# A Review of Experimental and Numerical Studies of Lithium Ion Battery Fires
## 4 Thermal Runaway
### Mitigating the Risk of Thermal Runaway and Fire
#### 4.3.1 Inherent Safety  
The mixture of organic carbonates and lithium salts commonly used in LIBs means that the electrolyte is highly flammable and cause for major safety concerns. FR electrolyte additives mixed with the solvents or completely replacing the flammable solvents is seen as the ideal way to mitigate the risk of an unwanted fire scenario. For an additive to be a viable option, the following characteristics are critical:  
1. Good chemical stability, no chemical reaction with battery components;
2. Electrochemical inertia, no adverse electrochemical reactions within the normal operating voltage range of the LIB;
3. Suitable physical properties including conductivity, viscosity, boiling point, density, solubility, etc. ;
4. Low toxicity, good machinability, and appropriate cost. Currently, the most common FR electrolyte additives and alternatives include:  
* Phosphorus additives [100; 101; 102; 103];
* Fluoride additives [104; 105];
* Brominated compound additives [106];
* Composite additives [107; 108];
* Ionic liquid [109; 110; 111; 112; 113; 114; 115; 116]. Detailed information of each FR electrolyte and additives can also be found in [PERSON] et al. 's [64] review paper. Another component of LIB that has a critical role in securing battery safety is the separator [55; 117; 118; 119]. There are a number of critical properties associated with separator technology that must be accomplished simultaneously in order to improve the safety and efficiency of LIBs:  
* Fire resistance;
* Mechanical properties;
* Thermomechanical stability;
* Ion-transport resistance. Figure 3: Tab configurations of a battery pack: (**a**) branched style of tabbing (M-type) and (**b**) serpentine style of tabbing (S-type) [92]. In situ SiO2@(PI/SiO2) was reported to be fire-resistant/nonflammable [58] as it would not ignite when exposed to a fire source. The mechanical properties are considered paramount in device safety. Nonwoven fabric separators are renowned for their weak mechanical properties due to the loose interactions between the different fibers. The in situ SiO2@(PI/SiO2) hybrid separator results [58] show a tensile strength increase at least five times greater than nanofabric devoid of the silica. Furthermore, the storage modulus (maintain structural integrity under compression) was increased at least five times [58]. Thermomechanical stability (the ability to maintain mechanical properties under thermal stress) is a critical parameter for high-power and high-energy-density LIB applications. Providing a thermally and mechanically stable separator improves battery safety significantly when undergoing TR. It was reported that the in situ SiO2@(PI/SiO2) hybrid separator remains thermomechanically stable up to 371 \({}^{\circ}\)C [58], which is significantly higher than fabric-based separators. The electrochemical performance of LFP half-cell batteries assembled with the in situ SiO2@(PI/SiO2) hybrid separator showed high capacity and excellent cycling stability with no decay of the battery capacity over 100 cycles at a high temperature of 120 \({}^{\circ}\)C [58]. [PERSON] [57] published results from experimental work on the development of plasma-treated polypropylene nonwoven-based composites for high-performance LIB separators. When fluorinated segments are introduced onto the surface of PP nonwoven through plasma treatment, the composite separators display increased electrolyte uptake and ionic conductivity in addition to the electrochemical stability and discharge capacity. [PERSON] [58] presented an in situ formed silica@silica-embedded polyimide (in situ SiO2@(PI/SiO2)) nanofabric as a new high-performance inorganic-organic hybrid separator. Various synthesized flame retardant composites, including Al2O3-sputtered PE separator, Al(OH)2-composite, and Mg(OH)3-composite, showed better flame retardancy compared to the conventional separators [120].