Judgment method of positive plate coating pit

Abstract

The invention relates to the technical field of batteries, and discloses a positive plate coating pit judgment method which comprises the following steps: judging the defect type of a coating pit on the basis of characteristic parameters of the coating pit on a positive plate; preparing a positive plate sample; assembling each positive plate sample, the corresponding diaphragm and the corresponding negative plate to form a pole group, assembling the pole group to form a battery cell, and performing charge and discharge test on the battery cell; disassembling the tested battery cell, detecting whether a dotted lithium precipitation defect appears on a negative plate interface corresponding to the position of the coating pit, and establishing a corresponding relation between the size of each coating pit and the dotted lithium precipitation defect; according to the corresponding relation between the sizes of the various coating pits and the dot-shaped lithium precipitation defects, determining the qualification standards of the various coating pits; and judging whether the positive plate is qualified based on the qualification standard. According to the method for judging the coating pits of the positive plate, the coating pits on the positive plate are classified, the qualified standard is relaxed, and the waste of the positive plate is reduced, so that the use cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a method for determining the pits in the coating of a positive electrode sheet. Background Technology

[0002] In the manufacturing process of lithium-ion batteries, the coating process of the positive and negative electrodes is a key step that determines the battery's performance and safety. During the coating process, due to factors such as the properties of the slurry, equipment conditions, and environmental control, circular pits are easily formed on the coating surface of the positive electrode, which are collectively referred to in the industry as "coating pits".

[0003] Coating pits on the positive electrode sheet result in a relatively thin negative electrode coating at the corresponding location after rolling, leading to insufficient negative electrode active material in that localized area. This reduces the number of lithium-ion insertion sites during charging, making it easier for lithium plating to occur on the negative electrode surface. The growth of lithium dendrites may puncture the separator, causing an internal short circuit and triggering thermal runaway, seriously jeopardizing the battery's safety performance.

[0004] In existing technologies, the presence of coating pits is generally determined based on their appearance (such as black spots or bubble defects). Furthermore, when coating pits are present on the positive electrode, the electrode is deemed defective, or only coating pits with extremely small diameters (e.g., ≤1 mm) are allowed as acceptable products. This results in a large number of coating pits that do not substantially harm the cell's performance and safety being judged as defective, leading to electrode waste and increased manufacturing costs. Summary of the Invention

[0005] This invention provides a method for determining the coating pits of positive electrode sheets, which solves the problem that existing methods have vague criteria for determining coating pit defects of positive electrode sheets, which can easily lead to electrode waste and high manufacturing costs.

[0006] This invention provides a method for determining the coating pits on a positive electrode, comprising:

[0007] Based on the characteristic parameters of the pits coated on the positive electrode sheet, the defect type of the pits is determined. Prepare a positive electrode sample comprising a variety of the aforementioned coating pits, wherein each of the aforementioned positive electrode samples comprises at least one of the aforementioned coating pits, and each of the aforementioned coating pits has multiple sizes; Each of the positive electrode samples is sequentially stacked and assembled with the corresponding separator and negative electrode to form an electrode group, and the electrode group is assembled to form a battery cell. The battery cell is then subjected to a charge-discharge test. After disassembling and testing the battery cell, check whether there are point-like lithium plating defects at the interface of the negative electrode corresponding to the position of the coating pit, and establish the correspondence between the size of various coating pits and point-like lithium plating defects. Based on the correspondence between the size of various coating pits and point-like lithium plating defects, the qualification standards for various coating pits are determined. Based on the aforementioned qualification criteria, it is determined whether the positive electrode sheet with coating pits is qualified.

[0008] Beneficial Effects: The method for determining coating pits on the positive electrode sheet of this invention classifies the coating pits into defects based on their characteristic parameters. This allows for the development of improvement measures for various coating pits based on their formation mechanisms and morphologies, and provides a theoretical basis for subsequent point-like lithiation analysis of the cell interface. Based on the correspondence between the size of various coating pits and point-like lithium deposition defects, acceptance criteria for various coating pits are established. Compared to traditional determination methods, this invention reasonably relaxes the acceptance criteria, avoiding the identification of a large number of coating pits that do not substantially harm the performance and safety of the cell as defective products, thus preventing electrode waste and significantly reducing manufacturing costs.

[0009] In one optional implementation, determining the defect type of the coating pits based on characteristic parameters of the coating pits on the positive electrode sheet includes: The element types in the pits on the positive electrode are detected by an energy dispersive spectroscopy (EDS) instrument. If foreign elements are detected in the pits, they are identified as foreign object pit defects.

[0010] Beneficial effects: Based on the elemental detection results of the energy dispersive spectrometer, foreign object pit defects can be accurately identified and distinguished from other pits with similar morphology. This allows for the development of improvement measures for foreign object pit defects, such as maintaining the cleanliness of the production environment and using multiple filtration stages during the production process to filter out foreign objects, thereby improving the production qualification rate of positive electrode sheets.

[0011] In one optional implementation, determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium plating defects includes: If the coating pit is a foreign object pit defect, the acceptance standard is that the diameter D1 < 5 mm.

[0012] Beneficial effects: When the coating pits are foreign object pit defects, the qualified standard is determined by the correspondence between the size of the foreign object pit defects and the point-like lithium plating defects. That is, the positive electrode sheet with foreign object pit defects with a diameter of D1 < 5 mm is judged as a qualified product, thus reducing the waste of such positive electrode sheets.

[0013] In one optional implementation, determining the defect type of the coating pits based on characteristic parameters of the coating pits on the positive electrode sheet includes: The element types in the pits on the positive electrode were detected by energy dispersive spectroscopy, and the edge thickness T1 of the pits and the normal area thickness T2 of the positive electrode were measured by scanning electron microscopy. If the edge thickness T1 of the coating pit is 3 μm to 5 μm greater than the normal area thickness T2 of the positive electrode sheet, and no foreign elements are detected at the coating pit, it is judged to be a bubble pit defect.

[0014] Beneficial effects: By increasing the edge thickness of the coating pit by 3 μm to 5 μm and ensuring the absence of foreign elements, bubble pit defects can be accurately identified and distinguished from foreign object pit defects. This allows for the development of improvement measures for bubble pit defects, such as defoaming the slurry, thereby further improving the production qualification rate of positive electrode sheets.

[0015] In one optional implementation, determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium plating defects includes: If the coating pit is the bubble pit defect, the acceptance standard is diameter D2 < 5 mm.

[0016] Beneficial effects: When the coating pits are bubble pit defects, the qualified standard is determined by the correspondence between the size of the bubble pit defects and the point-like lithium plating defects. That is, the positive electrode sheet with bubble pit defects with a diameter of D2 < 5 mm is judged as a qualified product, so as to reduce the overkill ratio and reduce the waste of such defective positive electrode sheets.

[0017] In one optional implementation, determining the defect type of the coating pits based on characteristic parameters of the coating pits on the positive electrode sheet includes: The diameter of the pits on the positive electrode is measured using a scanning electron microscope. If the diameter D3 of the pit is less than or equal to 2 mm, it is determined to be a colloid bubble pit defect.

[0018] Beneficial effects: When there are undispersed micelles in the adhesive solution, the micelles occupy the position of the slurry. Due to the high solvent content of the micelles, there is little residue after drying. Therefore, the position of the micelles is prone to forming coating pits. The appearance is a circular deep pit with clear edges. The diameter D3 is generally ≤2 mm. Therefore, by directly measuring the diameter of the coating pits with a scanning electron microscope, the defects of micelles, bubbles and pits can be quickly and accurately identified.

[0019] In one optional implementation, determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium plating defects includes: If the coating pits are defects caused by the colloidal bubbles, then the positive electrode sheet is determined to be a qualified product.

[0020] Beneficial effects: The areas with micelle bubble pit defects are not significantly different from normal areas in terms of morphology and elemental composition, and have almost no impact on the interface safety of the cell. They do not cause point-like lithium plating defects. Therefore, they can be directly qualified products without the need to formulate improvement measures, thereby avoiding the positive electrode sheet with micelle bubble pit defects from being judged as defective products.

[0021] In one optional implementation, determining the defect type of the coating pits based on characteristic parameters of the coating pits on the positive electrode sheet includes: The shape of the coating pits on the positive electrode sheet is obtained using a scanning electron microscope. If the coating pits are found to be annular, they are identified as gel shrinkage pit defects.

[0022] Beneficial effects: If the slurry is not dispersed and agglomerated particles are formed (polar macromolecular binders crosslink with small lithium iron phosphate particles to form a gel), then particles with low surface tension will exist in the flowable film surface before the slurry solidifies. This will easily result in a lower surface tension in the center, and the fluid will migrate outward from the particle center, eventually forming a circular depression with the edge higher than the center, i.e., multi-ring pits. By using a scanning electron microscope to obtain the shape of the coating pits as rings, the gel shrinkage pit defect can be accurately identified. Subsequently, it is necessary to improve the dispersion effect of the slurry to avoid the occurrence of such defects.

[0023] In one optional implementation, determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium plating defects includes: If the coating pit is a gel shrinkage pit defect, the acceptance standard is a diameter D4 < 5 mm.

[0024] Beneficial effects: When the coating pits are gel shrinkage pit defects, the qualified standard is determined by the correspondence between the size of the gel shrinkage pit defects and the point-like lithium plating defects. That is, the positive electrode sheet with gel shrinkage pit defects with a diameter of D4 < 5 mm is judged as a qualified product, so as to reduce the overkill ratio and reduce the waste of such defective positive electrode sheets.

[0025] In one optional implementation, after disassembly and testing, the negative electrode interface corresponding to the location of the coating pit is checked for point-like lithium plating defects, and a correspondence between the size of various coating pits and point-like lithium plating defects is established, including: After disassembly and testing, the morphology, number and size of lithium deposition points at the negative electrode interface corresponding to the location of the coating pits were obtained using a scanning electron microscope. Based on the morphology, number, and size of lithium deposits at the negative electrode interface, determine whether point-like lithium deposition defects occur, and establish the correspondence between the size of various coating pits and point-like lithium deposition defects.

[0026] Beneficial effects: By analyzing the morphology, quantity, and size of lithium deposits at the negative electrode interface using scanning electron microscopy, precise data support is provided for establishing the correspondence between the size of various coating pits and point-like lithium deposit defects. Based on this correspondence, the size relaxation standard for coating pits can be accurately determined, thereby reducing electrode waste and lowering usage costs while ensuring cell safety. Attached Figure Description

[0027] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0028] Figure 1 This is a flowchart illustrating a method for determining the coating pits of a positive electrode sheet according to an embodiment of the present invention. Figure 2 This is a sample image of a foreign object pit defect according to an embodiment of the present invention; Figure 3 This is a sample image of a bubble pit defect according to an embodiment of the present invention; Figure 4 This is a sample image of a gum bubble pit defect according to an embodiment of the present invention; Figure 5 This is a sample image of a gel shrinkage pit defect according to an embodiment of the present invention; Figure 6 This is a schematic diagram of point-like lithium plating defects according to an embodiment of the present invention.

[0029] Explanation of reference numerals in the attached figures: 1. Positive electrode; 2. Coated pits; 3. Negative electrode. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] The following is combined Figures 1 to 5 The following describes embodiments of the present invention.

[0032] According to embodiments of the present invention, such as Figure 1 As shown, a method for determining the pits in the coating of a positive electrode is provided, including: S100. Based on the characteristic parameters of the pits 2 coated on the positive electrode 1, determine the defect type of the pits 2.

[0033] S200. Prepare a positive electrode sample containing multiple coating pits 2, wherein each positive electrode sample contains at least one type of coating pit 2, and each type of coating pit 2 has multiple sizes.

[0034] S300. Each positive electrode sample is sequentially stacked and assembled with the corresponding separator and negative electrode 3 to form an electrode group, and the electrode group is assembled to form a battery cell. A charge and discharge test is performed on the battery cell.

[0035] S400, after disassembling and testing the battery cell, check whether there are point-like lithium plating defects on the interface of the negative electrode 3 corresponding to the position of the coating pit 2, and establish the correspondence between the size of various coating pits 2 and point-like lithium plating defects.

[0036] S500. Based on the correspondence between the size of various coating pits 2 and the point-like lithium plating defects, determine the qualification standards for various coating pits 2.

[0037] S600. Based on the qualification standard, determine whether the positive electrode 1 with coating pits 2 is qualified.

[0038] Therefore, the method for determining coating pits on the positive electrode sheet provided in this embodiment of the invention classifies the coating pits 2 as defects based on the characteristic parameters of the coating pits 2 on the positive electrode sheet 1. This allows for the formulation of improvement measures for the coating pits 2 based on their formation mechanisms and morphologies, and provides a theoretical basis for subsequent point-like lithiation analysis of the cell interface. Based on the correspondence between the size of various coating pits 2 and point-like lithium deposition defects, acceptance criteria for various coating pits 2 are established. Compared to traditional determination methods, this embodiment of the invention reasonably relaxes the acceptance criteria, which can avoid classifying a large number of coating pits 2 that do not substantially harm the performance and safety of the cell as defective products, thus avoiding electrode waste and significantly reducing manufacturing costs.

[0039] Specifically, the types of coating pits 2 mainly include foreign matter pit defects, bubble pit defects, and gel pit defects, among which gel pit defects include micelle bubble pit defects and gel shrinkage pit defects. In step S200, multiple sizes of positive electrode samples of each type of coating pit 2 need to be prepared in order to subsequently determine the qualification criteria for each type of coating pit 2. Each positive electrode sample may have one type of coating pit 2 or multiple types of coating pits 2.

[0040] The positive electrode sheet 1 includes a positive current collector and a positive active material. The positive active material, also known as a slurry, is formed on the surface of the positive current collector. The positive active material includes positive active materials. The coating process refers to uniformly coating the slurry onto the positive current collector, followed by drying, rolling, slitting, and other processes to form the positive electrode sheet 1.

[0041] The positive electrode active material may include at least one of the following materials: lithium phosphate, lithium transition metal oxide, and their respective modified compounds. However, the embodiments of the present invention are not limited to these materials, and other conventional materials that can be used as positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium phosphate include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, lithium iron manganese phosphate and carbon composites, and nickel-cobalt-manganese ternary materials and their derivatives may also be used.

[0042] The positive current collector can be made of metal foil. For example, the metal foil can be made of silver-treated aluminum or stainless steel, copper, aluminum, nickel, carbon electrodes, carbon, nickel or titanium, etc.

[0043] Positive electrode 1 and negative electrode 3 are stacked alternately to form an electrode group, and a separator is provided between positive electrode 1 and negative electrode 3.

[0044] The negative electrode 3 includes a negative electrode current collector and a negative electrode active material. The negative electrode current collector can be a metal foil. For example, the metal foil can be silver-treated aluminum or stainless steel, stainless steel, copper, aluminum, nickel, carbon electrodes, or carbon, nickel, or titanium. The negative electrode active material can be graphite, silicon, or a mixture of graphite and silicon.

[0045] The diaphragm can be made of polypropylene (PP), polyethylene (PE), or composite materials.

[0046] It should be noted that, as Figure 6 As shown, the central area of ​​the coating pit 2 of the positive electrode 1 is thinner than the normal area, while the edge of the pit is thicker than the normal area. Therefore, there is more positive electrode material at the edge of the pit, providing more lithium ions. However, the lithium insertion sites of the negative electrode 3 at the corresponding position are limited, resulting in a local N / P ratio (the capacity of lithium ions that can be inserted and removed from the negative electrode 3 / the capacity of lithium ions that can be inserted and removed from the positive electrode 1) being insufficient compared to the normal area. During charging, when lithium ions extracted from the positive electrode 1 are inserted into the surface of the negative electrode 3, the negative electrode 3 cannot provide enough lithium insertion sites, causing excess lithium ions to be deposited on the surface of the negative electrode, resulting in point-like lithium deposition defects.

[0047] Since the coating pits 2 of the positive electrode 1 are usually circular, the qualified standard can be prepared by the size of their diameter.

[0048] In one embodiment, step S100, based on the characteristic parameters of the coating pits 2 on the positive electrode 1, determines the defect type of the coating pits 2, including: S110. Use an energy dispersive spectroscopy (EDS) instrument to detect the elemental composition of the pits 2 on the positive electrode 1. If foreign elements are detected in the pits 2, it is determined to be a foreign object pit defect. Foreign elements refer to elements that are different from those in the normal area of ​​the positive electrode 1. The energy dispersive spectroscopy (EDS) instrument is used to analyze the elemental composition and content of micro-areas of the material, and is used in conjunction with scanning electron microscopy and transmission electron microscopy.

[0049] According to the element detection results of the energy dispersive spectrometer, the foreign object pit defect can be accurately identified and distinguished from other pits with similar morphology, so as to formulate improvement measures for the foreign object pit defect. For example, maintaining the cleanliness of the production environment and using multiple filters to filter foreign objects during the production process can improve the production qualification rate of the positive electrode 1.

[0050] Specifically, improvements are made to address the foreign object pit defect by focusing on two aspects: preventing its formation and preventing its outflow.

[0051] Prevention of contamination: The cleanliness standard for the production environment is ≤100,000 particles with a diameter ≥0.5 μm per cubic foot of air. Production equipment must be cleaned and sampled for testing before use. Production can only proceed after the test results are qualified.

[0052] Preventing overflow: Multiple filter cartridges are used from the homogenization to the coating process to remove foreign matter.

[0053] like Figure 2 As shown, if the slurry is contaminated by a foreign object with low surface tension, the slurry around the foreign object will flow towards the direction with higher surface tension. Compared to bubble pit defects, foreign object pit defects are shallower, and their edge thickness is not much different from that of the normal area. The diameter of foreign object pit defects is generally ≥3 mm. Under the characterization of energy dispersive spectroscopy, foreign objects can be clearly observed or obvious foreign elements can be identified.

[0054] In traditional judgment methods, foreign object pit defects are usually not allowed. That is, positive electrode 1 with foreign object pit defects will be directly judged as a defective product.

[0055] Further, in one embodiment, in step S500, the qualification criteria for various coating pits 2 are determined based on the correspondence between the size of various coating pits 2 and the point-like lithium plating defects, including: S510. If the coating pit 2 is a foreign object pit defect, the acceptance standard is that the diameter D1 < 5 mm.

[0056] like Figure 2As shown, when the coating pit 2 is a foreign matter pit defect, the qualified standard is determined by the correspondence between the size of the foreign matter pit defect and the point-like lithium plating defect. That is, the positive electrode sheet 1 with foreign matter pit defects with a diameter of D1 < 5 mm is judged as a qualified product, thereby reducing the waste of such positive electrode sheets 1.

[0057] In one embodiment, step S100, based on the characteristic parameters of the coating pits 2 on the positive electrode 1, determines the defect type of the coating pits 2, including: S120. Use an energy dispersive spectroscopy (EDS) instrument to detect the element types at the pits 2 on the positive electrode 1. Use a scanning electron microscope (SEM) to measure the edge thickness T1 of the pits 2 and the normal area thickness T2 of the positive electrode 1.

[0058] If the edge thickness T1 of the coating pit 2 is 3 μm to 5 μm greater than the normal area thickness T2 of the positive electrode 1, and no foreign elements are detected at the coating pit 2, then it is judged to be a bubble pit defect.

[0059] By increasing the edge thickness of the coating pit 2 by 3 μm to 5 μm and ensuring the absence of foreign elements, bubble pit defects can be accurately identified and distinguished from foreign object pit defects. This allows for the development of improvement measures for bubble pit defects, such as defoaming the slurry, thereby further improving the production qualification rate of the positive electrode sheet 1.

[0060] like Figure 3 and Figure 6 As shown, if the slurry contains air bubbles, meaning the air bubbles occupy the slurry's position, small air bubbles will float to the surface after drying, or large air bubbles will burst open when the material exits through the lip, forming air bubble pit defects. The morphology of the air bubble pit defects is volcano-like, meaning the edge thickness T1 is 3 μm to 5 μm greater than the normal area thickness T2 of the positive electrode 1, and the diameter is generally greater than or equal to 3 mm.

[0061] To address air bubble and pit defects, a reverse defoaming process is performed after homogenization and before discharge. The slurry undergoes vacuum defoaming in the transfer tank to ensure pressure maintenance and prevent air leakage. The slurry is circulated through the coating die for at least 30 minutes to further remove air bubbles.

[0062] In one embodiment, in step S500, the acceptance criteria for various coating pits 2 are determined based on the correspondence between the sizes of various coating pits 2 and point-like lithium plating defects, including: S520. If the coating pit 2 is a bubble pit defect, the acceptance standard is that the diameter D2 < 5 mm.

[0063] like Figure 3As shown, when the coating pit 2 is a bubble pit defect, the qualified standard is determined by the correspondence between the size of the bubble pit defect and the point-like lithium plating defect. That is, the positive electrode sheet 1 with bubble pit defects with a diameter of D2 < 5 mm is judged as a qualified product, so as to reduce the overkill ratio and reduce the waste of such defective positive electrode sheets 1.

[0064] In one embodiment, step S100, based on the characteristic parameters of the coating pits 2 on the positive electrode 1, determines the defect type of the coating pits 2, including: S130. Measure the diameter of the pit 2 on the positive electrode 1 using a scanning electron microscope. If the diameter D3 of the pit 2 is ≤ 2 mm, it is determined to be a colloid bubble pit defect.

[0065] like Figure 4 As shown, when there are undispersed micelles in the adhesive solution, the micelles occupy the slurry position. Due to the high solvent content of the micelles, there is little residue after drying. Therefore, the position of the micelles is prone to forming coating pits 2. The appearance is a circular deep pit with clear edges and a diameter of D3≤2 mm. Therefore, by directly measuring the diameter of the coating pits 2 of the positive electrode 1 using a scanning electron microscope, the defects of the micelle bubbles and pits can be quickly and accurately identified.

[0066] Further, in one embodiment, in step S500, the qualification criteria for various coating pits 2 are determined based on the correspondence between the size of various coating pits 2 and the point-like lithium plating defects, including: S530. If the coating pit 2 is a defective particle bubble pit, then the positive electrode sheet 1 is judged to be a qualified product.

[0067] The morphology and elemental composition of the areas with micellar bubble pit defects are not significantly different from those of normal areas, and they hardly affect the interface safety of the cell and do not cause point-like lithium plating defects. Therefore, they can be directly relaxed to qualified products without the need to formulate improvement measures, thereby avoiding the positive electrode sheet 1 with micellar bubble pit defects from being judged as a defective product.

[0068] In one embodiment, step S100, based on the characteristic parameters of the coating pits 2 on the positive electrode 1, determines the defect type of the coating pits 2, including: S140. Use a scanning electron microscope to obtain the shape of the coating pit 2 on the positive electrode 1. If the coating pit 2 is found to be annular, it is judged to be a gel shrinkage pit defect.

[0069] If the slurry is not dispersed and agglomerated particles are formed (polar macromolecular binders crosslink with small lithium iron phosphate particles to form a gel), then particles with low surface tension will exist in the flowable film surface before the slurry solidifies. This will easily result in a lower surface tension in the center, and the fluid will migrate outward from the particle center, eventually forming a circular depression with the edge higher than the center, i.e., a multi-ring pit. By using a scanning electron microscope to obtain the shape of the coating pit 2 as a ring, the gel shrinkage pit defect can be accurately identified. Subsequently, it is necessary to improve the dispersion effect of the slurry to avoid the occurrence of such defects.

[0070] In one embodiment, in step S500, the acceptance criteria for various coating pits 2 are determined based on the correspondence between the sizes of various coating pits 2 and point-like lithium plating defects, including: S540. If the coating pit 2 is a gel shrinkage pit defect, the acceptance standard is that the diameter D4 < 5 mm.

[0071] like Figure 5 As shown, when the coating pit 2 is a gel shrinkage pit defect, the qualified standard is determined by the correspondence between the size of the gel shrinkage pit defect and the point-like lithium plating defect. That is, the positive electrode sheet 1 with a diameter D4 < 5 mm is judged as a qualified product, so as to reduce the overkill ratio and reduce the waste of such defective positive electrode sheets 1.

[0072] It should be noted that both micelle bubble pits and gel crater pits can be considered gel pit defects, primarily related to poor dispersion of the adhesive and slurry. These two defects can be improved by increasing the dispersion speed and dispersion time; for example, increasing the dispersion speed from 18 m / s to 20 m / s and the dispersion time from 60 min to 90 min. The dispersion effect can also be improved by increasing the slurry kneading solids content (tailored to each material) and increasing shear force. Furthermore, multiple filter cartridges can be used during the homogenization and coating processes to remove agglomerates.

[0073] Traditional coating pits are identified and removed during the die-cutting process, primarily as black spots and air bubbles. Air bubble pits, gel pits, and foreign object pits occur during the coating process. After the rolling process, the raised parts of the pits are flattened, appearing as black spots or air bubbles. The standard for coating pits is: black spot / air bubble diameter ≤ 3 mm, quantity ≤ 2 per sheet.

[0074] This invention relaxes the acceptance criteria for foreign matter pit defects, bubble pit defects, and gel pit defects to a diameter of less than 5 mm, with no quantity limit, which can significantly reduce electrode waste and thus lower manufacturing costs. Furthermore, by measuring the diameter of the coating pit 2 using a microscope and comparing it with the acceptance criteria, the quality of the positive electrode 1 can be determined. The detection method is simple, convenient to use, and easy to promote.

[0075] In one embodiment, in step S400, the tested battery cell is disassembled, and the interface of the negative electrode 3 corresponding to the location of the coating pit 2 is checked for point-like lithium plating defects. A correspondence is established between the size of various coating pits 2 and point-like lithium plating defects, including: S410. After disassembling and testing the battery cell, the morphology, number and size of lithium deposits at the interface of the negative electrode 3 corresponding to the position of the coating pit 2 are obtained using a scanning electron microscope.

[0076] S420. Based on the morphology, quantity, and size of lithium deposits at the interface of the negative electrode 3, determine whether point-like lithium deposit defects have occurred, and establish the correspondence between the size of various coating pits 2 and point-like lithium deposit defects.

[0077] Specifically, the normal area of ​​the negative electrode 3 interface is usually smooth and golden under a scanning electron microscope. If the scanning electron microscope detects more than four grayish-white or silvery-white metal spots with a diameter greater than 1 mm per 100 μm² on the negative electrode 3 interface, it is determined to be a point-like lithium deposition defect.

[0078] Based on the morphology, number, and size of lithium deposits at the interface of the negative electrode 3 detected by scanning electron microscopy, precise data support was provided for establishing the correspondence between the size of various coating pits 2 and point-like lithium deposit defects. According to the correspondence between the size of various coating pits 2 and point-like lithium deposit defects, the size relaxation standard for coating pits 2 can be accurately obtained, thereby reducing electrode waste and lowering usage costs while ensuring cell safety.

[0079] In one embodiment, step S600, determining whether the positive electrode 1 with the coating pits 2 is qualified based on the acceptance criteria, includes: S601. If the diameter of the coating pit 2 is < 5 mm, the positive electrode 1 is judged to be a qualified product; otherwise, it is a defective product.

[0080] The following detailed description of the battery cell of the present invention, in conjunction with specific embodiments, is intended to limit the scope of protection claimed by the present invention.

[0081] Example 1: The diameter of the pits 2 on the positive electrode 1 was measured to be 1 mm using a scanning electron microscope, indicating that the positive electrode 1 is a qualified product.

[0082] The positive electrode 1 is sequentially stacked and assembled with the corresponding separator and negative electrode 3 to form an electrode group, and the electrode group is assembled to form a battery cell. The battery cell is then subjected to a charge-discharge test.

[0083] After disassembling and testing the battery cell, no point-like lithium plating defects were found at the interface of the negative electrode 3 corresponding to the location of the coating pit 2.

[0084] Example 2: The diameter of the pits 2 on the positive electrode 1 was measured to be 4 mm using a scanning electron microscope, indicating that the positive electrode 1 is a qualified product.

[0085] The positive electrode 1 is sequentially stacked and assembled with the corresponding separator and negative electrode 3 to form an electrode group, and the electrode group is assembled to form a battery cell. The battery cell is then subjected to a charge-discharge test.

[0086] After disassembling and testing the battery cell, no point-like lithium plating defects were found at the interface of the negative electrode 3 corresponding to the location of the coating pit 2.

[0087] Comparative Example 1: The diameter of the pit 2 on the positive electrode 1 was measured to be 5 mm using a scanning electron microscope, indicating that the positive electrode 1 was a defective product.

[0088] The positive electrode 1 is sequentially stacked and assembled with the corresponding separator and negative electrode 3 to form an electrode group, and the electrode group is assembled to form a battery cell. The battery cell is then subjected to a charge-discharge test.

[0089] After disassembling and testing the battery cell, point-like lithium plating defects were found at the interface of the negative electrode 3 corresponding to the location of the coating pit 2.

[0090] As can be seen, in Examples 1 and 2, the diameter of the coating pits 2 is less than 5 mm, which meets the qualification standard and no point lithium plating defects are observed. In Comparative Example 1, the diameter of the coating pits 2 is 5 mm, which does not meet the qualification standard and point lithium plating defects are observed.

[0091] Charge and discharge test: At room temperature, the battery cell is charged and discharged once at a current of 0.5 C (i.e., the current value at which the theoretical capacity is completely discharged within 2 hours). The charging is constant current and constant voltage charging, with a termination voltage of 4.2 V, a cutoff current of 0.05 C, and a discharge termination voltage of 2.8 V.

[0092] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for determining pits in a positive electrode coating, characterized in that, include: Based on the characteristic parameters of the pits coated on the positive electrode sheet, the defect type of the pits is determined. Prepare a positive electrode sample comprising a variety of the aforementioned coating pits, wherein each of the aforementioned positive electrode samples comprises at least one of the aforementioned coating pits, and each of the aforementioned coating pits has multiple sizes; Each of the positive electrode samples is sequentially stacked and assembled with the corresponding separator and negative electrode to form an electrode group, and the electrode group is assembled to form a battery cell. The battery cell is then subjected to a charge-discharge test. After disassembling and testing the battery cell, check whether there are point-like lithium plating defects at the interface of the negative electrode corresponding to the position of the coating pit, and establish the correspondence between the size of various coating pits and point-like lithium plating defects. Based on the correspondence between the size of various coating pits and point-like lithium plating defects, the qualification standards for various coating pits are determined. Based on the aforementioned qualification criteria, it is determined whether the positive electrode sheet with coating pits is qualified.

2. The method for determining the coating pits on the positive electrode sheet according to claim 1, characterized in that, The method of determining the defect type of the coating pits based on the characteristic parameters of the pits on the positive electrode sheet includes: The element types in the pits on the positive electrode are detected by an energy dispersive spectroscopy (EDS) instrument. If foreign elements are detected in the pits, they are identified as foreign object pit defects.

3. The method for determining the coating pits on the positive electrode sheet according to claim 2, characterized in that, The method for determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium deposition defects includes: If the coating pit is a foreign object pit defect, the acceptance standard is that the diameter D1 < 5 mm.

4. The method for determining the coating pits on the positive electrode sheet according to claim 1, characterized in that, The method of determining the defect type of the coating pits based on the characteristic parameters of the pits on the positive electrode sheet includes: The element types in the pits on the positive electrode were detected by energy dispersive spectroscopy, and the edge thickness T1 of the pits and the normal area thickness T2 of the positive electrode were measured by scanning electron microscopy. If the edge thickness T1 of the coating pit is 3 μm to 5 μm greater than the normal area thickness T2 of the positive electrode sheet, and no foreign elements are detected at the coating pit, it is judged to be a bubble pit defect.

5. The method for determining the coating pits on the positive electrode sheet according to claim 4, characterized in that, The method for determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium deposition defects includes: If the coating pit is the bubble pit defect, the acceptance standard is diameter D2 < 5 mm.

6. The method for determining the coating pits on the positive electrode sheet according to claim 1, characterized in that, The method of determining the defect type of the coating pits based on the characteristic parameters of the pits on the positive electrode sheet includes: The diameter of the pits on the positive electrode is measured using a scanning electron microscope. If the diameter D3 of the pit is less than or equal to 2 mm, it is determined to be a colloid bubble pit defect.

7. The method for determining the coating pits on the positive electrode sheet according to claim 6, characterized in that, The method for determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium deposition defects includes: If the coating pits are defects caused by the colloidal bubbles, then the positive electrode sheet is determined to be a qualified product.

8. The method for determining the coating pits on the positive electrode sheet according to claim 1, characterized in that, The method of determining the defect type of the coating pits based on the characteristic parameters of the pits on the positive electrode sheet includes: The shape of the coating pits on the positive electrode sheet is obtained using a scanning electron microscope. If the coating pits are found to be annular, they are identified as gel shrinkage pit defects.

9. The method for determining the coating pits of the positive electrode sheet according to claim 8, characterized in that, The method for determining the acceptance criteria for various coating pits based on the correspondence between the size of various coating pits and point-like lithium deposition defects includes: If the coating pit is a gel shrinkage pit defect, the acceptance standard is a diameter D4 < 5 mm.

10. The method for determining the coating pits on the positive electrode sheet according to claim 1, characterized in that, After disassembly and testing, the battery cell is inspected for point-like lithium plating defects at the interface of the negative electrode corresponding to the location of the coating pits. A correspondence is established between the size of various coating pits and point-like lithium plating defects, including: After disassembly and testing, the morphology, number and size of lithium deposition points at the negative electrode interface corresponding to the location of the coating pits were obtained using a scanning electron microscope. Based on the morphology, number, and size of lithium deposits at the negative electrode interface, determine whether point-like lithium deposition defects occur, and establish the correspondence between the size of various coating pits and point-like lithium deposition defects.

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