Theoretical study of interactions of a Li+(CF3SO2)2N- ion pair with CR3(OCR2CR2)nOCR3 (R = H or F)

Interactions of a lithium bis(trifluoromethane sulfonyl)imide (Li+Tf2N-) ion pair with oligoethers are investigated via density functional theory (DFT). As a model for polymer electrolytes polyethyleneoxide (PEO) and perfluoropolyether (PFPE), CR3(OCR2CR2)(n=1-5)OCR3 (R = H or F) is considered. Topographical analysis of the molecular electrostatic potential (MESP) is performed to determine preferential binding sites of Li+. Our study shows that the MESP value near the oxygen sites of the polymer backbone is more negative for PEO than for PFPE. This result indicates that substitution of hydrogen by fluorine in polyethers leads to reduction in Li+-polymer interactions, in concert with the experimental ionic conductivity results. S-O stretching vibrations of Tf2N- are calculated for the lithium salt in the presence and absence of electrolytes. The blue and red shifts predicted for S-O stretching are further explained by natural bond orbital analysis and molecular electron density topography. The S-O stretching vibrations can be used as a useful tool to understand the ion pair interactions and thus ion transport phenomena in polymer electrolytes.