Diaphragm, preparation method thereof and battery

Abstract

The invention provides a diaphragm, a preparation method thereof and a battery, and particularly relates to the technical field of batteries. The diaphragm comprises a porous base material and a gluing layer arranged on at least one side of the porous base material; the gluing layer comprises a first adhesive and a second adhesive, and the softening point of the second adhesive is smaller than that of the first adhesive; the distribution weight ratio of the same elements in the second adhesive on the surface layer of the gluing layer is greater than or equal to the distribution weight ratio of the same elements in the inner layer, and the difference value between the distribution weight ratio and the distribution weight ratio is 0-10%; and in any 1 square centimeter area of the diaphragm, the area of the first adhesive accounts for 5-30% of the area of the porous base material. According to the diaphragm disclosed by the invention, the bonding strength between the diaphragm and the pole piece can be reasonably controlled, enough bonding strength can be ensured, bad risks such as naked battery cell opening caused by too weak bonding strength can be effectively avoided, a proper pole piece rebound space can be reserved, and the reliability of the diaphragm is improved. And the problems of winding core interface wrinkles, large impedance and the like caused by too strong bonding strength are prevented.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a separator, its preparation method, and a battery. Background Technology

[0002] With the rapid development of new energy technologies, lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, and large-scale energy storage systems due to their advantages such as high energy density, high operating voltage, long cycle life, and environmental friendliness. The separator, as one of the key materials in lithium-ion batteries, allows lithium ions to pass through and plays a crucial role in isolating the positive and negative electrode materials and preventing short circuits, thus ensuring the safety of lithium-ion batteries.

[0003] Currently, commonly used separators mainly consist of polypropylene (PP) and polyethylene (PE) base films or coatings of base films. The coatings are primarily divided into two categories: ceramic coatings (ceramic separators) and polymer adhesive layers (adhesive separators). Compared to ceramic separators, adhesive separators not only improve the adhesion between the positive electrode and the separator but also allow space for electrode rebound. This effectively mitigates the problems of electrode wrinkling and localized thickening of the cell caused by negative electrode expansion during charging and discharging of lithium-ion batteries, improves interface flatness, and significantly optimizes the molding performance of bare cells. Therefore, the ability of the adhesive layer to bond the separator to the positive electrode is crucial.

[0004] However, existing coated separators cannot precisely control the adhesion strength between the separator and the positive electrode: if the adhesion is too weak, it can easily lead to openings in the bare cell after hot pressing, resulting in difficulties in casing, tab redundancy, and other adverse risks, making it impossible to fully achieve the purpose of hot pressing shaping; but if the adhesion is too strong, there is insufficient space for the electrode to rebound, and the core is prone to interface wrinkles, which in turn affects the overall performance of the lithium-ion battery. Therefore, there is an urgent need to provide a new type of separator to improve the above problems. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the present invention provides a separator, a method for preparing the separator and a battery, so as to improve the technical problem that the adhesion force between the separator and the positive electrode cannot be reasonably controlled.

[0006] To achieve the above and other related objectives, the present invention provides a diaphragm comprising: a porous substrate and an adhesive layer, the adhesive layer being disposed on at least one side of the porous substrate; the adhesive layer comprising a first adhesive and a second adhesive, wherein the softening point of the second adhesive is lower than that of the first adhesive; the weight ratio of the same element in the second adhesive distributed on the surface layer of the adhesive layer is greater than or equal to the weight ratio distributed in the inner layer, and the difference between the two is 0% to 10%; wherein the surface layer is the portion away from the porous substrate along the thickness direction of the adhesive layer, the inner layer is the portion close to the porous substrate along the thickness direction of the adhesive layer, and the thickness of the surface layer accounts for 1 / 2 of the total thickness of the adhesive layer; within any 1 square centimeter area of ​​the diaphragm, the area ratio of the first adhesive relative to the porous substrate is 5% to 30%.

[0007] In one embodiment of the present invention, the first adhesive includes at least one of polyacrylates, hydroxymethyl acrylates, carboxymethyl cellulose salts, and polyvinylidene fluoride adhesives.

[0008] In one embodiment of the present invention, the second adhesive includes at least one of polyether polyurethane, polyester polyurethane, polyether-polyester hybrid polyurethane, acrylic modified polyurethane, acrylic modified acrylonitrile, and acrylic modified polybenzimidazole.

[0009] In one embodiment of the present invention, the porous substrate includes a base film and a ceramic layer disposed on the base film, the adhesive layer is disposed on the ceramic layer, and the longest diameter d obtained by connecting any two points on the edge of the aggregate adhesive dots of the first adhesive in the adhesive layer ranges from 100μm to 800μm.

[0010] In one embodiment of the present invention, the porous substrate includes a base film, the adhesive layer is disposed on the base film, the adhesive layer further includes a ceramic material, and the longest diameter d obtained by connecting any two points on the edge of the aggregate adhesive dots of the first adhesive in the adhesive layer is in the range of 3μm~10μm.

[0011] In one embodiment of the present invention, the mass ratio of the first adhesive to the second adhesive is (5~20):(1~6).

[0012] The present invention also provides a method for preparing a diaphragm, the method comprising the following steps:

[0013] Provide porous substrates; The first adhesive, the second adhesive, and the solvent are mixed evenly to obtain the adhesive paste. The adhesive slurry is applied to at least one side of the porous substrate and dried to obtain the diaphragm.

[0014] In one embodiment of the present invention, the mass of the first adhesive is 5% to 20% of the total mass of the coating slurry, the mass of the second adhesive is 1% to 6% of the total mass of the coating slurry, and the solvent includes deionized water.

[0015] In one embodiment of the present invention, the adhesive slurry is applied to the porous substrate by a spraying process, and the viscosity of the adhesive slurry is 70~110 mpa·s, and the drying temperature is 70~85℃.

[0016] In one embodiment of the present invention, the adhesive slurry further includes ceramic material, and the adhesive slurry is coated on the porous substrate by a roller coating process. The viscosity of the adhesive slurry is 80~120 mpa·s, and the drying temperature is 70~85℃.

[0017] The present invention also provides a battery comprising a positive electrode, a negative electrode, and a separator as described above and a separator prepared by any of the above preparation methods, wherein the separator is disposed between the positive electrode and the negative electrode, and the adhesive coating layer of the separator faces the positive electrode.

[0018] The beneficial effects of the present invention are as follows: By adjusting the difference in the weight ratio of the same element distribution between the surface layer and the inner layer of the second adhesive in the coating layer along the thickness direction of the coating layer, and the area ratio of the first adhesive relative to the substrate within any 1 square centimeter of the separator, the bonding strength between the separator and the positive electrode sheet can be precisely controlled to keep it within a reasonable range. This ensures sufficient bonding strength, effectively avoiding adverse risks such as bare cell opening, difficulty in casing, and tab redundancy caused by insufficient bonding strength. At the same time, it can reserve appropriate electrode sheet rebound space to prevent problems such as core interface wrinkles and high impedance caused by excessive bonding strength.

[0019] Furthermore, by limiting the range of the longest diameter obtained by connecting any two points on the edge of the first adhesive aggregate, the surface morphology and pore structure of the coating layer can be optimized. This not only improves the wettability of the electrolyte to the coating layer and reduces ion transport impedance, but also further alleviates the problem of cell interface wrinkles, thereby improving the cycle stability, rate performance and other electrical performance indicators of lithium-ion batteries. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0021] In the attached diagram: Figure 1This is a schematic diagram of the structure of the diaphragm provided in one embodiment of the present invention; Figure 2 This is a schematic diagram of the diaphragm provided in another embodiment of the present invention; Figure 3 A scanning electron microscope (SEM) image of the diaphragm coating layer provided in one embodiment of the present invention; Figure 4 A scanning electron microscope (SEM) image of the diaphragm coating layer provided in another embodiment of the present invention; Figure 5 This is a flowchart of a method for preparing a diaphragm according to an embodiment of the present invention; Figure 6 The figure shows the state of the bare cell after hot pressing in Comparative Example 1 of the present invention. Figure (a) shows the apparent state of the bare cell, and Figure (b) shows the interface state of the bare cell after disassembly. Figure 7 The figure shows the state of the bare cell after hot pressing in Comparative Example 3 of the present invention. Figure (a) shows the apparent state of the bare cell, and Figure (b) shows the interface state of the bare cell after disassembly. Figure 8 The figure shows the state of the bare cell after hot pressing in Embodiment 1 of the present invention. Figure (a) shows the apparent state of the bare cell, and Figure (b) shows the interface state of the bare cell after disassembly. Figure 9 The figure shows the state of the bare battery cell after hot pressing in Embodiment 8 of the present invention. Figure (a) shows the apparent state of the bare battery cell, and Figure (b) shows the interface state of the bare battery cell after disassembly. Figure 10 The figure shows the state of the bare cell after hot pressing in Comparative Example 4 of the present invention. Figure (a) shows the apparent state of the bare cell, and Figure (b) shows the interface state of the bare cell after disassembly. Figure 11 The figure shows the state of the bare cell after hot pressing in Comparative Example 6 of the present invention. Figure (a) shows the apparent state of the bare cell, and Figure (b) shows the interface state of the bare cell after disassembly. Figure 12 The figure shows the state of the bare battery cell after hot pressing in Embodiment 9 of the present invention. Figure (a) shows the apparent state of the bare battery cell, and Figure (b) shows the interface state of the bare battery cell after disassembly. Figure 13 The figure shows the state of the bare battery cell after hot pressing in Embodiment 16 of the present invention. Figure (a) shows the apparent state of the bare battery cell, and Figure (b) shows the interface state of the bare battery cell after disassembly.

[0022] The attached figures are labeled as follows: 100. Separator; 110. Porous substrate; 111. Base membrane; 112. Ceramic layer; 120. Coating layer. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0024] It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. The drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0026] The terms or phrases used in this article have the following meanings: In this article, the terms "multiple," "various," and "multiple times" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0027] In this document, terms such as “preferred,” “ideal,” “further,” “even more,” and “particularly” are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

[0028] In this document, when referring to numerical ranges, unless otherwise specified, the distribution of selectable values ​​within a numerical range is considered continuous, including the two endpoints of the range (i.e., the minimum and maximum values), and every value between these two endpoints. When multiple numerical ranges are provided to describe a feature or property, these numerical ranges can be combined.

[0029] In battery structure, the separator is positioned between the positive and negative electrodes, serving to insulate and isolate them, preventing short circuits caused by contact between the positive and negative electrodes. During battery charge-discharge cycles, the volume expansion of the negative electrode is much greater than that of the positive electrode. When the negative electrode expands significantly, the separator, through its stable adhesion to the positive electrode, provides a buffer space for the expansion, preventing overall deformation and interface wrinkling of the cell due to negative electrode expansion. Therefore, the adhesion between the separator and the positive electrode is a prerequisite for maintaining the overall structural stability of the cell. Insufficient adhesion between the separator and the positive electrode will directly lead to problems such as exposed bare cells and redundant tabs. A moderate, non-rigid adhesion is required between the separator and the negative electrode. If the adhesion is too strong, it will restrict the normal expansion of the negative electrode, causing the active material to pulverize and detach, accelerating battery capacity decay. Therefore, the adhesive function design of the diaphragm will prioritize matching the needs of the positive electrode: the adhesive force between the diaphragm and the positive electrode is moderate, which can improve the adhesive force between the positive electrode and the diaphragm, and also reserve expansion space for the negative electrode.

[0030] Currently, it is difficult to precisely control the bonding strength between the separator and the positive electrode sheet: if the bonding strength is too weak, it is easy to cause the bare cell to open after hot pressing, which in turn leads to adverse risks such as difficulty in inserting into the casing and tab redundancy, making it impossible to fully achieve the purpose of hot pressing and shaping; if the bonding strength is too strong, there is insufficient space for the electrode sheet to rebound, and the core is prone to interface wrinkles, which in turn affects the overall performance of the lithium-ion battery.

[0031] Based on this, this application provides a separator, its preparation method, and a battery. The separator has a moderate bonding strength with the positive electrode sheet, which can ensure sufficient bonding strength and effectively avoid adverse risks such as bare cell opening, difficulty in casing, and tab redundancy caused by weak bonding strength. It can also reserve a suitable electrode sheet rebound space to prevent problems such as core interface wrinkles and high impedance caused by excessive bonding strength.

[0032] Please see Figure 1 and Figure 2 The separator 100 provided by the present invention includes a porous substrate 110 and an adhesive layer 120. The porous substrate 110 serves as the base of the separator 100 and can be selected from a porous material that is insulating and allows lithium ions to pass through. The adhesive layer 120 can improve the adhesion between the separator 100 and the electrode. It is disposed on at least one side of the porous substrate 110, that is, the porous substrate 110 has a first surface and a second surface disposed opposite to each other along its thickness direction. The adhesive layer 120 can be disposed on either the first surface or the second surface of the porous substrate 110 (only the case of single-sided disposal is shown in the figure), or it can be disposed on both the first surface and the second surface. It should be noted that when the adhesive layer 120 is disposed on only one surface of the porous substrate 110, the side on which the adhesive layer 120 is disposed is bonded to the positive electrode.

[0033] In this application, the adhesive layer 120 includes a first adhesive and a second adhesive. The softening point of the second adhesive is lower than that of the first adhesive. The first adhesive is used for bonding between the separator and the positive electrode sheet, and the second adhesive is used for bonding between the particles of the first adhesive and between the adhesive layer 120 and the porous substrate 110. In the second adhesive, the weight ratio of the same element in the surface layer of the adhesive layer 120 is greater than or equal to the weight ratio in the inner layer, and the difference between the two is 0% to 10%. That is, the weight ratio of the same element in the surface layer and the weight ratio in the inner layer of the second adhesive can be equal or unequal. When the two are unequal, the weight ratio of the same element in the surface layer of the second adhesive is greater than the weight ratio in the inner layer, but the difference between the two is less than or equal to 10%. The surface layer is the part away from the porous substrate 110 along the thickness direction of the adhesive layer 120, and the inner layer is the part close to the porous substrate 110 along the thickness direction of the adhesive layer 120. The thickness of the surface layer accounts for 1 / 2 of the total thickness of the adhesive layer 120.

[0034] In some optional embodiments, the difference between the weight ratio of the same element in the second adhesive distributed on the surface layer and the inner layer of the coating layer 120 can be 0%, 3%, 5%, 8%, or 10%, etc. The smaller the difference in the weight ratio of the same element in the second adhesive distributed on the surface layer and the inner layer of the coating layer 120, the better the uniformity of the distribution of the second adhesive within the coating layer 120, and the better the corresponding adhesive strength within the coating layer 120 and the adhesive strength between the coating layer 120 and the porous substrate 110. If the difference in the weight ratio of the same characteristic element in the second adhesive distributed on the surface layer and the inner layer of the coating layer 120 is greater than 10%, the adhesive will be less effective. This indicates that the second adhesive has migrated to the surface of the coating layer 120, and the effective content of the second adhesive in the inner layer of the coating layer 120 has been significantly reduced, resulting in a weakening of the bonding effect between the first adhesive particles. At the same time, the interfacial bonding force between the coating layer 120 and the porous substrate 110 has decreased, which leads to a poor overall bonding effect between the separator and the positive electrode sheet. The bare cell after hot pressing and shaping is prone to opening defects, causing subsequent problems such as difficulty in casing and tab redundancy.

[0035] Furthermore, within any 1 square centimeter area of ​​the separator 100, the area ratio of the first adhesive to the porous substrate 110 is 5% to 30%; that is, within any 1 square centimeter area of ​​the separator 100, the distribution area of ​​the first adhesive is 5% to 30% of the total area of ​​the porous substrate 110. When the area ratio of the first adhesive is within the above range, the adhesion between the adhesive layer 120 and the positive electrode sheet is moderate, which can ensure sufficient adhesion strength, effectively avoid adverse risks such as bare cell opening, difficulty in casing, and tab redundancy caused by insufficient adhesion strength, and also reserve suitable electrode sheet rebound space to prevent problems such as core interface wrinkles and high battery impedance caused by excessive adhesion strength.

[0036] In some optional embodiments, in any 1 square centimeter of separator 100, the area ratio of the first adhesive relative to the porous substrate 110 can be 5%, 10%, 20%, 30%, etc. If the area ratio of the first adhesive is less than 5%, the adhesion between the adhesive layer 120 and the positive electrode sheet is insufficient, which can easily cause opening problems in the bare cell after hot pressing and shaping, leading to adverse risks such as difficulty in casing and tab redundancy, thus preventing the expected purpose of hot pressing and shaping from being fully achieved. If the area ratio of the first adhesive is higher than 30%, the adhesion between the adhesive layer 120 and the positive electrode sheet is too strong, the space required for the electrode sheet to rebound is excessively compressed, and the cell is prone to interface wrinkles, ultimately affecting the overall performance of the lithium-ion battery, such as cycle performance and safety performance.

[0037] In this application, the second adhesive is used to bond the coating layer 120 to the porous substrate 110, and the first adhesive particles inside the coating layer 120. The first adhesive is used to bond the coating layer 120 (separator) to the positive electrode sheet. Therefore, the softening point of the second adhesive must ensure that it softens when the coating slurry forms the coating layer 120 to achieve bonding between the coating layer 120 and the porous substrate 110. The first adhesive does not soften when the coating layer 120 is formed, but softens during the hot pressing and shaping of the battery cell to achieve bonding between the separator and the positive electrode sheet. Further, the softening point of the second adhesive is 40~60℃, for example, it can be 40℃, 50℃, or 55℃, etc. The softening point of the first adhesive is 60~120℃, for example, it can be 60℃, 75℃, 95℃, 100℃, or 120℃, etc.

[0038] In some optional embodiments, the first adhesive includes at least one of polyacrylates, hydroxymethyl acrylates, carboxymethyl cellulose salts, and polyvinylidene fluoride adhesives. That is, the first adhesive can be any one of the adhesives listed above, or any combination of two or more of the adhesives listed above. For example, the first adhesive is polyacrylate, or hydroxymethyl acrylate, or polyvinylidene fluoride; or a combination of polyacrylate and polyvinylidene fluoride, but is not limited thereto.

[0039] The second adhesive includes at least one of polyether-type polyurethane, polyester-type polyurethane, polyether-polyester hybrid polyurethane, acrylic-modified polyurethane, acrylic-modified acrylonitrile, and acrylic-modified polybenzimidazole. That is, the second adhesive can be used alone or in combination. For example, the second adhesive can be polyether-type polyurethane, acrylic-modified acrylonitrile, acrylic-modified polyurethane, or a combination of acrylic-modified acrylonitrile and polyester-type polyurethane, etc.

[0040] The porous substrate 110 can be any porous material in the art that is insulating and allows lithium ions to pass through. In some alternative embodiments, the porous substrate 110 can be a base film 111 (e.g., Figure 2 It can also be a combination layer of the base film 111 and a functional layer disposed on the base film 111 (e.g. Figure 1 The base film 111 includes, but is not limited to, polypropylene (PP), polyethylene (PE), nonwoven fabric, and polypropylene-polyethylene-polypropylene composite layers. Functional layers can be, for example, ceramic layers, heat-resistant polymer layers, three-dimensional porous layers, etc.

[0041] Please see Figure 1 and Figure 3 In one specific embodiment, the porous substrate 110 includes a base film 111 and a ceramic layer 112 disposed on the base film 111. The base film 111 has two surfaces disposed opposite to each other along its thickness. The ceramic layer 112 can be disposed on either side of the base film 111 or on both sides of the base film 111. The ceramic layer 112 is formed by coating the surface of the base film 111 with an inorganic ceramic material as its core and a small amount of adhesive. The inorganic ceramic material includes one or more of alumina (Al2O3), silicon dioxide (SiO2), and boehmite (γ-AlOOH). An adhesive layer 120 is disposed on the surface of the ceramic layer 112 opposite to the base film 111. In the adhesive layer 120, the mass ratio of the first adhesive to the second adhesive is (5~20):(1~6), for example, it can be 5:1, 10:3, or 20:6, etc. In this embodiment, the first adhesive is distributed throughout the entire coating layer 120 in the form of aggregated adhesive dots. The longest diameter obtained by connecting any two points on the edge of the aggregated adhesive dots of the first adhesive is denoted as d. The value of d ranges from 100μm to 800μm. Exemplary values ​​can be 100μm, 300μm, 500μm, 700μm, or 800μm, etc. The specific value of d can be adjusted by controlling the viscosity of the slurry and the coating process parameters. When d is within the above-mentioned range, it can further improve the problem of poor electrolyte wettability and high ion impedance leading to deterioration of electrical performance. If d is too large, the bonding strength between the separator and the positive electrode will be too high, and the first adhesive will be excessively squeezed into the microporous structure of the separator, causing some pores to be blocked, thereby increasing the resistance to lithium ion transmission. In addition, excessive bonding strength will also reduce the pore volume between the adhesive layer 120 and the positive electrode, which will not only cause the electrolyte wettability to deteriorate, but also compress the electrode rebound space, causing wrinkles at the core interface, and ultimately affecting the cycle performance and safety performance of the battery.

[0042] Please see Figure 2 and Figure 4In another specific embodiment, the porous substrate 110 is a base film 111, and the adhesive layer 120 is disposed on at least one side of the base film 111. That is, the base film 111 has a first surface and a second surface disposed opposite to each other along its thickness direction. The adhesive layer 120 can be disposed on either one or both surfaces of the base film 111. In this embodiment, the adhesive layer 120 includes a first adhesive, a second adhesive, and a ceramic material. The mass ratio of the first adhesive to the second adhesive is (5~20):(1~6). For example, it can be 5:1, 10:3, or 20:6, etc. The mass content of the ceramic material in the adhesive layer 120 can be adjusted adaptively according to actual needs. In this embodiment, the first adhesive is distributed throughout the coating layer 120 in the form of aggregated adhesive dots. The longest diameter obtained by connecting any two points on the edge of the aggregated adhesive dots of the first adhesive is denoted as d, and the value of d ranges from 3μm to 10μm, for example, it can be 2μm, 5μm, 8μm or 10μm, etc. The specific value of d can be adjusted by controlling the viscosity of the slurry and the coating process parameters. When d is within the above value range, it can further improve the problem of poor electrolyte wettability and high ion impedance leading to electrical performance degradation. If the value of d is too large, the bonding strength between the separator and the positive electrode is too high, and the first adhesive will be excessively squeezed into the microporous structure of the separator, causing some pores to be blocked, thereby increasing the lithium ion transmission resistance. In addition, excessive bonding strength will also reduce the pore volume between the coating layer 120 and the positive electrode, which not only causes poor electrolyte wettability, but also compresses the electrode rebound space, causing core interface wrinkles, and ultimately affecting the cycle performance and safety performance of the battery.

[0043] In summary, this application can precisely control the bonding strength between the separator and the positive electrode by adjusting the difference in the weight ratio of the same elements in the outer and inner layers of the second adhesive in the coating layer along the thickness direction of the coating layer, and the area ratio of the first adhesive relative to the substrate within any 1 square centimeter of the separator. This ensures that the bonding strength is within a reasonable range, guaranteeing sufficient bonding strength and effectively avoiding adverse risks such as bare cell opening, difficulty in casing, and tab redundancy caused by insufficient bonding strength. At the same time, it can reserve appropriate electrode rebound space to prevent problems such as core interface wrinkles and high impedance caused by excessive bonding strength.

[0044] Please see Figure 5 This application also provides a method for preparing the above-mentioned diaphragm, the method comprising the following steps: S1. Provide a porous substrate; S2. Mix the first adhesive, the second adhesive, and the solvent evenly to obtain the adhesive paste; S3. Apply the adhesive slurry to at least one side of the porous substrate and dry it to obtain a diaphragm.

[0045] Specifically, the porous substrate in step S1 can be a base film, or a base film and a functional layer disposed on the base film. For the specific selection of materials for the base film and the functional layer, please refer to the detailed description above. The porous substrate can be obtained directly by purchase, or it can be prepared using conventional techniques in the art, and there are no restrictions here.

[0046] Step S2 involves preparing the adhesive slurry by mixing the first adhesive, the second adhesive, and the solvent in a specific ratio until homogeneous. The solvent is an aqueous solvent, such as deionized water. In the adhesive slurry, the first adhesive accounts for 5% to 20% of the mass, the second adhesive accounts for 1% to 6% of the mass, and the solvent accounts for 60% to 95% of the mass. For example, the mass percentage of the first adhesive in the adhesive slurry can be 5%, 10%, 15%, or 20%, etc.; the mass percentage of the second adhesive can be 1%, 3%, 5%, or 6%, etc.; and the mass percentage of the solvent can be 60%, 70%, 80%, 90%, or 95%, etc.

[0047] In some embodiments, the adhesive slurry also includes ceramic materials, including but not limited to one or more of alumina, silica, and boehmite. The content of ceramic materials in the adhesive slurry can be limited according to actual needs.

[0048] Step S3 is the coating step, in which the adhesive slurry obtained in step S2 is coated onto the porous substrate, and then transferred to an oven and dried at a set temperature to obtain the diaphragm. This application does not limit the coating process, as long as a diaphragm that meets the foregoing definition can be obtained.

[0049] In one embodiment, a spraying process is used to coat the adhesive slurry onto a porous substrate. In this embodiment, the viscosity of the adhesive slurry is 70~110 mPa·s; exemplaryly, it can be 70 mPa·s, 80 mPa·s, 90 mPa·s, 100 mPa·s, or 110 mPa·s, etc. The viscosity of the adhesive slurry can be controlled by adjusting the ratio of the first adhesive, the second adhesive, and the solvent. The drying temperature after coating is 70~85°C; for example, it can be 70°C, 80°C, or 85°C, etc. The drying temperature after coating directly affects the distribution of the second adhesive in the coating layer. If the temperature is too high, the solvent in the adhesive slurry may evaporate from the inner layer to the outer layer, causing the second adhesive in the slurry to migrate to the surface of the coating layer. This results in an increase in the weight ratio of the same element of the second adhesive in the surface and inner layers of the coating layer. As the second adhesive migrates to the surface, the adhesion between the first adhesive particles in the coating layer and between the coating layer and the porous substrate deteriorates. If the drying temperature is too low, the drying time is longer, extending the entire process. Adjusting process parameters such as the spray flow rate during spraying can regulate the area ratio of the first adhesive. In this embodiment, the spray flow rate is 3~5L / min, for example, 3L / min, 3.5L / min, or 5L / min, etc.

[0050] In another embodiment, a roller coating process is used to coat the adhesive slurry onto a porous substrate. In this embodiment, the adhesive slurry also includes ceramic material. The amount of ceramic material added can be selected according to actual needs. For example, the content of ceramic material in the adhesive slurry is 20%~40%, such as 20%, 30%, or 40%, etc. The viscosity of the adhesive slurry is 80~120 mPa·s. For example, it can be 80 mPa·s, 90 mPa·s, 100 mPa·s, 110 mPa·s, or 120 mPa·s, etc. The viscosity of the adhesive slurry can be controlled by adjusting the ratio of the first adhesive, the second adhesive, and the solvent. The drying temperature is 70~85℃. For example, it can be 70℃, 80℃, or 85℃, etc. Similarly, in this embodiment, the drying temperature also affects the distribution of the second adhesive in the coating layer. If the temperature is too high, the solvent in the adhesive slurry may evaporate from the inner layer to the outer layer, causing the second adhesive in the slurry to migrate towards the surface of the coating layer. This results in an increase in the weight ratio of the same element of the second adhesive in the surface and inner layers of the coating layer. As the second adhesive migrates towards the surface, the adhesion between the first adhesive particles in the coating layer and between the coating layer and the porous substrate deteriorates. If the drying temperature is too low, the drying time is longer, extending the entire process. The area ratio of the first adhesive in the coating layer can be adjusted by regulating the roller coating flow rate. In this embodiment, the roller coating flow rate is 4~5 L / min, for example, it can be 4 L / min, 4.5 L / min, or 5 L / min.

[0051] The diaphragm preparation method provided in this application can produce a diaphragm that meets the requirements by adjusting the viscosity of the adhesive slurry, the coating process, the process parameters and the drying temperature.

[0052] This invention also provides a battery comprising a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The separator is the separator provided in this application or a separator prepared by the method of this application, with the adhesive coating layer of the separator facing the positive electrode. The type of battery of this invention is not limited; it can be a lithium-ion battery or a sodium-ion battery. The positive and negative electrodes are not particularly limited, and the corresponding materials can be selected according to the battery type. The structure of the battery is described in detail below using a lithium-ion battery as an example.

[0053] In some optional embodiments, the positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector. The positive current collector is, for example, aluminum foil, carbon-coated aluminum foil, etc. The positive current collector has a first surface and a second surface disposed opposite to each other along its thickness direction. The positive active material layer can be disposed on either one surface or on both surfaces. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. The positive active material can be any material used in lithium-ion batteries, i.e., compounds capable of reversibly inserting and deintercalating lithium ions can be used. For example, the positive active material is selected from one or more of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium manganese iron phosphate. These materials can be used alone or in combination, such as lithium cobalt oxide, or lithium nickel cobalt manganese oxide, or a combination of lithium iron phosphate and lithium nickel cobalt manganese oxide, etc. The positive electrode binder is selected from one or more of the following: polyvinylidene fluoride (PVDF), poly(ethylene oxide) (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate (polyacrylate), polyvinyl ether (polyvinyl ether), polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexanefluoropropylene (polyhexafluoropropylene), or polymerized styrene-butadiene rubber (SBR). The positive electrode conductive agent is selected from one or more of the following: conductive carbon black (Super P), acetylene black, graphene, carbon nanotubes, carbon fiber (VGCF), Ketjen black, etc.

[0054] The negative electrode includes a negative current collector and a negative active material layer disposed on at least one side of the negative current collector. The negative current collector is selected, for example, from copper foil, carbon-coated copper foil, or foamed copper. The negative current collector has two surfaces disposed opposite to each other along its thickness direction, and the negative active material layer is disposed on either or both of the opposite surfaces of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, a negative binder, and a thickener. The negative active material is selected from materials capable of inserting and extracting lithium ions; exemplaryly, the negative active material includes, but is not limited to, artificial graphite, natural graphite, elemental silicon, silicon oxide compounds, and silicon-carbon composite materials. The negative conductive agent is selected from one or more of acetylene black, conductive carbon black (Super P), carbon fiber (VGCF), carbon nanotubes (CNTs), and Ketjen black. The negative electrode binder is selected from any one of polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR) or a combination of several in any proportion; the thickener is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0055] The battery described above also includes an electrolyte for transporting lithium ions. In one embodiment, the electrolyte includes a lithium salt and an organic solvent. This application does not limit the specific types of lithium salt and organic solvent; lithium salts and organic solvents well-known in the art can be selected. As an example, the lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(oxalateborate)borate (LiBOB), and lithium difluorooxalateborate (LiBF2C2O4). The organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). The lithium salts and solvents listed above can be used individually or in combination, preferably in combination, which can improve the overall performance of the electrolyte. Specifically, the lithium salt can be a combination of lithium hexafluorophosphate and lithium difluorosulfonylimide, and the organic solvent can be a combination of diethyl carbonate, dimethyl carbonate and ethylene carbonate, etc.

[0056] Furthermore, functional additives can be added to the electrolyte according to actual needs, such as ethylene sulfate (DTD), 1,3-propanesultone (PS), vinylene carbonate (VC), or vinyl ethylene carbonate (VEC).

[0057] The battery also includes a casing. The shape and material of the casing depend on the type of lithium-ion battery. For example, for pouch batteries, the casing can be made of aluminum-plastic film; for prismatic or cylindrical batteries, the casing can be made of a square or cylindrical shell that matches the shape of the bare cell, and the material can be stainless steel or other materials.

[0058] The battery can be prepared according to methods known in the art, which will not be described in detail here.

[0059] The technical solution of the present invention will be described in detail below through several specific embodiments and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following embodiments and comparative examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the embodiments are all commercially available.

[0060] Comparative Example 1 This comparative example provides a diaphragm comprising a porous substrate and an adhesive layer disposed on the porous substrate. The porous substrate includes a PE base film and a ceramic layer disposed on the surface of the PE base film. The PE base film has a thickness of 7 μm, and the ceramic layer has a thickness of 2 μm. The ceramic material of the ceramic layer is boehmite. The adhesive layer is disposed on the ceramic layer and includes a first adhesive PVDF and a second adhesive, a copolymer of acrylic acid and acrylonitrile. The thickness of the adhesive layer is 3 μm. In the surface and inner layers of the adhesive layer, the weight ratio difference of the same characteristic element N in the second adhesive is 12%. The diaphragm is oriented at any 1 cm depth. 2 The area ratio of the first adhesive PVDF to the substrate is 20%, and the longest diameter obtained by connecting any two points on the edge of the PVDF aggregate dots is 850μm.

[0061] The diaphragm was prepared using the following method: (1) Preparation of the adhesive slurry: Mix the first adhesive PVDF particles and deionized water in a certain proportion and stir until the PVDF particles are evenly dispersed. Then add the second adhesive (a copolymer of acrylic acid and acrylonitrile) and continue stirring until the first and second adhesives are completely and evenly dispersed. The second adhesive accounts for 6% of the total mass of the adhesive slurry, the first adhesive accounts for 8% of the total mass of the adhesive slurry, and the viscosity of the adhesive slurry is 105 mPa·s.

[0062] (2) Coating: The prepared adhesive slurry is coated onto the ceramic layer of the porous substrate by spraying, wherein the spraying flow rate is controlled at 4L / min.

[0063] (3) Drying: The diaphragm with the adhesive slurry is transferred to an oven at 85°C for drying to remove the moisture in the slurry and obtain the diaphragm.

[0064] Comparative Example 2 The difference between this comparative example and Comparative Example 1 is that the weight ratio of the same characteristic element N in the second adhesive of the surface and inner layers of the adhesive coating is 0.8%; the difference is also 0.8% between any 1 cm section of the diaphragm. 2 The area ratio of the first adhesive PVDF to the substrate is 3%.

[0065] In the preparation process of the diaphragm, the spraying flow rate is controlled at 3.2 L / min during spraying in step (2); and the oven temperature in step (3) is 75℃.

[0066] Comparative Example 3 The difference between this comparative example and Comparative Example 1 is that the weight ratio of the same characteristic element N in the second adhesive of the surface and inner layers of the adhesive coating is 0.8%; the diaphragm is per 1 cm 2 The area ratio of the first adhesive PVDF to the substrate is 32%.

[0067] In the preparation process of the diaphragm, the spraying flow rate is controlled at 5L / min during spraying in step (2); the oven temperature in step (3) is 75℃.

[0068] Example 1 The difference between this embodiment and Comparative Example 1 is that the weight ratio of the same characteristic element N in the surface and inner layers of the adhesive layer is 0.8%; and the oven temperature in step (3) of the preparation process is 75°C.

[0069] Example 2 The difference between this embodiment and Embodiment 1 is that the weight ratio difference of the same characteristic element N in the surface and inner layers of the adhesive layer is 10%, and the longest diameter obtained by connecting any two points on the edge of the PVDF aggregate adhesive dots of the first adhesive is 400 μm.

[0070] In the preparation process, the second adhesive in step (1) accounts for 4% of the total mass of the coating slurry, and the viscosity of the slurry is 92 mpa·s; the oven temperature in step (3) is 80℃.

[0071] Example 3 The difference between this embodiment and Embodiment 2 is that the weight ratio of the same characteristic element N in the surface and inner layers of the adhesive layer differs by 4%.

[0072] In the preparation process, the oven temperature in step (3) is 78℃.

[0073] Example 4 The difference between this embodiment and Embodiment 1 is that the longest diameter obtained by connecting any two points on the edge of the first adhesive PVDF aggregate glue dot in the adhesive layer is 800 μm.

[0074] In the preparation process, the second adhesive in step (1) accounts for 5% of the total mass of the coating slurry, and the viscosity of the slurry is 100 MPa·s.

[0075] Example 5 The difference between this embodiment and Embodiment 1 is that the longest diameter obtained by connecting any two points on the edge of the first adhesive PVDF aggregate glue dot in the adhesive layer is 100μm.

[0076] In the preparation process, the second adhesive in step (1) accounts for 3% of the total mass of the coating slurry, and the viscosity of the slurry is 80 mpa·s.

[0077] Example 6 The difference between this embodiment and Embodiment 1 is that: the diaphragm is at any 1cm... 2 The area ratio of the first adhesive PVDF to the substrate is 30%, and the longest diameter obtained by connecting any two points on the edge of the first adhesive PVDF aggregate dots is 400μm.

[0078] In the preparation process, the second adhesive accounts for 4% of the total mass of the coating slurry in step (1), and the viscosity of the slurry is 92 mpa·s; in step (2), the spraying flow rate is controlled at 4.8 L / min.

[0079] Example 7 The difference between this embodiment and Embodiment 6 is that: the diaphragm is at any 1cm... 2 The area ratio of the first adhesive PVDF to the substrate is 5%.

[0080] In the preparation process, the spraying flow rate is controlled at 3.5L / min during step (2) spraying.

[0081] Example 8 The difference between this embodiment and Embodiment 6 is that: the diaphragm is at any 1cm... 2 The area ratio of the first adhesive PVDF to the substrate is 20%.

[0082] In the preparation process, the spraying flow rate is controlled at 4L / min during step (2) of spraying.

[0083] This application also provides comparative examples 4-6 and examples 9-16 using a roller coating process, as detailed below: Comparative Example 4 The difference between this comparative example and Comparative Example 1 is that the porous substrate is a 7μm PE base film, and the adhesive layer includes a first adhesive PMMA, a second adhesive copolymer of acrylic acid and acrylonitrile, and a ceramic material boehmite.

[0084] The longest diameter obtained by connecting any two points on the edge of the first adhesive dot is 12 μm.

[0085] In the preparation process, step (1) of preparing the coating slurry includes: mixing the first adhesive PMMA, boehmite, and deionized water in a certain proportion, stirring until the particles are evenly dispersed, then adding the second adhesive, a copolymer of acrylic acid and acrylonitrile, and continuing to stir until the first and second adhesives are completely and evenly dispersed. The second adhesive accounts for 6% of the total mass of the coating slurry, the first adhesive accounts for 8% of the total mass of the coating slurry, boehmite accounts for 32%, and the remainder is deionized water. The viscosity of the coating slurry is 112 mPa·s; the roller coating flow rate during step (2) is 4.5 L / min.

[0086] Comparative Example 5 The difference between this comparative example and Comparative Example 4 is that the weight ratio of the same characteristic element N in the second adhesive differs by 0.8%; the difference in weight ratio of the diaphragm at any 1 cm depth is also less than that of Comparative Example 4. 2 The area ratio of the first adhesive PVDF to the substrate is 3%.

[0087] In the membrane preparation process, the roller coating flow rate in step (2) is 4 L / min. The oven temperature in step (3) is 75℃.

[0088] Comparative Example 6 The difference between this comparative example and Comparative Example 4 is that the weight ratio of the same characteristic element N in the second adhesive differs by 0.8%; the difference in weight ratio of the diaphragm at any 1 cm depth is also less than that of Comparative Example 4. 2 The area ratio of the first adhesive PVDF to the substrate is 32%.

[0089] In the membrane preparation process, the roller coating flow rate in step (2) is 5 L / min. The oven temperature in step (3) is 75℃.

[0090] Example 9 The difference between this embodiment and Comparative Example 4 is that the weight ratio of the same characteristic element N in the surface and inner layers of the adhesive layer is 0.8%; and the oven temperature in step (3) of the preparation process is 75°C.

[0091] Example 10 The difference between this embodiment and embodiment 9 is that the weight ratio difference of the same characteristic element N in the surface and inner layers of the adhesive layer is 10%, and the longest diameter obtained by connecting any two points on the edge of the PMMA glue dot of the first adhesive is 7μm.

[0092] In the preparation process, the second adhesive accounts for 4% of the total mass of the coating slurry in step (1), and the viscosity of the slurry is 97 mpa·s; the oven temperature in step (3) is 80℃.

[0093] Example 11 The difference between this embodiment and embodiment 10 is that the weight ratio of the same characteristic element N in the surface and inner layers of the adhesive layer is 4%.

[0094] In the preparation process, the oven temperature in step (3) is 78℃.

[0095] Example 12 The difference between this embodiment and embodiment 9 is that the longest diameter obtained by connecting any two points on the edge of the first adhesive PMMA glue dot in the adhesive layer is 10 μm.

[0096] In the preparation process, the second adhesive in step (1) accounts for 5% of the total mass of the coating slurry, and the viscosity of the slurry is 106 MPa·s.

[0097] Example 13 The difference between this embodiment and embodiment 9 is that the longest diameter obtained by connecting any two points on the edge of the first adhesive PMMA glue dot in the adhesive layer is 3μm.

[0098] In the preparation process, the second adhesive in step (1) accounts for 3% of the total mass of the coating slurry, and the viscosity of the slurry is 85 mpa·s.

[0099] Example 14 The difference between this embodiment and Embodiment 9 is that: the diaphragm is at any 1cm... 2 The area ratio of the first adhesive PMMA to the substrate is 30%, and the longest diameter obtained by connecting any two points on the edge of the first adhesive PMMA dot is 7μm.

[0100] In the preparation process, the second adhesive accounts for 4% of the total mass of the coating slurry in step (1), and the viscosity of the slurry is 97 mpa·s; the roller coating flow rate is 4.8 L / min during roller coating in step (2).

[0101] Example 15 The difference between this embodiment and Embodiment 14 is that: the diaphragm is at any 1cm... 2 The area ratio of the first adhesive PMMA relative to the substrate is 5%.

[0102] In the preparation process, the roller coating flow rate during step (2) is 4.2 L / min.

[0103] Example 16 The difference between this embodiment and Embodiment 14 is that: the diaphragm is at any 1cm... 2 The area ratio of the primary adhesive PMMA relative to the substrate is 20%.

[0104] In the preparation process, the roller coating flow rate during step (2) is 4.5 L / min.

[0105] Table 1: Characteristics, process parameters, and performance of comparative examples 1-3 and examples 1-8, and their diaphragms.

[0106] Table 2: Characteristics, process parameters, and performance of diaphragms in Comparative Examples 4-6 and Examples 9-16

[0107] The diaphragms of Examples 1-16 and Comparative Examples 1-6 were tested according to the following methods. The test results are shown in Table 1 and Table 2.

[0108] (1) Characterization of the distribution ratio of the second adhesive in the adhesive layer in the thickness direction of the adhesive layer The cross-sectional micromorphology (CP) of the diaphragm was observed using a scanning electron microscope. For the cross-section of the adhesive layer, the proportion of the same element N in the second adhesive layer in the outer layer far from the base film and the inner layer close to the base film was analyzed by energy dispersive X-ray spectroscopy at the halfway point of the total adhesive layer thickness. The difference between the two was then calculated to determine the distribution of the second adhesive in the thickness direction of the adhesive layer.

[0109] (2) Any 1 cm of the diaphragm 2 Test of the area ratio of the first adhesive relative to the porous substrate within the specified area: The diaphragm sample was cut into 100cm × 100cm pieces and placed under a microscope, with the test area set to 1cm². 2 Afterwards, a coverage test is conducted. The image analysis software processes the image to separate the adhesive layer area from the base film background and automatically calculates the coverage (%) = adhesive layer pixel area / total field of view pixel area) × 100%.

[0110] (3) Test the longest diameter obtained by connecting any two points on the edge of the glue application point. The diaphragm sample was cut to a suitable size, fixed to the sample post with conductive adhesive, and then subjected to a gold sputtering process to increase the conductivity of the sample. Scanning electron microscopy (SEM) was then used to image the sample. The morphology of the adhesive-coated diaphragm dots was observed at low magnification, and the longest diameter obtained by connecting any two points on the edge of the adhesive-coated dots was marked and measured.

[0111] (4) Viscosity test of the adhesive paste Transfer the prepared adhesive slurry into a centrifuge tube and fill it to a certain height. Select rotor No. 4 (range 50~2000 mpa.s) and start the instrument at a speed of 60 rpm to perform viscosity testing.

[0112] To test the impact of the separator on battery performance, the applicant formed bare cells from the separators of each embodiment and comparative example according to the following steps, and conducted performance tests on each cell: (1) Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate (LiFePO4), the positive electrode conductive agent acetylene black and the positive electrode binder polyvinylidene fluoride (PVDF) are stirred in N-methylpyrrolidone (NMP) at a weight ratio of 98:1:1 to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector aluminum foil, and after drying, cold pressing and other processes, the positive electrode sheet is obtained.

[0113] (2) Preparation of negative electrode sheet: The negative electrode active material artificial graphite, conductive agent acetylene black, adhesive styrene-butadiene rubber (SBR) and thickener sodium carboxymethyl cellulose (CMC) are thoroughly mixed in deionized water at a weight ratio of 96:1:2:1 to form a uniform negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector copper foil, and after drying, cold pressing and other processes, the negative electrode sheet is obtained.

[0114] (3) Bare cell assembly: The prepared positive electrode, separator and negative electrode are placed in sequence, so that the separator is in the middle of the positive and negative electrode to play a role in isolation. The bare cell is obtained by winding and hot pressing. The adhesive layer of the separator faces the positive electrode. Hot pressing conditions: hot pressing temperature is 95℃, pressure is 13.5T, and time is 90s.

[0115] (4) Characterization of electrolyte wettability: The wound bare battery cell was tilted and fixed, with one end immersed in a tank containing electrolyte. Based on the working principle that the electrolyte spontaneously rises along the tiny pores between the electrode and the separator, as well as the pores of the separator and electrode materials themselves, due to capillary force, the height of the electrolyte rising along the bare battery cell within 10 hours was recorded. The electrolyte composition was as follows: In an argon atmosphere glove box with a water content of <10ppm, EC, PC, and DEC were mixed in a volume ratio of EC:PC:DEC = 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the organic solvent and mixed evenly to obtain an electrolyte with a lithium salt concentration of 1mol / L.

[0116] (5) Ion transport impedance test The bare cell obtained in step (3) was encapsulated using an aluminum-plastic film, and an electrolyte was injected (the specific composition is described in the electrolyte wetting characterization section). The cell was then allowed to stand and vacuum-sealed to obtain the cell. The positive and negative electrodes of the cell were connected to the working electrode and counter electrode of an electrochemical workstation, respectively, with the reference electrode placed between the separator and the positive / negative electrodes. The amplitude was set to 5 mV, and the frequency range was set to 10^-2~10^6 Hz. The test was started, and the resistance of the bare cell was obtained by observing and processing the resulting Nyquist plot.

[0117] (6) Cell apparent condition test: Observe and record the appearance of the bare cell after hot pressing. Then, disassemble the bare cell obtained after hot pressing in step (3) and observe the disassembly interface.

[0118] The following conclusions can be drawn from Table 1: In Comparative Examples 1, 2-3, and 8, after the adhesive slurry was applied, the drying temperature was gradually reduced from 85°C to 75°C. As the drying temperature decreased, the difference in the nitrogen element weight ratio between the second adhesive layer in the adhesive layer, on the surface layer away from the porous substrate and the inner layer near the porous substrate, gradually decreased from 12% to 0.8% in the thickness direction of the adhesive layer. Correspondingly, only the battery cell with a nitrogen element weight ratio difference of 12% (Comparative Example 1) exhibited an open-cell phenomenon (see...). Figure 6 At other temperatures, the cells are well-formed without openings, and the disassembly interface is good with no obvious wrinkles. This is because excessively high oven temperatures cause the solvent in the adhesive slurry to evaporate from the inner layer to the outer layer, thereby causing the second binder in the adhesive slurry to migrate to the surface of the adhesive layer, resulting in an increase in the difference in the nitrogen element weight ratio. As the second binder migrates to the surface of the adhesive layer, the adhesion between the first binder particles and the porous substrate deteriorates, further leading to a poorer adhesion between the separator and the positive electrode, thus causing the bare cells after hot pressing to have openings.

[0119] Comparative Examples 2-3, Examples 6-7, and Example 8, when the difference in the weight ratio of nitrogen element in the surface and inner layers of the second adhesive layer remains constant, the spraying flow rate during spraying is adjusted to allow the membrane to be sprayed at any 1 cm depth. 2 The area ratio of the first adhesive relative to the porous substrate increased from 3% to 32%. Test results showed that when the area ratio of the first adhesive was 3%, the corresponding cell exhibited openings, mainly due to weak adhesion between the separator and the positive electrode. When the area ratio of the first adhesive was 5%, 20%, and 30%, the corresponding cells were well-formed without openings and had good disassembly interfaces without obvious wrinkles. However, when the area ratio of the first adhesive further increased to 32% (Comparative Example 3), wrinkles appeared at the cell disassembly interface (see...). Figure 7 Furthermore, the cell impedance increases and the electrolyte rise height decreases. Therefore, when the weight ratio of the same element in the second adhesive layer in the coating layer is between 0 and 10% in the coating layer thickness direction, and in any 1cm section of the separator, the cell impedance increases and the electrolyte rise height decreases. 2 When the area ratio of the first adhesive to the porous substrate is 5% to 30%, the diaphragm that meets both of these conditions has sufficient adhesive strength, which can effectively avoid the problem of cell opening caused by weak adhesive strength.

[0120] In Examples 1, 4-5, and 8, while maintaining the difference in the weight ratio of the same element in the surface and inner layers of the coating layer of the second adhesive, and keeping the area ratio of the first adhesive relative to the porous substrate per unit area constant, the viscosity and component content of the coating slurry were adjusted to regulate the longest diameter obtained from any two points on the edge of the first adhesive dot in the coating layer. When this diameter was increased from 100 μm to 400 μm and then to 800 μm, the corresponding cell surface was regular without openings, and the disassembly interface was good with no obvious wrinkles (see...). Figure 9 When the longest diameter continued to increase to 850 μm (Example 1), the electrolyte climbing height decreased significantly and the ionic impedance increased greatly. At the same time, obvious wrinkling was observed at the cell disassembly interface (see...). Figure 8 This is because when the bonding strength between the separator and the positive electrode is too high, the first adhesive will be excessively squeezed into the microporous structure of the separator, causing some pores to be blocked, thus increasing the resistance to lithium-ion transmission. In addition, excessive bonding strength will also cause the pore volume between the adhesive layer and the positive electrode to become smaller, resulting in poor electrolyte wetting and interface wrinkles.

[0121] In summary, when the weight ratio of the same element in the second adhesive layer, located away from the base film and in the inner layer, is between 0 and 10% in the thickness direction of the adhesive coating, and the membrane is at any 1 cm depth... 2 When the first adhesive accounts for 5% to 30% of the area of ​​the porous substrate, it can effectively avoid the problem of cell opening caused by insufficient adhesive strength. When the longest diameter of any two points on the edge of the first adhesive aggregate dots is 100 to 800 μm, it can also avoid problems such as poor electrolyte wettability, high ionic impedance, and cell interface wrinkling caused by excessive adhesive strength, which in turn leads to poor electrical performance.

[0122] As shown in Table 2, when using a separator coated with a mixture of PMMA and ceramic, similar results are achieved as with spray-coated separators: when the weight ratio difference of the same element in the second adhesive layer along the thickness direction of the coating is between 0% and 10%, and the area ratio of the first adhesive relative to the porous substrate is between 5% and 30%, the problem of cell opening due to weak adhesion can be avoided. In Comparative Example 4, the weight ratio difference of the same element in the second adhesive layer between the surface and inner layers of the coating is greater than 10%, resulting in cell opening (see...). Figure 10 In Comparative Example 5, the area ratio of the first adhesive in the coating layer was too low, resulting in an opening in the battery cell; in Comparative Example 6, the area ratio of the first adhesive relative to the porous substrate in the coating layer was too large, and although the battery cell did not open, wrinkles appeared at the disassembly interface of the battery cell (see...). Figure 11If the longest diameter of any two points on the edge of the first adhesive aggregate dot is within the range of 3~10μm, problems such as poor electrolyte wetting and interface wrinkling caused by excessive adhesive strength can be avoided. In the coating layer of Example 9, the longest diameter of any two points on the edge of the first adhesive aggregate dot is greater than 10μm, the cell is regular and without openings, but there are slight wrinkles at the disassembly interface; while in the coating layer of Example 16, the longest diameter of any two points on the edge of the first adhesive aggregate dot is within the limited range, the cell is regular and without openings, and the disassembly interface is good with no obvious wrinkles.

[0123] This invention precisely controls the bonding strength between the separator and the positive electrode by controlling the weight ratio difference of the same elements in the surface and inner layers of the second adhesive in the separator coating layer to be 0%~10% in the thickness direction of the coating layer, and by controlling the area ratio of the first adhesive relative to the substrate within any 1 square centimeter of the separator to be 5%~30%. This ensures that the bonding strength is within a reasonable range, guaranteeing sufficient bonding strength and effectively avoiding adverse risks such as bare cell opening, difficulty in casing, and tab redundancy caused by insufficient bonding strength. Simultaneously, it allows for appropriate electrode rebound space, preventing problems such as core interface wrinkling and excessive impedance caused by excessive bonding strength. Therefore, this invention effectively overcomes some practical problems in the prior art, thus possessing high utilization value and practical significance.

[0124] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A diaphragm, characterized in that, include: Porous substrate; An adhesive layer is disposed on at least one side of the porous substrate; The adhesive layer includes a first adhesive and a second adhesive, wherein the softening point of the second adhesive is lower than that of the first adhesive. In the second adhesive, the weight ratio of the same element in the surface layer of the coating layer is greater than or equal to the weight ratio in the inner layer, and the difference between the two is 0% to 10%; wherein, the surface layer is the portion away from the porous substrate along the thickness direction of the coating layer, the inner layer is the portion close to the porous substrate along the thickness direction of the coating layer, and the thickness of the surface layer accounts for 1 / 2 of the total thickness of the coating layer; Within any 1 square centimeter area of ​​the diaphragm, the area ratio of the first adhesive to the porous substrate is 5% to 30%.

2. The diaphragm according to claim 1, characterized in that, The first adhesive includes at least one of polyacrylates, hydroxymethyl acrylates, carboxymethyl cellulose salts, and polyvinylidene fluoride adhesives; The second adhesive includes at least one of polyether polyurethane, polyester polyurethane, polyether-polyester hybrid polyurethane, acrylic modified polyurethane, acrylic modified acrylonitrile, and acrylic modified polybenzimidazole.

3. The diaphragm according to claim 1, characterized in that, The porous substrate includes a base film and a ceramic layer disposed on the base film. The adhesive layer is disposed on the ceramic layer. The longest diameter d obtained by connecting any two points on the edge of the aggregate adhesive dots of the first adhesive in the adhesive layer ranges from 100μm to 800μm.

4. The diaphragm according to claim 1, characterized in that, The porous substrate includes a base film, the adhesive layer is disposed on the base film, the adhesive layer also includes a ceramic material, and the longest diameter d obtained by connecting any two points on the edge of the aggregate adhesive dots of the first adhesive in the adhesive layer ranges from 3μm to 10μm.

5. The diaphragm according to claim 1, characterized in that, The mass ratio of the first adhesive to the second adhesive is (5~20):(1~6).

6. A method for preparing the diaphragm according to any one of claims 1 to 5, characterized in that, Includes the following steps: Provide porous substrates; The first adhesive, the second adhesive, and the solvent are mixed evenly to obtain the adhesive paste. The adhesive slurry is applied to at least one side of the porous substrate and dried to obtain the diaphragm.

7. The method for preparing the diaphragm according to claim 6, characterized in that, The first adhesive accounts for 5% to 20% of the total mass of the adhesive slurry, the second adhesive accounts for 1% to 6% of the total mass of the adhesive slurry, and the solvent includes deionized water.

8. The method for preparing the diaphragm according to claim 6, characterized in that, The adhesive slurry is applied to the porous substrate by a spraying process. The viscosity of the adhesive slurry is 70~110 mpa·s, and the drying temperature is 70~85℃.

9. The method for preparing the diaphragm according to claim 6, characterized in that, The adhesive slurry also includes ceramic materials. The adhesive slurry is coated onto the porous substrate by a roller coating process. The viscosity of the adhesive slurry is 80~120 mPa·s, and the drying temperature is 70~85℃.

10. A battery, characterized in that, The membrane includes a positive electrode, a negative electrode, and a separator as described in any one of claims 1 to 5, or a separator prepared by any one of claims 6 to 9, wherein the separator is disposed between the positive electrode and the negative electrode, and the coating layer of the separator faces the positive electrode.

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