Effect and control method of thickness rebound of lithium battery electrode on production of lithium battery

In the process of lithium ion battery production, electrode manufacturing belongs to the previous stage and occupies an important position in the entire process. The quality of the electrode is related to the middle assembly process of the lithium battery, and it also affects the electrochemical performance of the posterior segment and the lithium battery.

In actual production, it is sometimes found that after a few hours or after other processes, the thickness of the pole increases compared to the thickness of the pole after the roller pressure. This is the rebound of the pole. The electrode is at different stages, and the reasons for its thickness rebound(such as roller pressure rebound, dry rebound, charging and discharging rebound, etc.) are different. The most fundamental reason is that the choice of compaction density is unreasonable.

I . the film rebound

In the process of lithium ion battery production, electrode manufacturing belongs to the previous stage and occupies an important position in the entire process. The quality of the electrode is related to the middle assembly process of the lithium battery, and it also affects the electrochemical performance of the posterior segment and the lithium battery.

The pole piece manufacturing is mainly composed of slurry preparation, current collector coated living material, pole piece rolling and pole piece slitting. The uniformly dispersed lithium battery slurry is coated on the positive and negative electrode current collector aluminum foil or copper foil through a slit extrusion type or a predictive amount transfer coating head, and is dried in an oven in different stages of temperature to remove excess in the slurry. The first lithium battery electrode pole piece is formed by moisture or NMP solvent.

The initial porosity of the coated electrode piece is high, and its adhesion is low. It has disadvantages such as infiltration of electrolytes, large contact resistance between living material particles, and easy separation of liquid from living materials during the use of lithium batteries, which seriously affects the electrochemical performance of lithium batteries. Play. Therefore, after the completion of the coating, the electrode needs to go through the roll pressure process to improve its performance.

From coating to rolling, the pole piece undergoes a process of thinning from thick to large and decreasing in porosity. The electrode pads we expect are based on the morphology of the positive and negative active material particles, with appropriate porosity and minimum interfacial contact resistance. 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 the pole piece is found after a few hours or after other processes, the pole piece thickness is higher than that after rolling. The thickness of the sheet has increased, which is the rebound of the pole piece. The pole pieces are at different stages, and the reasons for the rebound of thickness (such as roll rebound, dry rebound, charge and discharge rebound, etc.) are different, and the most fundamental reason is that the selection of compaction density is unreasonable.

Under normal circumstances, raw material suppliers will provide a range of maximum compaction density for lithium battery companies. This compaction density range is based on parameters such as the true density, material composition, and hardness of the material. It is not correct to select too large or too small compaction density. The compaction density is high in the porosity of the small electrode, the contact between the living material particles is not close, and the internal resistance of the battery affects the electrochemical performance of the lithium battery. Too large compaction density can cause damage to the living material structure, there is not enough gap between the particles, the internal repulsion force is too large, and the thickness bounces after roller pressure.

II. The Influence of Polarization on lithium battery Production

It is also normal for the film thickness to rebound slightly after the roller pressure, but if the rebound value is large, it may affect the core packaging and battery performance in the middle section. After roller pressure, the electrode will continue to rebound after 1-2 H, after which the thickness value will tend to stabilize. In addition to considering the thickness change of the stationary medium pole after roller pressure, the pole will also have a rebound value change after baking. After the core shell or aluminum plastic film is encapsulated, after the electrolyte is injected inwards, with the infiltration of the electrolyte, the solvent molecule enters the void of the electrode particle and occupies the inner space of the electrode, resulting in an increase in the volume of the electrode. It will also lead to an increase in the overall thickness. During the use of lithium batteries, due to the decomposition of electrolytes to produce gas, and the expansion of lithium ion de-embedded electrode sheets, the overall thickness of the battery may exceed the design value of the battery, and drum packets may appear, causing safety problems.

III. Methods to control the rebound of the polar film

1. Choose a reasonable compaction density

For polar rebound, select the appropriate compaction density value. Only by selecting the appropriate compaction density value can the electrochemical properties of the battery and the thickness of the battery be considered. For example, three types of compaction density positive polar tablets are used, which are polar A compaction density of 3.75 g/cc, Polar B compaction density of 3.85 g/cc, and polar C compaction density of 3.95 g/cc. After drying and pre-charging, the thickness of the electrode at the corresponding position is tested, and the rebound rate of the electrode thickness relative to the roller pressure is calculated.

With the increase of the compact density of the electrode, the rebound thickness of the electrode before pre-charging is greater. Although at a lower compaction density(3.75 g/cc), the thickness rebound rate before pre-charging after baking is minimal, at 2.33 %. However, after pre-charging, the thickness rebound rate of the pole A exceeds that of the pole B. During the pre-charging process, lithium ions are removed from the positive electrode material and embedded into the negative electrode through the electrolyte. For the electrode C, the contact between the particles is close, the electrolyte infiltration is more difficult, and the lithium ion can not be successfully removed from the positive electrode to the embedded negative electrode material during pre-charging. At the same time, there is stress rejection between the particles, so the rebound rate of the electrode C. The highest. For too small compacted density electrode A, due to the fact that the particles in A are not in close contact with each other, the porosity is large, and the electrolyte enters the gap between the electrodes, causing the thickness of the electrode to expand. Therefore, choosing the right compaction density is crucial.

2. Improve Roller Pressure Process

The best compaction density of living material was confirmed by the test of the electrochemical properties of lithium battery. After that, it is necessary to improve the roller pressure precision of the roll pressure process to ensure that the experimental design value and the actual roller pressure value are consistent. In order to improve the precision of roll pressure and reduce the rebound value of the polar plate, one is to use the second roll pressure, and the other is to use the hot roll process.

In the process of lithium battery production, due to the relatively fast roller pressure, the rebound properties of different materials are also different. One roller pressure often can not meet the requirements of the thickness, compaction density and quality of the electrode. The second roller pressure can effectively improve the roller pressure accuracy. problem. In general, the secondary roll pressure needs to be carried out in the drying room after 2 hours of roll pressure. The interval of 2 hours is to ensure that the electrode has a sufficiently long time elastic deformation and remains in a stable state.

The plate hot rolling process is also one of the effective solutions to reduce the plate rebound. The main purposes of the hot rolling process are:

1 Remove moisture from the pole;

2To reduce the rebound rate of the surface material after rolling, hot rolling can reduce the rebound of the electrode by about 50 %;

3 The internal stress of the electrode is reduced, because when the cutting or membrane cutting process is carried out, the electrode often produces serpentine, flip and other undesirable phenomena due to the release of internal stress;

4 Because the adhesive in the battery material is in a molten state during the hot rolling of the electrode, it can enhance the adhesion between the active material and the collecting fluid. Otherwise, the membrane layer can easily fall off and fall off the powder when the roller is pressed.

5 to reduce the deformation resistance of the battery electrode, so that the structure of the active material on the electrode is not destroyed, which is conducive to improving the absorption index of the active material.

There are two main ways of hot rolling technology, one is to heat the electrode and press it through the roller. The electrode is heated to 180 degrees Celsius before the roll pressure, and then rolled. The electrode thickness can be controlled at ± 2 microns. Another way is to heat the roller of the roller press to carry out roll pressure. This kind of hot roll technology needs to take into account the temperature distribution in the roller and the problem of the roller being heated.

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