1-Butyl-1-methylpyrrolidinium hexafluorophosphate (BMP-PF6) was used as a flame-retarding additive in the liquid electrolyte, and the influence of BMP-PF6 content on cycling performance and thermal properties of lithium-ion batteries was investigated. Self-extinguishing time and DSC studies demonstrated that the addition of BMP-PF6 to the electrolyte provided a significant suppression in the flammability of the electrolyte and an improvement in the thermal stability of the cell. The optimum BMP-PF6 content in the electrolyte was found to be 10wt.% for improving safety without degrading cycling performance of the cell.