We report the preparation of solvent-free polymer electrolytes via a free radical copolymerization of methacrylate-based oligomers in the presence of LiTFSI. Properties of the electrolytes were then studied as a function of their compositions. Furthermore, the incorporation of a room temperature ionic liquid (RTIL) into the copolymer electrolyte to form a gel polymer electrolyte (GPE) is also reported. The high miscibility of the oligomers in the RTIL enables the preparation of the GPEs by a one-step method using the in situ free radical copolymerization. The GPEs have a dry aspect and are free-standing; they also exhibit an ionic conductivity close to 4 × 10–4 S cm–1 at 25 °C and to 1.45 × 10–3 S cm–1 at 65 °C. Furthermore, the GPEs have been used as electrolytes in a Li/Electrolyte/LiNi1/3Mn1/3Co1/3O2 battery. Specific capacities of 79 mAh g–1 and 118 mAh g–1 were reached at the C/5 and C/10 rate, respectively.