A secondary formation method for reducing gas generation in soft packages

Abstract

The present invention relates to the field of battery preparation. Aiming at the problem of insufficient exhaust of soft-pack batteries, a secondary formation method for reducing soft-pack gas generation is provided, including the following steps: cell encapsulation, battery liquid injection, sealing, primary formation, primary air extraction and sealing, secondary formation, and secondary air extraction and sealing. The cut-off SOC of the primary formation is 30-40%, and the cut-off SOC of the secondary formation is 90-100%. By controlling the cut-off SOC of the primary formation, increasing the secondary formation, and secondary air extraction, the exhaust of the soft-pack battery is made more thorough.

Description

Technical Field

[0001] The present invention relates to the field of battery preparation, and in particular to a secondary formation method for reducing gas generation in soft-pack batteries. Background Art

[0002] Soft-pack lithium-ion batteries have great advantages in terms of high energy density or high endurance mileage. However, soft-pack batteries are very afraid of gas generation. If the moisture control is not in place during the battery manufacturing process, moisture is likely to form HF gas with the electrolyte, resulting in the phenomenon of the battery bulging or swelling. Or after the battery is normally formed and then evacuated and sealed, if the gas is not fully evacuated during the evacuation, and there is gas remaining in the battery body, it will cause the battery to become soft and swollen. Once the soft-pack battery generates gas, there will be adverse phenomena such as bulging, deterioration of cycling, and even a sharp decline in life. Therefore, solving the gas generation problem of soft-pack batteries has become a very critical control point.

[0003] The classic solution is to add additives that reduce gas generation to the electrolyte to inhibit gas generation during the use of the battery, such as patent CN112467214B. There is also a method of premixing a gas adsorbent with the slurry, and after the battery pre-charges and generates gas, adsorbing the generated gas to achieve the effect of controlling the gas generation of the battery, such as patent CN113054190A. However, adding additives or gas adsorbents may interact with other components in the battery, having a negative impact on the battery performance, and it is not environmentally friendly. Therefore, an ideal solution is needed. Summary of the Invention

[0004] In order to overcome the problem of insufficient exhaust of soft-pack batteries, the present invention provides a secondary formation method for reducing gas generation in soft-pack batteries, which makes the soft-pack battery exhaust more thoroughly by controlling the cut-off SOC of the first formation and increasing the secondary evacuation during the secondary formation.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A secondary formation method for reducing gas generation in soft-pack batteries, comprising the following steps: cell encapsulation, battery liquid injection, sealing, first formation, first evacuation and sealing, second formation, second evacuation and sealing, wherein the cut-off SOC of the first formation is 30-40%, and the cut-off SOC of the second formation is 90-100%.

[0007] The process flow of a conventional soft-pack lithium-ion battery is as follows: positive and negative electrode slurry mixing, positive and negative electrode coating, positive and negative electrode rolling, stacking or winding, encapsulation, liquid injection, sealing, formation, air extraction and sealing. While the process flow of the soft-pack lithium-ion battery of the present invention is: positive and negative electrode slurry mixing, positive and negative electrode coating, positive and negative electrode rolling, stacking or winding, encapsulation, liquid injection, sealing, primary formation, primary air extraction and sealing, secondary formation, secondary air extraction and sealing, N times of cyclic formation and air extraction. Generally, the number of times of formation and air extraction and sealing is 2 times. When the cut-off SOC of the primary formation is ≥30%, sufficient gas can be generated to avoid a large amount of gas generation again during the secondary formation; the secondary formation makes the battery fully charged and generates gas for the second time. After the cut-off SOC exceeds 90%, the battery basically stops generating gas.

[0008] Preferably, the width of the reserved airbag is 0.95-1.05 times the width of the battery cell body. More preferably, the width of the reserved airbag is the same as the width of the battery cell body. The width of the reserved airbag in the traditional battery cell encapsulation structure is 2 / 3 of the width of the battery cell body. The present invention expands the width of the reserved airbag, and the other parts of the battery remain unchanged. The volume of the airbag increases, providing sufficient gas storage space for the battery with abnormal gas generation. Different from the conventional process, when the gas generated by the battery exceeds the volume of the airbag, the gas cannot be discharged from the battery body and remains inside the battery electrodes, affecting the cycle life of the battery.

[0009] Preferably, the sealing is carried out by using a copper sealing head with a width of 3-8 mm to seal the edge.

[0010] Preferably, the specific operation of the primary formation is as follows: clamp the soft-pack battery with a fixture, and the charging rate in the charging process is 0.1-0.15C (C represents the capacity of a single battery cell. For example, for a 10AH battery, 0.1C is 1A). The purpose of using a current of 0.1-0.15C is that the battery has a relatively small polarization internal resistance and a relatively sufficient reaction under small-current charging. When the cut-off SOC ≥30%, sufficient gas can be generated to avoid a large amount of gas generation again during the secondary formation.

[0011] Preferably, the specific operation of the primary air extraction and sealing is as follows: place the battery in a negative pressure environment with a vacuum degree ≤ -90 kPa, then puncture the airbag and extract the gas generated during the primary formation. The air extraction time is 15-30 s.

[0012] Preferably, after the primary air extraction is completed, seal at 9 / 20-11 / 20 of the width of the airbag. The sealing is carried out by using a steel sealing head with a width of 3-8 mm. More preferably, the sealing is carried out at the midpoint, i.e., 1 / 2 of the width of the airbag.

[0013] Preferably, the charging current during the secondary formation is 0.3-0.7C. Since the preliminary pre-charging has been carried out for the first time, the charging current is increased during the secondary formation. This step is mainly to make the battery fully charged and generate gas for the second time.

[0014] Preferably, the width of the head of the secondary air extraction and sealing is 5-10 mm.

[0015] Preferably, after the air extraction and sealing of the battery, the airbag is cut to obtain the finished battery.

[0016] Therefore, the beneficial effects of the present invention are as follows: (1) controlling the charging rate of the first formation, the cut-off SOC, and the charging rate of the second formation to make the exhaust of the soft-pack battery more thorough; (2) increasing the volume of the airbag to provide sufficient gas storage space for the battery with abnormal gas production, unlike the conventional process, when the gas produced by the battery exceeds the volume of the airbag, the gas cannot be discharged from the battery body and remains inside the electrode sheet of the battery, affecting the cycle of the battery. Description of the Drawings

[0017] Figure 1 is the traditional cell packaging structure;

[0018] Figure 2 is the cell packaging structure of the present invention;

[0019] Figure 3 is the cell packaging structure after the first air extraction and sealing of the present invention;

[0020] Figure 4 is the cell packaging structure after the second air extraction and sealing of the present invention;

[0021] Figure 5 is the graph of the 60°C cycle capacity attenuation trend of the cells in each example and comparative example;

[0022] Figure 6 is the graph of the gas production test results of the batteries at 60°C for 120 days in each example and comparative example.

[0023] In the figures, 1 is the cell, 2 is the tab, 3 is the airbag, 4 is the first head, and 5 is the second head. Detailed Embodiments

[0024] The technical solutions of the present invention will be further described below through specific examples.

[0025] In the present invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the art. The methods in the examples are conventional methods in the art unless otherwise specified.

[0026] General Example

[0027] A secondary formation method for reducing the gas generation of soft packs. The process flow of soft pack lithium-ion batteries is as follows: positive and negative electrode slurry stirring, positive and negative electrode coating, positive and negative electrode rolling, stacking or winding, encapsulation, liquid injection, sealing, primary formation, primary gas extraction and sealing, secondary formation, secondary gas extraction and sealing, and N times of cyclic formation and gas extraction to obtain the finished battery. Generally, the number of times of formation and gas extraction and sealing is 2 times.

[0028] The cell structure is as Figure 2 shown. Compared with Figure 1 the traditional cell encapsulation structure, in the present invention, the width of the reserved airbag bag 3 is expanded to 0.95 - 1.05 times the width of the cell 1, and the other parts of the battery remain unchanged. The width of the cell 1 refers to the length of the side where the tab 2 is located.

[0029] After encapsulating the cell 1, the battery is filled with liquid, and a copper sealing head with a width of 3 - 8 mm is used to seal the edge.

[0030] During the primary formation, the soft pack battery is clamped by a fixture and charged according to the set charging process. The charging rate in the charging process is 0.1 - 0.15 C, and the cut-off SOC is 30 - 40%. After the primary formation, the battery is subjected to the first gas extraction and sealing. The battery is placed in a negative pressure environment with a vacuum degree ≤ -90 kPa, and then several bayonets composed of steel nails are used to pierce the airbag bag 3 to extract the gas generated during the primary formation. The gas extraction time is 15 - 30 s. After the gas extraction is completed, as Figure 3 shown, sealing is performed at 9 / 20 - 11 / 20 of the width of the airbag bag. The sealing also uses a steel sealing head, and the width of the primary sealing head 4 is 3 - 8 mm.

[0031] The battery after the first gas extraction and sealing is subjected to secondary formation. Since the first preliminary pre-charging has been carried out, the charging current is increased to 0.3 - 0.7 C during the secondary formation, and the cut-off SOC is 90 - 100%. The secondary formation is mainly to make the battery fully charged and generate gas for the second time. As Figure 4 shown, the battery after the secondary formation is subjected to the second gas extraction and sealing, and the process is the same as the first gas extraction and sealing. The secondary sealing head 5 uses a copper sealing head with a width of 5 - 10 mm.

[0032] After cutting the airbag bag of the battery after the second gas extraction and sealing, the battery manufacturing is completed to obtain the finished battery.

[0033] Example 1

[0034] A secondary formation method for reducing the gas generation of soft packs. The designed capacity of the battery cell is 10 Ah, the positive electrode is lithium iron phosphate, the negative electrode is graphite, a PP separator with a thickness of 14 μm is used, and it is encapsulated through an aluminum-plastic film (the single-layer thickness of the aluminum-plastic film is 0.15 mm, and the total thickness on both sides is 0.3 mm, and this value is the same hereinafter). The width of the reserved airbag is the same as the width of the battery cell body, and after baking at 95 °C for 12 h and injecting the electrolyte, it is sealed with a 5-mm head. After sealing, the battery cell is placed in an environment at a high temperature of 45 ± 3 °C for 24 h. After the placement is completed, a primary formation is carried out. The formation conditions are charging to 40% SOC at 45 °C and 0.3 MPa, and the bulging thickness H1 of the battery airbag is recorded at this time. Then, after the first formation is completed, it is evacuated for 15 s in a vacuum environment of -90 kPa, and the gas generation amount is recorded by the water displacement method. This value is recorded as the gas generation amount 1. Then, the airbag is sealed at the 1 / 2 position with a head with a sealing width of 5 mm. The battery cell after the first sealing is formed again. The formation conditions are charging to 100% SOC at 0.5 C, and the bulging thickness H2 of the battery airbag is recorded at this time. The evacuation and sealing are carried out under the same conditions as the first evacuation and sealing, and the gas generation amount is also recorded by the water displacement method. This value is recorded as the gas generation amount 2. After sealing, the preparation of the battery cell is completed.

[0035] Example 2

[0036] A secondary formation method for reducing the gas generation of soft packs, which is different from Example 1 in that the primary formation is charged to 35% SOC.

[0037] Example 3

[0038] A secondary formation method for reducing the gas generation of soft packs, which is different from Example 1 in that the primary formation is charged to 30% SOC.

[0039] Example 4

[0040] A secondary formation method for reducing the gas generation of soft packs, which is different from Example 3 in that the width of the reserved airbag is 0.95 times the width of the battery cell body.

[0041] Example 5

[0042] A secondary formation method for reducing the gas generation of soft packs, which is different from Example 3 in that the secondary formation is charged to 90% SOC

[0043] Comparative Example 1

[0044] The designed cell capacity is 10 Ah, with lithium iron phosphate used for the positive electrode and graphite used for the negative electrode. A PP separator with a thickness of 14 μm is adopted, and it is encapsulated with an aluminum-plastic film. The width of the reserved airbag is 2 / 3 of the width of the cell body. After baking at 95 °C for 12 h, injecting the electrolyte, it is sealed with a 5-mm seal head. After sealing, the cell is left standing in an environment at a high temperature of 45 ± 3 °C for 24 h. After the standing is completed, a primary formation is carried out. The formation conditions are in an environment of 45 °C and 0.3 MPa, charging at 0.1C until 40% SOC and then switching to 0.5C to charge to 100% SOC, and record the swelling thickness H1 of the battery airbag at this time. Then, after the formation is completed, it is evacuated in a vacuum environment of -90 kPa for 15 s, and the gas production is recorded by the water displacement method for the extracted gas, and this value is recorded as the gas production 1. After sealing, the cell preparation is completed.

[0045] Comparative Example 2

[0046] The difference from Example 1 is that the cut-off SOC for the primary formation is 20%.

[0047] Comparative Example 3

[0048] The difference from Example 1 is that the cut-off SOC for the primary formation is 50%.

[0049] Comparative Example 4

[0050] The difference from Example 1 is that the width of the reserved airbag is 2 / 3 of the width of the cell body.

[0051] Comparative Example 5

[0052] The difference from Example 1 is that the width of the reserved airbag is 4 / 3 of the width of the cell body.

[0053] Perform performance tests on the finished batteries prepared in the above examples and comparative examples. The test method is as follows:

[0054] Performance Test 1 (Normal Temperature Cycling Test)

[0055] ① Standing: 1800 s;

[0056] ② Constant current discharge: Set the discharge current to 1C, and the discharge cut-off voltage to 2.5V;

[0057] ③ Standing: Set the time to 600 s;

[0058] ④ Constant current and constant voltage charging: Set the charging current to 1C, the charging limit voltage to 3.65V, and the cut-off current to 0.05C;

[0059] ⑤ Standing: Set the time to 600 s;

[0060] ⑥ Constant current discharge: Set the discharge current to 1C, and the discharge termination voltage to 2.5V;

[0061] ⑦ Repeat steps ③ - ⑥. When the discharge capacity of the battery reaches 80% of the initial capacity, stop the test and record the number of cycles; record the discharge capacity of the fifth cycle as the initial capacity.

[0062] Performance Test 2 (Gas Generation Test during Storage at 60°C)

[0063] For the prepared battery cells, record the initial volume of the battery cells using the water displacement method, that is, immerse the battery in a container filled with water, introduce the volume of water discharged after immersing the battery cells into a graduated cylinder, and record the volume of the discharged water as V1.

[0064] Place the battery in an environment at 60°C for 120 days of storage.

[0065] After 120 days of battery storage, take out the battery and test the battery volume again using the water displacement method. At this time, record the volume of the discharged water as V2; calculate the gas generation rate as V = (V2 - V1) / V1. If the gas generation rate ≤ 5%, it is judged as qualified.

[0066] The results are as shown in the following table and Figure 5 、 6 as follows.

[0067] Thickness H1 / mm Gas production 1 / ml Thickness H2 / mm Gas production 2 / ml Test 1 / times Test 2 / % Example 1 0.45 400 0.31 150 2950 0.5 Example 2 0.43 390 0.32 156 2730 0.55 Example 3 0.44 385 0.31 158 2780 0.53 Example 4 0.48 397 0.31 148 2670 0.52 Example 5 0.44 403 0.32 145 2680 0.54 Comparative Example 1 2.5 470 / / 2300 4.6 Comparative Example 2 0.35 260 0.44 295 2260 3.5 Comparative Example 3 0.44 440 0.31 100 3000 0.3 Comparative Example 4 2.1 405 1.2 150 2300 4.3 Comparative Example 5 0.45 398 0.31 145 2700 0.5

[0068] As can be seen from the above table, the soft-pack batteries prepared in each embodiment of the present invention have sufficient exhaust, and no bad phenomena such as bulging and a sharp decline in cycle life due to cycle deterioration will occur.

[0069] For Comparative Example 1, only one formation was carried out. Using the battery cells prepared by the present invention, from the test 1 data, the number of cycles was increased by 650 times, and from the test 2 data, it can be obtained that there is no gas generation or swelling phenomenon in the battery cells after storage at 60°C, indicating that by exhausting the gas inside the battery at the initial stage of formation and avoiding the gas remaining inside the battery cells, the service life of the battery can be improved.

[0070] Compared with Example 1, the cut-off SOC for one-time formation in Comparative Example 2 is 20%, which is lower than the preferred range of 30 - 40%. It can be seen from the test data that a large amount of gas was still generated during the second-stage formation in Comparative Example 2, indicating that when the SOC is lower than 30%, the formation of the battery cells is not sufficient, the negative electrode film formation is poor, and the cycle and storage performance of the battery cells are also deteriorated.

[0071] Compared with Example 1, the cut-off SOC of the first formation in Comparative Example 3 is 50%, which is higher than the preferred range of 30-40%. It can be seen from the data of the first formation that when the cut-off is at 40% SOC and 50% SOC, the difference in the gas production volume in the first time is relatively small, indicating that the battery cell has completed the stage of a large amount of gas production. Extending the cut-off SOC further will increase the production cost and affect the production efficiency. After 40% SOC, the battery can accept high-rate charging and quickly reach the fully charged state.

[0072] Compared with Example 1, the width of the battery cell packaging structure in Comparative Example 4 is 2 / 3 of the width of the battery cell body, which is lower than the preferred range of 0.95-1.05. It can be seen from the formation state of the battery that the failure of the airbag bag in the second stage of the battery is relatively obvious, indicating that a large amount of gas is produced in the first stage of the battery. Although the secondary formation method is adopted, the gas generated in the first stage is not discharged to the airbag bag in time and accumulates inside the battery cell body. The residual gas during the secondary formation affects the layer spacing between the electrode plates, resulting in black spots or lithium deposition on the negative electrode. In the later tests, the cycling and storage do not meet the requirements.

[0073] Compared with Example 1, the width of the battery cell packaging structure in Comparative Example 5 is 4 / 3 of the width of the battery cell body, which is higher than the preferred range of 0.95-1.05. It can be seen from the formation state of the battery that the phenomena during the first / second formation of the battery cell are basically the same as those in the example, and the test data are also the same. Excessively increasing the width of the battery cell packaging structure, although it increases the space for storing gas and is beneficial to the discharge of the gas generated inside the battery cell, when the width of the battery cell packaging structure is kept consistent with the width of the battery cell body, this effect can already be achieved, and increasing the additional gas storage space will increase the production cost of the battery cell.

[0074] It can be seen that only the best results can be obtained within the preferred range of the present invention. The parameters affect each other, and any modification of any parameter of the present invention will lead to a decline in performance.

[0075] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A secondary formation method for reducing the gas generation of soft packages, characterized in that, Including the steps: cell encapsulation → battery injection → sealing → first formation → first evacuation and sealing → second formation → second evacuation and sealing → finished battery; the reserved airbag width is 0.95 - 1.05 times the width of the cell body; the first formation charging rate is 0.1 - 0.15C, and the cut-off SOC is 30 - 35%; after the first evacuation is completed, seal at 9 / 20 - 11 / 20 of the airbag width; the second formation charging current is 0.3 - 0.7C, and the cut-off SOC is 90 - 100%.

2. A secondary formation method for reducing gas generation in soft packages according to claim 1, characterized in that The sealing is carried out by using a copper head with a width of 3 - 8 mm to seal the edge.

3. A secondary formation method for reducing gas generation in soft packages according to claim 1, characterized in that, The specific operation of the first formation is: clamping the soft-pack battery with a fixture.

4. A secondary formation method for reducing gas generation in soft packages according to claim 1, characterized in that The specific operation of the first evacuation and sealing is: placing the battery in a negative pressure environment with a vacuum degree ≤ -90 kPa, and then puncturing the airbag to extract the gas generated during the first formation.

5. A secondary formation method for reducing gas generation in soft packages according to claim 4, characterized in that, The evacuation time is 15 - 30 s.

6. A secondary formation method for reducing gas generation in soft packages, according to claim 1 or 4 or 5, characterized in that The first evacuation and sealing uses a steel head with a width of 3 - 8 mm.

7. A secondary formation method for reducing gas generation in soft packages according to claim 1, characterized in that, The width of the head for the second evacuation and sealing is 5 - 10 mm.

8. A secondary formation method for reducing gas generation in soft packages according to claim 1, characterized in that, Cut the airbag of the battery after the second evacuation and sealing to obtain the finished battery.

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