Abstract:
Solid polymer electrolytes have been proven to be prospective candidates for
advanced electrochemical applications on basis of their characteristics such
as flexibility, viscoelasticity and ionic conductivity. The ionic conductivity of
solid polymer electrolytes is very low at ambient temperature. Several
attempts have been carried out to improve ionic conductivity of solid polymer
electrolyte systems. The ionic conductivity of these solid polymer electrolytes
can be improved by incorporation of plasticizers, which impart the necessary
salt solvating power and ionic mobility.
Solid polymer electrolyte system based on PVC containing ethylene
carbonate and propylene carbonate as plasticizers have also found
applications in lithium secondary batteries. PVC shows immiscibility with
plasticizer thereby acts as mechanical stiffener in solid polymer electrolyte
system. In the present work solid polymer electrolyte systems based on PVC
containing alkali metal salts (LiClO4, LiClO3, LiBF4, Li2SO4, NaClO4),
plasticizers(EC, PC), inorganic metallic oxides (ZnO, TiO2, Al2O3, ZrO2) and
PVC blended with PMMA and PEO were prepared and examined as solid
polymer electrolyte with improved desired properties. The prepared
polyelectrolytes were characterized by conductivity, Thermogravimetric,
Differential Thermogravimetric Analysis, X-ray diffraction, Scanning electron
microscopy and viscosity methods.
The conductance studies of different polymer electrolyte systems showed that
conductivity values exhibit enhancement with increase in concentration of
salts which may be due to the development of amorphous regions in the
polymer matrix which facilitates the mobility of ions through the polymer
matrix. The increase in ionic conductivity with salt may be attributed to the
increase in the number of ions of salts.
The effects of different plasticizers on the behavior of polymer electrolytes of
different compositions at various temperatures were also observed. The
addition of PC was found more effective as compared to EC.
The ionic
conductivity increased with rise in temperature for all different polymer
electrolyte systems containing various content of PVC, salts and plasticizers.
The studies of incorporation of various inorganic fillers showed increase in
ionic conductivity with increase in concentration of filler up to certain limits of
filler concentration beyond which ionic conductivity decreased. At higher
concentration of fillers, the formation of crystallites may be responsible for the
decrease in ionic conductivity of the polymer electrolyte systems.
The activation energies of different polymer electrolyte systems of various
compositions at various temperatures were calculated and found that Ea value
showed decrease with rise in temperature. Similarly Ea also decreased by
addition of salts or plasticizers thereby increasing the ionic conductivity of the
polymer electrolyte systems.
The effects of addition of salts, plasticizers and polymer on the mechanical
properties of polymer electrolytes were also studied. It was found that
mechanical strength of polymer electrolytes deteriorated with increase in
concentration of salts. The effect of EC and PC as plasticizers on the
mechanical strength were studied and found that Young’s modulus and stress
at peak values decreased by incorporation of low molecular weight
plasticizers while elongation at peak values revealed that elongation
decreased by addition of salt. The variation of modulus and tensile strength of
PVC-PMMA blend polymer electrolyte system regarding concentration of
PMMA was also studied and it was found that Young’s modulus and tensile
strength decreased with increase in concentration of PMMA up to 15 wt%
concentration of PMMA beyond which both these parameters showed
increase. The influence of silica exhibits improvement in mechanical
properties. SEM studies of various PVC based polymer electrolyte systems
showed that by incorporation of inorganic fillers, the surface becomes rougher
as compared to the films without any filler. The effects of salts on the SEM
micrographs were also studied. SEM micrographs showed that surface
morphology of pure PVC to be similar to rigid and glassy surfaces while
blends of PVC with PMMA showed two phase morphology without any sharp
boundary
between
these
two
phases.