Main components of lithium battery electrolyte
1. Ethylene carbonate (molecular formula: C3H4O3)
A transparent, colorless liquid (>35℃), which becomes a crystalline solid at room temperature. Boiling point: 248℃/760mmHg, 243-244℃/740mmHg; Flash point: 160℃; Density: 1.3218; Refractive index: 1.4158 (50℃); Melting point: 35-38℃. This product is a good solvent for polyacrylonitrile and polyvinyl chloride. It can be used as a spinning solution in textiles; it can also be used directly as a solvent for removing acidic gases and as an additive in concrete; in medicine, it can be used as a component and raw material for pharmaceuticals; it can also be used as a foaming agent for plastics and a stabilizer for synthetic lubricants; in the battery industry, it can be used as an excellent solvent for lithium battery electrolytes.
2. Propylene carbonate (molecular formula: C4H6O3)
It is a colorless and odorless, or pale yellow, transparent liquid, soluble in water and carbon tetrachloride, and miscible with ether, acetone, benzene, etc. It is an excellent polar solvent. It is mainly used in polymer processing, gas separation processes, and electrochemistry. In particular, it is used to absorb carbon dioxide from natural gas and ammonia synthesis feedstock in petrochemical plants. It can also be used as a plasticizer, spinning solvent, and extractant for olefins and aromatics.
3. Diethyl carbonate (molecular formula: C5H10O3)
Colorless liquid with a slight odor; vapor pressure 1.33 kPa/23.8℃; flash point 25℃ (flammable liquids can evaporate into vapor and enter the air. Evaporation accelerates with increasing temperature. When the mixture of vapor and air comes into contact with an ignition source and sparks, this brief combustion process is called flash combustion, and the lowest temperature at which flash combustion occurs is called the flash point. The lower the flash point, the greater the risk of fire.); melting point -43℃; boiling point 125.8℃; solubility: insoluble in water, miscible with most organic solvents such as alcohols, ketones, and esters; density: relative density (water=1) 1.0; relative density (air=1) 4.07; stability: stable; hazard label 7 (flammable liquid); main uses: used as a solvent and in organic synthesis.
4. Dimethyl carbonate (molecular formula: C3H6O3)
Dimethyl carbonate (DMC) is a non-toxic, environmentally friendly, and widely used chemical raw material. It is an important organic synthesis intermediate, containing carbonyl, methyl, and methoxy functional groups in its molecular structure, exhibiting diverse reactivity. Its production is characterized by safety, convenience, low pollution, and ease of transportation. Due to its relatively low toxicity, dimethyl carbonate is a promising "green" chemical product.
DMC possesses excellent solubility properties, with a narrow melting and boiling point range, high surface tension, low viscosity, and a small interfacial electrical constant. It also exhibits a high evaporation temperature and a fast evaporation rate, making it suitable as a low-toxicity solvent in the coatings and pharmaceutical industries. As shown in Table 1, DMC not only has low toxicity but also features a high flash point, low vapor pressure, and a high lower explosive limit in air, thus making it a green solvent that combines cleanliness and safety.
5. Ethyl methyl carbonate
Molecular weight: 104.1, density 1.00 g/cm3, colorless and transparent liquid, boiling point 107℃, melting point -14℃. It is a high-tech, high-value-added chemical product that has emerged in recent years, and an excellent solvent for lithium-ion battery electrolytes. It is a new product developed with the increasing production of dimethyl carbonate and lithium-ion batteries. Because it contains both methyl and ethyl groups, it possesses characteristics of both dimethyl carbonate and diethyl carbonate, and is also a solvent for specialty fragrances and intermediates. Due to the imbalance of methyl and ethyl groups, this product is unstable and not suitable for long-term storage.
6. Lithium hexafluorophosphate
White crystals or powder, relative density 1.50. Highly hygroscopic; readily soluble in water, and also soluble in low concentrations of methanol, ethanol, acetone, carbonates, and other organic solvents. Decomposes upon exposure to air or heating. When exposed to air or heated, lithium hexafluorophosphate decomposes rapidly in the air due to the action of water vapor, releasing PF5 and producing white fumes.
7. Phosphorus pentafluoride
Phosphorus pentafluoride (chemical formula: PF5) is a phosphorus halide compound with phosphorus atoms having an oxidation state of +5 and containing one three-center, four-electron bond. Phosphorus pentafluoride is a colorless, malodorous gas at room temperature and pressure, and it is highly irritating to the skin, eyes, and mucous membranes. It is a highly reactive compound that violently produces a toxic and corrosive white fumes of hydrogen fluoride in moist air. Phosphorus pentafluoride is used as a catalyst in polymerization reactions.
8. Hydrofluoric acid
A colorless, transparent, fuming liquid. It is an aqueous solution of hydrogen fluoride gas. It is weakly acidic and has a pungent odor. It reacts with silicon and silicon compounds to form gaseous silicon tetrafluoride, but it does not corrode plastics, paraffin wax, lead, gold, or platinum. It is miscible with water and ethanol. Its relative density is 1.298. 38.2% hydrofluoric acid is an azeotropic mixture with an azeotropic point of 112.2℃. It is toxic; the minimum lethal dose (rat, intraperitoneal) is 25 mg/kg. It is corrosive and can strongly corrode metals, glass, and silicon-containing materials. Inhalation of vapors or skin contact can cause ulcers that are difficult to heal.
The main electrolytes used in lithium batteries include lithium perchlorate and lithium hexafluorophosphate. However, batteries made with lithium perchlorate do not perform well at low temperatures and pose a risk of explosion. Batteries made with fluorinated lithium salts, on the other hand, have good performance, no risk of explosion, and are widely applicable. In particular, batteries made with lithium hexafluorophosphate, in addition to the above advantages, will have relatively simple disposal procedures and be environmentally friendly. Therefore, the market prospects for this type of electrolyte are very broad.
