Method for evaluating cohesion of lithium ion battery negative electrode sheet

Abstract

The present application relates to a kind of evaluation methods of the cohesion of lithium ion battery negative electrode sheet, comprising the following steps: (1) the preparation of negative electrode sheet: after roller pressing, negative electrode sheet is cut into certain size, and weighed;(2) the preparation of separator: take single-side rubber-coated separator, cut 6 same size;(3) the preparation of sample: negative electrode sheet is in the middle, and separator upper and lower layers cover negative electrode sheet;(4) three times hot pressing: after three times hot pressing, the mass ratio of negative electrode sheet reduction is calculated;(5) evaluation analysis: according to the size relationship of the mass ratio of different negative electrode sheet reduction, to evaluate their cohesion size relationship.The present application can greatly reduce the test volatility caused by human factors, improve the stability and accuracy of test, and the method is simple and easy to operate, and does not increase additional cost.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode technology, specifically relating to a method for evaluating the cohesive force of a lithium-ion battery negative electrode. Background Technology

[0002] During the charging and discharging process of a lithium-ion battery, the negative electrode expands, resulting in significant volume changes. This can cause the negative electrode to detach, making the cohesive force of the negative electrode very important.

[0003] The existing method for testing the cohesive force of negative electrode sheets typically involves applying adhesive tape to the electrode surface, pressing the tape down, and then using a tensile testing machine to pull the tape apart. The magnitude of the cohesive force is then assessed by measuring the tensile strength. This method involves manually applying and pressing the tape, resulting in significant fluctuations in the test results and failing to accurately reflect the cohesive force of the electrode sheet.

[0004] Chinese patent CN109994708A discloses a method for testing the cohesive force of an electrode coating. Specifically, one side of the negative electrode is adhered to a stainless steel plate with double-sided tape, and the other side is adhered with green adhesive. A tensile testing machine is used to peel the green adhesive off the surface of the negative electrode at a 180-degree angle. The surface of the green adhesive is then observed. If a complete negative electrode film remains, the force measured at this point is considered the cohesive force of the negative electrode. However, this testing method requires that the adhesive strength of the green adhesive be higher than the actual cohesive force of the electrode but lower than the adhesion force between the electrode and the current collector. This is difficult to achieve in practice. Often, the part after the green adhesive adheres is a discontinuous film with island-like distribution, which does not reach the level of the actual cohesive force. Moreover, this testing method is particularly inaccurate for electrodes with high cohesive force.

[0005] Chinese patent CN105203451A discloses a method for characterizing the cohesive force of a coating. Specifically, it involves attaching adhesive tape to the coated side of a test piece with a single-sided coating, leaving one end of the tape unattached to the test piece. The test piece is then fixed, and the tape is peeled off the coating surface using the unattached end. The peeling force after peeling is measured, and this peeling force characterizes the cohesive force of the coating. This method uses masking tape to test the cohesive force, requiring the tape's adhesiveness to be higher than the actual cohesive force of the electrode but lower than the adhesion force between the electrode and the current collector. This is difficult to achieve in practice. Furthermore, this patent determines the cohesive force by the proportion of the residual weight of the coating on the current collector to the total weight of the coating after tape adhesion. However, the portion after tape adhesion is a discontinuous membrane with an island-like distribution, which can lead to significant testing errors.

[0006] Therefore, it is necessary to develop an evaluation method for the cohesive force of lithium-ion battery anode sheets to reduce human interference, reduce test fluctuations, and improve test stability and accuracy. Summary of the Invention

[0007] The purpose of this invention is to provide a method for evaluating the cohesive force of lithium-ion battery negative electrode sheets, which can improve the stability and accuracy of the test, is simple and easy to operate, and does not increase the cost.

[0008] The technical solution adopted by this invention to solve the above problems is: a method for evaluating the cohesive force of a lithium-ion battery negative electrode sheet, comprising the following steps:

[0009] (1) Preparation of negative electrode sheet: Take the negative electrode sheet after rolling, cut it into a certain size, weigh it, and record the weight m0;

[0010] (2) Preparation of the diaphragm: Take a single-sided coated diaphragm and cut 6 pieces of the same size, ensuring that the size of the diaphragm is larger than the size of the negative electrode sheet;

[0011] (3) Sample preparation: Place the negative electrode in the middle, and place the two separators on the upper and lower layers of the negative electrode respectively, ensuring that the negative electrode is completely covered by the separators, and ensuring that the side of the separator coated with adhesive faces the negative electrode.

[0012] (4) First hot pressing: Place the sample from step (3) into the hot pressing equipment, set the temperature, pressure and time, and after the hot pressing is completed, tear open the diaphragm and take out the negative electrode sheet;

[0013] (5) Second hot pressing: Place the negative electrode sheet after the first hot pressing in the middle, and place the two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the adhesive side of the separators faces the negative electrode sheet. Place the sample in the hot pressing equipment, set the temperature, pressure and time. After the hot pressing is completed, tear open the separators and take out the negative electrode sheet.

[0014] (6) Third hot pressing: Place the negative electrode sheet after the second hot pressing in the middle, and place the two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the adhesive-coated side of the separators faces the negative electrode sheet. Place the sample in the hot pressing equipment, set the temperature, pressure and time. After the hot pressing is completed, tear off the separators, take out the negative electrode sheet, weigh it, and record the weight m3.

[0015] (7) Calculation results: After three hot pressings, the weight reduction of the negative electrode sheet is (m0- m3) / m0;

[0016] (8) Evaluation and analysis: Take different negative electrode sheets and obtain the weight reduction ratio of the negative electrode sheets through the above method. Based on this, evaluate the relationship between the cohesive forces of each negative electrode sheet. The smaller the weight reduction ratio, the greater the cohesive force.

[0017] Preferably, the negative electrode is a rolled negative electrode. A reference negative electrode can be set for comparison. During evaluation, two or more types can be evaluated.

[0018] Preferably, the base film of the single-sided coated separator is one of PE, PP, or PE and PP composite film.

[0019] Preferably, the adhesive used for the single-sided coated diaphragm is one of PVDF or PMMA.

[0020] Preferably, the coating method for the single-sided adhesive-coated diaphragm is gravure coating.

[0021] Preferably, the hot pressing temperature is ≥90℃.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] 1. Existing methods for testing and evaluating cohesion are subject to significant human interference and fluctuations. However, the testing method of this invention is simple, can greatly reduce the test fluctuations caused by human factors, improve the stability and accuracy of the test, and is simple and easy to operate.

[0024] 2. Existing methods for testing and evaluating cohesion require the use of tensile testing machines, necessitating the purchase of additional equipment. However, the testing method of this invention can utilize readily available hot presses on the R&D line, eliminating the need for additional investment and reducing costs. Detailed Implementation

[0025] The present invention will be further described in detail below with reference to the embodiments.

[0026] Example 1

[0027] A method for evaluating the cohesive strength of a lithium-ion battery negative electrode sheet includes the following steps:

[0028] Test method for negative electrode A:

[0029] (1) Preparation of negative electrode sheet: Take the negative electrode sheet A after rolling (the composition of negative electrode sheet A is: 95% graphite, 1.5% carbon black, 0.5% CMC and 3% water-based adhesive; among which the graphite is FSN-1 of Shanshan, the carbon black is SP of Temigao, the CMC is 2200 of Dassault, and the water-based adhesive is LA133 of Sichuan Yindile), cut it into a size of 10*10cm, weigh it, and record the weight m0=2597 mg;

[0030] (2) Preparation of the separator: Take a 16+1 (the base membrane is 16μm PP, and one side is coated with 1μm PVDF) single-sided coated separator, cut 6 sheets of 12*12cm size, and ensure that the size of the separator is larger than the size of the negative electrode sheet.

[0031] (3) Sample preparation: Place the negative electrode sheet in the middle, and place two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the side of the separator coated with adhesive faces the negative electrode sheet.

[0032] (4) First hot pressing: Place the sample in the hot and cold pressing equipment, set the temperature to 95℃, the pressure to 3000Kgf and the time to 60s. After the hot pressing is completed, tear open the diaphragm and take out the negative electrode sheet.

[0033] (5) Second hot pressing: Place the negative electrode sheet after the first hot pressing in the middle, and place the two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the adhesive side of the separators faces the negative electrode sheet. Place the sample in the hot pressing equipment, set the temperature to 95℃, the pressure to 3000Kgf and the time to 60s. After the hot pressing is completed, tear open the separators and take out the negative electrode sheet.

[0034] (6) Third hot pressing: Place the negative electrode sheet after the second hot pressing in the middle, and place the two diaphragms on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the diaphragms and that the adhesive side of the diaphragms faces the negative electrode sheet. Place the sample in the hot pressing equipment, set 95℃, pressure 3000Kgf and time 60s. After the hot pressing is completed, tear open the diaphragm, take out the negative electrode sheet, weigh it, and record the weight m3=2320 mg.

[0035] (7) Calculation results: After three hot pressings, the weight reduction of negative electrode A is (m0- m3) / m0=10.7%;

[0036] Test method for negative electrode B:

[0037] Take the rolled negative electrode B (composition of negative electrode B: 95% graphite, 1.5% carbon black, 0.5% CMC and 3% water-based adhesive; where the graphite is Shanshan's FSN-1, the carbon black is Temigo's SP, the CMC is Daicel's 2200, and the water-based adhesive is Sichuan Yindile's LA136D), and the remaining methods and conditions are the same as the test method for negative electrode A; it can be found that the weight reduction of negative electrode B is 7.9%;

[0038] (8) Evaluation and analysis: Because 7.9% < 10.7%, the cohesive force of negative electrode B is greater.

[0039] Verification and comparison:

[0040] The verification method for testing cohesion includes the following steps:

[0041] (1) Add 0.8 kg of fixed amount of negative electrode slurry to a rectangular polytetrafluoroethylene mold (the length, width and height of the rectangular polytetrafluoroethylene are 20 cm, 10 cm and 5 cm respectively), and bake at 100℃ for 24 h to obtain a rectangular block (the length and width are 20 cm and 10 cm respectively), which is denoted as C1;

[0042] (2) Place C1 on a polytetrafluoroethylene film and press it down with a block larger than C1 (with a layer of polytetrafluoroethylene film attached to the bottom of the block). The weight of the block is fixed at 50 kg and the pressing time is fixed at 10 minutes. The block obtained after pressing is called C2.

[0043] (3) Place C2 on a polytetrafluoroethylene film with the 20*10cm side facing down. Cut the block with a cutter. The cutting direction of the cutter is perpendicular to the 20*10cm side. Make a cut every 2cm along the length of the cuboid, for a total of 9 cuts. Collect the powder that falls off the cut surface and the powder that sticks to the cutter. Weigh the total amount of powder.

[0044] The total amount of powder obtained from the slurry of negative electrode A and B according to the above steps is 50g and 30g respectively. Since 30g < 50g, the cohesive force of negative electrode B is greater than that of negative electrode A.

[0045] The results obtained by the verification method are consistent with the test results of the present invention.

[0046] Example 2

[0047] Negative electrode sheets A', B', and C' were prepared, and the testing method was the same as in Example 1, wherein:

[0048] Preparation of negative electrode A': Take the rolled negative electrode A' (composition of negative electrode A': 95% graphite, 1.5% carbon black, 0.5% CMC and 3% water-based adhesive; where the graphite is Shanshan's FSN-1, the carbon black is Temigao's SP, the CMC is Daicel's 2200, and the water-based adhesive is Zhejiang Zhongke Lide's NV-1D), cut it into 10*10cm dimensions, and weigh it.

[0049] Preparation of negative electrode B': Take the rolled negative electrode B' (composition of negative electrode B': 95% graphite, 1.5% carbon black, 0.5% CMC and 3% water-based adhesive; where the graphite is Shanshan's FSN-1, the carbon black is Temigo's SP, the CMC is Daicel's 2200, and the water-based adhesive is Zhejiang Zhongke Lide's NV-1A), cut it into 10*10cm dimensions, and weigh it.

[0050] Preparation of negative electrode C': Take the rolled negative electrode C' (the composition of negative electrode C' is: 95% graphite, 1.5% carbon black, 0.5% CMC and 3% water-based adhesive; where the graphite is Shanshan's FSN-1, the carbon black is Temigao's SP, the CMC is Daicel's 2200, and the water-based adhesive is Zhejiang Zhongke Lide's NV-1T), cut it into 10*10cm size, and weigh it.

[0051] After three hot-pressing processes, the weight reduction percentages were 18.3%, 13.2%, and 7.8%, respectively.

[0052] Evaluation and analysis show that the cohesive force of the negative electrode varies depending on the type of water-based adhesive used. The cohesive force of negative electrode A' is less than that of negative electrode B', which is less than that of negative electrode C'.

[0053] Verification and comparison:

[0054] The slurries corresponding to the negative electrode sheets A', B', and C' were processed according to the verification method steps in Example 1, and the total powder amounts were 82g, 66g, and 35g, respectively. Conclusion: Since 82g > 66g > 35g, the cohesive force of the negative electrode sheets A' < B' < C'.

[0055] The results obtained by the verification method are consistent with the test results of the present invention.

[0056] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A method for evaluating the cohesive force of a lithium-ion battery negative electrode sheet, characterized in that, Includes the following steps: (1) Preparation of negative electrode sheet: Take the negative electrode sheet after rolling, cut it into a certain size, weigh it, and record the weight m0; (2) Preparation of the diaphragm: Take a single-sided coated diaphragm and cut 6 pieces of the same size, ensuring that the size of the diaphragm is larger than the size of the negative electrode sheet; (3) Sample preparation: Place the negative electrode sheet in the middle, and place two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the side of the separator coated with adhesive faces the negative electrode sheet. (4) First hot pressing: Place the sample in the hot pressing equipment, set the temperature, pressure and time. After the hot pressing is completed, tear open the diaphragm and take out the negative electrode sheet; (5) Second hot pressing: Place the negative electrode sheet after the first hot pressing in the middle, and place the two separators on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the separators and that the adhesive side of the separators faces the negative electrode sheet. Place the sample in the hot pressing equipment, set the temperature, pressure and time. After the hot pressing is completed, tear open the separators and take out the negative electrode sheet. (6) Third hot pressing: Place the negative electrode sheet after the second hot pressing in the middle, and place the two diaphragms on the upper and lower layers of the negative electrode sheet respectively, ensuring that the negative electrode sheet is completely covered by the diaphragms and that the adhesive side of the diaphragms faces the negative electrode sheet. Place the sample in the hot pressing equipment, set the temperature, pressure and time. After the hot pressing is completed, tear off the diaphragm, take out the negative electrode sheet, weigh it, and record the weight m3. (7) Calculation results: After three hot pressings, the weight reduction of the negative electrode sheet is (m0- m3) / m0; (8) Evaluation and analysis: For different negative electrode sheets, the weight reduction ratio of the negative electrode sheet can be obtained by the same method mentioned above. Based on this, the cohesive force of different negative electrode sheets can be evaluated. The smaller the weight reduction ratio, the greater the cohesive force. The hot pressing temperature is ≥90℃.

2. The method for evaluating the cohesive force of the negative electrode sheet of a lithium-ion battery according to claim 1, characterized in that: When comparing, a reference negative electrode is set.

3. The method for evaluating the cohesive strength of the negative electrode sheet of a lithium-ion battery according to claim 1, characterized in that: The base film of the single-sided coated diaphragm is one of PE, PP, or PE and PP composite films.

4. The method for evaluating the cohesive force of the negative electrode sheet of a lithium-ion battery according to claim 1, characterized in that: The adhesive used in single-sided coated diaphragms is either PVDF or PMMA.

5. The method for evaluating the cohesive force of the negative electrode sheet of a lithium-ion battery according to claim 1, characterized in that: The coating method for single-sided coated diaphragms is gravure coating.

6. The method for evaluating the cohesive force of the negative electrode sheet of a lithium-ion battery according to claim 1, characterized in that: The negative electrode sheet can be of two or more types.