Detailed explanation of lithium battery separator production process

Lithium ion batteries are representative of modern high-performance batteries, consisting of four main parts: positive electrode material, negative electrode material, separator, and electrolyte. Among them, the separator is a thin film with a microporous structure, which is a key inner component with technical barriers in the lithium-ion battery industry chain. It plays the following two main roles in lithium-ion batteries:

a、 Separate the positive and negative electrodes of the lithium battery to prevent short circuits caused by contact between the positive and negative electrodes;

b、 The micropores in the film can allow lithium ions to pass through, forming a charge discharge circuit.

Cost composition of lithium batteries

High performance lithium batteries require separators with uniform thickness, excellent mechanical properties (including tensile strength and puncture resistance), breathability, and physicochemical properties (including wettability, chemical stability, thermal stability, and safety). It is understood that the excellence of the separator directly affects the capacity, cycling ability, and safety performance of lithium batteries. A high-performance separator plays an important role in improving the overall performance of the battery.

The many characteristics of lithium battery separators and the difficulty in balancing their performance indicators determine the high technological barriers and research and development difficulties in their production process. The membrane production process includes many processes such as raw material formulation and rapid formulation adjustment, microporous preparation technology, and independent design of complete equipment. Among them, microporous preparation technology is the core separator of lithium battery separator preparation process. According to the difference in microporous pore formation mechanism, the separator process can be divided into dry and wet methods.

The dry membrane process is the most commonly used method in the preparation of membranes. This process involves mixing polymer, additives, and other raw materials to form a uniform melt. During extrusion, a lamellar structure is formed under tensile stress, and the lamellar structure is heat-treated to obtain a hard and elastic polymer film. Then, it is stretched at a certain temperature to form narrow slit shaped micropores, and after heat setting, a microporous film is prepared. At present, the dry process mainly includes two types of processes: dry uniaxial stretching and biaxial stretching.

Dry process single pull

Dry spinning is the use of polyethylene (PE) or polypropylene (PP) polymers with good fluidity and low molecular weight. By utilizing the manufacturing principle of hard elastic fibers, high orientation and low crystallinity polyolefin castings are first prepared. After low-temperature stretching to form micro defects such as silver lines, high-temperature annealing is used to pull apart the defects, thereby obtaining a microporous film with uniform pore size and uniaxial orientation.

The dry single pull process flow is as follows:

• Feeding: PE or PP and additives are pre treated according to the formula and transported to the extrusion system.

• Casting: Pre treated raw materials are melted and plasticized in an extrusion system, and then extruded from a die to form a molten membrane. The molten material is delayed to form a specific crystalline structure of the base film.

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

• Stretching: Cold stretching and hot stretching of a hard elastic film to form a nanoporous membrane.

• Cutting: Cut the nano microporous membrane into finished films according to customer specifications.

Dry Double Pull

The dry double pull process is a process developed by the Institute of Chemistry, Chinese Academy of Sciences, with independent intellectual property rights, and is also a unique membrane manufacturing process in China. Due to the hexagonal crystal structure of PP, the single crystal nucleates and the arrangement of the chips is loose. It has a divergent bundle like plate-like structure growing radially, but does not have a complete spherical crystal structure. Under the action of heat and stress, it will transform into denser and more stable alpha crystals, and after absorbing a large amount of impact energy, pores will be generated inside the material. This process involves adding a β - crystal modifier with nucleating properties to PP, utilizing the density difference between different phases of PP to undergo crystal transformation and form micropores during the stretching process.

The dry double pull process flow is:

• Feeding: Pre treat PP and pore forming agents according to the formula and transport them to the extrusion system.

• Casting: Obtain PP cast sheets with high content of β crystals and good uniformity of β crystal morphology.

• Longitudinal stretching: Stretching the cast piece longitudinally at a certain temperature, utilizing the characteristic of beta crystals that are prone to pore formation under tensile stress to create pores.

• Lateral stretching: Stretching the sample horizontally at a higher temperature to enlarge the hole and improve the uniformity of pore size distribution.

• Forming and winding: By heat treating the diaphragm at high temperatures, the thermal shrinkage rate is reduced and the dimensional stability is improved.

Is the wet process diaphragm divided into asynchronous and synchronous according to the stretching orientation

The wet process utilizes the principle of thermally induced phase separation to mix plasticizers (high boiling point hydrocarbon liquids or substances with relatively low molecular weight) with polyolefin resins. The phenomenon of solid-liquid or liquid-liquid phase separation occurs during the cooling process of the molten mixture, and the membrane is pressed. After heating to a temperature close to the melting point, the molecular chains are stretched to align. After holding for a certain period of time, volatile solvents (such as dichloromethane and trichloroethylene) are used to extract plasticizers from the membrane, thereby producing interconnected submicron sized microporous membrane materials.

The wet process is suitable for producing thinner single-layer PE membranes and is a preparation process with better thickness uniformity, physical and chemical properties, and mechanical properties of membrane products. According to whether the orientation during stretching is the same, wet process can also be divided into wet bi-directional asynchronous stretching process and bi-directional synchronous stretching process.

The wet asynchronous stretching process is as follows:

• Feeding: Pre treat PE, pore forming agents and other raw materials according to the formula and transport them to the extrusion system

• Casting: The pre treated raw materials are melted and plasticized in a twin-screw extrusion system, and the molten material is extruded from the die. The molten material is delayed to form thick sheets containing pore forming agents.

• Longitudinal stretching: Stretching the cast thick sheet longitudinally.

• Lateral stretching: Stretching the longitudinally stretched thick sheet horizontally to obtain a base film containing pore forming agents.

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

• Shaping: Drying and shaping the base film without pore forming agents to obtain a nano microporous membrane.

• Cutting: Cut the nano microporous membrane into finished films according to customer specifications.

The process flow of wet synchronous stretching technology is basically the same as that of asynchronous stretching technology, except that it can be oriented in both horizontal and vertical directions during stretching, eliminating the need for separate longitudinal stretching and enhancing the uniformity of diaphragm thickness. However, the problem with synchronous stretching is that the vehicle speed is slow, and secondly, the adjustability is slightly poor. Only the lateral stretching ratio is adjustable, while the longitudinal stretching ratio is fixed.

Wet synchronous stretching process

The overall performance of wet process diaphragm is superior to that of dry process diaphragm products, which is influenced by both the substrate material and the 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. Compared to dry separators, wet separators have better material properties such as thickness uniformity, mechanical properties (tensile strength, puncture resistance), breathability, and physicochemical properties (wetting, chemical stability, safety), which are beneficial for electrolyte absorption and retention, and improve battery charging, discharging, and cycling capabilities, making them suitable for high-capacity batteries. From the perspective of product strength, wet process membranes have stronger comprehensive performance than dry process membranes.