A lignin-based hard carbon composite active material and its preparation and application in sodium ion batteries

Abstract

The present invention belongs to the field of sodium ion batteries, and specifically relates to a method for preparing a lignin-based hard carbon composite active material, wherein a solution containing a lignin raw material and an organic acid is heat-treated, then filtered while hot, and the filtrate is cooled and solid-liquid separated to obtain pretreated lignin; wherein, the concentration of the organic acid in the starting solution of the heat treatment is 30-70%; the temperature of the heat treatment is 40-95°C; the sodium content in the pretreated lignin is 50-200ppm; the pretreated lignin is carbonized to obtain hard carbon, and then subjected to fluidized-state heat treatment in an organic gas to obtain the lignin-based hard carbon composite active material. The present invention also includes materials obtained by the preparation method and their applications. The present invention combines the special sodium control process and the fluidized-state treatment process of the organic gas source to synergistically improve the electrochemical performance of the prepared lignin-based active material in sodium ion batteries.

Description

Technical Field

[0001] The present invention belongs to the field of battery materials, and in particular relates to the field of sodium ion battery negative electrode materials. Background Art

[0002] Lithium-ion batteries are considered one of the most advanced energy storage technologies and can be applied to different devices. However, lithium resources are limited and unevenly distributed around the world, so alternatives are needed. Sodium is very abundant in the earth's crust and seawater, so the development of sodium-ion batteries has received particularly widespread attention. However, due to some thermodynamic factors, graphite, a common negative electrode for lithium ions, cannot store sodium. Therefore, the development of sodium-ion negative electrodes requires another approach. Since Dohn et al. discovered that hard carbon can store sodium in 2000, hard carbon has been considered to be a more ideal negative electrode material for the commercialization of sodium ions. Hard carbon can be produced by pyrolysis of biomass or polymers at a certain temperature. Due to the relatively high cost of polymers, it is urgent to use low-cost biomass to develop high-performance hard carbon materials for sodium-ion batteries.

[0003] Lignin is the second most abundant biopolymer in the world and is rich in aromatic compounds. Currently, the paper and pulp industry concentrates lignin, with the kraft pulping process accounting for 95% of the market share. During the pulping process, the bonds connecting lignin and cellulose in the plant cell wall are broken by a hot aqueous solution of sodium hydroxide and sodium sulfide, dissolving most of the lignin and removing the cellulose fibers from the wood matrix into pulp for papermaking. The resulting solution, a byproduct known as black liquor, represents a significant potential source of lignin. Typically, this lignin-rich black liquor is burned in recovery boilers for its calorific value. Currently, only approximately 0.2% of the lignin produced by pulp mills is recovered for non-fuel uses. However, lignin could have greater value as a renewable biomaterial or biopolymer. Lignin has a high carbon content, and its phenolic hydroxyl groups can be chemically modified to form quinone groups. This reversible conversion between the two provides pseudocapacitance, while also facilitating electron conduction and ion transport. Carbon materials function by providing surface adsorption sites for charge storage. By reducing the cost of hard carbon and improving its energy density, lignin-based carbon materials can be effectively used as electrode materials for sodium-ion batteries.

[0004] Although lignin is a conventional carbon source, the electrochemical performance of lignin-derived carbon materials is far below expectations. For example, Jean-Marie Tarascon et al. explored the most common commercial lignin raw material as a renewable resource for the preparation of hard carbon materials for sodium ion batteries. By controlling impurities and surface area through a water washing process, a capacity of 309 mAh g was obtained from industrial lignin. -1, the first cycle irreversible capacity of hard carbon is 20%. However, hard carbon materials with such performance are still some distance away from commercialization. Summary of the Invention

[0005] In response to the problem that materials prepared from lignin raw materials are difficult to adapt to the needs of sodium ion batteries and have unsatisfactory electrochemical properties, the first purpose of the present invention is to provide a method for preparing lignin-based hard carbon composite active materials, aiming to improve the electrochemical performance of the prepared materials in sodium ion batteries based on a combined sodium control-fluidization process.

[0006] The second object of the present invention is to provide a lignin-based hard carbon composite active material obtained by the preparation method, and its application as a negative electrode active material.

[0007] The third object of the present invention is to provide a negative electrode material, a negative electrode and a sodium secondary battery comprising the lignin-based hard carbon composite active material.

[0008] Although lignin raw materials are a common and widely used carbon source in industries such as activated carbon, the high purity requirements of battery-grade materials make it difficult for existing lignin raw materials to adapt to the needs of sodium ion battery anodes, making it difficult to obtain high-performance sodium ion battery anode materials. To address this problem, the present invention provides the following solutions:

[0009] A method for preparing a lignin-based hard carbon composite active material, comprising the following steps:

[0010] Step (1): Sodium control pretreatment:

[0011] A solution containing lignin raw material and organic acid is heat-treated, then filtered while hot, and the filtrate is cooled and solid-liquid separated to obtain pretreated lignin; wherein the concentration of the organic acid in the starting solution of the heat treatment is 30-70%; the heat treatment temperature is 40-95° C.; and the sodium content in the pretreated lignin is 50-200 ppm;

[0012] Step (2):

[0013] Carbonizing the pretreated lignin to obtain hard carbon, and then subjecting the pretreated lignin to a fluidized heat treatment in an organic gas to obtain the lignin-based hard carbon composite active material;

[0014] The carbonization process includes a first carbonization process and a second carbonization process, wherein the temperature of the first carbonization process is 500-1000°C; the temperature of the second carbonization process is 1000-1800°C;

[0015] The temperature of the fluidized state heat treatment process is 750-950℃.

[0016] The present invention unexpectedly discovered that the excessive residual sodium in the lignin raw material is the main cause of the electrochemical performance of the sodium ion battery prepared therefrom. Based on the innovative discovery of the root cause of this technical problem, the present invention innovatively provides a new idea for improving the electrochemical performance of lignin-based active materials based on a sodium control process, and further research has found that the innovative use of an organic acid-assisted heat treatment-cooling treatment sodium control idea, further combined with the concentration of organic acid, the temperature of heat treatment and the control concentration of sodium, helps to synergistically improve the electrochemical performance of lignin-derived active materials in sodium ion batteries. Further innovatively combining the special sodium control process, the two-stage carbonization mechanism and the fluidized bed treatment process of the organic gas source can further synergize and further improve the electrochemical performance of the prepared lignin-based active materials in sodium ion batteries. Not only that, it can also reduce equipment corrosion in the preparation process and significantly reduce preparation costs.

[0017] In the present invention, the lignin raw material is industrial grade lignin; preferably includes at least one of organic solvent lignin, sodium lignin sulfonate, sodium lignin sulfate, and alkali lignin;

[0018] Preferably, the sodium content in the lignin raw material is less than 10 wt.%, and can be further preferably 5 to 9 wt.% in consideration of material composition, industrial value and performance.

[0019] In the present invention, the organic acid-assisted heat treatment-cooling sodium control process and the control of the organic acid concentration, heat treatment temperature and sodium content after treatment during the sodium control treatment are the key to synergistically improving the gram capacity of the negative electrode material and constructing a suitable SEI, thereby synergistically improving its electrochemical performance in sodium ion batteries.

[0020] In the present invention, the organic acid is a C1-C10 carboxylic acid compound, preferably at least one of acetic acid, propionic acid, citric acid, and oxalic acid; more preferably, acetic acid. Research has found that combining acetic acid with other processes can further achieve synergy and improve the electrochemical properties of the resulting material.

[0021] Preferably, the concentration of the organic acid in the starting solution of the heat treatment is 40-60%, more preferably 45-50%. In the present invention, the combined control of the organic acid and its concentration helps to further synergistically improve the electrochemical properties of the prepared material.

[0022] Preferably, the heat treatment temperature is 80-90°C.

[0023] Preferably, the heat treatment time is, for example, 0.1 to 3 hours.

[0024] Preferably, in step (1), the cooling temperature is less than or equal to 25°C, preferably 4-20°C.

[0025] In the present invention, the obtained pretreated raw material is subjected to carbonization treatment, and then heat treatment is performed while gas phase fluidization is performed to make the hard carbon in a fluidized state.

[0026] In the present invention, under the sodium control process, further coordinating the processes and mechanisms of carbonization and fluidized state heat treatment helps to further synergistically improve the electrochemical properties of the prepared material.

[0027] In the present invention, the carbonization process includes a first carbonization stage and a second carbonization stage. The temperature of the first carbonization stage is 500-700°C, more preferably 600-650°C; the temperature of the second carbonization stage is 1300-1450°C, more preferably 1400-1450°C. In the present invention, under the preferred process, the electrochemical performance of the lignin-based negative electrode material is further synergistically improved.

[0028] In the present invention, the first carbonization treatment may last for 1 to 15 hours, preferably 1 to 10 hours, and more preferably 2 to 5 hours. The second carbonization treatment may last for 1 to 5 hours, preferably 2 to 2.5 hours.

[0029] The carbonization stage is carried out under a protective atmosphere, and the protective atmosphere is, for example, at least one of nitrogen, inert gas, ammonia, and a hydrogen-argon mixed gas.

[0030] In the present invention, the D50 of the hard carbon is controlled to be 3-15 μm. Subsequently, a fluidized heat treatment is performed in an organic gas.

[0031] Preferably, the organic gas is at least one of C1-C4 alkanes, C2-C4 alkenes, C2-C4 alkynes, and benzene.

[0032] In the present invention, the fluidized state heat treatment stage may also contain a carrier gas, such as at least one of nitrogen and an inert gas. Preferably, the volume content of the organic gas in the fluidized state heat treatment stage is 5-20% by volume.

[0033] Preferably, the fluidized state heat treatment process is carried out in a fluidized bed;

[0034] Preferably, during the sulfurization heat treatment stage, the flow rate of the atmosphere is greater than or equal to 100 L / min, preferably 100-1000 L / min, and more preferably 100-300 L / min.

[0035] Preferably, the temperature of the fluidized state heat treatment is 750-850°C, more preferably 800-850°C.

[0036] The time of fluidized state heat treatment can be adjusted as needed, for example, it can be 0.5 to 5 hours, preferably 1 to 3 hours.

[0037] The preferred preparation process of the present invention comprises the following steps:

[0038] (1) Sodium control;

[0039] Industrial lignin is dissolved in a hot acetic acid solution to obtain a homogeneous solution, which is then filtered and cooled to produce sodium-controlled lignin. Industrial lignin includes organic solvent lignin, sodium lignin sulfonate, sodium lignin sulfate, and alkali lignin. The concentration of the acetic acid aqueous solution is 30-70%; the temperature of the hot acid aqueous solution is 45-95°C; and the stirring speed is 100-500 rpm. The temperature of the hot acid lignin mixture, upon cooling to precipitate the lignin, is below 25°C. The sodium content of the treated lignin is 50-200 ppm.

[0040] (2) Pre-carbonizing the sodium-controlled lignin under a protective atmosphere to obtain a primary carbon material, which is then crushed. The pre-carbonization temperature is 500-1000°C, the heating rate is 1-10°C / min, and the time is 1-10 hours. The particle size D50 of the powder after initial crushing is 40-60 μm.

[0041] (3) Grinding and screening the primary powder, and selecting the powder with appropriate particle size as the raw material for carbonization;

[0042] The crushed powder is air-milled and sieved to a particle size D50 of 3-15 μm. The carbonization process is carried out under a protective atmosphere, selected from nitrogen, ammonia, and a hydrogen-argon mixture. The carbonization process includes a first carbonization stage and a second carbonization stage. The temperature of the first carbonization stage is 500-1000°C, while the temperature of the second carbonization stage is 1000-1800°C. The heating rate during the carbonization stages is 1-10°C / min, and the holding time of the first carbonization stage is 1-15 hours, while the holding time of the second carbonization stage is 1-3 hours.

[0043] (4) carbonizing the screened powder at high temperature under an inert atmosphere to prepare a hard carbon material;

[0044] (5) The carbonized material is subjected to gas phase surface purification treatment to obtain low-cost, high-performance hard carbon material.

[0045] The surface purification treatment mentioned above refers to adding the material into a fluidized bed and introducing a nitrogen-carrying treatment gas to modify the material surface. The treatment gas mentioned above can be methane, acetylene, benzene vapor, etc.

[0046] The present invention also provides a lignin-based hard carbon composite active material prepared by the preparation method.

[0047] In the present invention, thanks to the combination of the processes, the special physical and chemical properties of the prepared products can be effectively controlled, and special negative electrode active materials can be obtained. Moreover, the negative electrode active materials prepared by the preparation method can have excellent electrochemical properties in sodium ion batteries.

[0048] The material described in the present invention has a core-shell structure, wherein the core is a lignin-based amorphous carbon with a sub-nanometer pore structure (which is formed by cross-linking of various functional active groups in lignin, such as aromatic groups, phenolic hydroxyl groups, alcoholic hydroxyl groups, carbon-based conjugated double bonds, and other active groups), with a pore size of ≤1nm. The shell is a thin layer of amorphous carbon. The specific surface area of ​​the lignin-based hard carbon composite active material is less than or equal to 10m 2 / g; preferably 2 to 4 m 2 / g. The tap density of hard carbon materials is ≥0.5g / cm 3 ; preferably ≥0.6g / cm 3 ID / IG is 0.2 to 2, preferably 0.8 to 1. In the present invention, the material has excellent gram capacity and first-cycle coulombic efficiency, and has less sodium loss during the first charge and discharge process.

[0049] The present invention also provides a sodium ion battery negative electrode material, which comprises a lignin-based hard carbon composite active material prepared by the preparation method of the present invention;

[0050] Preferably, the battery further comprises a conductive agent and a binder. In the present invention, the conductive agent and binder can be components known in the field of sodium-ion batteries, such as carbon black and PVDF. The content of the conductive agent and binder can also be adjusted according to industry requirements, for example, 2 to 15 wt % each.

[0051] The present invention also provides a sodium ion battery negative electrode, comprising a current collector and a negative electrode material composited on the surface of the current collector, wherein the negative electrode material is the negative electrode material of the present invention.

[0052] The present invention also provides a sodium ion battery, the negative electrode of which is the negative electrode described in the present invention.

[0053] In the present invention, except for the lignin-based hard carbon composite active material prepared by the preparation method described in the present invention as the active material, other components, structures, materials and preparation methods of the sodium ion battery can be well known in the industry.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) The present invention reveals for the first time that the inappropriate residual sodium in lignin is the main problem leading to the unsatisfactory electrochemical performance of lignin-based active materials.

[0056] (2) The present invention proposes for the first time an idea to improve the electrochemical performance of lignin-based negative electrode materials in sodium-ion batteries based on the idea of ​​sodium control, and further finds that the innovative idea of ​​sodium control using organic acid-assisted heat treatment-cooling treatment, combined with the concentration of organic acid, the temperature of heat treatment and the controlled concentration of sodium, helps to synergistically improve the electrochemical performance of lignin-derived active materials in sodium-ion batteries.

[0057] (3) Combining the special sodium control process with the organic gas source fluidization treatment process can further synergize and improve the electrochemical performance of the prepared negative electrode in sodium ion batteries.

[0058] (4) The preparation process of the present invention is simple and can use industrial-grade raw materials, which is low in cost. In addition, high-performance materials can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 The relationship between the first cycle charge and discharge voltage and capacity of the hard carbon prepared in Example 1 applied to sodium ion batteries

[0060] Figure 2 This is a graph showing the long cycle performance of the hard carbon prepared in Example 1 when applied to sodium ion batteries; DETAILED DESCRIPTION

[0061] The present invention will be further described in detail below with reference to the embodiments, but the invention is not limited thereto.

[0062] In the following cases, the sodium content in lignin refers to the weight percentage of sodium element.

[0063] Example 1

[0064] Step (1):

[0065] Prepare an acetic acid aqueous solution with an acetic acid content of 50wt.%, add industrial grade alkali lignin (sodium content of 7.8wt.%) to the 50wt.% acetic acid aqueous solution, heat and stir until the temperature of the mixed system is 80°C, keep warm for 0.5h until the lignin is completely dissolved, filter while hot, collect the filtrate and continue stirring, and place it in an ice-water bath to reduce the temperature of the mixed system to 15°C until a solid lignin phase precipitates. The mixed solution is filtered again to obtain a filter residue phase, and the filter residue is dried in an 80°C oven for 24h. After drying, the sodium content of the sodium lignin is controlled to be 180ppm.

[0066] Step (2):

[0067] The dried material (sodium-controlled lignin obtained in step (1)) was preliminarily crushed to a particle size of D50 = 50 μm, added to a pyrolysis furnace, and heated to 600°C at a heating rate of 1°C / min under the protection of an argon atmosphere and kept at this temperature for 2 hours. The cooled material was crushed, air-milled, and sieved to select a material with a particle size range of D50 = 3-15 μm, and then loaded into a graphite crucible for high-temperature carbonization at a heating rate of 5°C / min to 1400°C (carbonization temperature) for 2 hours.

[0068] Step (3):

[0069] The carbonized material is placed in a fluidized bed reactor, where methane gas carried by an Ar inert gas is introduced to fluidize the material (at a flow rate of 200 L / min). The methane content in the mixed gas is 10%. The reactor is gradually heated to a temperature of 850°C in the reaction zone. The reaction is continued for one hour to treat the material surface. This treatment results in a low-cost, high-performance hard carbon material.

[0070] The hard carbon material obtained in Example 1, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were uniformly mixed in a mass ratio of 90:5:5 and dispersed in a predetermined mass of deionized water to form a slurry. The mixture was then coated onto an aluminum foil current collector and vacuum-dried at 60°C to produce a sodium-ion battery negative electrode. The battery was assembled and tested by punching the negative electrode sheet into a 10 mm diameter electrode sheet. Metallic sodium was used as the counter electrode, and the electrolyte was 1 M NaPF6 / EC:DEC (1:1). CR2032 button cells were assembled in an argon-filled glove box. Constant current charge and discharge tests were conducted at room temperature (25°C) at a current density of 30 mA / g, with a charge and discharge cutoff voltage of 0-3 V. The initial discharge capacity was 432 mAh / g, the initial coulombic efficiency was 90.32%, and the specific capacity remained at 376 mAh / g after 100 cycles.

[0071] Example 2

[0072] Prepare an acetic acid aqueous solution with an acetic acid content of 30wt.%, add sodium lignin sulfate (sodium content of 7.2wt.%) to the 30wt.% acetic acid aqueous solution, heat and stir until the temperature of the mixed system is 95°C, keep warm for 0.5h until the lignin is completely dissolved, filter while hot, collect the filtrate and continue stirring, and put it into an ice water bath to reduce the temperature of the mixed system to 10°C until a solid lignin phase precipitates. Filter the mixed solution again to obtain a filter residue phase, and dry the filter residue in an 80°C oven for 24h. The sodium content of the sodium-controlled lignin after drying is 90ppm.

[0073] The dried material is initially crushed to a particle size of D50 = 50 μm and placed in a pyrolysis furnace. Under the protection of an argon atmosphere, the temperature is increased to 1000°C at a heating rate of 10°C / min and maintained at this temperature for 1 hour. The cooled material is crushed, air-milled, and sieved to select material with a particle size range of D50 = 3-15 μm. The material is then loaded into a graphite crucible for high-temperature carbonization at a heating rate of 1°C / min to 1000°C and carbonized for 3 hours. The carbonized material is then placed in a fluidized bed reactor, and acetylene gas carried by an inert gas (Ar) is introduced to fluidize the material (atmosphere flow rate of 100 L / min). The acetylene content in the mixed gas is 5%. The reactor is gradually heated to a temperature of 750°C in the reaction zone. The reaction is carried out for 1 hour, and the material surface is treated. After the treatment is completed, a low-cost, high-performance hard carbon material is obtained.

[0074] The hard carbon material obtained in Example 1, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were uniformly mixed in a mass ratio of 90:5:5 and dispersed in a predetermined mass of deionized water to form a slurry. The mixture was then coated onto an aluminum foil current collector and vacuum-dried at 60°C to produce a sodium-ion battery negative electrode. The battery was assembled and tested by punching the negative electrode sheet into a 10 mm diameter electrode sheet. Metallic sodium was used as the counter electrode, and the electrolyte was 1 M NaPF6 / EC:DEC (1:1). CR2032 button cells were assembled in an argon-filled glove box. Constant current charge and discharge tests were conducted at room temperature (25°C) at a current density of 30 mA / g, with a charge and discharge cutoff voltage of 0 to 3 V. The initial discharge capacity was 384 mAh / g, the initial coulombic efficiency was 88.28%, and the specific capacity remained at 332 mAh / g after 100 cycles.

[0075] Example 3

[0076] Step (1):

[0077] Prepare an acetic acid aqueous solution with an acetic acid content of 70wt.%, add sodium lignin sulfonate (sodium content of 5.1wt.%) to the 70wt.% acetic acid aqueous solution, heat and stir until the temperature of the mixed system is 45°C, keep warm for 1.5 hours until the lignin is completely dissolved, filter while hot, collect the filtrate and continue stirring, and place it in an ice-water bath to reduce the temperature of the mixed system to 4°C until a solid lignin phase precipitates. Filter the mixed solution again to obtain a filter residue phase, and dry the filter residue in an 80°C oven for 24 hours. After drying, the sodium content of the sodium-controlled lignin is 50ppm.

[0078] Step (2):

[0079] The dried material was initially crushed to a particle size of D50 = 50 μm and placed in a pyrolysis furnace. Under the protection of an argon atmosphere, the temperature was raised to 500°C at a heating rate of 1°C / min and maintained at this temperature for 10 hours. The cooled material was crushed, air-milled, and sieved. The material with a particle size range of D50 = 3-15 μm was selected and placed in a graphite crucible for high-temperature carbonization at a heating rate of 10°C / min to 1800°C (carbonization temperature) for 1 hour.

[0080] Step (3):

[0081] The carbonized material is placed in a fluidized bed reactor and introduced with benzene vapor carried by an inert gas (Ar) at a flow rate of 1000 L / min to fluidize the material. The benzene vapor content in the mixed gas is 15%. The reactor is then gradually heated to a temperature of 950°C in the reaction zone. The reaction is continued for one hour to treat the material surface. This completes the treatment process to produce a low-cost, high-performance hard carbon material.

[0082] The hard carbon material obtained in Example 3, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were uniformly mixed in a mass ratio of 90:5:5 and dispersed in a predetermined mass of deionized water to form a slurry. The mixture was then coated onto an aluminum foil current collector and vacuum-dried at 60°C to produce a sodium-ion battery negative electrode. The battery was assembled and tested by punching the negative electrode sheet into a 10 mm diameter electrode sheet. Metallic sodium was used as the counter electrode, and the electrolyte was 1 M NaPF6 / EC:DEC (1:1). CR2032 button cells were assembled in an argon-filled glove box. Constant current charge and discharge tests were conducted at room temperature (25°C) at a current density of 30 mA / g, with a charge and discharge cutoff voltage of 0-3 V. The initial discharge capacity was 395 mAh / g, the initial coulombic efficiency was 87.90%, and the specific capacity remained at 331 mAh / g after 100 cycles.

[0083] Example 4

[0084] Prepare an oxalic acid aqueous solution with an oxalic acid content of 50wt.%, add alkali lignin (sodium content of 7.8wt.%) to the 50wt.% oxalic acid aqueous solution, heat and stir until the temperature of the mixed system is 65°C, keep warm for 1 hour until the lignin is completely dissolved, filter while hot, collect the filtrate and continue stirring, and place it in an ice water bath to reduce the temperature of the mixed system to below 20°C until a solid lignin phase precipitates. Filter the mixed solution again to obtain a filter residue phase, and dry the filter residue in an 80°C oven for 24 hours. The sodium content of the sodium-controlled lignin after drying is 80ppm.

[0085] The dried material is initially crushed to a particle size of D50 = 50um, and then added to a pyrolysis furnace. Under the protection of an argon atmosphere, the temperature is raised to 500°C at a heating rate of 1°C / min and kept warm for 10 hours. The cooled material is crushed, air-milled, and sieved to select materials with a particle size range of D50 = 3-15um. The material is then loaded into a graphite crucible for high-temperature carbonization at a heating rate of 10°C / min to 1800°C and carbonized for 1 hour. The carbonized material is loaded into a fluidized bed reactor, and methane carried by inert gas (Ar) is introduced to fluidize the material (the flow rate of the atmosphere is 500L / min). The methane content in the mixed gas is 15%, and the reactor is gradually heated so that the temperature of the reaction zone reaches 950°C. The reaction is carried out for 1 hour, and the surface of the material is treated. After the treatment is completed, a low-cost, high-performance hard carbon material can be obtained.

[0086] The hard carbon material obtained in Example 4, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were uniformly mixed in a mass ratio of 90:5:5 and dispersed in a predetermined mass of deionized water to form a slurry. The mixture was then coated onto an aluminum foil current collector and vacuum-dried at 60°C to produce a sodium-ion battery negative electrode. The battery was assembled and tested by punching the negative electrode sheet into a 10 mm diameter electrode sheet. Metallic sodium was used as the counter electrode, and the electrolyte was 1 M NaPF6 / EC:DEC (1:1). CR2032 button cells were assembled in an argon-filled glove box. Constant current charge and discharge tests were conducted at room temperature (25°C) at a current density of 30 mA / g, with a charge and discharge cutoff voltage of 0-3 V. The initial discharge capacity was 382 mAh / g, the initial coulombic efficiency was 86.50%, and the specific capacity remained at 313 mAh / g after 100 cycles.

[0087] Example 5

[0088] A citric acid aqueous solution was prepared with a citric acid content of 30 wt%, and industrial grade alkali lignin (sodium content of 7.8 wt.%) was added to the 30 wt% citric acid aqueous solution. The mixture was heated and stirred until the temperature of the mixture was 80°C. The mixture was kept warm for 0.5 h until the lignin was completely dissolved, and then filtered while hot. The filtrate was collected and stirred continuously, and the mixture was placed in an ice-water bath to reduce the temperature of the mixture to 20°C until a solid lignin phase was precipitated. The mixed solution was filtered again to obtain a filter residue phase, and the filter residue was dried in an 80°C oven for 24 h. The sodium content of the sodium-controlled lignin after drying was 195 ppm.

[0089] The dried material is initially crushed to a particle size of D50 = 50um, and then added to a pyrolysis furnace. Under the protection of an argon atmosphere, the temperature is raised to 600°C at a heating rate of 1°C / min and kept warm for 5 hours. The cooled material is crushed, air-milled, and sieved to select materials with a particle size range of D50 = 3-15um. The material is then loaded into a graphite crucible for high-temperature carbonization at a heating rate of 5°C / min to 1400°C and carbonized for 2 hours. The carbonized material is loaded into a fluidized bed reactor, and methane gas carried by inert gas (Ar) is introduced to fluidize the material (the flow rate of the atmosphere is 200L / min). The methane content in the mixed gas is 10%, and the reactor is gradually heated so that the temperature of the reaction zone reaches 850°C. The reaction is carried out for 1 hour, and the surface of the material is treated. After the treatment is completed, a low-cost, high-performance hard carbon material can be obtained.

[0090] The hard carbon material obtained in Example 1, conductive carbon black, and sodium carboxymethyl cellulose (CMC) were uniformly mixed in a mass ratio of 90:5:5 and dispersed in a predetermined mass of deionized water to form a slurry. The mixture was then coated onto an aluminum foil current collector and vacuum-dried at 60°C to produce a sodium-ion battery negative electrode. The battery was assembled and tested by punching the negative electrode sheet into a 10 mm diameter electrode sheet. Metallic sodium was used as the counter electrode, and the electrolyte was 1 M NaPF6 / EC:DEC (1:1). CR2032 button cells were assembled in an argon-filled glove box. Constant current charge and discharge tests were conducted at room temperature (25°C) at a current density of 30 mA / g, with a charge and discharge cutoff voltage of 0-3 V. The initial discharge capacity was 412 mAh / g, the initial coulombic efficiency was 90.33%, and the specific capacity remained at 349 mAh / g after 100 cycles.

[0091] Comparative Example 1

[0092] Compared with Example 1, the main difference is that the organic acid originally used is replaced by sulfuric acid during the sodium control treatment of industrial alkali lignin, while other process conditions remain unchanged.

[0093] The hard carbon material obtained in Comparative Example 1 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 322 mAh / g, an initial coulombic efficiency of 71.53%, and a capacity retention of 185 mAh / g after 100 cycles. This comparison with Example 1 demonstrates that the use of inorganic acid to control sodium treatment of the raw material is detrimental to the polymerization of lignin macromolecules, thereby affecting the electrochemical performance of the prepared hard carbon material.

[0094] Comparative Example 2

[0095] Compared with Example 1, the main difference is that the concentration of acetic acid is controlled at 20 wt.% when preparing the acetic acid solution. Other process conditions remain unchanged. The sodium content in the raw material after the sodium control treatment is 220 ppm.

[0096] The hard carbon material obtained in Comparative Example 2 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 384 mAh / g, an initial coulombic efficiency of 89.63%, and a capacity retention of 278 mAh / g after 100 cycles. Compared to Example 1, the organic acid concentration directly affects the sodium content of the material after sodium control. A lower organic acid content results in an excessively high sodium content after sodium control, thus affecting the material's capacity.

[0097] Comparative Example 3

[0098] Compared with Example 1, the main difference is that the lignin after the initial sodium control treatment is added to a newly prepared acetic acid aqueous solution for a secondary sodium control treatment, so that the sodium content of the lignin after sodium control is 20 ppm.

[0099] The hard carbon material obtained in Comparative Example 3 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 402 mAh / g, an initial coulombic efficiency of 88.79%, and a capacity retention of 272 mAh / g after 100 cycles. Compared to Example 1, secondary sodium control treatment of lignin significantly reduced the sodium content in the post-treatment lignin, which, to a certain extent, impacted the battery's cycling stability.

[0100] Comparative Example 4

[0101] Compared with Example 1, the main difference is that the carbonization temperature is increased to 2000° C., and other operations and parameters are the same as Example 1.

[0102] The hard carbon material obtained in Comparative Example 4 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 298 mAh / g, an initial coulombic efficiency of 89.65%, and a capacity retention of 236 mAh / g after 100 cycles. Compared to Example 1, carbonization and pyrolysis at higher temperatures lead to the closure of the nanopores within the material, hindering the storage of sodium ions and resulting in a decrease in sodium storage capacity.

[0103] Comparative Example 5

[0104] Compared with Example 1, the main difference is that the carbonization temperature is lowered to 800° C. Other process conditions remain unchanged.

[0105] The hard carbon material obtained in Comparative Example 5 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 268 mAh / g, an initial coulombic efficiency of 78.63%, and a capacity retention of 184 mAh / g after 100 cycles. Compared to Example 1, carbonization and pyrolysis at lower temperatures prevent the formation of nanoscale pores within the material, resulting in reduced sodium storage capacity and a lower initial coulombic efficiency.

[0106] Comparative Example 6

[0107] Compared with Example 1, the main difference is that step (3) is not performed, that is, the surface of the hard carbon material prepared in step (2) is not treated. Other process conditions remain unchanged.

[0108] The hard carbon material obtained in Comparative Example 6 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 432 mAh / g, an initial coulombic efficiency of 68.63%, and a capacity retention of 245 mAh / g after 100 cycles. Compared to Example 1, the lack of surface treatment of the hard carbon material after high-temperature carbonization results in a decrease in the initial coulombic efficiency. Furthermore, the highly reactive carbon surface thickens the SEI film, reducing the cycling stability of the material.

[0109] Comparative Example 7

[0110] Compared with Example 1, the main difference is that in step (3), the hard carbon material is not in a fluidized state (suspended state; the flow rate of the mixed gas is 30 L / min), and other process parameters remain the same.

[0111] The hard carbon material obtained in Comparative Example 7 was electrochemically evaluated using the same process as in Example 1. The results showed an initial discharge capacity of 425 mAh / g, an initial coulombic efficiency of 79.73%, and a capacity retention of 271 mAh / g after 100 cycles. Compared to Example 1, the lack of fluidized-state treatment of the high-temperature carbonized hard carbon material results in limited improvement in initial coulombic efficiency.

Claims

1. A method for preparing a lignin-based hard carbon composite active material, characterized in that the steps include: Step (1): Sodium control pretreatment: A solution containing lignin raw material and organic acid is heat-treated, then filtered while hot, and the filtrate is cooled and solid-liquid separated to obtain pretreated lignin; wherein the concentration of the organic acid in the starting solution of the heat treatment is 30-70 wt.%; the heat treatment temperature is 40-95°C; and the sodium content in the pretreated lignin is 50-200 ppm; Step (2): Carbonizing the pretreated lignin to obtain hard carbon, and then subjecting the pretreated lignin to a fluidized heat treatment in an organic gas to obtain the lignin-based hard carbon composite active material; The carbonization process includes a first carbonization process and a second carbonization process, wherein the temperature of the first carbonization process is 500-1000°C; the temperature of the second carbonization process is 1000-1800°C; The temperature of the fluidized bed heat treatment process is 750~950℃.

2. The method for preparing the lignin-based hard carbon composite active material according to claim 1, wherein: The lignin raw material is industrial grade lignin.

3. The method for preparing the lignin-based hard carbon composite active material according to claim 2, wherein: The lignin raw material is at least one of organic solvent lignin, sodium lignin sulfonate, sodium lignin sulfate and alkali lignin.

4. The method for preparing a lignin-based hard carbon composite active material according to claim 2, wherein: The sodium content in the lignin raw material is less than 10 wt.%.

5. The method for preparing the lignin-based hard carbon composite active material according to claim 1, wherein: The organic acid is C1~C 10 of carboxylic acid compounds.

6. The method for preparing the lignin-based hard carbon composite active material according to claim 5, wherein: The organic acid is at least one of acetic acid, propionic acid, citric acid and oxalic acid.

7. The method for preparing the lignin-based hard carbon composite active material according to claim 1, wherein: In step (1), the cooling temperature is less than or equal to 25°C.

8. The method for preparing the lignin-based hard carbon composite active material according to claim 7, wherein: In step (1), the cooling temperature is 4-20°C.

9. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The temperature of the first carbonization process is 500~700℃.

10. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The temperature of the first carbonization process is 600~650℃.

11. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The temperature of the second carbonization process is 1300~1450℃.

12. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The temperature of the second carbonization process is 1400~1450℃.

13. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The first carbonization treatment time is 1~15h.

14. The method for preparing a lignin-based hard carbon composite active material according to claim 13, wherein: The first carbonization treatment time is 2~5h.

15. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The second carbonization treatment time is 1~5h.

16. The method for preparing a lignin-based hard carbon composite active material according to claim 15, wherein: The second carbonization treatment time is 2~2.5h.

17. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The organic gas is at least one of C1-C4 alkanes, C2-C4 alkenes, C2-C4 alkynes, and benzene.

18. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The fluidized state heat treatment process is carried out in a fluidized bed.

19. The method for preparing a lignin-based hard carbon composite active material according to claim 18, wherein: The temperature of fluidized bed heat treatment is 750~850℃.

20. The method for preparing a lignin-based hard carbon composite active material according to claim 18, wherein: The temperature of fluidized bed heat treatment is 800~850℃.

21. The method for preparing a lignin-based hard carbon composite active material according to claim 18, wherein: The time of fluidized bed heat treatment is 0.5~5h.

22. The method for preparing a lignin-based hard carbon composite active material according to claim 21, wherein: The time of fluidized bed heat treatment is 1~3h.

23. The method for preparing a lignin-based hard carbon composite active material according to claim 1, wherein: The flow rate of the atmosphere during the fluidized state heat treatment process is greater than or equal to 100 L / min.

24. The method for preparing a lignin-based hard carbon composite active material according to claim 23, wherein: The flow rate of the atmosphere during the fluidized state heat treatment process is 100~1000 L / min.

25. The method for preparing a lignin-based hard carbon composite active material according to claim 23, wherein: The flow rate of the atmosphere during the fluidized state heat treatment process is 150~300 L / min.

26. A lignin-based hard carbon composite active material prepared by the preparation method according to any one of claims 1 to 25.

27. A sodium ion battery negative electrode material, characterized in that A lignin-based hard carbon composite active material prepared by the preparation method according to any one of claims 1 to 25.

28. The sodium ion battery negative electrode material according to claim 27, wherein Also contains a conductive agent and a binder.

29. A sodium ion battery negative electrode, comprising a current collector and a negative electrode material composited on its surface, characterized in that: The negative electrode material is the negative electrode material according to claim 27 or 28.

30. A sodium ion battery, characterized in that: The negative electrode is the negative electrode according to claim 29.

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