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Modeling of Thermal Runaway Propagation in a Pouch Cell Stack Serhat Bilyaz, Kevin C. Marr* and Ofodike A. Ezekoye, Mechanical Engineering, University of Texas at Austin, Austin, TX 78712, USA Received: 1 October 2019/Accepted: 29 February 2020

Abstract. Characterizing propagation of a thermal runaway hazard in cell arrays and modules is critical to understanding fire hazards in energy storage systems. In this paper, the thermal runaway propagation of a pouch cell array has been examined by developing a 1D finite difference model. The results are compared with experimental data. First, the thermal runaway reactions found in the literature are reviewed. Using the insight of the literature review and premixed flame propagation theory, a global first order Arrhenius type reaction is characterized. While applying the multiple kinetic reactions, an ‘‘onset temperature’’ of the combustion reactions has been determined by performing an induction time analysis on ethylene. The propagation speeds are predicted with a 1D finite difference model by using both multi-reaction kinetics and one step reduced-order kinetics. These results are in a good agreement with experiments for both 10 Ah and 5 Ah cell arrays. Keywords: Thermal runaway propagation, Pouch cell array, Reaction kinetics, Ignition time, Thermal abuse

1. Introduction Lithium-ion batteries are now widely used in various applications such as electronic devices, electric vehicles and large energy storage systems. Despite the increasing usage, their thermal-runaway failure and fire and explosion hazard behavior has not been completely understood. In energy storage applications, multiple cells are used to form compact modules which are designed for capacity and the power requirements of the system. However, the propagation of thermal runaway in such systems is not well characterized. Fundamentally, thermal runaway is the process in which the energy release rate from exothermic reactions exeeds the rate at which energy is lost from the system through heat transfer. This process is a cell-specific process that can trigger a propagation process between adjacent cells. Thermal runaway propagation may occur by different heat transfer mechanisms in the module depending on the design. Typically, modules that use prismatic and pouch cells contain stacks of cells that are in contact with each other. Therefore, it is critical to understand the

* Correspondence should be addressed to: Ofodike A. Ezekoye, E-mail: [email protected]

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Fire Technology 2020 heat transfer processes through the cells that affect thermal runaway propagation, called thermal propagation in the remainder of the paper. Various experimental and numerical studies have been conducted for thermal propagation through cell arrays. Lopez et al. [1] examined the thermal propagation in cylindrical and prismatic cell stacks with various electrical configurations. They concluded that increasing the spacing between cells or inserting insulation material around the triggered cell decreases the thermal propagation risk and