Polyimide lithium battery diaphragm and preparation method thereof

Abstract

The invention provides a polyimide lithium battery diaphragm and a preparation method thereof, and relates to the technical field of lithium battery diaphragms.The diaphragm is provided with spherical or ellipsoidal holes formed by cracking of thermal expansion microspheres; the thermal expansion microspheres and a dispersing agent are dispersed in a polyamide acid solution, then the mixed slurry is prepared into a wet film, polyamic acid gelation and expansion kettle breaking of the thermal expansion microspheres are synchronously completed in cooperation with multi-stage heating heat treatment, the average pore size and porosity of pores of the diaphragm meeting the requirements can be obtained, and the process is simple and easy to implement. The method is suitable for large-scale production and preparation; the average pore size and the porosity of the pores of the diaphragm are highly controllable in process.

Description

Technical Field

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

[0002] Lithium-ion batteries have become the mainstream choice for energy storage and power sources due to their advantages such as high energy density and long cycle life. As one of the key internal components of the battery, the separator's main function is to isolate the positive and negative electrodes to prevent short circuits, while allowing lithium ions to pass through freely. Its performance directly determines the battery's interface stability, rate performance, and safety margin.

[0003] Currently, commercially available separators are mainly polyolefin (such as polyethylene PE and polypropylene PP) microporous membranes, which have good mechanical properties and electrochemical stability, but poor heat resistance (melting point is usually below 170℃) and high thermal shrinkage rate. During battery thermal abuse, polyolefin separators are prone to shrinkage and melting at high temperatures, leading to large-area contact between positive and negative electrodes, causing internal short circuits, thermal runaway, and even fire and explosion, which is a bottleneck for the safety of high-energy-density batteries. At present, the field of high-energy-density lithium batteries is seeking a material that can withstand high temperatures and has good mechanical properties and electrochemical stability to replace polyolefin microporous membranes.

[0004] Polyimide (PI) is considered an ideal material for next-generation high-safety membranes due to its excellent heat resistance (decomposition temperature > 500℃), good chemical stability, and designable molecular structure. Currently, the mainstream methods for preparing porous PI membranes include electrospinning and phase inversion methods. Electrospinning can prepare PI nanofiber membranes with high specific surface area, but the membranes obtained by this method usually have the inherent contradiction of "high porosity and low mechanical strength". The physical overlap between the fibers results in insufficient puncture and tensile strength of the membrane, making it susceptible to dendrite puncture or damage during battery assembly or cycling. Furthermore, the excessively high porosity (typically >80%) and wide pore size distribution exacerbate the self-discharge phenomenon of the battery. The phase inversion method can prepare dense, high-strength PI porous membranes. Although this method can achieve mechanical properties superior to electrospun membranes, the resulting pore structure is "finger-like," with a wide pore size distribution and poor structural uniformity. Larger finger-like channels may become rapid pathways for dendrite growth. Moreover, this process involves complex coagulation bath and extraction steps, resulting in a long process, high solvent consumption, and the pore structure is affected by multiple thermodynamic and kinetic factors, making precise control difficult.

[0005] For example, Chinese invention patent CN115636976A discloses a method for improving membrane performance by molecular grafting modification and surface silica coating of polyimide. Although this method improves the overall performance of the membrane to some extent, it is essentially still a chemical modification and post-processing of the PI material itself, which is complex and fails to fundamentally solve the problem of insufficient controllability of the porous structure.

[0006] Currently, lithium-ion batteries, especially power batteries with high energy density and high safety requirements, urgently need a separator and preparation method that meets the following requirements: 1. High thermal and chemical stability; 2. Balanced and excellent mechanical properties (especially puncture resistance) and ion transport performance; 3. Uniform pore size and controllable porous structure; 4. Preparation process that can be mass-produced. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a polyimide lithium battery separator and its preparation method, solving the problems of wide pore size distribution, poor structural uniformity, insufficient mechanical properties, and uncontrollable production process of existing polyimide lithium battery separators.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A polyimide lithium battery separator has spherical or ellipsoidal pores formed by the rupture of thermally expanded microspheres, the average pore diameter of the pores being 5μm-30μm and the porosity being 35%-60%, and the ionic conductivity of the separator in the electrolyte being not less than 1.0 mS / cm.

[0010] The membrane is based on spherical or ellipsoidal pores, which allow electrolyte to flow into the interior, increasing ionic conductivity. Therefore, larger average pore diameters can be designed, such as 5μm-30μm, 15μm, 20μm, 28μm, etc., and smaller porosities can be designed, such as 35%-60%, 45%, 55%, 58%, etc., to achieve an ionic conductivity of not less than 1.0 mS / cm in the electrolyte.

[0011] Understandably, polyimide lithium battery separators have high thermal and chemical stability, and can withstand high temperatures up to 400℃. The separator design with larger average pore size and smaller porosity enables it to have balanced and excellent mechanical properties (especially puncture resistance). The internal spherical or ellipsoidal pores can allow electrolyte to flow in / store, increasing ionic conductivity.

[0012] Preferably, the spherical or ellipsoidal pores inside the diaphragm are interconnected, forming a three-dimensional network of channels.

[0013] Understandably, a three-dimensional interconnected pore network can further improve the internal connectivity of the diaphragm, increase ionic conductivity, and have little impact on the mechanical properties of the diaphragm.

[0014] Another object of the present invention is to provide a method for preparing a polyimide lithium battery separator, comprising the following steps:

[0015] S1. Disperse the thermally expanded microspheres and dispersant in a polyamic acid solution to obtain a mixed slurry;

[0016] The thermally expandable microspheres are Expancel® 091 DU 40 microspheres, with a thermal expansion temperature of approximately 80°C and a rupture temperature of approximately 130°C.

[0017] S2. The mixed slurry is made into a wet film, for example by coating, casting, printing or other processes.

[0018] One possible approach is to pour the mixed slurry onto a polyethylene terephthalate (PET) release film or a clean glass plate placed on a horizontal coating machine, and then coat it at a constant speed with a doctor blade or an automatic coater to control the wet film thickness to be between 100 μm and 400 μm (this thickness range can be followed by drying and heat treatment to obtain a final membrane of about 10-50 μm thickness).

[0019] S3. Perform a multi-stage heating heat treatment on the wet film, the multi-stage heating heat treatment including:

[0020] First stage: Heat the wet film to 80℃-130℃ and maintain it, so that the thermally expanding microspheres expand but remain intact, while the polyamic acid partially gels.

[0021] Second stage: Continue heating to 130℃-200℃ and maintain it to rupture the expanded thermally expanded microspheres and further gel the polyamic acid to obtain a polyimide membrane formed by incomplete imidization.

[0022] Third stage: Continue heating to 250℃-400℃ and maintain it to completely imidize the polyamic acid, thus obtaining a fully imidized polyimide membrane.

[0023] In step S3, after the thermally expanded microspheres expand and rupture in the second stage, they form spherical or ellipsoidal pores inside the polyimide membrane. The pore size of these spherical or ellipsoidal pores is related to the size of the thermally expanded microspheres and the expansion ratio. For example, the average diameter of the thermally expanded microspheres Expancel® 091 DU 40 is 10-16 μm. After expanding 3-4 times, they rupture (this diameter and expansion ratio range, after subsequent drying and heat treatment, can yield membrane pore sizes of approximately 5 μm-30 μm). The porosity of these spherical or ellipsoidal pores is related to the amount of thermally expanded microspheres added.

[0024] Understandably, multi-stage heating heat treatment can fully utilize the thermal gelation process of the wet film and the expansion and rupture process of the thermally expanded microspheres. For example, in the first stage, the wet film is heated to 80℃-130℃ and held, specifically to around 120℃ (below the rupture temperature of the thermally expanded microspheres, which is around 130℃). At this temperature range, the thermally expanded microspheres begin to expand, and the polyamic acid begins to partially gel. The gelation rate of polyamic acid at this temperature is relatively slow and can be maintained for 13 minutes. The initially gelled polyamic acid membrane has basically taken shape and has a certain mechanical strength. The temperature is then heated to 250℃-40℃. The temperature is maintained at 0℃, for example, heated to 300℃ (approximately 130℃ higher than the temperature at which the thermally expanded microspheres rupture). During further heating, the polyamic acid further gels, and the thermally expanded microspheres begin to gradually expand and rupture. At this point, the expansion and rupture of the thermally expanded microspheres does not damage the partially gelled polyamic acid membrane, and can form spherical or ellipsoidal pores with fixed shapes on the polyamic acid membrane. Heating is continued to 250℃-400℃ and maintained to completely imidize the polyamic acid, resulting in a fully imidized polyimide membrane; the average pore size and porosity of the pores can be obtained that meet the requirements.

[0025] Preferably, the process further includes: stretching the incompletely imidized polyimide membrane obtained in the second stage of step S3 under nitrogen protection, with a stretching ratio of 1.1-5.0:1 and a stretching ambient temperature of 250℃-400℃, before proceeding to the third stage of step S3.

[0026] It is understandable that nitrogen protection is applied to the incompletely imidized polyimide membrane obtained in the second stage of step S3 to avoid oxidation due to the presence of incompletely imidized material. Furthermore, stretching in this state allows similar spherical or ellipsoidal pores to connect within the incompletely imidized polyimide membrane, forming a three-dimensional interconnected pore network structure, since the mechanical properties of the incompletely imidized polyimide membrane are not fully finalized.

[0027] The stretching environment temperature is 250℃-400℃, and the polyamic acid is further gelled simultaneously during this process.

[0028] Another preferred embodiment further includes: stretching the polyimide diaphragm formed by complete imidization obtained in the third stage of step S3, with a stretching ratio of 3.0-7.0:1.

[0029] The stretching process can be uniaxial stretching, biaxial stretching or multiaxial stretching.

[0030] The stretching treatment of the fully imidized polyimide diaphragm is also to enable similar spherical or ellipsoidal pores to connect inside the fully imidized polyimide diaphragm, forming a three-dimensional interconnected pore network structure.

[0031] Preferably, the polyamic acid solution is obtained by reacting a diamine monomer and a dianhydride monomer in a polar aprotic solvent.

[0032] Preferably, the diamine monomer is one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl.

[0033] Preferably, the dianhydride monomer is one or more of pyromellitic dianhydride, biphenyl dianhydride, and 4,4'-oxobisphthalic anhydride.

[0034] A preferred embodiment is as follows: a polyamic acid solution is obtained by reacting the diamine monomer 4,4'-diaminodiphenyl ether, benzoic acid dianhydride, and 4,4'-oxobisphthalic acid anhydride in a polar aprotic solvent.

[0035] Preferably, the dispersant is a polymer block copolymer dispersant or a silane coupling agent;

[0036] The mass of the dispersant is 0.1%-5% of the mass of the thermally expanded microspheres.

[0037] Understandably, dispersants are used to ensure that thermally expanded microspheres are fully and uniformly dispersed in polyamic acid solutions.

[0038] Preferably, in multi-stage heating heat treatment,

[0039] First stage: Increase the temperature to 90℃ at a rate of 2℃ / min and maintain it for 10-20 minutes;

[0040] Second stage: Increase the temperature to 150℃ at a rate of 5℃ / min and hold for 2-5 minutes;

[0041] Third stage: Increase the temperature to 300℃ at 3℃ / min and maintain it for 60 minutes.

[0042] If the incompletely imidized polyimide membrane obtained in the second stage of step S3 is subjected to stretching treatment under nitrogen protection, the temperature holding time in the third stage can be appropriately shortened.

[0043] This invention provides a polyimide lithium battery separator and its preparation method. It has the following beneficial effects:

[0044] 1. This invention designs spherical or ellipsoidal pores on a polyimide membrane with high thermal and chemical stability. These spherical or ellipsoidal pores allow electrolyte to flow into them, increasing ionic conductivity. The membrane can be designed with a large average pore diameter and a small porosity, enabling it to have balanced and excellent mechanical properties (especially puncture resistance). The spherical or ellipsoidal pores inside can flow into / store electrolyte, increasing ionic conductivity.

[0045] 2. In this invention, thermally expandable microspheres and a dispersant are dispersed in a polyamic acid solution. The mixed slurry is then formed into a wet film, and a multi-stage heating heat treatment is used to simultaneously complete the gelation of the polyamic acid and the rupture of the expansion vessel of the thermally expandable microspheres. This method can obtain a membrane with the required average pore size and porosity. Furthermore, the process is simple and suitable for large-scale production and preparation. The average pore size and porosity of the membrane are highly controllable in the process. Detailed Implementation

[0046] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1:

[0048] This invention provides a polyimide lithium battery separator, which has spherical or ellipsoidal pores formed by the rupture of thermally expanded microspheres. The average pore diameter of the pores is 5μm-30μm, the porosity is 35%-60%, and the ionic conductivity of the separator in the electrolyte is not less than 1.0 mS / cm.

[0049] Lithium-ion battery separators made of polyimide have high thermal and chemical stability, and can withstand high temperatures up to 400℃. The separator design features large average pore size and low porosity, which enables it to have balanced and excellent mechanical properties (especially puncture resistance). The internal spherical or ellipsoidal pores can allow electrolyte to flow in / store, increasing ionic conductivity.

[0050] In one embodiment, the spherical or ellipsoidal pores inside the membrane are interconnected, forming a three-dimensional interconnected pore network. This three-dimensional interconnected pore network can further improve the connectivity inside the membrane, increase ionic conductivity, and has minimal impact on the mechanical properties of the membrane. For example, the membrane can be stretched to connect adjacent spherical or ellipsoidal pores inside the membrane, forming a three-dimensional interconnected pore network structure.

[0051] Example 2:

[0052] 1. Raw materials and formula

[0053] Polyamic acid (PAA) solution: prepared by reacting 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers in N-methylpyrrolidone (NMP) solvent under nitrogen protection, with a solid content of 15% (mass fraction) and a viscosity of 4500 cP at 25°C.

[0054] Thermally expandable microspheres: Expancel® 091 DU 40 (Noron), unexpanded average particle size 10-16 μm, initial expansion temperature approximately 80°C, shell rupture temperature approximately 130°C, volume expansion ratio approximately 3.5 times.

[0055] Dispersant: High molecular weight block copolymer dispersant BYK-180.

[0056] 2. Preparation steps

[0057] S1. Preparation of mixed slurry

[0058] Weigh 100g of PAA solution and add 20% of the PAA solid mass of thermally expanded microspheres and 2% of the microsphere mass of dispersant BYK-180. Place the mixture in a planetary mixer and stir at 300 rpm for 2 hours. Then degas in a vacuum degassing machine at -0.08MPa for 30 minutes to obtain a uniform and stable slurry.

[0059] S2. Wet film forming

[0060] The mixed slurry is transferred to a precision doctor blade coater. Using polyethylene terephthalate (PET) release film as the substrate, the doctor blade gap is set to 200 μm and the coating speed is 5 mm / s. A wet film is then cast onto the substrate.

[0061] The wet film, along with the substrate, is then placed in an 80°C hot air circulating oven for 10 minutes to pre-dry, allowing most of the solvent to evaporate and forming a pre-formed film with a certain degree of self-support.

[0062] S3. Multi-stage heating heat treatment

[0063] Transfer the substrate with the pre-formed film to a programmable temperature stretching oven and process it according to the following procedure:

[0064] First stage (gelation and microsphere pre-expansion): The temperature is increased from room temperature to 90°C at a rate of 2°C / min and held at this temperature for 15 minutes. At this point, PAA partially gels, and the microspheres expand due to heat, but the outer shell remains intact.

[0065] The second stage (microsphere rupture and pore formation): The temperature is increased to 150°C at a rate of 5°C / min and held for 3 minutes. During this stage, the microsphere shell ruptures, gas escapes, and pores are left in the gel network.

[0066] The third stage (imidization structure fixation): The temperature is increased to 280°C at a rate of 3°C / min. When the film temperature reaches 280°C, it is held at this temperature for about 60 minutes to completely imidize the polyamic acid into polyimide and permanently fix the porous structure.

[0067] The membrane was allowed to cool naturally to room temperature and then peeled off from the substrate to obtain a self-supporting porous polyimide membrane, denoted as PI-20.

[0068] Example 3:

[0069] 1. Raw materials and formula

[0070] Polyamic acid (PAA) solution: prepared by reacting 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers in N-methylpyrrolidone (NMP) solvent under nitrogen protection, with a solid content of 15% (mass fraction) and a viscosity of 4500 cP at 25°C.

[0071] Thermally expandable microspheres: Expancel® 091 DU 40 (Noron), unexpanded average particle size 10-16 μm, initial expansion temperature approximately 80°C, shell rupture temperature approximately 130°C, volume expansion ratio approximately 3.5 times.

[0072] Dispersant: High molecular weight block copolymer dispersant BYK-180.

[0073] 2. Preparation steps

[0074] S1. Preparation of mixed slurry

[0075] Weigh 100g of PAA solution and add 20% of the PAA solid mass of thermally expanded microspheres and 2% of the microsphere mass of dispersant BYK-180. Place the mixture in a planetary mixer and stir at 300 rpm for 2 hours. Then degas in a vacuum degassing machine at -0.08MPa for 30 minutes to obtain a uniform and stable slurry.

[0076] S2. Wet film forming

[0077] The mixed slurry is transferred to a precision doctor blade coater. Using polyethylene terephthalate (PET) release film as the substrate, the doctor blade gap is set to 200 μm and the coating speed is 5 mm / s. A wet film is then cast onto the substrate.

[0078] The wet film, along with the substrate, is then placed in an 80°C hot air circulating oven for 10 minutes to pre-dry, allowing most of the solvent to evaporate and forming a pre-formed film with a certain degree of self-support.

[0079] S3. Multi-stage heating heat treatment and stretching

[0080] Transfer the substrate with the pre-formed film to a programmable temperature stretching oven and process it according to the following procedure:

[0081] First stage (gelation and microsphere pre-expansion): The temperature is increased from room temperature to 90°C at a rate of 2°C / min and held at this temperature for 15 minutes. At this point, PAA partially gels, and the microspheres expand due to heat, but the outer shell remains intact.

[0082] The second stage (microsphere rupture and pore formation): The temperature is increased to 150°C at a rate of 5°C / min and held for 3 minutes. During this stage, the microsphere shell ruptures, gas escapes, and pores are left in the gel network.

[0083] The third stage (imidization, structure fixation, and stretching): The temperature is increased to 280°C at a rate of 3°C / min. When the film temperature reaches 280°C, the synchronous biaxial stretching device (nitrogen protection) is activated, and stretching is performed simultaneously in both the X and Y directions at a rate of 10% / min until the stretching ratio in both directions reaches 2.0:1. Maintaining the stretching state, the temperature is further increased to 300°C and held at this temperature for approximately 60 minutes to completely imidize the polyamic acid into polyimide and permanently fix the porous structure and stretching orientation.

[0084] After heat treatment, the membrane is allowed to cool naturally to room temperature under tension, and then peeled off from the substrate to obtain a self-supporting porous polyimide membrane, denoted as PI-30.

[0085] Example 4:

[0086] 1. Raw materials and formula

[0087] Polyamic acid (PAA) solution: prepared by reacting 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers in N-methylpyrrolidone (NMP) solvent under nitrogen protection, with a solid content of 15% (mass fraction) and a viscosity of 4500 cP at 25°C.

[0088] Thermally expandable microspheres: Expancel® 091 DU 40 (Noron), unexpanded average particle size 10-16 μm, initial expansion temperature approximately 80°C, shell rupture temperature approximately 130°C, volume expansion ratio approximately 3.5 times.

[0089] Dispersant: High molecular weight block copolymer dispersant BYK-180.

[0090] 2. Preparation steps

[0091] S1. Preparation of mixed slurry

[0092] Weigh 100g of PAA solution and add 20% of the PAA solid mass of thermally expanded microspheres and 2% of the microsphere mass of dispersant BYK-180. Place the mixture in a planetary mixer and stir at 300 rpm for 2 hours. Then degas in a vacuum degassing machine at -0.08MPa for 30 minutes to obtain a uniform and stable slurry.

[0093] S2. Wet film forming

[0094] The mixed slurry is transferred to a precision doctor blade coater. Using polyethylene terephthalate (PET) release film as the substrate, the doctor blade gap is set to 200 μm and the coating speed is 5 mm / s. A wet film is then cast onto the substrate.

[0095] The wet film, along with the substrate, is then placed in an 80°C hot air circulating oven for 10 minutes to pre-dry, allowing most of the solvent to evaporate and forming a pre-formed film with a certain degree of self-support.

[0096] S3. Multi-stage heating heat treatment and stretching

[0097] Transfer the substrate with the pre-formed film to a programmable temperature stretching oven and process it according to the following procedure:

[0098] First stage (gelation and microsphere pre-expansion): The temperature is increased from room temperature to 90°C at a rate of 2°C / min and held at this temperature for 15 minutes. At this point, PAA partially gels, and the microspheres expand due to heat, but the outer shell remains intact.

[0099] The second stage (microsphere rupture and pore formation): The temperature is increased to 150°C at a rate of 5°C / min and held for 3 minutes. During this stage, the microsphere shell ruptures, gas escapes, and pores are left in the gel network.

[0100] The third stage (imidization and structural fixation): The temperature is increased to 280°C at a rate of 3°C / min, and held at this temperature for about 60 minutes to completely imidize the polyamic acid into polyimide.

[0101] The membrane is transferred to a synchronous biaxial stretching device, which is then activated to stretch the membrane simultaneously in both the X and Y directions at a rate of 10% / min until the stretch ratio in both directions reaches 2.0:1. The membrane is then allowed to cool naturally to room temperature while under stretching conditions and is then peeled off from the substrate to obtain a self-supporting porous polyimide membrane, denoted as PI-40.

[0102] Comparative Example 1:

[0103] 1. Raw materials and formula

[0104] Polyamic acid (PAA) solution: prepared by reacting 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers in N-methylpyrrolidone (NMP) solvent under nitrogen protection, with a solid content of 15% (mass fraction) and a viscosity of 4500 cP at 25°C.

[0105] Thermally expandable microspheres: Expancel® 091 DU 40 (Noron), unexpanded average particle size 10-16 μm, initial expansion temperature approximately 80°C, shell rupture temperature approximately 130°C, volume expansion ratio approximately 3.5 times.

[0106] Dispersant: High molecular weight block copolymer dispersant BYK-180.

[0107] 2. Preparation steps

[0108] S1. Preparation of mixed slurry

[0109] Weigh 100g of PAA solution and add 20% of the PAA solid mass of thermally expanded microspheres and 2% of the microsphere mass of dispersant BYK-180. Place the mixture in a planetary mixer and stir at 300 rpm for 2 hours. Then degas in a vacuum degassing machine at -0.08MPa for 30 minutes to obtain a uniform and stable slurry.

[0110] S2. Wet film forming

[0111] The mixed slurry is transferred to a precision doctor blade coater. Using polyethylene terephthalate (PET) release film as the substrate, the doctor blade gap is set to 200 μm and the coating speed is 5 mm / s. A wet film is then cast onto the substrate.

[0112] The wet film, along with the substrate, is then placed in an 80°C hot air circulating oven for 10 minutes to pre-dry, allowing most of the solvent to evaporate and forming a pre-formed film with a certain degree of self-support.

[0113] S3. Heating and heat treatment

[0114] The substrate with the pre-formed film is transferred to a programmable temperature stretching oven:

[0115] The temperature is increased to 280°C at a rate of 3°C / min. When the film temperature reaches 280°C, it is held at this temperature for about 60 minutes to allow the polyamic acid to be completely imidized into polyimide.

[0116] The membrane was allowed to cool naturally to room temperature and then peeled off from the substrate to obtain a self-supporting porous polyimide membrane, which was designated as PI-NIPS1.

[0117] Comparative Example 2:

[0118] 1. Raw materials and formula

[0119] Polyamic acid (PAA) solution: prepared by reacting 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) as monomers in N-methylpyrrolidone (NMP) solvent under nitrogen protection, with a solid content of 15% (mass fraction) and a viscosity of 4500 cP at 25°C.

[0120] 2. Preparation steps

[0121] The PAA solution was coated onto a glass plate with a wet film thickness of 200 μm. The glass plate was immediately immersed in a coagulation bath composed of NMP and deionized water (NMP:water = 3:7, v / v) for phase separation. After 30 minutes, the plate was removed and washed with deionized water for 48 hours to replace the solvent.

[0122] Heat treatment: The washed membrane is peeled off the glass plate and dried and imidized in an oven according to the program of 80℃ / 1h, 150℃ / 1h, and 300℃ / 1h. The resulting membrane is denoted as PI-NIPS2.

[0123] Performance comparison:

[0124] The membranes PI-20, PI-30, PI-40, PI-NIPS1, and PI-NIPS2 obtained in Examples 2-4 and Comparative Examples 1-2 were tested, and the data are shown in Table 1 below:

[0125]

[0126] Test method description:

[0127] Porosity: determined by n-butanol absorption method.

[0128] Average pore size: Measured by statistical analysis of scanning electron microscope (SEM) images of at least 100 pores.

[0129] Gurley value: The time (in seconds) required for 100 ml of air to pass through the diaphragm, according to JIS P8117 standard.

[0130] Ionic conductivity: After the membrane was immersed in 1M LiPF6 / EC:DEC:EMC (1:1:1, v / v / v) electrolyte for 24 hours, a stainless steel|membrane|stainless steel symmetric cell was assembled and the conductivity was calculated by electrochemical impedance spectroscopy (EIS).

[0131] Puncture intensity: Using a hemispherical probe with a diameter of 1.0 mm, the diaphragm was punctured at a speed of 100 mm / min, and the maximum force value was recorded.

[0132] Heat shrinkage rate: Place a 10cm×10cm diaphragm sample in a 150℃ oven for 1 hour and measure the percentage change in length of each side.

[0133] Battery cycle performance: Using lithium iron phosphate as the positive electrode and graphite as the negative electrode, a 2032 coin cell was assembled and cycled 500 times at a 1C rate, and the capacity retention rate was recorded.

[0134] Based on the data in Table 1 above, at least the following conclusions can be drawn:

[0135] In Example 2, the polyimide lithium battery separator prepared by using thermally expanded microspheres and combined with a staged heating heat treatment process can achieve a polyimide lithium battery separator with a smaller porosity and a larger average pore size. The pore shape of the polyimide lithium battery separator is consistent with the shape of the thermally expanded microspheres, which are spherical or ellipsoidal pores. While meeting the requirement of high ionic conductivity (not less than 1 mS / cm), its mechanical properties are also improved, which is superior to the battery separator prepared by the existing technology (phase inversion method).

[0136] A comparison of Example 2 with Comparative Document 1 shows that the introduction of the staged heating heat treatment process can reduce the thermal rate of the battery separator and increase the puncture strength. The possible reason is that the second stage of heating to about 150°C and holding for 3 minutes can cause most of the microspheres to expand and rupture during the PAA partial gelation (incomplete gelation) stage. This stage can ensure the uniformity of the pore shape and ensure the complete gelation of the separator in the subsequent third stage, thus ensuring that the separator has excellent mechanical properties.

[0137] A comparison of Examples 3 and 4 with Example 2 shows that the introduction of the separator stretching process can further reduce the thermal rate of the battery separator and the battery cycle capacity retention rate, while the mechanical properties of the separator remain essentially unchanged. This may be because the stretching process allows the spherical or ellipsoidal pores inside the separator to connect, forming a three-dimensional interconnected pore network. Furthermore, a comparison of Examples 3 and 4 shows that stretching between the second and third stages (corresponding to stretching the incompletely imidized polyimide separator) yields better battery cycle capacity retention and ionic conductivity data compared to stretching after the third stage (corresponding to stretching the fully imidized polyimide separator). This may be because stretching the incompletely imidized polyimide separator allows the similar spherical or ellipsoidal pores to connect inside the separator, forming a more effective three-dimensional interconnected pore network structure.

[0138] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A polyimide lithium battery separator, characterized in that: The diaphragm has spherical or ellipsoidal pores formed by the rupture of thermally expanded microspheres. The average pore diameter of the pores is 5μm-30μm, the porosity is 35%-60%, and the ionic conductivity of the diaphragm in the electrolyte is not less than 1.0 mS / cm.

2. The polyimide lithium battery separator according to claim 1, characterized in that: The spherical or ellipsoidal pores inside the diaphragm are interconnected, forming a three-dimensional network of channels.

3. A method for preparing a polyimide lithium battery separator, characterized in that, Includes the following steps: S1. Disperse the thermally expanded microspheres and dispersant in a polyamic acid solution to obtain a mixed slurry; S2. Prepare a wet film from the mixed slurry; S3. Perform a multi-stage heating heat treatment on the wet film, the multi-stage heating heat treatment including: First stage: Heat the wet film to 80℃-130℃ and maintain it, so that the thermally expanding microspheres expand but remain intact, while the polyamic acid is partially gelled; Second stage: Continue heating to 130℃-200℃ and maintain it to rupture the expanded thermally expanded microspheres and further gel the polyamic acid to obtain a polyimide membrane formed by incomplete imidization. Third stage: Continue heating to 250℃-400℃ and maintain it to completely imidize the polyamic acid, and obtain a fully imidized polyimide membrane; Among them, the thermally expandable microspheres are Expancel® 091 DU 40 microspheres. After the thermally expandable microspheres expand and rupture in the second stage of step S3, they form spherical or ellipsoidal pores inside the polyimide membrane.

4. The method for preparing a polyimide lithium battery separator according to claim 3, characterized in that, Also includes: The incompletely imidized polyimide membrane obtained in the second stage of step S3 is subjected to stretching treatment under nitrogen protection, with a stretching ratio of 1.1-5.0:1 and a stretching ambient temperature of 250℃-400℃, and then the third stage of step S3 is carried out.

5. A method for preparing a polyimide lithium battery separator according to claim 3 or 4, characterized in that, Also includes: The polyimide diaphragm formed by complete imidization obtained in the third stage of step S3 is subjected to stretching treatment with a stretching ratio of 3.0-7.0:

1.

6. The method for preparing a polyimide lithium battery separator according to claim 3, characterized in that, The polyamic acid solution is obtained by reacting diamine monomers and dianhydride monomers in a polar aprotic solvent.

7. The polyimide lithium battery separator and its preparation method according to claim 6, characterized in that: The diamine monomer is one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl.

8. The method for preparing a polyimide lithium battery separator according to claim 6, characterized in that: The dianhydride monomer is one or more of pyromellitic dianhydride, biphenyl dianhydride, and 4,4'-oxobisphthalic anhydride.

9. The method for preparing a polyimide lithium battery separator according to claim 3, characterized in that: The dispersant is a polymeric block copolymer dispersant or a silane coupling agent; The mass of the dispersant is 0.1%-5% of the mass of the thermally expanded microspheres.

10. The method for preparing a polyimide lithium battery separator according to claim 3, characterized in that: In multi-stage heating heat treatment, First stage: Increase the temperature to 90℃ at a rate of 2℃ / min and maintain it for 10-20 minutes; Second stage: Increase the temperature to 150℃ at a rate of 5℃ / min and hold for 2-5 minutes; Third stage: Increase the temperature to 300℃ at 3℃ / min and maintain it for 60 minutes.

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