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 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(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 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(CF₃SO₂)₂-PC of 3 M strength results in an insignificant conductivity decline (as σ₂₅>10⁻³ S cm⁻¹) which is simultaneously accompanied by a massive increase in its macroscopic viscosity (as η₂₅>10⁸ 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(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.