Electrolyte is an important part of lithium-ion batteries and the "blood" of lithium-ion batteries. It is the medium of Li⁺ transmission during the working process of lithium-ion batteries, and is composed of organic solvents, electrolyte lithium salts, and additives.

Electrolyte lithium salt is a key component of the electrolyte, and its physical and chemical properties have an important impact on the performance of the electrolyte. According to the different central atoms of the anion in the lithium salt, common lithium salts can be divided into the following categories:

1. Phosphorus-based lithium salt with P as the central atom: LiPF₆ is a typical phosphorus-based lithium salt. In its molecular structure, the P central atom is covalently connected to the 6 F atoms that are attracting electricity, making the charge dispersion on the P central atom large, and Li⁺ dissociates easily. LiPF₆-based electrolyte has the best overall performance in terms of ionic conductivity, SEI film formation and passivation of aluminum current collectors. The disadvantage is that the salt has poor thermal stability and is prone to decomposition reactions. When the ambient temperature exceeds 60°C, the salt will decompose significantly and release the 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 difluorooxalate borate (LiDFOB), and lithium bisoxalate borate (LiBOB). This type of lithium salt Li⁺ is difficult to dissociate, so the ionic conductivity of the corresponding electrolyte is relatively low. Among them, LiBOB is easy to be reduced in the negative electrode, and it is easy to form excessive film on the negative electrode when used alone in the electrolyte;

3. Lithium imide salts with N as the central atom: Lithium imide salts mainly include lithium bisfluorosulfonimide (LiFSI), Lithium bis((trifluoromethyl)sulfonyl)azanide (LiTFSI) and derivatives of these salts. In this type of lithium salt, The N atom in this type of lithium salt is connected to two electrically attracting sulfonyl groups, and the charge on the N atom is sufficiently delocalized. Therefore, the electrolyte exhibits ionic conductivity comparable to that of the LiPF_6-based electrolyte. In addition, The thermal decomposition temperature of these salts is above 200 ℃, which is considered to be a promising new type of lithium salt to replace LiPF₆ ^[2].

Other lithium salts such as potassium perchlorate (LiClO₄) and lithium hexafluoroarsenate (LiAsF₆) are limited to experimental studies due to major hidden dangers. Among them, the Cl element in LiClO₄ is in the highest oxidation state and there is a risk of explosion, while LiAsF₆ is reduced The product, arsenic trioxide, is highly carcinogenic.

References

[1] Li Qianhui, Zhang Ya, Zheng Dandan, etc. The performance of lithium bisfluoroxanthimide and its application in lithium-ion batteries [J]. Henan Chemical Industry, 2020(37): 10-13.