Lithium battery electrolyte composition
Electrolyte is the 'blood' of lithium-ion batteries. It conducts electrons between the positive and negative electrodes in the battery. It is a guarantee for lithium-ion batteries to obtain high voltage, high specific energy and other advantages. Electrolyte is generally prepared from high-purity organic solvents, electrolyte lithium salt (lithium hexafluorophosphate, LiFL6), necessary additives and other raw materials under certain conditions in a certain proportion.
1. Organic solvent
Organic solvent is the main part of the electrolyte, and the performance of the electrolyte is closely related to the performance of the solvent. Commonly used solvents in lithium ion battery electrolytes include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. Generally, propylene carbonate (PC) is not used. ), ethylene glycol dimethyl ether (DME) and other solvents mainly used in lithium primary batteries. PC is used in secondary batteries and has poor compatibility with the graphite negative electrode of lithium ion batteries. During charging and discharging, PC decomposes on the surface of the graphite negative electrode, causing the graphite layer to peel off at the same time, causing the cycle performance of the battery to decrease. However, a stable SEI membrane can be established in the 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-ion batteries, such as EC+DMC, EC+DEC and so on. The same electrolyte lithium salt, such as LiPF6 or LiC104, the PC+DME system always shows the worst charge and discharge performance for mesocarbon microsphere C-MCMB material (compared to EC+DEC, EC+DMC system). But it is not absolute. When PC and related additives are used in lithium-ion batteries, it is beneficial to improve the low-temperature performance of the battery.
The quality of organic solvents must be strictly controlled before use. If the purity is required to be above 99.9%, the moisture content must be below 10*106. There is a close relationship between the purity of the solvent and the stable voltage. The oxidation potential of organic solvents with up to standard purity is about 5V. The oxidation potential of organic solvents is of great significance for the research on preventing battery overcharge and safety. Strictly controlling the water content of organic solvents has a decisive influence on the preparation of qualified electrolytes.
The moisture drops below 10*106, which can reduce the decomposition of LiPF6, slow down the decomposition of SEI film, and prevent air swelling.
Using molecular sieve adsorption, atmospheric or vacuum distillation, and inert gas introduction, the moisture content can meet the requirements.
2. Electrolyte lithium salt
LiPF6 is the most commonly used electrolyte lithium salt, and it is the direction of lithium salt development in the future. Although LiClO4, LiAsF6, etc. are also used as electrolytes in the laboratory, the high-temperature performance of the battery using LiC104 is not good, plus LiC10: it is easy to explode by impact, and it is also a strong oxidant. It is not suitable for industrialized large-scale use of lithium-ion batteries.
LiPF6 is stable to the negative electrode, has large discharge capacity, high conductivity, low internal resistance, fast charge and discharge speed, but it is extremely sensitive to moisture and HF acid, and is prone to react. It can only be operated in a dry atmosphere ( For example, the environmental moisture is less than 20×10 in a glove box), and is not resistant to high temperatures. The decomposition reaction occurs at 80°C to IO0°C to produce phosphorus pentafluoride and lithium fluoride, which is difficult to purify. Therefore, when preparing the electrolyte, control the dissolution of LiPF6 Self-decomposition caused by heat and thermal decomposition of solvents. The percentage of LiPF produced in China can generally meet the standard, but the HF acid content is too high to be directly used in the preparation of electrolytes and must be purified. In the past, LiPF6 relied on imports, but now some domestic manufacturers can also provide good quality products, such as Shantou Jinguang High-tech Co., Ltd., Tianjin Chemical Design and Research Institute, and Shandong Feicheng Xingtai Chemical Plant. The quality of LiPF6 produced abroad is better. It is formulated into electrolyte with stable moisture and HF acid content, and the electrolyte will not become sticky or red.
There are many types of additives. Different lithium battery pack manufacturers have different requirements for battery use and performance, and the focus of the selected additives is also different. Generally speaking, the additives used mainly have three functions:
(1) Adding anisole to the electrolyte improves the performance of the SEI membrane. Adding anisole or its halogenated derivatives to the electrolyte of lithium-ion batteries It 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 anisole reacts with the reduction product of the solvent to produce LiOCH, which is conducive to 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 that the battery can release above 3.6V, reflecting the high-current discharge characteristics of the battery to a certain extent. In actual operation, we found that adding anisole to the electrolyte can prolong the discharge platform of the battery and increase the discharge capacity of the battery.
(2) Adding metal oxides to reduce the trace water and HF acid in the electrolyte As mentioned above, lithium ion batteries have very strict requirements on the water and acid in the electrolyte. Carbodiimide compounds can prevent LiPFs from being hydrolyzed into acid. In addition, some metal oxides such as Al2O3, MgO, BaO, Li2Co3, CaCO3, etc. are used to remove HF. However, compared with the hydrolysis of LiPFs, the acid removal rate is too slow, and it is difficult to filter cleanly. The total content of the three elements Li, P, and F in the lithium battery pack electrolyte is 96.3%, and the total content of other main impurity elements such as Fe, K, Na, CI, and A1 is 0.055%.
(3) Prevent overcharge and overdischarge
Battery manufacturers have very urgent requirements for battery overcharge and discharge resistance. Traditional anti-overcharge passes through the internal protection circuit of the battery. Now it is hoped to add additives to the electrolyte, such as sodium imidazole ring, biphenyls, carbazoles and other compounds, which are in the research stage.
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