A bifunctional three-dimensional porous composite material and a preparation method and application thereof

Abstract

This invention discloses a bifunctional three-dimensional porous composite material and its preparation method and application. The method includes the following steps: (1) adding a carbon source to deionized water and stirring to obtain a uniformly dispersed solution; (2) dissolving a rare earth metal source, a transition metal source, and a sulfur source in deionized water and stirring to obtain a precursor solution; (3) mixing the solution obtained in step (1) and the precursor solution to obtain a mixed solution and performing a hydrothermal reaction; (4) repeatedly washing the product obtained in step (3), drying it, and then performing heat treatment to prepare the bifunctional three-dimensional porous composite material. The bifunctional three-dimensional porous composite material can effectively buffer the volume expansion effect of the sulfur cathode during charging and discharging and promote the full wetting of the electrolyte. On the other hand, it also improves the conductivity and catalytic activity of the composite material through the doping of rare earth elements, effectively improving the electrochemical performance of lithium-sulfur batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery technology, specifically to a dual-functional three-dimensional porous composite material, its preparation method, and its application. Background Technology

[0002] Lithium-sulfur batteries are characterized by their high energy density (2600 Wh kg). -1 With its low cost and environmentally friendly characteristics, sulfur has become a highly promising candidate for next-generation energy storage systems. However, the sluggish redox kinetics of sulfur and the "shuttle effect" of lithium polysulfides (LiPSs) severely limit the cycle capacity and lifespan of batteries, hindering their practical application.

[0003] To suppress the shuttle effect of LiPSs and improve the conversion efficiency of polysulfides, researchers have developed various catalytic systems, such as metal oxides, nitrides, and transition metal dichalcogenides. Among them, the layered structure of dichalcogenides formed by weak van der Waals forces exposes abundant active edge sites and can anchor polysulfides through dipole interactions of metal-sulfur (MS) and sulfur-lithium (S-Li) bonds, accelerating their redox conversion. However, the poor conductivity and basal inertness of dichalcogenides limit their catalytic efficiency in lithium-sulfur batteries. Therefore, further improving the conductivity and catalytic activity of dichalcogenides for LiPSs has become a key research focus. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a novel method for synthesizing lithium-sulfur battery cathode materials. This invention provides a bifunctional three-dimensional porous composite material, its preparation method, and its application, which solves the above problem by utilizing rare earth ions (such as Ce). 3+ Impurity energy levels are introduced near the Fermi level of transition metal dichalcogenide catalysts, inducing electronic structure reconstruction and widening interlayer spacing, thereby enhancing the intrinsic conductivity and catalytic activity of the catalyst. Simultaneously, a three-dimensional porous conductive framework constructed from two-dimensional carbon materials is used to prepare a bifunctional three-dimensional porous composite material. The composite material synthesized under this general method exhibits excellent conductivity, lithium polysulfide adsorption capacity, and catalytic conversion performance, and can be applied to the cathode of lithium-sulfur batteries, significantly improving the electrochemical performance of the battery.

[0005] In one aspect of the invention, a bifunctional three-dimensional porous composite material is proposed. According to an embodiment of the invention, the composite material consists of a three-dimensional porous framework constructed of two-dimensional carbon material and a rare-earth element-doped two-dimensional nanosheet catalyst supported thereon.

[0006] In addition, a dual-functional three-dimensional porous composite material according to the above embodiments of the present invention may also have the following additional technical features:

[0007] In some embodiments of the present invention, the mass ratio of the three-dimensional porous main framework to the rare earth element-doped two-dimensional nanosheet catalyst is (2~4):1; the two-dimensional nanosheet is a transition metal dichalcogenide; and the rare earth element is at least one of cerium, samarium, lanthanum, and neodymium.

[0008] On the one hand, the three-dimensional porous framework constructed from two-dimensional carbon materials can uniformly disperse two-dimensional catalysts, exposing more active sites, and stacking them to provide a three-dimensional conductive network, which can buffer the volume change of sulfur and promote electrolyte wetting. On the other hand, rare earth ions (such as Ce) doped into the dichalcogenide lattice... 3+ This effectively modulates the electronic structure of the catalyst, improving its intrinsic conductivity and adsorption capacity for lithium polysulfides. Furthermore, the introduction of rare earth elements expands the interlayer spacing of the dichalcogenides and induces more defects, exposing abundant catalytically active edge sites and enhancing its catalytic performance.

[0009] In another aspect, the present invention provides a method for preparing a bifunctional three-dimensional porous composite material, which, according to an embodiment of the present invention, includes the following steps:

[0010] (1) Add the carbon source to deionized water and stir to obtain a uniformly dispersed solution;

[0011] (2) Dissolve the rare earth metal source, the transition metal source and the sulfur source in deionized water and stir to obtain a precursor solution;

[0012] (3) Mix the solution obtained in step (1) with the precursor solution to obtain a mixed solution, and carry out a hydrothermal reaction;

[0013] (4) The product obtained in step (3) is repeatedly washed, dried and then heat-treated to prepare a bifunctional three-dimensional porous composite material.

[0014] The method for preparing a bifunctional three-dimensional porous composite material according to the embodiments of the present invention may further have the following additional technical features:

[0015] In some embodiments of the present invention, in step (1), the carbon source is single-layer or multi-layer graphene oxide with a sheet diameter of 0.5~20 μm and a thickness of 0.5~5 nm. The carbon source serves as the support for the catalyst generated in subsequent steps and as the main framework of the composite material. The surface of the graphene oxide is rich in oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy groups, which can achieve stable and uniform dispersion in aqueous solution, laying the foundation for the in-situ uniform growth of the subsequent catalyst. Furthermore, the reduced graphene oxide has high electrical conductivity and can be constructed into a continuously conductive framework.

[0016] In some embodiments of the present invention, in step (2), the metal source and the sulfur source are thoroughly dissolved and mixed in deionized water to carry out subsequent hydrothermal reactions to synthesize rare earth-doped catalysts; the molar ratio of the rare earth metal source to the transition metal source in the solution is (0.5~1):9, and the molar ratio of the sum of the rare earth metal source and the transition metal source to the sulfur source is 1:(2~3); the transition metal source is ammonium molybdate; the sulfur source is L-cysteine; the rare earth metal source is at least one of the nitrates of cerium, samarium, lanthanum, and neodymium. Ammonium molybdate has good water solubility as a molybdenum source and releases molybdate ions under hydrothermal conditions, which react with the sulfur source to generate molybdenum disulfide; L-cysteine ​​has the triple functions of reducing agent, sulfur source, and structure directing agent. Its thiol group (-SH) can slowly release S under hydrothermal conditions. 2- It reacts with ammonium molybdate to form molybdenum disulfide; at the same time, its reducing properties help to reduce graphene oxide. In addition, L-cysteine ​​has the advantages of being green and environmentally friendly; rare earth nitrates can provide rare earth ions, have high water solubility, and are easily and uniformly dispersed in precursor solutions.

[0017] In some embodiments of the present invention, in step (3), the concentration of the carbon source in the mixed solution is 1~3 mg / mL. -1 The hydrothermal reaction temperature is 180–210 °C, and the reaction time is 16–20 h. Under hydrothermal conditions, the molybdenum source and the sulfur source undergo a sulfidation reaction, and rare earth elements are simultaneously doped and modified to generate a rare earth-doped molybdenum disulfide two-dimensional catalyst. Simultaneously, L-cysteine ​​reduces graphene oxide to reduced graphene oxide (rGO). The rGO catalyst undergoes heterogeneous nucleation, growth, and self-assembly on its surface, ultimately forming a composite structure in one step. The selected reaction temperature and time promote complete decomposition of the sulfur source, ensure complete catalyst crystallization, and achieve efficient reduction of graphene oxide, while avoiding excessive grain growth.

[0018] In some embodiments of the present invention, in step (4), the heat treatment is carried out in a tube furnace under an argon atmosphere, at a temperature of 300-450 °C for 1-2 h, with a heating rate of 5 °C / min; the drying is carried out by freeze drying for 24-48 h. Freeze drying is used to rapidly dehydrate under low temperature and vacuum, avoiding structural stacking and pore collapse caused by surface tension during liquid-phase drying, thereby maximizing the preservation of the porous structure and high specific surface area of ​​the material, which is beneficial for subsequent electrolyte wetting and rapid ion diffusion; the main purpose of calcining the dried sample in a tube furnace is to further improve the crystallinity of the catalyst and eliminate residual amorphous impurities and structural defects in the hydrothermal products. The material exhibits good crystal integrity and structural stability under the heat treatment temperature and time.

[0019] In some embodiments of the present invention, the mass ratio of the three-dimensional porous main framework to the rare earth element-doped two-dimensional nanosheet catalyst is (2~4):1; the two-dimensional nanosheet is a transition metal dichalcogenide; and the rare earth element is at least one of cerium, samarium, lanthanum, and neodymium.

[0020] In another aspect, the present invention provides a lithium-sulfur battery cathode. According to an embodiment of the invention, it is prepared using the aforementioned bifunctional three-dimensional porous composite material.

[0021] In another aspect, the present invention provides a method for preparing a lithium-sulfur battery cathode. According to an embodiment of the present invention, the method includes the following steps:

[0022] (1) The bifunctional three-dimensional porous composite material is ground and mixed with sulfur powder, and the cathode material is obtained by melt impregnation method;

[0023] (2) Mix the positive electrode material and conductive carbon black evenly, add it to the N-methylpyrrolidone solution in which polyvinylidene fluoride is dissolved, stir evenly to make a positive electrode slurry, coat the positive electrode slurry evenly on the current collector, dry it and cut it into a positive electrode sheet.

[0024] In addition, the method for preparing a lithium-sulfur battery cathode according to the above embodiments of the present invention may also have the following additional technical features:

[0025] In some embodiments of the present invention, in step (1), the mass ratio of the bifunctional three-dimensional porous composite material to sulfur powder is 2~3:1.

[0026] The lithium-sulfur battery includes a negative electrode, an electrolyte, and the positive electrode of the present invention; the electrolyte may be a mixed solution of 1,3-dioxolane (DOL) / 1,2-dimethoxyethane (DME) (volume ratio of 1:1) with 1%~2% by mass of LiNO3 additive and 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LITFSI) added; the negative electrode may be a lithium metal sheet.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) Rare earth doping improves the conductivity and catalytic activity of the catalyst. According to the density functional theory calculation, rare earth doping introduces a new energy level near the Fermi level of the catalyst. The original catalyst has a large band gap (about 1.2-1.8 eV), making it difficult for electrons to transition from the valence band to the conduction band, resulting in poor conductivity. The impurity energy level introduced by rare earth doping helps electrons to be excited from the valence band to the conduction band more easily, thereby effectively reducing the effective band gap and improving the intrinsic conductivity of the material.

[0029] (2) Rare earth doping enhances the chemisorption capacity of the catalyst for lithium polysulfides. In this invention, the adsorption energies of Li2S8, Li2S6, and Li2S4 on the surface of the rare earth-doped catalyst are -1.313 eV, -1.416 eV, and -1.853 eV, respectively, which are significantly higher than the adsorption energies on the surface of the undoped catalyst (-0.827 eV, -1.012 eV, and -1.061 eV). This is mainly due to the orbital hybridization between rare earth elements and lithium polysulfides (e.g., Ce). 3+ The 4f electron orbital of lithium polysulfide (which interacts with the p orbital of lithium polysulfide) significantly enhances chemisorption.

[0030] (3) Due to their large ionic radius, the doping of rare earth elements will induce the expansion of the interlayer spacing of catalyst nanosheets and promote lithium ion diffusion. At the same time, the doping of large-radius rare earth ions will cause lattice distortion, induce more defects, expose abundant edge catalytic active sites, and improve catalytic performance.

[0031] (4) The functional groups on the surface of graphene oxide can coordinate or electrostatically adsorb with catalyst precursor ions, promote the uniform distribution of nucleation sites, and make the catalyst nanosheets uniformly dispersed, thereby exposing more active sites. At the same time, the reduced graphene oxide stacks to form a three-dimensional porous conductive framework, which can effectively alleviate the impact of the volume expansion of sulfur materials during charging and discharging, and can also serve as a conductive network to improve the conductivity of the positive electrode.

[0032] (5) The composite cathode prepared by the present invention has a capacity retention rate of up to 71% after 500 cycles at 1C rate, and the average capacity decay per cycle is as low as 0.058%. Compared with the undoped cerium system, the capacity retention rate is improved by more than 10%. This shows that the doping of rare earth elements and the dual-functional three-dimensional porous composite material improve the cycling stability of the battery at high current density through the above-mentioned beneficial effects.

[0033] In summary, this invention effectively buffers the volume expansion of the sulfur cathode using a three-dimensional porous conductive framework, while significantly enhancing the conductivity of the catalyst and its adsorption and catalytic conversion capabilities for lithium polysulfides through rare earth element doping. With these dual-functional characteristics, this invention significantly improves the electrochemical performance of lithium-sulfur batteries, providing a feasible and superior cathode material system for high-performance lithium-sulfur batteries. Attached Figure Description

[0034] Figure 1 This is a scanning electron microscope morphology observation result of Ce-MoS2 / rGO in Example 1 of the present invention;

[0035] Figure 2This is a scanning transmission electron microscope image and elemental distribution diagram of Ce-MoS2 / rGO in Embodiment 1 of the present invention, wherein red represents carbon, blue represents sulfur, pink represents molybdenum, and green represents cerium.

[0036] Figure 3 These are X-ray powder diffraction characterization results of Ce-MoS2 / rGO in Example 1, MoS2 / rGO in Comparative Example 1, and rGO in Comparative Example 2 of the present invention.

[0037] Figure 4 These are transmission electron microscope morphology observation results of Ce-MoS2 / rGO (right figure) in Example 1 of the present invention and MoS2 / rGO (left figure) in Comparative Example 1, as well as a comparison of interplanar spacing.

[0038] Figure 5 These are the nitrogen adsorption-desorption isotherms and pore size distribution curves of Ce-MoS2 / rGO (a) in Example 1 and rGO (b) in Comparative Example 2 of the present invention.

[0039] Figure 6 This is a comparison of the projected density of states calculated by density functional theory for the catalyst MoS2 (a) contained in Comparative Example 1 MoS2 / rGO and the catalyst Ce-MoS2 (b) contained in Example 1 Ce-MoS2 / rGO.

[0040] Figure 7 The graph shows the theoretical calculation results of the adsorption energies of Li2S8, Li2S6 and Li2S4 by the catalyst MoS2 contained in Comparative Example 1 MoS2 / rGO and the catalyst Ce-MoS2 contained in Example 1 Ce-MoS2 / rGO.

[0041] Figure 8 This invention compares the adsorption effects of Ce-MoS2 / rGO in Example 1, MoS2 / rGO in Comparative Example 1, and rGO in Comparative Example 2 in Li2S6 solution, as well as the UV-Vis absorption spectra of the Li2S6 solution after adsorption.

[0042] Figure 9 This is a comparison of the high-resolution X-ray photoelectron spectroscopy results of Ce 3d before and after Li2S6 adsorption in Ce-MoS2 / rGO in Example 1 of this invention;

[0043] Figure 10 This is a comparison of the electrochemical impedance of lithium-sulfur batteries assembled using the composite materials described in Example 1, Comparative Example 1, and Comparative Example 2 in Application Example 2 of the present invention.

[0044] Figure 11This is a comparison of the cyclic voltammetry curves of lithium-sulfur batteries assembled using the composite materials described in Example 1, Comparative Example 1, and Comparative Example 2 in Application Example 2 of the present invention;

[0045] Figure 12 The charge-discharge curves of lithium-sulfur batteries assembled using the composite materials described in Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown at 0.2 C.

[0046] Figure 13 This is a comparison of the rate performance of lithium-sulfur batteries assembled using the composite materials described in Example 1, Comparative Example 1, and Comparative Example 2 in Application Example 2 of the present invention.

[0047] Figure 14 This is a comparison graph showing the long-cycle performance of lithium-sulfur batteries assembled using the composite materials described in Example 1, Comparative Example 1, and Comparative Example 2 of this invention. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0049] Example 1

[0050] A method for preparing a bifunctional three-dimensional porous composite material includes the following steps:

[0051] (1) First, 120 mg of monolayer graphene oxide (GO) was dispersed in 40 mL of deionized water and sonicated for 1 h to obtain a uniformly dispersed GO colloidal solution (3 mg / mL). -1 Subsequently, (NH4)6Mo7O 24 ·4H2O (0.02 mmol), Ce(NO3)3·6H2O (0.014 mmol), and L-cysteine ​​(0.56 mmol) were dissolved in 20 mL of deionized water and stirred until completely dissolved. The resulting solution was then mixed thoroughly with the GO solution. The mixture was transferred to a 100 mL stainless steel reactor lined with polytetrafluoroethylene and hydrothermally reacted at 200 °C for 20 h to obtain a blocky gel product.

[0052] (2) The lumpy product was repeatedly washed with deionized water to remove impurities, and then freeze-dried for 48 h. Finally, the above product was heated in a tube furnace under Ar atmosphere at 5 °C for 1 min. -1The heating rate was increased to 400℃ and heated for 2 h to obtain the final cerium-doped molybdenum disulfide / reduced graphene oxide composite (Ce-MoS2 / rGO) sample.

[0053] Figure 1 The image shown is a scanning electron microscope image of the Ce-MoS2 / rGO prepared in Example 1, revealing an interconnected porous structure composed of rGO nanosheets. This structure has abundant and continuous pores, which can effectively buffer the volume changes of sulfur species during charging and discharging, while promoting full wetting of the electrolyte and enhancing ion and electron conduction inside the electrode.

[0054] Figure 2 The elemental distribution of Ce-MoS2 / rGO prepared in Example 1 was characterized. Mo, S and Ce elements were uniformly distributed on the surface of rGO nanosheets, confirming that Ce-MoS2 was uniformly anchored on the rGO support.

[0055] Example 2

[0056] A method for preparing a bifunctional three-dimensional porous composite material differs from Example 1 only in that the molar number of rare earth metal source cerium nitrate is increased to 0.028 mmol, while the other steps and parameters are the same.

[0057] Example 3

[0058] A method for preparing a bifunctional three-dimensional porous composite material, which differs from Example 1 only in that the rare earth metal source is replaced with neodymium nitrate.

[0059] Comparative Example 1

[0060] The preparation method of MoS2 / rGO composite material includes the following steps:

[0061] (1) First, 120 mg of monolayer graphene oxide (GO) was dispersed in 40 mL of deionized water and sonicated for 1 h to obtain a uniformly dispersed GO colloidal solution (3 mg / mL). -1 Subsequently, (NH4)6Mo7O 24 • 4H2O (0.02 mmol) and L-cysteine ​​(0.56 mmol) were dissolved in 20 mL of deionized water and stirred until completely dissolved. The resulting solution was then mixed thoroughly with the GO solution. The mixture was transferred to a 100 mL stainless steel reactor lined with polytetrafluoroethylene and hydrothermally reacted at 200 °C for 20 h to obtain a blocky gel product.

[0062] (2) The lumpy product was repeatedly washed with deionized water to remove impurities, and then freeze-dried for 48 h. Finally, the above product was heated in a tube furnace under Ar atmosphere at 5 °C for 1 min. -1The heating rate was increased to 400℃ and heated for 2 h to obtain the final molybdenum disulfide / reduced graphene oxide composite (MoS2 / rGO) sample.

[0063] Comparative Example 2

[0064] The preparation method of rGO material includes the following steps:

[0065] (1) First, 120 mg of monolayer graphene oxide (GO) was dispersed in 60 mL of deionized water and sonicated for 1 h to obtain a uniformly dispersed GO colloidal solution (2 mg / mL). -1 The solution was transferred to a 100 mL stainless steel reactor lined with polytetrafluoroethylene and hydrothermally reacted at 200 °C for 20 h to obtain a blocky gel product, namely reduced graphene oxide (rGO) material.

[0066] (2) The lumpy product was repeatedly washed with deionized water to remove impurities, and then freeze-dried for 48 h. Finally, the above product was heated in a tube furnace under Ar atmosphere at 5 °C for 1 min. -1 The heating rate was increased to 400℃, and the final rGO sample was obtained after heating for 2 hours.

[0067] Figure 3 The X-ray powder diffraction patterns of Ce-MoS2 / rGO, MoS2 / rGO, and rGO prepared in Example 1, Comparative Example 1, and Comparative Example 2 are shown. The MoS2 / rGO pattern exhibits typical characteristic peaks belonging to the (002), (100), and (110) crystal planes of MoS2, confirming the successful loading of MoS2 onto the rGO surface. The diffraction peaks of the materials did not change significantly after the introduction of cerium, indicating that the addition of cerium did not generate new impurity phases.

[0068] Figure 4 Transmission electron microscope (TEM) images of Ce-MoS2 / rGO and MoS2 / rGO prepared in Example 1 and Comparative Example 1, respectively. The (002) interplanar spacing of Ce-MoS2 increased from 0.641 nm to 0.678 nm, which may be due to the presence of Ce. 3+ The lattice expansion caused by doping increases the interlayer spacing, which is beneficial for lithium-ion transport. At the same time, lattice distortion also induces more defects, thereby exposing more catalytically active sites.

[0069] Figure 5The N2 adsorption-desorption curves and pore size distribution curves of Ce-MoS2 / rGO and rGO prepared in Example 1 and Comparative Example 2 are shown. Specifically, an appropriate amount of sample (approximately 100 mg) was placed in a sample tube and degassed under vacuum at 150 °C for 12 hours to remove surface adsorbates. Subsequently, the N2 adsorption-desorption isotherms (relative pressure P / P0 = 0.005~0.995) were measured at liquid nitrogen temperature (77.35 K) using a Micromeritics ASAP 2460 analyzer. The specific surface area was calculated using the BET model, and the pore size distribution curve was obtained from the desorption branch using the BJH model. The results show that the specific surface area of ​​Ce-MoS2 / rGO is 186.68 m². 2 g -1 It is greater than 126.21 m of rGO. 2 g -1 This difference may stem from the fact that the insertion of Ce-MoS2 prevents the tight stacking of rGO sheets, thus jointly constructing a three-dimensional porous structure rich in mesopores (3~4 nm).

[0070] Figure 6 The projected density of states (PDOS) of the catalysts in Example 1 and Comparative Example 1 (Ce-MoS2 / rGO and MoS2 / rGO) are shown. For MoS2, the density of states near the Fermi level is extremely low, with almost no electron occupation, exhibiting typical semiconductor characteristics. After introducing Ce doping, the density of states near the Fermi level increases significantly (the orbitals of Mo, S, and Ce overlap significantly in this region), indicating the generation of more electronic states that can participate in the reaction near the Fermi level. Simultaneously, Ce doping forms impurity levels in the band structure, narrowing the band gap between the conduction and valence bands and enhancing the material's conductivity. Therefore, Ce-MoS2 can accelerate electron transfer during the reaction process, thereby significantly improving the overall catalytic efficiency of the material.

[0071] Figure 7 The adsorption energies of Li2S8, Li2S6, and Li2S4 in the Ce-MoS2 / rGO and MoS2 / rGO catalysts of Example 1 and Comparative Example 1, obtained from theoretical calculations, are shown. To determine the optimal adsorption sites, various lithium polysulfide adsorption configurations were evaluated, and the site with the lowest energy was selected as the lithium polysulfide adsorption site for further calculations. The results show that the adsorption energies of Li2S8, Li2S6, and Li2S4 on the Ce-MoS2 surface are -1.313 eV, -1.416 eV, and -1.853 eV, respectively, all higher than those on the MoS2 surface (-0.827 eV, -1.012 eV, and -1.061 eV), indicating that cerium doping promotes the adsorption of LiPSs.

[0072] Figure 8This invention compares the adsorption effects of Ce-MoS2 / rGO, MoS2 / rGO, and rGO in Li2S6 solution in Examples 1, 1, and 2 of the present invention, and also shows the UV-Vis absorption spectra of the Li2S6 solution after adsorption. First, Li2S and sulfur powder were uniformly mixed at a molar ratio of 1:5 and then dissolved in DOL / DME (volume ratio 1:1) solvent to obtain a concentration of 5 mmol / L. -1 Li₂S₆ solution was prepared by weighing 20 mg of rGO, MoS₂ / rGO, and Ce-MoS₂ / rGO samples respectively, and placing them in transparent sealed glass bottles containing an equal volume of Li₂S₆ solution. After standing for 2 hours, it was observed that the Li₂S₆ solution with added rGO remained deep yellow, while the solutions with added MoS₂ / rGO and Ce-MoS₂ / rGO showed significantly lighter colors. UV-Vis absorption spectroscopy was performed on the Li₂S₆ supernatant after each adsorption experiment, and the results showed that Ce-MoS₂ / rGO had the strongest adsorption effect on Li₂S₆.

[0073] Figure 9 This comparison shows the high-resolution X-ray photoelectron spectroscopy (HPS) results of Ce-MoS2 / rGO before and after Li2S6 adsorption in Example 1. The Ce-MoS2 / rGO powder samples before and after Li2S6 adsorption were repeatedly washed with DME solvent in a glove box and dried. X-ray photoelectron spectroscopy was then performed using Al-Kα as the X-ray source in the binding energy range of 870–930 eV to investigate the changes in their surface chemical state. A new Ce 3d spectrum appeared after Li2S6 adsorption. 4+ Characteristic peaks indicate that some Ce in the material... 3+ Oxidized to Ce 4+ The results confirmed Ce 3+ There is a significant interaction between the 4f electrons of Ce and the p-orbital electrons of polysulfides. This dynamic electron transfer process elucidates another mechanism by which Ce-MoS2 / rGO enhances the reaction kinetics of lithium-sulfur batteries.

[0074] Application Example 1

[0075] A method for preparing a lithium-sulfur battery cathode includes the following steps:

[0076] (1) High-purity sulfur powder was thoroughly ground and mixed with the Ce-MoS2 / rGO composite material prepared in Example 1, the MoS2 / rGO composite material prepared in Comparative Example 1, and the rGO material prepared in Comparative Example 2 at a mass ratio of 7:3 to obtain an initial mixture. The mixture was then placed in a hydrothermal reactor with a polytetrafluoroethylene liner and kept at 155 °C for 12 h in an oven to ensure that the sulfur could fully melt and diffuse. After the heat treatment was completed, the mixture was allowed to cool naturally to room temperature to obtain S@Ce-MoS2 / rGO, S@MoS2 / rGO, and S@rGO composite materials, respectively.

[0077] (2) The S@Ce-MoS2 / rGO composite material, S@MoS2 / rGO composite material, and S@rGO composite material prepared in (1) above were mixed with Super P and added to an NMP solution containing dissolved PVDF. The mixture was stirred evenly to obtain a positive electrode slurry, wherein the mass ratio of each component was "S@Ce-MoS2 / rGO : Super P : PVDF=7:2:1". The slurry was coated on aluminum foil and dried in a vacuum oven at 60 ℃ for 12 h. The dried electrode was cut into round pieces with a diameter of 12 mm to obtain the positive electrode.

[0078] Application Example 2

[0079] A method for preparing a lithium-sulfur battery: In an argon-filled glove box, three types of positive electrodes prepared in Application Example 1 are used to assemble the battery, with lithium metal sheet as the negative electrode, Celgard 2400 as the separator, and 2 wt.% LiNO3 additive and 1.0 M LITFSI DOL / DME (volume ratio 1:1) solution added as electrolyte to complete the assembly of the lithium-sulfur battery.

[0080] The following analysis of the performance of the cathode materials prepared in Example 1 and Comparative Examples 1-2 is presented in conjunction with the accompanying drawings:

[0081] Figure 10 These are the electrochemical impedance spectroscopy (EIS) spectra of lithium-sulfur batteries assembled from the composite materials of Examples 1, 1, and 2 of this invention. The EIS measurements were performed in the frequency range of 0.01–100 kHz. The impedance response of the battery system was obtained by scanning AC signals at different frequencies. The S@Ce-MoS2 / rGO cathode exhibited the lowest charge transfer impedance (40 Ω), indicating that this material possesses superior charge transfer kinetics at the electrode / electrolyte interface.

[0082] Figure 11 The lithium-sulfur batteries assembled from the composite materials of Embodiment 1, Comparative Example 1, and Comparative Example 2 of this invention operate at a voltage range of 1.7-2.8V with a current of 0.1 mV / s. -1The cyclic voltammetry curves obtained from the scan rate test showed a significant positive shift in the reduction peak potential of the S@Ce-MoS2 / rGO cathode, while the oxidation peak potential shifted negatively accordingly, indicating the highest current density and the lowest degree of polarization.

[0083] Figure 12 These are the charge-discharge curves of lithium-sulfur batteries assembled from the composite materials of Examples 1, 1, and 2 of this invention, tested at a rate of 0.2 C within a voltage range of 1.7-2.7V. Among them, the S@Ce-MoS2 / rGO cathode exhibits the highest initial specific capacity (1230 mAh g⁻¹). -1 ) and the longest low-voltage discharge platform (882 mAh g) -1 This can more effectively promote the deep conversion of polysulfides and improve sulfur utilization.

[0084] Figure 13 This comparison examines the rate performance of lithium-sulfur batteries assembled from the composite materials of Example 1, Comparative Example 1, and Comparative Example 2 of this invention, with a test voltage range of 1.7-2.8 V. As the current density increases from 0.2 C to 3 C, the S@Ce-MoS2 / rGO cathode consistently exhibits the highest discharge specific capacity (1379.8, 1112, 1061.4, 963, and 879.3 mAh g⁻¹). -1 This indicates that it has good redox reaction kinetics of lithium polysulfides.

[0085] Figure 14 The lithium-sulfur batteries assembled from the composite materials of Examples 1, 1, and 2 of this invention exhibit long-cycle performance at 1 C, with a test voltage range of 1.7-2.8 V. The S@Ce-MoS2 / rGO cathode showed a specific capacity of 747.2 mAh g after 500 cycles. -1 With a capacity retention rate of 71% and a capacity decay rate of only 0.058% per cycle, its excellent cycling performance signifies its value in practical applications.

[0086] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the present invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A dual-functional three-dimensional porous composite material, characterized in that: The composite material consists of a three-dimensional porous main framework constructed from two-dimensional carbon material and a rare earth element-doped two-dimensional nanosheet catalyst loaded thereon.

2. The dual-functional three-dimensional porous composite material according to claim 1, characterized in that: The mass ratio of the three-dimensional porous main framework to the rare earth element-doped two-dimensional nanosheet catalyst is (2~4):1; The two-dimensional nanosheets are transition metal dichalcogenides; The rare earth element is at least one of cerium, samarium, lanthanum, and neodymium.

3. A method for preparing a bifunctional three-dimensional porous composite material according to any one of claims 1-2, characterized in that, Includes the following steps: (1) Add the carbon source to deionized water and stir to obtain a uniformly dispersed solution; (2) Dissolve the rare earth metal source, the transition metal source and the sulfur source in deionized water and stir to obtain a precursor solution; (3) Mix the solution obtained in step (1) with the precursor solution to obtain a mixed solution, and carry out a hydrothermal reaction; (4) The product obtained in step (3) is repeatedly washed, dried and then heat-treated to prepare the dual-function three-dimensional porous composite material.

4. The method for preparing a bifunctional three-dimensional porous composite material according to claim 3, characterized in that: In step (1), the carbon source is a single-layer or multi-layer graphene oxide with a sheet diameter of 0.5~20 μm and a thickness of 0.5~5 nm.

5. The method for preparing a bifunctional three-dimensional porous composite material according to claim 3, characterized in that: In step (2), the molar ratio of rare earth metal source to transition metal source in the solution is (0.5~1):9, and the molar ratio of the sum of rare earth metal source and transition metal source to sulfur source is 1:(2~3); the transition metal source is ammonium molybdate; the sulfur source is L-cysteine; and the rare earth element in the rare earth metal source is at least one of cerium, samarium, lanthanum, and neodymium.

6. The method for preparing a bifunctional three-dimensional porous composite material according to claim 3, characterized in that: In step (3), the concentration of the carbon source in the mixed solution is 1~3 mg / mL. -1 The hydrothermal reaction temperature is 180~210 ℃, and the hydrothermal reaction time is 16~20 h.

7. The method for preparing a bifunctional three-dimensional porous composite material according to claim 3, characterized in that: In step (4), the heat treatment is carried out in a tube furnace under an argon atmosphere, at a temperature of 300~450 ℃ and held for 1~2 h, with a heating rate of 5 ℃ / min; the drying is carried out by freeze drying for 24~48 h.

8. A lithium-sulfur battery cathode, characterized in that: It is prepared using the bifunctional three-dimensional porous composite material according to any one of claims 1-2.

9. A method for preparing a lithium-sulfur battery cathode, characterized in that, Includes the following steps: (1) The bifunctional three-dimensional porous composite material according to any one of claims 1-2 is ground and mixed with sulfur powder, and the positive electrode material is obtained by melt impregnation method; (2) Mix the positive electrode material and conductive carbon black evenly, add it to the N-methylpyrrolidone solution in which polyvinylidene fluoride is dissolved, stir evenly to make a positive electrode slurry, coat the positive electrode slurry evenly on the current collector, dry it and cut it into a positive electrode sheet.

10. The method for preparing a lithium-sulfur battery cathode according to claim 9, characterized in that: In step (1), the mass ratio of the bifunctional three-dimensional porous composite material to sulfur powder is 2~3:1.

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