## 1 Introduction  
Li-ion battery (LIB) technology has attracted extensive research interest, as it offers an efficient mechanism for energy conversion and storage, thus contributing towards decarbonization and energy electrification [1]. LIBs offer very high conversion efficiency between chemical and electrical energy. If the electricity comes from clean and renewable energy resources, LIB technology with sufficiently long cycle life will not further increase the carbon footprint from electricity generation and utilization. Moreover, LIBs can be seamlessly integrated with existing infrastructure and ongoing efforts in the energy sector--for example, the electricity grid, centralized carbon capture and sequestration, etc. Li-ion battery-based energy storage systems include large-scale Li-ion battery packs with additional control and thermal management modules, which enable load-shifting, frequency regulation, long-term storage, and decentralized deployment. In addition, LIB-based energy storage systems facilitate synergistic integration of other technologies toward decarbonization, such as battery regeneration and material recycling. Through the combination of existing grid infrastructure, diversified power generation, centralized carbon capture and sequestration, increased renewable energy, and efficient battery regeneration, a clean and sustainable energy sector can be expected in the future with LIB-based energy storage systems. However, the operation of LIBs is extremely sensitive to temperature. Thermal runaway and subsequent fires can be triggered when LIBs are exposed to abuse conditions such as overheating [2,3], internal short circuit [4,5], collision, and nail penetration [6,7]. Thermal runaway of LIBs has attracted extensive research interest in the recent past. These research efforts include characterization of the thermal stability of battery materials [8,9], battery and pack-level thermal runaway testing [10,11], development of kinetic models of thermal runaway [12,13], optimized cooling system design to mitigate thermal runaway and its propagation [14,15,16,17,18], modelling and simulation of thermal runaway using detailed and reduced-order models [19,20], and venting gas generation and subsequent fires [21,22,23]. Figure 1 shows the thermal runaway mechanism in a typical LIB with a LiCoO\({}_{2}\) cathode, an electrolyte of ethylene carbonate (C\({}_{3}\)H\({}_{4}\)O\({}_{3}\)) and lithium hexafluorophosphate (LiPF\({}_{6}\)), and a graphite anode. The solid electrolyte interphase (SEI) layer is thermally unstable and can decompose when subjected to high temperature or other abuse. The lithium in the anode will then directly contact the electrolyte and form lithium carbonate and ethylene. In addition, the cathode and electrolyte can directly react through the oxygen generated from cathode decomposition. Finally, the electrolyte material carbonate and salt can decompose under elevated temperatures. These reactions are largely exothermic and accompanied by the formation of gases such as ethylene and oxygen, thereby creating a self-sustained, destructive chain of highly exothermic reactions. Understanding the factors that impact the propagation of thermal runaway in a large battery pack remains an important research challenge. Past work has shown that multiple factors including cell-to-cell gap, thermal properties of the interstitial material, and kinetics of the decomposition reactions all play a key role in determining whether thermal runaway propagation occurs or not [24]. However, the role of radiative heat transfer in thermal runaway propagation has remained poorly understood, primarily due to the strong nonlinearity of radiation and its coupling with thermal runaway chemistry. Radiative heat transfer is usually dominant in high-temperature processes, such as combustion, which are, in principle, similar to thermal runaway. In the context of Li-ion cells, radiation is known to play a critical role in increasing the battery temperature and activating thermal runaway reactions [25,26,27,28,29,30].