## 3 Results and Discussions  
### Temperature Change during LIB Cycle  
When the LIB is charging and discharging, the reaction occurs inside the LIB, which produces heat and shows the increase of battery temperature. Figure 3 shows the temperature change during charging and discharging of Sanyo LIB under four different working conditions: extreme conditions, natural convection cooling, PCM cooling, and PCM combined with heat dissipation fins cooling at the ambient temperature of 25 \({}^{\circ}\)C. Figure 3a shows the temperature curve of LIB in two cycles. That can be seen from the figure, the temperature of LIB in the enclosed space rises from 30 \({}^{\circ}\)C to about 77.3 \({}^{\circ}\)C during discharging, which obviously exceeds the normal operating temperature of the battery. The high temperature has certain bad influence on the LIB. The long-term working under the extreme condition will cause the LIB internal electrolyte decomposition, the positive and negative electrode reaction, \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Processing**} & \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Current (A)**} & \multicolumn{2}{c}{**Voltage (V)**} \\ \cline{3-6}  & & **Sanyo Battery** & **Sany Battery** & **Sanyo Battery** & **Sany Battery** \\ \hline Discharging & Constant current & 2.6 (1C), 5.2 (2C) & 1 (1C),2 (2C) & 4.2 & 3.6 \\ shelving & 5 min & & & & \\ \multirow{2}{*}{Charging} & Constant voltage & & & & \\  & and current & & & & \\ shelving & 5 min & & & & \\ \hline \end{tabular}
\end{table} 
Table 3: Design of charging and discharging experimental conditions. Figure 2: Schematic diagram of the experiment. and so on, which may cause the LIB thermal runaway and produce the safety risks. The highest temperature of the LIB is about 54.5 \({}^{\circ}\)C under PCM cooling, 66.2 \({}^{\circ}\)C under natural convection, 77.3 \({}^{\circ}\)C under extreme condition. Compared to the air cooling, the temperature of LIB using PCM cooling has a much smaller increase rate, and the highest temperature drops by 11.7 \({}^{\circ}\)C. Therefore, the results show that PCM can absorb some heat during the LIB discharging process, effectively reducing the LIB temperature. For the analysis of the cooling condition of the PCM with heat dissipation fins on the LIB, the heat dissipation fins have been adopted. The volume ratio of PCM and heat dissipation fins in the device is about 96%:4%. According to the diagram, the maximum temperature difference between the LIB in this working condition and PCM cooling condition is about 0.2 \({}^{\circ}\)C. The combination of heat dissipation fins can slightly reduce the temperature of LIB. The reason may be the fin structure cannot dissipate the latent heat in time due to its small volume ratio. In the research on the combination of PCM with heat dissipating fins, while giving full play to the maximum effect of heat dissipating fins, its cost and influence on the whole LIB structure should also be considered. [PERSON] et al. (2018) investigated heat transfer with PCMs in passive thermal management of electric and hybrid electric vehicles. Their results showed that the temperature distribution became about 10% more uniform when the PCM was applied in a 3 mm thickness around the LIB. The PCM with 12 mm thickness decreased the maximum temperature about 6.6 \({}^{\circ}\)C. In this research, the PCM not only has better cooling performance, but also makes the device having the advantages of small size and lightweight. In addition, the PCM in this study has a solid-solid phase change process, which avoids the liquid leakage. The device structure in this study is simpler and more practical, which may has a potential for commercialization. Figure 3b shows the temperature change curve of LIB during six cycles. The temperature of LIB tends to be stable under various working conditions. However, the average of the maximum temperatures for the six cycles of LIB reaches about 77.8 \({}^{\circ}\)C in the enclosed space, which is in an unsafe state.