Production and application of lithium battery diaphragm

1. Introduction of lithium ion battery diaphragm
    Batteries can be divided into primary batteries and secondary batteries according to their working properties. Primary batteries are batteries that cannot be recycled. Secondary batteries refer to batteries that can be recharged and reused repeatedly, such as lead-acid batteries, nickel cadmium batteries, nickel hydrogen batteries and lithium-ion batteries. Lithium-ion battery has the characteristics of high average output voltage, large specific energy, no memory effect, long service life, good safety performance and no toxic substances. Therefore, it is also called "green battery".

    The cathode materials, anode materials, diapgragm and electrolyte materials for lithium-ion battery manufacturing are known as the four key materials for lithium-ion batteries.

    Among them, the main function of the diaphragm is to separate the positive and negative electrodes of the battery, prevent the short circuit caused by contact between the two poles, meanwhile, it can make electrolyte ions pass through. Its performance determines the interface structure and internal resistance of the battery, which directly affects the capacity, cycle and safety performance of the battery. The idaphragm with excellent performance plays an important role in improving the comprehensive performance of the battery.

    Lithium battery separator is the highest part of technical barriers among the four major materials, and its cost proportion is about 10% - 14% next to the cathode material. In some high-end batteries, the diaphragm cost even accounts for 20%.

隔膜产品主要有单层PP、单层PE、PP+陶瓷涂覆、PE+陶瓷涂覆、双层PP/PE、双层PP/PP和三层PP/PE/PP等，其中前两类产品主要用于3C消费电池，后几类产品主要用于动力锂电池。

Diaphragm products mainly include single-layer PP, single-layer PE, PP + ceramic coating, PE + ceramic coating, double-layer PP / PE, double-layer PP / PP and three-layer PP / PE / PP, etc. among them, the first two categories are mainly used for 3C consuming batteries, and the latter are mainly used for power lithium batteries. Chinese companies mainly produce double-layer PP / PP separator, while the diaphragm used in global automotive power lithium battery mainly includes three-layer PP / PE / PP, double-layer PP / PE, PP + ceramic coating, PE + ceramic coating and other diaphragm materials. This is mainly because China does not have the technology and ability to make PP and PE into double-layer composite membrane.

   The technical process of diaphragm is divided into dry process and wet process. The principle of dry process is physical stretching, crystallization and quenching. Compared with the dry process, the principle of wet process is phase separation. The diaphragm prepared by wet process has good uniformity of micropore distribution, appropriate pore size, low closed cell temperature, high biaxial tensile strength and high puncture strength, so thin diaphragm can be prepared. Compared with dry process, wet process is more complex, more capital investment, longer production cycle, higher technical barriers, and the complexity of production equipment is much higher than that of dry process.

Schematic diagram of dry process and wet process

2. Analysis of existing technologies:

    The difficulty of lithium battery separator lies in the preparation of matrix material and the engineering technology of pore forming. Besides the influence of substrate, additives and film-making process, the production equipment also has a great impact on the stability of hole making. The surface temperature uniformity and mechanical accuracy of heating and cooling rolls are important factors.

    The cost of traditional heat transfer oil heating roller is low, but after using for a period of time, the temperature difference of roller surface increases sharply due to the carbon deposition of heat transfer oil.The non-uniform temperature will increase the pore size distribution, which will affect the consistency of the diaphragm. It is characterized by porosity consistency, pore size and distribution, mechanical strength and so on。

    In the same way, the cooling roller is used as the heat exchange tool in the film cooling process. The temperature uniformity of cooling roller is also very important. The uniformity of temperature will also affect the final performance of the product. Meanwhille, the two ends of cooling roller will dewing in the production process under low environmental dew point, which will bring many troubles to the production.

3. Solutions:
    The induction heating roller of Lianjing adopts a special uniform temperature structure, which can keep the surface temperature of the roller within ± 1 ℃, so as to ensure the consistency of porosity, pore size and distribution, and mechanical strength, and improve the overall performance of the product. Considering the general thickness of the film is 0.015 mm, it is suggested to increase the run-out index to 0.005mm to reduce the mechanical energy affecting the consistency of the film.

    Meanwhile, the induction heating roller adopts a single independent closed-loop loop temperature control, which allows users to realize more process possibilities. At the same time, the electromagnetic heating roller adopts a single independent closed-loop loop temperature control, which allows users to realize more process possibilities. For diaphragm drawing, take 16 ~ 20 roller group as an example. In order to improve the versatility of the roller, the power design and other aspects are all consistent. However, when working, it is suggested that the surface temperature should be set gradually according to the needs of the process, rather than the same temperature setting for each roller. This gives a more even stretch.

    In order to make diapgragm heated evenly and avoid the situation that the heat load of the front rollers is particularly high and that of the rear rollers is particularly low, the temperature setting of the roller group should be gradually increased from low to high, so that the load of each roller can be balanced as much as possible. Among them, about 14 rollers in the front mainly play the role of heating, while the temperature of the last 6 rollers is set to the highest, which mainly plays the role of heat preservation and appropriate heat supplement. The total temperature rise Δ t of the film is equal to the sum of Δ Ti of each roller.

 ΔT=ΔT₁+…ΔT_i +…+ΔT_n。

    Generally, the temperature rise range (Δ Ti) of each roller used for heating is relatively close, so the actual load is close. The temperature setting of the roller and the actual temperature of the film are shown in the figure below.

    Note: the above temperature gradient suggestions are only for reference and should be determined by the process conditions。

    For the cooling of diaphragm, the existing cooling roll is replaced by the non-dewing cooling roller. The unique structure design and advanced heat exchange technology are adopted for the non-dewing cooling roller of Lianjing, which makes the heat in the working area quickly transfer to both ends, which can not only make the temperature of the roll surface uniform, but also eliminate the dewing problem at both ends of the roller body, and greatly improve the product quality.

4. Product parameters:

    1). Induction heating roller:

    2). non-dewing cooling roller: