An ultrathin high-strength diaphragm, a preparation method thereof and a battery

Abstract

This invention provides an ultrathin, high-strength separator comprising a first polyolefin resin and a second polyolefin resin, wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10min. Furthermore, a method for preparing the ultrathin, high-strength separator and a lithium-ion battery are provided. The separator exhibits significantly superior performance compared to existing technologies in terms of needle penetration strength, molecular weight distribution (MD), and tensile strength (TD), and the process is stable and controllable, offering advantages for application in power lithium-ion batteries and consumer electronics lithium batteries.

Description

Technical Field

[0001] This invention belongs to the technical field of manufacturing lithium-ion batteries, specifically providing a method for preparing an ultrathin high-strength separator, the ultrathin high-strength separator, and the battery. Background Technology

[0002] With the rapid development of new energy vehicles and large-scale energy storage technologies, the market has placed higher demands on the energy density, cycle life, and safety performance of lithium batteries. Among these, the separator, as a key internal component of lithium batteries, directly affects the battery's interface structure, internal resistance, and safety. To improve battery energy density, thinner and lighter separators have become an important trend.

[0003] However, as the membrane thickness decreases, it brings with it significant technical challenges: 1. Reduced strength: According to the basic laws of materials mechanics, the reduction in the thickness of the separator directly leads to a decrease in its absolute mechanical strength. When the needle penetration strength is insufficient, lithium dendrites generated inside the battery can easily pierce the separator, leading to micro-short circuits or thermal runaway; when the tensile strength is insufficient, deformation or damage is likely to occur during battery winding and assembly.

[0004] 2. High difficulty in process control: The extrusion molding of ultra-thin films requires extremely high melt flowability and uniformity. Single-component polypropylene resin is prone to problems such as melt fracture and uneven thickness when extruded at low thickness.

[0005] 3. Pore uniformity: When the membrane thickness is extremely thin, it is difficult to control the uniformity of the stretched pores. If the distribution of crystalline and amorphous regions is unreasonable, it will result in an excessively wide pore size distribution, affecting the uniformity of ion conduction.

[0006] To improve membrane performance, common methods in existing technologies include multilayer composite coating of the membrane (such as ceramic coating or PVDF coating) to enhance mechanical strength and thermal stability. However, coatings increase the overall thickness of the membrane or require additional equipment investment, and the interfacial adhesion between the coating and the substrate is also a potential risk.

[0007] Another type of technical solution involves adjusting the raw material formulation of the diaphragm. For example, some patents disclose the use of polypropylene blends with different molecular weights, but these often focus on improving film-forming properties or pore size, with few studies specifically addressing how to systematically improve the MD tensile strength, TD tensile strength, and crucial needle-punch strength of ultrathin diaphragms while maintaining high porosity. In particular, existing dry-process-prepared thinner diaphragms, limited by the characteristics of the uniaxial stretching process, typically have weak TD tensile strength, leading to easy tearing in the transverse direction and severely impacting production yield.

[0008] In view of the problems mentioned above, developing a dry membrane preparation method that does not require complex coating and focuses on substrate modification to achieve high needle penetration strength and uniaxial tensile strength in thin films has significant industrial value. Summary of the Invention

[0009] Based on the shortcomings of the prior art, the main objective of this invention is to provide an ultra-thin, high-strength diaphragm, which includes a first polyolefin resin and a second polyolefin resin, wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10 min, and the melt index is tested at 230°C.

[0010] In some embodiments, the polyolefin resin includes polyethylene, polypropylene, or copolymers of each or each other.

[0011] In some embodiments, when the thickness of the diaphragm is 8μm to 12μm, it has at least one of the following performance characteristics: Needle penetration intensity ≥280 gf; Longitudinal tensile strength (MD Tensile Strength) ≥260 MPa; Transverse tensile strength (TD Tensile Strength) ≥ 18 MPa; Porosity: 30%~40%; or Air permeability (Gurley value): 210s / 100mL~290s / 100mL.

[0012] In some embodiments, the thickness of the diaphragm is 9 μm to 11 μm.

[0013] In some embodiments, the thickness of the diaphragm is 10 μm.

[0014] In some embodiments, the melt index of the diaphragm is 0.42 g / 10 min to 0.58 g / 10 min.

[0015] Another object of the present invention is to provide a lithium-ion battery, including the ultrathin high-strength separator as described above.

[0016] Another objective of this invention is to provide a method for preparing an ultrathin, high-strength membrane, comprising the following steps: S1: Mix the first polyolefin resin and the second polyolefin resin to obtain a mixed raw material; wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10min, and the melt index test condition is 230℃. S2: The mixed raw materials are melted and plasticized in a melt extruder, extruded through a die, and cooled by a quenching roller to form a cast sheet; S3: Heat-treat and anneal the cast sheet; S4: Uniaxial stretching of the annealed castings to form a microporous film; uniaxial stretching includes low-temperature cold stretching and high-temperature hot stretching; and S5: The stretched microporous membrane is heat-set and wound up to obtain a diaphragm with a thickness of 8μm~12μm.

[0017] In some embodiments, the polyolefin resin includes polyethylene, polypropylene, or copolymers of each or each other.

[0018] In some embodiments, the first polyolefin resin is a first polypropylene resin, and the second polyolefin resin is a second polypropylene resin.

[0019] In some embodiments, the melt index of the first polypropylene resin is 0.7 g / 10 min to 0.9 g / 10 min, and / or the melt index of the second polypropylene resin is 0.15 g / 10 min to 0.35 g / 10 min.

[0020] In some embodiments, the melt index of the first polypropylene resin is 0.8 g / 10 min.

[0021] In some embodiments, the melt index of the second polypropylene resin is 0.25 g / 10 min.

[0022] In some embodiments, the mass ratio of the first polypropylene resin is 30% to 60%, and the mass ratio of the second polypropylene resin is 40% to 70%.

[0023] In some embodiments, in step S1, the mixed raw materials do not contain a crosslinking agent.

[0024] In some embodiments, in step S2, the temperature of the melt extruder is 190°C to 250°C, the temperature of the quench roll is 30°C to 100°C, and the cooling rate is ≥80°C / min.

[0025] In some embodiments, in step S3, the heat treatment annealing temperature is 140°C to 160°C, and the treatment time is 5 minutes to 30 minutes.

[0026] In some embodiments, in step S4, the cold stretching temperature is 60°C to 120°C, and the cold stretching ratio is 10% to 40%; the hot stretching temperature is 120°C to 150°C, and the hot stretching ratio is 100% to 200%.

[0027] In some embodiments, in step S5, the heat setting temperature is 150°C to 165°C, and the setting time is 5 minutes to 20 minutes.

[0028] In some embodiments, in step S5, the melt index of the diaphragm is 0.42 g / 10 min to 0.58 g / 10 min.

[0029] The ultrathin, high-strength separator provided by this invention comprises a first polyolefin resin and a second polyolefin resin, wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10 min, which is beneficial to improving the various test performances of the finished membrane. Furthermore, the preparation method of this invention, by precisely controlling the blending ratio of high melt index polyolefin resin and low melt index polyolefin resin, combined with an optimized dry uniaxial stretching process, successfully prepared a high-strength lithium battery separator with a thickness of 8 μm to 12 μm. The separator significantly outperforms existing technology products in terms of needle penetration strength, MD (medium density), and TD (diagonal tensile strength), and the process is stable and controllable, providing a key separator solution for next-generation high-safety lithium-ion batteries. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a flowchart illustrating the preparation method of an ultrathin, high-strength diaphragm provided in an embodiment of the present invention.

[0032] Figure 2A ,and Figure 2B This is a scanning electron microscope (SEM) image of the ultrathin, high-strength diaphragm provided in Embodiment 1 of the present invention.

[0033] The SEM-related conditions are as follows: Electron beam accelerating voltage 15.0 kV, Working distance 14.9 mm, Magnification 10,000 times ( Figure 2A ) and 20,000 times ( Figure 2B ), Detector: Secondary electron (SE) Scale: 5 µm Figure 2A ), 2 µm ( Figure 2B ).

[0034] Component labeling explanation: S110: Steps S120: Steps S130: Steps S140: Steps S150: Steps Detailed Implementation

[0035] To make the above and / or other objects, effects, and features of the present invention more apparent and understandable, preferred embodiments are described in detail below: In some embodiments, a method for preparing an ultrathin, high-strength diaphragm is provided, specifically involving a diaphragm with excellent needle-punching strength and longitudinal and transverse tensile strength prepared by a dry uniaxial stretching process using a blend system of high melt flow index polyolefin resin and low melt flow index polyolefin resin.

[0036] In some embodiments, the polyolefin resin includes polyethylene, polypropylene, or copolymers thereof or each other.

[0037] In some embodiments, the membrane fabrication process is mainly divided into two categories: dry process and wet process. Among them, the dry process includes melt extrusion, annealing and stretching to form pores. Due to its relatively simple process, low cost and no solvent pollution, it occupies an important position in the field of power batteries (such as lithium iron phosphate batteries) that pursue high safety.

[0038] In some embodiments, a method for preparing ultrathin, high-strength diaphragms using dry uniaxial stretching is employed. This method involves precisely proportioning two polypropylene (PP) resins with specific melt index (MFI) differences, utilizing their microstructure evolution during melt extrusion and stretching processes to form a fiber network possessing both strength and toughness. This improves the mechanical properties of the diaphragm without significantly increasing its thickness.

[0039] In some embodiments, the melt index of both the first polypropylene resin and the second polypropylene resin is below 1 g / 10 min. The resin with the lower melt index is referred to as a low melt index polypropylene resin, and the resin with the higher melt index is referred to as a high melt index polypropylene resin. In some embodiments, the melt index of the first polypropylene resin is any value or a range between any two of the following: 0.7 g / 10 min, 0.75 g / 10 min, 0.8 g / 10 min, 0.85 g / 10 min, and 0.9 g / 10 min, but is not limited thereto. The first polypropylene resin has good flowability and, as a continuous phase during die extrusion, helps to reduce melt viscosity and improve the stability of the molten plasticized mixture after die extrusion. The melt index of the second polypropylene resin is any value or a range between any two values ​​of 0.15 g / 10 min, 0.2 g / 10 min, 0.25 g / 10 min, and 0.35 g / 10 min, but is not limited to these values. This component has a high molecular weight and high melt strength. During the stretching process, it acts as a reinforcing phase, forming "tethered molecular chains" that connect the lamellar crystals and forming a denser microfiber network, which greatly improves the tensile strength of the diaphragm.

[0040] Please see Figure 1 This is a flowchart illustrating the preparation method of an ultrathin, high-strength membrane. (Step S110) Mix the first polyolefin resin and the second polyolefin resin to obtain a mixed raw material; wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10 min, and the melt index test condition is 230 °C. (Step S120) The mixed raw materials are melted and plasticized in a melt extruder, extruded through a die, and cooled by a quenching roller to form a cast sheet; (Step S130) The cast sheet is heat-treated and annealed; (Step S140) The annealed casting is subjected to uniaxial stretching to form a microporous film; uniaxial stretching includes low-temperature cold stretching and high-temperature hot stretching; and (Step S150) The stretched microporous membrane is heat-set and wound up to obtain a diaphragm with a thickness of 8μm~12μm.

[0041] In some embodiments, the first polyolefin resin is a first polypropylene resin, and the second polyolefin resin is a second polypropylene resin.

[0042] In some embodiments, the mass ratio of the first polypropylene resin (high melt index polypropylene resin) is, for example, but not limited to, any value or a range between any two values ​​of 30%, 35%, 40%, 45%, 50%, 55%, 60%; and the mass ratio of the second polypropylene resin (low melt index polypropylene resin) is, for example, but not limited to, any value or a range between any two values ​​of 40%, 45%, 50%, 55%, 60%, 65%, 70%.

[0043] In some embodiments, the preparation method of the ultrathin high-strength diaphragm includes the following steps, mainly steps one through five: Step 1: Raw material pretreatment and mixing High melt flow index (MFI) polypropylene resin (0.7 g / 10 min to 0.9 g / 10 min) and low melt flow index (MFI) polypropylene resin (0.15 g / 10 min to 0.35 g / 10 min) were mixed at specific weight percentages to obtain a mixed raw material. To achieve the best reinforcement effect and pore uniformity, the mass percentage of component A was 30% to 60%, and the mass percentage of component B was 40% to 70%. At this ratio, the high melt flow index component A forms the matrix, ensuring flowability; the low melt flow index component B is uniformly dispersed within it, forming a denser resin microfiber network, creating microscopic resin microfiber network structural reinforcement points.

[0044] In some embodiments, the first polypropylene resin is a homopolymer polypropylene resin, and / or the second polypropylene resin is a homopolymer polypropylene resin.

[0045] In some embodiments, the melt index of the first polypropylene resin is 0.8 g / 10 min.

[0046] In some embodiments, the melt index of the second polypropylene resin is 0.25 g / 10 min.

[0047] In some embodiments, the mass ratio of the first polypropylene resin to the second polypropylene resin in the mixed raw materials is any ratio or any two ratios within the range of 3:7 to 6:4.

[0048] In some embodiments, the mixed raw materials do not contain a crosslinking agent.

[0049] Step 2: Melt extrusion and casting The above-mentioned mixed raw materials are fed into a single-screw extruder for shear melting, with the extrusion temperature controlled between 190℃ and 250℃ to ensure complete plasticization and uniform mixing of the resin. After extrusion through the die, the melt is cast into sheets using a rapid cooling roller, with the cooling temperature controlled between 30℃ and 100℃ and the cooling rate not less than 80℃ / min. Rapid cooling helps to form a tightly packed lamellar structure arranged perpendicular to the extrusion direction (MD direction), which is the basis for subsequent stretching and pore formation.

[0050] In some embodiments, the extrusion temperature is controlled within the range of any two or more values ​​of 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, and 250°C, but is not limited thereto. The cooling temperature is controlled within the range of any two or more values ​​of 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, and 100°C, but is not limited thereto. The cooling rate is within the range of any two or more values ​​of 80°C / min, 81°C / min, 82°C / min, 83°C / min, 84°C / min, 85°C / min, 86°C / min, 87°C / min, 88°C / min, 89°C / min, and 90°C / min, but is not limited thereto.

[0051] Step 3: Heat treatment (annealing) The multilayer base film obtained by casting (with a total thickness of approximately 120μm-300μm) is subjected to online heat treatment (annealing). The annealing temperature is controlled between 140℃ and 160℃ (between the glass transition temperature and melting point of polypropylene), and the treatment time is 5 to 30 minutes. The purpose of annealing is to eliminate internal stress, improve the lamellar structure, thicken the lamellars, improve the regularity of the lamellars, and provide a uniform "hard and elastic" skeleton for subsequent stretching and pore formation.

[0052] In some embodiments, the annealing temperature is controlled within any value or a range between any two of 140°C, 145°C, 150°C, 155°C, and 160°C, but is not limited thereto. The annealing time is within any value or a range between any two of 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, and 30 minutes, but is not limited thereto.

[0053] Step 4: Unidirectional stretching to form a hole The heat-treated thick sheet (annealed casting) is subjected to uniaxial stretching. The stretching process consists of two stages: cold stretching and hot stretching. Cold stretching stage: At a temperature below the crystallization dispersion temperature of polypropylene resin (60℃~120℃), the cold stretching ratio is 10%~40%. This stage mainly causes the aforementioned lamellar crystals to deform and separate, forming silver streaks and micropores.

[0054] Hot stretching stage: A high-ratio stretching process (100%~200%) is performed at a temperature higher than the crystallization and dispersion temperature of polypropylene resin but lower than its melting point. This stage expands the micropores and draws out microfibers, forming a three-dimensionally interconnected porous structure. Due to the presence of a low melt index (high molecular weight) polypropylene resin component in the system, its long molecular chains are highly oriented during stretching, forming a strong microfiber framework, significantly enhancing the stability of the pore structure and the overall strength of the film.

[0055] In some embodiments, in the uniaxial stretching hole-forming step, a cold stretching stage is performed first, followed by a hot stretching stage.

[0056] In some embodiments, the cold drawing temperature is any value or a range between any two of 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, and 120°C, but is not limited thereto. The cold drawing ratio is any value or a range between any two of 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 25%, 30%, 35%, and 40%, but is not limited thereto.

[0057] In some embodiments, the hot stretching temperature is any value or a range between any two of 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, and 150°C, but is not limited thereto.

[0058] In some embodiments, the thermal stretching ratio is any value or a range between any two values ​​of 100%, 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190%, and 200%, but is not limited thereto.

[0059] Step 5: Heat setting and winding After stretching to form holes, the membrane is heat-set at a temperature higher than the hot stretching temperature but lower than the melting point (150℃~165℃) for 5 to 20 minutes to eliminate internal stress and reduce thermal shrinkage. Finally, through traction winding, layering, and slitting, an ultra-thin high-strength membrane with a thickness of 8μm~12μm is obtained.

[0060] In some embodiments, the heat setting temperature is any value or a range between any two of the following: 150°C, 151°C, 152°C, 153°C, 154°C, 155°C, 156°C, 157°C, 158°C, 159°C, 160°C, and 165°C, but is not limited thereto. The heat setting time is any value or a range between any two of the following: 5 minutes, 10 minutes, 15 minutes, and 20 minutes, but is not limited thereto.

[0061] In some embodiments, the melt index of the mixture of the first polypropylene resin and the second polypropylene resin is below 1 g / 10 min. In some embodiments, the melt index of the ultrathin high-strength diaphragm is any value or a range between any two values ​​selected from 0.42 g / 10 min, 0.43 g / 10 min, 0.44 g / 10 min, 0.45 g / 10 min, 0.46 g / 10 min, 0.47 g / 10 min, 0.48 g / 10 min, 0.49 g / 10 min, 0.50 g / 10 min, 0.51 g / 10 min, 0.52 g / 10 min, 0.53 g / 10 min, 0.54 g / 10 min, 0.55 g / 10 min, 0.56 g / 10 min, 0.57 g / 10 min, and 0.58 g / 10 min, but is not limited thereto. In some embodiments, the melt index of the diaphragm is calculated using the formula: M1 * mass ratio + M2 * mass ratio, where M1 is the melt index of the first polyolefin resin and M2 is the melt index of the second polyolefin resin. In some embodiments, the polyolefin resin is polypropylene.

[0062] Furthermore, in some embodiments, an ultrathin, high-strength separator is provided, comprising a first polyolefin resin and a second polyolefin resin, wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10 min, wherein the melt index is tested at 230°C. In some embodiments, the polyolefin resin comprises polyethylene, polypropylene, or copolymers thereof or together. In some embodiments, the polyolefin resin is polypropylene.

[0063] In some embodiments, an ultrathin high-strength diaphragm is provided, manufactured using the method for preparing an ultrathin high-strength diaphragm as described above. The thickness of the diaphragm is any value or a range between any two values ​​selected from 8 μm, 8.5 μm, 9 μm, 9.5 μm, 9.6 μm, 9.7 μm, 9.8 μm, 9.9 μm, 10 μm, 10.1 μm, 10.2 μm, 10.3 μm, 10.4 μm, 10.5 μm, 11 μm, 11.5 μm, and 12 μm, preferably 9 μm to 11 μm, but not limited thereto. Preferably, the thickness of the diaphragm is 10 μm, but not limited thereto.

[0064] In some embodiments, the diaphragm, at a thin thickness level (e.g., 8 μm to 12 μm), has at least one of the following performance characteristics: good mechanical strength, high needle penetration strength, good transverse (TD) tensile strength, good longitudinal (MD) tensile strength, porosity, or air permeability (Gurley value).

[0065] In some embodiments, when the thickness of the diaphragm is 8 μm to 12 μm, it has at least one of the following performance characteristics: Needle penetration intensity ≥280 gf; Longitudinal tensile strength (MD Tensile Strength) ≥260 MPa; Transverse tensile strength (TD Tensile Strength) ≥ 18 MPa; Porosity: 30%~40%; or Air permeability (Gurley value): 210s / 100mL~290s / 100mL.

[0066] In some embodiments, when the thickness of the ultrathin high-strength diaphragm is 8μm to 12μm, its needle penetration strength is any value or a range between any two of ≥280 gf, 285 gf, 290 gf, 295 gf, 300 gf, and 305 gf, but is not limited thereto.

[0067] In some embodiments, when the thickness of the ultrathin high-strength diaphragm is 8μm to 12μm, the longitudinal tensile strength is any value or a range between any two values ​​of ≥260MPa, 270 MPa, 280 MPa, 281 MPa, 282 MPa, 283MPa, 284 MPa, 285 MPa, 286 MPa, 287 MPa, 288 MPa, 289 MPa, 290 MPa, 291 MPa, 292 MPa, 293 MPa, 294 MPa, 295 MPa, 296 MPa, 297 MPa, 298 MPa, 299 MPa, and 300 MPa, but is not limited thereto.

[0068] In some embodiments, when the thickness of the ultrathin high-strength diaphragm is 8μm to 12μm, the transverse tensile strength is any value or a range between any two values ​​of ≥18MPa, 19MPa, 20MPa, 21MPa, 22MPa, 23MPa, 24MPa, 25MPa, but is not limited thereto.

[0069] In some embodiments, when the thickness of the ultrathin high-strength diaphragm is 8μm to 12μm, the porosity of the diaphragm is any value or a range between any two values ​​of 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, and 40%, but is not limited thereto.

[0070] In some embodiments, when the thickness of the ultrathin high-strength diaphragm is 8μm to 12μm, the air permeability of the diaphragm is any value or a range between any two of the following: 210s / 100mL, 220s / 100mL, 230s / 100mL, 240s / 100mL, 250s / 100mL, 260s / 100mL, 270s / 100mL, 280s / 100mL, and 290s / 100mL, but is not limited thereto.

[0071] Additionally, in some embodiments, a lithium-ion battery is provided, including the ultrathin, high-strength separator as described above.

[0072] In some embodiments, ultrathin high-strength separators are primarily suitable for power lithium-ion batteries and consumer electronics lithium batteries that require high energy density and high safety.

[0073] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0074] The above-described embodiments of the present invention are illustrated by the following examples: Test method: 1. Melt index measurement According to the national standard measurement method: GB / T3682.1-2018, the test conditions for dry-process diaphragm raw materials are 230℃, and after adding the raw materials, a 2.16kg weight is applied to compact and degas the material.

[0075] 2. Thickness Measurement The thickness of the diaphragm was measured using a Marl thickness gauge. For the same sample, the thickness of the diaphragm was measured at three random locations, and the average of the three thicknesses was taken.

[0076] 3. Needle puncture intensity measurement Using a universal testing machine with a needle tip diameter of 1.0 mm and a puncture speed of 100 mm / min, clamp the diaphragm and record the maximum puncture force.

[0077] 4. Tensile strength measurement According to ASTM D882 standard, the specimen width is 15 mm, the tensile speed is 50 mm / min, and the longitudinal (MD) and transverse (TD) fracture strength are tested respectively.

[0078] 5. Porosity Measurement According to the provisions of GB / T 6673-2001 and GB / T 6672-2001, measure the length, width and thickness of the sample. Weigh the sample using an analytical balance, wind it up and place it in a sample cup for true density testing. Finally, calculate the apparent volume, true volume and porosity of the membrane according to the calculation formulas in the aforementioned standard measurement method.

[0079] 6. Air permeability measurement The test was conducted using a Wang Yan-style air permeability meter, according to the national standard GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries". The test principle is as follows: under a constant pressure difference of 1.21 kPa, the time required for 100 mL of air to permeate through a fixed area separator is recorded (Gerley method).

[0080] The formula for calculating the melt flow index of the finished diaphragm is: M1 * mass ratio + M2 * mass ratio.

[0081] Table 1. Diaphragm characteristics and preparation process of the examples and comparative examples

[0082] PP is polypropylene resin.

[0083] A: B represents high melt index polypropylene resin and low melt index polypropylene resin.

[0084] Table 2. Measurement Results

[0085] Film formation stability is evaluated by assessing the "membrane surface appearance", where "excellent" means that the finished membrane surface has no defects.

[0086] Example 1

[0087] The preparation method of an ultrathin, high-strength membrane includes the following steps: Step 1: Mixing raw materials Weigh 400 kg of homopolymer polypropylene (H-PP) granules with a melt index of 0.8 g / 10 min (as component A) and 600 kg of homopolymer polypropylene granules with a melt index of 0.25 g / 10 min (as component B). Mix the above materials in batches in a high-speed mixer for 5 minutes to ensure uniform dispersion.

[0088] Step 2: Melt extrusion and casting The mixed materials are fed into a single-screw extruder equipped with a metering pump. The extrusion temperature is slightly increased to accommodate the higher content of low melt index components. The temperatures of each section of the extruder are set as follows: Zone 1 200℃, Zone 2 230℃, Zone 3 235℃, and the die head 235℃. After the melt is extruded through the T-die, it immediately comes into contact with a quenching roller at a temperature of 75℃ and is cooled at a cooling rate of 80℃ / min to enhance the quenching effect and form denser lamellar crystals.

[0089] Step 3: Heat treatment The cast sheet was heat-treated, with an annealing temperature of 152℃ and a time of 10 minutes.

[0090] Step 4: Unidirectional stretching to form a hole The heat-treated thick sheet (annealed casting) was subjected to uniaxial stretching. The initial cold stretching temperature was 70℃, with a cold stretching ratio of 18%; the subsequent hot stretching temperature was 138℃, with a hot stretching ratio of 160%.

[0091] Step 5: Heat setting and winding After stretching and forming holes, the membrane undergoes heat setting at 157℃ for 8 minutes. Finally, through traction winding, layering, and slitting, a finished separator with a thickness of 10.0 μm is obtained. The remaining performance test results of the ultra-thin high-strength separator are shown in Table 2. Air permeability is slightly increased, and mechanical strength reaches a higher level, making it particularly suitable for power battery scenarios with stringent safety requirements.

[0092] Figure 2A ,and Figure 2B The image shows a scanning electron microscope (SEM) image of the aforementioned ultrathin, high-strength diaphragm. Its microstructure is characterized by the following features: the diaphragm consists of highly oriented microfibers along the tensile direction (MD) and nodules connecting the microfibers, with slit-like micropores. Due to the presence of low melt flow index components, the microfiber network is more robust and the connections are more secure, giving the diaphragm extremely high mechanical properties.

[0093] In addition, the aforementioned ultrathin high-strength separator was used to prepare lithium batteries.

[0094] Example 2

[0095] The preparation method of an ultrathin, high-strength membrane includes the following steps: Step 1: Mixing raw materials Weigh 500 kg of homopolymer polypropylene granules with a melt index of 0.8 g / 10 min (as component A) and 500 kg of homopolymer polypropylene granules with a melt index of 0.25 g / 10 min (as component B). Mix the above materials in a high-speed mixer for 5 minutes to ensure uniform dispersion.

[0096] Step 2: Melt extrusion and casting The mixed materials are fed into a single-screw extruder equipped with a metering pump. The temperatures of each section of the extruder are set as follows: Zone 1 190℃, Zone 2 220℃, Zone 3 225℃, and the die head 225℃. After the melt is extruded through the T-die, it immediately comes into contact with a quenching roller at a temperature of 85℃ and is cooled at a cooling rate of 80℃ / min to form a cast film with a thickness of 11.5μm.

[0097] Step 3: Heat treatment The multilayer cast film is simultaneously stretched and unwound in one direction and fed into an online heat treatment oven. It is then treated online at an annealing temperature of 150℃ for about 10 minutes to perfect the lamellar structure.

[0098] Step 4: Unidirectional stretching to form a hole After heat treatment, the multilayer base film continues to enter the longitudinal stretching production line.

[0099] First, cold stretching is performed at 70℃ to form micropores, with a cold stretching ratio of 15%. Then, it enters the hot stretching zone at 135℃ and is stretched with a hot stretching ratio of 140%, during which micropores are formed and expanded.

[0100] Step 5: Heat setting and winding After being stretched and perforated, the film enters the heat-setting zone and is held at 155°C for approximately 8 minutes to eliminate internal stress. Finally, through traction winding, layering, and slitting, a finished separator with a thickness of 10.1 μm is obtained. The remaining performance test results of the ultrathin high-strength separator are shown in Table 2.

[0101] Comparative Example 1 The preparation method is roughly the same as in Example 2, except that the raw material mass ratio is 1,000 kg of 100 wt% homopolymer polypropylene resin (2 g / 10 min) particles as raw material.

[0102] Comparative Example 2 Step 1: Raw material preparation The raw material was 1,000 kg of homopolymer polypropylene resin (0.25 g / 10 min) particles with a mass ratio of 100 wt%.

[0103] Step 2: Melt extrusion and casting The raw materials were fed into a single-screw extruder equipped with a metering pump. The temperatures of each section of the extruder were set as follows: Zone 1 220℃, Zone 2 230℃, Zone 3 235℃, and the die head 235℃. After the melt was extruded through the T-die, it immediately came into contact with a quenching roller at a temperature of 85℃ and was cooled and shaped at a cooling rate of 80℃ / min. The resulting cast film had uneven thickness and severe surface patterns, making it impossible to obtain a cast film with an thickness of 11.5μm. Therefore, the preparation steps were recorded up to this stage.

[0104] Comparative Example 3 The preparation method is largely the same as in Example 2, except that 500 kg of homopolymer polypropylene (H-PP) granules with a melt index of 1.2 g / 10 min and 500 kg of homopolymer polypropylene granules with a melt index of 2.0 g / 10 min are used as raw materials and in the following mass ratios. The above materials are mixed in a high-speed mixer for 5 minutes to ensure uniform dispersion.

[0105] As can be seen from the above examples, by adjusting the raw material ratio, this invention can stably produce ultra-thin diaphragms with a thickness of 8μm~12μm without adding extra coatings or changing the main structure of existing dry production lines, and significantly improve their mechanical properties, especially the simultaneous improvement of needle punching strength and TD tensile strength. It solves the safety problems caused by the ultra-thinning of diaphragms and has extremely high industrial promotion value.

[0106] In summary, this invention successfully prepared a high-strength lithium-ion battery separator with a thickness of 8μm to 12μm by precisely controlling the blending ratio of high melt flow index polypropylene resin (MFI 0.7g / 10min~0.9g / 10min) and low melt flow index polypropylene resin (MFI 0.15g / 10min~0.35g / 10min) and combining it with an optimized dry uniaxial stretching process. This separator significantly outperforms existing products in terms of needle penetration strength, molecular weight (MD), and tensile strength (TD), and the process is stable and controllable, making it advantageous for application in power lithium-ion batteries and consumer electronics lithium batteries.

[0107] The above content involving common knowledge will not be described in detail, as those skilled in the art will understand.

[0108] The embodiments described above are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. An ultra-thin, high-strength diaphragm, characterized in that, The diaphragm comprises a first polyolefin resin and a second polyolefin resin, wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10min.

2. The ultra-thin high-strength diaphragm according to claim 1, characterized in that, The polyolefin resin includes polyethylene, polypropylene, or copolymers thereof or each other.

3. The ultra-thin high-strength diaphragm according to claim 1, characterized in that, When the thickness of the diaphragm is 8μm to 12μm, it has at least one of the following performance characteristics: Needle penetration intensity ≥280 gf; Longitudinal tensile strength (MD Tensile Strength) ≥260 MPa; Transverse tensile strength (TD Tensile Strength) ≥ 18 MPa; Porosity: 30%~40%; or Air permeability (Gurley value): 210s / 100mL~290s / 100mL.

4. The ultra-thin high-strength diaphragm according to claim 1, characterized in that, The thickness of the diaphragm is 9μm~11μm.

5. The ultra-thin high-strength diaphragm according to claim 1, characterized in that, The thickness of the diaphragm is 10 μm.

6. The ultra-thin high-strength diaphragm according to claim 1, characterized in that, The melt flow index of the diaphragm is 0.42 g / 10 min to 0.58 g / 10 min.

7. A lithium-ion battery comprising an ultrathin, high-strength separator as described in any one of claims 1 to 6.

8. A method for preparing an ultrathin, high-strength diaphragm, characterized in that, Includes the following steps: S1: Mix the first polyolefin resin and the second polyolefin resin to obtain a mixed raw material; wherein the melt index M1 of the first polyolefin resin is greater than the melt index M2 of the second polyolefin resin, and both M1 and M2 are less than 1.0 g / 10min, and the test condition for the melt index is 230℃. S2: The mixed raw materials are melted and plasticized in a melt extruder, extruded through a die, and cooled by a quenching roller to form a cast sheet; S3: The cast sheet is subjected to heat treatment annealing; S4: The annealed casting is subjected to uniaxial stretching to form a microporous membrane; the uniaxial stretching includes low-temperature cold stretching and high-temperature hot stretching. and S5: The stretched microporous membrane is heat-set and wound up to obtain a diaphragm with a thickness of 8μm~12μm.

9. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, The polyolefin resin includes polyethylene, polypropylene, or copolymers thereof or each other.

10. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, The first polyolefin resin is a first polypropylene resin, and the second polyolefin resin is a second polypropylene resin.

11. The method for preparing the ultrathin high-strength diaphragm according to claim 10, characterized in that, The melt index of the first polypropylene resin is 0.7 g / 10 min to 0.9 g / 10 min, and / or the melt index of the second polypropylene resin is 0.15 g / 10 min to 0.35 g / 10 min.

12. The method for preparing the ultrathin high-strength diaphragm according to claim 10, characterized in that, The melt flow index of the first polypropylene resin is 0.8 g / 10 min.

13. The method for preparing the ultrathin high-strength diaphragm according to claim 10, characterized in that, The melt index of the second polypropylene resin is 0.25 g / 10 min.

14. The method for preparing an ultrathin high-strength diaphragm according to claim 10, characterized in that, The mass ratio of the first polypropylene resin is 30% to 60%, and the mass ratio of the second polypropylene resin is 40% to 70%.

15. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, The mixed raw materials do not contain crosslinking agents.

16. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, In step S2, the temperature of the melt extruder is 190℃~250℃, the temperature of the quench roll is 30℃~100℃, and the cooling rate is ≥80℃ / min.

17. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, In step S3, the heat treatment annealing temperature is 140℃~160℃, and the treatment time is 5 minutes~30 minutes.

18. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, In step S4, the cold stretching temperature is 60℃~120℃, and the cold stretching ratio is 10%~40%; the hot stretching temperature is 120℃~150℃, and the hot stretching ratio is 100%~200%.

19. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, In step S5, the heat setting temperature is 150℃~165℃, and the setting time is 5 minutes~20 minutes.

20. The method for preparing an ultrathin high-strength diaphragm according to claim 8, characterized in that, In step S5, the melt index of the diaphragm is 0.42 g / 10 min to 0.58 g / 10 min.

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