At present, there are two main electrolytes used in lithium batteries: one is liquid electrolyte, and the other is solid electrolyte. The liquid electrolyte is commonly known as electrolyte, which is mainly used in cylindrical and square lithium batteries; solid electrolyte, also known as polymer electrolyte, is mainly used in soft-pack batteries, and a very small number of square lithium batteries use solid electrolyte. Because solid electrolyte is more expensive than liquid electrolyte, it has not been widely used in a large number of lithium batteries. Next, the editor will briefly introduce the specific information of liquid electrolyte and solid electrolyte.

1. Liquid electrolyte

The choice of electrolyte has a great influence on the performance of lithium batteries. It must be chemically stable, especially at higher potentials and in higher temperature environments. It is not easy to decompose, has a high ionic conductivity (>10-3 S/cm), and must be inert to the cathode and anode materials and cannot invade them. Due to the high charge and discharge potential of lithium-ion batteries and the chemically active lithium embedded in the anode material, the electrolyte must use organic compounds and cannot contain water. However, the ionic conductivity of organic matter is not good, so a soluble conductive salt is added to the organic solvent to improve the ionic conductivity. At present, lithium ion batteries mainly use liquid electrolytes, and their solvents are anhydrous organic substances such as EC, PC, DMC and DEC. Most of them use mixed solvents such as EC/DMC and PC/DMC.

Conductive salts include LiClO4, LiPF6, LiBF6, LiAsF6, etc., and their conductivity is LiAsF6>LiPF6>LiClO4>LiBF6. LiClO4 is prone to explosion and other safety problems due to its high oxidation, which is generally limited to experimental research. LiAsF6 has high ionic conductivity, easy purification and good stability, but contains toxic As, and its use is limited. LiBF6 has poor chemical and thermal stability and low conductivity. Although LiPF6 will undergo decomposition reaction, it has high ionic conductivity. Therefore, LiPF6 is basically used in lithium ion batteries at present. At present, most of the electrolytes used in commercial lithium batteries use LiPF6 EC/DMC, which has high ionic conductivity and good electrochemical stability.

2. Solid electrolyte

The use of lithium metal directly as an anode material has a high reversible capacity, its theoretical capacity is as high as 3862mAh · g, which is more than ten times that of graphite material, and the price is also low. It is regarded as the most attractive anode material for a new generation of lithium batteries, but it will produce dendritic lithium. The use of a solid electrolyte as ionic conduction suppresses the growth of dendrite lithium, making it possible to use metallic lithium as an anode material. In addition, the use of solid electrolyte can avoid the shortcomings of liquid electrolyte leakage, and the battery can be made into a thinner (thickness only 0.1mm), higher energy density, and smaller high-energy battery.

Destructive experiments show that solid-state lithium batteries have high safety performance. After destructive experiments such as nail penetration, heating (200 ℃), short circuit and overcharge (600%), liquid electrolyte lithium-ion batteries will have safety problems such as leakage and explosion, while solid-state batteries do not have any other safety problems except for a slight increase in internal temperature (<20 ℃). Solid polymer electrolyte has good flexibility, film-forming, stability, low cost, can be used as a positive and negative electrode spacer film can also be used as an electrolyte for the transfer of ions.

Solid polymer electrolytes can be generally classified into dry form solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE). The SPE solid polymer electrolyte is mainly based on polyethylene oxide (PEO), which has the disadvantage of low ionic conductivity, which can only reach 10-40cm at 100 ℃. In SPE, ion conduction mainly occurs in the amorphous region, and the transfer and migration are carried out by the movement of the polymer chain.

Adding a liquid organic solvent with high dielectric constant and low relative molecular mass such as PC to the solid polymer electrolyte can greatly improve the solubility of the conductive salt. The electrolyte is composed of GPE gel polymer electrolyte, which has high ionic conductivity at room temperature, but it will produce liquid separation and failure during use. Gel polymer lithium ion batteries have been commercialized.