# Investigation into the Fire Hazards of Lithium-Ion Batteries under Overcharging
## 3 Results and Discussion
### Battery Surface Temperature  
The histories of surface temperatures for all the configurations are plotted in Figure 7, where the five stages divided in Section 3.2 can also be observed. The variations of surface temperature during tests were discussed to analyze the fire behaviors of LIB. At the heating stage, the surface temperature rises smoothly due to continuous external heating, and the packing of the battery melts gradually. Subsequently, it can be seen that the safety vent cracks were accompanied by some smoke ejected. The smoke ejected is ignited later and a flame appears, followed by an accelerated temperature rise. Hereafter, thermal runaway occurs and it can be observed that the temperature increases sharply to a peak. Figure 5: Burning process of LIBs in tests: (**a**) 4.2 V NMC; (**b**) 5.0 V NMC; (**c**) 4.2 V LFP; (**d**) 5.0 V LFP. Figure 6: Photographs of batteries before and after burning. Key parameters, such as the time to safety vent cracks, the temperature to safety vent cracks, the time to ignition, the temperature to ignition, the time to thermal runaway, the temperature to thermal runaway, and the maximum temperature can be extracted from the curves and are listed in Table 2. Compared to the normal LIB, the overcharged LIB showed earlier safety vent cracks, ignition, and thermal runaway. Meanwhile, with the increasing voltage, the fire emerges earlier and the maximum surface temperature has an uptrend. Namely, an LIB overcharged beyond the cut-off voltage is more prone to be unstable and lose control. The phenomenon will become worse for a highly overcharged LIB. On the other hand, after comparing the details of these two types of LIBs, it is revealed that the NMC fails earlier than the LFP under external heating conditions, which is consistent with a previous report [19]. Figure 7: The typical curves of LIB surface temperature during tests: (**a**) NMC; (**b**) LFP. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**LIB Type** & **Voltage/V** & \begin{tabular}{c} **Time to** \\ **Cracks/s** \\ \end{tabular} & \begin{tabular}{c} **Temperature to** \\ **Cracks/°C** \\ \end{tabular} & \begin{tabular}{c} **Time to** \\ **Ignition/s** \\ \end{tabular} & \begin{tabular}{c} **Temperature to** \\ **Ignition/°C** \\ \end{tabular} & \begin{tabular}{c} **Time to Thermal** \\ **Runaway/s** \\ \end{tabular} & \begin{tabular}{c} **Temperature to Thermal** \\ **Runaway/s** \\ \end{tabular} & \begin{tabular}{c} **The Maximum** \\ **Runaway/s** \\ \end{tabular} \\ \hline \multirow{4}{*}{\
\begin{tabular}{c} NMC \\ 5.0 \\ \end{tabular} } & 4.2 & 197 & 127 & 299 & 158 & 317 & 232 & 553 \\  & 4.5 & 196 & 129 & 230 & 162 & 280 & 226 & 606 \\  & 4.8 & 191 & 133 & 222 & 160 & 273 & 228 & 630 \\  & 5.0 & 190 & 132 & 219 & 163 & 262 & 230 & 673 \\ \hline \multirow{4}{*}{
\begin{tabular}{c} LEP \\ \end{tabular} } & 4.2 & 201 & 115 & 300 & 182 & 358 & 229 & 571 \\  & 4.5 & 202 & 115 & 266 & 175 & 310 & 218 & 585 \\ \cline{1-1}  & 4.8 & 185 & 121 & 299 & 178 & 250 & 224 & 630 \\ \cline{1-1}  & 5.0 & 181 & 127 & 251 & 181 & 280 & 227 & 647 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specifications of the battery surface temperature. As shown in Equation (3), time to ignition (TTI) depends on the release rate of combustible gases inside the battery and the critical mass flow rate when gases are ignited [20]. Except for the radiation strength, TTI is affected by the pyrolysis kinetics of materials [20; 21]. With the increase in voltage, the electroactive materials of the battery will become more reactive, and therefore it is easier to reach the critical condition of ignition. \[\frac{1}{t_{ig}}=\frac{1}{\sqrt{\pi}}\frac{q_{e}^{\prime\prime}-aq_{\sigma \tau}^{\prime\prime}}{\sqrt{k\rho C(T_{ig}-T_{0})}}, \tag{4}\]  
where \(q_{e}^{\prime\prime}\) is the radiant heat flux, \(q_{\sigma\tau}^{\prime\prime}\) is the initial heat flux for ignition, \(k\rho C\) is the material thermal inertia, and \(T_{ig}\) and \(T_{0}\) are the ignition temperature and initial temperature, respectively.