For all the battery measurements, lithium metal (100 µm, Gelon) was used as anode material. Battery cells were then assembled using a Li metal anode, a free-standing designed SPE membrane, and the prepared LFP/LMO cathodes. Electrochemical measurements were performed using a multi-channel potentiostat VMP3 (BioLogic) with all measurements conducted in CR2032 coin cells (Hohsen Corp.). Linear sweep voltammetry (LSV) experiments were performed using a Li||SS (stainless steel) cell at 50 °C. The scans ran from the open-circuit voltage (OCV) up to 6 V (vs. Li/Li + ) at a scan rate of 0.1 mV s -1 . The galvanostatic cycling for Li||Li, LFP||Li, and LMO||Li cells were also carried out. Cell cycling was performed at 50 °C, with voltage limits set to 2.5-4 V vs. Li + /Li for LFP||Li cells and 3-4.3 V vs. Li + /Li for LMO||Li cells. Two formation cycles were used before long-term cycling unless mentioned otherwise. The reproducibility of the results has been checked by multiple tests. Both linear and crosslinked PCIL-SPEs were synthesized from homogeneous mixtures of PEO, LiFSI and C3mpyrFSI (EO:Li:IL molar ratio=10:1:1) by solvent casting and UVcrosslinking processes, namely SPE10:1:1 and SPE10:1:1-CL respectively (Figure ). Thermal gravimetric analysis (TGA) of both SPE10:1:1 and SPE10:1:1-CL confirms the thermal stability up to 220 °C which is sufficiently high for the safe operation of solid-state LMB (Figure , Support Information).