Vol.2, No.3, 190-196 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.23029
Solid polymeric electrolyte of poly(ethylene)oxide-50%
epoxidized natural rubber-lithium triflate
(PEO-ENR50-LiCF3SO3)
Siti Aminah bt. Mohd Noor1, Azizan Ahmad1, Mohd. Yusri bin Abd. Rahman2*, Ibrahim Abu Talib 3
1School of Chemical Science and Food Technology, Faculty of Science and Technology, National University of Malaysia, Selangor,
Malaysia;
2College of Engineering, Universiti Tenaga Nasional, Selangor, Malaysia; yusri@uni te n.ed u.my
3School of Applied Physics, Faculty of Science and Technology, National University of Malaysia, Selangor, Malaysia;
Received 19 November 2009; revised 28 December 2009; accepted 25 January 2010.
ABSTRACT
A solid polymer electrolyte (SPE) films consist-
ing of polyethylene oxide (PEO), 50% epoxi-
dized natural rubber (ENR50) and LiCF3SO3 with
various compositions of PEO-ENR50 and vari-
ous weight percentage of LiCF3SO3 were pre-
pared by solution casting technique. The poly-
mer electrolyte films were characterized using
DSC, XRD and AC impedance spectroscopy.
The SPE with the PEO-ENR50 composition of
70-30 shows the highest conductivity of 4.2 ×
10-5 Scm-1 at the 15 wt.% of LiCF3SO3 compared
with the other composition of PEO/ENR50. This
composition was then chosen to investigate the
effect of LiCF3SO3 on the thermal property,
structure and conductivity of the electrolyte.
The highest room temperature conductivity of
1.4 × 10-4 Scm-1 was achieved at 20 wt.% of
LiCF3SO3. The conductivity result is supported
by the DSC and XRD analysis which showed the
semi- crystalline nature of PEO turning to amor-
phous state due to the increase in LiCF3SO3
content.
Keywords: A. Polymers; C. Differential Scanning
Calorimetry (DSC); C. X-Ray Diffraction;
D. Electrical Conductivity; D. Electrical Properties
1. INTRODUCTION
Main application of polymer electrolytes lies in electro-
chemical device such as display, sensor, electrochemical
window, super-capacitor, rechargeable battery and photo-
electrochemical cell. However, the main attention of
many solid state researchers is the development of sec-
ondary lithium batteries [1-4]. Ionic conducting polymer
electrolytes were first suggested by Fenton et al. in 1973
[5], who reported that PEO-salt complexes are capable of
exhibiting ionic conductivity and subsequently substan-
tial activities were directed towards the development of a
wide variety of solid polymer electrolytes using different
combinations of polymer and salt. The most reported and
extensively studied polymer host for solid polymer elec-
trolyte is PEO [6-12]. This is due to a high electrochemi-
cal stability of PEO in comparison with other polyethers,
copolymers or PEO branched polymers [9]. Besides,
PEO has a good solubility for many salts such as LiTFSI,
Li2SO4, NH4ClO4 and LiCF3SO3. Even though PEO has a
good electrochemical stability, PEO exists in semi-crys-
talline state at room temperature and does not have an
excellent contact with the electrodes.
Solid polymer electrolytes (SPE) for lithium batter-
ies have many advantages over its counterpart liquid
electrolyte, such as mechanical stability and processing
flexibility. However, the conductivity at room tem-
perature is usually too low to be utilized in ionic device.
There are various techniques that can be employed to
enhance the conductivity of SPE at room temperature.
The recent technique developed for solid polymer elec-
trolytes is the dispersion of nanosize inorganic ceramic
filler particles such as Al2O3, SiO2, TiO2, SnO2, ZnO
and ZrO in the electrolyte systems [6-10]. Propylene
carbonate (PC), ethylene carbonate (EC) and dibutyl
phthalate (DBP) are also used as plasticizers for con-
ventional polymer electrolyte systems [11-13]. An al-
ternative to ceramic fillers or plasticizers is to employ a
complexing polymer such as modified natural rubber.
This is due to their distinctive characteristics such as
low glass transition temperature, soft elastomeric char-
acteristic at room temperature, good elasticity and ad-
hesion that makes them a suitable candidate as a filler
for polymeric electrolyte systems [1,11,12]. Figure 1
shows the structure of ENR50 dimmer.

S. A. M. Noor et al. / Natural Science 2 (2010) 190-196
191
Figure 1. Structure of ENR50 dimmer.
This work describes the effect of ENR50 content on
the conductivity of PEO-ENR50-LiCF3SO 3 electrolyte at
the fixed concentration of the LiCF3SO3 salt. The PEO-
ENR50 composition that gave the highest room tem-
perature conductivity was chosen to study the effect of
the salt content on the conductivity of the electrolyte.
The blended polymer of PEO-ENR50 as a polymer host
was doped with LiCF3SO3 using the solution casting
technique [14] to obtain a solid polymeric electrolyte
film. This work also reports the effect of LiCF3SO3 con-
tent on thermal property and structure of the electrolyte
fixed at PEO-ENR50 composition of 70/30.
2. EXPERIMENTAL
2.1. Sample Preparation
The SPE films were prepared by means of the solu-
tion-cast technique using a single solvent of THF. The
host materials, poly(ethylene oxide) (PEO) (MW =
600,000) was obtained from Sigma Aldrich, while 50%
epoxidized natural rubber (ENR50) (MW = 592,487) was
supplied by Malaysian Rubber Board. The required
amounts of PEO and ENR50 were dissolved in tetrahy-
drofuran (THF) that was obtained from SYSTERM
ChemAR. Lithium triflate (LiCF3SO3), obtained from
Fluka was used as the doping salt. All the materials were
used without further purification. Subsequently, the de-
sired amounts of PEO-ENR50 and various weight per-
centage of LiCF3SO3 were dissolved separately in THF,
mixed together and stirred efficiently for 24 hours in
order to achieve a homogeneous mixture. The solutions
obtained was cast on a teflon mould and was allowed to
evaporate completely at room temperature to produce an
electrolyte film. Then, the film was dried in a vacuum
oven at 60oC for 24 hours to remove the residual solvent.
This procedure provided a mechanically stable, free
standing and flexible films with the thickness ranging
from 150 to 250 μm. The preparation of SPE was carried
out in an atmosphere environment at room temperature
[12,14].
2.2. Sample Characterizations
The AC impedance measurement was carried out at
room temperature using a high frequency response ana-
lyzer (HFRA Solartron 1256, Schlumberger) in the fre-
quency range of 100 Hz-1 MHz with 30 mV amplitude.
The electrolyte was sandwiched between the stainless
steel ion-blocking electrodes with a surface contact area
of 2.0 cm2. The bulk resistance, (Rb) of the electrolyte
was determined from the equivalent circuit analysis as-
sisted with Z-View software. The conductivity, (σ) have
been calculated from the equation σ = (1/Rb)(t/A), where
t is the film thickness and A is the active area of the
electrode. The thermal property measurement was con-
ducted using a differential scanning calorimeter model
8822e Mettler Toledo from -60oC and + 150oC at a scan-
ning rate of 10oC min-1 under nitrogen atmosphere. Pre-
viously, a pure indium and tin were used for temperature
and enthalpy calibrations for DSC measurements. Ap-
proximately, 1 to 3 mg of the electrolyte film specimen
was used for each DSC measurement. The X-ray diffrac-
tion technique was performed on the electrolyte films at
room temperature using a Siemens model D5000
(Cu-Kα; α = 1.5418 Å). The diffraction angle, 2θ was
10o to 45o using a step size of 0.025o to determine the
crystallinity of the electrolyte samples. The Scherrer
length was then automatically calculated by evaluation
(EVA) software.
3. RESULTS AND DISCUSSION
3.1. Effect of ENR50 Content on
Conductivity of PEO- ENR50-LiCF3SO3
Electrolyte
The freestanding electrolyte films of PEO-ENR50-
LiCF3SO3 have been produced by using solution casting
technique. Table 1 shows the conductivity the electrolyte
with various compositions of PEO-ENR50 at the fixed
concentration of LiCF3SO3 (15 wt.%). The percentage
by weight of ENR50 was chosen up to 30% since after
this level, the electrolyte became a gel electrolyte and
was not a free standing film. It made the electrolyte film
Table 1. Room temperature conductivity for various com-
position of PEO-ENR50 at 15 wt.% LiCF3SO3.
Composition Ionic conductivity (Scm-1)
100-0 2.1 × 10-6
90-10 4.1 × 10-6
80-20 1.0 × 10-5
70-30 4.2 × 10-5

S. A. M. Noor et al. / Natural Science 2 (2010) 190-196
192
was difficult to peel off from the mould. From Table 1,
the conductivity of the electrolyte without ENR50 was
2.1 × 10-6 Scm-1. The conductivity was found to in-
crease with the increasing percentage of ENR50 in the
polymer host. The highest room temperature conduc-
tivity of 4.2 × 10-5 Scm-1 was achieved at 70-30 com-
position of PEO- ENR50. This observation shows that
the ENR50 content affected the conductivity of the
electrolyte. It shows that the elastomeric characteristic
of ENR50 reduced the glassy nature of PEO. This con-
dition will enhance the segmental motion of the poly-
mer host. The effect of segmental flexibility is respon-
sible for the formation of free volume which enhances
the ionic conductivity. The strong adhesive property
can give an efficient contact between the electrodes in
electrochemical contacts [12]. This behavior can be
seen in the electrolyte system which is due to a good
elasticity and adhesion property of ENR50. Razali et al.
[12] reported that ENR50-LiTf with 100% wt. of
EC/PC gave ionic conductivity of 4.0 × 10-5 Scm-1 at
room temperature. This conductivity value was almost
similar with our electrolyte system for 70-30 composi-
tion without any plasticizer. It shows that with the ab-
sence of a costly plasticizer, the conductivity of the
electrolyte has been improved by blending PEO with
ENR50. In addition, this electrolyte system produced a
low cost and an environmental friendly electrolyte.
Besides, the use of PC and EC can corrode the lithium
metal electrode in electrochemical cell [16]. Glasse et
al. [11] reported the incorporation of PEO with ENR50
can reduce the sticky property of ENR50 and made the
film could be peeled off easily from the mould.
3.2. Effect of LiCF3SO3 Salt on Thermal
Property, Structure and Conductivity of
PEO-ENR50-LiCF3SO3 Electrolyte
The effect of LiCF3SO3 salt on the above properties of a
PEO-ENR50-LiCF3SO3 electrolyte was investigated by
choosing the electrolyte with the 70-30 composition of
PEO-ENR50. This is because this composition has
shown the best performance in term of ionic conductiv-
ity as compared with the other compositions of
PEO-ENR50. The electrolytes were prepared by varying
the percentage by weight of LiCF3SO3 from 5% to 25%
at 5% interval. In order to investigate the effect of the
salt content on the thermal property of the electrolyte
such as melting point, DSC measurement was performed
on the prepared electrolyte samples. Figure 2 shows the
DSC thermographs for all samples. The sharp endother-
mic peak observed at 68oC, corresponds to the crystal-
line melting temperature (Tm) of the pure PEO [15]. The
endothermic peak for pure PEO showed the transition
from 68oC to 60oC by addition of 5 wt.% of the salt. The
Tm value decreases dramatically to 49oC with addition of
20 wt.% of the salt. This observation shows the reduc-
tion in Tm value by addition of the salt. The endothermic
curves also indicate a reduction of PEO crystallinity. The
relative percentage of crystallinity (λ) of PEO has been
calculated by using the relation, λ = (ΔHm/ΔHm
o) x 100%,
where, ΔHm is the melting enthalpy estimated experi-
mentally and ΔHm
o used as referenced is the melting
enthalpy for 100% crystalline PEO (213.7 Jg-1) [16]. The
calculated values of λ are summarized in Table 2. The
crystallinity degree of the electrolyte decreases with the
wt.% of the salt which causes an increase in the amor-
phous phase. The polymeric chain in the amorphous
phase is more flexible, which results in the enhancement
of segmental motion of the polymer [17]. The Tm and λ
values obtained from this work for polymer electrolyte
based on PEO closely agree with the values reported in
literature [8,18]. The reduction of Tm and λ suggest that
the interaction between the polymer host backbone and
LiCF3SO3 affects the dynamic main chain of the polymer.
This will promote the amorphous phase which is ex-
Figure 2. DSC thermograms of PEO,PEO-ENR50 (70-30) with different wt.% LiCF3SO3.

S. A. M. Noor et al. / Natural Science 2 (2010) 190-196
Copyright © 2010 SciRes. Openly accessible at http://www.scirp.org/journal/NS/
193
Table 2. O/Li mole ratio, crystallinity, (λ) and melting temperature, (Tm) of PEO-ENR50-LiCF3SO3 electrolyte.
LiCF3SO3 (wt%) to
PEO/ENR50 (70/30) O/Li Tm of PEO (oC)
Relative percentage of
crystalline phase (λ)
(%)
Pure PEO - 68 78.5
Pure ENR - -
0% 0/1 67 49.9
5% 16/1 60 35.9
10% 8/1 53 31.6
15% 5/1 50 24.5
20% 4/1 49 19.1
25% 3/1 - -
2 theta
20 25 30 35 40
Intensity (a.u)
2 theta
20 2530 35 40
Intensity (a.u)
(b)
(e)
(d)
(c)
(g)
(f)
(h)
(a)
Figure 3. XRD patterns of PEO-ENR50 (70/30) with LiCF3SO3 with different weight ratios
(a) Pure salt (b) Pure PEO (c) 0% (d) 5% (e) 10% (f) 15% (g) 20% (h) 25%.
pected to favour ion transport, thus enhancing the elec-
trolyte conductivity [2]. DSC investigation confirmed
that PEO crystallinity is reduced with the addition of
ENR50 and the salt which has a plasticizing effect on the
polymeric chain.
The structure of the electrolyte samples was investi-
gated by XRD analysis. The XRD pattern of the electro-
lyte samples with various concentrations of LiCF3SO3,
and that for pure PEO and pure LiCF3SO3 salt are shown
in Figure 3. The indexing of the two PEO peaks are re-
ported to be (120) at 19.5o and (032) + (112) at 23.5o of
the monoclinic system [19]. The characteristic diffraction
peaks of crystalline PEO are apparent between 2θ = 15o –
40o [3,20]. The two prominent peaks for PEO at 2θ =
19.2o and 23.2o are present in all patterns, indicating the
presence of pure polymer. Nevertheless, the patterns
show that the intensities of the prominent peaks decrease
and become broader with the increasing wt.% of LiCF3SO3.
The broadening of sharp peaks of PEO could be due to the
disruption of the PEO crystalline structure by LiCF3SO3
and indicates that the complexation has taken place in the
amorphous phase [10]. These results confirm both the
decrease in degree of crystallinity and the lamellae size of
PEO with the presence of plasticizing LiCF3SO3 [15]. The
LiCF3SO3 peaks are not seen in any of the polymer-salt
complexes and this signifies that LiCF3SO3 solvates very
well in PEO-ENR50 matrix, resulting in the absence of
pure salt phase in the complexes. The main XRD peaks of
PEO are characterized by significant changes in the full
width at half maximum (FWHM) with the increasing salt
concentration. The peak broadening estimated in terms of
FWHM has been used to evaluate the Scherrer length (l)

S. A. M. Noor et al. / Natural Science 2 (2010) 190-196
194
of main PEO peak which was 22.90, 18.20, 18.12, 18.07,
15.84, 12.51 and 8.14 nm for the electrolytes shown in
Figure 3(b)-3(h), respectively. The length was automati-
cally calculated using evaluation (EVA) software with the
formula (l) = [0.9λ]/[BcosθB] where λ is the wavelength of
the X-rays used and B is FWHM in radians [21]. The l of
main PEO peak was 22.9 nm. This value was decreased to
18.20 nm when PEO was blended with ENR50 and be-
come 8.14 nm at 20 wt.% of LiCF3SO3. The Scherrer
length characterizes the crystallite size of PEO and it gives
a picture of changes in the PEO crystallinity as a polymer
host upon the salt addition. These results suggested that
the degree of crystallinity of PEO decreases with the
LiCF3SO3 content.
The ionic conductivity measurement was carried out
with the aim to observe the effect of LiCF3SO3 addi-
tion on the ionic conductivity of the electrolyte. The
AC spectra for two of the PEO-ENR50-LiCF3S03
electrolyte films are shown in Figure 4 in form of
Cole-Cole plot. The high frequency semicircle gives
the information about the bulk properties of the elec-
trolyte, such as bulk resistance (Rb) and bulk capaci-
tance (Cb) which arises from the migration of lithium
ions and the dielectric polarization of the electrolyte
film, respectively. In the low frequency response re-
gion, the appearance of a non vertical spike is attrib-
uted to the additional capacitance and resistance,
arising from dielectric relaxation and ion trapping in
PEO-ENR50-LiCF3SO3 electrolyte [13]. At room
temperature, the conductivity of the electrolyte with-
out LiCF3SO3 is 1.5 × 10-9 Scm-1. The ionic conduc-
tivity increases rapidly by two orders of magnitude
with addition of 5 wt.% of LiCF3SO3 as Li+ charge
carriers were added to the system. It was observed
that the conductivity increases gradually as the wt.%
of LiCF3SO3 increases to 20 wt.% of LiCF3SO3 and
began to decrease at 25 wt.% of LiCF3SO3. The maxi
Figure 4. Cole-cole plot for two PEO-ENR50 (70/30)-LiCF3SO3
electrolytes.
PEO-ENR 50-LiCF3SO3 (wt. % of LiCF3SO3)
   
log /Scm-1








Figure 5. Room temperature conductivity of PEO-ENR50 (70/30)-LiCF3SO3
electrolyte a function of LiCF3SO3 concentration.

S. A. M. Noor et al. / Natural Science 2 (2010) 190-196
195
mum conductivity of 1.4 × 10-4 Scm-1 was achieved at 20
wt.% of LiCF3SO3 as presented in Figure 5. An addi-
tional factor that favoured the salt dissociation was that
the salt with a large monovalent anions such as LiCF3SO3
has a low lattice energy and as a result, it will easily dis-
solve in polyether [15]. The conductivity is reduced after
the maximum value due to an ever-increasing number of
transient crosslink in the system, thus reducing the chain
mobility. Besides, the formation of immobile aggregated
species will contribute to the fall in conductivity.
The conductivity increases with the lithium salt con-
centration due to the increase in charge carrier density and
also due to the plasticizing affects of the LiCF3SO3 which
decreases PEO crystallinity, thus increasing the pathway
for the polymer segmental motion [21]. Ramesh et al. [14]
reported that the highest conductivity achieved for
PEO-LiCF3SO3 electrolyte was 1.1 × 10-6 Scm-1 at room
temperature. However, the result is slightly lower than the
ionic conductivity obtained from this work. In conclusion,
the electrolyte based on PEO blended with modified natu-
ral rubber possess higher ionic conductivity than conven-
tional PEO-based electrolytes at ambient temperature
[11,12]. This result is due to the molecular structure of
ENR50. In term of molecular structure of the polymer,
ENR50 has active oxygen in the epoxy group attached to
the main chain as shown in Figure 1. It is assumed that the
ENR50 oxygen atom takes a role similar to the ether group
in the PEO polymer structure and provide co-ordination
sites for Li+ ion conduction [1,11,12]. Glasse et al. [11]
reported a gel electrolyte of PEO-ENR-LiTf-EC/PC and
found the highest conductivity around 10-4 Scm-1 at the
fixed salt concentration. This value was also in the range of
the electrolyte with 20 wt.% of LiCF3SO3. It shows that the
conductivity was influenced by the salt concentration.
However, the excessive of salt will increase the transient
crosslink in the electrolyte, thus reducing its chain mobility.
This observation also shows without using the EC/PC, high
room temperature conductivity can be achieved for solid
electrolyte system by increasing the wt.% of salt. The in-
crease in ionic conductivity with the wt.% of LiCF3SO3
clearly suggests that the major contribution to the conduc-
tivity enhancement is associated with structural modifica-
tion. This can be observed by the increase in amorphous
phase with Li salt content as observed from the DSC and
XRD results. According to Chandra & Chandra [20], there
is a correlation between Scherrer length and conductivity.
As the crystallite size decreases, the ionic conductivity in-
creases. Hence, it can be proven that the conductivity of our
electrolyte is greatly influenced by the wt.% of LiCF3SO3
through a consequential change in the crystallite size of the
host matrix.
4. CONCLUSIONS
A solid polymeric electrolyte of PEO-ENR50-LiCF3SO 3
has been successfully prepared by solution casting tech-
nique. The effect of ENR50 content on the conductivity
of the electrolyte was found to increase with the weight
percentage of ENR50. The highest room temperature
conductivity of 4.2 × 10-5 Scm-1 was achieved at 70-30
composition of PEO-ENR50 host and at 15 wt.%
LiCF3SO3 salt. The effect of the salt content on the ther-
mal property, structure and conductivity of the electro-
lyte was investigated for the electrolyte with 70-30 com-
position of PEO-ENR50. The maximum room tempera-
ture conductivity was 1.4 × 10-4 Scm-1 corresponding to
20 wt.% of LiCF3SO3. This result was supported by the
lowest relative percentage of crystalline phase and the
decrease in crystalline melting temperature of the elec-
trolyte. Also, XRD analysis showed that the semi-crys-
talline of the electrolyte became more amorphous with
the increase in LiCF3SO3 content.
5. ACKNOWLEDGEMENT
The authors would like to thank UKM and MOSTI for the provision of
grant 03-01-02-SF0423.
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