Solid polymer electrolytes are of significant current interest due to their potential use in rechargeable lithium metal batteries. The most widely studied polymer electrolyte system is poly(ethylene oxide) (PEO) mixed with a lithium salt (LiTFSI), PEO/LiTFSI. It is well known that the ether oxygens of the PEO backbone selectively solvate Li+ ions, which leads to high ionic conductivity and allows for the use of nanostructured block copolymer electrolytes to increase their mechanical properties. Through small angle neutron scattering (SANS) experiments, we have determined the effect of salt on the statistical segment length of PEO/LiTFSI mixtures in the melt state. We show that the effect of salt is non-monotonic and is correlated to the coordination between Li+ ions and EO monomers. In addition, neutron spin echo (NSE) spectroscopy experiments were conducted on nanostructured block copolymer electrolytes to determine the effect of salt on the segmental dynamics. The isotopic labeling allowed for the dynamics of only the ion-containing block, PEO, to be measured. We quantified the dynamics by comparing the data to predictions based on the Rouse model at low times as well as the standard tube model of de Gennes and Edwards at long times. There is a direct relationship between segmental dynamics measured on the monomer length scale and ion transport under an applied field on macroscopic length scales.
Whitney Loo (University of California, Berkeley)