A thermally stable and flame-retardant separator is proposed to improve the safety of lithium ion batteries. The separator is prepared by dip-coating both sides of a conventional tri-layer polyolefin separator with brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO). Significantly reduced thermal shrinkage and flammability are exhibited without decreases in the pore size or porosity of the conventional separator. By using the BPPO-coated separator, an electrochemical half-cell composed of a lithium metal anode and a LiCoO2 cathode are successively tested. The resulting stable cycle performances are demonstrated. It is expected that the BPPO-coated separator can be a capable candidate as a separator for the safe of lithium ion batteries.