Organosilicon functionalized glycerol carbonates as electrolytes for lithium-ion batteries

Article abstract of DOI:10.1039/c4ra15854g

Two triethoxyl-/trimethoxyl-silyl functionalized glycerol carbonates and one disiloxanyl functionalized glycerol carbonate were synthesized through a cycloaddition reaction of carbon dioxide with allyl glycidyl ether followed by a hydrosilylation with the corresponding hydrosilanes. Their chemical structures were fully characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy and their basic physicochemical properties including dielectric constant, viscosity, ionic conductivity, apparent lithium transference number and electrochemical window, were systematically measured. Trimethoxysilyl functionalized glycerol carbonate as electrolyte solvent with LiPF₆ (0.6 M) and lithium oxalyldifluoroborate (0.4 M) binary salts exhibited good cycling stability over 2.7-4.4 V in high-voltage-LiCoO₂/graphite full cells. Disiloxane functionalized glycerol carbonate acted as an efficient electrolyte additive to improve the wetting property on the separator in Li/LiCoO₂ cells. This journal is

Full text of DOI:10.1039/c4ra15854g

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Organosilicon functionalized glycerol carbonates
as electrolytes for lithium-ion batteries†
Cite this: RSC Adv., 2015, 5, 17660
a
ab
c
d
Jinglun Wang, Tianqiao Yong, Jianwen Yang, Chuying Ouyang
and Lingzhi Zhang*a
Two triethoxyl-/trimethoxyl-silyl functionalized glycerol carbonates and one disiloxanyl functionalized
glycerol carbonate were synthesized through a cycloaddition reaction of carbon dioxide with allyl
glycidyl ether followed by a hydrosilylation with the corresponding hydrosilanes. Their chemical
1
13
structures were fully characterized by H and C nuclear magnetic resonance (NMR) spectroscopy and
their basic physicochemical properties including dielectric constant, viscosity, ionic conductivity,
apparent lithium transference number and electrochemical window, were systematically measured.
Received 5th December 2014
Accepted 3rd February 2015
Trimethoxysilyl functionalized glycerol carbonate as electrolyte solvent with LiPF
oxalyldifluoroborate (0.4 M) binary salts exhibited good cycling stability over 2.7–4.4 V in high-voltage-
LiCoO /graphite full cells. Disiloxane functionalized glycerol carbonate acted as an efficient electrolyte
additive to improve the wetting property on the separator in Li/LiCoO cells.
6
(0.6 M) and lithium
DOI: 10.1039/c4ra15854g
2
www.rsc.org/advances
2
1
8–23
environmentally benign characters.
On the other hand, it
1
. Introduction
has also been reported that the electrochemical performances
The current state-of-the-art LiPF
6
-based carbonate electrolyte of lithium-ion batteries can be improved remarkably by adding
systems are commercially used in lithium-ion batteries, due to a small amount of organosilicon additives to act as acid
their high conductivity, excellent solubility with lithium salts, scavenger, ame retardant, over charge protecting agent, and
1,2
and ability to form a stable solid electrolyte interface (SEI), etc.
Research efforts have continued in an attempt to develop new carbonate electrolytes.
SEI lm-forming reagent in the conventional LiPF
6
-based
2
4–27
Regarding organosilicon functional
electrolyte systems with fewer hazards and improved features carbonates as electrolyte materials, a few research achieve-
concerning safety, operation voltage ranges, temperature limits, ments have been documented over the past decades. For
3–5
irreversible capacity, ion transportation, and so on. However, example, disiloxane functionalized carbonates were synthe-
organic carbonates are still an excellent choice as the solvent for sized and characterized as a potential solvent/additive for
electrolyte systems, and great advances have been documented electrolytes in lithium-ion batteries, although no cell tests
2
8,29
for functional carbonates, such as uoro functional carbon- were carried out for these compounds.
X. Huang et al.
6–9
10–13
ates, vinyl functional carbonates,
and ionic liquid func- reported pentamethyldisiloxanylethylene and trimethylsilyl-
14
15
tional carbonates, sulfonate functional carbonates, alkyl ethylene functionalized carbonate, EC-CH
2
CH
, which showed efficient
ates, as efficient electrolyte solvent/additive to address the SEI forming capability on the surface of mesocarbon
2 3 2
Si(CH ) -
16
functional carbonates, and organosilicon functional carbon- OSi(CH ) and EC-CH CH Si(CH )
3 3 2 2 3 3
17
3
0
problems mentioned above.
Organosilicon compounds have received much attention as EC-CH
microbead anode. Later, S. Tobishima and co-works studied
CH Si(CH OSi(CH and heptamethyltrisiloxanyl-
functionalized carbonate (EC-CH CH
CH ) as electrolyte co-solvents for improving the
2
2
3
)
2
3 3
)
electrolytes for energy storage devices due to their thermal ethylene
2
2
-
and electrochemical stability, low ammability and Si(OSi(CH
3
)
3
)
2
3
cycling performance of lithium metal, graphite and Si–C
3
1,32
anode.
We recently reported that trialkoxylsilyl compound
a
CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion,
with di(ethylene oxide) substituent (TESM2) in 1 M LiPF6
Chinese Academy of Sciences, Guangzhou 510640, China. E-mail: lzzhang@
ms.giec.ac.cn; Fax: +86 20 37246026; Tel: +86 20 37246025
exhibited good cycling stability in a LiCoO /Li half cell cycled
2
over 3.0–4.35 V, retaining 90% capacity aer 80 cycles. More
recently, we reported a series of new organosilicon compounds
containing nitrile and oligo(ethylene oxide) unit which
showed an excellent cyclability with a discharge capacity of 152
b
University of Chinese Academy of Sciences, Beijing 100039, China
c
College of Chemistry & Bioengineering, Guilin University of Technology, 12 Jiangan
Road, Guilin, Guangxi, 541004, China
d
Department of Physics, Jiangxi Normal University, Nanchang 330022, China
ꢀ
1
†
Electronic supplementary information (ESI) available: PFG-NMR tting curves mA h g for 185 cycles at a higher cut-off voltage of 4.4 V in
and comparison data of LiCoO /graphite performance in commercial
electrolyte. See DOI: 10.1039/c4ra15854g
33,34
2
2
LiCoO /graphite full cell.
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ꢀ
In this work, we introduce two triethoxyl-/trimethoxyl-silyl with 4 A molecular sieves. These compounds had a water
functionalized glycerol carbonates and one disiloxanyl func- content of approximately 17 ppm as measured by coulometric
tionalized glycerol carbonate as electrolyte solvent and additive Karl Fischer titration (Model 831, Metrohm). The NMR data are
for the application of lithium-ion batteries. The electrochemical as followed:
1
performances of trimethoxylsilyl functionalized glycerol
carbonate and disiloxanyl functionalized glycerol carbonate SiCH
have been investigated as electrolyte solvent and additive for (s, 9H, (CH
high-voltage application (upper cut-off voltage of 4.4 V) in (q, 1H, CHCH
TMGC. H NMR (600 MHz, CDCl
3
): d 0.63 (m, 2H, (CH
), 3.47 (m, 2H, CH CH CH ), 3.56
Si), 3.63 (dd, 2H, –CH CH CH OCH ), 4.38
O), 4.48 (q, 1H, CHCH O), 4.78 (q, 1H, CHCH O);
C NMR (150.9 MHz, CDCl ): d 5.06, 22.65, 50.51, 66.29, 69.57,
73.89, 75.05, 154.96.
3 3
O) -
2
), 1.66 (m, 2H, SiCH
O)
2
CH
2
2
2
2
3
3
2
2
2
2
2
2
2
1
3
LiCoO
2
/graphite full cells and for improving wetting property
3
on the separator in Li/LiCoO half cells, respectively.
2
1
TEGC. H NMR (600 MHz, CDCl ): d 0.62 (m, 2H, (CH CH -
3
3
2
3
O) SiCH ), 1.22 (t, 9H, J ¼ 7.2 Hz, (CH CH O) Si), 1.68 (m, 2H,
3
2
3
2
3
2
. Experimental
SiCH
CH
2
CH
2
), 3.48 (m, 2H, SiCH
2
CH
2
CH
2
), 3.63 (dd, 2H, –CH
CH O) Si), 4.39 (q,
O), 4.48 (q, 1H, J ¼ 8.4 Hz, OCHCH O),
): d 6.37,
2
-
2.1. Materials
3
2
CH
2
OCH
Trimethoxysilane, triethoxysilane, 1,1,1-trimethyl-2,2-dimethyl- 1H, J ¼ 8.4 Hz, OCHCH
disiloxane and N-methyl-2-pyrrolidone (NMP, Aladdin Reagent 4.79 (m, 1H, OCHCH O). C NMR (150.9 MHz, CDCl
Co., China), ethylene carbonate (EC, Shanshan Co., China) and 18.28, 22.88, 58.39, 66.33, 69.57, 74.13, 74.99, 154.89.
2
), 3.80 (q, 6H, J ¼ 7.2 Hz, (CH
3
2
3
3
3
2
2
1
3
2
3
1
dimethyl carbonate (DMC, Shanshan Co., China), and LiPF6
Guangzhou Tinci High-Tech Materials Co., China) were used as 0.06 (s, 9H, –Si(CH
received. Articial graphite powder and LiCoO
powder were OSi(CH CH CH ), 3.46 (m, 2H, –CH
obtained from Amperex Technology Limited (China). A Celgard –CH CH CH OCH ), 4.39 (q, 1H, CHCH
400 microporous polypropylene membrane was used as the CHCH O), 4.80 (q, 1H, CHCH
DSGC. H NMR (600 MHz, CDCl
), 0.49 (m, 2H, OSi(CH
CH CH
O), 4.49 (q, 1H,
O). C NMR (150.9 MHz, CDCl ):
3
): d 0.05 (s, 6H, OSi(CH
CH ), 1.59 (m, 2H,
O), 3.64 (dd, 2H,
) ),
3 2
(
3
)
3
3
)
2
2
)
2
2
3
2
2
2
2
2
2
2
2
2
2
1
3
2
2
2
3
separator.
d 0.24, 1.95, 14.09, 23.31, 66.34, 69.58, 74.79, 75.06, 154.94.
2.2. Apparatus
2.4. Pulsed magnetic-eld gradient-NMR (PFG-NMR)
1
13
H NMR, C NMR, and PFG-NMR experiments were taken on a The solutions for PFG-NMR analysis was prepared through
Bruker Avence 400 spectrophotometer in CDCl
3
. Viscosity (h) dissolving 0.5 M LiTFSI into the synthesized organosilicon
measurements were tested on SPb-2 Viscometer (Nirun Intelli- functionalized glycerol carbonates. The samples were trans-
gent Technology Co., Ltd.). The dielectric constants (3) were ferred into the coaxial NMR tubes, and the diffusion coefficients
measured on an 870 Liquid Dielectric Constant Meter (Scien- of the cationic and anionic species were measured by the
ꢁ
35
tica). Ionic conductivities were determined by DDS-310 application of the PFG-NMR technique at 25 C.
conductivity instrument, variable temperature conductivity
measurements were conducted in an oil bath for temperature 2.5. Electrode preparation and cell tests
ꢁ
ꢁ
ranging between 0 C and 90 C. Linear scanning voltammo-
grams measurements were performed on a platinum working
electrode, with counter and reference electrodes of lithium foils
using an electrochemical workstation (BAS-ZAHNER IM6, Ger-
2
Working electrodes were prepared by mixing LiCoO , carbon
black and polyvinylidene uoride (PVDF) at a weight ratio of
9
0 : 5 : 5 in NMP, then the slurry was spread onto an Al foil and
ꢁ
dried at 80 C under vacuum for 6 h. The graphite anodes were
made by coating a mixture of graphite, carbon black and PVDF
with weight ratio 90 : 5 : 5 on copper foil. The 2025 type coin
cells were assembled for electrochemical tests under a dry argon
ꢀ1
many) with the scanning rate of 5 mV s between ꢀ0.5 to 6.7 V.
The static contact angle measurements were taken by a CCD
camera on Dataphysics OCA40 Micro.
atmosphere in the glove box (Mikrouna, H
The LiCoO /graphite full cells were galvanostatically charged
General procedure: (1) carbon dioxide (3 MPa) was purged into a and discharged with the current density of C/10 in a voltage
00 mL stainless steel autoclave which was placed with allyl range of 2.7–4.4 V, the LiCoO /Li half cells were galvanostati-
glycidyl ether (0.2 mol) and tetra-butylammonium bromide cally charged and discharged on a multi-channel battery test
2 2
O and O < 1 ppm).
2.3. Synthesis
2
1
2
(
0.02 mol), and the reaction mixture was stirring for 16 hours at system (NEWARE BTS-610) in a voltage range of 2.7–4.2 V.
ꢁ
8
0 C. Aer the reaction was completed, the reactor was quickly
ꢁ
cooled to 0 C in ice water, the excess of CO
2
was depressurized
3
. Results and discussion
slowly. The resulting mixture was distilled under vacuum
condition giving allyl glycerol carbonate (AGC) with a purity of
3.1. Synthesis of organosilicon functionalized glycerol
carbonate
99% and a yield of 95%. (2) The synthesized AGC (0.2 mol) was
added slowly to silane (0.2 mol, trimethoxysilane, triethoxy- Considering the availability of the starting material, glycerol
silane, or pentamethyldisiloxane) with PtO (1 wt%) as catalyst. carbonate which can be obtained through an environmental
2
ꢁ
Aer stirring at 60 C for 12 h, the product was obtained by friend route from biomass was chose as a starting material
30–32
fractional distillation. The obtained organosilicon functional- rather than butylene carbonate.
On the other side, the
ized glycerol carbonate was dried over with CaH
2
and stored traditional phosgene route for the synthesis of cyclic carbonates
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has been replaced by cycloaddition of carbon dioxide with Table 1 Physicochemical parameters of organosilicon functionalized
a
glycerol carbonate
epoxides in the view point of carbon dioxide chemical xation
36,37
and sustainable chemistry.
Therefore, the target organo-
b
s /
s
o
/
a
E /
KJ mol
silicon functionalized glycerol carbonates have been efficiently
ꢀ1
ꢀ1
Compound
3
h/mPa S mS cm
S cm
0
T /K
synthesized through a two-step route of cycloaddition carbon
dioxide with allyl glycidyl ether to generate allyl glycerol AGC
71.1
7.4
1.09
0.49
0.59
0.07
1.45
1.22
1.56
0.76
3.89
4.35
4.71
5.90
190.6
188.6
189.1
187.8
TMGC
37.8 12.8
31.6 12.5
25.8 14.9
carbonate (AGC), followed by hydrosilylation of AGC with cor-
responding hydrosilanes (Scheme 1, denoted as TMGC, TEGC,
and DSGC). Compared with the reported methodologies,
esterication of 3-(allyloxy)-propane-2,2-diol with linear
TEGC
DSGC
a
b
Dates are measured at room temperature. Doping with 1 M LiTFSI.
28
carbonate followed by hydrosilylation with hydrosilanes or by
hydrosilylation of a laborious synthesized butylene carbonate
32
and TEGC. As a consequence, these organosilicon functional-
ized glycerol carbonate compounds have somewhat lower
conductivities than the conventional electrolytes, e.g. propylene
with hydrosilanes, the two-step route is undoubtedly the most
efficient for preparing organosilicon functionalized carbonate.
The synthesized organosilicon functionalized glycerol carbon-
ates with the overall yield over 90% are colourless and clear
ꢀ
1
38
carbonate has conductivity value of 5.1 mS cm with LiTFSI.
1
In order to analyze the mechanism of ionic conduction, the
ionic conductivity dependence on temperature was studied in
the temperature range of 273–363 K. Fig. 1a shows the Arrhe-
nius plot of temperature dependent conductivity (ln s) versus
the inverse absolute temperature for the organosilicon func-
liquid at room temperature and are characterized through H
1
3
NMR and C NMR.
3.2. Physicochemical and electrochemical properties
The physical properties for these compounds, such as dielectric tionalized glycerol carbonates. From the Arrhenius plot, it can
constant (3), viscosity (h), and ionic conductivity (s) are collected be seen that the ionic conductivity shows curvature indicating
in Table 1. It seems that the introduction of organosilicon group that Vogel–Tammann–Fulcher (VTF) behavior is involved.
into the framework of AGC results in decreasing their 3 and Usually, VTF behavior is valid for polymer and glassy
increasing their h compared with those of AGC. The value of
their dielectric constants decrement in the order of 3AGC > 3TMGC
>
3
TEGC > 3DSGC (with increasing volume of silicon functional
group) and the value of their viscosities decline in the order of
> hTMGC > hTEGC > hAGC (exhibiting similar viscosity to the
h
DSGC
reported siloxane functionalized butylene cyclic carbonate (4-
(
2-trimethylsilyloxydimethylsilylethyl)-1,3-dioxolan-2-one), 18.0
ꢁ
mPa S, 20 C (ref. 32)). Usually, high dielectric constant facili-
tates the dissociation of lithium salts, thus improving ionic
conductivity; while high viscosity limits the mobility of Li and
+
decreases the ionic conductivity. It is easy to understand that
the value of their ionic conductivity decrease in the order of
s
AGC
> sTEGC > sTMGC > sDSGC, TEGC has the higher conductivity
than that of TMGC probably because viscosity dominates the
conductivity for these organosilicon functionalized glycerol
carbonates (hTMGC > hTEGC). DSGC has the lowest conductivity
for its higher viscosity and lower dielectric constant than TMGC
Fig. 1 (a) Arrhenius plots of conductivity versus temperature for
organosilicon functionalized glycerol carbonates with 1 M LiTFSI. (b)
Vogel–Tammann–Fulcher (VTF) plots of conductivity versus temper-
Scheme 1 The synthetic route for organosilicon functionalized glyc- ature for organosilicon functionalized glycerol carbonates with 1 M
erol carbonate. LiTFSI.
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electrolytes, or concentrated electrolyte solutions, and has also apparent lithium-ion transference number (tLi
+) could be
been widely observed for organosilicon functionalized electro- calculated by eqn (4):
4
0
ꢀ1
lytes. The ln(sT) versus (T ꢀ T ) plots of the organosilicon
0
t
Li
+
¼ DLi
+
+
F
/(DLi + D ꢀ)
(4)
functionalized glycerol carbonates t well linearly (Fig. 1b)
according to the VTF eqn (1):
It has been found that the tLi
+
+
/AGC
increased in the order tLi <
ꢀ
1/2
t
Li
+
/DSGC < tLi
+
/TEGC < tLi
+
/TMGC (in Table 2). Although possessing
s ¼ s
o
T
exp[ꢀE
a
/R(T ꢀ T
0
)]
(1)
the highest dielectric constant, lowest viscosity, and the
where s , E and T is derived from the VTF equation and listed resulting highest ionic conductivity, AGC exhibits the lowest tLi
+
o
a
0
in Table 1. s
o
and E
a
are parameters correlated to the charge of the four compounds, which may originate from the
+
carrier number and the activation energy, respectively, whereas complexation ability of the C]C double bond with Li , thus
+
T
0
represents the ideal glass transition temperature. The VTF inhibiting the mobility of Li .
diagrams exhibits a linear behaviour with a higher slope for the The electrochemical stability windows (ESW) of the
DSGC–LiTFSI solution among the functionalized carbonate synthesized compounds with 1 M LiTFSI salt were deter-
family, suggesting that DSGC–LiTFSI mixture possesses higher mined by linear sweep voltammetry (Fig. 2). TMGC, TEGC,
activation energy for the ion transport resulting in slower and DSGC compounds oxidatively decomposes at around 4.8
+
transport property.
V (vs. Li/Li ), except for AGC at 5.8 V. The trend in reductive
7
19
ꢀ
PFG-NMR tests of Li and F (TFSI ) was carried out to degradation of these compounds increases in the order of
measure the diffusion coefficients of Li and TFSI . The diffu- TMGC < TEGC y AGC < DSGC. Based on their physico-
sion coefficients of Li and F were determined according to chemical and electrochemical data, these organosilicon
+
ꢀ
7
19
the relation:
functionalized glycerol carbonate compounds are possible to
be used as electrolyte solvent or additive for lithium-ion
batteries.
2
I ¼ I[0] ꢂ exp[ꢀD ꢂ (2 ꢂ p ꢂ g ꢂ G ꢂ d) ꢂ (D ꢀ d/3)] (2)
where g is the gyromagnetic ratio, D is the diffusion coefficient,
d is the pulse width of the eld width of the eld gradient, G
denotes the magnetic eld gradient, and D is the duration time.
Then the diffusion coefficient values could be obtained by
3
.3. TMGC as electrolyte solvent
The high voltage performance for lithium-ion batteries using
LiCoO as the cathode and articial graphite as the anode was
2
investigated using TMGC as electrolyte solvent owing to its
high electrochemical window, high apparent lithium trans-
ference number and moderate ionic conductivity. 0.4 M
lithium oxalyldiuoroborate (LiODFB) and 0.6 M LiPF
chosen as the binary lithium slats because LiODFB provides
an effective passivation lm on the graphite anode and LiPF

tting the intensity data I(G) by eqn (2) (the tting curves are
displayed in ESI, Fig. S1†). Several author related the self coef-
cient to the Stokes–Einstein eqn (3):

6
are
D ¼ K
B
T/(cphr)
(3)
6
where h represents the viscosity, r is the effective hydrody- exhibits a good dissolving property in organosilicon func-
namics radius. The constant c varies between 4 and 6, tionalized glycerol carbonate. The cycling performance of
depending on the shape of the diffusing entity. As illustrated graphite/0.4 M LiODFB + 0.6 M LiPF in TMGC/LiCoO full
6
2
ꢀ
in Table 2, the diffusion coefficients of TFSI are generally cells with a high cut-off voltage of 4.4 V is plotted in Fig. 3.
large than those of the Li despite the smaller size of Li . The cell was charged and discharged at a rate of C/10 and
Thus, the solvation of the Li can be assumed. The diffusion cycled between 2.7 and 4.4 V at room temperature. It
+
+
+
coefficient of anion in the organosilicon functionalized glyc-
erol carbonates are correlated to the variation of their
viscosity (hAGC > hTEGC > hTMGC > hDSGC), while the diffusion
coefficient of cation may correlate to the variation of their
viscosity and the coordinated solvent radius (rDSGC > rTEGC
>
r
TMGC > rAGC).
In order to analyze the individual contributions of the cation
and anion to the overall ionic conductivity of the mixture, the
Table 2 Results of the PFG-NMR measurements
D
Li
+
F
D ꢀ
Li
t +
ꢀ12
ꢀ12
ꢀ12
ꢀ12
2
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ12
ꢀ12
ꢀ12
ꢀ12
2
ꢀ1
ꢀ1
ꢀ1
ꢀ1
AGC
3.253 ꢂ 10
3.012 ꢂ 10
2.332 ꢂ 10
1.844 ꢂ 10
m s
6.420 ꢂ 10
3.666 ꢂ 10
4.064 ꢂ 10
3.482 ꢂ 10
m s
0.336
0.451
0.365
2
2
TMGC
TEGC
DSGC
m s
m s
2
2
m s
m s
2
2
m s
m s
0.346 Fig. 2 Linear scanning voltammograms of the electrolytes with 1 M
LiTFSI in the organosilicon functionalized glycerol carbonates.
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Fig. 4 Static contact angle measurement on separator with different
electrolytes, without addition of DSGC (left) and with addition of DSGC
(right).
2
Fig. 3 Cycling performance of graphite/LiCoO full cell with 0.4 M
LiODFB + 0.6 M LiPF
off voltage of 4.4 V.
6
in TMGC as electrolyte at C/10 with a high cut- DSGC additive aer C-rate test under the charged state at
the voltage of 4.2 V are provided in Fig. 6. All the Nyquist plots
are composed of two partially overlapping semi-circles at
high and medium frequency range and a straight sloping line
at the low frequency end, representing respectively three
exhibits excellent cycling performance with discharge
resistance components: lm resistance, charge transfer
ꢀ
1
capacity over 148 mA h g aer 80 cycles, while the cell
using conventional electrolyte of 1 M LiPF in EC/DEC/DMC
1/1/1, in vol) fades quickly (showed in ESI, Fig. S2†). The
+
45
resistance, and Li diffusion resistance. The radii of the two
semi-circles were much smaller than those without DSGC,
indicating the lower surface lm resistance and charge
transfer resistance for the cells with DSGC in the electrolyte.
Therefore, the improved rate performances for the electrolyte
with 5 vol% DSGC can be attributed to this improved wetting
on separator by DSGC and the correspondingly enhanced
kinetics.
6
(
low discharge capacity may relate to the high viscosity of the
electrolyte, which needs optimization near future.
3.4. DSGC as electrolyte additive
Among the merits of organosilicon compounds mentioned
above, another unique property is their extraordinary low
surface free energy of siloxane compounds (surface tension of
ꢀ
1
41
hexamethyldisiloxane is 16 mN m ), so that they have been
widely used as surface active agents for surface treatment.
The polyether modied siloxanes was reported as an elec-
trolyte additive for improving the surface lm stability on Li
4
2
metal electrode. In this study, the wetting property of DSGC
in the electrolyte of 1 M LiPF in EC/DEC (1/1, in vol) was
6
investigated by static contact angle measurement. The static
contact angle of the electrolyte with/without the addition of 5
vol% DSGC on electrode is too small to be measured (the
electrolyte spreads over quickly on the surface of LiCoO
2
electrode). However, we found that the electrolyte with an
optimized amount of 5 vol% DSGC showed a smaller contact
ꢁ
ꢁ
angle (35.2 ) on separator compared with 46.2 for the elec-
trolyte without the addition of DSGC (Fig. 4), indicating the
efficient wetting capability on separator for DSGC as additive
in electrolyte. It is well known that the separator placed
between cathode and anode is one of critical components in
lithium-ion batteries, which could effectively transport ionic
charge carriers as well as to prevent the electric contact
4
3,44
between two electrodes.
lyte with 5 vol% DSGC in Li/LiCoO
capacities at high rate conditions, e.g. 63 mA h g at 5 C as
In our experiments, the electro-
2
cells exhibited higher
ꢀ
1
ꢀ
1
compared with 0.7 mA h g
for the cell without DSGC
additive. When cycled at 0.5 C aer 5 C tests, the cells fully
recovered its capacity to the former 0.5 C level of about 120
mA h g (Fig. 5a and b). The electrochemical impedance
Fig. 5 C-rate performances and coulombic efficiency of Li/LiCoO
cells in different electrolytes (a) with DSGC, (b) without DSGC in the
spectroscopy (EIS) measurements of the cells with/without voltage range of 2.7–4.2 V.
2
ꢀ1
17664 | RSC Adv., 2015, 5, 17660–17666
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3
T. R. Jow, K. Xu, O. Borodin and M. Ue, Electrolytes for
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Fig. 6 EIS impedance of Li/LiCoO cell after C-rate tests with or
without DSGC addition.
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4
. Conclusions
Two triethoxyl-/trimethoxyl-silyl functionalized glycerol
carbonates and one disiloxanyl functionalized glycerol
carbonate were synthesized under mild conditions with high
yield by cycloaddition carbon dioxide with allyl glycidyl ether to
form allyl glycerol carbonate, followed by hydrosilylation with
corresponding hydrosilanes. Although the dielectric constant,
viscosity, and ionic conductivity decreased with the introduc-
tion of organosilicon group, the apparent lithium transference
number increased compared with allyl glycerol carbonate.
Trimethoxyl-silyl functionalized glycerol carbonate in 0.4 M
LiODFB + 0.6 M LiPF
performance compared with the conventional electrolyte (1 M
LiPF in EC/DEC/DMC, 1/1/1 in vol) at a cut-off voltage of 4.4 V
in graphite/LiCoO full cells. Disiloxanyl functionalized glycerol
carbonate with 5 vol% content in the electrolyte of 1 M LiPF in
EC/DEC (1/1 in vol) improved signicantly wetting property on
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This work is supported by the Hundred Talents Program of
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