Ion-pairing effects and ion-solvent-polymer interactions in LiN(CF3SO2)2-PC-PMMA electrolytes:: a FTIR study

FTIR spectroscopic investigations coupled with ionic conductivity and viscosity measurements on lithium imide (LiN(CF3SO2)(2))-propylene carbonate (PC)-poly(methyl methacrylate) (PMMA) based liquid and gel electrolytes over a wide range of salt (0.025-3 M) and polymer (5-25 wt.%) concentration range furnish a novel insight into the ion-ion and ion-solvent-polymer interactions. Vibrational spectral data for LiN(CF3SO2)(2)-PC electrolytes reveal that the solvation of lithium ions manifests from Li+-O=C and Li+-O (ring oxygens) interactions as the nu(s)(C=O), the ring breathing and the delta(CH) modes of the pentagonal solvent ring are strongly perturbed for all salt concentrations. The split of the nu(SO2) mode (that appears at 1355 cm(-1) for the free imide ion) into two components at 1337 and 1359 cm(-1) confirms the existence of contact ion-pairs possessing two different stable optimized geometries wherein the Li+ ion coordinates in a bidentate fashion in liquid and gel electrolytes of 3 M LiN(CF3SO2)(2)-PC strength. Perturbations observed for the nu(a)(SNS) and nu(s)(SNS) modes of the imide ion and the symmetric ring deformation mode of PC confirms the presence of ion-pairs in both 2 and 3 M electrolytes. Incorporation of even upto 25 wt.% of PMMA in a solution of LiN(CF3SO2)(2)-PC of 3 M strength results in an insignificant conductivity decline (as sigma(25) > 10(-3) S cm(-1)) which is simultaneously accompanied by a massive increase in its macroscopic viscosity (as eta(25) > 10(8) cSt). Gels containing 25 wt.% of PMMA exhibit a complex pattern of Li+-PMMA interactions through the carbonyl oxygen of its ester group which is evidenced from the perturbations observed for the nu(s)(C=O) mode of PMMA. Ionic conductivity decline that occurs at salt concentrations greater than or equal to 1.25 M LiN(CF3SO2)(2)-PC in both liquid and gel electrolytes. is therefore attributable to (i) ion-pairing phenomenon and (ii) an enhancement in the solution viscosity due to a high salt proportion. (C) 2003 Elsevier Ltd. All rights reserved.