Electrode-Level Modeling of Thermal Decomposition Reactions as a Foundation for Multiscale Thermal Runaway Analysis in Lithium-Ion Batteries

Thermal runaway in lithium-ion batteries represents a critical safety challenge, particularly in high-voltage battery systems used in electric vehicles and stationary energy storage. A comprehensive understanding of the multi-scale processes that initiate and propagate thermal runaway is essential for the development of effective safety measures and design strategies. This study provides a structured theoretical overview of the thermal runaway phenomenon across four hierarchical levels: electrode, single cell, module, and high-voltage battery system. At the electrode level, thermal runaway initiation is linked to electrochemical and chemical degradation mechanisms such as solid electrolyte interphase decomposition, separator breakdown, and internal short circuits. These processes lead to highly exothermic reactions that, at the cell scale, can result in rapid temperature increases, gas generation, and overpressure. On the module and system levels, thermal runaway can propagate through thermal and mechanical coupling between neighboring cells, influenced by layout, cooling design, and enclosure properties. The core contribution of this study lies in a detailed modeling approach that focuses exclusively on the chemical and thermal decomposition reactions occurring at the electrode scale. These reactions form the foundational layer of a broader simulation framework to be developed in subsequent work. A semi-empirical model is proposed, capturing key phenomena such as electrolyte decomposition, solid electrolyte interphase breakdown, and active material reactions. The model integrates thermal conduction and heat generation from exothermic reactions to characterize the local temperature evolution during the early stages of thermal runaway. By isolating and accurately representing these fundamental decomposition pathways, this modeling approach provides a critical building block for future extensions toward higher-scale thermal runaway simulations. It offers valuable insights into the onset mechanisms of thermal instability, supporting battery design optimization and risk assessment from the ground up.