Long-range graphite domain-rich hard carbon negative electrode material and preparation method therefor

Abstract

A long-range graphite domain-rich hard carbon negative electrode material and a preparation method therefor, aiming to regulate and control the growth of a long-range graphite domain microcrystalline structure in hard carbon so as to realize high-performance sodium-ion storage. The preparation of the long-range graphite domain-containing hard carbon negative electrode material is realized by means of activation / catalysis by a mixed salt. The preparation method mainly comprises the following steps: using biomass coconut shell as a raw material, using low-cost and weak-corrosive mixed salts (Na2CO3 and K2CO3) as activators / catalysts, destroying a C-sp2 / sp3 bond by means of molten salt activation / catalytic modification, cutting off a carbon chain, introducing defects, and activating carbon atoms so as to obtain an activated carbon intermediate; then performing high-temperature treatment and controlling crystal form conversion to prepare a hard carbon negative electrode material which contains a long-range graphite domain having a large interlayer spacing, and has a rich closed pore structure. The reasonable matching of the interlayer spacing, the long-range graphite domain length, and the closed pores is realized, so that the sodium ions can effectively enter, thereby obtaining excellent sodium storage performance.

Description

A hard carbon negative electrode material rich in long-range graphite domains and preparation method thereof Technical Field

[0001] The present invention relates to the technical field of energy materials, and in particular to a hard carbon negative electrode material rich in long-range graphite domains and a preparation method thereof. Background Art

[0002] With the increasingly serious energy crisis and environmental problems caused by the excessive consumption of fossil fuels, there is an urgent need to explore high-power, low-cost energy storage devices. Among the various existing electrochemical energy storage systems, lithium-ion batteries dominate the portable electronics and electric vehicle markets due to their advantages such as high energy density and long cycle life. However, the limited reserves and uneven distribution of lithium resources restrict their application in large-scale energy storage. In addition, due to the abundant reserves and low prices of sodium resources, sodium-ion batteries are widely considered to be the most competitive candidate for lithium-ion batteries. However, compared with lithium-ion batteries, In comparison, the radius of sodium ions This poses a challenge to the development of electrode materials with high sodium storage capacity and fast ion diffusion, especially for sodium-ion battery anode materials. For example, graphite anodes that have been commercially used in lithium-ion batteries have been proven to be unfeasible in sodium-ion batteries because Na + Insertion into the graphite interlayer (0.335 nm) is a thermodynamically unstable process.

[0003] Among the various carbon materials, hard carbon with long-range disorder and large interlayer spacing is considered to be the most promising negative electrode material for sodium ion batteries due to its large capacity, low cost, high abundance, and non-toxicity. Hard carbon is mainly composed of curved and irregularly distributed graphite domains, which are difficult to graphitize even at temperatures above 2800°C. The interlayer spacing of graphite domains (d 002 ) and size have a great influence on sodium ion storage. Therefore, controlling the graphitization of hard carbon and regulating the growth of graphite domains at the molecular level is the key to achieving high-performance hard carbon anode materials.

[0004] The charge-discharge curve of sodium storage in hard carbon is divided into a slope region above 0.1V and a plateau region below 0.1V. Theoretically, achieving greater plateau capacity below 0.1V requires the synergistic effects of long-range graphitic domains and closed-pore sodium storage. Previous studies have shown that the plateau capacity of hard carbon increases with increasing carbonization temperature, but when the carbonization temperature is too high, the plateau capacity actually decreases. The high degree of graphitization of carbon materials at elevated carbonization temperatures significantly reduces the number of edge and in-plane defects formed within the graphite layers, forming larger and more numerous pseudographitic domains, which promote the capacity contribution from interlayer intercalation of sodium ions. Furthermore, continued graphitization is always accompanied by an increase in the number of graphite layers, narrowing the interlayer spacing, hindering sodium ion intercalation and leading to a decrease in plateau capacity. Furthermore, the closed pores formed between long-range graphitic domains, which provide space for Na clusters to fill, are also crucial for achieving plateau capacity. Excessively high carbonization temperatures can cause these pores to melt and collapse, hindering Na cluster filling. Therefore, a proper balance between long-range graphitic domain length, interlayer spacing, and closed pores is key to achieving long plateau capacity and high capacity. There is an urgent need to form long-range graphite domains without excessive graphitization, so that the hard carbon structure can maintain a large interlayer spacing and rich closed pores at the same time. In the recent paper "Catalytic Defect-Repairing Using Manganese Ions for Hard Carbon Anode with High-Capacity and High-Initial-Coulombic-Efficiency in Sodium-Ion Batteries (Advanced Energy Materials, 2023, 2300444.)", the authors used metal ion catalysis to pyrolyze flexible paper towels to prepare hard carbon negative electrodes, and successfully regulated the defects and ion channels of hard carbon by coordinating a variety of metal ion catalysts with oxygen-containing defects. However, metal ion catalysis alone often leads to transitional graphitization of graphite domains, thereby affecting its rate performance.

[0005] In addition, the patent application filed by the applicant: Publication No. CN113307254A, discloses a method and application for preparing three-dimensional porous graphene sheets using low-temperature double salt compounds, the purpose of which is to solve the problem of preparing graphene sheets at low temperatures. Compared with traditional chemical or physical pore-forming methods, this method can control its pore size distribution and degree of graphitization at low temperatures; the present invention uses coconut shell as a biomass carbon source, uses K2CO3 and Na2CO3 as activators, and utilizes a double salt melting process to lower the activation temperature. The gas (CO) and K and Na released during the heating process intervene in the activation process, thereby causing the cross-linked sp in the hard carbon phase to 3Carbon atoms release graphite crystallites, which are then recrystallized to form graphene layers, ultimately forming unique three-dimensional porous graphene-like sheets. However, this prior art uses an ultra-high excess of mixed salts to directly convert biomass carbon into a graphene structure, rather than gradually regulating the growth of graphite crystallites, and also increases energy consumption and costs. At the same time, the microstructure of this type of graphene sheet has a high degree of graphitization, a narrow interlayer spacing, and an open-pore structure, and no closed-pore structure is formed. It is a porous graphene material, which has an essential structural difference from hard carbon negative electrode materials. The entire sodium storage process of this three-dimensional porous graphene-like sheet is equivalent to graphene sodium storage, with only the adsorption sodium storage capacity contribution and no platform area capacity contribution, resulting in a lower sodium storage capacity.

[0006] Therefore, how to adopt a suitable strategy to prepare hard carbon anode materials that simultaneously meet the requirements of suitable long-range graphite domain length, large interlayer spacing and abundant closed pores is one of the focuses in the field of hard carbon anode materials.

[0007] Summary of the Invention

[0008] The purpose of the present invention is to provide a hard carbon negative electrode material rich in long-range graphite domains and a preparation method thereof. The present invention uses biomass coconut shell as raw material, adopts low-cost, weakly corrosive Na2CO3 and K2CO3 mixed salt as activation / catalyst, and destroys C-sp through mixed salt activation / catalytic modification. 2 / sp 3 By removing bonds, cutting carbon chains, introducing defects, and activating carbon atoms, an activated carbon intermediate is obtained. This is then combined with high-temperature crystal transformation to produce a long-range graphite domain hard carbon anode material. This hard carbon anode material has a long-range graphite domain microcrystalline structure and abundant closed pores, allowing sodium ions to enter efficiently, achieving excellent sodium storage performance.

[0009] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0010] The present invention provides a hard carbon negative electrode material rich in long-range graphite domains and a preparation method thereof, comprising the following steps:

[0011] (1) Weigh a certain amount of coconut shell, wash it with pure water, and then place it in a blast drying oven for drying;

[0012] (2) placing the dried coconut shell in a grinder, crushing and sieving;

[0013] (3) The crushed coconut shells were placed in a tube furnace under an argon atmosphere for pre-carbonization;

[0014] (4) uniformly mixing the pre-carbonized carbonized material with mixed salt (Na2CO3 and K2CO3) according to mass percentage;

[0015] (5) The mixed material obtained in step (4) is heated to 700-850°C in a tube furnace under argon atmosphere at a certain heating rate for activation / catalytic reaction. The C-sp 2 / sp 3 bonds, cut carbon chains, introduce defects, activate carbon atoms, and then cool to room temperature to obtain an activated carbon mixed material.

[0016] (6) The carbon mixed material obtained in step (5) is washed with a hydrochloric acid solution and deionized water until the solution is neutral to obtain an activated carbon intermediate.

[0017] (7) The activated carbon intermediate obtained in step (6) is placed in a high-temperature furnace under an argon atmosphere for high-temperature crystal transformation, wherein the highly active carbonaceous intermediate is conducive to the orderly rearrangement of carbon atoms to form a large number of C=C bonds, promoting the growth of long-range graphite domain microcrystals, thereby obtaining a hard carbon negative electrode material rich in long-range graphite domains.

[0018] Preferably, the particle size of the coconut shell powder after sieving in step (2) is 1-10 μm, so as to ensure that the prepared long-range graphite domain hard carbon negative electrode material has fast kinetics and high tap density.

[0019] Preferably, the pre-carbonization temperature in step (3) is 350-750° C. (preferably 650° C.) to ensure that the lignin is fully carbonized and the cellulose and hemicellulose are fully decomposed.

[0020] Preferably, the pre-carbonization heating rate in step (3) is 5-10°C / min, and the pre-carbonization time is 2-5h. Preferably, the mass percentage of the carbonized material after pre-carbonization in step (4) to the activation / catalyst is 25%-100%, ensuring the activation / catalytic degree of the activated carbon intermediate, wherein Na2CO3 and K2CO3 are in a suitable molar ratio of 0.55:0.45 for optimal formation of molten eutectic salt.

[0021] Preferably, the activation / catalytic reaction temperature in step (5) is 700-850°C (preferably 750°C), the reaction time is 2-5h, and the heating rate is 5°C / min, based on the optimal temperature for generating molten eutectic salt (melting point 710°C), to ensure that the carbonized material is not over-activated / catalyzed.

[0022] Preferably, the concentration of the hydrochloric acid solution in step (6) is 0.5-2 mol / L, and the mixed salt is fully removed according to the percentage of mixed salt added.

[0023] Preferably, in step (7), the high-temperature crystal transformation temperature is 1200-1600°C, the holding time is 3-5h, and the heating rate is 5-10°C / min. The growth of long-range graphite domains is regulated according to the hard carbon crystal transformation temperature range to ensure a reasonable combination of interlayer spacing and long-range graphite domain length.

[0024] The present invention also provides a sodium ion battery hard carbon negative electrode material, which is characterized by being a long-range graphite domain hard carbon negative electrode material prepared by activation / catalysis using any combination of the above technical solutions.

[0025] The present invention adopts the above-mentioned hard carbon negative electrode material electrode plates and assembles them into a sodium ion battery. Beneficial effects:

[0026] 1. The long-range graphite domain hard carbon negative electrode material prepared by the present invention is based on biomass coconut shell as raw material, and adopts a hard carbon preparation method that is simple, low-cost, and easy to mass-produce.

[0027] 2. The long-range graphite domain hard carbon negative electrode material prepared by the present invention is first activated / catalyzed by mixed salt (Na2CO3 and K2CO3) to destroy the C-sp 2 / sp 3 By removing bonds, cutting carbon chains, introducing defects, and activating carbon atoms, an activated carbon intermediate is obtained. During the high-temperature crystal transformation, long-range graphite domains with large interlayer spacing and abundant closed pores are formed.

[0028] 3. The long-range graphite domain-rich hard carbon anode material of this invention has the following advantages: the long-range graphite domains in the hard carbon microstructure have excellent conductivity; the optimal balance of interlayer spacing and long-range graphite domain length facilitates enhanced platform dynamics; and the abundant closed pores provide a good platform capacity for pore filling with sodium ions. These advantages synergistically enhance the sodium storage performance of the biomass coconut shell-based hard carbon anode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG1 is an XRD diffraction pattern of the activated carbon intermediate after catalysis / activation in Examples 1-4 of the present invention;

[0030] FIG2 is an XRD diffraction pattern of the long-range graphite domain hard carbon negative electrode material prepared in Examples 1-4 of the present invention;

[0031] FIG3 is a SEM image of a long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention;

[0032] FIG4 is a TEM image of a long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention;

[0033] FIG5 is a second cycle charge-discharge curve of the long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention;

[0034] FIG6 shows the rate performance of the long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0035] The present invention uses biomass coconut shell as raw material, adopts low-cost, weakly corrosive mixed salt (Na2CO3 and K2CO3) as activation / catalyst, and destroys C-sp through mixed salt activation / catalytic modification. 2 / sp 3 By removing carbon atoms, breaking carbon chains, introducing defects and disrupting the arrangement of carbon layers, the activated carbon intermediates are obtained, and then combined with high-temperature crystal transformation to prepare hard carbon negative electrode materials rich in long-range graphite domains.

[0036] The long-range graphite domains in the microstructure of the hard carbon negative electrode material prepared by the present invention have good conductivity; a reasonable combination of interlayer spacing and long-range graphite domain length is achieved, which is conducive to improving the dynamics of the platform area; the abundant closed pores also provide a good platform capacity for pore filling of sodium ions, thereby improving the sodium storage performance of the hard carbon negative electrode material.

[0037] The present invention provides a hard carbon negative electrode material rich in long-range graphite domains and a preparation method thereof, comprising the following steps:

[0038] (1) Weigh a certain amount of coconut shell, wash it with pure water, and then place it in a blast drying oven for drying;

[0039] (2) placing the dried coconut shell in a grinder, crushing and sieving;

[0040] (3) The crushed coconut shells were placed in a tube furnace under an argon atmosphere for pre-carbonization;

[0041] (4) uniformly mixing the pre-carbonized carbonized material with mixed salt (Na2CO3 and K2CO3) according to mass percentage;

[0042] (5) The mixed material obtained in step (4) is heated to 700-850°C in a tube furnace under argon atmosphere at a certain heating rate for activation / catalytic reaction. The C-sp 2 / sp 3 bonds, cut carbon chains, introduce defects, activate carbon atoms, and then cool to room temperature to obtain an activated carbon mixed material.

[0043] (6) The carbon mixed material obtained in step (5) is washed with a hydrochloric acid solution and deionized water until the solution is neutral to obtain an activated carbon intermediate.

[0044] (7) The activated carbon intermediate obtained in step (6) is placed in a high-temperature furnace under an argon atmosphere for high-temperature crystal transformation, wherein the highly active carbon intermediate is conducive to the orderly rearrangement of carbon atoms to form a large number of C=C bonds, promoting the growth of long-range graphite domain microcrystals, thereby obtaining a hard carbon negative electrode material rich in long-range graphite domains.

[0045] Example 1

[0046] (1) 500 g of washed and dried coconut shells were weighed and crushed and sieved, wherein the particle size was less than 10 μm, which could be in good contact with the mixed salt for full activation / catalysis. At the same time, the prepared long-range graphite domain hard carbon negative electrode material had fast kinetics and high tap density.

[0047] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a temperature of 650°C for 2 h at a heating rate of 5°C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained. 5°C / min is the optimal heating rate. Carbonization at 650°C for 2 h is mainly to ensure that lignin is fully carbonized and cellulose and hemicellulose are fully decomposed. Excessive heating rates, high temperatures, and long times will affect the formation of carbon precursors. (3) Weigh 1g of pre-carbonized carbonized material and mix it evenly with mixed salts (Na2CO3 and K2CO3) according to mass percentage, with the total mass of Na2CO3 and K2CO3 accounting for 50%, wherein the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419g of Na2CO3 and 0.2581g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, with a melting point of 710℃. When the total mass percentage of the mixed salt is 50%, the carbon precursor can achieve the optimal activation degree and catalytic effect. When the total mass percentage of the mixed salt is too high or too low, the activation / catalytic degree of carbon atoms will be affected, thereby affecting the growth of long-range graphite domains during high-temperature crystal transformation. (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0048] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0049] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept at this temperature for 3 hours, and the long-range graphite domain hard carbon negative electrode material is obtained after cooling to room temperature. Experimental verification shows that keeping the temperature at 1300°C for 3 hours is the optimal condition for regulating the long-range graphite domain. If the temperature is too low, the graphite domain grows shorter and more defects are increased; if the temperature is too high, the long-range graphite domains are stacked and the closed pores are reduced.

[0050] (7) Preparation of electrode plates. The prepared long-range graphite domain hard carbon material, conductive carbon black, and binder PVDF were weighed in a mass ratio of 80:10:10 and mixed evenly in a mortar. An appropriate amount of N-methylpyrrolidone (NMP) solvent was added and stirred evenly to prepare a slurry. The slurry was evenly coated on aluminum (Al) foil and dried in a vacuum drying oven at 120°C for 12 h. The slurry was cut into 12 mm round negative electrode plates for later use.

[0051] (8) The button cell assembly was carried out in a double-station glove box filled with argon atmosphere. The prepared hard carbon material electrode was used as the negative electrode, the commercial electrolyte 1M NaPF6 DIGLYME=100Vol% was used as the electrolyte, and the metal Na sheet was used as the counter electrode to assemble the button cell.

[0052] (9) The assembled 2032 button cell was subjected to charge and discharge tests on a constant current test system with a voltage range of 0.001-2.4V.

[0053] Figure 1 is an XRD pattern of the activated carbon intermediate obtained after catalysis / activation reaction under different mixed salt ratios of the present invention; it can be seen from the figure that as the mixed salt ratio increases, the (002) peak position moves to a lower angle, the graphite microcrystalline interlayer spacing increases significantly, the crystallinity decreases, and more active disordered carbon atoms are obtained.

[0054] Figure 2 shows XRD patterns of the activated carbon intermediate after high-temperature crystal transformation at different mixed salt ratios. As can be seen from the figure, the (002) peak position does not shift to higher angles after high-temperature crystal transformation, indicating that the graphite crystallites maintain a large interlayer spacing. A faint broadened graphite peak appears at 25.6°, indicating that some of the activated carbon intermediate has grown into long-range graphite domains.

[0055] FIG3 is an SEM image of the long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention; the particles are irregular.

[0056] Figure 4 is a TEM image of the long-range graphite domain hard carbon negative electrode material prepared by embodiment 1 of the present invention; it can be clearly seen that there is a long-range graphite domain microcrystalline structure with high electrical conductivity; the long-range graphite domain is turbine-shaped and forms abundant closed pores, which is conducive to sodium storage in the platform area.

[0057] Figure 5 is the second cycle charge-discharge curve of the long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention; at 20 mA g -1 At the current density, the charge capacity is as high as 350.1mAh g -1 , of which the platform capacity contributes 201.9mAh g -1 , accounting for 57.7%. The high plateau capacity is attributed to the long-range graphite domains with large interlayer spacing and abundant closed pores.

[0058] FIG6 is the rate performance of the long-range graphite domain hard carbon negative electrode material prepared in Example 1 of the present invention; As can be seen from the figure, at 1A g -1 At the current density, the reversible specific capacity is 168.2 mAh g -1 The excellent rate performance is attributed to the long-range graphene domains with large interlayer spacing, high conductivity and fast kinetics.

[0059] Example 2

[0060] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0061] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 2 h, and a heating rate of 5 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0062] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 25%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.1210 g of Na2CO3 and 0.1290 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0063] (4) The mixed material obtained in step (3) was activated / catalyzed in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0064] (5) The carbon mixed material taken out in step (4) was washed with 0.5 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 h to obtain an activated carbon intermediate.

[0065] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a long-range graphite domain hard carbon negative electrode material is obtained.

[0066] By analyzing the above examples, it can be seen that the mass percentage of the mixed salt is too low, resulting in insufficient activation / catalytic reaction, insufficient activation / catalytic degree, and few active carbon atoms, which is not conducive to the growth of long-range graphite domain microcrystals during high-temperature crystal transformation.

[0067] Example 3

[0068] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0069] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 2 h, and a heating rate of 5 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0070] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 75%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45. Take 0.3629 g of Na2CO3 and 0.3871 g of K2CO3 respectively. The most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45 after experimental verification, and the melting point is 710 °C.

[0071] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0072] (5) The carbon mixed material taken out in step (4) was washed with 2 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 h to obtain an activated carbon intermediate.

[0073] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0074] Through analysis of the above embodiments, it can be seen that: the mass percentage of the mixed salt is high, the activation / catalytic degree is high, the defects are increased, and the active carbon atoms are excessive, resulting in serious stacking of long-range graphite domain crystallites during high-temperature crystal transformation, an increase in the degree of graphitization, and being unfavorable for the formation of closed pores.

[0075] Example 4

[0076] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0077] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 2 h, and a heating rate of 5 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0078] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 100%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.4838 g of Na2CO3 and 0.5162 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0079] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0080] (5) The carbon mixed material taken out in step (4) was washed with 2 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 h to obtain an activated carbon intermediate.

[0081] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0082] Through analysis of the above embodiments, it can be seen that: the mass percentage of the mixed salt is too high, the activation degree is too high, the defects are serious, and the activated carbon atoms are highly active, which leads to further growth and stacking of long-range graphite domain crystals during high-temperature crystal transformation, a high degree of graphitization, and closed pores, which is not conducive to platform capacity sodium storage.

[0083] Example 5

[0084] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0085] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 2 h, and a heating rate of 5 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0086] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 50%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419 g of Na2CO3 and 0.2581 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0087] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0088] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0089] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1400°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0090] Through analysis of the above embodiments, it can be seen that increasing the high-temperature crystal transformation temperature of the activated carbon intermediate will also lead to further growth of the long-range graphite domains, thereby affecting the closed-pore sodium storage capacity.

[0091] Example 6

[0092] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0093] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 2 h, and a heating rate of 5 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0094] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 50%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419 g of Na2CO3 and 0.2581 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0095] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0096] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0097] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1500°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0098] Through analysis of the above embodiments, it can be seen that further increasing the high-temperature crystal transformation temperature of the activated carbon intermediate will further lead to the growth of long-range graphite domains, increase the degree of graphitization, and affect the closed-pore sodium storage capacity.

[0099] Example 7

[0100] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0101] (2) Weigh 10 g of crushed coconut shell and place it in a tube furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650°C, a carbonization time of 2 h, and a heating rate of 5°C / min. After cooling to room temperature, a pre-carbonized carbonized material was obtained.

[0102] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 100%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419 g of Na2CO3 and 0.2581 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0103] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 5 h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0104] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0105] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1600°C at a heating rate of 5°C / min for high-temperature crystal transformation. The temperature is kept for 3 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0106] Through analysis of the above embodiments, it can be seen that when the high-temperature crystal transformation temperature of the activated carbon intermediate is too high, the length of the long-range graphite domain will be too long, the stacking will be serious, the degree of graphitization will be too high, and the closed pores will disappear.

[0107] Example 8

[0108] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0109] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 5 h, and a heating rate of 10 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0110] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 50%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419 g of Na2CO3 and 0.2581 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0111] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 3 hours, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0112] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0113] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 10°C / min for high-temperature crystal transformation. The temperature is kept for 5 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0114] Through analysis of the above embodiments, it can be seen that: shortening the activation / catalysis time of the mixed salt will also lead to a decrease in the degree of activation / catalysis; although the high-temperature crystal transformation time of the activated carbon intermediate is extended, it will also lead to a shorter length of the long-range graphite domain, serious defects, and a low degree of graphitization, and it is impossible to form a good long-range graphite domain microcrystalline structure and closed pores.

[0115] Example 9

[0116] 1) Weigh 500 g of washed and dried coconut shells, crush them, and sieve them to obtain a particle size of less than 10 μm.

[0117] (2) Weigh 10 g of crushed coconut shell and place it in a tubular furnace under argon atmosphere for pre-carbonization at a carbonization temperature of 650 °C, a carbonization time of 3 h, and a heating rate of 6 °C / min. After cooling to room temperature, the pre-carbonized carbonized material was obtained.

[0118] (3) Weigh 1 g of pre-carbonized carbonized material and mix it evenly with mixed salt (Na2CO3 and K2CO3) according to the mass percentage. The total mass of Na2CO3 and K2CO3 accounts for 50%, of which the molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and 0.2419 g of Na2CO3 and 0.2581 g of K2CO3 are taken respectively. Experimental verification shows that the most suitable molar ratio of Na2CO3 and K2CO3 is 0.55:0.45, and the melting point is 710 °C.

[0119] (4) The mixed material obtained in step (3) was subjected to a catalytic / activation reaction in a tube furnace under argon atmosphere protection at a reaction temperature of 750°C, a reaction time of 2h, and a heating rate of 5°C / min. After the reaction was completed, it was cooled to room temperature and taken out for washing.

[0120] (5) The carbon mixed material taken out in step (4) was washed with 1 mol / L hydrochloric acid solution and deionized water until the solution was neutral, and then dried in a forced air drying oven for 12 hours to obtain an activated carbon intermediate.

[0121] (6) The activated carbon intermediate obtained in step (5) is placed in a high-temperature furnace under an argon atmosphere, and the temperature is raised to 1300°C at a heating rate of 7°C / min for high-temperature crystal transformation. The temperature is kept for 4 hours, and after cooling to room temperature, a hard carbon negative electrode material rich in long-range graphite domains is obtained.

[0122] By analyzing the above embodiments, it can be seen that when the mixed salt activation / catalysis time is shortened, increasing the heating rate of high-temperature treatment will not form a long-range graphite domain microcrystalline structure and rich closed pores, thereby affecting its sodium storage performance.

[0123] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions merely illustrate the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A hard carbon negative electrode material rich in long-range graphite domains and a preparation method thereof, characterized in that: The following steps are involved: (1) Weigh a certain amount of coconut shell, wash it with pure water, and then dry it in a forced air drying oven; (2) placing the dried coconut shell in a pulverizer, crushing and sieving; (3) placing the crushed coconut shell in a tube furnace and pre-carbonizing it in an argon atmosphere; (4) uniformly mixing the pre-carbonized carbonized material with a mixed salt of Na2CO3 and K2CO3 according to mass percentage; (5) The uniformly mixed material obtained in step (4) is heated to 700-850° C. at a certain heating rate in a tube furnace under the protection of argon atmosphere for catalytic / activation reaction, and then taken out after cooling to room temperature; (6) washing the carbon mixed material obtained in step (5) with a hydrochloric acid solution and deionized water until the solution is neutral to obtain an activated carbon intermediate; (7) placing the activated carbon intermediate obtained in step (6) in a high-temperature furnace under an argon atmosphere to perform high-temperature crystal transformation to obtain a hard carbon negative electrode material rich in long-range graphite domains.

2. The preparation method according to claim 1, characterized in that: The particle size of the coconut shell powder is 1-10 μm.

3. The preparation method according to claim 1, characterized in that: In step 3, the coconut shell is placed in a tubular furnace under an argon atmosphere for pre-carbonization at a temperature of 350-750°C.

4. The pre-carbonization treatment according to claim 3, characterized in that: The pre-carbonization heating rate is 5-10°C / min, and the pre-carbonization time is 2-5h.

5. The preparation method according to claim 1, characterized in that: The mass percentage of the carbonized material and the activation / catalyst in step 4 is 25%-100%, wherein the molar ratio of Na2CO3 and K2CO3 to form a molten eutectic salt is 0.55:0.

45.

6. The preparation method according to claim 1, characterized in that: The reaction time of step 5 is 2-5h, and the heating rate is 5°C / min.

7. The preparation method according to claim 1, characterized in that: The concentration of the hydrochloric acid solution in step 6 is 0.5-2 mol / L.

8. The preparation method according to claim 1, characterized in that: The activated carbon intermediate obtained in step 6 is placed in a high-temperature furnace under an argon atmosphere, and the temperature for high-temperature crystal transformation is 1100-1600°C, the insulation time is 3-5h, and the heating rate is 5-10°C / min.

9. A long-range graphite domain hard carbon negative electrode material, the material being obtained by the preparation method described in any one of claims 1 to 8; wherein: The long-range graphite domain microcrystalline structure of the hard carbon negative electrode material gives it good conductivity, and the abundant closed pores provide a good platform capacity, which can effectively de / intercalate sodium ions and achieve excellent sodium storage performance.

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