As a core material of lithium-ion batteries, the performance of the diaphragm directly determines the interface structure and internal resistance of the battery, which directly affects the electrical performance of the battery. So what are the quality testing methods for the lithium battery diaphragm?

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thickness

With the increase of battery energy density, battery diaphragms are getting thinner and thinner, and the accuracy of measurement is also getting higher and higher. General companies use micrometers for measurement, and there is also “GB/T 6672-2001 Plastic film and sheet thickness measurement_mechanical measurement “Method” is a standard measurement method. There are also corresponding international standards for measurement, but these standards are not formulated for diaphragms, so there are problems such as wide testing range and low accuracy. Therefore, companies that require accuracy generally use precision thickness gauges. However, due to the soft diaphragm material, the pressure during measurement is too large and the measurement data is inaccurate. Therefore, some companies use non-contact thickness gauges for measurement. However, the diaphragm has a porous structure, so it will also be used for non-contact measurement. This leads to the problem of inconsistent thickness measurement, so in the actual measurement process, you need to choose different test methods according to the type of diaphragm, and try to measure as many points as possible to ensure the consistency of diaphragm thickness.

Curvature

Some companies also call the degree of arching, which refers to the arc formed after the diaphragm is slit. When the arc is obvious, the laminations will be uneven, and the vortex will be generated during winding, which will cause the pole pieces to be exposed and short-circuit. The test method is to lay the diaphragm flat on the desktop and compare the parallelism with the edge of the steel ruler to get the curvature of the diaphragm.

Air permeability

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 the battery. Generally, the ASTM test method (ASTM D726 Standard Test Method for Resistance of Nonporous Paper to Passage of Air)

Porosity

The ratio of the volume of the void to the entire volume. The test methods include liquid absorption calculation method and test method. The liquid absorption calculation method is to immerse the diaphragm in a known solvent, and calculate the gap occupied by the diaphragm by measuring the mass difference before and after the infiltration of the diaphragm. Volume, the calculation formula is as follows:

The mercury intrusion test method uses an external force to apply pressure to the diaphragm to press mercury into the pores of the diaphragm, and then calculate the porosity of the diaphragm by measuring the volume of the injected mercury, and take the average value after multiple measurements.

Pore size distribution

The mercury intrusion meter can also be used for measurement. The mercury intrusion method is to measure the pressure applied by the mercury intrusion hole to calculate the pore size parameters. However, it should be noted that the results measured by the mercury intrusion meter include through holes and non-through holes, and dry method When mercury is immersed in the diaphragm, stress will damage the microporous structure of the diaphragm. Therefore, the actual test will also be measured by a capillary flow analyzer. The inert gas is used to break through the wetted diaphragm, and the pressure value of the gas flowing out is measured, and the pore size is calculated. parameter.

Infiltration

The contact angle measurement method is generally used, and its principle has been described in detail in the introduction of negative electrode related knowledge, so I will not repeat it here.

surface condition

The surface state of the diaphragm can be seen through the SEM, and the type of the diaphragm can be distinguished.

Mechanical behavior

1) Tensile strength and elongation: reflect the mechanical properties of the diaphragm in the transverse direction (TD) and the longitudinal direction (MD). The force that stretches to the diaphragm until it breaks is generally measured with a tensile tester. It can be seen that there are Significant difference

2) Puncture strength: Evaluate the force of external sharp objects penetrating the diaphragm, which is strongly related to the safety performance of the battery and can be measured with special testing equipment.

Thermal performance

1) Thermal shrinkage rate: the change rate of the diaphragm size before and after heating is also divided into transverse (TD) and longitudinal (MD) shrinkage rates. Now the test temperature and time of each manufacturer are different, including 85°C 2h, 90°C 24h, 105℃2h, 130℃0.5h, etc., different tests can be carried out according to different needs; with the application of ceramic diaphragm, the thermal shrinkage rate of the diaphragm is getting lower and lower;

2) DSC test: Mainly test the temperature of closed pores and membrane rupture of the diaphragm, measured with a differential scanning calorimeter.

Electrical properties

Assemble the separator, the positive and negative electrodes, and the electrolyte to compare the performance of different separators by testing the ratio, high and low temperature, storage, circulation, internal resistance, and safety, which will not be repeated here.

As one of the four main materials, the function of the separator is to separate the positive and negative electrodes of the battery to prevent short circuit problems caused by the contact between the positive and negative electrodes. At the same time, its microporous structure can allow electrolyte ions to pass through. In addition, the longitudinal and transverse tensile strength of the diaphragm ensures that the diaphragm does not deform when receiving a certain external force, and its thermal stability can also ensure the safety performance of the battery when the battery fails at high temperatures.

Although its composition is relatively simple, there are still many items tested. With the development of technology, ceramic diaphragms have been widely used, and new types of diaphragms such as rubber-coated diaphragms, functional coated diaphragms, and non-woven diaphragms have gradually Applied to lithium-ion batteries, I believe that in the near future, more high-safety, high-mechanical performance separators will gradually enter the lithium-ion battery industry.