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Two ways to prepare biaxially stretched lithium battery separator film by wet method
2024-10-06
Lithium battery separator is an important material for lithium batteries. It is mainly used to separate the positive and negative plates of the battery to prevent the two poles from contacting and causing a short circuit, while allowing the ions in the electrolyte to pass through. The performance of the separator directly affects the capacity, cycle and safety performance of the battery, so its preparation process has a great influence on the battery performance. Since the uniaxially stretched separator is only stretched longitudinally during the preparation process without being stretched transversely, the strength in the transverse direction is relatively weak, and transverse cracking is prone to occur. In addition, low transverse strength will also limit the further reduction of the film thickness, so this article focuses on the analysis of biaxially stretched lithium battery separators.
Nowadays, we mainly use dry and wet methods to produce biaxially stretched lithium battery separators. The wet method is a preparation process carried out in a polymer solution. The melting point of a single-layer membrane prepared by the wet method can reach up to 170°C, but under normal circumstances it can only reach 140°C. The dry method refers to the preparation process carried out in a non-aqueous solution. As early as a few years ago, Mitsubishi Resin obtained a separator product with a melting point of up to 220°C by coating a high heat-resistant inorganic filler on the separator. Although the wet method can also be coated with a heat-resistant coating, it costs 40% more to achieve the same melting point.
During the preparation process of the dry biaxial stretching process, although biaxial stretching is carried out, the uniformity, consistency and stability of its pore size are still relatively poor compared to the wet process. This may lead to inconsistent ion transfer rates in different areas during the use of the battery, thereby affecting the performance and life of the battery. Although the dry biaxial stretching separator has certain advantages over the wet separator in terms of cost, with the advancement of technology and the play of scale effect, the cost of the wet separator is also constantly decreasing, and this advantage is shrinking. Therefore, this article mainly introduces two methods of wet double-stretching for preparing lithium battery separators: synchronous stretching and asynchronous stretching.
Synchronous stretching, as the name implies, is stretching in the longitudinal and transverse directions at the same time. The process of bidirectional synchronous stretching mainly includes feeding, casting, double stretching, and solution extraction. The process of pre-treating raw materials such as PE and plasticizers according to a specific formula and conveying them to the extrusion system is called feeding. The casting step is to melt and plasticize the pre-treated raw materials through a twin-screw extruder, extrude the melt from the die head, and then form a cast thick sheet containing plasticizer. After that, at a specific temperature and stretching rate, the cast thick sheet is subjected to bidirectional synchronous stretching to make the molecular chain orientation consistent and form a microporous structure. Finally, a volatile solvent is used to extract the plasticizer from the film to obtain the final lithium battery separator product. The lithium battery separator prepared by the above steps has the advantages of suitable and uniform pore size, few defects, and thin thickness. In addition, because the stretching in two directions can be completed in the same device at one time, the production efficiency is greatly improved and the equipment cost is reduced. However, synchronous stretching requires the control of stretching parameters in two directions at the same time, which places high demands on equipment and operators. And because stretching in two directions must be carried out simultaneously, the process flexibility is slightly inferior and may not meet the needs of all types of diaphragms
The process of wet asynchronous stretching is basically the same as that of synchronous stretching, except that longitudinal stretching is performed first and then transverse stretching. Through two-step stretching, the mechanical properties of the diaphragm can be optimized in the longitudinal and transverse directions respectively. Compared with synchronous stretching, the production method is also more flexible and the process can be adjusted according to different product requirements and material properties. The disadvantage of this stretching method is mainly that it involves more operating steps and equipment, which may lead to relatively low production efficiency; and two sets of independent stretching equipment are required, which relatively increases the investment and operating costs of the equipment.
There are significant differences between wet synchronous stretching and asynchronous stretching in the production process of lithium battery diaphragms. These differences are mainly reflected in the stretching method, technical characteristics and applicable fields.
1. In terms of stretching method, the wet asynchronous stretching process first performs longitudinal stretching and then transverse stretching during the preparation process. This step-by-step stretching method allows the diaphragm to be stretched in the longitudinal and transverse directions respectively, thereby achieving the desired stretching effect. The wet synchronous stretching process can be oriented in both the horizontal and vertical directions during stretching, eliminating the process of longitudinal stretching alone. This synchronous stretching method enhances the thickness uniformity of the diaphragm.
2. In terms of technical characteristics, asynchronous stretching has advantages in control accuracy, mechanical properties and process flexibility, but may face problems such as low production efficiency, high equipment cost and high complexity. Synchronous stretching is characterized by its high efficiency, uniformity of pore structure and low equipment cost, but faces considerable challenges in terms of control difficulty, flexibility and investment threshold.
3. In terms of applicable fields, the diaphragm prepared by the wet asynchronous stretching process is more suitable for use in the field of power batteries with higher performance requirements due to its high yield rate. Power batteries need to withstand large charge and discharge currents and cycles, so the diaphragm is required to have high strength and stability. The diaphragm prepared by the wet synchronous stretching process is more suitable for the field of consumer batteries with high requirements for thickness uniformity because of its good thickness uniformity.
In summary, wet synchronous stretching and asynchronous stretching each have their own advantages and disadvantages in the production process of lithium battery separators. When selecting the process, it is necessary to make comprehensive considerations based on specific product requirements, material properties and cost-effectiveness. There is no absolute superiority or inferiority between the two stretching methods.
