Porous composite separator and method for manufacturing the same, secondary battery

Abstract

The present application relates to the technical field of battery, especially to a porous composite diaphragm, a preparation method thereof and a secondary battery. The porous composite diaphragm comprises a base film, a first coating layer and a second coating layer. The first coating layer and the second coating layer are oppositely arranged on two surfaces of the base film. The first coating layer and the second coating layer comprise a copolymer containing a first monomer unit and a second monomer unit. The first monomer unit contains a cyano group on a side chain, and the second monomer unit contains a carbonyl group on a side chain. In the copolymer, the mole percentage of the second monomer unit is 1% to 20%. The first coating layer and the second coating layer comprise a skeleton structure and a first type of pore structure formed between the skeleton structure. A second type of pore structure is formed on the body of the skeleton structure. The average pore diameter of the first type of pore structure is greater than the average pore diameter of the second type of pore structure. The porous composite diaphragm can improve the adhesion performance with the pole piece while improving the lithium ion conductivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a porous composite separator, a preparation method thereof and a secondary battery. BACKGROUND

[0002] With the rapid development of high energy density, large capacity and high rate lithium battery system, the performance requirements of key materials (including positive and negative electrodes, separators and electrolytes) are increasingly stringent. As a bridge between the positive and negative electrodes, the separator has an important influence on lithium ion transmission. In commercial production, the surface of the separator is usually coated with a polyvinylidene fluoride (PVDF) glue layer. This design has multiple advantages: first, PVDF can effectively maintain the structural integrity of the electrode / separator interface due to its excellent thermal stability, chemical resistance, strong wear resistance and strong impact resistance; second, in the semi-crystalline structure of PVDF, the crystalline region provides mechanical support, and the amorphous region provides adhesion and lithium ion conductivity. By optimizing the molecular weight and crystallinity of PVDF, the adhesion performance can be adjusted.

[0003] However, with the rapid development of high rate battery systems, traditional separators face severe challenges. In high silicon / carbon system batteries, the electrode material undergoes large volume changes during cycling, and the electrode and the separator are prone to faulting, resulting in a sharp increase in battery resistance. Although the introduction of acrylic ester comonomers into PVDF can moderately improve the adhesion strength of the separator and the electrode, it still cannot meet the actual demand. In addition, PVDF swells significantly in strong polar electrolyte, which can significantly weaken the wet adhesion between the separator and the electrode; and as the cycle progresses, the swollen PVDF tends to migrate to the edge of the battery, eventually leading to PVDF overflow, which not only affects the stability of the interface, but also accelerates the performance degradation of the battery.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The present application aims to provide a porous composite separator, a preparation method thereof and a secondary battery. The porous composite separator of the present application can improve the adhesion performance with the electrode while improving the lithium ion conductivity.

[0006] To achieve the above-mentioned purpose of the present application, the first aspect of the present application provides a porous composite separator, comprising a base film, a first coating layer and a second coating layer; the first coating layer and the second coating layer are oppositely arranged on both sides of the base film;

[0007] The first coating layer and the second coating layer each independently comprise a copolymer containing first monomer units and second monomer units; the first monomer units contain cyano groups on the side chain, and the second monomer units contain carbonyl groups on the side chain; in the copolymer, the molar proportion of the second monomer units is 1% to 20%;

[0008] The first coating and the second coating each independently comprise a skeleton structure and a first type of pore structure formed between the skeleton structures, a body of the skeleton structure being formed with a second type of pore structure; the first type of pore structure has an average pore diameter greater than an average pore diameter of the second type of pore structure.

[0009] In some embodiments, the second monomer unit has a mole percentage of 8% to 20% in the copolymer.

[0010] In some embodiments, the first monomer unit is derived from acrylonitrile, and the second monomer unit is derived from an unsaturated carbonyl compound. Further, the unsaturated carbonyl compound comprises at least one of an acrylate monomer, an acrylic monomer, and an acrylamide monomer.

[0011] In some embodiments, the acrylate monomer comprises at least one of methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate.

[0012] In some embodiments, the acrylic monomer comprises (meth)acrylic acid.

[0013] In some embodiments, the acrylamide monomer comprises at least one of acrylamide, diacetone acrylamide, and N-methylol acrylamide.

[0014] In some embodiments, the unsaturated carbonyl compound comprises:

[0015] (i) an acrylate monomer; and,

[0016] (ii) at least one of an acrylic monomer and an acrylamide monomer.

[0017] In some embodiments, the copolymer has a weight average molecular weight of 200,000 to 1,000,000, and a glass transition temperature Tg of 40 to 120°C.

[0018] In some embodiments, the first type of pore structure has an average pore diameter of 1 to 8 μm, and the second type of pore structure has an average pore diameter of 30 to 150 nm.

[0019] In some embodiments, the first type of pore structure has a number of 40 to 120 in any 25.6 μm x 19.2 μm field of view of the first coating and the second coating.

[0020] In some embodiments, the first coating layer and the second coating layer are scanned and imaged by a scanning electron microscope (SEM) to obtain a SEM secondary electron image, based on the SEM secondary electron image, the first type of pore structure edge is fitted as a two-dimensional figure, a longest line segment and a shortest line segment formed by passing through a geometric center of the two-dimensional figure and intersecting the two-dimensional figure are set as M and N respectively, M < 15 μm, and 0.2 ≤ N / M ≤ 1.

[0021] In some embodiments, a direction from the first coating layer to the base film and perpendicular to the base film is set as a first direction, a height difference between a highest point and a lowest point of the first type of pore structure in the first direction is set as h, and h is 0.1-4 μm.

[0022] In some embodiments, the porous composite separator is pressed for 5 min at 25 °C, and a change value Δh of h before and after the pressing and a pressure P of the pressing satisfy: 0.01 ≤ Δh / P ≤ 0.2; wherein P is 0.1-20 MPa.

[0023] In some embodiments, an areal density of the first coating layer and the second coating layer is independently 0.1-3 g / m 2 .

[0024] In some embodiments, the first coating layer and the second coating layer independently comprise an inorganic heat-resistant material. Further, the inorganic heat-resistant material comprises at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, and silicon dioxide.

[0025] In some embodiments, a mass ratio of the copolymer to the inorganic heat-resistant material in the first coating layer and the second coating layer is independently 1: (0.5-2).

[0026] In some embodiments, the porous composite separator further comprises a heat-resistant coating layer, which is arranged between the base film and the first coating layer.

[0027] In some embodiments, the heat-resistant coating layer comprises inorganic heat-resistant particles and a binding polymer; the inorganic heat-resistant particles comprise at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, and silicon dioxide, and a D50 particle size of the inorganic heat-resistant particles is 1-1.5 μm.

[0028] In some embodiments, the base film comprises at least one of a polyethylene-based film and a polypropylene-based film.

[0029] The second aspect of the present application provides a method for preparing the porous composite separator provided by the first aspect of the present application, comprising the following steps: preparing a slurry of the first coating and a slurry of the second coating, coating the slurry of the first coating and the slurry of the second coating on two sides of the base film respectively, then immersing in a coagulation bath, and then performing a drying treatment.

[0030] The second aspect of the present application provides another method for preparing the porous composite separator provided by the first aspect of the present application, comprising the following steps:

[0031] (a) preparing a slurry of the heat-resistant coating, coating the slurry of the heat-resistant coating on one side of the base film, and performing a drying treatment to form the heat-resistant coating;

[0032] (b) preparing a slurry of the first coating and a slurry of the second coating, coating the slurry of the first coating and the slurry of the second coating on two sides of the base film on which the heat-resistant coating is formed respectively, then immersing in a coagulation bath, and then performing a drying treatment.

[0033] In some embodiments, the coagulation bath comprises a first coagulation bath, a second coagulation bath and a third coagulation bath; the first coagulation bath, the second coagulation bath and the third coagulation bath are all aqueous solutions of N,N-dimethylacetamide, and the concentration of N,N-dimethylacetamide decreases in turn. Further, the concentration of N,N-dimethylacetamide in the first coagulation bath, the second coagulation bath and the third coagulation bath is 55wt%-65wt%, 8wt%-12wt% and 3wt%-7wt% respectively.

[0034] The third aspect of the present application provides a secondary battery comprising the porous composite separator provided by the first aspect of the present application.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] (1) In the porous composite separator of the present application, the copolymer with cyano and carbonyl groups in the side chain is used as the bonding component, the cyano group of the first monomer unit forms an intermolecular interaction network through hydrogen bonding and dipole interaction, which endows it with excellent oxidation resistance; the introduction of the second monomer unit appropriately reduces the regularity of the molecular chain formed by the first monomer unit, which not only makes the copolymer have good dispersibility and solubility in solvents, which is helpful to form a uniform and stable coating structure, but also can improve the bonding performance of the coating and the pole piece containing active materials;

[0037] (2) In the porous composite separator of the present application, the first type of pore structure and the second type of pore structure with different pore diameters are arranged in the first coating and the second coating, and the double-scale pore structure forms a network structure that is more conducive to the transmission of lithium ions, thereby improving the conductivity of lithium ions;

[0038] (3) In the porous composite diaphragm, the size of the first type of pore structure in the first coating and the second coating in the thickness direction is further adjusted to form a certain liquid retention area between the diaphragm and the pole piece, which is helpful to the liquid retention capacity under high pressure / high viscosity, and improves the cycle performance of the battery;

[0039] (4) The secondary battery has excellent cycle stability and rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the specific embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0041] Figure 1 A structure diagram of a porous composite diaphragm provided by the present application is shown in the figure.

[0042] Figure 2 A structure diagram of another porous composite diaphragm provided by the present application is shown in the figure.

[0043] Figure 3 An SEM image of the second coating surface of the porous composite diaphragm prepared in Example 1 of the present application is shown in the figure.

[0044] Figure 4 An SEM image of the second coating surface of the porous composite diaphragm prepared in Example 1 of the present application at another magnification is shown in the figure.

[0045] Figure 5 An image after adjusting the threshold of the SEM image of the second coating surface of Example 1 of the present application using ImageJ is shown in the figure.

[0046] Figure 6 A cross-sectional SEM of the second coating of the porous composite diaphragm prepared in Example 1 of the present application is shown in the figure.

[0047] Reference signs:

[0048] 10 - base film; 20 - first coating; 30 - second coating; 40 - heat-resistant coating. DETAILED DESCRIPTION

[0049] The technical solutions of the present application will be described clearly and completely below in combination with the drawings and specific embodiments, but those skilled in the art will understand that the following described embodiments are part of the embodiments of the present application, not all the embodiments, and are only used to illustrate the present application, and should not be regarded as limiting the scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application. The specific conditions are not specified in the embodiments, and are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not specified by the manufacturer, and are conventional products that can be purchased on the market.

[0050] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second", "third" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0051] Figure 1 A schematic structural diagram of a porous composite diaphragm provided by the present application, the first aspect of the present application provides a porous composite diaphragm, comprising a base film 10, a first coating layer 20 and a second coating layer 30; the first coating layer 20 and the second coating layer 30 are oppositely arranged on both sides of the base film 10;

[0052] The first coating layer 20 and the second coating layer 30 each independently comprise a copolymer containing first monomer units and second monomer units; the first monomer units contain cyano groups on the side chains, and the second monomer units contain carbonyl groups on the side chains; in the copolymer, the molar proportion of the second monomer units is 1% to 20%;

[0053] The first coating layer and the second coating layer each independently comprise a skeleton structure and a first type of pore structure formed between the skeleton structures, and a second type of pore structure is formed on the body of the skeleton structure; the average pore size of the first type of pore structure is greater than the average pore size of the second type of pore structure.

[0054] The porous composite diaphragm of the present application has a copolymer with cyano and carbonyl in the side chain as a bonding component. The cyano of the first monomer unit forms an intermolecular interaction network through hydrogen bonding and dipole interaction, giving excellent oxidation resistance. The introduction of the second monomer unit appropriately reduces the regularity of the molecular chain formed by the first monomer unit, making the copolymer have good dispersibility and solubility in solvents, helping to form a uniform and stable coating structure, and improving the adhesion performance of the coating to the pole piece containing active materials. The first coating and the second coating of the present application can be the same or different.

[0055] The higher the content of cyano in the copolymer, the higher the proportion of intermolecular interaction network formed by cyano through hydrogen bonding and dipole interaction, and the stronger the rigidity of the copolymer, which is beneficial to the improvement of oxidation resistance. However, if the rigidity of the copolymer is too strong, the molecular chain movement is limited and the flexibility is poor, making it difficult to fully wet the pole piece to be bonded, affecting the adhesion performance. By introducing a certain amount of second monomer unit into the copolymer, the present application appropriately disrupts the crystalline region formed by the first monomer unit, improves the adhesion performance of the copolymer, and widens the temperature and pressure range that can be achieved, which is beneficial to actual production. The present application controls the molar ratio of the second monomer unit in the copolymer to be 1% to 20% to balance the oxidation resistance and adhesion performance of the copolymer. For example, in different embodiments, the molar ratio of the second monomer unit in the copolymer can be 1%, 3%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, or a range formed by any two of them. The molar ratio of the second monomer unit in the copolymer of the present application is calculated by nuclear magnetic resonance characterization of the copolymer.

[0056] In the porous composite diaphragm of the present application, the first coating and the second coating are provided with first and second pore structures with different pore sizes, and the double-scale pore structure forms a network structure that is more conducive to the transmission of lithium ions, improving the conductivity of lithium ions. The body of the skeleton structure includes the body of the skeleton structure and the surface of the body.

[0057] In some embodiments, the first monomer unit is derived from acrylonitrile, and the second monomer unit is derived from an unsaturated carbonyl compound. Further, the unsaturated carbonyl compound includes at least one of an acrylate monomer, an acrylic monomer, and an acrylamide monomer.

[0058] In some embodiments, the first monomer unit is derived from acrylonitrile, and the second monomer unit is derived from an unsaturated carbonyl compound. Further, the unsaturated carbonyl compound includes at least one of an acrylate monomer, an acrylic monomer, and an acrylamide monomer.

[0059] In some embodiments, the content of nitrogen element in the copolymer is 15wt%-26wt%, for example, it can be 15wt%, 16wt%, 18wt%, 20wt%, 22wt%, 24wt%, 26wt% or a range consisting of any two of them. The nitrogen element in the copolymer is mainly provided by the cyano group, and the content of the nitrogen element in the copolymer is controlled in the above range, which is more helpful to provide appropriate rigidity, thereby improving the oxidation resistance; and avoiding the poor adhesion performance caused by the too high content of cyano group.

[0060] In some embodiments, the acrylate monomer includes at least one of methyl (meth)acrylate, ethyl (meth)acrylate and butyl (meth)acrylate.

[0061] In some embodiments, the acrylic monomer includes (meth)acrylic acid.

[0062] In some embodiments, the acrylamide monomer includes at least one of acrylamide, diacetone acrylamide and N-methylol acrylamide.

[0063] In some embodiments, the unsaturated carbonyl compound includes:

[0064] (i) an acrylate monomer; and,

[0065] (ii) at least one of an acrylic monomer and an acrylamide monomer.

[0066] The introduction of the acrylic monomer and / or the acrylamide monomer in the unsaturated carbonyl compound of the present application is more helpful to improve the dispersibility, solubility and adhesion performance of the copolymer.

[0067] In some embodiments, the molar proportion of the acrylate monomer in the unsaturated carbonyl compound is 90%-99%, for example, it can be 90%, 92%, 95%, 97%, 99% or a range consisting of any two of them, and the total molar proportion of the acrylic monomer and the acrylamide monomer is 1%-10%, for example, it can be 1%, 3%, 5%, 8%, 10% or a range consisting of any two of them.

[0068] In some embodiments, the weight average molecular weight (Mw) of the copolymer is 200,000-1,000,000, for example, it can be 200,000, 350,000, 500,000, 650,000, 800,000, 1,000,000 or a range consisting of any two of them, for example, it can be 300,000-650,000; the glass transition temperature Tg of the copolymer is 40-120℃, for example, it can be 40℃, 50℃, 60℃, 80℃, 100℃, 120℃ or a range consisting of any two of them, for example, it can be 60-100℃. The molecular chain and the glass transition temperature of the copolymer are controlled in the above range, which is more helpful to improve the oxidation resistance of the coating and the adhesion performance to the pole piece.

[0069] In some embodiments, the copolymer is prepared by polymerization of acrylonitrile and an unsaturated carbonyl compound. Further, the polymerization is initiated by an initiator and is carried out in water.

[0070] In some embodiments, the initiator includes, but is not limited to, persulfate salt, such as ammonium persulfate. The amount of the initiator can be 0.1% to 0.5% of the total mass of the monomers used in the polymerization.

[0071] In actual operation, the amount of water used in the polymerization is determined to ensure that the polymerization proceeds stably, for example, it can be 0.5 to 2 times the total mass of the monomers used in the polymerization. In some embodiments, after the polymerization is completed, the copolymer powder is obtained by spray drying.

[0072] In some embodiments, the polymerization system further includes a chain transfer agent. The chain transfer agent includes, but is not limited to, thiol chain transfer agent, such as dodecyl mercaptan. The amount of the chain transfer agent can be 0.01% to 1% of the total mass of the monomers used in the polymerization, which can be adjusted according to the target molecular weight of the copolymer.

[0073] In some embodiments, the temperature of the polymerization is 60 to 80°C, but is not limited thereto, as long as the monomers can be polymerized. The time of the polymerization can be 24 to 48h, but is not limited thereto, which can be adjusted according to the target molecular weight of the copolymer.

[0074] In some embodiments, the average pore size of the first type of pore structure is 1 to 8 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or a range consisting of any two of them, and the average pore size of the second type of pore structure is 30 to 150 nm, for example, it can be 30 nm, 40 nm, 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, 150 nm, or a range consisting of any two of them.

[0075] In some embodiments, the number of the first type of pore structures in any 25.6 pm x 19.2 pm field of view of the first coating and the second coating is independently 40-120, for example, 40, 60, 80, 100, 120, or a range defined by any two of them, thereby further improving the bonding performance and liquid retention performance. Among them, the number of the first type of pore structures in the 25.6 pm x 19.2 pm field of view is obtained by scanning electron microscopy to obtain the surface image of the first coating and the second coating. Specifically, the statistical method includes: adjusting the picture threshold range of the surface image (0≤MinThr≤10, 60≤MaxThr≤150, for example, MinThr is 7, and MaxThr is 115) using ImageJ, making the first type of pore structure edge in the picture stand out, using the software to draw the pore structure profile (specifically, drawing along the part with higher gray value, which appears white or close to white), obtaining the drawing image, and the minimum unit in the drawing image is counted as one pore structure. The minimum unit located at the edge of the drawing image that is not completely displayed is also counted as one pore structure.

[0076] In some embodiments, the first coating and the second coating are scanned and imaged by SEM to obtain a SEM secondary electron image, the edge of the first type of pore structure is fitted as a two-dimensional graph based on the SEM secondary electron image, the length of the longest line segment and the length of the shortest line segment formed by passing through the geometric center of the two-dimensional graph and intersecting the two-dimensional graph are set as M and N, respectively, M<15 pm, and 0.2≤N / M≤1, for example, N / M can be 0.2, 0.4, 0.5, 0.6, 0.8, 1, or a range defined by any two of them. The first type of pore structure satisfying the above conditions is more helpful to fully exert the transmission performance of lithium ions and the liquid retention capacity. Among them, the drawing image is obtained according to the statistical method of the number of the first type of pore structures, and the maximum Feret diameter MaxFeret and the minimum Feret diameter MinFeret of the two-dimensional graph fitted by the edge of each first type of pore structure in the drawing image are measured by ImageJ software, i.e., M and N.

[0077] In some embodiments, the direction perpendicular to the base film and from the first coating to the base film is set as the first direction, the height difference between the highest point and the lowest point of the first type of pore structure in the first direction is h, and h is 0.1-4 pm, for example, 0.1 pm, 0.5 pm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm, 3.5 pm, 4 pm, or a range defined by any two of them.

[0078] In the porous composite separator of the present application, the size of the first type of pore structure in the thickness direction of the first coating and the second coating is further adjusted to form a certain liquid retention area between the separator and the pole piece, which is helpful to the liquid retention capacity under high pressure / high viscosity, etc., and improves the rate cycle performance of the battery cell.

[0079] In some embodiments, the change of h before and after the pressure is applied to the porous composite separator for 5 min at 25°C satisfies: 0.01≤△h / P≤0.2, wherein P is 0.1-20 MPa. Further, the pressure is a constant pressure.

[0080] It should be noted that when calculating △h / P, the units represented by △h and P are not considered, and only the corresponding numerical values when the unit of △h is μm and the unit of P is MPa are used to calculate the corresponding results of the corresponding relationship.

[0081] As in different embodiments, the porous composite separator of the present application has △h / P of 0.01, 0.05, 0.08, 0.1, 0.12, 0.15, 0.18, 0.2 or a range formed by any two of them under the above test conditions, indicating that the porous composite separator of the present application has a relatively stable structure and is not easy to deform under the action of external force after being applied to the battery, which can ensure the transmission of lithium ions.

[0082] In some embodiments, the first coating and the second coating each independently comprise an inorganic heat-resistant material. Further, the inorganic heat-resistant material comprises at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide and silicon dioxide; the D50 particle size of the inorganic heat-resistant material is 0.01-800 nm, for example, it can be 0.01 nm, 0.1 nm, 1 nm, 10 nm, 50 nm, 100 nm, 300 nm, 500 nm, 600 nm, 800 nm or a range formed by any two of them. The further introduction of inorganic heat-resistant material in the first coating and the second coating is more helpful to improve the thermal stability of the separator and ensure the safety of the battery.

[0083] In some embodiments, the mass ratio of the copolymer to the inorganic heat-resistant material in the first coating and the second coating is each independently 1:(0.5-2), for example, it can be 1:0.5, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2 or a range formed by any two of them.

[0084] In some embodiments, the areal density of the first coating and the second coating is each independently 0.1-3 g / m 2 , for example, it can be 0.1 g / m 2 , 0.5 g / m 2 , 0.8 g / m 2 , 1 g / m 2 , 1.5 g / m 2 , 2 g / m 2 , 2.5 g / m 2 , 3 g / m 2or a range between any two of them. The suitable areal density is more conducive to balance the membrane strength, lithium ion transport capacity, and adhesive strength, etc.

[0085] Figure 2 Another schematic diagram of a porous composite membrane provided by the present application is shown. The porous composite membrane further comprises a heat-resistant coating layer 40 disposed between the base film 10 and the first coating layer 20. Specifically, the porous composite membrane comprises a base film 10, a heat-resistant coating layer 40 disposed on one surface of the base film 10, a first coating layer 20 disposed on the surface of the heat-resistant coating layer 40, and a second coating layer 30 disposed on the other surface of the base film 10.

[0086] In some embodiments, the heat-resistant coating layer comprises inorganic heat-resistant particles and a binding polymer; the inorganic heat-resistant particles comprise at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide, and silicon dioxide, and the D50 particle size of the inorganic heat-resistant particles is 1-1.5 μm, for example, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, or a range between any two of them.

[0087] In some embodiments, the binding polymer comprises at least one of styrene-butadiene rubber, polymethyl methacrylate, polybutyl methacrylate, polyacrylic acid, polymethacrylic acid, polyacrylamide, and acrylic-acrylonitrile copolymer.

[0088] In some embodiments, the mass ratio of the inorganic heat-resistant particles and the binding polymer in the heat-resistant coating layer is (2-4) : 1, for example, 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, 3.2:1, 3.5:1, 3.8:1, 4:1, or a range between any two of them.

[0089] In some embodiments, the heat-resistant coating layer further comprises at least one of a thickening agent, a dispersing agent, etc. The thickening agent, the dispersing agent, and other auxiliary agents can be selectively added according to the actual process, and the amount of addition can be routinely adjusted, which will not be described here.

[0090] In some embodiments, the thickness of the heat-resistant coating layer is 0.1-3 μm, for example, 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or a range between any two of them.

[0091] In some embodiments, the base film comprises at least one of a polyethylene-based film and a polypropylene-based film. Further, the thickness of the base film is 3-10 μm. Specifically, the base film can be a porous base film.

[0092] The second aspect of the present application provides a method for preparing the porous composite separator according to the first aspect of the present application, comprising the following steps: preparing a slurry of the first coating and a slurry of the second coating, coating the slurry of the first coating and the slurry of the second coating on two sides of the base film respectively, then immersing in a coagulation bath, and then drying.

[0093] The second aspect of the present application provides another method for preparing the porous composite separator according to the first aspect of the present application, comprising the following steps:

[0094] (a) preparing a slurry of the heat-resistant coating, coating the slurry of the heat-resistant coating on one side of the base film, and drying to form the heat-resistant coating;

[0095] (b) preparing a slurry of the first coating and a slurry of the second coating, coating the slurry of the first coating and the slurry of the second coating on two sides of the base film with the heat-resistant coating respectively, then immersing in a coagulation bath, and then drying.

[0096] In actual operation, the coating method of the slurry of the first coating, the slurry of the second coating, and the slurry of the heat-resistant coating can include but is not limited to micro-gravure roll coating. Further, during coating, the relative humidity RH of the environment is 10% to 70%, for example, it can be 10%, 20%, 40%, 50%, 60%, 70%, or a range formed by any two of them. The relative humidity of the environment during coating in the subsequent examples is 50% ± 2%.

[0097] The slurry of the first coating, the slurry of the second coating, and the slurry of the heat-resistant coating can be prepared by conventional stirring according to the composition of each coating. For example, the slurry of the first coating and the slurry of the second coating each independently include a copolymer, a solvent, and optionally an inorganic heat-resistant material. Among them, the amount of solvent can be adjusted according to the solubility of the copolymer and the coating performance of the slurry to ensure the dissolution of the copolymer and obtain a suitable viscosity, thereby providing good coating performance. The types of solvents include but are not limited to N,N-dimethylacetamide.

[0098] When the first coating and the second coating of the first porous composite separator do not contain inorganic heat-resistant materials, no inorganic heat-resistant materials are added to the slurry of the first coating and the slurry of the second coating; when the first coating and the second coating of the first porous composite separator contain inorganic heat-resistant materials, inorganic heat-resistant materials are added to the slurry of the first coating and the slurry of the second coating, and the specific amount of addition is adjusted according to the content of inorganic heat-resistant materials in the first coating and the second coating; for example, when preparing the slurry of the first coating and the slurry of the second coating, the mass ratio of the copolymer to the inorganic heat-resistant material can be controlled to be 1: (0.5-2).

[0099] In some embodiments, when the porous composite separator contains a heat-resistant coating, the slurry of the heat-resistant coating comprises inorganic heat-resistant particles, a binding polymer, and optionally a thickening agent and optionally a dispersing agent. Among them, the binding polymer is added in the form of a liquid such as an emulsion, a solution, a dispersion, etc., and the solid content can be adjusted according to actual conditions to ensure the uniformity and coatability of the slurry; the thickening agent includes but is not limited to carboxymethyl cellulose, guar gum, polyacrylamide, etc., and the amount of the thickening agent can be 0% to 15% of the mass of the solid content of the binding polymer; the dispersing agent includes but is not limited to ammonium polyacrylate, polyethylene glycol, polyvinylpyrrolidone, etc., and the amount of the dispersing agent can be 0% to 15% of the mass of the solid content of the binding polymer.

[0100] In some embodiments, the coagulation bath comprises a first-stage coagulation bath, a second-stage coagulation bath, and a third-stage coagulation bath; the first-stage coagulation bath, the second-stage coagulation bath, and the third-stage coagulation bath are all aqueous solutions of N,N-dimethylacetamide, and the concentration of N,N-dimethylacetamide decreases in turn.

[0101] In the preparation of the porous composite separator of the present application, after coating the first coating and the second coating, the porous composite separator is immersed in the coagulation bath, and the exchange between the solvent and the non-solvent is utilized to form a suitable pore structure in cooperation with the composition of the copolymer. At the same time, the concentration of N,N-dimethylacetamide in the coagulation bath is regulated, and the porous composite separator is first passed through the first-stage coagulation bath with high N,N-dimethylacetamide concentration, which is helpful to form the second type of pores with small size and uniformity; and then the concentration of N,N-dimethylacetamide in the subsequent coagulation bath is decreased in turn, the water content is increased, the phase separation speed is increased, and the formation of the first type of pores with large pore size is promoted, thereby forming a double-scale pore structure and improving the lithium ion conduction performance.

[0102] In some embodiments, the concentration of N,N-dimethylacetamide in the first-stage coagulation bath, the second-stage coagulation bath, and the third-stage coagulation bath is 55wt% to 65wt%, 8wt% to 12wt%, and 3wt% to 7wt%, respectively. This is more helpful to form the first type of pores and the second type of pores with suitable size and quantity, and further improves the lithium ion conduction performance and liquid retention capacity of the separator, etc.

[0103] In some embodiments, the drying treatment comprises drying at 50 to 80℃. The specific drying method can include but is not limited to oven drying.

[0104] The third aspect of the present application provides a secondary battery comprising the porous composite separator of the first aspect of the present application. The secondary battery assembled using the porous composite separator of the present application has excellent cycle stability and rate performance.

[0105] In some embodiments, the secondary battery further comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte. The positive active material of the positive electrode sheet can include, but is not limited to, high-nickel ternary positive electrode material; the negative active material of the negative electrode sheet can include, but is not limited to, silicon-carbon negative electrode material; and the electrolyte can include commercial liquid electrolyte.

[0106] The adhesion of the porous composite separator to the positive electrode sheet is 10-40 N / m, for example, can be 10 N / m, 15 N / m, 20 N / m, 25 N / m, 30 N / m, 35 N / m, 40 N / m, or a range formed by any two of them.

[0107] The following examples are used to illustrate the case where the first coating and the second coating are the same, but not to limit the first coating and the second coating must be the same; it can be understood that the first coating and the second coating are different and each meet the conditions defined in the present application. The examples also belong to the protection scope of the present application. In addition, the same slurry is used to prepare the first coating and the second coating in the following examples, and the areal density of the second coating, the size and number of the first type of pore structure, the size and number of the second type of pore structure, and the adhesion to the positive electrode sheet are characterized. For the related parameters of the first coating and the parameters of the second coating, the parameters of the second coating are the same or within an acceptable deviation range, for example, the acceptable deviation range is within ±5% of the parameters of the second coating.

[0108] Example 1

[0109] The present embodiment provides a method for preparing a porous composite separator, comprising the following steps:

[0110] (1) Put 502 g of butyl acrylate, 1511 g of acrylonitrile, and 8 g of acrylic acid into a reaction kettle containing 2 kg of deionized water, heat to 70°C, and then gradually add 12 g of ammonium persulfate and 20 g of n-dodecyl mercaptan after stirring uniformly. React for 36 h to obtain an emulsion. Spray dry the emulsion to obtain a copolymer powder.

[0111] (2) Put 1 kg of N,N-dimethylacetamide into a 2L stirred kettle, add 40 g of copolymer powder, and stir at 1500 rpm for 4 h at 25°C to obtain a copolymer solution. Then add 40 g of alumina particles with a D50 particle size of 0.34 μm, and continue to stir for 1 h to obtain a ceramic copolymer mixture.

[0112] (3) Put 13.5 g of alumina particles with a D50 particle size of 0.92 μm into a 1L stirred kettle, add 0.5 g of polyacrylamide, 0.8 g of sodium carboxymethyl cellulose, and 20 g of polyacrylic acid solution with a solid content of 25%, and stir at 1000 rpm for 4 h at 25°C to obtain an alumina particle dispersion.

[0113] (4) The alumina particle dispersion liquid is coated on one side of a polyethylene film with a thickness of 5 μm by means of micro gravure roll coating, and dried in an oven at 60-65°C to obtain a heat-resistant coating layer with a thickness of 1 μm.

[0114] (5) The ceramic copolymer mixture is coated on the other side of the polyethylene film treated in step (4) and on the surface of the heat-resistant coating layer by means of micro gravure roll coating, and then sequentially passed through N,N-dimethylacetamide aqueous solutions with concentrations of 60 wt%, 10 wt% and 5 wt%, each with a coagulation time of 60 s; and then dried in an oven at 60-65°C to obtain a porous composite separator; wherein the areal densities of the first coating layer on the surface of the heat-resistant coating layer and the second coating layer on the surface of the polyethylene film are both 1.69 g / m 2 .

[0115] Figure 3 SEM image of the second coating layer surface of the porous composite separator prepared in this example, Figure 4 SEM image of the second coating layer surface of the porous composite separator prepared in this example at another magnification, from which it can be seen that the second coating layer comprises skeleton structures and first-type pore structures formed between the skeleton structures, and second-type pore structures formed on the bodies of the skeleton structures.

[0116] Examples 2-6

[0117] Examples 2-6 refer to the preparation method of the porous composite separator of Example 1, except that the amounts of the monomers used in step (1) for preparing the copolymer are different, and the areal densities of the first coating layer / second coating layer in step (5) are different, the specific differences being shown in Table 1.

[0118] Table 1 Amounts of monomers in step (1) and areal densities of the coating layers of Examples 2-6

[0119]

[0120] Example 7

[0121] Example 7 refers to the preparation method of the porous composite separator of Example 1, except that step (2) is different.

[0122] Step (2) of this example comprises: adding 1 kg of N,N-dimethylacetamide into a 2 L stirred tank, adding 80 g of copolymer powder, and stirring at 25°C at a stirring speed of 1500 rpm for 4 h to obtain a copolymer solution. Correspondingly, in step (5), the copolymer solution is used instead of the ceramic copolymer mixture in Example 1; the areal densities of the first coating layer and the second coating layer are both 0.71 g / m 2 .

[0123] Comparative Examples 1-8

[0124] Comparative Examples 1-8 refer to the preparation method of the porous composite separator of Example 1, the difference is that the amount of monomer used in step (1) for preparing copolymer is different, the areal density of the first coating / second coating in step (5) is different, see Table 1 for specific differences.

[0125] Table 2 Monomer amount information in step (1) of Comparative Examples 1-8

[0126]

[0127] Comparative Example 9

[0128] Comparative Example 9 refers to the preparation method of the porous composite separator of Example 1, the difference is that PVDF ceramic mixed solution is used instead of ceramic copolymer mixed solution in Example 1, and the porous composite separator is prepared according to the method of steps (3)-(5), in step (5), the areal density of the first coating and the second coating is 1.68 g / m 2 .

[0129] The preparation of the PVDF ceramic mixed solution includes: 1 kg of N,N-dimethylacetamide is added to a 2 L stirred tank, 40 g of PVDF resin (containing 4 mol% HFP, which can be specifically from Ruisheng Dongyang Guang Fluorine Resin Co., Ltd., model HEVER2601) is added, and stirred at 25℃ with a stirring speed of 1500 rpm for 4 h to obtain a PVDF resin solution. Then 40 g of alumina particles with a D50 particle size of 0.34 μm is added and continue to stir for 1 h to form a PVDF ceramic mixed solution.

[0130] Experimental Example

[0131] I. Detection of monomer ratio in copolymer

[0132] The copolymer powder is dissolved in dimethyl sulfoxide, and the 13 C-NMR, 1 H-NMR is measured respectively, and the molar ratio of unsaturated carbonyl compounds in the copolymer is calculated according to the test results.

[0133] II. Detection of nitrogen content in copolymer

[0134] EDS test method: tested by Japanese electron scanning microscope, electron microscope model IT800, EDS spectrum model Oxford AZtecLive UltimMax 65. Specifically, the long x wide = 30 mm x 8 mm conductive adhesive was pasted on the sample holder, the copolymer powder was placed on the surface of the conductive adhesive, and the ion sputtering instrument was used for gold spraying treatment (current 20 mA, time 45 s); the working distance was 8 mm, the working voltage was 15 kv, the beam current was 50 nA, the sample was focused for EDS point scanning, and the spectrum of the scanned image was collected and analyzed to calculate the nitrogen content.

[0135] III. Detection of weight average molecular weight of copolymer

[0136] The copolymer powder was dissolved in N,N-dimethylformamide, and a gel permeation chromatograph (PL-GPC50) was used to detect the copolymer powder at 50°C by differential refractive detector with 2x PLgel Mixed-B 7.5x300mm chromatographic column.

[0137] IV. Detection of glass transition temperature Tg of copolymer

[0138] Differential scanning calorimetry was used, 5-10 mg of copolymer powder to be tested was weighed in a 40 μL aluminum crucible, nitrogen atmosphere was selected, the temperature test range was -80-300°C, the temperature was raised at a rate of 10°C / min, the curve was obtained, and the glass transition temperature Tg was analyzed and obtained.

[0139] V. Adhesion test of second coating and positive electrode sheet containing active material

[0140] 1. The positive electrode active material lithium iron phosphate, the conductive agent acetylene black and the binder PVDF were mixed uniformly in a suitable amount of N-methyl pyrrolidone at a weight ratio of 97:1.5:1.5, then coated on the positive electrode current collector aluminum foil, dried and cold pressed to obtain a positive electrode sheet;

[0141] 2. The positive electrode sheet and the porous composite separator to be tested were cut to a size of 41 mm x 60 mm, and were aligned and stacked in the order of positive electrode sheet-porous composite separator to be tested-polyvinyl film-positive electrode sheet (the second coating of the porous composite separator to be tested was attached to the positive electrode sheet, and the first coating was attached to the polyvinyl film); then placed in a hot press and hot pressed at 95°C and 3MPa for 5min to obtain a test sample;

[0142] 3. The test sample was peeled off at 180° at a speed of 50 mm / min and a length of 20 mm; the adhesion F (N / m) = average peeling force (N) / width of the test sample (m).

[0143] VI. Average pore size of first type of pore structure and average pore size of second type of pore structure in second coating

[0144] The average pore size of the first type of pore structure is as follows: five samples of the porous composite separator to be tested are taken in parallel, and the average value is obtained after testing according to the following method. Under a SEM microscope, 5KX multiple images of the second coating side are obtained, with a field range of 25.6 μm x 19.2 μm. The picture threshold range of the surface image is adjusted using ImageJ, with MinThr being 7 and MaxThr being 115, so that the edges of the first type of pore structure in the picture stand out. The profile along the higher gray value of the pore structure is drawn to obtain a drawing image. The minimum unit in the drawing image is counted as one pore structure, and the minimum unit located at the edge of the drawing image that is not completely displayed is also counted as one pore structure. Thus, the number n of the first type of pore structure in the field range of 25.6 μm x 19.2 μm is obtained. The area S of the two-dimensional image fitted by the edges of each first type of pore structure in the drawing image is measured using ImageJ software, and then the equivalent diameter d of each first type of pore structure (the diameter of a circle with an area equal to S) is calculated according to S. The average pore size d of the first type of pore structure is calculated according to the following formula. n The average pore size of the first type of pore structure is as follows: five samples of the porous composite separator to be tested are taken in parallel, and the average value is obtained after testing according to the following method. Under a SEM microscope, 5KX multiple images of the second coating side are obtained, with a field range of 25.6 μm x 19.2 μm. The picture threshold range of the surface image is adjusted using ImageJ, with MinThr being 7 and MaxThr being 115, so that the edges of the first type of pore structure in the picture stand out. The profile along the higher gray value of the pore structure is drawn to obtain a drawing image. The minimum unit in the drawing image is counted as one pore structure, and the minimum unit located at the edge of the drawing image that is not completely displayed is also counted as one pore structure. Thus, the number n of the first type of pore structure in the field range of 25.6 μm x 19.2 μm is obtained. The area S of the two-dimensional image fitted by the edges of each first type of pore structure in the drawing image is measured using ImageJ software, and then the equivalent diameter d of each first type of pore structure (the diameter of a circle with an area equal to S) is calculated according to S. The average pore size d of the first type of pore structure is calculated according to the following formula.

[0145]

[0146] Specifically, taking the porous composite separator prepared in Example 1 as an example, the picture obtained after adjusting the picture threshold of the surface image using ImageJ is as shown in FIG. 1. Figure 5

[0147] The average pore size of the second type of pore structure is as follows: five samples of the porous composite separator to be tested are taken in parallel, and the average value is obtained after testing according to the following method. Under a SEM microscope, 50KX multiple images of the second coating side are obtained, with a field range of 2.56 μm x 1.92 μm. The image is imported into ImageJ software, and the profile along the second type of pore structure is drawn to obtain a drawing image. The minimum unit in the drawing image is counted as one pore structure, and the minimum unit located at the edge of the drawing image that is not completely displayed is also counted as one pore structure. Thus, the number n1 of the second type of pore structure in the field range of 2.56 μm x 1.92 μm is obtained. The area S1 of the two-dimensional image fitted by the edges of each second type of pore structure in the drawing image is measured using ImageJ software, and then the equivalent diameter d1 of each second type of pore structure (the diameter of a circle with an area equal to S1) is calculated according to S1. The average pore size d of the second type of pore structure is calculated according to the following formula. n1 The average pore size of the second type of pore structure is as follows: five samples of the porous composite separator to be tested are taken in parallel, and the average value is obtained after testing according to the following method. Under a SEM microscope, 50KX multiple images of the second coating side are obtained, with a field range of 2.56 μm x 1.92 μm. The image is imported into ImageJ software, and the profile along the second type of pore structure is drawn to obtain a drawing image. The minimum unit in the drawing image is counted as one pore structure, and the minimum unit located at the edge of the drawing image that is not completely displayed is also counted as one pore structure. Thus, the number n1 of the second type of pore structure in the field range of 2.56 μm x 1.92 μm is obtained. The area S1 of the two-dimensional image fitted by the edges of each second type of pore structure in the drawing image is measured using ImageJ software, and then the equivalent diameter d1 of each second type of pore structure (the diameter of a circle with an area equal to S1) is calculated according to S1. The average pore size d of the second type of pore structure is calculated according to the following formula.

[0148]

[0149] Seven, determination of related parameters of the first type of pore structure in the second coating

[0150] ​1. Take five parallel samples of the porous composite separator to be tested, and obtain 5KX multiple images of the second coating side under a SEM microscope, with a field range of 25.6 μm x 19.2 μm. Calculate the number of first type of pore structures in a single sample according to the method for average pore diameter of the first type of pore structure. Then calculate the average value according to the number of five parallel samples.

[0151] 2. Height difference h: cut the porous composite separator to be tested using an ion polisher IB-19530CP, with a voltage of 3.5 kV and a time of 4 h. After cutting, spray gold on the sample, and take pictures under an electron microscope (model IT800) with a field range of 16 μm x 12 μm. The height difference between the highest point and the lowest point of each first type of pore structure in the first direction is h. The average height difference of the first type of pore structure is which is calculated according to the following formula, where ∑h is the sum of the height differences of each first type of pore structure, and x is the number of first type of pore structures in the above field range of 16 μm x 12 μm.

[0152]

[0153] Specifically, taking the porous composite separator prepared in Example 1 as an example, the cross-sectional SEM of the second coating is shown in Figure 6 .

[0154] 3. Δh / P: test the height difference h of the porous composite separator to be tested according to the test method for height difference h. Then press the porous composite separator to be tested at 25°C and P = 7 MPa for 5 min, and test the height difference h1 of the porous composite separator to be tested after pressing according to the test method for height difference h. The Δh / P of each first type of pore structure is (h-h1) / 7, and the average value of Δh / P is Δh / P / x.

[0155] 4. M and N: obtain 5KX multiple images of the second coating side under a SEM microscope, with a field range of 25.6 μm x 19.2 μm. Draw the images and calculate the number of first type of pore structures n according to the method for average pore diameter of the first type of pore structure. Measure the maximum Feret diameter MaxFeret and the minimum Feret diameter MinFeret of the two-dimensional figure fitted by the edges of each first type of pore structure in the drawn figure using ImageJ software, which are M and N. Calculate the average values of M and N of the first type of pore structure according to the number n of first type of pore structures in the above field range.

[0156] Eight, battery capacity retention rate corresponding to the porous composite separator

[0157] 1. Preparation of positive electrode sheet: the positive electrode active material NCM811, conductive carbon black and binder PVDF are mixed uniformly in a weight ratio of 97:2:1 in a proper amount of N-methyl pyrrolidone, forming a positive electrode slurry, then coated on both sides of the positive electrode current collector aluminum foil, dried, rolled, cut to obtain the positive electrode sheet;

[0158] 2. Preparation of negative electrode sheet: the negative electrode active material containing 10% silicon artificial graphite, sodium carboxymethyl cellulose and butadiene rubber are mixed uniformly in a weight ratio of 97:1.5:1.5 in a proper amount of deionized water, forming a negative electrode slurry, then coated on both sides of the negative electrode current collector copper foil, dried, rolled, cut to obtain the negative electrode sheet;

[0159] 3. The positive electrode sheet, porous composite separator and positive electrode sheet are stacked in turn, the tab is welded, the aluminum plastic film is packaged, the hot pressing pre-sealing is performed, the electrolyte (1 mol / L LiPF6, EC, PC, DEC and EP in a volume ratio of 3:1:3:3) is injected, vacuum packaging is performed, standing is performed, hot pressing formation is performed, exhaust is performed, and final sealing is performed to obtain the lithium ion battery.

[0160] 4. Capacity retention rate test

[0161] The lithium ion battery is placed at 25 DEG C for 30 min, so that the lithium ion battery reaches a constant temperature state, is charged at 1C constant current to a voltage of 4.5V, is charged at constant voltage to a current of 0.05C, so that the lithium ion battery reaches a full charge state, is discharged at 1C constant current to a voltage of 3.0V, and the discharge capacity at this time is recorded as C1; after 1000 cycles of charging and discharging, the lithium ion battery is charged at 0.5C constant current to a voltage of 4.5V, is charged at constant voltage to a current of 0.05C, so that the lithium ion battery reaches a full charge state, is discharged at 1C constant current to a voltage of 3.0V, and the discharge capacity at this time is recorded as C2; then the discharge capacity retention rate after 1C / 1C charging and discharging at 25 DEG C = C2 / C1 x 100%.

[0162] The above test results are shown in Tables 3 and 4.

[0163] Table 3 Test results one

[0164]

[0165] Table 4 Test results two

[0166]

[0167] Note: Partially porous means that the pores are not completely formed, and closed pores appear.

[0168] The height difference h of the single first type of pore structure in each embodiment of the present application is in the range of 0.1-4 μm, M is less than 15 μm, N / M is in the range of 0.2-1, and Ah / P is in the range of 0.01-0.2.

[0169] In the porous composite separator of the present application, the copolymer with a side chain having a cyano group and a carbonyl group is used as a binding component, the cyano group of the first monomer unit forms an intermolecular interaction network through hydrogen bonding and dipole interaction, and excellent oxidation resistance is imparted; the introduction of the second monomer unit appropriately reduces the regularity of the molecular chain formed by the first monomer unit, so that the copolymer has good dispersibility and solubility in a solvent, which is helpful to form a uniform and stable coating structure, and the adhesion between the coating and the pole piece containing active materials can be improved. Moreover, the first coating and the second coating contain first type pore structures and second type pore structures with different pore sizes, and the bi-scale pore structure forms a network structure that is more conducive to the transmission of lithium ions and improves the conductivity of lithium ions. The first type pore structure has a certain size in the thickness direction, which helps to form a certain liquid retention area between the separator and the pole piece, thereby improving the rate cycle performance of the battery.

[0170] As can be seen from Comparative Example 1, Comparative Example 1, Comparative Example 4 to Comparative Example 8, when the copolymer used in the first coating and the second coating does not contain acrylonitrile or contains too little acrylonitrile, the rigidity of the copolymer is insufficient, which not only reduces the oxidation resistance, but also cannot guarantee sufficient adhesion strength with the positive pole piece.

[0171] As can be seen from Comparative Example 1, Comparative Example 2 to Comparative Example 3, when the copolymer used in the first coating and the second coating contains too little unsaturated carbonyl compound, the rigidity of the copolymer is too strong, which makes it difficult to wet the positive pole piece and affects the adhesion performance.

[0172] As can be seen from Comparative Example 1 and Comparative Example 9, when PVDF resin is used to prepare the first coating and the second coating, the rigidity of the PVDF resin is insufficient, which cannot guarantee the oxidation resistance and the adhesion strength with the positive pole piece.

[0173] At the same time, the composition of the copolymers of Comparative Example 1 to Comparative Example 9 results in that they cannot form a bi-scale pore structure after coating and coagulation bath treatment, which affects the lithium ion transmission performance.

[0174] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A porous composite separator, characterized by, The porous composite diaphragm comprises a base film, a first coating layer and a second coating layer; the first coating layer and the second coating layer are oppositely arranged on two surfaces of the base film; The first coating layer and the second coating layer each independently comprise a copolymer containing first monomer units and second monomer units; the first monomer units contain cyano groups on side chains, and the second monomer units contain carbonyl groups on side chains; in the copolymer, the molar proportion of the second monomer units is 1% to 20%; The first coating layer and the second coating layer each independently comprise a skeleton structure and a first type of pore structure formed between the skeleton structures, and a second type of pore structure is formed on the body of the skeleton structure; the average pore diameter of the first type of pore structure is greater than the average pore diameter of the second type of pore structure; The average pore diameter of the first type of pore structure is 1 to 8 μm; and the average pore diameter of the second type of pore structure is 30 to 150 nm.

2. The porous composite separator of claim 1, wherein, At least one of the following characteristics is possessed: (1) in the copolymer, the molar proportion of the second monomer units is 8% to 20%; (2) the first monomer units are derived from acrylonitrile; (3) the second monomer units are derived from an unsaturated carbonyl compound; (4) the weight average molecular weight of the copolymer is 200,000 to 1,000,000; (5) the glass transition temperature Tg of the copolymer is 40 to 120℃.

3. The porous composite separator of claim 2, wherein, The unsaturated carbonyl compound comprises at least one of an acrylate monomer, an acrylic monomer and an acrylamide monomer; The acrylate monomer comprises at least one of methyl (meth)acrylate, ethyl (meth)acrylate and butyl (meth)acrylate; The acrylic monomer comprises (meth)acrylic acid; The acrylamide monomer comprises at least one of acrylamide, diacetone acrylamide and N-methylol acrylamide.

4. The porous composite separator of claim 1, wherein, At least one of the following characteristics is possessed: (1) in any 25.6 μm×19.2 μm field of view of the first coating layer and the second coating layer, the number of the first type of pore structure is 40 to 120; (2) a scanning electron microscope is used to scan and image the first coating layer and the second coating layer to obtain a SEM secondary electron image, based on the SEM secondary electron image, the edge of the first type of pore structure is fitted as a two-dimensional figure, and the length of the longest line segment and the length of the shortest line segment formed by passing through the geometric center of the two-dimensional figure and intersecting the two-dimensional figure are set as M and N respectively, M < 15 μm, and 0.2 ≤ N / M ≤ 1.

5. The porous composite separator of claim 1, wherein, At least one of the following characteristics is possessed: (1) a first direction is set as a direction from the first coating layer to the base film and perpendicular to the base film, the height difference between the highest point and the lowest point of the first type of pore structure in the first direction is h, and h is 0.1 to 4 μm; (2) the change value △h of h before and after the porous composite diaphragm is pressed at 25℃ for 5 min satisfies 0.01 ≤ △h / P ≤ 0.2, wherein P is 0.1 to 20 MPa.

6. The porous composite separator of claim 1, wherein, At least one of the following characteristics is possessed: (1) the areal density of the first coating and the second coating are each independently 0.1 to 3 g / m 2 ; (2) the first coating layer and the second coating layer each independently comprise an inorganic heat-resistant material; (3) the mass ratio of the copolymer to the inorganic heat-resistant material in the first coating and the second coating is independently 1:(0.5-2); (4) the base film comprises at least one of a polyvinyl film and a polypropylene film.

7. The porous composite separator according to claim 1 or 6, wherein The porous composite separator further comprises a heat-resistant coating layer, which is arranged between the base film and the first coating layer; The heat-resistant coating layer comprises inorganic heat-resistant particles and a bonding polymer; the inorganic heat-resistant particles comprise at least one of alumina, boehmite, barium sulfate, barium titanate, magnesium hydroxide and silicon dioxide; The D50 particle size of the inorganic heat-resistant particles is 1-1.5 μm.

8. The method of producing a porous composite separator according to any one of claims 1 to 6, characterized by, The method comprises the following steps: preparing a slurry of the first coating layer and a slurry of the second coating layer, coating the slurry of the first coating layer and the slurry of the second coating layer on both sides of the base film respectively, then immersing in a coagulation bath, and then performing a drying treatment; The coagulation bath comprises a first coagulation bath, a second coagulation bath and a third coagulation bath; the first coagulation bath, the second coagulation bath and the third coagulation bath are all aqueous solutions of N,N-dimethylacetamide, and the concentration of N,N-dimethylacetamide decreases in turn.

9. The method of claim 7, wherein the porous composite separator is prepared by the steps of: The method comprises the following steps: (a) preparing a slurry of the heat-resistant coating layer, coating the slurry of the heat-resistant coating layer on one side of the base film, and performing a drying treatment to form the heat-resistant coating layer; (b) preparing a slurry of the first coating layer and a slurry of the second coating layer, coating the slurry of the first coating layer and the slurry of the second coating layer on both sides of the base film on which the heat-resistant coating layer is formed respectively, then immersing in a coagulation bath, and then performing a drying treatment; The coagulation bath comprises a first coagulation bath, a second coagulation bath and a third coagulation bath; the first coagulation bath, the second coagulation bath and the third coagulation bath are all aqueous solutions of N,N-dimethylacetamide, and the concentration of N,N-dimethylacetamide decreases in turn.

10. A secondary battery characterized by The porous composite separator comprises the porous composite separator according to any one of claims 1-7.

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