Method for preparing negative electrode material, negative electrode material and sodium ion battery

Abstract

This application relates to a method for preparing an anode material, the anode material itself, and a sodium-ion battery. The method for preparing the anode material includes: placing metallic sodium in a first temperature zone of a furnace chamber and placing a porous hard carbon material in a second temperature zone of the furnace chamber; the porous hard carbon material includes micropores, mesopores, and macropores; controlling the temperature of the first temperature zone to be higher than that of the second temperature zone, causing the metallic sodium to form sodium vapor; driven by the temperature gradient between the first and second temperature zones, the sodium vapor migrates to the second temperature zone and enters the pores of the porous hard carbon material; maintaining the first and second temperature zones at their respective set temperatures for a set time, followed by cooling treatment to form nano-sized metallic sodium at least in the micropores, obtaining a pre-sodiumized hard carbon material; placing the pre-sodiumized hard carbon material at a set temperature, introducing a gaseous carbon source, and performing chemical vapor deposition to coat it with carbon, thus obtaining the anode material. This method can improve the initial coulombic efficiency, cycle life, and capacity utilization of the anode material.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for preparing a negative electrode material, the negative electrode material, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have become a research hotspot in the field of large-scale energy storage due to the abundance of sodium resources and their low cost. Hard carbon materials are widely used as anode materials for sodium-ion batteries because of their low crystallinity, high sodium storage capacity, and wide availability of raw materials.

[0003] However, the initial coulombic efficiency of hard carbon materials is generally low (typically below 85%), limiting their practical application in batteries. The main factors contributing to this problem include: 1. Hard carbon materials have a complex pore structure (especially open pores) and numerous surface defects, which cause irreversible decomposition with the catalytic electrolyte during the first charge-discharge process, forming an excessively thick solid electrolyte interface film, thus irreversibly consuming a large amount of sodium ions; 2. Residual oxygen- and hydrogen-containing functional groups and trace amounts of ash (such as metal oxide impurities) in the hard carbon material can induce additional side reactions, further exacerbating the irreversible capacity loss.

[0004] In existing technologies, physical or chemical activation methods are used to regulate the pore structure of hard carbon, or high-temperature graphitization is employed to improve the conductivity of the material, addressing the aforementioned problems. However, both of these approaches have significant limitations: physical or chemical activation often introduces more open-pore structures and active sites while creating pores, potentially negatively impacting the first-cycle efficiency; while high-temperature graphitization is not only energy-intensive but also reduces defect sites in hard carbon that are conducive to sodium storage, hindering the material's capacity utilization. Therefore, balancing the first-cycle efficiency, cycle life, and capacity utilization of hard carbon materials has become a pressing technical challenge. Summary of the Invention

[0005] In view of this, the present application provides a method for preparing a negative electrode material, a negative electrode material, and a sodium-ion battery to solve at least one problem existing in the background art.

[0006] In a first aspect, embodiments of this application provide a method for preparing a negative electrode material, the method comprising the following steps:

[0007] S1: Placing metallic sodium in the first temperature zone of the furnace cavity and placing porous hard carbon material in the second temperature zone of the furnace cavity; the porous hard carbon material includes micropores, mesopores and macropores, the pore diameter of the micropores is less than 2nm, the pore diameter of the mesopores is 2nm~50nm, and the pore diameter of the macropores is greater than 50nm.

[0008] S2: Control the temperature of the first temperature zone to be higher than the temperature of the second temperature zone, so that the metallic sodium forms sodium vapor. Driven by the temperature gradient between the first temperature zone and the second temperature zone, the sodium vapor migrates to the second temperature zone along the direction from the first temperature zone to the second temperature zone and enters the pores of the porous hard carbon material. After the first temperature zone and the second temperature zone are maintained at their respective set temperatures for a set time, a cooling process is performed to form nano-metallic sodium at least in the micropores, thereby obtaining a pre-sodiumized hard carbon material.

[0009] S3: The pre-sodium hard carbon material is placed at a set temperature, and a gaseous carbon source is introduced to perform chemical vapor deposition to carbon-coat the pre-sodium hard carbon material, thereby obtaining the negative electrode material.

[0010] In conjunction with the first aspect of this application, in an optional embodiment, in step S1, the distance between the center of the porous hard carbon material placement area and the center of the sodium metal placement area is 10cm to 15cm.

[0011] In conjunction with the first aspect of this application, in an optional embodiment, in step S2, the temperature of the first temperature zone is 250°C to 300°C, and the temperature of the second temperature zone is 180°C to 220°C; and / or, the set time is 5h to 8h.

[0012] In conjunction with the first aspect of this application, in an optional embodiment, step S2, the cooling process includes:

[0013] The temperature of the first temperature zone is reduced to 200℃~240℃, and the temperature of the second temperature zone is reduced to 150℃~170℃. The first temperature zone and the second temperature zone are maintained at their respective set temperatures for 3h~4h.

[0014] The temperatures of the first and second temperature zones are reduced to 60°C to 80°C.

[0015] In conjunction with the first aspect of this application, in an alternative embodiment, step S3 satisfies at least one of the following features:

[0016] (1) The gaseous carbon source includes at least one of methane, ethane, and propane;

[0017] (2) The set temperature is 600℃~800℃;

[0018] (3) The chemical vapor deposition time is 1h~3h;

[0019] (4) In the negative electrode material, the mass fraction of the coated carbon deposited on the surface of the pre-sodium hard carbon material is 3%~8%.

[0020] In conjunction with the first aspect of this application, in an optional embodiment, the method for preparing the porous hard carbon material includes:

[0021] The hard carbon precursor is pulverized, and then subjected to acid washing, water washing and drying to obtain precursor powder.

[0022] The precursor powder is mixed with a low-temperature pore-forming agent and heated to 30°C~300°C for a first heat treatment. During the first heat treatment, the low-temperature pore-forming agent decomposes to generate gas, which forms macropores in the first heat treatment product.

[0023] The first heat-treated product is mixed with a medium-temperature pore-forming agent and heated to a temperature greater than 300°C and less than or equal to 600°C for a second heat treatment. During the second heat treatment, the medium-temperature pore-forming agent decomposes to generate gas, thereby forming mesopores in the second heat-treated product.

[0024] The second heat-treated product is mixed with a high-temperature pore-forming agent and heated to 700°C~800°C for a third heat treatment. During the third heat treatment, the high-temperature pore-forming agent reacts with the carbon layer in the second heat-treated product through a chemical etching reaction, thereby forming micropores in the third heat-treated product.

[0025] The third heat treatment product is subjected to acid washing, water washing and drying to obtain the porous hard carbon material.

[0026] In conjunction with the first aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0027] (1) The hard carbon precursor includes biomass precursor and / or resin-based precursor;

[0028] (2) The low-temperature pore-forming agent includes at least one of ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, and polyethylene glycol;

[0029] (3) The medium-temperature pore-forming agent includes at least one of polystyrene microspheres, zinc chloride, ammonium dihydrogen phosphate, melamine, and polyvinylpyrrolidone;

[0030] (4) The high-temperature pore-forming agent includes potassium hydroxide and / or sodium hydroxide.

[0031] In conjunction with the first aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0032] (1) The mass of the low-temperature pore-forming agent is 5% to 10% of the mass of the precursor powder;

[0033] (2) The mass of the medium-temperature pore-forming agent is 3% to 8% of the mass of the first heat-treated product;

[0034] (3) The mass of the high-temperature pore-forming agent is 0.5% to 2% of the mass of the second heat-treated product;

[0035] (4) In the porous hard carbon material, based on the total pore volume, the micropore volume accounts for 60%~75%, the mesopore volume accounts for 20%~30%, and the macropore volume accounts for 5%~10%.

[0036] Secondly, embodiments of this application provide a negative electrode material, which is prepared using the negative electrode material preparation method described in any one of the first aspects.

[0037] Thirdly, embodiments of this application provide a sodium-ion battery, including a negative electrode sheet, wherein the negative electrode sheet includes the negative electrode material described in the second aspect.

[0038] Compared with the prior art, the embodiments of this application have the following beneficial effects:

[0039] In the method for preparing the negative electrode material provided in this application embodiment, by placing metallic sodium and porous hard carbon material in the first and second temperature zones of the furnace cavity respectively, and controlling the temperature of the first temperature zone to be higher than that of the second temperature zone, metallic sodium can form sodium vapor. Driven by the temperature gradient, the sodium vapor migrates from the first temperature zone to the second temperature zone and enters the pores of the porous hard carbon material. Due to the nano-confinement effect, capillary condensation, and adsorption of high-energy sites of micropore defects, sodium vapor preferentially deposits sodium in the form of atomic clusters or nanoclusters in the micropores, forming nano-quasi-metallic clusters, avoiding the aggregation of free sodium metal, and constructing stable sodium storage sites. At the same time, sodium preferentially occupies the highly active defect sites on the carbon surface, which can reduce the continuous decomposition of the electrolyte and promote the formation of a thin, dense, and stable SEI film. Then, carbon coating is performed by chemical vapor deposition, which not only prevents the metallic sodium deposited in the pores of the porous hard carbon material from being oxidized, but also further suppresses interfacial side reactions. In this embodiment, by pre-sodiumizing the porous hard carbon material, stable sodium storage sites are constructed within the micropores, the mesopores provide channels for sodium ion transport, ensuring that sodium ions are rapidly and uniformly transported to the deposition sites in the micropores, reducing local current density, and the macropores provide space for electrolyte storage. This also alleviates the volume expansion stress of the electrode material during charging and discharging, inhibits the pulverization and cracking of the electrode material, and maintains the integrity of the electrode structure. As a result, the initial coulombic efficiency, cycle life, and capacity utilization of the hard carbon anode material can be improved.

[0040] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0041] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0042] Figure 1 This is a schematic flowchart illustrating a method for preparing a negative electrode material according to an embodiment of this application. Detailed Implementation

[0043] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0044] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. The terms “comprising” and / or “including,” when used in this specification, identify the presence of said features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0046] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0047] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0048] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0049] This application provides a method for preparing a negative electrode material. Please refer to... Figure 1 The method for preparing the negative electrode material provided in this application includes the following steps:

[0050] S1: Place metallic sodium in the first temperature zone of the furnace cavity and place porous hard carbon material in the second temperature zone of the furnace cavity; the porous hard carbon material includes micropores, mesopores and macropores, the pore diameter of micropores is less than 2nm, the pore diameter of mesopores is 2nm~50nm, and the pore diameter of macropores is greater than 50nm.

[0051] S2: Control the temperature of the first temperature zone to be higher than that of the second temperature zone, so that metallic sodium forms sodium vapor. Driven by the temperature gradient between the first and second temperature zones, the sodium vapor migrates to the second temperature zone along the direction from the first temperature zone to the second temperature zone and enters the pores of the porous hard carbon material. After the first and second temperature zones are maintained at their respective set temperatures for a set time, a cooling process is performed to form nano-sized metallic sodium at least in the micropores, thus obtaining a pre-sodiumized hard carbon material.

[0052] S3: Place the pre-sodium hard carbon material at a set temperature, introduce a gaseous carbon source, and perform chemical vapor deposition to carbon-coat the pre-sodium hard carbon material to obtain the negative electrode material.

[0053] In this embodiment, by placing metallic sodium and porous hard carbon material in the first and second temperature zones of the furnace cavity, respectively, and controlling the temperature of the first temperature zone to be higher than that of the second temperature zone, metallic sodium can form sodium vapor. Driven by the temperature gradient, the sodium vapor migrates from the first temperature zone to the second temperature zone and enters the pores of the porous hard carbon material. Due to the nano-confinement effect, capillary condensation, and adsorption of high-energy sites on micropore defects, sodium vapor preferentially deposits sodium in the form of atomic clusters or nanoclusters within the micropores, forming nano-quasi-metallic clusters (nano-metallic sodium), thus preventing the aggregation of free sodium metal and constructing stable sodium storage sites. At the same time, sodium preferentially occupies the highly active defect sites on the carbon surface, which can reduce the continuous decomposition of the electrolyte and promote the formation of a thin, dense, and stable SEI film (rich in NaF / Na2O). Then, carbon coating is performed by chemical vapor deposition, which not only prevents the metallic sodium deposited in the pores of the porous hard carbon material from being oxidized, but also further suppresses interfacial side reactions. In this embodiment, by pre-sodiumizing the porous hard carbon material, stable sodium storage sites are constructed within the micropores, the mesopores provide channels for sodium ion transport, ensuring that sodium ions are rapidly and uniformly transported to the deposition sites in the micropores, reducing local current density, and the macropores provide space for electrolyte storage. This also alleviates the volume expansion stress of the electrode material during charging and discharging, inhibits the pulverization and cracking of the electrode material, and maintains the integrity of the electrode structure. As a result, the initial coulombic efficiency, cycle life, and capacity utilization of the hard carbon anode material can be improved.

[0054] In step S1, metallic sodium is placed in the first temperature zone of the furnace cavity, and porous hard carbon material is placed in the second temperature zone of the furnace cavity.

[0055] Here, sodium metal can be a block of sodium metal. In the actual preparation process, small pieces of sodium metal (fresh cross-section) can be cut, immersed in anhydrous hexane to remove the oxide layer on the surface, and then placed in the ark (also known as the upstream ark) in the first temperature zone.

[0056] In the actual preparation process, before placing the porous hard carbon material in the second temperature zone of the furnace, it can be placed in a vacuum environment and dehydrated at 100℃~120℃ for 3h~6h. This removes adsorbed water from the pores of the porous hard carbon material, preventing an explosive reaction between the sodium metal and water during the subsequent deposition of metallic sodium. The dehydrated porous hard carbon material can then be spread flat in the ark (also known as the downstream ark) of the second temperature zone.

[0057] In some embodiments, the distance between the center of the porous hard carbon material placement area and the center of the metallic sodium placement area can be 10cm to 15cm, for example, 10cm, 11cm, 12cm, 13cm, 14cm, 15cm, or any value between any two of the above ranges. This allows for easier control of the temperatures of the first and second temperature zones while shortening the migration path of sodium vapor in subsequent steps.

[0058] In step S2, the temperature of the first temperature zone can be 250℃~300℃, for example, 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, or any value between any two of the above ranges. The temperature of the second temperature zone can be 180℃~220℃, for example, 180℃, 190℃, 200℃, 210℃, 220℃, or any value between any two of the above ranges. This allows metallic sodium to be stably converted into sodium vapor, generating a high saturated vapor pressure, while also facilitating the stable migration of sodium vapor to the porous hard carbon material location under the temperature gradient between the two temperature zones.

[0059] In step S2, the set time can be 5h to 8h, for example, 5h, 6h, 7h, 8h, or any value between any two of the above ranges. This can better ensure that sodium vapor fully enters the pores of the porous hard carbon material, especially the micropores.

[0060] In the actual preparation process, the furnace cavity of the dual-temperature zone tube furnace can be heated to 110℃~130℃ (heating rate can be 3℃ / min~5℃ / min), and argon gas can be continuously introduced for 1h~3h. Vacuuming is then performed, and this process is repeated three times to remove residual oxygen and water from the dual-temperature zone tube furnace. After placing metallic sodium and porous hard carbon material, before sealing, argon gas at 0.1kPa~0.2kPa can be introduced for pressure testing to check the equipment's sealing performance. If the pressure drop is <5% after 30min~50min, the equipment is considered to be in good condition and subsequent processing can proceed. If the pressure drop is ≥5% after 30min~50min, the equipment is considered to be in poor condition and requires inspection, repair, or replacement.

[0061] In step S2, the cooling process includes the following steps:

[0062] S21: Reduce the temperature of the first temperature zone to 200℃~240℃ and the temperature of the second temperature zone to 150℃~170℃. Maintain the first and second temperature zones at their respective set temperatures for 3h~4h.

[0063] S22: Reduce the temperature of the first and second temperature zones to 60℃~80℃.

[0064] In step S21, the temperatures of the first and second temperature zones are appropriately reduced to medium temperatures to lower the sodium vapor pressure. This allows sodium vapor to slowly sublimate and uniformly fill the micropores and mesopores within the hierarchical channels of the porous hard carbon material. Simultaneously, the medium temperature conditions facilitate solid-phase micro-diffusion of sodium atoms, balancing the sodium concentration in the surface and inner layers of the material. This promotes the confined growth of sodium in the form of nanoclusters within the micropores, completing the initial shaping of the pre-sodium structure. In step S22, the temperatures of both the first and second temperature zones are further reduced to 60℃~80℃. This temperature is below the melting point of metallic sodium (97.8℃), allowing the sodium clusters within the micropores to completely solidify and fix, resulting in a near-zero sodium vapor pressure and no volatilization loss, thus locking the metallic sodium within the porous structure. If the temperature is directly reduced to room temperature, sodium vapor may directly form a sodium film on the outer surface of the material instead of entering the pores, leading to sodium precipitation on the material surface. In this application, sodium vapor preferentially deposits as atomic clusters or nanoclusters within the micropores due to nanoconfinement effects, capillary condensation, and adsorption at high-energy sites in micropore defects. This forms nano-quasi-metallic clusters. The sodium clusters are spatially confined by the micropores, limiting their aggregation and growth, resulting in uniform size and the construction of stable "platform regions" for sodium storage. Unlike free sodium, which diffuses, migrates, and accumulates freely, thus agglomerating into large micron-sized metallic particles and forming "dead sodium," the sodium vapor remains largely unfilled. Mesopores are primarily filled with sodium only in a thin layer on the pore walls and in small amounts locally; macropores are essentially unfilled with sodium.

[0065] In step S3, the pre-sodium hard carbon material is placed at a set temperature, and a gaseous carbon source is introduced to perform chemical vapor deposition to carbon-coat the pre-sodium hard carbon material, thereby obtaining the negative electrode material.

[0066] In some embodiments, the gaseous carbon source may include at least one of methane, ethane, and propane. Exemplarily, the gaseous carbon source may be at least one of methane, ethane, and propane.

[0067] In some embodiments, the set temperature can be 600℃~800℃, for example, it can be 600℃, 650℃, 700℃, 750℃, 800℃ or any value between any two of the above ranges.

[0068] In some embodiments, the chemical vapor deposition time can be 1h to 3h, for example, 1h, 1.5h, 2h, 2.5h, 3h or any value between any two of the above ranges.

[0069] In some embodiments, the mass fraction of coated carbon deposited on the surface of the pre-sodium hard carbon material in the obtained negative electrode material can be 3% to 8%, for example, 3%, 4%, 5%, 6%, 7%, 8% or any value between any two of the above ranges.

[0070] In the actual preparation process, the equipment used in step S2 can be used for chemical vapor deposition. For example, the temperature of the first temperature zone of the furnace can be maintained at 60℃~80℃ to suppress the re-evaporation of metallic sodium; the second temperature zone can be heated to 600℃~800℃ at a rate of 3℃ / min~5℃ / min; a high-purity inert gas (such as argon, nitrogen, helium, etc.) is used as the carrier gas to carry the gaseous carbon source into the furnace for chemical vapor deposition. By adjusting the concentration of the gaseous carbon source and the deposition time, the amorphous carbon coating on the surface can be controlled to be 3wt%~8wt%. After the chemical vapor deposition is completed, the gaseous carbon source is stopped, and pure inert gas is continued to purge away the residual organic vapor; then, under the protection of inert gas, the furnace is naturally cooled to room temperature to obtain a well-coated pre-sodiumized gradient porous hard carbon material (anode material).

[0071] In some embodiments, the preparation of the porous hard carbon material described in any of the foregoing embodiments includes the following steps:

[0072] S11: The hard carbon precursor is pulverized, and then subjected to acid washing, water washing and drying to obtain precursor powder.

[0073] The hard carbon precursors described herein may include biomass precursors and / or resin-based precursors. For example, the hard carbon precursor may be a biomass precursor and / or a resin-based precursor.

[0074] In some specific examples, the hard carbon precursor is a biomass precursor. In the actual preparation process, the biomass precursor (such as peanut shells, coconut shells, walnut shells, etc.) can be crushed, acid-washed (the acid washing solution can be hydrochloric acid, sulfuric acid, oxalic acid, acetic acid, etc.), washed with water until neutral, and then dried for later use.

[0075] S12: Mix the precursor powder with a low-temperature pore-forming agent and heat it to 30°C~300°C (for example, it can be 30°C, 50°C, 100°C, 150°C, 200°C, 250°C, 300°C or any value between any two of the above ranges) to perform a first heat treatment. During the first heat treatment, the low-temperature pore-forming agent decomposes to generate gas, which forms macropores in the first heat treatment product.

[0076] The low-temperature pore-forming agent here may include at least one of ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, and polyethylene glycol. For example, the low-temperature pore-forming agent may be at least one of ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, and polyethylene glycol.

[0077] Ammonium bicarbonate (NH4HCO3) decomposes at temperatures between 30°C and 60°C. Ammonium oxalate ((NH4)2C2O4) begins to lose its water of crystallization at approximately 95°C and completely decomposes at approximately 230°C, producing oxalic acid (H2C2O4), ammonia (NH3), and water (H2O). Polymethyl methacrylate (PMMA) begins to lose its water of crystallization at approximately 95°C and begins to break chemical bonds at around 270°C. Polyethylene glycol (PEG-2000) begins to thermally decompose at approximately 200°C, with the main decomposition temperature around 270°C, during which chemical bonds break.

[0078] During the first heat treatment, the low-temperature pore-forming agent decomposes to generate gas, which expands and forms macropores inside the material. After holding at this temperature for 0.5 to 1 hour, it is cooled to room temperature. In this stage, the precursor powder still maintains a complete and tough cross-linked solid structure of biomass fibers. Only moisture and oxygen-containing functional groups on the surface are removed. The macromolecular chains and interparticle fiber overlaps and hydrogen bond connections are intact. The skeleton has good flexibility and overall structural strength. The temperature generated is mild, and macropores are only created inside the particles without damaging the connections between particles.

[0079] S13: The first heat treatment product is mixed with a medium-temperature pore-forming agent and heated to a temperature greater than 300°C and less than or equal to 600°C (for example, it can be 350°C, 400°C, 450°C, 500°C, 550°C, 600°C or any value between any two of the above ranges) to perform a second heat treatment. During the second heat treatment, the medium-temperature pore-forming agent decomposes to generate gas, thereby forming mesopores in the second heat treatment product.

[0080] The intermediate-temperature pore-forming agent here may include at least one of polystyrene microspheres (particle size can be 50nm~500nm), zinc chloride, ammonium dihydrogen phosphate, melamine, and polyvinylpyrrolidone. For example, the intermediate-temperature pore-forming agent may be at least one of polystyrene microspheres, zinc chloride, ammonium dihydrogen phosphate, melamine, and polyvinylpyrrolidone.

[0081] The initial decomposition temperature of polystyrene microspheres (PS) is about 300℃, and the decomposition is mainly concentrated between 400℃ and 500℃. The decomposition mainly produces styrene monomer, benzene, toluene and other volatile organic compounds, with no solid residue. It is a "sacrificial template" type pore-forming agent.

[0082] Zinc chloride (ZnCl2) decomposes at temperatures between 400℃ and 500℃. As a Lewis acid catalyst, it coordinates with phenolic hydroxyl groups and methylene bridges in hard carbon precursors, promoting the formation of cross-linked networks. During pyrolysis, ZnCl2 melts and encapsulates the carbon precursor, forming a "soft template" that restricts the disordered shrinkage of carbon chains. As the temperature increases, ZnCl2 volatilizes after 500℃, leaving behind a highly interconnected, uniformly thick mesoporous structure after the template is removed.

[0083] The decomposition temperature of ammonium dihydrogen phosphate (NH4H2PO4) is 400℃~500℃.

[0084] Melamine (C3H6N6) has a main decomposition temperature of 400℃~500℃. The gas slowly accumulates and expands inside the carbon precursor. Due to the moderate release rate, violent gas explosions can be avoided, allowing the bubbles to grow stably to the mesopore size level.

[0085] The decomposition temperature of polyvinylpyrrolidone (PVP) is 450℃~500℃.

[0086] During the second heat treatment process, the medium-temperature pore-forming agent thermally decomposes to create pores during material carbonization. After the gas volatilizes and overflows, it forms through-pores in the carbon skeleton. In this stage, the flexible biomass material is transformed into a semi-carbonized rigid solid, which basically loses its plasticity and no longer collapses. The pore walls of the macropores formed at low temperature in the previous stage are completely hardened and shaped, and the interparticle connections are carbonized into rigid carbon bonds, forming a relatively stable structure.

[0087] S14: Mix the second heat treatment product with a high-temperature pore-forming agent and heat it to 700℃~800℃ for a third heat treatment. During the third heat treatment, the high-temperature pore-forming agent reacts with the carbon layer in the second heat treatment product through a chemical etching reaction, thereby forming micropores in the third heat treatment product.

[0088] The high-temperature pore-forming agent here may include potassium hydroxide and / or sodium hydroxide. For example, the high-temperature pore-forming agent may be potassium hydroxide and / or sodium hydroxide.

[0089] During the third heat treatment process, the carbon layer undergoes high aromatization, and graphite microcrystals are stacked to form a stable hard carbon structure, resulting in a high-rigidity hard carbon framework. The carbon framework is highly dense, and the overall structural morphology of the material, including its macropores and mesopores, is essentially locked and no longer altered. The high-temperature pore-forming agent selectively chemically etches micropores only within the carbon layer, performing only internal micro-etching and having minimal impact on other channels.

[0090] Specifically, taking potassium hydroxide as the high-temperature pore-forming agent as an example, potassium hydroxide melts and penetrates into the gaps in the carbon layer, reacting with carbon as follows: 6KOH + 2C → 2K + 3H₂ + 2K₂CO₃. Potassium vapor (purple fumes) wedges into the carbon layers, expanding the nanoscale space to form micropore cores. K₂CO₃ decomposes to create secondary pores, with the reactions: K₂CO₃ → K₂O + CO₂, CO₂ + C → 2CO, thereby expanding the pore size of the micropores to the ideal sodium storage size of 1nm~2nm.

[0091] The third heat treatment temperature is the dominant temperature range for the chemical reaction between potassium hydroxide and carbon, rather than a simple thermal decomposition temperature. The pore-forming effect of potassium hydroxide is essentially a multi-step chemical reaction-driven selective etching of carbon atoms, dominated by the directional etching of carbon atoms by KOH. The reaction selectively removes disordered carbon while preserving ordered graphite microcrystals, forming a microporous network with a high specific surface area. Potassium vapor has extremely high permeability, allowing it to insert into the carbon interlayers, lowering the graphitization energy barrier, promoting microcrystal rearrangement, and enhancing the stability of the pore structure.

[0092] The reaction temperature and pore-forming mechanism of sodium hydroxide with carbon are similar to those of potassium hydroxide, but the activity of sodium vapor (boiling point 883℃) is lower than that of potassium vapor (762℃), resulting in relatively lower etching efficiency.

[0093] In this embodiment, the process employs a temperature-zone gradient pore-forming method, with the pore-forming agent added in stages. At low temperatures, macropores are formed first, and the carbon framework pore walls are pre-crosslinked and solidified. At medium and high temperatures, mesopores and micropores are formed sequentially. Subsequent heat treatment does not soften or melt the solidified pore walls. Simultaneously, the pore-forming agent is added in relatively low amounts, opening through-channels only in locally weak areas of the pore walls without damaging the carbon framework. Macropores, mesopores, and micropores are formed through three different mechanisms: gas generation, sacrificial template, and chemical etching, respectively. Each has a thermodynamically stable intrinsic size range, with pore size zones locked. Pore connectivity is achieved only through the formation of interconnected microchannels on the pore walls, representing "windowed connectivity," rather than pore wall ablation or pore merging.

[0094] S15: The product of the third heat treatment is acid-washed, water-washed and dried to obtain porous hard carbon material.

[0095] In the actual preparation process, the third heat treatment product (carbonization product) can be acid-washed first, then washed with water until neutral, and then dried at 100℃~120℃ for 4h~6h to obtain gradient porous hard carbon material.

[0096] In some embodiments, the mass of the low-temperature pore-forming agent can be 5% to 10% of the precursor powder mass, for example, 5%, 6%, 7%, 8%, 9%, 10%, or any value between any two of the above ranges. This facilitates controlling the proportion of macropores in the porous hard carbon material within a suitable range, ensuring the stability of the overall structure.

[0097] In some embodiments, the mass of the intermediate-temperature pore-forming agent can be 3% to 8% of the mass of the first heat-treated product, for example, 3%, 4%, 5%, 6%, 7%, 8%, or any value between any two of the above ranges. This facilitates controlling the proportion of mesopores in the porous hard carbon material within a suitable range.

[0098] In some embodiments, the mass of the high-temperature pore-forming agent is 0.5% to 2% of the mass of the second heat-treated product, for example, it can be 0.5%, 1%, 1.5%, 2%, or any value between any two of the above ranges. This facilitates controlling the proportion of micropores in the porous hard carbon material within a suitable range.

[0099] In some embodiments, in porous hard carbon materials, based on the total pore volume, the micropore volume accounts for 60% to 75%, the mesopore volume accounts for 20% to 30%, and the macropore volume accounts for 5% to 10%.

[0100] This application also provides a negative electrode material, which is prepared by the preparation method of the negative electrode material described in any of the foregoing embodiments.

[0101] This application also provides a sodium-ion battery, including a negative electrode sheet, wherein the negative electrode sheet includes the negative electrode material described in the foregoing embodiments.

[0102] It should be understood that the beneficial effects of the negative electrode material described in any of the foregoing embodiments apply to this battery. This application does not limit the type of sodium-ion battery. For example, it can be a cylindrical battery, an aluminum-cased battery, a pouch battery, etc.

[0103] In some embodiments, the negative electrode sheet may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer may include the negative electrode material described in the foregoing embodiments.

[0104] In some embodiments, the sodium-ion battery further includes a positive electrode and a separator, with the separator disposed between the positive electrode and the negative electrode.

[0105] During battery charging and discharging, sodium ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0106] In some embodiments, the positive electrode sheet may include a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive active material layer may include the positive electrode material described in the foregoing embodiments.

[0107] The positive electrode active material layer includes a positive electrode active material. This application does not limit the type of positive electrode active material, and it can be selected according to needs. For example, the positive electrode active material can be one or more combinations of layered positive electrode active materials, layered oxides, Prussian blue compounds, polyanionic compounds, etc., including but not limited to layered positive electrode active materials, layered oxides, Prussian blue compounds, polyanionic compounds, etc.

[0108] The separator can be any type of separator well known to those skilled in the art suitable for sodium-ion batteries. The separator material can be, for example, one or a combination of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0109] The methods for preparing sodium-ion batteries are known to those skilled in the art and will not be described in detail here. For example, the positive electrode, separator, and negative electrode can all be layers, which can be cut to the target size and stacked sequentially to form a cell, or further wound to the target size to form a cell, and can be further combined with an electrolyte to form a sodium-ion battery.

[0110] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0111] Example 1

[0112] The preparation of the negative electrode material in this embodiment includes the following steps:

[0113] Step S101: Crush the biomass raw material coconut shell (hard carbon precursor), wash it with 3M hydrochloric acid, wash it with water until neutral, and then dry it at 100℃ for 3 hours to obtain precursor powder.

[0114] Step S102: Weigh 50g of the precursor powder obtained in step S101 and mix it with 3.5g of the low-temperature pore-forming agent ammonium bicarbonate. Heat the mixture to 50°C. Ammonium bicarbonate decomposes in this temperature range to produce gases (NH3, CO2, H2O), which expand and form macropores inside the material. After keeping the mixture at this temperature for 1 hour, cool it to room temperature.

[0115] Step S103: Add 2.5g of polystyrene microspheres, a medium-temperature pore-forming agent, to the product obtained in step S102. After uniform mixing, heat to 400℃ to cause chain scission: C6H5CH=CH2 → aromatic fragments + gaseous hydrocarbons (styrene, toluene, etc.). After the gas volatilizes and overflows, it forms a through-pore in the carbon skeleton. Cool to room temperature.

[0116] Step S104: Add 0.5g of high-temperature pore-forming agent KOH to the product obtained in step S103, heat to 800℃, and react for 5 hours. At this point, the carbon skeleton is highly dense, and the high-temperature pore-forming agent forms micropores inside the carbon layer through chemical etching.

[0117] Step S105: The product from step S104 is acid-washed and water-washed until neutral, and then dried at 100°C for 5 hours to obtain gradient porous hard carbon material.

[0118] Step S106: Cut a small piece of metallic sodium (fresh cross-section), immerse it in anhydrous hexane to remove the surface oxide layer; place the metallic sodium in the upstream boat (first temperature zone positioning point) of the dual-temperature zone tube furnace; place the porous hard carbon material obtained in step S105 under vacuum conditions, dehydrate it at 100°C for 4 hours, and then spread it flat on the downstream boat (second temperature zone positioning point, the center distance between the upstream and downstream boats is 12cm) of the dual-temperature zone tube furnace.

[0119] Step S107: The furnace chamber of the dual-temperature zone tubular furnace is heated from room temperature to 120℃ (heating rate 5℃ / min), and argon gas is continuously introduced for 2 hours. Vacuuming is then performed, and this process is repeated three times to remove residual oxygen and water from the tubular furnace. Before sealing, 0.1 kPa of argon gas is introduced for pressure testing (pressure drop < 5% for 30 minutes) to confirm the equipment's sealing performance is qualified. The temperature of the high-temperature zone (first temperature zone) is raised to 250℃, causing the sodium block to generate a high saturated vapor pressure. The temperature of the low-temperature zone (second temperature zone) is raised to 180℃, and sodium vapor flows directionally along the temperature gradient to the second temperature zone and enters the pores of the porous hard carbon material. This temperature is maintained for 6 hours. Subsequently, the temperature of the first temperature zone is lowered to 200℃, and the temperature of the second temperature zone is lowered to 150℃, maintaining this temperature for 3 hours. Finally, the temperatures of the first and second temperature zones are simultaneously lowered to 60℃, locking metallic sodium in the porous structure to obtain pre-sodiumized hard carbon material.

[0120] Step S108: Maintain the temperature of the first temperature zone of the furnace chamber at 60℃~80℃ to suppress the re-evaporation of metallic sodium; raise the temperature of the second temperature zone to 700℃ at 5℃ / min; use high-purity argon as the carrier gas to carry the gaseous carbon source methane into the furnace chamber for chemical vapor deposition for 2 hours; after the chemical vapor deposition is completed, stop the introduction of the gaseous carbon source and continue to purge with pure argon to remove residual organic vapors; then cool down naturally to room temperature with the furnace under argon protection to obtain the negative electrode material (amorphous carbon mass ratio of 4%).

[0121] Example 2

[0122] The preparation method of the negative electrode material in this embodiment is basically the same as that in Example 1, except that:

[0123] (1) Replace the low-temperature pore-forming agent in step S102 with ammonium oxalate; replace the medium-temperature pore-forming agent in step S103 with zinc chloride; replace the high-temperature pore-forming agent in step S104 with NaOH;

[0124] (2) In step S107, the temperature of the high temperature zone is raised to 300°C and the temperature of the low temperature zone is raised to 220°C.

[0125] Example 3

[0126] The preparation method of the negative electrode material in this embodiment is basically the same as that in Example 1, except that:

[0127] (1) Replace the gaseous carbon source in step S108 with ethane;

[0128] (2) In step S107, the temperature of the high temperature zone is raised to 270°C and the temperature of the low temperature zone is raised to 200°C.

[0129] Comparative Example 1

[0130] The preparation method of the negative electrode material in this comparative example is basically the same as that in Example 1, except that:

[0131] Steps S102-S104 are omitted. Correspondingly, in step S105, the precursor powder from step S101 is placed in a tube furnace and heated to 800°C (heating rate 5°C / min) for 5 hours. The resulting product is then acid-washed, water-washed until neutral, and dried at 100°C for 5 hours to obtain the gradient porous hard carbon material. The difference from Example 1 is that there is no gradient pore formation.

[0132] Comparative Example 2

[0133] The preparation method of the negative electrode material in this comparative example is basically the same as that in Example 1, except that:

[0134] Steps S106-S107 are omitted. In step S108, the porous hard carbon material obtained in step S105 is directly placed in a tube furnace and heated to 700°C at a rate of 5°C / min. High-purity argon is used as the carrier gas to carry the gaseous carbon source methane into the furnace cavity for chemical vapor deposition (CVD) for 2 hours. After CVD, the gaseous carbon source is stopped, and pure argon is continuously purged to remove residual organic vapors. Subsequently, the furnace is allowed to cool naturally to room temperature under argon protection to obtain the negative electrode material. The difference from Example 1 is that no pre-sodium treatment is performed.

[0135] Using the negative electrode materials obtained in the above embodiments and comparative examples, negative electrode sheets were prepared and coin cells were assembled for electrochemical performance testing.

[0136] The methods for preparing batteries include:

[0137] Preparation of negative electrode sheet: The negative electrode material, conductive agent (single-walled carbon nanotubes and conductive carbon black in a mass ratio of 1:19), and binder (sodium carboxymethyl cellulose, polypropylene and styrene-butadiene rubber in a mass ratio of 1.5:0.5:1) prepared in the above embodiments and comparative examples are mixed in a mass ratio of 96:2.0:2.0, deionized water is added and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry is coated on the negative electrode current collector copper foil, and after drying, cold pressing, cutting and slitting, a negative electrode sheet is obtained.

[0138] Preparation of the positive electrode sheet: The positive electrode active material sodium nickel iron manganese oxide, the conductive agent (single-walled carbon nanotubes and conductive carbon black in a mass ratio of 1:2), and the binder (polyvinylidene fluoride and polyacrylonitrile in a mass ratio of 5:1) are mixed in a mass ratio of 96.5:1.5:2.0. N-methylpyrrolidone is added and stirred evenly to prepare a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector aluminum foil, and then dried, rolled, cut, and slit to obtain the positive electrode sheet.

[0139] Preparation of sodium-ion batteries: The above-mentioned positive electrode sheet, polyethylene separator coated with alumina and negative electrode sheet are wound into a cell, the cell is placed in a battery case, baked and then injected with electrolyte. After encapsulation, formation and capacity testing, sodium-ion batteries are made.

[0140] The prepared sodium-ion battery was tested as follows:

[0141] (1) Battery capacity test: At 25℃, the voltage range is 2.0V~4.0V. Charge to 4.0V at 1C, then charge at 4.0V constant voltage until the current decreases to 0.05C, and discharge to 2.0V at 0.1C. Record the discharge capacity at 25℃.

[0142] (2) First Coulombic Efficiency Test of Battery: At 25℃, with a voltage range of 2.0V~4.0V, charge at 1C to 4.0V, then charge at a constant voltage of 4.0V until the current decreases to 0.05C, and discharge at 0.1C to 2.0V. Record the discharge capacity at 25℃. First Coulombic Efficiency = (Discharge Capacity / Charge Capacity) × 100%;

[0143] (3) Battery cycle performance test: At 25℃, the battery is charged from the initial voltage of 2.0V at a constant current of 1C to the cutoff voltage of 2.0V, and then charged at a constant voltage of 4.0V until the current decreases to 0.05C; after standing for 5 minutes, the battery is discharged at a constant current of 0.1C to 2.0V, and the discharge capacity at this time is recorded, which is the discharge capacity C0 of the first cycle. Cycle test is performed according to this process. The capacity at the 100th cycle is recorded as C100, and the capacity at the 500th cycle is recorded as C500. The capacity retention rate after 100 cycles is obtained by using the formula (C100 / C0)×100%, and the capacity retention rate after 500 cycles is obtained by using the formula (C500 / C0)×100%.

[0144] The test results are shown in Table 1.

[0145] Table 1

[0146] As shown in Table 1, the capacities of the batteries corresponding to Examples 1 to 3 range from 338 mAh / g to 340 mAh / g, the initial coulombic efficiency ranges from 87.9% to 89.1%, the capacity retention rate after 100 cycles ranges from 87.3% to 88.7%, and the capacity retention rate after 500 cycles ranges from 81.6% to 84.8%. The performance differences between the examples are small, and all electrochemical performance data show significant improvements compared to Comparative Example 1 and Comparative Example 2. This indicates that in this application, by pre-sodiumizing the porous hard carbon material, stable sodium storage sites are constructed within the micropores, the mesopores provide channels for sodium ion transport, ensuring rapid and uniform transport of sodium ions to the deposition sites in the micropores, reducing local current density, and the macropores provide space for electrolyte storage. Furthermore, this process alleviates the volume expansion stress of the electrode material during charging and discharging, inhibits electrode material pulverization and cracking, and maintains the integrity of the electrode structure, thereby improving the initial coulombic efficiency, cycle life, and capacity performance of the hard carbon anode material.

[0147] It should be noted that the embodiments of the preparation method of the negative electrode material, the embodiments of the negative electrode material, and the embodiments of the sodium-ion battery provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0148] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A method for preparing a negative electrode material, characterized in that, The method includes the following steps: S1: Placing metallic sodium in the first temperature zone of the furnace cavity and placing porous hard carbon material in the second temperature zone of the furnace cavity; the porous hard carbon material includes micropores, mesopores and macropores, the pore diameter of the micropores is less than 2nm, the pore diameter of the mesopores is 2nm~50nm, and the pore diameter of the macropores is greater than 50nm. S2: Control the temperature of the first temperature zone to be higher than the temperature of the second temperature zone, so that the metallic sodium forms sodium vapor. Driven by the temperature gradient between the first temperature zone and the second temperature zone, the sodium vapor migrates to the second temperature zone along the direction from the first temperature zone to the second temperature zone and enters the pores of the porous hard carbon material. After the first temperature zone and the second temperature zone are maintained at their respective set temperatures for a set time, a cooling process is performed to form nano-metallic sodium at least in the micropores, thereby obtaining a pre-sodiumized hard carbon material. S3: The pre-sodium hard carbon material is placed at a set temperature, and a gaseous carbon source is introduced to perform chemical vapor deposition to carbon-coat the pre-sodium hard carbon material, thereby obtaining the negative electrode material.

2. The method for preparing the negative electrode material according to claim 1, characterized in that, In step S1, the distance between the center of the porous hard carbon material placement area and the center of the sodium metal placement area is 10cm~15cm.

3. The method for preparing the negative electrode material according to claim 1, characterized in that, In step S2, the temperature of the first temperature zone is 250℃~300℃, and the temperature of the second temperature zone is 180℃~220℃; and / or, the set time is 5h~8h.

4. The method for preparing the negative electrode material according to claim 1, characterized in that, In step S2, the cooling process includes: The temperature of the first temperature zone is reduced to 200℃~240℃, and the temperature of the second temperature zone is reduced to 150℃~170℃. The first temperature zone and the second temperature zone are maintained at their respective set temperatures for 3h~4h. The temperatures of the first and second temperature zones are reduced to 60°C to 80°C.

5. The method for preparing the negative electrode material according to claim 1, characterized in that, Step S3 satisfies at least one of the following characteristics: (1) The gaseous carbon source includes at least one of methane, ethane, and propane; (2) The set temperature is 600℃~800℃; (3) The chemical vapor deposition time is 1h~3h; (4) In the negative electrode material, the mass fraction of the coated carbon deposited on the surface of the pre-sodium hard carbon material is 3%~8%.

6. The method for preparing the negative electrode material according to claim 1, characterized in that, The method for preparing the porous hard carbon material includes: The hard carbon precursor is pulverized, and then subjected to acid washing, water washing and drying to obtain precursor powder. The precursor powder is mixed with a low-temperature pore-forming agent and heated to 30°C~300°C for a first heat treatment. During the first heat treatment, the low-temperature pore-forming agent decomposes to generate gas, which forms macropores in the first heat treatment product. The first heat-treated product is mixed with a medium-temperature pore-forming agent and heated to a temperature greater than 300°C and less than or equal to 600°C for a second heat treatment. During the second heat treatment, the medium-temperature pore-forming agent decomposes to generate gas, thereby forming mesopores in the second heat-treated product. The second heat-treated product is mixed with a high-temperature pore-forming agent and heated to 700°C~800°C for a third heat treatment. During the third heat treatment, the high-temperature pore-forming agent reacts with the carbon layer in the second heat-treated product through a chemical etching reaction, thereby forming micropores in the third heat-treated product. The third heat treatment product is subjected to acid washing, water washing and drying to obtain the porous hard carbon material.

7. The method for preparing the negative electrode material according to claim 6, characterized in that, The method satisfies at least one of the following characteristics: (1) The hard carbon precursor includes biomass precursor and / or resin-based precursor; (2) The low-temperature pore-forming agent includes at least one of ammonium bicarbonate, ammonium oxalate, polymethyl methacrylate, and polyethylene glycol; (3) The medium-temperature pore-forming agent includes at least one of polystyrene microspheres, zinc chloride, ammonium dihydrogen phosphate, melamine, and polyvinylpyrrolidone; (4) The high-temperature pore-forming agent includes potassium hydroxide and / or sodium hydroxide.

8. The method for preparing the negative electrode material according to claim 6, characterized in that, The method satisfies at least one of the following characteristics: (1) The mass of the low-temperature pore-forming agent is 5% to 10% of the mass of the precursor powder; (2) The mass of the medium-temperature pore-forming agent is 3% to 8% of the mass of the first heat-treated product; (3) The mass of the high-temperature pore-forming agent is 0.5% to 2% of the mass of the second heat-treated product; (4) In the porous hard carbon material, based on the total pore volume, the micropore volume accounts for 60%~75%, the mesopore volume accounts for 20%~30%, and the macropore volume accounts for 5%~10%.

9. A negative electrode material, characterized in that, The negative electrode material is prepared by any one of claims 1-8.

10. A sodium-ion battery, characterized in that, It includes a negative electrode sheet, wherein the negative electrode sheet comprises the negative electrode material as described in claim 9.

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