Silicon and polypyrrole composites with different microstructures of polypyrrole were synthesized using high-energy mechanical milling. The nano-wires polypyrrole and micro-particles polypyrrole act as a matrix to hold the active silicon grains as they repeatedly alloy with lithium during the operation of a lithium ion battery. The composite containing 10 wt.% polypyrrole nano-wires exhibits a relative better reversibility and cycle life than bare silicon, while the silicon/micro-particles polypyrrole composite shows no improvement. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.