Mg4C 60 Applications of lithium-sulfur batteries, lithium-sulfur batteries and their assembly methods

Abstract

This invention provides Mg4C 60 Applications of Mg4C as a catalyst in lithium-sulfur batteries, and lithium-sulfur batteries containing this catalyst and their assembly methods. 60 When used as a catalyst in lithium-sulfur batteries, it can significantly reduce the activation energy of S-S bond breaking in polysulfides, accelerate the liquid-solid reduction reaction of Li2S4→Li2S, and shorten the residence time of Li2S4 in the electrolyte. On the other hand, it can promote the direct conversion reaction of Li2S6→Li2S, avoid the formation pathway of Li2S4, thereby reducing the accumulation of highly soluble and easily shuttled Li2S4 intermediates in the system, suppressing the shuttle effect, and improving the performance of lithium-sulfur batteries.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery material preparation, specifically, it relates to a high-stability lithium-sulfur battery and its assembly method, particularly to a battery based on Mg4C. 60 Lithium-sulfur batteries with synergistic effects of catalyst and LLMDD electrolyte. Background Technology

[0002] Lithium-sulfur batteries are considered strong candidates for the next generation of high-specific-energy storage systems due to their extremely high theoretical specific capacity (1675 mAh / g) and energy density (2600 Wh / kg). However, their commercialization has long been limited by bottlenecks such as irreversible loss of active materials caused by the shuttle effect of lithium polysulfides (LiPSs), corrosion of lithium metal anodes, and rapid degradation of battery cycle life.

[0003] To suppress the shuttle effect, the industry generally explores solutions from two directions: cathode catalyst design and electrolyte engineering. Regarding cathode catalysts, the research focus is on developing materials that can efficiently adsorb and catalyze the conversion of polysulfides.

[0004] Current research on cathode catalysts mainly includes carbon-based materials, transition metal compounds, and single-atom catalysts. While carbon-based materials (such as porous carbon, graphene, and carbon nanotubes) can anchor polysulfides and improve conductivity through physical adsorption, their adsorption effect is weak, making it difficult to effectively suppress polysulfide shuttle. Furthermore, the volume expansion of sulfur during cycling can easily lead to the collapse of the conductive network. Transition metal compounds (such as oxides and sulfides) can enhance the binding of polysulfides and accelerate conversion through chemical adsorption, but they suffer from problems such as reduced electrode energy density when added in excessive amounts, high-temperature synthesis costs, and easy dissolution of metal ions that contaminate the electrolyte. Single-atom catalysts, while possessing high atom utilization and strong catalytic activity, have complex large-scale preparation processes, and their active sites are prone to aggregation during long-term cycling, requiring further stability improvement.

[0005] In the field of lithium-sulfur battery electrolyte design, traditional ether-based electrolytes, with their excellent solvation capabilities, can impart superior redox reaction kinetics to the battery system. However, at the same time, the ether solvent molecules, due to their strong electron-donating ether-oxygen bonds, form strong coordination interactions with polysulfide ions, lowering the deposition energy barrier of polysulfides in the electrolyte and promoting their dissolution. Furthermore, this strong solvation coordination promotes the diffusion of polysulfides in the electrolyte, accelerating their migration from the positive to the negative electrode, thus exacerbating the shuttle effect. In recent years, controlling the solvation structure of the electrolyte to directionally guide the reaction pathways of polysulfides has become a new research approach.

[0006] Existing lithium-sulfur battery technologies, whether focusing on the physical confinement and chemical adsorption of polysulfides by porous supports / catalysts (e.g., the paper "Rational Design and General Synthesis of Multimetallic Metal-Organic Framework Nano-Octahedra for Enhanced Li-S Battery", Adv. Mater. 2021, 33(45), 2105163) or on the single-dimensional regulation of the electrolyte solvation environment, primarily target the entire polysulfide group. However, polysulfides of different chain lengths exhibit significant differences in solubility, migration rate, and reactivity. In particular, low-chain-length Li2S4, due to its rapid formation, slow consumption, and small molecular weight, is considered a key species for triggering the shuttle effect. Currently, there is a lack of effective technical solutions that can precisely regulate the formation and consumption processes of Li2S4, a specific key intermediate, through the synergistic design of catalysts and electrolytes. Summary of the Invention

[0007] The present invention aims to solve the above-mentioned problems of the prior art, and its purpose is to provide Mg4C 60 Use as a catalyst in lithium-sulfur batteries; lithium-sulfur batteries.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: Firstly, Mg4C is provided. 60 Its use as a catalyst in lithium-sulfur batteries.

[0009] Secondly, a lithium-sulfur battery is provided, comprising a sulfur positive electrode, a lithium metal negative electrode, a separator, and an electrolyte, wherein the sulfur positive electrode contains Mg4C. 60 catalyst.

[0010] Thirdly, a method for assembling the lithium-sulfur battery described in the second aspect is provided, comprising: In an argon glove box with oxygen and water content both below 0.1 ppm, a lithium metal anode, a separator, and a sulfur cathode are stacked sequentially in a battery case, an electrolyte is injected, and the battery is then encapsulated to obtain a lithium-sulfur battery.

[0011] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: Mg4C 60As a catalyst in lithium-sulfur batteries, it can significantly reduce the activation energy of SS bond breaking in polysulfides, accelerate the liquid-solid reduction reaction of Li2S4→Li2S, and shorten the residence time of Li2S4 in the electrolyte. On the other hand, it can promote the direct conversion reaction of Li2S6→Li2S, avoid the formation pathway of Li2S4, thereby reducing the accumulation of highly soluble and easily shuttled Li2S4 intermediates in the system, suppressing the shuttle effect, and improving the performance of lithium-sulfur batteries.

[0012] Lithium-sulfur batteries employ a specific electrolyte composition. On one hand, the selected lithium salt weakens the chemical bond between Li and S in Li₂S₆, promoting its disproportionation to form S₃· - Free radicals stabilize the species, forming "Li2S6→S3· - →Li2S” free radical reaction pathway, avoiding the formation of Li2S4, and LiNO3 can be reduced to generate Li3N and Li with excellent lithium conductivity. x NO y The components, combined with inorganic components such as LiF formed by the reduction of LiOTF, jointly construct a stable electrode-electrolyte interface film, inhibiting lithium metal corrosion; on the other hand, the appropriate ionic conductivity provided in the electrolyte ensures the stability of Li... + Rapid transport to catalytically active sites provides sufficient reactants for the reduction reaction, improving consumption efficiency. Furthermore, this electrolyte reduces the overall solubility of polysulfides and slows down the liquid-liquid reaction rate of Li₂S₆→Li₂S₄; while Mg₄C 60 The strong adsorption capacity of Li2S6 causes Li2S6 to preferentially accumulate on the cathode surface, thus avoiding the formation of Li2S4 (Li2S6 → S3· - → Li2S), instead of dissolving in the electrolyte to follow the Li2S6→Li2S4 pathway, reducing the probability of liquid-liquid reactions. Both work synergistically to inhibit the formation of Li2S4, and the electrolyte can also stabilize S3· - Free radicals help to circumvent the Li2S4 formation pathway; Mg4C 60 Through interfacial catalytic conversion, the Li2S6 enriched on its surface is directly converted into Li2S (Li2S6→Li2S). Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1The Mg4C synthesized in Example 1 60 The high-resolution transmission electron microscopy morphology and structural characterization of the catalyst includes (a) morphology, (b) selected area electron diffraction pattern, (c) high-resolution transmission electron microscopy image, and (d) corresponding reciprocal fast Fourier transform and atomic position labeling.

[0015] Figure 2 Graphite or Mg4C synthesized in Example 1 60 Crystal orbital Hamiltonian population analysis to evaluate the catalyst’s ability to break SS bonds in Li2S4.

[0016] Figure 3 Mg4C based on graphite or prepared in Example 1 60 The adsorption energy of the adsorption configuration between Mg4C and sulfur species was used to evaluate Mg4C. 60 Or the strength of the interaction between graphite and sulfur species.

[0017] Figure 4 Mg4C based on graphite or prepared in Example 1 for DFT calculation 60 Gibbs free energies for catalytic stepwise reaction pathways and direct conversion pathways were used to assess the ease or difficulty of the reaction process.

[0018] Figure 5 Molecular dynamics simulations of the LLMDD electrolyte system synthesized in Example 1 were performed to describe the radial and cumulative distribution functions of oxygen around lithium ions, showing the corresponding changes of these two distribution functions as the distance between lithium ions and oxygen atoms changes.

[0019] Figure 6 This is an in-situ Raman characterization diagram of the electrolyte on the negative electrode side during the discharge process of the lithium-sulfur battery in Example 1, used to identify the types of polysulfides in the electrolyte and their content changes.

[0020] Figure 7 The graph shows the rate performance of the lithium-sulfur coin cell in Example 1 at different current densities.

[0021] Figure 8 The graph shows the long-cycle performance of the lithium-sulfur coin cell in Example 1 at a current density of 1 C.

[0022] Figure 9 The graph shows the cycle performance of the lithium-sulfur pouch cell in Example 2.

[0023] Figure 10 The graph shows the rate performance of the lithium-sulfur coin cell in Comparative Example 1 at different current densities.

[0024] Figure 11Impedance comparison diagrams of Li2S6-based symmetric cells used to evaluate the rate of conversion of Li2S6 to Li2S4 in LLMDD and BE electrolytes, for Comparative Examples 2 and 3.

[0025] Figure 12 Electron paramagnetic resonance detection for comparative examples 2 and 3, used to evaluate the presence of sulfur triradicals in LLMDD and BE electrolytes containing Li2S6.

[0026] Figure 13 The graph shows the rate performance of the lithium-sulfur coin cell in Comparative Example 2 at different current densities.

[0027] Figure 14 The graph shows the rate performance of the lithium-sulfur coin cell in Comparative Example 3 at different current densities.

[0028] Figure 15 The image shows the in-situ Raman characterization of the electrolyte on the negative electrode side during the discharge process of the lithium-sulfur coin cell of Comparative Example 3, which is used to identify the types and content changes of polysulfides in the electrolyte.

[0029] Figure 16 The graph shows the long-cycle performance of the lithium-sulfur coin cell in Comparative Example 3 at a current density of 1 C.

[0030] Figure 17 Mg4C as in Example 1 60 A schematic diagram illustrating the synergistic regulation of the lithium polysulfide reaction pathway by the catalyst and LLMDD electrolyte to reduce Li2S4. Detailed Implementation

[0031] Some implementations provide Mg4C 60 Its use as a catalyst in lithium-sulfur batteries.

[0032] The Mg4C 60 The catalyst is a two-dimensional metallofullerene polymer, consisting of alternating polymeric C atoms. 60 Alternating layers of Mg atoms form a periodic two-dimensional framework; the polymeric C 60 Adjacent C in the layer 60 Clusters are connected by covalent bridge bonds to form a regular two-dimensional network, wherein the covalent bridge bonds are C-C single bonds or [2+2] cycloaddition bonds, C 60 The pseudo-tetrahedral and octahedral voids formed between clusters are filled with Mg atoms.

[0033] Some embodiments provide a lithium-sulfur battery, including a sulfur positive electrode, a lithium metal negative electrode, a separator, and an electrolyte, wherein the sulfur positive electrode comprises Mg4C. 60 catalyst.

[0034] In some preferred embodiments, the electrolyte comprises a lithium salt and a mixed solvent, wherein the lithium salt comprises lithium trifluoromethanesulfonate (LiOTF) and lithium nitrate (LiNO3), and the mixed solvent is methyl propyl ether (MPE) as the core component.

[0035] In some preferred embodiments, the mixed solvent further includes 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME).

[0036] In some preferred embodiments, the volume percentage of MPE in the mixed solvent is 50% to 60%, for example, 50%, 52%, 54%, 56%, 58%, 60%, etc.

[0037] In some preferred embodiments, the volume ratio of DOL to DME in the mixed solvent is (0.8~1.2):1, for example, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, etc.

[0038] In some preferred embodiments, the concentration of LiOTF in the electrolyte is 0.8~1.2 M, such as 0.8M, 0.9M, 1M, 1.1M, 1.2M, etc., and the concentration of LiNO3 is 0.3~0.7 M, such as 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, etc.

[0039] In some preferred embodiments, the sulfur cathode comprises sulfur powder and Mg4C. 60 Catalyst, conductive agent, and binder; the preparation of the sulfur cathode includes: sulfur powder, Mg4C... 60 A catalyst, conductive agent, and binder are mixed, and N-methylpyrrolidone (NMP) is added as a dispersant. The mixture is then ground until homogeneous to obtain a slurry. This slurry is coated onto a current collector (e.g., an aluminum foil current collector), and after drying, a sulfur cathode is obtained. The conductive agent can be one or more of Ketjen black, conductive carbon black, carbon nanotubes, etc., and the binder can be polyvinylidene fluoride (PVDF). Sulfur powder and Mg4C are also included. 60 The catalyst, conductive agent, and binder can be mixed in a mass ratio of 6:2:1:1, or the content of the components can be adjusted based on this ratio.

[0040] In some preferred embodiments, Mg4C 60 The catalyst has a mass percentage of 15-40% in the sulfur cathode, such as 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, etc., preferably 15-25%.

[0041] In some preferred embodiments, the separator is a polypropylene (PP) separator, a polyethylene (PE) separator, or a glass fiber separator; the lithium metal negative electrode is a lithium foil or a lithium sheet, the lithium foil thickness can be 100 μm, and the lithium sheet thickness can be 0.6 mm.

[0042] Some embodiments provide a method for assembling a lithium-sulfur battery, including: In an argon glove box with oxygen and water content both below 0.1 ppm, a lithium metal anode, a separator, and a sulfur cathode are stacked sequentially in a battery case, an electrolyte is injected, and the battery is then encapsulated to obtain a lithium-sulfur battery.

[0043] In some preferred embodiments, in a button cell system, the mass ratio of electrolyte injection to sulfur is (20~50):1, for example, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1, 50:1, etc., and in a pouch cell system, the mass ratio of electrolyte injection to sulfur is (2~4):1, for example, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, etc.

[0044] Mg4C 60 Catalyst preparation: Mg4C was synthesized using a chemical vapor phase mass transfer method. 60 High-purity C 60 The powder and magnesium metal powder are uniformly mixed at a mass ratio of 14~16:1 (e.g., 14:1, 14.5:1, 15:1, 15.5:1, 16:1, etc.), vacuum-sealed in a quartz tube, and placed in the high-temperature zone (500~580℃) of a dual-temperature zone tube furnace. After high-temperature treatment, Mg4C is finally collected in the low-temperature zone. 60 product.

[0045] Electrolyte preparation: First, LiOTF and LiNO3 are added to a mixture of DOL and DME. After stirring until the lithium salt is completely dissolved, MPE is added and mixed evenly to obtain the composite electrolyte. The solvent reduces the solvation of polysulfides, thereby reducing the dissolution of species such as Li2S4 and inhibiting their diffusion and shuttle into the electrolyte bulk from the source. DOL can form a polymer-dominated electrode-electrolyte interface film on the lithium metal surface through ring-opening polymerization. DME has a high ability to dissociate lithium salts to maintain the necessary ionic conductivity of the electrolyte and ensure battery reaction kinetics.

[0046] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0047] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0048] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0049] Example 1 (1) Mg4C 60 Catalyst preparation: Mg4C 60 The synthesis was achieved via chemical vapor phase mass transfer. High-purity C... 60 The powder and magnesium powder were uniformly mixed at a mass ratio of 15:1, placed in a quartz tube, vacuum-sealed, and then transferred to the high-temperature zone (500~580 ℃) of a dual-temperature zone tube furnace for reaction. After the high-temperature process, Mg4C was deposited in the low-temperature zone of the quartz tube. 60 product.

[0050] Mg4C 60 High-resolution transmission electron microscopy morphology and structural characterization of the product are as follows: Figure 1 As shown, from Figure 1 It can be seen that the Mg4C 60 The morphology and structural characteristics of the catalyst are clearly defined: Figure 1 The transmission electron microscope (TEM) image shows that it is a sheet-like crystal at the hundred-nanometer scale, and the area marked by the dashed circle in the image is the area for structural analysis. Figure 1 The selected area electron diffraction pattern corresponding to the [0 0 1] zone axis of b shows clear and regular diffraction spots. Matching the diffraction signals of crystal planes such as (2 0 0), (1 1 0), and (1 -1 0), it indicates that it has a good periodic ordered structure. Figure 1 The high-resolution transmission electron microscope (TEM) image c clearly shows the lattice fringes of the (3-10) and (310) crystal planes, with a spacing of 0.481 nm, further confirming its regular crystal structure. The reciprocal fast Fourier transform pattern and atomic position markings ([0 0 1] zone axis) in Figure 1d not only identify the 0.618 nm lattice spacing of the (210) crystal plane, but also intuitively demonstrate its atomically ordered arrangement. Furthermore, the marked crystal plane angle of 44° is consistent with that of Mg4C. 60 The theoretical structural features of the crystal structure are consistent.

[0051] Mg4C 60 Crystal orbital Hamiltonian population (COHP) analysis was used to assess the ability of catalysts or graphite to catalyze SS bond breaking in Li₂S₄. Figure 2 As shown. In COHP analysis, the bond strength between atoms can be quantified by calculating the energy contribution of orbital interactions. For example... Figure 2 As shown, the horizontal axis represents the Hamiltonian population (-COHP) of the negative crystal orbital. This is due to the fact that at the Fermi level (E... F Within the energy range below 0.5, peaks exist simultaneously on both the left and right sides of the vertical axis, making it difficult to visually compare the overall bonding state graphically. Therefore, numerical fitting and integration methods are needed for overall quantitative analysis; by using E F By mathematically integrating the areas of all left and right peaks below, the total COHP integral value (ICOHP) can be obtained. The negative and positive regions represent the contributions of bonding and antibonding, respectively. A more positive ICOHP value indicates a higher proportion of electrons in the antibonding orbitals and weaker bonding, thus favoring bond breaking. Mg4C 60 The antibonding electron occupancy of the terminal SS bond in the -Li2S4 configuration (ICOHP = 0.03 eV) is significantly higher than that in the C-Li2S4 system (ICOHP = -1.64 eV). This quantitative total integral result clearly indicates that, compared to graphite, Mg4C 60 It can lower the energy barrier for SS bond breaking, thereby catalyzing the decomposition of lithium polysulfides, such as Li2S4.

[0052] Based on Mg4C 60 Or the adsorption energy of the adsorption configuration between graphite and sulfur species, such as Figure 3 As shown, Mg4C 60 It exhibits significantly enhanced negative adsorption energy for all sulfur species compared to graphite, indicating that it has a higher anchoring capacity.

[0053] To understand Mg4C from a thermodynamic perspective 60 The enhancing effect on sulfur species transformation was calculated based on Mg4C using DFT. 60 Or the Gibbs free energy of stepwise reaction pathways and direct conversion pathways catalyzed by graphite, such as Figure 4 As shown, the Gibbs free energy at each stage in both pathways is negative, confirming that the sulfur reduction reaction is a thermodynamically spontaneous process. In all reaction stages, Mg₄C… 60 Both exhibit significantly more negative Gibbs free energy values ​​than graphite, proving that Mg4C... 60 It significantly enhances sulfur reduction reactions (both stepwise and direct conversion pathways), accelerating the conversion of lithium polysulfides and suppressing the shuttle effect. Furthermore, in the direct conversion pathway, Mg4C… 60The Gibbs free energy of the catalytic Li₂S₆→Li₂S conversion reaction (-3.26 eV) is more negative than that of the stepwise Li₂S₆→Li₂S₄ conversion (-0.68 eV) and Li₂S₄→Li₂S conversion (-3.09 eV), highlighting that the thermodynamic driving force for Li₂S₆ reduction via the direct conversion pathway is stronger than that via the stepwise pathway. Overall, Mg₄C 60 It exhibits a strong chemical affinity for lithium polysulfides and a significant catalytic effect on their conversion. It inhibits the accumulation of Li2S4 in the electrolyte by accelerating the Li2S4→Li2S conversion (stepwise reaction pathway), while promoting the direct conversion of Li2S6 to Li2S to bypass the formation of Li2S4 (pathway II). These two mechanisms together effectively alleviate the shuttle effect of Li2S4.

[0054] (2) Preparation of LLMDD electrolyte: In an argon glove box with oxygen and water content both below 0.1 ppm, a mixed solvent of DOL and DME with a volume ratio of 1:1 was taken, and LiOTF and LiNO3 were added to it. The mixture was stirred continuously until the lithium salt was completely dissolved to obtain a homogeneous solution. Then, MPE was added to the solution to adjust the final volume ratio of MPE, DOL and DME in the system to 2:1:1. At this time, the concentrations of LiOTF and LiNO3 in the solution were 1 M and 0.5 M, respectively, thus obtaining the target electrolyte.

[0055] from Figure 5 It can be seen that the molecular dynamics simulation of the solvation structure of this LLMDD electrolyte system reveals that lithium ions react with NO3-, respectively. - OTF - The radial distribution function and coordination number (cumulative distribution function) characteristics between oxygen atoms in DME, DOL, and MPE were analyzed. The radial distribution functions of lithium ions and each oxygen-containing species all exhibit characteristic peaks at an interatomic distance of approximately 2 Å, reflecting close-range interactions. With increasing interatomic distance, the coordination numbers corresponding to different oxygen-containing species gradually increase and tend to stabilize, intuitively demonstrating the differences in solvation between lithium ions and each oxygen-containing species in this electrolyte system and the characteristics of the coordination environment. Furthermore, the simulation results at the molecular level confirm that, on the one hand, MPE exhibits extremely weak solvation ability (corresponding to an extremely low coordination number). This weak solvation characteristic is expected to reduce the solubility of polysulfides in the electrolyte, effectively suppressing the shuttle effect from the source. On the other hand, the anion NO3... - OTF - The first solvation layer exhibits strong coordination, and this coordination environment provides the possibility for efficient control of sulfur reaction pathways, especially the sulfur triradical pathway. It is precisely based on this unique microscopic coordination environment that lithium-sulfur batteries are expected to possess excellent electrochemical performance.

[0056] (3) Preparation and performance testing of lithium-sulfur button cells: The process of preparing the sulfur cathode is as follows: sulfur powder is directly mixed with Mg4C 60 Conductive agent, binder (mass ratio 6:2:1:1), and dispersant NMP were added and ground evenly to obtain a slurry. The slurry was coated onto an aluminum foil current collector and vacuum dried at 60 °C for 12 hours. The sulfur loading of the resulting positive electrode was 1.0 mg / cm³. 2 The assembly process of button cells involves using Mg4C in an argon glove box. 60 A 2032-type coin cell lithium-sulfur battery was assembled using a sulfur cathode, a lithium anode (0.6 mm thick), a polypropylene separator (Celgard 2400), and LLMDD electrolyte (the injection amount met the mass ratio of electrolyte to sulfur E / S=20:1).

[0057] In this embodiment, Mg4C 60 The in-situ Raman characterization of the electrolyte during the discharge process of the LLMDD system lithium-sulfur battery is shown in the figure below. Figure 6 As shown, Figure 6 In the middle, the Raman displacement is 398 cm. -1 445 cm -1 The characteristic signals at the locations correspond to polysulfides Li2S6 and Li2S4, respectively; the color intensity (signal strength) in the figure intuitively reflects the content distribution characteristics of the above polysulfides in the electrolyte during the discharge process.

[0058] The rate performance of the lithium-sulfur battery assembled in this embodiment at different current densities is as follows: Figure 7 As shown, from Figure 7 It can be seen that Mg4C 60 - The LLMDD system lithium-sulfur coin cell battery exhibits excellent rate performance: at different current densities of 0.1C, 0.2C, 0.5C, 1C, and 2C, the battery displays discharge capacities of 1387.3 mAh / g, 1157.7 mAh / g, 1063.3 mAh / g, 962.7 mAh / g, and 829.2 mAh / g, respectively; as the current density switches between different rates, the capacity can be effectively recovered when the current density is subsequently adjusted back to a lower rate, demonstrating good rate performance and capacity stability.

[0059] The long-cycle performance of the lithium-sulfur coin cell assembled in this embodiment at a 1 C current density is shown in the figure below. Figure 8 As shown, from Figure 8 It can be seen that Mg4C 60 - The LLMDD system lithium-sulfur coin cell exhibits excellent long-cycle performance at a 1 C current density. During 700 cycles, the battery's discharge specific capacity slowly decreases from 993.7 mAh / g to 747.4 mAh / g, with a capacity retention rate as high as 75.2%.

[0060] Example 2 (1) The preparation method of the sulfur cathode is the same as in Example 1, but the sulfur loading is increased to 5.0 mg / cm³. 2 ; (2) Assembly of soft-pack batteries: In an argon glove box, lithium-sulfur soft-pack batteries with a rated capacity of 0.8 Ah are assembled using a high sulfur loading positive electrode, a lithium foil negative electrode (thickness 100 μm), a polypropylene separator (Celgard 2400), and LLMDD prepared in Example 1 as electrolyte (the injection amount meets the mass ratio of electrolyte to sulfur E / S=4:1).

[0061] The cycle performance diagram of the lithium-sulfur pouch battery assembled in this embodiment is shown in the figure below. Figure 9 As shown, from Figure 9 It can be seen that after 5 cycles of activation at 0.1C, the capacity retention rate of the soft-pack battery is 71% after 50 cycles at 0.5C, and the overall energy density of the battery reaches 509.7Wh / kg, demonstrating the good cycle stability of this high sulfur loading and low electrolyte addition system.

[0062] Comparative Example 1: The difference between this comparative example and Example 1 is that the electrolyte composition in step (2) is different. Specifically, the electrolyte is replaced with the basic electrolyte (1 M LiTFSI, DOL / DME (v:v, 1:1) + 2 wt% LiNO3), while the other conditions are the same.

[0063] The cycle performance diagram of the lithium-sulfur coin cell assembled in this comparative example is shown in the figure below. Figure 10 As shown, from Figure 10 It can be seen that Mg4C 60 The catalyst-based electrolyte system lithium-sulfur coin cells exhibited discharge capacities of 1377.6 mAh / g, 1106.1 mAh / g, 1007.3 mAh / g, 911.6 mAh / g, and 786.8 mAh / g at 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively, which is a decrease in rate performance compared to Example 1.

[0064] Assembly method of Li₂S₆ / Li₂S₆ symmetric cell: The symmetric cell (Li₂S₆ symmetric cell) was assembled in an argon-filled glove box, using a 2032 coin cell casing as the research carrier and carbon paper as the electrode substrate, serving as the working electrode and counter electrode, respectively. The electrolyte solution contained 0.2M Li₂S₆ (specifically, two electrolyte systems were used: the first was the LLMDD electrolyte base system from Example 1 + Li₂S₆, and the second was the base electrolyte base system from Comparative Example 1 + Li₂S₆), with an addition volume of 50 μL. After assembly, the cell was used for subsequent electrochemical testing to evaluate the reversible redox conversion behavior of lithium polysulfides between the two electrodes.

[0065] Figure 11 Impedance comparison of Li2S6-based symmetric cells used to evaluate the rate of Li2S6 to Li2S4 conversion in LLMDD and BE electrolytes. Figure 11 It can be seen that the charge transfer resistance of LLMDD electrolyte (157.3Ω) is higher than that of BE electrolyte (118.0Ω). Analysis shows that this makes the conversion process of Li2S6 more difficult, which to some extent inhibits the rapid liquid-liquid conversion of Li2S6 to Li2S4. This helps to reduce the short-term excessive accumulation of soluble Li2S4 at low potential and reduce the shuttle tendency.

[0066] Figure 12 Electron paramagnetic resonance (EPR) detection results were obtained to assess the presence of sulfur triradicals in LLMDD and BE electrolytes containing Li₂S₆. The results showed the presence of S₃· The free radical peak is characterized by a quantitative g tensor of 2.032. Analysis shows that S3· The free radical peak may be due to Mg4C 60 The strong adsorption capacity of Li2S6 leads to the preferential enrichment of Li2S6 on the cathode surface, resulting in the following conversion pathway: Li2S6 → S3· - → Li2S).

[0067] Based on this, the conversion network of lithium polysulfides was reconstructed: the LLMDD electrolyte suppressed the formation of Li2S4 at the solvation level and activated a new free radical generation pathway, while Mg4C 60 The catalyst provides highly efficient catalytic sites at the interface, accelerating the consumption and direct one-step conversion of polysulfides, including Li2S4. The synergistic effect of both catalysts leads to a reduction in Li2S4, thereby confining the active material to the cathode region for efficient conversion. This targeted reduction of Li2S4 pathway regulation mechanism is the fundamental reason why the batteries in Examples 1 and 2 achieve high specific capacity, high coulombic efficiency, and long-term cycle stability.

[0068] Comparative Example 2: The difference between this comparative example and Example 1 is that Mg4C was not added to the sulfur cathode in step (3). 60 Catalyst, all other conditions are the same.

[0069] The cycle performance diagram of the lithium-sulfur coin cell assembled in this comparative example is shown in the figure below. Figure 13 As shown, from Figure 13It can be seen that the catalyst-LLMDD system lithium-sulfur coin cell exhibits discharge capacities of 1208.3 mAh / g, 898.5 mAh / g, 807.1 mAh / g, 747.2 mAh / g, and 683 mAh / g at 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively, which is a decrease in rate performance compared to Example 1.

[0070] Comparative Example 3: The differences between this comparative example and Example 1 include: the electrolyte composition in step (2) is different; specifically, the electrolyte in this comparative example uses a basic electrolyte (with the same composition as Comparative Example 1); in step (3), Mg4C is not added to the sulfur cathode. 60 The catalyst was used, and the other conditions were the same as in Example 1.

[0071] The cycle performance diagram of the lithium-sulfur coin cell assembled in this comparative example is shown in the figure below. Figure 14 As shown, from Figure 14 It can be seen that the catalyst-based electrolyte system lithium-sulfur coin cell has worse rate performance than Example 1, Comparative Example 1 and Comparative Example 2; at 0.1C, 0.2C, 0.5C, 1C and 2C, the battery exhibits discharge capacities of 1284.8 mAh / g, 835.9 mAh / g, 730.4 mAh / g, 682.8 mAh / g and 613.8 mAh / g, respectively.

[0072] The in-situ Raman characterization of the electrolyte in the lithium-sulfur coin cell of this comparative example during discharge is shown in the following figure. Figure 15 As shown, from Figure 15 It can be seen that, in this comparative example, the in-situ Raman characterization results of the electrolyte in the catalyst-basic electrolyte system during discharge, compared with the Raman shift signals of Li2S6 and Li2S4 in Example 1, directly reflect the increased content distribution of the above-mentioned polysulfides in the electrolyte during discharge. This is consistent with the Mg4C content in Example 1. 60 The -LLMDD system provides a clear control over its effect in inhibiting polysulfide accumulation. Combined with the foregoing analysis, the Mg4C system in Example 1, with its catalyst and optimized electrolyte, demonstrates this effect. 60 -LLMDD system due to Mg4C 60 Through synergistic regulation of the catalytic sites and LLMDD electrolyte, sulfur conversion was systematically regulated along three reaction pathways: ① direct conversion of long-chain polysulfides (Li2S6) to Li2S (Li2S6→ Li2S), bypassing the formation of Li2S4; ② accelerating the reduction reaction of Li2S4 to the final product Li2S (Li2S4→ Li2S); and ③ slowing down the reduction of Li2S6 to Li2S4. This resulted in the production of polysulfide S4. 2- It is consumed quickly, which alleviates the accumulation in the electrolyte.

[0073] The long-cycle performance of the lithium-sulfur coin cell in this comparative example at a 1 C current density is shown in the figure below. Figure 16 As shown, from Figure 16 It can be seen that the catalyst-based electrolyte system of lithium-sulfur coin cells exhibits poor long-cycle performance at 1 C current density. During 700 cycles, the battery's discharge specific capacity rapidly decreased from 747.1 mAh / g to 134.4 mAh / g, with a capacity retention rate of 17.9%.

[0074] The performance comparison of the above embodiments and comparative examples shows that the Mg4C of the present invention... 60 The synergistic system of catalyst and LLMDD electrolyte can significantly improve the specific capacity and cycle stability of lithium-sulfur batteries, effectively suppressing the shuttle effect of lithium polysulfides, thereby overcoming the problem of rapid capacity decay in traditional lithium-sulfur batteries. In contrast, comparative examples 1-3, which lack synergistic design and pathway regulation, exhibit significant capacity decay.

[0075] In the various embodiments and comparative examples of this invention, to ensure the scientific validity of the comparison of catalytic effects, the mass ratio of each component (sulfur, conductive agent, binder, and catalyst) in the sulfur cathode was kept consistent. Based on this, this invention emphasizes that as long as the sulfur cathode contains Mg4C... 60 By combining a catalyst with an LLMDD electrolyte system, battery performance can be improved through their synergistic effect, which constitutes the core technical solution of this invention.

[0076] Based on the above analysis, it was found that the catalyst-electrolyte synergistic mechanism in this invention, compared with the traditional reaction mechanism, is... Figure 17 As shown, the Mg4C in this invention is explained. 60 The synergistic mechanism between the catalyst and the LLMDD electrolyte is key to improving the performance of lithium-sulfur batteries. Figure 17 Clearly demonstrated, traditional sulfur conversion follows the "S8" principle. 2- →S4 2- →S2 2- / S 2- The gradual reaction path leads to Li2S4 accumulation and capacity loss due to slow reaction; while the present invention utilizes Mg4C... 60 A triple synergistic pathway involving catalysis and LLMDD electrolyte is constructed to achieve rapid conversion of sulfur species and reduce the formation and accumulation of Li2S4. The core of this invention lies in proposing and implementing a novel strategy for "reducing soluble lithium tetrasulfide (Li2S4) to stabilize lithium-sulfur batteries" through system design, systematically manipulating the sulfur conversion pathway.

[0077] like Figure 17 As shown, the Mg4C of the present invention 60 The synergistic effect of the catalyst and LLMDD electrolyte is achieved through innovation at the following three levels: (1) Systematic manipulation of reaction pathway: The synergistic system designed in this invention aims to increase the relative rate of Li2S4 consumption / generation and circumvent the Li2S4 generation pathway, thereby fundamentally reducing the accumulation of highly soluble and easily shuttled Li2S4 intermediates and stabilizing lithium-sulfur batteries.

[0078] (2) Mg4C 60 Catalysis at all active sites of the catalyst: Mg4C prepared as in Example 1 60 This material is used for the first time as a sulfur conversion catalyst in this invention. Its unique two-dimensional structure provides all catalytic active sites, which can efficiently accelerate the reduction reaction of Li2S4 to the final product Li2S (Li2S4→ Li2S), while promoting the direct conversion of long-chain polysulfides (Li2S6) to Li2S (Li2S6→ Li2S), bypassing the step of generating Li2S4.

[0079] (3) Functional design of LLMDD electrolyte: As shown in Example 2, the LLMDD electrolyte can slow down the reduction of Li2S6 to Li2S4 (Li2S6→ Li2S4) on the one hand, and accelerate the dissociation of Li2S6 to generate highly reactive S3· - Free radicals alter reaction pathways thermodynamically and kinetically, complementing the effects of catalysts.

[0080] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. Mg4C 60 Use as catalyst in lithium-sulfur batteries.

2. Lithium-sulfur battery, characterized in that A sulfur cathode, a lithium metal anode, a separator, and an electrolyte, the sulfur cathode comprising Mg4C 60 catalyst.

3. The lithium-sulfur battery of claim 2, wherein, The electrolyte comprises a lithium salt and a mixed solvent, wherein the lithium salt comprises lithium trifluoromethanesulfonate (LiOTF) and lithium nitrate (LiNO3), and the mixed solvent is based on methyl propyl ether (MPE).

4. The lithium-sulfur battery of claim 3, wherein, The mixed solvent also includes 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME).

5. The lithium-sulfur battery according to claim 3 or 4, wherein In the mixed solvent, the volume percentage of MPE is 50% to 60%; In the mixed solvent, the volume ratio of DOL to DME is (0.8~1.2):1; The concentration of LiOTF in the electrolyte is 0.8~1.2M, and the concentration of LiNO3 is 0.3~0.7M.

6. The lithium-sulfur battery of claim 2, wherein, The sulfur cathode includes sulfur powder and Mg4C. 60 Catalyst, conductive agent, and binder; the preparation of the sulfur cathode includes: sulfur powder, Mg4C... 60 The catalyst, conductive agent and binder are mixed, and the dispersant N-methylpyrrolidone (NMP) is added and ground evenly to obtain a slurry. The slurry is coated on the current collector and dried to obtain a sulfur cathode.

7. The lithium-sulfur battery of claim 2, wherein, Mg4C 60 The mass ratio of the catalyst in the sulfur positive electrode is 15-40%, preferably 15-25%.

8. The lithium-sulfur battery of claim 2, wherein, The diaphragm is a polypropylene (PP) diaphragm, a polyethylene (PE) diaphragm, or a glass fiber diaphragm; the lithium metal negative electrode is a lithium foil or a lithium sheet.

9. The method of assembling a lithium-sulfur battery according to any one of claims 2 to 8, wherein include: In an argon glove box with oxygen and water content both below 0.1 ppm, a lithium metal anode, a separator, and a sulfur cathode are stacked sequentially in a battery case, an electrolyte is injected, and the battery is then encapsulated to obtain a lithium-sulfur battery.

10. The method of assembling a lithium-sulfur battery of claim 9, wherein, In the button cell system, the mass ratio of electrolyte injection to sulfur is (20~50):1, and in the pouch cell system, the mass ratio of electrolyte injection to sulfur is (2~4):1.

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