The main factors affecting the performance of 18650 lithium battery separators include thickness, air permeability, wettability, chemical stability, pore size, puncture strength, thermal shutdown temperature, and porosity. These factors directly affect the quality of lithium battery products. Let's take a look at the performance parameters required for these 18650 lithium battery separators.
1. Thickness
For consumable lithium-ion batteries (batteries used in mobile phones, laptops, and digital cameras), a 25 micron separator is gradually becoming the standard. However, due to the increasing use of portable products, thinner diaphragms, such as 20 microns, 18 microns, 16 microns, and even thinner diaphragms, have begun to be widely used.
For power batteries, due to the mechanical requirements of the assembly process, thicker separators are often required. Of course, safety is also very important for large power batteries, and thicker separators often mean better safety. EV/HEV use separators with a thickness of about 40 micrometers.
2. Air permeability
MacMullin number: The ratio between the resistivity of a membrane containing electrolyte and the resistivity of the electrolyte itself. The smaller the value, the better. The value for a consumer 18650 lithium-ion battery is close to 8.
Gurley number: The time required for a certain volume of gas to pass through a certain area of diaphragm under certain pressure conditions. The internal resistance of the battery assembled with a diaphragm is directly proportional, that is, the larger the value, the greater the internal resistance.
It is meaningless to simply compare the Gurley numbers of two different diaphragms, as the microstructure of the two diaphragms may be completely different; But the Gurley number of the same type of diaphragm can well reflect the magnitude of internal resistance, because the microstructure of the same diaphragm is relatively the same or comparable.
3. Wettability
To ensure that the internal resistance of the battery is not too high, it is required that the separator can be completely soaked by the electrolyte used in the battery, which is related to the separator material itself and the surface and internal microstructure of the separator.
Rough judgment: Take a typical electrolyte (such as EC: DMC=1:1, 1M LiPF) and drop it on the surface of the diaphragm to see if the droplets will quickly disappear and be absorbed by the diaphragm.
Accurate judgment: Use an ultra-high time resolution camera to record the process from the contact of the droplet with the diaphragm to the disappearance of the droplet, calculate the time, and compare the infiltration degree of the two diaphragms based on the length of time.
4. Chemical stability of 18650 lithium battery separator
The membrane is required to be inert in electrochemical reactions, and not reactive to strong reduction and oxidation, with no attenuation of mechanical strength and no generation of impurities. It is generally believed that the current materials used for membranes, such as PE or PP, can meet the requirements for chemical inertness.
5. Aperture of 18650 lithium battery separator
To prevent electrode particles from directly passing through the diaphragm, a diaphragm aperture of 0.01 is required-
0.1um, when less than 0.01um, the lithium ion penetration ability is too small, greater than
At 0.1um, the battery is prone to short circuit when dendrites are generated inside the battery.
The electrode particles currently used are generally in the range of 10 microns, while the conductive additives used are in the range of 10 nanometers. Fortunately, carbon black particles tend to aggregate to form larger particles. Generally speaking, a diaphragm with a sub micron pore size is sufficient to prevent the direct passage of electrode particles. However, it cannot be ruled out that some electrode surfaces may not be well treated, and there may be situations such as micro short circuits caused by excessive dust.
6. Puncture strength of 18650 lithium battery separator
Puncture strength: At a certain speed (3-5 meters per minute), the maximum force applied to the needle to penetrate the annular fixed diaphragm through a 1mm diameter needle without sharp edges.
Due to the significant difference between the testing method used and the actual situation in the battery, it is not particularly reasonable to directly compare the puncture strength of the two types of separators. However, under a certain microstructure, those with high puncture strength have a relatively low assembly defect rate. But simply pursuing high puncture strength will inevitably lead to a decrease in other performance of the diaphragm. ⑦ thermal stability
The separator needs to maintain thermal stability within the temperature range used by the battery (-20 ℃~60 ℃). Generally speaking, the PE or PP materials currently used for diaphragms can meet the above requirements.
Normally, under vacuum conditions, with a constant temperature of 90 ℃ for 60 minutes, the transverse and longitudinal contraction of the diaphragm should be less than 5%.
7. Thermal closing temperature of 18650 lithium battery separator
Hot shutdown temperature: The temperature at which the simulated battery (with a separator sandwiched between two flat electrodes and electrolyte used for general-purpose lithium-ion batteries) is heated, when the internal resistance increases by three orders of magnitude.
Closed cell temperature: The temperature at which the heat generated by external short circuits or abnormally high currents causes the micropores in the diaphragm to close.
Melt rupture temperature: The temperature at which the diaphragm is heated to exceed the melting point of the specimen, causing it to rupture.
8. The porosity of 18650 lithium battery separator
The porosity of most lithium-ion battery separators ranges from 30% to 50%. There is a certain relationship between the size of porosity and internal resistance, but the absolute value of porosity between different types of membranes cannot be directly compared.