Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm and a preparation method thereof, and a secondary battery, and belongs to the field of secondary batteries, the battery diaphragm comprises a base membrane and a nanofiber layer stacked on at least one side surface of the base membrane, the nanofiber layer comprises modified nanofibers, the modified nanofiber comprises nanofiber and a thermosensitive layer covering the surface of the nanofiber, and the melting point of the thermosensitive layer is smaller than that of the nanofiber. The battery diaphragm has the advantages of reliable thermal hole closing performance, good heat resistance and relatively low thickness.

Description

Technical Field

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

[0002] In a secondary battery, a battery separator mainly functions to isolate the positive electrode and the negative electrode and allow ions to pass through freely. To improve the safety of the secondary battery, a battery separator with a thermal shut-off function is usually adopted at present. When the temperature of the battery rises due to reasons such as internal short circuit, overcharge, and over-discharge, 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 further prevent the spread of thermal runaway.

[0003] In related technologies, a battery separator with a thermal response includes a base film, reinforcing coatings provided on both sides of the base film, and a thermosensitive coating. The reinforcing coating ensures the dimensional stability of the separator at high temperatures, and the thermosensitive coating imparts the thermal shut-off performance to the battery separator. However, for the above-mentioned battery separator, in order to balance the thermal shut-off performance and the dimensional stability at high temperatures, different functional coatings need to be provided separately, resulting in a relatively large thickness, which will cause the separator to occupy too much of the limited internal space of the battery and is not conducive to the improvement of the battery energy density. Summary of the Invention

[0004] In view of this, the present invention provides a battery separator, a preparation method thereof, and a secondary battery, which can solve the technical problems that it is difficult for the battery separator in related technologies to have both thermal shut-off performance, heat resistance, and a relatively low thickness. Specifically, the following technical solutions are included:

[0005] On the one hand, a battery separator is provided. The battery separator includes: a base film and a nanofiber layer laminated on at least one surface of the base film. The nanofiber layer includes modified nanofibers, and the modified nanofibers include: nanofibers and a thermosensitive layer coated on the surface of the nanofibers, wherein the melting point of the thermosensitive layer is less than the melting point of the nanofibers.

[0006] In some possible implementation manners, the difference between the melting point of the nanofibers and the melting point of the thermosensitive layer is greater than or equal to 100 °C.

[0007] In some possible implementation manners, the melting point of the thermosensitive layer is 90 °C - 130 °C; the melting point of the nanofibers is greater than or equal to 200 °C.

[0008] In some possible implementation manners, the average thickness of the thermosensitive layer is D1, and the average diameter of the nanofibers is D2, and D1 and D2 satisfy: 0.3 ≤ D1 / D2 ≤ 1.

[0009] In some possible implementation manners, 0.5 ≤ D1 / D2 ≤ 1.

[0010] In some possible implementation manners, the average thickness of the thermosensitive layer is 0.5 μm to 4 μm, and the average diameter of the nanofibers is 1 to 5 μm.

[0011] In some possible implementation manners, the average thickness of the thermosensitive layer is 1.5 μm to 2 μm, and the average diameter of the nanofibers is 2 μm to 3 μm.

[0012] In some possible implementation manners, the nanofibers are surface-modified to enhance the bonding force between the nanofibers and the thermosensitive layer, wherein the surface modification treatment includes at least one of chemical grafting treatment and plasma treatment.

[0013] In some possible implementation manners, the thermosensitive layer includes: a thermosensitive polymer, and the thermosensitive polymer is selected from at least one of polyethylene, ethylene-vinyl acetate copolymer, and monomer-doped polyvinylidene fluoride.

[0014] In some possible implementation manners, the nanofibers are selected from at least one of cellulose nanofibers, algal cellulose nanofibers, bacterial cellulose nanofibers, and polymer nanofibers.

[0015] In some possible implementation manners, the nanofiber layer further includes a binder, and the binder is selected from at least one of polyethylene glycol, polyvinyl alcohol, a polymer of butadiene and styrene, a polymer of acrylonitrile, polyvinylidene fluoride, and epoxy resin.

[0016] In some possible implementation manners, the nanofiber layer further includes at least one of inorganic particles, a dispersant, a thickener, and a wetting agent.

[0017] In some possible implementation manners, the average thickness of the base film is 5 μm to 10 μm, and the average thickness of the nanofiber layer is 1 μm to 5 μm.

[0018] On the other hand, a method for preparing a battery separator is provided, and the battery separator is as described in any one of the above. The preparation method includes: preparing modified nanofibers, including: providing a thermosensitive polymer solution for forming a thermosensitive layer, coating the thermosensitive polymer solution on the surface of the nanofibers, and after curing treatment, forming a thermosensitive layer on the surface of the nanofibers to obtain the modified nanofibers; forming a nanofiber layer on at least one surface of the base film through the modified nanofibers to obtain the battery separator.

[0019] In some possible implementation manners, the forming a nanofiber layer on at least one surface of the base film through the modified nanofibers includes:

[0020] A slurry for preparing a nanofiber layer is provided. The slurry comprises the following components in mass percentages: modified nanofibers: 8%-15%, inorganic particles: 9%-20%, dispersant: 0.05%-0.10%, thickener 0.30%-0.50%, binder 1.0%-2.0%, wetting agent 0.05%-0.10%, and deionized water as the balance.

[0021] The slurry is coated on at least one surface of a base film and subjected to a curing treatment to form the nanofiber layer.

[0022] On the other hand, a secondary battery is provided. The secondary battery comprises: a housing, an electrolyte accommodated inside the housing, a negative electrode plate, a positive electrode plate, and a battery separator. The negative electrode plate and the positive electrode plate are separated by the battery separator, and the battery separator is as described in any one of the above.

[0023] The beneficial effects of the technical solution provided by the embodiments of the present invention at least include:

[0024] In the battery separator provided by the embodiments of the present invention, its thermal sensitive layer is pre-coated on the surface of nanofibers in advance to form modified nanofibers. The nanofiber layer is prepared based on the modified nanofibers, which can ensure the uniform distribution of the thermal sensitive layer in the nanofiber layer, and further ensure that the battery separator maintains stable and reliable thermal shut-off performance. In the embodiments of the present invention, the thermal sensitive layer is coated on the nanofibers and then the nanofiber layer is prepared, which allows part of the thermal sensitive layer to be in the internal area of the nanofiber layer. Compared with the thermal sensitive layer existing in the form of an additional coating on the surface of the nanofiber layer, this is beneficial to reducing the average thickness of the battery separator in the embodiments of the present invention. In addition, the melting point of the nanofibers is greater than the melting point of the thermal sensitive layer. When the thermal sensitive layer melts and closes the pores, the nanofibers will not melt, endowing the battery separator with dimensional stability at high temperatures, curbing the shrinkage and deformation of the battery separator during thermal shut-off, and ensuring the high strength and heat resistance of the battery separator. It can be seen that the battery separator provided by the embodiments of the present invention not only has good heat resistance and safety performance, but also has reliable thermal shut-off performance and a lower average thickness, which is beneficial to improving the safety and battery energy density of the battery. Detailed embodiments

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0026] Aiming at the technical problem that it is difficult for current battery separators to have both thermal shut-off performance, heat resistance and a low thickness at the same time, the embodiments of the present invention provide a new type of battery separator, which can solve the above technical problems and is beneficial to the improvement of battery energy density.

[0027] The battery separator provided by the embodiment of the present invention includes: a base film and a nanofiber layer laminated on at least one surface of the base film. The nanofiber layer includes modified nanofibers, and the modified nanofibers include nanofibers and a thermosensitive layer coated on the surface of the nanofibers. The melting point of the thermosensitive layer is less than that of the nanofibers.

[0028] It should be noted that when the temperature exceeds its melting point, the thermosensitive layer can melt, changing from a solid state to a flow state (which can be a liquid state or a viscous flow state), so as to block the microporous structure of the nanofiber layer and achieve the thermal shut-off function of the battery separator.

[0029] One example is that a nanofiber layer can be arranged on one side of the base film. Another example is that nanofiber layers can be arranged on both sides of the base film. The more the number of nanofiber layers, the correspondingly greater the average thickness of the battery separator and the more excellent the thermal shut-off performance. The number of nanofiber layers can be selected according to the requirement for the average thickness of the battery separator in different thermal management scenarios.

[0030] In the battery separator provided by the embodiment of the present invention, the thermosensitive layer is pre-coated on the surface of the nanofibers to form modified nanofibers, and the nanofiber layer is prepared based on the modified nanofibers. This can ensure the uniform distribution of the thermosensitive layer in the nanofiber layer, and further ensure that the battery separator maintains stable and reliable thermal shut-off performance. In the embodiment of the present invention, the thermosensitive layer is coated on the nanofibers and then the nanofiber layer is prepared. This allows part of the thermosensitive layer to be in the internal area of the nanofiber layer. Compared with the thermosensitive layer existing in the form of an additional coating on the surface of the nanofiber layer, this is beneficial to reducing the average thickness of the battery separator in the embodiment of the present invention. In addition, the melting point of the nanofibers is greater than that of the thermosensitive layer. When the thermosensitive layer melts to close the pores, the nanofibers will not melt, endowing the battery separator with dimensional stability at high temperatures, curbing the shrinkage and deformation of the battery separator during thermal shut-off, and ensuring the high strength and heat resistance of the battery separator. It can be seen that the battery separator provided by the embodiment of the present invention not only has good heat resistance and safety performance, but also has reliable thermal shut-off performance and a lower average thickness, which is beneficial to improving the safety and energy density of the battery.

[0031] It should be noted that in the nanofiber layer, multiple modified nanofibers are stacked, interwoven, and wound with each other in directions perpendicular and parallel to the base film to form a nanofiber layer with a network structure, so that the nanofiber layer maintains the desired porosity.

[0032] Define the exposed surface area of the nanofibers as S1, and define the coated area of the thermosensitive layer on the nanofibers as S2. The ratio of S2 to S1 can be at least 95%, preferably 100%, that is, all the exposed surfaces of the nanofibers are coated with the thermosensitive layer, so as to ensure that the battery separator obtains reliable thermal shut-off performance.

[0033] In some examples, the difference between the melting point of the nanofibers and the melting point of the thermosensitive layer is greater than or equal to 100 °C, and further can be greater than or equal to 150 °C, 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, etc. Based on this large temperature difference, when the thermosensitive layer melts at its melting point to achieve closed pores, the nanofibers will not melt and shrink, ensuring that the battery separator has excellent thermal dimensional stability (i.e., heat resistance).

[0034] Suitable for the temperature when thermal runaway occurs in current secondary batteries, the melting point of the thermosensitive layer can be 90 °C - 130 °C. For example, this can be the following values or the range formed by any two values: 90 °C, 91 °C, 92 °C, 93 °C, 94 °C, 95 °C, 96 °C, 97 °C, 98 °C, 99 °C, 100 °C, 101 °C, 102 °C, 103 °C, 104 °C, 105 °C, 106 °C, 107 °C, 108 °C, 109 °C, 110 °C, 111 °C, 112 °C, 113 °C, 114 °C, 115 °C, 116 °C, 117 °C, 118 °C, 119 °C, 120 °C, 121 °C, 122 °C, 123 °C, 124 °C, 125 °C, 126 °C, 127 °C, 128 °C, 129 °C, 130 °C, etc.

[0035] By setting the phase change temperature of the thermosensitive layer to 90 °C - 130 °C, 90 °C is higher than the normal operating temperature of the secondary battery to avoid mis-closed pores, and 130 °C is usually lower than the melting point of the base film of the battery separator to avoid the base film melting and causing pore collapse failure.

[0036] It should be noted that based on the melting point range of the thermosensitive layer, the thermosensitive layer can be configured to start melting (i.e., start to soften) at the first temperature and achieve complete melting (i.e., complete phase change) when the temperature rises to the second temperature. The temperature window composed of the first temperature and the second temperature matches the battery thermal runaway process. Or, the thermosensitive layer can also be configured to achieve both starting melting and complete melting at the third temperature, and it can be selected according to the requirements of the actual scenario.

[0037] For the melting point of the nanofibers, it can be greater than or equal to 200 °C, further greater than or equal to 300 °C, and even 400 °C to further optimize the dimensional stability of the battery separator at high temperatures.

[0038] In some examples, the porosity of the nanofiber layer is 60% - 80% to ensure that the nanofiber layer can provide a suitable ion transport channel and has good liquid absorption and liquid retention properties.

[0039] In the embodiments of the present invention, the average thickness of the thermal-sensitive layer is defined as D1, and the average diameter of the nanofibers is defined as D2. It has been found that when 0.3 ≤ D1 / D2 ≤ 1, on the one hand, it ensures that the nanofibers can maintain good dimensional stability at high temperatures, and on the other hand, it is also beneficial for the thermal-sensitive layer to sufficiently fill the pores of the nanofiber layer to achieve reliable thermal pore closing performance.

[0040] Further, 0.5 ≤ D1 / D2 ≤ 1. For example, the value of D1 / D2 includes but is not limited to the following values or the intervals formed by any two of the following values: 0.5, 0.6, 0.7, 0.8, 0.9, 1, etc., in order to achieve the purpose of optimizing the above effects.

[0041] On the basis of satisfying the above solutions, the average thickness of the thermal-sensitive layer can be 0.5 μm to 4 μm, and the average diameter of the nanofibers can be 1 to 5 μm. Further, the average thickness of the thermal-sensitive layer is 1.5 μm to 2 μm, and the average diameter of the nanofibers is 2 μm to 3 μm.

[0042] For example, the average thickness of the thermal-sensitive layer includes but is not limited to the following values or the intervals formed by any two of the following values: 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, etc., and the average diameter of the nanofibers includes but is not limited to the following values or the intervals formed by any two of the following values: 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, etc.

[0043] The above solutions are not only suitable for the current specifications of secondary batteries, but also more excellent in improving the dimensional stability of battery separators at high temperatures and reliable pore closing performance.

[0044] For the battery separator mentioned in the embodiments of the present invention, its thermal-sensitive layer contains a thermal-sensitive polymer, and the thermal-sensitive polymer melts when the temperature is greater than the melting point, causing its phase state to change from a solid state to a flowing state. As described above, the melting point of the thermal-sensitive polymer can be 90°C - 130°C.

[0045] Select the corresponding thermal-sensitive polymer according to the required thermal pore closing temperature. In some examples, some suitable thermal-sensitive polymers are selected from at least one of polyethylene (PE), ethylene-vinyl acetate copolymer (EV nanofiber layer), and monomer-doped polyvinylidene fluoride (PVDF).

[0046] For example, polyethylene, especially low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE), has a phase change temperature (i.e., melting point) that can usually be lower than 130 °C. The phase change temperature of ethylene-vinyl acetate copolymer can usually be 90 °C - 110 °C. The phase change temperature of polyvinylidene fluoride can usually be 160 °C - 170 °C. By introducing different monomers into it to lower its melting point, for example, when introducing hexafluoropropylene or tetrafluoroethylene monomers into it, its melting point can be reduced to below 130 °C. According to the doping amount of the monomer, the melting point of polyvinylidene fluoride can be adjusted adaptively.

[0047] For example, polyethylene and ethylene-vinyl acetate copolymer are suitable for secondary batteries in vehicles, polymethyl methacrylate is suitable for high-voltage batteries, and monomer-doped polyvinylidene fluoride is suitable for batteries in energy storage and extreme scenarios.

[0048] According to the actual application scenario, each of the above-mentioned thermosensitive polymers can be selected singly to prepare the thermosensitive layer, or a blend system of the above-mentioned multiple thermosensitive polymers can be used to prepare the thermosensitive layer, enabling the thermosensitive layer to undergo a phase change based on a certain temperature gradient and achieving synergistic protection. For the above blend system, the types of thermosensitive polymers can be two, three, or four.

[0049] In some examples, ethylene-vinyl acetate copolymer can be blended with polyethylene. At 90 °C - 110 °C, ethylene-vinyl acetate copolymer melts first to seal some of the micropores. As the temperature rises, when the temperature reaches 120 °C - 130 °C, polyethylene melts second to seal the remaining micropores.

[0050] In other examples, monomer-doped polyvinylidene fluoride can be blended with ethylene-vinyl acetate copolymer. Monomer-doped polyvinylidene fluoride provides high-temperature elasticity, and ethylene-vinyl acetate copolymer is responsible for low-temperature response, thus forming a gradient melting.

[0051] For nanofibers, their length can be 1 μm - 5 μm. For example, this includes but is not limited to the following values or the intervals formed by any two values: 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc. Based on this length, the network structure of the nanofiber layer formed by the nanofibers is relatively tight, having stronger support and integrity.

[0052] Exemplarily, some suitable nanofibers can be selected from at least one of cellulose nanofibers, seaweed cellulose nanofibers, bacterial cellulose nanofibers, and polymer nanofibers.

[0053] For cellulose nanofibers (CNF), which are nanofibers obtained by mechanically / chemically treating natural cellulose and removing lignin materials. On the one hand, they have high axial strength, which is beneficial for serving as a mechanical support framework to withstand the volume changes during battery charging and discharging. On the other hand, they also have good electrolyte affinity, which can improve the ion conductivity. Moreover, they have excellent thermal stability support. When the thermosensitive layer melts, the cellulose nanofiber network can maintain the pore structure and avoid "thermal runaway short circuit".

[0054] For algal cellulose nanofibers, which refer to cellulose extracted from brown algae (such as giant kelp). They have excellent thermal stability support to avoid "thermal runaway short circuit", and moreover, they are beneficial for reducing the interfacial impedance and enhancing the flexible characteristics of the battery separator.

[0055] For bacterial cellulose nanofibers, which refer to fibers obtained by culturing strains (such as Acetobacter xylinum). They not only have excellent thermal stability support but also have good interfacial stability.

[0056] For polymer nanofibers, which can be prepared from polymer materials and have the advantages of chemical compatibility, adjustable pore size, and being able to cooperate with the thermosensitive layer. In some examples, it is desired that the melting point of the polymer nanofibers is greater than or equal to 400 °C to ensure excellent dimensional stability of the battery separator at high temperatures.

[0057] Polymer nanofibers that can provide a stable mechanical framework can, for example, include at least one of aramid nanofibers (such as meta / para aramid), polyimide (PI) nanofibers, polybenzimidazole (PBI) nanofibers, polyacrylonitrile (PAN) nanofibers, polyvinyl alcohol (PVA) nanofibers, polylactic acid (PLA) nanofibers, polyurethane (PU) nanofibers, polyvinylidene fluoride - hexafluoropropylene (PVDF - HFP) nanofibers.

[0058] Among them, aramid nanofibers not only have advantages such as excellent thermal stability, stronger mechanical properties, and stronger chemical stability, but also, the polar groups on the surface of aramid nanofibers are more likely to form hydrogen bonds or covalent bonds with the thermosensitive polymer, enhancing the adhesion of the thermosensitive layer. When the thermosensitive layer melts and blocks the pores, the aramid skeleton still maintains the overall strength of the separator, preventing the deformation of the separator after the pores are closed.

[0059] Further, for example, aramid nanofibers can be poly(p - phenylene terephthalamide) nanofibers, also known as PPT nanofibers or aramid 1414 nanofibers, which have any of the effects mentioned above.

[0060] In summary, the battery separator provided by the embodiment of the present invention is based on the mechanical framework provided by nanofibers. In addition, the thermal-sensitive layer responds to thermal runaway through melting and closing pores. The two achieve the characteristics of "the framework does not collapse and the switch is controllable at high temperature" of the battery separator through interfacial interaction, which is beneficial to improving the safety threshold of the battery.

[0061] For any of the above-mentioned battery separators, the nanofibers can be further surface-modified to enhance the binding force between the nanofibers and the thermal-sensitive layer. Among them, the surface modification treatment includes at least one of chemical treatment and plasma treatment.

[0062] Exemplarily, the chemical treatment can be acid-base treatment, redox treatment, cross-linking agent treatment, etc. The acid-base treatment can be implemented using, for example, hydrochloric acid, sodium hydroxide solution, etc., which can achieve roughening of the fiber surface, increase the contact area between the nanofibers and the thermal-sensitive layer, improve the mechanical interlocking effect, and enhance the binding force. The redox treatment can be implemented using oxidants such as potassium permanganate, hydrogen peroxide, etc., and reducing agents such as sodium borohydride. Active groups such as hydroxyl groups, carboxyl groups, and amino groups can be introduced on the surface of the nanofibers. These active groups can form covalent bonds with the corresponding groups on the thermal-sensitive layer to enhance the binding force. The cross-linking agent treatment can use cross-linking agents containing multiple reactive functional groups, such as glutaraldehyde, epichlorohydrin, etc. The cross-linking agent can form a covalent bond bridge between the nanofibers and the thermal-sensitive layer, connect the two, and form a three-dimensional network structure to enhance the binding force.

[0063] Exemplarily, the plasma treatment can be, for example, radio frequency plasma treatment (the plasma source can be, for example, oxygen, nitrogen, argon, etc.), or it can also be microwave plasma treatment, to form active sites or introduce polar groups (hydroxyl groups, carboxyl groups, amino groups, etc.) on the surface of the nanofibers, so as to facilitate the combination of the nanofibers and the thermal-sensitive layer through chemical bonds or hydrogen bonds, etc., and enhance the binding force.

[0064] In some examples, the nanofiber layer of the embodiment of the present invention is bonded to the base film through an adhesive. Correspondingly, the nanofiber layer also includes an adhesive. That is to say, the nanofiber layer includes modified nanofibers and an adhesive. The adhesive is selected from at least one of polyethylene glycol, polyvinyl alcohol, polymers of butadiene and styrene, polymers of acrylonitrile, polyvinylidene fluoride, and epoxy resin.

[0065] By using the adhesive, the binding strength between the nanofiber layer and the base film is enhanced, ensuring that the battery separator maintains excellent structural stability and safety.

[0066] In addition to the binding function, the above-mentioned binder can also endow other functions to the battery separator. For example, polyethylene glycol and polyvinyl alcohol are also beneficial to improving the absorption and retention capacity of the separator for the electrolyte, and enhancing the ion conduction efficiency of the battery. The polymer of butadiene and styrene is also beneficial to endowing the battery separator with good mechanical properties and enabling it to maintain good flexibility and binding properties in a low-temperature environment. The polymer of acrylonitrile is beneficial to improving the overall strength of the separator and its stability in the complex chemical environment of the battery. Polyvinylidene fluoride and epoxy resin are beneficial to enhancing the chemical stability, thermal stability, mechanical properties and corrosion resistance of the battery separator.

[0067] As described above, the nanofiber layer includes modified nanofibers and a binder. Among them, the mass percentage of the binder in the nanofiber layer can be 1%-2%, which can avoid affecting the effect of the modified nanofibers on the premise of ensuring reliable binding performance.

[0068] Not excluded is that the nanofiber layer can further have other functional materials, such as at least one of inorganic particles (ceramic particles), dispersants, thickeners, and wetting agents, to achieve the purpose of further enhancing the adhesiveness, strength, thermal stability, electrolyte affinity, etc. of the nanofiber layer, which can be selected according to actual needs.

[0069] For example, the nanofiber layer simultaneously includes modified nanofibers, inorganic particles, a binder, a dispersant, a thickener, and a wetting agent. Correspondingly, the slurry for preparing the nanofiber layer can include the following components in mass percentages: modified nanofibers: 8%-15%; inorganic particles: 9%-20%; dispersant: 0.05%-0.10%; thickener 0.30%-0.50%; binder 1.0%-2.0%; wetting agent 0.05%-0.10%; deionized water as the balance. For the nanofiber layer obtained based on this slurry, except for not containing deionized water, other components all exist. According to the proportion of each component in the slurry, the proportion of each component in the nanofiber layer can be calculated adaptively.

[0070] As described above, the mass percentage of the modified nanofibers includes but is not limited to: 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0071] Exemplarily, the inorganic particles include at least one of boehmite, alumina, magnesium hydroxide, barium carbonate, magnesium carbonate, and zirconia. Its mass percentage includes but is not limited to: 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc.

[0072] The dispersant may include at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and methyl polyacrylate. Its mass percentage includes but is not limited to: 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, etc.

[0073] The thickener may be sodium carboxymethyl cellulose. During application, sodium carboxymethyl cellulose is mixed with pure water to form a glue solution with a mass concentration of 0.5% - 1.0%. Its mass percentage includes but is not limited to: 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, etc.

[0074] The adhesive may be an acrylic adhesive, including at least one of methyl polyacrylate, ethyl polyacrylate, butyl polyacrylate, organosilicon-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester. Its mass percentage includes but is not limited to: 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, etc.

[0075] The wetting agent may be at least one of polyether wetting agents, wetting agents that are mixtures of silicone and polyether, and alcohol alkoxylate wetting agents. Its mass percentage includes but is not limited to: 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, etc.

[0076] For any of the above-mentioned battery separators, the base film may be a polyolefin base film (such as a polyethylene base film, a polypropylene base film, etc.), a polymer composite base film (such as a polyvinylidene fluoride composite base film).

[0077] In the embodiments of the present invention, the average thickness of the base film may be 2 to 10 times the average thickness of the nanofiber layer. For example, this may be 2 times, 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, 10 times, etc.

[0078] Adapted to currently known secondary batteries, the average thickness of the base film may be 5 μm to 10 μm. For example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc., and the average thickness of the nanofiber layer may be 1 μm to 5 μm. For example, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 5 μm, etc., and further may be 1 m to 3 μm. Compared with the related art, the average thickness of the battery separator is effectively reduced.

[0079] For any of the battery separators described above, in some examples, the battery separator may further include other functional coatings. The functional coatings are stacked on the surface of the nanofiber layer. And the functional coating can be, for example, an adhesive layer, a ceramic layer, a flame retardant layer, etc. Among them, the adhesive layer mainly plays a bonding role, the ceramic layer mainly plays a role in safety reinforcement, and the flame retardant layer mainly plays a flame retardant function.

[0080] On the other hand, the embodiment of the present invention also provides a method for preparing a battery separator. The battery separator is as described in any of the above. The method for preparing the battery separator includes: preparing modified nanofibers, including: providing a thermosensitive polymer solution for forming a thermosensitive layer, coating the thermosensitive polymer solution on the surface of the nanofibers, and through a curing process, forming a thermosensitive layer on the surface of the nanofibers to obtain modified nanofibers. Through the modified nanofibers, a nanofiber layer is formed on at least one surface of the base film to obtain a battery separator.

[0081] When preparing the modified nanofibers, the thermosensitive polymer solution can be a molten thermosensitive polymer or can include a thermosensitive polymer and a solvent, and the solvent is used to dissolve and disperse the thermosensitive polymer.

[0082] For example, in the embodiment of the present invention, a solvent is used to dissolve the thermosensitive polymer to form a thermosensitive polymer solution. The solvent can be tetrahydrofuran, toluene, etc. The thermosensitive polymer is fully dissolved in the solvent under certain temperature and stirring conditions to form a uniform thermosensitive polymer solution. The preparation temperature is usually controlled at 50°C - 100°C, and the stirring speed can be 100 r / min - 500 r / min.

[0083] The thermosensitive polymer solution can be coated on the surface of the nanofibers by means of infiltration, spraying, knife coating, etc. to form a modified nanofiber precursor. For the infiltration scheme, the impregnation time can be 1 - 10 minutes to ensure that the thermosensitive layer obtains a desired average thickness. For the spraying and knife coating schemes, the average thickness and uniformity of the thermosensitive layer can meet specific requirements by controlling the coating operation parameters.

[0084] After the thermosensitive polymer solution is coated on the surface of the nanofibers, a curing process is carried out. The modified nanofiber precursor can be dried at room temperature or under heating conditions to cure the thermosensitive polymer. At the same time, when the solvent is present, the solvent is volatilized, and finally a thermosensitive layer is coated on the surface of the nanofibers. Exemplarily, the heating temperature can be 60°C - 120°C, and the drying time depends on the coating thickness and drying conditions and can be 1 hour - 24 hours.

[0085] For example, an exemplary preparation step of the modified nanofibers is as follows: Immerse the nanofibers in a thermosensitive polymer solution for 1 - 10 minutes to form a modified nanofiber precursor. Dry the modified nanofiber precursor at room temperature or under heating conditions to cure the thermosensitive polymer and volatilize the solvent, thereby preparing the modified nanofibers.

[0086] In some examples, the method for forming a nanofiber layer on at least one surface of a base film by using modified nanofibers includes: providing a slurry for preparing the nanofiber layer. Coating the slurry on at least one surface of the base film and subjecting it to a curing treatment to form the nanofiber layer, thereby preparing the battery separator.

[0087] For example, the slurry for preparing the nanofiber layer may include the following components in mass percentages: modified nanofibers: 8% - 15%; inorganic particles: 9% - 20%; dispersant: 0.05% - 0.10%; thickener 0.30% - 0.50%; binder 1.0% - 2.0%; wetting agent 0.05% - 0.10%; deionized water as the balance.

[0088] Applicable coating processes may include spraying, doctor blading, spin coating, etc. By controlling the coating operation parameters, the average thickness of the nanofiber layer can meet specific requirements.

[0089] On the other hand, an embodiment of the present invention provides a secondary battery, which includes: a housing, an electrolyte accommodated inside the housing, a negative electrode plate, a positive electrode plate, and a battery separator. The negative electrode plate and the positive electrode plate are separated by the battery separator, and the battery separator is as described in any one of the above.

[0090] The secondary battery provided by the embodiment of the present invention has all the advantages of the battery separator described above, which will not be elaborated here. Exemplarily, the secondary battery may be a lithium - ion battery, a sodium - ion battery, etc.

[0091] An embodiment of the present invention further provides an electrical device, which includes the secondary battery described above. For example, the electrical device may be a portable electronic device (such as a mobile phone, a laptop computer, a smart wearable device, etc.), a new - energy transportation device (such as a new - energy vehicle, etc.), an energy storage system, etc.

[0092] The exemplary embodiments of the present invention will be described in more detail below. Although the exemplary embodiments of the present invention are described below, 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. For those without specific technical or conditions noted in the examples, the techniques or conditions described in the literature in the art or according to the product specifications are followed. For reagents or instruments without the manufacturer noted, they are all conventional products that can be obtained through commercial purchase.

[0093] In each of the following embodiments, the slurry for preparing the nanofiber layer comprises the following components in mass percentages: 15% of modified nanofibers; 10% of boehmite particles; 0.10% of polyvinyl alcohol; 0.50% of sodium carboxymethyl cellulose; 2.0% of polymethyl acrylate; 0.10% of polyether wetting agent; and the balance is deionized water.

[0094] Example 1

[0095] Example 1 provides a battery separator, which comprises: a base film and a nanofiber layer laminated on one surface of the base film. The modified nanofibers in the nanofiber layer include nanofibers and a thermal-sensitive layer coated on the surface of the nanofibers. Among them, the base film is a PP base film with an average thickness of 8 μm, the average thickness of the nanofiber layer is 4 μm, the nanofibers are poly(p-phenylene terephthalamide) nanofibers with an average diameter of 2 μm, and the thermal-sensitive layer is polyethylene with an average thickness of 1.5 μm.

[0096] The preparation method of this battery separator is as follows:

[0097] Prepare modified nanofibers, including: adding polyethylene resin to tetrahydrofuran, and fully dissolving it under stirring conditions of 80 °C and 300 r / min to form a uniformly textured polyethylene solution. Immerse poly(p-phenylene terephthalamide) nanofibers in the polyethylene solution for 10 minutes to make the surface of the fibers uniformly adsorb the polyethylene solution. Take out and heat it under heating conditions of 100 °C to volatilize the tetrahydrofuran and cure the polyethylene on the fiber surface to form a thermal-sensitive layer, thus obtaining modified nanofibers.

[0098] Based on the prepared modified nanofibers, according to the foregoing formula, obtain the slurry for preparing the nanofiber layer, coat the slurry on one surface of the base film, and after curing treatment, prepare this battery separator.

[0099] Example 2

[0100] Example 2 provides a battery separator. The structure and preparation method of the battery separator can be referred to in Example 1. The difference between Example 2 and Example 1 is that:

[0101] The average diameter of the poly(p-phenylene terephthalamide) nanofibers is 2 μm, the thermal-sensitive layer is a polyethylene-vinyl acetate copolymer with an average thickness of 2 μm, and the average thickness of the nanofiber layer is 5 μm.

[0102] Example 3

[0103] Example 3 provides a battery separator. The structure and preparation method of the battery separator can be referred to in Example 1. The difference between Example 3 and Example 1 is that:

[0104] The nanofibers are composed of cellulose nanofibers, with an average diameter of 2.5 μm, the average thickness of the thermosensitive layer is 2 μm, and the average thickness of the nanofiber layer is 5.5 μm.

[0105] Example 4

[0106] Example 4 provides a battery separator. The structure and preparation method of the battery separator can be referred to Example 1. The difference between Example 4 and Example 1 is that:

[0107] The average diameter of the poly(p-phenylene terephthalamide) nanofibers is 3 μm, the average thickness of the thermosensitive layer is 0.5 μm, and the average thickness of the nanofiber layer is 4 μm.

[0108] Comparative Example 1

[0109] Comparative Example 1 provides a battery separator. The difference between this battery separator and the battery separator of Example 1 is that only nanofibers are used in the nanofiber layer, and no thermosensitive polymer is used. The nanofibers are poly(p-phenylene terephthalamide) fibers with an average diameter of 2 μm.

[0110] Test Example 1

[0111] Test Example 1 conducts the following performance tests on the battery separators provided in Examples 1 - 4 and Comparative Example 1. The performance test items and test results are shown in Table 1.

[0112] (1) Porosity test: Weigh the separator as Wd. Immerse it in n-butanol at room temperature and 200 °C for 2 h respectively, then take it out, gently blot 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 separator, and ρ b is the density of n-butanol.

[0113] (2) Air permeability value: The test method of the air permeability value refers to GB / T458 - 2008. Measure the initial air permeability value (i.e., at room temperature) and the air permeability value at 130 °C @ 5 min respectively: Take 5 battery separator samples, fix their two ends with heat-resistant tape, and then put them into an oven at 130 °C for 5 min. The unit is s / 100 mL.

[0114] (3) Heat shrinkage rate at 180 °C @ 1 h: The test method refers to "GB / T17 - 2004". Take 5 battery separator samples, measure the size before heating, sandwich the samples between A4 papers and put them into an oven at 180 °C for 1 hour, measure the size of the samples after heating, and calculate the heat shrinkage rate.

[0115] Table 1

[0116]

[0117] It can be seen that the battery separators provided in Embodiments 1-4 of the present invention, compared with the battery separator provided in Comparative Example 1, exhibit more excellent thermal shut-off performance on the premise of their lower thickness. At the same time, based on the application of nanofibers, the battery separators of each embodiment and Comparative Example 1 can maintain good dimensional stability at high temperatures and exhibit good heat resistance.

[0118] Test Example 2

[0119] In Test Example 2, lithium-ion batteries of the same specification were prepared from the battery separators provided in Embodiments 1-4 and Comparative Example 1. Specifically, a composite separator of a certain size was cut and the positive and negative electrode plates were isolated by winding to form a battery core body. The battery core body was subjected to a short-circuit evaluation to screen for high-quality battery cores; then it was placed in a battery case, the battery cover was covered, and the battery was welded and sealed. Electrolyte was injected into the battery case, formed, and then secondarily sealed, baked with a jig, and capacitanced to obtain a lithium-ion battery.

[0120] The following tests were performed on the lithium-ion batteries, and the test results are shown in Table 2.

[0121] Hot box test: After the battery was fully charged, it was placed in a temperature chamber and heated from room temperature to 150 ± 2°C at a rate of 5°C / min, and heating was stopped after maintaining this temperature for 30 min, and it was observed for 1 h.

[0122] Pinprick test: After the battery was fully charged, a high-temperature resistant steel needle with a diameter of Φ3 mm (the conical angle of the needle tip was 45-60°, the needle surface was smooth, free of rust, oxide layer and oil stain) was used to penetrate from the direction perpendicular to the battery plate at a speed of (25 ± 5) mm / s. The penetration position should be close to the geometric center of the pierced surface, and the steel needle remained in the battery and was observed for 1 h.

[0123] Table 2

[0124] Project Hot box test Needle puncture test Example 1 Does not catch fire, does not explode Does not catch fire, does not explode Example 2 Does not catch fire, does not explode Does not catch fire, does not explode Example 3 Does not catch fire, does not explode Does not catch fire, does not explode Example 4 Smokes, does not explode Smokes, does not explode Comparative example 1 Catches fire, does not explode Catches fire, does not explode

[0125] It can be seen that for the battery separator provided in the embodiment of the present invention, it exhibits good safety. In particular, when the average thickness D1 of the thermal sensitive layer and the average diameter D2 of the nanofibers satisfy a specific range, the effect is more excellent.

[0126] The above is only for the convenience of those skilled in the art to understand the technical solution of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A battery separator, characterized in that: The battery separator includes: a base film and a nanofiber layer stacked on at least one side of the base film, the nanofiber layer includes modified nanofibers, and the modified nanofibers include: nanofibers and a thermosensitive layer coated on the surface of the nanofibers, wherein the melting point of the thermosensitive layer is lower than the melting point of the nanofibers.

2. The battery separator according to claim 1, characterized in that: The difference between the melting point of the nanofiber and the melting point of the thermosensitive layer is greater than or equal to 100°C.

3. The battery separator according to claim 2, characterized in that: The melting point of the heat-sensitive layer is 90°C-130°C; The melting point of the nanofiber is greater than or equal to 200°C.

4. The battery separator according to any one of claims 1 to 3, characterized in that: The average thickness of the thermosensitive layer is D1, the average diameter of the nanofibers is D2, and D1 and D2 satisfy: 0.3≤D1 / D2≤1.

5. The battery separator according to claim 4, characterized in that: 0.5≤D1 / D2≤1.

6. The battery separator according to claim 4, characterized in that: The average thickness of the thermosensitive layer is 0.5 μm to 4 μm, and the average diameter of the nanofiber is 1 to 5 μm.

7. The battery separator according to claim 6, characterized in that: The average thickness of the thermosensitive layer is 1.5 μm to 2 μm, and the average diameter of the nanofiber is 2 μm to 3 μm.

8. The battery separator according to claims 1-7, characterized in that: The nanofibers are subjected to surface modification treatment to enhance the bonding force between the nanofibers and the thermosensitive layer, wherein the surface modification treatment includes at least one of chemical grafting treatment and plasma treatment.

9. The battery separator according to any one of claims 1 to 8, characterized in that: The thermosensitive layer comprises: a thermosensitive polymer, and the thermosensitive polymer is selected from at least one of polyethylene, polyethylene-vinyl acetate copolymer, and monomer-doped polyvinylidene fluoride.

10. The battery separator according to any one of claims 1 to 9, characterized in that: The nanofiber is selected from at least one of cellulose nanofiber, algae cellulose nanofiber, bacterial cellulose nanofiber and polymer nanofiber.

11. The battery separator according to any one of claims 1 to 10, characterized in that: The nanofiber layer further comprises a binder, and the binder is selected from at least one of polyethylene glycol, polyvinyl alcohol, a polymer of butadiene and styrene, a polymer of acrylonitrile, polyvinylidene fluoride and epoxy resin.

12. The battery separator according to claim 11, characterized in that The nanofiber layer further comprises at least one of inorganic particles, a dispersant, a thickener, and a wetting agent.

13. The battery separator according to any one of claims 1 to 12, characterized in that: The average thickness of the base film is 5 μm to 10 μm, and the average thickness of the nanofiber layer is 1 μm to 5 μm.

14. A method for preparing a battery separator, characterized in that: The battery separator is as claimed in any one of claims 1 to 13, wherein the preparation method comprises: The modified nanofiber is prepared, comprising: providing a thermosensitive polymer solution for forming a thermosensitive layer, coating the thermosensitive polymer solution on the surface of the nanofiber, and forming a thermosensitive layer on the surface of the nanofiber through a curing treatment, thereby preparing the modified nanofiber; The modified nanofibers are used to form a nanofiber layer on at least one surface of the base film to prepare the battery separator.

15. The method for preparing a battery separator according to claim 14, characterized in that: The method of forming a nanofiber layer on at least one side of the base film by modifying the nanofibers comprises: A slurry for preparing a nanofiber layer is provided, the slurry comprising the following components in percentage by mass: modified nanofibers: 8%-15%, inorganic particles: 9%-20%, dispersant: 0.05%-0.10%, thickener 0.30%-0.50%, binder 1.0%-2.0%, wetting agent 0.05%-0.10%, and deionized water as the balance. The slurry is applied to at least one side of the surface of the base film and cured to form the nanofiber layer.

16. 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 1-13.

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