A lithium-ion battery consists of four parts: positive electrode material, negative electrode material, electrolyte, and lithium battery separator. The diaphragm is a functional membrane material with a microporous structure, generally 8-40μm in thickness, which plays a role in separating the positive and negative electrodes in the battery system, blocking the passage of electrons in the circuit during charge and discharge, and allowing lithium ions in the electrolyte to pass freely. It can selectively close the micropores when the battery is charging and discharging or the temperature rises, so as to limit the excessive current and prevent short circuit. The performance of the battery directly determines the overall performance of the battery. Traditional lithium-ion battery separators are polyolefin separators, mostly single-layer or three-layer structures, such as single-layer PE, single-layer PP, PP/PE/PP composite films, etc. According to the conventional preparation process, it can be divided into dry process and wet process.
1. Dry process: dry process is the most commonly used method, the use of extrusion, blowing film method, will melt polyalkene resin into flake crystalline film, and through one-way stretching or two-way stretching at high temperature to form a slit porous structure. The microporous structure of the thin films prepared by unidirectional drawing process is long and flat, and the permeability is good. No solvent is used in the production process, and the process environment is friendly. The longitudinal strength of the film is better than that of the transverse, and there is no thermal shrinkage in the transverse.
biaxial tension process is a process developed by the Institute of Chemistry of Chinese Academy of Sciences with independent intellectual property rights. By adding nucleating β-crystal improver into PP, using the density difference between different phases of PP, crystal transformation occurs during the drawing process to form micropores. The films prepared by bidirectional drawing process have certain strength, microporous size and uniform distribution.
2. Wet process of lithium battery diaphragm manufacturer: The wet process is also known as phase separation or thermogenic phase separation in industry. Its preparation principle is to heat and melt the mixture of incompatible low molecular weight substances (liquid hydrocarbon, paraffin, etc.) and high fraction substances (polyolefin resin) at normal temperature, so that the mixture forms a uniform mixture of liquid, and through cooling phase separation and pressing to obtain microporous membrane materials. The 3D structure of wet film is more complex than that of dry film, and the micropore buckling degree is higher. However, compared with dry method, wet method has relatively less advantages in environmental protection due to the use of solvents in the production process, and has poor thermal stability and relatively complex process flow.
According to the different membrane stretching process of lithium battery when pressing the diaphragm, it can be divided into bidirectional synchronous stretching and bidirectional asynchronous stretching. The difference between the two stretching processes lies in whether the stretching is carried out simultaneously in vertical and horizontal direction when pressing the diaphragm. The properties of the films prepared by bidirectional synchronous stretching, such as tensile strength and thermal shrinkage, are basically the same in vertical and horizontal directions. Bidirectional asynchronous drawing is the melting polymer cooling to make a film, the first longitudinal stretching, and then transverse stretching.
