Base membrane for composite current collector, preparation method of base membrane, composite current collector, pole piece and battery

Abstract

The invention relates to the technical field of batteries, and provides a base membrane for a composite current collector, a preparation method of the base membrane, the composite current collector, a pole piece and a battery. The base membrane for the composite current collector comprises a porous matrix and a coating layer. Wherein the coating layer is arranged on the surface of at least one side of the porous matrix, the coating layer contains nanocellulose crystals, and the aperture of the maximum hole in the surface of the base membrane is smaller than that of the maximum hole in the porous matrix. The surface of the porous base membrane is coated with the nano cellulose crystals, a layer of compact, uniform and firm nano network is constructed on the surface of the base membrane by utilizing the unique one-dimensional nano fiber structure and the membrane forming characteristic of the nano cellulose crystals, the surface aperture is remarkably reduced, and the penetration problem in the metal deposition process is effectively prevented.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to the base film for composite current collectors and its preparation method, composite current collectors, electrodes and batteries. Background Technology

[0002] As lithium-ion batteries develop towards higher energy density and higher safety, composite current collectors have become a research hotspot due to their advantages of light weight and high safety. Some composite current collectors typically use porous polymer materials (such as polyethylene terephthalate, polypropylene, or polyethylene) as the intermediate supporting film, and prepare metal conductive layers (such as copper or aluminum) on both sides using physical vapor deposition methods such as magnetron sputtering or evaporation. The metal conductive layers are partially embedded in the pores of the supporting film, improving the adhesion between the metal conductive layer and the base film. However, porous polymer materials usually have a network porous structure with a wide pore size distribution range (30nm~300nm). The surface of the intermediate supporting film is prone to large-diameter defects, which can lead to metal atoms or clusters penetrating or completely entering the pores during the deposition of the metal layer. This results in the metal layer not completely covering the surface of the porous polymer material, leading to discontinuous metal layers, poor surface uniformity, and poor adhesion, causing problems such as uneven conductivity and large sheet resistance variations in the composite current collector. During battery cycling, uneven lithium-ion deposition can easily occur, posing a safety risk. Summary of the Invention

[0003] Therefore, it is necessary to provide a base film for composite current collectors with good stability and reduced surface pore size, as well as a method for preparing the same, the composite current collector, the electrode, and the battery.

[0004] In a first aspect, this application provides a base film for composite current collectors, comprising:

[0005] Porous matrix;

[0006] A coating layer is disposed on at least one side surface of a porous substrate, the coating layer contains nanocellulose crystals, and the pore size of the largest pore on the substrate surface is smaller than the pore size of the largest pore in the porous substrate.

[0007] In some embodiments, the base film for composite current collectors satisfies at least one of the following conditions:

[0008] (1) The porosity of the porous matrix is ​​40%~60%;

[0009] (2) The diameter of the largest pore in the porous matrix is ​​less than or equal to 300 nm;

[0010] (3) The porosity of the base membrane for composite current collectors is 30%~50%;

[0011] (4) The maximum pore diameter on the surface of the composite current collector base film is less than or equal to 50 nm;

[0012] (5) The pore size distribution (D95-D05) / D50 of the base membrane for composite current collectors is less than 0.5.

[0013] In some embodiments, the nanocellulose crystals satisfy at least one of the following conditions:

[0014] (1) The aspect ratio of the nanocellulose crystals is (5~20):1;

[0015] (2) The length of the nanocellulose crystals is 100nm~400nm and the diameter is 15nm~40nm;

[0016] (3) Nanocellulose crystals include carboxyl-modified nanocellulose crystals, and the proportion of carboxyl groups in the carboxyl-modified nanocellulose crystals is 0.5 mmol / g to 1.2 mmol / g;

[0017] (4) The crystallinity of nanocellulose crystals is greater than or equal to 80%.

[0018] In some embodiments, the base film for composite current collectors also satisfies at least one of the following conditions:

[0019] (1) The mass percentage of nanocellulose crystals contained in the coating layer is 60%~95%;

[0020] (2) The coating layer also contains a binder, and the mass of the binder in the coating layer is 5% to 30% of the mass of the nanocellulose crystals;

[0021] (3) The thickness of the coating layer is 0.5μm~5μm;

[0022] (4) The thickness of the porous matrix is ​​2μm~8μm;

[0023] (5) The porous substrate includes one or more of PP porous membrane, PE porous membrane, and PET porous membrane.

[0024] Secondly, this application provides a method for preparing a base film for composite current collectors, the method comprising:

[0025] A mixture containing nanocellulose crystals is applied to at least one side of a porous substrate and dried to form a coating layer on the surface of the porous substrate. The diameter of the largest pore on the substrate surface is smaller than the diameter of the largest pore in the porous substrate.

[0026] In some embodiments, the method of forming the mixture includes: mixing nanocellulose crystals with a solvent to form a dispersion, then adding a binder and a dispersant to the dispersion, and stirring to obtain the mixture.

[0027] In some embodiments, the method for forming the mixture also satisfies at least one of the following conditions:

[0028] (1) The solid content of the dispersion is 0.5%~5%;

[0029] (2) The amount of binder added is 5% to 30% of the mass of the nanocellulose crystals;

[0030] (3) The amount of dispersant added is 0.1% to 5% of the mass of the nanocellulose crystals;

[0031] (4) The stirring speed is 1000rpm~5000rpm and the stirring time is 2h~6h.

[0032] In some embodiments, before forming a coating layer on the surface of the porous substrate, the preparation method further includes: surface treatment of the porous substrate; the surface treatment includes at least one of corona treatment and plasma treatment.

[0033] In some embodiments, the preparation method also satisfies at least one of the following conditions:

[0034] (1) A coating layer is formed by at least one of the following methods: gravure coating, slot extrusion coating, and dip coating;

[0035] (2) The preparation method also includes: hot pressing the porous substrate with the coating layer; the hot pressing temperature is 80℃~130℃ and the pressure is 0.5MPa~5MPa;

[0036] (3) The drying temperature is 60℃~100℃.

[0037] Thirdly, this application provides a composite current collector, which includes a base film for composite current collector prepared by the method of preparing a base film for composite current collector according to the first aspect or the method of preparing a base film for composite current collector according to the second aspect.

[0038] The composite current collector also includes at least a metal layer disposed on the surface of the coating layer away from the base film for the composite current collector.

[0039] Fourthly, this application also provides an electrode, which includes a composite current collector as described in the third aspect.

[0040] Fifthly, this application also provides a battery, which includes the electrode sheet of the fourth aspect.

[0041] Compared with traditional technologies, this application has at least the following beneficial effects:

[0042] This application involves depositing a coating layer containing nano-cellulose crystals on the surface of a porous substrate. The nano-cellulose crystals interweave to form a dense three-dimensional nanonetwork on the surface of the porous substrate, effectively covering and filling the large pores on the substrate surface. This reduces the pore size of the base film and avoids the uneven deposition of metal on the surface caused by poor pore uniformity and large pore defects in the porous substrate during the initial stage of metal deposition. Simultaneously, the nanoporous structure and surface charge of the three-dimensional nanonetwork of nano-cellulose crystals facilitate uniform current distribution, thereby ensuring uniform lithium ion distribution. This can suppress the nucleation and growth of lithium dendrites from the source, further improving battery safety.

[0043] Furthermore, the flexibility and hydroxyl-rich surface of nanocellulose crystals give the coating sufficient strength and stable bonding with the porous matrix, making it less prone to cracking or peeling during subsequent processing. It also effectively improves the mechanical strength, tensile strength, and modulus of the base film, reducing the risk of lithium dendrite puncture and deformation during processing and use. Simultaneously, slurries containing nanocellulose crystals are more suitable for precision coating processes such as microgravure coating and slot extrusion, making it easier to control coating thickness and uniformity, suitable for large-scale production. Moreover, the thermal stability of nanocellulose crystals is superior to that of the porous matrix, forming a heat-resistant nanonetwork framework within the composite current collector. This suppresses thermal shrinkage of the base film, improving its thermal dimensional stability and safety, and delaying melting and collapse under high-rate charge / discharge or thermal runaway scenarios.

[0044] In addition, nanocellulose crystals are derived from renewable biomass (such as wood pulp and cotton pulp), and the raw materials are widely available and environmentally friendly. Attached Figure Description

[0045] Figure 1 A schematic diagram of the structure of a composite current collector base film provided in one embodiment;

[0046] Figure 2 A schematic diagram of the structure of a composite current collector provided in one embodiment.

[0047] Among them, 100-base membrane for composite current collector; 110-porous substrate; 120-coating layer; 200-metal layer. Detailed Implementation

[0048] The present application will be further described in detail below with reference to the embodiments and examples. These embodiments and examples are only for illustrating the present application and are not intended to limit the scope of the present application. The purpose of providing these embodiments and examples is to make the disclosure of the present application more thorough and comprehensive. It should also be understood that the present application can be implemented in many different forms and is not limited to the embodiments and examples described herein. Those skilled in the art can make various modifications or alterations without departing from the spirit of the present application, and the equivalent forms obtained also fall within the protection scope of the present application. In addition, numerous specific details are set forth in the following description to provide a fuller understanding of the present application. It should be understood that the present application can be implemented without one or more of these details.

[0049] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it means that it is selected from either "with" or "without." If there are multiple "optional" entries in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, each "optional" entry shall be independent.

[0050] In this application, the terms "first aspect," "second aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first aspect," "second aspect," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0051] In traditional technologies, polyolefin porous films have a wide pore size distribution (typically 30nm~300nm), making them prone to containing a small number of large defect pores. When depositing a metal layer, metal atoms or clusters can easily penetrate these pores, leading to the following problems: 1. Discontinuous metal layer: The metal layer deposited at the pore edges is prone to cracking and discontinuity, resulting in uneven conductivity and large sheet resistance variations in the composite current collector. 2. Low deposition efficiency: To prevent light transmission after sputtering, very slow thin-layer deposition is often required to "seal" the large pores, severely impacting production efficiency and cost. 3. Poor surface uniformity of the coating: During battery cycling, this can easily lead to uneven lithium-ion deposition, posing a safety risk. 4. Poor adhesion between the metal layer and this porous film.

[0052] Although ceramic coating (such as alumina or silicon dioxide) can be used to reduce the pore size of the base membrane, ceramic particles are hard and are prone to wear and powdering during the membrane winding process, and the interfacial bonding with the polymer matrix is ​​sometimes poor.

[0053] Based on this, this application uses nanocellulose crystals coated on the surface of a porous substrate. By utilizing the unique one-dimensional nanofiber structure and film-forming properties of nanocellulose crystals, a dense and uniform nanonetwork is constructed on the substrate surface, which significantly reduces the surface pore size and effectively prevents the problem of metals not being uniformly distributed on the substrate surface due to easy penetration of the substrate during the metal deposition process.

[0054] The first aspect of this application provides a base membrane for composite current collectors, such as... Figure 1 As shown, the composite current collector base membrane 100 includes a porous substrate 110 and a coating layer 120. The coating layer 120 is disposed on at least one side surface of the porous substrate 110, and contains nano-cellulose crystals. The pore size of the largest pore on the surface of the composite current collector base membrane 100 is smaller than the pore size of the largest pore in the porous substrate 110.

[0055] In some embodiments, the diameter of the largest pore on the surface of the base film for composite current collector is less than or equal to 50 nm, for example, it can be 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm or 50 nm; optionally, the diameter of the largest pore is less than 30 nm.

[0056] In some embodiments, the porosity of the porous matrix is ​​40% to 60%, for example, it can be 40%, 45%, 50%, 55% or 60%. The diameter of the largest pore in the porous matrix is ​​less than or equal to 300 nm, for example, it can be 60 nm, 90 nm, 120 nm, 150 nm, 180 nm, 210 nm, 240 nm, 270 nm or 300 nm.

[0057] In some embodiments, the porosity of the composite current collector base membrane is 30% to 50%, for example, it can be 30%, 35%, 40%, 45%, or 50%. This application prepares the composite current collector base membrane by setting a coating layer on a porous substrate, which reduces the pore size on the surface of the porous substrate while allowing the base membrane to retain sufficient porosity, thereby facilitating the bonding between the base membrane and the metal layer.

[0058] The pore size distribution (D95-D05) / D50 of the base membrane for composite current collectors is less than 0.5, for example, it can be 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, or 0.50; it can be selected as less than 0.25; and further selected as less than 0.2. Here, D05 refers to the pore size at which 5% of the composite current collector's pores are cumulatively distributed, indicating that 5% of the pore sizes are smaller than this value; D50 refers to the pore size at which 50% of the composite current collector's pores are cumulatively distributed, indicating that 50% of the pore sizes are smaller than this value; and D95 ​​refers to the pore size at which 95% of the composite current collector's pores are cumulatively distributed, indicating that 95% of the pore sizes are smaller than this value. The pore size distribution (D95-D05) / D50 can be determined using a semi-dry method (ASTM E1294-89).

[0059] The narrower pore size distribution of the base film selected in this application effectively avoids the problem of large pores in certain areas causing uneven metal deposition across the base film surface. This pore distribution also provides anchor points for the metal layer formation, enhancing adhesion. Furthermore, this application solves the problems of light transmission due to larger pore sizes and water absorption during electroplating, preventing electroplating solution penetration and severe blue-green discoloration of the composite current collector.

[0060] In some embodiments, the aspect ratio of the nanocellulose crystals is (5~20):1, for example, it can be 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or 20:1.

[0061] Optionally, the length of the cellulose nanocrystals is 100 nm to 400 nm, for example, 100 nm, 130 nm, 160 nm, 190 nm, 220 nm, 250 nm, 280 nm, 310 nm, 340 nm, 370 nm, or 400 nm. The diameter of the cellulose nanocrystals is 15 nm to 40 nm, for example, 15 nm, 18 nm, 21 nm, 24 nm, 27 nm, 30 nm, 33 nm, 36 nm, 39 nm, or 40 nm.

[0062] The present application selects the size of the nanocellulose crystals as described above, so that they have excellent network formation ability, which can effectively cover and penetrate into the defects of the porous matrix to achieve "nanosealing". This can reduce the maximum pore size of the base film to below 50nm, thereby forming a dense and continuous three-dimensional nanofiber network on the surface of the porous matrix, preventing metal atoms from penetrating the porous base film during magnetron sputtering and causing the metal to not be uniformly covered on the base film surface.

[0063] In some embodiments, the nanocellulose crystals include carboxyl-modified nanocellulose crystals. Optionally, the proportion of carboxyl groups in the carboxyl-modified nanocellulose crystals is 0.5 mmol / g to 1.2 mmol / g, for example, 0.5 mmol / g, 0.6 mmol / g, 0.7 mmol / g, 0.8 mmol / g, 0.9 mmol / g, 1.0 mmol / g, 1.1 mmol / g, or 1.2 mmol / g. Taking a carboxyl group content of 0.5 mmol / g as an example, it means that the amount of carboxyl groups in each 1g of carboxyl-modified nanocellulose crystals is 0.5 mmol.

[0064] Specifically, this application employs carboxyl-modified nanocellulose crystals, prepared by introducing carboxyl groups (-COOH) onto the surface of the nanocellulose crystals. These cellulose crystals contain abundant carboxyl and hydroxyl groups, which endow them with excellent hydrophilicity and electronegativity, thereby improving the material's dispersion performance, preventing aggregation within the system, and exhibiting good compatibility with the system. They are particularly suitable for water-based binders, working together to form a coating with excellent uniformity. Simultaneously, the numerous carboxyl groups within the cellulose crystals can form hydrogen bonds or van der Waals forces with polar groups on the surface of porous substrates (such as PET), enhancing the interlayer bonding strength between the coating layer, the porous substrate, and the metal layer, and promoting interfacial bonding.

[0065] In some embodiments, the crystallinity of the nanocellulose crystals is greater than or equal to 80%, for example, it can be 80%, 82%, 84%, 86%, 88%, 90%, 92% or 94%.

[0066] This application uses highly crystalline nanocellulose crystals, which can maintain the three-dimensional nanonetwork of the coating layer from deformation during coating and drying, thereby maintaining the stability of the nanopore structure. It also helps to maintain the pore size stability of the coating under temperature changes, thereby avoiding pore size changes caused by temperature changes, which in turn affect the overall performance of the composite current collector.

[0067] In some embodiments, the mass percentage of nanocellulose crystals contained in the coating layer is 60% to 95%, for example, it can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%. By controlling the content of nanocellulose crystals in the coating layer within the above range, this application ensures the formation of a three-dimensional nanonetwork structure on the surface of the porous substrate, thereby effectively reducing the surface pore size and improving the surface pore size uniformity.

[0068] In some embodiments, the coating layer further comprises a binder, wherein the mass of the binder in the coating layer is 5% to 30% of the mass of the nanocellulose crystals, for example, 5%, 10%, 15%, 20%, 25%, or 30%. Optionally, the binder comprises at least one of styrene-butadiene rubber, polyvinylidene fluoride, and polyacrylate.

[0069] In some embodiments, the thickness of the coating layer is 0.5 μm to 5 μm, for example, it can be 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm or 5.0 μm.

[0070] In some embodiments, the thickness of the porous substrate is 2 μm to 8 μm, for example, it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm or 8 μm. Optionally, the porous substrate includes a PP porous membrane, a PE porous membrane, or a PET porous membrane. For example, the porous substrate can be a laminated structure of one or at least two layers of PP porous membrane, PE porous membrane, or PET porous membrane.

[0071] The second aspect of this application provides a method for preparing a base film for composite current collectors, the method comprising:

[0072] A mixture containing nanocellulose crystals is applied to at least one side of a porous substrate and dried to form a coating layer on the surface of the porous substrate. The diameter of the largest pore on the substrate surface is smaller than the diameter of the largest pore in the porous substrate.

[0073] In some embodiments, the method for forming the mixture includes: mixing nanocellulose crystals with a solvent to form a dispersion, then adding a binder and a dispersant to the dispersion, and stirring to obtain the mixture. Optionally, deionized water can be used as the solvent.

[0074] The solid content of the dispersion is 0.5% to 5%, for example, it can be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5% or 5.0%.

[0075] The amount of binder added is 5% to 30% of the mass of the nanocellulose crystals, for example, 5%, 10%, 15%, 20%, 25%, or 30%. The amount of dispersant added is 0.1% to 5% of the mass of the nanocellulose crystals, for example, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, or 5.0%. Optionally, the dispersant includes polyoxyethylene ether dispersants, for example, at least one of fatty alcohol polyoxyethylene ether AEO-9 and lauryl alcohol polyoxyethylene ether Brij 35.

[0076] Optionally, the stirring speed is 1000rpm~5000rpm, for example, 1000rpm, 1500rpm, 2000rpm, 2500rpm, 3000rpm, 3500rpm, 4000rpm, 4500rpm or 5000rpm; the time is 2h~6h, for example, 2.0h, 2.5h, 3.0h, 3.5h, 4.0h, 4.5h, 5.0h, 5.5h or 6.0h.

[0077] In some embodiments, before forming a coating layer on the surface of the porous substrate, the preparation method further includes: surface treatment of the porous substrate.

[0078] Optionally, the surface treatment includes at least one of corona treatment and plasma treatment.

[0079] This application performs surface treatment on a porous substrate, which can increase the polar groups on the surface of the porous substrate and increase its surface tension (to more than 40 dyn / cm), thereby improving the affinity between the porous substrate and the coating layer.

[0080] In some embodiments, the coating layer is formed using at least one of microgravure coating, slot extrusion coating, and dip coating. In some embodiments, the drying temperature is 60°C to 100°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, or 100°C. Optionally, staged drying at different temperatures can be used. This application achieves complete removal of moisture and solvent from the coating layer through drying, thereby forming a dense coating layer containing nano-cellulose crystals on the surface of a porous substrate.

[0081] In some embodiments, the preparation method further includes hot-pressing the porous substrate with the coating layer. Optionally, the hot-pressing temperature is 80°C to 130°C, for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, or 130°C; the pressure is 0.5MPa to 5MPa, for example, 0.5MPa, 1.0MPa, 1.5MPa, 2.0MPa, 2.5MPa, 3.0MPa, 3.5MPa, 4.0MPa, 4.5MPa, or 5.0MPa. For example, a set of heating rollers can be used to hot-press the porous substrate with the coating layer, and the linear speed of the heating rollers can be 1m / min to 10m / min.

[0082] This application involves hot-pressing a porous substrate with a coating layer, which can further smooth the nanocellulose crystals and promote their tighter bonding with the binder and the porous substrate, forming a more dense coating layer.

[0083] Exemplarily, a method for preparing the above-mentioned composite current collector base film is provided, comprising the following steps:

[0084] Nanocellulose crystals were dispersed in a solvent to prepare a dispersion with a solid content of 0.5% to 5.0%. Then, a binder and a dispersant were added to the dispersion, with the amount of binder being 5% to 30% of the mass of the nanocellulose crystals and the amount of dispersant being 0.5% to 5.0% of the mass of the nanocellulose crystals. The mixture was stirred at a speed of 1000 rpm to 5000 rpm for 2 to 6 hours to obtain a uniform and stable mixture.

[0085] A porous substrate is provided, and the surface of the porous substrate is subjected to corona treatment or plasma treatment.

[0086] The above mixture is coated on at least one side of a porous substrate; then dried at 60°C to 100°C to form a coating layer with a thickness of 0.5 μm to 5 μm.

[0087] A porous substrate with a coating layer is hot-pressed using a heated roller. The hot-pressing temperature is 80℃~130℃, the pressure is 0.5MPa~5MPa, and the linear speed of the heated roller is 1m / min~10m / min, thus preparing a base film for composite current collectors.

[0088] A third aspect of this application provides a composite current collector, such as... Figure 2 As shown, the composite current collector includes a composite current collector base film 100 prepared by the preparation method of the first aspect of the composite current collector base film 100 or the second aspect of the composite current collector base film 100.

[0089] The composite current collector also includes at least a metal layer 200 disposed on the surface of the base film 100 for the composite current collector having a coating layer 120.

[0090] Understandably, the material of the metal layer can be selected based on the type of current collector. When the current collector is a negative electrode current collector, copper can be used for the metal layer; when the current collector is a positive electrode current collector, aluminum can be used for the metal layer.

[0091] In some embodiments, a metal layer may be formed by plating methods such as magnetron sputtering, vapor deposition, or electroplating.

[0092] A fourth aspect of this application also provides an electrode, which includes a composite current collector as described in the third aspect.

[0093] In this application, the electrode can be either a positive or negative electrode. The metal layer of the composite current collector in the positive electrode can be made of aluminum. The positive electrode material can be a ternary positive electrode material or a lithium iron phosphate positive electrode material. The metal layer of the composite current collector in the negative electrode material can be made of copper. The negative electrode material can be at least one of artificial graphite, natural graphite, silicon carbide, and hard carbon.

[0094] The fifth aspect of this application also provides a battery, which includes the electrode sheet of the fourth aspect.

[0095] It should be noted that the battery of this application can be assembled from a positive electrode, a negative electrode, a separator, and an electrolyte. The current collectors in the positive and negative electrodes can be the composite current collectors described above. The separator can be made of polypropylene as the base material, and the electrolyte can be a commonly used electrolyte in the art, composed of organic solvents, lithium salts, and additives.

[0096] The embodiments of this application will be described in detail below with reference to examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application. For experimental methods in the following embodiments where specific conditions are not specified, please refer to the guidelines given in this application, or follow experimental manuals or conventional conditions in the art, or follow the conditions recommended by the manufacturer, or refer to experimental methods known in the art.

[0097] Example 1

[0098] 1g of nanocellulose crystals were dispersed in 99g of deionized water to form a dispersion. The nanocellulose crystals had a length of about 200nm, a diameter of about 20nm, and a crystallinity of about 90%. Then, 0.2g of polyvinylidene fluoride and 0.01g of AEO-9 were added to the dispersion. The mixture was stirred at 3000rpm for 4h to obtain a uniform and stable mixture.

[0099] A PE (polyethylene) porous membrane with a thickness of 2 μm was selected as the porous substrate. Its porosity was 50%, the average pore size was about 50 nm, and the maximum pore size was about 150 nm. The porous substrate was subjected to corona treatment. The working parameters of the corona treatment were a power density of 1.0 W·min / m² and an electrode gap of 1.5 mm, so that the surface tension of the substrate after treatment was 50 dyn / cm.

[0100] The above mixture was coated onto the opposite two surfaces of the porous substrate using a slot extrusion coating method; then dried at 80°C to form coating layers with a thickness of 3 μm.

[0101] A porous substrate with a coating layer was hot-pressed at 100℃ and 2MPa to obtain a composite current collector base film with a thickness of 5μm and a porosity of 25%. The maximum pore size on the surface was measured to be 50nm by the bubble point method, and the particle size distribution was 0.48.

[0102] Example 2

[0103] 2g of nanocellulose crystals were dispersed in 98g of deionized water to form a dispersion. The nanocellulose crystals had a length of about 100nm, a diameter of about 15nm, and a crystallinity of about 80%. Then, 0.3g of polyvinylidene fluoride and 0.05g of AEO-9 were added to the dispersion. The mixture was stirred at 3000rpm for 4h to obtain a uniform and stable mixture.

[0104] PET (polyethylene terephthalate) with a thickness of 6 μm was selected as the porous substrate, with an average pore size of about 80 nm, a maximum pore size of about 200 nm, and a porosity of 60%. The porous substrate was then subjected to plasma treatment with the following parameters: power density of 3.0 W·min / m² and electrode gap of 2.5 mm, so that the surface tension of the treated substrate reached 60 dyn / cm.

[0105] The above mixture was coated onto the opposite two surfaces of the porous substrate using a microgravure coating method; then dried at 90°C to form coating layers with a thickness of 2 μm.

[0106] A porous substrate with a coating layer was hot-pressed at 120℃ and 5MPa to obtain a composite current collector base film with a thickness of 8μm and a porosity of 28%. The maximum pore size on the surface was measured to be 48nm by the bubble point method, and the particle size distribution was 0.45.

[0107] Example 3

[0108] The difference from Example 1 is that 5g of nanocellulose crystals were dispersed in 95g of deionized water to form a dispersion. The nanocellulose crystals had a length of approximately 400nm, a diameter of approximately 40nm, and a crystallinity of approximately 85%. Then, 1.5g of polyvinylidene fluoride and 0.25g of AEO-9 were added to the dispersion. The mixture was stirred at 3000rpm for 4 hours to obtain a homogeneous and stable mixture, thus preparing a composite current collector base film with a thickness of 5μm and a porosity of 32%. The maximum pore size on the surface was measured to be 45nm using the bubble point method, and the particle size distribution was 0.42.

[0109] Example 4

[0110] The composite current collector base membrane was prepared according to the method of Example 1, except that the nanocellulose crystals were replaced with an equal mass of carboxyl-modified nanocellulose crystals, and the carboxyl groups in the carboxyl-modified nanocellulose crystals accounted for 0.5 mmol / g. A composite current collector base membrane with a thickness of 5 μm and a porosity of 40% was obtained. The maximum pore size was measured to be 32 nm using the bubble point method, and the particle size distribution was 0.25.

[0111] Example 5

[0112] The composite current collector base membrane was prepared according to the method in Example 1, except that the nanocellulose crystals were replaced with an equal mass of carboxyl-modified nanocellulose crystals, and the proportion of carboxyl groups in the carboxyl-modified nanocellulose crystals was 1.2 mmol / g. A composite current collector base membrane with a thickness of 5 μm and a porosity of 42% was obtained. The maximum pore size was measured to be 25 nm using the bubble point method, and the particle size distribution was 0.28.

[0113] Example 6

[0114] The composite current collector base membrane was prepared according to the method in Example 1, except that the nanocellulose crystals were replaced with an equal mass of carboxyl-modified nanocellulose crystals, and the carboxyl groups in the carboxyl-modified nanocellulose crystals accounted for 0.8 mmol / g. A composite current collector base membrane with a thickness of 5 μm and a porosity of 45% was obtained. The maximum pore size was measured to be 20 nm using the bubble point method, and the particle size distribution was 0.2.

[0115] Comparative Example 1

[0116] The difference from the previous example is that a commercially available alumina ceramic was used to coat a PE porous membrane to prepare a base film with a porosity of 15%. The maximum pore size on the surface was measured to be 100 nm using the bubble point method, and the particle size distribution was 0.7.

[0117] Comparative Example 2

[0118] The composite current collector base membrane was prepared according to the method in Example 4, except that the carboxyl-modified nanocellulose crystals were replaced with nanocellulose fibers, resulting in a base membrane with a porosity of 35%. The maximum pore size of the base membrane surface was measured to be 60 nm using the bubble point method, and the particle size distribution was 0.52.

[0119] Cellulose nanocrystals (CNC) are rigid rod-shaped crystals with a negatively charged surface; while cellulose nanofibers (CNF) are flexible filamentous or mesh-like structures. As can be seen from Comparative Example 2, the cellulose nanocrystals used in this application, compared to cellulose nanofibers, can be tightly stacked during the coating and drying process to form a denser and more uniform nano-network sealing layer; while cellulose nanofibers (CNF), due to their flexible long fiber structure, are prone to forming a looser and more random network, resulting in a decrease in sealing effect and pore size distribution uniformity.

[0120] In summary, this application involves depositing a coating layer containing nanocellulose crystals on the surface of a porous substrate. The nanocellulose crystals interweave to form a dense three-dimensional nanonetwork on the surface of the porous substrate, effectively covering and filling the large pores on the substrate surface, reducing the pore size of the base film, and avoiding uneven metal coverage of the base film surface due to penetration risk in the early stages of metal deposition. Simultaneously, the nanoporous structure and surface charge of the three-dimensional nanonetwork of nanocellulose crystals can guide lithium ions to distribute more evenly on the surface of the composite current collector, thereby enabling more uniform deposition / dissolution of lithium metal during charging and discharging, effectively suppressing the nucleation and growth of lithium dendrites from the source.

[0121] In particular, compared with the use of nanocellulose crystals in Examples 1-3, the use of carboxyl-modified nanocellulose crystals in Examples 4-6 has multiple synergistic enhancement effects. It greatly reduces the pore size on the surface of the base film and the pore size distribution is more uniform. At the same time, the porosity of the base film is closer to that of the porous matrix before coating, and it still retains more pores, which has a positive effect on enhancing the bonding strength of the subsequent deposited metal layer. First, it has an enhanced electrostatic stabilizing effect. The strong electrostatic repulsion between particles effectively overcomes van der Waals forces and hydrogen bond attraction, thereby inhibiting the aggregation of nanocellulose. Second, for ester-containing matrices such as PET, carboxyl groups can form strong hydrogen bonds with them. Even for non-polar polyolefins (PP / PE), after corona or plasma treatment, oxygen-containing polar groups (such as carbonyl and hydroxyl groups) will be introduced on their surface. Carboxyl groups can form strong hydrogen bonds or van der Waals forces with them, significantly improving the adhesion between the coating layer and the substrate. Finally, during subsequent magnetron sputtering deposition of metal (Cu / Al), the negatively charged carboxyl groups can serve as an ideal "anchor point," generating strong physicochemical interactions with the initially deposited metal ions or atoms, enhancing the bonding force between the metal layer and the coating layer, reducing sheet resistance, and preventing the metal layer from falling off.

[0122] 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.

[0123] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these modifications and improvements all fall within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the appended claims.

Claims

1. A base film for a composite current collector, characterized by, The composite current collector base film comprises: a porous base body; a coating layer provided on at least one side surface of the porous base body, the coating layer containing nanocellulose crystals, and the maximum pore diameter of the surface of the base film being smaller than the maximum pore diameter of the porous base body.

2. The base film for a composite current collector according to claim 1, wherein The composite current collector base film satisfies at least one of the following conditions: (1) the porosity of the porous base body is 40% to 60%; (2) the diameter of the maximum pore in the porous base body is less than or equal to 300 nm; (3) the porosity of the composite current collector base film is 30% to 50%; (4) the maximum pore diameter of the surface of the composite current collector base film is less than or equal to 50 nm; (5) the pore size distribution (D95-D05) / D50 of the composite current collector base film is 0.5 or less.

3. The base film for a composite current collector according to claim 1, wherein The nanocellulose crystals satisfy at least one of the following conditions: (1) the aspect ratio of the nanocellulose crystals is (5-20):1; (2) the length of the nanocellulose crystals is 100 nm to 400 nm, and the diameter is 15 nm to 40 nm; (3) the nanocellulose crystals include carboxyl-modified nanocellulose crystals, and the proportion of carboxyl groups contained in the carboxyl-modified nanocellulose crystals is 0.5 mmol / g to 1.2 mmol / g; (4) the crystallinity of the nanocellulose crystals is greater than or equal to 80%.

4. The base film for a composite current collector according to any one of claims 1 to 3, wherein The composite current collector base film further satisfies at least one of the following conditions: (1) the mass proportion of the nanocellulose crystals contained in the coating layer is 60% to 95%; (2) the coating layer further contains a binder, and the mass of the binder contained in the coating layer is 5% to 30% of the mass of the nanocellulose crystals; (3) the thickness of the coating layer is 0.5 μm to 5 μm; (4) the thickness of the porous base body is 2 μm to 8 μm; (5) the porous base body is one or more of a PP porous film, a PE porous film, and a PET porous film.

5. A method for producing a base film for a composite current collector, characterized by, The preparation method comprises: arranging a mixed solution containing nanocellulose crystals on at least one side surface of a porous base body, and drying to form a coating layer on the surface of the porous base body, the maximum pore diameter of the surface of the base film being smaller than the maximum pore diameter of the porous base body.

6. The method for producing a base film for a composite current collector according to claim 5, characterized in that, The method for forming the mixed solution comprises: mixing the nanocellulose crystals with a solvent to form a dispersion solution, then adding a binder and a dispersant to the dispersion solution, and stirring to obtain the mixed solution.

7. The method for producing a base film for a composite current collector according to claim 6, characterized in that, The method for forming the mixed solution further satisfies at least one of the following conditions: (1) the solid content of the dispersion solution is 0.5% to 5%; (2) the amount of the binder added is 5% to 30% of the mass of the nanocellulose crystals; (3) the amount of the dispersant added is 0.1% to 5% of the mass of the nanocellulose crystals; (4) the stirring speed is 1000 rpm to 5000 rpm, and the stirring time is 2 h to 6 h.

8. The method for preparing a base film for a composite current collector according to claim 5, characterized by, Before forming the coating layer on the surface of the porous substrate, the preparation method further comprises: surface treatment of the porous substrate; the surface treatment comprises at least one of corona treatment and plasma treatment.

9. The method for producing a base film for a composite current collector according to any one of claims 5 to 8, wherein The preparation method further satisfies at least one of the following conditions: (1) the coating layer is formed by at least one of microgravure coating, slot die coating and dip coating; (2) the preparation method further comprises: heat pressing treatment of the porous substrate with the coating layer; the heat pressing treatment is at a temperature of 80-130 DEG C and a pressure of 0.5-5 MPa; (3) the drying temperature is 60-100 DEG C.

10. A composite current collector, characterized by, The composite current collector comprises the composite current collector base film according to any one of claims 1-4 or the composite current collector base film prepared by the preparation method according to any one of claims 5-9; The composite current collector further comprises a metal layer arranged at least on the side surface of the coating layer away from the composite current collector base film.

11. A pole piece characterized by, The electrode tab comprises the composite current collector according to claim 10.

12. A battery, characterized by The battery comprises the electrode tab according to claim 11.

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