A coal-based hard carbon with a gradient structure, and a preparation method and application thereof

Abstract

The application provides a coal-based hard carbon with a gradient structure and a preparation method and application thereof, and belongs to the technical field of sodium ion battery electrode materials. The coal-based hard carbon is prepared by the following steps: performing oxidative crosslinking on a deashed coal-based precursor in a wet environment with less solvent, then performing gradient reduction treatment in a reducing atmosphere, realizing gradient reduction from the surface to the inside of the particles, and finally performing carbonization treatment, so that the coal-based hard carbon material with the amorphous degree gradually increasing from the surface to the inside is prepared. The coal-based hard carbon prepared by the application has a gradient structure, the structural disorder degree gradually increases from the surface to the inside of the material particles, and there is no two-phase mutation interface. The inside is a closed pore curved around, which is a rich sodium storage site; and the near-surface is a microcrystalline carbon inert structure with high order degree, which reduces the surface side reaction, enhances the solid-phase diffusion and surface charge transfer, and makes the coal-based hard carbon exhibit excellent low-temperature performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sodium ion battery electrode materials, and particularly relates to a coal-based hard carbon with a gradient structure and a preparation method and application thereof. BACKGROUND

[0002] With the growing demand for clean energy and the increasing demand for reliable energy storage solutions in extreme environments, the development of low-temperature batteries is particularly important. Sodium-ion batteries, due to their abundant resources and low cost, have shown unique advantages in large-scale energy storage applications. However, the performance of sodium-ion batteries under low-temperature conditions is insufficient, which limits their widespread use in cold regions, such as electric vehicles and outdoor energy storage facilities. Scientists are working to improve the energy density, cycle stability and safety of these batteries by improving the electrolyte material, positive and negative electrode materials and their structural design, in order to overcome this challenge and broaden the application range of sodium-ion batteries. Among them, the negative electrode material is of great significance to the energy density of sodium-ion batteries, especially the platform capacity at low potential, which directly affects the energy density of the full battery.

[0003] Carbon materials are widely used as negative electrode materials for sodium-ion batteries due to their high stability, good electrical conductivity, and adjustable microstructure. Hard carbon materials have a low degree of graphitization, mainly composed of a large number of disordered carbon microcrystals interlaced and stacked, with large interlayer spacing and a large number of nanometer micropores, which provide ideal active sites for sodium ion storage. At the same time, hard carbon materials have low sodium storage potential, high reversible capacity and excellent cycle stability in sodium-ion batteries, and are considered to be the best potential negative electrode material for sodium-ion batteries.

[0004] China is rich in coal resources, which contains a large amount of graphite-like microcrystals and condensed aromatic clusters in its composition structure, and has a high carbon yield, making it an excellent carbonaceous raw material. Although coal-based carbon materials with high carbon content and strong electrical conductivity can be prepared by coal selection and carbonization, the coal-based carbon materials prepared by direct carbonization of coal have high order degree and are prone to form graphite-like structures, lacking closed pore sodium storage sites. Researchers have improved the disorder degree of the material through pre-oxidation and template methods, but the high disorder degree of the surface is prone to cause side reactions and is not conducive to the charge transfer on the surface of the electrode at low temperature. The viscosity of the electrolyte increases at low temperature, the liquid phase diffusion is delayed, the solid phase diffusion in the electrode material is slow, and the charge transfer at the interface between the electrolyte and the electrode material surface is slow. Surface coating can reduce the occurrence of surface side reactions and improve the electrical conductivity of the material surface, but due to the existence of the two-phase interface between the surface coating layer and the interior, the resistance of solid phase diffusion is increased, and there are also problems such as reducing the volume energy density of the negative electrode material, and the uniformity of the coating is difficult to control, which cannot meet the demand for high-performance low-temperature sodium-ion battery negative electrode materials.

[0005] Therefore, it is urgent to develop a preparation method of coal-based hard carbon with gradient structure, which can provide rich ion storage sites, reduce the occurrence of surface side reactions and phase interface problems, and improve the low-temperature sodium storage capacity and ion diffusion rate. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a coal-based hard carbon with gradient structure and a preparation method and application thereof, which solves the technical problems of the prior art, such as complicated preparation method of coal-based hard carbon, few ion storage sites, easy occurrence of surface side reactions, and phase interface and uniformity in coating treatment.

[0007] In order to achieve the above-mentioned purpose of the application, the present application provides the following technical solutions.

[0008] The present application provides a preparation method of coal-based hard carbon with gradient structure, comprising the following steps:

[0009] (1) oxidizing and cross-linking the de-ashed coal-based precursor in a solvent to obtain an over-oxidized coal-based precursor;

[0010] (2) performing gradient reduction treatment on the over-oxidized coal-based precursor in a reducing atmosphere to obtain a gradient-reduced precursor;

[0011] (3) finally, sequentially performing carbonization treatment on the gradient-reduced precursor to obtain a coal-based hard carbon material with gradient structure.

[0012] Further, the coal-based precursor includes one or more of lignite, bituminous coal, anthracite, coal pitch, petroleum pitch, coal coke and petroleum coke.

[0013] The particle size of the coal-based precursor is 100-800 mesh.

[0014] Further, the solvent includes one or more of water, ethanol, propanol and diethyl ether.

[0015] Further, the oxidizing agent used in the oxidizing and cross-linking process includes one or more of peroxide, persulfide, oxidizing acid and acid anhydride.

[0016] Further, the temperature of the oxidizing and cross-linking is 80-200 DEG C, the pressure of the oxidizing and cross-linking is 0.1-2.3 MPa, and the time of the oxidizing and cross-linking is 3-10 h.

[0017] The mass-volume ratio of the coal-based precursor, the oxidizing agent and the solvent is 8-12 g:2 g:10-70 mL.

[0018] Further, the gas of the reducing atmosphere includes one or more of hydrogen, carbon monoxide and methane.

[0019] The temperature of the gradient reduction treatment is 100-500 DEG C, and the time of the gradient reduction treatment is 1-5 hours.

[0020] Further, the temperature of the carbonization treatment is 1000-2000 DEG C, the heating rate is 0.5-20 DEG C / min, and the time of the carbonization treatment is 0.5-10 hours.

[0021] The carbonization treatment is carried out in a protective atmosphere, and the protective atmosphere comprises an inert gas or a vacuum environment.

[0022] The application provides a gradient-structure coal-based hard carbon prepared by the preparation method of the gradient-structure coal-based hard carbon.

[0023] The application further provides an application of the gradient-structure coal-based hard carbon in preparation of a sodium ion battery negative electrode material.

[0024] The application has the following beneficial effects:

[0025] (1) The application prepares a coal-based hard carbon material with an increase in amorphous degree from the surface to the inside by gradient reduction after over-oxidation, and the gradient microstructure of the coal-based hard carbon with controllability is prepared by controlling the temperature, pressure, solution concentration of the oxidation cross-linking reaction in the solvent wetting environment, the temperature, time and gas atmosphere of the gradient reduction treatment, and the carbonization temperature, time and heating rate.

[0026] (2) The negative electrode material of the coal-based hard carbon sodium ion battery prepared in the application has a gradient function distribution, the inside is a curved and surrounded disordered carbon sheet layer, which is stacked to form a closed pore, creating abundant sites for sodium ion filling at a low potential; and the near-surface is a higher-order microcrystalline carbon structure, reducing the surface side reaction, thereby showing excellent electrochemical performance.

[0027] (3) The negative electrode material of the coal-based hard carbon sodium ion battery prepared in the application has abundant sodium-storing sites inside and a sodium-inert structure on the surface, and the carbon structure gradually transitions from the surface to the inside without obvious phase separation interface, thereby enhancing the internal solid-phase diffusion and reducing the surface charge transfer resistance, and thus showing excellent low-temperature performance.

[0028] (4) The coal-based hard carbon prepared by the method has excellent electrochemical performance as a negative electrode material of a sodium ion battery, including a high sodium storage capacity (303.3 mAh·g -1 ), a long low-potential platform sodium storage capacity (the platform capacity below 0.1 V is 222 mAh / g), a high initial coulombic efficiency (84.6%) and high cycle stability, which can meet the commercialization needs of high-energy density and high-power density sodium ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Process flow chart for preparing the gradient structure coal-based hard carbon negative material in Example 1 of the present application.

[0030] Figure 2 XRD chart of the gradient structure coal-based hard carbon negative material of sodium ion battery prepared in Example 1.

[0031] Figure 3 Scanning electron microscope chart of the gradient structure coal-based hard carbon negative material of sodium ion battery prepared in Example 1 of the present application.

[0032] Figure 4 Transmission electron microscope chart and carbon structure model of the gradient structure coal-based hard carbon negative material of sodium ion battery prepared in Example 1 of the present application.

[0033] Figure 5 First three circle charge-discharge curve chart of the gradient structure coal-based hard carbon negative material prepared in Example 1 of the present application.

[0034] Figure 6 Low temperature performance chart of the gradient structure coal-based hard carbon negative material prepared in Example 1 of the present application.

[0035] Figure 7 Transmission electron microscope chart of the coal-based hard carbon negative material prepared in Comparative Examples 1, 2 and 3 of the present application.

[0036] Figure 8 Cyclic voltammetry curve of the sodium ion battery prepared in Example 1 and Comparative Examples 1-3, wherein (a) is the cyclic voltammetry curve of the sodium ion battery prepared in Example 1, (b) is the cyclic voltammetry curve of the sodium ion battery prepared in Comparative Example 1, (c) is the cyclic voltammetry curve of the sodium ion battery prepared in Comparative Example 2, and (d) is the cyclic voltammetry curve of the sodium ion battery prepared in Comparative Example 3. DETAILED DESCRIPTION

[0037] The present application provides a preparation method of a gradient structure coal-based hard carbon, comprising the following steps:

[0038] (1) oxidizing and cross-linking the de-ashed coal-based precursor in a solvent to obtain an over-oxidized coal-based precursor;

[0039] (2) performing gradient reduction treatment on the over-oxidized coal-based precursor in a reducing atmosphere to obtain a gradient-reduced precursor;

[0040] (3) finally sequentially performing carbonization treatment on the gradient-reduced precursor to obtain a coal-based hard carbon material with gradient structure.

[0041] In the present application, the coal-based precursor is obtained by heating and mixing a carbon precursor and a mixed acid solution and then standing;

[0042] The coal-based precursor includes one or more of lignite, bituminous coal, anthracite, coal pitch, petroleum pitch, coal coke and petroleum coke, preferably one or more of lignite, bituminous coal, anthracite, coal pitch and petroleum pitch, and further preferably one or more of lignite, bituminous coal, anthracite and coal pitch.

[0043] In the present application, the particle size of the coal-based precursor is 100-800 mesh, preferably 200-500 mesh.

[0044] In the present application, the solvent includes one or more of water, ethanol, propanol and diethyl ether, preferably one or more of water, ethanol and diethyl ether.

[0045] In the present application, the oxidizing agent used in the oxidative crosslinking process includes one or more of peroxide, persulfide, oxidizing acid and acid anhydride, preferably one or more of peroxide, oxidizing acid and acid anhydride, and further preferably peroxide and / or acid anhydride.

[0046] In the present application, the temperature of the oxidative crosslinking is 80-200℃, preferably 100-180℃, and further preferably 140-160℃; the pressure of the oxidative crosslinking is 0.1-2.3 MPa, preferably 0.11-2.0 MPa, and further preferably 0.12-1.5 MPa; and the time of the oxidative crosslinking is 3-10 h, preferably 3-8 h, and further preferably 3-5 h.

[0047] In the present application, the mass-volume ratio of the coal-based precursor, the oxidizing agent and the solvent is 8-12 g:2 g:10-70 mL, preferably 9-11 g:2 g:15-60 mL, and further preferably 10 g:2 g:20-50 mL.

[0048] In the present application, the temperature of the gradient reduction treatment is 100-500℃, preferably 150-450℃, and further preferably 200-400℃; and the time of the gradient reduction treatment is 1-5 h, preferably 1-4.5 h, and further preferably 1-4 h.

[0049] In the present application, the gas of the reducing atmosphere includes one or more of hydrogen, carbon monoxide and methane, preferably hydrogen and / or methane, and further preferably hydrogen.

[0050] In the present application, the temperature of the carbonization treatment is 1000-2000℃, the heating rate is 0.5-20℃ / min, and the time of the carbonization treatment is 0.5-10 h.

[0051] Preferably, the temperature of the carbonization treatment is 1100-1800℃, the heating rate is 1-15℃ / min, and the time of the carbonization treatment is 1-9 h.

[0052] Further preferably, the temperature of the carbonization treatment is 1200-1600℃, the heating rate is independently 2-10℃ / min, and the carbonization treatment time is 2-8h.

[0053] In the present application, the carbonization treatment is performed in a protective atmosphere, which comprises an inert gas or a vacuum environment, preferably an inert gas.

[0054] In the present application, the carbonization treatment is followed by acid washing.

[0055] In the present application, by controlling the temperature, pressure, solution concentration of the oxidative crosslinking reaction under high temperature and high pressure in the solvent wetting environment, the temperature, time, and gas atmosphere of the gradient reduction treatment, and the carbonization temperature, time, and heating rate, the oxygen content and oxygen configuration of the coal-based precursor can be controlled, and the size, distribution state, and crystallization degree of the graphitic microcrystalline region of the coal-based hard carbon material can be regulated, so as to obtain a coal-based hard carbon negative electrode material with a microstructure gradient that can be adjusted.

[0056] In the present application, the wetting environment for oxidative crosslinking is to wet each 10g of precursor with 10-70mL of solvent, so as to promote oxidation and crosslinking through solvent wetting and high temperature and high pressure environment.

[0057] The present application provides a gradient-structured coal-based hard carbon prepared by the preparation method of the gradient-structured coal-based hard carbon.

[0058] In the present application, the coal-based hard carbon has a gradient carbon structure, the inside is a curved and winding disordered carbon sheet layer, which is stacked to form closed pores, forming abundant sodium-storing sites; and the near-surface is a microcrystalline carbon structure with a higher degree of order, forming a sodium-averse inert structure, reducing the surface side reaction, thereby enhancing the internal solid-phase diffusion and reducing the surface charge transfer resistance.

[0059] The present application also provides a use of the gradient-structured coal-based hard carbon in the preparation of a sodium ion battery negative electrode material.

[0060] The technical solutions provided by the present application will be described in detail below in conjunction with the embodiments, but they should not be understood as limiting the scope of protection of the present application.

[0061] Example 1

[0062] The present embodiment provides a preparation method of a gradient-structured coal-based hard carbon negative electrode material, and a process flow chart thereof is shown in Figure 1 The present embodiment provides a preparation method of a gradient-structured coal-based hard carbon negative electrode material, and a process flow chart thereof is shown in

[0063] Take 10 g of the ash-removed lignite, crush it and pass it through a 200-mesh sieve, then mix it with 2 g of maleic anhydride and add it to a reaction kettle, then add 40 mL of deionized water, and react at 160℃ and 0.12 MPa for 3 h, then wash, filter and dry to obtain over-oxidized and cross-linked lignite;

[0064] The over-oxidized lignite is subjected to gradient reduction treatment in an argon-hydrogen mixed gas atmosphere, and is heated to 300℃ at a rate of 5℃ / min, and the gradient reduction treatment is performed for 1 h, wherein the volume ratio of the flow rates of hydrogen and argon in the argon-hydrogen mixed gas is V H2 / V Ar = 10 / 90 mL min -1 ;

[0065] The over-oxidized lignite is subjected to gradient reduction treatment in an argon-hydrogen mixed gas atmosphere, and is heated to 300℃ at a rate of 5℃ / min, and the gradient reduction treatment is performed for 1 h, wherein the volume ratio of the flow rates of hydrogen and argon in the argon-hydrogen mixed gas is V

[0066] The gradient-structured coal-based hard carbon prepared in Example 1 has a powder morphology, and the transmission electron microscope image shows that there are abundant sp2 graphite region microcrystals and sp3 defect structures, and the degree of amorphousness increases from the surface to the interior, and the ID / IG value is 1.18.

[0067] The gradient-structured coal-based hard carbon prepared in Example 1 is used as a negative electrode material to prepare a sodium ion battery, and the preparation method of the sodium ion battery comprises the following steps:

[0068] A gradient-structured coal-based dense negative electrode material, acetylene black and carboxymethylcellulose sodium are uniformly mixed in a mass ratio of 92:3:5, and then uniformly coated on a copper foil, and the battery electrode is prepared after drying at 60℃ for 12 h, and the active material loading of the negative electrode is greater than 1 mg·cm -2 The obtained battery electrode is used as a working electrode, a sodium sheet is used as a counter electrode, a GF / D glass fiber is used as a separator, and a 1M NaPF6 (solvent including dimethyl ether) is used as an electrolyte, and all assembly processes are carried out in a glove box.

[0069] The sodium ion battery prepared in Example 1 is tested.

[0070] Figure 2 The XRD pattern of the gradient-structured coal-based hard carbon sodium ion battery negative electrode material prepared in Example 1 is shown in the figure, and the XRD pattern of the gradient-structured coal-based hard carbon shows two wide diffraction peaks at 22° and 43°, which are typical diffraction peaks of (002) and (100) crystal planes in disordered carbon.

[0071] Figure 3The scanning electron microscope image of the gradient structure coal-based hard carbon sodium ion battery negative electrode material prepared in Example 1 can be seen from the electron microscope image that the coal-based hard carbon negative electrode material presents an irregular particle shape.

[0072] Figure 4 The transmission electron microscope image of the gradient structure coal-based hard carbon sodium ion battery negative electrode material prepared in Example 1 can be seen from the figure that the coal-based hard carbon with a gradient structure has a curved and winding disordered carbon sheet layer inside, which is stacked to form a closed pore, and the near-surface is a higher-order microcrystalline carbon structure.

[0073] Figure 5 The first three weeks of charge-discharge curve of the gradient structure coal-based hard carbon negative electrode material prepared in Example 1 can be seen from the figure that the discharge curve of the coal-based hard carbon negative electrode material presents a typical slope region and a platform region, the reversible capacity of the negative electrode material is 303.3 mAh g -1 , the first efficiency is 84.6%, and the 0.1V platform capacity is 222 mAh·g -1 .

[0074] Figure 6 The sodium storage capacity of the gradient structure coal-based hard carbon negative electrode material prepared in Example 1 at different low temperatures can be seen from the figure that at 25, 0, -10, -20, -30, -40℃, the reversible capacity of the gradient structure coal-based hard carbon negative electrode material is 300, 271, 254, 232, 214, 195 mAh g -1 , which shows excellent performance at low temperature.

[0075] Example 2

[0076] 10g of deashed lignite is weighed, crushed and then passed through a 200 mesh sieve, then mixed with 2g of maleic anhydride and added to a reaction kettle, then 40mL of deionized water is added, and the reaction is carried out at 100℃, 0.1MPa for 3h, after washing, filtering and drying, the over-oxidized lignite is obtained;

[0077] The over-oxidized lignite is subjected to gradient reduction treatment under an argon-hydrogen mixed gas atmosphere, heated to 300℃ at a rate of 5℃ / min, and the gradient reduction treatment time is 1h, wherein the volume ratio of hydrogen and argon in the argon-hydrogen mixed gas is V H2 / V Ar =10 / 90mL min -1 ;

[0078] Carbonization treatment is carried out in an argon atmosphere at a rate of 5℃ / min to 1600℃, and the carbonization treatment time is 3h, and finally the gradient structure coal-based hard carbon is obtained after acid washing.

[0079] The gradient structure coal-based hard carbon prepared in Example 2 is in powder morphology, and the transmission electron microscopy image shows that there are abundant sp2graphite region microcrystals and sp3defect structures, and the amorphous degree increases from the surface to the inside, and the ID / IG value is 1.15.

[0080] The sodium ion battery is prepared according to the method of Example 1, and the sodium ion battery prepared in Example 2 is subjected to electrochemical test, and the test result is that the sodium ion battery has high initial coulomb efficiency (85.4%), high reversible capacity (294.5mAh·g -1 ), and the capacity of the 0.1V platform is 226mAh·g -1 The reversible capacity of the sodium ion battery is 268mAh·g -1 at 0℃, and the reversible capacity is 216mAh·g -1 at-30℃.

[0081] Example 3

[0082] 10g of the lignite treated by ash removal is crushed and then passed through a 200 mesh sieve, and then mixed with 2g of maleic anhydride and added to a reaction kettle, and then 40mL of deionized water is added, and the reaction is carried out at 200℃ and 1.5MPa for 3h, and after washing, filtration and drying, the over-oxidized lignite is obtained;

[0083] The over-oxidized lignite is subjected to gradient reduction treatment in an argon-hydrogen mixed gas atmosphere, and the temperature is raised to 300℃ at a rate of 5℃ / min, and the gradient reduction treatment time is 1h, wherein the volume ratio of the flow rates of hydrogen and argon in the argon-hydrogen mixed gas is V H2 / V Ar =10 / 90mL min -1 ;

[0084] The carbonization treatment is carried out in an argon atmosphere at a rate of 5℃ / min to 1400℃, and the carbonization treatment time is 3h, and finally the gradient structure coal-based hard carbon is obtained after acid washing.

[0085] The gradient structure coal-based hard carbon prepared in Example 3 is in powder morphology, and the transmission electron microscopy image shows that there are abundant sp2graphite region microcrystals and sp3defect structures, and the amorphous degree increases from the surface to the inside, and the ID / IG value is 1.23.

[0086] The sodium ion battery is prepared according to the method of Example 1, and the sodium ion battery prepared in Example 3 is subjected to electrochemical test, and the test result is that the sodium ion battery has high initial coulomb efficiency (82.4%), high reversible capacity (291.3mAh·g -1 ), and the capacity of the 0.1V platform is 193mAh·g -1 The reversible capacity of the sodium ion battery is 264mAh·g -1The reversible capacity at 0°C is 256 mAh·g -1 .

[0087] Example 4

[0088] 10 g of the deashed lignite is crushed and sieved through a 200 mesh sieve, then mixed with 2 g of maleic anhydride and added to a reaction kettle, 40 mL of ethanol is added, and the reaction is carried out at 160°C and 0.12 MPa for 3 h. After washing, the product is filtered and dried to obtain over-oxidized lignite;

[0089] The over-oxidized lignite is subjected to low-temperature heat treatment in an argon-hydrogen mixed gas atmosphere, and the temperature is raised to 300°C at a rate of 5°C / min, and the low-temperature heat treatment time is 1 h. In the argon-hydrogen mixed gas, the volume ratio of the flow rates of hydrogen and argon is V H2 / V Ar = 10 / 90 mL / min -1 .

[0090] Carbonization treatment is carried out in an argon atmosphere at a rate of 5°C / min to 1500°C, and the carbonization treatment time is 3 h. Finally, acid washing is carried out to obtain a gradient structure coal-based hard carbon.

[0091] The gradient structure coal-based hard carbon prepared in Example 4 has a powder morphology, and the transmission electron microscopy image shows that there are abundant sp2 type graphite region microcrystallites and sp3 defect structures. The amorphous degree increases from the surface to the inside, and the ID / IG value is 1.20.

[0092] A sodium ion battery is prepared according to the method of Example 1, and the sodium ion battery prepared in Example 4 is subjected to electrochemical testing. The test results are as follows: the sodium ion battery has a high initial coulombic efficiency (85.2%), a high reversible capacity (298.4 mAh·g -1 ), and a platform capacity of 214 mAh·g -1 at 0°C. The reversible capacity of the sodium ion battery is 256 mAh·g -1 at 0°C, and the reversible capacity is 205 mAh·g -1 .

[0093] Example 5

[0094] The difference between this example and Example 1 is that the gradient reduction treatment temperature is 200°C.

[0095] The rest of the preparation method and parameters are consistent with Example 1.

[0096] Example 6

[0097] The difference between this example and Example 1 is that the gradient reduction treatment temperature is 400°C.

[0098] The rest of the preparation method and parameters are consistent with Example 1.

[0099] Example 7

[0100] The difference between this example and Example 1 is that the reducing gas passed in the gradient reduction treatment is methane.

[0101] The rest of the preparation method and parameters are consistent with Example 1.

[0102] Example 8

[0103] The difference between this example and Example 1 is that the coal-based precursor used is anthracite.

[0104] The rest of the preparation method and parameters are consistent with Example 1.

[0105] Comparative Example 1

[0106] The difference between this comparative example and Example 1 is that the liquid phase oxidation and gradient reduction operations are not performed.

[0107] The rest of the preparation method and parameters are consistent with Example 1.

[0108] Comparative Example 2

[0109] The difference between this comparative example and Example 1 is that the liquid phase oxidation is not performed.

[0110] The rest of the preparation method and parameters are consistent with Example 1.

[0111] Comparative Example 3

[0112] The difference between this comparative example and Example 1 is that the gradient reduction treatment is not performed.

[0113] The rest of the preparation method and parameters are consistent with Example 1.

[0114] The sodium-ion batteries prepared in Examples 1-8 and Comparative Examples 1-3 were subjected to electrochemical performance tests, including:

[0115] Cyclic voltammetry test: the voltage range was 0-2.5V, the scanning speed was 0.1mV s -1 , and 3 cycles of scanning were performed.

[0116] Charge-discharge test: the voltage range was 0-2.5V, and the charge-discharge test was performed at a current density of 20mA / g to obtain the specific charge-discharge capacity and the initial efficiency.

[0117] Low temperature performance test: the test temperature was 0, -10, -20, -30, -40℃, the voltage range was 0-2.5V, and the charge-discharge test was performed at a current density of 20mA / g.

[0118] The test results are shown in Table 1.

[0119] Table 1 Performance test results of sodium ion batteries prepared from Examples 1-8 and Comparative Examples 1-3

[0120]

[0121]

[0122] As can be seen from Examples 1 and 2, 3, and 4, if the oxidation cross-linking reaction temperature is increased or solvent thermal treatment is performed using ethanol, that is, the oxidation cross-linking strength is increased, the modification of the polycyclic aromatic hydrocarbon molecular structure in the coal is too high, resulting in a large number of polycyclic aromatic hydrocarbon molecules being cross-linked, and in the later carbonization process, the oxygen-containing functional groups escape seriously, and the hard carbon has many defects. If the oxidation temperature is too low, the oxidation cross-linking effect is not sufficient, the carbon sheet layers in the carbon particles cannot be bent and stacked to form closed pore sites, which will all result in a decrease in the reversible capacity of the hard carbon, a decrease in the initial efficiency, and poor cycle performance.

[0123] As can be seen from Examples 1 and 5, 6, and 7, if the temperature in the gradient reduction process is increased, that is, the reduction strength is increased, the oxygen-containing groups in the coal are few, and most of the polycyclic aromatic hydrocarbon molecular structures tend to be ordered in the later carbonization process, resulting in a low reversible capacity of the coal-based hard carbon and poor cycle performance. If the gradient reduction reaction temperature is decreased or methane is used as the reducing gas, the gradient reduction effect is not obvious, which is not conducive to the ordering of the near-surface and the reduction of side reactions, resulting in a decrease in the initial efficiency.

[0124] As can be seen from Examples 1 and 8, the process method described in the present application is not only limited to lignite as the raw material, but also has excellent modification effects on other coal-based precursors such as anthracite, that is, the general principle of the present application is universal for different coal-based precursors.

[0125] As can be seen from Examples 1 and Comparative Example 1, if the lignite is directly carbonized without any pretreatment, the polycyclic aromatic hydrocarbon molecules in the coal tend to be ordered, and a graphite-like structure is easily formed, resulting in a hard carbon with a high degree of order and lacking closed pore sodium storage sites.

[0126] As can be seen from Examples 1 and Comparative Example 2, if the liquid-phase oxidation is not followed by gradient reduction treatment, the oxygen atoms between the polycyclic aromatic hydrocarbon molecules escape seriously during the carbonization process, the hard carbon has many defects, the near-surface disorder degree is too high, the long-range ordered structure cannot be formed, the specific surface area is increased, and the initial efficiency is decreased.

[0127] As can be seen from Examples 1 and Comparative Example 3, if only hydrogen reduction treatment is performed before carbonization, the precursor is excessively deoxidized, the carbon sheet layers are excessively developed during the carbonization process, and a highly ordered graphite-like structure is formed.

[0128] The above merely describes the preferred embodiments of the present application, and it should be pointed out that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.

Claims

1. A method for producing a coal-based hard carbon of a gradient structure, characterized by, The method comprises the following steps: (1) oxidatively cross-linking the de-ashed coal-based precursor in a solvent to obtain an over-oxidized coal-based precursor; (2) gradient-reducing the over-oxidized coal-based precursor in a reducing atmosphere to obtain a gradient-reduced precursor; (3) finally carbonizing the gradient-reduced precursor to obtain a coal-based hard carbon material with a gradient structure; The temperature of the gradient-reducing treatment is 100-500 ℃, and the time of the gradient-reducing treatment is 1-5 h; The oxidizing agent used in the oxidative cross-linking process is maleic anhydride; The coal-based precursor comprises one or more of lignite, bituminous coal, anthracite, coal coke and petroleum coke.

2. The method of claim 1, wherein the coal-based hard carbon having a gradient structure is prepared by the steps of: The particle size of the coal-based precursor is 100-800 mesh.

3. The method for producing a coal-based hard carbon of a gradient structure according to claim 1 or 2, characterized by, The solvent comprises one or more of water, ethanol, propanol and diethyl ether.

4. The method of claim 3, wherein the gradient structured coal-based hard carbon is prepared by the steps of: The temperature of the oxidative cross-linking is 80-200 ℃, the pressure of the oxidative cross-linking is 0.1-2.3 MPa, and the time of the oxidative cross-linking is 3-10 h; The mass-volume ratio of the coal-based precursor, the oxidizing agent and the solvent is 8-12 g: 2 g: 10-70 mL.

5. The method of claim 4, wherein the gradient structured coal-based hard carbon is prepared by the steps of: The gas of the reducing atmosphere comprises one or more of hydrogen, carbon monoxide and methane.

6. The method of claim 5, wherein the gradient structured coal-based hard carbon is prepared by the steps of: The temperature of the carbonization treatment is 1000-2000 ℃, the heating rate is 0.5-20 ℃ / min, and the time of the carbonization treatment is 0.5-10 h; The carbonization treatment is carried out in a protective atmosphere, and the protective atmosphere comprises an inert gas or a vacuum environment.

7. The coal-based hard carbon with a gradient structure prepared by the method of any one of claims 1-6.

8. The application of the coal-based hard carbon with a gradient structure of claim 7 in the preparation of a negative electrode material for a sodium ion battery.

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