A preparation method of a borate-etched flower-shaped bimetallic loaded porous carbon material for a lithium-sulfur battery

Abstract

The application relates to a preparation method of a borate-etched flower-shaped bimetallic loaded porous carbon material for a lithium-sulfur battery, and relates to a preparation method of a lithium-sulfur battery positive electrode material. The application aims to solve the problems of slow sulfur positive electrode redox reaction rate and shuttle effect in the lithium-sulfur battery. The method is as follows: first, preparation of a nickel-cobalt bimetallic organic framework compound (NiCo-ZIF8); second, preparation of nickel-cobalt bimetallic atom fixed nitrogen-doped porous carbon (NiCo / NPC); and third, preparation of a nickel-cobalt bimetallic atom-based composite positive electrode material (S / NiCo / NPC). The application uses cobalt nitrate hexahydrate and nickel nitrate hexahydrate as metal salts, 2-methyl imidazole as an organic ligand, and boric acid as a pore-forming agent, and through a one-step pyrolysis method, a nickel-cobalt doped flower-shaped porous carbon is prepared, thereby effectively solving the problems of slow sulfur positive electrode redox rate, positive electrode volume expansion and shuttle effect in the lithium-sulfur battery. Meanwhile, the application proposes a spin delocalized electron effect control, which represents a new strategy for promoting bidirectional catalysis of a lithium-sulfur system. The raw materials used in the application are rich and low in cost, and the preparation process is simple and easy to implement, so the application has great commercial application potential.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur battery positive electrode materials, and in particular to a method for preparing a boric acid-etched flower-shaped bimetallic-loaded porous carbon material for lithium-sulfur batteries. Background Art

[0002] Lithium-sulfur batteries have high energy density (2600Wh kg -1 ) and low cost. However, during discharge, the electrochemical reaction at the sulfur cathode involves multiple conversion steps. Polysulfides produced by the dissolution of the active material migrate to the lithium anode under the influence of a concentration gradient, forming insoluble short-chain lithium sulfides. During charging, these polysulfides migrate back to the cathode. The shuttling effect caused by this reciprocating behavior severely depletes the cathode material, and the slow conversion kinetics also cause the actual energy density of lithium-sulfur batteries to fall far below its theoretical value. Therefore, it is crucial to find suitable catalysts to accelerate the bidirectional catalytic sulfur redox process and effectively improve the reaction kinetics. In recent years, various catalytic concepts have been introduced into lithium-sulfur cathodes to enhance the activity of the sulfur cathode. Among them, atomic-scale catalysts have been widely used in lithium-sulfur batteries due to their high catalytic activity. However, single-atom catalysts still suffer from poor selectivity. Therefore, rationally adjusting the active sites at the atomic scale is key to improving the selective catalytic performance of the electrode. Constructing catalytic materials using diatomic sites is feasible. However, existing reports have only confirmed the catalytic mechanism of lithium-sulfur from the perspective of diatomic selective catalysis, and little research has been conducted on the electronic state and microstructure between the active sites and sulfur. The spin polarization and electron transfer capabilities of diatomic centers facilitate the dynamic transformation of sulfur and provide a continuous site for the complex transformation process of sulfur. Spin-polarized catalysis has attracted widespread attention in the field of electrochemistry as an effective catalytic strategy. Researchers have widely recognized that spin polarization of metal active sites is very effective in accelerating the spin state transition between intermediates in chemical reactions. In particular, spin-polarized metal active sites can promote quantum spin exchange interactions and provide pathways for spin electron transfer in electrocatalytic reactions. Due to the excellent spin polarization and multi-electron transfer capabilities of dimetallic centers, dimetallic active sites have been widely studied. In addition, dimetallic atomic sites can selectively catalyze and improve the electrochemical performance of lithium-sulfur. Therefore, the preparation of a diatomic catalyst that can effectively increase the electrochemical reaction rate of the sulfur cathode is the key to enhancing the cycling stability and energy density of lithium-sulfur batteries. Summary of the Invention

[0003] The main purpose of the present invention is to provide a method for preparing a boric acid-etched flower-shaped bimetallic-loaded porous carbon material for lithium-sulfur batteries, so as to overcome the problem of slow reaction rates of oxidation and reduction reactions of the sulfur positive electrode of existing lithium-sulfur batteries and effectively suppress the shuttle effect.

[0004] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0005] A method for preparing a boric acid-etched flower-shaped bimetallic-loaded porous carbon material for lithium-sulfur batteries comprises the following steps:

[0006] 1. Preparation of Nickel-Cobalt Bimetallic Organic Framework (NiCo-ZIF8)

[0007] 7.50-8.00 mmol zinc nitrate hexahydrate and 30.00-33.00 mmol 2-methylimidazole were dissolved in 80 ml of a mixed solvent (ethanol / N,N-dimethylformamide, volume ratio 1:1) and magnetically stirred for 30 minutes; 0.45-0.60 mmol nickel nitrate hexahydrate and 0.15-0.25 mmol cobalt nitrate hexahydrate were added and stirred for another 30 minutes, and then 7.90-8.20 mmol boric acid was added and stirred for another 15-20 minutes to obtain a suspension; the resulting solution was transferred to an autoclave and heated to 150-190°C for 12 hours; after cooling to room temperature, centrifuged, washed repeatedly with ethanol and water 5-10 times, and then dried in an oven at 80°C for 24 hours to obtain white powder NiCo-ZIF8;

[0008] 2. Preparation of NiCo diatoms fixed on nitrogen-doped porous carbon (NiCo / NPC)

[0009] The NiCo-ZIF8 obtained in step 1 was placed in a porcelain boat and heated at 40 mL min -1 The porcelain boat was then heated to 850-950°C (heating rate of 3-5°C min-1) in a nitrogen atmosphere. -1 ) annealed for 6 hours; the black powder obtained after cooling was washed several times with water and ethanol, and then dried in an oven at 80°C for 24 hours to obtain NiCo / NPC;

[0010] 3. Preparation of Nickel-Cobalt Diatom-Based Composite Cathode Material (S / NiCo / NPC)

[0011] Sulfur and NiCo / NPC obtained in step 2 were mixed in a ratio of 7:3 (wt:wt) and ground in a ball mill for 2-5 hours; the resulting material was then heated in an oven at 155° C. for 12 hours to obtain a S / NiCo / NPC composite material.

[0012] The amount of zinc nitrate hexahydrate in step 1 is 7.50-8.00 mmol, and the amount of 2-methylimidazole is 30.00-33.00 mmol. The ratio of zinc nitrate hexahydrate to 2-methylimidazole is crucial to the product morphology. Too little zinc nitrate hexahydrate may prevent the metal framework from forming; too much zinc nitrate hexahydrate may prevent the zinc template from being completely removed.

[0013] Furthermore, the amount of nickel nitrate hexahydrate in step 1 is 0.45-0.60 mmol, and the amount of cobalt nitrate hexahydrate is 0.15-0.25 mmol.

[0014] Furthermore, the amount of boric acid in step 1 is 7.90-8.20 mmol. Too little boric acid may result in insufficient etching, and the product may not exhibit a flower-like porous structure; excessive boric acid may result in large and uneven pores, failing to provide suitable metal active sites.

[0015] Furthermore, the stirring time after adding boric acid in step 1 is 15-20 minutes. If the stirring time is insufficient, the boric acid may be unevenly distributed, thereby affecting the uniform porous morphology of the etched product; if the stirring time is too long, the probability of side reactions may increase, affecting the purity of the product.

[0016] Furthermore, the heating temperature of the reactor in step 1 is 150-190°C.

[0017] Furthermore, the number of repeated washings with ethanol and water in step 1 is 5-10 times. Too few washings may result in the inability to completely remove the remaining boric acid impurities; too many washings may cause changes in the surface properties of the material.

[0018] Furthermore, the heating temperature in step 2 is 850-950° C. The boiling point of zinc is 907° C. Too low a temperature may result in incomplete removal of zinc impurities, while too high a temperature may cause the carbon skeleton to collapse.

[0019] Furthermore, the heating rate in step 2 is 3-5°C min -1 A heating rate that is too slow may lead to side reactions; a heating rate that is too fast may result in uneven pore structure.

[0020] Furthermore, the grinding time in step 3 is 2-5 hours. Too short a grinding time may result in the sulfur and the prepared carbon material not being evenly mixed; too long a grinding time may destroy the flower-like porous structure of the prepared carbon material.

[0021] Based on the above technical solutions, the following beneficial effects are achieved:

[0022] By designing a diatomic spin polarization modulation strategy, this paper proposes a method for preparing a boric acid-etched flower-shaped bimetallic-loaded porous carbon material for lithium-sulfur batteries. The Ni-Co diatomic catalyst is immobilized on nitrogen-doped porous carbon, enabling high-performance lithium-sulfur battery applications. First, using an organic metal framework (OMF) as a template and boric acid chemical etching, the present invention successfully prepared a uniform flower-shaped porous carbon material. The porous structure provides abundant active sites for Ni-Co diatomic doping, while the nitrogen-doped carbon framework provides a continuous electron transport channel, further enhancing the material's conductivity. This porous structure also helps reduce volume expansion of the positive electrode and allows for high sulfur loading. Second, the present invention utilizes the spin polarization effect of the Ni-Co diatoms to construct an electron spin channel, effectively promoting electron transport and exchange during the electrochemical reaction. The coupling of the Ni-Co diatoms modifies the distribution of electron spin states, increasing the number of spin-polarized electrons. During the reaction, the formation of the spin channel enhances dp orbital hybridization between the Ni-Co diatoms and sulfur, reducing the activation energy of the Li-S redox reaction. The spin polarization effect not only improves the adsorption capacity of the catalytic material for intermediate reactants, but also further promotes dp orbital hybridization, thereby significantly accelerating the reaction kinetics of the battery and improving its cycle performance. In addition, the present invention enhances the electronic interaction between the nickel and cobalt atoms and the reaction intermediates by regulating the d-band centers. The upward shift of the d-band center gives the nickel-cobalt diatomic catalyst more unoccupied 3d orbitals, which can better improve adsorption with polysulfides and reduce the reaction activation energy. This d-band regulation strategy significantly improves the selectivity and catalytic activity of the catalytic material, effectively promotes charge transfer and reaction kinetics in lithium-sulfur batteries, and ultimately improves the battery's energy density and cycle stability. Finally, the present invention achieves bidirectional catalysis of redox reactions (SRR and SER) in lithium-sulfur batteries. The Ni-Co diatomic site can accelerate the redox kinetics of sulfur by enhancing electron transfer and multi-electron transfer capabilities, thereby improving the reaction rate and cycle stability of the battery and suppressing the polysulfide shuttling effect. This bidirectional catalytic mechanism effectively improves the battery's energy density and charge-discharge efficiency. In summary, the synergistic effect of Ni-Co diatoms and their nitrogen-doped carbon skeleton gives this material significant advantages in inhibiting the polysulfide shuttle effect and improving the capacity and cycle stability of lithium-sulfur batteries. Specific performance tests show that the initial discharge capacity of this lithium-sulfur battery at a rate of 0.1 is as high as 1186.4 mAh g -1 After 500 cycles, the capacity decay rate per cycle was only 0.06%, showing excellent long-cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the modified results of the embodiments of the present invention, the accompanying drawings required for describing the comparative examples and embodiments will be briefly introduced below.

[0024] Figure 1 Schematic diagram of the synthesis mechanism of nickel and cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0025] Figure 2 This is a scanning electron micrograph of nickel-cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0026] Figure 3 This is a transmission electron micrograph of nickel-cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0027] Figure 4 This is a selected area electron diffraction pattern of nickel and cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0028] Figure 5 This is an X-ray diffraction spectrum of nickel and cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0029] Figure 6 This is the X-ray diffraction spectrum of nickel single atoms fixed on nitrogen-doped porous carbon materials prepared in Comparative Example 1;

[0030] Figure 7 This is the X-ray diffraction spectrum of the cobalt single atom fixed on the nitrogen-doped porous carbon material prepared in Comparative Example 2;

[0031] Figure 8 This is a Raman spectrum of nickel-cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0032] Figure 9 This is a Raman spectrum of nickel single atoms fixed on nitrogen-doped porous carbon materials prepared in Comparative Example 1;

[0033] Figure 10 This is a Raman spectrum of a cobalt single atom fixed on a nitrogen-doped porous carbon material prepared in Comparative Example 2;

[0034] Figure 11 Full scan X-ray photoelectron spectroscopy of nickel and cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0035] Figure 12 This is a C1s graph of the X-ray photoelectron spectrum of nickel and cobalt diatoms fixed on the nitrogen-doped porous carbon material prepared in Example;

[0036] Figure 13 This is the N1s graph of the X-ray photoelectron spectrum of nickel and cobalt diatoms fixed on the nitrogen-doped porous carbon material prepared in Example;

[0037] Figure 14The Tafel curves of different positive electrodes of lithium-sulfur batteries prepared and assembled in Example, Comparative Example 1 and Comparative Example 2 are shown. A is the Tafel curve of Example, B is the Tafel curve of Comparative Example 1, and C is the Tafel curve of Comparative Example 2.

[0038] Figure 15 Cyclic voltammetry curves of lithium-sulfur batteries prepared and assembled for Example, Comparative Example 1, and Example 2, A is the cyclic voltammetry curve of Example, B is the cyclic voltammetry curve of Comparative Example 1, and C is the cyclic voltammetry curve of Comparative Example 2;

[0039] Figure 16 This is a rate performance diagram of the lithium-sulfur battery assembled in Example;

[0040] Figure 17 Figure 3 is a graph of the cycle performance of lithium-sulfur batteries assembled in Example, Comparative Example 1, and Comparative Example 2 at a rate of 0.5, where A is the cycle performance of Example, B is the cycle performance of Comparative Example 1, and C is the cycle performance of Comparative Example 2;

[0041] Figure 18 The charge and discharge curves of the lithium-sulfur batteries prepared and assembled are shown in Figures 1 and 2. Figure A shows the charge and discharge curve of the embodiment, B shows the charge and discharge curve of Comparative Example 1, and C shows the charge and discharge curve of Comparative Example 2.

[0042] Figure 19 This is a performance diagram of the lithium-sulfur battery assembled in the embodiment after 500 cycles at 1 rate;

[0043] Figure 20 Impedance diagrams of lithium-sulfur batteries assembled in Example, Comparative Example 1, and Example 2 after 500 cycles. Figure A is the impedance diagram of Example after 500 cycles, Figure B is the impedance diagram of Comparative Example 1 after 500 cycles, and Figure C is the impedance diagram of Comparative Example 2 after 500 cycles.

[0044] Figure 21 The projected density of states (PDOS) calculation diagram of nickel and cobalt diatoms fixed on nitrogen-doped porous carbon materials prepared in Example;

[0045] Figure 22 Figures 1 and 2 are the differential charge density calculation diagrams of NiCo / NPC, Ni / NPC, and Co / NPC prepared in Example, Comparative Example 1, and Comparative Example 2, respectively. Figure A is the differential charge density calculation diagram of Example, Figure B is the differential charge density calculation diagram of Comparative Example 1, and Figure C is the differential charge density calculation diagram of Comparative Example 2.

[0046] Figure 23 Magnetic hysteresis loops of NiCo / NPC, Ni / NPC and Co / NPC prepared in Example, Comparative Example 1 and Comparative Example 2, respectively. A is the hysteresis loop of Example, B is the hysteresis loop of Comparative Example 1, and C is the hysteresis loop of Comparative Example 2.

[0047] Figure 24 Gibbs free energy curves of NiCo / NPC, Ni / NPC and Co / NPC prepared in Example, Comparative Example 1 and Comparative Example 2, respectively. A is the Gibbs free energy curve of Example, B is the Gibbs free energy curve of Comparative Example 1, and C is the Gibbs free energy curve of Comparative Example 2. DETAILED DESCRIPTION

[0048] The above contents of the present invention are further described in detail below through examples, but the subject matter of the present invention is not limited to the following examples, and all technologies realized based on the above contents of the present invention belong to the scope of the present invention.

[0049] Experimental drugs

[0050]

[0051] Experimental equipment

[0052]

[0053] The above contents of the present invention are further described in detail below through examples, but the subject matter of the present invention is not limited to the following examples, and all technologies realized based on the above contents of the present invention belong to the scope of the present invention.

[0054] Example:

[0055] 1. Preparation of Nickel-Cobalt Bimetallic Organic Framework (NiCo-ZIF8)

[0056] 7.50-8.00 mmol zinc nitrate hexahydrate and 30.00-33.00 mmol 2-methylimidazole were dissolved in 80 ml of a mixed solvent (ethanol / N,N-dimethylformamide, volume ratio 1:1) and magnetically stirred for 30 minutes; 0.45-0.60 mmol nickel nitrate hexahydrate and 0.15-0.25 mmol cobalt nitrate hexahydrate were added and stirred for another 30 minutes, and then 7.90-8.20 mmol boric acid was added and stirred for another 15-20 minutes to obtain a suspension; the resulting solution was transferred to an autoclave and heated to 150-190°C for 12 hours; after cooling to room temperature, centrifuged, washed repeatedly with ethanol and water 5-10 times, and then dried in an oven at 80°C for 24 hours to obtain white powder NiCo-ZIF8;

[0057] 2. Preparation of NiCo diatoms fixed on nitrogen-doped porous carbon (NiCo / NPC)

[0058] The NiCo-ZIF8 obtained in step 1 was placed in a porcelain boat and heated at 40 mL min -1The porcelain boat was then heated to 850-950°C (heating rate of 3-5°C min-1) in a nitrogen atmosphere. -1 ) annealed for 6 hours; the black powder obtained after cooling was washed several times with water and ethanol, and then dried in an oven at 80°C for 24 hours to obtain NiCo / NPC;

[0059] 3. Preparation of Nickel-Cobalt Diatom-Based Composite Cathode Material (S / NiCo / NPC)

[0060] Sulfur and NiCo / NPC obtained in step 2 were mixed in a ratio of 7:3 (wt:wt) and ground in a ball mill for 2-5 hours; the resulting material was then heated in an oven at 155° C. for 12 hours to obtain a S / NiCo / NPC composite material.

[0061] Comparative Example 1:

[0062] 1. Preparation of Nickel Metal-Organic Framework (Ni-ZIF8)

[0063] 7.50-8.00 mmol of zinc nitrate hexahydrate and 30.00-33.00 mmol of 2-methylimidazole were dissolved in 80 ml of a mixed solvent (ethanol / N,N-dimethylformamide, volume ratio 1:1) and magnetically stirred for 30 minutes; 0.45-0.60 mmol of nickel nitrate hexahydrate was added and stirred for another 30 minutes, followed by 7.90-8.20 mmol of boric acid and stirring for another 15-20 minutes to obtain a suspension; the resulting solution was transferred to an autoclave and heated to 150-190°C for 12 hours; after cooling to room temperature, the solution was centrifuged, washed repeatedly with ethanol and water 5-10 times, and then dried in an oven at 80°C for 24 hours to obtain white powder Ni-ZIF8;

[0064] 2. Preparation of Nickel Single Atoms Fixed on Nitrogen-Doped Porous Carbon (Ni / NPC)

[0065] The Ni-ZIF8 obtained in step 1 was placed in a porcelain boat and heated at 40 mL min -1 The porcelain boat was then heated to 850-950°C (heating rate of 3-5°C min-1) in a nitrogen atmosphere. -1 ) annealed for 6 hours; the black powder obtained after cooling was washed several times with water and ethanol, and then dried in an oven at 80°C for 24 hours to obtain Ni / NPC;

[0066] 3. Preparation of Nickel Single Atom-Based Composite Cathode Material (S / Ni / NPC)

[0067] Sulfur and Ni / NPC obtained in step 2 were mixed in a ratio of 7:3 (wt:wt) and ground in a ball mill for 2-5 hours; the resulting material was then heated in an oven at 155° C. for 12 hours to obtain a S / Ni / NPC composite material.

[0068] Comparative Example 2:

[0069] 1. Preparation of Cobalt Metal-Organic Framework Compound (Co-ZIF8)

[0070] 7.50-8.00 mmol of zinc nitrate hexahydrate and 30.00-33.00 mmol of 2-methylimidazole were dissolved in 80 ml of a mixed solvent (ethanol / N,N-dimethylformamide, volume ratio 1:1) and magnetically stirred for 30 minutes; 0.45-0.60 mmol of cobalt nitrate hexahydrate was added and stirred for another 30 minutes, followed by 7.90-8.20 mmol of boric acid and stirring for another 15-20 minutes to obtain a suspension; the resulting solution was transferred to an autoclave and heated to 150-190°C for 12 hours; after cooling to room temperature, the solution was centrifuged, washed repeatedly with ethanol and water 5-10 times, and then dried in an oven at 80°C for 24 hours to obtain white powder Co-ZIF8;

[0071] 2. Preparation of Cobalt Single Atoms Fixed on Nitrogen-Doped Porous Carbon (Co / NPC)

[0072] The Co-ZIF8 obtained in step 1 was placed in a porcelain boat and heated at 40 mL min -1 The porcelain boat was then heated to 850-950°C (heating rate of 3-5°C min-1) in a nitrogen atmosphere. -1 ) annealed for 6 hours; the black powder obtained after cooling was washed several times with water and ethanol, and then dried in an oven at 80°C for 24 hours to obtain Co / NPC;

[0073] 3. Preparation of Cobalt Single Atom-Based Composite Cathode Material (S / Co / NPC)

[0074] Sulfur and the Co / NPC obtained in step 2 were mixed in a ratio of 7:3 (wt:wt) and ground in a ball mill for 2-5 hours; the resulting material was then heated in an oven at 155° C. for 12 hours to obtain a S / Co / NPC composite material.

[0075] The main difference between Comparative Example 1, Comparative Example 2 and the embodiment is that the metal atoms fixed on the nitrogen-doped porous carbon are different. The composite materials synthesized in Comparative Example 1, Comparative Example 2 and the embodiment and the lithium-sulfur batteries assembled therefrom were characterized as follows:

[0076] Figure 1Schematic diagram of the synthesis mechanism of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. With cobalt nitrate hexahydrate and nickel nitrate hexahydrate as metal salts, 2-methyl imidazole as organic ligand, boric acid as pore forming agent, nickel-cobalt doped hierarchical porous carbon was prepared by one-step pyrolysis method.

[0077] Figure 2 Scanning electron microscope image of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. The scanning electron microscope image shows that NiCo / NPC has hierarchical porous morphology, showing a unique spherical flower-like structure, which provides rich sites for the doping of nickel atoms and cobalt atoms.

[0078] Figure 3 Transmission electron microscope image of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. The transmission electron microscope image further observes that NiCo / NPC has a clear spherical porous structure.

[0079] Figure 4 Selected area electron diffraction pattern of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. The selected area electron diffraction pattern further confirms that NiCo / NPC is a porous carbon sphere, and no lattice fringes are found, confirming that the metal atoms do not have obvious agglomeration.

[0080] Figure 5 X-ray diffraction spectrum of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. The X-ray diffraction pattern of NiCo / NPC presents a graphitized broad diffraction peak at (0 0 2) plane 26°.

[0081] Figure 6 X-ray diffraction spectrum of nickel monatomic atom immobilized in nitrogen-doped porous carbon material prepared for Comparative Example 1. The X-ray diffraction pattern of Ni / NPC presents a graphitized broad diffraction peak at (0 0 2) plane 26°.

[0082] Figure 7 X-ray diffraction spectrum of cobalt monatomic atom immobilized in nitrogen-doped porous carbon material prepared for Comparative Example 2. The X-ray diffraction pattern of Co / NPC presents a graphitized broad diffraction peak at (0 0 2) plane 26°.

[0083] Figure 8 Raman spectrum of nickel-cobalt bimetallic atom immobilized in nitrogen-doped porous carbon material prepared for the example. The Raman spectrum of NiCo / NPC has two different peaks at 1335 cm -1 and 1580 cm -1 , which correspond to the D peak and G peak of the carbon material, respectively.

[0084] Figure 9This is the Raman spectrum of the nickel single atom fixed on the nitrogen-doped porous carbon material prepared in Comparative Example 1. The Raman spectrum of Ni / NPC is at 1335 cm -1 and 1580cm -1 There are two different peaks, corresponding to the D peak and G peak of carbon materials.

[0085] Figure 10 This is the Raman spectrum of the cobalt single atom fixed on the nitrogen-doped porous carbon material prepared in Comparative Example 2. The Raman spectrum of Co / NPC is at 1335 cm -1 and 1580cm -1 There are two different peaks, corresponding to the D peak and G peak of carbon materials.

[0086] Figure 11 This is a full-scan X-ray photoelectron spectrum of the nickel-cobalt diatomic anchored to the nitrogen-doped porous carbon material prepared in this example. Analysis of the full-scan X-ray photoelectron spectrum of the NiCo / NPC reveals the presence of nickel, cobalt, nitrogen, and carbon, confirming that the prepared NiCo / NPC meets expectations.

[0087] Figure 12 This is the C1s spectrum of the X-ray photoelectron spectrum of the NiCo / NPC prepared in Example 1, which shows distinct peaks at 284.8 and 286.2 eV, corresponding to the C—C bond and the C—N bond, respectively.

[0088] Figure 13 This is the N1s spectrum of the X-ray photoelectron spectrum of the nickel-cobalt diatoms fixed on the nitrogen-doped porous carbon material prepared in Example. According to the N1s spectrum of NiCo / NPC, the N1s spectrum of the sample can be decomposed into four types, with different peaks at 400.6, 401.2, 398.6, and 399.8 eV, corresponding to pyridinic-N, graphite-N, metal-N, and pyridinic-N, respectively. These findings confirm that nickel and cobalt are coordinated with nitrogen as one atom to form Ni-Co-N6.

[0089] Figure 14 Figures 1 and 2 show Tafel plots of different positive electrodes for lithium-sulfur batteries prepared and assembled using Example, Comparative Example 1, and Comparative Example 2. A shows the Tafel plot for Example, B shows the Tafel plot for Comparative Example 1, and C shows the Tafel plot for Comparative Example 2. The Tafel plot for the lithium-sulfur battery assembled with S / NiCo / NPC shows that, compared with S / Ni / NPC and S / Co / NPC, the lithium-sulfur battery assembled with NiCo / NPC has the highest exchange current density and the lowest Tafel slope, indicating accelerated sulfur redox conversion.

[0090] Figure 15Cyclic voltammetry curves of lithium-sulfur batteries prepared and assembled for Example, Comparative Example 1, and Comparative Example 2. A is the cyclic voltammetry curve of Example, B is the cyclic voltammetry curve of Comparative Example 1, and C is the cyclic voltammetry curve of Comparative Example 2. The figure shows the cyclic voltammetry curve at a scan rate of 0.1 mV s -1 The cyclic voltammetry curves of different lithium-sulfur batteries assembled with three materials, S / NiCo / NPC, S / Ni / NPC and S / Co / NPC, are shown. Due to the sufficient and rapid electrochemical redox reaction of sulfur, the S / NiCo / NPC cathode has the highest current density and the smallest polarization voltage gap.

[0091] Figure 16 The figure shows the rate performance of the lithium-sulfur battery assembled in the example. The maximum discharge capacity of the lithium-sulfur battery assembled with S / NiCo / NPC at 0.1, 0.2, 0.5, 1 and 2 rates were 1253.6, 1046.2, 945.0, 885.3 and 727.0 mAh g -1 When the rate is restored to 0.5 times, 944.3 mAh g -1 High capacity and good reversibility.

[0092] Figure 17 Figures 1 and 2 show the cycling performance of lithium-sulfur batteries assembled at 0.5x rate, with A showing the cycling performance of the embodiment, B showing the cycling performance of comparative example 1, and C showing the cycling performance of comparative example 2. Compared to S / Ni / NPC and S / Co / NPC, S / NiCo / NPC exhibits better cycling stability, significantly lower capacity decay, and higher initial capacity.

[0093] Figure 18 The charge and discharge curves of the lithium-sulfur batteries prepared and assembled in Example, Comparative Example 1, and Comparative Example 2 are shown in Figure 1. A is the charge and discharge curve of Example, B is the charge and discharge curve of Comparative Example 1, and C is the charge and discharge curve of Comparative Example 2. Compared with S / Ni / NPC and S / Co / NPC, S / NiCo / NPC has an initial discharge capacity of up to 1099 mAh g at a rate of 0.5. -1 , the overpotential is also smaller.

[0094] Figure 19 The performance diagram of the lithium-sulfur battery assembled in the embodiment after 500 cycles at 1 rate. The lithium-sulfur battery assembled with S / NiCo / NPC provides 1186.4 mAh g -1 The high initial capacity and capacity retention rate after 500 cycles are 78%.

[0095] Figure 20Impedance plots of lithium-sulfur batteries assembled and prepared for Example, Comparative Example 1, and Comparative Example 2 after 500 cycles are shown. Figure A shows the impedance plot for Example after 500 cycles, Figure B shows the impedance plot for Comparative Example 1 after 500 cycles, and Figure C shows the impedance plot for Comparative Example 2 after 500 cycles. The EIS impedances of the different cathodes after cycling indicate that the S / NiCo / NPC cathode has a lower charge transfer impedance after 500 cycles, as the redox reaction at the sulfur cathode is well maintained with increasing cycle number.

[0096] Figure 21 This is the projected density of states (PDOS) calculated for the nickel-cobalt diatoms immobilized on nitrogen-doped porous carbon prepared in Example 1. The 3d orbitals of nickel and cobalt in NiCo / NPC strongly resonate near the Fermi level, indicating dd orbital coupling between Ni and Co.

[0097] Figure 22 Figures 1 and 2 show the calculated differential charge density of NiCo / NPC, Ni / NPC, and Co / NPC prepared in Example, Comparative Example 1, and Comparative Example 2, respectively. Figure A shows the calculated differential charge density of Example, B shows the calculated differential charge density of Comparative Example 1, and C shows the calculated differential charge density of Comparative Example 2. Compared to Ni / NPC and Co / NPC, the 3d electrons of nickel and cobalt in NiCo / NPC are delocalized and attracted by the surrounding nitrogen atoms, forming electron-rich regions. This is due to the partial transfer of nickel and cobalt electrons to the coordinating nitrogen atoms.

[0098] Figure 23 The hysteresis loops of NiCo / NPC, Ni / NPC and Co / NPC prepared in Example, Comparative Example 1 and Comparative Example 2, respectively, are shown in Figure 1. A is the hysteresis loop of Example, B is the hysteresis loop of Comparative Example 1, and C is the hysteresis loop of Comparative Example 2. The saturation magnetization of NiCo / NPC is 4emu g -1 , which is 20 times higher than that of Ni / NPC and Co / NPC, because NiCo / NPC has more spin electrons.

[0099] Figure 24 Figures 2 and 3 show the Gibbs free energy curves for NiCo / NPC, Ni / NPC, and Co / NPC prepared in Example, Comparative Example 1, and Comparative Example 2, respectively. A shows the Gibbs free energy curve for Example, B shows the Gibbs free energy curve for Comparative Example 1, and C shows the Gibbs free energy curve for Comparative Example 2. The ΔG value for the conversion of Li2S2 to Li2S on NiCo / NPC is 0.820 eV, compared to 0.894 eV for Ni / NPC and 0.990 eV for Co / NPC. NiCo / NPC exhibits the lowest Gibbs free energy, demonstrating its synergistic and selective catalytic activity.

[0100] In summary, integrating Ni-Co atomic active sites into layered porous carbon helps to increase the redox reaction rate of the sulfur positive electrode and suppress the shuttle effect. The enhanced electrochemical activity of NiCo / NPC is due to the upward shift of the d band caused by the transfer of electrons from nickel to nitrogen, which reduces the antibonding orbital filling and promotes the adsorption of polysulfides. In addition, the coupling of nickel and cobalt sites in the cathode induces the difference in spin states, creating a spin channel for electron transfer in the sulfur redox reaction, thereby improving the stability of lithium-sulfur battery cycles. This manipulation of spin-delocalized electron effects represents a new strategy to promote bidirectional catalysis in lithium-sulfur systems. Therefore, based on the above advantages, the nickel-cobalt diatomic-based composite positive electrode material for lithium-sulfur batteries applied in the present invention shows significant application potential and broad development value in various actual energy storage systems.

Claims

1. A method for preparing a flower-shaped bimetallic loaded porous carbon material etched by boric acid for lithium-sulfur batteries, characterized in that A method for preparing a boric acid-etched flower-shaped bimetallic-loaded porous carbon material for lithium-sulfur batteries is carried out according to the following steps:

1. Preparation of NiCo-ZIF8 Bimetallic Organic Framework 7.50-8.00 mmol of zinc nitrate hexahydrate and 30.00-33.00 mmol of 2-methylimidazole were dissolved in 80 ml of a mixed solvent of ethanol / N,N-dimethylformamide in a volume ratio of 1:1, and magnetically stirred for 30 minutes; 0.45-0.60 mmol of nickel nitrate hexahydrate and 0.15-0.25 mmol of cobalt nitrate hexahydrate were added, and the mixture was stirred for another 30 minutes, and then 7.90-8.20 mmol of boric acid was added, and the mixture was stirred for another 15-20 minutes to obtain a suspension; the resulting solution was transferred to an autoclave, heated to 150-190°C for 12 hours; after cooling to room temperature, centrifuged, repeatedly washed with ethanol and water 5-10 times, and then dried in an oven at 80°C for 24 hours to obtain white powder NiCo-ZIF8; 2. Preparation of NiCo / NPC with Nitrogen-doped Porous Carbon The NiCo-ZIF8 obtained in step 1 was placed in a porcelain boat and heated at 40 mL min -1 The porcelain boat was purged with nitrogen at a flow rate of 1 hour to remove air; then the porcelain boat was heated to 850-950℃ in a nitrogen atmosphere and annealed for 6 hours at a heating rate of 3-5℃min -1 The black powder obtained after cooling was washed several times with water and ethanol, and then dried in an oven at 80°C for 24 hours to obtain NiCo / NPC.

3. Preparation of Nickel-Cobalt Diatom-Based Composite Cathode Material S / NiCo / NPC Sulfur and NiCo / NPC obtained in step 2 were mixed in a mass ratio of 7:3 and ground in a ball mill for 2-5 hours; the resulting material was then heated in an oven at 155°C for 12 hours to obtain a S / NiCo / NPC composite positive electrode material.

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