A base film for a composite current collector, a method for preparing the same, and an application thereof

Abstract

The application discloses a base film for a composite current collector and a preparation method and application thereof, and belongs to the technical field of film materials. The base film is periodically arranged with a plurality of repeating units along the thickness direction, and a single repeating unit sequentially comprises a first thermoplastic layer, an intermediate functional layer and a second thermoplastic layer, and the base film is periodically arranged with the repeating units along the thickness direction; wherein the intermediate functional layer is prepared by compounding dopamine modified nanomaterials and a base resin into a functional resin. Through the structural design of micro-nano layering and functional layer intercalation, the synergistic layer multiplication design and the secondary stretching process, not only the efficient conversion and multi-dimensional directional strengthening of the performance of the nanomaterials are realized, but also the technical problems of the traditional composite material, such as large brittleness, easy warping and poor size stability, are solved simultaneously by means of the built-in toughening mechanism of the high-density alternating interface and the stress rebalancing effect of the symmetrical structure, so that the base film has excellent mechanical reliability, thermal size stability and processing adaptability.

Description

Technical Field

[0001] This invention relates to the field of thin film materials technology, and more specifically, to a base film for composite current collectors, its preparation method, and its application. Background Technology

[0002] Composite current collectors, as a novel battery current collector material, offer an important pathway to achieving battery lightweighting and improved safety through their "metal-polymer-metal" sandwich structure design (typically using a PET or PP polymer film as the intermediate base layer, with aluminum or copper conductive layers deposited on both sides). However, the widely used PP and PET base film materials have significant limitations in mechanical and thermal properties, becoming a key bottleneck restricting the engineering application of this technology. On the one hand, these polymer base films have low rigidity, making them prone to wrinkles, creases, and other appearance defects during multiple roll-to-roll deposition processes due to tension and mechanical stress, significantly reducing product yield. Simultaneously, the significant difference in thermal expansion coefficients and elastic moduli between the softer base film and the rigid metal coating can easily lead to delamination, film peeling, and other instability issues at the interface, weakening structural integrity. On the other hand, in subsequent battery manufacturing processes such as coating, rolling, and high-temperature baking, the base film, due to insufficient heat resistance, is prone to macroscopic thermal shrinkage or creep deformation under thermo-mechanical coupling loads. This can even induce microscopic damage such as microcracks and pores in the metal coating, thereby affecting the integrity of the current collector's conductive network and the electrode interface contact characteristics. Ultimately, this poses a potential risk to the process yield, cycle stability, and safety performance of the battery product. Therefore, how to simultaneously improve the rigidity, thermal stability, and interfacial bonding reliability of the composite current collector through base film material modification or structural optimization has become a core technical problem that urgently needs to be solved in this field.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide a composite current collector base film, its preparation method, and its application. By combining the excellent physicochemical properties of thermoplastic resin materials and dopamine-modified nanosheet materials, the rigidity and structural stability of the composite current collector base film are improved.

[0005] This invention is implemented as follows: In a first aspect, the present invention provides a base film for composite current collectors, the base film comprising at least one repeating unit; the repeating unit sequentially comprises a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer; the base film is formed by periodically repeating the repeating unit along the thickness direction; In the repeating unit, the thickness ratio of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is (0.5-1.5):2:(0.5-1.5); in the repeating unit, the thickness of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is independently 10nm-100nm; the intermediate functional layer includes a matrix resin and dopamine-modified nanomaterials in a mass ratio of (5-7):(3-5).

[0006] In an optional embodiment, the base film comprises 2 n There are repeating units, where n is an integer between 2 and 8; The first thermoplastic layer and the second thermoplastic layer are each independently selected from at least one of polyethylene terephthalate, polypropylene, polyamide or polyethylene; And / or, the matrix resin is polyimide; And / or, the thickness of the base film is 2 μm - 10 μm; And / or, the dopamine-modified nanomaterial is selected from at least one of dopamine-modified hexagonal boron nitride, aluminum nitride, aluminum oxide, or graphene oxide.

[0007] Secondly, the present invention provides a method for preparing a base film for composite current collectors, comprising the following steps: A precursor for a functional resin is prepared by blending a precursor of a matrix resin with dopamine-modified nanomaterials. The first thermoplastic resin, the functional resin precursor, and the second thermoplastic resin are melted and co-extruded to form a molten co-extruded structural layer with a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer along the thickness direction. The molten co-extruded structural layer is then subjected to n-fold multiplication processes to finally form a 3×2 n Layered nanolayered melt; The nanolayered melt was cooled and shaped to obtain a resin casting. The resin casting is subjected to longitudinal and transverse stretching in sequence, followed by imidization treatment and post-treatment to obtain a base film for composite current collectors.

[0008] In an optional embodiment, the preparation of the functional resin precursor includes the following steps: dissolving the precursor of the matrix resin in an organic solvent, adding the dopamine-modified nanomaterial, mixing thoroughly, removing the solvent, and granulating to obtain the functional resin. And / or, the mass ratio of the matrix resin precursor to the dopamine-modified nanomaterial in the functional resin precursor is (5-7):(3-5). And / or, the precursor of the matrix resin is polyamic acid.

[0009] In an optional embodiment, the extrusion temperature of the first thermoplastic resin and the second thermoplastic resin is 240°C-280°C; the extrusion temperature of the functional resin precursor is 150°C-200°C. And / or, the cooling and shaping temperature is 20℃-30℃; And / or, the thickness of the resin casting is 100μm-150μm; And / or, n is an integer between 2 and 8.

[0010] In an optional embodiment, the preheating temperature for the longitudinal stretching is 80℃-120℃, the stretching temperature is 95℃-125℃, the cooling temperature is 20℃-50℃, and the stretching ratio is 4-5 times. And / or, the preheating temperature for the transverse stretching is 90℃-120℃, the stretching temperature is 100℃-140℃, the setting temperature is 150℃-200℃, the cooling temperature is 90℃-110℃, and the stretching ratio is 2.5 times-3.0 times.

[0011] In an optional embodiment, the imidization treatment includes subjecting the stretched film to a gradient temperature: sequentially holding at 60℃-100℃ for 30min-50min, at 120℃-180℃ for 30min-90min, and at 180℃-200℃ for 90min-150min, and then cooling to room temperature. And / or, the imidization treatment is carried out under an inert atmosphere.

[0012] Thirdly, the present invention provides a composite current collector, comprising the base film for the composite current collector described above or the base film for the composite current collector obtained by the above preparation method.

[0013] Fourthly, the present invention provides an electrode comprising the aforementioned composite current collector.

[0014] Fifthly, the present invention provides an electrochemical device comprising the aforementioned electrode.

[0015] The present invention has the following beneficial effects: (1) In this invention, a functional resin blended with dopamine-modified nanomaterials and polyamic acid is used as an intermediate functional layer. After co-extrusion with the thermoplastic layer, the layers are multiplied in multiple stages to form an alternating nano-layer structure. During the multiplication process, the dopamine-modified nanomaterials are confined within the nanoscale functional layers, which effectively inhibits their aggregation and significantly improves the dispersion uniformity of the dopamine-modified nanomaterials in the matrix. At the same time, through the biaxial stretching process, the dopamine-modified nanomaterials are oriented in a controllable manner in the planar direction, which further enhances the mechanical properties and dimensional stability of the base film.

[0016] (2) In this invention, polyamic acid is converted into polyimide in situ through gradient heating imidization treatment. The polyimide molecular chain interacts with the polar groups on the surface of the dopamine-modified two-dimensional nanosheets by forming hydrogen bonds, thereby achieving a firm entanglement and fixation of the nanosheets in the polyimide matrix. This avoids the migration and aggregation of the nanosheets during subsequent use and significantly improves the interfacial bonding force. In addition, the upper and lower surface layers of the base film are the same thermoplastic resin layer, which ensures the symmetry and consistency of the adhesion of the coating on both sides during the subsequent coating process and effectively avoids the problem of film warping caused by thermal shrinkage differences.

[0017] (3) The preparation method described in this invention is continuous and highly controllable, and is suitable for large-scale industrial production. The resulting composite current collector base film has good application prospects in the field of battery current collectors. Attached Figure Description

[0018] 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.

[0019] Figure 1 This is a schematic diagram of the micro / nano laminated co-extrusion system in this application; Figure 2 This is a schematic diagram of the flow channel structure of the middle layer multiplier in this application. Detailed Implementation

[0020] 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.

[0021] Existing composite current collector films (such as PP and PET films) generally suffer from common problems such as insufficient rigidity, poor interfacial compatibility between the polymer matrix and the metal coating, and poor overall structural stability, which restrict their performance improvement and application expansion. In contrast, nanosheet materials with natural layered structures and nanoscale effects exhibit superior comprehensive performance in terms of thermal, electrical, mechanical, and interfacial chemistry. Therefore, this application aims to combine the excellent physicochemical properties of thermoplastic resins and nanomaterials to synergistically improve the rigidity and structural stability of composite current collector films.

[0022] However, conventional nanomaterials, due to their extremely high specific surface area, are prone to severe agglomeration when directly blended, resulting in limited filling ratios and poor modification effects. To address this challenge, this application introduces a micro-nano stacking process and combines it with a nanomaterial intercalation strategy to transform the random distribution of nanosheets at the micrometer thickness scale into an ordered arrangement at the nanometer thickness scale, fundamentally improving their dispersion state. Furthermore, a biaxial stretching process is employed to significantly enhance the orientation of the nanomaterials in the planar direction, forming a regular structure of planar stacking and intercalation. This achieves a complementary advantage between the ultrathin layered structure and the functional properties of the nanosheets, thereby effectively improving the overall performance of the base film.

[0023] The first aspect of this application provides a base film for composite current collectors, the base film comprising at least one repeating unit; the repeating unit sequentially comprises a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer; the base film is formed by periodically repeating the repeating unit along the thickness direction.

[0024] In the repeating unit, the thickness ratio of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is (0.5-1.5):2:(0.5-1.5), for example, but not limited to, at least one of 0.5:2:0.5, 0.5:2:1, 0.5:2:1.5, 1:2:0.5, 1:2:1, 1:2:1.5, 1.5:2:0.5, 1.5:2:1, or 1.5:2:1.5, or any two of these numerical ranges.

[0025] The first thermoplastic layer and the second thermoplastic layer are selected from at least one of polyethylene terephthalate (PET), polypropylene (PP), polyamide (PA), or polyethylene (PE), preferably PET. In some preferred embodiments, considering the depth of molecular chain diffusion entanglement at the interface between the thermoplastic layer and the functional resin, the thicknesses of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer in the repeating unit are each independently controlled within the range of 10nm-100nm, for example, but not limited to, at least one of 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, or 100nm, or any two of these numerical ranges. If the thickness of each layer in the repeating unit is too large, the degree of polymer chain diffusion entanglement between dissimilar interfaces is insufficient, which easily leads to interface delamination. If the thickness of each layer in the repeating unit is too small, this requires extremely high control precision of the equipment, and the structural integrity of the independent unit layer is difficult to stabilize, which easily leads to interface structural imbalance, increased interlayer defects, and affects the overall performance of the material.

[0026] Furthermore, the thickness of the base film is 2μm-10μm, for example, but not limited to, at least one of 2μm, 4μm, 6μm, 8μm or 10μm or any numerical range consisting of two of them, and the base film comprises 2 nThere are repeating units, where n is an integer between 2 and 8.

[0027] In some preferred embodiments, the dopamine-modified nanomaterial is selected from at least one of dopamine-modified hexagonal boron nitride, aluminum nitride, aluminum oxide, or graphene oxide.

[0028] Specifically, the modification method of inorganic nanomaterials includes the following steps: dispersing inorganic nanomaterials in a tris(hydroxymethyl)aminomethane buffer solution, adjusting the pH value to 8.5, adding dopamine hydrochloride buffer (the mass ratio of dopamine to hydrochloric acid in the dopamine hydrochloride buffer is 1:1), stirring at room temperature for 24 h to complete the in-situ polymerization modification of dopamine, collecting the modified product by centrifugation, washing it three times with deionized water, and then vacuum drying to obtain dopamine-modified nanomaterials.

[0029] A second aspect of this application provides a method for preparing a base film for a composite current collector, comprising the following steps: S1. Dopamine-modified nanomaterials are mixed with matrix resin precursors to obtain functional resin precursors. The functional resin precursor granules and thermoplastic resin granules are pretreated separately.

[0030] In some preferred embodiments, the preparation of the functional resin precursor includes the following steps: dissolving the matrix resin precursor (polyamic acid resin) in an organic solvent to prepare a solution with a mass concentration of 15wt%-25wt%, adding the dopamine-modified nanomaterial, and dispersing, mixing, removing the solvent, and granulating to obtain functional resin precursor granules.

[0031] For example, the organic solvents include, but are not limited to, N,N-dimethylformamide (DMF), N,N-dimethylacetamide and N-methylpyrrolidone; the mass concentration of the polyamic acid resin solution is, for example, but not limited to, at least one of 15 wt%, 20 wt% or 25 wt% or a numerical range consisting of any two of them.

[0032] The mass ratio of polyamic acid to dopamine-modified nanomaterials in the functional resin precursor is (5-7):(3-5), for example, but not limited to, at least one of 5:3, 2:1, 6:5, 7:3, 7:4, 7:5, 5:4, 1:1, 3:2 or any two of the following numerical ranges.

[0033] Specifically, the dispersion is achieved by ultrasonic dispersion for 30-40 minutes followed by mechanical stirring for 2-2.5 hours to obtain a uniform blend solution. After removing the solvent, the solution is granulated to obtain blended resin granules.

[0034] Preferably, the pretreatment of the thermoplastic resin includes: vacuum drying the thermoplastic resin granules at 100°C to 130°C for 8 to 16 hours, preferably at 120°C for 12 hours.

[0035] S2. The pretreated matrix resin precursor granules and thermoplastic resin granules are added to separate extruders for melt plasticization and then co-extruded. The extrusion system is as follows: Figure 1 As shown, a co-extruded structural layer is formed, comprising a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer.

[0036] In the co-extruded structural layer, the thickness ratio of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is (0.5-1.5):2:(0.5-1.5).

[0037] In this embodiment of the application, there are three extruders, one of which feeds the functional resin precursor, and the other two extruders feed the thermoplastic resin.

[0038] In some preferred embodiments, the extrusion temperature of the first thermoplastic resin and the second thermoplastic resin is 240°C-280°C, for example, but not limited to, at least one of 240°C, 250°C, 260°C, 270°C or 280°C or any two of these values; the extrusion temperature of the functional resin precursor is 150°C-200°C, for example, but not limited to, at least one of 150°C, 160°C, 170°C, 180°C, 190°C or 200°C or any two of these values, to ensure that the polyamic acid does not undergo premature imidization while achieving uniform plasticization.

[0039] S3. The co-extruded structural layer is fed into a series of layer multiplication units for multiplication processing. Each layer multiplication unit divides the current flow layer and then stacks them along the thickness direction, so that the total number of stacked layers increases step by step, and finally forms a nano-layered melt.

[0040] In this embodiment of the application, the base film is a 3-layer extrusion, such as... Figure 2 As shown, layers A and C are thermoplastic layers, layer B is an intermediate functional layer, the co-extruded structure is an ABC structure, using n layer multiplier units, and the base film is 3×2 n In the co-extruded structural layers, the thickness ratio of A, B, and C is (0.5-1.5):2:(0.5-1.5); the downstream multiplication makes its layup structure (ABC) 2. n That is, ABCABC.......

[0041] Preferably, the thicknesses of adjacent resin layers and functional resin precursor layers are equal. A and C are both thermoplastic resins, chosen independently, and preferably the same thermoplastic resin; B is the functional resin precursor. This design, with both upper and lower surface layers being thermoplastic layers, facilitates consistency in subsequent coating processes.

[0042] However, it should be noted that when the base film is extruded in two layers, the basic structure is thermoplastic resin-matrix resin / two-dimensional nanosheet intercalation. After passing through n layer multiplier units, the base film becomes 2×2 n Layer; its layering structure is (AB) 2 n+1 The ABAB... pattern results in both the upper and lower surface layers being heterogeneous thermoplastic layers and matrix resin / two-dimensional nanosheet intercalation layers. The difference between the upper and lower surface layers is not conducive to the consistency of subsequent coatings.

[0043] Furthermore, thanks to the use of layer multiplier units, the production of base films with more layers can be achieved on conventional multilayer (2- or 3-layer) co-extrusion base film production lines. At the same time, the thickness of the smallest unit layer can be controlled at the nanometer level, which is unattainable by multilayer extrusion die designs. The original ratio of the thickness of each layer can be quickly and easily adjusted by the corresponding extruder extrusion rate, and the biaxial stretching process enables the continuous production of high-performance film materials.

[0044] S4. The nano-layered melt is cast through a die onto a cooling roller for cooling and shaping to obtain a resin sheet.

[0045] In some preferred embodiments, the temperature of the cooling roller is 20°C-30°C, for example, but not limited to at least one of 20°C, 22°C, 24°C, 26°C, 28°C or 30°C or any two of the following numerical ranges.

[0046] Furthermore, the linear speed of the casting roller is controlled between 1 m / min and 5 m / min, and can be 1 m / min, 2 m / min, 3 m / min, 4 m / min or 5 m / min, etc. The thickness of the resin sheet is controlled between 100 μm and 150 μm by adjusting the speed of the melt pump, and can be 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, etc.

[0047] S5. The resin sheet is stretched longitudinally and laterally in sequence to align the dopamine-modified nanomaterials along the planar direction.

[0048] In a preferred embodiment of this application, the resin sheet first enters a longitudinal stretching machine, with the preheating temperature controlled at 80°C-120°C, the stretching temperature at 95°C-125°C, the cooling temperature at 20°C-50°C, and the stretching ratio at 4-5 times.

[0049] For example, the preheating temperature is, for example, but not limited to, a numerical range consisting of at least one or any two of 80°C, 90°C, 100°C, 110°C, or 120°C; the stretching temperature is a numerical range consisting of at least one or any two of 95°C, 105°C, 115°C, or 125°C; the cooling temperature is a numerical range consisting of at least one or any two of 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, or 50°C; and the stretching ratio is 4 times, 4.5 times, or 5 times.

[0050] In a preferred embodiment of this application, the resin sheet is longitudinally stretched and then enters a transverse stretching machine. The preheating temperature is 90℃-120℃, the stretching temperature is 100℃-140℃, the setting temperature is 150℃-200℃, the cooling temperature is 90℃-110℃, and the stretching ratio is 2.5 times-3.0 times.

[0051] For example, the preheating temperature may be, but is not limited to, a numerical range consisting of at least one or any two of 90°C, 100°C, 110°C, or 120°C; the stretching temperature may be a numerical range consisting of at least one or any two of 100°C, 110°C, 120°C, 130°C, or 140°C; the setting temperature may be a numerical range consisting of at least one or any two of 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C; the cooling temperature may be a numerical range consisting of at least one or any two of 90°C, 95°C, 100°C, 105°C, or 110°C; and the stretching ratio may be 2.5 times, 2.6 times, 2.7 times, 2.8 times, 2.9 times, or 3.0 times, etc.

[0052] During biaxial stretching, thanks to the spatial confinement of the dopamine-modified nanomaterials by the nanolayered structure, combined with the effect of the stretching force field, the dopamine-modified nanomaterials are oriented along the planar direction, and their distribution gradually tends to planar stacking and intercalation. At the same time, the multiple layer multiplication process pre-segments the dopamine-modified nanomaterials into different nanolayers, effectively inhibiting the aggregation of the dopamine-modified nanomaterials and significantly improving the dispersion of the nanomaterials in the matrix.

[0053] S6. The biaxially stretched film is subjected to imidization under gradient heating conditions to convert polyamic acid into polyimide in situ, while simultaneously achieving entanglement and fixation of polyimide and dopamine-modified nanomaterials. The imidized film is cooled, flattened, trimmed, and then wound up and slit to obtain a composite current collector base film.

[0054] In some preferred embodiments, the gradient heating includes the following steps: First stage: Temperature 60℃-100℃, hold for 30min-50min to remove residual solvent from the film; Second stage: Temperature 120℃-180℃, hold for 30min-90min, to initiate the initial imidization reaction of polyamic acid; Third stage: Temperature 180-200℃, hold for 90-150 minutes to complete imidization; Cool to room temperature and then roll up to obtain the final nano-layered composite film.

[0055] During the imidization process, the polyamic acid molecular chains gradually cyclize to form polyimide molecular chains. The polar groups on the surface of the dopamine-modified nanomaterials can form hydrogen bonds with the polyimide molecular chains, inducing the polyimide molecular chains to entangle on the surface of the nanomaterials. This achieves the fixation of the nanomaterials in the polyamic acid matrix, preventing the migration and aggregation of nanosheets during subsequent use, and at the same time improving the interfacial bonding force between polyimide and nanomaterials.

[0056] In this application, after all the above processes are completed, the film first enters the traction pretreatment unit. The traction winding speed is 150-250 m / min, which can be 150 m / min, 200 m / min, or 250 m / min, etc. The winding method is contact winding or gap winding. The film is cooled to room temperature a second time by cooling rollers to eliminate internal stress and prevent warping after winding. The film is stretched by flattening rollers to eliminate wrinkles and ensure a flat film surface during winding. The edge trimming device removes the uneven edges held by the clamps during stretching to obtain a regular width. The thickness uniformity is detected in real time by an online thickness gauge, and the out-of-tolerance areas are marked for subsequent slitting and rejection. The base film master roll is obtained by traction winding. The slitting speed is usually controlled at 100 m / min-300 m / min, which can be 100 m / min, 200 m / min, or 300 m / min. m / min, etc.; For thin film materials, a circular knife rolling method is used. During the slitting process, online re-inspection and verification of dimensional deviations and film surface defects are carried out. After rejecting unqualified sections, the finished film of the target specification is obtained by slitting, and then it is rolled up again, packaged and stored. After final inspection, it is a qualified finished product.

[0057] Therefore, this invention combines micro-nano stacking technology with functional layer intercalation design containing nanomaterials, fully leveraging the processing advantages of thermoplastic resins and the performance advantages of nanomaterials. Through the synergy of layer multiplication design and secondary stretching process, a deep coupling of the performance of nanomaterials and the advantages of micro-nano stacked structures is achieved. This process is adaptable to different nanomaterial and thermoplastic resin systems, and through flexible material selection, it directionally enhances the performance in different dimensions such as thermal, electrical, mechanical, and interfacial properties. In terms of mechanical reliability, the high-density alternating multilayer structure can effectively disperse tip stress, inhibit crack propagation, form an internal toughening mechanism, and significantly improve the material's resistance to bending fatigue and cracking, overcoming the problem of high brittleness in traditional nanocomposite materials. In terms of dimensional stability, this multilayer alternating symmetrical structure can achieve rebalancing of internal stress—the functional layer containing nanomaterials dominates thermal dimensional stability, while the thermoplastic phase absorbs and dissipates processing and thermal stresses through viscoelastic relaxation, effectively reducing the risk of warpage, wrinkling, and dimensional changes, and endowing the base film with excellent comprehensive performance and processing adaptability.

[0058] Thirdly, the present invention provides a composite current collector, comprising the base film for the composite current collector described above or the base film for the composite current collector obtained by the above preparation method.

[0059] Fourthly, the present invention provides an electrode comprising the aforementioned composite current collector.

[0060] Fifthly, the present invention provides an electrochemical device comprising the aforementioned electrode.

[0061] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0062] Example 1 This embodiment provides a method for preparing a composite current collector-based film, including the following steps: 1. Raw material preparation: (1) BN nanosheet modification: BN nanosheets were dispersed in tris(hydroxymethyl)aminomethane buffer solution, the pH was adjusted to 8.5, hydrochloric acid and dopamine hydrochloride buffer solution with a mass ratio of 1:1 were added, and the mixture was stirred at room temperature for 24 h to complete the in-situ polymerization modification of dopamine. The modified product was collected by centrifugation, washed three times with deionized water and then vacuum dried to obtain dopamine modified nanomaterials with a mass ratio of 1:1 of dopamine and BN nanosheets. (2) Preparation of blended resin: Polyamic acid resin was dissolved in N,N-dimethylformamide (DMF) solvent to prepare a polyamic acid solution with a mass concentration of 15wt%. The above-mentioned dopamine modified nanomaterials were added, ultrasonically dispersed for 30 min, and mechanically stirred for 2 h to obtain a uniformly dispersed functional resin precursor solution. After removing the solvent, the solution was granulated to obtain the functional resin precursor. The mass ratio of polyamic acid to nanomaterials in the functional resin precursor was 5:3. (3) Both the first thermoplastic resin and the second thermoplastic resin are PET resins. The PET resin granules are vacuum dried at 120°C for 12 hours to remove residual moisture and are ready for use.

[0063] 2. Multilayer co-extrusion and layer multiplication: (1) The pretreated PET resin and functional resin precursor were fed into three independent single-screw extruders. Two extruders were fed with PET resin (corresponding to the raw materials of layers C and A on both sides), and one extruder was fed with functional resin precursor (corresponding to the raw material of layer B). The PET extrusion temperature was 265℃ and the functional resin precursor extrusion temperature was 180℃.

[0064] (2) After the three resins are plasticized by the extruder, they enter the initial distribution module, where a three-layer co-extrusion structure is first formed: a first thermoplastic layer (PET), a functional resin precursor layer (PAA + nanomaterials), and a second thermoplastic layer (PET). Then, the structure enters a series of n-stage multiplier units. Each multiplier unit cuts the existing flow layer into two layers along the casting direction, and then stacks them along the thickness direction. The number of layers doubles with each stage of the unit. After n stages of multiplication, a final 3×2 layer is obtained. n A nano-layered structure with alternating layers.

[0065] (3) The multiplied nano-layered melt enters the T-type casting die head. The die head temperature is controlled at 270℃. After the melt flows out of the die head, it is cooled and shaped on the casting cooling roller at 25℃ to obtain a resin sheet. The linear speed of the casting roller is controlled at 2.5m / min. The thickness of the resin sheet is controlled to 120μm by adjusting the speed of the melt pump.

[0066] 3. Biaxial stretching: (1) The cooled cast sheet is fed into the longitudinal stretching machine. The preheating temperature is controlled at 100℃, the stretching temperature is 100℃, the cooling temperature is 30℃, and the stretching ratio is 4 times. (2) After the resin sheet is stretched longitudinally, it enters the transverse stretching machine. The preheating temperature is 100℃, the stretching temperature is 110℃, the setting temperature is 170℃, the cooling temperature is 95℃, and the stretching ratio is 3.0 times.

[0067] 4. Gradient temperature imidization treatment: First stage: temperature 80℃, hold for 40 min; Second stage: temperature 150℃, hold for 60 min; Third stage: temperature 180℃, hold for 120 min, complete imidization; Cool to room temperature and wind up to obtain a composite current collector base film with a thickness of 6 μm. Where n is 5, there are 32 repeating units, and the total number of layers is 96; in the co-extrusion structure, the thickness ratio of the first thermoplastic layer (PET) / functional resin precursor layer (PAA + nanomaterials) / second thermoplastic layer (PET) is 1:2:1; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 47nm, and the thickness of the functional resin precursor layer is 94nm; in the repeating unit, the thickness ratio of the first thermoplastic layer (PET), the functional resin precursor layer and the second thermoplastic layer is 1:2:1; the base film structure is ABCABC..., therefore, except for the surface thermoplastic layer of the base film, the thicknesses of adjacent thermoplastic layers and functional layers inside are equal.

[0068] 5. Winding and Slitting: The base film first enters the traction pretreatment unit, with a traction winding speed of 200m / min and a contact winding method. After traction winding, the base film master roll is obtained. The slitting speed is usually controlled at 200m / min. For thin film materials, a circular knife rolling method is used. During the slitting process, online re-inspection and verification of dimensional deviations and film surface defects are carried out. After rejecting unqualified sections, the finished film of the target specification is obtained by slitting. After being wound into small rolls again, it is packaged and stored. After final inspection, it becomes a qualified composite current collector base film.

[0069] Example 2 This embodiment provides a base film for composite current collectors, which differs from Embodiment 1 only in that: n is 6, there are 64 repeating units, and the total number of layers is 192; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 23nm, and the thickness of the functional layer resin precursor layer is 46nm.

[0070] Example 3 This embodiment provides a base film for composite current collectors, which differs from Embodiment 1 only in that: n is 7, there are 128 repeating units, and the total number of layers is 384; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 12nm, and the thickness of the functional layer resin precursor layer is 24nm.

[0071] Example 4 This embodiment provides a base film for composite current collectors, which differs from Embodiment 1 only in that: the base film thickness is 2μm; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 16nm, and the thickness of the functional layer resin precursor layer is 32nm.

[0072] Example 5 This embodiment provides a base film for composite current collectors, which differs from Embodiment 2 only in that: The base film thickness is 10 μm; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 39 nm, and the thickness of the functional layer resin precursor layer is 78 nm.

[0073] Example 6 This embodiment provides a composite current collector base film, which differs from Embodiment 1 only in that the intermediate functional layer is a matrix resin and dopamine-modified nanomaterials in a mass ratio of 7:3.

[0074] Example 7 This embodiment provides a composite current collector base film, which differs from Embodiment 1 only in that the intermediate functional layer is a matrix resin and dopamine-modified nanomaterials in a mass ratio of 7:5.

[0075] Example 8 This embodiment provides a composite current collector base film, which differs from Embodiment 2 only in that: the thickness ratio of the first thermoplastic layer (PET) / functional resin precursor layer (PAA + nanomaterials) / second thermoplastic layer (PET) in the co-extruded structure is 0.5:2:0.5; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 16nm, and the thickness of the functional layer resin precursor layer is 62nm; the base film structure is ABCABC..., therefore, except for the surface thermoplastic layer of the base film, the thickness ratio of adjacent thermoplastic layers and functional layers inside is 2:1.

[0076] Example 9 This embodiment provides a composite current collector base film, which differs from Embodiment 1 only in that: the thickness ratio of the first thermoplastic layer (PET) / functional resin precursor layer (PAA + nanomaterials) / second thermoplastic layer (PET) in the co-extruded structure is 1.5:2:1.5; in the repeating unit, the thickness of the first thermoplastic layer and the second thermoplastic layer is 56nm, and the thickness of the functional layer resin precursor layer is 75nm; the base film structure is ABCABC..., therefore, except for the surface thermoplastic layer of the base film, the thickness ratio of adjacent thermoplastic layers and functional layers inside is 3:2.

[0077] Comparative Example 1 This embodiment provides a base film for composite current collectors, which differs from Embodiment 1 only in that it does not use a layer multiplier unit.

[0078] Experimental Example 1 The composite current collector base films prepared in Examples 1-9 and Comparative Example 1 were subjected to tensile strength and F-5 value performance verification; the composite current collector base films prepared in Examples 1-9 and Comparative Example 1 were used to prepare electrode sheets and then subjected to crack resistance tests.

[0079] Tensile strength test: The composite current collector base films prepared in Examples 1-9 and Comparative Example 1 were cut into standard dumbbell-shaped or strip-shaped specimens along the longitudinal direction (MD), with a specimen width of 15 mm and a gauge length of 50 mm. At least 5 specimens were tested in each direction, and the arithmetic mean was taken. The thickness of three points within the gauge line of the specimen was measured, and the arithmetic mean was taken, accurate to 1 μm. The specimen was placed in the upper and lower clamps of the universal tensile testing machine, so that the longitudinal axis of the specimen coincided with the line connecting the centers of the clamps. The testing machine was started at a tensile speed of 200 mm / min (or 250±50 mm / min) to perform tensile testing until the specimen broke. The maximum load F at the time of specimen breakage was read. max (Unit: N), calculate the tensile strength using the following formula: σ = F max / (b × d), where σ is the tensile strength (MPa), b is the specimen width (mm), and d is the specimen thickness (mm); record the tensile strength in the MD direction respectively, and take the average value of 5 specimens in each direction.

[0080] F-5 Value Test: The F-5 value represents the stress required for the film to be stretched by 5%. Using the same specimen specifications and cutting method as the tensile strength test, standard specimens are cut along the MD direction, with at least 5 specimens tested in each direction. The testing machine is started at a tensile speed of 200 mm / min, and the load-elongation curve is recorded in real time. The load F_5% (unit: N) corresponding to the specimen being stretched by 5% is read, and the F-5 value is calculated using the following formula: F-5 value = F_5% / (b × d), where F-5 value (MPa), b is the specimen width (mm), and d is the specimen thickness (mm). The F-5 values ​​in the MD direction are recorded, and the average value of the 5 specimens in each direction is taken as the final test result. Other procedures are the same as the tensile strength test method.

[0081] Setting the conductive layer: A roll-to-roll vacuum evaporation machine was used, and the vacuum level was increased to 5×10. -3 Pa; using aluminum wire with a purity of 99.90% as raw material, wire feeding speed: 250 mm / min, evaporation temperature controlled at 1200℃; film travel speed 15 m / min, composite aluminum foil with single-sided aluminum layer thickness controlled at 1 μm.

[0082] Battery electrode preparation and crack resistance testing: A Kejingzhida EI-200 coating machine was used to coat the rolled composite aluminum foil and electrolytic copper foil of the above examples and comparative examples. The material formulation of the positive electrode was 95% lithium cobalt oxide (LiCoO2), 3% PVDF, and 2% SP. The positive electrode was baked and subjected to a tear resistance roll test (compaction density set to 4.2 g / cm³).

[0083] CP-SEM observation and analysis: The prepared cross-sectional sample of the above-mentioned compacted electrode was placed on the sample stage of a field emission scanning electron microscope and observed using backscattered electron imaging mode. This mode is sensitive to the difference in atomic number of materials and can clearly distinguish aluminum foil, polymer layer, lithium cobalt oxide particles and cracks and voids, and observe the cracking of the base film.

[0084] The test results are shown in Table 1.

[0085] Table 1 Performance test results of each embodiment and comparative example

[0086] As shown in Table 1, the micro-nano stacked structure can effectively improve the dispersion and orientation of nanosheets, and significantly enhance the rigidity and strength of the base film. Furthermore, the ultrathin confinement effect helps the nanosheets to arrange in an orderly manner; the symmetrical structure is more conducive to the balance of internal stress; at the same time, the multilayer alternating structure of the present invention effectively inhibits crack propagation and significantly improves durability.

[0087] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A base membrane for composite current collectors, characterized in that, The base film comprises at least one repeating unit; the repeating unit sequentially includes a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer; the base film is formed by periodically repeating the repeating unit along the thickness direction; In the repeating unit, the thickness ratio of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is (0.5-1.5):2:(0.5-1.5); in the repeating unit, the thickness of the first thermoplastic layer, the intermediate functional layer, and the second thermoplastic layer is independently 10nm-100nm; the intermediate functional layer includes a matrix resin and dopamine-modified nanomaterials in a mass ratio of (5-7):(3-5).

2. The composite current collector base membrane according to claim 1, characterized in that, The base membrane comprises 2 n There are repeating units, where n is an integer between 2 and 8; The first thermoplastic layer and the second thermoplastic layer are each independently selected from at least one of polyethylene terephthalate, polypropylene, polyamide or polyethylene; And / or, the matrix resin is polyimide; And / or, the thickness of the base film is 2 μm - 10 μm; And / or, the dopamine-modified nanomaterial is selected from at least one of dopamine-modified hexagonal boron nitride, aluminum nitride, aluminum oxide, or graphene oxide.

3. A method for preparing a composite current collector base film as described in any one of claims 1-2, characterized in that, The process includes the following steps: blending a precursor of a matrix resin with dopamine-modified nanomaterials to obtain a precursor of a functional resin; The first thermoplastic resin, the functional resin precursor, and the second thermoplastic resin are melted and co-extruded to form a molten co-extruded structural layer with a first thermoplastic layer, an intermediate functional layer, and a second thermoplastic layer along the thickness direction. The molten co-extruded structural layer is then subjected to n-fold multiplication processes to finally form a 3×2 n Layered nanolayered melt; The nanolayered melt was cooled and shaped to obtain a resin casting. The resin casting is subjected to longitudinal and transverse stretching in sequence, followed by imidization treatment and post-treatment to obtain a base film for composite current collectors.

4. The preparation method according to claim 3, characterized in that, The preparation of the functional resin precursor includes the following steps: The precursor of the matrix resin is dissolved in an organic solvent, the dopamine-modified nanomaterial is added, and after thorough mixing, the solvent is removed and the mixture is granulated to obtain the functional resin. And / or, the mass ratio of the matrix resin precursor to the dopamine-modified nanomaterial in the functional resin precursor is (5-7):(3-5). And / or, the precursor of the matrix resin is polyamic acid.

5. The preparation method according to claim 3, characterized in that, The extrusion temperature of the first thermoplastic resin and the second thermoplastic resin is 240℃-280℃; the extrusion temperature of the functional resin precursor is 150℃-200℃. And / or, the cooling and shaping temperature is 20℃-30℃; And / or, the thickness of the resin casting is 100μm-150μm; And / or, n is an integer between 2 and 8.

6. The preparation method according to claim 4, characterized in that, The preheating temperature for the longitudinal stretching is 80℃-120℃, the stretching temperature is 95℃-125℃, the cooling temperature is 20℃-50℃, and the stretching ratio is 4-5 times. And / or, the preheating temperature for the transverse stretching is 90℃-120℃, the stretching temperature is 100℃-140℃, the setting temperature is 150℃-200℃, the cooling temperature is 90℃-110℃, and the stretching ratio is 2.5 times-3.0 times.

7. The preparation method according to claim 4, characterized in that, The imidization treatment includes subjecting the stretched film to a gradient temperature: sequentially holding at 60℃-100℃ for 30min-50min, at 120℃-180℃ for 30min-90min, and at 180℃-200℃ for 90min-150min, and then cooling to room temperature. And / or, the imidization treatment is carried out under an inert atmosphere.

8. A composite current collector, characterized in that, The composite current collector includes the base film for composite current collectors as described in any one of claims 1 to 2, or the base film for composite current collectors prepared by the preparation method described in any one of claims 3 to 7.

9. An electrode sheet, characterized in that, The electrode includes the composite current collector.

10. An electrochemical device, characterized in that, The electrochemical device includes an electrode as described in claim 9.

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