Triphenyl phosphite (TPPi) is adopted as a flame retardant to improve the safety of rechargeable lithium batteries with sulfur composite cathodes. The thermal stability of the electrolyte is greatly enhanced after the addition of TPPi, which also has a positive impact on the electrochemical performance of the Li-S batteries. TPPi facilitates the formation of SEI, resulting in a smaller interfacial impedance and a better rate performance. The addition of about 5 wt% TPPi greatly reduces the polarization voltage, stabilizing the cycle performance of the battery. This indicates that an optimized addition of TPPi is a favorable additive in conventional liquid electrolytes for rechargeable Li-S batteries with high performances and good safety.