# 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  
Battery packs are generally consisted of single batteries, in which the quantity of batteries depends on the application ranging from several to thousands. For example, one charge-pal usually contains 2-4 batteries (18,650), while there are more than 7000 batteries for the pack in a Tesla Model S. Consequently, battery pack possesses the thermal hazards of a single battery as described in Section 4.2.1. Whereas, different to a single battery, a battery pack possesses the hazard of thermal failure propagation, that is, the thermal failure of one or several batteries will propagate to the neighboring ones, resulting in catastrophic consequences. The thermal hazard will be heightened during the propagation; hence it is essential to pay attention to the issue of failure propagation within a battery pack [105; 125; 126; 127; 128; 129; 130; 131; 132; 133]. [PERSON] et al. carried out a set of failure propagation experiments in 18,650 packs with a size of \(3\times 3\)[128]. As depicted in Figure 5, the thermal failure of the battery in the lower right corner of the figure was induced by external heating, and then thermal failure propagated with a domino effect within the pack. Typically, the thermal failure of battery pack could be divided into several phases, i.e., the failure of the batteries in the former phase affected the batteries in the next phase and then caused the hierarchical propagation. Besides, they also researched the influence of several key factors such as the SOC, battery gap, failure location and pack size on the failure propagation. Results showed that higher SOC would worsen the propagation behavior of pack, in which the propagation speed grew linearly with the increasing pack SOC. It can be attributed to the fact that battery SOC affects the internal lithium ions distribution which will further influence the chain-reactions during thermal runaway. By increasing the battery gap, it was found that the risk of failure propagation within pack could be greatly reduced and the propagation speed exhibited a square index decline relationship with the battery gap, which was the result of radiation decay with squared distance. Upon further increasing the battery gap, the propagation behavior would eventually be interrupted. If the battery close to the center of pack underwent thermal failure, the failure propagation was revealed to be aggravated significantly since the failure of the central battery would affect the surrounding batteries with the largest scope and lead to the thermal failure of the whole pack in a relatively short time. Moreover, it was exhibited that with the increase of pack size, it would take more time for the failure propagation to reach the outermost phase, i.e., the thermal failure propagation would be delayed. Due to the application scenarios, it is common to see that batteries are generally arranged in various shapes or modes within battery packs. [PERSON] et al. [125] compared the thermal failure propagation of battery packs with various shapes, where the triangle pack, rectangle pack, parallelogram pack, line pack, hexagon pack, and square pack were included. The results of failure propagation speed  
Figure 5: Schematic of thermal failure propagation. are summarized in Figure 6, where the triangular pack and the linear pack are much safer as their propagation speeds are smaller than the others. After combining with the space utilization, it was revealed that the triangular pack might be the best choice of battery module due to its lower propagation hazard and higher space utilization. Additionally, resulting from the disadvantages of traditional PCM such as low conductivity and diffusivity [134; 135], it was found that the failure propagation behavior of a pack wrapped with PCM would be severer. The low conductivity and diffusivity of PCM caused a great deal of heat to be accumulated inside pack, and the close stacking of PCM reduced the heat release, therefore the propagation of thermal failure was accelerated.