Electrolyte is an important part of lithium-ion battery and the "blood" of lithium-ion battery. It is the li+ transmission medium in the working process of lithium ion battery, and is composed of organic solvent, electrolyte lithium salt and additives.
Electrolyte lithium salt is a key component of electrolyte. Its physical and chemical properties have an important impact on the performance of electrolyte. According to the different central atoms of anions in lithium salt, common lithium salt can be divided into the following categories:
1. phosphorus lithium salt with P as the central atom: LiPF6 is a typical phosphorus lithium salt. In its molecular structure, the P central atom is covalently connected with the six F atoms of electricity absorption, which makes the charge dispersion on the P central atom large and li+ easy to dissociate. LiPF6 based electrolyte has the best comprehensive performance in ionic conductivity, SEI film formation and passivation of aluminum collector. The disadvantage is that the salt has poor thermal stability and is prone to decomposition reaction. When the ambient temperature exceeds 60 ℃, the salt will decompose significantly and release toxic gas HF [1].
2. Boron based lithium salts with B as the central atom: boron based lithium salts mainly include lithium tetrafluoroborate (LiBF4), lithium difluoroacetate borate (lidfob), lithium Bisoxalate borate (LiBOB). This kind of lithium salt is difficult to dissociate li+, so the ionic conductivity of the corresponding electrolyte is relatively low. LiBOB is easy to be reduced on the negative electrode, and it is easy to form excessive film on the negative electrode when used alone in electrolyte.
3. imine lithium salt with n as the central atom: imine lithium salt mainly includes bifluorosulfimide lithium salt (lifsi), bifluoromethane sulfonimide lithium (LiTFSI) and their derivatives. In this kind of lithium salt, the N atom is connected with two electric absorbing sulfonyl groups, and the charge on the N atom is fully delocalized. Therefore, its electrolyte shows ionic conductivity comparable to that of LiPF6 based electrolyte. In addition, the thermal decomposition temperature of these salts is above 200 ℃, which is considered as a new lithium salt promising to replace LiPF6 [2].
Other lithium salts such as potassium perchlorate (LiClO4) and lithium Hexafluoroarsenate (liasf6) are limited to experimental research due to major hidden dangers. Among them, the CL element in LiClO4 is in the highest oxidation state, which has the risk of explosion, while the reduction product arsenic trioxide of liasf6 has high carcinogenicity.
reference
[1] Liqianhui, Zhang Ya, zhengdandan, et al Properties of lithium difluoropheroimide and its application in lithium ion batteries [j] Henan chemical industry, 2020 (37): 10-13
[2] Zeng Shuangwei Synthesis of lithium difluorosulfonate and Study on the corrosion performance of its electrolyte on aluminum foil [d] Lanzhou: Lanzhou University of technology, 2020:4-5