Method for preparing biomass-derived carbon / bimetallic phosphide and application thereof

Abstract

This invention discloses a method for preparing biomass-derived carbon / bimetallic phosphide and its application in lithium-sulfur battery cathode materials. First, biomass-derived carbon material (FBC) is prepared using a KOH alkalization method and a tubular furnace high-temperature carbonization method with *Ficus pumila* interlayer as raw material. Then, FBC / CoMoO4 material is prepared using a hydrothermal method with ammonium molybdate tetrahydrate and cobalt nitrate hexahydrate as raw materials. Next, FBC / CoMoP composite material is prepared by gas-phase phosphating using sodium hypophosphite monohydrate as the phosphorus source. Finally, the composite material is fused with sublimed sulfur powder to form the cathode material for use in lithium-sulfur batteries. The biomass carbon material, upon activation, can form a three-dimensional conductive carbon framework, promoting efficient electron / ion transport. Simultaneously, the unique electronic properties of metal phosphides can modulate the electronic structure of lithium polysulfides on the surface, mitigating polysulfide diffusion and shuttle behavior. The combination of bimetallic phosphides and biomass porous carbon materials has a unique effect on accelerating the conversion kinetics of lithium-sulfur batteries, exhibiting a synergistic effect of "1+1>2".

Description

Technical Field

[0001] This invention relates to the field of lithium-sulfur battery materials technology, specifically to a method for preparing biomass-derived carbon / bimetallic phosphide and the application of the biomass-derived carbon / bimetallic phosphide prepared by this method in lithium-sulfur battery cathode materials. Background Technology

[0002] Lithium-sulfur batteries are considered a next-generation electrochemical energy storage system due to their environmental friendliness, low cost, high theoretical specific capacity (1675 mAh / g), and energy density (2600 Wh / kg). However, lithium-sulfur batteries face a series of problems and challenges in practical applications, such as the low conductivity of sulfur, the large volume expansion of sulfur during charging and discharging, and the presence of soluble polysulfides (Li₂S₂). n The shuttle effect (4≤n≤8) and other issues contribute to the decline in coulombic efficiency, sulfur utilization, and cycle stability of lithium-sulfur batteries, hindering their further development. To address these problems, researchers have devoted considerable effort to improving the performance of lithium-sulfur batteries.

[0003] Biomass raw materials are used to prepare porous carbon because they are abundant in nature, readily available, inexpensive, and possess a unique and original microstructure. The prepared porous carbon exhibits tunable pore structure, large specific surface area, and high adsorption capacity, which helps enhance the specific capacity and cycle stability of batteries. Biomass-derived porous carbon contains some macropores to achieve high sulfur loading, uniform sulfur distribution, provide rapid ion transport channels, and sufficient space to buffer volume changes. However, the geometric confinement and strong physical adsorption of lithium polysulfides (LiPSs) require a large number of micropores. Transition bimetallic phosphides possess excellent conductivity and superior electrocatalytic performance. Their phosphine groups have lone pairs of electrons in the 3p and 3d orbitals, which can improve electronic conductivity. Appropriate phosphine group content is crucial to the conductivity of transition metal phosphides. The electronic structure of polysulfides can be adjusted on the surface through the electronic properties of transition metal phosphides, thereby accelerating reaction kinetics.

[0004] Given the aforementioned shortcomings, although some studies have shown that the "shuttle effect" can be suppressed through carbon materials or metal phosphide strategies, the combination of nonpolar porous carbon materials and polysulfides has a weak effect and cannot effectively adsorb and catalyze the conversion of polysulfides. Therefore, an ideal host material should possess both physical barriers and chemical adsorption capabilities to prevent lithium polysulfides from diffusing into the electrolyte. Biomass carbon materials can form a three-dimensional conductive carbon framework through activation, promoting efficient electron / ion transport. By combining biomass porous carbon materials with transition metal phosphides, lithium polysulfides can be physically adsorbed and chemically regulated. Simultaneously, the unique electronic properties of transition bimetallic phosphides can modulate the electronic structure of lithium polysulfides on the surface, mitigating polysulfide diffusion and shuttle. The combination of three-dimensional porous carbon materials and transition metal phosphides achieves synergistic adsorption and catalytic conversion of LiPSs. Summary of the Invention

[0005] To address the aforementioned problems in lithium-sulfur batteries, this invention provides a method for preparing biomass-derived carbon / bimetallic phosphide and its application in lithium-sulfur battery cathode materials. When this composite material is used in lithium-sulfur batteries, its porous structure can physically encapsulate the polysulfides, and the numerous highly catalytically active sites can improve the conversion reaction kinetics of the polysulfides, thereby significantly enhancing the discharge specific capacity, cycle performance, and rate performance of lithium-sulfur batteries.

[0006] This invention addresses the challenges faced in the lithium-sulfur battery field by providing a method for preparing FBC / CoMoP as a cathode material for lithium-sulfur batteries. The method employs a potassium hydroxide alkalization method and a tube furnace high-temperature carbonization method, using ficus microcarpa interlayer as raw material to prepare biomass porous carbon materials. Then, ammonium molybdate tetrahydrate (H... 24 Mo7N6O 24 FBC / CoMoO4 composite material was synthesized via a hydrothermal method using cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and sodium hypophosphite monohydrate as the phosphorus source. Finally, FBC / CoMoP composite material was prepared via a gas-phase phosphating method. The prepared FBC / CoMoP composite material was then melt-composited with sublimed sulfur powder to obtain the cathode material for lithium-sulfur batteries.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] This invention first discloses a method for preparing biomass-derived carbon / bimetallic phosphide, comprising the following steps:

[0009] (1) Weigh 5-10g of the interlayer of Ficus pumila, add it to anhydrous ethanol for washing, dry it, add 20-40g of KOH and 400-800mL of deionized water, stir evenly, let it stand for 24h, filter the alkaline solution, and obtain the pretreated interlayer of Ficus pumila.

[0010] (2) The pretreated creeping fig intermediate layer was dried and heated in a tube furnace at a heating rate of 5℃ / min. The temperature was raised to 600, 700 and 800℃ and calcined for 2 hours. The product was then taken out, ground evenly in a mortar, washed and centrifuged with deionized water in a centrifuge, and after the pH was measured to be neutral, it was freeze-dried for 12 hours to obtain FBC.

[0011] (3) Weigh out a certain proportion of FBC and H 24 Mo7N6O 24 ·4H2O and Co(NO3)2·6H2O were added to a mixed solution of ethanol and deionized water, and the mixture was sonicated for 30 min and stirred for 1 h; the FBC concentration was 80–160 mg, H 24 Mo7N6O 24 The molar ratio of ·4H2O and Co(NO3)2·6H2O is 1:1, and the volume ratio of ethanol and deionized water is 1:1, to obtain a mixed aqueous solution;

[0012] (4) After the mixed aqueous solution is stirred evenly, it is placed in a reaction vessel and sealed. It is then placed in an oven at 180°C for 12 hours to obtain the FBC / CoMoO4 composite material.

[0013] (5) Weigh FBC / CoMoO4 composite material and sodium hypophosphite monohydrate in a mass ratio of 1:20, and phosphate them in a tube furnace at a high temperature of 5℃ / min. The temperature is raised to 800℃ and the phosphate treatment time is 2h to obtain FBC / CoMoP, namely biomass-derived carbon / bimetallic phosphide.

[0014] The present invention also discloses a biomass-derived carbon / bimetallic phosphide prepared according to the above preparation method.

[0015] The present invention also discloses the application of the above-mentioned biomass-derived carbon / bimetallic phosphide in the preparation of lithium-sulfur battery cathodes.

[0016] Furthermore, the application includes:

[0017] (1) Sublimed sulfur and FBC / CoMoP composite material are uniformly mixed in a certain proportion, and FBC / CoMoP / S composite cathode material is obtained by melt-filling sulfur under vacuum or inert gas atmosphere; the mass ratio of biomass-derived carbon / bimetallic phosphide FBC / CoMoP in FBC / CoMoP / S composite cathode material is 20% to 30%; the melt-filling sulfur temperature is 155℃ and the time is 12 to 16h;

[0018] (2) The FBC / CoMoP / S composite cathode material, conductive agent and binder are mixed in a mass ratio of 8:1:1 or 7:2:1. The mixture is prepared by using NMP as solvent and then coated evenly on the current collector. After vacuum drying, the cathode sheet of lithium-sulfur battery is obtained.

[0019] (3) Assemble the lithium-sulfur battery positive electrode, lithium negative electrode, separator, electrolyte and battery case obtained in step (2) into a lithium-sulfur battery and perform electrochemical tests.

[0020] Further, the conductive agent in step (2) is one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene, and the binder is polyvinylidene fluoride.

[0021] Furthermore, the homogenization mixing method for the slurry prepared in step (2) is ball milling, with a milling time of 1-2 hours, and the sulfur loading of the lithium-sulfur battery positive electrode sheet is 1-5 mg / cm³. 2 The positive current collector used is carbon-coated aluminum foil.

[0022] The beneficial effects of this invention are as follows:

[0023] This invention provides a method for preparing biomass-derived carbon / bimetallic phosphide materials. Porous carbon materials are obtained by potassium hydroxide alkalization and high-temperature carbonization, and then FBC / CoMoP composite materials are prepared by high-temperature phosphating in an inert gas environment using hydrothermal and gas-phase phosphating methods.

[0024] The biomass carbon material prepared in this invention possesses excellent electrical conductivity, a large specific surface area, and a large pore structure, maximizing ion transport rate and improving sulfur utilization. Simultaneously, the introduction of a highly catalytically active transition bimetallic phosphide catalyst increases the catalytically active sites in the FBC / CoMoP composite material, enhancing the adsorption capacity of the FBC / CoMoP cathode material for polysulfides. This improves the redox kinetics of polysulfides during the electrochemical reaction, alleviates the shuttle effect of polysulfides, and reduces battery polarization. Experimental results show that when the FBC / CoMoP composite material is used as the cathode material in lithium-sulfur batteries, it can significantly improve the charge-discharge capacity, rate performance, and cycle performance of lithium-sulfur batteries. The preparation method in this invention features mild reaction conditions and a simple process, providing guidance for the future commercial application of high-performance lithium-sulfur batteries. Attached Figure Description

[0025] Figure 1 Scanning electron microscope (SEM) image of the FBC / CoMoP material prepared for Example 1;

[0026] Figure 2Nitrogen isothermal adsorption / desorption curves of the FBC material prepared in Example 2;

[0027] Figure 3 This is a comparison chart of the cycle performance at 0.5C when applied to lithium-sulfur batteries. Detailed Implementation

[0028] The present invention will be described in detail below with reference to specific embodiments.

[0029] Example 1:

[0030] A method for preparing biomass-derived carbon / bimetallic phosphide

[0031] (1) Weigh 5g of the middle layer of Ficus pumila, add anhydrous ethanol to wash it, dry it, add 20g of KOH and 400mL of deionized water, soak it and let it stand for 24h;

[0032] (2) Dry the middle layer of the creeping fig in step (1) at 110℃, carbonize it in a tube furnace at 600℃ for 2h with a heating rate of 5℃ / min. After calcination, take it out and grind it evenly in a mortar. Wash it with deionized water and centrifuge it 3 times. After the pH value is measured to be 7, freeze dry it for 12h to obtain FBC.

[0033] (3) Weigh out 80mg FBC and 0.72g H 24 Mo7N6O 24 ·4H2O and 0.12g Co(NO3)2·6H2O were added to a mixed solution of 20mL ethanol and 20mL deionized water, sonicated for 30min and stirred for 1h.

[0034] (4) After stirring the aqueous solution from step (3) evenly, put it into a reaction vessel and seal it. Then place it in an oven at 180°C for 12 hours to obtain the FBC / CoMoO4 composite material.

[0035] (5) Weigh out FBC / CoMoO4 powder and sodium hypophosphite monohydrate (mass ratio 1:20). Place the purple FBC / CoMoO4 precursor in the downstream region of a quartz tube, and place sodium hypophosphite monohydrate in the upstream side of a tube furnace. Phosphate at 750℃ for 2 hours in a tube furnace with a heating rate of 5℃ / min. After calcination, remove the powder and grind it evenly in a mortar to obtain FBC / CoMoP.

[0036] Example 2:

[0037] A method for preparing biomass-derived carbon / bimetallic phosphide

[0038] (1) Weigh 5g of the middle layer of Ficus pumila, add anhydrous ethanol to wash it, dry it, add 20g of KOH and 400mL of deionized water, soak it and let it stand for 24h;

[0039] (2) Dry the middle layer of the creeping fig in step (1) at 110°C, carbonize it in a tube furnace at 700°C for 2 hours with a heating rate of 5°C / min. After calcination, take it out and grind it evenly in a mortar. Wash it with deionized water and centrifuge it 3 times. After the pH value is measured to be 7, freeze dry it for 12 hours to obtain FBC.

[0040] (3) Weigh out 80mg FBC and 0.72g H 24 Mo7N6O 24 ·4H2O and 0.12g Co(NO3)2·6H2O were added to a mixed solution of 20mL ethanol and 20mL deionized water, sonicated for 30min and stirred for 1h.

[0041] (4) After stirring the aqueous solution from step (3) evenly, put it into a reaction vessel and seal it. Then place it in an oven at 180°C for 12 hours to obtain the FBC / CoMoO4 composite material.

[0042] (5) Weigh out FBC / CoMoO4 powder and sodium hypophosphite monohydrate (mass ratio 1:20). Place the purple FBC / CoMoO4 precursor in the downstream region of a quartz tube, and place sodium hypophosphite monohydrate in the upstream side of a tube furnace. Phosphate at 750℃ for 2 hours in a tube furnace with a heating rate of 5℃ / min. After calcination, remove the powder and grind it evenly in a mortar to obtain FBC / CoMoP.

[0043] Example 3:

[0044] A method for preparing biomass-derived carbon / bimetallic phosphide

[0045] (1) Weigh 5g of the middle layer of Ficus pumila, add anhydrous ethanol to wash it, dry it, add 20g of KOH and 400mL of deionized water, soak it and let it stand for 24h;

[0046] (2) Dry the middle layer of the creeping fig in step (1) at 110℃, carbonize it in a tube furnace at 800℃ for 2h with a heating rate of 5℃ / min. After calcination, take it out and grind it evenly in a mortar. Wash it with deionized water and centrifuge it 3 times. After the pH is measured to be 7, freeze dry it for 12h to obtain FBC.

[0047] (3) Weigh out 80mg FBC and 0.72g H 24 Mo7N6O 24 ·4H2O and 0.12g Co(NO3)2·6H2O were added to a mixed solution of 20mL ethanol and 20mL deionized water, sonicated for 30min and stirred for 1h.

[0048] (4) After stirring the aqueous solution from step (3) evenly, put it into a reaction vessel and seal it. Then place it in an oven at 180°C for 12 hours to obtain the FBC / CoMoO4 composite material.

[0049] (5) Weigh out FBC / CoMoO4 powder and sodium hypophosphite monohydrate (mass ratio 1:20). Place the purple FBC / CoMoO4 precursor in the downstream region of a quartz tube, and place sodium hypophosphite monohydrate in the upstream side of a tube furnace. Phosphate at 750℃ for 2 hours in a tube furnace with a heating rate of 5℃ / min. After calcination, remove the powder and grind it evenly in a mortar to obtain FBC / CoMoP.

[0050] Figure 1 The image shows a scanning electron microscope (SEM) image of the FBC / CoMoP material prepared in Example 1; CoMoP is a rod-shaped structure distributed on the surface of biomass-derived porous carbon, proving that the material was successfully synthesized.

[0051] Figure 2 The nitrogen isotherm adsorption / desorption curves and pore size distribution of the FBC material prepared in Example 2 are shown. The carbonized FBC exhibits a type I adsorption-desorption isotherm. From the figure, when P / P0 < 0.1, it can be seen that the type I adsorption isotherm shows a steep upward trend, proving the presence of a large number of micropores in the FBC. When P / P0 > 0.1, the desorption curve shows a slight hysteresis, proving the presence of mesopores. The FBC has a high specific surface area of ​​1487.46 m² / g. It possesses both micropores and mesopores. This allows for uniform sulfur distribution, provides rapid ion transport channels, and sufficient space to buffer volume changes.

[0052] Application Example 1

[0053] Application of a biomass-derived carbon / bimetallic phosphide in the preparation of lithium-sulfur battery cathodes

[0054] (1) Sublimed sulfur and FBC / CoMoP composite material are uniformly mixed in a certain proportion, and FBC / CoMoP / S composite cathode material is obtained by melt-filling sulfur under vacuum or inert gas atmosphere; the mass ratio of biomass-derived carbon / bimetallic phosphide FBC / CoMoP in FBC / CoMoP / S composite cathode material is 30%; the melt-filling sulfur temperature is 155℃ and the time is 12h.

[0055] (2) The FBC / CoMoP / S composite cathode material, conductive agent and binder are mixed in a mass ratio of 7:2:1. The mixture is prepared by uniformly mixing with NMP as solvent and then uniformly coated on the current collector. After vacuum drying, the cathode sheet of lithium-sulfur battery is obtained.

[0056] (3) Assemble the lithium-sulfur battery positive electrode, lithium negative electrode, separator, electrolyte and battery case obtained in step (2) into a lithium-sulfur battery and perform electrochemical tests.

[0057] Further, the conductive agent in step (2) is conductive carbon black, and the binder is polyvinylidene fluoride.

[0058] Furthermore, the homogenization method for preparing the slurry in step (2) is ball milling, with a milling time of 1 hour, and the sulfur loading of the lithium-sulfur battery positive electrode sheet is 1.2 mg / cm³. 2 The positive current collector used is carbon-coated aluminum foil.

[0059] Figure 3 The graph shows a comparison of the cycling performance of Application Example 1 in a lithium-sulfur battery at 0.5C. After 300 cycles, the discharge capacity of FBC / CoMoP / S as the positive electrode in a lithium-sulfur battery is 847 mAh / g, with a capacity retention of 88%. This indicates that the FBC / CoMoP / S composite material can effectively improve electrochemical reaction kinetics, suppress the shuttle effect of polysulfides, and enhance battery stability.

Claims

1. A method for preparing biomass-derived carbon / cobalt-molybdenum bimetallic phosphide, characterized in that, Includes the following steps: (1) The middle layer of the creeping fig was soaked in anhydrous ethanol, dried in an oven, stirred evenly with potassium hydroxide and deionized water, placed at room temperature for 24 hours, and the alkaline solution was filtered; the product was then dried, heated to 600~800℃ in a tube furnace at 5℃ / min for 2 hours, and after calcination, it was taken out and ground evenly in a mortar, washed and centrifuged with deionized water in a centrifuge, and after the pH was measured to be neutral, it was freeze-dried to obtain FBC; (2) Add FBC, ammonium molybdate tetrahydrate and cobalt nitrate hexahydrate to a mixed solution of ethanol and deionized water, sonicate and stir magnetically to obtain a mixed aqueous solution; wherein the molar ratio of FBC, ammonium molybdate tetrahydrate and cobalt nitrate hexahydrate is 1:1:1, and the volume ratio of ethanol and deionized water is 1:

1. (3) After stirring the mixed aqueous solution evenly, put it into the reaction vessel and seal it. Place it in an oven at 180℃ and react for 12 hours. After natural cooling, wash it with deionized water and anhydrous ethanol alternately 3 to 6 times, and then vacuum dry it to obtain FBC / CoMoO4 composite material. (4) Take FBC / CoMoO4 composite material and sodium hypophosphite monohydrate, place FBC / CoMoO4 composite material in the downstream area of ​​the quartz tube, place sodium hypophosphite monohydrate on the upstream side of the tube furnace, and perform high-temperature phosphating treatment with a heating rate of 5℃ / min, a phosphating temperature of 750℃, and a phosphating time of 2h to obtain FBC / CoMoP, namely biomass-derived carbon / cobalt-molybdenum bimetallic phosphide.

2. The preparation method according to claim 1, characterized in that, In step (1), the amount of the intermediate layer of Ficus pumila is 5~10g, the amount of potassium hydroxide is 20~40g, and the amount of deionized water is 400~800mL.

3. The preparation method according to claim 1, characterized in that, The carbonization temperature in step (1) is 800℃.

4. The preparation method according to claim 1, characterized in that, In step (4), the mass ratio of the FBC / CoMoO4 composite material to sodium hypophosphite monohydrate is 1:

20.

5. A biomass-derived carbon / cobalt-molybdenum bimetallic phosphide prepared by the preparation method according to any one of claims 1 to 4.

6. The application of the biomass-derived carbon / cobalt-molybdenum bimetallic phosphide according to claim 5 in the preparation of lithium-sulfur battery cathodes, characterized in that, include: (1) Sublimed sulfur and biomass-derived carbon / cobalt-molybdenum bimetallic phosphide are uniformly mixed at a mass ratio of 3:

1. FBC / CoMoP / S composite cathode material is obtained by melting and injecting sulfur under vacuum or inert gas atmosphere. The heating temperature is 155℃ and the holding time is 12~16 hours. (2) The FBC / CoMoP / S composite cathode material, conductive agent and binder are mixed in a mass ratio of 7:2:1 or 8:1:

1. The mixture is prepared by using N-methylpyrrolidone as solvent and then coated evenly on the current collector. After vacuum drying, the lithium-sulfur battery cathode sheet is obtained. (3) Assemble the lithium-sulfur battery positive electrode, lithium negative electrode, separator, electrolyte and battery case obtained in step (2) to obtain the lithium-sulfur battery.

7. The application according to claim 6, characterized in that, The conductive agent in step (2) is one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene, and the binder is polyvinylidene fluoride.

8. The application according to claim 6, characterized in that, The sulfur loading of the positive electrode sheet of the lithium-sulfur battery in step (2) is 1~5 mg / cm². 2 .

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