The effect of diphenyloctyl phosphate (DPOF) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) as flame-retardant (FR) additives in the liquid electrolyte of Li-ion cells is evaluated at both elevated temperature (40°C) and room temperature (RT, 25°C). The tested cells use mesocarbon microbeads (MCMB) and LiCoO2 as the anode and cathode materials, respectively. Cell characteristics are investigated by means of electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The results of the cycle performance tests demonstrate the superior discharge capacity and capacity retention of the DPOF-containing cell compared will TTFP after cycling at both RT and 40°C. Therefore, these results confirm the promising potential of DPOF as an FR additive for improving the electrochemical performance of Li-ion batteries.