A diaphragm, its preparation method and use

Abstract

The application provides a diaphragm and a preparation method and application thereof. The diaphragm comprises a diaphragm base and a coating layer on the surface of the diaphragm base; the coating layer comprises modified aramid fiber and a binder; and the modified aramid fiber contains amino groups. The diaphragm provided by the application contains aramid fiber with amino groups in the coating layer, the adhesion and high-temperature resistance of the coating layer are improved, the problems of positive and negative electrode expansion during the use of the battery are solved, and the high-temperature resistance of the diaphragm is improved, so that the thermal stability and cycle performance of the battery are improved.

Description

Technical Field

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

[0002] In recent years, batteries have experienced rapid development due to their superior performance, including high energy density, long cycle life, and high voltage. They are now widely used in digital devices such as mobile phones, laptops, digital cameras, and camcorders, with new energy vehicles and energy storage representing their largest potential future applications. Batteries typically consist of positive and negative electrodes, a separator, and an electrolyte. The separator, acting as a barrier between the positive and negative electrodes, plays a crucial role in battery safety. Therefore, research aimed at improving battery safety by enhancing separator performance has attracted considerable attention.

[0003] Currently, the separators widely used in the market are mainly polyolefin organic separators. For example, CN101271967A discloses a thermally stable lithium-ion battery separator, whose composition is 50-80% by weight of polyolefin, 10-35% by weight of polysulfone, and 1%-15% by weight of a block copolymer of acrylamide and ether. Another example is CN111525076A, which discloses a method for preparing a high-mechanical-strength polyolefin separator. Before the heat-setting process of preparing the polyolefin separator, a crosslinking aid and a crosslinking agent are sequentially sprayed onto the semi-finished polyolefin separator, and finally, it is heated and set at 100-150°C to obtain the high-mechanical-strength polyolefin separator. However, polyolefin separators (uncoated) have a low heat resistance temperature. Once a short circuit, overcharge, or compression occurs and heat is generated, the polyolefin separator is prone to shrinkage, further aggravating the battery safety issues. In addition, there is no bonding bridge between the polyolefin separator and the positive and negative electrode plates, and the distance between the separator and the positive and negative electrodes may change with use, which will also affect the battery cycle performance.

[0004] Therefore, how to provide a separator that can improve the safety and cycle performance of batteries is an urgent technical problem to be solved. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a separator, its preparation method, and its applications. The separator provided by the present invention comprises a separator substrate and a coating on the surface of the separator substrate. The coating incorporates aramid fibers containing amino groups, which enhances the coating's adhesion and high-temperature resistance, thus mitigating the problem of positive and negative electrode expansion during battery use. Furthermore, it improves the separator's high-temperature resistance, thereby enhancing the battery's thermal stability and cycle performance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a diaphragm, the diaphragm comprising a diaphragm substrate and a coating on the surface of the diaphragm substrate; the coating comprising modified aramid and an adhesive; the modified aramid containing amino groups.

[0008] The coating provided by this invention can be located on one side or both sides of the diaphragm substrate, depending on the actual needs.

[0009] The separator provided by the present invention includes a separator substrate and a coating on the surface of the separator substrate. The coating contains aramid containing amino groups. After the amino groups are introduced into the aramid, its adhesion is improved. Aramid itself has excellent high temperature resistance, thereby improving the adhesion and high temperature resistance of the coating, enhancing the adhesion between the separator and the electrode, and improving the problem of positive and negative electrode expansion during battery use. At the same time, it also improves the high temperature resistance of the separator, and ultimately improves the thermal stability and cycle performance of the battery.

[0010] In this invention, if the aramid does not contain amino groups, it is impossible to achieve better adhesion between the separator and the electrode, which will lead to premature battery failure.

[0011] Preferably, the coating further includes inorganic powder, dispersant, and emulsifier.

[0012] In this invention, inorganic powder is added to the coating to further enhance the high temperature resistance of the diaphragm. The addition of inorganic powder can also improve the diaphragm's ability to wet the electrolyte. At the same time, the inorganic powder embedded in the modified aramid also improves the puncture resistance of the diaphragm and improves the thermal runaway problem of the diaphragm.

[0013] Preferably, the thickness of the diaphragm substrate is 4 to 20 μm, such as 4 μm, 5 μm, 7 μm, 10 μm, 12 μm, 15 μm, 18 μm or 20 μm.

[0014] Preferably, the thickness of the coating is 0.5 to 4 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm or 4 μm.

[0015] It should be noted that the phrase "the coating thickness is 0.5 to 4 μm" in this invention refers to the thickness of the coating on a single surface of the diaphragm substrate. That is, if the coating is provided on both surfaces of the diaphragm substrate, the thickness of the coating on each surface of the diaphragm substrate is 0.5 to 4 μm.

[0016] In this invention, if the coating is too thick, it will lead to increased internal resistance, while if it is too thin, it will not be conducive to suppressing battery expansion.

[0017] In a second aspect, the present invention provides a method for preparing a diaphragm as described in the first aspect, the method comprising the following steps:

[0018] (1) Aramid, modifier and first solvent are mixed and reacted, and the product after the mixed reaction is reacted with the reaction solvent to obtain modified aramid.

[0019] (2) Mix the modified aramid, binder and solvent described in step (1) to obtain a coating slurry, and coat the coating slurry onto the surface of the diaphragm substrate to obtain the diaphragm.

[0020] The preparation method provided by this invention involves modifying aramid with a modifier, in which the groups in the modifier replace the active hydrogen in the aramid. After mixing with a reaction solvent, an amino group is obtained, thereby achieving the purpose of introducing amino groups into the aramid and improving the bonding performance of the modified aramid. After simple slurry coating, a separator with good adhesion and high temperature resistance can be obtained. When used in batteries, it improves the thermal stability and cycle performance of the batteries.

[0021] In this invention, amino groups cannot be obtained unless the mixed reaction product is reacted again with the reaction solvent.

[0022] Preferably, the aramid is pretreated before the mixing reaction in step (1).

[0023] In this invention, impurities are removed from the aramid fibers by pretreatment, thereby improving the purity of the aramid fibers.

[0024] Preferably, the preprocessing includes:

[0025] The aramid fibers were mixed with a pretreatment solution, soaked, and then dried.

[0026] Preferably, the pretreatment agent comprises carboxymethyl cellulose and sodium dodecylbenzene sulfonate.

[0027] Preferably, the mass ratio of carboxymethyl cellulose to sodium dodecylbenzene sulfonate is (1.5-2.5):1, for example, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1 or 2.5:1, etc.

[0028] Preferably, the soaking time is 18 to 24 hours, such as 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, or 24 hours.

[0029] Preferably, the drying temperature is 50-80°C, for example, 50°C, 60°C, 70°C or 80°C.

[0030] Preferably, the drying time is 12 to 18 hours, such as 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, or 18 hours.

[0031] Preferably, the mass ratio of aramid to modifier in step (1) is (1-2.5):1, for example, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1 or 2.5:1, etc.

[0032] In this invention, if the mass ratio of aramid to modifier in step (1) is too small, that is, too much modifier is added, it will result in too much reactant remaining. If the mass ratio is too large, that is, too little modifier is added, it will not be conducive to a full reaction.

[0033] Preferably, the temperature of the mixing reaction in step (1) is 120 to 135°C, for example, 120°C, 121°C, 122°C, 123°C, 124°C, 125°C, 126°C, 127°C, 128°C, 129°C, 130°C, 131°C, 132°C, 133°C, 134°C or 135°C.

[0034] In this invention, if the temperature of the mixing reaction in step (1) is too low or too high, byproducts are easily generated and the product is impure.

[0035] Preferably, the mixing reaction time in step (1) is 1 to 2 hours, for example, 1 hour, 1.3 hours, 1.5 hours, 1.8 hours or 2 hours.

[0036] In this invention, by controlling the temperature and time of the mixing reaction in step (1), the amino groups are effectively introduced without generating side reactions.

[0037] Preferably, the modifier in step (1) includes diisocyanate.

[0038] In this invention, the isocyanate group (NCO) is replaced by the active hydrogen of the aramid fiber, and then reacted with a reaction solvent to obtain an amino group.

[0039] Preferably, the diisocyanate includes any one or a combination of at least two of toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, and dicyclohexylmethane diisocyanate.

[0040] Preferably, the first solvent in step (1) includes any one or a combination of at least two of N,N-dimethylformamide, N-methylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and N-ethylpyrrolidone.

[0041] Preferably, the reaction solvent in step (1) includes water.

[0042] Preferably, the time for the secondary reaction in step (1) is 3 to 6 hours, for example, 3 hours, 4 hours, 5 hours or 6 hours.

[0043] Preferably, the product after the secondary reaction in step (1) is washed.

[0044] Preferably, the adhesive in step (2) includes any one or a combination of at least two of styrene-butadiene rubber, acrylate, polytetrafluoroethylene, polymethyl methacrylate, and acrylonitrile.

[0045] Preferably, the membrane matrix in step (2) includes any one or a combination of at least two of the following: polyolefin matrix, polyethylene matrix, and polypropylene matrix.

[0046] Preferably, the coating slurry in step (2) further includes inorganic powder, dispersant, wetting agent and emulsifier.

[0047] Preferably, the inorganic powder comprises any one or a combination of at least two of the following: alumina, magnesium hydroxide, barium oxide, magnesium oxide, silicon oxide, boehmite, titanium oxide, or zirconium oxide.

[0048] Preferably, the D50 of the inorganic powder is 0.2 to 2.5 μm, for example, 0.2 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm, 1.3 μm, 1.5 μm, 1.8 μm, 2 μm, 2.3 μm or 2.5 μm.

[0049] Preferably, the dispersant comprises any one or a combination of at least two of polyvinylpyrrolidone, hydroxyethyl methylcellulose, carboxymethyl cellulose, ethyl cellulose, benzyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, and sodium polyacrylate.

[0050] Preferably, the emulsifier includes any one or a combination of at least two of sodium polyacrylate, polyacrylate, and polyacrylamide.

[0051] Preferably, the wetting agent includes at least one selected from ethanol, propylene glycol, glycerol, and butanediol.

[0052] Preferably, in step (2), the modified aramid fiber accounts for 55% to 80% of the mass of the solvent-free coating slurry, such as 55%, 58%, 60%, 63%, 65%, 68%, 70%, 73%, 75%, 78%, or 80%.

[0053] Preferably, in step (2), the adhesive accounts for 3 to 7.5% of the mass of the solvent-free coating slurry, for example, 3%, 4%, 4.3%, 4.5%, 4.8%, 5%, 5.3%, 5.5%, 5.8%, 6%, 6.6%, 6.5%, 6.8%, 7%, 7.3%, or 7.5%.

[0054] Preferably, in step (2), the inorganic powder accounts for 10 to 18.5% of the mass of the solvent-free coating slurry, for example, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, or 18.5%.

[0055] Preferably, in step (2), the dispersant accounts for 0.5% to 2% of the mass of the solvent-free coating slurry, for example, 0.5%, 1%, 1.5% or 2%.

[0056] Preferably, the coating is dried after application.

[0057] As a preferred technical solution, the preparation method includes the following steps:

[0058] (1) First, mix the aramid with the pretreatment agent solution, soak and dry it. Then, mix the dried and pretreated aramid, diisocyanate and solvent at 120-135℃ for 1-2 hours. The mass ratio of the dried and pretreated aramid to diisocyanate is (1-2.5):1. The product after mixing and reaction is reacted with the reaction solvent for 3-6 hours. After washing, the modified aramid is obtained.

[0059] (2) Mix the modified aramid, binder, inorganic powder, dispersant, wetting agent, emulsifier and solvent described in step (1) to obtain a coating slurry, coat the coating slurry on the surface of the diaphragm substrate and dry it to obtain the diaphragm;

[0060] The modified aramid fiber accounts for 55-80% of the mass of the solvent-free coating slurry; the binder accounts for 3-7.5% of the mass of the solvent-free coating slurry; the inorganic powder accounts for 10-18.5% of the mass of the solvent-free coating slurry; and the dispersant accounts for 0.5-2% of the mass of the solvent-free coating slurry.

[0061] Thirdly, the present invention also provides a secondary battery, the secondary battery comprising the separator as described in the first aspect.

[0062] Preferably, the secondary battery includes a lithium-ion battery, a potassium-ion battery, or a sodium-ion battery.

[0063] Compared with the prior art, the present invention has the following beneficial effects:

[0064] The separator provided by this invention includes a separator substrate and a coating on the surface of the separator substrate. Modified aramid is added to the coating. The modified aramid is obtained by mixing and reacting a modifier with aramid, which introduces amino groups into the modified aramid, thereby improving the adhesion of the modified aramid. Aramid itself has excellent high-temperature resistance, thereby improving the adhesion and high-temperature resistance of the coating, enhancing the adhesion between the separator and the electrode, and improving the problem of positive and negative electrode expansion during battery use. At the same time, it also improves the high-temperature resistance of the separator, ultimately improving the thermal stability and cycle performance of the battery. Detailed Implementation

[0065] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments are merely illustrative of the present invention and should not be construed as limiting the invention.

[0066] Example 1

[0067] This embodiment provides a diaphragm comprising a polyethylene matrix (9 μm thick) and a coating (double-sided, with a single-sided thickness of 2 μm) on the surface of the polyethylene matrix. The coating contains modified aramid with amino groups, polytetrafluoroethylene, alumina (D50 of 1 μm), polyvinylpyrrolidone (dispersant), and sodium polyacrylate (emulsifier).

[0068] The membrane is prepared as follows:

[0069] (1) Place the aramid in a pretreatment solution (carboxymethyl cellulose and sodium dodecylbenzene sulfonate are mixed in a mass ratio of 3:2 and dissolved in deionized water), soak for 20 h, then wash with water, and dry at 80 °C for 15 h to obtain the pretreated aramid.

[0070] (2) The pretreated aramid and toluene diisocyanate were added to a reaction vessel containing N,N dimethylformamide (solvent) at a mass ratio of 2.5:1 and reacted at a reaction temperature of 125°C for 1.5 h. The reaction product was then mixed with water for 4 h and washed with ethanol to obtain modified aramid (with amino groups).

[0071] (3) Modified aramid, alumina, polyvinylpyrrolidone, sodium polyacrylate and ethanol (wetting agent) are mixed and added to deionized water to obtain a coating slurry (in the solvent-free coating slurry, the mass percentage of modified aramid is 65%, the mass percentage of alumina is 18.5%, the mass percentage of polytetrafluoroethylene is 7.5%, the mass percentage of dispersant is 2%, and the total mass percentage of wetting agent and emulsifier is 7%). The coating slurry is coated on both sides of the membrane substrate (polyethylene substrate) and dried to obtain the membrane.

[0072] Example 2

[0073] This embodiment provides a diaphragm comprising a polyethylene matrix (9 μm thick) and a coating (double-sided, with a single-sided thickness of 2 μm) on the surface of the polyethylene matrix. The coating contains modified aramid with amino groups, polyacrylate, zirconium oxide (D50 of 0.4 μm), hydroxyethyl methyl cellulose (dispersant), and polyacrylate (emulsifier).

[0074] The membrane is prepared as follows:

[0075] (1) Place the aramid in a pretreatment solution (carboxymethyl cellulose and sodium dodecylbenzene sulfonate are mixed in a mass ratio of 5:2 and dissolved in deionized water), soak for 18 hours, then wash with water and dry at 80°C for 18 hours to obtain the pretreated aramid.

[0076] (2) The pretreated aramid and hexamethylene diisocyanate were added to a reaction vessel containing N,N dimethylformamide (solvent) at a mass ratio of 1.5:1 and reacted at a reaction temperature of 120°C for 2 hours. Then the reaction product was mixed with water for 3 hours and washed with ethanol to obtain modified aramid (with amino groups).

[0077] (3) Modified aramid, zirconium oxide, hydroxyethyl methyl cellulose, polyacrylate and glycerol (wetting agent) are mixed and added to deionized water to obtain a coating slurry (in the solvent-free coating slurry, the mass percentage of modified aramid is 80%, the mass percentage of alumina is 10%, the mass percentage of polytetrafluoroethylene is 3%, the mass percentage of dispersant is 0.5%, and the total mass percentage of wetting agent and emulsifier is 6.5%). The coating slurry is coated on both sides of the diaphragm substrate (polyethylene substrate) and dried to obtain the diaphragm.

[0078] Example 3

[0079] This embodiment provides a diaphragm comprising a polyethylene matrix (12 μm thick) and a coating (double-sided, with a single-sided thickness of 2 μm) on the surface of the polyethylene matrix. The coating contains modified aramid with amino groups, polytetrafluoroethylene, alumina (D50 of 1.5 μm), polyvinylpyrrolidone (dispersant), and sodium polyacrylate (emulsifier).

[0080] The membrane is prepared as follows:

[0081] (1) Place the aramid in a pretreatment solution (carboxymethyl cellulose and sodium dodecylbenzene sulfonate are mixed in a mass ratio of 3:2 and dissolved in deionized water), soak for 20 h, then wash with water, and dry at 80 °C for 15 h to obtain the pretreated aramid.

[0082] (2) The pretreated aramid and diphenylmethane diisocyanate were added to a reaction vessel containing N,N dimethylformamide (solvent) at a mass ratio of 1:1 and reacted at a reaction temperature of 135°C for 1 h. The reaction product was then mixed with water for 6 h and washed with ethanol to obtain modified aramid (with amino groups).

[0083] (3) Modified aramid, alumina, polyvinylpyrrolidone, sodium polyacrylate and ethanol (wetting agent) are mixed and added to deionized water to obtain a coating slurry (in the solvent-free coating slurry, the mass percentage of modified aramid is 70%, the mass percentage of alumina is 15%, the mass percentage of polytetrafluoroethylene is 5%, the mass percentage of dispersant is 1%, and the total mass percentage of wetting agent and emulsifier is 9%). The coating slurry is coated on both sides of the membrane substrate (polyethylene substrate) and dried to obtain the membrane.

[0084] Example 4

[0085] The difference between this embodiment and embodiment 1 is that the reaction temperature in step (2) of this embodiment is 110℃ (the reaction temperature after the modifier and aramid are mixed).

[0086] The remaining preparation methods and parameters are consistent with those in Example 1.

[0087] Example 5

[0088] The difference between this embodiment and embodiment 1 is that the mass ratio of the pretreated aramid to toluene diisocyanate in step (2) of this embodiment is 0.5:1.

[0089] The remaining preparation methods and parameters are consistent with those in Example 1.

[0090] Example 6

[0091] The difference between this embodiment and embodiment 1 is that the mass ratio of the pretreated aramid to toluene diisocyanate in step (2) of this embodiment is 3:1.

[0092] The remaining preparation methods and parameters are consistent with those in Example 1.

[0093] Comparative Example 1

[0094] The difference between this comparative example and Example 1 is that the aramid provided in this comparative example is not modified, that is, step (2) is not performed in the preparation process, and no amino groups are introduced into the aramid.

[0095] The remaining preparation methods and parameters are consistent with those in Example 1.

[0096] Comparative Example 2

[0097] The difference between this comparative example and Example 1 is that the coating in this comparative example is a ceramic coating, that is, it does not contain modified aramid and emulsifier (excluding steps 1 and 2 of Example 1). Specifically, alumina, polyvinylpyrrolidone and ethanol (wetting agent) are mixed and added to deionized water to obtain a coating slurry (in the solvent-free coating slurry, the mass percentage of alumina is 83.5%, the mass percentage of polytetrafluoroethylene is 7.5%, the mass percentage of dispersant is 3.0%, and the total mass percentage of wetting agent is 6.0%). The coating slurry is coated on both sides of the membrane substrate (polyethylene substrate), and the membrane is obtained after drying.

[0098] The diaphragms provided in Examples 1-6 and Comparative Examples 1-2 were tested to assess their thermal nailing and needle-punching strength, as well as their adhesive strength. The test results are shown in Table 1, and the specific test methods are as follows:

[0099] Hot nailing: At 150 degrees Celsius, the needle is suspended 2 mm above the diaphragm and held for 60 seconds. The diameter of the closed pore area of ​​the diaphragm is then measured by taking pictures with a microscope.

[0100] Needle penetration strength: The test method refers to GB / T36363-2018;

[0101] Diaphragm adhesion test method: The diaphragms obtained in each embodiment and comparative example are prepared into core rolls and hot-pressed. After hot pressing, the diaphragm will adhere tightly to the electrode. Then, the electrode with the diaphragm is cut into strips of 24mm*15cm. Then, 3M release tape is attached to a stainless steel plate, and the side of the strip (diaphragm and electrode) with the electrode is attached to the 3M release tape. The strip is rolled with a flat roller to remove air bubbles between the strip and the 3M release tape. Then, one end of the diaphragm layer is torn open, about 5cm. The strip is clamped in the fixture of the testing machine and a 180-degree peel test is performed.

[0102] Table 1

[0103]

[0104]

[0105] The data from Examples 1 and 4 show that during the mixing and modification process, the temperature of the modification reaction was too low, which led to an increase in the closed-pore area of ​​the diaphragm and reduced thermal stability.

[0106] The data results from Examples 1 and 5 or 6 show that during the mixing and modification process, if the mass ratio of aramid to modifier is too small, that is, too much modifier is added, the needle punching strength will decrease. If the mass ratio is too large, that is, too little modifier is added, it will not be conducive to the adhesion and bonding of the diaphragm and the electrode.

[0107] The data from Example 1 and Comparative Example 1 show that without modification of the aramid fiber, i.e. without amino groups, the adhesion between the diaphragm and the electrode sheet will be significantly worse.

[0108] The data from Example 1 and Comparative Example 2 show that if the coating does not contain modified aramid, the adhesion between the diaphragm and the electrode is significantly reduced, and the heat resistance of the diaphragm is also greatly reduced.

[0109] In summary, the separator provided by this invention incorporates modified aramid in its coating. The modified aramid is obtained by mixing and reacting a modifier with aramid, which introduces amino groups into the modified aramid, improving its adhesion. Furthermore, aramid itself has excellent high-temperature resistance, thereby enhancing the coating's adhesion and high-temperature resistance, strengthening the bond between the separator and the electrode, and mitigating the problem of positive and negative electrode expansion during battery use. It also improves the separator's high-temperature resistance, ultimately enhancing the battery's thermal stability and cycle performance.

[0110] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A diaphragm, characterized in that, The diaphragm includes a diaphragm substrate and a coating on the surface of the diaphragm substrate; the coating includes modified aramid and an adhesive; the modified aramid contains amino groups, wherein the modified aramid is obtained by mixing and reacting aramid, a modifier and a first solvent, and then reacting the product of the mixture with the reaction solvent in a secondary reaction; the modifier includes diisocyanate and the reaction solvent includes water.

2. The diaphragm according to claim 1, characterized in that, The coating also includes inorganic powder, dispersant and emulsifier.

3. The diaphragm according to claim 1, characterized in that, The thickness of the diaphragm substrate is 4~20μm; And / or, the thickness of the coating is 0.5~4μm.

4. A method for preparing a diaphragm as described in any one of claims 1-3, characterized in that, The preparation method includes the following steps: (1) Aramid, modifier and first solvent are mixed and reacted, and the product after the mixed reaction is reacted with the reaction solvent to obtain modified aramid. The modifier includes diisocyanate and the reaction solvent includes water. (2) The modified aramid, binder and solvent described in step (1) are mixed to obtain a coating slurry. The coating slurry is then coated onto the surface of the diaphragm substrate to obtain the diaphragm.

5. The method for preparing the diaphragm according to claim 4, characterized in that, Before the mixing reaction in step (1), the aramid fiber is pretreated.

6. The preparation method according to claim 5, characterized in that, The preprocessing includes: The aramid fibers were mixed with a pretreatment solution, soaked, and then dried.

7. The preparation method according to claim 6, characterized in that, The pretreatment agent includes carboxymethyl cellulose and sodium dodecylbenzene sulfonate.

8. The preparation method according to claim 7, characterized in that, The mass ratio of carboxymethyl cellulose to sodium dodecylbenzenesulfonate is (1.5~2.5):

1.

9. The preparation method according to claim 6, characterized in that, The soaking time is 18-24 hours; And / or, the drying temperature is 50~80℃; And / or, the drying time is 12-18 hours.

10. The method for preparing the diaphragm according to claim 4, characterized in that, The mass ratio of aramid to modifier in step (1) is (1~2.5):1; And / or, the temperature of the mixing reaction in step (1) is 120~135℃; And / or, the mixing reaction time in step (1) is 1~2 hours; And / or, the diisocyanate includes any one or a combination of at least two of toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethane diisocyanate, and dicyclohexylmethane diisocyanate; And / or, in step (1), the first solvent includes any one or a combination of at least two of N,N-dimethylformamide, N-methylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and N-ethylpyrrolidone.

11. The method for preparing the diaphragm according to claim 4, characterized in that, The time for the secondary reaction in step (1) is 3~6 hours; And / or, the product after the secondary reaction described in step (1) is washed.

12. The method for preparing the diaphragm according to claim 4, characterized in that, The adhesive in step (2) includes any one or a combination of at least two of the following: styrene-butadiene rubber, acrylate, polytetrafluoroethylene, polymethyl methacrylate, and acrylonitrile. And / or, the membrane matrix in step (2) includes any one or a combination of at least two of the following: polyolefin matrix, polyethylene matrix, and polypropylene matrix; And / or, the coating slurry in step (2) also includes inorganic powder, dispersant, wetting agent and emulsifier.

13. The method for preparing the diaphragm according to claim 12, characterized in that, The inorganic powder includes any one or a combination of at least two of the following: aluminum oxide, magnesium hydroxide, barium oxide, magnesium oxide, silicon oxide, boehmite, titanium oxide, and zirconium oxide. And / or, the D50 of the inorganic powder is 0.2~2.5μm; And / or, the dispersant comprises any one or a combination of at least two of polyvinylpyrrolidone, hydroxyethyl methylcellulose, carboxymethyl cellulose, ethyl cellulose, benzyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, and sodium polyacrylate. And / or, the emulsifier includes any one or a combination of at least two of sodium polyacrylate, polyacrylate, and polyacrylamide.

14. The method for preparing the diaphragm according to claim 12, characterized in that, In step (2), the modified aramid fiber accounts for 55-80% of the mass of the solvent-free coating slurry; And / or, in step (2), the adhesive accounts for 3-7.5% of the mass of the solvent-free coating slurry; And / or, in step (2), the inorganic powder accounts for 10-18.5% of the mass of the solvent-free coating slurry; And / or, in step (2), the dispersant accounts for 0.5-2% of the mass of the solvent-free coating slurry.

15. The method for preparing the diaphragm according to claim 4, characterized in that, The coating is then dried.

16. The method for preparing the diaphragm according to claim 4, characterized in that, The preparation method includes the following steps: (1) First, mix the aramid with the pretreatment agent solution, soak and dry it. Then, mix the dried and pretreated aramid, diisocyanate and the first solvent at 120~135℃ for 1~2h. The mass ratio of the dried and pretreated aramid to diisocyanate is (1~2.5):

1. The product after mixing and reaction is reacted with the reaction solvent for 3~6h. After washing, the modified aramid is obtained. (2) The modified aramid, binder, inorganic powder, dispersant, wetting agent, emulsifier and solvent described in step (1) are mixed to obtain a coating slurry. The coating slurry is coated on the surface of the diaphragm substrate and dried to obtain the diaphragm. The modified aramid fiber accounts for 55-80% of the mass of the solvent-free coating slurry; the binder accounts for 3-7.5% of the mass of the solvent-free coating slurry; the inorganic powder accounts for 10-18.5% of the mass of the solvent-free coating slurry; and the dispersant accounts for 0.5-2% of the mass of the solvent-free coating slurry.

17. A secondary battery, characterized in that, The secondary battery includes the separator as described in any one of claims 1-3.

18. The secondary battery according to claim 17, characterized in that, The secondary battery includes a lithium-ion battery, a potassium-ion battery, or a sodium-ion battery.

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