The use of conventional lithium-ion batteries in high temperature applications (>50°C) is currently inhibited by the high reactivity and volatility of liquid electrolytes. Solvent-free, solid-state polymer electrolytes allow for safe and stable operation of lithium-ion batteries, even at elevated temperatures. Recent advances in polymer synthesis have led to the development of novel materials that exhibit solid-like mechanical behavior while providing the ionic conductivities approaching that of liquid electrolytes. Here we report the successful charge and discharge cycling of a graft copolymer electrolyte (GCE)-based lithium-ion battery at temperatures up to 120°C. The GCE consists of poly(oxyethylene) methacrylate-g-poly(dimethyl siloxane) (POEM-g-PDMS) doped with lithium triflate. Using electrochemical impedance spectroscopy (EIS), we analyze the temperature stability and cycling behavior of GCE-based lithium-ion batteries comprised of a LiFePO4 cathode, a metallic lithium anode, and an electrolyte consisting of a 20-μm-thick layer of lithium triflate-doped POEM-g-PDMS. Our results demonstrate the great potential of GCE-based Li-ion batteries for high-temperature applications.