Aug 14, 2019 Pageview：23
In the production process of lithium ion battery, after the production of the electrode plate, the positive and negative electrode plates and the diaphragm are assembled and manufactured by means of winding or lamination to form the basic cell. Subsequently, the cell is usually thermally pressed. This paper mainly introduces the purpose and process control points of hot press shaping process.
I. purpose of the process
No matter in the winding or Z fold, must exert a certain tension on the diaphragm, in order to ensure positive plate, diaphragm, neat degree between the cathode piece, however, the technological process of tension can make the diaphragm in the direction of tape elongated, the diaphragm shrinkage is very big, at the direction of tape will make serious extrusion sheet diaphragm, which leads to batteries after assembly process, especially the coiling process, batteries, deformation, deformation after the batteries not only appearance smoothness is poor, also exist inside the diaphragm defects such as fold, as shown in figure 1 and figure 2, which leads to low capacity, fast cycle performance is poor and self-discharge of quality problem, especially the winding of the thicker batteries, The deformation problem after winding is particularly prominent. In addition, the thickness consistency of the cell in the loose state is also poor, which will affect the process of the cell entering the shell, increase the difficulty of the shell entering process, and even cause the cell damage when entering the shell.
Therefore, the main purposes of cell thermocompression shaping include:
(1) improve the flatness of lithium ion battery, so that the cell thickness meets the requirements and has a high consistency;
(2) eliminate the membrane folds, and drive out the air inside the cell, so that the membrane and the positive and negative poles are closely attached together, shorten the lithium ion diffusion distance, and reduce the internal resistance of the battery.
In order to enable the lithium ion battery cell after the plastic straight and avoid retraction and recovery. The process of cell hot-pressing shaping is as follows: put the wound or laminated cells on the template, set the pressure of the pressurized cylinder and the template temperature, and then the upper and lower templates under a certain pressure and temperature to make the cell finalize the design, to achieve the same thickness of the cell, so that the cell elasticity is reduced, reduce the qualified rate of the core and protect
Verify the consistency of the thickness of finished cells.
For the square battery, the hot-press shaping device is shown in figure 3. The upper and lower templates are flat plates, which are pressed together to form flat cells. For cylindrical batteries, the hot-press shaping device is shown in figure 4. It is fixed on the base to generate two cylinders moving in the opposite direction, and two semicircle dies with cylindrical groove at the end of two cylinders piston rod. The radius of cylindrical groove of two semicircle dies is equal to or less than the preset coil core radius. The coil core is clamped by a manipulator or a clamp at the center of the two semicircle dies, and the two cylinders are controlled to drive the two semicircle dies to move towards each other, so that the two semicircle dies can squeeze the coil core, thus shaping the winding to the preset size, so that it can be put into the matching shell.
In general, when the cell hot-voltage shaping, at the same time on the cell insulation voltage test, test whether there is a micro short circuit inside the cell. If there are tiny metal foreign matter particles in the cell, it is easy to detect unqualified products with micro-short circuit when the cell is under pressure and the cell diaphragm is pierced. On the test of voltage resistance of battery cell insulation, I have sorted out an article before: basic knowledge of the test of voltage resistance of lithium battery cell insulation (click to read).
Three, process points
The main technological parameters of cell hot press shaping are pressure, time and template-plate temperature. Under the appropriate process parameters, there is almost no air inside the thick cell, the diaphragm and the pole are tightly bonded together, the loose cell can become a hard state. However, for the ceramic diaphragm used in recent years, it is difficult to bond the diaphragm with the plate to form such a state due to the existence of the ceramic layer. In the process determination test, the test items include the permeability of diaphragm, thickness change; Whether the cell thickness meets the requirements for shell entry; Whether the pole piece breaks, etc.
Battery diaphragm as the core component of the battery, give play to the isolation are negative electrons, while allowing lithium ion between the poles of reciprocating through the key role of the microporous structure of the diaphragm is the importance of these ions is shuttling between the cathode channel, its permeability will directly affect the performance of the battery diaphragm permeability refers to the diaphragm in a certain amount of time through the amount of gas under pressure. If the permeability of diaphragm is not good, it will affect the transfer of lithium ions between the positive and negative poles, and then affect the charge and discharge of lithium battery. The test process is as follows: fix the battery diaphragm, apply the air pressure on the side of the diaphragm, measure the pressure drop and the time used, and test the permeability of the diaphragm. The shorter the time used, the better the air permeability. During the hot-pressing process, the diaphragm may be severely compressed and the thickness of the diaphragm may change greatly, resulting in the blockage of the micropores. The diaphragm will become transparent when observed by the naked eye. This situation indicates that the hot-pressing shaping can overlimit the action of the cell and affect the transmission of lithium ions. The winding cell is shown in figure 2. If the electrode sheet is brittle, the cell is prone to powder loss or even fracture during the hot-pressing shaping, which will result in limited electronic transmission and increase the internal resistance of the battery. Therefore, the cell hot press shaping must also avoid this situation. These two aspects require hot shaping pressure as small as possible, the shorter the better. But on the other hand, the hot pressing shaping must make the cell finalize the design, the cell thickness satisfies the technological requirements, the cell elasticity reduces, and guarantees the product cell thickness consistency. Therefore, process parameters such as pressure, time and temperature need to be optimized.
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