Long flame coal-based hard carbon negative electrode material, preparation method and secondary battery

Abstract

The invention provides a long flame coal-based hard carbon negative electrode material, a preparation method and a secondary battery, and belongs to the technical field of battery materials. The preparation method comprises the following steps: mixing long flame coal and water, introducing oxygen-containing gas, and sealing; heating to convert the water into supercritical water, and carrying out wet oxidation reaction to obtain an intermediate product; and carrying out carbonization treatment on the intermediate product to obtain the long flame coal-based hard carbon negative electrode material. The supercritical water and the dissolved oxygen cooperate with each other and are subjected to dual oxidation, so that the strength and the uniformity of the oxidation reaction between the surfaces of the long flame coal particles and oxygen free radicals are enhanced, and the problem of non-uniform reaction caused by particle accumulation is avoided, so that efficient oxidation of the long flame coal is realized, more ion storage sites and rapid ion transmission channels can be formed, and the service life of the long flame coal is prolonged. The long flame coal-based hard carbon negative electrode material has the advantages of being simple and easy to implement, low in cost, free of pollution, environmentally friendly and the like, and has good development potential.

Description

Technical Field

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

[0002] As the world pays more and more attention to clean energy and sustainable development, secondary batteries, as a highly efficient energy storage device, have been widely used in electric vehicles, portable electronic devices, etc. Among them, the negative electrode material is one of the key factors that determine the performance of secondary batteries.

[0003] At present, although commonly used secondary battery negative electrode materials such as graphite have certain performance advantages, they also have some problems. For example, graphite resources are limited, the cost is high, and the performance in some specific application scenarios needs to be improved. Therefore, finding new high-performance, low-cost negative electrode materials has become a top priority.

[0004] Hard carbon is regarded as the most promising negative electrode material for sodium-ion / lithium-ion batteries due to its high capacity, low working potential, wide material source, and low cost. At present, the hot spot in the field of coal deep processing is the research and development of negative electrode materials using anthracite with a carbon content of more than 90% as raw materials. However, the product obtained by direct high-temperature carbonization of anthracite has a narrow interlayer spacing of carbon atoms, which leads to slow kinetics of the electrode energy storage process, resulting in negative effects such as small capacity and low rate, making the prepared coal-based hard carbon itself have defects such as low specific capacity and power performance deviation. In addition, the carbon materials prepared by direct carbonization of coal are mostly soft carbons with large-layer polycyclic aromatic hydrocarbon molecular structures, which have a certain gap with hard carbon in terms of capacity and electrochemical properties. In addition, for coal oxidation, air oxidation and acid oxidation methods are mostly used, and the above methods have the problems of flammability and high cost.

[0005] Therefore, how to achieve efficient oxidation of coal so that the coal can be fully converted into a hard carbon structure while improving the capacity performance and initial efficiency of coal-based hard carbon is a technical problem that needs to be solved urgently. Summary of the invention

[0006] In view of the deficiencies of the prior art, the present invention aims to provide a long flame coal-based hard carbon negative electrode material, a preparation method and a secondary battery. The preparation method provided by the present invention has the advantages of being simple and easy to operate, low cost, pollution-free, and green and environmentally friendly. By using the oxidation characteristics of supercritical water under high temperature and high pressure and the dual oxidation effect of dissolved oxygen in water, the uniformity of the reaction between the particle surface and oxygen free radicals can be ensured, the problem of uneven reaction caused by particle accumulation can be avoided, and efficient oxidation of coal can be achieved, so that the coal forms a large number of "house of cards"-shaped hard carbon structures, which significantly improves the capacity performance and initial efficiency of the long flame coal-based hard carbon negative electrode material, and has good development potential.

[0007] One of the purposes of the present invention is to provide a method for preparing a coal-based hard carbon negative electrode material, the preparation method comprising the following steps: (1) After mixing long flame coal and water, oxygen-containing gas with a pressure of 0.2-1 MPa is introduced, and the reaction system is sealed; (2) The reaction system is heated to 300-500°C to convert water into supercritical water, and a wet oxidation reaction is performed to obtain an intermediate product; (3) carbonizing the intermediate product to obtain the long flame coal-based hard carbon negative electrode material; Wherein, the long flame coal is in powder form.

[0008] Preferably, during the wet oxidation reaction, the pressure of the reaction system is 20-25 MPa.

[0009] Preferably, the holding time of the wet oxidation reaction is 0.5-5h.

[0010] Preferably, the stirring speed of the wet oxidation reaction is 50-1000 r / min.

[0011] Preferably, the heating rate of the wet oxidation reaction in step (2) is 1-2°C / min.

[0012] Preferably, the mass ratio of the long flame coal to water is 1:(2-10).

[0013] Preferably, step (1) further comprises the step of adding a solvent during the mixing process of the long flame coal and water.

[0014] Preferably, the mass ratio of the co-solvent to water is (1-10):100.

[0015] Preferably, the long flame coal is fine particles with a particle size D50 of 4-20 μm.

[0016] Preferably, step (2) further comprises the steps of separation and drying after the wet oxidation reaction is completed.

[0017] Preferably, the temperature of the carbonization treatment is 1200-1600°C.

[0018] Preferably, the heat preservation time of the carbonization treatment is 1-10h.

[0019] The second object of the present invention is to provide a long flame coal-based hard carbon negative electrode material, wherein the long flame coal-based hard carbon negative electrode material is prepared by the preparation method as described above.

[0020] A third object of the present invention is to provide a secondary battery, wherein the negative electrode of the secondary battery includes the long flame coal-based hard carbon negative electrode material as described above.

[0021] The beneficial effects of the present invention include: The present invention first mixes long flame coal and water, then introduces oxygen-containing gas, and converts water into supercritical water under high temperature and high pressure, performs wet oxidation reaction, and then performs carbonization reaction. Supercritical water and dissolved oxygen in water cooperate with each other and perform dual oxidation, which can enhance the intensity and uniformity of the oxidation reaction between the surface of long flame coal particles and oxygen free radicals, avoid the problem of uneven reaction caused by particle accumulation, and thus achieve efficient oxidation of long flame coal. On the one hand, rich oxygen-containing functional groups can be introduced to increase the surface activity and ion storage capacity of hard carbon negative electrode materials. On the other hand, the organic components in the long flame coal undergo oxidative decomposition to form a large number of micropores and mesopore structures, optimize the crystal structure and micromorphology of the long flame coal, make it easier to form a layered "house of cards" structure during the carbonization process, realize the regulation of the hard carbon structure, and provide more ion storage sites and fast ion transmission channels. The preparation method of the present invention can significantly improve the capacity performance and first effect of long flame coal-based hard carbon negative electrode materials, and has the advantages of being simple, low-cost, pollution-free, green and environmentally friendly, and has good development potential. Furthermore, the preparation method provided by the present invention is simple and easy to implement during industrial scale-up, thus avoiding a large number of complicated process steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A flow chart of the preparation process of the long flame coal-based hard carbon negative electrode material provided in Example 1; Figure 2 This is the SEM image of the long flame coal-based hard carbon negative electrode material prepared in Example 2. DETAILED DESCRIPTION

[0023] In the following description, certain specific details are included to provide a comprehensive understanding of each disclosed embodiment. However, one skilled in the relevant art will recognize that the embodiments can be implemented without one or more of these specific details, but with other methods, components, materials, etc.

[0024] Unless otherwise required herein, the words "include" and "comprising" should be interpreted as having an open and inclusive meaning, ie, "including but not limited to".

[0025] References to "one embodiment" or "an embodiment" or "a preferred embodiment" or "certain embodiments" throughout this specification are intended to include, in at least one embodiment, the specific referenced elements, structures or features described in connection with the embodiment. Thus, the phrases "in one embodiment" or "in an embodiment" or "in a preferred embodiment" or "in certain embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, the specific elements, structures or features may be combined in any appropriate manner in one or more embodiments.

[0026] According to a first aspect of the present invention, a method for preparing a coal-based hard carbon negative electrode material is provided, the preparation method comprising the following steps: (1) After mixing long flame coal and water, oxygen-containing gas with a pressure of 0.2-1 MPa is introduced, and the reaction system is sealed; (2) The reaction system is heated to 300-500°C to convert water into supercritical water, and a wet oxidation reaction is performed to obtain an intermediate product; (3) carbonizing the intermediate product to obtain the long flame coal-based hard carbon negative electrode material; Among them, long flame coal is in powder form.

[0027] Specifically, the wet oxidation reaction is carried out in a high temperature and high pressure reaction device (such as a high temperature and high pressure reactor), and the pressure of the oxygen-containing gas can be, for example, 0.2MPa, 0.4MPa, 0.6MPa, 0.8MPa or 1MPa, and any value between the above two. The temperature of the reaction system is, for example, 350°C, 375°C, 400°C, 425°C or 450°C, and any value between the above two. The oxygen-containing gas includes air and / or oxygen.

[0028] In the present invention, supercritical water refers to water when the density of water expanded due to high temperature and the density of water vapor compressed due to high pressure are exactly the same when the air pressure and temperature reach a certain value. At this time, there is no difference between the liquid and gas of water, which are completely blended together to become a new fluid in a high-pressure and high-temperature state. The viscosity of supercritical water is very low and the diffusion coefficient is very high. Supercritical water has two significant characteristics: one is that it has strong oxidizing ability, which can be used as a strong oxidant to promote the oxidative decomposition of organic matter, and can also be used as a catalyst to accelerate the progress of chemical reactions; the other characteristic is strong solubility, which has a strong solubility for organic matter and a high solubility for gas.

[0029] The present invention first mixes the long flame coal, then introduces oxygen-containing gas, and converts the water into supercritical water under high temperature and high pressure, performs a wet oxidation reaction, and then performs a carbonization reaction. On the one hand, the high temperature and high pressure environment of supercritical water can promote the dissolution of oxygen-containing gas in water, and the dissolved oxygen can be more evenly distributed in the reaction system. The dissolved oxygen in the water can work together with the supercritical water to form more oxygen free radicals; on the other hand, supercritical water has the characteristics of high diffusion coefficient and low viscosity, which can make the oxygen free radicals diffuse quickly and evenly distributed in the reaction system; on the third hand, supercritical water can also dissolve part of the organic matter in the long flame coal, so that the surface of the coal particles exposes more active sites, which is conducive to the full contact and reaction of the oxygen free radicals with the surface of the coal particles. The supercritical water and the oxygen-containing gas can cooperate with each other to enhance the intensity and uniformity of the oxidation reaction. The supercritical water and the dissolved oxygen in the water cooperate with each other and double oxidation, which can enhance the intensity and uniformity of the oxidation reaction between the surface of the long flame coal particles and the oxygen free radicals, avoid the problem of uneven reaction caused by particle accumulation, and thus achieve efficient oxidation of the long flame coal.

[0030] Through efficient oxidation of long flame coal, rich oxygen-containing functional groups can be introduced into the long flame coal to increase the surface activity and ion storage capacity of the hard carbon negative electrode material. It can also cause the organic components in the long flame coal to undergo oxidative decomposition to form a large number of micropores and mesopore structures, optimize the crystal structure and micromorphology of the long flame coal, and make it easier to form a layered "house of cards" structure during the carbonization process, thereby achieving regulation of the hard carbon structure and providing more ion storage sites and fast ion transmission channels. In addition, the capacity performance and first effect of the long flame coal-based hard carbon negative electrode material are improved, and it has the advantages of being simple, low-cost, pollution-free, and green and environmentally friendly, and has good development potential.

[0031] In a preferred embodiment of the present invention, during the wet oxidation reaction, the pressure of the reaction system is 20-25 MPa, for example, it can be 20 MPa, 21 MPa, 22 MPa, 23 MPa, 24 MPa or 25 MPa, and any point value between the above two.

[0032] In the present invention, under the conditions of a pressure of 20-25MPa and a reaction temperature of 350-450°C, it is beneficial to fully convert water into supercritical water. The supercritical state of water under high temperature and high pressure can ensure the uniformity of the reaction between the particle surface and oxygen free radicals, avoiding the problem of uneven reaction caused by particle accumulation. At the same time, with the help of the oxidation characteristics of supercritical water and the dual oxidation effect of dissolved oxygen in water, efficient oxidation of coal can be achieved. If the pressure or reaction temperature is too low, it is difficult for water to fully transform into a supercritical water state, the intensity of the oxidation reaction is weakened, and the efficiency is reduced, and it is difficult to promote the necessary structural adjustment of the long flame coal, thereby affecting the capacity performance and primary effect of the hard carbon negative electrode material. If the pressure or reaction temperature is too high, excessive temperature or pressure may deteriorate the stability of the reaction system, there is a risk of excessive oxidation of coal, destroying its original structure, and also posing challenges to reaction equipment and safety.

[0033] Preferably, the holding time of the wet oxidation reaction is 0.5-5 h, for example, it can be 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h or 5 h, and any value between any two of the above.

[0034] Preferably, the stirring speed of the wet oxidation reaction is 50-1000r / min, for example, it can be 50r / min, 100r / min, 150r / min, 200r / min, 400r / min, 600r / min, 800r / min or 1000r / min, and any point value between any two of the above, preferably 100-200r / min.

[0035] In a preferred embodiment of the present invention, the heating rate of the wet oxidation reaction in step (2) is 1-2°C / min, for example, it can be 1°C / min, 1.2°C / min, 1.4°C / min, 1.5°C / min, 1.6°C / min, 1.8°C / min or 2°C / min, and any point value between the above two.

[0036] In the present invention, if the heating rate is too slow and the time is too long, the preparation efficiency of coal-based hard carbon will be affected. At the same time, if the heating rate is too slow, it will cause it to stay in a certain temperature range for a long time, affecting the formation of effective functional group structure on the coal surface, resulting in poor electrochemical properties of the hard carbon finally formed. If the heating rate is too fast, it may lead to uneven temperature distribution inside the reaction system, and some areas may be over-oxidized while other areas are under-oxidized. It may also damage the structure of the long flame coal, thereby reducing the quality and performance of the hard carbon negative electrode material.

[0037] In a preferred embodiment of the present invention, the mass ratio of the long flame coal to water is 1:(2-10), for example, it can be 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10, and any point value between any two of the above.

[0038] In the present invention, if the mass ratio of long flame coal to water is too small, the amount of water used is too large, which may change the properties of supercritical water, causing it to deviate from the optimal reaction state, and may also dilute the oxygen-containing substances dissolved in the water, reducing the intensity of the oxidation reaction.

[0039] If the mass ratio of long flame coal to water is too large, that is, when there is relatively less water, this may lead to closer contact between the long flame coal particles, making it easy for particles to accumulate, making it difficult to ensure sufficient reaction between the surface of the long flame coal particles and supercritical water and dissolved oxygen. It may also make it difficult to fully transform into a supercritical water state under given temperature and pressure conditions, affecting the reaction rate and uniformity.

[0040] Specifically, the water may be deionized water.

[0041] In a preferred embodiment of the present invention, step (1) further comprises the step of adding a solvent during the mixing of the long flame coal and water.

[0042] In the present invention, the solvent can improve the hydrophilicity of the surface of the long flame coal, help promote the dispersion of the long flame coal in water, prevent the aggregation of particles, and thus help ensure that in the subsequent reaction process, the long flame coal particles can fully contact with the supercritical water and the oxygen-containing gas, thereby improving the uniformity and efficiency of the reaction. The solvent can also interact with the long flame coal to introduce additional active sites on the surface of the long flame coal. It can also increase the solubility of the oxygen-containing gas in water and promote its diffusion in the reaction system, making the oxidation reaction easier to control.

[0043] Preferably, the cosolvent comprises any one of ethanol, acetic acid, hydrogen peroxide or Tween, or a combination of at least two of them, preferably a combination of ethanol, Tween, hydrogen peroxide and acetic acid.

[0044] It should be noted that Tween is a non-ionic surfactant with strong hydrophilicity, a pH of about 6-8, and is stable to electrolytes, weak acids and weak bases.

[0045] Preferably, the mass ratio of the co-solvent to the water is (1-10):100, for example, 1:100, 2:100, 3:100, 4:100, 5:100, 6:100, 7:100, 8:100, 9:100 or 10:100, and any value between the above two.

[0046] In a preferred embodiment of the present invention, the long flame coal is fine particles with a particle size D50 of 4-20μm, for example, 4μm, 6μm, 8μm, 10μm, 12m, 14μm, 16μm, 18μm or 20μm, and any point value between the above two.

[0047] In the present invention, if the particle size is too small, the long flame coal particles may easily agglomerate and be difficult to disperse evenly, affecting the uniformity of the overall reaction; if the particle size is too large, the specific surface area of ​​the long flame coal particles is relatively small, which will reduce the rate and efficiency of the oxidation reaction and may lead to insufficient oxidation reaction. Larger particles may find it difficult to form a uniform "house of cards"-like hard carbon structure during subsequent carbonization treatment.

[0048] In a preferred embodiment of the present invention, step (2) further comprises the steps of separation and drying after the wet oxidation reaction is completed.

[0049] In the present invention, the separation is carried out by centrifugation, and the centrifuge liquid obtained by centrifugation can be recycled.

[0050] Preferably, after the wet oxidation reaction is completed, the mixture is centrifuged at a speed of 500-1500 rpm (for example, 500 rpm, 750 rpm, 1000 rpm, 1250 rpm or 1500 rpm, etc.) for 20-60 min (for example, 20 min, 30 min, 40 min, 50 min or 60 min, etc.), and then dried at 100-120° C. (for example, 100° C., 105° C., 110° C., 115° C. or 120° C., etc.) for 8-12 h to obtain an intermediate product.

[0051] In a preferred embodiment of the present invention, the temperature of the carbonization treatment is 1200-1600°C, for example, it can be 1200°C, 1300°C, 1400°C, 1500°C or 1600°C, and any value between the above two.

[0052] Preferably, the holding time of the carbonization treatment is 1-10 h, for example, it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, and any value between any two of the above.

[0053] Preferably, the carbonization treatment is performed in an inert atmosphere, such as nitrogen or argon.

[0054] Preferably, the heating rate of the carbonization treatment is 1-10°C / min, for example, it can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min or 10°C / min, and any point value between any two of the above.

[0055] In a preferred embodiment of the present invention, the preparation method comprises the following steps: (1) The long flame coal is crushed to obtain fine particles with a particle size D50 of 4-20 μm, which are then mixed with water and a solvent and added to a high temperature and high pressure reaction device, and then an oxygen-containing gas with a pressure of 0.2-1 MPa is introduced and sealed; Among them, the mass ratio of long flame coal to water is 1:(2-10), and the mass ratio of solvent to water is (1-10):100; (2) heating the sealed high temperature and high pressure reaction device of step (1) to 350-450°C at a heating rate of 1-2°C / min, while the pressure of the high temperature and high pressure reaction device is 20-25MPa, so that water is converted into supercritical water, and a wet oxidation reaction is carried out for 0.5-5h. After the reaction is completed, centrifugation is carried out at a speed of 500-1500rpm for 20-60min, and then drying is carried out at 100-120°C for 8-12h to obtain an intermediate product; (3) In an inert atmosphere, the temperature is raised to 1200-1600° C. at a heating rate of 1-10° C. / min, and the intermediate product is carbonized for 1-10 h to obtain the long flame coal-based hard carbon negative electrode material.

[0056] According to a second aspect of the present invention, a long flame coal-based hard carbon negative electrode material is provided. The long flame coal-based hard carbon negative electrode material is prepared by the preparation method described in the first aspect.

[0057] According to a third aspect of the present invention, a secondary battery is provided, wherein the negative electrode of the secondary battery comprises the long flame coal-based hard carbon negative electrode material as described above.

[0058] Preferably, the secondary battery is a lithium ion battery or a sodium ion battery.

[0059] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0060] Example The present invention is further described in detail below in conjunction with the embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0061] In the following examples, unless otherwise specified, all raw materials are commercially available products.

[0062] Example 1 This embodiment provides a method for preparing a long flame coal-based hard carbon negative electrode material, and its preparation process flow chart is as follows: Figure 1 As shown, the preparation method comprises the following steps: (1) The long flame coal is crushed to obtain fine particles with a particle size D50 of 12 μm, which are then mixed with deionized water and a co-solvent and added to a high temperature and high pressure reaction device, and then oxygen with a pressure of 0.6 MPa is introduced and sealed; The mass ratio of long flame coal to deionized water is 1:8, the co-solvent is Tween, and the mass ratio of the co-solvent to deionized water is 5:100; (2) The sealed high-temperature and high-pressure reaction device of step (1) is heated to 374°C at a heating rate of 1.5°C / min, and the pressure of the high-temperature and high-pressure reaction device is 22MPa, so that the water is converted into supercritical water, and the wet oxidation reaction is carried out for 3 hours at a stirring speed of 150r / min. After the reaction, the obtained reaction product is centrifuged at a speed of 1000rpm for 40min, and then dried at 110°C for 10h to obtain an intermediate product; The centrifuge liquid obtained by centrifugation can be refluxed into the wet oxidation reaction for recycling; (3) In a nitrogen atmosphere, the temperature is raised to 1400° C. at a heating rate of 5° C. / min, and the intermediate product is carbonized for 5 h to obtain the long flame coal-based hard carbon negative electrode material.

[0063] Example 2 This embodiment provides a method for preparing a long flame coal-based hard carbon negative electrode material, the preparation method comprising the following steps: (1) The long flame coal is crushed to obtain fine particles with a particle size D50 of 4 μm, which are then mixed with deionized water and a co-solvent and added to a high temperature and high pressure reaction device, and then oxygen with a pressure of 0.2 MPa is introduced and sealed; The mass ratio of long flame coal to deionized water is 1:5, the co-solvent is acetic acid, and the mass ratio of the co-solvent to deionized water is 1:100; (2) The sealed high-temperature and high-pressure reaction device of step (1) is heated to 374°C at a heating rate of 1°C / min, and the pressure of the high-temperature and high-pressure reaction device is 22 MPa, so that the water is converted into supercritical water, and the wet oxidation reaction is carried out for 1 hour at a stirring speed of 150 r / min. After the reaction is completed, the reaction is centrifuged at a speed of 500 rpm for 60 minutes, and then dried at 100°C for 12 hours to obtain an intermediate product; (3) In an argon atmosphere, the temperature is raised to 1200° C. at a heating rate of 1° C. / min, and the intermediate product is carbonized for 10 hours to obtain the long flame coal-based hard carbon negative electrode material.

[0064] Example 3 This embodiment provides a method for preparing a long flame coal-based hard carbon negative electrode material, the preparation method comprising the following steps: (1) The long flame coal is crushed to obtain fine particles with a particle size D50 of 15 μm, which are then mixed with deionized water and a co-solvent and added to a high temperature and high pressure reaction device, and then air with a pressure of 1 MPa is introduced and sealed; The mass ratio of long flame coal to deionized water is 1:10, the co-solvent is hydrogen peroxide, and the mass ratio of the co-solvent to deionized water is 10:100; (2) The sealed high-temperature and high-pressure reaction device of step (1) is heated to 374°C at a heating rate of 2°C / min, and the pressure of the high-temperature and high-pressure reaction device is 22 MPa, so that the water is converted into supercritical water, and the wet oxidation reaction is carried out for 1 hour at a stirring speed of 150 r / min. After the reaction is completed, the reaction is centrifuged at a speed of 1500 rpm for 20 minutes, and then dried at 120°C for 8 hours to obtain an intermediate product; (3) In a nitrogen atmosphere, the temperature is raised to 1600° C. at a heating rate of 10° C. / min, and the intermediate product is carbonized for 1 hour to obtain the long flame coal-based hard carbon negative electrode material.

[0065] Example 4 The difference between this embodiment and embodiment 1 is that the temperature of the high temperature and high pressure reaction device after heating in step (2) is 350° C., and the pressure of the high temperature and high pressure reaction device is 15 MPa, so that water is converted into near-supercritical water.

[0066] The rest of the preparation methods and parameters were the same as those in Example 1.

[0067] Example 5 The difference between this embodiment and embodiment 1 is that the temperature of the high temperature and high pressure reaction device after heating in step (2) is 450° C., and the pressure of the high temperature and high pressure reaction device is 40 MPa, so that water is converted into supercritical water.

[0068] The rest of the preparation methods and parameters were the same as those in Example 1.

[0069] Example 6 The difference between this embodiment and embodiment 1 is that no co-solvent is added in step (1).

[0070] The rest of the preparation methods and parameters were the same as those in Example 1.

[0071] Example 7 The difference between this embodiment and embodiment 1 is that the temperature of the high temperature and high pressure reaction device after heating in step (2) is 300°C.

[0072] The rest of the preparation methods and parameters were the same as those in Example 1.

[0073] Example 8 The difference between this embodiment and embodiment 1 is that the temperature of the high temperature and high pressure reaction device after heating in step (2) is 500°C.

[0074] The rest of the preparation methods and parameters were the same as those in Example 1.

[0075] Example 9 The difference between this embodiment and embodiment 1 is that the mass ratio of the long flame coal to water in step (1) is 1:10.

[0076] The rest of the preparation methods and parameters were the same as those in Example 1.

[0077] Example 10 The difference between this embodiment and embodiment 1 is that the mass ratio of the long flame coal to water in step (1) is 1:5.

[0078] The rest of the preparation methods and parameters were the same as those in Example 1.

[0079] Embodiment 11 The difference between this embodiment and embodiment 1 is that the mass ratio of the co-solvent to water in step (1) is 1:100.

[0080] The rest of the preparation methods and parameters were the same as those in Example 1.

[0081] Example 12 The difference between this embodiment and embodiment 1 is that the mass ratio of the co-solvent to water in step (1) is 10:100.

[0082] The rest of the preparation methods and parameters were the same as those in Example 1.

[0083] Comparative Example 1 The difference between this comparative example and Example 1 is that in step (1), the high temperature and high pressure reaction device is not sealed, so that after the high temperature and high pressure reaction device is heated, water is completely converted into water vapor.

[0084] The rest of the preparation methods and parameters were the same as those in Example 1.

[0085] Comparative Example 2 The difference between this comparative example and Example 1 is that the oxygen introduced in step (1) is replaced by nitrogen.

[0086] The rest of the preparation methods and parameters were the same as those in Example 1.

[0087] Comparative Example 3 This comparative example provides a method for preparing a long flame coal-based hard carbon negative electrode material, the preparation method comprising the following steps: The long flame coal is crushed to obtain fine particles with a particle size D50 of 15m; The long flame coal and nitric acid were subjected to acid oxidation treatment in a mass ratio of 1:10, the acid oxidation time was 1 hour, the reaction temperature was 80°C, and the concentration of nitric acid was 5 mol / L.

[0088] After the reaction, the mixture was centrifuged at 1500 rpm for 20 min and then dried at 120 °C for 8 h to obtain an intermediate product; The dried intermediate product was heated to 1600° C. at a heating rate of 10° C. / min in a nitrogen atmosphere, and the intermediate product was carbonized for 1 hour to obtain the long flame coal-based hard carbon negative electrode material.

[0089] Performance Testing (1) The microstructure of the long flame coal-based hard carbon negative electrode material prepared in Example 2 was characterized, and its SEM image is shown in FIG. Figure 2 shown.

[0090] (2) First, the long flame coal-based hard carbon negative electrode material is made into a negative electrode sheet, and the preparation method of the negative electrode sheet includes: mixing the prepared hard carbon, conductive carbon black (Super P), carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) according to a formula ratio of 91:2:2:5 to prepare a slurry. The prepared slurry is scraped onto the carbon-coated copper foil, and then transferred to a 65°C forced air drying oven until the electrode sheet is dry (about 1-2 h), and then transferred to a vacuum drying oven for drying for 12 h to obtain a negative electrode sheet.

[0091] Then the negative electrode plate is assembled with the positive electrode plate sodium plate, a polyethylene separator and a carbonate electrolyte to obtain a sodium ion battery.

[0092] The electrochemical performance of the sodium ion battery was tested, including capacity performance and initial efficiency. The conditions for charge and discharge tests were: voltage range 0-2V, current density 30mA·g −1 Charge and discharge tests were carried out under the conditions to obtain the reversible capacity and initial efficiency.

[0093] The test results are shown in Table 1.

[0094] Table 1 Test results of Examples 1-12 and Comparative Examples 1-3

[0095] analyze: It can be seen from the above table that the preparation method provided by the present invention has the advantages of being simple, easy to operate, low-cost, pollution-free, and green and environmentally friendly. By making use of the oxidation characteristics of supercritical water under high temperature and high pressure and the dual oxidation effect of dissolved oxygen in water, the uniformity of the reaction between the particle surface and oxygen free radicals can be ensured, avoiding the problem of uneven reaction caused by particle accumulation, and achieving efficient oxidation of coal, so that the coal forms a large number of "house of cards"-shaped hard carbon structures, which significantly improves the capacity performance and initial efficiency of long flame coal-based hard carbon negative electrode materials, and has good development potential.

[0096] It can be seen from Example 1, Examples 4-5, and Examples 7-8 that if the temperature or pressure of the high-temperature and high-pressure reaction device after heating is too low, the oxidation effect on the long flame coal particles will be poor, which may cause insufficient formation of the "house of cards" structure of the hard carbon, and the reversible capacity and initial efficiency of the hard carbon will be low; if the temperature or pressure of the high-temperature and high-pressure reaction device after heating is too high, the oxidation effect on the long flame coal particles will be too sufficient, resulting in the existence of a large number of pore structures, which will make it difficult to close the pores after subsequent carbonization treatment, resulting in low reversible capacity and initial efficiency of the hard carbon.

[0097] It can be seen from Examples 1 and 6 that if a solvent is not added in step (1), the long flame coal and water cannot be completely infiltrated and mixed evenly, resulting in poor reaction uniformity of the long flame coal in the subsequent wet oxidation process.

[0098] It can be seen from Examples 1 and 9-10 that if the mass ratio of long flame coal and water is too small, the reaction will easily deviate from the optimal reaction state, and may also dilute the oxygen-containing substances dissolved in the water, reducing the intensity of the oxidation reaction, resulting in lower reversible capacity and first efficiency of the hard carbon; if the mass ratio of long flame coal and water is too large, particle accumulation will easily occur, and it may also make it difficult to fully transform into a supercritical water state under given temperature and pressure conditions, affecting the reaction rate and uniformity, resulting in lower reversible capacity and first efficiency of the hard carbon.

[0099] It can be seen from Example 1 and Examples 11-12 that if the amount of solvent used is too small, it is easy to cause poor uniformity of the long flame coal reaction; if the amount of solvent used is too large, it may induce side reactions and may interfere with the structure formation of the long flame coal during the subsequent carbonization treatment process; if the amount of solvent used is too small or too large, it is easy to form an inappropriate hard carbon structure and thus lead to a decrease in the capacity performance and cycle stability of the material.

[0100] It can be seen from Example 1 and Comparative Example 1 that if the high temperature and high pressure reaction device is not sealed in step (1), the water in the high temperature and high pressure reaction device will be completely converted into water vapor after the temperature is increased, and a large amount of water vapor will escape and fail to oxidize the long flame coal.

[0101] It can be seen from Example 1 and Comparative Example 2 that if the oxygen introduced in step (1) is replaced by nitrogen, the long flame coal will show surface inertness during the wet oxidation process under high temperature and high pressure, and the oxidation effect on the interior of the long flame coal particles is poor.

[0102] It can be seen from Example 1 and Comparative Example 3 that if conventional technology is used to prepare long flame coal-based hard carbon negative electrode materials, the electrochemical properties and capacity properties of the prepared hard carbon are poor, and the acid waste liquid generated during the preparation process is difficult to treat, causing pollution to the environment.

[0103] The applicant declares that the present invention illustrates the process method of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned process steps, that is, it does not mean that the present invention must rely on the above-mentioned process steps to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of the raw materials selected by the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a long flame coal-based hard carbon negative electrode material, characterized in that: The preparation method comprises the following steps: (1) After mixing long flame coal and water, oxygen-containing gas with a pressure of 0.2-1 MPa is introduced, and the reaction system is sealed; (2) The reaction system is heated to 300-500°C to convert water into supercritical water, and a wet oxidation reaction is performed to obtain an intermediate product; (3) carbonizing the intermediate product to obtain the long flame coal-based hard carbon negative electrode material; Wherein, the long flame coal is in powder form.

2. The preparation method according to claim 1, characterized in that: During the wet oxidation reaction, the pressure of the reaction system is 20-25 MPa, the insulation time of the wet oxidation reaction is 0.5-5 h, and the stirring speed is 50-1000 r / min.

3. The preparation method according to claim 1, characterized in that: The heating rate of the wet oxidation reaction in step (2) is 1-2°C / min.

4. The preparation method according to claim 1, characterized in that: The mass ratio of the long flame coal to water is 1:(2-10).

5. The preparation method according to claim 1, characterized in that: Step (1) also includes the step of adding a solvent during the mixing process of the long flame coal and water, wherein the mass ratio of the solvent to water is (1-10):

100.

6. The preparation method according to claim 1, characterized in that: The long flame coal is fine particles with a particle size D50 of 4-20 μm.

7. The preparation method according to claim 1, characterized in that: Step (2) also includes the steps of separation and drying after the wet oxidation reaction is completed.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The temperature of the carbonization treatment is 1200-1600° C., and the insulation time of the carbonization treatment is 1-10 hours.

9. A long flame coal-based hard carbon negative electrode material, characterized in that: The long flame coal-based hard carbon negative electrode material is prepared by the preparation method according to any one of claims 1 to 8.

10. A secondary battery, characterized in that: The negative electrode of the secondary battery includes the long flame coal-based hard carbon negative electrode material as claimed in claim 9.

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