The polymer-in-concentrated ionic liquid SPEs (PCIL-SPEs) were prepared using a solvent casting technique within an Ar-filled glovebox. The preparation steps are shown in Figure . First, predetermined amounts of LiFSI salt and C3mpyrFSI IL (Li + :IL molar ratio = 1:1) were mixed and dissolved overnight at room temperature. Then the obtained solutions were mixed with various amount of PEO in acetonitrile with EO:Li + :IL molar ratios equal to 10:1:1. The obtained solutions were then cast into silicon moulds and rested for 48 hours to ensure slow evaporation of solvent. The obtained free-standing solid membranes were then sandwiched between two Mylar foils and hot-pressed (70°C, 2 MPa for 10 mins) to obtain homogenous membranes with a thickness of 150 ± 10μm. Crosslinked PCIL-SPEs were prepared by using benzophenone (BP, 5 wt% of PEO) as photo-initiator and crosslinked in a TBK 905 UV curing box (power 200W) for 3 mins right after hot-pressing. The prepared SPEs were then transferred to the Buchi oven for a secondary vacuum drying process (50°C for 24 h) before further characterization. All prepared SPEs were kept in argon-filled glovebox.