According to incomplete statistics: the battery failure caused by the pole piece coating process accounts for more than 10% of all lithium battery failures, which is also one of the hot topics in the lithium battery frontier group. Therefore, Microcharge has summarized the lithium battery coating process and equipment selection for everyone. Interested partners can go to see it with Microcharge.
1 The significance of the lithium battery coating process
Lithium battery slurry coating is the next process after the preparation of the slurry. The main purpose of this process is to uniformly coat the slurry with good stability, good viscosity, and good fluidity on the positive and negative current collectors of the lithium battery. The coating of the pole piece is of great significance to lithium batteries' capacity, consistency, and safety.
2 Influence of coating process on the performance of lithium battery
Lithium battery pole piece coating generally refers to a process in which a uniformly stirred slurry is evenly coated on the current collector, and the organic solvent in the slurry is dried. The effect of coating has an important impact on the capacity, internal resistance, cycle life, and safety of lithium batteries, ensuring that the pole pieces are evenly coated. The choice of coating method and control parameters have an important impact on the performance of lithium batteries, mainly in:
1) Control of coating and drying temperature: If the drying temperature during coating is too low, the complete drying of the pole piece cannot be guaranteed. If the temperature is too high, the surface coating of the pole piece may appear tortoises due to the rapid evaporation of the organic solvent inside the pole piece. cracking, falling off, etc.;
2) Coating surface density: If the coating surface density is too small, the lithium battery capacity may not reach the nominal capacity. If the coating surface density is too large, it is easy to cause waste of ingredients. The precipitation of lithium forms lithium dendrites that pierce the lithium battery separator and cause a short circuit, causing potential safety hazards;
3) Coating size: If the coating size is too small or too large, the positive electrode inside the lithium battery may not be completely covered by the negative electrode. During the charging process, the lithium ions are embedded from the positive electrode and move to the electrolyzer which is not completely covered by the negative electrode. In the liquid, the actual capacity of the positive electrode cannot be efficiently exerted. In severe cases, lithium dendrites will be formed inside the lithium battery, which will easily pierce the diaphragm and lead to the internal circuit of the lithium battery;
Further reading: Lithium battery pole piece design basis
4) Coating thickness: If the coating thickness is too thin or too thick, the next pole piece rolling process will be affected, and the performance consistency of the lithium battery pole piece cannot be guaranteed.
In addition, pole piece coating is of great significance to the safety of lithium batteries. 5S work should be done before coating to ensure that no particles, sundries, dust, etc. are mixed into the pole piece during the coating process. If the sundries are mixed, it will cause a micro-short circuit inside the lithium battery, and in severe cases, the lithium battery will catch fire and explode.
3 Lithium battery coating equipment and coating process selection
(1) Lithium battery coating process
The generalized lithium battery coating process includes:
Unwinding → splicing → pulling → tension control → coating → drying → deviation correction → tension control → deviation correction → winding and other processes. The coating process is complex, and there are many factors that affect the coating effect, such as: the manufacturing accuracy of the coating equipment, the smoothness of the equipment operation, the control of the dynamic tension during the coating process, the size of the air volume during the drying process, and the temperature control The curve will affect the coating effect, so it is extremely important to choose the appropriate coating process.
(2) Selection of coating method for lithium battery
Generally, the following aspects need to be considered when choosing a coating method, including the number of layers to be coated, the thickness of the wet coating, the rheological properties of the coating solution, the required coating accuracy, the coating support or substrate, the coating cloth speed, etc.
In addition to the above factors, the specific conditions and characteristics of the pole piece coating must also be combined.
(3) Characteristics of lithium battery pole piece coating:
①Double-sided single-layer coating;
②The wet coating of the slurry is thicker (100～300μm);
③The slurry is a non-Newtonian high-viscosity fluid;
④The coating precision of the pole piece is high, which is similar to the coating precision of the film;
⑤ The coating support is aluminum foil and copper foil with a thickness of 10-20 μm;
⑥Compared with the film coating speed, the pole piece coating speed is not high.
In view of the above factors, the general laboratory equipment often adopts the scraper type, the consumer lithium battery mostly adopts the roller coating transfer type, and the power lithium battery mostly adopts the slit extrusion method.
(4) Selection of lithium battery coating equipment
1) Lithium battery scraper coating:
Working principle: The foil substrate passes through the coating roller and is in direct contact with the slurry tank, the excess slurry is coated on the foil substrate, and when the substrate passes between the coating roller and the scraper, the gap between the scraper and the substrate The coating thickness is determined, while the excess slurry is scraped off and reflowed, thereby forming a uniform coating on the surface of the substrate. Scraper Type Main comma scraper. The comma scraper is one of the key components in the coating head. Generally, a comma-like edge is formed along the busbar on the surface of the round roll. This scraper has high strength and hardness and is easy to control the coating amount and coating accuracy. It is suitable for High solids and high viscosity slurries.
2) Lithium battery roll coating transfer type:
The rotation of the coating roller drives the slurry, adjusts the transfer amount of the slurry through the gap of the comma scraper, and transfers the slurry to the substrate by the rotation of the back roller and the coating roller. The process is shown in Figure 2. Roll transfer coating consists of two basic processes: (1) The rotation of the coating roller drives the slurry to pass through the gap between the metering rolls to form a slurry layer of a certain thickness; (2) The slurry layer of a certain thickness passes through the opposite coating roller and The back roll rotates to transfer the slurry to the foil to form the coating.
3) Lithium battery slit extrusion coating:
As a precise wet coating technology, as shown in Figure 3, the working principle is that the coating liquid is extruded and sprayed along the gap of the coating die under certain pressure and a certain flow rate and transferred to the substrate. Compared with other coating methods, it has many advantages, such as fast coating speed, high precision, and uniform wet thickness; the coating system is closed, which can prevent the entry of pollutants during the coating process, and has high slurry utilization rate and can maintain the slurry. The properties are stable and can be multi-layered at the same time. And it can adapt to different slurry viscosity and solid content range and has stronger adaptability compared with the transfer coating process.
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