In this work, we present a mathematical model and associated experiments for describing the performance of porous electrodes under high rates of charge and discharge. By increasing the physical accuracy of porous battery modeling, we hope to enable improved design of cells for high-power applications, such as hybrid and plug-in-hybrid electric vehicles. The model includes an improved accounting of electron transfer between different-size particles or materials, including the conductive carbon additive, as well as a modified Bruggeman relation to handle liquid-phase ion transport through porous electrodes. Both types of resistance, electronic and liquid-phase ionic, are strongly coupled to particle properties, including size and volume-fraction distributions. The model is used to better understand the cause for decreased utilization of active material for relatively highly loaded lithium-ion cathodes at high discharge rates. It was found for LixCoO2 cathodes with loading around 1.6 mAh/cm(2) that voltage losses at 1C discharge rate are mostly governed by local interparticle resistances. At 5C discharge rate, diffusional resistance in the liquid electrolyte had the greatest influence on cell performance. (c) 2007 The Electrochemical Society.