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Electrolyte is one of the four key materials in lithium-ion batteries, known as the "blood" of lithium-ion batteries. Its function is to conduct electrons between the positive and negative electrodes in the battery, and it is also an important guarantee for lithium-ion batteries to obtain advantages such as high voltage and high specific energy. Do you know the composition of electrolyte components in lithium-ion batteries? Below, the editor will introduce in detail the components of lithium battery electrolytes and the types of lithium battery electrolytes.

1、 What are the components of lithium battery electrolyte?

The electrolyte composition of lithium batteries mainly consists of three parts:

(1) Solvent: cyclic carbonates (PC, EC); chain carbonates (DEC, DMC, EMC); carboxylic esters (MF, MA, EA, MA, MP, etc.) (used to dissolve lithium salts).

(2) Lithium salts: LiPF6, LiClO4, LiBF4, LiAsF6, etc.

(3) Additives: film-forming additives, conductive additives, flame retardant additives, overcharge protection additives, additives for controlling H2O and HF content in electrolytes, additives for improving low-temperature performance, multifunctional additives.

Electrolytes used for lithium-ion batteries should generally meet the following basic requirements:

a. High ionic conductivity should generally reach 1x10-3-2x10-2S/cm.

b. High thermal and chemical stability, no separation occurs over a wide voltage range.

c. A wider electrochemical window maintains stable electrochemical performance over a wider voltage range.

d. It has good compatibility with other parts of the battery such as electrode materials, electrode current collectors, and separators.

e. Safe, non-toxic, and non polluting.

2、 Types of electrolytes for lithium batteries

1. Liquid lithium battery electrolyte

The selection of electrolyte has a significant impact on the performance of lithium-ion batteries. It must have good chemical stability, especially in high potential and high temperature environments, not easily decompose, and have high ion conductivity (>10-3S/cm). In addition, it must be inert to the anode and cathode materials and cannot corrode them.

Due to the high charge and discharge potential of lithium-ion batteries and the presence of chemically active lithium embedded in the anode material, the electrolyte must use organic compounds and cannot contain water. But the ion conductivity of organic compounds is not good, so soluble conductive salts need to be added to organic solvents to improve ion conductivity.

At present, lithium-ion batteries mainly use liquid electrolytes, and their solvents are anhydrous organic compounds such as EC, PC, DMC, DEC. Most of them use mixed solvents such as EC/DMC and PC/DMC. Conductive salts include LiClO4, LiPF6, LiBF6, LiAsF6, etc. Their conductivity is in the order of LiAsF6>LiPF6>LiClO4>LiBF6. LiClO4 is generally limited to experimental research due to its high oxidizing properties, which can lead to safety issues such as explosions; LiAsF6 has high ion conductivity, is easy to purify, and has good stability, but it contains toxic As, which limits its use; LiBF6 has poor chemical and thermal stability and low conductivity. Although LiPF6 undergoes decomposition reactions, it has high ionic conductivity. Therefore, currently lithium-ion batteries mostly use LiPF6. At present, most of the electrolytes used in commercial lithium-ion batteries adopt LiPF6 EC/DMC, which has high ion conductivity and good electrochemical stability.

2. Solid electrolyte

The direct use of metallic lithium as an anode material has a high reversible capacity, with a theoretical capacity of up to 3862mAh · g-1, which is more than ten times that of graphite materials. The price is also relatively low, and it is considered the most attractive anode material for the new generation of lithium-ion batteries, but it will produce dendritic lithium. The use of solid electrolytes as ion conductors can suppress the growth of dendritic lithium, making it possible for metallic lithium to be used as an anode material. In addition, using solid electrolytes can avoid the disadvantage of liquid electrolyte leakage, and can also make the battery thinner (with a thickness of only 0.1mm), with higher energy density and smaller volume. Destructive experiments have shown that solid-state lithium-ion batteries have high safety performance. After destructive experiments such as piercing, heating (200 ℃), short circuit, and overcharging (600%), liquid electrolyte lithium-ion batteries may experience safety issues such as leakage and explosion, while solid-state batteries have no other safety issues except for a slight increase in internal temperature (<20 ℃). Solid polymer electrolytes have good flexibility, film-forming properties, stability, and low cost, and can be used as separators between positive and negative electrodes as well as electrolytes for ion transfer.

Solid polymer electrolyte can be generally divided into dry solid polymer electrolyte (SPE) and gel polymer electrolyte (GPE). SPE solid polymer electrolytes are mainly based on polyethylene oxide (PEO), which has the disadvantage of low ionic conductivity, reaching only 10-40cm at 100 ℃. In SPE, ion conduction mainly occurs in the amorphous region, through the movement of polymer chains for transfer and migration. PEO is prone to crystallization due to the high regularity of its molecular chains, and crystallization reduces ionic conductivity. Therefore, to improve ion conductivity, on the one hand, the crystallinity of the polymer can be reduced to increase the mobility of the chain, and on the other hand, the solubility of conductive salts in the polymer can be increased. The use of grafting, blocking, crosslinking, copolymerization and other methods to disrupt the crystallization properties of polymers can significantly improve their ionic conductivity. In addition, adding inorganic composite salts can also improve ion conductivity. Adding liquid organic solvent with high dielectric constant and low relative molecular weight, such as PC, into solid polymer electrolyte can greatly improve the solubility of conductive salt. The electrolyte formed is GPE gel polymer electrolyte, which has high ionic conductivity at room temperature, but will fail due to liquid separation during use. Gel polymer lithium-ion batteries have been commercialized.

The above is the content compiled by the editor about the components of lithium battery electrolytes and the types of lithium battery electrolytes. The role of lithium battery electrolytes is quite significant. Lithium battery electrolytes are carriers of ion transport in batteries, mainly composed of high-purity organic solvents, electrolyte lithium salts, necessary additives and other raw materials. Electrolytes are generally prepared from high-purity organic solvents, electrolyte lithium salts (lithium hexafluorophosphate, LiFL6), necessary additives and other raw materials in a certain proportion under certain conditions.

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