A high-efficiency flame-retarding additive, (Ethoxy)pentafluorocyclotriphosphazene (N3P3F5OCH2CH3, PFPN), has been synthesized and explored as a safer protection additive for rechargeable lithium batteries. The flammability tests indicate that only the addition of 5 wt% PFPN can make the electrolyte be totally non-flammable. As far as we know, the PFPN additive is the most efficient one of any flame-retarding additive ever synthesized and reported in the literature. The charge–discharge results demonstrate that the PFPN additive shows a good electrochemical compatibility on the graphitic anode and LiCoO2 cathode. Meanwhile, the incorporated PFPN additive can greatly improve the cyclic performance of LiCoO2 electrode at a high cut-off voltage of 4.5 V, showing a prospective flame-retarding additive for the applications in high-voltage lithium ion batteries.