Lithium-ion battery electrolyte liquid
I. Overview of lithium ion electrolyte
Electrolyte is one of the four key materials of lithium-ion batteries （anode, cathode, separator, electrolyte）, known as the "blood" of lithium-ion batteries, plays a role in the transmission of
electrons between anode and cathode in the battery, and is the guarantee of lithium-ion batteries to obtain high voltage, high specific energy and other advantages。Electrolytes are generally made of high-purity organic solvents, electrolyte lithium salts （ lithium hexafluorophosphate , LiFL6）, necessary additives and other raw materials, under certain conditions, in a certain proportion.
Organic solvents are the main part of electrolyte and are closely related to the performance of electrolytes, and are generally used with high dielectric constant solvents and low viscosity solvents; Commonly used lithium-ion salts include lithium perchlorate, lithium hexafluoride, and lithium tetrafluoroborate, but considering cost, safety and other aspects, l - hexafluoride is the main electrolyte used in commercial lithium ion batteries. The use of additives has not yet been commoditized, but has always been one of the hot spots in the research of electrolytes.
Since the development of lithium-ion battery electrolyte in 1991, lithium-ion battery quickly entered the notebook computer, mobile phones and other electronic information products market, and gradually occupy a dominant position. At present, lithium-ion battery electrolyte product technology is also in further development. In terms of lithium-ion battery electrolyte research and production, the companies engaged in the development and development of special electrolytes for lithium ion batteries internationally are mainly concentrated in Japan, Germany, South Korea, the United States, Canada and other countries, with Japan's electrolyte developing the fastest and accounting for the largest market share.
The commonly used electrolyte systems in China are EC + DMC, EC + DEC, EC + DMC + EMC, EC + DMC + DEC, etc。The use conditions of different electrolytes are different, the compatibility with the anode and cathode of the battery is different, the decomposition voltage is also different。The electrolyte composition is lmol / LLiPF6 / EC + DMC + DEC + EMC, which has better cycle life, low temperature performance and safety performance than ordinary electrolyte, can effectively reduce gas production and prevent battery expansion. The decomposition voltage of the EC / DEC and EC / DMC electrolyte systems is 4.25V and 5.10V, respectively.According to Bellcore research, LiPF6 / EC + DMC has good compatibility with carbon anodes, for example, in LixC6 / LiMnO4 batteries，With LiPF6 / EC + DMC as the electrolyte, it can be stabilized to 4.9V at room temperature, 4.8V at 55 ℃, and the liquid phase is from -20 ℃ to 130 ℃. Its outstanding advantages are wide temperature range, good compatibility with carbon anode, high safety index, good cycle life and discharge characteristics.
II. Electrolyte composition
2.1 Organic Solvents
The organic solvent is the main part of the electrolyte, and the performance of the electrolysis is closely related to the performance of solvent. Solvents commonly used in lithium battery electrolyte are vinyl carbonate (EC), diethyl carbonate (DEC), Dimethyl Carbonate (DMC), methyl ethyl carbonate (EMC), etc., generally do not use propylene carbonate (PC), Ethylene dimethyl ether (DME), etc. are mainly used as solvents for lithium primary batteries. The PC is used in secondary batteries and has poor compatibility with the graphite negative electrode of lithium-ion batteries. During charging and discharging, the PC decomposes on the surface of the graphite-negative electrode, causing the peeling of the graphitic layer, causing a decrease in the cycle performance of the battery. However, stable SEI films can be established in EC or EC + DMC composite electrolyte. It is generally believed that the mixed solvent of EC and a chain carbonate is an excellent electrolyte for lithium batteries, such as EC + DMC, EC + DEC, etc. The same electrolyte lithium salts, such as LiPF6 or LiC104, PC + DME systems always show the worst charge and discharge performance for intermediate-phase carbon microsphere C-MCMB materials (as opposed to EC + DEC, EC + DMC systems). But not absolutely, when PC and related additives are used for lithium batteries, it is conducive to improving the low temperature performance of the battery.
The quality of organic solvents must be strictly controlled before use, such as the purity of 99.9% or more, and the moisture content must reach 10 * l0-6. There is a close relationship between the purity of the solvent and the stable voltage. The oxidation potential of organic solvents with a standard purity is about 5V, and the oxidation potential is of great significance for research on preventing battery overcharge and safety. Strict control of the moisture of organic solvents has a decisive effect on the formulation of qualified electrolytes. The water content is reduced to 10 * l0-6, which can reduce the decomposition of LiPF6, slow down the decomposition of SEI film, and prevent gas swelling. The method of molecular siphonation, constant pressure or decompression distillation, and insertion of inert gas can enable the water content to meet the requirements.
2.2 Electrolyte lithium salt
LiPF6 is the most commonly used lithium salt of electrolysis, and is the direction of the future development of lithium salt. Although LiClO4, LiAsF6, etc. are also used as electrolytes in the laboratory, because the high temperature performance of batteries using LiC104 is not good, and LiCl04 itself is easy to explode when impacted, and it is also a strong oxidizer, which is not suitable for the industrialized large-scale use of lithium-ion batteries.
LiPF is stable to the negative electrode, has a large discharge capacity, high conductivity, small internal resistance, and fast charge and discharge speed, but is extremely sensitive to moisture and HF acid, and is easy to react. It can only be operated in a dry atmosphere (e.g. in glove cases where ambient moisture is less than 20x10).It is not resistant to high temperature, and it occurs a decomposition reaction from 80 ° C to IO0 ° C, producing phosphoryl pentafluoride and lithium fluoride, and it is difficult to purify, so the thermal decomposition of LiPF6 solution should be controlled when preparing the electrolyte.The content of LiPF produced in China is generally up to the standard, but the content of HF acid is too high to be used directly for the preparation of electrolyte and must be purified. In the past, LiPF relied on imports, but now there are some domestic manufacturers that can also provide good quality products, such as Shantou Jinguang Gaoke Co. Ltd., Tianjin Chemical Design Research Institute, and Xingtai Chemical Plant in Fuhai City, Shandong. The quality of LiPF produced abroad is better, and it is formulated into electrolyte, and the content of water and HF acid is stable, and the electrolyte will not become sticky and red.
2.3 Additives
There are many kinds of additives, and different lithium-ion battery manufacturers have different requirements for the purpose and performance of the battery, and the focus of the additives selected also differs. Generally, the additives used have three main roles:
(1) Improve the performance of SEI membrane
The addition of benzoether or its halting derivatives to the electrolyte of lithium-ion batteries can improve the cycle performance of the battery and reduce the irreversible capacity loss of the battery. Huang Wenhuang did research on its mechanism and found that the phenyl ether reacted with the solvent reduction products, and the resulting LiOCH facilitated the formation of an efficient and stable SEI film on the electrode surface, thereby improving the cycle performance of the battery. The discharge platform of the battery can measure the energy released by the battery above 3.6V, which reflects the high-current discharge characteristics of the battery to a certain extent. In practice, we found that the addition of benzoether to the electrolyte can extend the battery's discharge platform and improve the battery'd discharge capacity.
(2) Reduce trace water and HF acid in the electrolyte
As mentioned earlier, lithium-ion batteries have very strict requirements for water and acid in the electrolyte. Carbonated diamines can prevent LiPF6 from hydrolysis into acid, in addition, some metal oxides such as Al2O3, MgO, BaO, Li2CO3, CaCO3, etc. are used to remove HF, but the acid-removal rate is too slow compared to LiPF6 hydrolysis and is difficult to filter clean.
(3) Prevent overcharging and overdischarge
Battery manufacturers are very anxious about the battery resistance to overcharge and discharge. Traditional anti-overcharging passes through the protection circuits inside the battery, and it is now desirable to add additives to the electrolyte, such as midazole sodium chloride, biphenyls, and cazoles, which are in the research stage.
III. Types of electrolyte for lithium-ion batteries
3.1 Liquid electrolyte
The choice of electrolyte has a very large impact on the performance of lithium-ion batteries, which must be chemically stable and not easily decomposed under higher potential and higher temperature environments.It has a high ionic conductivity (> 10-3 S / cm), and it must be inert to the cathode and cathode materials and cannot invade them.Because the charge and discharge potential of lithium-ion batteries is high and the anode material is embedded with lithium with greater chemical activity, the electrolyte must be organic compounds and cannot contain water. However, the ionic conductivity of organic matter is not good, so it is necessary to add soluble conductive salts to the organic solvent to improve the ionic cductivity. At present, lithium batteries mainly use liquid electrolysis, the solvent is anhydrous organic matter such as EC, PC, DMC, DEC, most of the mixed solvents, such as EC / DMC and PC / DMC.Conductive salts include LiClO4, LiPF6, LiBF6, LiAsF6, etc. The conductivity of them is LiAsF6 > LiPF6 > LiClO4 > LiBF6.Because LiClO 4 has high oxidability, it is prone to explosion and other safety problems, which is generally limited to experimental research. The ionic conductivity of LiAsF6 is high, easy to purify and stable, but it contains toxic As, which limits its use. LiPF6 has poor chemical and thermal stability against short circuit and low conductivity. Although LiPF6 decomposes, it has high ionic conductivity. At present, most of the electrolytes used in commercial lithium-ion batteries are EC / DMC of LiPF6, which has high ionic conductivity and good electrochemical stability.
3.2 Solid electrolyte liquid
Using lithium metal directly as an anode material has a high reversible capacity, with a theoretical capacity of 3862 mAh · g-1, which is more than ten times that of graphite materials, and is also relatively low in price, and is considered the most attractive anode material for the new generation of lithium-ion batteries, but will produce lithium trifluoride.The use of solid electrolytes as ion conduction inhibits the growth of lithium branched crystals, making the use of metallic lithium as anode material possible. In addition, the use of solid electrolysis can avoid the shortcomings of liquid electrolyte leakage, and the battery can also be made into a thinner (thickness of only 0.1 mm), higher energy density, and smaller high-energy battery.Disruptive experiments show that solid-state lithium-ion batteries have a high safety performance when used, pierced, heated (200 ° C),During destructive experiments such as short circuit and overcharge (60 percent), liquid electrolyte lithium-ion batteries will experience leakage, explosion and other safety problems, while solid-state batteries do not have any other safety issues except a slight increase in internal temperature (< 20 ° C).Solid polymer electrolytes have good flexibility, film formation, stability and low cost, and can be used both as positive and negative electrode gap membranes and as ion transfer electrolytes.
Solid polymer electrolysis can generally be divided into dry solid polymer electrolysis (SPE) and gel polymer electrolysis (GPE). SPE solid polymer electrolysis is mainly based on polyethylene oxide (PEO), its disadvantage is low ionic conductivity, at 100C can only reach 10-40cm. In SPE, the ion conduction mainly occurs in the amorphous region and is carried out by the movement of the polymer chain. PEO is easy to crystallize because of its high regularity of molecular chains, and crystallization will reduce ionic conductivity. Therefore, in order to improve the ionic conductivity, we can reduce the degree of crystallinity to improve the mobility of the chain, on the other hand, we can improve the solubility of the conductive salt. Using branches, splices, cross-linking, copolymerization and other means to disrupt the crystallization properties of high polymers can significantly improve their ion conductivity. In addition, the addition of inorganic compound salts can also improve ion conductivity. Adding a liquid organic solvent such as PC with high dielectric constant and low relative molecular weight to the solid polymer electrolysis can greatly improve the solubility of the conductive salt. The GPE gel polymer electrolysis has high ionic conductivity at room temperature, but it will fail during use. Gel-polymer lithium-ion batteries have been commercialized.
IV. Conditions for the electrolyte of lithium-ion batteries
The electrolyte used in a lithium-ion battery is an ion-type conductor with lithium salts of the electrolyte in an organic solvent. Generally, the electrolytic solution used as a practical lithium-ion battery should have the following properties:
(1) High ionic conductivity, generally should reach 10-3 ~ 2 * 10-3 S / cm; The number of lithium ion transitions should be close to 1;
(2) The range of electrochemically stable potentials is wide; There must be a 0-5 V electrochemical stability window;
(3) The heat is well stable, the use temperature range is wide;
(4) Chemical properties are stable, and no chemical reaction occurs with the concentrated fluids and ephemeral substances in the battery;
(5) Safe and low-toxicity, preferably biodegradable.
A suitable solvent needs a high dielectric constant and a small viscosity. Commonly used are alkyl carbonates such as PC, EC and others with a strong polarity, high dielectric Constant but a large viscosity, a large intermolecular force, and a slow movement of lithium ion in them. Linear esters, such as DMC (dimethyl carbonate), DEC (diethyl carbonate), etc., have a low viscosity, but also a low dielectric constant, so in order to obtain a solution with high ionic conductivity, mixed solvents such as PC + DEC, EC + DMC are generally used. These organic solvents have some taste, but overall, they can meet the requirements of the EU's RoHS and REACH, and are very toxic and environmentally friendly materials.
LiBF4, LiPF6 and LiAsF6 are the main inorganic anionic conductive salts. Their conductivity, thermal stability against short circuit oxidation are as follows:
Conductivity: LiAsF6 ≥ LiPF6 > LiClO4 > LiBF4
Thermal stability against short circuit: LiAsF6 > LiBF4 > LiPF6
Oxidation resistance: LiAsF6 ≥ LiPF6 ≥ LiBF4 > LiClO4
LiAsF6 has very high electrical conductivity, stability and battery charge-discharge rate, but its application is limited due to arsenic toxicity. The most commonly used is LiPF6.
V. Global status of lithium-ion battery electrolyte development
In recent years, the global lithium-ion battery electrolyte industry has developed steadily, and the market is mainly concentrated in Japan UMBC and ECOPRO (Korea First Textile City), which account for about 50% of the global market share. The following companies are: Mitsubishi Chemical, Takayama Chemical, Mitsui Chemical, Kishida Chemical, Zhangjiagang Guotai Ronghua and others.
Domestically produced electrolytes have gradually replaced imported products since entering the market in 2002, and through continuous improvement and improvement, product quality has reached an international advanced level. At present, domestic battery manufacturers' electrolyte support has basically been domesticated, and only a small part of them use imported electrolytes.
The main units of domestic production of electrolytes are Guotai Ronghua, affiliated with Jiangsu Guotai (002091), Tianjin Taurus, Dongguan Sugian, Shantou Jinguang, Zhuhai Saiwei Electronics, Guangzhou Tiandi, and Beijing Zhenya Chemical Company. The annual production capacity is above 1,000 tons, covering various high, medium and low-end markets, and can meet the needs of China's lithium-ion battery production and some exports.
Table 1: Main domestic electrolyte producers
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