Coating separator and preparation method therefor, and secondary battery

Abstract

A coating separator and a preparation method therefor, and a secondary battery, relating to the technical field of battery separators. The coating separator comprises a base membrane and a coating layer covered on one side or two sides of the base membrane. The coating later comprises inorganic particles. After the coating separator is cut off by a serrated knife, cracks appear in the coating near the fracture and / or the coating near the fracture falls off. Any crack and / or falling satisfies the following characteristics: 1) the length of the crack in the MD direction is less than 20 μm, and the width of the crack is less than 20 nm; and 2) the area of the falling area is less than 1 μm2. By changing the preparation raw materials and process parameters of the base membrane in a base membrane section, and adjusting the ratio of two binders and the relationship between the BET of the inorganic particles to the total mass ratio of the binders in a coating section, the changing and the adjustments are used in cooperation to ameliorate the problems of cracks and falling of the cut separator, such that the cracks and falling are significantly ameliorated, thereby avoiding short circuits of a battery cell.

Description

A coated separator and its preparation method, and a secondary battery Technical Field

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

[0002] Currently, lithium-ion battery separators are typically made of polyolefins, which have poor heat resistance. When the battery temperature reaches the separator's melting point, the separator melts and breaks, causing direct contact between the positive and negative electrodes and resulting in an internal short circuit. Simultaneously, polyolefin separators have poor electrolyte wetting properties, leading to poor electrolyte filling and retention in lithium-ion batteries, thus negatively impacting battery performance. The main solution to these problems is to coat the separator surface with appropriate functional coatings, such as inorganic ceramic coatings. Applying a single-sided or double-sided inorganic ceramic coating to the separator surface can effectively improve its heat resistance, support the separator base film at high temperatures, and reduce thermal shrinkage. Simultaneously, the inorganic ceramic coating also improves the separator's wetting effect with the electrolyte.

[0003] Winding is a crucial process in lithium-ion battery manufacturing. First, the tabs are ultrasonically welded to the current collector. Then, the positive and negative electrode sheets and the separator are arranged in the order of positive electrode—separator—negative electrode—separator. Finally, they are assembled into a cylindrical or prismatic cell through winding. During the cell winding process, the separator roll is cut after each winding (usually using a serrated cutter). Summary of the Invention

[0004] The inventors discovered that defects appear near the cut edge of the coated separator after it is cut. Although these defects are not visible to the naked eye, microscopic observation reveals cracks and even powdering in the coating. During continuous battery cell production, each time the separator is cut, the cracks and coating peeling generate dust on-site. This dust accumulates and can damage the battery cell, increasing the risk of short circuits. Furthermore, severe cracks and peeling can significantly increase the thermal shrinkage of the separator, potentially leading to short circuits between the positive and negative electrodes during battery charging and discharging.

[0005] To address the aforementioned problems, this invention provides a coated separator and its preparation method, as well as a secondary battery, which alleviates the occurrence of cracks and detachment of the coated separator after cutting.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0007] The first aspect of the present invention provides a coated membrane, comprising: a base membrane and a coating layer covering one or both sides of the base membrane, the coating layer comprising inorganic particles;

[0008] After the coated diaphragm is cut with a serrated knife (along the TD direction), cracks and / or peeling of the coating appear near the fracture. Any cracks and / or peeling satisfy the following characteristics:

[0009] 1) The crack length in the MD direction is less than 20 μm, and the crack width is less than 20 nm; and

[0010] 2) The area of ​​the detached region is less than 1 μm 2 .

[0011] It should be noted that the serrated blade described in this invention refers to a knife with a blade covered with serrations, as opposed to a flat-edged knife without serrations. The serration pitch of the serrated blade is between 0.5 and 10 mm, and it can be a serrated blade of various models or specifications adapted to different battery manufacturers.

[0012] The following is a detailed explanation:

[0013] Base membrane:

[0014] There are no particular restrictions on the type of base film, which can be selected according to actual needs; preferably, the base film can be selected from one or more of polyethylene film, polypropylene film, polyethylene / polypropylene composite film, polyimide film, polyvinylidene fluoride film, polyvinylidene fluoride-hexafluoropropylene film, polyamide film, and polyethylene terephthalate film, preferably polyethylene film.

[0015] The thickness of the base film can be 4-20 μm, preferably 5-12 μm.

[0016] The porosity of the base membrane can be 25-70%, preferably 30-65%, and more preferably 35-42%.

[0017] The base film has a minimum elongation at break (MD) of ≤100%, preferably 50-90%.

[0018] Elongation at break refers to the ratio between the increase in length of the base film after being stretched along the MD direction until it breaks at room temperature and the initial length, expressed as a percentage (%). This elongation at break can be measured by a tensile test.

[0019] Coating layer:

[0020] The coating layer comprises inorganic particles;

[0021] The inorganic particles are selected from one or more of the following: α-alumina, γ-alumina, boehmite, calcium carbonate, barium sulfate, hydrotalcite, montmorillonite, spinel, titanium dioxide, silicon dioxide, zirconium dioxide, magnesium oxide, calcium oxide, beryllium oxide, magnesium hydroxide, calcium hydroxide, and silicon carbide.

[0022] The average particle size D50 of the inorganic particles is preferably 0.1-1.5 μm, more preferably 0.2-0.8 μm.

[0023] The specific surface area S of the inorganic particles is preferably 1-25 m². 2 / g.

[0024] Specific surface area (BET) can be measured by a specific surface area tester and refers to the total specific surface area per unit weight.

[0025] In some embodiments, the coating layer further includes an organic polymer material;

[0026] The organic polymer material is selected from one or more of the following organic powders or suspensions: polyimide, polyetherimide, aramid, aramid sulfone, polyvinylidene fluoride, polymethyl methacrylate, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polyvinylidene fluoride-hexafluoropropylene.

[0027] In some embodiments, the coating layer further includes a first adhesive and a second adhesive;

[0028] The first adhesive is an acid-based adhesive, which may be acrylic, polyacrylic acid, or carboxylic acid.

[0029] The second adhesive is an ester-based adhesive, which may be acrylate, polyacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, or ethyl methacrylate.

[0030] Preferably, the coating layer comprises the following components by weight percentage: 80-96% inorganic particles, 1-6% first binder, 1-6% second binder, and 3-12% organic polymer material.

[0031] The above percentages refer to the dry weight of the components. Binders and organic polymer materials may also be added in emulsion or solution form.

[0032] Preferably, the thickness of the coating layer is 0.1-5 μm.

[0033] The coated diaphragm of the present invention was placed on a tensile testing machine (tensioned in the MD direction) and cut perpendicularly with a serrated blade (the blade face was perpendicular to the diaphragm surface). The cut diaphragm was observed under a 1K magnification electron microscope. Near the fracture surface (within a 30μm length perpendicular to the fracture boundary from the outside in), cracks and / or flaking (powdering) appeared in the diaphragm coating, as shown in Figure 1. Cracks refer to ridge-like fissures in the coating, and flaking refers to blocky powdery areas on the coating. The cracks and / or flaking have the following characteristics:

[0034] 1) The length of any crack perpendicular to the cutting (fracture surface, or cross-section) direction (i.e., the processing direction, or the MD direction, or the longitudinal direction) is less than 20 μm, preferably 5-15 μm, for example 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 μm, etc.; the crack usually starts from the fracture surface (starting point) and extends inward. When measuring, draw a line parallel to the fracture surface at the end of the crack. The perpendicular distance from the crack starting point to this line is the specified length, as shown in Figure 1a.

[0035] The width of any crack is less than 20 nm, preferably 5-15 nm, such as 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 nm, etc.; the width of the widest point in the crack extension direction can be measured under an electron microscope.

[0036] 2) The area of ​​any powder-shedding area perpendicular to the cutting (fracture surface, or cross-section) direction (i.e., the processing direction, or the MD direction, or the longitudinal direction) is less than 1μm. 2 For example, 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1μm 2 etc.; ImageJ software can be used for measurement (insert the electron microscope image into the software, draw the trajectory of the powder falling off area, and then obtain the area inside the trajectory), as shown in Figure 1b.

[0037] In some implementations, any crack and / or spalling satisfies the following characteristics:

[0038] 1) The length of the crack along the MD direction is less than 15 μm, and the crack width is less than 15 nm; and

[0039] 2) The area of ​​the coating peeling zone along the MD direction is less than 1μm. 2 .

[0040] The above characteristics reflect some inherent features of the coated diaphragm, including but not limited to the slip adhesion between the base film and the coating, the bonding force between the base film and the coating, and the cohesive force between particles within the coating. These characteristics are quantitatively manifested after being cut by a serrated blade. When the coated diaphragm of the present invention is cut, the proportion and size of the crack and / or detachment areas are within the above range, and the thermal shrinkage performance of the diaphragm does not change significantly during the serrated blade cutting process in cell processing, avoiding the risk of short circuits in the cell. Beyond the above range, cracks and detachment become obvious, resulting in poor stability of the diaphragm's thermal shrinkage performance and a higher risk of short circuits.

[0041] In this embodiment of the invention, the difference between the MD direction thermal shrinkage rate of the sample cut with a serrated knife after coating the diaphragm and the MD direction thermal shrinkage rate of the sample cut with a flat-edged knife after coating the diaphragm and the sample kept at 150°C for 1 hour does not exceed 10%.

[0042] Preferably, the thermal shrinkage rate in the MD direction of the coated diaphragm after being kept at 150°C for 1 hour is <15%;

[0043] The MD-direction thermal shrinkage rate of the coated diaphragm after being cut with a flat-bladed knife and kept at 150°C for 1 hour is <18%.

[0044] The MD-direction thermal shrinkage rate of the coated diaphragm after being cut with a serrated knife and kept at 150°C for 1 hour is <25%.

[0045] Heat shrinkage rate can be tested using methods known in the art. As a specific example, the test shall be conducted in accordance with the requirements of GB / T36363-2018.

[0046] MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0047] Cutting the coated separator with a sharp, flat-edged blade does not result in significant changes in thermal shrinkage compared to an uncut coated separator. However, battery manufacturers commonly use serrated blades to cut battery cells, which leads to noticeable powder shedding and cracking, and a significant increase in thermal shrinkage. The coated separator of this invention controls powder shedding and cracking within a certain range, and the increase in thermal shrinkage performance after cutting with a serrated blade is not significant.

[0048] In this embodiment of the invention, the powder removal rate after the coated diaphragm is cut with a serrated knife (each time) is less than 0.6%, preferably 0.2-0.4%.

[0049] The powder loss rate during cutting is the powder loss rate after the coating membrane is applied, compared to before cutting.

[0050] The coated diaphragm of the present invention has a low powder loss rate during cutting.

[0051] A second aspect of the present invention provides a method for preparing a coated diaphragm, comprising the following steps:

[0052] Base film preparation:

[0053] (1) Extrusion: The diluent and polyethylene are heated and melted in an extruder to form a uniform mixed melt; the viscosity-average molecular weight of polyethylene is ≤2 million;

[0054] (2) Casting: The mixed melt obtained in (1) flows out to the casting roller to cool down, perform phase separation, cool and form a film;

[0055] (3) Pre-stretching: The film obtained in (2) is heated and biaxially stretched to orient the molecular chains; MD stretching ratio > 1, 1 < MD stretching ratio / TD stretching ratio < 2.

[0056] (4) Biaxial stretching: The film obtained in (3) is heated and biaxially stretched to orient the molecular chains; the stretching results of steps (3) and (4) are: MD stretching ratio > 8 times, 1 < MD stretching ratio / TD stretching ratio < 2.

[0057] (5) Extraction and drying: Elute the diluent remaining in the biaxially stretched membrane from step (4) with an extractant and dry;

[0058] (6) Lateral stretching and shaping: The dried film obtained in step (5) is laterally stretched and shaped, and then wound up to obtain the base film.

[0059] The obtained base film has a MD elongation at break of ≤100%;

[0060] Coating:

[0061] (7) Disperse the inorganic particles, the first binder, the second binder and the optional organic polymer material in a solvent, mix them evenly to obtain a coating slurry;

[0062] Based on the dry weight of the coating slurry as 100%, the coating slurry comprises the following components by mass percentage: 80-96% inorganic particles, 1-6% first binder, 1-6% second binder, and 0-12% organic polymer material;

[0063] The specific surface area S of the inorganic particles and the total mass ratio M of the first and second binders satisfy the following relationship: S / 0.5 ≥ M ≥ S / 5, where S is in meters. 2 / g, M is in %;

[0064] (8) Coating slurry is applied to the surface of at least one side of the base film and dried to obtain a coated diaphragm.

[0065] The following is a detailed explanation:

[0066] Base film preparation:

[0067] According to an embodiment of this disclosure, the diluent in step (1) is a mixture of hydrocarbons from C18 to C30;

[0068] Preferably, the diluent is a mixture of C18-C30 liquid hydrocarbons with a molecular weight of 200-600;

[0069] More preferably, the diluent is paraffin oil.

[0070] According to an embodiment of this disclosure, the mass ratio of polyethylene to diluent in step (1) is (10-30):(70-90);

[0071] According to the embodiments of this disclosure, the heating and melting temperature in step (1) is 160°C to 210°C.

[0072] According to the embodiments of this disclosure, the temperature of the casting roller in step (2) is ≤30°C, preferably ≤25°C.

[0073] According to the embodiments of this disclosure, the biaxial stretching temperature in step (4) is 119°C to 135°C.

[0074] According to an embodiment of this disclosure, the extractant in step (5) is dichloromethane.

[0075] According to the embodiments of this disclosure, the drying temperature in step (5) is 25-80°C and the drying time is 10-100s.

[0076] According to the embodiments of this disclosure, the transverse stretching temperature in step (6) is 100℃~140℃, and the TD (transverse) stretching ratio is 1.1~1.8 times.

[0077] According to the embodiments of this disclosure, the setting temperature in step (6) is 120℃~140℃, and the setting time is 0.1s~60s.

[0078] MD or TD stretch ratio = length after stretching / length before stretching.

[0079] Coating:

[0080] This invention controls the numerical relationship between the specific surface area S of inorganic particles and the total mass ratio M of the first and second binders to ensure that: S / 0.5≥M≥S / 5. Excessive binder content will lead to excessive cohesion, which will easily cause inorganic particles to clump together and fall off. Insufficient binder content will easily cause cracks.

[0081] The above relationship is in S units m 2 When using / g and M as units (%), the relationship between the values ​​(without units) substituted. M refers to the mass percentage of the total dry weight of the first and second binders in the coating layer to the dry weight of the coating slurry.

[0082] According to the embodiments of this disclosure, the solvent in step (7) is at least one selected from water, N-methylpyrrolidone, and N,N-dimethylacetamide.

[0083] According to the embodiments of this disclosure, the mixing speed in step (7) is 800-2000 rpm and the mixing time is 20-100 min.

[0084] In addition to the raw materials mentioned above, the coating slurry may also contain any other components. Examples include viscosity modifiers, dispersants, wetting agents, and electrolyte dispersion inhibitors. There are no particular restrictions on the above components as long as they do not adversely affect the lithium battery. Any of the above components may be one, two, or more.

[0085] There are no particular restrictions on the coating method for the slurry; methods such as coating by application or dipping can be selected. Examples of coating methods include doctor blade coating, reverse roller coating, direct roller coating, microgravure roller coating, extrusion coating, spray coating, and dot coating. Considering the uniformity of the porous film thickness, microgravure roller coating is preferred.

[0086] There are no particular restrictions on the drying method; hot air, low-humidity air, vacuum drying, spray drying, freeze drying, and other drying methods can be selected.

[0087] During the base film preparation stage, adjusting the relative slippage between the base film and the coating is crucial. If the difference between the elastic deformation of the base film and the coating is too large, the coating will break. Therefore, the elongation at break (MD) needs to be reduced. This invention achieves an MD elongation at break of ≤100% for the base film by adjusting the raw materials and process parameters.

[0088] During the coating stage, it is necessary to improve the adhesion between the coating and the base film, as well as the cohesive bonding force between particles. This invention improves the problems of cracking and powder shedding after cutting by adjusting the ratio of two different binders and controlling the relationship between the BET of inorganic particles and the total mass ratio of the binder.

[0089] Through the improvements in the base film and coating stages, the relative slippage between the base film and coating after cutting is controlled, and the adhesion between the coating and the base film and the cohesion between particles inside the coating are improved. The two stages work together to significantly improve the cracking and detachment problems of the diaphragm after cutting.

[0090] According to the method provided in the second aspect of the present invention, a coated diaphragm according to any embodiment of the first aspect of the present invention can be obtained.

[0091] The diaphragm provided by the present invention is not limited to the preparation method provided by the present invention.

[0092] In a third aspect, the present invention provides a secondary battery comprising a coated separator provided in the first aspect of the present invention or a coated separator prepared by the preparation method provided in the second aspect of the present invention.

[0093] The coated separator provided in this invention can be used in different types of batteries, therefore there is no particular limitation on the type of battery, and it can be selected according to actual needs. Preferably, it can be used in secondary batteries, especially secondary batteries containing liquid electrolytes, such as lithium-ion batteries, sodium-ion batteries, etc. Beneficial effects:

[0094] The diaphragm provided by this invention can alleviate the cracking and detachment of the coated diaphragm after cutting. Furthermore, this invention also provides a preparation method that, by changing the raw materials and process parameters in the base film preparation stage, and adjusting the ratio of the two binders and the relationship between the BET of the inorganic particles and the total mass percentage of the binder in the coating stage, these factors work together to alleviate the cracking and detachment problems of the diaphragm after cutting, significantly improving cracking and detachment and preventing short circuits in the battery cell.

[0095] The present invention has been described in detail above; however, the above embodiments are merely illustrative in nature and are not intended to limit the invention. Furthermore, this document is not limited to the foregoing prior art or the invention itself, or to any theory described in the following embodiments. Attached Figure Description

[0096] Figure 1 shows the cracks and peeling off of the coated diaphragm after cutting. Detailed Implementation

[0097] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0098] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0099] Unless otherwise specified, the raw materials, reagents, and methods used in the embodiments are all conventional raw materials, reagents, and methods in the art.

[0100] Test method:

[0101] Elongation at break: Tested according to GB / T 1040.3-2006. Cut a 2.5cm×20cm specimen and mark it in the MD / TD direction of the diaphragm. Use a tensile testing machine (Jinan Sike Testing Technology Co., Ltd., TSL-1002) to test. Fix the specimen between the upper and lower clamps of the tensile testing machine (the distance between the clamps is 100±5mm). Ensure that the specimen is flat and wrinkle-free, and vertical and not skewed. The tensile speed is 250mm / min. Based on the width and thickness of the specimen, test 3 times and take the average value as the elongation at break (%) in the MD / TD direction.

[0102] Heat shrinkage rate: Cut a 10cm×10cm sample, mark the transverse TD and longitudinal MD on the sample, measure the transverse and longitudinal widths using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C), hold the sample between two sealed A4 sheets of paper, place the sample in a 150℃ oven and let it stand for 1 hour. After the sample returns to room temperature, measure the transverse and longitudinal widths again using the fully automatic image measuring projector. Take the average value of the measurements three times. The heat shrinkage rate can be obtained by the following formula: MD heat shrinkage rate (%) = (MD length before heating - MD length after heating) ÷ MD length before heating × 100;

[0103] Powder shedding rate during cutting: Before cutting, weigh the diaphragm on an analytical balance and record the weight as m1. After cutting with a serrated knife, weigh the cut diaphragm on an analytical balance and record the weight as m2. Powder shedding amount M (mg) = m1 - m2, Powder shedding rate = M / m1 × 100%.

[0104] Preparation of base films in Examples 1-9

[0105] In the preparation example, the polyethylene used was polyethylene with a molecular weight average (viscosity average) of 1.5 million (manufacturer: Celanese, model: GURX223), and the equipment used was a wet-process separator line of Sinoma Lithium Membrane Co., Ltd.

[0106] (1) Extrusion: Paraffin oil and polyethylene are fed into the extruder in proportion. The paraffin oil-polyolefin resin mixture is heated and melted in the extruder at 180°C to form a uniform mixed melt.

[0107] (2) Casting: The mixed melt obtained in (1) flows out to the casting roll to cool down. The temperature of the casting roll is ≤20℃. Phase separation is performed, and the mixture is cooled and formed.

[0108] (3) Pre-stretching: The film obtained in (2) is heated and biaxially stretched to orient the molecular chains; the stretching ratio of MD is 1.4 times and the stretching ratio of TD is 1 time.

[0109] (4) Biaxial stretching: The film obtained in (3) is heated and biaxially stretched to orient the molecular chains; the stretching results of steps (3) and (4) meet the requirements in Table 1.

[0110] (5) Extraction: Elute the extractant remaining in the biaxially stretched membrane from step (4) with dichloromethane; dry the extracted membrane at 35°C for 21 seconds;

[0111] (6) Lateral stretching: The dried film obtained in step (4) is stretched laterally and shaped.

[0112] The relevant parameters of the base film for each preparation example are shown in Table 1.

[0113] Preparation Examples 1-9: Preparation of Coating Slurry

[0114] A water-based coating slurry was prepared by stirring boehmite, PMMA emulsion (Shenzhen Haodian Technology Co., Ltd., SWA709, 20% solids content), polyacrylic acid emulsion (Hunan Gaorui Power Materials Co., Ltd., LIB-S105B, 21% solids content), polyacrylate emulsion (Japan ZEON Co., Ltd., BM-900B, 40% solids content), sodium carboxymethyl cellulose, polyether-modified siloxane, and water at 1300 rpm for 60 minutes.

[0115] The relevant parameters of the coating slurry for each preparation example are shown in Table 1.

[0116] Table 1

[0117] Preparation of coated diaphragms in Experiments 1-13

[0118] The base film and coating slurry for each embodiment are shown in Table 2.

[0119] Coating process: The above-mentioned water-based coating slurry is applied to one side of the base film by gravure coating using an MCD type coating machine, and finally dried in an oven at 70°C for 12 seconds.

[0120] The coated diaphragms of the examples and comparative examples were placed on a tensile testing machine to be stretched in the MD direction, and then vertically cut with a serrated knife. The cut diaphragms were observed under a 1K electron microscope, and the size of the maximum crack and powder shedding area within a 30μm range near the fracture was recorded (method as before).

[0121] After cutting the coated diaphragms of the Examples and Comparative Examples with a serrated blade using the method described above, the heat shrinkage rate of the cut samples was tested at 150°C for 1 hour. After cutting the coated diaphragms of the Examples and Comparative Examples with a flat blade using the method described above, the heat shrinkage rate of the cut samples was tested at 150°C for 1 hour.

[0122] After cutting the coated diaphragms of the examples and comparative examples with the serrated knife described above, the weight loss before and after cutting the samples was tested, and the powder loss rate was calculated.

[0123] The test results of each embodiment and comparative embodiment are shown in Table 2.

[0124] Table 2

[0125] As can be seen from the above test examples 1-5 and 7-12, the maximum crack length in the MD direction of the diaphragm of this application is less than or equal to 12 nm, the maximum crack width is less than or equal to 4 nm, and the area of ​​the powder shedding region is less than or equal to 0.8 μm. 2The heat shrinkage rate after serrated cutting is less than 15%, and the powder loss rate after serrated cutting is less than 0.5%. In particular, when the elongation at break of the base film is 50-90%, it has an even smaller heat shrinkage rate after serrated cutting.

[0126] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A coated diaphragm, characterized in that, include: A base film and a coating layer covering one or both sides of the base film, the coating layer comprising inorganic particles; After the coated diaphragm is cut with a serrated knife, cracks and / or peeling of the coating appear near the cut edge. Any cracks and / or peeling satisfy the following characteristics: 1) The crack length in the MD direction is less than 20 μm, and the crack width is less than 20 nm; and 2) The area of ​​the detached region is less than 1 μm 2 .

2. The coated diaphragm according to claim 1, characterized in that, The difference between the MD-direction thermal shrinkage rate of the coated diaphragm after being cut with a serrated knife and kept at 150°C for 1 hour and the MD-direction thermal shrinkage rate of the coated diaphragm after being cut with a flat-bladed knife and kept at 150°C for 1 hour does not exceed 10%.

3. The coated diaphragm according to claim 1, characterized in that, The powder loss rate after the coated diaphragm is cut with a serrated blade is less than 0.6%.

4. The coated diaphragm according to any one of claims 1-3, characterized in that, The base film has a MD elongation at break of ≤100%.

5. The coated diaphragm according to any one of claims 1-3, characterized in that, The coating layer further includes a first adhesive, a second adhesive, and an optional organic polymer material; The first adhesive is at least one selected from acrylic acid, polyacrylic acid, and carboxylic acid adhesives; The second adhesive is selected from at least one of acrylate, polyacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, and ethyl methacrylate; The organic polymer material is selected from at least one of polyimide, polyetherimide, aramid, aramid sulfone, polyvinylidene fluoride, polymethyl methacrylate, polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polyvinylidene fluoride-hexafluoropropylene.

6. The coated diaphragm according to claim 5, characterized in that, The coating comprises the following components by weight percentage: 80-96% inorganic particles, 1-6% first binder, 1-6% second binder, and 0-12% organic polymer material.

7. A method for preparing a coated diaphragm according to any one of claims 1-6, characterized in that, Includes the following steps: Base film preparation: (1) Extrusion: The diluent and polyethylene are heated and melted in an extruder to form a uniform mixed melt; the viscosity-average molecular weight of polyethylene is ≤2 million; (2) Casting: The mixed melt obtained in (1) flows out to the casting roller to cool down, perform phase separation, cool and form a film; (3) Pre-stretching: The film obtained in (2) is heated and biaxially stretched to orient the molecular chains; MD stretching ratio > 1, 1 < MD stretching ratio / TD stretching ratio < 2. (4) Biaxial stretching: The film obtained in (3) is heated and biaxially stretched to orient the molecular chains; the stretching results of steps (3) and (4) are: MD stretching ratio > 8 times, 1 < MD stretching ratio / TD stretching ratio < 2. (5) Extraction and drying: Elute the diluent remaining in the biaxially stretched membrane from step (4) with an extractant and dry; (6) Lateral stretching and shaping: The dried film obtained in step (5) is laterally stretched and shaped, and then wound up to obtain the base film. The obtained base film has a MD elongation at break of ≤100%; Coating: (7) Disperse the inorganic particles, the first binder, the second binder and the optional organic polymer material in a solvent, mix them evenly to obtain a coating slurry; Based on the dry weight of the coating slurry as 100%, the coating slurry comprises the following components by mass percentage: 80-96% inorganic particles, 1-6% first binder, 1-6% second binder, and 0-12% organic polymer material; The specific surface area S of the inorganic particles and the total mass ratio M of the first and second binders satisfy the following relationship: S / 0.5 ≥ M ≥ S / 5, where S is in meters. 2 / g, M is in %; (8) Coating slurry is applied to the surface of at least one side of the base film and dried to obtain a coated diaphragm.

8. The preparation method according to claim 7, characterized in that, In step (1), the mass ratio of polyethylene to diluent is (10-30): (70-90).

9. The preparation method according to claim 7, characterized in that, The biaxial stretching temperature in step (4) is 119℃~135℃; In step (6), the transverse stretching temperature is 100℃~140℃ and the TD stretching ratio is 1.1~1.8 times.

10. A secondary battery, characterized in that, The coating includes the coating membrane according to any one of claims 1-6 or the coating membrane prepared by the preparation method according to any one of claims 7-9.

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