In the production process of lithium-ion batteries, after the production of the pole pieces is completed, the positive and negative pole pieces and diaphragms are assembled and manufactured by winding or laminating to form the basic cells. Subsequently, the cells are generally subjected to hot-pressing and shaping. The main purpose of this article is to briefly introduce the purpose and process control points of the hot press forming process.
First, the purpose of the process
Whether in winding or Z-type lamination, a certain tension must be applied to the diaphragm to ensure the orderliness of the positive electrode sheet, diaphragm, and negative electrode sheet. However, tension in the process causes the separator to be pulled in the direction of travel. Long, the diaphragm has a large amount of shrinkage in the direction of travel, which can cause the diaphragm to seriously squeezing the pole pieces, resulting in a deformation of the battery core after the assembly process of the battery, especially the winding process, and not only the appearance flatness of the deformed battery core. Bad, there will be defects such as diaphragm folds, as shown in Figure 1 and Figure 2, which will lead to low capacity, poor cycle performance and self-discharge and other quality problems, especially winding thicker batteries, after winding The problem of deformation is particularly prominent. In addition, the consistency of the core thickness of the loose state is also poor, which will affect the process of cell core entry, increase the difficulty of the shelling process, and even cause damage to the cell core when entering the shell.
Therefore, the main purpose of hot-pressing plastic cores includes:
(1) Improve the flatness of lithium-ion batteries so that the cell thickness meets the requirements and has high consistency;
(2) Eliminating the wrinkles of the diaphragm, driving out the air inside the cell, making the separator and the positive and negative pole pieces stick together, shortening the diffusion distance of lithium ions, and reducing the internal resistance of the battery.
Second, the process
In order to make the lithium-ion battery cells straight after shaping and avoid retraction. The process of hot-pressing the core is as follows: Place the wound or laminated cores on the template and set the booster pressure. And the template temperature, then the upper and lower template under a certain pressure and temperature to make the cell shape, to achieve the same thickness of the cell, so that the elasticity of the cell decreases, reduce the core pass rate and guarantee
Verify the consistency of the finished product core thickness.
Figure 3 Square electric core hot pressing and shaping device
For a square battery, the hot press-shaping device is as shown in Fig. 3. The upper and lower platens are flat plates, and the plate is closed and flattened by the pressure. In the case of a cylindrical battery, the hot-pressing and shaping device is fixed on the base as shown in Fig. 4. Two cylinders moving toward each other, two semi-circular molds with cylindrical grooves at the end of the two-cylinder piston rods, and the radius of the cylindrical grooves of the two semi-circular molds are equal and equal to or smaller than the preset core radius. The roll is held by a robot or a jig The core is placed at the center of the two semi-circular molds, and the two cylinders are controlled to drive the two semi-circular molds to move towards each other, so that the two semi-circular molds press the cores, thereby shaping the windings to a predetermined size. Can fit into a matching shell.
Fig. 4 Schematic diagram of a hot-pressing and shaping device for a cylindrical battery cell
In general, when the cell is hot-pressed and shaped, the cell is subjected to an insulation withstand voltage test at the same time to detect the presence of a micro-short circuit within the cell. If there is a minute metal foreign particle inside the cell, the cell is pressed down. Piercing, it is easy to detect micro-short circuit failure products. About the battery insulation withstand voltage test, before finishing an article: Lithium battery core insulation voltage test basic knowledge (click to read).
Third, the process points
The main process parameters of the hot-pressing and shaping of the core include the pressurization pressure, the pressurization time and the die temperature. Under the appropriate process parameters, there is almost no air inside the thick cell, the separator and the pole piece are closely bonded together, and the loose cell is It can be turned into a hard block state. However, for ceramic separators used in recent years, it is difficult for the separators to adhere to the pole pieces to form this state due to the presence of the ceramic layer. In the process determination test, the test item includes the permeability of the separator, Change in thickness; whether the core thickness meets the requirements for shelling; whether the pole piece breaks or not.
The battery separator serves as the core component of the battery, and plays a key role in isolating the positive and negative electrode electron conduction while allowing lithium ions to reciprocate between the two electrodes. The microporous structure on the separator is an important channel for the ions to and from the positive and negative electrodes. , Its air permeability will directly affect the performance of the battery, diaphragm permeability refers to the amount of gas through the diaphragm under a certain time pressure. If the air permeability of the diaphragm is not good, will affect the lithium ion between the positive and negative Transmission, and then affect the charge and discharge of lithium batteries. Test process is: Fixed battery separator, apply air pressure on the side of the diaphragm, measure pressure drop and time, measure the air permeability of the diaphragm, the shorter the time, the better the breathability. During the hot pressing process, the diaphragm may be severely compressed, and the thickness of the diaphragm may vary greatly, causing the micro-holes to become clogged, and the diaphragm may become transparent when viewed with the naked eye. This condition indicates that the heat-pressing shaping has an excessive effect on the battery core and affects lithium ions. Transmission. As shown in Figure 2, if the pole piece is fragile, the bent portion of the core is likely to be powdered or even broken in the hot-press shaping, which will result in the limitation of electron transmission and increase. The internal resistance of the battery. Therefore, the hot-pressing of the battery core must also avoid this happening. These two requirements require that the pressure of the hot-pressing plastic is as small as possible, and the shorter the time, the better. On the other hand, the hot-pressing and shaping must be made The cell core is shaped, the cell thickness meets the process requirements, the cell elasticity is reduced, and the consistency of the cell core thickness is ensured. Therefore, the process parameters such as pressure, time, and temperature need to be optimized.
