To improve the safety of lithium ion battery, cresyl diphenyl phosphate (COP) is used as a flame-retardant additive in a LiPF6 based electrolyte. The electrochemical performances of LiCoO(2)/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li(0.5)CoO(2) and Li(0.5)CoO(2)-CDP-electrolyte, and Li(x)C(6) and Li(x)C(6)-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO(2)/CDP-electrolyte/Li cells, the onset temperature of single Li(0.5)CoO(2) is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li(0.5)CoO(2) coexists with CDP-electrolyte, the thermal stability is enhanced. COP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO(2)/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% COP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% COP to the electrolyte almost does not worsen the electrochemical performance of LiCoO(2) cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive. (C) 2011 Published by Elsevier B.V.