The passivating solid electrolyte interphase (SEI) layer forms at the surface of the negative-electrode active material in lithium-ion cells. A continuum-scale mathematical model has been developed to simulate the growth of the SEI and transport of lithium and electrons through the film. The model is used to estimate the film growth rate, film resistance, and irreversible capacity loss due to film formation. We show that film growth at the negative electrode is faster for charged batteries than for uncharged batteries and that higher electron mobility in the film leads to faster growth. If electron mobility is low, the rate of film growth is limited by transport of electrons through the film, and the rate decreases as the thickness increases. We examine the dependence of film resistance upon both film thickness and defect concentration in the film. We also show that the concentration polarization in the film increases as it grows at open circuit, even though the concentration gradient may decrease. (C) 2004 The Electrochemical Society.