Panoramic scanning of lithium ion battery pole piece coating process
的生产制造，是由一个个工艺步骤严密联络起来的过程。 The production of lithium-ion batteries is a process closely linked by process steps. 锂电池 的生产包括极片制造工艺、电池组装工艺以及最后的注液、预充、化成、老化工艺。 Overall, the production of lithium batteries includes pole piece manufacturing processes, battery assembly processes, and final liquid injection, pre-charge, formation, and aging processes. In these three stages of the process, each process can be divided into several key processes, each step will have a great impact on the final performance of the battery.
In the pole piece manufacturing process stage, it can be divided into five processes: slurry preparation, slurry coating, pole piece rolling, pole piece cutting, and pole piece drying. In the battery assembly process, according to the different battery specifications, it is roughly divided into winding, casing, welding and other processes. In the final injection stage, various processes including injection, exhaust, sealing, pre-charging, forming, and aging are included.
Every process in the battery manufacturing process will cause a certain amount of waste. The reasons for wastage include employee errors, equipment errors, environmental reasons, etc. In order to ensure that the cost rate of the product is good enough, every step of the product is guaranteed to be qualified.
First, the significance of coating
Slurry coating is the next step after the preparation of the slurry. The main purpose of this step is to uniformly coat the positive and negative electrode current collectors with good stability, good viscosity, and good fluidity. 锂电 池具有重要的意义，主要体现在以下几点： The pole piece coating has important significance for lithium batteries , mainly reflected in the following points:
1. It is of great significance to the capacity of the finished battery. During the coating process, if the thicknesses of the positive and negative electrode slurry coatings at the front, middle, and rear sections of the pole piece are inconsistent, it is easy to cause the battery capacity to be too low and too high, and it is easier to form lithium precipitation during the battery cycle, affecting the battery life.
2. It is of great significance to the safety of the battery. Before coating, do 5S work to ensure that no particles, debris, dust, etc. are mixed into the pole piece during the coating process. If the debris is mixed, it will cause a micro-short circuit inside the battery, which will cause the battery to explode in serious cases.
3. It is of great significance to the consistency of battery performance. Battery factories are relatively taboo because in a batch of batteries, the difference in capacity and cycle life are large, so it is necessary to ensure the same parameters before and after the pole piece coating process.
4. Significant to battery life. The large difference before and after the slurry coating, the dust mixed into the pole pieces, and the uneven thickness of the pole pieces, etc., are all related to the electrochemical performance of the battery.
Therefore, the requirements for slurry coating in this process are: when the slurry is good enough, the density of the active material before, during, and after the pole piece remains the same, and no impurities are mixed in the coating process. Of course, the definition of the quality of the pole piece is not only determined by the effect of coating. If the pole piece has serious powder fall, is not resistant to bending, and there are white bubbles in the pole piece, this is the problem of the slurry. Need to go back to the first step to resolve.
Second, the coating method
Coating equipment is mainly composed of unwinding unit, feeding unit, tension control system, coating machine head, oven and other parts. Coating can be divided into transfer coating and extrusion coating, both of which have advantages and disadvantages.
Transfer coating: The coating roller rotates to drive the slurry, adjust the amount of slurry transfer by adjusting the blade gap, and use the rotation of the back roller or coating roller to transfer the slurry to the substrate. According to the process requirements, control the coating layer The thickness is to meet the weight requirement. At the same time, the solvent in the slurry spread on the substrate is removed by drying and heating, so that the solid matter is well adhered to the substrate. As shown in Figure 1.
The advantage is that the viscosity of the slurry is not high, it is easy to adjust the coating parameters, there is no blockage, etc. The disadvantage is that the coating accuracy is poor for power batteries, and the consistency of the pole pieces cannot be guaranteed. The slurry is exposed to the air between the rolls, which has a partial effect on the properties of the slurry.
Extrusion coating: the feeding system feeds the coating to the screw pump, and then sends the slurry power to the extrusion head. The slurry is made into a liquid film by extrusion and applied to a moving current collector. After drying, a uniform coating is formed, as shown in Figure 2. Its advantages are that the pole piece is very uniform and highly accurate after coating, the flatness of the coating edge is high, and the sealed operating system is not affected by foreign objects, which is suitable for mass production. The disadvantages are high equipment accuracy, high maintenance requirements, high slurry viscosity range, and the need to replace new gaskets when changing specifications.
题 Third, pay attention to problems in coating
Reducing coating defects during the coating process, improving coating quality and yield, and reducing costs are important aspects of the coating process that need to be studied. The waste generated during the coating process is mainly the initial adjustment of the machine (related to the operator's operation level), coating interruption foil, mixed foreign matter, etc., each time the shutdown will produce a certain distance of waste.
The problems that often occur in coating are: raw material contamination, unstable coating process, irregular operation, incorrect setting of drying procedures, etc. These problems often cause the following problems for pole pieces:
Mainly comes from air bubbles and mixed foreign matter in the slurry. Air bubbles can come from incomplete degassing during agitation, during feeding, or during coating. Foreign matter mainly comes from errors or environmental problems during operation.
During the coating process, the air bubbles inside the slurry are sprayed on the pole pieces. When dried in an oven, the air bubbles burst and white round spots are formed on the pole pieces. Here, the active material coating is thin, and it is also most likely to cause micro short-circuits during the battery charging and discharging process. When there is a foreign body in the pole piece, the coating film around the particles is a low surface tension area, and the liquid film migrates to the surroundings to form a point defect. The main measures to prevent such defects are: controlling the operating environment, optimizing the slurry stirring, controlling the coating speed, and ensuring that the substrate is clean.
2. Thick-Edge Defects
During the rolling process of the pole pieces, the thick edges are subjected to greater pressure, which not only causes the pole pieces to have different transverse densities, but also causes the live material particles at the thick edges to be crushed. After pressing the pole pieces with thick edge defects, a serious warping phenomenon will occur, which will also have a great impact on the subsequent slitting and winding processes. After the live material particles at the thick edges are crushed, the lithium ion and electron transmission paths become farther during the charge and discharge process, which will cause the battery internal resistance to increase and deepen polarization, which will affect the battery life and safety.
At the same time, because the edge of the pole piece is thicker and forms a stress concentration point inside the winding core, lithium precipitation and micro short-circuiting are very likely to occur here, which is also extremely detrimental to battery performance. The cause of the thick edges is driven by the surface tension of the slurry, which causes the slurry to migrate to the uncoated edge of the pole piece and form thick edges after drying.
Some studies have found that the coating speed has no significant effect on the width and height of the edges. The edge gradient increases with increasing coating speed, and reducing the gap ratio (coating gap / film thickness) can reduce the edge effect. Related gap coating research results show that thick edges can also be reduced by adjusting the coating gap and pre-adjusting the pressure. Adding surfactants to reduce the surface tension of the slurry can also reduce the occurrence of thick edges to a certain extent.
Development Trend of Future Coating Process
Extrusion coating is widely used in the field of power batteries due to its advantages such as high precision, uniform coating, and suitable for large-width coating. It has also gradually replaced the transfer coating machine suitable for pilot lines. In the future, the coating process will develop in the direction of high equipment performance, high crop rate, on-line thickness measurement control accuracy, and improved drying efficiency.