Hyperbranched network-based gel copolymer electrolytes are synthesized by in situ free radical polymerization. This research is separated into two parts: the first is an investigation of modified bismaleimide oligomer (MBMI) as a free volume additive, and the second investigates the salt concentration effect on high power application. A polymer electrolyte with MBMI additive provided more free volume space, and the ionic conductivity of gel copolymer electrolytes was measured as a function of the salt concentration of lithium hexafluorophosphate (LiPF6). The highest ionic conductivity and the lowest activation energy of hyperbranched-network gel copolymer electrolytes were determined to be 7.72×10−3 S/cm at 23°C and 5.41kJ/mol, respectively. Furthermore, the MBMI additive and the optimal concentration of lithium salt increased the free space for carrier ions and contributed to increasing capacity and working voltage at a high rate discharge (8C). The reliability and cycling ability of lithium polymer batteries are as good as lithium ion batteries for potential electric vehicle (EV) application.