A magnesium fluoride nanofiber film, a preparation method and application thereof

Abstract

The application provides a magnesium fluoride nanofiber film and a preparation method and application thereof. The application utilizes the film forming property of nanocellulose material and the mesoporous network of nanofiber, combines the characteristics of lithium metal and lithium fluoride spontaneously forming a lithiumophilic site and lithium fluoride, and can be used as a functional interlayer of lithium metal battery to protect the lithium metal negative electrode. When used as a thin protection layer of lithium metal, a uniform ion transmission path and a lithium fluoride interface protection layer are formed on the surface of the lithium metal, the uniform deposition of lithium metal is promoted and the growth of lithium dendrites is inhibited, thereby the cycle stability of the lithium metal negative electrode is greatly improved, and the lithium metal battery with long service life and high energy density can be constructed. The nanocellulose, magnesium fluoride and lithium metal battery are organically combined, the unique role of the magnesium fluoride nanofiber in the lithium metal protection interface is explored, the cycle performance of the lithium metal battery is greatly improved, the rate performance of the battery is improved, and the preparation process is simple and the energy consumption is low.

Description

Technical Field

[0001] This invention relates to the field of lithium metal battery technology, and in particular to a magnesium fluoride nanofiber thin film, its preparation method, and its application. Background Technology

[0002] Rechargeable lithium metal batteries are widely used in energy storage due to their advantages such as high theoretical energy density, low electrochemical potential, light weight, and good electrochemical cycle stability. However, during the cycling process of secondary metal batteries, such as lithium metal batteries, the high reactivity of organic electrolytes and the uncontrolled dendrite growth of lithium metal anodes leading to dendrite penetration of the separator seriously affect the lifespan and safety of lithium metal batteries.

[0003] To mitigate persistent side reactions and lithium dendrite growth between the lithium metal anode and organic electrolyte, the design of robust and chemically stable solid electrolyte interphase (SEI) films on the lithium metal anode surface has attracted widespread attention and research. Among these, lithium fluoride-rich inorganic SEI films are considered key passivation films for stabilizing the lithium metal system. Due to the strong electro-withdrawal properties of fluorine-containing functional groups, and the fact that the lowest unoccupied molecular orbital energy of fluorinated substances is lower than that of organic solvents, they preferentially decompose and react with lithium on the anode surface, hindering contact and reaction between the lithium metal surface and the organic electrolyte. Furthermore, studies have shown that the presence of a metallic alloy phase at the lithium metal anode interface can promote uniform lithium metal deposition and increase the lithium-ion diffusion coefficient. Magnesium metal particles, with their infinite solubility in lithium metal, can achieve ultra-low or zero nucleation overpotentials and facilitate uniform lithium deposition / stripping, thereby suppressing lithium dendrite growth.

[0004] Based on the synergistic effects of fluorine-containing functional groups and metal fluorides in lithium metal batteries, many researchers have combined these two substances in hopes of achieving a synergistic effect greater than the sum of its parts (1+1>2). Magnesium fluoride can undergo an in-situ electrochemical reaction with lithium metal to generate lithium fluoride, a component that forms a lithium-magnesium alloy and stabilizes the SEI layer, thereby inducing uniform lithium metal deposition and inhibiting lithium dendrite growth. To achieve this effect, many researchers have recently used magnesium fluoride nanoparticles to coat polypropylene membranes for membrane modification or directly coat them onto the surface of lithium metal anodes to construct LiF-rich SEIs. However, due to the powder characteristics of magnesium fluoride particles, they often rely on binders to coat the membrane or lithium metal surface, resulting in a weakening of the activity of the magnesium fluoride particles due to the binder covering them. Simultaneously, the uneven particle distribution and voids lead to uneven reaction sites and ion distribution, affecting the formation of a highly uniform SEI layer. Therefore, the method of directly using magnesium fluoride particles to improve the stability of lithium metal anodes has limitations.

[0005] Previous work by the inventors has shown that cellulose materials, derived from forests and grasslands, possess uniformly distributed hydroxyl groups on their surface, a three-dimensional network structure with uniform pore size distribution, papermaking and film-forming properties, and biodegradability, providing a possibility for designing flexible, self-supporting interface protective layers with uniform porosity. For example, patent authorization number CN115149209B discloses the use of cellulose extracted from common natural plants combined with zirconium ion modification to prepare modified nanocellulose membranes, which significantly improves the rate performance of aqueous zinc-ion batteries or lithium-ion batteries. However, the aforementioned patent only utilizes the ionic cross-linking effect between zirconium ions of soluble zirconium salts and hydroxyl groups on the surface of nanocellulose, essentially still a nanocellulose material. While it utilizes a porous three-dimensional network to improve ion transport, it does not react with the lithium metal anode, making it difficult to control the physicochemical properties of the lithium metal interface electrolyte membrane and unable to effectively protect the lithium metal. Therefore, its application in high-energy-density lithium metal batteries is limited.

[0006] Therefore, we hope to develop a self-supporting magnesium fluoride nanofiber film with uniform pore size, resistance to lithium dendrites, and ability to strengthen the lithium metal electrolyte interface membrane to protect lithium metal. Summary of the Invention

[0007] This invention provides a magnesium fluoride nanofiber thin film, its preparation method, and its application, with the aim of solving the aforementioned problems existing in the background art.

[0008] To achieve the above objectives, embodiments of the present invention provide a magnesium fluoride nanofiber film, its preparation method, and its application. This invention, for the first time, uses nanocellulose as a template to uniformly coat magnesium fluoride nanoparticles onto the nanocellulose, preparing a flexible, self-supporting magnesium fluoride nanofiber film. It fully utilizes the intrinsic mesoporous network and film-forming properties of nanocellulose to obtain the magnesium fluoride nanofiber film. Used as a lightweight, self-supporting protective film for lithium metal, it can induce uniform lithium metal deposition and generate a LiF-rich solid electrolyte interphase (SEI), significantly improving the cycle stability of lithium metal batteries. This invention utilizes biomass cellulose to prepare magnesium fluoride nanofiber films at high cost. The uniformly distributed hydroxyl functional groups on the cellulose ensure the uniform coating of magnesium fluoride nanoparticles. The resulting nanocellulose film with uniformly coated magnesium fluoride nanoparticles is flexible, self-supporting, lightweight, and possesses uniform porosity. This magnesium fluoride nanofiber film can homogenize lithium ions, induce uniform lithium metal deposition, and simultaneously react with lithium metal in situ to form a lithiophilic lithium-magnesium alloy with a high lithium-ion diffusion coefficient and a uniform LiF-rich solid electrolyte interphase (SEI), thereby inhibiting lithium dendrite growth and improving the cycle stability of lithium metal batteries.

[0009] One object of the present invention is to provide a method for preparing magnesium fluoride nanofiber thin films, comprising the following steps:

[0010] S1: Add nanocellulose powder and soluble magnesium salt to deionized water, and disperse by ultrasonication to obtain a nanocellulose suspension containing magnesium ions.

[0011] S2: Add soluble fluoride to the magnesium ion-containing nanocellulose suspension and stir to react. After filtration, washing and drying, obtain magnesium fluoride nanofiber film.

[0012] The magnesium fluoride nanofiber film has a uniform and dense magnesium fluoride coating layer on the surface of the nanocellulose.

[0013] Preferably, the soluble magnesium salt is any one of magnesium acetate, magnesium chloride, magnesium nitrate, or magnesium sulfate, and the concentration is 1–60 mmol / L.

[0014] Preferably, the soluble fluoride is any one of potassium fluoride, ammonium fluoride, and sodium fluoride, with a concentration of 1–60 mmol / L.

[0015] Preferably, the stirring reaction time is 1 to 24 hours.

[0016] Preferably, the drying temperature is 40–120°C and the drying time is 6–48 hours.

[0017] Based on a general inventive concept, embodiments of the present invention provide a magnesium fluoride nanofiber film obtained by the above-described preparation method. This magnesium fluoride nanofiber film aims to form a uniform and dense magnesium fluoride coating layer on the surface of nanocellulose, which can undergo an in-situ electrochemical reaction with lithium metal to form a lithium-magnesium alloy with lithiophilic properties, promoting uniform nucleation and growth of lithium metal. It also generates a LiF-rich solid electrolyte interphase (SEI) film, inhibiting lithium dendrite growth, effectively regulating lithium metal deposition behavior, and further improving the rate cycling characteristics of lithium metal batteries.

[0018] Preferably, the porosity of the magnesium fluoride nanofiber film is 30% to 60%.

[0019] Preferably, the thickness of the magnesium fluoride nanofiber film is 1–60 μm.

[0020] Preferably, the diameter of the nanofibers in the magnesium fluoride nanofiber film is between 30 and 80 nm.

[0021] The embodiments of the present invention also provide a magnesium fluoride nanofiber film obtained by the above preparation method or the application of the above magnesium fluoride nanofiber film in lithium metal batteries.

[0022] Reaction mechanism

[0023] This invention uses nanocellulose as a template and modifies it with magnesium fluoride to produce a magnesium fluoride nanofiber film. This film has the characteristics of high mechanical strength, good flexibility, high thermal stability, controllable porosity, uniform and flat surface, light weight and good hydrophilicity. It can also be used as a battery separator interlayer to form a dense and uniform deposition layer on the surface of the lithium anode, forming a lithium-magnesium alloy with a high lithium-ion diffusion coefficient and a solid electrolyte interphase (SEI) film rich in lithium fluoride with certain mechanical strength. This can hinder the direct contact reaction between lithium metal and organic electrolyte and inhibit the formation of lithium dendrites during cycling, thereby extending the cycle stability of lithium metal batteries. The magnesium fluoride nanofiber film of this invention inherits the platform advantages of nanocellulose solution processability and paper film formation, and directly forms a dense magnesium fluoride nanoparticle coating layer on the surface of nanocellulose. When the magnesium fluoride nanofiber film comes into contact with lithium metal, it can react in situ at the lithium metal anode to generate a lithium-magnesium alloy with a high lithium-ion diffusion coefficient and a solid electrolyte interface film rich in lithium fluoride with good mechanical strength, thereby inducing uniform lithium metal deposition and inhibiting lithium dendrite growth. It has been verified that the magnesium fluoride nanofiber film can suppress side reactions on the electrode surface, reduce lithium sheet corrosion, and inhibit lithium dendrite growth.

[0024] The above-described solution of the present invention has the following beneficial effects:

[0025] (1) Compared with the original nanocellulose film, the magnesium fluoride nanofiber film of the present invention has a unique uniform network of magnesium fluoride that can react with lithium metal in situ to form a lithium-magnesium alloy with lithium affinity, which promotes the uniform nucleation and growth of lithium metal and generates a solid electrolyte interphase (SEI) film rich in LiF, inhibiting the growth of lithium dendrites, effectively regulating the lithium metal deposition regulation behavior, and further improving the rate cycle characteristics of lithium metal batteries.

[0026] (2) Compared with traditional lithium metal interface protection methods based on magnesium fluoride particles, the magnesium fluoride nanofiber film of the present invention is flexible and self-supporting, with a three-dimensional nanofiber network structure and uniform mesoporous distribution. It is lightweight and thin, which conforms to the current development trend of lightweight and functional lithium metal protective films. The thickness and porosity of the magnesium fluoride nanofiber film of the present invention can be adjusted, down to 1 μm, which is significantly lower than that of conventional particle-based functional coatings, thus helping to improve the overall energy density of lithium metal batteries.

[0027] (3) The preparation process of the magnesium fluoride nanofiber film described in this invention is simple and fully inherits the intrinsic platform advantages of nanocellulose materials. The above-mentioned scheme of this invention reveals that inorganic non-metallic magnesium fluoride nanofibers can be prepared using nanocellulose as a one-dimensional template. The intrinsic hydroxyl functional groups on nanocellulose are beneficial to the uniform coating of magnesium fluoride nanoparticles. The prepared magnesium fluoride nanofiber film has uniform porosity, which can promote electrolyte wetting, homogenize lithium ion distribution and accelerate lithium ion transport, induce uniform lithium metal deposition, and improve the cycle performance of lithium metal batteries. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a scanning electron microscope image of the magnesium fluoride nanofiber thin film provided in the embodiments of the present invention;

[0030] Figure 2 This is a scanning electron microscope image of nanocellulose provided in the comparative example of this invention;

[0031] Figure 3 XRD patterns of magnesium fluoride nanofiber films compared to original nanocellulose films provided in this embodiment of the invention;

[0032] Figure 4 The following are schematic diagrams showing the contact angles of the magnesium fluoride nanofiber film provided in the embodiments of the present invention compared with the original nanocellulose film; (a) is a schematic diagram of the contact angles of the magnesium fluoride nanofiber film, and (b) is a schematic diagram of the contact angles of the original nanocellulose film.

[0033] Figure 5 The magnesium fluoride nanofiber film provided in this embodiment of the invention, compared with the original nanocellulose film, is used in lithium metal symmetric batteries at 1 mA cm⁻¹. -2 Current density and 1mAh cm -2 Cyclic performance diagram under area capacity. Detailed Implementation

[0034] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0035] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0036] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0037] In this article, "CNF" refers to the original nanocellulose, and "CNF@MgF2" refers to the magnesium fluoride nanofiber film.

[0038] This invention addresses existing problems by providing a magnesium fluoride nanofiber thin film, its preparation method, and its applications.

[0039] Example 1

[0040] This embodiment provides a method for preparing magnesium fluoride nanofiber films.

[0041] The method in this embodiment specifically includes the following steps:

[0042] (1) Weigh 50mg of nanocellulose powder and 200mg of magnesium chloride powder, mix them, add a certain amount of water and ultrasonically disperse them to obtain a nanocellulose suspension containing magnesium ions.

[0043] (2) Add excess ammonium fluoride to the magnesium ion-containing nanocellulose suspension, stir for 6 hours, set up a vacuum filtration device, directly filter to form a film, heat and dry in an oven, and peel off to obtain a magnesium fluoride nanofiber film. Its SEM image is shown below. Figure 1 As shown, its thickness is basically the same throughout, at 27 μm, and its porosity is ~60%.

[0044] Comparative Example 1

[0045] Without the participation of nanocellulose in the synthesis, 200mg of magnesium chloride powder was weighed, mixed and dissolved in a certain amount of water, and then excess ammonium fluoride was added. After stirring for 6 hours, a vacuum filtration device was set up, and the mixture was directly filtered and dried. Only powdered magnesium fluoride was obtained, with particles of varying sizes. Without a binder, a smooth magnesium fluoride interface protective layer could not be obtained, and it could only exist in powder form.

[0046] Comparative Example 2

[0047] The difference from Example 1 is that: 50 mg of nanocellulose powder was weighed, added to a certain amount of water, and ultrasonically dispersed. A vacuum filtration device was set up, and the mixture was directly filtered. The film was then dried in an oven to obtain a nanocellulose film, the SEM image of which is shown below. Figure 2 As shown.

[0048] Comparative Example 3

[0049] The difference from Example 1 is that 50 mg of nanocellulose powder and 200 mg of magnesium fluoride powder were weighed, a certain amount of deionized water was added, ultrasonic dispersion was carried out, stirring was carried out for 6 hours, a vacuum filtration device was built, and the film was directly filtered and dried in an oven. The composite film of magnesium fluoride and nanocellulose was peeled off, and its thickness was uneven, about 36-45 μm.

[0050] Comparative Example 4

[0051] The difference from Example 1 is that magnesium chloride powder was replaced with zinc chloride powder. All other steps and parameters were the same as in Example 1. Ultimately, zinc fluoride nanoparticles could not be coated onto the surface of nanocellulose, and no zinc fluoride particles were obtained. Because the intrinsic hydroxyl functional groups on the surface of nanocellulose interact differently with different metal ions, the hydroxyl functional groups on nanocellulose are beneficial for the uniform coating of magnesium fluoride nanoparticles, while those for zinc fluoride are not.

[0052] Comparative Example 5

[0053] The difference from Example 1 is that magnesium chloride powder was replaced with aluminum chloride powder. All other steps and parameters were the same as in Example 1. Ultimately, aluminum fluoride nanoparticles could not be coated onto the surface of nanocellulose, nor were zinc fluoride particles obtained. The intrinsic hydroxyl functional groups on the surface of nanocellulose interact differently with different metal ions. The hydroxyl functional groups on cellulose are beneficial for the uniform coating of magnesium fluoride nanoparticles, while those for aluminum fluoride are not.

[0054] like Figures 1 to 2 As shown, where Figure 2 It is a nanocellulose film, in which the fibers are interwoven to form a uniform three-dimensional cross-linked network with a fiber diameter of 10-30 nm. Figure 1 The film is a magnesium fluoride nanofiber film with significantly thicker fibers, ranging from 30 to 80 nm in diameter. A uniformly distributed nanoparticle coating layer appears on the surface of the nanocellulose, preserving the original three-dimensional porous structure of the nanocellulose.

[0055] from Figure 3 The XRD pattern shows that the magnesium fluoride nanofiber film of the present invention has a lower characteristic peak of cellulose compared with the original nanocellulose. This is because the magnesium fluoride particles are coated on the fiber surface, which shields the signal of some of the characteristic peaks of cellulose. According to the characteristic peaks of the magnesium fluoride XRD standard card PDF#41-1443, it can be shown that the magnesium fluoride is successfully coated on the surface of nanocellulose.

[0056] like Figure 4 As shown in the contact angle diagram, the contact angle of the magnesium fluoride nanofiber film of the present invention is 19°, while the contact angle of the original cellulose film is 37°, indicating that the magnesium fluoride nanofiber film has better electrolyte wettability.

[0057] The magnesium fluoride nanofiber film and the original nanocellulose film obtained in the examples were used as a sandwich layer in a lithium metal symmetric battery. The lithium metal symmetric battery achieved a 1 mA cm⁻¹ performance. -2 Current density and 1mAh cm -2 Under the area capacity, such as Figure 5As shown, lithium metal batteries with magnesium fluoride nanofiber films as interlayers have lower polarization voltages and longer cycle life, indicating that magnesium fluoride nanofiber films can promote uniform lithium metal deposition and suppress dendrite growth.

[0058] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a magnesium fluoride nanofiber thin film, characterized in that, Includes the following steps: S1: Add nanocellulose powder and soluble magnesium salt to deionized water, and disperse by ultrasonication to obtain a nanocellulose suspension containing magnesium ions. S2: Add soluble fluoride to the magnesium ion-containing nanocellulose suspension and stir to react. After filtration, washing and drying, obtain magnesium fluoride nanofiber film. The magnesium fluoride nanofiber film has a uniform and dense magnesium fluoride coating layer on the surface of the nanocellulose.

2. The method for preparing a magnesium fluoride nanofiber thin film according to claim 1, characterized in that, The soluble magnesium salt is any one of magnesium acetate, magnesium chloride, magnesium nitrate, or magnesium sulfate, with a concentration of 1–60 mmol / L.

3. The method for preparing a magnesium fluoride nanofiber thin film according to claim 1, characterized in that, The soluble fluoride is any one of potassium fluoride, ammonium fluoride, and sodium fluoride, with a concentration of 1–60 mmol / L.

4. The method for preparing a magnesium fluoride nanofiber thin film according to claim 1, characterized in that, The stirring reaction time is 1 to 24 hours.

5. The method for preparing a magnesium fluoride nanofiber thin film according to claim 1, characterized in that, The drying temperature is 40–120°C, and the drying time is 6–48 hours.

6. A magnesium fluoride nanofiber film obtained by the preparation method according to any one of claims 1 to 5.

7. A magnesium fluoride nanofiber film according to claim 6, characterized in that, The porosity of the magnesium fluoride nanofiber film is 30% to 60%.

8. A magnesium fluoride nanofiber film according to claim 6, characterized in that, The thickness of the magnesium fluoride nanofiber film is 1–60 μm.

9. A magnesium fluoride nanofiber film according to claim 6, characterized in that, The diameter of the nanofibers in the magnesium fluoride nanofiber film is between 30 and 80 nm.

10. The application of a magnesium fluoride nanofiber film obtained by the preparation method according to any one of claims 1 to 5 or a magnesium fluoride nanofiber film according to any one of claims 6 to 9 in a lithium metal battery.

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