Method for preparing coal-based hard carbon material by oxygen-assisted low-temperature etching of amorphous components in coal precursor

Abstract

The invention discloses a method for preparing a coal-based hard carbon material by oxygen-assisted low-temperature etching of amorphous components in a coal precursor, and belongs to the technical field of electrode material preparation. The method is characterized in that a mild pre-etching process is introduced before conventional high-temperature carbonization treatment: under the protection of an inert atmosphere carrying a small amount of oxygen, the oxygen participates in a pre-etching reaction for 10 minutes to 1 hour, so that the sodium ion storage performance of the coal-based hard carbon negative electrode material can be comprehensively improved; comprising high reversible capacity, high platform capacity, high first-circle coulombic efficiency and excellent rate capability. Compared with a carbon material obtained by the process, the coal-based hard carbon obtained by the method disclosed by the invention has the advantages that precursors such as cane sugar and biomass do not need to be additionally added, and the technical difficulty caused by the complexity of a coal-based structure to regulation and control of a high-performance hard carbon negative electrode can be effectively solved only by means of mild pre-etching treatment; and the preparation method can be quickly matched with an existing hard carbon batch production line, and has important practical application potential.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode material preparation, and specifically relates to a method for preparing coal-based hard carbon, which targets the bottleneck of high complexity of the composition structure in coal-based precursors, utilizes low-temperature etching of amorphous components in coal in an oxygen-containing atmosphere, and optimizes the microenvironment of coal-based carbon crystals. The method is used to solve the technical problems of low reversible capacity, low platform capacity, low first-cycle coulombic efficiency, and low rate performance of coal-based hard carbon caused by its complex structure in one step. Background Art

[0002] Sodium-ion batteries have attracted much attention due to their abundant resources and low cost, and the carbon negative electrode is a key component that determines its energy density. Among them, using coal as a low-cost, high-carbon natural carbon source to directly prepare carbon negative electrode materials with high low-voltage platform capacity, high first-cycle coulombic efficiency and excellent rate performance is an important direction for the development of low-cost sodium-ion batteries.

[0003] Compared with other carbon anode precursors with clear structural units (such as cellulose / lignin, sucrose / starch, resin and asphalt), the coal structure is usually a highly complex mixture of aromatic macromolecules, fat molecules and fluid mesophases connected by cross-linking structures and side chains. The high complexity of this structural composition makes the hard carbon obtained by direct thermal conversion of coal usually present a complex mixed state, including long-range ordered graphite sheet structure and amorphous carbon components randomly mixed in it; this has led to its sodium ion storage performance being limited to a low level for a long time, including low first-cycle coulomb efficiency, low reversible capacity (<300mAh / g) and poor rate performance. Reducing the complexity of coal structure to develop short-range ordered carbon microcrystals and sub-nanopores is the key to achieving high platform capacity and high-rate sodium ion storage of coal-based hard carbon.

[0004] Existing strategies including coal rock microcomponent sorting, air / liquid phase pre-oxidation, pre-cross-linking with other precursors such as sucrose and citric acid, and phosphorylation have been used to block the long-range growth of carbon crystallites to optimize the sodium ion storage performance of coal-based hard carbon (Carbon Energy, 2024, 6, 12, e584; Energy Storage Mater. 2024, 67, 103282; Advanced Science, 2022, 9, 20, 2200023). By introducing additional carbon sources such as sucrose and starch (Advanced Science, 2022, 9, 20, 2200023), exogenous cross-linking agents such as citric acid (Carbon Energy, 2024, 6, 12, e584), and exogenous hydrogen donors such as microplastics PE and PET (CN114853003B) into the coal precursor, the crystal structure of coal-based carbon can be regulated and its long-range ordered development can be inhibited, thereby obtaining coal-based hard carbon materials with high reversible specific capacity. However, there are problems such as high raw material cost or difficulty in collection, complicated process, and severe furnace slagging. Another technical route is to introduce air or oxygen atmosphere during the thermal conversion process for pre-crosslinking reaction (also known as pre-oxidation treatment), or to pre-oxidize the coal precursor by hydrogen peroxide or high-pressure hydrothermal reaction, specifically by introducing a large number of oxygen-containing functional groups to increase the degree of crosslinking between coal macromolecular structures, thereby inhibiting the splicing of coal-based carbon crystals and developing long-range ordered graphitized structures (CN118479455A; Fuel 2021,310,122072). Due to the complex structure of carbon microcrystals and amorphous components intertwined in the obtained coal-based hard carbon, its platform capacity and rate performance are still greatly limited. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problem of poor sodium storage capacity of carbon negative electrode materials prepared by direct thermal conversion of coal precursors, with the goal of reducing the complexity of coal precursors and optimizing the microenvironment of coal-based carbon microcrystals, and to provide a method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in coal precursors. This method uses sub-bituminous coal / bituminous coal as raw materials, and introduces a mild oxygen-containing pre-etching process in the thermal conversion path of coal to hard carbon to effectively decompose amorphous components such as aliphatic molecules and hydrogen-rich side chains in the coal matrix, thereby obtaining a low steric hindrance microenvironment that is conducive to the homogeneous formation, rearrangement and growth of coal-based carbon microcrystals, and preparing hard carbon materials that can be used for high-performance sodium ion battery negative electrodes. Specifically, the purpose of the "oxygen pre-etching" process is to regulate the microenvironment of carbon crystals. Its technical idea is: amorphous carbon is the main reason for the low sodium ion storage capacity and slow diffusion in coal-based hard carbon materials. Through oxygen pre-etching at a lower temperature of 350-550°C and a lower oxygen concentration of 3-6%, the amorphous components are selectively consumed to construct a coal-based skeleton with more resonance-stabilized π radicals, fewer amorphous components, and more sub-nanometer pores. Subsequently, during the high-temperature carbonization process, on the one hand, abundant π-type radicals and clear amorphous components are used to promote the homogeneous formation, isotropic arrangement and short-range orderly development of coal-based carbon microcrystals, and on the other hand, the spontaneous contraction of sub-nanometer pores is used to induce the formation of closed pore structures, thereby obtaining optimized microcrystals and pore matching patterns of hard carbon materials. Different from the above-mentioned background technical features, the present invention directly intervenes in the thermal conversion path of coal through precise control of heat treatment atmosphere and temperature, introduces aromatic π radicals, and eliminates amorphous components to induce the homogeneous development of coal-based carbon crystals and the formation of closed pores.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a coal-based hard carbon material by oxygen-assisted low-temperature etching of amorphous components in a coal precursor, the method comprising:

[0008] Step 1: Grinding and screening: crushing and screening the raw coal to obtain powder with target particle size;

[0009] Step 2: Pre-etching: Introduce an inert gas with a small amount of oxygen, and the total flow rate should be controlled at 0.6-4.5L / (m 3 ·min), the volume flow ratio of oxygen to inert gas is 1:9-1:50, the temperature is increased to 350-550°C at a heating rate of 2-20°C / min, the reaction time is 10min-1h, and a pre-etched product is obtained; the purpose is to selectively consume amorphous components in coal precursors such as fatty carbon, introduce more π radicals and sub-nanometer pores, so as to optimize the crystal environment for the evolution of coal-based precursors to hard carbon structures.

[0010] Step 3: Carbonization: Under the protection of an inert atmosphere, the pre-etched product obtained in step 2 is heated to 800-1800°C at a heating rate of 2-20°C / min, and the carbonization time is 0.5-10 hours to obtain a coal-based hard carbon material.

[0011] The reversible specific capacity of the obtained coal-based hard carbon used as the negative electrode of sodium ion battery should be higher than 300mAh / g, the platform capacity should be higher than 225mAh / g, and the first-cycle coulombic efficiency should reach 80% or above.

[0012] The present invention performs a pre-etching step before the carbonization step, and selectively eliminates amorphous carbon components and introduces resonance-stabilized free radicals and nanopores to induce homogeneous nucleation, isotropic arrangement and growth during the high-temperature carbonization of coal, thereby obtaining a coal-based hard carbon negative electrode material with high reversible capacity, high platform capacity and high first-cycle coulombic efficiency.

[0013] Furthermore, the step one also includes a pickling step, specifically, using hydrochloric acid, hydrofluoric acid and water in sequence for cleaning and drying at the end of grinding.

[0014] Furthermore, in step 1, the concentration of the hydrochloric acid is 2-5M, the concentration of the hydrofluoric acid is 5-20wt%, and the ratio of the acid volume to the powder mass is 20-40:1.

[0015] Furthermore, in step one, considering the actual effect of the "pre-etching" process on the regulation of coal-based microcrystalline structure, the raw coal is a mixture of one or more of lignite, sub-bituminous coal or bituminous coal, preferably sub-bituminous coal.

[0016] Furthermore, in step one, in order to ensure the etching strength and depth in the pre-etching step and to facilitate the preparation of the carbon electrode after the carbonization step, the target particle size is 80 to 400 meshes, preferably 100 to 200 meshes.

[0017] Furthermore, in step 1, the water is distilled water or deionized water, and the final effect of water washing is that the supernatant of the solution is neutral or weakly acidic.

[0018] Furthermore, in steps 2 and 3, the inert gas is nitrogen and / or argon, preferably nitrogen.

[0019] Furthermore, in step 2, the pre-etching reaction temperature is 400-500° C., and the pre-etching reaction time is 25-40 min.

[0020] Furthermore, in step 2, considering the structural control strength of the subsequent carbonization step and the electrochemical performance when applied to the carbon negative electrode, the pre-etching degree needs to be controlled, and the specific surface area of ​​the material after pre-etching treatment is controlled to be 100-500m 2 / g, preferably 150 to 300 m 2 / g.

[0021] Furthermore, in step three, the carbonization temperature is 1150-1350° C., and the carbonization time is 1-3 hours.

[0022] The beneficial effects of the present invention compared with the existing preparation technology of coal-based carbon materials are:

[0023] (1) The present invention proposes a method for preparing coal-based hard carbon by introducing a low-temperature oxygen etching process in the thermal conversion path to solve the problem of poor sodium storage electrochemical performance caused by the structural complexity of the coal-based precursor. Specifically, only by mild oxygen-containing pre-etching at a relatively low temperature, amorphous components such as aliphatic compounds and hydrogen-rich side chains in the coal-based precursor are selectively consumed to create a carbon microcrystal growth environment with more spatial freedom, so that the prepared coal-based hard carbon material has short-range ordered layers, wide interlayer spacing, isotropically arranged carbon microcrystals and rich sub-nanometer pore structures, which can solve the bottleneck problems of low reversible capacity, small platform capacity and low first coulombic efficiency of coal-based hard carbon negative electrode materials caused by the high complexity of the coal precursor.

[0024] (2) Compared with carbon materials obtained by other processes, the coal-based hard carbon obtained by the present invention based on the concept of "oxygen-containing pre-etching to purify the carbon crystal microenvironment" has high reversible capacity, high platform capacity, high first coulombic efficiency and excellent rate performance in sodium ion storage. It has the best comprehensive performance among the currently publicly reported coal direct conversion hard carbon materials and has important application prospects. Specifically, pre-etching at 400℃~500℃ can make the reversible specific capacity of coal-based hard carbon materials reach 320~335mAh / g, and the first coulombic efficiency can be controlled at 83~86%, of which the capacity in the low voltage platform area reaches 238~248mAh / g.

[0025] (3) The process of the present invention is highly compatible with existing hard carbon preparation equipment and can be extended to heavy precursors with complex chemical components such as coal. Compared with the preparation of coal-based hard carbon materials by combining pre-oxidation (or pre-crosslinking) and high-temperature carbonization, no additional crosslinking agent or other precursors are required, and it has higher practical potential and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the XRD spectrum of the coal-based hard carbon material carbonized at 1200°C described in Comparative Example 1.

[0027] Figure 2 This is the XRD peak spectrum of the coal-based hard carbon material carbonized at 1200°C described in Comparative Example 1.

[0028] Figure 3 This is a charge and discharge curve diagram of the coal-based hard carbon material carbonized at 1200°C described in Comparative Example 1.

[0029] Figure 4This is a rate performance diagram of the coal-based hard carbon material carbonized at 1200°C described in Comparative Example 1.

[0030] Figure 5 This is the XRD spectrum of the coal-based hard carbon described in Comparative Example 3 that has been subjected to oxygen pre-etching at 600°C and carbonization treatment at 1200°C.

[0031] Figure 6 This is the XRD peak spectrum of the coal-based hard carbon described in Comparative Example 3 that has undergone oxygen pre-etching at 600°C and carbonization treatment at 1200°C.

[0032] Figure 7 This is a charge and discharge curve diagram of the coal-based hard carbon described in Comparative Example 3 that has undergone oxygen pre-etching at 600°C and carbonization treatment at 1200°C.

[0033] Figure 8 This is a rate performance diagram of the coal-based hard carbon described in Comparative Example 3 that has undergone oxygen pre-etching at 600°C and carbonization treatment at 1200°C.

[0034] Fig. 9 This is the XRD spectrum of the coal-based hard carbon described in Comparative Example 4 that has undergone oxygen deep pre-etching at 600°C and carbonization treatment at 1200°C.

[0035] Fig.10 This is the XRD peak spectrum of the coal-based hard carbon described in Comparative Example 4 that has undergone oxygen deep pre-etching at 600°C and carbonization treatment at 1200°C.

[0036] Fig.11 This is a charge and discharge curve diagram of the coal-based hard carbon described in Comparative Example 4 that has undergone oxygen deep pre-etching at 600°C and carbonization treatment at 1200°C.

[0037] Fig.12 This is a rate performance diagram of the coal-based hard carbon described in Comparative Example 4 that has undergone 600°C oxygen deep pre-etching and 1200°C carbonization treatment.

[0038] Fig.13 It is a graph of nitrogen isothermal adsorption and desorption curves of the coal-derived structure after oxygen pre-etching treatment at different temperatures of 400°C, 500°C and 600°C as described in Example 1, Example 2, Comparative Example 3 and Comparative Example 4.

[0039] Fig.14 This is a comparison chart of the micropore and mesopore volumes of the coal-derived structure after oxygen pre-etching treatment at different temperatures of 400°C, 500°C, and 600°C as described in Example 1, Example 2, Comparative Example 3, and Comparative Example 4.

[0040] Fig.15FTIR spectra of the coal-derived structures described in Example 1, Example 2, Comparative Example 3 and Comparative Example 4 after oxygen pre-etching treatment at different temperatures of 400°C, 500°C and 600°C.

[0041] Fig.16 This is a comparison chart of the relative proportion of fat carbon and the hydrogen-carbon ratio of the coal-derived structure after oxygen pre-etching treatment at different temperatures of 400°C, 500°C, and 600°C as described in Example 1, Example 2, Comparative Example 3, and Comparative Example 4.

[0042] Fig.17 This is a distribution diagram of free radical species of the coal-derived structure after oxygen pre-etching treatment at different temperatures of 400°C, 500°C, and 600°C as described in Example 1, Example 2, Comparative Example 3, and Comparative Example 4.

[0043] Fig.18 This is the XRD spectrum of the coal-based hard carbon described in Example 1 that has been oxygen pre-etched at 400°C and carbonized at 1200°C.

[0044] Fig.19 This is the XRD peak spectrum of the coal-based hard carbon described in Example 1 that has been oxygen pre-etched at 400°C and carbonized at 1200°C.

[0045] Fig. 20 This is a charge and discharge curve of the coal-based hard carbon described in Example 1 that has been oxygen pre-etched at 400°C and carbonized at 1200°C.

[0046] Fig.21 This is a rate performance diagram of the coal-based hard carbon described in Example 1 that has undergone oxygen pre-etching at 400°C and carbonization treatment at 1200°C.

[0047] Fig. 22 This is the XRD spectrum of the coal-based hard carbon described in Example 2 that has been subjected to oxygen pre-etching at 500°C and carbonization treatment at 1200°C.

[0048] Fig.23 This is the XRD peak spectrum of the coal-based hard carbon described in Example 2 that has been oxygen pre-etched at 500°C and carbonized at 1200°C.

[0049] Fig.24 This is a charge and discharge curve diagram of the coal-based hard carbon described in Example 2 that has been oxygen pre-etched at 500°C and carbonized at 1200°C.

[0050] Fig.25 This is a rate performance diagram of the coal-based hard carbon described in Example 2 that has undergone oxygen pre-etching at 500°C and carbonization treatment at 1200°C.

[0051] Fig.26 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0052] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0053] The present invention aims at the technical bottleneck of poor sodium storage capacity of coal-based carbon materials prepared by traditional direct carbonization or introduction of pre-crosslinked precursors, with the goal of eliminating amorphous carbon components and inducing short-range order and isotropic development of carbon microcrystals. The technical advancement of the method lies in the introduction of a mild pre-etching process before conventional high-temperature carbonization treatment: under the protection of an inert atmosphere carrying a small amount of oxygen, oxygen is involved in the pre-etching reaction for 10min to 1h, so that the comprehensive improvement of the sodium ion storage performance of coal-based hard carbon negative electrode materials can be achieved, including high reversible capacity, high platform capacity, high first-cycle coulomb efficiency and excellent rate performance. Compared with the carbon materials obtained by the above process, the coal-based hard carbon obtained by the present invention based on the idea of ​​"oxygen-containing pre-etching to eliminate amorphous components in complex precursors and induce homogeneous carbonization" does not need to add precursors such as sucrose and biomass. Only with the help of mild pre-etching treatment, the technical difficulties brought by the complexity of the coal-based structure to the regulation of high-performance hard carbon negative electrodes can be effectively solved, and this preparation method can be quickly matched with the existing hard carbon batch production line, and has important practical application potential.

[0054] The principle of the present invention is that the bituminous coal has a low degree of aromatic condensation (usually 2 to 4 aromatic rings), a large number of side chains, and is rich in amorphous carbon precursors such as aliphatic compounds. During the thermal conversion process, on the one hand, a local long-range ordered graphite-like structure is formed through the polycondensation of aromatic sheets, and at the same time, aliphatic compounds are cross-linked with aromatic sheets through alkylation to form an amorphous domain structure between high-graphitization carbon crystals. The present invention targets the amorphous carbon derived from aliphatic compounds in a large number of bituminous coal-based hard carbons, which hinders the filling, storage and diffusion of sodium ions. The present invention selectively consumes amorphous carbon precursors such as aliphatic compounds and hydrogen-rich side chains in bituminous coal by low-temperature etching treatment at 350 to 550°C in an oxygen-containing atmosphere, and at the same time introduces a large number of aromatic π free radicals and sub-nanometer pores to induce the homogeneous growth and isotropic arrangement of coal-based carbon microcrystals in the high-temperature carbonization stage.

[0055] Comparative Example 1:

[0056] The method for preparing the coal-based hard carbon material provided in this comparative example is carried out according to the following steps:

[0057] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 100-200 mesh.

[0058] Acid washing: the powder is acid washed with 4M hydrochloric acid, washed with water, acid washed with 20% hydrofluoric acid, washed with water, and dried.

[0059] Carbonization: Under the protection of argon atmosphere, the acid-washed product was heated to 1200° C. at a rate of 5° C. / min and kept at the constant temperature for 2 h to obtain a coal-based hard carbon material.

[0060] The microstructure of the coal-based hard carbon material was characterized by X-ray diffraction (XRD) technology ( Figure 1 ) and by performing peak separation on the spectral lines ( Figure 2 ), the calculated crystallite interlayer spacing is 0.368nm, the average number of stacked layers is 6.7, and the average crystallite width is 2.85nm. When the hard carbon material is used as a negative electrode for a sodium ion battery, the reversible capacity is 274mAh / g at 0.1C (1C = 250mA / g), of which the platform capacity is 170mAh / g, and the first coulombic efficiency is 83% ( Figure 3 ), and the rate performance is poor, with a capacity of only 120mAh / g at 1C ( Figure 4 ).

[0061] Comparative Example 2:

[0062] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 100-200 mesh.

[0063] Acid washing: the powder is acid washed with 4M hydrochloric acid, washed with water, acid washed with 20% hydrofluoric acid, washed with water, and dried.

[0064] Carbonization: Under the protection of argon atmosphere, the acid-washed product was heated to 1300°C at a rate of 5°C / min and carbonized for 2h to obtain a coal-based hard carbon material. When the hard carbon material was used as the negative electrode of a sodium ion battery, the reversible capacity was 257 mAh / g at 0.1C (1C = 250 mA / g) and the first coulombic efficiency was 79%.

[0065] Comparative Example 3:

[0066] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 160-200 mesh. The sieved powder is pickled with 4M hydrochloric acid, washed with water, pickled with 20% hydrofluoric acid, washed with water, and dried.

[0067] Pre-etching: In a nitrogen and oxygen atmosphere with a volume ratio of 24:1, the temperature was raised to 600° C. at a rate of 10° C. / min, and the reaction time was 0.5 h to obtain a pre-etched product.

[0068] Carbonization: Under the protection of nitrogen or argon atmosphere, the pre-etched product is heated to 1200° C. at a rate of 5° C. / min for 2 hours to obtain a coal-based hard carbon material.

[0069] The microcrystalline structure parameters of the coal-based hard carbon material are determined by XRD spectra ( Figure 5 ) and its fine peak separation ( Figure 6) calculated, the interlayer spacing is 0.367nm, the number of microcrystalline stacking layers is 4.2, and the microcrystalline width is 2.62nm. The constant current charge-discharge test curve and rate performance of the coal-based hard carbon material at 0.1C (1C = 250mA / g) are as follows Figure 7 and Figure 8 The first cycle coulombic efficiency of the coal-based hard carbon material is 69%, the reversible specific capacity is 252 mAh / g, the platform capacity at a voltage between 0.001 and 0.138 V is 169 mAh / g, and the reversible specific capacity at 1C is 75 mAh / g.

[0070] Comparative Example 4:

[0071] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 160-200 mesh. The sieved powder is pickled with 4M hydrochloric acid, washed with water, pickled with 20% hydrofluoric acid, washed with water, and dried.

[0072] Pre-etching: In a nitrogen and oxygen atmosphere with a volume ratio of 9:1, the temperature was raised to 600° C. at a rate of 10° C. / min, and the reaction time was 1 h to obtain a pre-etched product.

[0073] Carbonization: Under the protection of nitrogen or argon atmosphere, the pre-etched product is heated to 1200° C. at a rate of 5° C. / min for 2 hours to obtain a coal-based hard carbon material.

[0074] The microcrystalline structure parameters of the coal-based hard carbon material are determined by XRD spectra ( Fig. 9 ) and its fine peak separation ( Fig.10 ) calculated, the interlayer spacing is 0.368nm, the number of microcrystalline stacking layers is 4.3, and the microcrystalline width is 2.63nm. The constant current charge-discharge test curve and rate performance of the coal-based hard carbon material at 0.1C (1C = 250mA / g) are as follows Fig.11 and Fig.12 The first cycle coulombic efficiency of the coal-based hard carbon material is 53.4%, the reversible specific capacity is 221 mAh / g, the platform capacity at a voltage between 0.001 and 0.138 V is 148 mAh / g, and the reversible specific capacity at 1C is 108 mAh / g.

[0075] Embodiment 1:

[0076] The method for preparing the coal-based carbon material provided in this embodiment is carried out according to the following steps:

[0077] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 160-200 mesh. The sieved powder is pickled with 4M hydrochloric acid, washed with water, pickled with 20% hydrofluoric acid, washed with water, and dried.

[0078] Pre-etching: In a nitrogen and oxygen atmosphere with a volume ratio of 24:1, the temperature was raised to 400° C. at a rate of 10° C. / min, and the reaction time was 0.5 h to obtain a pre-etching product A.

[0079] Carbonization: Under the protection of nitrogen or argon atmosphere, the pre-etched product A is heated to 1200° C. at a rate of 5° C. / min for 2 h to obtain a coal-based hard carbon material A.

[0080] Embodiment 2:

[0081] The method for preparing the coal-based carbon material provided in this embodiment is carried out according to the following steps:

[0082] Grinding and selection: The raw coal is crushed and sieved to obtain powder of 160-200 mesh. The sieved powder is pickled with 4M hydrochloric acid, washed with water, pickled with 20% hydrofluoric acid, washed with water, and dried.

[0083] Pre-etching: In a nitrogen and oxygen atmosphere with a volume ratio of 24:1, the temperature was raised to 500° C. at a rate of 10° C. / min, and the reaction time was 0.5 h to obtain a pre-etched product B.

[0084] Carbonization: Under the protection of nitrogen or argon atmosphere, the pre-etched product B is heated to 1200° C. at a rate of 5° C. / min for 2 hours to obtain a coal-based hard carbon material B.

[0085] The difference between the pre-etched products A and B described in Example 1 and Example 2 and the pre-etched products described in Comparative Examples 3 and 4 is that the etching depth is different. Precisely controlling the oxygen content, processing time and processing temperature is a necessary condition for achieving the technical effect of the present invention. The pore structure of the pre-etched product is determined by nitrogen isothermal adsorption and desorption test ( Figure 5 ), the micropore volumes of the pre-etched products A, B, and C are calculated to be 0.09 cm 3 / g, 0.15cm 3 / g, 0.21cm 3 / g, and the mesopore volume is 0.06cm 3 / g, 0.07cm 3 / g, 0.08cm 3 / g. The aliphatic carbon and aromatic carbon bonding of the pre-etched products A, B, C, and D were characterized by FTIR ( Figure 7 ), the proportion of fatty carbon in the pre-etched products A, B, C, and D was calculated to be 2.8%, 6.4%, 5.0%, and 4.3%, respectively, which are much lower than 19.1% of the raw coal ( Figure 8 ), in addition, the hydrogen-carbon ratio of the pre-etched products A and B measured by the organic element analyzer is significantly lower than that of the raw coal ( Figure 8 ). Electron paramagnetic resonance is used to characterize the distribution of free radicals, such as Fig. 9As shown in the figure, the concentration of π radicals on aromatic carbon in pre-etched products A and B first increases and then decreases, among which the concentration of π radicals in pre-etched products A and B is the highest, 71.3 au and 74.6 au, respectively, which are much higher than 24 a.u. of raw coal. In summary, the pre-etching process can significantly change the physical and chemical structure of coal precursors, especially its carbon and hydrogen element composition and aliphatic-aromatic carbon distribution, and introduce more aromatic π-type radicals.

[0086] The microcrystalline structure parameters of the coal-based hard carbon material A are determined by XRD spectra ( Fig.10 ) and its fine peak separation ( Fig.11 ) calculated, the interlayer spacing is 0.377nm, the number of microcrystalline stacking layers is 3.9, and the microcrystalline width is 2.43nm. The constant current charge-discharge test curve and rate performance of the coal-based hard carbon material A at 0.1C (1C = 250mA / g) are as follows Fig.12 and Fig.13 As shown. The first cycle coulombic efficiency of the coal-based hard carbon material A is 84.8%, the reversible specific capacity is 333 mAh / g, of which the platform capacity at a voltage between 0.001 and 0.138 V is 248 mAh / g, and it has excellent rate performance, and the reversible specific capacity at 1C is 210 mAh / g. It can be seen that the comprehensive performance of the coal-based hard carbon material A is comprehensively improved compared with Comparative Example 1.

[0087] The microcrystalline structure parameters of the coal-based hard carbon material B are determined by XRD spectra ( Fig.10 ) and its fine peak separation ( Fig.11 ) calculated, the interlayer spacing is 0.375nm, the number of microcrystalline stacking layers is 4.1, and the microcrystalline width is 2.50nm. The constant current charge-discharge test curve and rate performance of the coal-based hard carbon material B at 0.1C (1C = 250mA / g) are as follows Fig.12 and Fig.13 The first cycle coulombic efficiency of the coal-based hard carbon material B is 85.4%, the reversible specific capacity is 321 mAh / g, the platform capacity at a voltage between 0.001 and 0.138 V is 238 mAh / g, and it has excellent rate performance, with a reversible specific capacity of 147 mAh / g at 1C.

[0088] The relative content of oxygen-containing functional groups in the carbon structure was characterized by elemental analysis and XPS testing. The oxygen contents of the coal-based hard carbon materials A and B described in Examples 1 and 2 were 9.5% and 10.3%, respectively, which were slightly different from the 8.9% oxygen content in the coal-based hard carbon material obtained in Comparative Example 1. Compared with other chemical structure changes, the introduction of oxygen-containing functional groups should be a non-critical influencing factor in the present invention.

[0089] Embodiment 3:

[0090] The difference between the method for preparing the coal-based carbon material provided in this embodiment and that in embodiment 1 is that the carbonization temperature is 1300° C. The reversible capacity of the obtained coal-based hard carbon material is 307 mAh / g, and the first-cycle coulombic efficiency is 83%.

[0091] Embodiment 4:

[0092] The difference between the method for preparing the coal-based carbon material provided in this embodiment and that in embodiment 2 is that the carbonization temperature is 1300° C. The reversible capacity of the obtained coal-based hard carbon material is 312 mAh / g, and the first-cycle coulombic efficiency is 81%.

Claims

1. A method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor, characterized in that: The method is: Step 1: Grinding and screening: crushing and screening the raw coal to obtain powder with target particle size; Step 2: Pre-etching: Introduce an inert gas with a small amount of oxygen, and the total flow rate should be controlled at 0.6-4.5L / (m 3 ·min), the volume flow ratio of oxygen to inert gas is 1:9-1:50, the temperature is increased to 350-550°C at a heating rate of 2-20°C / min, the reaction time is 10min-1h, and a pre-etched product is obtained; Step 3: Carbonization: Under the protection of an inert atmosphere, the pre-etched product obtained in step 2 is heated to 800-1800°C at a heating rate of 2-20°C / min, and the carbonization time is 0.5-10 hours to obtain a coal-based hard carbon material.

2. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1, characterized in that: The step one also includes a pickling step, specifically, using hydrochloric acid, hydrofluoric acid, and water to clean and dry in sequence at the end of grinding.

3. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1, characterized in that: In step 1, the concentration of the hydrochloric acid is 2-5M, and the concentration of the hydrofluoric acid is 5-20wt%.

4. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In step 1, the raw coal is a mixture of one or more of lignite, sub-bituminous coal or bituminous coal.

5. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In step 1, the target particle size is 80-400 mesh.

6. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In step 1, the water is distilled water or deionized water, and the final effect of water washing is that the supernatant of the solution is neutral or weakly acidic.

7. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In steps 2 and 3, the inert gas is nitrogen and / or argon.

8. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In step 2, the pre-etching reaction temperature is 400-500° C., and the pre-etching reaction time is 25-40 minutes.

9. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1 or 2, characterized in that: In step 2, the specific surface area of ​​the material after pre-etching is controlled to be 100-500m 2 / g, preferably 150 to 300 m 2 / g.

10. The method for preparing coal-based hard carbon materials by oxygen-assisted low-temperature etching of amorphous components in a coal precursor according to claim 1, characterized in that: In step 3, the carbonization temperature is 1150-1350° C., and the carbonization time is 1-3 hours.

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