A gradient pore size composite membrane, its preparation method and application

Abstract

This invention belongs to the field of battery separator technology, specifically relating to a gradient pore size composite separator, its preparation method, and its application. The gradient pore size composite separator includes a base layer, a ceramic layer A, and a ceramic layer B, wherein the pore size of ceramic layer A is larger than that of ceramic layer B. The composite separator is bonded in the following order: ceramic layer A, base layer, ceramic layer B, base layer, ceramic layer A. This optimizes the lithium-ion transport channel, significantly improving ion conductivity and transport efficiency, and greatly enhancing the fast-charging capability of the battery cell. It effectively blocks and inhibits lithium dendrite growth, fundamentally avoiding the risk of internal short circuits caused by lithium dendrites. It significantly reduces the safety hazards of thermal runaway, fire, and explosion during fast charging of the battery cell, and significantly improves the safety and reliability of the battery cell during high-rate charging.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, specifically relating to a gradient pore size composite separator, its preparation method, and its application. Background Technology

[0002] In recent years, significant breakthroughs have been made in high-power charging technology. However, certain technological barriers still exist in the main materials such as the positive electrode, negative electrode, electrolyte, and separator. Among these, impedance is a key factor affecting fast charging capability. Under high-rate fast charging conditions, it is difficult to simultaneously ensure the safety performance and fast charging performance of the battery cell. Specifically, this manifests as insufficient fast charging capability of the battery cell and significant safety hazards during the fast charging process. In terms of fast charging capability, the ion transport efficiency of existing battery cells is difficult to match the lithium-ion flux requirements of high-rate charging. The unreasonable pore structure design of lithium-ion transport channels leads to low ion conductivity and high interface impedance, which limits the migration rate of lithium ions between the positive and negative electrodes. At the same time, the electrolyte system is prone to insufficient ion conduction capacity during high-rate charging. The ion transport efficiency of the solid electrolyte interface film is low, and the polarization voltage rises sharply during charging, which restricts the improvement of the fast charging rate of the battery cells and cannot meet the application requirements of ultra-fast charging.

[0003] In terms of safety performance, the polarization reaction inside the battery cell intensifies under high-rate fast charging conditions, generating a large amount of heat in a short period of time, causing the battery cell temperature to rise rapidly. Traditional separators have insufficient thermal stability and mechanical strength, and a high thermal shrinkage rate, making them prone to deformation and cracking at high temperatures, and unable to effectively isolate the positive and negative electrodes. At the same time, the separator is unable to suppress the growth and piercing of lithium dendrites, which can easily cause short circuits inside the battery cell. In addition, the electrolyte is prone to decomposition reaction at high temperatures, and the solid electrolyte interface film is prone to cracking, further aggravating the risk of lithium plating. This significantly increases the safety hazards of thermal runaway, fire, or even explosion of the battery cell during fast charging, seriously restricting the large-scale application of fast charging technology. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defects of insufficient fast charging capability and poor safety of existing composite membranes, thereby providing a gradient pore size composite membrane, its preparation method and application.

[0005] Therefore, the present invention provides the following technical solution: The first aspect of this invention protects a gradient pore size composite membrane, wherein the gradient pore size composite membrane includes a base layer, a ceramic layer A and a ceramic layer B, wherein the pore size of the ceramic layer A is larger than the pore size of the ceramic layer B; The bonding method of the gradient pore size composite diaphragm is ceramic layer A, base layer, ceramic layer B, base layer, ceramic layer A.

[0006] This invention achieves optimized reconstruction of lithium-ion transport channels through a gradient pore size design of an outer large-pore ceramic layer A and an inner small-pore ceramic layer B, significantly improving ion conductivity and transport rate.

[0007] In one optional embodiment, the average pore size of the ceramic layer A is 20-150 nm. As an example, the average pore size of the ceramic layer A can be 20 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 150 nm, or within any of the above values; it can be 60-100 nm.

[0008] In one optional embodiment, the average pore size of the ceramic layer B is 8-30 nm. As an example, the average pore size of the ceramic layer B can be 8 nm, 12 nm, 13 nm, 14 nm, 15 nm, 20 nm, 30 nm, or within any of the above values; it can be 12-15 nm.

[0009] In one optional embodiment, the porosity of the ceramic layer A is 40-70%. For example, the porosity of the ceramic layer A can be 40%, 50%, 52%, 55%, 58%, 60%, 70%, or within any range of the above values; 50-60% is optional.

[0010] In one optional embodiment, the porosity of the ceramic layer B is 45-80%. For example, the porosity of the ceramic layer B can be 45%, 50%, 53%, 55%, 58%, 60%, 80%, or within any range of the above values; 50-60% is optional.

[0011] In this invention, porosity and average pore size are tested using a high-performance, fully automated mercury porosimeter, the AutoPore V model, from Micromeritics, Inc., USA.

[0012] In one alternative embodiment, the thickness of the ceramic layer A is 1-2 μm. For example, the thickness of the ceramic layer A can be 1 μm, 1.2 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.8 μm, 2 μm, or within any of the above values.

[0013] In one alternative embodiment, the thickness of the ceramic layer B is 1-2 μm. For example, the thickness of the ceramic layer B can be 1 μm, 1.2 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.8 μm, 2 μm, or within any of the above values.

[0014] In one optional embodiment, the material of the ceramic layer A includes at least one of alumina, silicon carbide, silicon nitride, zirconium oxide, chromium oxide, spinel, and aluminum titanate; optionally, it includes at least one of alumina, silicon nitride, and aluminum titanate.

[0015] In one optional embodiment, the material of the ceramic layer B includes at least one of boehmite, aluminum phosphate, and hydroxyapatite, with boehmite being an option.

[0016] In one alternative embodiment, the average pore size of the substrate is 50-80 nm.

[0017] In one alternative embodiment, the porosity of the substrate is 40-50%.

[0018] In one alternative embodiment, the thickness of the base layer is 5-10 μm.

[0019] In one alternative embodiment, the base material includes at least one of PP (polypropylene) and PE (polyethylene).

[0020] A second aspect of this invention protects a method for preparing the aforementioned gradient pore size composite membrane, wherein the preparation method includes the following steps: S1, ceramic raw material a, first solvent and first dispersant are mixed in the first mixing; first binder is added and mixed in the second mixing to obtain slurry i; slurry i is coated on one side of the base layer to obtain intermediate product 1-1, intermediate product 1-1 includes ceramic layer A and base layer; S2, ceramic raw material b, second solvent and second dispersant are mixed in the third mixture; second binder is added and mixed in the fourth mixture to obtain slurry ii; slurry ii is coated on the uncoated side of the intermediate product 1-1 base layer to obtain intermediate product 2, intermediate product 2 includes ceramic layer A, base layer 1 and ceramic layer B; S3, repeat step S1 to obtain another intermediate product 1-2; laminate intermediate product 1-2 with intermediate product 2 to obtain a gradient pore size composite membrane.

[0021] In this invention, the preparation method, through stepwise slurry preparation, sequential coating on one side, and symmetrical lamination, can form a bonding pattern of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A. This allows ceramic layer A and ceramic layer B to maintain a stable pore size gradient distribution, improving the bonding strength between each layer and preventing delamination, peeling, or deformation during use. At the same time, it ensures rapid lithium ion transport and sufficient electrolyte wetting, effectively inhibits lithium dendrite growth, and improves the thermal stability of the separator. Ultimately, a gradient pore size composite separator with excellent fast charging performance and safety performance is obtained. The overall process is simple and controllable, suitable for industrial mass production.

[0022] In this invention, the first mixing, the second mixing, the third mixing, and the fourth mixing are each independent and operated in accordance with conventional practices in the field. They can be selected according to actual conditions. Typically, without limitation, they are carried out by stirring, with specific conditions including stirring at a rotation speed of 1500-2500 r / min and a revolution speed of 15-30 r / min for 60-120 min.

[0023] In one optional embodiment, the ceramic raw material a has an average particle size of 0.3-3 μm.

[0024] In one optional embodiment, the ceramic raw material a has a Mohs hardness of 6-9, optionally 7-8.

[0025] In one optional embodiment, based on slurry i, the mass fraction of ceramic raw material a is 10-25 wt%, the mass fraction of the first dispersant is 0.1-1.5 wt%, the mass fraction of the first binder is 0.1-1.5 wt%, and the mass fraction of the first solvent is 69-89 wt%.

[0026] In one optional embodiment, the ceramic raw material b has an average particle size of 0.2-1 μm.

[0027] In one optional embodiment, the ceramic raw material b has a Mohs hardness of 3-4, optionally 3.5-4.

[0028] In one optional embodiment, based on slurry ii, the mass fraction of ceramic raw material b is 10-25 wt%, the mass fraction of the second dispersant is 0.1-1.5 wt%, the mass fraction of the second binder is 0.1-1.5 wt%, and the mass fraction of the second solvent is 69-89 wt%.

[0029] In this invention, the first dispersant and the second dispersant are conventional in the art, each independently comprising at least one of sodium polyacrylate, sodium hexametaphosphate, and sodium polymethacrylate; the first binder and the second binder are conventional in the art, each independently comprising at least one of polyvinyl alcohol (PVA), aqueous carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF); the first solvent and the second solvent are conventional in the art, each independently comprising at least one of water, methanol, ethanol, isopropanol, and acetone, and may be water.

[0030] In one optional embodiment, in step S3, the adhesive is sandwiched between the base layer side of intermediate product 1-2 and the ceramic layer B side of intermediate product 2; it plays a role in interface bonding and structural shaping during the lamination process; at the same time, it can reduce the interface impedance between the base layer and the ceramic layer B, improve the smoothness of ion transport, and enhance the mechanical properties of the membrane.

[0031] In one alternative embodiment, the adhesive comprises at least one of polyvinylidene fluoride, polyvinyl alcohol, and polytetrafluoroethylene.

[0032] In one optional embodiment, the amount of adhesive used is 0.5-1.5 g / m³. 2 .

[0033] In one optional embodiment, the lamination temperature is 70-110°C, the pressure is 1-8 MPa, and the time is 0.5-5 min.

[0034] A third aspect of the present invention protects a secondary battery, wherein the secondary battery comprises the aforementioned gradient pore size composite membrane or the gradient pore size composite membrane prepared by the aforementioned preparation method.

[0035] The technical solution of this invention has the following advantages: This invention provides a gradient pore size composite separator, wherein the gradient pore size composite separator includes a base layer, a ceramic layer A, and a ceramic layer B, wherein the pore size of the ceramic layer A is larger than the pore size of the ceramic layer B; the bonding method of the gradient pore size composite separator is ceramic layer A, base layer, ceramic layer B, base layer, ceramic layer A; the gradient pore size composite separator of this invention adopts a gradient structure design with a ceramic layer A with a large average pore size on the outer side and a ceramic layer B with a small average pore size on the inner side, which optimizes the lithium ion transport channel and significantly improves ion conductivity and transport efficiency; The large pore size of ceramic layer A increases the electrolyte retention capacity of the separator, allowing the electrolyte to uniformly wet the separator and electrode surfaces, reducing the polarization voltage during charging, and effectively improving the negative electrode-reference safety potential. This enables the battery cell to maintain a stable potential state even under high-rate charging conditions, significantly improving the fast charging capability of the battery cell. At the same time, the large average pore size of ceramic layer A on the outer side improves the thermal stability and mechanical strength of the separator, reduces the thermal shrinkage rate of the separator at high temperatures, and effectively avoids high-temperature deformation. The ceramic layer B with a small average pore size on the inner side can absorb the local stress transmitted from the outer layer, prevent damage to the internal structure, effectively block and inhibit the growth of lithium dendrites, and fundamentally avoid the risk of internal short circuit caused by lithium dendrites; it greatly reduces the safety hazards of thermal runaway, fire and explosion during the fast charging process of the battery cell, and significantly improves the safety and reliability of the battery cell during high-rate charging. Detailed Implementation

[0036] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0037] In the description of the embodiments of the present invention, the technical terms "first", "second", etc. are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0038] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0039] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This range can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. In this invention, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein; "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to disclosing that the parameter can be, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0040] In the description of the embodiments of the present invention, the term "and / or" is merely a description of the relationship between associated objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0041] In the description of the embodiments of the present invention, the term "at least one" refers to one or more (including two).

[0042] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0043] Porosity and average pore size were tested using a high-performance, fully automated mercury porosimeter, the AutoPore V model, from Micromeritics, Inc.

[0044] The film thickness was tested using a Mitutoyo MDH-25M high-precision digital micrometer.

[0045] Method for testing the Mohs hardness of ceramic powder film: Loose ceramic powder is pressed into dense blocks, and the solidified blocks are scratched with a standard mineral. The surface scratches are then observed. The standard hardness sample here is a standard mineral with a known Mohs hardness.

[0046] Base layer 1: The material is a polypropylene diaphragm (PP-based diaphragm) with a porosity of 50%, a pore size of 80nm, and a thickness of 8μm. It was purchased from Shanghai Enjie New Materials Co., Ltd. Base layer 2: The material is a polypropylene diaphragm (PP-based diaphragm), with a porosity of 40%, a pore size of 50nm, and a thickness of 8μm. It was purchased from Shanghai Enjie New Materials Co., Ltd. Base layer 3: The material is a polypropylene diaphragm (PP-based diaphragm), with a porosity of 45%, a pore size of 65nm, and a thickness of 8μm. It was purchased from Shanghai Enjie New Materials Co., Ltd. The weight-average molecular weight of sodium polyacrylate is 3000 g / mol; The weight-average molecular weight of polyacrylic acid is 400,000 g / mol.

[0047] Example 1 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, alumina, water, and sodium polyacrylate are mixed and stirred at a rotation speed of 2000 r / min and a revolution speed of 30 r / min for 60 min to form a uniform dispersion; polyacrylic acid is added and stirred at 2000 r / min for 60 min to obtain slurry i; wherein, the median particle size of alumina is 1.0 μm and the Mohs hardness is 7; based on slurry i, the mass fraction of alumina is 15.00 wt%, the mass fraction of sodium polyacrylate is 0.79 wt%, the mass fraction of polyacrylic acid is 0.79 wt%, and the mass fraction of water is 83.42 wt%; slurry i is coated on one side of the base layer 1 at a coating speed of 10 m / min and a drying temperature of 70℃ to obtain intermediate product 1-1, which includes an alumina ceramic layer and a base layer, the porosity of the alumina ceramic layer is 60%, the average pore size is 100 nm, and the thickness is 1.5 μm; S2, Boehmite, water, and sodium polyacrylate are mixed and stirred at a rotation speed of 2000 r / min and a revolution speed of 30 r / min for 60 min; polyacrylic acid is added and stirred at 2000 r / min for 60 min to obtain slurry ii; wherein, the median particle size of boehmite is 0.5 μm and the Mohs hardness is 3.5; based on slurry ii, the mass fraction of boehmite is 15.00 wt%, the mass fraction of polyacrylic acid is 0.79 wt%, the mass fraction of sodium polyacrylate is 0.79 wt%, and the mass fraction of water is 83.42 wt%; coated on one side of intermediate product 1-1 base layer 1 at a coating speed of 10 m / min and a drying temperature of 70 °C to obtain intermediate product 2, intermediate product 2 includes an alumina ceramic layer, base layer 1, and a boehmite ceramic layer, the porosity of the boehmite ceramic layer is 60%, the average pore size is 15 nm, and the thickness is 1.5 μm; S3, Repeat step S1 to obtain another intermediate product 1-2; Laminate intermediate product 1-2 with intermediate product 2, wherein polyvinylidene fluoride (PVDF) is sandwiched between the base layer side of intermediate product 1-2 and the boehmite ceramic layer side of intermediate product 2, and the amount of PVDF used is 0.5 g / m³. 2 The lamination temperature was 100℃, the pressure was 2MPa, and the time was 30s to obtain a gradient pore size composite membrane.

[0048] Example 2 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, following the method of S1 in Example 1, except that the median particle size of alumina is 2.0 μm, and based on slurry i, the mass fraction of alumina is 25.00 wt%, the mass fraction of sodium polyacrylate is 1.32 wt%, the mass fraction of polyacrylic acid is 1.32 wt%, and the mass fraction of water is 72.36 wt%, resulting in intermediate product 1-1. Intermediate product 1-1 includes an alumina ceramic layer and a base layer 2. The porosity of the alumina ceramic layer is 40%, the average pore size is 20 nm, and the thickness is 1.5 μm. S2, following the S2 method of Example 1, except that the median particle size of boehmite is 1.0 μm, and based on slurry ii, the mass fraction of boehmite is 25.00 wt%, the mass fraction of polyacrylic acid is 1.32 wt%, the mass fraction of sodium polyacrylate is 1.32 wt%, and the mass fraction of water is 72.36 wt%, resulting in intermediate product 2. Intermediate product 2 includes an alumina ceramic layer, a base layer 2, and a boehmite ceramic layer, wherein the porosity of the boehmite ceramic layer is 45%, the average pore size is 8 nm, and the thickness is 1.5 μm; S3, in accordance with the method of S3 in Example 1.

[0049] Example 3 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, following the S1 method of Example 1, except that the median particle size of alumina is 1.5 μm, and based on slurry i, the mass fraction of alumina is 20.00 wt%, the mass fraction of sodium polyacrylate is 1.05 wt%, the mass fraction of polyacrylic acid is 1.05 wt%, and the mass fraction of water is 77.90 wt%, resulting in intermediate product 1-1. Intermediate product 1-1 includes an alumina ceramic layer and a base layer 3. The porosity of the alumina ceramic layer is 50%, the average pore size is 60 nm, and the thickness is 1.5 μm. S2, following the S2 method of Example 1, except that the median particle size of boehmite is 0.8 μm, and based on slurry ii, the mass fraction of boehmite is 20.00 wt%, the mass fraction of polyacrylic acid is 1.05 wt%, the mass fraction of sodium polyacrylate is 1.05 wt%, and the mass fraction of water is 77.90 wt%, resulting in intermediate product 2. Intermediate product 2 includes an alumina ceramic layer, a base layer 3, and a boehmite ceramic layer, wherein the porosity of the boehmite ceramic layer is 53%, the average pore size is 12 nm, and the thickness is 1.5 μm; S3, in accordance with the method of S3 in Example 1.

[0050] Example 4 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, according to the S1 method of Example 1, the difference is that the Mohs hardness of alumina is 9, and intermediate product 1-1 is obtained. Intermediate product 1-1 includes an alumina ceramic layer and a base layer 1. The porosity of the alumina ceramic layer is 60%, the average pore size is 100 nm, and the thickness is 1.5 μm. S2, following the S2 method of Example 1, except that the Mohs hardness of boehmite is 4, and intermediate product 2 is obtained. Intermediate product 2 includes an alumina ceramic layer, a base layer 1, and a boehmite ceramic layer. The porosity of the boehmite ceramic layer is 60%, the average pore size is 15 nm, and the thickness is 1.5 μm. S3, in accordance with the method of S3 in Example 1.

[0051] Example 5 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, according to the S1 method of Example 2, the difference is that the Mohs hardness of alumina is 9, and intermediate product 1-1 is obtained. Intermediate product 1-1 includes an alumina ceramic layer and a base layer 2. The porosity of the alumina ceramic layer is 40%, the average pore size is 20nm, and the thickness is 1.5μm. S2, following the S2 method of Example 2, except that the Mohs hardness of boehmite is 4, intermediate product 2 is obtained. Intermediate product 2 includes an alumina ceramic layer, a base layer 2, and a boehmite ceramic layer. The porosity of the boehmite ceramic layer is 45%, the average pore size is 8 nm, and the thickness is 1.5 μm. S3, in accordance with the manner of S3 in Example 2.

[0052] Example 6 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, according to the S1 method of Example 3, the difference is that the Mohs hardness of alumina is 9, and intermediate product 1-1 is obtained. Intermediate product 1-1 includes an alumina ceramic layer and a base layer 3. The porosity of the alumina ceramic layer is 50%, the average pore size is 60nm, and the thickness is 1.5μm. S2, following the S2 method of Example 3, except that the Mohs hardness of boehmite is 4, and intermediate product 2 is obtained. Intermediate product 2 includes an alumina ceramic layer, a base layer 3, and a boehmite ceramic layer. The porosity of the boehmite ceramic layer is 53%, the average pore size is 12 nm, and the thickness is 1.5 μm. S3, in accordance with the method of S3 in Example 3.

[0053] Example 7 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, following the S1 method of Example 1, except that the median particle size of alumina is 0.5 μm, and based on slurry i, the mass fraction of alumina is 10.00 wt%, the mass fraction of sodium polyacrylate is 0.53 wt%, the mass fraction of polyacrylic acid is 0.53 wt%, and the mass fraction of water is 88.94 wt%; intermediate product 1-1 is obtained, which includes an alumina ceramic layer and a base layer 1. The porosity of the alumina ceramic layer is 70%, the average pore size is 150 nm, and the thickness is 1.5 μm; S2, following the S2 method of Example 1, except that the median particle size of boehmite is 0.2 μm, and based on slurry ii, the mass fraction of boehmite is 10.00 wt%, the mass fraction of polyacrylic acid is 0.53 wt%, the mass fraction of sodium polyacrylate is 0.53 wt%, and the mass fraction of water is 88.94 wt%; intermediate product 2 is obtained, which includes an alumina ceramic layer, a base layer 1, and a boehmite ceramic layer, wherein the porosity of the boehmite ceramic layer is 80%, the average pore size is 30 nm, and the thickness is 1.5 μm; S3, in accordance with the method of S3 in Example 1.

[0054] Example 8 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, in accordance with the manner of S1 in Example 1; S2, following the method of Example 1, except that the median particle size of boehmite is 0.2 μm, and based on slurry ii, the mass fraction of boehmite is 10.00 wt%, the mass fraction of polyacrylic acid is 0.53 wt%, the mass fraction of sodium polyacrylate is 0.53 wt%, and the mass fraction of water is 88.94 wt%; intermediate product 2 is obtained, which includes an alumina ceramic layer, a base layer 1, and a boehmite ceramic layer, wherein the porosity of the boehmite ceramic layer is 80%, the average pore size is 30 nm, and the thickness is 1.5 μm; S3, in accordance with the method of S3 in Example 1.

[0055] Example 9 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, following the method of Example 1, except that the median particle size of alumina is 0.5 μm, and based on slurry i, the mass fraction of alumina is 10.00 wt%, the mass fraction of sodium polyacrylate is 0.53 wt%, the mass fraction of polyacrylic acid is 0.53 wt%, and the mass fraction of water is 88.94 wt%; intermediate product 1-1 is obtained, which includes an alumina ceramic layer and a base layer 1. The porosity of the alumina ceramic layer is 70%, the average pore size is 150 nm, and the thickness is 1.5 μm; S2, in accordance with the manner of S2 in Example 1; S3, in accordance with the method of S3 in Example 1.

[0056] Example 10 This embodiment provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, base layer, ceramic layer B, base layer, and ceramic layer A again. The preparation method includes the following steps: S1, following the method of S1 in Example 1, except that alumina is replaced with an equal mass of zirconium oxide, the median particle size of the zirconium oxide is 1.0 μm, and the Mohs hardness is 8.5; the final intermediate product 1-1 includes a zirconium oxide ceramic layer and a base layer, wherein the porosity of the zirconium oxide ceramic layer is 65%, the average pore size is 120 nm, and the thickness is 1.5 μm; S2, following the method of S2 in Example 1, except that boehmite is replaced with an equal mass of aluminum phosphate, the median particle size of aluminum phosphate is 0.5 μm, and the Mohs hardness is 4; finally, intermediate product 2 is obtained; intermediate product 2 includes a zirconia ceramic layer, a base layer 1, and an aluminum phosphate ceramic layer, wherein the porosity of the aluminum phosphate ceramic layer is 65%, the average pore size is 20 nm, and the thickness is 1.5 μm; S3, in accordance with the method of S3 in Example 1.

[0057] Comparative Example 1 This comparative example provides a gradient pore size composite membrane. The gradient pore size composite membrane is bonded in the form of ceramic layer A, ceramic layer B, base layer, ceramic layer B, and ceramic layer A. The preparation method includes the following steps: S1, according to the method of S1 in Example 1; the porosity of the alumina ceramic layer is 60%, the average pore size is 100 nm, and the thickness is 1.5 μm; S2, following the method of S2 in Example 1; the porosity of the boehmite ceramic layer is 60%, the average pore size is 15 nm, and the thickness is 1.5 μm; S3, apply slurry ii and slurry i sequentially to both sides of the base layer 1, and bond the layers together with an adhesive. The adhesive used is polyvinylidene fluoride, and the amount of adhesive used is 0.5 g / m². 2 Gradient pore size composite membranes were obtained.

[0058] Comparative Example 2 This comparative example provides a gradient pore size composite membrane, the preparation method of which includes the following steps: S1, following the method of S1 in Example 1, except that the median particle size of alumina is 2.0 μm, and based on slurry i, the mass fraction of alumina is 25.00 wt%, the mass fraction of sodium polyacrylate is 1.32 wt%, the mass fraction of polyacrylic acid is 1.32 wt%, and the mass fraction of water is 72.36 wt%, resulting in intermediate product 1-1; intermediate product 1-1 includes an alumina ceramic layer and a base layer 2, the porosity of the alumina ceramic layer is 40%, the average pore size is 20 nm, and the thickness is 1.5 μm; S2, following the S2 method of Example 1, except that the median particle size of boehmite is 0.2 μm, to obtain intermediate product 2. Intermediate product 2 includes an alumina ceramic layer, a base layer 1, and a boehmite ceramic layer. The porosity of the boehmite ceramic layer is 80%, the average pore size is 30 nm, and the thickness is 1.5 μm. S3, following the method in S3 of Example 1, a gradient pore size composite membrane is obtained.

[0059] Test case NCM622 (LiNi) with a mass ratio of 97:1:0.5:1.5 0.6 Co 0.2 Mn 0.2 O2), conductive carbon black, carbon nanotubes, and polyvinylidene fluoride were added to a mixing tank, followed by N-methylpyrrolidone. The mixture was stirred at high speed to prepare a positive electrode slurry with a solid content of 72%. The positive electrode slurry was then coated onto aluminum foil, achieving an areal density of 350 g / m³. 2 The positive electrode sheet is produced by baking, rolling, and cutting at 90℃. A slurry containing graphite, silicon carbide, conductive carbon black, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, and carbon nanotubes in a mass ratio of 76.4:18:1:0.8:2:1.5:0.3 was added to a mixing tank, along with deionized water. The mixture was stirred at high speed to prepare a negative electrode slurry with a solid content of 38%. This negative electrode slurry was then coated onto copper foil, resulting in an areal density of 144.2 g / m³. 2 The negative electrode sheet is produced by baking, rolling, and cutting at 85℃. Fabrication of 2Ah pouch cells: The aluminum-plastic film has a thickness of 153μm, with an inner layer of polypropylene (PP), a middle layer of aluminum, and an outer layer of nylon (PA). The positive electrode tab is made of aluminum, with a thickness of 0.3mm, a width of 25mm, and a height of 34mm; the adhesive of the positive electrode tab is 35mm wide, 8mm high, and 0.3mm thick. The negative electrode tab is a copper-plated nickel tab with a thickness of 0.2mm, a width of 25mm, and a height of 34mm; the tab adhesive of the negative electrode tab is 35mm wide, 8mm high, and 0.3mm thick. The gradient pore size composite separator, positive electrode, negative electrode, aluminum-plastic film, and tabs prepared in the examples and comparative examples were assembled into a 2Ah soft-pack battery cell.

[0060] The thermal runaway test procedure is as follows: Let the 2Ah soft-pack battery cell stand for 30 minutes; Discharge the battery cell at a constant current of 0.5C to 2.7V; let it stand for 30 minutes. Charge the battery cell to 4.35V with a constant current and constant voltage of 0.2C, and cut off the current at 0.02C, i.e., 100% SOC (state of charge), and let it stand for 30 minutes. Place the battery cell in a temperature chamber and heat it to 130℃, 140℃, and 150℃ respectively at a rate of 5℃ / min, and maintain it within the target temperature range for 60 minutes. After the end, observe for another 60 minutes and record the test temperature and voltage. Fabrication of a 2Ah soft-pack three-electrode battery cell: First, the gradient pore size composite separator, positive electrode, and negative electrode prepared in the examples and comparative examples are assembled into a 2Ah bare cell. One 10mm end of a 0.02mm thick copper wire was immersed in 5mL of concentrated sulfuric acid (70% by mass) for 2 hours, and then cleaned with anhydrous ethanol. The copper wire treated with concentrated sulfuric acid was wrapped with a diaphragm and placed in the middle of the bare cell. The untreated copper wire end extended out of the cell. Subsequently, this cell underwent liquid injection, formation, and capacity testing together with ordinary cells. After the battery cell officially rolls off the production line, an untreated copper wire is soldered onto another tab. Here, the reference electrode is a copper wire, and the reference electrode tab is a nickel-plated copper tab with a thickness of 0.12 mm, a width of 8 mm, and a length of 40 mm. This yields a 2Ah soft-pack three-electrode battery cell. The obtained 2Ah soft-pack three-electrode battery cell was subjected to lithium plating. The lithium plating process was as follows: Forward lithium plating, with the electrode connection method being positive electrode + reference electrode, charging current of 30uA, time of 10h, and resting time of 12h; Reverse lithium plating: The electrode connection method is negative electrode + reference electrode, the discharge current is 30uA, the time is 10h, and the resting time is 12h; Three-electrode cell testing: The following test procedure and MAP table were used to test the three-electrode battery cells; Let the battery cells stand for 30 minutes; Discharge to 2.75V at a discharge rate of 1C. Let stand for 1 hour; Pulse charging is performed according to the following MAP table, with the final step being constant voltage charging to 0.05C; Let stand for 1 hour; Discharge to 2.75V at a discharge rate of 1C; After standing for 1 hour, the fresh cells were discharged to 0% SOC and then the pulse charging capability was tested according to Table 1. Table 1. SOC-Charging Rate Mapping Relationship (MAP)

[0061] Test data are shown in Tables 2 and 3; Table 2 Thermal runaway test

[0062] Table 3 Comparison of Negative-Reference Safety Potential Data for Three-Electrode Cells

[0063] The bonding method of the gradient pore size composite separator in Comparative Example 1 is ceramic layer A, ceramic layer B, base layer, ceramic layer B, and ceramic layer A; the pore size of ceramic layer A in Comparative Example 2 is smaller than that of ceramic layer B. Compared with Comparative Examples 1 and 2, the present invention enables the battery cell to maintain excellent thermal stability in high temperature environment, and maintains a stable negative electrode potential during high-rate charging, significantly reducing the risk of lithium plating and thermal runaway, and has good safety. When the SOC is 70%, Comparative Examples 1 and 2 show negative values ​​because the lithium-ion gradient transport channel is disrupted. Under high-rate charging, lithium-ion transport is blocked, the polarization of the negative electrode increases sharply, the negative electrode potential drops below the lithium intercalation safety threshold and enters the lithium plating potential range, and a large number of lithium ions are deposited on the negative electrode surface in the form of metallic lithium, which leads to the negative value.

[0064] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A gradient pore size composite membrane, characterized in that, The gradient pore size composite membrane includes a base layer, a ceramic layer A, and a ceramic layer B, wherein the pore size of the ceramic layer A is larger than the pore size of the ceramic layer B. The bonding method of the gradient pore size composite diaphragm is ceramic layer A, base layer, ceramic layer B, base layer, ceramic layer A.

2. The gradient pore size composite membrane according to claim 1, characterized in that, The average pore size of the ceramic layer A is 20-150 nm, and can be selected as 60-100 nm; And / or, the average pore size of the ceramic layer B is 8-30 nm, optionally 12-15 nm.

3. The gradient pore size composite membrane according to claim 1 or 2, characterized in that, The porosity of the ceramic layer A is 40-70%, and can be selected as 50-60%; And / or, the porosity of the ceramic layer B is 45-80%, optionally 50-60%; And / or, the thickness of the ceramic layer A is 1-2 μm; And / or, the thickness of the ceramic layer B is 1-2 μm.

4. The gradient pore size composite membrane according to any one of claims 1-3, characterized in that, The material of the ceramic layer A includes at least one of alumina, silicon carbide, silicon nitride, zirconium oxide, chromium oxide, spinel, and aluminum titanate; optionally, it includes at least one of alumina, silicon nitride, and aluminum titanate. And / or, the material of the ceramic layer B includes at least one of boehmite, aluminum phosphate, and hydroxyapatite, with boehmite being an option.

5. The gradient pore size composite membrane according to any one of claims 1-4, characterized in that, The average pore size of the substrate is 50-80 nm; And / or, the porosity of the substrate is 40-50%; And / or, the thickness of the base layer is 5-10 μm; And / or, the material of the base layer includes at least one of PP and PE.

6. A method for preparing a gradient pore size composite membrane according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: S1, ceramic raw material a, first solvent and first dispersant are mixed in the first mixing; first binder is added and mixed in the second mixing to obtain slurry i; slurry i is coated on one side of the base layer to obtain intermediate product 1-1, intermediate product 1-1 includes ceramic layer A and base layer; S2, ceramic raw material b, second solvent and second dispersant are mixed in the third mixture; second binder is added and mixed in the fourth mixture to obtain slurry ii; slurry ii is coated on the uncoated side of the intermediate product 1-1 base layer to obtain intermediate product 2, intermediate product 2 includes ceramic layer A, base layer 1 and ceramic layer B; S3, repeat step S1 to obtain another intermediate product 1-2; laminate intermediate product 1-2 with intermediate product 2 to obtain a gradient pore size composite membrane.

7. The preparation method according to claim 6, characterized in that, The average particle size of the ceramic raw material a is 0.3-3 μm; And / or, the Mohs hardness of the ceramic raw material a is 6-9, and can be 7-8; And / or, based on slurry i, the mass fraction of ceramic raw material a is 10-25 wt%, the mass fraction of the first dispersant is 0.1-1.5 wt%, the mass fraction of the first binder is 0.1-1.5 wt%, and the mass fraction of the first solvent is 69-89 wt%.

8. The preparation method according to claim 6 or 7, characterized in that, The average particle size of the ceramic raw material b is 0.2-1 μm; And / or, the Mohs hardness of the ceramic raw material b is 3-4, optionally 3.5-4; And / or, based on slurry ii, the mass fraction of ceramic raw material b is 10-25 wt%, the mass fraction of the second dispersant is 0.1-1.5 wt%, the mass fraction of the second binder is 0.1-1.5 wt%, and the mass fraction of the second solvent is 69-89 wt%.

9. The preparation method according to any one of claims 6-8, characterized in that, In step S3, the adhesive is sandwiched between the base layer side of intermediate product 1-2 and the ceramic layer B side of intermediate product 2; Optionally, the adhesive includes at least one of polyvinylidene fluoride, polyvinyl alcohol, and polytetrafluoroethylene; Optionally, the amount of the adhesive is 0.5-1.5 g / m³. 2 ; And / or, the lamination temperature is 70-110℃, the pressure is 1-8MPa, and the time is 0.5-5min.

10. A secondary battery, characterized in that, The secondary battery includes the gradient pore size composite membrane according to any one of claims 1-5 or the gradient pore size composite membrane prepared by the preparation method according to any one of claims 6-9.

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