Lithium iron phosphate battery separator production process flow and principle. The production process of lithium iron phosphate battery separators is complex and the technical barriers are high. High-performance lithium batteries require separators with uniform thickness and excellent mechanical properties (including tensile strength and puncture resistance), breathability, and physical and chemical properties (including wettability, chemical stability, thermal stability, and safety). The excellence of the separator directly affects the capacity, cycle ability, safety performance and other characteristics of lithium batteries.

　　Production process principle of lithium-ion battery separator

　　The production process of lithium iron phosphate battery separator is mainly divided into dry process and wet process. The main steps and principles of dry and wet process are:

　　1. Dry method - first melt, extrusion and blowing the polyolefin resin to form a crystalline polymer film, then perform crystallization heat treatment and annealing operations to obtain a highly oriented film structure, and then stretch it at high temperature , test the crystal cross-section separation to form a porous structure battery separator. The dry process can also be divided into uniaxial stretching and biaxial stretching.

　　2. Wet method - Traditional wet method preparation is mainly based on phase inversion method, and in recent years, TIPS thermally induced phase separation method is mainly used. The principle is to mix crystalline polymers, thermoplastic polymers and small molecule chemical diluents with high boiling points (such as paraffin oil) to form a homogeneous solution at high temperature, and then lower the solution temperature to cause solid-liquid phase separation of the mixture. Or liquid-liquid separation, extracting and removing small molecule chemical diluents to form a porous membrane of thermoplastic and crystalline polymers.

　　Lithium iron phosphate battery separator production process flow

　　The many characteristics of lithium iron phosphate battery separators and the difficulty in balancing their performance indicators determine that the technical barriers to its production process are high and its research and development is difficult. The separator production process includes raw material formula and rapid formula adjustment, micropore preparation technology, independent design of complete sets of equipment, and many other processes. Among them, micropore preparation technology is the core of the lithium iron phosphate battery separator preparation process. According to the difference in micropore pore formation mechanism, the separator process can be divided into two types: dry method and wet method.

　　Dry separators are divided into single-stretch and double-stretch according to the stretching orientation.

　　The dry separator process is the most commonly used method in the separator preparation process. This process is to mix high molecular polymers, additives and other raw materials to form a uniform melt. During extrusion, a lamellar crystal structure is formed under tensile stress. The lamellar crystal structure is heat treated. A hard elastic polymer film is obtained, which is then stretched at a certain temperature to form slit-like micropores, and then heat-set to obtain a microporous film. At present, dry processes mainly include dry uniaxial stretching and biaxial stretching.

　　The dry single drawing process flow is:

　　1) Feeding: After preprocessing raw materials such as PE or PP and additives according to the formula, they are transported to the extrusion system.

　　2) Casting: The pretreated raw materials are melted and plasticized in the extrusion system and then the melt is extruded from the die. The melt forms a base film with a specific crystal structure after casting.

　　3) Heat treatment: The base film is heat treated to obtain a hard elastic film.

　　4) Stretching: The hard elastic film is cold stretched and hot stretched to form a nano-porous film.

　　5) Cutting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

　　The dry double drawing process flow is:

　　1) Feeding: Raw materials such as PP and pore-forming agent are pretreated according to the formula and then transported to the extrusion system.

　　2) Tape casting: Obtain PP cast sheets with high β-crystal content and good uniformity of β-crystal morphology.

　　3) Longitudinal stretching: The cast sheet is stretched longitudinally at a certain temperature, and the characteristics of β crystals that are prone to pore formation under tensile stress are used to cause pores.

　　4) Transverse stretching: The sample is stretched transversely at a higher temperature to expand the pores while improving the uniformity of pore size distribution.

　　5) Shaping and winding: By heat-treating the separator at high temperatures, the thermal shrinkage rate is reduced and the dimensional stability is improved.

　　Whether wet separators are divided into asynchronous and synchronous according to the stretching orientation?

　　The wet process is suitable for producing thinner single-layer PE separators. It is a preparation process with better thickness uniformity, physical, chemical and mechanical properties of separator products. Depending on whether the orientations are simultaneous during stretching, the wet process can also be divided into two types: wet bidirectional asynchronous stretching process and bidirectional synchronous stretching process. The wet asynchronous stretching process flow is:

　　1) Feeding: Preprocess PE, pore-forming agent and other raw materials according to the formula and transport them to the extrusion system.

　　2) Casting: The pretreated raw materials are melted and plasticized in a twin-screw extrusion system and then the melt is extruded from the die. After casting, the melt forms a cast thick sheet containing a pore-forming agent.

　　3) Longitudinal stretching: Longitudinal stretching of the cast thick sheet.

　　4) Transverse stretching: The cast thick sheet after longitudinal stretching is stretched transversely to obtain a base film containing a pore-forming agent.

　　5) Extraction: The base film is extracted with a solvent to form a base film without pore-forming agents.

　　6) Shaping: Dry and shape the base film without pore-forming agent to obtain a nano-porous membrane.

　　7) Cutting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

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