First, lithium ion battery electrolyte
Electrolyte is one of the four key materials for lithium-ion batteries. The blood of lithium-ion batteries is the guarantee for the high-voltage and high-energy energy of lithium-ion batteries. The electrolyte is mainly composed of a high-purity organic solvent, an electrolyte lithium salt, and a raw material of a necessary additive, and is prepared according to a certain ratio under certain conditions.
1.1 organic solvent
The organic solvent is generally mixed with a high dielectric constant solvent in a low viscosity solvent. Commonly used electrolyte lithium salts are potassium perchlorate, potassium hexafluorophosphate, potassium tetrafluoroborate, etc., in view of cost, safety and the like, potassium hexafluorophosphate It is the main electrolyte used in commercial lithium-ion batteries.
Commonly used organic solvents in lithium ion battery electrolytes are ethylene carbonate (EC) diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), acrylic acid B. Ester (EA), methyl acrylate (MA), and the like. The organic solvent must be strictly controlled before use. The purity of the solvent is closely related to the stable voltage. The moisture of the organic solvent plays a decisive role in formulating the qualified electrolyte. Lowering the water to below 10-6 can reduce the decomposition of lithium hexafluorophosphate, slow down the decomposition of the SEI film, and prevent gas rise. The moisture content can be achieved by molecular sieve adsorption, atmospheric or vacuum distillation, and introduction of an inert gas. In order to obtain a solution with high ionic conductivity so that lithium ions move rapidly therein, the solvent is generally a mixed material such as ethylene carbonate (EC) + dimethyl carbonate (DMC), ethylene carbonate (EC) + diethyl carbonate. Ester (DEC).
1.2 electrolyte lithium salt
The electrolyte lithium salt accounts for the largest cost of the electrolyte, accounting for about 40% of the electrolyte cost. LiPF6 is the most commonly used electrolyte lithium salt, which is stable to the negative electrode, has high electrical conductivity, large discharge capacity, small internal resistance, and fast charge and discharge speed. However, it is sensitive to moisture and HF, and its reaction should be carried out in a dry atmosphere (such as a glove box). It is not resistant to high temperatures, and decomposition reaction occurs at 80 ° C to 100 ° C to form phosphorus pentafluoride and lithium fluoride. . Considering cost, safety and other aspects, lithium hexafluorophosphate has the advantages of outstanding ionic conductivity, superior oxidation stability and low environmental pollution. It is currently the preferred lithium ion battery electrolyte and is also used in commercial lithium ion batteries. The main electrolyte. In addition, LiBF4, LiPF6, LiBOB, LiFSI, LiPF2, LiTDI and other series of lithium salt electrolyte systems with high safety and good cycle performance have attracted attention.
1.2.1 Lithium hexafluorophosphate
At present, the related research on the preparation process of LiPF6 is mainly divided into two categories: HF solvent method and ion exchange method. The HF? Solvent method is the most traditional method for preparing LiPF6 by dissolving LiF in an HF solvent and then directly introducing a substance containing phosphorus or fluorine, and evaporating or cooling the crystal after the reaction to obtain a final product. The method is the main method of industrial equipment, and the prepared LiPF6 has high purity and good quality, and is suitable for high-end lithium battery production demand. However, the preparation process has a high demand for equipment and operation, and the HF remaining in LiPF6 has a great influence on the performance of the battery.
Another major production method for LiPF6 is the chestnut exchange method. Refers to a method of ion exchange of a hexafluorophosphate with a lithium-containing compound in an organic solvent to obtain LiPF6. The main feature of the ion exchange method is that it is simple and easy, but the LiPF6 purity problem limits its wide application.
1.2.2 New lithium salt
At present, a series of lithium salt electrolyte systems with high safety and good cycle performance have attracted attention. Compared with the traditional electrolyte lithium salt LiPF6, although the comprehensive ability can not compete with LiPF6, they have obvious advantages in different aspects, such as , LiBOB? has good electrochemical stability and thermal stability, can react with specific solvents to form a stable ?SEI? membrane, which can be attenuated after repeated cycles of energy. LiFSI is a lithium battery electrolyte with excellent performance. It has excellent conductivity and good compatibility with electrode materials. LiBF4 has better chemical and thermal stability than LiPF6, and its safety performance is more remarkable. However, a large number of experimental data prove that there are always some unavoidable determinations using a single lithium salt. For example, LiFSI is easy to cause aluminum corrosion. LiBF4 has a relatively small anion radius, strong interaction with lithium ions, and weak conductivity. It is inferior in performance as a lithium ion battery for use as an electrolyte lithium salt alone. Therefore, lithium salts of different structures and different structures are compounded, so that the composite electrolyte exhibits excellent properties not possessed by simple electrolytes, thereby improving electrolyte performance in various aspects.
1.2.3 Advantages and disadvantages of various lithium salts
LiBF4: low temperature performance is better, but expensive and less soluble;
LiPF6: The comprehensive performance is better, and the disadvantage is easy water absorption and hydrolysis;
LiBOB: high temperature performance is better, especially inhibiting the insertion damage of the solvent to the negative electrode, but the solubility is too low;
LiFSI: not only environmentally friendly, but also has good thermal stability, sensitivity to moisture, and electrical conductivity;
LiPF2: Improves high temperature cycle performance and storage performance, low temperature output performance, and overcharge protection and balanced capacity performance of lithium batteries;
LiTFSI: good electrochemical stability, high ionic conductivity, good thermal stability, and difficult to hydrolyze;
LiTDI: Has a very high lithium ion migration number, reducing the amount of lithium salt and reducing battery cost.
1.3.1 Additives
There are many kinds of additives, and different lithium ion battery manufacturers have different requirements on the use and performance of the battery, and the focus of the selected additives is also different. In general, the additives used mainly have the following effects:
(1) Film forming additive
Inorganic film-forming additives: small molecules such as SO2, CO2, and CO can promote the formation of a passivation film, and the addition of a halide such as LiI or LiBr can also improve the passivation film.
Organic film-forming additives: fluorinated, chlorinated and brominated organic compounds such as anisole or its halogenated derivatives can improve the cycle performance of the battery and reduce the irreversible capacity loss of the battery. Among them, vinylene carbonate (VC) is a very good film-forming additive.
(2) reducing trace water and HF acid additives in the electrolyte
The carbodiimide compound can prevent the hydrolysis of LiPF6 to an acid. In addition, some metal oxides such as Al2O1, MgO, BaO, Li2CO1, CaCO1 and the like are used to remove HF.
(3) Prevent overcharge and overdischarge additives
Compounds such as organic amines and imines, biphenyls, and carbazoles are used as additives to prevent overcharge and overdischarge.
(4) Flame retardant additives
Organophosphorus compounds such as tetrapropoxysilane (TPOS), tetramethoxysilane (TMOS), organofluorine compounds, and halogenated alkyl phosphates are used as flame retardant additives in high boiling point high flash point non-flammable compounds.
(5) Improve low temperature performance additives
N, N-dimethyltrifluoroacetamide, organic boride, fluorine-containing carbonate and other low viscosity, high flash point is beneficial to the improvement of low temperature performance of the battery.
(6) Multifunctional additives
After 12-crown-4 was added to the PC solvent, the SEI film at the electrode interface was optimized to reduce the first irreversible capacity loss of the electrode. The addition of fluorinated organic solvents and halogenated phosphates such as BTE and TTFP to the electrolyte not only contributes to the formation of an excellent SEI film, but also has a certain or even significant flame retardancy to the electrolyte.