Lithium Ion Batteries (LIBs) have emerged as the leading technology in energy storage, with potential applications ranging from consumer electronics to hybrid vehicles and storage of energy produced by photovoltaics. State-of-the-art LIBs utilize a liquid electrolyte that poses issues such as flammability, potential leakage from battery casings, and dendrite growth when used with lithium metal. Solid polymer electrolytes (SPEs) show promise in replacing liquid electrolytes, as they offer decreased flammability and the potential for dendrite prevention, enabling the use of lithium metal. However, SPEs are often hindered by low ionic conductivities and poor contact with electrodes. In this work, a SPE is proposed that consists of a high molecular weight polyethylene oxide (PEO) matrix with an ionic liquid and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as plasticizers. A range of different compositions of SPE were fabricated and their electrochemical and thermal properties were characterized using cyclic voltammetry, electrical impedance spectroscopy, and differential scanning calorimetry. Our experiments demonstrated a high lithium conductivity and wide electrochemical and thermal stability windows for the electrolyte. Cycling cells, constructed using lithium metal and a composite Lithium Iron Phosphate (LFP) electrode, demonstrated high coulombic efficiency and minimal capacity fade through cycling at both elevated temperature and room temperature. As a whole, the SPE compositions with a higher ratio of ionic liquid to polymer showed higher conductivity and improved cycling performance. These electrolytes with high ionic liquid content show promise as safe, stable, and conductive alternatives to liquid electrolytes for lithium ion batteries and are also commercially relevant due to their ability to cycle at a wide range of temperatures with commercially available electrodes.