The synthesis and characterization of novel ionic liquid (ILs) based gel polymer electrolytes for application in sodium-ion (Na+) batteries is reported. It comprises a 0.2M solution of NaN(CF3SO2)2 (sodium bis(trifluoromethylsulfonyl) imide) in mono and dicationic imidazolium-based ionic liquids immobilized in a poly(vinylidene fluoride)-hexafluoropropylene copolymer matrix. The membranes offer acceptable ionic conductivity, up to 2.2×10−4 S cm−1 at room temperature, with excellent thermal and electrochemical stabilities. The useful thermal stability reached up 150 °C and decomposition only starts at about 400 °C, while electrochemical stability window comprises from 1.5 V to 5.0 V vs. Na/Na+. Na+ transference values ( t N a + ) are in the range of 0.1 to 0.5, indicating a contribution of anionic transport (TFSI− anion) and component ions of ionic liquid to the total ionic conductivity. The nature of Na+ coordination and the formation of Na+-complexes were elucidated through Raman spectroscopy. The average number of TFSI coordinated to Na+ (n) allowed to know the contribution of the formed complexes in ionic mobility mechanism in the gel electrolytes. The effect of side chain length in oxyethylene-functionalized imidazolium mono and dicationic RTILs on both structural and electrochemical properties were discussed. Results obtained demonstrate promising characteristics of the membranes, being suitable candidates to be used in rechargeable sodium batteries.