AbstractLithium-ion batteries have achieved huge success in the field of energy storage, but they use flammable electrolytes that may cause safety issue. Solid polymer electrolytes have been considered as the final solution to this safety issue, which have received many researchers’ attention. However, solid polymer electrolytes still face several challenges, such as low ionic conductivity. Herein, we report a quaternary solid polymer electrolyte to achieve high ionic conductivity, which was synthesized by one-step casting method. This quaternary electrolyte contains two polymers (poly(vinylidene fluoride-co-hexafluoropropylene) and poly(ethylene oxide) diacrylate), one plasticizer (succinonitrile), and one lithium salt (lithium bis(trifluoromethane)sulfonimide). Long-chain polymer is used as matrix, and short-chain polymer and plasticizer are used to increase ionic conductivity. This quaternary electrolyte achieves a high ionic conductivity of 1.49 × 10⁻⁴S cm⁻¹at room temperature. When assembled into coin cell with lithium iron phosphate cathode and lithium metal anode, this quaternary electrolyte achieves a capacity of 139 mAh g⁻¹at 0.2 C after 50 cycles and 106 mAh g⁻¹at 1 C after 160 cycles. Moreover, this facile one-step casting method provides a new route towards the development of solid polymer electrolytes.