Lithium battery flame-retardant cellulose diaphragm and preparation method thereof

Abstract

The invention discloses a lithium battery flame-retardant cellulose diaphragm and a preparation method thereof, and belongs to the technical field of modified cellulose.The cellulose diaphragm is prepared from flame-retardant cellulose through a papermaking method, and the flame-retardant cellulose is prepared by grafting cellulose with a monomer 1, a monomer 2, a monomer 3 and a monomer 4 through atom transfer radicals, the monomer 1 is an acrylate compound, the monomer 2 is a phosphaphenanthrene compound with an alkenyl group, the monomer 3 is obtained by reacting arginine with glycidyl methacrylate under an alkaline condition at 50-80 DEG C, and the monomer 4 is obtained by reacting bis (trihydroxymethyl) propane with 10-undecylenoyl chloride under the action of a catalyst. The monomer 1-4 is grafted on the main chain of the cellulose, so that the tensile strength and the thermal stability of the cellulose are improved, and the application prospect of the cellulose in the lithium battery diaphragm is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of modified cellulose, and particularly relates to a flame-retardant cellulose separator for lithium batteries and a preparation method thereof. Background Art

[0002] A lithium-ion battery is an electrochemical battery that converts chemical energy into electrical energy, and has advantages such as high energy density, high power density, no memory effect, long cycle life, and low self-discharge rate. A lithium-ion battery mainly consists of a positive electrode, a negative electrode, an electrolyte, and a separator. The lithium-ion battery separator can provide a physical barrier for the positive and negative electrodes of the battery. Because it has a certain mechanical strength and thermal stability, it can maintain dimensional stability under extreme conditions and prevent the diaphragm from rupturing, resulting in physical contact between the two electrodes and causing a battery short circuit. The separator has porosity and the ability to absorb and retain the electrolyte, which can provide a path for the lithium ions in the electrolyte to be transmitted between the positive and negative electrodes, transmit ions during the battery charge and discharge cycle, and ensure the normal operation of the battery. An ideal separator should have a uniform thickness, low cost, high mechanical properties, good electrolyte absorption and retention performance, a curved porous structure that prevents the growth of lithium dendrites, high dimensional stability in a highly redox and high-temperature environment, and good tolerance to the degradation of impurities and chemical reagents in the electrolyte. Currently, most commercial lithium-ion batteries use microporous polyolefin separators, and their greatest advantage is that they have good tensile strength and puncture strength and an appropriate thickness. However, polyolefin has poor thermal stability. During the repeated charge and discharge cycles of the battery, a large amount of heat is generated. The softening and melting temperatures of polyolefin materials are relatively low. When the temperature rises slightly, the polyolefin separator can protect the battery by closing the pore structure. However, when the temperature inside the battery is too high, the polyolefin separator will continue to shrink and melt, and even cause a short circuit or explosion inside the battery, leading to safety problems.

[0003] Cellulose is an excellent biopolymer and sustainable raw material, and is also an important source of industrial-grade renewable materials. Cellulose is the largest natural biomass raw material in the world, and its annual output is estimated to be 10 10 -10 11 tons. In addition to mainly coming from wood, it also comes from other plants such as cotton and hemp, marine animals (tunicates), algae, and fungi. The cellulose extraction and processing process is simple and mature, and has good practical properties, such as good physical and chemical properties, biocompatibility, non-toxicity, light weight, recyclability, environmental friendliness, etc. Especially its excellent surface hydrophilicity and thermal dimensional stability make it the best candidate to replace the traditional polyolefin separator for lithium batteries. However, cellulose also has problems such as poor mechanical strength, large thickness, and poor flame retardancy, which have hindered the popularization and application of cellulose-based separators in lithium batteries. Summary of the Invention

[0004] In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a flame-retardant cellulose separator for lithium batteries and a preparation method thereof. By grafting toughening monomers, flame-retardant monomers and cross-linking monomers onto the cellulose main chain, flame-retardant cellulose is prepared, and then a cellulose separator with appropriate thickness, high mechanical strength and flame-retardant effect is prepared by the papermaking method.

[0005] The technical solutions for achieving the objectives of the present invention are as follows: A flame-retardant cellulose separator for lithium batteries, wherein the cellulose separator is prepared by the papermaking method from flame-retardant cellulose, and the flame-retardant cellulose is prepared by atom transfer radical grafting of monomers 1, 2, 3 and 4 onto cellulose. Monomer 1 is an acrylate compound, monomer 2 has the structure shown in formula 1 or formula 2, monomer 3 has the structure shown in formula 3, and monomer 4 has the structure shown in formula 4: Formula 1, Formula 2, Formula 3, Formula 4.

[0006] Preferably, the acrylate compound is selected from at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate, tert-butyl acrylate, isobornyl acrylate, 2-ethylhexyl acrylate, dimethylaminoethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate, isobornyl methacrylate, 2-ethylhexyl methacrylate, dimethylaminoethyl methacrylate.

[0007] Preferably, monomer 3 is obtained by reacting arginine with glycidyl methacrylate under alkaline conditions at 50-80 °C.

[0008] Preferably, monomer 4 is prepared by reacting bis(trimethylol)propane with 10-undecenoyl chloride under nitrogen protection using triethylamine as a catalyst and reacting at room temperature for 8-16 hours.

[0009] Preferably, the molar ratio of monomers 1, 2, 3 and 4 is (3-8):(2-5):(2-5):(2-3).

[0010] Preferably, the cellulose is selected from at least one of microcrystalline cellulose or nanocellulose.

[0011] Preferably, the cellulose is pretreated by solvation and esterified to obtain esterified cellulose as an initiator, and then copolymerized with monomers 1-4 by atom transfer radical grafting.

[0012] Preferably, the solvation pretreatment method is to dissolve cellulose in deionized water, then perform solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolve it in DMAc / LiCl solvent to obtain a cellulose solution; the esterification treatment is to react cellulose with trichloroacetyl chloride under the action of an acid-binding agent and a catalyst to obtain cellulose trichloroacetate.

[0013] Preferably, the conditions for atom transfer radical grafting are as follows: under nitrogen protection, using cellulose trichloroacetate as an initiator, CuBr / bipyridine as a catalyst, N,N-dimethylformamide as a solvent, the reaction temperature is 80°C to 90°C, and the reaction time is 24 to 48 h.

[0014] The present invention also protects a preparation method of a flame-retardant cellulose separator for lithium batteries, which includes the following steps: dispersing flame-retardant cellulose in a solvent to prepare a suspension, stirring and dispersing, grinding, and homogenizing, and then performing vacuum filtration to obtain a wet film. The wet film is hot-pressed at 80°C for 6 h and peeled off to obtain a flame-retardant cellulose separator with a thickness of 15 to 50 μm.

[0015] The present invention also protects the application of the flame-retardant cellulose separator for lithium batteries in lithium batteries.

[0016] Beneficial effects

[0017] The present invention provides a flame-retardant cellulose separator for lithium batteries and a preparation method thereof, which have the following beneficial effects: (1) Cellulose is a renewable resource. Compared with traditional petroleum-based polyolefin separator materials, it reduces the dependence on limited fossil resources, and cellulose separators are biodegradable in the natural environment, which helps to reduce the environmental pressure after lithium batteries are discarded.

[0018] (2) Cellulose itself has a porous structure. After grafting modification, the pore structure can be further optimized. The introduction of acrylate monomers can increase the hydrophilicity of the separator, enabling the separator to absorb more electrolyte, improving the ion transport efficiency. The high porosity is conducive to the rapid transport of lithium ions in the separator, reducing the internal resistance of the battery, and thus improving the charge and discharge performance of the battery.

[0019] (3) The introduction of multi-reaction-site crosslinking monomers can form a covalent crosslinked network structure between cellulose molecular chains, enhancing the mechanical strength and dimensional stability of the separator. This enables the separator to withstand greater stress during the charge and discharge process of the battery, and is not easily deformed or ruptured, improving the safety and reliability of the battery. At the same time, the guanidine group on the arginine monomer can form hydrogen bonds with cellulose to further enhance the mechanical properties of the separator.

[0020] (4) The phosphaphenanthrene monomer has high thermal stability. After being grafted onto cellulose, it can significantly improve the thermal stability of the separator. When the battery is used at high temperatures or in abnormal situations such as internal short circuits, the phosphorus-nitrogen synergistic effect of phosphaphenanthrene and arginine promotes the formation of a carbonized layer, which blocks the transfer of oxygen and heat at high temperatures, effectively preventing phenomena such as separator shrinkage and melting, avoiding thermal runaway of the battery, and enhancing the safety performance of the battery. Description of the Drawings

[0021] Figure 1 is the 1H NMR spectrum of monomer 3; Figure 2 is the 1H NMR spectrum of monomer 4; Figure 3 is the infrared spectrum of cellulose and the flame-retardant cellulose of Example 1. Specific Embodiments

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0023] In the embodiments, unless otherwise specified, the experimental methods used are all conventional methods, and the materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial channels.

[0024] Now, the raw materials and equipment used in the examples and comparative examples are described as follows: Cellulose: Microcrystalline cellulose, with a degree of polymerization of 280, purchased from Changshu Pharmaceutical Excipients Co., Ltd., Jiangsu Province; Arginine: L-arginine, with a purity of 99%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Glycidyl methacrylate: With a purity of 97%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; Bis(trimethylol)propane: With a purity of 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.; 10-Undecenoyl chloride: With a purity of 98%, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; Trichloroacetyl chloride: With a purity of 98%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Polyolefin separator: Commercial polyolefin separator, Celgard 2325, purchased from Shenzhen Kejing Zhida Technology Co., Ltd.; Monomer 1: Ethyl methacrylate, CAS: 97-63-2, with a purity of 99%, purchased from Shanghai Macklin Biochemical Co., Ltd.; Monomer 2-A: 6H-dibenz[c,e][1,2]oxaphosphinine, 6-[(4-vinylphenyl)methyl]-6-oxide, CAS: 916899-25-7, purity 95%, purchased from Shandong Xingshun New Materials Co., Ltd.; Monomer 2-B: 9,10-dihydro-9-oxa-10-allylphosphaphenanthrene-10-oxide, CAS: 311342-65-1, purity 98%, purchased from Henan Weiti Xi Chemical Technology Co., Ltd.; Monomer 3: Self-made, and the preparation method is as follows: Dissolve 0.046 mol of arginine and 0.05 mol of sodium hydroxide in 50 mL of deionized water to form a homogeneous solution. Dropwise add 0.051 mol of glycidyl methacrylate to the above solution and stir and react at 65 °C for 1 hour. During the reaction process, the solution gradually changes from milky white to transparent, indicating that the epoxy group of glycidyl methacrylate reacts with the amino group of arginine to generate the target product. It should be noted that under alkaline conditions, the α-amino group on arginine mainly exists in a neutral (non-protonated) form, while the guanidine group remains in a protonated state. This difference makes the α-amino group more likely to react with the epoxy group of GMA, thereby achieving regioselectivity and obtaining a single reaction product; after the reaction is completed, add 50 mL of water to dilute the solution, and then wash it 3 times with 300 ml of acetone in a 500 mL separatory funnel to obtain a pale yellow aqueous phase product solution. Add an equal volume of concentrated hydrochloric acid for acidification treatment to protonate the monomer. Add 35 mL of acetone and centrifuge at 6000 rpm for 5 minutes to separate the NaCl precipitate, and obtain a clear solution. Remove acetone by vacuum distillation at 30 °C, and then freeze-dry for 24 hours to remove water. Finally, a white solid product is obtained with a yield of 70.3%. The reaction structural formula is as follows, and the nuclear magnetic resonance hydrogen spectrum ( 1 H NMR, 400 MHz, Bruker Avance-400, CDCl 3 ) is as Figure 1 shown. From the chemical shifts and integrals of the Figure 1 H that have been assigned in the nuclear magnetic resonance hydrogen spectrum in 1 , it can be known that Monomer 3 is successfully prepared.

[0025]

[0026] Monomer 4: Self-made, and the preparation method is as follows: Place 0.2 mmol of bis(trimethylol)propane in a 50 ml Schlenk tube dried in an oven, evacuate the tube and fill it with nitrogen, add 20 ml of dichloromethane and 4.8 mmol of triethylamine, and dropwise add 3.98 mmol of 10-undecenoyl chloride into the tube at 0 °C. Stir overnight at room temperature, slowly pour the reaction solution into the stirred ice water, and stir for 10 - 15 minutes to hydrolyze the residual acyl chloride and neutralize the acid-binding agent at the same time. Extract with 50 ml of dichloromethane three times respectively, separate the liquid to obtain the organic phase, wash the organic phase three times with saturated brine, remove the aqueous phase, dry the organic phase with anhydrous magnesium sulfate, filter to remove the desiccant, concentrate the organic solution under reduced pressure, and dry it under vacuum to obtain monomer 4. The reaction structural formula is as shown below, and the proton nuclear magnetic resonance spectrum (1H NMR, 400 MHz, Bruker Avance-400, CDCl 3 ) is as Figure 2 shown. From the chemical shifts and integrals of the Figure 2 H that have been assigned in the proton nuclear magnetic resonance spectrum in 1 , it can be known that monomer 3 was successfully prepared.

[0027]

[0028] Unless otherwise specified, the component raw materials used in each example and comparative example of the present invention are all commercially available raw materials, and the component raw materials used in each parallel experiment are all of the same kind.

[0029] Example 1

[0030] A flame-retardant cellulose separator for lithium batteries, and the preparation method is as follows: Pretreatment: Pour 250 ml of deionized water into a 500 ml beaker, add 30 g of microcrystalline cellulose under the stirring of a magnetic stirrer, stir for half an hour to form a slurry, let it stand for stratification, filter, disperse the solid in 250 ml of N,N-dimethylacetamide and stir for 6 h, let it stand for stratification and filter, and repeat the same operation once. Then disperse the solid in 250 ml of anhydrous methanol and repeat the above steps twice to complete the whole process. Place the finally obtained solid in a vacuum drying oven at 80 °C for 48 h to obtain solvent-exchanged cellulose; Add 125 ml of N,N-dimethylacetamide and 10.5 g of anhydrous lithium chloride dried at 110 °C under vacuum into a 250 ml three-necked flask with nitrogen protection, stir magnetically for 1 h at 70 - 80 °C in an oil bath until LiCl is completely dissolved, cool down to 50 °C, add 4 g of solvent-exchanged cellulose, stir at room temperature for 12 h, and the solution becomes transparent to obtain a cellulose solution; Esterification: Under magnetic stirring and nitrogen protection, 8 ml of pyridine as an acid-binding agent and 0.26 g of 4-dimethylaminopyridine as a catalyst were added to the above-mentioned cellulose solution. After stirring for 30 min to disperse evenly, 6 ml of trichloroacetyl chloride was added dropwise within 60 - 90 min using a constant-pressure dropping funnel, and the reaction was carried out at room temperature for 12 h. Under rapid stirring, the solution was slowly poured into a 1000 mL solution of distilled water and anhydrous methanol in a 1:1 mixture, and a large amount of white fibrous precipitate was formed. It was filtered with a Buchner funnel, washed several times with distilled water first, then washed several times with a mixed washing solution of methanol and distilled water in a 1:1 ratio, and finally washed twice with methanol. The washed product was extracted with methanol in a Soxhlet extractor for 48 h, and the extracted product was dried in a vacuum drying oven at 50 °C for 48 h to obtain a white solid powder. The degree of substitution of the cellulose ester was measured to be 0.72 by chemical analysis method;

[0031] Grafting: 2.2 g of cellulose trichloroacetate and 50 mL of dried and pretreated N,N-dimethylformamide were added to a three-necked flask, and magnetically stirred at room temperature for 1 - 2 h to disperse. Under nitrogen protection, 15 mg of CuBr, 35 mg of 2,2'-bipyridine, 80 mmol of monomer 1, 30 mmol of monomer 2-A, 30 mmol of monomer 3, and 20 mmol of monomer 4 were added. The system was evacuated, filled with nitrogen, and this was repeated three times, then sealed under vacuum and placed in an oil bath at a set temperature and slowly heated to 80 - 90 °C, and the reaction was carried out for 48 h. After the experiment ended, it was immediately cooled to room temperature, the solution was poured into a large amount of acetone, and a yellow solid powder was precipitated. The solid powder was obtained by suction filtration, dissolved in water, filled into a dialysis bag with a molecular weight cut-off of 7000 and dialyzed with water. After dialysis, the solution was freeze-dried to obtain flame-retardant cellulose.

[0032] Paper-making film formation: The flame-retardant cellulose was dispersed in tert-butanol to prepare a suspension with a concentration of 1 mg / ml, and magnetically pre-stirred for 10 min for preliminary dispersion. Then, a high-speed stirrer was used to stir at 700 rpm for 4 h to make the flame-retardant cellulose evenly dispersed in the dispersion medium. Then, it was ground 40 times with an ultrafine grinder (MKZA6-2J, Masuko Sangyo Co.), and then the ground suspension was diluted to a concentration of 0.5 mg / ml. Then, it was homogenized 3 times (D8 pore size) with a nano-homogenizer (Mini, Noozle). Finally, the homogenized flame-retardant cellulose slurry was diluted to a concentration of 0.05 mg / ml, and vacuum filtration was carried out to obtain a wet film. The wet film was hot-pressed at 80 °C for 6 h and peeled off to obtain a flame-retardant cellulose separator with a thickness of about 20 μm.

[0033] The infrared spectra of cellulose and flame-retardant cellulose were tested using an infrared spectrometer with the model of IR / Nicolet 6700. During sample preparation, a small amount of sample powder was mixed and ground with KBr. The ground sample should be transparent to ensure that light can pass through the sample. In the experiment, the wavelength range of the spectral analyzer was set to 4000 cm -1 ~500 cm -1 ; The results are as Figure 3 shown. Among them, the stretching vibration absorption peak of cellulose -OH is at 3417 cm -1 . In addition, compared with the infrared absorption spectrum of cellulose, for the flame-retardant cellulose, the stretching vibration peak of the methylene C-H of the monomer 4 long carbon chain appears at 2967 cm -1 , the stretching vibration peaks of the ester group C=O of monomers 1 and 3 appear at 1729 cm -1 , the bending vibration absorption peak of the guanidine group in monomer 3 appears at 1555 cm -1 , the absorption peaks of the benzene ring in monomer 2 appear at 1603 cm -1 and 1489 cm -1 , the asymmetric and symmetric vibration absorption peaks of CH -1 in monomer 4 appear at 1431 - 1390 cm 3 , the absorption peak of the P=O bond in monomer 2 appears at 1176 cm -1 , the absorption peak of P-O-C in monomer 2 appears at 876 cm -1 , and the bending vibration absorption peak of C -1 -H appears at 674 - 812 cm Ar , which proves that monomers 1 to 4 are successfully grafted onto cellulose.

[0034] Example 2

[0035] Compared with the preparation method of Example 1, the difference lies in that the addition amounts of monomers 1 to 4 in the grafting step are 30 mmol of monomer 1, 50 mmol of monomer 2, 50 mmol of monomer 3, and 30 mmol of monomer 4.

[0036] Example 3

[0037] Compared with the preparation method of Example 1, the difference lies in that monomer 2-A in the grafting step is replaced by monomer 2-B.

[0038] Comparative Example 1 Compared with the preparation method of Example 1, the difference lies in that monomer 3 is not added in the grafting step.

[0039] Comparative Example 2 Compared with the preparation method of Example 1, the difference lies in that monomer 4 is not added in the grafting step.

[0040] Comparative Example 3 Microcrystalline cellulose was dispersed in tert-butanol to prepare a suspension with a concentration of 1 mg / ml. It was pre-stirred magnetically for 10 min for preliminary dispersion, and then stirred at 700 rpm for 4 h using a high-speed stirrer to uniformly disperse the flame-retardant cellulose in the dispersion medium. Then, it was ground 40 times using an ultrafine grinder (MKZA6-2J, Masuko Sangyo Co.). Subsequently, the ground suspension was diluted to a concentration of 0.5 mg / ml, and then homogenized 3 times (D8 aperture) using a nano-homogenizer (Mini, Noozle). Finally, the homogenized cellulose slurry was diluted to a concentration of 0.05 mg / ml, and vacuum filtration was carried out to obtain a wet film. The wet film was hot-pressed at 80 °C for 6 h and peeled off to obtain a cellulose separator with a thickness of approximately 50 μm.

[0041] Comparative Example 4 Commercial polyolefin separator, Celgard 2325, was purchased from Shenzhen Kejing Zhida Technology Co., Ltd.

[0042] The separators prepared in the examples and comparative examples were subjected to the following tests, and the results are shown in Table 1: (1) Thickness: The thickness of the separator was measured by referring to the method of GB / T 451.3-2002.

[0043] (2) Mechanical strength: The tensile strength of the separator was tested by referring to GB / T 12914-2018.

[0044] (3) Thermal stability: The thermal properties of the separator were detected using a thermogravimetric analyzer, and the mass loss curve of the separator sample from room temperature to 600 °C was recorded. The mass of the separator sample added for each detection was 10 mg, the heating rate was 10 °C / min, the protective gas was nitrogen, and the nitrogen flow rate was 25 mL / min. The temperature at which the separator began to decompose was recorded as the thermal decomposition temperature.

[0045] (4) Thermal dimensional stability: The separator was cut into circular pieces with a diameter of 15.8 mm using a cutting machine, and the shrinkage area of the circular pieces was measured after heating in an electrothermal constant-temperature forced-air drying oven at 160 °C for 1 h.

[0046] (5) Porosity: It was measured by the liquid absorption method. The separator was completely immersed in n-butanol for 2 h, and the weight of the separator before and after absorbing n-butanol was weighed. The porosity P = M / ρ / (M / ρ + M m / ρ m ), where M is the mass (g) of n-butanol absorbed in the separator, M m is the mass (g) of the pure separator, ρ is the density of n-butanol (g / cm 3 ), and ρ m is the density of the substrate (g / cm 3 ).

[0047] (6)Liquid absorption rate: In the glove box Lab2000, the weighed separator is immersed in the electrolyte for 2 h and then taken out. The surface of the separator is wiped dry with filter paper, and the weight of the separator before and after is weighed. The liquid absorption rate is calculated by the formula ξ=(M - M 0 ) / M 0 , where M 0 and M are the masses (g) of the dry film and the wet film impregnated with electrolyte, respectively.

[0048] (7)Wettability: The wettability of the separator is measured by a contact angle tester. The contact angle is measured when distilled water is dropped onto the surface of the separator for 5 s.

[0049] (8)Ionic conductivity: The separator is used to assemble a lithium-ion battery, and the electrochemical impedance spectroscopy (EIS) of the battery sample is detected by an electrochemical workstation, with a frequency of 0.1 MHz - 0.1 Hz and an amplitude of 10 mV. The ionic conductivity (σ, mS / cm) of the separator is calculated by the following formula: σ = L / AR, where L (cm) is the thickness of the separator, A (cm 2 ) is the area of the stainless steel sheet electrode, and R (Ω) is the resistance obtained from the Nyquist plot.

[0050] Table 1 Performance tests of examples and comparative examples

[0051] It can be seen from the table that compared with the traditional polyolefin separator, the prepared flame-retardant cellulose separator has higher electrolyte absorption rate, thermal dimensional stability and hydrophilicity, and has great application potential in the field of lithium-ion batteries.

[0052] Compared with unmodified cellulose, the tensile strength, thermal decomposition temperature and liquid absorption rate of the prepared flame-retardant cellulose separator are significantly improved, overcoming the short-board problems of cellulose separators in the application of lithium battery separators and further improving the application prospects.

[0053] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principle and practical application of the present invention, so that those skilled in the art can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A flame-retardant cellulose separator for a lithium battery, characterized in that: The cellulose membrane is prepared from flame-retardant cellulose by a papermaking method. The flame-retardant cellulose is prepared by grafting monomer 1, monomer 2, monomer 3 and monomer 4 with cellulose through atom transfer free radicals. The monomer 1 is an acrylic ester compound, the monomer 2 is a structure shown in Formula 1 or Formula 2, the monomer 3 is a structure shown in Formula 3, and the monomer 4 is a structure shown in Formula 4: Formula 1, Formula 2, Formula 3, Formula 4.

2. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The acrylic acid ester compound is selected from at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate, tert-butyl acrylate, isobornyl acrylate, 2-ethylhexyl acrylate, dimethylaminoethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate, isobornyl methacrylate, 2-ethylhexyl methacrylate, and dimethylaminoethyl methacrylate.

3. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The monomer 3 is obtained by reacting arginine with glycidyl methacrylate at 50-80° C. under alkaline conditions; the monomer 4 is obtained by reacting di(trimethylol)propane with 10-undecenoyl chloride.

4. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The molar ratio of the monomer 1, the monomer 2, the monomer 3 and the monomer 4 is (3-8): (2-5): (2-5): (2-3).

5. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The cellulose is selected from at least one of microcrystalline cellulose and nanocellulose.

6. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The cellulose is pretreated by solventization and esterification to obtain esterified cellulose, which is then used as an initiator and then grafted copolymerized with monomers 1 to 4 through atom transfer free radicals.

7. The flame-retardant cellulose separator for lithium batteries according to claim 6, characterized in that: The solvation pretreatment method is to dissolve cellulose in deionized water, then perform solvent exchange with N,N-dimethylacetamide and anhydrous methanol, and finally dissolve in DMAc / LiCl solvent to obtain a cellulose solution; the esterification treatment is to react cellulose with trichloroacetyl chloride under the action of an acid binding agent and a catalyst to obtain cellulose trichloroacetate.

8. The flame-retardant cellulose separator for lithium batteries according to claim 1, characterized in that: The conditions of the atom transfer radical grafting are: under nitrogen protection, cellulose trichloroacetate is used as an initiator, CuBr / bipyridine is used as a catalyst, N,N-dimethylformamide is used as a solvent, the reaction temperature is 80° C. to 90° C., and the reaction time is 24 to 48 hours.

9. A method for preparing a flame-retardant cellulose separator for a lithium battery, characterized in that: The following steps are involved: The flame-retardant cellulose is dispersed in a solvent to prepare a suspension, and a wet film is obtained by vacuum filtration after stirring, dispersing, refining and homogenizing. The wet film is hot-pressed at 80°C for 6 hours and peeled off to obtain a flame-retardant cellulose separator with a thickness of 15-50 μm.

10. Use of the flame-retardant cellulose separator for lithium batteries according to any one of claims 1 to 8 in lithium batteries.

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