Detailed explanation of the performance parameter requirements of lithium-ion battery separators

Regarding consumable lithium-ion batteries (batteries used in mobile phones, notebook computers, and digital cameras), 25-micron separators are gradually becoming the standard. However, due to the increasing use of portable products, thinner diaphragms, such as 20 micrometers, 18 micrometers, 16 micrometers, and even thinner diaphragms, have begun to be used in a wide range of applications. Regarding power lithium batteries, due to the mechanical requirements of the assembly process, thicker diaphragms are often required. Of course, safety is also very important for large power batteries, and thicker diaphragms often mean better safety at the same time. EV/HEV uses a diaphragm with a thickness of about 40 microns. ② Air permeability MacMullin number: the ratio between the resistivity of the electrolyte-containing diaphragm and the resistivity of the electrolyte itself. The smaller the value, the better, and the value of consumable lithium-ion batteries is close to 8. Gurley number: the time required for a certain volume of gas to pass through a certain area of u200bu200ba diaphragm under certain pressure conditions. It is proportional to the internal resistance of the battery assembled with the diaphragm, that is, the larger the value, the greater the internal resistance. It is meaningless to simply compare the Gurley numbers of two different diaphragms, because the microstructures of the two types of diaphragms may be completely different; but the Gurley number of the same type of diaphragm can reflect the size of the internal resistance, because the same type of diaphragm Relatively speaking, the microstructure is the same or comparable. ③The degree of wettability is to ensure that the internal resistance of the battery is not too large. It is required that the diaphragm be completely infiltrated by the electrolyte used in the battery. This is related to the diaphragm material itself and the surface and internal microstructure of the diaphragm. Rough judgment: take a typical electrolyte (such as EC:DMCu003d1:1, 1MLiPF) and drop it on the surface of the diaphragm to see if the droplets will disappear quickly and be absorbed by the diaphragm. Accurate judgment: Use an ultra-high time-resolution camera to record the process from the droplet contacting the diaphragm to the disappearance of the droplet, calculate the time, and compare the infiltration degree of the two diaphragms through the length of time. ④The chemical stability requires that the diaphragm is inert in the electrochemical reaction, and is inactive to strong reduction and strong oxidation, the mechanical strength is not attenuated, and no impurities appear. It is generally believed that the current PE or PP materials for diaphragms can meet the requirements of chemical inertness. ⑤The pore size prevents the electrode particles from directly passing through the diaphragm. The diaphragm pore size is required to be 0.01-0.1um. When it is less than 0.01um, the lithium ion penetration ability is too small. When it is greater than 0.1um, the battery is easy to short-circuit when dendrites are generated inside the battery. The electrode particles currently used are generally on the order of 10 microns, while the conductive additives used are on the order of 10 nanometers. Fortunately, the general carbon black particles tend to agglomerate to form large particles. Generally speaking, the sub-micron pore size membrane is sufficient to prevent the direct passage of electrode particles. Of course, it is not ruled out that some electrode surfaces are not treated well, and some situations such as micro short circuits caused by more dust are not excluded. ⑥ Puncture strength Puncture strength: at a certain speed (3-5 meters per minute), a needle with a diameter of 1mm without sharp edges is pierced to the ring-shaped fixed septum, which is the maximum value applied to the needle to penetrate the septum force. Because the method used in the test is very different from the actual battery situation, it is not very reasonable to directly compare the puncture strength of the two types of separators. However, when the microstructure is fixed, the puncture strength is relatively high, and its assembly The defective rate is low. However, the pure pursuit of high puncture strength will inevitably lead to the degradation of other properties of the diaphragm. ⑦The thermally stable diaphragm should be thermally stable within the temperature range of the battery (-20℃~60℃). Generally speaking, PE or PP materials currently used in diaphragms can meet the above requirements. Normally, under vacuum conditions, at a constant temperature of 90°C for 60 minutes, the transverse and longitudinal shrinkage of the diaphragm should be less than 5%. ③Thermal shut-off temperature The thermal shut-off temperature: the temperature at which the simulated battery (with a diaphragm between two planar electrodes and the electrolyte for general lithium-ion batteries) is heated, when the internal resistance is increased by three orders of magnitude. Closed cell temperature: The temperature at which the diaphragm pores are blocked by the heat generated when an external short circuit or abnormal large current passes. Melting rupture temperature: The temperature at which the diaphragm is heated when the temperature exceeds the melting point of the sample to cause the sample to rupture. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. 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