Separator and preparation method therefor, and secondary battery

Abstract

A separator and a preparation method therefor, and a secondary battery. The separator comprises a substrate and a polymer coating provided on at least one side of the substrate. The polymer coating comprises at least two coating strips spaced apart. Each coating strip contains a main polymer and an auxiliary polymer. The main polymer forms a porous structure layer; and the auxiliary polymer is embedded in the porous structure layer, and forms a protrusion on the surface of the porous structure layer. The secondary battery prepared by means of hot pressing of the separator has a good electrolyte retention capability, a battery cell has a low expansion rate and a high capacity retention rate, the problem of dendrite precipitation in a corner region of the battery cell is significantly alleviated, the performance of the battery cell is balanced, and the present invention has high application value.

Description

A separator and its preparation method and a secondary battery Technical Field

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

[0002] Currently, in existing technologies, the separator in lithium-ion batteries is typically a ceramic separator, with a polymer coating applied to one or both sides to ensure adhesion between the separator and the electrode. Common polymer coatings include aqueous PMMA (polymethyl methacrylate), aqueous PVDF (polyvinylidene fluoride), and conventional oil-based PVDF. These polymer coatings have low porosity, poor electrolyte retention, and low ionic conductivity, leading to rapid capacity decay in lithium-ion batteries containing these separators during fast charging.

[0003] Furthermore, aqueous separator coatings have lower adhesion, which can easily lead to significant cell thickness expansion. Conventional oil-based PVDF coatings have higher adhesion, resulting in less cell expansion. However, conventional oil-based PVDF coatings typically have an average pore size of less than 0.5 μm and low porosity. Therefore, during fast charging of lithium-ion batteries, oil-based PVDF coatings can cause metal dendrites to precipitate at the negative electrode interface. Sharp dendrites, once they grow to a certain size, can pierce the separator, thus affecting safety. In lithium-ion batteries, the precipitation of metal dendrites is commonly known as "lithium deposition."

[0004] The main reasons affecting the performance of traditional separators are the rapid electrolyte consumption and cell expansion during the fast charging process of secondary batteries. This reduces the gap between the electrodes and leads to insufficient electrolyte replenishment, resulting in insufficient electrolyte retention in the later stages and consequently a short cycle life of the secondary battery. Furthermore, the expansion stress at the cell corners is the greatest within the entire cell, leading to the most significant electrolyte consumption at these corners and a particularly high risk of lithium plating.

[0005] Therefore, in order to address the aforementioned problems, there is an urgent need to develop a secondary battery based on a novel separator, in order to enhance the electrolyte retention capacity, reduce the cell thickness expansion, and improve the cell structure, thereby reducing the risk of dendrite precipitation; at the same time, the secondary battery should have superior cycle performance and high application value. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a separator, its preparation method, and a secondary battery. The separator provided by this invention has excellent liquid retention capacity, and when used in secondary batteries, it can significantly improve the cell expansion rate and capacity retention rate, and reduce the occurrence of metal dendrite precipitation, thus having high application value.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] In a first aspect, the present invention provides a diaphragm comprising a substrate and a polymer coating disposed on at least one side of the substrate, the polymer coating comprising at least two spaced coating strips;

[0009] The coating strip in the polymer coating contains a main polymer and an auxiliary polymer; the main polymer forms a porous structure layer; the auxiliary polymer is embedded in the porous structure layer and forms protrusions on the surface of the porous structure layer.

[0010] To improve the electrolyte retention capacity of the cell corner area of ​​a secondary battery, the present invention provides a separator in which the polymer coating is provided with at least two coating strips arranged in an intermittent manner, so that the polymer coating does not completely cover the side of the separator substrate, but leaves a certain gap between the at least two coating strips, thereby forming a groove between two adjacent polymer coating strips to accommodate more electrolyte.

[0011] Specific spacing and width of the coating strips can significantly increase the electrolyte storage space, thereby improving cell performance. If the coating width is inappropriate, it will affect the application of the polymer coating, thus impacting the separator's electrolyte retention capacity, leading to a significant deterioration in separator performance and a serious risk of dendrite precipitation. If the coating strips are too narrow, it will affect the coating's adhesion, resulting in significant cell thickness expansion; if the coating strips are not spaced out, the separator's electrolyte retention capacity will be poor, with a serious risk of metal dendrite precipitation.

[0012] Furthermore, this invention designs a polymer coating with a specific structure. This specific structure is compressible and is formed by a porous main polymer and an auxiliary polymer embedded in the main polymer and protruding on its surface. This specific structure can increase the gap between the electrodes and the separator at the corners of the secondary battery, effectively increasing the electrode spacing in the cell corner region of the secondary battery containing the separator of this invention (the spacing is controlled by the separator between the electrodes), and improving the electrolyte retention capacity of the separator in the cell corner region, thereby improving the problem of lithium deposition due to electrolyte deficiency in the cell corner region after long-term cycling. At the same time, this specific structure also has compressibility, which can reduce the thickness of the main cell region through hot pressing during battery manufacturing, making the cell as thin as possible.

[0013] The separator provided by this invention, when applied to secondary batteries, exhibits excellent liquid retention capabilities, resulting in low cell expansion and high capacity retention. It significantly improves the problem of metal dendrite precipitation in corner regions, achieving balanced cell performance without requiring equipment modifications or negatively impacting the chemical system. Furthermore, the technical solution of this invention is low-cost, has no performance loss, is highly scalable, and has promising application prospects.

[0014] Preferably, the substrate is at least one of polyethylene, polypropylene, polyethylene terephthalate, polyimide, and aramid.

[0015] As a preferred embodiment of the present invention, a ceramic material layer may be coated first on one side of the substrate where the polymer coating is provided, or on both sides of the substrate, and then the polymer coating may be provided. The ceramic material in the ceramic material layer is at least one of alumina, boehmite, magnesium oxide, magnesium hydroxide, barium titanate, aluminum hydroxide, silicon dioxide, aluminum nitride, lithium titanium aluminum phosphate, and lithium lanthanum zirconium oxide solid electrolyte.

[0016] Preferably, the average pore size of the micropores in the porous structure layer is 0.5-1.5 μm;

[0017] And / or, the median particle size D of the secondary particles of the auxiliary polymer. 50 Conforms to 3μm≤D 50 ≤15μm.

[0018] As a preferred embodiment of the present invention, the average pore size of the micropores in the porous structure layer is any one of 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, and 1.5μm, or a range of any two of these values.

[0019] As a preferred embodiment of the present invention, the median particle size D of the secondary particles of the auxiliary polymer is... 50 It is a range of any one or any two of the following: 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, and 15μm.

[0020] The auxiliary polymer is preferably a secondary particle auxiliary polymer. After compression, the secondary particles of the auxiliary polymer still have gaps between them, which can retain ion channels and not deteriorate the cell cycle performance.

[0021] By controlling the concentration of the main polymer in the preparation steps, different average micropore sizes can be achieved in the porous structure layer. Controlling the average micropore size of the polymer coating's porous structure layer to 0.5-1.5 μm ensures a high electrolyte retention capacity of the separator. When the average micropore size is less than 0.5 μm, the polymer coating of the separator has poor electrolyte retention capacity, increasing the risk of lithium plating; when the average micropore size is greater than 1.5 μm, the adhesion of the polymer coating of the separator deteriorates, and the cell thickness expansion rate increases.

[0022] Preferably, the compressibility coefficient R1 of the porous structure layer formed by the main polymer under hot pressing conditions of 85°C, 1.2MPa, and 50min meets the condition of 0.05≤R1≤0.3;

[0023] And / or, the compressibility coefficient R2 of the auxiliary polymer under hot pressing conditions of 85°C, 1.2MPa, and 50min meets the condition of 0.6≤R2<1.0;

[0024] And / or, the thickness of the porous structure layer formed by the main polymer is 0.5-5 μm, and the protrusion height of the auxiliary polymer is 1-10 μm.

[0025] The compressibility coefficient R refers to the ratio of the thickness of the polymer that can be compressed to the original thickness of the polymer. The auxiliary polymer is preferably a polymer with good compressibility, and gaps may exist between the secondary particle aggregates of the auxiliary polymer, making it easy to compress. Under hot-pressing conditions of 85°C, 1.2 MPa, and 50 min, the compressibility coefficient R2 of the auxiliary polymer conforms to 0.6 ≤ R2 < 1.0. The porous structure layer formed by the main polymer is more difficult to compress, and under the same hot-pressing conditions, its compressibility coefficient R1 conforms to 0.05 ≤ R1 ≤ 0.3. When the compressibility coefficient R2 of the auxiliary polymer is within the above-mentioned range, the auxiliary polymer will be distributed in the porous structure layer with a specific pore size, forming protrusions on its surface. That is, during the fabrication of the secondary battery, after hot-pressing, the auxiliary polymer can be compressed to a certain extent, but it will not be compressed to the same thickness as the main polymer, nor will it be completely embedded in the porous structure layer formed by the main polymer; instead, it will inevitably produce a certain degree of protrusion. Furthermore, compared to the main body of the cell, the separator coating in the cell corner area experiences less hot-pressing pressure, resulting in less compression of the auxiliary polymer. This allows for larger gaps to form in the corner area, providing storage space for the electrolyte. Therefore, for a secondary battery using this separator, after hot pressing, the thickness reduction of the polymer coating in the main body of the cell will remain within the range of 3-5 μm. This creates a certain difference between the electrode interlayer spacing in the main body and the electrode interlayer spacing in the cell corner area, ensuring that the electrode interlayer spacing in the cell corner area of ​​the secondary battery is within the required range.

[0026] On the one hand, the greater the compression of the polymer coating in the main body of the cell, the smaller the proportion of polymer coating thickness, and the thinner the secondary battery. On the other hand, the greater the compression of the polymer coating in the main body of the cell, the higher the flatness of the coating after compression, and the higher the flatness of the secondary battery. However, if no auxiliary polymer is added, or the particle size of the auxiliary polymer is too small, the polymer coating may be compressed too much during the preparation of the secondary battery, resulting in the closure of the gaps between the secondary particles and the blockage of the substrate pores, ultimately leading to a higher risk of metal dendrite precipitation in the main body of the cell.

[0027] Preferably, in the polymer coating, the interval between two adjacent coating strips is 0.15-2.5 mm;

[0028] And / or, the width of each of the coating strips is 0.5-3 mm.

[0029] More preferably, the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is 60-85%. The "coated side" of the substrate refers to the area of ​​one or both sides of the substrate coated with the polymer coating. When the polymer coating is applied only to one side of the substrate, the coated side is the area of ​​that side of the substrate; when the polymer coating is applied to both sides of the substrate, the coated side is the sum of the areas of both sides of the substrate.

[0030] As a preferred embodiment of the present invention, the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is any one of 60%, 65%, 70%, 75%, 80%, and 85%, or a range of any two. However, it should be noted that the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is not limited to the specific values ​​listed above, and those skilled in the art can choose other suitable ratios within the range according to actual needs.

[0031] When the distribution area of ​​the polymer coating on the coated side of the substrate is too small, the adhesion of the polymer coating is weak, and the cell thickness will expand and increase after multiple charge-discharge cycles. When the distribution area of ​​the polymer coating on the coated side of the substrate is greater than 85%, the gap between at least two coating bands of the polymer coating will be too small, resulting in poor liquid retention capacity of the separator and a greater risk of metal dendrite precipitation.

[0032] More preferably, in the polymer coating, the interval between two adjacent coating strips is 0.2-0.4 mm;

[0033] And / or, the width of each of the coating strips is 0.6-0.85 mm.

[0034] More preferably, the thickness of the polymer coating is 5-10 μm;

[0035] And / or, the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is S%, and the median particle size D of the secondary particles of the auxiliary polymer is... 50 =dμm, and the quantitative relationship between S and d conforms to 10≤0.1S+d≤12.

[0036] Median particle size D of secondary particles of auxiliary polymer 50The value of d is dμm, and the magnitude of d is related to the degree of protrusion of the auxiliary polymer on the main polymer layer, which affects the longitudinal height of the gap between at least two coating bands in the polymer coating. The ratio of the polymer coating area to the area of ​​the coated side of the substrate is S%, and the magnitude of S affects the cross-sectional area of ​​the gap. When the polymer coating thickness is controlled within the above-mentioned preferred range, the above two parameters (i.e., d and S) will jointly regulate the spatial capacity of the gap formed by at least two coating bands in the polymer coating in each region of the battery. When the above-mentioned relationship between these two parameters is within the above-defined range, the separator can provide suitable storage space for the electrolyte, better avoid the risk of metal dendrite precipitation in the battery, reduce the thickness expansion of the cell, and improve the overall performance of the battery.

[0037] Preferably, the coating strip forms an inclined angle with the width direction of the substrate (i.e., the direction perpendicular to the length direction of the substrate), and the angle between the coating strip and the width direction of the substrate is 30°-60°.

[0038] To improve liquid retention while accommodating the stacking and winding processes in secondary battery fabrication, the coating strips are applied in an alternating pattern as inclined stripes. This means the extension direction of the coating strips forms an angle with the width direction of the substrate, such as 30 degrees, 45 degrees, or 60 degrees, rather than being parallel. Preferably, the extension direction of the coating strips forms an angle of 45 ± 3 degrees with the width direction of the substrate. In the stacking cell process, the extension direction of the coating strips can form an angle with the width direction of the substrate, or they can be parallel or perpendicular to each other.

[0039] Preferably, the main polymer is at least one selected from polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-trifluorochloroethylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-trifluorochloroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer, and polymethyl methacrylate.

[0040] And / or, the auxiliary polymer is at least one of polyethylene, polypropylene, polybutene, poly4-methyl-1-pentene, polytetrafluoroethylene, polyethylene wax, and oxidized polyethylene wax.

[0041] Secondly, the present invention provides a method for preparing the above-mentioned diaphragm, comprising the following steps:

[0042] (1) Dissolve the main polymer in an organic solvent to obtain a main polymer solution, then add an auxiliary polymer and disperse to obtain a polymer coating;

[0043] (2) The polymer coating is applied to at least one side of the substrate in an intermittent manner, and then the substrate is immersed in a coagulation bath solution. After removal, it is washed and dried to obtain the diaphragm.

[0044] The preparation method of the present invention provides a membrane containing a polymer coating. This polymer coating comprises a main polymer and an auxiliary polymer, wherein the main polymer forms a porous structure layer, and the auxiliary polymer is embedded in the porous structure layer and forms protrusions on its surface. This specific structure of the polymer coating enables the membrane to achieve the desired performance characteristics.

[0045] Preferably, the organic solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and acetone; the coagulation bath solution includes at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, ethanol, isopropanol, and water.

[0046] More preferably, the organic solvent is N,N-dimethylacetamide, and the coagulation bath solution is an aqueous solution of N,N-dimethylacetamide.

[0047] Preferably, the main polymer solution has a main polymer concentration of 4%-15%; and / or the mass ratio of the main polymer to the auxiliary polymer is (3-4):(1-2).

[0048] Thirdly, the present invention provides a secondary battery, comprising, in sequence, a positive electrode, the aforementioned separator, and a negative electrode; the interlayer spacing of the electrode in the corner region of the cell of the secondary battery is A, and the interlayer spacing of the electrode in the main body region of the cell of the secondary battery is B, wherein A and B conform to the relationship: 3μm≤AB≤5μm.

[0049] The electrode interlayer spacing A in the cell corner region and the electrode interlayer spacing B in the cell body region of the secondary battery provided by this invention satisfy the relationship: 3μm≤AB≤5μm. This invention increases the electrode spacing in the cell corner region of the secondary battery by setting a specific separator between the electrodes, thereby improving the electrolyte wetting rate and electrolyte retention in the cell corner region, and better addressing the problem of electrolyte deficiency and metal dendrite precipitation in the cell corner region after long-term cycling.

[0050] When the difference between the interlayer spacing A of the electrode in the cell corner region and the interlayer spacing B of the electrode in the main body region of the cell is less than 3 μm, the interlayer gap in the cell corner region is too small, resulting in insufficient electrolyte retention in the cell corner region and poor improvement effect on the problem of metal dendrite precipitation in the cell corner region. When AB > 5 μm, the interlayer gap in the cell corner region is too large, the ion transport path is too long, and the secondary battery will have the problem of poor metal ion insertion after cycling, thus precipitating metal dendrites, which will have a negative impact on the cell corner region.

[0051] It should be noted that the secondary batteries mentioned in this article include, but are not limited to, lithium-ion batteries, sodium-ion batteries, potassium-ion batteries, zinc-ion batteries, lithium metal batteries, lithium-oxygen batteries, and lithium-sulfur batteries.

[0052] Fourthly, the present invention also provides an electrical device, the electrical device comprising the aforementioned secondary battery.

[0053] The secondary battery of this invention can be applied to electrical devices that use batteries as a power source, or to various energy storage systems that use batteries as energy storage elements. These electrical devices include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, and spacecraft.

[0054] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0055] This invention designs the polymer coating of the separator to have a compressible structure within a certain range for use in the fabrication of secondary battery cells, thereby increasing the interlayer gap in the cell corner region within a suitable range. Simultaneously, by optimizing the spacing, distribution distance, and distribution area of ​​the polymer coating, a polymer coating with a specific structure (a compressible structure formed by a porous main polymer and auxiliary polymers embedded in the main polymer and protruding on its surface) is prepared. This allows the areas not covered by the polymer coating to form grooves with higher electrolyte retention capacity, further improving the electrolyte retention capacity in the cell corner region, extending the cycle life of the secondary battery, mitigating the problem of metal dendrite precipitation, and reducing the cell thickness expansion rate. The separator provided by this invention has low manufacturing cost, no performance loss, and high applicability, showing great promise for application in secondary batteries. Attached Figure Description

[0056] Figure 1 is a schematic diagram of the distribution of the polymer coating on the diaphragm in Embodiment 1 of the present invention;

[0057] Figure 2 is a schematic diagram of the diaphragm structure in Embodiment 1 of the present invention;

[0058] Figure 3 is a schematic diagram of the secondary battery structure in Embodiment 1 of the present invention. Detailed Implementation

[0059] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0060] The polyethylene used in the following examples and comparative examples was purchased from Yanshan Petrochemical Co., Ltd. of China Petroleum & Chemical Corporation (brand name: LD605); the polyvinylidene fluoride-hexafluoropropylene used was purchased from Arkema (brand name: LBG).

[0061] Example 1

[0062] An embodiment of the separator and secondary battery of the present invention, wherein the preparation method of the separator in this embodiment is as follows:

[0063] (1) The main polymer (i.e., polyvinylidene fluoride-hexafluoropropylene) was added to the solvent N,N-dimethylacetamide (DMAC), and after being stirred and completely dissolved at 60°C, a polyvinylidene fluoride-hexafluoropropylene solution with a mass concentration of 8% was obtained; then the auxiliary polymer (i.e., the median particle size D of the secondary particles) was added. 50 Polyethylene powder with a particle size of 4 μm was prepared, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene to polyethylene was 65:35. After thorough stirring and dispersion, a polymer coating was obtained.

[0064] (2) A polyethylene microporous membrane prepared by wet method with boehmite ceramic coating is used as the substrate. The total thickness of the substrate is 6 μm and the thickness of the ceramic coating is 1 μm. The polymer coating prepared above is uniformly coated on one side of the substrate by intermittent coating with a micro-concave roller, so that the polymer coating presents an inclined stripe pattern. The thickness of the polymer coating is 8 μm, the width of the coating strip in the coated area is 0.7 mm, the spacing between the uncoated areas between each coating strip is 0.3 mm, and the area of ​​the polymer coating accounts for 70% of the area of ​​the polymer-coated side of the substrate.

[0065] (3) The coated substrate is immersed in a mixed coagulation bath solution containing DMAC and water (the mass percentage of DMAC in the mixed coagulation bath solution is 30%), and left to stand for 5 minutes; then it is taken out and washed with water, and then dried at 60°C to obtain the diaphragm.

[0066] Figure 1 shows a schematic diagram of the distribution of the polymer coating after applying the polymer coating in step (2) above. It can be seen that the angle between the coated stripes and the width direction of the substrate is approximately 45°.

[0067] Figure 2 shows a schematic diagram of the diaphragm structure described in this embodiment. As can be seen, since the auxiliary polymer cannot be dissolved in the solvent of the coating and due to the special particle size difference, the auxiliary polymer can be embedded in the main polymer after drying and form protrusions on its surface; while the main polymer can form a porous structure after drying due to the specific preparation process.

[0068] The preparation method of the secondary battery described in this embodiment is as follows:

[0069] S1. Preparation of positive electrode sheet

[0070] Lithium cobalt oxide (CCO) as the positive electrode active material, conductive carbon black (CCO) as the conductive agent, and polyvinylidene fluoride (PVDF) as the binder were mixed in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) was added, and the mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was uniformly coated onto one surface of an 8 μm thick aluminum foil, and the aluminum foil was dried at 125 °C for 1 h to obtain a positive electrode sheet with a single-sided coating of positive electrode material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After cold pressing, cutting, and slitting, the sheet was dried under vacuum at 125 °C for 2 h to obtain a positive electrode sheet with a size of 65 mm × 860 mm.

[0071] S2, Preparation of negative electrode sheet

[0072] The negative electrode active material graphite, the binder styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed at a mass ratio of 98:1:1, deionized water was added, and the mixture was stirred evenly under vacuum to obtain a negative electrode slurry with a solid content of 60 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil, and the copper foil was dried at 120 °C to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The above steps were repeated on the other surface of an aluminum foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing, cutting, and slitting, the sheet was dried under vacuum at 120 °C for 2 hours to obtain a negative electrode sheet with a size of 69 mm × 868 mm.

[0073] S3. Preparation of electrolyte

[0074] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate, propylene carbonate and dimethyl carbonate are mixed in a mass ratio of 3:2:4 to obtain an organic solvent. Lithium hexafluorophosphate is then added to the organic solvent and mixed evenly to obtain an electrolyte. The concentration of lithium hexafluorophosphate is 1 mol / L.

[0075] S4. Assembly and preparation of secondary batteries

[0076] Preparation of lithium-ion batteries: The positive electrode, separator, and negative electrode obtained above are stacked in sequence, with the separator positioned between the positive and negative electrode to provide isolation. The electrode assembly is then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and secondary sealing, a lithium-ion battery is obtained, resulting in the secondary battery. The formation conditions are 85℃, 1.2MPa, and 50min. Under these conditions, the compressibility coefficient of the porous structure layer formed by the main polymer in the separator is 0.1, and the compressibility coefficient of the auxiliary polymer is 0.8.

[0077] The cell structure of the secondary battery in this embodiment, in the thickness direction, is shown in Figure 3, including the cell body region and the cell corner region. After hot pressing, the polymer coating thickness of the separator in the cell body region is compressed and reduced; the electrode interlayer spacing in the cell corner region of the resulting secondary battery is A, and the electrode interlayer spacing in the cell body region is B, where AB = 4 μm.

[0078] Examples 2-5

[0079] Examples 2-5 are multiple embodiments of the separator and secondary battery of the present invention. The difference between Examples 2-5 and Example 1 is that, during the preparation of the separator, the median particle size D of the secondary particles of the auxiliary polymer polyethylene powder is changed respectively. 50 The median particle size D of the secondary particles of the polyethylene powder is thus reduced. 50 The corresponding sizes are 5μm, 6μm, 3μm, and 2μm.

[0080] Example 6

[0081] Examples 6-7 are two embodiments of the separator and secondary battery of the present invention. The difference between Examples 6-7 and Example 1 is that:

[0082] During the preparation of the diaphragm, the amount of polyvinylidene fluoride-hexafluoropropylene was changed so that the mass concentration of polyvinylidene fluoride-hexafluoropropylene in the solution was 13% and 15%, respectively.

[0083] Meanwhile, the ratio of the coagulation bath solution was changed so that the mass percentage of DMAC in the mixed coagulation bath solution was 25%.

[0084] Examples 8-9

[0085] Examples 8-9 are two embodiments of the separator and secondary battery of the present invention. The difference between Examples 8-9 and Example 1 is that the amount of polyvinylidene fluoride-hexafluoropropylene is changed during the preparation of the separator so that the mass concentration of polyvinylidene fluoride-hexafluoropropylene in the solution is 5% and 4.5%, respectively.

[0086] Example 10

[0087] Example 10 is an embodiment of the separator and secondary battery of the present invention. The only difference between Example 10 and Example 1 is that: during the preparation of the separator, the width of the coating strip in the coating area is 0.6 mm, the spacing between the uncoated areas between each coating strip is 0.4 mm, and the area of ​​the polymer coating accounts for 60% of the area of ​​the coated side of the substrate.

[0088] Example 11

[0089] Example 11 is an embodiment of the separator and secondary battery of the present invention. The only difference between Example 11 and Example 1 is that: during the preparation of the separator, the width of the coating strip in the coating area is 0.8 mm, the spacing between the uncoated areas between each coating strip is 0.2 mm, and the area of ​​the polymer coating accounts for 80% of the area of ​​the coated side of the substrate.

[0090] Example 12

[0091] Example 12 is an embodiment of the separator and secondary battery of the present invention. The only difference between Example 12 and Example 1 is that: during the preparation of the separator, the width of the coating strip in the coating area is 0.85 mm, the spacing between the uncoated areas between each coating strip is 0.15 mm, and the area of ​​the polymer coating accounts for 85% of the area of ​​the coated side of the substrate.

[0092] Comparative Example 1

[0093] The difference between Comparative Example 1 and Example 1 lies in the preparation of the polymer coating on the diaphragm, and the steps are as follows:

[0094] S1. Using the same substrate as in Example 1, polyvinylidene fluoride-hexafluoropropylene and isobutyl acrylate polymer (mass ratio 90:6) were added to deionized water and dispersed to obtain a slurry with a mass concentration of 8%; then, secondary particles with a median particle size D were added. 50 Polyethylene powder with a thickness of 4 μm was prepared, and the mass ratio of polyvinylidene fluoride-hexafluoropropylene to polyethylene was 65:35. After thorough mixing, a polymer coating slurry was obtained.

[0095] S2. The polymer coating is uniformly coated on one side of the substrate using a micro-concave roller intermittent coating method, so that the polymer coating presents an inclined stripe-like intermittent distribution. The thickness of the polymer coating is 8μm, the width of the coating strip in the coated area is 0.7mm, the spacing between the uncoated areas between each coating strip is 0.3mm, and the area of ​​the polymer coating accounts for 70% of the area of ​​the coated side of the substrate. After drying at 60℃, a diaphragm with a polymer coating on one side is obtained.

[0096] Comparative Example 2

[0097] The only difference between Comparative Example 2 and Example 1 is that no auxiliary polymer, polyethylene powder, is added during the preparation of the diaphragm.

[0098] Comparative Example 3

[0099] The only difference between Comparative Example 3 and Example 1 is that, in the preparation of the diaphragm, the polymer coating is not applied in an intermittent coating manner, but rather the area of ​​the polymer coating accounts for 100% of the area of ​​the coated side of the substrate.

[0100] Comparative Example 4

[0101] The only difference between Comparative Example 4 and Example 1 is that, during the preparation of the diaphragm, the width of the coating strip in the coated area is 7 mm, and the spacing between the uncoated areas between each coating strip is 3 mm.

[0102] Comparative Example 5

[0103] The only difference between Comparative Example 5 and Example 1 is that during the preparation of the diaphragm, the width of the coating strip in the coated area is 0.55 mm, the spacing between the uncoated areas between each coating strip is 0.45 mm, and the area of ​​the polymer coating accounts for 55% of the area of ​​the coated side of the substrate.

[0104] The structural parameters of the separator and secondary battery in the above embodiments and comparative examples are shown in Table 1.

[0105] In the above embodiments and comparative examples, the method for characterizing the average pore size of the porous structure layer of the polymer coating is as follows: samples are taken from the diaphragm, five samples are randomly selected, the surface morphology is observed and photographed by scanning electron microscopy, and the pore size of the porous structure layer of the coating in the image is analyzed using scanning electron microscopy analysis software (the pore size of at least 20 micropores is measured in each image), and the average pore size is calculated.

[0106] Table 1. Structural parameters of the separator and secondary battery in Examples 1-12 and Comparative Examples 1-5.

[0107] Example 1

[0108] To investigate the performance of the separator and secondary battery of the present invention, the initial thickness and flatness of the secondary battery cell were recorded, and the capacity retention rate and lithium plating of the negative electrode were tested after 800 cycles at 4C rate at room temperature. The details are as follows.

[0109] (1) Cell flatness test: The cell thickness distribution is tested using a 3D profile measuring instrument. Cell flatness = maximum cell thickness - minimum cell thickness.

[0110] (2) Cyclic test of the cell at room temperature and 4C rate: Place the secondary battery in a 25℃ environment, let it stand for 30 minutes, charge it at a constant current of 4C rate to 4.50V, then charge it at a constant voltage of 4.50V to 0.05C rate, let it stand for 5 minutes, discharge it at a constant current of 0.7C rate to 3.0V, and cycle the charge and discharge 800 times.

[0111] (3) Determination of the surface lithium plating level of the negative electrode sheet in the main body area of ​​the battery cell:

[0112] No lithium deposition: There is no lithium deposition on the surface of the negative electrode in the main body area of ​​the battery cell;

[0113] Slight lithium plating: The surface lithium deposition area of ​​the negative electrode in the main body area of ​​the battery cell is less than 5%;

[0114] Moderate lithium plating: The surface lithium deposition area of ​​the negative electrode sheet in the main body area of ​​the battery cell is 5% to 20%;

[0115] Severe lithium plating: The surface area of ​​the negative electrode in the main body area of ​​the battery cell has a lithium deposition area greater than 20%.

[0116] (4) Determination of the surface lithium plating level of the negative electrode in the corner area of ​​the battery cell:

[0117] No lithium deposition: There is no lithium deposition on the surface of the negative electrode in the corner area of ​​the battery cell;

[0118] Slight lithium plating: The surface lithium deposition area of ​​the negative electrode in the corner area of ​​the cell is less than 10%, and it does not grow into a flat area;

[0119] Moderate lithium deposition: The surface lithium deposition area of ​​the negative electrode in the corner area of ​​the cell is 10% to 40%, and it does not grow into a flat area;

[0120] Severe lithium plating: The surface lithium deposition area of ​​the negative electrode in the corner area of ​​the cell is greater than 40%, or it grows into a flat area.

[0121] The performance test results of the secondary batteries in each embodiment and comparative example are shown in Table 2.

[0122] Based on the performance test results of the secondary batteries in Examples 1-12 and Comparative Examples 1-5, it can be seen that in the cell structure, the difference in interlayer spacing between the electrode layers in the cell corner region and the cell body region, the average pore size of the porous structure (micropores) of the polymer coating, the median particle size of the secondary particles of the auxiliary polymer, and the ratio of the area of ​​the polymer coating to the area of ​​the substrate coating side all have a significant impact on the performance improvement of the secondary battery cell. Compared with Comparative Examples 1-5, the separator and secondary battery in Examples 1-12, through the specific polymer coating distribution method of this invention, under specific distribution area, specific coating composition and structure, can better improve the lithium plating problem and reduce the risk of lithium plating, and have a lower cell thickness expansion rate, better capacity retention rate and cell flatness, and balanced performance in all aspects. At the same time, the polymer coating ratio (S%) and the median particle size (D) of the secondary particles of the auxiliary polymer can be optimally selected from the above Examples 1-12. 50=dμm) satisfies the relationship 10≤0.1S+d≤12. This is because the ratio of the polymer coating area to the substrate coating side surface area (S%) and the median particle size of the auxiliary polymer secondary particles (D) are related. 50 =dμm) can coordinately adjust the space capacity of the gap, which can improve the liquid retention capacity of the cell while effectively avoiding the risk of lithium plating.

[0123] As shown in Examples 1-5, when the reduction in polymer coating thickness (AB) in the main body region of the battery cell is 3-5 μm, the smaller the cell thickness, the higher the flatness, given the same original polymer coating thickness. When AB > 5 μm, the polymer material is excessively compressed, closing the gaps between secondary polymer particles and clogging the substrate pores, leading to a certain risk of lithium plating. Examples 1 and 3 show that when AB < 3 μm, the interlayer spacing in the cell corner region is too small, making lithium plating prone to occur in the cell corner region after long cycling. Examples 1 and 5 show that when AB > 5 μm, the interlayer spacing in the cell corner region is too large, resulting in a long ion transport path, making lithium plating due to poor lithium intercalation prone to occur in the main body region of the cell after long cycling, but not in the cell corner region. In contrast, when 3 μm ≤ AB ≤ 5 μm, the problem of lithium plating due to lack of liquid in the cell corner region after long cycling is significantly improved.

[0124] As shown in Examples 1, 10-12 and Comparative Examples 3 and 5, when the area of ​​the polymer coating is less than 60% of the area of ​​the coated side surface of the substrate (Comparative Example 5), the adhesion of the polymer coating is weak, and the cell thickness expands significantly. When the polymer coating is applied without intermittent coating, and the area of ​​the polymer coating is 100% of the area of ​​the coated side surface of the substrate (Comparative Example 3), there is no gap between the polymer coating layers, resulting in poor liquid retention and a serious risk of lithium plating. Preferably, the area of ​​the polymer coating is 60-80% of the area of ​​the coated side surface of the substrate. Controlling the area of ​​the polymer coating within the above-mentioned range can more significantly improve the lithium plating phenomenon.

[0125] As demonstrated in Examples 1, 6-9, and Comparative Example 1, altering the membrane preparation steps to prevent the polymer coating from forming a specific porous structure (Comparative Example 1) severely impacts cell performance, leading to severe lithium plating and a significant increase in cell thickness expansion rate after long cycling. When the average pore size of the polymer coating's porous structure is <0.5 μm (Example 7), the polymer coating exhibits poor liquid retention capacity, making lithium plating risk more likely. When the average pore size of the polymer coating's porous structure is >1.5 μm (Example 9), the polymer coating has poor adhesion, resulting in a significant increase in cell thickness expansion rate. The preferred average pore size of the porous structure layer of the polymer coating is 0.5-1.5 μm.

[0126] In summary, this invention improves the liquid retention capacity of the battery cell by optimizing the spacing, distribution area, composition, and specific coating structure of the polymer coating in the separator, while maintaining a relatively thin separator. This invention also provides a secondary battery prepared using this separator, which exhibits significantly improved cell performance and effectively addresses the problem of lithium deposition due to liquid shortage in the cell corner area after long-term cycling. At room temperature and 4C rate, after 800 cycles, the secondary battery still maintains high capacity retention and low cell thickness expansion rate, demonstrating excellent electrochemical performance. Furthermore, the separator's preparation method is simple and easy to implement, showing great promise for widespread application.

[0127] Table 2. Performance test results of the secondary batteries in Examples 1-12 and Comparative Examples 1-5.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A diaphragm comprising a substrate and a polymer coating disposed on at least one side of the substrate, said polymer coating comprising at least two spaced-apart coating strips; The coating strip in the polymer coating contains a main polymer and an auxiliary polymer; the main polymer forms a porous structure layer; the auxiliary polymer is embedded in the porous structure layer and forms protrusions on the surface of the porous structure layer.

2. The diaphragm as claimed in claim 1, wherein the average pore size of the micropores in the porous structure layer is 0.5-1.5 μm; And / or, the median particle size D of the secondary particles of the auxiliary polymer. 50 Conforms to 3μm≤D 50 ≤15μm.

3. The diaphragm as claimed in claim 1 or 2, wherein the compressibility coefficient R1 of the porous structure layer formed by the main polymer under hot-pressing conditions of 85°C, 1.2 MPa, and 50 min meets the condition of 0.05 ≤ R1 ≤ 0.3; And / or, the compressibility coefficient R2 of the auxiliary polymer under hot pressing conditions of 85°C, 1.2MPa, and 50min meets the condition of 0.6≤R2<1.0; And / or, the thickness of the porous structure layer formed by the main polymer is 0.5-5 μm, and the protrusion height of the auxiliary polymer is 1-10 μm.

4. The diaphragm according to any one of claims 1-3, wherein the interval between two adjacent coating strips in the polymer coating is 0.15-2.5 mm; And / or, the width of each of the coating strips is 0.5-3 mm.

5. The diaphragm according to any one of claims 1-4, wherein the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is 60%-85%.

6. The diaphragm of claim 4, wherein the interval between two adjacent coating strips in the polymer coating is 0.2-0.4 mm; And / or, the width of each of the coating strips is 0.6-0.85 mm.

7. The diaphragm according to any one of claims 1-6, wherein the thickness of the polymer coating is 5-10 μm; And / or, the ratio of the area of ​​the polymer coating to the area of ​​the coated side of the substrate is S%, and the median particle size D of the secondary particles of the auxiliary polymer is... 50 =dμm, and the quantitative relationship between S and d conforms to 10≤0.1S+d≤12.

8. The diaphragm according to any one of claims 1-7, wherein the coating strip forms an inclined angle with the width direction of the substrate, and the angle between the coating strip and the width direction of the substrate is 30°-60°.

9. The diaphragm according to any one of claims 1-8, wherein the main polymer is at least one selected from polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-trifluoroethylene copolymer, polyvinylidene fluoride-trifluorochloroethylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-trifluorochloroethylene copolymer, polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene copolymer, and polymethyl methacrylate; And / or, the auxiliary polymer is at least one of polyethylene, polypropylene, polybutene, poly4-methyl-1-pentene, polytetrafluoroethylene, polyethylene wax, and oxidized polyethylene wax.

10. A method for preparing the diaphragm according to any one of claims 1-9, comprising the following steps: (1) Dissolve the main polymer in an organic solvent to obtain a main polymer solution, then add an auxiliary polymer and disperse to obtain a polymer coating; (2) The polymer coating is applied to at least one side of the substrate in an intermittent manner, and then the substrate is immersed in a coagulation bath solution. After removal, it is washed and dried to obtain the diaphragm.

11. The method for preparing the diaphragm according to claim 10, wherein the organic solvent is at least one selected from N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and acetone; and the coagulation bath solution includes at least one selected from N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, acetone, ethanol, isopropanol, and water.

12. The method for preparing the diaphragm according to claim 10 or 11, wherein the mass concentration of the main polymer in the main polymer solution is 4%-15%; And / or, the mass ratio of the main polymer to the auxiliary polymer is (3-4):(1-2).

13. A secondary battery, comprising, in sequence, a positive electrode, a separator as described in any one of claims 1-9, and a negative electrode; wherein the interlayer spacing of the electrode layers in the corner region of the secondary battery cell is A, and the interlayer spacing of the electrode layers in the main body region of the secondary battery cell is B, wherein A and B satisfy: 3μm≤AB≤5μm.

14. An electrical device comprising the secondary battery of claim 13.

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