Diaphragm is an important part of lithium-ion battery. Its performance determines the internal resistance of the battery’s interface structure, which directly affects the battery’s capacity, cycle performance and other key characteristics. Excellent performance of the diaphragm is important for improving the overall performance of the battery. effect.
Lithium-ion battery separator materials are mainly porous polyolefins, and there are two main preparation methods: one is wet, that is, phase separation; the other is dry, that is stretch-induced pores law. Regardless of the method, the purpose is to increase the porosity and strength of the separator.
The wet production process refers to mixing liquid hydrocarbons or some small molecular substances with polyolefin resin, heating and melting to form a uniform mixture, then phase separation with a volatile solvent, and pressing to obtain a membrane; then the membrane is heated to close to crystallization Melting point, keep it for a certain period of time, then use volatile substances to elute the residual solvent, add inorganic plasticizer powder to form a film, and then further use solvent to elute the inorganic plasticizer, and finally extrude it into a sheet. The diaphragm made by this method can change its performance and structure by controlling the composition of the solution and the volatilization of the solvent during the gel curing process. The raw materials used are generally polyethylene (UHMWPE) with good mechanical properties and ultra-high molecular weight. The wet method can better control the pore size and porosity, but it requires the use of solvents, which may cause pollution and increase costs. The dry method is to melt the polyolefin resin, and then extrude and blow it into a crystalline polymer film. After crystallization heat treatment and annealing, a highly oriented multilayer structure is obtained, and then further stretched at a high temperature to peel off the crystal interface. Porous structure, which can increase the pore size of the diaphragm just right. The porous structure is related to the crystallinity and orientation of the polymer.
The performance of the battery depends on the overall performance of the separator and other materials. As the design requirements of the battery are different, the requirements for the separator are also different. The main properties of the diaphragm include air permeability, pore size and distribution, porosity, mechanical properties, thermal properties, automatic closing mechanism and electrical conductivity.
Air permeability is an important physical and chemical index of air permeable membranes. It is determined by the pore size distribution and porosity of the membrane. The Gurley method is often used to characterize air permeability. The size and distribution of porosity and pore size are related to the preparation method of the microporous membrane. However, some commercial separators (such as surface active agent treatment) have a porosity lower than 30%, and some separators have a higher porosity, up to about 60%. When the temperature is close to the melting point of the polymer, the porous ion-conducting polymer film becomes a non-porous insulating layer, and the micropores are closed and self-closing occurs. At this time, the impedance rises significantly, and the current through the battery is also limited, which prevents explosions caused by overheating. Most polyolefin membranes have a melting temperature lower than 200°C (for example, the self-closing temperature of polyethylene membrane is 130~140°C, and the self-closing temperature of polypropylene membrane is about 170°C). Of course, in some cases, even if it has been “Self-closing”, the temperature of the battery may also continue to rise, so the separator is required to withstand higher temperatures and have a sufficiently high strength.
The development of diaphragm manufacturing technology and process is an important factor that affects the performance of lithium-ion batteries. With the advancement and diversification of battery technology, a variety of good diaphragms can be designed according to different requirements. In addition, the performance-price ratio needs to be further improved. The current development trend of diaphragms is to require higher porosity, lower electrical resistance, higher tear strength, better acid and alkali resistance and good elasticity.
Ion battery electrolyte is an important part of the battery. It is responsible for the transfer of charge between the positive and negative electrodes in the battery. It is vital to the specific capacity, operating temperature range, cycle efficiency and safety performance of the battery. Lithium-ion battery organic electrolyte is composed of organic solvent, electrolyte lithium salt and necessary additives. The electrochemical stability of organic electrolyte is not only related to the composition of organic solvent, but also related to the type of electrolyte lithium salt. Organic solvents are the main part of the electrolyte and are closely related to the performance of the electrolyte. Generally, high-dielectric constant solvents and low-viscosity solvents are mixed for use. Commonly used electrolyte lithium salts include lithium perchlorate, lithium hexafluorophosphate, lithium tetrafluoroborate, etc. However, in terms of cost and safety, lithium hexafluorophosphate is the main electrolyte used in commercial lithium-ion batteries; the use of additives has not been commercialized, but has been It is one of the research hotspots of organic electrolyte.
The organic electrolyte of lithium ion batteries generally requires high ion conductivity, generally 10-3~2×10-3S·cm-l; the migration number of lithium ions should be close to 1; the electrochemically stable potential range is wide; there must be 0 ~5V electrochemical stability window; good thermal stability, wide operating temperature range; stable chemical performance, no chemical reaction with active materials and current collectors in the battery; safe and low toxicity, preferably biodegradable.