**2. Thermal runaway at the single-cell level**

Utilizing a multi-physics and multi-scale model is essential for comprehensively understanding the intricate dynamics of battery performance. Xu and Hendricks [34] developed a model of a lithium-ion battery (LIB) to investigate both its electrochemical and thermal-mechanical processes. The model's accuracy was confirmed through comparison with experimental data obtained from large-format battery cells subjected to diverse load and boundary conditions [35]. The simulation model configuration progresses through several stages:


The multi-physics model is simulated by solving a series of individual physics consecutively, and the obtained results will be passed on to the next module to account for the coupling effect. This is done to ensure that the interdependencies and interactions between different physical phenomena within the battery are appropriately accounted for. This physics focuses on different areas such as fluid dynamics, heat transfer, and solid mechanics to individually address heat generation, thermal expansion, and structural deformation.

**Figure 1(a)** showcases the outcome of the overcharging simulation, providing insights into the temperature distribution within the lithium-ion battery (LIB). By the end of the time-dependent simulation, specifically at 3960 seconds, the highest temperature is localized at the core of the cell, gradually diminishing toward the outer circumference.

**Figure 1(b)** presents the simulation results under overcharging conditions, delineating the structural deformation and stress distribution within the LIB cell. At 3960 seconds, signifying the conclusion of the time-dependent simulation, stress concentration emerges along the junction between the upper and side walls of the cell, attaining its maximum intensity. Moreover, **Figure 1** highlights that the upper portion of the LIB cell experiences the most pronounced deformation, in line with empirical observations. Additionally, the simulation indicates that the entire battery cell expands due to heightened internal pressure, as depicted in **Figure 1**.

**Figure 1(c)** exhibits the outcome illustrating the temperature distribution within the LIB cell, with a focus on the overheating test simulation where exothermic

#### **Figure 1.**

*(a) Temperature distribution of the LIB in the overcharging simulation at 3960 s; (b) stress distribution and structural deformation of the LIB in the overcharging simulation at 3960 s; (c) temperature distribution of the LIB in the oven heating test without exothermic reaction at 8560 s; (d) stress distribution and structural deformation of the LIB in the oven heating test without exothermic reactions at 8550 s.*
