Coal-based hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention relates to the technical field of battery materials, in particular to a coal-based hard carbon negative electrode material and a preparation method and application thereof.The coal-based hard carbon negative electrode material is of a porous structure with pore channels in bimodal pore distribution, the porous structure comprises micropores and mesopores, the pore diameter of the micropores is 0.5-2 nm, the pore diameter of the mesopores is 2-50 nm, and the volume ratio of the micropores is 11%-13%; the coal is used as a raw material and is obtained through carbonization, the coal raw material contains Ca element and Fe element, the content a of the Ca element is greater than or equal to 4000 ppm and less than or equal to 9100 ppm, the content b of the Fe element is greater than or equal to 7200 ppm and less than or equal to 10000 ppm, and a / b is greater than or equal to 0.5 and less than or equal to 1. The coal-based hard carbon negative electrode material product provided by the invention has relatively good rate capability, and the preparation method not only realizes high-value conversion of coal, but also enables the hard carbon negative electrode material to be continuously produced, and is beneficial to promoting the commercial process of sodium ion batteries.

Description

Technical Field

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

[0002] As an emerging energy storage system, sodium-ion batteries have gradually become a powerful supplement to lithium-ion batteries due to their advantages such as abundant resources and low cost, and are particularly suitable for large-scale energy storage applications. However, in order to truly commercialize sodium-ion batteries and widely use them in various fields, especially those application scenarios that require fast charging and discharging, such as electric vehicles and smart grids, improving their rate performance is an essential and key step. Therefore, the development of high-performance high-rate negative electrode materials is not only an effective way to solve the current technical bottleneck, but also the key to promoting the industrialization of sodium-ion batteries. The development of high-rate negative electrode materials is directly related to the improvement of the power density of sodium-ion batteries. By developing new high-rate negative electrode materials, the diffusion path of sodium ions can be effectively shortened, the electrochemical reaction kinetics can be optimized, and the power density of the battery can be greatly improved to meet the needs of practical applications.

[0003] Coal-based materials, especially hard carbon formed by pyrolysis carbonization, have received widespread attention in the field of sodium-ion batteries due to their abundant resources, low cost, excellent sodium storage performance and good rate performance. Coal as a raw material is not only widely distributed, but also relatively cheap, which makes hard carbon materials prepared based on coal have significant cost advantages compared to other negative electrode materials. Moreover, the preparation process of coal-based materials is relatively simple, easy to industrialize, and can utilize existing sodium-ion battery production equipment without the need for additional equipment investment, which is conducive to rapid industrialization. Therefore, coal-based materials are ideal materials for sodium-ion battery negative electrode materials.

[0004] However, to optimize the rate performance and fully realize the potential of coal-based hard carbon anode materials, existing technologies generally use steam activation. While steam activation can optimize the rate performance of coal-based hard carbon anode materials, it requires precise control of temperature, time, and steam flow rate. This not only increases process complexity and production costs, but also reduces production efficiency. It can also damage the material structure, affect performance, and have adverse effects on the environment. Future development of more efficient and environmentally friendly preparation processes is needed.

[0005] Therefore, the present invention is proposed to solve the above-mentioned problems. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the present invention provides a coal-based hard carbon negative electrode material and a preparation method and application thereof.

[0007] In order to achieve the above objectives, the main technical solutions adopted by the present invention include:

[0008] On the one hand, the present invention proposes a coal-based hard carbon negative electrode material, which is a porous structure with a bimodal pore distribution. The porous structure includes micropores and mesopores. The pore diameter of the micropores is 0.5-2nm, the pore diameter of the mesopores is 2-50nm, and the volume of the micropores in the porous structure accounts for 11%-13%.

[0009] Furthermore, the coal-based hard carbon negative electrode material satisfies at least one of the following conditions:

[0010] a) The interlayer spacing of the (002) crystal plane is 0.37-0.39 nm;

[0011] b) Grain size is

[0012] c) Specific surface area <11m 2 / g.

[0013] Specifically, the ash content is ≤0.5wt%, the Fe content is ≤100ppm, and the Ca content is ≤100ppm.

[0014] On the other hand, the present invention also proposes a method for preparing a coal-based hard carbon negative electrode material, which is obtained by carbonization using coal as raw material. The coal raw material contains Ca and Fe elements, the content of Ca element is a, and the content of Fe element is b, and a and b satisfy: 4000ppm≤a≤9100ppm, 7200ppm≤b≤10000ppm, 0.5≤a / b≤1.

[0015] The present invention unexpectedly discovered that when coal is used as raw material and the Ca and Fe content in the coal raw material meets the above requirements, the Ca and Fe elements work together to induce the formation of porous structures with different pore sizes. The reason may be that calcium compounds decompose and release gas products during the high-temperature carbonization process to form micropores, providing more active sites for sodium ion embedding / de-embedding, increasing sodium ion transmission channels, and improving rate performance. At the same time, iron catalyzes the dehydrogenation condensation of organic components and promotes the graphitization of carbon materials. The increase in the degree of graphitization helps to enhance the conductivity and structural stability of hard carbon materials. Under the synergistic effect, the coal raw material is carbonized to form a coal-based hard carbon negative electrode material with a mesoporous structure and good electrochemical properties.

[0016] Furthermore, when a and b satisfy: 4600ppm≤a≤8200ppm, 7400ppm≤b≤8100ppm, the synergistic effect of Ca and Fe elements in promoting graphitization, regulating pore structure, and improving the electrochemical properties of hard carbon materials is more significant, and the performance of the resulting coal-based hard carbon negative electrode material is better; preferably, when a and b satisfy: 7600ppm≤a≤8200ppm, 7500ppm≤b≤8000ppm, the synergistic effect of Ca and Fe elements is most significant, and the performance of the resulting coal-based hard carbon negative electrode material is particularly good.

[0017] Specifically, the ash content of the coal raw material is 7%-10% and the volatile matter content is 35%-42.5%.

[0018] Ash is a non-combustible inorganic mineral in coal. Excessive ash content will affect the battery preparation process and electrochemical performance. It is necessary to use methods such as impurity removal, modification, and conversion to eliminate the negative effects of ash. However, Ca and Fe elements mainly exist in ash. Too low ash content will lead to too low Ca and Fe content, which is not conducive to catalyzing carbonization reactions and optimizing hard carbon structures.

[0019] Volatile matter is an important source of pore structure during the carbonization of coal raw materials to form hard carbon negative electrode materials. If the volatile matter is too high, the volatile matter will be released too much and too quickly during carbonization. Excessive release will lead to large raw material loss and low hard carbon yield during carbonization, while too fast release will lead to uneven carbonization reaction and the formation of structural defects. If the volatile matter is too low, the carbonization reaction activity is insufficient, and a disordered carbon structure suitable for sodium ion embedding / de-embedding cannot be formed. The product has low specific surface area and porosity, and poor electrochemical performance.

[0020] In summary, the volatile matter mainly affects the uniformity of the carbonization reaction, the hard carbon yield and the pore structure, while the ash content mainly affects the purity, conductivity and structural uniformity of the hard carbon. The present invention has found through several creative experiments that when the ash content in the coal raw material is 7%-10% and the volatile matter content in the coal raw material is 35%-42.5%, the volatile matter is catalyzed by the appropriate amount of Ca and Fe elements in the ash and is uniformly released, thereby obtaining a specific surface area greater than 80m 2 / g of the first carbonization product, which has many surface active sites, helps to improve the pickling effect and remove impurities more thoroughly. The pores left after pickling to remove impurities form micropores or closed pores in the second carbonization, which can further increase the sodium storage sites. On the other hand, it promotes the better discharge of volatiles in the second carbonization, making the hard carbon material have a more stable structure and the interlayer spacing greater than 0.38nm, which is conducive to the shuttle of sodium ions, and finally obtains a hard carbon material with excellent electrochemical properties.

[0021] Specifically, the coal raw materials are subjected to pulverization, first carbonization, pickling and second carbonization in sequence, wherein the temperature of the first carbonization is 500-900°C and the time is 2-5 hours, and the temperature of the second carbonization is 1100-1400°C and the time is 2-5 hours; preferably, the temperature of the first carbonization is 600°C and the time is 3 hours; more preferably, the temperature of the second carbonization is 1300°C and the time is 3 hours.

[0022] The first carbonization temperature is relatively low, and its main goal is to remove volatiles and some impurities, and initially form a carbon skeleton structure with microporous and mesoporous structures and a certain mechanical strength. The second carbonization temperature is relatively high, which can further condense and rearrange the carbon structure, regulate and optimize the pore structure, increase the proportion of micropores, and increase the specific surface area, forming a more stable and orderly hard carbon material that is conducive to electrolyte infiltration and ion transport. In addition, it further removes residual impurities and ash, improves the purity of the material, and further improves the mechanical strength and hardness of the material. Acid washing is performed between the two carbonizations to remove metal impurities such as Ca and Fe and ash, and to introduce oxygen-containing functional groups to modify the hard carbon surface, improve its surface chemical properties and hydrophilicity. Furthermore, acid washing can also promote and optimize the formation of micropores, increase the specific surface area, and facilitate electrolyte infiltration and ion transport. The pore size distribution is regulated by acid washing, and the electrochemical performance of the negative electrode material is further optimized.

[0023] After testing: the ash content in the coal-based hard carbon negative electrode material prepared by the above preparation method is ≤0.5wt%, the Fe element content is ≤100ppm, and the Ca element content is ≤100ppm, which avoids the introduction of excessive impurity phases by the residues of the two, resulting in increased side reactions of the negative electrode material and increased structural defects, affecting the performance of the negative electrode material.

[0024] Furthermore, the coal raw material is specifically bituminous coal, and the Dv50 of the bituminous coal after pulverization is 1-10 μm; the coal raw material is dried before the first carbonization; the reagent used in the pickling is selected from one or more of hydrochloric acid, nitric acid, and hydrofluoric acid; preferably, the Dv50 of the bituminous coal after pulverization is 4-6 μm; more preferably, the reagent used for pickling is a mixed solution of hydrochloric acid, nitric acid, and hydrofluoric acid, and the mixed solution is conducive to introducing multiple types of active oxygen-containing functional groups such as carboxyl (-COOH), hydroxyl (-OH), nitro (-NO) and carbonyl (=O) into the material.

[0025] In this embodiment, bituminous coal is pulverized to a Dv50 of 1-10 μm, particularly 4-6 μm, to improve carbonization reaction uniformity, optimize heat transfer, promote volatile release, and enhance pore structure. Excessively large coal particle size can lead to uneven reaction, poor heat transfer, and an undesirable pore structure. While too small a particle size can lead to particle agglomeration, increased energy and yield losses, and an excessively small pore structure, controlling the particle size within this range allows the production of a hard carbon material with superior performance.

[0026] In this embodiment, since the synergistic effect of Ca and Fe elements is mainly reflected in the first carbonization, after the first carbonization is completed, the product is acid-washed, which can not only remove Ca, Fe and harmful impurities, but also optimize the pore structure, and increase active sites through surface modification, thereby significantly improving the electrochemical performance of the hard carbon negative electrode material, which is of great significance for improving the performance of sodium ion batteries.

[0027] It should be noted that: (1) Other coal raw materials containing the Ca and Fe elements required by the present invention, such as anthracite, lignite, coal tar pitch, coal gangue, coal-based activated carbon, etc., can also be used in the present invention. (2) During pickling, the concentration of the reagent used should not be too high or too low. If the concentration is too high, although the pickling efficiency is high and the impurity removal effect is good, it will cause excessive etching of the product surface after the first carbonization, which will not only destroy the material structure, but may also introduce new side reactions and corrode the equipment. If the concentration is too low, although the surface structure of the material can be finely controlled and it is friendly to the equipment, the pickling efficiency is low and the impurity removal effect is poor.

[0028] In addition, the present invention also proposes a sodium ion battery comprising the above-mentioned coal-based hard carbon negative electrode material.

[0029] Furthermore, its 0.1C first-week reversible capacity is above 295mAh / g, its 0.5C first-week reversible capacity is above 233.5mAh / g, and its 1C first-week reversible capacity is above 172mAh / g.

[0030] Furthermore, its 0.5C capacity retention rate after 50 cycles is above 92.5%, and its 1C capacity retention rate after 50 cycles is above 85%.

[0031] Compared with the existing technology, the preparation method of the present invention is conventional, simple, and easy to scale up. It can carbonize coal-based raw materials into hard carbon materials with excellent electrochemical properties without steam activation. It not only realizes the high-value conversion of coal, but also enables the sustainable production of hard carbon negative electrode materials, which helps to promote the commercialization of sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0033] Figure 1 XRD patterns of Example 1-1, Example 2-1, Comparative Example 1-1, Comparative Example 2-1, and Comparative Example 3-1;

[0034] Figure 2 SEM of Example 1-1 Figure 1 ;

[0035] Figure 3 SEM of Example 1-1 Figure 2 ;

[0036] Figure 4 SEM of Example 1-1 Figure 3 ;

[0037] Figure 5 This is a pore size distribution diagram of the coal-based hard carbon negative electrode materials obtained in Example 1-1, Comparative Example 1-1, Comparative Example 2-1, and Comparative Example 3-1;

[0038] Figure 6 0.5C charge and discharge curves of the sodium ion batteries assembled from Example 1-1, Example 2-1, Example 3-1, Example 4-1, Example 5-1, Comparative Example 1-1, Comparative Example 2-1, and Comparative Example 3-1. DETAILED DESCRIPTION

[0039] The following will be combined with the accompanying drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0040] In the following examples and comparative examples, the reagents used for pickling were a mixed solution of 1 mol / L hydrochloric acid, 1 mol / L nitric acid, and 0.4 mol / L hydrofluoric acid in a volume ratio of 3:2:5, and the pickling time was 15 min.

[0041] Example 1-1

[0042] Coal-based hard carbon anode material was prepared as follows:

[0043] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 7956 ppm, the Fe content was 7642 ppm, the Ca / Fe ratio was about 1, the volatile matter was about 40.7%, and the ash content was about 8.5%;

[0044] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0045] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0046] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0047] Example 1-2

[0048] Coal-based hard carbon anode material was prepared as follows:

[0049] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 8123 ppm, the Fe content was 7954 ppm, the Ca / Fe ratio was about 1, the volatile matter was about 41.5%, and the ash content was about 8.3%;

[0050] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0051] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0052] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0053] Examples 1-3

[0054] Coal-based hard carbon anode material was prepared as follows:

[0055] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 7634 ppm, the Fe content was 7536 ppm, the Ca / Fe ratio was about 1, the volatile matter was about 42.2%, and the ash content was about 8.1%;

[0056] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0057] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0058] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0059] Example 2-1

[0060] Coal-based hard carbon anode material was prepared as follows:

[0061] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 4892 ppm, the Fe content was 7743 ppm, the Ca / Fe ratio was about 0.5, the volatile matter was about 36.5%, and the ash content was about 8.4%;

[0062] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0063] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0064] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0065] Example 2-2

[0066] Coal-based hard carbon anode material was prepared as follows:

[0067] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 5136 ppm, the Fe content was 8081 ppm, the Ca / Fe ratio was about 0.5, the volatile matter was about 35.9%, and the ash content was about 7.1%;

[0068] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0069] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0070] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0071] Example 2-3

[0072] Coal-based hard carbon anode material was prepared as follows:

[0073] (1) The bituminous coal raw material was crushed to a Dv50 of 4-6 μm. After testing, the Ca content of the bituminous coal raw material was 4631 ppm, the Fe content was 7456 ppm, the Ca / Fe ratio was about 0.5, the volatile matter was about 37.3%, and the ash content was about 9.9%;

[0074] (2) The product obtained in the previous step was dried first, and then carbonized for the first time under nitrogen atmosphere protection. The heating rate of the first carbonization was 3°C / min, the first carbonization temperature was 600°C, and the time was 3h;

[0075] (3) The product obtained in the previous step is acid-washed, washed with pure water and filtered until the pH of the filtered water is 7;

[0076] (4) The product obtained in the previous step was dried and then carbonized for the second time under nitrogen atmosphere protection. The heating rate of the second carbonization was 3°C / min, the second carbonization temperature was 1300°C, and the time was 3h to obtain the final product, coal-based hard carbon negative electrode material.

[0077] Example 3-1

[0078] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 4025 ppm, the Fe element content is 7208 ppm, the Ca / Fe ratio is about 0.56, the volatile matter is about 45.0%, and the ash content is about 9.4%. The rest are consistent with Example 1-1.

[0079] Example 3-2

[0080] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 8154 ppm, the Fe element content is 9560 ppm, the Ca / Fe ratio is about 0.85, the volatile matter is about 38.7%, and the ash content is about 8.5%. The rest are consistent with Example 1-1.

[0081] Example 3-3

[0082] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 9068 ppm, the Fe element content is 9987 ppm, the Ca / Fe ratio is about 0.91, the volatile matter is about 35.1%, and the ash content is about 7.6%. The rest are consistent with Example 1-1.

[0083] Example 4-1

[0084] Compared with Example 1-1, the temperature of the first carbonization was 500° C. and the time was 5 h, and the rest were consistent with Example 1-1.

[0085] Example 4-2

[0086] Compared with Example 1-1, the temperature of the first carbonization was 900° C. and the time was 2 h, and the rest were consistent with Example 1-1.

[0087] Example 4-3

[0088] Compared with Example 1-1, the temperature of the first carbonization was 700° C. and the time was 4 h, and the rest were consistent with Example 1-1.

[0089] Example 5-1

[0090] Compared with Example 1-1, the temperature of the second carbonization is 1100° C. and the time is 5 h, and the rest are consistent with Example 1-1.

[0091] Example 5-2

[0092] Compared with Example 1-1, the temperature of the second carbonization is 1400° C. and the time is 2 h, and the rest are consistent with Example 1-1.

[0093] Example 5-3

[0094] Compared with Example 1-1, the temperature of the second carbonization is 1200° C. and the time is 4 h, and the rest are consistent with Example 1-1.

[0095] Comparative Example 1-1

[0096] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 8456 ppm, the Fe element content is 4387 ppm, the Ca / Fe ratio is about 2, the volatile matter is about 32.5%, and the ash content is about 7.5%. The rest are consistent with Example 1-1.

[0097] Comparative Example 1-2

[0098] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 8174 ppm, the Fe element content is 3842 ppm, the Ca / Fe ratio is about 2, the volatile matter is about 31.8%, and the ash content is about 7.7%. The rest are consistent with Example 1-1.

[0099] Comparative Examples 1-3

[0100] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 7891 ppm, the Fe element content is 3312 ppm, the Ca / Fe ratio is about 2, the volatile matter is about 31.2%, and the ash content is about 7.2%. The rest are consistent with Example 1-1.

[0101] Comparative Examples 1-4

[0102] Compared with Example 1-1, the Ca element, Fe element, volatile matter and ash content of the bituminous coal raw material in step (1) have changed. After testing, the Ca element content in the bituminous coal raw material is 4025 ppm, the Fe element content is 8921 ppm, the Ca / Fe ratio is about 0.45, the volatile matter is about 34.6%, and the ash content is about 9.2%. The rest are consistent with Example 1-1.

[0103] Comparative Example 2-1

[0104] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 8080 ppm, the Fe content was 7898 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 40.5%, and the ash content was approximately 8.2%. In step (2), the first carbonization temperature was adjusted to 450°C, and the time was unchanged. All other aspects remained the same as in Example 1-1.

[0105] Comparative Example 2-2

[0106] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 7892 ppm, the Fe content was 7605 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 39.8%, and the ash content was approximately 8.5%. In step (2), the first carbonization temperature was adjusted to 450° C., and the time was unchanged. All other aspects remained the same as in Example 1-1.

[0107] Comparative Examples 2-3

[0108] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 8254 ppm, the Fe content was 8043 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 41.7%, and the ash content was approximately 8.1%. In step (2), the first carbonization temperature was adjusted to 450°C, and the time was unchanged. All other conditions remained the same as in Example 1-1.

[0109] Comparative Examples 2-4

[0110] Compared with Comparative Example 2-1, the first carbonization temperature in step (2) was adjusted to 950° C. and the time remained unchanged.

[0111] Comparative Example 3-1

[0112] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 8678 ppm, the Fe content was 8524 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 40.1%, and the ash content was approximately 7.2%. In step (4), the second carbonization temperature was adjusted to 1450° C., and the time was unchanged. All other aspects remained the same as in Example 1-1.

[0113] Comparative Example 3-2

[0114] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 8395 ppm, the Fe content was 7965 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 41.3%, and the ash content was approximately 7.8%. In step (4), the second carbonization temperature was adjusted to 1450° C., and the time was unchanged. All other aspects remained the same as in Example 1-1.

[0115] Comparative Example 3-3

[0116] Compared with Comparative Example 3-1, the second carbonization temperature in step (4) was adjusted to 1000° C. and the time remained unchanged. The rest remained the same as in Example 1-1.

[0117] Comparative Examples 3-4

[0118] Compared with Example 1-1, the Ca, Fe, volatile matter, and ash content of the bituminous coal raw material in step (1) were changed. After testing, the Ca content of the bituminous coal raw material was 8278 ppm, the Fe content was 8124 ppm, the Ca / Fe ratio was approximately 1, the volatile matter was approximately 40.1%, and the ash content was approximately 7.2%. In step (4), the second carbonization temperature was adjusted to 1000° C., and the time was unchanged. All other conditions remained the same as in Example 1-1.

[0119] Comparative Example 4-1

[0120] Compared with Example 1-1, the pickling operation shown in step (3) is adjusted to be performed before the first carbonization, that is, pickling is performed first, and then the first carbonization and the second carbonization are performed in sequence. The rest are consistent with Example 1-1.

[0121] The products of each embodiment and comparative example were subjected to XRD test (eg Figure 1 The XRD patterns of some embodiments and some comparative examples are shown in Table 1. The interlayer spacing of the product (002) is calculated according to the Bragg equation, and the grain size is calculated according to the Scherrer formula, as shown in Table 1. Figure 6 Example 1-1 is represented as S1-1, and so on; Comparative Example 1-1 is represented as D1-1, and so on.

[0122] Table 1

[0123]

[0124]

[0125] The products obtained from each embodiment and comparative example are used as button battery negative electrode active materials, and assembled into sodium ion batteries for various electrochemical performance tests. The test results are shown in Table 2. The specific assembly method is: the obtained product is used as the main material of the button battery negative electrode material, and the negative electrode sheet is prepared according to the ratio of main material: CMC: SP = 90:5:5, the metal sodium sheet is used as the positive electrode, and 1 mol / L NaPF6 / EC+DEC (volume ratio 1:1) is used as the electrolyte. The CR2032 button battery is assembled in the glove box, and the contents of H2O and O2 in the glove box are guaranteed to be <0.1ppm throughout the process. The main test conditions for the relevant electrochemical properties are: the CR2032 button battery is tested using the Blue Electric Battery Test System, and the charge and discharge test is carried out at a current density of 0.1C / 0.5C / 1C. The discharge cut-off voltage is 0V, and the charge cut-off voltage is 2.0V. The 0.5C charge and discharge curves of some embodiments and comparative examples are shown as follows: Figure 6 shown.

[0126] Table 2

[0127]

[0128]

[0129] As shown in Table 1, Table 2 and Figures 1-6 Commonly shown:

[0130] The present invention limits the Ca content in the coal raw material to 4000-9100ppm, the Fe content to 7200-10000ppm and the Ca / Fe ratio to 0.5-1, thereby obtaining a coal-based hard carbon negative electrode material with a 0.1C first discharge specific capacity of 295.01-306.38mAh / g, a 0.5C first discharge specific capacity of 233.62-240.35mAh / g, a 1C first discharge specific capacity of 172.95-180.6mAh / g, a 0.5C capacity cycle 50-week retention rate of more than 92.5%, and a 1C capacity cycle 50-week retention rate of more than 85%. Figure 5 As shown: the coal-based hard carbon negative electrode material has a porous structure with a bimodal pore distribution, and the volume of micropores accounts for 11%-13%; Figure 2-Figure 4 As shown, the internal micropore diameter is 0.5-2nm and the mesopore diameter is 2-50nm; Figure 1 The XRD patterns of some embodiments and some comparative examples are shown. Further tests show that the interlayer spacing of the (002) crystal plane of the coal-based hard carbon negative electrode material is about 0.37-0.39 nm and the grain size is about Specific surface area <11m 2 / g.

[0131] From the comparison of the test results of Comparative Example 1-1, Comparative Example 1-2, Comparative Example 1-3, Comparative Example 1-4 with Example 1-1, it can be seen that if the content of one element among Ca and Fe changes, resulting in Ca / Fe being greater than 1 (D1-1, D1-2, D1-3) or less than 0.5 (D1-4), even if the other element is still within the preferred content range, the energy density and rate performance of the product will be significantly deteriorated.

[0132] From the comparison of the test results of Comparative Example 2-1, Comparative Example 2-2, Comparative Example 2-3, Comparative Example 2-4 with Example 1-1, it can be seen that: when Ca, Fe, and Ca / Fe are all within the specified range, the temperature of the first carbonization during preparation should be controlled at 500-900°C. If it is higher than 900°C (such as 950°C shown in Comparative Example 2-4) or lower than 500°C (such as 450°C shown in Comparative Example 2-1, Comparative Example 2-2, and Comparative Example 2-3), it will lead to unreasonable distribution of micropores and mesopore structures of the carbon skeleton and insufficient mechanical strength of the carbon skeleton, which will in turn affect the infiltration of the electrolyte and ion transport, which is not conducive to improving the electrochemical performance. From the comparison of the test results of Comparative Examples 3-1, 3-2, 3-3, and 3-4 with those of Example 1-1, it can be seen that when Ca, Fe, and Ca / Fe are all within the specified range, the temperature of the second carbonization during preparation should be controlled at 1100-1400°C. If it is higher than 1400°C (such as 1450°C shown in Comparative Examples 3-1 and 3-2) or lower than 1100°C (such as 1000°C shown in Comparative Examples 3-3 and 3-4), it is not conducive to the condensation, rearrangement, and optimization of the pore structure of the carbon structure, and will also affect the infiltration of the electrolyte and ion transport, resulting in cracking of the electrochemical performance. Obviously, the temperature control of the first and second carbonizations has a great influence on the energy density and rate performance of the product.

[0133] From the comparison of the test results of Comparative Example 4-1 and Example 1-1, it can be seen that the order of pickling has a significant impact on the performance of the product. For example, pickling is performed first and then two-step carbonization is performed. Although Ca, Fe and harmful impurities can be removed, it is not conducive to optimizing the pore structure and increasing the active sites. Therefore, it is not conducive to improving the energy density and rate performance of the material.

[0134] In summary: The preparation method proposed in the present invention limits the Ca element content range, Fe element content range and Ca / Fe range in the coal raw material. The Ca and Fe elements work together to induce the formation of porous structures with different pore sizes. Combined with two carbonizations and acid washing between the two carbonizations, the rate performance of the coal-based hard carbon negative electrode material is significantly improved.

[0135] The present invention not only realizes the high-value conversion of coal, but also enables the sustainable production of hard carbon negative electrode materials, which helps to advance the commercialization of sodium-ion batteries.

[0136] Although embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are illustrative and are not to be construed as limiting the present invention. Those skilled in the art may alter, modify, replace, and modify the above embodiments within the scope of the present invention. Furthermore, those skilled in the art may combine and incorporate the various embodiments or examples described in this specification, as well as features thereof, without conflicting requirements.

Claims

1. A coal-based hard carbon negative electrode material, characterized in that: The coal-based hard carbon negative electrode material is a porous structure with a bimodal pore distribution. The porous structure includes micropores and mesopores. The pore diameter of the micropores is 0.5-2nm, the pore diameter of the mesopores is 2-50nm, and the volume of the micropores in the porous structure accounts for 11%-13%.

2. The coal-based hard carbon negative electrode material according to claim 1, characterized in that: The coal-based hard carbon negative electrode material meets at least one of the following conditions: a) The interlayer spacing of the (002) crystal plane is 0.37-0.39 nm; b) Grain size is c) Specific surface area <11m 2 / g.

3. The coal-based hard carbon negative electrode material according to claim 1, characterized in that: Therein: ash content ≤ 0.5wt%, Fe element content ≤ 100ppm, Ca element content ≤ 100ppm.

4. A method for preparing the coal-based hard carbon negative electrode material according to any one of claims 1 to 3, characterized in that: It is obtained by carbonizing coal as raw material. The coal raw material contains Ca and Fe elements. The content of Ca element is a, and the content of Fe element is b. a and b meet the following conditions: 4000ppm≤a≤9100ppm, 7200ppm≤b≤10000ppm, and 0.5≤a / b≤1.

5. The preparation method according to claim 4, characterized in that: a, b meet: 4600ppm≤a≤8200ppm, 7400ppm≤b≤8100ppm; More preferably, 7600 ppm≤a≤8200 ppm, 7500 ppm≤b≤8000 ppm.

6. The preparation method according to claim 4, characterized in that: The ash content of the coal raw materials is 7%-10%, and the volatile matter content is 35%-42.5%.

7. The preparation method according to claim 4, characterized in that The process includes sequentially performing pulverization, first carbonization, pickling and second carbonization on the coal raw materials. The first carbonization temperature is 500-900°C and the time is 2-5 hours. The second carbonization temperature is 1100-1400°C and the time is 2-5 hours. More preferably, the temperature of the first carbonization is 600°C and the time is 3 hours; More preferably, the temperature of the second carbonization is 1300° C. and the time is 3 h.

8. The preparation method according to claim 7, characterized in that: The coal raw material is specifically bituminous coal, and the Dv50 of the bituminous coal after pulverization is 1-10 μm; the coal raw material is dried before the first carbonization; the reagent used in the pickling is selected from one or more of hydrochloric acid, nitric acid, and hydrofluoric acid; Further preferably, the Dv50 of the bituminous coal after pulverization is 4-6 μm; More preferably, the reagent used for pickling is a mixed solution of hydrochloric acid, nitric acid and hydrofluoric acid.

9. A sodium ion battery, characterized in that: The invention comprises the coal-based hard carbon negative electrode material according to any one of claims 1 to 3.

10. The coal-based hard carbon negative electrode material according to claim 9, characterized in that: Its 0.1C first-week reversible capacity is above 295mAh / g, 0.5C first-week reversible capacity is above 233.5mAh / g, and 1C first-week reversible capacity is above 172mAh / g; Preferably, the 0.5C capacity retention rate after 50 cycles is above 92.5%, and the 1C capacity retention rate after 50 cycles is above 85%.

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