Influence of Ether-Functionalized Pyrrolidinium Ionic Liquids on Properties and Li+ Cation Solvation in Solvate Ionic Liquids

Ionic liquids are tunable solvents composed entirely of ions that have properties desirable as electrolytes for lithium batteries such as nonflammability and a large electrochemical stability window. Solvate ionic liquids are a subclass of ionic liquids that consist of a glyme-based solvent and lithium salt in an equimolar ratio, where Li⁺ cation-glyme solvation interactions result in ionic liquid-like properties. LiG4TFSI is a well-studied solvate ionic liquid consisting of equimolar amounts of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and tetraglyme (G4). In this work, pyrrolidinium ionic liquids with ether-functionalized side chains were synthesized, containing either one ether (EO1) moiety or three ether (EO3) moieties and mixed with LiG4TFSI to form a new class of electrolyte mixtures. Their physical and transport properties, as well as ion solvation structures, were characterized by electrochemical, thermal, rheological, and spectroscopic measurements. The conductivity of the electrolyte mixture composed of EO1:LiTFSI:G4 in a 1:1:1 molar ratio is 2.54 mS/cm at 30 °C, compared to 1.53 mS/cm for LiG4TFSI, an increase of 67%. A significant decrease in the conductivity to 0.279 mS/cm is observed for the EO3:LiTFSI:G4 mixture in a 1:1:0.4 molar ratio. Pulsed-field gradient nuclear magnetic resonance (PFG-NMR) measurements revealed that the EO1 cation diffuses significantly faster than the EO3 cation in their respective mixtures. Liquid-state ¹³C NMR experiments indicate that Li⁺ cations preferentially coordinate with tetraglyme. Li⁺ cations do not coordinate with the EO1 cation and coordinate with the EO3 ether side chains only at lower concentrations of tetraglyme. We hypothesize that the oligoether EO3 cation competes with G4 and TFSI^– for lithium cation solvation in G4-deficient compositions, leading to a largely adverse effect on the mass transport properties of the electrolyte.