A method for preparing tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon materials

Abstract

This invention relates to a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material and its preparation method, belonging to the technical field of lithium-sulfur battery cathode materials. Tannic acid and chitosan are dissolved in an acetic acid solution, introducing Fe... 3+ It reacts with glutaraldehyde to form a gel, which is then subjected to high-temperature carbonization to form carbon materials. Fe 3+ The strong coordinating ability of the ortho-phenolic hydroxyl groups of tannic acid is utilized to achieve atomic-level uniform anchoring of iron ions. A stable three-dimensional covalently cross-linked network gel is constructed by the Schiff base reaction between the dialdehyde groups of glutaraldehyde and the amino groups of chitosan, locking tannic acid within the framework. This effectively constrains the shrinkage path of the carbon skeleton during carbonization, inhibiting the disordered collapse of the skeleton during pyrolysis and achieving controllable construction of interconnected channels. (The carbonization process involves Fe...) 3+ Highly dispersed iron single atoms or ultrasmall nanoclusters are formed through carbothermal reduction, and synergistically with nitrogen atoms that enter the carbon framework in situ to construct Fe–N. X Catalytic active sites endow the material with abundant polar active centers.

Description

Technical Field

[0001] This invention belongs to the field of lithium-sulfur battery cathode material technology, specifically relating to a method for preparing tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material, which can be used to prepare sulfur support for lithium-sulfur battery cathodes. Background Technology

[0002] Lithium-sulfur batteries have a theoretical specific capacity as high as 1675 mAh·g -1 Energy density approximately 2600 Wh·kg -1 Lithium-sulfur batteries are considered strong contenders for next-generation high-energy-density energy storage systems. However, soluble polysulfides are generated during charging and discharging, and their migration in the electrolyte leads to the loss of active materials and decreased cycle stability. Furthermore, sulfur and its discharge products have poor conductivity, resulting in slow reaction kinetics. Existing technologies typically achieve physical confinement by constructing porous carbon materials or enhance chemisorption capacity through heteroatom doping, but the resulting materials generally suffer from problems such as disordered pore structures, uneven distribution of active sites, and discontinuous conductive networks.

[0003] Although research on the use of biochar in lithium-sulfur battery cathodes is relatively extensive, existing biochar cathode materials still have significant defects. Specifically, these defects include: (1) Resource waste: Carbon materials obtained by direct pyrolysis of tannic acid polyphenols have disordered pores and uneven distribution of active sites, resulting in low initial specific capacity at 0.1 C rate and a capacity retention rate of only ~50% after 100 cycles, indicating significant room for performance improvement. (2) Performance bottleneck: Carbon obtained by simple physical composite of chitosan and pomegranate peel, although incorporating nitrogen doping, lacks chemical cross-linking, resulting in insufficient pore structure control and poor continuity of the conductive network. It often requires the addition of conductive agents such as Mxenes and other conductive agents to improve material performance. Due to poor compatibility between the two, uneven distribution of nitrogen atoms during carbonization, and severe pore structure collapse, the specific surface area and active site density of the final carbon material are limited, making it impossible to simultaneously achieve physical confinement and chemical anchoring of polysulfides.

[0004] Therefore, developing a structurally stable, well-connected, and uniformly distributed iron / nitrogen / oxygen co-doped porous carbon material has become a crucial technical problem that needs to be solved to improve the performance of the sulfur carrier in the cathode of lithium-sulfur batteries. Summary of the Invention

[0005] In view of this, the present invention aims to overcome the problems of disordered pore structure, uneven distribution of active sites, and discontinuous conductive network in the prior art, and provides a tannic acid-stable iron / nitrogen / oxygen co-doped porous carbon material and its preparation method. The invention achieves synergistic regulation of the pore structure, heteroatom doping configuration and electrochemical performance of the carbon material through molecular-scale chemical bonding design.

[0006] Plant polyphenols, as a type of renewable biomass, possess rigid aromatic skeletons that can be reconstructed into conductive carbon networks through pyrolysis. Furthermore, their abundant phenolic hydroxyl groups endow them with excellent metal coordination ability and structural stabilization. Hydrolyzed tannins, represented by tannic acid, can utilize their ortho-phenolic hydroxyl groups to coordinate Fe... 3+ Transition metal ions are uniformly anchored in the precursor network and stabilize the carbon skeleton structure during subsequent crosslinking and carbonization, providing a molecular basis for the formation of uniformly distributed iron / nitrogen / oxygen multi-polar sites.

[0007] This invention provides a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material. This material utilizes a cross-linking network to regulate the structural stability of the precursor, thereby limiting the carbon skeleton's shrinkage path during carbonization and achieving controllable pore connectivity. In the precursor stage, a pre-assembled structure is formed through intermolecular interactions between chitosan, polyphenols, and iron ions, and a three-dimensional network is formed through glutaraldehyde cross-linking. This network constrains the carbon skeleton's shrinkage path during carbonization, resulting in a carbon material with a continuous porous structure. The preparation method includes polyphenol extraction, phenol-amine pre-assembly, glutaraldehyde cross-linking to form a gel, freeze-drying, and high-temperature carbonization. The resulting carbon material has a microporous-mesoporous composite structure with interconnected pathways between micropores and mesopores, and the carbon skeleton contains uniformly dispersed iron / nitrogen / oxygen heteroatom doping sites. This material, used as a sulfur carrier in the positive electrode of lithium-sulfur batteries, exhibits good specific capacity, rate performance, and cycle stability in lithium-sulfur batteries through the regulation of precursor structural stability via a cross-linking network.

[0008] To achieve the above objectives, the present invention first provides a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material. The iron / nitrogen / oxygen co-doped porous carbon material has a microporous-mesoporous composite structure with a three-dimensional interconnected porous carbon framework. Nitrogen is in situ doped into the carbon framework, and iron species are distributed in the carbon matrix in the form of highly dispersed Fe3O4 nanoparticles, iron single atoms, or combinations thereof, forming Fe-N synergistic and / or Fe-O polar active sites. The surface of the iron / nitrogen / oxygen co-doped porous carbon material is also rich in iron / nitrogen / oxygen multi-polar sites.

[0009] Preferably, the total nitrogen content in the iron / nitrogen / oxygen co-doped porous carbon material is 2-10 wt%, wherein pyridine nitrogen and pyrrole nitrogen together account for 40-80 wt% of the total nitrogen content.

[0010] Secondly, this invention provides a method for preparing tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon materials, specifically including the following steps:

[0011] (1) Add chitosan and tannic acid to acetic acid solution, heat and stir until completely dissolved to obtain solution A.

[0012] (2) Dissolve the iron salt in deionized water to obtain solution B; under stirring conditions, slowly add solution B to solution A to form a metal-organic composite precursor system.

[0013] Its beneficial effects are: Fe 3+ It coordinates with the ortho-phenolic hydroxyl groups of tannic acid and the amino groups of chitosan to form a metal-organic complex precursor system.

[0014] (3) Glutaraldehyde solution was slowly added dropwise to the metal-organic composite precursor system, and then the pH was adjusted with NaOH solution. The reaction was stirred to obtain a homogeneous gel.

[0015] Its beneficial effects are as follows: coordination and cross-linking occur simultaneously during the gel formation process: Fe 3+ The iron species are uniformly dispersed in the precursor system by forming a metal-polyphenol coordination structure with the ortho-phenolic hydroxyl group of tannic acid; the dialdehyde group of glutaraldehyde undergoes Schiff base condensation reaction with the amino group of chitosan to construct a three-dimensional covalent cross-linked network, and the precursor skeleton is stabilized by tannic acid.

[0016] (4) The gel was thoroughly washed with deionized water and then freeze-dried to obtain Fe-chitosan tannic acid carbon precursor. The precursor powder was then ground. (5) The precursor powder is carbonized at high temperature under inert gas protection and then naturally cooled to room temperature to obtain black powder. After being thoroughly washed with deionized water and dried, tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material is obtained, denoted as Fe-GCTC.

[0017] Its beneficial effects are as follows: the three-dimensional cross-linked network constrains the shrinkage path of the carbon skeleton during the carbonization process, inhibits the disordered collapse of the skeleton, and thus forms iron / nitrogen / oxygen co-doped carbon materials with interconnected pore structures and Fe-N polar active sites.

[0018] Preferably, in step (1), the degree of deacetylation of the chitosan is ≥95%, and the viscosity is 100~200 mPa·s; the mass ratio of chitosan to tannic acid is 0.5~2:0.4~1.6; the mass fraction of the acetic acid solution is 2.0~4.0 wt%; the mass ratio of chitosan to acetic acid solution is 0.5~2g:80~150mL; and the heating and stirring temperature is 50~70℃.

[0019] Preferably, the iron salt in step (2) is Fe(NO3)3·9H2O; the mass ratio of Fe(NO3)3·9H2O to the volume ratio of deionized water is 0.1~0.5g:5~20mL; the slow addition of solution B to solution A requires that the iron ion concentration in the final metal-organic composite precursor system be 0.001~0.008 mol·L. -1 .

[0020] Preferably, the mass fraction of the glutaraldehyde solution in step (3) is 50 wt%; the molar ratio of glutaraldehyde in the glutaraldehyde solution to amino groups in chitosan is 1:0.5~1:3; the concentration of the NaOH solution is 1 mol / L, and the pH is adjusted to 5.0~6.0; the stirring reaction time is 0.5~3 h.

[0021] Preferably, the specific operation of freeze-drying in step (4) is as follows: first in 60~ Pre-freeze at 20℃ for 2-12 hours, then freeze under vacuum conditions. 60~ Freeze-dry at 40℃ for 12~72 h until the sample is completely dry.

[0022] Preferably, the inert gas in step (5) is nitrogen or argon; the high-temperature carbonization treatment is specifically: heating to 800-1000℃ at a heating rate of 2-10℃ / min and holding for 1-5 h; the drying conditions are drying at 60℃ under vacuum for 12-36 h.

[0023] Finally, the application of the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material provided by this invention in lithium-sulfur batteries is as follows: Tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material (Fe-GCTC) is mixed with elemental sulfur, allowing sulfur to be loaded into the pores of the Fe-GCTC, resulting in a Fe-GCTC mixed sulfur material; the mass fraction of sulfur in the Fe-GCTC mixed sulfur material is 50~80wt%; the Fe-GCTC mixed sulfur material is mixed and ground with Super-P and PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP is added to prepare a positive electrode slurry; the slurry is uniformly coated onto aluminum foil, vacuum dried, and cut to obtain the lithium-sulfur battery positive electrode.

[0024] The method for mixing Fe-GCTC with elemental sulfur is to directly mix and grind to obtain Fe-GCTC mixed sulfur material.

[0025] Alternatively, the method for mixing Fe-GCTC with elemental sulfur involves mixing and grinding the powder, then drying it in an oven at 60°C for 6 hours. Subsequently, the dried powder is placed in a reaction vessel and heated at 155°C for 12 hours under an argon atmosphere, followed by heating to 200°C for 30 minutes to obtain the Fe-GCTC mixed sulfur material.

[0026] In summary, this invention provides a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material and its preparation method. Compared with the prior art, this invention has the following significant advantages: (1) The hierarchical porous structure of the carbonization precursor is precisely controllable: A three-dimensional covalent network is constructed through a two-stage strategy of chitosan-tannic acid coordination complex and glutaraldehyde Schiff base crosslinking. Iron salt is introduced into the precursor system, Fe... 3+ The strong coordination with the ortho-phenolic hydroxyl group of tannic acid enables the metal species to be uniformly dispersed, and the ice crystal template induces the formation of a through-hole microporous-mesoporous hierarchical structure during freeze-drying.

[0027] (2) Efficient construction of multiple polar sites of iron / nitrogen / oxygen: Fe 3+ After coordination and anchoring with the phenolic hydroxyl groups of tannic acid, highly dispersed iron species are formed through high-temperature carbonization. These species, along with nitrogen introduced by chitosan and oxygen introduced by tannic acid, form iron / nitrogen / oxygen multi-polar sites. These polar sites enhance the chemisorption of polysulfides and promote their redox transformation. Electrochemical impedance spectroscopy (EIS) tests show that the charge transfer impedance (Rct) of Fe-GCTC is 80 Ω, which is approximately 60.6% lower than that of undoped GCTC (202.8 Ω).

[0028] (3) Excellent lithium-sulfur battery performance: Fe-GCTC, as the sulfur carrier of the positive electrode in lithium-sulfur batteries, achieves an initial discharge specific capacity of 1250 mAh·g at a 0.1 C rate. -1 After 100 charge-discharge cycles, the specific capacity remained at 737 mAh·g. -1 .

[0029] (4) Green and Sustainable: Using chitosan as the biomass nitrogen source and tannic acid as a renewable polyphenol raw material, the production process is green and environmentally friendly, which is conducive to the high-value utilization of forestry residues. It can improve the effective utilization rate of iron ions, accelerate the redox rate of batteries, and increase the power density of lithium-sulfur batteries. Overall, this method is simple and easy to implement, has high production efficiency, and has excellent development prospects. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0031] Figure 1 A schematic diagram of the preparation of tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon materials.

[0032] Figure 2 Scanning electron microscope (SEM) images of GCTC and Fe-GCTC samples.

[0033] Figure 3X-ray diffraction (XRD) spectra of GCTC and Fe-GCTC samples.

[0034] Figure 4 X-ray photoelectron spectroscopy (XPS) spectrum of Fe-GCTC (Fe 2p, C 1s, O 1s, N 1s).

[0035] Figure 5 Comparison of charge-discharge performance between GCTC and Fe-GCTC based lithium-sulfur batteries.

[0036] Figure 6 Comparison of cycle performance between GCTC and Fe-GCTC based lithium-sulfur batteries.

[0037] Figure 7 Comparison of rate performance between GCTC and Fe-GCTC based lithium-sulfur batteries.

[0038] Figure 8 Comparison of electrochemical impedance spectroscopy between GCTC and Fe-GCTC based lithium-sulfur batteries.

[0039] Figure 9 Comparison of polysulfide adsorption properties between GCTC and Fe-GCTC samples.

[0040] Figure 10 Cyclic voltammetry curves of Fe-GCTC-based lithium-sulfur batteries at different scan rates. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Comparative Example 1 (1) Weigh 1 g of chitosan (degree of deacetylation ≥ 95%, viscosity 100~200 mPa·s) and 0.8 g of tannic acid and dissolve them in 100 mL of 2.5 wt% acetic acid solution. Stir at 60℃ until completely dissolved and record as solution A.

[0043] (2) While stirring continuously, slowly add 0.5 mL of 50% glutaraldehyde solution to solution A obtained in step (1) (the molar ratio of glutaraldehyde to the amino group in chitosan is about 1:2). The dialdehyde group of glutaraldehyde reacts with the free amino group in chitosan molecule to form a Schiff base reaction (forming –C=N-imine bond), integrating the precursor solution into a three-dimensional covalent cross-linked network.

[0044] (3) Adjust the pH of the system obtained in step (2) to 5.5 using 1 M NaOH solution, and continue stirring at room temperature for 1 h to form a homogeneous gel.

[0045] (4) Wash the gel obtained in step (3) thoroughly with deionized water several times to remove unreacted glutaraldehyde and other impurities; first, place the gel in... Pre-freeze at 30°C for 12 hours, then freeze under vacuum conditions. Freeze-dry at 45℃ for 48 h until the sample is completely dry to obtain chitosan tannic acid carbon precursor, which is then ground into powder.

[0046] (5) Place the precursor powder obtained in step (4) into a tube furnace and heat it at 5 °C·min under a N2 atmosphere. -1 The temperature was increased to 900 °C at a rising rate and held for 2 h. After naturally cooling to room temperature, the resulting black powder was removed, washed three times with deionized water, and dried at 60 °C under vacuum for 24 h to obtain nitrogen / oxygen co-doped porous carbon material, denoted as GCTC.

[0047] (6) The GCTC obtained in step (5) is mixed and ground with elemental sulfur at a mass ratio of 3:7 to obtain GCTC mixed sulfur material. The mixed sulfur material is mixed and ground with Super-P and PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP is added to prepare a positive electrode slurry. The slurry is uniformly coated on aluminum foil, vacuum dried at 55°C for 12 h, and cut into 14 mm diameter discs to obtain the positive electrode of the lithium-sulfur battery.

[0048] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a rate of 0.1 C, the initial specific capacity of the prepared chitosan / tannic acid carbon-based lithium-sulfur battery was 1109 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 52%.

[0049] Comparative Example 2 Referring to Comparative Example 1, the only difference between this comparative example and Comparative Example 1 is that the mass ratio of chitosan to tannic acid in step (1) is 1:1 (i.e., the amount of tannic acid used is 1 g), and the other steps and parameters are the same.

[0050] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a 0.1 C rate, the initial specific capacity of the prepared chitosan / tannic acid carbon-based lithium-sulfur battery was 434 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 40%.

[0051] Comparative Example 3 Referring to Comparative Example 1, the only difference between this comparative example and Comparative Example 1 is that the mass ratio of chitosan to tannic acid in step (1) is 0.8:1 (i.e., the amount of chitosan used is 0.8 g), and the other steps and parameters are the same.

[0052] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a rate of 0.1 C, the initial specific capacity of the prepared chitosan / tannic acid carbon-based lithium-sulfur battery was 497 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 36%.

[0053] Comparative Example 4 Referring to Comparative Example 1, the only difference between this comparative example and Comparative Example 1 is the preparation method of the GCTC mixed sulfur material in step (6). GCTC and elemental sulfur were ground and mixed in a mass ratio of 3:7 and dried in an oven at 60°C for 6 h. Subsequently, the dried mixed powder was placed in a reaction vessel and heated at 155°C for 12 h under an argon atmosphere, followed by heating to 200°C for 30 min to obtain the GCTC mixed sulfur material. The mixed sulfur material was mixed and ground with Super-P and PVDF in a mass ratio of 8:1:1, and an appropriate amount of NMP was added to prepare the positive electrode slurry. The slurry was uniformly coated on aluminum foil, vacuum dried at 55°C for 12 h, and cut into 14 mm diameter discs to obtain the lithium-sulfur battery positive electrode.

[0054] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a rate of 0.1 C, the initial specific capacity of the prepared chitosan / tannic acid carbon lithium-sulfur battery was 648 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate was 43%.

[0055] Example 1 (1) Weigh 1 g of chitosan (degree of deacetylation ≥95%, viscosity 100~200 mPa·s) and 0.8 g of tannic acid and dissolve them in 100 mL of 2.5 wt% acetic acid solution. Stir at 60 ℃ until completely dissolved and record as solution A.

[0056] (2) Weigh 0.2 g of Fe(NO3)3·9H2O and dissolve it in 10 mL of deionized water to obtain iron salt solution B. Under stirring conditions, slowly add iron salt solution B to solution A. 3+ It coordinates with the ortho-phenolic hydroxyl groups of tannic acid and the amino groups of chitosan to form a metal-organic complex precursor system.

[0057] (3) Slowly add 0.5 mL of 50% glutaraldehyde solution (the molar ratio of glutaraldehyde to the amino group in chitosan is 1:2) to the precursor system obtained in step (2). The dialdehyde group of glutaraldehyde reacts with the free amino group in the chitosan molecule to form a Schiff base reaction (forming a –C=N–imine bond), integrating the precursor into a three-dimensional covalent cross-linked network (see Appendix). Figure 1 The system was adjusted to pH 5.5 using 1 M NaOH solution, and stirred for 1 h at room temperature to form a homogeneous gel.

[0058] (4) Wash the gel obtained in step (3) thoroughly with deionized water several times to remove unreacted glutaraldehyde and other impurities; first, place the gel in... Pre-freeze at 30°C for 12 hours, then freeze under vacuum conditions. Freeze-dry at 45℃ for 48 h until the sample is completely dry to obtain Fe-chitosan tannin carbon precursor, which is then ground into powder.

[0059] (5) Place the precursor powder obtained in step (4) into a tube furnace and heat it at 5 °C·min under a N2 atmosphere. -1 The temperature was increased to 900 °C and held for 2 h. After cooling to room temperature, the black powder was removed, washed three times with deionized water, and dried at 60 °C for 24 h under vacuum to obtain tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material, denoted as Fe-GCTC.

[0060] (6) The Fe-GCTC obtained in step (5) is mixed and ground with elemental sulfur at a mass ratio of 3:7 to obtain Fe-GCTC mixed sulfur material. The mixed sulfur material is mixed and ground with Super-P and PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP is added to prepare a positive electrode slurry. The slurry is uniformly coated on aluminum foil, vacuum dried at 55 ℃ for 12 h, and cut into 14 mm diameter discs to obtain the positive electrode of the lithium-sulfur battery.

[0061] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a 0.1 C rate, the initial specific capacity of the lithium-sulfur battery prepared using the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as the sulfur support for the cathode was 1250 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 59%.

[0062] Example 2 Referring to Example 1, the only difference between this example and Example 1 is that in step (3), 0.25 mL of 50% glutaraldehyde solution (the molar ratio of glutaraldehyde to the amino group in chitosan is 1:1) is slowly added dropwise to the precursor system obtained in step (2). All other steps are the same.

[0063] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a 0.1 C rate, the initial specific capacity of the lithium-sulfur battery prepared using the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as the cathode sulfur support was 605 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 47%.

[0064] Example 3 Referring to Example 1, the only difference between this example and Example 1 is that in step (3), 1.0 mL of 50% glutaraldehyde solution (the molar ratio of glutaraldehyde to the amino group in chitosan is 1:0.5) is slowly added dropwise to the precursor system obtained in step (2), and all other steps are the same.

[0065] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a 0.1 C rate, the initial specific capacity of the lithium-sulfur battery prepared using the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as the sulfur support for the cathode was 394 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 52%.

[0066] Example 4 Referring to Example 1, the only difference between this example and Example 1 is the preparation method of the Fe-GCTC mixed sulfur material in step (6). Fe-GCTC and elemental sulfur were ground and mixed at a mass ratio of 3:7 and dried in an oven at 60°C for 6 hours. Subsequently, the dried mixed powder was placed in a reaction vessel and heated at 155°C for 12 hours under an argon atmosphere, followed by heating to 200°C for 30 minutes to obtain the Fe-GCTC mixed sulfur material. The mixed sulfur material was mixed and ground with Super-P and PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP was added to prepare the positive electrode slurry. The slurry was uniformly coated on aluminum foil, vacuum dried at 55°C for 12 hours, and cut into 14 mm diameter discs to obtain the lithium-sulfur battery positive electrode.

[0067] Testing: The resulting lithium-sulfur battery cathode was used to assemble CR-2032 coin cells. At a 0.1 C rate, the initial specific capacity of the lithium-sulfur battery prepared using the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as the cathode sulfur support was 788 mAh·g. -1 After 100 charge-discharge cycles, the capacity retention rate is 56%.

[0068] Furthermore, to further verify the superiority of the technology of this invention, the inventors conducted a series of characterizations on GCTC and Fe-GCTC materials and performed performance tests on the GCTC-based lithium-sulfur battery of Comparative Example 1 and the Fe-GCTC-based lithium-sulfur battery of Example 1. The specific details are as follows: Material characterization (1) Scanning electron microscopy (SEM) analysis: The microstructure of the samples was observed using a Gemini-300 scanning electron microscope (test results are attached). Figure 2 GCTC exhibits a typical three-dimensional interconnected macroporous network structure, with relatively smooth and dense pore wall surfaces and irregular polyhedral pore morphology. The pore walls are interconnected to form an open channel system. Fe-GCTC retains the three-dimensional porous framework of GCTC, but the pore wall structure is thicker and more irregular, indicating that the introduction of iron species induces surface roughening and structural complexity.

[0069] (2) X-ray diffraction (XRD) analysis was performed using an XRD-7000 X-ray diffractometer (Cu Kα, 40 kV, 30 mA), with a scanning range of 5°~90° (test results are attached). Figure 3 The XRD pattern of GCTC shows a broadened diffuse peak at approximately 23° 2θ, indicating that GCTC is mainly composed of amorphous carbon. The Fe-GCTC pattern shows a series of sharp diffraction peaks at 2θ = 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, 62.6°, and 74.0°, which are in high agreement with the Fe3O4 standard card (PDF#65-3107), corresponding to the (220), (311), (400), (422), (511), (440), and (533) crystal planes of Fe3O4, respectively. The Fe3O4@C structure can provide strong chemisorption of polysulfides through Fe-O bonds and promote the redox reaction process of polysulfides.

[0070] (3) X-ray photoelectron spectroscopy (XPS) analysis was performed using a K-Alpha type X-ray photoelectron spectroscopy analyzer with monochromatic Al Kα as the X-ray source (test results are attached). Figure 4 In the Fe 2p spectrum, Fe 2p 3 / 2 and Fe 2p 1 / 2 The main peaks are located at 709.22 eV and 724.52 eV, respectively, with a spin-orbit splitting energy of 15.3 eV. The satellite peak is located at 713.31 eV, with a distance of 4.09 eV from the main peak, confirming that the iron species exhibits reduced state characteristics, which is beneficial for improving the material's interfacial charge transfer capability. In the C 1s spectrum, the position at 284.0 eV corresponds to the C / C=C bond (sp). 2The graphitized carbon framework shows a defect carbon structure at 284.8 eV and a carboxyl group (OC=O) at 288.4 eV. The O 1s spectrum can be deconvoluted into three characteristic peaks: 529.77 eV (lattice oxygen, originating from amorphous iron oxide), 532.16 eV (CO / C=O), and 533.5 eV (surface adsorbed hydroxyl / water molecules). In the N 1s spectrum, the main peak at 399.81 eV is highly consistent with the binding energy of pyrrole nitrogen, indicating that nitrogen is mainly embedded in the carbon framework in the form of pyrrole nitrogen.

[0071] Lithium-sulfur battery performance testing (1) Charge and discharge performance

[0072] At a 0.1C rate, the Fe-GCTC-based battery exhibits a 12.7% increase in initial discharge specific capacity compared to the GCTC-based battery, an increased low-potential plateau length, a flatter voltage plateau, and a significantly reduced voltage hysteresis between charge and discharge curves (see Appendix). Figure 5 ).

[0073] (2) Cyclic performance

[0074] At 0.1C rate, the capacity retention of Fe-GCTC-based lithium-sulfur batteries is approximately 21% higher than that of GCTC. The coulombic efficiency of both materials remains close to 100% during cycling (see Appendix). Figure 6 ).

[0075] (3) Ratio performance Compared to GCTC, Fe-GCTC-based lithium-sulfur batteries exhibit improved rate performance (see appendix). Figure 7 At rates of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, its specific capacity reaches 1203 mAh·g. -1 997 mAh·g -1 873 mAh·g -1 735mAh·g -1 and 374 mAh·g -1 When the current density recovers to 1 C, 0.5 C, and 0.2 C, its specific capacity can still recover to 608 mAh·g. -1 826 mAh·g -1 and 990 mAh·g -1 The capacity recovery rates were 82.7%, 94.6%, and 99.3%, respectively. In contrast, the rate performance of GCTC-based lithium-sulfur batteries was poor.

[0076] (4) Electrochemical impedance spectroscopy

[0077] Comparing the two materials reveals that the Rct of the Fe-GCTC-based solar cell is approximately 80 Ω, a decrease of about 60.6% compared to the 202.8 Ω of the GCTC-based solar cell (see appendix). Figure 8 The Fe-GCTC-based battery exhibits a low charge transfer resistance, indicating that the Fe3O4-related polar sites contribute to enhanced chemisorption of polysulfides and improved interfacial charge transfer processes. Fe-GCTC can more effectively promote the migration of lithium ions and electrons, thereby helping to improve the rate performance of the battery.

[0078] (5) Polysulfide adsorption experiment Equal amounts of GCTC and Fe-GCTC were added to a Li₂S₆ solution. After standing for 24 h, the solution with Fe-GCTC almost completely decolorized to a colorless and transparent state, while the solution with GCTC remained pale yellow. Quantitative analysis using UV-Vis absorption spectroscopy confirmed that Fe-GCTC had a significantly stronger adsorption capacity for Li₂S₆ than GCTC (see Appendix). Figure 9 ).

[0079] (5) Cyclic voltammetry curve As the scan rate changes, the cyclic voltammetry curves of the Fe-GCTC-based lithium-sulfur battery exhibit the characteristic redox properties typical of lithium-sulfur batteries (see Appendix). Figure 10 Its oxidation peak is located near 2.41 V (Peak A), corresponding to the process of polysulfides being oxidized to S8 during battery charging; its two reduction peaks are located near 2.29 V (Peak B) and 2.01 V (Peak C), respectively, corresponding to the process of battery discharging. This can be attributed to the transformation of S8 molecules into higher-order polysulfides and further reduction to Li2S2 and Li2S.

[0080] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material, characterized in that, The iron / nitrogen / oxygen co-doped porous carbon material has a microporous-mesoporous composite structure with a three-dimensional interconnected porous carbon framework. Nitrogen is in situ doped into the carbon framework, and iron species are distributed in the carbon matrix in the form of highly dispersed Fe3O4 nanoparticles, iron single atoms, or combinations thereof, forming Fe-N synergistic and / or Fe-O polar active sites. The surface of the iron / nitrogen / oxygen co-doped porous carbon material is also rich in iron / nitrogen / oxygen multi-polar sites.

2. The tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material according to claim 1, characterized in that, The total nitrogen content in the iron / nitrogen / oxygen co-doped porous carbon material is 2-10 wt%, of which pyridine nitrogen and pyrrole nitrogen account for 40-80 wt% of the total nitrogen content.

3. A method for preparing a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as described in any one of claims 1 or 2, characterized in that, Specifically, the steps include the following: (1) Add chitosan and tannic acid to acetic acid solution, heat and stir until completely dissolved to obtain solution A; (2) Dissolve the iron salt in deionized water to obtain solution B; under stirring conditions, slowly add solution B to solution A to form a metal-organic composite precursor system; (3) Glutaraldehyde solution was slowly added dropwise to the metal-organic composite precursor system, and then the pH was adjusted with NaOH solution. The reaction was stirred to obtain a homogeneous gel. (4) The gel was thoroughly washed with deionized water and then freeze-dried to obtain Fe-chitosan tannic acid carbon precursor. The precursor powder was then ground. (5) The precursor powder is carbonized at high temperature under inert gas protection and then naturally cooled to room temperature to obtain black powder. After being thoroughly washed with deionized water and dried, tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material is obtained, denoted as Fe-GCTC.

4. The preparation method according to claim 3, characterized in that, The degree of deacetylation of the chitosan mentioned in step (1) is ≥95%, and the viscosity is 100~200 mPa·s; The mass ratio of chitosan to tannic acid is 0.5~2:0.4~1.6; The acetic acid solution has a mass fraction of 2.0~4.0 wt%; The mass ratio of chitosan to acetic acid solution is 0.5~2g:80~150mL; The heating and stirring temperature is 50~70℃.

5. The preparation method according to claim 3, characterized in that, The iron salt mentioned in step (2) is Fe(NO3)3·9H2O; the mass ratio of Fe(NO3)3·9H2O to the volume ratio of deionized water is 0.1~0.5g:5~20mL; The step involves slowly adding solution B to solution A, aiming to achieve an iron ion concentration of 0.001–0.008 mol·L⁻¹ in the final metal-organic composite precursor system. -1 .

6. The preparation method according to claim 3, characterized in that, The mass fraction of the glutaraldehyde solution in step (3) is 50 wt%; the molar ratio of glutaraldehyde in the glutaraldehyde solution to the amino group in chitosan is 1:0.5~1:3; The concentration of the NaOH solution is 1 mol / L, and the pH is adjusted to 5.0~6.0; The stirring reaction time is 0.5~3h.

7. The preparation method according to claim 3, characterized in that, The specific operation of freeze-drying in step (4) is as follows: First, in 60~ Pre-freeze at 20℃ for 2-12 hours, then freeze under vacuum conditions. 60~ Freeze-dry at 40℃ for 12~72 h until the sample is completely dry.

8. The preparation method according to claim 3, characterized in that, The inert gas mentioned in step (5) is nitrogen or argon; The high-temperature carbonization treatment specifically involves heating to 800-1000℃ at a heating rate of 2-10℃ / min and holding at that temperature for 1-5 hours. The drying conditions are vacuum drying at 60°C for 12-36 hours.

9. The application of a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material as described in any one of claims 1-2, or a tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material prepared by the method described in any one of claims 3-8, in a lithium-sulfur battery, characterized in that, The specific application is as follows: Tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material is mixed with elemental sulfur, so that sulfur is loaded into the pores of the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material to obtain a sulfur-mixed material; the mass fraction of sulfur in the sulfur-mixed material is 50~80 wt%. Tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material mixed with sulfur is mixed and ground with Super-P and PVDF at a mass ratio of 8:1:1, and an appropriate amount of NMP is added to prepare a positive electrode slurry. The slurry is uniformly coated on aluminum foil, vacuum dried, and cut to obtain the positive electrode of lithium-sulfur battery.

10. The application according to claim 9, characterized in that, The method for mixing tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material with elemental sulfur is direct mixing and grinding to obtain tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material mixed with sulfur.

11. The application according to claim 9, characterized in that, The method for mixing tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material with elemental sulfur involves grinding the mixture and drying it in an oven at 60°C for 6 hours. Subsequently, the dried mixed powder is placed in a reaction vessel and heated at 155°C for 12 hours under an argon atmosphere, followed by heating to 200°C for 30 minutes to obtain the tannic acid-stabilized iron / nitrogen / oxygen co-doped porous carbon material mixed with sulfur.

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