A lithium battery composite separator and its preparation method

Abstract

The present invention discloses a highly safe lithium battery composite separator and a preparation method thereof. The highly safe lithium battery composite separator includes a base film and a coating layer. The coating layer is obtained by coating a coating slurry including a heat-resistant polymer solution, core-shell structured microspheres, a pore-forming agent, and a silane coupling agent on the base film. The content of the heat-resistant polymer in the heat-resistant polymer solution is 0.5 to 15 wt%. The core-shell structured microspheres include a shell layer and a core: the material forming the shell layer includes hydrolyzable and condensable ester substances; the material forming the core includes polymer microspheres, a surfactant, and a swelling agent. In the coating slurry of the present invention, core-shell structured microspheres are added, and the obtained separator has a lower closed pore temperature and a higher film-breaking temperature, thereby significantly improving the safety of the separator and providing double protection for the battery. At the same time, the core-shell structured microspheres encapsulate the polymer microspheres in the core, avoiding the influence of chemically inert polymer microspheres on the battery performance.

Description

Technical Field

[0001] The present invention relates to a lithium-ion battery, and more particularly to a high-safety lithium battery composite separator and a preparation method thereof. Background Art

[0002] The separator is a barrier that plays a safety function inside the lithium-ion battery. Currently, the commonly used separator material is polyolefin material, which has a relatively low melting point, generally <150 °C, and poor high-temperature heat resistance. When the internal temperature of the battery approaches the thermal deformation temperature of the separator, the separator will undergo shrinkage deformation, which will cause the positive and negative electrodes to come into direct contact and trigger internal thermal runaway. With the increase in the energy density of the battery, the separator used in the battery has gradually become thinner and lighter, but this will greatly increase the risk and harm degree of short circuit inside the battery.

[0003] In order to improve the deficiencies of the polyolefin film and increase the endurance mileage and heat resistance, ceramic modification is usually carried out on the polyolefin separator. The ceramic coating is used to improve the wettability and heat resistance of the separator and the electrolyte. However, since the base film is polyolefin in this way, it can only improve the heat resistance of the separator to a certain extent, and cannot increase the membrane-breaking temperature of the separator to above 200 °C.

[0004] When thermal runaway occurs inside the battery, a large amount of heat will be generated. At this time, if the separator can quickly close the pores in the initial stage of triggering thermal runaway and then block the continuous transmission of ions to form an open circuit, it can play a role in protecting the battery. The temperature at which the micropores close is the pore-closing temperature. For lithium batteries, it is better that the pore-closing temperature is lower. When the temperature rises, the micropores can be closed to prevent short circuit phenomena, thereby improving the safety of battery use. If the battery triggers further thermal runaway, if the used separator has a higher membrane-breaking temperature, it can have better integrity within a certain temperature range, and will not cause the positive and negative electrode plates to contact due to thermal shrinkage at high temperature or deformation due to a lower membrane-breaking temperature, thereby triggering further battery runaway.

[0005] Generally, for polyolefin materials, their pore-closing temperature is 130 - 140 °C, and the pore-closing temperature does not change much after coating with a heat-resistant inorganic ceramic coating. Their membrane-breaking (thermal shrinkage deformation) temperature is 150 - 170 °C. The safe use pore-closing - membrane-breaking temperature range of the separator is relatively small, only about <40 °C, which does not provide much guarantee for the safety performance of the battery.

[0006] Chinese Patent No. CN 113013547 A discloses a lithium battery composite separator and a preparation method thereof, but its safety needs to be further improved.

[0007] Therefore, it is necessary to develop a composite separator and a preparation method thereof that can simultaneously greatly increase the membrane-breaking temperature of the separator and lower the pore-closing temperature, thereby doubling the safety performance of the battery. Summary of the Invention

[0008] The object of the present invention is to solve the deficiencies of the above-mentioned background technology, and provide a composite separator and its preparation method that can simultaneously greatly increase the separator's film-breaking temperature and reduce the closing temperature, thereby doubly improving the battery safety performance.

[0009] The technical solution of the present invention is as follows: a lithium battery composite separator, the thermal shrinkage rates of the separator in the MD direction and TD direction after baking at 250 °C for 1 h are both less than 1.5%, the closing temperature of the separator is ≤ 125 °C, and the film-breaking temperature is ≥ 300 °C.

[0010] Preferably, the ionic conductivity of the separator is greater than 0.7 mS / cm;

[0011] The separator includes a base film and a coating layer. The coating layer includes core-shell structure microspheres. The core-shell structure microspheres include a shell layer and a core: the material forming the shell layer includes hydrolyzable polymerizable ester substances; the material forming the core includes polymer microspheres and an expander. The melting point of the polymer microspheres is 90 - 120 °C, and the expander is an organic solvent with a boiling point lower than 120 °C.

[0012] Furthermore, the coating layer further includes at least one of a heat-resistant polymer solution, a pore-forming agent, and a silane coupling agent;

[0013] and / or the material forming the core further includes a surfactant;

[0014] The ester substances are selected from at least one of tetraethyl orthosilicate, tetrabutyl orthotitanate, tetraethyl orthotitanate, triisopropyl aluminate, and trimethyl aluminate;

[0015] and / or the content of the heat-resistant polymer in the heat-resistant polymer solution is 0.5 - 15 wt%; the heat-resistant polymer is selected from at least one of para-aramid and meta-aramid;

[0016] and / or the mass ratio of the core-shell structure microspheres, the heat-resistant polymer, the pore-forming agent, and the silane coupling agent is 1:(0.3 - 2.5):(0.04 - 0.2):(0.02 - 0.35).

[0017] Even further, the polymer microspheres are selected from at least one of polystyrene, polyethylene, polymethyl methacrylate, polypropylene, poly(acrylate-butadiene-styrene), polylactic acid, polyvinyl chloride, ethylene-vinyl acetate copolymer, and polyvinyl butyral;

[0018] The surfactant is selected from at least one of octadecylamide ethyl diethyl benzyl ammonium chloride, octadecylamide ethyl trimethyl ammonium sulfate, dodecyl trimethyl ammonium bromide, and cetyl trimethyl ammonium bromide;

[0019] The blowing agent is selected from at least one of n-butanol, isobutanol, neopentyl alcohol, heptane, isooctane, and petroleum ether;

[0020] The mass ratio of the polymer microspheres, surfactant, and blowing agent is 1:(0.5 - 1):(0.08 - 0.2);

[0021] The mass ratio of the surfactant in the core material to the ester substance in the shell material is 1:(0.25 - 2);

[0022] The D50 particle size of the core-shell structured microspheres is 200 - 600 nm, and the shell thickness is 10 - 40 nm.

[0023] Furthermore, the pore-forming agent is selected from at least one of dimethyl carbonate, ethyl acetate, cyclohexane, or dimethyl phosphate;

[0024] The silane coupling agent is selected from at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, and N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane.

[0025] Preferably, the thickness of the coating layer is 0.5 - 4 μm, and the closed pore temperature of the lithium battery composite separator is 100 - 125 °C, and the film-breaking temperature is 300 - 600 °C.

[0026] The composite separator prepared by the present invention has a thermal shrinkage rate in both the MD direction and the TD direction of less than 1.5% after baking at 250 °C for 1 h. The closed pore temperature of the separator is ≤125 °C, and the film-breaking temperature is ≥300 °C. Thus, it can be seen that the composite separator prepared by the present invention has a lower closed pore temperature and a higher film-breaking temperature. At the same time, the composite separator prepared by the present invention takes into account that the thermal shrinkage rates in both the MD direction and the TD direction are relatively low, achieving a good balance in terms of closed pore temperature, film-breaking temperature, and thermal shrinkage rate, thereby significantly improving the safety of the separator and ultimately providing better dual protection for the battery.

[0027] The present invention also provides a preparation method for any one of the above lithium battery composite separators, including the following steps:

[0028] S1. Dissolve the co-solvent in the first solvent to obtain a first solution, and add the heat-resistant polymer to the first solution and dissolve it completely to obtain a heat-resistant polymer solution with a heat-resistant polymer content of 0.5 - 15 wt%. The heat-resistant polymer is selected from at least one of para-aramid and meta-aramid;

[0029] S2. Ultrasonically disperse polymer microspheres, surfactant and swelling agent in an absolute ethanol medium to form a core. After adding an ester substance and stirring evenly, add ammonia water for catalysis, and keep stirring until the reaction is complete to hydrolyze and polymerize the ester substance on the surface of the core to form a shell. Filter, and wash, filter and dry the obtained precipitate to obtain core-shell structured microspheres;

[0030] S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structured microspheres obtained in S2, then add a pore-forming agent and a silane coupling agent, stir evenly, and adjust the viscosity of the slurry to 20 - 600 mPa·S to obtain a coating slurry;

[0031] S4. Coat the coating slurry obtained in S3 on a base film, immerse it in water for film formation by the phase inversion method, then dry and cool and shape it to obtain a lithium battery composite separator.

[0032] Preferably, in step S1, the co-solvent is selected from at least one of calcium chloride, lithium chloride, potassium chloride, magnesium chloride, strontium chloride, barium chloride, the first solvent is selected from at least one of N-methylpyrrolidone NMP, N,N-dimethylacetamide DMAc, N,N-dimethylformamide DMF or dimethyl sulfoxide DMSO, and the mass ratio of the co-solvent to the first solvent is (1 - 4):100;

[0033] In step S2, the polymer microspheres are selected from at least one of polystyrene, polyethylene, polymethyl methacrylate, polypropylene, polyacrylate-butadiene-styrene, polylactic acid, polyvinyl chloride, ethylene-vinyl acetate copolymer, polyvinyl butyral;

[0034] The surfactant is selected from at least one of octadecylamide ethyl diethyl benzyl ammonium chloride, octadecylamide ethyl trimethyl ammonium sulfate, dodecyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide;

[0035] The swelling agent includes at least one of n-butanol, isobutanol, neopentyl alcohol, heptane, isooctane, petroleum ether;

[0036] The mass ratio of polymer microspheres, surfactant, and swelling agent is 1:(0.5 - 1):(0.08 - 0.2);

[0037] The mass ratio of the surfactant in the core material to the ester substance in the shell material is 1:(0.25 - 2), the mass concentration of the ammonia water is 23 - 28%, and the mass ratio of the ester substance to the ammonia water is 1:(2 - 4).

[0038] Preferably, in step S3, the mass ratio of the core-shell structured microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent is 1:(0.3 - 2.5):(0.04 - 0.2):(0.02 - 0.35);

[0039] The pore-forming agent is selected from at least one of dimethyl carbonate, ethyl acetate, cyclohexane or dimethyl phosphate;

[0040] The silane coupling agent is selected from at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane.

[0041] Preferably, in step S4, the thickness of the base film is 4-20 μm, and the base film is one of a polyethylene microporous membrane, a polypropylene microporous membrane, a polyethylene-polypropylene-polyethylene three-layer composite porous membrane, a polyimide membrane or a non-woven fabric membrane;

[0042] The thickness of the coating layer on the surface of the base film in the prepared high-safety lithium battery composite separator is 0.5-4 μm, and the closed pore temperature of the prepared high-safety lithium battery composite separator is 100-125 °C, and the film-breaking temperature is 300-600 °C.

[0043] Preferably, in step S1, when the heat-resistant polymer is dissolved in the first solution, it is heated in a water bath at 70-80 °C.

[0044] Preferably, in step S2, the D50 particle size of the obtained core-shell structured microspheres is 200-600 nm, and the shell layer thickness is 10-40 nm.

[0045] Preferably, in step S2, the polymer microspheres, the surfactant and the swelling agent are ultrasonically dispersed in an anhydrous ethanol medium to form a core, and after stirring at 60-70 °C for 1-2 h, an ester substance is added. After keeping warm and stirring evenly, ammonia water is added for catalysis, and the mixture is continuously stirred while keeping warm until the reaction is complete to hydrolyze and polymerize the ester substance on the surface of the core to form a shell layer. The obtained precipitate is filtered, washed, filtered and dried to obtain the core-shell structured microspheres.

[0046] Preferably, in step S3, the solid content of the coating slurry is 5%-40%.

[0047] In step S2 of the present invention, in the core material of the core-shell structured microspheres, when the polymer microspheres are dispersed in ethanol containing a surfactant, the surfactant forms an emulsifying layer covering the surface of the polymer microspheres and simultaneously encapsulates the swelling agent therein, forming a stable polymer microsphere / ethanol emulsion. In step S2, when the ester substance is added to the polymer microsphere / ethanol emulsion, due to the amphiphilic property of the ester substance, the ester substance will be captured by the surfactant layer and the ester substance exists on the surface of the polymer microspheres. When ammonia water is added to the emulsion containing polymer microspheres and ester substances, the ammonia water catalyzes the hydrolysis and polycondensation of the ester substances to form an inorganic oxide coating layer.

[0048] The present invention has the following beneficial effects:

[0049] (1) In the coating slurry of the present invention, core-shell structured microspheres are added. When the internal temperature of the battery exceeds 90 °C, the swelling agent in the core of the core-shell structured microspheres rapidly swells, causing the shell layer to crack and releasing the low-melting-point polymer microspheres in the core. The low-melting-point polymer melts and blocks the micropores of the base film, making the separator have a low closed-pore temperature and improving the safety performance of the separator.

[0050] (2) The silane coupling agent in the coating slurry of the present invention contains both polar and non-polar groups, enabling it to bind to both the core-shell structured microspheres, heat-resistant polymers, and the base film, thereby increasing the bonding firmness between the coating and the base film, strengthening the cohesive force of the coating, increasing the breakdown temperature of the separator, and improving the thermal stability performance of the separator. Therefore, the separator prepared by the present invention has a low closed-pore temperature and a high breakdown temperature, and can significantly improve the safety of the separator, providing double protection for the battery.

[0051] (3) The shell layer of the core-shell structured microspheres of the present invention is made of inorganic material and has an affinity for the electrolyte, which can reduce the direct contact between the core that is not wetted and not compatible with the electrolyte and the electrolyte, reducing the internal resistance of the battery and ensuring the performance of the battery capacity while improving the safety of the battery.

[0052] (4) The composite separator prepared by the present invention has a thermal shrinkage rate in both the MD direction and the TD direction of less than 1.5% after baking at 250 °C for 1 h. The closed-pore temperature of the separator ≤ 125 °C, and the breakdown temperature ≥ 300 °C. It can be seen that the composite separator prepared by the present invention has a low closed-pore temperature and a high breakdown temperature. At the same time, the composite separator prepared by the present invention also takes into account the low thermal shrinkage rate in both the MD direction and the TD direction, achieving a good balance in terms of closed-pore temperature, breakdown temperature, and thermal shrinkage rate, and significantly improving the safety of the separator, ultimately providing better double protection for the battery. Detailed implementation mode

[0053] The following specific examples further illustrate the present invention. Unless otherwise specified, the drugs used in the examples are commercially available products, and the methods used are conventional methods in the art unless otherwise specified.

[0054] Example 1

[0055] The present invention provides a method for preparing a highly safe lithium battery composite separator, comprising the following steps:

[0056] S1. Prepare a heat-resistant polymer solution: Dissolve the cosolvent CaCl2 in the first solvent NMP to obtain a first solution, with the mass ratio of CaCl2 to NMP being 1:100. Add the heat-resistant polymer (meta-aramid) to the first solution, with the mass ratio of meta-aramid to the first solution being 5.5:100. Heat and stir in a water bath at 70°C for 3 h until the meta-aramid fibers are completely dissolved. Isolate from air and let it stand to room temperature to obtain the heat-resistant polymer solution;

[0057] S2. Prepare core-shell structured microspheres: Mix the polymer microspheres (powdered polyethylene, 2 g, melting point 100°C), surfactant (cetyltrimethylammonium bromide CTAB, 1.5 g), and swelling agent (isobutanol, density 0.803 g / cm3, 0.4 mL, boiling point 105°C) with 100 mL of absolute ethanol, and ultrasonically disperse in a 250 mL three-necked flask;

[0058] Stir at 60°C for 60 minutes, drop in 1.4 g (1.5 mL, density 0.94 g / cm 3 ) of tetraethyl orthosilicate, then maintain the temperature at 60°C and stir for 30 minutes. Then drop in 4.5 g of 25% ammonia water by mass concentration, continue to stir at 60°C for 2 h until the reaction is complete. Filter the obtained precipitate, wash it thoroughly with ethanol, collect it by vacuum filtration, and finally dry it at 50°C for 12 h to obtain the core-shell structured microspheres. After measurement, the D50 particle size of the obtained core-shell structured microspheres is 428 nm, and the thickness of the shell layer is 25 nm.

[0059] S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structured microspheres obtained in S2, and then add a pore-forming agent (dimethyl carbonate) and a silane coupling agent (γ-aminopropyltriethoxysilane APTES). The respective amounts used of the core-shell structured microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent are 200 g, 300 g, 30 g, and 40 g. After stirring evenly, adjust the viscosity of the slurry to obtain a coating slurry with a solid content of 18.9% and a viscosity of 103 mPa·S;

[0060] S4. Coat the coating slurry obtained in S3 on a base film (polyvinyl film, thickness 7 μm), immerse it in water for phase inversion film formation, then dry and cool and shape it to obtain a highly safe lithium battery composite separator. After measurement, the thickness of the coating layer formed by the coating slurry is 2.98 μm.

[0061] Example 2

[0062] The present invention provides a method for preparing a highly safe lithium battery composite separator, comprising the following steps:

[0063] S1. Prepare a heat-resistant polymer solution: Dissolve the co-solvent CaCl2 in the first solvent NMP to obtain a first solution, with the mass ratio of CaCl2 to NMP being 1:100. Add the heat-resistant polymer (meta-aramid) to the first solution, with the mass ratio of meta-aramid to the first solution being 5.5:100. Heat and stir in a water bath at 70°C for 3 h until the meta-aramid fibers are completely dissolved. Isolate from air and let it stand to room temperature to obtain the heat-resistant polymer solution;

[0064] S2. Prepare core-shell structure microspheres: Mix the polymer microspheres (powdered polyethylene, 2 g, melting point 100°C), surfactant (cetyltrimethylammonium bromide CTAB, 1.5 g), and swelling agent (isobutanol, 0.803 g / cm 3 , 0.4 mL, boiling point 105°C) with 100 mL of absolute ethanol, and ultrasonically disperse in a 250 mL three-necked flask;

[0065] Stir at 60°C for 60 minutes, drop in 1.4 g (1.5 mL, density 0.94 g / cm 3 ) of tetraethyl orthosilicate, keep the temperature at 60°C, stir for 30 minutes, then drop in 4.5 g of 25% ammonia water by mass concentration, continue to stir at 60°C for 2 h until the reaction is complete, filter the resulting precipitate, wash it thoroughly with ethanol, collect it by vacuum filtration, and finally dry it at 50°C for 12 h to obtain the core-shell structure microspheres. After measurement, the D50 particle size of the obtained core-shell structure microspheres is 428 nm, and the thickness of the shell layer is 25 nm.

[0066] S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structure microspheres obtained in S2, and then add a pore-forming agent (dimethyl carbonate) and a silane coupling agent (γ-aminopropyltriethoxysilane). The respective dosages of the core-shell structure microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent are 200 g, 67 g, 30 g, and 40 g. After stirring evenly, adjust the viscosity of the slurry to obtain a coating slurry with a solid content of 19.1% and a viscosity of 152 mPa·S;

[0067] S4. Coat the coating slurry obtained in S3 on a base film (polyvinyl film, thickness 7 μm), immerse it in water for phase inversion film formation, and then dry and cool and shape it to obtain a highly safe lithium battery composite separator. After measurement, the thickness of the coating layer formed by the coating slurry is 3.02 μm.

[0068] Example 3

[0069] The present invention provides a method for preparing a highly safe lithium battery composite separator, comprising the following steps:

[0070] S1. Prepare a heat-resistant polymer solution: Dissolve the co-solvent CaCl2 in the first solvent NMP to obtain a first solution, with the mass ratio of CaCl2 to NMP being 1:100. Add the heat-resistant polymer (para-aramid) to the first solution, with the mass ratio of para-aramid to the first solution being 5.5:100. Heat and stir in a water bath at 70 °C for 3 h until the meta-aramid fiber is completely dissolved. Isolate from air and let it stand to room temperature to obtain the heat-resistant polymer solution;

[0071] S2. Prepare core-shell structure microspheres: Mix the polymer microspheres (powdered polyethylene, 2 g, melting point 100 °C), surfactant (cetyltrimethylammonium bromide CTAB, 1.5 g), and swelling agent (isobutanol, 0.803 g / cm 3 , 0.4 mL, boiling point 105 °C) with 100 mL of absolute ethanol, and ultrasonically disperse in a 250 mL three-necked flask;

[0072] Stir at 60 °C for 60 minutes, drop in 1.4 g (1.5 mL, density 0.94 g / cm 3 ) of tetraethyl orthosilicate, maintain the temperature at 60 °C, stir for 30 minutes, then drop in 4.5 g of 25% ammonia water by mass concentration, continue to stir at 60 °C for 2 hours until the reaction is complete. Filter the resulting precipitate, wash it thoroughly with ethanol, collect it by vacuum filtration, and finally dry it at 50 °C for 12 h to obtain the core-shell structure microspheres. After measurement, the D50 particle size of the obtained core-shell structure microspheres is 428 nm, and the thickness of the shell layer is 25 nm.

[0073] S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structure microspheres obtained in S2, and then add a pore-forming agent (dimethyl carbonate) and a silane coupling agent (γ-aminopropyltriethoxysilane APTES). The respective amounts used of the core-shell structure microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent are 200 g, 300 g, 30 g, and 40 g. After stirring evenly, adjust the viscosity of the slurry to obtain a coating slurry with a solid content of 18.3% and a viscosity of 114 mPa·S;

[0074] S4. Coat the coating slurry obtained in S3 on a base film (polyvinyl film, thickness 7 μm), immerse it in water for phase inversion film formation, and then dry and cool to shape to obtain a highly safe lithium battery composite separator. After measurement, the thickness of the coating layer formed by the coating slurry is 3.01 μm.

[0075] Example 4

[0076] The present invention provides a method for preparing a highly safe lithium battery composite separator, comprising the following steps:

[0077] S1. Preparation of heat-resistant polymer solution: Dissolve the cosolvent CaCl2 in the first solvent NMP to obtain the first solution. The mass ratio of CaCl2 to NMP is 1:100. Add the heat-resistant polymer (para-aramid) to the first solution. The mass ratio of para-aramid to the first solution is 5.5:100. Heat and stir in a water bath at 70 °C for 3 h until the meta-aramid fiber is completely dissolved. Isolate from air and let it stand to room temperature to obtain the heat-resistant polymer solution;

[0078] S2. Preparation of core-shell structure microspheres: Mix the polymer microspheres (powdered polyethylene, 2 g, melting point 100 °C), surfactant (cetyltrimethylammonium bromide CTAB, 1.5 g), and swelling agent (isobutanol, 0.4 mL, boiling point 105 °C) with 100 mL of absolute ethanol, and ultrasonically disperse in a 250 mL three-necked flask;

[0079] Stir at 60 °C for 60 minutes, drop in 1.4 g (1.5 mL, density 0.94 g / cm 3 ) of tetraethyl orthosilicate, then maintain the temperature at 60 °C and stir for 30 minutes. Then drop in 4.5 g of 25% ammonia water by mass concentration, and continue to stir at 60 °C for 2 h until the reaction is complete. Filter the obtained precipitate, wash it thoroughly with ethanol, collect it by vacuum filtration, and finally dry it at 50 °C for 12 h to obtain the core-shell structure microspheres. After measurement, the D50 particle size of the obtained core-shell structure microspheres is 428 nm, and the thickness of the shell layer is 25 nm.

[0080] S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structure microspheres obtained in S2, and then add a pore-forming agent (dimethyl carbonate) and a silane coupling agent (γ-aminopropyltriethoxysilane). The dosages of the core-shell structure microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent are 200 g, 67 g, 30 g, and 40 g respectively. After stirring evenly, adjust the viscosity of the slurry to obtain a coating slurry with a solid content of 19.5% and a viscosity of 126 mPa·S;

[0081] S4. Coat the coating slurry obtained in S3 on a base film (polyvinyl film, thickness 7 μm), immerse it in water for phase inversion film formation, then dry and cool to shape to obtain a high-safety lithium battery composite separator. After measurement, the thickness of the coating layer formed by the coating slurry is 2.97 μm.

[0082] Comparative Example 1

[0083] The specific preparation method of the composite separator in this comparative example is as follows:

[0084] 1) Preparation of heat-resistant polymer solution: The same as in Example 1

[0085] 2) Preparation of the slurry: Dissolve polyethylene microspheres (melting point: 100 °C) in 250 ml of NMP, and mix with the heat-resistant polymer solution obtained in step 1) and alumina (D50 = 0.8 μm). The mass ratio of polyethylene microspheres: aramid: alumina is 2:6:4. Stir the resulting mixture evenly to obtain the slurry;

[0086] 3) Preparation of the coating: Coating the slurry on a 7-μm polyethylene film, immersing it in water for phase inversion film formation, followed by drying and cooling for shaping to obtain the composite separator of Comparative Example 1, with the formed coating thickness of 3 μm.

[0087] Comparative Example 2

[0088] The specific preparation method of the composite separator in this comparative example is as follows:

[0089] 1) Preparation of the heat-resistant polymer solution: The same as in Example 1

[0090] 2) Preparation of the slurry: Mix the heat-resistant polymer solution obtained in step 1) and alumina (D50 = 0.8 μm). The mass ratio of aramid: alumina is 6:4. Stir the resulting mixture evenly to obtain the slurry;

[0091] 3) Preparation of the coating: Coating the slurry on a 7-μm polyethylene film, immersing it in water for phase inversion film formation, followed by drying and cooling for shaping to obtain the composite separator of Comparative Example 1, with the formed coating thickness of 3 μm.

[0092] Performance testing

[0093] Assemble the composite separators of Examples 1 to 4 and Comparative Examples 1 to 2 with the positive and negative electrode sheets respectively to obtain the lithium-ion batteries of Examples 1 to 4 and Comparative Examples 1 to 2.

[0094] The specific preparation method includes the following steps: Stir, coat, roll, and slit the positive and negative electrode slurries to obtain the positive electrode sheet and the negative electrode sheet. The active material of the positive electrode sheet is lithium iron phosphate, and the active material of the negative electrode sheet is artificial graphite. Stack the positive electrode sheet, the composite separator, and the negative electrode sheet, and then complete the processes of stacking and top sealing to make a soft-pack lithium-ion battery. Then, place the soft-pack lithium-ion battery in a vacuum oven at 80 °C for baking for 12 - 24 h; when the mixed water content of the negative electrode sheet of the bare battery cell < 150 ppm, perform the processes of automatic liquid injection, high-temperature standing, negative-pressure formation, sealed welding, grading, and testing on the soft-pack lithium battery. Finally, obtain the soft-pack lithium-ion battery. Collect and test the following data for the composite separators and lithium-ion batteries of Examples 1 to 4 and Comparative Examples 1 to 2:

[0095] 1. Diaphragm closure temperature

[0096] Using the temperature-rising internal resistance method, record the resistance values at different temperatures, and take the temperature corresponding to the maximum resistance value as the closure temperature.

[0097] 2. Diaphragm rupture temperature

[0098] Use TMA instrument to test and record the curve of diaphragm length changing with temperature until the diaphragm breaks; the temperature when the diaphragm length increases instantly is the diaphragm rupture temperature.

[0099] 3. Diaphragm ion conductivity

[0100] Place the diaphragm in an electrolyte at a temperature of 23±2℃, keep it sealed, and soak for 2h. Inject the electrolyte into the resistance test mold, connect it to the electrochemical workstation, and set the test parameters. Place one layer of diaphragm in turn, test its impedance spectrum, then place another layer, test its impedance spectrum, until four layers are placed, measure four impedance spectra, and read the resistance values ​​R1, R2, R3 and R4 for layers 1 to 4 from the impedance spectra. Where σ=d / (R*S). σ: ionic conductivity; d: thickness of a single-layer diaphragm; R: resistance value; S: test diaphragm area.

[0101] 4.250℃&1h thermal shrinkage

[0102] The composite diaphragm was clamped with printing paper and placed in an oven at 250°C for 1 hour. The dimensions of the diaphragm in the MD and TD directions before and after baking were recorded. Thermal shrinkage = 1 - dimension after baking / dimension before baking.

[0103] 5. Battery cycle capacity retention rate

[0104] Test method: At 25°C, charge the divided battery at 1.0C constant current and constant voltage to 3.65V, with a cut-off current of 0.05C, and then discharge it at 1.0C constant current to 2.5V. Repeat this cycle. After 2500 cycles of charge and discharge, calculate the capacity retention rate at the 2500th week. The calculation formula is as follows: 2500th cycle capacity retention rate (%) = (2500th cycle discharge capacity / first cycle discharge capacity) × 100%.

[0105] The test results of the above performances are shown in Table 1.

[0106] Table 1

[0107]

[0108] As can be seen from Table 1 above, the functional diaphragms with safety performance prepared in Examples 1-4 of the present invention have higher heat resistance, with a thermal shrinkage of <1.5% at 250°C for 1 h, while the composite diaphragms prepared in Comparative Examples 1-2 have a thermal shrinkage rate of >5% at 250°C for 1 h; in addition, as can also be seen from Table 1 above, the closed pore temperatures of the functional diaphragms with safety performance prepared in Examples 1-4 of the present invention are all less than 125°C, and the film-breaking temperatures are all >300°C. Although the closed pore temperature of the composite diaphragm prepared in Comparative Example 1 is also less than 125°C, since the electrolyte-inert polyethylene is in direct contact with the electrolyte, it affects the ion transport performance of the diaphragm; in Comparative Example 2, there is only a porous layer of heat-resistant polymer, and no low-melting polymer is added, so the diaphragm has no closed pore temperature, and the diaphragm will directly break at a certain temperature.

[0109] In summary, it can be seen that the use safety windows of the composite diaphragms prepared in Comparative Examples 1-2 are significantly smaller than those of the functional diaphragms with safety performance prepared in Examples 1-4 of the present invention, and the composite diaphragms prepared in Examples 1-4 do not affect the electrical performance of the battery.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium battery composite separator, characterized in that, The thermal shrinkage rates of the diaphragm in the MD direction and TD direction after baking at 250 °C for 1 h are both less than 1.5%, the closed pore temperature of the diaphragm is ≤125 °C, and the film breaking temperature is ≥300 °C; The ionic conductivity of the diaphragm is greater than 0.7 mS / cm; The diaphragm includes a base film and a coating layer. The coating layer includes core-shell structure microspheres. The core-shell structure microspheres include a shell layer and a core. The shell layer is formed by hydrolysis polymerization of an ester substance. The ester substance is selected from at least one of tetraethyl orthosilicate, tetrabutyl orthotitanate, tetraethyl orthotitanate, triisopropyl aluminate, and trimethyl aluminate; The material forming the core includes polymer microspheres and an expanding agent. The melting point of the polymer microspheres is 90-120 °C, and the expanding agent is an organic solvent with a boiling point lower than 120 °C.

2. The lithium battery composite separator according to claim 1, wherein The raw materials of the coating layer further include at least one of a heat-resistant polymer solution, a pore-forming agent, and a silane coupling agent; and / or the material forming the core further includes a surfactant; and / or the content of the heat-resistant polymer in the heat-resistant polymer solution is 0.5-15 wt%; the heat-resistant polymer is selected from at least one of para-aramid and meta-aramid; and / or the mass ratio of the core-shell structure microspheres, heat-resistant polymer, pore-forming agent, and silane coupling agent is 1:(0.3-2.5):(0.04-0.2):(0.02-0.35).

3. The lithium battery composite separator according to claim 2, wherein The polymer microspheres are selected from at least one of polystyrene, polyethylene, polymethyl methacrylate, polypropylene, polyacrylate-butadiene-styrene, polylactic acid, polyvinyl chloride, ethylene-vinyl acetate copolymer, and polyvinyl butyral; The surfactant is selected from at least one of octadecanamide ethyl diethyl benzyl ammonium chloride, octadecanamide ethyl trimethyl ammonium sulfate, dodecyl trimethyl ammonium bromide, and cetyl trimethyl ammonium bromide; The expanding agent is selected from at least one of n-butanol, isobutanol, neopentyl alcohol, heptane, isooctane, and petroleum ether; The mass ratio of polymer microspheres, surfactant, and expanding agent is 1:(0.5-1):(0.08-0.2); The mass ratio of the surfactant in the core material to the ester substance in the shell material is 1:(0.25-2); The D50 particle size of the core-shell structure microspheres is 200-600 nm, and the shell layer thickness is 10-40 nm.

4. The lithium battery composite separator according to claim 2, wherein, The pore-forming agent is selected from at least one of dimethyl carbonate, ethyl acetate, cyclohexane, or dimethyl phosphate; The silane coupling agent is selected from at least one of γ-aminopropyltriethoxysilane, γ-glycidyletheroxypropyltrimethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, and N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane.

5. The lithium battery composite separator according to claim 1, wherein, The thickness of the coating layer is 0.5-4 μm, and the closed pore temperature of the lithium battery composite diaphragm is 100-125 °C, and the film breaking temperature is 300-600 °C.

6. A method for preparing a lithium battery composite separator according to any one of claims 1-5, characterized in that, Including the following steps: S1. Dissolve the cosolvent in the first solvent to obtain a first solution, and add the heat-resistant polymer to the first solution and dissolve it completely to obtain a heat-resistant polymer solution with a heat-resistant polymer content of 0.5-15 wt%, and the heat-resistant polymer is selected from at least one of para-aramid and meta-aramid; S2. Ultrasonically disperse the polymer microspheres, surfactant and swelling agent in an absolute ethanol medium to form a core, add the ester substance and stir evenly, then add ammonia water for catalysis, and keep stirring until the reaction is complete to hydrolyze and polymerize the ester substance on the surface of the core to form a shell layer. Filter, wash, filter and dry the obtained precipitate to obtain core-shell structured microspheres; S3. Mix the heat-resistant polymer solution obtained in S1 and the core-shell structured microspheres obtained in S2, then add a pore former and a silane coupling agent, stir evenly and adjust the viscosity of the slurry to 20-600 mPa·S to obtain a coating slurry; S4. Coating the coating slurry obtained in S3 on a base film, immersing it in water for phase inversion film formation, and then drying and cooling for shaping to obtain a lithium battery composite separator.

7. The preparation method of the lithium battery composite separator according to claim 6, characterized in that, In step S1, the cosolvent is selected from at least one of calcium chloride, lithium chloride, potassium chloride, magnesium chloride, strontium chloride and barium chloride, and the first solvent is selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), and the mass ratio of the cosolvent to the first solvent is (1-4):100; In step S2, the polymer microspheres are selected from at least one of polystyrene, polyethylene, polymethyl methacrylate, polypropylene, poly(acrylate-butadiene-styrene), polylactic acid, polyvinyl chloride, ethylene-vinyl acetate copolymer and polyvinyl butyral; The surfactant is selected from at least one of octadecylamide ethyldiethylbenzylammonium chloride, octadecylamide ethyltrimethylammonium sulfate, dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide; The swelling agent includes at least one of n-butanol, isobutanol, neopentyl alcohol, heptane, isooctane and petroleum ether; The mass ratio of the polymer microspheres, surfactant and swelling agent is 1:(0.5-1):(0.08-0.2); The mass ratio of the surfactant in the core material to the ester substance in the shell material is 1:(0.25-2), the mass concentration of the ammonia water is 23-28%, and the mass ratio of the ester substance to the ammonia water is 1:(2-4).

8. The preparation method of the lithium battery composite separator according to claim 6, characterized in that, In step S3, the mass ratio of the core-shell structured microspheres, heat-resistant polymer, pore former and silane coupling agent is 1:(0.3-2.5):(0.04-0.2):(0.02-0.35); The pore former is selected from at least one of dimethyl carbonate, ethyl acetate, cyclohexane or dimethyl phosphate; The silane coupling agent is selected from at least one of γ-aminopropyltriethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, γ-(acryloyloxy)propyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltriethoxysilane, and N-β-(aminoethyl)-γ-aminopropylmethyldimethoxysilane.

9. The preparation method of the lithium battery composite separator according to claim 6, characterized in that, In step S4, the thickness of the base film is 4-20 μm, and the base film is one of a polyethylene microporous membrane, a polypropylene microporous membrane, a polyethylene-polypropylene-polyethylene three-layer composite porous membrane, a polyimide membrane, or a non-woven fabric membrane; The thickness of the coating layer on the surface of the base film in the prepared lithium battery composite separator is 0.5-4 μm, and the closed pore temperature of the prepared lithium battery composite separator is 100-125 °C, and the membrane breaking temperature is 300-600 °C.

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