A method for improving surface dishing of an electrochemical cell

Abstract

The application discloses a method for improving the surface depression of an electric core, belongs to the field of new energy batteries, and aims at improving the appearance depression of a square lithium ion electric core. The electric core to be capped and welded is subjected to laser capping and welding, air tightness detection, primary liquid injection, formation, secondary liquid injection and sealing nail welding in sequence, so that the surface of the electric core is flat, the electric core assembly is not affected, and the safety hidden danger of the electric core in a module is reduced.

Description

Technical Field

[0001] This invention belongs to the field of new energy batteries, and specifically relates to a method for improving surface depressions of battery cells. Background Technology

[0002] A lithium-ion battery cell is a rechargeable cell that can be repeatedly charged and discharged. It consists of main components such as anode and cathode electrodes, a separator, electrolyte, and mechanical parts. Throughout the entire lifecycle of a lithium-ion battery cell, particularly after it has been stored in its battery compartment, large-scale, localized dents are prone to occur. This phenomenon mainly causes the following impacts:

[0003] 1. Uneven appearance leads to reduced customer satisfaction;

[0004] 2. Uneven surface makes the coating prone to air bubbles, which increases the rework rate of the coating.

[0005] 3. Due to the large-area local depression, the mutual squeezing force between the cells is uneven during the assembly of the module. To be precise, the force is mainly concentrated at the welding point of the top cover. When the module is used later, it is easy to break the blue film under the influence of vibration and other working conditions, which will damage the insulation of the cell and thus cause safety risks to the module.

[0006] Research has found that the problem of large-area localized dents in hard-shell battery cells mainly occurs in cells with large surface areas and thin shells. Furthermore, because the current trend in hard-shell battery cell products is towards larger sizes and thinner shells, this problem becomes even more pronounced. Given the numerous adverse effects of large-area localized dents, this issue has become a critical problem that urgently needs to be addressed. Summary of the Invention

[0007] The technical problem to be solved by this invention:

[0008] Improvements have been made to address the issue of large-area localized depressions in square lithium-ion cells, resulting in a smooth cell surface that does not affect cell assembly and reduces safety hazards of the cells within the module.

[0009] To address the aforementioned technical problems, this invention provides targeted improvements based on the causes of large-area localized depressions. Firstly, the operating trajectory of the tooling for laser welding the top cover is adjusted. The current method, where the two sides are fixed and the other two sides are pushed and pressed together, is changed to first pressing the two sides of the large surface together, then pushing and pressing together using the side tooling. (See schematic diagram). Figure 1 and Figure 2 . Figure 1 This is a schematic diagram showing the movement trajectory of the laser welding fixture for the top cover before the change. Figure 2This is a schematic diagram showing the modified movement trajectory of the laser welding fixture for the top cover. The purpose of this modification is to ensure even stress distribution along the edges of the aluminum shell, preventing deformation after laser welding. Another cause of deformation is the excessively high temperature of the laser-welded portion of the top cover compared to the lower temperature of the non-welded portions, which exacerbates the deformation. To mitigate this, a heating element needs to be embedded in the middle of the laser welding fixture for the top cover, reducing the temperature difference between the main shell surface and the welding area of ​​the top cover. The temperature of the heating element is controlled between 80 and 110°C; too low a temperature is ineffective, while too high a temperature can negatively impact the battery cells.

[0010] Special clamps are used in all processes that require negative pressure extraction. Figure 3 This is a top view of the fixture used in the negative pressure process. Figure 4 This is a side view of the fixture used in the negative pressure process.

[0011] Processes requiring negative pressure removal: In the conventional production of hard-shell battery cells, processes requiring negative pressure removal include airtightness testing, vacuum baking, electrolyte injection, and formation. These processes create a large pressure difference between the inside and outside of the cell, easily causing the hard shell to dent inwards. Due to the large pressure difference and the thin wall of the hard shell, plastic deformation is likely to occur. Generally, hard shells are mainly made of aluminum, although some are made of steel; this patent also applies to these processes.

[0012] Fixtures for negative pressure processes: When the flow drawing reaches the process that requires negative pressure, a special tooling fixture is added to the outside of the battery cell. The fixture needs to have the following functions: (1) It can fit the battery cell perfectly and create uniform air holes on both sides of the large surface of the battery cell to adsorb the hard shell wall of the large surface; (2) The two sides of the large surface of the fixture need to be equipped with air pipes that connect to the negative pressure and can be connected to the air pipes that draw negative pressure. This ensures that negative pressure is drawn inside the battery cell while negative pressure is drawn on the outer surface of the large surface, so as to achieve a balance of internal and external air pressure and prevent deformation.

[0013] A method for improving surface depressions in battery cells includes the following specific steps:

[0014] S1. Laser top cover welding process: First, push the assembled battery cell to be welded into the top cover laser welding process. Then, movable fixture 1 and movable fixture 2 clamp the battery cell simultaneously. Next, movable fixture 3 and movable fixture 4 continue to clamp the battery cell. Movable fixture 1 and movable fixture 2 advance and clamp synchronously. Movable fixture 3 and movable fixture 4 also advance and clamp synchronously. The start time of movable fixture 1 and movable fixture 2 differs from the start time of movable fixture 3 and movable fixture 4 by 0.5 to 2 seconds. Before clamping the battery cell, the heating and constant temperature functions of heating tube 1 and heating tube 2 are turned on in advance, and the temperatures of heating tube 1 and heating tube 2 are set. After the four fixtures are clamped, laser top cover welding is performed. After welding is completed, the four fixtures are withdrawn synchronously, and the battery cell with the completed top cover welding is slowly pushed out of this station and flows to the next station.

[0015] S2. Perform airtightness testing: The battery cell with the top cover welded is first clamped by a robot into a pre-made battery cell fixture with negative pressure air holes. Then, the battery cell with the fixture is pushed into the airtightness testing station for airtightness testing. The negative pressure extraction inside the battery cell and the negative pressure extraction on the large surface of the fixture are started simultaneously, and the negative pressure time is consistent. In order to ensure that the negative pressure value inside the battery cell and the large surface of the battery cell are consistent, the negative pressure air pipe on the fixture is connected to the negative pressure air pipe inside the battery cell and is controlled by the same switch.

[0016] S3. Perform the first injection, formation, and second injection processes in sequence: all of which are based on the original process, with the addition of a special fixture with negative pressure vents, while the other process flows remain unchanged.

[0017] S4. Finally, the sealing nail welding process is carried out to completely seal the battery cell and finally obtain a square lithium-ion battery cell with a flat surface.

[0018] The beneficial effects obtained by this invention are as follows:

[0019] (1) The special fixture is simple to make, does not require special technical skills, and has low production costs;

[0020] (2) The problem of the concave appearance of square lithium-ion cells has been significantly improved, and the surface of the cells is smoother after the process reform.

[0021] (3) The current shell space is larger than before, and the injection speed is significantly improved. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the movement trajectory of the laser welding fixture for the top cover before the change.

[0023] Figure 2 This is a schematic diagram showing the changed movement trajectory of the laser welding fixture for the top cover.

[0024] Among them, 1. Movable fixture one; 2. Movable fixture two; 3. Movable fixture three; 4. Movable fixture four; 5. Heating tube one; 6. Heating tube two; 7. Battery cell to be welded to the top cover.

[0025] Figure 3 This is a top view of the fixture used in the negative pressure process.

[0026] Figure 4 This is a side view of the fixture used in the negative pressure process. Detailed Implementation

[0027] The following description of the embodiments will provide a more detailed explanation of the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0028] Control group 1: 100Ah lithium iron phosphate aluminum-cased cells (before process improvement)

[0029] A 100Ah lithium iron phosphate aluminum-cased cell was used as the experimental sample cell. Thirty-two assembled cells, awaiting top cover welding, were selected for the experiment. This was repeated for three sets.

[0030] Process flow:

[0031] Figure 1 This is a schematic diagram of the movement trajectory of the laser welding fixture for the top cover before the change. The battery cell to be welded is pushed into the laser welding process. The cell is first pushed close to the fixed fixture ①, and then the movable fixtures ② and ③ are simultaneously activated to continue clamping the cell. Laser welding of the top cover is performed after clamping. After welding, the movable fixtures ② and ③ simultaneously retract, and the cell is slowly pushed out of the station and flows to the next station. The next process is airtightness testing. The cell is first clamped into a standard limiting fixture by a robotic arm. Then, the cell, with the fixture in place, is pushed into the airtightness testing station for testing. The next processes are primary liquid injection, formation, and secondary liquid injection, all performed by first using a standard limiting fixture before liquid injection and formation. During the experiment, to confirm the effect of each process, a coordinate measuring machine was used to measure the flatness of the large surface after each process, and the time consumed at each station was recorded. The final average data is shown in Table 1. Finally, the sealing nails are welded, and the battery cell is completely sealed. Subsequent processes will not significantly alter the flatness of the battery cell. Data shows that the average flatness of the battery cell is 1.56mm, while the total time for the first / secondary electrolyte injection is 32.8 seconds.

[0032] Example 1: 100Ah lithium iron phosphate aluminum-cased cell (after process improvement)

[0033] A 100Ah lithium iron phosphate aluminum-cased cell was used as the experimental sample cell. Thirty-two assembled cells, awaiting top cover welding, were selected for the experiment. This was repeated for three sets.

[0034] Specific process flow:

[0035] Figure 2 This is a schematic diagram showing the changed movement trajectory of the laser welding fixture for the top cover. The battery cell 7 to be welded to the top cover is pushed into the laser welding process. Then, movable fixture 1 and movable fixture 2 simultaneously clamp the battery cell. Next, movable fixture 3 and movable fixture 4 are activated to continue clamping the battery cell. It is important to emphasize that movable fixture 1 and movable fixture 2 need to advance and clamp synchronously, as do movable fixture 3 and movable fixture 4. The activation time of movable fixture 1 and movable fixture 2 differs from that of movable fixture 3 and movable fixture 4 by 0.5–2 seconds. The heating and temperature control functions of heating tube 1 and heating tube 2 are activated before clamping the battery cell to reduce wasted time during heating. The heating tube temperature is set to 85℃. After clamping, laser welding of the top cover is performed. After welding, all four clamps retract synchronously, and the battery cell is slowly pushed out of the station and flows to the next station. The next process is airtightness testing. The battery cell is first clamped by a robotic arm into a pre-made battery cell clamp with negative pressure vents. Then, the battery cell with the clamp is pushed into the airtightness testing station for airtightness testing. It is important to note that the negative pressure extraction inside the battery cell and the negative pressure extraction on the clamp surface are started simultaneously, and the negative pressure time is consistent. To ensure that the negative pressure values ​​inside the battery cell and on the battery surface are consistent, the negative pressure vent on the clamp must be connected to the negative pressure vent inside the battery cell and controlled by the same switch. Other processes remain unchanged. The next process is primary liquid injection, formation, and secondary liquid injection, all of which are based on the original process with the addition of specially made clamps with negative pressure vents. Other processes remain unchanged. During the experiment, to confirm the effect of each process, a coordinate measuring machine is used to measure the flatness of the large surface after each process, and the time consumed at each station is recorded. The final average data is shown in Table 1. Finally, the sealing nails are welded, and the battery cell is completely sealed. Subsequent processes will not significantly change the flatness of the battery cell. The data shows that the average flatness of the battery cell is 0.1mm, which is a significant improvement compared to control group 1; and the total time for the first / secondary electrolyte injection is 24.8s, which is 24.4% less time than control group 1.

[0036] Control group 2: 280Ah lithium iron phosphate aluminum-cased cells (before process improvement)

[0037] Using 280Ah lithium iron phosphate aluminum-cased cells as experimental sample cells, 32 assembled cells awaiting top cover welding were selected for the experiment. This was repeated for 3 sets.

[0038] Process flow:

[0039] Figure 1 This is a schematic diagram of the movement trajectory of the laser welding fixture for the top cover before the change. The battery cell to be welded is pushed into the laser welding process. The cell is first pushed close to the fixed fixture ①, and then the movable fixtures ② and ③ are simultaneously activated to continue clamping the cell. Laser welding of the top cover is performed after clamping. After welding, the movable fixtures ② and ③ simultaneously retract, and the cell is slowly pushed out of the station and flows to the next station. The next process is airtightness testing. The cell is first clamped into a standard limiting fixture by a robotic arm. Then, the cell, with the fixture in place, is pushed into the airtightness testing station for testing. The next processes are primary liquid injection, formation, and secondary liquid injection, all performed by first using a standard limiting fixture before liquid injection and formation. During the experiment, to confirm the effect of each process, a coordinate measuring machine was used to measure the flatness of the large surface after each process, and the time consumed at each station was recorded. The final average data is shown in Table 1. Finally, the sealing nails are welded, and the battery cell is completely sealed. Subsequent processes will not significantly alter the flatness of the battery cell. Data shows that the average flatness of the battery cell is 1.48mm, while the total time for the first / secondary electrolyte injection is 43.4 seconds.

[0040] Example 2: 280Ah lithium iron phosphate aluminum-cased battery cell (after process improvement)

[0041] Using 280Ah lithium iron phosphate aluminum-cased cells as experimental sample cells, 32 assembled cells awaiting top cover welding were selected for the experiment. This was repeated for 3 sets.

[0042] Process flow:

[0043] Figure 2This is a schematic diagram showing the changed movement trajectory of the laser welding fixture for the top cover. The battery cell 7 to be welded to the top cover is pushed into the laser welding process. Then, movable fixture 1 and movable fixture 2 simultaneously clamp the battery cell. Next, movable fixture 3 and movable fixture 4 are activated to continue clamping the battery cell. It is important to emphasize that movable fixture 1 and movable fixture 2 need to advance and clamp synchronously, as do movable fixture 3 and movable fixture 4. The activation time of movable fixture 1 and movable fixture 2 differs from that of movable fixture 3 and movable fixture 4 by 0.5–2 seconds. The heating and temperature control functions of heating tube 1 and heating tube 2 are activated before clamping the battery cell to reduce wasted time during heating. The temperature of the heating tubes is set to 107℃. After clamping, laser welding of the top cover is performed. After welding, all four clamps retract synchronously, and the battery cell is slowly pushed out of the station and flows to the next station. The next process is airtightness testing. The battery cell is first clamped by a robotic arm into a pre-made battery cell clamp with negative pressure vents. Then, the battery cell with the clamp is pushed into the airtightness testing station for airtightness testing. It is important to note that the negative pressure extraction inside the battery cell and the negative pressure extraction on the large surface of the clamp are started simultaneously, and the negative pressure time is consistent. To ensure that the negative pressure value inside the battery cell and on the large surface of the battery cell is consistent, the negative pressure air pipe on the clamp must be connected to the negative pressure air pipe inside the battery cell and controlled by the same switch. Other processes remain unchanged. The next process is primary liquid injection, formation, and secondary liquid injection, all of which are based on the original process with the addition of specially made clamps with negative pressure vents. Other processes remain unchanged. During the experiment, to confirm the effect of each process, a coordinate measuring machine is used to measure the flatness of the large surface after each process, and the time consumed at each station is recorded. The final average data is shown in Table 1. Finally, the sealing nails are welded, and the battery cell is completely sealed. Subsequent processes will not significantly change the flatness of the battery cell. The data shows that the average flatness of the battery cell is 0.06mm, which is a significant improvement compared to control group 2; and the total time for the first / secondary electrolyte injection is 35.3s, which is 18.7% less than control group 2.

[0044] Table 1

[0045]

[0046] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention. Technologies not covered in this invention can be implemented using existing technologies.

Claims

1. A method for improving surface depressions in battery cells, characterized in that, Includes the following steps: S1. Perform laser top cover welding process: First, push the assembled battery cell (7) to be welded into the top cover laser welding process. Then, movable fixture one (1) and movable fixture two (2) simultaneously clamp the battery cell. Then, start movable fixture three (3) and movable fixture four (4) to continue clamping the battery cell. Movable fixture one (1) and movable fixture two (2) advance and clamp synchronously. Movable fixture three (3) and movable fixture four (4) also advance and clamp synchronously. The start time of fixture 1 (1) and movable fixture 2 (2) differs from the start time of fixture 3 (3) and movable fixture 4 (4) by 0.5 to 2 seconds. Before clamping the battery cell, the heating constant temperature function of heating tube 1 (5) and heating tube 2 (6) is turned on in advance, and the temperature of heating tube 1 (5) and heating tube 2 (6) is set. After the four fixtures are clamped, laser top cover welding is performed. After the welding is completed, the four fixtures are withdrawn synchronously, and the battery cell with the top cover welded is slowly pushed out of the station and flows to the next station. S2. Perform airtightness testing: The battery cell with the top cover welded is first clamped by a robot into a pre-made battery cell fixture with negative pressure air holes. Then, the battery cell with the fixture is pushed into the airtightness testing station for airtightness testing. The negative pressure extraction inside the battery cell and the negative pressure extraction on the large surface of the fixture are started simultaneously, and the negative pressure time is consistent. In order to ensure that the negative pressure value inside the battery cell and the large surface of the battery cell are consistent, the negative pressure air pipe on the fixture is connected to the negative pressure air pipe inside the battery cell and is controlled by the same switch. S3. Perform the first injection, formation, and second injection processes in sequence: all of which are based on the original process, with the addition of a special fixture with negative pressure vents, while the other process flows remain unchanged. S4. Finally, the sealing nail welding process is carried out to completely seal the battery cell and finally obtain a square lithium-ion battery cell with a flat surface.

2. The method for improving surface depressions of a battery cell according to claim 1, characterized in that, The temperature of heating tube one (5) and heating tube two (6) is controlled between 80 and 110°C.

3. The method for improving surface depressions of a battery cell according to claim 1, characterized in that, The hard shell of the battery cell (7) to be welded to the top cover is made of aluminum or steel.

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