AG10 battery

Detailed explanation of the rolling process of AG10 battery pole pieces

Pole piece rolling is an important step in the production process of lithium batteries. Its purpose is to obtain pole pieces that meet the design requirements. Rolling is a very necessary process. After the pole pieces are coated and dried, the peel strength between the active material and the current collector foil is very low. At this time, they need to be rolled to enhance the bonding strength between the active material and the foil to prevent peeling during electrolyte immersion and battery use. The following will introduce in detail:

1. Rolling equipment

The basic functions of rolling equipment are:

(1) Roller pressure adjustment and rapid response function: The pressure adjustment between the two rollers of the roller press is a necessary condition for improving the compaction density of the active material of the battery pole piece. Due to the influence of factors such as coating intervals and single-sided and double-sided staggered, the pressure adjustment between the two rollers must respond quickly.

(2) Roller gap adjustment and accurate reset function: The gap adjustment between the two rollers of the roller press is a necessary condition for obtaining the thickness of the battery pole piece. Due to the changes in the pole piece coating method and the need for pole piece tape connection, the gap between the two rollers must be accurately reset after rapid adjustment.

(3) Adjustment of tension before and after rolling of pole pieces and quick response function: Adjustment of tension before and after rolling of battery pole pieces can control the flatness of the battery pole pieces. The rolling line speed often changes suddenly during rolling, and quick response of tension control is an important means to prevent belt breakage.

(4) Roller stepless speed regulation and line speed synchronization function: During the start-up and stop process or according to the process requirements, the two rollers of the battery pole piece rolling machine must be steplessly changed, and the line speed of the two rollers must be consistent.

(5) Pole piece rolling temperature adjustment function: Adjusting the rolling temperature of the battery pole piece can directly affect the deformation resistance and plastic deformation of the battery pole piece during rolling.

(6) Roller deformation correction function: During the rolling process or temperature adjustment of the battery pole piece, the two rollers will inevitably have axial flexural deformation and radial bulging deformation. Correcting the roller deformation directly affects the thickness uniformity and compaction density uniformity of the battery pole piece.

(7) Intelligent control function of the electrode sheet rolling process: With the continuous improvement of the electrode sheet rolling speed and the degree of automation, automatic loading and unloading, automatic belt connection, automatic pressurization, automatic gap adjustment, online monitoring, etc. all require closed-loop control or even intelligent control.

(8) Roller cleaning and maintenance function: It is common for the two rollers to stick powder during the battery electrode sheet rolling process. Keeping the roller surface clean can not only reduce roller wear, but also improve the surface quality of the battery electrode. It is also a very necessary function for the roller press to be easy to maintain.

2. Rolling process and control

(1) Basic mechanism of battery electrode sheet rolling: Battery electrode sheet rolling belongs to powder rolling, and its purpose is to improve the compaction density and uniformity of the active material of the battery electrode sheet, improve the adhesion of the active material, and improve the surface roughness. The rolling process follows the law of weight invariance.

(2) Vertical compaction and longitudinal extension. During the rolling process, the pressure of the two rollers on the battery electrode sheet is actually the combined force of vertical pressure and horizontal pressure, and its size depends on the compression amount of the active material of the electrode sheet and the roller bite angle. Under the premise that the compression amount of the active material of the electrode sheet is constant, the magnitude of the vertical pressure and the horizontal pressure depends on the bite angle of the two rollers. The larger the bite angle, the larger the horizontal pressure, and the smaller the bite angle, the larger the vertical pressure. The compaction density depends on the magnitude of the vertical pressure, and the longitudinal elongation depends on the magnitude of the horizontal pressure.

(3) The uniformity of the compaction density and surface roughness of the electrode sheet. Assuming that the electrode sheet coating thickness is uniform, the uniformity of the compaction density of the battery electrode sheet depends on the parallelism of the contact busbars between the two rollers. The important influencing factors are the coaxiality of the rollers, the cylindricality of the roller body, the bearing accuracy, the rigidity stability of the equipment, and the gap adjustment at both ends of the rollers. The roughness of the electrode sheet rolling surface depends on the size of the active material particles and the roughness of the roller surface.

(4) The extension of the current collector and the slippage of the active material particles. Aluminum foil or copper foil current collectors are difficult to extend when rolled on a large roller diameter roller rolling equipment, but the active material bonded to the current collector will slip under the push of horizontal pressure, thereby driving the battery electrode sheet current collector to extend. The elongation rate affects the flatness and conductivity of the electrode sheet.

(5) Local extension and compression of the battery pole piece and uneven internal stress. There are errors in the coating thickness of the battery pole piece, and there are also errors in the parallelism of the two rollers contacting the busbars. For this reason, the local compaction density of the active material on the battery pole piece is not uniform. The coexistence of local extension and peripheral compression causes uneven internal stress of the pole piece, which in turn affects the flatness of the battery pole piece plate.

(6) Pole piece compaction density, elongation and roller neck. As mentioned earlier, the bite angle of the two rollers directly affects the compaction density and elongation of the active material of the pole piece, and the size of the roller body diameter directly determines the bite angle. The larger the roller diameter, the smaller the bite angle, and the smaller the roller diameter, the larger the bite angle.

(7) Pole piece rolling thickness rebound, rolling speed and ambient humidity. Slow rolling speed will reduce the elastic deformation of the active material of the pole piece, that is, the thickness rebound after rolling will become smaller. However, the fact is that when the rolling speed is increased to a certain value, the thickness rebound of the pole piece after rolling will become smaller, which is caused by ambient humidity. The amount of water absorbed by the active material not only affects the surface alkalinity of the active material, but also affects the thickness rebound.

(8) Uneven internal stress and tension control of the pole piece rolling process. The pole piece rolling process is a process of compression deformation and extension deformation. In this process, the inlet tension affects the internal stress distribution of the pole piece, and the outlet tension affects the plate flatness of the pole piece.

(9) Hot rolling and deformation resistance of the pole piece. Generally speaking, the deformation resistance of the material will decrease with the increase of temperature, and the amount of plastic deformation will also increase accordingly. Hot rolling of the pole piece is also beneficial to reduce the surface wear of the roller. However, the comparison of hot and cold rolling of the pole piece has not had a significant effect, which shows the complexity of the factors affecting the rolling of the pole piece.

3. Common problems and solutions in the rolling process

(1) Uneven thickness of the pole piece. There are many factors that cause uneven thickness of the pole piece rolling, such as uneven coating thickness of the pole piece, coaxiality error of the roller, cylindrical error of the roller, non-parallel contact generatrix of the roller, axial deflection deformation of the roller, poor rigidity and stability of the rolling equipment, etc. These problems are difficult to adjust, and are caused by the incoming materials and the equipment itself.

(2) The pole piece has sickle bend. This situation is mainly caused by the two rollers not contacting the busbars in parallel or the thickness of the pole piece coating on both sides is different.

(3) The pole piece has wavy edges. This situation is mainly caused by the large elongation rate during the pole piece rolling process. The causes are small roller diameter, small tension before pole piece rolling, large compression of pole piece thickness, bulges on both sides of the pole piece coating, etc.

(4) Dark stripes appear on the pole piece surface. This situation is mainly caused by vibration marks on the roller surface, large roller cylindricality error, small and uneven front tension.

(5) The pole piece has curling. This situation is caused by excessive elongation of the pole piece. The solution is mainly to increase the roller diameter, reduce the compression of the pole piece, adjust the front and rear tension of the pole piece, etc.

(6) The pole piece has broken belt. This situation is mainly caused by uneven and unstable tension, lack of tension rapid response mechanism, and serious bulges on the edge of the pole piece coating.

(7) The tension on both sides of the pole piece is different. This situation is mainly caused by the non-parallelism of the roller axis and the axis of each roller. The problem can be solved by adjusting the parallelism of each roller axis.

(8) There are pitting on the roller surface. This situation is fatigue pitting on the roller surface. It is mainly caused by uneven roller material and heat treatment metallographic structure, poor roller surface fatigue strength, and is also related to the surface roughness of the roller.

(9) The thickness of the pole piece rebounds after rolling. This situation is mainly caused by large residual elastic deformation of the pole piece after rolling and high environmental humidity. You can try hot rolling, slow rolling, high-speed rolling, and reducing the relative humidity of the environment.

(10) The pole piece is uneven. This situation is mainly caused by uneven rolling deformation of the pole piece, small and uneven front and rear tension, or error in the thickness of the pole piece coating.

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