Brief Analysis of Impurities in Lithium-ion Battery Electrolyte and Their Purification

Impurities in the electrolyte of lithium-ion batteries and their purification. The unpurified organic electrolyte usually contains water and organic matter with active hydrogen and a small amount of metal ions such as iron, sodium, aluminum and nickel. The impurity content will have a serious impact on the performance of the lithium-ion battery. High-purity lithium salt and carbonate are the guarantee for the normal operation of lithium-ion batteries. There are three important sources of impurities in organic electrolytes: a. The preparation of lithium electrolyte (such as lithium hexafluorophosphate) inevitably contains a certain amount of metal impurity ions such as HF and H2O. These substances may form strong chemical adsorption, which is difficult to remove, and it is difficult to prepare a completely pure lithium salt. B. Common carbonate solvents inevitably contain by-products such as water, organic acids, alcohols, aldehydes, ketones, amines, amides, metal ion impurities and ester substances with similar structures during the preparation process. C. Lithium salts and solvents are mostly hydrophilic, and a small amount of water and dust in the working environment can also pollute the electrolyte. Some lithium has poor thermal stability, is easy to decompose, and lowers purity. In general, the impurities in organic electrolytes are mainly trace amounts of water, molecules containing active hydrogen and metal ion impurities, such as HF. A small amount of water can decompose lithium salts, such as lithium hexafluorophosphate, and may react with organic solvents to produce corresponding alcohols. When the battery is working, the presence of water will consume limited lithium ions, and there will be substances such as LiOH, Li2O, HF that are not conducive to improving the electrochemical performance of the electrode. At the same time, the large amount of gas generated by the reaction will increase the internal pressure of the battery, which will bring unsafe factors. As the moisture content in the organic electrolyte increases, the charge-discharge performance and cycle efficiency of lithium-ion batteries will significantly decrease. When the water content exceeds 0.1%, the lithium-ion battery is basically useless. The content of HF in the organic electrolyte also has an important impact on the performance of lithium-ion batteries, and HF is difficult to remove during the production process of lithium hexafluorophosphate. In addition to the accelerated decomposition method and the organic solvent for polymerizing lithium, high frequency also generates life. By reducing the reaction of the carbon negative electrode in the first charge and discharge process, the electrode interface resistance is added, the reversibility of the passivation film is reduced, and the specific capacity of the battery And cycle efficiency, etc. Other organic molecules containing active hydrogen, such as alcohol and amines, will also consume electrolytes and affect battery performance. For example, Zhuang Quanchao and others studied the influence of methanol impurities in organic electrolytes on the performance of graphite electrodes, and found that when the content of methanol in the organic electrolyte exceeds 0.5%, the flammability of the graphite electrode during charge and discharge cycles will be completely destroyed. During the charging process, metal impurity ions are first deposited on the surface of the carbon cathode, which affects the electrolytic lithium conductivity. Obviously, the organic electrolyte must be configured in a clean, dry (humidity less than 10×10-6) environment (such as a glove box commonly used in laboratories). At the same time, use a feasible method to eliminate impurities in the lithium salt and organic reagents as much as possible, and slowly configure the electrolyte to prevent the thermal decomposition reaction caused by the exotherm during the dissolution of the lithium salt. Before the electrolyte is used, scientific methods should be used for testing to ensure that the electrolyte meets the working requirements of lithium-ion batteries. The most common impurity in organic electrolytes is water. By removing most of the water by distillation, and then adding 0.4nm or 0.5nm active zeolite for sealing, the purpose of reducing the water content in the laboratory can be achieved. This method can not only reduce water, but also absorb some small molecules of solvent, which is beneficial to improve the purity of the electrolyte. But it should be noted that because molecular sieves have ion exchange function, if ordinary sodium molecular sieves are placed in the electrolyte for a long time, sodium ions may be introduced into the organic electrolyte and affect the battery performance. Therefore, in the process of using molecular sieve to remove impurities, it is best to use lithium molecular sieve. By exchanging 1mol/L LiC1O4/ethanol or 1mol/L LiF/ethanol sol, you can get a lithium zeolite statement: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete the previous one: analysis and polymerization Production process of lithium-ion battery