Diphenyloctyl phosphate (DPOF) was used as a flame-retardant additive to improve the thermal stability of electrolyte for lithium-ion batteries. Effects of DPOF on the flammability and conductivity of electrolyte were investigated. A series of electrochemical measurements and scanning electron microscopy (SEM) were employed to investigate the compatibility of DPOF with LiNi1/3Co1/3Mn1/3O2 (NCM) and artificial graphite (AG), respectively. The results show that DPOF can effectively reduce the flammability of the electrolytes, corresponding with a small decrease of conductivity. Additionally, DPOF has no distinct effect on SEI film formation of artificial graphite, and the capacity of Li/AG half-cell with 5% DPOF is higher than that of base electrolyte, while the cycling stability of NCM/Li cells becomes worse with the increasing ratio of DPOF. Electrochemical impedance spectroscopy shows that the charge-transfer resistance of NCM/Li cells with DPOF is much larger than that of cells without DPOF in the 50th cycle, while there is no distinct variation on resistances of Li/AG cells with or without DPOF. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.058208jes] All rights reserved.