For coating, the core technology lies in the slurry formulation, which accounts for a significant portion of the cost in separator coating, typically around 65% of the total cost. The development trend of power lithium-ion batteries is towards high output, high capacity, and fast charging, which will lead to more diverse requirements for separator coating technology and processes. Continuous innovation in formulations will become the core competitiveness of coating.
The ceramic coating slurry for separators mainly consists of four key components: ceramic particles, binder, solvent, and additives. Different slurry ratios are required for different applications. The rigid support function of ceramics gives ceramic-modified separators excellent thermal stability and dimensional integrity at high temperatures, providing support for maintaining the separator structure at high temperatures. At high temperatures, the separator substrate becomes closed-cell and transparent. Uncoated separators shrink significantly, while coated lithium-ion battery separators have significantly improved shrinkage resistance due to the ceramic layer, resulting in better mechanical properties and safety.
The coating of the diaphragm is not technically difficult and can be achieved by gravure roller coating, dip coating, narrow coating or spray coating, which can be achieved by the mainstream base film manufacturers and a large number of OEMs in China.
Gravure roller coating
Gravure roller coating is a commonly used coating method. A micro-gravure roller carries the slurry from the slurry box, and an excess coating is scraped off by a thickness-controlled doctor blade. Then, under pressure, the coating is applied to a diaphragm. This method is relatively simple in principle, the groove capacity of the gravure roller is easy to control, and the product precision is good. The disadvantages are that due to the use of high-hardness alumina ceramic particles, the anilox roller is prone to wear, causing the coating to gradually thin during continuous coating, requiring replacement with a new roller; and the roller must be replaced whenever the target coating thickness changes, making it inflexible.
Narrow-slot extrusion coating
Narrow-slot extrusion coating, or narrow coating for short, is a process that uses pressure to force a slurry from an extrusion nozzle onto a diaphragm. The coating thickness is controlled by the nozzle slit width and a metering pump. This process is suitable for a wide viscosity range, produces high-precision products, can coat thin layers, and has a high coating speed, but the nozzle is relatively difficult to maintain.
Dip coating
The basic principle of dip coating is to immerse the lithium-ion battery separator in a slurry of a certain concentration for a certain period of time, then remove it and air-dry or bake it to obtain a coating of a certain thickness on the separator surface. This method is simple in principle and operation, but controlling the thickness is relatively difficult.
electrospinning
Electrospinning is a process that uses an electric field to spray slurry onto a diaphragm, giving the slurry an electric charge. Under the influence of the electric field, the slurry overcomes its surface tension and forms nanofibers that are then sprayed onto the diaphragm.
