Technical requirements for 18650 battery flat top separators

　　The main indicators that determine the performance of 18650 battery flat top separators include separator thickness, mechanical properties, porosity, air permeability, pore size and distribution, thermal properties and self-closing properties. The thinner the separator, the less resistance the solvated lithium ions encounter when crossing it, and the better the ionic conductivity is. The lower the impedance, but when the separator is too thin, its liquid retention capacity and electronic insulation are reduced, which will also have an adverse impact on battery performance. Currently, the thickness of the separator actually used is usually 25~35um.

　　Lithium batteries also have high requirements for the mechanical properties of the separator. The separator prepared by uniaxial stretching has anisotropy in mechanical properties. The strength along the stretching direction is about 50N, while the strength in the vertical stretching direction is only about 5N. . The strength of the separator prepared by biaxial stretching is basically the same in both directions. Multi-layer separators have uniform strength in different directions and are more suitable as 18650 battery flat top separators.

　　Increasing the porosity of the separator can reduce the resistance of the separator to the migration of lithium ions. The greater the porosity, the smaller the curvature of the pores, the better the penetration of the pores, and the stronger the penetrating ability of lithium ions. However, the increase in the number of pores will This leads to a decrease in the mechanical properties and electronic insulation of the material, and even causes a physical short circuit when the active material of the electrode crosses the separator. Therefore, the porosity of most 18650 battery flat top separators is between 4% and 5%.

　　The size of the pore size is closely related to the porosity. The pore size of commercial separators is generally between 0.03~0.12uA, and the pore size distribution is narrow, the pore size is uniform, and the difference between the maximum pore size and the average pore size distribution does not exceed 0.011um.

　　Air permeability is also a physical quantity closely related to ion migration properties. It is determined by many factors such as membrane thickness, pore rate, pore size, pore size distribution, etc. For separators made of the same material, the smaller the thickness, the greater the porosity, the larger the pore size, and the more uniform the pore size distribution, the higher the air permeability of the material.

　　For lithium batteries and 18650 battery flat top separator materials, hot meltability is a particularly important performance indicator because it is an important guarantee for battery safety. Lithium batteries suffer from self-heating and oxidation of electrolyte due to abuse and other reasons. The temperature of the battery rises sharply, which becomes a safety hazard of lithium batteries. To eliminate this hidden danger, the separator must be able to melt at the required temperature to close the micropores and turn into a non-porous ion insulating layer to interrupt the battery and prevent the battery from burning or even exploding due to the continuous increase in temperature. This is Self-closing phenomenon of diaphragm.

　　The self-closing temperature of polypropylene diaphragms is about 170°C, the self-closing temperature of polyethylene diaphragms is 130-140°C, and the self-closing temperature of polypropylene and polyethylene multi-layer composite diaphragms developed in recent years is 80-120°C. between. Its strength and safety are better than single-layer films, and its impedance is high when self-closing, making lithium batteries safer. However, if the self-closing temperature is too low, the battery will easily lose performance and be scrapped.

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