Local structures in crystalline and amorphous phases of diglyme-LiCF3SO3 and poly(ethylene oxide)-LiCF3SO3 systems: Implications for the mechanism ofionic transport

The ability of ethylene oxide-based materials to dissolve salts and form io nically conducting systems has attracted widespread interest. Our molecular level understanding of ionic conductivity in these systems critically depe nds on our understanding of local structures. The crystal structure of diet hylene glycol dimethyl ether:LiCF3SO3 or diglyme:LiCF3SO3 consists of digly me-salt dimers with the lithium ion in a 5-fold coordination strikingly sim ilar to that in crystalline poly(ethylene oxide)(3): LiCF3SO3. However, spe ctroscopic studies of diglyme-LiCF3SO3 solutions indicate that lithium ion is coordinated by only three oxygen atoms from a diglyme molecule and one o xygen atom from a CF3SO3- anion as part of a contact ion pair. A parallel s pectroscopic study of high molecular weight PEO-LiCF3SO3 suggests that the ionically conducting amorphous phase also contains four-coordinate lithium ions in a local structure similar to that in the diglyme solution. Analysis of Raman and X-ray data of PEO-LiCF3SO3 films suggests that the amorphous phase contains local structures which resemble the structure present in cry stalline P(EO)(3):LiCF3SO3. The significance of local structures for the me chanism of ion transport is discussed.