## 3 Results and Discussions  
### General Features of Radiation-Induced Thermal Runaway  
Figure 5a shows the evolution of maximum temperature of Cell 2 with different temperature of Cell 1, T\({}_{1}\). The results show that for T\({}_{1}\) > 602 K, thermal runaway occurs in Cell 2, as indicated by the sharp increase in T\({}_{2}\). For all these cases, the higher the T\({}_{1}\), the earlier the thermal runaway is triggered in Cell 2, as expected. When T\({}_{1}\) is increased to 900 K, the thermal runaway delay time is reduced to around 1 min. There is a sharp change in the nature of curves in Figure 5a when T\({}_{1}\) is below a threshold value of 602 K, wherein it is found that thermal runaway is not triggered in Cell 2, even at much longer times. The maximum temperature of Cell 2 in such cases is found to eventually reduce to around 450 K, where a quasi-thermal equilibrium is reached due to the balance between the net radiation heat flux and chemical heat generation. Figure 5b presents plots of T\({}_{2}\) as well as various dimensionless concentrations for two cases very close to the threshold found in Figure 5a: T\({}_{1}\) = 601 K and T\({}_{1}\) = 602 K. As shown in Figure 5b, the dimensionless SEI concentration is completely consumed around 2000 s for both T\({}_{1}\) = 602 K and 601 K cases, much earlier than the complete consumptions of other species, which occur at the instant of thermal runaway. Results suggest that the four chemical reactions may occur at different stages of thermal runaway, and their occurrence can be decoupled depending on the thermal runaway time scale. Evolution of the other dimensionless concentrations shows distinct behaviors, with much stronger consumption under thermal runaway conditions, as expected. Figure 5: (**a**) Maximum temperature of Cell 2 as a function of time for different T\({}_{1}\). (**b**) Comparison of evolution of average dimensionless concentration and temperature for two cases close to the thermal runaway threshold T\({}_{1}\) = 601 K (solid lines) and T\({}_{1}\) = 602 K (dashed lines). By defining the onset of thermal runaway as the instant with the maximum rate of temperature rise, Figure 6a,b plot spatial distributions of temperature and net heat generation rate, respectively, within Cell 2 at the onset of thermal runaway. It can be seen that the maximum temperature and peak heat release occurs in the vicinity of the surface of Cell 2 for high triggering temperature T\({}_{1}\) due to the large radiation heat flux from Cell 1. For extremely high T\({}_{1}\), the surface temperature of Cell 2 will rise so rapidly that thermal runaway will occur at or very close to the surface. Given that thermal runaway for high T\({}_{1}\) occurs so rapidly, as shown in Figure 5a, it is expected that radiation plays the sole dominant role for temperature rise during thermal runaway. At lower values of T\({}_{1}\), the location corresponding to the maximum temperature and peak heat release gradually moves to the internal region of the battery, indicating increased influence of heat conduction and heat generation from the chemical reactions during the much longer thermal runaway. For the threshold case with T\({}_{1}\) = 602 K, the peak heat release occurs very close to the center of Cell 2. The distinction between high T\({}_{1}\) and low T\({}_{1}\) behavior shown in Figure 6 can be explained by the increase of radiation time scales at reduced temperatures, where thermal conduction--a much slower, diffusive process--tends to play an increasingly significant role. To further characterize the different thermal chemical processes for thermal runaway triggered by high and low temperature radiation, Figure 7 shows the evolution of dimensionless concentration and temperature in Cell 2 for a relatively high triggering temperature of T\({}_{1}\) = 900 K. Compared to the low triggering temperature case with T\({}_{1}\) = 602 K shown in Figure 5b, the thermal runaway is triggered much quicker.