Porous separator and preparation method therefor, and lithium-ion battery

Abstract

The present invention belongs to the technical field of battery separators, and provides a porous separator and a preparation method therefor, and a lithium-ion battery. The porous separator is mainly prepared from a polyamide material; and the pore structure of the porous separator is mainly elliptical or ellipse-like, and the relationship between the length a of a major axis and the length b of a minor axis in the pore structure satisfies a≥1.5b, wherein a is in the range of 0.3-6 μm, and b is in the range of 0.1-3 μm. The method for preparing the porous separator comprises the following step: subjecting a semi-solidified film to simultaneous drawing and pore-forming, wherein the semi-solidified film contains a polyamide material. By means of the process, a unique oriented microporous structure can be formed, which is conducive to improving the porosity and air permeability of the separator; moreover, the separator has a good thermal stability and is more suitable for a high-power lithium-ion battery.

Description

A porous diaphragm and its preparation method and lithium ion battery

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This disclosure claims priority to application number 2023116022279 filed with the Patent Office of China on November 28, 2023, entitled “A porous diaphragm, its preparation method and lithium-ion battery”, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0003] The present disclosure relates to the technical field of battery separators, and in particular to a porous separator, a preparation method thereof, and a lithium-ion battery. Background Art

[0004] Currently, the separators used in lithium-ion batteries are primarily polyolefin (PP) and polyethylene (PE) separators. However, polyolefins have a low intrinsic melting point, and separators formed from them exhibit poor thermal stability. This poses significant risks to lithium-ion batteries when used in high-temperature environments. To improve the high-temperature safety performance of lithium-ion batteries, it is necessary to develop new separators with high thermal stability.

[0005] The melting point of polyamide materials is generally higher than that of polyolefin materials, and they have good mechanical strength, so they can be used as diaphragm materials. Unlike low-melting-point polyolefin materials, polyamide materials with high melting points and rich polar functional groups cannot be prepared into porous diaphragms through a high-temperature melting process. Instead, they need to be prepared using a phase inversion method. There are two main traditional phase inversion membrane making processes, one is coating-steam pore formation, and the other is cast-coagulation bath pore formation. The former easily forms a dense cortex on the surface of the film, affecting the permeability of the porous membrane. The latter process is time-consuming, inefficient, and has poor product consistency.

[0006] In view of this, the present disclosure is proposed.

[0007] Summary of the Invention

[0008] The purpose of the present disclosure is to provide a porous diaphragm and a preparation method thereof and a lithium ion battery to solve or improve the above technical problems.

[0009] The present disclosure can be implemented as follows:

[0010] In the first aspect, the present disclosure provides a porous diaphragm, which is mainly made of polyamide material. The pore structure in the porous diaphragm is elliptical or quasi-elliptical, and the relationship between the major axis length a and the minor axis length b in the pore structure is a≥1.5b, and the range of a is 0.3-6μm, and the range of b is 0.1-3μm.

[0011] In an optional embodiment, the porous membrane further has at least one of the following features:

[0012] Feature 1: The pore structure is oriented;

[0013] Feature 2: The thickness of the porous diaphragm is 9-20 μm;

[0014] Feature 3: The tensile strength of the porous diaphragm is 30-70MPa;

[0015] Feature 4: The elongation at break of the porous diaphragm is 50-150%;

[0016] Feature 5: The air permeability of the porous membrane is 50-180s / 100mL;

[0017] Feature 6: After the porous membrane is left to stand at 250°C for 15 minutes, the thermal shrinkage in both the TD and MD directions is less than 2%;

[0018] Feature 7: The porosity of the porous membrane is 42-70%;

[0019] Feature 8: The main components of the porous membrane include nylon and aramid; preferably, the main components of the porous membrane include at least one of nylon 6, nylon 66, meta-aramid and para-aramid.

[0020] In a second aspect, the present disclosure provides a method for preparing a porous membrane as described in the aforementioned embodiment, comprising the following steps: simultaneously stretching and forming pores in a semi-solidified film; wherein the semi-solidified film contains a polyamide material.

[0021] In an optional embodiment, the semi-solidified film contains 10-30 wt% of polyamide material, 25-70 wt% of solvent and 20-40 wt% of non-solvent.

[0022] In an optional embodiment, the preparation of the semi-solidified film includes: coating a slurry containing a polyamide material on a surface of a substrate, removing part of the solvent, and then pre-solidifying to obtain a semi-solidified film.

[0023] In an optional embodiment, the preparation of the semi-solidified film includes at least one of the following features:

[0024] Feature 1: Coating thickness is 100-500μm;

[0025] Feature 2: The mass percentage of polyamide material in the slurry is 10-30%;

[0026] Feature 3: The rotational viscosity of the slurry is 50,000 Pa·s-700,000 Pa·s;

[0027] Feature 4: The substrate includes at least one of release film, PP, stainless steel, PET, POM and PTFE;

[0028] Feature 5: removing part of the solvent by evaporation; preferably, evaporation is performed at 40-80°C for 1-5 minutes;

[0029] Feature 6: The portion of solvent removed accounts for 20-60 wt% of the total solvent in the slurry; preferably, the solvent includes at least one of DMF, DMAC, NMP, formic acid and acetic acid;

[0030] Feature 7: Pre-solidification is to expose the coating after partial removal of the solvent to conditions containing non-solvent gas and non-solvent atomizing vapor for 30-120 seconds; the non-solvent includes at least one of water, ethanol, methanol, isopropanol, acetone and DMSO.

[0031] In an optional embodiment, during the stretching and pore-forming process, a non-solvent atomized vapor is introduced into both the upper and lower surfaces of the semi-solidified film to form a pore structure on the semi-solidified film; the non-solvent comprises at least one of water, ethanol, methanol, isopropanol, acetone, and DMSO;

[0032] In an alternative embodiment, the temperature of the non-solvent atomizing gas is 50-80°C.

[0033] In an alternative embodiment, the stretch ratio during the stretching process is 1.2-4.

[0034] In an alternative embodiment, the major axis direction in the pore structure of the porous membrane is the same as the stretching direction.

[0035] In an optional embodiment, the semi-solidified film is subjected to a film-forming treatment before stretching and pore-forming.

[0036] In an optional embodiment, the temperature of the film-entry treatment is 30-50° C., and the stretching ratio is 1-1.2.

[0037] In an optional embodiment, after stretching and pore-forming, the process further includes subjecting the stretched and pore-forming film to a film removal process.

[0038] In an optional embodiment, during the film removal process, non-solvent atomized gas is introduced into both the upper and lower surfaces of the film after stretching and pore formation.

[0039] In an alternative embodiment, the temperature of the non-solvent atomizing vapor is 4-15°C.

[0040] In an optional embodiment, the stretching ratio during the film discharge process is 1-1.5.

[0041] In an optional embodiment, the method further includes: cleaning, drying and shaping the film after the film removal treatment.

[0042] In an optional embodiment, the cleaning temperature is 25-100° C., and the cleaning time is 2-10 minutes.

[0043] In an optional embodiment, the drying temperature is 100-200° C., and the drying time is 2-10 minutes.

[0044] In an optional embodiment, the setting temperature is 220-270° C., and the setting time is 1-5 minutes.

[0045] In a third aspect, the present disclosure provides a lithium ion battery comprising the porous separator of the aforementioned embodiment.

[0046] The beneficial effects of the present disclosure include:

[0047] The porous membrane provided herein is primarily made of polyamide material. The pore structure of the porous membrane is primarily elliptical or quasi-elliptical, with a major axis length a and a minor axis length b being in a relationship of a ≥ 1.5b, with a ranging from 0.3 to 6 μm and b ranging from 0.1 to 3 μm. A porous membrane with such a pore structure exhibits excellent thermal stability and mechanical strength.

[0048] The preparation method involves simultaneously stretching and pore-forming a semi-solidified film. This simultaneous stretching and pore-forming process prevents the formation of a dense layer on the membrane surface, resulting in a unique, oriented microporous structure. This improves the membrane's porosity and air permeability, imparting excellent thermal stability and making it more suitable as a battery separator for lithium-ion batteries, particularly high-power lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0050] FIG1 is a SEM image of the porous diaphragm prepared in Example 1;

[0051] FIG2 is a SEM image of the porous diaphragm prepared in Example 2;

[0052] FIG3 is a SEM image of the porous membrane prepared in Example 3;

[0053] FIG4 is a SEM image of the porous diaphragm prepared in Comparative Example 2;

[0054] FIG5 is a SEM image of the porous membrane prepared in Comparative Example 5. DETAILED DESCRIPTION

[0055] The embodiments of the present disclosure will be described in detail below with reference to the examples. However, those skilled in the art will appreciate that the following examples are intended only to illustrate the present disclosure and should not be construed as limiting the scope of the present disclosure. Where specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0056] The endpoints of the ranges and any values ​​disclosed in this disclosure are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0057] The present disclosure provides a porous separator primarily made of a polyamide material. The porous separator is understood to be primarily composed of a highly thermally stable polymer containing amide groups, such as at least one of nylon and aramid. The nylon may, by way of example but not limitation, include at least one of nylon 6 and nylon 66, and the aramid may, by way of example but not limitation, include at least one of meta-aramid and para-aramid.

[0058] In some embodiments, the porous membrane is formed only of polyamide material. In some other embodiments, it is not excluded that the porous membrane can be formed on the basis of polyamide material and in combination with other materials.

[0059] The pore structure in the porous membrane is mainly elliptical or quasi-elliptical. The so-called "quasi-elliptical" is basically elliptical, but not an absolutely standard elliptical shape.

[0060] It should be noted that the pore structure of the porous membrane disclosed herein does not exclude the possibility of a small portion of circular or other shapes. However, the elliptical or quasi-elliptical pore structure accounts for at least 50% of the total pore structure. In some typical embodiments, the elliptical or quasi-elliptical pore structure accounts for at least 85% by weight of the total pore structure.

[0061] For reference, the relationship between the major axis length a and the minor axis length b in the pore structure is a≥1.5b; the length of the major axis in the pore structure (which can also be understood as the length of the pore) can be 0.3-6μm, such as 0.3μm, 0.5μm, 0.8μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm or 6μm, etc., or it can be any other value within the range of 0.3-6μm. The length of the short axis (which can also be understood as the width of the hole) can be 0.1-3μm, such as 0.1μm, 0.2μm, 0.5μm, 0.8μm, 1μm, 1.2μm, 1.5μm, 1.8μm, 2μm, 2.2μm, 2.5μm, 2.8μm or 3μm, etc., or it can be any other value within the range of 0.1-3μm.

[0062] It should be noted that the major and minor axis lengths of the pore structure of the porous membrane in this disclosure refer to the area of ​​pores with such size within a defined range on the same plane accounting for 50% or more of the defined area. Therefore, pores of other pore sizes within the defined range are permitted in the pore structure.

[0063] In some typical embodiments, the length of the major axis of the pore structure is 1-2 μm, and the length of the minor axis is 0.1-0.3 μm.

[0064] In the present disclosure, the above-mentioned elliptical pore structure has orientation, that is, its orientation is basically consistent.

[0065] The porosity of the porous membrane in the present disclosure is about 42-70%, such as 42%, 50%, 55%, 60%, 62%, 65%, 68% or 70%, etc., and can also be any other value within the range of 42-70%. In some typical embodiments, the porosity of the porous membrane is 60%.

[0066] If the porosity is too low, it is not conducive to the migration of lithium ions; if the porosity is too high, it is not conducive to the mechanical strength and puncture resistance of the diaphragm.

[0067] The porous membrane having the pore structure meeting the above conditions can have better thermal stability and mechanical strength.

[0068] For reference, the thickness of the porous membrane can be, by way of example but not limitation, 9-20 μm, such as 9 μm, 10 μm, 12 μm, 15 μm, 18 μm, or 20 μm, or any other value within the range of 9-20 μm. In some typical embodiments, the thickness of the porous membrane is 14-18 μm.

[0069] In some embodiments, the porous separator may have a tensile strength of 30-70 MPa, such as 30 MPa, 35 MPa, 40 MPa, 45 MPa, 50 MPa, 55 MPa, 60 MPa, 65 MPa, or 70 MPa.

[0070] In some embodiments, the elongation at break of the porous separator can be 50-150%, such as 50%, 60%, 70%, 80%, 90%, 100%, 110%, 120%, 130%, 140% or 150%, etc.

[0071] In some embodiments, the porous membrane may have an air permeability of 50-180 s / 100 mL, such as 50 s / 100 mL, 60 s / 100 mL, 80 s / 100 mL, 100 s / 100 mL, 120 s / 100 mL, 140 s / 100 mL, 160 s / 100 mL, or 180 s / 100 mL.

[0072] In some embodiments, after the porous diaphragm is allowed to stand at 250°C for 15 minutes, the thermal shrinkage rates in the TD direction (transverse direction) and the MD direction (longitudinal direction) are both less than 2%, such as 1.8%, 1.6%, 1.4%, 1.2%, 1%, 0.8% or 0.5%, etc.

[0073] The porous separator with the above characteristics not only has good thermal stability and mechanical strength, but also has good air permeability, and is suitable for use as a battery separator.

[0074] Correspondingly, the present disclosure also provides a method for preparing the above-mentioned porous diaphragm, comprising the following steps: simultaneously stretching and forming pores in a semi-solidified film; wherein the semi-solidified film contains a polyamide material.

[0075] By using high thermal stability polyamide material as the membrane substrate and carrying out the stretching and pore-forming processes simultaneously, the formation of a dense layer on the membrane surface can be avoided, which is conducive to the formation of a unique oriented microporous structure, improving the porosity and permeability of the membrane, and giving it excellent thermal stability and mechanical properties.

[0076] For reference, the preparation of the semi-solidified film may include: coating a slurry containing a polyamide material on a surface of a substrate, removing a portion of the solvent, and then pre-solidifying to obtain a semi-solidified film.

[0077] By replacing the casting-coagulation bath process in the traditional phase inversion film-making process with a coating film-forming method, the film-making speed can be greatly increased and the production efficiency can be improved.

[0078] For example, the substrate may illustratively but not limitatively include at least one of a release film, PP, stainless steel, PET, POM and PTFE.

[0079] The mass percentage of the polyamide material in the slurry can be 10-30%, such as 10%, 15%, 20%, 25% or 30%, or any other value within the range of 10-30%. In addition, the slurry also contains a solvent for dissolving the polyamide material, which can exemplarily but not limitatively include at least one of DMF, DMAC, NMP, formic acid and acetic acid. In some embodiments, the slurry can also contain some auxiliary ingredients as needed, such as co-solvents, pore-forming agents and acid-binding agents commonly used in the art, which are not further limited herein.

[0080] If the mass percentage of the polyamide material in the slurry is lower than 10%, it is not conducive to the formation of a semi-solidified film and cost control; if the mass percentage of the polyamide material in the slurry is higher than 30%, it is not conducive to the formation of a uniform slurry.

[0081] In the present disclosure, the rotational viscosity of the slurry may be 50,000 Pa·s-700,000 Pa·s, such as 50,000 Pa·s, 10,000 Pa·s, 15,000 Pa·s, 20,000 Pa·s, 25,000 Pa·s, 30,000 Pa·s, 35,000 Pa·s, 40,000 Pa·s, 45,000 Pa·s, 50,000 Pa·s, 55,000 Pa·s, 60,000 Pa·s, 65,000 Pa·s or 70,000 Pa·s, or any other value within the range of 50,000 Pa·s-700,000 Pa·s. In some typical embodiments, the rotational viscosity of the slurry may be 200,000 Pa·s.

[0082] If the rotational viscosity of the slurry is less than 50,000 Pa·s, it is not conducive to maintaining the non-fluidity of the coating during the cast coating process. If the rotational viscosity of the slurry is greater than 700,000 Pa·s, the cast coating operation cannot be performed effectively.

[0083] The coating thickness may be 100-500 μm, such as 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm or 500 μm, or any other value within the range of 100-500 μm. In some typical embodiments, the coating thickness may be 100-300 μm.

[0084] If the coating thickness is less than 100 μm, it is not conducive to stretching and pore-making operations, and membrane breakage is likely to occur; if the coating thickness is greater than 500 μm, it is not conducive to the preparation of thin membranes.

[0085] In the present disclosure, evaporation is primarily used to remove a portion of the solvent. For reference, evaporation can be performed at 40-80°C (e.g., 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, or 80°C) for 1-5 minutes (e.g., 1 minute, 1.5 minutes, 2 minutes, 2.5 minutes, 3 minutes, 3.5 minutes, 4 minutes, 4.5 minutes, or 5 minutes). Furthermore, the evaporation temperature and time can be adjusted according to actual circumstances.

[0086] The removed portion of solvent may account for 20-60 wt% of the total solvent in the slurry, such as 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt% or 60 wt%, etc.

[0087] If the amount of solvent removed is too much, it is not conducive to the stretchability of the semi-solidified film. If the amount of solvent removed is too little, it is not conducive to peeling the semi-solidified film off the substrate surface.

[0088] For reference, pre-solidification can involve exposing the coating, after partial solvent removal, to a non-solvent gas and / or non-solvent atomizing vapor (e.g., in a constant humidity chamber) for 30-120 seconds, such as 30 seconds, 50 seconds, 80 seconds, 100 seconds, or 120 seconds, followed by cooling to room temperature, so that the polymer coating becomes a semi-solidified film. The non-solvent can include at least one of water, ethanol, methanol, isopropanol, acetone, and DMSO; in some typical embodiments, the non-solvent is water. The non-solvent atomizing vapor can be understood as a non-solvent in the form of droplets.

[0089] In some preferred embodiments, the coating after partial solvent removal is exposed to the non-solvent gas and / or non-solvent atomized vapor for no more than 3 minutes. If the pre-solidification time is too long, the stretchability of the semi-solidified film will be impaired.

[0090] As mentioned above, the preparation of the above-mentioned semi-solidified film is mainly to convert the coating into a ductile film, wherein the purpose of removing part of the solvent is to increase the proportion of polyamide material in the coating; the purpose of exposing the coating to non-solvent gas and non-solvent atomized vapor for pre-solidification is mainly to partially precipitate the polyamide material in the coating, so as to achieve the degree that the coating can independently form a film, thereby facilitating the processing of subsequent processes.

[0091] In the present disclosure, the semi-solidified film obtained after pre-solidification may contain, for example, 10-30% (e.g., 10%, 15%, 20%, 25%, or 30%) of polyamide material, 25-70% (e.g., 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%) of solvent, and 20-40% (e.g., 20%, 25%, 30%, 35%, or 40%) of non-solvent. If auxiliary components are present, the content of the auxiliary components in the semi-solidified film does not exceed 5%.

[0092] The semi-solidified film with the above characteristics is easy to peel off from the surface of the substrate and maintains a certain strength. It has ductility and can be stretched. It contains some non-solidified polyamide materials to form a porous structure.

[0093] Furthermore, the semi-solidified film is peeled off from the surface of the substrate for subsequent operations.

[0094] First, the semi-solidified film is subjected to a film-forming treatment.

[0095] For reference, the film entry temperature can be 30-50°C (e.g., 30°C, 35°C, 40°C, 45°C, or 50°C), meaning the semi-solidified film is heated from room temperature to within the aforementioned range. The film entry stretch ratio can be 1-1.2, such as 1, 1.05, 1.1, 1.15, or 1.2. The film entry time can be 30 seconds to 2 minutes.

[0096] Subsequently, the film after the film-entry treatment is stretched and pore-formed.

[0097] In the present disclosure, during the stretching and pore-forming process, non-solvent atomized gas is introduced into both the upper and lower surfaces of the semi-solidified film to form a pore structure on the semi-solidified film.

[0098] For reference, the temperature of the non-solvent atomizing gas used in the stretching and pore-forming process can be 50-80°C, such as 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, or any other value within the range of 50-80°C.

[0099] If the temperature of the non-solvent atomizing gas used in the stretching and pore-forming process is lower than 50°C, the membrane will be too hard and prone to tearing or breaking during the stretching process. If the temperature of the non-solvent atomizing gas used in the stretching and pore-forming process is higher than 80°C, the membrane will be easily over-softened, resulting in membrane breakage.

[0100] The stretch ratio during the stretching process is 1.2-4, such as 1.2, 1.5, 1.8, 2, 2.5, 3, 3.5 or 4, or any other value within the range of 1.2-4. In some typical embodiments, the stretch ratio during the stretching process is 1.2-2.5, which can produce a film with better performance and pore structure.

[0101] If the stretching ratio during the stretching process is less than 1.2, the introduced water vapor cannot effectively play a pore-forming role; if the stretching ratio during the stretching process is greater than 4, it is easy to cause the film to break.

[0102] The time for stretching and pore creation is 2-10 minutes, which is usually related to the equipment capacity and is adjusted according to the characteristics of the diaphragm product.

[0103] In the present disclosure, the major axis direction in the pore structure of the porous separator is the same as the stretching direction, so that a pore structure with substantially uniform orientation can be obtained.

[0104] Finally, the film after stretching and pore forming is subjected to film removal treatment.

[0105] During the film removal process, a non-solvent atomizing gas is introduced into both the upper and lower surfaces of the film after stretching and pore formation. For example, the temperature of the non-solvent atomizing gas is lower than room temperature, for example, 4-15°C. In some typical embodiments, the temperature of the non-solvent atomizing gas can be 10-15°C.

[0106] It should be noted that after stretching and pore-forming, the film has a high degree of elasticity. If atomized water vapor is not introduced during the film removal process, the film will easily shrink after leaving the stretching equipment, and a film product with good porosity cannot be obtained. By introducing cold steam at a lower temperature during the film removal process, the stretched film can be solidified upon exiting the film, preventing shrinkage, improving the film's porosity, and ensuring process stability.

[0107] In some embodiments, stretching is not required during the film-exiting process. In other embodiments, stretching may be performed during the film-exiting process, and the corresponding stretch ratio may be 1-1.5, such as 1, 1.1, 1.2, 1.3, 1.4, or 1.5. The film-exiting process may last from 30 seconds to 2 minutes, depending on the actual equipment capacity.

[0108] It should be noted that the above-mentioned film entry processing, stretching and pore making, and film exit processing are all carried out in a stretching device. For example, the stretching device can be any one of a TD direction stretching device, an MD direction stretching device, and a biaxial stretching device.

[0109] During specific operations, the stretching and pore-forming stages can be performed once, twice or more times between the film entry process and the film exit process as required.

[0110] As mentioned above, in the preparation process of the porous diaphragm provided by the present invention, the film is creatively pore-formed while being stretched, wherein the film entry treatment and film exit treatment mainly affect the process stability rather than being a factor determining the properties of the diaphragm. The preheating performed during the film entry treatment can play the role of preheating the film and softening the film. On the one hand, it prevents the film from experiencing drastic thermal contraction due to excessive temperature difference when entering the steam stretching stage. On the other hand, it moderately improves the ductility of the film, making it easier for the film to be stretched, which is beneficial to improving the tensile elongation of the film. The role of the stretching and pore-forming treatment is mainly to steam-form the semi-solidified film while stretching it. In the process of continuously expanding the width of the film, the unsolidified part inside the film is exposed to the surface. The surface is invaded by steam, causing solvent separation and triggering phase inversion to precipitate solid polymer components, forming a porous polymer film in the process of continuous surface exposure-solidification. Therefore, the hot steam temperature and stretch ratio used in this stage are the main conditions for controlling the pore structure in the porous membrane. Using water mist for cooling during the film exit treatment can further promote film solidification, prevent elastic contraction when the film leaves the stretching equipment, improve film strength, and reduce tearing.

[0111] Furthermore, the film after the film removal treatment is cleaned, dried and shaped.

[0112] The film may be rinsed with water in a sink to remove residual solvent components. The cleaning temperature may be 25-100°C (e.g., 25°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or 100°C), and the cleaning time may be 2-10 minutes (e.g., 2 minutes, 4 minutes, 6 minutes, 8 minutes, or 10 minutes).

[0113] The drying can be carried out in an oven at a temperature of 100-200° C. (e.g., 100° C., 120° C., 150° C., 180° C., or 200° C.), and a drying time of 2-10 min. (e.g., 2 min, 4 min, 6 min, 8 min, or 10 min.).

[0114] The shaping can be carried out in a heat setting device. The shaping temperature can be 220-270°C (such as 220°C, 230°C, 240°C, 250°C, 260°C or 270°C), and the shaping time can be 1-5 minutes (such as 1 minute, 2 minutes, 3 minutes, 4 minutes or 5 minutes).

[0115] As mentioned above, the preparation method of the porous diaphragm provided by the present disclosure is simple, easy to operate, short in time and highly efficient, and the prepared product has high thermal stability, high mechanical strength and good consistency.

[0116] In addition, the present disclosure also provides a lithium-ion battery, which includes the above-mentioned porous diaphragm, and can have good thermal stability and improve the safety of the lithium-ion battery under high temperature conditions.

[0117] The features and performance of the present disclosure are further described in detail below with reference to the embodiments.

[0118] Example 1

[0119] This embodiment provides a porous diaphragm, the preparation method of which includes:

[0120] S1: Prepare a slurry with a meta-aramid mass fraction of 14% (the specific components of the slurry are 14wt% meta-aramid, 83wt% dimethylacetamide and 3wt% calcium chloride, and the rotational viscosity of the slurry is 150000Pa·s), apply the slurry on the surface of the release film, the coating thickness is 150μm, and then enter the 50℃ oven area for baking for 1 minute to remove 26wt% of the solvent. After baking, enter the water vapor box (constant humidity box) with 80% humidity for treatment for 45s, and then stand at room temperature for 1 minute. The coating solidifies to form a semi-solidified film. The semi-solidified film contains 14% polymer, 57% solvent, 26% water and 3% calcium chloride. At this time, the film is easy to peel off from the surface of the release film.

[0121] S2: The semi-solidified film is peeled off from the release film surface and pulled into the TD stretching equipment, which goes through three stages:

[0122] The first stage (film treatment): in indoor air, the temperature is 45 ° C, the stretch ratio is 1.1, and the treatment time is 30 seconds;

[0123] The second stage (stretching and pore formation): in the water mist formed by hot steam, the hot steam temperature is 65 ° C, the stretching ratio is 1.5, and the treatment time is 1 min;

[0124] The third stage (film removal treatment): in 15°C water mist, the stretching ratio is 1.1, and the treatment time is 30s.

[0125] The film treated by the stretching equipment is solid and has a certain toughness, with an elongation at break of about 180%.

[0126] S3: The film was rinsed in room-temperature pure water for 3 minutes, then in 70°C pure water for 2 minutes. The rinsed film was then baked in an oven at 120°C for 5 minutes. Finally, the film was heat-set at 230°C for 1 minute. After cooling, it was rolled up to obtain a porous membrane, designated Sample 1.

[0127] By observing sample 1 through scanning electron microscopy (as shown in Figure 1), it can be found that a large number of elliptical pores are developed on the surface and inside of the membrane. The long axis direction of the elliptical pore points to the TD direction, and the long axis length of the pore is mainly about 1 to 2 μm, and the short axis length is mainly about 0.1 to 0.3 μm.

[0128] Example 2

[0129] The difference between this embodiment and embodiment 1 is that in S2, the hot steam temperature of the second stage is 65° C., the stretching ratio is 1.9, and the processing time is 1 min.

[0130] The sample obtained in this step is recorded as sample 2. By observing sample 2 through an electron microscope (as shown in Figure 2), it can be found that the pore size of sample 2 is larger than that of sample 1. This may be due to the formation of the second-stage dominant pores in S2. A larger stretching ratio can produce larger pores.

[0131] Example 3

[0132] The difference between this embodiment and embodiment 2 is that in S2, the hot steam temperature is 80° C., the stretching ratio is 1.9, and the processing time is 1 min.

[0133] The sample obtained in this step is recorded as sample 3. By observing sample 3 through an electron microscope (as shown in Figure 3), it can be found that sample 3 has more large-sized pores compared with sample 2. This may be because in the second stage of S2, higher temperatures promote the development of large pores, resulting in an increase in the number of large-sized pores.

[0134] Example 4

[0135] The difference from Example 1 is that in S2, the temperature of the hot steam is 55° C., the stretching ratio is 1.3, and the processing time is 1 min.

[0136] The remaining steps were consistent with those in Example 1. The properties of the membrane of this example are shown in Table 1.

[0137] Example 5

[0138] The difference from Example 1 is that in S1, the coating thickness is 100 μm, and then it enters the 50°C oven area for baking for 45 seconds to remove 30wt% of the solvent. After baking, it enters the water vapor box (constant humidity box) with 80% humidity for treatment for 30 seconds, and then stands at room temperature for 1 minute. The coating solidifies to form a semi-solidified film. The semi-solidified film contains 14wt% of polymer, 53wt% of solvent, 30wt% of water and 3wt% of calcium chloride. At this time, the film is easy to peel off from the surface of the release film.

[0139] The remaining steps were consistent with those in Example 1. The properties of the membrane of this example are shown in Table 1.

[0140] Example 6

[0141] The difference from Example 1 is that in S1, the coating thickness is 250 μm, and then it enters the 750°C oven area for baking for 70 seconds to remove 27wt% of the solvent. After baking, it enters the water vapor box (constant humidity box) with 80% humidity for treatment for 65 seconds, and then stands at room temperature for 1 minute. The coating solidifies to form a semi-solidified film. The semi-solidified film contains 14wt% of polymer, 56wt% of solvent, 27wt% of water and 3wt% of calcium chloride. At this time, the film is easy to peel off from the surface of the release film.

[0142] The remaining steps were consistent with those in Example 1. The properties of the membrane of this example are shown in Table 1.

[0143] Example 7

[0144] S1: Prepare a slurry with a nylon 6 mass fraction of 12% (the specific components of the slurry are 12wt% nylon 6 and 88wt% formic acid, and the rotational viscosity of the slurry is 200000Pa·s), apply the slurry on the surface of the release film, the coating thickness is 150μm, and then enter the 50℃ oven area for baking for 1 minute to remove 30wt% of the solvent. After baking, enter the water vapor box (constant humidity box) with 60% humidity for treatment for 60s, and then stand at room temperature for 1 minute. The coating solidifies to form a semi-solidified film. The semi-solidified film contains 15wt% nylon 6, 72.5wt% formic acid and 12.5wt% water. At this time, the film is easy to peel off from the surface of the release film.

[0145] The remaining steps were consistent with those in Example 1. The properties of the membrane of this example are shown in Table 1.

[0146] Comparative Example 1

[0147] The difference between this comparative example and Example 1 is that: in S2, the semi-solidified film first enters into 65°C hot steam for treatment for 1 min, and then enters into the TD stretching equipment, and goes through three stages. The first stage is in indoor air, the temperature is 45°C, the stretching ratio is 1.1, and the treatment time is 30s; the second stage is in indoor air, the temperature is 65°C, the stretching ratio is 1.5, and the treatment time is 1 min; the third stage is in room temperature water mist, the water vapor amount is, the stretching ratio is 1.1, and the treatment time is 30s.

[0148] That is, in the comparative example, the pore-forming treatment is performed before the membrane-entering treatment. In other words, in the second stage of S2, only the stretching treatment is performed without the pore-forming treatment.

[0149] In this comparative example, since the film was treated with hot steam, the degree of solidification of the film was too high, and the stretchability decreased, resulting in tearing during the second stage stretching process, and the TD stretching treatment with a stretching ratio of 1.5 could not be performed.

[0150] Comparative Example 2

[0151] The difference between this comparative example and comparative example 1 is that in S2, the stretching ratio of the second stage is 1.2 and the processing time is 1 min.

[0152] In this comparative example, the TD stretching ratio in the second stage of S2 is reduced compared with comparative example 1, so a diaphragm can be prepared, which is recorded as sample 4.

[0153] SEM analysis of Sample 4 (Figure 4) reveals low porosity, a small number of pores, and a relatively large circular diameter, indicating a pore structure formed by steam-induced pore formation. Compared to Sample 1, due to the lack of a stretching step during the steam pore formation process, the surface non-solvent components are more abundant, tending to form a dense cortical layer, which reduces the membrane's air permeability. Furthermore, due to the inability to complete a high-ratio stretching step, the film is thicker and has lower TD tensile strength.

[0154] Comparative Example 3

[0155] ①. Comparative Example 3-1

[0156] The difference between this comparative example and Example 1 is that there is no first stage in S2, that is, the semi-solidified film is directly stretched and pores are formed.

[0157] In this comparative example, the semi-solidified film directly entered the steam stretching zone. Due to the high steam temperature, the film surface shrank and wrinkled, and tore during the stretching process, making it impossible to prepare a diaphragm.

[0158] ②Comparative Example 3-2

[0159] The difference between this comparative example and Example 1 is that in the first stage of S2, the semi-solidified film is not preheated, that is, the film-entering process is carried out at room temperature. The results are the same as those of Comparative Example 3-1.

[0160] Comparative Example 4

[0161] ①. Comparative Example 4-1

[0162] The difference between this comparative example and Example 1 is that there is no third stage in S2, that is, no film removal treatment is performed after the stretching and pore-forming treatments.

[0163] In this comparative example, the film treated with hot steam shrinks at a ratio of 0.2 immediately after leaving the clamp of the stretching equipment. The film is relatively soft and easily deformed, which has a great impact on process stability.

[0164] ②. Comparative Example 4-2

[0165] The difference between this comparative example and Example 1 is that in the third stage of S2, no water mist treatment was performed, and the results were the same as those of Comparative Example 4-1.

[0166] Comparative Example 5

[0167] The difference between this comparative example and Example 1 is that there is no first stage and third stage in S2, and in the second stage, the hot steam temperature is 40° C., the stretching ratio is 1.3, and the time is 3 minutes.

[0168] This comparative example can be used to prepare a diaphragm. Due to the low steam temperature and low stretching ratio, a dense layer with gloss is formed on the surface of the membrane, which is airtight. This diaphragm is recorded as sample 5. The scanning electron microscope image of sample 5 is shown in Figure 5, which shows that the surface of the membrane is dense and has a non-porous structure.

[0169] This indicates that when the hot steam temperature of S2 is low, the temperature difference with room temperature is small, and the preparation of the diaphragm can be completed without preheating and post-curing, but the properties of the diaphragm formed under this condition are poor.

[0170] Comparative Example 6

[0171] The difference between this comparative example and Example 1 is that in S1, the coated coating does not enter the oven, but directly enters the steam box for treatment at room temperature.

[0172] That is, after coating, the comparative example did not remove part of the solvent, but was directly pre-solidified.

[0173] After the S1 treatment, the surface layer of the coating solidified, but a large portion of the interior remained in a fluid state, resulting in the coating being unable to be peeled off from the surface of the release film and unable to enter the subsequent process.

[0174] Comparative Example 7

[0175] The difference between this comparative example and Example 1 is that in S1, the coated coating is placed in a 50° C. oven and baked for 10 minutes to completely remove the solvent; and then subsequent operations are performed.

[0176] That is, in this comparative example, all solvents were removed before pre-solidification.

[0177] After the comparative example was treated in S1, the coating was in a solidified state and was easily peeled off from the surface of the release film. When the film entered S2 for stretching, the film tore during the stretching process due to its poor ductility, and film production could not be achieved.

[0178] Comparative Example 8

[0179] The difference between this comparative example and Example 1 is that in S1, the water vapor box is removed.

[0180] That is, no pre-solidification treatment was performed.

[0181] In this comparative example, after S1 is completed, the coating still remains in a fluid state and cannot be peeled off from the surface of the release film, so the subsequent process steps cannot be entered.

[0182] Test example

[0183] The films of Examples 1-7 and Comparative Examples 2 and 5 were subjected to performance tests, and the results are shown in Table 1.

[0184] The tensile strength and elongation at break were measured in accordance with the provisions of GB / T1040.3-2006. Type 2 specimens with a width of (15 + 0.1) mm were used. The initial distance between the clamps of the MTL (PC) tensile testing machine was (100 + 5) mm. The test speed was (250 + 10) mm / min and the test was completed on the tensile testing machine.

[0185] The membrane's air permeability was measured using a Wangyan air permeability tester. For the heat shrinkage test, a 100mm x 100mm specimen was placed in a 250°C oven and timed. After 15 minutes, the specimen was removed and allowed to rest horizontally for 10 minutes. The longitudinal and transverse dimensions of the specimen were measured and calculated using the following formula: S = (L0 - L) / L0 × 100%, where S is the shrinkage rate; L0 is the specimen length before heating (in millimeters); and L is the specimen length after shrinkage (in millimeters).

[0186] The porosity is measured using a calculation method using the following formula: porosity = 1 - surface density / (thickness x membrane skeleton material density).

[0187] Table 1 Test results

[0188] As can be seen from Table 1, the film provided by the embodiments of the present disclosure has excellent mechanical properties, air permeability and thermal stability.

[0189] In summary, the porous membrane provided by the present disclosure has excellent thermal stability and mechanical strength, and good air permeability, which makes it have excellent thermal stability and is suitable as a battery membrane for preparing lithium-ion batteries to improve the safety of lithium-ion batteries under high temperature conditions.

[0190] The above are merely preferred embodiments of the present disclosure and are not intended to limit the present disclosure. Those skilled in the art will readily appreciate that various modifications and variations of the present disclosure are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present disclosure shall be included within the scope of protection of the present disclosure. Industrial Applicability

[0191] The porous separator provided by the present disclosure has good thermal stability and mechanical strength, and is suitable for use as a battery separator in the preparation of lithium-ion batteries, especially high-power lithium-ion batteries.

Claims

1. A porous membrane, characterized in that: The porous membrane is mainly made of polyamide material, the pore structure in the porous membrane is mainly elliptical or quasi-elliptical, the relationship between the major axis length a and the minor axis length b in the pore structure is a≥1.5b, and the range of a is 0.3-6μm, and the range of b is 0.1-3μm.

2. The porous membrane according to claim 1, characterized in that The porous membrane also has at least one of the following characteristics: Feature 1: The pore structure is oriented; Feature 2: The thickness of the porous membrane is 9-20 μm; Feature 3: The tensile strength of the porous diaphragm is 30-70MPa; Feature 4: The elongation at break of the porous diaphragm is 50-150%; Feature 5: The air permeability of the porous membrane is 50-180s / 100mL; Feature 6: After the porous diaphragm is left to stand at 250° C. for 15 minutes, the thermal shrinkage rates in the TD direction and the MD direction are both less than 2%; Feature 7: The porosity of the porous membrane is 42-70%; Feature 8: The main components of the porous membrane include nylon and aramid; preferably, the main components of the porous membrane include at least one of nylon 6, nylon 66, meta-aramid and para-aramid.

3. A method for preparing a porous membrane according to claim 1 or 2, characterized in that: The following steps are involved: The semi-solidified film is stretched and pore-formed simultaneously; wherein the semi-solidified film contains a polyamide material; Preferably, the semi-solidified film contains 10-30% of the polyamide material, 25-70% of the solvent and 20-40% of the non-solvent.

4. The preparation method according to claim 3, characterized in that: The preparation of the semi-solidified film comprises: coating a slurry containing a polyamide material on the surface of a substrate, removing part of the solvent, and then pre-solidifying to obtain a semi-solidified film.

5. The preparation method according to claim 4, characterized in that: The preparation of the semi-solidified film includes at least one of the following features: Feature 1: Coating thickness is 100-500μm; Feature 2: The mass percentage of the polyamide material in the slurry is 10-30%; Feature 3: The rotational viscosity of the slurry is 50000Pa·s-700000Pa·s; Feature 4: The substrate includes at least one of a release film, PP, stainless steel, PET, POM and PTFE; Feature 5: removing the part of the solvent by evaporation; preferably, the evaporation is carried out at 40-80° C. for 1-5 min; Feature 6: The removed part of the solvent accounts for 20-60wt% of the total solvent in the slurry; preferably, the solvent includes DMF, at least one of DMAC, NMP, formic acid, and acetic acid; Feature 7: Pre-solidification is to expose the coating after partial solvent removal to a non-solvent gas and / or non-solvent atomizing gas for 30-120 seconds, wherein the non-solvent includes at least one of water, ethanol, methanol, isopropanol, acetone and DMSO.

6. The preparation method according to claim 3, characterized in that: During the stretching and pore-forming process, a non-solvent atomized gas is introduced into both the upper and lower surfaces of the semi-solidified film to form a pore structure on the semi-solidified film; the non-solvent comprises at least one of water, ethanol, methanol, isopropanol, acetone and DMSO; Preferably, the temperature of the non-solvent atomizing gas is 50-80°C; Preferably, the stretching ratio is 1.2-4; Preferably, the long axis direction in the pore structure of the porous membrane is the same as the stretching direction.

7. The preparation method according to claim 6, characterized in that: Before stretching and pore forming, the semi-solidified film is subjected to film treatment; Preferably, the film processing temperature is 30-50°C, and the stretching ratio is 1-1.

2.

8. The preparation method according to claim 6, characterized in that: After stretching and pore-forming, the film after stretching and pore-forming is also subjected to film-out treatment; Preferably, during the film removal process, non-solvent atomized gas is introduced into both the upper and lower surfaces of the film after stretching and pore formation; Preferably, the temperature of the non-solvent atomizing gas is 4-15°C; Preferably, the stretching ratio during the film discharge process is 1-1.

5.

9. The preparation method according to claim 8, characterized in that: Also includes: Cleaning, drying and shaping the film after film processing; Preferably, the cleaning temperature is 25-100°C and the cleaning time is 2-10min; Preferably, the drying temperature is 100-200°C and the drying time is 2-10min; Preferably, the setting temperature is 220-270° C., and the setting time is 1-5 min.

10. A lithium ion battery, characterized in that: The porous separator comprises the porous separator according to claim 1 or 2.

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