# A Review on the Thermal Hazards of the Lithium-Ion Battery and the Corresponding Countermeasures
## 5 Management and Countermeasures
### Fire Retardant
#### 5.2.1 Fire-retardant Electrolyte
[208] introduced tris(2,2,2-trifluoroethyl) phosphate (TFP) as the FR additive into a LiPF6-based electrolytes containing EC/PC/EMC, and their results showed that the addition of TFP caused a continuous decrease in the dielectric constant, and a continuous increase in the glass transition and boiling temperatures. Therefore, the flammability of electrolytes was substantially reduced. [PERSON] et al. [209] synthesized a novel fluorinated alkyl phosphonate bis(2, 2, 2-trifluoroethyl) methylphosphonate (TFMP) as the FR additive to reduce the flammability of electrolyte, see Figure 12. Their experiments demonstrated that the addition of 20 vol.% TFMP could yield an electrolyte that was hardly flammable while having the least impact on the electrochemical performance. 3. Ionic Liquid  
Figure 11: (**a**) Schemes for the three kinds of phosphorus. Adopted from Ref. [206]; (**b**) The synthetic route of DEMEMPA. Adopted from Ref. [207]. Figure 12: The synthetic route of bis(2, 2, 2-trifluoroethyl) methylphosphonate (TFMP). Adopted from Ref. [209]. An ionic liquid usually refers to a liquid salt at room temperature solely composed of anions and cations. It possesses certain characteristics such as it is non-volatile, nonflammable, pollution free and has a wide electrochemical window. Hence, the electrolyte with ionic liquid added is hoped to replace the traditional organic electrolyte to improve the safety of LIBs [210; 211; 212; 213; 214]. [PERSON] et al. [215] reported the successful design of a new class of lithium salts, phosphoryl-rich FR ions (FRIONs) for safer LIB, see Figure 13. The thermogravimetric analysis of these lithium salts showed that they were thermally stable up to around 200 \({}^{\circ}\)C. Moreover, the pyrolysis combustion flow calorimetry revealed that these salts would produce high char yields upon combustion which indicated the excellent FR property. Besides, the promising ionic liquid pentafluorocyclotriphosphazene (FPPN) is known to exhibit both an excellent FR performance and electrochemical properties. [PERSON] et al. [216] investigated a standard electrolyte mixed with 5 wt.% FPPN towards abuse tolerance in a 5 Ah battery and their results showed that FPPN significantly reduced the self-heating rate of the battery in the temperature range from 80 to 110 \({}^{\circ}\)C. Further, they conducted nail penetration and external short circuit experiments but found no significant difference between standard and FPPN-containing batteries. (4) Composite Additives  
Use of a single flame retardant generally requires adding a large amount. In addition, the solubility of the additive and the electrolyte compatibility are also relatively limited. Thus, it is necessary to use a composite approach by comparing the advantages and disadvantages of various types of retardants to configure more suitable additives for LIB. [PERSON] et al. [217] synthesized a phosphazene compound triethoxymosphazen-N-phosphorylyldiethylester (PNP) by a facile method and characterized it as a FR electrolyte additive for LIB, see Figure 14. They found that the self-ext  
Figure 13: Conversion of 1,2-diphosphinatocatech into phosphoryl-rich FR ions (FRIONs). Adopted from Ref. [215]. of 10% PNP in the electrolyte was decreased by 40% compared to the base electrolyte, implying a strong inhabitation to the flammability. Additionally, [PERSON] et al. [218] synthesized a multi-functional FR electrolyte additive pentafluoro cyclotriphosphazene (PFPN). The electrolyte with 5 vol.% of PFPN was found to be a non-flammable electrolyte, moreover it could reduce the charge transfer resistance of battery resulting in a decreased electrode polarization and enhanced electrochemical performance at low temperature. Finally, several representatives belonging to the four kinds of additives above are listed in Figure 10.0  
Commonly, the choice of FR electrolyte additive requires a comprehensive consideration including conductivity, solubility, retardant efficiency and cost etc.