FTIR characterization of PEO + LiN(CF₃SO₂)₂ electrolytes

FTIR transmission spectra of thin films of polyethylene oxide (PEO) + lithium trifluoromethanesulfonimide (LiN(CF₃SO₂)₂, LiTFSI) mixtures have been obtained for ethylene oxideLi ratios from 64:1 to 2:1. The phase information from these spectra is compared with the reported phase diagrams based on thermal measurements. The infrared spectrum of LiTFSI is assigned by analogy with those of related compounds. Using the spectral subtraction technique, the effects of Li⁺ solvation on the PEO matrix and of ion pairing and aggregate formation on the TFSI anion are revealed. An explanation is offered for the variation in ionic conductivity for compositions within the “crystallinity gap”.