As another core material in lithium-ion batteries , the separator's performance directly determines the battery's interface structure and internal resistance, thus directly affecting its electrical performance. As is well known, the separator's function is to separate the positive and negative electrodes, preventing contact that could lead to safety issues. Simultaneously, its microporous structure allows electrolyte ions to pass through. Furthermore, the separator's longitudinal and transverse tensile strength ensures it doesn't deform under certain external forces, and its thermal stability guarantees the battery's safety performance in the event of high-temperature failure. This second article in the series will begin with the testing principles and methods of separators, providing a clear and concise understanding of lithium-ion battery separators.
1. Thickness:
As battery energy density increases, battery separators are becoming thinner, requiring higher measurement accuracy. While companies typically use micrometers, there's also a standard measurement method: "GB/T6672-2001 Determination of Thickness of Plastic Films and Sheets - Mechanical Measurement Method." International standards also exist, but these are not specifically designed for separators, resulting in wide testing ranges and low accuracy. Companies requiring high accuracy generally use precision thickness gauges. However, due to the softness of the separator material, excessive pressure during measurement can lead to inaccurate data. Some companies use non-contact thickness gauges, but the porous structure of separators can cause inconsistent thickness measurements with non-contact methods. Therefore, in actual measurement, different testing methods should be selected based on the type of separator, and multiple measurement points should be taken to ensure consistent separator thickness.
2. Curvature:
Some companies also call it arching, which refers to the arc shape produced after the diaphragm is cut. A significant arc can cause uneven stacking of the sheets, create vortices during winding, and lead to exposed electrodes and short circuits. The testing method involves laying the diaphragm strip flat on a table and comparing its parallelism with the edge of a steel ruler to obtain the diaphragm's arc.
3. Breathability:
The time required for a given volume of air to pass through the diaphragm under certain conditions is also known as the Gurley value. Its value has a certain impact on battery performance and is generally tested using the ASTM test method (ASTM D726 Standard Test Method for Resistance of Nonporous Paper to Passage of Air).
4. Porosity:
The proportion of the void volume to the total volume can be determined by two methods: the liquid absorption calculation method and the test method. The liquid absorption calculation method involves immersing the diaphragm in a known solvent and calculating the void volume occupied by the liquid by measuring the mass difference of the diaphragm before and after immersion. The calculation formula is as follows:
Mercury intrusion porosimetry (MIP) involves applying pressure to a diaphragm to force mercury into its pores. The porosity of the diaphragm is then calculated by measuring the volume of mercury injected. The average value is taken after multiple measurements.
5. Aperture distribution:
Mercury porosimetry can also be used for measurement. Mercury porosimetry involves measuring the pressure applied when mercury is injected into the pore to calculate the pore size parameters. However, it should be noted that the results measured by mercury porosimetry include both through-hole and non-through-hole measurements. Furthermore, the mercury immersion process in dry membranes can cause stress that damages the microporous structure of the membrane. Therefore, in actual testing, a capillary flow analyzer is also used for measurement. An inert gas is used to break through the wetted membrane, and the pressure of the gas outflow is measured. The pore size parameters are then calculated.
6. Wetting properties:
The contact angle measurement method is generally used. The principle of this method has been explained in detail in the introduction of negative electrode related knowledge, and will not be repeated here.
7. Surface condition:
SEM can reveal the surface condition of the membrane and distinguish the types of membranes.
8. Mechanical properties
1) Tensile strength and elongation: These reflect the mechanical properties of the diaphragm in the transverse (TD) and longitudinal (MD) directions. They represent the force required to stretch the diaphragm until it breaks. They are generally measured using a tensile tester, and there is a significant difference between dry and wet methods.
2) Puncture strength: This assesses the force exerted by a sharp external object when it penetrates the separator. It is strongly correlated with the safety performance of the battery and can be measured using specialized testing equipment.
9. Thermal properties
1) Heat shrinkage rate: The rate of change in diaphragm dimensions before and after heating, which is also divided into transverse (TD) and longitudinal (MD) shrinkage rates. Currently, different manufacturers use different test temperatures and times, such as 85℃ for 2 hours, 90℃ for 24 hours, 105℃ for 2 hours, 130℃ for 0.5 hours, etc. Different tests can be performed according to different needs. With the application of ceramic diaphragms, the heat shrinkage rate of diaphragms is also getting lower and lower.
2) DSC test: This test mainly examines the pore closure and rupture temperatures of the diaphragm, and is performed using a differential scanning calorimeter.
10. Electrical properties
The performance of different diaphragms was compared by assembling them together with the positive and negative electrodes and electrolyte, and the tests included rate capability, high and low temperature, storage, cycling, internal resistance, and safety. These details will not be elaborated here.
summary:
As one of the four main materials, the separator, although its composition is relatively simple, still has a lot of testing items. With the development of technology, ceramic separators have been widely used, and new types of separators such as coated separators, functional coated 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 performance will gradually enter the lithium-ion battery industry.
