High-performance thin film for composite current collector as well as preparation method and application of high-performance thin film

Abstract

The invention discloses a high-performance thin film for a composite current collector and a preparation method and application of the high-performance thin film. The high-performance thin film for the composite current collector sequentially comprises a first surface layer, a matrix layer and a second surface layer from outside to inside, the preparation method comprises the following steps: pretreating raw materials in a high-temperature vacuum environment; the preparation method comprises the following steps: uniformly mixing the raw materials in proportion at a high speed, and carrying out melt blending, extrusion, cooling and grain-sized dicing through a double-screw extruder to obtain master batches; the first surface layer master batch, the base body master batch and the second surface layer master batch are added into three independent single-screw extruders respectively, and co-extrusion is conducted through a three-layer co-extrusion die head with an ABA runner structure; drafting and cooling the melt to form a primary film; and the three-layer co-extrusion composite film is formed through two-way stretching / heat setting and rolling. Maleic anhydride is mixed in the outer surface layer of the film, so that the binding force of the film and metal can be improved, polyethylene and polypropylene are mixed in the matrix layer, the tensile strength is improved, and the whole film has the advantages of high tensile strength, high adhesive force and high heat resistance.

Description

Technical Field

[0001] This invention relates to the field of composite current collector technology, and in particular to a high-performance thin film for composite current collectors, its preparation method, and its application. Background Technology

[0002] As a key auxiliary material in lithium-ion batteries, the current collector's main function is to conduct electricity. It serves as both a carrier for the positive and negative electrode active materials and a collector and conductor for electrons in both electrodes. Its role is to collect the current generated by the battery's active materials to produce a larger output current. An ideal current collector needs to possess a combination of properties, including high conductivity, good stability, good mechanical strength, and low cost.

[0003] With the development needs of the battery industry, the demands for weight reduction and improved safety are increasing daily. Composite current collectors have become one of the most promising new lithium battery current collector materials. Structurally, composite current collectors resemble a "sandwich," with a polymer layer in the middle and metal coatings on the top and bottom. Polyethylene terephthalate (PET) has advantages such as high tensile strength and good adhesion to metals, but disadvantages such as poor corrosion resistance and high thermal shrinkage. Polyimide has advantages such as excellent mechanical and electrical properties, but disadvantages such as high cost and complex processing. Polypropylene has advantages such as high temperature resistance and good rigidity, but its low-temperature brittleness and low toughness limit its application in certain fields. Polyethylene has excellent toughness and impact resistance, but poor heat resistance. Blending polyethylene and polypropylene can achieve complementary properties to some extent, but simple physical blending has problems such as poor compatibility and low interfacial strength, resulting in poor film mechanical properties. In summary, in the application scenarios of composite current collectors, in order to impart specific surface properties to the thin film, post-treatment of the thin film surface is usually required. These methods have problems such as short-lasting effects, complex processes, and potential pollution or damage to the substrate.

[0004] Therefore, developing a multilayer composite film that combines high strength, high toughness, high temperature resistance, and a surface with permanent highly active functional groups is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention proposes a high-performance thin film material for composite current collectors, its preparation method and application, which synergistically enhances mechanical properties, excellent interfacial bonding and surface properties, improves mechanical properties and ensures thermal stability.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A high-performance thin film for composite current collectors includes, from the outside to the inside, a first surface layer, a substrate layer, and a second surface layer; The first and second surface layers are composed of a maleic anhydride-grafted modified polyolefin composition. The matrix layer is composed of a composite composition of polyethylene, polypropylene and inorganic fillers; The total thickness of the high-performance film is 2~8 μm, the thickness of the first and second surface layers is 0.2 μm~1 μm, and the thickness of the substrate layer is 1.6~6 μm.

[0007] Further technology of the present invention: Preferably, the maleic anhydride-grafted modified polyolefin composition has a grafting rate of 0.5% to 2.0%.

[0008] Preferably, the composite composition of the matrix layer comprises, by weight: 40-70 parts of polypropylene, 20-40 parts of polyethylene, 5-20 parts of inorganic filler, and 1-5 parts of compatibilizer.

[0009] Preferably, the inorganic filler is one or more of nano- or micro-sized talc, silica, calcium carbonate, barium sulfate, or boehmite, and the average particle size of the inorganic filler is 0.5~5 μm.

[0010] Preferably, the polyethylene is high-density polyethylene or linear low-density polyethylene.

[0011] Preferably, the polypropylene is homopolymer polypropylene or copolymer polypropylene.

[0012] Preferably, the compatibilizer is an ethylene-vinyl acetate copolymer or a styrene-ethylene-butene-styrene block copolymer.

[0013] This invention also provides a method for preparing a high-performance thin film for composite current collectors, comprising the following steps: Step 1. Raw material pretreatment and drying: Vacuum dry each raw material used for the first surface layer, the second surface layer and the substrate layer at 80-100℃ for 4-6 hours respectively; Step 2. Material mixing and granulation: The matrix layer raw materials are mixed evenly at high speed in a certain proportion, and then melt-blended, extruded, cooled, and pelletized by a twin-screw extruder to obtain the B-layer composite masterbatch; Mix the first and second surface layer materials evenly, or use commercially available maleic anhydride-grafted polyolefin granules directly. Step 3. Three-layer co-extrusion molding: The first and second surface layer masterbatches and the matrix layer masterbatch are respectively added to three independent single-screw extruders and co-extruded through a three-layer co-extrusion die with an ABA flow channel structure; Step 4. Melt stretching and cooling: The extruded molten film is stretched at high speed and cooled and shaped through a narrow air gap to form a nascent film; Step 5. Biaxial stretching: The nascent film is fed into a biaxial stretching machine and subjected to simultaneous or stepwise biaxial stretching at a temperature higher than the glass transition temperature of polypropylene but lower than its melting point, with a stretching ratio of 3*3 to 9*9. Step 6. Heat setting and winding: The stretched film is heat set at 140-160℃, then cooled, trimmed, wound, and cured. The curing conditions are 35℃±5℃ / 72h to obtain a high-performance composite current collector film of three layers co-extruded.

[0014] The present invention also provides an application of the high-performance film for composite current collectors as claimed in the present invention, wherein the high-performance film for composite current collectors is a three-layer co-extruded composite current collector as the intermediate layer, and a metal layer is provided on each side of the intermediate layer.

[0015] The metal layers on both sides are either copper or aluminum.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: The high-performance composite current collector film provided by this invention improves the adhesion between the film and the metal by mixing maleic anhydride grafts in the outer layer of the film. In addition, the addition of inorganic materials by mixing polyethylene and polypropylene in the matrix layer can improve the thermal stability and overall tensile strength of the film. The thickness of the first and second surface layers is controlled within 25%, which ensures surface functionalization and maximizes the use of low-cost, high-strength matrix core materials, achieving the best balance between cost and performance.

[0017] 1. Synergistically enhanced mechanical properties: The polyethylene / polypropylene blend system in the matrix layers achieves a tensile strength of up to 400-600 MPa through the reinforcement of inorganic fillers and biaxial stretching orientation; at the same time, the introduction of polyethylene ensures an excellent elongation of 50-120% for the film, avoiding the brittleness of pure polypropylene film.

[0018] 2. Excellent interfacial adhesion and surface properties: The anhydride groups of maleic anhydride and other compounds in the first and second surface layers can chemically react with the polyolefin molecular chains in the matrix layer (especially in the presence of a compatibilizer) during co-extrusion, forming a strong interfacial bond and preventing interlayer delamination. Simultaneously, the exposed maleic anhydride functional groups in the first and second surface layers provide the film with a permanently high surface energy, allowing it to be directly used for printing, coating, or laminating with other materials without post-treatment.

[0019] 3. Outstanding heat resistance: Using highly heat-resistant polypropylene as the continuous phase and adding heat-resistant inorganic fillers, followed by a heat setting process, the heat distortion temperature of the film is stably above 140℃, meeting the requirements of high-temperature application scenarios.

[0020] 4. Ingenious combination of structural design and process: The thickness of the first and second surface layers is controlled within 25%, ensuring surface functionalization while maximizing the use of low-cost, high-strength matrix materials, achieving the best balance between cost and performance. The biaxial stretching process greatly improves the orientation of molecular chains, which is key to obtaining high strength, high modulus, and dimensional stability. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of a high-performance thin film for composite current collectors provided by the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] like Figure 1 The present invention provides a high-performance thin film for composite current collectors, comprising, from the outside to the inside, a first surface layer 1, a substrate layer 2, and a second surface layer 3.

[0025] Example 1: The first surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 3.0%. The matrix layer consists of 55 parts homopolymer polypropylene, 25 parts high-density polyethylene (HDPE), 15 parts talc with an average particle size of 1 μm, and 5 parts styrene-ethylene-butene-styrene block copolymer (SEBS) compatibilizer. The second surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 3.0%. The total thickness of the film is 4 μm, the thickness of the first and second surface layers is 0.4 μm, and the thickness of the substrate layer is 3.2 μm. Its preparation method: Step 1. Raw material pretreatment and drying: The raw materials used for the first surface layer, the second surface layer and the substrate layer are vacuum dried at 90°C for 5 hours.

[0026] Step 2. Material mixing and granulation: • The matrix layer raw materials are mixed evenly at high speed in a certain proportion, and then melt-blended, extruded, cooled and pelletized by a twin-screw extruder to obtain the B-layer composite masterbatch.

[0027] The first and second surface layers use maleic anhydride-grafted polyolefin granules.

[0028] Step 3. Three-layer co-extrusion molding: The first and second surface layer masterbatches and the matrix layer masterbatch are added to three independent single-screw extruders and co-extruded through a three-layer co-extrusion die with an ABA flow channel structure.

[0029] Step 4. Melt stretching and cooling: The extruded molten film is stretched and cooled at high speed through a narrow air gap to form a nascent film.

[0030] Step 5. Biaxial stretching: The nascent film is fed into a biaxial stretching machine and subjected to synchronous or step-by-step biaxial stretching at 125°C, with a stretching ratio of 6*8.

[0031] Step 6. Heat setting and winding: The stretched film is heat set at 150°C, then cooled, trimmed, wound, and cured (curing conditions 35°C±5°C / 72h) to obtain the three-layer co-extruded high-performance composite film.

[0032] Example 2: The first surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 3.0%. The matrix layer consists of 60 parts of copolymer polypropylene, 25 parts of linear low-density polyethylene (LLDPE), 10 parts of nano-silica, and 3 parts of ethylene-vinyl acetate copolymer (EVA) compatibilizer. The second surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 3.0%. The total thickness of the film is 4 μm, the thickness of the first and second surface layers is 0.4 μm, and the thickness of the substrate layer is 3.2 μm. The preparation method is the same as in Example 1.

[0033] Example 3: The first surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH) with a grafting rate of 5%. The matrix layer consists of 55 parts homopolymer polypropylene, 25 parts high-density polyethylene (HDPE), 15 parts talc with an average particle size of 1 μm, and 5 parts styrene-ethylene-butene-styrene block copolymer (SEBS) compatibilizer. The second surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 5%. The total thickness of the film is 6 μm, the thickness of the first and second surface layers is 1 μm, and the thickness of the substrate layer is 4 μm. The preparation method is the same as in Example 1.

[0034] Example 4: The first surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH) with a grafting rate of 1.5%. The matrix layer consists of 60 parts of copolymer polypropylene, 25 parts of linear low-density polyethylene (LLDPE), 10 parts of nano-silica, and 3 parts of ethylene-vinyl acetate copolymer (EVA) compatibilizer. The second surface layer is composed of polypropylene grafted with maleic anhydride (PP-g-MAH), with a grafting rate of 1.5%. The total thickness of the film is 6 μm, the thickness of the first and second surface layers is 1 μm, and the thickness of the substrate layer is 4 μm. The preparation method is the same as in Example 1.

[0035] Comparative Example 1: The comparison film is a biaxially oriented polypropylene film with a thickness of 4 μm. Its preparation method: Melt extrusion: Polypropylene resin is mixed with additives and then heated and melted through an extruder to form a continuous film-like material; Longitudinal stretching: The molten film is stretched in the vertical direction (longitudinal direction) through a roller system, and the stretching ratio is usually 4 to 5.5 times; Lateral stretching: The film after longitudinal stretching is stretched in a special stretching machine in the horizontal direction (lateral direction), and the stretching ratio is generally 7 to 9 times; Post-processing: The stretched film needs to be cooled, heat-treated or specially processed (such as corona treatment, coating, etc.), trimmed, and wound up.

[0036] Comparative Example 2: The comparative film is a biaxially oriented polypropylene film with a thickness of 6 μm. Its preparation method is the same as that of Comparative Example 1.

[0037] Comparative Example 3: The comparative film is a biaxially oriented polyethylene terephthalate film with a thickness of 4 μm; Its preparation method: Pre-crystallization and drying: Polyethylene terephthalate is highly hygroscopic and must be pre-crystallized before drying to prevent hydrolysis and air bubbles during melt extrusion. Typical conditions: temperature 150–170℃, time 3.5–4h; moisture content after drying should be controlled to ≤50ppm (30–50ppm recommended for high-end applications). Melt extrusion and flake casting: The dried chips are fed into a single-screw extruder (Barrier type screw is preferred to ensure plasticization and melt uniformity, or exhaust-type twin screw can be used to enhance dehumidification), and are formed into thick sheets through melt metering pump, melt filter, static mixer and T-die; Thick sheets are rapidly cooled on quenching rollers to form amorphous castings, which inhibits crystallization and provides a good foundation for subsequent stretching; the cooling rate and temperature directly affect transparency and crystallinity.

[0038] Biaxially oriented (MDO+TDO) Longitudinal stretching (MDO): Preheat to approximately 86–87°C, stretching ratio approximately 3×, to improve orientation and crystallinity.

[0039] Transverse stretching (TDO): Preheating 98–100℃, stretching 100–120℃, stretching ratio 2.5–4.0×; biaxial stretching enables the film to obtain high strength and flatness in both longitudinal and transverse directions.

[0040] 4. Post-processing: Heat setting under tension to eliminate internal stress and improve dimensional stability; typical heat setting temperature is approximately 230–240℃, followed by cooling to the winding temperature; then winding, slitting, inspection, and packaging; ultra-thin specifications have higher requirements for tension, guide roller cleanliness, and static electricity control to prevent wrinkling, film breakage, and adhesion. Comparative Example 4: The comparative film is a 6µm thick biaxially oriented polyethylene terephthalate film. Its preparation method is the same as that of Comparative Example 3.

[0041] Detection and analysis: The films prepared in Examples 1 to 4, and the films prepared in Comparative Examples 1 to 4, were subjected to performance tests. 1. Thin film thickness testing: The specific operation involved using a Mahr high-precision thin film thickness gauge (model: Digi3Plus) to measure the film thickness. The sampling size was 100mm x 100mm, and a total of 10 values ​​were measured. The average value of the results was taken. The test results are shown in Table 1 below.

[0042] 2. Film tensile strength and elongation test: The specific operation is as follows: The tensile strength and elongation are tested using a Labthink tensile puncture machine (model: C610H). The sample size is 15mm*200mm. The prepared sample is placed in the test area of ​​the tensile machine. The speed is set to 50mm / min and the running distance is 100mm. The maximum value is taken. The test results of tensile strength and elongation are shown in Table 1.

[0043] 3. Heat shrinkage test: The specific operation is as follows: the sample size is 100mm*100mm, and the baking test is carried out using an oven (NMT-1001) at 120℃ for 15 minutes. The heat shrinkage test results are shown in Table 1.

[0044] Table 1 shows the test results of the original film's related performance. Tensile strength and elongation tests of composite copper foil: The specific operation is as follows: The tensile strength and elongation are tested using a Labthink tensile puncture machine (model: C610H). The sample size is 15mm*200mm. The prepared sample is placed in the test area of ​​the tensile machine. The speed is set to 50mm / min and the running distance is 100mm. The maximum value is taken. The test results of tensile strength and elongation are shown in Table 2.

[0045] 5. Composite copper foil peel strength test: The specific operation was as follows: A Labthink tensile puncture machine (model: C610H) was used to test the peel force between the adhesive layer and the metal layer. The sample size was 15mm*200mm. 3M-9448A double-sided tape was attached to a 30mm*50mm steel plate. One end of the sample to be tested was then evenly attached to the double-sided tape. A 2kg standard roller was used to roll back and forth 3 times. The prepared sample was then placed on the tensile testing machine and peeled at 180°. The speed was set to 50mm / min and the running distance was 100mm. The maximum value was taken as the result. The peel force test results are shown in Table 2.

[0046] Table 2 shows the test results of the composite current collector. As shown in Table 1, the films prepared using the existing technology have significant advantages in tensile strength and thermal stability. As shown in Table 2, compared with Comparative Examples 1-4, the films prepared in Examples 1-4 have strong bonding with metal and superior mechanical properties, making them particularly suitable for use in composite current collectors. The high-performance thin film for composite current collectors prepared by this invention can be applied to the preparation of composite current collectors. The thin film prepared by this invention can be used as the middle layer, with a metal layer on each side, which can be one or a combination of copper and aluminum.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-performance thin film for composite current collectors, characterized in that, It consists of, from the outside in, the first surface layer, the base layer, and the second surface layer; The first and second surface layers are composed of a maleic anhydride-grafted modified polyolefin composition. The matrix layer is composed of a composite composition of polyethylene, polypropylene and inorganic fillers; The total thickness of the high-performance film is 2~8 μm, the thickness of the first and second surface layers is 0.2 μm~1 μm, and the thickness of the substrate layer is 1.6~6 μm.

2. The high-performance thin film for composite current collectors according to claim 1, characterized in that, The maleic anhydride-grafted modified polyolefin composition has a grafting rate of 0.5% to 2.0%.

3. The high-performance thin film for composite current collectors according to claim 1, characterized in that, The composite composition of the matrix layer comprises, by weight: 40-70 parts of polypropylene, 20-40 parts of polyethylene, 5-20 parts of inorganic filler, and 1-5 parts of compatibilizer.

4. The high-performance thin film for composite current collectors according to claim 3, characterized in that, The inorganic filler is one or more of nano- or micro-sized talc, silica, calcium carbonate, barium sulfate, or boehmite, and the average particle size of the inorganic filler is 0.5~5 μm.

5. A high-performance thin film for composite current collectors according to claim 3, characterized in that, The polyethylene is high-density polyethylene or linear low-density polyethylene.

6. The high-performance thin film for composite current collectors according to claim 3, characterized in that, The polypropylene is either homopolymer polypropylene or copolymer polypropylene.

7. The high-performance thin film for composite current collectors according to claim 3, characterized in that, The compatibilizer is an ethylene-vinyl acetate copolymer or a styrene-ethylene-butene-styrene block copolymer.

8. A method for preparing a high-performance thin film for composite current collectors as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1. Raw material pretreatment and drying: Vacuum dry each raw material used for the first surface layer, the second surface layer and the substrate layer at 80-100℃ for 4-6 hours respectively; Step 2. Material mixing and granulation: The matrix layer raw materials are mixed evenly at high speed in a certain proportion, and then melt-blended, extruded, cooled, and pelletized by a twin-screw extruder to obtain the B-layer composite masterbatch; Mix the first and second surface layer materials evenly, or use commercially available maleic anhydride-grafted polyolefin granules directly. Step 3. Three-layer co-extrusion molding: The first and second surface layer masterbatches and the matrix layer masterbatch are respectively added to three independent single-screw extruders and co-extruded through a three-layer co-extrusion die with an ABA flow channel structure; Step 4. Melt stretching and cooling: The extruded molten film is stretched at high speed and cooled and shaped through a narrow air gap to form a nascent film; Step 5. Biaxial stretching: The nascent film is fed into a biaxial stretching machine and subjected to simultaneous or stepwise biaxial stretching at a temperature higher than the glass transition temperature of polypropylene but lower than its melting point, with a stretching ratio of 3*3 to 9*9. Step 6. Heat setting and winding: The stretched film is heat set at 140-160℃, then cooled, trimmed, wound, and cured. The curing conditions are 35℃±5℃ / 72h to obtain a high-performance composite current collector film of three layers co-extruded.

9. The application of a high-performance thin film for composite current collectors as described in any one of claims 1-7 in composite current collectors, characterized in that, The composite current collector uses a high-performance thin film co-extruded in three layers as the intermediate layer, with a metal layer on each side of the intermediate layer.

10. The application of the high-performance thin film for composite current collectors according to claim 9 in composite current collectors, characterized in that, The metal layers on both sides are either copper or aluminum.

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