Lithium rechargeable batteries featuring solvent-free highly conductive solid polymer electrolytes (SPEs) will make a dramatic impact on the electric and hybrid-electric vehicles (EV/HEV) industry by eliminating hazards related to the use of liquid electrolytes. In this paper, we report the synthesis and characterization of a star-shaped borate ester plasticizer, which was then incorporated into the poly(ethylene oxide) polymer matrix in different proportions. Significant improvement was observed in conductivity, with the best value of 9.1 × 10−5 S/cm at 30 °C. These borate ester plasticized SPEs also exhibited excellent thermal and electrochemical stabilities.