Positive plate active material layer nondestructive separation method and cohesion measurement method

Abstract

The invention relates to the technical field of lithium batteries, in particular to a positive plate active material layer nondestructive separation method and a cohesion measurement method. The lossless separation method comprises the following steps: immersing the positive plate in deionized water for a water immersion time t, and physically separating the active material layer and the current collector; the time t is enough to weaken the binding force between the active material layer and the current collector so as to realize separation, but is not enough to substantially swell the active material layer; the time t does not exceed 60 s, and the swelling rate of the active substance layer obtained through separation does not exceed 0.1%. According to the preparation method, water molecules permeate into the interface of the binder and the current collector by using water immersion for an extremely short time, so that the adhesive force of the interface is reduced, the internal part of the active substance layer is not damaged by swelling and the like, the lossless separation of the interface is realized, and the complete active substance layer without influence on cohesion is obtained; the active material layer is used as a cohesion measurement object, and the detection result is accurate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, and relates to a positive electrode sheet active material layer nondestructive separation method and a cohesion force measurement method, in particular to a positive electrode sheet active material layer nondestructive separation method and a cohesion force measurement method with lithium iron phosphate as an active material and PVDF as a binder. BACKGROUND

[0002] The cohesion force of the lithium battery electrode sheet coating (i.e. the adhesion between active material particles) is a key factor affecting the battery performance (such as internal resistance, cycle life) and structural integrity. During the charging and discharging process, the repeated expansion and contraction of the positive and negative electrode coatings can easily lead to coating peeling, and uniform distribution of the binder is crucial to ensuring sufficient cohesion. In actual manufacturing (especially the drying link) and subsequent electrolyte infiltration, formation, and long-term cycling, uneven distribution of the binder, insufficient amount, or its adhesion decay can significantly reduce the cohesion, increase the risk of material falling, and affect production and battery life.

[0003] To detect the cohesion of the active material layer, the widely used adhesive tape tension test method (manual adhesive tape sticking and pulling) has the following defects: first, the peeling force between the current collector and the active material is often measured; second, the same material has different peeling forces for different thicknesses of the electrode sheet, which cannot objectively quantify the inherent cohesion level of the electrode sheet.

[0004] The patent document with the publication number CN114544486A discloses a test method for the adhesion of a binder to active material in a lithium battery electrode sheet, which includes the following steps: A. uniformly mixing the active material and the binder to prepare a slurry, then coating the slurry on the electrode sheet substrate and drying to obtain an electrode sheet E1; B. sticking one side of the first double-sided adhesive tape to the fixed substrate and the other side to the active material layer of the electrode sheet E1; C. peeling the electrode sheet substrate from the active material layer to expose the reverse side of the active material layer; D. sticking one side of the second double-sided adhesive tape to the reverse side of the active material layer; E. fixing the fixed substrate to the base of the tensile testing machine, fixing one end of the second double-sided adhesive tape to the tensile testing machine clamp, and starting the tensile testing machine to test and obtain the tensile value. The prior art is used to analyze the cohesion between the binder and the active material.

[0005] However, the prior art does not specify how to separate the pole piece substrate and the active material layer. If the two are directly physically separated, the active material layer will be pulled by the pole piece substrate due to the interface connection between the active material layer and the pole piece substrate, and problems such as adhesive filamentation may occur during separation. After separation, there may be problems such as uneven thickness of the active material layer and changes in the distribution of active particles, which will change the cohesion of the active material layer, so that even if the active material layer is subsequently detected for cohesion, the original cohesion of the active material layer is not measured, and the measurement result is inaccurate.

[0006] In addition, some methods in the prior art separate the pole piece substrate and the active material by solvent soaking or long-term water soaking, but the ultimate goal of these methods is the recovery of active substances, and the separated active material layer is not complete and cannot be used for cohesion detection of the active material layer. SUMMARY

[0007] The present application solves the above problems and provides a positive pole piece active material layer non-destructive separation method and cohesion measurement method which can separate a complete active material layer without affecting its cohesion for cohesion detection of the active material layer.

[0008] The technical solution of the present application is to provide a positive pole piece active material layer non-destructive separation method, the positive pole piece including a current collector and an active material layer, comprising the following steps: immersing the positive pole piece in deionized water, physically separating the active material layer and the current collector after water immersion time t; the time t is sufficient to weaken the binding force between the active material layer and the current collector to achieve separation, but not enough to cause substantial swelling of the active material layer; the time t is not more than 60s, and the swelling rate of the active material layer obtained by separation is not more than 0.1%.

[0009] Within 60s, non-destructive peeling can be achieved without substantial swelling, and as a preferred embodiment of the present application, the time t is not more than 10s; preferably, the time t is not more than 5s; further preferably, the time t is not more than 2s to improve separation efficiency. The time t may, for example, be 1.0s, 1.5s, 2.0s, 2.5s, 3.0s, 3.5s, 4.0s, 4.5s, 5.0s, 5.5s, 6.0s, 6.5s, 7.0s, 7.5s, 8.0s, 8.5s, 9.0s, 9.5s, 10.0s.

[0010] As a preferred embodiment of the present application, the swelling rate of the active material layer obtained by separation is not more than 0.02%; preferably, the swelling rate of the active material layer obtained by separation is not more than 0.01%.

[0011] The present inventors have found that, when certain positive electrode sheets are immersed in water for a very short time, water molecules can penetrate into the interface between the binder and the current collector within a very short time, causing the interface adhesion to decrease by more than 90%. Since the time is very short, no damage such as swelling occurs to the interior of the active material layer, and lossless separation of the interface is achieved.

[0012] The present application first recognizes that there is a "critical time window for lossless peeling" in the process of separating the active material layer: within a threshold time that is much lower than the time required to initiate swelling of the active material layer, there is a short time window during which the interface adhesion has been greatly weakened, but water has not yet penetrated to a sufficient extent to damage the structure of the active material layer. Based on this, it is verified that the use of this window can solve the technical problem of "obtaining a complete active material layer". Although the prior art often uses immersion to recover the active material, it should be understood that in order to achieve recovery, the prior art will inevitably cause swelling, damage or dissolution of the active material layer. Under the guidance of the prior art, those skilled in the art will not have the motivation to greatly shorten the immersion time, which usually means that the separation fails. Even if those skilled in the art try to shorten the time, they expect to find a "just right peeling time", but they do not expect to find a narrow window for "successful peeling and lossless active material layer".

[0013] As a preferred embodiment of the present application, the positive electrode sheet uses lithium iron phosphate as the active material and polyvinylidene fluoride (PVDF) as the binder. The present application is particularly suitable for lithium iron phosphate positive electrode sheets, reduces the interface adhesion between the PVDF binder and the current collector, and achieves complete peeling of the active material layer.

[0014] As a preferred embodiment of the present application, the process further comprises the step of drying the separated active material layer at 60-120°C and a vacuum degree of ≤-0.08 MPa for at least 0.5 h. The specific drying process can completely remove the residual water in the active material layer, ensuring the accuracy of subsequent measurements; and the vacuum environment helps to accelerate the evaporation of water, while avoiding the impact of high temperature on the structure of the active material layer. In the present application, further combined with the specific drying process, the integrity of the internal structure of the active material layer can be maintained, laying the foundation for accurate measurement of the cohesive force. For example, the drying temperature can be 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C; preferably 60°C. The vacuum degree can be -0.08 MPa, -0.09 MPa, -0.10 MPa, -0.11 MPa, -0.12 MPa, -0.13 MPa; preferably -0.08 MPa. The drying time can be 0.5-5 h, such as 0.5 h, 1.0 h, 1.5 h, 2.0 h, 2.5 h, 3.0 h, 3.5 h, 4.0 h, 4.5 h, 5.0 h; preferably 3.0 h.

[0015] Specifically in the separation: when water immersion, as the preferred embodiment of the present application, the positive electrode sheet is cut into a strip sample with a width of 20-30 mm, and the length is not limited. For example, the width can be 20 mm, 21 mm, 22 mm, 23 mm, 24 mm, 25 mm, 26 mm, 27 mm, 28 mm, 29 mm, 30 mm; preferably 25 mm.

[0016] As a preferred embodiment of the present application, the deionized water is at room temperature of 20-28℃; preferably 25℃.

[0017] As a preferred embodiment of the present application, the positive electrode sheet is immersed in the deionized water vertically; preferably, the positive electrode sheet is immersed in the deionized water vertically.

[0018] As a preferred embodiment of the present application, the immersion depth of the positive electrode sheet in the deionized water is not less than 5 cm. Specifically, the distance between the highest point of the positive electrode sheet and the water surface is not less than 5 cm, preferably 5-10 cm; for example, it can be 5 cm, 6 cm, 7 cm, 8 cm, 9 cm, 10 cm; preferably 5 cm.

[0019] After water immersion, as a preferred embodiment of the present application, the lithium iron phosphate positive electrode sheet is taken out vertically after water immersion and is vertically placed for 3.0-7.0 s, and then the active material layer and the current collector are separated. By vertical placement, the water on the surface of the sample is drained, avoiding excessive water affecting the subsequent operation; at the same time, vertical placement avoids water penetration into the active material layer. The standing time can be 3.0 s, 3.5 s, 4.0 s, 4.5 s, 5.0 s, 5.5 s, 6.0 s, 6.5 s, 7.0 s; preferably 5.0 s.

[0020] During the separation, as a preferred embodiment of the present application, the active material layer is pulled in parallel at 180° to separate the active material layer and the current collector.

[0021] As a preferred embodiment of the present application, the active material layer is pulled in parallel at a speed of not more than 10 mm / s to separate the active material layer and the current collector. A slower pulling speed helps to ensure the smoothness of the separation process and reduce the tearing of the active material layer. The stretching speed is preferably 1-10 mm / s, for example, it can be 1 mm / s, 2 mm / s, 3 mm / s, 4 mm / s, 5 mm / s, 6 mm / s, 7 mm / s, 8 mm / s, 9 mm / s, 10 mm / s; preferably 5 mm / s.

[0022] As a preferred embodiment of the present application, the edge of the active material layer is gently clamped with a round-head tweezers during the separation, and attention should be paid to avoid damage to the active material layer during operation.

[0023] In a second aspect, the present application also aims to provide a method for measuring the cohesion of an active material layer of a positive electrode sheet, using the active material layer obtained by the non-destructive separation method as the measurement object.

[0024] In the present application, the active material layer with intact cohesion is used as the measurement object, and the measured cohesion result is accurate.

[0025] As a preferred embodiment of the present application, the following steps are included:

[0026] S1. Measure the thickness of the active material layer, denoted as H;

[0027] S2. Stretch the active material layer and measure the force at the time of fracture, denoted as F;

[0028] S3. The cohesion σ of the active material layer is F / (W×H); wherein W is the width of the active material layer.

[0029] In step S1, as a preferred embodiment of the present application, at least three thicknesses are measured and averaged. Preferably, the thickness of at least three points on each active material layer is tested and averaged, and each test point needs to avoid the edge by 1-2 mm to avoid measurement errors caused by edge effects.

[0030] In step S2, as a preferred embodiment of the present application, at least three sets of stretching forces are measured and averaged to reduce the influence of accidental errors on the results.

[0031] As a preferred embodiment of the present application, a flat rubber clamp is used for stretching. The clamp made of rubber material can increase the friction force and at the same time avoid damage to the active material layer during clamping.

[0032] As a preferred embodiment of the present application, the stretching speed is 4.5-5.5 mm / min. For example, it can be 4.5 mm / min, 4.6 mm / min, 4.7 mm / min, 4.8 mm / min, 4.9 mm / min, 5.0 mm / min, 5.1 mm / min, 5.2 mm / min, 5.3 mm / min, 5.4 mm / min, 5.5 mm / min; preferably 5.0 mm / min.

[0033] The present application has the following beneficial effects:

[0034] 1. The present application provides a non-destructive separation method for an active material layer of a positive electrode sheet. By immersing in water for a very short time, water molecules penetrate into the interface between the binder and the current collector, reducing the adhesion of the interface, but without causing swelling and other damage to the inside of the active material layer, achieving non-destructive separation of the interface, and obtaining an intact active material layer with no influence on cohesion.

[0035] 2. The application provides a positive electrode sheet active material layer cohesion measurement method, which takes the above active material layer as the measurement object and has accurate detection results. The measurement method is reliable and accurate, which has core significance for scientifically evaluating the performance of the binder, optimizing the dosage, developing new high-efficiency binders, and ultimately improving the stability and life of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 is a schematic diagram of the lithium iron phosphate sample in Example 1;

[0037] FIG. 2 is a schematic diagram of the water immersion of the lithium iron phosphate sample in Example 1;

[0038] FIG. 3 is a schematic diagram of the current collector and active material layer separated in Example 1;

[0039] FIG. 4 is a schematic diagram of the cohesion measurement in Example 1. DETAILED DESCRIPTION

[0040] The following is a specific embodiment of the application, and the technical solutions of the application are further described in conjunction with the drawings, but the application is not limited to these embodiments.

[0041] Example 1

[0042] A method for nondestructively separating an active material layer of a positive electrode sheet The positive electrode sheet takes lithium iron phosphate as the active material and PVDF as the binder, and includes a current collector and an active material layer. The following steps are included:

[0043] (1) As shown in FIG. 1 , cut the lithium iron phosphate positive electrode sheet into a strip-shaped sample with a width W = 25.00 mm. Measure and adjust the mass W of the sample.

[0044] (2) As shown in FIG. 2 , immerse the strip-shaped sample in deionized water at room temperature, with an immersion depth greater than 5 cm, and immerse for 1.5 s.

[0045] (3) Use tweezers to hold the current collector end and vertically lift the strip-shaped sample, then let it stand for 5 seconds to drain the water on the surface of the sample.

[0046] (4) Gently hold the edge of the active material layer with a round-tipped tweezer, and pull the active material layer in parallel at a speed of 180°, 5 mm / s, as shown in FIG. 3 , to achieve complete separation of the active material layer from the current collector.

[0047] The mass W1' of the active material layer and the mass W2 of the current collector after drying are measured. The original mass W1 of the active material layer is calculated as W1 = W - W2, and the swelling rate of the active material layer is calculated as (W1' - W1) / W1. In this embodiment, the swelling rate of the active material layer does not exceed 0.01%.

[0048] (5) Place the separated active material layer in a vacuum oven for vacuum drying. The drying conditions are: 60℃, vacuum degree -0.08MPa, drying for 3h to obtain the active material layer.

[0049] A method for measuring cohesion of an active material layer of a positive electrode sheet Using the active substance layer obtained above as the measurement object, the following steps are included:

[0050] S1. Measure the thickness at three points on the active material layer, avoiding the edge by 1 mm at each test point. The measured thicknesses are 0.0984 mm, 0.0988 mm, and 0.0985 mm, respectively. Calculate the average value H = 0.0986 mm.

[0051] S2. For example FIG. 4 The active material layer was held at both ends by a flat-mouthed rubber clamp and stretched at a speed of 5 mm / min. The force at which the active material layer broke was recorded. Three sets of tests were conducted, and the measured forces were 6.8124 N, 6.2888 N, and 6.5085 N, respectively. The average value F was calculated to be 6.5366 N.

[0052] S3. The cohesive force of the active material layer σ = F / (W×H) = 6.5366 / (25.00×0.0986) = 2.6518 MPa.

[0053] Example 2

[0054] This embodiment is basically the same as Embodiment 1, except that:

[0055] In step (2), the water is soaked for 2 seconds.

[0056] The swelling rate of the active material layer was measured and calculated to be no more than 0.01%, and the cohesive force was measured and calculated to be 2.6506 MPa.

[0057] Example 3

[0058] This embodiment is basically the same as Embodiment 1, except that:

[0059] In step (2), the water is soaked for 60 seconds.

[0060] The swelling rate of the active material layer was measured and calculated to be 0.02%, and the cohesive force was measured and calculated to be 2.6510 MPa.

[0061] Example 4

[0062] This embodiment is basically the same as Embodiment 1, except that:

[0063] In step (4), the pulling speed when peeling off the active material layer is 20 mm / s.

[0064] The swelling rate of the active material layer was measured and calculated to be no more than 0.01%, and the cohesive force was measured and calculated to be 2.1208 MPa.

[0065] In this embodiment, due to the excessive pulling speed when peeling off the active material layer, on the one hand, the separation is not complete and there are defects such as serrations and burrs; on the other hand, the active material layer may be damaged by tensile force, resulting in a decrease in the subsequent tensile strength at break; both of these will result in low cohesion.

[0066] Example 5

[0067] This embodiment is basically the same as Embodiment 1, except that:

[0068] In step (5), the separated active material layer was placed in an oven and dried at normal pressure under the following conditions: 60℃ for 3 hours, to obtain the active material layer. The cohesive force was measured and calculated to be 2.6175 MPa.

[0069] In this embodiment, since the drying is carried out under normal pressure, there may be a problem that the outer part of the active material layer is dried but the inner part is not completely dried. The inner and outer parts of the active material layer cannot deform synchronously and uniformly, which causes internal stress in the active material layer, resulting in a decrease in the tensile strength at break and thus a lower cohesive force.

[0070] Example 6

[0071] This embodiment is basically the same as Embodiment 1, except that:

[0072] In step S2, the stretching speed was 10 mm / min. The measured and calculated cohesive force was 2.5004 MPa.

[0073] In this embodiment, due to excessive stretching speed, the weak links in the active material layer may be damaged prematurely, resulting in a decrease in tensile strength at break and thus a lower cohesive force.

[0074] Comparative Example 1

[0075] This comparative example is basically the same as Example 1, except that:

[0076] In step (2), the soaking time is 70 seconds.

[0077] Swelling was observed at the edge of the active material layer, indicating that the active material layer has begun to swell and water molecules no longer act solely on the interface between the active material layer and the current collector. The swelling rate of the active material layer was measured and calculated to be greater than 0.5%. The comparative example was able to separate the active material layer, but the edges of the separated active material layer were irregular, which led to a significant decrease in the measured cohesive strength value.

[0078] Comparative Example 2

[0079] This comparative example is basically the same as Example 1, except that:

[0080] In step (2), the soaking time is 120 seconds.

[0081] The active material layer was observed to be significantly swollen, with slight damage on the surface. The swelling rate of the active material layer was measured and calculated to be greater than 10%.

[0082] Comparative Example 3

[0083] This comparative example is basically the same as Example 1, except that:

[0084] In step (2), the soaking time is 300s.

[0085] It was observed that the active material layer was broken and dispersed in the water, and the active material layer could not be completely recovered.

[0086] A comparison of Examples 1-3 and Comparative Examples 1-3 reveals that this application discovers a critical window for the first time, which weakens the binding force between the active material layer and the current collector, thereby achieving separation, but is insufficient to cause substantial swelling of the active material layer. When the water immersion time exceeds 60 seconds, the swelling rate of the active material layer increases significantly and non-linearly. This swelling leads to changes in the cohesion and thickness of the separated active material layer, affecting the accuracy of the measurement results.

[0087] 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 non-destructive separation of the active material layer of a positive electrode, wherein the positive electrode comprises a current collector and an active material layer, characterized in that: Includes the following steps: The positive electrode is immersed in deionized water for a time t, after which the active material layer and the current collector are physically separated. The time t is sufficient to weaken the bonding force between the active material layer and the current collector, thereby achieving separation, but not sufficient to cause substantial swelling of the active material layer; The time t does not exceed 60s, and the swelling rate of the separated active material layer does not exceed 0.1%.

2. The method for non-destructive separation of the active material layer of a positive electrode sheet according to claim 1, characterized in that: The time t shall not exceed 2s, or the swelling rate of the separated active material layer shall not exceed 0.02%.

3. The method for non-destructive separation of the active material layer of a positive electrode sheet according to claim 1, characterized in that: The positive electrode uses lithium iron phosphate as the active material and PVDF as the binder.

4. A method for non-destructive separation of the active material layer of a positive electrode sheet according to claim 1 or 3, characterized in that: It also includes the following steps: The separated active material layer is dried at 60~120℃ and vacuum degree ≤-0.08MPa for at least 0.5h.

5. A method for non-destructive separation of the active material layer of a positive electrode sheet according to claim 1 or 3, characterized in that: After immersion in water, the positive electrode sheet is taken out vertically and left to stand vertically for 3.0~7.0s, and then the active material layer and the current collector are separated.

6. A method for non-destructive separation of the active material layer of a positive electrode sheet according to claim 1 or 3, characterized in that: The active material layer is pulled parallel to each other at a speed of 180° and not exceeding 10 mm / s to separate the active material layer from the current collector.

7. A method for measuring the cohesive force within the active material layer of a positive electrode, characterized in that: The active material layer obtained by the non-destructive separation method as described in any one of claims 1 to 6 is used as the measurement object.

8. The method for measuring the cohesive force within the active material layer of a positive electrode according to claim 7, characterized in that: Includes the following steps: S1. Measure the thickness of the active material layer and record it as H; S2. The active material layer is stretched and the force at which it breaks is measured and denoted as F; S3. The cohesive force of the active material layer is σ = F / (W×H); where W is the width of the active material layer.

9. The method for measuring the cohesive force within the active material layer of a positive electrode according to claim 8, characterized in that: In step S2, the stretching speed is 4.5~5.5 mm / min.

10. A method for measuring the cohesive force within the active material layer of a positive electrode according to claim 8, characterized in that: In steps S1 and S2, at least three sets of data are measured and the average value is taken.

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