The main factors limiting the popularity of electric vehicles include prices, infrastructure, mileage and safety. In fact, prices, infrastructure and endurance can be made up and improved in other ways, and security, which is the most important concern of the users, is also an important problem that the manufacturers of electric vehicles and lithium batteries must solve.
The electrolyte of lithium battery usually uses organic solvents and is flammable under overheating conditions. Therefore, adding Flame Retardant additives in the electrolyte is one of the most economical and effective ways to improve the safety of the battery. At present, the main components of flame retardants for lithium ion batteries include phosphates, phosphite esters, organic halogenates and phosphonitriles.
Alkyl phosphates (such as TMP, TEP, etc.) are the first to study the flame retardants used in lithium ion batteries, but their viscosity is larger and the electrochemical stability is poor. It will cause negative effects on the ionic conductivity and the recycling reversibility of the electrolyte while improving the flame retardancy of the electrolyte. Using F element to replace H on alkyl to get fluorinated alkyl phosphate (such as TFP, BMP, TDP), it can improve the reduction stability and flame retardance effect of the compound. In addition, phosphoric acid cresol two phenyl ester (CDP) and phosphoric acid two phenyl octyl ester (DPOF) and other aromatic phosphate ester also has good flame retardant effect.
In addition to phosphorus (V) compounds, P (III) compounds are also effective flame retardant additives. Compared with phosphorus (V) compounds, P (III) compounds are more conducive to the generation of SEI films, and the former can deactivate five PF5. In the phosphorus (III) compounds, three (2, 2, 2- three fluoro ethyl) phosphite (TTFP) can not only reduce the combustibility of the electrolyte, but also improve the cycle performance of the lithium ion battery, which is a potential flame retardant.
Organic halogenated flame retardants mainly refer to fluorinated organic compounds, including fluorocarbon carbonate, fluorocarbon carbonate and alkyl perfluoroalkyl ether. Fluorinated organic compounds have high flash point, while fluorine substituted hydrogen atoms will reduce the hydrogen content and combustibility of the solvent molecules. Adding them to the organic electrolyte can improve the safety of the electrolyte. The fluorinated cyclic carbonate compounds, such as CH2F-EC, CHF2-EC and CF3-EC, have good chemical and physical stability, high flash point and dielectric constant, which can dissolve lithium salt electrolyte well and mix with other organic solvents.
Phosphonitrile compounds are small molecules ring or polymer linear phosphorus nitrogen compounds. Some of the phosphonitrile compounds have good ionic conductivity themselves. They can be used as the electrolyte of lithium ion batteries, such as linear polyphosphoritrile containing the side chain of the ethylene oxide, and the ionic conductivity can reach 10-5S/cm. Moreover, these polymers have relatively high decomposition temperature (about 235 degrees centigrade) and moderate heat release. For example, the cyclic phosphonitrile trimer of the oligomeric ethylene oxide side chain has good flame retardancy while maintaining ionic conductivity.
Although many kinds of electrolyte additives have effect on flame retardancy to some extent, the addition of additives often has negative effects on other aspects of the battery because of the properties of the additives, such as the viscosity, chemical or electrochemical instability. Therefore, in keeping the electrochemical performance of the battery in all aspects, it is the direction for the future development of the flame retardant additives for the lithium ion battery electrolyte to develop the additive with effective flame retardancy.
In addition, thermal polymerization additives are also an important research direction for the safety additives of lithium ion batteries. The thermal polymerization additives can be polymerized at a certain temperature (such as 110~150 C), thus blocking the charge and discharge of the battery, preventing the increase of the battery temperature and avoiding the occurrence of "heat out of control". The monomers of thermal polymerization should meet the requirements of good electrochemical stability and good thermal stability without affecting the lithium ion conduction in Li ion batteries. At present, there are few studies on this type of thermal polymerization additives in China, and a lot of research work needs to be done before they can be applied.
