A co-doped carbon nanocage loaded VN / Mo2C heterostructure material, a preparation method thereof and application thereof in lithium-sulfur batteries

Abstract

This invention discloses a Co-doped carbon nanocage material (labeled VN / Mo2C / Co-NC) with a VN / Mo2C heterostructure loaded on its surface, its preparation method, and its application in lithium-sulfur batteries. The specific surface area of ​​the VN / Mo2C / Co-NC material is 151.03 m². 2 g⁻ 1 The material has a size of 200-250 nm. The preparation process of this VN / Mo2C / Co-NC material is as follows: surface coating of CoZn-ZIF material, followed by high-temperature carbonization to obtain Co-doped carbon nanocages (labeled Co-NC); and loading a VN / Mo2C heterostructure catalyst onto the Co-NC surface. VN / Mo2C / Co-NC can achieve highly efficient catalytic conversion of polysulfides. A battery separator modified with VN / Mo2C / Co-NC material is labeled VN / Mo2C / Co-NC@PP. The lithium-sulfur battery using the VN / Mo2C / Co-NC@PP modified separator exhibits excellent comprehensive performance, especially due to the high catalytic activity of the VN / Mo2C heterostructure in this material for polysulfides, which effectively suppresses the shuttle effect of polysulfides, thus endowing the lithium-sulfur battery with excellent long-cycle stability and high-rate performance, meeting the performance requirements of energy storage or power batteries.

Description

Technical Field

[0003] This invention relates to a Co-doped nanocage-supported VN / Mo2C heterostructure material, its preparation method, and its application in lithium-sulfur batteries, belonging to the field of lithium-sulfur battery technology. Background Technology

[0005] With the increasing demand for high-energy-density energy storage systems in new energy vehicles, aerospace, and large-scale energy storage, lithium-sulfur batteries have become a promising next-generation energy storage battery system due to their extremely high theoretical energy density (five times that of commercial lithium-ion batteries), abundant sulfur resources, low cost, and environmental friendliness. However, the practical application of lithium-sulfur batteries is constrained by many inherent problems: sulfur and its discharge products Li2S / Li2S2 have poor electronic conductivity; soluble lithium polysulfides generated during charging and discharging migrate between the positive and negative electrodes, causing a severe shuttle effect, resulting in loss of active material, negative electrode passivation, and rapid capacity decay; the slow kinetics of sulfur redox reactions lead to low sulfur utilization and short battery cycle life; in addition, uneven Li⁺ deposition can easily induce lithium dendrite growth, posing a battery safety hazard.

[0006] Among numerous solutions, functional coating modification of the separator is a simple and efficient method. By constructing a functional layer on the separator surface, both the blocking of lithium polysulfides and the enhancement of redox kinetics can be achieved simultaneously. Currently, coating materials for separator modification are mainly divided into three categories: 1) Carbon materials (graphene, carbon nanotubes, MOF-derived carbon, etc.), which provide physical confinement due to their high conductivity and large specific surface area, but their non-polar surfaces have no chemical interaction with lithium polysulfides, resulting in limited blocking effects; 2) Polar materials (polymers, transition metal oxides, etc.), which can capture lithium polysulfides through strong chemisorption, but excessive adsorption can easily lead to the accumulation of active substances and lack catalytic conversion ability; 3) Catalytic materials such as transition metal chalcogenides, which can significantly improve the conversion kinetics of lithium polysulfides, but the lack of catalytic active sites often limits their catalytic effect. Therefore, it is evident that single-component materials cannot simultaneously meet the multiple requirements of high conductivity, strong adsorption, and high catalytic efficiency.

[0007] Heterostructured materials, through rational composition and interface design, can possess both adsorption and catalytic functions. By adjusting the interfacial electronic structure, they exhibit excellent performance in the fixation and conversion of polysulfides, becoming a research hotspot for lithium-sulfur battery separator modification layers. In recent years, heterostructures with synergistic adsorption-catalysis effects have been widely used in separator modification materials. For example, reference 1 (Zhou, F., Yuan, X., Xiong, X. et al. In-situ grown Ti3C2@TiO2heterostructure enables high-performance lithium-sulfur batteries with polysulfides capture and catalytic conversion mechanism [J]. Journal of Central South University, 2024, 31, 4397–4410.) describes the in-situ ultraviolet photocatalytic synthesis method to generate TiO2 on the surface of Ti2C1 nanosheets, forming a Ti2C1@TiO2 heterostructure. This catalyst exhibits strong adsorption and rapid catalytic conversion capabilities for lithium polysulfides. However, the supports used to load heterostructures lack sufficient active sites, making it difficult to maximize the catalytic activity of the heterostructures and resulting in poor battery cycle stability. Reference 2 (Hou J, Yang M, Guo X, et al. Chemisorption-catalysis synergy driven VN-V2O3 anchored on reduced graphene oxide heterostructure for stable lithium-sulfur battery separators[J]. Journal of Power Sources, 2025, 645, 237197.) synthesized VN-V2O3 heterostructures on graphene substrates via a hydrothermal method. While the graphene substrate used in this VN-V2O3@rGO material has high conductivity, its anchoring ability for lithium polysulfides is weak. This structure cannot enable efficient catalytic conversion of lithium polysulfides on the catalyst, resulting in low utilization of the active material. Furthermore, the hydrothermal synthesis method is costly, and the high-temperature, high-pressure experimental conditions are dangerous, making it unsuitable for large-scale production. Therefore, it is necessary to explore a lithium-sulfur battery separator modification material that has a high catalytic effect on lithium polysulfides, a simple synthesis process, and good stability.

[0008] This invention proposes using cobalt-doped nitrogen-containing hollow carbon nanocages (labeled Co-NC) derived from MOFs as a carrier to load VN / Mo2C heterostructures to form VN / Mo2C / Co-NC composite materials, achieving synergistic effects of physical confinement, chemical adsorption, and catalytic conversion; effectively suppressing the shuttle effect of lithium polysulfides and accelerating sulfur redox kinetics in lithium-sulfur batteries, while inhibiting lithium dendrite growth, thus achieving stable long cycle life. Summary of the Invention

[0010] To address the aforementioned problems in existing technologies, the first objective of this invention is to provide a battery separator modified with a VN / Mo2C / Co-NC composite material. The functional layer constructed on the surface of this modified separator combines physical confinement, chemical adsorption, and catalytic conversion functions. On one hand, the Co-NC hollow carbon nanocage structure constructs an interconnected conductive network, ensuring efficient electron and Li⁺ transport and providing structural support for the efficient adsorption and conversion of lithium polysulfides. On the other hand, the VN / Mo2C heterostructure exerts a synergistic effect of "strong adsorption and high catalysis." The interface optimization of the heterostructure promotes charge transfer, enhances the chemical anchoring of soluble lithium polysulfides, and lowers the reaction energy barrier for the liquid-solid conversion of lithium polysulfides.

[0011] The second objective of this invention is to provide a method for preparing the aforementioned VN / Mo2C / Co-NC composite material. First, using CoZn-ZIF as a precursor, a dopamine hydrochloride protective layer is coated onto its surface. This layer is then carbonized at high temperature under nitrogen to form cobalt-doped hollow carbon nanocages (labeled Co-NC). Subsequently, a VN / Mo2C heterostructure is loaded onto the surface to form the VN / Mo2C / Co-NC composite material. This process is simple and suitable for large-scale industrial production.

[0012] A third objective of this invention is to provide the application of the modified separator described above in lithium-sulfur batteries, using it as a battery separator in lithium-sulfur battery assembly. Based on the special structure and high catalytic activity of the modified separator material of this invention, the lithium-sulfur battery prepared as a battery separator exhibits excellent overall performance, significantly improving the cycle stability, rate performance, and electrochemical performance under high sulfur loading, thus meeting the practical application requirements of lithium-sulfur batteries.

[0013] To achieve the above technical objectives, the present invention provides a VN / Mo2C / Co-NC composite material modified lithium-sulfur battery separator, which consists of a battery separator (preferably Celgard 2500 PP, denoted as PP) and a functional modification layer covering the surface of the PP base film. The functional modification layer is a VN / Mo2C / Co-NC composite material, wherein Co-NC is a Co-doped hollow carbon nanocage derived from CoZn-ZIF, providing a high specific surface area loading substrate for the VN / Mo2C heterostructure, while simultaneously forming a physical barrier to alleviate the shuttle effect of lithium polysulfides. The VN / Mo2C heterojunction is loaded inside and on the surface of the Co-NC cage, enhancing the chemisorption of lithium polysulfides through interfacial charge transfer and reducing the energy barrier for their conversion, thus achieving efficient catalysis. The areal density of the functional modification layer is 0.5~0.7 mg cm⁻. 2 This thickness ensures functionality without excessively increasing the battery's size and weight.

[0014] As a preferred embodiment, in the VN / Mo2C / Co-NC composite material, the average particle size of the hollow carbon nanocage Co-NC is 200 nm, and carbon nanotubes grow on the surface due to the catalytic effect of Co, which further improves the conductivity and specific surface area of ​​the composite material.

[0015] As a preferred embodiment, in the VN / Mo2C / Co-NC composite material, the characteristic crystal plane of VN in the heterojunction VN / Mo2C is (200), and the characteristic crystal plane of Mo2C is (101). The two form a clear hetero interface, which can enhance the charge transfer during the battery charging and discharging process and promote the catalytic conversion of lithium polysulfides.

[0016] As a preferred embodiment, the hollow carbon nanocages in the VN / Mo2C / Co-NC composite material have a specific surface area of ​​151.03 m². 2 g⁻ 1 It has a mesoporous porous structure, which is beneficial for electrolyte wetting and Li⁺ transport; the contact angle of the modified membrane with the electrolyte is close to 0°, which is much lower than the 43.7° of the pure PP base membrane.

[0017] The present invention also provides a method for preparing the above-mentioned VN / Mo2C / Co-NC composite material, wherein a layer of dopamine hydrochloride protective layer is coated on the surface of CoZn-ZIF to form a CoZn-ZIF@PDA precursor, which is then carbonized at high temperature under nitrogen to form cobalt-doped hollow carbon nanocages Co-NC, and subsequently a VN / Mo2C heterostructure is loaded on the surface to form VN / Mo2C / Co-NC.

[0018] As a preferred embodiment, the preparation method of the CoZn-ZIF@PDA precursor is as follows: Zn(NO3)2·6H2O, Co(NO3)2·6H2O, and dodecyltrimethylammonium chloride are ultrasonically dispersed in deionized water at a mass ratio of 505:87:10 to obtain solution A; 2-methylimidazole is dissolved in deionized water to obtain solution B; solution A is slowly poured into solution B; after magnetic stirring at room temperature for 24 h, the solid substance is obtained by centrifugation and drying to obtain CoZn-ZIF; 100 mg of CoZn-ZIF is uniformly dispersed in 50 mL of methanol, 10 mL of methanol solution containing 60 mg of dopamine hydrochloride is added, and after stirring at room temperature for 12 h, the mixture is washed with methanol and dried to obtain the CoZn-ZIF@PDA precursor.

[0019] As a preferred embodiment, the preparation method of Co-NC is as follows: CoZn-ZIF@PDA is placed in a tube furnace and heated at 5 °C min⁻ under a nitrogen atmosphere. 1 The temperature was increased to 900 °C at a certain rate, held for 3 h, and then naturally cooled to obtain Co-NC.

[0020] As a preferred embodiment, the preparation method of the VN / Mo2C / Co-NC is as follows: 100 mg Co-NC and 50 mg (NH4)6Mo7O 24 • Mix 4H2O, 50 mg NH4VO3, and 150 mg melamine and grind them thoroughly. Place the mixture in a tube furnace and heat it at 5 °C for 1 min under a nitrogen atmosphere. 1 The temperature was increased to 900 °C at a certain rate, held for 3 h, and then naturally cooled to obtain VN / Mo2C / Co-NC.

[0021] As a preferred embodiment, the preparation method of the VN / Mo2C / Co-NC composite material modified membrane is as follows: VN / Mo2C / Co-NC composite material, Super P conductive agent, and PVDF binder are dispersed in NMP solvent at a mass ratio of 7:2:1 and stirred to obtain a uniform slurry; the slurry is coated onto the surface of Celgard 2500 PP base film by a blade coating method and dried overnight in a vacuum oven at 60 ℃ to obtain the VN / Mo2C / Co-NC composite material modified membrane.

[0022] The present invention also provides the application of the above-mentioned VN / Mo2C / Co-NC composite material modified separator in lithium-sulfur batteries, which is used as a separator component in lithium-sulfur batteries, wherein the lithium-sulfur batteries are composed of a positive electrode, a negative electrode, the above-mentioned modified separator and an electrolyte.

[0023] As a preferred embodiment, the cathode is an S / CNTs composite cathode, prepared by melt diffusion method: carbon nanotubes (CNTs) and sulfur powder are mixed and ground at a mass ratio of 3:7, and kept at 155 ℃ for 12 h under a nitrogen atmosphere to obtain S / CNTs composite material; S / CNTs, Super P, and PVDF are dispersed in NMP at a mass ratio of 8:1:1 to form a slurry, which is then coated onto the surface of carbon-coated aluminum foil (carbon cloth for high sulfur loading), vacuum dried at 60 ℃, and then sheeted, with a sulfur loading of 1~3.18 mg cm⁻. 2 .

[0024] As a preferred embodiment, the negative electrode is a lithium metal foil; the electrolyte is a mixed solution of 1.0 M LiTFSI and 0.2 wt.% LiNO3, with DOL and DME in a 1:1 volume ratio as solvents; and conventional sulfur loading (1~1.5 mg cm⁻). 2 When the electrolyte / sulfur ratio is 20 μL mg⁻ 1 High sulfur loading (3.18 mg cm⁻) 2 When the electrolyte / sulfur ratio is 13 μL mg⁻ 1 .

[0025] Compared with the prior art, the beneficial technical effects of the present invention are reflected in the following aspects:

[0026] 1) Triple functional synergy for efficient regulation of lithium polysulfide electrochemical processes: Co-NC hollow carbon nanocages provide a large specific surface area and porous structure to achieve physical confinement of lithium polysulfides; VN / Mo2C heterojunctions form strong chemisorption with lithium polysulfides through interfacial charge transfer, while lowering the energy barrier for lithium polysulfide reduction, thus achieving efficient catalytic conversion; the triple functions of physical confinement, chemisorption, and catalytic conversion work synergistically to improve sulfur oxidation-reduction kinetics and suppress the shuttle effect.

[0027] 2) Improve battery interface performance and suppress lithium dendrites: The mesoporous structure and high polarity of the modified layer significantly improve electrolyte wettability and reduce Li⁺ transport resistance; the conductive network of Co-NC promotes uniform deposition of Li⁺, effectively suppresses the growth of lithium dendrites, and improves battery safety.

[0028] 3) The preparation process is simple and suitable for industrialization: This invention uses MOFs as precursors to prepare VN / Mo2C / Co-NC composite materials through one-step carbonization and one-step high-temperature annealing. Then, a modified membrane is prepared by a simple coating method. The whole process does not require expensive equipment, the raw materials are readily available, the operation is simple, and it is suitable for industrial production.

[0029] 4) Lithium-sulfur batteries modified with VN / Mo2C / Co-NC composite materials have excellent comprehensive performance, which can effectively suppress the shuttle effect, improve the cycle stability, discharge specific capacity and cycle life of the battery, and meet the performance requirements of energy storage or power batteries. Attached Figure Description

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0032] Figure 1 This is a schematic diagram illustrating the preparation of the membrane modified layer materials obtained in Examples 1 and 2;

[0033] Figure 2 These are TEM images of Examples 1-4;

[0034] Figure 3 The XRD patterns are for Examples 1-4;

[0035] Figure 4 The BET diagram for Example 2;

[0036] Figure 5 Examples 1-4 show the adsorption tests and UV-Vis spectra of lithium polysulfides.

[0037] Figure 6 SEM images of the PP separator and Example 6;

[0038] Figure 7 The contact angle test results are for the PP diaphragm and Example 6.

[0039] Figure 8 Permeation test results for the PP separator and lithium polysulfide in Example 6;

[0040] Figure 9 This is a schematic diagram of the button cell structure assembled according to the present invention;

[0041] Figure 10 Cycling performance of the lithium-sulfur batteries assembled with PP separator and Examples 5-8 at a current density of 1.0 C;

[0042] Figure 11 Cycle performance graphs of PP separators and lithium-sulfur batteries assembled in Examples 5-8 at different current densities;

[0043] Figure 12 SEM images of the lithium anode after a certain number of cycles for the PP separator and the lithium-sulfur battery assembled in Example 6.

[0044] Figure 13 The PP separator and the Li‖Li symmetric cell assembled in Example 6 were used at 0.5 mA cm⁻ 20.5 mAh cm⁻ 2 Cyclic performance under certain conditions. Detailed Implementation

[0046] The concept and technical effects of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the protection scope of the present invention.

[0047] Example 1

[0048] A1. Disperse 505 mg Zn(NO3)2・6H2O, 87 mg Co(NO3)2・6H2O, and 10 mg dodecyltrimethylammonium chloride in 20 mL of deionized water using ultrasonication to obtain solution A; dissolve 9 g of 2-methylimidazole in 140 mL of deionized water to obtain solution B; slowly pour solution A into solution B, stir magnetically at room temperature for 24 h, centrifuge, wash with deionized water, and dry at 80 ℃ to obtain CoZn-ZIF;

[0049] A2. Disperse 100 mg CoZn-ZIF in 50 mL of methanol, add 10 mL of methanol solution containing 60 mg dopamine hydrochloride, stir at room temperature for 12 h, wash three times with methanol, and dry at 60 °C to obtain CoZn-ZIF@PDA.

[0050] A3. Place the CoZn-ZIF@PDA in a tube furnace and incubate at 5 °C min⁻ under a nitrogen atmosphere. 1 The temperature was raised to 900 °C, held for 3 hours, and then allowed to cool naturally to room temperature. A schematic diagram of the preparation process is shown below. Figure 1 As shown.

[0051] The membrane modification material prepared in this embodiment is denoted as Co-NC.

[0052] Example 2

[0053] The Co-NC obtained in Example 1, 50 mg of (NH4)6Mo7O 24 • Grind 4H2O, 50 mg NH4VO3, and 150 mg melamine thoroughly, and place in a tube furnace under a nitrogen atmosphere for 5 °C min⁻ 1 The temperature was raised to 900 °C, held for 3 hours, and then allowed to cool naturally to room temperature. A schematic diagram of the preparation process is shown below. Figure 1 As shown.

[0054] The membrane modification material prepared in this embodiment is denoted as VN / Mo2C / Co-NC.

[0055] Example 3

[0056] The Co-NC obtained in Example 1 was mixed with 100 mg of (NH4)6Mo7O 24 • Mix thoroughly with 4H2O and grind, then heat in a tube furnace under a nitrogen atmosphere for 5 ℃ min⁻ 1 The temperature is raised to 900 ℃, held for 3 hours, and then naturally cooled to room temperature.

[0057] The membrane modification material prepared in this embodiment is denoted as Mo2C / Co-NC.

[0058] Example 4

[0059] The Co-NC obtained in Example 1 was mixed with 100 mg NH4VO3 and 150 mg melamine and ground thoroughly. The mixture was then heated in a tube furnace under a nitrogen atmosphere for 5 °C min⁻. 1 The temperature is raised to 900 ℃, held for 3 hours, and then naturally cooled to room temperature.

[0060] The membrane modification material prepared in this embodiment is denoted as VN / Co-NC.

[0061] Example 5

[0062] The Co-NC, Super P, and PVDF obtained in Example 1 were dispersed in N-methylpyrrolidone solvent at a mass ratio of 7:2:1 and ultrasonically stirred to form a slurry. The slurry was coated onto a Celgard 2500 PP base film and vacuum dried overnight at 60 °C. After drying, it was cut into round pieces with a diameter of 19 mm.

[0063] The Co-NC modified diaphragm prepared in this embodiment is denoted as Co-NC@PP.

[0064] Example 6

[0065] The VN / Mo2C / Co-NC, Super P, and PVDF obtained in Example 2 were dispersed in N-methylpyrrolidone solvent at a mass ratio of 7:2:1 and ultrasonically stirred to form a slurry. The slurry was coated onto a Celgard 2500 PP base film and vacuum dried overnight at 60 °C. After drying, it was cut into discs with a diameter of 19 mm.

[0066] The VN / Mo2C / Co-NC modified separator prepared in this embodiment is denoted as VN / Mo2C / Co-NC@PP.

[0067] Example 7

[0068] The Mo2C / Co-NC, Super P, and PVDF obtained in Example 3 were dispersed in N-methylpyrrolidone solvent at a mass ratio of 7:2:1 and ultrasonically stirred to form a slurry. The slurry was coated onto a Celgard 2500 PP base film and vacuum dried overnight at 60 °C. After drying, it was cut into discs with a diameter of 19 mm.

[0069] The Mo2C / Co-NC modified diaphragm prepared in this embodiment is designated as Mo2C / Co-NC@PP.

[0070] Example 8

[0071] The VN / Co-NC, Super P, and PVDF obtained in Example 4 were dispersed in N-methylpyrrolidone solvent at a mass ratio of 7:2:1 and ultrasonically stirred to form a slurry. The slurry was coated onto a Celgard 2500 PP base film and vacuum dried overnight at 60 °C. After drying, it was cut into discs with a diameter of 19 mm.

[0072] The VN / Co-NC modified diaphragm prepared in this embodiment is denoted as VN / Co-NC@PP.

[0073] Results Analysis

[0074] The morphology, structure, and adsorption performance of Examples 1-4 were characterized and tested in this invention, and the process and results are analyzed as follows:

[0075] 1) Morphological and structural characterization of materials

[0076] The composite materials of Examples 1-4 were characterized by TEM, XRD, and BET tests:

[0077] Depend on Figure 2 As can be seen from the TEM test results, Examples 1-4 all exhibit hollow carbon nanocage structures with an average particle size of 200 nm, and carbon nanotubes are grown on the surface. In Example 2, VN (200) and Mo2C (101) form a clear heterojunction interface. The microstructure of Example 2 confirms the formation of hollow nanocages and the successful construction of the VN / Mo2C heterostructure.

[0078] Depend on Figure 3 As can be seen from the XRD test results, the Co-NC obtained in Example 1 has the characteristic diffraction peak of Co, the VN / Mo2C / Co-NC obtained in Example 2 has the characteristic diffraction peaks of VN, Mo2C and Co, the Mo2C / Co-NC obtained in Example 3 has the characteristic diffraction peaks of Mo2C and Co, and the VN / Co-NC obtained in Example 4 has the characteristic diffraction peaks of VN and Co. This indicates that the Co-NC, VN / Mo2C / Co-NC, Mo2C / Co-NC and VN / Co-NC obtained in Examples 1 to 4 were successfully synthesized.

[0079] Depend on Figure 4 As can be seen from the BET test results, the specific surface area of ​​Example 2 is 151.03 m². 2 g⁻ 1 Its pore size distribution is relatively wide and mainly consists of mesopores, classifying it as a high specific surface area mesoporous material. This porous framework structure can improve electrolyte wetting and reduce ion transport resistance, providing a structural basis for the VN / Mo2C heterostructure to exert a synergistic "adsorption-catalysis" effect.

[0080] 2) Adsorption performance test of lithium polysulfides of the materials obtained in Examples 1-4

[0081] Static adsorption test: Equal masses of Co-NC obtained in Example 1, VN / Mo2C / Co-NC obtained in Example 2, Mo2C / Co-NC obtained in Example 3, and VN / Co-NC obtained in Example 4 were added to 2 mL of 5 mM Li2S6 solution. After standing for 6 h, the color change of the solution was recorded and the supernatant was taken for UV-Vis test.

[0082] Depend on Figure 5 As can be seen, the solution treated with VN / Mo2C / Co-NC obtained in Example 2 is almost colorless, and the UV-Vis spectrum shows that its supernatant has the lowest absorbance. This indicates that the VN / Mo2C / Co-NC obtained in Example 2 can efficiently adsorb lithium polysulfides dissolved in the electrolyte, which is beneficial to the subsequent catalytic conversion of lithium polysulfides and improves the utilization rate of active materials.

[0083] The present invention conducted corresponding tests on the microstructure, electrolyte wettability, lithium polysulfide blocking effect, and the performance of the assembled button batteries of the modified separators obtained in Examples 5-8. The process and results are analyzed as follows:

[0084] 1) Microstructure of the diaphragm, contact angle, and lithium polysulfide permeation test

[0085] Depend on Figure 6 As can be seen from the SEM test results, the Celgard 2500 PP membrane surface exhibits a uniform mesh-like porous structure, which is insufficient to effectively suppress the shuttle effect of lithium polysulfides. SEM analysis of the modified membrane VN / Mo2C / Co-NC@PP obtained in Example 6 shows that VN / Mo2C / Co-NC forms a uniform and dense modification layer covering the PP base membrane, with a thickness of approximately 10.4 μm. This dense modification layer can anchor lithium polysulfides through strong chemisorption and also acts as a catalyst to accelerate the conversion kinetics of lithium polysulfides, thus suppressing their shuttle effect.

[0086] Depend on Figure 7As can be seen from the contact angle test, after the electrolyte was added and allowed to stand for a period of time, the contact angle of Example 6 with the electrolyte was close to 0°, while that of the Celgard 2500 PP membrane was 43.7°, indicating that Example 6 has better electrolyte wettability. Excellent electrolyte wettability can better accelerate ion migration during the redox reaction, ensuring the rapid conversion of lithium polysulfides.

[0087] Permeation test: In an H-type electrolytic cell, the modified membranes VN / Mo2C / Co-NC@PP and Celgard 2500 PP obtained in Example 6 were used as membranes to separate the Li2S6 solution and the blank electrolyte, respectively. After 24 h, the color change on the blank electrolyte side was recorded.

[0088] Depend on Figure 8 It can be seen that in the H-type electrolytic cell using the PP membrane, the blank electrolyte side turns dark yellow. However, in the H-type electrolytic cell using the modified membrane VN / Mo2C / Co-NC@PP obtained in Example 6, the blank electrolyte side shows almost no color change. This indicates that the modified membrane VN / Mo2C / Co-NC@PP obtained in Example 6 can effectively inhibit the permeation of lithium polysulfides. This is because the hollow carbon nanocage structure in the modified membrane VN / Mo2C / Co-NC@PP obtained in Example 6 can play a certain physical barrier role. Simultaneously, it indicates that the loaded VN and Mo2C also have a strong polar interaction force with lithium polysulfides, enhancing the chemical adsorption of lithium polysulfides.

[0089] 2) Lithium-sulfur battery assembly and electrochemical performance testing

[0090] In an argon glove box (O2, H2O < 0.5 ppm) according to Figure 9 Assemble CR2025 coin cell lithium-sulfur batteries in the following sequence: The positive electrode is an S / CNTs composite positive electrode (sulfur loading 1~1.5 mg cm⁻). 2 The negative electrode is lithium metal foil, and the separators are Celgard 2500 PP separators and modified separators obtained in Examples 5-8, respectively. The electrolyte is a 1.0 M LiTFSI + 0.2 wt.% LiNO3 DOL / DME (1:1) solution, and the electrolyte / sulfur ratio is 20 μL mg⁻⁻⁶. 1 Under constant temperature conditions of 30 ℃, constant current charge-discharge tests were conducted using the Blue Electric testing system. The test voltage range was 1.7~2.8 V, and the performance test data for the coin cell full cell with a test current density of 1 C are as follows: Figure 10 As shown in Table 1; the rate performance test data for button cell full cells are as follows: Figure 11 As shown in Table 2.

[0091] Table 1 Cycle performance of coin cell lithium-sulfur batteries at a current density of 1 C

[0092]

[0093] From Table 1 and Figure 10 It can be seen that, at a 1 C rate, the reversible capacity of the lithium-sulfur coin cell obtained in Example 6 after 1000 cycles using the VN / Mo2C / Co-NC@PP method is 547 mAh g⁻. 1 The capacity decay rate was only 0.047% per cycle; while the lithium-sulfur batteries using the Co-NC@PP obtained in Example 5, the Mo2C / Co-NC@PP obtained in Example 7, the VN / Co-NC@PP obtained in Example 8, and the Celgard 2500PP separators, respectively, showed capacities of 362.3 mAh g⁻ after 1000 cycles at 1 C. 1 460.5 mAh g⁻ 1 420.9mAh g⁻ 1 245.4 mAh g⁻ 1 The degradation rates were 0.058%, 0.052%, 0.055%, and 0.064% per cycle, respectively. This indicates that the lithium-sulfur battery assembled using the VN / Mo2C / Co-NC@PP obtained in Example 6 can ensure that lithium polysulfides can undergo redox reactions smoothly, exhibiting high specific capacity and good long-term cycle stability.

[0094] Table 2 Rate Performance of Button Lithium-Sulfur Batteries

[0095]

[0096] From Table 2 and Figure 11 It can be seen that the specific capacities of the lithium-sulfur coin cell using the VN / Mo2C / Co-NC@PP obtained in Example 6 are 1364.3, 1215.6, 1058.2, 926.5, and 878 mAh g⁻¹ at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. When the current recovers to 0.1 C, the capacity recovers to 1164.7 mAh g⁻¹, with a recovery rate of 85.36%, which is better than that of the modified separators obtained in Examples 5, 7, and 8, and the Celgard 2500 PP separator, respectively. This indicates that the lithium-sulfur coin cell using the VN / Mo2C / Co-NC@PP obtained in Example 6 has high electrochemical reversibility.

[0097] 3) SEM image of the lithium anode in the cycled lithium-sulfur battery

[0098] Given that the migration of lithium polysulfides to the negative electrode can trigger lithium metal side reactions and corrosion, lithium-sulfur batteries assembled using the VN / Mo2C / Co-NC@PP and Celgard 2500 PP separators obtained in Example 6 were further disassembled after the same number of cycles, and the morphology of the lithium negative electrode was compared. The characterization results are as follows: Figure 12 As shown.

[0099] Depend on Figure 12 It can be seen that the lithium anode surface of the lithium-sulfur coin cell using the VN / Mo2C / Co-NC@PP obtained in Example 6 is smoother and more even, while the lithium anode of the lithium-sulfur coin cell assembled using the Celgard 2500 PP separator is severely corroded, with not only a large amount of lithium polysulfides adhering to it, but also lithium dendrite growth caused by uneven lithium deposition. The results indicate that the VN / Mo2C / Co-NC@PP obtained in Example 6 can effectively suppress the shuttle effect of lithium polysulfides and alleviate the side reactions of the lithium anode, thereby improving the long-cycle stability of the battery.

[0100] 4) Assembly and performance testing of Li||Li symmetric cells

[0101] The Li||Li symmetric battery used lithium-ion sheets as both positive and negative electrodes, and the separator was the VN / Mo2C / Co-NC@PP and Celgard 2500 PP obtained in Example 6. The electrolyte was a lithium-sulfur battery electrolyte. Testing was conducted using a blue electric shock testing system under a constant temperature of 30 °C. The Li||Li symmetric battery assembled using the VN / Mo2C / Co-NC@PP and Celgard 2500 PP separator obtained in Example 6 achieved a performance of 0.5 mA cm⁻¹. 2 0.5 mAh cm⁻ 2 Test data under the conditions such as Figure 13 As shown.

[0102] Depend on Figure 13 It can be seen that the symmetric cell assembled using the VN / Mo2C / Co-NC@PP obtained in Example 6 can be stably cycled for 1100 h with an overpotential of about 20 mV; while the symmetric cell assembled using the Celgard 2500 PP separator only cycled for 236 h before the overpotential rose to 60 mV and the voltage fluctuated significantly. This indicates that the VN / Mo2C / Co-NC@PP obtained in Example 6 can effectively promote uniform Li⁺ deposition and suppress lithium dendrite growth.

[0103] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principles and spirit of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A Co-doped carbon nanocage-supported VN / Mo2C heterostructure material (labeled VN / Mo2C / Co-NC), characterized in that: The carbon nanocages (labeled Co-NC) have an average particle size of 100–400 nm, and carbon nanotubes are grown on their surface. In the VN / Mo2C heterostructure, the exposed crystal plane of VN is (200), and the exposed crystal plane of Mo2C is (101), forming a clear heterostructure interface. The VN / Mo2C heterostructure is loaded inside and on the surface of the Co-NC cages. The specific surface area of ​​the VN / Mo2C / Co-NC composite material is 50–300 m². 2 g⁻ 1 The pore size distribution is relatively wide, and it is mainly mesopore.

2. The VN / Mo2C / Co-NC composite material of claim 1, wherein the preparation process is as follows: (1) Zn(NO3)2·6H2O, Co(NO3)2·6H2O, and dodecyltrimethylammonium chloride (CTAB) are dissolved in deionized water in a mass ratio preferably 505:87:10 to obtain solution A, and 2-methylimidazole is dissolved in deionized water to obtain solution B. Solution A and solution B are mixed, and after magnetic stirring, the solid product is obtained by centrifugation and dried to obtain CoZn-ZIF; (2) CoZn-ZIF and dopamine hydrochloride are dispersed in methanol in a mass ratio preferably 5:3, and after stirring, the solid product is obtained by centrifugation and dried to obtain CoZn-ZIF@PDA; (3) The CoZn-ZIF@PDA precursor is carbonized at 900 °C for 3 h in a nitrogen atmosphere to obtain Co-doped carbon nanocages (labeled as Co-NC); (4) Co-NC and molybdenum source (preferably (NH4)6Mo7O) are mixed. 24 • 4H2O), vanadium source (preferably NH4VO3), and melamine are mixed and ground evenly in a mass ratio of 100:50:50:150, and then carbonized at 900 °C for 3 h under a nitrogen atmosphere to obtain VN / Mo2C / Co-NC composite material.

3. A VN / Mo2C / Co-NC modified battery separator (labeled as VN / Mo2C / Co-NC@PP), characterized in that: The modified separator consists of a battery separator (preferably Celgard 2500 PP, denoted as PP) and a functionally modified layer containing VN / Mo2C / Co-NC composite material covering the surface of the PP base film. The modified separator is prepared by: dispersing the VN / Mo2C / Co-NC material described in claims 1 and 2, a conductive agent (preferably Super P), and a binder (preferably PVDF) in a mass ratio preferably 7:2:1 in N-methylpyrrolidone (NMP), and stirring to obtain a uniform slurry; coating the slurry onto the surface of the PP base film, and vacuum drying (preferably at a temperature of 60 °C, preferably at a drying time of 12 h or more) to obtain VN / Mo2C / Co-NC@PP; the areal density of the functionally modified layer is 0.5~0.7 mgcm⁻ 2 .

4. The application of the VN / Mo2C / Co-NC@PP modified separator as described in claim 3 in lithium-sulfur batteries, characterized in that: The VN / Mo2C / Co-NC@PP modified separator is used as a separator component in a lithium-sulfur battery. The lithium-sulfur battery consists of a positive electrode, a negative electrode, the VN / Mo2C / Co-NC@PP modified separator as described in claim 3, and an electrolyte; the positive electrode is a sulfur electrode (preferably an S / CNTs composite positive electrode, preferably with a sulfur mass percentage of 70 wt% and a sulfur loading of 1~1.5 mg cm⁻). 2 The negative electrode is a lithium metal foil; the electrolyte is preferably a mixed solution of 1.0 M LiTFSI and 0.2 wt.% LiNO3, and the preferred solvent is 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) in a volume ratio of 1:1.

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