Simulating onset and evolution of thermal runaway in Li-ion cells using a coupled thermal and venting model

Abstract

A coupled, thermal and gas generation/venting model has been developed for simulating the onset and evolution of thermal runaway in 18650 format Li-ion battery cells. The model simulates heat and gas generation during external heating of an electrically isolated cell that results in thermal runaway. Gas generation within the cell leads to pressure build up until the point at which the vent mechanism opens and relieves the internal pressure. Compressible flow of gases is modeled through the vent cap as a function of pressure ratio across the vent. The energy balance of the battery cell includes: heat generated from decomposition reactions and electrical short, external heat transfer to the surroundings, heat absorbed with vaporization and melting processes, as well as the energy loss as material is vented from the cell. The model was able to capture features of the temperature evolution of the battery cell well and generate detailed information about the progression of thermal runaway. The model was exercised to simulate time-to-venting and time-to-thermal-runaway for various changes in cell design parameters such as: amount of free liquid electrolyte, external convection coefficient, and electrolyte evaporation rate after vent opening.