Processes that cannot be ignored in lithium 18650 li ion battery production

　　In the production process of lithium-ion batteries, after the production of electrode plates is completed, the positive and negative electrode plates and separators are assembled and manufactured by winding or lamination to form the basic battery core. Subsequently, the battery core is generally hot-pressed and shaped. This article mainly briefly introduces the purpose of the hot press shaping process and the key points of process control.

　　1. Process purpose

　　Whether in winding or Z-shaped lamination, a certain tension must be applied to the separator to ensure the neatness between the positive electrode sheet, separator, and negative electrode sheet. However, the tension during the process will cause the separator to be pulled in the tape transport direction. long, the diaphragm shrinks greatly in the direction of tape transport, which will cause the diaphragm to severely squeeze the pole pieces, resulting in deformation of the battery core after the assembly process of the battery core, especially the winding process. The deformed battery core not only has a smooth appearance Poor, there will also be defects such as separator wrinkles inside, as shown in Figures 1 and 2, which will lead to quality problems such as low capacity, poor cycle performance, and fast self-discharge, especially for thicker wound cells. The deformation problem is particularly prominent. In addition, the thickness consistency of loose battery cells is also poor, which will affect the battery insertion process, increase the difficulty of the battery insertion process, and even cause battery core damage during insertion.

　　Therefore, the main purposes of hot pressing and shaping of battery cells include:

　　(1) Improve the flatness of lithium-ion batteries so that the cell thickness meets the requirements and has high consistency;

　　(2) Eliminate the wrinkles of the separator and drive out the air inside the battery core, so that the separator and the positive and negative electrode plates are closely connected, shortening the lithium ion diffusion distance and reducing the internal resistance of the battery.

　　2. Process

　　In order to make the lithium-ion battery cells straight and avoid shrinkage and recovery after shaping. The battery hot pressing shaping process is as follows: Place the wound or laminated battery core on the template, set the pressure of the booster cylinder and the temperature of the template, and then the upper and lower templates shape the battery core under a certain pressure and temperature to achieve The thickness of the battery core is consistent, which reduces the elasticity of the battery core, reduces the core installation qualification rate and ensures

　　Verify the consistency of finished battery core thickness.

　　Figure 3 Square battery core hot pressing shaping device

　　For square batteries, the hot pressing shaping device is shown in Figure 3. The upper and lower templates are flat plates, and the flat plates are clamped under pressure to flatten the cells. For cylindrical batteries, the hot pressing shaping device is shown in Figure 4. Two cylinders are fixed on the base to produce relative motion. Two semi-circular molds with cylindrical grooves at the ends of the piston rods of the two cylinders. The two semi-circular molds have cylindrical grooves. The radius is equal to and equal to or smaller than the preset core radius. Use a manipulator or clamp to clamp the winding core and place it at the center of the two semi-circular dies. Control the two cylinders to drive the two semi-circular dies to move toward each other, causing the two semi-circular dies to squeeze the winding core, thereby shaping the winding to Preset dimensions to fit into matching housing.

　　Figure 4 Schematic diagram of the hot pressing and shaping device for cylindrical battery cells

　　Generally, when the battery core is hot pressed and shaped, the insulation withstand voltage test of the battery core is performed at the same time to detect whether there is a micro short circuit inside the battery core. If there are tiny metal foreign matter particles inside the battery core, the battery core diaphragm will be pierced when the battery core is pressed down, and it is easy to detect micro-short-circuit defective products. Regarding the cell insulation withstand voltage test, I have compiled an article before: Basic knowledge of lithium 18650 li ion battery cell insulation withstand voltage test (click to read).

　　3. Key points of process

　　The main process parameters of hot pressing and shaping of battery cores include pressurization pressure, pressurization time and template temperature. Under appropriate process parameters, there is almost no air inside the thick battery core, the separator and the pole piece are tightly bonded together, and the loose battery core can become a hard block. However, for the ceramic diaphragms used in recent years, due to the presence of the ceramic layer, it is difficult for the diaphragm to bond with the pole pieces to form this state. In the process determination test, the test items include the air permeability and thickness change of the separator; whether the thickness of the battery core meets the requirements for shell insertion; whether the pole piece is broken, etc.

　　As the core component of the battery, the battery separator plays a key role in isolating electron conduction between the positive and negative electrodes and allowing lithium ions to pass back and forth between the two poles. The microporous structure on the separator is an important channel for these ions to travel to and from the positive and negative electrodes. , its air permeability will directly affect the performance of the battery. The air permeability of the separator refers to the amount of gas permeable by the separator under a certain time pressure. If the separator has poor air permeability, it will affect the transfer of lithium ions between the positive and negative electrodes, thereby affecting the charging and discharging of lithium batteries. The test process is as follows: fix the battery separator, apply air pressure on one side of the separator, measure the air pressure drop and the time used, and detect the air permeability of the separator. The shorter the time used, the better the air permeability. During the hot pressing process, the separator may be severely compressed, and the thickness of the separator changes greatly, causing the micropores to be blocked, and the separator will become transparent when viewed with the naked eye. This situation indicates that the hot pressing shaping effect exceeds the limit on the battery core and will affect lithium ions. transmission. The wound battery core is shown in Figure 2. If the pole piece is relatively brittle, powder will easily fall off or even break at the bend of the battery core during hot pressing and shaping, which will result in limited electron transmission and increase the internal resistance of the battery. Therefore, hot pressing and shaping of the battery core must also avoid this situation. These two aspects require that the hot press shaping pressure should be as small as possible and the time as short as possible. On the other hand, hot pressing shaping must shape the battery core, ensure the thickness of the battery core meets the process requirements, reduce the elasticity of the battery core, and ensure the consistency of the thickness of the finished battery core. Therefore, process parameters such as pressure, time and temperature need to be optimized.

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