## 4 Conclusions  
Based on carefully designed and validated calculations and numerical simulation, the interplay between radiation and pre-runaway chemistry in battery thermal runaway propagation is identified in the current study. Results evaluate the role of radiative heat transfer from a hot trigger cell to colder neighboring cells in the propagation of thermal runaway. Results indicate that radiation-induced thermal runaway exhibits intrinsic differences between low and high trigger temperature regimes; at high triggering temperatures, thermal runaway is triggered very rapidly and close to the battery surface, and the thermal runaway reactions occur simultaneously in space and time. In this case, radiation plays a solely dominant and direct role. In contrast, at a low trigger temperature, thermal runaway  
Figure 16: Radiation heat flux in an orthogonal 3 \(\times\) 3 battery array with localized hot spot. is triggered much more slowly and deeper in the cell, and the thermal runaway reactions can separate in spatial and temporal domains due to the depletion of certain species. By investigating the evolution of the net radiation flux during thermal runaway, we have found that the net radiation heat flux has a cross-over instant, beyond which the net radiation heat flux becomes negative. Therefore, radiation can either facilitate or mitigate thermal runaway. For high temperature conditions, the cross-over state is very close to the onset of thermal runaway--as such, radiation constantly facilities thermal runaway. In contrast, under low temperature conditions, the cross-over state can be far ahead of the onset of thermal runaway; therefore, radiation facilitates thermal runaway in the earlier stage and mitigates thermal runaway in the later stage. Results from this work identify the non-monotonic role of radiation in thermal runaway triggering and provide direct insights into the complex interaction of two highly nonlinear phenomena: radiation heat transfer and chemical reactions during thermal runaway. The time-integrated radiation heat flux exhibits a non-monotonic dependence on the triggering temperature. The underlying reasons can be twofold: for one thing, the delay time of thermal runaway is substantially longer for low temperature conditions, albeit with a lower radiation heat flux; for another, the pre-runaway chemistry plays an increasingly significant role in increasing the battery temperature under lower triggering temperature conditions, which must be considered for radiation-induced thermal runaway propagation. We analytically evaluated the view factor, accounting for the equal-spaced homogenous arrangement and the orthogonal arrangement of cells in practical battery packs. It was shown that the blocking effect in the equally spaced homogenous arrangement can decrease the radiation heat flux by as much as 8.2%. While in the orthogonal arrangement, a threshold cell separation distance exists, within which the closest cells receive higher radiation while beyond which the corner batteries receive higher radiation. The results indicate that radiation-induced thermal runaway propagation in a pack is not necessarily along the trajectory among closest cells. Radiation effects from a localized hot spot with varying sizes were also investigated, showing the increased focusing effects and necessitating specific thermal runaway prevention methods. In the context of relatively little past work on radiative heat transfer during thermal runaway propagation, this work makes fundamental contributions in understanding the role of radiation and its interplay with chemical reactions that cause thermal runaway. The insights gained from this work may help develop strategies for reduced thermal runaway propagation and, hence, safer battery packs. Conceptualization, P.Z. and A.J. ; methodology, P.Z., H.G. and L.Z. ; software, L.Z. ; validation, P.Z., H.G. and A.J. ; formal analysis, P.Z. and L.Z. ; investigation, P.Z., Y.C. and L.Z. ; resources, P.Z. and L.Z. ; data curation, Y.C. and L.Z.