Preparation method and application of self-supporting dual-functional membrane material based on aluminum-based heterogeneous nanosheets

Abstract

The present invention relates to a preparation method and application of a self-supporting bifunctional membrane material based on aluminum-based heterojunction nanosheets. This method prepares a two-dimensional aluminum-based metal organic framework nanosheet array on a carbon nanofiber film by a seed secondary growth method, and then obtains a self-supporting membrane material based on aluminum-based heterojunction nanosheets by high-temperature carbonization in an ammonia atmosphere; this membrane material can be used as a self-supporting positive and negative electrode material in lithium-sulfur batteries to inhibit the shuttle effect and lithium dendrites of lithium polysulfide. The product obtained by the present invention combines the multiple advantages of three-dimensional carbon materials, nanosheet arrays, and heterostructures, can promote the adsorption and catalytic conversion of lithium polysulfide, and can also induce the uniform deposition of lithium ions and inhibit the growth of lithium dendrites, thereby improving the cycle life and safety of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and in particular relates to a preparation method and application of a self-supporting dual-functional membrane material based on aluminum-based heterogeneous nanosheets. Background Art

[0002] Lithium-sulfur battery is a new type of secondary battery with sulfur as positive electrode and lithium as negative electrode. It has an ultra-high theoretical specific capacity (1675mAh g -1 ) and energy density (2600Wh kg -1 ), and the sulfur element is abundant and environmentally friendly, so lithium-sulfur batteries are expected to become the next generation of energy storage systems. However, the large-scale application of lithium-sulfur batteries still faces some technical bottlenecks, such as the "shuttle effect" of lithium polysulfide and the excessive growth of lithium dendrites. Lithium polysulfide produced in the sulfur positive electrode is easily soluble in the electrolyte and migrates through the diaphragm to the lithium negative electrode, causing a shuttle effect, resulting in a decrease in coulombic efficiency and a decay of the battery cycle life. In addition, the uneven diffusion and deposition of lithium ions on the negative electrode will cause the formation and excessive growth of dendrites, resulting in "dead lithium", volume expansion effect and explosion safety hazards. Therefore, how to suppress the shuttle effect and lithium dendrites is a key problem in realizing the commercial application of lithium-sulfur batteries.

[0003] In order to solve the above technical problems, researchers have made a lot of attempts, and the strategies they adopted include developing positive and negative electrode host materials, membrane modification, introduction of interlayers, electrolyte design, etc. Among them, three-dimensional carbon-based materials represented by carbon cloth and carbon nanofiber (CNF) have excellent conductivity and good flexibility, and are ideal candidates for positive and negative electrode materials of lithium-sulfur batteries. However, non-polar carbon materials have weak affinity for lithium polysulfide and lithium ions, and their surface is uneven, which cannot inhibit the shuttle effect of lithium polysulfide and it is difficult to induce uniform deposition of lithium ions. It is often necessary to introduce polar components on its surface to improve its adsorption capacity for lithium polysulfide and lithium ions.

[0004] Polar materials such as transition metal oxides and sulfides have been used in research on positive and negative electrode materials for lithium-sulfur batteries due to their strong adsorption capacity for both lithium polysulfides and lithium ions. Transition metal oxides are widely available and easy to prepare. Their unique structural and surface properties allow for excellent adsorption or catalytic conversion of lithium polysulfides, and some materials can even be used to induce uniform lithium ion deposition. Currently, metal oxides such as zinc oxide, titanium dioxide, and cobalt oxide have been reported as positive or negative electrode host materials for lithium-sulfur batteries. However, the poor intrinsic electronic conductivity of transition metal oxides hinders the rapid conversion of adsorbed lithium polysulfides, reducing catalytic efficiency and resulting in battery capacity decay and low coulombic efficiency. For the lithium negative electrode, the poor conductivity of metal oxides leads to increased local current density, hindering uniform lithium ion deposition and causing the formation and growth of lithium dendrites. Furthermore, existing research on positive and negative materials for lithium-sulfur batteries has mostly focused on either the shuttle effect or the lithium dendrite problem, with limited research addressing both simultaneously. This is due to the different physicochemical properties and reaction processes of the positive and negative electrodes in lithium-sulfur batteries. Therefore, developing positive and negative electrode host materials based on new polar materials and simultaneously solving the shuttle effect and lithium dendrite problems in lithium-sulfur batteries are of great significance for breaking through the bottleneck of their commercial application. Summary of the Invention

[0005] In response to the shortcomings of the prior art, the present invention proposes a method for preparing and applying a self-supporting bifunctional membrane material based on aluminum-based heterogeneous nanosheets. This method uses a seeded secondary growth method to prepare an aluminum-based metal-organic framework (MOF) MIL-101 (Al) series two-dimensional nanosheet array on a CNF film substrate. This array is then subjected to high-temperature carbonization in ammonia to produce a self-supporting membrane material based on aluminum oxide-aluminum nitrogen (Al2O3-AlN) heterogeneous nanosheets. This material is then used as the host material for the sulfur positive electrode and lithium negative electrode in lithium-sulfur batteries, suppressing the shuttle effect and lithium dendrite problem. The membrane material has a three-dimensional porous structure, facilitating material transport and the storage of sulfur and lithium. Its two-dimensional nanosheet structure ensures full exposure of active sites, and its heterogeneous structure accelerates electron transport, improving the poor conductivity of Al2O3. When used as a positive electrode material, it can effectively anchor lithium polysulfide and accelerate its catalytic conversion; when used as a negative electrode material, the Al2O3-AlN heterogeneous nanosheet array can provide sufficient lithium-philic sites and reduce the local charge density, thereby inducing rapid and uniform deposition of lithium ions, inhibiting the formation of lithium dendrites, and improving the safety of the battery during cycling.

[0006] The technical solution of the present invention is:

[0007] A method for preparing a self-supporting dual-functional membrane material based on aluminum-based heterogeneous nanosheets comprises the following steps:

[0008] (1) The CNF film was placed in a film-forming solution and reacted at 210-230°C for 6-10 hours to obtain a CNF film with MOF seeds. After the above-mentioned film material was cleaned, a secondary hydrothermal growth was performed to obtain a CNF film-supported MOF nanosheet array (MOF@CNF). The film-forming solution and reaction conditions used were the same as those in the process of introducing seeds.

[0009] (2) MOF@CNF was placed in a muffle furnace and kept at 600-800 °C for 2-6 h in an ammonia atmosphere to obtain CNF film-supported Al2O3-AlN heterogeneous nanosheet arrays (Al2O3-AlN@CNF).

[0010] Preferably, the CNF film has a thickness of 50 to 200 μm and a diameter of 10 to 16 mm;

[0011] Preferably, the MOF material is MIL-101(Al) or MIL-101(Al)-NH2;

[0012] Preferably, the membrane-forming solution comprises a metal salt aluminum chloride hexahydrate, a ligand and deionized water in a molar ratio of 1:1:(500-800).

[0013] Preferably, the molar ratio of the metal salt aluminum chloride hexahydrate, the ligand and the deionized water in the membrane-forming solution is 1:1:(600-700).

[0014] Preferably, the ligand is one of terephthalic acid and aminoterephthalic acid.

[0015] The aluminum-based heterogeneous nanosheet self-supporting film material prepared by the method is used as positive electrode and negative electrode materials in lithium-sulfur batteries.

[0016] The raw materials involved in the above preparation method are all commercially available, and the equipment and processes used are well known to those skilled in the art.

[0017] The essential features of the present invention are:

[0018] The key steps of the present invention's technical solution are to fabricate MOF nanosheet arrays on CNF thin films using a seeded secondary growth method. These arrays are then carbonized in ammonia at high temperatures to produce a self-supporting film material based on Al2O3-AlN heterogeneous nanosheets. This material's unique characteristics allow it to function as both a positive and negative electrode material for lithium-sulfur batteries.

[0019] The present invention uses the reactants of aluminum chloride hexahydrate, a ligand (terephthalic acid or aminoterephthalic acid), and deionized water as a solvent. The inductive effect of the seed crystals from the initial growth, the curvature of the support, and changes in the ligand concentration cause the MOF crystals to change their growth direction on the CNF fiber surface. As a result, the MIL-101 (Al) series material grows along a specific direction, forming a two-dimensional nanosheet array structure. By carbonizing this material at high temperature in an ammonia atmosphere and incorporating nitrogen, an aluminum-based heterogeneous nanosheet array can be obtained.

[0020] The beneficial effects of the present invention are:

[0021] (1) The Al2O3-AlN heterogeneous nanosheets prepared in this invention are reported for the first time and are also the first to be used in lithium-sulfur batteries. Electrochemical test results show that this material can significantly improve the capacity and cycle stability of lithium-sulfur batteries when used as both positive and negative electrode materials.

[0022] (2) The membrane material has a three-dimensional porous structure, which facilitates material transport and the storage of sulfur and lithium. Its two-dimensional nanosheet structure can ensure the full exposure of active sites, and its heterogeneous structure can accelerate electron transport and improve the shortcomings of poor conductivity of Al2O3. When used as a sulfur positive electrode material, it can effectively anchor lithium polysulfide and accelerate its catalytic conversion. The reversible capacity at a current density of 0.2C reaches 1287mAhg -1 , and the cycle performance is stable, with a capacity retention rate of 89.7% after 100 cycles. When used as a lithium negative electrode material, the Al2O3-AlN heterogeneous nanosheet array can provide sufficient lithium-philic sites and reduce the local charge density, thereby inducing rapid and uniform deposition of lithium ions and inhibiting the formation of lithium dendrites. The assembled symmetrical battery has a capacity of 1 mAh cm -2 and 1 mA cm -2 Under normal conditions, it can cycle stably for 800 hours, significantly improving the safety of the battery during cycling.

[0023] (3) The present invention adopts a hydrothermal method, and the experimental operation is simple and easy; the metal salts and ligands used are inexpensive; the solvent used is deionized water, which reduces the use of organic solvents, is green and environmentally friendly, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a scanning electron microscope (SEM) image of the CNF film with seeds prepared in Example 1.

[0025] Figure 2 This is the SEM image of MOF@CNF prepared in Example 1.

[0026] Figure 3 This is the SEM image of Al2O3-AlN@CNF prepared in Example 1.

[0027] Figure 4 This is the X-ray diffraction (XRD) pattern of Al2O3-AlN@CNF prepared in Example 1.

[0028] Figure 5 The charge and discharge curves of the Al2O3-AlN@CNF loaded with sulfur prepared in Example 1 as the positive electrode material for lithium-sulfur batteries at a current density of 0.2C.

[0029] Figure 6 The symmetrical battery assembled by lithium-plated Al2O3-AlN@CNF prepared in Example 1 as the negative electrode of lithium-sulfur battery has a high efficiency of 1 mAh cm -2 and 1 mA cm -2 Constant current cycle test curve under 37°C.

[0030] Figure 7 This is the SEM image of Al2O3-AlN@CNF prepared in Example 2.

[0031] Figure 8 This is the SEM image of MOF@CNF prepared in Comparative Example 1.

[0032] Figure 9 This is the SEM image of MOF@CNF prepared in Comparative Example 2. DETAILED DESCRIPTION

[0033] The present invention will be further described below with reference to the accompanying drawings and examples, but the protection scope of the present invention is not limited thereto.

[0034] In the nomenclature of the MIL-101(Al) series two-dimensional nanosheet arrays described in the present invention, MIL stands for Materials of the Institute Lavoisier, which refers to the organization that developed this material, and 101 is the customary serial number in the industry.

[0035] Example 1

[0036] Synthesis of MIL-101(Al) nanosheet arrays on CNF films and their application in lithium-sulfur batteries:

[0037] (1) Preparation of MIL-101@CNF membrane materials:

[0038] 0.783g of aluminum chloride hexahydrate and 0.539g of terephthalic acid were mixed in 35mL of deionized water (molar ratio of 1:1:600) and thoroughly mixed by ultrasonication for 30 minutes. The mixture was then transferred to a 50mL polytetrafluoroethylene-lined autoclave, and the 10mm or 14mm diameter CNF film disc was placed vertically. The mixture was heated to 210°C for 6 hours, cooled naturally, and then rinsed with deionized water to obtain the seeded CNF film. The material obtained in the first step was placed in a film-forming solution for secondary growth. The composition and reaction conditions of the film-forming solution were the same as those for the seeding process. After the reaction was complete and the temperature was cooled naturally, the film was washed several times with deionized water and anhydrous ethanol, and then dried at 60°C to obtain the MIL-101@CNF material.

[0039] The MIL-101@CNF material was placed in a tube furnace and heated at 1.5 °C min -1 The temperature was raised to 600℃ at a heating rate of 100℃ and maintained for 2h to obtain Al2O3-AlN heterojunction nanosheet arrays supported by CNF film, namely Al2O3-AlN@CNF.

[0040] (2) Application of Al2O3-AlN@CNF in lithium-sulfur batteries:

[0041] The resulting Al2O3-AlN@CNF was used as a cathode material for lithium-sulfur batteries. The S / Al2O3-AlN@CNF cathode was prepared via a melt-diffusion method: 0.128 g of sulfur was dissolved in 1 mL of CS2 and stirred thoroughly until clear and transparent. The solution was slowly dripped onto a 10 mm diameter Al2O3-AlN@CNF disc and then dried in a vacuum oven at 60°C for 12 h. Subsequently, the sample was placed in a Teflon-lined stainless steel reactor and heated in a vacuum oven at 155°C for 12 h to obtain the S / Al2O3-AlN@CNF self-supporting cathode material. The assembled battery used a CR2032 battery case in the following assembly order: positive electrode case, positive electrode sheet (S / Al2O3-AlN@CNF), commercial separator Celgard 2400, lithium sheet, gasket, spring sheet, and negative electrode case.

[0042] The obtained Al2O3-AlN@CNF was used as the negative electrode material for lithium-sulfur batteries. Pre-lithium plating: The battery shell model used for the assembled battery was CR2032. The assembly order was positive electrode shell, Al2O3-AlN@CNF, commercial separator Celgard2400, Li sheet, gasket, spring sheet, and negative electrode shell. The assembled battery was then tested on an electrochemical workstation at 1 mA cm -2The Li / Al2O3-AlN@CNF anode was obtained by electroplating under the same conditions for 3 h. Symmetrical cells were then assembled using a CR2032 battery case. The assembly order was as follows: positive electrode case, Li / Al2O3-AlN@CNF, commercial separator Celgard 2400, Li / Al2O3-AlN@CNF, gasket, spring, and negative electrode case.

[0043] Figure 1 This is a SEM image of the sample prepared in this example. In the image, a small amount of MOF nanosheets can be seen deposited on the CNF film, which can serve as seeds for the second synthesis and induce the growth of MOF nanosheet arrays.

[0044] Figure 2 This is a SEM image of the MIL-101@CNF membrane material prepared in this example. It can be seen that a MOF nanosheet array was successfully formed on the CNF film, with the tubular CNF fibers completely encapsulated by the two-dimensional MOF nanosheets.

[0045] Figure 3 This is the SEM image of the Al2O3-AlN@CNF prepared in this example. It can be seen that after calcination, the morphology of the nanosheets can remain unchanged. Although its thickness is significantly reduced compared with the original MOF nanosheets, it still completely covers the CNF fibers.

[0046] Figure 4 The XRD pattern of Al2O3-AlN@CNF prepared in this example shows obvious structural peaks of Al2O3 and AlN.

[0047] Figure 5 The electrochemical charge-discharge curve of Al2O3-AlN@CNF prepared in this example as a positive electrode material for lithium-sulfur batteries. As can be seen from the figure, at a current density of 0.2C, the first discharge capacity of the material is as high as 1287mAhg -1 After 100 cycles, the discharge capacity is 1154 mAh g -1 , the capacity retention rate is 89.7%. It also shows two discharge platforms and one charge platform, and the polarization of each cycle is small, which shows that the cathode material can effectively suppress the shuttle effect.

[0048] Figure 6 The Al2O3-AlN@CNF film prepared in this example was plated with lithium and used as the negative electrode of lithium-sulfur battery to assemble symmetrical cells. -2 and 1 mA cm -2The constant current cycling test curves under these conditions are shown. The initial overpotential of the Li / Al2O3-AlN@CNF prepared by the present invention is 9 mV, lower than that of pure Li sheets, and remains stable at 10 mV after 800 hours. In contrast, the overpotential of pure Li sheets increases sharply after 185 hours of cycling, leading to short circuits. This comparison demonstrates that the negative electrode material obtained by the present invention exhibits stable lithium plating and stripping performance.

[0049] The Al2O3-AlN@CNF material obtained in this embodiment has a three-dimensional porous structure, which is convenient for material transport and storage of sulfur and metallic lithium. Its two-dimensional nanosheet structure can ensure the full exposure of active sites, and its heterogeneous structure can accelerate electron transport and improve the shortcomings of poor conductivity of Al2O3, thus having the dual function of inhibiting the shuttle effect and lithium dendrites. When used as a sulfur positive electrode material, it can effectively anchor lithium polysulfide and accelerate its catalytic conversion, thereby inhibiting the shuttle effect. This is also Figure 5 The electrochemical cycle curve of the lithium-sulfur battery using this cathode material has been confirmed to have an initial discharge capacity of up to 1287 mAh g -1 After 100 cycles, the discharge capacity is 1154 mAh g -1 , with a capacity retention rate of 89.7%. When used as a lithium negative electrode material, Al2O3-AlN heterogeneous nanosheets can provide sufficient lithium-philic sites and reduce the local charge density, thereby inducing rapid and uniform deposition of lithium ions, inhibiting the formation of lithium dendrites, and improving the safety of the battery during cycling. A symmetrical battery assembled with this nanostructure as the negative electrode has a capacity retention rate of 89.7% at 1 mAh cm -2 and 1 mA cm -2 In the cycle test under the conditions, the initial potential was 9mV, and its overpotential could remain stable, still at 10mV after 800h. However, the initial potential of the symmetrical battery assembled with pure Li sheets was higher, and an increasingly large polarization phenomenon appeared, and a short circuit occurred after 185h of cycling.

[0050] Example 2

[0051] The preparation process of the material differs from that of Example 1 in that 0.539 g of terephthalic acid is replaced with 0.587 g of aminoterephthalic acid, and 35 mL of deionized water is replaced with 40.9 mL of deionized water, resulting in a molar ratio of 1:1:700. In addition, the temperature during the preparation of MOF@CNF is adjusted to 230°C for 10 h; the temperature during the calcination is adjusted to 800°C for 6 h. The other steps are the same as those of Example 1.

[0052] Figure 7 This is the SEM image of Al2O3-AlN@CNF prepared in this example, which shows that Al2O3-AlN nanosheet arrays are obtained.

[0053] When it is used as the positive electrode material for lithium-sulfur batteries, the initial discharge capacity is 1284 mAh g -1 After 100 cycles, the discharge capacity is 1147 mAh g -1 The capacity retention rate is 89.3%. As the negative electrode, the symmetrical battery assembled at 1 mAh cm -2 and 1mAcm -2 In the constant current cycle test under the conditions of 100 nm, the overpotential was stable at 10 mV and the cycle lasted for 800 h.

[0054] Comparative Example 1

[0055] The synthesis steps of MIL-101@CNF were the same as those in Example 1, except that the amount of deionized water was adjusted to 29.2 mL (the molar ratio of the substances was 1:1:500).

[0056] Figure 8 This is the SEM image of the MIL-101@CNF membrane prepared in Comparative Example 1. It can be found that there are only a small amount of MOF nanosheets on the CNF fibers, and no MOF nanosheet array structure is formed.

[0057] Comparative Example 2

[0058] The synthesis steps of the MIL-101@CNF membrane were the same as those in Example 1, except that the amount of deionized water was adjusted to 46.7 mL (the molar ratio of the substances was 1:1:800).

[0059] Figure 9 This is the SEM image of the MIL-101@CNF membrane prepared in Comparative Example 2. It can be found that there are only a small amount of MOF nanosheets on the CNF fibers, and no MOF nanosheet array structure is formed.

[0060] In Comparative Examples 1 and 2, the concentration of the reactants was not within the preferred range. When the concentration was low, the nutrient solution in the solution was insufficient; when the concentration was higher than the preferred concentration, the reaction rate was too fast, which resulted in the inability to obtain a MIL-101 (Al) nanosheet array structure on the CNF film, let alone an Al2O3-AlN heterogeneous nanosheet array structure.

[0061] The steps in the examples are all preferred steps. From the SEM image, it can be seen that the Al2O3-AlN heterogeneous nanosheet array obtained in Example 1 is the most complete. When used as a positive electrode material, the first discharge specific capacity reaches 1287 mAh g -1 , the capacity retention rate is 89.7%; when used as the negative electrode material, the symmetrical battery assembled after lithium plating has a capacity retention rate of 89.7% at 1 mAh cm -2 and 1 mA cm -2 Under these conditions, the overpotential can remain stable for 800 hours. These performances are at a relatively high level in the current literature.

[0062] Matters not covered by the present invention are known in the art. The present invention has been described in detail above with reference to specific embodiments and exemplary examples, but these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, and all of these fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for preparing a self-supporting dual-functional membrane material based on aluminum-based heterogeneous nanosheets, characterized by: The method comprises the following steps: (1) A carbon nanofiber (CNF) film was placed in a film-forming solution and reacted at 210-230 °C for 6-10 h to obtain a CNF film with MOF seeds. After the above-mentioned film material was cleaned, a secondary hydrothermal growth was performed to obtain an aluminum-based MOF nanosheet array MOF@CNF supported by the CNF film. The film-forming solution and reaction conditions used were the same as those in the process of introducing the seeds. (2) MOF@CNF was placed in a muffle furnace and kept at 600-800 °C for 2-6 h in an ammonia atmosphere to obtain a support membrane material based on aluminum oxide-aluminum nitrogen heterogeneous nanosheets; The MOF material is MIL-101 (Al) or MIL-101 (Al)-NH2; The membrane-forming solution includes a metal salt aluminum chloride hexahydrate, a ligand and deionized water in a molar ratio of 1:1:(600-700); The ligand is one of terephthalic acid and aminoterephthalic acid.

2. The method for preparing a self-supporting dual-functional membrane material based on aluminum-based heterogeneous nanosheets according to claim 1, wherein: The CNF film has a thickness of 50-200 μm and a diameter of 10-16 mm.

3. Application of the aluminum-based heterogeneous nanosheet self-supporting film material prepared by the method according to any one of claims 1 to 2, characterized in that: Used as positive and negative electrode materials in lithium-sulfur batteries.

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