Para-aramid composite diaphragm as well as preparation method and application thereof

Abstract

The invention provides a para-aramid composite diaphragm as well as a preparation method and application thereof, and belongs to the technical field of battery diaphragms. The para-aramid composite diaphragm comprises a base film layer with a first surface and a second surface which are opposite to each other; the first para-aramid gel layer is formed on the first surface of the base film layer; the second para-aramid gel layer is formed on the second surface of the base film layer; the first para-aramid nanofiber layer is formed on the first para-aramid gel layer; the second para-aramid nanofiber layer is formed on the second para-aramid gel layer; each of the first para-aramid nanofiber layer and the second para-aramid nanofiber layer comprises para-aramid nanofiber, a polysaccharide bonding substance and ceramic particles. The para-aramid composite diaphragm provided by the embodiment of the invention has good heat resistance, air permeability and wettability, and is less in warping; the method is suitable for lithium ion batteries and other products.

Description

Technical Field

[0001] The present invention relates to a para-aramid composite separator and its preparation method and application, belonging to the technical field of battery separators. Background Art

[0002] The separator is a key component in lithium-ion batteries, used to separate the positive and negative electrodes. Its main function is to prevent short circuits while allowing lithium ions to pass freely during charge and discharge. Traditional battery separator materials are mainly polyolefins, such as polyethylene and polypropylene. In recent years, the development of new separator materials and the optimization of separator preparation processes have become a research hotspot in the field of lithium-ion batteries. Among them, separators containing para-aramid have received increasing attention due to their excellent thermal stability, electrochemical stability, and good wettability, etc.

[0003] Para-aramid is usually a high-performance synthetic fiber with heat resistance and high chemical stability, which can be made by processes such as polycondensation of terephthaloyl chloride and p-phenylenediamine. Separators containing para-aramid are mainly made by a composite process. For example, materials such as polyolefins are used as substrates, and para-aramid solutions are coated on their surfaces to form composite separators. Currently, considering cost savings and other aspects, the preparation process of single-sided coating of para-aramid nanofibers is used more frequently. However, single-sided coating of para-aramid easily leads to large warping at the edges of the separator. Due to the stress effect, the separator will bend towards the coated side, and the thicker the coating thickness, the more serious the stress phenomenon, which is not conducive to application. At the same time, single-sided coating also results in a relatively thin para-aramid coating, making it difficult to effectively optimize the heat resistance of the separator, etc.

[0004] Therefore, it is imperative to find a suitable method to prepare high-performance separators containing para-aramid. Summary of the Invention

[0005] In view of this, the present invention provides a para-aramid composite separator and its preparation method and application. The para-aramid composite separator provided by the present invention has good heat resistance and can reduce warping, which is beneficial to applications in lithium-ion batteries and the like.

[0006] The present invention provides a para-aramid composite separator, comprising:

[0007] A base film layer having opposite first and second surfaces;

[0008] A first para-aramid gel layer formed on the first surface of the base film layer;

[0009] A second para-aramid gel layer formed on the second surface of the base film layer;

[0010] A first para-aramid nanofiber layer formed on the first para-aramid gel layer;

[0011] The second para-aramid nanofiber layer formed on the second para-aramid gel layer;

[0012] Both the first para-aramid nanofiber layer and the second para-aramid nanofiber layer contain para-aramid nanofibers, polysaccharide binder substances, and ceramic particles.

[0013] In an embodiment of the present invention, the base film layer is a polyolefin film layer, further a polypropylene film layer or a polyethylene film layer; and / or, the thickness of the base film layer is 12 - 35 μm.

[0014] In an embodiment of the present invention, the thicknesses of the first para-aramid gel layer and the second para-aramid gel layer are respectively 0.5 - 1 μm; and / or, the thicknesses of the first para-aramid nanofiber layer and the second para-aramid nanofiber layer are respectively 1 - 4 μm.

[0015] In an embodiment of the present invention, the thickness ratio of the first para-aramid gel layer to the first para-aramid nanofiber layer, and the thickness ratio of the second para-aramid gel layer to the second para-aramid nanofiber layer are both 1:1 - 8.

[0016] In an embodiment of the present invention, in the first para-aramid nanofiber layer and the second para-aramid nanofiber layer, the polysaccharide binder substance is chitosan; and / or, the ceramic particles are one or more of alumina, silica, and boehmite.

[0017] The present invention provides a method for preparing the para-aramid composite separator as described above, comprising the following steps:

[0018] S1. Coat the poly(p-phenyleneterephthalamide) solution on two surfaces of the base film respectively, and perform a phase inversion treatment to form a first para-aramid gel layer and a second para-aramid gel layer respectively;

[0019] S2. Coat the mixed coating solution on the first para-aramid gel layer and the second para-aramid gel layer respectively. The mixed coating solution contains para-aramid nanofibers, polysaccharide binder substances, and ceramic particles. After drying, a para-aramid composite separator is obtained, and the two surfaces thereof correspond to the first para-aramid nanofiber layer and the second para-aramid nanofiber layer formed by the mixed coating solution.

[0020] In an embodiment of the present invention, in step S1, the poly(p-phenyleneterephthalamide) solution is prepared by reacting p-phenylenediamine, p-phthaloyl chloride, and polyvinylpyrrolidone at low temperature in an N-methylpyrrolidone-calcium chloride solution; the temperature of the low-temperature reaction does not exceed 10°C.

[0021] In an embodiment of the present invention, in step S1, the speed of the drawing is 50 - 500 mm / min; the phase inversion treatment uses water as a solvent.

[0022] In an embodiment of the present invention, in step S2, the preparation of the mixed coating solution includes:

[0023] Dissolve poly-p-phenyleneterephthalamide in NMP, and then mix it with the fiber-forming solution. The fiber-forming solution is one or more of ethanol, acetic acid, and acetone, and non-aqueous phase - para-aramid nanofibers are obtained through washing.

[0024] Mix the non-aqueous phase - para-aramid nanofibers, polysaccharide binder, ceramic particles with NMP, and homogenize and stir to obtain a mixed coating solution.

[0025] The mass ratio of the polysaccharide binder to the non-aqueous phase - para-aramid nanofibers is 1:3 - 5, and the mass concentration of the non-aqueous phase - para-aramid nanofibers is 0.5 - 2‰; the mass ratio of the polysaccharide binder to the ceramic particles is 1:0.5 - 2; the speed of the homogenizing and stirring is 5000 - 6000 rpm.

[0026] In addition, the present invention provides the application of the para-aramid composite separator as described above in a battery.

[0027] Compared with the prior art, the para-aramid composite separator provided by the present invention is mainly a separator with double-sided coating of para-aramid. On the two surfaces of the base film, a first layer can be formed by para-aramid gel first, which can be used as a bottom layer, and then para-aramid nanofibers are used as the main component of the second layer on both sides. The second layer also contains a polysaccharide binder and a ceramic particle framework material. In the present invention, the para-aramid gel layer on each surface of the base film can form a physical and chemical combined force with it. After priming, the para-aramid nanofiber layer can be further combined respectively; and these two layers form a three-dimensional network structure through bond energy, and the structure becomes fluffy, which to a certain extent contributes to the pore structure, promotes the increase of the liquid retention rate and porosity, and then optimizes the air permeability and wettability. Moreover, the high temperature resistance of para-aramid itself increases the heat resistance of the separator, and the double-sided coating ensures the stress uniformity and reduces the occurrence of warping. Therefore, the para-aramid composite separator described in the embodiments of the present invention has good heat resistance, air permeability, and wettability, and less warping; it is suitable for applications in products such as lithium-ion batteries. Description of the Drawings

[0028] Figure 1 It is a schematic structural diagram of the para-aramid composite separator provided by the embodiment of the present invention. Detailed Embodiments

[0029] In order to more clearly understand the technical features, objectives, and effects of the present invention below, the technical solutions of the present invention will be described in detail with reference to specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0030] The terms "first", "second", etc. in the description, claims and above-mentioned drawings of this application are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances, which is only a way of distinguishing objects with the same attributes when describing the embodiments of this application. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, so that a process, method, system, product or device including a series of units does not have to be limited to those units, but may include other units not clearly listed or inherent to these processes, methods, products or devices.

[0031] The present invention provides a para-aramid composite separator, comprising:

[0032] A base film layer having opposite first and second surfaces;

[0033] A first para-aramid gel layer formed on the first surface of the base film layer;

[0034] A second para-aramid gel layer formed on the second surface of the base film layer;

[0035] A first para-aramid nanofiber layer formed on the first para-aramid gel layer;

[0036] A second para-aramid nanofiber layer formed on the second para-aramid gel layer;

[0037] Both the first para-aramid nanofiber layer and the second para-aramid nanofiber layer contain para-aramid nanofibers, polysaccharide binder substances and ceramic particles.

[0038] The para-aramid composite separator provided by the present invention has good heat resistance and can reduce warping, which is beneficial to its application in lithium-ion batteries and the like.

[0039] See Figure 1 , Figure 1 is a schematic structural diagram of the para-aramid composite separator provided by the embodiment of the present invention, wherein 00 is the base film layer, 11 is the first para-aramid gel layer, 12 is the second para-aramid gel layer, 21 is the first para-aramid nanofiber layer, and 22 is the second para-aramid nanofiber layer.

[0040] The base film layer 00 described in the embodiments of the present invention is a thin film layer with a certain porous structure, which has two opposite surfaces, namely the first surface and the second surface. The base film layer 00 is usually a polyolefin film layer, that is, a film made of polyolefin material. The thickness of the film is preferably 12 - 35 μm, such as 12 μm, 15 μm, 20 μm, 23 μm, 30 μm, 34 μm, 35 μm; it is further a polypropylene (PP) film layer or a polyethylene (PE) film layer.

[0041] In the embodiments of the present invention, the first surface of the base film layer 00 is the first para-aramid gel layer 11, and the second surface correspondingly is the second para-aramid gel layer 12. The components of these two layers are para-aramid gel, that is, gel-state poly(p-phenyleneterephthalamide) (PPTA), which has a physically and chemically combined force with the base film layer, and can be called the bottom layer or the primer layer. The thicknesses of the first para-aramid gel layer 11 and the second para-aramid gel layer 12 can be denoted as d1, and are respectively preferably 0.5 - 1 μm, such as 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, and the thicknesses of the two layers are the same; PPTA is relatively dense and easily weakens the gas permeability of the film layer, and its thickness should not be too thick.

[0042] Moreover, on the first para-aramid gel layer 11 in the embodiments of the present invention is the first para-aramid nanofiber layer 21, and on the second para-aramid gel layer 12 is the second para-aramid nanofiber layer 22; these two layers, as the second layer, both contain para-aramid nanofibers, polysaccharide binder substances, and ceramic particles. The second layer containing para-aramid nanofibers has a certain pulling effect on the bottom layer, making the separator form a certain stability, and under the pulling effect of the second layer, it maintains a certain porosity and has good gas permeability. In the present invention, the synergistic advantages of the bottom layer and the second layer are mainly reflected in gas permeability and wettability. The superposition of the two layers will increase the porosity of the separator. These two layers form a fluffy three-dimensional network structure through bond energy, which to a certain extent contributes to the pore structure and promotes the increase of the liquid retention rate and porosity; at the same time, it has good heat resistance and can also correspondingly reduce warping.

[0043] The thicknesses of the first para-aramid nanofiber layer 21 and the second para-aramid nanofiber layer 22 can be denoted as d2, and are respectively preferably 1 - 4 μm, more preferably 2 - 4 μm; the thicknesses of the two layers are the same. As a preference, the thickness ratio of the first para-aramid gel layer 11 to the first para-aramid nanofiber layer 21, and the thickness ratio of the second para-aramid gel layer 12 to the second para-aramid nanofiber layer 22 are both 1:1 - 8.

[0044] In an embodiment of the present invention, the para-aramid nanofibers are the main component of the second layer, so that the para-aramid nanofiber layer not only has good heat resistance but also has certain flame resistance. The polysaccharide binder substances are preferably all chitosan; preferably, the presence of chitosan is mainly to increase the binding force with the bottom layer. Chitosan has diverse functional groups on its surface, which can crosslink with the bottom layer to a certain extent, thereby increasing the binding force between the second layer and the bottom solution. The functional groups of chitosan and cellulose have certain similarities, but chitosan has better solubility than cellulose and can play a better binding role.

[0045] The ceramic particles can act as a skeleton to support the para-aramid nanofibers to a certain extent, which is beneficial to increasing the porosity of the membrane layer and also has certain heat resistance; preferably, they are one or more of alumina (aluminum trioxide), silica (silicon dioxide), and boehmite. Among them, boehmite belongs to the aluminum hydroxide mineral and has a certain porosity. In addition, the particle size of the ceramic particles can be 1-30 μm.

[0046] The overall thickness of the para-aramid composite separator described in the embodiment of the present invention can be 15-45 μm; the contact angle is below 30° (such as between 13-20°), with good wettability, and at the same time has optimized air permeability, heat resistance, and structural stability.

[0047] Correspondingly, the embodiment of the present invention provides a preparation method of the para-aramid composite separator as described above, including the following steps:

[0048] S1. Coat the poly(p-phenyleneterephthalamide) solution on both surfaces of the base film respectively, and perform a phase inversion treatment to form a first para-aramid gel layer and a second para-aramid gel layer respectively.

[0049] S2. Coat the mixed coating solution on the first para-aramid gel layer and the second para-aramid gel layer respectively. The mixed coating solution contains para-aramid nanofibers, polysaccharide binder substances, and ceramic particles. After drying, a para-aramid composite separator is obtained, and the two surfaces thereof correspond to the first para-aramid nanofiber layer and the second para-aramid nanofiber layer formed by the mixed coating solution.

[0050] The preparation method of the para-aramid composite separator described in the embodiment of the present invention is mainly a method for coating a para-aramid double-sided separator. The specific preparation steps of this technical solution are as follows:

[0051] (1) Synthesis of the para-aramid bottom solution for the first layer: Prepare PPTA in an N-methylpyrrolidone (NMP)-calcium chloride solution in a bottom-up manner. Preferably, polyvinylpyrrolidone (PVP), p-phenylenediamine (PPD), and terephthaloyl chloride (TPC) are used for synthesis at low temperature.

[0052] (2) Coating of the primer liquid: The primer liquid obtained in the first step is coated on both sides of the base film using a coating machine and subjected to phase inversion treatment to form primer layers (para-aramid gel layers) to a corresponding thickness d1. If the thickness cannot be reached in one application, multiple applications can be performed and the film can be dried.

[0053] (3) Preparation of para-aramid nanofibers for the second layer: The PPTA synthesized in the first step can be dissolved in NMP, then transferred to the fiber-forming solution, and washed multiple times to remove hydrogen chloride and other substances to obtain non-aqueous phase-para-aramid nanofibers;

[0054] (4) Preparation of a mixed coating liquid for the second layer: the non-aqueous phase-para-aramid nanofiber obtained in the previous step is configured to a certain concentration; at the same time, a chitosan-ceramic solution is prepared, a certain amount of chitosan and ceramic powder is dissolved in a certain amount of acetic acid, and after the dissolution is completed, it is replaced with an NMP solution to obtain a chitosan-ceramic-NMP solution, and a certain amount of the chitosan-ceramic-NMP solution is added to a certain concentration of non-aqueous phase-para-aramid nanofiber, and homogenized and stirred to obtain a mixed coating liquid for forming the second layer;

[0055] (5) Coating of the mixed coating liquid: A coating machine is used to perform double-sided coating of a certain size on the base layer to form a second layer (para-aramid nanofiber layer) and reach the corresponding thickness d2. If the thickness cannot be reached in one time, multiple coatings can be performed and a para-aramid double-sided coated diaphragm is obtained after drying.

[0056] In step (1) of the embodiment of the present invention, when PPTA is synthesized, the bottom-up method is to gradually polymerize to form poly(p-phenylene terephthalamide), wherein p-phenylenediamine (PPD) and terephthaloyl chloride (TPC) are mixed in substantially equal molar ratios. Specifically, PVP (g): TPC (mol): PPD (mol) = (0.001-0.009): (0.990-1): 1. NMP-calcium chloride is used as the solvent for the reaction system (preferably, the solubilizing salt calcium chloride: NMP = 2-8g: 100ml), which helps dissolve the monomer and polymer and improve the reaction uniformity; preferably, PPD (mol): NMP (ml) = 1: 400-500. At the same time, the temperature of the low-temperature polymerization reaction is generally 0-10°C, preferably 0°C, to minimize side reactions. The existing synthetic PPTA is extremely difficult to preserve and will be oxidized and deteriorated (about 2 hours) under certain temperature and air conditions. However, the embodiment of the present application uses PVP in the reaction system to prepare gel-state PPTA, which can be used for coating within 1 day.

[0057] Exemplarily, in an embodiment of the present invention, a certain amount of PVP is weighed and dissolved in a certain amount of NMP, where PVP (g): NMP (ml) = (0.001 - 0.009) = (400 - 500); after stirring for a certain time for dissolution, the time is about 1 - 2 h, and then a certain amount of PPD is added. After dissolution, a certain amount of TPC is added for polymerization. Preferably, PVP (g): TPC (mol): PPD (mol) = (0.001 - 0.009): (0.990 - 1): 1, the polymerization temperature is 0 °C, and a para-aramid bottom-forming solution containing gel is obtained after the reaction. The reaction time of the polymerization is about 15 min, and the reaction product is in a gel state.

[0058] In step (2) of the embodiment of the present invention, double-sided coating of the bottom-forming solution is performed to form bottom-forming layers respectively, which specifically includes: first, the bottom-forming solution obtained in the first step is coated on both sides of the base film. At this time, a coater is used for coating. The base film is a commercially available conventional PP film or PE film, and its film thickness is preferably 12 - 35 μm. The thickness of the coater used is usually 50 - 200 μm, and the coating speed can be 50 - 500 mm / min, further preferably 200 - 500 mm / min; then a phase inversion treatment is performed. The phase inversion treatment is the occurrence of phase separation, and the polymer-rich phase solidifies to form a certain porous structure, mainly solvent-induced phase inversion; specifically, water is used as the solvent in the phase inversion treatment, and the volume ratio of the para-aramid bottom-forming solution to water can be 1: (100 - 200), and the phase inversion time can be 1 - 10 min. And a drying treatment is performed; the thickness d1 of the film layer after the drying treatment is preferably 0.5 - 1 μm. If the thickness cannot be achieved at one time, multiple coatings can be performed (usually, a single coating is relatively uneven and at least 2 coatings are required).

[0059] In step (3), the PPTA synthesized in the first step can be first dissolved in NMP, where PPTA: NMP = 1 g: (100 - 200) ml, and then transferred to the fiber-forming solution. The fiber-forming solution is mainly common organic solvents such as acetic acid, ethanol, acetone, etc., and PPTA: fiber-forming solution = 1 g: (50 - 400) ml. After washing multiple times to remove hydrogen chloride and calcium chloride, the number of washing times is about 1 - 5 times, and non-aqueous phase - para-aramid nanofibers (such as NMP - para-aramid nanofibers, and the nanofiber size is between 1 - 60 nanometers) are obtained. The PPTA solution in the embodiment of the present invention is precipitated by other solvents, and under high-speed shearing conditions, para-aramid nanofibers (ANFs) are directly obtained, which is convenient to prepare and has a low cost.

[0060] In step (4), the non-aqueous phase - para-aramid nanofibers obtained in the previous step are configured at a certain concentration, and the concentration is preferably (0.5 - 2)‰, more preferably 1 - 2‰. Meanwhile, a chitosan - ceramic solution is prepared. Chitosan is a natural polysaccharide, generally obtained by deacetylation reaction of chitin, and its particle size can be 80 - 100 mesh; the ceramic is one or more of common powders such as alumina, silica, and boehmite. Chitosan and ceramic powders can be dissolved in acetic acid, with a mass ratio of chitosan: ceramic = 1:(0.5 - 2), and a ratio of chitosan: acetic acid = 1 g:(200 - 400) ml. After complete dissolution, it is subjected to NMP solution replacement, with a volume ratio of acetic acid: NMP = 1:1 - 3, and the replacement is carried out about 2 - 5 times to obtain a chitosan - ceramic - NMP solution. Finally, the chitosan - ceramic - NMP solution is added to the non-aqueous phase - para-aramid nanofibers at a certain concentration, with a mass ratio of chitosan: non-aqueous phase - para-aramid nanofibers = 1:(3 - 5), and homogenously stirred at a speed preferably of 5000 - 6000 rpm to obtain a mixed coating solution.

[0061] In step (5), the coating machine can be continuously used for coating. On the basis of the first layer, double-sided coating of the mixed coating solution is carried out to respectively form a para-aramid nanofiber layer, which is the second layer, that is, the surface layer of the separator. In the embodiment of the present invention, PPTA is used as the base layer and ANFs are further coated. Under the action of amide bonds, PPTA and ANFs can generate a certain force between the two layers, achieving a certain total coating thickness and being relatively uniform. Moreover, the double-sided coating reduces the coating stress of the overall film layer. The thickness used by the coating machine can be 50 - 200 μm, and the coating speed can be 50 - 500 mm / min; preferably, d2 is about 1 - 4 μm, and d1:d2 = 1:(1 - 8). If the thickness cannot be achieved in one time, multiple coatings can be carried out, and then through conventional drying (such as 50 - 60 °C), the para-aramid double-sided coated separator (i.e., para-aramid composite separator) is obtained.

[0062] In the preparation method of the embodiment of the present invention, first, the para-aramid gel of the first layer is used as the bottom layer, and in the second step, the para-aramid nanofiber main body is used as the second layer, and two layers are respectively coated on the two surfaces of the base film in sequence (the total thickness of the separator is the thickness of the separator + the thickness of the coating); the composite separator obtained thereby has good heat resistance, air permeability, and wettability, and less warping.

[0063] In addition, the present invention also provides the application of the para-aramid composite separator as described above in a battery, which can be used as a separator component in a lithium-ion battery to assemble a corresponding battery device.

[0064] To better illustrate the present invention, further examples are given below by way of embodiments. In the embodiments, all the original reagents and materials are commercially available, and the experimental methods without specific experimental conditions are conventional methods and conditions well known in the art.

[0065] The main reagents used in the following examples: Para-aramid nanofibers and PPTA were synthesized and prepared by ourselves; the PE-based film was purchased from Yihang (Quanzhou) Optical Materials Co., Ltd., with specifications of 12μm and 35μm; chitosan was purchased from Shaanxi Shize Tiancheng Biotechnology Co., Ltd., with a particle size of 80-100 mesh; alumina was purchased from Zhejiang Zhitaina Micro New Materials Co., Ltd., product number 8559647, with a particle size of 1-30μm; TPC and PPD were both purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., and PVP was purchased from Shanghai Macklin Biochemical Technology Co., Ltd., product number P816205-25g, with a molecular weight of 58000.

[0066] Example 1

[0067] (1) Synthesis of para-aramid bottom coating solution: The synthesis of PPTA was carried out in a bottom-up manner. Weighed 0.001g of PVP and 2g of calcium chloride and dissolved them in 100ml of NMP. After stirring for 1h, they were dissolved. Subsequently, 1mol of PPD was added. After dissolution, 0.99mol of TPC was added for polymerization. The synthesis temperature was 0°C, and a para-aramid gel solution was obtained. The polymerization time was about 15-20min; the reaction ended when the climbing rod did not stick to the wall; the product mass was about 2g.

[0068] (2) Coating of the bottom coating solution: First, the bottom coating solution obtained in the first step was coated on both sides of the base film. At this time, a coater was used for coating. The base film was a commercial PE film with a film thickness of 12μm, and the thickness used by the coater was 200μm, and the coating speed was 500mm / min; subsequently, a phase inversion treatment was carried out. The phase inversion used an aqueous solvent with a water volume of 400ml and a time of 1min, and a drying treatment was carried out. The film layer thickness d1 after the phase inversion treatment and drying was 0.5μm.

[0069] (3) Preparation of para-aramid nanofibers: First, all the reaction products of the PPTA synthesized in the first step were dissolved in 800ml of NMP, and then transferred to a fiber-forming solution. The fiber-forming solution was 800ml of acetone. After washing with NMP three times to remove hydrogen chloride and calcium chloride, NMP-para-aramid nanofibers were obtained, and the length of the nanofibers was 1-60 nanometers.

[0070] (4) Preparation of the mixed coating solution: Prepare 2‰ of NMP - para - aramid nanofibers obtained in the previous step (prepared based on pure para - aramid, the same in the following examples). At the same time, prepare a chitosan - alumina solution. Dissolve 1 g of chitosan and 1 g of alumina in 200 ml of acetic acid. After complete dissolution, replace it with 200 ml of NMP solution about 2 times to obtain a chitosan - alumina - NMP solution. Add the chitosan - alumina - NMP solution to 2‰ of NMP - para - aramid nanofibers and perform homogenizing stirring at a speed of 5000 rpm to obtain the mixed coating solution.

[0071] (5) Coating of the mixed coating solution: Continue to use a coater for coating. Perform double - sided coating on the basis of the first layer (bottom - building layer - para - aramid gel layer). The thickness used by the coater is 200 μm, the coating speed is 500 mm / min, and the coating is performed 4 times to obtain para - aramid nanofiber layers with d2 both being 4 μm. After drying, a para - aramid double - sided coated separator is obtained.

[0072] Example 2

[0073] (1) Synthesis of the para - aramid bottom - building solution: Synthesize PPTA in a bottom - up manner. Weigh 0.005 g of PVP and 4 g of calcium chloride and dissolve them in 100 ml of NMP. Stir for 2 h until dissolved. Then add 1 mol of PPD. After dissolution, add 1 mol of TPC for polymerization at a synthesis temperature of 0 °C to obtain a para - aramid gel solution.

[0074] (2) Coating of the bottom - building solution: First, coat the bottom - building solution obtained in the first step on both sides of the base film. At this time, use a coater for coating. The base film is a commercially available PE film with a film thickness of 35 μm. The thickness used by the coater is 100 μm, and the coating speed is 300 mm / min. Subsequently, perform a phase - inversion treatment. The phase - inversion uses an aqueous solvent with a water volume of 500 ml and a time of 5 min, and then perform a drying treatment. The thickness d1 of the film layer after the phase - inversion treatment and drying is 0.8 μm.

[0075] (3) Preparation of para - aramid nanofibers: First, dissolve the PPTA synthesized in the first step in 800 ml of NMP, and then transfer it to a fiber - forming solution. The fiber - forming solution is 800 ml of acetic acid. Wash it 3 times with NMP to remove hydrogen chloride and calcium chloride to obtain NMP - para - aramid nanofibers.

[0076] (4) Preparation of the mixed coating solution: Prepare 1‰ of the NMP - para - aramid nanofibers obtained in the previous step. At the same time, prepare a chitosan - boehmite solution by dissolving 1 g of chitosan and 1.5 g of boehmite in 200 ml of acetic acid. After complete dissolution, replace it with 300 ml of NMP solution about 2 times to obtain a chitosan - boehmite - NMP solution. Add the chitosan - boehmite - NMP solution to 1‰ of the NMP - para - aramid nanofibers and carry out homogeneous stirring at a speed of 6000 rpm to obtain the mixed coating solution.

[0077] (5) Coating of the mixed coating solution: Continue to use a coater for coating. Based on the first layer (bottom - building layer - para - aramid gel layer), perform double - sided coating. The thickness used by the coater is 100 μm, the coating speed is 300 mm / min, and the coating is carried out 4 times to obtain para - aramid nanofiber layers with d2 both being 3 μm. After drying, a para - aramid double - sided coated separator is obtained.

[0078] Comparative Example 1

[0079] (1) Synthesis of the para - aramid bottom - building solution: Synthesize PPTA in a bottom - up manner. Weigh 0.001 g of PVP and 2 g of calcium chloride and dissolve them in 100 ml of NMP. After stirring for 1 h to dissolve, then add 1 mol of PPD. After dissolution, add 0.99 mol of TPC for polymerization at a synthesis temperature of 0 °C to obtain a para - aramid gel solution.

[0080] (2) Coating of the bottom - building solution: First, coat the bottom - building solution obtained in the first step on both sides of the base film. At this time, use a coater for coating. The base film uses a commercially available PE film with a film thickness of 12 μm, and the thickness used by the coater is 200 μm, and the coating speed is 500 mm / min. Subsequently, carry out a phase - inversion treatment. The phase - inversion uses an aqueous solvent with a water volume of 400 ml and a time of 1 min, and then carry out a drying treatment to obtain a para - aramid double - sided coated separator, and the film layer thickness d1 after the phase - inversion treatment and drying is both 0.5 μm.

[0081] Comparative Example 2

[0082] (1) Preparation of para - aramid nanofibers: Synthesize PPTA in a bottom - up manner. Weigh 0.005 g of PVP and 4 g of calcium chloride and dissolve them in 100 ml of NMP. After stirring for 2 h to dissolve, then add 1 mol of PPD. After dissolution, add 1 mol of TPC for polymerization at a synthesis temperature of 0 °C. First, dissolve the synthesized PPTA in 800 ml of NMP, and then transfer it to a fiber - forming solution, which is 800 ml of acetic acid. Wash it 3 times with NMP to remove hydrogen chloride and calcium chloride to obtain NMP - para - aramid nanofibers.

[0083] (2) Preparation of coating solution: Prepare 1‰ of the NMP - para - aramid nanofibers obtained in the previous step. Meanwhile, prepare a chitosan - boehmite solution by dissolving 1 g of chitosan and 1.5 g of boehmite in 200 ml of acetic acid. After complete dissolution, replace it with 300 ml of NMP solution about 2 times to obtain a chitosan - boehmite - NMP solution. Add the chitosan - boehmite - NMP solution to 1‰ of the NMP - para - aramid nanofibers and conduct homogenizing stirring at a speed of 6000 rpm to obtain the coating solution.

[0084] (5) Coating of the coating solution: Use a coater for coating. The base film is a commercially available PE film with a film thickness of 35 μm. Conduct double - sided coating. The thickness used by the coater is 100 μm, the coating speed is 300 mm / min, and coat 4 times to obtain a d2 of 3 μm. After drying, obtain a para - aramid double - sided coated separator.

[0085] Perform conventional performance tests on the separators of the above examples and comparative examples, and the results are as follows.

[0086] Table 1 Separator performance

[0087]

[0088] Judging from the data of the examples and counter - examples (examples and comparative examples), the superposition of two layers will increase the porosity of the separator. The two layers form a three - dimensional network structure through bond energy, and the structure becomes fluffy, which to a certain extent contributes to the pore structure, promotes the increase of the liquid retention rate and porosity, and then optimizes the air permeability (the air permeability and pore size are opposite) and wettability. Moreover, the high - temperature resistance of para - aramid itself increases the heat resistance of the separator; and double - sided coating ensures the stress uniformity, and it can be observed that the occurrence of warping is reduced.

[0089] The above - described examples are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary engineering and technical personnel in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A para-aramid composite separator, characterized in that, Comprising: A base film layer having opposite first and second surfaces; A first para-aramid gel layer formed on the first surface of the base film layer; A second para-aramid gel layer formed on the second surface of the base film layer; A first para-aramid nanofiber layer formed on the first para-aramid gel layer; A second para-aramid nanofiber layer formed on the second para-aramid gel layer; Both the first para-aramid nanofiber layer and the second para-aramid nanofiber layer contain para-aramid nanofibers, polysaccharide binder substances, and ceramic particles.

2. The para-aramid composite separator according to claim 1, wherein The base film layer is a polyolefin film layer, further a polypropylene film layer or a polyethylene film layer; and / or, the thickness of the base film layer is 12 - 35 μm.

3. The para-aramid composite separator according to claim 1, wherein The thicknesses of the first para-aramid gel layer and the second para-aramid gel layer are respectively 0.5 - 1 μm; and / or, the thicknesses of the first para-aramid nanofiber layer and the second para-aramid nanofiber layer are respectively 1 - 4 μm.

4. The para-aramid composite separator according to any one of claims 1-3, characterized in that The thickness ratios of the first para-aramid gel layer to the first para-aramid nanofiber layer, and the second para-aramid gel layer to the second para-aramid nanofiber layer are both 1:1 - 8.

5. The para-aramid composite separator according to claim 4, wherein In the first para-aramid nanofiber layer and the second para-aramid nanofiber layer, the polysaccharide binder substance is chitosan; and / or, the ceramic particles are respectively one or more of alumina, silica, and boehmite.

6. A method for preparing a para-aramid composite separator as described in any one of claims 1-5, characterized in that, Including the following steps: S1. Coat a poly(p-phenyleneterephthalamide) solution on the two surfaces of the base film respectively, and perform a phase inversion treatment to form a first para-aramid gel layer and a second para-aramid gel layer respectively; S2. Coat the mixed coating solution on the first para-aramid gel layer and the second para-aramid gel layer respectively. The mixed coating solution contains para-aramid nanofibers, polysaccharide binder substances, and ceramic particles. After drying, a para-aramid composite separator is obtained, and the two surfaces thereof correspond to the first para-aramid nanofiber layer and the second para-aramid nanofiber layer formed by the mixed coating solution.

7. The preparation method according to claim 6, characterized in that, In step S1, the poly(p-phenyleneterephthalamide) solution is prepared by reacting p-phenylenediamine, p-phthaloyl chloride, and polyvinylpyrrolidone at low temperature in an N-methylpyrrolidone-calcium chloride solution; the temperature of the low-temperature reaction does not exceed 10°C.

8. The preparation method according to claim 6, characterized in that, In step S1, the speed of the coating is 50 - 500 mm / min; the phase inversion treatment uses water as a solvent.

9. The preparation method according to claim 6, characterized in that, In step S2, the preparation of the mixed coating solution includes: Dissolve poly(p-phenyleneterephthalamide) in NMP, and then mix it with a fiber-forming solution. The fiber-forming solution is one or more of ethanol, acetic acid, and acetone. After washing, a non-aqueous phase - para-aramid nanofiber is obtained; Mix the non-aqueous phase - para-aramid nanofiber, polysaccharide binder substance, ceramic particles with NMP, and homogenize and stir to obtain a mixed coating solution; The mass ratio of the polysaccharide binder substance to the non-aqueous phase - para-aramid nanofiber is 1:3 - 5, the mass concentration of the non-aqueous phase - para-aramid nanofiber is 0.5 - 2‰; the mass ratio of the polysaccharide binder substance to the ceramic particles is 1:0.5 - 2; the speed of the homogenizing and stirring is 5000 - 6000 rpm.

10. Application of the para-aramid composite separator according to any one of claims 1-5 in a battery.

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