Mesoporous alumina composite diaphragm, preparation method and application

Abstract

The invention discloses a mesoporous alumina composite diaphragm, a preparation method and application. The composite diaphragm comprises a polar polymer layer and a porous alumina layer which are laminated. The polar polymer layer can reduce the charge transfer impedance of the lithium battery and improve the cycle capacity of the lithium battery; meanwhile, the polar polymer layer is rich in polar groups and can absorb a large amount of electrolyte; the porous aluminum oxide layer can reduce the stacking density of the diaphragm coating layer, so that the diaphragm is light; meanwhile, the problem that the porosity of the base membrane and the ionic conductivity are reduced can be solved; in addition, the porous aluminum oxide layer has a flat surface and can induce lithium nucleation homogenization, so that the growth of lithium dendrites is inhibited.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery separators, and more specifically, relates to a porous alumina composite separator, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, commercial lithium batteries use graphite anodes, whose theoretical capacity (372 mAh g -1 ) and energy density (300 Wh kg -1 ) are both relatively low, making it difficult to meet the long endurance requirements of electronic devices, electric vehicles, drones, etc. Commercial lithium batteries mainly consist of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the separator plays a role in isolating the positive and negative electrodes of the battery and providing a transmission channel for lithium ions, and is one of the key components determining the safety and electrochemical performance of lithium-ion batteries. Polyolefin separators are currently the mainstream separators for lithium batteries. However, commercial polyolefin separators have poor affinity with electrolytes, and the resistance suffered during the ion transmission process is large, which limits the improvement of the battery's electrochemical performance. At the same time, polyolefin separators have poor thermal stability and are prone to thermal shrinkage at high temperatures, resulting in contact between the positive and negative electrodes and battery short circuits, triggering the risk of battery thermal runaway. In addition, the poor thermal conductivity of commercial separators causes the current density in the high-temperature area of the battery to be higher than that in the low-temperature area, and the uneven diffusion of lithium ions promotes the growth of lithium dendrites, affecting the safety performance of the battery.

[0003] Grafting or coating modification on the surface of polyolefin separators is a common method to improve the above problems. For example, Patent CN111969161A discloses a porous alumina ceramic-coated separator; Patent CN118712663A discloses a graft-modified alumina ceramic-coated separator. Surface grafting or coating modification can improve the high-temperature thermal shrinkage of the separator to a certain extent, thus avoiding safety problems such as battery combustion and explosion caused by thermal runaway. However, the surface grafting or coating modification layer cannot fundamentally solve the problems of low porosity and poor thermal stability of polyolefin separators. Selecting a polar polymer separator with excellent thermal stability and high electrolyte affinity to replace the polyolefin separator is a common method to improve the above problems. For example, Patent CN116315451A discloses an alumina-modified para-aramid fiber-based lithium-sulfur battery separator; Patent CN109411682B discloses an alumina-modified polyimide separator and a preparation method thereof. This alumina surface-modified polar polymer separator can further improve the heat resistance of the separator. However, coating alumina with a relatively low specific surface area results in a large packing density and low porosity of the separator coating, restricting the improvement of the overall performance of the battery. Summary of the Invention

[0004] Aiming at the defects of the prior art, the purpose of the present invention is to provide a porous alumina composite separator, a preparation method thereof, and an application thereof, aiming to solve the problems such as low thermal conductivity and insufficient thermal safety of existing separators.

[0005] To achieve the above object, the present invention provides a porous alumina composite separator, characterized in that the composite separator comprises a polar polymer layer and a porous alumina layer arranged in a stacked manner.

[0006] According to an embodiment of the present invention, the composite separator comprises a polar polymer layer and a porous alumina layer located above the polar polymer layer.

[0007] According to an embodiment of the present invention, the thickness of the polar polymer layer is 20 - 60 μm, and the thickness of the alumina layer is 2 - 10 μm.

[0008] According to an embodiment of the present invention, the particle size of the porous alumina is 100 - 400 nm; the pore size is 2 - 15 nm; the specific surface area is 10 - 80 m 2 / g, preferably 30 - 60 m 2 / g, more preferably 40 - 50 m 2 / g.

[0009] The present invention also provides a method for preparing the above composite separator, characterized by comprising the following steps:

[0010] S1. Dispersing porous alumina and a binder in a solvent to obtain a coating slurry;

[0011] S2. Mixing and reacting a polar polymer, a strong base, a proton transfer agent and a solvent to obtain a polar polymer fiber dispersion;

[0012] S3. Coating the coating slurry described in step S1 on a substrate, drying to obtain an alumina layer; then, coating the polar polymer fiber dispersion described in step S2 on the alumina layer, subjecting it to phase inversion in water and drying to obtain a composite separator.

[0013] According to an embodiment of the present invention, in step S1, the binder is one or more of polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol or carboxymethyl cellulose.

[0014] According to an embodiment of the present invention, in step S1, the mass ratio of the porous alumina to the binder is (3 - 9):1.

[0015] According to an embodiment of the present invention, in step S1, the solid content of the coating slurry is 10 - 30 wt%.

[0016] According to an embodiment of the present invention, in step S2, the polar polymer comprises one or more of para - aramid, meta - aramid, polyimide or polyacrylonitrile fiber.

[0017] According to an embodiment of the present invention, in step S2, the strong base includes one or more of sodium hydroxide, potassium hydroxide, or potassium tert-butoxide.

[0018] According to an embodiment of the present invention, in step S2, the proton transfer agent includes one or more of water, absolute ethanol, or absolute methanol.

[0019] According to an embodiment of the present invention, in step S1 or step S2, the solvent is selected from one or more of water, 1-methyl-2-pyrrolidone, dimethyl sulfoxide, or N,N-dimethylformamide.

[0020] According to an embodiment of the present invention, in step S2, the polar polymer, strong base, proton transfer agent, and solvent are in a mass ratio of 3:(3-6):(3-6):(50-91).

[0021] According to an embodiment of the present invention, in step S3, the material of the substrate is not particularly limited as long as the coating slurry can form a film. For example, the substrate is a glass plate.

[0022] According to an embodiment of the present invention, in step S3, the solid content of the coating slurry is 10-30 wt%, for example, 10 wt%, 20 wt%, or 30 wt%.

[0023] According to an embodiment of the present invention, in step S3, the coating thickness of the alumina layer is 2-10 μm.

[0024] According to an embodiment of the present invention, in step S3, the phase inversion time is 5-30 min, for example, 5 min, 10 min, 15 min, 20 min, or 30 min. If the phase inversion time is too short, it will cause the collapse of the porous structure of the composite diaphragm during the subsequent drying process.

[0025] According to an embodiment of the present invention, step S3 further includes a post-treatment step: the phase-inverted polymer porous membrane is successively soaked in ethanol and n-hexane for solvent exchange. Among them, the soaking time in ethanol and n-hexane is 5-15 min, for example, 5 min, 10 min, 15 min; then the porous membrane containing n-hexane is hot-pressed and dried on a hot stage at 90-120 °C to obtain the composite diaphragm.

[0026] According to an embodiment of the present invention, in step S1, the preparation method of the porous alumina is as follows:

[0027] (1) Mix and react an aluminum salt, a base, and water to prepare an aluminum hydroxide precipitate.

[0028] (2) After heating and calcining the aluminum hydroxide precipitate obtained in the above step (1), the porous alumina is obtained.

[0029] According to an embodiment of the present invention, in step (1), the aluminum salt is one or more of aluminum nitrate, aluminum sulfate, aluminum chloride or aluminum isopropoxide;

[0030] According to an embodiment of the present invention, in step (1), the base is one or more of urea, hexamethylenetetramine or ammonia water.

[0031] According to an embodiment of the present invention, in step (1), the molar ratio of the aluminum salt to the base is 1:(10-100).

[0032] According to an embodiment of the present invention, in step (1), the temperature of the mixing reaction is 60-120° C., preferably 70-100° C., and the mixing time is 1-8 h, preferably 1-6 h.

[0033] According to an embodiment of the present invention, in step (2), the heating and calcining temperature is 100-1200°C, the heating and calcining time is 1-10h; the heating rate during heating and calcining is 2-10°C / min, preferably 2-5°C / min.

[0034] According to an embodiment of the present invention, in step (2), the calcination is carried out at a gradient rising temperature, for example, first maintaining at 100-700°C for 0-1h (preferably 0.1-1h), and then maintaining at 1100-1200°C for 1-4h.

[0035] According to an embodiment of the present invention, in step (2), the calcination temperature is increased to 100-700°C at 2-5°C / min and maintained for 1-2 hours; then increased to 1100-1200°C at 2-5°C / min and maintained for 2-3 hours.

[0036] As an exemplary embodiment of the present invention, the method for preparing the composite diaphragm is characterized by comprising the following steps:

[0037] (1) Aluminum salt, alkali and water are fully mixed and reacted at 95°C for 3-5 hours to generate aluminum hydroxide precipitate;

[0038] (2) filtering, washing, and drying the aluminum hydroxide precipitate obtained in step (1), and then calcining the precipitate once or multiple times to obtain porous alumina;

[0039] (3) dispersing the porous alumina obtained in step (2) and a binder in a solvent to obtain a coating slurry;

[0040] (4) mixing a polar polymer, a strong base, a proton transfer agent, and a solvent in a certain proportion and reacting for a period of time to obtain a polymer fiber dispersion;

[0041] (5) Uniformly coat the porous alumina coating slurry described in step (3) on the substrate, and obtain a porous alumina layer after sufficient drying;

[0042] (6) Uniformly coat the polymer fiber dispersion liquid described in step (4) on the surface of the porous alumina layer described in step (5), and perform phase inversion in water. After 10 minutes, the water-containing composite separator falls off from the substrate, and a composite separator is obtained after drying.

[0043] The present invention also provides a battery separator material, which includes the above-mentioned porous alumina composite separator.

[0044] The present invention also provides the application of the above-mentioned porous alumina composite separator or battery separator material in lithium batteries.

[0045] Advantages of the present invention:

[0046] (1) The porous alumina provided by the present invention is obtained by reacting an aluminum salt and an alkali in an aqueous solution to obtain a precipitate of aluminum hydroxide, and then calcining it one or more times. By regulating the ratio of the aluminum salt to the alkali, the processes of burst nucleation and slow growth are carried out separately, so that the obtained spherical aluminum hydroxide particles are composed of small aluminum hydroxide particles. After calcination at different temperatures, the following transformations will occur:

[0047] Aluminum hydroxide → γ-alumina → α-alumina

[0048] Among them, during the calcination process, the spherical small aluminum hydroxide particles first lose water and shrink. Because this process is an amorphous reconstruction-type transformation, the obtained γ-alumina is still composed of spherical small grains. Then, the γ-alumina small grains are transformed into α-alumina small grains. This process is a lattice reconstruction-type transformation, so the obtained α-alumina is still spherical; after long-term high-temperature calcination, the spherical grains gradually form worm-shaped grains, and finally further sintering forms porous α-alumina.

[0049] (2) The present invention provides a porous alumina composite separator, which includes a polar polymer layer and a porous alumina layer arranged in a stacked manner. The polar polymer layer can reduce the charge transfer impedance of the lithium battery and improve the cycle capacity of the lithium battery; at the same time, the polar polymer layer is rich in polar groups and can absorb a large amount of electrolyte; the porous alumina layer can reduce the packing density of the separator coating layer and make the separator lightweight; at the same time, it can improve the problem of reducing the porosity and ionic conductivity of the base film; in addition, the porous alumina layer has a flat surface, which can induce uniform lithium nucleation and thus inhibit the growth of lithium dendrites.

[0050] (3) The porous alumina composite separator of the present invention uses a polar polymer with excellent thermal stability and porous alumina with high thermal conductivity, making the separator have excellent thermal stability and thermal diffusivity, effectively avoiding thermal shrinkage or melting of the separator at high temperatures, serious dendrite growth piercing the separator in the case of uneven battery temperature distribution, and further causing safety accidents, and effectively improving the cycle performance and safety performance of the battery.

[0051] (4) The preparation method of the composite separator provided by the present invention has the advantages of simple process and high production efficiency, and is convenient for large-scale production and application; specifically, in the preparation of porous alumina, a stable porous structure is effectively formed through a one-stage or multi-stage gradient temperature calcination process, avoiding pore collapse during high-temperature treatment; different thicknesses of porous alumina layers and polymer layers are obtained by means of doctor blade coating, which is convenient for mass production; a phase inversion and solvent exchange synergistic process (step-by-step replacement of water → ethanol → n-hexane) is adopted to construct a porous network structure, and combined with a 120 °C hot stage 10 s flash drying process, while realizing rapid prototyping, the drying time is greatly shortened, and the deformation of the film layer structure caused by traditional long-time high-temperature drying is avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 SEM cross-sectional view of the porous alumina composite separator prepared in Example 1.

[0053] Figure 2 Transmission electron microscope image of the porous alumina prepared in Example 1.

[0054] Figure 3 Nitrogen adsorption and desorption curve of the porous alumina prepared in Example 1.

[0055] Figure 4 Infrared thermal imaging diagram of the porous alumina composite separator prepared in Example 1.

[0056] Figure 5 For the Li||Li symmetric battery assembled in Example 1 and Comparative Example 1 at a current density of 0.5 mA / cm 2 and a capacity of 0.5 mAh / cm 2 The lithium deposition curve obtained by cycling.

[0057] Figure 6 Specific capacity - cycle number curve obtained by cycling 200 times at a cycle rate of 1C at 70 °C for the lithium metal||lithium iron phosphate batteries assembled in Example 1, Comparative Example 1 and Comparative Example 2 respectively. DETAILED DESCRIPTION OF THE INVENTION

[0058] The technical solution of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the following embodiments are only for exemplarily illustrating and explaining the present invention, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0059] Unless otherwise specified, the raw materials and reagents used in the following embodiments are all commercially available products or can be prepared by known methods.

[0060] A porous alumina composite separator provided by the present invention is characterized in that the composite separator includes a polar polymer layer and a porous alumina layer which are stacked. When this asymmetric separator works, its porous alumina layer faces the negative electrode side of the battery, and the polar polymer layer faces the positive electrode side of the electrode.

[0061] In some embodiments, the thickness of the polymer layer of the composite separator is 20 - 60 μm, and the alumina layer is 2 - 10 μm.

[0062] In some embodiments, the particle size of the porous alumina is 100 - 400 nm; the pore size is 2 - 15 nm; the specific surface area is 10 - 80 m 2 / g.

[0063] The present invention also provides a preparation method of the above-mentioned porous alumina composite separator, which is characterized by including the following steps:

[0064] (1) After mixing an aluminum salt, an alkali and water, stir at room temperature for 30 min, then heat to 95 °C at a heating rate of 2 - 5 °C / min, and react for 3 - 5 h to generate aluminum hydroxide precipitate;

[0065] (2) After filtering, washing and drying the aluminum hydroxide precipitate obtained in step S1, porous alumina is obtained after one or more calcinations;

[0066] (3) Disperse the porous alumina obtained in step S2 and a binder in a solvent to obtain a coating slurry; wherein the solvent is one or more of water, ethanol, 1 - methyl - 2 - pyrrolidone, dimethyl sulfoxide, N,N - dimethylformamide or N,N - dimethylacetamide;

[0067] (4) Mix a polar polymer, a strong base, a proton transfer agent and a solvent according to a mass ratio of 3:(3 - 6):(3 - 6):(50 - 91), and react at 25 - 100 °C for 12 h to obtain a polar polymer nanofiber dispersion;

[0068] (5) Uniformly coat the coating slurry described in step S3 on one side of a clean glass plate with a 0 - 50 μm doctor blade, and after drying for 6 h, a glass plate containing an alumina layer is obtained;

[0069] (6) The polar polymer fiber dispersion described in S4 is uniformly coated on the surface of the porous alumina layer described in step S5 with a doctor blade of 100-500 μm to obtain a coated alumina layer;

[0070] (7) The coated alumina layer described in step S6 is placed in water for phase inversion. After 10 min, the strong interaction between the polymer porous membrane and the alumina layer causes the alumina layer to fall off from the glass plate, obtaining a polymer-coated film containing water. Then, it is successively immersed in ethanol and n-hexane liquids for 10 min. After solvent exchange, it is dried on a hot stage at 95-120 °C for 10 s to obtain a porous alumina composite separator.

[0071] In some embodiments, in step (1), the aluminum salt is one or more of aluminum nitrate, aluminum sulfate, aluminum chloride, or aluminum isopropoxide.

[0072] In some embodiments, the above base is one or more of urea, hexamethylenetetramine, or ammonia water.

[0073] In some embodiments, in step (1), the molar ratio of the aluminum salt to the base is 1:(10-100).

[0074] In some embodiments, in step (2), the temperature for heating and calcining is 100-1200 °C, and the time for heating and calcining is 1-10 h; the heating rate during heating and calcining is 2-10 °C / min, preferably 2-5 °C / min.

[0075] According to the embodiment of the present invention, in step (2), the calcination is carried out at a gradually increasing temperature. For example, it is first maintained at 100-700 °C for 0-1 h (preferably 0.1-1 h), and then maintained at 1100-1200 °C for 1-4 h.

[0076] In some embodiments, in step (3), the binder is one or more of polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol, or carboxymethyl cellulose.

[0077] In some embodiments, in step (3), the mass ratio of the porous alumina to the binder is (3-9):1.

[0078] In some embodiments, in step (4), the polar polymer includes one or more of para-aramid, meta-aramid, polyimide, or polyacrylonitrile fiber.

[0079] In some embodiments, in step (4), the strong base includes one or more of sodium hydroxide, potassium hydroxide, or potassium tert-butoxide.

[0080] In some embodiments, in step (4), the proton transfer agent includes one or more of water, absolute ethanol, or absolute methanol.

[0081] In some embodiments, in step (4), the solvent comprises one or more of water, 1-methyl-2-pyrrolidone, dimethyl sulfoxide or N,N-dimethylformamide.

[0082] In some embodiments, in step (5), the solid content of the coating slurry is 10-30 wt%.

[0083] In some embodiments, in step (5), the thickness of the alumina layer is 2-10 μm.

[0084] Example 1

[0085] A porous alumina composite separator is composed of a polar polymer layer and a porous alumina layer, wherein the polymer layer is composed of para-aramid, and the porous alumina layer is composed of a binder polyvinylidene fluoride and porous alumina with a particle size of 200 nm; wherein, the thickness of the polymer layer is 40±5 μm, and the thickness of the alumina layer is 3±0.5 μm.

[0086] The preparation method of the porous alumina composite separator is characterized by comprising the following steps:

[0087] (1) Aluminum sulfate and aluminum nitrate are added to ultrapure water according to a molar ratio of 4:6. Meanwhile, a quantitative amount of urea is added according to a molar ratio of base:Al 3+ molar ratio of 60, and stirred evenly. Then it is heated to 95 °C at a heating rate of 2 °C / min and reacted for 3 h to obtain aluminum hydroxide precipitate.

[0088] (2) After the above-mentioned aluminum hydroxide precipitate is filtered, washed and dried, it is heated to 1100 °C at a rate of 5 °C / min and calcined continuously for 3 h to obtain porous alumina.

[0089] (3) The porous alumina and the binder polyvinylidene fluoride are dispersed in 1-methyl-2-pyrrolidone according to a mass ratio of 9:1 to prepare a slurry, wherein the solid content of the slurry is 10 wt%; then the above slurry is evenly coated on one side of a clean glass plate with a 25-μm doctor blade, and placed in an oven at 60 °C for drying for 6 h to obtain a glass plate with an alumina layer.

[0090] (4) Para-aramid, potassium hydroxide, water and dimethyl sulfoxide are mixed according to a mass ratio of 3:3:3:91, and reacted at 95 °C for 12 h to obtain a para-aramid nanofiber dispersion.

[0091] (5) The para-aramid nanofiber dispersion was uniformly coated on the alumina layer obtained in (3) with a 250-μm doctor blade, and then quickly placed into water for phase inversion. After 10 min, the strong interaction between the para-aramid porous membrane and the alumina layer caused the alumina layer to peel off from the glass plate, obtaining a water-containing para-aramid coated film. Then, it was successively immersed in ethanol and n-hexane for 10 min. After solvent exchange, it was dried on a hot stage at 120 °C for 10 s to obtain a porous alumina composite separator.

[0092] The porous alumina was characterized by transmission electron microscopy and a fully automatic specific surface area and porosity analyzer. As Figure 2 shown, the diameter of the porous alumina was about 200 nm, and the pore size was about 2 - 8 nm. As Figure 3 shown, the specific surface area of the porous alumina was 45.7 m 2 / g. The thermal diffusion property of the separator was tested by an infrared thermal imager. As Figure 4 shown, it can be seen that after coating with porous alumina with high thermal conductivity, the temperature difference on the surface of the separator decreased, that is, the above-mentioned separator had excellent thermal diffusion function. Figure 4 In

[0093] Example 2

[0094] A porous alumina composite separator consists of a polar polymer layer and a porous alumina layer. The polymer layer is composed of meta-aramid, and the porous alumina layer is composed of a binder polyacrylic acid and porous alumina with a particle size of 250 nm. Among them, the thickness of the polymer layer is 20 ± 5 μm, and the thickness of the alumina layer is 2 ± 0.5 μm.

[0095] The preparation method of the porous alumina composite separator is characterized by including the following steps:

[0096] (1) Aluminum sulfate, aluminum nitrate, and aluminum chloride were added to ultrapure water in a molar ratio of 10:7:1. At the same time, a quantitative amount of hexamethylenetetramine was added in a molar ratio of base:Al 3+ of 10, and stirred evenly. Then, it was heated to 95 °C at a heating rate of 5 °C / min and reacted for 4 h to obtain aluminum hydroxide precipitate.

[0097] (2) The above-mentioned aluminum hydroxide precipitate was heated to 200 °C at a rate of 3 °C / min and calcined for 1 h, and then heated to 1200 °C at a rate of 5 °C / min and calcined for 3 h to obtain porous alumina.

[0098] (3) The porous alumina and the binder polyacrylic acid are dispersed in water and ethanol according to a mass ratio of 3:1 to prepare a slurry, where the solid content of the slurry is 15 wt%; then the above slurry is evenly coated on one side of a clean glass plate with a 20-μm doctor blade, and after being placed in an oven at 60 °C for drying for 6 h, a glass plate with an alumina layer is obtained.

[0099] (4) Meta-aramid, potassium tert-butoxide, potassium hydroxide, anhydrous methanol and water are mixed according to a mass ratio of 3:3:3:6:85, and after reacting at 25 °C for 12 h, a meta-aramid nanofiber dispersion is obtained.

[0100] (5) The meta-aramid nanofiber dispersion is evenly coated on the alumina layer obtained in (3) with a 500-μm doctor blade, and is quickly put into water for phase inversion. After 10 min, the strong interaction between the meta-aramid porous membrane and the alumina layer causes the alumina layer to fall off from the glass plate, and a water-containing meta-aramid coated film is obtained. Then it is soaked in ethanol and n-hexane liquids for 10 min in sequence. After solvent exchange, it is dried on a hot stage at 95 °C for 10 s to obtain a porous alumina composite separator.

[0101] The above-mentioned porous alumina has a diameter of about 250 nm, a pore diameter of about 8 - 10 nm, and a specific surface area of 40.7 m 2 / g;. The thermal diffusion property of the separator is tested by an infrared thermal imager. The temperature difference on the surface of the separator decreases, that is, the above-mentioned separator has excellent thermal diffusion function.

[0102] Example 3

[0103] A porous alumina composite separator is composed of a polar polymer layer and a porous alumina layer, where the polymer layer is composed of polyacrylonitrile, and the porous alumina layer is composed of the binder carboxymethyl cellulose and alumina with a particle size of 400 nm; among them, the thickness of the polymer layer is 60 ± 5 μm, and the thickness of the alumina layer is 4 ± 0.5 μm.

[0104] The preparation method of the porous alumina composite separator is characterized by including the following steps:

[0105] (1) Aluminum sulfate, aluminum nitrate and aluminum chloride are added to ultrapure water according to a molar ratio of 10:10:10. At the same time, a quantitative amount of ammonia water is added according to a molar ratio of base:Al 3+ with a molar ratio of 60, and stirred evenly; then it is heated to 95 °C at a heating rate of 3 °C / min and reacted for 5 h to obtain aluminum hydroxide precipitate.

[0106] (2) The above-mentioned aluminum hydroxide precipitate is heated to 200 °C at a rate of 3 °C / min and calcined continuously for 1 h, and then heated to 1200 °C at a rate of 5 °C / min and calcined continuously for 3 h to obtain porous alumina.

[0107] (3) The porous alumina and the binder carboxymethyl cellulose are dispersed in N,N-dimethylformamide and N,N-dimethylacetamide according to a mass ratio of 3:1 to prepare a slurry, where the solid content of the slurry is 15 wt%; then the above slurry is uniformly coated on one side of a clean glass plate with a 20-μm doctor blade, and after being placed in an oven at 60 °C for drying for 6 h, a glass plate with an alumina layer is obtained.

[0108] (4) Polyacrylonitrile, potassium tert-butoxide, anhydrous methanol and N,N-dimethylformamide are mixed according to a mass ratio of 3:6:6:50, and after reacting at 90 °C for 12 h, a polyacrylonitrile fiber dispersion is obtained.

[0109] (5) The polyacrylonitrile dispersion is uniformly coated on the alumina layer obtained in (3) with a 100-μm doctor blade, and is quickly put into water for phase inversion. After 10 min, the strong interaction between the polyacrylonitrile porous membrane and the alumina layer causes the alumina layer to fall off from the glass plate, and a water-containing polyacrylonitrile coated film is obtained. Then, it is successively soaked in ethanol and n-hexane for solvent exchange and then dried on a hot stage at 95 °C for 10 s to obtain a porous alumina composite separator.

[0110] The above-mentioned porous alumina has a diameter of about 400 nm, a pore diameter of about 15 nm, and a specific surface area of 10 m 2 / g; The thermal diffusion property of the separator is tested by an infrared thermal imager, and the surface temperature difference of the separator decreases, that is, the above-mentioned separator has excellent thermal diffusion function.

[0111] Example 4

[0112] A porous alumina composite separator is composed of a polar polymer layer and a porous alumina layer, where the polymer layer is composed of polyimide, and the porous alumina layer is composed of the binder polyvinyl alcohol and porous alumina with a particle size of 300 nm; among them, the thickness of the polymer layer is 20 ± 5 μm, and the thickness of the alumina layer is 10 ± 0.5 μm.

[0113] The preparation method of the porous alumina composite separator is characterized by including the following steps:

[0114] (1) Aluminum sulfate and aluminum isopropoxide are added to ultrapure water according to a molar ratio of 10:10. At the same time, a quantitative amount of ammonia water is added according to a ratio of base:Al 3+ molar ratio of 100, and stirred evenly; then it is heated to 95 °C at a heating rate of 2 °C / min and reacted for 3 h to obtain aluminum hydroxide precipitate.

[0115] (2) The above-mentioned aluminum hydroxide precipitate is then heated to 1100 °C at a rate of 5 °C / min and calcined continuously for 3 h to obtain porous alumina.

[0116] (3) The porous alumina and the binder polyvinyl alcohol are dispersed in dimethyl sulfoxide according to a mass ratio of 7:1 to prepare a slurry, where the solid content of the slurry is 30 wt%; then the above slurry is evenly coated on one side of a clean glass plate with a 50-μm doctor blade, and after being placed in an oven at 60 °C for drying for 6 h, a glass plate with an alumina layer is obtained.

[0117] (4) Polyimide, potassium hydroxide, absolute ethanol and dimethyl sulfoxide are mixed according to a mass ratio of 3:3:3:91, and after reacting at 100 °C for 12 h, a polyimide fiber dispersion is obtained.

[0118] (5) The polyimide dispersion is evenly coated on the alumina layer obtained in (3) with a 500-μm doctor blade, and is quickly put into water for phase inversion. After 10 min, the strong interaction between the polyimide porous membrane and the alumina layer causes the alumina layer to fall off from the glass plate, and a water-containing polyimide-coated film is obtained. Then it is successively soaked in ethanol and n-hexane liquids for 10 min. After solvent exchange, it is dried on a hot stage at 95 °C for 10 s to obtain a porous alumina composite separator.

[0119] The above-mentioned porous alumina has a diameter of about 300 nm, a pore diameter of about 10 nm, and a specific surface area of 30.3 m 2 / g; The thermal diffusion property of the separator is tested by an infrared thermal imager, and the surface temperature difference of the separator is reduced, that is, the above-mentioned separator has excellent thermal diffusion function.

[0120] Example 5

[0121] A porous alumina composite separator is composed of a polar polymer layer and a porous alumina layer, wherein the polymer layer is composed of para-aramid, and the porous alumina layer is composed of the binder polyvinylidene fluoride and porous alumina with a particle size of 150 nm; wherein, the thickness of the polymer layer is 40 ± 5 μm, and the thickness of the alumina layer is 2 ± 0.5 μm.

[0122] The preparation method of the porous alumina composite separator is characterized by including the following steps:

[0123] (1) Aluminum sulfate and aluminum isopropoxide are added to ultrapure water according to a molar ratio of 10:1. At the same time, a certain amount of ammonia water, urea and hexamethylenetetramine are added according to a molar ratio of base:Al 3+ molar ratio of 100, and the molar ratio of the three is 1:1:1, and they are stirred evenly; then it is heated to 95 °C at a heating rate of 4 °C / min and reacted for 5 h to obtain aluminum hydroxide precipitate.

[0124] (2) The above-mentioned aluminum hydroxide precipitate is heated to 700 °C at a rate of 5 °C / min and calcined continuously for 1 h, and then heated to 1200 °C at a rate of 2 °C / min and calcined continuously for 3 h to obtain porous alumina.

[0125] (3) The porous alumina and the binder polyvinylidene fluoride are dispersed in 1-methyl-2-pyrrolidone according to a mass ratio of 5.5:1 to prepare a slurry, wherein the solid content of the slurry is 20 wt%; then the above slurry is evenly coated on one side of a clean glass plate with a 50-μm doctor blade, and after being placed in an oven at 60 °C for drying for 6 h, a glass plate with an alumina layer is obtained.

[0126] (4) Para-aramid, potassium hydroxide, water and dimethyl sulfoxide are mixed according to a mass ratio of 3:6:5:86, and after reacting at 100 °C for 12 h, a para-aramid fiber dispersion is obtained.

[0127] (5) The para-aramid dispersion is evenly coated on the alumina layer obtained in (3) with a 100-μm doctor blade and quickly put into water for phase inversion. After 10 min, the strong interaction between the para-aramid porous membrane and the alumina layer causes the alumina layer to fall off from the glass plate, and a water-containing para-aramid coated film is obtained. Then, it is soaked in ethanol and n-hexane liquids for 10 min in sequence; after solvent exchange, it is dried on a hot stage at 95 °C for 10 s to obtain a porous alumina composite separator.

[0128] The above alumina has a diameter of about 150 nm, a pore size of about 2-3 nm, and a specific surface area of 80 m 2 / g; The thermal diffusion property of the separator is tested by an infrared thermal imager. The temperature difference on the surface of the separator decreases, that is, the above separator has excellent thermal diffusion function.

[0129] Comparative Example 1

[0130] A commercial alumina composite separator is composed of a polar polymer layer and a commercial alumina layer. The polymer layer is composed of para-aramid, and the commercial alumina layer is composed of the binder polyvinylidene fluoride and commercial alumina with a particle size of 200 nm; wherein, the thickness of the polymer layer is 40 ± 5 μm, and the thickness of the alumina layer is 2 ± 0.5 μm.

[0131] The preparation method of the commercial alumina composite separator is characterized by including the following steps:

[0132] (1) Commercial alumina and the binder polyvinylidene fluoride are dispersed in 1-methyl-2-pyrrolidone according to a mass ratio of 9:1 to prepare a slurry, wherein the solid content of the slurry is 10 wt%; then the above slurry is evenly coated on one side of a clean glass plate with a 25-μm doctor blade, and after being placed in an oven at 60 °C for drying for 6 h, a glass plate with an alumina layer is obtained.

[0133] (2) Para-aramid, potassium hydroxide, water and dimethyl sulfoxide are mixed according to a mass ratio of 3:3:3:91, and after reacting at 95 °C for 12 h, a para-aramid nanofiber dispersion is obtained.

[0134] (3) The para-aramid nanofiber dispersion is uniformly coated on the alumina layer obtained in (1) with a 250-μm doctor blade, and then quickly put into water for phase inversion. After 10 min, the strong interaction between the para-aramid polymer porous membrane and the alumina layer causes the alumina layer to peel off from the glass plate, obtaining a water-containing para-aramid coated membrane, which is then successively immersed in ethanol and n-hexane for 10 min. After solvent exchange, it is dried on a hot stage at 120 °C for 10 s to obtain a commercial alumina composite separator.

[0135] The above-mentioned commercial alumina has a diameter of about 200 nm, no porous structure, and a specific surface area of 5.25 m 2 / g; The thermal diffusion property of the separator is tested by an infrared thermal imager. The temperature difference on the separator surface decreases, indicating that the above-mentioned separator has excellent thermal diffusion function.

[0136] Comparative Example 2

[0137] A para-aramid separator, wherein the thickness of the polymer layer is 40 ± 5 μm.

[0138] The preparation method of the para-aramid separator is characterized by including the following steps:

[0139] (1) Para-aramid, potassium hydroxide, water and dimethyl sulfoxide are mixed in a mass ratio of 3:3:3:91, and reacted at 95 °C for 12 h to obtain a para-aramid nanofiber dispersion;

[0140] (2) The para-aramid nanofiber dispersion is uniformly coated on a clean glass plate with a 250-μm doctor blade, and then quickly put into water for phase inversion. After 10 min, the para-aramid separator peels off from the glass plate, obtaining a water-containing para-aramid coated membrane, which is then successively immersed in ethanol and n-hexane for 10 min; after solvent exchange, it is dried on a hot stage at 120 °C for 10 s to obtain a para-aramid fiber-based composite separator.

[0141] The thermal diffusion property of the separator is tested by an infrared thermal imager. Due to the poor heat conduction of the polymer-based membrane, the temperature difference on the separator surface is large, indicating that the above-mentioned separator has poor thermal diffusion function.

[0142] The separators in Example 1 and Comparative Examples 1-2 above are assembled into a lithium metal||lithium iron phosphate battery. The assembly process is as follows: In a glove box, the separator prepared above is placed between the lithium metal negative electrode sheet and the lithium iron phosphate positive electrode sheet, and 40 μL of electrolyte is added to assemble a CR2032 coin cell. Among them, the lithium metal negative electrode sheet has a diameter of 15 mm, a thickness of 1 mm, and a purity of 99.95%; the lithium iron phosphate positive electrode sheet has a diameter of 9 mm and a lithium iron phosphate loading of 1.5 mg·cm -2; The volume ratio of ethylene carbonate to diethyl carbonate in the electrolyte is 1 / 1. The performance of the battery was tested using a CT2001A type Blue Electric test system from Wuhan Blue Electric Co., Ltd. The test voltage range was 2.5 V to 3.8 V, and the temperature was 30 °C. The test results are shown in Table 1.

[0143] Table 1 Performance of the separators prepared in Example 1 and Comparative Examples 1-2 and the performance of the assembled batteries

[0144] Performance Example 1 Comparative Example 1 Comparative Example 2 Lithium ion transference number 0.47 0.31 0.42 <![CDATA[Ionic conductivity (mS·cm -1 )]]> 0.83 0.53 0.85 Capacity retention rate after 1000 cycles 43.5％ 33.6％ 35.4％ Coulombic efficiency after 1000 cycles 99.96％ 98.82％ 99.8％

[0145] According to the results in Table 1, it was found that coating with commercial alumina with a specific surface area of 5.25 m 2 / g in Comparative Example 1 reduced the porosity of the separator and increased the separator thickness, resulting in a lower ion transference number, ionic conductivity, and capacity retention rate after 1000 cycles at 1 C at 30 °C for Comparative Example 1 compared to Comparative Example 2. By coating with the porous alumina with a larger specific surface area in Example 1, on the one hand, it can effectively avoid the problems of reduced separator porosity and decreased specific surface area, and on the other hand, the uniformly coated alumina layer can also make the lithium-ion transport more uniform, making the battery cycle performance better than that of Comparative Example 1 and Comparative Example 2; at the same time, the dense alumina layer can also prevent the risk of dendrite piercing the separator and improve the battery safety.

[0146] Figure 1 Figure 15 is the cross-sectional SEM image of the porous alumina composite separator prepared in Example 1.

[0147] Figure 2 Figure 19 is the transmission electron microscope image of the porous alumina prepared in Example 1. It can be seen that the porous alumina is spherical, with a particle size of about 200 nm and a pore size of about 2-8 nm.

[0148] Figure 3 Figure 23 is the nitrogen adsorption-desorption isotherm curve of the porous alumina prepared in Example 1, which was obtained by testing its specific surface area with a fully automatic specific surface area and porosity analyzer. After calculation, the specific surface area of the porous alumina is 45.7 m 2 / g.

[0149] Figure 4 Figure 29 is the infrared thermal imaging diagram of Example 1, which was obtained by recording the surface temperature change of the separator when the same local hot spot was given by an infrared thermal imager. It can be seen that by coating with a highly thermally conductive alumina layer, the thermal conductivity of the separator can be improved to a certain extent.

[0150] Figure 5 Figure 33 shows the lithium metal||lithium metal symmetric batteries assembled in Example 1 and Comparative Example 1 at 30 °C with a current density of 0.5 mA / cm 2 and a capacity of 0.5 mAh / cm 2From the lithium deposition curve obtained by the capacity cycle, it can be seen that the Li||Li symmetric battery assembled in Example 1 has a stable voltage and a small overvoltage value, indicating that the polymer-coated separator provided by the present invention has a good effect of inhibiting the growth of lithium dendrites.

[0151] Figure 6 It is the specific capacity-cycle number curve obtained by cycling the lithium metal||lithium iron phosphate batteries assembled in Example 1, Comparative Example 1, and Comparative Example 2 at a cycling rate of 1C for 200 times at 70°C. From Figure 6 It can be clearly seen that at high temperatures, Example 1 and Comparative Example 1 have better thermal conductivity than Comparative Example 2. At the same time, at 30°C, Example 1 has more excellent electrochemical performance and cycling performance than Comparative Example 1 and Comparative Example 2. It can be seen that the battery of the present invention has more excellent cycling performance at high temperatures.

[0152] As mentioned above, the embodiments of the present invention have been described by way of example. However, the protection scope of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc. made by those skilled in the art within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A porous alumina composite separator, characterized in that, The composite separator includes a polar polymer layer and a porous aluminum oxide layer which are stacked.

2. The composite separator according to claim 1, wherein The thickness of the polar polymer layer is 20 to 60 μm, and the thickness of the aluminum oxide layer is 2 to 10 μm. Preferably, the particle size of the porous alumina is 100 to 400 nm; the pore size is 2 to 15 nm; the specific surface area is 10 to 80 m 2 / g.

3. The preparation method of the composite separator according to claim 1 or 2, characterized in that, The following steps are involved: S1, dispersing porous alumina and a binder in a solvent to obtain a coating slurry; S2, mixing a polar polymer, a strong base, a proton transfer agent, and a solvent to react to obtain a polar polymer fiber dispersion; S3. Apply the coating slurry described in step S1 on the substrate, and obtain an aluminum oxide layer after drying; then, apply the polar polymer fiber dispersion described in step S2 on the aluminum oxide layer, place it in water for phase conversion and dry it to obtain a composite diaphragm.

4. The method according to claim 3, characterized in that, In step S1, the binder is one or more of polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol or carboxymethyl cellulose. Preferably, in step S1, the mass ratio of the porous alumina to the binder is (3-9):

1. Preferably, in step S1, the solid content of the coating slurry is 10-30 wt%.

5. The method according to claim 3, wherein In step S2, the polar polymer includes one or more of para-aramid, meta-aramid, polyimide or polyacrylonitrile fiber. Preferably, in step S2, the strong base comprises one or more of sodium hydroxide, potassium hydroxide or potassium tert-butoxide. Preferably, in step S2, the proton transfer agent comprises one or more of water, anhydrous ethanol or anhydrous methanol. Preferably, in step S2, the solvent is selected from one or more of water, 1-methyl-2-pyrrolidone, dimethyl sulfoxide or N,N-dimethylformamide. Preferably, in step S2, the mass ratio of the polar polymer, the strong base, the proton transfer agent and the solvent is 3:(3-6):(3-6):(50-91).

6. The method according to claim 3, wherein In step S3, the solid content of the coating slurry is 10-30 wt%. Preferably, in step S3, the phase inversion time is 5-20 min.

7. The method according to claim 3, characterized in that, In step S1, the preparation method of the porous alumina is as follows: (1) Aluminum salt, alkali and water are mixed and reacted to prepare aluminum hydroxide precipitate. (2) The aluminum hydroxide precipitate obtained in the above step (1) is heated and calcined to obtain the porous alumina. Preferably, in step (2), the heating and calcining temperature is 100-1200° C., the heating and calcining time is 1-10 h; the heating rate during heating and calcining is 2-10° C. / min, preferably 2-5° C. / min. Preferably, in step (2), the calcination is carried out at a gradient rising temperature, for example, first maintaining at 100-700°C for 0-1 hour, and then maintaining at 1100-1200°C for 1-4 hours.

8. The method according to claim 3, wherein The preparation method of the composite diaphragm comprises the following steps: (1) Aluminum salt, alkali and water are fully mixed and reacted at 95°C for 3-5 hours to generate aluminum hydroxide precipitate; (2) filtering, washing, and drying the aluminum hydroxide precipitate obtained in step (1), and then calcining the precipitate once or multiple times to obtain porous alumina; (3) dispersing the porous alumina obtained in step (2) and a binder in a solvent to obtain a coating slurry; (4) Mix a polar polymer, a strong base, a proton transfer agent, and a solvent in a certain proportion, and after reacting for a period of time, obtain a polymer fiber dispersion; (5) Uniformly coat the porous alumina coating slurry described in step (3) on a substrate, and obtain a porous alumina layer after sufficient drying; (6) Uniformly coat the polymer fiber dispersion described in (4) on the surface of the porous alumina layer described in step (5), and perform phase inversion in water. After 10 minutes, the water-containing composite separator falls off from the substrate, and a composite separator is obtained after drying.

9. A battery separator material, which comprises the porous alumina composite separator described in claim 1 or 2.

10. The application of the porous alumina composite separator described in claim 1 or 2 or the battery separator material described in claim 9 in a lithium battery.

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