# Thermal Failure Propagation in Lithium-Ion Battery Modules with Various Shapes
## 3 Results and Discussion  
### Triangle Module  
The typical surface temperature curves of batteries in triangular modules were plotted in Figure 4. Generally, the thermal failure process of LIBs could be divided into several representative stages: (a) heating stage, (b) gases releasing stage, and (c) thermal runaway stage. At the heating stage, the surface temperature of the batteries rose stably under the effect of the heater. With the continuous growth of temperature, the chemical reactions inside the batteries occurred, accompanied by the generation of numerous gases and heat. The inner pressure of the batteries increased rapidly, leading to the opening of the safety vent. Hereafter, the batteries entered the gases releasing stage, where a large amount of combustible gases were released and the battery temperature increased unceasingly. When the inner temperature and pressure approached the critics, the thermal runaway of the batteries happened with large quantities of gases ejected from the batteries. The combustible gases premixed with air and were further ignited by the external high temperature, resulting in continuous combustion. Subsequently, it could be observed that the surface temperature had a sharp increase to the peak in the range of 500-600 \({}^{\circ}\)C. Between the gases releasing and the thermal runaway, there existed an interval which could be utilized to escape or take measures to prevent the deterioration to failure. Finally, the combustion extinguished with the consumption of combustibles and the surface temperature declined gradually. In addition, the photographs of packs after tests are also presented in Figure 4. It could be seen that the plastic shell of the batteries was entirely burned away after tests and there were some electrolyte residuals on the surface of the batteries. Besides, it was clear that the positive pole of the batteries was carbonized totally. The nickel belts joining the batteries were ripped because of the violent ejection. For the thermal failure propagation in the triangular module, an evident domino effect could be observed, and the two phases of thermal failure were demonstrated. For dominoes, where the collapse of the previous card would lead to the collapse of the next one, the thermal failure in battery modules also exhibited a similar phenomenon. The two batteries close to the heater appeared to release gases at first and then took thermal failure at about 344 s. The thermal failure of the first phase caused an accelerated temperature rise of the batteries in the second phase. Hereafter, their gases releasing emerged and the thermal failure took place at around 555 s. Namely, the thermal failure propagated from the first phase to the second phase. From the heater to the second phase of batteries, the propagation distance could be estimated by the geometry (i.e., \(\sqrt{3}\)d), as shown in Figure 5. Therefore, the propagation speed could be calculated as 0.056 mm/s. On the other hand, Figure 4a,b reveals the surface temperature curves of batteries in modules with and without discharging treatment. After comparing, it can be found that the thermal failure propagation in the battery module with discharging treatment was worse than that without discharging \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Group No.** & **Test No.** & **Module Shape** & **SOC** & **With Discharging Treatment** \\ \hline \multirow{5}{*}{1} & 1 & triangle & & \\  & 2 & rectangle & & \\  & 3 & parallelogram & & \\  & 4 & line & & \\  & 5 & hexagon & & \\  & 6 & square & & \\ \hline \multirow{5}{*}{2} & 1 & triangle & & \\  & 2 & rectangle & & \\  & 3 & parallelogram & & \\  & 4 & line & & \\  & 5 & hexagon & & \\  & 6 & square & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The experimental configurations. treatment. For the latter, its thermal failure corresponding to the first and second phases appeared at 387 s and 575 s, respectively. Both of them were more hysteretic than that of the former. Correspondingly, the propagation speed was 0.054 mm/s.