Assessment of Thermal Runaway propagation in lithium-ion battery modules with different separator materials

The current work aims to understand and model thermal runaway (TR) events in lithium-ion (LIB) 18650 cells within the context of aircraft battery applications. The primary goal is to comprehend the phenomenon and discuss strategies for mitigating its consequences during aircraft operation. TR is modeled using Arrhenius kinetic equations and is implemented in both lumped parameters (Matlab SimulinkTM), and 3D CFD simulations (Ansys FluentTM) using User Defined Functions. To validate the thermochemical model, cells are initially simulated in an oven test, where a cell is exposed to a temperature-controlled atmosphere, triggering exothermic reactions. With a strong correlation between lumped parameters and 3D models, the latter is simulated under battery module installation conditions. An internal short-circuit is then implemented within the cell to observe how thermal runaway is triggered by an internal heat source. The trigger cell is subsequently placed in a battery module assembly to assess the dominant heat transfer modes and the likelihood of TR induction from one cell to its neighbors. This work's main objective and innovation are to compare different materials in which cells are immersed while observing the main heat transfer parameters for each material. Three conditions are tested: ceramic paper fiber and G7 as solid separators, and no separator material, where air fills the gaps between cells. The analysis of heat transfer modes reveals radiation's dominance in the case of air interstice, suggesting the possibility of using a special coating to reduce the cell surface emissivity as an alternative to decrease the likelihood of TR propagation. Thus, two values of surface emissivity were tested in the case of air. Considering a cell triggered by an internal short-circuit, a thermal runaway temperature spike is not observed in any of the four cases. However, the air interstice case with regular emissivity is the most critical one, with the closest cell reaching peak temperatures as high as 136 °C in 490 s. The ceramic paper fiber is considered the best separator material, as it postpones the temperature increase in the closest cell while also being lighter than G7. The results and discussions concerning heat propagation presented herein can serve as guidelines for developing strategies to mitigate thermal runaway in battery modules.