Coal-based composite hard carbon material, and preparation method and application thereof

Abstract

The present application belongs to the technical field of carbon materials, and particularly relates to a coal-based composite hard carbon material, a preparation method thereof and application thereof. The coal-based composite hard carbon material has a core-shell double-layer structure: the inner core is coal-based carbon after Fenton oxidation activation and acid pickling, the outer shell is a pitch carbon layer, and graphene quantum dots are dispersed in the inner core. The preparation method comprises Fenton oxidation activation treatment, acid pickling impurity removal, graphene quantum dot assisted granulation, pitch coating and carbonization. The application of the coal-based composite hard carbon material in the preparation of a sodium ion battery negative electrode is also disclosed. The present application discloses a coal-based composite hard carbon material, a preparation method thereof and application thereof, and aims to solve the technical problems of high impurity content, poor electronic conductivity, poor rate performance and easy agglomeration of coal powder in the existing coal-based hard carbon material, and provides a coal-based composite hard carbon material with low ash content, high conductivity, high rate performance and excellent cycle stability, and a preparation method thereof.

Description

Technical Field

[0001] This invention belongs to the field of carbon materials technology, specifically relating to a coal-based composite hard carbon material and its preparation method and application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance some understanding of the overall background of this disclosure and is not necessarily to be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art.

[0003] Hard carbon materials are considered the most promising anode materials for sodium-ion batteries due to their large interlayer spacing, rich pore structure, and low cost. Coal resources are abundant, have high carbon content, and well-developed aromatic ring structures, making them ideal raw materials for preparing hard carbon precursors. However, directly carbonizing coal to obtain hard carbon materials presents the following technical challenges: (1) The high content of ash (mainly silicon and aluminum oxides), sulfur and other impurities in coal leads to serious electrochemical side reactions and low initial coulombic efficiency. (2) Coal has poor microstructure order, poor electronic conductivity after carbonization, and poor rate performance; (3) Nanoparticles of coal powder tend to agglomerate during the electrode preparation process, making it difficult to form a uniform electrode coating.

[0004] In existing technologies, strong acid / base chemical activation methods are commonly used to pretreat coal to create pores and remove impurities. However, this method is highly corrosive, difficult to treat wastewater, and requires sophisticated equipment. There are also reports of using polymers or asphalt for coating modification, but single coating layers are prone to microcracks during high-temperature carbonization, and the improvement in conductivity after polymer carbonization is limited. Therefore, there is an urgent need to develop a green and efficient method for coal activation and structure control, simultaneously addressing the three major issues of purity, conductivity, and dispersibility. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention discloses a coal-based composite hard carbon material, its preparation method, and its application. It aims to solve the technical problems of high impurity content, poor electronic conductivity, poor rate performance, and easy agglomeration of coal powder in existing coal-based hard carbon materials, and to provide a coal-based composite hard carbon material with low ash content, high conductivity, high rate performance, and excellent cycle stability, as well as its preparation method.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A coal-based composite hard carbon material has a core-shell double-layer structure: the core is coal-based carbon activated by Fenton oxidation and acid washing, the outer shell is a pitch carbon layer, and graphene quantum dots are dispersed in the core.

[0007] A method for preparing a coal-based composite hard carbon material includes the following steps: (1) Fenton oxidation activation treatment: Coal powder is subjected to Fenton oxidation activation treatment to obtain activated coal powder; (2) Acid washing to remove impurities: The activated coal powder is acid washed to remove impurities, and purified coal powder is obtained; (3) Graphene oxide quantum dot-assisted granulation: Purified coal powder is mixed with graphene oxide quantum dots and spray-dried to obtain precursor microspheres for primary granulation; (4) Asphalt coating and carbonization: The precursor microspheres are mixed with asphalt and coated with high temperature carbonization at 1200-1500℃ under an inert atmosphere to obtain coal-based composite hard carbon material.

[0008] Preferably, in step (1), the Fenton oxidation activation conditions are: pH=3-4, the mass ratio of hydrogen peroxide to coal powder is 0.5:1 to 5:1, the molar ratio of ferrous ions to hydrogen peroxide is 0.01:1 to 0.2:1, the reaction temperature is 10-40℃, and the reaction time is 2-5 hours.

[0009] Preferably, in step (2), the pickling includes washing with hydrochloric acid and hydrofluoric acid in sequence.

[0010] Preferably, in step (3), the average particle size of the purified coal powder is 100-300 nanometers, the oxygen content of the graphene oxide quantum dots is 30%-55%, the amount of graphene oxide quantum dots added is 1%-20% of the mass of the coal powder, and the average particle size of the microspheres after spray drying is 6-10 micrometers.

[0011] Preferably, in step (4), the high-temperature carbonization temperature is 1300-1500℃, and the amount of asphalt added is 0.5%-3% of the mass of the precursor microspheres.

[0012] Preferably, before step (1), the method further includes a step of pulverizing the raw coal into coal powder with an average particle size of 100-300 nanometers.

[0013] This invention also discloses the application of coal-based composite hard carbon materials in the preparation of sodium-ion battery anodes.

[0014] The beneficial effects of this invention, which describes a coal-based composite hard carbon material, its preparation method, and its application, are as follows: 1. Fenton Oxidation: Mild and Efficient Pore Formation and Capacity Enhancement: Compared with traditional strong acid-base activation methods, Fenton oxidation is carried out at room temperature and pressure, with mild reaction conditions and easily treatable wastewater. Its core function lies in forming nanopores on and inside the coal surface through hydroxyl radical oxidation, providing more storage sites for sodium ions. Example 3 (Fenton reaction for 5 hours) achieved a specific capacity of 361 mAh / g, a 33% increase compared to Comparative Example 1 (without Fenton oxidation, 271 mAh / g), confirming the significant contribution of Fenton oxidation to capacity formation. Simultaneously, Fenton oxidation oxidizes organic impurities into soluble species, which is beneficial for subsequent deep impurity removal through acid washing, resulting in purified coal powder with an ash content below 0.3%.

[0015] 2. In-situ Construction of Conductive Network by Graphene Quantum Dots Enhances Rate Performance: GOQDs are reduced to GQDs through high-temperature carbonization. Their sp² crystalline carbon structure is uniformly embedded in the interlayer of coal-based carbon, forming a three-dimensional conductive pathway. Example 1 achieved a 73% rate retention rate at 5C, while Comparative Example 2 (without GOQDs) only achieved 48%, representing a 52% improvement. This confirms the decisive role of the GQDs conductive network in rate performance. The nanoscale size and in-situ formation characteristics of GQDs enable atomic-level contact with the coal-based carbon matrix, resulting in a significantly improved conductivity compared to traditional conductive carbon black addition methods.

[0016] 3. Dual-functional integration of graphene oxide quantum dots (GOQDs): GOQDs act as both dispersants and binders, eliminating the need for additional polymers or surfactants and simplifying formulation. During spray granulation, they assemble nano-coal powder into spherical micron-sized particles, whose spherical morphology is beneficial for electrode coating and ion transport.

[0017] 4. Highly efficient coating with extremely low asphalt content: Because GOQDs act as binders and granulators, only 0.5%-3% asphalt is needed to form a dense coating layer, far lower than the 10%-40% asphalt content required for conventional coatings. Low asphalt content means less volume expansion and a more stable interface structure, while also reducing costs.

[0018] 5. Excellent comprehensive electrochemical performance: The composite hard carbon material obtained by this invention has high specific capacity (up to 366 mAh / g), high initial coulombic efficiency (up to 90.5%) and excellent rate performance (5C capacity retention up to 76%). Attached Figure Description

[0019] Figure 1 : Process flow diagram of this invention.

[0020] Figure 2 Scanning electron microscope (SEM) images of nano-coal powder before step (1) of this invention (particle size 100-300 nm).

[0021] Figure 3SEM image of the precursor microspheres obtained in step (3) of this invention (spherical, particle size 6-10 μm).

[0022] Figure 4 Comparison of rate performance of Example 1 of the present invention with Comparative Examples 1-3 (capacity retention at 0.1C, 0.5C, 1C, 2C, and 5C).

[0023] Figure 5 Fenton reaction time versus specific capacity curve.

[0024] Figure 6 : Curve showing the relationship between the amount of GOQDs added and the retention rate of 5C multiplier. Detailed Implementation

[0025] The following description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0026] This invention provides the following technical solution: A coal-based composite hard carbon material has a core-shell double-layer structure: the core is coal-based carbon activated by Fenton oxidation and acid washing, the outer shell is a pitch carbon layer, and graphene quantum dots are dispersed in the core.

[0027] A method for preparing a coal-based composite hard carbon material includes the following steps: (1) Fenton oxidation activation treatment: Coal powder is subjected to Fenton oxidation activation treatment to obtain activated coal powder; (2) Acid washing to remove impurities: The activated coal powder is acid washed to remove impurities, and purified coal powder is obtained; (3) Graphene oxide quantum dot-assisted granulation: Purified coal powder is mixed with graphene oxide quantum dots and spray-dried to obtain precursor microspheres for primary granulation; (4) Asphalt coating and carbonization: The precursor microspheres are mixed with asphalt and coated with high temperature carbonization at 1200-1500℃ under an inert atmosphere to obtain coal-based composite hard carbon material.

[0028] Specifically, in step (1), the Fenton oxidation activation conditions are: pH=3-4, the mass ratio of hydrogen peroxide to coal powder is 0.5:1 to 5:1, the molar ratio of ferrous ions to hydrogen peroxide is 0.01:1 to 0.2:1, the reaction temperature is 10-40℃, and the reaction time is 2-5 hours.

[0029] In step (2), pickling includes washing with hydrochloric acid and hydrofluoric acid in sequence.

[0030] In step (3), the average particle size of the purified coal powder is 100-300 nanometers, the oxygen content of the graphene oxide quantum dots is 30%-55%, the amount of graphene oxide quantum dots added is 1%-20% of the mass of the coal powder, and the average particle size of the microspheres after spray drying is 6-10 micrometers.

[0031] In step (4), the high-temperature carbonization temperature is 1300-1500℃, and the amount of asphalt added is 0.5%-3% of the mass of the precursor microspheres.

[0032] Before step (1), the method further includes pulverizing the raw coal into coal powder with an average particle size of 100-300 nanometers.

[0033] This invention also discloses the application of coal-based composite hard carbon materials in the preparation of sodium-ion battery anodes.

[0034] This invention achieves a dual effect through Fenton oxidation, with the main objective of improving the specific capacity of the material: On the one hand, the hydroxyl radicals (·OH) generated by the Fenton reaction oxidize the aromatic ring structure in coal, forming nanopores on and inside the coal surface, providing more active sites for sodium ion storage, thereby significantly increasing the specific capacity of the battery; on the other hand, Fenton oxidation oxidizes impurities such as organic sulfur and organometallic complexes in coal into soluble species, while destroying the encapsulation structure of inorganic minerals in coal, which is beneficial for subsequent acid washing to remove ash, obtain high-purity coal-based carbon precursors, and reduce the negative impact of impurities on capacity.

[0035] Furthermore, this invention introduces graphene oxide quantum dots (GOQDs) to achieve a dual function, primarily aimed at improving the rate performance of the material: On the one hand, the surface of GOQDs is rich in oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy groups, which can act as a highly efficient dispersant in aqueous systems, ensuring uniform dispersion of purified coal powder without agglomeration; simultaneously, these functional groups form hydrogen bonds with the surface of the coal powder, acting as a binder during spray drying, assembling nano-coal powder into micron-sized spherical particles; on the other hand, the carbon atoms of GOQDs exist in the form of sp² hybridized crystalline carbon, uniformly interspersed between coal layers, and after high-temperature carbonization, they are reduced to graphene quantum dots (GQDs). Their highly crystalline carbon structure forms a three-dimensional conductive network in the coal-based hard carbon, significantly improving the electronic conductivity of the composite material, thereby greatly enhancing the rate performance.

[0036] In summary, this invention achieves a dual improvement in the specific capacity and rate performance of coal-based hard carbon materials through a synergistic strategy of "Fenton oxidation for pore formation and capacity enhancement + graphene quantum dots for constructing a conductive network to improve rate performance".

[0037] The following provides examples illustrating the implementation of the above technical solutions.

[0038] Example 1 (Standard Scheme): Anthracite was coarsely crushed and wet ball-milled to an average particle size of 200 nm (D50=205 nm, D90=280 nm).

[0039] Add 100g of coal powder to 1L of water, and while stirring, add 30g of 30% H2O2 and 5g of FeSO4·7H2O. Adjust the pH to 3.5 with dilute H2SO4 and react at 25℃ for 3 hours. Filter, wash with water until neutral, and dry at 80℃.

[0040] The activated coal powder was sequentially acid-washed with 1M HCl and 1M HF (solid-liquid ratio 1:10) at 60℃ for 2 hours each, filtered, washed with water until pH>5, and dried to obtain purified coal powder (ash content 0.25%, specific surface area increased by 3.2 times compared with raw coal).

[0041] Take 80g of purified coal powder and disperse it in 200g of deionized water. Add 4g of graphene oxide quantum dots (oxygen content 42%, particle size 2-5nm), ultrasonically disperse for 30 minutes, stir for 1 hour, and spray dry (inlet air temperature 180℃, outlet air temperature 90℃, atomization pressure 0.2MPa) to obtain spherical microspheres with D50≈8μm.

[0042] The microspheres were mixed with 1.2g of coal tar pitch (softening point 120℃, added at 1.5% of the precursor mass), placed in a tube furnace, heated to 1400℃ at 5℃ / min under Ar atmosphere, held for 3 hours, and then naturally cooled to obtain the composite hard carbon material.

[0043] Electrochemical testing: As a sodium-ion battery anode, the initial reversible specific capacity at 0.1C is 352 mAh / g, with an initial coulombic efficiency of 89.2%; the capacity retention rate at 5C is 73% of that at 0.1C; and the capacity retention rate after 500 cycles is 94%.

[0044] Example 2 (GOQDs Dosage Optimization - High Addition Amount): It is basically the same as Example 1, except that the amount of GOQDs added in step 4 is increased to 16g (20% of the coal powder mass).

[0045] Electrochemical test results: initial reversible specific capacity 341 mAh / g, initial coulombic efficiency 88.1%; 5C capacity retention 75%.

[0046] Note: Higher GOQDs addition creates a denser conductive network, further improving rate performance to 75%, but with a slight decrease in capacity.

[0047] Example 3 (Optimization of Fenton Reaction Time - Long Reaction Time): It is basically the same as Example 1, except that the Fenton reaction time in step 2 is extended to 5 hours.

[0048] Electrochemical test results: initial reversible specific capacity 361 mAh / g, initial coulombic efficiency 87.6%; 5C capacity retention 70%.

[0049] Note: Extending the Fenton reaction time increases the degree of pore formation, and the specific capacity reaches 361 mAh / g (2.6% higher than in Example 1), confirming the positive effect of Fenton oxidation pore formation on capacity.

[0050] Example 4 (Optimization of Asphalt Dosage - High Dosage): It is basically the same as Example 1, except that the amount of asphalt added in step 5 is increased to 2.4g (3% of the precursor mass).

[0051] Electrochemical test results: initial reversible specific capacity 338 mAh / g, initial coulombic efficiency 90.5%; 5C capacity retention 71%.

[0052] Example 5 (Carbonization Temperature Optimization - High Temperature): It is basically the same as Example 1, except that the carbonization temperature in step 5 is increased to 1500°C.

[0053] Electrochemical test results: initial reversible specific capacity 331 mAh / g, initial coulombic efficiency 89.8%; 5C capacity retention 68%.

[0054] Example 6 (Coal Type Variation - Lignite): It is basically the same as Example 1, except that anthracite is replaced with lignite.

[0055] Electrochemical test results: initial reversible specific capacity 340 mAh / g, initial coulombic efficiency 87.1%; 5C capacity retention 71%.

[0056] Example 7 (GOQDs Oxygen Content Optimization - Low Oxygen Content): It is basically the same as Example 1, except that the oxygen content of the graphene oxide quantum dots used in step 4 is 32% (instead of 42%).

[0057] Electrochemical test results: initial reversible specific capacity 345 mAh / g, initial coulombic efficiency 88.6%; 5C capacity retention 71%.

[0058] Example 8 (GOQDs Oxygen Content Optimization - High Oxygen Content): It is basically the same as Example 1, except that the oxygen content of the graphene oxide quantum dots used in step 4 is 53% (instead of 42%).

[0059] Electrochemical test results: initial reversible specific capacity 358 mAh / g, initial coulombic efficiency 89.5%; 5C capacity retention 74%.

[0060] Example 9 (Optimized hydrogen peroxide dosage - high proportion): The method is basically the same as in Example 1, except that the amount of hydrogen peroxide used in step 2 is increased to 80g (hydrogen peroxide:coal powder mass ratio = 0.8:1).

[0061] Electrochemical test results: initial reversible specific capacity 366 mAh / g, initial coulombic efficiency 86.8%; 5C capacity retention 69%.

[0062] Note: Higher hydrogen peroxide dosage increased the degree of pore formation, and the capacity reached the highest value of 366 mAh / g among all examples, confirming the key role of Fenton oxidation pore formation in capacity.

[0063] Example 10 (Optimization of Fenton Reaction Temperature): It is basically the same as Example 1, except that the Fenton reaction temperature is increased to 35°C (instead of 25°C) in step 2.

[0064] Electrochemical test results: initial reversible specific capacity 349 mAh / g, initial coulombic efficiency 88.3%; 5C capacity retention 72%.

[0065] Example 11 (Optimization of Asphalt Dosage - Extremely Low Dosage): It is basically the same as Example 1, except that the amount of asphalt added in step 5 is reduced to 0.4g (0.5% of the precursor mass).

[0066] Electrochemical test results: initial reversible specific capacity 355 mAh / g, initial coulombic efficiency 87.5%; 5C capacity retention 70%.

[0067] Example 12 (Optimization of spray drying parameters): It is basically the same as Example 1, except that in step 4 the spray drying inlet air temperature is adjusted to 200°C and the outlet air temperature is 105°C.

[0068] Electrochemical test results: initial reversible specific capacity 348 mAh / g, initial coulombic efficiency 88.9%; 5C capacity retention 73%.

[0069] Comparative Example 1 (Fenton oxidation omitted): The raw coal was directly acid washed (same as step 3 in Example 1), GOQDs-assisted granulation was performed, and asphalt coating and carbonization were carried out. The rest was the same as in Example 1.

[0070] Electrochemical test results: initial reversible specific capacity 271 mAh / g, initial coulombic efficiency 82.3%; 5C capacity retention 52%.

[0071] Note: Without Fenton oxidation, impurities in the coal were not completely removed (ash content 1.2%), and the specific capacity was reduced by 23% compared to Example 1, confirming that Fenton oxidation pore formation is the core contribution to capacity.

[0072] Comparative Example 2 (graphene oxide quantum dots omitted): After Fenton oxidation and acid washing, without adding GOQDs, the purified coal powder is directly spray-dried (it cannot form regular spherical shapes, but irregular agglomerates), and then mixed with asphalt for carbonization. The rest is the same as in Example 1.

[0073] Electrochemical test results: initial reversible specific capacity 302 mAh / g, initial coulombic efficiency 80.5%; 5C capacity retention 48%.

[0074] Note: Without GOQDs, the lack of GQDs conductive network resulted in a 34% decrease in 5C rate retention compared to Example 1, confirming the core contribution of the GQDs conductive network to rate performance.

[0075] Comparative Example 3 (asphalt coating omitted): After Fenton oxidation, pickling, and GOQDs-assisted granulation, it is directly carbonized (1400℃) without adding asphalt.

[0076] Electrochemical test results: initial reversible specific capacity 318 mAh / g, initial coulombic efficiency 81.2%; 5C capacity retention 65%.

[0077] Comparative Example 4 (pH deviation during Fenton oxidation): It is basically the same as Example 1, except that the pH is adjusted to 6.0 in step 2.

[0078] Electrochemical test results: initial reversible specific capacity 283 mAh / g, initial coulombic efficiency 80.8%; 5C capacity retention 54%.

[0079] Comparative Example 5 (without hydrofluoric acid pickling): It is basically the same as Example 1, except that in step 3 only HCl is used for acid washing, and HF is not used.

[0080] Electrochemical test results: initial reversible specific capacity 295 mAh / g, initial coulombic efficiency 83.5%; 5C capacity retention 58%.

[0081] Comparative Example 6 (coal powder particle size too large): It is basically the same as Example 1, except that in step 1 the coal powder is only pulverized to D50=2μm (2000nm).

[0082] Electrochemical test results: initial reversible specific capacity 305 mAh / g, initial coulombic efficiency 85.1%; 5C capacity retention 55%.

[0083] Comparative Example 7 (pickling step omitted): After Fenton oxidation, hydrochloric acid and hydrofluoric acid pickling are not performed. GOQDs-assisted granulation and asphalt coating carbonization are carried out directly, and the rest is the same as in Example 1.

[0084] Electrochemical test results: initial reversible specific capacity 248 mAh / g, initial coulombic efficiency 75.6%; 5C capacity retention 42%.

[0085] Comparative Example 8 (using ordinary graphene oxide instead of GOQDs): The method is basically the same as in Example 1, except that in step 4, ordinary graphene oxide (GO, with a sheet diameter of 200-500 nm) is used instead of graphene oxide quantum dots, and the amount added is the same.

[0086] Electrochemical test results: initial reversible specific capacity 298 mAh / g, initial coulombic efficiency 83.1%; 5C capacity retention 51%.

[0087] Comparative Example 9 (excessive asphalt addition): It is basically the same as Example 1, except that the amount of asphalt added in step 5 is increased to 8g (10% of the precursor mass).

[0088] Electrochemical test results: initial reversible specific capacity 286 mAh / g, initial coulombic efficiency 86.2%; 5C capacity retention 45%.

[0089] Comparative Example 10 (without spray granulation): After undergoing Fenton oxidation, pickling, and GOQDs mixing, the slurry is directly filtered and dried to obtain powder without spray drying, and then mixed with asphalt and carbonized. The rest is the same as in Example 1.

[0090] Electrochemical test results: initial reversible specific capacity 288 mAh / g, initial coulombic efficiency 79.8%; 5C capacity retention 46%.

[0091] Comparative Example 11 (coal powder particle size too small): It is basically the same as Example 1, except that in step 1, the coal powder is pulverized to an average particle size of 50 nm.

[0092] Electrochemical test results: initial reversible specific capacity 312 mAh / g, initial coulombic efficiency 82.4%; 5C capacity retention 56%.

[0093] Comparative Example 12 (Carbonization temperature too low): It is basically the same as Example 1, except that the carbonization temperature in step 5 is reduced to 1100°C.

[0094] Electrochemical test results: initial reversible specific capacity 269 mAh / g, initial coulombic efficiency 78.3%; 5C capacity retention 47%.

[0095] Table 1. Performance comparison between the example and comparative products:

[0096] Data Analysis: 1. Verification of the pore-forming and volume-enhancing effect of Fenton oxidation: Comparative Example 1: With Fenton oxidation vs. without Fenton oxidation; Comparison: Example 1 (352 mAh / g) vs. Comparative Example 1 (271 mAh / g); Analysis results: After Fenton oxidation activation treatment, the specific capacity of the material increased from 271 mAh / g to 352 mAh / g, an increase of 30%; the first coulombic efficiency increased from 82.3% to 89.2%, an increase of 6.9 percentage points; and the 5C rate retention increased from 52% to 73%, an increase of 21 percentage points.

[0097] Mechanism Explanation: The hydroxyl radicals (·OH) generated by the Fenton reaction form nanopores on and inside the coal surface, providing more storage sites for sodium ions. Simultaneously, it oxidizes organic impurities into soluble species, which is beneficial for subsequent deep impurity removal through acid washing. Without Fenton oxidation, impurities in the coal are not completely removed (ash content 1.2%), and the lack of nanopore structure results in a significant decrease in both capacity and rate performance.

[0098] 2. Verification of the rate-increasing effect of graphene quantum dot conductive network: Comparative Example 2: With GOQDs vs. Without GOQDs; Comparison: Example 1 (5C retention rate 73%) vs. Comparative Example 2 (5C retention rate 48%); Analysis results: After adding graphene oxide quantum dots, the 5C rate retention rate increased significantly from 48% to 73%, an improvement of 52%. Specific capacity increased from 302 mAh / g to 352 mAh / g, an improvement of 50 mAh / g. First-time coulombic efficiency increased from 80.5% to 89.2%, an improvement of 8.7 percentage points.

[0099] Mechanism Explanation: GOQDs are reduced to GQDs through high-temperature carbonization. The sp² crystalline carbon structure of GQDs is uniformly embedded in the interlayer of coal-based carbon, forming a three-dimensional conductive network that significantly improves electronic conductivity. Without GOQDs, the material lacks a conductive network, resulting in discontinuous electron transport paths and severely degraded rate performance.

[0100] 3. Verification of the effect of asphalt coating: Comparative Example 3: With asphalt coating vs. without asphalt coating; Comparison: Example 1 (first-efficacy 89.2%) vs. Comparative Example 3 (first-efficacy 81.2%). Analysis conclusions: After adding asphalt for secondary coating, the initial coulombic efficiency increased from 81.2% to 89.2%, an increase of 8 percentage points; the specific capacity increased from 318 mAh / g to 352 mAh / g, an increase of 34 mAh / g; and the 5C rate retention rate increased from 65% to 73%, an increase of 8 percentage points.

[0101] Mechanism Explanation: Carbonization of asphalt forms a dense coating layer, reducing the specific surface area of ​​the material, minimizing direct contact between the electrolyte and active substances, and inhibiting irreversible side reactions. Without asphalt coating, the material surface has more defects, the electrolyte decomposes severely, and the initial efficiency and cycle stability are poor.

[0102] 4. Verification of the optimization effect of Fenton reaction conditions: Comparative Example 4: Prolonged Fenton reaction time (3h vs 5h); Comparison: Example 1 (352 mAh / g) vs Example 3 (361 mAh / g); Analysis results: The Fenton reaction time was extended from 3 hours to 5 hours, and the specific capacity increased from 352 mAh / g to 361 mAh / g, an increase of 9 mAh / g; the first coulombic efficiency decreased slightly from 89.2% to 87.6% (a decrease of 1.6 percentage points); and the 5C rate retention decreased from 73% to 70% (a decrease of 3 percentage points).

[0103] Mechanism explanation: Extending the Fenton reaction time increases the degree of oxidation, forming more nanopores, which is beneficial for capacity improvement. However, excessive oxidation may lead to a decrease in the orderliness of the carbon structure, slightly affecting first-efficiency and rate performance. The optimal reaction time is 3 hours.

[0104] Comparative Example 5: Increased hydrogen peroxide dosage (0.3:1 vs 0.8:1); Comparison: Example 1 (352 mAh / g) vs Example 9 (366 mAh / g); Analysis conclusions: Increasing the hydrogen peroxide ratio from 0.3:1 to 0.8:1 increased the specific capacity from 352 mAh / g to 366 mAh / g, reaching the highest value among all examples; the initial coulombic efficiency decreased from 89.2% to 86.8% (a decrease of 2.4 percentage points); and the 5C rate retention rate decreased from 73% to 69% (a decrease of 4 percentage points).

[0105] Mechanism explanation: Higher hydrogen peroxide dosage generates more hydroxyl radicals, increasing pore formation and further improving capacity. However, excessive oxidation introduces too many oxygen-containing functional groups, increasing residual defects after carbonization and slightly affecting initial efficiency and rate performance.

[0106] Comparative Example 6: pH deviation of the Fenton reaction (pH=3.5 vs pH=6.0); Comparison: Example 1 (352 mAh / g) vs. Comparative Example 4 (283 mAh / g); Analysis conclusions: When the pH deviated from 3.5 to 6.0, the specific capacity decreased significantly from 352 mAh / g to 283 mAh / g, a decrease of 20%; the first coulombic efficiency decreased from 89.2% to 80.8%, a decrease of 8.4 percentage points; and the 5C rate retention rate decreased from 73% to 54%, a decrease of 19 percentage points.

[0107] Mechanism explanation: The Fenton reaction yields the highest hydroxyl radicals at pH 3-4. At pH 6.0, Fe... 2+ It is easily hydrolyzed and precipitated, which significantly reduces the efficiency of the Fenton reaction and weakens the oxidative activation effect. The pH must be strictly controlled within the range of 3-4.

[0108] Comparative Example 7: Increased Fenton reaction temperature (25℃ vs 35℃); Comparison: Example 1 (352 mAh / g) vs Example 10 (349 mAh / g); Analysis conclusions: When the reaction temperature was increased from 25℃ to 35℃, the specific capacity decreased slightly from 352 mAh / g to 349 mAh / g, which was not a significant change; the initial coulombic efficiency and the 5C rate retention rate remained basically the same.

[0109] Mechanism explanation: The Fenton reaction can be carried out effectively in the range of 10-40℃, and the temperature has little effect on the performance, with a wide process window; moderately increasing the temperature can accelerate the reaction rate, but excessively high temperatures may lead to the ineffective decomposition of H2O2.

[0110] 5. Verification of the optimization effect of GOQDs related parameters: Comparative Example 8: Optimization of GOQDs addition (5% vs 20%); Comparison: Example 1 (5% addition) vs. Example 2 (20% addition); Analysis findings: Increasing the GOQDs addition from 5% to 20% improved the 5C rate retention from 73% to 75%, reaching the highest value among all examples. Specific capacity decreased from 352 mAh / g to 341 mAh / g (a decrease of 11 mAh / g). Initial coulombic efficiency decreased from 89.2% to 88.1% (a decrease of 1.1 percentage points).

[0111] Mechanism explanation: Higher GOQDs addition amounts form a denser graphene quantum dot conductive network, further improving rate performance to 75%. However, the capacity of GOQDs after carbonization is slightly lower than that of coal-based carbon; therefore, excessive addition will slightly dilute the active material, resulting in a slight decrease in specific capacity. The optimal addition amount is 5%-10%.

[0112] Comparative Example 9: Effect of GOQDs on oxygen content (32% vs 42% vs 53%): Comparison objects: Example 7 (32%), Example 1 (42%), Example 8 (53%); Analysis results: 32% oxygen content: specific capacity 345 mAh / g, 5C retention 71%; 42% oxygen content: specific capacity 352 mAh / g, 5C retention 73%; 53% oxygen content: specific capacity 358 mAh / g, 5C retention 74%. As the oxygen content of GOQDs increases, both specific capacity and rate performance show an upward trend.

[0113] Mechanism explanation: High oxygen content GOQDs have more oxygen-containing functional groups on their surface, better dispersibility in aqueous systems, and tighter bonding with the surface of coal powder. At the same time, the GQDs formed by reduction after carbonization have fewer defects, higher crystallinity, and better conductivity.

[0114] Comparative Example 10: GOQDs vs. Ordinary Graphene Oxide Comparison: Example 1 (GOQDs) vs. Comparative Example 8 (Ordinary GO); Analysis conclusions: Replacing GOQDs with ordinary GO resulted in a decrease in specific capacity from 352 mAh / g to 298 mAh / g, a drop of 54 mAh / g. The 5C rate retention rate plummeted from 73% to 51%, a decrease of 22 percentage points. The initial coulombic efficiency decreased from 89.2% to 83.1%, a drop of 6.1 percentage points.

[0115] Mechanism Explanation: Ordinary GO flakes (200-500 nm) have a diameter much larger than GOQDs (2-5 nm), making them unable to disperse uniformly within the gaps of nano-coal powder (100-300 nm), resulting in irregular particle morphology after spraying. Furthermore, the graphene sheets formed by GO reduction after carbonization exhibit severe stacking, preventing the formation of a uniform conductive network at the nanoscale. Only nanoscale GOQDs can achieve the technical effects of this invention.

[0116] 6. Verification of the effectiveness of acid pickling for impurity removal: Comparative Example 11: Pickling with HF vs. Pickling without HF; Comparison: Example 1 (HCl + HF) vs. Comparative Example 5 (HCl only); Analysis results: When only HCl was used for washing (without HF), the specific capacity decreased from 352 mAh / g to 295 mAh / g, a decrease of 57 mAh / g. The initial coulombic efficiency decreased from 89.2% to 83.5%, a decrease of 5.7 percentage points. The 5C rate retention decreased from 73% to 58%, a decrease of 15 percentage points.

[0117] Mechanism explanation: HCl primarily dissolves metal oxides, while HF specifically dissolves silicate minerals (the main component of ash in coal). Without HF, silicate impurities remain (approximately 1.5% of ash). These impurities are non-conductive and clog pores, severely affecting ion transport and electronic conductivity. Only the combined use of HCl and HF can achieve deep impurity removal.

[0118] Comparative Example 12: With pickling vs. without pickling: Comparison: Example 1 (complete pickling) vs. Comparative Example 7 (no pickling); Analysis conclusions: Completely omitting the acid washing step resulted in a sharp drop in specific capacity from 352 mAh / g to 248 mAh / g, a decrease of 30%. The initial coulombic efficiency decreased from 89.2% to 75.6%, a drop of 13.6 percentage points, the lowest among all comparative studies. The 5C rate retention decreased from 73% to 42%, a drop of 31 percentage points, also the lowest among all comparative studies.

[0119] Mechanism explanation: Without acid washing, a large amount of ash (>5%) and impurities remain in the coal. These impurities do not participate in the sodium storage reaction and will clog the pores and increase the interfacial resistance. Acid washing is the core step in impurity removal and cannot be omitted.

[0120] 7. Verification of particle size optimization effect: Comparative Example 13: Particle size 100-300nm vs 2μm; Comparison: Example 1 (200nm) vs. Comparative Example 6 (2μm); Analysis results: As the coal powder particle size increased from 200 nm to 2 μm, the specific capacity decreased from 352 mAh / g to 305 mAh / g, a drop of 47 mAh / g. The 5C rate retention rate decreased from 73% to 55%, a decrease of 18 percentage points.

[0121] Mechanism Explanation: When the coal powder particle size is too large, GOQDs cannot uniformly coat all coal powder particles, resulting in insufficiently modified areas inside the particles after spray granulation. After carbonization, the electron transport path inside large particles is long, the conductive network of GQDs is not completely covered, and the rate performance is significantly reduced.

[0122] Comparative Example 14: Particle size 100-300nm vs 50nm; Comparison: Example 1 (200nm) vs. Comparative Example 11 (50nm); Analysis results: As the coal powder particle size decreased from 200 nm to 50 nm, the specific capacity decreased from 352 mAh / g to 312 mAh / g, a decrease of 40 mAh / g. The 5C rate retention rate decreased from 73% to 56%, a decrease of 17 percentage points.

[0123] Mechanism explanation: When the particle size is too small (50nm), the specific surface area of ​​the coal powder is too large, resulting in extremely high surface energy. Even with GOQDs dispersion, irreversible agglomeration is still likely to occur. Simultaneously, excessively fine particles experience increased loss during acid washing and spray drying, making the overall performance inferior to the 100-300nm range. The optimal particle size range is 100-300nm.

[0124] 8. Verification of the effect of optimized asphalt dosage: Comparative Example 15: Asphalt 0.5% vs 1.5% vs 3% vs 10%; Comparison examples: Example 11 (0.5%), Example 1 (1.5%), Example 4 (3%), Comparative Example 9 (10%); Analysis conclusions: 0.5% asphalt content: specific capacity 355 mAh / g, initial efficiency 87.5%, 5C retention rate 70%; 1.5% asphalt content: specific capacity 352 mAh / g, initial efficiency 89.2%, 5C retention rate 73%; 3% asphalt content: specific capacity 338 mAh / g, initial efficiency 90.5%, 5C retention rate 71%; 10% asphalt content: specific capacity 286 mAh / g, initial efficiency 86.2%, 5C retention rate 45%; with increasing asphalt content, the initial efficiency first increases and then decreases (optimal at 1.5%-3%); specific capacity and rate retention rate continuously decrease, showing significant deterioration at 10%.

[0125] Mechanism Explanation: Asphalt carbonization forms a dense coating layer, and an appropriate amount (0.5%-3%) can reduce the specific surface area and improve the initial efficiency. However, when the asphalt content is too high (10%), an excessively thick and dense carbon layer is formed after carbonization, which hinders the diffusion and transport of sodium ions from the electrolyte to the internal active substances. At the same time, the excessively thick inactive carbon layer dilutes the content of active substances, resulting in a significant decrease in capacity and rate performance. This invention achieves highly efficient coating with an extremely low asphalt content of 0.5%-3%.

[0126] 9. Verification of the effect of carbonization temperature optimization: Comparative Example 16: Carbonization temperatures 1400℃ vs 1500℃ vs 1100℃; Comparative examples: Example 1 (1400℃), Example 5 (1500℃), Comparative Example 12 (1100℃); Analysis conclusions: 1100℃: specific capacity 269 mAh / g, initial efficiency 78.3%, 5C retention 47%; 1400℃: specific capacity 352 mAh / g, initial efficiency 89.2%, 5C retention 73%; 1500℃: specific capacity 331 mAh / g, initial efficiency 89.8%, 5C retention 68%; performance is significantly improved when carbonization temperature increases from 1100℃ to 1400℃; capacity decreases slightly when increasing to 1500℃.

[0127] Mechanism explanation: At excessively low temperatures (1100℃), the carbonization of pulverized coal and pitch is incomplete, resulting in insufficient reduction of GOQDs. This leaves a large number of oxygen-containing functional groups in the material, increasing irreversible capacity loss and leading to low carbon layer order and poor electronic conductivity. At excessively high temperatures (1500℃), the material tends towards graphitization, reducing interlayer spacing and hindering sodium ion insertion / extraction, thus causing a decrease in capacity. The optimal carbonization temperature is 1300-1400℃.

[0128] 10. Verification of spray granulation effect: Comparative Example 17: Spray granulation vs. non-spray granulation; Comparison: Example 1 (spray granulation) vs. Comparative Example 10 (non-spray granulation); Analysis conclusions: Without spray granulation, the specific capacity decreased from 352 mAh / g to 288 mAh / g, a decrease of 64 mAh / g; the initial coulombic efficiency decreased from 89.2% to 79.8%, a decrease of 9.4 percentage points; and the 5C rate retention decreased from 73% to 46%, a decrease of 27%.

[0129] Mechanism Explanation: Without spray granulation, the coal powder / GOQDs mixture consists of irregular fine powder (100-300nm), with loose interparticle contact and uneven asphalt coating (some particles are not coated or are coated too thickly). After carbonization, the internal electron transport path of the material is discontinuous, and the irregular morphology makes it prone to cracking and powder shedding during electrode coating. Spray granulation is a key step in forming regular spherical particles and ensuring uniform coating.

[0130] The above comparative analysis fully verifies the necessity of each core step of the present invention (Fenton oxidation, GOQDs addition, acid washing and impurity removal, asphalt coating, and spray granulation) and the optimal range of key process parameters (pH, particle size, temperature, and dosage).

Claims

1. A coal-based composite hard carbon material, characterized in that, The material has a core-shell double-layer structure: the core is coal-based carbon activated by Fenton oxidation and acid washing, and the outer shell is a layer of pitch carbon. Graphene quantum dots are also dispersed in the core.

2. The method for preparing a coal-based composite hard carbon material as described in claim 1, characterized in that, Includes the following steps: (1) Fenton oxidation activation treatment: Coal powder is subjected to Fenton oxidation activation treatment to obtain activated coal powder; (2) Acid washing to remove impurities: The activated coal powder is acid washed to remove impurities, and purified coal powder is obtained; (3) Graphene oxide quantum dot-assisted granulation: Purified coal powder is mixed with graphene oxide quantum dots and spray-dried to obtain precursor microspheres for primary granulation; (4) Asphalt coating and carbonization: The precursor microspheres are mixed with asphalt and coated with high temperature carbonization at 1200-1500℃ under an inert atmosphere to obtain coal-based composite hard carbon material.

3. The method for preparing a coal-based composite hard carbon material as described in claim 2, characterized in that, In step (1), the Fenton oxidation activation conditions are: pH=3-4, the mass ratio of hydrogen peroxide to coal powder is 0.5:1 to 5:1, the molar ratio of ferrous ions to hydrogen peroxide is 0.01:1 to 0.2:1, the reaction temperature is 10-40℃, and the reaction time is 2-5 hours.

4. The method for preparing a coal-based composite hard carbon material as described in claim 2, characterized in that, In step (2), pickling includes washing with hydrochloric acid and hydrofluoric acid in sequence.

5. The method for preparing a coal-based composite hard carbon material as described in claim 2, characterized in that, In step (3), the average particle size of the purified coal powder is 100-300 nanometers, the oxygen content of the graphene oxide quantum dots is 30%-55%, the amount of graphene oxide quantum dots added is 1%-20% of the mass of the coal powder, and the average particle size of the microspheres after spray drying is 6-10 micrometers.

6. The method for preparing a coal-based composite hard carbon material as described in claim 2, characterized in that, In step (4), the high-temperature carbonization temperature is 1300-1500℃, and the amount of asphalt added is 0.5%-3% of the mass of the precursor microspheres.

7. The method for preparing a coal-based composite hard carbon material as described in claim 2, characterized in that, Before step (1), the method further includes pulverizing the raw coal into coal powder with an average particle size of 100-300 nanometers.

8. The application of the coal-based composite hard carbon material as described in claim 1 or 2 in the preparation of sodium-ion battery anodes.

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