Prevention of thermal runaway and its propagation remains a technical barrier to the application of lithium-ion batteries. To mitigate thermal runaway in lithium-ion battery packs, heat sinks have been designed using various materials, such as phase-change materials or metal plates. In this study, aluminum plates were assembled into battery modules as heat sinks and the effect of plate thickness on thermal runaway mitigation was numerically investigated. A three-dimensional integrated multiphysics model was validated and calibrated with experimental data. It identified the mechanism and sequence of thermal runaway propagation in detail. Thermal mass and contact resistance are found to be the key design parameters for preventing thermal runaway propagation for the studied battery module configuration. In addition, this study provides further insights into the design of aluminum heat sinks for lithium-ion battery packs.