Monatomic catalyst Co / mofc composite material, preparation method and preparation method of room temperature sodium-sulfur battery anode material thereof

Abstract

The application discloses a single-atom catalyst Co / MOFc composite material, a preparation method and a preparation method of a room-temperature sodium-sulfur battery positive electrode material, and the composite material is expressed as Co-N2O2 / MOFc. The catalyst is synthesized by taking a metal organic framework material (MOF) as a precursor and a single-atom cobalt and two N and two O coordination forms (Co-N2O2). The metal organic framework compound derived carbon material accounts for 98.85-99.04 wt.%, Co accounts for 0.96-1.15 wt.%, and the molar ratio of Co:N:O is 1:2:2. The single-atom cobalt and two N and two O coordination forms (Co-N2O2) catalyst is designed and synthesized by taking the metal organic framework material as the precursor, and the catalyst is firstly used as a sulfur storage material and applied to a room-temperature sodium-sulfur battery positive electrode, so that the electrochemical performance of the room-temperature sodium-sulfur battery is effectively improved.

Description

Technical Field

[0001] This invention relates to a single-atom catalyst Co / MOFc composite material, its preparation method, and its preparation method for a room-temperature sodium-sulfur battery cathode material, belonging to the technical field of room-temperature sodium-sulfur batteries (RT Na-S). Background Technology

[0002] Room temperature sodium-sulfur batteries (RT Na-S) hold promise as a potential alternative to lithium-ion batteries due to their low cost and high energy density. However, despite these advantages, research on RT Na-S batteries is still in its early stages, and several pressing issues hinder their electrochemical performance. These issues primarily include the following: 1-3 :

[0003] (1) Poor conductivity: The sulfur, the starting material of the positive electrode reaction in RT Na-S batteries, and the short-chain sodium polysulfide, a discharge product, have poor conductivity, resulting in low electronic and ionic conductivity in the positive electrode. Simultaneously, the long-chain sodium polysulfide Na2S generated during the discharge process... x (4≤x≤8) The intermediate has high solubility in organic electrolytes and can pass through the membrane to the negative electrode with the electrolyte. This eventually leads to the deposition of some insulating Na2S products on the surface of the negative electrode metallic sodium, forming an insulating interface and reducing the conductivity of the negative electrode.

[0004] (2) Shuttle effect: sulfur and Na + The reaction intermediate, long-chain sodium polysulfide (Na2S), x (4≤x≤8) can dissolve in organic electrolytes and pass smoothly through the membrane, freely shuttling between the positive and negative electrodes. Upon reaching the negative electrode, the long-chain sodium polysulfide reacts directly with metallic Na and is reduced to the short-chain sodium polysulfide Na₂S. x (1≤x<4). As the reaction proceeds, only a portion of the short-chain sodium polysulfides can shuttle back to the sulfur cathode and be oxidized to long-chain sodium polysulfides. This process occurs repeatedly with battery cycling and is called the "shuttle effect." Since only a portion of the long-chain sodium polysulfides that "shuttle" to the negative electrode can be oxidized again to participate in the electrochemical reaction after reduction, the remainder cannot participate in the electrochemical cycle as an active material. This leads to the gradual loss of active materials during charge and discharge, causing the battery capacity to decay during electrochemical cycling and significantly reducing the cycle stability of the RT Na-S battery.

[0005] (3) Volume expansion of elemental sulfur: Due to the large density difference between elemental sulfur and the final product of the electrochemical cycle (Na2S) (2.36 g / cm³, respectively) 3 and 1.86 g / cm 3Therefore, elemental sulfur expands significantly in volume during discharge and gradually returns to its original volume during charging. This expansion and contraction occurs repeatedly at the positive electrode during electrochemical cycling, easily leading to the collapse and damage of the positive electrode material structure, thus significantly deteriorating battery performance.

[0006] To address the aforementioned problems with RT Na-S batteries, researchers have actively explored and improved upon them. The main existing strategy involves using different types of conductive carbon materials (such as carbon nanotubes, hollow carbon spheres, and porous carbon) as sulfur-carrying materials to composite with the active sulfur, thereby enhancing the conductivity of the sulfur cathode. 4-7 Meanwhile, by encapsulating and confining elemental sulfur with porous materials, the volume change of elemental sulfur can be effectively mitigated, stabilizing the structure of the RT Na-S cathode. Some porous materials have a certain adsorption capacity for long-chain sodium polysulfides, which is also very beneficial for reducing the "shuttle effect" of RT Na-S batteries. For example, Wang et al. designed a mesoporous hollow carbon nanosphere sulfur carrier (iMCHS), whose hollow structure can provide sufficient space for sulfur storage and help mitigate the volume change of elemental sulfur during charge and discharge. At the same time, the outer carbon shell can restrict the dissolution of polysulfides, thereby mitigating the influence of the "shuttle effect". Therefore, as the cathode of RT Na-S batteries, S@iMCHS exhibits good cycle stability at 100 mA g -1 The capacity retention rate after 200 cycles at a given current density is ~88.8%. However, due to the limited physical constraints of carbon materials on sodium polysulfides, simply using carbon materials is insufficient to solve the problem of sodium polysulfide dissolution. 8 .

[0007] Introducing catalysts to accelerate the conversion rate of long-chain sodium polysulfides and reduce their residence time in the electrolyte is also an effective way to solve the "shuttle effect." The slow reaction kinetics of the conversion process of long-chain sodium polysulfides lead to their long residence time in the electrolyte, which further exacerbates their dissolution in the electrolyte. Related studies have shown that incorporating catalytic components such as metal nitrides, metal sulfides, metal oxides, metal nanoclusters, and single-atom metals into carbon supports can effectively reduce the energy barrier of reaction intermediates in the conversion of long-chain sodium polysulfides and significantly improve the electrochemical reaction kinetics. 9-14 For example, Qi et al. introduced Fe3N catalyst into a nitrogen-doped carbon network and used it as a sulfur-supporting material. Their results showed that Fe3N effectively promoted the conversion of polysulfides, reduced the battery's "shuttle effect," and improved the battery's cycle stability. The battery achieved a cycle stability of 8375 mAg. -1 It can still maintain a high capacity after 2800 cycles under certain conditions. 15Lai et al. synthesized a series of single-atom catalysts (Pt, Ru, Fe, Ni, Ge, Mn) by modifying polypyrrole (PPy) with sodium toluenesulfonate and applied these "single-atom / carbon" composites to RT Na-S batteries. The results showed that these catalysts can significantly reduce the reaction energy barrier during the conversion of long-chain sodium polysulfides and decrease their solubility in the electrolyte. 16 .

[0008] These improvements have mitigated the problems of RT Na-S batteries to varying degrees. However, further enhancing their electrochemical performance and ultimately achieving commercial application remains a challenging task for researchers.

[0009] References:

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[0019]

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[0020]

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[0021]

[12] Luo,S.N.;Ruan,J.F.;Wang,Y.;Hu,J.M.;Song,Y.;Chen,M.;Wu,L.M.,Flower-Like Interlayer-Expanded MoS 2-x Nanosheets Confined in Hollow CarbonSpheres with High-Efficiency Electrocatalysis Sites for Advanced Sodium-Sulfur Battery.Small 2021,17(37),10.

[0022]

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[0023]

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[0024]

[15] Qi,Y.R.;Li,Q.J.;Wu,Y.K.;Bao,S.J.;Li,C.M.;Chen,Y.M.;Wang,G.X.;Xu,M.W.,A Fe3N / carbon composite electrocatalyst for effective polysulfidesregulation in room-temperature Na-S batteries.Nat.Commun.2021,12(1),6347.

[0025]

[16] Lai,WH;Wang,H.;Zheng,LR;Jiang,Q.;Yan,ZC;Wang,L.;Yoshikawa,H.;Matsumura,D.;Sun,Q.;Wang,YX;Gu,QF;Wang,JZ;Liu,HK;Chou,SL;Dou,SX Room-Temperature Na-S Batteries.Angew.Chem.,Int.Ed.2020,59(49),22171-22178. Summary of the Invention

[0026] The purpose of this invention is to provide a single-atom catalyst Co / MOFc composite material, its preparation method, and a method for preparing a room-temperature sodium-sulfur battery cathode material.

[0027] This invention uses metal-organic framework (MOF) materials as precursors to synthesize a novel single-atom catalyst Co / MOFc composite material, which is then applied for the first time as a sulfur storage material in a room-temperature sodium-sulfur battery. Electrochemical tests show that the composite material of this invention can efficiently catalyze the conversion of polysulfides, prevent polysulfide shuttle and loss, change the slow kinetics of multi-step reactions, and improve the energy storage performance of sodium-sulfur batteries.

[0028] The technical solution of this invention is as follows:

[0029] A single-atom catalyst Co / MOFc composite material, denoted as Co-N2O2 / MOFc, is a catalyst synthesized from a single-atom cobalt in a coordination form of two N and two O atoms (Co-N2O2) using a metal-organic framework (MOF) as a precursor. The metal-organic framework compound-derived carbon material accounts for 98.85–99.04 wt.%, Co accounts for 0.96–1.15 wt.%, and the molar ratio of Co:N:O is 1:2:2.

[0030] A method for preparing a single-atom catalyst Co / MOFc composite material includes the following steps:

[0031] (1) Weigh 0.736 g of 4,4-dimethyl-2,2-bipyridine and 1.032 g of 4,4-diphenyl ether dicarboxylic acid and disperse them in 130 mL of ethanol and 72 mL of NaOH (0.1 mol / L). After ultrasonic treatment, solution A is obtained.

[0032] (2) Dissolve 1 mmol of Zn(NO3)2 and 1 mmol of Co(NO3)2 in 20 mL of ethanol to obtain solution B;

[0033] (3) Place solution A in a round-bottom flask, heat to boiling point, then add solution B, reflux for 30 minutes, and filter under reduced pressure to obtain pink Co-Zn MOF;

[0034] (4) Place the above Co-Zn MOF in a quartz boat, and then carbonize it in a tube furnace at 600°C under argon protection;

[0035] (5) Take out the carbonized sample and place it in the lining of a 20 mL hydrothermal reactor. Add the prepared dilute hydrochloric acid (4 mL HCl + 8 mL H2O). Tighten the hydrothermal reactor and place it in an electric thermostatic drying oven at 85 °C for 5 h to remove ZnO and unfixed Co. After the reaction, filter and dry to obtain Co-N2O2 / MOFc.

[0036] A method for preparing a room-temperature sodium-sulfur battery cathode material includes:

[0037] (1) Grind sulfur powder and Co-N2O2 / MOFc material in a quartz mortar at a mass ratio of 1:1 for a period of time to ensure that they are fully mixed.

[0038] (2) Then add it to a small test tube, sonicate it in an ultrasonic instrument for 10 min, then put it in an oven at 60℃ for 12 h, take it out and put it in an eggplant bottle, heat it to 155℃ and keep it warm for 12 h under argon protection.

[0039] (3) After the sample is removed, it is heated at 700°C for 30 min in a tube furnace under an argon atmosphere to obtain a sulfur-carbon composite material.

[0040] (4) The prepared sulfur-carbon composite material, the binder polyvinylidene fluoride (PDVF) and acetylene black are mixed in a ratio of 7:1.5:1.5, and the solvent N-methylpyrrolidone (NMP) is added. The mixture is stirred in a quartz mortar to form a uniform slurry. The slurry is uniformly coated on copper foil and dried in a vacuum drying oven at 60°C for 12 hours to obtain the positive electrode of the sodium-sulfur battery.

[0041] (5) After cooling, remove the electrode sheet and use a stamping machine to prepare a sulfur-carbon positive electrode sheet (Co-N2O2 / MOFc / S) with a diameter of 12mm. Weigh it using an electronic balance and set it aside for later use.

[0042] An assembly of a room-temperature sodium-sulfur battery, comprising:

[0043] The coin cell was prepared and packaged in a glove box under a high-purity argon atmosphere, where the H2O and O2 contents were both below 0.01 ppm. A sodium sheet was used as the negative electrode, and the electrolyte was 1.0 M NaCF3SO3 dissolved in 100 vol% diethylene glycol dimethyl ether (DEGDME). A Whatman glass fiber separator was used. The battery was assembled using a CR2025 battery case, with the coin cell installed sequentially in the following order: positive electrode case, carbon-sulfur composite positive electrode sheet containing the room-temperature sodium-sulfur battery positive electrode material of this invention, separator, electrolyte, sodium sheet, gasket, spring, and negative electrode case. First, the positive electrode case was placed on the assembly platform, with the sulfur positive electrode sheet placed in the center. Then, the separator was evenly placed on the sulfur positive electrode, a certain amount of electrolyte was added, followed by the sodium negative electrode, gasket, and spring, and then the negative electrode case was attached. Finally, the coin cell was placed in a manual sealing machine and sealed under pressure to form a coin cell. Then, the coin cells were left to stand in a constant temperature and humidity environment for 12 hours before electrochemical performance testing was performed.

[0044] The mechanism and beneficial effects of this invention include:

[0045] This invention uses metal-organic frameworks (MOFs) as precursors to synthesize a novel single-atom catalyst Co-N₂O₂ / MOFc composite material, and applies it as a sulfur storage material in a room-temperature sodium-sulfur battery for the first time. The principle of preparing the single-atom catalyst Co-N₂O₂ / MOFc composite material in this invention is as follows: Figure 1 As shown. Electrochemical tests show that the composite material of this invention can efficiently catalyze the conversion of polysulfides, prevent polysulfide shuttle and loss, change the slow kinetics of multi-step reactions, and improve the energy storage performance of sodium-sulfur batteries.

[0046] Currently reported single-atom catalysts mainly anchor the single-atom active center through the coordination of N. The room-temperature sodium-sulfur battery cathode material of this invention introduces oxygen to regulate the first shell of the single-atom catalyst. Utilizing the different electronegativity and electron configurations of various heteroatoms, the material is modified from multiple angles. Using a metal-organic framework (MOF) as a precursor, a catalyst with a single-atom cobalt coordinated to two N atoms and two O atoms (Co-N₂O₂) was designed and synthesized. This catalyst was then used for the first time as a sulfur storage material in the cathode of a room-temperature sodium-sulfur battery, effectively improving the electrochemical performance of the battery. Attached Figure Description

[0047] Figure 1 : Schematic diagram of the principle of preparing single-atom catalyst Co / MOFc composite material according to the present invention.

[0048] Figure 2 Example 1: X-ray powder diffraction (XRD) pattern of the single-atom catalytic material prepared.

[0049] Figure 3 Scanning electron microscope (SEM) image of the single-atom catalytic material prepared in Example 1.

[0050] Figure 4 Example 1: High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the single-atom catalytic material prepared.

[0051] Figure 5 Example 1: Synchrotron radiation absorption spectrum fitting diagram of the single-atom catalytic material prepared.

[0052] Figure 6 Thermogravimetric analysis (TGA) diagram of the room temperature sodium-sulfur battery cathode material (Co-N2O2 / MOFc / S) prepared in Example 3.

[0053] Figure 7 Example 5: Test and analysis of room temperature sodium-sulfur battery cathode materials (Co-N2O2 / MOFc / S), Example 2 (Co / MOFc), and Comparative Example 1 (MOFc / S) at 0.1 A·g -1 Cyclic performance at current density.

[0054] Figure 8 Example 5: Test and analysis of the rate performance of room temperature sodium-sulfur battery cathode materials (Co-N2O2 / MOFc / S) and Example 2 (Co / MOFc) at different current densities. Detailed Implementation

[0055] Example 1

[0056] A method for preparing a single-atom catalyst Co-N2O2 / MOFc composite material includes the following steps:

[0057] (1) Weigh 0.736 g of 4,4-dimethyl-2,2-bipyridine and 1.032 g of 4,4-diphenyl ether dicarboxylic acid and disperse them in 130 mL of ethanol and 72 mL of NaOH (0.1 mol / L). After ultrasonic treatment, solution A is obtained.

[0058] (2) Dissolve 1 mmol of Zn(NO3)2 and 1 mmol of Co(NO3)2 in 20 mL of ethanol to obtain solution B;

[0059] (3) Place solution A in a round-bottom flask, heat to boiling point, then add solution B, reflux for 30 minutes, and filter under reduced pressure to obtain pink Co-Zn MOF;

[0060] (4) Place the above Co-Zn MOF in a quartz boat, and then carbonize it in a tube furnace at 600°C under argon protection;

[0061] (5) Take out the contents of the 20mL hydrothermal reactor liner and add the prepared dilute hydrochloric acid (4mL HCl + 8mL H2O). Tighten the hydrothermal reactor and place it in an electric thermostatic drying oven at 85℃ for 5 hours to remove ZnO and unfixed Co. After the reaction, filter and dry to obtain Co-N2O2 / MOFc. This composite material is a catalyst synthesized from a metal-organic framework (MOF) material as a precursor, using a coordination form of single-atom cobalt with two N and two O atoms (Co-N2O2). The metal-organic framework compound-derived carbon material accounts for 99.04 wt.%, and Co accounts for 0.96 wt.%; the molar ratio of Co:N:O is 1:2:2.

[0062] like Figure 2 As stated above, within the 10-60° range, Co-N2O2 / MOFc only exhibits obvious diffraction peaks of graphitic carbon at around 23° and 45°, without any Co metal peaks. This indicates that no Co metal is present, thus concluding that Co has high dispersion in the Co-N2O2 / MOFc material.

[0063] like Figure 3 As shown, the SEM images indicate that the Co-N2O2 / MOFc material has the morphology of a three-dimensional nanomaterial resembling a lychee shell.

[0064] like Figure 4 As shown, HAADF-STEM testing indicates that Co in the Co-N2O2 / MOFc composite material is distributed in the carbon matrix in the form of single atoms with a size of approximately 0.2 nm. Figure 5 Synchrotron radiation fitting data for Co-N2O2 / MOFc composite materials.

[0065] from Figure 5 The results show that the main peak has a high degree of fit, and the fitting effect is highly reliable.

[0066] Table 1 shows the fitting results, which indicate that Co and O / N exist in a tetracoordinate form.

[0067] The fitting of the constrained coordination environment shows that the coordination mode of Co with 2 N and 2 O is a reasonable result (see Table 2).

[0068] Table 1. Synchrotron Radiation Fitting Results

[0069]

[0070] Table 2. Fitting results of synchrotron radiation on the Co coordination environment

[0071]

[0072] Example 2

[0073] A method for preparing a single-atom catalyst Co / MOFc composite material includes the following steps:

[0074] (1) Weigh 0.736 g of 4,4-dimethyl-2,2-bipyridine and 1.032 g of 4,4-diphenyl ether dicarboxylic acid and disperse them in 130 mL of ethanol and 72 mL of NaOH (0.1 mol / L). After ultrasonic treatment, solution A is obtained.

[0075] (2) Dissolve 2 mmol of Zn(NO3)2 and 1 mmol of Co(NO3)2 in 20 mL of ethanol to obtain solution B;

[0076] (3) Place solution A in a round-bottom flask, heat to boiling point, then add solution B, reflux for 30 minutes, and filter under reduced pressure to obtain pink Co-Zn MOF;

[0077] (4) Place the above Co-Zn2 MOF in a quartz boat and then carbonize it in a tube furnace at 600°C;

[0078] (5) Take out the 20mL hydrothermal reactor liner and add the prepared dilute hydrochloric acid (4mL HCl + 8mL H2O). After tightening the hydrothermal reactor, put it into an electric thermostatic drying oven and keep it at 85℃ for 5h to remove ZnO. Filter and dry to obtain a single-atom Co / MOFc catalyst with a Co content of 0.5wt%.

[0079] Example 3

[0080] A method for preparing a room-temperature sodium-sulfur battery cathode material includes:

[0081] (1) Grind the sulfur powder and Co-N2O2 / MOFc material in a quartz mortar in a certain ratio (1:1) by mass for a period of time to make them fully mixed and uniform;

[0082] (2) Then add it to a small test tube, sonicate it in an ultrasonic instrument for 10 min, then put it in an oven at 60℃ for 12 h, take it out and put it in an eggplant bottle, heat it to 155℃ and keep it warm for 12 h under argon protection.

[0083] (3) After being removed, it is heated at 700°C for a period of time in a tube furnace under an argon atmosphere to obtain a sulfur-carbon composite material;

[0084] (4) The prepared sulfur-carbon composite material, the binder polyvinylidene fluoride (PDVF) and acetylene black are mixed in a ratio of 7:1.5:1.5, and the solvent N-methylpyrrolidone (NMP) is added. The mixture is stirred in a quartz mortar to form a uniform slurry. The slurry is uniformly coated on copper foil and dried in a vacuum drying oven at 60°C for 12 hours to obtain the positive electrode of the sodium-sulfur battery.

[0085] (5) After cooling, remove the electrode sheet and use a stamping machine to prepare a sulfur-carbon positive electrode sheet (Co-N2O2 / MOFc / S) with a diameter of 12mm. Weigh it using an electronic balance and set it aside for later use.

[0086] like Figure 6 As shown, the weight loss of the Co-N2O2 / MOFc / S cathode material under N2 is mainly concentrated between 200-300℃, which can be attributed to the loss of S. Therefore, the S content in this cathode material can be determined to be 60wt%.

[0087] Example 4

[0088] An assembly of a room-temperature sodium-sulfur battery, comprising:

[0089] The coin cells were prepared and packaged in a glove box under a high-purity argon atmosphere, where the H2O and O2 contents were both below 0.01 ppm. A sodium sheet was used as the negative electrode, and the electrolyte was 1.0 M NaCF3SO3 dissolved in 100 vol% diethylene glycol dimethyl ether (DEGDME). A Whatman glass fiber separator was used. The battery was assembled using a CR2025 battery case, with the coin cell installed in the following order: positive electrode case, carbon-sulfur composite positive electrode sheet, separator, electrolyte, sodium sheet, gasket, spring, and negative electrode case. First, the positive electrode case was placed on the assembly platform, with the sulfur positive electrode sheet placed in the center. Then, the separator was evenly placed on the sulfur positive electrode, a certain amount of electrolyte was added, followed by the sodium negative electrode, gasket, and spring, and then the negative electrode case was attached. Finally, the coin cell was placed in a manual sealing machine and sealed under pressure to form a coin cell. Then, the coin cells were left to stand in a constant temperature and humidity environment for 12 hours before their electrochemical performance was tested in a constant temperature and humidity room at 25°C to determine the catalytic ability of different amounts of single-atom Co on room temperature sodium-sulfur batteries.

[0090] Example 5

[0091] This embodiment characterizes the electrochemical performance of the cathode material for room temperature sodium-sulfur batteries.

[0092] Electrostatic charge-discharge (GCD) measurements were performed on Co-N2O2 / MOFc / S and Co / MOFc / S cathode materials.

[0093] like Figure 7 As shown, Co-N2O2 / MOFc / S at 0.1 A·g -1 The initial discharge specific capacity under the conditions is 807 mA h·g -1 (or calculated based on sulfur mass as 1345 mA h·g) -1 The first-cycle coulombic efficiency (CE) was 84.3%, indicating high sulfur utilization. After 100 cycles, the Co-N2O2 / MOFc / S capacity remained at 590 mA h·g.-1 The capacity is higher than that of Co / MOFc / S (535mAh·g). -1 This demonstrates that a high loading of single-atom Co is more conducive to catalyzing the conversion of sodium polysulfides and immobilizing them, thereby suppressing the "shuttle effect." Meanwhile, in contrast, the capacity of control sample 1 rapidly decayed in the first 20 cycles, with a reversible capacity of only 405 mA h·g at the 100th cycle. -1 This demonstrates that the Co single-atom catalyst plays an effective catalytic role in the conversion reaction of sodium polysulfide.

[0094] like Figure 8 As shown, the rate performance of the Co-N2O2 / MOFc / S cathode is also higher than that of Co / MOFc / S, at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A·g -1 The average capacities at the following values ​​were 649, 612, 578, 521, 464, 429, and 370 mA h·g, respectively. -1 .

[0095] Comparative Example 1

[0096] Unlike Example 1, Comparative Example 1 uses carbon materials without the introduction of a single-atom catalyst. Specifically, following the method of Example 1, only 1 mmol of Zn(NO3)2 was added to prepare a Zn-MOF. The Zn-MOF was then heat-treated with HCl at 120°C for 12 hours to remove the Zn portion, yielding the MOFc material. Then, following the method of Example 3, sulfur was introduced into the MOFc material to prepare the MOFc / S cathode material.

Claims

1. A single-atom catalyst Co / MOFc composite material, characterized in that, The composite material, referred to as Co-N2O2 / MOFc, is a catalyst synthesized from a metal-organic framework material as a precursor, using a single-atom cobalt coordinated with two N and two O atoms in the form of Co-N2O2. The metal-organic framework compound-derived carbon material accounts for 98.85~99.04 wt.%, Co accounts for 0.96~1.15 wt.%, and the molar ratio of Co:N:O is 1:2:

2. The preparation method of the single-atom catalyst Co / MOFc composite material includes the following steps: (1) Weigh 0.736 g of 4,4-dimethyl-2,2-bipyridine and 1.032 g of 4,4-diphenyl ether dicarboxylic acid and disperse them in 130 mL of ethanol and 72 mL of NaOH. After ultrasonic treatment, solution A is obtained. (2) Dissolve 0.8 g of Zn(NO3)2 and 0.4 g of Co(NO3)2 in 20 mL of ethanol to obtain solution B; (3) Place solution A in a round-bottom flask, heat to boiling point, then add solution B, reflux for 30 minutes, and filter under reduced pressure to obtain pink Co-Zn MOF; (4) Place the above Co-Zn MOF in a quartz boat, and then carbonize it in a tube furnace at 600 °C under argon protection; (5) Take out the carbonized sample and put it into the lining of a 20 mL hydrothermal reactor. Add 12 mL of prepared dilute hydrochloric acid. Tighten the hydrothermal reactor and put it into an electric thermostatic drying oven at 85℃ for 5 h to remove ZnO and unfixed Co. After the reaction, filter and dry to obtain Co-N2O2 / MOFc.

2. The single-atom catalyst Co / MOFc composite material according to claim 1, characterized in that, The molar concentration of NaOH mentioned in step (1) is 0.1 mol / L.

3. The single-atom catalyst Co / MOFc composite material according to claim 1, characterized in that, The concentration of the dilute hydrochloric acid mentioned in step (5) is 50 vol.

4. A method for preparing a room-temperature sodium-sulfur battery cathode material, characterized in that, Includes the following steps: (1) Grind the sulfur powder and the single-atom catalyst Co / MOFc composite material as described in claim 1 in a quartz mortar at a mass ratio of 1:1 for a period of time to make them fully and evenly mixed; (2) Then add it to a small test tube, disperse it by ultrasonication in an ultrasonic instrument, dry it in an oven at 60 ℃, take it out and put it in an eggplant bottle, heat it to 155 ℃ under argon protection and keep it warm for a certain time; (3) After the sample is taken out, it is heated at 700°C for a certain time in a tube furnace under an argon atmosphere to obtain a sulfur-carbon composite material. (4) The prepared sulfur-carbon composite material, the binder polyvinylidene fluoride PDVF and acetylene black are mixed in a ratio of 7:1.5:1.5, and the solvent N-methylpyrrolidone NMP is added. The mixture is stirred in a quartz mortar to form a uniform slurry. The slurry is uniformly coated on copper foil and dried in a vacuum drying oven to obtain the positive electrode of the sodium-sulfur battery. (5) After cooling, remove the electrode and use a stamping machine to prepare a sulfur-carbon positive electrode Co-N2O2 / MOFc / S with a diameter of 12 mm.

5. The preparation method according to claim 4, characterized in that, In step (2), the ultrasonic dispersion time is 10 min.

6. The preparation method according to claim 4, characterized in that, In step (2), the drying time in the oven and the heat preservation time under argon are both 12 h.

7. The preparation method according to claim 4, characterized in that, In step (3), the heating time is 30 minutes.

8. The preparation method according to any one of claims 4-7, characterized in that, In step (4), the product is dried in a vacuum drying oven at 60 °C for 12 h.

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