Nanocellulose separator and method for its production and use

Abstract

The application relates to the technical field of battery diaphragm materials, and particularly discloses a preparation method of a nanocellulose diaphragm, which comprises the following steps: mixing nanocellulose and acrylic acid, adding a solvent and a catalyst, performing esterification, obtaining nanocellulose acrylate, adding a second monomer, adding an initiator to perform polymerization, filtering, adding a lithium hydroxide or lithium carbonate solution to stir, filtering and washing, and drying to obtain nanocellulose lithium grafted polyacrylic acid salt; dispersing the nanocellulose lithium grafted polyacrylic acid salt in water, performing vacuum suction filtration by using a sand core funnel provided with a polytetrafluoroethylene filter membrane to obtain a wet film; moving the wet film into anhydrous ethanol to perform solvent replacement, and drying to obtain the nanocellulose diaphragm. The nanocellulose diaphragm prepared by the application has electrolyte wettability, thermal stability and tensile property which are superior to those of a commercial polyolefin diaphragm, the diaphragm porosity is moderate, the nanocellulose diaphragm is used for a lithium ion battery diaphragm, and lithium ion migration is facilitated, so that the ion conductivity of the diaphragm is improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator materials technology, and in particular to a nanocellulose separator, its preparation method and application. Background Technology

[0002] The battery separator, a porous insulating layer, is a key component of lithium-ion batteries. It plays a crucial role in preventing contact between the positive and negative electrodes, preventing short circuits, absorbing electrolyte, ensuring high ionic conductivity, and stopping the battery reaction in case of abnormalities, thus improving battery safety. Currently, lithium-ion batteries primarily use polyolefin separators. These separators suffer from poor thermal stability and poor electrolyte wettability, easily causing contact between the positive and negative electrodes, leading to short circuits, instantaneous heat generation, and potentially fires or explosions, compromising battery safety. Furthermore, polypropylene separators are mainly derived from non-renewable fossil fuels, making them unsuitable for the green and low-carbon development of high-performance lithium-ion batteries. Cellulose, as a natural polymer, possesses excellent structural stability, strong thermal stability, good electrolyte wettability, abundant reserves, wide availability, and biodegradability, making it a promising candidate for next-generation lithium-ion battery separators. However, separators made from ordinary cellulose fibers used in papermaking suffer from problems such as low strength, poor flexibility, and large pore size. In particular, the fine fibrous portions, after fibrillation, are easily dissolved and selectively disappear, thus reducing the separator's insulating properties and making the battery prone to internal short circuits. Separators made from nanocellulose with a smaller aspect ratio have an overly dense structure, making it difficult to meet the high conductivity requirements of lithium-ion battery separators. Therefore, how to provide a green, safe lithium-ion battery separator with good electrolyte wettability, strong thermal stability, and high conductivity has become a research hotspot.

[0003] Chinese patent CN105926347A discloses a method for producing lithium-ion battery separator paper. The method involves uniformly mixing wood pulp, dissolving pulp, and nanocellulose as raw materials, feeding them into a flow system, adding polyethylene oxide before the headbox or high-level tank in the flow system, forming the paper on a paper machine, and then drying and calendering it to obtain nanocellulose lithium-ion battery separator paper. A highly hydrophilic and high-temperature resistant ceramic coating is then applied to the surface of the nanocellulose lithium-ion battery separator paper to obtain a lithium-ion battery separator. This lithium-ion battery separator requires the combination of nanocellulose lithium-ion battery separator paper and ceramic coating to improve the separator's stability, wettability, mechanical properties, and heat resistance. However, the nanocellulose-based separator has low porosity, which restricts lithium-ion movement. A complex pore-forming process is usually required to increase the porosity. Furthermore, the battery separator prepared by this method exhibits anisotropic properties, which is not conducive to its application. Summary of the Invention

[0004] To address the above shortcomings, this invention provides a method for preparing a nanocellulose membrane. The prepared nanocellulose membrane exhibits good electrolyte wettability, high mechanical strength, and high ionic conductivity. The specific technical solution is as follows:

[0005] A method for preparing a nanocellulose membrane includes the following steps:

[0006] (1) Mix nanocellulose with acrylic acid, add solvent, add catalyst, and carry out esterification reaction at 30-100℃ to obtain nanocellulose acrylate.

[0007] (2) Add a second monomer to the nanocellulose acrylate obtained in step (1), then add an initiator to carry out a polymerization reaction, filter, add lithium hydroxide or lithium carbonate solution and stir, then filter, wash, dry and pulverize to obtain nanocellulose lithium grafted polyacrylate.

[0008] (3) Disperse lithium-grafted polyacrylate nanocellulose in water to obtain a suspension, and use a sand core funnel equipped with a polytetrafluoroethylene filter membrane to filter the suspension under reduced pressure to obtain a wet nanocellulose membrane.

[0009] (4) Remove the filter membrane and the wet nanofiber membrane together and transfer them into anhydrous ethanol for solvent replacement. After replacement, dry the wet membrane to obtain the nanofiber membrane.

[0010] Preferably, in the above-mentioned method for preparing the nanocellulose membrane, the amount of acrylic acid used is 0.1-5% of the mass of the nanocellulose.

[0011] Preferably, in the above-mentioned method for preparing the nanocellulose membrane, the solvent is one or more of water, methanol, acetonitrile, isopropanol, tetrahydrofuran, N,N-dimethylformamide, and dioxane, and the amount of solvent used is 2 to 10 times the mass of the nanocellulose; the catalyst is 4-dimethylaminopyridine, dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and the amount of catalyst used is 0.1 to 2% of the mass of the acrylic acid.

[0012] Preferably, in the above-mentioned method for preparing nanocellulose membranes, the esterification reaction time in step (1) is 1 to 10 hours.

[0013] Preferably, in the above-mentioned method for preparing nanocellulose membrane, in step (2), the second monomer is acrylic acid, methacrylic acid, acrylonitrile, acrylamide or hydroxyethyl acrylate, and the amount of the second monomer is 0.1 to 10 times the mass of acrylic acid; the initiator is azobisisobutyronitrile, benzoyl peroxide or diisopropylbenzene peroxide.

[0014] Preferably, in the above-mentioned method for preparing the nanocellulose membrane, the amount of lithium hydroxide or lithium carbonate used is 2 to 12 times the mass of the acrylic acid.

[0015] Preferably, in the above-mentioned method for preparing the nanocellulose membrane, the polymerization reaction temperature is 30-100°C and the reaction time is 2-8 hours.

[0016] Preferably, in the above-mentioned method for preparing nanocellulose membrane, in step (3), the mass percentage of nanocellulose lithium grafted polyacrylate in the suspension is 0.1-0.5%; in step (4), the solvent is replaced for 1.5-2.5 hours, the wet membrane after replacement is sandwiched with a filter membrane, placed in a glass plate to keep it flat, and dried in a constant temperature drying oven at 50-70°C for 20-36 hours.

[0017] On the other hand, the present invention also provides a nanocellulose membrane prepared by the above-described preparation method.

[0018] On the other hand, the present invention also provides the application of the above-mentioned nanocellulose membrane in lithium-ion battery separators.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1. The method for preparing the nanocellulose separator of the present invention modifies the structure of nanocellulose molecules by grafting polyacrylate to create flexible chains, and applies it to lithium battery separators to improve the mechanical properties and heat resistance of the separators, thereby improving the safety of the batteries.

[0021] 2. The method for preparing the nanocellulose membrane of the present invention involves preparing the nanocellulose membrane by vacuum filtration, and the membrane is isotropic.

[0022] 3. In the preparation method of the nanofiber cellulose membrane of the present invention, the hydrogen bonding effect is weakened by replacing water with anhydrous ethanol when the nanofiber membrane is wet, so that the nanofiber membrane can maintain a certain three-dimensional network structure during the drying process, thereby increasing the porosity of the membrane, increasing the absorption of electrolyte by the membrane, and thus improving the ionic conductivity of the membrane.

[0023] 4. The preparation method of the nanocellulose membrane of the present invention is simple, easy to produce, low in cost, and easy to promote and use. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 The tensile strength of the diaphragms prepared in Examples 1-3 and Comparative Example 1 of the present invention and the Celgard 2400 diaphragm;

[0026] Figure 2 The membranes prepared in Examples 1-3 and Comparative Example 1 of the present invention, and the porosity of the Celgard 2400 membrane;

[0027] Figure 3 Photographs of the diaphragms prepared in Example 2 and Comparative Example 2 of this invention;

[0028] Figure 4 Electrolyte wetting diagrams of the membranes prepared in Examples 1-3 and Comparative Example 1 of the present invention, and the Celgard 2400 membrane;

[0029] Figure 5 The electrochemical impedance spectroscopy of the membranes prepared in Examples 1-3 and Comparative Example 1 of the present invention, and the Celgard 2400 membrane. Detailed Implementation

[0030] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise defined, all technical terms used below have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of the present invention. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in the present invention are commercially available or can be prepared by existing methods.

[0031] Example 1

[0032] A method for preparing a nanocellulose membrane includes the following steps:

[0033] (1) Mix 100g of nanocellulose with 0.1g of acrylic acid, then add 700g of solvent (volume fraction of 20% purified water, 50% methanol and 30% isopropanol), add 0.0005g of 4-dimethylaminopyridine, and esterify by reacting at 58℃ for 5h to obtain nanocellulose acrylate.

[0034] (2) Add 0.5g of methacrylic acid and azobisisobutyronitrile and polymerize at 65°C for 6h. After polymerization, filter to separate the polymer and solvent, disperse the separated polymer in 200g of 0.5 wt% lithium hydroxide solution, stir for 15min, filter to separate the solid and liquid, wash the separated solid with 700g of solvent (volume fraction of 20% water, 50% methanol and 30% isopropanol), and filter. Repeat washing-filtration three times to obtain the final product. Dry the final product at 55°C for 12 hours and pulverize to obtain nanocellulose lithium grafted polyacrylate.

[0035] (3) The lithium-grafted polyacrylate nanocellulose obtained in step (2) is dispersed in deionized water to obtain a suspension with a concentration of 0.1 wt%. The suspension is then filtered under reduced pressure using a sand core funnel and a hydrophilic polytetrafluoroethylene filter membrane. The filtration is stopped when the water in the funnel is drained, resulting in a wet nanocellulose membrane. The hydrophilic polytetrafluoroethylene filter membrane has stable hydrophilic properties and a high retention rate.

[0036] (4) The filter membrane was removed along with the wet nanocellulose membrane and placed in anhydrous ethanol for solvent replacement for 2 hours. The replaced wet membrane was then sandwiched with another filter membrane, placed flat in a glass plate, and dried at 60°C for 24 hours in a constant-temperature drying oven. The filter membrane was then peeled off to obtain the dried nanocellulose membrane. The final membrane thickness was controlled by adjusting the volume of lithium nanocellulose grafted polypropylene used in the filtration process. A nanocellulose membrane with a thickness of 25 μm, denoted as CNF-1, was prepared. The prepared nanocellulose membrane was sealed in a self-sealing bag for electrochemical characterization and testing.

[0037] Example 2

[0038] A method for preparing a nanocellulose membrane includes the following steps:

[0039] (1) Mix 100g of nanocellulose with 1g of acrylic acid, then add 700g of solvent (volume fraction of 20% purified water, 50% methanol and 30% isopropanol), add 0.005g of 4-dimethylaminopyridine, raise the temperature to 76℃ and react for 7h to esterify, and obtain nanocellulose acrylate.

[0040] (2) Then add 6g of methacrylic acid and azobisisobutyronitrile and polymerize at 70℃ for 7h. After polymerization, filter to separate the polymer and solvent. Disperse the separated polymer in 700g of 1wt% lithium hydroxide solution and stir for 15min. Filter to separate the solid and liquid. Wash the separated solid with 700g of solvent (volume fraction of 20% water, 50% methanol and 30% isopropanol), filter, and repeat washing-filtration three times to obtain the final product. Dry the final product at 55℃ for 12 hours, pulverize, and obtain nanocellulose lithium grafted polyacrylate.

[0041] (3) Disperse the nanocellulose lithium grafted polyacrylate obtained in step (2) in deionized water to obtain a suspension with a concentration of 0.1 wt%. Use a sand core funnel and a hydrophilic polytetrafluoroethylene filter membrane for vacuum filtration. Stop filtration when the water in the funnel is dried to obtain a wet nanocellulose membrane.

[0042] (4) The filter membrane was removed along with the wet nanocellulose membrane and placed in anhydrous ethanol for solvent replacement for 2 hours. After replacement, the wet membrane was clamped with another filter membrane and placed flat in a glass plate. It was then dried in a constant temperature drying oven at 60°C for 24 hours. The filter membrane was then peeled off to obtain the dried nanocellulose membrane. The thickness of the final membrane was controlled by adjusting the volume of lithium nanocellulose grafted polypropionate used in the filtration. A nanocellulose membrane with a thickness of 25 μm was prepared and designated CNF-2.

[0043] Example 3

[0044] A method for preparing a nanocellulose membrane includes the following steps:

[0045] (1) Mix 100g of nanocellulose with 5g of acrylic acid, then add 700g of solvent (volume fraction of 20% purified water, 50% methanol and 30% isopropanol), add 0.025g of 4-dimethylaminopyridine, and esterify by reacting at 58℃ for 5h to obtain nanocellulose acrylate.

[0046] (2) Then add 25g of methacrylic acid and azobisisobutyronitrile and polymerize at 85℃ for 10h. After polymerization, filter to separate the polymer and solvent. Disperse the separated polymer in 1000g of 4wt% lithium hydroxide solution and stir for 15min. Filter to separate the solid and liquid. Wash the separated solid with 700g of solvent (volume fraction of 20% water, 50% methanol and 30% isopropanol), filter, and repeat washing-filtration three times to obtain the final product. Dry the final product at 55℃ for 12 hours, pulverize, and obtain nano-cellulose lithium grafted polyacrylate.

[0047] (3) Disperse the nanocellulose lithium grafted polyacrylate obtained in step (2) in deionized water to obtain a suspension with a concentration of 0.1 wt%. Use a sand core funnel and a hydrophilic polytetrafluoroethylene filter membrane for vacuum filtration. Stop filtration when the water in the funnel is dried to obtain a wet nanocellulose membrane.

[0048] (4) The filter membrane was removed along with the wet nanocellulose membrane and placed in anhydrous ethanol for solvent replacement for 2 hours. The wet membrane after replacement was clamped with another filter membrane, placed in a glass plate to keep it flat, and dried in a constant temperature drying oven at 60°C for 24 hours. The filter membrane was then peeled off to obtain the dried nanocellulose membrane. The thickness of the final membrane was controlled by controlling the volume of lithium nanocellulose grafted polypropionate used for filtration. A nanocellulose membrane with a thickness of 25 μm was prepared and designated CNF-3.

[0049] Comparative Example 1

[0050] The nanocellulose membrane of this comparative example was prepared as follows: nanocellulose was dispersed in deionized water to obtain a suspension with a concentration of 0.1 wt%. The suspension was then filtered under reduced pressure using a sand core funnel and a hydrophilic polytetrafluoroethylene filter membrane. The filtration was stopped when the water in the funnel was drained, resulting in a wet nanocellulose membrane.

[0051] (4) The replaced wet membrane was sandwiched with a filter membrane and placed flat in a glass plate. It was then dried at 60°C for 24 hours in a constant temperature drying oven to obtain a dried nanocellulose membrane. The volume of nanocellulose used for filtration was used to control the thickness of the final membrane. A nanocellulose membrane with a thickness of 25 μm was prepared and designated CNF-4.

[0052] Comparative Example 2

[0053] The difference between this comparative example and Example 2 is as follows: Step (4): The filter membrane is removed together with the wet nanocellulose membrane, a layer of filter membrane is added to hold it in place, it is placed in a glass plate to keep it flat, and it is dried at 60°C for 24 hours in a constant temperature drying oven. The filter membrane is then peeled off to obtain a dried nanocellulose membrane. The amount of suspension used to prepare the nanocellulose membrane is the same as in Example 2, and the other steps are the same as in Example 2, resulting in a nanocellulose membrane with a thickness of only 6 μm.

[0054] Lithium-ion battery assembly

[0055] Before assembling the battery, the prepared nanofiber cellulose membrane and Celgard 2400 membrane samples were cut into Ф16mm discs using a slicer. The assembly and testing involved 2032 coin cell half-cells, requiring the assembly of different battery samples to test different performance characteristics of the membrane or battery. Taking a lithium-ion cell / membrane / electrode battery as an example, the assembly process is basically as follows:

[0056] Because lithium metal is sensitive to water and oxygen, the entire assembly process of lithium-ion batteries is completed in a glove box. The lithium-ion half-cell assembly uses an inverted method: first, the negative electrode case of the lithium battery is removed and laid upside down. Then, spring sheets, gaskets, lithium plates, a separator pre-soaked in EC / PC, and LFP (or LTO) electrode plates are placed inside in sequence. When placing the electrode plates, care must be taken to ensure that the side coated with the active material faces down. Finally, the positive electrode case of the lithium battery is covered and sealed with a sealing machine at a pressure of 10 MPa. The assembled battery is then placed in an oven at 60°C and left to stand for 12 hours to allow the separator and electrode materials to fully contact each other.

[0057] The performance of the nanocellulose membranes prepared in Examples 1-3 and Comparative Example 1 was characterized using the following specific test methods:

[0058] (1) Physicochemical property tests of the diaphragm:

[0059] Tensile strength test:

[0060] The tensile strength of nanocellulose membranes and Celgard 2400 membranes was tested using an electronic universal testing machine. The test parameters were: membrane width 15 mm, clamping distance 20 mm, and tensile rate 5 mm / min.

[0061] Thermal dimensional stability test:

[0062] A high-temperature environment, provided by an electrically heated constant-temperature drying oven, was used to test the thermal dimensional stability of the diaphragms. First, the prepared poplar cellulose nanofiber diaphragms and Celgard 2400 diaphragms were cut into 2×2 cm squares. Then, the diaphragm samples were placed in glass petri dishes and placed in the electrically heated constant-temperature drying oven. The morphology and dimensions of the diaphragms were recorded after being placed at 120℃ for 1 hour, and their thermal shrinkage rate was calculated. The thermal shrinkage rate characterizes the thermal dimensional stability.

[0063] Porosity determination:

[0064] Nanocellulose membrane and Celgard 2400 membrane samples were cut into Ф16mm round slices using a slicer. After recording the initial mass of the samples, they were immersed in n-butanol for 1 hour. The immersed membranes were then removed, the n-butanol on the surface was absorbed with filter paper, and the weight was recorded. The porosity P (%) of the membranes was calculated by the gravimetric method, using the following formula:

[0065]

[0066] Among them, W m W b The figures represent the membrane mass before soaking in n-butanol and the membrane mass after soaking in n-butanol for 1 hour (including n-butanol), respectively, in grams; ρ m ρ bThese are the densities of the diaphragm and n-butanol, respectively, in g / cm³. 3 .

[0067] (2) Electrolyte wettability test:

[0068] The wettability of electrolyte on the membrane surface was analyzed by a droplet experiment. First, nanocellulose membrane and Celgard 2400 membrane samples were cut into Ф16mm discs using a slicer and placed on a horizontal platform. Then, 3μL of electrolyte was transferred by pipette and dropped onto the membrane surface. The diffusion of the electrolyte was recorded by photograph after 10 seconds.

[0069] (3) Electrochemical property testing of the diaphragm

[0070] Ionic conductivity measurement:

[0071] The battery samples were subjected to AC impedance testing using a CHI660E electrochemical workstation (amplitude 5 mV, test frequency 0.1-100 Hz). The ionic conductivity (δ, mS / cm) of the separator was calculated using the following formula:

[0072] δ=L / RS

[0073] Where L is the thickness of the diaphragm, in cm; and S is the area of ​​the electrode, in cm². 2 R is the resistance obtained through the Nyquist plot, and its unit is Ω.

[0074] The results are shown in Table 1 and Figures 1-5 .

[0075] Table 1. Performance of the diaphragm

[0076]

[0077]

[0078] (1) Tensile strength analysis

[0079] To withstand the winding and other operations during battery assembly, the separator needs to possess a certain tensile strength. Typically, the tensile strength of the separator is tested in both the longitudinal and transverse directions. However, the nanocellulose separator prepared in this experiment was fabricated using a vacuum filtration method, making it isotropic; therefore, testing its tensile strength does not require differentiation between directions. Figure 1 As shown in Table 1, the transverse tensile strength of the CNF separator prepared by this invention is higher than that of the Celgard 2400 separator (14.21 MPa), and the longitudinal tensile strength of the nanocellulose lithium-grafted polyacrylate separator is higher than that of the CNF separator. Although the longitudinal tensile strength is lower than that of the Celgard 2400 separator (141.95 MPa), it is sufficient to meet the tensile strength requirements for lithium-ion battery separators.

[0080] (2) Analysis of the thermal dimensional stability of the diaphragm

[0081] When lithium-ion batteries operate at high temperatures, the separator should maintain dimensional stability and not undergo significant shrinkage or curling deformation; otherwise, it can easily cause a short circuit and lead to serious safety problems. Table 1 shows that the Celgard 2400 separator begins to change size at 120°C, with a shrinkage rate of 5.3%, while the ungrafted polyacrylate nanocellulose separator has a shrinkage rate of 1.1%. In contrast, the nanocellulose separators prepared using this invention maintain good dimensional morphology.

[0082] (3) Porosity analysis

[0083] A dense membrane structure can restrict or even hinder the movement of ions between the membranes, leading to a decrease in battery performance. However, an overly loose structure may cause micro-short circuits that cause the membrane to fail, resulting in safety issues. Therefore, the membrane needs to have a suitable porosity to ensure that ions can pass through smoothly while avoiding short circuits between the positive and negative electrodes. Figure 2 The porosity of the CNF membrane and the Celgard 2400 membrane of the present invention is shown in the figure. It can be seen from the figure that the porosity of the nanocellulose membrane-nanocellulose grafted polyacrylate membrane of the present invention is between 38% and 48%. The porosity of CNF-2 and CNF-3 is higher than that of the Celgard 2400 membrane. The porosity of the nanocellulose membrane of the present invention is suitable.

[0084] Figure 3 The nanocellulose membranes prepared in Example 2 and Comparative Example 2 were prepared by vacuum filtration of the same suspension, using the same amount of solvent and the same membrane preparation process. The difference was that the membrane in Example 2 was solvent-displaced with anhydrous ethanol, while the membrane in Comparative Example 2 was not. As can be seen from the figure, the membrane on the right has obvious wrinkles and a thickness of only 6 μm. Immersion in n-butanol showed no significant mass change, indicating that its porosity is almost zero. The membrane on the left, however, is white like cellulose paper, with a smooth surface, a thickness of approximately 25 μm, and a porosity of 48%. This is because the replacement of water with anhydrous ethanol weakens the hydrogen bonding, allowing the cellulose membrane to maintain a certain three-dimensional network structure during drying, thus giving the cellulose membrane a certain porosity.

[0085] (4) Analysis of wettability of diaphragm electrolyte

[0086] Most separators are insulators by themselves, only becoming ionicly conductive after being immersed in electrolyte. Therefore, the electrolyte wettability of a separator is an important indicator of its performance. The easier it is for the electrolyte to wet the separator, the faster it can fill the separator during battery assembly; in addition, good electrolyte wettability helps to improve the ionic conductivity of the separator, thereby reducing battery impedance. Figure 4The figure shows the electrolyte wetting area in the droplet experiment. As can be seen from the figure, the electrolyte wetting area of ​​the Celgard 2400 membrane is much smaller than that of the CNF membrane, indicating that the nanocellulose membrane prepared in this invention has excellent electrolyte wettability.

[0087] (5) Electrochemical impedance spectroscopy and ionic conductivity analysis of the membrane

[0088] Figure 5 The AC impedance spectra of lithium-ion battery samples assembled with CNF and Celgard 2400 separators are shown, from which the ionic conductivity of the separators can be obtained. Batteries using lithium nanocellulose-grafted polyacrylate separators exhibit relatively low charge transfer resistance, while batteries using Celgard 2400 separators and unmodified CNF separators show higher charge transfer resistance. This is because the modified CNF separator possesses better electrolyte wettability and higher porosity, ensuring easier passage of lithium ions. Consequently, lithium-ion batteries using CNF-grafted polyacrylate separators exhibit superior ionic conductivity.

[0089] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A method for preparing a nanocellulose membrane, characterized in that, Includes the following steps: (1) Mix nanocellulose with acrylic acid, add solvent, add catalyst, and carry out esterification reaction at 30~100℃ to obtain nanocellulose acrylate; (2) Add a second monomer to the nanocellulose acrylate obtained in step (1), then add an initiator to carry out the polymerization reaction, filter; add lithium hydroxide or lithium carbonate solution and stir, then filter, wash, dry and pulverize to obtain nanocellulose lithium grafted polyacrylate. (3) Disperse lithium-grafted polyacrylate nanocellulose in water to obtain a suspension, and use a sand core funnel equipped with a polytetrafluoroethylene filter membrane to filter the suspension under reduced pressure to obtain a wet nanocellulose membrane. (4) Remove the filter membrane and the wet nanofiber membrane together and transfer them into anhydrous ethanol for solvent replacement. After replacement, dry the wet membrane to obtain the nanofiber membrane. In step (1), the amount of acrylic acid used is 0.1-5% of the mass of the nanocellulose; In step (2), the second monomer is acrylic acid, methacrylic acid, acrylonitrile, acrylamide, or hydroxyethyl acrylate.

2. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, The solvent is one or more of water, methanol, acetonitrile, isopropanol, tetrahydrofuran, N,N-dimethylformamide, and dioxane, and the amount of solvent used is 2 to 10 times the mass of the nanocellulose; the catalyst is 4-dimethylaminopyridine, dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and the amount of catalyst used is 0.1 to 2% of the mass of the acrylic acid.

3. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, In step (1), the esterification reaction time is 1~10h.

4. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, In step (2), the amount of the second monomer is 0.1 to 10 times the mass of acrylic acid in step (1); the initiator is azobisisobutyronitrile, benzoyl peroxide or diisopropylbenzene peroxide.

5. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, The amount of lithium hydroxide or lithium carbonate used is 2 to 12 times the mass of acrylic acid in step (1).

6. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, The polymerization reaction temperature is 30~100℃, and the reaction time is 2~8h.

7. The method for preparing the nanocellulose membrane according to claim 1, characterized in that, In step (3), the mass percentage of nanocellulose lithium grafted polyacrylate in the suspension is 0.1~0.5%; in step (4), the solvent is replaced for 1.5~2.5h, the wet membrane after replacement is clamped with a filter membrane, placed in a glass plate to keep it flat, and dried in a constant temperature drying oven at 50~70℃ for 20~36h.

8. A nanocellulose membrane prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the nanocellulose separator as described in claim 8 in lithium-ion battery separators.

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