# Investigation into the Fire Hazards of Lithium-Ion Batteries under Overcharging
## 3 Results and Discussion  
### Thermal Behaviors of LIB during Charging and Discharging  
Figure 3 shows the discharging curves of LIBs subjected to different cut-off voltages. They were discharged at 2 C rate to the discharging cut-off voltage (2.5 V). From the curves, it can be seen that the discharging process of LIB can be divided into three segments. At the initial segment of discharging, there exists a short platform where voltage drops sharply. With the increase of voltage, the platform will be more obvious and last longer. The duration of this segment is short and will not exceed 100 s generally. After that, the battery enters a stable discharging segment where voltage decreases at a relatively stable rate until the battery gets into the epilogue of discharging. At the third segment, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**LIB Type** & **Test No.** & **Voltage/V** & **Capacity/mAh** & _SOC/\%_ \\ \hline \multirow{4}{*}{NMC} & 1 & 4.2 & 956.9 & 73.6 \\  & 2 & 4.5 & 1334.6 & 102.7 \\ \cline{1-1}  & 3 & 4.8 & 1507.2 & 115.9 \\ \cline{1-1}  & 4 & 5.0 & 1538.0 & 118.3 \\ \hline \multirow{4}{*}{LFP} & 1 & 4.2 & 906.2 & 69.7 \\ \cline{1-1}  & 2 & 4.5 & 1302.0 & 100.2 \\ \cline{1-1}  & 3 & 4.8 & 1416.5 & 109.0 \\ \cline{1-1}  & 4 & 5.0 & 1501.3 & 115.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The experimental configurations. Figure 2: **(a)** Schematic of experimental setup; **(b)** the thermocouples setup around the battery (TC1 was used to detect the battery surface temperature; TC2, 3, 4, 5 were used to detect the flame temperature). battery appears another platform and the voltage declines sharply. Soon after, the discharging ends when the voltage falls to 2.5 V. From the curves, it is found that the initial discharging voltage increases with the growth of cut-off voltage. The battery with higher cut-off voltage possesses higher initial discharging voltage. Moreover, the higher the cut-off voltage, the longer the discharging time to reach 2.5 V. Namely, the overcharged battery stores more energy and releases more energy in return. Finally, it is interesting to find that the battery will enter the third segment after the voltage drops to around 3.2 V, no matter what the initial state is. The surface temperature curves of LIBs during charging and discharging are presented in Figure 4. The batteries with a voltage of 2.5 V were firstly charged to 5 V at a 2 C rate and then discharged at the same rate to 2.5 V again. Both were measured at room temperature simultaneously. It can be seen that the surface temperature rises at a similar rate in the initial segment of charging. After around 980 s, the temperature increases to about 23.8 \({}^{\circ}\)C when the voltage grows to 4.2 V. Hereafter, the temperature difference between them becomes more obvious. The LFP battery possesses a higher surface temperature and reaches the end of charging sooner, when the temperature is 26.5 \({}^{\circ}\)C. Afterwards, the charging of the NMC battery also ends with a surface temperature of 25.7 \({}^{\circ}\)C. During the discharging process, there exist two obvious temperature rises at the initial and final stages. At the initial stage of discharging, the temperature has a huge increase and gets to a peak temperature of 27.9 or 26.2 \({}^{\circ}\)C, respectively. Hereafter, the temperature drops continually and fluctuates around 25 \({}^{\circ}\)C. With the end of discharging approaching, the temperature presents another rise and has an end temperature of 26.8 or 26.7 \({}^{\circ}\)C, respectively. Figure 4: The surface temperature curves of LIBs during charging and discharging at 2 \({}^{\circ}\)C rate. Figure 3: Discharging curves of LIBs subjected to different cut-off voltage at 2 C rate. The surface temperature variation mainly results from the heat generated by the battery, including partly irreversible (\(Q_{irr}\)) and partly reversible (\(Q_{rev}\)), especially for the Joule heat included in \(Q_{irr}\)[15].