Analytical power cell diaphragm material

Due to the increasingly prominent international issues such as energy crisis and environmental pollution, new energy vehicles have become an active development and promotion project in various countries around the world. Lithium-ion batteries are widely used in new energy vehicles as relatively good power/energy storage batteries.

Lithium-ion batteries are mainly composed of positive electrode materials, negative electrode materials, membranes, and electrolytes. The diaphragm is one of the four key raw materials for lithium-ion batteries. The diaphragm acts as an electronic isolation, preventing direct contact between positive and negative poles. Allow the free passage of lithium ions in the electrolyte. At the same time, the diaphragm also plays a crucial role in ensuring the safe operation of the battery. The advantages and disadvantages of the diaphragm performance directly affect the internal resistance, discharge capacity, recycling life, and battery safety performance of the battery. The lithium battery diaphragm is the highest technical barrier among the four major materials. Its cost ratio is second only to that of positive materials, which is about 10 % to 14 %. In some high-end batteries, the cost of diaphragm accounts for even 20 %.

At present, the commercialized lithium battery diaphragm on the market is mainly a microporous polyolefin membrane dominated by polyethylene (PE) and polypropylene (PP). This type of diaphragm is widely used in lithium battery diaphragm due to its low cost, good mechanical properties, excellent chemical stability and electrochemical stability. The practical applications include a single-layer PP or PE diaphragm, a double-layer PE/PP composite diaphragm, and a three-layer PP/PE / PP composite diaphragm.

The preparation method of lithium-ion battery diaphragm is mainly divided into two categories: dry and wet. The dry method is to melt, squeeze, and blow the polyolefin resin into a crystalline polymer film. After crystallization and annealing, a highly oriented multi-layer structure is obtained, and the crystal surface is further stretched at a high temperature, and the crystal surface is stripped to form a porous structure. The aperture of the film can be increased. The wet method, also known as phase separation or thermal phase separation, mixes liquid hydrocarbons or small molecular substances with polyolefin resins, heats and melts to form a uniform mixture, and then cools to separate the phases to suppress the diaphragm. Heating to near the melting point temperature, Two-way stretching leads to the orientation of the molecular chain, and finally holds the heat for a certain period of time. The solvent of the residue is washed off with volatile substances to prepare a micro-porous membrane that runs through each other.

Dry process is relatively simple, but the aperture and porosity are difficult to control, and the product is difficult to thin; Wet process film aperture is small and uniform, thin film, but the investment is large and the process is complex; The lithium-ion power batteries needed for new energy vehicles often require batteries with greater capacity, higher voltage, longer cycle performance, higher safety performance, long-term stable output of uniform performance, and the provision of instantaneous acceleration of large power discharge performance for cars. Therefore, the diaphragm is required to have high porosity, good infiltration, high strength, good thermal dimensional stability, suitable thermal closing temperature and high thermal melting temperature; Because of its high porosity, high strength and low closed temperature, the wet diaphragm has gradually become the mainstream in power cells.

At present, domestic diaphragms are mainly used in the middle and low-end markets. The domestic high-end lithium battery separator market is mainly occupied by imported manufacturers such as Asahi Kasei of Japan, Toho Chemical of Japan, UBE of Japan, W-Scope of Korea, and Celgard of the United States.

The gap between domestic diaphragm products and foreign countries is due to a large part of the raw materials, foreign well-known diaphragm manufacturers have their own special materials, either to produce raw materials, or to jointly develop diaphragm special materials with raw materials manufacturers; Polyolefin diaphragm mainly requires mechanical properties, solubility, dispersion, cleanliness, etc. of raw materials. The substrate used for diaphragm production belongs to the polyolefin special material with relatively high molecular weight. At present, imported materials still occupy a large share., domestic material use is low.

Celanese's Ticona has introduced ultra-high molecular weight polyethylene for lithium diaphragm production with high tensile strength, chemical inertness and low melt flow. The product specification is 50-100 million molecular weight, and can be customized according to customer requirements.

ExxonMobil Chemical Co., Ltd. is a special custom-made diaphragm material for the East combustion chemistry. With the technology of the East combustion chemistry itself, the resulting diaphragm can effectively improve the safety and reliability of lithium batteries. Lyondellbasell, the international chemical giant, has also been successfully introduced into China, including GX5028 for wet process HDPE and E4462 for dry process PP.

China's petrochemical enterprises have also made a lot of achievements in the research and development and production of polyolefin special materials. In August 2010, the PPH-F02-H, a special polypropylene material independently developed by Yangzi Petrochemical Company, successfully achieved industrial production. In April 2011, China Petroleum Lanzhou Petrochemical Company also successfully achieved the production of L402, a special material for lithium battery diaphragm. In July 2012, Luoyang Petrochemical Polypropylene Company conducted a trial production of the newly developed lithium battery diaphragm special material F301L, and the main technical performance indicators of the product reached user demand. In September 2012, Huajin in the North improved the lithium diaphragm material from the polypropylene F401 brand and passed the production and application performance test of the lithium diaphragm.

What is even more striking is that on May 27, 2017, Yangzi Petrochemical successfully tested and produced the new YEV-4500, a special polymer weight polyethylene product for lithium ion battery diaphragm, and officially began marketing recently. This new product fills the domestic gap. Breaking the monopoly of imported products, it is another breakthrough made by Yangzi Petrochemical in the high-end polyolefin field.

Due to its excellent performance, UHMWPE can improve the membrane high temperature melt integrity (HTMI) and significantly increase the membrane breaking temperature. HTMI is essential for improving the safety of plug-in hybrid vehicles; At the same time, it will also give the diaphragm higher puncture strength, higher porosity and mechanical strength. It is an ideal raw material for manufacturing high-end lithium battery diaphragm.

In short, the revolution has not yet been successful. Comrades still need to work hard. We believe that with the joint efforts of the upper and lower reaches of the lithium battery barrier, the status quo in this industry will soon be broken.

The page contains the contents of the machine translation.