Hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention discloses a hard carbon negative electrode material and a preparation method and application thereof. Sawdust particles are smashed, sieved and dried for 1-2 h at the temperature of 115-125 DEG C; the preparation method comprises the following steps: in a nitrogen atmosphere, heating to 500-600 DEG C at a rate of 40-50 DEG C / min, cooling to 200 DEG C at a rate of 6-10 DEG C / min, then heating to 300 DEG C at a rate of 2-3 DEG C / min, preserving heat for 6-8 hours, continuously heating to 700 DEG C at a rate of 2-3 DEG C / min, preserving heat for 1-2 hours to obtain hard carbon particles, cooling to less than or equal to 40 DEG C, crushing, pickling with hydrochloric acid, washing with water until the pH value is greater than or equal to 6.9, filtering, drying, stirring with a sodium-doped material in water, dispersing, and drying to obtain the sodium-doped carbon material. And heating to 1100-1200 DEG C at the rate of 2-3 DEG C / min, preserving heat for 6-8 hours, and cooling to obtain the hard carbon negative electrode material. The hard carbon particles prepared by the method can better act with sodium salt, show better gram volume and first efficiency in the battery, avoid the use of metal sodium, improve the operation safety, and show high specific capacity, high first efficiency and high cycle performance when being used for manufacturing the negative electrode of the sodium ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery negative electrode materials, and in particular relates to a hard carbon negative electrode material and a preparation method and application thereof. Background Art

[0002] With the development of economy and society, the demand for new energy batteries is increasing, which makes the mainstream product lithium-ion batteries increasingly unable to meet the needs of large-scale electrochemical energy storage. However, the reserves of sodium resources are much greater than those of lithium resources, and sodium and lithium are in the same family and have many commonalities. Researchers have conducted research on sodium-ion batteries, which are expected to become an effective supplement to lithium-ion batteries. At present, many problems need to be solved in order to realize the application of sodium-ion batteries, among which the low first coulomb efficiency is one of the important technical barriers restricting its application.

[0003] The key to solving the first coulombic efficiency of sodium-ion batteries lies in the selection and preparation of negative electrode materials. Amorphous carbon, including hard carbon, soft carbon and graphite, is the first choice for negative electrode materials. Since the radius of sodium ions is more than 35% larger than that of lithium ions, graphite materials that can be well embedded with lithium ions cannot carry sodium ions well due to too small voids. Soft carbon with graphitized structure also has the same problem. The interlayer spacing and pores of hard carbon are larger than those of graphite and soft carbon, resulting in more active sites for sodium storage, higher specific capacity, smaller volume expansion after sodium embedding, and faster charging. Therefore, hard carbon is the first choice for negative electrodes of sodium-ion batteries. As a negative electrode, hard carbon also has the following defects: During the first charge and discharge, the hard carbon negative electrode will form a SEI film due to excessive consumption of sodium ions, resulting in a large amount of sodium ion loss; the residual hydrogen-terminated aromatic fragments in the hard carbon negative electrode will combine with sodium ions, resulting in high-voltage potential movement and voltage hysteresis. At present, in order to solve the above defects, the mainstream treatment method is to perform pre-sodium treatment during the preparation of hard carbon negative electrode, and certain results have been achieved, such as: Patent CN115249795A discloses a pre-sodiumized negative electrode plate and its pre-sodium method, which is to mix an aromatic hydrocarbon compound with an ether solvent, add metallic sodium to obtain a pre-sodiumized solution, and then soak the negative electrode plate to obtain a pre-sodiumized negative electrode plate. Patent CN115893369A discloses a hard carbon negative electrode material and its preparation method, which is to perform a first sintering of the hard carbon raw material under an inert atmosphere to obtain a precursor, and then mix it with a lithium source or a sodium source for a second sintering, so that the precursor contains lithium or sodium element, and obtain a modified hard carbon precursor.

[0004] Patent CN117466282A discloses a pre-sodiumized hard carbon material and a pre-sodiumization method thereof, wherein an organic carbon source such as glucose and starch, a surfactant, a weak base and a soluble sodium salt are added to a solvent alcohol and mixed evenly, and then sintered and carbonized in an ion sintering furnace to obtain a pre-sodiumized hard carbon material. Patent CN117954629A discloses a negative electrode material for an ion battery and a preparation method thereof, wherein hard carbon, soft carbon and a pore-forming agent are mixed and sintered in a certain proportion to obtain a negative electrode material intermediate with a porous structure, and then pre-sodiumized with metallic sodium to obtain a negative electrode material.

[0005] The existing technology mainly performs pre-sodium treatment on the hard carbon negative electrode to improve the initial coulombic efficiency of the material. The adsorption of sodium ions during the pre-sodium treatment process depends largely on the pore conditions of the hard carbon. Therefore, if you want to improve the effect of pre-sodium treatment, it is not only about the treatment of the pre-sodium step, but also closely related to the pore size of the hard carbon. Although the above patents also provide some preparation methods for multi-aperture hard carbon, these methods either require too high operating precision or have high raw material costs, which are not conducive to industrial promotion. Summary of the invention

[0006] In view of the problem of low first coulombic efficiency of hard carbon negative electrode materials in the prior art, the present invention provides a hard carbon negative electrode material and a preparation method and application thereof. The sodium ion battery prepared with the hard carbon negative electrode material exhibits high gram capacity and high first efficiency.

[0007] The present invention is achieved through the following technical solutions: A method for preparing a hard carbon negative electrode material comprises the following steps (1) Crush the sawdust particles, pass them through a 40-mesh sieve, and dry the sieved sawdust particles at 115-125°C for 1-2 hours; (2) In a nitrogen atmosphere, the sawdust particles dried in step (1) are heated to 500-600°C at a rate of 40-50°C / min, cooled to 200°C at a rate of 6-10°C / min, then heated to 300°C at a rate of 2-3°C / min, kept at this temperature for 6-8 hours, and then heated to 700°C at a rate of 2-3°C / min, kept at this temperature for 1-2 hours, to obtain hard carbon particles; (3) The hard carbon particles are naturally cooled to ≤40°C and then taken out and crushed. The crushed hard carbon powder is acid-washed with hydrochloric acid, then washed with water until the pH value is ≥6.9, filtered and dried; (4) The dried hard carbon powder and sodium-doped material are stirred and dispersed in water, dried, heated to 1100-1200°C at a rate of 2-3°C / min, kept at this temperature for 6-8h, and cooled to obtain a hard carbon negative electrode material.

[0008] Furthermore, the particle size distribution range of the hard carbon powder after crushing in step (3) is D10≥2μm, 4μm<D50<15μm, and D90≤45μm.

[0009] Furthermore, in step (3), the concentration of hydrochloric acid is 1-2 mol / L, and the amount of hydrochloric acid used is 2-3 times the mass of the crushed hard carbon powder.

[0010] Furthermore, the sodium doping material in step (4) is one of sodium acetate, sodium dihydrogen phosphate, sodium citrate, sodium bromate, sodium glucose, sodium citrate, and sodium ethylenediaminetetramethylenephosphonate.

[0011] Furthermore, in step (4), the mass ratio of the sodium-doped material to the dried hard carbon powder is 0.1-0.3:100; and the amount of water used is 2-3 times the mass of the dried hard carbon powder.

[0012] Furthermore, the sawdust particles in step (1) are sawdust particles from low-oil trees, and the low-oil trees are poplar and / or locust wood.

[0013] Furthermore, in step (3), the drying condition is 95-105° C. for 1-2 h; and in step (4), the drying condition is 90-95° C. until dry.

[0014] In the present invention, the hard carbon negative electrode material is prepared by the above-mentioned preparation method.

[0015] In the present invention, the hard carbon negative electrode material is used in manufacturing the negative electrode of a sodium ion battery.

[0016] Furthermore, the sodium ion battery negative electrode comprises 96 wt % of hard carbon negative electrode material, 1 wt % of conductive carbon black, 1.5 wt % of dispersant and 1.5 wt % of binder.

[0017] Beneficial effects of the present invention: The present invention adopts a method of first rapidly heating, then cooling, and then slowly heating to prepare hard carbon negative electrode materials. Compared with direct slow heating, the prepared hard carbon particles can better react with sodium salts, and the prepared hard carbon negative electrode materials show better gram capacity and first efficiency in the battery; and avoid the use of metallic sodium, thereby improving the safety of operation. The hard carbon negative electrode material prepared by the present invention is used to make the negative electrode of a sodium ion battery, and shows high specific capacity, high first efficiency and high cycle performance. DETAILED DESCRIPTION

[0018] The above contents of the present invention are further described in detail below through examples, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following examples, and all technologies implemented based on the above contents of the present invention belong to the scope of the present invention. Example 1

[0019] (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 116±1°C for 1 h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 40°C / min to rapidly heat up to 500°C, then cooling down to 200°C at a rate of 6°C / min, and then heating up to 300°C at a rate of 2°C / min, keeping the temperature for 6 h, and then heating up to 700°C at a rate of 2°C / min, keeping the temperature for 1 h, to obtain hard carbon particles; (3) Turn off the power supply, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10=3.5μm, D50=10.3μm, D90=37.8μm), weigh 200g of the above hard carbon powder, pour in 400g of 1mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 6.92, filter and dry at 96±1°C for 1h; (4) Add 100 g of dried hard carbon powder and 0.52 g of sodium dihydrogen phosphate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 90±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 2°C / min. Keep it warm for 6 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Example 2

[0020] (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 124±1°C for 2 h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 50°C / min to rapidly heat up to 600°C, then cooling down to 200°C at a rate of 10°C / min, and then heating up to 300°C at a rate of 3°C / min, keeping the temperature for 8 hours, and then heating up to 700°C at a rate of 3°C / min, keeping the temperature for 2 hours, to obtain hard carbon particles; (3) Turn off the power, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10=3.2μm, D50=10.5μm, D90=36.6μm), weigh 200g of the above hard carbon powder, pour in 600g of 2mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 7.01, filter and dry at 104±1°C for 2h; (4) Add 100 g of dried hard carbon powder and 1.57 g of sodium dihydrogen phosphate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 94±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 3°C / min. Keep it warm for 8 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Example 3

[0021] (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 116±1°C for 1 h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 40°C / min to rapidly heat up to 500°C, then cooling down to 200°C at a rate of 6°C / min, and then heating up to 300°C at a rate of 2°C / min, keeping the temperature for 6 h, and then heating up to 700°C at a rate of 2°C / min, keeping the temperature for 1 h, to obtain hard carbon particles; (3) Turn off the power supply, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10 = 2.9 μm, D50 = 8.8 μm, D90 = 39.2 μm), weigh 200 g of the above hard carbon powder, pour in 400 g of 1 mol / L hydrochloric acid, stir and soak for 12 h, then wash with pure water until the pH value of the carbon powder filtrate is 6.97, filter and dry at 96 ± 1°C for 1 h; (4) Add 100 g of dried hard carbon powder and 0.59 g of sodium acetate trihydrate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a forced air drying oven and dry at 90±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 2°C / min. Keep it warm for 6 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Example 4

[0022] (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 124±1°C for 2 h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 50°C / min to rapidly heat up to 600°C, then cooling down to 200°C at a rate of 10°C / min, and then heating up to 300°C at a rate of 3°C / min, keeping the temperature for 8 hours, and then heating up to 700°C at a rate of 3°C / min, keeping the temperature for 2 hours, to obtain hard carbon particles; (3) Turn off the power, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10 = 2.7μm, D50 = 10.1μm, D90 = 40.9μm), weigh 200g of the above hard carbon powder, pour in 600g of 2mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 7.04, filter and dry at 104±1°C for 2h; (4) Add 100 g of dried hard carbon powder and 1.77 g of sodium acetate trihydrate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a forced air drying oven and dry at 94±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 3°C / min. Keep it warm for 8 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Example 5

[0023] (1) 300 g of poplar sawdust particles were crushed in a high-speed Chinese medicine grinder for 20 min, sieved with a 40-mesh screen, and the sieved powder was poured into a crucible and dried in a forced air drying oven at 116±1°C for 1 h; (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 40°C / min to rapidly heat up to 500°C, then cooling down to 200°C at a rate of 6°C / min, and then heating up to 300°C at a rate of 2°C / min, keeping the temperature for 6 h, and then heating up to 700°C at a rate of 2°C / min, keeping the temperature for 1 h, to obtain hard carbon particles; (3) Turn off the power supply, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10 = 3.0 μm, D50 = 9.5 μm, D90 = 41.3 μm), weigh 200 g of the above hard carbon powder, pour in 400 g of 1 mol / L hydrochloric acid, stir and soak for 12 h, then wash with pure water until the pH value of the carbon powder filtrate is 6.99, filter and dry at 96 ± 1°C for 1 h; (4) Add 100 g of dried hard carbon powder and 0.66 g of sodium bromate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 90±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 2°C / min. Keep it warm for 6 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Example 6

[0024] (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 124±1°C for 2 h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 50°C / min to rapidly heat up to 600°C, then cooling down to 200°C at a rate of 10°C / min, and then heating up to 300°C at a rate of 3°C / min, keeping the temperature for 8 hours, and then heating up to 700°C at a rate of 3°C / min, keeping the temperature for 2 hours, to obtain hard carbon particles; (3) Turn off the power, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10 = 2.5μm, D50 = 6.7μm, D90 = 35.3μm), weigh 200g of the above hard carbon powder, pour in 600g of 2mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 6.97, filter and dry at 104±1°C for 2h; (4) Add 100 g of dried hard carbon powder and 1.97 g of sodium bromate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 94±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 3°C / min. Keep it warm for 8 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Comparative Example 1

[0025] There is no rapid heating process, and the rest is the same as Example 1, specifically: (1) Put 300 g of poplar sawdust particles into a high-speed Chinese medicine grinder and grind them for 20 min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 116±1°C for 1 h. (2) placing the crucible in an atmosphere muffle furnace, under nitrogen protection, setting the operation to heat up to 300°C at a rate of 2°C / min, keeping the temperature for 6 hours, and then continue to heat up to 700°C at a rate of 2°C / min, keeping the temperature for 1 hour, to obtain hard carbon particles; (3) Turn off the power supply, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10=3.5μm, D50=10.3μm, D90=37.8μm), weigh 200g of the above hard carbon powder, pour in 400g of 1mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 6.92, filter and dry at 96±1°C for 1h; (4) Add 100 g of dried hard carbon powder and 0.52 g of sodium dihydrogen phosphate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 90±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 2°C / min. Keep it warm for 6 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Comparative Example 2

[0026] Pine sawdust was used instead of poplar sawdust, and the rest was the same as in Example 1, specifically: (1) Put 300g of pine sawdust particles into a high-speed Chinese medicine grinder and grind them for 20min. Sieve them with a 40-mesh screen. Pour the sieved powder into a crucible and dry it in a forced air drying oven at 116±1℃ for 1h. (2) placing the crucible in an atmosphere muffle furnace, and under nitrogen protection, setting the operation rate to 40°C / min to rapidly heat up to 500°C, then cooling down to 200°C at a rate of 6°C / min, and then heating up to 300°C at a rate of 2°C / min, keeping the temperature for 6 h, and then heating up to 700°C at a rate of 2°C / min, keeping the temperature for 1 h, to obtain hard carbon particles; (3) Turn off the power supply, wait until the temperature in the atmosphere muffle furnace drops to 40°C, take out the crucible, pour the hard carbon particles in the crucible into a high-speed Chinese medicine grinder, and grind them to a particle size (D10=3.5μm, D50=10.3μm, D90=37.8μm), weigh 200g of the above hard carbon powder, pour in 400g of 1mol / L hydrochloric acid, stir and soak for 12h, then wash with pure water until the pH value of the carbon powder filtrate is 6.92, filter and dry at 96±1°C for 1h; (4) Add 100 g of dried hard carbon powder and 0.52 g of sodium dihydrogen phosphate into a crucible, pour in 200 g of pure water, stir for 2 h, then place the crucible in a blast drying oven and dry at 90±1°C for 2 h. Transfer the crucible into an atmosphere muffle furnace and heat it to 1200°C at a rate of 2°C / min. Keep it warm for 6 h, turn off the power, wait until the temperature in the furnace drops below 40°C, take out the crucible, and the crucible contains the target hard carbon negative electrode material. Performance Testing

[0027] The physical and chemical properties of the hard carbon negative electrode materials prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were tested in accordance with the national standard GB / T43114-2023 for hard carbon negative electrodes. The results are shown in Table 1.

[0028] Table 1 Physical and chemical properties of hard carbon negative electrode materials prepared in Examples 1 to 6 and Comparative Examples 1 to 2

[0029] Electrochemical performance testing of hard carbon negative electrode materials: According to the method provided by the national standard GB / T43114~2023 for hard carbon negative electrodes, button cells were prepared and performance tested. When making the negative electrode of the button cell, the following raw materials and mass ratios were used: 96% hard carbon negative electrode material, 1 wt% Cabot carbon black REGAL330R, 1.5wt% carboxymethyl cellulose (CMC) and 1.5 wt% styrene butadiene latex (SBR). When the negative electrode of the button cell was prepared using the hard carbon negative electrode material prepared in Examples 1~6 and Comparative Examples 1~2 as raw materials, the electrochemical performance test results are shown in Table 2 below: Table 2 Electrochemical performance test results of button batteries

[0030] It can be seen from Tables 1 and 2 that the hard carbon negative electrode material prepared by first rapidly heating, then cooling, and then slowly heating, shows better gram capacity and first efficiency after being made into the negative electrode of the sodium ion battery than the hard carbon negative electrode material prepared by directly slowly heating. The hard carbon negative electrode material prepared by using pine sawdust has a lower first efficiency performance than the hard carbon negative electrode material prepared by using poplar sawdust.

[0031] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing a hard carbon negative electrode material, characterized in that: The following steps are included (1) Crush the sawdust particles, pass them through a 40-mesh sieve, and dry the sieved sawdust particles at 115-125°C for 1-2 hours; (2) In a nitrogen atmosphere, the sawdust particles dried in step (1) are heated to 500-600°C at a rate of 40-50°C / min, cooled to 200°C at a rate of 6-10°C / min, then heated to 300°C at a rate of 2-3°C / min, kept at this temperature for 6-8 hours, and then heated to 700°C at a rate of 2-3°C / min, kept at this temperature for 1-2 hours, to obtain hard carbon particles; (3) The hard carbon particles are naturally cooled to ≤40°C and then taken out and crushed. The crushed hard carbon powder is acid-washed with hydrochloric acid, then washed with water until the pH value is ≥6.9, filtered and dried; (4) The dried hard carbon powder and sodium-doped material are stirred and dispersed in water, dried, heated to 1100-1200°C at a rate of 2-3°C / min, kept at this temperature for 6-8h, and cooled to obtain a hard carbon negative electrode material.

2. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The particle size distribution range of the hard carbon powder after crushing in step (3) is D10≥2μm, 4μm<D50<15μm, and D90≤45μm.

3. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The concentration of hydrochloric acid in step (3) is 1-2 mol / L, and the amount of hydrochloric acid used is 2-3 times the mass of the crushed hard carbon powder.

4. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The sodium doping material in step (4) is one of sodium acetate, sodium dihydrogen phosphate, sodium citrate, sodium bromate, sodium glucose, sodium citrate, and sodium ethylenediaminetetramethylenephosphonate.

5. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: In step (4), the mass ratio of the sodium-doped material to the dried hard carbon powder is 0.1-0.3:100; and the amount of water used is 2-3 times the mass of the dried hard carbon powder.

6. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The sawdust particles in step (1) are sawdust particles from low-oil trees, and the low-oil trees are poplar and / or locust wood.

7. The method for preparing a hard carbon negative electrode material according to claim 1, characterized in that: The drying condition in step (3) is drying at 95-105° C. for 1-2 h; and the drying condition in step (4) is drying at 90-95° C. until dry.

8. A hard carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the hard carbon negative electrode material according to claim 8 in manufacturing a negative electrode for a sodium ion battery.

10. The use according to claim 9, characterized in that The sodium ion battery negative electrode comprises 96 wt % of hard carbon negative electrode material, 1 wt % of conductive carbon black, 1.5 wt % of dispersant and 1.5 wt % of binder.

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