Polymer Gel Electrolyte Prepared by “Salting-In” Poly(ethylene glycol) into a Phosphonium-Based Ionic Liquid with a Lithium Salt

We report the synthesis of an ion gel electrolyte based on a tetra-arm poly(ethylene glycol) (TetraPEG) network in a phosphonium-based ionic liquid (IL) solution utilizing the “salting-in” effect with a Li salt. Poly(ethylene glycol) (PEG) chains are insoluble in phosphonium-based ILs (such as triethylpentylphosphonium bis(trifluoromethanesulfonyl)amide, [P2225][TFSA]); however, the addition of LiTFSA enabled complete solvation of TetraPEG into a [P2225][TFSA] solution. The Li salt and polymer concentration dependences for PEG solubility in a [P2225][TFSA] system were investigated, and the optimum concentrations for polymer gel electrolyte preparation were elucidated. The gelation reaction (Michael addition between tetra-maleimide-terminated and tetra-thiol-terminated PEGs) of TetraPEG prepolymers in LiTFSA/[P2225][TFSA] solutions was characterized via chemical reaction kinetics to determine the apparent gelation rate constant (kgel′), and the reaction efficiency (p) was estimated to be p ∼ 95% at the end of the reaction. The resulting nearly defect-free TetraPEG gel electrolyte polymer network showed high ionic conductivity, comparable to the corresponding solution electrolyte. The TetraPEG gel electrolyte containing LiTFSA/[P2225][TFSA] also possessed a wide electrochemical window (∼4.2 V vs Li/Li+), thus allowing the reversible Li deposition/dissolution reaction to be performed using typical metal negative electrodes.