Two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function, and preparation method and application thereof

Abstract

The application discloses a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride with bidirectional catalytic function and a preparation method and application thereof. A two-dimensional transition metal nitride precursor mixed phase is obtained by nitriding a transition metal sulfide and a decomposable alkali metal compound at high temperature, and after acid washing treatment, the precursor is heat-treated in a reducing atmosphere, part of nitrogen elements are reduced to form a nitrogen-deficient transition metal compound, by adjusting the reaction time and temperature, a transition metal nitride material with a heterostructure constructed by nitrogen-deficient and nitrogen-rich compounds is obtained. The two-dimensional heterostructure transition metal nitride prepared by the application can promote the reduction deposition of liquid phase polysulfide and simultaneously promote the oxidative dissociation of solid-state lithium sulfide as a discharge end product, has excellent bidirectional catalytic function for lithium-sulfur batteries, and can significantly improve the capacity and cycle stability of the lithium-sulfur battery, improve the utilization rate of sulfur and prolong the service life of the battery when applied to a lithium-sulfur battery positive electrode.

Description

Technical Field

[0001] This invention belongs to the field of nitride material preparation and application technology, specifically relating to a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function, its preparation method and application. Background Technology

[0002] Lithium-sulfur batteries are known for their high specific capacity (1675 mAh g). -1 High energy density (2600Wh kg) -1 Due to its low material cost, it has received widespread attention and is considered one of the high-energy battery technologies with the greatest potential for practical application. (The last sentence appears to be incomplete and unrelated to the preceding text. It likely refers to Li-based intercalation / deintercalation.) + Unlike lithium-ion batteries, which store energy, lithium-sulfur batteries utilize the electrochemical conversion between elemental sulfur and metallic lithium to store and convert chemical energy into electrical energy. The overall reaction is as follows: However, the actual conversion process is more complex. The charge-discharge process of lithium-sulfur batteries involves a multiphase conversion from solid to liquid to solid, characterized by multi-step, multi-electron reactions. This generates soluble intermediate lithium polysulfides, accompanied by the decomposition and formation of elemental sulfur and lithium sulfides, which have extremely poor conductivity. This multi-electron, multi-step reaction, whether liquid-liquid or liquid-solid, is a kinetically slow redox process. This slow kinetics leads to two problems: firstly, a large amount of intermediate liquid-phase lithium polysulfides remains in the electrolyte, exacerbating the shuttle effect and causing continuous loss of active materials; secondly, it makes rapid charge-discharge conversion difficult, deteriorating the battery's rate performance and limiting its performance under high sulfur loads and long cycle times.

[0003] Catalysts play a crucial role in accelerating the conversion kinetics of lithium polysulfides in the electrochemical reactions of lithium-sulfur batteries, significantly alleviating the challenges they face in achieving practical application. However, using only unidirectional catalysts targeting sulfur reduction or oxidation reactions cannot meet the practical needs of lithium-sulfur batteries. On the one hand, relying solely on the unidirectional catalytic function of oxidation or reduction catalysts makes it difficult to achieve rapid sulfur recycling. During the overall charge-discharge process of lithium-sulfur batteries, using catalysts with only oxidation or reduction catalytic functions may result in incomplete conversion in the reverse process, limiting the overall performance improvement during charge-discharge cycles. On the other hand, the slow solid-liquid-solid conversion leads to the covering of active sites on the catalyst surface, causing catalyst poisoning and deactivation, thus restricting its catalytic effect. Furthermore, catalysts relying solely on unidirectional catalysis cannot achieve rapid solid-liquid bidirectional conversion, resulting in the catalyst surface being continuously covered by solid-phase sulfur-containing components, leading to the loss of catalytic function. Therefore, the design and research of bidirectional catalysts are particularly important.

[0004] Transition metal nitrides, as interstitial solid solutions, have nitrogen atoms occupying interstitial spaces in face-centered cubic, hexagonal, or hexagonal close-packed metal lattices. Introducing nitrogen atoms forms MN bonds (M being a metal atom), expanding the original intermetallic spacing and contracting the metal d-band, leading to increased d-orbital electron density and a higher Fermi state density. This results in metal-like conductivity and active electron transfer properties, making them highly promising catalysts. As polysulfide conversion catalysts, transition metal nitrides possess high electrical and thermal conductivity, high density, and good chemical stability, along with multiple active sites. Compared to other transition metal compounds, they exhibit better conductivity and catalytic kinetics, thus showing great promise for applications in lithium-sulfur batteries. To date, various nitride structures have been reported, such as nanoparticles, nanosheets, and nanowires. However, the existence of these nitride materials necessitates an effective and controllable method to prepare metal nitride-based materials with high specific surface areas and enhance their bidirectional catalytic capabilities. This is the material design principle and goal for the practical application of metal nitride catalysts in lithium-sulfur batteries. Summary of the Invention

[0005] This invention aims to enrich the current lithium-sulfur battery catalyst system and solve the problem that most catalysts only perform single catalysis for the oxidation or reduction of polysulfides. Its purpose is to develop two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitrides with bidirectional catalytic function, as well as their preparation methods and applications. This enables the controllable preparation of two-dimensional nanosheet structures, thereby increasing the effective catalytic area per unit mass of the material and achieving bidirectional catalytic function.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function includes the following steps:

[0008] (i) A decomposable alkali metal compound was used as an auxiliary agent and a bulk layered transition metal sulfide was ball-milled and mixed to obtain a uniformly mixed nitride precursor powder.

[0009] (ii) The nitriding precursor powder obtained in step (i) is transferred to a high-temperature reactor, and an ammonia-containing gaseous precursor is introduced to further peel off the layered nitriding precursor powder; the decomposable alkali metal compound is controlled to melt at high temperature so that it can penetrate into the layers of the layered transition metal sulfides, and the bulk layered transition metal sulfides are desulfurized and nitrided to generate controllable alkali metal sulfides, and finally a mixed phase of by-products (alkali metal sulfides) and single-crystal two-dimensional transition metal nitrides is formed;

[0010] (iii) After the furnace temperature has cooled to room temperature, the mixed phase obtained in step (ii) is taken out and subjected to ultrasonic acid washing to completely remove the reaction byproducts and obtain a two-dimensional single crystal transition metal nitride precursor.

[0011] (iv) The two-dimensional single-crystal transition metal nitride nanosheets obtained in step (iii) are placed in a high-temperature reactor and a hydrogen-containing gaseous precursor is introduced for heat treatment. The treatment time is adjusted to control the degree of reduction, and two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride materials are obtained.

[0012] In the above technical solution, the bulk layered transition metal sulfide is any one of bulk layered molybdenum sulfide, molybdenite concentrate, or bulk layered tungsten sulfide.

[0013] In the above technical solution, the decomposable alkali metal compound is any one or more of sodium carbonate, potassium carbonate, lithium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, potassium oxide, sodium oxide, lithium oxide, sodium sulfite, or potassium sulfite.

[0014] In the above technical solution, the mixing in step (i) is carried out by dry ball milling, with a ball milling speed range of 1 rpm to 1000 rpm and a ball milling time of 0.5 h to 10 h.

[0015] In the above technical solution, the molar ratio of bulk layered transition metal sulfides and decomposable alkali metal compounds in step (i) is 1:0.5 to 1:6.

[0016] In the above technical solution, the ammonia-containing gaseous precursor in step (ii) is a mixture of ammonia and argon or a mixture of ammonia and nitrogen, wherein the volume fraction of ammonia is not less than 50%; the conditions for the desulfurization and nitriding treatment are: the flow rate of the ammonia-containing gaseous precursor is controlled at 50-200 sccm, preferably 160 sccm, the nitriding temperature is 650℃-900℃, and the heating rate is 1℃ / min. -1 ~30℃min -1 The thermal nitriding time is 1h to 20h.

[0017] In the above technical solution, the acidic solution in step (iii) is any one or more of HCl, H2SO4, or HNO3, and the H2SO4 of the acidic solution is... + The concentration is 1M to 6M; the ultrasound time is 0.25 to 2 hours, preferably 1 hour; the mass ratio between the acidic solution and the mixed phase is not less than 10:1.

[0018] In the above technical solution, the hydrogen-containing gaseous precursor in step (iv) is H2 or a mixture of H2 and Ar; the heat treatment conditions are as follows: the hydrogen-containing gaseous precursor in step (iv) is H2 or a mixture of H2 and Ar; the heat treatment conditions are as follows: the flow rate of the hydrogen-containing gaseous precursor is controlled at 50 sccm to 200 sccm, preferably 150 sccm, the heat treatment temperature is 400℃ to 1000℃, and the heating rate is 1℃ / min. -1 ~30℃ min -1 The heat treatment time is 4h to 24h.

[0019] A two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material is obtained by a method for preparing two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride materials with bidirectional catalytic function.

[0020] A two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material is disclosed, wherein the transition metal nitride is composed of MoN and Mo2N, and a heterostructure is formed between MoN and Mo2N; the thickness of the MoN-Mo2N heterojunction is approximately 7 nm, the size is 1 μm to 2 μm, and the specific surface area is not less than 35 m². 2 g -1 Its electronic conductivity is not less than 8 S cm⁻¹ -1 Hole enrichment occurs on the Mo2N side and electron enrichment occurs on the MoN side at the MoN-Mo2N heterojunction interface.

[0021] A two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride is used in lithium-sulfur batteries, wherein the two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride serves as an anode catalyst.

[0022] A lithium-sulfur battery, wherein the anode catalyst of the lithium-sulfur battery adopts the aforementioned two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material.

[0023] The beneficial effects of this invention are:

[0024] This invention provides a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function, its preparation method, and its application. The prepared material, as a catalyst, can promote the reduction and deposition of liquid-phase polysulfides and simultaneously promote the oxidative dissociation of solid lithium sulfide, the final product of discharge, thus exhibiting excellent bidirectional catalytic function in lithium-sulfur batteries. When applied to the cathode of lithium-sulfur batteries, this material can significantly improve the capacity and cycle stability of lithium-sulfur batteries, enhance sulfur utilization, and extend battery life.

[0025] This invention prepares a heterostructure catalyst by heat-treating two-dimensional transition metal nitride nanosheets, providing the catalyst with bidirectional catalytic function. This facilitates the rapid diffusion and adsorption of ions during charging and discharging, as well as the redox reactions of surface polysulfides and lithium sulfide. It also increases the ion contact surface area and energy storage active sites, allowing for more efficient utilization of active sulfur in the electrode. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the reaction mechanism of the present invention;

[0027] Figure 2 These are XRD patterns of MoN-Mo2N prepared in Example 1, Mo2N prepared in Comparative Example 1, and MoN prepared in Comparative Example 2 of the present invention.

[0028] Figure 3 This is the XRD pattern of Mo2N prepared using the MoO3 nitridation precursor in Comparative Example 3 of the present invention.

[0029] Figure 4 These are TEM images of MoN-Mo2N prepared in Example 1 and MoN prepared in Comparative Example 2 of the present invention;

[0030] Figure 5 These are SEM images of MoN-Mo2N prepared in Example 1, Mo2N prepared in Comparative Example 1, and MoN prepared in Comparative Example 2 of the present invention.

[0031] Figure 6 This is a SEM image of the Mo2N prepared using the MoO3 nitridation precursor in Comparative Example 3 of this invention.

[0032] Figure 7 These are the XPS spectra of MoN-Mo2N prepared in Example 1, Mo2N prepared in Comparative Example 1, and MoN prepared in Comparative Example 2 of the present invention.

[0033] Figure 8 These are the BET plot and the fractional conductivity IV test plot of MoN-Mo2N prepared in Example 1 of this invention;

[0034] Figure 9 The UV-vis absorption spectra of the solutions obtained by the interaction of MoN-Mo2N prepared in Example 1, Mo2N prepared in Comparative Example 1, MoN prepared in Comparative Example 2, and CNTs prepared in Comparative Example 4 with 0.1M Li2S6 solution are shown.

[0035] Figure 10 The XPS spectrum of the MoN-Mo2N prepared in Example 1 of this invention after interaction with 0.1M Li2S6 solution is shown.

[0036] Figure 11These are half-cell electrochemical data graphs for comparative examples 4, 5, 6, and 7 of this invention;

[0037] Figure 12 These are the LSV curves and Tafel slope curves of the half-cells of comparative examples 4, 5, 6, and 7 of this invention at different scan rates.

[0038] Figure 13 These are the CV curves of the half-cells of Comparative Examples 4, 5, 6, and 7 of this invention at different scan rates, and the Li... + Diffusion rate analysis diagram;

[0039] Figure 14 These are the electrochemical deposition curves of polysulfides in the half-cells of comparative examples 5, 6, and 7 of this invention.

[0040] Figure 15 These are the electrochemical dissociation curves of Li2S in the half-cells of Comparative Examples 5, 6, and 7 of this invention.

[0041] Figure 16 These are the charge-discharge curves of the half-cells of Comparative Examples 4, 5, 6, and 7 of this invention at a current density of 0.1C.

[0042] Figure 17 This is a graph showing the rate performance data of the half-cells of comparative examples 4, 5, 6, and 7 of this invention.

[0043] Figure 18 This is a graph showing the long-cycle electrochemical stability data of half-cells in comparative examples 4, 5, 6, and 7 of this invention.

[0044] For those skilled in the art, other related figures can be obtained from the above figures without any creative effort. Detailed Implementation

[0045] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0046] Example 1

[0047] A method for preparing a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function, specifically comprising:

[0048] (i) Weigh out bulk molybdenum sulfide and sodium carbonate at a molar ratio of 1:2.5, and ball mill them at 800 rpm for 1 hour to obtain a uniformly mixed nitrided precursor powder.

[0049] (ii) Place the nitrided precursor powder from step (i) into a high-temperature furnace, introduce NH3 at a flow rate controlled at 160 sccm, and then heat at 5°C for 1 minute. -1 The temperature was increased to 750℃ at a heating rate and held for 3 hours.

[0050] (iii) After the furnace temperature has cooled to room temperature, the sample is taken out, 100 mL of 3M dilute HCl is added for acid washing, and ultrasonic treatment is performed for 1 h to completely remove the reaction byproducts and obtain two-dimensional single crystal transition metal nitride nanosheets (two-dimensional nitrogen-rich phase molybdenum nitride).

[0051] (iv) The two-dimensional nitrogen-rich molybdenum nitride obtained in step (iii) is placed in a high-temperature reactor and H2 / Ar mixed gas with a volume fraction of 5% is introduced. After high-temperature reduction at 800℃ for 6 hours, it is allowed to cool naturally to room temperature to obtain the two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material MoN-Mo2N.

[0052] To compare the superior functions, such as bidirectional catalysis, of the nitrogen-deficient / nitrogen-rich heterostructures of this application, the following comparative examples were set up:

[0053] Comparative Example 1

[0054] This comparative example used essentially the same experimental materials and conditions as Example 1, except that the reduction time was increased during high-temperature heat treatment, ultimately obtaining a two-dimensional nitrogen-deficient Mo2N single phase, as follows:

[0055] (i) Weigh out bulk molybdenum sulfide and sodium carbonate at a molar ratio of 1:2.5, and ball mill them at 800 rpm for 1 hour to obtain a uniformly mixed nitrided precursor powder.

[0056] (ii) Place the nitrided precursor powder from step (i) into a high-temperature furnace, introduce NH3 at a flow rate controlled at 160 sccm, and then heat at 5°C for 1 minute. -1 The temperature was increased to 750℃ at a heating rate and held for 3 hours.

[0057] (iii) After the furnace temperature has cooled to room temperature, the sample is taken out, 100 mL of 3M dilute HCl is added for acid washing, and ultrasonic treatment is performed for 1 h to completely remove the reaction byproducts and obtain two-dimensional single crystal transition metal nitride nanosheets (two-dimensional nitrogen-rich phase molybdenum nitride MoN).

[0058] (iv) The two-dimensional nitrogen-rich molybdenum nitride obtained in step (iii) is placed in a high-temperature reactor, and Ar / H2 (5%) gas is introduced. After high-temperature reduction at 800℃ for 12 hours, it is allowed to cool naturally to room temperature to obtain a two-dimensional nitrogen-deficient Mo2N single phase.

[0059] Comparative Example 2

[0060] This comparative example uses essentially the same experimental materials and conditions as Example 1, but without reduction treatment after step (iii), ultimately obtaining two-dimensional nitrogen-rich molybdenum nitride (MoN), specifically:

[0061] (i) Weigh out bulk layered molybdenum sulfide and sodium carbonate at a molar ratio of 1:2.5, and ball mill them at 800 rpm for 1 h to obtain a uniformly mixed nitrided precursor powder.

[0062] (ii) Place the nitrided precursor powder from step (i) into a high-temperature furnace, introduce NH3 at a flow rate controlled at 160 sccm, and then heat at 5°C for 1 minute. -1 The temperature was increased to 750℃ at a heating rate and held for 3 hours.

[0063] (iii) After the furnace temperature has cooled to room temperature, the sample is taken out, 100 mL of 3M dilute HCl is added for acid washing, and ultrasonic treatment is performed for 1 h to completely remove the reaction byproducts and obtain two-dimensional nitrogen-rich molybdenum nitride (MoN).

[0064] Comparative Example 3

[0065] (i) Weigh a certain amount of MoO3 powder, place it in a high-temperature furnace, and introduce NH3 at a flow rate controlled at 200 sccm, and then introduce it at 5℃ min. -1 The temperature was increased to 700℃ at a certain heating rate and held for 2 hours to obtain bulk powdered nitrogen-deficient molybdenum nitride (Mo2N).

[0066] Comparative Example 4 (Electrical Properties of CNTs)

[0067] In this comparative example, conventional CNTs@S materials were prepared as the anode active material, specifically as follows:

[0068] (i) Weigh S and CNTs at a mass ratio of 8:2, grind them in an agate mortar for 30 min until they become a homogeneous gray powder. Transfer the powder to a 50 mL polytetrafluoroethylene liner in a glove box filled with an inert atmosphere and seal it. Then place it in a constant temperature oven at 155℃ for 24 h, allow it to cool naturally, and grind it again for 10 min to prepare CNTs@S.

[0069] (ii) Grind CNTs@S and the conductive agent in an agate mortar at a mass ratio of 8:1 for 15 minutes. Add the ground powder to a homogeneous NMP solution containing PVDF, and stir and degas in a planetary degassing machine for 1 hour to obtain a homogeneous slurry. Use carbon-coated aluminum foil as the electrode current collector, and coat the slurry evenly on the current collector using a doctor blade. Place the coated electrode sheet in a vacuum drying oven at 60℃ for 12 hours. After drying, cut it into a circular positive electrode with a diameter of 12 mm, which is the CNTs anode.

[0070] Comparative Example 5 (Electrical Properties of MoN)

[0071] In this comparative example, conventional CNTs@S materials were prepared and mixed with MoN as an anode catalyst to investigate the effect of the MoN prepared in Comparative Example 2 on the performance of lithium-sulfur batteries. Specifically:

[0072] (i) Weigh S and CNTs at a mass ratio of 7:3, grind them in an agate mortar for 30 min until they become a homogeneous gray powder. Transfer the powder to a 50 mL polytetrafluoroethylene liner in a glove box filled with an inert atmosphere and seal it. Then place it in a constant temperature oven at 155℃ for 24 h, allow it to cool naturally, and grind it again for 10 min to prepare CNTs@S.

[0073] (ii) CNTs@S, conductive agent, and MoN prepared in Comparative Example 2 were ground in an agate mortar for 15 min at a mass ratio of 7:1:1. The ground mixture was added to a homogeneous NMP solution containing dissolved PVDF, and the mixture was stirred and degassed in a planetary degassing machine for 1 h to obtain a homogeneous slurry. Carbon-coated aluminum foil was used as the electrode current collector, and the slurry was evenly coated onto the current collector using a doctor blade coater. The coated electrode sheet was placed in a vacuum drying oven at 60℃ for 12 h, and after drying, it was cut into a circular positive electrode with a diameter of 12 mm, which is the MoN anode.

[0074] Comparative Example 6 (Electrical Properties of MoN-Mo2N)

[0075] This comparative example prepared conventional CNTs@S materials and mixed them with MoN-Mo2N as an anode catalyst to investigate the effect of the MoN-Mo2N prepared in Example 1 on lithium-sulfur batteries. Specifically:

[0076] (i) Weigh S and CNTs at a mass ratio of 7:3, grind them in an agate mortar for 30 min until they become a homogeneous gray powder. Transfer the powder to a 50 mL polytetrafluoroethylene liner in a glove box filled with an inert atmosphere and seal it. Then place it in a constant temperature oven at 155℃ for 24 h, and after natural cooling, grind it again for 10 min to prepare CNTs@S.

[0077] (ii) CNTs@S, conductive agent, and MoN-Mo2N prepared in Example 1 were ground in an agate mortar for 15 min at a mass ratio of 7:1:1. The ground mixture was added to a homogeneous NMP solution containing dissolved PVDF, and stirred and degassed in a planetary degassing machine for 1 h to obtain a homogeneous slurry. Carbon-coated aluminum foil was used as the electrode current collector, and the slurry was evenly coated onto the current collector using a doctor blade. The coated electrode sheet was placed in a vacuum drying oven at 60℃ for 12 h, and after drying, it was cut into a circular positive electrode with a diameter of 12 mm, which is the MoN-Mo2N anode.

[0078] Comparative Example 7 (Electrical Properties of Mo2N)

[0079] In this comparative example, conventional CNTs@S materials were prepared and mixed with Mo2N as an anode catalyst to investigate the effect of Mo2N prepared in Comparative Example 1 on lithium-sulfur batteries. Specifically:

[0080] (i) Weigh S and CNTs at a mass ratio of 7:3, grind them in an agate mortar for 30 min until they become a homogeneous gray powder. Transfer the powder to a 50 mL polytetrafluoroethylene liner in a glove box filled with an inert atmosphere and seal it. Then place it in a constant temperature oven at 155℃ for 24 h, and after natural cooling, grind it again for 10 min to prepare CNTs@S.

[0081] (ii) CNTs@S, conductive agent, and Mo2N prepared in Comparative Example 1 were ground in an agate mortar for 15 min at a mass ratio of 7:1:1. The ground mixture was added to a homogeneous NMP solution containing dissolved PVDF, and stirred and degassed in a planetary degassing machine for 1 h to obtain a homogeneous slurry. Carbon-coated aluminum foil was used as the electrode current collector, and the slurry was evenly coated onto the current collector using a doctor blade. The coated electrode sheet was placed in a vacuum drying oven at 60℃ for 12 h, and after drying, it was cut into a circular positive electrode with a diameter of 12 mm, which is the Mo2N anode.

[0082] The following is a description of the detection and analysis of the above embodiments and comparative examples:

[0083] XRD analysis was performed on the MoN-Mo2N prepared in Example 1, the Mo2N prepared in Comparative Example 1, and the MoN prepared in Comparative Example 2. The XRD results are as follows: Figure 2 , 3 As shown, the prepared two-dimensional transition metal nitride MoN gradually forms a nitrogen-rich / nitrogen-deficient transition metal compound heterostructure MoN-Mo2N under a reducing atmosphere, and eventually transforms into a nitrogen-deficient Mo2N single phase. However, nitriding using MoO3 as a precursor can only ultimately transform into a nitrogen-deficient Mo2N single phase.

[0084] The morphology of MoN-Mo2N prepared in Example 1, Mo2N prepared in Comparative Example 1, and MoN prepared in Comparative Example 2 was observed, and the results are as follows: Figure 4 , 5 As shown in Figures 6 and 7, the catalytic materials prepared in Examples 1, 1, and 2 all maintain a two-dimensional nanosheet structure, and the precursor MoN remains in a single crystal state. Under suitable conditions, heat treatment of nitrogen-rich MoN can yield a MoN-Mo2N heterostructure catalyst. However, the catalyst prepared in Comparative Example 3 only maintains the bulk powder morphology of the MoO3 precursor.

[0085] XPS analysis was performed on the MoN-Mo2N prepared in Example 1, the Mo2N prepared in Comparative Example 1, and the MoN prepared in Comparative Example 2. The XPS spectra are shown below. Figure 7 As shown, the main Mo and N elements in MoN exist in the form of Mo-N bonds, while a small amount of O elements exist in the form of surface oxidation and adsorption. Subsequent thermal reduction treatment will further reduce the oxygen content. Electron transfer occurs at the interface of the MoN-Mo2N heterojunction, i.e., electrons transfer from Mo2N to MoN, resulting in hole enrichment on the Mo2N side and electron enrichment on the MoN side at the interface. This means that the MoN-Mo2N heterojunction can simultaneously act as an electron donor and electron acceptor, providing electrons / holes in the conversion reaction, thereby improving the kinetic efficiency of the reaction.

[0086] The MoN-Mo2N prepared in Example 1 was subjected to BET and conductivity tests, and the results are as follows: Figure 8 As shown, its adsorption-desorption isotherm curves exhibit type IV isotherms, indicating a mesoporous structure, and its specific surface area is 38.4 m². 2 g -1 Its powder conductivity is 8.93 S cm⁻¹ -1 .

[0087] After the MoN-Mo2N prepared in Example 1, the Mo2N prepared in Comparative Example 1, the MoN prepared in Comparative Example 2, and the CNTs prepared in Comparative Example 4 were interacted with a 0.1M Li2S6 solution, the UV-vis absorption curves of the solutions were then measured. The results are as follows: Figure 9 As shown, the reduced Mo2N exhibits stronger adsorption capacity than MoN, and the MoN-Mo2N heterojunction prepared by partial reduction demonstrates better lithium polysulfide adsorption capacity compared to the original MoN. Furthermore, XPS analysis of the MoN-Mo2N material surface after polysulfide adsorption yields the following results: Figure 10 As shown, the MoN-Mo2N heterojunction can achieve chemical fixation of lithium polysulfides, exhibiting strong polysulfide adsorption performance and accelerating subsequent electrochemical catalytic behavior, thereby improving the polysulfide conversion rate.

[0088] The performance of the half-cells prepared in Comparative Examples 4 / 5 / 6 / 7 was compared, and the electrochemical data of the half-cells are shown in the figure below. Figure 11As shown, the MoN-Mo2N heterojunction catalyst exhibits the catalytic properties of both MoN and Mo2N, promoting the conversion of lithium polysulfides to S8 and Li2S. It also demonstrates excellent catalytic promotion effects on both the forward reduction and negative oxidation processes of lithium polysulfides. The MoN-Mo2N heterojunction catalyst exhibits optimal electrochemical catalytic activity, displaying the catalytic characteristics of both MoN and Mo2N. It significantly reduces electrochemical polarization during the lithium polysulfide conversion process, effectively accelerating the redox conversion kinetics.

[0089] Comparing the Tafel slopes of the half-cells in comparative examples 4 / 5 / 6 / 7, the results are as follows: Figure 12 As shown, the MoN-Mo2N heterojunction can reduce reaction polarization in both oxidation and reduction processes, exhibiting a dual catalytic function that promotes the rapid oxidation / reduction conversion of lithium polysulfides.

[0090] Li-5 half-cells of comparative examples 4 / 5 / 6 / 7 + A comparison of diffusion capabilities yielded the following results: Figure 13 As shown, due to the electron transfer at the interface of the MoN-Mo2N heterojunction catalyst, a potential difference is generated across the interface, thus creating a built-in electric field for Li. + Migration provides the driving force for transfer, exhibiting the highest Li in both sulfur oxidation / reduction processes. + The diffusion rate is significantly improved, especially in the two processes of oxidative dissociation of Li2S to lithium polysulfides and reduction deposition of lithium polysulfides to Li2S.

[0091] The catalytic abilities of the half-cells in Comparative Examples 3 / 4 / 5 / 6 for Li2S deposition were compared, and the results are as follows: Figure 14 As shown, the MoN-Mo2N heterojunction has a catalytic effect on promoting the reduction and deposition process of Li2S. Similar to Mo2N, it exhibits strong catalytic activity in the deposition and transformation process of liquid-phase LiPSs to solid-phase Li2S, which can enhance the deposition kinetics of Li2S and guide the three-dimensional growth of Li2S.

[0092] The catalytic abilities of the half-cells in Comparative Examples 4 / 5 / 6 / 7 for Li2S dissociation were compared, and the results are as follows: Figure 15 As shown, the MoN-Mo2N heterojunction has a catalytic effect on promoting the oxidative dissociation process of Li2S. Similar to MoN, it exhibits strong catalytic activity in the dissociation transformation of solid-phase Li2S to liquid-phase LiPSs, thereby enhancing the dissociation kinetics and accelerating the dissociation process.

[0093] The constant current charge-discharge curves of the half-cells in Comparative Examples 4 / 5 / 6 / 7 were compared, and the results are as follows: Figure 16As shown, the MoN-Mo2N heterojunction exhibits strong catalytic activity for both sulfur oxidation dissociation and reduction deposition processes, demonstrating its dual catalytic function.

[0094] The rate performance of the half-cells in Comparative Examples 3 / 4 / 5 / 6 was compared, and the results are as follows: Figure 17 As shown, the MoN-Mo2N heterojunction significantly improves the electrode polarization phenomenon under high current, enhances the conversion degree of sulfur, and still has a strong catalytic effect during high-rate charge and discharge.

[0095] The long-cycle performance of the half-cells in Comparative Examples 4 / 5 / 6 / 7 was compared, and the results are as follows: Figure 18 As shown, the bidirectional catalytic function of the MoN-Mo2N heterojunction improves the deposition / dissociation behavior of solid-phase Li2S, thereby effectively inhibiting its surface poisoning and loss of active sites, enabling it to maintain high catalytic activity over a long period of time and improving the cycle stability of the sulfur cathode.

[0096] The reaction mechanism of this invention:

[0097] Nitrogen-deficient transition metal nitrides exhibit significantly different polysulfide conversion catalytic activity compared to polynitrides due to the higher charge density distribution of the Mo atoms at their catalytic active sites. Organically combining nitrogen-deficient and nitrogen-rich transition metal compounds can specifically enhance the catalytic rates of oxidation and reduction kinetics in lithium-sulfur batteries. Furthermore, the heterostructure combining the different electronic structure energy level distributions of the two transition metal nitrides causes electron rearrangement and creates electron distribution defects, further enhancing the interaction between the bidirectional catalytic material and lithium polysulfides. This improves the catalytic reaction kinetics of both oxidation and reduction processes in lithium-sulfur batteries, reduces polysulfide spillover, weakens the 'shuttle effect,' and further improves the battery's rate performance and cycle life.

[0098] like Figure 1The schematic diagram of the reaction mechanism shows that this invention uses an alkali metal compound-assisted method to simply and efficiently transform bulk layered sulfides into two-dimensional nitrides. Taking sulfide MoS2 as an example, without the assistance of an alkali metal compound, the strong Mo-S covalent bond and the low affinity between S and H atoms make it difficult to break the Mo-S bond with NH3. Therefore, complete nitridation of bulk MoS2 in ammonia at 750°C requires at least 40 hours, which is inefficient and cannot yield a two-dimensional structure. This invention, with the assistance of a decomposable alkali metal compound, only requires 3 hours of nitridation at 750°C. After simple acid washing, byproducts can be removed to obtain two-dimensional nitride nanosheets, thus exposing more Mo metal catalytic sites and increasing the effective active area per unit mass of catalyst. Meanwhile, considering the further thermal nitridation of the thermally unstable Mo2N, it is difficult to ensure that Mo2N maintains structural stability during nitridation, easily forming a single-phase Mo. 1+x N (0≤x≤1) material. This invention involves the partial reduction of highly thermally stable MoN. By adjusting the reduction conditions, only some defect states in the material are reduced, thereby obtaining a MoN-Mo2N heterostructure material. Compared to the relatively abundant stoichiometric nitrogen content of MoN, the heterostructure is formed by combining with the nitrogen-deficient compound Mo2N. Simultaneously, the charge distribution of the catalytically active Mo in both the MoN and Mo2N phases is modulated, enhancing its chemical bonding ability with polysulfides and reducing the charge transport rate in the subsequent electrochemical catalytic oxidation and reduction processes of polysulfides. This improves the reaction kinetics of the catalyst in the oxidation and reduction processes of polysulfides, enhances the chemisorption effect of the material on polysulfides, and ultimately improves the electrochemical performance of lithium-sulfur batteries.

[0099] This invention utilizes an alkali metal compound-assisted nitridation-reduction method to prepare two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalysts to enhance the electrochemical performance of lithium-sulfur batteries. Using a salt-assisted nitridation method with bulk transition metal sulfides as precursors, a single-crystal two-dimensional transition metal nitride nanosheet precursor was synthesized through thermal nitridation in an ammonia atmosphere. This precursor was then heat-treated in a reducing atmosphere to prepare the two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalyst.

[0100] The two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalytic material prepared in this invention exhibits bidirectional catalytic function, promoting both the reduction deposition process of liquid-phase polysulfides to solid-phase Li₂S and the oxidative dissociation process of solid-phase Li₂S to liquid-phase polysulfides. The prepared two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalytic material demonstrates extremely high deposition capacity and a very fast deposition rate in the depositional transformation of liquid-phase polysulfides to solid-phase Li₂S; and extremely high dissociation current and a very fast dissociation rate in the reverse dissociation transformation of Li₂S to liquid-phase polysulfides.

[0101] This invention utilizes an alkali metal compound-assisted preparation method, requiring only simple steps such as nitriding, acid washing, and heat treatment to obtain two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalytic materials. During nitriding, the alkali metal compound accelerates the formation of the two-dimensional transition metal nitride and lowers the reaction temperature; in the subsequent heat treatment, the content of the heterostructure components can be controlled by adjusting the reduction time and temperature. This invention achieves low-cost, green, simple, efficient, and scalable preparation of two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride catalytic materials.

[0102] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function, characterized in that: Includes the following steps: (i) Mix the decomposable alkali metal compound and the bulk layered transition metal sulfide evenly to obtain the nitridation precursor; In step (i), the molar ratio of bulk layered transition metal sulfides to decomposable alkali metal compounds is 1:0.5 to 1:

6. (ii) The nitriding precursor obtained in step (i) is subjected to desulfurization and nitriding treatment in an ammonia-containing gaseous precursor to obtain a mixed phase including by-products and two-dimensional transition metal nitrides; The ammonia-containing gaseous precursor in step (ii) is a mixture of ammonia and argon or a mixture of ammonia and nitrogen, wherein the volume fraction of ammonia is not less than 50%; the conditions for the desulfurization and nitriding treatment are: the flow rate of the ammonia-containing gaseous precursor is controlled at 50 sccm to 200 sccm, the nitriding temperature is 650℃ to 900℃, and the heating rate is 1℃·min. -1 ~30℃·min -1 The thermal nitriding time is 1h to 20h; (iii) The mixed phase obtained in step (ii) is subjected to ultrasonic acid washing in an acidic solution to obtain two-dimensional single-crystal transition metal nitride nanosheets; (iv) The two-dimensional single-crystal transition metal nitride nanosheets obtained in step (iii) are heat-treated in a hydrogen-containing gaseous precursor and the treatment time is adjusted to obtain two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitrides. The hydrogen-containing gaseous precursor in step (iv) is H2 or a mixture of H2 and Ar; the heat treatment conditions are as follows: the flow rate of the hydrogen-containing gaseous precursor is controlled at 50 sccm to 200 sccm, the heat treatment temperature is 400℃ to 1000℃, and the heating rate is 1℃·min. -1 ~30℃·min -1 The heat treatment time is 4h to 24h.

2. The method for preparing a two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material with bidirectional catalytic function according to claim 1, characterized in that: The mixing in step (i) is carried out by dry ball milling, with a ball milling speed range of 800 rpm to 1000 rpm and a ball milling time of 0.5 h to 10 h.

3. The method for preparing two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride materials with bidirectional catalytic function according to claim 1, characterized in that: The acidic solution in step (iii) is any one or more of HCl, H2SO4, or HNO3, and the H2SO4 content of the acidic solution is... + The concentration is 1M to 6M; the ultrasound time is 0.25h to 2h; the mass ratio between the acidic solution and the mixed phase is ≥10:

1.

4. A two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material, characterized in that: Prepared by the method described in any one of claims 1 to 3.

5. A two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material, characterized in that: The transition metal nitride is composed of MoN and Mo2N, and a heterostructure is formed between MoN and Mo2N; the thickness of the MoN-Mo2N heterojunction is 5nm to 20nm, the size is 1μm to 2μm, and the specific surface area is ≥35m². 2 ·g -1 Its electronic conductivity is ≥8 S·cm -1 Hole enrichment occurs on the Mo2N side and electron enrichment occurs on the MoN side at the MoN-Mo2N heterojunction interface.

6. The application of the two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride as described in claim 4 or 5 in lithium-sulfur batteries, characterized in that: The two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride is used as an anode catalyst.

7. A lithium-sulfur battery, characterized in that: The anode catalyst of the lithium-sulfur battery adopts the two-dimensional nitrogen-deficient / nitrogen-rich heterostructure transition metal nitride material as described in claim 4 or 5.

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