Safety has become an increasingly pressing issue in large-format, energy-dense Li-ion batteries for automotive applications. Among various abusive scenarios for Li-ion batteries, internal short-circuit is most dangerous and has been the root cause for several highly publicized catastrophic accidents in recent years. Nail penetration and crush tests are commonly used as experimental proxy for internal shorting, but fail to truly emulate the internal short-circuits seen in field accidents. Also, experimental methods only give a simple pass/fail result, providing little insight into fundamental mechanisms governing the battery thermal and electrochemical response during internal shorting. In this study, a 3D electrochemical-thermal coupled model is used to scrutinize the internal short-circuit process in a large-format Li-ion cell with a stacked-electrode design. The model reveals the 3D electrochemical and thermal processes inside the battery cell during internal shorting. A parametric study is carried out, showing that the short-circuit resistance and the number of shorted electrode layers have the most significant influence on cell electrochemical and thermal behavior. Novel experimental methods, designed to precisely control these key parameters, must be developed in order to advance the understanding and improvement of Li-ion battery safety. The Author(s) 2015. Published by ECS. All rights reserved.