Electrolyte is an important part of lithium-ion batteries. It not only transports and conducts current between the positive and negative electrodes, but also determines the working mechanism of the battery to a large extent, affecting the specific energy, safety performance, rate charge and discharge performance, cycle life and production cost of the battery.
The electrolyte conducts electrons between the positive and negative electrodes of the lithium-ion battery, which is the guarantee for the lithium-ion battery to obtain the advantages of high voltage and high specific energy. Electrolyte is generally prepared from high-purity organic solvent, electrolyte lithium salt (lithium hexafluorophosphate, LiFL6), necessary additives and other raw materials under certain conditions in a certain proportion.
1. Organic solvent
The organic solvent is the main component 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 electrolyte are ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. Generally, propylene carbonate is not used (PC). ), ethylene glycol dimethyl ether (DME) and other important solvents used in lithium primary batteries. PC is used in secondary batteries and has poor compatibility with graphite negative electrodes of lithium-ion batteries. During the charge and discharge process, PC decomposes on the surface of the graphite negative electrode, and at the same time causes the graphite layer to peel off, resulting in a decrease in battery cycle performance. 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 chain carbonate is an excellent electrolyte for lithium-ion batteries, such as EC+DMC, EC+DEC and so on. For the same electrolyte lithium salt, such as LiPF6 or LiC104, the PC+DME system always shows the worst charge-discharge performance compared to the mesophase carbon microsphere C-MCMB material (compared to the EC+DEC, EC+DMC system). But this 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 purity up to the standard is about 5V. The oxidation potential of organic solvents is of great significance to the study of battery overcharge prevention and safety. Strictly controlling the water content of organic solvents has a decisive influence on the preparation of qualified electrolytes.
The moisture drop below 10*106 can reduce the decomposition of LiPF6, slow down the decomposition of SEI film, and prevent air expansion.
The use of molecular sieve adsorption, atmospheric or vacuum distillation, and inert gas introduction can make the moisture content 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 electrolyte in the laboratory, the high-temperature performance of the battery using LiC104 is not good, plus LiC10: it is easy to explode when impacted, and it is a strong oxidant, which is not safe when used in batteries. It is not suitable for industrialized large-scale use of lithium-ion batteries.
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 electrolyte in the laboratory, the high-temperature performance of the battery using LiC104 is not good, plus LiC10: it is easy to explode when impacted, and it is a strong oxidant, which is not safe when used in batteries. It is not suitable for industrialized large-scale use of lithium-ion batteries.
3. Additives
There are many types of additives, and different lithium-ion battery manufacturers have different requirements for battery use and performance, and the focus of the additives selected is also different. Generally speaking, the additives used mainly have the following three purposes:
(1) Add anisole to the electrolyte to improve the performance of the SEI membrane. Adding anisole or its halogenated derivatives to the electrolyte of a lithium ion battery can improve the cycle performance of the battery and reduce the irreversible capacity loss of the battery. Huang Wenhuang studied its mechanism and found that LiOCH is formed by the reaction of anisole with the reduction product of the solvent, which is conducive to the formation of an efficient and stable SEI film on the surface of the electrode, thereby improving the cycle performance of the battery. The battery discharge platform can measure the energy that the battery can release above 3.6V, which reflects 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 trace amounts of 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. The carbodiimide compound can prevent LiPFs from being hydrolyzed into acid. In addition, some metal oxides such as Al2O3, MgO, BaO, Li2Co3, CaCO3, etc. are also used to remove HF. However, in 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-ion battery electrolyte is 96.3%, and the total content of other important impurity elements such as Fe, K, Na, Cl, and A1 is 0.055%.
(3) Prevent overcharge and overdischarge. Battery manufacturers have very urgent requirements for battery overcharge and discharge resistance. The traditional anti-overcharge is achieved through the protection circuit inside the battery. Now it is hoped to add additives to the electrolyte, such as sodium imidazole ring, biphenyl, carbazole and other compounds, which is in the research stage.
