In order to investigate the influence of the biphenyl (BH) as a polymerizable electrolyte additive on the properties of Al-plastic film lithium-ion batteries, we examined the electrochemical properties of batteries which containing different amounts of BH. The main analysis tools were overcharge tests, linear sweep voltammetry, cycling tests, rate capability, thermal stability, AC impedance, etc. The results showed that the BH can electrochemically polymerize at the overcharge potential of 4.5-5.5 V to form a layer on the cathode surface, the internal resistance was increased rapidly after the electrode covered with the black electro-polymerization production, and the internal short-circuit was occurred with enough polymerization product, all of these caused to low overcharge current and good overcharge performance, meanwhile, the overcharge performance was increased with the increasing of BH content. However, the cycling performance was deteriorated with an increase in BH content, but not seriously, so the content of additive is ought to be adjusted to practical need in production. (C) 2008 Elsevier B.V. All rights reserved.