In actual production, it is sometimes found that after a few hours or after other processes, the thickness of the pole piece increases compared to the thickness of the pole piece after rolling, which is the rebound of the pole piece. The pole piece is at different stages, and its thickness rebound (such as roll rebound, dry rebound, charge and discharge rebound, etc.) has different reasons. The most fundamental reason is that the choice of compaction density is unreasonable.
One, pole piece rebound
In the production process of lithium-ion batteries, the manufacture of pole pieces belongs to the previous process, occupying an important position in the entire process. The quality of the pole piece is related to the progress of the assembly process of the middle stage of the lithium battery, and also affects the electrochemical performance of the rear stage and the lithium battery.
The pole piece manufacturing is mainly composed of slurry preparation, current collector coating of active material, pole piece rolling and pole piece cutting. The uniformly dispersed lithium battery slurry is coated on the positive and negative current collector aluminum foil or copper foil through a slit extrusion or predictive transfer coating head, and is dried in an oven at different temperature zones to remove excess in the slurry Moisture or NMP solvent forms the initial lithium battery electrode pole piece.
The coated pole piece has high initial porosity and low adhesion, which is not conducive to electrolyte wetting, large contact resistance between active material particles, and easy removal of current collector during the use of lithium batteries. The electrochemical performance of lithium batteries. Therefore, the pole piece after coating needs to go through a rolling process to improve its performance.
From coating to rolling, the pole piece has gone from thick to thin, and porosity from large to small. We expect the electrode pole piece to have proper porosity and minimum interface contact resistance without destroying the morphology of the positive and negative active material particles. It means that on the basis of the above, the thinner the thickness of the pole piece is, the better. However, in actual production, sometimes it will be found that after a few hours or after other processes, the thickness of the pole piece is higher than that of the pole piece after rolling There is an increase in the thickness of the piece, which is the rebound of the pole piece. The pole piece is at different stages, and its thickness rebound (such as roll rebound, dry rebound, charge and discharge rebound, etc.) has different reasons. The most fundamental reason is that the choice of compaction density is unreasonable.
Under normal circumstances, raw material suppliers will provide lithium battery companies with a range of maximum compaction density. This compaction density range is based on the actual density, material composition, hardness and other parameters of the material. It is not correct to choose too large or too small compaction density. If the compaction density is too small, the porosity in the pole piece is high, the contact of active material particles is not close, and the internal resistance of the battery greatly affects the electrochemical performance of the lithium battery; compaction If the density is too large, the structure of the living material will be destroyed. There is not enough space between the particles. The internal repulsive force is too large, and the thickness will rebound after rolling.
Second, the impact of pole piece rebound on lithium battery production
It is normal for the thickness of the pole piece to rebound slightly after rolling, but if the rebound value is large, it may affect the middle cell packaging and battery performance. After rolling, the pole piece will continue to rebound after 1-2h, and then the thickness value will tend to stabilize. In addition to considering the thickness change of the pole piece at rest after rolling, the pole piece will also have a rebound value change after baking. After the cell is enclosed in the shell or aluminum plastic film, after the electrolyte is injected inward, as the electrolyte penetrates, the solvent molecules enter the gap of the pole piece particles, occupying the internal space of the pole piece, causing the volume of the pole piece to increase, and Resulting in an increase in overall thickness. In the process of using lithium batteries, due to the decomposition of the electrolyte and gas production, the expansion of the lithium ion extraction electrode, etc., it may cause the overall thickness of the battery to exceed the design value of the battery, causing bulging, causing safety problems.
Third, the method of controlling the rebound of the pole piece
1. Choose a reasonable compaction density
For the rebound of the pole piece, an appropriate compaction density value should be selected. Only by choosing an appropriate compaction density value can we take into account both the electrochemical performance of the battery and the thickness of the battery. For example: three positive densities of compacted densities are used: the compact density of pole piece A is 3.75g / cc, the compact density of pole piece B is 3.85g / cc, and the compact density of pole piece C is 3.95g / cc. Test the thickness of the pole piece at the corresponding position after drying and pre-charging, and calculate the rebound rate relative to the thickness of the pole piece after rolling. The results of the rebound rate are shown in Table 1:
Table 1. Bounce rate of pole pieces at different compaction densities and stages (%)
It can be seen from the data in the above table that as the compact density of the pole piece increases, the rebound thickness of the pole piece before pre-charging increases. Although at a lower compaction density (3.75g / cc), the thickness rebound rate of the pole piece before baking after pre-filling is the smallest, at 2.33%. However, after pre-charging, the thickness rebound rate of the pole piece A exceeds that of the pole piece B. During the pre-charging process, lithium ions are released from the positive electrode material and inserted into the negative electrode through the electrolyte. For the pole piece C, the particles are in close contact, and it is difficult to infiltrate the electrolyte. During pre-charging, lithium ions cannot be smoothly released from the positive electrode and inserted into the negative electrode material. At the same time, there is a stress repulsion between the particles, so the rebound rate of the pole piece C highest. For the extremely small compact density pole piece A, since the active material particles in A are not in close contact and the porosity is large, the electrolyte enters the electrode gap, causing the thickness of the pole piece to expand. Therefore, it is very important to choose an appropriate compaction density.
2. Improve the rolling process
After the lithium battery production and electrochemical performance evaluation, the best compacted density of the active material was confirmed. After that, it is necessary to improve the rolling accuracy of the rolling process to ensure that the experimental design value matches the actual rolling value. In order to improve the rolling accuracy and reduce the rebound value of the pole piece, one is to use the second rolling method, and the other is to use the hot roller process.
In the production process of lithium batteries, due to the fast rolling speed, the rebound properties of the pole pieces are different for different materials. The primary rolling often fails to meet the requirements for the thickness, compaction density and quality of the pole pieces. The secondary rolling can be effective Improve the problem of rolling accuracy. Generally speaking, the secondary rolling needs to be carried out in a drying room after 2 hours of primary rolling. The interval of 2 hours is to ensure that the pole pieces have sufficient time to elastically deform and maintain a stable state.
The pole piece hot rolling process is also one of the effective solutions to reduce pole piece rebound. The main purposes of the hot rolling process are:
â‘ Remove the water in the pole piece;
â‘¡ Reduce the rebound rate of the surface material of the pole piece after rolling, hot rolling can reduce the rebound of the pole piece by about 50%;
â‘¢Reducing the internal stress of the pole piece, because when the slitting or film cutting process is performed, the pole piece often produces snakes, flips and other undesirable phenomena due to the release of the internal stress;
â‘£During the hot rolling of the pole piece, the binder in the battery material is in a molten state, which can enhance the adhesion between the active material and the current collector, otherwise the film layer will easily fall off and fall off powder when rolled.
⑤ Reduce the deformation resistance of the battery pole piece, so that the pore structure of the active material on the pole piece is not destroyed, which is conducive to improving the increase of the liquid absorption index of the active material.
There are two main ways of hot rolling process. One is to heat the pole piece and then compact it by the roller. The pole piece is heated to 180 degrees Celsius before rolling, and then rolled, and the thickness of the pole piece can be controlled within ± 2 microns. Another method is to heat the rollers of the roller press to perform rolling. This hot roller process needs to consider the temperature distribution in the roller and the deformation of the roller when heated.
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