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 and a heat-resistant layer, the heat-resistant layer is arranged on at least one surface of the porous base membrane, the heat-resistant layer comprises inorganic particles and an adhesive, and the surface static friction coefficient of the heat-resistant layer is smaller than or equal to 0.8; the stacking density A of the heat-resistant layer meets the formula: A = (0.4-0.5) * rho, rho represents the real density of the inorganic particles, and the unit is g / cm < 3 >. By controlling the static friction coefficient and the stacking density of the heat-resistant layer, the heat-resistant layer with compact stacking and flat surface is obtained, and the effective contact area between the bonding particles in the heat-resistant layer and the pole piece can be increased, so that the bonding force between the whole diaphragm and the pole piece is 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 separator 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 separators are polyolefin separators. These materials have relatively low melting points and are prone to large shrinkage at high temperatures, which leads to contact between the positive and negative electrodes and rapid accumulation of a large amount of heat, causing the battery to catch fire or explode due to short circuits, affecting the safety performance of the battery.

[0003] At present, in order to improve the heat resistance and safety of the diaphragm, a heat-resistant layer is usually coated on the diaphragm substrate, and the heat-resistant material in the heat-resistant layer is used to improve the heat resistance of the diaphragm. However, after multiple cycles of the battery, the diaphragm with this structure will cause the positive and negative electrodes to fall off from the diaphragm due to the expansion of the pole piece, which will manifest as insufficient adhesion between the pole piece and the diaphragm as a whole, and then affect the battery capacity cycle retention rate. 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 insufficient adhesion between the diaphragm and the electrode.

[0005] To achieve the above-mentioned purpose and other related purposes, the first aspect of the present invention provides a diaphragm, the diaphragm comprises a porous base film and a heat-resistant layer, the heat-resistant layer is arranged on at least one surface of the porous base film, wherein the heat-resistant layer comprises inorganic particles and an adhesive, and the surface static friction coefficient of the heat-resistant layer is ≤0.8; the bulk density A of the heat-resistant layer satisfies: A = (0.4-0.5) × ρ, ρ represents the true density of the inorganic particles, and the unit is g / cm 3 By controlling the surface static friction coefficient and stacking density of the heat-resistant layer, the diaphragm can obtain a heat-resistant layer with dense stacking and a gentle surface, so that the effective contact area between the adhesive in the heat-resistant layer and the pole piece is increased, thereby improving the adhesion between the heat-resistant layer and the pole piece, obtaining a diaphragm air permeability in the range of 180 to 203 s / 100 cc, and improving the capacity retention rate of the battery after 300 cycles at 25°C.

[0006] In one embodiment of the present invention, the diaphragm further includes an adhesive layer, and the adhesive layer is at least arranged on the other side of the contact surface between the heat-resistant layer and the porous base film. After the battery is hot-pressed, the adhesive force between the adhesive layer and the pole piece is stimulated, which has the effect of battery shaping. At the same time, it can also inhibit the wrinkling and deformation of the battery cell caused by the expansion of the pole piece after the cycle, thereby improving the cycle life of the battery. In addition, the above-mentioned densely stacked and flat-surfaced heat-resistant layer also increases the effective contact area between the adhesive particles in the adhesive layer and the heat-resistant layer, further improves the adhesion between the diaphragm as a whole and the pole piece, and improves the wrinkling of the battery cell after 100 cycles, so as to improve the capacity retention rate after 300 cycles at 25°C.

[0007] In one embodiment of the present invention, the difference between the maximum and minimum values ​​of the effective adhesion R value of the side of the diaphragm provided with the heat-resistant layer is ≤ 20%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area. Controlling the difference in the effective adhesion R value within 20% can make the bonding force between the diaphragm and the pole piece evenly distributed, thereby improving the wrinkling phenomenon of the pole piece.

[0008] In one embodiment of the present invention, the effective adhesion R value of the side of the diaphragm provided with the heat-resistant layer is 60% to 80%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area. The effective adhesion of the diaphragm is controlled at 60% to 80%, so that the effective bonding point between the diaphragm and the pole piece has sufficient bonding strength, thereby improving the bonding force between the diaphragm and the pole piece.

[0009] In one embodiment of the present invention, the volume ratio of the inorganic particles in the heat-resistant layer is 80% to 99%. If the volume ratio of the inorganic particles in the heat-resistant layer is lower than 80%, on the one hand, its heat resistance will be reduced. On the other hand, if the volume ratio of the adhesive is too high, it will easily lead to closed pores in the coating due to the swelling of the adhesive during the battery cell cycle, thereby affecting the cycle capacity of the battery cell. If the volume ratio of the inorganic particles in the heat-resistant layer is higher than 99%, the adhesive content in the heat-resistant layer is too low, which is not conducive to the molding of the heat-resistant layer and the shaping effect of the overall diaphragm and the pole piece.

[0010] In one embodiment of the present invention, the inorganic particles are selected from at least one of the compositions consisting of oxides, hydroxides and nitrides of silicon, aluminum, zirconium and titanium, and can also be formed using a complex containing at least one of oxides, hydroxides and nitrides of silicon, aluminum, zirconium and titanium. These inorganic particles can come from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, mica, etc., or can be artificially manufactured. In addition, these inorganic particles can be used alone or in combination of two or more. From a cost perspective, aluminum oxide, hydrated aluminum oxide, aluminum hydroxide, silicon dioxide, titanium dioxide, magnesium hydroxide, magnesium oxide, etc. are preferred. The above-mentioned inorganic particles have good high temperature resistance and will not have an adverse effect on the battery.

[0011] In one embodiment of the present invention, the average particle size of the adhesive is smaller than the average particle size of the inorganic particles. The adhesive can be filled between the inorganic particles to improve the flatness and adhesion of the heat-resistant layer.

[0012] In one embodiment of the present invention, the single-sided coating amount of the adhesive layer 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, carboxymethyl cellulose, cellulose ethylene-vinyl acetate copolymer, polyvinyl chloride, polyacrylic acid and its copolymers, polyacrylate and its copolymers, styrene-butadiene copolymers, styrene-acrylate copolymers and fluorine-based polymer materials.

[0014] In one embodiment of the present invention, the porous base film is selected from one or more of polyethylene, polypropylene, non-woven fabric, polyethylene terephthalate, polyimide, and polypropylene-polyethylene-polypropylene composite film.

[0015] In one embodiment of the present invention, the single layer thickness of the heat-resistant layer is 0.5-5 μm, and the thickness of the porous base film is 3-20 μm.

[0016] 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

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description 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 creative work.

[0018] Figure 1 It is a schematic structural diagram of a diaphragm of the present invention in one embodiment;

[0019] Figure 2 FIG. 1 is a schematic diagram of the internal particle arrangement of the diaphragm of the present invention in one embodiment.

[0020] Component number description:

[0021] 100, diaphragm; 101, base film; 102, heat-resistant layer; 103, adhesive layer. DETAILED DESCRIPTION

[0022] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied by other different specific embodiments, and the details in this specification can also be modified or changed in various ways 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 embodiments can be combined with each other without conflict. It should also be understood that the terms used in the embodiments of the present invention are to describe specific specific embodiments, rather than to limit the scope of protection of the present invention.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0024] In this article, when it comes to numerical ranges, unless otherwise specified, the distribution of optional values ​​within the numerical range is considered continuous and includes the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as every value between the two numerical endpoints. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be combined.

[0025] In this document, "multiple", "multiple", "multiple times", etc., unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "one or more" means one or greater than or equal to two. "Further", "further", "particularly", etc. are used for descriptive purposes to indicate differences in content, but should not be construed 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] Surface static friction coefficient: refers to the physical quantity of friction resistance between two objects at rest, usually represented by the letter μ. The static friction coefficient determines the minimum external force required to make an object start sliding. If the external force is less than the static friction, the object will remain stationary.

[0028] Bulk density: refers to the mass per unit volume of the material, including the material entity, internal pores (open pores and closed pores) and spaces between particles. Bulk density can be calculated by A = coating surface density / coating thickness.

[0029] True density: Also known as real density, it refers to the mass per unit volume of a material in an absolutely dense state. It refers to the density obtained by dividing the mass of the powder by the volume (true volume) excluding the voids inside and outside the particles. The calculation formula for 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.

[0030] 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 bonding layer, that is, the surface area of ​​the diaphragm bonding layer, and the effective bonding area refers to the area where the diaphragm bonding layer exerts a bonding effect, that is, the surface area retained on the adherend after the diaphragm bonding layer is separated from the adherend; the effective adhesion R value difference refers to the difference between the maximum and minimum values ​​of R.

[0031] Single-sided coating amount: refers to the coating amount of slurry on one surface during the preparation process.

[0032] Diaphragm air permeability: It is a parameter to measure the air permeability of the diaphragm, which indicates the time required for a specific volume of gas to pass through the diaphragm under a certain pressure.

[0033] Particle size: refers to the size of the 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.).

[0034] 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.

[0035] The calculation formula of particle size standard deviation σ is:

[0036] 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.

[0037] Herein, the surface static friction coefficient of the heat-resistant layer, the bulk density of the heat-resistant layer, the effective adhesion R value, the difference between the maximum and minimum values ​​of the effective adhesion R value and other parameters are affected by the membrane production and preparation 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 control can be used to adjust the membrane structure and obtain the corresponding membrane. It should be noted that the following description is an example of a manufacturing method for obtaining a corresponding membrane, and is not limited to this method.

[0038] See also Figure 1 In a first aspect, the present invention provides a separator 100, which includes a porous substrate 101 and a heat-resistant layer 102, wherein the porous substrate 101 is used as a substrate of the separator 100, and is made of an insulating porous material that allows lithium ions to pass through. The heat-resistant layer 102 is disposed on at least one surface of the porous substrate 101, for example, the porous substrate 101 has a first surface and a second surface disposed opposite to each other along its thickness direction, and the heat-resistant layer 102 can be disposed on one of the first surface and the second surface of the porous substrate 101, or can be disposed on both the first surface and the second surface ( Figure 1 Only the case of single-sided arrangement is shown as an example. Those skilled in the art can make a selection according to actual production needs. The arrangement of the heat-resistant layer 102 can effectively improve the heat-resistant performance of the diaphragm 100 .

[0039] In some embodiments, the porous base film 101 can be selected from any porous material that can be used in the diaphragm in the art. As an example, the porous base film 101 is selected from one or more of polyethylene, polypropylene, non-woven fabric, polyethylene terephthalate, polyimide, polypropylene-polyethylene-polypropylene composite film. That is, the porous base film 101 can be selected from any one of the above-mentioned multiple materials, such as polyethylene, non-woven fabric, polypropylene or polyimide, etc., or a mixture of the same materials with different weight average molecular weights, viscosity average molecular weights, and number average molecular weights; the porous base film 101 can also be selected from any combination of two or more of the above-mentioned materials, for example, the porous base film 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 film 101 as long as a porous base film 101 with the above-mentioned characteristics can be obtained. The following is an example of a method for manufacturing the porous base membrane 101: 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 a composite membrane of two or more layers of a substrate prepared by the above two methods, etc.

[0040] The thickness and porosity of the porous base film 101 can be set according to actual production needs, and are not limited here. As an example, the thickness of the porous base film 101 is 3 to 20 μm, for example, the thickness of the porous base film 101 can be 3 μm, 5 μm, 10 μm, 15 μm or 20 μm, etc. The porosity of the porous base film 101 is 20% to 70%, for example, the thickness of the porous base film 101 can be 20%, 40%, 60% or 70%, etc.

[0041] 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.

[0042] The heat-resistant layer 102 includes inorganic particles and adhesives, wherein the inorganic particles are selected from high-temperature resistant ceramic materials and will not have adverse effects on the battery. In some embodiments, the inorganic particles include but are 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 listed materials, such as aluminum oxide, or titanium dioxide, or magnesium oxide, etc., and the inorganic particles can also be selected from any combination of two or more of the listed materials, such as a composition of barium sulfate and barium titanate, or a composition of aluminum oxide, hydrated aluminum oxide and aluminum hydroxide, and so on. It should be noted that when the inorganic particles are a composition of two or more, there is no restriction on the ratio between the components in the composition, and they can be mixed in any proportion. The adhesive is used to bond the inorganic particles and provide bonding force for the heat-resistant layer 102 and the porous base film 101. In some embodiments, the adhesive includes at least one of polyacrylonitrile and its copolymers, polyacrylic acid and its copolymers, polyacrylate and its copolymers, acrylates, styrene-butadiene copolymers, styrene-acrylate copolymers and fluorine-based polymer materials. That is, the types of adhesives listed above can be used alone or in combination. For example, the adhesive is polyacrylonitrile, or polyacrylic acid, or polyacrylate, or acrylates, or styrene-butadiene copolymers, or a combination of styrene-acrylate copolymers and fluorine-based polymer materials. Further, the adhesive in the heat-resistant layer 102 is an emulsion adhesive, and the average particle size of the emulsion adhesive is smaller than the average particle size of the inorganic particles. This type of adhesive can be filled between the inorganic particles to improve the flatness and adhesion of the heat-resistant layer 102.

[0043] In one embodiment, the heat-resistant layer 102 can be prepared in the following manner: first, inorganic particles, adhesives and solvents are mixed and stirred in proportion to obtain a heat-resistant layer slurry, 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. Since the solvent is mainly used to disperse the inorganic particles and adhesives, and the inorganic heat-resistant layer 102 needs to be dried, the amount of the 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 adhesives. As an example, the amount of solvent used satisfies the solid content of the heat-resistant layer slurry of 32%wt~45wt%, such as 32wt%, 35wt%, 38wt% or 45wt%. The solvent is, for example, deionized water. The thickness of the heat-resistant layer 102 can be set according to actual needs. As an example, the single-sided thickness of the heat-resistant layer 102 is 0.5-5 μm. For example, the single-sided thickness of the heat-resistant layer 102 can be 0.5 μm, 2 μm, 3 μm or 5 μm.

[0044] There is no particular limitation on the method for coating the heat-resistant layer 102 as long as the method can achieve the necessary layer thickness and coating area. Examples thereof include gravure coater method, small-diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss roll coater method, dip coater method, blade coater method, air knife coater method, blade coater method, rod coater method, extrusion coater method, caster method, die coater method, screen printing method, and spray coating method.

[0045] During the battery cycle, if the adhesion between the positive and negative electrodes and the separator is insufficient, the expansion of the pole piece will cause the positive and negative electrodes and the separator to fall off or misalign, causing the entire cell to wrinkle and deform, thereby reducing the battery cycle life. The inventors of this application found in their research that by controlling the surface static friction coefficient μ of the heat-resistant layer to ≤0.8 and the bulk density of the heat-resistant layer to (0.4-0.5)×ρ, ρ represents the true density of the inorganic particles, and the unit is g / cm 3 , which can improve the adhesion between the entire diaphragm and the electrode, thereby improving the problem of insufficient bonding between the electrode and the diaphragm, and the problem of poor air permeability, thereby improving the capacity retention rate after 300 cycles at 25°C.

[0046] In the present invention, the surface static friction coefficient μ of the heat-resistant layer 102 is ≤0.8. In some embodiments, the surface static friction coefficient μ of the heat-resistant layer 102 may be 0.8, 0.5, 0.3 or 0.1, etc. The smaller the static friction coefficient μ, the higher the surface flatness of the heat-resistant layer 102, the larger the effective contact area between the heat-resistant layer 102 and the porous base film 101, and the greater the adhesion between the entire diaphragm and the pole piece. At the same time, the bulk density A of the heat-resistant layer 102 satisfies: A = (0.4-0.5) × ρ, where ρ represents the true density of the inorganic particles, expressed in g / cm 3That is, the bulk density of the heat-resistant layer 102 is related to the type of inorganic particles. For example, the inorganic particles are boehmite, and the true density of boehmite is 3.07 g / cm 3 , the bulk density should be limited to 1.228~1.535g / cm 3 . The stacking density A of the heat-resistant layer 102 can be specifically listed as 0.4ρ, 0.45ρ or 0.5ρ, etc. When the surface static friction coefficient of the heat-resistant layer 102 and the stacking density of the heat-resistant layer 102 are within the above-mentioned specific range, a heat-resistant layer 102 with dense stacking and relatively flat surface can be obtained. In this way, the effective bonding area between the adhesive particles in the heat-resistant layer 102 and the porous base film 101 is increased, thereby increasing the bonding force between the heat-resistant layer 102 and the porous base film 101, and increasing the bonding force between the diaphragm 100 as a whole and the pole piece. If the static friction coefficient μ satisfies the above-mentioned limited range, but the stacking density A does not satisfy it, the obtained heat-resistant layer 102 is not dense enough; if the stacking density A satisfies the above-mentioned limited range, but the static friction coefficient μ does not satisfy it, the surface of the obtained heat-resistant layer 102 is not flat enough. Both of these situations will lead to insufficient bonding between the heat-resistant layer 102 and the porous base film 101, thereby affecting the bonding force between the diaphragm 100 and the pole piece. Here, at least the transfer rate during coating can be achieved by regulating the particle size of inorganic particles in the heat-resistant layer, the solid content of the heat-resistant layer slurry, the viscosity of the heat-resistant layer slurry, and the transfer of the heat-resistant layer 102 slurry to the porous base membrane 101, so as to adjust the stacking, leveling, and shaping effects of the heat-resistant layer during coating to adjust the surface static friction coefficient μ of the heat-resistant layer and the stacking density of the heat-resistant layer.

[0047] To ensure that the heat resistance of the diaphragm 100 meets the heat resistance requirements, the volume ratio of the inorganic particles in the heat-resistant layer 102 is preferably maintained at 80% and above to 99% and below. If the volume ratio of the inorganic particles in the heat-resistant layer 102 is lower than 80%, the heat resistance of the heat-resistant layer 102 will be reduced, and the volume ratio of the adhesive in the heat-resistant layer 102 will also increase, which may easily lead to the coating being closed due to the swelling of the adhesive during the battery cell cycle, thereby affecting the cycle capacity of the battery cell. Further, the volume ratio of the inorganic particles in the heat-resistant layer 102 is 90% and above, and further, the volume ratio of the inorganic particles in the heat-resistant layer 102 is 90% to 99%, which can be specifically listed as 93%, 95%, 97%, etc.

[0048] In one embodiment, the diaphragm further includes an adhesive layer 103, and the adhesive layer 103 is at least arranged on the other side of the contact surface between the heat-resistant layer 102 and the porous base film 101. After the battery is hot-pressed, the adhesive force between the adhesive layer 103 and the pole piece is stimulated, which has the effect of battery shaping, and can also inhibit the wrinkling and deformation of the battery cell caused by the expansion of the pole piece after the cycle, thereby improving the cycle life of the battery. The adhesive layer 103 is at least arranged on the other side of the contact surface between the heat-resistant layer 102 and the porous base film 101, including the following situations: If the heat-resistant layer 102 is arranged on one surface of the porous base film 110, the adhesive layer 103 can be arranged only on the surface of the heat-resistant layer 102 away from the porous base film 101, or it can be arranged on the surface of the heat-resistant layer 102 away from the porous base film 101 and on one surface of the porous base film 110 where the heat-resistant layer 102 is not arranged. If the heat-resistant layer 102 is disposed on both surfaces of the porous base film 110, the adhesive layer 103 is disposed on both surfaces of the heat-resistant layer 102, and the adhesive layer 103 is located on the surface of the heat-resistant layer 102 away from the porous base film 101. The specific arrangement of the adhesive layer 103 can be selected according to actual conditions. Preferably, the heat-resistant layer 102 is disposed on both surfaces of the porous base film 101, and the adhesive layer 103 is disposed on both surfaces of the heat-resistant layer 102 away from the porous base film.

[0049] The adhesive layer 103 can be set according to conventional components in the art. As an example, the adhesive layer 103 includes 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, that is, the adhesive layer 103 can be selected from any one of the above-listed materials, such as polyacrylonitrile, or polyacrylic acid, or polyacrylate, or styrene-acrylate copolymer, etc. The adhesive can also be selected from any combination of two or more of the above-listed materials, such as a combination of styrene-acrylate copolymer and fluorine-based polymer materials.

[0050] 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 .

[0051] There is no particular limitation on the method for coating the adhesive layer 103 as long as it is a method that can achieve the necessary layer thickness and coating area. Examples thereof include gravure coater method, small-diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss roll coater method, dip coater method, blade coater method, air knife coater method, blade coater method, rod coater method, extrusion coater method, caster method, die coater method, screen printing method, and spray coating method.

[0052] In one embodiment, the coating amount of the adhesive layer 103 on one side is 0.1-2 g / m 2 That 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-sided 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 0.8 g / m 2 , 1.0g / m 2 or 1.5g / m 2 etc.

[0053] In one embodiment, the difference between the maximum and minimum values ​​of the effective adhesion R on the side of the heat-resistant layer 102 of the diaphragm 100 (R value difference) is less than or equal to 20%. For example, the R value difference can be 20%, 15%, 10%, 5% or 3%, etc. The smaller the difference in R value, the more uniform the distribution of the adhesion force between the diaphragm 100 and the pole piece, which can effectively improve the wrinkling and deformation of the battery cell caused by the expansion of the pole piece after the cycle, and improve the flatness. Here, at least the weight ratio of the adhesive in the adhesive layer 103, the coverage of the adhesive layer 103 to the heat-resistant layer 102, the surface density of the adhesive layer 103, and the parameters of the adhesive layer 103 coating equipment can be adjusted to adjust the structural morphology and distribution of the adhesive layer 103 on the side of the heat-resistant layer 102, and adjust the effective adhesion R value difference on the side of the diaphragm 100 with the heat-resistant layer 102.

[0054] Furthermore, the effective adhesion R value of the heat-resistant layer 102 side of the diaphragm 100 is 60% to 80%, which can be specifically listed as 60%, 70% or 80%. When the effective adhesion R value is within the above range, the bonding strength of the effective bonding point between the diaphragm 100 and the pole piece is sufficient, thereby improving the bonding force between the diaphragm 100 and the pole piece. If the R value is lower than 60%, there is a certain bonding force between the diaphragm and the pole piece, the shaping effect of the bare battery cell is relatively poor, and the battery cell is relatively easy to deform and expand. The relatively uneven resistance on the interface will lead to increased polarization, and the capacity loss generated during the cycle will increase, which will narrow the lithium precipitation window of the battery cell. If the R value is too high, the remaining space between the diaphragm and the pole piece will be relatively small, and the amount of electrolyte retained will be relatively reduced. In the later stage of the cycle, the electrolyte is relatively insufficient, which is prone to purple spot problems. Here, 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 can be adjusted to adjust the structural morphology and distribution of the adhesive layer 103 on the heat-resistant layer 102 side, and adjust the effective adhesion R value of the diaphragm 100 provided with the heat-resistant layer 102 side.

[0055] A second aspect of the present invention provides an electrochemical device, wherein the electrochemical device comprises the separator 100 described above.

[0056] The electrochemical device of the present invention may 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.

[0057] The lithium-ion battery includes a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet, and forms a bare battery cell with the positive electrode sheet and the negative electrode sheet by lamination or winding.

[0058] Specifically, the positive electrode sheet 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, a foil formed by surface treatment of nickel, titanium, aluminum, silver, stainless steel or carbon. In addition to the foil, the positive 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 current collector is, for example, 8μm to 15μm. In this embodiment, the positive current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm. The positive active material layer includes a positive active material, a positive conductor and a positive adhesive. There is no specific restriction on the positive active material, the positive conductor and the positive adhesive, and those skilled in the art can choose according to actual needs.

[0059] 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, and nickel cobalt manganese metal oxide (NCM), but is not limited thereto. The positive electrode adhesive is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene (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 fiber (VGCF), Ketjen black, and the like.

[0060] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the surface 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 selects 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 disposed on any one 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.

[0061] The negative electrode active material is selected from compounds that can embed and deintercalate 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 of several mixed in any proportion; the thickener is selected from sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0062] 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 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). Further, the organic solvent is a non-aqueous organic solvent, and the non-aqueous organic solvent may include any type of carbonate and / or carboxylate. The carbonate may include a cyclic carbonate or a chain carbonate. The non-aqueous organic solvent may also include a halogenated compound of a carbonate. 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, ethyl methyl carbonate, γ-butyrolactone, methyl formate, ethyl formate, ethyl propionate, propyl propionate, and tetrahydrofuran.

[0063] Furthermore, some additives may be added to the electrolyte according to actual needs, such as vinyl sulfate (DTD), 1,3-propane sultone (PS), vinyl carbonate (VC) or vinyl ethylene carbonate (VEC).

[0064] The lithium-ion secondary battery also includes a shell. The shape and material of the shell are related to the type of lithium-ion battery. For example, if the lithium-ion battery is a soft-pack battery, its shell can be packaged 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.

[0065] The lithium-ion secondary battery can be prepared according to methods known in the art, which will not be described in detail here.

[0066] The technical scheme of the present invention is described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are all commercially available products, or can be prepared by conventional methods in the art, and the instruments used in the examples are all commercially available.

[0067] [Example 1]

[0068] This embodiment provides a diaphragm, which includes a porous base film and a heat-resistant layer, wherein the porous base film is a 7μm porous polyethylene film, and the heat-resistant layer includes inorganic particles (boehmite) and a binder (polyacrylonitrile). In this embodiment, the heat-resistant layer is arranged on both surfaces of the porous base film. The types and amounts of each substance in the heat-resistant layer refer to the preparation method and Table 1.

[0069] The preparation process of the diaphragm is as follows:

[0070] First, deionized water and boehmite (particle size standard deviation σ=1.06μm) are added to a double planetary mixer and dispersed at high speed at 40°C for 1 hour; then an adhesive (polyacrylonitrile) is added and stirred at low speed at room temperature for 1 hour to obtain a heat-resistant layer slurry, the solid content of the heat-resistant layer slurry is 42%, wherein the dry matter ratio of boehmite to the adhesive is 93:6; then the heat-resistant layer slurry is coated on both surfaces of a porous polyethylene film with a thickness of 7μm by a micro-gravure roller coating method, and then dried at 45°C to obtain a diaphragm with a heat-resistant layer, wherein the micro-gravure surface mesh is designed to have a wide opening and a shallow depth, the mesh size is a mesh spacing of 145μm, a mesh depth of 39μm, and the single-sided thickness of the heat-resistant layer is 2μm.

[0071] [Example 2]

[0072] During the preparation of the diaphragm, compared with Example 1, the solid content of the heat-resistant layer slurry was adjusted to 38%, and the other processes and parameters remained consistent with Example 1.

[0073] [Example 3-4]

[0074] During the preparation of the diaphragm, compared with Example 2, a micro-concave plate with different cell size designs was used for heat-resistant layer coating. Among them, the cell size of Example 3 was 120 μm for the mesh spacing and 47 μm for the mesh depth; the micro-concave plate surface cell design used in Example 4 was narrow opening, large depth, cell size was 106 μm for the mesh spacing, and cell depth was 53 μm; the cell design in Example 3 was between Example 4 and Example 1. The unit cell volume was consistent in the three designs. Other processes and parameters were consistent with Example 2.

[0075] [Example 5]

[0076] During the preparation of the diaphragm, compared with Example 1, the solid content of the heat-resistant layer slurry was adjusted to 32%, and the other processes and parameters were consistent with Example 4.

[0077] [Example 6]

[0078] During the preparation of the diaphragm, compared with Example 4, the solid content of the heat-resistant layer slurry was adjusted to 35%, and the other processes and parameters remained consistent with Example 4.

[0079] [Examples 7-9]

[0080] During the preparation of the diaphragm, compared with Example 5, the inorganic particles were replaced with aluminum oxide, titanium dioxide and barium titanate, respectively, and the other processes and parameters remained consistent with Example 5.

[0081] [Example 10]

[0082] On the basis of the diaphragm in Example 5, an adhesive layer coating is performed. The adhesive layer coating process is as follows: first, deionized water and an auxiliary adhesive of the polyacrylonitrile type 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, the solid content of the adhesive layer slurry is 12%, and the dry matter ratio of PVDF to the auxiliary adhesive is 9:1. Then, the adhesive layer slurry is coated on the two surfaces of the above-mentioned diaphragm with a heat-resistant layer by spray coating. The spraying speed is set to 5000rpm, and the spacing of the spraying rotors increases from left to right. In Example 10, the rotor spacing increases from left to right in increments of 24mm, and the initial spacing is 150mm. Then, it is 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 , and the adhesive layer is coated on the surface of the heat-resistant layer. Other processes and parameters are consistent with those in Example 5.

[0083] [Examples 11-13]

[0084] During the preparation of the diaphragm, compared with Example 10, it is necessary to adjust the spacing between the rotors of the rotary spray coating head to prepare diaphragms with different uniformity of adhesive layer coating. Among them, the rotor spacing increases from left to right, the rotors in Example 12 are equally spaced, the rotor spacings of Examples 11 and 13 are at a level between the two, and the rotor spacing of Example 11 is greater than that of Example 13. The rotor spacing parameters of each embodiment are as follows: the rotors in Example 12 are equally spaced 150mm, the rotor spacing in Example 11 increases from left to right in increments of 16mm, the initial spacing is 150mm, and the rotor spacing in Example 13 increases from left to right in increments of 8mm. Other processes and parameters remain consistent with Example 10.

[0085] [Example 14]

[0086] During the preparation of the diaphragm, compared with Example 13, the rotation speed of the rotor during rotary spraying was increased, and the standard deviation of the single spray point area was reduced, so that the number of effective bonding points increased. The spraying speed was 8000 rpm, and the other processes and parameters were consistent with Example 13.

[0087] [Examples 15-16]

[0088] During the diaphragm preparation process, compared with Example 14, the spacing between the rotors of the rotary spray coating head was adjusted, wherein the rotor spacing in Example 16 was the same as that in Example 12, and the rotor spacing in Example 15 increased from left to right in increments of 4 mm, with an initial spacing of 150 mm, and other processes and parameters remained consistent with Example 14.

[0089] [Comparative Example 1]

[0090] During the preparation of the diaphragm, compared with Example 3, the solid content of the heat-resistant layer slurry was adjusted to 45%, and the other processes and parameters remained consistent with Example 3.

[0091] [Comparative Example 2]

[0092] During the preparation of the diaphragm, compared with Example 2, the cell size selected was a cell pitch of 190 μm and a cell depth of 32 μm, and the other processes and parameters were consistent with Example 2.

[0093] [Comparative Example 3]

[0094] During the preparation of the diaphragm, the micro-gravure roller cell design is further narrowed and deepened on the basis of Example 4. The cell size is 92 μm for the mesh spacing and 61 μm for the cell depth. At the same time, the cell volume remains unchanged. Other processes and parameters remain consistent with Example 4.

[0095] In order to verify the effect of the separators of each embodiment, the separators of embodiments 1-16 and comparative examples 1-3 are respectively assembled in lithium ion secondary batteries. The preparation process of the lithium ion secondary batteries is as follows:

[0096] (1) Cathode preparation

[0097] The positive electrode active material NCM523 (nickel, cobalt and manganese have 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) are 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 is coated on the positive electrode current collector aluminum foil, and after drying, cold pressing and other processes, a positive electrode sheet is obtained, and the sheet density of the positive electrode sheet is controlled to be 2.58g / cm 3 .

[0098] (2) Negative electrode preparation

[0099] The negative electrode active material artificial graphite, the conductive agent acetylene black, the adhesive styrene butadiene rubber (SBR), and the 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, and after drying, cold pressing and other processes, the negative electrode sheet was obtained, and the negative electrode sheet density was controlled to be 1.65g / cm 3 .

[0100] (3) Preparation of electrolyte

[0101] In an argon atmosphere glove box with a water content of <10ppm, EC, PC, and DEC are mixed in a volume ratio of EC:PC:DEC=1:1:1 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the organic solvent. After mixing evenly, an electrolyte with a lithium salt concentration of 1 mol / L is obtained.

[0102] (4) Battery assembly

[0103] The prepared positive electrode sheet, separator, and negative electrode sheet are placed in sequence, so that the separator is placed between the positive and negative electrode sheets to play an isolating role, and a bare cell is obtained by winding, and the thickness of the bare cell is 12±0.3mm. The bare cell is hot-pressed by a hot press (hot-pressing temperature is 95±2°C, hot-pressing pressure is 4.5Mpa, and hot-pressing time is 35s) and packaged with an aluminum-plastic film, and the prepared electrolyte is injected into the dried bare cell, and a lithium-ion secondary battery is obtained after vacuum packaging, standing, forming, shaping and other processes.

[0104] The lithium ion secondary batteries prepared in Examples 1 to 16 and Comparative Examples 1 to 3 were subjected to performance tests respectively. The test process is as follows. The test results are shown in Tables 1 and 2:

[0105] (1) True density test:

[0106] The inorganic particles used to prepare the heat-resistant layer in Examples 1-16 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;

[0107] Alternatively, the diaphragms prepared in Examples 1-16 and Comparative Examples 1-3 are used as samples, or the batteries prepared in Examples 1-16 and Comparative Examples 1-3 are disassembled to obtain diaphragms, which are cleaned, dried by natural ventilation and used as samples, and the inorganic particles in the samples are separated, and the true density of the samples is obtained by referring to the test method of the national standard: QB / T1010-2015.

[0108] The aforementioned separation method is not particularly limited, as long as it can meet the QB / T1010-2015 test requirements. At least the following solutions can be used:

[0109] Solution 1: Dissolve the sample in DMAC (dimethylacetamide) or NMP (N-methylpyrrolidone) and perform ultrasonic cleaning for 12 hours until the inorganic particles are separated from the base film, filter and dry to obtain the inorganic particles, and repeatedly use DMAC or NMP for ultrasonic cleaning until the inorganic particles meet the test requirements of QB / T1010-2015;

[0110] 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 repeatedly for calcination until the inorganic particles meet the test requirements of QB / T1010-2015.

[0111] (2) Bulk density test:

[0112] The diaphragms prepared in Examples 1-9 and Comparative Examples 1-3 are used as samples, or the batteries prepared in Examples 1-9 and Comparative Examples 1-3 are disassembled to obtain diaphragms, and the diaphragms are cleaned, dried by natural ventilation, and used as samples. The total thickness of the diaphragm samples is measured by a Mahr thickness gauge, and the total weight of the diaphragm samples is obtained by weighing the diaphragm samples using a precision electronic balance. The area of ​​the diaphragm samples is measured and recorded as the total area. The above-mentioned diaphragms are soaked in NMP solvent and ultrasonically washed away 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. The density of the heat-resistant layer is: W = (total weight - base film weight) / (2 × total area), the thickness of the heat-resistant layer = (total thickness - base film thickness) / 2, and the bulk density of the heat-resistant layer is calculated by the heat-resistant layer density / the thickness of the heat-resistant layer. The unit of the bulk density is: g / cm 3 .

[0113] Alternatively, the diaphragms prepared in Examples 10-16 are used as samples, or the batteries prepared in Examples 10-16 are disassembled to obtain diaphragms, the diaphragms are cleaned, dried by natural ventilation and used as samples, the adhesive layer on the surface of the samples is removed, the total thickness of the diaphragm samples is measured by a Mahr thickness gauge, the total weight of the diaphragm samples is weighed by a precision electronic balance, the area of ​​the diaphragm samples is measured and recorded as the total area, the aforementioned diaphragms are 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 with 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 .

[0114] The aforementioned method of removing the adhesive layer on the surface of the sample is not particularly limited, as long as the adhesive layer can be removed as much as possible without destroying the morphology of the heat-resistant layer. At least the following methods can be used:

[0115] Use a Keyence VHX-7000 scanning mirror to scan and record the coverage area of ​​the adhesive layer on the sample surface, which is recorded as the initial area. Use an adhesive strip to repeatedly 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. The bulk density of the heat-resistant layer surface is tested. When the adhesive layer is removed with an adhesive tape, compared with directly using a diaphragm without an adhesive layer as a sample, a portion 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 and have a 5% error.

[0116] It should be noted that the diaphragms satisfying a certain stacking density range provided in the foregoing text of the present application are based on the characterization results obtained from samples of diaphragms prepared in Examples 1-9 and Comparative Examples 1-3 with the heat-resistant layer as the outermost layer.

[0117] (3) Static friction coefficient test:

[0118] Use the diaphragms prepared in Examples 1-9 and Comparative Examples 1-3 as samples, or disassemble the batteries prepared in Examples 1-9 and Comparative Examples 1-3 to obtain diaphragms, find diaphragms with relatively intact relative surfaces, clean the diaphragms, and use them as samples after natural ventilation and drying. Fix the aforementioned positive electrode sheet on a certain inclined plane, and then lay the diaphragm to be tested flat on the surface of the positive electrode sheet, gradually increase the angle of the inclined plane, and when the diaphragm starts to slide, record the angle θ at this time. The static friction coefficient μ=tanθ of the diaphragm to the positive electrode sheet can be calculated through the angle.

[0119] Alternatively, use Examples 10-16 as samples, or disassemble the batteries prepared in Examples 1-16 to obtain a diaphragm, clean the diaphragm, dry it by natural ventilation and use it as a sample, remove the adhesive layer on the surface of the sample, find a diaphragm with a relatively intact relative surface, clean the diaphragm, dry it by natural ventilation and use it as a sample, fix the aforementioned positive electrode sheet on a certain inclined plane, and then lay the diaphragm to be tested on the surface of the aforementioned positive electrode sheet, gradually increase the angle of the inclined plane, and when the diaphragm starts to slide, record the angle θ at this time. The angle can be used to calculate the static friction coefficient μ = tanθ test value of the diaphragm to the positive electrode sheet, and after correcting the characterization error caused by removing the adhesive layer on the sample surface, the static friction coefficient value is obtained.

[0120] The above method for removing the adhesive layer on the surface of the sample is not particularly limited, as long as the adhesive layer can be removed as much as possible without destroying the morphology of the heat-resistant layer. At least the following methods can be used:

[0121] Use a Keyence VHX-7000 scanning mirror to scan and record the coverage area of ​​the adhesive layer on the sample surface, which is recorded as the initial area. Use an adhesive strip to repeatedly 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. The static friction coefficient of the heat-resistant layer surface is tested. When the adhesive layer is removed with an adhesive tape, compared with directly using a diaphragm without an adhesive layer as a sample, a portion 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 and have a 10% error.

[0122] It should be noted that the diaphragm satisfying a certain range of static friction coefficient provided in the foregoing text of the present application is based on the characterization results obtained from samples of the diaphragms prepared in Examples 1-9 and Comparative Examples 1-3, with the heat-resistant layer being the outermost layer.

[0123] (4) Test of effective adhesion R value and R value difference:

[0124] The diaphragms prepared in Examples 10-16 were used as samples, or the diaphragms prepared in Examples 10-16 and Comparative Examples 1-3 were disassembled to obtain diaphragms. The diaphragms were cleaned and dried by natural ventilation as samples. The area close to the center of the diaphragm width and covering 90% of the length was used as the sampling area. The sampling area was divided into five equal parts in the width direction to obtain five equal sampling areas with 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 the diaphragm surfaces were scanned by Keyence VHX-7000 to output the viscosity. The initial unit area of ​​the bonding layer is S1; after being stacked 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, and the size of the external force is not particularly limited, 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 and 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.

[0125] (5) Adhesion test between isolation film and electrode:

[0126] The diaphragms prepared in Examples 1-16 and Comparative Examples 1-3 were used as samples. First, the diaphragms and pole pieces were cut into small strips with a size of 50 mm long and 15 mm wide, respectively. Then, the small samples of the diaphragm and pole piece were stacked up and down, and hot-pressed into one piece by a flat hot press. The hot pressing temperature was 95±2°C, the hot pressing pressure was 4.5 MPa, and the hot pressing time was 35 seconds. Then, the hot-pressed diaphragm / pole piece composite strips were passed through a universal testing machine, and the adhesion between the diaphragm and the pole piece was tested by a 180°C peeling method.

[0127] (6) Air permeability test of diaphragm

[0128] The separators prepared in Examples 1-16 and Comparative Examples 1-3 were used as samples. The separators were punched into samples of 50 mm×50 mm and placed on the test platform of an OHKEN permeability tester. The value displayed by the tester was the air permeability of the separator.

[0129] (7) Testing of the cycle performance of lithium-ion secondary batteries

[0130] The batteries prepared in Examples 1-16 and Comparative Examples 1-3 were used as samples. At 25°C, they were charged to 4.25V at a constant current of 0.5C, then charged to a current lower than 0.05C at a constant voltage of 4.25V, and then discharged to 2.8V at a constant current of 0.5C. This was the first cycle process, and the discharge capacity of the lithium-ion secondary battery in the first cycle was recorded. Then, 300 charging and discharging cycles were performed according to the above method. Three lithium-ion secondary batteries were taken from each group, and the average value was calculated.

[0131] 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%.

[0132]

[0133]

[0134] Referring to Table 1, comparing Examples 1-6 and Comparative Examples 1-3, under the premise of keeping other conditions the same, by adjusting the solid content of the heat-resistant layer slurry and the mesh size used during coating, the surface static friction coefficient of the heat-resistant layer and the bulk density of the heat-resistant layer can be adjusted, thereby adjusting the adhesion between the diaphragm and the electrode sheet, the air permeability of the diaphragm and the cycle performance of the battery. From the test results, it can be concluded that: when the surface static friction coefficient μ≤0.8 of the heat-resistant layer, and the bulk density A of the heat-resistant layer is (0.4~0.5)ρ, the adhesion between the diaphragm and the positive electrode sheet is greater than that not within the above-defined range, the air permeability of the diaphragm is moderate, and the cycle performance of the battery is better. If the bulk density A of the heat-resistant layer is within the above range, but the static friction coefficient μ is high (Comparative Example 1), the adhesion between the diaphragm and the positive electrode sheet is also small. This is because: the larger the static friction coefficient, the more uneven the surface of the heat-resistant layer is, and the smaller the effective bonding area between the bonding particles in the heat-resistant layer and the porous base film is, resulting in insufficient bonding between the diaphragm and the pole piece. If the static friction coefficient satisfies μ≤0.8, but the bulk density of the heat-resistant layer is A<0.4ρ (Comparative Example 2), it will also lead to insufficient bonding between the diaphragm and the pole piece; if the bulk density of the heat-resistant layer is A>0.5ρ (Comparative Example 3), although the bonding between the diaphragm and the pole piece can meet the standard, the test value of the air permeability of the diaphragm is high, that is, the time for lithium ions to pass through the diaphragm is increased, which reduces the transmission efficiency of lithium ions, thereby affecting the cycle performance of the battery.

[0135] Taking Examples 1-5 as an example, for the heat-resistant layer composed of boehmite as a heat-resistant material, if the surface static friction coefficient μ≤0.8 and the stacking density A of the heat-resistant layer is (0.4~0.5)ρ, the flat and dense heat-resistant layer provides a diaphragm with an adhesion force of more than 0.65N / m to the positive electrode sheet and an air permeability value of 180-203s / 100cc, so as to achieve the effect of improving the battery cycle retention rate. Under the coordinated adjustment of the static friction coefficient and the stacking density, Example 5 is taken as the optimal example, and a battery with a capacity retention rate of 97.8% after 300 cycles at 25°C is obtained.

[0136] In Examples 7-9, by replacing different inorganic particles, better effects can be achieved when the surface static friction coefficient of the heat-resistant layer is less than or equal to 0.8 and the packing density of the heat-resistant layer satisfies (0.4-0.5)ρ.

[0137] Compared with Examples 1-9, Examples 10-16 add an adhesive layer on the heat-resistant layer, and the adhesion between the separator and the positive electrode plate is better, and the adhesion between the separator and the negative electrode plate is also better. This is because after the battery cell is hot-pressed, the adhesive layer and the electrode plate stimulate the adhesion, further improving the bonding strength between the separator as a whole and the electrode plate.

[0138] Comparing Examples 10-13, adjusting the spacing of the coating head rotors when spraying the adhesive layer can adjust the distance between each rotor in the spray coverage area and the morphology of the spraying points to achieve the purpose of adjusting the effective adhesion R value and the effective adhesion difference of the diaphragm. The test results show that when the effective adhesion R value difference of the diaphragm is less than 20%, compared with when the R value difference is greater than 20% (Example 10), the adhesion between the diaphragm and the pole piece is further improved, and the cycle performance of the battery cell is further improved. This is because the smaller the difference in the effective adhesion R value, the better the uniformity of the adhesion between the diaphragm and the pole piece, and the greater the adhesion; the larger the difference in the effective adhesion R, the worse the uniformity of the adhesion between the diaphragm and the pole piece, and the smaller the adhesion.

[0139] Comparing Examples 11-13 and Examples 14-16, the process parameters of the spacing of the coating head rotors and the spraying speed during the adhesive layer spraying are adjusted so that the effective adhesion R value of the diaphragm is 60-80% while the effective adhesion R value difference is less than 20%. The test results show that: when the effective adhesion R value difference of the diaphragm is less than 20%, the effective adhesion R value is 60-80%, which further improves the adhesion between the diaphragm and the pole piece, and further improves the cycle performance of the battery cell. This is because the higher the effective adhesion R value, the larger the effective bonding area between the diaphragm and the pole piece, the more bonding points, and the greater the bonding force.

[0140] The diaphragm provided by the present invention can obtain a heat-resistant layer with dense stacking and a gentle surface by controlling the static friction coefficient and stacking density of the heat-resistant layer on the surface of the base film, so that the effective contact area between the adhesive particles in the heat-resistant layer and the porous base film is increased, thereby improving the bonding force between the heat-resistant layer and the porous base film, and increasing the bonding force between the entire diaphragm and the pole piece. An adhesive layer is arranged on the heat-resistant layer. After the battery is hot-pressed, an adhesive force is formed between the adhesive layer and the pole piece, which has the effect of battery shaping. At the same time, it can also inhibit the wrinkling and deformation of the battery cell caused by the expansion of the pole piece after the cycle, thereby improving the cycle life of the battery. In addition, the heat-resistant layer with dense stacking and a gentle surface also increases the effective contact area between the adhesive particles in the adhesive layer and the heat-resistant layer, further improving the bonding force between the entire diaphragm and the pole piece. In addition, by controlling the effective adhesion difference of the diaphragm to be less than 20%, the adhesion distribution between the pole piece and the diaphragm can be more uniform, and the effective adhesion is controlled within 60%-80%, ensuring that the density of the bonding points between the diaphragm and the pole piece is sufficient, thereby further improving the adhesion between the diaphragm and the pole piece. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and use significance.

[0141] 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 familiar with 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 a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still 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; The heat-resistant layer comprises inorganic particles and an adhesive, and the surface static friction coefficient of the heat-resistant layer is ≤0.8; the bulk density A of the heat-resistant layer satisfies: A=(0.4-0.5)×ρ, where ρ represents the true density of the inorganic particles, in units of g / cm 3 .

2. The diaphragm according to claim 1, characterized in that It also includes an adhesive layer, which is disposed at least on the other side of the contact surface between the heat-resistant layer and the porous base film.

3. The diaphragm according to claim 2, characterized in that The difference between the maximum and minimum values ​​of the effective adhesion R value of the diaphragm provided with the heat-resistant layer is ≤ 20%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area.

4. The diaphragm according to claim 2 or 3, characterized in that: The effective adhesion R value of the diaphragm provided with the heat-resistant layer is 60% to 80%; wherein the effective adhesion R value represents the ratio of the effective bonding area of ​​the diaphragm to the theoretical bonding area.

5. The diaphragm according to claim 1, 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 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.

7. The diaphragm according to claim 1, characterized in that The average particle size of the binder is smaller than the average particle size of the inorganic particles.

8. The diaphragm according to claim 2, 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 1, 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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