With the increase of energy density of lithium battery, the diaphragm of the battery is thinner and thinner, and the measurement accuracy is higher and higher. Micrometers used in general enterprises also have the standard measurement method of "GB/T6672-200 1 Plastic film and sheet thickness measurement _ mechanical measurement method", and there are corresponding international standards for measurement, but these standards are not formulated for diaphragms, so the test range is relatively wide.
However, due to the soft material of diaphragm, the measurement data will be inaccurate due to excessive pressure, so some enterprises use non-contact thickness gauge for measurement. However, the diaphragm has a porous structure, and non-contact measurement will also lead to inconsistent thickness measurement. Therefore, it is necessary to choose different testing methods according to the type of diaphragm in the actual measurement process.
What are the detection methods of lithium battery diaphragm technology?
1. thickness
There are two main online control technologies for the thickness of lithium battery diaphragm, one is MD (longitudinal) control, and the other is CD (transverse) control. The second is the on-line machine vision inspection technology of diaphragm. The so-called MD control and CD control refer to the closed-loop thickness control in MD direction by controlling the rotating speed or traction speed of the feeding screw; The closed-loop control of data in CD direction is finally realized by measuring data through scanning frame.
2. Bending degree
Some enterprises are also called camber, which refers to the arc generated after the lithium battery diaphragm is cut. When the arc shape is obvious, it will cause uneven lamination, and when it is wound, it will produce vortex, which will cause the pole piece to be exposed and then short-circuited. The test method is to put the diaphragm strip flat on the desktop and compare it with the parallelism of the edge of the steel ruler to get the radian of the diaphragm.
3. Breathability
The time required for a certain volume of air to pass through the separator under certain conditions, also known as the Gurley value, has a certain influence on the performance of lithium batteries, and ASTM testing method is generally used.
4. Porosity
The ratio of void volume to total volume can be measured by liquid absorption calculation method and test method. The calculation method of liquid absorption is to soak the lithium battery diaphragm in a known solvent, and calculate the void volume occupied by the liquid by measuring the mass difference before and after the diaphragm is soaked. Mercury injection test method is to use external force to press mercury into the pores of the diaphragm, and then calculate the porosity of the diaphragm by measuring the volume of mercury injection, and take the average value after many measurements.
5. Stomatal distribution
It can also be measured by mercury intrusion meter, and the pore size parameters can be calculated by mercury intrusion method. However, it should be noted that the results measured by mercury porosimeter include through holes and non-through holes, and the separator of dry lithium battery will produce stress to destroy the microporous structure of the separator when mercury is immersed, so capillary flow analyzer will also be used for measurement in actual test, and inert gas will be used to break through the wet separator to measure the pressure value of gas outflow, and the aperture parameters can be obtained through calculation.
Step 6 infiltrate
Generally, the contact angle measurement method is used, and its principle has been introduced in detail in the introduction of negative electrode related knowledge, so I will not repeat it here.
7. Electrical performance
The lithium battery separator, anode and cathode and electrolyte were assembled together, and the performance of different separators was compared by testing the rate, high and low temperature, storage, circulation, internal resistance and safety.
As one of the four main materials, the lithium battery separator has a single composition, but there are still many items to be tested. With the development of technology, ceramic separators have been widely used, and new separators such as coated separators, functional coated separators and non-woven separators have been gradually applied to lithium-ion batteries. It is believed that in the near future, more separators with high safety and high mechanical properties will gradually enter the lithium-ion battery industry.