Composite diaphragm as well as preparation method and application thereof

Abstract

The invention relates to a composite diaphragm and a preparation method and application thereof, and the preparation method comprises the following steps: mixing a diamine monomer and a dianhydride monomer for the first time to obtain a polyamide acid solution; obtaining a water-in-oil emulsion from the second mixed polyamide acid solution, deionized water and a surfactant, and sequentially carrying out heat treatment and imidization treatment on the water-in-oil emulsion to obtain polyimide microspheres; and stirring and mixing a binder and the polyimide microspheres to obtain polyimide coating slurry, coating the polyimide coating slurry on the surface of a polyethylene or polypropylene base membrane, and drying to obtain the composite diaphragm. The polyimide microspheres which are uniform in size and complete in particle are coated on the surface of the base membrane to prepare the composite membrane, and the composite membrane not only has excellent electrochemical performance, but also has excellent temperature resistance and safety.

Description

[0001] This application is a divisional application. The parent application number is 202510638506.3, the application date is May 19, 2025, and the invention title is "A composite membrane modified with polyimide microspheres and its preparation method and application". Technical Field

[0002] This invention relates to the field of lithium-ion battery technology, and to a composite separator, its preparation method and application, particularly to a composite separator, its preparation method and application. Background Technology

[0003] Lithium-ion batteries, as highly efficient energy storage devices, possess advantages such as high energy density, long cycle life, and no memory effect, and are widely used in portable electronic devices, electric vehicles, and energy storage systems. The separator, as a key component of lithium-ion batteries, primarily functions to isolate the positive and negative electrodes to prevent short circuits, while also providing a channel for the transport of lithium ions in the electrolyte.

[0004] Currently, commercial lithium-ion batteries widely use polyolefin separators, such as polyethylene and polypropylene. Although polyolefin separators have advantages such as excellent chemical stability, high mechanical strength, and low cost, their thermal stability and electrolyte wettability are poor. They are prone to thermal shrinkage at high temperatures, which can lead to short circuits and safety hazards. They also affect the uniform transport of lithium ions inside the battery, limiting their application in high-energy-density and high-rate lithium batteries.

[0005] To improve the heat resistance and electrolyte wettability of separators, existing technologies typically coat the surface of polyolefin separators with ceramic or polymer materials. However, ceramic coatings have poor compatibility with polyolefin-based membranes, ceramic particles are prone to detachment, and the high density of ceramic coatings increases the weight of the separator, leading to a decrease in the energy density of lithium-ion batteries. While polymer coatings can improve the thermal stability of the separator, their temperature resistance still needs further improvement. Therefore, developing a novel lithium-ion battery separator material that combines high temperature resistance, high wettability, and lightweight properties is of great significance for meeting the high energy density and safety performance requirements of power batteries.

[0006] CN116315447A discloses a polyimide porous microsphere coated separator for lithium batteries and its preparation method. This patent prepares polyimide porous microspheres using a non-aqueous emulsion system and partially replaces the diamine monomer with water-soluble chitosan during the preparation process, significantly improving the liquid absorption rate of the coated separator and reducing the areal density of the coated membrane. However, the polyimide porous microspheres prepared by this patent have relatively low porosity and specific surface area, which need further improvement to enhance the ion conductivity and electrochemical performance of the separator.

[0007] CN114883746A discloses a novel polyimide microsphere slurry and its coated separator. The polyimide microsphere slurry-coated separator in this patent exhibits excellent resistance to lithium dendrite penetration and high-temperature pore-closure to prevent internal short circuits in the battery, and also has a lower areal density than ceramic-coated separators. However, the polyimide microspheres prepared by this patent have poor dispersibility and dimensional uniformity, and poor thermal conductivity uniformity, making it difficult to meet the higher safety requirements of high-performance lithium-ion batteries.

[0008] CN117645723A discloses a method for preparing polyimide microspheres with carboxylic acid groups on their surface. This patent describes a process where polyamic acid microspheres are prepared by electrospinning, followed by imidization to obtain polyimide microspheres. Subsequently, the polyimide microspheres are etched with an alkaline solution, causing a ring-opening reaction of the imide rings, resulting in carboxylic acid groups on the surface of the polyimide microspheres. This method is lengthy and overly complex, and the ring-opening process is difficult to control, which is detrimental to stable batch production.

[0009] CN103570946A discloses a method for preparing polyimide microspheres using an emulsion method. This patent involves preparing a polyamic acid slurry with a certain solid content (less than 5%), then adding a certain amount of water droplets to the polyamic acid slurry solution, and finally preparing nanospheres with a particle size between 37.4 nm and 100 nm through imidization. This method uses a low concentration of polyamic acid, resulting in extremely low efficiency in microsphere preparation. Furthermore, the small particle size of the microspheres can easily cause pore blockage in membranes when applied to them.

[0010] In the existing technology, the preparation efficiency of polyimide microspheres is low, and there are problems such as small particle size, incomplete particles, and wide particle size distribution. Therefore, how to efficiently prepare polyimide microspheres with moderate particle size, complete particles, and narrow particle size distribution, and further coat them on the base film to obtain a high-performance composite membrane, is an urgent problem to be solved. Summary of the Invention

[0011] To address the aforementioned technical problems, this invention provides a composite separator, its preparation method, and its applications. By coating a base membrane with uniformly sized, intact polyimide microspheres, not only can the temperature resistance of the separator and battery safety be significantly improved, but the coating thickness of the base membrane can also be reduced, thereby increasing the energy density and cycle life of the lithium battery. Furthermore, the preparation method described in this invention is simple, has a high yield, and is highly efficient, meeting the needs of large-scale industrial production. Simultaneously, the polyimide coating prepared by this invention has high compatibility with the base membrane, improving the wettability of the composite separator in the electrolyte, and thus enhancing the safety of the composite separator.

[0012] To achieve this objective, the present invention adopts the following technical solution:

[0013] In a first aspect, the present invention provides a method for preparing a composite membrane, the method comprising the following steps: first, mixing a diamine monomer and a dianhydride monomer, and polymerizing them to obtain a polyamic acid solution; second, mixing the polyamic acid solution, deionized water, and a surfactant to obtain a water-in-oil emulsion, subjecting the water-in-oil emulsion to heat treatment and imidization treatment sequentially to obtain polyimide microspheres; stirring and mixing a binder and the polyimide microspheres to obtain a polyimide coating slurry, then coating the polyimide coating slurry onto the surface of a polyethylene or polypropylene base membrane, and drying to obtain the composite membrane; wherein the diamine monomer includes carboxylic acid diamine monomers and non-carboxylic acid diamine monomers; the viscosity of the polyamic acid solution is 450 cps-4000 cps; and the particle size D50 of the polyimide microspheres is 190 nm-800 nm.

[0014] This invention coats a base membrane with uniformly sized, intact polyimide microspheres, significantly improving the temperature resistance of the separator and battery safety. It also reduces the coating thickness, thereby increasing the energy density and cycle life of the lithium battery. Furthermore, the preparation method described in this invention is simple, yields high output, and is highly efficient, meeting the needs of large-scale industrial production. Simultaneously, the polyimide coating prepared by this invention exhibits high compatibility with the base membrane, improving the wettability of the composite separator in the electrolyte and thus enhancing its safety.

[0015] Specifically, the viscosity of the polyamic acid solution can be 450 cps, 500 cps, 1000 cps, 2000 cps, 3000 cps, 3500 cps or 4000 cps, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Specifically, the particle size D50 of the polyimide microspheres can be 190nm, 200nm, 300nm, 500nm, 700nm or 800nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0017] This invention further regulates the porosity of the composite membrane by controlling the particle size of the polyimide microspheres, ensuring ion permeability and simultaneously improving the heat resistance of the composite membrane. Within a preferred polyimide microsphere particle size range, the prepared composite membrane exhibits uniform porosity and does not clog the pores of the base membrane; more preferably, it further enhances the heat resistance of the composite membrane.

[0018] Preferably, the carboxylic acid diamine monomer comprises any one or a combination of at least two of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, 3,5-diaminobenzoic acid, or 6,6'-diamino-3,3'-methylenedibenzoic acid. Typical but non-limiting combinations include a combination of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 3,5-diaminobenzoic acid, or a combination of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid, 3,5-diaminobenzoic acid, and 6,6'-diamino-3,3'-methylenedibenzoic acid.

[0019] Preferably, the non-carboxylic acid diamine monomer includes 2,2-bis(3-amino-4-hydroxyphenyl)dimethylpropane, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, 4,4'-diaminobiphenyl-2,2'-diol, 2,2-bis(4-hydroxy-3-aminophenyl)propane, 2,2-bis(4-hydroxy-3-aminophenyl)hexafluoropropane, 3,3'-dihydroxybiphenyldiamine, or 6,6'-bisamino-3, Any one or at least two of 3'-methylenedibenzoic acid, typically but not limitingly, include combinations of 2,2-bis(3-amino-4-hydroxyphenyl)dimethylpropane and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, or combinations of 2,2-bis(4-hydroxy-3-aminophenyl)hexafluoropropane, 3,3'-dihydroxybenzidine and 6,6'-bisamino-3,3'-methylenedibenzoic acid.

[0020] Preferably, the dianhydride monomer comprises pyromellitic dianhydride, biphenyl-3,3',4,4'-tetracarboxylic acid dianhydride, 4,4'-oxobisphthalic anhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 4,4'-(hexafluoroisopropene)diphthalic anhydride, 2,3,3',4'-biphenyltetracarboxylic dianhydride, 2,3,3',4'-diphenyl ether tetracarboxylic dianhydride, and 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) The combination of any one or at least two of 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, or 1,4,5,8-naphthalenetetracarboxylic anhydride, typically but not limitingly, includes the combination of pyromellitic dianhydride and biphenyl-3,3',4,4'-tetracarboxylic dianhydride, or the combination of 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and 1,4,5,8-naphthalenetetracarboxylic anhydride.

[0021] Preferably, the molar ratio of the dianhydride monomer to the diamine monomer is 1:(1-1.5), for example, it can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0022] Preferably, the molar ratio of the carboxylic acid diamine monomer to the non-carboxylic acid diamine monomer is 1:(1-5), for example, it can be 1:1, 1:2, 1:3, 1:4 or 1:5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] Preferably, the holding temperature for the polymerization reaction is 0℃-65℃, for example, it can be 0℃, 20℃, 40℃, 50℃, 60℃ or 65℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0024] Preferably, the holding time for the polymerization reaction is 12h-36h, for example, it can be 12h, 18h, 24h, 30h, 33h or 36h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] Preferably, the solvent of the polyamic acid solution includes N,N-dimethylformamide.

[0026] Preferably, the mass concentration of the polyamic acid solution is 8wt%-18wt%, for example, it can be 8wt%, 10%, 12%, 14%, 16% or 1wt%, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0027] This invention allows for further control of the polyimide microsphere particle size by adjusting the mass concentration of the polyamic acid solution. Within a preferred mass concentration range, the prepared polyimide microspheres exhibit moderate and uniform particle size.

[0028] Preferably, the surfactant comprises any one or a combination of at least two of sulfonates, phosphates, quaternary ammonium salts, ammonium carboxylates, or polyethylene glycol. Typical but non-limiting combinations include combinations of sulfonates and phosphates, or combinations of quaternary ammonium salts, ammonium carboxylates, and polyethylene glycol.

[0029] Preferably, the sulfonate comprises any one or a combination of at least two of alkylbenzene sulfonates, α-olefin sulfonates, alkyl sulfonates, α-sulfonyl monocarboxylic acids, fatty acid sulfonyl esters, succinate sulfonates, alkylnaphthalene sulfonates, petroleum sulfonates, lignin sulfonates, or alkyl glycerol ether sulfonates. Typical but non-limiting combinations include combinations of alkylbenzene sulfonates and α-olefin sulfonates, or combinations of petroleum sulfonates, lignin sulfonates, and alkyl glycerol ether sulfonates.

[0030] Preferably, the phosphate salt includes any one or a combination of at least two of the following: potassium monoalkyl phosphate, sodium monoalkyl phosphate, potassium dialkyl phosphate, or AEO phosphate. Typical but non-limiting combinations include a combination of potassium monoalkyl phosphate and sodium monoalkyl phosphate, or a combination of sodium monoalkyl phosphate, potassium dialkyl phosphate, and AEO phosphate.

[0031] Preferably, the quaternary ammonium salt comprises any one or a combination of at least two of hexadecyltrimethylammonium bromide, octadecyltrimethylammonium chloride, dioctadecyldimethylammonium chloride, didodecyldimethylammonium bromide, hexadecylpyridine chloride, or imidazoline quaternary ammonium salt. Typical but non-limiting combinations include a combination of hexadecyltrimethylammonium bromide and octadecyltrimethylammonium chloride, or a combination of didodecyldimethylammonium bromide, hexadecylpyridine chloride, and imidazoline quaternary ammonium salt.

[0032] Preferably, the ammonium carboxylate salt includes any one or a combination of at least two of the following: cocoyl ammonium salt, lauroyl ammonium salt, cocoyl glutamate ammonium salt, lauroyl sarcosine ammonium salt, polyacrylate ammonium salt, taurine ammonium salt, or siloxane carboxylate ammonium salt. Typical but non-limiting combinations include a combination of cocoyl ammonium salt and lauroyl ammonium salt, or a combination of polyacrylate ammonium salt, taurine ammonium salt, and siloxane carboxylate ammonium salt.

[0033] Preferably, the mass ratio of the polyamic acid solution, deionized water and surfactant is 1:(4-6):(0.1-0.9), for example, it can be 1:4:0.1, 1:5:0.1, 1:6:0.1, 1:4:0.9, 1:5:0.9 or 1:6:0.9, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the heat treatment holding temperature is 20℃-55℃, for example, it can be 20℃, 30℃, 40℃, 50℃ or 55℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0035] This invention further regulates the particle size distribution of polyimide microspheres by adjusting the heat treatment holding temperature. Within the preferred holding temperature range, the prepared polyimide microspheres have a narrow particle size distribution and high microsphere dispersion, which is beneficial for uniform coating in the later stage.

[0036] Preferably, the heat treatment holding time is 1.5h-3h, for example, it can be 1.5h, 1.8h, 2h, 2.2h, 2.5h or 3h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, stirring is also performed during the heat treatment process.

[0038] Preferably, the stirring speed is 1500 r / min to 2500 r / min, for example, it can be 1500 r / min, 1800 r / min, 2000 r / min, 2200 r / min, 2400 r / min or 2500 r / min, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0039] Preferably, the environmental pressure for the imidization treatment is atmospheric pressure.

[0040] Preferably, the ambient atmosphere for the imidization treatment includes a nitrogen atmosphere or an air atmosphere.

[0041] Preferably, the holding temperature for the imidization treatment is 80℃-350℃, for example, it can be 80℃, 100℃, 200℃, 300℃ or 350℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0042] Preferably, the holding time for the imidization treatment is 100 min to 200 min, for example, it can be 100 min, 120 min, 140 min, 160 min, 180 min or 200 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the imidization treatment includes a first imidization treatment, a second imidization treatment, a third imidization treatment, a fourth imidization treatment, and a fifth imidization treatment.

[0044] Preferably, the holding temperature for the first imidization treatment is 80℃-85℃, for example, it can be 80℃, 81℃, 82℃, 83℃, 84℃ or 85℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] Preferably, the holding time for the first imidization treatment is 15 min to 25 min, for example, it can be 15 min, 18 min, 20 min, 22 min, 24 min or 25 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0046] Preferably, the holding temperature for the second imidization treatment is 100℃-105℃, for example, it can be 100℃, 101℃, 102℃, 103℃, 104℃ or 105℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0047] Preferably, the holding time for the second imidization treatment is 15 min to 25 min, for example, it can be 15 min, 18 min, 20 min, 22 min, 24 min or 25 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the holding temperature for the third imidization treatment is 150℃-160℃, for example, it can be 150℃, 152℃, 154℃, 156℃, 158℃ or 160℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0049] Preferably, the holding time for the third imidization treatment is 15 min to 25 min, for example, it can be 15 min, 18 min, 20 min, 22 min, 24 min or 25 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0050] Preferably, the holding temperature for the fourth imidization treatment is 200℃-210℃, for example, it can be 200℃, 202℃, 204℃, 206℃, 208℃ or 210℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0051] Preferably, the holding time for the fourth imidization treatment is 15 min to 25 min, for example, it can be 15 min, 18 min, 20 min, 22 min, 24 min or 25 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0052] Preferably, the holding temperature for the fifth imidization treatment is 350℃-360℃, for example, it can be 350℃, 352℃, 354℃, 356℃, 358℃ or 360℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0053] Preferably, the holding time for the fifth imidization treatment is 15 min to 25 min, for example, it can be 15 min, 18 min, 20 min, 22 min, 24 min or 25 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0054] The first and second mixing methods of this invention are conventional operations. The first mixing method, the first mixing time, the second mixing method, and the second mixing time obtained by those skilled in the art within a reasonable range are all applicable to this invention.

[0055] Preferably, the stirring speed is 800 r / min to 1200 r / min, for example, it can be 800 r / min, 900 r / min, 1000 r / min, 1100 r / min or 1200 r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0056] Preferably, the mixing time is 8 min to 12 min, for example, it can be 8 min, 9 min, 10 min, 11 min or 12 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0057] Preferably, the adhesive comprises carboxymethyl cellulose and / or polyacrylic acid.

[0058] Preferably, the solvent for the polyimide coating slurry includes deionized water.

[0059] Preferably, the mass ratio of the polyimide microspheres to the binder is (90-99):(1-10), for example, it can be 90:1, 90:10, 95:5, 99:1 or 99:10, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] Preferably, the coating thickness is 1μm-3μm, for example, it can be 1μm, 1.5μm, 2μm, 2.5μm or 3μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0061] Preferably, the drying and heat preservation temperature is 100℃-150℃, for example, it can be 100℃, 110℃, 120℃, 130℃, 140℃ or 150℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0062] Preferably, the drying and heat preservation time is 0.5h-1.5h, for example, it can be 0.5h, 0.8h, 1h, 1.2h or 1.5h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0063] As a preferred technical solution of the present invention, the preparation method includes the following steps:

[0064] (1) Preparation of polyamic acid solution: Dissolve dianhydride monomer and diamine monomer in N,N-dimethylformamide at a molar ratio of 1:(1-1.5) and polymerize at 0℃-65℃ for 12h-36h to obtain a polyamic acid solution with a mass concentration of 8wt%-18wt% and a viscosity of 500cps-4000cps.

[0065] (2) Preparation of polyimide microspheres: A water-in-oil emulsion was obtained by mixing a polyamic acid solution, deionized water, and a surfactant at a mass ratio of 1:(4-6):(0.1-0.9). The water-in-oil emulsion was then subjected to heat treatment and imidization treatment to obtain polyimide microspheres with a particle size D50 of 200nm-1200nm. The heat treatment temperature was 20℃-55℃, and the holding time was 1.5h-3h. Stirring was also carried out during the heat treatment process at a stirring speed of 150 rpm. The imidization treatment, ranging from 0 r / min to 2500 r / min, includes a first imidization treatment at 80℃-85℃ for 15 min-25 min, a second imidization treatment at 100℃-105℃ for 15 min-25 min, a third imidization treatment at 150℃-160℃ for 15 min-25 min, a fourth imidization treatment at 200℃-210℃ for 15 min-25 min, and a fifth imidization treatment at 350℃-360℃ for 15 min-25 min.

[0066] (3) Preparation of composite membrane: The binder and polyimide microspheres are dispersed in deionized water at a mass ratio of (90-99):(1-10), and stirred at a speed of 800r / min-1200r / min for 8min-12min to obtain a polyimide coating slurry. The polyimide coating slurry is then coated on the surface of a polyethylene or polypropylene base membrane and dried at 100℃-150℃ for 0.5h-1.5h to obtain the composite membrane; the coating thickness is 1μm-3μm.

[0067] Secondly, the present invention provides a composite membrane, which is prepared by the preparation method described in the first aspect.

[0068] Thirdly, the present invention provides an application of the composite separator as described in the second aspect, wherein the composite separator is used in a lithium-ion battery.

[0069] Compared with the prior art, the present invention has at least the following beneficial effects:

[0070] (1) By coating the surface of the base film with polyimide microspheres of uniform size and intact particles, this invention can not only significantly improve the temperature resistance of the separator and the safety of the battery, but also reduce the coating thickness of the base film, thereby improving the energy density and cycle life of the lithium battery.

[0071] (2) The preparation method described in this invention has a simple process, high yield, and high preparation efficiency, which can meet the needs of large-scale industrial production.

[0072] (3) The polyimide coating prepared by the present invention has high compatibility with the base film, which can improve the wettability of the composite membrane in the electrolyte and thus improve the safety of the composite membrane. Attached Figure Description

[0073] Figure 1 This is a SEM image of the polyimide microspheres described in Example 1 of this invention;

[0074] Figure 2 This is a SEM image of the polyimide microspheres described in Example 2 of this invention;

[0075] Figure 3 This is a SEM image of the polyimide microspheres described in Example 3 of this invention;

[0076] Figure 4 This is a SEM image of the polyimide microspheres described in Comparative Example 1 of the present invention;

[0077] Figure 5 This is a SEM image of the polyimide microspheres described in Comparative Example 2 of this invention. Detailed Implementation

[0078] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0079] Example 1

[0080] This embodiment provides a method for preparing a composite membrane, the method comprising the following steps:

[0081] (1) Preparation of polyamic acid solution: 10 moles of pyromellitic dianhydride, 8 moles of 4,4'-diaminodiphenyl ether and 2 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine were dissolved in N,N-dimethylformamide and polymerized at 35°C for 12 h to obtain a polyamic acid solution with a mass concentration of 14.5 wt% and a viscosity of 1822 cps.

[0082] (2) Preparation of polyimide microspheres: The polyamic acid solution, deionized water and sodium dodecyl sulfate were mixed at a mass ratio of 1:5:0.15 to obtain a water-in-oil emulsion. The water-in-oil emulsion was subjected to heat treatment and imidization treatment in sequence to obtain polyimide microspheres with a particle size D50 of 252 nm. The heat treatment temperature was 45 °C and the heat treatment time was 2 h. The heat treatment was also stirred at a speed of 2000 r / min. The imidization treatment included a first imidization treatment at 80 °C for 20 min, a second imidization treatment at 100 °C for 20 min, a third imidization treatment at 150 °C for 20 min, a fourth imidization treatment at 200 °C for 20 min, and a fifth imidization treatment at 350 °C for 20 min.

[0083] (3) Preparation of composite membrane: Sodium carboxymethyl cellulose and the polyimide microspheres were dissolved in ethanol at a mass ratio of 95:5 and stirred at 1000 r / min for 10 min to obtain a polyimide coating slurry. The polyimide coating slurry was then coated on the surface of a polyethylene base film with a thickness of 5 μm and dried at 120 °C for 1 h to obtain the composite membrane; the coating thickness was 2 μm.

[0084] Figure 1 This is a SEM image of the polyimide microspheres described in Example 1 of this invention. Figure 1 It can be seen that the polyimide microspheres have a uniform particle size and a narrow particle size distribution.

[0085] Example 2

[0086] This embodiment provides a method for preparing a composite membrane, the method comprising the following steps:

[0087] (1) Preparation of polyamic acid solution: 10 moles of 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 10 moles of 4,4'-diaminobiphenyl-2,2'-diphenol, and 2 moles of 4,4'-diaminodiphenyl ether were dissolved in N,N-dimethylformamide and polymerized at 0°C for 36 h to obtain a polyamic acid solution with a mass concentration of 8 wt% and a viscosity of 456 cps.

[0088] (2) Preparation of polyimide microspheres: The polyamic acid solution, deionized water and fatty alcohol polyoxyethylene ether were mixed at a mass ratio of 1:4:0.1 to obtain a water-in-oil emulsion. The water-in-oil emulsion was subjected to heat treatment and imidization treatment in sequence to obtain polyimide microspheres with a particle size D50 of 196 nm. The heat treatment temperature was 20 °C and the heat treatment time was 3 h. The heat treatment was also carried out by stirring at a speed of 2500 r / min. The imidization treatment included a first imidization treatment at 85 °C for 15 min, a second imidization treatment at 105 °C for 15 min, a third imidization treatment at 160 °C for 15 min, a fourth imidization treatment at 210 °C for 15 min, and a fifth imidization treatment at 360 °C for 15 min.

[0089] (3) Preparation of composite membrane: Sodium carboxymethyl cellulose and polyimide microspheres were dissolved in ethanol at a mass ratio of 90:10 and stirred at 800 r / min for 12 min to obtain polyimide coating slurry. The polyimide coating slurry was then coated on the surface of a polyethylene base membrane with a thickness of 4 μm and dried at 100 °C for 1.5 h to obtain the composite membrane; the coating thickness was 3 μm.

[0090] Figure 2 This is a SEM image of the polyimide microspheres described in Example 2 of this invention. Figure 2It can be seen that the polyimide microspheres have a uniform particle size and a narrow particle size distribution.

[0091] Example 3

[0092] This embodiment provides a method for preparing a composite membrane, the method comprising the following steps:

[0093] (1) Preparation of polyamic acid solution: 10 moles of biphenyl-3,3',4,4'-tetracarboxylic acid dianhydride, 7.5 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid and 7.5 moles of 2,2-bis(3-amino-4-hydroxyphenyl)dimethylpropane were dissolved in N,N-dimethylformamide and polymerized at 65°C for 12 h to obtain a polyamic acid solution with a mass concentration of 18 wt% and a viscosity of 3935 cps;

[0094] (2) Preparation of polyimide microspheres: The polyamic acid solution, deionized water and polyol polyoxyethylene ether fatty acid ester were mixed at a mass ratio of 1:6:0.9 to obtain a water-in-oil emulsion. The water-in-oil emulsion was subjected to heat treatment and imidization treatment in sequence to obtain polyimide microspheres with a particle size D50 of 785 nm. The heat treatment temperature was 55 °C and the heat treatment time was 1.5 h. The heat treatment was also carried out by stirring at a speed of 1500 r / min. The imidization treatment included a first imidization treatment at 80 °C for 25 min, a second imidization treatment at 100 °C for 25 min, a third imidization treatment at 150 °C for 25 min, a fourth imidization treatment at 200 °C for 25 min, and a fifth imidization treatment at 350 °C for 25 min.

[0095] (3) Preparation of composite membrane: The binder and polyimide microspheres were dissolved in ethanol at a mass ratio of 99:1 and stirred at 1200 r / min for 8 min to obtain a polyimide coating slurry. The polyimide coating slurry was then coated on the surface of a polyethylene base film with a thickness of 6 μm and dried at 150 °C for 0.5 h to obtain the composite membrane; the coating thickness was 1 μm.

[0096] Figure 3 This is a SEM image of the polyimide microspheres described in Example 3 of this invention. Figure 3 It can be seen that the polyimide microspheres have a uniform particle size and a narrow particle size distribution.

[0097] Example 4

[0098] The only difference between this embodiment and Example 1 is that, except that the viscosity of the polyamic acid solution is 453 cps, resulting in polyimide microspheres with a particle size D50 of 183 nm, everything else is the same as in Example 1.

[0099] Example 5

[0100] The only difference between this embodiment and Example 1 is that, except that the viscosity of the polyamic acid solution is 4177 cps, resulting in polyimide microspheres with a particle size D50 of 845 nm, everything else is the same as in Example 1.

[0101] Example 6

[0102] The only difference between this embodiment and Example 1 is that, except that in step (1), 8 moles of 4,4'-diaminodiphenyl ether and 2 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine are replaced with 4 moles of 4,4'-diaminodiphenyl ether and 1 mole of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine, i.e., the molar ratio of dianhydride monomer to diamine monomer is 1:0.5, everything else is the same as in Example 1.

[0103] Example 7

[0104] The only difference between this embodiment and Example 1 is that, except that in step (1), 8 moles of 4,4'-diaminodiphenyl ether and 2 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine are replaced with 16 moles of 4,4'-diaminodiphenyl ether and 4 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine, i.e., the molar ratio of dianhydride monomer to diamine monomer is 1:2, everything else is the same as in Example 1.

[0105] Example 8

[0106] The only difference between this embodiment and embodiment 1 is that, except that the heat treatment temperature in step (2) is 15°C, everything else is the same as in embodiment 1.

[0107] Example 9

[0108] The only difference between this embodiment and embodiment 1 is that, except that the heat treatment temperature in step (2) is 60°C, everything else is the same as in embodiment 1.

[0109] Comparative Example 1

[0110] The only difference between Comparative Example 1 and Example 1 is that, except that the viscosity of the polyamic acid solution is 8755 cps, resulting in polyimide microspheres with a particle size D50 of 1.5 μm, everything else is the same as in Example 1.

[0111] Figure 4 This is a SEM image of the polyimide microspheres described in Comparative Example 1 of this invention. Figure 4 It can be seen that the polyimide microspheres have a large particle size, and the integrity of some particles is reduced.

[0112] Comparative Example 2

[0113] The only difference between Comparative Example 1 and Example 1 is that, except that the viscosity of the polyamic acid solution is 9115 cps, resulting in polyimide microspheres with a particle size D50 of 6 μm, everything else is the same as in Example 1.

[0114] Figure 5 This is a SEM image of the polyimide microspheres described in Comparative Example 2 of this invention. Figure 5 It can be seen that the polyimide microspheres have a large particle size, and the integrity of some particles is reduced.

[0115] Comparative Example 3

[0116] The only difference between this comparative example and Example 1 is that, except that 8 moles of 4,4'-diaminodiphenyl ether and 2 moles of 4,4'-diaminobiphenyl-2,2'-dicarboxylic acid diamine in step (1) are replaced with 10 moles of 4,4'-diaminodiphenyl ether, everything else is the same as in Example 1.

[0117] Test methods

[0118] The composite membranes described in Examples 1-9 and Comparative Examples 1-3 were used, and the data on areal density, air permeability, needle penetration strength, shrinkage rate, and teardrop angle were recorded in Table 1.

[0119] Table 1

[0120]

[0121] The test results show that:

[0122] (1) As can be seen from Examples 1-9 and Comparative Examples 1-3, the present invention, by coating the surface of the base membrane with uniformly sized and intact polyimide microspheres, can not only significantly improve the temperature resistance of the separator and the safety of the battery, but also reduce the coating thickness of the base membrane, thereby improving the energy density and cycle life of the lithium battery. Furthermore, the preparation method described in the present invention has a simple process, high yield, and high preparation efficiency, which can meet the needs of large-scale industrial production. At the same time, the polyimide coating prepared by the present invention has high compatibility with the base membrane, which can improve the wettability of the composite separator in the electrolyte, thereby improving the safety of the composite separator.

[0123] (2) As can be seen from Examples 1, 4-5 and Comparative Examples 1-2, the present invention can further control the particle size of polyimide microspheres by further controlling the viscosity of the polyamic acid solution, thereby improving the wettability and air permeability of the composite membrane, reducing the shrinkage rate of the composite membrane, and thus improving the electrochemical performance and safety performance of the membrane.

[0124] (3) As can be seen from Examples 1 and 6-7, by further adjusting the molar ratio of dianhydride monomer and diamine monomer, the present invention can further improve the wettability and air permeability of the composite membrane, reduce the shrinkage rate of the composite membrane, and thus improve the electrochemical performance and safety performance of the membrane.

[0125] (4) As can be seen from Examples 1 and 8-9, by further controlling the heat treatment temperature, the present invention can further improve the wettability and air permeability of the composite membrane, reduce the shrinkage rate of the composite membrane, and thus improve the electrochemical performance and safety performance of the membrane.

[0126] (5) As can be seen from Example 1 and Comparative Example 3, without the use of carboxylic acid diamine monomer, the diamine monomer and dianhydride monomer cannot be effectively dispersed when the polyamic acid solution is prepared during the polymerization reaction and further polymerized by emulsion, so polyimide microspheres cannot be obtained.

[0127] In summary, this invention, by coating the surface of a base membrane with uniformly sized, intact polyimide microspheres, not only significantly improves the temperature resistance of the separator and battery safety but also reduces the coating thickness of the base membrane, thereby increasing the energy density and cycle life of the lithium battery. Furthermore, the preparation method described in this invention is simple, has a high yield, and is highly efficient, meeting the needs of large-scale industrial production. Simultaneously, the polyimide coating prepared by this invention exhibits high compatibility with the base membrane, improving the wettability of the composite separator in the electrolyte and thus enhancing its safety.

[0128] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a composite diaphragm, characterized in that, The preparation method includes the following steps: The first mixture of diamine monomer and dianhydride monomer is polymerized to obtain a polyamic acid solution; the second mixture of polyamic acid solution, deionized water, and surfactant is used to obtain a water-in-oil emulsion, which is then subjected to heat treatment and imidization treatment to obtain polyimide microspheres; the binder and the polyimide microspheres are stirred and mixed to obtain a polyimide coating slurry, which is then coated on the surface of a polyethylene or polypropylene base film and dried to obtain the composite membrane; The diamine monomer includes carboxylic acid diamine monomers and non-carboxylic acid diamine monomers; The viscosity of the polyamic acid solution is 450 cps-4000 cps; The particle size D50 of the polyimide microspheres is 190nm-800nm; The heat treatment is performed at a temperature of 20℃-45℃ for 1.5h-3h. The molar ratio of the dianhydride monomer to the diamine monomer is 1:(1-1.5); The molar ratio of the carboxylic acid diamine monomer to the non-carboxylic acid diamine monomer is 1:(1-5).

2. The preparation method according to claim 1, characterized in that, The mass concentration of the polyamic acid solution is 8wt%-18wt%.

3. The preparation method according to claim 1, characterized in that, The mass ratio of the polyamic acid solution, deionized water, and surfactant is 1:(4-6):(0.1-0.9).

4. The preparation method according to claim 1, characterized in that, The iminoization treatment includes a first iminoization treatment, a second iminoization treatment, a third iminoization treatment, a fourth iminoization treatment, and a fifth iminoization treatment; The holding temperature for the first imidization treatment is 80℃-85℃, and the holding time is 15min-25min; The holding temperature for the first imidization treatment is 100℃-105℃, and the holding time is 15min-25min; The holding temperature for the third imidization treatment is 150℃-160℃, and the holding time is 15min-25min; The holding temperature for the fourth imidization treatment is 200℃-210℃, and the holding time is 15min-25min; The fifth imidization treatment is held at a temperature of 350℃-360℃ for 15min-25min.

5. The preparation method according to claim 1, characterized in that, The mass ratio of the polyimide microspheres to the binder is (90-99):(1-10).

6. The preparation method according to any one of claims 1-5, characterized in that, The drying process is carried out at a temperature of 100℃-150℃ for 0.5h-1.5h.

7. A composite diaphragm, characterized in that, The composite diaphragm is prepared using the preparation method described in any one of claims 1-6.

8. An application of the composite diaphragm as described in claim 7, characterized in that, The composite separator is used in lithium-ion batteries.

Images