A novel flame retardant, Resorcinol bis(diphenyl phosphate) (RDP), is successfully synthesized and investigated as electrolyte additive for lithium ion batteries. It is found that RDP can be electro-polymerized at 4.4 V (vs. Li/Li+), which will protect the batteries from voltage runaway by consuming the overcharge current. Moreover, RDP addition could lower the self-extinguishing time (SET) of the electrolyte. The influence of proper RDP addition on the cycling performance and capacity is negligible. Thus, RDP could be used as a novel electrolyte additive for lithium ion batteries functioned as both overcharge protection and flame retardant. The results may also be helpful for the further design of advanced lithium ion battery electrolytes.