World Journal of Engineering
EFFECT OF MODIFIED-MONTMORILLONITE AND POSS ON
ELECTRICAL PROPERTIES OF SOLID POLYMER ELECROLYTES
Keun-Byoung Yoon1 and Dong-ho Lee2*
1
Department of Polymer Science & Engineering, Kyungpook National University,
Daegu 701-702, Korea
2
Dongyang Mirae University, Seoul 152-714, Korea
Introduction
in this work.
A solid polymer electrolyte (SPE) is a solid medium
that permits ion transport. An SPE in its original sense
is a (liquid) solvent-free system where the ionically
conducting phase is formed by dissolving salts in a
high-molecular-weight polymer matrix. A polymer
that strongly coordinates with cations is necessary for
SPE formation. Among a variety of polymers that
solvate lithium salts, the poly(ethylene oxide) (PEO)based systems are the most comprehensively
investigated due to the beneficial structure for
supporting fast ion transport. The main reason for the
use of PEO is its ability to show a higher conductivity
than any commercially available polymer.
Conventional PEO–Li salt complexes show
conductivities of the order of 10−4 S/cm at about 100
◦C; however, widespread applications of these
electrolytes require a conductivity of about 10−4 S/cm
at ambient temperature. A high crystalline phase
concentration limits the conductivity of PEO-based
electrolytes, so an important amorphous phase is
desired. Apart from high crystallinity, PEO-based
electrolytes suffer from low cation transport number t+,
ion-pair formation and inferior mechanical properties.
To counter these problems various chemical and
physical modifications have been tested, aimed at
reducing the crystallinity. One such approach is to add
plasticisers to polymer–salt complexes such as
poly(ethylene glycol) (PEG) and propylene carbonate
(PC). The problem encountered with such systems is
that higher conductivities are obtained at very high
plasticiser concentration, which in turn leads to poorer
mechanical properties and reduces the compatibility
of the electrolyte film with the electrode. Chu et al.
reported the modification of PEO with a phenolic
resin to improve the mechanical properties and
conductivity.
The study reported in the present work aimed at the
synthesis and characterisation of PEO/MMT/POSS
nanocomposite electrolytes using apolar alkylammonium cation modified nanoclay. The characterisation
of the nanocomposite electrolyte samples using XRD
and DSC and the evaluation of the electrical
properties of such composites as SPEs are presented
Experimental
Materials and Procedures
PEO (Mw=20,000 and 400,000, Aldrich), Na+MMT(Southern Clay) and POSS (1,3,5,7,9,11,14heptaisobutyltricycloheptasiloxane-endo-3,7,14-triol
(Hybrid Plastics) were uses. An organo-modified
MMT made using an ion exchange method.
The electrolytes were prepared by coratating twinscrew mixer (MiniLab, Hakke Rheomex) on pre-dried
PEO, organo-modified MMT, POSS and LiClO4
([EO]/[Li+]=8) under a nitrogen blanket.
Characterization
XRD experiments were conducted using a Philips
X’pert Pro X-ray diffraction instrument that employed
Kα radiation (λ = 1.5405A° ). Scans were carried out
from 0◦ to 30◦ at a scanning rate of 0.2◦ min−1. The
thermal behaviour of electrolytes was characterized
using a DSC(TA Q100). The crystallinity of the
samples was calculated as the ratio of the heat
enthalpy of the crystalline phase to the heat enthalpy
of pure crystalline PEO.
The ionic conductivity of the electrolyte films was
measured using alternating current (AC) impedance
spectroscopy. Measurements were conducted by a
frequency response analysis (Autolab from Eco
Chemie). The experiments were carried out at room
temperature in the frequency range 1–10MHz. The
AC amplitude was 5 mV. Samples of rectangular
shape were cut and clamped between two ITO
electrode having an area 1 cm × 1 cm. The resistivity
was obtained from the intercept of the impedance
curve with the real axis at high-frequency range.
Results and Discussion
In order to study their crystalline behavior and thermal
properties, PEO/MMT/LiClO4 electrolytes were
subjected to DSC analysis. It was found that the heat
of melting decrease with addition of orfano-modified
MMT. The crystallinity of electrolytes containing
organo-modified MMT was 26%, whereas the
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World Journal of Engineering
crystallinity of PEO/LiClO4 was 39%. It was observed
that PEO/Li+ electrolyte showed a lower crystallinity
than undoped PEO. This is because the lithium cation
coordinate with the flexible CH2-O- chain of PEO.
When the organo-modified MMT is loaded into the
PEO/Li+ electrolyte the crystallinity slightly decreases.
In order to study the effect of MMT contents on the
room temperature conductivity of PEO/MMT/LiClO4
composite electrolytes, solid electrolyte films were
analyzed using AC impedance analyser.(Figure 1)
10
+
(a) HW PEO/LiClO4/Na -MMT
(b) HW PEO/LiClO4/Dodecylamine-MMT
(c) HW PEO/LiClO4/1,12-diaminododecane-MMT
(f) HW PEO/LiClO4/Coisite 6A
(h) HW PEO/LiClO4/Coisite 30B
10
2
4
6
8
MMT Contents (wt %)
Figure 1. Ionic conductivity of PEO/LiClO4/MMT
composites for various MMT contents at 30oC
It is observed that ion conductivity increased with
incorporation of organo-modified MMT, reaches at 5
wt% of MMT and slightly decreases with further
increase in MMT content. Addition of MMT leads to a
higher ionic mobility of the ions by reducing the
crystallinity. Interaction of lithium cations with
silicate layers disrupts the strong intra-association
between lithium cations and anions. Since silicate
layers cannot move, only Li+ cations move under an
electric field.
To reduce the crystallinity of PEO, POSS was added
at PEO/LiClO4 electrolytes. Low molecular weight
PEG was also used for plasticizer. The crystallinity of
electrolytes is summarized at Table 1.
The crystallinity of electrolyte composites was
dramatically decreased by addition of POSS (5 wt%),
reached at 17%. Generally, POSS and low molecular
weight of PEG (LW-PEG) were well-known
plasticizer for thermoplastics. Therefore, LW-PEG
also added to electrolyte, the crystallinity of
electrolytes decreased with increasing the amount of
LW-PEG. The lowest crystallinity of electrolytes was
11%. It is observed that the plasticizability of POSS
was higher than that of LW-PEG at PEO-based
electrolyte composites.
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(b) HW-PEO/LiClO4/POSS
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17
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51
16
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54
15
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52
14
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53
11
The ionic conductivity changes as a function of
crystallinity at 30oC was shown in Figure 2.
-7
0
(a) HW-PEO/LiClO4
(c) HW-PEO/LW-PEO(5wt%)
/LiClO4/POSS
(d) HW-PEO/LW-PEO(10wt%)
/LiClO4/POSS
(e) HW-PEO/LW-PEO(15wt%)
/LiClO4/POSS
(f) HW-PEO/LW-PEO(12wt%)
/LiClO4/POSS
-5
-6
Tg(oC) Tm(oC) Xc(%)
Sample
Ionic Conductivity (S/Cm)
Ionic Conductivity (S/Cm)
10
Table 1. Thermal properties of HW-PEO/LWPEO/POSS 5wt% Composites
10
-3
10
-4
10
-5
10
-6
10
-7
15
20
25
30
35
40
Crystallinity (Xc)
Figure 2. Ion conductivity changes as a function of
crystallinity of electrolyte composites.
The ionic conductivity of the solid electrolyte
composites increased with decreasing that of
crystallinity, exhibiting a high-degree slope from 35 to
25 % crystallinity. After that point, ionic conductivity
was almost constant. The highest value of ionic
conductivity of solid electrolyte composite was
9.8ｘ10-3 S/cm. This value is feasible for commercial
applications.
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