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

• DOI: 10.1016/j.electacta.2003.08.020
• Publication Date: 2003-10-11T02:42:02Z
• Authors: M Deepa; S.A Agnihotry; D Gupta; R Chandra
• Keywords: Propylene carbonate; Imide

Abstract FTIR spectroscopic investigations coupled with ionic conductivity and viscosity measurements on lithium imide (LiN(CF 3 SO 2 ) 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(CF 3 SO 2 ) 2 –PC electrolytes reveal that the solvation of lithium ions manifests from Li + OC and Li + O (ring oxygens) interactions as the ν s (CO), the ring breathing and the δ (CH) modes of the pentagonal solvent ring are strongly perturbed for all salt concentrations. The split of the ν (SO 2 ) 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(CF 3 SO 2 ) 2 –PC strength. Perturbations observed for the ν a (SNS) and ν 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(CF 3 SO 2 ) 2 –PC of 3 M strength results in an insignificant conductivity decline (as σ 25 >10 −3 S cm −1 ) which is simultaneously accompanied by a massive increase in its macroscopic viscosity (as η 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 ν s (CO) mode of PMMA. Ionic conductivity decline that occurs at salt concentrations ≥1.25 M LiN(CF 3 SO 2 ) 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.