Lithium ion battery electrolyte type
1.1 liquid electrolyte
The choice of electrolyte has a great influence on the performance of lithium ion batteries. It must have good chemical stability, especially at higher potentials and higher temperature environments, and has higher ionic conductivity (>10-3). ?S / cm), and must be inert to the anode and cathode materials, can not invade them. Since the lithium ion battery has a high charge and discharge potential and the anode material is embedded with a chemically active lithium, the electrolyte must use an organic compound 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 increase the ionic conductivity. At present, lithium-ion batteries are mainly used as electrolytes. The solvents are anhydrous organic substances such as EC, PC, DMC, DEC, and most of them use mixed solvents such as EC/DMC and PC/DMC. The conductive salts are LiClO?4, LiPF6, LiBF6, LiAsF6, etc., and their conductivity is once LiAsF6>?LiPF6>?LiClO?4>LiBF6. LiClO4 is vulnerable to explosion and other safety problems due to its high oxidizing property. It is generally limited to experimental research. LiAsF6 has high ion conductivity and is easy to purify and has good stability, but contains toxic As, which is limited in use; LiBF6 chemistry And the thermal stability is not good and the conductivity is not high. Although LiPF6 will undergo decomposition reaction, it has high ionic conductivity, so lithium ion batteries are basically using LiPF6. At present, most of the electrolytes used in commercial lithium ion batteries use the EC/DMC of LiPF6, which has high ionic conductivity and good electrochemical stability.
2.2 solid electrolyte
The use of metallic lithium directly as an anode material has a high reversible capacity, and its theoretical capacity is as high as 3862 mAh·g-1, which is more than ten times that of graphite materials, and the price is also low, which is regarded as the optimal attraction of a new generation of lithium ion batteries. The anode material will produce dendritic lithium. The use of a solid electrolyte as the conduction of ions can always grow dendritic lithium, making it possible to use metallic lithium as an anode material. In addition, the use of a solid electrolyte avoids the disadvantage of too much electrolyte leakage, and the battery can be made into a thinner (only 0.1 mm thick) high-energy battery with a higher energy density and a smaller volume. Destructive experiments show that solid-state lithium-ion batteries have high safety performance. After liquidation, heating (200 ° C), short circuit and overcharge (600%) and other destructive experiments, liquid electrolyte lithium-ion batteries will leak and explode. Sexual problems, while solid-state batteries have no other safety problems except for a slight increase in internal temperature (<20 °C). Solid polymer electrolytes have good flexibility, moldability, stability, and low cost. It can be used as a positive and negative electrode spacer film and as an electrolyte for ion transport.
Solid polymer electrolytes are generally classified into a dry solid polymer electrolyte (SPE) and a gel polymer electrolyte (GPE). SPE solid polymer electrolytes are mainly based on polyethylene oxide (PEO), which has the disadvantage of low ionic conductivity and can only reach 10-40 cm at 100 °C. In SPE, ion conduction occurs mainly in the amorphous region, and transport is transferred by the movement of the polymer chain. PEO is easily crystallized due to the high regularity of its molecular chain, and crystallisation reduces ionic conductivity. Therefore, in order to increase the ionic conductivity, on the one hand, it is possible to reduce the ionic conductivity. Therefore, to improve the ionic conductivity, on the one hand, by reducing the solubility of the polymer. Grafting, block, cross-linking, copolymerization and the like are used to destroy the starvation crystal properties of the polymer, and the ionic conductivity thereof can be remarkably improved. In addition, the addition of an inorganic composite salt can also increase the ionic conductivity. Adding a high dielectric constant low molecular weight liquid organic solvent such as PC to the solid polymer electrolyte can greatly improve the solubility of the conductive salt. The electrolyte formed is a GPE gel polymer electrolyte, which has an improved temperature at room temperature. Ionic conductivity, but liquidation will fail during use. Gel polymer lithium ion batteries have been commercialized.