Dry-process separator, preparation method therefor and use thereof

Abstract

The present application discloses a dry-process separator, comprising a porous substrate layer and a tear-resistant coating coated on at least one surface of the porous substrate layer. The tear-resistant coating comprises a tear-resistant slurry, and the tear-resistant slurry comprises a polymer, an inorganic material, a binder, and a solvent, wherein the inorganic material contains a hydroxyl group, and the binder contains a hydroxyl group. The inorganic material comprises at least one of a tubular structure and a sheet-shaped structure; when the inorganic material is of the tubular structure, the length-diameter ratio of the inorganic material is 5:1-10:1; and when the inorganic material is of the sheet-shaped structure, the thickness of the inorganic material is 0.6-1 μm.

Description

A dry-process diaphragm and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 19, 2023, with application number 2023117552836. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium-ion batteries, and in particular to a dry-process diaphragm and a preparation method and application thereof. Background Art

[0003] Lithium-ion batteries primarily consist of a positive electrode, a negative electrode, a separator, and an electrolyte. The separator plays a crucial role, crucially determining the battery's chemical properties, safety, and cost. With increasing demand for lithium-ion battery energy density across various sectors, separators are becoming thinner. However, this thinning poses challenges to battery safety. Separators currently used in the industry are categorized by process, with dry-process and wet-process separators being used.

[0004] Wet-process diaphragms are prepared by a wet process. The wet process is achieved by mixing liquid hydrocarbons or some small molecular substances with polyolefins, heating and melting to form a uniform mixture, and then cooling to separate the phases to obtain a membrane containing white oil. The membrane is then heated to a temperature close to the melting point and subjected to biaxial stretching and molecular orientation. Finally, it is extracted and kept warm for a certain period of time to prepare a polyethylene film material with micropores. In contrast, dry-process diaphragms are prepared by a dry process. The dry process is a method of producing hard elastic fibers to prepare a low-crystallinity, highly oriented polypropylene film. This film is first stretched at low temperature to form micro defects, and then stretched at high temperature to form micropores, resulting in a microporous diaphragm with a certain orientation. It is precisely because of the difference in the diaphragm preparation process that the dry-process diaphragm exhibits excellent thermal shrinkage properties in the TD transverse direction. Technical issues

[0005] The dry-process diaphragm will tear along the MD longitudinal direction, and the strength of the dry-process diaphragm in the TD transverse direction is much smaller than that of the wet-process diaphragm in the TD transverse direction. Moreover, since the dry-process diaphragm is easy to tear, after being used in the battery cell, when the battery cell is accidentally deformed, the diaphragm will not be able to resist the transverse tearing force and will rupture. The contact between the positive and negative electrodes in the battery cell will cause a short circuit, which will lead to serious safety accidents. Technical Solutions

[0006] In a first aspect, the present application provides a dry-process diaphragm, comprising a porous base layer and an anti-tear coating coated on at least one surface of the porous base layer;

[0007] The tear-resistant coating comprises a tear-resistant slurry, which comprises a polymer, an inorganic substance, a binder and a solvent; wherein the inorganic substance contains a hydroxyl group, and the binder contains a hydroxyl group;

[0008] The inorganic material includes at least one of a tubular structure and a sheet structure; when the inorganic material is a tubular structure, the aspect ratio of the inorganic material is 5:1-10:1; when the inorganic material is a sheet structure, the thickness of the inorganic material is 0.6-1 μm.

[0009] In a second aspect, the present application provides a method for preparing a dry-process diaphragm, comprising the following steps:

[0010] S1: mixing the polymer, the inorganic substance, the binder, and the solvent under stirring to obtain the tear-resistant slurry; wherein the tear-resistant slurry further comprises a dispersant accounting for 0.5 wt% to 3 wt% of the mass of the tear-resistant slurry;

[0011] S2: coating the tear-resistant slurry on at least one surface of the porous base layer, then removing the solvent and drying to obtain the dry-process diaphragm.

[0012] In a third aspect, the present application provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and the dry-process separator as described above. Beneficial effects

[0013] Since both inorganic substances and binders contain a certain number of hydroxyl groups, during the preparation of tear-resistant slurry, the hydroxyl groups in the inorganic substances can react with the hydroxyl groups in the binder to generate a large number of hydrogen bonds, thereby enhancing the intermolecular forces in the tear-resistant slurry. On the one hand, it can improve the adhesion ability of the tear-resistant slurry on the surface of the porous substrate layer, so that the tear-resistant coating formed on the surface of the porous substrate layer can improve the ductility of the diaphragm to a certain extent. On the other hand, this interaction between the binder and the inorganic substance can also enable the inorganic substance to be better filled into the polymer, and work together with the polymer to form a network structure, thereby increasing the flexibility of the diaphragm and improving the elongation at break of the diaphragm in the TD transverse direction, thereby avoiding the situation in which the dry-process diaphragm is easily torn in the TD transverse direction during application, thereby improving the tear resistance of the diaphragm and significantly improving the thermal shrinkage rate of the diaphragm.

[0014] Moreover, when the inorganic material is a tubular structure and meets the above-mentioned aspect ratio limit range, the inorganic material is more likely to overlap in the tear-resistant slurry to form a three-dimensional network structure. At this time, the inorganic material has a larger specific surface area and can better combine with the binder, so that the inorganic material can be filled into the polymer chain segment in a staggered manner, further promoting the formation of a three-dimensional network structure in the tear-resistant coating and improving the tear resistance of the diaphragm; and when the inorganic material is a sheet structure and meets the above-mentioned thickness limit, the inorganic material has a larger specific surface area, which can not only better combine with the binder, but also the inorganic material is relatively loosely accumulated in the tear-resistant coating, thereby enhancing the wettability of the dry-process diaphragm. After applying it in the battery cell, the electrochemical performance of the battery cell can be further improved. Modes for Carrying Out the Invention

[0015] In some embodiments, the inorganic material includes at least one of boehmite, nanocellulose, and halloysite tubes.

[0016] In some embodiments, the binder includes at least one of polyacrylic acid, polyacrylate, polyacrylonitrile, and a terpolymer binder.

[0017] The above adhesives used in this application have a certain flexibility, which is beneficial to further improve the tear resistance of the dry-process diaphragm.

[0018] In some embodiments, the polymer includes at least one of polyaramid, polyimide, polymethyl methacrylate, and polyvinylidene fluoride.

[0019] In some embodiments, the polymer has a molecular weight of 200,000-1,000,000 Da.

[0020] Through the molecular weight of the polymer, the entanglement between the polymer molecular chains is made tighter, and the three-dimensional network structure formed after the inorganic matter is filled into the polymer molecules is denser, and the density of the tear-resistant coating is also correspondingly improved, thereby improving the tear strength and toughness of the diaphragm.

[0021] In some embodiments, the solid content of the tear-resistant slurry is 28%-32%, the viscosity of the tear-resistant slurry is less than 80 cp, the pH of the tear-resistant slurry is 7.5-9.0, and the average particle size D50 of the tear-resistant slurry is less than 1 μm.

[0022] By controlling the viscosity and solid content of the tear-resistant slurry, the tear-resistant slurry can be easily coated on the surface of the porous base layer, and the slurry is thin. The tear-resistant coating formed on the surface of the porous base layer can better combine with the porous base layer while remaining light and thin, which is conducive to the lightweight development of the diaphragm and makes the diaphragm exhibit good tear resistance.

[0023] In some embodiments, the thickness of the dry-process separator is 10-18 μm, and the thickness ratio of the porous base layer to the anti-tearing coating layer is 5-8:1.

[0024] In some embodiments, the solvent is an aqueous solvent.

[0025] In some embodiments, the dispersant includes at least one of a nonionic dispersant and a nonionic dispersant.

[0026] In some embodiments, the dispersant includes at least one of polyvinyl alcohol, polyvinyl ether, sodium dodecylbenzene sulfonate, and polyvinyl pyrrolidone.

[0027] In some embodiments, the stirring speed is 800-1000 rpm, and the duration of the stirring process is 25-35 minutes; the temperature during the drying process is 55-65° C., and the duration of the drying process is 15-25 minutes.

[0028] The dry-process diaphragm in this application has certain advantages in terms of cost. Less solvent is introduced or no solvent is used during the production process, which improves the environmental performance of the diaphragm.

[0029] In some embodiments, in the tear-resistant slurry, the mass ratio of the polymer, the inorganic substance, and the binder is 10-70:40-80:0.1-10.

[0030] Example

[0031] Example 1

[0032] 1. Preparation of Dry Diaphragm

[0033] The tear-resistant slurry includes the following raw materials in parts by weight:

[0034] 13 parts of polyvinylidene fluoride with a molecular weight of 200,000-1,000,000 Da,

[0035] 70 parts of boehmite, 7 parts of halloysite tubes with an aspect ratio of 5:1,

[0036] 8 parts of polyacrylic acid,

[0037] 2 parts of polyvinyl alcohol,

[0038] 300 parts of deionized water;

[0039] Polyvinylidene fluoride, boehmite, halloysite tube, polyacrylic acid, polyvinyl alcohol and deionized water were stirred at 1000 rpm for 25 minutes to obtain a tear-resistant slurry with a solid content of 30%, a pH value of 7.5, a viscosity of 60 cp and an average particle size of 0.8 μm.

[0040] The above-mentioned tear-resistant slurry is coated on one side of a polypropylene diaphragm and dried at 60°C for 20 minutes to form a tear-resistant coating on the surface of the polypropylene diaphragm, thereby obtaining a dry-process diaphragm with a thickness of 18 μm; wherein, in the dry-process diaphragm, the thickness of the polypropylene diaphragm and the thickness of the tear-resistant coating are 9:1.

[0041] 2. Preparation of battery cells

[0042] 2.1. Preparation of positive electrode

[0043] The positive electrode slurry was prepared as follows: LFP positive electrode active material: conductive agent acetylene black: binder PVDF were added into a vacuum mixer in a mass ratio of 97.9:0.9:1.2 for mixing, and then solvent NMP was added to the mixed slurry. The mixed slurry was stirred until it was uniform under the action of the vacuum mixer, thereby obtaining the positive electrode slurry of this embodiment.

[0044] The above-mentioned positive electrode slurry is evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet is obtained, and then the semi-finished positive electrode sheet is cold pressed and cut to obtain the positive electrode sheet to be assembled.

[0045] 2.2. Preparation of negative electrode sheet

[0046] The negative electrode slurry was prepared as follows: artificial graphite: conductive agent acetylene black: thickener CMC: binder SBR were added to a vacuum mixer in a mass ratio of 96.4:1:1.2:1.4 and mixed, and then deionized water was added as a solvent to the mixture. The mixed slurry was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0047] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0048] 2.3 Assembly of battery cells

[0049] The positive electrode sheet, the negative electrode sheet and the dry-process separator are wound together to obtain a bare battery cell.

[0050] 3. Preparation of lithium batteries

[0051] The bare cell is packaged, injected, left to stand, formed, and divided into different volumes to produce a soft-pack battery, wherein the electrolyte used is a commercially available electrolyte.

[0052] Example 2

[0053] 1. Preparation of Dry Diaphragm

[0054] The tear-resistant slurry includes the following raw materials in parts by weight:

[0055] 11 parts of polymethyl methacrylate with a molecular weight of 200,000-1,000,000 Da,

[0056] 65 parts of boehmite, 12 parts of nanocellulose,

[0057] 10 parts of polyacrylic acid,

[0058] 2 parts of polyethylene ether,

[0059] 330 parts of deionized water;

[0060] Polymethyl methacrylate, boehmite, nanocellulose, polyacrylic acid, polyethylene ether and deionized water were mixed under stirring at 800 rpm and stirred for 35 minutes to obtain a tear-resistant slurry with a solid content of 28%, a pH value of 8.0, a viscosity of 70 cp and an average particle size of 0.5 μm.

[0061] The above-mentioned tear-resistant slurry is coated on one side of a polyethylene diaphragm and dried at 55°C for 25 minutes to form a tear-resistant coating on the surface of the polyethylene diaphragm, thereby obtaining a dry diaphragm with a thickness of 12 μm; wherein, in the dry diaphragm, the thickness of the polyethylene diaphragm and the thickness of the tear-resistant coating are 5:1.

[0062] 2. Preparation of battery cells

[0063] 2.1. Preparation of positive electrode

[0064] The positive electrode slurry was prepared as follows: LFP positive electrode active material: conductive agent acetylene black: binder PVDF were added into a vacuum mixer in a mass ratio of 97.9:0.9:1.2 for mixing, and then solvent NMP was added to the mixed slurry. The mixed slurry was stirred until it was uniform under the action of the vacuum mixer, thereby obtaining the positive electrode slurry of this embodiment.

[0065] The above-mentioned positive electrode slurry is evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet is obtained, and then the semi-finished positive electrode sheet is cold pressed and cut to obtain the positive electrode sheet to be assembled.

[0066] 2.2. Preparation of negative electrode sheet

[0067] The negative electrode slurry was prepared as follows: artificial graphite: conductive agent acetylene black: thickener CMC: binder SBR were added to a vacuum mixer in a mass ratio of 96.4:1:1.2:1.4 and mixed, and then deionized water was added as a solvent to the mixture. The mixed slurry was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0068] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0069] 2.3 Assembly of battery cells

[0070] The positive electrode sheet, the negative electrode sheet and the dry-process separator are wound together to obtain a bare battery cell.

[0071] 3. Preparation of lithium batteries

[0072] The bare cell is packaged, injected, left to stand, formed, and divided into different volumes to produce a soft-pack battery, wherein the electrolyte used is a commercially available electrolyte.

[0073] Example 3

[0074] 1. Preparation of Dry Diaphragm

[0075] The tear-resistant slurry includes the following raw materials in parts by weight:

[0076] 11 parts of polyaramid with a molecular weight of 200,000-1,000,000 Da,

[0077] 65 parts of boehmite, 12 parts of nanocellulose,

[0078] 10 parts of polyacrylate,

[0079] 2 parts of polyvinyl alcohol,

[0080] 280 parts of N-methylpyrrolidone,

[0081] Polyaramide, boehmite, nanocellulose, polyacrylate, polyvinyl alcohol, and N-methylpyrrolidone were mixed under stirring at 900 rpm, and after stirring for 30 minutes, a tear-resistant slurry with a solid content of 32%, a pH value of 9.0, a viscosity of 50 cp, and an average particle size of 0.3 μm was obtained.

[0082] The above-mentioned tear-resistant slurry was coated on one side of a polypropylene diaphragm, and then the excess solvent N-methylpyrrolidone was removed in a water bath, and then dried at 65°C for 15 minutes to form a tear-resistant coating on the surface of the polypropylene diaphragm, thereby obtaining a dry diaphragm with a thickness of 16 μm; wherein, in the dry diaphragm, the thickness of the polypropylene diaphragm and the thickness of the tear-resistant coating were 7:1.

[0083] 2. Preparation of battery cells

[0084] 2.1. Preparation of positive electrode

[0085] The positive electrode slurry was prepared as follows: LFP positive electrode active material: conductive agent acetylene black: binder PVDF were added into a vacuum mixer in a mass ratio of 97.9:0.9:1.2 for mixing, and then solvent NMP was added to the mixed slurry. The mixed slurry was stirred until it was uniform under the action of the vacuum mixer, thereby obtaining the positive electrode slurry of this embodiment.

[0086] The above-mentioned positive electrode slurry is evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet is obtained, and then the semi-finished positive electrode sheet is cold pressed and cut to obtain the positive electrode sheet to be assembled.

[0087] 2.2. Preparation of negative electrode sheet

[0088] The negative electrode slurry was prepared as follows: artificial graphite: conductive agent acetylene black: thickener CMC: binder SBR were added to a vacuum mixer in a mass ratio of 96.4:1:1.2:1.4 and mixed, and then deionized water was added as a solvent to the mixture. The mixed slurry was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0089] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0090] 2.3 Assembly of battery cells

[0091] The positive electrode sheet, the negative electrode sheet and the dry-process separator are wound together to obtain a bare battery cell.

[0092] 3. Preparation of lithium batteries

[0093] The bare cell is packaged, injected, left to stand, formed, and divided into different volumes to produce a soft-pack battery, wherein the electrolyte used is a commercially available electrolyte.

[0094] Example 4

[0095] 1. Preparation of Dry Diaphragm

[0096] The tear-resistant slurry includes the following raw materials in parts by weight:

[0097] 11 parts of polyamide polymer with a molecular weight of 200,000-1,000,000 Da,

[0098] 65 parts of boehmite, 7 parts of halloysite tubes with an aspect ratio of 10:1,

[0099] 10 parts of polyacrylic acid,

[0100] 2 parts of polyethylene ether,

[0101] 300 parts of deionized water,

[0102] The polyamide polymer, boehmite, halloysite tube, polyacrylic acid, polyethylene ether and deionized water were mixed under stirring at 1000 rpm and stirred for 25 minutes to obtain a tear-resistant slurry with a solid content of 31%, a pH value of 8.5, a viscosity of 50 cp and an average particle size of 0.7 μm.

[0103] The above-mentioned tear-resistant slurry was coated on both sides of a polyethylene diaphragm and dried at 60°C for 25 minutes to form a tear-resistant coating on the surface of the polyethylene diaphragm, thereby obtaining a dry diaphragm with a thickness of 15 μm; wherein, in the dry diaphragm, the thickness of the polyethylene diaphragm and the thickness of the tear-resistant coating were 7:1.

[0104] 2. Preparation of battery cells

[0105] 2.1. Preparation of positive electrode

[0106] The positive electrode slurry was prepared as follows: LFP positive electrode active material: conductive agent acetylene black: binder PVDF were added into a vacuum mixer in a mass ratio of 97.9:0.9:1.2 for mixing, and then solvent NMP was added to the mixed slurry. The mixed slurry was stirred until it was uniform under the action of the vacuum mixer, thereby obtaining the positive electrode slurry of this embodiment.

[0107] The above-mentioned positive electrode slurry is evenly coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature, and then transferred to an oven for further drying. After drying in the oven, a semi-finished positive electrode sheet is obtained, and then the semi-finished positive electrode sheet is cold pressed and cut to obtain the positive electrode sheet to be assembled.

[0108] 2.2. Preparation of negative electrode sheet

[0109] The negative electrode slurry was prepared as follows: artificial graphite: conductive agent acetylene black: thickener CMC: binder SBR were added to a vacuum mixer in a mass ratio of 96.4:1:1.2:1.4 and mixed, and then deionized water was added as a solvent to the mixture. The mixed slurry was stirred in a vacuum mixer until it became uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0110] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0111] 2.3 Assembly of battery cells

[0112] The positive electrode sheet, the negative electrode sheet and the dry-process separator are wound together to obtain a bare battery cell.

[0113] 3. Preparation of lithium batteries

[0114] The bare cell is packaged, injected, left to stand, formed, and divided into different volumes to produce a soft-pack battery, wherein the electrolyte used is a commercially available electrolyte.

[0115] Example 5

[0116] The difference between this embodiment and embodiment 1 is that, in the process of preparing the tear-resistant slurry, the molecular weight of the polyvinylidene fluoride is 100,000-180,000 Da;

[0117] The anti-tear slurry in this embodiment has a solid content of 30%, a viscosity of 30 cp, and an average particle size of 0.8 μm; the rest are consistent with Example 1.

[0118] Example 6

[0119] The difference between this embodiment and embodiment 1 is that, in the process of preparing the tear-resistant slurry, the molecular weight of the polyvinylidene fluoride is 2100000-2500000Da;

[0120] The solid content of the tear-resistant slurry in this embodiment is 30%, the viscosity is 120 cp, and the average particle size is 0.8 μm; the rest are consistent with Example 1.

[0121] Comparative Example 1

[0122] The difference between this comparative example and Example 1 is that the dry-process diaphragm of this comparative example does not contain an anti-tear coating, and the rest of the parts are consistent with Example 1.

[0123] Comparative Example 2

[0124] The difference between this comparative example and Example 1 is that an equal weight of alumina is used to replace the halloysite tube in Example 1; the rest of the parts are the same as in Example 1.

[0125] Comparative Example 3

[0126] The difference between this comparative example and Example 1 is that, during the preparation of the tear-resistant slurry, the aspect ratio of the halloysite tube is 2:1; the rest of the parts are consistent with Example 1.

[0127] Comparative Example 4

[0128] The difference between this comparative example and Example 1 is that, during the preparation of the tear-resistant slurry, the aspect ratio of the halloysite tube is 15:1; the rest of the parts are consistent with Example 1.

[0129] Comparative Example 5

[0130] The difference between this comparative example and Example 3 is that, in the process of preparing the tear-resistant slurry, the thickness of the boehmite lamellae with a lamellae structure is 0.2 μm; the rest is consistent with Example 3.

[0131] Comparative Example 6

[0132] The difference between this comparative example and Example 3 is that, in the process of preparing the tear-resistant slurry, the thickness of the boehmite lamellae with a lamellae structure is 1.5 μm; the rest is consistent with Example 3.

[0133] Test Method

[0134] 1. Tensile strength and elongation at break test

[0135] The tensile strength test was performed on the dry-process separators in the above embodiments and comparative examples. The specific test method was based on GB / T 36363-2018 "Polyolefin separators for lithium-ion batteries".

[0136] 2. Thermal Shrinkage Test

[0137] The dry-process diaphragms in the above embodiments and comparative examples were subjected to a heat shrinkage test. The specific test method was as follows: the dry-process diaphragms were stacked into three layers, flattened, the air between the films was exhausted, the cutting plate (300×100) was removed, the diaphragm samples were cut out, and the length A1 and width B1 of the cut samples were measured; the oven temperature was set to 90°C, and after the temperature reached 90°C, the temperature was kept at 90°C for 1 hour, and the sample was placed in the oven and kept at this temperature for 2 hours; after the holding time was over, the diaphragm was removed, and after cooling for 10 minutes, the length A2 and width B2 of the sample were measured;

[0138] Diaphragm transverse TD thermal shrinkage = (A1-A2) / A1×100%,

[0139] The longitudinal MD thermal shrinkage of the diaphragm = (B1-B2) / B1×100%.

[0140] 3. Heavy Object Impact Test

[0141] The lithium batteries in the above embodiments and comparative examples were subjected to a heavy object impact test. The specific test method refers to GB 31241-2014 "Safety Requirements for Ion Batteries and Battery Packs for Portable Electronic Products"; after the lithium battery is fully charged according to the prescribed test method, the battery is placed on a platform surface, a metal rod with a diameter of 15.8mm±0.2mm is placed horizontally on the upper surface of the geometric center of the battery, and a weight with a mass of 9.1kg±0.1kg is used to freely fall from a height of 610mm±25mm to impact the battery surface where the metal rod is placed, and the battery is observed for 6 hours; during the impact test, cylindrical batteries are required to have their longitudinal axis parallel to the surface of the weight, and the metal rod is perpendicular to the longitudinal axis of the battery. Prismatic batteries and soft-package batteries are only impacted on the wide surface. When button batteries are impacted, the metal rod is placed across the center of the battery surface. Only one impact test is performed on one sample.

[0142] 4. Overcharge performance test

[0143] The lithium batteries in the above embodiments and comparative examples were subjected to overcharge performance tests. The specific test methods were based on the overcharge test in GB / T 36276-2018 "Lithium-ion Batteries for Power Energy Storage".

[0144] Table 1 Diaphragm performance test table

[0145]

[0146] In combination with Examples 1-4, Comparative Examples 1-2 and Table 1, it can be seen that by using an inorganic substance containing hydroxyl groups in the tear-resistant slurry, it can react with the binder to generate hydrogen bonds, thereby adjusting the intermolecular forces in the tear-resistant slurry. At the same time, after the inorganic substance reacts with the binder, it can be more evenly distributed between the high molecular polymers to form a structure, which increases the flexibility of the diaphragm and improves the elongation at break of the diaphragm in the TD transverse direction, thereby solving the tearing problem of the dry-process diaphragm. Accordingly, the lithium battery has strong impact resistance and stable overcharge performance.

[0147] In combination with Example 1, Examples 3-6 and Table 1, it can be seen that when the aspect ratio of the tubular structured inorganic material is too high or too low, or the thickness of the lamellar structured inorganic material is too high or too low, it is not conducive to the combination of the inorganic material in the tear-resistant slurry and the binder, and the dispersion uniformity in the tear-resistant slurry decreases, which is not conducive to the uniform distribution of the inorganic material in the molecular chain segments of the polymer, causing the relevant performance of the diaphragm to decline. Accordingly, the safety performance and overcharge performance of the lithium battery deteriorate.

[0148] In combination with Example 1, Examples 5-6 and Table 1, it can be seen that when the molecular weight of the polymer is too high or too low, it is not conducive to the formation of a stable three-dimensional network structure in the tear-resistant slurry, which causes the relevant performance of the diaphragm to decline, and accordingly, the safety performance and overcharge performance of the lithium battery deteriorate.

[0149] In combination with Examples 1-4, Comparative Examples 7-8 and Table 1, it can be seen that when the solid content and viscosity of the tear-resistant slurry do not meet the above-mentioned limited range, on the one hand, the degree of bonding between the components in the tear-resistant slurry is poor, and on the other hand, the coating operation of the tear-resistant slurry on the polypropylene diaphragm becomes more difficult, the degree of bonding between the tear-resistant coating and the polypropylene diaphragm is poor, and the improvement effect of the tensile strength, elongation at break and thermal insulation shrinkage of the diaphragm is not significant, thereby reducing the impact resistance and overcharge performance of the lithium battery.

[0150] In combination with Example 1, Examples 9-10 and Table 1, it can be seen that when the thickness ratio of the tear-resistant coating to the porous base layer is too low, the contribution to the improvement of the tensile strength of the dry-process diaphragm is limited. When the thickness ratio of the tear-resistant coating to the porous base material is too high, although the various properties of the dry-process diaphragm can still be maintained at a relatively excellent level, the wettability of the diaphragm is slightly reduced, which is not conducive to the improvement of the electrochemical performance of the lithium battery.

Claims

1. A dry-process diaphragm, wherein: The dry-process separator includes a porous substrate layer and an anti-tear coating layer coated on at least one surface of the porous substrate layer; The tear-resistant coating comprises tear-resistant slurry, and the tear-resistant slurry comprises polymer, inorganic substance, binder and solvent; wherein the inorganic substance contains hydroxyl group, and the binder contains hydroxyl group; The inorganic material includes at least one of a tubular structure and a sheet structure; when the inorganic material is a tubular structure, the aspect ratio of the inorganic material is 5:1-10:1; when the inorganic material is a sheet structure, the thickness of the inorganic material is 0.6-1 μm.

2. A dry-process diaphragm according to claim 1, wherein: The inorganic substance includes at least one of boehmite, nanocellulose, and halloysite tubes.

3. A dry-process diaphragm according to claim 1, wherein: The binder includes at least one of polyacrylic acid, polyacrylate, polyacrylonitrile, and a terpolymer binder.

4. A dry-process diaphragm according to claim 1, wherein: The polymer includes at least one of polyaromatic amide, polyimide, polymethyl methacrylate and polyvinylidene fluoride.

5. A dry-process diaphragm according to claim 1, wherein: The molecular weight of the polymer is 200,000-1,000,000 Da.

6. A dry-process diaphragm according to claim 1, wherein: The solid content of the tear-resistant slurry is 28%-32%, the viscosity of the tear-resistant slurry is less than 80cp, the pH of the tear-resistant slurry is 7.5-9.0, and the average particle size D50 of the tear-resistant slurry is less than 1 μm.

7. A dry-process diaphragm according to claim 1, wherein: The thickness of the dry-process separator is 10-18 μm, and the thickness ratio of the porous substrate layer to the anti-tearing coating is 5-20:

1.

8. A method for preparing a dry-process diaphragm according to any one of claims 1 to 7, comprising the following steps: S1: Mixing the polymer, the inorganic substance, the binder, and the solvent under stirring to obtain the tear-resistant slurry; wherein: The tear-resistant slurry further includes a dispersant accounting for 0.5wt%-3wt% of the mass of the tear-resistant slurry; S2: coating the tear-resistant slurry on at least one surface of the porous substrate layer, then removing the solvent and drying to obtain the dry-process diaphragm.

9. A method for preparing a dry-process diaphragm according to claim 8, wherein: In the tear-resistant slurry, the mass ratio of the polymer, the inorganic substance and the binder is 10-70:40-80:0.1-10.

10. A lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet and the dry-process separator according to any one of claims 1 to 7.

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