Lithium-Ion Battery Electrode: Process Control and Precautions of Rolling Process

1. Introduction to roller press machine

Battery roller press machine is a device for rolling battery electrode, and is one of the key equipment for lithium battery manufacturing. Its main purpose is to increase the discharge capacity of the battery, reduce internal resistance, reduce polarization loss, extend the cycle life of the battery and improve the utilization rate of lithium-ion batteries by compacting the active materials on the battery electrode. In the production process, lithium battery electrode are generally compacted by battery rolling press machine, so that the electrode can better adapt to the charging and discharging process of the battery, thereby improving the energy density and stability of the battery. The roller press machine plays a key role in the manufacture of lithium batteries, and its technical parameters directly affect the quality and performance of the battery.

2.Purpose of rolling

Necessity of rolling: After the electrode is coated and dried, the peeling strength between the active material and the current collector foil is very low. At this time, it is necessary to roll it to enhance the bonding strength between the active material and the foil to prevent peeling during electrolyte immersion and battery use.

The purposes of rolling are as follows:

Ensure that the surface of the electrode is smooth and flat to prevent the burrs on the coating surface from piercing the diaphragm and causing a short circuit.

Compact the electrode coating material to reduce the volume of the electrode to increase the energy density of the battery.

Make the active material and conductive agent particles contact more closely to improve the electronic conductivity.

Enhance the bonding strength between the coating material and the current collector, reduce the occurrence of powder loss during the cycle of the battery electrode, and improve the cycle life and safety performance of the battery.

3. Pole piece rolling process and control

Rolling process

The process of rolling the battery electrode is that the battery electrode is pulled into the rotating roller by the friction between the roller and the battery electrode, and the battery electrode is compressed and deformed. The rolling of the battery electrode is different from the rolling of the steel block. The process of rolling steel is a process in which the iron molecules extend longitudinally and expand horizontally, and its density does not change during the rolling process; while the rolling of the battery electrode is a process in which the battery material on the positive and negative plates is compacted. When the battery electrode is rolled, the rolling force should not be too large or too small, and should be in line with the characteristics of the battery electrode material.

Region I is the first stage: in this stage, when the rolling force begins to increase gradually, the density of the electrode increases rapidly. This is because in this stage, the electrode material particles are displaced and the pore structure is filled. The first stage is generally called the sliding stage. This stage is the stage with the fastest rate of increase in electrode density among the three stages.

Region II is the second stage: because the pore structure of the electrode material is filled in the first stage, the density of the electrode coating material has reached a fixed value, and a certain compression deformation resistance appears when the electrode is rolled in the second stage. Compared with the first stage, although the rolling force continues to increase in this stage, the rate of increase in electrode density has decreased. From a microscopic point of view, this is because the displacement of the electrode coating material particles in this stage is already very small, but the large displacement movement of the coating material particles has not yet begun.

Region III is divided into the third stage: when the rolling force exceeds a certain size, the density of the battery electrode begins to increase with the increase in rolling force, and then the rate of increase gradually decreases. This is because when the rolling force exceeds a certain value, the displacement of the electrode coating material particles gradually begins, and the density of the electrode begins to increase again. When the rolling force increases to a certain value, the deformation of the electrode coating material is more severe, resulting in work hardening. If the rolling force continues to increase at this time, it is more difficult for the electrode coating material to deform further. Therefore, in the end, as the rolling force continues to increase, the density of the electrode does not increase much, and the increase is also reduced.

4. Problems and solutions in the rolling process

Uneven thickness of the electrode: There are many factors that cause uneven thickness of the electrode rolling, such as uneven thickness of the electrode coating, coaxiality error of the roller, cylindrical error of the roller, non-parallel contact busbar of the roller, axial deflection deformation of the roller, poor rigidity and stability of the rolling equipment, etc. The lateral thickness is uneven. During the rolling process of the electrode, the thickness of the left and right electrode sheets is often inconsistent. When the thickness of the electrode is inconsistent on the left and right sides, the influence of the electrode coating process must be eliminated first. When the thickness of the electrode that has not been rolled is consistent on the left and right sides, the rolling pressure must be adjusted on the left and right sides to ensure that the compaction density of the electrode is consistent after rolling. The electrode should be tested on time during the rolling process to prevent the pressure from changing during the rolling process. The longitudinal thickness is uneven. Sometimes the electrode meets the requirements after rolling, but the thickness increases during slitting. This is the rebound phenomenon of the electrode. The electrode rebounds because there is more water inside the electrode and the speed is too fast during rolling. The electrode rebound problem can be solved by using a hot roller process and controlling the rolling speed.

The electrode has sickle bend: This situation is mainly caused by the two rollers contacting the busbars not being parallel or the thickness of the electrode coating on both sides being different. Since the edge thickness is several microns or more than ten microns larger than the middle part, when the roller pressure is applied to the electrode, the area with large edge thickness bears greater rolling force, which leads to inconsistent lateral density of the electrode rolling compaction, resulting in serious warping of the electrode after rolling, and will also have an adverse effect on the subsequent slitting process. To control the warping, the key is to control the quality of the electrode coating. By controlling the parameters such as the surface tension of the slurry, pump pressure, belt speed, and rolling pressure, the warping of the electrode can be effectively reduced. Of course, this is under the condition of meeting the design requirements.

The electrode has wavy edges: This situation is mainly caused by the large elongation rate during the rolling process of the electrode. The inducement is the small diameter of the roller body, the small tension before the electrode rolling, the large compression of the electrode thickness, the bulges on both sides of the electrode coating, etc. When the electrode is rolled, the active substances squeeze each other and exert a certain pressure on the copper foil and aluminum foil, which will cause a certain extension. During rolling, the part without active material coating did not stretch, while the electrode with active material stretched under the roller pressure. The uneven stretching formed wavy wrinkles on the edge of the foil strip in appearance, and the parallel wave marks were perpendicular to the direction of movement of the foil strip.

Dark stripes appeared on the surface of the electrode: This situation is mainly caused by vibration marks on the surface of the roller, large cylindrical error of the roller body, small and uneven front tension.

Curling of the electrode: This situation is caused by excessive elongation of the electrode. The solution is mainly to increase the roller body diameter, reduce the compression of the electrode, adjust the front and rear tension of the electrode, etc.

The electrode has broken belt: This situation is mainly caused by uneven and unstable tension, lack of tension rapid response mechanism, and serious bulge of the coating edge of the electrode. For example, during the coating process, if there are small particles and other uneven textures on the surface of the electrode, the small particles will be squeezed toward the foil strip under the pressure of the double rollers during rolling. Softer particles can be crushed into powder and then fall off, while harder particles will squeeze the foil strip, causing holes in the foil strip or even breakage of the foil strip. During the coating process, if the surface density of the electrode is different, one piece will be over-rolled and the other piece will be under-rolled during the rolling process. During the electrode tape running process, under the same tension control, some active materials will fall off or even break the foil in places where the roller pressure is insufficient. Controlling the winding tension and preventing large particles from falling on the electrode surface can effectively reduce the breakage of the electrode.

The tension on both sides of the electrode is different: This situation is mainly caused by the non-parallelism of the roller axis and the axis of each roller, which can be solved by adjusting the parallelism of each roller axis.

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

Pole sheet rolling thickness rebound: This situation is mainly caused by large residual elastic deformation of the pole sheet after rolling and high environmental humidity. You can try hot rolling, slow rolling, high-speed rolling, reducing environmental relative humidity and other measures.

Pole sheet plate unevenness: This situation is mainly caused by uneven rolling deformation of the pole sheet, small and uneven front and rear tension or pole sheet coating thickness error.

In summary, lithium battery roller press machine plays a vital role in lithium battery manufacturing. It not only improves the performance and quality of the battery, but also reduces production costs, and provides support and guarantee for the development of lithium battery technology.

Acey new energy is a professional supplier specialized in lithium battery cylindrical cell lab equipments and pouch cell lab equipments, such as tube furnace, vacuum mixer, film coating machine, rolling press machine, die cutting machine, stacking machine, pouch cell forming machine, glove box, battery testing machine, etc and we provide one-stop solution for lithium ion battery production line. If any interested, please feel free to contact us.