MXene composite metal monatomic material as sulfur positive electrode catalyst and application of MXene composite metal monatomic material in all-solid-state lithium-sulfur battery

Abstract

The invention provides an MXene composite metal monatomic material which is characterized in that MXene with a general formula of MxCyTz is used as a substrate, and Co monatomic is anchored on a T functional group of the MXene substrate through a Co-T coordinate bond; the Co single atoms are in atomic-scale dispersion; wherein M is selected from one of titanium, vanadium, chromium, niobium and molybdenum; wherein Tz is selected from one of halogen element groups, y is an integer of 1-4, z is an integer of 1-4, and x is equal to y + 1. The problem that Li < + > transmission is limited due to the fact that a solid electrolyte cannot freely diffuse on the surface of a catalyst is solved, the problem that active sites are limited due to the fact that sulfur species adsorbed on the specific active sites are fixed and difficult to desorb is also solved, on the basis, a solid-phase sulfur conversion path is regulated, solid-phase sulfur reaction kinetics is accelerated, and the solid-phase sulfur conversion efficiency is improved. And a high-performance MXene composite metal monatomic material is obtained by utilizing deep sulfur conversion capacity and is applied to an all-solid-state lithium-sulfur battery.

Description

Technical Field

[0001] This invention relates to the field of all-solid-state lithium-sulfur battery technology, specifically to an MXene composite metal single-atom material, its use as a sulfur cathode catalyst, and its application in all-solid-state lithium-sulfur batteries. Background Technology

[0002] All-solid-state lithium-sulfur (Li-S) batteries (ASSLSBs), employing non-flammable inorganic solid electrolytes, are considered a promising next-generation energy storage technology. ASSLSBs possess extremely high theoretical energy density and intrinsic safety, fundamentally eliminating the harmful polysulfide shuttle effect found in liquid lithium-sulfur batteries, and effectively addressing safety issues arising from lithium dendrite growth, parasitic reactions between lithium metal and organic liquid electrolytes (LE), and the inherent flammability of LE. However, ASSLSBs still face significant challenges, particularly the high reaction energy barrier and slow redox kinetics during the solid-state S8 / Li2S conversion process. This results in low sulfur utilization, and the complete release of ASSLSB capacity is severely hampered by incomplete sulfur conversion at the cathode. Researchers have explored various strategies to overcome these obstacles, including nano-sizing of S8 / Li2S, introducing novel carbon support materials, and using redox mediators or electrocatalysts.

[0003] Despite significant progress in improving the capacity and cycling performance of all-solid-state lithium-sulfur batteries (ASSLSBs), the redox mechanism of S8 / Li2S remains unclear. Previous studies typically assumed that ASSLSBs undergo a one-step reduction of S8 without forming any intermediate products; however, recent evidence suggests the presence of Li2S2 as an intermediate product, indicating that some S8 is ultimately reduced only to Li2S2. Notably, the Li2S2 to Li2S conversion contributes half of the theoretical capacity of lithium-sulfur batteries. In reality, the conversion reaction in ASSLSBs is often incomplete, with a large amount of Li2S2 remaining unreduced due to the high energy barrier. Therefore, fully exploring the conversion capacity of sulfur is crucial. Introducing electrocatalysts to lower the reaction energy barrier and improve the redox kinetics of the sulfur cathode offers a promising solution. However, catalytic research on ASSLSBs is still in its early stages, and the underlying catalytic mechanisms remain poorly understood. Some catalysts from liquid lithium-sulfur batteries have been directly applied to ASSLSBs, but key solid-state catalytic challenges and fundamental differences from liquid-phase systems are often overlooked. In particular, because solid reactants and solid electrolytes cannot diffuse freely on the catalyst surface, sulfur species adsorbed at specific active sites are firmly fixed and almost impossible to desorb and transfer to another site; meanwhile, Li + The transport of [Li] is also greatly limited, which contrasts sharply with liquid lithium-sulfur catalysis. Therefore, the abundant electroactive sites on the catalyst surface and the rapid Li [Li] transport [are crucial].+ Diffusion is essential for achieving adequate sulfur conversion in ASSLSBs.

[0004] Therefore, a tandem catalysis strategy needs to be developed, employing Co single-atom (Co SAs) catalysts anchored on a conductive MXene substrate, to segment the reaction pathway, lower the reaction energy barrier, and accelerate Li... + Surface diffusion enables deep sulfur conversion in ASSLSBs, laying a solid foundation for the practical application of ASSLSBs and obtaining a high-performance lithium-sulfur battery. Summary of the Invention

[0005] This invention discloses an MXene composite metal single-atom material as a sulfur cathode catalyst and its application in all-solid-state lithium-sulfur batteries. All-solid-state lithium-sulfur batteries (ASSLSBs) possess extremely high theoretical energy density and intrinsic safety, but their full capacity release is severely hampered by incomplete sulfur conversion at the cathode. Here, we propose a tandem catalytic method to achieve a deep conversion of S8 to Li2S via the intermediate product Li2S2, and its application in high-capacity all-solid-state lithium-sulfur batteries. We prepared a composite cathode material using a Co single-atom catalyst anchored on a conductive MXene substrate. Through tandem catalysis, we achieved the stepwise reduction of S8 to Li2S. In this process, the atomically dispersed Co sites in this invention promote the breaking of the S8-S bond, while the polar MXene surface facilitates lithium-ion diffusion, thereby significantly reducing the energy barrier for solid-phase sulfur conversion. This overcomes the limitation caused by the inability of the solid electrolyte to freely diffuse on the catalyst surface, which hinders the conversion of Li2S. + The problem of transport limitation was also overcome, as well as the problem of sulfur species adsorbed on specific active sites being fixed and difficult to desorb, which led to the limitation of active sites. Based on the above, the solid-phase sulfur conversion pathway was regulated, the solid-phase sulfur reaction kinetics were accelerated, and the deep sulfur conversion capacity was utilized to obtain a high-performance MXene composite metal single-atom material, which was applied to all-solid-state lithium-sulfur batteries.

[0006] This invention is achieved through the following technical solution: This invention provides an MXene composite metal single-atom material, wherein the MXene composite metal single-atom material (Co@MX) has the general formula M x C y T z MXene is used as a substrate, and Co single atoms (Co SAs) are anchored to the T functional groups of the MXene substrate through Co-T coordination bonds; the Co single atoms are atomically dispersed. M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), and molybdenum (Mo); T zIt is selected from one of the halogen element groups, where y is an integer from 1 to 4, z is an integer from 1 to 4, and x = y + 1.

[0007] As a further embodiment, the MXene composite metal single-atom material possesses at least one of the following characteristics: (1) The XRD pattern of the MXene composite metal single-atom material shows no Co metal peak; (2) The MXene composite metal single-atom material shows a lattice without Co particles in the transmission electron microscope (TEM) image, but the energy dispersive X-ray spectroscopy (EDS) elemental distribution image shows that the Co element is uniformly distributed; (3) The aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the MXene composite metal single-atom material shows bright spots representing Co single atoms (Co SAs) uniformly dispersed on MXene.

[0008] As a further preferred embodiment, the MXene composite metal single-atom material also possesses at least one of the following characteristics: (1) The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of the MXene composite metal single-atom material has only one significant peak at a radial distance of 1.98 Å; (2) The X-ray absorption near-edge structure spectrum of the Co K-side of the MXene composite metal single-atom material shows that the Co valence state is between Co K and Co K. 0 and Co 2+ between; (3) The EXAFS wavelet transform (EXAFS-WT) results of the MXene composite metal single-atom material show that Co@MX and CoT z They exhibit similar or identical maximum values ​​in k and R spaces.

[0009] As a further embodiment, the MXene composite metal single-atom material (Co@MX) with the general formula Ti x C y Cl z The substrate is MXene, and Co single atoms (Co SAs) are anchored to the Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co single atoms are atomically dispersed; where y is an integer from 1 to 4, z is an integer from 1 to 4, and x = y + 1.

[0010] As a further preferred embodiment, the MXene composite metal single-atom material (Co@MX) uses Ti3C2Cl2 MXene as a substrate, and Co single atoms (Co SAs) are anchored to the Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co single atoms are atomically dispersed.

[0011] As a further embodiment, the loading amount of Co in the MXene composite metal single-atom material is 0.5~2 wt%.

[0012] As a further preferred embodiment, the loading amount of Co in the MXene composite metal single-atom material is 1~2 wt%.

[0013] The present invention also provides a method for preparing the MXene composite metal single-atom material, comprising the following steps: S1: Etching of the general formula M in an inert gas atmosphere at 500~900 ℃ in a mixture of CoCl2 and alkali halide molten salts. n+1 AC n The MAX phase, wherein M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), and molybdenum (Mo); n is an integer from 1 to 4; A is selected from one of aluminum (Al) and silicon (Si); and the reaction time is 0.5 to 10 hours. S2: Soak the product obtained in S1 in a 5-12 M acid or alkaline solution and stir for 1-24 hours to remove Co particles that are not in single-atom form; S3: After multiple centrifugations and washing with deionized water until the pH is neutral, the precipitate is dried at 40~80 ℃ for 6~24 hours to obtain the MXene composite metal single-atom material.

[0014] As a further option, the general formula is M n+1 AC n The molar ratio of the MAX phase to CoCl2 and alkali halide molten salt is 1:(2~6):(2~8).

[0015] As a further example, the alkali metal halide molten salt is selected from one or more of lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, sodium bromide, and potassium bromide.

[0016] As a further preferred example, the alkali metal halide molten salt is selected from one or more of sodium chloride and potassium chloride.

[0017] As a further preferred embodiment, the preparation method of the MXene composite metal single-atom material includes the following steps: S1: Mix Ti3AlC2 MAX phase, CoCl2, NaCl and KCl uniformly in a molar ratio of 1:(2~6):(1~4):(1~4) and grind for 10~30 min; react for 1~6 h in an inert gas atmosphere at 600~750 ℃. S2: Soak the product obtained in S1 in a 5-9 M acid or alkali solution and stir for 1-24 h to remove Co particles that are not in single-atom form; S3: After multiple centrifugation and deionized water washing until the pH is neutral, the precipitate is dried at 50~70 ℃ for 10~15 h to obtain the MXene composite metal single-atom material.

[0018] As a further example, the base in S2 is exemplary selected from one or more of potassium hydroxide (KOH), sodium hydroxide (NaOH), and ammonia water.

[0019] As a further example, the acid in S2 is exemplary selected from one or more of hydrochloric acid and sulfuric acid.

[0020] As a further preferred embodiment, the molar ratio of MAX to CoCl2, NaCl and KCl in S1 is 1:3:2:2; the reaction time is 1 h in an inert gas atmosphere at 700 ℃.

[0021] As a further preferred embodiment, step S2 involves soaking the product obtained in step S1 in a 5-9 M acid solution and stirring for 24 hours to remove Co particles that are not in a single-atom form.

[0022] The present invention also provides a method for preparing a composite cathode comprising the MXene composite metal single-atom material, comprising the following steps: S1: Li2S and the MXene composite metal single-atom material are ball-milled at 400-700 rpm for 3-7 hours in an inert gas atmosphere at a mass ratio of (1-5):(1-5) to obtain Li2S / Co@MX; S2: The Li2S / Co@MX, conductive agent, and composite cathode prepared in S1 are ball-milled with electrolyte at a mass ratio of (1~10):(1~5):(1~7) and ball-milled at 400~700 rpm for 3~7 hours in an inert gas atmosphere to obtain the composite cathode.

[0023] As a further embodiment, the conductive agent is selected from one or more of Ketjen black, carbon black, graphite, graphene, carbon nanotubes, acetylene black, activated carbon, conductive carbon spheres, conductive carbon fibers, and reduced graphene oxide.

[0024] As a further option, the electrolyte for the composite positive electrode is selected from one or more of the silver-germanium sulfide type electrolytes.

[0025] As a further preferred embodiment, the method for preparing the composite positive electrode includes the following steps: S1: Li2S and the MXene composite metal single-atom material were ball-milled at 550 rpm for 5 hours in an inert gas atmosphere at a mass ratio of 4:2 to obtain Li2S / Co@MX; S2: The Li2S / Co@MX, Ketjen Black and Li6PS5Br were ball-milled at a mass ratio of 6:2:4 and then ball-milled at 550 rpm for 5 hours in an inert gas atmosphere to prepare the composite cathode.

[0026] The present invention also provides a method for preparing an all-solid-state lithium-sulfur battery including the composite cathode, comprising the following steps: S1: Press the electrolyte powder with a pressure of 100~400 MPa for 1~5 minutes to obtain an electrolyte sheet, then spread the composite positive electrode evenly on the surface of the electrolyte sheet, and press it again with a pressure of 100~400 MPa for 1~5 minutes; S2: Place a lithium alloy layer on the other side of the electrolyte sheet, apply a pressure of 50~200 MPa for 1~5 minutes.

[0027] As a further option, the lithium-containing alloy is selected from one of Li-In alloy, Li-Al alloy, Li-Si alloy, Li-Sn alloy, and Li-Bi alloy.

[0028] As a further preferred embodiment, the lithium-containing alloy is a Li-In alloy.

[0029] As a further preferred embodiment, the preparation method of the all-solid-state lithium-sulfur battery includes the following steps: S1: Press the electrolyte powder with a pressure of 250 MPa for 3 minutes to obtain an electrolyte sheet, then spread the composite positive electrode evenly on the surface of the electrolyte sheet, and press it with a pressure of 250 MPa for another 3 minutes. S2: Place a layer of Li-In alloy on the other side of the electrolyte sheet, apply a pressure of 125 MPa for 3 minutes.

[0030] As an example, the electrolyte powder is selected from one or more of the following: silver-germanium sulfide electrolytes (Li6PS5X, where X is a halogen element).

[0031] As an example, the sulfide-germanium ore type electrolyte includes one or more of Li6PS5Cl and Li6PS5Br.

[0032] The present invention also provides a composite cathode prepared by a composite cathode preparation method and an all-solid-state lithium-sulfur battery prepared by a method for preparing an all-solid-state lithium-sulfur battery, including the MXene composite metal single-atom material.

[0033] The features and beneficial effects of this invention are as follows: This invention achieves deep conversion of S8 to Li2S via intermediate product Li2S2 through tandem catalysis, which can be used in high-capacity all-solid-state lithium-sulfur batteries. This invention achieves stepwise reduction of S8 to Li2S through tandem catalysis of Co single-atom (Co@MX) catalyst anchored on a conductive MXene substrate. In this process, the invention uses atomically dispersed Co sites to promote the breaking of SS bonds, while the polar MXene surface facilitates lithium-ion diffusion, thereby significantly reducing the energy barrier of solid-phase sulfur conversion.

[0034] This invention is based on Co@MX's ASSLSB at room temperature with a current of 2.8 mA cm⁻¹ -2 The current density remains at 1329 mAh g after 2000 cycles. S -1 It exhibits high specific capacity without capacity decay. The Co@MX tandem catalyst of this invention demonstrates superior advantages in regulating the solid-phase sulfur conversion pathway and utilizing the deep sulfur conversion capacity. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 The structure of the MXene composite metal single-atom material prepared in Example 1 and the interaction diagram of Li2S / Co@MX; Figure 1 Image a is the energy dispersive X-ray spectroscopy (EDS) elemental distribution image of the MXene composite metal single-atom material prepared in Example 1; Figure 1 In Figure b, the image is a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the MXene composite metal single-atom material prepared in Example 1 with spherical aberration correction. Co SAs are marked with red circles. Figure 1 In the image, c represents the synchrotron radiation-based Co-K edge X-ray absorption spectrum (XANES) of the MXene composite metal single-atom material prepared in Example 1. Figure 1In the figure, d represents the Fourier transform extended X-ray absorption fine structure spectrum (FT-EXAFS) of the MXene composite metal single-atom material (Co@MX), Co foil, and CoCl2 prepared in Example 1. Figure 1 In Figure e, the image is a transmission electron microscope (TEM) image of Li2S / Co@MX prepared in Example 1. Figure 1 In the image, f represents the Raman spectra of Li2S / Co@MX, Co@MX, Li2S, and S8 prepared in Example 1; Figure 1 g represents the solid-state Li2S / Co@MX and Li2S prepared in Example 1. 6 Li magic angle rotation nuclear magnetic resonance spectrum ( 6 LiMAS-NMR). Figure 1 In this context, h represents the Li₂S / Co@MX and Li₂S Li₂S prepared in Example 1. 1s X-ray photoelectron spectroscopy (XPS); Figure 1 In the image, i represents the FT-EXAFS spectra of Li2S / Co@MX, Co@MX, CoS2, and CoS prepared in Example 1; Figure 1 In the middle j is the EXAFS wavelet transform spectrum (EXAFS-WT) of Li2S / Co@MX prepared in Example 1; Figure 1 In this context, k represents the EXAFS wavelet transform (EXAFS-WT) spectrum of CoS2. Figure 1 Figure 1 shows the differential charge density plot of Li2S / Co@MX prepared in Example 1, where the yellow and blue areas represent charge accumulation and loss, respectively (isosurface: 0.018 e Å). -3 ).

[0037] Figure 2 Analysis of the reaction pathway of ASSLSBs and the tandem catalytic mechanism of Co@MX; Figure 2 In this context, 'a' represents the binding energy between Li2S2 and Li2S with Co@MX prepared in Example 1, MXene-Cl in Comparative Example 8, and graphene in Comparative Example 6. Figure 2 In the figure, b represents MXene-Cl (comparative Example 8) and graphene Li (comparative Example 6). + The energy curve of the migration, and the corresponding Li + A top-down view of the migration path; Figure 2In the figure, c represents the SS bond strength in the adsorbed Li2S2 calculated from the integral crystal orbital Hamiltonian population (-ICOHP), and the top-view optimized adsorption conformation of Li2S2 on the Co@MX prepared in Example 1, MXene-Cl in Comparative Example 8, and graphene substrates in Comparative Example 6. Figure 2 In the figure, d represents the free energy diagram of sulfur reduction reaction (SRR) on graphene prepared in Example 1, MXene-Cl in Comparative Example 8, Co@Gr in Comparative Example 5, and graphene in Comparative Example 6. Figure 2 In the first cycle, e represents the non-in-situ positive electrode S of Co@MX in Example 1. 2p XPS spectrum; Figure 2 In the first cycle, f represents the non-in-situ positive electrode S of Comparative Example 2 without Co@MX. 2p XPS spectrum.

[0038] Figure 3 The reaction kinetics of the Co@MX-based sulfur cathode are shown in the figure. Figure 3 In Figure 'a', the dQ / dV curves of ASSLSBs with and without Co@MX are shown for Example 1 and Comparative Example 2. Figure 3 In the figure, b represents the apparent activation energy of the positive electrode with / without Co@MX in Example 1 and Comparative Example 2, which was obtained by temperature-dependent CV testing. Figure 3 In the middle, c is a contour plot of the CV curves of the positive electrode based on Co@MX in Example 1 at different scan rates; Figure 3 In the figure, d represents the relationship between the peak current value and the square root of the sweep rate of the CV curves of Example 1 and Comparative Example 2. Figure 3 In the figure, e represents the effective ionic conductivity of the positive electrode composites with / without Co@MX in Example 1 and Comparative Example 2, which was measured by DC polarization. Figure 3 In the figure, f represents the presence / absence of Co@MX ASSLSBs in Example 1 and Comparative Example 2 at different discharge potentials. R s , R int and R ct The evolution was measured by in-situ EIS; Figure 3 In the figure, g represents the DRT curve of the discharge process of Co@MX-based ASSLSBs in Example 1, calculated from in-situ EIS testing; Figure 3In the figure, h is the relaxation function γ(τ) obtained from the DRT spectra of the discharge processes of ASSLSBs with and without Co@MX in Example 1 and Comparative Example 2.

[0039] Figure 4 The graph shows the electrochemical performance of ASSLSBs at room temperature. Figure 4 In Example a, Comparative Examples 1, 7, 9, and 2, the Li₂S surface loading was 2.4 mg / cm². -2 The current density is 0.2 mA cm⁻¹ -2 The loop performance of ASSLSBs; Figure 4 In Example b, we have Example 1 and Comparative Example 2, with a Li₂S surface loading of 1.1 mg / cm³. -2 The current density is 0.4 mAcm. -2 Constant current charge-discharge curves of ASSLSBs at that time; Figure 4 c represents Example 1 and Comparative Example 2, with a Li2S surface loading of 1.1 mg / cm³. -2 The current density is 0.4 mAcm. -2 Cyclic performance of ASSLSBs with / without Co@MX, the battery was pretreated with several cycles at a lower current density; Figure 4 In Example 1 and Comparative Example 2, d represents the Li₂S surface loading of 1.1 mg / cm³. -2 The current density is 0.8 mAcm. -2 Cyclic performance of ASSLSBs at that time; Figure 4 In Example 1 and Comparative Example 2, the isothermal loading of Li2S was 1.2 mg / cm³. -2 Rate performance of ASSLSBs at that time; Figure 4 In Example 1 and Comparative Example 2, f represents the Li₂S surface loading of 1.2 mg / cm³. -2 The current density is 2.8 mAcm. -2 Cyclic performance of ASSLSBs at that time; Figure 4 In Example 1, g represents the case with a Li₂S surface loading of 1.2 mg / cm³. -2 The current density is 2.8 mA cm⁻¹ -2 Constant current charge-discharge curves at different cycle numbers.

[0040] Figure 5 Electron microscopy images of the MXene composite metal single-atom material prepared in Example 1; Figure 5In Figure a, the image is a SEM image of the MXene composite metal single-atom material (Co@MX) prepared in Example 1 at a scale of 20 μm. Figure 5 Image b is a SEM image of the MXene composite metal single-atom material (Co@MX) prepared in Example 1 at a scale of 5 μm; Figure 5 In the middle c, there is a TEM image of the MXene composite metal single-atom material (Co@MX) prepared in Example 1; Figure 5 In the middle, d is a high-resolution TEM image of the MXene composite metal single-atom material (Co@MX) prepared in Example 1; Figure 5 In the image, e is a HAADF-STEM image of the MXene composite metal single-atom material (Co@MX) prepared in Example 1 at a scale of 5 nm. Co SAs are marked with red circles. Figure 6 XRD patterns, Raman spectra, and EDS elemental distribution images of MXene with different Co contents in Examples 1 and Comparative Examples 3-4; Figure 6 In Figure a, the XRD patterns of MXene with different Co contents are from Examples 1 and Comparative Examples 3-4. Figure 6 b represents the Raman spectra of MXene with different Co contents in Example 1 and Comparative Examples 3-4; Figure 6 c is the EDS element distribution image of Comparative Example 3; Figure 6 In the middle, d is the EDS element distribution image of Comparative Example 4.

[0041] Figure 7 EXAFS-WT spectra of the MXene composite metal single-atom material (Co@MX), Co metal foil, and CoCl2 prepared in Example 1; Figure 7 In Figure 'a', the EXAFS wavelet transform (EXAFS-WT) spectrum of the MXene composite metal single-atom material (Co@MX) prepared in Example 1 is shown. Figure 7 In the image, b is the EXAFS-WT spectrum of Co metal foil; Figure 7 c is the EXAFS-WT spectrum of CoCl2.

[0042] Figure 8Examples 1 and 2-4 show MXene-based ASSLSBs with different Co contents and ASSLSBs without a catalyst at 0.2 mA cm⁻¹. -2 At current density, 2.4 mg cm -2 Cyclic performance of Li2S under canopy loading. Detailed Implementation

[0043] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below, and embodiments of the present invention will be provided, but this does not limit the scope of the present invention.

[0044] This invention discloses an MXene composite metal single-atom material as a sulfur cathode catalyst and its application in all-solid-state lithium-sulfur batteries. All-solid-state lithium-sulfur batteries (ASSLSBs) possess extremely high theoretical energy density and intrinsic safety, but their full capacity release is severely hampered by incomplete sulfur conversion at the cathode. Here, we propose a tandem catalytic method to achieve a deep conversion of S8 to Li2S via the intermediate product Li2S2, and its application in high-capacity all-solid-state lithium-sulfur batteries. We prepared a composite cathode material using a Co single-atom catalyst anchored on a conductive MXene substrate. Through tandem catalysis, we achieved the stepwise reduction of S8 to Li2S. In this process, the atomically dispersed Co sites in this invention promote the breaking of the S8-S bond, while the polar MXene surface facilitates lithium-ion diffusion, thereby significantly reducing the energy barrier for solid-phase sulfur conversion. This overcomes the limitation caused by the inability of the solid electrolyte to freely diffuse on the catalyst surface, which hinders the conversion of Li2S. + The problem of transport limitation was also overcome, as well as the problem of sulfur species adsorbed on specific active sites being fixed and difficult to desorb, which led to the limitation of active sites. Based on the above, the solid-phase sulfur conversion pathway was regulated, the solid-phase sulfur reaction kinetics were accelerated, and the deep sulfur conversion capacity was utilized to obtain a high-performance MXene composite metal single-atom material, which was applied to all-solid-state lithium-sulfur batteries.

[0045] This invention is achieved through the following technical solution: This invention provides an MXene composite metal single-atom material, wherein the MXene composite metal single-atom material (Co@MX) has the general formula M x C y T z MXene is used as a substrate, and Co single atoms (Co SAs) are anchored to the T functional groups of the MXene substrate through Co-T coordination bonds; the Co single atoms are atomically dispersed. M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), and molybdenum (Mo); T z It is selected from one of the halogen element groups, where y is an integer from 1 to 4, z is an integer from 1 to 4, and x = y + 1.

[0046] This invention addresses the problem of incomplete conversion of S8 to Li2S via the intermediate product Li2S2 in existing ASSLSBs. Due to the high energy barrier, a large amount of Li2S2 remains unreduced, leading to low capacity in lithium-sulfur batteries. A novel MXene composite metal single-atom material is proposed. This material employs a Co single-atom catalyst anchored on a conductive MXene substrate to segment the reaction pathway and lower the reaction energy barrier, thereby achieving deep sulfur conversion in ASSLSBs. Compared to other elements, this invention leverages the strong covalent interaction between the d electrons of Co and the p orbitals of S atoms in the active material. This interaction not only effectively adsorbs the active material but also activates the SS bond, significantly reducing the energy barrier for its breaking. Furthermore, since Co is also a good conductor, when anchored on the highly conductive MXene, it effectively maintains the overall conductive network of the electrode, promoting rapid electron transfer to the reaction sites. Under the action of a tandem catalyst composed of Co SAs and polar MXene, atomically dispersed Co sites bind sulfur species to promote SS bond breaking, while the polar MXene surface rich in halogen groups enables rapid Li2S conversion. + The redox kinetics of S8 to Li2S, which are gradually reduced to Li2S via the Li2S2 intermediate, are accelerated. Therefore, the overall conversion rate of S8→Li2S2→Li2S is improved, especially the Li2S2→Li2S reduction step is effectively promoted. Furthermore, the abundant Co SAs active sites on MXene enable it to continuously interact with more pulverized active material particles during long-term cycling, further improving the battery's specific capacity and overcoming the capacity decay problem during cycling.

[0047] As a further embodiment, the MXene composite metal single-atom material possesses at least one of the following characteristics: (1) The XRD pattern of the MXene composite metal single-atom material shows no Co metal peak; (2) The MXene composite metal single-atom material shows a lattice without Co particles in the transmission electron microscope (TEM) image, but the energy dispersive X-ray spectroscopy (EDS) elemental distribution image shows that the Co element is uniformly distributed; (3) The aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the MXene composite metal single-atom material shows bright spots representing Co single atoms (Co SAs) uniformly dispersed on MXene.

[0048] As a further preferred embodiment, the MXene composite metal single-atom material also possesses at least one of the following characteristics: (1) The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of the MXene composite metal single-atom material has only one significant peak at a radial distance of 1.98 Å; (2) The X-ray absorption near-edge structure spectrum of the Co K-side of the MXene composite metal single-atom material shows that the Co valence state is between Co K and Co K. 0 and Co 2+ between; (3) The EXAFS wavelet transform (EXAFS-WT) results of the MXene composite metal single-atom material show that Co@MX and CoT z They exhibit similar or identical maximum values ​​in k and R spaces.

[0049] This invention limits the MXene composite metal single-atom material to atomically dispersed Co metal single atoms, and observes that metallic Co is not observed in Co@MX. 0 The characteristic Co-Co peaks confirm the presence of atomically dispersed Co rather than cobalt nanoparticles (Co NPs); furthermore, the presence of Co@MX and CoT peaks was revealed by EXAFS wavelet transform (EXAFS-WT). z The similar or identical maxima in k and R spaces indicate that Co SAs coordinate with the halogen functional group T on MXene via Co-T interactions. Based on this, the atomically dispersed cobalt monoatoms enable stronger chemisorption of Li₂S / Li₂S₂ molecules on Co@MX compared to MXene alone (e.g., MXene-Cl) and graphene. The strong interaction between Co@MX with atomically dispersed Co and Li₂S weakens the Li-S bond and promotes Li… + Dissociation from Li2S increases the energy density of the battery.

[0050] As a further embodiment, the MXene composite metal single-atom material (Co@MX) with the general formula Ti x C y Cl z The substrate is MXene, and Co single atoms (Co SAs) are anchored to the Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co single atoms are atomically dispersed; where y is an integer from 1 to 4, z is an integer from 1 to 4, and x = y + 1.

[0051] As a further preferred embodiment, the MXene composite metal single-atom material (Co@MX) uses Ti3C2Cl2 MXene as a substrate, and Co single atoms (Co SAs) are anchored to the Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co single atoms are atomically dispersed.

[0052] The present invention further preferably uses M x C y T z In this formula, T stands for chlorine. Chlorine atoms (Cl) have lone pairs of electrons, making them excellent electron donors that form strong Co-Cl coordinate bonds with the transition metal Co. This bonding method is very stable, firmly stabilizing Co single atoms (Co SAs) on the MXene surface and preventing them from migrating and agglomerating into nanoparticles during cycling, thus maintaining high catalytic activity.

[0053] As a further embodiment, the loading amount of Co in the MXene composite metal single-atom material is 0.5~2 wt%.

[0054] As a further preferred embodiment, the loading amount of Co in the MXene composite metal single-atom material is 1~2 wt%.

[0055] This invention can further limit the loading of Co in MXene composite metal single-atom materials. On this basis, it further avoids the reduction of catalytic activity caused by the aggregation of Co atoms into nanoparticles due to excessive loading. At this time, the advantages of single-atom dispersion are further utilized, and stable Co-Cl bonds are formed, which further improves the specific capacity of the battery and overcomes the capacity decay problem during cycling.

[0056] The present invention also provides a method for preparing the MXene composite metal single-atom material, comprising the following steps: S1: Etching of the general formula M in an inert gas atmosphere at 500~900 ℃ in a mixture of CoCl2 and alkali halide molten salts. n+1 AC n The MAX phase, wherein M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), and molybdenum (Mo); n is an integer from 1 to 4; A is selected from one of aluminum (Al) and silicon (Si); and the reaction time is 0.5 to 10 hours. S2: The product obtained in S1 is soaked in a 5-12 M acid or alkaline solution and stirred for 1-24 hours to remove Co particles that are not in single-atom form; S3: After multiple centrifugations and washing with deionized water until the pH is neutral, the precipitate is dried at 40~80 ℃ for 6~24 hours to obtain the MXene composite metal single-atom material.

[0057] This invention innovatively employs alkali halide molten salt etching technology to successfully etch metals with the general formula M. n+1 AC n The MAX phase was obtained by yielding a halogenated group, with the general formula Mx C y T z Based on the MXene substrate, this invention can achieve sufficient etching by limiting the etching reaction time and the type and amount of alkali halide metal molten salts used in the etching process. This facilitates control over the Co loading, enabling atomic-level dispersion of Co after washing with acid or alkali solutions within a certain concentration range. This avoids the aggregation of cobalt single atoms into nanoparticles. The resulting MXene composite metal single-atom material, based on a tandem catalytic strategy, utilizes a Co SAs catalyst anchored on MXene nanosheets to achieve deep reduction of S8 to Li2S via the Li2S2 intermediate. This fully utilizes the sulfur conversion capacity in ASSLSBs. The strong chemical interaction between sulfur species and the atomically dispersed Co sites and polar Cl groups on MXene, coupled with rapid lithium-ion surface diffusion, jointly promotes the redox reaction of solid-phase sulfur. The Co@MX tandem catalyst provides abundant active sites and rapid electron / ion conduction for the all-solid-state sulfur cathode, accelerating the key steps of Li2S2 to Li2S conversion during discharge and synergistically lowering the energy barrier of each reaction step, thus achieving a deep sulfur conversion process. Furthermore, the high density of Co SAs active sites on MXene ensures that its interaction with broken active material particles is continuously enhanced during long-term cycling, regulating the reaction pathway of solid-phase sulfur redox process, thereby realizing high-capacity and long-life lithium-sulfur batteries, and has great potential to promote the practical development of ASSLSBs.

[0058] As a further option, the general formula is M n+1 AC n The molar ratio of the MAX phase to CoCl2 and alkali halide molten salt is 1:(2~6):(2~8).

[0059] As a further example, the alkali metal halide molten salt is selected from one or more of lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, sodium bromide, and potassium bromide.

[0060] As a further preferred example, the alkali metal halide molten salt is selected from one or more of sodium chloride and potassium chloride.

[0061] This invention, based on a lower etching temperature, preferentially uses sodium chloride and / or potassium chloride as alkali metal halide molten salts. Their lower melting points reduce the etching temperature, which in turn reduces side reactions during etching and avoids potential structural changes in the MXene substrate at high temperatures, thus facilitating the formation of high-quality products. One of the core innovations of this invention is the preparation of Co single-atom materials anchored to halogen functional groups on the MXene substrate via Co-T coordination bonds. NaCl and KCl, in their molten state, provide stable, abundant, and chemically suitable chloride ions, offering a more stable structure compared to other halogen elements and establishing crucial coordination relationships.

[0062] As a further preferred embodiment, the preparation method of the MXene composite metal single-atom material includes the following steps: S1: Mix Ti3AlC2 MAX phase, CoCl2, NaCl and KCl uniformly in a molar ratio of 1:(2~6):(1~4):(1~4) and grind for 10~30 min; react for 1~6 h in an inert gas atmosphere at 600~750 ℃. S2: Soak the product obtained in S1 in a 5-9 M acid or alkali solution and stir for 1-24 h to remove Co particles that are not in single-atom form; S3: After multiple centrifugation and deionized water washing until the pH is neutral, the precipitate is dried at 50~70 ℃ for 10~15 h to obtain the MXene composite metal single-atom material.

[0063] This invention can further regulate the loading of Co single atoms in MXene composite metal single-atom materials by adjusting the molar ratio of the MAX phase of Ti3AlC2 and the molten alkali halide salt as a Lewis acid molten salt, so that the MAX phase of Ti3AlC2 is completely etched, and by controlling the etching kinetics during the reaction process.

[0064] As a further example, the base in S2 is exemplary selected from one or more of potassium hydroxide (KOH), sodium hydroxide (NaOH), and ammonia water.

[0065] As a further example, the acid in S2 is exemplary selected from one or more of hydrochloric acid and sulfuric acid.

[0066] As a further preferred embodiment, the molar ratio of MAX to CoCl2, NaCl and KCl in S1 is 1:3:2:2; the reaction time is 1 h in an inert gas atmosphere at 700 ℃.

[0067] As a further preferred embodiment, step S2 involves soaking the product obtained in step S1 in a 5-9 M acid solution and stirring for 24 hours to remove Co particles that are not in a single-atom form.

[0068] In this invention, the content of the catalyst Co in MXene is adjusted by controlling the type and concentration of the acid or alkali solution in the washing process of step S2. Using the same synthesis method, the loading of Co can be increased by reducing the concentration of acid or alkali, but a higher Co loading will cause Co to aggregate and form Co particles. Therefore, optimizing the parameters in the etching process and the washing process at the same time is of great significance to ensure that Co coordinates with the halogen groups on MXene and exists in the form of single atoms in the MXene composite metal single-atom material.

[0069] The present invention also provides a method for preparing a composite cathode comprising the MXene composite metal single-atom material, comprising the following steps: S1: Li2S and the MXene composite metal single-atom material are ball-milled at 400-700 rpm for 3-7 hours in an inert gas atmosphere at a mass ratio of (1-5):(1-5) to obtain Li2S / Co@MX; S2: The Li2S / Co@MX, conductive agent, and composite cathode prepared in S1 are ball-milled with electrolyte at a mass ratio of (1~10):(1~5):(1~7) and ball-milled at 400~700 rpm for 3~7 hours in an inert gas atmosphere to obtain the composite cathode.

[0070] As a further embodiment, the conductive agent is selected from one or more of Ketjen black, carbon black, graphite, graphene, carbon nanotubes, acetylene black, activated carbon, conductive carbon spheres, conductive carbon fibers, and reduced graphene oxide.

[0071] As a further option, the electrolyte for the composite positive electrode is selected from one or more of the silver-germanium sulfide type electrolytes.

[0072] As a further preferred embodiment, the method for preparing the composite positive electrode includes the following steps: S1: Li2S and the MXene composite metal single-atom material were ball-milled at 550 rpm for 5 hours in an inert gas atmosphere at a mass ratio of 4:2 to obtain Li2S / Co@MX; S2: The Li2S / Co@MX, Ketjen Black and Li6PS5Br were ball-milled at a mass ratio of 6:2:4 and then ball-milled at 550 rpm for 5 hours in an inert gas atmosphere to prepare the composite cathode.

[0073] The present invention also provides a method for preparing an all-solid-state lithium-sulfur battery including the composite cathode, comprising the following steps: S1: Press the electrolyte powder with a pressure of 100~400 MPa for 1~5 minutes to obtain an electrolyte sheet, then spread the composite positive electrode evenly on the surface of the electrolyte sheet, and press it again with a pressure of 100~400 MPa for 1~5 minutes; S2: Place a lithium alloy layer on the other side of the electrolyte sheet, apply a pressure of 50~200 MPa for 1~5 minutes.

[0074] As a further option, the lithium-containing alloy is selected from one of Li-In alloy, Li-Al alloy, Li-Si alloy, Li-Sn alloy, and Li-Bi alloy.

[0075] As a further preferred embodiment, the lithium-containing alloy is a Li-In alloy.

[0076] As a further preferred embodiment, the preparation method of the all-solid-state lithium-sulfur battery includes the following steps: S1: Press the electrolyte powder with a pressure of 250 MPa for 3 minutes to obtain an electrolyte sheet, then spread the composite positive electrode evenly on the surface of the electrolyte sheet, and press it with a pressure of 250 MPa for another 3 minutes. S2: Place a layer of Li-In alloy on the other side of the electrolyte sheet, apply a pressure of 125 MPa for 3 minutes.

[0077] As an example, the electrolyte powder is selected from one or more of the following: silver-germanium sulfide electrolytes (Li6PS5X, where X is a halogen element).

[0078] As an example, the sulfide-germanium ore type electrolyte includes one or more of Li6PS5Cl and Li6PS5Br.

[0079] Those skilled in the art can select a suitable preparation method to prepare the electrolyte of this invention as needed. An electrolyte that is the same as or different from the electrolyte used in the composite positive electrode of this invention can be selected as the electrolyte powder constituting the electrolyte sheet. For example, when the electrolyte powder is Li6PS5Br, it can be synthesized by ball milling Li2S, LiBr, and P2S5 in a molar ratio at 550 rpm for 10 hours in an inert gas atmosphere, such as argon. Afterwards, the mixture is pressed into sheets and annealed in a quartz tube under vacuum at 550 °C for 10 hours to obtain Li6PS5Br.

[0080] The present invention also provides a composite cathode prepared by a composite cathode preparation method and an all-solid-state lithium-sulfur battery prepared by a method for preparing an all-solid-state lithium-sulfur battery, including the MXene composite metal single-atom material.

[0081] As a specific example of the implementation of this invention, detailed cases are provided below.

[0082] Example 1: (1) Synthesis of Co@MX: First, the MAX phase of Ti3AlC2 was ground with CoCl2, NaCl, and KCl in a mortar for 20 min to achieve uniform mixing. The molar ratio of MAX to CoCl2, NaCl, and KCl was 1:3:2:2. The reaction was carried out at 700 °C under an argon atmosphere for 1 hour. The Co loading in the MXene composite metal single-atom material was 1 wt%. The solid product obtained above was soaked in 9 M hydrochloric acid solution and stirred for 24 h to remove metal particles. After several washes with deionized water and centrifugation at 8000 rpm for 5 min until the pH of the supernatant was neutral, the precipitate was dried at 60 ℃ for 12 h to obtain MXene composite metal single-atom material (Co@MX); (2) Synthesis of LiPSBr electrolyte: Li6PS5Br was synthesized by ball milling Li2S, LiBr and P2S5 according to the molar ratio of each element in Li6PS5Br, reacting at 550 rpm for 10 hours in an argon atmosphere. Afterwards, the mixture was pressed into sheets and annealed in a quartz tube at 550 °C for 10 hours under vacuum. (3) Preparation of composite cathode: Li2S / Co@MX was prepared by ball milling Li2S and the MXene composite metal single-atom material Co@MX at a mass ratio of 4:2 at 550 rpm for 5 hours in an argon atmosphere. The Li2S / Co@MX composite cathode was prepared by ball milling the above Li2S / Co@MX, Ketjen Black (KB) and Li6PS5Br at a mass ratio of 6:2:4 at 550 rpm for 5 hours in an argon atmosphere. (4) Assembly of ASSLSBs: ASSLSBs were assembled in an argon-filled glove box using a cold pressing method in a mold battery with a diameter of 10 mm. Specifically, 90 mg of Li6PS5Br was first pressed into a disc at 250 MPa for 3 minutes. Then, the required amount of composite cathode powder was evenly spread on the surface of the electrolyte disc and pressed again at 250 MPa for 3 minutes. Next, a layer of Li-In alloy was placed on the other side of the electrolyte disc at 125 MPa for 3 minutes. Finally, the mold was tightened with three bolts to obtain the ASSLSBs.

[0083] Example 2: Unlike Example 1, the concentration of hydrochloric acid in the synthesis of Co@MX was 12M, and the loading of Co in the MXene composite metal single-atom material was 0.5 wt%.

[0084] Example 3: Unlike Example 1, the concentration of hydrochloric acid in the synthesis of Co@MX was 10M, and the loading of Co in the MXene composite metal single-atom material was 0.75 wt%.

[0085] Example 4: Unlike Example 1, the concentration of hydrochloric acid in the synthesis of Co@MX was 7M, and the loading of Co in the MXene composite metal single atom material was 1.5 wt%.

[0086] Example 5: Unlike Example 1, the concentration of hydrochloric acid in the synthesis of Co@MX was 5M. The loading of Co in the MXene composite metal single-atom material was 2 wt%.

[0087] Example 6: Unlike Example 1, the MAX phase of Ti3AlC2, CoCl2, and NaCl were first ground in a mortar for 20 minutes to achieve uniform mixing. The molar ratio of MAX to CoCl2 and NaCl could be 1:3:4. The reaction was carried out at 700 °C under an argon atmosphere for 1 hour. The loading of Co in the MXene composite metal single-atom material was 1.7 wt%.

[0088] Example 7: Unlike Example 1, the MAX phase of Ti3AlC2 was first ground with CoCl2, NaCl, and KCl in a mortar for 10-30 min to achieve uniform mixing. The molar ratio of MAX to CoCl2, NaCl, and KCl could be 1:2:1:1. The reaction was carried out at 700 °C under an argon atmosphere for 1 hour. The loading of Co in the MXene composite metal single-atom material was 0.8 wt%.

[0089] Example 8: Unlike Example 1, the MAX phase of Ti3AlC2 was first ground with CoCl2, NaBr, and KBr in a mortar for 10-30 min to achieve uniform mixing. The molar ratio of MAX to CoCl2, NaBr, and KBr could be 1:3:2:2. The reaction was carried out at 700 °C under an argon atmosphere for 1 hour. The loading of Co in the MXene composite metal single-atom material was 0.9 wt%.

[0090] Example 9: Unlike Example 1, Li2S / Co@MX was prepared by ball milling Li2S and Co@MX at 550 rpm for 5 hours in an argon atmosphere with a mass ratio of 4:4. The loading of Co in the MXene composite metal single-atom material was 1 wt%.

[0091] Comparative Example 1: Unlike Example 1, CoCl2 was replaced with ZnCl2. The loading of Zn in the MXene composite metal single-atom material was 1 wt%.

[0092] Comparative Example 2: Unlike Example 1, a positive electrode without Co@MX was prepared using the same steps, wherein the mass ratio of Li2S:KB:Li6PS5Br was 4:2:4.

[0093] Comparative Example 3: Unlike Example 1, the concentration of hydrochloric acid was 0.5 M. The loading of Co in the MXene composite metal single-atom material was 7 wt%.

[0094] Comparative Example 4: Unlike Example 1, hydrochloric acid was replaced with deionized water. The Co loading in the MXene composite metal single-atom material was 19 wt%.

[0095] Comparative Example 5: Unlike Example 1, Ti3AlC2 MAX was replaced with graphene. The loading of Co in Co-supported graphene (Co@Gr) was 1 wt%.

[0096] Comparative Example 6: Unlike Example 1, Co@MX was replaced with graphene.

[0097] Comparative Example 7: Unlike Example 1, the reference is Adv. Mater. 2019, 31, 1902432, https: / / doi.org / 10.1002 / adma.201902432 Fast Gelation of Ti3C2Tx MXene Initiated by Metal Ions for the preparation of Co-free MXene-O (Ti3C2O2).

[0098] Comparative Example 8: Unlike Example 1, the reference is Adv. Energy Mater. 2023, 13, 2202709, https: / / doi.org / 10.1002 / aenm.202202709 Tuning the Surface Chemistry of MXene to Improve Energy Storage: Example of Nitrification by Salt Melt for the preparation of Co-free single-atom MXene-Cl (Ti3C2Cl2).

[0099] Comparative Example 9: Unlike Example 1, Ti3AlC2 MAX was replaced with porous carbon. The loading of Co in the Co-supported porous carbon (Co@C) was 1 wt%.

[0100] The following tests were performed on Examples 1-9 and Comparative Examples 1-8: (1) At room temperature, 2.8 mA cm -2 At high current densities, with a Li₂S surface loading of 1.2 mg cm⁻¹, -2 Run, and perform a reversible specific capacity test with 2000 cycles; (2) At room temperature, with a surface loading of 1.2 mg cm⁻¹ of Li₂S… -2 The current density ranges from 0.1 to 2.8 mA cm⁻¹. -2 Rate performance testing within the specified range; (3) At room temperature, with a surface loading of 2 to 3 mg cm⁻¹ of high Li₂S. -2 The current density is 0.4 mA cm⁻¹ -2 Cyclic stability test.

[0101] Table 1 shows the results of Examples 1-9 and Comparative Examples 1-9 at 2.8 mA cm⁻¹. -2 It operates at high current density and has a reversible specific capacity of 2000 cycles.

[0102]

[0103] As can be seen from the comparison between Example 1 and Comparative Example 1, the present invention uses Co as a single-atom material because the d electrons of Co can undergo strong covalent interactions with the p orbitals of S atoms in the active material. Compared with other single atoms, this interaction can not only effectively adsorb the active material, but also activate the S-S bond, significantly reducing its breaking energy barrier. Furthermore, since Co is also a good conductor, when anchored on MXene with excellent conductivity, it can well maintain the overall conductive network of the electrode, promoting the rapid transfer of electrons to the reaction site.

[0104] As can be seen from the comparison between Example 1 and Comparative Examples 3-4, the content of catalyst metal elements in MXene can be adjusted by controlling the concentration of acid or alkali solutions during the acid or alkali washing process. For MXene loaded with Co, the same synthesis method was used, except that the concentration of 9 M hydrochloric acid in Example 1 was reduced to 0.5 M, or deionized water was used instead of hydrochloric acid, and the mixture was soaked and stirred for 24 h to synthesize MXene with a higher Co content. Correspondingly, as the acid washing concentration decreased, Co aggregated from single atoms to form metal particles. In Comparative Examples 3 and 4, the loading of Co in the MXene composite metal single-atom material was 7 wt% and 19 wt%, respectively. However, because Co aggregated from single atoms to form metal particles, the long-cycle performance significantly decreased.

[0105] A comparison of Example 1 and Comparative Example 2 shows that Comparative Example 2, which does not contain the MXene composite metal single-atom material of this invention, exhibits extremely poor cycling performance. This demonstrates that the introduction of Co@MX is crucial for catalyzing solid-phase sulfur conversion, improving reaction kinetics, and achieving deep sulfur conversion. Without a catalyst, sulfur conversion is incomplete, and capacity and cycling performance are severely limited.

[0106] A comparison of Example 1 and Comparative Examples 5-6 shows that Comparative Example 5 uses graphene as a substrate to prepare graphene loaded with Co single atoms, while Comparative Example 6 directly uses graphene as the positive electrode carrier material. This comparison highlights the irreplaceable advantages of the MXene substrate in this invention. The abundant halogen functional groups (such as Cl) on the MXene surface can serve as strong anchoring sites, forming stable Co-T coordination bonds with Co, effectively preventing the migration and aggregation of single atoms during synthesis and cycling. Furthermore, the polarity and metallic conductivity of MXene itself are more conducive to Li… + The transport and rapid transfer of electrons are facilitated. Graphene substrates are weaker than MXene in both stabilizing single atoms and promoting ion transport. Stable Co-Cl bonds cannot be formed.

[0107] A comparison of Example 1 with Comparative Examples 7 and 8 shows that when using MXene-O or MXene-Cl without Co single atoms, the cycle performance and reversible specific capacity are lower due to the lack of catalytic activity from Co single atoms. This further illustrates that Co single atoms are the core source of catalytic activity. Although the MXene-O substrate itself has a certain degree of polarity, its surface lithium-ion conduction ability is lower than that of Cl. It also lacks atomically dispersed Co sites to efficiently catalyze the breaking of SS bonds and the key conversion step from Li₂S₂ to Li₂S. Furthermore, its conductivity is low, and its catalytic ability is far inferior to that of Co@MX in Example 1.

[0108] As can be seen from the comparison between Examples 1 and Examples 2-8, the content of Co element in the catalyst of MXene in this invention is adjusted by controlling the concentration of acid or alkali solution in the washing process of step S2. Using the same synthesis method, the loading of Co can be increased by reducing the concentration of acid or alkali, but a higher Co loading will cause Co to aggregate and form Co particles. At the same time, optimizing the parameters in the etching process and the washing process is of great significance to ensure that Co exists in the coordinated and single-atom form in the MXene composite metal single-atom material under a higher Co loading.

[0109] A comparison of Examples 1 and 2-5 shows that treatment with 9 M hydrochloric acid effectively removes non-target cobalt nanoparticles (Co NPs) that may form during synthesis, while retaining Co single atoms anchored to MXene via strong Co-Cl coordination bonds, achieving a superior Co loading of approximately 1 wt%. The material under these conditions exhibits higher reversible specific capacity and optimal cycle stability in battery testing.

[0110] When treated with higher concentrations of acid (such as the 12 M hydrochloric acid in Example 2), the excessively acidic environment may hinder the formation of stable Co-Cl coordination structures, leading to the elution of excess Co species and resulting in a low loading of Co SAs. Although the material can still maintain a single-atom morphology, the insufficient density of active sites limits its full catalytic efficiency.

[0111] Therefore, by optimizing the acid washing concentration, a balance can be achieved between the efficient purification of Co SAs and the ideal loading, thereby preparing Co@MX materials with maximized catalytic active site density and optimal structure, ultimately endowing all-solid-state lithium-sulfur batteries with excellent electrochemical performance.

[0112] A comparison of Examples 1 and 6-8 shows that, based on a lower etching temperature, this invention can preferentially use sodium chloride and potassium chloride as alkali metal halide molten salts. Their lower melting points reduce the etching temperature, which in turn reduces side reactions during etching and avoids potential structural changes in the MXene substrate at high temperatures, thus facilitating the formation of high-quality products. One of the core innovations of this invention is the preparation of Co single-atom materials anchored to halogen functional groups on the MXene substrate via Co-T coordination bonds. NaCl and KCl, in their molten state, provide stable, abundant, and chemically suitable chloride ions, offering a more stable structure compared to other halogen elements and establishing crucial coordination relationships.

[0113] A comparison of Examples 1 and 9 shows that the present invention can further optimize the mass ratio of Li2S and the MXene composite metal single-atom material, thereby balancing the relationship between active material loading and the catalytic / conductive network. When the mass ratio of Li2S to Co@MX is 4:2, sufficient catalytic active sites and efficient electron / ion conduction pathways can be provided while ensuring sufficient sulfur loading, resulting in better battery performance. For example, the excessively high Co@MX ratio in Example 9, while providing richer catalytic sites and a more efficient conductive network, correspondingly reduces the proportion of the active material Li2S, limiting the overall energy density improvement; while an excessively low Co@MX ratio may lead to insufficient catalytic sites and decreased conductivity, failing to effectively promote deep sulfur conversion and affecting capacity utilization and cycle stability. Therefore, the optimized mass ratio can further realize high-capacity, long-life all-solid-state lithium-sulfur batteries.

[0114] Depend on Figure 1 As can be seen from the energy dispersive X-ray spectroscopy (EDS) elemental distribution diagram of a, the MXene composite metal single-atom material prepared in Example 1 shows a uniform distribution of Ti, Cl and Co elements; this indicates that the MXene composite metal single-atom material in this invention has undergone sufficient etching, and the Co element is uniformly distributed on the Ti3C2Cl2-based MXene obtained after etching.

[0115] Depend on Figure 1 b and Figure 5 The aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the medium clearly shows the isolated Co SAs uniformly dispersed on the MXene prepared in Example 1, presenting clear bright spots.

[0116] like Figure 5 c and Figure 5 As shown in Figure d, no Co particle lattice was detected on the MXene substrate prepared in Example 1 in the transmission electron microscope (TEM) image, which is consistent with... Figure 6 The X-ray diffraction (XRD) results for a were consistent with those for a, confirming the absence of Co nanoparticles (NPs).

[0117] Depend on Figure 1 As can be seen from c, this invention further utilizes synchrotron X-ray absorption spectroscopy (XANES) to investigate the properties of Co SAs. The X-ray absorption near-edge structure spectra of the Co K-side indicate that the Co valence state in Co@MX is intermediate between that of Co. 0 and Co 2+ between.

[0118] like Figure 1As can be seen from Figure d, the Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of Co@MX prepared in Example 1 of this invention has only one significant peak at a radial distance of 1.98 Å, similar to CoCl2. No metallic Co was observed in Co@MX. 0 The characteristic Co-Co peak further confirms the presence of atomically dispersed Co nanoparticles rather than cobalt (Co NPs).

[0119] In addition, such as Figure 7 As can be seen from the results, the Co SAs prepared in Example 1 of this invention may coordinate with the Cl functional group on MXene through Co-Cl interaction. This is supported by the EXAFS wavelet transform (EXAFS-WT) results, in which Co@MX and CoCl2 show similar maximum values ​​in k and R spaces.

[0120] This invention obtains a Li2S / Co@MX mixture using a high-energy ball milling method. For example... Figure 1 The TEM image of Li2S shows that Li2S and Co@MX are uniformly mixed, with Li2S maintaining its initial crystalline state, while Co@MX has undergone amorphization. Figure 1 As can be seen from f, a small amount of amorphous sulfur was detected in the Li₂S / Co@MX mixture by Raman spectroscopy. This phenomenon may be attributed to a slight side reaction between Li₂S and Co@MX, which may contribute to improved electrochemical capacity. Figure 1 In medium g, Li2S / Co@MX solid state 6 The magic-angle rotation nuclear magnetic resonance (MAS-NMR) spectrum of Li shows that, compared with pure Li₂S, its 6 The broadening of the Li resonance peaks indicates a more diversified local environment in Li due to its interaction with Co@MX; for example Figure 1 The results showed that X-ray photoelectron spectroscopy (XPS) further confirmed the chemical interaction between Li₂S and Co@MX. The Li₂S / Co@MX Li₂S / Co@MX 1s The peak position shifted by 0.45 eV towards higher binding energies, attributed to the electron-withdrawing effect of the Cl-based functional groups on the MXene surface, an effect that favors the decomposition of Li₂S during electrochemical oxidation. Figure 1 The FT-EXAFS results for i show that Li2S / Co@MX exhibits a main peak at approximately 1.85 Å, which is slightly different from Co@MX (1.98 Å), but consistent with the Co-S bond position in CoS2. Furthermore, as... Figure 1 j and Figure 1 The EXAFS-WT spectrum further confirms the possible Co-S coordination, as evidenced by the approximate maximum values ​​in both k and R spaces; for example... Figure 1 The differential charge density calculations also demonstrate the charge transfer between Li₂S and Co@MX. The strong interaction between the S atom and the Co site, accompanied by the relatively weak Li-Cl bond, plays a crucial role in promoting the redox process of sulfur.

[0121] like Figure 2 As can be seen from Figure a, through density functional theory (DFT) calculations, we have gained a deeper understanding of the catalytic mechanism of Co@MX in the redox reaction of ASSLSBs. The chemisorption of Li2S / Li2S2 molecules on the Co@MX prepared in Example 1 is stronger than that of MXene alone (such as MXene-Cl in Comparative Example 8) and graphene in Comparative Example 6. The strong interaction between Co@MX and Li2S is conducive to weakening the Li-S bond and promoting Li + Dissociation from Li2S, such as Figure 2 As shown in Figure b, the Li content on the surfaces of polar MXene and non-polar graphene was calculated. + Diffusion barrier. Li on MXene-Cl + The diffusion barrier is 0.206 eV, lower than that of graphene (0.294 eV). The lower Li... + The diffusion barrier increased Li + It diffused and facilitated the reaction between lithium and sulfur. For example... Figure 2 As shown in Figure c, the SS bond strength in adsorbed Li₂S₂ was quantitatively evaluated using the crystal orbital Hamiltonian population (COHP) method. The SS bond strength of Li₂S₂ on the Co@MX surface is much weaker than that on MXene and graphene alone, indicating that the Co@MX prepared in Example 1 can promote the breaking of SS bonds and the reduction of Li₂S₂. Figure 2 As can be seen from Figure d, the increase in Gibbs free energy for the Li₂S₂→Li₂S reaction is the largest, indicating that the reduction reaction from Li₂S₂ to Li₂S is the rate-limiting step. The free energy change on MXene-Cl is lower than that on graphene, which is consistent with the results of COHP analysis, highlighting the catalytic potential of MXene-Cl. Furthermore, Co@MX not only promotes the conversion of S₈ / Li₂S₂ through a significantly lower free energy change, but also makes the Li₂S₂→Li₂S reduction reaction thermodynamically more advantageous. In particular, the energy-intensive Li₂S₂ reduction reaction becomes a thermodynamically spontaneous reaction with the assistance of Co@MX, making Li₂S nucleation easier. Moreover, the free energy change of each step on Co@MX is significantly lower than that on the surface of graphene (Co@Gr) loaded with Co SAs. Therefore, the synergistic effect of atomically dispersed Co sites and the polar Cl-rich nature of the MXene surface reveals the high catalytic activity of Co@MX, jointly realizing a tandem catalytic process.

[0122] This invention assembled ASSLSBs to study the tandem catalytic sulfur conversion process in a Co@MX-based sulfur cathode. The solid electrolyte used in this invention is a sulfide-germanium sulfide electrolyte, a highly promising candidate material due to its high ionic conductivity, good mechanical deformability, and good stability to both lithium metal and the sulfur cathode. This invention uses XPS to probe the evolution of sulfur species in the composite cathode under different charge-discharge states, such as... Figure 2 China and Figure 2 As can be seen from f, S 2p XPS spectra show that Li₂S(2) is present in the initial state. p3 / 2 At 160.6 eV and PS4 3- (2) p3 / 2 At 162.0 eV). During charging to 3.6 V (C - 3.6 V, all potentials relative to Li). + After Li), abundant elemental sulfur (S) was detected in the Co@MX-based cathode. 0 Furthermore, less residual Li2S was observed in the cathode without the Co@MX catalyst, i.e., in Comparative Example 2, where less S was observed. 0 After discharging to 1.2 V (D-1.2 V), the S in the positive electrode based on Co@MX... 0 The S₈ almost disappears and transforms into Li₂S₂ / Li₂S, as it exhibits a 0.20 eV shift towards a higher binding energy compared to pure Li₂S. Upon further discharge to 0.6 V (D-0.6 V), the Li₂S peak returns to its initial position and becomes the dominant peak. It can be inferred that S₈ first transforms into Li₂S₂, and then Li₂S₂ is reduced back to Li₂S. Even at D-0.6 V, the residual S₈ in the absence of Co@MX remains... 0 The presence of Li₂S₂ indicates incomplete sulfur reduction and limited utilization of the active material. This suggests that Co@MX lowers the reaction energy barrier and increases the reaction rate at each step, thereby enabling a deeper S₈→Li₂S₂→Li₂S conversion process.

[0123] To further elucidate the effect of tandem catalysis, we systematically investigated the electrochemical behavior of the Co@MX-based sulfur cathode and analyzed its redox kinetics. In the constant current charge-discharge curves, Comparative Example 2, the ASSLSB without Co@MX, exhibited lower capacity and more severe polarization, which is further reflected in the corresponding differential capacity versus voltage (dQ / dV) curves. Figure 3The two reduction peaks (peaks i and ii) in α were observed at both positive electrodes, indicating that the reduction process involves two steps: first, the formation of the Li₂S₂ intermediate, and then the formation of Li₂S. The apparent activation energy of the sulfur redox reaction was determined by cyclic voltammetry (CV) curves at different temperatures. E a ).like Figure 3 In b, the positive electrode based on Co@MX E a The value is significantly lower than that of the cathode without Co@MX, indicating that the reaction barrier is lower due to the effect of Co@MX. Figure 3 c and Figure 3 In section d, we further investigated the effect of the Co@MX tandem catalyst on ion transport in the solid-state cathode during sulfur redox. For both cathodes, a linear relationship was observed between the CV peak current and the square root of the scan rate; however, the Co@MX-based cathode exhibited a higher current response and a steeper slope, indicating enhanced lithium-ion diffusion. Figure 3 As can be seen from Figure e, compared with Comparative Example 2 (which does not contain Co@MX), the ionic resistance of the Co@MX-based cathode composite material is significantly reduced (as determined by Ohm's law), as measured by DC polarization technology. Correspondingly, the effective ionic conductivity of the Co@MX-based composite material is significantly increased, reaching as high as 1.1 × 10⁻⁶. -5 Scm -1 These results highlight the crucial role of Co@MX, particularly the Cl-rich polar surface of MXene in promoting lithium-ion diffusion and enhancing lithium-ion migration at the Li2S-Li6PS5Br (LiPSBr) interface. Figure 3 In addition, this invention also extracts and compares the system resistance at various charge and discharge potentials. R s ), interface resistance ( R int ) and charge transfer resistance ( R ct The results were obtained by fitting in-situ electrochemical impedance spectroscopy (EIS). Based on the Co@MX cathode... R ct and R int The values ​​were all lower and more stable, attributed to Co@MX promoting lithium-ion / electron diffusion and the robust MXene substrate providing structural integrity, thus helping to maintain the tightness of the cathode / electrolyte interface. Figure 3 Zhongg and Figure 3 As can be seen from h, we use the relaxation time distribution (DRT) to distinguish the resistance changes of each part over the time scale. For example... Figure 3As shown in Figure g, the D3 peak (relaxation time: 0.1–1 s) is related to the ion transport impedance across the cathode interface, while the D4 peak, with the largest time constant, is attributed to charge transfer processes within the cathode. For both charging and discharging processes, the intensities of these peaks are almost all lower in the Co@MX-based cathode than in the cathode without Co@MX. The DRT results indicate that Co@MX enhances charge transfer in the composite cathode and promotes lithium-ion transport at the cathode / electrolyte interface, consistent with the EIS analysis results.

[0124] Depend on Figure 4 a, Figure 4 c, Figure 4 d, Figure 4 As can be seen from Figure f, the capacity of the composite cathode using the MXene composite metal single-atom material of this invention gradually increases during cycling, eventually exceeding its initial capacity, especially at high current densities. This indicates that the reversible lithiation / delithiation of the LiPSBr electrolyte or the MXene composite metal single-atom material may generate some additional capacity, but this limited contribution cannot explain the sustained capacity increase. More importantly, the aforementioned capacity increase can be explained at the microscopic and molecular levels through a unique catalytic mechanism in an all-solid-state system. At the microscopic level, the initial micron-sized Li2S particles are electrochemically inert, resulting in a low initial capacity. Furthermore, these large particles undergo significant volume changes and severe pulverization during cycling, leading to fragmentation, separation from the ionic / electronic conductors, and consequently, the accumulation of deactivated S8 / Li2S. However, the Co@MX matrix in this invention mitigates this contact loss and allows for efficient Li + Diffusion and high electron conductivity enable the full conversion and maximum utilization of fragmented active material particles. At the molecular level, for solid-state catalysis, sulfur species adsorbed on specific active sites are almost fixed and rarely undergo site desorption, while solid electrolytes cannot move freely, leading to Li... + Transport is limited, which is quite different from liquid Li-S catalysis. Therefore, Co@MX has abundant surface active sites and Li + Rapid diffusion on the surface plays a crucial role. With increasing cycle count, the utilization of Co@MX catalytic sites gradually improves as more fragmented active material particles come into contact with and interact with Co SAs active sites, which helps to increase battery capacity.

[0125] Depend on Figure 4As can be seen from Figure a, the discharge capacity and cycle stability of the Co@MX-based lithium-sulfur battery prepared in Example 1 of this invention are significantly higher than those of MXene-O (Comparative Example 7) without Co SAs or Co SAs (Co@C) with porous carbon as the support (Comparative Example 9). These results confirm the synergistic effect of the Co@MX tandem catalyst.

[0126] Depend on Figure 4 As can be seen from b, Example 1 uses a Co@MX all-solid-state lithium-sulfur battery at 0.4 mA cm⁻¹. -2 The initial discharge capacity at the current density is 1176 mAh g. S -1 This is significantly better than batteries that do not use Co@MX. This is due to the large energy barrier of the S8→Li2S2→Li2S conversion and the incomplete conversion reaction.

[0127] Depend on Figure 4 As can be seen from c, the Co@MX-based battery in Example 1 still maintains an excellent 1249 mAh g after more than 440 cycles. S -1 The capacity.

[0128] Depend on Figure 4 As can be seen from d, the battery of Example 1 performs well even at higher current densities (0.8 mA cm⁻¹). -2 Under these conditions, the Co@MX-based cathode can also provide an initial reversible capacity of 1071 mAh g. S -1 And there was no capacity decay during 500 cycles.

[0129] Depend on Figure 4 As can be seen from Figure e, due to the tandem catalytic effect, the rate performance of the Co@MX-based cathode is significantly better than that of the cathode without Co@MX.

[0130] Depend on Figure 5 a, Figure 5 b, Figure 5 As shown in Figure c, SEM and TEM images reveal that the Co@MX prepared in Example 1 exhibits a multilayered sheet-like structure. High-resolution TEM and... Figure 6 XRD analysis of a in the image verified the crystal properties of the MXene substrate. For example... Figure 5 As can be seen from the TEM image, no Co particle lattice was detected on the MXene substrate; only the lattice spacing of the (002) plane of MXene itself was approximately 0.26 nm, confirming that no nanoparticles were present in the Co@MX prepared in Example 1. Accordingly, as Figure 5The spherical aberration electron microscope (HAADF-STEM) image shown in Figure e clearly shows isolated Co SAs uniformly dispersed on MXene nanosheets (clear bright spots).

[0131] Depend on Figure 6 As can be seen, by reducing the concentration of hydrochloric acid during the preparation process, in Comparative Examples 3 and 4, Co single atoms are transformed into Co metal particles with typical Co metal XRD characteristic peaks. Simultaneously, the EDS elemental distribution images also detect more Co element signals and exhibit aggregation. Therefore, excessively low acid washing concentrations are not conducive to obtaining Co single-atom dispersion.

[0132] Depend on Figure 8 As can be seen, to confirm the key role of Co surface active sites in improving electrochemical performance, we studied and compared a series of MXene samples with different Co loadings. With increasing Co content, Co SAs tended to aggregate into nanoparticles, resulting in a decrease in discharge capacity compared to Co@MX. These findings indicate that the abundant, atomically dispersed Co active sites on the polar MXene surface are more capable of effectively tandemly catalyzing sulfur species during long-term cycling compared to Co nanoparticles.

[0133] In summary, this invention utilizes a tandem catalytic strategy, employing a Co SAs catalyst anchored on MXene nanosheets, to achieve deep reduction of S8 to Li2S via the Li2S2 intermediate, thereby fully leveraging the sulfur conversion capacity of ASSLSBs. The strong chemical interaction between the sulfur species and the atomically dispersed Co sites and polar Cl groups on MXene, coupled with rapid Li... + Surface diffusion jointly promotes the redox reaction of sulfur. The Co@MX tandem catalyst provides abundant active sites and rapid electron / ion conduction for the all-solid-state sulfur cathode, accelerating the key step of Li₂S₂ to Li₂S conversion during discharge and synergistically lowering the energy barrier of each reaction step, thus achieving a deep sulfur conversion process. Furthermore, the high density of Co SAs active sites on MXene ensures continuously enhanced interaction with fragmented active material particles during long-term cycling. A battery matched with Co@MX achieves a performance of 2.8 mA cm⁻¹. -2 After 2000 cycles at a current density, its reversible capacity reached 1329 mAh g. S -1 The high-load positive electrode still maintains 3.8 mAh cm⁻¹ after 100 cycles at room temperature. -2Our research demonstrates that tandem catalysis can modulate the reaction pathway of the solid-phase sulfur redox process, thereby achieving high-capacity and long-life Li-S batteries, which have great potential to promote the practical application of ASSLSBs.

[0134] It should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A MXene composite metal monatomic material, characterized in that, The MXene composite metal monatomic material (Co@MX) has a general formula of M x C y T z The MXene is a base, and Co monatomic atoms (Co SAs) are anchored on T functional groups of the MXene base through Co-T coordination bonds; the Co monatomic atoms are atomically dispersed; wherein M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), molybdenum (Mo); wherein T z is selected from one of a halogen element group, wherein y is an integer from 1 to 4, z is an integer from 1 to 4, x = y + 1.

2. The MXene composite metal monatomic material of claim 1, wherein, The MXene composite metal monatomic material at least has one of the following characteristics: (1) The XRD pattern of the MXene composite metal monatomic material has no Co metal peak; (2) The MXene composite metal monatomic material has no Co particle lattice in the transmission electron microscope (TEM) image, but the energy dispersive X-ray spectrum (EDS) element distribution image shows that the Co element is uniformly distributed; (3) The spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the MXene composite metal monatomic material shows bright spots representing Co single atoms (Co SAs) uniformly dispersed on the MXene; (4) The Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectrum of the MXene composite metal monatomic material has only one significant peak at a radial distance of 1.98 Å; (5) The X-ray absorption near-edge structure spectrum of MXene composite metal monatomic material Co K edge shows that the valence state of Co is between Co 0 and Co 2+ ; (6) The EXAFS wavelet transform (EXAFS-WT) results of the MXene composite metal monatomic material EXAFS show that Co@MX and CoT z Show similar or same maximum in k and R space.

3. The MXene composite metal monatomic material of claim 1, wherein, The MXene composite metal monatomic material (Co@MX) has a general formula of Ti x C y Cl z MXene as a substrate, and Co monatomic atoms (Co SAs) are anchored on Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co monatomic atoms are atomically dispersed; wherein y is an integer of 1-4, z is an integer of 1-4, and x=y+1; Preferably, the MXene composite metal monatomic material (Co@MX) takes Ti3C2Cl2 MXene as a substrate, and Co single atoms (Co SAs) are anchored on the Cl functional groups of the MXene substrate through Co-Cl coordination bonds; the Co single atoms are atomically dispersed.

4. The MXene composite metal monatomic material of claim 1, wherein, The loading amount of Co in the MXene composite metal monatomic material in the MXene composite metal monatomic material is 0.5-2 wt%; Preferably, the loading amount of Co in the MXene composite metal monatomic material in the MXene composite metal monatomic material is 1-2 wt%.

5. A method for preparing the MXene composite metal monatomic material according to any one of claims 1-4, characterized in that, The method comprises the following steps: S1: etching a MAX phase of general formula M n+1 AC n in a mixture of CoCl2 and halogenated alkali metal molten salt in an inert gas atmosphere at 500-900 ℃, wherein M is selected from one of titanium (Ti), vanadium (V), chromium (Cr), niobium (Nb), and molybdenum (Mo); n is an integer of 1-4, A is selected from one of aluminum (Al) and silicon (Si), and the reaction time is 0.5-10 hours; S2: The product obtained in S1 is soaked and stirred in an acid solution or a base solution with a concentration of 5-12 M for 1-24 hours to remove Co particles in a non-single-atom form; S3: After multiple centrifugation-deionized water washing until the pH is neutral, the precipitate is dried at 40-80 ℃ for 6-24 hours to obtain the MXene composite metal monatomic material.

6. The preparation method according to claim 5, characterized in that, The general formula is M n+1 AC n MAX phase and CoCl2, alkali halide mol ratio of molten salt is 1: (2 ~ 6): (2 ~ 8); Preferably, the halogenated alkali metal molten salt is selected from one or more of lithium chloride, sodium chloride, potassium chloride, rubidium chloride, cesium chloride, sodium bromide, and potassium bromide; Further preferably, the halogenated alkali metal molten salt is selected from one or more of sodium chloride and potassium chloride.

7. The preparation method according to claim 5, characterized in that, The method comprises the following steps: S1: uniformly mix Ti3AlC2 MAX phase, CoCl2, NaCl, and KCl in a molar ratio of 1:(2-6):(1-4):(1-4) and grind for 10-30 min; react in an inert gas atmosphere at 600-750 ℃ for 1-6 h; S2: The product obtained in S1 is soaked and stirred in an acid or base solution with a concentration of 5-9 M for 1-24 h to remove Co particles in a non-single-atom form; S3: After multiple centrifugation-deionized water washing until the pH is neutral, the precipitate is dried at 50-70 ℃ for 10-15 h to obtain the MXene composite metal monatomic material; Preferably, the molar ratio of MAX to CoCl2, NaCl and KCl in S1 is 1:3:2:2; the reaction time is 1h in an inert gas atmosphere at 700℃; Preferably, S2 is to immerse and stir the product obtained from S1 in an acid solution of 5-9 M for 24 hours to remove Co particles in non-atomic form.

8. A method for preparing a composite cathode comprising the MXene composite metal monatomic material of any one of claims 1-4 or the MXene composite metal monatomic material prepared by the method of any one of claims 5-7, characterized in that, The method comprises the following steps: S1: ball milling Li2S and the MXene composite metal monatomic material at a mass ratio of (1-5):(1-5) in an inert gas atmosphere at 400-700 rpm for 3-7 hours to obtain Li2S / Co@MX; S2: ball milling Li2S / Co@MX prepared in S1, a conductive agent and a composite positive electrode electrolyte at a mass ratio of (1-10):(1-5):(1-7) to prepare the composite positive electrode; Preferably, the method for preparing the composite positive electrode comprises the following steps: S1: ball milling Li2S and the MXene composite metal monatomic material at a mass ratio of 4:2 in an inert gas atmosphere at 550 rpm for 5 hours to obtain Li2S / Co@MX; S2: ball milling the Li2S / Co@MX, Ketjenblack and Li6PS5Br at a mass ratio of 6:2:4 in an inert gas atmosphere at 550 rpm for 5 hours to prepare the composite positive electrode.

9. A method of producing a full solid-state lithium-sulfur battery comprising the composite positive electrode produced by the production method according to any one of claims 8, characterized by, The method comprises the following steps: S1: pressing the electrolyte powder at a pressure of 100-400 MPa for 1-5 minutes to obtain an electrolyte sheet, then uniformly laying the composite positive electrode on the surface of the electrolyte sheet and pressing again at a pressure of 100-400 MPa for 1-5 minutes; S2: placing a layer of lithium-containing alloy on the other side of the electrolyte sheet at a pressure of 50-200 MPa for 1-5 minutes; Preferably, the method for preparing the all-solid-state lithium-sulfur battery comprises the following steps: S1: pressing the electrolyte powder at a pressure of 250 MPa for 3 minutes to obtain an electrolyte sheet, then uniformly laying the composite positive electrode on the surface of the electrolyte sheet and pressing again at a pressure of 250 MPa for 3 minutes; S2: placing a layer of Li-In alloy on the other side of the electrolyte sheet at a pressure of 125 MPa for 3 minutes.

10. A composite cathode prepared by the method of claim 8 or a full solid-state lithium-sulfur battery prepared by the method of claim 9, characterized in that, The MXene composite metal monatomic material is included.

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