Effect of zwitterionic salt on the electrochemical properties of a solid polymer electrolyte with high temperature stability for lithium ion batteries

Article abstract of DOI:10.1016/j.electacta.2010.09.068

In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N′-methyl imidazolium tetrafluoroborate (EMIBF₄), LiBF₄ and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 °C and high room temperature ionic conductivity near to 10^-3 S cm^-1. Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3- sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10^-3 S cm^-1 at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol ^-1. The charge-discharge performance of a Li₄Ti ₅O₁₂/SPE/LiCoO₂ cell indicates the potential application of the as-prepared SPE in lithium ion batteries.

Full text of DOI:10.1016/j.electacta.2010.09.068

Electrochimica Acta 56 (2010) 804–809
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effect of zwitterionic salt on the electrochemical properties of a solid polymer
electrolyte with high temperature stability for lithium ion batteries
Z.H. Lia,∗,1, Q.L. Xiaa, L.L. Liua, G.T. Leia, Q.Z. Xiaoa, D.S. Gaoa, X.D. Zhoub
a Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Hunan 411105 China
b State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 China
a r t i c l e i n f o
a b s t r a c t
In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-Nꢀ-methyl imi-
dazolium tetrafluoroborate (EMIBF4), LiBF4 and poly(vinylidene difluoride-co-hexafluoropropylene)
[P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 ◦C and high room
temperature ionic conductivity near to 10−3 S cm−1. Its electrochemical properties are improved with
incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When
the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10−3 S cm−1 at room
temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent
activation energy for ions transportation of 30.9 kJ mol−1. The charge–discharge performance of a
Li4Ti5O12/SPE/LiCoO2 cell indicates the potential application of the as-prepared SPE in lithium ion bat-
teries.
Article history:
Received 9 June 2010
Received in revised form
15 September 2010
Accepted 18 September 2010
Available online 25 September 2010
Keywords:
Ionic liquid
Polymer electrolyte
Lithium ion batteries
Ionic conductivity
Zwitterionic salt
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
alkyl carbonates. Moreover, it shows an electrochemical window
about 4.0 V and a high thermal decomposition temperature more
Lithium ion batteries (LIBs) have been widely used as power
sources for many portable electronics such as laptop computer, dig-
ital camera and mobile phone owing to their high energy density
and long life. In general, the conventional LIBs use the solution of
lithium salt in alkyl carbonates as electrolyte, which will be evap-
orated into gas at elevated temperature. The aggregation of gas in
battery case results in some serious accidents such as burn and
explosion. Therefore, the LIBs suffer from a safety problem when
they operate at elevated temperature that might come from their
operation at larger current density or improper uses such as over-
charge, over-discharge and short-circuit. The safety problem of the
LIBs would be solved if the nonaqueous electrolyte were replaced
with solid polymer electrolyte or room temperature ionic liquid
(RTIL) [1]. RTIL, which is nonvolatile and nonflammable, has been
used as a plasticizer to incorporate the SPE to enhance the ionic
conductivity. Of the most important, the safety of LIBs can be guar-
anteed with use of RTIL [2–15].
than 300 ◦C. The incorporation of EMIBF4 into polymer matrix can
enhance the ionic conductivity of the resultant SPE [17–22], but the
SPE incorporated with much EMIBF4 is poor in mechanical strength
[23]. In addition, lithium salt must be added to the polymer elec-
trolyte because it is Li+ ions not EMI+ ions intercalate the electrode
materials in LIBs. In this case, the migration cations in the lithium
salt–RTIL–polymer ternary system (i.e. the RTIL-plasticized SPE)
include both Li+ and EMI+ ions in a potential gradient [2]. The com-
petition of EMI+ ions with Li+ ions does decrease the lithium ions
transference number of the obtained polymer electrolyte [24]. Ogi-
hara et al. reported that the addition of lithium salt led to a decrease
of ionic conductivity of the RTIL-plasticized SPE [25]. Fortunately, it
has been reported that the addition of the zwitterionic compounds
based on pyrrolidinium or imidazolium assists in the dissocia-
tion of Li+ ions [12,26] and the formation of the solid electrolyte
interface (SEI) film in the RTIL–zwitterions-based electrolyte or
polymer electrolyte [27–29]. However, few of papers discuss the
effect of zwitterionic salt (MIm3S) on the electrochemical proper-
ties of the P(VdF-HFP)-based polymer electrolyte that is plasticized
with EMIBF4.
EMIBF4 with low viscosity has a high ionic conductivity of
14 × 10−3 S cm−1 at 25 ◦C [16], which is similar to those of the
classical electrolytes based on lithium salts in the mixtures of
Here we synthesize a zwitterionic salt, MIm3S to incorporate
the P(VdF-HFP)-based SPE plasticized with EMIBF4, and investi-
gate its effect on the electrochemical properties of the obtained
SPE such as ionic conductivity, lithium ions transference number
and apparent activation energy for ions transportation. In addition,
∗
Corresponding author. Tel.: +86 731 58292206; fax: +86 731 58292251.
E-mail address: lzh69@xtu.edu.cn (Z.H. Li).
ISE member.
1
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.068

Z.H. Li et al. / Electrochimica Acta 56 (2010) 804–809
805
O
The alter current (AC)/direct current (DC) method, which has
been elucidated in detail by Bruce and Vincent [36], was widely
used to determine the lithium ions transference number (t+) of the
RTIL–lithium salt–polymer system [37–42] thus the t+ of the SPE in
this study can be calculated from Eq. (1):
O
S
+
N
N
N
N
SO3
O
Scheme 1. Structure of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate.
Is(ꢁV − I0Rc0t
I0(ꢁV − IsRcst
)
)
t
+
=
(1)
some Li4Ti5O12/SPE/LiCoO2 cells are fabricated to evaluate the pos-
sible application of the as-prepared SPE in LIBs.
In Eq. (1), I0 and Is are the initial and the steady state currents
measured by a DC polarization method, respectively. Rct and Rct
0
s
are the charge transfer resistances at the initial and steady states,
respectively. Their values can be determined from the Nyquist
curves of the SPE before and after the DC polarization measure-
ment. ꢁV (50 mV) is an imposed potential difference between two
lithium electrodes.
Linear sweep voltammetry (LSV) experiments were performed
to investigate the electrochemical stability of the SPE using a three-
electrode cell with stainless steel working electrode and lithium
reference and counter electrodes. All experiments were carried out
at a sweep rate of 10 mV s−1 in the potential range of 0–6 V (vs.
Li/Li+) at 25 ◦C.
Galvanostatic curves of the Li4Ti5O12/SPE/LiCoO2 cells were
achieved using Arbin BT2000 battery tester. A slurry of LiCoO2,
acetylene black and poly(vinylidene difluoride) with a mass ratio
of 85:10:5 in NMP was coated onto an aluminum sheet, and dried
at 120 ◦C overnight under vacuum forming the cathode. The anode,
which consists of 80 wt% of Li4Ti5O12, 15 wt% of acetylene black
and 5 wt% of PVDF, was prepared using copper sheet according to
the process of cathode. The SPE containing 1 wt% of the MIm3S was
sandwiched between the cathode and anode and encapsulated into
a 2016-type coin cell. The cell was charged and discharged at a con-
stant current in the output potential difference range of 1.1–2.6 V
at 25 ◦C.
2. Experimental
1-(1-Methyl-3-imidazolium)propane-3-sulfonate was synthe-
sized according to a similar method described in the literature
[30]. Typically, 4.0 g (42.0 mmol) 1-methyl imidazole (Alfa Aesar)
was added dropwise to the solution of 5.0 g (42.0 mmol) 1,3-
propanesultone (Alfa Aesar) in 100 ml acetone at first (the solution
has been degassed with N2 at 0 ◦C for 1 h at prior). The mixture
was then stirred at room temperature for 100 h in a N2 atmosphere.
After that, the solution was filtered and the precipitate was purified
twice with acetone and dried at 80 ◦C for 24 h in vacuum. Finally,
8.3 g white powders (MIm3S) were obtained (yielding ratio: 93%).
The synthesis route is described in Scheme 1.
1H NMR data of the as-prepared white powder: 2.5 (s, 3H), 7.67
(s, 1H), 7.76 (s, 1H), 9.09 (s, 1H), 2.43–2.40 (t, 2H), 2.13–2.06 (m,
2H), 4.33–4.29 (t, 2H).
FTIR data of the as-prepared white powder: 3154 cm−1
(C–H stretching), 3014 cm−1 (CH2 stretching vibrations of the
alkane) [31], 1646 cm−1 (stretching vibrations of the quaternary
ammonium salt), 1575 cm−1(C–C, C–N in-plane stretching vibra-
tions of the cyclic imidazole) [32], 1392 cm−1 (CH2 wagging),
1362 cm−1 (shoulder peaks, stretching vibrations of cyclic imi-
dazole), 1337 cm−1 (CH2 twisting) [33], 1194, 1043 cm−1 (S
O
stretching vibrations of sulfate group) [34], 1172 cm−1 (C–H in-
plane deformed vibration of cyclic imidazole) [32], 847 and
623 cm−1 (C–H out-plane bending vibrations of cyclic imidazole),
745 and 648 cm−1 (C–H out-plane bending vibrations of EMI+) [35].
A mixed solution of 0.50 g EMIBF4 (Alfa Aesar), 0.05 g LiBF4
(Shanghai China Lithium Industry Co., Ltd.) and 0.50 g P(VdF-HFP)
(Kynar Flex® 2801) in 4.50 ml N-methyl pyrrolidone (NMP) was
produced at first. Various amounts (0, 0.016, 0.032 and 0.048 g,
corresponding to 0, 1.0, 2.0 and 3.0 wt% of in the obtained SPE,
respectively.) of the MIm3S was then dissolved in the above solu-
tion. The resultant viscous solution was spread on a glass plate and
dried at 80 ◦C for 24 h in vacuum. Due to evaporation of the NMP, a
self-supporting polymer membrane formed on the glass plate and
was immediately transferred into a dry glove-box (H2O content
less than 5 ppm). The elastic polymer membrane was pouched into
discs forming the SPE samples finally.
The melting point of the synthesized MIm3S was measured
using the differential scanning calorimeter (DSCQ10, Perkin-Elmer)
in the temperature range of 80–250 ◦C at a scan rate of 10 ◦C min−1
in a nitrogen atmosphere. The thermal decomposition tempera-
tures of the as-prepared SPE and the MIm3S were determined
through the thermogravimetric (TG) analysis at a heating rate of
10 ◦C min−1 in a nitrogen atmosphere.
3. Results and discussion
Fig. 1 shows the differential scanning calorimetry (DSC) and
thermogravimetric (TG) curves of the synthesized MIm3S (a) and
the TG curves of the pure P(VdF-HFP) membrane and the as-
prepared SPE (b). It can be seen that, from Fig. 1a, the melting
temperature and the thermal decomposition temperature of the
MIm3S are 222 and 357 ◦C, respectively. The thermal decomposi-
tion temperature of the pure P(VdF-HFP) is 478 ◦C and those of the
solid polymer electrolytes (SPEs) incorporated without and with
1 wt% of the MIm3S are 353 and 341 ◦C, respectively. The thermal
decomposition temperatures of the SPEs containing other amounts
of the MIm3S are summarized in Table 1. It suggests that the ther-
mal decomposition temperature of the SPE hardly varies with the
amount of the zwitterionic salt, indicating that the lithium ion bat-
teries using this kind of SPE would be safe even at high temperature
(more than 300 ◦C).
The bulk resistance (Rb) and the charge transfer resistance (Rct
)
of the SPEs can be obtained from the Nyquist curves (Fig. 2). The
semicircle relating to the resistance component corresponds to the
charge transfer and the capacity component to the electric dou-
ble layer when non-blocking electrodes (i.e. lithium electrodes) are
used [43]. In the low frequency range, the straight line declining
about 45 degrees represents the Warburg impedance (Zw) based
on the diffusion of ions. The left intercept of the semicircle on the Zꢀ
axis represents the bulk resistance (Rb) of the SPE, while the right
one equals to the sum of the Rb and Rct. The equivalent circuit of the
Li/SPE/Li cell is depicted in Fig. 2 (insertion). Table 1 presents the
room temperature ionic conductivities of the SPEs containing var-
ious amounts of the MIm3S. It is found that the ionic conductivity
of the SPE without the MIm3S is 0.59 × 10−3 S cm−1 and increases
In a dry glove-box, the SPE sample was sandwiched between
two symmetrical lithium electrodes and encapsulated into a mod-
ule cell. The electrochemical impedance spectra (EIS) were taken
using an EG&G M273 Potentiostat/Galvanostat in conjunction with
M5210 Lock-in amplifier electrochemical analysis system in the
frequency range of 10−1–105 Hz. The ionic conductivity of the SPE
can be calculated from the equation ꢀ = d/(Rb × S), where ꢀ, Rb, d and
S are the ionic conductivity, the bulk resistance, the thickness of the
SPE membrane and the area of the lithium electrode, respectively.

806
Z.H. Li et al. / Electrochimica Acta 56 (2010) 804–809
-2
-4
a
-10
0
b
10
20
30
40
50
60
70
80
90
100
-6
341
C
478
C
-8
353
C
-10
-12
-14
-16
-18
-20
The SPE without the MIm3S
The SPE containing 1 wt% of the MIm3S
Pure P(VdF-HFP)
80 100 120 140 160 180 200 220 240 260
100
200
300
400
500
600
Endo up
T / C
T / C
Fig. 1. (a) Differential scanning calorimetry and thermogravimetric curves of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate measured in a N2 atmosphere at a scan
rate 10 ◦C min−1. (Insert: thermogravimetric curve); (b) Thermogravimetric curves of the solid polymer electrolytes incorporated without and with 1.0 wt% of the 1-(1-
methyl-3-imidazolium)propane-3-sulfonate.
Table 1
The thermal decomposition temperatures and the electrochemical properties of the solid polymer electrolytes incorporated with various amounts of the 1-(1-methyl-3-
imidazolium)propane-3-sulfonate (MIm3S).
Content of the MIm3S
(wt%)
Thermal
Room temperature
ionic conductivity
Apparent activation energy
for ions transportation
Lithium ions
transference number
decomposition
temperature (◦C)
(10−3 S cm−1
)
(kJ mol−1
)
0
353
347
341
338
330
0.59
0.78
1.57
1.01
0.41
39.0
38.7
30.9
37.7
43.3
0.19
0.24
0.36
0.25
0.10
0.5
1.0
2.0
3.0
firstly with the amount of MIm3S and then decreases with excessive
incorporation. The ionic conductivity reaches the maximum value,
1.57 × 10−3 S cm−1 when the SPE contains 1.0 wt% of the MIm3S.
To elucidate the stability of the SPE with respect to metallic
lithium, the dependence of the interfacial resistances (Ri) of the
Li/SPE/Li cell on storage time is shown in Fig. 3. The interfacial resis-
tance of the SPE without the MIm3S increases gradually within 24 h,
and changes hardly in the time range of 24–104 h, then increases
slightly with further storage. With incorporation of the MIm3S,
the SPE reaches a stable value of the interfacial resistance after 8 h
storage because the zwitterionic salt might react with the lithium
electrode to form a solid electrolyte interface (SEI) film on its sur-
face [29]. Thus, all the electrochemical measurements of the overall
SPE samples were carried out after 24 h storage in this study.
Fig. 4 displays the chronoamperometry profile (a) and the
Nyquist plots (b) of the SPE containing 1.0 wt% of the MIm3S. From
Fig. 4a, we can find that I0 and Is of the SPE are 51 and 25 ␮A,
0
s
respectively. Fig. 4b reveals that Rct and Rct are 750 and 1368 ꢂ,
respectively. Thus, the lithium ions transference number (t+) is cal-
culated to be 0.36 according to Eq. (1). The other t+ values of the SPEs
are summarized in Table 1. It suggests that the lithium ions transfer-
ence number increases at first and then decreases with the amount
of the MIm3S. The lithium ions transference number reaches the
maximum value when the SPE contains 1.0 wt% of the MIm3S.
In the as-prepared SPE, Li+ ions are surrounded by a large num-
−
ber of BF4 anions forming an ionic cluster, which absorbs many
EMI+ ions forming a large ionic atmosphere. Li+ ions are trapped
in the ionic atmospheres and migrate difficultly resulting in a low
ionic conductivity and small lithium ions transference number.
With incorporation of the zwitterionic salt (MIm3S), the compe-
7000
Mass fraction of
the MIm3S in the SPE
Cdl
Mass fraction of the MIm3S in the SPE
6000
6000
0 wt%
0.5 wt%
1.0 wt%
2.0 wt%
3.0 wt%
3 wt%
5000
5000
Rb
w
Rct
Zw
0 wt%
4000
3000
2000
1000
0
4000
3000
0.5 wt%
2000
2 wt%
1000
0
1 wt%
0
1000
2000
3000
4000
5000
6000
7000
0
30
60
90
120
150
180
210
Z' / Ω
t / h
Fig. 2. Nyquist curves of the solid polymer electrolytes containing various
amounts of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate at room temper-
ature. (Insert: equivalent circuit of the Li/SPE/Li cell).
Fig. 3. Interfacial resistances of the solid polymer electrolytes incorporated with
various amounts of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate at room
temperature after different storage time.

Z.H. Li et al. / Electrochimica Acta 56 (2010) 804–809
807
1800
b
I0
a
before dc polarization
after dc polarization
50
40
30
20
10
0
1500
1200
900
Is
600
300
0
R
+R
R
+R
-10
0
300
600
900
1200
1500
1800
0
200
400
600
800
1000
t / min
Z'/ Ω
Fig. 4. Chronoamperometry profile (a) and Nyquist plots (b) of the solid polymer electrolytes containing 1.0 wt% of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate at
room temperature.
tition of the dissociated SO3− anions with Li+ and EMI+ ions maybe
destroys the ionic atmospheres and thus Li+ ions migrate easily. As
a result, the ionic conductivity and the lithium ions transference
number increase. Nevertheless, Li+ ions will be surrounded by a
large numbers of SO3− anions forming another ionic atmosphere if
excessive MIm3S is incorporated into the SPE. Thus, the ionic con-
ductivity and the lithium ions transference number decrease on the
contrary. The incorporation of a suitable amount of the MIm3S into
the SPE facilitates the dissociation of the lithium salt in fact.
Fig. 5 shows the dependence of the ionic conductivity of the
SPE on temperature (log ꢀ–T−1 plots). We can see that the ionic
conductivity increases with the increasing temperature and all
the plots appear linear in the temperature range measured. In a
wide temperature range, the conductive behavior of ions in poly-
mer electrolyte usually obeys the VTF (Vogel–Tamman–Fulcher)
relation ꢀ = ꢀ0 exp(−Ea/R(T − T0), where T0 is the idealized glass
transition temperature of the polymer and is lower than that of
the polymer by 20–50 K. Since the glass transition temperature of
P(VdF-HFP) is −100 ◦C [44], T0 will be far below the measure tem-
perature range of 25–55 ◦C. Therefore, VTF behavior can be modeled
as Arrhenius behavior whose equation is:
respectively. Ea calculated from Eq. (2) is 39.0 kJ mol−1 for the SPE
without the MIm3S and decreases with the amount of the MIm3S
(see Table 1) suggesting that the ions in the SPE containing the
MIm3S migrate more easily than those in the SPE without the
MIm3S. When the SPE contains 1.0 wt% of the MIm3S, Ea reaches the
minimum value, 30.9 kJ mol−1. On the contrary, it increases with
excessive addition of the MIm3S (more than 1.0 wt%).
Fig. 6 shows the linear sweep votammogroms of the SPEs
incorporated with various amounts of the MIm3S at a scan rate
of 10 mV s−1 at room temperature. The SPE without the MIm3S
exhibits a cathodic limit at 0.72 V (vs. Li/Li+) and an anodic limit at
4.91 V (vs. Li/Li+). When incorporated with 0.5, 1.0, 2.0, and 3.0 wt%
of the MIm3S, the SPEs display the cathodic limits of 0.67, 0.49, 0.28
and 0.18 V (vs. Li/Li+), respectively, and the corresponding anodic
limits are 5.07, 5.36, 5.35 and 5.25 V (vs. Li/Li+). It can be seen that
the cathodic stability versus lithium of the SPE is improved with
incorporation of the zwitterionic salt. The zwitterionic salt (MIm3S)
facilitates the formation of the SEI film on the surface of Li electrode
and thus the EMI+ ions can hardly contact with Li electrode. Conse-
quently, the electrochemical stability of this kind of SPE is improved
although EMIBF4 is used.
ꢀ
ꢁ
It has been reported that the cathodic limit of the pure EMIBF4 is
about 1.0 V (vs. Li/Li+), which is caused by the presence of the acidic
C-2 hydrogen atom in the imidazolium heterocycle [45–49]. Some
improvements on the electrochemical stability of imidazolium
have been achieved by substituting C-2 hydrogen with an alkyl
−Ea
ꢀ = ꢀ0 exp
(2)
RT
where Ea, R and T are the apparent activation energy for ion trans-
portation, the gas constant and the absolute test temperature,
Mass fraction of the MIm3S in the SPE
3.0 wt%
1.0
0.5
Mass fraction of the MIm3S in the SPE
0.5 wt%
3.0 wt%
0 wt%
1.0 wt%
2.0 wt%
-2.6
2.0 wt%
1.0 wt%
-2.8
-3.0
-3.2
-3.4
-3.6
0.5 wt%
0 wt%
0.49V
0.0
5.36V
-0.5
-1.0
Scanning rate: 10 mV -s1
0
1
2
3
4
5
6
+
3.05
3.10
3.15
3.20
103 T -1/ K-1
3.25
3.30
3.35
3.40
E vs (Li/Li) / V
Fig. 6. Linear sweep voltammograms of the solid polymer electrolytes incorporated
with various amounts of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate at a
sweep rate of 10 mV s−1 at room temperature. (Working electrode: stainless steel;
reference and counter electrodes: lithium).
Fig. 5. log ꢀ–T−1 curves of the solid polymer electrolytes incorporated with vari-
ous amounts of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate in the range
of 25–55 ◦C.

808
Z.H. Li et al. / Electrochimica Acta 56 (2010) 804–809
SPE contained 1.0 wt% of the MIm3S, it had a room temperature
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
ionic conductivity of 1.57 × 10−3 S cm−1 and a lithium ions trans-
ference number up to 0.36. At this amount, the SPE possessed
the minimum value of the apparent activation energy for ions
transportation, 30.9 kJ mol−1 and displayed an electrochemical
stability in the potential range of 0.49–5.36 V (vs. Li/Li+). The results
of charge–discharge measurements of the Li4Ti5O12/SPE/LiCoO2
cell suggest a potential application of this kind of SPE in LIBs.
0.5 mA cm (0.5C)
0.2 mA cm (0.2C)
0.1 mA cm (0.1C)
Acknowledgements
Financial support from the Natural Science Fund from Hunan
Province (09JJ5008), the Open Fund from State Key Laboratory
of Chemical Engineering in East China University of Science and
Technology (SKL-ChE-08C06), and the Doctoral Starting Fund from
Xiangtan University (05QDZ18) is greatly appreciated. We also
thank Ms. M.N. Liu for her help in the DSC and TG measurements.
0
10
20
30
40
50
60
70
80
90
100
Specific capacity / mAh g-1
Fig. 7. Galvanostatic curves of the Li4Ti5O12/SPE/LiCoO2 cell at various current den-
sities of 0.1, 0.2 and 0.5 mA cm−2 at room temperature. The used solid polymer
electrolyte contains 1 wt% of the 1-(1-methyl-3-imidazolium)propane-3-sulfonate.
(Insert: cycle performance of the test cell at 0.1 ◦C).
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´
Some persons have reported that the addition of lithium salt
would make the cathodic limit of EMI+ ions shift more negatively
[59–61] and the polymer electrolyte containing EMI+ ions showed
high electrochemical stability [58,62]. In this study, the formed SEI
film on the surface of electrode prevents EMI+ ions from contacting
lithium electrode. As a result, EMI+ ions might not be reduced on the
surface of lithium electrode and thus the electrochemical stability
of SPE is improved.
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Fig.
7
shows
the
galvanostatic
curves
of
the
Li4Ti5O12/SPE/LiCoO2 cell at various current densities at room
temperature and its cycling performance (inset). The SPE con-
taining 1 wt% of the MIm3S was used as the separator of the
Li4Ti5O12/SPE/LiCoO2 cell. It reveals that the test cell delivers
the capacities of 78, 65, and 57 mAh g−1 (based on the mass of
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,
respectively. These capacities are much lower than the theoretical
capacity of 175 mAh g−1. Since lithium ions migrate more diffi-
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4. Conclusions
A
zwitterionic-type
ionic
liquid,
1-(1-methyl-3-
imidazolium)propane-3-sulfonate (MIm3S) has been synthesized
successfully via one step and was incorporated into the P(VdF-
HFP)-based SPE containing EMIBF4 and LiBF4. The resultant SPE
showed a thermal decomposition temperature more than 300 ◦C.
Since the zwitterionic salt would facilitate the formation of the SEI
film on the surface of Li electrode and enhance the disassociation
of lithium salt in the SPE, the electrochemical properties of the
SPE were improved by incorporation of the MIm3S. When the
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