The galvanostatic charge and discharge of a silicon composite electrode/separator/lithium foil cell is modeled using porous electrode theory and concentrated solution theory. The one-dimensional (flow) model is solved with COMSOL 3.5a software. Porosity changes that accompany the large molar volume changes in the lithium-silicon electrode during operation are included and analyzed. The concept of reservoir is introduced for lithium-ion cells to accommodate the displaced electrolyte (i.e. the liquid phase). Simulation results quantitatively show the importance of a high initial porosity in silicon electrodes for better utilization of active material, especially at low rates. At higher rates, the utilization becomes similar for both thicker, porous electrodes and thinner, less porous electrodes. The insensitivity to porosity and thickness at high rates is attributed to the slow electrode kinetics for the Li-Si system. Therefore, the application dictates the optimum thickness and porosity of the electrode. Moreover, the importance and need for faster electrode kinetics relative to transport limitations is quantitatively shown. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3589301] All rights reserved.