Composite separator and method for manufacturing the same, secondary battery

Abstract

The application provides a composite diaphragm, a preparation method thereof and a secondary battery, the composite diaphragm comprising a base film and a coating layer arranged on at least one surface of the base film; the coating layer comprises filler particles 50-95 wt%, nanocellulose particles 3-40 wt% and adhesive polymer 2-10 wt%; the filler particles are organic filler particles and / or inorganic filler particles; and the nanocellulose particles are hydrophobically modified nanocellulose. The composite diaphragm, the preparation method thereof and the secondary battery can form a high-strength three-dimensional network through the nanocellulose particles and the filler particles, can improve the heat resistance of the coating layer, can enhance the strength of the coating layer, can improve the ion conductivity of the coating layer, and have good air permeability.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, and more specifically, to a composite separator, its preparation method, and a secondary battery. Background Technology

[0002] Traditional nanocellulose membranes typically employ vacuum filtration and ethanol-to-deionized-water replacement processes to stack nanocellulose dispersions from top to bottom, forming a high-density membrane with a certain strength due to hydrogen bonding. However, this type of nanocellulose membrane has low overall permeability (high permeability value), which is detrimental to lithium-ion transport in the battery. Patent CN117175138A discloses a method for preparing a functionalized nanocellulose membrane. By introducing substances such as citric acid to hydrophobically modify the nanocellulose particles, the hydrogen bonding forces between the nanocellulose particles are effectively weakened, improving the permeability of the membrane. However, this can lead to a certain degree of reduction in the strength of the modified nanocellulose membrane.

[0003] Currently, the mainstream nanocellulose composite separator is prepared by homogenizing nanocellulose particles and fillers into a slurry and then coating it onto a porous substrate such as PE or PP. Patent CN107799702A discloses a ceramic separator and lithium-ion battery and their preparation method, using nanocellulose particles with an average diameter of 50-1000 nm and an average aspect ratio of 20-200, and ceramic fillers with an average particle size of 100-800 mm, homogenized and coated onto a porous substrate to form a nanocellulose composite separator. In patent CN107799702A, the average diameter of the nanocellulose particles and the average particle size of the ceramic filler are similar, which to some extent reduces the interaction sites between them, resulting in a weaker bonding strength between the nanocellulose particles and the ceramic filler.

[0004] Patent CN119231097A discloses a coating composition, a separator, an electrochemical device, and an electronic device. The preparation process involves homogenizing and coating a fiber structure and particulate fillers (extending to combinations between inorganic and organic materials, or between inorganic and organic materials) onto a porous substrate to prepare a functional composite separator. Patent CN119231097A simply uses the fiber structure as a supporting framework to improve the separator's stability, heat resistance, and the overall rigidity of the battery cell; while the filler particles are merely used to fill the gaps in the fiber structure, reducing the coating porosity and thus improving the separator's mechanical strength.

[0005] In summary, the fibrous membranes prepared by patents CN117175138A, CN107799702A, and CN119231097A all have difficulty simultaneously possessing properties such as high air permeability, high strength, and high temperature resistance.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing fiber-structured separators, which are difficult to combine high air permeability, high strength, and high temperature resistance. This invention provides a composite separator, its preparation method, and a secondary battery. By forming a high-strength three-dimensional network through nanocellulose particles and filler particles, the heat resistance of the coating can be improved, the coating strength can be enhanced, the ionic conductivity of the coating can be increased, and good air permeability can also be achieved.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: A composite membrane includes a base membrane and a coating disposed on at least one surface of the base membrane; the coating comprises 50-95 wt% filler particles, 3-40 wt% nanocellulose particles, and 2-10 wt% binder polymer; the filler particles are organic filler particles and / or inorganic filler particles; the nanocellulose particles are hydrophobically modified nanocellulose.

[0009] Furthermore, the base film is at least one of PE base film, PP base film, PP and PE composite multilayer film, non-woven fabric, and cellulose film.

[0010] Furthermore, the filler particles are organic filler particles, which include organic compounds containing polar groups.

[0011] Furthermore, the filler particles are inorganic filler particles, and the inorganic filler particles contain polar groups.

[0012] Furthermore, the nanocellulose has hydrophobic groups, and the degree of substitution of the hydrophobic groups in the nanocellulose particles is 0.10~0.35.

[0013] Furthermore, the diameter of the nanocellulose particles is 10~30nm and the aspect ratio is 20~100:1.

[0014] Furthermore, the adhesive polymer includes at least one of polyacrylate, polyurethane acrylate, SBR, polyvinyl alcohol, polyacrylamide, acrylamide-acrylonitrile copolymer, and carboxymethyl cellulose.

[0015] Furthermore, the organic filler particles comprise organic compounds containing hydrogen bond donor groups and / or hydrogen bond acceptor groups, preferably containing both hydrogen bond donor groups and hydrogen bond acceptor groups.

[0016] Furthermore, the particle size of the organic filler is 0.1~2.0μm.

[0017] Furthermore, the organic filler particles have a true density ≤ 2.0 g / cm³. 3 Organic compounds with a decomposition temperature > 250℃.

[0018] Furthermore, the organic filler particles include at least one of the following organic compounds modified with polar groups: ammonium polyphosphate, piperazine pyrophosphate, melamine polyphosphate, polyacrylonitrile, melamine, melamine thiocyanate, melamine cyanurate, polyimide, or other organic compounds modified with polar groups.

[0019] Furthermore, the inorganic filler particles are alumina and / or boehmite.

[0020] Furthermore, the hydrophobic group includes at least one of alkane group, aromatic hydrocarbon group, and cycloalkyl group; preferably an aromatic hydrocarbon group.

[0021] Furthermore, the hydrophobic group includes at least one of benzoyl, phenylacetyl, and phenylpropionyl.

[0022] Furthermore, compared to the base membrane, the composite diaphragm exhibits a tensile strength increase of ≥300 kgf / cm² in both the MD and TD directions. 2 .

[0023] Furthermore, the peel force of the composite membrane coating detaching from the base membrane surface is ≥85N / m; preferably ≥155N / m.

[0024] Furthermore, the air permeability of the composite membrane is ≤250s / 100cc; preferably ≤240s / 100cc.

[0025] Furthermore, the electrolyte diffusion length of the composite diaphragm is ≥70mm; preferably ≥80mm.

[0026] Furthermore, the composite membrane has an ionic conductivity ≥ 0.90 mS / cm.

[0027] Furthermore, the density of the composite membrane is ≤0.9 g / cm³. 3 .

[0028] A method for preparing a composite membrane includes the following steps: Step S1: React nanocellulose particles with a modifier to obtain modified nanocellulose; Step S2: Wet-mix the filler particles, the modified nanocellulose particles, and the binder polymer in a certain proportion to form a slurry; Step S3: Coat the slurry onto at least one side of the base membrane and dry it to obtain a composite membrane.

[0029] Further, in step S1, the nanocellulose particles are reacted with the modifier at pH 4.3-4.5 and temperature 60-140°C to obtain hydrophobic modified nanocellulose; the modifier includes hydrophobic groups.

[0030] Further, in step S2, the amounts of the filler particles, the nanocellulose particles, and the adhesive polymer added are 50~95wt%, 3~40wt%, and 2~10wt%, respectively.

[0031] Furthermore, the areal density of the coating after drying in step S3 is 0.8~3 g / m². 2 .

[0032] Furthermore, the modifier mentioned in step S1 includes at least one of citric acid, maleic acid, benzoic acid, and phenylacetic acid.

[0033] Furthermore, in step S2, after the filler particles and the nanocellulose particles are premixed and dispersed, the adhesive polymer is added and dispersed.

[0034] Furthermore, in step S3, the coating is performed using at least one of the following methods: roller coating, spin coating, and vacuum filtration.

[0035] A secondary battery includes the composite separator described above or the composite separator obtained by the above preparation method.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The composite membrane of the present invention forms a high-strength three-dimensional network through nanocellulose particles and filler particles, which can improve the heat resistance of the coating, enhance the coating strength, improve the ionic conductivity of the coating, and also have good air permeability.

[0037] 2. The coating of the composite membrane of the present invention includes low-density organic filler particles with polar groups on the surface and nanocellulose modified with an appropriate amount of hydrophobic groups with a certain aspect ratio. The polar groups on the surfaces of the two form a three-dimensional strong network structure similar to "steel bars + concrete" on at least one side of the porous base membrane through hydrogen bonding, so that the composite membrane has the characteristics of low weight, high air permeability, high strength, high temperature resistance and high membrane rupture temperature.

[0038] In a "reinforced concrete" structure, nanocellulose acts like the reinforcing steel (having a one-dimensional structure), while the organic filler resembles the concrete (having a granular state). The "reinforced concrete" structure involves three binding forces: intermolecular forces (hydrogen bonds and van der Waals forces, etc.), friction, and mechanical interlocking. The structure formed by nanocellulose and organic fillers possesses all of these forces simultaneously, with hydrogen bonds playing a more prominent role.

[0039] 3. The ratio of organic filler particles to nanocellulose particles in the coating of the composite membrane of this invention is optimal, effectively improving coating density and strength while maintaining good air permeability. Increased nanocellulose particle content enhances coating density and increases hydrogen bond strength, thus strengthening the coating. When organic filler particles are present alone, the low inter-particle correlation leads to low overall coating strength. When nanocellulose is present alone, the numerous hydrogen bonds result in a tight arrangement of nanocellulose particles, leading to high coating air permeability, which is detrimental to lithium-ion transport after cell assembly. Furthermore, the lack of organic filler as bonding and rigid support points weakens the bond between nanocellulose and the porous base membrane.

[0040] 4. When the hydrophobic group-modified nanocellulose and organic filler particles in the coating of the composite separator of this invention are combined, a suitable aspect ratio is required to simultaneously ensure the coating's permeability and the bonding strength between the two. A too small aspect ratio results in weak bonding between the nanocellulose and organic filler particles, making it impossible to construct a three-dimensional network structure; a too large aspect ratio leads to the nanocellulose particles easily forming a tight arrangement, which is detrimental to lithium-ion transport in the battery. The aspect ratio of the nanocellulose particles in this invention is within the optimal range, improving coating continuity and strength while also providing good permeability.

[0041] 5. When the hydrophobic groups modified with the present invention are combined with organic filler particles, a certain amount of hydrophobic groups reduce the number of hydroxyl groups on the nanocellulose through esterification, mainly weakening the tight arrangement between nanocellulose particles, while having a limited effect on weakening the hydrogen bonding between nanocellulose and organic filler particles (the polar groups of organic filler particles are not reduced), thus enhancing air permeability. In addition, the priority of hydrogen bonding formation between nanocellulose particles is higher than that between nanocellulose and organic filler particles, resulting in the weak point of the entire three-dimensional network structure being the interaction between organic filler particles and nanocellulose. Therefore, the modification of nanocellulose with an appropriate amount of hydrophobic groups does not significantly deteriorate the three-dimensional network structure.

[0042] 6. The degree of substitution of the nanocellulose particles in this invention is controlled within an optimal range, which can improve the strength, heat resistance, and ionic conductivity of the composite separator, while also providing good air permeability. If the degree of substitution is too low, the nanocellulose particles in the coating, as well as the nanocellulose particles and organic filler particles, become tightly entangled, resulting in low porosity and high air permeability, which is detrimental to lithium-ion transport in the battery. If the degree of substitution is too high, the dispersion of nanocellulose particles in the slurry system deteriorates, and the strength of the three-dimensional network structure in the coating decreases, leading to a deterioration in the strength, heat resistance, and other properties of the composite separator. The purpose of modifying with hydrophobic groups is to reduce the number of hydrogen bonds. The lower the degree of substitution of the hydrophobic groups, the more hydrogen bonds there are, and the stronger the hydrogen bonds, resulting in better heat resistance of the coating (stronger hydrogen bond strength improves the heat resistance of the coating); however, if the degree of substitution is too high, the heat resistance of the composite separator will deteriorate. Increasing the content of hydrophobic groups can effectively increase the voids between nanocellulose particles and between nanocellulose particles and organic filler particles, increasing ion conduction channels and improving ionic conductivity. Attached Figure Description

[0043] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0044] Figure 1 This is a schematic diagram of the composite diaphragm structure of the present invention. Detailed Implementation

[0045] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. 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. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0046] A composite membrane comprising a base membrane and a coating disposed on at least one surface of the base membrane; the coating comprising 50-95 wt% filler particles, 3-40 wt% nanocellulose particles and 2-10 wt% binder polymer.

[0047] Optionally, the base film is at least one of PE base film, PP base film, PP and PE composite multilayer film, non-woven fabric, cellulose film, etc.

[0048] In some embodiments, the filler particles contain polar groups; the filler particles are organic filler particles and / or inorganic filler particles.

[0049] Preferably, the filler particles are organic filler particles, which include organic compounds containing polar groups that can form hydrogen bonds.

[0050] This invention utilizes nanocellulose particles and organic fillers to form a high-strength three-dimensional network. The hydrogen bonding energy improves the heat resistance and enhances the strength of the coating. The presence of organic filler particles significantly increases the ionic conductivity of the coating. The interface between the organic filler and nanocellulose particles provides effective channels for ion conduction.

[0051] Preferably, the organic filler particles comprise organic matter containing hydrogen bond donor groups and / or hydrogen bond acceptor groups, and more preferably contain both hydrogen bond donor groups and hydrogen bond acceptor groups.

[0052] Optionally, the hydrogen bond donor group includes at least one of hydroxyl, amino, imino, fluorohydrogen, etc.

[0053] Optionally, the hydrogen bond acceptor group includes at least one of carbonyl, cyano, nitro, etheroxy, and nitroso groups.

[0054] In some embodiments, the particle size of the organic filler is 0.1~2.0μm, including but not limited to 0.1μm, 0.3μm, 0.5μm, 0.7μm, 0.9μm, 1.0μm, 1.1μm, 1.3μm, 1.5μm, 1.7μm, 1.9μm, and 2.0μm.

[0055] In some embodiments, the organic filler particles have a true density ≤ 2.0 g / cm³. 3 Organic compounds with a decomposition temperature > 250℃.

[0056] Optionally, the organic filler particles include at least one of the following organic compounds modified with polar groups: ammonium polyphosphate, piperazine pyrophosphate, melamine polyphosphate, polyacrylonitrile, melamine, melamine trithiocyanate, melamine cyanurate (MCA), polyimide (PI), or organic compounds modified with polar groups. Preferably, the organic filler particles contain both hydrogen bond donor and hydrogen bond acceptor groups, including at least one of the following organic compounds modified with polar groups: ammonium polyphosphate, piperazine pyrophosphate, melamine polyphosphate, melamine, melamine trithiocyanate, melamine cyanurate (MCA), or organic compounds modified with polar groups.

[0057] In some embodiments, the filler particles are inorganic filler particles containing polar groups that can form a high-strength three-dimensional network with nanocellulose particles.

[0058] Optionally, the inorganic filler particles are alumina and / or boehmite, both of which contain polar groups such as hydroxyl groups.

[0059] Preferably, the nanocellulose particles are hydrophobically modified nanocellulose with hydrophobic groups. Increasing the content of nanocellulose particles leads to better coating density; it also increases hydrogen bond strength, thus enhancing coating strength. However, a higher nanocellulose particle content results in higher coating permeability. Therefore, the ratio of organic filler particles to nanocellulose particles in this invention is optimal, effectively improving coating density and strength while maintaining good permeability.

[0060] In some embodiments, the diameter of the nanocellulose particles is 10-30 nm (including but not limited to 10 nm, 15 nm, 20 nm, 25 nm, 30 nm), and the aspect ratio is 20-100:1, including but not limited to 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1, 100:1.

[0061] Increasing the aspect ratio of nanocellulose particles improves the heat resistance of the coating to some extent; the three-dimensional network formed by longer nanocellulose particles has better continuity and strength, is easier to entangle, and has fewer vacancies; however, increasing the aspect ratio of nanocellulose particles also leads to an increase in the air permeability of the coating to some extent. The aspect ratio of the nanocellulose particles in this invention is within the optimal range, improving the continuity and strength of the coating while also providing good air permeability.

[0062] Preferably, the degree of substitution of hydrophobic groups in the nanocellulose particles is 0.10 to 0.35, including but not limited to 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35.

[0063] When the degree of substitution is too low, the nanocellulose particles in the coating are tightly entangled with each other, as well as with the organic filler particles, resulting in low porosity and high permeability, which is detrimental to the transport of lithium ions in the battery. When the degree of substitution is too high, the dispersion of nanocellulose particles in the slurry system deteriorates, and the strength of the three-dimensional network structure in the coating decreases, leading to a deterioration in the strength, heat resistance, and other properties of the composite separator.

[0064] The purpose of modifying hydrophobic groups is to reduce the number of hydrogen bonds. The smaller the degree of substitution of hydrophobic groups, the more hydrogen bonds there are, the stronger the hydrogen bonds are, and the better the heat resistance of the coating (the binding energy of strong hydrogen bonds can improve the heat resistance of the coating); however, if the degree of substitution is too high, it will lead to a decrease in the heat resistance of the composite membrane.

[0065] The increased content of hydrophobic groups can effectively increase the gaps between nanocellulose particles and between nanocellulose particles and organic fillers, thereby increasing the ion conduction channels and the ion conductivity.

[0066] The degree of substitution of nanocellulose particles in this invention is controlled within the optimal range, which can improve the strength, heat resistance, and ionic conductivity of the composite membrane, while also providing good air permeability.

[0067] In some embodiments, the hydrophobic group includes at least one of alkane group, aromatic group, and cycloalkyl group; preferably aromatic group.

[0068] In some embodiments, the hydrophobic group includes at least one of aromatic hydrocarbon groups such as benzoyl, phenylacetyl, and phenylpropionyl.

[0069] In some embodiments, the tensile strength increase of the composite separator in both the MD and TD directions (composite separator strength in the MD and TD directions minus base membrane strength) is ≥300 Kgf / cm² compared to the base membrane. 2 .

[0070] In some embodiments, the peel force of the composite diaphragm coating from the base film surface is ≥85 N / m (the typical value for the steel plate adhesion test of the tape used to test the peel force is 55 N / m); preferably ≥155 N / m.

[0071] Optionally, the adhesive polymer includes at least one of polyacrylate, polyurethane acrylate, SBR, polyvinyl alcohol, polyacrylamide, acrylamide-acrylonitrile copolymer, and carboxymethyl cellulose.

[0072] In some embodiments, the air permeability of the composite diaphragm is ≤250s / 100cc; preferably ≤240s / 100cc.

[0073] In some embodiments, the electrolyte diffusion length of the composite diaphragm is ≥70 mm; preferably ≥80 mm.

[0074] In some embodiments, the composite membrane has an ionic conductivity ≥0.90 mS / cm.

[0075] In some embodiments, the density of the composite membrane is ≤0.9 g / cm³. 3 In this invention, the density of organic filler particles in the composite membrane coating is lower than that of inorganic filler particles, which gives the composite membrane a low weight characteristic.

[0076] A method for preparing a composite membrane includes the following steps: Step S1: React nanocellulose particles with a modifier to obtain modified nanocellulose; Step S2: Wet-mix filler particles, modified nanocellulose particles, and binder polymer in a certain proportion to form a slurry; Step S3: Coat the slurry onto at least one side of the base membrane and dry it to obtain a composite membrane.

[0077] In some embodiments, step S1 involves reacting nanocellulose particles with a modifier at a pH of 4.3 to 4.5 (including but not limited to 4.3, 4.4, and 4.5) and a temperature of 60 to 140°C (including but not limited to 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, and 140°C) to obtain hydrophobically modified nanocellulose.

[0078] Preferably, the modifier in step S1 includes hydrophobic groups.

[0079] In some embodiments, the modifier in step S1 includes at least one of citric acid, maleic acid, benzoic acid, and phenylacetic acid.

[0080] Preferably, in step S2, the amounts of filler particles, nanocellulose particles, and binder polymer added are 50-95 wt%, 3-40 wt%, and 2-10 wt%, respectively.

[0081] In some embodiments, after the filler particles and nanocellulose particles are premixed and dispersed in step S2, the binder polymer is added and dispersed.

[0082] Optionally, the coating in step S3 may be performed using at least one of the following methods: roller coating, spin coating, vacuum filtration, etc.

[0083] In some embodiments, the areal density (area density of the coating on one side) of the coating after drying in step S3 is 0.8~3 g / m². 2 including but not limited to 0.8g / m 2 1.0g / m 2 1.2g / m 2 1.4g / m 2 1.6g / m 2 1.8g / m 2 2.0g / m 2 2.2g / m 2 2.4g / m 2 2.6g / m 2 2.8g / m 2 3.0g / m 2 .

[0084] A secondary battery includes the composite separator described above or the composite separator obtained by the above preparation method.

[0085] Example 1 A composite diaphragm, comprising: Organic filler particles: MCA median particle size 0.5μm, true density 1.7g / cm³ 3 Decomposition temperature > 370℃; Cellulose nanoparticles (hydrophobically modified with phenylacetic acid): degree of substitution 0.20, hydrophobic group is phenylacetyl; diameter 20nm, aspect ratio 50:1; Adhesive polymer: Acrylamide-acrylonitrile copolymer with a molecular weight of 95,000 and a copolymerization ratio of acrylamide to acrylonitrile of 8:2.

[0086] The method for preparing the composite membrane includes the following steps: S1. At pH (4.3~4.5) and temperature 80°C, the raw material nanocellulose particles and phenylacetic acid were dispersed at a mass ratio of 1:2 for 120 min; and centrifugation was performed to purify the nanocellulose particles modified with hydrophobic groups with a degree of substitution of 0.20. S2. Add 80 parts of MCA and 15 parts of hydrophobic group-modified nanocellulose particles to deionized water and disperse at 60°C for 120 min; then add 5 parts of acrylamide-acrylonitrile copolymer (in aqueous solution state, the number of 5 parts is calculated according to the dry component state) and disperse at 25°C for 30 min. S3. Using a roller coating method, the slurry is coated on both sides of a 7μm PE base film, and then dried to obtain a coating surface density of 1.3g / m² on each side. 2 Composite membrane.

[0087] Example 2 The difference between this embodiment and Embodiment 1 is that: Only the composition of nanocellulose particles in the coating of Example 1 was changed.

[0088] Cellulose nanoparticles (hydrophobically modified with phenylacetic acid): degree of substitution 0.20, hydrophobic group is phenylacetyl; diameter 20 nm, aspect ratio 25:1. The rest are the same as in Example 1.

[0089] Example 3 The difference between this embodiment and Embodiment 1 is that: Only the composition of nanocellulose particles in the coating of Example 1 was changed.

[0090] Cellulose nanoparticles (hydrophobically modified with phenylacetic acid): degree of substitution 0.20, hydrophobic group is phenylacetyl; diameter 20 nm, aspect ratio 90:1. The rest are the same as in Example 1.

[0091] Example 4 The difference between this embodiment and Embodiment 1 is that: Only the composition of nanocellulose particles in the coating of Example 1 was changed.

[0092] Cellulose nanoparticles (hydrophobically modified with phenylacetic acid): degree of substitution 0.30, hydrophobic group is phenylacetyl; diameter 20nm, aspect ratio 50:1.

[0093] In Example 1, the phrase "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:2" in S1 is adjusted to "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:5". The rest is the same as in Example 1.

[0094] Example 5 The difference between this embodiment and Embodiment 1 is that: Only the composition of nanocellulose particles in the coating of Example 1 was changed.

[0095] Cellulose nanoparticles (hydrophobically modified with phenylacetic acid): degree of substitution 0.12, hydrophobic group is phenylacetyl; diameter 20nm, aspect ratio 50:1.

[0096] In Example 1, the phrase "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:2" in S1 is adjusted to "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:1". The rest is the same as in Example 1.

[0097] Example 6 The difference between this embodiment and Embodiment 1 is that: Variation of Example 1: Nanocellulose particles in the coating and components of the modifier in the preparation method.

[0098] Cellulose nanoparticles (hydrophobically modified with benzoic acid): degree of substitution 0.20, hydrophobic group is benzoyl; diameter 20nm, aspect ratio 50:1.

[0099] In Example 1, the setting "pH (4.3~4.5), temperature 80°C" in S1 is adjusted to "pH (4.3~4.5), temperature 130°C". The rest is the same as in Example 1.

[0100] Example 7 The difference between this embodiment and Embodiment 1 is that: In Example 1, the "80 parts MCA and 15 parts hydrophobic group modified nanocellulose particles" in S2 were adjusted to "55 parts MCA and 40 parts hydrophobic group modified nanocellulose particles". The rest are the same as in Example 1.

[0101] Example 8 The difference between this embodiment and Embodiment 1 is that: In Example 1, the "80 parts MCA and 15 parts hydrophobic group-modified nanocellulose particles" in S2 were adjusted to "90 parts MCA and 5 parts hydrophobic group-modified nanocellulose particles". The rest are the same as in Example 1.

[0102] Example 9 The difference between this embodiment and Embodiment 1 is that: Only the composition of the filler in the coating of Example 1 was changed.

[0103] Organic filler particles: Polyacrylonitrile with a median particle size of 0.5 μm and a true density of 1.1 g / cm³. 3 Decomposition temperature > 250℃.

[0104] In Example 1, “MCA” in S2 is changed to “polyacrylonitrile”. The rest is the same as in Example 1.

[0105] Example 10 The difference between this embodiment and Embodiment 1 is that: In Example 1, S3 was coated on both sides of the base film, with each side having a coating density of 1.3 g / m³. 2 The "composite diaphragm" was adjusted to "coated on one side of the base membrane, with a coating surface density of 2.6 g / m³". 2 "A composite diaphragm." The rest is the same as in Example 1.

[0106] Example 11 The difference between this embodiment and Embodiment 1 is that: In Example 1, S3 was coated on both sides of the base film, with each side having a coating density of 1.3 g / m³. 2 The "composite diaphragm" was adjusted to "coated on both sides of the base membrane, with a coating surface density of 0.9 g / m² on each side". 2 "A composite diaphragm." The rest is the same as in Example 1.

[0107] Example 12 The difference between this embodiment and Embodiment 1 is that: Only the composition of nanocellulose particles in the coating of Example 1 was changed.

[0108] Nanocellulose particles (hydrophobically modified with phenylacetic acid): degree of substitution 0.45, hydrophobic group is phenylacetyl; diameter 20nm, aspect ratio 50:1.

[0109] In Example 1, the phrase "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:2" in S1 is adjusted to "raw material nanocellulose particles and phenylacetic acid in a mass ratio of 1:10". The rest is the same as in Example 1.

[0110] Example 13 The difference between this embodiment and Embodiment 1 is that only the composition of the filler in the coating of Embodiment 1 is changed.

[0111] Inorganic filler particles: alumina with a median particle size of 0.5 μm and a true density of 3.7 g / cm³. 3 Decomposition temperature > 2500℃. The rest is the same as in Example 1.

[0112] Example 14 The difference between this embodiment and Embodiment 1 is that only the composition of the filler in the coating of Embodiment 1 is changed.

[0113] Inorganic filler particles: Boehmite with a median particle size of 0.5 μm and a true density of 3.0 g / cm³. 3 Decomposition temperature > 400℃. The rest is the same as in Example 1.

[0114] Comparative Example 1 A composite diaphragm, comprising: Organic filler particles: MCA median particle size 0.5μm, true density 1.7g / cm³ 3 Decomposition temperature > 370℃; Cellulose nanoparticles: degree of substitution 0.00; diameter 20 nm; aspect ratio 50:1; Adhesive polymer: Acrylamide-acrylonitrile copolymer with a molecular weight of 95,000 and a copolymerization ratio of acrylamide to acrylonitrile of 8:2.

[0115] The method for preparing the composite membrane includes the following steps: S1. Add 80 parts of MCA and 15 parts of nanocellulose particles to deionized water and disperse at 60℃ for 120 min; then add 5 parts of acrylamide-acrylonitrile copolymer (in aqueous solution state, the number of 5 parts is calculated according to the dry component state) and disperse at 25℃ for 30 min. S2. Using a roller coating method, the slurry is coated on both sides of a 7μm PE base film, and then dried to obtain a coating surface density of 1.3g / m² on each side. 2 Composite membrane.

[0116] Comparative Example 2 The difference between this comparative example and Example 1 is that: Remove the nanocellulose particle component and the modifier component from the preparation method in the coating of Example 1.

[0117] S1. Add 95 parts of MCA with a particle size of 0.5 μm and 5 parts of acrylamide-acrylonitrile copolymer to deionized water and disperse at 25°C for 30 min; S2. Using a roller coating method, the slurry is coated on both sides of a 7μm PE base film, and then dried to obtain a coating surface density of 1.3g / m² on each side. 2 Composite membrane.

[0118] Comparative Example 3 The difference between this comparative example and Example 1 is that: Remove the organic filler particles from the coating of Example 1.

[0119] S1. At pH (4.3~4.5) and temperature 130°C, raw material cellulose nanoparticles with a diameter of 20 nm and an aspect ratio of 50:1 were dispersed with phenylacetic acid at a mass ratio of 1:2 for 120 min; and then purified by centrifugation to obtain cellulose nanoparticles modified with hydrophobic groups with a degree of substitution of 0.20. S2. Add 95 parts of hydrophobic group-modified nanocellulose particles to deionized water and 5 parts of acrylamide-acrylonitrile copolymer to deionized water, and disperse at 25°C for 30 min. S3. Using a roller coating method, the slurry is coated on both sides of a 7μm PE base film, and then dried to obtain a coating surface density of 1.3g / m² on each side. 2 Composite membrane.

[0120] Test case I. Testing Methods 1. Degree of substitution of hydrophobic groups in nanocellulose particles: The degree of substitution (DS) test was performed using elemental analysis. The specific formula is as follows: DS=(C AGU -X C AGU) / (X C CA-C CA ) In the above formula, DS represents the degree of substitution of hydrophobic groups in nanocellulose particles, and C... AGU X is the molar mass of carbon in anhydrous glucose units. C The carbon weight fraction in the hydrophobic group-modified nanocellulose particles is AGU, which refers to the total molar mass of all atoms in a unit of anhydrous glucose, and CA is the total molar mass of all atoms in the modifier. CA This refers to the total molar mass of all carbon atoms in the modifier.

[0121] 2. Composite membrane air permeability test: The air permeability performance of the composite membrane was tested using a Wang Yanshi air permeability meter EG01-55-1MR. Three values ​​were tested for each group of membranes and the average value was taken.

[0122] 3. Composite diaphragm heat shrinkage test: The composite diaphragm is cut into 10cm pieces. Six diaphragm samples were taken from each group, 10cm in length (MD direction: length L, TD direction: width W), and baked at 150℃ for 1 hour. The thermal shrinkage rate of the diaphragm was recorded (MD = (L...). 烘烤前 -L 烘烤后 ) / L 烘烤前 100%; TD = (W 烘烤前 -W 烘烤后 ) / W 烘烤前 (100%), take the average.

[0123] 4. Coating electrolyte diffusion length test: The composite diaphragm was cut into 5mm pieces along the TD direction. Cut 4 strips of diaphragm for each group, each strip measuring 200mm (MD: 5mm, TD: 200mm). Take 25μm of electrolyte (EC:EMC:DEC=3:5:2, 1M LiPF6) and drop it onto the center of the strip diaphragm. Allow it to diffuse for 1 minute, record the diffusion length, and take the average value.

[0124] 5. Coating peel strength test: The peel strength was tested using a tensile testing machine (RZ-DW100). The composite diaphragm was cut into 3cm pieces. A 10cm strip was used, with transparent tape (typically 55 N / m for steel plate adhesion testing) adhered to the coating. The tape was then peeled off using a 180° peel method at a peeling speed of 50 mm / min. Six sets of values ​​were tested, and the average value was taken.

[0125] 6. Coating strength test: The membrane strength was tested using a tensile testing machine (RZ-DW100). Both the composite membrane and the base membrane were cut into 15mm pieces. A 100mm long strip (6 sets along the MD direction and 6 sets along the TD direction) was stretched upwards along its long side at a test speed of 50mm / min. Twelve sets of values ​​were measured, and the average value was taken. Average coating strength = Average composite diaphragm strength - Average base film strength.

[0126] 7. Composite Membrane Ionic Conductivity Test: In an argon-filled glove box, the composite membrane was fabricated into a 2016 button cell. An appropriate amount of electrolyte (EC:EMC:DEC = 3:5:2, 1M LiPF6) was added. The AC impedance was measured using an electrochemical workstation, yielding σ = L / (Rb). A), where σ is the ionic conductivity (mS / cm); L is the membrane thickness (cm); Rb is the intrinsic resistance of the membrane (Ω); and A is the effective area (cm²). 2 ).

[0127] II. The performance test data of the composite membranes of Examples 1-14 and Comparative Examples 1-3 are shown in Table 1.

[0128] Table 1 Performance test data of composite membranes in Examples 1-14 and Comparative Examples 1-3

[0129] III. Test Results of Composite Separator Performance In Examples 1-14, the components and process parameters are all within the scope of the technical solution of the present invention, and the composite membranes obtained have good performance in all aspects.

[0130] (1) Comparison of air permeability values ​​of composite membranes: In Examples 1-3, the increased aspect ratio of nanocellulose particles leads to an increase in the air permeability value of the coating to a certain extent; the three-dimensional network formed by longer nanocellulose particles has better continuity and is easier to entangle, with fewer vacancies. In Examples 1, 4, 5 and Comparative Examples 1-2, the smaller the degree of substitution of hydrophobic groups, the larger the air permeability value of the coating; the increase in the amount of hydrophobic group modification can effectively reduce the tightness of the connection between nanocellulose particles and between nanocellulose particles and fillers; however, after the degree of substitution reaches a certain level, the improvement in air permeability value is significantly reduced. In Examples 1, 6 and Comparative Example 1, nanocellulose particles modified with benzoic acid and phenylacetic acid can effectively reduce the air permeability value of the coating. In Examples 1, 7 and 8, the higher the content of nanocellulose particles, the greater the air permeability value of the coating; the increase in the content of nanocellulose particles leads to better coating density. In Examples 1, 13 and 14, the air permeability values ​​of the coatings are basically at the same level, and only the change in filler type does not affect the air permeability value of the coating.

[0131] (2) Comparison of heat shrinkage of composite membranes: In Examples 1-3, the increased aspect ratio of nanocellulose particles improves the heat resistance of the coating to a certain extent; the three-dimensional network formed by longer nanocellulose particles has better continuity and strength. In Examples 1, 4, 5, 12 and Comparative Example 1, the lower the degree of substitution of hydrophobic groups, the better the heat resistance of the coating (strong hydrogen bond strength can improve the heat resistance of the coating); however, if the degree of substitution is too high, it will lead to a decrease in the heat resistance of the composite membrane (Example 12). In Examples 1 and 10, double-sided coating can more effectively improve the heat resistance of the coating. In Examples 1 and 9, different fillers have different effects on the heat shrinkage performance of the composite membrane. It is possible that the overall hardness of polyacrylonitrile is relatively low, which leads to a decrease in the heat resistance of the composite membrane. In addition, the small number of hydrogen bonds formed by polyacrylonitrile and nanocellulose will also deteriorate the heat resistance. Comparing Examples 1, 13 and 14, the overall heat resistance of fillers containing alumina and boehmite is slightly better. The overall hardness of alumina and boehmite is higher, which can play a better supporting role when the membrane is heat-shrinking.

[0132] (3) Comparison of electrolyte diffusion length in composite membranes: Examples 1-12 and Comparative Examples 1-2 showed little difference in overall electrolyte wettability, but Example 10 exhibited superior electrolyte diffusion length; the single-sided coating process was more conducive to electrolyte wettability diffusion. Compared to Comparative Example 3, the absence of organic filler particles in Example 1 actually weakened the electrolyte diffusion length; the capillary action between organic filler particles may have promoted electrolyte diffusion. Comparing Examples 1, 13, and 14, the surfaces of alumina and boehmite had more hydroxyl groups, which slightly promoted electrolyte diffusion.

[0133] (4) Comparison of coating peel strength: In Examples 1, 4-6 and Comparative Example 1, the presence of hydrophobic groups, and the increase in their content, leads to a weakening of the bonding strength between the coating and the base film within a suitable range, thus slightly reducing the coating peel strength. In Examples 1, 7, 8 and Comparative Examples 2-3, there is an optimal ratio of organic filler particles and nanocellulose particles, which can most effectively improve the coating peel strength. Comparing Examples 1, 13 and 14, the peel strength of the coatings formed by the three fillers, MCA, alumina and boehmite, is basically at the same level; only the change in filler type does not affect the peel strength of the coating.

[0134] (5) Coating strength comparison: In Examples 1-3, when the length of the nanocellulose particles is short, the three-dimensional network strength formed by the nanocellulose particles and organic fillers is low, resulting in weaker coating strength. In Examples 1, 4, 5, 12 and Comparative Example 1, the increase in the content of hydrophobic groups weakens the strength of hydrogen bonds to a certain extent, leading to a decrease in coating strength. In Examples 1, 7, 8 and Comparative Examples 2-3, the increase in the content of nanocellulose particles increases the strength of hydrogen bonds, thus enhancing the coating strength. Comparing Examples 1, 13 and 14, the surfaces of alumina and boehmite have more hydroxyl groups, which slightly increases the strength of hydrogen bonds bonded to nanocellulose.

[0135] (6) Ionic conductivity of the composite membrane: In Examples 1, 4, 5, 12 and Comparative Example 1, the increased content of hydrophobic groups effectively increases the voids between nanocellulose particles and between nanocellulose particles and organic fillers, resulting in more ion conduction channels and increased ionic conductivity. In Examples 1, 9 and Comparative Example 3, the presence of organic fillers significantly improves the ionic conductivity of the coating. The difference between the interface of organic fillers and nanocellulose particles effectively provides channels for ion conduction. In comparison of Examples 1, 13 and 14, the surfaces of alumina and boehmite have more hydroxyl groups, which better promote lithium ion migration.

Claims

1. A composite diaphragm, characterized in that, The invention comprises a base film and a coating disposed on at least one surface of the base film; the coating comprises 50-95 wt% filler particles, 3-40 wt% nanocellulose particles, and 2-10 wt% binder polymer; the filler particles are organic filler particles; the nanocellulose particles are hydrophobically modified nanocellulose. The hydrophobic modified nanocellulose has hydrophobic groups, and the degree of substitution of the hydrophobic groups in the hydrophobic modified nanocellulose is 0.10~0.

35. The organic filler particles are organic compounds containing hydrogen bond donor groups and / or hydrogen bond acceptor groups, including at least one of piperazine pyrophosphate, melamine polyphosphate, polyacrylonitrile, melamine, melamine thiocyanate, melamine cyanurate, and polyimide. The hydrophobic group includes at least one of benzoyl, phenylacetyl, and phenylpropionyl.

2. The composite diaphragm according to claim 1, characterized in that, Includes at least one of the following technical features: (1) The base film is at least one of PE base film, PP base film, PP and PE composite multilayer film, non-woven fabric, and cellulose film; (2) The diameter of the hydrophobic modified nanocellulose is 10~30nm and the aspect ratio is 20~100:1; (3) The adhesive polymer includes at least one of polyacrylate, polyurethane acrylate, SBR, polyvinyl alcohol, polyacrylamide, acrylamide-acrylonitrile copolymer, and carboxymethyl cellulose.

3. The composite diaphragm according to claim 2, characterized in that, The organic filler particles have a particle size of 0.1~2.0μm.

4. The composite diaphragm according to any one of claims 1 to 3, characterized in that, Includes at least one of the following technical features: (1) Compared with the base membrane, the tensile strength increase of the composite diaphragm in both the MD and TD directions is ≥300 Kgf / cm. 2 ; (2) The peel force of the composite membrane coating detaching from the base membrane surface is ≥85N / m; (3) The air permeability of the composite diaphragm is ≤250s / 100cc; (4) The electrolyte diffusion length of the composite diaphragm is ≥70mm; (5) The ionic conductivity of the composite membrane is ≥0.90 mS / cm; (6) The density of the composite membrane is ≤0.9 g / cm³. 3 .

5. The method for preparing the composite separator according to any one of claims 1 to 4, characterized in that, The steps include the following: Step S1: React unmodified nanocellulose particles with a modifier to obtain hydrophobically modified nanocellulose; Step S2: Wet-mix the filler particles, the hydrophobic modified nanocellulose, and the binder polymer in a certain proportion to form a slurry; Step S3: Coat the slurry onto at least one side of the base membrane and dry it to obtain a composite membrane.

6. The method for preparing the composite diaphragm according to claim 5, characterized in that, Includes at least one of the following technical features: (1) In step S1, the unmodified nanocellulose particles are reacted with the modifier at pH 4.3-4.5 and temperature 60-140°C to obtain hydrophobic modified nanocellulose; the modifier includes hydrophobic groups; (2) The amounts of the filler particles, the hydrophobically modified nanocellulose, and the adhesive polymer added in step S2 are 50~95wt%, 3~40wt%, and 2~10wt%, respectively; (3) The areal density of the coating after drying in step S3 is 0.8~3 g / m³. 2 .

7. The method for preparing the composite diaphragm according to claim 6, characterized in that, Includes at least one of the following technical features: (1) The modifier mentioned in step S1 includes at least one of benzoic acid and phenylacetic acid; (2) After the filler particles and the hydrophobic modified nanocellulose are premixed and dispersed in step S2, the adhesive polymer is added and dispersed. (3) In step S3, the coating is performed by at least one of the following methods: roller coating, spin coating, and vacuum filtration.

8. A secondary battery, characterized in that, This includes the composite membrane according to any one of claims 1 to 4 or the composite membrane obtained by the preparation method according to any one of claims 5 to 7.

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