Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm, a preparation method thereof and a secondary battery, and belongs to the field of secondary batteries, and the battery diaphragm comprises a base membrane, a nanofiber layer and a thermosensitive layer which are stacked in sequence. The base film is prepared from polyethylene with the molecular weight of 1.5 million to 3 million. The thermosensitive layer comprises 5-10 parts of a thermosensitive polymer, 15-20 parts of first inorganic particles, 0.05-0.1 part of a first dispersing agent, 0.3-0.5 part of a first thickening agent, 2-5 parts of a first binder and 0.05-0.1 part of a first wetting agent, and the thermosensitive polymer is selected from polyethylene wax and / or an ethylene copolymer. The nanofiber layer comprises nanocellulose and second inorganic particles, and the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals and bacterial nanocellulose; the inorganic particles are selected from at least one of boehmite, aluminum oxide, magnesium hydroxide, barium carbonate, magnesium carbonate and zirconium oxide. The diaphragm has low-temperature closed pores and high-temperature dimensional stability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a battery separator and a preparation method thereof, and a secondary battery. Background Art

[0002] The separator in a secondary battery primarily separates the positive and negative electrodes and allows ions to pass freely. Currently, separators are typically made of polyethylene-based film, which has a low melting point (approximately 140°C-145°C), making it less heat-resistant. When the battery temperature rises due to internal short circuits, overcharge, or over-discharge, thermal runaway can occur.

[0003] In order to improve the safety of secondary batteries, relevant technologies usually use battery separators with thermal closure function. When the internal temperature of the battery rises, the battery separator can respond to the high temperature and close the pores to prevent ion transmission, cut off the electrochemical reaction path inside the battery, and prevent the further spread of thermal runaway.

[0004] However, the thermal closure temperature of currently known battery separators is relatively high, even close to the melting point of the base film, making it impossible to curb thermal runaway in the early stages. Summary of the Invention

[0005] In view of this, the present invention provides a battery separator and a method for preparing the same, as well as a secondary battery, which can solve the technical problems in the related art. Specifically, the present invention includes the following technical solutions:

[0006] In one aspect, a battery separator is provided, comprising: a base film, a nanofiber layer laminated on at least one surface of the base film, and a thermosensitive layer laminated on a surface of the nanofiber layer;

[0007] Wherein, the base film is prepared from polyethylene with a molecular weight of 1.5 million to 3 million;

[0008] The heat-sensitive layer comprises the following components in parts by weight: 5-10 parts of a heat-sensitive polymer, 15-20 parts of first inorganic particles, 0.05-0.1 parts of a first dispersant, 0.3-0.5 parts of a first thickener, 2-5 parts of a first binder, and 0.05-0.1 parts of a first wetting agent, wherein the heat-sensitive polymer is selected from at least one of polyethylene wax and ethylene copolymer;

[0009] The nanofiber layer comprises nanocellulose and second inorganic particles, wherein the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose;

[0010] The first inorganic particles and the second inorganic particles are each independently selected from at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

[0011] In some possible implementations, the average particle size D50 of the thermosensitive polymer is 0.3 μm-5 μm.

[0012] In some possible implementations, the ethylene copolymer is selected from at least one of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, and ethylene-methyl methacrylate copolymer.

[0013] In some possible implementations, the specific surface area of ​​the first inorganic particles and the second inorganic particles are each independently 8 m 2 / g-16m 2 / g, the particle size distributions of the first inorganic particles and the second inorganic particles are independently as follows: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; and D99 ≤ 3.0 μm.

[0014] In some possible implementations, the first dispersant is selected from at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate.

[0015] In some possible implementations, the first thickener is sodium hydroxymethyl cellulose.

[0016] In some possible implementations, the first binder is selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

[0017] In some possible implementations, the first wetting agent is selected from at least one of a polyether wetting agent, a silicone and polyether mixture wetting agent, and an alcohol alkoxylate wetting agent.

[0018] In some possible implementations, the nanofiber layer includes the following components in parts by mass: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent.

[0019] In some possible implementations, the battery separator satisfies at least the following characteristics:

[0020] The battery separator has an air permeability of 200s / 100ml-400s / 100ml at room temperature;

[0021] The battery separator has an air permeability of ≥5000s / 100ml when maintained at a temperature greater than or equal to 100°C for at least 1 minute;

[0022] The thermal shrinkage rate of the battery separator at a temperature of 200° C. is less than or equal to 5%;

[0023] The puncture strength of the basement membrane is greater than or equal to 8N.

[0024] On the other hand, a method for preparing any of the above-mentioned battery separators is provided. The battery separator is as described above, and the method for preparing the battery separator comprises:

[0025] Applying a nanofiber slurry on at least one side of a base film and drying the nanofiber slurry to obtain a nanofiber layer, wherein the nanofiber slurry comprises a composition for constituting the nanofiber layer and deionized water;

[0026] The battery separator is obtained by coating a heat-sensitive slurry on the surface of the nanofiber layer and drying the slurry. The heat-sensitive slurry includes a composition for constituting the heat-sensitive layer and deionized water.

[0027] On the other hand, a secondary battery is provided, comprising: a shell, an electrolyte contained inside the shell, a negative electrode plate, a positive electrode plate and a battery separator, wherein the negative electrode plate and the positive electrode plate are separated by the battery separator, and the battery separator is as described above.

[0028] The beneficial effects of the technical solution provided by the embodiment of the present invention include at least:

[0029] The battery separator provided by an embodiment of the present invention includes a nanofiber layer and a thermosensitive layer stacked in sequence, wherein the nanofiber layer contains nanocellulose and a second inorganic particle, nanocellulose with high heat resistance is used as a filler material, and inorganic particles with high heat resistance are used as a skeleton, and the two act synergistically to give the nanofiber layer higher heat resistance. Its thermosensitive layer includes polyethylene wax or ethylene copolymer with thermosensitive characteristics and first inorganic particles with high heat resistance. Polyethylene wax or ethylene copolymer is conducive to enabling the thermosensitive layer to achieve thermal closure at a relatively low temperature (e.g., 100°C-110°C), and the first inorganic particles are conducive to enhancing the dimensional stability of the thermosensitive layer at high temperatures, thereby ensuring that the battery separator maintains dimensional stability at high temperatures and curbs the shrinkage and deformation of the battery separator during thermal closure. In addition, the polyethylene wax or ethylene copolymer in the thermosensitive layer is stably combined with the first inorganic particles through a binder, and through the synergistic effect of the first dispersant, the first thickener, and the first wetting agent, the polyethylene wax or ethylene copolymer and the first inorganic particles are evenly distributed in the thermosensitive layer, ensuring that the low-temperature thermal closure performance and high-temperature dimensional stability of the thermosensitive layer are more excellent. In summary, the battery separator provided by the embodiment of the present invention has low-temperature closed-pore properties and high-temperature heat-resistant properties based on its improved nanofiber layer and thermosensitive layer. Its low-temperature closed-pore property is conducive to timely curbing thermal runaway in the early stages, and its high-temperature heat-resistant property is conducive to enhancing the rupture temperature of the battery separator and reducing its thermal shrinkage rate at high temperatures, so as to achieve reliable high-temperature isolation between the positive and negative electrode sheets and improve the safety of the battery. DETAILED DESCRIPTION

[0030] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0031] In one aspect, embodiments of the present invention provide a battery separator comprising: a base film; a nanofiber layer laminated on at least one surface of the base film; and a thermosensitive layer laminated on the surface of the nanofiber layer. The base film is made of polyethylene with a molecular weight of 1.5 million to 3 million. The thermosensitive layer comprises the following components in parts by weight: 5-10 parts thermosensitive polymer, 15-20 parts first inorganic particles, 0.05-0.1 parts first dispersant, 0.3-0.5 parts first thickener, 2-5 parts first binder, and 0.05-0.1 parts first wetting agent. The thermosensitive polymer is selected from at least one of polyethylene wax and ethylene copolymer. The nanofiber layer comprises nanocellulose and second inorganic particles, the nanocellulose being selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose. The first and second inorganic particles are each independently selected from at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

[0032] The battery separator provided by an embodiment of the present invention includes a nanofiber layer and a thermosensitive layer stacked in sequence, wherein the nanofiber layer contains nanocellulose and a second inorganic particle, nanocellulose with high heat resistance is used as a filler material, and inorganic particles with high heat resistance are used as a skeleton, and the two act synergistically to give the nanofiber layer higher heat resistance. Its thermosensitive layer includes polyethylene wax or ethylene copolymer with thermosensitive characteristics and first inorganic particles with high heat resistance. Polyethylene wax or ethylene copolymer is conducive to enabling the thermosensitive layer to achieve thermal closure at a relatively low temperature (e.g., 100°C-110°C), and the first inorganic particles are conducive to enhancing the dimensional stability of the thermosensitive layer at high temperatures, thereby ensuring that the battery separator maintains dimensional stability at high temperatures and curbs the shrinkage and deformation of the battery separator during thermal closure. In addition, the polyethylene wax or ethylene copolymer in the thermosensitive layer is stably combined with the first inorganic particles through a binder, and through the synergistic effect of the first dispersant, the first thickener, and the first wetting agent, the polyethylene wax or ethylene copolymer and the first inorganic particles are evenly distributed in the thermosensitive layer, ensuring that the low-temperature thermal closure performance and high-temperature dimensional stability of the thermosensitive layer are more excellent. In summary, the battery separator provided by the embodiment of the present invention has low-temperature closed-pore properties and high-temperature heat-resistant properties based on its improved nanofiber layer and thermosensitive layer. Its low-temperature closed-pore property is conducive to timely curbing thermal runaway in the early stages, and its high-temperature heat-resistant property is conducive to enhancing the rupture temperature of the battery separator and reducing its thermal shrinkage rate at high temperatures, so as to achieve reliable high-temperature isolation between the positive and negative electrode sheets and improve the safety of the battery.

[0033] For the battery separator, one example is to arrange a nanofiber layer on one side of the base film, and arrange a thermosensitive layer on the surface of the nanofiber layer. Another example is to arrange nanofiber layers on both sides of the base film, and arrange a thermosensitive layer on the surface of each nanofiber layer.

[0034] In the embodiment of the present invention, polyethylene wax or ethylene copolymer is used as the thermosensitive polymer. The low-melting-point polyethylene wax or ethylene copolymer can melt and spread in the gaps of the thermosensitive layer at a temperature of 100°C-110°C to achieve closed pores in the thermosensitive layer, thereby isolating the positive and negative electrodes and preventing heat accumulation inside the battery.

[0035] The mass fraction of the thermosensitive polymer in the thermosensitive layer can be any of the following values ​​or an interval consisting of two of the values: 5 parts, 5.5 parts, 6 parts, 6.5 parts, 7 parts, 7.5 parts, 8 parts, 8.5 parts, 9 parts, 9.5 parts, 10 parts, etc.

[0036] Exemplarily, the ethylene copolymer is selected from at least one of ethylene-vinyl acetate copolymer (EVA), ethylene-acrylic acid copolymer (EAA), and ethylene-methyl methacrylate copolymer (EMMA), all of which have sensitive thermal properties.

[0037] Ethylene-vinyl acetate copolymer (EVA) is a random copolymer of ethylene and vinyl acetate, and the mass fraction of vinyl acetate (VA) is generally 5%-40%. The melting point of ethylene-vinyl acetate copolymer decreases with the increase of vinyl acetate content, and the melting point can be selected according to actual needs. For example, the melting point of ethylene-vinyl acetate copolymer can be made into 70°C-100°C, and accordingly, the mass fraction of vinyl acetate is 15%-25%.

[0038] Ethylene-acrylic acid copolymer (EAA) is a copolymer of ethylene and acrylic acid, with the mass fraction of acrylic acid monomer being 10%-30%, so that the melting point of the ethylene-acrylic acid copolymer can be 100°C-110°C.

[0039] Ethylene-methyl methacrylate copolymer (EMMA) is a copolymer of ethylene and methyl methacrylate. As the methyl methacrylate content increases, the melting point of the ethylene-methyl methacrylate copolymer increases accordingly. The melting point of the ethylene-methyl methacrylate copolymer can be selected according to actual needs. For example, the melting point of the ethylene-methyl methacrylate copolymer can be 80°C-100°C.

[0040] In some examples, the average particle size D50 of the thermosensitive polymer is 0.3 μm-5 μm, which can be any of the following values ​​or an interval consisting of two of them: 0.3 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, etc.

[0041] By defining the average particle size D50 of the thermosensitive polymer as above, the thermosensitive polymer will not block the porous base film and can also achieve rapid pore closure.

[0042] The first inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles. For example, the first inorganic particles may be boehmite particles. These inorganic particles all have high hardness and strength, as well as high heat resistance, which is beneficial for improving the strength and heat resistance of battery separators.

[0043] The mass fraction of the first inorganic particles in the thermosensitive layer can be any of the following values ​​or a range consisting of two of the values: 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, 20 parts, etc.

[0044] In some examples, the specific surface area of ​​the first inorganic particles is 8 m 2 / g-16m 2 / g and the particle size distribution of the first inorganic particles is as follows: D10≥0.1 μm, D50 is 0.3-0.6 μm; D90≤1.8 μm; D99≤3.0 μm.

[0045] The particle sizes D10, D50, D90, and D99 refer to the particle sizes corresponding to the cumulative distributions reaching 10%, 50%, 90%, and 99% respectively in the particle size distribution.

[0046] The high specific surface area of ​​the first inorganic particles strengthens the bond between them and other components, such as nanocellulose and the binder, improving the structural stability and integrity of the nanofiber layer. Furthermore, it increases the contact area between the first inorganic particles and the electrolyte, thereby enhancing the separator's ability to adsorb and retain the electrolyte, making the electrolyte more evenly distributed within the separator. This improves the battery's ionic conductivity, and consequently, enhances its charge-discharge efficiency and cycle performance.

[0047] For the particle size distribution of the first inorganic particles, D10 ≥ 0.1 μm ensures the presence of a certain number of first inorganic particles with smaller particle sizes. These small particles can fill the gaps between large particles, making the pore structure of the separator more uniform and dense. D50 is 0.3-0.6 μm, indicating that the average particle size of the first inorganic particles is moderate, which helps to form pores of appropriate size, which is conducive to the rapid transmission of lithium ions and prevents direct conduction of electrons, thereby improving the ion selective permeability of the separator. D90 ≤ 1.8 μm and D99 ≤ 3.0 μm indicate that the proportion of particles with larger particle sizes is relatively small, avoiding the presence of particles with excessively large particle sizes that lead to a decrease in the mechanical properties of the separator, such as reduced puncture resistance and poor flexibility. At the same time, this narrow particle size distribution range helps to reduce the agglomeration of the first inorganic particles. Particles with uniform particle size distribution are easier to disperse evenly in the separator, avoiding local performance differences caused by particle agglomeration, and ensuring the consistency and stability of the separator performance. Moreover, a suitable particle size distribution helps to improve the overall strength and toughness of the separator, enabling it to withstand external forces during battery assembly and use.

[0048] The first dispersant serves to disperse the thermosensitive polymer particles and the first inorganic particles and prevent particle agglomeration. In some examples, the mass fraction of the first dispersant in the thermosensitive layer can be any of the following values ​​or a range consisting of two of these values: 0.05 parts, 0.06 parts, 0.07 parts, 0.08 parts, 0.095 parts, 0.1 parts, etc. Exemplarily, some suitable first dispersants are selected from at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate. Each of these dispersants can synergistically act with the other components in the thermosensitive layer to achieve a good dispersion effect.

[0049] The first thickener forms a cross-linked network within the particles after curing, maintaining the shape of the heat-sensitive layer. This improves the membrane's liquid retention and air permeability. In some examples, the first thickener content in the heat-sensitive layer can be any of the following values, or a range of two of these values: 0.3 part, 0.35 part, 0.4 part, 0.45 part, 0.5 part, etc.

[0050] Illustratively, the first thickener is sodium hydroxymethyl cellulose. When preparing the heat-sensitive layer, sodium hydroxymethyl cellulose can be pre-mixed with water to form a sodium hydroxymethyl cellulose glue with a mass concentration of 0.5%-1%. Subsequently, the sodium hydroxymethyl cellulose glue and other components are dissolved in water to form a slurry.

[0051] The first binder enables a tight bond between the thermosensitive polymer and the first inorganic particles, and also facilitates stable adhesion of the thermosensitive layer to the surface of the nanofiber layer. In some examples, the mass fraction of the first binder in the thermosensitive layer can be any of the following values, or a range of two of these values: 2 parts, 2.5 parts, 3 parts, 3.5 parts, 4 parts, 4.5 parts, 5 parts, etc.

[0052] Illustratively, the first adhesive is an acrylic adhesive, and the first adhesive can be selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

[0053] The first adhesive can not only enhance the adhesive force of the heat-sensitive layer, but also facilitate the formation of an elastic network to inhibit deformation and cracking of the heat-sensitive layer.

[0054] The first wetting agent can reduce the surface tension of the particles and improve the wettability and adhesion of the thermal layer. In some examples, the mass fraction of the first wetting agent in the thermal layer can be any of the following values ​​or a range of two of these values: 0.05 parts, 0.06 parts, 0.07 parts, 0.08 parts, 0.095 parts, 0.1 parts, etc.

[0055] Exemplarily, the first wetting agent is selected from at least one of a polyether wetting agent, a mixture wetting agent of silicone and polyether, and an alcohol alkoxylate wetting agent. Among them, the polyether wetting agent can be, for example, a fatty alcohol polyoxyethylene ether (lauryl alcohol polyoxyethylene ether, etc.), a polyoxyethylene polyoxypropylene block copolymer (poloxamer, etc.), and the mixture wetting agent of silicone and polyether can be, for example, the commercially available product Digo Wet 270, Evonik Wet 290, etc., and alcohol alkoxylate wetting agents can be, for example, isotridecanol polyoxyethylene ether, etc.

[0056] For any of the battery separators mentioned above, in some examples, its nanofiber layer includes the following components in parts by mass: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent.

[0057] Nanocellulose and the second inorganic particles work synergistically in a certain ratio to improve the heat resistance of the battery separator, increase the membrane rupture temperature of the battery separator and the thermal shrinkage rate at high temperatures. At the same time, the surface of nanocellulose contains rich hydroxyl functional groups and has a strong affinity with the electrolyte, which makes the battery separator have strong liquid absorption and retention properties for the electrolyte. By doping a certain amount of the second dispersant, the second thickener, the second wetting agent and the second binder into the nanofiber layer, it is not only beneficial to enhance the bonding strength between the nanofiber layer and the base film, but also to improve the distribution uniformity of the nanocellulose and the second inorganic particles in the nanofiber layer, thereby optimizing its heat resistance and liquid absorption and retention properties.

[0058] The nanocellulose is selected from at least one of cellulose microfibrils (CNF), nanocellulose crystals (CNC), and bacterial nanocellulose (BNC).

[0059] Cellulose microfibrils are mainly extracted from plant fibers (such as wood and crop straw). The hydrogen bonds between cellulose fibers are destroyed through mechanical treatment (such as high-pressure homogenization and ball milling) or chemical pretreatment (such as TEMPO oxidation) to peel off the cellulose microfibrils. In addition to having strong heat resistance, they also have high strength, modulus and large specific surface area.

[0060] Nanocellulose crystals are obtained by hydrolyzing plant cellulose with strong acids (such as sulfuric acid), selectively removing the amorphous regions, and retaining rigid nanoparticles with high crystallinity (70-95%). In addition to having strong heat resistance, they also have high strength, hardness, good solubility and processability.

[0061] Bacterial nanocellulose is synthesized by fermentation of specific bacteria (such as Acetobacter xylinum) in a sugar-containing culture medium. In addition to its strong heat resistance, it also has high water retention and air permeability.

[0062] Nanocellulose can be made from cellulose microfibrils, nanocellulose crystals, or bacterial nanocellulose, or from two or all of them. The appropriate nanocellulose and its content can be selected based on actual needs.

[0063] In some examples, the diameter of the nanocellulose is 5 nm to 50 nm, and the length is 1 μm to 10 μm. For example, the diameter of the nanocellulose can be any of the following values ​​or an interval consisting of two of them: 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc., and the length of the nanocellulose can be any of the following values ​​or an interval consisting of two of them: 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0064] By limiting the size of the nanocellulose as above, so that it is at the nanoscale diameter, micron-scale length and large specific surface area, the nanofiber layer has at least the following advantages: enhancing the mechanical properties of the nanofiber layer, increasing the porosity and air permeability of the nanofiber layer, improving the lyophilicity and wettability of the nanofiber layer, enhancing the interfacial interaction between the nanocellulose and other components, and enabling the components to work better synergistically.

[0065] For example, the mass fraction of nanocellulose in the nanofiber layer can be any of the following values ​​or an interval consisting of two of the following values: 8 parts, 8.5 parts, 9 parts, 9.5 parts, 10 parts, 10.5 parts, 11 parts, 11.5 parts, 12 parts, 12.5 parts, 13 parts, 13.5 parts, 14 parts, 14.5 parts, 15 parts, etc.

[0066] The second inorganic particles in the nanofiber layer include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles. For example, the second inorganic particles may be boehmite particles. These second inorganic particles all have high hardness and strength, as well as high heat resistance, which is beneficial for improving the strength and heat resistance of the battery separator.

[0067] The mass fraction of the second inorganic particles in the nanofiber layer can be any of the following values ​​or an interval consisting of two of the values: 9 parts, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, 20 parts, etc.

[0068] In some examples, the specific surface area of ​​the second inorganic particles is 8 m 2 / g-16m 2 / g and the particle size distribution of the second inorganic particles is as follows: D10≥0.1 μm, D50 is 0.3-0.6 μm; D90≤1.8 μm; D99≤3.0 μm.

[0069] The particle sizes D10, D50, D90, and D99 refer to the particle sizes corresponding to the cumulative distributions reaching 10%, 50%, 90%, and 99% respectively in the particle size distribution.

[0070] The high specific surface area of ​​the second inorganic particles strengthens the bond between them and other components, such as the nanocellulose and the second binder, improving the structural stability and integrity of the nanofiber layer. Furthermore, this increases the contact area between the second inorganic particles and the electrolyte, thereby enhancing the separator's ability to adsorb and retain the electrolyte, making the electrolyte more evenly distributed within the separator. This improves the battery's ionic conductivity, and consequently, enhances its charge-discharge efficiency and cycle performance.

[0071] For the particle size distribution of the second inorganic particles, D10 ≥ 0.1 μm ensures the presence of a certain number of smaller second inorganic particles. These small particles can fill the gaps between larger particles, making the pore structure of the separator more uniform and dense. D50 is 0.3-0.6 μm, indicating that the average particle size of the second inorganic particles is moderate, which helps to form pores of appropriate size, which is beneficial for the rapid transmission of lithium ions while preventing direct conduction of electrons, thereby improving the ion selective permeability of the separator. D90 ≤ 1.8 μm and D99 ≤ 3.0 μm indicate that the proportion of larger particles is relatively small, avoiding the presence of particles with excessively large sizes that lead to a decrease in the mechanical properties of the separator, such as reduced puncture resistance and decreased flexibility. At the same time, this narrow particle size distribution range helps to reduce the agglomeration of the second inorganic particles. Particles with a uniform particle size distribution are easier to disperse evenly in the separator, avoiding local performance differences caused by particle agglomeration, and ensuring the consistency and stability of the separator performance. Moreover, a suitable particle size distribution helps to improve the overall strength and toughness of the separator, enabling it to withstand external forces during battery assembly and use.

[0072] The mass fraction of the second dispersant in the nanofiber layer can be any of the following values ​​or a range consisting of two of these values: 0.05 part, 0.06 part, 0.07 part, 0.08 part, 0.095 part, 0.1 part, etc. Some suitable second dispersants can be at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate. Each of the above second dispersants can synergistically act with other components in the nanofiber layer to achieve a good dispersion effect.

[0073] The mass fraction of the second thickener in the nanofiber layer can be any of the following values ​​or a range consisting of two of these values: 0.3 part, 0.35 part, 0.4 part, 0.45 part, 0.5 part, etc. In some examples, the second thickener is sodium hydroxymethyl cellulose. When preparing the nanofiber layer, the sodium hydroxymethyl cellulose can be pre-mixed with water to form a sodium hydroxymethyl cellulose glue with a mass concentration of 0.5%-1%. Subsequently, the sodium hydroxymethyl cellulose glue and other components are dissolved in water to form a slurry.

[0074] The mass fraction of the second binder in the nanofiber layer can be any of the following values ​​or an interval consisting of two of these values: 1 part, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2 parts, etc. For example, the second binder is an acrylic binder, and the second binder can be selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

[0075] The second binder can not only further enhance the bonding force of the nanofiber layer, but also facilitates the formation of an elastic network to inhibit deformation and cracking of the nanofiber layer.

[0076] The second wetting agent can reduce the surface tension of the particles and enhance the bonding strength between the nanofiber layer, the base film, and the heat-sensitive layer. In some examples, the weight percentage of the second wetting agent can be any of the following values, or a range of two of them: 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, etc. The weight percentage of the second wetting agent in the heat-sensitive layer can be any of the following values, or a range of two of them: 0.05 parts, 0.06 parts, 0.07 parts, 0.08 parts, 0.095 parts, 0.1 parts, etc.

[0077] Exemplarily, the second wetting agent is selected from at least one of a polyether wetting agent, a mixture wetting agent of silicone and polyether, and an alcohol alkoxylate wetting agent. Among them, the polyether wetting agent can be, for example, a fatty alcohol polyoxyethylene ether (lauryl alcohol polyoxyethylene ether, etc.), a polyoxyethylene polyoxypropylene block copolymer (poloxamer, etc.), and the mixture wetting agent of silicone and polyether can be, for example, the commercially available product Digo Wet 270, Evonik Wet 290, etc., and alcohol alkoxylate wetting agents can be, for example, isotridecanol polyoxyethylene ether, etc.

[0078] For any of the battery separators mentioned above, the thickness of its nanofiber layer can be greater than or equal to 5μm, for example, 5μm-15μm, which includes but is not limited to 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc., and the thickness of its thermosensitive layer can be 1μm-5μm, and further can be 1μm-4μm, for example, 1μm, 2μm, 3μm, 4μm, etc.

[0079] For any of the battery separators mentioned above, the base film is a polyethylene base film and is prepared using polyethylene with a molecular weight of 1.5 million g / mol-3 million g / mol. The ultra-high molecular weight polyethylene has a longer molecular chain and a high entanglement density, which is beneficial to improving the tensile strength and puncture strength of the base film. In addition, the above-mentioned ultra-high molecular weight polyethylene material also has strong heat resistance, ultimately achieving the purpose of improving the strength and heat resistance of the base film.

[0080] In some examples, the base film may have a thickness of 3 μm-12 μm, and the puncture strength of the base film may be ≥8.0 N, while the puncture strength of the battery separator may be greater than or equal to 8 N. Thus, the base film has the advantages of both low thickness and high strength.

[0081] For any of the battery separators mentioned above, at room temperature, the air permeability of the battery separator is 200s / 100ml-400s / 100ml, and when the temperature is greater than or equal to 100°C and maintained for at least 1 minute, the air permeability of the battery separator is ≥5000s / 100ml. It can be seen that the thermosensitive layer uses polyethylene wax or ethylene copolymer as the thermosensitive material, and when the temperature is 100°C-110°C, the heat-induced closed-cell mechanism can be completed, so that the air permeability is ≥5000s / 100ml when the temperature is greater than or equal to 100°C and maintained for at least 1 minute. For example, the air permeability at 110°C@1min is ≥5000s / 100ml, achieving rapid low-temperature (100°C-110°C) closed-cell and blocking the transfer of lithium ions between the positive and negative electrodes, reducing heat spread inside the battery.

[0082] The nanofiber layer of the battery separator adopts the synergistic effect of nanocellulose and inorganic particles, which has the advantages of strong density, good structural stability, and not easy to shrink when heated. In particular, based on the high melting point of nanocellulose and inorganic particles (for example, the melting point can be greater than or equal to 250°C), the membrane rupture temperature of the battery separator is ≥250°C, and the thermal shrinkage rate at 200°C is ≤5.0%. According to tests, after the battery separator is baked at 200°C for 1 hour, its thermal shrinkage rate is less than or equal to 5%, which includes: longitudinal thermal shrinkage rate MD ≤5%, transverse thermal shrinkage rate TD ≤3%.

[0083] On the other hand, an embodiment of the present invention further provides a method for preparing a battery separator, wherein the battery separator is as described above, and the method for preparing the battery separator comprises:

[0084] The nanofiber slurry is coated on at least one side of the surface of the base film and dried to obtain a nanofiber layer, wherein the nanofiber slurry includes a composition for constituting the nanofiber layer and deionized water.

[0085] The heat-sensitive slurry is coated on the surface of the nanofiber layer and dried to obtain a battery separator; wherein the heat-sensitive slurry includes a composition for constituting the heat-sensitive layer and deionized water.

[0086] As described above, the composition for constituting the nanofiber layer includes the following components in parts by mass: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent.

[0087] In some examples, the solid content of the nanofiber slurry is 18%-38%, and the content of deionized water in the slurry is adaptively determined according to the solid content.

[0088] The composition for constituting the heat-sensitive layer includes the following components in parts by mass: 5-10 parts of heat-sensitive polymer, 15-20 parts of first inorganic particles, 0.05-0.1 parts of first dispersant, 0.3-0.5 parts of first thickener, 2-5 parts of first binder, and 0.05-0.1 parts of first wetting agent.

[0089] In some examples, the solid content of the thermal sensitive slurry is 22%-36%, and the content of deionized water in the slurry is adaptively determined according to the solid content.

[0090] When preparing the nanofiber layer, the nanofiber slurry is applied to at least one surface of the base film and dried to form the nanofiber layer. During this process, deionized water is removed, that is, the nanofiber layer does not contain deionized water, but only includes the components in its corresponding composition.

[0091] When preparing the thermosensitive layer, the thermosensitive material is coated on the surface of the nanofiber layer and dried to form the thermosensitive layer. During this process, deionized water is removed. That is, the thermosensitive layer does not contain deionized water, but only includes the components in its corresponding composition.

[0092] The coating process of the nanofiber slurry and the thermosensitive slurry can be gravure coating, micro gravure coating, transfer coating, spray coating or spin coating, etc. By controlling the coating process parameters, the average thickness of the nanofiber layer and the thermosensitive layer can meet the corresponding requirements.

[0093] On the other hand, an embodiment of the present invention provides a secondary battery, which includes: a shell, an electrolyte contained inside the shell, a negative electrode plate, a positive electrode plate and a battery separator, wherein the negative electrode plate and the positive electrode plate are separated by a battery separator, and the battery separator is as described above.

[0094] The secondary battery provided by the embodiment of the present invention has all the advantages of the battery separator mentioned above, which will not be described in detail here. For example, the secondary battery can be a lithium ion battery, a sodium ion battery, etc.

[0095] An embodiment of the present invention also provides an electrical device, which includes the secondary battery involved above. For example, the electrical device can be a portable electronic device (mobile phone, laptop computer, smart wearable device, etc.), new energy transportation equipment (new energy vehicle, etc.), energy storage system, etc.

[0096] Below will be described in more detail exemplary embodiments of the present invention. Although the following describes exemplary embodiments of the present invention, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. In the examples, if specific techniques or conditions are not indicated, they are carried out according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments used that do not indicate the manufacturer are conventional products that can be obtained commercially.

[0097] Example 1

[0098] Example 1 provides a battery separator, which includes: a polyethylene-based film, a nanofiber layer stacked on both sides of the polyethylene-based film, and a thermosensitive layer stacked on the surface of the nanofiber layer.

[0099] The polyethylene-based film is made of polyethylene with a molecular weight of 2 million and a thickness of 7 μm. The thickness of the nanofiber layer is 5 μm, and the thickness of the heat-sensitive layer is 3 μm.

[0100] The nanofiber layer includes the following components in parts by weight: 12 parts of nanocellulose, 15 parts of boehmite particles, 0.08 parts of polyvinyl alcohol, 0.5 parts of sodium hydroxymethyl cellulose, 1.5 parts of polymethyl acrylate, and a wetting agent. Wet 270 0.06 parts. Nanocellulose is cellulose microfibrils with a diameter of 20nm and a length of 5μm. The specific surface area of ​​boehmite particles is 10m 2 / g and the particle size distribution meets the following requirements: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; D99 ≤ 3.0 μm.

[0101] The thermosensitive layer includes the following components by weight: 8 parts thermosensitive polymer, 17 parts boehmite particles, 0.07 parts polyvinyl alcohol, 0.4 parts sodium hydroxymethylcellulose, 4 parts polymethyl acrylate, and 0.1 parts first wetting agent. The thermosensitive polymer is polyethylene wax with a particle size D50 of 4 μm, and the specific surface area of ​​the boehmite particles is 10 m 2 / g and the particle size distribution meets the following requirements: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; D99 ≤ 3.0 μm.

[0102] The battery separator of Example 1 was prepared by the following method:

[0103] The raw materials for the nanofiber layer were mixed with deionized water to prepare a nanofiber slurry with a solid content of 20%. This slurry was then applied to both sides of the base film and dried to form the nanofiber layer. The raw materials for the heat-sensitive layer were mixed with deionized water to prepare a heat-sensitive slurry with a solid content of 30%. This slurry was then applied to the surface of the nanofiber layer and dried to form a heat-sensitive layer, thus completing the preparation of the battery separator.

[0104] Example 2

[0105] Example 2 provides a battery separator, which differs from Example 1 in the following aspects:

[0106] (1) Nanofiber layer: The nanocellulose is selected from nanocellulose crystals with a diameter of 10 nm and a length of 3 μm.

[0107] (2) Thermosensitive layer: The thermosensitive polymer is selected from ethylene-vinyl acetate copolymer, and the mass fraction of ethylene-vinyl acetate copolymer is 6 parts.

[0108] Example 3

[0109] Example 3 provides a battery separator, which differs from Example 1 in the following aspects:

[0110] (1) Nanofiber layer: The nanocellulose is selected from bacterial nanocellulose, with a diameter of 30 nm and a length of 8 μm.

[0111] (2) Thermosensitive layer: The thermosensitive polymer thereof is selected from ethylene-acrylic acid copolymer, and the mass portion of the ethylene-acrylic acid copolymer is 10 parts.

[0112] Example 4

[0113] Example 4 provides a battery separator, which differs from Example 1 in that the heat-sensitive layer is different. The heat-sensitive layer comprises the following components in parts by weight: 10 parts of polyethylene wax with a particle size D50 of 2 μm, 15 parts of boehmite particles, 0.06 parts of polyvinyl alcohol, 0.5 parts of sodium hydroxymethyl cellulose, 5 parts of polymethyl acrylate, and a wetting agent. Wet 270 0.1 parts.

[0114] Comparative Example 1

[0115] Comparative Example 1 provides a battery separator, which differs from Example 1 in that the thermal sensitive layer is different and no inorganic particles are used in the thermal sensitive layer. Its formula is as follows: 25 parts of polyethylene wax, 0.07 parts of polyvinyl alcohol, 0.4 parts of sodium hydroxymethyl cellulose, 4 parts of polymethyl acrylate, and 0.1 parts of a first wetting agent.

[0116] Comparative Example 2

[0117] Comparative Example 2 provides a battery separator, which differs from Example 1 in that the base film is different and the base film adopts polyethylene with a molecular weight of 1 million.

[0118] Test Example 1

[0119] The following performance tests were performed on the battery separators provided in Examples 1 to 4, and Comparative Examples 1 and 2. The performance test items and test results are shown in Table 1.

[0120] (1) Porosity test: weigh the membrane as Wd, soak it in n-butanol at room temperature for 2 hours, take it out, gently absorb the liquid on its surface with filter paper, and weigh it as Ww. The porosity P% calculation formula is: P% = (Ww-Wd) / (Vp*ρ b ), where Vp is the volume of the dry diaphragm, ρ b is the density of n-butanol, and the unit of porosity is %.

[0121] (2) Air permeability: The test method for air permeability is carried out in accordance with GB / T36363-2018. The initial air permeability (i.e., at room temperature) and the air permeability at 110°C @ 5 min are measured respectively: 5 battery separator samples are taken, both ends of which are fixed with heat-resistant tape and then placed in a 110°C oven for 5 min for testing. The unit is s / 100mL.

[0122] (3) 250°C @ 1h Thermal Shrinkage: The test method is based on GB / T36363-2018. Five battery separator samples are taken and their dimensions before heating are measured. The samples are sandwiched between 6mm glass plates and placed in a 250°C oven for 1 hour. The dimensions of the separator samples after heating are measured and their thermal shrinkage is calculated. The longitudinal shrinkage (MD) and transverse shrinkage (TD) of the separator samples are measured separately, and the unit is %.

[0123] Table 1

[0124]

[0125] As can be seen from Table 1, the porosity of Examples 1 to 4 and Comparative Examples 1 and 2 in the initial state is greater than 40% and less than 50%, and the porosity is moderate, meeting the basic requirements of battery separators.

[0126] The battery separators provided in Examples 1 to 4 are based on an improved thermosensitive layer and a nanofiber layer arranged on the surface of the base membrane. The synergistic effect of the two enables the battery separator to have both low-temperature closed-cell properties (completed closed-cell properties at 110°C) and high-temperature heat-resistant properties (low thermal shrinkage rate at 250°C), which is beneficial for timely curbing thermal runaway in the early stages, and is beneficial for enhancing the membrane rupture temperature of the battery separator, so as to achieve reliable high-temperature isolation between the positive and negative electrode sheets, thereby improving the safety of the battery.

[0127] The thermosensitive layer in Comparative Example 1 does not use inorganic particles. Although it achieves the low-temperature closed-cell function (closed-cell at 110°C), its thermal shrinkage rate at high temperature is higher than that of Examples 1 to 4, indicating that its battery separator has poor high-temperature heat resistance.

[0128] Comparative Example 2 uses a polyethylene-based film with a lower molecular weight. Compared with Example 1, its high-temperature heat resistance is deteriorated, which shows that the molecular weight of the polyethylene-based film has a certain influence on the heat resistance of the battery separator.

[0129] Test Example 2

[0130] The battery separators provided by Examples 1 to 4 and Comparative Examples 1 and 2 were used to prepare lithium-ion batteries of the same specifications. The lithium-ion batteries were subjected to the following tests. The test results are shown in Table 2.

[0131] Hot box test: After the battery is fully charged, place it in a temperature box and heat it from room temperature to 180±2℃ at a rate of 5℃ / min. Maintain this temperature for 60 minutes, then stop heating and observe for 1 hour.

[0132] Needle puncture test: After the battery is fully charged, use a Φ10mm high-temperature resistant steel needle (the cone angle of the needle tip is 45-60°, the needle surface is smooth, free of rust, oxide layer and oil), at a speed of (40±5)mm / s, to penetrate the battery plate in a direction perpendicular to the battery plate. The penetration position should be close to the geometric center of the punctured surface. The steel needle stays in the battery and is observed for 1 hour.

[0133] Table 2

[0134] project Hot box test Acupuncture test Example 1 No fire, no explosion No fire, no explosion Example 2 No fire, no explosion No fire, no explosion Example 3 No fire, no explosion No fire, no explosion Example 4 No fire, no explosion No fire, no explosion Comparative Example 1 Fire, explosion Fire, explosion Comparative Example 2 Smoke, no explosion Smoke, no explosion

[0135] It can be seen that the battery separators provided by Examples 1 to 4 have enhanced rupture temperatures compared to Comparative Examples 1 and 2, thereby achieving reliable high-temperature insulation between the positive and negative electrode sheets and improving battery safety.

[0136] The above description is only for the purpose of facilitating those skilled in the art to understand the technical solution of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A battery separator, characterized in that: The battery separator comprises: a base film, a nanofiber layer stacked on at least one side of the base film, and a heat-sensitive layer stacked on the surface of the nanofiber layer; Wherein, the base film is prepared from polyethylene with a molecular weight of 1.5 million to 3 million; The heat-sensitive layer comprises the following components in parts by weight: 5-10 parts of a heat-sensitive polymer, 15-20 parts of first inorganic particles, 0.05-0.1 parts of a first dispersant, 0.3-0.5 parts of a first thickener, 2-5 parts of a first binder, and 0.05-0.1 parts of a first wetting agent, wherein the heat-sensitive polymer is selected from at least one of polyethylene wax and ethylene copolymer; The nanofiber layer comprises nanocellulose and second inorganic particles, wherein the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose; The first inorganic particles and the second inorganic particles are each independently selected from at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

2. The battery separator according to claim 1, characterized in that The average particle size D50 of the thermosensitive polymer is 0.3 μm-5 μm.

3. The battery separator according to claim 1, characterized in that The ethylene copolymer is selected from at least one of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, and ethylene-methyl methacrylate copolymer.

4. The battery separator according to claim 1, characterized in that The specific surface area of ​​the first inorganic particles and the second inorganic particles are each independently 8 m 2 / g-16m 2 / g, the particle size distributions of the first inorganic particles and the second inorganic particles are independently as follows: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; and D99 ≤ 3.0 μm.

5. The battery separator according to claim 1, characterized in that The first dispersant is selected from at least one of polyvinyl alcohol, sodium dodecylbenzene sulfonate, glycerol, and polymethyl acrylate.

6. The battery separator according to claim 1, characterized in that The first thickener is sodium hydroxymethyl cellulose.

7. The battery separator according to claim 1, characterized in that The first binder is selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

8. The battery separator according to claim 1, characterized in that The first wetting agent is selected from at least one of a polyether wetting agent, a mixture wetting agent of silicone and polyether, and an alcohol alkoxylate wetting agent.

9. The battery separator according to any one of claims 1 to 8, characterized in that: The nanofiber layer includes the following components in parts by weight: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent.

10. The battery separator according to any one of claims 1 to 9, characterized in that: The battery separator at least meets the following characteristics: The battery separator has an air permeability of 200s / 100ml-400s / 100ml at room temperature; The battery separator has an air permeability of ≥5000s / 100ml when maintained at a temperature greater than or equal to 100°C for at least 1 minute; The thermal shrinkage rate of the battery separator at a temperature of 200° C. is less than or equal to 5%; The puncture strength of the basement membrane is greater than or equal to 8N.

11. A method for preparing a battery separator, characterized in that: The battery separator according to any one of claims 1 to 10, wherein the preparation method of the battery separator comprises: Applying a nanofiber slurry on at least one side of a base film and drying the nanofiber slurry to obtain a nanofiber layer, wherein the nanofiber slurry comprises a composition for constituting the nanofiber layer and deionized water; The battery separator is obtained by coating a heat-sensitive slurry on the surface of the nanofiber layer and drying the slurry. The heat-sensitive slurry includes a composition for constituting the heat-sensitive layer and deionized water.

12. A secondary battery, characterized in that: The secondary battery comprises: a shell, an electrolyte contained in the shell, a negative electrode sheet, a positive electrode sheet and a battery separator, wherein the negative electrode sheet and the positive electrode sheet are separated by the battery separator, and the battery separator is as described in any one of claims 1-10.

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