Different ionic liquids based on the 1-butyl-1-methylpyrrolidinium cation are added as a flame-retarding additive to a standard organic electrolyte, and their influences on the cycling performance and thermal stability of lithium-ion cells are investigated. The self-extinguishing time and DSC results demonstrate that the addition of the ionic liquids to the base electrolyte is effective in reducing the flammability of the electrolyte solution and improving the thermal stability of the cell. Among the ionic liquids evaluated in this study, 1-butyl-1-methylpyrrolidinium hexafluorophosphate was the most desirable flame-retarding additive for achieving the best cycling performance of lithium-ion cell.