Ion association and ion solvation effects at the crystalline-amorphous phase transition in PEO-LiTFSI

From band shape analyses of the strong 740 cm(-1) Raman mode of the TFSI anion, we deduce that the crystalline state of the P(EO)(n)LiTFSI system contains a very small amount of ion pairs (approximate to 7%), most probably belonging to the minor amorphous phase, for the n greater than or equal to 6 range. For all-amorphous samples, we found a small amount of ion pairs for n greater than or equal to 8, but a significant amount (approximate to 24%) for n = 6. This increase in ion pair formation coincides with a decrease in the relative solvation of lithium cations by polymeric ether oxygens, as detected from a careful study of the 863 cm(-1) polymer-cation "breathing mode". We therefore propose that the local ionic structure is preserved during the melting of n greater than or equal to 8 compositions, but that there is a changeover from a predominately ether oxygen lithium coordination to a combined ether oxygen and anionic coordination of the lithium cations for more concentrated samples upon melting.