# 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
#### 4.2.1 Thermal Hazards of a Single Battery  
With the proceeding of chain reactions inside battery during thermal runaway such as the decomposition of SEI layer, the decomposition of electrode and electrolyte, the reactions between electrode and electrolyte, and the combustion of electrolyte [101; 102], substantial heat will be generated which leads to the sharp increase of battery temperature. Figure 2 displays the typical curves of battery surface temperature and mass during the thermal runaway process, where the thermal runaway is triggered with a 2.5 kW electronic heater. As observed, the surface temperature of battery increases stably under the effect of the heater. The SEI layer decomposes at about 69 \({}^{\circ}\)C [90], which enables the reaction between electrolyte and anode to take palace. As the temperature builds up, the intercalated lithium in the anode can also react with electrolyte and release heat [102]. At around 130 \({}^{\circ}\)C, the polymer separator will melt [103; 104], resulting in the internal short circuit between cathode and anode, therefore a number of reactions take place that contribute to the accumulation of heat and pressure. At approximately 150 \({}^{\circ}\)C, the safety vent of battery opens to decrease the pressure, hereafter gases releasing appears. As the continuous rising of battery temperature, the released combustible gases will be ignited, and then thermal runaway occurs. It can be observed that the surface temperature has a sharp increase to the peak value after thermal runaway, which ranges from 400-700 \({}^{\circ}\)C and it is related to battery chemistry, state of charge (SOC) and capacity etc. [105; 106; 107]. Once the safety vent cracks, quantities of gases will be ejected, which is accompanied by a clear sound [108; 109]. The ejected gases are usually high-temperature, toxic and combustible, which will be ignited soon and exhibited as the first combustion. Furthermore, after thermal runaway appears, the second combustion will be generated. It is much more violent than the first one and is usually accompanied with the ejection of flame. The experimental phenomena during thermal runaway can be seen in Figure 3. According to the result of [PERSON] et al., the axial flame temperature could reach as high as 800 \({}^{\circ}\)C [110]. Especially for the condition of overcharge, high-capacity and closed/semi-closed space, tremendous amounts of energy cannot be released effectively which may even result in the explosion of battery. [PERSON] et al. researched the thermal runaway behavior of an overcharged 18,650 battery, and they found that the thermal runaway behavior of overcharged battery was much more violent than the normal battery. As depicted in Figure 4, where the safety vent of battery was blown away, the jellyroll was brought out and exposed to air [111]. With the help of an adiabatic calorimeter, vent sizing package 2 (VSP2), [PERSON] et al. found that the charged battery was prone to experiencing a thermal explosion compared to the uncharged battery, and the heat of reaction was calculated to be 26.2 kJ [112]. Figure 1: The combustion heat distribution of lithium-ion battery: the overall combustion heat on the left was determined using experimental results, while the combustion heat of battery components on the right was obtained by the thermodynamic calculations. Adopted from Ref. [76]. Commonly, quantities of toxic gases can be generated during the thermal runaway process such as HF, carbon monoxide (CO), nitric oxide (NO), sulfur dioxide (SO\({}_{2}\)), hydrogen chloride (HCI) and hydrogen (H\({}_{2}\)) [76,113,114,115,116]. Due to the limited thermal stabilization of LiPF\({}_{6}\), it may decompose at elevated temperatures and it can be described as [117,118]:  
\[LiPF_{6}\to LiF+PF_{5} \tag{1}\]  
\[PF_{5}+H_{2}O\to POF_{3}+2 HF \tag{2}\]  
\[LiPF_{6}+H_{2}O\to LiF+POF_{3}+2 HF \tag{3}\]  
Figure 4: The photographs of the overcharged battery after thermal runaway test.