Design and Characterization of Gel Polymer Electrolytes Incorporating Highly Concentrated Electrolytes: Impact of Solidification on Bulk and Electrochemical Properties

Gel polymer electrolytes (GPEs) are considered promising candidates for next-generation lithium batteries (LIBs) due to their high ionic conductivity, shape flexibility, wide electrochemical stability window, and compatibility with electrodes. In this study, chemically cross-linked poly(ethylene oxide-co-propylene oxide)-based GPEs incorporating highly concentrated lithium bis(fluorosulfonyl)amide (LiFSA) electrolytes in ethylene carbonate (EC) were systematically investigated. The thermal properties, solvation structure, ionic transport properties, and electrochemical performance of the GPEs were evaluated by thermal analysis, Raman spectroscopy, ionic conductivity measurements, pulsed-field gradient nuclear magnetic resonance, interfacial stability tests and deposition/dissolution reversibility with metallic lithium electrode, direct-current polarization for the Li+-transference number measurements, and charge-discharge cycling with a graphite electrode. The results revealed that at high salt concentrations, Li⁺-ion transport becomes faster than FSA- and EC transport, predominantly involving contact ion pairs and aggregates. Electrochemical evaluations confirmed the formation of stable interfacial layers and good compatibility with metallic lithium. Also, high capacity and high Coulombic efficiency was demonstrated for the charge/discharge of a graphite electrode. All the revealed properties show the potential of this GPE system for safer, high-performance lithium batteries.