What Is Liquid electrolyte?
Liquid electrolyte
(1) Chemical composition Liquid electrolyte is also called electrolyte solution. It is a commonly used organic liquid electrolyte in lithium-ion batteries, also known as non-aqueous liquid electrolyte. Organic liquid electrolyte is composed of lithium salt, organic solvent, and additives.
① Lithium salt: mainly provides conductive ions. Lithium hexafluorophosphate is the most widely used lithium salt in commercial lithium-ion batteries. Pure LiPF6 is a white crystal, soluble in organic solvents such as low-alkyl ethers, nitriles, pyridine esters, ketones, and alcohols, but sparingly soluble in organic solvents such as alkanes and benzene. LiPF6 electrolyte has a relatively high conductivity; at 20℃, the conductivity of EC+DMC (volume ratio 1:1) can reach 10×10-²S/cm. The conductivity usually reaches its maximum when the electrolyte concentration is close to 1mol/L. LiPF6 has stable electrochemical performance and does not corrode the current collector. However, LiPF6 has poor thermal stability and decomposes easily in water, requiring strict control of environmental moisture content during preparation and use.
② Organic solvents: Their main function is to dissolve lithium salts, enabling the lithium salt electrolyte to form conductive ions.
Commonly used solvents include propylene carbonate, dimethyl carbonate, and diethyl carbonate.
Organic solvents are generally chosen for their high dielectric constant and low viscosity. A higher dielectric constant makes lithium salts easier to dissolve and dissociate; lower viscosity results in faster ion migration. However, in reality, solvents with high dielectric constants have high viscosity, and solvents with low viscosity have low dielectric constants. Therefore, it is difficult for a single solvent to simultaneously meet all these requirements. Lithium-ion batteries typically use a mixture of organic solvents with high dielectric constants and those with low viscosity to compensate for the shortcomings of each component. For example, EC carbonates have high dielectric constants, which is beneficial for lithium salt dissociation, while DMC, DEC, and EMC carbonates have low viscosity, which helps improve the migration rate of lithium ions.
③ Additives: They generally play a role in improving and enhancing the electrical and safety performance of the electrolyte. Generally speaking, additives have three main functions: a. a. Improve the performance of the SEI membrane, such as by adding vinylene carbonate (VC), vinyl sulfite (ES), and SO2; b. Prevent overcharging (by adding biphenyl) and over-discharging; c. Flame retardant additives can prevent the battery from burning or exploding under overheating conditions, such as adding halogenated flame retardants, phosphorus-based flame retardants, and composite flame retardants; d. Reduce the trace water and HF content in the electrolyte.
