Coated separator, method for producing a coated separator, and battery

Abstract

A kind of coated diaphragm, the preparation method of coated diaphragm and battery, by mixing each first nanofiber (21) with similar size in first coating (20), mixing each second nanofiber (31) with similar size in second coating (30), thereby avoid the problem that the size difference between different nanofibers is too large, and the nanofibers with similar surface energy are prone to agglomeration due to the large difference in surface energy, greatly improve the mixing uniformity between nanofibers.And by setting first coating (20) as large-scale first nanofiber (21) mixed with large-particle-size first ceramic particles (22), by setting second coating (30) as small-scale second nanofiber (31) mixed with small-particle-size second ceramic particles (32), so that the pore size of first coating (20) is large and has a strong skeleton structure, can make coated diaphragm maintain high heat resistance and high permeability;Small pore size second coating (30) can improve the transmission capacity of lithium ion, reduce the risk of electrochemical stability of high temperature storage or long time cycle.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and more particularly to a coated separator, a method for preparing the coated separator, and a battery. Background Technology

[0002] The separator is one of the core components of a lithium battery, and its performance has a significant impact on the overall performance of the battery, making it a key technology restricting the development of lithium batteries. As the application fields of lithium batteries continue to expand and the influence of lithium battery products on people's lives deepens, the requirements for lithium battery performance are also increasing. To meet the development requirements of lithium batteries, the separator, as an important component, should not only possess good chemical stability and low manufacturing costs, but improving the safety performance of lithium batteries is also a crucial trend in current lithium battery development.

[0003] In existing related technologies, the coated separator may include a base membrane and a coating applied to at least one surface of the base membrane, and a ceramic layer may be applied to the surface of the coated separator to improve the heat resistance of the separator. However, the two-phase interface between the ceramic layer and the separator is a weak bond, which is prone to separation at high temperatures, leading to the peeling of the ceramic layer, melting of the separator substrate, and a decrease in heat resistance. In addition, the surface ceramic layer increases the lithium ion transport distance, thereby reducing the lithium ion conductivity, and the large pores cannot effectively retain the liquid electrolyte. Summary of the Invention

[0004] This invention provides a coated separator, a method for preparing the coated separator, and a battery, to improve the performance of the separator.

[0005] According to a first aspect of the present invention, a coated membrane is provided, comprising a base membrane and a coating structure disposed on at least one surface of the base membrane, the coating structure comprising at least a first coating and a second coating, wherein the first coating is disposed on the surface of the base membrane, and the second coating is disposed on the side of the first coating away from the base membrane; the first coating comprises a first nanofiber material and a first ceramic particle, and the second coating comprises a second nanofiber material and a second ceramic particle;

[0006] The first nanofiber material includes a plurality of first nanofibers, and the second nanofiber material includes a plurality of second nanofibers;

[0007] The length difference between the plurality of first nanofibers is no greater than 500 nm, the length difference between the plurality of second nanofibers is no greater than 200 nm, and the length of the first nanofiber is greater than the length of the second nanofiber, and the average particle size of the first ceramic particle is greater than the average particle size of the second ceramic particle.

[0008] Optionally, the length of the first nanofiber is 500-1000 nm; the average particle size of the first ceramic particle is 100-600 nm.

[0009] Optionally, the mass ratio of the first nanofiber material to the first ceramic particles is 5:1 to 1:5.

[0010] Optionally, the length of the second nanofiber is 100-300 nm; the average particle size of the second ceramic particle is 10-60 nm.

[0011] Optionally, the mass ratio of the second nanofiber material to the second ceramic particles is 5:1 to 1:5.

[0012] Optionally, the diameter of the first nanofiber is 5-50 nm, and the diameter of the second nanofiber is 5-50 nm.

[0013] Optionally, the thickness ratio of the first coating to the second coating is greater than 2.

[0014] Optionally, at least one additional coating may be included between the first coating and the second coating.

[0015] Optionally, the surface of the second ceramic particle is grafted with lithium-ion fast conductor functional groups;

[0016] Furthermore, the lithium-ion fast conductor functional group includes any one of hydroxyl (-OH), carbonyl (-C=O), fluorine (-F), and carboxyl (-COOH).

[0017] Optionally, the first ceramic particles and / or the second ceramic particles are inorganic materials with a melting point above 200°C, electrical insulation properties, and electrochemical stability within the operating range of lithium batteries.

[0018] Optionally, the coated diaphragm satisfies at least one of the following conditions:

[0019] a) Ionic conductivity ≥ 1.2 mS / cm;

[0020] b) Capacity retention ≥ 98%;

[0021] c) Heat shrinkage at 180℃ / h ≤5%.

[0022] According to a second aspect of the present invention, a method for preparing a coated diaphragm is provided for preparing the coated diaphragm involved in the first aspect and its alternative embodiments, the method comprising:

[0023] Preparation of slurry: preparing a first slurry comprising first ceramic particles and first nanofiber material, and preparing a second slurry comprising second ceramic particles and second nanofiber material;

[0024] Coating film formation: The first slurry is coated onto at least one surface of the base film to form a first coating, and a second slurry is coated onto the side of the first coating away from the base film to form a second coating;

[0025] Furthermore,

[0026] The preparation of the first slurry includes dispersing the first ceramic particles in a first solvent to obtain a first ceramic dispersion, dispersing the first nanofiber material in a second solvent to obtain a first nanofiber dispersion, and mixing the first ceramic dispersion and the first nanofiber dispersion to obtain the first slurry.

[0027] The preparation of the second slurry includes dispersing the second ceramic particles in a third solvent to obtain a second ceramic dispersion, dispersing the second nanofiber material in a fourth solvent to obtain a second nanofiber dispersion, and mixing the second ceramic dispersion and the second nanofiber dispersion to obtain the second slurry.

[0028] Optionally, prior to preparing the slurry, the method further includes:

[0029] Ceramic screening: Select first ceramic particles and second ceramic particles of different sizes, wherein the average particle size of the first ceramic particles is 100-600 nm and the average particle size of the second ceramic particles is 10-60 nm.

[0030] Furthermore, it also includes ceramic pretreatment: the second ceramic particles are subjected to a grafting reaction with the material to be grafted, so that lithium-ion fast conductor functional groups are grafted onto the surface of the second ceramic particles.

[0031] Optionally, the graft material includes any one of polycarbonate, polylactic acid, polyurethane, and perfluoropropyl vinyl ether.

[0032] According to a third aspect of the present invention, a battery is provided, comprising a coated separator as described in the first aspect and its alternatives, or a coated separator prepared by a method comprising a coated separator as described in the second aspect and its alternatives.

[0033] Furthermore, the battery is a lithium battery.

[0034] The coated separator, its preparation method, and the battery provided by this invention, by mixing first nanofibers of similar size in a first coating and second nanofibers of similar size in a second coating, avoids the problem of agglomeration of nanofibers with similar surface energies due to excessive size differences between different nanofibers, thus significantly improving the mixing uniformity between nanofibers. Furthermore, by setting the first coating as a mixture of large-scale first nanofibers and large-diameter first ceramic particles to form a first mixed material layer with large and uniform pore size, this first mixed material layer, located on the surface of the base membrane, maintains high heat resistance and high permeability of the coated separator due to its large pore size and strong framework structure. Similarly, by setting the second coating as a mixture of small-scale second nanofibers and small-diameter second ceramic particles to form a second mixed material layer with small and uniform pore size, the small pores have a strong adsorption capacity for electrolyte due to capillary action, thereby improving the transport capacity of lithium ions and reducing the possibility of lithium dendrite formation, thus reducing the risk of electrochemical stability during high-temperature storage or long-term cycling. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a partial structural diagram of the coated diaphragm in one embodiment of the present invention;

[0037] Figure 2 This is a schematic flowchart of a method for preparing a coated diaphragm according to an embodiment of the present invention.

[0038] Explanation of reference numerals in the attached figures:

[0039] 10-Base film;

[0040] 20 - First coating;

[0041] 21-First Nanofiber

[0042] 22-First ceramic particle;

[0043] 30 - Second coating;

[0044] 31-Second nanofiber;

[0045] 32 - Second ceramic particles. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] In the description of this invention, it should be understood that the terms "upper part", "lower part", "upper end", "lower end", "lower surface", "upper surface", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention.

[0048] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature.

[0049] In the description of this invention, "a plurality of" means multiple, such as two, three, four, etc., unless otherwise explicitly specified.

[0050] In the description of this invention, unless otherwise explicitly specified and limited, the term "connection" and other such terms should be interpreted broadly. For example, it can refer to a fixed connection, a detachable connection, or an integral connection; it can refer to a mechanical connection, an electrical connection, or a connection that allows communication between the components; it can refer to a direct connection or an indirect connection through an intermediate medium; it can refer to the internal communication between two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0051] The technical solution of the present invention will be described in detail below with reference to specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments.

[0052] Prior to filing this application, the applicant conducted a series of studies and experiments on existing diaphragms:

[0053] In order to address the problem of easy peeling off of ceramic layers, the art proposes to add one-dimensional nanomaterials to the layers.

[0054] To address the problem of disordered stacking of one-dimensional nanomaterials in coatings, which easily leads to excessive voids and insufficient contact points, the applicant proposed a corresponding solution. This solution involves progressively decreasing the length of the one-dimensional nanomaterials with each layer of the coating (reflected in the decreasing length description of the one-dimensional nanomaterials in each layer). Since longer nanomaterials create larger voids, these voids become larger at the bottom layers. The shorter nanomaterials in the upper layers can fill these voids to some extent. After layer-by-layer deposition, the coating has fewer voids and more contact points. Therefore, the dense structure of the coating can suppress thermal deformation of the diaphragm under heat, thereby improving the heat resistance of the coated diaphragm. The applicant has also filed a patent application for this solution (application number: PCT / CN2022 / 077087, application date: 2022.02.21). This application incorporates the entire contents of PCT / CN2022 / 077087, meaning that the entire contents of PCT / CN2022 / 077087 serve as the basis for this application.

[0055] To address the problem that the excessively high density of one-dimensional nanofibers, resulting from the layer-by-layer decreasing stacking method, hinders lithium-ion transport channels, affects lithium-ion transport efficiency, and limits membrane performance, the applicant proposed a solution: introducing ceramic particles into the one-dimensional nanomaterial coating can effectively improve ion conductivity. The applicant has also filed a patent application for this solution (application number: PCT / CN2022 / 108551, application date: July 28, 2022). The entire contents of PCT / CN2022 / 108551 are incorporated herein by reference, and thus, the entire contents of PCT / CN2022 / 108551 serve as the basis for this application.

[0056] However, further research by the applicant revealed that while the aforementioned approach can utilize ceramic particles to avoid excessive stacking density of one-dimensional nanomaterials, thereby forming more efficient lithium-ion transport channels and improving lithium-ion transport efficiency, the overall performance of the separator remains unsatisfactory. Through research and experimentation, the applicant discovered that the reason lies in the significant size differences between different nanofibers. This leads to the tendency for nanofibers with similar surface energies to aggregate, meaning smaller nanofibers with higher surface energies tend to agglomerate, forming clusters that then mix with larger nanofibers. This prevents smaller nanofibers from fully utilizing their potential. Furthermore, the ceramic particles were not differentiated for nanofibers of different sizes, further limiting the separator's performance.

[0057] Based on this discovery, the applicant obtained the technical solution of this application through a series of studies, experiments, and verifications. Since the ordered arrangement of one-dimensional nanofibers is a major innovation of the applicant, the problems further studied based on the ordered arrangement and the solutions obtained are also major innovations. The entire research process should be regarded as an integral part of this solution and should be considered as a whole when evaluating the inventiveness of this application.

[0058] Please refer to Figure 1 This invention provides a coated diaphragm, comprising: a base membrane 10 and a coating structure disposed on the base membrane 10, the coating structure comprising a first coating 20 and a second coating 30, wherein the first coating 20 is disposed on the surface of the base membrane 10, and the second coating 30 is disposed on the side of the first coating 20 away from the base membrane 10; the first coating 20 contains a first nanofiber material and a first ceramic particle 22, and the second coating 30 contains a second nanofiber material and a second ceramic particle 32; the first nanofiber material comprises a plurality of first nanofibers 21, and the second nanofiber material comprises a plurality of second nanofibers 31; the length difference between each first nanofiber 21 is not greater than 500 nm, the length difference between each second nanofiber 31 is not greater than 200 nm, and the length of the first nanofiber 21 is greater than the length of the second nanofiber 31, and the average particle size of the first ceramic particle 22 is greater than the average particle size of the second ceramic particle 32.

[0059] This invention addresses the issue that significant size differences between nanofibers can lead to agglomeration of nanofibers with similar surface energies due to their large surface energy variations. Smaller nanofibers, with their higher surface energy, readily aggregate to form clusters, which then mix with larger nanofibers. This prevents the smaller nanofibers from fully realizing their potential. By mixing nanofibers of similar size, the mixing uniformity is significantly improved, thereby reducing these differences. Furthermore, by mixing large-scale first nanofibers with large-diameter first ceramic particles to form a first mixed material layer with large and uniform pore size, this first mixed material layer is disposed on the surface of the base membrane. Due to its large pore size and strong framework structure, it can maintain the high heat resistance and high permeability of the coated separator. By mixing small-scale second nanofibers with small-diameter second ceramic particles to form a second mixed material layer with small and uniform pore size, the small pores have a strong adsorption capacity for electrolyte due to capillary action, thereby improving the transport capacity of lithium ions and reducing the possibility of lithium dendrite formation, thus reducing the risk of electrochemical stability during high-temperature storage or long-term cycling. This effectively improves the performance of the separator.

[0060] In a preferred embodiment, the length of the first nanofiber 21 is 500-1000 nm, for example, it can be 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, etc.; the average particle size of the first ceramic particle 22 is 100-600 nm, for example, it can be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, etc. The first coating containing the first nanofiber and the first ceramic particle is disposed on the surface of the base film to form a large pore size and a strong skeleton structure, which can maintain the high heat resistance and high permeability of the coated diaphragm.

[0061] In a preferred embodiment, the mass ratio of the first nanofiber material to the first ceramic particles 22 is 5:1 to 1:5. By setting the mass ratio of the first nanofiber material to the first ceramic particles 22 within this range, the coating can maintain good heat resistance and lithium-ion conductivity. Specifically, the mass ratio of the first nanofiber material to the first ceramic particles 22 can be any one of 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, or 1:5, or a range between any two. Of course, the mass ratio of the first nanofiber material to the first ceramic particles can also take other values. Specific values ​​are not considered limitations of this invention; as long as the ratio is between 5:1 and 1:5, it is within the scope of protection of this invention.

[0062] In a preferred embodiment, the length of the second nanofiber 31 is 100-300 nm, for example, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, etc.; the average particle size of the second ceramic particle 32 is 10-60 nm, for example, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, etc. The second coating is disposed on the side away from the base film relative to the first coating, and is a mixture of the second nanofiber and the second ceramic particle. Due to capillary action, the small pore size has a strong adsorption capacity for the electrolyte, improving the transport capacity of lithium ions, thereby reducing the possibility of lithium dendrite formation and reducing the risk of electrochemical stability during high-temperature storage or long-term cycling.

[0063] In a preferred embodiment, the mass ratio of the second nanofiber material to the second ceramic particles 32 is 5:1 to 1:5. By setting the mass ratio of the second nanofiber material to the second ceramic particles 32 within this range, the coating exhibits better battery capacity retention and lithium-ion conductivity. Specifically, the mass ratio of the second nanofiber material to the second ceramic particles 32 can be any one of 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, or 1:5, or a range between any two. Of course, the mass ratio of the second nanofiber material to the second ceramic particles can also take other values. Specific values ​​are not considered limitations of this invention; as long as the ratio is between 5:1 and 1:5, it is within the scope of protection of this invention.

[0064] In a preferred embodiment, the diameter of the first nanofiber is 5-50 nm, and the diameter of the second nanofiber is 5-50 nm.

[0065] In a preferred embodiment, the thickness ratio of the first coating 20 to the second coating 30 is greater than 2; thereby further and effectively improving the heat resistance of the coated diaphragm.

[0066] In a preferred embodiment, at least one additional coating layer is further included between the first coating 20 and the second coating 30. For example, an additional coating layer, such as a third coating, may be included between the first coating 20 and the second coating 30; or two additional coating layers may be included, such as a third coating and a fourth coating. Of course, other numbers of additional coating layers may also be included. The additional coating layers may be selected from the same material and size as the first or second coating layer, or they may be selected from different materials and sizes. That is, a third coating layer is provided between the first and second coating layers, the third coating layer comprising third ceramic particles with an average particle size of 100-600 nm and third nanofibers with an aspect ratio ≤200, and the material and size of the third ceramic particles and the third nanofibers are different from those of the second coating layer; or a third coating layer is provided between the first and second coating layers, the third coating layer comprising third ceramic particles with an average particle size of 10-60 nm and third nanofibers with an aspect ratio ≤60, and the material and size of the third ceramic particles and the third nanofibers are different from those of the first coating layer. It should be noted that the number of coating layers, the selection of different interlayer materials, and the setting of dimensions in the coated diaphragm can be adjusted according to actual needs, and should not be construed as a limitation of the present invention.

[0067] In a preferred embodiment, the surface of the second ceramic particle 32 is grafted with lithium-ion fast conductor functional groups. The lithium-ion fast conductor functional groups abundant on the surface of the second ceramic particle can significantly improve the lithium-ion transport capability. Further, the lithium-ion fast conductor functional groups include any one of hydroxyl (-OH), carbonyl (-C=O), fluorine (-F), and carboxyl (-COOH).

[0068] In a preferred embodiment, the first ceramic particle 22 and / or the second ceramic particle 32 are inorganic materials with a melting point above 200°C, electrical insulation, and electrochemical stability within the operating range of lithium batteries. Examples include oxide ceramics such as alumina, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium oxide, cerium oxide, yttrium oxide, and zinc oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; and ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, basic alumina, potassium titanate, talc, kaolinite, clay, pearl clay, hydrous kaolinite, pyrophyllite, montmorillonite, sericite, mica, magnesia chlorite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand. These ceramics can be used individually or in combination.

[0069] In a preferred embodiment, the coated diaphragm satisfies at least one of the following conditions:

[0070] a) Ionic conductivity ≥ 1.2 mS / cm;

[0071] b) Capacity retention ≥ 98%;

[0072] c) Heat shrinkage at 180℃ / h ≤5%.

[0073] In addition, please refer to Figure 2 The present invention also provides a method for preparing a coated diaphragm, used to prepare the coated diaphragm involved in the first aspect and its alternative solutions, the preparation method comprising:

[0074] S1: Preparation of the first slurry: Preparation of a first slurry containing first ceramic particles and first nanofiber material;

[0075] S2: Preparation of the second slurry: Preparation of a second slurry containing second ceramic particles and second nanofiber materials;

[0076] S3: Coating film formation: The first slurry is coated on at least one surface of the base film to form a first coating, and the second slurry is coated on the side of the first coating away from the base film to form a second coating.

[0077] Specifically, the preparation of the first slurry includes dispersing the first ceramic particles in a first solvent to obtain a first ceramic dispersion, dispersing the first nanofiber material in a second solvent to obtain a first nanofiber dispersion, and mixing the first ceramic dispersion and the first nanofiber dispersion to obtain the first slurry.

[0078] The preparation of the second slurry includes dispersing the second ceramic particles in a third solvent to obtain a second ceramic dispersion, dispersing the second nanofiber material in a fourth solvent to obtain a second nanofiber dispersion, and mixing the second ceramic dispersion and the second nanofiber dispersion to obtain the second slurry.

[0079] The first solvent, the second solvent, the third solvent, and the fourth solvent can be the same type of solvent, or they can be different; this invention does not impose any restrictions.

[0080] In this invention, there is no restriction on the order of preparing the first slurry and the second slurry; the second slurry can be prepared first, followed by the first slurry.

[0081] In one example, the first and third solvents are selected from water, N-methylpyrrolidone, ethanol, acetone, etc. The first and second ceramic particles are dispersed uniformly in the solvent by means of high-speed stirring, high-pressure homogenization, sand milling, etc., and the mass concentration of the first and second ceramic particles in the dispersion is 2%-40%.

[0082] In one example, the second and fourth solvents are selected from water, N-methylpyrrolidone, ethanol, acetone, etc. The first and second nanofiber materials are dispersed uniformly in the dispersant by means of high-speed stirring, high-pressure homogenization, sand milling, etc., and the mass concentration of the first and second nanofibers in the dispersant is 2%-30%.

[0083] The lengths of the first nanofibers can vary, as long as the length difference is no greater than 500 nm, i.e., within a roughly equal scale range. Similarly, the lengths of the second nanofibers can vary, as long as the length difference is no greater than 200 nm, i.e., within a roughly equal scale range.

[0084] The different lengths of nanofibers are achieved by selecting the raw materials for the nanofibers. For example, when different materials are selected for the nanofibers, nanofibers of different lengths may be formed.

[0085] In other examples, nanofibers of different lengths can also be formed using appropriate technical means.

[0086] The raw materials or broken nanofiber materials are broken apart, and nanofibers of different lengths are formed by breaking them apart once or multiple times.

[0087] In this way, by breaking the nanofiber material, a shorter nanofiber material can be formed based on a raw material of a certain length of nanofiber material. For example, the nanofiber material can be broken into half of the raw material. In other examples, it can also be achieved without breaking it in half.

[0088] Any existing or improved means that can break nanofiber materials can be used as a specific example of an embodiment of the present invention. In a specific example, this can be achieved by etching the nanofiber material; for example, the nanofibers can be etched to half the length of the raw material.

[0089] Based on the required layering, the raw material can be broken in one step or multiple times. For example, the nanofiber material can be etched to half the length of the raw material, then half the length of the nanofiber material can be retained, and then the other half of the length of the nanofiber material can be broken to obtain a quarter-length nanofiber material. In other examples, a quarter-length portion of the nanofiber material can be further broken. The number of breaks can be configured arbitrarily according to the requirements.

[0090] Of course, at least one of the following bonding processes can also be implemented, and nanofiber materials of different lengths can be formed through one or more bonding processes:

[0091] The raw material of the nanofiber material is bonded to one end of another raw material;

[0092] The joined nanofiber material is joined to one end of the raw material;

[0093] The joined nanofiber material is joined to one end of another joined nanofiber material.

[0094] It can be seen that the objects to be joined can be one end of a raw material and one end of another raw material, one end of a raw material and one end of the joined nanofiber material, or one end of the joined nanofiber material and one end of another joined nanofiber material.

[0095] In this regard, by joining nanofiber materials, a longer nanofiber material can be formed based on a raw material of one length. For example, two raw materials can be joined to form a nanofiber material of twice the length. In other examples, raw materials (or joined nanofiber materials) of different lengths can also be used.

[0096] Any existing or improved means that can achieve the bonding of nanofiber materials can be used as a specific example of an embodiment of the present invention.

[0097] For example, the bonding of nanofiber materials can be achieved based on materials rich in hydroxyl functional groups (such as polyethylene glycol PEG). Furthermore, the bonding mentioned above can include:

[0098] The nanofiber material to be bonded is mixed with a material rich in hydroxyl functional groups (such as PEG) in a solution;

[0099] Molecular sieve particles are added to the solution as a catalyst;

[0100] The solution is heated, then cooled, and the molecular sieve is removed by filtration to obtain the bonded nanofiber material.

[0101] In a specific example, let's take nanocellulose. The hydroxyl content at the ends of nanocellulose is significantly higher than in the middle region. To increase the length of nanocellulose, the activity of the hydroxyl groups can be fully utilized, as follows:

[0102] (1) Mix nanocellulose with polyethylene glycol (PEG). The molecular weight of PEG is 50,000-1,000,000 g / mol. PEG accounts for 1% of nanocellulose. Stir the two thoroughly to form the corresponding solution.

[0103] (2) Add 13A molecular sieve particles with a particle size of 1mm-10mm to the above solution. The molecular sieve is a catalyst.

[0104] (3) Heat the water to 80 degrees Celsius for 1-2 hours;

[0105] (4) Cool to room temperature and filter to remove 13A molecular sieve particles to obtain conjugated nanocellulose.

[0106] In the above scheme, by adding materials rich in hydroxyl functional groups (such as PEG), bridging can be achieved between nanocellulose fibers. Using molecular sieves (such as 13A molecular sieves) as catalysts can accelerate the polymerization reaction between PEG and nanocellulose, ultimately forming a nanocellulose-PEG-nanocellulose structure. Based on the desired layering, single or multiple bonding of the raw materials can be achieved. For example, the raw material of the nanofiber material can be bonded to twice its original length, then a portion of the nanofiber material of twice its original length can be retained, and another portion of the nanofiber material of twice its original length can be bonded to the raw material or the nanofiber material of twice its original length to obtain nanofiber material of three or four times its original length. In other examples, further bonding can be performed, and the number of bonding operations can be arbitrarily configured according to requirements.

[0107] The process of mixing a first ceramic dispersion with a first nanofiber dispersion to obtain a first slurry and mixing a second ceramic dispersion with a second nanofiber dispersion to obtain a second slurry specifically includes: mixing the first ceramic dispersion with the first nanofiber dispersion to obtain a first mixed solution; and mixing the second ceramic dispersion with the second nanofiber dispersion to obtain a second mixed solution. The mixing methods include high-speed stirring, high-pressure homogenization, and sand milling dispersion. Subsequently, an adhesive is added to both the first and second mixed solutions to obtain the first and second slurries, respectively. The adhesive is at least one selected from polyvinyl alcohol, polyacrylonitrile, polyacrylic acid, styrene-butadiene rubber, carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, and polyimide. The adhesive accounts for 1%-10% of the solid mass in both the first and second mixed solutions.

[0108] In one example, the coating process involves first coating a first slurry (prepared above) onto a base film and then drying it to obtain a first coating layer. Subsequently, a second slurry is coated onto the first coating layer and dried to obtain the coated diaphragm. The coating method can be spraying, dip coating, gravure coating, printing coating, extrusion coating, or wire rod coating. The base film is, for example, a polyolefin base film with a thickness of 3-30 micrometers, a drying temperature of 40-130 degrees Celsius, and a coating speed of 10-200 m / min.

[0109] Of course, it should be recognized that drying is only one way for the first / second slurry to form the first / second coating; the first / second slurry can also form the first / second coating through phase inversion. This invention does not impose any limitations on how the first / second slurry forms the first / second coating.

[0110] As a preferred method, ceramic screening and pretreatment are included before ceramic dispersion, specifically:

[0111] The ceramic screening process specifically involves selecting first ceramic particles and second ceramic particles of different sizes, wherein the average particle size of the first ceramic particles is 100-600 nm and the average particle size of the second ceramic particles is 10-60 nm.

[0112] The ceramic pretreatment specifically involves adding the second ceramic particles to a reaction vessel, adding the substrate to be grafted, and performing a grafting reaction to graft lithium-ion fast conductor functional groups onto the surface of the second ceramic particles. Specifically, the second ceramic particles are added to a solvent, then placed together in a reaction vessel, followed by the substrate to be grafted. Through a specific reaction temperature, pressure, and time, lithium-ion fast conductor functional groups can be grafted onto the second ceramic particles.

[0113] There are no particular limitations on the second ceramic particle, but it is preferred to be a material with a melting point above 200°C, high electrical insulation, and electrochemical stability within the operating range of lithium batteries. Examples include: oxide ceramics such as alumina, silicon dioxide, titanium dioxide, zirconium dioxide, magnesium oxide, cerium oxide, yttrium oxide, and zinc oxide; nitride ceramics such as silicon nitride, titanium nitride, and boron nitride; and ceramics such as silicon carbide, calcium carbonate, magnesium sulfate, aluminum sulfate, basic alumina, potassium titanate, talc, kaolinite, clay, pearl clay, hydrous kaolinite, pyrophyllite, montmorillonite, sericite, mica, magnesia chlorite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, and silica sand. These can be used individually or in combination.

[0114] The first ceramic particle is not particularly limited, but is preferably a material with a melting point above 200℃, high electrical insulation, and electrochemical stability within the operating range of lithium batteries. It can be an inorganic material that is the same as or different from the second ceramic particle. The solvent is water, ethanol, acetone, NMP, etc., and the grafting material is polycarbonate, polylactic acid, polyurethane, perfluoropropyl vinyl ether, etc. The reaction temperature is 100-200℃. o C, the reaction pressure is 0.1-0.5 MPa, and the reaction time is 10-50 min. After the reaction is complete, the liquid solvent is removed, leaving the solid, which yields the second ceramic particles grafted with lithium-ion fast conductors.

[0115] Based on this, the dispersion of ceramics is specifically as follows:

[0116] The first ceramic particles and the pretreated second ceramic particles were dispersed in a first solvent and a third solvent, respectively, to obtain a first ceramic dispersion and a second ceramic dispersion.

[0117] Furthermore, embodiments of the present invention also provide a battery, including the coated separator according to the first aspect of the present invention and its alternative solutions, or the coated separator prepared using the second aspect of the present invention and its alternative solutions. The battery is, for example, a lithium battery.

[0118] The product performance of some embodiments of the present invention will be analyzed through experiments below.

[0119] Example 1

[0120] This embodiment prepares the coated diaphragm through the following steps:

[0121] Ceramic screening: Select first ceramic particles and second ceramic particles of different sizes, wherein the average particle size of the first ceramic particles is 300 nm and the average particle size of the second ceramic particles is 40 nm; both the first ceramic and the second ceramic are alumina.

[0122] Ceramic dispersion: The first ceramic particles and the second ceramic particles are uniformly dispersed in N-methylpyrrolidone by high-speed stirring to obtain a first ceramic dispersion and a second ceramic dispersion, wherein the mass concentration of the first ceramic particles in the first ceramic dispersion and the mass concentration of the second ceramic particles in the second ceramic dispersion are both 20%.

[0123] Dispersion of nanomaterials: First nanofibers and second nanofibers of different lengths were dispersed in N-methylpyrrolidone to obtain first nanofiber dispersion and second nanofiber dispersion, respectively. The length L50 of the first nanofiber was 600 nm, and the length L50 of the second nanofiber was 300 nm. The dispersion method was high-speed stirring, and the mass concentration of both the first and second nanofibers in the dispersant was 15%.

[0124] Mixing and slurry preparation: The first ceramic dispersion and the first nanofiber dispersion are mixed to obtain a first mixed solution; the second ceramic dispersion and the second nanofiber dispersion are mixed to obtain a second mixed solution; the mixing method is high-speed stirring; then, an adhesive is added to the first mixed solution and the second mixed solution respectively to obtain a first slurry and a second slurry. The adhesive is polyvinyl alcohol; the adhesive accounts for 8% of the solid mass in the first mixed solution and the second mixed solution respectively; the mass ratio of the first nanofiber to the first ceramic particle is 1:1, and the mass ratio of the second nanofiber to the second ceramic particle is 1:1.

[0125] Coating process: A first slurry is coated onto one surface of the base film and dried to form a first coating layer, resulting in a semi-finished coated separator. A second slurry is then coated onto the first coating layer and dried to form a second coating layer, thus obtaining the coated separator. The base film is a polyethylene film with a thickness of 9 micrometers. The drying temperature is 100 degrees Celsius, and the coating speed is 100 m / min. The thickness of the first coating layer is 0.8 micrometers, and the thickness of the second coating layer is 0.3 micrometers.

[0126] Specifically, the first and second nanofibers are one-dimensional nanofibers that are deposited on the surface of the base film to form a layered structure (i.e., nanomaterials of different lengths in layers). During the deposition process on the base film, due to surface energy, the longest one-dimensional nanomaterial (which has the highest surface energy and is the least stable, and is most likely to adhere to an interface with lower surface energy) is deposited first, followed by longer ones, and the shortest nanowires are deposited last, thus forming a layered structure that gradually accumulates from long to short. This invention significantly improves the mixing uniformity by mixing nanofibers of similar dimensions, thereby reducing differences.

[0127] Examples 2-12 and Comparative Examples 1-5 were obtained in the same manner. For the performance of the corresponding membranes obtained in Examples 1-12 and Comparative Examples 1-5, please refer to Table 1. Except for the differences in the preparation parameters listed in Table 1, all examples and comparative examples are consistent with Example 1 in other aspects, and therefore will not be repeated.

[0128] Example 12, shown in Table 1, differs from Example 1 only in that a ceramic pretreatment step was performed on the second ceramic particles before S2 ceramic dispersion: the second ceramic particles were added to a solvent, then placed together in a reaction vessel, followed by the grafting material. A specific reaction temperature, pressure, and time were maintained to graft lithium-ion fast conductor functional groups onto the second ceramic particles. The solvent was acetone, and the grafting material was polycarbonate. The reaction temperature was 150°C. o C, the reaction pressure is 0.3 MPa, and the reaction time is 20 min. After the reaction is complete, the liquid solvent is removed, leaving a solid, which yields the second ceramic particles grafted with lithium-ion fast conductors.

[0129]

[0130] Table 1

[0131] Table 1, comparing Examples 1-12, shows that when the first nanofiber is mixed with the first ceramic particle, and the second nanofiber is mixed with the second ceramic particle, the thermal shrinkage at 180°C is less than 5%, the lithium-ion conductivity is greater than 1.2 mS / cm, and the battery capacity retention rate after 30 days is greater than 98%. When the thickness of the first coating decreases and the thickness of the second coating increases, the heat resistance decreases (Examples 1, 2, 3). This is mainly because the first nanofiber and the first ceramic particle form the main framework and are in direct contact with the base film. Therefore, the first coating, composed of the first nanofiber and the first ceramic particle, directly improves the heat resistance. If the thickness of the first coating decreases, the decrease in heat resistance is directly reflected. The second coating, composed of the second nanofiber and the second ceramic particle, is in direct contact with the electrode, and its uniformity directly determines the battery capacity retention rate. The higher the uniformity, the higher the battery capacity retention rate. The thickness of the second coating can significantly improve the uniformity of the entire coating structure. Therefore, as the thickness of the second coating increases, the battery capacity retention rate improves (Examples 1, 2, 3).

[0132] A comparison of Examples 1, 5, and Comparative Example 3 shows that when the particle size of the second ceramic particles in the second coating is too large, the capacity retention rate of the battery after high-temperature storage decreases (Comparative Example 3); when the particle size of the second ceramic particles in the second coating decreases, the lithium-ion conductivity decreases (Examples 1 and 5). A comparison of Examples 1 and 7 shows that when the length of the second nanofibers in the second coating decreases, the lithium-ion conductivity decreases. This is because an increase in the length of the second nanofibers leads to an increase in the disorder of the pore size distribution in the second coating, resulting in a decrease in battery capacity retention. A decrease in the length of the second nanofibers leads to a narrower pore size, which in turn leads to a smaller lithium-ion transport channel. However, by grafting fast lithium-ion conductor functional groups onto the second ceramic, the lithium-ion conductivity can be further improved, as shown in Example 12.

[0133] As can be seen from the comparison of Examples 1, 2, and 4, the heat resistance of the diaphragm decreases when the particle size of the first ceramic in the first coating increases; the heat resistance of the diaphragm also decreases when the length of the first nanofiber in the first coating increases. The main reason for this phenomenon is that when the size increases too much, the gaps between the coating materials increase, resulting in limited contact points with the base membrane. This manifests as the diaphragm being unable to effectively suppress its shrinkage when heated.

[0134] As can be seen from the comparison of Example 1, Comparative Example 1, and Comparative Example 5, when the nanofibers and ceramic particles are randomly distributed, the high-temperature storage battery capacity retention rate is only 95%, far lower than the result of layering. Furthermore, when the first nanofiber is mixed with the second ceramic particle, and the second nanofiber is mixed with the first ceramic particle, the lithium-ion conductivity, high-temperature storage battery capacity retention rate, and heat resistance further decrease. This phenomenon is caused by the disordered distribution of nanofibers and ceramics, resulting in poor pore size uniformity and inconsistent pore sizes, thus leading to a decrease in the high-temperature storage battery capacity retention rate. When the first coating is composed of the second nanofiber and the second coating is composed of the first nanofiber, the increased size difference between the nanofibers and ceramics leads to decreased mixing consistency. In addition, the introduction of the second nanofiber in the second coating significantly increases the pore size and its inconsistency, resulting in a substantial decrease in the high-temperature storage battery capacity retention rate. The introduction of the first nanofiber in the first coating reduces the rigidity of the framework, leading to a decrease in high-temperature heat resistance.

[0135] In the description of this specification, the references to terms such as "an embodiment," "an example," "a specific implementation process," and "an example" indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A coated diaphragm, characterized in that, include: A base film and a coating structure disposed on at least one surface of the base film, the coating structure comprising at least a first coating and a second coating, wherein the first coating is disposed on the surface of the base film, and the second coating is disposed on the side of the first coating away from the base film; the first coating contains a first nanofiber material and a first ceramic particle, and the second coating contains a second nanofiber material and a second ceramic particle; The first nanofiber material includes a plurality of first nanofibers, and the second nanofiber material includes a plurality of second nanofibers; The length difference between the plurality of first nanofibers is no greater than 500 nm, the length difference between the plurality of second nanofibers is no greater than 200 nm, and the length of the first nanofiber is greater than the length of the second nanofiber, and the average particle size of the first ceramic particle is greater than the average particle size of the second ceramic particle. The length of the first nanofiber is 500-1000 nm; the average particle size of the first ceramic particle is 100-600 nm. The length of the second nanofiber is 100-300 nm; the average particle size of the second ceramic particle is 10-60 nm.

2. The coated diaphragm according to claim 1, characterized in that, The mass ratio of the first nanofiber material to the first ceramic particles is 5:1 to 1:

5.

3. The coated diaphragm according to claim 1, characterized in that, The mass ratio of the second nanofiber material to the second ceramic particles is 5:1 to 1:

5.

4. The coated diaphragm according to claim 1, characterized in that, The diameter of the first nanofiber is 5-50 nm, and the diameter of the second nanofiber is 5-50 nm.

5. The coated diaphragm according to claim 1, characterized in that, The thickness ratio of the first coating to the second coating is greater than 2.

6. The coated diaphragm according to any one of claims 1-5, characterized in that, At least one additional coating layer is also included between the first coating layer and the second coating layer.

7. The coated diaphragm according to any one of claims 1-5, characterized in that, The surface of the second ceramic particle is grafted with lithium-ion fast conductor functional groups; The functional groups of the lithium-ion fast conductor include any one of hydroxyl (-OH), carbonyl (-C=O), fluorine (-F), and carboxyl (-COOH).

8. The coated diaphragm according to any one of claims 1-5, characterized in that, The first ceramic particle and / or the second ceramic particle are inorganic materials with a melting point above 200°C, electrical insulation properties, and electrochemical stability within the operating range of lithium batteries.

9. The coated diaphragm according to claim 1, characterized in that, The coated diaphragm must satisfy at least one of the following conditions: a) Ionic conductivity ≥ 1.2 mS / cm; b) Capacity retention ≥ 98%; c) Heat shrinkage at 180℃ / h ≤5%.

10. A method for preparing a coated diaphragm, characterized in that, The method for preparing the coated diaphragm according to any one of claims 1 to 9 comprises: Preparation of slurry: preparing a first slurry comprising first ceramic particles and first nanofiber material, and preparing a second slurry comprising second ceramic particles and second nanofiber material; Coating film formation: The first slurry is coated onto at least one surface of the base film to form a first coating, and a second slurry is coated onto the side of the first coating away from the base film to form a second coating.

11. The method for preparing the coated diaphragm according to claim 10, characterized in that, The preparation of the first slurry includes dispersing the first ceramic particles in a first solvent to obtain a first ceramic dispersion, dispersing the first nanofiber material in a second solvent to obtain a first nanofiber dispersion, and mixing the first ceramic dispersion and the first nanofiber dispersion to obtain the first slurry. The preparation of the second slurry includes dispersing the second ceramic particles in a third solvent to obtain a second ceramic dispersion, dispersing the second nanofiber material in a fourth solvent to obtain a second nanofiber dispersion, and mixing the second ceramic dispersion and the second nanofiber dispersion to obtain the second slurry.

12. The method for preparing the coated diaphragm according to claim 10, characterized in that, Before preparing the slurry, the method further includes: Ceramic screening: Select first ceramic particles and second ceramic particles of different sizes, wherein the average particle size of the first ceramic particles is 100-600 nm and the average particle size of the second ceramic particles is 10-60 nm.

13. The method for preparing the coated diaphragm according to claim 12, characterized in that, It also includes ceramic pretreatment: the second ceramic particles are grafted with the material to be grafted, so that lithium-ion fast conductor functional groups are grafted onto the surface of the second ceramic particles.

14. The method for preparing the coated diaphragm according to claim 13, characterized in that, The graft material includes any one of polycarbonate, polylactic acid, polyurethane, and perfluoropropyl vinyl ether.

15. A battery, characterized in that, The coating includes the coating membrane according to any one of claims 1 to 9, or the coating membrane prepared by the method for preparing the coating membrane according to any one of claims 10 to 14.

16. The battery according to claim 15, characterized in that, The battery is a lithium battery.

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