Copolymerized para-aramid coating liquid as well as preparation method and application thereof

Abstract

The invention relates to the field of lithium ion batteries, in particular to copolymerized para-aramid coating liquid as well as a preparation method and application thereof, by introducing a naphthalene ring structural unit and a stabilizer in a polymerization process, the solubility of para-aramid is improved, self-aggregation among molecules is prevented, the stability of the coating liquid is improved, and the coating liquid can be stored for a long time. The controllability of the polymerization reaction can be remarkably improved by a diamine monomer segmented feeding method, and the thermal stability and solubility of the copolymerized para-aramid fiber are improved. The combination of the inorganic nanofibers and the para-aramid nanofibers is adopted as filler in the coating liquid, a fiber network structure is provided, the internal stress of the coating film during phase change of the polymerization stock solution is relieved, the problem of edge curling of the diaphragm is avoided, and the problem that the bonding force is poor due to direct coating of the inorganic nanofibers and the para-aramid nanofibers is solved. After the copolymerized para-aramid fiber coating layer is added, the heat resistance of the diaphragm is obviously improved, the longitudinal thermal shrinkage at 180 DEG C for 1 hour is less than or equal to 3%, the transverse thermal shrinkage is less than or equal to 2%, and the safety of the lithium battery is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a copoly-paraphenylene aramid coating solution, a preparation method thereof, and an application thereof. Background Art

[0002] The separator of a lithium-ion battery is a core component in the battery. Its main function is to isolate the positive and negative electrodes to prevent short circuits, and at the same time allow lithium ions to pass through freely to achieve battery charging and discharging. With the expansion of the application of lithium-ion batteries in the new energy field, the requirements for the performance of the separator are also increasing day by day. At present, polyethylene (PE) and polypropylene (PP) microporous membranes and their composite materials, as the mainstream separator materials, have problems of insufficient thermal stability and relatively low mechanical strength. When the local temperature inside the battery rises or it suffers external impacts, the separator may undergo rapid thermal shrinkage and breakdown, resulting in the loss of the isolation function of the separator, short circuit between the positive and negative electrodes causing a violent electrochemical reaction, and ultimately leading to the thermal runaway phenomenon of the battery. Therefore, researchers are actively exploring methods to improve the heat resistance and mechanical strength of the separator to meet the requirements of high performance and high safety of lithium-ion batteries.

[0003] Patent CN 119176937 A discloses a modified aramid separator coating slurry, a battery separator, and a preparation method thereof. In a solvent-cosolvent mixed system, p-phenylenediamine is first added, and then p-phthaloyl chloride is added in batches for a polymerization reaction to obtain the modified aramid separator coating slurry. However, the amino group is in an excessive state during the prepolymerization process, forming an amino-terminated oligomer (prepolymerization product). At the same time, there is a delay effect in the addition of acyl chloride, and it is necessary to redisperse it into the high-viscosity system of the amino-terminated oligomer, making it difficult to avoid secondary microzone explosion polymerization. The amino-terminated oligomer has relatively low reactivity of the amino group in the subsequent reaction, and a higher reaction temperature or a longer reaction time is required to ensure complete reaction. Moreover, the amino group is prone to side reactions under certain conditions, such as oxidation, etc., which may affect the stability of the prepolymer and the performance of the final product. Patent CN 119161572A proposes to perform precondensation, block copolymerization, and neutralization treatment on the polymerization solution to obtain a modified para-aramid polymerization solution. However, the added p-phenylenediamine and p-phthaloyl chloride also have a delay effect, and there is a risk of inactivation of the amino-terminated oligomer. Terminal oxidation / hydrolysis may occur during the waiting period for the addition of acyl chloride, reducing the subsequent reaction efficiency.

[0004] Para-aramid, namely poly(p-phenyleneterephthalamide) (PPTA), has important application value in the field of lithium battery separator coating due to its excellent heat resistance, mechanical strength and chemical stability. However, as a liquid crystal polymer, PPTA is difficult to dissolve in organic solvents. The polymerized para-aramid stock solution can only stably exist in air for a few hours, and then it will precipitate and lose its coating property, which limits its industrial application. Patent CN 112694610B successfully improved the solubility of PPTA and solved the long-term storage problem of the polymerized stock solution by introducing ether bonds during the PPTA polymerization process. However, this improvement comes at a cost, because the introduction of ether bonds increases the flexibility of the molecular chain, which to a certain extent reduces the rigidity of PPTA and thus affects its heat resistance. In view of this, the research focus has shifted to the development of a new type of para-aramid coating solution that can improve the solubility of its polymerized stock solution without sacrificing the thermal stability of PPTA.

[0005] Para-aramid nanofibers (ANFs) inherit the excellent properties of aramid fibers and possess the characteristics of nanomaterials, so they have become a research hotspot in the existing technology. However, the adhesion between the para-aramid nanofiber coating layer and the base film is poor and it is easy to shed slag. Although adding polyvinylidene fluoride can increase the binding force, it will reduce the heat resistance of the coated separator. Therefore, it is still necessary to improve the binding force between para-aramid nanofibers and the base film while maintaining its heat resistance. Summary of the Invention

[0006] The present invention is committed to overcoming the problems of the performance of lithium battery separators and significantly improving the stability and safety of separators through innovative methods. Specifically, a copolymerized para-aramid coating solution, its preparation method and application are provided. During the para-aramid polymerization process, a monomer containing a naphthalene ring and a stabilizer are introduced, and long-term stable storage of the polymerized stock solution is achieved without sacrificing the thermal stability of the polymer, thus solving the storage problem of the product at one stroke. In addition, the present invention uses a composition of inorganic nanofibers and oily para-aramid nanofibers as fillers to construct a fiber network structure, which effectively slows down the internal stress during the liquid phase transition of the polymerized stock solution and avoids the problem of separator curling. At the same time, the fixing and bonding effect of the polymerized stock solution on the inorganic nanofibers and oily para-aramid nanofibers further reduces the thermal shrinkage rate of the coated separator. In the separator coating process, the stabilizer introduced into the coating solution is water-soluble. By utilizing its dissolution and migration characteristics during the subsequent water washing process, this component can be removed directionally, thereby forming a uniform porous structure on the separator matrix, increasing the air permeability of the separator, and significantly improving the overall performance of the separator.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A preparation method of a copolymerized para-aramid coating solution, the specific steps are as follows: (1) Under the protection of inert gas, inorganic nanofibers are added to a reaction solvent and subjected to ultrasonic dispersion treatment to obtain a reaction solvent with uniformly dispersed inorganic nanofibers. The mass fraction of inorganic nanofibers in the obtained reaction solvent is 0.2% - 3%. (2) The reaction solvent containing inorganic nanofibers obtained in step (1) is added to a reaction kettle, and a cosolvent and a stabilizer are added. After heating to 60°C - 80°C and stirring to dissolve, the temperature is lowered to -10°C - 5°C. Then, a diamine monomer is added to make the molar concentration of the diamine monomer in the reaction solvent reach 0.1 mol / L - 0.6 mol / L. The temperature of the system is controlled at -10°C - 5°C and stirred to dissolve, and then it is evenly divided into two parts, denoted as mixed solvents A1 and A2 respectively. (3) First-stage polymerization: According to the molar ratio of the dicarbonyl chloride monomer to the diamine monomer in step (2) being (0.98 - 1.05):1, the dicarbonyl chloride monomer is taken and added to mixed solvent A1 for copolycondensation reaction to obtain a prepolymerization stock solution C. The prepolymerization reaction time is 2 - 15 min. The dicarbonyl chloride monomer is composed of terephthaloyl chloride and a naphthalene ring-containing dicarbonyl chloride monomer. (4) Second-stage polymerization: Mixed solvent A2 is gradually added dropwise to the prepolymerization stock solution C obtained in step (3), and stirring is carried out for second-stage polycondensation reaction to obtain a copolymerization stock solution. The reaction time is 10 min - 30 min. (5) An alkaline substance is added to the copolymerization stock solution obtained in step (4) to neutralize the by-product hydrogen chloride generated during the copolymerization process. (6) An oily para-aramid nanofiber dispersion liquid is taken and added to the copolymerization stock solution treated in step (5) for uniform mixing. The mixed liquid is placed in a vacuum tank for degassing treatment to obtain a copolymerization para-aramid coating liquid for diaphragm coating.

[0008] Further, the inorganic nanofibers in step (1) are selected from one or more of alumina nanofibers, silica nanofibers, and titanium oxide nanofibers. The diameter of the inorganic nanofibers is 4 nm - 50 nm, and the length is 200 nm - 10 μm. The reaction solvent is selected from one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylacetamide, and N,N-dimethylformamide. The ultrasonic dispersion treatment time is 30 min - 90 min.

[0009] The cosolvent described in step (2) is calcium chloride or lithium chloride, and the addition ratio is 5% to 10% of the mass of the reaction solvent; the stabilizer is one or more of polyvinylpyrrolidone, polyethylene glycol, poly(N-vinylcaprolactam), and polyacrylamide; the addition ratio of the stabilizer is 0.5% to 6% of the mass of the reaction solvent; the heating and cooling are achieved by using the low-temperature circulating liquid and high-temperature circulating liquid provided by a circulating cooler. The inventor introduced a stabilizer as an auxiliary during the polymerization process, which isolates the alignment units by forming hydrogen bonds with PPTA molecules and controls the self-aggregation between PPTA molecules.

[0010] The diamine monomer is composed of p-phenylenediamine and a naphthalene ring-containing diamine monomer, and the molar ratio of p-phenylenediamine to the naphthalene ring-containing diamine monomer is (0.7 to 1.0):(0.05 to 0.3); wherein the naphthalene ring-containing diamine monomer is selected from one or more of 2,6-naphthalenediamine, 1,8-naphthalenediamine, 2,3-naphthalenediamine, and 1,5-naphthalenediamine; the stirring method is mechanical stirring, and the stirring speed is 100 r / min to 800 r / min; the inventor introduced a parallel-axis naphthalene ring structural unit during the polymerization process, which causes multiple turns in the molecular chain axis, reduces the coaxiality, partially changes the regularity of the molecular chain, and weakens the hydrogen bonds between molecules, thereby improving the solubility of PPTA.

[0011] The dicarbonyl chloride monomer described in step (3) is composed of terephthaloyl chloride and 2,6-naphthaloyl chloride; the molar ratio of terephthaloyl chloride to 2,6-naphthaloyl chloride is (0.8 to 1.0):(0.05 to 0.2); the initial reaction temperature of the first-stage polymerization is -10°C to 10°C; the polymerization is carried out in a way that the diamine monomer is fed in batches to obtain an oligomer capped with an acyl chloride group, aiming to reduce the explosion polymerization phenomenon in the initial stage of polymerization, control the heat release rate of the reaction system, give the reaction system sufficient heat removal time, and reduce the generation of side reactions.

[0012] The time for gradually dropping the mixed solvent A2 in step (4) is 5 s to 30 s; the initial reaction temperature of the second-stage polymerization is -10°C to 15°C; the stirring speed is 600 r / min to 1800 r / min; the inherent viscosity of the obtained copoly(p-phenylene terephthalamide) (hereinafter simply referred to as copoly PPTA) is 0.8 dL / g to 2.8 dL / g.

[0013] During the two-stage polymerization process, the mixed solvent A2 has previously fully dissolved the remaining diamine monomers to form a homogeneous solution, and is precisely mixed with the prepolymer stock solution C at a volume ratio close to 1:1. This enables the diffusion rates of the diamine monomer solution and the oligomer terminated with an acyl chloride group to match during mixing, thereby eliminating the delay effect of mixing caused by the lag in the dissolution of solid reagents during the traditional secondary feeding process. Moreover, through the pre-constructed equal-volume liquid-liquid mixing system described above, the risk of explosive polymerization caused by an instantaneously excessive monomer concentration in the local micro-reaction zone during the direct addition of solid materials is avoided. By coordinating the prepolymerization control in the first-stage polymerization and the chain growth in the second-stage polymerization, the reaction heat is released in stages and quasi-steady-state conditions are maintained to ensure the orderly growth of molecular chains, ultimately achieving the controllability of the polymerization reaction and the uniformity of the product molecular weight distribution.

[0014] The basic substance described in step (5) is one of calcium hydroxide and calcium oxide; the molar ratio of the basic substance to the dicarbonyl chloride monomer is (1.0~1.1):1.0; stirring can be carried out during the neutralization process to promote the reaction, and the stirring speed is 600r / min~1800r / min.

[0015] The solid content of the oily para-aramid nanofiber dispersion liquid described in step (6) is 2%~4%, that is, the percentage of the dry fiber mass in the total mass of the dispersion liquid; the solvent system is the same as the solvent used in step (1); the morphological parameters of the oily para-aramid nanofibers are: diameter 10nm~100nm, length 5μm~90μm; the water content of the oily para-aramid nanofiber dispersion liquid <200ppm. This is because the copoly para-aramid stock solution undergoes phase separation and film formation when encountering water, resulting in the loss of coating function. Therefore, an oily nanofiber system with strictly controlled water is selected; based on the dry fiber mass of the oily para-aramid nanofibers, its addition amount in the coating liquid is 0.1%~2% of the total mass of the coating liquid; it should be noted that the defoaming process is implemented using a vacuum dryer, controlling the vacuum degree at -0.08MPa~-0.1MPa, and the treatment time is 3min~30min. The termination standard is to confirm that there are no visible tiny bubbles in the system through visual inspection.

[0016] The main active components of the copoly para-aramid coating liquid prepared by the above scheme, calculated by mass fraction, include: copoly para-aramid 1.7%~9.1%, stabilizer 0.3%~4.3%, inorganic nanofibers 0.1%~2.2%, para-aramid nanofibers 0.1%~2%; the rotational viscosity of the copoly aramid coating liquid is 2000mPa.s~80000mPa.s.

[0017] The above solution combines inorganic nanofibers and para-aramid nanofibers as fillers to provide a fiber network structure in the coating solution, which slows down the internal stress in the coating film during the phase transition of the polymerization stock solution and avoids the curling problem during the phase transition and drying of the separator coating; the polymerization stock solution plays an adhesive and fixing role for the inorganic nanofibers and para-aramid nanofibers.

[0018] Furthermore, the present invention also claims the application of the copolyparaphenylene terephthalamide coating solution prepared by the above method in the preparation of lithium battery separators. The copolyparaphenylene terephthalamide coating solution obtained above is uniformly coated on one side or both sides of the substrate, and the coating film is preliminarily phase-transformed in high-temperature and high-humidity air, and then left standing in a coagulation bath, washed with water and dried to obtain a lithium-ion battery separator coated with copolyparaphenylene terephthalamide.

[0019] Preferably, the coating substrate is any one of a polyethylene microporous membrane, a polypropylene microporous membrane, and a polypropylene / polyethylene / polypropylene three-layer composite microporous membrane.

[0020] Preferably, the coating method is one of doctor blade coating and slot die coating.

[0021] Preferably, the humidity of the high-humidity air purge is controlled at 60% - 90%RH, the temperature is 50°C - 80°C, and the residence time is 60s - 300s.

[0022] Preferably, the coagulation bath is a mixed solution of N-methylpyrrolidone and water with a volume ratio of (0.1 - 0.3):(0.7 - 0.9), and it is left standing in the coagulation bath for 10min - 20min; then it is subjected to overflow rinsing in a water tank for 60s - 300s.

[0023] Preferably, the drying device is one of an oven or a computerized coater; the drying temperature is 50°C - 80°C, and the drying time is 10min - 30min. The thickness of the copolyparaphenylene terephthalamide coating layer after drying is 1μm - 8μm.

[0024] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention first provides a copolyparaphenylene terephthalamide coating solution. During the polymerization process, a parallel-axis naphthalene ring structural unit (i.e., a diamine monomer containing a naphthalene ring and a naphthalene dicarbonyl chloride monomer containing a naphthalene ring) is introduced, which causes multiple turns in the molecular chain axis, the coaxiality decreases, and part of the change in the molecular chain regularity weakens the intermolecular hydrogen bond and improves the solubility of PPTA; a stabilizer is introduced as an auxiliary during the polymerization process, which isolates the alignment units by forming hydrogen bonds with PPTA molecules and controls the self-aggregation between PPTA molecules. Compared with the problem that pure PPTA polymers are difficult to dissolve and not easy to store in polar solvents, the copolyparaphenylene terephthalamide coating solution obtained in the present invention has good stability, can be stored for a long time without precipitation, and is conducive to industrial production.

[0025] The polymerization method adopts a segmented feeding strategy for diamine monomers, decomposing the polymerization process into a prepolymerization stage (first-stage polymerization) and a chain growth stage (second-stage polymerization). By using the prepolymerized acyl chloride-terminated oligomer as a reaction intermediate, the initial reaction activity is significantly reduced, enabling the dynamic matching of the heat release rate of the system and the heat removal capacity, and fundamentally suppressing the tendency of explosion polymerization. In the second-stage polymerization, the remaining diamine monomers are pre-dissolved in the mixed solvent A2 to form a homogeneous solution, which is precisely mixed with the prepolymerized stock solution C at a volume ratio of 1:1. By constructing an equal-volume diffusion system, the molecular mass transfer rate between the monomers and the prepolymers is coordinated, completely eliminating the dissolution lag caused by solid feeding and the instantaneous concentration gradient in the local micro-reaction zone. The first-stage polymerization restricts the instantaneous release of reaction heat through prepolymerization degree regulation, and the second-stage polymerization completes the directional growth of molecular chains under quasi-steady-state conditions. The synergistic effect of the two stages breaks through the heat accumulation bottleneck of traditional single-pass polymerization, ultimately achieving the controllability of the polymerization reaction and the uniformity of the product molecular weight distribution.

[0026] (3) During the copolymerization process, a third monomer containing a naphthalene ring is introduced. Due to the conjugated structure of the naphthalene ring further enhancing the molecular stability, the heat resistance of the obtained copolymerized PPTA is superior to that of pure para-aramid. Compared with introducing ether bonds to improve the solubility of PPTA at the expense of thermal stability, the present invention improves the solubility of PPTA on the basis of further enhancing the thermal stability of PPTA. In the present invention, the naphthalene ring exists in both the diamine monomer and the acyl chloride monomer. The synergistic introduction of the naphthalene ring in the diamine and acyl chloride makes the distribution of the naphthalene ring in the polymer main chain more uniform. This uniformity can avoid local structural defects and improve the mechanical properties and thermal stability of the material.

[0027] (4) The combination of inorganic nanofibers and para-aramid nanofibers is used as a filler in the coating solution to provide a fiber network structure, reducing the internal stress in the coating film during the phase transition of the polymerization stock solution and avoiding the curling problem during the phase transition and drying of the separator coating; the polymerization stock solution plays an adhesive and fixing role for the inorganic nanofibers and para-aramid nanofibers, solving the problem of insufficient adhesion when directly coating the inorganic nanofibers and para-aramid nanofibers.

[0028] (5) By adding a copolymerized para-aramid coating layer, the heat resistance of the separator is significantly improved. The longitudinal thermal shrinkage at 180 °C @ 1 h ≤ 3%, and the transverse thermal shrinkage ≤ 2%, greatly improving the safety of lithium batteries. Description of the Drawings

[0029] Figure 1 It is a thermogravimetric analysis test chart of copolymerized PPTA in Example 1 and polymerized PPTA in Comparative Example 1; Figure 2 It is a curve graph of the change of the reaction system temperature T and the reaction time t during the first-stage polymerization and the second-stage polymerization in Example 1; Figure 3It is a graph showing the changes in the reaction system temperature T and reaction time t during the polymerization process in Comparative Example 1. Detailed implementation mode

[0030] The technical solutions of the present invention will be further described below in conjunction with specific embodiments. These embodiments are only used to describe the technical solutions of the present invention in more detail and should not be construed as limiting the protection scope of the present invention.

[0031] Example 1 A preparation method of a copolymerized para-aramid coating solution, the specific steps are as follows: (1) Under the protection of inert gas, weigh 0.6 g of alumina nanofibers and add them to 100 mL of N-methylpyrrolidone solvent. Use a high-power probe-type ultrasonic instrument with a power setting of 960 W and perform ultrasonic dispersion treatment for 80 min to obtain a uniformly dispersed alumina nanofiber NMP solution as a para-aramid polymerization solvent for standby; the diameter of the used alumina nanofibers is 4 nm - 8 nm, and the length is 200 - 300 nm.

[0032] (2) Take 100 mL of the reaction solvent containing alumina nanofibers obtained in step (1) and add it to the reaction kettle. Add 8 g of calcium chloride and 1.6 g of stabilizer polyvinylpyrrolidone respectively, connect the cooling circulator, first set the temperature to 70 °C and stir. After the co-solvent calcium chloride and stabilizer polyvinylpyrrolidone are both dissolved, change the temperature parameter of the circulating cooler to lower the solution temperature to 5 °C; then add 0.02 mol of diamine monomer to make its molar concentration in the reaction solvent reach 0.2 mol / L, control the system temperature at 5 °C and stir to dissolve, and then divide it into two equal parts and record them as mixed solvents A1 and A2 respectively; The total feed amount of the diamine monomer is 0.02 mol, which is composed of p-phenylenediamine and 2,6-naphthalenediamine in a molar ratio of 0.85:0.15, that is, the molar amount of p-phenylenediamine is 0.017 mol, and the molar amount of 2,6-naphthalenediamine is 0.003 mol.

[0033] (3) One-stage polymerization, based on the total feed amount of 0.02 mol of the diamine monomer in step (2), add 0.0203 mol of dicarbonyl chloride monomer to the system according to the molar ratio of n(acyl chloride):n(diamine)=1.015:1; the dicarbonyl chloride monomer is composed of terephthaloyl chloride and 2,6-naphthaloyl chloride in a molar ratio of 0.9:0.1, that is, the amount of terephthaloyl chloride is 0.01827 mol, and the amount of 2,6-naphthaloyl chloride is 0.00203 mol; add the dicarbonyl chloride monomer to the mixed solvent A1 obtained in step (2) for pre-copolycondensation reaction, the initial reaction temperature is 5 °C, adjust the stirring speed to 1000 r / min, and obtain a pre-copolymerized stock solution C after reacting for 5 min.

[0034] (4) Second-stage polymerization: Gradually add the mixed solution A2 obtained in step (2) dropwise to the prepolymerization stock solution C in step (3). The initial reaction temperature is 10 °C, the dropping time is 10 s, and a second-stage co-condensation polymerization reaction is carried out. Adjust the stirring speed to 1500 r / min. After reacting for 12 min, stop stirring to obtain a copolymerized PPTA stock solution.

[0035] (5) Add 0.0203 mol of calcium oxide to the copolymerized PPTA stock solution obtained in step (4), and continue to stir at a speed of 1000 r / min for 15 min to neutralize the by-product hydrogen chloride generated during the copolymerization process; the inherent viscosity of the copolymerized PPTA is 1.95 dL / g.

[0036] (6) Weigh 20 g of an oily para-aramid nanofiber dispersion and add it to the copolymerized PPTA stock solution obtained in step (5) and mix and stir. The stirring speed is 800 r / min, and the stirring time is 15 min to obtain a uniformly mixed solution; place the mixed solution in a vacuum dryer for degassing treatment. The vacuum degree is -0.098 MPa, and the treatment time is about 20 min. Stop when it is visually confirmed that there are no tiny bubbles in the system, that is, obtain a copolymerized para-aramid coating solution; The oily para-aramid nanofiber dispersion used is commercially available, a dispersion with N-methylpyrrolidone as the solvent system, its solid content is 3.5 wt%, the water content is <200 ppm, the diameter of the nanofibers is 10 nm - 50 nm, and the length is 5 μm - 30 μm; For the finally obtained copolymerized para-aramid coating solution, calculated as a percentage of the total mass of the coating solution, the copolymerized PPTA is about 3.5%, the para-aramid nanofibers are about 0.5%, and the alumina nanofibers are about 0.4%; the rotational viscosity of the coating solution is 28586 mPa·s.

[0037] A preparation method of a lithium battery separator: Select a commercially available wet synchronous double-drawn polyethylene membrane (PE) as the coating base film, and use a doctor blade coating method to uniformly coat the above-mentioned copolymerized para-aramid coating solution on the first surface of the base film; the coated film stays in the air with a humidity of 90% RH and a temperature of 60 °C for 180 s for slow phase transition, and then stands in a coagulation bath with a volume ratio of N-methylpyrrolidone to water of 2:8 for 10 min, then overflows and rinses in a water tank for 60 s, and then enters an oven at 60 °C for drying for 15 min to obtain a lithium-ion battery separator with a single-sided coating of copolymerized para-aramid; then repeat the above coating, water washing, and drying steps on the second surface of the base film to finally obtain a lithium-ion battery separator with a double-sided coating of copolymerized para-aramid. The total thickness of the two-sided coatings is 4.4 μm, and the air permeability is 173 s / 100 mL.

[0038] Example 2 A preparation method of a copolymerized para-aramid coating solution, the specific steps are as follows: The preparation method is the same as that of Example 1, except for the different addition amounts of alumina nanofibers and oily para-aramid nanofibers; Under the protection of inert gas, weigh 1.8 g of alumina nanofibers and add them to 100 mL of N-methylpyrrolidone solvent, and perform ultrasonic dispersion treatment to obtain a uniformly dispersed alumina nanofiber NMP solution as a para-aramid polymerization solvent for standby; Weigh 40 g of oily para-aramid nanofiber dispersion and add it to the copolymerized PPTA stock solution, mix and stir to obtain a uniformly mixed solution; place the mixed solution in a vacuum dryer for degassing treatment, with a vacuum degree of -0.098 MPa and a treatment time of about 20 min. Stop when no tiny bubbles can be observed in the mixed solution, and thus obtain the copolymerized para-aramid coating solution.

[0039] In the copolymerized para-aramid coating solution obtained in the above steps, calculated by the percentage of the total mass of the coating solution, the copolymerized PPTA is about 3.1%, the para-aramid nanofibers are about 0.9%, and the alumina nanofibers are about 1.1%; the inherent viscosity of the copolymerized PPTA is 2.05 dL / g, and the rotational viscosity of the coating solution is 26304 mPa·s.

[0040] A preparation method of a lithium battery separator, and the coating preparation steps are the same as those of Example 1. The finally obtained coating thickness is 4.8 μm, and the air permeability is 151 s / 100 mL.

[0041] Comparative Example 1 A preparation method of a para-aramid coating solution, and the specific steps are as follows: The differences in the preparation method and reagent selection from Example 1 are that no stabilizer is added during the polymerization process, no structure containing a naphthalene ring is introduced, and no stepwise polymerization is carried out; Under the protection of inert gas, weigh 0.6 g of alumina nanofibers and add them to 100 mL of N-methylpyrrolidone solvent. Use a high-power probe-type ultrasonic instrument with a power setting of 960 W for ultrasonic dispersion treatment for 80 min to obtain a uniformly dispersed alumina nanofiber NMP solution as a para-aramid polymerization solvent for standby; the alumina nanofibers used are the same as those in Example 1; Add the 100 mL of alumina nanofiber NMP solution obtained in step (1) to the reaction kettle, add 8 g of calcium chloride, heat to 70 °C, stir and dissolve, and then cool to 6 °C; then add 0.02 mol of p-phenylenediamine to make its molar concentration in the reaction solvent reach 0.2 mol / L, and control the system temperature at 5 °C and stir to dissolve; (3) Based on the total dosage of 0.02 mol of p-phenylenediamine in step (2), 0.203 mol of terephthaloyl chloride was added to the system obtained in step (3) according to the molar ratio of n(acyl chloride):n(diamine)=1.015:1. The stirring speed was adjusted to 1000 r / min, the initial reaction temperature was 6 °C, and the reaction was stopped after stirring for 5 min to obtain a para-aramid polymerization stock solution; (4) 0.203 mol of calcium oxide was added to the para-aramid stock solution, and stirring was continued at a speed of 1000 r / min for 15 min to neutralize the by-product hydrogen chloride generated during the copolymerization process, and the pH value was adjusted to neutral; (5) 20 g of oily para-aramid nanofibers were weighed and added to the para-aramid polymerization stock solution obtained in step (4) and mixed and stirred. The stirring speed was 800 r / min, and the stirring time was 15 min to obtain a uniformly mixed solution (the oily para-aramid nanofiber dispersion used was the same as in Example 1); The mixed solution was placed in a vacuum dryer for degassing treatment. The vacuum degree was -0.098 MPa, and the treatment time was about 20 min until no tiny bubbles were observed in the mixed solution, and then the para-aramid coating solution was obtained.

[0042] In the para-aramid coating solution obtained in the above steps, by mass fraction, PPTA was about 3.4%, para-aramid nanofibers were about 0.5%, and alumina nanofibers were about 0.4%; The inherent viscosity of PPTA was 2.15 dL / g, and the rotational viscosity of the coating solution was 31500 mPa·s.

[0043] The preparation steps of a lithium battery separator coating were the same as in Example 1. The coating thickness was 4.5 μm, and the air permeability was 357 s / 100 mL.

[0044] Comparative Example 2 A method for preparing a para-aramid coating solution, the specific steps are as follows: Compared with Example 2, the difference was that no alumina nanofibers and oily para-aramid nanofibers were added during the copolymerization process.

[0045] In the copolymerized para-aramid coating solution prepared above, by mass fraction, the copolymerized PPTA was 4.2%, the inherent viscosity of the copolymerized PPTA was 2.2 dL / g, and the rotational viscosity of the coating solution was 35087 mPa·s.

[0046] The lithium battery coated separator prepared using this coating solution had a coating thickness of 3.9 μm and an air permeability of 182 s / 100 mL.

[0047] Experimental Example The separators prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention were respectively tested for storage stability, thickness, air permeability value, thermal shrinkage, and separator flatness. The test results are shown in Table 1.

[0048] Table 1 Comparative data of diaphragm test results for Examples 1-2 and Comparative Examples 1-2 Stable storage time Coating thickness (μm) Air permeability (s / 100mL) Longitudinal thermal shrinkage rate at 180°C for 1h (%) Transverse thermal shrinkage rate at 180°C for 1h (%) Flatness of the coated separator Example 1 > 90 days 4.4 173 2.8 1.8 Flat Example 2 > 90 days 4.8 151 2.1 1.3 Flat Comparative Example 1 Precipitation after 8h 4.5 357 4.6 2.9 Flat Comparative Example 2 > 90 days 3.9 182 3.2 1.9 Edge curling From the comparison between Example 1 and Comparative Example 1, it can be seen that after introducing monomers and stabilizers containing naphthalene rings during the polymerization process of the copolyparaphenylene terephthalamide coating solution of the present invention, the solubility of PPTA is improved, the self-aggregation between PPTA molecules is reduced, and the storage stability of the coating solution is significantly enhanced. Specifically, compared with the coating solution in Comparative Example 1, the stable existence time of the coating solution in Example 1 is greatly extended from 8 hours to more than 90 days.

[0049] According to Figure 1 the thermogravimetric analysis test results, we found that the thermal stability of the copoly-PPTA prepared in Example 1 is better than that of the PPTA prepared in Comparative Example 1. While improving the solubility, the thermal stability of PPTA is not sacrificed, but further enhanced. Specifically, when comparing the coated diaphragms obtained in Example 1 and Comparative Example 1, we observed that the copoly-PPTA coated diaphragm obtained in Example 1 exhibited lower thermal shrinkage rates under the conditions of 180 °C @ 1 h, with a longitudinal thermal shrinkage rate < 3% and a transverse thermal shrinkage rate < 2%, which meets the stringent requirements for the heat resistance of diaphragms in high-performance lithium-ion batteries.

[0050] Through Figure 2 and Figure 3 the experimental data analysis in shows that the method of segmental feeding of diamine monomer solution proposed by the present invention can significantly improve the controllability of the polymerization reaction. As Figure 3 shown, when Comparative Example 1 adopted conventional direct polymerization, a violent explosion polymerization phenomenon occurred at the initial stage of the reaction, and the temperature rise △T of the system reached 27 °C. This out-of-control thermodynamic behavior is prone to cause side reactions, resulting in a wider molecular weight distribution of the product. In contrast, as Figure 2 shown, in Example 1 of the present invention, through the segmented feeding strategy of diamine monomers, the polymerization process is decoupled into two kinetic stages: in the first stage, by controlling the monomer concentration, the temperature rise △T is limited within 9 °C, making the reaction rate match the heat removal efficiency; in the second stage, the temperature rise amplitude is further reduced to 3 °C, successfully controlling the overall reaction under quasi-steady state conditions. This segmented control method not only extends the heat removal window period but also reduces the by-product content, significantly enhancing the stability of the polymerization process and the product homogeneity.

[0051] From the comparison between Example 2 and Comparative Example 2, it can be seen that the present invention uses a combination of alumina nanofibers and para-aramid nanofibers as fillers in the coating solution. This combination effectively alleviates the stress inside the coated film and avoids the curling problem that occurred in Comparative Example 2. In addition, the polymerization stock solution plays an adhesive and fixing role for the inorganic nanofibers and para-aramid nanofibers, solving the problem of insufficient adhesion during direct coating.

[0052] The embodiments described above 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 spirit of the present invention's design, various deformations and improvements made by those of ordinary skill 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 method for preparing a copolymerized para-aramid coating liquid, characterized in that: The preparation method is as follows: (1) Under the protection of inert gas, inorganic nanofibers are added to a reaction solvent and subjected to ultrasonic dispersion treatment to obtain a reaction solvent containing uniformly dispersed inorganic nanofibers, wherein the mass fraction of inorganic nanofibers in the obtained reaction solvent is 0.2% to 3%; (2) Add the reaction solvent containing inorganic nanofibers obtained in step (1) into a reaction kettle, add a cosolvent and a stabilizer, heat to 60°C~80°C, stir and dissolve, and then cool to -10°C~5°C; then add a diamine monomer to make the molar concentration of the diamine monomer in the reaction solvent reach 0.1 mol / L~0.6 mol / L; control the system temperature to -10°C~5°C, stir and dissolve, and then divide it into two equal parts, which are respectively counted as mixed solvents A1 and A2; (3) a one-stage polymerization, wherein the molar ratio of the diformyl chloride monomer to the diamine monomer in step (2) is (0.98-1.05):1, the diformyl chloride monomer is added to the mixed solvent A1 for co-condensation reaction to obtain a pre-copolymerization stock solution C, and the pre-copolymerization reaction time is 2-15 min; (4) Two-stage polymerization, the mixed solvent A2 is gradually added dropwise to the pre-copolymer stock solution C obtained in step (3), and stirred to carry out a two-stage polycondensation reaction to obtain a copolymer stock solution, and the reaction time is 10 min to 30 min; (5) adding an alkaline substance to the copolymerization stock solution obtained in step (4) to neutralize the hydrogen chloride by-product generated during the copolymerization process; (6) Add the oily para-aramid nanofiber dispersion to the copolymer stock solution treated in step (5) and mix them evenly. Place the mixed solution in a vacuum tank for degassing to obtain a copolymer para-aramid coating solution for diaphragm coating.

2. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The inorganic nanofibers in step (1) are selected from one or more of aluminum oxide nanofibers, silicon oxide nanofibers, and titanium oxide nanofibers; the diameter of the inorganic nanofibers is 4 nm to 50 nm, and the length is 200 nm to 10 μm; the reaction solvent is selected from one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylacetamide, and N,N-dimethylformamide; and the ultrasonic dispersion treatment time is 30 min to 90 min.

3. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: In step (2), the cosolvent is calcium chloride or lithium chloride, and the addition ratio is 5% to 10% of the mass of the reaction solvent; the stabilizer is one or more of povidone, polyethylene glycol, poly(N-vinyl caprolactam), and polyacrylamide; the addition ratio of the stabilizer is 0.5% to 6% of the mass of the reaction solvent.

4. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The diamine monomer in step (2) is composed of p-phenylenediamine and a diamine monomer containing a naphthalene ring, and the molar ratio of p-phenylenediamine to the diamine monomer containing a naphthalene ring is (0.7-1.0): (0.05-0.3); wherein the diamine monomer containing a naphthalene ring is selected from one or more of 2,6-naphthalene diamine, 1,8-naphthalene diamine, 2,3-naphthalene diamine, and 1,5-naphthalene diamine; and the stirring method is mechanical stirring, and the stirring speed is 100 r / min-800 r / min.

5. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The diformyl chloride monomers in step (3) are composed of terephthaloyl chloride and 2,6-naphthalene dichloride; the molar ratio of terephthaloyl chloride to 2,6-naphthalene dichloride is (0.8-1.0): (0.05-0.2); and the initial reaction temperature of the first stage polymerization is -10°C-10°C.

6. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The time for gradually adding the mixed solvent A2 in step (4) is 5s to 30s; the initial reaction temperature of the second-stage polymerization is -10°C to 15°C; the stirring speed is 600r / min to 1800r / min; and the logarithmic viscosity of the obtained copolymerized para-aramid is 0.8dL / g to 2.8dL / g.

7. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The alkaline substance in step (5) is one of calcium hydroxide and calcium oxide; the molar ratio of the alkaline substance to the diformyl chloride monomer is (1.0-1.1):1.0; stirring can be performed during the neutralization process to promote the reaction, and the stirring speed is 600 r / min-1800 r / min.

8. The method for preparing the copolymerized para-aramid coating liquid according to claim 1, characterized in that: The solid content of the oily para-aramid nanofiber dispersion in step (6) is 2% to 4%; the solvent system is the same as the solvent used in step (1); the morphological parameters of the oily para-aramid nanofiber are: diameter 10nm to 100nm, length 5μm to 90μm; the water content of the oily para-aramid nanofiber dispersion is less than 200ppm; based on the dry fiber mass of the oily para-aramid nanofiber, the amount added to the coating liquid is 0.1% to 2% of the total mass of the coating liquid.

9. The method for preparing the copolymerized para-aramid coating liquid according to claim 1 or 8, characterized in that: The degassing process in step (6) is carried out in a vacuum dryer, the vacuum degree is controlled at -0.08MPa to -0.1MPa, the treatment time is 3min to 30min, and the termination standard is to confirm by visual inspection that there are no visible tiny bubbles in the system.

10. A para-aramid copolymer coating liquid prepared by the preparation method according to any one of claims 1 to 9, characterized in that: The main active components, measured by mass fraction, include: 1.7% to 9.1% of copolymerized para-aramid, 0.3% to 4.3% of stabilizer, 0.1% to 2.2% of inorganic nanofiber, and 0.1% to 2% of para-aramid nanofiber; the rotational viscosity of the copolymerized aramid coating liquid is 2000 mPa.s to 80000 mPa.s.

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