Today's Li-ion battery stringent requirements include high electric currents, large format cells and possibly the use of carbon based current collectors, which would enhance the electrochemical-thermal non-uniformities in the cell. Although a number of models have been implemented in order to study such non-uniformities, no efforts have been made to describe the distribution of side reaction rates and their effect on aging distribution. To fill that gap, we developed a pseudo-3D porous electrode electrochemical model that incorporates via first principles the solid-electrolyte interphase (SEI) growth on the anode, which is thought to be a dominant aging mechanism. The model was used to simulate the cyclic behavior of a large format (40 Ah) Li-ion polymer pouch cell with the assumption of a carbon based positive current collector. It was found that SEI growth localization would form in the electrode thickness and planar dimensions during discharging, but would be destroyed during the subsequent charging, so that the associated aging would be uniform upon cycling. This suggests that computationally efficient lumped models could be used to describe the cell aging process associated with the SEI growth on the graphitic anode, which would be ideal for onboard implementations such as in electric vehicle applications. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.022302jes] All rights reserved.