A MOF precursor nitrogen-doped porous carbon / sulfur composite material and its preparation method and application

Abstract

The present invention provides a MOF precursor nitrogen-doped porous carbon / sulfur composite material, its preparation method, and application. This invention relates to the field of material preparation. MIL-88(B)-NH2 is used as a precursor for preparing porous carbon. After calcination, the iron element in the MOF is oxidized to Fe3O4, obtaining nitrogen-doped carbon-coated Fe3O4 nanoparticles. Finally, the Fe3O4 nanoparticles within the carbon shell are etched by acid washing to obtain N-MPC. Sulfur powder is then introduced into the micropores of the porous carbon by melt diffusion to obtain N-MPC@S, which serves as the positive electrode of a RT-Na / S battery. The synergistic effect of the porous structure and nitrogen doping results in excellent battery performance and outstanding rate reversibility in RT-Na / S batteries.

Description

Technical Field

[0001] The present invention belongs to the field of material preparation, and in particular relates to a MOF precursor nitrogen-doped porous carbon / sulfur composite material and a preparation method and application thereof. Background Art

[0002] As sodium-ion batteries are about to enter the commercial stage, the search for safer and more efficient energy storage materials remains a pressing goal. The high abundance of sodium in the Earth's crust makes it relatively cost-effective, and its use in conjunction with sulfur cathodes enables high energy density and large-scale deployment. However, for the large-scale deployment of room-temperature sodium-sulfur batteries, the following issues must be addressed regarding the cathode: (1) capacity loss caused by the dissolution of active materials into soluble intermediates; and (2) the inherent low electrical conductivity of sulfur itself.

[0003] Currently, the most widely used strategy is to use polar carrier materials to bind polar polysulfides while providing a conductive framework; or to store sulfur in microporous carbon (MPC) with a porous structure to achieve physical confinement. Porous carbon materials have a very broad prospect in the application of room-temperature sodium-sulfur batteries. Many studies have reported on porous carbon materials prepared by various methods, such as biomass carbon and carbon composites derived from metal-organic framework (MOF) precursors. For example, by adding polar nitrogen-containing groups to MOF, uniformly distributed micropores can be formed after treatment to restrict the shuttling of polysulfides.

[0004] Due to the differences in nuclear charge number and outermost electronic structure, doping nitrogen atoms in the carbon structure can introduce defects and increase the polarity of the material. It has been reported that polar nitrogen doping has a binding effect on polysulfides, which can improve the dissolution of polysulfides and inhibit the occurrence of the shuttle effect. Summary of the Invention

[0005] The present invention aims to provide a nitrogen-doped porous carbon / sulfur composite material based on a MOF precursor and its preparation method. MIL-88(B)-NH2 is used as the porous carbon precursor. After calcination, the iron in the MOF is oxidized to Fe3O4, producing nitrogen-doped carbon-coated Fe3O4 nanoparticles. Finally, the Fe3O4 nanoparticles within the carbon shell are etched by acid washing to obtain N-MPC. Sulfur powder is then introduced into the micropores of the porous carbon via melt diffusion to produce N-MPC@S. This simple and efficient preparation method is employed.

[0006] Another object of the present invention is to provide an application of a MOF precursor nitrogen-doped porous carbon / sulfur composite material for sodium batteries.

[0007] The specific technical solutions of the present invention are as follows:

[0008] A method for preparing a MOF precursor nitrogen-doped porous carbon / sulfur composite material comprises the following steps:

[0009] 1) calcining MIL-88(B)-NH2, and then heating and acid washing the obtained N-Fe3O4@C to obtain N-MPC;

[0010] 2) N-MPC and sulfur are mixed and heated to react to obtain an N-MPC@S composite material.

[0011] In step 1), the calcination is performed at 600-800°C for 2h; the calcination atmosphere is high-purity argon; the heating rate is 5°C min -1 , raised to 600-800℃; during calcination, the air flow rate was 20mL min -1 .

[0012] In step 1), the preparation method of MIL-88(B)-NH2 is:

[0013] N,N-dimethylformamide and an iron source are mixed and evenly mixed, and then 2-aminoterephthalic acid and a sodium hydroxide solution are added. After stirring, the mixture is heated for reaction to obtain MIL-88(B)-NH2.

[0014] In the preparation method of MIL-88(B)-NH2, the amount ratio of the iron source and N,N-dimethylformamide is 0.028-0.032 mol / L, preferably 0.031 mol / L; the molar ratio of the iron source and 2-aminoterephthalic acid is 1:1; the concentration of the added sodium hydroxide solution is 2M, the volume ratio of N,N-dimethylformamide and sodium hydroxide solution is 35-40:1, and the iron source is selected from ferric chloride hexahydrate; the stirring is carried out, and the solution turns dark brown after stirring for 0.5 hours; the heating reaction is carried out, the mixture is loaded into a Teflon reactor, and the reaction is heated at 120°C in an oven for 8 hours; after the reaction is completed, the reactor is completely cooled, the product is collected by centrifugation, and washed alternately with deionized water and anhydrous ethanol three times each, and the product is dried at 60°C overnight, and the product turns brown.

[0015] In step 1), the heated pickling process uses N-Fe3O4@C and an acid solution in a ratio of 1:30 g / mL; the acid solution used is concentrated hydrochloric acid with a mass concentration of 36%.

[0016] In step 1), the conditions for the heated pickling are: heating at 120° C. for 48 h;

[0017] In step 1), heated pickling is performed in a Teflon reactor. After the heated pickling is completed, the reactor is completely cooled and the product is collected by centrifugation. The product is washed with deionized water until the pH of the clear solution is 7, and finally washed once with anhydrous ethanol. The product is placed in a 60° C. oven and dried overnight to obtain N-MPC.

[0018] In step 2), the mass ratio of N-MPC to sulfur is 1:1;

[0019] In step 2), the heating reaction refers to: heating at 155°C for 15h under vacuum conditions, cooling, and then heating at 5°C / min -1 Heat to 200°C, nitrogen range, gas flow rate 20mL min -1 After 0.5h, the sulfur on the surface of the material is removed to obtain the protective gas flow which is 99.999% pure argon.

[0020] The present invention first adopts 155℃ treatment for 15 hours because the solid orthorhombic sulfur powder will melt into liquid, at which point its viscosity is lowest, which is most conducive to the molten sulfur penetrating into the matrix material under the action of capillary force. The treatment at 200℃ for 0.5 hours is to remove the unmelted sulfur on the surface of the material.

[0021] The present invention provides a MOF precursor nitrogen-doped porous carbon / sulfur composite material, which is prepared by the above method. The morphology of the MOF precursor nitrogen-doped porous carbon / sulfur composite material is a multi-porous hexagonal prism. The nanoscale hollow pores distributed in the material are cross-linked with each other. The multi-level structure composed of macropores >50nm and micropores <2nm is conducive to improving the sulfur loading efficiency during the electrochemical reaction, and is also conducive to the penetration of the electrolyte and improving the ion transmission rate. The specific surface area of ​​N-MPC reaches 225m 2 g -1 .

[0022] The nanoscale hollow pores distributed in the material of the present invention are cross-linked with each other in the material. This multi-level structure composed of macropores (>50nm) and micropores (<2nm) is conducive to improving the sulfur loading efficiency during the electrochemical reaction. The macropores can provide sufficient space to buffer the volume expansion generated during the discharge process, and the small pores can effectively inhibit the occurrence of the polysulfide shuttle effect; at the same time, this structure is also conducive to the penetration of the electrolyte and improves the ion transfer rate. During the melt diffusion process, the S8 molecules diffuse into the microporous space of the material, so that the soluble polysulfides formed during the electrochemical reaction are confined to this space, achieving the effect of physical restriction. This nitrogen-doped multi-level pore cross-linked structure is an ideal carrier material for active sulfur in RT-Na / S batteries.

[0023] The present invention provides an application of a MOF precursor nitrogen-doped porous carbon / sulfur composite material for sodium batteries.

[0024] The method first prepares the precursor MIL-88(B)-NH2. After calcination, the iron in the MOF is oxidized to Fe3O4, resulting in nitrogen-doped carbon-coated Fe3O4 nanoparticles. Finally, the Fe3O4 nanoparticles within the carbon shell are etched by acid washing to obtain N-MPC. Sulfur powder is then incorporated into the micropores of the porous carbon via a melt diffusion method. High-temperature calcination evaporates zinc to produce nitrogen-doped porous carbon.

[0025] The chemical equation is:

[0026]

[0027] Compared with the existing technology, the present invention uses MIL-88(B)-NH2 as a precursor for preparing porous carbon. After calcination, the iron element in the MOF is oxidized to Fe3O4, obtaining nitrogen-doped carbon-coated Fe3O4 nanoparticles. Finally, the Fe3O4 nanoparticles in the carbon shell are etched by acid washing to obtain N-MPC; sulfur powder enters the micropores of the porous carbon through the melt diffusion method. The nitrogen-doped porous carbon N-MPC@S is obtained by high-temperature calcination and evaporation of zinc element, and is used as the positive electrode of the RT-Na / S battery. Under the synergistic effect of the porous structure and nitrogen doping, the RT-Na / S battery exhibits excellent battery performance and outstanding rate reversibility, at 0.1A g -1 At a current density of 1000 mA hg, it shows a high cycle capacity, which can reach 1000 mA hg -1 ; At the same time in 2A g -1 Under the high current density, the capacity is still 700mA hg after a long cycle of 3500 times. -1 . BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of the preparation of the N-MPC@S composite material of the present invention;

[0029] Figure 2 SEM images of MIL-88(B)-NH2 prepared in Example 1 of the present invention with different N,N-dimethylformamide solvent dosages and different reaction times; (ab) 40 mL solvent reaction for 8 h; (cd) 40 mL solvent reaction for 10 h; (ef) 40 mL solvent reaction for 12 h; (gh) 30 mL solvent reaction for 8 h;

[0030] Figure 3SEM and TEM images of MIL-88(B)-NH2, N-Fe3O4@C, and N-MPC prepared in Example 1; (ab) are SEM and TEM images of MIL-88(B)-NH2; (cd) are SEM and TEM images of N-Fe3O4@C; (ef) are SEM and TEM images of N-MPC;

[0031] Figure 4 SEM and TEM images of MIL-88 (B), Fe3O4@C, and MPC prepared in Comparative Example 1; (ab) are SEM and TEM images of MIL-88 (B); (cd) are SEM and TEM images of Fe3O4@C; (ef) are SEM and TEM images of MPC;

[0032] Figure 5 Element distribution diagrams of MIL-88(B)-NH2 prepared in Example 1 and MIL-88(B) prepared in Comparative Example 1; (ad) is the element distribution diagram of MIL-88(B)-NH2; (eh) is the element distribution diagram of MIL-88(B);

[0033] Figure 6 is the N-MPC EDS spectrum;

[0034] Figure 7 The XRD spectra, Raman spectra, thermogravimetric analysis spectra, nitrogen adsorption-desorption isotherms and pore size distribution diagrams of the products of Example 1 and Comparative Example 1 are shown; (a) is the XRD spectra during the preparation process; (b) is the Raman spectrum; (c) is the thermogravimetric analysis spectra; (d) is the nitrogen adsorption-desorption isotherms; and (e) is the pore size distribution diagram.

[0035] Figure 8 Energy spectrum analysis of N-MPC; (a) is the full XPS spectrum of N-MPC; (b) is the C1s high-resolution spectrum; (c) is the N1s high-resolution spectrum; (d) is the O1s high-resolution spectrum;

[0036] Figure 9 The positive electrode of RT-Na / S battery was prepared from the product N-MPC@S in Example 1 and the performance test and electrochemical characterization of the half-cell were assembled; (a) shows the electrochemical characterization of N-MPC@S at a scan rate of 0.1 mV s -1 CV curves of N-MPC@S at 100mA g -1 (c) is the constant current charge and discharge curve of N-MPC@S at different current densities; (d) is the rate performance of N-MPC@S; (ef) is the constant current charge and discharge curve of N-MPC@S at 100mA g -1 、1A g-1 Cycling performance under current density; (g) is N-MPC@S at 2A g -1 Long cycle performance at different current densities;

[0037] Figure 10 The voltage curve and ex situ XRD spectrum of N-MPC@S in Example 1, wherein (a) is the voltage curve of N-MPC@S; (b) is the ex situ XRD spectrum;

[0038] Figure 11 The performance test and electrochemical characterization diagram of the RT-Na / S battery prepared with the MPC@S of Comparative Example 1; (a) is the performance test of MPC@S at a scan rate of 0.1 mV s -1 (b) CV curve of MPC@S at 100mA g -1 The first three cycles of constant current charge and discharge curves at the current density; (c) is the electrochemical impedance spectroscopy; (d) is the rate performance of MPC@S; (ef) is the MPC@S at 100mA g -1 , 500mA g -1 Cycling performance under different current densities; (g) is the cycling performance of MPC@S at 1A g -1 Long cycle performance at different current densities;

[0039] Figure 12 Cyclic voltammetry curves of MPC@S and N-MPC@S at different scan rates; (a, d) are the CV curves of MPC@S and N-MPC@S at different scan rates; (b, e) are the log(ν)–log(i) plots corresponding to the peak positions of MPC@S and N-MPC@S; (c, f) are the contributions of pseudocapacitance of MPC@S and N-MPC@S at different scan rates. DETAILED DESCRIPTION

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] Unless otherwise specified, the test materials and reagents used in the following examples can be obtained from commercial sources.

[0042] If no specific techniques or conditions are specified in the examples, they can be carried out according to the techniques or conditions described in the literature in the field or according to the product instructions.

[0043] The reagents used in the present invention are shown in Table 1. The instruments used are shown in Table 2.

[0044] Table 1 Reagents of the present invention

[0045] Reagents Specification Production Unit Ferric chloride analytically pure Sinopharm Chemical Reagent Co., Ltd. Terephthalic acid analytically pure Shanghai Aladdin Biochemical Technology Co., Ltd. 2-Aminoterephthalic acid analytically pure Shanghai Aladdin Biochemical Technology Co., Ltd. N,N-dimethylformamide analytically pure Sinopharm Chemical Reagent Co., Ltd. concentrated hydrochloric acid Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol analytically pure Sinopharm Chemical Reagent Co., Ltd. Sublimed sulfur Chemically pure Sinopharm Chemical Reagent Co., Ltd. Sodium carboxymethyl cellulose analytically pure Shanghai Aladdin Biochemical Technology Co., Ltd. SBR rubber 50％ Nanjing Mojies Energy Technology Co., Ltd. Superconductive carbon black Electronic grade CABOT Corporation of the United States Glass fiber diaphragm GF / D British Waterman Company Sodium trifluoromethanesulfonate Electronic grade Shanghai Aladdin Biochemical Technology Co., Ltd. Diethylene glycol dimethyl ether Electronic grade Shanghai Aladdin Biochemical Technology Co., Ltd. sodium ≥99％ Sinopharm Chemical Reagent Co., Ltd. copper foil Electronic grade Nanjing Mojies Energy Technology Co., Ltd.

[0046] Table 2 Instruments used in the present invention

[0047]

[0048]

[0049] Example 1

[0050] A method for preparing a MOF precursor nitrogen-doped porous carbon / sulfur composite material comprises the following steps:

[0051] 1) Preparation of MIL-88(B)-NH2:

[0052] Measure 30 mL of N,N-dimethylformamide, add 0.25 g of ferric chloride hexahydrate, stir to dissolve, then add 0.17 g of 2-aminoterephthalic acid (PTA-NH2). Add 0.8 mL of 2M NaOH solution, stir for 0.5 h, and the solution turns dark brown. Pour the mixture into a Teflon reactor and heat in an oven at 120°C for 8 h. After the reactor has completely cooled, collect the product by centrifugation, wash it three times with deionized water and then with anhydrous ethanol, and then dry it overnight at 60°C until it turns tan.

[0053] The process was carried out in accordance with step 1) of Example 1, except that 40 mL of N,N-dimethylformamide was used and the heating reaction time was 8, 10, and 12 h. The morphology of the product was detected. The results are as follows: Figure 2 shown.

[0054] Figure 2 (af) show scanning electron microscope (SEM) images of the product morphology obtained when the reaction temperature is 120°C and the amount of N,N-dimethylformamide is 40 mL, and the reaction time is 8-12 h respectively; Figure 2 The SEM image (gh) shows an 8-hour reaction at 120°C with 30 mL of N,N-dimethylformamide. At 40 mL of solvent, the reduced concentration leads to a tendency for smaller particles to form. As the reaction time increases, the particles gradually break due to extrusion. Reducing the solvent dosage, at the same temperature, produces more uniform particles with no breakage.

[0055] 2) Preparation of N-Fe3O4@C:

[0056] The dried MIL-88(B)-NH2 was crushed and placed in a porcelain boat, which was then placed in a tube furnace. After three pumping and three fillings, high-purity argon was introduced for 0.5 h at a heating rate of 5 °C min -1 , calcined at 650℃ for 2h, with an air flow of 20mL min -1 After the tube furnace was completely cooled down, the samples were taken out and they were all magnetic black powder, namely N-Fe3O4@C.

[0057] 3) Preparation of N-MPC:

[0058] The black powder N-Fe3O4@C prepared in step 2) was ground evenly, 1 g was weighed and placed in a 40 mL Teflon reactor, 30 mL of concentrated hydrochloric acid was added, and the mixture was heated at 120°C in an oven for 48 h. After the reactor was completely cooled, the product was collected by centrifugation, washed with deionized water until the clear solution had a pH of 7, and finally washed once with anhydrous ethanol. The product was placed in a 60°C oven and dried overnight to obtain N-MPC.

[0059] 4) Preparation of N-MPC@S:

[0060] Weigh 0.1g of dried N-MPC and 0.1g of sublimed sulfur, grind them evenly, and put them into a quartz tube. Vacuum the tube and seal both ends. Place the tube in an oven and heat it at 155℃ for 15h. After cooling completely, open the tube and place it in a tube furnace. Heat it at 5℃min under a nitrogen protective gas flow. -1 Heat to 200℃, protective gas flow size 20mL min -1 The sulfur on the surface of the material was removed after 0.5 h of working. Finally, a black solid with a slightly pungent odor and a slightly whiter color than before compounding was obtained, which is N-MPC@S.

[0061] like Figure 1 Figure 2 shows the preparation process of N-MPC. The precursor, MIL-88(B)(-NH2), is prepared in a reactor using a simple solvothermal method. After calcination, the iron within the MOF is oxidized to Fe3O4, yielding nitrogen-doped carbon-coated Fe3O4 nanoparticles. Finally, the Fe3O4 nanoparticles within the carbon shell are etched away by acid washing to yield N-MPC. Sulfur powder is incorporated into the micropores of the porous carbon by melt diffusion. Nitrogen-doped porous carbon is then obtained by evaporating zinc through high-temperature calcination.

[0062] The chemical equation is:

[0063]

[0064]

[0065] Comparative Example 1

[0066] A method for preparing a MOF precursor porous carbon / sulfur composite material comprises the following steps:

[0067] 1) Preparation of MIL-88 (B): The 2-aminoterephthalic acid in Example 1 was replaced with 0.156 g of terephthalic acid (PTA). After stirring, the solution turned orange-red. Other preparations were the same as in Example 1. The dried product showed orange-red MIL-88 (B).

[0068] 2) Preparation of Fe3O4@C: The same procedures as in Example 1 were followed except that MIL-88(B)-NH2 was replaced by MIL-88(B).

[0069] 3) Preparation of MPC: The same procedures as in Example 1 were followed except that N-Fe3O4@C was replaced with Fe3O4@C.

[0070] 4) Preparation of MPC@S: MPC@S was prepared according to Example 1 except that N-MPC was replaced by MPC.

[0071] Comparison of detection and characterization of products in each step of Example 1 and Comparative Example 1:

[0072] Figure 3 (a-b) are SEM and TEM photos of MIL-88(B)-NH2. From the photos, we can see that the size of MIL-88(B)-NH2 is about 0.9μm and MIL-88(B)-NH2 is a solid hexagonal prism. Figure 4 As can be seen from the photo, MIL-88(B) is a solid octahedron.

[0073] Figure 3 The middle (cd) are SEM and TEM photos of N-Fe3O4@C. The size is similar to that before calcination. The SEM photo shows that the surface is uneven, not as smooth as before calcination, and there are varying degrees of damage due to calcination; the TEM photo clearly shows the existence of bright and dark fields in the material. The darker color is Fe3O4 particles with poor conductivity, and the lighter color is the carbon layer with relatively good conductivity. The template particles of metal oxide are obtained by calcination.

[0074] Figure 3 Middle (ef) are SEM and TEM images of N-MPC. Figure 4SEM and TEM images of the corresponding MPC in (ef). The SEM image clearly shows further damage to the material after acid etching, while the N-MPC maintains its general morphology. The MPC is nearly destroyed, with uneven pores visible on its surface, a phenomenon consistent with the TEM image. The TEM image of the N-MPC reveals a distributed hollow structure within the material, with these nanoscale pores interconnected within the material. This hierarchical structure, composed of macropores (>50nm) and micropores (<2nm), improves sulfur loading efficiency during electrochemical reactions. The macropores provide ample space to buffer the volume expansion during discharge, while the small pores effectively suppress the polysulfide shuttling effect. This structure also facilitates electrolyte penetration and improves ion transport. Specifically, during the melt-diffusion process, S8 molecules diffuse into the microporous spaces of the material, confining the soluble polysulfides formed during the electrochemical reaction within these spaces, achieving a physical confinement effect. This nitrogen-doped, multi-level porous cross-linked structure is an ideal carrier material for active sulfur in RT-Na / S batteries. The present invention prepared nitrogen-doped and nitrogen-free porous carbon materials. Although the morphology was partially damaged after high-temperature calcination and acid treatment, the multi-level porous structure was still formed. This demonstrates that preparing a carbon-coated oxide template from a MOF precursor and then etching the oxide particles is a reliable method for preparing porous structures.

[0075] like Figure 5 The element distribution diagram shown in (ah) corresponds to MIL-88(B)-NH2 Figure 5 In (ad), the outlines of carbon and nitrogen elements can be clearly observed. The corresponding MIL-88 (B) Figure 5 In (eh), only the outline of carbon elements can be observed, and the distribution of N elements is not observed, indicating that changing the group of the ligand can introduce nitrogen doping into MOF. The EDS energy spectrum corresponding to N-MPC and MPC is shown in Figure 2. Figure 6 As shown in the figure, N-MPC has a characteristic energy spectrum peak of N element in the range of 0.35-0.42KeV, the atomic proportion reaches 6.32%, and the normalized mass reaches 5.78% (the mass proportion of N doped in N-MPC).

[0076] The X-ray diffraction (XRD) of the N-MPC preparation process is as follows Figure 7In (a), MIL-88(B)-NH2 and MIL-88(B) have almost identical spectra, proving that by changing the groups on the ligand, MOF crystals can grow in almost the same pattern, but there are differences in the growth orientation of different crystal faces. After calcination, both formed cubic Fe3O4, space group Fd-3m(227), with distinct diffraction peaks (PDF#97-002-8664). The three strongest peaks at 30.124°, 35.483°, and 62.629° correspond to the (220), (311), and (440) crystal faces, respectively. At the same time, a characteristic diffraction peak of amorphous carbon appears at around 20°. N-MPC obtained by acid etching only has a broad peak of amorphous carbon at around 20° in XRD.

[0077] like Figure 7 (b) shows the Raman spectrum of N-MPC, 1300 cm -1 and 1500cm -1 The two strong peaks correspond to the carbon D band (disordered vibration) and G band (sp 2 In-plane vibration of carbon), by calculating the I D / I G , due to the presence of nitrogen doping in N-MPC, I D / I G The value of is larger than that of MPC, which indicates that more defects are generated due to nitrogen doping. After N-MPC is compounded with sublimed sulfur, it can be clearly observed in the XRD spectrum (PDF#97-002-7261). The strongest peak at 23.124° corresponds to the (222) crystal plane. In the corresponding Raman spectrum, the peak at 469cm -1 、217cm -1 , 150cm -1 Peaks corresponding to sulfur can be observed.

[0078] As shown in the thermogravimetric (TGA) curve of N-MPC@S in Figure 7 (c), a small amount of mass loss occurs during the initial heating phase, likely due to the adsorption of small amounts of water and gases within the material. After 150°C, elemental sulfur evaporates due to its boiling point, reaching a maximum rate around 250°C. Most of the sulfur is completely lost before 300°C, with the subsequent small mass loss due to the volatilization of sulfur that has diffused into the micropores. In contrast, N-MPC exhibits a slower mass loss, likely initially due to a small amount of adsorbed water and gases. Continued heating may be caused by the loss of a small amount of chloride ions that were not completely removed. Furthermore, this is due to incomplete pyrolysis during the material preparation phase, further pyrolysis, and graphitization. Calculations indicate that the sulfur loading of N-MPC@S is approximately 44.5%.

[0079] Figure 7 The figure (d) corresponds to the nitrogen adsorption-desorption isotherm of N-MPC@S. The specific surface area of ​​N-MPC without sulfur composite reaches 225m 2 g -1 During the test, the curve rises rapidly at relatively low pressure (P / P0≤0.015) and relatively high pressure (P / P0=0.4-1), which respectively represent the presence of macropores and micropores. In the intermediate pressure range, the curve also rises slowly, indicating the presence of mesopores in addition to macropores and micropores. After compounding with sulfur, the specific surface area of ​​N-MPC@S drops rapidly to only 9m 2 g -1 .from Figure 7 From the pore size distribution diagram corresponding to (e), it can be analyzed that the pore size before compounding is mainly distributed in the micropore and mesopore sizes; after compounding with sulfur, the micropore part completely disappears and the number of mesopores is significantly reduced, indicating that during the compounding process, sulfur enters the micropores and mesopores, achieving the storage effect.

[0080] The elemental composition and bonding type of N-MPC were studied by X-ray photoelectron spectroscopy (XPS). Figure 8 The full spectrum of (a) can determine the presence of three elements: carbon, nitrogen, and oxygen, as well as a small amount of residual iron. Figure 8 In the high-resolution spectrum of C1s corresponding to (b), the three peaks are located at 287.3eV, 284.8eV, and 283.9eV, corresponding to C=O, CO, and CC bonds, respectively. Figure 8 The high-resolution spectrum of N1s corresponding to (c) can divide nitrogen elements into three types: oxidized nitrogen (404eV), pyrrolic nitrogen (399.8eV), and pyridinic nitrogen (397.5eV), which also matches the results of EDS energy spectrum and mapping characterization. Figure 8 The corresponding O1s high-resolution spectrum in (d) shows that the peaks at 532.5eV, 531.3eV, and 530.1eV correspond to CO, C=O, and Fe3O4, respectively. Polar nitrogen doping facilitates electron transfer and the adsorption of polysulfides by the carrier, thereby effectively improving the capacity.

[0081] Example 2

[0082] An application of a MOF precursor nitrogen-doped porous carbon / sulfur composite material for sodium batteries, specifically comprising:

[0083] S1: Battery Assembly

[0084] After grinding 70 mg of N-MPC@S prepared in Example 1 with 20 mg of conductive carbon black for 0.5 h, 200 mg of 6.54% sodium carboxymethyl cellulose, 0.1 mL of 5% SBR rubber solution, and 0.5 mL of deionized water were added and stirred to a homogenous slurry. The mixture was then coated onto copper foil and dried overnight in a vacuum oven at 60°C. The resulting mixture was then cut into 12 mm diameter circular electrode sheets. The active mass loaded on the electrode sheets was measured on an electronic balance with an accuracy of 1 part per 100,000 and found to be 0.9-1.1 mg. The cut electrode sheets were assembled into CR2032 coin-type half-cells in a high-purity argon glove box (H₂O <0.01 ppm, O₂ <0.01 ppm). A GF / D glass fiber separator was used, and a 1 M sodium trifluoromethanesulfonate solution in diethylene glycol dimethyl ether was used as the electrolyte. The prepared electrode sheet was placed at the positive electrode, while metallic sodium served as the negative electrode.

[0085] S2: Performance Test

[0086] On the Xinwei battery testing system, the operating window was set to 0.5-2.8V to complete constant current charge and discharge, rate performance, constant current intermittent titration (GITT) testing, and the charge and discharge steps required for ex-situ testing. Cyclic voltammetry, electrochemical impedance spectroscopy, ion mobility, and pseudocapacitance tests were completed on the electrochemical workstation.

[0087] The N-MPC@S composite material was used as the positive electrode of the RT-Na / S battery and a half-cell was assembled for performance testing and electrochemical characterization. MPC@S was used as a comparison to analyze the effect of nitrogen doping on the performance. Figure 9 (a) shows N-MPC@S at 0.1mV s -1 At the scan rate, the first five CV curves within the electrochemical window of 0.5-2.8V are taken. There is an obvious reduction peak at around 0.75V in the first cycle. On the one hand, it is due to the side reaction caused by the formation of the SEI film. No peak appears at the same position afterwards, which proves this explanation. On the other hand, it is due to the peak shift caused by irreversible capacity loss, and then stabilizes at 0.9V. In the subsequent scan process, four reduction peaks appear at 2.0V, 1.6V, 1.2V, and 0.9V respectively during the reduction process, and Figure 9 (b) in 100mAg -1The first three cycles of GCD curves under current density correspond to each other. The reduction reaction corresponding to 2.0V is the conversion of S8 to Na2S8. This process is the key step in the occurrence of the shuttle effect of RT-Na / S batteries. Solid Na2S8 is converted to liquid Na2S4, but this reaction process is not obvious in CV and completely disappears in the fourth and fifth cycles, corresponding to the GCD curve. This shows that this conversion only occurs in the first two or three cycles. This is because it is only generated in the first cycle and all dissolved after participating in the first few cycles of reaction. The reduction peaks at 1.6V and 1.2V correspond to the conversion of Na2S4 to various short-chain polysulfides (Na2S x , 2≤x≤4) is very stable in the CV scan, indicating that this reaction continues stably during subsequent cycles. The reduction peak at 0.9 V and the corresponding inflection point in the GCD curve at this voltage correspond to the conversion of Na2S2 to Na2S. In the subsequent oxidation process, the oxidation peak at 1.5 V and the corresponding plateau in the GCD curve correspond to the conversion of various short-chain polysulfides to Na2S4. The oxidation peak around 1.7 V and the inflection point in the GCD curve correspond to the conversion of Na2S4 to Na2S8. The significantly weaker oxidation peak signal here is due to the rapid kinetics in the ether electrolyte. The final oxidation peak around 2.2 V corresponds to the reaction from Na2S8 to S8. No peaks or plateaus for the S8 to Na2S8 reaction appear in the subsequent CV and GCD curves, and the intermediate process from N2S8 to Na2S4 is also very rapid, demonstrating that the reaction proceeds almost in a single step from S8 to Na2S4. The stability and overlap of the corresponding reaction peaks indicate that the electrochemical reaction in this system is fully reversible. like Figure 9 (c) shows the constant current charge and discharge curves at different current densities. It can be observed that the current density increases from 50 mA g -1 Increase to 2A g -1 During the charging and discharging process, the charge and discharge curves can maintain almost the same shape, which proves that the N-MPC@S composite material can also maintain excellent electrochemical performance under different current densities.

[0088] The main tests were conducted on the performance of N-MPC@S composite battery, such as Figure 9 (d) shows its rate performance, showing the current density from 0.05A g -1 , 0.1A g -1 , 0.2A g -1 , 0.5A g -1 、1A g -1 、2A g -1 , and then return to 0.05A g -1The discharge capacity step diagram during the process shows that the discharge capacity decreases with the increase of current density. This is because the faster kinetic process under high current may lead to incomplete electrochemical reaction, resulting in lower capacity. The average capacity in this process reached 1452 mA h g -1 、1192mA hg -1 、1063mA hg -1 、962mA hg -1 、868mA hg -1 、744mA hg -1 、861mA hg -1 、955mA hg -1 、1040mA hg -1 、1102mA hg -1 、1142mA hg -1 The corresponding capacity retention rates are 78.6%, 92.4%, 97.8%, 99.3% and 99.2% respectively. At the same time, the discharge capacity of the first cycle reaches 1715mA hg -1 , which is very close to the theoretical capacity of sulfur, 1763 mA hg -1 , except for the first few cycles, due to the production of a small amount of soluble intermediates, the subsequent cycles showed excellent rate performance, which indicates that the prepared N-MPC@S composite material has excellent performance in adapting to continuously changing current density.

[0089] The N-MPC@S composite material was subjected to constant current cycling tests at different current densities, such as Figure 9 (eg) shown in 100mA g -1 、1A g -1 、2A g -1 The discharge capacity diagram of the constant current cycle at three currents. Figure 9 As shown in (e), at 100mA g -1 After 50 cycles at a current density of 1000 mA hg -1 , and there is a slow upward trend. This is due to the slow addition of sulfur deep inside the material to the electrochemical reaction process. Except for the first few cycles, the coulombic efficiency is always close to 100%, which shows that the increase in discharge capacity is not due to side reactions; 1Ag -1 At this current density, the capacity is still 850mA hg after 350 cycles. -1During the cycle, the capacity remained relatively stable, and the coulombic efficiency was close to 100%. The capacity suddenly increased around 170 cycles and then returned to normal. This was because the ambient temperature rose, which increased the cycle capacity. When the ambient temperature returned to 25°C, the capacity returned to normal. This phenomenon also occurred at 2A g -1 In the long cycle of current density; at 2A g -1 In the long cycle under the current density, N-MPC@S showed an ultra-high cycle life of up to 3500 cycles, and it can still maintain 700mA hg after 3500 cycles. -1 The discharge capacity of the N-MPC@S composite material is approximately 100 cycles high. The capacity increase over the first 100 cycles is due to the gradual participation of deep-seated sulfur in the electrochemical reaction. After this process, the capacity decreases slightly and then stabilizes. Through the above constant current charge and discharge tests, the N-MPC@S composite material exhibits high cycle capacity, good rate performance, excellent cycle life, and excellent capacity retention in terms of battery performance.

[0090] In RT-Na / S batteries, sulfur conversion is carried out in multiple steps. In order to further study the electrochemical reaction process of N-MPC@S composite materials, ex situ XRD characterization was performed. The purpose is to determine the phase change during the electrochemical reaction by measuring the XRD of electrodes in different states and comparing them. The position of the CV corresponding peak was selected, and the battery was discharged / charged to the specified voltage. After disassembling the battery, XRD characterization was performed, such as Figure 10 (a) shows the voltage position selected during the charge and discharge process. Figure 10 In (b), the 41.5° and 50° angles correspond to Cu (PDF#97-005-3758). During discharge, the diffraction peaks of Na2S2 (PDF#89-2753) and Na2S (PDF#77-2149) gradually become more pronounced and numerous. During the subsequent charge process, these peaks gradually weaken and disappear. This corresponds to the final discharge products, Na2S2 and Na2S, demonstrating the complete process of sulfur charge and discharge. During the subsequent charge process, sulfur gradually transforms into amorphous sulfur, resulting in the absence of distinct signal peaks.

[0091] In order to prove the improvement of RT-Na / S battery performance by nitrogen doping, the MPC@S composite material prepared in Comparative Example 1 was also assembled into RT-Na / S battery according to the method of Example 2 to study its electrochemical behavior and battery performance. Figure 11 (a) is MPC@S at 0.1mV s -1Scan rate, the first five CV curves with 0.5-2.8V as the electrochemical window. Only in the first cycle of reduction process, a sharp reduction peak appeared at around 1.3V, which was due to the peak position shift caused by irreversible loss, and then stabilized at around 1.6V. In the subsequent scans, the position of the reduction peak was almost the same as that of N-MPC@S, showing a similar electrochemical process during the reduction process; but in the subsequent charge oxidation process, the oxidation peak at around 1.7V and the platform in the GCD curve corresponded to the conversion of Na2S4 to Na2S8, but the CV peak here often shifted during the scan, which may be due to the irreversible loss of active substances. It is worth noting that MPC@S did not show the S8 to Na2S8 conversion peak at 2.0V. As shown in the figure, Figure 11 From the first three cycles of GCD curves in (b), it can be observed that MPC@S experienced more capacity loss during the first three cycles of charge and discharge, and this loss still occurred from the second to the third cycle, but it did not occur in N-MPC@S, indicating that nitrogen doping has significantly improved the binding ability of polysulfides.

[0092] The differences in impedance between N-MPC@S, MPC@S, N-MPC and MPC are compared. Figure 11 Electrochemical impedance spectroscopy (EIS) in (c) can be used to study their electrochemical kinetics. The semicircular portion of the figure, representing high-frequency oscillations, is related to charge transfer in the electrode, while the straight line portion corresponds to the low-frequency oscillations, which are related to the Warbug resistance of sodium ions. Fitting shows that the charge transfer resistance of N-MPC is approximately 20Ω, slightly larger than the 10Ω of MPC. This is due to the presence of nitrogen doping, which leads to more defects and reduces the conductivity of the material. After N-MPC and MPC are composited with sulfur, the corresponding charge transfer resistance increases, reaching 35Ω and 20Ω, respectively, due to the non-conductive sulfur reducing the conductivity of the carrier.

[0093] Some battery performance tests were also conducted on MPC@S, such as Figure 11 (d) shows its rate performance, showing the current density from 0.05A g -1 , 0.1A g -1 , 0.2A g -1 , 0.5A g -1 、1A g -1 、2A g -1 , and then return to 0.05A g -1 The discharge capacity step diagram during the process shows that the discharge capacity decreases with the increase of current density. This is because the faster kinetic process under high current may lead to incomplete electrochemical reaction, resulting in lower capacity. The average capacity in this process reached 986 mA h g-1 、774mA hg -1 、693mA hg -1 、640mA hg -1 , 523mAh g -1 、450mA hg -1 、510mA hg -1 、585mA hg -1 、658mA hg -1 、717mA hg -1 、786mA hg -1 , the corresponding capacity retention rates are 79.7%, 92.6%, 94.9%, 91.4%, and 97.5%, respectively, showing good rate performance, but the discharge capacity is slightly lower than that of nitrogen-doped N-MPC, and there is a rapid loss of capacity in the first few cycles, which may be due to the irreversible capacity loss caused by the shuttle effect between the load and the surface sulfur. Figure 11 100mAg shown in (e) -1 The cycle test under current density, the first cycle discharge capacity reached 1360mA hg -1 After a large capacity loss in the first few cycles, the capacity stabilized at 950 mA hg after 100 cycles. -1 and has an upward trend during the cycle, which is caused by the gradual participation of deep sulfur in the reaction; Figure 11 500mA g shown in (f) -1 Cycling test under current density, the remaining capacity is about 870mA hg after 650 cycles -1 . Tests with higher currents were conducted, such as Figure 11 1A g shown in (g) -1 The long cycle test under the current density shows that there is a long process of capacity increase, which indicates that the electrolyte infiltration in the MPC is not good. It takes a long time for the deep sulfur to participate in the electrochemical reaction. After that, the capacity continues to decline. From the highest point of capacity at 150 cycles to 1600 cycles, the remaining capacity is about 600mA hg -1 The capacity retention rate was only 83.4%, and the capacity loss rate per cycle reached 0.058%. The above performance tests show that the lack of polar nitrogen in MPC and the lack of binding effect on polysulfides make the cycle stability and capacity retention of MPC unsatisfactory.

[0094] After that, the electrochemical dynamic process of the material was studied, and the diffusion rate of sodium ions (D Na+ ). The formula used is:

[0095]

[0096] In this formula: τ is the set relaxation time, which is 600s in the present invention; mB is the mass of active material loaded on the electrode sheet; V M is the molar volume of the active substance; M B is the relative molar mass of the active substance; S is the area of ​​the electrode sheet, which is 1.131 cm in the present invention. 2 , ΔE S is the voltage change caused by the charge and discharge process, and ΔE t is the voltage change caused by the relaxation time. Usually, since the thickness of the active material after compaction is L<<100μm, if τ is related to L 2 / D GITT The GITT of N-MPC@S was tested at the first, second, and fourth cycles respectively to compare the effect of nitrogen doping on the ion migration rate and the change of GITT during the cycle. GITT The average values ​​in different processes are shown in Table 3.

[0097] Table 3 Average values ​​of DGITT in different processes

[0098]

[0099]

[0100] From the calculated data, it can be seen that only during the first cycle of discharge, D GITT The value of D is small, which is due to the formation of SEI film during the first discharge process. In the subsequent cycles, whether charging or discharging, the measured D GITT The average value difference is not large and remains at a similar level. Compared with N-MPC@S composite materials, the D GITT It is relatively small, which may be due to the fact that nitrogen doping produces more defects, which form more ion transmission channels, and the multi-level porous structure also makes ion transmission easier. In addition, through the distribution of scattered points, it is not difficult to find that at the position of the voltage platform corresponding to the constant current charge and discharge curve, D GITT The value of increases significantly and a more obvious peak shape appears, which may be caused by the large-scale migration of ions during the electrochemical process.

[0101] The N-MPC@S and MPC@S composites were subjected to 0.2 mV s -1 , 0.4mV s -1 , 0.6mV s -1 , 0.8mVs -1 , 1.0mV s -1 These cyclic voltammetry curves at different scan rates. Figure 12 By comparing the CV graphs at different scan rates shown in (a), it is not difficult to find that even at different scan rates, the shapes of the curves can still correspond well, which indicates that the polarization degree of the electrode is low. Generally speaking, in battery devices, there are two behaviors of electrodes to realize charge storage, namely Faraday behavior and capacitance. Faraday behavior is the energy transfer achieved by diffusion reaction in a general sense; the second describes the process of Faraday charge transfer through surface atoms or atoms near the surface (pseudocapacitance) or non-Faraday double layer. Since the induced current (i) is related to the power of the scan rate (ν), the following assumptions can be made, and the system can be analyzed through relevant calculations to determine the electrochemical behavior in the battery:

[0102] i=av b ;

[0103] log(i)=b log(v)+log(a);

[0104] Where a and b are constants, and the value of b can, to some extent, represent the capacity contribution due to capacitive behavior and intercalation behavior. If the b value is close to 1, the battery is controlled by capacitive behavior because the capacitor current is proportional to the scan rate (ν). When the b value is close to 0.5, it means that the battery is controlled by faradaic diffusion behavior, where the current is proportional to the power of ν / 2.

[0105] like Figure 12 In (a), the locations where the electrochemical reactions occur are marked as peak 1 / 2 / 3 / 4 / 5. The values ​​of the corresponding peak positions at different scan rates are fitted, such as Figure 12 The log(ν)–log(i) graph of the peak current is given in (b). The b values ​​obtained by linear fitting the slopes of the peak values ​​of the five peaks are 0.644590, 0.69736, 0.74125, 0.4778, and 0.70913, respectively, which indicates that diffusion behavior dominates the electrochemical reaction process; by error analysis of the fitted data, the variances obtained are 0.98628, 0.99779, 0.99940, 0.99065, and 0.99417, respectively, and the variance values ​​are close to 1, indicating that the data obtained by fitting are highly reliable.

[0106] By dividing the response current (i) at a certain voltage (V) into two parts, namely capacitance contribution and diffusion, the contribution of pseudocapacitive behavior at different scan speeds is further analyzed. e Dun n Research: i(V)=k1v+k2v 1 / 2 ;

[0107] Through calculation, we can know that Figure 12 As shown in (c), at the corresponding scanning speeds, the corresponding pseudocapacitance contributions are 17.9%, 29.1%, 38.8%, 47.6% and 53.5%, respectively, which indicates that the charge stored in the battery of this system mainly comes from the contribution of diffusion behavior.

[0108] like Figure 12 As shown in (df), the MPC@S composite material was tested in the same manner as above. Figure 12 The peaks 1 / 2 / 3 / 4 / 5 marked in (d) are as follows: Figure 12 The log(ν)–log(i) graph of the peak current is given in (e). The b values ​​obtained by calculating the slopes of the five peak fitting lines are 0.40977, 0.78637, 0.33072, 0.72181, and 0.69530, respectively. Compared with the N-MPC@S composite material, the contribution of the pseudocapacitive behavior at the corresponding peak position has increased; by performing error analysis on the fitted data, the variances obtained are 0.99401, 0.99670, 0.98935, 0.99263, and 0.99913, respectively. The variance values ​​are very close to 1, which proves that the fitted data are highly reliable. Figure 12 The pseudocapacitive contributions at different scanning speeds in (f) are 75.1%, 81.2%, 86.9%, 89.2%, and 94.0%, respectively, indicating that pseudocapacitive behavior dominates in this system.

[0109] From the above performance comparison of N-MPC@S and MPC@S, it can be seen that the change in behavior may be due to the large number of defects generated by nitrogen doping in the prepared N-MPC@S composite material. At the same time, there are a large number of porous structures, which construct a large number of channels for ion transport. During the electrochemical reaction, the kinetic process reaction in the electrochemical hardening process is accelerated, and the electrochemical reaction has more capacity contribution at a high rate and has better rate performance.

[0110] In the present invention, nitrogen-doped MOF precursors and porous carbon are obtained by using amino-containing ligands; the composite material prepared by combining the prepared nitrogen-doped porous carbon with sulfur is applied to the positive electrode of the RT-Na / S battery, and compared with the composite material without nitrogen, it is proved that it has a certain effect on binding polysulfides; by changing the ligand to introduce N doping in MOF and forming a comparison, the use of EDS, XPS and other characterizations proves that this method can introduce nitrogen doping in porous carbon; and the results of non-in situ characterization of the electrochemical charge and discharge process show that porous carbon has a physical restriction effect on polysulfides. That is, the synergistic effect generated by the physical structure and polarity is used to inhibit the occurrence of polysulfide "shuttling", which effectively improves the cycle life and capacity retention of the battery. Under the synergistic effect of porous structure and nitrogen doping, excellent battery performance and outstanding rate reversibility are demonstrated in RT-Na / S batteries, at 0.1A g -1 At a current density of 1000 mA hg, it shows a high cycle capacity, which can reach 1000 mA hg -1 ; At the same time in 2A g -1 Under the high current density, the capacity is still 700mA hg after a long cycle of 3500 times. -1 .

[0111] The above embodiments are described to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing a nitrogen-doped porous carbon / sulfur composite material as a MOF precursor for sodium-sulfur batteries, characterized in that: The preparation method comprises the following steps: 1) The dried MIL-88(B)-NH2 was crushed and placed in a porcelain boat, which was then placed in a tube furnace. After three pumping and three fillings, high-purity argon was introduced for 0.5 h at a heating rate of 5 °C min. - 1 , calcined at 650 °C for 2 h, with an air flow of 20 mL min - 1 After the tube furnace has completely cooled down, the samples are taken out, and all of them are magnetic black powders, namely N-Fe3O4@C. The N-Fe3O4@C is ground evenly, and 1 g is weighed and placed in a 40 mL Teflon reactor. 30 mL of concentrated hydrochloric acid is added, and the mixture is heated at 120°C in an oven for 48 h. After the reactor is completely cooled, the product is collected by centrifugation and washed with deionized water until the clear solution has a pH of 7. Finally, the product is washed once with anhydrous ethanol and dried in a 60°C oven overnight to obtain N-MPC. 2) Weigh 0.1 g of dried N-MPC and 0.1 g of sublimed sulfur, grind them evenly, and place them in a quartz tube. Evacuate the tube and seal both ends. Place the tube in an oven and heat it at 155 °C for 15 h. After cooling completely, open the tube and place it in a tube furnace. Heat it at 5 °C min under a nitrogen atmosphere. - 1 Heat to 200 °C, protective gas flow rate 20 mL min - 1 , the sulfur on the surface of the material was removed after working for 0.5 h, and N-MPC@S was obtained.

2. A MOF precursor nitrogen-doped porous carbon / sulfur composite material prepared by the preparation method according to claim 1, characterized in that: The MOF precursor nitrogen-doped porous carbon / sulfur composite material has a morphology of porous hexagonal prisms.

3. An application of the MOF precursor nitrogen-doped porous carbon / sulfur composite material according to claim 2, characterized in that: Used in sodium-sulfur batteries.

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