Lithium iron battery separator production process, lithium battery separator production process principle. As one of the four core materials of lithium batteries, separators are an important part of determining the performance, safety and cost of lithium batteries. The production process of lithium battery separators is divided into two categories: dry production process and wet production process. Dry production process It is further subdivided into one-way stretching process and two-way stretching process.

　　Lithium iron battery separator production process

　　The production process of lithium iron battery separators is mainly divided into dry stretching and wet stretching. The many characteristics that lithium battery separators need to possess put forward special requirements for its production process, which includes raw material formula and rapid formula adjustment, micropore preparation technology, and independent design of complete sets of equipment.

　　The dry uniaxial stretching process is a method of producing hard elastic fibers to prepare highly oriented polypropylene or polyethylene films with low crystallinity. During the high-temperature annealing process, a film with high crystallinity is obtained. This film is first stretched at low temperatures to form micro-defects, and then at high temperatures the defects are pulled apart to form micropores. At present, more than one-third of China's production capacity uses the dry double drawing process, and its products occupy a large proportion of the mid- to low-end market.

　　Wet production process, also known as phase separation method or thermal phase separation method, wet process mixes liquid hydrocarbons or some small molecular substances with polyolefin resin. After heating and melting, a uniform mixture is formed, and then the temperature is cooled for phase separation and pressing. The membrane is obtained, and then the membrane is heated to a temperature close to the melting point, biaxially stretched to orient the molecular chains, and finally kept warm for a certain period of time. The residual solvent is eluted with volatile substances to prepare interconnected microporous membrane materials.

　　The wet production process can not only prepare interconnected microporous membrane materials, but also produce lithium battery separators with high longitudinal and transverse strength. At present, the wet production process is mainly used to produce single-layer lithium battery separators.

　　Lithium battery separator production process principle

　　1. Dry method - The polyolefin resin is first melted, extruded and blown to form a crystalline polymer film, then crystallized heat treatment and annealing operations are performed to obtain a highly oriented film structure, and then stretched 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.

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

　　The raw material of dry process is generally PP, while the raw material of wet process is generally PE. Generally speaking, the melting temperature of PP is around 170°C, and the melting temperature of PE is around 140°C. Therefore, although the separator produced by the wet process is thinner, the lower melting temperature makes the separator easy to shrink at high temperatures, causing a short circuit in the battery, and the safety of the battery cannot be guaranteed. Coating a layer of inorganic nanoparticles or high-temperature-resistant organic chemicals on the surface of the separator can improve the high-temperature safety performance of the separator and can well make up for this shortcoming of the wet process.

　　Lithium battery separator dry single drawing process flow

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

　　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.

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

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

　　Slitting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

　　Wet asynchronous stretching process

　　Feeding: PE, pore-forming agent and other raw materials are pretreated according to the formula and transported to the extrusion system.

　　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.

　　Longitudinal stretching: Longitudinal stretching of cast thick sheets.

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

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

　　Shaping: The base film without pore-forming agent is dried and shaped to obtain a nano-porous membrane.

　　Slitting: Cut the nano-microporous membrane into finished membranes according to customer specifications.

　　The performance of diaphragm products is affected by the base material and manufacturing process. The stability, consistency, and safety of the separator have a decisive impact on the discharge rate, energy density, cycle life, and safety of lithium batteries. . From the perspective of product strength, the comprehensive performance of wet separators is stronger than that of dry separators.

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