A hard carbon material for sodium-ion batteries and a preparation method thereof

Abstract

The application belongs to the technical field of sodium ion batteries, and particularly relates to a hard carbon material for a sodium ion battery and a preparation method thereof. The preparation method comprises the following steps: adding sawdust, starch and ammonium persulfate into water to uniformly disperse, carrying out hydrothermal reaction, filtering and drying to obtain a hard carbon precursor; adding the hard carbon precursor and yttrium-doped sodium hafnium oxide into ethanol to uniformly disperse, carrying out hydrothermal reaction, filtering and drying to obtain a composite hard carbon precursor; uniformly mixing the composite hard carbon precursor and soft carbon-coated silicon boride, and calcining under an argon atmosphere to obtain the hard carbon material. By using the yttrium-doped sodium hafnium oxide and the soft carbon-coated silicon boride, the specific capacity, rate performance and good cycle stability of the hard carbon material for the sodium ion battery are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a hard carbon material for sodium-ion batteries and its preparation method. Background Technology

[0002] Sodium-ion batteries, due to their abundant resources and low cost, are considered a powerful complement to lithium-ion batteries, and the anode material is crucial in determining their electrochemical performance. Currently, hard carbon is the most commonly used anode material for sodium-ion batteries. Hard carbon is an amorphous carbon that is difficult to graphitize even after high-temperature heat treatment. Its interior consists of randomly stacked curved graphene sheets forming numerous nanopores and defects, giving it high hardness, high strength, and good conductivity. Thanks to this unique disordered structure, hard carbon anodes exhibit high reversible specific capacity (typically reaching 300-350 mAh / g) and a wide interlayer spacing, which is beneficial for sodium ion insertion / extraction, thus becoming the mainstream choice for sodium-ion battery anodes.

[0003] Despite the superior capacity performance of hard carbon anodes, they still have significant drawbacks in practical applications. The numerous defects, oxygen-containing functional groups, and micropores within hard carbon irreversibly consume the electrolyte, resulting in generally low initial efficiency (typically below 80%) and severe sodium source loss. Simultaneously, the sodium storage mechanism in its plateau region relies on the slow diffusion and quasi-metallic deposition of sodium ions within the nanopores, leading to a sharp increase in polarization at high current densities, significant voltage hysteresis, and rapid rate capacity decay. More critically, the excessively low discharge plateau (<0.1 V vs. Na) further exacerbates the problem. + Sodium dendrites (Na) can easily induce sodium deposition. The resulting sodium dendrites can not only puncture the separator and cause safety hazards, but also cause repeated rupture and reconstruction of the solid electrolyte interphase (SEI) membrane, accelerating electrolyte consumption and active material shedding. In addition, irreversible sodium residues gradually block the pores in each cycle, resulting in a long-cycle capacity retention rate that is far lower than theoretically expected, which seriously restricts the practical lifespan of the battery.

[0004] Therefore, there is an urgent need to develop a new type of hard carbon material that combines high initial efficiency, excellent rate performance, and long cycle stability to meet the practical application requirements of sodium-ion batteries for high safety and long lifespan anodes. Summary of the Invention

[0005] The purpose of this invention is to provide a hard carbon material for sodium-ion batteries and a method for preparing the same. When used as the negative electrode of a sodium-ion battery, the hard carbon material exhibits excellent specific capacity, rate performance, and good cycle stability.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first aspect of this invention provides a method for preparing a hard carbon material for sodium-ion batteries, comprising the following steps: (1) Add sawdust, starch and ammonium persulfate to water and disperse evenly. Perform hydrothermal reaction, filter and dry to obtain hard carbon precursor; (2) The hard carbon precursor and yttrium-doped sodium hafnium oxide were added to ethanol and dispersed evenly. After hydrothermal reaction, the composite hard carbon precursor was obtained by filtration and drying. (3) The composite hard carbon precursor and soft carbon-coated silicon hexaboride are mixed evenly, calcined in an inert gas atmosphere, and then cooled to obtain hard carbon material.

[0007] Further, the yttrium-doped sodium hafnium oxide mentioned in step (2) is prepared by the following steps: Hafnium hydroxide, sodium nitrate, and yttrium oxide were added to water and dispersed evenly. After evaporating the solvent, the reaction was carried out under heat to obtain yttrium-doped sodium hafnium oxide.

[0008] Furthermore, the molar ratio of yttrium oxide, hafnium hydroxide, and sodium nitrate is 1:(3-4):(13-16); the temperature of the heat preservation reaction is 800-900 °C, and the time is 3-6 h.

[0009] Furthermore, the soft carbon-coated silicon hexaboride described in step (3) is prepared by the following steps: (a) Silicon dioxide, boron anhydride, boron carbide and graphite were ball-milled and mixed, pressed into shape and vacuum sintered, cooled and ground to obtain silicon hexaboride; (b) Silicon hexaboride and pitch are ball-milled and mixed, sintered in an inert gas atmosphere, cooled and carbonized to obtain soft carbon-coated silicon hexaboride.

[0010] Further, in step (a), the molar ratio of silicon dioxide, boron anhydride, boron carbide and graphite is 1:(1-1.2):(1.2-1.4):(6-6.5); the vacuum sintering temperature is 1100-1300 ℃ and the time is 2-5 h.

[0011] Further, in step (b), the mass ratio of silicon hexaboride to pitch is 1:(0.04-0.1); the sintering temperature is 500-600 ℃ and the time is 2-5 h; the carbonization temperature is 1000-1100 ℃ and the time is 4-6 h.

[0012] Further, in step (1), the mass ratio of sawdust, starch and ammonium persulfate is 1:(0.8-1):(0.08-0.12); the hydrothermal reaction temperature is 160-190 ℃, the time is 3-6 h, and the pressure is 2-5 MPa.

[0013] Further, in step (2), the mass ratio of the hard carbon precursor to yttrium-doped sodium hafnium is 1:(0.04-0.08); the hydrothermal reaction temperature is 150-180 °C, the time is 2-4 h, and the pressure is 1-3 MPa.

[0014] Further, in step (3), the mass ratio of soft carbon-coated silicon hexaboride and composite hard carbon precursor is 1:(65-85); the calcination temperature is 800-1000 ℃ and the time is 2-5 h.

[0015] A second aspect of the present invention provides a hard carbon material for sodium-ion batteries prepared by the above-described preparation method.

[0016] The beneficial technical effects of this invention are as follows: 1. This invention significantly improves the first-pass efficiency and rate performance of hard carbon materials by adding yttrium-doped sodium hafnium. Sodium hafnium has a stable three-dimensional ion channel structure, and its addition to hard carbon materials can enhance the sodium ion transport capability of the materials. On this basis, doping with yttrium will, on the one hand, produce lattice distortion and charge imbalance, increase the active insertion sites of sodium ions and reduce the diffusion barrier, thereby further improving the sodium ion transport rate; on the other hand, it will enhance the structural strength and chemical stability of sodium hafnium, effectively suppress sodium precipitation and reduce the volume expansion and structural damage of hard carbon materials during cycling, thus improving the cycling performance of the materials.

[0017] 2. This invention significantly improves the long-term cycling stability of hard carbon materials by adding soft carbon-coated silicon hexaboride. Silicon hexaboride itself possesses the high strength and excellent electrical properties of ceramic materials, and has a low volume expansion rate. Doping it into hard carbon materials promotes sodium ion transport and suppresses volume changes, thereby improving the material's cycling stability. This invention further utilizes soft carbon to coat the silicon hexaboride, which on the one hand improves conductivity and optimizes the ion transport network, and on the other hand forms a stable shell structure, improving the interfacial compatibility between silicon hexaboride and hard carbon materials, reducing side reactions caused by contact between silicon hexaboride and the electrolyte, and simultaneously acting as a flexible buffer interface to further alleviate cycling stress. Furthermore, the synergistic effect of the rigid framework and the flexible shell effectively ensures the structural integrity of the hard carbon material during long-term cycling. Attached Figure Description

[0018] Figure 1 A scanning electron microscope image of the yttrium-doped sodium hafnium oxide prepared in Example 1 of this invention; Figure 2 The image shown is a scanning electron microscope image of the soft carbon-coated silicon hexaboride prepared in Example 5 of this invention. Figure 3 This is a scanning electron microscope image of the hard carbon material prepared in Example 1 of the present invention. Detailed Implementation

[0019] The following is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention. Specific conditions not specified in the embodiments are performed according to conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, all reagents or instruments used are conventional products obtained through commercial channels.

[0020] (a) Preparation example Preparation Example 1 Preparation Example 1 provides a yttrium-doped sodium hafnium oxide, prepared by the following steps: Yttrium oxide, hafnium hydroxide, sodium nitrate, and water were added to water in a ratio of 1 mol:3.5 mol:14 mol:17 L, and the mixture was ultrasonically dispersed until homogeneous. After evaporation and drying, the solution was kept at 850 ℃ for 4 h and then cooled to obtain yttrium-doped sodium hafnium oxide. A scanning electron microscope image of the yttrium-doped sodium hafnium oxide is shown below. Figure 1 As shown.

[0021] Preparation Example 2 Preparation Example 2 provides a yttrium-doped sodium hafnium oxide, prepared by the following steps: According to the ratio of yttrium oxide, hafnium hydroxide, sodium nitrate and water as 1 mol: 3 mol: 13 mol: 15 L, hafnium hydroxide, sodium nitrate and yttrium oxide were added to water and ultrasonically dispersed evenly. After evaporation and drying, the mixture was kept at 800 ℃ for 6 h and cooled to obtain yttrium-doped sodium hafnium oxide.

[0022] Preparation Example 3 Preparation Example 3 provides a yttrium-doped sodium hafnium oxide, prepared by the following steps: According to the ratio of yttrium oxide, hafnium hydroxide, sodium nitrate and water as 1 mol: 4 mol: 16 mol: 20 L, hafnium hydroxide, sodium nitrate and yttrium oxide were added to water and ultrasonically dispersed evenly. After evaporation and drying, the mixture was kept at 900 ℃ for 3 h and cooled to obtain yttrium-doped sodium hafnium oxide.

[0023] Preparation Example 4 Preparation Example 4 provides a sodium hafnium salt, which is prepared by the following steps: Sodium hafnium hydroxide was obtained by ultrasonically dispersing hafnium hydroxide and sodium nitrate in water at a ratio of 3.5 mol: 14 mol: 17 L, evaporating and drying the mixture, and then keeping it at 850 ℃ for 4 h.

[0024] Preparation Example 5 Preparation Example 5 provides a soft carbon-coated silicon hexaboride, prepared by the following steps: (a) Silicon dioxide, boron anhydride, boron carbide and graphite were ball-milled and mixed evenly according to the molar ratio of silicon dioxide, boron anhydride, boron carbide and graphite 1:1.1:1.3:6.2. The mixture was pressed into shape using a hydraulic press, and then sintered under vacuum at 1200 °C for 3 h. After cooling and grinding, silicon hexaboride was obtained. (b) Silicon hexaboride and pitch were ball-milled and mixed evenly at a mass ratio of 1:0.08. The mixture was sintered at 600 °C for 3 h under an argon atmosphere, cooled, and then placed in a carbonization apparatus for carbonization at 1100 °C for 5 h under an argon atmosphere. After cooling, soft carbon-coated silicon hexaboride was obtained. A scanning electron microscope image of this soft carbon-coated silicon hexaboride is shown below. Figure 2 As shown.

[0025] Preparation Example 6 Preparation Example 6 provides a soft carbon-coated silicon hexaboride, prepared by the following steps: (a) Silicon dioxide, boron anhydride, boron carbide and graphite were ball-milled and mixed evenly according to the molar ratio of silicon dioxide, boron anhydride, boron carbide and graphite 1:1:1.2:6. The mixture was pressed into shape using a hydraulic press, and then sintered under vacuum at 1100 °C for 5 h. After cooling and grinding, silicon hexaboride was obtained. (b) Silicon hexaboride and pitch were ball-milled and mixed evenly according to the mass ratio of silicon hexaboride to pitch of 1:0.04. After sintering at 500 °C for 5 h in an argon atmosphere, the mixture was cooled and then placed in a carbonization device and carbonized at 1000 °C for 6 h in an argon atmosphere. After cooling, soft carbon-coated silicon hexaboride was obtained.

[0026] Preparation Example 7 Preparation Example 7 provides a soft carbon-coated silicon hexaboride, prepared by the following steps: (a) Silicon dioxide, boron anhydride, boron carbide and graphite were ball-milled and mixed evenly according to the molar ratio of silicon dioxide, boron anhydride, boron carbide and graphite 1:1.2:1.4:6.5. The mixture was pressed into shape using a hydraulic press, and then sintered under vacuum at 1300 °C for 2 h. After cooling and grinding, silicon hexaboride was obtained. (b) Silicon hexaboride and pitch were ball-milled and mixed evenly according to the mass ratio of silicon hexaboride to pitch of 1:0.1. After sintering at 600 °C for 2 h in an argon atmosphere, the mixture was cooled and then placed in a carbonization device and carbonized at 1100 °C for 4 h in an argon atmosphere. After cooling, soft carbon-coated silicon hexaboride was obtained.

[0027] (II) Implementation Examples Example 1 A method for preparing a hard carbon material for sodium-ion batteries includes the following steps: (1) According to the ratio of sawdust, starch, ammonium persulfate and water, 1 g: 0.9 g: 0.1 g: 5.5 mL, sawdust, starch and ammonium persulfate were added to water and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized at 3 MPa. The mixture was hydrothermally reacted at 180 °C for 4 h. After filtration and drying, the hard carbon precursor was obtained. (2) According to the ratio of hard carbon precursor, yttrium-doped sodium hafnium oxide and ethanol, 1 g: 0.06 g: 1.1 mL, the hard carbon precursor and the yttrium-doped sodium hafnium oxide from Preparation Example 1 were added to ethanol and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized to 2 MPa. The mixture was then hydrothermally reacted at 160°C for 3 h. After filtration and drying, the composite hard carbon precursor was obtained. (3) The soft carbon-coated silicon hexaboride and the composite hard carbon precursor of Preparation Example 5 were ball-milled and mixed evenly according to the mass ratio of soft carbon-coated silicon hexaboride and composite hard carbon precursor of 1:75. The mixture was then calcined at 900 °C for 3 h in an argon atmosphere and cooled to obtain hard carbon material.

[0028] This embodiment also provides a hard carbon material for sodium-ion batteries prepared by the above method, and a scanning electron microscope image of the hard carbon material is shown below. Figure 3 As shown.

[0029] Example 2 A method for preparing a hard carbon material for sodium-ion batteries includes the following steps: (1) According to the ratio of sawdust, starch, ammonium persulfate and water, 1 g: 0.8 g: 0.08 g: 5 mL, sawdust, starch and ammonium persulfate were added to water and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized to 2 MPa. The mixture was hydrothermally reacted at 160 °C for 6 h. After filtration and drying, the hard carbon precursor was obtained. (2) According to the ratio of hard carbon precursor, yttrium-doped sodium hafnium oxide and ethanol, 1 g: 0.04 g: 1 mL, the hard carbon precursor and the yttrium-doped sodium hafnium oxide from Preparation Example 2 were added to ethanol and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized to 1 MPa. The mixture was then hydrothermally reacted at 150°C for 4 h. After filtration and drying, the composite hard carbon precursor was obtained. (3) The soft carbon-coated silicon hexaboride and the composite hard carbon precursor of Preparation Example 6 were ball-milled and mixed evenly according to the mass ratio of soft carbon-coated silicon hexaboride and composite hard carbon precursor of 1:65. The mixture was then calcined at 800 °C for 5 h in an argon atmosphere and cooled to obtain hard carbon material.

[0030] This embodiment also provides a hard carbon material for sodium-ion batteries prepared by the above method.

[0031] Example 3 A method for preparing a hard carbon material for sodium-ion batteries includes the following steps: (1) According to the ratio of sawdust, starch, ammonium persulfate and water, 1 g: 1 g: 0.12 g: 6 mL, sawdust, starch and ammonium persulfate were added to water and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized at 5 MPa. The mixture was then hydrothermally reacted at 190 °C for 3 h. After filtration and drying, the hard carbon precursor was obtained. (2) According to the ratio of hard carbon precursor, yttrium-doped sodium hafnium oxide and ethanol, 1 g: 0.08 g: 1.2 mL, the hard carbon precursor and the yttrium-doped sodium hafnium oxide of Preparation Example 3 were added to ethanol and dispersed evenly. The mixture was placed in a high-pressure reactor and pressurized at 3 MPa. The mixture was then hydrothermally reacted at 180°C for 2 h. After filtration and drying, the composite hard carbon precursor was obtained. (3) The soft carbon-coated silicon hexaboride and the composite hard carbon precursor of Preparation Example 7 were ball-milled and mixed evenly according to the mass ratio of soft carbon-coated silicon hexaboride and composite hard carbon precursor of 1:85. The mixture was then calcined at 1000 °C for 2 h under an argon atmosphere and cooled to obtain hard carbon material.

[0032] This embodiment also provides a hard carbon material for sodium-ion batteries prepared by the above method.

[0033] (III) Comparative Example Comparative Example 1 Comparative Example 1 is basically the same as Example 1, except that the yttrium-doped sodium hafnium oxide in Example 1 is omitted.

[0034] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that the yttrium-doped sodium hafnium in Example 1 is replaced with sodium hafnium in Preparation Example 4.

[0035] Comparative Example 3 Comparative Example 3 is basically the same as Example 1, except that the soft carbon coating silicon hexaboride in Example 1 is omitted.

[0036] Comparative Example 4 Comparative Example 4 is basically the same as Example 1, except that the soft carbon-coated silicon hexaboride in Example 1 is replaced with silicon hexaboride.

[0037] (iv) Test Examples The hard carbon materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to the following performance tests.

[0038] Using the hard carbon materials prepared in Examples 1-3 or Comparative Examples 1-4 as the negative electrode, the battery was assembled in an argon glove box in the following order: positive electrode shell, positive electrode, separator, electrolyte, negative electrode, gasket, spring, and negative electrode shell.

[0039] The negative electrode is prepared by mixing hard carbon material, PVDF and conductive carbon black in a ratio of 8:1:1, dissolving it in an appropriate amount of N-methylpyrrolidone, grinding and dispersing it evenly again, coating it on the surface of aluminum foil, drying it in a vacuum drying oven, and then cutting it to obtain the negative electrode; the positive electrode is the cut metallic sodium; the electrolyte is 1M NaPF6-EC / DMC (v / v, 1:1).

[0040] The assembled battery was tested using the Blue Electric test system with a voltage of 0-2 V. After standing for 12 hours, it was activated by cycling at 0.1 C for 5 cycles. After activation, it was stood for 1 minute and then 0.1 C and 1 C charge-discharge cycle tests were performed. The initial efficiency, rate performance and cycle stability were recorded. The rate performance was reflected by the initial discharge specific capacity at 0.1 C and 1 C. The cycle stability was reflected by the capacity retention rate after 50 cycles at 1 C.

[0041] As shown in Table 1, the sodium-ion battery hard carbon materials prepared in Examples 1-3 of this invention have high initial discharge specific capacity at both 0.1C and 1C rates, and excellent capacity retention after 50 cycles. This indicates that the hard carbon material has both high initial efficiency, good rate performance and excellent cycle stability.

[0042] Compared to Example 1, Comparative Example 1 omitted yttrium-doped sodium hafnium, resulting in a significant decrease in its initial efficiency, 0.1C initial discharge specific capacity, and 1C initial discharge specific capacity. Furthermore, the 1C / 0.1C capacity ratio decreased from 76.0% to 52.1%. This indicates that the addition of yttrium-doped sodium hafnium significantly improves the initial efficiency and rate performance of hard carbon materials, enhancing cycle performance. This is because the three-dimensional structure of the doped sodium hafnium provides a rapid transport channel for sodium ions. Simultaneously, the lattice distortion and charge imbalance generated by yttrium doping increase the number of active sites, significantly improving the diffusion rate of sodium ions and the rate performance of the material. In addition, yttrium doping enhances the structural stability of the material, effectively reducing irreversible capacity loss, thereby improving the initial efficiency.

[0043] Compared to Example 1, Comparative Example 2, which replaced yttrium-doped sodium hafnium oxide with undoped sodium hafnium oxide, showed significantly lower initial efficiency, 0.1C capacity, and 1C capacity compared to Example 1. The capacity retention after 50 cycles also decreased from 95.2% to 89.6%. This indicates that while undoped sodium hafnium oxide possesses certain three-dimensional ion channels, it lacks the lattice distortion and charge imbalance effects introduced by yttrium doping, failing to provide sufficient sodium ion transport sites. Furthermore, its insufficient mechanical strength and stability lead to accelerated structural degradation during long-term cycling. These results demonstrate that yttrium doping is crucial for improving the electrochemical activity and structural stability of sodium hafnium oxide.

[0044] Compared to Example 1, Comparative Example 3 omitted the soft carbon coating on silicon hexaboride. Although the initial efficiency remained at 88.7%, the 0.1C and 1C capacities were significantly reduced, especially the capacity retention after 50 cycles, which plummeted to 77.6%, far lower than in Example 1. This indicates that without the soft carbon coating on silicon hexaboride, the hard carbon material experienced severe volume expansion during cycling, damaging its structural integrity and leading to rapid capacity decay. Comparative Example 4 added uncoated silicon hexaboride, increasing the cycle retention to 81.8%, but still significantly lower than in Example 1. This demonstrates that while silicon hexaboride alone provides some structural buffering and ion conduction, its interfacial compatibility with the hard carbon matrix is ​​poor, and its conductivity is insufficient. Soft carbon coating, on the other hand, not only forms a stable shell structure and improves interfacial compatibility but also constructs a highly efficient conductive network. Simultaneously, the flexible buffering effect of the soft carbon effectively suppresses the volume effect, thereby synergistically improving rate performance and long-cycle stability.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A method for preparing a hard carbon material for sodium-ion batteries, characterized in that, Includes the following steps: (1) Add sawdust, starch and ammonium persulfate to water and disperse evenly. Perform hydrothermal reaction, filter and dry to obtain hard carbon precursor; (2) The hard carbon precursor and yttrium-doped sodium hafnium oxide were added to ethanol and dispersed evenly. After hydrothermal reaction, the composite hard carbon precursor was obtained by filtration and drying. (3) The composite hard carbon precursor and soft carbon-coated silicon hexaboride are mixed evenly, calcined in an inert gas atmosphere, and then cooled to obtain hard carbon material.

2. The method for preparing hard carbon material for sodium-ion batteries according to claim 1, characterized in that, The yttrium-doped sodium hafnium oxide mentioned in step (2) is prepared by the following steps: Hafnium hydroxide, sodium nitrate, and yttrium oxide were added to water and dispersed evenly. After evaporating the solvent, the reaction was carried out under heat to obtain yttrium-doped sodium hafnium oxide.

3. The method for preparing hard carbon material for sodium-ion batteries according to claim 2, characterized in that, The molar ratio of yttrium oxide, hafnium hydroxide, and sodium nitrate is 1:(3-4):(13-16); the temperature of the heat preservation reaction is 800-900 ℃, and the time is 3-6 h.

4. The method for preparing hard carbon material for sodium-ion batteries according to claim 1, characterized in that, The soft carbon-coated silicon hexaboride described in step (3) is prepared by the following steps: (a) Silicon dioxide, boron anhydride, boron carbide and graphite were ball-milled and mixed, pressed into shape and vacuum sintered, cooled and ground to obtain silicon hexaboride; (b) Silicon hexaboride and pitch are ball-milled and mixed, sintered in an inert gas atmosphere, cooled and carbonized to obtain soft carbon-coated silicon hexaboride.

5. The method for preparing hard carbon material for sodium-ion batteries according to claim 4, characterized in that, The molar ratio of silicon dioxide, boron anhydride, boron carbide and graphite in step (a) is 1:(1-1.2):(1.2-1.4):(6-6.5); the vacuum sintering temperature is 1100-1300 ℃ and the time is 2-5 h.

6. The method for preparing hard carbon material for sodium-ion batteries according to claim 4, characterized in that, In step (b), the mass ratio of silicon hexaboride to pitch is 1:(0.04-0.1); the sintering temperature is 500-600 ℃ and the time is 2-5 h; the carbonization temperature is 1000-1100 ℃ and the time is 4-6 h.

7. The method for preparing hard carbon material for sodium-ion batteries according to claim 1, characterized in that, The mass ratio of sawdust, starch and ammonium persulfate in step (1) is 1:(0.8-1):(0.08-0.12); the hydrothermal reaction temperature is 160-190 ℃, the time is 3-6 h, and the pressure is 2-5 MPa.

8. The method for preparing hard carbon material for sodium-ion batteries according to claim 1, characterized in that, In step (2), the mass ratio of the hard carbon precursor to yttrium-doped sodium hafnium is 1:(0.04-0.08); the hydrothermal reaction is carried out at a temperature of 150-180 °C for 2-4 h and a pressure of 1-3 MPa.

9. The method for preparing hard carbon material for sodium-ion batteries according to claim 1, characterized in that, In step (3), the mass ratio of soft carbon-coated silicon hexaboride and composite hard carbon precursor is 1:(65-85); the calcination temperature is 800-1000 ℃ and the time is 2-5 h.

10. Hard carbon material for sodium-ion batteries prepared by the preparation method according to any one of claims 1-9.

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