What are the additives to improve electrolyte conductivity?

Improving the electrical performance of the electrolyte is mainly to improve the dissociation and dissolution of the conductive lithium salt and to prevent the damage of the electrode caused by the co-intercalation of the solvent. Additives can be divided into two types, cationic and anionic, according to their interaction with electrolyte salt. Cationic additives are mainly crown ethers and cryptic compounds, as well as amines and aromatic heterocyclic compounds with more than two oxygen atoms in the molecule. These compounds can have a strong chelating or coordination effect with lithium ions to promote the dissolution of lithium salts. Crown ethers and cryptic compounds can form coated integrants with lithium ions, thereby improving the solubility of lithium salts in organic solvents, realizing the effective separation of anion and cation pairs and the separation of lithium ions from the solvent. These crown ethers and cryptic compounds can not only improve the conductivity of the electrolyte, but also may reduce the co-intercalation and reductive decomposition of solvent molecules during the charging process. For example, 12-crown-4 ether can significantly improve the electrochemical stability of carbon electrodes in electrolytes such as PC. However, the expensive price and high toxicity of crown ether compounds limit their application in commercial lithium-ion batteries. NH3 and some low-molecular-weight amine compounds can have a strong coordination effect with lithium ions, reduce the solvation radius of lithium ions, and significantly increase the conductivity of the electrolyte. However, such additives are often accompanied by coordination during electrode charging. The co-embedding of the body will cause great damage to the electrode. Adding 1% to 5% of acetamide or its derivatives to the electrolyte can significantly improve the cycle performance of the battery. In lithium ion battery electrolytes, anionic complexes are more important than cationic complexes, because the formation of anionic complexes is not only beneficial to increase the conductivity of the electrolyte, but also can increase the migration number of lithium ions.

The additives that interact with anions are mainly some anion acceptor compounds such as azaethers or boron-based compounds, which can form complexes with lithium salts such as F- to improve the solubility and solubility of lithium salts in organic solvents. Conductivity. One is based on the N electron defect on the azaether, and the H on N is replaced by a strong electron withdrawing group such as CF₃SO₃; the other is based on borane or boride with various fluorinated aryl or alkyl groups The upper B electron defect is the basis, as shown in Figure 1 and Figure 2. Using such compounds as additives can increase the conductivity of 0.2mol/L CF₃COOLi and C₂F₅COOLi dissolved in DME from 3.3×10^-5S/cm and 2.1×10^-5S/cm to 3.3×10^-3S/cm and 3.7×10^-3S/cm. The solution can even dissolve LiF, which is completely insoluble in DME, with a concentration of up to 1.0 mol/L and a conductivity of 6.8×10^-3S/cm. Since this type of complexing agent complexes an anion, it is expected to increase the number of lithium ion migration.

Figure 1 Schematic diagram of the structure of various azaether-based anion receptors

Figure 2 Schematic diagram of the structure of various boron-based anion receptors

Inorganic nano oxides such as SiO₂, TiO₂, Al₂O₃ and other insulating phases are added to the liquid electrolyte to form a "soggy sand" composite electrolyte, which can significantly improve its electrical conductivity. The study found that the addition of acidic oxide SiO₂ has the most obvious effect on improving the conductivity of the electrolyte. When adding 25% SiO₂ with a volume fraction of about 300 nm to 0.1 mol/L LiClO₄-CH₃OH, the conductivity can be 2.68×10^-3S/L cm increased to 1.2×10^-2S/cm. The main reason for the increase in conductivity is that the addition of acidic oxide SiO₂ makes the anions in the lithium salt adsorb on the surface of the oxide and destroys the ion pairs that originally existed in the electrolyte, which can increase the dissociation of LiClO₄ in the solvent and enhance the space around the oxide. Free Li⁺ concentration in the charge layer region.

Boron-based compounds include various boranes and boronic acid compounds with different fluorinated aryl and fluorinated alkyl functional groups, which are used as acceptors for lithium-ion battery electrolytes. Before using additives, the solubility of LiF in DME and other non-aqueous solvents was very low. After using additives, the concentration of LiF/DME solution can reach 1mol/L. The use of additives can also increase the solubility of other salts such as LiCl, LiBr, LiI, CF₃COOLi and C₂F₅COOLi in DME and increase the conductivity of the electrolyte. Therefore, as an additive for DME [1,2—Dimethoxy(yl)ethane] solutions containing various lithium salts, anion acceptors can increase the ionic conductivity of the solution. Studies have shown that Cl^- and I^- anions are complexed with LiCl or LiF in DME solution, and the degree of complexation is closely related to the structure of the electron-withdrawing fluorinated aryl and alkyl functional groups.