Nanocellulose-based lithium ion battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a nanocellulose-based lithium ion battery diaphragm, a preparation method thereof and a secondary battery, and relates to the technical field of battery diaphragms. According to the preparation method, chitosan and a branched chain monomer containing multiple amino groups are subjected to a grafting reaction to prepare branched chitosan, then the branched chitosan is grafted to the surface of nanocellulose, through a two-step method strategy of chitosan branching chain extension-then grafting to the nanocellulose, branched chains which extend outwards and are rich in amino groups are introduced while the nanocellulose is not excessively damaged, so that the surface of the nanocellulose is modified, and the surface of the nanocellulose is modified. In other words, high effective group introduction is obtained with the low modification degree, the lithium ion transference number is remarkably increased, and the pore structure of the diaphragm is optimized. The nitrogen content of the prepared lithium ion battery diaphragm reaches 0.6-1.5%, the porosity is 65-70%, the tensile strength is greater than 12 MPa, the ionic conductivity is greater than 1.8 mS cm <-1 >, and the lithium ion transference number is greater than 0.6.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery separator, in particular to a nanocellulose-based lithium ion battery separator, a preparation method thereof and a secondary battery. BACKGROUND

[0002] As a key component of lithium ion batteries, the separator largely determines the performance of the lithium ion battery. In recent years, nanocellulose-based lithium ion battery separators have attracted widespread attention due to their good mechanical strength, thermal stability, and electrolyte wettability. In order to improve the performance of the separator, grafting modification of nanocellulose is a common choice. For example, researchers graft chitosan onto the surface of nanocellulose, and use the interaction between the amino groups on the chitosan molecular chain and the anions (such as PF6 - ) in the electrolyte to inhibit the migration of anions, thereby increasing the lithium ion transference number (t + ) of the separator and improving the battery performance.

[0003] However, the prior art has obvious limitations. On the one hand, since both chitosan and cellulose are long-chain molecules, direct grafting results in a large steric hindrance, leading to a low grafting degree and a small number of introduced amino groups. On the other hand, forcibly increasing the grafting degree often damages the structure of nanocellulose and significantly reduces the mechanical strength of the separator.

[0004] Therefore, it is urgent to develop a new structural design strategy that can achieve high-density, three-dimensional extension of amino functionalization without excessively damaging the strength of the nanocellulose skeleton, thereby significantly improving the ion selectivity and electrochemical performance of the separator.

[0005] In view of this, the present application is proposed. SUMMARY

[0006] The present application aims to provide a nanocellulose-based lithium ion battery separator, a preparation method thereof and a secondary battery, which can achieve high-density, three-dimensional extension of amino functionalization without excessively damaging the strength of the nanocellulose skeleton, thereby significantly improving the ion selectivity and electrochemical performance of the separator.

[0007] The present application is achieved as follows: In a first aspect, the present application provides a preparation method of a nanocellulose-based lithium ion battery separator, comprising: grafting chitosan and a branched monomer containing multiple amino groups to obtain branched chitosan; wherein the branched chitosan has a branched structure rich in amino groups extending outward from the main chain; grafting the branched chitosan to the surface of nanocellulose to form a composite material with a three-dimensional amino functional layer. Battery separators are prepared using composite materials.

[0008] In an optional embodiment, the branched monomer is selected from at least one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine and low molecular weight polyethyleneimine; wherein the weight-average molecular weight of the low molecular weight polyethyleneimine is 300-1200. And / or, the molar ratio of repeating units to branched monomers of chitosan is 1:(1-5).

[0009] In an optional embodiment, the preparation process of branched chitosan includes: mixing chitosan, branched monomers and coupling agents, and reacting them at a pH of 5-6 and a temperature of 35℃-45℃ for a reaction time of 4h-12h.

[0010] In an optional embodiment, the coupling agent is selected from at least one of glutaraldehyde and xylose dialdehyde; And / or, the molar ratio of chitosan repeating units to coupling agent is 1:(0.2-0.7). And / or, the preparation process of branched chitosan includes: adjusting the pH of chitosan acetic acid solution to 5-6, then mixing it with branched monomers, adding a coupling agent dropwise, reacting under an inert atmosphere, quenching unreacted aldehyde groups after the reaction is completed, pouring the reaction solution into an alcohol solvent to precipitate, collecting the precipitate, washing it, redissolving it in acetic acid solution, then dialysis with water and drying it.

[0011] In an optional embodiment, branched chitosan is covalently grafted onto the surface of nanocellulose; Preferably, the nanocellulose is oxidized nanocellulose with aldehyde groups on its surface; Preferably, the covalent bond is a Schiff base bond, formed by the reaction of aldehyde groups on the surface of activated nanocellulose with the amino groups of branched chitosan.

[0012] In an optional embodiment, the process of preparing the composite material from branched chitosan includes: A nanocellulose suspension with a mass fraction of 0.5%-2.0% was mixed with an oxidant and reacted. The reaction was then terminated, purified by dialysis, and then ultrasonically dispersed to obtain an oxidized nanocellulose suspension. Branched chitosan was dissolved in buffer solution and then added dropwise to an oxidized nanocellulose suspension. The reaction was carried out under an inert atmosphere, with the reaction temperature controlled at 30℃-40℃, the reaction time at 3h-10h, and the reaction pH at 5.0-6.0. After the reaction was completed, the reaction mixture was washed with water to obtain a composite material suspension with a three-dimensional amino functional layer.

[0013] In an optional embodiment, the oxidant is NaIO4. The reaction with the oxidant is carried out under light-protected conditions at a temperature of 40°C-50°C for 3-10 hours. Ethylene glycol is then added to continue the reaction for 20-40 minutes to terminate the reaction. The reaction mixture is then placed in a dialysis bag, dialyzed with water, and then ultrasonically dispersed. And / or, the buffer is an acetate buffer; And / or, control the ratio of nanocellulose to branched chitosan to be 1:(0.7-1.1).

[0014] In an optional embodiment, the composite material suspension is diluted with water and then filtered through a vacuum to form a wet film, which is then transferred to anhydrous ethanol for solvent replacement and then dried.

[0015] Secondly, the present invention provides a nanocellulose-based lithium-ion battery separator, which is prepared by any of the preparation methods described in the foregoing embodiments; Preferably, the nanocellulose-based lithium-ion battery separator has at least one of the following characteristics 1 to 5: Feature 1: Nitrogen content reaches 0.6%-1.5%; Feature 2: Porosity is 65%-70%; Feature 3: Tensile strength greater than 12 MPa; Feature 4: Ionic conductivity greater than 1.8 mS / cm -1 ; Feature 5: Lithium-ion transference number greater than 0.6.

[0016] Thirdly, the present invention provides a secondary battery, including the nanocellulose-based lithium-ion battery separator of the aforementioned embodiments.

[0017] The present invention has the following beneficial effects: The present invention first performs a grafting reaction between chitosan and branched monomers containing multiple amino groups to obtain branched chitosan, and then grafts it onto the surface of nanocellulose. Through the two-step strategy of "chitosan branching and chain extension - grafting onto nanocellulose", branched amino groups are introduced outward without excessively damaging nanocellulose. That is, a high effective group introduction is obtained with a low degree of modification, which significantly improves the lithium ion migration number and optimizes the membrane pore structure. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a photograph of the diaphragm during the anhydrous ethanol immersion and replacement process in the preparation process. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] This invention employs a two-step strategy of "chitosan branching and chain extension - regrafting to nanocellulose" to introduce outward-extending, amino-rich branched chains without excessively damaging nanocellulose. The preparation method provided by this invention achieves a high level of effective group introduction with a relatively low degree of modification, significantly improving lithium-ion transference number and optimizing membrane pore structure.

[0022] This invention provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: S1. Preparation of branched chitosan Branched chitosan is prepared by grafting chitosan with branched monomers containing multiple amino groups. The branched chitosan has an amino-rich branched structure extending outwards from the main chain.

[0023] In some embodiments, the branched monomer is selected from at least one of diethylenetriamine, triethylenetetraamine, tetraethylenepentamine, and low molecular weight polyethyleneimine, and the branched monomer can be any one or more of the above. The weight-average molecular weight of the low molecular weight polyethyleneimine is 300-1200. The molar ratio of the repeating unit of chitosan to the branched monomer is 1:(1-5), such as 1:1, 1:2, 1:3, 1:4, 1:5, etc.

[0024] In some embodiments, the preparation process of branched chitosan includes: mixing chitosan, branched monomers, and a coupling agent, and reacting them at a pH of 5-6 and a temperature of 35℃-45℃ for 4-12 hours. The branched monomers and coupling agent react, followed by reaction with chitosan. Specifically, the reaction pH can be 5.0, 5.3, 5.5, 5.8, 6.0, etc.; the reaction temperature can be 35℃, 38℃, 40℃, 43℃, 45℃, etc.; and the reaction time can be 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, etc.

[0025] Furthermore, the coupling agent is selected from at least one of glutaraldehyde and xylose dialdehyde, and the coupling agent can be any one or more of the above. The molar ratio of the repeating unit of chitosan to the coupling agent is 1:(0.2-0.7), such as 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, etc. Specifically, chitosan is a linear polysaccharide linked by β-(1→4) glycosidic bonds, and its repeating unit is a copolymer of two monosaccharides, N-acetyl-D-glucosamine and D-glucosamine.

[0026] In practice, the preparation process of branched chitosan includes: adjusting the pH of the chitosan acetic acid solution to 5-6, then mixing it with the branched monomer, adding a coupling agent dropwise, and reacting under an inert atmosphere (the reaction is carried out at a pH of 5-6 and a temperature of 35℃-45℃ for 4-12 hours). After the reaction is completed, unreacted aldehyde groups are quenched, the reaction solution is poured into an alcohol solvent to precipitate, the precipitate is collected, washed, and then redissolved in acetic acid solution. After dialysis with water, it is dried.

[0027] Specifically, there are no restrictions on the method of quenching unreacted aldehyde groups, such as using glycine solution, but it is not limited to this.

[0028] Taking diethylenetriamine, glutaraldehyde, and chitosan as raw materials as an example, the reaction principle is as follows: The reaction of diethylenetriamine with glutaraldehyde: ; Branched chitosan formation: .

[0029] S2, grafted onto the surface of nanocellulose Branched chitosan is grafted onto the surface of cellulose nanofibers to form a composite material with a three-dimensional amino functional layer. The grafting method is unrestricted; for example, branched chitosan can be grafted onto the surface of cellulose nanofibers via covalent bonds. These covalent bonds can be Schiff base bonds, formed by activating the aldehyde groups on the cellulose nanofiber surface and reacting with the amino groups of the branched chitosan.

[0030] In some embodiments, the nanocellulose can be oxidized nanocellulose with aldehyde groups on its surface. The preparation process involves first oxidizing the nanocellulose and then reacting it with branched chitosan. The specific steps are as follows: a nanocellulose suspension with a mass fraction of 0.5%-2.0% is mixed with an oxidant and reacted. The reaction is then terminated, purified by dialysis, and ultrasonically dispersed to obtain an oxidized nanocellulose suspension. Branched chitosan is dissolved in a buffer solution and added dropwise to the oxidized nanocellulose suspension. The reaction is carried out under an inert atmosphere, with the reaction temperature controlled at 30℃-40℃, the reaction time at 3h-10h, and the reaction pH at 5.0-6.0. After the reaction, the reaction mixture is washed with water to obtain a composite material suspension with a three-dimensional amino functional layer. The ratio of nanocellulose to branched chitosan is controlled to be 1:(0.7-1.1), such as 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, etc.

[0031] Specifically, the mass fraction of the nanocellulose suspension can be 0.5%, 1.0%, 1.5%, 2.0%, etc. During the reaction between branched chitosan and oxidized nanocellulose suspension, the reaction temperature can be controlled at 30℃, 33℃, 35℃, 38℃, 40℃, etc.; the reaction time can be 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc.; and the reaction pH value can be 5.0, 5.3, 5.5, 5.8, 6.0, etc.

[0032] In some embodiments, the oxidant can be NaIO4, but is not limited thereto. The reaction with the oxidant can be carried out under light-protected conditions at a temperature of 40℃-50℃, such as 40℃, 43℃, 45℃, 48℃, 50℃, etc.; and for a reaction time of 3h-10h, such as 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, etc. After the reaction is complete, ethylene glycol is added and the reaction is continued for 20min-40min to terminate the reaction. The reaction mixture is then placed in a dialysis bag, dialyzed with water, and then ultrasonically dispersed to obtain an oxidized nanocellulose suspension.

[0033] In some embodiments, the buffer solution used may be an acetate buffer, but is not limited thereto.

[0034] The reaction process is as follows: Oxidized nanocellulose generation: ; The reaction of oxidized nanocellulose with branched chitosan:

[0035] Note: CS represents chitosan, DETA represents the grafted portion, and NH2 is a group in branched chitosan used for reaction with oxidized cellulose.

[0036] S3, forming a thin film A self-supporting thin film is fabricated using composite materials to obtain a lithium-ion battery separator. The specific film formation method is not limited.

[0037] In some embodiments, the composite material suspension is diluted with water and then vacuum filtered to form a wet film, followed by solvent replacement in anhydrous ethanol (e.g., ...). Figure 1 (As shown in the figure), and then dried to obtain a lithium-ion battery separator.

[0038] This invention provides a nanocellulose-based lithium-ion battery separator, prepared using the method described in this invention. A bi-level grafting strategy is employed to achieve multi-level amplification of amino groups, increasing the surface amino density to 2-3 times that of the unbranched state. Due to the high amino density, the separator prepared by this invention exhibits excellent lithium-ion transference number. Under the same amino density conditions, the preparation method provided in this invention causes less damage to the nanocellulose matrix and produces a stronger separator.

[0039] The nanocellulose-based lithium-ion battery separator provided in this invention has a nitrogen content of 0.6%-1.5%, a porosity of 65%-70%, a tensile strength greater than 12 MPa, and an ionic conductivity greater than 1.8 mS / cm. -1 The lithium-ion transference number is greater than 0.6.

[0040] This invention provides a secondary battery, including a nanocellulose-based lithium-ion battery separator provided in this invention. Due to the optimization of the battery separator, the electrochemical performance of the secondary battery is improved.

[0041] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0042] Example 1 This embodiment provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of a 1% (w / v) chitosan acetate solution (chitosan is dissolved in a 1% acetic acid aqueous solution) and adjust the pH to 5.5 (adjust with 1 mol / L NaOH solution, the same below). Then, add 1.28 g of diethylenetriamine while stirring, and then slowly add 1.0 mL of 25% glutaraldehyde solution at a rate of about 0.5 mL / min. The molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is 1:2:0.4. The reaction system is protected with nitrogen and stirred at 40°C for 6 hours, with the pH maintained at 5.0-6.0. After the reaction time is up, add 5 mL of 10% glycine solution and stir for 30 minutes to quench unreacted aldehyde groups. After the reaction is complete, pour the reaction solution into 800 mL of anhydrous ethanol to precipitate, and centrifuge to collect the precipitate (8000 rpm, 10 minutes). The precipitate was washed and redissolved in 1% acetic acid solution, and then dialyzed with deionized water for 48 hours before freeze-drying to obtain branched chitosan.

[0043] (2) Take 100 mL of 1% nanocellulose suspension (obtained by mixing 1 g of nanocellulose with 100 mL of water; the nanocellulose was purchased from Zhejiang Jinjiahao Green Nanomaterials Co., Ltd., hereinafter the same), add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then ultrasonically disperse to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0044] Accurately weigh 200 mg of branched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the reaction mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a composite material suspension with a three-dimensional amino functional layer. Note: In this example, the mass ratio of nanocellulose to branched chitosan is 1:1.

[0045] (3) Take 5 mL of the composite material suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0046] Example 2 This embodiment provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of 1% (w / v) chitosan acetate solution and adjust the pH to 5.5. Then, add 1.92 g of diethylenetriamine under stirring, followed by slow dropwise addition of 1.0 mL of 25% glutaraldehyde solution at a rate of approximately 0.5 mL / min. The molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is 1:3:0.4. The reaction system is protected with nitrogen and stirred at 40°C for 6 hours, with the pH maintained at 5.0-6.0. After the reaction time is up, add 5 mL of 10% glycine solution and stir for 30 minutes to quench unreacted aldehyde groups. After the reaction is complete, pour the reaction solution into 800 mL of anhydrous ethanol to precipitate the precipitate and collect it by centrifugation (8000 rpm, 10 minutes). After washing the precipitate, redissolve it in 1% acetic acid solution and dialyze it with deionized water for 48 hours before freeze-drying to obtain branched chitosan.

[0047] (2) Take 100 mL of 1% nanocellulose suspension, add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then sonicate to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0048] Accurately weigh 200 mg of branched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the reaction mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a composite material suspension with a three-dimensional amino functional layer.

[0049] (3) Take 5 mL of the composite material suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0050] Example 3 This embodiment provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of 1% (w / v) chitosan acetate solution and adjust the pH to 5.5. Then, add 2.56 g of diethylenetriamine under stirring, and slowly add 1.0 mL of 25% glutaraldehyde solution at a rate of about 0.5 mL / min. The molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is 1:4:0.4. The reaction system is protected with nitrogen and stirred at 40°C for 6 hours, with the pH maintained at 5.0-6.0. After the reaction time is up, add 5 mL of 10% glycine solution and stir for 30 minutes to quench unreacted aldehyde groups. After the reaction is complete, pour the reaction solution into 800 mL of anhydrous ethanol to precipitate the precipitate and collect it by centrifugation (8000 rpm, 10 minutes). After washing the precipitate, redissolve it in 1% acetic acid solution and dialyze it with deionized water for 48 hours before freeze-drying to obtain branched chitosan.

[0051] (2) Take 100 mL of 1% nanocellulose suspension, add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then sonicate to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0052] Accurately weigh 200 mg of branched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the reaction mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a composite material suspension with a three-dimensional amino functional layer.

[0053] (3) Take 5 mL of the composite material suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0054] Example 4 This embodiment provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of 1% (w / v) chitosan acetate solution and adjust the pH to 5.5. Then, add 1.92 g of diethylenetriamine under stirring, and slowly add 1.0 mL of 25% glutaraldehyde solution at a rate of about 0.5 mL / min. The molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is 1:3:0.5. The reaction system is protected with nitrogen and stirred at 40°C for 6 hours, with the pH maintained at 5.0-6.0. After the reaction time is up, add 5 mL of 10% glycine solution and stir for 30 minutes to quench unreacted aldehyde groups. After the reaction is complete, pour the reaction solution into 800 mL of anhydrous ethanol to precipitate the precipitate and collect it by centrifugation (8000 rpm, 10 minutes). After washing the precipitate, redissolve it in 1% acetic acid solution and dialyze it with deionized water for 48 hours before freeze-drying to obtain branched chitosan.

[0055] (2) Take 100 mL of 1% nanocellulose suspension, add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then sonicate to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0056] Accurately weigh 200 mg of branched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the reaction mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a composite material suspension with a three-dimensional amino functional layer.

[0057] (3) Take 5 mL of the composite material suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0058] Example 5 The only difference from Example 1 is that diethylenetriamine is replaced with an equimolar amount of triethylenetetramine.

[0059] Example 6 The only difference from Example 1 is that diethylenetriamine is replaced with an equimolar amount of tetraethylenepentamine.

[0060] Example 7 The only difference from Example 1 is that the amount of branched chitosan in step (2) is changed so that the mass ratio of nanocellulose and branched chitosan in this example is 1:0.7.

[0061] Example 8 The only difference from Example 1 is that the amount of branched chitosan in step (2) is changed so that the mass ratio of nanocellulose to branched chitosan in this example is 1:1.1.

[0062] Comparative Example 1 (using unbranched chitosan) This comparative example provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of 1% nanocellulose suspension, add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then sonicate to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0063] Accurately weigh 200 mg of unbranched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a chitosan-grafted nanocellulose suspension.

[0064] (2) Take 5 mL of the above suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0065] Comparative Example 2 This comparative example provides a method for preparing a nanocellulose-based lithium-ion battery separator, the steps of which are as follows: (1) Take 100 mL of 1% (w / v) chitosan acetate solution and adjust the pH to 5.5. Then, add 3.84 g of diethylenetriamine under stirring, and slowly add 0.5 mL of 25% glutaraldehyde solution at a rate of about 0.5 mL / min. The molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is 1:6:0.2. The reaction system is protected with nitrogen and stirred at 40°C for 6 hours, with the pH maintained at 5.0-6.0. After the reaction time is up, add 5 mL of 10% glycine solution and stir for 30 minutes to quench unreacted aldehyde groups. After the reaction is complete, pour the reaction solution into 800 mL of anhydrous ethanol to precipitate the precipitate and collect it by centrifugation (8000 rpm, 10 minutes). After washing the precipitate, redissolve it in 1% acetic acid solution and dialyze it with deionized water for 48 hours. Then freeze-dry to obtain branched chitosan.

[0066] (2) Take 100 mL of 1% nanocellulose suspension, add 1.07 g of NaIO4, wrap with aluminum foil to protect from light, and react in a 45°C water bath shaker for 6 hours. Then add 2 mL of ethylene glycol and continue the reaction for 30 minutes to terminate the reaction. Put the reaction mixture into a dialysis bag, dialyze with deionized water for 72 hours, and then sonicate to obtain a homogeneous oxidized nanocellulose suspension (concentration of about 0.5%).

[0067] Accurately weigh 200 mg of branched chitosan, dissolve it in 40 mL of pH 5.0 acetate buffer, and then add it dropwise to 40 mL of the above oxidized nanocellulose suspension at a rate of approximately 1 mL / min. Under nitrogen protection, stir the reaction mixture at 35°C for 6 hours, maintaining the pH at 5.0-6.0. After the reaction is complete, repeatedly centrifuge and wash the reaction mixture with deionized water to obtain a composite material suspension with a three-dimensional amino functional layer.

[0068] (3) Take 5 mL of the composite material suspension and dilute it with deionized water to 100 mL. Then, prepare a wet membrane by vacuum filtration. Then, transfer it into anhydrous ethanol for solvent replacement. Finally, dry it at 100°C to obtain the high amino density nanocellulose-based lithium-ion battery separator.

[0069] The only difference between Comparative Example 2 and Example 1 is that the molar ratio of chitosan repeating units, diethylenetriamine, and glutaraldehyde was changed.

[0070] Comparative Example 3 The only difference from step (1) of Example 1 is that the molar ratio of chitosan repeating unit: diethylenetriamine: glutaraldehyde is set to 1:0.8:1 to obtain cross-linked chitosan gel, which cannot be used for subsequent membrane preparation.

[0071] Test case The physical and electrochemical properties of various cellulose-based lithium-ion battery separators prepared in Examples 1-4 and Comparative Examples 1-3 were compared. Ionic conductivity and lithium-ion transference number were tested after further assembling the above battery separators into batteries: R2032 was used as the standard test battery. Cut lithium iron phosphate positive electrode sheets (LF0609), the prepared battery separators, and graphite negative electrode sheets (SM0206) were concentrically stacked and placed in an R2032 battery case. 30 μL of lithium-ion battery electrolyte (model LFP01, lithium hexafluorophosphate, dissolved as EC (ethylene carbonate):DMC (dimethyl carbonate) = 1:1 (volume ratio)) was added, and the battery was then sealed under pressure to obtain the lithium-ion battery required for testing. The positive electrode sheet, negative electrode sheet, battery case, and electrolyte were all purchased from KELOD Technology Co., Ltd. The results are shown in Table 1 below.

[0072] Table 1 Performance testing of cellulose-based battery separators in the examples and comparative examples.

[0073] The above experiments revealed that the high-amino-density nanocellulose-based lithium-ion battery separator prepared by the present invention, through a two-step strategy of "chitosan branching and chain extension - regrafting to nanocellulose", introduces outward-extending amino-rich branches without excessively damaging nanocellulose. That is, a high effective group introduction is obtained with a low degree of modification, which significantly improves the lithium-ion migration number and optimizes the pore structure of the separator.

[0074] Comparative experiments showed that the cellulose-based membrane grafted with unbranched chitosan had limited nitrogen content, few amino groups, and weak inhibition of lithium anions. Furthermore, excessive addition of cross-linking agents during chitosan branching could lead to excessive cross-linking of chitosan itself or with amino branches, forming a gel, making it unsuitable for grafting onto the surface of nanocellulose.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a nanocellulose-based lithium-ion battery separator, characterized in that, include: Chitosan is grafted with branched monomers containing multiple amino groups to obtain branched chitosan; wherein the branched chitosan has an amino-rich branched structure extending outward from the main chain. The branched chitosan was grafted onto the surface of nanocellulose to form a composite material with a three-dimensional amino functional layer. Battery separators are prepared using the composite material.

2. The preparation method according to claim 1, characterized in that, The branched monomer is selected from at least one of diethylenetriamine, triethylenetetramine, tetraethylenepentamine and low molecular weight polyethyleneimine; wherein the weight average molecular weight of the low molecular weight polyethyleneimine is 300-1200. And / or, the molar ratio of the repeating unit of the chitosan to the branched monomer is 1:(1-5).

3. The preparation method according to claim 1 or 2, characterized in that, The preparation process of the branched chitosan includes: mixing chitosan, the branched monomer and the coupling agent, and reacting them at a pH of 5-6 and a temperature of 35℃-45℃ for a reaction time of 4h-12h.

4. The preparation method according to claim 3, characterized in that, The coupling agent is selected from at least one of glutaraldehyde and xylose dialdehyde; And / or, the molar ratio of the repeating units of chitosan to the coupling agent is 1:(0.2-0.7). And / or, the preparation process of the branched chitosan includes: adjusting the pH of the chitosan acetic acid solution to 5-6, then mixing it with the branched monomer, then adding the coupling agent dropwise, reacting under an inert atmosphere, quenching unreacted aldehyde groups after the reaction is completed, pouring the reaction solution into an alcohol solvent to precipitate, collecting the precipitate, washing it, redissolving it in acetic acid solution, then dialysis with water, and drying it.

5. The preparation method according to claim 1, characterized in that, The branched chitosan was covalently grafted onto the surface of the nanocellulose; The nanocellulose is oxidized nanocellulose with aldehyde groups on its surface; The covalent bond is a Schiff base bond, formed by the reaction of aldehyde groups on the surface of activated nanocellulose with the amino groups of branched chitosan.

6. The preparation method according to claim 5, characterized in that, The process of preparing the composite material from the branched chitosan includes: A nanocellulose suspension with a mass fraction of 0.5%-2.0% was mixed with an oxidant and reacted. The reaction was then terminated, purified by dialysis, and then ultrasonically dispersed to obtain an oxidized nanocellulose suspension. The branched chitosan was dissolved in a buffer solution and then added dropwise to the oxidized nanocellulose suspension. The reaction was carried out under an inert atmosphere, with the reaction temperature controlled at 30℃-40℃, the reaction time at 3h-10h, and the reaction pH at 5.0-6.

0. After the reaction was completed, the reaction mixture was washed with water to obtain a composite material suspension with a three-dimensional amino functional layer.

7. The preparation method according to claim 6, characterized in that, The oxidant is NaIO4. The reaction is carried out under light-protected conditions at a temperature of 40℃-50℃ for 3-10 hours. Ethylene glycol is then added to continue the reaction for 20-40 minutes to terminate the reaction. The reaction mixture is then placed in a dialysis bag, dialyzed with water, and then ultrasonically dispersed. And / or, the buffer solution is an acetate buffer solution; And / or, control the ratio of the nanocellulose to the branched chitosan to be 1:(0.7-1.1).

8. The preparation method according to claim 6, characterized in that, The composite material suspension was diluted with water and then filtered through a vacuum to form a wet film. It was then transferred to anhydrous ethanol for solvent replacement and dried.

9. A nanocellulose-based lithium-ion battery separator, characterized in that, Prepared by the preparation method according to any one of claims 1-8; The nanocellulose-based lithium-ion battery separator has at least one of the following characteristics 1 to 5: Feature 1: Nitrogen content reaches 0.6%-1.5%; Feature 2: Porosity is 65%-70%; Feature 3: Tensile strength greater than 12 MPa; Feature 4: Ionic conductivity greater than 1.8 mS / cm -1 ; Feature 5: Lithium-ion transference number greater than 0.

6.

10. A secondary battery, characterized in that, Including the nanocellulose-based lithium-ion battery separator as described in claim 9.

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