Method for preparing hard carbon material from coal and application thereof

Abstract

The application belongs to the field of sodium ion battery negative electrode materials, and particularly relates to a method for preparing hard carbon material from coal and application thereof. The hard carbon material is prepared by using high-reserve coal as raw material and adopting a low-cost process. The hard carbon material obtained through special gas-phase carbon deposition process treatment has the characteristics of high initial coulombic efficiency, high reversible specific capacity and the like. The material is used as a negative electrode and applied to a sodium ion battery, so that the cost of the sodium ion battery is reduced, the energy density of the sodium ion battery is improved, and the full battery shows excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, and specifically relates to a method for preparing hard carbon materials using coal, as well as the hard carbon materials and their applications. Background Technology

[0002] With energy upgrades and the rapid development of renewable energy, large-scale energy storage technology has become crucial for achieving green and sustainable development. Electrochemical energy storage has also become a key technology for solving the problems of instability and discontinuity in renewable energy generation such as wind and solar power. Sodium-ion batteries have advantages such as abundant raw material resources, low cost, high cost-effectiveness, and excellent performance, and have broad application prospects in electric bicycles, low-speed electric vehicles, distributed energy storage, and large-scale energy storage. Currently, lithium-ion batteries are widely used in electric vehicles, energy storage products, and large-scale energy storage. However, the contradiction between the limited reserves of lithium, cobalt, and nickel resources in lithium-ion batteries and the market demand for large-scale electrochemical energy storage is becoming increasingly prominent. Therefore, the development of low-cost sodium-ion battery technology is urgent.

[0003] Sodium-ion batteries convert chemical energy into electrical energy by allowing sodium ions to shuttle back and forth between the positive and negative electrode materials, a principle similar to that of lithium-ion batteries. In recent years, driven by market demand, sodium-ion battery technology has matured significantly. As a crucial component of sodium-ion batteries, the positive and negative electrode materials play a decisive role in the battery's operating voltage, energy density, cycle performance, and rate performance. While various sodium-ion battery positive electrode materials have entered the exploration phase for large-scale production, research progress on sodium-ion battery negative electrode materials has been relatively slower. Excellent sodium-ion battery negative electrode materials should meet the following criteria: low redox potential, high reversible specific capacity, high electron conductivity and ion mobility, high stability, simple preparation process, abundant raw materials, low cost, and environmental friendliness.

[0004] Carbon-based materials have become a research hotspot for anode materials due to their abundant resources, low cost, low sodium storage potential, and small volume change during charge and discharge. Extensive research on sodium-ion batteries has demonstrated that graphite materials are difficult to apply. Hard carbon materials, with their high reversible capacity and relatively low sodium storage potential, are considered to have the greatest practical application value and are currently one of the best choices for commercial anode materials in sodium-ion batteries. Hard carbon refers to carbon that is difficult to graphitize at temperatures above 2500℃, and is a type of amorphous carbon material. Hard carbon materials have fewer carbon sheets stacked along the c-axis of their crystal structure, exhibiting a random orientation. Their microstructure is characterized by long-range disorder and short-range order, with numerous nanopores, a structural feature that facilitates ion or electron transport. As a sodium-ion battery anode, hard carbon exhibits a ramp-like range between 1-0.1V and a plateau below 0.1V during charge and discharge. Currently, some studies have reported on the preparation of hard carbon materials using biomass feedstocks, organic polymers, coal, and bitumen as carbon sources. However, these methods still suffer from problems such as complex processes, high costs, inability to scale up production, low initial coulombic efficiency, and low reversible specific capacity. Therefore, developing low-cost hard carbon materials that can be scalably prepared, especially those with high initial coulombic efficiency and high reversible specific capacity, and that can be practically applied to sodium-ion batteries, is of great significance.

[0005] Currently, the cost of anode materials accounts for the largest proportion of the material cost of sodium-ion batteries. my country has abundant coal reserves and resources. This invention uses coal as a raw material to prepare hard carbon, which can reduce raw material costs. Optimizing low-rank coal with low value further reduces costs. Blast furnace gas or coking gas is used as a reducing gas source during the preparation process, improving material performance while reducing manufacturing costs. The preparation process is simple and inexpensive. This invention also uses a special gas-phase carbon deposition process to obtain hard carbon materials with advantages such as high initial coulombic efficiency and high reversible specific capacity. Using the hard carbon material prepared by the process method of this invention as an anode in sodium-ion batteries can reduce the cost of sodium-ion batteries, increase the energy density of sodium-ion batteries, and result in excellent overall battery performance. Summary of the Invention

[0006] This invention aims to reduce the cost and improve the performance of sodium-ion batteries by designing and developing a method for preparing hard carbon materials using coal, as well as the hard carbon materials and their applications. The hard carbon materials prepared using this method exhibit high initial coulombic efficiency and high reversible specific capacity. Using this material as the negative electrode in sodium-ion batteries can reduce the cost of sodium-ion batteries, increase their energy density, and result in excellent overall battery performance.

[0007] The technical solution of this invention is:

[0008] 1) After crushing and screening the coal, particles with a diameter of 1-5 mm are obtained, and the particles are washed with water to remove ash.

[0009] 2) Coal particle pre-burning treatment: Coal particles with 10-30% moisture content are placed in a heating furnace for roasting. The roasting atmosphere is an oxygen-containing atmosphere, which is a mixture of oxygen and an inert atmosphere gas, namely nitrogen or argon, or two or more of these gases. The oxygen volume percentage in the atmosphere is 2-8%. The heating rate from room temperature to the pre-burning temperature is 1-15℃ / min, the pre-burning temperature is 400-700℃, and the holding time is 1-5h. The gas space velocity of the oxygen-containing mixed atmosphere is 10-150mL / min / g. The loss on ignition of the coal particles is controlled to be 10-30%. Loss on ignition = (dry weight of coal particles before pre-burning treatment - weight of coal particles after pre-burning treatment) / dry weight of coal particles before pre-burning treatment.

[0010] 3) The pretreated coal particles from step 2) are ball-milled into powder with a particle size of 10-75 μm. The ball-milled powder is then acid-washed and then water-washed.

[0011] 4) The powder particles washed in step 3) are subjected to high-temperature calcination to obtain hard carbon material. The calcination process is divided into five stages in sequence: heating stage, high-temperature holding stage, cooling stage, medium-temperature holding stage, and cooling stage. In the heating stage, the temperature is increased from room temperature to 1000-1500℃ at a rate of 1-10℃ / min; in the high-temperature holding stage, the temperature is held at 1000-1500℃ for 2-5 hours; in the cooling stage, the temperature is naturally cooled to 700-1000℃; in the medium-temperature holding stage, the temperature is held at 700-1000℃ for 0.5-10 hours; and in the cooling stage, the temperature is naturally cooled to room temperature.

[0012] The four stages—heating stage, high-temperature holding stage, cooling stage, and cooling phase—are all mixed with an inert atmosphere and a reducing atmosphere. The volume ratio of the reducing gas is 20-40%. The inert gas used is one or a mixture of nitrogen and argon, and the reducing gas is one or a mixture of blast furnace gas, coking gas, carbon monoxide, and hydrogen. The mixed gas space velocity is 30-200 mL / min / g.

[0013] The intermediate temperature holding stage is a mixed atmosphere of carbon source gas and inert gas. The carbon source gas is one or more of the following: methane, ethane, propane, n-butane, isobutane, ethylene, propylene, n-butene, isobutene, butadiene, cis-dibutene, trans-dibutene, cyclopropane, acetylene, and propyne. The inert gas is one or more of the following: nitrogen and argon. The carbon source gas accounts for 10-80% of the total gas. After mixing the carbon source gas and the inert gas, the mixture is introduced into the furnace at a space velocity of 10-120 mL / min / g.

[0014] Further, the hard carbon material obtained in step 4) is subjected to ball milling, air jet milling and sieving to obtain hard carbon material with a particle size range of 0.5 to 10 μm.

[0015] Furthermore, the coal mentioned in step 1) includes low-rank coal, medium-rank coal, and high-rank coal, preferably low-cost, low-value low-rank coal, including one or more of lignite, bituminous coal, and sub-bituminous coal.

[0016] Furthermore, in step 3), the acid used in the pickling process is one or a mixture of two of the following: hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid. The mass concentration of the acid is 5-40%, and the number of pickling cycles is 1-3. After pickling, water is used for rinsing, and the number of water rinsing cycles is 2-6.

[0017] Further, in step 4), the main components of the blast furnace gas are 20-40% carbon monoxide, 50-70% nitrogen, and 1-10% carbon dioxide (volume composition); the coking gas consists of 45-60% methane, 30-45% carbon monoxide, 5-10% carbon monoxide, and 1-10% carbon dioxide (volume composition).

[0018] Furthermore, the hard carbon material is prepared using any one of the methods described in technical solutions 1-4 above.

[0019] Furthermore, the obtained hard carbon material has a pore size distribution range of 0.1–20 nm and an average pore size range of 0.4–8 nm; its specific surface area ranges from 1 to 40 m². 2 / g.

[0020] Furthermore, using any of the aforementioned hard carbon materials as the negative electrode active material in sodium-ion batteries can yield low-cost, high-energy-density, and high-performance sodium-ion batteries.

[0021] The method for preparing hard carbon materials using coal proposed in this invention has the following significant advantages:

[0022] 1) my country has large coal reserves and abundant resources. Using coal as a raw material to produce hard carbon can reduce costs. Prioritizing low-rank coal with low value further reduces costs while enabling the high-value utilization of low-rank coal. Using blast furnace gas or coking gas as a reducing gas source during the preparation process improves material performance while reducing manufacturing costs. The preparation process is simple and inexpensive.

[0023] 2) Using a mixed atmosphere of carbon source gas and reducing gas to achieve carbon deposition in the micropores of hard carbon particles greatly reduces the specific surface area and micropore size of hard carbon particles, which can significantly reduce the irreversible capacity loss during the first charge and discharge process of sodium-ion batteries, improve the first coulombic efficiency of the battery, and improve battery performance.

[0024] 3) The obtained hard carbon material has a high initial coulombic efficiency and a high specific capacity, which can significantly improve the energy density of sodium-ion batteries.

[0025] The outstanding advantage of this invention lies in the low cost of raw materials and manufacturing process, which significantly reduces the cost of hard carbon materials. At the same time, by using a mixed atmosphere of carbon source gas and reducing gas, carbon deposition is achieved in the micropores of hard carbon particles, which improves the first coulombic efficiency of hard carbon materials. The sodium-ion battery prepared as a negative electrode material has excellent charge and discharge performance. Attached Figure Description

[0026] Figure 1 This is a SEM image of the hard carbon material prepared in Example 1.

[0027] Figure 2 The image shows the XRD pattern of the hard carbon material prepared in Example 1.

[0028] Figure 3 The graph shows the first charge-discharge curve of a sodium coin cell using the hard carbon material prepared in Example 1. Detailed Implementation

[0029] The advantages of the present invention will be further explained below with reference to specific embodiments.

[0030] Example 1

[0031] 1) Weigh 500g of lignite as raw material, crush it with a crusher, screen it, and wash it with water to obtain lignite particles with a particle size of 1-5mm.

[0032] 2) Maintain 15% water content in the lignite particles. Place the moistened lignite particles into a heating furnace for pre-burning. The pre-burning atmosphere is a mixture of oxygen and nitrogen, with oxygen accounting for 5% by volume. The gas space velocity of the mixed atmosphere is 50 mL / min / g. The heating rate from room temperature to the pre-burning temperature is 4℃ / min, the heating termination temperature is 550℃, and the holding time is 2.5h. The loss on ignition of the coal particles is 18%.

[0033] 3) Crush the lignite particles from step 2) to obtain pre-calcined powder material with a particle size of 10-75 μm. Use sulfuric acid to perform one acid wash on the pre-calcined powder material with a sulfuric acid mass concentration of 20%, and then wash it four times with deionized water.

[0034] 4) The pre-calcined powder material washed in step 3) is subjected to high-temperature calcination. The process is as follows: Heating stage: temperature is increased from room temperature to 800℃ at 8℃ / min, then further increased to 1350℃ at 4℃ / min; High-temperature holding stage: 1350℃ is held for 2.5 hours; Cooling stage: temperature is naturally reduced to 800℃; Medium-temperature holding stage: temperature is maintained at 800℃ for 2 hours; Cooling stage: temperature is naturally cooled to room temperature. The atmosphere for the four stages (heating, high-temperature holding, cooling, and cooling) is a mixture of nitrogen and coking gas, with coking gas accounting for 30% of the volume. The gas space velocity (GSV) of the mixture is 50 mL / min / g. The volume composition of the coking gas is 50% hydrogen, 40% methane, 7% carbon monoxide, and 3% carbon dioxide. The medium-temperature holding stage at 800℃ is a mixture of ethylene and argon, with ethylene gas accounting for 30% of the volume. The GSV of the mixture is 30 mL / min / g. 5) The hard carbon material obtained in step 4) is ball-milled, air-jet pulverized and sieved to obtain hard carbon material with a particle size of 0.5 to 10 μm.

[0035] The specific surface area, average pore size, and micropore size range of the hard carbon material obtained in Example 1 are shown in Table 1.

[0036] The prepared hard carbon material was used to fabricate the negative electrode, which was then matched with a sodium vanadium phosphate positive electrode to prepare a pouch cell. Specific conditions for preparing the hard carbon negative electrode were: slurry ratio of hard carbon:Sp:CMC:SBR = 92:4:2:2; electrode surface loading of 4.5 mg / cm²; and compaction density of 0.93 g / m³. Based on a negative electrode capacity to positive electrode capacity ratio of 1.1, a sodium vanadium phosphate positive electrode was matched, and a 5.5 Ah pouch cell was fabricated using a Z-shaped stacking process for performance testing. Test conditions were: initial coulombic efficiency test of the full cell, voltage range: 2.0–4.3 V, ambient temperature: 45℃, and charge / discharge current rate of 0.1C; and energy density test of the full cell, voltage range: 2.0–4.3 V, ambient temperature: 25℃, and charge / discharge current rate of 0.5C.

[0037] The conditions for preparing coin half-cells of sodium metal using the prepared hard carbon material were as follows: slurry ratio of hard carbon:Sp:CMC:SBR = 92:4:2:2; electrode surface loading of 3 mg / cm2; compaction density of 0.90 g / m3; voltage range of 0–2.5 V; test ambient temperature of 25 °C; and initial coulombic efficiency and specific capacity tests at a charge / discharge rate of 0.1 C.

[0038] Example 2

[0039] The process and conditions are the same as in Example 1, except that in step 2 of this example, bituminous coal is used.

[0040] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 2 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0041] Example 3

[0042] The process and conditions are the same as in Example 1, except that in step 2 of this example, sub-bituminous coal is used.

[0043] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 3 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0044] Example 4

[0045] The process and conditions are the same as in Example 1. The difference is that in step 2 of this example, the oxygen content in the atmosphere during the pre-burning of lignite particles is 2%, the roasting temperature is 650℃, and the holding time is 4h.

[0046] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 4 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0047] Example 5

[0048] The process and conditions are the same as in Example 1. The difference is that step 2 is different in this example. The oxygen content in the pre-burning atmosphere of lignite particles is 7%, the roasting temperature is 450℃, and the holding time is 3h.

[0049] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 5 is tested, and the test methods, test conditions, and test contents are the same as in Example 1.

[0050] Example 6

[0051] The process and conditions are the same as in Example 1, except that step 2 is different in this example. The space velocity of the pre-burning atmosphere for lignite particles is 120 mL / min / g, and the holding time is 1 h.

[0052] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 6 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0053] Example 7

[0054] The process and conditions are the same as in Example 1, except that step 2 is different in this example. The space velocity of the pre-burning atmosphere for lignite particles is 100 mL / min / g, and the holding time is 1 h.

[0055] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 7 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0056] Example 8

[0057] The process and conditions are the same as in Example 1, except that step 2 is different in this example, where the lignite particles are kept at 25% moisture content.

[0058] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 8 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0059] Example 9

[0060] The process and conditions are the same as in Example 1, except that in step 3 of this example, hydrofluoric acid is used to acid wash the powder, and the mass concentration of hydrofluoric acid is 5%.

[0061] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 9 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0062] Example 10

[0063] The process and conditions are the same as in Example 1, except that in this example, step 4, the high-temperature heat preservation stage, has a temperature of 1200℃.

[0064] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 10 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0065] Example 11

[0066] The process and conditions are the same as in Example 1, except that in this example, step 4, the high-temperature heat preservation stage, has a temperature of 1450°C.

[0067] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 11 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0068] Example 12

[0069] The process and conditions are the same as in Example 1, except that in this example, step 4, the high-temperature heat preservation stage, is held for 4 hours.

[0070] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 12 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0071] Example 13

[0072] The process and conditions are the same as in Example 1, except that in this example, step 4, the medium-temperature heat preservation stage, has a temperature of 900°C.

[0073] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 13 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0074] Example 14

[0075] The process and conditions are the same as in Example 1, except that in this example, step 4, the medium-temperature heat preservation stage, has a temperature of 750°C.

[0076] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 14 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0077] Example 15

[0078] The process and conditions are the same as in Example 1, except that in this example, step 4 is the medium-temperature heat preservation stage, and the heat preservation time is 4 hours.

[0079] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 15 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0080] Example 16

[0081] The process and conditions are the same as in Example 1, except that in this example, step 4 is the medium-temperature heat preservation stage, and the heat preservation time is 8 hours.

[0082] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 16 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0083] Example 17

[0084] The process and conditions are the same as in Example 1, except that in step 4 of this example, the four stages—heating stage, high-temperature holding stage, cooling stage, and cooling phase—involve a mixture of nitrogen and blast furnace gas. The blast furnace gas consists of 33% carbon monoxide, 63% nitrogen, and 4% carbon dioxide.

[0085] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 17 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0086] Example 18

[0087] The process and conditions are the same as in Example 1. The difference is in step 4 of this example, where the atmosphere during the medium-temperature insulation stage is a mixture of methane and argon, with methane accounting for 60%.

[0088] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 18 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0089] Example 19

[0090] The process and conditions are the same as in Example 1. The difference is that in step 4 of this example, the atmosphere during the medium-temperature heat preservation stage is a mixture of methane and argon, with methane accounting for 70%.

[0091] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 19 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0092] Example 20

[0093] The process and conditions are the same as in Example 1. The difference is that in step 4 of this example, the atmosphere during the medium-temperature heat preservation stage is a mixture of methane and argon, with methane accounting for 20% and the medium-temperature heat preservation time being 5 hours.

[0094] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 20 is tested, and the test methods, conditions, and contents are the same as in Example 1.

[0095] Example 21

[0096] The process and conditions are the same as in Example 1. The difference is in step 4 of this example, where the atmosphere during the medium-temperature insulation stage is a mixture of acetylene and argon, with acetylene accounting for 20%.

[0097] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 21 is tested, and the test methods, test conditions, and test contents are the same as in Example 1.

[0098] Example 22

[0099] The process and conditions are the same as in Example 1. The difference is in step 4 of this example, where the atmosphere during the medium-temperature heat preservation stage after cooling is a mixture of propane and nitrogen, with propane accounting for 40%.

[0100] The remaining steps are the same as in Example 1. The hard carbon material prepared in Example 22 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0101] Comparative Example 1

[0102] The process and conditions are the same as in Example 1, except for step 2. In Comparative Example 1, the lignite is not pre-burned. The specific difference is as follows:

[0103] 2) The lignite particles cleaned in step 1) are dried in an oven at 80℃ without pre-burning.

[0104] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 1 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0105] Comparative Example 2

[0106] The process and conditions are the same as in Example 1, except for step 2. In Comparative Example 2, the lignite pre-burning process is carried out in a nitrogen atmosphere.

[0107] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 2 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0108] Comparative Example 3

[0109] The process and conditions are the same as in Example 1, except for step 2. In Comparative Example 3, the lignite is dried and then pre-burned, meaning the water content in the lignite is <10%.

[0110] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 3 was tested, and the test methods, conditions and contents were the same as in Example 1.

[0111] Comparative Example 4

[0112] The process and conditions were the same as in Example 1, except for step 3. In Comparative Example 4, the pre-burned lignite particles were not acid washed.

[0113] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 4 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0114] Comparative Example 5

[0115] The process and conditions are the same as in Example 1, except for step 4. In Comparative Example 5, the high-temperature calcination temperature is 800°C.

[0116] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 5 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0117] Comparative Example 6

[0118] The process and conditions are the same as in Example 1, except that in step 4, nitrogen atmosphere is used during the medium-temperature heat preservation stage in Comparative Example 6.

[0119] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 6 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0120] Comparative Example 7

[0121] The process and conditions are the same as in Example 1, except for step 4. In Comparative Example 7, the temperature during the medium-temperature insulation stage is 500°C.

[0122] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 7 was tested, and the test methods, conditions and contents were the same as in Example 1.

[0123] Comparative Example 8

[0124] The process and conditions are the same as in Example 1, except for step 4. In Comparative Example 8, there are five stages: heating, high-temperature holding, cooling, medium-temperature holding, and natural cooling, all using a nitrogen atmosphere.

[0125] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 8 is tested, and the test methods, test conditions, and test contents are the same as in Example 1.

[0126] Comparative Example 9

[0127] The process and conditions are the same as in Example 1, except that in step 4, in Comparative Example 9, the heating stage, the high-temperature holding stage, the cooling stage, and the cooling stage are all in a nitrogen atmosphere.

[0128] The remaining steps are the same as in Example 1. The hard carbon material prepared in Comparative Example 9 is tested, and the test methods, conditions and contents are the same as in Example 1.

[0129] Table 1 compares the performance test results of hard carbon materials obtained in Examples 1 to 22 and Comparative Examples 1 to 9.

[0130]

[0131]

[0132] The implementation data from Examples 1-23 above show that this invention uses coal as a raw material to prepare hard carbon, preferably low-rank coal with low value. During the preparation process, blast furnace gas or coking gas is used as a reducing gas source to improve material performance. This invention also employs a special gas-phase carbon deposition process to obtain hard carbon materials with high initial coulombic efficiency and high reversible specific capacity. When applied to sodium-ion pouch batteries, they exhibit high energy density. Furthermore, the preparation process of this invention is simple and the material preparation cost is low. The initial coulombic efficiency of the hard carbon material half-cells in Examples 1-23 is all higher than 80%, and the specific capacity is all higher than 300 mAh / g. When the hard carbon material prepared by this invention is used as the negative electrode in sodium-ion pouch batteries, the energy density is all higher than 120 Wh / kg, demonstrating the significant performance advantages of the hard carbon material prepared by this invention.

[0133] Comparative Example 1 illustrates that the pre-firing process has a significant impact on the specific surface area and average pore size of the material.

[0134] Comparative Example 2 shows that the oxygen-containing atmosphere has a significant impact on the specific surface area and micropore size distribution of the material during the pre-calcination process, and ultimately plays an important role in the sodium storage capacity and initial efficiency of the material.

[0135] Comparative Example 3 illustrates that if the water content in the raw materials is too low or there is no water at all during the pre-calcination process, it will have a significant impact on the performance of the final product.

[0136] Comparative Example 4 illustrates that acid washing plays an important role in the treatment of pre-calcined products. Acid washing can remove impurities after pre-calcination and has a significant impact on the initial coulombic efficiency and sodium storage capacity of the material.

[0137] Comparative Example 5 illustrates that when the high-temperature carbonization roasting temperature is too low, the carbon layer of the material cannot form an effective sodium storage structure at the microscopic level, resulting in low sodium storage capacity and poor performance. A reasonable roasting temperature is crucial for material performance.

[0138] Comparative Example 6 illustrates that introducing a carbon-source atmosphere during the intermediate-temperature holding stage after calcination can significantly improve the microporous structure of the material, reduce material defects, and play an important role in improving the material's performance.

[0139] Comparative Example 7 illustrates that during the intermediate temperature insulation stage, excessively low temperatures cannot achieve the regulation of the microporous structure of the material by the carbon source gas, which has a significant impact on the first coulombic efficiency and performance of the final product.

[0140] Comparative Examples 8 and 9 demonstrate that reducing gases and carbon source gases play a crucial role in regulating the microporous structure of the material and removing oxygen-containing functional groups throughout the high-temperature carbonization and roasting process, and have a significant impact on the first coulombic efficiency and performance of the final product.

[0141] This invention uses only low-rank coal as raw material, resulting in low raw material costs and a simple manufacturing process, which significantly reduces the cost of the prepared hard carbon material. Innovative processes, such as oxygen-containing atmosphere pre-calcination, staged use of a mixed atmosphere of reducing gas, carbon source gas, and inert gas, and high-temperature treatment of the material, improve the initial coulombic efficiency and specific capacity of the material. The resulting material, when used as a negative electrode, produces sodium-ion pouch batteries with high energy density. This invention demonstrates significant beneficial effects.

Claims

1. A method for preparing hard carbon materials using coal, characterized in that, Includes the following steps: 1) After crushing and screening the coal, obtain particles with a particle size of 1~5mm, and wash them with water to remove ash; 2) Coal particle pre-burning treatment: Coal particles with a moisture content of 10-30% by mass are placed in a heating furnace for roasting. The roasting atmosphere is an oxygen-containing atmosphere, which is a mixture of oxygen and an inert atmosphere gas, namely nitrogen or argon, or one or both. The oxygen volume percentage in the atmosphere is 2-8%. The heating rate from room temperature to the pre-burning temperature is 1-15℃ / min, the pre-burning temperature is 400-700℃, and the holding time is 1-5h. The gas space velocity of the oxygen-containing mixed atmosphere is 10-150mL / min / g. The loss on ignition of the coal particles is controlled to be 10-30%. Loss on ignition = (dry weight of coal particles before pre-burning treatment - dry weight of coal particles after pre-burning treatment) / dry weight of coal particles before pre-burning treatment. 3) The pretreated coal particles from step 2) are ball-milled into powder with a particle size of 10~75μm. The ball-milled powder is then acid-washed and then water-washed. 4) The powder particles washed in step 3) are subjected to high-temperature calcination to obtain hard carbon material. The calcination process is divided into five stages in sequence: heating stage, high-temperature holding stage, cooling stage, medium-temperature holding stage, and cooling stage. In the heating stage, the temperature is increased from room temperature to 1000~1500℃ at a rate of 1~10℃ / min. During the high-temperature heat preservation stage, the temperature is maintained at 1000~1500℃ for 2~5 hours; during the cooling stage, the temperature is naturally cooled to 700~1000℃; during the medium-temperature heat preservation stage, the temperature is maintained at 700~1000℃ for 0.5~10 hours; during the cooling stage, the temperature is naturally cooled to room temperature. The four stages—heating stage, high-temperature holding stage, cooling stage, and cooling phase—are a mixture of inert and reducing atmospheres. The volume percentage of the reducing gas is 20-40%. The inert gas used is one or a mixture of nitrogen and argon, and the reducing gas is one or a mixture of blast furnace gas, coking gas, carbon monoxide, and hydrogen. The mixed gas space velocity is 30-200 mL / min / g. The intermediate temperature holding stage is a mixed atmosphere of carbon source gas and inert gas. The carbon source gas is one or more of the following: methane, ethane, propane, n-butane, isobutane, ethylene, propylene, n-butene, isobutene, butadiene, cis-dibutene, trans-dibutene, cyclopropane, acetylene, and propyne. The inert gas is one or more of the following: nitrogen and argon. The carbon source gas accounts for 10-80% of the total gas. After mixing the carbon source gas and the inert gas, the mixture is introduced into the furnace at a space velocity of 10-120 mL / min / g.

2. The method according to claim 1, characterized in that: The hard carbon material obtained in step 4) is ball-milled, air-jet pulverized and sieved to obtain hard carbon material with a particle size range of 0.5~10μm.

3. The method according to claim 1, characterized in that: Step 1) The coal includes one or more of low-rank coal, medium-rank coal and high-rank coal.

4. The method according to claim 3, characterized in that: The low-rank coal includes one or both of lignite and bituminous coal.

5. The method according to claim 1, characterized in that: Step 3) The acid used in the pickling process is one or a mixture of two of the following: hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid. The mass concentration of the acid is 5-40%, and the number of pickling cycles is 1-3. After pickling, water is used for rinsing, and the number of water rinsing cycles is 2-6.

6. The method according to claim 1, characterized in that: Step 4) The main volume composition of the blast furnace gas is 20-40% carbon monoxide, 50-70% nitrogen, and 1-10% carbon dioxide; the volume composition of the coking gas is 45-60% hydrogen, 30-45% methane, 5-10% carbon monoxide, and 1-10% carbon dioxide.

7. A hard carbon material prepared by the method of any one of claims 1-6.

8. The hard carbon material according to claim 7, characterized in that: The pore size distribution of hard carbon materials ranges from 0.1 to 20 nm, with an average pore size ranging from 0.4 to 8 nm; the specific surface area ranges from 1 to 40 m². 2 / g.

9. The application of the hard carbon material according to any one of claims 7-8 as a negative electrode active material in a sodium-ion battery.

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