A kind of preparation method of multi-layer microporous film for lithium ion battery

Abstract

The invention provides a method for preparing a multilayer microporous membrane for lithium ion batteries. Polyolefin resin and auxiliary additives are used as raw materials to obtain a melt after melting and plasticizing, extruding through a die head, and tape-casting to obtain an intermediate Membrane: Take multiple rolls of the intermediate film, compound it through a composite device with transverse stretching function, obtain a composite intermediate, anneal it under bidirectional micro tension, and stretch it longitudinally to obtain a multilayer microporous membrane for lithium ion batteries . In the present invention, the compounding roll is heated during the compounding process of the intermediate film, and the crystallization of the polymer structure inside the intermediate film is further improved. At the same time, the transverse extension is carried out under slight tension, and the degree of transverse orientation is increased, so that the prepared microporous film has uniform micropores. , the lateral strength is improved, the ability to resist lithium dendrite piercing is effectively improved, and micro-short circuits are avoided, which is suitable for large-scale production. The multilayer microporous membrane prepared by the invention has high structural uniformity, and has obvious effect on improving the safety performance of the battery.

Description

technical field [0001] The invention relates to the technical field of lithium ion batteries, in particular to a preparation method of a multi-layer microporous film for lithium ion batteries. Background technique [0002] The lithium-ion battery separator is an important part of the lithium-ion battery. The microporous structure, physical properties, chemical properties, and thermal properties of the separator are closely related to the battery performance. Due to the high working voltage of lithium-ion batteries, the oxidizing properties of positive electrode materials and the reducibility of negative electrode materials, lithium-ion battery separator materials should have excellent compatibility with positive and negative electrode materials with high electrochemical activity. Excellent stability, solvent resistance, ionic conductivity, electronic insulation, good mechanical strength, high heat resistance and fuse isolation. The physical and chemical properties of the se...

AI Technical Summary

Problems solved by technology

The composite rollers used in the existing composite equipment are composited at room temperature, and the gas between the layers cannot be removed in time when the product is composited, forming small air bubbles, and there are differences in the degree of bonding of the composite films of each layer, resulting in bubbles during the stretching process. Part of it cannot be stretched into holes, forming a "blind hole" area, and the degree of lamination of the film is different. When stretched under force, it will form a serious "neck shrinkage". Big

[0004] In addition, the micropores of the separator prepared by the dry uniaxial stretching process are slit-shaped structures, because only a high degree of longitudinal stretching occurs during the stretching process, resulting in higher longitudinal strength of the finished microporous membrane and higher transverse strength. Small, easy to be torn by mechanical force during battery assembly and battery use, or because the lithium dendrites generated on the negative electrode material side penetrate the separator to form a short circuit or micro short circuit, which is likely to cause safety problems

The dry biaxial stretching process adds a β crystal form modifier with nucleating effect to polypropylene, and utilizes the difference in density between different phases of polypropylene to form micropores during the stretching process through crystal transformation, which is used for production Single-layer PP film, but because of the transition from β crystal form to α crystal form during its preparation process, the structure of the micropores is not easy to control, the micropore structure is uneven, and the permeability is low

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