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-Co3O(4) coating layer can accommodate the volume expansion of Si during Li+ insertion, which has its original thickness almost recovered after Li+ extraction. (c) 2006 Elsevier Ltd. All fights reserved.