# A Review on the Thermal Hazards of the Lithium-Ion Battery and the Corresponding Countermeasures
## 4 Thermal Hazards of Lithium-Ion Battery
### Thermal Hazards of a Single Battery and Battery Pack
#### 4.2.2 Thermal Hazards of a Battery Pack
Besides, traditional PCM was flammable and it would be ignited by external heating which further aggravated the thermal failure propagation. In short, it is demanding to avoid the pack wrapped with traditional PCM being exposed to external heating or high temperature, especially for the flammable PCM. On the other hand, more work should be done to improve the thermal conductivity, diffusivity and fire resistance of PCM. Apart from the factors above, it was determined that tab configuration also had a huge influence on the failure propagation behavior of the pack as a result of the heat transfer from the tab. According to the work of [PERSON] et al. [126], it was revealed that a branched style of tabbing as shown in Figure 7 improved the voltage retention as well as the safety of the pack over a serpentine style of tabbing as the shorted trigger battery was electrically better isolated from the rest of the pack when the tabs were branched. [PERSON] et al. [127] examined the failure propagation behavior of packs consisting of cylindrical and pouch batteries respectively, where the thermal failure was induced in a single battery. They observed that cylindrical batteries were less prone to propagation compared to the pouch batteries owing to the limited contact between neighboring batteries. On the other hand, numerical simulation methods were also applied to investigate the failure propagation behaviors of battery packs [136; 137; 138; 139; 140; 141; 142; 143; 144]. [PERSON] et al. [136] established a 3D thermal runaway propagation model for a large format LIB pack based on the energy balance equation. They proposed that the thermal failure propagation could be postponed by increasing the thermal runaway triggering temperature, which was generally reflected as the collapse temperature of battery separator. Furthermore, it was revealed that reducing the total electrical energy released during thermal  
Figure 6: The propagation speeds of packs with various shapes where the numbering 1, 2, 3, 4, 5, and 6 represents the triangle pack, rectangle pack, parallelogram pack, line pack, hexagon pack and square pack respectively. runaway and enhancing the heat dissipation level were beneficial to suppress the thermal failure propagation. [PERSON] et al. [137; 138] presented a numerical model to analyze the heat propagation in a custom-made battery pack in which venting of the electrolyte and jellyroll contents were considered. Their results demonstrated that the heat dissipation due to the ejection of electrolyte and jellyroll contents contributed significantly to the failure propagation. In addition, they declared that the combination of small insulating layers wrapped around the batteries and a conductive heat sink were beneficial to the design of a safer battery pack which could mitigate the thermal failure propagation. In short, the thermal failure propagation within a battery pack is severe and the corresponding hazard will be greatly enhanced during the propagation, which may be influenced by several key parameters of the LIB including the SOC, battery gap, package and so on. To decrease the thermal hazard of battery pack, some methods such as improving the separator property, enhancing the heat dissipation, adding an insulation board and decreasing the SOC may be considered.