During lithiation, the electrode of the lithium battery undergoes a large volume expansion, which exerts a compressive stress on the separator. The porosity of the separator will be dramatically reduced under a compressive state, which could be determined by the Carroll and Holt's pore-collapse relation. The reduced porosity increases the tortuosity of the separator as described by Bruggeman relation, so that the inevitable separator conductivity loss occurs. The physical process is analyzed in this work. Both porosity reduction and conductivity loss can be quantitatively described. The corresponding theoretical results also show a good agreement with the experimental data. (C) 2014 The Electrochemical Society. All rights reserved.