A self-standing, flexible and lithium dendrite growth-suppressing composite gel polymer electrolyte membrane was designed for the use of room-temperature lithium ion batteries. The multi-functional composite semi-interpenetrating polymer network (referred to as "Cs-IPN") electrolyte membrane was fabricated by combining a UV-cured ethoxylated trimethylolpropane triacrylate (ETPTA) macromer with alumina nanoparticles in the presence of liquid electrolyte and thermoplastic linear poly(ethylene oxide) (PEO). The polymer electrolyte membrane exhibits a semi-interpenetrating polymer network structure and a higher room temperature ionic conductivity, which impart the electrolyte with a significant cycling (120 mAh g⁻¹ after 200 cycles) and a remarkable rate (137 mAh g⁻¹ at 0.1 ℃, 130 mAh g⁻¹ at 0.5 ℃, 119 mAh g⁻¹ at 1 ℃ and 100 mAh g⁻¹ at 2 ℃) performance in Li/LiFePO₄ battery. More importantly, the polymer electrolyte possesses superior ability to inhibit the growth of lithium dendrites, which makes it promising for next generation lithium ion batteries.