Process principle of lithium ion battery separator

　　1. Drying method First, the polyolefin resin is melted, extruded, and blown into a crystalline polymer film. Then, crystallization heat treatment and annealing are performed to obtain a highly oriented film structure. The drying process can also be divided into unidirectional stretching and bidirectional stretching.

　　2. Wet method The traditional wet method mainly uses the phase conversion method, and the TIPS thermally induced phase separation method is an important method used in recent years. The principle of crystalline polymers, thermoplastic polymers and small molecule chemical substances with high boiling point diluent mixtures (such as paraffin oil), form a homogeneous solution at high temperature, then reduce the temperature of the solution, solid-liquid separation or liquid-liquid separation, extraction After the diluent of the small molecule chemical substance is removed, a thermoplastic and crystalline polymer porous film is formed.

　　The important characteristics of the wet lithium ion battery separator are high cost, large investment, high equipment requirements, long construction period, long production cycle, energy consumption and use of organic solvents in the production process. However, the wet method can better control the pore size, distribution and porosity, so it is usually used to make high-end films.

　　The dry raw material is PP, and the wet raw material is PE. Generally speaking, the melting point of PP is around 170°C, while the melting point of PE is around 140°C. Therefore, although the thickness of the separator produced by the wet method is relatively thin, the lower melting temperature makes the separator easy to shrink at high temperatures, resulting in a short circuit of the battery, and the safety of the battery cannot be guaranteed. Coating inorganic nanoparticles or high temperature resistant organic chemicals on the surface of the film can improve the high temperature safety performance of the film and make up for the defects of wet processing.