The electrochemical characterization and overcharge protection mechanism of cyclohexyl benzene as an additive in electrolyte for lithium ion battery was studied by microelectrode cyclic voltammetry, Galvanostatic charge–discharge measurements and SEM observation on both the cathode and separator of the overcharged cells. It was found that when the battery is overcharged, cyclohexyl benzene electrochemically polymerized to form polymer between separator and cathode at the potentials lower than that for electrolyte decomposition. The polymer blocks the overcharging process of the battery. The additive causes a small capacity loss and impedance increase in a real cell, but that can be mitigated if the operating voltage is much lower than the polymerization voltage.