Microporous membrane materials with high thermal stability, high mechanical strength, and thinning have become a hot spot in research, development and industrialization. Generally, the following conditions should be met: ① good ionic conductivity and electronic insulation; ② high chemical stability, It is usually inert to most acids, alkalis, salts and oxidants, basically insoluble in common electrolytes and does not swell significantly; 3. High electrochemical stability, does not participate in electrode reactions, and is within the voltage window of lithium-ion battery working conditions No redox reaction and no degradation; ④ High thermal stability, high melting point or melting temperature, and a small proportion of shrinkage at a certain temperature.
In addition to meeting the above requirements, the large-scale production of diaphragm products is in terms of porosity, pore size and pore size distribution, air permeability, three-dimensional structure of micropores, and electrolyte absorption and retention. Both are related to the structural design and manufacturing process of the diaphragm, and must also be guaranteed.
At present, polymer polyolefin microporous membranes are the mainstream choice for domestic and foreign power battery companies, mainly including polypropylene and polyethylene materials, mainly single-layer membranes and composite membranes. Among them, the composite membrane can effectively improve the diaphragm Mechanical strength and thermal stability of the product.
In view of the trend and requirements of increasing attention to the safety of power batteries, inorganic ceramic coatings and/or organic coatings are usually coated on the surface of the microporous membrane material, and surface modification treatment is performed to improve the temperature resistance and/or flame retardancy of the separator. , improve the wettability with electrolyte, etc.
In view of the substantial increase in the energy density of power batteries, the thinning of the diaphragm material is the development trend. Because the polyethylene diaphragm material can be thinned, it has been widely used. At the same time, diaphragm materials such as polyvinylidene fluoride, polyimide, aramid and cellulose have also been technologically developed and applied.
From the perspective of the diaphragm material industry, most of the polypropylene diaphragms of the dry process are produced in China, and the polyethylene diaphragms of the wet process have been mass-produced in many domestic and foreign companies, and the coating modified diaphragm products have been applied and promoted.
American Celgard Company, Shenzhen Xingyuan Material and Cangzhou Mingzhu and other companies mainly produce polypropylene diaphragms. Japanese companies such as Asahi Kasei and Dongran, Shanghai Enjie, Chongqing Niumi and Foshan Jinhui mainly produce polyethylene diaphragms, which can meet the needs of domestic and foreign power battery companies.
Demand for diaphragm materials. Focus on the technology and market development trend of diaphragm materials. In recent years, most domestic diaphragm companies have invested in the technological development and product marketing of polyethylene diaphragms, and have achieved batch exports.
High ionic conductivity and wide electrochemical window. Good thermal stability, stable chemical properties and good safety are the main requirements of the electrolyte.
At present, the electrolyte used for power batteries has the following outstanding problems: the main solvent components of the electrolyte are carbonates, which are highly flammable and have poor safety; the long-term stability of the electrolyte is not good, and the long-term cycle life of the battery (10 years). left and right) cannot be guaranteed; the narrow liquid temperature range of the electrolyte leads to poor high and low temperature performance of the battery.
New lithium salts such as bisfluorosulfonimide lithium salt (DFSI) have appeared in the market and have been initially used. Compared with traditional lithium hexafluorophosphate electrolyte salts, lithium bisfluorosulfonimide salts have higher solubility and conductivity in solvents, a wider operating temperature range and higher safety, but due to their high price and impurity content It is mainly used as an auxiliary material additive in combination with lithium hexafluorophosphate.
Lithium hexafluorophosphate electrolyte salt is used. Organic liquid electrolytes based on carbonate solvents are still the mainstream products of lithium-ion battery electrolytes. By adding functional additives, the compatibility of electrode materials and electrolytes is improved, and the stability and safety of electrolytes against oxidation are improved.
For lithium-ion power batteries, partially fluorinated organic solvents and mixed lithium salts are the mainstream directions for future development, which can improve the operating voltage, safety, and compatibility with silicon-based anode materials.