The lithium ion battery is mainly composed of a positive electrode, a negative electrode plate and a separator, an electrolyte, a casing and a positive and negative electrode terminal, wherein, the separator plays an important role in the internal of the lithium ion battery. In the interior of the lithium ion battery, the diaphragm should not only avoid contact between positive and negative electrode, achieve electronic insulation effect, but also maintain a certain porosity allowing ions in the electrolyte through the diaphragm, between positive and negative electrode in reciprocating motion. To meet these basic requirements at the same time, the diaphragm to reach safety requirements, such as during the charge discharge cycle at the cathode surface will form a lithium dendrite, sharp lithium dendrite development to a certain extent may penetrate the diaphragm cause a short circuit between the anode and release large amounts of heat, resulting in lithium ion battery thermal runaway, resulting in serious safety accidents. In the process of extrusion or battery acupuncture, due to the short release a lot of heat, resulting in diaphragm heat shrinkage, resulting in a large area of the positive and negative pole contact, directly caused by battery fire explosion, so the diaphragm performance and safety of lithium ion batteries have the important influence.
In order to meet the requirements of lithium-ion battery separator on the performance and safety of the people, the development of a variety of composite membrane, such as PP-PE-PP three layer composite diaphragm, the battery temperature is higher than 130 DEG C, PE layer middle layer will melt, and both sides of the PP diaphragm high melting point, played a supporting role, melted PE jam PP on the membrane pore, thereby blocking discharge effect. A ceramic coating diaphragm, diaphragm in the general matrix coated on Al2O3 inorganic oxide at high temperature to support action on the diaphragm, reduce the shrinkage of the diaphragm, so as to improve the safety of lithium ion battery.
Recently, Zhang Xiaowei of Massachusetts Institute of Technology, a variety of different processes and structures of the mechanical properties of the diaphragm, analysis of mechanical parameters resulting in the failure of the diaphragm.
The study shows that the main types of diaphragm in the market are now in the direction of tensile strength, compression strength and puncture strength, as well as the difference in failure mode. The study found that there is a big difference in the dry process of PE and the three layer composite membrane in various directions on the tensile strength, the longitudinal tensile strength of MD is much larger than the transverse tensile strength of TD, and the wet process preparation of diaphragm has a tensile strength similar in all directions, and higher than other types of diaphragm, in thickness due to the dry diaphragm compression experiments of anisotropic, which leads to the diaphragm core of large plastic deformation, and the wet in the puncture experiment diaphragm also showed the highest puncture strength, and only appeared in the local circular hemomediastinum, while PE is the diaphragm cracks strip.
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