What is Lithium battery electrolyte made of?

Battery Electrolyte is one of the four key materials of lithium-ion batteries. It is called 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 the high voltage for lithium-ion batteries. The important guarantee of high specific energy and other advantages, this article mainly explains the main composition of lithium battery electrolyte components and the advantages of lithium battery electrolyte.

The main composition of the lithium battery electrolyte

The electrolyte is mainly composed of three parts:

• Solvent: cyclic carbonate (PC, EC); chain carbonate (DEC, DMC, EMC); carboxylate (MF, MA, EA, MA, MP, etc.); (used to dissolve lithium salt )
• Lithium salt: LiPF6, LiClO4, LiBF4, LiAsF6, etc.;
•  Additives: film-forming additives, conductive additives, flame retardant additives, overcharge protection additives, additives to control H2O and HF content in the electrolyte, additives to improve low-temperature performance, multifunctional additives;

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

• High ionic conductivity, generally should reach 1×10-3~2×10-2 S/cm;
• High thermal stability and chemical stability, no separation occurs in a wide voltage range;
• Wider electrochemical window, stable electrochemical performance in a wider voltage range;
• Good compatibility with other parts of the battery such as electrode materials, electrode collectors and separators;
• Safe, non-toxic and non-polluting.

The type of lithium battery electrolyte

Liquid electrolyte

The choice of electrolyte has a great influence on the performance of lithium-ion batteries. It must have good chemical stability, especially in a high potential and high temperature environment, and it is not easy to decompose, and has a high ion conductivity (>10-3 S/cm), and must be inert to the cathode and anode materials, and must not corrode them. Due to the high charge and discharge potential of lithium-ion batteries and the anode material embedded with lithium with high chemical activity, the electrolyte must use organic compounds instead of water. However, the ionic conductivity of organic matter is not good, so a soluble conductive salt should be 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, and most of them use mixed solvents, such as EC/DMC and PC/DMC.

Conductive salts include LiClO 4 , LiPF6, LiBF6, LiAsF6, etc., and their conductivity is in the order of LiAsF6>LiPF6>LiClO 4>LiBF6. LiClO4 is prone to safety problems such as explosion due to its high oxidative property, which is generally limited to experimental research; LiAsF6 has high ionic conductivity, is easy to purify and has good stability, but it contains toxic As, so its use is limited; LiBF6 chemistry And the thermal stability is not good and the conductivity is not high. Although LiPF6 will undergo decomposition reaction, it has high ion conductivity. Therefore, the current lithium-ion batteries basically use LiPF6. At present, most of the electrolytes used in commercial lithium-ion batteries use LiPF6 EC/DMC, which has high ionic conductivity and good electrochemical stability.

Solid electrolyte

Using metallic lithium directly as an anode material has a very high reversible capacity, its theoretical capacity is as high as 3862mAh g-1, which is more than ten times that of graphite materials, and the price is also lower. It is regarded as the most attractive of the new generation of lithium-ion batteries. anode material, but will produce dendrite lithium. The use of solid electrolyte as ion conduction can inhibit 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 disadvantage of liquid electrolyte leakage, and the battery can also be made thinner (thickness is only 0.1mm), high-energy battery with higher energy density and smaller volume. Destructive experiments show that solid-state lithium-ion batteries have high safety performance. After destructive experiments such as nail penetration, heating (200°C), short circuit and overcharge (600%), liquid electrolyte lithium-ion batteries will leak, explode, etc. However, solid-state batteries do not have any other safety issues except for a slight rise in internal temperature (<20°C). Solid polymer electrolytes have good flexibility, film-forming properties, stability, and low cost. They can be used not only as positive and negative electrode separators but also as electrolytes for ion transfer.

Solid polymer electrolytes can generally be divided into dry solid polymer electrolytes (SPE) and gel polymer electrolytes (GPE). SPE are mainly based on polyethylene oxide (PEO), which has a disadvantage of low ionic conductivity and can only reach 10-40cm at 100°C. In SPE, ion conduction mainly occurs in the amorphous region, and transfer migration is carried out by the movement of polymer chains. The easy crystallization of PEO is due to the high regularity of its molecular chain, and crystallization will reduce the ionic conductivity. Therefore, in order to improve the ionic conductivity, on the one hand, the crystallinity of the polymer should be reduced to increase the mobility of the chain; and on the other hand, the solubility of the conductive salt in the polymer should be increased. Using grafting, block, crosslinking, copolymerization and other means to destroy the crystallization properties of polymers can significantly improve their ionic conductivity. In addition, the addition of inorganic compound salt can also improve the ionic conductivity. A liquid organic solvent with high dielectric constant and low relative molecular mass is added to the solid polymer electrolyte such as PC can greatly improve the solubility of conductive salts, and the electrolyte formed is GPE gel polymer electrolyte, which has high ionic conductivity at indoor temperature, but it will fail due to liquid separation during use. Gel-polymer lithium-ion batteries are commercially available.

Advantages of lithium battery electrolyte

The battery electrolyte plays the role of conducting electrons between the positive and negative electrodes of the lithium battery, which is the guarantee for the high voltage and high specific energy of the lithium ion battery. The electrolyte is generally prepared from high-purity organic solvents, electrolyte lithium salts, necessary additives and other raw materials under certain conditions and in a certain proportion.

Lithium batteries mainly use electrolytes such as lithium perchlorate and lithium hexafluorophosphate. However, batteries made of lithium perchlorate are not effective at low temperatures, and there is a danger of explosion, and Japan and the United States have banned it. The battery made of fluorine-containing lithium salt , especially the battery made of lithium hexafluorophosphate, has good performance, no explosion hazard, and it has a strong applicability. In addition to the above advantages, the disposal of waste batteries in the future is relatively simple and friendly to the ecological environment. The market prospect of this type of battery electrolyte is very broad.

Conditions for lithium battery electrolyte

The electrolyte used in lithium-ion batteries is an ionic type conductor in which electrolyte lithium salts are dissolved in organic solvents. Generally, as an organic electrolyte for a practical lithium-ion battery, it should have the following properties:

1. The ion conductivity is high, generally it should reach 10-3～2*10-3S/cm; the lithium ion migration should be close to 1;
2. The potential range of electrochemical stability is wide; there must be an electrochemical stability window of 0-5V;
3. Good thermal stability and wide operating temperature range;
4. The chemical performance is stable, and there is no chemical reaction with the current collector and the active substance in the battery;
5. Safe and low toxicity, preferably biodegradable.

A suitable solvent needs to have a high dielectric constant and low viscosity, commonly used are alkyl carbonates such as PC, EC, etc., which have strong polarity and high dielectric constant, but have high viscosity and strong intermolecular forces, and lithium ions are moving slowly in them. And linear esters, such as DMC (dimethyl carbonate), DEC (diethyl carbonate), etc. have low viscosity, but also low dielectric constant. Therefore, in order to obtain a solution with high ion conductivity, we generally use PC+DEC, EC+DMC and other mixed solvents. These organic solvents have some smell. But generally speaking, they are materials with little toxicity and good environmental protection, and can meet the EU’s RoHS and REACH requirements.

The currently developed inorganic anion conductive salts mainly include three categories: LiBF4, LiPF6, and LiAsF6. The order of their electrical conductivity, thermal stability and oxidation resistance is as follows:

• Electrical Conductivity: LiAsF6≥LiPF6>LiClO4>LiBF4
• Thermal stability: LiAsF6>LiBF4>LiPF6
• Oxidation resistance: LiAsF6≥LiPF6≥LiBF4>LiClO4

LiAsF6 has very high conductivity, stability and battery charge-discharge rate, but it is limited due to its toxicity. Currently the most commonly used is LiPF6.

In general, the role of lithium battery electrolyte is very large, and its development prospects are also very broad.Lithium battery electrolyte is the carrier of ion transmission in the battery. It is mainly composed of high-purity organic solvents, electrolyte lithium salts, necessary additives and other raw materials. The battery electrolyte is generally composed of high-purity organic solvents, electrolyte lithium salts (lithium hexafluorophosphate, LiFL6), necessary additives and other raw materials are prepared in a certain proportion under certain conditions.