Sodium-ion battery hard carbon material and preparation method thereof

Abstract

The application relates to the technical field of hard carbon materials, in particular to a sodium ion battery hard carbon material and a preparation method thereof. The preparation method comprises the following steps: (S1) uniformly mixing a pitch precursor and a low-temperature template agent to obtain a first mixture, and performing first-stage heat treatment under an inert atmosphere to obtain a primary stabilized precursor; the first-stage heat treatment temperature is 280-350 DEG C; the low-temperature template agent comprises ammonium sulfate; (S2) uniformly mixing the primary stabilized precursor and a high-temperature template agent to obtain a second mixture, performing second-stage heat treatment under an inert atmosphere, and then continuously heating to a high-temperature carbonization temperature to perform carbonization, so as to obtain the sodium ion battery hard carbon material. The method adopts a process idea of introducing modified components in stages, so that different modified components play roles in different heat treatment stages, excellent electrochemical performance is realized, and the yield of the hard carbon material is also improved.

Description

Technical Field

[0001] This invention relates to the field of hard carbon materials technology, and in particular to a hard carbon material for sodium-ion batteries and its preparation method. Background Technology

[0002] With the increasing global demand for renewable energy and large-scale energy storage systems, sodium-ion batteries, thanks to the widespread availability of sodium resources, low cost, and excellent low-temperature performance, have become an electrode system with great commercial potential after lithium-ion batteries. Among various anode materials, hard carbon (amorphous carbon) is considered the most promising anode material for sodium-ion batteries due to its low sodium storage potential, high specific capacity, and good cycle stability.

[0003] In the selection of hard carbon precursors, fossil minerals such as coal tar pitch have strong commercial potential due to their low price, abundant reserves, and relatively high carbonization yield. As a major byproduct of the coal coking industry, coal tar pitch not only possesses extremely high carbon content and a natural aromatic structure, but its cost advantage is also unmatched by biomass or synthetic resin precursors. Furthermore, the molecular structure of coal tar pitch has high plasticity; through reasonable physicochemical modification, it can theoretically be transformed into hard carbon materials with ideal interlayer spacing and rich microporous structure, thereby meeting the dual requirements of high-performance sodium-ion batteries for energy density and rate performance.

[0004] To tap the potential of pitch-based hard carbon, academia and industry have conducted extensive research on its modification:

[0005] CN118479452A discloses a pitch-derived sodium-ion battery hard carbon material and its preparation method. The preparation method of the pitch-derived sodium-ion battery hard carbon material includes the following steps: S1, preparation of hypercrosslinked pitch: Pitch, a crosslinking agent, and a crosslinking catalyst are added sequentially to an organic solvent, followed by a heated crosslinking reaction. The resulting product is then washed three times with anhydrous ethanol, extracted with tetrahydrofuran, and dried to obtain hypercrosslinked pitch; S2, preparation of hard carbon material: A certain amount of hypercrosslinked pitch is added to a high-temperature tube furnace, and carbonization is performed under an argon atmosphere to obtain the pitch-derived sodium-ion battery hard carbon material.

[0006] CN120841484A discloses a method for preparing hard carbon material for sodium-ion batteries, comprising the following steps: (S1) uniformly mixing asphalt-like substances with heteroatom-rich organic matter to obtain mixed precursor I; (S2) heat-treating the mixed precursor under an oxidizing atmosphere, cooling it down, and during the asphalt melting process, encapsulating the heteroatom-rich organic matter to obtain mixed precursor II; (S3) calcining, cooling, grinding, washing, and drying the mixed precursor II under an inert atmosphere or a hydrocarbon atmosphere to obtain an amorphous sodium-ion battery anode material. The obtained anode material achieves both high capacity and high energy density.

[0007] CN116553512A discloses a method for preparing a sodium-ion battery anode material, comprising the following steps: (S1) mixing asphalt-like substances uniformly in the presence of sulfur-containing compounds to obtain a mixed precursor I; (S2) pressing the mixed precursor into sheets, heat-treating it under an oxidizing atmosphere, and cooling it to obtain cross-linked modified asphalt; mixing the cross-linked modified asphalt, a pore-forming agent, and a pore-modifying agent uniformly to obtain a mixed precursor II; (S3) calcining, cooling, grinding, acid washing, washing, and drying the mixed precursor II under an inert atmosphere or a hydrocarbon atmosphere to obtain an amorphous sodium-ion battery anode material. The anode material prepared by this invention combines high capacity and high energy density, and proposes a low-cost, simple-process, tunable-degree-of-disorder, high-carbon-yield carbon material suitable for large-scale production, which is then applied as an anode material in sodium-ion secondary batteries.

[0008] CN119430148A discloses a method for preparing hard carbon from pre-oxidized heavy organic materials by in-situ doping, comprising the following steps: (1) Low-temperature oxidation: pre-oxidized carbon material is obtained by low-temperature oxidation of medium-temperature coal tar pitch; (2) Element doping: sulfur element doping material and medium-temperature coal tar pitch are reacted at low temperature to obtain a hard carbon precursor doped with sulfur element; (3) Low-temperature curing: a cured sulfur element doped pre-oxidized heavy organic hard carbon precursor is obtained; (4) High-temperature carbonization: a stable sulfur element doped pre-oxidized heavy organic hard carbon precursor material is obtained.

[0009] However, these efforts often fall short when dealing with low-softening-point coal tar pitch. Low-softening-point coal tar pitch, as a precursor, suffers from serious inherent defects during pyrolysis: from a molecular composition perspective, it is rich in low-molecular-weight components (such as light aromatics and aliphatic segments), which have poor chemical stability and low boiling points. In the early stages of the pyrolysis reaction, especially when the temperature rises to its plasticizing window, these light components volatilize violently before undergoing condensation reactions to form a carbon skeleton. This imbalance between "volatilization and carbon formation" leads to several technical challenges: on the one hand, the large amount of carbon source escaping as small-molecule gases results in low final carbon yield, losing the economic viability for industrial application; on the other hand, the disordered release of light components leaves the precursor lacking support during the critical phase transition stage, resulting in a dense, low-porosity hard carbon structure that limits sodium ion intercalation kinetics, leading to poor rate performance. How to capture these volatile components within this critical temperature window and simultaneously induce efficient pore formation remains a significant obstacle in the current research and development of pitch-based hard carbon.

[0010] Furthermore, the preparation of hard carbon materials from pitch-based precursors mostly requires pre-oxidation. The purpose of pre-oxidation is to introduce oxygen-containing functional groups, crosslinking the small molecules of pitch into a huge three-dimensional network that is not easily melted or flowed at high temperatures. This prevents the rearrangement of pitch molecules and inhibits the ordered graphitization of pitch, resulting in hard carbon materials with larger interlayer spacing and more defects. However, pre-oxidation also has drawbacks: First, the oxygen-containing functional groups introduced by air pre-oxidation are mostly located on the surface of the aromatic molecules of pitch, which may interact strongly with sodium ions, forming a "trapping effect" that hinders the free movement of sodium ions and negatively impacts the rate performance and first-cycle coulombic efficiency of the battery. Second, during heating, the pitch softens and melts, making it difficult for oxygen to penetrate evenly, resulting in uneven pre-oxidation and affecting the consistency of the final product. Third, excessive oxidation can lead to the breakage of aromatic rings, damaging the carbon skeleton, which not only significantly reduces carbon yield but may also introduce too many ineffective defects, similarly impairing electrochemical performance. Summary of the Invention

[0011] To address the technical problems in existing technologies, such as the easy volatilization of lightweight components and low carbon yield of low-softening-point coal tar pitch and other carbon source precursors during heat treatment, the difficulty in achieving a balance between disorder and suitable pore structure construction in the resulting carbon framework, and the drawbacks of pre-oxidation in the preparation of hard carbon materials from pitch, this invention provides a sodium-ion battery hard carbon material and its preparation method. This method employs a phased introduction of modifying components, allowing different modifying components to play their roles at different heat treatment stages, thereby improving the stabilization behavior of the precursor and the subsequent evolution of the hard carbon structure. The preparation method of this invention achieves excellent electrochemical performance without pre-oxidation, thus avoiding the drawbacks of the pre-oxidation process.

[0012] To achieve the above objectives, the present invention adopts the following technical solution:

[0013] A hard carbon material for sodium-ion batteries and its preparation method, comprising the following steps:

[0014] (S1) The asphalt precursor and the low-temperature template agent are mixed evenly to obtain a first mixture. The mixture is then subjected to a first-stage heat treatment under an inert atmosphere to obtain a primary stabilized precursor. The first-stage heat treatment temperature is 280-350℃, and the low-temperature template agent decomposes at 280-350℃.

[0015] (S2) The first stabilized precursor and the high-temperature template agent are mixed evenly to obtain a second mixture. The mixture is then subjected to a second stage of heat treatment under an inert atmosphere, and then the temperature is further increased to the high-temperature carbonization temperature for carbonization to obtain the sodium-ion battery hard carbon material.

[0016] Further, the softening point of the asphalt precursor is ≤135℃, for example ≤100℃, or ≤80℃, or ≤70℃. In a preferred embodiment of the present invention, the softening point of the asphalt precursor is between 60-70℃. The method for preparing hard carbon materials of the present invention is applicable to asphalt with a low softening point, i.e., asphalt with a softening point between 60-80℃, preferably asphalt with a softening point between 60-70℃. However, the method of the present invention is also applicable to asphalt with a higher softening point, such as medium-low softening asphalt. It should not be construed as the scope of protection of the present invention being limited to asphalt with a low softening point. The inventors have found that thermogravimetric analysis of different asphalts shows that, although their softening points and compositions differ significantly, their intense pyrolysis and crosslinking reactions are mainly concentrated in the temperature range of 280-350℃. We define this as the "critical window temperature." The present invention proposes a template strategy for matching pyrolysis kinetics, which posits that only when the decomposition temperature of the template is precisely matched with this critical pyrolysis window of the asphalt can the carbonization process be controlled and intervened. Low-temperature asphalt with a softening point of 60-70℃ is inexpensive, but it has a high content of lightweight aromatic components, which are easily volatilized during carbonization. This results in carbon materials with both high graphitization and low carbon yield, thus usually requiring complex modification processes to prepare hard carbon. Therefore, the low-temperature template agents of this invention are adapted to the aforementioned critical temperature window. These template agents react to generate gas at the critical temperature window and also undergo a certain cross-linking reaction with asphalt molecules.

[0017] Further, the low-temperature template agent is selected from one or more of ammonium sulfate, ammonium persulfate, ammonium perchlorate, ammonium nitrate, ammonium chloride, ammonium bicarbonate, calcium formate, potassium formate, magnesium formate, and boric acid-ethanolamine compounds. Preferably, the low-temperature template agent contains ammonium sulfate. The inventors discovered that the thermogravimetric curves of asphalt and ammonium sulfate exhibit excellent temperature synchronicity and highly overlapping reaction windows. Within this temperature range, asphalt macromolecules undergo chemical breakage and fragment recombination, reaching a transition state with the highest pyrolysis activity. Ammonium sulfate simultaneously decomposes, releasing active species that precisely and efficiently crosslink with asphalt fragments in situ and regulate their microstructure. This ideal thermal behavior matching ensures that the active species released by the low-temperature template agent can directly act on the highly reactive asphalt, avoiding premature or delayed reactions, maximizing the utilization of asphalt, improving yield, and simultaneously constructing a highly stable stabilized precursor, which is beneficial for subsequent second-stage reactions and carbonization to obtain hard carbon materials with excellent electrochemical performance.

[0018] Furthermore, when the low-temperature softening agent is ammonium sulfate, the first-stage heat treatment temperature is 280-310℃, preferably 300-310℃, with a holding time of 2-4 hours. The inventors discovered that the above-mentioned preferred first-stage heat treatment temperature represents the optimal reaction window for low-softening-point asphalt. At this temperature, the decomposition rate of the lightweight components of the asphalt and the rate of active species generation by the low-temperature template agent achieve optimal matching, enabling precise capture of the lightweight components of hard carbon. Simultaneously, this temperature ensures smooth pore closing without causing excessive pore collapse. In other words, at the above-mentioned first-stage heat treatment temperature, both the yield of hard carbon materials and their electrochemical performance can be improved simultaneously.

[0019] In a preferred embodiment of the present invention, the low-temperature template agent is a mixture of ammonium sulfate and a boric acid-ethanolamine compound; the boric acid-ethanolamine compound is selected from at least one of monoethanolamine-boric acid (CAS: 68586-07-2) and diethanolamine-boric acid (CAS: 68298-96-4). More preferably, the mass ratio of ammonium sulfate to boric acid-ethanolamine compound is 6-8:2-3. The inventors have found that by adding a small amount of boric acid-ethanolamine compound to ammonium sulfate, the mixture can maximize the yield and electrochemical performance of hard carbon materials, especially the rate performance, at 1000 mA g. -1It exhibits excellent long-term cycling stability at high current densities. The boric acid-ethanolamine compound decomposes upon heating, releasing active BH3 species, which react with the unsaturated double bonds on the aryl group via hydroboration to form a CBC cross-linked structure. Simultaneously, the hydroxyl and amino groups of the ethanolamine moiety may also participate in the reaction of the pitch molecules, forming a COC cross-linked structure. The synergistic effect of these two factors inhibits the ordered stacking of the pitch precursor, resulting in carbon materials with higher disorder and more exposed active sites after subsequent carbonization. Furthermore, the reduction sites of the boric acid-ethanolamine compound introduce co-doping of B and N, increasing the interlayer spacing of the carbon layers, disrupting the ordered arrangement of the carbon layers, increasing the disorder of the material, and improving electronic conductivity.

[0020] When the low-temperature template agent is a combination of ammonium sulfate and boric acid-ethanolamine compound, the first stage of heat treatment involves holding at 280-310℃ for 1-2 hours, followed by raising the temperature to 330-350℃ and holding for another 1-2 hours. When using the combination of ammonium sulfate and boric acid-ethanolamine compound as the template agent, the reaction temperature for ammonium sulfate to release active species and undergo cross-linking begins approximately at 290℃, while for boric acid-ethanolamine compound it begins at approximately 330℃. Therefore, when using the combination of ammonium sulfate and boric acid-ethanolamine compound, to fully utilize their synergistic effect, the first stage of heat treatment involves gradually holding at the activation temperature of ammonium sulfate for a period of time, then gradually raising the temperature to the activation temperature of boric acid-ethanolamine compound and holding for a period of time. This allows the low-temperature template agent to function throughout the entire first stage of heat treatment, which is more beneficial for stabilizing the carbon skeleton.

[0021] Further, the high-temperature template agent includes one or more of zinc chloride, basic magnesium carbonate, and magnesium carbonate; the second stage heat treatment involves heating to 400-500℃ at a heating rate of 1-5℃ / min and holding at that temperature for 1-3 hours; preferably, heating to 400-450℃ and holding at that temperature for 2-3 hours.

[0022] Furthermore, the high-temperature carbonization involves heating to 1200-1400℃ at a heating rate of 5-10℃ / min and holding at that temperature for 2-6 hours.

[0023] Furthermore, the mass ratio of asphalt precursor to low-temperature template agent is 1:0.3-0.4; the mass ratio of primary stabilized precursor to high-temperature template agent is 1:0.4-0.6.

[0024] Furthermore, the mixing methods in steps (S1) and (S2) include one or more of grinding, ball milling, and mechanical stirring.

[0025] The present invention also provides a sodium-ion battery hard carbon material prepared by the above preparation method.

[0026] The present invention also provides a sodium-ion battery negative electrode sheet, the negative electrode sheet comprising a current collector and a negative electrode active layer disposed on the surface of the current collector, the negative electrode active layer comprising the aforementioned sodium-ion battery hard carbon material.

[0027] The present invention also provides a sodium-ion secondary battery, wherein the negative electrode comprises the aforementioned negative electrode sheet.

[0028] This invention employs a stepwise introduction of high- and low-temperature template agents, rather than a method of uniformly heating after all additives are blended at once. The inventors believe that in the first-stage heat treatment, the low-temperature template agent preferentially undergoes a thermal stabilization process together with the carbon source precursor. This process helps to slow down the premature volatilization of lightweight components in the critical temperature range and promotes the formation of a more stable intermediate structure in the precursor. In the second-stage heat treatment, the first-stabilized precursor is then mixed with the high-temperature template agent, allowing it to participate in subsequent carbon framework evolution and pore structure adjustment under conditions where the precursor has already achieved a certain degree of structural stability.

[0029] Compared with existing technologies, this invention separates precursor stabilization and subsequent structural regulation in the process sequence by introducing low-temperature template agents and high-temperature template agents in stages, which is beneficial for targeted regulation of the behavior of precursors at different thermal evolution stages.

[0030] In a preferred embodiment of the present invention, the low-temperature template agent is a compound of ammonium sulfate and boric acid-ethanolamine compound, and the first stage of heat treatment is followed by two stages of atmospheric treatment to adapt to the two template agents respectively.

[0031] Furthermore, the process flow of this invention is relatively simple, mainly relying on solid-phase mixing and segmented heat treatment, without the need for complex organic solvent extraction steps, making it suitable for further scale-up and process optimization. This invention also eliminates the need for pre-oxidation steps to achieve crosslinking and modification of hard carbon precursors, avoiding problems caused by uneven or excessive pre-oxidation. Attached Figure Description

[0032] Figure 1 The Fourier transform infrared spectra of the precursors obtained in Examples 1 (S2) and (S5) of the present invention are shown.

[0033] Figure 2 This is a small-angle X-ray scattering (SAXS) pattern of the hard carbon material obtained in the essential embodiment 1 of the present invention.

[0034] Figure 3 This is a high-resolution transmission electron microscope (HRTEM) image of the hard carbon material obtained in Example 1. Detailed Implementation

[0035] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise stated, the raw materials, equipment and testing methods used in the embodiments can all adopt conventional techniques in the art.

[0036] Example 1

[0037] (S1) Weigh low softening point coal tar pitch (softening point about 60℃) and ammonium sulfate at a mass ratio of 1:0.4 (100g:40g), grind the materials to make them fully mixed and uniform, and obtain the first mixture. Place it in a tube furnace and heat it to 300℃ at a heating rate of 1℃ / min under a nitrogen atmosphere. Hold it at this temperature for 2 hours. After cooling, about 70g of primary stabilized precursor is obtained, which is denoted as PAS-300.

[0038] (S2) The primary stabilized precursor PAS-300 and zinc chloride were mixed at a mass ratio of 1:0.6 and ground to make them uniformly mixed to obtain a second mixture. The mixture was placed in a tube furnace and heated to 400°C at a heating rate of 5°C / min under a nitrogen atmosphere and held for 2 hours. Then, the temperature was further increased to 1300°C for high-temperature carbonization and held for 4 hours. The mixture was then cooled to room temperature to obtain the sodium-ion battery hard carbon material, denoted as PASZ-HC.

[0039] (S3) As a control, low softening point coal tar pitch was placed in a tubular furnace and heated to 300°C at a heating rate of 1°C / min under an inert atmosphere. The temperature was held for 2 hours and then cooled to obtain the precursor, which was denoted as Pitch-300.

[0040] Figure 1 Fourier transform infrared spectra of precursor PAS-300 obtained in step (S1) and precursor Pitch-300 obtained in step (S3) are used to characterize the changes in characteristic absorption peaks related to sulfur-containing functional groups after ammonium sulfate treatment, to illustrate the introduction of sulfur-related structures. The plasticizing window of asphalt is around 300 °C. At this temperature, sulfur-containing crosslinking bonds (such as COS or S=O) can be introduced at the edges of aromatic rings, bridging the originally discrete and volatile low molecular weight components through covalent bonds and solidifying them into a highly crosslinked three-dimensional oligomeric network framework. Simultaneously, the gas pressure generated by the in-situ release of NH3 acts as a physical foaming agent in the semi-fluid plastic framework, initially outlining the rudimentary hierarchical pore structure.

[0041] Example 2

[0042] The other operating steps are the same as in Example 1, except that in step (S1), the low softening point coal tar pitch is replaced with medium temperature coal tar pitch with a softening point of 165°C to obtain a hard carbon material, denoted as M-PASZ-HC.

[0043] Example 3

[0044] The other operating steps are the same as in Example 1, except that in step (S1), the temperature of the first stage heat treatment is changed from 300°C to 280°C.

[0045] Example 4

[0046] The other operating steps are the same as in Example 1, except that in step (S1), the temperature of the first stage heat treatment is changed from 300°C to 310°C.

[0047] Example 5

[0048] The other operating steps are the same as in Example 1, except that in step (S1), the temperature of the first stage heat treatment is changed from 300°C to 350°C.

[0049] Example 6

[0050] The other operating steps are the same as in Example 1, except that: in step (S1), low softening point coal tar pitch and ammonium sulfate are fed in a mass ratio of 1:0.3; in step (S2), the primary stabilization precursor is mixed with zinc chloride and fed in a mass ratio of 1:0.4; and in step S2, the second stage heat treatment temperature is changed from 400℃ to 450℃.

[0051] Example 7

[0052] The other operating steps are the same as in Example 1, except that step (S1) is changed to: weighing low softening point coal tar pitch (softening point about 60°C) and low temperature template agent at a mass ratio of 1:0.4. The low temperature template agent is a compound of ammonium sulfate and monoethanolamine-boric acid at a mass ratio of 2:1. Grind the materials to make them fully mixed and uniform to obtain the first mixture. Place it in a tube furnace and heat it to 300°C at a heating rate of 1°C / min under a nitrogen atmosphere, and hold it at that temperature for 1 hour. Then heat it to 320°C at a heating rate of 1°C / min and hold it at that temperature for 1 hour.

[0053] Example 8

[0054] The other operating steps are the same as in Example 1, except that step (S1) is changed to: weighing low softening point coal tar pitch (softening point about 60°C) and low temperature template agent at a mass ratio of 1:0.4. The low temperature template agent is a compound of ammonium sulfate and diethanolamine-boric acid at a mass ratio of 4:1. Grind the materials to make them fully mixed and uniform to obtain the first mixture. Place it in a tube furnace and heat it to 300°C at a heating rate of 1°C / min under a nitrogen atmosphere, and hold it at that temperature for 1 hour. Then heat it to 330°C at a heating rate of 1°C / min and hold it at that temperature for 1 hour.

[0055] Example 9

[0056] The other operating steps are the same as in Example 1, except that in step (S1), the low-temperature template agent is magnesium formate.

[0057] Comparative Example 1

[0058] (S1) Weigh low softening point coal tar pitch (softening point about 60℃) and ammonium sulfate and zinc chloride in a mass ratio of 1:0.4:0.42. Grind the materials to make them fully mixed and uniform. Place the mixture in a tube furnace and heat it to 300℃ at a heating rate of 1℃ / min under a nitrogen atmosphere. Hold it at this temperature for 2 hours. Then heat it to 400℃ and hold it for 2 hours. Then continue to heat it to 1300℃ for high-temperature carbonization and hold it for 4 hours. Cool it to room temperature to obtain sodium-ion battery hard carbon material.

[0059] Comparative Example 2

[0060] The other operating steps are the same as in Example 1, except that in step (S1), the temperature of the first stage heat treatment is changed from 300°C to 270°C.

[0061] Comparative Example 3

[0062] The other operating steps are the same as in Example 1, except that in step (S1), the temperature of the first stage heat treatment is changed from 300°C to 360°C.

[0063] Figure 2 The small-angle X-ray scattering (SAXS) pattern of the hard carbon material obtained in Example 1 of this invention. The closed-pore characteristic evaluation (SAXS) is the core structural basis for the contribution of hard carbon to the plateau capacity. The SAXS curve of the hard carbon material PASZ-HC obtained in Example 1 shows obvious scattering characteristics in a specific q range, which is a typical manifestation of the closed-pore related electron density fluctuation.

[0064] Figure 3 This is a high-resolution transmission electron microscope (HRTEM) image of the hard carbon material obtained in Example 1. The obtained hard carbon material exhibits highly disordered, randomized nanostructures. The interlayer spacing of PASZ-HC is increased to 0.384 nm, significantly exceeding the interlayer spacing of ideal graphite (0.335 nm). Notably, a large number of closed-pore structures can be clearly observed in the randomized structure of PASZ-HC. This increased interlayer spacing, combined with the abundant closed-pore system, lays the microstructural foundation for improving the plateau region capacity by enhancing the intercalation kinetics and pore-filling mechanism of sodium ions.

[0065] Application examples

[0066] The hard carbon material, conductive additive SP, and binder CMC / SBR obtained in the above examples and comparative examples were mixed at a weight ratio of 93:3:4, dissolved in water, and stirred to obtain a uniform slurry. The slurry was then uniformly coated onto carbon-coated aluminum foil using a 50 μm scraper, dried, and sliced ​​to obtain the negative electrode sheet.

[0067] The dried negative electrode sheet was assembled into a coin cell with a sodium metal negative electrode, a carboxylic acid cellulose membrane, and a DME / 1M NaPF6 ether electrolyte. The cell was then placed on the LAND test platform for testing, and the first cycle curve is shown in Table 1.

[0068] Constant current charge-discharge tests were conducted on different samples at a current density of 20 mA / g. Based on existing experimental results, the PASZ-HC sample exhibited high reversible capacity and first coulombic efficiency, with a reversible capacity of approximately 369.9 mAh / g at a high rate of 1 A / g. -1 At current density, the hard carbon material prepared by this invention achieves outstanding rate performance, exhibiting high capacity and long cycle performance.

[0069] The hard carbon yield is calculated based on the mass of hard carbon material that can be obtained from 100g of asphalt material.

[0070] Table 1 Electrochemical performance data

[0071]

[0072] As shown in Table 1, the test results of this invention demonstrate that the preparation of hard carbon materials for sodium-ion battery anodes using low-softening-point coal tar pitch as raw material, employing a sequential process of introducing low-temperature and high-temperature template agents in stages, simultaneously improves the sodium storage performance, electrochemical performance (especially rate performance), and yield of the resulting hard carbon anode material. The core of this performance improvement lies in the step-by-step process design of "early-stage stabilization – later-stage structural adjustment," achieving separate and directional adjustment in the process sequence. Low-softening-point coal tar pitch and other precursors are rich in low-molecular-weight components. In the first stage (280-350℃), this invention introduces low-temperature template agents, including ammonium sulfate, for thermal stabilization, successfully introducing sulfur-containing functional groups and effectively anchoring volatile components, constructing a stable precursor framework. In the second stage (400-500℃), metal chlorides are further mixed in, participating in subsequent evolution based on the already stabilized precursor structure, efficiently leveraging the role of pore structure regulators to construct ideal interlayer spacing and abundant microporous structures, providing a large number of sodium storage active sites for the low-pressure plateau region. If the additives are blended in a single step (Comparative Example 1), the sodium storage capacity will be limited due to component competition or premature loss of light components. If the first-stage heat treatment temperature is too low or too high, the electrochemical performance and / or yield will decrease due to poor matching with the critical temperature window of the asphalt.

Claims

1. A hard carbon material for sodium-ion batteries and its preparation method, characterized in that, Includes the following steps: (S1) The asphalt precursor and the low-temperature template agent are mixed evenly to obtain a first mixture. The mixture is then subjected to a first-stage heat treatment under an inert atmosphere to obtain a primary stabilized precursor. The first-stage heat treatment temperature is 280-350℃, and the low-temperature template agent decomposes at 280-350℃. (S2) The first stabilized precursor and the high-temperature template agent are mixed evenly to obtain a second mixture. The mixture is then subjected to a second stage of heat treatment under an inert atmosphere, and then the temperature is further increased to the high-temperature carbonization temperature for carbonization to obtain the sodium-ion battery hard carbon material.

2. The preparation method according to claim 1, characterized in that, The softening point of the asphalt precursor is ≤135℃, preferably ≤100℃, more preferably ≤80℃, and most preferably ≤70℃.

3. The preparation method according to claim 1, characterized in that, The low-temperature template agent is selected from one or more of the following: ammonium sulfate, ammonium persulfate, ammonium perchlorate, ammonium nitrate, ammonium chloride, ammonium bicarbonate, calcium formate, potassium formate, magnesium formate, and boric acid-ethanolamine compounds.

4. The preparation method according to claim 1, characterized in that, When the low-temperature softener is ammonium sulfate, the first stage heat treatment temperature is 280-310℃, preferably 300-310℃, and the holding time is 2-4 hours.

5. The preparation method according to claim 1, characterized in that, The low-temperature template agent is a compound of ammonium sulfate and boric acid-ethanolamine compound; the boric acid-ethanolamine compound is selected from at least one of monoethanolamine-boric acid and diethanolamine-boric acid; preferably, the mass ratio of ammonium sulfate to boric acid-ethanolamine compound is 6-8:2-3.

6. The preparation method according to claim 5, characterized in that, The first stage of heat treatment involves holding at 280-310℃ for 1-2 hours, followed by raising the temperature to 330-350℃ and holding for another 1-2 hours.

7. The preparation method according to claim 1, characterized in that, The high-temperature template agent includes one or more of zinc chloride, basic magnesium carbonate, and magnesium carbonate; the second stage heat treatment involves heating to 400-500℃ at a heating rate of 1-5℃ / min and holding at that temperature for 1-3 hours; preferably, heating to 400-450℃ and holding at that temperature for 2-3 hours. Furthermore, the high-temperature carbonization involves heating to 1200-1400℃ at a heating rate of 5-10℃ / min and holding at that temperature for 2-6 hours.

8. The preparation method according to claim 1, characterized in that, The mass ratio of asphalt precursor to low-temperature template agent is 1:0.3-0.4; the mass ratio of primary stabilized precursor to high-temperature template agent is 1:0.4-0.

6.

9. The sodium-ion battery hard carbon material prepared by the preparation method according to any one of claims 1-8.

10. A sodium-ion battery negative electrode sheet, the negative electrode sheet comprising a current collector and a negative electrode active layer disposed on the surface of the current collector, the negative electrode active layer comprising the sodium-ion battery hard carbon material prepared by the preparation method according to any one of claims 1-8.

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