What are the testing principles and detection methods of lithium battery separator technology? At present, almost all domestic lithium battery separator thickness measurement equipment only has detection functions, and few can achieve closed-loop control of separator thickness. In this article, the editor will start with the testing principles and methods of separators to gain an in-depth understanding of the separators of lithium-ion batteries.

　　Current status of lithium battery separator technology

　　Lithium battery separator technology is currently a pain point in my country's power battery industry. Among lithium-ion battery materials, positive and negative electrode materials and electrolytes are basically domestically produced, while separators started late, and the technological maturity of domestic companies is not high. Although the localization rate of my country's lithium battery separators has been increasing in recent years, it mainly occupies the low-end 3C battery separator market. The localization rate of high-end separators is still very low. High-end 3C batteries and power battery separators still rely heavily on imports. .

　　It is reported that separators are a high value-added material with the highest technical barriers among lithium battery materials, accounting for about 15% of the cost of lithium batteries. The technical difficulties lie in the engineering technology of pore creation, matrix materials, and manufacturing equipment. A common problem with domestic lithium battery separators is low consistency, which is mainly manifested in irregular defects, substandard porosity, uneven thickness, pore distribution, and pore size distribution. The porosity and pore size distribution of uniaxially stretched domestic PP separators are relatively close to foreign products; the porosity of bidirectional step-stretched PE separators is usually lower than that of foreign products, and the pore size distribution is not ideal.

　　In the field of wet separators, domestic separator companies are limited by many factors such as process and technology. The product level is still low, and production equipment relies heavily on imports. Like chips, high-end diaphragm technology has a very high threshold. It not only requires huge investments, but also requires a strong R&D and production team, sophisticated process technology and high-level production lines, and it will not be possible to break through in a short time.

　　Lithium battery separator technology testing principle

　　The principle of online detection of diaphragm thickness is mainly to convert the attenuation of rays after passing through the diaphragm into diaphragm thickness. This means that the accuracy and stability of the detection must be ensured during the detection process.

　　As the energy density of lithium batteries increases, the separator of the battery becomes thinner and thinner, and the measurement accuracy is also required to be higher and higher. Generally, enterprises use micrometers for measurement, and there is also "GB/T6672-2001 Plastic Film and Sheet Thickness Determination_Mechanical Measurement Method" "This standard measurement method also has corresponding international standards for measurement, but these standards are not formulated for diaphragms, so there are problems such as wide test range and low accuracy. Therefore, companies that require accuracy generally use precision thickness gauges. Measurement.

　　However, due to the soft material of the diaphragm, excessive pressure during measurement will lead to inaccurate measurement data. Therefore, some companies use non-contact thickness gauges for measurement. However, the diaphragm has a porous structure, and non-contact measurement will also cause thickness measurement. Inconsistency problem, so in the actual measurement process, different testing methods need to be selected according to the type of diaphragm.

　　What are the testing methods for lithium battery separator technology?

　　1.Thickness

　　There are two main online control technologies for lithium battery separator thickness, one is MD (vertical) control and CD (transverse) control; the other is separator online machine vision inspection technology. The so-called MD control and CD control refer to the closed-loop thickness control in the MD direction by controlling the feeding screw speed or traction speed; and the data measurement through the scanning frame, and finally the closed-loop data control in the CD direction.

　　2. Curvature

　　Some companies also call it camber, which refers to the arc produced after slitting the lithium battery separator. When the arc is obvious, it will cause the laminations to be uneven, and create a vortex during winding, causing the pole pieces to be exposed and cause short circuits. The test method is to lay the diaphragm strip flat on the table and compare the parallelism with the edge of the steel ruler to get the curvature of the diaphragm.

　　3. Breathability

　　The time required for a certain volume of air to pass through the diaphragm under certain conditions is also called the Gurley value. Its size has a certain impact on the performance of lithium batteries. The ASTM test method is generally used.

　　4.Porosity

　　The volume of the void accounts for the proportion of the entire volume. The test methods include the liquid absorption calculation method and the test method. The liquid absorption calculation method is to immerse the lithium battery separator in a known solvent, and calculate the amount of the separator occupied by the liquid by measuring the mass difference before and after the separator is infiltrated. Void volume, mercury intrusion test method uses external force to apply pressure on the diaphragm to press mercury into the pores of the diaphragm, and then calculates the porosity of the diaphragm by measuring the volume of mercury pressed into the diaphragm. After multiple measurements, the average value is taken.

　　5.Pore size distribution

　　A mercury porosimeter can also be used for measurement. The mercury porosimeter is used to measure the pressure exerted by mercury into the hole to calculate the pore diameter parameters. However, it should be noted that the results measured by the mercury porosimeter include through holes and non-through holes, and the dry method When mercury is immersed in the lithium battery separator, it will generate stress and damage the microporous structure of the separator. Therefore, a capillary flow analyzer will be used for measurement during actual testing. Inert gas is used to break through the wetted separator, and the pressure value of the gas outflow is measured and calculated. Aperture parameters.

　　6. Infiltration

　　The contact angle measurement method is generally used. Its principle has been introduced in detail in the introduction of negative electrode related knowledge and will not be repeated here.

　　7. Electrical properties

　　The lithium battery separator, positive and negative electrodes, and electrolyte are assembled together to conduct tests such as rate, high and low temperature, storage, circulation, internal resistance, and safety to compare the performance of different separators.

　　As one of the four main materials, lithium battery separators have a relatively single composition, but there are still many test items. With the development of technology, ceramic separators have been widely used, including rubber-coated separators and functional coatings. New separators such as separators and non-woven separators have also 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.

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