In this communication, a novel electrolyte additive, N,N-diallyic-diethyoxyl phosphamide (DADEPA), is described for the first time to improve the thermal stability of lithiated graphite anode in Li-ion batteries. The differential scanning calorimetry (DSC) measurement demonstrated that when the graphite was lithiated in the 5% DADEPA-containing electrolyte, the heat generation decreased sharply by half as compared with the reference, whereas the onset temperature for the main exothermic process was postponed by 20 °C. Electrochemical and XPS analyses indicated that the distinctive improvement in thermal safety came from a new interfacial chemistry, in which phosphorus-containing ingredients was embedded during the initial forming of the interphase.