What are the relevant detection methods and methods for lithium battery diaphragm?

As another core material of lithium-ion batteries, the performance of the diaphragm directly determines the interface structure and internal resistance of the battery, which in turn directly affects the battery's electrical performance. It is well known that the role of the diaphragm is to separate the positive and negative poles of the battery, prevent positive and negative polar contact from causing safety problems, and at the same time, its microporous structure can enable the function of electrolytic liquid ions to pass through. In addition, the longitudinal and transverse tensile strength of the diaphragm ensures that the diaphragm does not deform when it receives a certain amount of external force, and its thermal stability can also guarantee the safety performance of the battery at high temperature failure. The second part of this series will start with the testing principle and method of the diaphragm, and make a simple understanding of the diaphragm of lithium ion batteries.

1. Thickness:

With the increase of battery energy density, the diaphragm of the battery is also becoming thinner and thinner, and the accuracy of the measurement is also required to be higher and higher. The general enterprise measurement is used for micrometers. There is also a standard measurement method of "GB/T6672 -2001 determination of plastic film and sheet thickness _ mechanical measurement method". There are also corresponding standards for measurement in the international community, but these standards are not formulated for the diaphragm. Therefore, there is a problem of wide test scope and low accuracy. Therefore, accurate companies are generally required to use precision thickeners for measurement. However, due to the softness of the diaphragm material, the pressure at the time of measurement is excessive and the measurement data is inaccurate. Therefore, there are also companies that use non-contact thickeners to measure, but the diaphragm has a porous structure, and the use of non-contact measurements can also lead to inconsistent thickness measurements. Therefore, in the actual measurement process, it is necessary to select different test methods according to the type of diaphragm, and to measure as many points as possible to ensure the consistency of the diaphragm thickness.

2. Curving degree:

Some enterprises are also called arch degrees, which refer to the arcs produced after the diaphragm is cut. When the arcs are obvious, they will cause overlapping pieces, and when they are wound around, they will produce a vortex, causing the pole to expose and short-circuit. The test method is to lay the diaphragm strip on the desktop and compare the parallel degree with the edge of the steel plate ruler to obtain the curvature of the diaphragm.

3. Permeability:

The time it takes for a certain volume of air to pass through the diaphragm under certain conditions, also known as the Gurley value, and its size has a certain influence on the performance of the battery. The ASTMD 726 StandTest Metrometric Restoration Method is generally used(ASTMD 726 StardTestMetrometric Resaction Nonporoso PaperozoofAir)

4. Porosity:

The volume of the voids accounts for the entire volume. The test methods include the suction calculation method and the test method. The suction calculation method is to infiltrate the diaphragm into a known solvent. The gap volume occupied by the liquid is calculated by measuring the mass difference before and after the diaphragm infiltration. The calculation formula is as follows:

The Mercury pressure test method uses external pressure on the diaphragm to press the mercury into the pores of the diaphragm, and then calculates the porosity of the diaphragm by measuring the volume of pressed Mercury. After multiple measurements, the mean value is taken.

5. Aperture distribution:

The aperture parameters can also be calculated by means of a Mercury compressor, which measures the pressure applied by the pressure of the Mercury pressure hole, but it needs to be stated that the results measured by the Mercury compressor include both through and through holes, In addition, the dry diaphragm is a microporous structure that produces stress in the immersion of mercury, so the capillary flow analyzer is also used to measure the actual test. The inert gas is used to break through the wet diaphragm and the pressure value of the gas outflow is measured. The aperture parameters are calculated.

6. Permeability:

The contact angle measurement method is generally used. Its principle has been described in detail in the introduction of negative polar related knowledge. It is not repeated here.

7. Surface state:

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

8. Mechanical properties

1) tensile strength, elongation: Reflects the mechanical properties of the diaphragm transverse(TD) and longitudinal(MD), the force stretched to the diaphragm until it breaks, and is generally measured by a pull instrument. The dry and wet methods can be seen. There is a significant difference;

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

9. Thermal performance

1) Thermal shrinkage: The rate of change in the size of the diaphragm before and after heating is also divided into horizontal(TD) and longitudinal(MD) shrinkage rates. The test temperature and time of each manufacturer are now different. There are 85 °C 2H, 90 °C 24h, 105 °C 2H, 130 °C 0.5 H, etc., and different tests can be conducted according to different needs; With the application of ceramic diaphragm, the thermal shrinkage of the diaphragm is also lower and lower.

2) DSC test: The main test is the closure of the diaphragm and the temperature of the rupture film, measured by a differential scanning calorimeter.

10. Electrical performance

The performance of the diaphragm and the positive and negative poles, electrolytes, and other tests such as doubling, high low temperature, storage, cycling, internal resistance, and safety are compared and are not described here.

Summary:

As one of the four main materials, although its composition is relatively single, there are still many projects to test. With the development of technology, the ceramic diaphragm has been widely used. New diaphragm such as the coated diaphragm, functional coating diaphragm, and non-woven diaphragm have gradually been applied to lithium ion batteries. It is believed that in the future, more diaphragm with high safety and high mechanical properties will gradually enter the lithium ion battery industry..

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