Electrochemical properties and working mechanism of biphenyl as a polymerizable electrolyte additive for overcharge protection of lithium ion batteries are studied by microelectrode voltammetry, charge–discharge measurements and SEM characterization of the overcharged cell’s components. The experimental results reveal that biphenyl can electrochemically polymerize at the overcharge potential of 4.5–4.75V (versus Li/Li+) to form a layer of conductive film on the cathode surface and the polymer deposits may develop to penetrate the separator to reach the anode surface, resulting an internal short-circuit to prevent from the cell voltage runaway. On the other hand, the electro-oxidative polymerization of biphenyl produces excessive gas and heat, which help to enhance the sensitivities of electric disconnecting devices. In addition, it is also found that the use of biphenyl as an electrolyte additive does not significantly influence the normal performances of the lithium ion batteries.