Cyclohexyl benzene (CHB) is widely used as an electrolyte additive for overcharge protection of lithium-ion batteries. This study reports that CHB and biphenyl (BP) mixtures are much more effective than CHB. On the overcharging tests for graphite-LiCoO2 cells with a nominal capacity of 760 mAh, CHB and BP mixtures expand the safety region up to 12 V/2 A where CHB alone without BP can never reach. Using the linear sweep voltammetry and the electrochemical quartz crystal microbalance, it was found that the CHB and BP mixture exhibits much bigger oxidation current and forms a larger amount of polymeric film than the numeric sum of each component's response. The origin of the synergistic effects between CHB and BP has been speculated based on their different electrochemical characteristics.