To improve the safety of lithium-ion batteries, cresyl diphenyl phosphate (CDP) was used as a flame retardant additive in a LiPF6 electrolyte solution. The flammability of the electrolytes containing CDP and the electrochemical performances of the cells, LiCoO2/Li, graphite/Li and the battery LiCoO2/graphite with these electrolytes, were studied by measuring the self-extinguishing time of the electrolytes, the variation of surface temperature of the battery and the charge/discharge curve of the cells or battery. It is found that the addition of CDP to the electrolyte provides a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of battery. On the other hand, the electrochemical performances of the cells become slightly worse due to the application of CDP in the electrolyte. This alleviated trade-off between the flammability and thermal stability and cell performances provides a possibility to formulate a nonflammable electrolyte by using CDP.