Diaphragm and electrochemical device

Abstract

The invention provides a diaphragm and an electrochemical device, and particularly relates to the technical field of batteries. The diaphragm comprises a porous base membrane, a heat-resistant layer and an adhesive layer, wherein the heat-resistant layer is arranged on at least one surface of the porous base membrane; the bonding layer is at least arranged on the other surface of the contact surface of the heat-resistant layer and the porous base membrane; the effective bonding degree R value of the side, provided with the heat-resistant layer, of the diaphragm is 40%-80%, and the difference value between the maximum value and the minimum value of the effective bonding degree R value is smaller than 30%; wherein the effective bonding degree R value represents the ratio of the effective bonding area to the theoretical bonding area of the diaphragm. According to the present invention, the uniformity of the adhesion between the diaphragm and the pole piece is high, and the adhesion has a certain degree, such that the cell wrinkle deformation caused by the pole piece expansion after the circulation can be effectively improved, the flatness is improved, and the cycle performance of the battery is further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a diaphragm and an electrochemical device. Background Art

[0002] As one of the important components of the battery, the diaphragm not only isolates the positive and negative electrodes to avoid short circuits, but its porous structure also provides a channel for the migration and transmission of lithium ions. At present, most commercial diaphragms are polyolefin diaphragms. These materials have relatively low melting points and are prone to large shrinkage at high temperatures, which causes the positive and negative electrodes to contact and quickly accumulate a large amount of heat, causing the battery to catch fire or explode due to short circuits, affecting the safety performance of the battery. In order to improve the heat resistance and safety of the diaphragm, a heat-resistant layer is usually coated on one side of the diaphragm substrate. The heat-resistant material in the heat-resistant layer is used to improve the heat resistance of the diaphragm and increase the adhesion of the diaphragm to a certain extent. In addition, in some cases, an adhesive layer is also provided on the heat-resistant layer. The adhesive layer will stimulate the adhesion when hot-pressed with the positive and negative electrode sheets, which has the effect of battery shaping.

[0003] However, after multiple cycles of lithium-ion batteries, the active materials in the electrodes, especially the negative electrodes, tend to expand. If the bonding force between the separator and the positive and negative electrodes is unevenly distributed, and the binding force between the separator and the positive and negative electrodes is weak, then when the electrodes expand, the positive and negative electrodes and the separator are prone to detachment or dislocation points. This provides space for the stress generated by the expansion of the electrodes to be released, causing the entire battery cell to wrinkle and deform, thereby reducing the cycle life of the battery. If the amount of glue in the adhesive layer is stacked on this basis, although the adhesion between the electrode and the separator adhesive layer can be improved during the hot pressing stage of the battery cell, after hundreds of battery cycles, the separator will detach or dislocate points between the heat-resistant layer and the adhesive layer, resulting in the battery cell wrinkling and deforming as the electrode expands, affecting the battery cycle performance. Summary of the Invention

[0004] In view of the above problems existing in the prior art, the present invention provides a diaphragm and an electrochemical device to improve the problem of wrinkling and deformation of battery cells after long-term battery cycling.

[0005] To achieve the above-mentioned and other related objectives, the present invention provides, in a first aspect, a separator comprising a porous base film, a heat-resistant layer, and an adhesive layer, wherein the heat-resistant layer is disposed on at least one surface of the porous base film; the adhesive layer is disposed on at least one surface of the heat-resistant layer that is in contact with the porous base film; the effective adhesion R value of the separator on the side provided with the heat-resistant layer is 40% to 80%, and the difference between the maximum and minimum values ​​of the effective adhesion R value is less than 30%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the separator to the theoretical bonding area. The separator of the present invention maintains a certain bonding force between the separator and the electrode by adjusting the ratio of the effective bonding area to the theoretical bonding area. At the same time, the difference between the ratio of the effective bonding area to the theoretical bonding area is controlled to be less than 30%, thereby ensuring uniform bonding between the separator and the electrode, thereby improving the wrinkling and deformation of the battery cell caused by the expansion of the electrode after battery cycling, improving the flatness of the battery cell, and thereby improving the cycling performance of the battery.

[0006] In one embodiment of the present invention, the surface roughness Ra of the heat-resistant layer is less than 1 μm. Furthermore, by adjusting the surface roughness of the separator heat-resistant layer, the flatness of the microscopic surface topography of the heat-resistant layer can be adjusted. Within a limited range, the surface flatness of the heat-resistant layer is high. Compared with the uneven state of ups and downs, it can ensure the uniform distribution of effective contact points with the adhesive layer while increasing the overall effective contact area, thereby enhancing the adhesion of the adhesive layer to the surface of the heat-resistant layer and the uniform distribution of adhesion, thereby improving the overall flatness and wrinkle resistance of the battery cell.

[0007] In one embodiment of the present invention, the heat-resistant layer includes inorganic particles and a binder, and the bulk density of the heat-resistant layer is A=(0.4-0.5)×ρ, where ρ represents the true density of the inorganic particles in g / cm 3 .

[0008] In one embodiment of the present invention, the heat-resistant layer includes inorganic particles and a binder, and the standard deviation of the particle size of the inorganic particles is σ≤0.9*d, where d represents the thickness of the heat-resistant layer. In addition, further limiting the standard deviation of the particle size or the bulk density of the inorganic particles in the heat-resistant layer makes the heat-resistant layer more compact, and the effective bonding area between the adhesive layer and the heat-resistant layer is increased. When the separator and the positive and negative electrodes are subjected to hot pressing to stimulate the bonding force, the uniformity of the separator bonding is ensured, and the bonding strength of the dense heat-resistant layer is further increased, thereby achieving a better cell shaping effect and improving the performance of the cell.

[0009] In one embodiment of the present invention, the volume proportion of the inorganic particles in the heat-resistant layer is 80% to 99%.

[0010] In one embodiment of the present invention, the thickness of a single surface of the heat-resistant layer is 0.5-5 μm.

[0011] In one embodiment of the present invention, the inorganic particles are selected from at least one of aluminum oxide, hydrated aluminum oxide, aluminum hydroxide, silicon dioxide, titanium dioxide, barium sulfate, barium titanate, magnesium hydroxide, and magnesium oxide.

[0012] In one embodiment of the present invention, the coating amount of the adhesive layer on one side is 0.1 to 2 g / m 2 .

[0013] In one embodiment of the present invention, the adhesive is selected from at least one of polyacrylonitrile and its copolymers, polyacrylic acid and its copolymers, polyacrylate and its copolymers, styrene-butadiene copolymers, styrene-acrylate copolymers and fluorine-based polymer materials.

[0014] A second aspect of the present invention provides an electrochemical device, comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, wherein the separator is the separator in any of the above embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 Schematic diagram of the structure of the diaphragm of the present invention in one embodiment;

[0017] Figure 2 A schematic diagram of the internal particle structure of a diaphragm according to one embodiment of the present invention;

[0018] Figure 3 The flatness of the negative electrode surface after the battery assembled with the diaphragm of the present invention is disassembled in a cycle, wherein Figure 3 (a) is a photo of flatness grade A. Figure 3 (b) is a photo of flatness level B. Figure 3 (c) A photo showing flatness level C; Figure 3 (d) A photo showing level D flatness; Figure 3 (e) is a photo of flatness grade E.

[0019] Component number description:

[0020] 100. Diaphragm; 101. Porous base film; 102. Heat-resistant layer; 103. Adhesive layer. DETAILED DESCRIPTION

[0021] The following describes the embodiments of the present invention by means of specific examples. 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. The 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. It should be noted that the features in the following embodiments and examples can be combined with each other unless they conflict. It should also be understood that the terms used in the embodiments of the present invention are intended to describe specific embodiments, not to limit the scope of protection of the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] When referring to a numerical range herein, unless otherwise specified, the distribution of the values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe a feature or characteristic, these numerical ranges may be combined.

[0024] As used herein, "plurality," "multiple," "multiple times," etc., unless otherwise specified, refer to a quantity greater than or equal to 2. For example, "one or more" means one or more than or equal to two.

[0025] Herein, “further”, “further”, “particularly”, etc. are used for descriptive purposes to indicate differences in content, but should not be understood as limiting the scope of protection of the present invention.

[0026] The definitions of the professional terms used in this article are as follows:

[0027] Effective adhesion R value: refers to the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area, wherein the theoretical bonding area refers to the area that can be covered by the diaphragm adhesive layer, that is, the surface area of ​​the diaphragm adhesive layer; the effective bonding area refers to the area where the diaphragm adhesive layer exerts a bonding effect, that is, the surface area retained on the adherend after the diaphragm adhesive layer is separated from the adherend; the difference in the effective adhesion R value refers to the difference between the maximum and minimum R values.

[0028] Surface roughness: It is the unevenness of the surface with small spacing and tiny peaks and valleys; the rougher the surface, the smaller the effective contact area between the surface and other surfaces.

[0029] Bulk density: refers to the mass per unit volume of a material, including the material entity, internal pores (open pores and closed pores), and spaces between particles.

[0030] True density, also known as true density, refers to the mass per unit volume of a material in an absolutely dense state. It is calculated by dividing the mass of a powder by the volume excluding the voids inside and outside the particles (true volume). The formula for calculating true density is ρ = m / v, where ρ represents true density, m represents the mass of the sample, and v represents the volume of the sample.

[0031] Particle size: refers to the size of a particle, usually expressed as diameter. For spherical particles, the particle size is the diameter; for non-spherical particles, the particle size is described by the equivalent diameter (such as volume equivalent diameter, area equivalent diameter, etc.).

[0032] Particle size standard deviation: reflects the degree of dispersion of particle size in the particle system. The larger the standard deviation, the more uneven the particle size distribution; the smaller the standard deviation, the more concentrated the particle size distribution.

[0033] The calculation formula for the particle size standard deviation σ is: Among them, D i is the particle size of a single particle, is the average particle size, and N is the total number of particles.

[0034] Single-sided coating amount: refers to the amount of slurry coated on one surface during the preparation process.

[0035] The parameters herein, such as the effective adhesion R value, the difference between the maximum and minimum effective adhesion R values, the standard deviation of the particle size of the inorganic particles, the surface roughness of the heat-resistant layer, and the bulk density of the heat-resistant layer, are affected by various aspects of the diaphragm production process, such as material selection, pulping, coating method, selection of production line equipment, and setting of equipment parameters. Those skilled in the art should understand that at least the following single or combined controls can be used to adjust the diaphragm structure and obtain the corresponding diaphragm. It should be noted that the following description is an example of a manufacturing method for obtaining a corresponding diaphragm and is not limited to this method.

[0036] See also Figure 1 In a first aspect, the present invention provides a diaphragm 100, which includes a porous base film 101, a heat-resistant layer 102, and an adhesive layer 103. The porous base film 101 serves as the substrate of the diaphragm 100 and is made of an insulating porous material that allows lithium ions to pass through. The heat-resistant layer 102 is provided on at least one surface of the porous base film 101. For example, the porous base film 101 has a first surface and a second surface that are oppositely arranged along its thickness direction. The heat-resistant layer 102 can be provided on one of the first surface and the second surface of the porous base film 101, or can be provided on both the first surface and the second surface ( Figure 1Only the case of single-sided arrangement is shown as an example). The adhesive layer 103 is provided on at least the other side of the contact surface between the heat-resistant layer 102 and the porous base film 101, that is, the adhesive layer 103 can be provided only on the heat-resistant layer 102. In this case, the adhesive layer 103 is provided on the side of the heat-resistant layer 102 facing away from the porous base film 101.

[0037] As an example, the adhesive layer 103 can also be arranged on the heat-resistant layer 102 and the porous base membrane 101 at the same time. In this case, the heat-resistant layer 102 is only arranged on one surface of the porous base membrane 101, and the adhesive layer 103 is arranged on the side of the heat-resistant layer 102 facing away from the porous base membrane 101 and the side of the porous base membrane 101 where the heat-resistant layer 102 is not arranged.

[0038] In some embodiments, the porous base membrane 101 as the matrix of the diaphragm 100 can be any porous material that can be used in the diaphragm in the art. As an example, the porous base membrane 101 is selected from one or more of polyethylene, polypropylene, non-woven fabric, polyethylene terephthalate, polyimide, and polypropylene-polyethylene-polypropylene composite film. That is, the porous base membrane 101 can be selected from any one of the above-mentioned materials, such as polyethylene, non-woven fabric, polypropylene or polyimide, or a mixture of the same materials with different weight average molecular weight, viscosity average molecular weight, and number average molecular weight; the porous base membrane 101 can also be selected from a combination of any two or more of the above-mentioned materials, for example, the porous base membrane 101 is selected from a combination of polyethylene and polypropylene, or a combination of non-woven fabric, polyethylene terephthalate and polyimide, and so on. There is no particular limitation on the method for manufacturing the porous base membrane as long as a porous base membrane 101 with the above-mentioned characteristics can be obtained. The following are examples of methods for manufacturing porous base membranes: forming a porous membrane by melting and extruding a polymer resin; or forming a non-woven fabric by aggregating filaments obtained by melting / spinning a polymer resin; or forming a composite membrane of two or more layers of a substrate prepared by the above two methods, etc.

[0039] The thickness and porosity of the porous base membrane 101 can be set according to actual production needs and are not limited here. As an example, the thickness of the porous base membrane 101 is 3 to 20 μm. For example, the thickness of the porous base membrane 101 can be 3 μm, 5 μm, 10 μm, 15 μm, or 20 μm, etc. The porosity of the porous base membrane 101 is 20% to 70%. For example, the thickness of the porous base membrane 101 can be 20%, 40%, 60%, or 70%, etc.

[0040] The porous base film 101 has a low melting point and is prone to large shrinkage at high temperatures, thereby affecting the safety performance of the battery. Therefore, the diaphragm 100 of the present invention is provided with a heat-resistant layer 102 on at least one side of the porous base film 101 to improve the heat resistance of the diaphragm 100.

[0041] See also Figure 1 and Figure 2 In one embodiment, the heat-resistant layer 102 includes inorganic particles and a binder, wherein the inorganic particles can be selected from high-temperature resistant ceramic materials, including but not limited to at least one of aluminum oxide, hydrated aluminum oxide (boehmite), aluminum hydroxide, silicon dioxide, titanium dioxide, barium sulfate, barium titanate, magnesium hydroxide, and magnesium oxide. That is, the inorganic particles can be selected from any one of the materials listed above, such as aluminum oxide, titanium dioxide, or magnesium oxide, etc. The inorganic particles can also be selected from any combination of two or more of the listed materials, such as a combination of barium sulfate and barium titanate, or a combination of aluminum oxide, hydrated aluminum oxide, and aluminum hydroxide, etc. It should be noted that when the inorganic particles are a combination of two or more, there is no restriction on the ratio between the components in the composition, and they can be mixed in any ratio. The binder is selected from at least one of polyacrylonitrile and its copolymers, polyacrylic acid and its copolymers, polyacrylates and its copolymers, acrylic acid salts, styrene-butadiene copolymers, styrene-acrylate copolymers, and fluorine-based polymer materials. That is, the adhesive can be selected from any one of the materials listed above, such as polyacrylonitrile, or polyacrylic acid, or polyacrylate, or polyacrylate, or styrene-butadiene copolymer, etc. The adhesive can also be selected from a combination of any two or more of the materials listed above, such as a combination of styrene-acrylate copolymer and fluorine-based polymer material.

[0042] The heat-resistant layer 102 can be prepared by the following method: first, inorganic particles, adhesives and solvents are mixed and stirred in proportion to obtain a heat-resistant layer slurry, and then the heat-resistant layer slurry is coated on the surface of the porous base film 101, and the heat-resistant layer 102 is obtained after curing and drying. The amount of the above-mentioned solvent does not need to be particularly limited, and can be selected according to actual production needs, based on the ability to evenly disperse the inorganic particles and the adhesive. As an example, the amount of solvent used satisfies the solid content of the heat-resistant layer slurry of 35wt% to 45wt%, which can be specifically listed as 35wt%, 40wt% or 45wt%. The solvent is, for example, deionized water.

[0043] In one embodiment, the thickness of the heat-resistant layer 102 can be set according to actual needs. As an example, the thickness of a single side of the heat-resistant layer 102 is 0.5 to 5 μm. For example, the thickness of a single side of the heat-resistant layer 102 can be 0.5 μm, 2 μm, 3 μm or 5 μm.

[0044] Regarding the adhesive layer 103, the adhesive layer 103 is provided as the outermost layer of the separator on the porous base film 101 or the heat-resistant layer 102, and can be bonded to the electrode through the adhesive layer 103. There are no particular restrictions on the adhesive contained in the adhesive layer 103 as long as it can bond to the electrode. The adhesive in the adhesive layer 103 is selected from at least one of polyacrylonitrile and its copolymers, polyacrylic acid and its copolymers, polyacrylate and its copolymers, acrylic acid salts, styrene-butadiene copolymers, styrene-acrylate copolymers, and fluorine-based polymer materials. That is, the adhesive can be selected from any one of the materials listed above, such as polyacrylonitrile, polyacrylic acid, polyacrylate, polyacrylate, or styrene-butadiene copolymer, etc. The adhesive can also be selected from a combination of any two or more of the materials listed above, such as a combination of a styrene-acrylate copolymer and a fluorine-based polymer material.

[0045] The adhesive layer 103 can be prepared by the following method: an adhesive, a solvent or other auxiliary agent are mixed and stirred in proportion to obtain an adhesive layer slurry, which is then coated on the porous substrate 101 or the heat-resistant layer 102 , and dried to evaporate and remove the solvent, thereby obtaining the adhesive layer 103 .

[0046] The method for coating the heat-resistant layer 102 and the adhesive layer 103 is not particularly limited as long as the required layer thickness and coating area can be achieved. Examples include gravure coaters, small-diameter gravure coaters, reverse roll coaters, transfer roll coaters, kiss roll coaters, dip coaters, doctor coaters, air knife coaters, blade coaters, rod coaters, extrusion coaters, casters, die coaters, screen printing, and spray coating.

[0047] During the battery cycle, the expansion of the electrode sheet causes the positive and negative electrodes and the separator to fall off or misalignment points, which will cause the overall wrinkle deformation of the battery cell, thereby reducing the cycle life of the battery. The inventors of the present application found in their research that the overall wrinkle deformation of the battery cell can be reduced by controlling the effective adhesion R value of the separator with a heat-resistant layer within the range of 40% to 80%. When the R value difference (the difference between the maximum and minimum values) is controlled below 30%, the adhesion force between the separator and the electrode sheet is evenly distributed and has a certain adhesion force. When the negative electrode sheet expands after charging, the binding force between the positive and negative electrodes and the separator is increased, thereby improving the problems of wrinkle deformation of the battery cell and low battery cycle performance.

[0048] In the present invention, the effective adhesion R value of the side of the diaphragm provided with the heat-resistant layer is 40% to 80%. Furthermore, the R value can be 50% to 70%, and can be specifically listed as 50%, 60% or 80%. If the R value is too low, the bonding strength between the diaphragm and the electrode is poor, the shaping effect of the bare cell is poor, and the cell is prone to deformation and expansion. The uneven resistance on the interface leads to increased polarization, and the capacity loss during the cycle increases, which narrows the lithium plating window of the cell. If the R value is too high, there is too little space left between the diaphragm and the electrode, and the amount of electrolyte retained will be greatly reduced. In the later stage of the cycle, the electrolyte is insufficient, and purple spots are likely to occur. At the same time, the difference between the maximum and minimum values ​​of the effective adhesion R value is less than 30%; further, the R value difference is less than 20%, and can be specifically listed as 1%, 10%, 15%, 29%, etc. The smaller the R-value difference, the more uniform the adhesion distribution between the separator 100 and the electrode, which can effectively improve the wrinkling and deformation of the battery cell caused by electrode expansion after cycling and improve flatness. Here, the effective adhesion R value or R-value difference of the separator 100 on the side with the heat-resistant layer 102 can be adjusted by adjusting at least the weight ratio of the adhesive in the adhesive layer 103, the coverage of the adhesive layer 103 on the heat-resistant layer 102, the surface density of the adhesive layer 103, and the parameters of the adhesive layer 103 coating equipment, and adjusting the structure, morphology, and distribution of the adhesive layer 103 on the heat-resistant layer 102 side.

[0049] In one embodiment, the surface roughness Ra of the heat-resistant layer 102 is less than 1 μm. Furthermore, the surface roughness Ra of the heat-resistant layer 102 is less than 0.8 μm. The surface roughness of the heat-resistant layer 102 reflects the flatness of the surface of the heat-resistant layer 102, which affects the effective contact area of ​​the adhesive layer 103 attached to the surface of the heat-resistant layer 102. The smaller the roughness, the flatter the surface of the heat-resistant layer 102, the more evenly distributed the effective contact points between the heat-resistant layer 102 and the adhesive layer 103, and the more evenly distributed the bonding points between the separator and the positive and negative electrode sheets. At the same time, increasing the overall effective contact area also enhances the overall adhesion of the adhesive layer 103 to the surface of the heat-resistant layer 102, which can further effectively improve the wrinkling and deformation of the battery cell caused by the expansion of the electrode sheet after cycling, improve flatness, and further improve the battery cycle performance. If the surface roughness Ra of the heat-resistant layer 102 is greater than 1 μm, the surface of the heat-resistant layer 102 is relatively uneven and has pits. When the adhesive layer 103 is applied to the surface of the heat-resistant layer 102, the pits on the surface of the heat-resistant layer 102 cannot make good contact with the adhesive layer 103, and may only have partial contact or no contact at all. The uniformity and area of ​​the adhesion points of the adhesive layer 103 on the surface of the heat-resistant layer 102 will be relatively reduced. Here, at least the solid content of the heat-resistant layer 102 slurry, the particle size of the inorganic particles in the heat-resistant layer 102, and the drying process of the heat-resistant layer 102 can be adjusted to adjust the effect of the heat-resistant layer 102 slurry during leveling or after setting, so as to achieve the adjustment of the surface roughness Ra of the heat-resistant layer 102.

[0050] In one embodiment, the bulk density of the heat-resistant layer 102 is A=(0.4-0.5)×ρ, where ρ represents the actual density of the inorganic particles, in units of g / cm 3 That is, the bulk density of the heat-resistant layer 102 is related to the type of inorganic particles. For example, if the inorganic particles are boehmite, the actual density of boehmite is 3.07 g / cm 3 , the bulk density should be limited to 1.228~1.535g / cm 3 The bulk density A of the heat-resistant layer 102 can be specifically listed as 0.4ρ, 0.45ρ or 0.5ρ, etc. The bulk density of the heat-resistant layer 102 that meets the above conditions can make the bulk morphology of the heat-resistant layer 102 denser, further improve the bonding force between the heat-resistant layer 102 and the adhesive layer 103, obtain a better battery cell shaping effect, and improve the performance of the battery cell. Here, at least the bulk density of the heat-resistant layer 102 can be adjusted by transferring the heat-resistant layer 102 slurry to the porous base film 101 to achieve the transfer rate during coating, the viscosity of the heat-resistant layer 102 slurry, and the drying process of the heat-resistant layer 102.

[0051] In one embodiment, the particle size standard deviation σ of the inorganic particles is ≤ 0.9*d, where d represents the thickness of the heat-resistant layer. Heat-resistant particles with uniform particle size are selected for stacking to obtain a uniform, flat and dense stacking morphology, thereby further increasing the overall effective bonding area of ​​the diaphragm and increasing the overall bonding strength of the diaphragm. Specifically, for example, when the thickness of the heat-resistant layer is 2 μm, the particle size standard deviation σ of the inorganic particles can be 0.1 μm, 0.5 μm, 1.0 μm or 1.8 μm, etc. The smaller the particle size standard deviation σ of the inorganic particles, the more uniform the particle size. As methods for adjusting the particle size distribution of inorganic particles, for example, a method of crushing the inorganic filler using a ball mill, a bead mill, an air flow mill, etc., and adjusting it to the desired particle size distribution, a method of adjusting fillers with multiple particle size distributions and then mixing them, etc. can be cited. In one embodiment, the volume proportion of inorganic particles in the heat-resistant layer 102 of the present invention is 80% or more. If the volume proportion of inorganic particles in the heat-resistant layer 102 is less than 80%, the heat resistance of the heat-resistant layer 102 is limited. In addition, the volume ratio of the adhesive in the heat-resistant layer 102 is too high, which can easily lead to closed pores in the coating due to swelling of the adhesive during the battery cell cycle, thereby affecting the cycle capacity of the battery cell. Furthermore, the volume proportion of inorganic particles in the heat-resistant layer 102 is 90% or more. More specifically, the volume proportion of inorganic particles in the heat-resistant layer 102 is 90% to 99%, specifically 93%, 95%, 97%, etc. If the volume proportion of inorganic particles exceeds 99%, the heat-resistant layer 102 is difficult to form.

[0052] In one embodiment, the coating weight of the adhesive layer 103 on one side is 0.1-2 g / m 2That is to say, in the process of preparing the diaphragm, when preparing the adhesive layer 103, 0.1 to 2 g of adhesive layer slurry is applied per square meter of the coating surface. Furthermore, the single-side coating amount of the adhesive layer 103 is 0.5 to 1.5 g / m 2 Furthermore, the single-sided coating amount of the adhesive layer 103 can be 0.8g / m 2 , 1.0g / m 2 or 1.5g / m 2 etc.

[0053] A second aspect of the present invention provides an electrochemical device comprising the separator 100 described above. The electrochemical device of the present invention can be any energy storage device known in the art, such as a supercapacitor, a lithium-ion secondary battery, a sodium-ion secondary battery, etc. The structure of the electrochemical device is described below using a lithium-ion secondary battery as an example.

[0054] A lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and is stacked or wound with the positive electrode sheet and the negative electrode sheet to form a bare cell.

[0055] Specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on the surface of at least one side of the positive electrode current collector. The positive electrode current collector is, for example, a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel or carbon. In addition to foil, the positive electrode current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam or non-woven fabric. Among them, the thickness of the positive electrode current collector is, for example, 8μm to 15μm. In this embodiment, the positive electrode current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm. The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent and a positive electrode adhesive. There is no specific limitation on the positive electrode active material, the positive electrode conductive agent and the positive electrode adhesive here, and those skilled in the art can choose according to actual needs.

[0056] The positive electrode active material can be selected from any material used in lithium ion batteries, that is, any compound that can reversibly embed and deintercalate lithium ions can be used. In the present invention, the positive electrode active material is selected from one or more combinations of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, nickel cobalt manganese metal oxide (NCM), but is not limited thereto. The positive electrode binder is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene or styrene butadiene rubber (SBR). The positive electrode conductive agent is, for example, one or at least two selected from conductive carbon black (Super P, Super S, 350G, etc.), acetylene black, graphene, carbon nanotubes, carbon fibers (VGCF), Ketjen black, and the like.

[0057] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one side of the negative electrode current collector. The negative electrode current collector is, for example, selected from a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foam copper current collector or a stainless steel current collector, and the thickness of the negative electrode current collector is, for example, 8μm to 15μm. In this embodiment, the negative electrode current collector is copper foil, and the thickness of the copper foil is, for example, 13μm. The negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is provided on either or both of the two opposite surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode adhesive and a thickener. The specific types of the negative electrode active material, the negative electrode conductive agent and the negative electrode adhesive are not specifically limited here. Materials known in the art that can be used in lithium-ion batteries can be used, and those skilled in the art can select according to actual needs.

[0058] The negative electrode active material is selected from compounds capable of intercalating and deintercalating lithium ions. In this embodiment, the negative electrode active material includes but is not limited to artificial graphite, natural graphite, etc. The negative electrode conductive agent is selected from one or more of acetylene black, conductive carbon black (Super P, Super S, 350G), carbon fiber (VGCF), carbon nanotubes (CNT), 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 thereof in any proportion; the thickener is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0059] The lithium-ion battery also includes an electrolyte, which can be a solid electrolyte or a liquid electrolyte. In this embodiment, the electrolyte is a liquid electrolyte (i.e., an electrolyte), and the liquid electrolyte includes a lithium salt and an organic solvent. The present application does not limit the specific types of lithium salts and organic solvents, and 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 LiPF6, LiBF4, LiN(SO2F)2 (abbreviated as LiFSI), LiN(CF3SO2)2 (abbreviated as LiTFSI), LiClO4, LiAsF6, LiB(C2O4)2 (abbreviated as LiBOB), and LiBF2C2O4 (abbreviated as LiDFOB). Furthermore, the organic solvent is a non-aqueous organic solvent, and the non-aqueous organic solvent includes any type of carbonate and / or carboxylic acid ester. Carbonates include cyclic carbonates and / or chain carbonates. The non-aqueous organic solvent may also include halogenated compounds of carbonates. Specifically, the organic solvent is selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, pentylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, methyl ethyl carbonate, gamma-butyrolactone, methyl formate, ethyl formate, ethyl propionate, propyl propionate, and tetrahydrofuran. The above-mentioned solvent can be used singly or in combination, and is preferably used in combination to improve the overall performance of the electrolyte. Specifically, it can be a composition of diethyl carbonate, dimethyl carbonate and ethylene carbonate or a composition of methyl formate, fluoroethylene carbonate and propyl propionate, etc.

[0060] Furthermore, some additives may be added to the electrolyte according to actual needs, such as ethylene sulfate (DTD), 1,3-propane sultone (PS), vinylene carbonate (VC) or vinyl ethylene carbonate (VEC).

[0061] Lithium-ion secondary batteries also include a shell. The shape and material of the shell depend on the type of lithium-ion battery. For example, if the lithium-ion battery is a soft-pack battery, its shell can be encapsulated with an aluminum-plastic film; if the lithium-ion battery is a square-shell battery or a cylindrical battery, the shell can be a square shell or a cylindrical shell that matches the shape of the bare battery cell, and the material can be stainless steel or other materials.

[0062] The lithium-ion secondary battery can be prepared according to methods known in the art, which will not be described in detail here.

[0063] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0064] [Examples 1-3]

[0065] This embodiment provides a separator comprising a porous base membrane, a heat-resistant layer, and an adhesive layer. The porous base membrane is a 7μm porous polyethylene film, and the heat-resistant layer comprises inorganic particles and an adhesive. In this embodiment, the heat-resistant layers are disposed on both surfaces of the porous base membrane; the adhesive layer comprises an adhesive and is disposed on the surface opposite the contact surface between the heat-resistant layer and the porous base membrane. The types and amounts of the various substances in the heat-resistant layer and adhesive layer are described in the preparation method and Table 1.

[0066] The preparation method of the diaphragm is as follows:

[0067] In the first step, deionized water and boehmite with a particle size distribution standard deviation σ of 2.05 μm are added to a double planetary mixer and dispersed at high speed at 40°C for 1 hour. Then, the adhesive polyacrylonitrile is added and stirred at low speed at room temperature for 1 hour to obtain a heat-resistant layer slurry with a solid content of 45%, wherein the dry matter ratio of the boehmite and the adhesive is 95:5. The heat-resistant layer slurry is then coated on both surfaces of a porous polyethylene membrane with a thickness of 7 μm using a gravure roller coating method. The single-sided coating thickness of the heat-resistant layer is 2 μm. The cell size of Examples 1-3 is a mesh pitch of 195 μm and a cell depth of 29 μm. The membrane is then dried at 45°C to obtain a double-sided heat-resistant layer diaphragm.

[0068] In the second step, deionized water and polyacrylonitrile auxiliary adhesive are added to a double planetary mixer and stirred at low speed for 1 hour at room temperature; then polyvinylidene fluoride (PVDF) powder is added and dispersed at high speed at 40°C for 3 hours to obtain an adhesive layer slurry with a solid content of 12%, wherein the dry matter ratio of PVDF to the auxiliary adhesive is 9:1. The adhesive layer slurry is applied to both surfaces of the above-mentioned heat-resistant layer diaphragm by spray coating. The rotor speed of the rotary spraying in Examples 1-3 is 8000 rpm, and then dried at 45°C to obtain a diaphragm with a heat-resistant layer and an adhesive layer on both sides. The coating amount of the adhesive layer on each side is 0.5g / m 2 By adjusting the spacing between the rotors of the rotary spray coating head, diaphragms with different adhesive layer coating uniformity can be prepared. In Example 1, the rotor spacing increases from left to right in increments of 16 mm, with an initial spacing of 150 mm. In Example 2, the rotors are equally spaced 150 mm apart. In Example 3, the rotor spacing increases from left to right in increments of 8 mm. Comparison of adhesive layer coating uniformity: [Example 2] > [Example 3] > [Example 1].

[0069] [Examples 4-6]

[0070] During the preparation of the diaphragm, the solid content of the heat-resistant layer slurry was adjusted to 42%, 38% and 35% respectively, and the other processes and parameters remained consistent with those in Example 2.

[0071] [Examples 7-9]

[0072] During the membrane preparation process, micro-gravure plates with different cell size designs were used for heat-resistant coating. Example 7 employed a micro-gravure plate with narrow openings and large depths for the surface cells; Example 9 employed a micro-gravure plate with wide openings and shallow depths for the surface cells; and Example 8 employed a cell design somewhere in between. The unit cell volume was consistent across the three designs. Other processes and parameters remained consistent with Example 2. Example 7 employed a cell pitch of 106 μm and a cell depth of 53 μm; Example 8 employed a cell pitch of 120 μm and a cell depth of 47 μm; and Example 9 employed a cell pitch of 141 μm and a cell depth of 40 μm.

[0073] [Example 10]

[0074] During the preparation of the diaphragm, the particle size distribution standard deviation σ of the selected boehmite was 1.80 μm, and the other processes and parameters were consistent with those in Example 2.

[0075] [Example 11]

[0076] During the preparation of the diaphragm, the particle size distribution standard deviation σ of the selected boehmite was 0.25 μm, and the other processes and parameters were consistent with those in Example 2.

[0077] [Example 12]

[0078] During the preparation of the diaphragm, the particle size distribution standard deviation σ of the selected boehmite was 1.06 μm, and the other processes and parameters were consistent with those in Example 2.

[0079] [Example 13]

[0080] During the preparation of the diaphragm, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the heat-resistant coating was applied using a micro-gravure roller with the same cell design as in Example 8. Other processes and parameters were consistent with those in Example 2.

[0081] [Example 14]

[0082] During the preparation of the diaphragm, the solid content of the heat-resistant layer slurry was adjusted to 42%, and the heat-resistant layer was coated using a micro-gravure roller with the same cell design as in Example 9. Other processes and parameters remained the same as in Example 2.

[0083] [Example 15]

[0084] During the preparation of the diaphragm, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the heat-resistant layer was coated using a micro-gravure roller with the same cell design as in Example 7. Other processes and parameters remained the same as in Example 2.

[0085] [Example 16]

[0086] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.06 μm, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the heat-resistant coating was applied using a micro-gravure roller with the same mesh design as in Example 7. Other processes and parameters remained consistent with Example 2.

[0087] [Example 17]

[0088] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.80 μm, the solid content of the heat-resistant layer slurry was adjusted to 35%, a micro-gravure roller cell was used, the cell size was a mesh spacing of 92 μm, and a cell depth of 61 μm. Other processes and parameters remained consistent with Example 2.

[0089] [Example 18]

[0090] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.06 μm, the solid content of the heat-resistant layer slurry was adjusted to 42%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 7. Other processes and parameters remained consistent with Example 2.

[0091] [Example 19]

[0092] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.06 μm, the solid content of the heat-resistant layer slurry was adjusted to 45%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 8. Other processes and parameters remained consistent with Example 2.

[0093] [Example 20]

[0094] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.80 μm, the solid content of the heat-resistant layer slurry was adjusted to 45%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 7. Other processes and parameters remained consistent with Example 2.

[0095] [Example 21]

[0096] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 0.25 μm, the solid content of the heat-resistant layer slurry was adjusted to 45%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 9. Other processes and parameters remained consistent with Example 2.

[0097] [Example 22]

[0098] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.06 μm, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 8. Other processes and parameters remained consistent with Example 2.

[0099] [Example 23]

[0100] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 0.25 μm, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the heat-resistant layer was coated using a micro-gravure roller with the same mesh design as in Example 7. Other processes and parameters remained consistent with Example 2.

[0101] [Example 24]

[0102] During the preparation of the diaphragm, the standard deviation σ of the particle size distribution of the selected boehmite was 1.80 μm, the solid content of the heat-resistant layer slurry was adjusted to 35%, and the heat-resistant coating was applied using a micro-gravure roller with the same mesh design as in Example 9. Other processes and parameters remained consistent with Example 2.

[0103] [Examples 25-28]

[0104] During the preparation of the diaphragm, aluminum oxide, titanium dioxide, barium titanate and magnesium hydroxide were used instead of boehmite, and the corresponding particle size distribution standard deviations σ were 1.24 μm, 1.26 μm, 1.35 μm and 1.29 μm, respectively. The solid content of the heat-resistant layer slurry was 35%. The heat-resistant layer was coated with a micro-gravure roller with the same cell design as in Example 8. The adhesive layer was sprayed with a rotary spray with the same rotor spacing as in Example 2, and the coating amount of the adhesive layer on each side was 0.5 g / m 2 , and the adhesive layer is coated on the surface of the heat-resistant layer. Other processes and parameters are consistent with those in Example 2.

[0105] [Comparative Example 1]

[0106] Compared with Example 1, the spacing between the rotors of the rotary spray coating head is further increased, and the rotor spacing increases from left to right in increments of 24 mm. The initial spacing is 150 mm. Other processes and parameters remain consistent with Example 1.

[0107] [Comparative Example 2]

[0108] Compared with Example 3, the rotation speed of the rotor during rotary spraying was reduced to 5000 rpm, and the other processes and parameters remained consistent with Example 3.

[0109] [Comparative Example 3]

[0110] Compared with Example 1, the spacing between the rotors of the rotary spray coating head is further increased, and the rotor spacing is the same as that of Comparative Example 1. At the same time, the rotor speed during rotary spraying is reduced, and the rotor speed is the same as that of Comparative Example 2. Other processes and parameters remain consistent with Example 1.

[0111] To verify the effects of the separators of each embodiment, the separators of Examples 1-28 and Comparative Examples 1-3 were respectively assembled in lithium-ion secondary batteries. The preparation process of the lithium-ion secondary batteries is as follows:

[0112] (1) Positive electrode preparation

[0113] The positive electrode active material NCM523 (nickel, cobalt and manganese in a molar ratio of 5:2:3, single crystal material), the positive electrode conductive agent acetylene black and the positive electrode binder polyvinylidene fluoride (PVDF) were fully stirred in N-methylpyrrolidone (NMP) at a weight ratio of 96:2:2 to form a uniform positive electrode slurry; the positive electrode slurry was coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained, and the sheet density of the positive electrode sheet was controlled to be 2.58g / cm 3 .

[0114] (2) Negative electrode preparation

[0115] The negative electrode active material artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were fully stirred and mixed in deionized water at a weight ratio of 97:1:1:1 to form a uniform negative electrode slurry. The negative electrode slurry was coated on the negative electrode current collector copper foil. After drying and cold pressing, the negative electrode sheet was obtained. The negative electrode sheet density was controlled to be 1.65g / cm 3 .

[0116] (3) Preparation of electrolyte

[0117] In an argon atmosphere glove box with a water content of <10 ppm, EC, PC, and DEC were mixed in a volume ratio of EC:PC:DEC = 1:1:1 to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 was dissolved in the organic solvent. After mixing evenly, an electrolyte with a lithium salt concentration of 1 mol / L was obtained.

[0118] (4) Battery assembly

[0119] The prepared positive electrode sheet, separator, and negative electrode sheet are placed in sequence, with the separator positioned between the positive and negative electrode sheets to act as an insulator. The resulting bare cell is wound to form a thickness of 12±0.3 mm. The bare cell is then hot-pressed (at a temperature of 95±2°C, a pressure of 4.5 MPa, and a time of 35 seconds) and packaged with aluminum-plastic film. The prepared electrolyte is then injected into the dried bare cell. The cell undergoes vacuum packaging, resting, formation, and shaping to produce a lithium-ion secondary battery.

[0120] The lithium ion secondary batteries prepared in Examples 1 to 28 and Comparative Examples 1 to 3 were subjected to performance tests. The test process is as follows. The test results are shown in Table 1:

[0121] (1) True density test:

[0122] The inorganic particles used to prepare the heat-resistant layer in Examples 1-28 and Comparative Examples 1-3 were used as samples, and the true density of the samples was obtained by referring to the test method of the national standard: QB / T1010-2015;

[0123] Alternatively, the batteries prepared in Examples 1-28 and Comparative Examples 1-3 were disassembled to obtain separators, which were then cleaned, dried under natural ventilation, and used as samples. The inorganic particles in the samples were separated, and the true density of the samples was obtained by referring to the test method of the national standard: QB / T1010-2015;

[0124] The aforementioned separation methods are not particularly limited, as long as they can meet the QB / T1010-2015 test requirements. At least the following solutions can be used:

[0125] Option 1: Dissolve the sample in DMAC or NMP and ultrasonically clean it for 12 hours until the inorganic particles are separated from the base membrane. Filter and dry the obtained inorganic particles. Repeated ultrasonic cleaning with DMAC or NMP can be used until the inorganic particles meet the test requirements of QB / T1010-2015.

[0126] Option 2: calcine the sample in a muffle furnace at 650°C for 2 hours and then screen out the inorganic particles. The muffle furnace can be used for repeated calcination until the inorganic particles meet the test requirements of QB / T1010-2015.

[0127] (2) Bulk density test of heat-resistant layer:

[0128] The double-sided heat-resistant layer diaphragms obtained in the first step of Examples 1-28 and Comparative Examples 1-3 were used as samples. The total thickness of the diaphragm samples was measured using a Mahr thickness gauge, and the total weight was obtained by weighing the diaphragm samples using a precision electronic balance. The area of ​​the diaphragm samples was measured and recorded as the total area. The above-mentioned diaphragms were soaked in NMP solvent and ultrasonically washed away to obtain a base film. The thickness of the base film was measured and recorded, and the weight of the base film was obtained by weighing using a precision electronic balance. The heat-resistant layer density; W = (total weight - base film weight) / (2 × total area), the heat-resistant layer thickness = (total thickness - base film thickness) / 2, the heat-resistant layer density / heat-resistant layer thickness is used to calculate the bulk density of the heat-resistant layer, and the unit of bulk density is: g / cm 3 .

[0129] Alternatively, the diaphragm with a heat-resistant layer and an adhesive layer on both sides obtained in the second step of Examples 1-28 and Comparative Examples 1-3 is used as a sample; alternatively, the batteries prepared in Examples 1-28 and Comparative Examples 1-3 are disassembled to obtain a diaphragm, the diaphragm is cleaned, dried by natural ventilation and used as a sample, the adhesive layer on the surface of the sample is removed, the total thickness of the diaphragm sample is measured by a Mahr thickness gauge, the total weight of the diaphragm sample is weighed using a precision electronic balance, the area of ​​the diaphragm sample is measured and recorded as the total area, the above-mentioned diaphragm is soaked in NMP solvent to ultrasonically wash away the heat-resistant layer on the surface of the diaphragm to obtain a base film, the thickness of the base film is tested and recorded, and the weight of the base film is obtained by weighing using a precision electronic balance, then the heat-resistant layer density; W = (total weight - base film weight) / (2 × total area), heat-resistant layer thickness = (total thickness - base film thickness) / 2, the heat-resistant layer density / heat-resistant layer thickness is calculated to obtain the test value of the bulk density of the heat-resistant layer, and after correcting the characterization result error caused by removing the adhesive layer on the surface of the sample, the bulk density value is obtained, and the unit of the bulk density is: g / cm 3 .

[0130] The aforementioned method for removing the adhesive layer from the sample surface is not particularly limited, as long as the adhesive layer can be removed as much as possible without damaging the morphology of the heat-resistant layer. At least the following methods can be used:

[0131] Use a Keyence VHX-7000 scanning mirror to scan and record the area covered by the adhesive layer on the sample surface, which is recorded as the initial area. Repeatedly use adhesive strips to remove the adhesive layer on the diaphragm surface until the adhesive layer coverage area on the sample surface is less than 5% of the initial area. At this time, it is considered that the adhesive layer and the heat-resistant layer are fully peeled off, and the bulk density of the heat-resistant layer surface is tested. When the adhesive layer is removed with tape, compared with directly using a diaphragm without an adhesive layer as a sample, a part of the adhesive layer will inevitably remain on the diaphragm surface and there will be a certain degree of damage to the heat-resistant layer. Therefore, compared with directly using a diaphragm without an adhesive layer as a sample, the test result will be higher, with an error of 5%.

[0132] It should be noted that the diaphragms provided in the previous text of this application that meet a certain stacking density range are based on the characterization results obtained from the diaphragms with double-sided heat-resistant layers obtained in the first step of Examples 1-28 and Comparative Examples 1-3, with the heat-resistant layer being the outermost layer.

[0133] (3) Particle size standard deviation test of inorganic particles:

[0134] The inorganic particles used to prepare the heat-resistant layer in Examples 1-28 and Comparative Examples 1-3 were used as samples, and the particle size distribution of the inorganic particles was measured by a Malvern 3000 particle size analyzer and obtained through statistical calculation of the data.

[0135] Alternatively, the batteries prepared in Examples 1-28 and Comparative Examples 1-3 were disassembled to obtain diaphragms, which were cleaned, dried by natural ventilation, and used as samples. The inorganic particles in the samples were separated, and the particle size distribution of the inorganic particles was measured using a Malvern 3000 particle size analyzer, and obtained through statistical calculation of the data.

[0136] The aforementioned separation methods are not particularly limited, as long as they can meet the testing requirements of the Malvern 3000 particle size analyzer. At least the following solutions can be used:

[0137] Option 1: Dissolve the sample in DMAC or NMP and ultrasonically clean it for 12 hours until the inorganic particles are separated from the base membrane. Filter and dry the obtained inorganic particles. Repeated ultrasonic cleaning with DMAC or NMP can be used until the inorganic particles meet the test requirements of the Malvern 3000 particle size analyzer.

[0138] Option 2: calcine the sample in a muffle furnace at 650°C for 2 hours and then screen out the inorganic particles. The muffle furnace can be used for repeated calcination until the inorganic particles meet the testing requirements of the Malvern 3000 particle size analyzer.

[0139] (4) Surface roughness Ra test:

[0140] The double-sided heat-resistant layer separators obtained in the first step of Examples 1-28 and Comparative Examples 1-3 were used as samples and measured using a Mahr roughness tester to obtain the surface roughness Ra of the heat-resistant layer.

[0141] Alternatively, the diaphragms with both heat-resistant layers and adhesive layers on both sides obtained in the second step of Examples 1-28 and Comparative Examples 1-3 are used as samples; alternatively, the batteries prepared in Examples 1-28 and Comparative Examples 1-3 are disassembled to obtain diaphragms, which are cleaned and dried naturally through ventilation to prepare samples. After removing the adhesive layer on the surface of the sample, a diaphragm with a relatively intact surface is found, and the surface roughness Ra test value is obtained by measuring the surface roughness using a Mahr roughness tester. After correcting the characterization error caused by removing the adhesive layer on the surface of the sample, the surface roughness Ra value is obtained.

[0142] The aforementioned method for removing the adhesive layer from the sample surface is not particularly limited, as long as the adhesive layer can be removed as much as possible without damaging the morphology of the heat-resistant layer. At least the following methods can be used:

[0143] Use a Keyence VHX-7000 scanning mirror to scan and record the area covered by the adhesive layer on the sample surface, which is recorded as the initial area. Repeatedly use adhesive strips to remove the adhesive layer on the diaphragm surface until the adhesive layer coverage area on the sample surface is less than 5% of the initial area. At this time, it is considered that the adhesive layer and the heat-resistant layer are fully peeled off, and the bulk density of the heat-resistant layer surface is tested. When the adhesive layer is removed with tape, compared with directly using a diaphragm without an adhesive layer as a sample, a part of the adhesive layer will inevitably remain on the diaphragm surface and there will be a certain degree of damage to the heat-resistant layer. Therefore, compared with directly using a diaphragm without an adhesive layer as a sample, the test result will be larger and there will be a 10% error.

[0144] It should be noted that the diaphragm provided in the previous text of this application that meets a certain surface roughness range is based on the characterization results obtained from the diaphragm with a double-sided heat-resistant layer obtained in the first step of Examples 1-28 and Comparative Examples 1-3, with the heat-resistant layer as the outermost layer.

[0145] (5) Test of effective adhesion R value of diaphragm:

[0146] The diaphragms with heat-resistant layers and adhesive layers on both sides obtained in the second step of Examples 1-28 and Comparative Examples 1-3 were used as samples; alternatively, the batteries prepared in Examples 1-28 and Comparative Examples 1-3 were disassembled to obtain diaphragms, which were cleaned and dried under natural ventilation as samples. The area covering 90% of the width of the diaphragm near the center was used as the sampling area. The sampling area was divided into five equal parts along the width direction to obtain five equal sampling areas of the same width. In each equal sampling area, a diaphragm sheet with a diameter of 90% of the width was taken. Five diaphragm sheets were taken continuously along the same width direction and were respectively passed through a Keyence VHX-7 000 scans the surface of the diaphragm to output the initial unit area S1 of the bonding layer; after stacking with the aforementioned positive electrode sheet of the same area, hot pressing is performed, and the hot pressing parameters are: hot pressing temperature is 95±2℃, hot pressing pressure is 4.5Mpa, and hot pressing time is 35s; after hot pressing, external force is applied to gently peel off the diaphragm from the positive electrode sheet. There is no special restriction on the size of the external force, as long as the diaphragm and the positive electrode sheet can be separated, the KEYENCE VHX-7000 is used to scan the surface of the electrode sheet to output the unit bonding layer area S2, and the effective adhesion of the diaphragm is obtained as R=S2 / S1. The difference between the maximum R value and the minimum R value among the five diaphragm sheets is the effective adhesion difference.

[0147] (6) Adhesion test between diaphragm and electrode:

[0148] Using the diaphragms with both heat-resistant and adhesive layers on both sides, as prepared in Examples 1-28 and Comparative Examples 1-3, as samples, the diaphragms and the aforementioned electrode sheets were first cut into small strips 50 mm long and 15 mm wide. The diaphragm and electrode sheet samples were then stacked one on top of the other and heat-pressed together using a flat-plate hot press at a temperature of 95±2°C, a pressure of 4.5 MPa, and a time of 35 seconds. The heat-pressed diaphragm / electrode sheet composite strips were then tested on a universal testing machine using a 180°C peel test to determine the adhesion strength between the diaphragm and the electrode sheet.

[0149] (7) Testing of the cycle performance of lithium-ion secondary batteries:

[0150] Batteries prepared in Examples 1-28 and Comparative Examples 1-3 were used as samples. At 25°C, they were charged at a constant current of 0.5C to 4.25V. Then, they were charged at a constant voltage of 4.25V until the current dropped below 0.05C. Then, they were discharged at a constant current of 0.5C to 2.8V. This constituted the first cycle, and the discharge capacity of the lithium-ion secondary battery was recorded. The above method was then repeated for 300 charge and discharge cycles. Three lithium-ion secondary batteries were sampled from each group, and the average value was calculated.

[0151] Capacity retention rate of the lithium ion secondary battery after 300 cycles = (discharge capacity of the lithium ion secondary battery after 300 cycles / discharge capacity of the lithium ion secondary battery after the first cycle) × 100%.

[0152] (8) Flatness of battery cell:

[0153] The batteries made in Examples 1-28 and Comparative Examples 1-3 were used as samples. After 300cls of cycling, the batteries were disassembled in a fully charged state to confirm the surface flatness of the negative electrode sheets. The surface flatness was divided into five grades: A, B, C, D, and E. Among them, the surface of the A-grade battery cell was flat without obvious wrinkles. Figure 3 (a); There are slight wrinkles on the surface of the B-grade battery cell, see Figure 3 (b); The surface wrinkles of C-grade cells are more obvious, see Figure 3 (c); The surface of the D-grade battery cell is severely wrinkled, see Figure 3 (d) The surface wrinkles of E-grade cells are very serious, see Figure 3 (e).

[0154]

[0155]

[0156]

[0157] Referring to Table 1, comparing Examples 1-3 and Comparative Examples 1-3, while maintaining all other conditions the same, by adjusting the spacing between the coating rotors and the rotor speed during adhesive layer application, and adjusting the distance between each rotor in the spray coverage area and the spray point morphology, the effective adhesion R of the separator and the effective adhesion difference are adjusted. This, in turn, adjusts the distribution of the adhesion force between the separator and the electrode, thereby improving the flatness of the battery cell after multiple cycles and the battery's cycling performance. It can be seen that when the R value is between 40% and 70% and the R value difference is below 30%, the cell interface flatness after cycling is better and the battery's cycling performance is better. The smaller the difference in effective adhesion R value, the more uniform the effective adhesion force distribution between the separator and the electrode, and the higher the capacity retention rate of the corresponding battery after 300 cycles at 25°C. When the effective adhesion R value difference is higher than 30% (Comparative Example 1), the cell surface flatness is poor, resulting in reduced battery cycling performance. If the effective adhesion R value of the separator is less than 40%, even if the effective adhesion R difference is small (Comparative Example 2), it will affect the surface flatness of the battery cell and thus lead to poor cycle performance.

[0158] Example 4-6 adjusts the leveling speed of the heat-resistant layer by adjusting the solid content of the heat-resistant layer slurry to adjust the surface roughness Ra of the heat-resistant layer. By comparing the performance data of Example 2 and Example 4-6, it can be seen that: when the surface roughness Ra of the heat-resistant layer is further controlled to be less than 1μm, the R value of the diaphragm is improved relative to Example 1-3, and the R value difference is relatively reduced. The adhesion between the diaphragm and the electrode is further improved, the surface of the battery cell after the cycle is smoother, and the cycle performance of the battery is further improved; this is because the smaller the surface roughness of the heat-resistant layer, the higher the surface flatness of the heat-resistant layer, and the adhesive layer coated on the surface of the heat-resistant layer can obtain a larger contact area, thereby enhancing the bonding force between the adhesive layer and the surface of the heat-resistant layer, so that the bonding force and the binding force between the entire diaphragm and the electrode are further increased. When the negative electrode active material expands due to multiple cycles, the overall binding force of the bare battery cell can reduce the pores between the positive and negative electrodes and the diaphragm, and avoid wrinkling of the negative electrode sheet as it expands, thereby improving the cycle performance of the battery and the interface of the battery cell. Between Examples 4-6, as the surface roughness decreases, the proportion of the diaphragms with R values ​​exceeding 60% tends to increase, and the R value difference tends to decrease.

[0159] In Example 7-9, the packing density of the heat-resistant layer is adjusted by adjusting the mesh size used when coating the heat-resistant layer, adjusting the transfer rate of the heat-resistant layer slurry to the porous base film during coating. Comparing the test results of Example 2 and Example 7-9, it can be seen that: when the packing density of the heat-resistant layer is within the range of 0.4ρ-0.5ρ, the R value of the entire diaphragm is relatively improved, the adhesion between the diaphragm and the electrode is further improved, the surface of the battery cell after cycling is smoother, and the cycle performance of the battery is further improved; this is because the heat-resistant layer is more densely stacked on the basis of meeting the requirements of Example 2, and when the diaphragm adhesive layer and the positive and negative electrode sheets are subjected to hot pressing to form adhesion, the adhesion can be maximized, thereby achieving a better battery cell shaping effect and improving the performance of the battery cell.

[0160] Examples 10-12 adjust the particle size standard deviation of the inorganic particles in the heat-resistant layer, adjust the adhesion between the diaphragm and the pole piece, and thus affect the shaping effect and cycle performance of the battery cell. The test results show that when the particle size standard deviation σ of the inorganic particles is less than or equal to 0.9*d (d represents the thickness of the heat-resistant layer), the adhesion between the diaphragm and the pole piece is further improved, the surface of the battery cell after the cycle is smoother, and the cycle performance of the battery is further improved. This is because: the smaller the particle size standard deviation, the more uniform the particle size distribution of the inorganic particles. The use of inorganic particles with uniform particle size for stacking can obtain a uniform, flat and dense stacking morphology, further increasing the effective bonding area between the diaphragm and the pole piece, and the bonding strength between the diaphragm and the pole piece is greater.

[0161] Comparing Examples 4-6, 7-9 and Examples 13-15, the process parameters of the heat-resistant layer and the adhesive layer are adjusted so that the separator meets the requirements of an effective adhesion R of 40-80% and an R value difference of less than 30%, while the surface roughness Ra of the heat-resistant layer is less than 1 and the bulk density is within 0.4ρ-0.5ρ. Compared with only meeting a single condition (for example, 4-6 only meet the roughness, 7-8 only meet the bulk density), the adhesion between the separator and the electrode is further improved, the flatness of the battery cell after cycling is higher, the shaping effect is better, and the cycle performance of the battery cell is further improved. Comparing Example 15 with Example 2, it can be seen that by simultaneously controlling the bulk density A and the surface roughness of the heat-resistant layer, the R value can be further increased and the R value difference can be reduced, thereby improving the battery cycle performance.

[0162] Comparing Examples 4-6, 10-12, and 16-18, the process parameters of the heat-resistant layer and the adhesive layer are adjusted so that the diaphragm meets the effective adhesion R value of 40-80% and the R value difference is less than 30%. At the same time, the surface roughness of the heat-resistant layer Ra is less than 1 and the particle size standard deviation σ of the inorganic particles is less than 0.9*d (d represents the thickness of the heat-resistant layer). Compared with only meeting a single condition (for example, 4-6 only meet the roughness, 10-12 only meet the particle size standard deviation and specific surface area), the adhesion between the diaphragm and the electrode is further improved, the flatness of the battery cell after cycling is higher, the shaping effect is better, and the cycle performance of the battery cell is further improved. Comparing Example 18 with Example 2, it can at least be seen that by simultaneously controlling the surface roughness and the inorganic heat-resistant ion particle size standard deviation and specific surface area, the R value can be further improved and the R value difference can be reduced, thereby improving the battery cycle performance.

[0163] Comparing Examples 7-9, Examples 10-12 and Examples 19-21, the process parameters of the heat-resistant layer and the adhesive layer are adjusted so that the diaphragm meets the effective adhesion R value of 40-80% and the R value difference is less than 30%, while the bulk density of the heat-resistant layer meets 0.4ρ-0.5ρ and the particle size standard deviation σ≤0.9*d of the inorganic particles (d represents the thickness of the heat-resistant layer). Compared with only meeting a single condition (for example, 7-9 only meet the bulk density, 10-12 only meet the particle size standard deviation), the adhesion between the diaphragm and the electrode is further improved, the flatness of the battery cell after cycling is higher, the shaping effect is better, and the cycle performance of the battery cell is further improved.

[0164] Comparing Examples 13-15, Examples 16-18, Examples 19-21 and Examples 22-24, the process parameters of the heat-resistant layer and the bonding layer are adjusted so that the diaphragm meets the effective adhesion R value of 40-80% and the R value difference is less than 30%. At the same time, the surface roughness of the heat-resistant layer meets Ra<1, the particle size standard deviation σ≤0.9*d of the inorganic particles (d represents the thickness of the heat-resistant layer), and the stacking density of the heat-resistant layer meets 0.4ρ-0.5ρ. When the diaphragm bonding layer and the positive and negative electrode sheets are bonded after hot pressing, the bonding force can be maximized, thereby obtaining the best battery cell shaping effect and optimizing the battery cell cycle performance.

[0165] Comparing Examples 22-24 and Examples 25-28, different inorganic particles are replaced, while satisfying the effective adhesion R value of 40-80% and the R value difference of less than 30%, the surface roughness of the heat-resistant layer Ra <1, the particle size standard deviation σ of the inorganic particles ≤ 0.9*d (d represents the thickness of the heat-resistant layer), the heat-resistant layer stacking density 0.4ρ-0.5ρ, and the bonding force between the diaphragm bonding layer and the positive and negative electrode sheets can be maximized after hot pressing to form a bonding force, thereby obtaining the best battery cell shaping effect and optimizing the battery cell cycle performance.

[0166] The diaphragm of the present invention maintains a certain adhesion force between the diaphragm and the electrode by adjusting the ratio of the effective bonding area to the theoretical bonding area. At the same time, the difference in the ratio of the effective bonding area to the theoretical bonding area is controlled to be less than 30%, so that the bonding uniformity between the diaphragm and the electrode is improved, the wrinkling and deformation of the battery cell caused by the expansion of the electrode after the cycle is improved, the flatness is improved, and the cycle performance of the battery is improved. Furthermore, the flatness of the surface of the heat-resistant layer can be adjusted by adjusting the surface roughness of the diaphragm heat-resistant layer and the standard deviation of the particle size of the inorganic particles in the heat-resistant layer. Within the limited range, the surface flatness of the heat-resistant layer is high, and the adhesive layer attached to the surface of the heat-resistant layer can obtain a larger contact area, thereby enhancing the adhesion of the adhesive layer to the surface of the heat-resistant layer. In addition, the packing density of the heat-resistant layer is further limited, making the packing of the heat-resistant layer denser. In particular, the surface of the heat-resistant layer is free of unevenness caused by the loose accumulation of inorganic particles, thereby obtaining a heat-resistant layer surface with low roughness. When the adhesive layer and the positive and negative electrode sheets are bonded after hot pressing, the bonding force can be maximized, thereby better achieving the best battery cell shaping effect and improving the performance of the battery cell. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and practical significance.

[0167] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may 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 one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A diaphragm, characterized in that: include: Porous base membrane; A heat-resistant layer is disposed on at least one surface of the porous base film; An adhesive layer is provided at least on the other side of the contact surface between the heat-resistant layer and the porous base film; The effective adhesion R value of the diaphragm provided with the heat-resistant layer side is 40% to 80%, and the difference between the maximum and minimum values ​​of the effective adhesion R value is less than 30%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area.

2. The diaphragm according to claim 1, characterized in that The surface roughness of the heat-resistant layer Ra is less than 1 μm.

3. The diaphragm according to claim 1, characterized in that The heat-resistant layer comprises inorganic particles and a binder. The bulk density of the heat-resistant layer is A=(0.4-0.5)×ρ, wherein ρ represents the true density of the inorganic particles, in units of g / cm 3 .

4. The diaphragm according to claim 1, characterized in that The heat-resistant layer comprises inorganic particles and a binder, the particle size standard deviation σ of the inorganic particles is ≤0.9*d, and d represents the thickness of the heat-resistant layer.

5. The diaphragm according to claim 3 or 4, characterized in that: The volume proportion of the inorganic particles in the heat-resistant layer is 80% to 99%.

6. The diaphragm according to claim 1, characterized in that The single-side thickness of the heat-resistant layer is 0.5 to 5 μm.

7. The diaphragm according to claim 3 or 4, characterized in that: The inorganic particles are selected from at least one of aluminum oxide, hydrated aluminum oxide, aluminum hydroxide, silicon dioxide, titanium dioxide, barium sulfate, barium titanate, magnesium hydroxide and magnesium oxide.

8. The diaphragm according to claim 1, characterized in that The single-sided coating amount of the adhesive layer is 0.1 to 2 g / m 2 .

9. The diaphragm according to claim 3 or 4, characterized in that: The adhesive is selected from at least one of polyacrylonitrile and its copolymers, polyacrylic acid and its copolymers, polyacrylate and its copolymers, styrene-butadiene copolymers, styrene-acrylate copolymers and fluorine-based polymer materials.

10. An electrochemical device, characterized in that: It comprises a positive electrode sheet, a negative electrode sheet and a separator as claimed in any one of claims 1 to 9 arranged between the positive electrode sheet and the negative electrode sheet.

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