Diaphragm detection method and method for lithium ion battery

   

1. Thickness:

As the energy density of the battery increases, the diaphragm of the battery is also thinner and thinner, and the measurement accuracy is also required to be higher and higher. The general enterprise uses a micrometer for measurement, and also has the "GB/T 6672-2001 plastic film and sheet thickness measurement _ mechanical measurement "The law" is a standard measurement method, and there are corresponding standards in the world for measurement. However, these standards are not formulated for the diaphragm, so there are problems such as wide test range and low precision. Therefore, enterprises that require accurate use generally use precision thickness gauges. The measurement is made, but because the diaphragm material is soft, the pressure at the time of measurement is too large, resulting in inaccurate measurement data. Therefore, some companies use non-contact thickness gauges for measurement, but the diaphragm has a porous structure and can be used for non-contact measurement. The problem of inconsistent thickness measurement is caused, so in the actual measurement process, different test methods are selected according to the type of the diaphragm, and several points are measured as much as possible to ensure the consistency of the diaphragm thickness.

2. Bending degree:

Some enterprises also call the arch shape, which refers to the arc shape produced by the slitting of the diaphragm. When the arc shape is obvious, the lamination is uneven, and the spiral shape occurs when winding, which causes the pole piece to be exposed and short-circuited. The test method is to flatten the diaphragm strip on the table top and compare the parallelism with the edge of the steel ruler to obtain the curvature of the diaphragm.

3. Air permeability:

The time required for a certain volume of air to pass through the diaphragm under certain conditions, also known as the Gurley value, the size of which has a certain influence on the performance of the battery. The ASTM test method is generally used for ASTM D726 Standard Test Method for Resistance of Nonporous Paper to Passage of Air)

4. Porosity:

The volume of the void accounts for the proportion of the whole volume. The test method includes a liquid absorption calculation method and a test method. The liquid absorption calculation method is to infiltrate the separator in a known solvent, and calculate the void occupied by the liquid by measuring the difference in mass before and after the diaphragm is wetted. Volume, the 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 calculates the porosity of the diaphragm by measuring the volume of the pressed mercury, and takes an average after multiple measurements.

5. Pore size distribution:

It can also be measured by a mercury intrusion meter. The pore size parameter is calculated by the mercury intrusion method, that is, the pressure applied by measuring the mercury intrusion hole, but it should be noted that the results measured by the mercury intrusion meter include through holes and non-through holes, and the dry method When the membrane is immersed in mercury, it will cause stress to damage the microporous structure of the membrane. Therefore, the actual measurement will also be carried out by capillary flow analyzer. The inert gas is used to break the wetted membrane, the pressure value of the gas is measured, and the pore diameter is calculated. parameter.

6. Infiltration:

The contact angle measurement method is generally adopted, and the principle thereof has been described in detail in the introduction of the negative electrode related knowledge, and is not repeated here.

7. Surface status:

The surface state of the separator can be seen by SEM, and the type of the separator is distinguished.

8. Mechanical properties

1) Tensile strength and elongation: reflect the mechanical properties of the transverse direction (TD) and the longitudinal direction (MD) of the diaphragm. The force of stretching until the diaphragm is broken is generally measured by a tensile tester. The dry and wet methods can be seen. Significant difference

2) Puncture strength: The force of the sharp object passing through the diaphragm is evaluated, which is related to the safety performance of the battery. It can be measured with special test equipment.

9. Thermal performance

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

2) DSC test: The temperature of the closed cell of the diaphragm and the temperature of the membrane are mainly tested, and the measurement is performed by a differential scanning calorimeter.

10. Electrical performance

The separator, the positive and negative electrodes, and the electrolyte are assembled together to perform the tests of magnification, high and low temperature, storage, circulation, internal resistance, safety, etc., and the performance of different separators is compared, and will not be described here.

summary:

As one of the four main materials, although the composition of the diaphragm is relatively simple, there are still many test items. With the development of technology, ceramic diaphragms have been widely used, plastic coated diaphragms and functional coated diaphragms. New separators such as non-woven membranes have also been gradually applied to lithium-ion batteries. It is believed that in the future, more high-security, high-mechanical diaphragms will gradually enter the lithium-ion battery industry.