Predoped medium metamorphic coal-based carbon microcrystal modulation method

Abstract

The invention discloses a preparation method of pre-doped coal-based carbon microcrystals with a medium metamorphic degree. The method comprises the following steps: 1, grinding and selecting: crushing and screening raw coal to obtain powder with a target particle size; 2, pre-crosslinking: modifying an oxygen source, and performing doping modification on the powder obtained in the step 1 and an oxidizing agent under a solvothermal condition to obtain pre-crosslinked powder; 2, modifying a nitrogen source, a sulfur source and a phosphorus source, and performing doping modification on the powder obtained in the step 1 and an organic heterocyclic structure under a solvothermal condition to obtain pre-crosslinked powder; and (3) carbonization: under the protection of inert atmosphere, heating the pre-crosslinked powder obtained in the step (2) to 800-1800 DEG C, and preserving heat for 0.5-10 hours to obtain the coal-based modified hard carbon material with the medium metamorphic degree. Compared with a carbon material obtained by a direct carbonization process, the medium metamorphic coal-based hard carbon obtained on the basis of pre-crosslinking modification has high reversible capacity and high initial coulombic efficiency in sodium ion storage and transportation, and has important application prospects.

Description

Technical Field

[0001] This invention belongs to the field of electrode material preparation technology, and relates to a method for modulating pre-crosslinked, moderately metamorphic coal-based carbon microcrystals. Background Technology

[0002] Sodium / potassium ion batteries are considered an important option for next-generation rechargeable ion batteries. Developing carbon anode materials with low-cost, natural carbon sources, using coal as a low-cost, top-down directional method, and exhibiting low voltage plateaus (widening the full-cell voltage window), high first-cycle coulombic efficiency, and long cycle life is a crucial direction for developing low-cost sodium / potassium ion batteries.

[0003] Medium-grade metamorphic coal (maximum reflectance R of vitrinite group) max Coal with a carbon content of 1.2-2.0% (including gas-rich coal, coking coal, and bituminous coal) is characterized by high carbon content (75-90%), low price, and easy pretreatment, making it an important choice for developing high-performance carbon-based electrode materials. Medium-rank coal contains abundant medium-sized molecular active components (with both aromatic and aliphatic characteristics, and the aromatic centers averaging 2-4 condensed aromatic rings), possessing bridging bonds, functional groups, and heteroatoms that readily combine with other components in the coal. Therefore, it exhibits thermoplasticity between 350-500℃, with coal particles fusing together to form a fluid mesophase. Mesophase nucleation and growth are crucial steps in the transformation of coal-based carbon microcrystals from an amorphous to a crystalline state, significantly impacting the transformation of carbon microcrystals to crystalline graphite structures after high-temperature treatment and the quality of carbon formation.

[0004] Currently, the main approach to intervene in mesophase nucleation and growth is by adding nucleating agents. Chang Hongyan et al. used carbon black, petroleum coke, graphite, and needle coke as seed crystals to regulate the uniformity of particle size distribution and surface morphology of mesophase carbon microspheres (Coal Conversion, 2017, 40(05): 39-44); Mao Zhaokun et al. used naphthyl mesophase pitch as seed crystals to shorten the formation time of mesophase carbon microspheres and form more anisotropic regions (Fuel, 2019, 243: 390-397). Liu Dongdong et al. used air pre-oxidation treatment on coal to reduce the fluidity of coal when heated and reduce the content of anisotropic components in carbides (Energy & Fuels, 2017, 31(2): 1406-1415). The above methods belong to macroscopic scale structure regulation and are difficult to target the microscopic functional structure design and regulation for different requirements of carbon anode materials (energy density, high current discharge performance).

[0005] On the other hand, medium-rank coal undergoes a liquid-phase carbonization process, and the graphite-like crystals in coal-based carbon materials prepared by direct carbonization exhibit long-range order, which is detrimental to improving ion diffusion storage capacity and rate. Adding seed crystals promotes mesophase nucleation and growth, forming more anisotropic crystalline structures. However, highly graphitized structures have been proven unsuitable for sodium / potassium ion batteries (Chem. Soc. Rev., 2019, 48, 4655-4687), and the construction of short-range ordered graphite-like crystal structures is key to rapid, high-density sodium ion insertion and storage. Air pre-oxidation can increase disorder by forming cross-links, but it does not change the ion storage and transport mechanism in the coal-based carbon structure and fails to fundamentally improve high-current discharge performance. As the above analysis shows, existing methods cannot meet the regulatory requirements for high-performance medium-rank coal-based carbon microcrystals. Summary of the Invention

[0006] This invention addresses the technical bottleneck of poor sodium / potassium storage capacity in medium-rank coal-based carbon materials prepared by traditional direct carbonization or seed crystal addition. It aims to suppress the long-range microcrystal formation during the high-temperature carbonization process of medium-rank coal while introducing functional active groups, providing a pre-doped method for modulating medium-rank coal-based carbon microcrystals. This method aims to synergistically optimize the microcrystalline framework and functional active structure of medium-rank coal-based hard carbon, thereby improving sodium / potassium storage capacity. It pre-introduces organic heterocyclic structures corresponding to adjacent / similar elements (nitrogen, oxygen, phosphorus, sulfur) of carbon, and then uses electrophilic substitution and hydrogen bonding self-assembly to form a cross-linked structure with good thermal stability with the medium-rank coal structure. This regulates the distribution of amorphous and crystalline carbon in the coal during high-temperature carbonization, and introduces functional active groups in situ into the bulk phase, thereby obtaining hard carbon materials with short-range ordered graphite-like crystal structures and directional functional modification, achieving synergistic regulation of the microcrystalline structure and functional active structure.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for modulating pre-doped medium-rank coal-based carbon microcrystals, using medium-rank coal as raw material, involves intervening in the coal-based microcrystal growth process through a pre-crosslinking-post-carbonization process to simultaneously introduce functional groups, thereby preparing hard carbon materials that can be used as anode materials for sodium / potassium secondary ion batteries. The method specifically includes the following steps:

[0009] Step 1, Grinding and Screening: The raw coal is crushed and screened to obtain powder of the target particle size;

[0010] In this step, the raw coal is of medium metamorphic grade, i.e., the maximum reflectance R of the vitrinite group. maxIt is a mixture of one or more types of gas-rich coal, coking coal and bituminous coal, with a content of 1.2 to 2.0%. Within the cost limits, in order to ensure the depth and uniformity of crosslinking modification and to facilitate the production of carbon electrodes after the carbonization step, the target particle size is 160 to 400 mesh.

[0011] This step also includes an acid washing step: at the end of the sieving process, hydrochloric acid, water, hydrofluoric acid, and water are used sequentially for washing and drying. The concentration of hydrochloric acid is 2~5M, the concentration of hydrofluoric acid is 5~10wt%, and the ratio of acid volume to powder mass is 20~40:1. The water is distilled water or deionized water with a resistivity of not less than 10MΩ·cm. The final effect of water washing is that the supernatant of the solution is neutral or weakly acidic.

[0012] Step 2, Pre-crosslinking: For oxygen modification, the powder obtained in Step 1 is mixed with an oxidant at a mass ratio of 1:4 to 1:20, and doping modification is carried out under solvothermal conditions to obtain pre-crosslinked powder; for nitrogen and sulfur modification, the powder obtained in Step 1 is mixed with an organic heterocyclic structure at a mass ratio of 1:10 to 1:0.1, and doping modification is carried out under solvothermal conditions to obtain pre-crosslinked powder;

[0013] In this step, the ratio of solvent volume to coal powder mass is 6~40, the final heat treatment temperature of solvothermal treatment is 60~260℃, and the holding time is 2~12h;

[0014] In this step, the oxidant is one or more of furans, pyrans, hydrogen peroxide, peracetic acid, nitric acid, sulfuric acid, potassium permanganate, and alkali metal hydroxides, preferably one or more of alkali metal hydroxides, hydrogen peroxide, and peracetic acid; the organic heterocyclic structure is one or more of pyrroles and polypyrroles, pyridines, pyrimidines, imidazoles, pyrazoles, melamine, quinolines, purines, thiophenes, and thiazoles, preferably pyrroles;

[0015] In this step, for organic heterocyclic structures, an oxidant with a mass ratio of 0.1:1 to 2:1 can be added simultaneously to enhance the cross-linking effect with coal-based precursor molecules.

[0016] Step 3, carbonization: Under an inert atmosphere, the pre-crosslinked powder obtained in step 2 is heated to 800~1800℃ at a heating rate of 2~20℃ / min and held for 0.5~10h to obtain coal-based modified hard carbon material with medium metamorphism.

[0017] In this step, the inert gas is one or more of nitrogen and argon.

[0018] Compared with existing methods for preparing coal-based carbon materials, this invention has the following advantages:

[0019] (1) The medium-rank coal-based hard carbon obtained by pre-crosslinking modification in this invention has both high reversible capacity and high first coulombic efficiency in sodium ion storage and transportation compared with carbon materials obtained by direct carbonization process, and has important application prospects. Specifically, compared with the traditional preparation of coal-based porous carbon materials by direct carbonization, a pre-crosslinking step is added before the traditional carbonization step. By forming crosslinking bonds in the coal char structure, the long-range formation of graphite-like crystals during high-temperature carbonization is suppressed, and the distribution of amorphous carbon and crystalline carbon in the carbon structure is controlled, so that the prepared medium-rank coal-based hard carbon material has a short-range ordered graphite-like crystal structure. At the same time, the pre-crosslinking structure acts as an in-situ doping source, uniformly introducing target functional active groups, controlling the interface characteristics of amorphous carbon and crystalline carbon, and playing a role in guiding the directional transport and storage of ions. This can synergistically improve the low reversible capacity and low first coulombic efficiency of medium-rank coal-based hard carbon anode materials.

[0020] (2) The medium-grade coal-based carbon anode prepared by pre-crosslinking to inhibit the growth of coal-based microcrystals in this invention has a significantly improved reversible capacity compared with direct carbonization materials, and the initial coulombic efficiency is still maintained at a high level of 75-85%. Attached Figure Description

[0021] Figure 1 The image shows the XRD pattern of the moderately metamorphic coal-based hard carbon material from Comparative Example 1, which underwent carbonization at 1400℃.

[0022] Figure 2 The rate performance of the medium-grade coal-based hard carbon material carbonized at 1400℃ in Comparative Example 1 is shown.

[0023] Figure 3 The image shows the XRD pattern of medium-grade coal-based porous carbon that underwent liquid-phase oxygen crosslinking and 1200℃ carbonization treatment in Example 1.

[0024] Figure 4 This is a rate performance diagram of medium-grade coal-based porous carbon subjected to liquid-phase oxygen crosslinking and 1200℃ carbonization treatment in Example 1. Detailed Implementation

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0026] Comparative Example 1:

[0027] In this comparative example, the medium-metamorphic coal-based hard carbon material was prepared according to the following steps:

[0028] Step 1, Grinding and Screening: The raw coal of Jixi bituminous coal is crushed and screened to obtain powder that passes through a 160-mesh sieve.

[0029] Step 2, pickling: The powder obtained in Step 1 is pickled with 4M hydrochloric acid, washed with water, pickled with 10% hydrofluoric acid, washed with water, and dried.

[0030] Step 3, carbonization: Under the protection of argon atmosphere, the pickling product obtained in step 2 is heated to 1400℃ at a rate of 5℃ / min and held for 1h to obtain coal-based hard carbon material with medium metamorphism.

[0031] In this comparative example, the microcrystalline structure of the medium-metamorphic coal-based hard carbon material was characterized by X-ray diffraction (XRD). Figure 1 When hard carbon materials are used as the anode in sodium-ion batteries, the initial coulombic efficiency is 77% at 30 mA / g. Figure 2 Furthermore, its rate capability is poor, with a reversible capacity of only 88 mAh / g at 200 mA / g. Figure 3 ).

[0032] Example 1:

[0033] In this embodiment, the medium-metamorphic coal-based carbon material is prepared according to the following steps:

[0034] Step 1, Grinding and Screening: The raw coal of Jixi bituminous coal is crushed and screened to obtain powder that passes through a 160-mesh sieve.

[0035] Step 2, pickling: The powder obtained in Step 1 is pickled with 2M hydrochloric acid, washed with water, pickled with 10% hydrofluoric acid, washed with water, and dried.

[0036] Step 3, Pre-crosslinking: Add hydrogen peroxide solution, control the mass ratio of coal powder to hydrogen peroxide to be 1:10, and the ratio of solvent volume to coal powder mass to be 30, and keep at 80℃ for 6 hours to obtain the pre-crosslinked product.

[0037] Step 4, carbonization: Under nitrogen atmosphere protection, the pre-crosslinked product is heated to 1200℃ at a rate of 5℃ / min and held for 1 hour to obtain a medium-modified coal-based hard carbon material.

[0038] In this embodiment, the microcrystalline structure of the medium-metamorphic coal-based hard carbon material was characterized by X-ray diffraction (XRD). Figure 4 When medium-metamorphic coal-based hard carbon materials are used as anodes in sodium-ion batteries, the initial coulombic efficiency is 81% at 30 mA / g, and the reversible capacity is 174 mAh / g at 200 mA / g. Compared with the directly carbonized medium-metamorphic coal-based hard carbon materials prepared in Comparative Example 1, the reversible capacity at low current is basically the same, while the initial coulombic efficiency is improved; the reversible capacity at high current increases by 86 mAh / g.

[0039] Example 2:

[0040] In this embodiment, the medium-metamorphic coal-based carbon material is prepared according to the following steps:

[0041] Step 1, Grinding and Screening: The raw coal of Jixi bituminous coal is crushed and screened to obtain powder that passes through a 160-mesh sieve.

[0042] Step 2, pickling: The powder obtained in Step 1 is pickled with 2M hydrochloric acid, washed with water, pickled with 10% hydrofluoric acid, washed with water, and dried.

[0043] Step 3, Pre-crosslinking: Take the dried coal powder obtained in Step 2, mix it with the nitrogen source pyrrole at a mass ratio of 1:1, add ethanol as a solvent (the ratio of solvent volume to coal powder mass is 20), and add ammonium persulfate as an oxidant (its amount is 0.5 times that of pyrrole). React at 0~5℃ for 12h to obtain the pre-crosslinked product.

[0044] Step 4, carbonization: Under the protection of argon atmosphere, the pre-crosslinked product is heated to 1000℃ at a rate of 5℃ / min and held for 2 hours to obtain a medium-grade coal-based hard carbon material.

[0045] In this embodiment, the medium-metamorphic coal-based hard carbon material was pre-crosslinked with a pyrrole nitrogen source, achieving effective nitrogen doping and microcrystalline structure modulation. When used as the anode of a sodium-ion battery, it exhibited an initial coulombic efficiency of 80% at 50 mA / g and a reversible capacity of 165 mAh / g at 500 mA / g, demonstrating a significant improvement in rate performance compared to Comparative Example 1.

[0046] Example 3:

[0047] In this embodiment, the medium-metamorphic coal-based carbon material is prepared according to the following steps:

[0048] Step 1, Grinding and Screening: The raw coal of Jixi bituminous coal is crushed and screened to obtain powder that passes through a 160-mesh sieve.

[0049] Step 2, pickling: The powder obtained in Step 1 is pickled with 2M hydrochloric acid, washed with water, pickled with 10% hydrofluoric acid, washed with water, and dried.

[0050] Step 3, Pre-crosslinking: Take the dried coal powder obtained in Step 2, mix it with nitrogen source polypyrrole at a mass ratio of 1:0.5, add N-methylpyrrolidone as solvent (the ratio of solvent volume to coal powder mass is 25), and keep it at 120℃ for 6 hours to obtain the pre-crosslinked product.

[0051] Step 4, carbonization: Under the protection of argon atmosphere, the pre-crosslinked product is heated to 1000℃ at a rate of 5℃ / min and held for 2 hours to obtain a medium-grade coal-based hard carbon material.

[0052] In this embodiment, efficient nitrogen doping and conductive network construction were achieved through polypyrrole pre-crosslinking. When used as a sodium-ion battery anode, it exhibits an initial coulombic efficiency of 79% at 50 mA / g and retains a reversible capacity of 135 mAh / g at 1000 mA / g, demonstrating excellent high-rate performance.

[0053] Example 4:

[0054] This embodiment provides a method for modulating coal-based carbon microcrystals by pre-crosslinking thiophene and combining it with high-temperature carbonization, which is carried out according to the following steps:

[0055] Step 1, Grinding and Screening: The Datong bituminous coal is crushed and screened to obtain powder that passes through a 160-mesh sieve.

[0056] Step 2, pickling: The powder obtained in Step 1 is pickled with 2M hydrochloric acid, washed with water, pickled with 10% hydrofluoric acid, washed with water, and dried.

[0057] Step 3, Pre-crosslinking: Take the dried coal powder obtained in Step 2 and mix it with the sulfur source thiophene at a mass ratio of 1:0.5. Add toluene as a solvent (the solvent volume to coal powder mass ratio is 20 mL / g). Keep the mixture at a solvothermal temperature of 180℃ for 4 h. After the reaction is complete, filter, wash and dry to obtain the pre-crosslinked product.

[0058] Step 4, carbonization: Under nitrogen atmosphere protection, the pre-crosslinked product is slowly heated to 1300℃ at a heating rate of 2℃ / min and held at that temperature for 2 h. After natural cooling, the sulfur-doped medium metamorphic coal-based hard carbon material of Example 3 is obtained.

[0059] Performance Characterization: This embodiment utilizes a cross-linked structure between a sulfur source and a coal-based precursor. During subsequent high-temperature carbonization, sulfur atoms escape and form closed pores within the carbon structure, resulting in a highly disordered carbon structure. When used as a negative electrode in a sodium-ion battery, it achieves a reversible capacity of 307 mAh / g at a current density of 100 mA / g.

[0060] The embodiments of this invention achieve precise control and functional design of the microcrystalline structure of coal-based hard carbon by pre-crosslinking coal with different pathways. The core mechanism lies in the crosslinking reaction between oxidants, heteroatom compounds, and coal molecules to construct a stable three-dimensional network structure. This effectively suppresses excessive growth and ordered arrangement of microcrystals during carbonization, simultaneously achieving heteroatom (nitrogen, sulfur) doping and the construction of unique pore structures (such as closed pores). Oxidation pretreatment (such as hydrogen peroxide) breaks some chemical bonds, paving the way for subsequent doping; nitrogen source (such as pyrrole, polypyrrole) doping mainly improves the conductivity of the material and creates more defect sites; sulfur source (such as thiophene) crosslinking and high-temperature treatment generate closed pores through escape from the carbon layer, providing additional sodium storage space. The synergistic evolution of these microstructures jointly promotes the improvement of sodium ion diffusion kinetics and the formation of a stable electrode-electrolyte interface, ultimately enabling the material to exhibit high initial efficiency, high reversible capacity, and excellent rate performance.

Claims

1. A method for modulating pre-doped, moderately metamorphic coal-based carbon microcrystals, characterized in that... The method includes the following steps: Step 1, Grinding and Screening: The raw coal is crushed and screened to obtain powder of the target particle size; Step 2, Pre-crosslinking: For oxygen modification, the powder obtained in Step 1 is mixed with an oxidant at a mass ratio of 1:4 to 1:20, and doping modification is carried out under solvothermal conditions to obtain pre-crosslinked powder; for nitrogen and sulfur modification, the powder obtained in Step 1 is mixed with an organic heterocyclic structure at a mass ratio of 1:10 to 1:0.1, and doping modification is carried out under solvothermal conditions to obtain pre-crosslinked powder; Step 3, carbonization: Under an inert atmosphere, the pre-crosslinked powder obtained in step 2 is heated to 800~1800℃ and held for 0.5~10h to obtain coal-based modified hard carbon material with medium metamorphism.

2. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1, characterized in that... In step one, the raw coal is of medium metamorphic grade, i.e., the maximum reflectance R of the vitrinite group. max It is a mixture of one or more types of gas-rich coal, fat coal and coking coal, with a target particle size of 1.2 to 2.0% and a target particle size of 160 to 400 mesh.

3. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1, characterized in that... Step one also includes an acid washing step: at the end of the sieving process, hydrochloric acid, water, hydrofluoric acid, and water are used sequentially for washing and drying.

4. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 3, characterized in that... The concentration of hydrochloric acid is 2-5 M, the concentration of hydrofluoric acid is 5-10 wt%, and the ratio of acid volume to powder mass is 20-40:1; the water is distilled water or deionized water with a resistivity of not less than 10 MΩ·cm, and the final effect of water washing is that the supernatant of the solution is neutral or weakly acidic.

5. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1, characterized in that... In step two, the ratio of solvent volume to coal powder mass is 6 to 40, the final heat treatment temperature of solvothermal treatment is 60 to 260°C, and the holding time is 2 to 12 hours.

6. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1, characterized in that... In step two, for organic heterocyclic structures, an oxidant with a mass ratio of 0.1:1 to 2:1 is added to enhance the cross-linking effect with coal-based precursor molecules.

7. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1 or 6, characterized in that... The oxidant is one or more of furans, pyrans, hydrogen peroxide, peracetic acid, nitric acid, sulfuric acid, potassium permanganate, and alkali metal hydroxides, and the organic heterocyclic structure is one or more of pyrroles, polypyrroles, pyridines, pyrimidines, imidazoles, pyrazoles, melamine, quinolines, purines, thiophenes, and thiazoles.

8. The method for pre-doped medium-rank coal-based carbon microcrystal modulation according to claim 1, characterized in that... In step three, the heating rate is 2~20℃ / min; the inert gas is one or more of nitrogen and argon.

9. A medium-metamorphic coal-based modified hard carbon material prepared by the method of any one of claims 1-8.

10. The application of a moderately metamorphic coal-based modified hard carbon material prepared by the method of any one of claims 1-8 in the anode material of sodium / potassium secondary ion batteries.

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