Diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of batteries, in particular to a diaphragm, a preparation method thereof and a lithium ion battery. The diaphragm comprises a base film, the base film is made of bamboo fibers, and the bamboo fibers form a three-dimensional network structure with pores; and zeolitic imidazate framework material particles, at least a portion of which is disposed within the pores. According to the diaphragm provided by some embodiments of the invention, the zeolite imidazate framework material (ZIFs) particles are arranged in the pores of the base membrane which is formed by the bamboo fibers and has the three-dimensional network structure, and the migration rate of lithium ions is increased and the compatibility of the diaphragm and an electrolyte is improved by utilizing the synergistic effect of the bamboo fiber three-dimensional network structure and the ZIFs particles; and the thermal stability and the mechanical strength of the diaphragm are improved, so that the rate capability, the cycle performance and the safety performance of the battery are improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to separators and their preparation methods, and lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries have significant advantages such as high energy density, long cycle life, and low self-discharge rate, and have received widespread attention in the field of new energy vehicles in recent years. Among them, the separator, as an important component of lithium-ion batteries, plays a role in isolating the positive and negative electrodes to prevent short circuits, maintaining the electrolyte, and allowing lithium-ion transport.

[0003] Traditional lithium-ion battery separator materials, such as polypropylene (PP) or polyethylene (PE), suffer from poor thermal stability and low mechanical strength. Therefore, biomass bamboo membranes, with their excellent thermal stability and mechanical strength, are considered a highly promising alternative material.

[0004] However, directly applying natural bamboo membranes to lithium-ion battery separator materials presents the following problems: (1) bamboo membranes have irregular pore structures, resulting in tortuous ion transport paths; (2) the hydroxyl groups abundant on the surface of bamboo fibers adsorb Li + (3) Bamboo membranes are prone to absorbing moisture and swelling in the electrolyte, resulting in poor compatibility with the electrolyte. The above problems affect the rate performance and cycle performance of the battery, limiting the large-scale application of bamboo membrane-based separators. Summary of the Invention

[0005] Therefore, it is necessary to provide a separator and its preparation method, and a lithium-ion battery, to improve the migration rate of lithium ions in the bamboo membrane-based separator, improve the compatibility between the bamboo membrane-based separator and the electrolyte, and thus improve the rate performance and cycle performance of the battery.

[0006] A first aspect of this application provides a diaphragm comprising: a base membrane made of bamboo fiber forming a three-dimensional network structure with pores; and zeolite imidazole ester skeleton material particles, at least a portion of which are disposed within the pores.

[0007] In some embodiments, the bamboo fiber satisfies at least one of the following conditions: (1) the tensile strength of the bamboo fiber is 150 MPa to 200 MPa; (2) the aspect ratio of the bamboo fiber is 500 to 1000; (3) the Young's modulus of the bamboo fiber is 10 GPa to 70 GPa; (4) the thermal decomposition temperature of the bamboo fiber is greater than or equal to 200°C.

[0008] In some embodiments, at least some of the zeolite imidazole ester framework material particles are also disposed on the surface of the base film.

[0009] In some embodiments, the diaphragm satisfies at least one of the following conditions: (1) the particle size of the zeolite imidazole ester framework material particles is 20 nm to 200 nm; (2) the surface loading of the zeolite imidazole ester framework material particles on the base membrane is 20% to 90%; (3) the pore size of the base membrane is 10 nm to 100 nm; (4) the thickness of the base membrane is 5 μm to 50 μm; (5) the porosity of the base membrane is 30% to 60%; and (6) the three-dimensional network structure formed by bamboo fiber has an oriented pore structure.

[0010] A second aspect of this application provides a method for preparing a diaphragm, the method comprising the following steps: subjecting a bamboo fiber dispersion to vacuum freeze-drying to form a three-dimensional network structure, thereby preparing a base membrane; and sequentially immersing the base membrane in a metal source solution and an imidazole ligand solution to form zeolite imidazole ester framework material particles within the pores of the base membrane, thereby preparing a diaphragm.

[0011] In some embodiments, the bamboo fiber dispersion is prepared by the following steps: subjecting the bamboo membrane to alkali treatment and oxidation treatment to remove hemicellulose and lignin; mixing the alkali-treated and oxidized bamboo membrane with a solvent to prepare the bamboo fiber dispersion.

[0012] In some embodiments, the vacuum freeze-drying process includes pre-freezing the bamboo fiber dispersion and then drying it; the vacuum freeze-drying process meets at least one of the following conditions: (1) the temperature of the pre-freezing process is -80℃ to -60℃; (2) the temperature of the drying process is -60℃ to -50℃; (3) the vacuum degree of the drying process is 1 Pa to 10 Pa.

[0013] In some embodiments, before immersing the base film sequentially in the metal source solution and the imidazole ligand solution, the method further includes the step of activating the base film with an alcohol solvent.

[0014] In some embodiments, the preparation method satisfies at least one of the following conditions: (1) the concentration of the bamboo fiber dispersion is 5 wt% to 10 wt%; (2) the molar ratio of metal ions in the metal source solution to imidazole ligands in the imidazole ligand solution is 1:(2~5); (3) the concentration of the metal source solution is 0.05 mol / L to 0.2 mol / L; (4) the concentration of the imidazole ligand solution is 0.2 mol / L to 0.8 mol / L; (5) the total immersion time of the base membrane in the metal source solution is 30 min to 60 min; (6) the total immersion time of the base membrane in the imidazole ligand solution is 6 h to 24 h.

[0015] A third aspect of this application provides a lithium-ion battery comprising the separator provided in the first aspect above, or a separator prepared by the method for preparing the separator provided in the second aspect above.

[0016] Compared with traditional technologies, this application has at least the following beneficial effects:

[0017] The membranes provided in some embodiments of this application utilize zeolite imidazole ester framework (ZIFs) particles embedded in the pores of a base membrane with a three-dimensional network structure formed from bamboo fibers. On one hand, metal ions in the ZIFs particles interact electrostatically or coordinately with hydroxyl groups on the bamboo fiber surface, reducing the adsorption of lithium ions by hydroxyl groups. Simultaneously, the ZIFs bridge the gaps between bamboo fibers, further reducing the tortuosity of lithium ion transport within the bamboo fiber network structure. Furthermore, the pore size of the ZIFs is between that of lithium ions and solvated lithium ions. Under the influence of an electric field, part of the solvation layer peels off and rearranges at the pore entrances of the ZIFs, allowing lithium ions to pass through the pores with a lower energy barrier, while larger solvent molecules are partially blocked or need to bypass the ZIFs particles. This lowers the activation energy of ion migration and increases the efficiency of lithium ion transport. + The migration number increases the migration rate of lithium ions; on the other hand, ZIF particles have a large specific surface area and their microporous structure can generate strong capillary forces, providing a large number of adsorption sites in the pores of the base film to adsorb and store electrolyte molecules, thereby improving the compatibility between the separator and the electrolyte.

[0018] Furthermore, ZIFs particles have a high decomposition temperature. The ZIFs particles are dispersed in the network structure formed by bamboo fibers and combine with the hydroxyl groups on the surface of bamboo fibers through electrostatic or coordination interactions to form a stable skeleton structure. When the temperature rises, the ZIFs particles can restrict the movement and shrinkage of the surrounding bamboo fibers, maintain the overall size and pore structure of the base membrane, and thus improve the thermal stability of the membrane. When subjected to external force, the stress can be transferred from the base membrane to the ZIFs particles. The particles hinder the slippage and crack propagation of bamboo fibers, thereby improving the mechanical strength of the membrane.

[0019] In summary, the separators provided in some embodiments of this application improve the lithium-ion migration rate and enhance the compatibility between the separator and the electrolyte by setting zeolite imidazole ester framework (ZIFs) particles in the pores of a base membrane with a three-dimensional network structure formed by bamboo fibers. This is achieved through the synergistic effect of the bamboo fiber three-dimensional network structure and the ZIFs particles. Simultaneously, the thermal stability and mechanical strength of the separator are improved, thereby enhancing the rate performance, cycle performance, and safety performance of the battery.

[0020] The membrane preparation method provided in some embodiments of this application involves, firstly, vacuum freeze-drying a bamboo fiber dispersion to form a regular three-dimensional network structure, improving the tortuosity of the lithium ion transport path. Simultaneously, the vacuum condition provides very little oxygen, preventing the oxidation of the hydroxyl groups in the bamboo fiber chain structure, thus facilitating the subsequent adsorption of metal ions from the metal source solution. Secondly, the base membrane is sequentially immersed in a metal source solution and an imidazole ligand solution. Metal ions in the metal source solution diffuse and are adsorbed into the pores of the bamboo fiber through electrostatic or coordination interactions. Subsequently, they self-assemble with the imidazole ligands in the imidazole ligand solution in situ to form ZIF particles, thereby forming zeolite imidazole ester framework material particles within the pores of the base membrane. The above-mentioned method for preparing the separator involves a combination of steps, including vacuum freeze-drying to form a three-dimensional network structure of bamboo fiber and sequential immersion in a metal source solution and an imidazole ligand solution to form ZIFs particles in situ. This effectively anchors the ZIFs particles within the pores of the three-dimensional network structure formed by the bamboo fiber. Through the synergistic effect of the bamboo fiber three-dimensional network structure and the ZIFs particles, the migration rate of lithium ions is improved, the compatibility between the separator and the electrolyte is enhanced, and the thermal stability and mechanical strength of the separator are increased. This, in turn, improves the rate performance, cycle performance, and safety performance of the battery. Attached Figure Description

[0021] Figure 1 This is a schematic flowchart of a method for preparing a diaphragm according to one embodiment of this application. Detailed Implementation

[0022] Reference will now be made to detailed embodiments of this application, one or more of which are described below. Each example is provided for explanation and not for limitation of this application. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to this application without departing from its scope or spirit. For example, features described or illustrated as part of one embodiment may be used in another embodiment to produce further embodiments.

[0023] Therefore, this application is intended to cover such modifications and variations falling within the scope of the appended claims and their equivalents. Other objects, features, and aspects of this application are disclosed in or will be apparent from the following detailed description. It will be understood by those skilled in the art that this discussion is merely a description of exemplary embodiments and is not intended to limit the broader aspects of this application.

[0024] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0025] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0026] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0027] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0028] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0029] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0030] A first aspect of this application provides a diaphragm comprising a base membrane and zeolite imidazole ester framework material particles. The base membrane is made of bamboo fiber, which forms a porous three-dimensional network structure. At least a portion of the zeolite imidazole ester framework material particles are disposed within the pores.

[0031] The membranes provided in some embodiments of this application utilize zeolite imidazole ester framework (ZIFs) particles embedded in the pores of a base membrane with a three-dimensional network structure formed from bamboo fibers. On one hand, metal ions in the ZIFs particles interact electrostatically or coordinately with hydroxyl groups on the bamboo fiber surface, reducing the adsorption of lithium ions by hydroxyl groups. Simultaneously, the ZIFs bridge the gaps between bamboo fibers, further reducing the tortuosity of lithium ion transport within the bamboo fiber network structure. Furthermore, the pore size of the ZIFs is between that of lithium ions and solvated lithium ions. Under the influence of an electric field, part of the solvation layer peels off and rearranges at the pore entrances of the ZIFs, allowing lithium ions to pass through the pores with a lower energy barrier, while larger solvent molecules are partially blocked or need to bypass the ZIFs particles. This lowers the activation energy of ion migration and increases the efficiency of lithium ion transport. + The migration number increases the migration rate of lithium ions; on the other hand, ZIF particles have a large specific surface area and their microporous structure can generate strong capillary forces, providing a large number of adsorption sites in the pores of the base film to adsorb and store electrolyte molecules, thereby improving the compatibility between the separator and the electrolyte.

[0032] Furthermore, ZIFs particles have a high decomposition temperature. The ZIFs particles are dispersed in the network structure formed by bamboo fibers and combine with the hydroxyl groups on the surface of bamboo fibers through electrostatic or coordination interactions to form a stable skeleton structure. When the temperature rises, the ZIFs particles can restrict the movement and shrinkage of the surrounding bamboo fibers, maintain the overall size and pore structure of the base membrane, and thus improve the thermal stability of the membrane. When subjected to external force, the stress can be transferred from the base membrane to the ZIFs particles. The particles hinder the slippage and crack propagation of bamboo fibers, thereby improving the mechanical strength of the membrane.

[0033] In summary, the separators provided in some embodiments of this application improve the lithium-ion migration rate and enhance the compatibility between the separator and the electrolyte by setting zeolite imidazole ester framework (ZIFs) particles in the pores of a base membrane with a three-dimensional network structure formed by bamboo fibers. This is achieved through the synergistic effect of the bamboo fiber three-dimensional network structure and the ZIFs particles. Simultaneously, the thermal stability and mechanical strength of the separator are improved, thereby enhancing the rate performance, cycle performance, and safety performance of the battery.

[0034] In this article, "bamboo membrane" or "natural bamboo membrane" refers to a layer of natural biomass film existing on the inner wall of bamboo stems (bamboo tubes). Its chemical composition is mainly natural high molecular polymers, including cellulose nanofibers (CNF, about 45%~50%), lignin (about 20%~25%), hemicellulose (about 15%~20%), a small amount of ash and silicates (about 5%), etc.

[0035] In this article, "bamboo fiber" refers to cellulose nanofiber material obtained by extracting natural bamboo membrane as raw material and removing non-cellulose components such as lignin and hemicellulose.

[0036] In this article, "zeolite imidazole ester framework material" or "ZIFs" refers to a material with a zeolite-like topology formed by the self-assembly of metal ions and imidazole ligands. The crystals of ZIFs consist of a hexagonal network, with the metal ions coordinated to the nitrogen atoms of the imidazole rings, forming a three-dimensional porous network structure with high specific surface area and tunable pore size. For example, the zeolite imidazole ester framework material can be at least one of ZIF-8, ZIF-11, ZIF-67, and ZIF-90. Among them, the metal ion of ZIF-8 is Zn. 2+ The imidazole ligand is 2-methylimidazole; the metal ion of ZIF-11 is Zn. 2+ The imidazole ligand is benzimidazole; the metal ion of ZIF-67 is Co. 2+ The imidazole ligand is 2-methylimidazole; the metal ion of ZIF-90 is Zn. 2+ The imidazole ligand is 2-formylimidazole.

[0037] In some embodiments, the tensile strength of bamboo fiber is 150 MPa to 200 MPa. Exemplarily, the tensile strength of bamboo fiber can be, but is not limited to, 150 MPa, 160 MPa, 170 MPa, 180 MPa, 190 MPa, and 200 MPa.

[0038] In some embodiments, the aspect ratio of bamboo fiber is 500 to 1000. Exemplarily, the aspect ratio of bamboo fiber can be, but is not limited to, 500, 600, 700, 800, 900, or 1000.

[0039] In some embodiments, the Young's modulus of bamboo fiber is 10 GPa to 70 GPa. Exemplarily, the Young's modulus of bamboo fiber can be, but is not limited to, 10 GPa, 20 GPa, 30 GPa, 40 GPa, 50 GPa, 60 GPa, or 70 GPa.

[0040] In some implementations, the thermal decomposition temperature of bamboo fiber is greater than or equal to 200°C.

[0041] Bamboo fibers prepared from unmodified natural bamboo membranes meet the above requirements for tensile strength, aspect ratio, Young's modulus, and thermal decomposition temperature. They exhibit excellent mechanical properties, strong puncture resistance, and maintain structural stability at high temperatures, thus delaying thermal runaway in the battery. The high aspect ratio of bamboo fibers ensures sufficient overlap points when forming a three-dimensional network structure, which is beneficial for improving the mechanical properties of the separator.

[0042] Furthermore, the thermal decomposition temperature of bamboo fiber is 200℃~300℃. For example, the thermal decomposition temperature of bamboo fiber can be, but is not limited to, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, and 300℃.

[0043] In some embodiments, at least some of the zeolite imidazole ester framework material particles are also disposed on the surface of the base film.

[0044] In some embodiments, the particle size of the zeolite imidazole ester framework material particles is 20 nm to 200 nm. Exemplarily, the particle size of the zeolite imidazole ester framework material particles can be, but is not limited to, 20 nm, 50 nm, 100 nm, 150 nm, and 200 nm. Modifying the base film with ZIFs of suitable particle size is beneficial for improving the rate performance and cycle performance of the battery. If the ZIFs particles are too large, they cannot grow inside the pores of the base film, resulting in the ZIFs particles being mainly distributed on the surface of the base film. The particles on the surface of the base film will create a certain resistance to the diffusion and transport of lithium ions, which will affect the rate performance and cycle performance of the battery to some extent. If the ZIFs particles are too small, the particles attached to the surface of the bamboo fiber inside the pores of the base film cannot effectively bridge the gaps between the fibers, resulting in an insignificant improvement in the tortuosity of the separator, affecting the ion transport rate, and thus affecting the rate performance and cycle performance of the battery. Regardless of whether the particle size is too large or too small, the mechanical strength and thermal safety performance will decrease to varying degrees due to the imperfect matching between the ZIFs particles and the base film.

[0045] In some embodiments, the surface loading of zeolite imidazole ester framework material particles on the base film is 20% to 90%. Exemplarily, the surface loading of zeolite imidazole ester framework material particles on the base film can be, but is not limited to, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%. Modifying the base film with an appropriate loading of ZIFs is beneficial for improving the rate performance and cycle performance of the battery. If the loading of ZIFs is too high, the particles on the base film surface will create some resistance to the diffusion and transport of lithium ions, affecting the rate performance and cycle performance of the battery to some extent; if the loading of ZIFs is too low, the ability to improve lithium ion diffusion and transport decreases, thereby causing a decline in battery performance.

[0046] In this article, "surface loading" refers to the percentage of the two-dimensional projected area of ​​zeolite imidazole ester (ZIFs) particles in the field of view of the diaphragm surface image obtained by scanning electron microscopy (SEM) relative to the two-dimensional projected area occupied by the base film in the same image.

[0047] In some embodiments, the pore size of the base film is 10 nm to 100 nm. Exemplarily, the pore size of the base film can be, but is not limited to, 10 nm, 30 nm, 50 nm, 70 nm, and 100 nm. A suitable pore size range is beneficial for improving the rate performance, cycle performance, and safety performance of the battery. If the pore size of the base film is too small, it hinders the free transport path of lithium ions. Simultaneously, the smaller space leads to a reduction in the ZIF particles loaded within the pores of the base film, thus affecting the electrochemical performance of the battery. If the pore size of the base film is too large, although it facilitates the diffusion and transport of lithium ions, it can also easily generate weak short circuits when the negative electrode (e.g., silicon negative electrode) expands, leading to a decrease in cycle performance. Furthermore, the mechanical strength and thermal safety performance of the separator also show a certain degree of decline.

[0048] In some embodiments, the thickness of the base film is 5 μm to 50 μm. Exemplarily, the thickness of the base film can be, but is not limited to, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, and 50 μm. A suitable thickness of the base film is beneficial for improving the rate performance and cycle performance of the battery. If the base film is too thin, the lithium-ion diffusion distance is shortened, which can also lead to a risk of weak short circuits, causing a certain degree of decrease in the battery's cycle performance or even a sharp drop. If the base film is too thick, the lithium-ion diffusion distance is lengthened, resulting in a decrease in the battery's rate performance, which in turn leads to a decrease in cycle performance.

[0049] In some embodiments, the porosity of the base film is 30% to 60%. Exemplarily, the porosity of the base film can be, but is not limited to, 30%, 40%, 50%, and 60%. A suitable range of porosity is beneficial for improving the rate performance, cycle performance, and safety performance of the battery. It is understood that the effect of the porosity of the base film on battery performance is generally consistent with the trend of changes in the pore size of the base film, and will not be elaborated upon here.

[0050] In some embodiments, the three-dimensional network structure formed by bamboo fibers has an oriented pore structure. Compared to an irregular three-dimensional network structure, the oriented pore structure provides a more direct transport path for lithium ions to migrate within the separator, reduces the tortuosity of lithium ion transport, increases the diffusion transport rate of lithium ions, and further improves the rate performance of the battery.

[0051] Furthermore, the oriented channel structure exhibits a honeycomb network structure with uniform pore size.

[0052] A second aspect of this application provides a method for preparing a diaphragm, such as... Figure 1 As shown, the preparation method includes the following steps:

[0053] S1. The bamboo fiber dispersion is subjected to vacuum freeze-drying to form a three-dimensional network structure and prepare a base film.

[0054] S2. The base membrane is sequentially immersed in a metal source solution and an imidazole ligand solution to form zeolite imidazole ester framework material particles in the pores of the base membrane, thereby preparing a diaphragm.

[0055] The membrane preparation method provided in some embodiments of this application involves, firstly, step S1, vacuum freeze-drying of bamboo fiber dispersion to form a regular three-dimensional network structure, improving the tortuosity of the lithium ion transport path. Simultaneously, the vacuum condition provides very little oxygen, preventing the hydroxyl groups of the bamboo fiber chain structure from being oxidized, thus facilitating the subsequent adsorption of metal ions from the metal source solution. Secondly, in step S2, the base membrane is sequentially immersed in the metal source solution and the imidazole ligand solution. Metal ions in the metal source solution diffuse and are adsorbed into the pores of the bamboo fiber through electrostatic or coordination interactions. Subsequently, they self-assemble with the imidazole ligands in the imidazole ligand solution in situ to form ZIF particles, thereby forming zeolite imidazole ester framework material particles within the pores of the base membrane. The above-mentioned method for preparing the separator involves a combination of steps, including vacuum freeze-drying to form a three-dimensional network structure of bamboo fiber and sequential immersion in a metal source solution and an imidazole ligand solution to form ZIFs particles in situ. This effectively anchors the ZIFs particles within the pores of the three-dimensional network structure formed by the bamboo fiber. Through the synergistic effect of the bamboo fiber three-dimensional network structure and the ZIFs particles, the migration rate of lithium ions is improved, the compatibility between the separator and the electrolyte is enhanced, and the thermal stability and mechanical strength of the separator are increased. This, in turn, improves the rate performance, cycle performance, and safety performance of the battery.

[0056] It is understandable that the step of sequentially immersing the base membrane in the metal source solution and the imidazole ligand solution is used for sequential impregnation. This sequential impregnation method has the following advantages: ZIFs grow only on the surface and within the pores of the base membrane, allowing for precise control of the reaction rate and particle distribution; adsorption occurs through electrostatic interactions or coordination with cellulose hydroxyl groups, forming stronger interactions and resulting in higher stability; and the original porous structure of the membrane can be preserved to the maximum extent while introducing ZIF functionality, ensuring efficient lithium-ion transport. In some embodiments, the bamboo fiber dispersion is prepared using the following steps:

[0057] S01. The bamboo membrane is subjected to alkali treatment and oxidation treatment to remove hemicellulose and lignin.

[0058] S02. Bamboo film treated with alkali and oxidation is mixed with solvent to prepare bamboo fiber dispersion.

[0059] In this embodiment, hemicellulose and some lignin are dissolved by alkali treatment, and the residual lignin is removed by oxidation treatment, thereby obtaining bamboo fiber material with cellulose nanofiber material as the main component, so as to form a regular three-dimensional network structure in the future.

[0060] In some specific embodiments, the bamboo membrane has a thickness of 10 μm to 100 μm, a length of 20 cm to 50 cm, and a width of 8 cm to 10 cm.

[0061] In some specific embodiments, the alkali treatment includes immersing the bamboo membrane in an alkaline solution.

[0062] Furthermore, the alkaline substances in the alkaline solution include at least one of sodium hydroxide, lithium hydroxide, potassium hydroxide, calcium hydroxide, and barium hydroxide.

[0063] Furthermore, the concentration of the alkaline solution is 5wt%~10wt%.

[0064] Furthermore, the alkali treatment soaking temperature is 60℃~90℃, and the soaking time is 3h~6h.

[0065] In some specific embodiments, the oxidation treatment includes immersing the bamboo membrane in an oxidizing agent solution.

[0066] Furthermore, the oxidant in the oxidant solution is selected from at least one of hydrogen peroxide, cobalt fluoride, sodium persulfate, potassium persulfate, potassium dichromate, potassium permanganate, potassium chlorate, sodium chlorate, and fuming sulfuric acid.

[0067] Furthermore, the concentration of the oxidant solution is 5wt%~10wt%.

[0068] Furthermore, the immersion temperature for the oxidation treatment is 30℃~40℃, and the immersion time is 1h~8h.

[0069] In some embodiments, the bamboo membrane after alkali treatment and oxidation treatment is mixed with a solvent, specifically including the following steps: dissolving bamboo fibers in deionized water and performing ultrasonic treatment.

[0070] In some specific implementations, the frequency of ultrasonic treatment is 30 kHz to 40 kHz, and the duration is 0.5 h to 2 h.

[0071] In some embodiments, vacuum freeze-drying involves pre-freezing the bamboo fiber dispersion before drying it.

[0072] In some embodiments, the pre-freezing temperature is -80°C to -60°C. Exemplarily, the pre-freezing temperature can be, but is not limited to, -80°C, -75°C, -70°C, -65°C, and -60°C.

[0073] In some embodiments, the drying temperature is -60°C to -50°C. Exemplarily, the drying temperature can be, but is not limited to, -60°C, -58°C, -56°C, -54°C, -52°C, and -50°C.

[0074] In some embodiments, the vacuum degree of the drying process is 1 Pa to 10 Pa. Exemplarily, the vacuum degree of the drying process can be, but is not limited to, 1 Pa, 3 Pa, 5 Pa, 7 Pa, 9 Pa, or 10 Pa.

[0075] In some embodiments, before sequentially immersing the base membrane in the metal source solution and the imidazole ligand solution, the method further includes the step of activating the base membrane with an alcohol solvent. Activating the base membrane removes residual air, opens the pores of the base membrane, and enhances the subsequent interaction between hydroxyl groups and metal ions.

[0076] Furthermore, the alcohol solvent includes at least one of methanol, ethanol, propanol, and isopropanol.

[0077] Furthermore, the alcohol solvent is methanol.

[0078] In some embodiments, the concentration of the bamboo fiber dispersion is 5 wt% to 10 wt%. Exemplarily, the concentration of the bamboo fiber dispersion can be, but is not limited to, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%.

[0079] In some embodiments, the molar ratio of metal ions in the metal source solution to imidazole ligands in the imidazole ligand solution is 1:(2~5). Exemplarily, the molar ratio of metal ions in the metal source solution to imidazole ligands in the imidazole ligand solution can be, but is not limited to, 1:2, 1:3, 1:4, or 1:5.

[0080] In some embodiments, the total immersion time of the base film in the metal source solution is 30 min to 60 min. Exemplarily, the immersion time of the base film in the metal source solution can be, but is not limited to, 30 min, 40 min, 50 min, or 60 min.

[0081] In some embodiments, the total immersion time of the base membrane in the imidazole ligand solution is 6 h to 24 h. Exemplarily, the immersion time of the base membrane in the imidazole ligand solution can be, but is not limited to, 6 h, 12 h, 18 h, or 24 h.

[0082] In some embodiments, the concentration of the metal source solution is 0.05 mol / L to 0.2 mol / L. Exemplarily, the concentration of the metal source solution can be, but is not limited to, 0.05 mol / L, 0.06 mol / L, 0.08 mol / L, 0.1 mol / L, 0.12 mol / L, 0.14 mol / L, 0.16 mol / L, 0.18 mol / L, and 0.2 mol / L.

[0083] In some embodiments, the concentration of the imidazole ligand solution is 0.2 mol / L to 0.8 mol / L. Exemplarily, the concentration of the imidazole ligand solution can be, but is not limited to, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, and 0.8 mol / L.

[0084] A third aspect of this application provides a lithium-ion battery comprising the separator provided in the first aspect above, or a separator prepared by the method for preparing the separator provided in the second aspect above.

[0085] A fourth aspect of this application provides an electrical device comprising the lithium-ion battery provided in the third aspect above.

[0086] The electrical device used in this application is not particularly limited and can be any electrical device known in the prior art. For example, the electrical device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0087] The present application will be further described below with reference to specific embodiments and comparative examples.

[0088] Example 1

[0089] Diaphragm:

[0090] (1) Bamboo membrane preparation: Cut off both ends of the bamboo section with a saw, leaving only the bamboo tube, and split the bamboo tube in half. Soak it in 30℃ warm water for about 40 minutes. Take it out and use tweezers to gently pry up the bamboo membrane at one end of the bamboo tube to separate the membrane from the bamboo tube. After air drying, you will get a natural bamboo membrane.

[0091] (2) Preparation of bamboo fiber dispersion: Bamboo film (thickness: 50 μm, length: 45 cm, width: 9 cm) was soaked in 10wt% NaOH solution for alkali treatment at 80℃ for 4h to dissolve hemicellulose and some lignin; then soaked in 5wt% H2O2 solution for oxidation treatment at 30℃ for 6h to remove residual lignin. After purification, 8wt% bamboo fiber was dissolved in deionized water and ultrasonically treated at 40kHz for 1h to obtain bamboo fiber dispersion.

[0092] The parameters of bamboo fiber are as follows: tensile strength of 180 MPa, aspect ratio of 900, Young's modulus of 60 GPa, and thermal decomposition temperature of 230℃.

[0093] (3) Preparation of the base film: The bamboo fiber dispersion was placed on a clean and flat petri dish, and the desired dispersion layer thickness of 20 μm was obtained by scraping. The dispersion was sealed with the petri dish lid and then placed in a high and low temperature test chamber at -80℃ for pre-freezing for 15h. The pre-frozen sample was taken out, the petri dish lid was removed, and then it was placed in a vacuum freeze dryer at -50℃ and vacuum was drawn with a vacuum degree of 1.5 Pa. The moisture in the sample was completely removed by low temperature vacuum and the drying time was 24h to form a base film with a three-dimensional porous structure.

[0094] (4) Modification of ZIF-8 particles: A certain amount of Zn(NO3)2·6H2O and 2-methylimidazole were weighed and dissolved in a certain volume of methanol to prepare 0.1 mol / L and 0.4 mol / L solutions, respectively. The solutions were then stirred magnetically until completely dissolved. At this point, Zn 2+ The molar ratio of Zn(NO3)2 to 2-methylimidazole was 1:4; the above-mentioned base membrane was activated by immersing it in methanol for 20 min; while keeping the membrane moist, excess methanol on the surface was absorbed with filter paper; the activated base membrane was then completely immersed in Zn(NO3)2 solution for 60 min, so that Zn 2+ The Zn diffuses to the surface and internal pores of the base film and is adsorbed by coordinating with the hydroxyl groups of bamboo fibers through electrostatic interactions; 2+ The base film was removed and quickly immersed in methanol for 4 seconds to remove excess Zn adsorbed on the surface. 2+ Excess methanol was removed with filter paper. The treated base membrane was completely immersed in 2-methylimidazole solution and soaked at room temperature for 12 hours, with slight shaking or stirring to ensure uniform solution. After the reaction was completed, the membrane was quickly removed and immediately immersed in methanol for washing. This was repeated 4 times, each time for 8 minutes, with stirring or sonication (150W, 45s) to thoroughly remove unreacted raw materials, by-products and loosely adsorbed ZIF-8 particles.

[0095] (5) Post-treatment of the diaphragm: Take out the base membrane after thorough washing and use filter paper to absorb the surface liquid; freeze dry again under the same drying conditions as in step (3), and hot press for 30s at a temperature of 60°C and a pressure of 0.5 MPa to make it flat.

[0096] The physical properties of the prepared diaphragm are as follows: the pore size of the base membrane is 50 nm, the thickness is 20 μm, and the porosity is 50% (considering only the base membrane itself); the particle size of the ZIF-8 particles is 50 nm, and the surface loading is 60%.

[0097] Lithium-ion batteries:

[0098] A lithium-ion battery is prepared by sequentially stacking, baking, injecting electrolyte, forming, and grading the positive electrode, the separator, and the negative electrode. The positive electrode is prepared by sequentially homogenizing, coating, and rolling an NCM811 ternary positive electrode, a conductive agent SP, and a binder PVDF, with a mass percentage of NCM811, SP, and PVDF of 97.5:1:1.5. The negative electrode is prepared by sequentially homogenizing, coating, and rolling an silicon-oxygen-doped graphite negative electrode (90% graphite content), a conductive agent SP, a binder SBR, and a dispersant CMC, with a mass percentage of silicon-oxygen-doped graphite, SP, SBR, and CMC of 93.5:1.5:4.5:0.5. The electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC:DEC:EMC = 1:1:1 (volume ratio).

[0099] Example 2

[0100] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0101] In step (4), the concentrations of Zn(NO3)2·6H2O solution and 2-methylimidazole solution are 0.15 mol / L and 0.6 mol / L, respectively, and the particle size of ZIF-8 particles prepared therefrom is 100 nm.

[0102] Example 3

[0103] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0104] In step (4), the concentrations of Zn(NO3)2·6H2O solution and 2-methylimidazole solution are 0.05 mol / L and 0.2 mol / L, respectively, and the particle size of ZIF-8 particles prepared therefrom is 20 nm.

[0105] Example 4

[0106] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0107] In step (4), the concentrations of Zn(NO3)2·6H2O solution and 2-methylimidazole solution are 0.2 mol / L and 0.8 mol / L, respectively, and the particle size of ZIF-8 particles prepared therefrom is 200 nm.

[0108] Example 5

[0109] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0110] In step (2), the bamboo fiber dispersion was ultrasonically treated for 1.6 h, and the concentration of the bamboo fiber dispersion was 8.8 wt%. The pore size of the base film prepared in this way was 20 nm.

[0111] The parameters of bamboo fiber are as follows: tensile strength of 175 MPa, aspect ratio of 870, Young's modulus of 55 GPa, and thermal decomposition temperature of 226℃.

[0112] Example 6

[0113] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0114] In step (2), the dispersion is ultrasonically treated for 2 hours, the concentration of the bamboo fiber dispersion is 10 wt%, and the pore size of the base film prepared therefrom is 10 nm.

[0115] The parameters of bamboo fiber are as follows: tensile strength of 170 MPa, aspect ratio of 840, Young's modulus of 50 GPa, and thermal decomposition temperature of 220℃.

[0116] Example 7

[0117] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0118] In step (2), the dispersion is ultrasonically treated for 0.5 h, the concentration of the bamboo fiber dispersion is 5 wt%, and the pore size of the base film prepared therefrom is 100 nm.

[0119] The parameters of bamboo fiber are as follows: tensile strength of 185 MPa, aspect ratio of 920, Young's modulus of 63 GPa, and thermal decomposition temperature of 223℃.

[0120] Example 8

[0121] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0122] In step (2), the dispersion was ultrasonically treated for 1.8 h, the concentration of the bamboo fiber dispersion was 8.5 wt%, and the porosity of the base membrane prepared therefrom was 40%.

[0123] The parameters of bamboo fiber are as follows: tensile strength of 178 MPa, aspect ratio of 890, Young's modulus of 57 GPa, and thermal decomposition temperature of 228℃.

[0124] Example 9

[0125] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0126] In step (2), the dispersion was ultrasonically treated for 2 hours, the concentration of the bamboo fiber dispersion was 9.2 wt%, and the porosity of the base membrane prepared therefrom was 30%.

[0127] The parameters of bamboo fiber are as follows: tensile strength of 165 MPa, aspect ratio of 830, Young's modulus of 48 GPa, and thermal decomposition temperature of 218℃.

[0128] Example 10

[0129] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0130] In step (2), the dispersion was ultrasonically treated for 0.5 h, the concentration of the bamboo fiber dispersion was 7.6 wt%, and the porosity of the base membrane prepared therefrom was 60%.

[0131] The parameters of bamboo fiber are as follows: tensile strength of 188 MPa, aspect ratio of 930, Young's modulus of 65 GPa, and thermal decomposition temperature of 235℃.

[0132] Example 11

[0133] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0134] In step (2), the thickness of the dispersion layer is 10 μm, and the thickness of the base film prepared therefrom is 10 μm.

[0135] Example 12

[0136] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0137] In step (2), the dispersion layer is coated with a thickness of 5 μm, and the thickness of the base film prepared thereby is 5 μm.

[0138] Example 13

[0139] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0140] In step (2), the dispersion layer is coated to a thickness of 50 μm, and the thickness of the base film prepared therefrom is 50 μm.

[0141] Example 14

[0142] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0143] In step (3), the membrane is immersed in 2-methylimidazole solution at room temperature for 10 h, and the surface loading of ZIF-8 particles in the membrane prepared thereby is 40%.

[0144] Example 15

[0145] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0146] In step (3), the membrane is immersed in 2-methylimidazole solution at room temperature for 6 hours, and the surface loading of ZIF-8 particles in the membrane prepared thereby is 20%.

[0147] Example 16

[0148] The separator and lithium-ion battery provided in this embodiment are basically the same as those in Embodiment 1, except that:

[0149] In step (3), the membrane is immersed in 2-methylimidazole solution at room temperature for 24 h, and the surface loading of ZIF-8 particles in the membrane prepared thereby is 90%.

[0150] Example 17

[0151] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0152] In step (4), the concentrations of Zn(NO3)2·6H2O solution and 2-methylimidazole solution are 0.02 mol / L and 0.08 mol / L, respectively, and the particle size of ZIF-8 particles prepared therefrom is 5 nm.

[0153] Example 18

[0154] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0155] In step (4), the concentrations of Zn(NO3)2·6H2O solution and 2-methylimidazole methanol solution are 0.8 mol / L and 3.2 mol / L, respectively, and the particle size of ZIF-8 particles prepared therefrom is 500 nm.

[0156] Example 19

[0157] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0158] In step (2), the dispersion was ultrasonically treated for 3 hours, the concentration of the bamboo fiber dispersion was 12 wt%, and the pore size of the base film prepared therefrom was 5 nm.

[0159] The parameters of bamboo fiber are as follows: tensile strength of 162 MPa, aspect ratio of 820, Young's modulus of 45 GPa, and thermal decomposition temperature of 215℃.

[0160] Example 20

[0161] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0162] In step (2), the dispersion is ultrasonically treated for 0.1 h, the concentration of the bamboo fiber dispersion is 3 wt%, and the pore size of the base film prepared therefrom is 300 nm.

[0163] The parameters of bamboo fiber are as follows: tensile strength of 192 MPa, aspect ratio of 940, Young's modulus of 66 GPa, and thermal decomposition temperature of 240℃.

[0164] Example 21

[0165] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0166] In step (2), the dispersion was ultrasonically treated for 2.8 h, the concentration of the bamboo fiber dispersion was 11.2 wt%, and the porosity of the base film prepared therefrom was 5%.

[0167] The parameters of bamboo fiber are as follows: tensile strength of 163 MPa, aspect ratio of 825, Young's modulus of 46 GPa, and thermal decomposition temperature of 217℃.

[0168] Example 22

[0169] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0170] In step (2), the dispersion was ultrasonically treated for 0.1 h, the concentration of the bamboo fiber dispersion was 2.3 wt%, and the porosity of the base membrane prepared therefrom was 95%.

[0171] The parameters of bamboo fiber are as follows: tensile strength of 190 MPa, aspect ratio of 936, Young's modulus of 64 GPa, and thermal decomposition temperature of 238℃.

[0172] Example 23

[0173] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0174] In step (2), the thickness of the dispersion layer is 1 μm, and the thickness of the base film prepared therefrom is 1 μm.

[0175] Example 24

[0176] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0177] In step (2), the thickness of the dispersion layer is 150 μm, and the thickness of the base film prepared therefrom is 150 μm.

[0178] Example 25

[0179] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0180] In step (3), the ZIF-8 particles were immersed in a 2-methylimidazole solution at room temperature for 2 hours, and the surface loading of the particles was 5%.

[0181] Example 26

[0182] The separator and lithium-ion battery provided in this comparative example are basically the same as those in Example 1, except that:

[0183] In step (3), the ZIF-8 particles were immersed in a 2-methylimidazole solution at room temperature for 48 hours, and the surface loading of the ZIF-8 particles was 100%. At the same time, due to the excess of ZIF-8 particles, the particles were located not only in the pores of the base film but also on the surface of the base film, presenting a multilayer structure.

[0184] Comparative Example 1

[0185] Diaphragm: Commercially available polyethylene diaphragm.

[0186] Lithium-ion batteries:

[0187] A lithium-ion battery is prepared by sequentially stacking, baking, injecting electrolyte, forming, and grading the positive electrode, the separator, and the negative electrode. The positive electrode is prepared by sequentially homogenizing, coating, and rolling an NCM811 ternary positive electrode, a conductive agent SP, and a binder PVDF, with a mass percentage of NCM811, SP, and PVDF of 97.5:1:1.5. The negative electrode is prepared by sequentially homogenizing, coating, and rolling an silicon-oxygen-doped graphite negative electrode (90% graphite content), a conductive agent SP, a binder SBR, and a dispersant CMC, with a mass percentage of silicon-oxygen-doped graphite, SP, SBR, and CMC of 93.5:1.5:4.5:0.5. The electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC:DEC:EMC = 1:1:1 (volume ratio).

[0188] Comparative Example 2

[0189] Diaphragm:

[0190] (1) Bamboo membrane preparation: Cut off both ends of the bamboo section with a saw, leaving only the bamboo tube, and split the bamboo tube in half. Soak it in 30℃ warm water for about 40 minutes. Take it out and use tweezers to gently pry up the bamboo membrane at one end of the bamboo tube to separate the membrane from the bamboo tube. After air drying, you will get a natural bamboo membrane.

[0191] (2) Preparation of bamboo fiber dispersion: Bamboo film (thickness: 50 μm, length: 45 cm, width: 9 cm) was soaked in 10wt% NaOH solution for alkali treatment at 80℃ for 4h to dissolve hemicellulose and some lignin; then soaked in 5wt% H2O2 solution for oxidation treatment at 30℃ for 6h to remove residual lignin. After purification, 8wt% bamboo fiber was dissolved in deionized water and ultrasonically treated at 40kHz for 1h to obtain bamboo fiber dispersion.

[0192] The parameters of bamboo fiber are as follows: tensile strength is 180 MPa, aspect ratio is 900, Young's modulus is 60 GPa, and thermal decomposition temperature is 230℃. (3) Preparation of diaphragm: bamboo fiber dispersion is placed on a clean and flat petri dish, and the required dispersion layer thickness of 20 μm is obtained by scraping; the dispersion is sealed with a petri dish cover, and then placed in a high and low temperature test chamber at -80℃ for pre-freezing for 15h; the pre-frozen sample is taken out, the petri dish cover is removed, and then placed in a vacuum freeze dryer at -50℃, and a vacuum of 1.5 Pa is drawn to completely remove the moisture in the sample through low temperature vacuum. The drying time is 24h to form a diaphragm with a three-dimensional porous structure.

[0193] Lithium-ion batteries:

[0194] A lithium-ion battery is prepared by sequentially stacking, baking, injecting electrolyte, forming, and grading the positive electrode, the separator, and the negative electrode. The positive electrode is prepared by sequentially homogenizing, coating, and rolling an NCM811 ternary positive electrode, a conductive agent SP, and a binder PVDF, with a mass percentage of 97.5:1:1.5. The negative electrode is prepared by sequentially homogenizing, coating, and rolling an oxygen-doped graphite negative electrode (90% graphite content), a conductive agent SP, a binder SBR, and a dispersant CMC, with a mass percentage of 93.5:1.5:4.5:0.5. The electrolyte is a 1 mol / L LiPF6 solution dissolved in a mixed solvent of EC:DEC:EMC = 1:1:1 (volume ratio).

[0195] Comparative Example 3

[0196] Diaphragm:

[0197] (1) Bamboo membrane preparation: Cut off both ends of the bamboo section with a saw, leaving only the bamboo tube, and split the bamboo tube in half. Soak it in 30℃ warm water for about 40 minutes, then take it out. Use tweezers to gently pry up the bamboo membrane at one end of the bamboo tube to separate the membrane from the bamboo tube. After air drying, you will get a natural bamboo membrane, which is the base membrane.

[0198] (2) Modification of ZIF-8 particles: A certain amount of Zn(NO3)2·6H2O and 2-methylimidazole were weighed and dissolved in a certain volume of methanol to prepare 0.1 mol / L and 0.4 mol / L solutions, respectively. The solutions were then stirred magnetically until completely dissolved. At this point, Zn 2+ The molar ratio of Zn to 2-methylimidazole was 1:4; the above-mentioned base membrane was activated by immersing it in methanol for 20 min; while keeping the membrane moist, excess methanol on the surface was absorbed with filter paper; the activated base membrane was completely immersed in Zn(NO3)2 solution for 60 min; the Zn-adsorbed base membrane was then immersed in the solution. 2+ The base film was removed and quickly immersed in methanol for 4 seconds to remove excess Zn adsorbed on the surface. 2+ Excess methanol was removed with filter paper. The treated base membrane was completely immersed in 2-methylimidazole solution and soaked at room temperature for 12 hours, with slight shaking or stirring to ensure uniform solution. After the reaction was completed, the membrane was quickly removed and immediately immersed in methanol for washing. This was repeated 4 times, each time for 8 minutes, with stirring or sonication (150W, 45s) to thoroughly remove unreacted raw materials, by-products and loosely adsorbed ZIF-8 particles.

[0199] (5) Post-treatment of the diaphragm: Take out the base membrane after thorough washing and use filter paper to absorb the surface liquid; perform hot pressing at 60°C and 0.5 MPa for 30s to make it flat.

[0200] Lithium-ion batteries:

[0201] A lithium-ion battery is prepared by sequentially stacking, baking, injecting electrolyte, forming, and grading the positive electrode, the separator, and the negative electrode. The positive electrode is prepared by sequentially homogenizing, coating, and rolling an NCM811 ternary positive electrode, a conductive agent SP, and a binder PVDF, with a mass percentage of NCM811, SP, and PVDF of 97.5:1:1.5. The negative electrode is prepared by sequentially homogenizing, coating, and rolling an silicon-oxygen-doped graphite negative electrode (90% graphite content), a conductive agent SP, a binder SBR, and a dispersant CMC, with a mass percentage of silicon-oxygen-doped graphite, SP, SBR, and CMC of 93.5:1.5:4.5:0.5. The electrolyte is a mixed solvent of 1 mol / L LiPF6 dissolved in EC:DEC:EMC = 1:1:1 (volume ratio).

[0202] The key parameters of the above embodiments and comparative examples are summarized in Table 1 below.

[0203] Table 1

[0204]

[0205] Performance testing

[0206] (1) Ratio performance test

[0207] Under a stable test environment temperature of 25℃±0.5℃, using the capacity Q0 obtained from a 0.33C rate test as a baseline, the cells were charged at a constant current of 0.33C to 4.2V, then charged at a constant voltage of 4.2V until the current decreased to 0.05C. After resting for 0.5 hours, the cells were discharged to 2.5V at different rates of 0.33C / 0.5C / 1C / 2C. Five batteries were tested, and the average capacity of the five batteries was taken. A clamp was used during the test. The torque of the clamp was 1.8 N·m. The formulas for calculating the capacity retention rate at different rates are shown below:

[0208] ;

[0209] In the formula, R represents the capacity retention rate at different magnifications, in units of %; Q t Q0 represents the average discharge capacity at different discharge rates, in mAh; Q0 represents the average discharge capacity at a 0.33C rate, in mAh.

[0210] (2) Cyclic performance test

[0211] Under a stable test environment temperature of 25℃±0.5℃, the battery was charged at a current of 0.5C to the charging cutoff voltage of 4.2V, then switched to constant voltage charging to the cutoff current of 0.05C, allowed to stand for 0.5 hours, and then discharged at a current of 0.5C to the cutoff voltage of 2.5V, allowed to stand for 0.5 hours. Batteries with different separator assemblies were subjected to cyclic charge-discharge cycles using the above method, and the discharge capacity of the battery in each cycle was recorded. The initial discharge capacity of the cycle was denoted as C0, and the discharge capacity C at the end of each cycle was recorded. n This process continues until the loop reaches 1000 iterations. The formula for calculating the capacity retention rate after n loops is shown below:

[0212] ;

[0213] In the formula, R' represents the capacity retention rate after 1000 cycles, in %; C n C0 represents the battery discharge capacity in the nth cycle, in mAh; C0 represents the battery discharge capacity in the initial cycle, in mAh.

[0214] (3) Puncture strength test

[0215] The puncture strength test procedure was performed according to section 6.5.3 of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries": The puncture strength of the separator was determined using a puncture tester. A needle with a diameter of 1.00 mm and a spherical tip with a radius of 0.5 mm was used. The inner diameter of the sample clamp was 10 mm. The separator was laid flat in the clamp and clamped. The puncture was performed at a rate of (100±10) mm / min. After the puncture was completed, the values ​​collected by the puncture tester were recorded.

[0216] (4) Thermal stability test

[0217] The test procedure for heat shrinkage rate was performed according to section 6.5.2 of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries": The prepared separator was cut into small squares of 2×2 cm; then, the separator samples were placed in glass petri dishes and placed in an electrically heated constant temperature drying oven. The separator dimensions were recorded after being placed at room temperature and 150℃ for 2 hours respectively. The heat shrinkage rate was calculated using the formula shown below:

[0218] ;

[0219] In the formula, ΔL t The lateral shrinkage rate of the diaphragm is expressed as a percentage (%), or TD; L0 represents the initial lateral dimension of the diaphragm, in mm; L t This indicates the size of the diaphragm after lateral shrinkage, in mm.

[0220] ;

[0221] In the formula, ΔL m The longitudinal shrinkage rate of the diaphragm is expressed as a percentage (%), or MD; L0 represents the initial longitudinal dimension of the diaphragm, in mm; L m This indicates the size of the diaphragm after longitudinal shrinkage, in mm.

[0222] The test data for each embodiment and comparative example are shown in Table 2 below.

[0223] Table 2

[0224]

[0225] As shown in Table 2, comparing Examples 1-26 and Comparative Examples 1-3, it can be seen that the separators provided in some embodiments of this application utilize the synergistic effect of the bamboo fiber three-dimensional network structure and ZIFs particles to improve the thermal stability and mechanical strength of the separator, thereby improving the rate performance, cycle performance and safety performance of the battery.

[0226] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0227] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical concept of this application, and these modifications and improvements all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A diaphragm, characterized in that, include: The base membrane is made of bamboo fiber, which forms a three-dimensional network structure with pores. as well as Zeolite imidazole ester framework material particles, at least a portion of the zeolite imidazole ester framework material particles are disposed within the pores.

2. The diaphragm according to claim 1, characterized in that, The bamboo fiber satisfies at least one of the following conditions: (1) The tensile strength of the bamboo fiber is 150 MPa~200 MPa; (2) The aspect ratio of the bamboo fiber is 500~1000; (3) The Young's modulus of the bamboo fiber is 10 GPa to 70 GPa; (4) The thermal decomposition temperature of the bamboo fiber is greater than or equal to 200℃.

3. The diaphragm according to claim 1, characterized in that, At least some of the zeolite imidazole ester framework material particles are also disposed on the surface of the base film.

4. The diaphragm according to any one of claims 1 to 3, characterized in that, At least one of the following conditions must be met: (1) The particle size of the zeolite imidazole ester framework material particles is 20 nm to 200 nm; (2) The surface loading of the zeolite imidazole ester framework material particles on the base film is 20%~90%; (3) The pore size of the base film is 10 nm to 100 nm; (4) The thickness of the base film is 5 μm to 50 μm; (5) The porosity of the base membrane is 30%~60%; (6) The three-dimensional network structure formed by the bamboo fiber has an oriented pore structure.

5. A method for preparing a diaphragm, characterized in that, Includes the following steps: Bamboo fiber dispersion was subjected to vacuum freeze-drying to form a three-dimensional network structure, and a base film was prepared. The base membrane is sequentially immersed in a metal source solution and an imidazole ligand solution to form zeolite imidazole ester framework material particles within the pores of the base membrane, thereby preparing a diaphragm.

6. The method for preparing the diaphragm according to claim 5, characterized in that, The bamboo fiber dispersion was prepared using the following steps: Bamboo membranes are subjected to alkali and oxidation treatments to remove hemicellulose and lignin. Bamboo fiber dispersion was prepared by mixing bamboo films treated with alkali and oxidation with solvent.

7. The method for preparing the diaphragm according to claim 5, characterized in that, The vacuum freeze-drying process includes pre-freezing the bamboo fiber dispersion followed by drying. The vacuum freeze-drying process satisfies at least one of the following conditions: (1) The temperature of the pre-freezing treatment is -80℃ to -60℃; (2) The drying temperature is -60℃ to -50℃; (3) The vacuum degree of the drying process is 1 Pa to 10 Pa.

8. The method for preparing the diaphragm according to any one of claims 5 to 7, characterized in that, Before sequentially immersing the base membrane in the metal source solution and the imidazole ligand solution, the following steps are also included: The base film was activated using an alcohol solvent.

9. The method for preparing the diaphragm according to any one of claims 5 to 7, characterized in that, At least one of the following conditions must be met: (1) The concentration of the bamboo fiber dispersion is 5 wt%~10 wt%; (2) The molar ratio of metal ions in the metal source solution to imidazole ligands in the imidazole ligand solution is 1:(2~5). (3) The concentration of the metal source solution is 0.05 mol / L to 0.2 mol / L; (4) The concentration of the imidazole ligand solution is 0.2 mol / L to 0.8 mol / L; (5) The immersion time of the base film in the metal source solution is 30 min to 60 min; (6) The base film is immersed in the imidazole ligand solution for 6h to 24h.

10. A lithium-ion battery, characterized in that, This includes the diaphragm as described in any one of claims 1 to 4, or the diaphragm prepared by the method described in any one of claims 5 to 9.

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