at
25 8C is shown in Fig. 8. Each conductivity in EC-DFDMC,
EC-MFDMC and EC-DMC solutions shows the maximum
values at 1 mol dmꢀ3 LiPF6. For example, the conductivity
in EC-DFDMC solution is 6.91 mS cmꢀ1, which means a
moderate conductivity for lithium batteries.
Fig. 8. Variation of specific conductivities (k) with various LiPF6
concentrations in EC-DFDMC, EC-MFDMC and EC-DMC equimolar
binary solutions at 25 8C.
Fig. 6. Temperature dependence on viscosity (Z) of DFDMC, MFDMC
and DMC.

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Y. Sasaki et al. / Journal of Fluorine Chemistry 125 (2004) 1205–1209
higher than 80%. The EC-DFDMC solution is a good
electrolyte for rechargeable lithium batteries.
4. Experimental
F2 gas was generated by the electrolysis of KFÁ2HF
(Fluorodec TM , Toyo Tanso Co., Ltd.) at 80–90 8C. Fluor-
30
ination of DMC was carried out using 15% F2 gas diluted by
N2 gas. The diluted gas was poured onto the DMC sample
surface at 22.4 ml minꢀ1 with stirring in PFAvessel at 25 8C.
The fluorinated DMC derivatives were confirmed by gas
chromatograph and mass spectrometer (GC–MS) (JMS-
SX102A) [5]. The purification of DFDMC was carried
out by fractional distillation of the fluorinated DMC sample.
The formation and structure of DFDMC were confirmed by
GC–MS and 1H (Fig. 10), 13C NMR (Fig. 11) (JEOL, JNM-
LA500) [6]. The purity of DFDMC was measured as 99.5%
by using GC (SHIMAZU, GC-1700). DMC, EC and LiPF6
were used as received (battery grade). DFDMC was dehy-
drated using purified molecular sieves (4A) before prepara-
tion of the solution. The preparation of the solution and the
assembly of the cell were carried out in an argon-filled dry
box. The apparatus and techniques for measurements of
specific conductivity [7–9], dielectric constant [10], density
[10] and viscosity [10] were reported previously. The
lithium electrode cycling efficiency (charge–discharge
coulombic cycling efficiency of lithium electrode) was
measured with a charge/discharge unit (Hokuto Denko,
HJ-201B) and estimated by a galvanostatic plating–stripping
method reported by Koch and Brummer [11]. The decom-
position voltage in electrolyte using a three-electrode system
Fig. 9. Variation of lithium electrode cycling efficiencies in DFDMC,
MFDMC, EC-DFDMC, EC-MFDMC and EC-DMC equimolar binary
solutions containing 1.0 mol dmꢀ3 LiPF6 at 25 8C.
dependent on the morphology of the films formed on the
electrode [4].
3. Conclusions
The dielectric constant and viscosity of DFDMC are
higher than those of MFDMC and DMC. The oxidative
decomposition voltage of DFDMC is higher than those of
DMC and MFDMC. The specific conductivity in DFDMC
solution is considerably lower than those in MFDMC and
DMC solutions because of high viscosity of DFDMC. The
lithium electrode cycling efficiency in EC-DFDMC equi-
molar binary solution containing 1 mol dmꢀ3 LiPF6 is
Fig. 10. 1H NMR spectra of DFDMC, MFDMC and DMC.

Y. Sasaki et al. / Journal of Fluorine Chemistry 125 (2004) 1205–1209
1209
Fig. 11. 13C NMR spectra of DFDMC, MFDMC and DMC.
(100), gem-DFDMC, CIMS: 127 [M þ H]þ, EIMS: 95 [M–OCH3]þ
(2.38), 59 [M–OCHF2]þ (30), 51 [M–OCOOCH3]þ (100), 31
[M–COOCHF2]þ (30.95), TFDMC, GCCI: 145 [M þ H]þ, GCEI:
33 [M–OCOOCHF2]þ (46.27), 51 [M–OCOOCH2F]þ (100), 77
[M–OCHF2]þ (7.05), 95 [M–OCH2F]þ (3.52).
(Pt working, Li counter and reference electrodes) were
measured at a scan rate of 5 mV sꢀ1 by means of an
ELECTROCHEMICAL INTERFACE (Solartron Analyti-
cal, Model SI-1287).
[6] DFDMC, 1H NMR: (500 MHz, CDCl3), d ¼ 6.07 (d, JHF ¼ 50 Hz),
13C NMR: (125.65 MHz, CDCl3), d¼ 97.31 (d, JCF ¼ 219.89),
153.01 (s), MFDMC, 1H NMR: (500 MHz, CDCl3), d ¼ 4.26 (s),
6.11 (d, JHF ¼ 50 Hz), 13C NMR: (125.65 MHz, CDCl3), d ¼ 55.14
(s), 97.01 (d, JCF ¼ 219.89), 154.72 (s).
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