Ni-mof derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material and preparation method and application thereof

Abstract

This invention relates to the field of new energy storage materials and electrochemical energy storage devices, and provides a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, its preparation method, and its application. The cathode material includes a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support and elemental sulfur loaded on the pores, interlamellar gaps, and / or surface of the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support. The Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is obtained by co-pyrolysis of Ni-MOF precursor and melamine, and includes a g-C3N4 framework, dispersed nickel species, and nitrogen-containing active sites. This invention significantly improves the cycle stability and rate performance of lithium-sulfur batteries and is suitable for the field of high-energy-density energy storage devices.

Description

Technical Field

[0001] This invention relates to the field of new energy storage materials and electrochemical energy storage devices, specifically a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material and its preparation method and application. Background Technology

[0002] With the development of new energy vehicles, portable electronic devices, unmanned systems, and large-scale energy storage systems, the demand for high-energy-density, low-cost, and long-life rechargeable batteries continues to rise. Traditional lithium-ion batteries are limited by the theoretical capacity of their intercalated cathode materials, restricting their potential for energy density improvement. Lithium-sulfur batteries, using elemental sulfur as the cathode active material, possess high theoretical specific capacity and theoretical energy density. Furthermore, sulfur resources are abundant, low-cost, and environmentally friendly, making them widely recognized as one of the core candidate systems for next-generation high-energy-density batteries. However, the practical application of lithium-sulfur batteries is still limited by several issues. First, elemental sulfur and its final discharge product Li2S / Li2S2 have poor electronic conductivity, leading to increased electrode polarization and decreased utilization of active materials. Second, the intermediate product, long-chain lithium polysulfides, is readily soluble in ether electrolytes and migrates between the positive and negative electrodes, generating a shuttle effect that results in the loss of active sulfur, reduced coulombic efficiency, and exacerbated side reactions at the lithium anode. Third, the conversion of sulfur to Li2S is accompanied by significant volume changes, which can easily cause electrode structure damage and interfacial instability. Fourth, the conversion kinetics between lithium polysulfides and Li2S / Li2S2 are relatively slow, limiting rate performance and long-cycle stability.

[0003] Constructing functionalized sulfur supports is one of the core strategies for solving the above problems. Graphite-phase carbon nitride (g-C3N4) possesses a graphite-like layered structure, abundant nitrogen coordination sites, good chemical stability, and low raw material cost, theoretically providing certain adsorption sites for lithium polysulfides. Therefore, g-C3N4 is considered a suitable material for the sulfur cathode support in lithium-sulfur batteries. However, traditionally thermally polymerized g-C3N4 typically exhibits a blocky, dense morphology with a low specific surface area and underdeveloped pore structure, resulting in insufficient sulfur loading space, limited electrolyte wetting, and restricted ion transport. Simultaneously, its intrinsic conductivity is poor, making it difficult to provide a fast electron channel for sulfur redox reactions. Furthermore, g-C3N4 alone mainly exhibits limited physical barrier or weak chemisorption for lithium polysulfides, lacking sufficient catalytic conversion capacity and failing to alleviate the shuttle effect from a kinetic perspective.

[0004] Metal doping is an effective means of modulating the electronic structure of g-C3N4 and introducing catalytic active sites, among which nickel species have potential advantages in the adsorption and conversion of lithium polysulfides. However, if nickel salts are directly mixed with melamine for pyrolysis, nickel species are prone to migration, reduction, or aggregation during heating and holding, forming large nickel particles or agglomerates. This not only reduces the number of effective active sites but may also clog pores, reduce the specific surface area of ​​the material, and decrease mass transfer efficiency. Therefore, in view of the above situation, there is an urgent need to provide a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, its preparation method, and its application to overcome the shortcomings in current practical applications. Summary of the Invention

[0005] The purpose of this invention is to provide a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, its preparation method and application, in order to improve the problems of bulk density, insufficient porosity, limited conductivity and catalytic ability of traditional g-C3N4 support, as well as the easy agglomeration of nickel species in conventional nickel salt doping.

[0006] The present invention is achieved as follows: a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, wherein the cathode material comprises a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support and elemental sulfur loaded in the pores, interlaminar spaces and / or surface of the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support;

[0007] The Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is obtained by co-pyrolysis of Ni-MOF precursor and melamine, and includes a g-C3N4 framework, dispersed nickel species and nitrogen-containing active sites.

[0008] The dispersed nickel species at least partially form Ni-N coordination sites with nitrogen in the g-C3N4 framework;

[0009] The Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support has a loose porous structure composed of particles, sheets, and pores.

[0010] As a further aspect of the present invention: the elemental sulfur is dispersed in an S8 crystal form in a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support;

[0011] The BET specific surface area of ​​the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is 10-50 m². 2 ·g -1 The average pore size is 10-30 nm.

[0012] As a further aspect of the present invention: the Ni-N coordination site and the nitrogen-containing active site together constitute a synergistic active site for the adsorption and conversion of lithium polysulfides;

[0013] The elemental sulfur is generated in situ by the reaction of sodium thiosulfate with acid, and enters the pores and / or interlayer gaps of the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support during the generation process.

[0014] As a further aspect of the present invention, the loading of elemental sulfur in the cathode material is 70wt%-80wt%.

[0015] This invention also provides a method for preparing the above-mentioned Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, comprising the following steps:

[0016] Step 1: Dissolve nickel salt, organic carboxylic acid ligand and polyvinylpyrrolidone in a mixed solvent, and then perform hydrothermal reaction, washing and drying to obtain Ni-MOF precursor;

[0017] Step 2: Mix the Ni-MOF precursor with melamine and pyrolyze it under an inert atmosphere to obtain Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support;

[0018] Step 3: Disperse the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support in an aqueous solution of sodium thiosulfate, add acid dropwise to decompose the sodium thiosulfate in situ to generate elemental sulfur, and obtain the cathode material by stirring, washing and drying.

[0019] As a further aspect of the present invention: the nickel salt mentioned in step 1 is one or more of nickel sulfate, nickel nitrate, nickel chloride, or their hydrates;

[0020] The organic carboxylic acid ligand is a benzotricarboxylic acid ligand;

[0021] The mixed solvent includes N,N-dimethylformamide, anhydrous ethanol and deionized water, and the volume ratio of N,N-dimethylformamide, anhydrous ethanol and deionized water is (0.5-2):(0.5-2):(0.5-2).

[0022] As a further aspect of the present invention: the hydrothermal reaction temperature in step 1 is 120-180℃, and the reaction time is 6-18h;

[0023] For every 30 mL of mixed solvent, the corresponding amount of nickel salt is 0.3-1.0 g, the amount of organic carboxylic acid ligand is 0.05-0.5 g, and the amount of polyvinylpyrrolidone is 0.5-3.0 g.

[0024] As a further aspect of the present invention: the mass ratio of Ni-MOF precursor to melamine in step 2 is 1:(8-20), the temperature is 500-600℃, the pyrolysis time is 1-4h, and the heating rate is 2-10℃·min. -1 .

[0025] As a further aspect of the present invention: the mass ratio of sodium thiosulfate to Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support in step 3 is (5-25):1;

[0026] The acid solution is an aqueous solution of inorganic acid. After adding the acid solution, the stirring time is 2-12 hours, and the drying temperature is 40-80℃.

[0027] The present invention also provides a lithium-sulfur battery, comprising a positive electrode, a lithium metal negative electrode, a separator, and an electrolyte; wherein the positive electrode comprises the above-mentioned Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) By using Ni-MOF as a nickel source and structural precursor, nickel species are confined and separated by the organic framework before pyrolysis, thereby reducing the migration and aggregation tendency of nickel species during the heat treatment process from the source.

[0030] (2) By co-pyrolyzing Ni-MOF with melamine, dispersed nickel species and Ni-N coordination sites can be constructed in the g-C3N4 framework, thereby enhancing the adsorption and catalytic conversion of lithium polysulfides by Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support.

[0031] (3) The obtained Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support has a loose and porous structure, which is more conducive to sulfur loading, electrolyte wetting, ion diffusion and volume change buffering than the traditional bulk g-C3N4.

[0032] (4) The in-situ sulfur precipitation method using sodium thiosulfate decomposition can promote the dispersion of sulfur in the pores and interlayers of Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support, improve the utilization rate of active materials and reduce the accumulation of large sulfur particles.

[0033] (5) The porous structure provides physical confinement, while the Ni-N coordination sites and nitrogen-containing active sites provide chemical adsorption and catalytic conversion. The two work together to suppress the lithium polysulfide shuttle effect.

[0034] (6) In a preferred embodiment, the sulfur loading of the obtained cathode material is approximately 75 wt%, and the initial discharge specific capacity at a 0.1C rate is 1011.7 mAh·g. -1 ;

[0035] The initial specific capacity at 1C rate is 742.9 mAh·g. -1 After 500 cycles, the capacity remains at 365.2 mAh·g. -1 The average capacity decay rate per revolution is approximately 0.102%. Attached Figure Description

[0036] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 The image shown is a low-magnification scanning electron microscope image of the Ni-g-C3N4 material prepared in Example 2, showing the overall particle packing and loose morphology of the material.

[0038] Figure 2 The image shown is a high-magnification scanning electron microscope image of the Ni-g-C3N4 material prepared in Example 2, revealing the local pores and lamellar / particle composite structure of the material.

[0039] Figure 3 The image shown is a scanning electron microscope image of the Ni-g-C3N4 / S composite material prepared in Example 3, showing the surface morphology of the composite material after sulfur loading.

[0040] Figure 4 The image shown is a low-magnification transmission electron microscope image of the Ni-g-C3N4 material prepared in Example 2, revealing a sheet-like structure.

[0041] Figure 5 The image shown is a high-magnification transmission electron microscope image of the Ni-g-C3N4 material prepared in Example 2, displaying lamellar and porosity features.

[0042] Figure 6 The image shown is a low-magnification transmission electron microscope image of the Ni-g-C3N4 / S composite material prepared in Example 3, showing the composite structure after sulfur loading.

[0043] Figure 7 The image shown is a high-magnification transmission electron microscope image of the Ni-g-C3N4 / S composite material prepared in Example 3, showing the distribution of sulfur species in a local region of the carrier.

[0044] Figure 8 The elemental mapping diagram of the Ni-g-C3N4 / S composite material prepared in Example 3 shows the distribution of C, N, Ni and S elements.

[0045] Figure 9 Thermogravimetric analysis (TGA) diagrams of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3 are used to illustrate the sulfur loading.

[0046] Figure 10 The nitrogen adsorption-desorption curves and pore size distribution of the Ni-g-C3N4 material prepared in Example 2 are shown.

[0047] Figure 11The nitrogen adsorption-desorption curves and pore size distribution of the Ni-g-C3N4 / S composite material prepared in Example 3 are shown.

[0048] Figure 12 The XRD patterns of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3 are shown to illustrate the carbon nitride framework and the S8 crystal form.

[0049] Figure 13 The image shows the Raman spectrum of the Ni-g-C3N4 material prepared in Example 2.

[0050] Figure 14 XPS full spectrum of Ni-g-C3N4 prepared in Example 2 and Ni-g-C3N4 / S composite material prepared in Example 3.

[0051] Figure 15 The C1s high-resolution XPS images of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3 are shown.

[0052] Figure 16 The N 1s high-resolution XPS images of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3 are shown.

[0053] Figure 17 High-resolution XPS images of Ni 2p of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3.

[0054] Figure 18 The O 1s high-resolution XPS images of the Ni-g-C3N4 prepared in Example 2 and the Ni-g-C3N4 / S composite material prepared in Example 3 are shown.

[0055] Figure 19 The image shows the S 2p high-resolution XPS image of the Ni-g-C3N4 / S composite material prepared in Example 3.

[0056] Figure 20 The image shows the electrochemical impedance spectroscopy of the Ni-g-C3N4 / S composite material battery prepared in Example 4.

[0057] Figure 21 The cyclic voltammogram is shown for the Ni-g-C3N4 / S composite material battery prepared in Example 4.

[0058] Figure 22 The rate performance diagram is shown for the Ni-g-C3N4 / S composite material battery prepared in Example 4.

[0059] Figure 23The image shows the rate charge-discharge curves of the Ni-g-C3N4 / S composite material battery prepared in Example 4.

[0060] Figure 24 The image shows the cycle performance of the Ni-g-C3N4 / S composite material battery prepared in Example 4. Detailed Implementation

[0061] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0062] The present invention will be further explained below with reference to specific embodiments.

[0063] The present invention provides a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, comprising a Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support and elemental sulfur loaded in the pores, interlayer gaps and / or surface of the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support.

[0064] The Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is obtained by co-pyrolysis of a Ni-MOF precursor and melamine under an inert atmosphere, and comprises a g-C3N4 framework, dispersed nickel species, and nitrogen-containing active sites. The dispersed nickel species at least partially form Ni-N coordination sites with nitrogen in the g-C3N4 framework. The nitrogen-containing active sites may include one or more of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen.

[0065] In a more specific example, the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support has a loose porous structure composed of particles, sheets, and pores, with a BET specific surface area of ​​10-50 m². 2 ·g -1 The average pore size is 10-30 nm. This pore structure can provide loading space for sulfur, buffer space for volume changes during charge and discharge, and improve electrolyte wetting and ion transport.

[0066] In a more specific example, the sulfur loading in the cathode material is 70wt%-80wt%, more preferably 73wt%-77wt%. The elemental sulfur is dispersed in the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support in the S8 crystal form.

[0067] This invention also provides a method for preparing the above-mentioned Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material, comprising the following steps:

[0068] Step 1, Preparation of Ni-MOF precursor: Nickel salt, organic carboxylic acid ligand and polyvinylpyrrolidone are dissolved in a mixed solvent, and after hydrothermal reaction, washing and drying, Ni-MOF precursor is obtained;

[0069] Step 2, Preparation of Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support: The Ni-MOF precursor is mixed with melamine and pyrolyzed under an inert atmosphere to obtain Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support;

[0070] Step 3, In-situ loading of sulfur: The Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is dispersed in an aqueous solution of sodium thiosulfate, and acid is added dropwise to decompose the sodium thiosulfate in situ to generate elemental sulfur. After stirring, washing and drying, the cathode material is obtained.

[0071] In a more specific example, in step 1, the nickel salt is one or more of nickel sulfate, nickel nitrate, nickel chloride, or their hydrates, preferably nickel sulfate hexahydrate; the organic carboxylic acid ligand is a benzoic acid ligand, preferably trimesic acid; the mixed solvent includes N,N-dimethylformamide, anhydrous ethanol, and deionized water, with a volume ratio of (0.5-2):(0.5-2):(0.5-2), preferably 1:1:1.

[0072] In a more specific example, the hydrothermal reaction temperature in step 1 is 120-180℃, and the reaction time is 6-18h; in an even more specific example, the hydrothermal reaction temperature is 150℃, and the reaction time is 10h. The amount of nickel salt used per 30mL of mixed solvent can be 0.3-1.0g, the amount of organic carboxylic acid ligand can be 0.05-0.5g, and the amount of polyvinylpyrrolidone can be 0.5-3.0g.

[0073] In a more specific example, in step 2, the mass ratio of Ni-MOF precursor to melamine is 1:(8-20), preferably 1:14; the pyrolysis temperature is 500-600℃, the pyrolysis time is 1-4h, and the heating rate is 2-10℃·min. -1 In a more specific example, the pyrolysis temperature is 550℃, the pyrolysis time is 2 hours, and the heating rate is 5℃·min. -1 The inert atmosphere is argon or nitrogen.

[0074] In a more specific example, in step 3, the mass ratio of sodium thiosulfate to the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support is (5-25):1, preferably 15:1; the acid solution is an aqueous solution of inorganic acid, preferably 0.5-2 mol·L⁻¹. -1 Hydrochloric acid, more preferably 1 mol·L -1 Hydrochloric acid; the stirring time after adding the acid solution is 2-12 hours, preferably 6 hours; the drying temperature is 40-80℃, preferably 60℃.

[0075] The reaction of sodium thiosulfate with hydrochloric acid can be represented as: Na₂S₂O₃ + 2HCl → S↓ + SO₂↑ + H₂O + 2NaCl. Since sulfur is generated in situ in the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support dispersion system, the resulting sulfur particles are more easily deposited in pores, interlaminar spaces, and on the surface, which is beneficial for improving the uniformity of sulfur dispersion.

[0076] In the embodiments of the present invention, this method is used to improve the problems of traditional g-C3N4 support being bulky and dense, having insufficient porosity, limited conductivity and catalytic ability, and the easy agglomeration of nickel species in conventional nickel salt doping.

[0077] This invention uses Ni-MOF as the nickel source and structural precursor. On one hand, the nickel ions in Ni-MOF are spatially separated by organic ligands, which can reduce the migration and aggregation tendency of nickel species during subsequent pyrolysis. On the other hand, when melamine is thermally polymerized to form the g-C3N4 framework, the skeletal nitrogen can anchor nickel species, forming Ni-N coordination sites, and synergistically interact with nitrogen-containing sites such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen. Subsequently, fine sulfur particles are generated in situ through the reaction of sodium thiosulfate with acid, allowing sulfur to preferentially deposit and disperse in the pores, interlaminar spaces, and surface of the support, thereby obtaining the Ni-g-C3N4 / S cathode composite material.

[0078] This invention also provides a lithium-sulfur battery, comprising a positive electrode, a lithium metal negative electrode, a separator, and an electrolyte; the positive electrode comprises the above-mentioned Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material.

[0079] In a more specific example, the positive electrode is composed of the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material, conductive carbon black, and polyvinylidene fluoride in a mass ratio of 8:1:1.

[0080] In a more specific example, the electrolyte includes LiTFSI, LiNO3, 1,3-dioxolane, and ethylene glycol dimethyl ether.

[0081] This technical solution can improve the utilization rate of sulfur cathode active materials, enhance the adsorption and conversion capacity of lithium polysulfides, suppress the lithium polysulfide shuttle effect, buffer the volume change of sulfur cathode during charging and discharging, and improve the rate performance and cycle stability of lithium-sulfur batteries.

[0082] In embodiments of the present invention, the battery still exhibits good utilization of active materials, rate response, and cycle stability even with a high sulfur loading ratio.

[0083] In summary, the mechanism of action of this invention can be summarized as a synergistic process of "MOF confined dispersion, carbon nitride framework anchoring, porous mass transfer confinement, in-situ sulfur deposition and filling, and adsorption-catalytic conversion." Specifically:

[0084] (1) MOF confined dispersion: The nickel centers in Ni-MOF are spatially separated by organic ligands. During the co-pyrolysis with melamine, the probability of direct contact and aggregation of nickel species can be reduced, which is conducive to the formation of dispersed nickel species.

[0085] (2) Skeleton anchoring and synergistic site construction: Melamine thermal polymerization forms a g-C3N4 skeleton, in which nitrogen atoms can coordinate with nickel species to form Ni-N coordination sites; at the same time, nitrogen-containing sites such as pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen can regulate the polarity and electronic structure of the material.

[0086] (3) Porous confinement and improved mass transfer: The particle / sheet / pore composite structure of the support can provide sulfur loading space and polysulfide physical confinement space, and promote electrolyte wetting and ion transport.

[0087] (4) In-situ precipitation and uniform loading of sulfur: The sulfur generated by the decomposition of sodium thiosulfate is precipitated in-situ in the dispersion system, which allows the sulfur to enter the pores and interlayer gaps more fully, reducing the utilization rate caused by the accumulation of large elemental sulfur particles.

[0088] (5) Adsorption-catalytic conversion: Ni-N coordination sites and nitrogen-containing active sites can enhance the chemical adsorption of lithium polysulfides and promote the redox conversion between lithium polysulfides and Li2S / Li2S2, thereby suppressing the shuttle effect and improving reaction kinetics.

[0089] To further illustrate the technical means and effects adopted in this invention, the following describes the preferred embodiments of this invention and... Figures 1-24 The technical solution of the present invention will be further explained as follows:

[0090] Example 1: Preparation of Ni-MOF precursor:

[0091] Weigh 0.582 g of nickel sulfate hexahydrate, 0.15 g of benzotricarboxylic acid ligand and 1.5 g of polyvinylpyrrolidone, add them to 30 mL of a mixed solvent of N,N-dimethylformamide, anhydrous ethanol and deionized water in a volume ratio of 1:1:1, and stir at room temperature for 30 min to ensure that the components are fully dissolved or uniformly dispersed.

[0092] The above mixture was transferred to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at a constant temperature of 150°C for 10 hours. After the reaction, it was allowed to cool naturally to room temperature and then reacted at a rate of 6000 r·min. -1 Centrifuge for 10 min, wash 5 times alternately with deionized water and ethanol to remove residual solvent, free ligands and surface adsorbates, and then vacuum dry at 60 °C for 12 h to obtain light green Ni-MOF precursor powder.

[0093] In this step, polyvinylpyrrolidone can play an auxiliary role in dispersion and morphology control; N,N-dimethylformamide in the mixed solvent is conducive to the dissolution of organic ligands and crystal growth, while anhydrous ethanol and deionized water are conducive to regulating the polar environment and nucleation process.

[0094] Example 2: Preparation of Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support (Ni-g-C3N4):

[0095] The Ni-MOF precursor prepared in Example 1 was ground and mixed thoroughly in a mortar with melamine at a mass ratio of 1:14, so that the Ni-MOF precursor was uniformly dispersed in the melamine. The mixture was then transferred to a quartz crucible and placed in a tube furnace.

[0096] Under argon protection, at 5℃·min -1 The temperature was raised to 550℃ and pyrolyzed for 2 hours, then naturally cooled to room temperature to obtain a brown porous Ni-MOF-derived nickel-nitrogen co-doped carbon nitride support, denoted as Ni-g-C3N4. During pyrolysis, melamine underwent condensation polymerization to form a g-C3N4 framework; the Ni-MOF precursor provided confined nickel species, which were anchored in the nitrogen-containing framework to form Ni-N coordination-related sites.

[0097] like Figure 1 and Figure 2 As shown, the prepared Ni-g-C3N4 exhibits a loose, porous particle / lamellae composite morphology, which differs from the traditional bulk, dense g-C3N4. Figure 4 and Figure 5 The transmission electron microscope image shown further demonstrates that the material has a sheet-like structure and porous features, which are beneficial for subsequent sulfur loading, electrolyte wetting, and ion transport.

[0098] like Figure 10 As shown, Ni-g-C3N4 exhibits the adsorption-desorption behavior common to mesoporous materials, with a BET specific surface area ranging from 10 to 50 m². 2 ·g -1 The average pore size is in the range of 10-30 nm. Figure 13 The Raman spectra shown reflect the presence of defects and disordered structures in the material, which is beneficial for exposing more active sites and improving interfacial reactions.

[0099] Example 3: Preparation of Ni-g-C3N4 / S cathode composite material:

[0100] Weigh 3g of sodium thiosulfate and dissolve it in 20mL of deionized water. Stir until completely dissolved. Add 0.2g of Ni-g-C3N4 prepared in Example 2 and ultrasonically disperse for 30min to ensure that the carrier is fully dispersed in the sodium thiosulfate aqueous solution.

[0101] Then, an excess of 1 mol·L⁻¹ was slowly added dropwise.-1 Hydrochloric acid causes sodium thiosulfate to decompose in situ, generating elemental sulfur, and the system gradually becomes turbid. After the addition is complete, magnetic stirring is continued at room temperature for 6 hours to allow the sulfur generated in situ to deposit and fully load in the pores, interlayer spaces, and surface of the support. The mixture is then washed with deionized water until neutral, and stirred at 6000 rpm. -1 The solid was collected by centrifugation for 10 min and then vacuum dried at 60 °C for 12 h to obtain the Ni-g-C3N4 / S cathode composite material.

[0102] like Figure 3 , Figure 6 and Figure 7 As shown, the composite material retains its particle / lamellae composite structure after sulfur loading, with some sulfur species distributed in the carrier lamellars and pore regions. Figure 8 The elemental mapping results show that C, N, Ni and S are relatively uniformly distributed in the composite material, indicating that sulfur was successfully introduced and no obvious macroscopic phase separation occurred.

[0103] Figure 9 Thermogravimetric analysis shows that the sulfur loading of the Ni-g-C3N4 / S composite material is about 75 wt%, which belongs to the category of composite cathode materials with high sulfur content. Figure 11 The nitrogen adsorption-desorption and pore size distribution results show that the pore volume and adsorption capacity of the composite material decreased after sulfur loading, indicating that sulfur entered or covered part of the pore structure. Figure 12 The XRD results shown indicate the presence of S8 crystal diffraction peaks in the composite material, proving that sulfur exists in the composite material in crystalline form.

[0104] Figures 14-19 The XPS results show the presence of C, N, Ni, O, and S elements in the material. Specifically, the C 1s and N 1s spectra confirm the presence of the g-C3N4 framework and nitrogen-containing sites; the Ni 2p spectrum shows nickel species-related signals; and the S 2p spectrum shows sulfur species signals. These results collectively demonstrate the successful construction of the Ni-g-C3N4 / S composite material.

[0105] Example 4: Lithium-sulfur battery assembly and electrochemical testing:

[0106] The Ni-g-C3N4 / S cathode composite material prepared in Example 3, Super P, and PVDF were mixed at a mass ratio of 8:1:1. N-methylpyrrolidone was added to prepare a uniform slurry, which was then uniformly coated onto an aluminum foil current collector. After the coated electrode was vacuum dried at 60°C for 12 hours, it was cut into cathode discs with a diameter of 12 mm.

[0107] A 2016-type coin cell was assembled in an argon-atmosphere glove box. The negative electrode was a lithium metal sheet, the separator was Celgard 2325, and the electrolyte contained 1 mol·L⁻¹. -1A mixed solution of LiTFSI and 1.0 wt% LiNO3 in DOL / DME, wherein the volume ratio of DOL to DME is 1:1.

[0108] like Figure 20 As shown, the lithium-sulfur battery containing Ni-g-C3N4 / S composite material exhibits significant charge transfer and ion diffusion responses, indicating that the composite support participates in the regulation of interfacial reactions. Figure 21 The cyclic voltammetry curves shown exhibit typical redox peaks of lithium-sulfur batteries. As the scan rate increases, the peak current increases accordingly, indicating that the electrode reaction has good reversibility and kinetic response.

[0109] Figure 22 and Figure 23 The rate test results show that the battery has a stable charge and discharge platform at different rates and exhibits capacity recovery capability after the rate is restored to 0.1C. Figure 24 The cycle test results shown indicate that the battery's initial specific capacity at a 1C rate is 742.9 mAh·g. -1 After 500 cycles, the capacity remains at 365.2 mAh·g. -1 The average capacity decay rate per cycle is approximately 0.102%; meanwhile, its initial discharge specific capacity at 0.1C rate is 1011.7 mAh·g. -1 .

[0110] The above results demonstrate that the Ni-MOF-derived nickel-nitrogen co-doped carbon nitride-based sulfur cathode material prepared in this invention can improve the utilization rate of sulfur active materials, reaction kinetics, and cycle stability in lithium-sulfur batteries through porous confinement, Ni-N / nitrogen-containing site synergistic adsorption catalysis, and in-situ sulfur loading strategies.

[0111] Example 5: Adjustable range of process conditions:

[0112] While maintaining the co-pyrolysis of Ni-MOF precursor with melamine and the in-situ sulfur precipitation route of sodium thiosulfate decomposition, the hydrothermal reaction temperature can be adjusted within the range of 120-180℃, and the reaction time can be adjusted within the range of 6-18h, in order to regulate the degree of crystallinity and morphology of Ni-MOF precursor.

[0113] The mass ratio of Ni-MOF precursor to melamine can be adjusted within the range of 1:(8-20) to regulate the nickel species content and carbon nitride skeleton ratio in the support. The pyrolysis temperature can be adjusted within the range of 500-600℃. Too low a temperature may lead to insufficient melamine polycondensation, while too high a temperature may lead to excessive skeleton shrinkage or an increased risk of nickel species migration.

[0114] The mass ratio of sodium thiosulfate to Ni-g-C3N4 can be adjusted within the range of (5-25):1 to obtain composite materials with different sulfur loadings. By adjusting the acid concentration, dropping rate, and reaction time, the sulfur formation rate and the degree of sulfur dispersion in the carrier can be further controlled.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A Ni-MOF derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material, characterized in that, The positive electrode material comprises a Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier and elemental sulfur loaded in the pores, interlayer gaps and / or surfaces of the Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier; The Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier is obtained by co-pyrolysis of a Ni-MOF precursor and melamine, and comprises a g-C3N4 skeleton, dispersed nickel species and nitrogen-containing active sites; The dispersed nickel species at least partially form Ni-N coordination sites with the nitrogen in the g-C3N4 skeleton; The Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier has a loose porous structure composed of particles, layers and pores.

2. The Ni-MOF derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material of claim 1, wherein, The elemental sulfur is dispersed in the Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier in the form of S8 crystals; The BET specific surface area of ​​Ni-MOF-derived nickel-nitrogen co-doped carbon nitride supports is 10-50 m². 2 ·g -1 The average pore size is 10-30 nm. 3.The Ni-MOF derived NixNy / C3N4-based sulfur positive electrode material of claim 1, wherein, The Ni-N coordination sites and the nitrogen-containing active sites together form synergistic active sites for lithium polysulfide adsorption and conversion; The elemental sulfur is generated in situ by reacting sodium thiosulfate with an acid solution and enters the pores and / or interlayer gaps of the Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier during the generation process. 4.The Ni-MOF derived NixNy / C3N4-based sulfur positive electrode material of claim 1, wherein, The loading amount of elemental sulfur in the positive electrode material is 70wt%-80wt%.

5. A method for preparing the Ni-MOF derived nickel and nitrogen co-doped carbon nitride-based sulfur positive electrode material according to any one of claims 1-4, characterized in that, The method comprises the following steps: Step 1: dissolving a nickel salt, an organic carboxylic acid ligand and polyvinylpyrrolidone in a mixed solvent, and then performing hydrothermal reaction, washing and drying to obtain a Ni-MOF precursor; Step 2: mixing the Ni-MOF precursor with melamine and pyrolyzing under an inert atmosphere to obtain a Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier; Step 3: dispersing the Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier in an aqueous sodium thiosulfate solution, adding an acid solution dropwise to decompose sodium thiosulfate in situ to generate elemental sulfur, and then performing stirring, washing and drying to obtain the positive electrode material.

6. The preparation method according to claim 5, characterized in that, The nickel salt in step 1 is one or more of nickel sulfate, nickel nitrate, nickel chloride or a hydrate thereof; The organic carboxylic acid ligand is a benzenetricarboxylic acid ligand; The mixed solvent comprises N,N-dimethylformamide, anhydrous ethanol and deionized water, and the volume ratio of N,N-dimethylformamide, anhydrous ethanol and deionized water is (0.5-2):(0.5-2):(0.5-2).

7. The preparation method according to claim 5, characterized in that, The hydrothermal reaction temperature in step 1 is 120-180°C, and the reaction time is 6-18h; The amount of the nickel salt, the organic carboxylic acid ligand and polyvinylpyrrolidone corresponding to 30mL of the mixed solvent is 0.3-1.0g, 0.05-0.5g and 0.5-3.0g, respectively.

8. The preparation method according to claim 5, characterized in that, In step 2, the mass ratio of Ni-MOF precursor to melamine is 1:(8-20), the temperature is 500-600℃, the pyrolysis time is 1-4h, and the heating rate is 2-10℃·min. -1 .

9. The preparation method according to claim 5, characterized in that, The mass ratio of the sodium thiosulfate to the Ni-MOF derived nickel-nitrogen co-doped carbon nitride carrier in step 3 is (5-25):1; The acid solution is an aqueous inorganic acid, the stirring time after adding the acid solution is 2-12h, and the drying temperature is 40-80°C.

10. A lithium-sulfur battery, characterized by, The battery comprises a positive electrode sheet, a metal lithium negative electrode, a separator and an electrolyte, and the positive electrode sheet comprises the Ni-MOF derived nickel-nitrogen co-doped carbon nitride-based sulfur positive electrode material according to any one of claims 1-4.

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