The separator itself is not only a poor conductor of electrons, but also has the characteristics of electrolyte ions passing through. The diaphragm material must have good chemical and electrochemical stability, good mechanical properties, and maintain a high degree of wettability to the electrolyte during repeated charge and discharge processes. The interfacial compatibility between the separator material and the electrode, and the retention of the separator to the electrolyte have important effects on the charge-discharge performance and cycle performance of lithium-ion batteries.
The separator is one of the most critical inner layer components in the structure of lithium-ion batteries. The important purpose of the separator is to separate the positive and negative plates to prevent the short circuit of the battery and to ensure the normal passage of ions during charging and discharging to ensure the normal operation of the battery. The quality of its performance directly affects the capacity, rate, life and safety of the battery.
Commonly used separator materials for lithium-ion batteries include fiber paper or non-woven fabrics, and porous membranes made of synthetic resins. Common separators are polypropylene and polyethylene porous membranes, and the basic requirement for the separator is high stability in the electrolyte. Because polyethylene and polypropylene microporous membranes have higher porosity, lower electrical resistance, higher tear strength, better acid and alkali resistance, good elasticity and retention of aprotic solvents, the commodity The diaphragm materials of lithium-ion batteries mainly use polyethylene and polypropylene microporous films.
1. Thickness
The uniformity of the thickness of the diaphragm is a particularly important quality index, which directly affects the appearance quality of the diaphragm roll as well as the internal performance, and must be strictly controlled in the production process. In the diaphragm production line with a high degree of automation, the diaphragm thickness is automatically detected and controlled by a high-precision online non-contact thickness gauge and a fast feedback control system. The thickness uniformity of the separator includes the longitudinal thickness uniformity and the transverse thickness uniformity, among which the transverse thickness uniformity is particularly important, and it is generally required to be controlled within 1 micron.
2. Aperture
The lithium-ion battery separator material itself has a microporous structure, and the distribution of the micropores in the entire separator material should be uniform. The electrode particles currently used are generally in the order of 10 microns, and the pore size is generally 0.03-0.12um. If the aperture is too small, resistance will be added, and if the aperture is too large, the positive and negative electrodes may come into contact or be short-circuited by dendrites. Generally speaking, the sub-micron pore size separator is enough to prevent the direct passage of electrode particles. Of course, some situations such as micro-short circuit caused by poor surface treatment of some electrodes and more dust are not excluded.
3. Porosity
Porosity is the volume percentage of pores in the volume of the monomer membrane, which is related to the density of the raw resin and membrane. There is a certain relationship between the size of porosity and internal resistance, but the absolute value of porosity between different kinds of separators cannot be directly compared. The porosity of existing lithium-ion battery separators is between 40% and 50%.
4. Infiltration
In order to ensure that the internal resistance of the lithium-ion battery is not too large, the separator is required to be completely infiltrated by the electrolyte used in the battery. On the one hand, the degree of wettability is related to the diaphragm material itself, and on the other hand, it is closely related to the surface and internal microstructure of the diaphragm. Better wettability is conducive to improving the affinity of the separator and the electrolyte, expanding the contact surface between the separator and the electrolyte, thereby increasing the ionic conductivity and improving the charge-discharge performance and capacity of the battery. Wetting can be measured by measuring its liquid uptake and liquid holdup.
5. Consistency
Due to differences in the preparation process, the consistency of the separator may vary greatly. The consistency includes its own characteristics such as closing temperature, as well as apparent consistency such as the consistency of holes observed under electron microscope and the consistency of thickness. Higher consistency of the diaphragm is beneficial for other performance aspects.
