Titanium dioxide-cobalt loaded nitrogen-doped carbon composite material based on double MOFs (Metal-Organic Frameworks) derivation

Abstract

The invention discloses a titanium dioxide-cobalt loaded nitrogen-doped carbon composite material based on double MOFs (Metal-Organic Frameworks) derivation, which is characterized in that MIL-125 is used as a substrate, ZIF-67 is grown on the MIL-125 substrate through a stirring reaction to obtain an MIL-125 / ZIF-67 composite material, and finally, Ti-Co / NC is obtained through carbonization; the basic microstructure of the Ti-Co / NC is in a round cake shape, and the size of a single round cake is 1-2 microns; the Ti-Co / NC takes cake-shaped Ti / NC as a substrate, Co / NC nanoparticles are loaded on the surface of the substrate, and the size of a single nanoparticle is 30-50 nm. The preparation method comprises the following steps: 1, preparing MIL-125; 2, preparation of a dual-MOFs (Metal Organic Frameworks) material MIL-125 / ZIF-67; and 3, preparation of Ti-Co / NC. When the positive electrode material is used as a lithium-sulfur battery positive electrode material, the initial discharge specific capacity is 1200-1300 mA h g <-1 > under the condition that the current density is 0.2 C; and under the condition that the current density is 0.2 C, after 200 times of charge-discharge cycles, the residual specific capacity is 700-800 mA h g <-1 >, and the capacity retention ratio is 60-65%.

Description

Technical Field

[0001] This invention relates to the field of cathode materials for lithium-sulfur batteries, specifically to a composite material based on dual MOFs-derived titanium dioxide-cobalt-supported nitrogen-doped carbon. Background Technology

[0002] The lithium-sulfur battery has a capacity of 1675 mA hg. -1 High theoretical capacity and 2600 W h Kg -1 It has the advantages of high energy density, and at the same time, it has the characteristics of low raw material cost and abundant resources of sulfur. However, sulfur has an electrical conductivity of only 10. -19 S m -1 In other words, lithium-sulfur batteries have insulating properties, and they also have problems with intermediate products such as polysulfides (LiPSs) generated during charging and discharging, as well as dissolution in the electrolyte. These issues lead to capacity reduction and rapid cycle decay in lithium-sulfur batteries.

[0003] To address the aforementioned issue of sulfur's insulating properties, carbon materials with high specific surface area and high electrical conductivity can be combined with active sulfur to improve the overall electrical conductivity of the electrode. For example, in the previous work of the inventors' research group, existing literature 1 (Sun Lixian. A porous carbon material based on waste monk fruit cores. CN202410071949.4[P]. 2024-01-18.) uses waste monk fruit cores as raw material, and after carbonization, prepares a material with a specific surface area as high as 1501 m². 2 A conductive carbon material with a density of / g was used to achieve an initial discharge specific capacity of 1310 mA hg at a current density of 0.1 C. -1 The inventors' subsequent research revealed that nonpolar carbon materials have a binding energy of only 0.5-1.5 eV with lithium polysulfides, while transition metal compounds have a binding energy of 2-4 eV. This means that the binding energy between transition metal compounds and lithium polysulfides is significantly higher than that of nonpolar carbon materials, thus preventing nonpolar carbon materials from solving the problem of limiting the dissolution of polysulfide LiPSs. Therefore, the technical solution in existing literature 1 can only achieve a capacity retention of only 70% after 100 charge-discharge cycles at a current density of 0.1 C.

[0004] To address the inability of carbon materials to dissolve lithium polysulfides, the shuttle effect can be suppressed by introducing transition metal compounds that interact with the polysulfides. For example, existing literature 2 (Jing L, Yong L, Tengfei L, et al. Efficient Regulation of Polysulfides by Anatase / Bronze TiO2Heterostructure / Polypyrrole Composites for High-Performance Lithium-SulfurBatteries[J]. Molecules, 2023, 28: 11.) uses the conductive polymer PPy as a substrate and introduces TiO2 nanoparticles. The polar TiO2 nanoparticles exhibit strong polar and chemisorption effects on polysulfide LiPSs. This results in a capacity retention of 61.6% after 200 charge-discharge cycles at a current density of 1 C. However, this technique uses PPy as a substrate, leading to a low specific surface area of ​​only 167 m². 2 g -1 Therefore, recombination with sulfur cannot increase the sulfur loading, ultimately resulting in an initial specific capacity of only 1250 mA hg at a current density of 0.1 C. -1 Technical issues.

[0005] To simultaneously enhance the adsorption capacity for polysulfides and achieve high sulfur loading, high specific surface area MOFs materials can be introduced. The principle is that since MOFs are formed from transition metals and organic ligands, they not only possess a specific microstructure with high specific surface area but also contain transition metal elements. Therefore, the transition metal compound composite carbon material derived from the carbonization of MOFs materials can not only maintain a high specific surface area but also achieve polar and chemisorption effects on polysulfide LiPSs through the transition metals. For example, in the inventors' previous work, existing literature 3 (Xu Fen. A TiO2 composite carbon material with a bicrystalline structure. CN202510640593.6[P]. 2025-05-19.) uses Ti-based MIL-125 as a precursor, introduces MXene, and then carbonizes to obtain TiO2(R) / C-TiO2(R+A), achieving a surface area of ​​385 μm. 2 g -1 The high specific surface area provides a significant technical advantage. Furthermore, the TiO2 derived from MIL-125 enhances the material's chemisorption capacity for polysulfides, resulting in an initial specific capacity of 1263 mA hg at a current density of 0.2C. -1The technical effect is good. However, the underlying principle of this technology is that TiO2 only performs chemical adsorption on dissolved polysulfide LiPSs, which cannot fundamentally solve the problem of polysulfide LiPSs dissolution. Therefore, after 200 charge-discharge cycles at a current density of 0.2 C, the capacity retention rate of this technology is only 56%.

[0006] To fundamentally address the dissolution problem of polysulfide LiPSs, highly catalytically active materials can be introduced to convert polysulfide LiPSs into the final product, thereby avoiding prolonged contact with the electrolyte. For example, existing literature 4 (Tongtao W, Enjie W, Changcheng W, et al. MOF-derived micro-mesoporous TiO2-based composite as sulfur host for high-performance lithium-sulfur batteries[J]. Electrochimica Acta, 2023, 439: 141570.) uses Ti-based MIL-125 as a substrate, adsorbs Co ions on the surface, and then carbonizes it to obtain Co / NC-TiO2. This technical solution not only achieves chemical adsorption capacity for polysulfides through the MIL-125-derived TiO2 reinforcement material, but also catalyzes the conversion of polysulfide LiPSs by loading elemental Co. Therefore, it achieves a capacity retention of 68.8% after 500 charge-discharge cycles at a current density of 1 C. However, this method based on the in-situ adsorption of Co by stirring has the potential for Co contamination. 2+ The inability to freely control the distribution of Co nanoparticles directly leads to their aggregation at high temperatures after calcination. Summary of the Invention

[0007] The purpose of this invention is to provide a nitrogen-doped carbon composite material based on dual MOF-derived titanium dioxide-cobalt supported composite material. Addressing the technical problems existing in the prior art, the technical principles and specific technical effects of this invention are reflected in the following two aspects:

[0008] 1. Both MIL-125 and ZIF-67 have high specific surface areas, and the derived TiO2 and Co elements have strong polarity and high catalytic activity, respectively. This not only improves the chemical adsorption capacity of the materials for polysulfide LiPSs, but also accelerates the reaction kinetics of the battery, promotes the faster conversion of catalytic polysulfide LiPSs to the final product, and effectively improves the sulfur loading and cycle performance of the battery.

[0009] 2. After adsorbing Co ions in MIL-125, ZIF-67 is grown in situ, which can adjust the micromorphology of ZIF-67. Specifically, the dodecahedral structure is transformed into a nanosheet structure. In the Co / NC nanoparticles obtained after carbonization of ZIF-67, elemental Co is encapsulated in situ in a highly graphitized N-doped carbon matrix, which effectively restricts the migration and aggregation of elemental Co during the cycling process.

[0010] To achieve the above objectives, the specific technical solution for realizing the objectives of this invention is as follows:

[0011] A dual MOF-derived titanium dioxide-cobalt supported nitrogen-doped carbon composite material is obtained by using MIL-125 as a substrate, growing ZIF-67 on the MIL-125 substrate through a stirring reaction to obtain the MIL-125 / ZIF-67 composite material, and finally carbonizing to obtain the dual MOF-derived titanium dioxide-cobalt supported nitrogen-doped carbon composite material, abbreviated as Ti-Co / NC.

[0012] The basic microstructure of the Ti-Co / NC is disc-shaped, with the size of a single disc being 1-2 μm.

[0013] The Ti-Co / NC uses a disc-shaped Ti / NC substrate as the base, with Co / NC nanoparticles loaded on the surface. The size of each nanoparticle is 30-50 nm.

[0014] A method for preparing a dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material includes the following steps:

[0015] Step 1, preparation of MIL-125: N,N dimethylformamide, methanol, terephthalic acid, and tetrabutyl titanate are mixed in a specific mass ratio. First, N,N dimethylformamide and methanol are stirred and mixed to obtain mixture A. Then, terephthalic acid is added to mixture A and stirred to obtain mixture B. Next, tetrabutyl titanate is added to mixture B and stirred to obtain reaction solution C. Then, reaction solution C is subjected to a hydrothermal reaction. After the reaction is complete, the reaction product is centrifuged, washed, and dried to obtain a disc-shaped MIL-125.

[0016] In step 1, the mass ratio of N,N dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate is 34:3:2:1.

[0017] In step 1, the hydrothermal reaction conditions are: hydrothermal temperature of 120-180℃ and hydrothermal time of 20-30 h.

[0018] In step 1, the centrifugal washing conditions are as follows: centrifugation speed of 3000-4000 rpm, centrifugation time of 5-10 min, and centrifugal washing three times. The washing solution is N,N dimethylformamide followed by methanol. The drying conditions are as follows: drying temperature of 60-80℃, and drying time of 8-12 h.

[0019] Step 2, preparation of the dual MOF material MIL-125 / ZIF-67: MIL-125, Co(NO3)2·6H2O and 2-methylimidazole are prepared in a certain mass ratio. First, MIL-125 obtained in Step 1 is added to methanol and ultrasonically dispersed to obtain mixture D. Then, Co(NO3)2·6H2O is added to mixture D and stirred to obtain mixture E. At the same time, 2-methylimidazole is dissolved in methanol and stirred to obtain mixture F. Next, mixture F is poured into mixture E and stirred to react. After the reaction is completed, the reaction product is centrifuged, washed and dried to obtain the dual MOF material MIL-125 / ZIF-67.

[0020] In step 2, the mass ratio of MIL-125, Co(NO3)2·6H2O and 2-methylimidazole is 2:1:1;

[0021] In step 2, the conditions for ultrasonic dispersion are: ultrasonic power of 1000-1500 W and ultrasonic time of 5-10 min; the conditions for stirring reaction are: stirring temperature of 30-40℃ and stirring reaction time of 24-36 h.

[0022] In step 2, the conditions for centrifugal washing are: centrifugation speed of 3000-4000 rpm, centrifugation time of 5-10 min, centrifugal washing 2-3 times, and the washing liquid is methanol; the conditions for drying are: drying temperature of 60-80℃, and drying time of 8-12 h.

[0023] Step 3, preparation of Ti-Co / NC: First, the MIL-125 / ZIF-67 obtained in Step 2 is pre-carbonized, and then high temperature treatment is performed to obtain Ti-Co / NC, a composite material based on dual MOFs-derived titanium dioxide-cobalt supported nitrogen-doped carbon, for short Ti-Co / NC.

[0024] In the preparation method, the stirring and mixing conditions are as follows: stirring temperature is 35-40℃, and stirring time is 10-15 min.

[0025] In step 3, the pre-carbonization conditions are as follows: under an argon atmosphere, the heating rate is 2-3℃ / min, the pre-carbonization temperature is 300-400℃, and the pre-carbonization time is 1-2 h; the high-temperature carbonization conditions are as follows: under an argon atmosphere, the heating rate is 4-5℃ / min, the carbonization temperature is 700-800℃, and the carbonization time is 2-3 h.

[0026] When the dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material is used as a cathode material for lithium-sulfur batteries, the initial discharge specific capacity is 1200-1300 mA hg under a current density of 0.2 C. -1 Under a current density of 0.2C, after 200 charge-discharge cycles, the remaining specific capacity is 700-800 mA hg. -1 The capacity retention rate is 60-65%.

[0027] The technical effects of this invention have been tested and are as follows:

[0028] XRD test results show that Ti-Co / NC contains both TiO2 and elemental Co.

[0029] SEM results show that the Ti-Co / NC microstructure is disc-shaped, with the size of a single disc being 0.5-1 μm. Furthermore, the Ti-Co / NC surface is loaded with a large number of nanoparticles, with the size of a single nanoparticle being 50-100 nm.

[0030] EIS test results show that the charge transfer resistance R of the Ti-Co / NC assembled battery is... ct Only 10-15 Ω.

[0031] Cyclic charge-discharge test results show that the Ti-Co / NC assembled battery, under a current density of 0.2 C, has an initial discharge specific capacity of 1200-1300 mA hg. -1 At 200 cycles, the residual discharge specific capacity is 700-800 mA hg. -1 The capacity retention rate is 60-65%.

[0032] Therefore, the present invention has the following advantages over the prior art:

[0033] 1. This invention grows ZIF-67 in situ on the surface of MIL-125 using a stirring reaction method. The Co / NC layer obtained by pre-carbonization and re-carbonization can form a stable conductive network structure, thereby improving the conductivity of the material.

[0034] 2. This invention uses high specific surface area dual MOF material MIL-125 / ZIF-67, which can be pre-carbonized and then carbonized to obtain TiO2 and Co elements, thereby enhancing the material's chemical adsorption capacity for lithium polysulfides and improving the material's electrical conductivity. Attached Figure Description

[0035] Figure 1 The XRD patterns of MIL-125, ZIF-67 and MIL-125 / ZIF-67 in Embodiment 1 of the present invention are shown below.

[0036] Figure 2 This is a SEM image of MIL-125 in Embodiment 1 of the present invention;

[0037] Figure 3 This is a SEM image of ZIF-67 in Embodiment 1 of the present invention;

[0038] Figure 4 This is a SEM image of MIL-125 / ZIF-67 in Embodiment 1 of the present invention;

[0039] Figure 5 The XRD patterns of Ti / NC, Co / NC, and Ti-Co / NC in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention are shown.

[0040] Figure 6 This is a SEM image of Ti-Co / NC in Example 1 of the present invention;

[0041] Figure 7 EIS plots of Ti / NC, Co / NC, and Ti-Co / NC in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0042] Figure 8 This is a charge-discharge cycle diagram of Ti / NC, Co / NC, and Ti-Co / NC in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention;

[0043] Figure 9 This is a SEM image of Ti / NC in Comparative Example 1 of this invention;

[0044] Figure 10 This is a SEM image of Co / NC in Comparative Example 2 of the present invention. Detailed Implementation

[0045] The present invention will be further described in detail through embodiments and with reference to the accompanying drawings, but this is not intended to limit the scope of the invention.

[0046] Example 1

[0047] A method for preparing a dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material includes the following steps:

[0048] Step 1, Preparation of MIL-125: First, 9 mL of N,N dimethylformamide and 1 mL of methanol were stirred and mixed to obtain mixture A. Then, 0.5 g of terephthalic acid was added to mixture A and stirred to obtain mixture B. After that, 0.26 mL of tetrabutyl titanate was added to mixture B and stirred to obtain reaction solution C. Next, reaction solution C was subjected to hydrothermal reaction at a hydrothermal temperature of 150℃ for 24 h. After the reaction was completed, the reaction product was centrifuged, washed and dried under certain conditions to obtain a disc-shaped MIL-125.

[0049] In step 1, the centrifugal washing conditions are as follows: centrifugation speed is 3500 rpm, centrifugation time is 5 min, centrifugation washing is performed 3 times, and the centrifugal washing solution is N,N dimethylformamide first and then methanol; the drying conditions are as follows: drying temperature is 60℃, drying time is 12 h.

[0050] To verify the composition of MIL-125, XRD tests were performed. The test results are as follows: Figure 1 As shown, MIL-125 exhibits characteristic peaks.

[0051] To demonstrate the microstructure of MIL-125, SEM testing was performed. The test results are as follows: Figure 2 As shown, the microstructure of MIL-125 is a disc-shaped structure with a size of 1-2 μm for each disc, and the surface of the disc-shaped structure is smooth.

[0052] Step 2, preparation of the dual MOF material MIL-125 / ZIF-67: First, 100 mg of MIL-125 obtained in Step 1 was added to 20 mL of methanol and ultrasonically dispersed to obtain mixture D. Then, under the conditions of stirring temperature of 30℃ and stirring time of 15 min, 50 mg of Co(NO3)2·6H2O was added to mixture D and stirred to obtain mixture E. At the same time, under the conditions of stirring temperature of 30℃ and stirring time of 5 min, 0.11 g of 2-methylimidazole was dissolved in 20 mL of methanol and stirred to obtain mixture F. Next, under the conditions of stirring reaction temperature of 30℃ and stirring reaction time of 24 h, mixture F was poured into mixture E and stirred to react. After the reaction was completed, the reaction product was centrifuged, washed and dried under certain conditions to obtain the dual MOF material MIL-125 / ZIF-67.

[0053] In step 2, the conditions for ultrasonic dispersion are: ultrasonic power of 1000 W and ultrasonic time of 10 min.

[0054] In step 2, the centrifugal washing conditions are: centrifugation speed of 3500 rpm, centrifugation time of 5 min, centrifugal washing number of times of 3 times, and the centrifugal washing liquid is methanol; the drying conditions are: drying temperature of 60℃, and drying time of 12 h.

[0055] To confirm the composition of MIL-125 / ZIF-67, XRD tests were performed. Simultaneously, for comparison, ZIF-67 was prepared directly without adding MIL-125 in step 2 and used as a reference for XRD testing. The test results are as follows... Figure 1 As shown,

[0056] ZIF-67 only has the characteristic peaks of ZIF-67;

[0057] MIL-125 / ZIF-67 exhibits characteristic peaks of both MIL-125 and ZIF-67.

[0058] By comparing the MIL-125 obtained in step 1, the reference ZIF-67, and the MIL-125 / ZIF-67, it can be seen that ZIF-67 was successfully prepared in situ on MIL-125 through step 2, thus forming a dual MOF material.

[0059] To demonstrate the microstructure of MIL-125 / ZIF-67, SEM testing was performed. Simultaneously, for comparison, SEM testing of ZIF-67 was conducted as a reference.

[0060] The SEM test results of ZIF-67 are as follows: Figure 3 As shown, the microstructure of ZIF-67 is a regular dodecahedral structure, with the size of a single dodecahedron being 1-2 μm.

[0061] SEM test results for MIL-125 / ZIF-67 are as follows: Figure 4 As shown, the basic microstructure of MIL-125 / ZIF-67 is the same as that of MIL-125, which is a disc-shaped structure with a size of 1-2 μm for each disc. However, there are a large number of nanosheet structures on the surface of the disc-shaped structure, with a size of 50-100 nm for each nanosheet. Combined with the XRD test results, it can be seen that the nanosheet structure is ZIF-67, that is, MIL-125 / ZIF-67 contains both MIL-125 and ZIF-67 MOFs.

[0062] By comparing the MIL-125, ZIF-67 and MIL-125 / ZIF-67 obtained in step 1, it can be seen that MIL-125 can adjust the microstructure of ZIF-67, specifically, transforming the dodecahedral structure into a nanosheet structure, and significantly reducing the size from 1-2 μm to 50-100 nm.

[0063] Step 3, preparation of the dual MOFs-derived material Ti-Co / NC: First, under argon conditions, the MIL-125 / ZIF-67 obtained in Step 2 was pre-carbonized at a heating rate of 2℃ / min, a pre-carbonization temperature of 300℃, and a pre-carbonization time of 1 h. Then, under argon conditions, carbonization was carried out at a heating rate of 5℃ / min, a carbonization temperature of 750℃, and a carbonization time of 2 h to obtain the dual MOFs-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material Ti-Co / NC, abbreviated as Ti-Co / NC.

[0064] In the preparation method described above, the stirring and mixing conditions are as follows: stirring temperature is 35℃ and stirring time is 10 min.

[0065] To confirm the composition of Ti-Co / NC, XRD tests were performed. The test results are as follows: Figure 5 As shown, Ti-Co / NC exhibits characteristic peaks not only for amorphous carbon but also for elemental TiO2 and Co. The test results indicate that Ti-Co / NC is composed of TiO2, elemental Co, and carbon materials. MIL-125, after carbonization, is a titanium dioxide-supported nitrogen-doped carbon composite material, while ZIF-67, after carbonization, is a cobalt-supported nitrogen-doped carbon composite material.

[0066] To verify the microstructure of Ti-Co / NC, SEM measurements were performed. The test results are as follows: Figure 6 As shown, the basic microstructure of Ti-Co / NC can still be maintained as a disc-shaped structure of MIL-125, with the size of a single disc being 1-2 μm. However, there are a large number of nanoparticle structures on the surface of the disc-shaped structure, with the size of a single nanoparticle being 30-50 nm. Combined with the XRD test results, it can be seen that the nanoparticle structure is Co / NC formed after ZIF-67 carbonization.

[0067] By comparing with the MIL-125 / ZIF-67 obtained in step 2, it can be seen that the carbonization process transforms the basic microstructure of MIL-125 into TiO2 / NC while maintaining the basic microstructure of MIL-125. At the same time, the nanosheet structure of ZIF-67 is transformed into the nanoparticle structure of Co / NC, and the size is further significantly reduced from 50-100 nm to 30-50 nm.

[0068] To demonstrate the performance of Ti-Co / NC as a cathode material for lithium-sulfur batteries, lithium-sulfur batteries were assembled and their electrochemical performance was tested.

[0069] The specific method for assembling a lithium-sulfur battery is as follows: First, sulfur powder S is ground with the sample to be tested at a mass ratio of 7:3 and a grinding time of 30 min to obtain a sulfur-loaded composite material using a conventional melt diffusion method. Then, the sulfur-loaded composite material is used as the positive electrode, a lithium sheet is used as the negative electrode with a diameter of 9 mm, Celgard 2500 is used as the separator, the lithium salt concentration in the electrolyte is 1 M LiTFSI, the solvent in the electrolyte is a mixture of DOL and DME, and the volume ratio of DOL:DME is 1:1. The additive in the electrolyte is 1.0 wt.% LiNO3, and the liquid sulfur ratio E / S is 25:1 to assemble the lithium-sulfur battery. Since there is no need to distinguish between them, the obtained lithium-sulfur battery is referred to as the positive electrode active material. That is, the lithium-sulfur battery obtained in Specific Example 1 based on Ti-Co / NC as the positive electrode material is also named Ti-Co / NC.

[0070] The EIS test results of Ti-Co / NC are as follows: Figure 7 As shown, the semicircular portion corresponds to the charge transfer resistance R during the battery charging and discharging process. ct The linear portion corresponds to the lithium-ion diffusion rate. Through equivalent circuit diagram fitting, the charge transfer resistance R of Ti-Co / NC can be determined. ct The resistance is only 12.08 Ω. Test results show that Ti-Co / NC has a small charge transfer resistance and a large lithium-ion diffusion rate.

[0071] The cyclic charge-discharge test results of Ti-Co / NC are as follows: Figure 8 As shown, under test voltage range of 1.7-2.8 V and current density of 0.2 C, the initial discharge specific capacity is 1204 mA hg. -1 At 200 cycles, the remaining discharge specific capacity is 736 mA hg. -1 The capacity retention rate is 61%; this indicates that 1 C = 1675 mA hg -1 .

[0072] To demonstrate the impact of MIL-125 and ZIF-67 on performance, Comparative Example 1 and Comparative Example 2 are provided, in which lithium-sulfur battery cathode composite materials are prepared using only MIL-125 or ZIF-67 as precursors.

[0073] Comparative Example 1

[0074] A method for preparing Ti / NC material based on MIL-125 is the same as that in Example 1 unless otherwise specified. The difference is that step 2 is not required, and in step 3, MIL-125 is used instead of MIL-125 / ZIF-67 for pre-carbonization and subsequent carbonization operations. The resulting material is named Ti / NC. Furthermore, the lithium-sulfur battery based on Ti / NC cathode is also named Ti / NC.

[0075] To confirm the composition of Ti / NC, XRD tests were performed. The test results are as follows: Figure 5 As shown, Ti / NC exhibits characteristic peaks of both amorphous carbon and TiO2. The test results indicate that Ti / NC is composed of TiO2 and carbon materials.

[0076] To verify the microstructure of Ti / NC, SEM testing was performed. The test results are as follows: Figure 9 As shown, the basic microstructure of Ti / NC is the same as that of MIL-125, which is a disc-shaped structure with a size of 1-2 μm for each disc.

[0077] The EIS test results of Ti / NC are as follows: Figure 7 As shown, by fitting the equivalent circuit diagram, it can be seen that the charge transfer resistance R of Ti / NC is... ct The resistance is 15.49 Ω. Compared with Example 1, it can be seen that Ti-Co / NC can reduce the charge transfer resistance by loading ZIF-67-derived Co / NC nanoparticles on the surface, that is, increase the diffusion rate of lithium ions, with a reduction of up to 22.00%.

[0078] The cyclic charge-discharge test results of Ti / NC are as follows: Figure 8 As shown, under test voltage range of 1.7-2.8 V and current density of 0.2 C, the initial discharge specific capacity is only 1034 mA hg. -1 It is only 85.8% of that in Example 1; after 200 cycles, the remaining discharge specific capacity is only 614 mA hg. -1 The capacity retention rate was 59%. Compared with Example 1, it can be seen that the ZIF-67-derived Co / NC nanoparticles can not only improve catalytic activity, but also improve the initial discharge specific capacity.

[0079] Comparative Example 2

[0080] A method for preparing Co / NC material based on ZIF-67 is provided. The steps, unless otherwise specified, are the same as those in Example 1. The difference is that step 1 is not required, and in step 3, ZIF-67 is used instead of MIL-125 / ZIF-67 for pre-carbonization and subsequent carbonization operations. The resulting material is named Co / NC. Furthermore, the lithium-sulfur battery based on the Co / NC cathode is also named Co / NC.

[0081] To confirm the composition of Co / NC, XRD tests were performed. The test results are as follows: Figure 5 As shown, Co / NC exhibits characteristic peaks of both amorphous carbon and elemental Co. The test results indicate that Co / NC is composed of elemental Co and carbon materials.

[0082] To demonstrate the microstructure of Co / NC, SEM testing was performed. The test results are as follows: Figure 10 As shown, Co / NC forms amorphous nanoparticle aggregates with individual particle sizes of 50-100 nm. Compared with Example 1, it can be seen that without using disc-shaped MIL-125 as a substrate, it is impossible to avoid the aggregation of elemental Co obtained after carbonization.

[0083] Co / NC's EIS test results are as follows: Figure 7 As shown, through equivalent circuit diagram fitting, the charge transfer resistance R of Co / NC is... ct The resistance is 30.09 Ω. Compared with Example 1, it can be seen that Ti-Co / NC, with Ti / NC as the substrate with high conductivity, can significantly reduce the charge transfer resistance, that is, significantly increase the diffusion rate of lithium ions, with a reduction of up to 59.85%.

[0084] The cyclic charge-discharge test results of Co / NC are as follows: Figure 8 As shown, under test voltage ranges of 1.7-2.8 V and current density of 0.2 C, the initial discharge specific capacity was only 974 mA h g⁻¹, only 81.1% of that in Example 1; after 200 cycles, the remaining discharge specific capacity was only 556 mA h g⁻¹, with a capacity retention rate of 57%. Compared with Example 1, it can be seen that Ti-Co / NC contains TiO₂, which has extremely strong polarity and chemisorption on polysulfide LiPSs, effectively improving sulfur utilization and giving the material a higher capacity retention rate.

Claims

1. A composite material based on dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon, characterized in that: Using MIL-125 as a substrate, ZIF-67 was grown on the original MIL-125 substrate through a stirring reaction to obtain a MIL-125 / ZIF-67 composite material. Finally, after carbonization, a nitrogen-doped carbon composite material based on dual MOFs-derived titanium dioxide-cobalt supported was obtained, which is referred to as Ti-Co / NC. The basic microstructure of the Ti-Co / NC is disc-shaped, with the size of a single disc being 1-2 μm. The Ti-Co / NC uses a disc-shaped Ti / NC substrate as the base, with Co / NC nanoparticles loaded on the surface. The size of each nanoparticle is 30-50 nm.

2. A method for preparing a dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material, characterized in that... Includes the following steps: Step 1, preparation of MIL-125: N,N dimethylformamide, methanol, terephthalic acid, and tetrabutyl titanate are mixed in a specific mass ratio. First, N,N dimethylformamide and methanol are stirred and mixed to obtain mixture A. Then, terephthalic acid is added to mixture A and stirred to obtain mixture B. Next, tetrabutyl titanate is added to mixture B and stirred to obtain reaction solution C. Then, reaction solution C is subjected to a hydrothermal reaction. After the reaction is complete, the reaction product is centrifuged, washed, and dried to obtain a disc-shaped MIL-125. Step 2, preparation of the dual MOF material MIL-125 / ZIF-67: MIL-125, Co(NO3)2·6H2O and 2-methylimidazole are prepared in a certain mass ratio. First, MIL-125 obtained in Step 1 is added to methanol and ultrasonically dispersed to obtain mixture D. Then, Co(NO3)2·6H2O is added to mixture D and stirred to obtain mixture E. At the same time, 2-methylimidazole is dissolved in methanol and stirred to obtain mixture F. Next, mixture F is poured into mixture E and stirred to react. After the reaction is completed, the reaction product is centrifuged, washed and dried to obtain the dual MOF material MIL-125 / ZIF-67. Step 3, preparation of Ti-Co / NC: First, the MIL-125 / ZIF-67 obtained in Step 2 is pre-carbonized, and then high temperature treatment is performed to obtain Ti-Co / NC, a composite material based on dual MOFs-derived titanium dioxide-cobalt supported nitrogen-doped carbon, for short Ti-Co / NC. In the preparation method described above, the stirring and mixing conditions are as follows: stirring temperature is 35-40℃, and stirring time is 10-15 min.

3. The preparation method according to claim 2, characterized in that: In step 1, the mass ratio of N,N dimethylformamide, methanol, terephthalic acid and tetrabutyl titanate is 34:3:2:

1. In step 1, the hydrothermal reaction conditions are: hydrothermal temperature of 120-180℃ and hydrothermal time of 20-30 h. In step 1, the centrifugal washing conditions are as follows: centrifugation speed is 3000-4000 rpm, centrifugation time is 5-10 min, centrifugation washing is performed 3 times, and the centrifugal washing solution is N,N dimethylformamide first, followed by methanol. The drying conditions are as follows: drying temperature is 60-80℃, drying time is 8-12 h.

4. The preparation method according to claim 2, characterized in that: In step 2, the mass ratio of MIL-125, Co(NO3)2·6H2O and 2-methylimidazole is 2:1:1; In step 2, the conditions for ultrasonic dispersion are: ultrasonic power of 1000-1500 W and ultrasonic time of 5-10 min; the conditions for stirring reaction are: stirring temperature of 30-40℃ and stirring reaction time of 24-36 h. In step 2, the conditions for centrifugal washing are: centrifugation speed of 3000-4000 rpm, centrifugation time of 5-10 min, centrifugal washing 2-3 times, and washing liquid of methanol; the conditions for drying are: drying temperature of 60-80℃, and drying time of 8-12 h.

5. The preparation method according to claim 2, characterized in that: In step 3, the pre-carbonization conditions are as follows: under an argon atmosphere, the heating rate is 2-3℃ / min, the pre-carbonization temperature is 300-400℃, and the pre-carbonization time is 1-2 h; the high-temperature carbonization conditions are as follows: under an argon atmosphere, the heating rate is 4-5℃ / min, the carbonization temperature is 700-800℃, and the carbonization time is 2-3 h.

6. The dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material according to claim 1, characterized in that: When used as a cathode material in lithium-sulfur batteries, the initial discharge specific capacity is 1200-1300 mA hg under a current density of 0.2 C. -1 .

7. The dual MOF-derived titanium dioxide-cobalt-supported nitrogen-doped carbon composite material according to claim 1, characterized in that: When used as a cathode material in lithium-sulfur batteries, under a current density of 0.2 C, the remaining specific capacity is 700-800 mA hg after 200 charge-discharge cycles. -1 The capacity retention rate is 60-65%.

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