Summarize the pole piece rolling process and key control elements

1. Working process

Pole piece rolling is generally completed by a double-roll press, which is composed of two cast steel compaction rolls, a motor and a drive shaft. The roller press needs to be coated with a thin layer of oil when it is not in operation to prevent it from rusting, wipe the oil layer with absolute ethanol before use, and clean the winding and unwinding mechanism, automatic correction mechanism, rollers and other pole pieces The contact may be contaminated with impurities. After the coated pole piece is fixed to the unwinding mechanism, the pole piece is correctly passed through the double roller gap and connected to the winding system. After the rolling mode is turned on, the motor drives the upper and lower rollers to rotate at the same time, and the winding mechanism pulls the pole piece to steadily pass through the rolling gap, and is finally pressed to the required compaction density.

2. The purpose of rolling

The ultimate goal of rolling is to obtain pole pieces that meet the design requirements. What is meeting the design requirements? It is to require the relevant parameters of the pole piece: how much area density, how much compaction density, what is the adhesion, particle integrity, warpage size, etc. to meet the design requirements of the cell.

Necessity of rolling: After the pole piece is coated and dried, the peeling strength of the active material and the current collector foil is very low. At this time, it needs to be rolled to enhance the bonding strength between the active material and the foil. Prevent peeling off during electrolyte immersion and battery use.

3. Problems and solutions in the process of rolling

1). The pole piece is broken

In the process of pole piece rolling, the pole piece is often broken suddenly when walking. The fracture not only affects the work efficiency, but also causes troubles for the subsequent slitting and winding processes. There are two reasons for this:

①During the coating process, if there are uneven textures such as small particles on the surface of the pole piece, the small particles will be squeezed in the direction of the foil belt when they are pressed by the two rollers during the rolling process. The softer particles can be It is crushed into powder and then falls off. The hard particles will squeeze the foil tape, causing the foil tape to break or even break.

② During the coating process, if the surface density of the pole piece is different, one piece will be over-rolled and the other piece will be under-rolled during the rolling process. In the course of the pole piece, if the tension control is the same, the part of the live material will fall off or even the foil will be broken in the place where the rolling pressure is insufficient. Controlling the winding tension and preventing large particles of impurities from falling on the surface of the pole piece can effectively reduce the pole piece fracture.

2). The pole piece warped seriously

After the pole piece is rolled, sometimes the pole piece will show a relatively large degree of internal concave and external convex phenomenon, which is not conducive to the slitting and winding of the pole piece. An important problem in the coating process is "thick edges", which is an important cause of pole piece warpage after rolling. Because the edge thickness is a few micrometers or more than ten micrometers larger than the middle part, when the pressure of the rolling roller is applied to the pole piece, the area with a large edge thickness bears greater rolling force, which leads to inconsistent horizontal density of the pole piece roller compaction. After the pole piece is rolled, the warpage is serious, which will also adversely affect the subsequent slitting process. To control the warpage, the key is to control the coating quality of the pole piece. By controlling the slurry surface tension, pump pressure, belt speed, roll pressure and other parameters, the pole piece warpage can be effectively reduced. Of course, it is under the condition of meeting the design requirements.

3). The horizontal and vertical thickness of the pole piece is different

Horizontal direction: In the process of pole piece rolling, the thickness of the left and right pole pieces is often inconsistent. When the left and right thickness of the pole piece is inconsistent, the influence of the pole piece coating process should be eliminated first. When the thickness of the unrolled pole piece is the same on the left and right sides of the test, the rolling pressure needs to be adjusted left and right to ensure that the pole piece is rolled. The compaction density is the same on the left and right. During the rolling process, the pole pieces should be tested regularly to prevent the pressure from changing during the rolling process.

Longitudinal: Sometimes, after the pole piece is rolled, the thickness of the test pole piece meets the requirements, but the thickness increases again during slitting. This is the rebound phenomenon of the pole piece. First, the pole piece rebounds because there is more water inside the pole piece, but the rolling speed is too fast. The problem of pole piece rebound can be solved by using hot roll technology and controlling the rolling speed. The benefits of hot rollers are:

①Remove the moisture inside the pole piece

②Reduce the material rebound of the pole piece after rolling.

③Since the pole piece is in a molten state during heating, hot rolling can increase the adhesion between the active material and the fluid

④Reduce the internal stress of the pole piece, because during the slitting and die-cutting, the negative effect of the release of the internal stress of the pole piece is reduced

⑤Reducing the deformation resistance of the battery pole pieces is beneficial to increase the liquid absorption of the active material.

4). The pole piece has serious wavy edges

After the pole piece is coated, a part of the copper and aluminum foil is exposed in addition to the active material. After the pole piece is rolled, a denser wave pattern appears on the edge of the exposed copper and aluminum foil. The occurrence of this phenomenon will affect the winding, hot pressing, tab welding and other processes of the battery, and must be eliminated. The reason for the wavy edge is that copper foil and aluminum foil are made of copper and aluminum blocks through extrusion and sheeting, which has good metal workability and ductility. When the pole piece is in the process of rolling, the living substances are squeezed each other, and a certain pressure is applied to the copper foil and aluminum foil, which will produce a certain degree of extension. During the rolling, the part without the coating of the living substance does not stretch, while the pole piece with the living substance is stretched under the action of the roller pressure. The different stretch forms the wavy wrinkles at the edge of the foil strip in appearance, and parallel waves. The trace is perpendicular to the direction of movement of the foil strip. The active material attached to the wrinkled foil tape is prone to cracks, bumps or fall off. Reasonably designed coating surface density and roll compaction density can reduce the occurrence of wave edges of pole pieces. In addition, uneven rollers can also cause wavy edges.

In addition, there are some operational errors, such as scraping when measuring the thickness of the pole piece, and the problem points are not marked in time, and other human errors can be solved by strengthening training and raising awareness.

4. The impact of rolling process on batteries

1). The influence of rolling condition on the processing state of pole piece

The ideal state of the pole piece after rolling is that the surface of the pole piece is flat, the gloss under the light is consistent, the blank part has no obvious waves, and the pole piece has no large degree of warping. However, operating proficiency and equipment operating conditions in actual production will cause some problems. The most direct impact is that it affects the pole piece slitting, the width of the slitting pole piece is inconsistent, and the pole piece appears burrs; the result of rolling affects the winding of the pole piece, and serious warpage will cause the pole piece and diaphragm during the winding process of the pole piece. Larger gaps are produced, and some parts of the multilayer diaphragm will be superimposed after hot pressing, which becomes a stress concentration point and affects the performance of the cell.

2). The influence of the rolling condition on the performance of the cell

Several typical problems in the rolling process are: ①The thickness of the pole piece is inconsistent. Inconsistent thickness means that the density of living materials is inconsistent, and the transmission and conduction rates of lithium ions and electrons in the pole piece will be different. When the current density is different, it is easy to cause the precipitation of dendritic lithium, which is detrimental to the performance of the battery cell. In addition, when the thickness of the pole piece is different, the contact resistance between the active material and the current collector is also different. The thicker the pole piece, the greater the internal resistance, and the more serious the battery polarization, which affects the cell capacity. ②There is overvoltage in the pole piece. Because the thickness is too thick at some locations during coating, overpressure may occur after rolling. The live material particles are broken at the position of overvoltage, and the contact between the live material particles is close. During the charging and discharging process of the cell, the electronic conductivity increases, but the ion movement channel is reduced or blocked, which is not conducive to the capacity development and the polarization during the discharge process. Larger, the voltage drops and the capacity decreases. At the same time, the overvoltage affects the infiltration effect of the electrolyte, and also has a great impact on the performance of the cell.

5. Summary

There are many influencing factors in the production process of lithium-ion batteries. After solving the process problems that may occur in each process, it will directly reduce the waste of production materials and improve the quality and efficiency of the subsequent assembly, injection, packaging and other processes. Improve the quality and consistency of the final product, reduce production costs, and then make lithium-ion battery products more competitive in the market.