H 1 , Li7, and F19 nuclear magnetic resonance and ionic conductivity studies for liquid electrolytes composed of glymes and polyetheneglycol dimethyl ethers of CH3O(CH2CH2O)nCH3 (n=3–50) doped with LiN(SO2CF3)2

Abstract

Solid polyethyleneoxides (PEO) are in effect “polymer liquids” due to their flexibility and high solubility for alkaline salts. To clarify the role of PEO in electrolyte systems, electrolytes composed of members of the “glyme family” [i.e., diglyme(DG), triglyme(TG), tetraglyme(TeG), pentaglyme(PG), and polyethyleneglycol dimethyl ethers(PEGDM)] doped with LiN(SO2CF3)2 were investigated. PEGDMs form a series of low molecular weight PEO-like homologues with molecular weights between 400 and 2500. Electrolytes of the glymes and PEGDMs were prepared for two salt concentrations (ether oxygen: lithium; O:Li) 20:1 and 10:1. The ionic conductivities and the self-diffusion coefficients of the solvent, anion and lithium ions in the electrolytes were measured using pulsed-field gradient spin-echo (PGSE) H1, F19, and Li7 NMR, respectively. From the spin-lattice relaxation times (T1) it was found that the segmental motions of the CH2CH2O moiety and the lithium hopping motions are correlated and that the rate of the segmental motions decreases as the molecular size of the solvent increases. The ionic conductivities calculated from the diffusion coefficients are compared with the experimental ac ionic conductivities. The diffusion and ion conduction mechanisms are discussed and the lithium ion–solvent interactions are shown to depend on the solvent size.