A coal-based hard carbon negative electrode sodium storage material and its preparation method and application

Abstract

The present invention relates to the field of electrode material technology, and in particular to a coal-based hard carbon negative electrode sodium storage material, and a preparation method and application thereof. The present invention comprises the following steps: S1, using anthracite as a substrate, activating it by an alkali activation method to obtain activated anthracite; S2, ball-milling the activated anthracite to obtain an anthracite precursor with a small microcrystal size; S3, carbonizing the anthracite precursor at high temperature to obtain a coal-based hard carbon negative electrode sodium storage material. The present invention adopts a combination of alkali activation and ball milling to prepare anthracite-based hard carbon. The alkali activation method can effectively remove the ash of the coal and activate the anthracite, so that the anthracite can be carbonized into hard carbon. The ball milling method effectively reduces the microcrystal size of the activated anthracite. During the high-temperature carbonization process, the cross-linking between oxygen-containing functional groups is promoted to form a large number of closed pores. The closed pore structure restricts the electrolyte from entering the pores, reducing the irreversible loss in the first cycle and thus improving the first coulombic efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of electrode materials, and in particular to a coal-based hard carbon negative electrode sodium storage material and a preparation method and application thereof. Background Art

[0002] The industrialization of sodium-ion batteries still faces bottlenecks, among which the negative electrode material is a key link restricting industrialization. At present, hard carbon is the most commercially promising negative electrode material for sodium-ion batteries, but it faces the severe challenge of low first coulombic efficiency, which limits the energy density of the battery. Among the many hard carbon precursors, biomass (such as coconut shells, walnut shells, etc.) has poor batch consistency and low carbon content; polymers are expensive and monomers are usually toxic; and anthracite has the advantages of high carbon content, low ash content, and large reserves, and has attracted much attention. However, anthracite forms a soft carbon structure after pyrolysis, and activation treatment is required to obtain hard carbon. However, the first efficiency of anthracite-based hard carbon treated only by activation is low and cannot meet commercial requirements (>90%). Therefore, there is an urgent need for a simple, clean and low-loss method to further improve the first efficiency of hard carbon. Summary of the Invention

[0003] The present invention aims to address the aforementioned shortcomings of the existing technology by providing a coal-based hard carbon anode sodium storage material, its preparation method, and its application. Using anthracite as a precursor, the present invention reduces the anthracite precursor's crystallite size through ball milling, modulating the formation of a closed-pore structure during high-temperature carbonization, and effectively improving the hard carbon's initial Coulombic efficiency.

[0004] A method for preparing a coal-based hard carbon negative electrode sodium storage material of the present invention comprises the following steps:

[0005] S1. Using anthracite as a base material, activating it by an alkali activation method to obtain activated anthracite;

[0006] S2. ball-milling the activated anthracite to obtain an anthracite precursor having a small crystallite size;

[0007] S3. Carbonize the anthracite precursor at high temperature to obtain a coal-based hard carbon negative electrode sodium storage material.

[0008] Furthermore, the anthracite precursor obtained in step S2 has a La of 14-16 nm and a Lc of 0.8-0.95 nm.

[0009] Furthermore, the specific operation of step S1 is: fully grinding the anthracite with a strong base, then transferring the powder to air or nitrogen for pre-pyrolysis, grinding the pre-pyrolysis product, acid washing, water washing, filtering, and drying to obtain activated anthracite.

[0010] Furthermore, step S1 also includes oxidizing the activated anthracite.

[0011] Furthermore, the oxidation treatment operation is to place the activated anthracite in air for heating.

[0012] Furthermore, the strong base includes one or more of sodium hydroxide and potassium hydroxide.

[0013] Furthermore, the pre-pyrolysis temperature is 200-400°C; and\or: the pre-pyrolysis temperature is 400°C.

[0014] Furthermore, the pickling is performed by stirring and immersing in hydrochloric acid or sulfuric acid with a concentration of 1 to 6 mol / L; and washing with water until the pH is neutral.

[0015] Furthermore, in step S2, the ball milling is wet milling, with a rotation speed of 300 to 800 r / min, a duration of 4 to 12 hours, and a ball-to-material ratio of 200:1.

[0016] Furthermore, in step S3, the temperature of high-temperature carbonization is 1000-2000°C, the time is 1-10 hours, and the heating rate is 1-5°C / min.

[0017] A coal-based hard carbon negative electrode sodium storage material prepared by the above-mentioned preparation method.

[0018] An application of the coal-based hard carbon negative electrode sodium storage material as described above is used as a negative electrode material for sodium ion batteries.

[0019] The present invention adopts a combination of alkali activation and ball milling to prepare anthracite-based hard carbon. The alkali activation method can effectively remove the ash of the coal and activate the anthracite, so that the anthracite can be carbonized into hard carbon. The ball milling method effectively reduces the crystallite size of the activated anthracite and controls the crystallite size La of the anthracite to be 14-16nm and Lc to be 0.8-0.95nm. During the high-temperature carbonization process, the cross-linking effect between oxygen-containing functional groups is promoted to form a large number of closed pores. The closed pore structure restricts the entry of the electrolyte into the pores, reduces the irreversible loss in the first cycle, and thus improves the first coulombic efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flow chart of the preparation method of the present invention;

[0021] Figure 2 1 is the XRD pattern of the precursor of each embodiment and the anthracite precursor of Comparative Example 1;

[0022] Figure 3 The Raman spectra of the precursors of each embodiment and the anthracite precursor of Comparative Example 1 are shown;

[0023] Figure 4 are the crystallite parameters of the precursors of each embodiment and the anthracite precursor of Comparative Example 1;

[0024] Figure 5This is the first cycle charge and discharge curve of Example 1;

[0025] Figure 6 This is the cycle performance diagram of Example 1. DETAILED DESCRIPTION

[0026] The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0027] Example 1

[0028] Step 1: Grind 1.5 g of anthracite and 3.0 g of potassium hydroxide thoroughly in an agate mortar, then transfer the powder to a muffle furnace for pre-pyrolysis at a temperature of 400 ° C and a heating rate of 5 ° C / min. Use an agate mortar to fully grind the pre-pyrolysis product into powder, soak it in 200 mL of 3 mol / L hydrochloric acid for pickling for 2 hours, then use a large amount of deionized water to wash it until the pH is neutral, and finally obtain the activated anthracite after filtration and drying.

[0029] Step 2: The activated anthracite obtained in step 1 was ball-milled at a speed of 500 r / min for 4 h, with a ball-to-material ratio of 200:1, and 20 mL of ethanol was added for wet grinding. After drying, an anthracite precursor with a small crystallite size was obtained.

[0030] Step 3: Transfer the anthracite with small microcrystal size obtained in step 2 to a high-temperature tube furnace, and calcine it at 1400°C at a heating rate of 2°C / min under a nitrogen atmosphere to obtain a high-efficiency sodium ion battery hard carbon negative electrode material.

[0031] Example 2

[0032] Step 1 is the same as in Example 1.

[0033] Step 2: The activated anthracite obtained in step 1 was ball-milled at a speed of 500 r / min for 12 h, with a ball-to-material ratio of 200:1, and 20 mL of ethanol was added for wet grinding. After drying, an anthracite precursor with a small crystallite size was obtained.

[0034] Step 3 is the same as in Example 1.

[0035] Example 3

[0036] Step 1: Same as Example 1.

[0037] Step 2: The activated anthracite obtained in Step 1 is ball-milled at 500 rpm for 4 hours, with a ball-to-batch ratio of 200:1. 20 mL of ethanol is added for wet milling. After drying, an anthracite precursor with small crystallite size is obtained. The anthracite precursor is then oxidized by heating it to 400°C in a muffle furnace for 2 hours and then oxidizing it with air. Finally, an anthracite precursor with small crystallite size and rich oxygen-containing functional groups is obtained.

[0038] Step 4: The oxidized small-crystal-sized anthracite obtained in step 3 is transferred to a high-temperature tube furnace and calcined at 1400°C at a heating rate of 2°C / min to obtain a high-efficiency sodium ion battery hard carbon negative electrode material.

[0039] Comparative Example 1

[0040] The difference from Example 1 is that Comparative Example 1 does not undergo the second ball milling process.

[0041] 1.5 g of anthracite and 3.0 g of potassium hydroxide were thoroughly ground in an agate mortar, and then the powder was transferred to a muffle furnace for pre-pyrolysis at a temperature of 400 °C and a heating rate of 5 °C / min. The pre-pyrolysis product was fully ground into powder using an agate mortar, soaked in 200 mL of 3 mol / L hydrochloric acid for 2 h, and then washed with a large amount of deionized water until the pH was neutral. Finally, the activated anthracite precursor was obtained after filtration and drying.

[0042] The activated anthracite precursor was transferred to a high-temperature tube furnace and calcined at 1400°C at a heating rate of 2°C / min to obtain a hard carbon negative electrode material for sodium ion batteries.

[0043] Comparative Example 2

[0044] Step 1 is the same as in Example 1.

[0045] Step 2: The activated anthracite obtained in step 1 is ball-milled at a speed of 500 r / min for 20 h, with a ball-to-material ratio of 200:1, and 20 mL of ethanol is added for wet grinding. After drying, an anthracite precursor with a small crystallite size is obtained.

[0046] Step 3 is the same as in Example 1.

[0047] Comparative Example 3

[0048] Step 1 is the same as in Example 1.

[0049] Step 2: The activated anthracite obtained in step 1 is ball-milled at a speed of 500 r / min for 28 h, with a ball-to-material ratio of 200:1, and 20 mL of ethanol is added for wet grinding. After drying, an anthracite precursor with a small crystallite size is obtained.

[0050] Step 3 is the same as in Example 1.

[0051] Comparative Example 4

[0052] Step 1: Same as Example 1.

[0053] Step 2: The activated anthracite obtained in step 1 is oxidized by heating it to 400°C in a muffle furnace for 2 hours and then oxidizing it with air. Finally, an activated anthracite precursor rich in oxygen-containing functional groups is obtained.

[0054] Step 4: Transfer the oxidized anthracite precursor to a high-temperature tube furnace and calcine it at 1400°C at a heating rate of 2°C / min to obtain a hard carbon negative electrode material for sodium ion batteries.

[0055] Electrode sheet preparation and battery assembly:

[0056] The electrochemical properties of hard carbon were tested by assembling a 2032 model half-cell. For slurry preparation, aqueous binder sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) were selected, and the materials with a mass ratio of hard carbon: conductive carbon black: CMC: SBR = 90:2:3:5 were weighed, mixed thoroughly with pure water, and evenly coated on copper foil, and then transferred to a vacuum oven at 80°C for drying for 12 hours. After that, circular electrode sheets with a diameter of 12 mm were cut to assemble button batteries, in which sodium metal was the counter electrode, the diaphragm was glass fiber (Whatman GF / D), and the electrolyte was an ether electrolyte 1M NaPF6 in DIGLYME = 100 Vol%. The oxygen and moisture contents in the glove box were both less than 0.1 ppm, and the test current was 0.3C (defined as 300 mAg -1 =1C), the test voltage range is 0.001~3V.

[0057] Table 1 shows the values ​​of the crystallite parameters La and Lc of each embodiment and comparative example, and Table 2 shows the electrochemical performance of each embodiment and comparative example 1 obtained by the above assembly test.

[0058] Table 1 Crystallite size of anthracite precursor

[0059] Precursor Precursor ball milling time / h La / nm Lc / nm Example 1 Precursor 4 14.40 0.91 Example 2 Precursor 12 14.11 0.88 Example 3 Precursor 4 14.51 0.94 Comparative Example 1 Precursor 0 16.02 1.00 Comparative Example 2 Precursor 20 12.95 0.84 Comparative Example 3 Precursor 28 11.98 0.81 Comparative Example 4 Precursor 0 15.89 0.98

[0060] Table 2 Electrochemical properties of samples

[0061] sample First coulombic efficiency / % Specific capacity / mAh g1 Platform specific capacity / mAh g1 Example 1 92.2 263.0 191.0 Example 2 91.0 263.6 194.7 Example 3 93.2 288.3 212.0 Comparative Example 1 84.4 249.4 180.2 Comparative Example 2 80.7 184.2 140.0 Comparative Example 3 79.2 179.0 135.5 Comparative Example 4 83.6 264.1 191.2

[0062] As can be seen from Table 1, as the ball milling time increases to 28 h, the lateral crystallite size La decreases from 16.02 nm to 11.98 nm, but the stacking height Lc does not change significantly, only decreasing from 1.0 nm to 0.81 nm. Therefore, ball milling mainly reduces the lateral crystallite size La.

[0063] It can be seen from Table 2 that when the ball milling time is no more than 12h (i.e., La>14.11nm), the first coulombic efficiency reaches more than 90%, which meets the commercial requirements (>90%). However, when the ball milling time is greater than 12h (i.e., La<14.11nm), the first coulombic efficiency and capacity both decrease significantly, and most of the capacity loss comes from the platform area. Since the platform area capacity of hard carbon during the sodium storage process is mainly contributed by the closed-pore structure, it indirectly shows that too long a ball milling time will reduce the closed-pore volume in hard carbon. This is because during the high-temperature calcination process, the pinning effect caused by the excessively small crystallite size of the precursor will hinder the rearrangement of carbon atoms and the movement of carbon layers, which is not conducive to the evolution of closed pores. Correspondingly, the excessively large crystallite size of the precursor makes it difficult for oxygen-containing functional groups (such as -COOH and C=O) to cross-link, which is not conducive to the induction of the formation of closed pores.

[0064] Figure 1 The flow chart of the present invention is shown in FIG. 1 , which is a flow chart of the present invention. Taking Example 1 as an example, the process mainly involves three steps: KOH activation, ball milling, and high-temperature calcination, to obtain hard carbon with high initial efficiency.

[0065] Figure 2 The XRD patterns of the Examples and Comparative Examples show that the (002) peaks do not broaden significantly for each sample, indicating that the stacking height Lc does not change significantly. Fitting calculations show that Lc for milling times of 0, 4, 12, 20, and 28 h are 1.00, 0.91, 0.88, 0.84, and 0.81 nm, respectively.

[0066] Figure 3 is the Raman spectrum of each embodiment and comparative example, wherein the peak area ratio A of the D peak to the G peak is D / A G This not only indicates the degree of defects but also allows calculation of the lateral crystallite size, La. Fitting calculations reveal that Lc for milling times of 0, 4, 12, 20, and 28 h is 16.02, 14.40, 14.11, 12.95, and 11.98 nm, respectively. Therefore, ball milling significantly reduces the lateral crystallite size, La, of the anthracite precursor.

[0067] Figure 5 This is the first cycle charge-discharge curve of Example 1. It can be seen that the hard carbon has an obvious low potential platform area (<0.1V), indicating the existence of a closed-pore sodium storage mechanism. The platform area capacity is 190.3mAh g -1 , accounting for 72.4% of the total capacity, indicating that the sodium storage capacity is mainly contributed by closed pores. Figure 6 This is the cycle performance diagram of Example 1. After 300 cycles, the capacity retention rate is as high as 90.3%, indicating that the closed-cell structure has good cyclability.

[0068] Ball milling can improve the first Coulombic efficiency of hard carbon materials. The principle is that the crystallite size of the anthracite precursor is reduced after ball milling. The smaller crystallite size promotes cross-linking between oxygen-containing functional groups (such as -COOH and C=O) during calcination, which induces the formation of a large number of closed microporous structures. Because the closed-pore structure restricts the electrolyte from entering the pores to form the SEI, it effectively reduces the irreversible decomposition of the electrolyte, thereby improving the first Coulombic efficiency of the hard carbon material. However, if the precursor crystallite size is too small, it will hinder the movement of the carbon layer, which is not conducive to the evolution and formation of closed pores.

[0069] Comparative Examples 1 and 4 show that the first efficiency cannot be improved by oxidation treatment alone. This may be because the additionally introduced oxygen-containing functional groups are difficult to carry out more cross-linking in the precursor with a larger crystallite size, so there is almost no improvement.

[0070] Example 3 demonstrates that introducing additional oxygen-containing functional groups and then reducing crystallite size can improve initial efficiency and capacity. This is because the oxidation treatment further increases the number of oxygen-containing functional groups, resulting in a sufficient number of oxygen-containing functional groups in the anthracite precursor to provide crosslinking. However, based on current performance data, in actual production, a single step of ball milling to reduce the anthracite precursor crystallite size is sufficient.

[0071] In summary, the anthracite-based hard carbon negative electrode sodium storage material prepared in Example 1 of the present invention has a rich closed microporous structure and its first charge capacity is 263.2 mAh g -1 , the first coulombic efficiency is as high as 92.2%, the cycle performance is good, and the capacity retention rate after 300 cycles is 90.3%. The strategy provided by the present invention to reduce the size of the anthracite precursor crystallites by ball milling can effectively increase the closed pores of hard carbon. This is because the small crystallites promote the formation of cross-linking between oxygen-containing functional groups. The present invention combines the alkali activation method and the ball milling method to prepare anthracite-based hard carbon negative electrode sodium storage material with high first coulombic efficiency, which meets the commercial hard carbon demand (>90%).

[0072] Any matters not mentioned above shall be subject to the existing technology.

[0073] Although some specific embodiments of the present invention have been described in detail through examples, those skilled in the art should understand that the above examples are for illustration only and are not intended to limit the scope of the present invention. Those skilled in the art of the present invention may make various modifications or additions to the described specific embodiments or replace them in similar ways, but they will not deviate from the direction of the present invention or exceed the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent replacements, improvements, etc. made to the above embodiments based on the technical essence of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a coal-based hard carbon negative electrode sodium storage material, characterized in that: The steps include: S1. Using anthracite as a base material, activating it by an alkali activation method to obtain activated anthracite; S2. ball-milling the activated anthracite to obtain an anthracite precursor having a small crystallite size; S3, carbonizing the anthracite precursor at high temperature to obtain a coal-based hard carbon negative electrode sodium storage material; The anthracite precursor obtained in step S2 has a La of 14-16 nm and a Lc of 0.8-0.95 nm; The specific operation of step S1 is: thoroughly grinding the anthracite with a strong base, then transferring the powder to air or nitrogen for pre-pyrolysis, grinding the pre-pyrolysis product, acid washing, water washing, suction filtering, and drying to obtain activated anthracite; In step S2, the ball milling is wet milling, with a rotation speed of 300-800 r / min, a duration of 4-12 h, and a ball-to-material ratio of 200:

1.

2. The preparation method according to claim 1, wherein: Step S1 further includes oxidizing the activated anthracite; the oxidation treatment operation is to place the activated anthracite in air and heat it.

3. The preparation method according to claim 1, wherein: The pre-pyrolysis temperature is 200-400°C; and\or: the pre-pyrolysis temperature is 400°C.

4. The preparation method according to claim 1, wherein: Acid pickling is to stir and soak in hydrochloric acid or sulfuric acid with a concentration of 1~6 mol / L; wash with water until the pH is neutral.

5. The preparation method according to claim 1, wherein: In step S3, the high-temperature carbonization temperature is 1000-2000°C, the time is 1-10 h, and the heating rate is 1-5°C / min.

6. A coal-based hard carbon negative electrode sodium storage material prepared by the preparation method according to any one of claims 1 to 5.

7. An application of the coal-based hard carbon negative electrode sodium storage material according to claim 6, characterized in that: Used as negative electrode material for sodium-ion batteries.

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