Unusual Polyether Dissolution in Salt-Concentrated Ionic Liquid Electrolytes: Application to High-Performance Ion Gels for Lithium-Ion Batteries

High Resolution Image Download MS PowerPoint Slide In this study, we report an anomalous dissolution behavior of poly(ethylene glycol) (PEG) in bis(fluorosulfonyl)amide (FSA)-based ionic liquids (ILs) containing LiFSA salt. Although PEG is initially soluble in FSA-based ILs, it turns to be insoluble upon adding LiFSA salt (conventional salting-out effect). Surprisingly, further addition of LiFSA salt results in PEG redissolution, yielding transparent PEG/IL solutions with high LiFSA concentrations (unusual salting-in effect). Combined high-energy X-ray scattering and all-atom molecular dynamics simulations revealed the molecular mechanism underlying this phenomenon. (1) At low Li + concentrations ( c Li ), Li + ions coordinate exclusively with PEG to form insoluble crown ether-type PEG–Li + complexes with contracted polymer chains. (2) At high c Li, the PEG structure changes drastically, forming shared PEG–Li + –FSA – complexes. Here, Li + ions act as solvation bridging sites between PEG and FSA –, facilitating polymer chain expansion and redissolution of PEG (transparent solution). Leveraging this redissolution effect, we synthesized homogeneous ion gels using tetra-arm PEG (TetraPEG) as a prepolymer. The resulting TetraPEG ion gel exhibit high mechanical strength, liquid-like ionic conductivity, and reversible Li + insertion/deinsertion at the graphite negative electrodes for lithium-ion batteries (LIBs).