# 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  
The electrolyte of a commercial LIB is a mixture of organic carbonates such as solvents and lithium salt, in which linear carbonates such as DMC, EMC or diethyl carbonate (DEC) and cyclic carbonates such as EC and propylene carbonate (PC) may be included. These organic carbonates are highly flammable which causes great concerns regarding their safety in LIB [200; 201]. To solve this problem, the most straightforward way is to add FR components into solvents or thoroughly replace the flammable solvents [202]. However, some conflicts still exist between the property of FR additive and the electrochemical performance of LIB. In order to ensure the electrochemical performance of battery as much as possible, additives are necessary to meet the following requirements as depicted in Figure 9: (1) good chemical stability, no chemical reaction with battery components; (2) electrochemical inertia, no adverse electrochemical reactions within the normal operating voltage range of LIB; (3) suitable physical properties including conductivity, viscosity, boiling point, density, solubility etc. ; (4) low toxicity, good machinability and appropriate cost [202]. Normally, the FR mechanism can be divided into two types: gas physical mechanism and chemical free-radical capture mechanism [203; 204]. The former is based on oxygen isolation via the formation of a FR vapor and absorbing heat simultaneously, while the latter one is based on the additives capturing active free radicals and acids produced by combustion reaction to stop combustion development. Based on the elements contained and the working mechanism, currently studied FR additives for electrolytes can be divided into four main categories: phosphorus FR additives, fluoride FR additives, ionic liquid FR additives and composite FR additives, as illustrated in Figure 10.  
(1) Phosphorus Additives  
Considering their advantages such as environmental persistence, ability to bioaccumulation and low toxicity, organophosphorus compounds are one of the most popular candidates for FR additives in LIBs [205]. They will decompose at high temperature to produce hydrogen phosphate (H\({}_{3}\)PO\({}_{4}\)), which will continue to transform into meta phosphorous acid (HPO\({}_{2}\)), PO- and PO\({}_{2}\). HPO\({}_{2}\) will promote the carbonation reaction of carbonate, PO- and PO\({}_{2}\): will capture H- and O- and; therefore suppress the combustion of electrolytes. Until now, [PERSON] et al. [206] investigated the electrolyte with three kinds of dimethyl methylphosphonate (DMMP) additives respectively which could significantly suppress the flammability while only causing a small decrease in the electrochemical performance of battery, see Figure 11a. [PERSON] et al. [207] synthesized a new phisohinamidate bis(N,N-diethyl) (2-methoxyethoxy)  
Figure 10: Typical FR additives in LIB electrolytes. Figure 9: The common constitutes of LIB electrolytes and the requirement of FR additives. methylphosphonamidate (DEMEMPA) as the N-P collaborative FR additive in electrolyte and found that 10 vol.% of DEMEMPA to 0.9 M LiPF6/EC/DMC could significantly inhibit the burning of electrolytes and provide a wide electrochemical window, see Figure 11b. Besides, [PERSON] et al. [203] performed studies on several phosphates with different fluorination concentrations to determine the influence of fluorination degree within organophosphate on the flammability and electrochemical performance. Their results depicted that the partially fluorinated phosphate offered the best cycling and FR results. 2. Fluoride Additives  
Fluorinated substituents in organic molecules are known to suppress the flammability of molecules and reduce the boiling temperature and viscosity. By replacing hydrogen atoms with fluorine atoms, H-radicals can be reduced or eliminated to yield a non-flammable electrolyte. [PERSON] et al.