Tri(β-chloromethyl) phosphate (TCEP) and cyclohexyl benzene (CHB) had been studied synchronously as the additives of the lithium ion batteries to improve the high temperature and overcharge safety. The study group used the cyclic voltammetry and environment scanning electron microscopy (ESEM) to investigate the oxidize potentials of the TCEP and CHB at normal environment and 150°C and the surface characteristics of the positive electrode of the graphite/LiCoO2(063465) batteries before and after overcharge. The self-extinguishing time (SET) tests were carried out to measure the effect of additives on the electrolyte combustibility. The oven and overcharge tests and other electrochemical testing methods were performed to test the reliability of the protection provided by the TCEP and CHB of the high temperature and overcharge safety and the effect of the additives on the electrochemical performance. The results show that the oxidize potential of the TCEP is about 4.75V and the CHB about 4.6V at normal environment and the oxidize potential of the TCEP drops to 4V and CHB about 4.1V at 150°C; the TCEP has favorable effect on the self-extinguish time of the electrolyte; the surface of the LiCoO2 positive electrode after overcharge formed a layer of CHB polymer; when the content of both additives are over 5wt%, the batteries show improved safety through the oven tests of 150°C and can stand 1.3A (1C) and constant 10V overcharge tests; the adverse effect of TCEP and CHB on cycle performance of battery is relatively small. In sum, the cooperative use of these two additives can improve the safety of lithium ion battery greatly.