Lewis acid metal ion-piperazine modified nano cellulose film as well as preparation method and application thereof

Abstract

The invention provides a Lewis acid metal ion-piperazine modified nanocellulose film as well as a preparation method and application thereof, and belongs to the technical field of battery materials. The preparation method comprises the following steps: mixing carboxylated nanocellulose with Lewis acidic metal salt in a solution, and carrying out pre-crosslinking reaction to obtain Lewis acidic metal ion modified carboxylated nanocellulose; a piperazine ligand compound and dopamine hydrochloride are added into the Lewis acidic metal ion modified carboxylated nano-cellulose, a cross-linking reaction is carried out with Lewis acidic metal ions as the center, suction filtration is carried out, and a film is formed. The metal in the Lewis acid metal salt is metal cations with the valence of more than bivalent. The carboxylated nanocellulose, the Lewis acid metal ion-piperazine complex and the solid polymer electrolyte are organically combined together for the first time and are applied to the lithium metal battery, the unique advantages of the lithium metal battery in the field of batteries are shown, and the rate capability and the cycling stability are greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a Lewis acidic metal ion-piperazine modified nanocellulose film, its preparation method, and its application. Background Technology

[0002] Lithium metal possesses extremely high theoretical specific capacity (3860 mAh / g) and extremely low redox potential, making it ideal for high-energy-density lithium metal batteries. However, the application of lithium metal batteries using lithium metal anodes is limited by many practical problems, such as the flammability and leakage risks of current liquid electrolytes, safety issues caused by lithium dendrite puncture, and battery life issues due to cycle stability. Therefore, the development of solid-state electrolytes to replace liquid electrolytes and the development of solid-state lithium metal batteries based on solid-state electrolytes are receiving increasing attention. Existing research has shown that solid-state electrolytes made of ceramic materials can suppress lithium dendrite growth and improve safety. However, these ceramic solid-state electrolytes suffer from poor interfacial compatibility and low lithium-ion conductivity, making them difficult to meet the needs of practical battery applications. In contrast, polymer electrolytes have good interfacial compatibility and controllable ionic conductivity. In particular, polymer solid-state electrolytes, which are formed by in-situ polymerization and solidification of traditional liquid electrolytes, have ionic conductivity comparable to liquid electrolytes, but with better anti-dendrite performance and non-flammability, and are expected to significantly improve the performance of lithium metal batteries.

[0003] In in-situ polymerizable electrolyte systems, a membrane or porous film placed between the positive and negative electrodes is an essential component. It plays a crucial role in improving the mechanical strength of the polymer electrolyte and enhancing the polymerization uniformity and stability. Currently, the most widely used membranes in in-situ polymerization technology are polyolefin membranes (PP / PE) and glass fiber membranes. Polyolefin membranes exhibit poor wettability to most organic precursor liquids, while glass fiber membranes are expensive and too thick. These porous matrices do not participate in the polymerization reaction; therefore, in-situ polymerization relies on external heating or UV light stimulation, which increases reaction costs and hinders large-scale production. Research has found that common polymerizable electrolyte precursors, such as 1,3-dioxolane (DOL), undergo a ring-opening reaction initiated by Lewis acidic substances, polymerizing to form long-chain polyether structures that play a vital role in lithium-ion conduction. Some studies have shown that substances such as Zn(TFSI)2 containing zinc ions and montmorillonite containing magnesium ions can initiate the ring-opening polymerization of DOL. However, the introduction of other substances during the in-situ polymerization process is not suitable for the battery manufacturing process that seeks simplicity and ease of operation.

[0004] Cellulose materials, derived from forests and grasslands, possess a uniformly distributed hydroxyl group structure, exhibiting three-dimensional network structure, papermaking and film-forming properties, and biodegradability. Furthermore, the abundant hydroxyl and carboxyl groups on the surface of nanocellulose can crosslink with some electrophilic Lewis acidic metal ions, providing a possibility for designing porous matrices with uniform porosity that can be polymerized in situ using DOL. For example, Chinese patent CN119875171A discloses a modified nanocellulose membrane prepared by combining cellulose extracted from common natural plants with magnesium fluoride and tin sulfide, significantly improving the cycle stability of lithium-ion batteries. However, this patent only utilizes the skeletal carrier effect of cellulose and the film-forming properties of inorganic materials at the negative electrode and electrolyte interface. It relies on the reaction of metal fluorides or sulfides with the lithium metal negative electrode to promote the formation of a tough and stable SEI layer. The crystalline magnesium fluoride or tin sulfide is difficult to expose Mg. 2+ or Sn 2+ The Lewis acid sites are difficult to exert the Lewis acid effect, and therefore will not have a fundamental impact on lithium salt dissociation and lithium ion transport capacity. They are also difficult to initiate in-situ polymerization of polymer electrolyte monomers in porous matrices, thus limiting their application in polymer solid-state lithium metal batteries.

[0005] Therefore, developing a nanocellulose film that can fully expose Lewis acid sites as a three-dimensional framework carrier for polymer solid electrolytes, especially a three-dimensional framework carrier for DOL-based polymer solid electrolytes, has become a challenge in the current development of lithium-ion battery separator technology. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, the first objective of this invention is to provide a Lewis acid metal ion-piperazine modified nanocellulose film. This film material, by coating the surface of nanocellulose with a Lewis acid metal ion-piperazine complex coating layer, not only improves the porosity of the film but also introduces Lewis acid centers through chemical bonding, thereby providing a basis for the ring-opening polymerization of polymer electrolytes.

[0007] The second objective of this invention is to provide a method for preparing Lewis acid metal ion-piperazine modified nanocellulose films. This method achieves the immobilization of Lewis acid metal ions on nanocellulose through a two-step reaction and enhances the acidity of Lewis acid metal ions. The preparation process is simple and controllable.

[0008] The third objective of this invention is to provide an application of Lewis acid metal ion-piperazine modified nanocellulose film. Through the uniformly distributed porous structure, large porosity, and the synergistic effect of Lewis acid metal ions and piperazine electron-withdrawing ligands, the film exhibits excellent resistance to lithium dendrite wettability and ionic conductivity when applied to polymer solid-state lithium metal batteries.

[0009] To achieve the above-mentioned technical objectives, this invention provides a method for preparing Lewis acid metal ion-piperazine modified nanocellulose films. The method involves mixing carboxylated nanocellulose with a Lewis acid metal salt in solution to undergo a pre-crosslinking reaction, thereby obtaining Lewis acid metal ion modified carboxylated nanocellulose. Piperazine ligand compounds and dopamine hydrochloride are added to the Lewis acid metal ion modified carboxylated nanocellulose to undergo a crosslinking reaction centered on the Lewis acid metal ion, followed by filtration to form a film. The metal in the Lewis acid metal salt is a divalent or higher metal cation.

[0010] In the preparation method of this invention, the co-introduction and sequence of introduction of Lewis acid metal ions and piperazine ligands are crucial to the modification effect of the nanocellulose film. Firstly, since the surface of carboxylated nanocellulose is rich in carboxyl and hydroxyl groups, the addition of Lewis acid metal salts allows for pre-crosslinking with the carboxylated nanocellulose through electrostatic interactions, changing the chemical properties of the nanocellulose from Lewis basic to Lewis acidic. Subsequently, piperazine ligands are further introduced into the coordination structure centered on Lewis acid metal ions to provide electron-withdrawing ligands. This not only anchors the mobile Lewis acid metal ions, forming a Lewis acid metal ion-piperazine coating layer, but also, because piperazine ligands can simultaneously provide multiple nitrogen-containing groups, further crosslinking forms a three-dimensional porous network structure with high mechanical strength and a smooth surface. Furthermore, in the system of this invention, dopamine hydrochloride undergoes oxidative polymerization with piperazine, ultimately existing as dopamine and connecting with piperazine, acting as a bridge in the complex. Dopamine assists piperazine molecules in forming a continuous structure, promoting the aggregation of the metal-piperazine complex. If dopamine hydrochloride is missing, the metal ion-piperazine molecular weight is too small to nucleate and aggregate for growth.

[0011] Experiments have shown that the introduction of a Lewis acid metal ion-piperazine coating layer on carboxylated nanocellulose not only enhances the Lewis acidity of the Lewis acid metal ions at the crosslinking center, but also avoids the adhesion problem of the carboxylated nanocellulose itself, further improving the porosity of the carboxylated nanocellulose film, thus giving it better lithium-ion conductivity and anti-dendritic properties when applied to lithium metal batteries.

[0012] In this invention, the metal in the Lewis acidic metal salt must be a divalent or higher metal cation. This is because the Lewis acidic metal ion needs to act as a crosslinking center to simultaneously bind carboxylated nanocellulose and piperazine complex ligands.

[0013] As a preferred embodiment, the Lewis acidic metal salt is a soluble copper salt; the carboxylated nanocellulose is obtained by treating nanocellulose using the TEMPO oxidation method. Compared to Ca... 2+Mg 2+ Alkaline earth metals, Cu 2+ Due to its small ionic radius and high charge density, it exhibits stronger Lewis acidity and can simultaneously form 2-4 chelate coordination numbers with the hydroxyl or carboxyl groups of cellulose, effectively cross-linking cellulose. Compared to common strong Lewis acids, Al... 3+ Although it has a high charge density, Al 3 + Copper salts readily form hydroxide precipitates. Furthermore, copper salts are widely available and do not interfere with lithium battery performance. Therefore, considering the Lewis acidity, stable chemical properties, and availability of copper salts for initiating DOL polymerization, this invention further prefers copper salts.

[0014] As a preferred embodiment, the soluble copper salt includes at least one of copper nitrate, copper chloride, copper sulfate, copper citrate, and copper acetate.

[0015] As a preferred embodiment, the mass ratio of the carboxylated nanocellulose to the Lewis acid metal salt is 1:(0.15~2.39); the pre-crosslinking reaction time is 6~24h. In this invention, the mass ratio of carboxylated nanocellulose to the Lewis acid metal salt determines the strength and density of the pre-crosslinking, while the pre-crosslinking reaction time determines the completeness and uniformity of the pre-crosslinking reaction, ensuring the stability and reliability of the modification effect. If the amount of Lewis acid metal salt is too small, the carboxyl groups on the surface of the carboxylated nanocellulose cannot be fully coordinated and covered; if the amount of Lewis acid metal salt is too large, the film surface will become rough and uneven.

[0016] As a preferred embodiment, the piperazine ligand compound includes at least one of N-aminoethylpiperazine and anhydrous piperazine.

[0017] As a preferred embodiment, the mass ratio of the piperazine ligand compound to dopamine hydrochloride is (20.6~103.2):(9.2~55.1). Within the preferred range of this invention, the uniformity and continuity of the crosslinked coating layer can be guaranteed. If the mass ratio is too low, it will result in an island-like distribution of the coating layer, causing inhomogeneity in the DOL polymerization, and the pores cannot be guaranteed to be uniform. If it exceeds the preferred range, it will lead to excessive brittleness of the film and pore closure.

[0018] As a preferred embodiment, the crosslinking reaction time is 0.5 to 3 hours. Within the crosslinking reaction time range of the present invention, it is possible to ensure the fixation of Lewis acid metal ions on the carboxylated nanocellulose, preventing them from falling off during subsequent washing, and also to ensure the continuous formation of the Lewis acid metal ion-piperazine coating layer.

[0019] The present invention also provides a Lewis acid metal ion-piperazine modified nanocellulose film, which is obtained by the above preparation method.

[0020] As a preferred embodiment, the nanocellulose film has a porous network structure, and the surface of the carboxylated nanocellulose in the film is coated with a Lewis acid metal ion-piperazine complex coating layer. This invention utilizes the three-dimensional mesoporous network and hydrogen-bonded functional groups of carboxylated nanocellulose to form a Lewis acid metal ion-piperazine complex coating layer on its surface through in-situ crosslinking of Lewis acid metal ions, piperazine ligand compounds, and dopamine hydrochloride, resulting in greater porosity, excellent wettability, and greater mechanical strength.

[0021] As a preferred embodiment, the film has a thickness of 10~60 µm and a porosity of 40~70%.

[0022] Finally, this invention also provides an application of a Lewis acid metal ion-piperazine modified nanocellulose film, which is used as a separator material in polymer solid-state lithium metal batteries. Compared with traditional polypropylene separator porous substrates and unmodified nanocellulose film substrates, the nanocellulose film of this invention has characteristics such as high porosity and Lewis acidity, which greatly reduces the energy barrier for lithium-ion migration, accelerates lithium-ion transport, and regulates lithium-ion flux. As a porous film substrate for filling polymer electrolytes, it can be applied to solid-state polymer lithium metal batteries, homogenize lithium metal deposition, significantly inhibit lithium dendrite growth, and extend the cycle life of polymer solid-state lithium metal batteries.

[0023] Furthermore, the Lewis acidic metal ion-piperazine modified nanocellulose film of the present invention is particularly suitable as a three-dimensional framework carrier for 1,3-dioxolane (DOL)-based polymer solid electrolytes in solid-state lithium metal batteries.

[0024] The Lewis acid metal ion-piperazine modified nanocellulose film of this invention, when applied to DOL-based polymer solid-state lithium metal batteries, can significantly improve the cycle stability and suppress dendrite growth of lithium metal batteries. The principle behind this invention's film material is that it possesses the characteristics of nanocellulose—high mechanical strength, smooth surface, easy film formation, good flexibility, controllable thickness, and dense and uniformly distributed pores—while also serving as a film with a three-dimensional framework structure to fully wet and absorb the polymer electrolyte, forming a "highway" of continuous lithium-ion migration channels with self-supporting reinforcement. Building upon the platform advantages of nanocellulose solution processability and paper-forming film-forming properties, through cross-linked Lewis acid metal ions and piperazine ligands with electron-withdrawing nitrogen-containing groups, the inherently Lewis basic cellulose is transformed into Lewis acid cellulose that undergoes cross-linking modification centered on Lewis acid metal ions. This transformation triggers cationic ring-opening polymerization of the polymer monomers, easily forming a high-polymerization-degree polymer electrolyte, improving the assembly convenience and safety of the battery system. On the other hand, the positively charged Lewis acidic metal ion centers formed by piperazine ligands competitively bind lithium salt anions, promoting the dissociation of lithium salts in solid polymers and weakening the strong binding between lithium ions and polymer ether oxygen. This further promotes precursor liquid wetting, triggers in-situ polymerization of the polymer electrolyte, increases lithium ion flux, and improves the stability of lithium metal batteries.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) This invention is the first to combine carboxylated nanocellulose, Lewis acid metal ion-piperazine complex and solid polymer electrolyte through chemical bonding and apply it to lithium metal battery, demonstrating its unique advantages in the battery field, greatly improving rate performance and cycle stability. Moreover, this method of combining Lewis acid metal ions as coordination centers with carboxylated nanocellulose and piperazine ligands respectively also achieves the anchoring of Lewis acid metal ions, avoiding the loss of Lewis acid metal ions during use.

[0027] (2) Unlike common commercial separators such as polyolefins, the copper ion-piperazine modified nanocellulose film matrix of the present invention has excellent affinity and wetting ability for polymer monomer precursor solutions used in in-situ polymerization technology due to the presence of hydroxyl and carboxyl groups on the surface of carboxylated nanocellulose and nitrogen-containing groups introduced in the ligands. The excellent wettability helps the solution to quickly fill the skeleton structure and form good interfacial contact and pathway conduction during the subsequent curing process, effectively controlling lithium ion transport and uniform lithium deposition, and further improving the rate performance of solid-state lithium metal batteries.

[0028] (3) This invention, by coating the surface of carboxylated nanocellulose with a Lewis acid metal ion-piperazine complex, simultaneously introduces multiple electron-withdrawing basic groups such as amino, pyridine nitrogen, and hydroxyl groups, which can enhance the electron attraction to the copper ion cation center, enhance the Lewis acidity of the Lewis acid metal ion center, and provide enhanced binding effect for the lithium salt anion, thereby accelerating the migration of lithium ions between the positive and negative electrodes; and the introduced piperazine ligands can synergistically regulate the conduction of lithium salt in the electrolyte with Lewis acid metal ions, promoting the rapid migration and uniform deposition of lithium ions. In addition, the introduction of the coating layer avoids the adhesion problem of the carboxylated nanocellulose itself, further improving the porosity of the carboxylated nanocellulose film.

[0029] (4) The thin film preparation process of the present invention is relatively simple and controllable, the material sources are very wide, it is environmentally friendly and pollution-free, and has ecological sustainability; through simple and controllable film thickness, porosity, and modified substance concentration, a longer battery life can be achieved, which helps to improve the overall battery performance. Compared with traditional commercial battery separators, the film of the present invention is thinner and lighter, requires less electrolyte, and is expected to improve energy density.

[0030] (5) The Lewis acidic metal ion-piperazine modified nanocellulose film of the present invention is particularly suitable as a three-dimensional framework carrier for 1,3-dioxolane (DOL) based polymer solid electrolytes in solid lithium metal batteries. Attached Figure Description

[0031] Figure 1 The images shown are scanning electron microscope (SEM) images and optical images of the copper ion-piperazine complex powder provided in Comparative Example 1 of this invention; (a) is a scanning electron microscope image, and (b) is an optical image.

[0032] Figure 2 The images shown are scanning electron microscope (SEM) images and optical images of the carboxylated cellulose nanofilm provided in Comparative Example 2 and the copper ion-piperazine modified cellulose nanofilm provided in Example 1, respectively; (a) is a scanning electron microscope (SEM) image and optical image of the carboxylated cellulose nanofilm, and (b) is a scanning electron microscope (SEM) image and optical image of the copper ion-piperazine modified cellulose nanofilm.

[0033] Figure 3 Optical images of the copper ion-piperazine modified nanocellulose film provided in Example 1 and the carboxylated nanocellulose film provided in Comparative Example 2 after 48 hours of initiation and curing with DOL electrolyte; (a) is an optical image of the copper ion-piperazine modified nanocellulose film after curing with electrolyte, and (b) is an optical image of the carboxylated nanocellulose film after curing with electrolyte for 48 hours.

[0034] Figure 4The images show the contact angles of the copper ion-piperazine modified nanocellulose film provided in Example 1 of the present invention, the carboxylated nanocellulose film provided in Comparative Example 2, and the commercial polypropylene separator; (a) is a contact angle image of the commercial separator; (b) is a contact angle image of the carboxylated nanocellulose film; and (c) is a contact angle image of the copper ion-piperazine modified nanocellulose film.

[0035] Figure 5 The solid polymer lithium metal batteries using the copper-ion-piperazine modified nanocellulose film as the separator provided in Example 1 and the carboxylated nanocellulose film as the separator provided in Comparative Example 2 operate at 0.2 mA / cm². 2 The current density and 0.2 mAh / cm 2 Cyclic performance diagram under areal capacity. Detailed Implementation

[0036] To more clearly illustrate the technical content of the present invention, it is described in detail here with reference to specific embodiments and accompanying drawings. Obviously, the listed embodiments are only preferred embodiments of the present technical solution, and other technical solutions that can be obviously derived by those skilled in the art based on the disclosed technical content still fall within the protection scope of the present invention.

[0037] In the embodiments of the present invention, unless otherwise specified, all raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0038] In this article, "PP" refers to commercial polypropylene membrane, "TCNF" refers to carboxylated nanocellulose membrane, and "Cu-Pi / TCNF" refers to the copper ion-piperazine modified nanocellulose membrane of this invention.

[0039] Example 1

[0040] This embodiment describes a method for preparing a Lewis acidic metal ion-piperazine modified cellulose nanofilm, which is obtained through the following steps:

[0041] The preparation process of carboxylated nanocellulose obtained by TEMPO oxidation is as follows: 1g of raw nanocellulose powder was added to 140ml of deionized water and sonicated at 350W to form a uniform suspension. 52mg of NaBr and 5mg of TEMPO were dissolved in 10ml of water beforehand. Under stirring, the above solution was added dropwise to the cellulose suspension, followed by the addition of 10ml of 10wt% NaClO solution. The pH of the system was adjusted to 10.3 by adding 0.5mol / L NaOH solution dropwise, and then stirred for 60min. Finally, 2ml of ethanol was added to terminate the reaction. A vacuum filtration device was set up, and the precipitate was filtered and washed repeatedly by adding deionized water. The collected precipitate was then sonicated in 200ml of deionized water to obtain uniform carboxylated nanocellulose.

[0042] 50 mg of carboxylated nanocellulose was ultrasonically dispersed in 60 mL of deionized water, and then 35 mg of copper sulfate was added. The mixture was stirred for 12 hours to induce a pre-crosslinking reaction. Subsequently, 24 mg of dopamine hydrochloride and 44 mg of anhydrous piperazine were added simultaneously, and the mixture was stirred for another hour to induce a crosslinking reaction. A vacuum filtration device was set up, and a nylon filter membrane was used to form a film. The membrane was repeatedly filtered and washed with deionized water, and finally filtered once with anhydrous ethanol. The resulting membrane was dried in an oven and then peeled off. It was named copper ion-piperazine modified nanocellulose film, and its thickness was measured to be 29 μm with a porosity of 60%.

[0043] To verify the self-initiating effect of copper ion-piperazine modified cellulose nanofiber film on DOL: 273 mg of LiTFSI was weighed into 1 ml of DOL in a water- and oxygen-free glove box and stirred vigorously until completely dissolved; then, a copper ion-piperazine modified cellulose nanofiber film with a diameter of 18 mm was placed in the solution and allowed to stand for 48 hours before its condition was observed. The optical image is shown below. Figure 3 In (a), it can be seen that the liquid state changes into a transparent solid state, indicating that the DOL monomer has been completely polymerized into PDOL solid polymer under the self-initiation of the film.

[0044] Example 2

[0045] Unlike Example 1, the content ratio of each substance in this example is as follows: 50 mg of carboxylated nanocellulose was weighed and ultrasonically dispersed in 60 mL of deionized water. Then, 35 mg of copper sulfate was added and stirred for 12 hours. Subsequently, 12 mg of dopamine hydrochloride and 22 mg of anhydrous piperazine were added simultaneously and stirred for another hour. A vacuum filtration device was set up, and a filter membrane was used to form a film. The membrane was repeatedly filtered and washed with deionized water. Finally, it was filtered once with anhydrous ethanol. The resulting membrane was heated and dried in an oven and then peeled off. It was named copper ion-piperazine modified nanocellulose film, and the thickness was measured to be 25 μm with a porosity of 65%.

[0046] Example 3

[0047] Unlike Example 1, the content ratio of each substance in this example is as follows: 50 mg of carboxylated nanocellulose was weighed and ultrasonically dispersed in 60 mL of deionized water. Then, 70 mg of copper sulfate was added and stirred for 12 hours. Subsequently, 48 mg of dopamine hydrochloride and 88 mg of anhydrous piperazine were added simultaneously, and stirring was continued for 1 hour. A vacuum filtration device was set up, and a filter membrane was used to form a film. The film was repeatedly filtered and washed with deionized water. Finally, it was filtered once with anhydrous ethanol. The resulting membrane was heated and dried in an oven and then peeled off. It was named copper ion-piperazine modified nanocellulose film, and the thickness was measured to be 35 μm and the porosity was 40%.

[0048] Example 4

[0049] The only difference between this embodiment and Example 1 is that an equal amount of N-aminoethylpiperazine is used as a ligand instead of anhydrous piperazine. All other steps and conditions are the same, and a copper ion-piperazine modified nanocellulose film is obtained with a thickness of 31 μm and a porosity of 57%.

[0050] Example 5

[0051] The difference between this embodiment and Example 1 is as follows: 50 mg of carboxylated nanocellulose was weighed and ultrasonically dispersed in 60 mL of deionized water. Then, 35 mg of copper sulfate was added and stirred to induce a pre-crosslinking reaction for 2 hours. Subsequently, 24 mg of dopamine hydrochloride and 44 mg of anhydrous piperazine were added simultaneously, and stirring was continued to induce a crosslinking reaction for 10 minutes. A vacuum filtration device was set up, and a nylon filter membrane was used to form a membrane. Deionized water was added repeatedly for filtration and washing. Finally, the membrane was filtered once with anhydrous ethanol. The resulting membrane was heated and dried in an oven and then peeled off. The thickness of the resulting membrane was measured to be 22 μm and the porosity was 53%.

[0052] Comparative Example 1

[0053] The difference from Example 1 is as follows: 35 mg of copper sulfate was weighed and dissolved in 60 ml of deionized water to prepare a homogeneous solution; 24 mg of dopamine hydrochloride and 44 mg of anhydrous piperazine were weighed and added simultaneously to the copper sulfate solution under vigorous stirring, and stirring was continued for 1 hour; a vacuum filtration apparatus was set up, and a filter membrane was used for filtration. The membrane was repeatedly washed with large amounts of deionized water to remove unreacted residual impurities. The resulting wet membrane was dried in an oven and peeled off to obtain copper ion-piperazine complex powder.

[0054] Its SEM image is as follows Figure 1As shown in (a), without the participation of carboxylated cellulose nanoparticles in the synthesis, a fully formed complex film cannot be obtained; only unevenly sized and aggregated particles are produced. A more uniform dispersion can be obtained by redispersing the powder in an aqueous solution, but after standing for several days, stratified sediments form, such as... Figure 1 As shown in (b) of the diagram.

[0055] Comparative Example 2

[0056] The difference from Example 1 is that 50 mg of carboxylated nanocellulose was weighed, ultrasonically dispersed in the same amount of deionized water as in Example 1, a vacuum filtration device was set up, a nylon filter membrane was used to form a film, the film was dried in an oven, and the carboxylated nanocellulose membrane was peeled off. The thickness was measured to be 18 μm and the porosity was 55%.

[0057] To verify the effect of carboxylated nanofiber films on DOL: 273 mg of LiTFSI was weighed into 1 ml of DOL in a glove box isolated from water and oxygen, and stirred vigorously until completely dissolved; then, a 18 mm diameter piece of the above-mentioned carboxylated nanofiber film was placed in the solution, and its condition was observed after standing for 48 hours. The optical image is shown below. Figure 3 As shown in (b), the solution remained liquid after 48 hours, indicating that the DOL monomer did not undergo ring-opening polymerization in the presence of the carboxylated cellulose nanofilm.

[0058] Comparative Example 3

[0059] The difference from Example 1 is as follows: 50 mg of carboxylated nanocellulose and 25 mg of copper ion-piperazine complex powder prepared according to Comparative Example 1 were weighed, added to 60 mL of deionized water, ultrasonically dispersed, stirred for 4 hours, a vacuum filtration device was set up, a nylon filter membrane was used to filter and form a film, heated and dried in an oven, and peeled off to obtain a copper ion-piperazine carboxylated nanocellulose composite membrane. The obtained composite membrane partially peeled off, and its color and thickness were uneven. The thickness was measured to be about 23~31 μm.

[0060] Comparative Example 4

[0061] Unlike Example 1, this comparative example did not include anhydrous piperazine and dopamine hydrochloride: 50 mg of carboxylated nanocellulose was weighed and ultrasonically dispersed in 60 ml of deionized water, then 35 mg of copper sulfate was added and stirred for 12 hours; subsequently, a vacuum filtration device was set up, and a filter membrane was used for filtration. The membrane was repeatedly filtered and washed with deionized water, and finally filtered once with anhydrous ethanol. The resulting membrane was heated and dried in an oven, and the dried film was peeled off.

[0062] Observations revealed that the resulting cellulose membrane did not exhibit the light blue color associated with copper ions, indicating that the copper ions were almost completely washed away during the filtration and washing process. The membrane thickness was measured at 18 μm, suggesting that it remains a carboxylated nanocellulose membrane. Combined with the copper ion-piperazine modified nanocellulose membranes obtained in Examples 1-3, this demonstrates that the addition of piperazine substances stabilizes the cross-linking of copper ions at the cellulose interface by forming a copper ion-piperazine coating layer, preventing their loss with water flow during washing.

[0063] like Figure 2 As shown, where Figure 2 (a) is a carboxylated cellulose nanofilm, in which the fibers are interwoven to form a three-dimensional network structure with uniform pores, a porosity of 55%, and a fiber diameter of 30~50 nm. Figure 2 (b) is the copper ion-piperazine modified nanocellulose film of the present invention. Its fibers are slightly thicker, while the three-dimensional network structure between the fibers is still maintained. However, some adhesion areas of the original cellulose are effectively relieved, so the porosity is improved to about 60%.

[0064] Table 1 compares the physical parameters and electrochemical performance of various cellulose films provided in the embodiments of the present invention and Comparative Example 2.

[0065]

[0066] As shown in Table 1, compared with the unmodified carboxylated cellulose nanofiber film, the copper ion-piperazine modified film has more abundant pores. The increased porosity helps to improve the liquid absorption capacity, thereby improving the continuity of the electrolyte in the three-dimensional network framework. Ultimately, this is reflected to a certain extent in the improvement of its ionic conductivity, which reaches as high as 3.28*10. -4 S / cm. (Combination) Figure 4 The contact angle test results showed that, among the PP, TCNF, and Cu-Pi / TCNF films, the copper ion-piperazine modified nanocellulose film exhibited the lowest contact angle to the polymer electrolyte, indicating its ultra-high affinity and wettability to the electrolyte. In contrast, the commercial PP membrane showed significantly poor wettability, thus confirming the significant advantages of the modified film of this invention. Furthermore, its mechanical strength reached 44 MPa, indicating that the introduction of the optimally proportioned complex ensured the high mechanical strength of the cellulose film matrix, helping to suppress lithium dendrite penetration. In contrast, when too few piperazine ligands or too much copper ion-piperazine complex was applied to the nanocellulose, Examples 2 and 3 both showed limited performance gains, or even deterioration, in different aspects of performance.

[0067] The copper-ion-piperazine modified cellulose nanofilm obtained in the examples and the carboxylated cellulose nanofilm obtained in the comparative examples were used as separators in a polymer solid-state lithium metal symmetric battery. 60 μl of 1,3-dioxolane (DOL)-based polymer electrolyte was added to the battery, followed by curing at 60 °C for 3 hours to obtain the polymer solid-state lithium metal symmetric battery. This battery achieved an efficiency of 0.2 mA / cm². 2 The current density and 0.2 mAh / cm 2 The capacity is used in a loop, such as Figure 5 As shown, polymer solid-state lithium metal batteries with Cu-Pi / TCNF separators exhibit a lower polarization voltage of approximately 20 mV, while lithium metal batteries with TCNF separators show a significant increase in polarization voltage with increasing cycle count, even exceeding 100 mV. This indicates that the copper-piperazine complex accelerates lithium-ion transport during battery cycling, which is consistent with its highest ionic conductivity. Furthermore, polymer solid-state lithium metal batteries with Cu-Pi / TCNF separators demonstrate superior cycle stability, indicating that copper-piperazine modified nanocellulose promotes uniform lithium deposition, inhibits dendrite growth, and significantly improves battery life.

[0068] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A method for preparing a Lewis-acidic metal ion-piperazine-modified nanocellulose film, characterized by: The carboxylated nanocellulose is mixed with a Lewis acidic metal salt in a solution to perform a pre-crosslinking reaction to obtain Lewis acidic metal ion modified carboxylated nanocellulose; the Lewis acidic metal ion modified carboxylated nanocellulose is added with a piperazine ligand compound and a dopamine hydrochloride to perform a crosslinking reaction centered on the Lewis acidic metal ion, and is extracted and filtered to form a film to obtain the Lewis acidic metal ion-piperazine modified nanocellulose film. The metal in the Lewis acidic metal salt is a divalent or higher metal cation.

2. The method for preparing a Lewis acidic metal ion-piperazine modified cellulose nanofilm according to claim 1, characterized in that: The Lewis acidic metal salt is a soluble copper salt; and the carboxylated nanocellulose is obtained by treating nanocellulose with a TEMPO oxidation method.

3. The method for preparing a Lewis acidic metal ion-piperazine modified cellulose nanofilm according to claim 2, characterized in that: The soluble copper salt includes at least one of copper nitrate, copper chloride, copper sulfate, copper citrate and copper acetate.

4. The method for preparing a Lewis acidic metal ion-piperazine modified cellulose nanofilm according to claim 1, characterized in that: The mass ratio of the carboxylated nanocellulose to the Lewis acidic metal salt is 1:(0.15-2.39); and the pre-crosslinking reaction is performed for 6-24 hours.

5. The preparation method of the Lewis acidic metal ion-piperazine modified nanocellulose film according to claim 4, characterized in that: The piperazine ligand compound includes at least one of N-aminoethylpiperazine and anhydrous piperazine; The mass ratio of the piperazine ligand compound to the dopamine hydrochloride is (20.6-103.2):(9.2-55.1).

6. The method for preparing a Lewis acidic metal ion-piperazine modified cellulose nanofilm according to claim 1, characterized in that: The crosslinking reaction is performed for 0.5-3 hours.

7. A Lewis-acidic metal ion-piperazine-modified nanocellulose film, characterized by: The preparation method of any one of claims 1-6.

8. The Lewis-acidic metal ion-piperazine-modified nanocellulose film according to claim 7, characterized by: The Lewis acidic metal ion-piperazine modified nanocellulose film has a porous network structure, and the surface of the carboxylated nanocellulose in the film is coated with a Lewis acidic metal ion-piperazine complex coating layer.

9. The Lewis-acidic metal ion-piperazine-modified nanocellulose film according to claim 8, characterized by: The thickness of the film is 10-60 µm, and the porosity is 40-70%.

10. Use of a Lewis-acidic metal ion-piperazine modified nanocellulose film according to any one of claims 7 to 9, characterized in that: The film is applied as a separator material in a polymer solid-state lithium metal battery.

Images