Preparation method of double-layer modified lithium-sulfur battery composite diaphragm

Abstract

The present invention discloses a method for preparing a double-layer modified lithium-sulfur battery composite separator. The method comprises sequentially preparing a two-dimensional material, a Co / CoN4@KB composite material, and finally a Co / CoN4@KB-two-dimensional material double-layer composite separator. The preparation method is simple, easy to control, and employs mild synthesis conditions at low cost. A polysulfide adsorption-catalytic layer is further constructed on the two-dimensional material. This material exhibits hierarchical polysulfide adsorption and conversion capabilities, abundant active sites, and enhanced ion and electron transport, thereby improving the rate performance and cycle stability of lithium-sulfur batteries. The double-layer composite separator prepared by the present invention is suitable for use as a modified separator in lithium-sulfur batteries.

Description

Technical Field

[0001] The invention belongs to the field of battery diaphragm materials and relates to a method for preparing a double-layer modified lithium-sulfur battery composite diaphragm. Background Art

[0002] The rapid development of science and technology has led to an increasing demand for energy. However, the use of traditional fossil fuels can pollute the environment, leading to a growing demand for green and clean energy. Lithium-ion batteries, as a leading example of green and clean energy, have rapidly dominated the mobile communications, laptop, digital camera, electric vehicle, and aerospace markets. However, due to limitations in the theoretical capacity of their battery materials, the energy density of lithium-ion batteries is approaching its limit, yet it still cannot fully meet the demand for long-lasting devices.

[0003] Lithium-sulfur batteries have high theoretical energy density (2675Wh kg -1 ), high theoretical capacity based on sulfur (1675 mAh g -1 ), high natural abundance and environmental friendliness, has become a research hotspot for the next generation of battery systems. Despite this, the application of lithium-sulfur batteries still faces severe challenges. These include the low conductivity of sulfur and its discharge products (Li2S2 / Li2S), huge volume changes, the intermediate polysulfides dissolving in the electrolyte and shuttling between the lithium negative electrode and the sulfur positive electrode, the low electron transfer of sulfur species and the sluggish multiphase transformation. Among them, the shuttling effect of polysulfide intermediates and poor reaction kinetics are the main problems, which lead to the accumulation of low-activity sulfur and insufficient utilization of active materials, further shortening the service life.

[0004] Currently, researchers are using carbon fibers and carbon nanotubes as modifications to polypropylene separators to improve electronic conductivity. However, the cross-linked one-dimensional structure and poor adsorption capacity still cannot effectively limit polysulfide shuttling. Two-dimensional materials such as graphene and MXene, however, suffer from van der Waals forces, leading to the stacking of large numbers of nanosheets, which impedes ion migration. Furthermore, their weak catalytic activity cannot further enhance the reaction kinetics of lithium-sulfur batteries, resulting in limited improvements in battery capacity. Summary of the Invention

[0005] In order to overcome the above technical problems, the present invention aims to provide a method for preparing a double-layer modified lithium-sulfur battery composite membrane, which is obtained by sequentially preparing a two-dimensional material, preparing a Co / CoN4@KB composite material, and preparing a Co / CoN4@KB-two-dimensional material double-layer composite membrane. The preparation method is simple, the process is easy to control, the synthesis conditions are mild, and the cost is low. A polysulfide adsorption-catalysis layer is further constructed on the two-dimensional material. The material has the ability of hierarchical adsorption and conversion of polysulfides and rich active sites, and improves the transport capacity of ions and electrons, thereby improving the rate performance and cycle stability of lithium-sulfur batteries.

[0006] In order to solve the above problems, the technical solution adopted by the present invention is:

[0007] A method for preparing a double-layer modified lithium-sulfur battery composite separator is carried out in the following order:

[0008] (1) Preparation of two-dimensional materials

[0009] Weigh the two-dimensional material and dilute the prepared MXene suspension with an ethanol diluent formed by ethanol and deionized water in a volume ratio of 1:1, which is recorded as solution A;

[0010] (2) Preparation of Co / CoN4@KB composites

[0011] Cobalt acetate tetrahydrate and phenanthroline were mixed in ethanol and stirred continuously at 50°C for 3 hours. Subsequently, Ketjen black was added to the above solution and stirred at 50°C for another 3 hours to form a precursor. The precursor was then transferred to a vacuum drying oven to evaporate the solvent. Finally, the obtained product was calcined under argon protection to obtain the Co / CoN4@KB composite material.

[0012] (3) Preparation of Co / CoN4@KB-2D material double-layer composite membrane

[0013] Co / CoN4@KB was dispersed in a mixed solution of ethanol and water to obtain solution B. Then, a vacuum filtration device was used with a polypropylene membrane as a filter membrane to dropwise add solution A. After the solvent was completely separated, solution B was added dropwise. After the solvent was completely separated, the loaded membrane Co / CoN4@KB-two-dimensional material was transferred to a vacuum drying oven for drying, and finally a Co / CoN4@KB-two-dimensional material double-layer composite membrane was obtained.

[0014] As a limitation of the present invention:

[0015] (1) In step (1), the concentration of the hydrochloric acid is 9 M; the mass volume ratio of the lithium fluoride to the hydrochloric acid is 0.1 g / mL.

[0016] (2) In step (1), the concentration of solution A is 0.3 mg / mL.

[0017] (3) In step (2), the mass ratio of cobalt acetate tetrahydrate, phenanthroline and Ketjen black is (12-60):(17-34):48;

[0018] The mass volume ratio of the Ketjen black to ethanol is 16 mg / mL.

[0019] In step (2), the mass ratio of cobalt acetate tetrahydrate and phenanthroline will directly affect the generation of Co clusters and atomically dispersed CoN4. If the ratio is too high, it will affect the generation of single-atom Co, and if the ratio is too low, no Co clusters will be generated. It is worth noting that metal Co clusters have better adsorption and catalytic capabilities for long-chain polysulfides, while CoN4 has better adsorption capabilities for short-chain polysulfides and effectively reduces the conversion energy barrier between short-chain polysulfides. Therefore, the ratio of cobalt acetate tetrahydrate and phenanthroline should be strictly controlled to ensure the coexistence of Co clusters and single-atom CoN4, so that both have good adsorption and catalytic capabilities for polysulfides; the mass volume ratio of Ketjen black to ethanol will affect the loading amount of Co / CoN4 active sites on the conductive carbon structure, which will affect the synergy between the active sites and the adsorption and catalytic capabilities for long-chain and short-chain polysulfides, thereby further affecting the electrochemical performance of the battery.

[0020] (iv) In step (2), the temperature of the evaporating solvent is 80°C.

[0021] (5) In step (2), the calcination temperature is 650°C and the calcination time is 2h.

[0022] The calcination heating rate and calcination temperature in the present invention have a certain influence on the morphology and structure formation of Co / CoN4@KB. When the calcination temperature is greater than 650°C and the heating rate is high (greater than 10°C / min), part of the Co will rapidly crystallize at high temperature and a large number of crystal structures will appear, which is not conducive to the generation and agglomeration of atomic-level Co; when the calcination temperature is less than 650°C and the heating rate is low (less than 10°C / min), the coordination of N atoms and atomically dispersed Co is insufficient, which is not conducive to its stable Co-4N cross-linking coordination to form CoN4 configuration.

[0023] (6) In step (3), the concentration of solution B is 0.5 mg / mL; the amount of solution A and solution B used is 2 mL.

[0024] The concentration of solution B and the amount of solution A and solution B will affect the thickness of the double-layer material formed by filtration, thereby affecting the electrochemical performance and mass energy density of the battery.

[0025] (VII) In step (1), the two-dimensional material is any one of a graphene material, a MXene material, and the like; when the two-dimensional material is a MXene material, the preparation process is carried out as follows:

[0026] Ti3AlC2 powder was slowly added to a mixed solution containing lithium fluoride and hydrochloric acid and stirred vigorously at 45 °C for 24 h. The mixture was then washed with deionized water by centrifugation at 3500 rpm until the pH value of the supernatant was about 6. Finally, the prepared neutral suspension was ultrasonically treated and centrifuged for 1 h to collect Ti3C2T x MXene supernatant.

[0027] The present invention also has a limitation. In step (3), the Co / CoN4@KB-two-dimensional material double-layer composite membrane has an upper layer of Co / CoN4@KB layer and a lower layer of two-dimensional material layer, and the thickness of the Co / CoN4@KB layer is about 5 μm.

[0028] It is well known that the thickness of the diaphragm material affects the migration and diffusion of ions and polysulfides. A Co / CoN4@KB layer that is too thick is conducive to blocking polysulfides but will limit the diffusion path of ions. A layer that is too thin is not conducive to the interception of polysulfides. Therefore, selecting the appropriate thickness has a significant impact on improving the electrochemical performance.

[0029] The beneficial effects of the above technical solution of the present invention are:

[0030] (1) The present invention uses a simple annealing method to prepare a conductive Ketjen black loaded with clustered Co and single-atom dispersed CoN4 as a highly efficient adsorption and catalytic functional material for polysulfides. The Co cluster has a better capture and catalytic ability for long-chain polysulfides, while CoN4 has a stronger adsorption effect on short-chain polysulfides and better reduces the conversion energy barrier between short-chain solid polysulfides. Therefore, by constructing a coexistence of Co clusters and CoN4, the synergistic effect of the two is exerted to enable the capture and conversion of polysulfides to be more efficiently achieved, the shuttle effect is reduced, the loss of active substances is alleviated, and the redox reaction kinetics of the electrochemical reaction is improved.

[0031] (2) The two-dimensional material MXene or graphene used in the present invention can further intercept the polysulfides missed by the Co / CoN4@KB layer through chemical adsorption and physical confinement, further alleviating the shuttle effect of the battery.

[0032] (3) The preparation process of the double-layer modified lithium-sulfur battery composite membrane is simple, the synthesis conditions are mild, the cost is low, and it can be extended to other low-dimensional material systems. The material has excellent ionic conductivity when used as a lithium-sulfur battery membrane material. -1The initial discharge capacity at the current density is 1233 mAh g -1 After 100 cycles, the discharge capacity is 940 mAh g -1 .

[0033] The invention is suitable for preparing a double-layer modified lithium-sulfur battery composite diaphragm.

[0034] The present invention will be further described in detail below with reference to specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Ti3C2T prepared in Example 1 x Scanning electron microscope image of MXene;

[0036] Figure 2 This is a cross-sectional scanning electron microscope image of the Co / CoN4@KB-MXene double-layer modified composite membrane prepared in Example 1;

[0037] Figure 3 X-ray diffraction spectra of Co / CoN4@KB material and KB material prepared in Example 1 and Comparative Example;

[0038] Figure 4 This is a high-resolution electron microscopy image of the Co / CoN4@KB material prepared in Example 1 after high-angle annular dark-field spherical aberration correction;

[0039] Figure 5 Graphs showing the ionic conductivity of different modified membranes prepared in Example 1 and Comparative Example;

[0040] Figure 6 ion diffusion activation energy diagrams of different modified membranes prepared in Example 1 and Comparative Example;

[0041] Figure 7 This is a constant potential lithium sulfide deposition curve of Co / CoN4@KB material prepared in Example 1;

[0042] Figure 8 This is the constant potential lithium sulfide deposition curve of MXene material;

[0043] Figure 9 This is the constant potential lithium sulfide deposition curve of KB material;

[0044] Figure 10 Graph showing the cycle performance of lithium-sulfur batteries using different modified separators prepared in Example 1 and Comparative Example;

[0045] Figure 11 The figure is a rate performance diagram of lithium-sulfur batteries using different modified separators prepared in Example 1 and the comparative example;

[0046] Figure 12 The cyclic voltammetry curves of lithium-sulfur batteries using the differently modified separators prepared in Example 1 and the comparative example are shown. DETAILED DESCRIPTION

[0047] The following embodiments of the present invention are described in further detail with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0048] Unless otherwise specified, the reagents described in the following examples are all existing reagents, and the methods described in the following examples are all existing preparation and detection methods unless otherwise specified.

[0049] Example 1 Preparation method of a double-layer modified lithium-sulfur battery composite separator

[0050] This embodiment is a method for preparing a double-layer modified lithium-sulfur battery composite separator Co / CoN4@KB-MXene, which is carried out in the following steps in sequence:

[0051] (1) Ti3C2T x Preparation of MXene nanosheets

[0052] 1 g of Ti3AlC2 powder was slowly added to a mixed solution containing 1 g of lithium fluoride and 9 M hydrochloric acid and vigorously stirred at 45 °C for 24 h. The mixture was then centrifuged and washed with deionized water until the pH value of the supernatant was about 6. Finally, the prepared neutral suspension was ultrasonically treated and centrifuged at 3500 rpm for 1 h to collect Ti3C2T x The MXene supernatant was recorded as the MXene suspension; finally, the prepared MXene suspension was diluted to 0.3 mg / mL with ethanol and deionized water (volume ratio of 1:1), recorded as solution A;

[0053] (2) Preparation of Co / CoN4@KB composites

[0054] 24 mg of cobalt acetate tetrahydrate and 17.3 mg of phenanthroline were mixed in 3 mL of ethanol and stirred continuously at 50°C for 3 hours. Subsequently, 48 mg of Ketjen black was added to the above solution and stirred at 50°C for another 3 hours to form a precursor. The precursor was then transferred to a vacuum drying oven at 80°C to evaporate the solvent. Finally, the obtained product was heated to 650°C at a rate of 10°C / min under argon protection and calcined for 2 hours to obtain a Co / CoN4@KB composite material.

[0055] (3) Preparation of Co / CoN4@KB-MXene double-layer composite membrane

[0056] Co / CoN4@KB was dissolved in a mixture of ethanol and water (volume ratio of 1:1) to obtain solution B with a concentration of 0.5 mg / mL. Then, a vacuum filtration device was used with a polypropylene membrane as a filter membrane to add 2 mL of solution A. After the solvent was completely separated, 2 mL of solution B was added. After the solvent was completely separated, the loaded membrane Co / CoN4@KB-MXene was transferred to a vacuum drying oven and dried at 60°C. Finally, the Co / CoN4@KB-MXene double-layer composite membrane was obtained.

[0057] The membrane prepared in this example was subjected to a series of morphology and electrochemical performance tests, and the specific results are as follows:

[0058] Figure 1 Ti3C2T prepared in this example x Scanning electron microscope image of MXene. It shows distinct edges and a typical two-dimensional nanosheet structure, which can physically hinder the migration of polysulfides.

[0059] Figure 2 This is a cross-sectional scanning electron microscope image of the Co / CoN4@KB-MXene bilayer modified composite separator prepared in this example. The upper layer is Co / CoN4@KB, and the lower layer is a MXene stacking layer, clearly distinguishing the bilayer structure between Co / CoN4-KB and MXene. The Co / CoN4@KB catalytic layer is ~5μm thick, ensuring an efficient electrochemical reaction interface and mitigating the accumulation of low-conductivity dead sulfur during charge and discharge. The cross-linked conductive network, rich in active sites, promotes rapid electron transfer and reaction kinetics. Due to van der Waals forces and hydrogen bonding interactions, the MXene nanosheets are well stacked into a two-dimensional layer with a thickness of only ~69.1nm. This unique two-dimensional MXene stacking layer, with its abundant terminal functional groups and Lewis acidic surface, acts as a microbarrier, further physically blocking and chemically anchoring polysulfides leaking from the pores and interstices of the upper Co / CoN4@KB layer.

[0060] Figure 3 The X-ray diffraction spectrum of the Co / CoN4@KB material obtained in this example is shown. Because the Co content is low and exists in the form of clusters and single atoms, there are no obvious Co metal characteristic diffraction peaks in the figure. Instead, the two clear broad diffraction peaks are attributed to the (002) and (101) crystal planes of carbon.

[0061] Figure 4 A high-resolution electron microscopy image of the Co / CoN4@KB material obtained in this example, corrected for spherical aberration, is shown. Numerous bright spots are visible, representing atomically dispersed Co atoms, marked by white circles. Black circles represent Co clusters, demonstrating the coexistence of isolated dispersed Co atoms and metallic Co clusters on the conductive Ketjen black.

[0062] Figure 5 The ionic conductivity of the Co / CoN4@KB-MXene double-layer modified composite membrane obtained in this example is shown. Due to the presence of the highly conductive and active Co / CoN4@KB layer, Co / CoN4@KB-MXene exhibits the highest ionic conductivity.

[0063] Figure 6 The diffusion activation energy of the Co / CoN4@KB-MXene double-layer modified composite membrane obtained in this example is shown. The results show that the activation energy of Co / CoN4@KB-MXene is 0.0341 eV.

[0064] Figure 7 The constant potential lithium sulfide deposition curve of the Co / CoN4@KB material obtained in this example is shown. The results show that the Co / CoN4@KB composite material prepared by the present invention makes lithium sulfide contribute 217.4 mAh g during this deposition process. -1 capacity, which promotes the kinetics of sulfur reduction reaction.

[0065] Figure 10 The cycling performance of the lithium-sulfur battery using Co / CoN4@KB-MXene obtained in this embodiment is shown. The results show that at 0.2C (1C = 1675mA g -1 The initial discharge capacity at the current density is 1233 mAh g -1 After 100 cycles, the discharge capacity is 940 mAh g -1 .

[0066] Figure 11 The results show the rate performance of the lithium-sulfur battery using Co / CoN4@KB-MXene obtained in this example. The results show that the average discharge capacity at current densities of 0.2C, 0.5C, 1C, 2C, and 4C is 1297, 1086, 984, 853, and 709 mAh g, respectively. -1 , and then return to the average discharge capacity of 0.2C to 1168mAh g -1 .

[0067] Example 2

[0068] (1) Preparation of graphene

[0069] The purchased graphene was dispersed in ethanol and deionized water (volume ratio of 1:1) to form a 0.3 mg / mL suspension, which was recorded as solution A;

[0070] (2) Preparation of Co / CoN4@KB composites

[0071] 30 mg of cobalt acetate tetrahydrate and 25 mg of phenanthroline were mixed in 3 mL of ethanol and stirred continuously at 50°C for 3 hours. Subsequently, 48 mg of Ketjen black was added to the above solution and stirred at 50°C for another 3 hours to form a precursor. The precursor was then transferred to a vacuum drying oven at 80°C to evaporate the solvent. Finally, the obtained product was heated to 650°C at a rate of 10°C / min under argon protection and calcined for 2 hours to obtain a Co / CoN4@KB composite material.

[0072] (3) Preparation of Co / CoN4@KB-graphene double-layer composite membrane

[0073] Co / CoN4@KB was dissolved in a mixture of ethanol and water (volume ratio of 1:1) to obtain solution B with a concentration of 0.5 mg / mL. Then, a vacuum filtration device was used with a polypropylene membrane as a filter membrane to add 2 mL of solution A. After the solvent was completely separated, 2 mL of solution B was added. After the solvent was completely separated, the loaded membrane Co / CoN4@KB-MXene was transferred to a vacuum drying oven and dried at 60°C. Finally, the Co / CoN4@KB-graphene double-layer composite membrane was obtained.

[0074] The electrochemical performance of the membrane prepared in this example was tested, and the specific results are as follows:

[0075] The cycling performance test results of the lithium-sulfur battery using Co / CoN4@KB-graphene obtained in this embodiment show that at 0.2C (1C = 1675mA g -1 The initial discharge capacity at the current density is 1216 mAh g -1 After 100 cycles, the discharge capacity is 882 mAh g -1 .

[0076] The rate performance test results of the lithium-sulfur battery using Co / CoN4@KB-graphene obtained in this example show that the discharge specific capacities at current densities of 0.2C, 0.5C, 1C, 2C, and 4C are 1224, 973, 889, 774, and 677 mAh g, respectively. -1 , and then return to the 0.2C discharge capacity of 1113mAh g -1 .

[0077] Examples 3-5

[0078] In Examples 3-5, a double-layer modified lithium-sulfur battery composite separator Co / CoN4@KB-MXene was prepared respectively. The specific preparation process was similar to that in Example 1, except that the technical parameters in the preparation process were different, as follows:

[0079]

[0080] Comparative Example

[0081] Preparation method of double-layer modified lithium-sulfur battery composite membrane without active sites in group A

[0082] This group prepared a double-layer modified lithium-sulfur battery composite membrane without active sites. The preparation process was similar to that of Example 1, with the only difference being that in step (2) of the preparation process, ordinary Ketjen black was directly dissolved in a solution formed by ethanol and deionized water (volume ratio of 1:1) to obtain a solution C with a concentration of 0.5 mg / mL. Solution C was used instead of solution B to perform the subsequent step (3). The prepared membrane was named KB-MXene.

[0083] A series of morphology and performance measurements were performed on the KB-MXene membrane prepared in this comparative example. The specific results are as follows:

[0084] Figure 3 The X-ray diffraction spectrum of the KB material of this group is shown in FIG. Two clear broad diffraction peaks are both attributed to the (002) and (101) crystal planes of carbon.

[0085] Figure 5 The ionic conductivity of the KB-MXene double-layer modified composite membrane obtained by this group is shown. The results show that the ionic conductivity of KB-MXene is lower than that of Co / CoN4@KB-MXene.

[0086] Figure 6 The diffusion activation energy of the KB-MXene double-layer modified composite membrane obtained by this group is shown. The results show that the activation energy of KB-MXene is 0.04 eV.

[0087] Figure 8 The constant potential lithium sulfide deposition curve of the MXene material used by this group is shown. The results show that the MXene material enables lithium sulfide to contribute 110.2mAh g during this deposition process. -1 capacity.

[0088] Figure 9 The constant potential lithium sulfide deposition curve of the KB material used in this group is shown. The results show that the KB material makes lithium sulfide contribute 54.2mAh g in this deposition process. -1 capacity.

[0089] Figure 10 The cycling performance of the lithium-sulfur battery using KB-MXene obtained by this group is shown. The results show that the initial discharge capacity at a current density of 0.2C is 1169 mAh g -1 After 100 cycles, the discharge capacity is 761 mAh g -1 .

[0090] Figure 11 The results show the rate performance of lithium-sulfur batteries using KB-MXene obtained by this group. The results show that the average discharge capacity at current densities of 0.2C, 0.5C, 1C, 2C, and 4C is 1228, 987, 848, 688, and 521 mAh g, respectively. -1 , and then return to 0.2C, the average discharge capacity is 1048mAh g -1 .

[0091] Group B MXene single layer modified lithium sulfur battery separator

[0092] This group prepared a MXene-modified lithium-sulfur battery separator and investigated the effect of the MXene layer on polysulfide formation. The material preparation process for this comparative example was similar to that of Example 1, with the only differences being that step (2) was omitted and only solution A was added during the filtration process in step (3). The resulting separator was named MXene.

[0093] Our group conducted performance tests on the prepared MXene single-layer modified lithium-sulfur battery separator, as follows:

[0094] Figure 5 The ionic conductivity of the MXene monolayer modified membrane obtained by this group is shown in Figure 2. The results show that MXene has the lowest ionic conductivity. Driven by van der Waals forces, the self-stacking effect of MXene nanosheets leads to the Li + The space is compressed, which is not conducive to the migration of ions. Therefore, the MXene-modified separator has the lowest ionic conductivity.

[0095] Figure 6 The diffusion activation energy of the MXene monolayer modified membrane obtained by this group is shown. The results show that the activation energy of KB-MXene is 0.0426 eV.

[0096] Figure 8 The constant potential lithium sulfide deposition curve of the MXene material used by this group is shown. The results show that the MXene material enables lithium sulfide to contribute 110.2mAh g during this deposition process. -1 capacity.

[0097] Figure 10 The cycling performance of the lithium-sulfur battery using MXene obtained by this group is shown. The results show that the initial discharge capacity at a current density of 0.2C is 992mAh g -1 After 100 cycles, the discharge capacity is 645 mAh g -1 .

[0098] Figure 11The results show the rate performance of the lithium-sulfur battery using MXene obtained by this group. The results show that the average discharge capacity at current densities of 0.2C, 0.5C, 1C, 2C, and 4C is 929, 740, 659, 545, and 346 mAh g, respectively. -1 , and then return to the average discharge capacity of 0.2C to 795mAh g -1 .

[0099] Group C KB single-layer modified lithium-sulfur battery separator

[0100] This group prepared a MXene monolayer-modified lithium-sulfur battery separator to investigate the effect of the MXene layer on polysulfide formation. The preparation process was similar to that of Example 1, with the only difference being that step (1) was omitted and step (2) involved directly dissolving ordinary Ketjen black in a solution of ethanol and deionized water (1:1 by volume) to obtain a solution C with a concentration of 0.5 mg / mL. In step (3), only solution C was added during the filtration process. The resulting separator was named KB.

[0101] Our group conducted performance tests on the prepared KB membrane, as follows:

[0102] Figure 5 The ionic conductivity of the KB monolayer modified separator obtained by this group is shown. The results show that the ionic conductivity of KB is the highest than that of MXene.

[0103] Figure 6 The diffusion activation energy of the KB monolayer modified membrane obtained by this group is shown. The results show that the activation energy of KB is 0.0345 eV.

[0104] Figure 9 The constant potential lithium sulfide deposition curve of the KB material used in this group is shown. The results show that the KB material makes lithium sulfide contribute 54.2mAh g in this deposition process. -1 capacity.

[0105] Figure 10 The cycling performance of the lithium-sulfur battery using KB obtained by this group is shown. The results show that the initial discharge capacity at a current density of 0.2C is 923mAh g -1 After 100 cycles, the discharge capacity is 470 mAh g -1 .

[0106] Figure 11 The results show the rate performance of the lithium-sulfur battery using KB obtained by this group. The results show that the average discharge capacity at current densities of 0.2C, 0.5C, 1C, 2C, and 4C is 819, 596, 458, 336, and 273 mAh g, respectively. -1, and then return to the average discharge capacity of 0.2C to 645mAh g -1 .

[0107] The above results demonstrate that the Co / CoN4@KB-MXene bilayer modified lithium-sulfur battery composite separator provided in Example 1 of the present invention exhibits a well-defined bilayer structure. The Co / CoN4@KB exhibits high conductivity, highly active catalytic sites that promote the conversion of long- and short-chain polysulfides, improving sulfur redox reaction kinetics, and possesses strong chemical adsorption capacity for polysulfides. The MXene layer adsorbs polysulfides through Lewis acid-base interactions, while the stacked two-dimensional nanosheets act as microbarriers for polysulfides, further mitigating the shuttling effect of polysulfides and preventing significant loss of active materials.

[0108] Figures 5-6 The results show that the ionic conductivity of the Co / CoN4@KB-MXene double-layer modified lithium-sulfur battery (the composite material shown in the present invention) is better than that of other samples. This advantage is attributed to the fact that the highly conductive and active site-rich Co / CoN4@KB can reduce the energy barrier for lithium ion diffusion and enrich the ion transmission channels, while the MXene-modified membrane compresses the ion migration space due to the dense stacking of nanosheets, and the lower activity leads to low ion diffusion capacity.

[0109] Figures 7-11 Results show that Co / CoN4@KB has the ability to selectively promote the rapid conversion of sulfur intermediates of varying lengths, thereby improving the specific capacity of lithium-sulfur batteries. The excellent adsorption capacity, combined with the MXene barrier layer, further reduces the loss of active materials, resulting in more stable cycling performance in lithium-sulfur batteries. Therefore, lithium-sulfur batteries using Co / CoN4@KB-MXene exhibit excellent electrochemical performance.

[0110] from Figure 12 From the comparison of the cyclic voltammetry curves, it can be seen that the battery using the Co / CoN4@KB-MXene membrane (the composite material shown in the present invention) has a stronger redox peak current, which proves its higher redox kinetic activity. At the same time, the potential difference between the oxidation peak and the reduction peak is smaller, indicating good reversibility and smaller electrochemical polarization.

[0111] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a double-layer modified lithium-sulfur battery composite diaphragm, characterized in that: Follow the steps below in order: (1) Preparation of two-dimensional materials Weigh the two-dimensional material and dilute the prepared MXene suspension with an ethanol diluent formed by ethanol and deionized water in a volume ratio of 1:1, which is recorded as solution A; (2) Preparation of Co / CoN4@KB composite materials Cobalt acetate tetrahydrate and phenanthroline were mixed in ethanol and stirred continuously at 50°C for 3 hours. Subsequently, Ketjen black was added to the above solution and stirred at 50°C for another 3 hours to form a precursor. The precursor was then transferred to a vacuum drying oven to evaporate the solvent. Finally, the obtained product was calcined under argon protection to obtain the Co / CoN4@KB composite material. (3) Preparation of Co / CoN4@KB-2D material double-layer composite membrane Co / CoN4@KB was dispersed in a mixed solution of ethanol and water to obtain solution B. Then, a vacuum filtration device was used with a polypropylene membrane as a filter membrane to dropwise add solution A. After the solvent was completely separated, solution B was added dropwise. After the solvent was completely separated, the loaded membrane Co / CoN4@KB-two-dimensional material was transferred to a vacuum drying oven for drying, and finally a Co / CoN4@KB-two-dimensional material double-layer composite membrane was obtained.

2. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (1), the concentration of solution A is 0.3 mg / mL.

3. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (2), the mass ratio of cobalt acetate tetrahydrate, phenanthroline and Ketjen black is (24-60): (17-34): 48; The mass volume ratio of the Ketjen black to ethanol is 16 mg / mL.

4. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (2), the temperature of the evaporating solvent is 80°C.

5. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (2), the calcination temperature is 650°C and the calcination time is 2h.

6. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (3), the concentration of solution B is 0.5 mg / mL; the amount of solution A and solution B used is 2 mL.

7. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to claim 1, characterized in that: In step (1), the two-dimensional material is any one of two-dimensional materials such as graphene material and MXene.

8. The method for preparing a double-layer modified lithium-sulfur battery composite separator according to any one of claims 1 to 7, characterized in that: In step (3), the Co / CoN4@KB-two-dimensional material double-layer composite membrane has an upper layer of Co / CoN4@KB layer and a lower layer of two-dimensional material layer, and the thickness of the Co / CoN4@KB layer is 5 μm.

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