Preparation method and application of two-dimensional MOF coated metal oxide composite material

Abstract

The present application relates to a kind of preparation method and application of two-dimensional MOF coated metal oxide composite, and the preparation method includes the surface charge modification of metal, the dispersion of modified metal oxide, two-dimensional MOF coated metal oxide preparation, the shell-core composite material (MO@MOF) with two-dimensional layered MOF coated metal oxide obtained by preparation.Compared with prior art, the present application is simple and easy to operate, and has good repeatability.The MO@MOF material obtained has novel morphology, and the combination of MOF and metal oxide is stable.The MOF with abundant catalytic sites is combined with polar metal oxide, which is beneficial to improve the conductivity and adsorption and catalytic performance of polysulfide of the material.The product can be combined with conductive agent and elemental sulfur as cathode material of lithium-sulfur battery, and has excellent performance of adsorbing and catalyzing polysulfide conversion.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a lithium-sulfur battery catalyst, in particular to a synthesis method of a two-dimensional MOF coated metal oxide and a two-dimensional material coated nanoparticle core-shell composite material and application thereof in the field of lithium-sulfur battery catalyst materials. BACKGROUND

[0002] In recent years, with the growing demand for energy in society, the use of new clean energy such as solar energy and water energy has attracted more and more attention. Developing advanced energy storage technology is an effective way to fully utilize such energy. Among various energy storage technologies, electrochemical energy storage technology, especially high specific energy rechargeable batteries, has the advantages of high efficiency, stability, cleanliness, flexible configuration and low cost, and has become a hot spot for the development of advanced energy storage technology. At present, lithium ion batteries have been widely commercialized, but they have the disadvantages of poor cycle stability and low energy density, which makes it difficult to meet the needs of high-density energy storage systems.

[0003] Among various potential high specific energy new chemical power systems, lithium-sulfur batteries with elemental sulfur and metal lithium as positive and negative electrodes have attracted much attention. The reduction of sulfur is based on a multi-electron transfer mechanism, and the theoretical specific capacity of elemental sulfur is as high as 1675 mAh g -1 The lithium-sulfur battery system with metal lithium can achieve an ultra-high theoretical specific energy of 2600 Wh kg -1 , and elemental sulfur is abundant in reserves, low in price and environmentally friendly. Therefore, the research on lithium-sulfur batteries with high energy density and high cycle stability is of great significance to meet the energy needs of society and improve energy utilization.

[0004] The low room-temperature conductivity of elemental sulfur and Li2S (discharge product) leads to a decrease in the utilization rate of active materials. The intermediate product Li2S n (n = 4-8) is easily soluble in lithium-sulfur electrolyte. The chain-like polysulfide will shuttle back and forth between the positive and negative electrodes. This process will cause loss of active materials, ultimately leading to problems such as capacity decay, low coulombic efficiency and short cycle life. Due to the difference in density between elemental sulfur and the final product Li2S (2.03 vs. 1.66 g cm -3 ), about 80% volume change occurs during charging and discharging.

[0005] In order to maximize the life of lithium-sulfur battery, the positive material needs to have a strong physical or chemical adsorption effect on polysulfide, and inhibit the occurrence of "shuttle effect" by capturing or dispersing polysulfide. MOF material is considered as an important candidate for lithium-sulfur battery positive carrier material due to its controllable pore size, highly ordered structure, ultra-high porosity and Lewis acid-base interaction with polysulfide. The existence of pores can effectively control the volume change of the positive electrode of lithium-sulfur battery during the cycle process, and the polysulfide can be reversibly captured and released by the pores of the MOF material, and the sulfur atoms can interact with the unsaturated metal sites in the material, so that the substrate has better polysulfide adsorption capacity, which can effectively inhibit the occurrence of "shuttle effect". In addition, the metal sites on the surface of the MOF are often catalytic sites for catalytic reactions, and by selecting different organic ligands and metal centers, the catalytic performance of the MOF material can be adjusted. However, considering that the conductivity of most pure MOFs is not ideal, it is usually necessary to further design MOF composites and MOF derivatives to meet the conductivity requirements of the positive electrode. SUMMARY

[0006] The purpose of the present application is to overcome the defects of the prior art and provide a novel, simple and controllable method for preparing two-dimensional MOF coated metal oxide composite material.

[0007] The purpose of the present application can be achieved by the following technical solutions:

[0008] In the conception process, it is considered that the flaky two-dimensional MOF has a larger specific surface area than the blocky MOF, can provide more active sites, and has a better adsorption and catalytic reaction effect on soluble polysulfide.

[0009] The adsorption effect of the metal active sites in the metal oxide on lithium polysulfide is much better than that of the non-polar carbon material, and some metal oxides even have good conductivity due to their inherent surface defects and unique energy band structure. Therefore, a strategy can be proposed to combine metal oxides and metal organic frameworks to construct composite materials and apply them to lithium-sulfur battery positive electrodes.

[0010] The first aspect of the present application provides a method for preparing a two-dimensional MOF coated metal oxide composite material, comprising the following steps:

[0011] (1) Surface charge modification of metal oxide: weigh the metal oxide and the surface charge modifier, add the metal oxide into the aqueous solution of the surface charge modifier, then ultrasonic mix, separate the solid in the mixed system after mixing, wash with alcohol and water for several times, pour off the liquid part to obtain the polar metal oxide after surface charge modification;

[0012] (2) dispersing the surface charge modified metal oxide in solution: taking the complex of metal oxide prepared in step (1), adding water, alcohol, N, N-dimethylformamide (DMF), and performing ultrasonic dispersion to obtain a polar metal dispersed solid-liquid mixture;

[0013] (3) adding terephthalic acid, metal salt, and stirring and dissolving in DMF to obtain a metal salt solution;

[0014] (4) adding the solid-liquid mixture in step (2) to the solution in step (3) and stirring to obtain a solid-liquid mixture;

[0015] (5) introducing nitrogen into the container of the solid-liquid mixture obtained in step (4) to remove air in the container;

[0016] (6) adding triethylamine to the mixture in step (5) and stirring under a sealed condition;

[0017] (7) placing the container in step (6) into an ultrasonic instrument for ultrasonic treatment, and controlling the temperature of the mixture in the bottle during ultrasonic treatment;

[0018] (8) centrifuging the solid-liquid mixture obtained in step (7), washing with water and alcohol, and vacuum drying the solid in an oven to obtain a MOF coated metal oxide;

[0019] Further, the metal oxide in step (1) is a polar transition metal oxide having good electrical conductivity and strong adsorption performance for polysulfides, and the surface charge modifier is one of polyvinyl benzene sulfonic acid sodium (PSS), polyethyleneimine (PEI), and polyvinylpyrrolidone (PVP).

[0020] Further, the mass concentration of the aqueous solution of the surface modifier is 0.1-1%.

[0021] Further preferably, the mass concentration of the aqueous solution of the surface modifier is 0.3%.

[0022] Further preferably, the metal oxide is selected from one of Fe3O4, MnO2, Co2O3, Co2O3, and NiO.

[0023] Further, the volume ratio of water, alcohol, and DMF in step (2) is 1:1:4-16.

[0024] In step (3), the volume of N, N-dimethylformamide added is the same as in step (2).

[0025] Further, in step (3), the ratio of terephthalic acid, metal salt, and DMF is 32.5 mg: 170-200 mg: 8-12 ml.

[0026] Further, the metal salt is one or any two selected from CoCl2·6H2O, NiCl2·6H2O, SrCl2·6H2O, CuCl2·6H2O, ZnCl2·6H2O, MnCl2·6H2O.

[0027] Further, the volume ratio of triethylamine added in step (6) to DMF added in the whole preparation process is 1:60-120.

[0028] Further preferably, the volume ratio of triethylamine added in step (6) to DMF added in the whole preparation process is 1:80-100.

[0029] The ultrasonic time in step (7) is 6-12 h, the container for the reaction is in a sealed state during ultrasonic, and the interior is in a state of being filled with nitrogen, and the temperature of the ultrasonic liquid in the ultrasonic machine is kept at no more than 25℃ during ultrasonic.

[0030] The drying temperature in step (8) is 50-80℃, and the drying time is 12-18 h.

[0031] The second aspect of the present application provides a two-dimensional MOF coated metal oxide prepared by the above preparation method, characterized in that the MOF coated metal oxide has a shell-core structure of two-dimensional layered MOF coating granular metal oxide.

[0032] Further, the two-dimensional MOF coated metal oxide composite material with a shell-core structure is prepared, and the thickness of the two-dimensional MOF is 10-30 nm.

[0033] The third aspect of the present application provides an application of the above two-dimensional MOF coated metal oxide in the preparation of a lithium-sulfur battery positive electrode material.

[0034] Compared with the prior art, the present application has the following technical advantages:

[0035] 1) The composite method adopted utilizes different electrical properties of materials, and electrostatically adsorbs and assembles a two-dimensional MOF and a composite shell-core structure material of nanometer metal oxide, the preparation process is simple, high-temperature sintering is not used, energy consumption is low, the prepared product has consistent composition and good stability. The prepared composite material has better performance improvement effect on lithium-sulfur batteries than single MOF and metal oxide materials.

[0036] 2) The use amount of surface charge modifier, DMF and triethylamine, whether nitrogen is introduced into the container for reaction to isolate air, the time and temperature of ultrasonic, and the drying temperature and other parameters in the preparation process of the application will affect the structure and performance of the synthesized material. The type of surface charge modifier directly determines whether the MOF can be compounded with metal oxide. The greater the difference in electronegativity between the MOF and the metal oxide on the surface, the stronger the attraction between them. The use amount of DMF will determine whether the metal salt can be completely dissolved, which determines the number of metal sites in the MOF material. The use of terephthalic acid can stabilize the metal ions in the solution, forming a metal ion-centered metal coordination structure with terephthalic acid as a ligand; triethylamine connects the dispersed metal coordination structure in the solution to form a metal-organic framework, and its use amount will directly determine whether a two-dimensional MOF can be formed. Excessive use of triethylamine will lead to excessive metal-terephthalic acid connection, making it difficult to separate the two-dimensional layered MOF during the later ultrasonic process. The ultrasonic time will determine the peeling effect of the layered MOF, and the ultrasonic temperature will affect the morphology of the MOF. If the ultrasonic temperature is too high, the two-dimensional MOF will curl. Sealing the container with nitrogen during the reaction and ultrasonic process can reduce the oxidation of divalent metal ions by oxygen during the formation of MOF, so that the metal active sites in the MOF remain unsaturated coordination structure.

[0037] 3) The prepared composite material has stable combination of metal oxide and two-dimensional MOF material, and a large number of active metal sites can be maintained in the MOF structure. The obtained composite material is coated on the nanometer metal oxide to form one or more coating structures. The material combines the high catalytic activity of two-dimensional MOF and the strong conductivity and adsorption capacity of polar metal oxide, and has a better promotion effect on the performance of lithium-sulfur battery than single MOF material and metal oxide. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 Transmission electron microscope (TEM) photo of Fe3O4@MOF composite material prepared in Example 1;

[0039] Figure 2 Scanning electron microscope (SEM) photo of Fe3O4@MOF composite material prepared in Example 2;

[0040] Figure 3 Rate performance chart of lithium-sulfur battery prepared by Fe3O4@MOF composite sulfur material, MOF composite sulfur material and Fe3O4 composite sulfur material prepared in Example 6. DETAILED DESCRIPTION

[0041] The application relates to a preparation method and application of a two-dimensional metal organic framework (MOF) coated metal oxide (MO) composite material, and comprises the following steps: surface charge modification of a metal, dispersion of the modified metal oxide, preparation of a two-dimensional MOF coated metal oxide, and preparation of a shell-core composite material (MO@MOF) with a two-dimensional layered MOF coated metal oxide. The application is simple, easy to implement, and good in repeatability. The obtained MO@MOF material has novel morphology, and the MOF and the metal oxide are combined stably. The MOF with rich catalytic sites is combined with a polar metal oxide, which is beneficial to improving the conductivity and the adsorption and catalytic performance of polysulfide of the material. The product can be combined with a conductive agent and elemental sulfur to serve as a positive electrode material of a lithium-sulfur battery, and has excellent performance of adsorbing and catalyzing the conversion of polysulfide.

[0042] The application will be described in detail below in combination with the drawings and specific embodiments. The following embodiments will help the skilled in the art to further understand the application, but do not limit the application in any form. It should be pointed out that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the application. These all belong to the protection scope of the application.

[0043] In the technical solution, the preparation means, materials, structures or component ratios of features not explicitly described are regarded as common technical features disclosed in the prior art.

[0044] Example 1

[0045] Preparation of Fe3O4@MOF composite material

[0046] Step 1: 150 mg Fe3O4 is added into 20 mL of a 0.3% PSS solution, and is ultrasonically dispersed for 30 min, is separated by a magnet, and is washed with alcohol and water twice. Step 2: the Fe3O4-PSS is added into a solution of 1 mL of H2O, 1 mL of alcohol (99.5% ethanol aqueous solution is used in the examples) and 8 mL of DMF, and is ultrasonically dispersed for 30 min. Step 3: 32.5 mg of terephthalic acid, 89.25 mg of CoCl2.6H2O and 89.25 mg of NiCl2.6H2O are added into 8 mL of DMF, and are stirred and dissolved. Step 4: the Fe3O4-PSS dispersion is added into the bottle in step 3, and is stirred uniformly by a glass rod. Step 5: after nitrogen is introduced into the bottle in step 4 to remove air, 0.2 mL of triethylamine is added, the bottle is sealed, and is mixed. Ultrasonic treatment is performed for 8 h, and the liquid temperature in the ultrasonic instrument is controlled to be not higher than 25 DEG C. After ultrasonic treatment, centrifugal separation is performed by a centrifuge, and the obtained solid is washed with water and alcohol for 3-4 times, is placed into a vacuum oven, and is vacuum dried at 60 DEG C for 12 h to obtain Fe3O4@MOF. The obtained Fe3O4@MOF material is characterized by TEM, and the TEM image is as shown in FIG. 1. Figure 1, showing a two-dimensional MOF coated particle-like Fe3O4 morphology.

[0047] Example 2

[0048] Preparation of Fe3O4@MOF composite material

[0049] Step 1: 150 mg Fe3O4 was added to 20 mL of 0.3% PEI solution, ultrasonic for 30 min, separated by magnet, washed with alcohol and water twice. Step 2: Fe3O4-PEI was added to 1 mL H2O, 1 mL alcohol, 8 mL DMF solution, ultrasonic dispersion for 30 min. Step 3: 32.5 mg terephthalic acid, 89.25 mg CoCl2·6H2O and 89.25 mg NiCl2·6H2O were added to 8 mL DMF, stirred and dissolved. Step 4: Fe3O4-PSS dispersion was added to the bottle of step 3, stirred uniformly with glass rod. Step 5: After purging the bottle in step 4 with nitrogen to remove air, 0.2 mL triethylamine was added, sealed and mixed. Ultrasonic for 8 h, control the liquid temperature in the ultrasonic instrument not more than 25℃. After ultrasonic, centrifuge with centrifuge, washed with water and alcohol for 3-4 times, the obtained solid was put into vacuum oven, vacuum dried at 60℃ for 12 h to obtain Fe3O4@MOF. The obtained Fe3O4@MOF material was characterized by scanning electron microscope, as shown in Figure 2 , showing a two-dimensional MOF coated particle-like Fe3O4 morphology.

[0050] Example 3

[0051] Preparation of MnO2@MOF composite material

[0052] Step 1: 150 mg MnO2 was added to 20 mL of 0.3% PSS solution, ultrasonic for 30 min, separated by centrifuge, washed with alcohol and water twice. Step 2: MnO2-PSS was added to 1 mL H2O, 1 mL alcohol, 8 mL DMF solution, ultrasonic dispersion for 30 min. Step 3: 32.5 mg terephthalic acid, 90.98 mg CuCl2·6H2O and 89.25 mg NiCl2·6H2O were added to 8 mL DMF, stirred and dissolved. Step 4: MnO2-PSS dispersion was added to the bottle of step 3, stirred uniformly with glass rod. Step 5: After purging the bottle in step 4 with nitrogen to remove air, 0.2 mL triethylamine was added, sealed and mixed. Ultrasonic for 8 h, control the liquid temperature in the ultrasonic instrument not more than 25℃. After ultrasonic, centrifuge with centrifuge, washed with water and alcohol for 3-4 times, the obtained solid was put into vacuum oven, vacuum dried at 60℃ for 12 h to obtain MnO2@MOF.

[0053] Example 4

[0054] Preparation of Co2O3@MOF composite

[0055] Step 1: 120 mg Co2O3 was added into 20 mL 0.3% PSS solution, ultrasonic dispersion for 30 min, centrifugal separation, alcohol and water washing twice. Step 2: Co2O3-PSS was added into 1 mL H2O, 1 mL alcohol, 8 mL DMF solution, ultrasonic dispersion for 30 min. Step 3: 32.5 mg terephthalic acid, 91.67 mg ZnCl2·6H2O and 89.25 mg NiCl2·6H2O were added into 12 mL DMF, stirring and dissolving. Step 4: Co2O3-PSS dispersion was added into the bottle of step 3, stirring uniformly with glass rod. Step 5: After nitrogen was introduced into the bottle of step 4 to remove air, 0.2 mL triethylamine was added, sealed, mixed uniformly. Ultrasonic dispersion for 12 h, the liquid temperature in the ultrasonic instrument was controlled not to exceed 25℃. After ultrasonic dispersion, centrifugal separation was carried out, water and alcohol washing was carried out for 3-4 times, the obtained solid was placed into vacuum oven, vacuum drying at 60℃ for 12 h to obtain Co2O3@MOF.

[0056] Example 5

[0057] Preparation of NiO@MOF composite

[0058] Step 1: 90 mg NiO was added into 20 mL 0.3% PSS solution, ultrasonic dispersion for 30 min, centrifugal separation, alcohol and water washing twice. Step 2: NiO-PSS was added into 2 mL H2O, 2 mL alcohol, 8 mL DMF solution, ultrasonic dispersion for 30 min. Step 3: 32.5 mg terephthalic acid, 89.25 mg CoCl2·6H2O and 99.98 mg SrCl2·6H2O were added into 8 mL DMF, stirring and dissolving. Step 4: NiO-PSS dispersion was added into the bottle of step 3, stirring uniformly with glass rod. Step 5: After nitrogen was introduced into the bottle of step 4 to remove air, 0.3 mL triethylamine was added, sealed, mixed uniformly. Ultrasonic dispersion for 8 h, the liquid temperature in the ultrasonic instrument was controlled not to exceed 25℃. After ultrasonic dispersion, centrifugal separation was carried out, water and alcohol washing was carried out for 3-4 times, the obtained solid was placed into vacuum oven, vacuum drying at 60℃ for 18 h to obtain NiO@MOF.

[0059] Verification example 1

[0060] Fe3O4@MOF prepared in example 1 was loaded with sulfur (Fe3O4@MOF / S) to prepare electrode sheet, which was assembled into lithium-sulfur battery to test the rate performance

[0061] Experimental instrument: new Wei charge-discharge tester

[0062] Battery parameters and assembly: Fe3O4@MOF accounts for 24wt% in the positive electrode material, and sulfur accounts for 48wt% in the positive electrode material. The positive electrode sheet takes 200μm thick Hercules carbon paper as the current collector, takes a diameter of 10mm, and the S loading is about 1mg / cm 2 The electrode sheet is used as the positive electrode sheet, a 19mm diameter round sheet Celgard separator is used as the battery separator, a 0.5mm thick 16mm diameter metal lithium sheet is used as the negative electrode, 20μL electrolyte is added on both sides of the separator, and a 2025 battery shell is used to assemble the battery.

[0063] Test parameters: The battery is left for two hours before charging and discharging, so that the electrolyte and the electrode sheet are fully soaked, the discharge cutoff voltage is 1.8V, and the charge cutoff voltage is 2.8V; the first circle is charged and discharged at a rate of 0.05C to activate the battery, and then the battery is charged and discharged at rates of 0.1C, 0.2C, 0.3C, 0.5C, 1C, 1.5C, 2C, and 0.5C, respectively.

[0064] The obtained rate performance graph is shown in Figure 3 Compared with Fe3O4 / S, the lithium-sulfur battery with MOF / S as the positive electrode material has higher discharge specific capacity at different rates.

[0065] The specific embodiments of the present application are described above. It should be understood that the present application is not limited to the above specific embodiments, and those skilled in the art can make various modifications or changes within the scope of the claims, which does not affect the essential content of the present application.

Claims

1. A method for preparing a two-dimensional MOF-coated metal oxide composite material, characterized by, The method comprises the following steps: (1) surface charge modification of metal oxide: weigh the metal oxide and the surface charge modifier, add the metal oxide into the water solution of the surface charge modifier, separate the solid in the mixed system after ultrasonic mixing, clean the solid with alcohol and water for several times, pour out the liquid part to obtain the surface charge modified metal oxide; (2) dispersion of the surface charge modified metal oxide in solution: take the compound of the metal oxide prepared in step (1), add water, alcohol, N, N-dimethylformamide, and perform ultrasonic dispersion to obtain a solid-liquid mixture in which the metal is dispersed; (3) add N, N-dimethylformamide to terephthalic acid and metal salt, stir and dissolve to obtain a metal salt solution; (4) add the solid-liquid mixture in step (2) into the solution in step (3) to stir and mix uniformly to obtain a solid-liquid mixture; (5) discharge the air in the container of the solid-liquid mixture obtained in step (4); (6) add triethylamine into the mixture in step (5) and seal the container; (7) place the container in step (6) into an ultrasonic instrument to perform ultrasonic treatment, and control the temperature of the mixture in the bottle during ultrasonic treatment; (8) perform centrifugal separation on the solid-liquid mixture obtained in step (7), wash with water and alcohol, place the solid into an oven for vacuum drying to obtain the MOF coated metal oxide.

2. The method for preparing a two-dimensional MOF-coated metal oxide composite material according to claim 1, characterized in that, In step (1), the surface charge modifier is one of polyvinylbenzenesulfonic acid sodium, polyethyleneimine and polyvinylpyrrolidone. The concentration of the surface charge modifier is 0.1 wt % to 1 wt %.

3. The method for preparing a two-dimensional MOF-coated metal oxide composite material according to claim 2, characterized in that, The surface charge modifier makes the surface of the charge modified metal oxide opposite to the electronegativity of the surface of the MOF, thereby producing electrostatic adsorption effect.

4. The method of claim 1, wherein the two-dimensional MOF-coated metal oxide composite is prepared by the steps of: In step (2), the volume ratio of water, alcohol and N, N-dimethylformamide is 1:1:6-12.

5. The method for preparing a two-dimensional MOF-coated metal oxide composite material according to claim 1, characterized in that, In step (3), the volume of the added N, N-dimethylformamide is the same as that in step (2). In step (3), the ratio of terephthalic acid, metal salt and DMF is 32.5 mg:170-200 mg:8-12 ml. The metal salt is one or two selected from CoCl2·6H2O, NiCl2·6H2O, SrCl2·6H2O, CuCl2·6H2O, ZnCl2·6H2O and MnCl2·6H2O.

6. The method of claim 1, wherein the two-dimensional MOF-coated metal oxide composite is prepared by the steps of: In step (5), protective gas is introduced into the container, and the protective gas is nitrogen or argon.

7. The method for preparing a two-dimensional MOF-coated metal oxide composite material according to claim 1, characterized in that, In step (7), the ultrasonic treatment time is 6-12 h, and the temperature of the ultrasonic container is controlled to be lower than 25℃ during ultrasonic treatment.

8. The two-dimensional MOF-coated metal oxide obtained by the production method according to any one of claims 1 to 7, characterized in that, The MOF coated metal oxide has a shell-core structure in which two-dimensional layered MOF coats the granular metal oxide.

9. Application of the two-dimensional MOF coated metal oxide as described in claim 8 in preparation of a positive electrode material for a lithium-sulfur battery.

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