Research progress on lithium-ion battery separator materials?

　　The difficulty of diaphragm technology lies in the engineering technology of creating holes and the preparation of matrix materials. The hole-making engineering technology includes membrane hole-making technology, production equipment and product stability. Matrix material preparation includes the preparation and modification technology of polypropylene, polyethylene materials and additives. The difficulties in hole-making engineering technology are mainly reflected in insufficient porosity, uneven thickness, and poor strength. This article mainly compares the performance of separators prepared by different processes.

　　In the structure of lithium-ion batteries, the separator is one of the key inner components. The performance of the separator determines the interface structure and internal resistance of the battery, which directly affects the battery's capacity, cycle and safety performance. Separators with excellent performance play an important role in improving the overall performance of batteries.

　　The difficulty of diaphragm technology lies in the engineering technology of creating holes and the preparation of matrix materials. The hole-making engineering technology includes membrane hole-making technology, production equipment and product stability. Matrix material preparation includes the preparation and modification technology of polypropylene, polyethylene materials and additives. The difficulties in hole-making engineering technology are mainly reflected in insufficient porosity, uneven thickness, and poor strength. This article mainly compares the performance of separators prepared by different processes.

　　2 Preparation method of lithium-ion battery separator

　　Polyethylene (PE) and polypropylene (PP) microporous membranes have higher porosity, lower resistance, higher tear strength, better acid and alkali resistance, good elasticity and resistance to aprotic solvents. It maintains performance, so it is used as a separator material in the early stages of lithium-ion battery research and development. The currently marketed lithium-ion battery separators mainly include single-layer PE, single-layer PP, and 3-layer PP/PE/PP composite films. Lithium-ion battery separators can be divided into two categories: dry method and wet method according to different preparation processes, and the pore formation mechanisms of the separator micropores are different [1~2].

　　2.1 Dry process

　　In the dry method, polyolefin resin is melted, extruded, and blown into a crystalline polymer film. After crystallization and annealing, a highly oriented multilayer structure is obtained, which is further stretched at high temperature to peel off the crystalline interface. , forming a porous structure that can increase the pore size of the film. Dry stretching can be divided into dry uniaxial stretching and biaxial stretching according to different stretching directions.

　　The dry uniaxial stretching process uses hard elastic fibers to prepare highly oriented PE or PP separators with low crystallinity, and then anneales them at high temperature to obtain oriented films with high crystallinity. This film is first stretched at low temperatures to form defects such as silver streaks, and then stretched at high temperatures to form micropores. At present, Celgard Company in the United States and Ube Company in Japan use this process to produce single-layer PE, PP and 3-layer PP/PE/PP composite films. The separator produced by this process has an elongated microporous structure. Since it is only stretched in one direction, the transverse strength of the separator is relatively poor, but there is almost no thermal shrinkage in the transverse direction.

　　2.2 Wet process

　　The wet method is also called phase separation method or thermally induced phase separation method. Liquid hydrocarbons or some small molecular substances are mixed with polyolefin resin. After heating and melting, a uniform mixture is formed. Then the temperature is lowered for phase separation, and the film is obtained by pressing. The membrane is heated to a temperature close to the melting point, stretched in two directions 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. This method is applicable to a wide range of materials. . Companies that adopt this method include Asahi Kasei, Toran, and Nitto of Japan and Entek of the United States [5]. The pore size range of the separator produced by the wet biaxial stretching method is in the order of magnitude of the phase microscopic interface, which is relatively small and uniform. The stretch ratio in both directions can reach 5 to 7, so the separator performance is isotropic, the transverse tensile strength is high, and the puncture strength is high. The normal process flow will not cause perforation, and the product can be made thinner, increasing the energy density of the battery. higher.

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