# A Review of Experimental and Numerical Studies of Lithium Ion Battery Fires
## 4 Thermal Runaway  
TR is the phenomenon that poses the most significant scientific obstacle regarding LIB safety research, the mechanisms of which are yet to be clearly comprehended [7]. [PERSON]'s plot [59], as shown in Figure 1, graphically depicts the rate of the reaction change as a function of the temperature (exponential function) and the heat removal rate (linear function) at different coolant temperatures. Vessel A is stable at the point E intersection and can absorb a rise until point F (T\({}_{1}\)) where it reaches instability. Vessel B is unstable and at the point D intersection, the temperature of no return (T\({}_{\text{NR}}\)) is reached. When the battery temperature increases to T\({}_{\text{NR}}\), the LIB will go to a thermal runaway. Vessel C is stable at no point in time. To get a clearer insight into the dynamic of TR in LIB, heat sources in LIB during normal operating conditions and elevated temperature should be comprehended. During the normal operating condition, the total heat released in LIB is the sum of reversible heat generated by the electrochemical reactions and irreversible heat produced by the charging/discharging process [60; 61; 62]:  
\[\text{Q}_{\text{tot}}=\text{Q}_{\text{rev}}+\text{Q}_{\text{rcon}} \tag{1}\]  
\[\text{Q}_{\text{rev}}=-I\times T\times(\partial E_{\text{oc}}/\partial T)=-a_ {\text{s}}\times I\times F\times T\times(\partial E_{\text{oc}}/\partial T) \tag{2}\]  
\[\text{Q}_{\text{rcon}}=I\times(E_{\text{oc}}-E)=a_{\text{s}}\times J\times F \times(E_{\text{oc}}-E) \tag{3}\]  
where \(I\) is the current, \(T\) is the absolute temperature, \(E_{\text{oc}}\) is the open circuit potential, \(E\) is the potential of cell, \((E_{\text{oc}}-E)\) is the overpotential accounting for irreversibilities such as ohmic losses, charge-transfer overpotential and mass-transfer limitation, \(a_{\text{s}}\) is a specific interfacial area of the electrode, \(J\) is the transfer current that resulted from the intercalation/deintercalation of lithium, and \(F\) is the Faraday constant. During abnormal operating conditions (abuse conditions), the temperature of LIB may increase and exceed the safe/normal operating condition, which may consequently lead  
Figure 1: [PERSON]’s plot depicting the reaction rate of change as a function of the temperature (exponential function) and the heat removal rate (linear function) at different coolant temperatures [59]. into TR. The abuse conditions include mechanical abuse, electrical abuse, and environmental abuse [63]. Table 2 shows the abuse conditions and possible causes. All three abuse conditions can lead to an ISC (most common pathway to TR), which initiates a sequence of chain reactions [64]. The exothermic reactions required to initiate TR have been documented to begin earlier, but temperatures in excess of 80 \({}^{\circ}\)C are typically required to begin the breakdown of the SEI [5; 6; 7]. Once this stage is complete and given further increases in temperature, reactions between the anode and electrolyte take place. These initial reactions are followed by larger cathode-electrolyte reactions. Most experimental studies indicate that the point at which the effects of TR cannot be reversed is in the range 150-200 \({}^{\circ}\)C [5; 64; 65; 66; 67]. An overview of the TR process from the abuse conditions leading to the ISC and initiation of TR, exothermic reactions, swelling/venting, and cell case rupturing/explosion/fire is shown in Figure 2. The released heat can propagate to adjacent cells and induce an additional thermal event in the battery pack/module/system. The heat released through each process is discussed and quantified in Section 5.1. In addition to the thermal abuse conditions that have been widely investigated [7; 65; 66; 67; 68; 69], the nail penetration [53; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79] (mechanical abuse) and overcharge [9; 10] (electrical abuse) for initiating failure mechanisms are also studied.