Lightweight high-sodium-affinity V2C modified diaphragm material, and preparation method and application thereof

Abstract

The invention belongs to the technical field of diaphragm materials of sodium metal batteries, and particularly relates to a lightweight high-sodium-affinity V2C modified diaphragm material as well as a preparation method and application thereof. Aiming at the problems that a polyolefin diaphragm is poor in surface sodium-affinity and has short-circuit hidden danger, the invention provides the lightweight high-sodium-affinity V2C modified diaphragm material, a polyolefin diaphragm is used as a matrix, one side of the polyolefin diaphragm is covered with a V2C nanosheet functional layer with a few-layer structure, and the functional layer is deposited in a vacuum filtration manner, is compact and uniform, and is used for preparing the lightweight high-sodium-affinity V2C modified diaphragm material. Wherein the two-dimensional material provides abundant sodium affinity active sites, guides sodium ions to uniformly deposit, effectively inhibits formation of dendritic crystals, improves the cycling stability of a sodium negative electrode, and effectively reduces the short-circuit risk of a sodium metal battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of separator materials for sodium metal batteries, and particularly relates to a lightweight and highly sodium-philic V2C modified separator material, a preparation method of the modified separator material, and an application thereof in sodium metal batteries. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the overall understanding of the present invention, and is not necessarily regarded as an admission or any form of implication that this information constitutes prior art already known to those of ordinary skill in the art.

[0003] Sodium metal batteries are widely regarded as important candidates for the next-generation high-energy density energy storage system due to their high theoretical specific capacity, low reduction potential, abundant sodium resources, and low cost. However, sodium metal anodes are prone to form dendrites, have unstable interfaces, and are accompanied by volume expansion during cycling, seriously affecting the safety and lifespan of the battery. To alleviate the above problems, researchers have proposed various anode regulation strategies, such as constructing an artificial solid electrolyte interface (artificial SEI), introducing a three-dimensional conductive framework, etc., aiming to optimize the sodium ion deposition behavior. Although these methods have made certain progress in improving the dendrite problem, they generally have limitations such as complex preparation, poor compatibility, and difficulty in scaling up.

[0004] In comparison, separator modification, as a general and scalable interface engineering method, can effectively regulate the sodium deposition behavior without changing the anode structure. By introducing a sodium-philic functional layer on the separator surface, the local current density can be reduced, and uniform sodium ion nucleation can be induced, thereby inhibiting dendrite growth and improving cycling stability, which has important application value. Currently, glass fiber separators are mostly used in sodium metal batteries. Although they have excellent wettability and thermal stability, they are heavy in mass, large in thickness, and poor in mechanical strength, significantly reducing the energy density of the battery and making it difficult to meet commercial requirements. In contrast, polyolefin separators such as Celgard have advantages such as light weight and stable structure, and are important candidate materials for high specific energy sodium metal batteries. However, their surface has poor sodium-philicity, making it difficult to inhibit dendrite growth and there is a risk of short circuit.

[0005] Therefore, developing a method for surface modification of Celgard separators, which combines light weight, high sodium-philicity, and structural stability, has become the key path to improving the energy density and safety performance of sodium metal batteries. Summary of the Invention

[0006] The present invention aims to overcome the problems of existing sodium metal battery diaphragms, such as large mass, poor sodium affinity, and unstable interfaces, and provides a lightweight and highly sodium-affine composite diaphragm, which not only retains the lightweight advantage of commercial polyolefin diaphragms but also endows them with excellent sodium affinity and interface stability through surface functionalization, significantly improving the energy density and cycle life of sodium metal batteries.

[0007] To achieve the above object, the present invention proposes the following technical solutions: In the first aspect of the present invention, a lightweight and highly sodium-affine V2C modified diaphragm material is provided, with a polyolefin diaphragm as the matrix, and a functional layer is covered on one side thereof. The functional layer is a few-layer structured V2C nanosheet, and its XRD diffraction pattern has a characteristic peak (002) at 2 θ at 6.74 ± 0.2°, and the crystal plane spacing is 1.30 - 1.32 nm.

[0008] Furthermore, feasible matrix materials are PE films, PP films, or two- or three-layer diaphragms after their combination; in an implementation mode verified by the present invention, the matrix uses a Celgard diaphragm. The advantage of using such a diaphragm as the matrix is that it is lighter in mass.

[0009] Furthermore, the above functional layer is composed of a few-layer structured V2C nanosheet and has abundant sodium-affine active sites. In an implementation mode of the present invention, the functional layer is uniformly loaded on one side of the diaphragm by a suction filtration method to form a dense and continuous interfacial layer; this functional layer does not significantly increase the mass and thickness of the diaphragm, thereby improving the interfacial wettability with the sodium metal negative electrode and the uniformity of ion channel distribution while maintaining the lightweight characteristics.

[0010] Therefore, in the second aspect of the present invention, a preparation method of the lightweight and highly sodium-affine V2C modified diaphragm material described in the first aspect is provided, including the following steps: (1) Selectively etch the V2AlC MAX phase to obtain a V2C MXene phase rich in functional groups on the surface, denoted as V2C-MXene. Disperse V2C-MXene in a tetramethylammonium hydroxide (TMAOH) solution, stir at room temperature for intercalation and exfoliation, wash, and then ultrasonically exfoliate in an aqueous phase to obtain a few-layer structured V2C nanosheet, denoted as d-V2C; (2) Deposit the dispersion of the above d-V2C on the surface of the Celgard diaphragm by vacuum suction filtration, and obtain the product after drying and curing.

[0011] Furthermore, step (1) also has the following preferred technical solutions: The method of the above "selective etching" is as follows: Use concentrated hydrofluoric acid (mass fraction of 45 - 52%) to react at 30 - 40 °C for 4 - 6 days to selectively etch the Al element therein; in an implementation mode verified by the present invention, the reaction is carried out at 35 °C for 5 days.

[0012] The concentration of the above-mentioned tetramethylammonium hydroxide (TMAOH) solution is 23-27 wt%, preferably 24-26 wt%, and more preferably 25 wt%.

[0013] The above-mentioned stirring time is 10-14 h, preferably 11-13 h, and more preferably 12 h.

[0014] The power of the ultrasonic wave is 450-480 W, the temperature is controlled between 0-10 °C, and the ultrasonic time is 30-60 min.

[0015] Furthermore, the above step (2) also has the following preferred technical solutions: During the above-mentioned suction filtration process, a circulating water type vacuum pump with an ultimate vacuum degree up to -0.098 MPa is preferably used. After ultrasonic treatment in the aqueous phase in step (1), a uniformly dispersed d-V2C dispersion is obtained. The separator material is placed on the suction filtration device, and the d-V2C dispersion is applied above the separator material. There is a vacuum environment below the separator material, and the aqueous phase in the dispersion is drawn away, while the d-V2C remains above the separator to form a uniform and dense functional layer. The loading amount of d-V2C on the separator is 0.03-0.13 mg cm -2 , and this loading amount can be achieved by adjusting the concentration of the dispersion or the application dosage.

[0016] In the third aspect of the present invention, an application of the above-mentioned lightweight and highly sodium-philic V2C modified Celgard separator in the preparation of sodium metal batteries is provided.

[0017] The modified separator material described above is disposed between the positive and negative electrodes of the sodium metal battery, wherein the functional layer is close to the sodium metal side.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Ordinary polyolefin materials have the advantages of low cost and light weight, but their low surface polarity leads to poor wettability of the electrolyte, and their insufficient mechanical strength makes it difficult to effectively inhibit the penetration of sodium dendrites. The present invention uses a few-layer structured d-V2C material as a functional modification layer, and constructs a functional layer on the surface of the polyolefin separator through vacuum filtration technology to play the role of a rectifying layer. Due to the rich oxygen-containing functional groups (-O, -F, etc.) on the surface of the d-V2C material, the sodium-philicity of the separator can be significantly improved; the atomically flat surface of the two-dimensional material can induce uniform deposition of sodium ions, significantly reducing the generation of sodium dendrites. In the prior art, the MXene material compounded with polyolefin membrane materials is mostly Ti3C2. The present invention uses d-V2C, and the advantages compared with the prior art are: small molecular weight, avoiding the large mass of the modified separator, thus avoiding affecting the energy density of the battery.

[0019] 2. During the preparation process of the modified separator material, high temperature and high pressure are not required, and no toxic or harmful reagents are introduced. The functional layer prepared by vacuum filtration is dense and uniform, reducing the risk of short circuit and being compatible with the large-scale production process of existing polyolefin separators. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments and descriptions thereof of the invention are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 It is a scanning electron microscope photograph of the functional layer V2C in Example 1; Figure 1 In Figure (a), it is bulk V2C-MXene, Figure 1 In Figure (b), it is the exfoliated two-dimensional flaky d-V2C sample.

[0022] Figure 2 It is a transmission electron microscope photograph of the d-V2C nanosheets in Example 1; Figure 2 In Figure (a), it is a photograph under a large field of view, Figure 2 In Figure (b), it is a high-resolution photograph, and it can be seen that V2C is exfoliated into a few layers with a layer spacing of 1.31 nm.

[0023] Figure 3 It is the XRD pattern of V2C in Example 1; Figure 3 In it, bulk V2C represents the XRD pattern of the V2C-MXene sample, and d-V2C is the XRD pattern of the exfoliated two-dimensional flaky sample.

[0024] Figure 4 It is the XRD pattern of the modified separator material in Example 1; Figure 4 In it, Celgard is the XRD pattern of the unmodified Celgard separator, and d-V2C / Celgard is the XRD pattern of the lightweight and highly sodium-philic V2C modified Celgard separator.

[0025] Figure 5 It is the optical photograph of the separator before and after modification in Example 1; Figure 5 In Figure (a), it is a photograph of the unmodified Celgard separator, Figure 5 In Figure (b), it is a photograph of the separator modified with d-V2C.

[0026] Figure 6 It is for the Na||Na symmetric cell using the unmodified Celgard separator at 10 mA cm -2Current density, 1 mAh cm -2 Time-voltage curve at areal capacity; Figure 7 For the Na||Na symmetric cell using a lightweight and highly sodium-philic V2C modified Celgard separator at 10 mA cm -2 Current density, 1 mAh cm -2 Time-voltage curve at areal capacity. Detailed implementation mode

[0027] It should be noted that the following detailed description is illustrative and aims to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0028] It should be noted that the terms used herein are only for describing specific implementation modes and are not intended to limit the exemplary implementation modes according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0029] In the context of this specification, the word "comprising" is considered to mean "including in particular". It should not be construed as "consisting only of...".

[0030] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0031] Example 1 In this example, a lightweight and highly sodium-philic V2C modified Celgard separator is provided, and the preparation method of the separator is as follows: (1) Few-layer V2C modified material: Add V2AlC MAX phase to concentrated hydrofluoric acid for selective etching at 35 °C to selectively remove Al in the material, and obtain V2C MXene with -OH, -F, and -O rich on the surface, denoted as V2C-MXene; Disperse V2C-MXene in a 25 wt% solution of tetramethylammonium hydroxide (TMAOH), stir at room temperature for 12 h to achieve intercalation and expansion. After washing the solid part with deionized water, ultrasonically exfoliate it in deionized water, with an ultrasonic power of 460 W, the temperature controlled between 5 °C, and the ultrasonic time of 45 min to obtain a dispersion of few-layer structured V2C nanosheets, denoted as d-V2C (as Figure 1 and Figure 2 ).

[0032] The XRD pattern of the above-mentioned bulk V2C-MXene is as Figure 3 shown by the black curve. The characteristic peak is located at 12.16°, corresponding to the (002) crystal plane of V2C, and the interplanar spacing is 0.73 nm.

[0033] The XRD pattern of the prepared d-V2C is as Figure 3 shown by the green curve. The characteristic peak shifts to a smaller angle of 6.74°, and the interplanar spacing expands to 1.31 nm. Its larger interlayer spacing, good dispersibility, and abundant surface active sites are suitable for the sodium-philic functional modification of the separator in high specific energy sodium metal batteries.

[0034] (2) d-V2C modified Celgard separator: The above d-V2C dispersion was deposited on the surface of the Celgard separator by vacuum filtration using a circulating water vacuum pump to obtain a composite separator, ensuring that the two-dimensional material forms a uniform and dense coating on the separator surface. The loading amount of d-V2C on the separator is 0.072 mg cm -2 .

[0035] After the filtration is completed, the composite separator is dried in a vacuum oven to remove moisture and solidify the structure, obtaining a lightweight and highly sodium-philic composite separator with surface modification. This preparation method is simple and efficient, which is conducive to achieving large-area consistent preparation.

[0036] Figure 4 Shown are the XRD patterns of the unmodified Celgard separator (labeled as Celgard in the figure) and the above-mentioned lightweight and highly sodium-philic V2C modified Celgard separator, Figure 5 Shown is the unmodified Celgard separator ( Figure 5 (a) in the figure) and the lightweight and highly sodium-philic V2C modified Celgard separator ( Figure 5 (b) in the figure) of the optical photos. After modification, the surface of the separator is evenly covered with a black functional layer, and the functional layer is dense and uniform.

[0037] Example 2 In this example, another lightweight and highly sodium-philic V2C modified Celgard separator is provided, and the preparation method of the separator is as follows: (1) Few-layer V2C modified material: V2AlC MAX phase was added to concentrated hydrofluoric acid (mass fraction of 45%) and reacted at 40 °C for 4 days for selective etching to obtain V2C MXene, denoted as V2C-MXene; V2C-MXene was dispersed in a 27 wt% solution of tetramethylammonium hydroxide (TMAOH) and stirred at room temperature for 14 h to achieve intercalation and expansion. After the solid part was washed with deionized water, it was ultrasonically exfoliated in the aqueous phase to obtain a few-layer structured V2C nanosheet dispersion, denoted as d-V2C (as Figure 1 and Figure 2), the power of the ultrasound is 450 W, the time is 60 min, and the temperature is 0 °C.

[0038] (2) Modifying the Celgard separator with d-V2C: The dispersion of the above d-V2C was deposited on the surface of the Celgard separator by vacuum filtration to obtain a composite separator. During the filtration process, the pressure, flow rate, and functional layer thickness were controlled to ensure that the two-dimensional material formed a uniform and dense coating on the surface of the separator. The loading of d-V2C on the separator was 0.040 mg cm -2 .

[0039] After the filtration was completed, the composite separator was dried in vacuum to remove moisture and solidify the structure, obtaining a lightweight and highly sodium-philic composite separator with surface modification.

[0040] Example 3 In this example, another lightweight and highly sodium-philic V2C-modified Celgard separator was provided, and the preparation method of the separator was as follows: (1) Few-layer V2C modified material: V2AlC MAX phase was added to concentrated hydrofluoric acid (mass fraction of 52%) and reacted at 30 °C for 6 days for selective etching to obtain V2C MXene, denoted as V2C-MXene; V2C-MXene was dispersed in a 23 wt% solution of tetramethylammonium hydroxide (TMAOH) and stirred at room temperature for 10 h to achieve intercalation and exfoliation. After the solid part was washed with deionized water, it was ultrasonically exfoliated in the aqueous phase to obtain a dispersion of few-layer structured V2C nanosheets, denoted as d-V2C (as Figure 1 and Figure 2 ), the power of the ultrasound was 480 W, the time was 30 min, and the temperature was 10 °C.

[0041] (2) Modifying the Celgard separator with d-V2C: The dispersion of the above d-V2C was deposited on the surface of the Celgard separator by vacuum filtration to obtain a composite separator. During the filtration process, the pressure and flow rate were controlled to ensure that the two-dimensional material formed a uniform and dense coating on the surface of the separator. The loading of d-V2C on the separator was 0.122 mg cm -2 .

[0042] After the filtration was completed, the composite separator was dried in vacuum to remove moisture and solidify the structure, obtaining a lightweight and highly sodium-philic composite separator with surface modification. This preparation method is simple and efficient, which is conducive to realizing large-area consistent preparation.

[0043] Example 4 In this example, a sodium metal battery was provided. The sodium metal battery used the lightweight and highly sodium-philic V2C-modified Celgard separator prepared in Example 1. The above dried separator was punched into small round pieces with a punch with a diameter of 16 mm and used as the separator of the Na||Na symmetric battery.

[0044] The sodium metal was rolled into thin sheets and then punched into small round pieces with a punch of 10 mm in diameter to serve as the positive and negative electrodes. Celgard and the above-mentioned modified separator were used as the separators respectively, and 1 M NaPF6-DME electrolyte was injected to assemble a symmetric cell. The assembled button cell was cycled under the conditions of a current density of 10 mA cm -2 and a areal capacity of 1 mAh cm -2 to test the cycling stability during its charge and discharge process. The results are as Figure 6 , Figure 7 shown.

[0045] According to Figure 6 , when using the unmodified Celgard separator, the cycle life of the battery was less than 8 h and the battery short-circuited and failed. According to Figure 7 , when using the modified separator prepared in Example 1, the cycle life of the battery was significantly improved at this high current density and it could still cycle stably after 450 h, indicating that this functional layer has the functions of homogenizing the Na + ion current and inhibiting the growth of sodium dendrites, thus greatly improving the cycle life of the battery.

[0046] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A lightweight and highly sodium-philic V2C modified separator material, characterized in that, Based on a polyolefin separator as the substrate, with a functional layer covering one side, the functional layer being few-layer V2C nanosheets, whose XRD diffraction pattern has a characteristic peak (002) at 2 θ at 6.74 ± 0.2°, and the interplanar spacing is 1.30 - 1.32 nm.

2. The lightweight and highly sodium-philic V2C modified separator material according to claim 1, wherein The matrix material is a Celgard separator.

3. The lightweight and highly sodiumophilic V2C modified separator material according to claim 1, wherein The functional layer is composed of few-layer V2C nanosheets and is uniformly loaded on one side of the separator by a vacuum filtration method.

4. The preparation method of the lightweight and highly sodium-philic V2C modified separator material according to any one of claims 1-3, characterized in that, It includes the following steps: S1. Selectively etch the V2AlC MAX phase to obtain a V2C MXene phase with functional groups rich on the surface, denoted as V2C-MXene. Disperse V2C-MXene in a tetramethylammonium hydroxide solution, stir at room temperature for intercalation and expansion, wash, and ultrasonically exfoliate in an aqueous phase to obtain a dispersion of few-layer V2C nanosheets, denoted as d-V2C; S2. Deposit the dispersion of the above d-V2C on the surface of the Celgard separator by vacuum filtration, and obtain it after drying and curing.

5. The preparation method according to claim 4, characterized in that, In step S1, the selective etching method is as follows: React with concentrated hydrofluoric acid with a mass fraction of 45 - 52% at 30 - 40 °C for 4 - 6 days to selectively etch the Al element therein.

6. The preparation method according to claim 4, wherein In step S1, the concentration of the tetramethylammonium hydroxide solution is 23 - 27 wt%.

7. The preparation method according to claim 4, characterized in that, In step S1, the stirring time is 10 - 14 h; the power of the ultrasonic wave is 450 - 480 W, the time is 30 - 60 min, and the temperature is 0 - 10 °C.

8. The preparation method according to claim 4, wherein In step S2, the drying is carried out by vacuum oven drying.

9. Use of the lightweight and highly sodium-philic V2C modified separator material according to any one of claims 1-3 in the preparation of a sodium metal battery, characterized in that, The modified separator material is arranged between the positive and negative electrodes of the sodium metal battery, wherein the functional layer is close to the sodium metal side.

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