Lithium ion battery is mainly composed of positive and negative electrode plate and diaphragm, electrolyte, shell and positive and negative extreme son, of which diaphragm plays a crucial role in the internal lithium ion battery. In the interior of a lithium-ion battery, the membrane not only avoids contact between the positive and negative electrodes to achieve the effect of electronic insulation, but also maintains a certain porosity to allow the ions in the electrolyte to pass through the membrane and reciprocate between the positive and negative electrodes. While satisfy the basic requirement of the above, the diaphragm to meet security requirements, such as in the process of charging and discharging cycle may form on the surface of the cathode for lithium dendrite, sharp lithium dendrite development to a certain extent can result in a membrane is short circuit between the cathode and release a lot of heat, causing the lithium ion battery thermal runaway, lead to serious safety accidents. Or in the process of battery extrusion and acupuncture, the local short circuit point releases a large amount of heat, which leads to thermal contraction of the diaphragm, leading to a large area of positive and negative pole contact and directly causing the battery to burst into flames. Therefore, the diaphragm has an important impact on the performance and safety of lithium ion battery.
In order to satisfy the lithium ion battery diaphragm for performance and safety requirements, people developed a variety of composite membrane, such as PP, PE, PP three-layer composite membrane, when the battery temperature higher than 130 ℃, the middle layer of PE melt layer will happen, and on both sides of the PP membrane high melting point, play a supporting role, molten PE jam the pore on the PP membrane, so as to block the effect of discharge. Another example is ceramic coating membrane. Al2O3 and other inorganic oxides are coated on the substrate of ordinary membrane to support the membrane at high temperature and reduce membrane contraction, thus improving the safety of lithium ion battery.
Recently, Xiaowei Zhang et al. from Massachusetts institute of technology studied the mechanical properties of a variety of membranes with different processes and structures, and analyzed the mechanical parameters that lead to diaphragm failure. These membranes include PE diaphragm prepared by dry process and three-layer composite diaphragm, ceramic coating membrane prepared by wet process and membrane prepared by non-woven process, which basically covers the common types of diaphragm in the current market. The uniaxial tensile strength, thickness compression test and axial puncture test of the above mentioned diaphragms in longitudinal (MD), transverse (TD) and diagonal (DD) directions were tested. These tests revealed the failure mechanical parameters of various diaphragms. Xiaowei Zhang et al. established a finite element model of PE diaphragm based on the above results, and accurately predicted the feedback results of PE diaphragm in unidirectional tensile test and thickness compression test.
Specifically, the test process was as follows. Firstly, a rectangular specimen with regular shape was made from the diaphragm materials tested according to the requirements of ASTM specification D882 for thin film materials. The tensile test was conducted by Instron 5944 unidirectional tensile machine, and the loading speed was 25mm/min. The test results showed that there was a big difference in the tensile strength of PE diaphragm and three-layer composite diaphragm prepared by dry process in all directions. For example, in the longitudinal MD, the tensile strength >120MPa, and in the transverse TD and diagonal direction only >20MPa. The wet process prepared by the diaphragm in all directions has a similar tensile strength (>140MPa), while the non-woven process prepared by the diaphragm has the worst tensile strength (<35MPa), non-woven diaphragm in the longitudinal and transverse have similar strength, but the diagonal direction of the tensile strength is much weaker.
To test the thickness and compression performance of the diaphragm, Xiaowei Zhang wound the diaphragm into a 40-layer cylindrical structure with a diameter of 16mm. First, the core was pressurized with 0.5mpa to ensure that there was no gap between the diaphragm layers, and then the pressure was gradually increased to 100MPa. After the compression experiment of the preparation of dry process and three layer PE composite membrane formed in the shaft up change ellipse, but wet ceramic coating membrane and non-woven process diaphragm is still maintained the circular structure after the test, this is mainly due to the dry diaphragm anisotropy is larger, the wet membrane and non-woven fabric membrane tensile strength caused by approximation in all directions. The strain test of diaphragm core also found that there was an obvious yield point at about 20MPa for PE and three-layer composite diaphragm prepared by dry process in the process of pressure loading, and the strain was also greater than that of wet process membrane and non-woven process membrane, which did not appear obvious yield point in the test.
It was found in the puncture strength experiment that PE diaphragm and three-layer composite diaphragm prepared by dry process would have a long crack along the longitudinal direction, while for the diaphragm of wet process and non-woven process, most of the failures would only occur locally and present a circular fracture.
This study shows us the differences in uniaxial tensile strength, thickness compression and puncture strength, and failure mode between the major types of diaphragm currently on the market. The study found that the dry process of preparation and three layer PE composite membrane in all directions of the tensile strength is quite different, the longitudinal tensile strength is greater than the horizontal MD TD tensile strength, and the wet process of the preparation of the diaphragm with similar tensile strength in all directions, and higher than that of other types of diaphragm, compression experiments in thickness due to dry diaphragm all the members of the opposite sex is very big, leading to the diaphragm core winding plastic deformation is larger, and wet diaphragm in puncture experiments also showed the highest puncture strength, and only a partial circular crevasse, whereas the rectangular cracks in PE membrane.