Si-carbon composite prepared by mechanical milling showed good cyclic capacity retention until the utilization of Si was limited below 32%, whereas the retention of a Si-Cu-carbon composite obtained by two-step mechanical milling was maintained up to 55%. A comparison between the first charge curves of a Si-carbon composite and a Si-Cu-carbon composite at 0.1C, indicated that the Si-carbon composite underwent a much higher polarization than the Si-Cu-carbon composite, leading to the difference in utilization of Si. Impedance spectroscopy let us confirm that the electrochemical alloying between Si and Li+ is much easier in the Si-Cu-carbon composite than in the Si-carbon composite. The superiority of the Si-Cu-carbon composite in kinetics enabled its electrode to have a more homogeneous Li+ concentration after Li+ insertion. Because this phenomenon means that the Si-Cu-carbon composite has a more homogeneous volume expansion than the Si-carbon composite, the disparity in electrochemical performance between the Si-carbon composite and the Si-Cu-carbon composite was attributed to enhanced Li+ transfer in the Si-Cu-carbon composite. (c) 2006 Elsevier B.V. All rights reserved.