The use of electric vehicles has significantly increased over the last decade. To face this challenge, Lithiumion (Li-ion) batteries must resist to high electrical currents, generating high thermal stresses that degrade their performance and lifespan. From a safety point of view, a key issue is to prevent the thermal runaway of battery cells. Safety requirements are necessary to demonstrate that in the case of a cell thermal runaway there is neither propagation to the neighboring cells nor propagation to the whole battery pack. The thermal management of battery packs is therefore a crucial element in dealing with these industrial challenges. Immersion cooling is an emerging solution, which promotes heat transfer, especially during two-phase flow, used in battery thermal management systems. The present study aims at simulating under "ANSYS Fluent" a postulated benchmark thermal runaway scenario in a Li-ion battery module which cooling system is based on a dielectric liquid direct contact immersion. The purpose of this simulation is to study the transient propagation of thermal runaway in the submerged module by the HFE 7100. The results show that the heat transfer of the liquid refrigerant mainly controls the temperature rise in the battery module. Phase change ensures that the neighboring cells temperatures stay under the saturation all along the duration of the transient. This result has been numerically compared to other cooling methods such as using an insulating inter-cell material, or on the contrary a solid with a high conductivity, or single-phase cooling. Calculations show that two-phase flow cooling is the most efficient approach with regard to thermal runaway and propagation risks.