Safety concerns have been the key problem in the practical application of lithium-ion batteries. In the present study, triphenylphosphate (TPP) is used as an electrolyte additive to improve the thermal safety and electrochemical performance of lithium-ion cells. Cyclic voltammetric measurements and thermal stability measurements by means of differential scanning calorimetry are undertaken. Rate capability and cycling performance are evaluated. The flame-retarding additive TPP is electrochemically stable up to 4.9V. The TPP-containing electrolytes display improved thermal stability compared with TPP-free electrolytes. The addition of 3% TPP to the ionic electrolyte is an optimum content for improvement of cell performance and suppression of electrolyte flammability.