# 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
Figure 3: The experimental phenomena during thermal runaway: (**a**) heating stage; (**b**) safety vent opening; (**c**) gases releasing; (**d**) thermal runaway; (**e**) ignition and combustion; (**f**) abatement and extinguishment. Figure 2: Typical curves of battery surface temperature and mass vs. time. \[POF_{3}+H_{2}O\to POF_{2}(OH)+HF \tag{4}\]  
On the other hand, the reaction between PVDF binder and lithiated graphite will occur in acidic medium and is as follows [119]:  
\[-CH_{2}-CF_{2}-\overset{acid}{\rightarrow}-CH=CF-+HF \tag{5}\]  
By means of fire calorimetry, [PERSON] et al. [76] carried out combustion experiments on pouch batteries and results revealed that an increasing amount of total HF emissions would be generated for the decreasing SOC value of battery. Besides, [PERSON] et al. found that the generation of HF increased linearly with the increasing battery quantity and the burning of about 100 batteries (18,650) would release an amount of HF causing the immediate death of humans, 1.5 g [120,121]. In addition, resulting from the incomplete combustion of battery electrolyte as described in Equation (6), there will be some CO generated during thermal runaway [114]:  
Figure 1:0  
It is well-known that CO can bind with hemoglobin causing oxygen poisoning. By varying battery SOC and ambient pressure, [PERSON] et al. [121] declared that with the increasing battery SOC, the amount of CO\({}_{2}\) decreased, while the amount of CO increased. The decreasing ratio of CO\({}_{2}\)/CO revealed that the combustion efficiency of battery would be lower for the high SOC battery. Besides, a low combustion efficiency could also be obtained under low-pressure conditions, and the battery with higher capacity would generate much more CO during thermal runaway [114]. Based on the research of [PERSON] et al. [76], NO may be produced as a reaction product of nitrogen (originating from air or fuel-bound nitrogen) and oxygen from air within the flame (thermal route of NO\({}_{\text{x}}\) production), which can be further expressed as:  
Figure 1:1  
Figure 1:2  
NO will damage the peogaster of human and destroy the ozonosphere. Furthermore, it can be oxidized as corrosive nitrogen dioxide (NO\({}_{2}\)). With the increase of battery SOC, the amount of NO first ascends and then descends. Additionally, sulfur-based compounds as additives are known to be used in electrolytes for their property in facilitating SEI formation [122,123]. Whereas, they will undergo degradation to form SO\({}_{2}\) at high temperatures. Similarly, SO\({}_{2}\) is harmful to the peogaster of human and it can also lead to acid rain. Moreover, the amount of SO\({}_{2}\) will ascend with the increasing battery SOC. The generation of HCl originates from the combustion of polymers inside battery containing a binder, separator and package [124]. It is found that the amount of HCl is irrelevant to battery SOC. Finally, one possible source of H\({}_{2}\) is the reaction between metal Li and the separator. Common separator materials are PVDF and carboxymethyl cellulose (CMC). As can be seen from the reaction detailed below, PVDF may react with metal Li and release H\({}_{2}\) under high-temperature conditions [124]:  
Figure 1:3  
A similar reaction of CMC and metal Li may take place to release H\({}_{2}\) which is as follows:  
Figure 1:4  
H\({}_{2}\) is highly reactive, and it may lead to violent combustion or explosion under external high-temperature.