In order to improve the electrochemical performance of Si as an anode material for Li ion secondary batteries, a biphasic layer composed of Co and Co3O4 was coated on Si particles by sol–gel method. Compared to Si, Co–Co3O4 coated Si showed the drastic improvement in several electrochemical properties, such as initial coulombic efficiency (55%→88%), cyclic efficiency and cycle life. The comparison between Co–Co3O4 coated Si and heat-treated Si without the coating let us know that the improvement of electrochemical properties only results from Co–Co3O4 coating layer. Little changed cyclic properties (cyclic efficiency and cycle life) of Co–Co3O4 coated Si even at a higher charge–discharge rate insinuated that Co–Co3O4 coating layer plays a crucial role in maintaining the electronic contacts between particles and conducting parts. When trying to measure a thickness variation of the electrodes each containing Si and Co–Co3O4 coated Si as active materials, it was notified that Co–Co3O4 coating layer can accommodate the volume expansion of Si during Li+ insertion, which has its original thickness almost recovered after Li+ extraction.