X.J. Wang et al. / Electrochemistry Communications 12 (2010) 386–389
387
make the reduction more difficult. If this hypothesis is correct,
2.4. Synthesis of 4-fluoromethyl-1,3-dioxlan-2-one (MFPC)
replacing the methyl group with similar steric structure but differ-
ent electron withdrawing groups should change the SEI formation
properties. Therefore, we have synthesized several compounds and
carried out the comparative SEI formation studies. The most inter-
esting compound would be the trifluoropropylene carbonate
(TFPC) which has similar structure as PC but substituting three
hydrogen atoms in the methyl group by three fluorine atoms. This
compound was studied by Inaba et al. reporting that the SEI forma-
tion of TFPC is better than PC [16], but poorer than EC. The origin of
the SEI formation improvement of TFPC was explained by forma-
tion of immobile products as the stable SEI along step edge as soon
as co-intercalation occurring. No synthesis procedure of the TFPC
was given in their paper. It was also reported that TFPC has weaker
solvation ability compared to PC [17].
The similar synthesis procedure for TFPC was used for this com-
pound: 18 g of 3-fluoro-1,2-propanediol was reacted with 105 g of
20% phosgene in toluene to give 15 g of final product. The product
shows a boiling point of 65 °C/3 mm. 1H NMR (CDCl3) result: d 4.2–
4.8 (m, 3H), 4.8–5.2 (m, 2H) (ppm).
2.5. Synthesis of 2-(4-(1,3-dioxolan-2-one)) ethylpentamethyl
disiloxane (Si-A)
Twenty-four grams of 4-vinyl-1,3-dioxolan-2-one and 30 g of
pentamethyldisiloxane were mixed in 50 mL of anhydrous ether
and then cooled in an ice bath. Twenty milligrams of catalyst plat-
inum-divinyltetramethyldisiloxane complex was added to the
mixture. After stirring the solution for 1.5 h, the ether was re-
moved by evaporation. Residue was distillated under vacuum.
The yield was 34 g. The product shows a boiling point of 98–
100°C/0.1 mm. 1H NMR (CDCl3) result: d 0.05 (s, 15H), 0.3–0.6
(m, 2H), 3.9–4.7 (m, 3H) (ppm).
In order to clarify the effects of electron withdrawing groups,
TFPC and MFPC were synthesized and their electrochemical perfor-
mance was compared to EC and PC. Two new cyclic carbonate com-
pounds with long silane chain were also synthesized for the study
of steric effects.
2.6. Synthesis of 4-(2-trimethyl silyl ethyl)-1,3-dioxolan-2-one (Si-B)
2. Experimental
A mixture of 24 g of 4-vinyl-1,3-dioxolan-2-one and 20 mg of
catalyst platinum–cyclovinylmethylsiloxane complex was intro-
duced in a pressure tube cooled with liquid nitrogen. Then, 19 g tri-
methylsilane was condensed into the tube under vacuum and the
tube was sealed. The solution in the tube was stirred at 55 °C bath
for 17 h. After cooling, the tube was opened and the solution was
distilled with a column. The yield was 15 g. The product shows a
boiling point of 90–92 °C/0.12 mm. 1H NMR (CDCl3): O (s, 9H),
0.3–0.6 (m, 2H), 3.9–4.7 (m, 3H) (ppm).
All LiPF6 based electrolytes were prepared in an Ar-filled glove
box and the ratios are volume ratios. The electrodes used in this
study were mixture of MCMB (Osaka Gas, Japan), polyvinylidene
fluoride (PVDF), and acetylene black with a weight ratio of
90:2:8. Cu foil was used as current collector. Area of electrode
was 2.85 cm2. The battery was a Sandwich-type two-electrode half
cell using a lithium foil anode and a Celgard 2300 separator. The
cell was cycled between 0 and 3 V with a current of 0.1 mA at room
temperature using a Land batteries tester (Wuhan KINGNUO Elec-
tronic Co. Ltd, China).
Pentamethyldisiloxane, catalyst platinum–divinyltetramethyl-
disiloxane trimethylsilane, and platinum–cyclovinylmethylsilox-
ane complexes were purchased from Gelest Inc.; ethyl
trifluoropyruvate was purchased from SynQuest. All other starting
materials were purchased from Sigma–Aldrich.
2.1. Synthesis of 3,3,3-trifluoropropane-1,2-diol
Twenty-five grams of lithium aluminum hydride was sus-
pended in 500 mL of anhydrous tetrahydrofuran (THF). Seventy-
five grams of ethyl pyruvate was dropped into the mixture under
ice bath. The dropping time was 1.5 h. The mixture was refluxed
for 1 h, and then reacted with 2 M HCl until a clear solution was
obtained. After evaporating the THF, the aqueous solution was ex-
tracted with ether. The extract was dried over magnesium sulfate
and the ether was removed through evaporation. Residue was dis-
tilled under vacuum. The yield was 54 g. The product shows a boil-
ing point of 60 °C/5.5 mm. 1H NMR (CD3OD) result: d 3.5–3.8 (m,
2H), 3.8–4.2 (m, 1H), 4.5 (s, 2H) (ppm).
3. Results and discussion
2.2. Synthesis of 4-Trifluoromethyl-1,3-dioxlan-2-one (TFPC)
The charge–discharge curves of Li/MCMB cells are shown in
Fig. 1(A) for PC/DMC (1:1), EC/DMC (1:1), MFPC/DMC (1:1),
TFPC/DMC (1:1) with enlarged view in Fig. 1(B); and in Fig. 1(C)
for EC/PC/DMC (1:4.5:4.5), MFPC/PC/DMC (1:4.5:4.5), and TFPC/
PC/DMC (1:4.5:4.5) electrolytes with enlarged view in Fig. 1(D).
The voltage plateau near 0.8 V may be originated from PC co-
intercalation and used as an indicator for poor SEI formation
[18]. It can be seen that for the PC/DMC electrolyte, this plateau
keep extending for a very long time and resulted with 0% Cou-
lomb efficiency. In contrast, EC/DMC, MFPC/DMC and TFPC/DMC
electrolytes all show a high voltage plateau above 1.2 V and a
suppressed low voltage plateau with 90%, 83%, and 87% Coulomb
efficiencies, respectively. For the three tri-solvent electrolytes of
EC/PC/DMC, MFPC/PC/DMC and TFPC/PC/DMC, the Coulomb effi-
ciencies are 69%, 66% and 81% with similar discharge curves for
EC/DMC and TFPC/DMC, respectively. These results show that
TFPC has better SEI formation capability than EC, especially in
PC containing tri-solvent electrolytes. This is quite different from
what had been reported in Ref. [16], where the SEI formation
capability of TFPC is better than PC but poorer than EC. This
One hundred and seventeen grams of 20% phosgene in toluene
was added into an ice cooled solution of 28 g 3,3,3-trifluoropro-
pane-1,2-diol and 48 g triethylamine in 400 mL anhydrous THF
for 1.5 h. The mixture was stirred at room temperature for 1 h to
complete the reaction. The by-products were filtered out and the
THF was removed by evaporation. Residue was dissolved in ether
and then washed with water. The yield was 28 g. The product
shows a boiling point of 65 °C/3 mm. 1H NMR (CDCl3) result: d
4.4–4.7 (m, 2H), 4.7–5.3 (m, 1H) (ppm).
2.3. Synthesis of 3-fluoro-1,2-propanediol
25 g of epifluorohydrin was mixed with 50 mL of water. 0.1 mL
of 98% sulfuric acid was added. The solution was stirred at room
temperature for 24 h. The solution was neutralized with diluted
sodium hydroxide solution and then the water was removed by
evaporation. Residue was distilled under vacuum. The yield was
20 g. The product shows a boiling point of 60 °C/5.5 mm. 1H NMR
(CDCl3) result: d 3.5–4.2 (m, 4H), 4.8–5.0 (m, 3H) (ppm).