Base membrane as well as preparation method and application thereof

Abstract

The invention relates to a base membrane and a preparation method and application thereof. The base film comprises a core layer, a first surface layer and a second surface layer, wherein the first surface layer and the second surface layer are located on two opposite surfaces of the core layer respectively. The materials of the core layer, the first surface layer and the second surface layer respectively and independently comprise matrix resin and a buffer material, the mass percent of the buffer material in the core layer is greater than the mass percent of the buffer material in any one of the first surface layer and the second surface layer, and the buffer material is a high polymer material with elastic recovery performance. The buffer material with elastic recovery performance in the base film is used as a stress dispersion point to promote the film to form more uniformly distributed crazes and shear bands in a stress area instead of penetrating cracks caused by concentrated stress; a large amount of pressure energy can be consumed in the craze and shear band forming process, and meanwhile, the elastic deformation capacity of the buffer material can further absorb residual stress, so that the shear stress peak value in the film is remarkably reduced, and film cracking is avoided.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to base films, their preparation methods, and applications. Background Technology

[0002] With the continuous upgrading of the range requirements of terminals such as 3C digital products and new energy electric vehicles, the compaction density of lithium cobalt oxide system electrode sheets has been increased to 4.35 g / cm³, corresponding to a roll forming pressure of over 600 MPa. However, as the key positive electrode material of the lithium cobalt oxide system electrode, the composite current collector is subjected to extremely high forming pressure corresponding to this compaction density. This will cause the PET film in the middle of the composite current collector to be subjected to severe Z-axis pressure. Under such enormous pressure, significant shear stress will be generated inside the PET film. When the shear stress exceeds the mechanical tolerance limit of the PET film itself, it will directly lead to cracking and damage of the film, seriously affecting the packaging integrity of the composite aluminum foil, and thus causing performance degradation and safety hazards of the lithium cobalt oxide electrode.

[0003] The mechanical buffering properties of PET film in existing composite current collectors are insufficient. Under the strong stress generated by high density, it cannot effectively disperse or absorb stress energy and can only passively withstand shear force, resulting in frequent cracking problems. This has become the core technical bottleneck restricting the application of composite current collectors in high density lithium cobalt oxide systems. Summary of the Invention

[0004] Therefore, it is necessary to provide a base film with crack resistance under high pressure density conditions, its preparation method, and its application.

[0005] In one aspect, the present invention provides a base film comprising: a core layer and a first surface layer and a second surface layer respectively located on two opposing surfaces of the core layer; the core layer, the first surface layer and the second surface layer are each independently composed of a matrix resin and a buffer material, and the mass percentage of the buffer material in the core layer is greater than the mass percentage of the buffer material in either the first surface layer or the second surface layer; the buffer material is a polymeric material with elastic recovery properties.

[0006] In some embodiments, the cushioning material satisfies at least one of the following characteristics:

[0007] The elastic recovery rate of the buffer material is ≥70% under conditions of 23℃ and 50% compression.

[0008] The buffer material has a processing temperature resistance ≥260℃;

[0009] The buffer material comprises any one or more of granular polymer materials and fibrous polymer materials; optionally, the granular polymer material comprises any one or more of polyurethane particles, polyolefin elastomer particles, and nitrile rubber microspheres; optionally, the particle size of the granular polymer material is 0.5-3 μm, and the particle size distribution span is ≤0.5; optionally, the fibrous polymer material comprises any one or more of aromatic polyamide fibers, polyetheretherketone fibers, and polyethylene fibers; optionally, the ratio of the average length of the fibrous polymer material to the total thickness of the base film is 0.1:1-1:1.

[0010] The buffer material accounts for 0.5%-20% of the total mass of the base film, and optionally, 8%-15%.

[0011] In some embodiments, the particle size of the particulate polymer material in the first and second surface layers is smaller than the particle size of the particulate polymer material in the core layer; and / or

[0012] The average length of the fibrous polymer material in the first and second outer layers is less than the average length of the fibrous polymer material in the core layer.

[0013] In some embodiments, the base film includes at least one of the following features:

[0014] The particle size of the granular polymer material in the core layer is 1.5 μm-3 μm;

[0015] The particle size of the particulate polymer material in the first and second surface layers is 0.5 μm-1.5 μm;

[0016] The ratio of the average length of the fibrous polymer material in the core layer to the total thickness of the base film is 0.3:1-0.6:1;

[0017] The average length of the fibrous polymer material in the first and second surface layers is independently in a ratio of 0.1:1 to 0.3:1 to the total thickness of the base film.

[0018] In some embodiments, the base film satisfies at least one of the following characteristics:

[0019] The mass ratio of the cushioning material in the first and second outer layers to the mass of the cushioning material in the core layer is 1:(2-5).

[0020] The total thickness of the base film is 3μm-15μm, and can be selected as 4μm-10μm;

[0021] The thickness of the first and second surface layers each independently accounts for 5%-25% of the total thickness of the base film, and can be selected as 5%-15%;

[0022] The matrix resin includes any one or more of PET, PPS, PEN, PP, PE, and their modified resins. In some embodiments, the base film further includes a third surface layer, which is located on the side of the first surface layer and / or the second surface layer away from the core layer; the mass percentage of the cushioning material in the third surface layer is less than the mass percentage of the cushioning material in the adjacent first or second surface layer.

[0023] The second aspect provides a method for preparing a base film as described above, comprising the following steps:

[0024] A core layer material, a first surface layer material, and a second surface layer material are provided. Each of the core layer material, the first surface layer material, and the second surface layer material independently includes a matrix resin and a buffer material. The mass percentage of the buffer material in the core layer material is greater than the mass percentage of the buffer material in either the first surface layer material or the second surface layer material. The core layer material, the first surface layer material, and the second surface layer material are melt-extruded using a multi-layer co-extrusion method to prepare the base film.

[0025] A third aspect provides a composite current collector, comprising a base film and a conductive metal layer located on at least one side of the base film; the base film is any of the base films described above.

[0026] A fourth aspect provides an electrode comprising a composite current collector and an active material layer located on at least one side of the composite current collector; wherein the composite current collector is the aforementioned composite current collector;

[0027] Optionally, the active material layer includes lithium cobalt oxide.

[0028] The fifth aspect provides an electrochemical device including the electrodes described above;

[0029] Optionally, the electrochemical device includes a battery.

[0030] The buffer material with elastic recovery properties in the base film provided by this invention acts as a stress dispersion point, promoting the formation of more uniformly distributed crazing and shear bands in the stressed area of ​​the base film, rather than through-cracks caused by concentrated stress. The formation process of crazing and shear bands consumes a large amount of pressure energy, and the elastic deformation capability of the buffer material can further absorb residual stress, thereby significantly reducing the peak shear stress inside the base film and preventing cracking. Furthermore, the core layer containing more buffer material in the base film undertakes the main stress absorption function, reducing the peak shear stress of the base film and preventing cracking. The low content of buffer material in the first and second surface layers makes the surface of the base film relatively smooth, preventing excessive buffer material from protruding from the base film surface and affecting the composite effect with the metal layer when applied to composite current collectors, which is beneficial to improving the bonding force with the metal layer. Attached Figure Description

[0031] Figure 1 The image shows a cross-sectional scanning electron microscope (SEM) image of the base film prepared in Example 1 after disassembly during cyclic testing.

[0032] Figure 2 Scanning electron microscope (SEM) image of a cross-sectional sample of the base film prepared for Comparative Example 2 after cyclic testing and disassembly. Detailed Implementation

[0033] To facilitate understanding of the present invention, a more complete description will be provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0035] Under the intense stress generated by high compaction density, composite current collectors cannot effectively disperse or absorb stress energy and can only passively withstand shear force, leading to frequent cracking problems and limiting their application in high compaction density systems such as lithium cobalt oxide. To solve this problem, this application provides at least one base film, its preparation method, and its application.

[0036] According to a typical embodiment of this application, a base film is provided, comprising: a core layer and a first surface layer and a second surface layer respectively located on two opposite surfaces of the core layer; the core layer, the first surface layer and the second surface layer are each independently composed of a matrix resin and a buffer material, and the mass percentage of the buffer material in the core layer is greater than the mass percentage of the buffer material in either the first surface layer or the second surface layer, wherein the buffer material is a polymer material with elastic recovery properties.

[0037] The elastically recoverable buffer material in the aforementioned base film acts as a stress dispersion point, promoting the formation of more uniformly distributed crazing and shear bands in the stressed area, rather than through-cracks caused by concentrated stress. The formation of crazing and shear bands consumes a significant amount of pressure energy, while the elastic deformation capacity of the buffer material further absorbs residual stress, thereby significantly reducing the peak shear stress inside the base film and preventing cracking. Furthermore, the core layer containing a large amount of buffer material in the base film undertakes the main stress absorption function, reducing the peak shear stress of the base film and preventing cracking. The lower content of buffer material in the first and second surface layers results in a smoother surface of the base film, preventing excessive buffer material from protruding from the film surface and affecting the bonding effect with the metal layer when applied to composite current collectors, thus improving the adhesion with the metal layer.

[0038] The amount of cushioning material added in the first surface layer can be the same as or different from the amount of cushioning material added in the second surface layer. In some embodiments, the amount of cushioning material added in the first surface layer is the same as the amount of cushioning material added in the second surface layer.

[0039] In some embodiments, the elastic recovery rate of the cushioning material at 23°C and 50% compression ratio is ≥70%, which can more effectively absorb residual stress and improve the crack resistance of the base film. Non-limitingly, the elastic recovery rate of the cushioning material at 23°C and 50% compression ratio can be 70%, 75%, 80%, 85%, 90%, 95%, etc. For example, the elastic recovery rate can be tested according to the method specified in GB / T 7759-2015. Optionally, the elastic recovery rate of the cushioning material can be 70%-95%, or 70%-95%, or 70%-85% or 75%-90%.

[0040] In some embodiments, the buffer material has a processing temperature of ≥260°C, which is well adapted to the film extrusion process and subsequent current collector and battery processing.

[0041] Optionally, the cushioning material includes any one or more of particulate polymer materials and fibrous polymer materials.

[0042] In some embodiments, the particulate polymer material includes any one or more of polyurethane particles, polyolefin elastomer (POE) particles, and nitrile rubber microspheres. These particulate polymer materials not only have high elastic recovery rate and processing temperature resistance, but also good compatibility with the matrix resin, especially with PET resin, with no obvious phase separation. Optionally, the nitrile rubber microspheres are cross-linked nitrile rubber microspheres with a cross-linking degree ≥60% and a melting temperature of 255℃-270℃, so that they can maintain the microsphere morphology in the base film.

[0043] In some embodiments, the particle size of the particulate polymer material is 1-15 μm, and the particle size distribution span is ≤0.5. The formula for calculating the particle size distribution span is as follows: .

[0044] In some embodiments, the fibrous polymer material includes any one or more of aromatic polyamide (aramid) fibers, polyetheretherketone (PEEK) fibers, and polyethylene fibers. Optionally, the polyethylene fiber is ultra-high molecular weight polyethylene (UHMWPE) with a molecular weight greater than 1 million.

[0045] In some embodiments, the ratio of the average length of the fibrous polymer material to the total thickness of the base film is 0.1:1 to 1:1. Determining the length of the fibrous polymer material based on the base film thickness can prevent fiber agglomeration or penetration through the film, which would affect the stress dispersion of the base film and lead to a decrease in the mechanical properties of the base film.

[0046] In some embodiments, the buffer material accounts for 0.5%-20% of the total mass of the base film, optionally 8%-15%. Non-limitingly, the buffer material may account for 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc., of the total mass of the base film.

[0047] In some embodiments, the particle size of the particulate polymer material in the first and second surface layers is smaller than the particle size of the particulate polymer material in the core layer; wherein, the particle size of the particulate polymer material in the first and second surface layers may be the same or different, preferably the same.

[0048] In some embodiments, the average length of the fibrous polymer material in the first and second surface layers is less than the average length of the fibrous polymer material in the core layer. The average lengths of the fibrous polymer material in the first and second surface layers may be the same or different, but are preferably the same.

[0049] Including a high content of large-particle-size (or long-fiber) buffer material in the core layer of the base film can better undertake the main stress absorption function and further reduce the peak shear stress of the base film; using a low content of small-particle-size (or short-fiber) buffer material in the surface layer can more effectively prevent the buffer material from protruding from the film surface, thereby improving the composite effect with the metal layer when preparing composite current collectors and further enhancing the bonding force with the metal layer.

[0050] In some embodiments, the particle size of the particulate polymer material in the core layer is 1.5 μm-3 μm. Non-limitingly, the particle size of the particulate polymer material in the core layer is 1.5 μm, 1.8 μm, 2 μm, 2.3 μm, 2.5 μm, 2.7 μm, 3 μm, etc. Optionally, the particle size of the particulate polymer material in the first and second surface layers is 0.5 μm-1.5 μm. Non-limitingly, the particle size of the particulate polymer material in the first and second surface layers is each independently 0.5 μm, 0.7 μm, 0.9 μm, 1.1 μm, 1.3 μm, 1.5 μm, etc.

[0051] In some embodiments, the ratio of the average length of the fibrous polymer material in the core layer to the total thickness of the base film is 0.3:1-0.6:1, and non-limitingly, it can be 0.3:1, 0.4:1, 0.5:1, 0.6:1, etc. Optionally, the ratio of the average length of the fibrous polymer material in the first and second surface layers to the total thickness of the base film is independently 0.1:1-0.3:1, and non-limitingly, it can be 0.1:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1, etc.

[0052] In some embodiments, the mass ratio of the cushioning material in the first and second outer layers to the cushioning material in the core layer is 1:(2-5). Non-limitingly, the mass ratio of the cushioning material in the first and second outer layers to the cushioning material in the core layer can be 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, etc.

[0053] In some embodiments, the total thickness of the base film is 3 μm-15 μm, and optionally 4 μm-10 μm.

[0054] In some embodiments, the thickness of the first surface layer and the second surface layer each independently accounts for 5%-25% of the total thickness of the base film, optionally 5%-15%.

[0055] In some embodiments, the thickness of the first surface layer and the second surface layer may be the same or different, but preferably the same.

[0056] In some embodiments, the base film further includes a third surface layer located on the side of the first and / or second surface layers away from the core layer; the mass percentage of the cushioning material in the third surface layer is less than the mass percentage of the cushioning material in the adjacent first or second surface layer.

[0057] In some embodiments, the cushioning material in the third surface layer is a particulate polymer and / or a fibrous polymer.

[0058] In some embodiments, the particle size of the particulate polymer material in the third surface layer is smaller than that of the particulate polymer material in the adjacent first or second surface layer.

[0059] In some embodiments, the particle size of the particulate polymer material in the third surface layer is 0.1 μm-1 μm, and can be, without limitation, 0.1 μm, 0.3 μm, 0.5 μm, 0.7 μm, 0.9 μm, 1 μm, etc.

[0060] In some embodiments, the average length of the fibrous polymer material in the third surface layer is less than the average length of the fibrous polymer material in the adjacent first or second surface layer.

[0061] In some embodiments, the ratio of the average length of the fibrous polymer material in the third surface layer to the total thickness of the base film is 0.01:1-0.1:1, and can be non-limitingly 0.01:1, 0.03:1, 0.05:1, 0.07:1, 0.09:1, 0.1:1, etc.

[0062] In some embodiments, the mass ratio of the cushioning material in each third outer layer to the mass of the cushioning material in the core layer is 1:(3-6).

[0063] In some embodiments, the thickness of each third surface layer is less than or equal to the thickness of the first or second surface layer adjacent to it.

[0064] In some embodiments, the thickness of each third surface layer is 3%-20% of the total thickness of the base film.

[0065] It should be noted that the types of cushioning materials in the core layer, the first surface layer, the second surface layer, and optionally the third surface layer can be the same or different, but are preferably the same.

[0066] It should be noted that the cushioning material in the core layer, the first surface layer, the second surface layer, and optionally the third surface layer can simultaneously include granular polymer materials and fibrous polymer materials, as long as the above requirements are met.

[0067] It should be noted that the maximum length of the fibers in the fibrous polymer material in the core layer, the first surface layer, the second surface layer, and the optional third surface layer is less than or equal to the thickness of the base film, ensuring that the fibrous polymer material does not penetrate the base film and affect the surface quality of the base film.

[0068] In some embodiments, the matrix resin in the core layer, the first surface layer, the second surface layer, and optionally the third surface layer includes any one or more of PET, PPS, PEN, PP, PE, and modified resins thereof.

[0069] According to another typical embodiment of this application, a method for preparing a base film as described above is provided, comprising the following steps: providing a core layer raw material, a first surface layer raw material, and a second surface layer raw material, wherein the core layer raw material, the first surface layer raw material, and the second surface layer raw material each independently include a matrix resin and a buffer material, and the mass percentage of the buffer material in the core layer raw material is greater than the mass percentage of the buffer material in either the first surface layer raw material or the second surface layer raw material; and melt-extruding the core layer raw material, the first surface layer raw material, and the second surface layer raw material by a multilayer co-extrusion method to prepare a base film.

[0070] The base film prepared by the above method is compatible with existing common film extrusion and stretching production lines. Only the addition of buffer material is required in the raw material mixing stage. When the base film is used to prepare composite current collectors, there is no need to carry out large-scale modification of the existing composite aluminum foil production equipment, and the production cost is controllable.

[0071] The buffer material with elastic recovery properties in the base film prepared by the above method acts as a stress dispersion point, promoting the formation of more uniformly distributed crazing and shear bands in the stressed area of ​​the base film, rather than through-cracks caused by concentrated stress. The formation process of crazing and shear bands consumes a large amount of pressure energy, and the elastic deformation capacity of the buffer material can further absorb residual stress, thereby significantly reducing the peak shear stress inside the base film and preventing cracking. Furthermore, the core layer of the base film containing more buffer material undertakes the main stress absorption function, reducing the peak shear stress of the base film and preventing cracking. The lower content of buffer material in the first and second surface layers makes the surface of the base film relatively smooth, preventing excessive buffer material from protruding from the base film surface and affecting the composite effect with the metal layer when applied to composite current collectors, which is beneficial to improving the bonding force with the metal layer.

[0072] In some embodiments, in order to further improve the dispersion effect of the buffer material in the matrix resin, the buffer material (particles or fibers) in the core layer raw material, the first surface layer raw material and the second surface layer raw material are premixed with the matrix resin at a mass ratio of 1:(0.5~1.5) to prepare core layer masterbatch, first surface layer masterbatch and second surface layer masterbatch. The core layer masterbatch, the first surface layer masterbatch and the second surface layer masterbatch are mixed evenly with the matrix resin according to the set base film thickness and the raw material ratio of each layer to obtain core layer raw material, first surface layer raw material and second surface layer raw material.

[0073] Furthermore, the core layer masterbatch, the first surface layer masterbatch, and the second surface layer masterbatch can be prepared respectively according to the following methods: the matrix resin and the buffer material (particles or fibers) are premixed in a twin-screw extruder at a mass ratio of 1:(0.5~1.5); optionally, the premixing temperature is 250-290℃ and the rotation speed is 300-400rpm; then dispersed in a high-speed disperser (optionally, the rotation speed is 1500-2000rpm and the time is 10-15min) to ensure that the uniformity of the buffer dispersion is ≥90%, and no agglomerates >50μm can be observed under a microscope. After uniform mixing, the core layer masterbatch, the first surface layer masterbatch, and the second surface layer masterbatch are obtained by melt granulation.

[0074] In some embodiments, the preparation method of the base film further includes: mixing the core layer masterbatch, the first surface layer masterbatch, and the second surface layer masterbatch with the matrix resin according to the set base film thickness and the raw material ratio of each layer; melting and extruding the core layer raw material, the first surface layer raw material, and the second surface layer raw material through a multi-layer co-extrusion method at a temperature of 210°C-280°C; then casting and stretching; and finally setting the film at a temperature of 200°C-240°C for 20-30 seconds to obtain the base film of this application. Optionally, the length-to-diameter ratio of the twin-screw extruder is 36:1. The casting process involves extruding the melt through a die (gap 0.8mm-1.2mm), casting it onto a 39℃±2℃ quenching roller, and electrostatically adsorbing and bonding it. The thickness of the casting is 10-22 times that of the finished product. The stretching process includes longitudinal stretching with preheating at 80℃-90℃, stretching 3.3-4.3 times, and cooling at 25℃-35℃; and transverse stretching with preheating at 90℃-100℃ and stretching 4.0-4.8 times at 95℃-120℃.

[0075] According to another typical embodiment of this application, a composite current collector is provided, comprising a base film and a conductive metal layer located on at least one side of the base film; wherein the base film is any of the base films described above.

[0076] Because of the use of the aforementioned base film, the composite current collector is less prone to cracking or damage under high forming roller pressure, which helps maintain the packaging integrity of the composite current collector and thus improves the performance and safety of high-density electrodes such as lithium cobalt oxide.

[0077] The material of the conductive metal layer can be selected from existing technologies, such as aluminum, copper, or their alloys. Optionally, the thickness of the conductive metal layer on one side is 0.5 μm-2 μm.

[0078] In some embodiments, the conductive metal layer of the composite current collector is made of aluminum; optionally, the composite current collector is prepared by a method comprising the following steps: preparing a base film by any of the above methods; and depositing a conductive metal layer on at least one side of the base film. Optionally, the process of depositing the conductive metal layer (taking an aluminum layer as an example) includes: using a roll-to-roll vacuum evaporation machine, and evacuating to a vacuum degree ≤ 5 × 10⁻⁶. -3 Pa; using aluminum wire with a purity ≥ 99.90% as raw material, wire feeding speed: 200-300 mm / min, evaporation temperature controlled at 1000-1600℃; base film speed 10-20 m / min, controlling aluminum layer thickness at 0.5 μm-2 μm.

[0079] According to another typical embodiment of this application, an electrode is provided, comprising a composite current collector and an active material layer located on at least one side of the composite current collector; wherein the composite current collector is the aforementioned composite current collector. The electrode of this application, due to the use of the aforementioned composite current collector, has superior performance and a longer service life.

[0080] Because the base film of the aforementioned composite current collector is not easily cracked under high pressure, it can be applied to cathode material systems with high compaction density, such as lithium cobalt oxide. Optionally, the compaction density of the active material layer is 4.2 g / cm³. 3 -4.5g / cm 3 .

[0081] According to another typical embodiment of this application, an electrochemical device is provided, including the electrode described above. Due to the use of the electrode described above, the electrochemical device of this application has better performance and a longer service life. Optionally, the electrochemical device includes a battery.

[0082] The beneficial effects that this application can achieve will be further illustrated below through examples and comparative examples.

[0083] Example 1

[0084] (1) Raw material preparation

[0085] The base film raw material consists of 0.5% cushioning material and 99.5% PET matrix resin by weight.

[0086] The base film comprises a first surface layer, a core layer, and a second surface layer; the mass ratio of the buffer material in the first surface layer to the buffer material in the core layer is 1:2, and the mass ratio of the buffer material in the second surface layer to the buffer material in the core layer is also 1:2; the buffer material in the first and second surface layers consists of 0.5 μm polyurethane particles (elastic recovery rate of 70%, particle size distribution span less than 0.5), and the buffer material in the core layer consists of 1.5 μm polyurethane particles (elastic recovery rate of 70%, particle size distribution span less than 0.5);

[0087] The total thickness of the base film is set to 3μm, the first surface layer is set to account for 25% of the total thickness of the base film, and the second surface layer is set to account for 25% of the total thickness of the base film.

[0088] (2) Masterbatch preparation

[0089] The buffer material and matrix resin in the core layer, first surface layer, and second surface layer were premixed at a 1:1 mass ratio in a twin-screw extruder at 280℃ and 350 rpm. They were then dispersed in a high-speed disperser at 1800 rpm for 12 minutes to ensure a buffer material dispersion uniformity of ≥90%. Microscopic observation revealed no agglomerates >50 μm. The resulting masterbatches were then melt-granulated to obtain core layer masterbatch, first surface layer masterbatch, and second surface layer masterbatch.

[0090] (3) Multilayer co-extrusion preparation of base film

[0091] Melt extrusion: According to the base film thickness and raw material ratio set in step (1), the core layer masterbatch, the first surface layer masterbatch, the second surface layer masterbatch, and the matrix resin are added to a twin-screw extruder (length-to-diameter ratio 36:1), melt extruded at 280°C, then cast and stretched. After stretching, it is shaped at 220°C for 25s to obtain the base film. Among them, the cast sheet: the melt is extruded through the die (1mm gap), cast to a 39°C quench roller, and electrostatically adsorbed and bonded. The thickness of the cast sheet is 18 times that of the finished product. The stretching process parameters are set as follows: longitudinal stretching preheated at 85°C, stretched 4 times, and cooled at 30°C; transverse stretching preheated at 95°C and stretched 4.4 times at 110°C.

[0092] Example 2

[0093] The difference from Example 1 is that in step (1), the base film raw material is 5% buffer material and 95% matrix resin PET resin at a 100% mass ratio.

[0094] The mass ratio of the cushioning material in the first outer layer to the cushioning material in the core layer is 1:3, and the mass ratio of the cushioning material in the second outer layer to the cushioning material in the core layer is 1:3.

[0095] The cushioning material in the first and second outer layers consists of 1 μm polyurethane particles (70% elastic recovery rate, particle size distribution span less than 0.5), while the cushioning material in the core layer consists of 2 μm polyurethane particles (70% elastic recovery rate, particle size distribution span less than 0.5).

[0096] The total thickness of the base film is set to 6 μm, the first surface layer is set to account for 20% of the total thickness of the base film, and the second surface layer is set to account for 20% of the total thickness of the base film.

[0097] Example 3

[0098] The difference from Example 1 is that in step (1), the base film raw material is 10% buffer material and 90% matrix resin (PET resin) by a 100% mass ratio.

[0099] The mass ratio of the cushioning material in the first outer layer to the cushioning material in the core layer is 1:4, and the mass ratio of the cushioning material in the second outer layer to the cushioning material in the core layer is 1:4.

[0100] The cushioning material in the first and second outer layers is polyurethane particles with a particle size of 1.5 μm (elastic recovery rate of 70% and particle size distribution span of less than 0.5), while the cushioning material in the core layer is polyurethane particles with a particle size of 2.5 μm (elastic recovery rate of 70% and particle size distribution span of less than 0.5).

[0101] The total thickness of the base film is set to 10 μm, the first surface layer is set to account for 15% of the total thickness of the base film, and the second surface layer is set to account for 15% of the total thickness of the base film.

[0102] Example 4

[0103] The difference from Example 1 is that in step (1), the base film raw material is 15% buffer material and 85% matrix resin (PET resin) by a 100% mass ratio.

[0104] The mass ratio of the cushioning material in the first outer layer to the cushioning material in the core layer is 1:5, and the mass ratio of the cushioning material in the second outer layer to the cushioning material in the core layer is 1:5.

[0105] The first and second outermost layers are made of 1.5 μm polyurethane particles (70% elastic recovery rate, particle size distribution span less than 0.5), while the core layer is made of 3 μm polyurethane particles (70% elastic recovery rate, particle size distribution span less than 0.5).

[0106] The total thickness of the base film is set to 15 μm, the first surface layer is set to account for 5% of the total thickness of the base film, and the second surface layer is set to account for 5% of the total thickness of the base film.

[0107] Example 5

[0108] The difference from Example 1 is that in step (1), the base film raw material is 20% buffer material and 80% matrix resin (PET resin) by a 100% mass ratio.

[0109] Example 6

[0110] The difference from Example 1 is that in step (1), the buffer material is a fibrous polymer material (polyether ether ketone fiber with an elastic recovery rate of 70%), the average length of the fibrous polymer material in the first and second outer layers is 0.3 μm, and the average length of the fibrous polymer material in the core layer is 0.9 μm.

[0111] Example 7

[0112] The difference from Example 2 is that in step (1), the buffer material is a fibrous polymer material (polyether ether ketone fiber with an elastic recovery rate of 70%), the average length of the fibrous polymer material in the first and second outer layers is 1.2 μm, and the average length of the fibrous polymer material in the core layer is 2.4 μm.

[0113] Example 8

[0114] The difference from Example 3 is that in step (1), the buffer material is a fibrous polymer material (polyether ether ketone fiber with an elastic recovery rate of 70%), the average length of the fibrous polymer material in the first and second outer layers is 3 μm, and the average length of the fibrous polymer material in the core layer is 6 μm.

[0115] Example 9

[0116] Compared with Example 2, the base film further includes a third surface layer, which is disposed on the side of the first surface layer away from the core layer and the side of the second surface layer away from the core layer. The total thickness of the base film remains 6 μm. The mass ratio of the buffer material in the third surface layer to the buffer material in the core layer is 1:4. The buffer material in the third surface layer is polyurethane particles with a particle size of 0.5 μm (elastic recovery rate of 70%, particle size distribution span of less than 0.5). The thickness of the third surface layer accounts for 15% of the total thickness of the base film.

[0117] Example 10

[0118] Compared with Example 7, the base film further includes a third surface layer, which is disposed on the side of the first surface layer away from the core layer and the side of the second surface layer away from the core layer. The total thickness of the base film remains 6 μm. The mass ratio of the buffer material (polyetheretherketone fiber with an elastic recovery rate of 70%) in the third surface layer to the buffer material in the core layer is 1:4. The average length of the fibrous polymer material in the third surface layer is 0.6 μm. The thickness of the third surface layer is set to account for 15% of the total thickness of the base film.

[0119] Example 11

[0120] The difference from Example 5 is that, by a 100% mass ratio, the base film raw material consists of 25% buffer material and 75% matrix resin (PET resin).

[0121] Comparative Example 1

[0122] The difference from Example 1 is that in step (1), the core material, the first surface material, and the second surface material do not contain any cushioning material.

[0123] Comparative Example 2

[0124] The difference from Example 3 is that in step (1), the mass ratio of the buffer material in the first surface layer to the buffer material in the core layer is 4:1, and the mass ratio of the buffer material in the second surface layer to the buffer material in the core layer is 4:1.

[0125] The base films prepared in the above examples and comparative examples were respectively prepared into composite current collectors according to the following methods, and then assembled into batteries for electrochemical performance testing:

[0126] 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.

[0127] Battery Assembly: Using a Kejingzhida EI-200 coating machine, the rolled composite aluminum foil and electrolytic copper foil from the above examples and comparative examples were coated. The positive electrode material formulation was 95% lithium cobalt oxide (LiCoO2), 3% PVDF, and 2% SP; the negative electrode material formulation was 95% graphite, 2% conductive SP, and 2% CMC. After baking, rolling (compacting at 25 tons / cm at room temperature), die-cutting, roll welding, stacking, spot welding, plastic film forming, electrolyte injection, formation, and capacity testing, a 50AH soft-pack cell was prepared. The die-cut positive and negative electrode sizes were 126mm × 162mm for the positive electrode and 128mm × 164mm for the negative electrode, with 60 stacked layers for the positive electrode and 61 for the negative electrode. CR2032 coin cells were assembled in an argon glove box with a dew point below -40℃. After sealing, the battery should be left to stand for 12 hours.

[0128] Electrochemical cycling: The packaged battery was charged and discharged according to the national standard "Lithium-ion Batteries for Power Storage" (GB / T 36276-2023) using a battery testing system, and the cycle was repeated 30 times.

[0129] Post-cycle disassembly and sample preparation: The battery was disassembled in a glove box, the positive electrode was removed and thoroughly washed with anhydrous DMC solvent to remove residual electrolyte. For CP-SSEM cross-sectional analysis, electrode cross-sectional samples were prepared using focused ion beam (FIB) etching or cold mounting-mechanical polishing.

[0130] CP-SSEM Observation and Analysis: The prepared cross-sectional sample was placed on the field emission scanning electron microscope stage and observed using backscattered electron (BSE) imaging mode. This mode is sensitive to differences in atomic number of materials and can clearly distinguish aluminum foil, polymer layer, lithium cobalt oxide particles, and crack pores, as well as observe the cracking of the base film. Figure 1 and 2 The images show scanning electron microscope (SEM) images of cross-sectional samples of the base films prepared in Example 1 and Comparative Example 2 after cyclic testing and disassembly, respectively. The area within the blue box represents the region where the base film is located.

[0131] The test results are listed in Table 1.

[0132] Table 1

[0133]

[0134] The test results above show that the composite current collectors prepared using the base film of the present application containing a buffer material with specific proportions of elastic recovery performance in Examples 1-11 are less prone to cracking when applied in batteries. The base film prepared in Comparative Example 1, without the addition of buffer material, showed severe cracking under the same testing conditions. The base film in Comparative Example 2, with the addition of buffer material, failed to resist cracking because the rolling process during battery assembly presses from both sides to the middle, and the force during battery cycling is transmitted from the surface layer to the core layer. Since the surface buffer material content is higher than that in the core layer, it does not provide adequate crack resistance.

[0135] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0136] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A base film, characterized in that, include: The core layer and a first surface layer and a second surface layer located on two opposite surfaces of the core layer; the materials of the core layer, the first surface layer and the second surface layer each independently include a matrix resin and a cushioning material, and the mass percentage of the cushioning material in the core layer is greater than the mass percentage of the cushioning material in either the first surface layer or the second surface layer; The buffer material is a polymer material with elastic recovery properties.

2. The base film according to claim 1, characterized in that, The cushioning material satisfies at least one of the following characteristics: The elastic recovery rate of the buffer material is ≥70% under conditions of 23℃ and 50% compression. The buffer material has a processing temperature resistance ≥260℃; The buffer material comprises any one or more of granular polymer materials and fibrous polymer materials; optionally, the granular polymer material comprises any one or more of polyurethane particles, polyolefin elastomer particles, and nitrile rubber microspheres; optionally, the particle size of the granular polymer material is 0.5-3 μm, and the particle size distribution span is ≤0.5; optionally, the fibrous polymer material comprises any one or more of aromatic polyamide fibers, polyetheretherketone fibers, and polyethylene fibers; optionally, the ratio of the average length of the fibrous polymer material to the total thickness of the base film is 0.1:1-1:

1. The buffer material accounts for 0.5%-20% of the total mass of the base film, and optionally, 8%-15%.

3. The base film according to claim 2, characterized in that, The particle size of the particulate polymer material in the first and second surface layers is smaller than the particle size of the particulate polymer material in the core layer; and / or, The average length of the fibrous polymer material in the first and second outer layers is less than the average length of the fibrous polymer material in the core layer.

4. The base film according to claim 3, characterized in that, Includes at least one of the following features: The particle size of the granular polymer material in the core layer is 1.5 μm-3 μm; The particle size of the particulate polymer material in the first and second surface layers is 0.5 μm-1.5 μm; The ratio of the average length of the fibrous polymer material in the core layer to the total thickness of the base film is 0.3:1-0.6:1; The average length of the fibrous polymer material in the first and second surface layers is independently in a ratio of 0.1:1 to 0.3:1 to the total thickness of the base film.

5. The base film according to claim 1, characterized in that, It meets at least one of the following characteristics: The mass ratio of the cushioning material in the first and second outer layers to the mass of the cushioning material in the core layer is 1:(2-5). The total thickness of the base film is 3μm-15μm, and can be selected as 4μm-10μm; The thickness of the first and second surface layers each independently accounts for 5%-25% of the total thickness of the base film, and can be selected as 5%-15%; The matrix resin includes any one or more of PET, PPS, PEN, PP, PE and their modified resins.

6. The base film according to any one of claims 1 to 5, characterized in that, The base film further includes a third surface layer, which is located on the side of the first surface layer and / or the second surface layer away from the core layer; the mass percentage of the cushioning material in the third surface layer is less than the mass percentage of the cushioning material in the adjacent first surface layer or second surface layer.

7. A method for preparing a base film as described in any one of claims 1 to 6, characterized in that, The steps include the following: A core layer material, a first surface layer material, and a second surface layer material are provided. Each of the core layer material, the first surface layer material, and the second surface layer material independently includes a matrix resin and a cushioning material, and the mass percentage of the cushioning material in the core layer material is greater than the mass percentage of the cushioning material in either the first surface layer material or the second surface layer material. The base film is prepared by melt extrusion of the core layer material, the first surface layer material, and the second surface layer material through a multi-layer co-extrusion method.

8. A composite current collector, characterized in that, It includes a base film and a conductive metal layer located on at least one side of the base film; the base film is the base film according to any one of claims 1 to 6.

9. An electrode, characterized in that, It includes a composite current collector and an active material layer located on at least one side of the composite current collector; wherein the composite current collector is the composite current collector according to claim 8; Optionally, the active material layer includes lithium cobalt oxide.

10. An electrochemical device, characterized in that, Includes the electrode as described in claim 9; Optionally, the electrochemical device includes a battery.

Images