The main function of the battery separator is to electrically insulate the ions, which prevents the direct electronic contact between the positive and negative electrodes in the battery, but the ions can pass freely. For lithium ion battery separators, the basic requirements are as follows:
1. thickness For consumable lithium-ion batteries (cells, laptops, batteries used in digital cameras), 25-micron diaphragms are becoming standard. However, due to the increasing use of portable products, thinner membranes, such as 20 micron, 18 micron, 16 micron, or even thinner, have begun to be used in a wide range of applications. For power batteries, thicker diaphragms, such as 30 micron, 40 micron, etc., are often required due to the mechanical requirements of the assembly process. Of course, for large power batteries, safety is also very important, and thicker diaphragms often mean better safety. 2. Air permeability From an academic point of view, the diaphragm is inert in the battery, that is, the diaphragm is not a necessary component of the battery, but only the requirements for industrial production of the battery. The existence of the separator must first satisfy the fact that it does not deteriorate the electrochemical performance of the battery, mainly in the internal resistance. The ratio between the resistivity of the electrolyte-containing separator and the resistivity of the electrolyte itself is called the MacMullin number. In general, this value for a consumable lithium-ion battery is close to 8, although the smaller the value, the better. In general, there is a gas permeability parameter in the lithium ion battery separator, or Gurley number. This number is defined as the volume of gas required to pass a certain area of the membrane under a certain pressure. The volume of the gas is generally 50 cc, and some companies will also mark 100 cc. The final result will be twice the difference. . The area should be 1 square inch and the pressure difference is not clear. In a certain sense, this value is proportional to the internal resistance of the battery assembled with the diaphragm, that is, the larger the value, the larger the internal resistance. However, for different diaphragms, the direct comparison of this number does not make any sense. Because the internal resistance in lithium-ion batteries is related to ion conduction, and the gas permeability is related to the gas transfer, the two mechanisms are different. In other words, it is meaningless to simply compare the Gurley numbers of two different membranes, because the microstructures of the two membranes may be completely different; but the size of the Gurley number of the same membrane can well reflect the internal resistance. Because the same membrane is relatively the same or comparable in microstructure. 3. Infiltration degree In order to ensure that the internal resistance of the battery is not too large, the diaphragm is required to be completely wetted by the electrolyte used in the battery. There is no recognized test standard in this regard. It can be judged by the following test: Take a typical electrolyte (such as EC: DMC = 1:1, 1M LiPF6), drop on the surface of the diaphragm to see if the droplets will quickly disappear and be absorbed by the diaphragm. If it is, the wettability is basically satisfied. Claim. A more accurate test can use a very high time-resolved camera to record the process from droplet contact to the disappearance of the droplet, calculate the time, and compare the infiltration of the two membranes over the length of time. Infiltration is related to the membrane material itself on the one hand, and the surface and internal microstructure of the membrane are closely related. 4. Chemical stability In other words, the diaphragm is required to be inert in the electrochemical reaction. After several years of industrial inspection, it is generally believed that the separator material PE or PP is currently required to meet chemical inertness requirements. 5. Aperture In general, in order to prevent direct contact of the electrode particles, it is important to prevent the electrode particles from passing directly through the separator. The electrode particles currently used are typically on the order of 10 microns, while the conductive additives used are on the order of 10 nanometers, but fortunately the general carbon black particles tend to agglomerate to form large particles. In general, the submicron pore size membrane is sufficient to prevent the direct passage of the electrode particles, and it is of course not excluded that some of the electrode surfaces are not well treated, and some of the dust is caused by some such as micro short circuit. 6. Puncture strength This parameter is actually a requirement due to the insufficient surface of the electrode and the limited level of technology in the assembly process. Therefore, the diaphragm is required to have a considerable puncture strength. The puncture strength test can be followed by industry standards, roughly at a certain speed (3-5 meters per minute), allowing a needle with a sharp edge of 1 mm diameter to be fixed to the ring, which is a penetrating diaphragm. The force exerted on the needle is called the puncture strength. Similarly, since the method used in the test differs greatly from the actual battery, it is not particularly reasonable to directly compare the puncture strength of the two membranes, but in the case of a certain microstructure, the puncture strength is relatively high. , its assembly failure rate is low. However, the pursuit of high puncture strength alone will inevitably lead to a decline in other properties of the diaphragm. 7. Thermal stability The diaphragm needs to be thermally stable within the temperature range of the battery (-20C~60C). In general, PE or PP materials used in the current diaphragm can meet the above requirements. Of course, there is also a problem that because the electrolyte is sensitive to moisture, most manufacturers will do about 80C before the injection, which will not have too much problem for the PP/PE diaphragm. 8. Thermal shutdown temperature Due to the serious safety of the content, the current lithium ion battery diaphragm can generally provide an additional function, that is, thermal shutdown. Generally, we heat the principle battery (a diaphragm between two planar electrodes, using a common lithium ion battery electrolyte), and the temperature when the internal resistance is increased by three orders of magnitude is called the thermal shutdown temperature. This feature provides an extra layer of protection for lithium-ion batteries. In fact, the shutdown temperature is closely related to the melting point of the material itself, such as PE near 135C. Of course, different microstructures have a certain effect on the thermal shutdown temperature. But for small batteries, the role of the thermal shutdown mechanism is limited. 9. Porosity At present, the porosity of the separator for lithium ion batteries is about 40%. There is a relationship between the size of the porosity and the internal resistance, but the value of the void ratio between the different types of membranes cannot be directly compared.
January 24, 2024
