Electrochemical properties of PEO/PMMA blend-based polymer electrolytes using imidazolium salt-supported silica as a filler

Article abstract of DOI:10.1007/s11164-012-0839-8

In this study, the composite polymer electrolytes (CPEs) were prepared by solution casting technique. The CPEs consisted of PEO/PMMA blend as a host matrix doped with LiClO₄. Propylene carbonate (PC) was used as plasticizer and a small amount of imidazolium salt-supported amorphous silica (IS-AS) as a filler was prepared by the sol-gel method. At room temperature, the highest conductivity was obtained for the composition having PEO-PMMA-LiClO₄-PC-4wt. % IS-AS with a value of 1.15 × 10 ^-4 S/cm. In particular, the CPE using the IS-AS filler showed a higher conductivity than any other sample (fumed silica, amorphous silica). Studies of differential scanning calorimetry and scanning electron microscopy indicated that the ionic conductivity increase was due to an expansion in the amorphous phase which enhances the flexibility of polymeric chains and the homogeneous structure of CPEs. It was found that the ionic conductivity and interfacial resistance stability of CPEs was significantly improved by the addition of IS-AS. In other words, the resistance stability and maximum ambient ionic conductivity of CPEs containing IS-AS filler were better than CPEs containing any other filler.

Full text of DOI:10.1007/s11164-012-0839-8

Res Chem Intermed
DOI 10.1007/s11164-012-0839-8
Electrochemical properties of PEO/PMMA blend-based
polymer electrolytes using imidazolium salt-supported
silica as a filler
•
•
•
Lyungyu Lee Ick-Jun Kim Sunhye Yang
Seok Kim
Received: 7 August 2012 / Accepted: 26 September 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract In this study, the composite polymer electrolytes (CPEs) were prepared
by solution casting technique. The CPEs consisted of PEO/PMMA blend as a host
matrix doped with LiClO4. Propylene carbonate (PC) was used as plasticizer and a
small amount of imidazolium salt-supported amorphous silica (IS-AS) as a filler
was prepared by the sol–gel method. At room temperature, the highest conductivity
was obtained for the composition having PEO–PMMA–LiClO4–PC–4wt. % IS-AS
with a value of 1.15 9 10-4 S/cm. In particular, the CPE using the IS-AS filler
showed a higher conductivity than any other sample (fumed silica, amorphous
silica). Studies of differential scanning calorimetry and scanning electron micros-
copy indicated that the ionic conductivity increase was due to an expansion in the
amorphous phase which enhances the flexibility of polymeric chains and the
homogeneous structure of CPEs. It was found that the ionic conductivity and
interfacial resistance stability of CPEs was significantly improved by the addition
of IS-AS. In other words, the resistance stability and maximum ambient ionic
conductivity of CPEs containing IS-AS filler were better than CPEs containing any
other filler.
Keywords Composite polymer electrolyte ꢀ PEO ꢀ PMMA ꢀ Imidazolium
salt-supported amorphous silica ꢀ Ionic conductivity
L. Lee ꢀ S. Kim (&)
Department of Chemical and Biochemical Engineering, Pusan National University, San 30,
Jangjeon-dong, Geumjeong-gu, Busan 609-735, South Korea
e-mail: seokkim@pusan.ac.kr
I.-J. Kim ꢀ S. Yang
Battery Research Group, Korea Electrotechnology Research Institute, 28-1, Seongju-dong,
Changwon 641-120, South Korea
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L. Lee et al.
Introduction
Lithium ion batteries are overwhelmingly used for various applications and the
usage is expected to expand in the future. However, the severe reactivity of lithium
metal on the electrodes greatly limits the choice of usable electrolytes and incurs
safety problems with the commercialization of rechargeable batteries. Because solid
state electrolytes are in demand for use in high-capacity Li batteries, an ideal way to
obtain a light-weight, leak-proof and flexible lithium ion battery is to utilize solid
polymer as an electrolyte [1–5].
Organic solid electrolytes, as well as inorganic ones, have been widely studied,
and polymeric solid electrolytes are especially expected to be excellent composites
because of their flexibility and processibility. Since the ionic conductivity of the
PEO/LiX complex was measured in 1973 [6], extensive research and development
has been done on solid polymer electrolytes (SPEs) during the past three decades.
Recently, SPEs containing dissolved metal salts have been proposed as an
alternative to a liquid electrolyte in rechargeable lithium batteries. The advantages
of SPEs are their capability for high-speed production of thin bipolar cells and
enhanced safety due to the absence of liquid electrolytes. This has promoted the
development of dry, gelled, and porous types of SPEs [7, 8]. PEO is one of the most
promising SPEs. However, due to the poor ionic conductivity (r = 10-8–10-7 S/cm)
of these polymer electrolytes at an ambient operating temperature, various modi-
fications have been carried out [9].
Blending of different polymers provides easy preparation and control of the
physical properties within the miscibility compositional region, and the blends often
exhibit properties that are superior to the properties of individual components of the
blend [10, 11]. Since the polymer blending technique can offer versatile and useful
routes for improving polymeric material properties, it has been employed and
applied for many polymeric systems [12]. A broad range of studies on miscibility
[13, 14], crystallization [15, 16], structure [17], and dynamics [18] for poly(ethylene
oxide) methyl(PEO)/poly(methyl methacrylate) (PMMA) systems have been
investigated. PEO/PMMA blends have been reported as marginally miscible [19]
with a weak enthalpic interaction, i.e., v12 ffi 0 [20], implying that they can often be
immiscible in certain temperature and composition ranges. The miscibility of this
blend is strongly dependent on the tacticity of PMMA [21].
Many kinds of ceramic fillers, such as Al2O3 [22–24], ZnO [25, 26], TiO2 [27],
ZrO2 [28], SiO2 [29–31] etc., layered clays, e.g., montmorillonite [32–34], and
organic–inorganic hybrid materials [35–37] have been used as a second phase for
polymer electrolytes to obtain composite polymer electrolytes (CPEs). The addition
of an inert ceramic filler has proved an effective approach to improve the overall
performances of polymer electrolytes, which usually showed improved mechanical
performances and even enhanced ionic conductivity and interface stability with
various electrode materials [38–40].
Ionic liquids (ILs) have been attracting much attention in view of their potential
applications as new types of electrolytes in many electrochemical devices. This
continuously growing interest is motivated by their unique properties such as non-
volatility, nonflammability, negligible vapor pressure, high ionic conductivity,
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Electrochemical properties of PEO/PMMA blend-based polymer
broad electrochemical window, and special characteristics in comparison with
conventional organic and inorganic solvents [41–43]. In the past decade, there has
been increasing use of ILs containing imidazolium and its counter anions in many
research fields such as Li batteries [44], dye-sensitized solar cells [45], separation
science, and material synthesis [46].
In this study, imidazolium salt-supported amorphous silica (IS-AS) was prepared
by the sol–gel method as described previously [47]. The as-prepared IS-AS was used
as a filler to prepare a series of PEO/PMMA blend-based CPEs with LiClO4. It was
anticipated that the mesoporous structures of IS-AS and the amorphous PEO/PMMA
matrix would provide shorter ion transport paths, resulting in higher conductivity and
better electrochemical stability. Therefore, the effects of the addition of IS-AS on the
micro-structure and electrochemical property of the CPEs were studied.
Experimental
Materials
Poly(ethylene oxide) and poly(methyl methacrylate) with average molecular
weights of 106 (Aldrich) and 1.2 9 105 (Aldrich), respectively, were used without
further purification. Lithium perchlorate (LiClO4) was obtained from (Aldrich) and
was dried under vacuum (\10-3 Torr) for 24 h at 80 °C before use. Acetonitrile
(ACN) and propylene carbonate (PC) were acquired from Junsei (Japan), and used
without further purification. The IS-AS was prepared according to the method
reported previously by Ref. [47]. Amorphous silica and fumed silica as fillers to
compare with IS-AS were acquired from Kanto (35 lm) and Aldrich (7 nm),
respectively, and used without further purification.
Preparation of IS-AS (Imidazolium salt supported amorphous silica)
Imidazole (5.1 g, 79.5 mmol) was dissolved in 75 mL of dry toluene, to which
3-chloropropyltriethoxysilane (ClPTES, 18 mL, 75 mmol) was added, and the mix-
ture was refluxed for 12 h under an argon atmosphere. An oil containing 3-(N-
imidazolyl)propyl triethoxysilane was obtained, and to it was added 1-bromobutane,
followed by refluxing for another 12 h under argon to yield 1-(triethoxysilylpropyl)-3-
n-butylimidazolium bromide. This is denoted as IL-functionalized organosilane
(IFOS). The molar ratio of the constituents of the synthetic mixture was optimized
according to a previous report [48]; 0.818TEOS:0.182IFOS:5.61HCl:153.9H2O,
where the ratio of tetraethyl orthosilicate (TEOS) to IFOS was 4.5. In a typical
synthesis, a 500-mL polypropylene (PP) bottle, fitted with a wheel-type magnetic bar
and brought to 40 °C, was charged with 100 mL of water, followed by 25 mL of 35 %
HCl. After 15 min of stirring, a clear solution was obtained. Before 8.01 mL of TEOS
was added, it was placed in the open PP bottle and stirred for 3 h at 40 °C for pre-
hydrolysis and expulsion of ethanol from the reaction mixture. Then, 4.64 mL of the
IFOS–ethanol mixture was added (initial organosilane mixed with 15 mL of ethanol to
avoid errors in the measurement of the more viscous IFOS) into the aforementioned
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L. Lee et al.
solution and stirred for 24 h at 40 °C; it was subsequently heated for another 24 h at
60 °C in a hot-air oven in the PP bottle. The solid product was recovered and mixed
with 50 mL of ethanol, filtered, and dried at 60 °C for 12 h.
Preparation of composite polymer electrolytes
All the CPEs were prepared using the solution casting technique. The CPEs system
entailed the use of PEO and PMMA blend polymer matrix in a molar ratio of 4:1,
PEO and LiClO4 salt in a molar ratio [EO]:[Li] of 16:1, and PEO and PC plasticizer
in a molar ratio 1:0.1, with a different weight ratio IS-AS. The preparation of CPE
films involved first the dissolution of PEO, PMMA and PC in ACN at room
temperature for several hours, after which LiClO4, filler was added to the PEO/
PMMA/PC polymer matrix solution prepared. After a completely homogenous
mixture was obtained, it was cast onto a Petri dish and any residual solvent such as
an ACN were evaporated in a dry oven at 60 °C for 24 h. Finally, resultant
electrolyte films were obtained. The obtained SPE was peeled from the laboratory
dish and stored inside the glove box.
Characterization
Surface area, pore volume, and pore size distribution were measured by nitrogen
adsorption at 77 K using a BET apparatus (ASAP 2010; Micromeritics). The
solvent-extracted samples were degassed at 110 °C and 10-5 Torr overnight before
adsorption experiments. The mesoporous volume was estimated from the amount of
nitrogen adsorbed at a relative pressure of 0.5 by assuming that all mesopores had
been filled with condensed nitrogen in the normal liquid state. The pore size
distribution was estimated using the Barrett, Joyner, and Halenda (BJH) algorithm
(ASAP 2010) built-in software from Micromeritics. Spectral measurements were
recorded between 4,000 and 400 cm-1
carried out at frequencies of 79.5 MHz.
.
29Si solid-state NMR experiments were
The morphology of the CPE films was investigated by Scanning electron
microscopy (SEM) (Hitachi S3500 N). Differential scanning calorimetry (DSC)
studies were performed in the 0–100 °C range with a Perkin-Elmer (Pyris 1) system,
at a heating rate of 5 °C/min and with purging nitrogen gas.
Measurements of electrochemical properties were carried out using a Iviumstat
(IVIUM Technologies, Netherlands). The ionic conductivity measurements of the
CPEs were carried out by means of AC impedance spectroscopy over the frequency
range from 200 kHz to 10 Hz. The impedance measurements were performed by
sandwiching the films between two stainless-steel (SS) electrodes. The conductivity
values (r) were calculated from the bulk resistance (Rb), which was determined by
equivalent circuit analysis software.
1
RbA
t
¼
ð1Þ
r
where Rb is the bulk resistance, and t and A are the thickness of the electrolytes film
and the areas of film, respectively.
123

Electrochemical properties of PEO/PMMA blend-based polymer
Results and discussion
Structure of IS-AS (imidazolium salt supported amorphous silica)
Figure 1a shows the solid state 29Si MAS NMR spectra of BMImBr-AS confirmed
the presence of organo-functionalized moieties ionically coupled with surface of
self-assembled porous silica. Distinct resonances can be clearly observed for the
siloxane [Qn = Si(OSi)n(OH)4-n, n = 2–4; Q3 at -121 ppm and Q4 at -130 ppm]
and organosiloxane [Tm = RSI(OSi)m(OH)3-m, m = 1–3; T3 at -86 ppm; T2 at
-76 ppm] units.
Figure 1b shows the morphology of the BMImBr-AS which were characterized
by SEM. It shows the presence of spherical particles with a regular shape. To study
the pore structure of the IS-AS (BMImBr-AS), the surface and adsorption
parameters, including the specific surface area (SBET), the BET’s constant
(CBET), the total pore volume (VT), and the BJH average pore diameter (DBJH),
and an immobilized amount of IL, were measured through the N2 adsorption–
desorption isotherm; the data are shown in Table 1. The BET surface area of the
2
˚
BMImBr-AS was 364 m /g and the pore diameter was 75.9 A, and average pore
volume was 0.7 cm3/g. Nitrogen content in the BMImBr-AS was analyzed by
elemental analysis, and the immobilized amount of IS was 1.92 mmol/g—SiO2. It
seems that the IFOS incorporation onto the silica materials is performed
successfully through the template-free route.
Ionic conductivity behaviors of CPEs containing IS-AS
In order to enhance the ionic conductivity, studies of the effects on the ion-
conducting behavior of CPEs containing specific amounts of fillers were carried out.
As a result, the ionic conductivity increased with increasing IS-AS content, and
attained the maximum value, 1.15 9 10-4 S/cm, when the IS-AS content is 4wt. %
(inset of Fig. 2a). Subsequently, the ionic conductivity decreases slightly with
further increase in IS-AS content over 4wt. %. These decreases in the conductivity
were due mainly to the decreased ion mobility, which was related to the micro-
structure and morphology of CPE. The decrease of the conductivity for filler
concentrations higher than 4wt. % can be caused not only by particles aggregation
but also by the dilution effect of introducing a non-conducting phase in a conducting
one. From this result, the optimum content of IS-AS filler was decided as 4wt. %.
Figure 2a shows the impedance plots for various filler (amorphous silica, fumed
silica) containing CPEs at 25 °C. In the case of no filler, the graph shows a rather
large-diameter semicircle. The approximate bulk resistance value can be obtained
with reference to the touch point at the x-axis. By contrast, the CPEs containing IS-
AS filler showed the smallest diameter semicircles. In other words, the CPEs
showing the smallest diameter semicircles demonstrated the most superior electrical
conductivity confirmed by the low resistance value.
Figure 2b shows the effect of IS-AS filler on the enhancement of the room
temperature ionic conductivity of CPEs. The enhancement of ionic conductivity was
found by an addition of fillers and reaches the maximum value of 1.15 9 10-4 S/cm
123

L. Lee et al.
Fig. 1 a Si-NMR and b SEM images of IS-AS
Table 1 Surface and adsorption parameters of synthesized IS-AS
ILs contenta (mmol/g)
SBET (m2/g)
DBJH (A)
75.9
VT (m2/g)
b
c
d
˚
Sample
BMImBr-AS
1.92
364
0.7
a
Amount of immobilized IL calculated by the nitrogen content of elemental analysis
Specific surface area from BET-method by N2 adsorption
BJH average pore diameter
b
c
d
Total pore volume
123

Electrochemical properties of PEO/PMMA blend-based polymer
Fig. 2 a Impedance plots and b ionic conductivity of CPEs with various fillers of equivalent weight
contents
at IS-AS filler. This value was double that of the no filler polymer blend (PEO/
PMMA) system. This reveals that the charge carriers moved more freely in the CPE
with IS-AS than that without IS-AS or other fillers, and this was attributed to the
extra interactions between the charge carriers and IS-AS having high surface area,
interconnected pores, and high porosity, which provided a more accessible and
shorter pathway for transporting the charge carriers. In other words, superior ion
behavior and homogeneous particles dispersion of IS-AS filler provided the most
suitable environment for the ionic transport and achieved the highest conductivity.
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L. Lee et al.
Fig. 3 SEM image of CPEs containing: a no filler, b amorphous SiO2, c fumed SiO2, d IS-AS
Structural properties of CPEs containing IS-AS
Figure 3 displays SEM images of the CPEs films containing various fillers. In the
case of the (a) sample, that is, a pristine film without filler, a rather homogeneous
morphology was shown, except for some pores and some white spots probably
related to the existence of salt species. By adding various filler content 4wt. %, the
film morphology was changed. First of all, at amorphous silica filler, the small phase
region had appeared. And, in the case of fumed silica, IS-AS filler, the more
homogeneous morphology was shown. Specially, CPEs film morphology containing
IS-AS filler appeared, with a wide range of amorphous phase. These phenomena
could be explained by improvement of the ion transport ability by miscibility
increase between PEO and PMMA polymer chains by uniform particle dispersion of
the IS-AS filler. By adding modified filler, the polymer homogeneous feature was
developed. Consequently, it can be stated that the presence of the IS-AS filler
improved the ionic conductivity of the CPEs. This conductivity could be related to
the improvement of the ion transport ability.
Figure 4 shows the DSC results for the pure PEO and CPEs containing various
fillers. The melting transitions, corresponding to crystalline to amorphous transition,
were shown in DSC data. The endothermic curves began to rise below the melting
point. The relative percentage of crystalline (vc) has been calculated by referencing
pure PEO as 100 % crystalline. It can be calculated with the equation:
123

Electrochemical properties of PEO/PMMA blend-based polymer
Fig. 4 DSC curves of CPEs containing: a pure PEO, b no filler, c amorphous SiO2, d fumed SiO2, e IS-AS
DHf
DHfꢁ
vC ¼
ð2Þ
where DHfꢁ the melting enthalpy of a completely crystalline PEO is the sample and
DHf is the experimental enthalpy.
Figure 4 shows the DSC curves for the effect of PEO/PMMA blending and
various fillers as an additive in the CPEs. It was found that the PEO showed
endothermic peaks due to the melting of the crystalline domain of the PEO with
increasing temperature, and the curves moved to a low temperature region by the
addition of fillers. It was found that the melting temperature (Tm) of crystalline PEO
was around 35*42 °C, which was slightly degraded when the filler was added.
Also, the crystallinity of CPEs containing IS-AS was decreased the most among the
fillers. Consequently, it can be stated that the presence of filler improved the ionic
conductivity of the CPEs by slightly decreasing the CPEs crystallinity [49].
DSC studies indicated that the PEO crystallinity was diminished by the addition
of filler. The melting temperature, heat of fusion, and relative crystallinity values are
listed in Table 2. The PEO itself shows a heat of fusion of 147.8 J/g, and no filler,
32.5 J/g, the estimated crystallinity being 22 %. This indicates that PMMA can
reduce the crystallinity of PEO by means of blending effects. With the addition of
the IS-AS filler, the Tm gradually fell to 36.24 °C, and the crystallinity fell to its
smallest value, 13 %.
Electrochemical stability of CPEs containing IS-AS
The variation of interfacial impedance of CPEs containing various filler has been
analyzed under prolonged exposure at room temperature and is presented in Fig. 5.
123

L. Lee et al.
Table 2 DSC data for CPEs with various fillers of equivalent weight contents
Code
Sample
Melting temperature
Tm (°C)
Heat of fusion
4H (J/g)
Relative crystallinity
XC (%)
1
2
3
4
5
Pure PEO
No filler
68.68
41.17
40.00
38.00
36.24
147.8
32.5
24.2
22.3
19.2
–
22
16
15
13
Amorphous SiO2
Fumed SiO2
IS-AS
Fig. 5 Variation of interfacial resistance of CPEs with a storage time (SS/CPEs/SS cells, frequency
range 200 kHz to 10 Hz, amplitude 10 mV)
It was found that the interfacial resistance of all CPEs increases with storage time.
The observed increase in the impedance implies that the electrode was passivated
for a long time due to the reactivity between the electrode interfacial and the CPEs.
The impedance of CPEs containing filler are smaller than the value of polymer
blends without fillers. The composite with IS-AS filler (compared with fumed silica
filler) showed better performance than any other samples. The composite with IS-
AS not only showed a lower interfacial resistance but also a lower rate of increase in
impedance. This can be attributed to the better interfacial stability offered by the
modified filler content. In other words, IS-AS filler enabled an improvement in the
stability of interfacial resistance and formation of stable SEI layers between
electrode and polymer electrolyte.
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Electrochemical properties of PEO/PMMA blend-based polymer
Conclusions
This study demonstrated that addition of the IS-AS into the PEO/PMMA blend-
based polymer electrolyte made the composite a better polymer electrolyte than
those without filler. This is related to the fact that the crystalline structure of PEO
was weakened due to the possible coordination of Li? ions with IS-AS, which was
also easily coordinated with the LiClO4 salt.
The addition of IS-AS not only increased the concentration of the solvated Li?
ions but the interconnected IS-AS filler with high surface area and high porosity also
provided a more accessible and shorter pathway, resulting in increasing the mobility
of the Li? ions in the electrolyte. This phenomenon indicated that the structures of
CPEs containing IS-AS filler showed stable morphology, having the larger
amorphous phase regions by homogeneous particles dispersion. Furthermore,
DSC study had shown the reduced crystallinity values of PEO due to structural
deterioration by the addition of PMMA or PC and further by the addition of filler.
Especially, with the addition of the IS-AS filler, the Tm and crystallinity values
were reduced to the smallest values, 36.24 °C and 13 %, respectively. The
ionic conductivity of the PEO/PMMA/IS-AS CPE appeared as the higher ionic
conductivity of 1.15 9 10-4 S/cm compared with any other fillers (no filler,
amorphous silica, fumed silica). In other words, the CPEs containing IS-AS filler
retained the low crystallinity, wide amorphous phase regions, high ionic conduc-
tivity, and stable interfacial properties, indicating a possible application in lithium
polymer batteries.
Acknowledgments This work was partly supported by the next generation secondary battery R&D
program of MKE/KEIT (Grant No. 10042575.2012, Development of 5 kW Zn-air battery for EV and
3.3 V-1,000F pouch type high-power supercapacitor). This research was partly supported by Basic
Science Research Program through the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (Grant No.: 2011-0009007).
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