The electrochemical properties and working mechanisms of dimethoxydiphenylsilane (DDS) as an electrolyte additive for overcharge protection of lithium ion batteries have been investigated by microelectrode cyclic voltammetry, galvanostatic charge-discharge cycling, and SEM observation on both the cathode and separator of the overcharged cells. DDS can be electrochemically polymerized when the cell was overcharged to 4.9 V (vs. Li/Li+), resulting in a polymer layer on the electrode and the separator, which increases the internal resistance of the cell and postpones the voltage rising up during the overcharge process. Therefore, the safety issue of lithium-ion batteries during overcharge state can be significantly improved by utilization of DDS. Furthermore, incorporation of DDS does not significantly degrade the performance of the cell as there is only a small capacity loss during the normal charge–discharge process.