Battery diaphragm parameter description

The main function of the battery separator is to separate the positive and negative plates in the battery to prevent direct contact between the positive and negative plates. At the same time, due to the large number of micropores in the diaphragm, the positive and negative ions in the battery can pass freely in the micropores. Between the positive and negative plates, the internal conductive circuit of the battery is formed, and the electrons are transferred between the positive and negative electrodes through the external circuit to form a current for use by the electric device.
(Note: At present, some people explain the diaphragm function, the argument that "the diaphragm can pass ions and the electrons cannot pass" is unfounded and does not conform to the basic principle of the primary battery, because only positive and negative ions exist in the electrolyte inside the battery. Without free electrons, the internal conduction of the battery is achieved by the migration of ions between the positive and negative electrodes.
For lithium ion battery separators, the basic performance parameters are as follows:
  1. thickness:
  2. Air permeability:
  3. Infiltration degree:
  4. Chemical stability:
  5. Aperture and distribution:
  6. Puncture strength:
  7. Thermal stability:
  8. Closed cell temperature, membrane rupture temperature
  9. Porosity:
Basic parameters
1, the thickness of the depletion type <br> for lithium ion battery (cell phones, laptop computers, digital cameras use a battery), a separator 25 microns becoming the 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 are often required due to the mechanical requirements of the assembly process. Of course, for large power batteries, safety is also very important, while 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, the 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 baking around 80C before the injection, which will not have too much problem for the PP/PE diaphragm.
8, hot off 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.
market outlook:
At present, the diaphragm suppliers are mainly the following:
United States: Celgard (three-layer PP/PE/PP), Entek (single-layer PE)
Netherlands: DSM (single layer PE)
Germany: Degussa (inorganic and organic composite film, thicker, mainly suitable for power-type large batteries)
Japan: Asahi, Tonen (single layer PE), UBE (three layer PP/PE/PP)
In addition, there are three to five domestic companies, but the current product performance is not satisfactory.
At present, there are mainly the following problems in domestic production:
1, the porosity is not enough
2, uneven thickness
3, there are pinholes
4, the uniformity is not enough
5, insufficient strength
Bellcore technology uses a copolymer and a solvent to form a gel, which is then scraped onto an electrode or other flat surface, and then the solvent is evaporated to form a porous film, which is then stacked or wound with an electrode sheet and encapsulated in an aluminum film. Then inject the liquid. The specific process is slightly different for each family. This battery uses a self-forming electrolyte that eliminates the need for a PP/PE separator.
Domestically: Gryen in Xinxiang, one in Zhejiang (Hua Rong), in addition to BYD and Shanshan. At present, there are samples of Xinxiang and Zhejiang in the market, but there is still a big gap between performance and imports.
In fact, any polymer film material inevitably has the problem of heat shrinkage, more or less. This is determined by the properties of the polymer, and the closer to the melting point of the polymer, the stronger the shrinkage. For example, in the diaphragm material, the melting point of PE is about 130C. If the diaphragm is heated to about 120 degrees, the shrinkage rate of the free state of the diaphragm will exceed 30%. For the PP/PE/PP three layers, due to the higher melting point of PP, the shrinkage at the high temperature is relatively better.
Some domestic manufacturers have linked the shrinkage rate and safety of the diaphragm. In fact, there is not much truth to be said. The safety of a large shrinkage diaphragm is not necessarily poor, and the safety of shrinkage is not necessarily good. Since the separator is fixed between the electrode sheets in the battery, generally, if the free shrinkage rate of the separator at 15 C for fifteen minutes is less than 5%, substantially no problem occurs.
The impact of other aspects is not obvious.
Of course, if the overcharge voltage exceeds 5V, basically the temperature of the battery will definitely rise, no doubt. If this is the case, then the diaphragm plays a role in it. For example, if the internal temperature of the battery exceeds 130C, the diaphragm should be closed to reduce the current and prevent the temperature from rising further. But if it is a small battery, more should be solved from the battery material and design.
However, it seems that UL's standards are not so high. What kind of customers need such a high overcharge test. Because the purpose of the overcharge test is to say that if the protection circuit is broken, the charging voltage of the battery may exceed 4.2V, reaching about 5V. This standard has some truth. But if it is more than 5V overcharge, it means that the charger transformer is broken. At this time, it is definitely not only 5V. It may be the mains. Does this make sense? Of course, here is a small battery, for example, the capacity <2Ah, the big battery is another matter.
In addition, the uniaxially stretched membrane generally has a smaller shrinkage ratio in the transverse direction than the two-way stretch, but this does not guarantee that the battery will be safer. This is especially true for batteries with high specific capacity (ie, using thinner diaphragms). I have done the corresponding tests.
When the acupuncture test is carried out, it is true that even if the other things are exactly the same, but only the diaphragm is changed, the result is different. However, this is still not directly related to the diaphragm. It can only be said that the two processes do not match. In other words, the diaphragm with problems is not suitable for the battery process, and it requires some adjustment. If it is directly attributed to the diaphragm, it is somewhat reluctant, resulting in a false and wrong case. The specifics are really not very good to say, depending on the specific situation.

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