Solvent extraction modified coal-based hard carbon material as well as preparation method and application thereof

Abstract

The invention discloses a solvent extraction modified coal-based hard carbon material as well as a preparation method and application thereof. The preparation method specifically comprises the following steps: selecting pulverized coal with the particle size of 75 microns or below; carrying out acid pickling and ash removal on the pulverized coal to obtain an ultra-pure coal sample with ash content lower than 0.3%; carrying out selective extraction on the ultra-pure coal sample by selecting a proper organic solvent; carrying out low-temperature pretreatment on the obtained raffinate coal sample to obtain a hard carbon precursor material; and placing the precursor in an inert atmosphere for high-temperature carbonization to obtain the coal-based hard carbon material. The principle that coal and an organic solvent are similar to each other and dissolves is utilized, organic small molecules in a coal carbon skeleton are directionally cut through solvent extraction, oxygen-containing functional groups are introduced by means of a pre-oxidation process, formation of a graphite microcrystal region in the subsequent high-temperature carbonization process is inhibited, the microstructure of the graphite microcrystal region is adjusted, and more excellent rate capability and sodium storage capacity are obtained. The preparation method has the advantages of wide raw material sources and controllable preparation process.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery material preparation, and specifically relates to a solvent extraction modified coal-based hard carbon material and a preparation method and application thereof. Background Art

[0002] Coal is a natural heterogeneous polymer material with a three-dimensional cross-linked structure, which is composed of a certain number of alicyclic and aromatic / hydrogenated aromatic units connected by ester bonds and ether bonds. Its complex organic components have a great influence on the evolution of the structure during the carbonization process, which restricts the electrochemical performance of the hard carbon anode prepared from it. Extraction technology has gradually attracted attention because it can accurately control the molecular structure of coal, remove low molecular weight compounds, and leave the structure required for hard carbon.

[0003] Compared with traditional coal utilization methods, coal extraction technology can not only significantly improve coal utilization and added value, but also effectively reduce environmental pollution and resource waste. In recent years, many experts and scholars have proposed many optimization and improvement methods for coal extraction. For example, CN105498279A proposed to use metal salts or acidic ionic liquids as catalysts to improve the thermal extraction yield of coal through catalysis; CN107510955A adopts a cyclic extraction method, using the low molecular weight extract solution as the organic solvent for the next extraction experiment, and performs multiple cyclic extractions to improve the yield and quality of high molecular weight extracts; CN112587957A uses a combination of liquid-liquid extraction and micro-solid-liquid extraction to deeply extract low-rank coal and improve the quality and yield of the extract. Of course, the development of coal extraction technology also faces challenges, such as the demand for high-precision equipment, the improvement of process control accuracy, and the treatment of extraction waste. However, the above scheme uses extraction as a means to extract the structure in coal, ignoring the impact of these structures on the hard carbon negative electrode. Therefore, it is urgent to combine extraction with the preparation of high-performance hard carbon, which is the key to realizing the widespread application of coal-based carbon materials in the field of energy storage. Summary of the invention

[0004] One of the purposes of the present invention is to provide a method for preparing a solvent-extracted modified coal-based hard carbon material, to achieve precise control of the coal structure, and to promote the application of coal materials.

[0005] The second object of the present invention is to provide a coal-based hard carbon material prepared by the above preparation method, which has high specific capacity and excellent rate performance.

[0006] The third object of the present invention is to provide the application of the above-mentioned coal-based hard carbon material as a negative electrode material for sodium ion batteries.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing a solvent-extracted modified coal-based hard carbon material, comprising the following steps:

[0009] (1) After crushing and screening the coal particles, select coal powder with a particle size of less than 75 μm;

[0010] (2) acid-washing and deashing the coal powder obtained in step (1), washing with water until neutral, and drying to obtain an ultrapure coal sample with an ash content of less than 0.3 wt %;

[0011] (3) placing the ultrapure coal sample obtained in step (2) in an organic solvent for stirring extraction, centrifuging, filtering, and drying to obtain a raffinate coal sample;

[0012] (4) subjecting the raffinate coal sample obtained in step (3) to low temperature pretreatment to obtain a hard carbon precursor sample;

[0013] (5) The hard carbon precursor sample obtained in step (4) is placed in an inert atmosphere for carbonization treatment to obtain a coal-based hard carbon material.

[0014] Preferably, the raw coal in step (1) is selected from one or more of anthracite, lean coal, lean coal, coking coal, fat coal, gas coal, slightly sticky coal, non-sticky coal, long flame coal and lignite.

[0015] Preferably, the pickling in step (2) uses one or two of hydrochloric acid, hydrofluoric acid, sulfuric acid and nitric acid, with a concentration of 5 to 12 mol / L; and the reaction time is 12 to 24 h.

[0016] Preferably, the organic solvent described in step (3) is selected from at least one of N-methyl-2-pyrrolidone, carbon disulfide, acetone, toluene, pyridine, quinoline, and N,N-dimethylformamide, and the solid-liquid ratio is 1:20-60 (g / mL); the centrifugal speed is 8000-10000 r / min, and the centrifugal time is 5-8 min; the drying temperature is 70-90°C, and the drying time is 8 h.

[0017] Preferably, the stirring speed in step (3) is 1000-1200 r / min, and the stirring time is 24-48 h.

[0018] Preferably, the low-temperature pretreatment method in step (4) is selected from an air crosslinking method and a low-temperature carbonization method; the treatment temperature is 200-300°C, the heating rate is 1-5°C / min, and the insulation time is 1-3h.

[0019] Preferably, the carbonization temperature in step (5) is 1100-1500° C., the heating rate is 1-10° C. / min, and the insulation time is 1-3 h.

[0020] In a second aspect, the present invention provides a coal-based hard carbon material prepared by the above preparation method.

[0021] The first cycle reversible capacity of the coal-based hard carbon material is 250-280 mAh / g, and the first charge and discharge efficiency is greater than 80%, showing excellent electrochemical performance. In particular, the first cycle reversible capacity of the coal-based hard carbon material is greater than 280 mAh / g, and the first charge and discharge efficiency is greater than 83%.

[0022] In a third aspect, the present invention provides the use of the above-mentioned coal-based hard carbon material as a negative electrode material for a sodium ion battery.

[0023] Compared with the prior art, the coal-based hard carbon material preparation technology provided by the present invention can precisely control the structure of coal, utilize the principle of similar miscibility between coal and organic solvents, remove low molecular weight compounds in coal by extraction method, adjust the molecular configuration of hard carbon precursor, and introduce oxygen-containing functional groups by combining with appropriate pre-oxidation method, inhibit the formation of graphite microcrystalline area in the subsequent high-temperature carbonization process, adjust its microstructure, and make the final prepared coal-based hard carbon material conducive to the transmission and storage of sodium ions. The hard carbon material prepared by this method has a rich pore-deficient structure and a higher sodium storage capacity, and its reversible specific capacity is greater than 252mAh / g, and the first charge and discharge coulomb efficiency is greater than 83%, showing excellent sodium ion battery performance. The present invention provides coal-based hard carbon materials with high energy density, long cycle life, excellent rate performance and low cost, which can well meet the needs of industrialization. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Scanning electron microscope (SEM) images of coal-based hard carbon materials prepared in Example 1 and Comparative Example 1: (a) Example 1; (b) Comparative Example 1;

[0025] Figure 2 This is a pore size distribution diagram of the coal-based hard carbon material prepared in Example 1;

[0026] Figure 3 The charge and discharge curves of the coal-based sodium ion batteries prepared in Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3;

[0027] Figure 4 This is the long cycle diagram of the coal-based sodium ion battery prepared in Example 1. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] Example 1

[0030] This embodiment provides a method for preparing a coal-based hard carbon material, comprising the following steps:

[0031] (1) The non-sticky coal is first crushed in a universal crusher, then finely ground in a rod mill to 200 mesh (75 μm), and the sieved material is sieved;

[0032] (2) Weigh 100 g of coal powder and place it in a 500 mL glass beaker, add 50 mL of anhydrous ethanol to wet the coal sample, then slowly add 250 mL of hydrochloric acid solution (5 mol / L), then put it in a water bath, stir at 60 ° C for 12 h, wash it with deionized water until pH ≈ 7, then transfer the sample to a 500 mL polytetrafluoroethylene beaker, add 250 mL of hydrofluoric acid solution (1.13 g / L), then put it in a water bath, stir at 60 ° C for 12 h, then wash it with deionized water until pH ≈ 7, and put it in an oven at 80 ° C for 8 h to obtain an ultra-pure coal sample with an ash content of less than 0.3%.

[0033] (3) Weigh 50g of ultrapure coal sample obtained in step (2), mix it evenly with organic solvent carbon disulfide at a solid-liquid ratio of 1:30g / mL, stir it in a magnetic stirrer at 1000r / min for 24h, take it out and divide it into 6 50mL centrifuge tubes, and then centrifuge it at 10000r / min for 5min. After filtration, place it in an 80℃ forced air oven and dry it for 8h to obtain an extract; extract the coal sample to trim the functional groups in the coal and improve the purity;

[0034] (4) placing the extracted coal obtained in step (3) in a tubular furnace, heating the temperature to 300° C. at a heating rate of 1° C. / min at an air flow rate of 0.3 L / min, keeping the temperature for 2 h, and cooling to room temperature;

[0035] (5) The precursor material obtained in step (4) is placed in the tubular furnace again, heated to 1300°C at a heating rate of 2°C / min under a nitrogen environment, kept at this temperature for 2 hours, and cooled to room temperature to obtain a coal-based hard carbon material.

[0036] Example 2

[0037] The difference between this embodiment and embodiment 1 is that the organic solvent used in step (3) is N-methyl-2-pyrrolidone. The other steps and parameters are the same as those in embodiment 1.

[0038] Example 3

[0039] The difference between this embodiment and embodiment 1 is that in step (4), the temperature is raised to 300° C. at a heating rate of 1° C. / min at a nitrogen flow rate of 0.3 L / min, kept at that temperature for 2 h, and cooled to room temperature. The other steps and parameters are the same as those in embodiment 1.

[0040] Example 4

[0041] The difference between this embodiment and embodiment 1 is that the organic solvent used in step (3) is N-methyl-2-pyrrolidone, and in step (4), the temperature is raised to 300° C. at a nitrogen flow rate of 0.3 L / min at a heating rate of 1° C. / min, kept at that temperature for 2 h, and cooled to room temperature. The other steps and parameters are the same as those in embodiment 1.

[0042] Comparative Example 1

[0043] (1) Firstly, the non-sticky coal is crushed in a universal crusher, then finely ground in a rod mill to 200 mesh, and the sieved material is sieved;

[0044] (2) Weigh 100 g of coal powder and place it in a 500 mL glass beaker, add 50 mL of anhydrous ethanol to wet the coal sample, then slowly add 250 mL of hydrochloric acid solution (5 mol / L), then put it in a water bath, stir at 60 ° C for 12 h, wash it with deionized water until pH ≈ 7, then transfer the sample to a 500 mL polytetrafluoroethylene beaker, add 250 mL of hydrofluoric acid solution (1.13 g / L), then put it in a water bath, stir at 60 ° C for 12 h, then wash it with deionized water until pH ≈ 7, and put it in an oven at 80 ° C for 8 h to obtain an ultra-pure coal sample with an ash content of less than 0.3%.

[0045] (3) The precursor material obtained in step (2) is placed in the tubular furnace again, heated to 1300°C at a heating rate of 2°C / min under a nitrogen environment, kept warm for 2 hours, and cooled to room temperature to obtain a coal-based hard carbon material.

[0046] Comparative Example 2

[0047] The difference from comparative example 1 is that after step (2), the ultrapure coal is heated to 300°C at an air flow rate of 0.3 L / min and a heating rate of 1°C / min, kept at that temperature for 2 hours, and cooled to room temperature. The other steps and parameters are the same as those in comparative example 1.

[0048] Comparative Example 3

[0049] The difference from comparative example 1 is that after step (2), the ultrapure coal is heated to 300°C at a nitrogen flow rate of 0.3 L / min and a heating rate of 1°C / min, kept at that temperature for 2 hours, and cooled to room temperature. The other steps and parameters are the same as those in comparative example 1.

[0050] See also Figure 1 , Figure 1 The following are scanning electron microscope (SEM) images of the coal-based hard carbon materials prepared in Example 1 and Comparative Example 1. Among them, Figure (a) is the coal-based hard carbon material prepared in Example 1, and Figure (b) is the coal-based hard carbon material prepared in Comparative Example 1. Figure 1 It can be seen that the coal-based hard carbon material presents an irregular block shape with a relatively flat surface, and its particle size is mostly distributed between 5 and 25 μm.

[0051] See also Figure 2 , Figure 2 This is the pore size distribution diagram of the coal-based hard carbon material prepared in Example 1. As can be seen from the figure, the pore sizes are mostly concentrated in 1-2 nm, with abundant micropores, and a small proportion of mesopores and macropores, which is conducive to Na + store.

[0052] Example 5

[0053] Battery assembly and testing

[0054] The hard carbon material for the negative electrode of the sodium ion battery prepared above was mixed evenly with carbon black, carboxymethyl cellulose and styrene-butadiene rubber in a mass ratio of 91:2:2:5, and then the slurry was coated on a carbon-coated copper foil, placed in a vacuum oven at 80°C and dried for 12 hours, and punched into a 12mm electrode sheet using a slicer. Commercial sodium sheets were used as positive electrodes, the materials of the embodiments and comparative examples were used as negative electrodes, GF / D type glass fiber diaphragms were used, and 1M NaPF6 in diglyme was used as electrolytes to assemble CR2032 button cells in a glove box. Constant current charge and discharge tests were performed using a high-precision battery tester (Wuhan Blue Electric) at a current density of 20mA / g and a voltage range of 0-3.0V. The test results are shown in Table 1.

[0055] Table 1

[0056] Charge specific capacity / (mAh / g) Discharge specific capacity / (mAh / g) First charge and discharge efficiency / (%) Example 1 281.21 338.5 83.09 Example 2 281.1 334.2 84.11 Example 3 256.75 290.5 88.38 Example 4 266.64 319.36 83.49 Comparative Example 1 228.58 292.77 78.08 Comparative Example 2 268.79 314.08 85.58 Comparative Example 3 236.24 272.51 86.69

[0057] From the data in Table 1, it can be seen that the extraction and 300°C pretreatment of coal have a certain influence on the first charge and discharge efficiency of the hard carbon negative electrode.

[0058] By comparing Example 1, Example 2 and Comparative Example 1, it is found that the charge-discharge specific capacity and the first coulombic efficiency of the hard carbon negative electrode can be improved by extraction and 300° C. air pretreatment.

[0059] By comparing Example 1, Example 2 and Comparative Example 2, it is found that the use of the extractant can increase the charge and discharge specific capacity of the hard carbon negative electrode, and then the first efficiency is slightly reduced.

[0060] By comparing Example 3, Example 4 and Comparative Example 3, it is found that the first coulombic efficiency of the hard carbon negative electrode after pretreatment with carbon disulfide and 300°C nitrogen is the highest, which is 88.38%, which is also the highest among all samples. This shows that extraction-nitrogen pretreatment is beneficial to improving the first efficiency of the hard carbon negative electrode.

[0061] Comparing Example 1 and Example 3 (Example 2 and Example 4), it is found that the air pretreatment at 300°C has a higher charge-discharge specific capacity, but the first efficiency is reduced. This is because the air pretreatment introduces oxygen functional groups into the coal sample, forming structural defects. These defects can attract more sodium ions to deposit, but these sodium ions are difficult to escape, resulting in a decrease in the first efficiency. In summary, extraction-air pretreatment produces a high-capacity battery, while extraction-nitrogen pretreatment produces a high-first-efficiency battery.

[0062] Figure 3 The charge and discharge curves of the sodium ion battery assembled from the coal-based hard carbon materials prepared in Example 1 and Comparative Example 1. It can be seen from the curves that when the current density of the prepared coal-based hard carbon material is 20 mA / g, the sodium storage capacity of the slope area and the platform area of ​​Example 1 is improved compared with that of Comparative Example 1, the first cycle reversible capacity is increased from 228.58 to 281.21 mAh / g, and the first coulombic efficiency is increased to 83.09%.

[0063] Figure 4 This is a long cycle performance diagram of the coal-based hard carbon material prepared in Example 1. Figure 4 It can be seen that at a current density of 100 mA / g, after 100 cycles, its capacity retention rate is close to 94%, showing good long-cycle performance.

[0064] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a solvent-extracted modified coal-based hard carbon material, characterized in that: The following steps are involved: (1) After crushing and screening the coal particles, select coal powder with a particle size of less than 75 μm; (2) acid-washing and deashing the coal powder obtained in step (1), washing with water until neutral, and drying to obtain an ultra-pure coal sample with an ash content of less than 0.3%; (3) placing the ultrapure coal sample obtained in step (2) in an organic solvent for stirring extraction, centrifuging, filtering, and drying to obtain a raffinate coal sample; (4) subjecting the raffinate coal sample obtained in step (3) to low temperature pretreatment to obtain a hard carbon precursor sample; (5) The hard carbon precursor sample obtained in step (4) is placed in an inert atmosphere for carbonization treatment to obtain a coal-based hard carbon material.

2. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The raw coal in step (1) is selected from one or more of anthracite, lean coal, lean coal, coking coal, fat coal, gas coal, slightly sticky coal, non-sticky coal, long flame coal and lignite.

3. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The pickling in step (2) uses one or two of hydrochloric acid, hydrofluoric acid, sulfuric acid and nitric acid, with a concentration of 5 to 12 mol / L; the reaction time is 12 to 24 hours.

4. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The organic solvent described in step (3) is selected from at least one of N-methyl-2-pyrrolidone, carbon disulfide, acetone, toluene, pyridine, quinoline, and N,N-dimethylformamide, and the solid-liquid ratio is 1:20-60 g / mL; the centrifugal speed is 8000-10000 r / min, and the centrifugal time is 5-8 min; the drying temperature is 70-90° C., and the drying time is 8 h.

5. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The stirring speed described in step (3) is 1000-1200 r / min, and the stirring time is 24-48 h.

6. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The low-temperature pretreatment method in step (4) is selected from one of an air crosslinking method and a low-temperature carbonization method; the treatment temperature is 200-300°C, the heating rate is 1-5°C / min, and the insulation time is 1-3h.

7. The method for preparing a solvent-extracted modified coal-based hard carbon material according to claim 1, characterized in that: The carbonization temperature is 1100-1500° C., the heating rate is 1-10° C. / min, and the heat preservation time is 1-3 hours.

8. A coal-based hard carbon material obtained by the preparation method according to any one of claims 1 to 7.

9. Use of the coal-based hard carbon material according to claim 8 as a negative electrode material for sodium ion batteries.

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