Lithium battery separator based on reaction control agent and preparation method therefor

Abstract

A lithium battery separator based on a reaction control agent and a preparation method therefor. The lithium battery separator is prepared on the basis of a low-apparent-viscosity and high-molecular-weight para-aramid fiber polymer solution. A method for preparing the low-apparent-viscosity and high-molecular-weight para-aramid fiber polymer solution comprises the following steps: adding a reaction control agent during the process of synthesizing poly(p-phenylene terephthamide) from p-phenylenediamine and paraphthaloyl chloride, so as to obtain a low-apparent-viscosity and high-molecular-weight para-aramid fiber polymer solution comprising the reaction control agent, wherein the concentration of the reaction control agent in the low-apparent-viscosity and high-molecular-weight para-aramid fiber polymer solution is less than 300 ppm, the concentration of water in a first solvent used for synthesizing the poly(p-phenylene terephthamide) is less than or equal to 100 ppm, and the reaction control agent is a solvent which can be miscible with the first solvent and also can cure the poly(p-phenylene terephthamide). The low-apparent-viscosity and high-molecular-weight para-aramid fiber polymer solution has a reduced apparent viscosity, coarsened fibers, an increased porosity, better wettability, and enhanced thermal stability.

Description

Lithium battery separator based on reaction control agent and preparation method thereof Technical Field

[0001] The present invention belongs to the technical field of battery separators, and in particular relates to a lithium battery separator based on a reaction control agent and a preparation method thereof. Background Art

[0002] With the development of social economy, people's demand for lithium-ion batteries is increasing, and safety issues during use have undoubtedly become a problem that needs to be solved urgently.

[0003] Lithium-ion batteries are prone to combustion and explosion during use. For example, under abnormal conditions such as collisions, the separator can be damaged, allowing the positive and negative electrodes to come into direct contact, resulting in a short circuit. This can lead to thermal runaway or even more serious consequences such as spontaneous combustion or explosion. Therefore, the separator must possess excellent thermal stability and mechanical properties, as well as high ion permeability, to ensure the electrochemical performance of lithium-ion batteries.

[0004] Para-aramid has high heat resistance (glass transition temperature above 300°C, thermal decomposition temperature as high as 560°C, and strength retention rate of 84% after being placed in air at 180°C for 48 hours), high tensile strength and initial elastic modulus (fiber strength 0.215 Newtons / denier, modulus 4.9-9.8 Newtons / denier, specific strength is 5 times that of steel), stable thermal shrinkage and creep properties, in addition to high insulation and chemical corrosion resistance, it is a very excellent diaphragm material. Therefore, the use of diaphragm slurry containing para-aramid to prepare battery diaphragms has become an important research direction.

[0005] CN115295961A proposes using a solvent combined with a solubilizing agent (strong base) to dissolve the finished para-aramid product, creating a slurry that is then coated onto a base film to form a composite membrane. Steam-induced phase separation is then used to solidify the para-aramid diaphragm slurry into pores within the composite membrane. This method uses a strong base as a solvent, which is highly hazardous, resulting in higher costs for the finished para-aramid product, and the use of steam-induced phase separation is cumbersome.

[0006] CN114388985A proposes to directly use exo-aramid polymerization liquid to make a wet-process aramid porous membrane layer, and coat a para-aramid nanofiber aqueous solution on both sides of the membrane layer to make a para-aramid diaphragm. The diaphragm can better reflect the performance advantages of para-aramid, but this method uses a large content of para-aramid, the cost is high, and the process is also relatively complicated.

[0007] CN111019124A proposes adding polyethylene glycol and gas-phase nano-ceramic particles during the synthesis of para-aramid, and adding dimethyl carbonate as a pore-forming agent after the polymerization is completed. This method effectively extends the shelf life of the slurry. However, the slurry prepared by this method has a high apparent viscosity, which is not conducive to coating. The coating thickness is large, and the diaphragm is thick, which is not conducive to the shuttling of ions. On the other hand, the introduction of the pore-forming agent also increases the difficulty of recovering the subsequent extractant.

[0008] CN109411676A proposes adding a non-solvent to the para-aramid solution as a pore-forming agent for the coating slurry, which can effectively improve the heat stability and heat shrinkage rate of the diaphragm. However, in actual production, this method cannot be smoothly fed due to the high apparent viscosity of the slurry, making it difficult to achieve mass production. In addition, the addition of the pore-forming agent is equivalent to introducing a new substance, which is not conducive to the separation and recovery of the extractant (solvent), thereby increasing production costs.

[0009] Summary of the Invention

[0010] In view of the shortcomings of the prior art, the object of the present invention is to provide a method for preparing a para-aramid polymer solution with low apparent viscosity and high molecular weight.

[0011] Another object of the present invention is to provide a slurry for lithium battery coating diaphragm.

[0012] Another object of the present invention is to provide a method for preparing a slurry for a lithium battery coating separator.

[0013] Another object of the present invention is to provide a method for preparing a lithium battery separator.

[0014] The purpose of the present invention is achieved through the following technical solutions.

[0015] A method for preparing a low-apparent-viscosity, high-molecular-weight para-aramid polymer solution comprises the following steps: adding a reaction control agent during the synthesis of poly(p-phenylene terephthalamide) (PPTA) from p-phenylenediamine and terephthaloyl chloride to obtain a low-apparent-viscosity, high-molecular-weight para-aramid polymer solution containing the reaction control agent, wherein the concentration of the reaction control agent in the low-apparent-viscosity, high-molecular-weight para-aramid polymer solution is less than or equal to 300 ppm (the concentration of the reaction control agent in the low-apparent-viscosity, high-molecular-weight para-aramid polymer solution is preferably less than or equal to 200 ppm, more preferably less than or equal to 150 ppm); the concentration of water in a first solvent used to synthesize poly(p-phenylene terephthalamide) is less than or equal to 100 ppm; and the reaction control agent is a solvent that is miscible with the first solvent and can simultaneously solidify poly(p-phenylene terephthalamide, para-aramid).

[0016] In the above technical solution, the reaction control agent is deionized water, alcohols, esters or ethers.

[0017] Specifically, the method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution comprises the following steps:

[0018] Step 1: In a nitrogen or inert gas atmosphere, mixing a solubilizing salt and a first solvent, stirring until the solubilizing salt is uniformly dispersed in the first solvent to obtain a first mixed solution, wherein the ratio of the first solvent to the solubilizing salt is (100-103): (3-8) by mass;

[0019] In step 1, the dissolution-advantageous salt is calcium chloride and / or lithium chloride.

[0020] In step 1, the first solvent is a non-aqueous solvent or an aqueous solvent, the non-aqueous solvent is a mixture of one or more of N-methylpyrrolidone, hexamethylphosphoramide, dimethylacetamide and tetramethylurea, and the aqueous solvent is a mixture of a non-aqueous solvent and water.

[0021] In step 1, the stirring speed is 300-1500 rpm, the stirring time is 60-120 min, and the stirring temperature is 50-90°C.

[0022] Step 2: In a nitrogen or inert gas atmosphere, the first mixed solution is cooled to 5-15° C., p-phenylenediamine is added to the first mixed solution, and the mixture is stirred until uniform to obtain a second mixed solution, wherein the ratio of the mass fraction of the first mixed solution to the amount of p-phenylenediamine is 100:(16-20), the unit of the amount of the substance is mol, and the unit of the mass fraction is kg;

[0023] In step 2, the stirring speed is 300 to 1500 rpm, and the stirring time is 20 to 50 minutes.

[0024] Step 3: In a nitrogen or inert gas atmosphere, the second mixed solution is cooled to -5 to 5° C., terephthaloyl chloride is added to the second mixed solution, and the mixture is stirred until uniform to obtain a low apparent viscosity and high molecular weight para-aramid polymer solution, wherein the ratio of p-phenylenediamine to the terephthaloyl chloride is (1 to 1.05):1, based on the amount of the substance;

[0025] In the steps 1 to 3, the reaction control agent is added once or multiple times in any step of the steps 1 to 3 so that the concentration of the reaction control agent in the low apparent viscosity high molecular weight para-aramid polymer solution is less than or equal to 300 ppm.

[0026] In step 3, the stirring speed is 300 to 1500 rpm, and the stirring time is 10 to 30 minutes.

[0027] A slurry for a lithium battery coating diaphragm comprises: a second solvent, ceramic solid particles and a low apparent viscosity high molecular weight para-aramid polymer solution, wherein, by weight, the ratio of the ceramic solid particles to the low apparent viscosity high molecular weight para-aramid polymer solution is (5-30):(50-80), and the ceramic solid particles are a mixture of one or more of alumina, fumed alumina, silica, zirconium oxide, aluminum hydroxide, magnesium hydroxide, barium sulfate, boehmite, boron nitride, silicon nitride and silicon carbide.

[0028] The above technical solution also includes: a dispersant, in which the ratio of the dispersant to the ceramic solid particles is (0.01 to 5): (5 to 30) by mass, and the dispersant is a mixture of one or more of a polymer block copolymer dispersant, polyacrylate sodium salt, polyacrylate potassium salt, polyacrylate ammonium salt and polycarboxylic acid sodium salt.

[0029] In the above technical solution, the second solvent is the non-aqueous solvent or the aqueous solvent. Preferably, the second solvent is the same as the first solvent.

[0030] In the above technical solution, the ratio of the second solvent to the ceramic solid particles is (60-90):(5-30) by mass.

[0031] In the above technical solution, the particle size of the ceramic solid particles is 0.05 to 10 μm, and the particle size of the fumed alumina is 50 to 600 nm.

[0032] The preparation method of the above-mentioned slurry for lithium battery coating membrane comprises the following steps: uniformly mixing the second solvent, ceramic solid particles and low apparent viscosity high molecular weight para-aramid polymer solution to obtain the slurry for lithium battery coating membrane.

[0033] When the slurry for lithium battery coating membrane also includes a dispersant, the preparation method of the above-mentioned slurry for lithium battery coating membrane includes the following steps: uniformly mixing the dispersant, the second solvent, ceramic solid particles and the low apparent viscosity high molecular weight para-aramid polymer liquid to obtain the slurry for lithium battery coating membrane.

[0034] Specifically, the preparation method of the slurry for lithium battery coating separator includes the following steps:

[0035] S1, preparing a solution A, wherein the solution A includes: a second solvent;

[0036] In S1, the solution A further comprises a dispersant, and the method for preparing the second solvent is: mixing the dispersant with the second solvent, stirring until the dispersant is uniformly dispersed in the second solvent, to obtain the solution A;

[0037] In S1, the stirring speed is 300-800 rpm, and the stirring time is 15-45 min.

[0038] S2, adding ceramic solid particles to the solution A obtained in S1, stirring, and sand-milling to obtain solution B;

[0039] In S2, the stirring speed is 300-800 rpm, and the stirring time is 15-50 min.

[0040] In S2, the sanding time is 30 to 60 minutes, and the sanding speed is 1000 to 2000 rpm.

[0041] S3, mixing the B solution described in S2 with a low apparent viscosity high molecular weight para-aramid polymer solution, stirring, and obtaining a slurry for a lithium battery coating separator.

[0042] In S3, the stirring speed is 500-2000 rpm, and the stirring time is 20-60 min.

[0043] A lithium battery separator comprises a base film and a coating covering one or both sides of the base film, wherein the coating is prepared from a slurry for lithium battery coating separators.

[0044] In the above technical solution, the base film is a polyethylene diaphragm, a PP / PE / PP three-layer diaphragm, a non-woven fabric or a polyimide diaphragm.

[0045] The preparation method of the above-mentioned lithium battery separator comprises the following steps: coating the lithium battery coating separator slurry on one or both sides of the base film, performing solvent step-by-step content extraction, and drying to obtain the lithium battery separator.

[0046] In the above technical solution, the coating method is micro-gravure coating, dip coating or slit coating, and the thickness of the single-sided coating after coating is 1 to 10 μm.

[0047] In the above technical solution, the thickness of the base film is 5 to 25 μm.

[0048] In the above technical solution, the solvent step content extraction is to sequentially use an extractant containing an extracting liquid and water for extraction. When the extractant containing an extracting liquid is used for extraction, the concentration of the extracting liquid in the extracting liquids used successively decreases.

[0049] In the above technical solution, the solvent step content extraction is performed in sequence using a first extractant, a second extractant, a third extractant and a fourth extractant, wherein the first extractant, the second extractant and the third extractant are each a mixture of an extract and water, the concentration of the extract in the first extractant is 80-95wt%, the concentration of the extract in the second extractant is 40-55wt%, the concentration of the extract in the third extractant is 20-35wt%, and the fourth extractant is water.

[0050] In the above technical solution, the drying temperature is 60-80° C., and the drying time is 30-120 seconds.

[0051] In the above technical solution, when the first solvent is a non-aqueous solvent, the extract is the same as the first solvent; when the first solvent is an aqueous solvent, the extract is the same as the non-aqueous solvent.

[0052] Application of the above-mentioned lithium battery separator in lithium batteries.

[0053] In the above technical solution, the positive electrode active material of the lithium battery is a ternary material composed of lithium cobalt oxide, lithium manganese oxide and lithium nickel oxide, the negative electrode material is one of graphite and SiC, and the electrolyte of the electrolyte in the lithium battery is one or a mixture of LiPF6, LiBF4, LiAsF6 and LiSbF6.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] 1. The present invention provides a low apparent viscosity, high molecular weight para-aramid polymer solution containing a reaction control agent at a concentration of 300 ppm or less. Through reverse calculation, a corresponding amount of the reaction control agent and the first solvent are added during the synthesis of the para-aramid. The reaction control agent can be added once or multiple times. The method for preparing a low apparent viscosity, high molecular weight para-aramid polymer solution of the present invention includes the step of adding the reaction control agent, thereby controlling the polymerization reaction rate, making the chain length of the polymerized single chains more uniform, and making the distribution of the polymer chains more orderly. This reduces the apparent viscosity of the low apparent viscosity, high molecular weight para-aramid polymer solution, coarsens the fibers, increases the porosity, improves the wettability, and enhances the thermal stability.

[0056] 2. Because the fibers are coarsened and the porosity is increased, there is no need to add additional pore-forming agents to the slurry, which is beneficial to reducing costs and increasing efficiency;

[0057] 3. Because no pore-forming agent is required, the extract only needs to be neutralized with alkali and then the solid and liquid can be separated by distillation. At the same time, water and the first solvent / second solvent are also separated due to the large difference in boiling points, which is beneficial to the separation and recycling of the extract. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] FIG1 is the electrolytic wettability data of the lithium battery separators prepared in Examples 1 and 2 and the lithium battery separators prepared in Comparative Examples 1 and 2;

[0059] FIG2 is a scanning electron microscope image of the lithium battery separator prepared in Example 1;

[0060] FIG3 is a scanning electron microscope image of the lithium battery separator prepared in Comparative Example 1;

[0061] FIG4 is a scanning electron microscope image of the para-aramid diaphragm in CN109411676A;

[0062] FIG5 is a scanning electron microscope image of the lithium battery separator prepared in Example 2;

[0063] FIG6 is a scanning electron microscope image of the lithium battery separator prepared in Comparative Example 2;

[0064] FIG7 is a cross-sectional electron microscope image of the lithium battery separator prepared in Example 1. DETAILED DESCRIPTION

[0065] The present invention obtains a para-aramid polymerization liquid with low apparent viscosity and high molecular weight by adding a reaction control agent. The reaction control agent can control the reaction rate of the polymerization reaction and reduce the apparent viscosity to the greatest extent under the premise that the weight-average molecular weight remains unchanged or slightly decreases, so as to facilitate the preparation of the slurry and better adapt to enterprise mass production.

[0066] By mixing a low apparent viscosity, high molecular weight para-aramid polymer liquid with ceramic solid particles to prepare a slurry for lithium battery coating diaphragms, the performance advantages of para-aramid can be maximized while ensuring that the coating can proceed smoothly. On the other hand, no pore-forming agent is used in the preparation of the slurry for lithium battery coating diaphragms. Therefore, after extraction, the extractant contains only water and extractant in addition to the acid generated by the synthesis reaction, which is conducive to the separation and recycling of the extractant.

[0067] The technical solution of the present invention is further described below with reference to specific embodiments.

[0068] The basic information of the raw materials and equipment involved in the following examples is as follows:

[0069] The electrolyte wettability was tested in an electrolyte solution containing LiPF6 (purchased from Zhuhai Guangrui), and the concentration of LiPF6 in the electrode solution was 1 mol / L.

[0070] Example 1

[0071] A method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution comprises the following steps:

[0072] Step 1: In a nitrogen atmosphere, a dissolving salt and a first solvent are mixed, and the mixture is stirred at 80° C. and 600 rpm for 80 min until the dissolving salt is uniformly dispersed in the first solvent to obtain a first mixed solution, wherein deionized water is added as a reaction controller during the stirring process to make the concentration of the reaction controller in the low apparent viscosity high molecular weight para-aramid polymer solution 100 ppm, the ratio of the first solvent to the dissolving salt is 100:6 by mass, the first solvent is N-methylpyrrolidone solution, the water content in the first solvent is 62.3 ppm, and the dissolving salt is calcium chloride;

[0073] Step 2: In a nitrogen atmosphere, the first mixed solution was cooled to 11° C., p-phenylenediamine was added to the first mixed solution, and the mixture was stirred at 600 rpm for 25 min until uniform, to obtain a second mixed solution, wherein the ratio of the mass fraction of the first mixed solution to the amount of p-phenylenediamine was 100:16.5, the unit of mass fraction was kg, and the unit of amount of substance was mol;

[0074] Step 3: In a nitrogen atmosphere, the second mixed solution was cooled to 1.2° C., terephthaloyl chloride was added to the second mixed solution, and the mixture was stirred at 600 rpm for 20 minutes until uniform, to obtain a low apparent viscosity and high molecular weight para-aramid polymer solution, wherein the ratio of p-phenylenediamine to terephthaloyl chloride was 16.5:16 based on the amount of the substance.

[0075] A slurry for lithium battery coating separators comprises a second solvent, a dispersant, ceramic solid particles, and a low-viscosity, high-molecular-weight para-aramid polymer solution. The ratio of the dispersant, ceramic solid particles, and low-viscosity, high-molecular-weight para-aramid polymer solution is 0.5:15:60 by mass, and the ratio of the second solvent to the ceramic solid particles is 83.5:15. In this embodiment, the dispersant is BYK-LPN21954, and the ceramic solid particles are aluminum oxide (solid particles) and fumed aluminum oxide (powder). The mass ratio of aluminum oxide to fumed aluminum oxide is 5:10. The particle size of the aluminum oxide is 0.05 to 1 μm, and the particle size of the fumed aluminum oxide is 50 to 150 nm. The second solvent is the same as the first solvent.

[0076] The method for preparing the slurry for the lithium battery coating separator comprises the following steps:

[0077] S1, mixing the dispersant and the second solvent, stirring at 500 rpm for 25 min until the dispersant is uniformly dispersed in the second solvent, to obtain solution A;

[0078] S2, adding alumina to solution A, stirring at 500 rpm for 25 minutes, then adding fumed alumina, stirring at 600 rpm for 25 minutes, adding all the ceramic solid particles, and sand milling at 1700 rpm for 50 minutes to obtain solution B;

[0079] S3, mixing the B solution with the low apparent viscosity high molecular weight para-aramid polymer solution, stirring at 600 rpm for 30 minutes to obtain a slurry for a lithium battery coating separator.

[0080] A method for preparing a lithium battery separator comprises the following steps:

[0081] A lithium battery separator coating slurry was coated on one side of a base film using micro-gravure coating at a speed of 8 m / min, and solvent step extraction was performed. The slurry was then dried at 80°C for 60 seconds to obtain a lithium battery separator. The thickness of the coating after coating was 3 μm, and the base film was a PE film with a thickness of 12 μm. The solvent step extraction was performed sequentially using a first extractant, a second extractant, a third extractant, and a fourth extractant (the separator was immersed in each extractant for 9 seconds). The first extractant, the second extractant, and the third extractant were each a mixture of an extractant and water. The extractant was N-methylpyrrolidone. The concentration of the extractant in the first extractant was 90 wt %, the concentration of the extractant in the second extractant was 50 wt %, the concentration of the extractant in the third extractant was 30 wt %, and the fourth extractant was water.

[0082] Comparative Example 1

[0083] A method for preparing a para-aramid polymer solution is basically the same as that of Example 1, except that deionized water as a reaction control agent is not added during the stirring process and the water content in the first solvent is 55.4 ppm.

[0084] A slurry for a lithium battery coating diaphragm is basically the same as the slurry for the lithium battery coating diaphragm in Example 1, the only difference being that the low apparent viscosity and high molecular weight para-aramid polymer liquid in the slurry for the lithium battery coating diaphragm in Example 1 is replaced by the "para-aramid polymer liquid" in this comparative example.

[0085] The preparation method of the slurry for lithium battery coating diaphragm in this comparative example is basically the same as the preparation method of the slurry for lithium battery coating diaphragm in Example 1. The only difference is that the low apparent viscosity and high molecular weight para-aramid polymer liquid in the preparation method of the slurry for lithium battery coating diaphragm in Example 1 is replaced by the "para-aramid polymer liquid" in this comparative example.

[0086] A method for preparing a lithium battery separator is basically the same as Example 1 "A method for preparing a lithium battery separator", except that the apparent viscosity of the lithium battery coating separator slurry obtained by using the para-aramid polymer solution prepared in Comparative Example 1 is too high, the feed speed is slow during the coating process on the machine, and the coating speed cannot be matched, resulting in serious coating leakage and the thickness of the single-side coating of the lithium battery separator after coating is 2 μm.

[0087] Example 2

[0088] A method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution comprises the following steps:

[0089] Step 1: In a nitrogen atmosphere, a dissolving salt and a first solvent are mixed, and the mixture is stirred at 80° C. and 600 rpm for 80 min until the dissolving salt is uniformly dispersed in the first solvent to obtain a first mixed solution, wherein the ratio of the first solvent to the dissolving salt is 100:6.5 by mass, the first solvent is an N-methylpyrrolidone solution, the water content in the first solvent is 61.2 ppm, and the dissolving salt is calcium chloride;

[0090] Step 2: In a nitrogen atmosphere, the first mixed solution is cooled to 9.2° C., p-phenylenediamine is added to the first mixed solution, and the mixture is stirred at 600 rpm for 35 minutes until uniform, to obtain a second mixed solution, wherein deionized water is added as a reaction controller during the stirring process so that the concentration of the reaction controller in the low apparent viscosity high molecular weight p-aramid polymer solution is 200 ppm, and the ratio of the mass fraction of the first mixed solution to the amount of p-phenylenediamine is 100:17.5, the unit of the mass fraction is kg, and the unit of the amount of substance is mol;

[0091] Step 3: In a nitrogen atmosphere, the second mixed solution is cooled to -1.0°C, terephthaloyl chloride is added to the second mixed solution, and the mixture is stirred at a speed of 600 rpm for 10 minutes until uniform, to obtain a low apparent viscosity and high molecular weight para-aramid polymer solution, wherein the ratio of p-phenylenediamine to terephthaloyl chloride is 17.5:17 based on the amount of substance.

[0092] A slurry for lithium battery coating separators comprises a second solvent, a dispersant, ceramic solid particles, and a low-viscosity, high-molecular-weight para-aramid polymer solution. The ratio of the dispersant, ceramic solid particles, and low-viscosity, high-molecular-weight para-aramid polymer solution is 1:10:55, and the ratio of the second solvent to the ceramic solid particles is 89:10. In this embodiment, the dispersant is BYK-LPN21954, and the ceramic solid particles are aluminum oxide (solid particles) and fumed aluminum oxide (particles). The weight ratio of the aluminum oxide to the fumed aluminum oxide is 5:5. The particle size of the aluminum oxide is 0.05 to 1 μm, and the particle size of the fumed aluminum oxide is 50 to 150 nm. The second solvent is the same as the first solvent.

[0093] The method for preparing the slurry for the lithium battery coating separator comprises the following steps:

[0094] S1, mixing the dispersant and the second solvent, stirring at 500 rpm for 25 min at room temperature until the dispersant is uniformly dispersed in the second solvent, to obtain solution A;

[0095] S2, adding alumina to solution A, stirring at 600 rpm for 25 minutes at room temperature after adding alumina, then adding fumed alumina, stirring at 600 rpm for 25 minutes, adding all the ceramic solid particles, and sand milling at 1700 rpm for 50 minutes to obtain solution B;

[0096] S3, mixing the B solution with the low apparent viscosity high molecular weight para-aramid polymer solution, stirring at 600 rpm for 30 minutes at room temperature to obtain a slurry for a lithium battery coating separator.

[0097] A method for preparing a lithium battery separator comprises the following steps:

[0098] The lithium battery coating separator slurry was coated on one side of the base film using micro-gravure coating at a speed of 8 m / min, and solvent step extraction was performed. The slurry was then dried at 80°C for 60 seconds to obtain a lithium battery separator. The thickness of the coating after coating was 3 μm, and the base film was a PE film with a thickness of 9 μm. The solvent step extraction was performed sequentially using a first extractant, a second extractant, a third extractant, and a fourth extractant (the separator was immersed in each extractant for 9 seconds). The first extractant, the second extractant, and the third extractant were each a mixture of an extractant and water. The extractant was N-methylpyrrolidone. The concentration of the extractant in the first extractant was 90 wt %, the concentration of the extractant in the second extractant was 50 wt %, the concentration of the extractant in the third extractant was 30 wt %, and the fourth extractant was water.

[0099] Comparative Example 2

[0100] A method for preparing a para-aramid polymer solution is basically the same as that in Example 2, except that deionized water as a reaction control agent is not added during the stirring process, the water content in the first solvent is 61.6 ppm, and "the second mixed solution is cooled to -1.0°C" in Example 2 is replaced by "the second mixed solution is cooled to -1.2°C".

[0101] A method for preparing a slurry for a lithium battery coating diaphragm is basically the same as the method for preparing a slurry for a lithium battery coating diaphragm in Example 2, with the only difference being that the low apparent viscosity and high molecular weight para-aramid polymer solution in the method for preparing a slurry for a lithium battery coating diaphragm in Example 2 is replaced by the "para-aramid polymer solution" in this comparative example.

[0102] A method for preparing a lithium battery separator comprises manually coating the lithium battery separator coating slurry described in this comparative example on the surface of a 9 μm thick PE base film, followed by solvent step extraction and drying to obtain a lithium battery separator. The "solvent step extraction and drying" steps in this comparative example are consistent with those in Example 2, "A method for preparing a lithium battery separator." (The apparent viscosity of the lithium battery separator coating slurry obtained using the para-aramid polymer solution prepared in Comparative Example 2 was too high, preventing smooth feeding and requiring manual coating. After coating, significant material shedding occurred during the solvent step extraction process.)

[0103] Example 3

[0104] A method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution comprises the following steps:

[0105] Step 1: In a nitrogen atmosphere, a dissolving salt and a first solvent are mixed, and the mixture is stirred at 80° C. and 600 rpm for 80 min until the dissolving salt is uniformly dispersed in the first solvent to obtain a first mixed solution, wherein deionized water is added as a reaction control agent during the stirring process, and the ratio of the first solvent to the dissolving salt is 100:7.5 by mass, the first solvent is an N-methylpyrrolidone solution, the water content in the first solvent is 56.4 ppm, and the dissolving salt is calcium chloride;

[0106] Step 2: In a nitrogen atmosphere, the first mixed solution is cooled to 8.6° C., p-phenylenediamine is added to the first mixed solution, and the mixture is stirred at 600 rpm for 40 minutes until uniform, to obtain a second mixed solution, wherein deionized water is added as a reaction control agent during the stirring process, and the ratio of the mass fraction of the first mixed solution to the amount of p-phenylenediamine is 100:18.2, the unit of mass fraction is kg, and the unit of amount of substance is mol;

[0107] Step 3: Cool the second mixed solution to -2.3°C in a nitrogen atmosphere, add terephthaloyl chloride to the second mixed solution, and stir at a speed of 600 rpm for 15 minutes until uniform, to obtain a low apparent viscosity and high molecular weight para-aramid polymer solution, wherein deionized water is added as a reaction control agent during the cooling process of the second mixed solution, and the ratio of p-phenylenediamine and terephthaloyl chloride is 18.2:18 in terms of molar fractions. In this embodiment, the ratio of the reaction control agent added in steps 1, 2, and 3 is 5:3:2 in terms of mass fractions, so that the concentration of the reaction control agent in the low apparent viscosity and high molecular weight para-aramid polymer solution is 300 ppm.

[0108] A slurry for a lithium battery coating separator comprises: a second solvent, ceramic solid particles, and a low-viscosity, high-molecular-weight para-aramid polymer solution. The ratio of the second solvent, ceramic solid particles, and low-viscosity, high-molecular-weight para-aramid polymer solution is 3:1:4 by weight. The ceramic solid particles are barium sulfate (solid particles) with a particle size of 0.05 to 1 μm. The second solvent is the same as the first solvent.

[0109] The method for preparing the slurry for the lithium battery coating separator comprises the following steps:

[0110] S1, preparing a second solvent as solution A;

[0111] S2, adding ceramic solid particles to the second solvent, stirring at 500 rpm for 35 min, and sand milling at 1700 rpm for 50 min to obtain solution B;

[0112] S3, mixing the B solution with the low apparent viscosity high molecular weight para-aramid polymer solution, stirring at 600 rpm for 30 minutes at room temperature to obtain a slurry for a lithium battery coating separator.

[0113] A method for preparing a lithium battery separator comprises the following steps:

[0114] The lithium battery coating separator slurry was coated on one side of the base film using micro-gravure coating at a speed of 8 m / min, and solvent step extraction was performed. The slurry was then dried at 80°C for 60 seconds to obtain a lithium battery separator. The thickness of the coating after coating was 3 μm, and the base film was a PE film with a thickness of 9 μm. The solvent step extraction was performed sequentially using a first extractant, a second extractant, a third extractant, and a fourth extractant (the separator was immersed in each extractant for 9 seconds). The first extractant, the second extractant, and the third extractant were each a mixture of an extractant and water. The extractant was N-methylpyrrolidone. The concentration of the extractant in the first extractant was 90 wt %, the concentration of the extractant in the second extractant was 50 wt %, the concentration of the extractant in the third extractant was 30 wt %, and the fourth extractant was water.

[0115] Comparative Example 3

[0116] A method for preparing a para-aramid polymer solution is basically the same as that in Example 3, except that deionized water as a reaction control agent is not added in this comparative example, the water content in the first solvent is 57.6 ppm, and "the second mixed solution is cooled to -2.3°C" in Example 3 is replaced by "the second mixed solution is cooled to -2.6°C".

[0117] A method for preparing a slurry for a lithium battery coating separator is described. The method for preparing a slurry for a lithium battery coating separator is performed according to Example 3, except that the low-viscosity, high-molecular-weight para-aramid polymer solution in Example 3 is replaced with the "para-aramid polymer solution" in this comparative example. During the preparation of the slurry for a lithium battery coating separator using the para-aramid polymer solution prepared in Comparative Example 3, the para-aramid polymer solution continued to polymerize, the apparent viscosity of the para-aramid polymer solution continued to increase, and it gradually solidified, preventing the slurrying process from proceeding smoothly. Therefore, the slurry for a lithium battery coating separator prepared in Comparative Example 3 was not successfully used to prepare a lithium battery separator for the following tests.

[0118] The low apparent viscosity and high molecular weight para-aramid polymer solutions prepared in Examples 1 to 3 and the para-aramid polymer solutions prepared in Comparative Examples 1 to 3 were subjected to apparent viscosity tests. The low apparent viscosity and high molecular weight para-aramid polymer solutions / para-aramid polymer solutions were extruded into films, washed and dried, and thermal weight loss and weight average molecular weight tests were performed. The comparative results are shown in Table 1.

[0119] Table 1

[0120] From the analysis in Table 1, it can be seen that after adding the reaction control agent during the synthesis of poly(p-phenylene terephthalamide) (PPTA), the apparent viscosity of the obtained low apparent viscosity and high molecular weight para-aramid polymer solution is significantly reduced, the weight average molecular weight is slightly reduced, but the weight average molecular weight distribution is more uniform, the percentage of decomposition above 600°C is increased, and the heat resistance is enhanced.

[0121] The performance tests of the lithium battery separators prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were performed. The results are shown in Table 2, Table 3 and Figure 1:

[0122] Table 2

[0123] Table 3

[0124] It can be seen from Tables 2 and 3 that the test data of the embodiment and its corresponding comparative example are close, or even better in performance, specifically, thermal stability is maintained and mechanical tensile strength is enhanced. It can be seen from Figure 1 that the embodiment has better electrolyte wettability than its corresponding comparative example.

[0125] The lithium battery separators prepared in Examples 1 to 3 and Comparative Example 1 were assembled into lithium-ion batteries. The positive electrode material was a ternary material composed of lithium cobalt oxide, lithium manganese oxide, and lithium nickel oxide (purchased from Huijie Lithium Energy), the negative electrode material was graphite, and the electrolyte in the electrolyte was LiPF6. The results are shown in Table 4:

[0126] Table 4

[0127] Comparative Example 4

[0128] A para-aramid diaphragm, the preparation method of which can be found in CN109411676A.

[0129] Compared with patent CN109411676A, the present invention does not use a pore-forming agent, is more conducive to the recovery of the extractant, and has improved air permeability, heat shrinkage, and needle puncture strength, thereby achieving the purpose of reducing costs and increasing efficiency.

[0130] As can be seen from Figures 2-3 and 5-6, the addition of the reaction control agent in the embodiment makes the fibers thicker and the ceramics and fibers evenly distributed, and the porosity is significantly increased, while the fibers in the comparative example are finer and agglomerated, and the ceramics and fibers are unevenly distributed.

[0131] As can be seen from Figure 4, the fibers of the para-aramid diaphragm prepared in CN109411676A are obvious and stacked layer by layer, but ceramic particles are only displayed in small quantities on the right side. The fibers and ceramics are unevenly distributed, and it is speculated that the fibers and ceramics may be stratified.

[0132] As can be seen from FIG. 7 , the fibers and ceramics of the para-aramid diaphragm prepared in Example 1 of the present invention are distributed alternately and evenly and orderly, so that the coated diaphragm can better combine the performance advantages of the fibers and ceramics.

[0133] Through the above analysis, it can be concluded that adding a reaction control agent in the process of synthesizing PPTA can effectively reduce the apparent viscosity of the synthetic product while ensuring the molecular weight of the synthetic product. After mixing with ceramics, the slurry for lithium battery coating diaphragm is coated on the base film. The obtained lithium battery diaphragm maintains thermal stability, enhances mechanical tensile strength, significantly increases porosity, and has better electrolyte wettability.

[0134] Example 4 (comparative)

[0135] A method for preparing a para-aramid polymer solution is basically the same as that of Comparative Example 1, with the only difference being that the water content in the first solvent of this embodiment is 20 ppm.

[0136] Example 5

[0137] A method for preparing a para-aramid polymer solution. This embodiment is basically the same as embodiment 4, except that the water content of the first solvent is different. In this embodiment, the water content of the first solvent is 103 ppm.

[0138] Example 6

[0139] A method for preparing a para-aramid polymer solution. This embodiment is basically the same as embodiment 4, with the only difference being that the water content of the first solvent is different. In this embodiment, the water content of the first solvent is 162 ppm.

[0140] Example 7

[0141] A method for preparing a para-aramid polymer solution. This embodiment is basically the same as embodiment 4, with the only difference being that the water content of the first solvent is different. In this embodiment, the water content of the first solvent is 240 ppm.

[0142] Example 8

[0143] A method for preparing a para-aramid polymer solution. This embodiment is basically the same as embodiment 4, with the only difference being that the water content of the first solvent is different. In this embodiment, the water content of the first solvent is 350 ppm.

[0144] Example 9

[0145] A method for preparing a para-aramid polymer solution. This embodiment is basically the same as embodiment 4, except that the water content of the first solvent is different. In this embodiment, the water content of the first solvent is 420 ppm.

[0146] Example 10

[0147] A method for preparing a low apparent viscosity, high molecular weight, para-aramid polymer solution. This embodiment is basically the same as Example 1, except that the amount of reaction controller added is different. In this embodiment, the amount of reaction controller added is 50 ppm (addition amount: the reaction controller is added to achieve the concentration of the reaction controller in the low apparent viscosity, high molecular weight, para-aramid polymer solution).

[0148] Example 11

[0149] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 100 ppm.

[0150] Example 12

[0151] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, except that the amount of reaction control agent added is different. In this embodiment, the amount of reaction control agent added is 150 ppm.

[0152] Example 13

[0153] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 200 ppm.

[0154] Example 14

[0155] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 250 ppm.

[0156] Example 15

[0157] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 300 ppm.

[0158] Example 16

[0159] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 350 ppm.

[0160] Example 17

[0161] A method for preparing a low apparent viscosity and high molecular weight para-aramid polymer solution. This embodiment is basically the same as Example 1, the only difference being the amount of reaction control agent added. In this embodiment, the amount of reaction control agent added is 400 ppm.

[0162] The para-aramid polymer solutions prepared in Examples 4 to 9 and the low apparent viscosity, high molecular weight para-aramid polymer solutions prepared in Examples 10 to 17 were tested for apparent viscosity. The para-aramid polymer solutions / low apparent viscosity, high molecular weight para-aramid polymer solutions were extruded into films, washed, and dried, and then tested for thermal weight loss and weight average molecular weight. The results are shown in Table 5.

[0163] Table 5

[0164] A method for preparing a slurry for a lithium battery coating diaphragm is the same as the method for preparing a slurry for a lithium battery coating diaphragm in Example 1, with the only difference being that the "low apparent viscosity high molecular weight para-aramid polymer solution" in the method for preparing a slurry for a lithium battery coating diaphragm in Example 1 is replaced by "one of the low apparent viscosity high molecular weight para-aramid polymer solutions prepared in Examples 10 to 17 and the para-aramid polymer solutions prepared in Examples 4 to 9."

[0165] A method for preparing a lithium battery separator is operated according to Example 1, "A method for preparing a lithium battery separator," wherein the slurry for lithium battery separator coating is one of those prepared in Examples 6, 7, and 10-16. (The slurry for lithium battery separator coating obtained using the para-aramid polymer solution prepared in Example 4 could not be slurried smoothly due to the excessive apparent viscosity of the polymer solution. The slurry for lithium battery separator coating obtained using the para-aramid polymer solution prepared in Example 5 had an apparent viscosity that was too high and could not be fed. The slurry for lithium battery separator coating obtained using the para-aramid polymer solution prepared in Example 6 had an apparent viscosity that was too high and fed slowly. The separator coating obtained using the para-aramid polymer solution prepared in Examples 8-9 shed during the coating process. The separator coating obtained using the low apparent viscosity, high molecular weight para-aramid polymer solution prepared in Example 17 shed severely during the coating process. Therefore, the slurries for lithium battery separator coating obtained in Examples 4-5, 8-9, and 17 were not used to prepare the lithium battery separator.)

[0166] The apparent viscosity test was performed on the slurries for lithium battery coating separators prepared in Examples 4 to 17 (Table 6), and the performance test was performed on the lithium battery separators prepared in Examples 6, 7, and Examples 10 to 16 (Table 7). The results are shown in Tables 6 and 7:

[0167] Table 6

[0168] Table 7

[0169] It can be concluded from the examples that the appropriate synthesis conditions are: the water content of the first solvent is less than or equal to 100 ppm, and the reaction controller is added so that the concentration of the reaction controller in the low apparent viscosity and high molecular weight para-aramid polymer solution is less than or equal to 300 ppm. At this time, the synthesized para-aramid polymer solution has a low apparent viscosity and a relatively high weight-average molecular weight, which is suitable for preparing lithium battery coating slurry.

[0170] From the above data analysis, by adjusting the ratio of the reaction control agent to the water content of the first solvent, the resulting lithium battery coating membrane slurry can well demonstrate the excellent thermal stability and high strength properties of para-aramid. The coating properties are directly related to the molecular weight of para-aramid, and can be suitable for preparing slurry. At the same time, the weight-average molecular weight of para-aramid with sufficient performance for lithium battery separators is in the range of 5000 to 10000. In the process of synthesizing the para-aramid polymer solution, the optimal ratio of the water content of the reaction control agent to the first solvent itself is determined to prepare a para-aramid polymer solution with low apparent viscosity and high molecular weight. The slurry for lithium battery coating membrane is prepared from the low apparent viscosity and high molecular weight para-aramid polymer solution. The battery separator obtained by coating with the slurry has excellent performance, and the thermal shrinkage, needle punch strength and air permeability are all better than those of patent CN109411676A.

[0171] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

1. A method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution, characterized in that: The following steps are involved: A reaction control agent is added during the process of synthesizing poly(p-phenylene terephthalamide) from p-phenylenediamine and terephthaloyl chloride to obtain a low apparent viscosity high molecular weight para-aramid polymer solution containing the reaction control agent, wherein the concentration of the reaction control agent in the low apparent viscosity high molecular weight para-aramid polymer solution is less than or equal to 300 ppm, the concentration of water in the first solvent used to synthesize poly(p-phenylene terephthalamide) is less than or equal to 100 ppm, and the reaction control agent is a solvent that is miscible with the first solvent and can simultaneously solidify the poly(p-phenylene terephthalamide).

2. The preparation method according to claim 1, characterized in that: The reaction control agent is deionized water, alcohols, esters or ethers.

3. The preparation method according to claim 1 or 2, characterized in that: The method for preparing a low apparent viscosity high molecular weight para-aramid polymer solution comprises the following steps: Step 1, in a nitrogen or inert gas atmosphere, mixing a dissolving salt and a first solvent, stirring until the dissolving salt is uniformly dispersed in the first solvent, to obtain a first mixed solution, wherein the ratio of the first solvent to the dissolving salt is (100-103): (3-8) by weight; Step 2, in a nitrogen or inert gas atmosphere, cooling the first mixed solution to 5-15° C., adding p-phenylenediamine to the first mixed solution, stirring until uniform, to obtain a second mixed solution, wherein the ratio of the mass fraction of the first mixed solution to the amount of p-phenylenediamine is 100:(16-20), the unit of the amount of the substance is mol, and the unit of the mass fraction is kg; Step 3, in a nitrogen or inert gas atmosphere, cooling the second mixed solution to -5 to 5°C, adding terephthaloyl chloride to the second mixed solution, and stirring until uniform, to obtain a low apparent viscosity high molecular weight para-aramid polymer solution, wherein the ratio of p-phenylenediamine to the terephthaloyl chloride is (1 to 1.05):1 in terms of molar fractions; In the steps 1 to 3, the reaction control agent is added once or multiple times in any step of the steps 1 to 3, so that the concentration of the reaction control agent in the low apparent viscosity high molecular weight para-aramid polymer solution is less than or equal to 300 ppm.

4. The preparation method according to any one of claims 1 to 3, characterized in that: In step 1, the first solvent is a non-aqueous solvent or an aqueous solvent, the non-aqueous solvent is a mixture of one or more of N-methylpyrrolidone, hexamethylphosphoramide, dimethylacetamide and tetramethylurea, and the aqueous solvent is a mixture of a non-aqueous solvent and water; In step 1, the stirring temperature is 50-90° C.; the dissolution-aiding salt is calcium chloride and / or lithium chloride.

5. A slurry for lithium battery coating diaphragm, characterized in that: include: A second solvent, ceramic solid particles and a low apparent viscosity high molecular weight para-aramid polymer solution obtained by the preparation method according to any one of claims 1 to 4, wherein, by mass, the ratio of the ceramic solid particles to the low apparent viscosity high molecular weight para-aramid polymer solution is (5 to 30): (50 to 80), and the ceramic solid particles are a mixture of one or more of alumina, fumed alumina, silica, zirconium oxide, aluminum hydroxide, magnesium hydroxide, barium sulfate, boehmite, boron nitride, silicon nitride and silicon carbide.

6. The slurry for lithium battery coating separator according to claim 5, characterized in that: Also includes: The dispersant is a mixture of one or more of a polymer block copolymer dispersant, a sodium salt of polyacrylate, a potassium salt of polyacrylate, an ammonium salt of polyacrylate and a sodium salt of polycarboxylate. The second solvent is the non-aqueous solvent or the aqueous solvent. Preferably, the second solvent is the same as the first solvent.

7. The slurry for lithium battery coating separator according to claim 6, characterized in that: In terms of mass fractions, the ratio of the second solvent to the ceramic solid particles is (60-90):(5-30).

8. A lithium battery separator, characterized in that: include: A base film and a coating covering one or both sides of the base film, wherein the coating is prepared from the slurry for lithium battery coating diaphragm according to any one of claims 5 to 7.

9. The method for preparing the lithium battery separator according to claim 8, comprising the following steps: The lithium battery coating separator is coated on one or both sides of the base film with a slurry, and a solvent step content extraction is performed, and the lithium battery separator is obtained after drying.

10. Application of the lithium battery separator obtained by the preparation method of claim 8 in lithium batteries.

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