POLY(ETHYLENE OXIDE) BASED SOLID ELECTROLYTES: PHASE DIAGRAM, KINETICS, AND IONIC CONDUCTIVITY (PEO)

This research concentrates on the quantitative study of the morphology of the poly(ethylene oxide), PEO, based solid state electrolytes, and its correlation with the ionic conductivity. The observed composition and temperature dependances of phase equilibria of PEO-NaSCN and PEO-NaI electrolytes are summarized in the eutectic phase diagrams. These phase diagrams propose the existence of a semicrystalline SPEO phase, which is comprised of 60% crystalline PEO and 40% of associated salt-containing amorphous region, in equilibrium with a crystalline complex phase of fixed stoichiometry.;Conductivities of PEO-NaSCN and PEO-NaI systems are successfully modeled by assuming the relaxation properties of the polymer matrix control the ionic transport. Below the eutectic temperature the conducting medium is the constrained amorphous regions associated with the SPEO phase; while at temperatures above the eutectic temperature, the liquid phase is the primary conducting path. With this single amorphous conduction mechanism, the model successfully accounts for the observed temperature and composition dependances of the ionic conductivity.;Not surprisingly, the kinetics of the phase separation processes determine the final morphology of the polymer electrolytes. A detailed study of the phase separation kinetics is conducted in order to understand the phase behavior of these systems. At temperatures below the eutectic temperature, the observed morphology and sequence of appearance of the constituents are rationalized by a proposed "pseudo-eutectic" mechanism. On the other hand, a diffusion-controlled precipitation mechanism is responsible for the phase separation at temperatures above the eutectic temperature. With proper assumptions and computer modeling, the latter study also provides a means for calculating the ionic diffusivity.