A relation between enhanced Li ion transfer and the improvement in electrochemical performance of a Si–Cu–carbon composite

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 0.1 C, 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.