Tetra-arm Poly(ethylene glycol)-Based Ion Gel Electrolytes via Salting-in Polymer Dissolution in Sodium Salt-Containing Ionic Liquids

We report the development of tetra-arm poly(ethylene glycol) (TetraPEG)-based ion gel electrolytes formed in a pyrrolidinium-based ionic liquid (IL) via salting-in-induced polymer dissolution using a sodium (Na) salt. The gelation reaction, based on cross-end linking between maleimide- and thiol-terminated TetraPEGs, in the IL containing sodium bis(trifluoromethanesulfonyl)amide (NaTFSA) was investigated using rheological measurements and kinetic analysis. The gelation rate exhibited a strong dependence on the metal ion species (Na⁺ vs Li⁺): gelation occurred more slowly in the Na system than in the Li system, as characterized by the gelation time (t_gel) and apparent rate constant (k_gel′). The resulting TetraPEG ion gels showed excellent mechanical integrity even at a low polymer concentration (5 wt %), though the Na-based gels were mechanically weaker than their Li-based counterparts. A comprehensive structural analysis combining experimental and theoretical approaches revealed the molecular-level origin of this ion-specific behavior: (1) in IL solutions without PEG, Na⁺ is coordinated by three TFSA^– anions to form the anionic complex [Na(TFSA)₃]^2–; (2) in the presence of PEG, partial TFSA^– release enables the formation of PEG–Na⁺–TFSA^– ternary complexes with expanded polymer conformations; and (3) in contrast, Li⁺ preferentially coordinates with PEG, forming compact Li⁺–PEG complexes with contracted polymer chains. These distinct coordination modes result in different chain conformations, which in turn influence the mechanical performance of the ion gels through entropy-driven elasticity. These findings highlight a viable molecular-level design strategy for ion gel electrolytes by tailoring metal–polymer interactions in IL-based systems.