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

The ability of ethylene oxide-based materials to dissolve salts and form ionically conducting systems has attracted widespread interest. Our molecular level understanding of ionic conductivity in these systems critically depends on our understanding of local structures. The crystal structure of diethylene glycol dimethyl ether:LiCF3SO3 or diglyme:LiCF3SO3 consists of diglyme-salt dimers with the lithium ion in a 5-fold coordination strikingly similar to that in crystalline poly(ethylene oxide)(3): LiCF3SO3. However, spectroscopic studies of diglyme-LiCF3SO3 solutions indicate that lithium ion is coordinated by only three oxygen atoms from a diglyme molecule and one oxygen atom from a CF3SO3- anion as part of a contact ion pair. A parallel spectroscopic 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 crystalline P(EO)(3):LiCF3SO3. The significance of local structures for the mechanism of ion transport is discussed.