Relating Structure, Viscoelasticity, and Local Mobility to Conductivity in PEO/LiTf Electrolytes

Abstract

The phase state, local structure, local mobility, and viscoelastic response have been studied in the archetypal polymer electrolyte (PEO)xLiCF3SO3 with ether oxygen to lithium ion ratio of 2 ≤ [EO]/[Li] ≤12 over a broad temperature range in an effort to explore the factors controlling ionic conduction. We confirm that the crystal structure of the complex is identical to the (PEO)3LiCF3SO3 polymer electrolyte independent of the [EO]:[Li] content. Heating the nonstoichiometric compositions result in progressive melting of the complex, whereas the complex formed at or near the stoichiometric composition remains stable up to the liquidus temperature. The temperature dependence of dc conductivity is neither Arrhenius nor VFT. Its temperature dependence is more complex reflecting the underlying structural changes. Surprisingly, ionic conduction takes place both within the crystalline complex and in the amorphous phase with the latter having the major contribution. The (PEO)12LiCF3SO3 polymer electrolyte is the one with the highest conductivity at all temperatures investigated. The linear viscoelastic properties were studied as a function of temperature at two compositions. The different phases have distinct viscoelastic signatures. The complex formed at or near stoichiometric composition has a predominantly elastic response, whereas the more dilute compositions (consisting of the crystalline complex and an ion-containing amorphous phase) have a viscoelastic response and an ultraslow relaxation. Local polymer relaxation and ionic mobility are completely coupled. It is suggested that local ion jumps at subsegmental level are responsible for the measured conductivity.