To enhance the resistance of lithium-ion battery components to ignition and to reduce the flammability of the electrolyte with minimal effect on performance, we added flame-retardant additives to the electrolyte. The flame retardants were selected from a group of organic phosphate compounds, triphenylphosphate (TPP) and tributylphosphate (TBP), to provide superior thermal safety in lithium-ion cells at the fully charged state. The cycling characteristics of the lithium-ion cells containing flame-retardant additives were found to be similar or superior to the cells that contained no additives. Horizontal burning tests of electrolytes were carried out in a flame test chamber referenced by Underwriters Laboratories (UL) test standard 94 (UL 94) and ASTM D4986-98 to evaluate the electrolyes’ flammability characteristics. The thermal stability characteristics of the electrodes and electrolytes with and without flame-retardant additives were investigated by accelerating rate calorimetry (ARC). Negative electrode samples with electrolytes containing flame-retardant additives revealed less heat generation and higher-onset decomposition temperatures. The results disclose that the thermal safety of lithium-ion cells can be improved by incorporating small amounts of suitable additives such as triphenylphosphate and tributylphosphate to the electrolyte.