Method for testing intrinsic bonding strength of active material layer and current collector of double-sided pole piece

Abstract

The invention relates to the technical field of battery pole piece detection, in particular to a method for testing intrinsic bonding strength of a double-sided pole piece active material layer and a current collector. Comprising the following steps: preparing a plurality of same double-sided pole pieces; each double-sided pole piece comprises a to-be-detected active substance layer, a current collector and a back active substance layer, and the to-be-detected active substance layers of the plurality of double-sided pole pieces are respectively scraped with different thicknesses; recording the thickness hn of the to-be-tested active substance layer after scraping in each double-sided pole piece, and testing the peel strength Fn between the to-be-tested active substance layer after scraping and the current collector; establishing a linear regression equation of hn and Fn, wherein the intrinsic bonding strength of the to-be-measured active material layer and the current collector is the intercept of the linear regression equation. According to the method, the thickness-stripping force linear relation curve is established, the longitudinal intercept is utilized to eliminate the interference of the mechanical property of the thickness of the active layer on the stripping force test result, and the obtained longitudinal intercept directly corresponds to the real bonding strength between the active material layer and the current collector.

Description

Technical Field

[0001] This invention relates to the field of battery electrode testing technology, and in particular to a method for testing the intrinsic bonding strength between the active material layer and the current collector of a double-sided electrode. Background Technology

[0002] In the lithium-ion battery manufacturing process, the bonding strength (i.e., peel force) between the active material of the electrode and the current collector (such as copper foil or aluminum foil) is a key quality indicator, directly affecting the battery's charge-discharge cycle stability, internal resistance consistency, and safety performance. For double-sided coated electrodes, when traditional peel force testing methods directly peel the target side, the thickness of the active material layer on the back side is affected by stress transmitted through the current collector, resulting in additional thickness-related influencing factors in the test results. This makes it impossible to accurately reflect the intrinsic bonding strength between the active material and the current collector. Current technologies lack specific methods to eliminate this interference, leading to insufficient accuracy in double-sided electrode peel force testing results and making it difficult to meet the requirements of high-precision production quality control.

[0003] Patent document CN114778441A discloses a method for testing the peel strength of battery electrodes. This method includes the following technical solution: removing the back coating from an electrode with a test coating on each side and a back coating on the other side, and then performing a peel test on the remaining structure to obtain test data. While this prior art removes the back coating and avoids its influence, on the one hand, the mechanical force applied during peeling (such as the roller pressing and rolling, or scraping mentioned above) is transmitted to the test coating on the other side through the current collector. This process may irreversibly change the physical state of the current collector and may cause hidden damage to the interface of the test coating. Furthermore, the peeling process itself is difficult to standardize, resulting in the final measured peel strength data not accurately reflecting the original bonding state between the test coating and the current collector in the actual product, leading to poor repeatability. On the other hand, it only obtains the apparent bonding strength between the test coating and the current collector, including interference factors such as internal stress in the coating. When the coating thickness or structure changes, this apparent strength value loses comparability and cannot be used to evaluate the inherent bonding performance of the interface treatment or the material itself. Summary of the Invention

[0004] The present invention aims to solve the above problems by providing a test method applicable to double-sided electrodes, which can eliminate interference from the back active material layer, reflect the original bonding state of the interface, and obtain the intrinsic bonding strength between the active material layer and the current collector.

[0005] The technical solution to the problem of this invention is to provide a method for testing the intrinsic bonding strength between the active material layer and the current collector of a double-sided electrode, wherein the double-sided electrode includes an active material layer to be tested, a current collector, and a back active material layer, and includes the following steps:

[0006] S1. Prepare several identical double-sided electrode sheets;

[0007] S2. The active material layers to be tested on several of the aforementioned bifacial electrodes are scraped off to different thicknesses; the thickness h of the active material layer to be tested after scraping is recorded in each bifacial electrode. n The peel strength F between the active material layer to be tested and the current collector in each double-sided electrode after scraping was also tested. n ;

[0008] S3. Establish h n and F n The linear regression equation F n =ah n +b, where the intrinsic binding strength between the active material layer to be tested and the current collector is the intercept b value of the linear regression equation.

[0009] In this invention, a linear regression model of peel strength and thickness is established by performing peel tests on double-sided electrodes with different active material layer thicknesses but identical other conditions. By extrapolating to the theoretical zero thickness point, the thickness-related stresses, including the influence of the back active material layer, are peeled off, thereby obtaining an intrinsic bonding strength evaluation of the active material layer and the current collector that is closer to the real situation. This parameter can be used to fairly compare the true performance of different interface treatment processes and different active material layer formulations.

[0010] In step S1, "several bifacial electrodes are identical" means that each bifacial electrode has the same microstructure, internal stress state, and interface morphology. Preferably, the bifacial electrodes are obtained by cutting from the same parent bifacial electrode. The only variable controlling the bifacial electrodes is the thickness of the active material layer to be tested.

[0011] In step S2, different thicknesses of the active material layers to be tested are obtained by scraping. First, this invention does not directly prepare double-sided electrodes with different thicknesses of the active material layers to be tested. This is because the solvent evaporation path, surface skinning rate, internal stress development process, and rolling process are different for thick films and thin films, which would introduce uncontrollable differences in the active coating to be tested, besides thickness. Second, since scraping only acts on the surface of the active material layer to be tested, the mechanical force is difficult to transmit to the interface with the current collector, or even the back active material layer. The disturbance to the interface is far less than a complete interface peeling, and it will not interfere with the current collector and the back active material layer, ensuring the consistency of other conditions and not affecting the true bonding state.

[0012] The scraping method is not limited. As a preferred embodiment of the present invention, high-precision scraping equipment is used for scraping, such as a focused ion beam (FIB) system, an argon ion polishing (CP) instrument, etc., or at least includes an electrode fixing structure, a scraper structure for fixing the scraper and driving the scraper to move in and out along the thickness direction of the active material layer to control the scraping thickness, and a scraping separation structure for making the scraper and the electrode move relative to each other in a direction parallel to the surface of the active material layer to be tested.

[0013] The scraping thickness of the various double-sided electrodes is different. Preferably, the scraping thickness of the various double-sided electrodes increases or decreases progressively. The tolerance for this increase or decrease is preferably limited. If the tolerance is too large, and the thickness of the active material layer to be tested is limited, it will result in too few test data sets, affecting the accuracy of regression analysis. If the tolerance is too small, the difference in peel strength measured at two adjacent thickness points may mainly reflect slight differences in surface damage, rather than differences caused by actual thickness changes, leading to data distortion. Therefore, preferably, the tolerance is 10% to 30% of the original thickness of the active material coating to be tested. For example, it can be 10%, 15%, 20%, 25%, or 30%; preferably 20%.

[0014] Although the scraping thickness was controlled, the actual thickness of the active material layer to be tested after scraping still needs to be measured again.

[0015] In some embodiments, as a preferred embodiment of the present invention, the thickness h of the active material layer to be tested after scraping is... n Obtained through the following steps:

[0016] In step S1, when preparing the double-sided electrode, the thickness T0 of the current collector is first measured; then the active material layer to be tested is coated and dried on the current collector, and the total thickness T1 is measured; then the active material layer on the back side is coated and dried on the other side of the current collector, and the total thickness T2 is measured.

[0017] In step S2, the total thickness T of the double-sided electrode sheet after scraping is measured. n ;

[0018] h n =T n -T0-(T2-T1).

[0019] In preparing the (master) double-sided electrode, by employing a "single-sided first, then double-sided" coating process, the initial thickness of the active material layer to be tested can be calculated as T1-T0; the thickness of the back active material layer is T2-T1. After scraping, the total thickness T is... n By successively subtracting the current collector thickness T0 and the back active material layer thickness T2-T1, the actual thickness h of the active material layer to be tested after scraping can be obtained. n .

[0020] This measurement and calculation method involves measuring and calculating the double-sided electrode sheet before it is coated and rolled.

[0021] In some embodiments, in order to further enhance the industrial application value of the present invention, a method for measuring and calculating the double-sided electrode sheet after roll forming is also provided.

[0022] As a preferred embodiment of the present invention, the thickness h of the active material layer to be tested after scraping n Obtained through the following steps:

[0023] In step S1, when preparing the double-sided electrode, the thickness T0 of the current collector is first measured; then the active material layer to be tested is coated and dried on the current collector, and the total thickness T1 is measured; then the active material layer on the back side is coated and dried on the other side of the current collector, and the total thickness T2 is measured; finally, the double-sided electrode is rolled and the total thickness t2 after rolling is measured.

[0024] Take a double-sided electrode sheet after roller pressing, peel off the active material layer to be tested and the back active material layer, and measure the thickness of the current collector as t0.

[0025] The compressibility of the active material layer and the back active material is as follows: ;

[0026] In step S2, the total thickness T of the double-sided electrode sheet after scraping is measured. n ;

[0027] h n =T n -t0-(T2-T1)(1-x).

[0028] The intrinsic bonding strength of the interface is mainly determined by the slurry of the active material layer to be tested and the current collector, and has little to do with the original thickness of the active material layer to be tested.

[0029] Therefore, the original thickness of the active material layer to be tested is not limited, for example, it can be 80~120μm, such as 80μm, 90μm, 100μm, 110μm, or 120μm.

[0030] However, the slurry used to form the active material layer to be tested affects the uniformity of the distribution of the active material layer in the thickness direction. If the distribution is not uniform, the linear relationship between the scraping thickness and the peel strength is poor.

[0031] The slurry used to form the active material layer to be tested typically includes the active material, a conductive agent, and a binder, with the binder being the main factor affecting the uniformity of the slurry.

[0032] For the negative electrode bifacial sheet, as a preferred embodiment of the present invention, the binder comprises carboxymethyl cellulose (binder CMC) and styrene-butadiene rubber (binder SBR) in a mass ratio of 1:(1~2). Preferably, the binder is composed of carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 1:(1~2). For example, the mass ratio may be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, or 1:2.

[0033] For the positive electrode double-sided electrode sheet, as a preferred embodiment of the present invention, the binder is polyvinylidene fluoride (PVDF).

[0034] The amount of binder should be limited. If the amount of binder is too low, the dispersibility will be poor and the active material layer to be tested will be brittle, which will easily introduce micro-cracks during the scraping process. If the amount of binder is too high, it will easily lead to large plastic deformation during the peeling process and affect the actual peel strength.

[0035] As a preferred embodiment of the present invention, the binder has a mass fraction of 2wt% to 6wt% in the slurry. For example, it can be 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, or 6wt%. For the negative electrode double-sided sheet, 4wt% is preferred. For the positive electrode double-sided sheet, 5wt% is preferred.

[0036] There are no restrictions on active materials and conductive agents.

[0037] For the negative electrode double-sided electrode sheet, as a preferred embodiment of the present invention, the slurry is composed of graphite, conductive agent SP, binder CMC, and binder SBR. Preferably, the mass ratio of graphite, conductive agent SP, binder CMC, and binder SBR is (91~99):(1~3):(0.5~2.5):(1.5~3.5), more preferably (93~95):(1.5~2.5):(1~2):(2~3), and even more preferably 94:2:1.5:2.5.

[0038] For the positive electrode double-sided electrode sheet, as a preferred embodiment of the present invention, the slurry is composed of positive electrode active material, conductive agent SP, and PVDF. Preferably, the mass ratio of positive electrode active material, SP, and PVDF is (85~96):(2~8):(2~8), more preferably (88~92):(4~6):(4~6), and even more preferably 90:5:5.

[0039] Since the present invention does not eliminate the interference of the back active material layer, but rather uses experimental design and mathematical extrapolation to incorporate this interference into a constant term, thereby separating the influence of the thickness of the active material layer under test. As long as the state of the back active material layer remains constant, this induction is effective, and the intrinsic bonding strength obtained based on this is meaningful as a relative parameter for comparing the quality of interfaces of different active material layers under test. To ensure the stability of the back active material layer state, that is, to reduce the change of the back active material layer during the peel test, it is preferable to control the overall bending deformation of the double-sided electrode sheet to be dominated by the current collector. This places certain requirements on the material of the current collector and the thickness ratio of the current collector to the back active material layer. Preferably, the material forming the current collector is at least one of copper and aluminum; the thickness ratio of the current collector to the back coating is not less than 0.06. If the current collector is too thin or too soft, the thickness change of the active material layer under test will have a significant impact on the overall structural stiffness.

[0040] The method for testing the peel strength between the scraped active material layer and the current collector in a scraped double-sided electrode is not limited; conventional and standard measurement methods in existing technologies can be used. Preferably, a 180° peel test is used to measure the peel strength F. n Specifically, under ambient temperatures of 23±1℃ and relative humidity of 50±5%, the double-sided electrode is fixed to one clamp of a tensile testing machine, while the other clamp of the machine holds the end of the active material layer to be tested at a 180° angle to the current collector, pulling the active material layer apart. The peeling force required to continuously peel the active material layer from the current collector is measured and converted into peel strength.

[0041] In this invention, the width of the double-sided electrode sheet is 24-26 mm, preferably 25 mm; the peeling speed is 80-120 mm / min, preferably 100 mm / min; and the peeling length is 130-150 mm, preferably 140 mm.

[0042] As a preferred embodiment of the present invention, during the peel test, a peel displacement range with stable peeling is selected as the effective test segment, and the average peel strength within the effective test segment is taken as the peel strength F. n To improve the accuracy of the test results, the effective test segment is preferably selected from the peel displacement range of 40-120mm.

[0043] In step S3, the established regression model should have a high goodness of fit to reduce the error of subsequent calculation results relative to the actual structure. Preferably, the goodness of fit of the regression equation is not less than 0.9. If it is less than 0.9, the thickness of the active material layer slurry binder and / or the current collector and / or the back active material layer should be adjusted before repeating the experiment.

[0044] The beneficial effects of this invention are:

[0045] 1. This invention establishes a linear relationship curve between thickness and peel force by using electrodes of different thicknesses. The longitudinal intercept eliminates the interference of the mechanical properties of the active layer thickness on the peel force test results. The obtained longitudinal intercept directly corresponds to the true bonding strength between the active material layer and the current collector, solving the problem that the thickness of the active material affects the test accuracy in traditional double-sided testing. The linear fit goodness is ≥0.9, indicating that the linear correlation is significant.

[0046] 2. The intrinsic bonding strength obtained by this invention can be used to compare the true performance of different interface treatment processes and different active material layer formulations. In addition, the slope in the obtained linear regression equation can be used to compare the compatibility of different active material layer formulations with thick coating processes (for example, a small slope means that the increase in thickness has a weak effect on the peel strength, and the formulation has great advantages in thick coating). The goodness of fit in the obtained linear regression equation can determine the uniformity of the slurry (for example, a small goodness of fit indicates poor uniformity). Therefore, this invention also provides a method for screening double-sided electrodes, using linear regression equations to guide the research and development and production of double-sided electrodes. Attached Figure Description

[0047] Figure 1 This is a graph showing the relationship between thickness and peel strength in Example 1;

[0048] Figure 2 This is a graph showing the relationship between peel strength and displacement in Example 1. Detailed Implementation

[0049] The following are specific embodiments of the present invention, and the technical solutions of the present invention will be further described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0050] Example 1

[0051] A method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector includes the following steps:

[0052] S1. Preparation of double-sided electrodes: Prepare a slurry according to the mass ratio of graphite: conductive agent SP: binder CMC: binder SBR = 94:2:1.5:2.5. Prepare a copper foil current collector with a thickness T0 = 6.3μm.

[0053] A "single-sided first, then double-sided" coating process was adopted. A slurry was coated on one side of the current collector, and after drying, the active material layer to be tested was obtained. The total thickness T1 was measured to be 105.9 μm, indicating that the initial thickness of the active material layer was 99.8 μm. Then, a slurry was coated on the other side of the current collector, and after drying, the back active material layer was obtained. The total thickness T2 was measured to be 205.7 μm, indicating that the thickness of the back active material layer was 99.6 μm.

[0054] The double-sided electrode sheet is cut into 5 pieces, each piece being 25mm wide and no less than 140mm long.

[0055] S2. A high-precision scraping device is used to scrape the active material layer of the five double-sided electrode sheets obtained by slitting. The scraping thickness is designed to be 0μm (no scraping), 20μm, 40μm, 60μm and 80μm respectively, to ensure that the surface is flat after scraping and that the current collector and the active material layer on the back are not damaged.

[0056] Measure the actual total thickness T of 5 double-sided electrode sheets after scraping. n And calculate the thickness h of the active material layer to be tested after scraping. n As shown in Table 1 below.

[0057] Five double-sided electrode sheets were subjected to 180° peel tests using an electronic peel testing machine. The test speed was set to 100 mm / min, the peel length to be 140 mm, and the electrode width to be 25 mm. The relationship between peel strength and displacement during the peel process is shown in the figure below. Figure 2 As shown, the 40-120mm range where the force value is stable during the peeling process is selected as the effective test section. The average peel strength F within this range is calculated. n As shown in Table 1 below.

[0058] Table 1.

[0059] Parallel sample number Scraping thickness (μm) <![CDATA[Actual total thickness T after scraping n (μm)]]> <![CDATA[The thickness h of the active material layer to be measured after scraping n (μm)]]> <![CDATA[Effective interval peel strength F n (N / m)]]> 1 80 125.8 19.9 10.60 2 60 145.6 39.7 12.68 3 40 166.1 60.2 12.84 4 20 185.9 80.0 14.04 5 0 205.7 99.8 16.15

[0060] S3. Establish h n and F n Linear relationship such as Figure 1 As shown, the linear regression equation is: y = 1.246x + 9.524; the goodness of fit R0 2 = 0.9381, indicating a significant linear correlation.

[0061] Therefore, the intrinsic binding strength between the active material layer and the current collector is 9.524 N / m.

[0062] Example 2

[0063] This embodiment is basically the same as embodiment 1, except that the double-sided electrode sheet is subjected to roll forming.

[0064] S1. Take the double-sided electrode sheet from Example 1, and after roll forming, measure the total thickness t2, which is 163.2 μm.

[0065] The double-sided electrode sheet is cut into 6 pieces, each piece being 25mm wide and no less than 140mm long.

[0066] After taking one sample and peeling off the active material layer to be tested and the back active material layer, the thickness t0 of the current collector was measured to be 6.1 μm. The compressibility of the active material layer to be tested and the back active material layer was... 20.1%;

[0067] S2. A high-precision scraping device is used to scrape the active material layer of the remaining 5 double-sided electrode sheets obtained by slitting. The scraping thickness is designed to be 0μm (no scraping), 10μm, 20μm, 30μm and 40μm respectively, to ensure that the surface is flat after scraping and that the current collector and the active material layer on the back are not damaged.

[0068] Measure the total thickness T of the double-sided electrode sheet after scraping. n And calculate the thickness h of the active material layer to be tested after scraping. n =T n -t0-(T2-T1)(1-x) is shown in Table 2 below.

[0069] Five double-sided electrodes were subjected to 180° peel tests using an electronic peel testing machine. The test speed was set to 100 mm / min, the peel length to 140 mm, and the electrode width to 25 mm. The 40–120 mm range where the force was stable during peeling was selected as the effective test segment. The average peel strength F within this range was calculated. n As shown in Table 2 below.

[0070] Table 2.

[0071] Parallel sample number Scraping thickness (μm) <![CDATA[Actual total thickness T after scraping n (μm)]]> <![CDATA[The thickness h of the active material layer to be measured after scraping n (μm)]]> <![CDATA[Effective interval peel strength F n (N / m)]]> 1 40 123.5 37.7 7.25 2 30 132.8 47.0 7.37 3 20 143.4 57.6 7.79 4 10 153.2 67.4 7.89 5 0 163.2 77.4 8.26

[0072] S3. Establish h n and F n The linear relationship is shown in the equation: y = 0.0255x + 6.2473; the goodness of fit R0 is 0.0255x + 6.2473. 2 = 0.9698.

[0073] Therefore, the intrinsic binding strength between the active material layer and the current collector is 6.247 N / m.

[0074] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for testing the intrinsic bonding strength between the active material layer and the current collector of a double-sided electrode, wherein the double-sided electrode comprises an active material layer to be tested, a current collector, and a back active material layer, characterized in that: Includes the following steps: S1. Prepare several identical double-sided electrode sheets; S2. The active material layers to be tested on several of the aforementioned double-sided electrodes are scraped off to different thicknesses; Record the thickness h of the active material layer to be tested after scraping in each bifacial electrode. n The peel strength F between the active material layer to be tested and the current collector in each double-sided electrode after scraping was also tested. n ; S3. Establish h n and F n The linear regression equation F n =ah n +b, where the intrinsic binding strength between the active material layer to be tested and the current collector is the intercept b value of the linear regression equation.

2. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: The thickness h of the active material layer to be tested after scraping n Obtained through the following steps: In step S1, when preparing the double-sided electrode, the thickness T0 of the current collector is first measured; then the active material layer to be tested is coated and dried on the current collector, and the total thickness T1 is measured; then the active material layer on the back side is coated and dried on the other side of the current collector, and the total thickness T2 is measured. In step S2, the total thickness T of the double-sided electrode sheet after scraping is measured. n ; h n =T n -T0-(T2-T1)。 3. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: Several of the aforementioned bifacial electrodes are obtained by cutting the same mother bifacial electrode.

4. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: The scraping thickness of several of the double-sided electrodes increases or decreases, with a tolerance of 10% to 30% of the original thickness of the coating of the active material to be tested.

5. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: The slurry used to form the layer of the active substance to be tested includes a binder; The adhesive comprises carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 1:(1~2), or the adhesive is polyvinylidene fluoride.

6. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 5, characterized in that: The adhesive has a mass fraction of 2wt% to 6wt% in the slurry.

7. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: The material forming the current collector is at least one of copper and aluminum; the thickness ratio of the current collector to the back active material layer is not less than 0.

06.

8. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: Peel strength F was tested using a 180° peel test. n .

9. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 8, characterized in that: During the peel test, the peel displacement range with stable peel is selected as the effective test segment, and the average peel strength within the effective test segment is taken as the peel strength F. n .

10. The method for testing the intrinsic bonding strength between the active material layer of a double-sided electrode and the current collector according to claim 1, characterized in that: The goodness of fit of the linear regression equation is not less than 0.9.

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