Nitrogen / sulfur-doped hard carbon negative electrode material and preparation method thereof

Abstract

The invention discloses a nitrogen / sulfur-doped hard carbon negative electrode material and a preparation method thereof. The preparation method comprises the following steps: 1) placing chitin particles on a cellulose pulp board, and carrying out medium and low temperature pyrolysis carbonization to obtain nitrogen-doped precursor carbon; 2) dispersing the nitrogen-doped precursor carbon in a strong oxidation solution, carrying out oxidation and doping reactions, and washing and drying to obtain nitrogen / sulfur-doped expanded carbon; and 3) performing high-temperature carbonization on the nitrogen / sulfur-doped expanded carbon, and performing washing, ball milling and drying to obtain the nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material. The nitrogen / sulfur doped hard carbon sodium ion battery negative electrode material has high specific capacity, excellent rate capability and cycling stability. The raw materials are renewable, the cost is low, and the preparation method is flexible and adjustable.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and more specifically, relates to a nitrogen / sulfur doped hard carbon negative electrode material and a preparation method thereof. Background Art

[0002] With the continuous exploitation of fossil fuels, energy transformation is imminent, and lithium-ion batteries are the mainstream batteries today. However, the reserves of lithium resources are mainly distributed overseas, and the price of lithium resources fluctuates greatly with international turmoil. In order to stabilize the needs of sustainable development strategies, sodium-ion batteries have gradually come into people's view and are expected to become a substitute for lithium-ion batteries. First of all, sodium is abundant on the earth, the price of sodium resources is lower than that of lithium resources, and sodium ions and lithium ions are both in the first main group and have similar chemical and physical properties. Therefore, sodium-ion batteries can be used as a new type of energy storage battery, and have broad application prospects in the field of large-scale energy storage and the low-speed new energy vehicle market.

[0003] The charging and discharging principles of sodium-ion batteries are similar to those of lithium-ion batteries, but the atomic radius of sodium ions is higher than that of lithium ions, which means that graphite materials cannot be directly applied to sodium-ion batteries. Hard carbon materials are considered to be one of the most promising negative electrode materials for sodium-ion batteries because of their high degree of disorder and the fact that they will not be converted into graphite under high-temperature carbonization. Compared with graphite, they have higher interlayer spacing and disordered pore structure. The precursors of hard carbon come from a wide range of sources, mainly including petroleum-based phenolic resins, asphalt, and biomass-based materials. Among them, chitin is a polysaccharide polymer biomass composed of β-(1-4)-poly-N-acetyl-D-glucosamine, which is widely found in nature, such as fungal cell walls and crustacean exoskeletons. Hao et al. prepared N-doped amorphous carbon nanofibers by simple direct pyrolysis of pure chitin. The hard carbon negative electrode has a reversible specific capacity of up to 320.6 mAh / g at a current of 0.05 A / g. At a current density of 1 A / g, the electrode still provides a reversible specific capacity of 105 mAh / g after 8000 cycles. Its electrochemical performance is mainly attributed to the unique one-dimensional mesoporous nanofibers that have a synergistic effect in promoting electron / electrolyte transport, and the N-doped amorphous nanostructure increases the conductivity and the number of active sites (Nano Energy 2018, 45, 220-228). However, the N element rich in chitin itself is rarely retained after high-temperature carbonization. The hard carbon negative electrode obtained by direct pyrolysis of chitin has the characteristics of low specific surface area, small (002) crystal plane spacing, low conductivity, and non-porous or less porous structure, resulting in poor rate performance and cycle stability. It is well known that heteroatom doping such as nitrogen, phosphorus, and sulfur is an effective way to improve the sodium storage performance of hard carbon. Nitrogen, phosphorus, and sulfur doping can improve the electronic conductivity of hard carbon or expand the (002) crystal plane spacing, thereby increasing the capacity of hard carbon. Hong et al. used asphalt as the carbon source and Na2S2O3 as the S source to synthesize S-doped hard carbon by molten salt method. The (002) crystal plane spacing of hard carbon materials is as high as The sulfur content of the carbon material is about 6.3wt%, and the reversible specific capacity reaches 420mAh / g at a current density of 0.05A / g; after 100 cycles at a current density of 0.1A / g, the reversible capacity of 320mAh / g remains stable; after 4000 cycles at a current density of 1A / g, the battery still retains a reversible specific capacity of about 200mAh / g (Advanced Materials 2018, 1802035). Summary of the invention

[0004] In view of the problem that the current hard carbon negative electrode materials have poor rate performance and cycle stability, one of the purposes of the present invention is to provide a nitrogen / sulfur doped hard carbon negative electrode material, which has high rate performance and excellent cycle stability. Another purpose of the present invention is to provide a method for preparing the above negative electrode material, wherein the main raw material chitosan used in the method is renewable and low in cost, and the preparation method is flexible and adjustable.

[0005] In order to achieve the above object, the technical solution of the present invention is specifically implemented as follows:

[0006] 1. A method for preparing a nitrogen / sulfur doped hard carbon negative electrode material, characterized in that it comprises the following steps:

[0007] (1) placing chitosan particle powder on a cellulose pulp board, and subjecting it to medium-low temperature pyrolysis and carbonization treatment under the protection of an inert gas to obtain nitrogen-doped precursor carbon;

[0008] (2) dispersing the nitrogen-doped precursor carbon obtained in step (1) in a strong oxidizing solution, stirring in an oil bath at an appropriate temperature to carry out oxidation and doping reactions, and obtaining nitrogen / sulfur-doped expanded carbon after washing and drying;

[0009] (3) The nitrogen / sulfur-doped expanded carbon obtained in step (2) is carbonized at high temperature under the protection of an inert gas, and then ball-milled, washed, and dried to obtain a nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material.

[0010] Furthermore, in the step (1), the chitosan is one or more of α-chitosan, β-chitosan, γ-chitosan and deacetylated chitosan; and the cellulose pulp board is one of a broadleaf pulp board and a coniferous pulp board.

[0011] Furthermore, in step (1), the inert gas is one of nitrogen (N2) and argon (Ar); the medium-low temperature carbonization treatment temperature is 350-900°C, and the carbonization time is 4-12h.

[0012] Furthermore, in step (2), the strong oxidizing solution is a mixed solution of ammonium persulfate, concentrated sulfuric acid (98wt%) and hydrogen peroxide (30wt%); the dosage ratio of the precursor, ammonium persulfate, concentrated sulfuric acid and hydrogen peroxide is 1g: (0.5-3)g: (3-15)mL: (0.5-3)mL.

[0013] Furthermore, in the step (2), the oil bath heating temperature is 40 to 95° C., the stirring rate is 600 to 1200 rpm, and the stirring time is 2 to 10 h.

[0014] Furthermore, in step (2), the detergent is deionized water, the drying temperature is 60 to 100° C., and the drying time is 4 to 10 hours.

[0015] Furthermore, in step (3), the inert gas is one of nitrogen (N2) and argon (Ar), the high-temperature carbonization temperature is 1000-1400°C, and the carbonization time is 4-12 hours; the ball milling speed is 800-1600rpm, and the ball milling time is 1-3 hours; the detergent is deionized water and ethanol, the drying temperature is 60-100°C, and the drying time is 2-10 hours.

[0016] The nitrogen / sulfur doped hard carbon negative electrode material of the present invention has a nitrogen content of 0.5-15wt%, a sulfur content of 0.1-10wt%, and a (002) crystal plane spacing of 0.38-0.45nm.

[0017] The present invention also relates to a sodium ion battery whose negative electrode material is the aforementioned nitrogen / sulfur doped hard carbon.

[0018] Compared with the prior art, the present invention has the following advantages and technical effects:

[0019] (1) The present invention compositely fires chitosan and cellulose. Since cellulose generates gases such as H2, CH4, CO and CO2 during high-temperature pyrolysis, the pore structure of chitosan-based hard carbon and the distribution of graphite crystallites can be effectively adjusted. More closed pores are beneficial to the storage of sodium ions.

[0020] (2) The present invention uses ammonium persulfate as a nitrogen / sulfur dopant to achieve nitrogen / sulfur co-doping of hard carbon. Compared with the hard carbon negative electrode obtained by direct pyrolysis of chitin, which has a low specific surface area, a small (002) crystal plane spacing (0.35nm), a low conductivity, and a non-porous or slightly porous structure, the nitrogen / sulfur doped hard carbon negative electrode material prepared by the present invention has a larger (002) crystal plane spacing (0.393nm), which makes the sodium ion insertion and extraction rate fast, showing excellent rate performance and cycle stability.

[0021] (3) The present invention selects biomass chitosan as the hard carbon precursor, which is abundant in reserves and renewable, is beneficial to the sustainable development of the environment, and has low cost, which is conducive to large-scale production. In addition, chitosan can also be used as a nitrogen doping agent because it contains acetamide groups. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0023] Figure 1 is the XRD pattern of the product obtained in Example 1;

[0024] Figure 2 is the SEM image of the product obtained in Example 1;

[0025] Figure 3 is the XRD pattern of the product obtained in Example 2;

[0026] Figure 4 is the SEM image of the product obtained in Example 2;

[0027] Figure 5 is the XRD pattern of the product obtained in Example 3;

[0028] Figure 6 This is the SEM image of the product obtained in Example 3;

[0029] Figure 7 is the XRD pattern of the product obtained in Comparative Example 1;

[0030] Figure 8 This is the SEM image of the product obtained in Comparative Example 1; DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] The embodiment of the present invention discloses a method for preparing a nitrogen / sulfur doped hard carbon negative electrode material. The nitrogen / sulfur doped hard carbon negative electrode material prepared in the following embodiment is tested as follows:

[0033] (1) X-ray diffraction (XRD) test:

[0034] The test was carried out using a Rigaku-D / max-2550pc X-ray powder diffractometer from Hitachi, Japan, using Cu-Kα as the radiation source and a wavelength of A Ni filter was used, the tube current was 40 mA, the tube voltage was 40 KV, the scanning range was 10° to 90°, the scanning speed was 20° / min, and the step length was 0.08°. The material was placed in a glass slide and flattened, and the glass slide was embedded in the center of the instrument experimental slot for testing; the identification of the phase and the crystal structure information were analyzed by JADE5.0 software.

[0035] (2) Scanning electron microscopy characterization:

[0036] The morphology of the sodium ion battery electrode material prepared in each embodiment was observed using a scanning electron microscope tester of model S-4800 produced by HITACHI with an acceleration voltage of 5KV.

[0037] Example 1

[0038] This embodiment includes the following specific steps:

[0039] (1) Chitosan particles are spread on a cellulose pulp board with a chitosan to cellulose mass ratio of 6:1, placed in a crucible and sent into a tube furnace, argon gas is introduced into the tube furnace at a gas flow rate of 100 sccm, and the temperature is raised to 350°C at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, the temperature is further raised to 700°C at a heating rate of 5°C / min and kept at this temperature for 2 hours to obtain nitrogen-doped composite precursor carbon;

[0040] (2) Weigh 1 g of nitrogen-doped composite precursor carbon and 6 ml of concentrated sulfuric acid solution, place them in an oil bath and mix them evenly, then take 2 g of ammonium persulfate and 2 ml of hydrogen peroxide (30 wt%) into the above mixture, react at a temperature of 60 ° C for 2 h, and stir at a speed of 600 rpm; then wash the above precipitate with deionized water until it is neutral, and then dry it in an oven at 65 ° C for 5 h to obtain nitrogen / sulfur-doped expanded carbon;

[0041] (3) The nitrogen / sulfur doped expanded carbon was placed in a high temperature tube furnace, argon was introduced at a gas flow rate of 100 sccm, the temperature was increased to 1000°C at a heating rate of 5°C / min, and then the temperature was increased to 1300°C at a heating rate of 5°C / min, and the high temperature was kept constant for 2 hours. The obtained product was ball milled to reduce the particle size, the ball milling speed was 800 rpm, the time was 1.5 hours, and the pyrolysis product was then washed by alternating centrifugation with deionized water and ethanol until neutral, and then dried in an oven at 65°C for 8 hours to obtain a hard carbon negative electrode material with a nitrogen content of 3.9wt% and a sulfur content of 0.5wt%.

[0042] Figure 1 This is an X-ray diffraction diagram of a nitrogen / sulfur doped hard carbon negative electrode material obtained in Example 1, wherein the ordinate is the X-ray intensity and the abscissa is the X-ray scanning angle. Figure 1 It can be seen that the negative electrode material has a characteristic peak on the (002) crystal plane at a scanning angle of 22.6°, with a crystal plane spacing of 0.393nm, and a characteristic peak on the (100) crystal plane at a scanning angle of 43°. There are no impurity peaks in the X-ray diffraction pattern, indicating that the negative electrode material is a pure phase substance. The peak intensity is weak and the peak shape is wide, indicating that the obtained material is an amorphous carbon material.

[0043] Figure 2 This is a scanning electron microscope image of a nitrogen / sulfur doped hard carbon negative electrode material obtained in Example 1. The chitin-based hard carbon of the sodium ion negative electrode material obtained in Example 1 was observed to have a particle size of 2-15 μm.

[0044] Example 2

[0045] This embodiment includes the following specific steps:

[0046] (1) Chitosan particles are spread on a cellulose pulp board with a mass ratio of chitosan to cellulose of 5:2, placed in a crucible and sent into a tube furnace, argon gas is introduced into the tube furnace at a gas flow rate of 100 sccm, and the temperature is raised to 350°C at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, the temperature is further raised to 850°C at a heating rate of 5°C / min and kept at this temperature for 2 hours to obtain nitrogen-doped composite precursor carbon;

[0047] (2) Weigh 1 g of nitrogen-doped composite precursor carbon and 6 ml of concentrated sulfuric acid solution, place them in an oil bath and mix them evenly, then take 2 g of ammonium persulfate and 2 ml of hydrogen peroxide (30 wt%) into the above mixture, react at a temperature of 60 ° C for 2 h, and stir at a speed of 600 rpm; then wash the above precipitate with deionized water until it is neutral, and then dry it in an oven at 65 ° C for 5 h to obtain nitrogen / sulfur-doped expanded carbon;

[0048] (3) The nitrogen / sulfur doped expanded carbon was placed in a high temperature tube furnace, argon was introduced at a gas flow rate of 100 sccm, the temperature was increased to 1000°C at a heating rate of 5°C / min, and then the temperature was increased to 1200°C at a heating rate of 5°C / min, and the high temperature was kept constant for 2 hours. The obtained product was ball milled to reduce the particle size, the ball milling speed was 800 rpm, the time was 1.5 hours, and the pyrolysis product was then washed by alternating centrifugation with deionized water and ethanol until neutral, and then dried in an oven at 65°C for 8 hours to obtain a hard carbon negative electrode material with a nitrogen content of 3.9wt% and a sulfur content of 0.5wt%.

[0049] Figure 3 This is an X-ray diffraction diagram of a nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material obtained in Example 2, wherein the ordinate is the X-ray intensity and the abscissa is the X-ray scanning angle. Figure 3 It can be seen that the negative electrode material has a characteristic peak on the (002) crystal plane at a scanning angle of 22.95°, with a crystal plane spacing of 0.387nm, and a characteristic peak on the (100) crystal plane at a scanning angle of 43.15°. The peak intensity is weak and the peak shape is wide, indicating that the obtained material is an amorphous carbon material.

[0050] Figure 4 This is a scanning electron microscope image of a nitrogen / sulfur doped hard carbon negative electrode material obtained in Example 2. The chitin-based hard carbon of the sodium ion negative electrode material obtained in Example 2 was observed to have a particle size of 2-15 μm.

[0051] Example 3

[0052] This embodiment includes the following specific steps:

[0053] (1) Chitosan particles are spread on a cellulose pulp board with a chitosan to cellulose mass ratio of 6:1, placed in a crucible and sent into a tube furnace, argon gas is introduced into the tube furnace at a gas flow rate of 100 sccm, and the temperature is raised to 350°C at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, the temperature is further raised to 900°C at a heating rate of 5°C / min and kept at this temperature for 2 hours to obtain nitrogen-doped composite precursor carbon;

[0054] (2) Weigh 1 g of nitrogen-doped composite precursor carbon and 10 ml of concentrated sulfuric acid solution, place them in an oil bath and mix them evenly, then add 2 g of ammonium persulfate and 2 ml of hydrogen peroxide (30 wt%) to the mixture, react at 60 ° C for 2 h, and stir at 600 rpm; then wash the precipitate with deionized water until it is neutral, and then dry it in an oven at 65 ° C for 5 h to obtain nitrogen / sulfur-doped expanded carbon;

[0055] (3) The nitrogen / sulfur doped expanded carbon was placed in a high temperature tube furnace, argon was introduced at a gas flow rate of 100 sccm, the temperature was increased to 1000°C at a heating rate of 5°C / min, and then the temperature was increased to 1400°C at a heating rate of 5°C / min, and the high temperature was kept constant for 2 hours. The obtained product was ball milled to reduce the particle size, the ball milling speed was 800 rpm, the time was 1.5 hours, and the pyrolysis product was then centrifuged and washed alternately with deionized water and ethanol to neutrality, and then dried in an oven at 65°C for 8 hours to obtain a hard carbon negative electrode material with a nitrogen content of 2.9wt% and a sulfur content of 1wt%.

[0056] Figure 5 This is an X-ray diffraction diagram of a nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material obtained in Example 3, wherein the ordinate is the X-ray intensity and the abscissa is the X-ray scanning angle. Figure 5 It can be seen that the negative electrode material has a characteristic peak on the (002) crystal plane at a scanning angle of 23.11°, with a crystal plane spacing of 0.385nm, and a characteristic peak on the (100) crystal plane at a scanning angle of 43.7°. The peak intensity is weak and the peak shape is wide, indicating that the obtained material is an amorphous carbon material.

[0057] Figure 6 This is a scanning electron microscope image of a nitrogen / sulfur doped hard carbon negative electrode material obtained in Example 3. The chitin-based hard carbon of the sodium ion negative electrode material obtained in Example 3 was observed to have a particle size of 2-15 μm.

[0058] Comparative Example 1

[0059] This comparative example comprises the following specific steps:

[0060] (1) Chitosan particles were placed in a crucible and sent into a tube furnace. Argon gas was introduced into the tube furnace at a gas flow rate of 100 sccm, and the temperature was raised to 350°C at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, the temperature was further raised to 700°C at a heating rate of 5°C / min and kept at this temperature for 2 hours. Nitrogen-doped precursor carbon was obtained;

[0061] (2) Weigh 1 g of precursor carbon and 6 ml of concentrated sulfuric acid solution and mix them evenly in an oil bath at 60° C. and 450 rpm. Then, add 2 g of ammonium persulfate and 2 ml of hydrogen peroxide (30 wt%) to the mixture. The components are reacted at 60° C. for 2 h and then allowed to stand for 24 h to obtain nitrogen / sulfur doped expanded carbon.

[0062] (3) The nitrogen / sulfur doped expanded carbon was placed in a high temperature tube furnace, argon was introduced at a gas flow rate of 100 sccm, the temperature was increased to 1000°C at a heating rate of 5°C / min, and then the temperature was increased to 1300°C at a heating rate of 5°C / min, and the high temperature was kept constant for 2 hours. The obtained product was ball milled to reduce the particle size, the ball milling speed was 800 rpm, the time was 1.5 hours, and the pyrolysis product was then washed by alternating centrifugation with deionized water and ethanol until neutral, and then dried in an oven at 65°C for 8 hours to obtain a hard carbon sodium ion battery negative electrode material with a nitrogen content of 4.4wt% and a sulfur content of 0.4wt%.

[0063] Figure 7 This is an X-ray diffraction diagram of a nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material obtained in Comparative Example 1, wherein the ordinate is the X-ray intensity and the abscissa is the X-ray scanning angle. Figure 7 It can be seen that there is a characteristic peak on the (002) crystal plane at 25.3°, the interplanar spacing is 0.35nm, there is a characteristic peak on the (100) crystal plane at a scanning angle of 44°, and there are no impurity peaks in the X-ray diffraction pattern, indicating that the negative electrode material is a pure phase substance, the peak intensity is weak, and the peak shape is wide, indicating that the obtained material is an amorphous carbon material.

[0064] Figure 8 This is a scanning electron microscope image of a nitrogen / sulfur doped hard carbon negative electrode material obtained in Comparative Example 1. The chitin-based hard carbon of the sodium ion negative electrode material obtained in Comparative Example 1 was observed to have a particle size of 1-10 μm.

[0065] In this specification, various embodiments and comparative examples are described in a parallel reference manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts between the various embodiments can be referenced to each other.

[0066] A nitrogen / sulfur-doped hard carbon sodium ion battery negative electrode material prepared in each embodiment and comparative example was used as the negative electrode active material, the negative electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent (Super P) were mixed evenly in a mass ratio of 8:1:1, evenly coated on aluminum foil into a thin layer, and cut into discs after drying as the negative electrode material, a metallic sodium sheet was used as the counter electrode, Whatman glass fiber was used as a diaphragm, and the electrolyte was a mixed solution of 1M sodium hexafluorophosphate (NaPF6) of fluoroethylene carbonate (FEC), diethyl carbonate (DEC) and ethylene carbonate (EC) (volume ratio of 5:47.5:47.5), and CR2032 button batteries were assembled in an argon glove box.

[0067] The button battery was tested using a blue battery tester produced by Wuhan Jinnuo Electronics Co., Ltd. The test conditions and results are as follows:

[0068] The button battery was subjected to constant current charge and discharge test with a charge and discharge voltage range of 0 to 2.5 V. At a current density of 30 mA / g, the initial specific capacity was 240 to 320 mAh / g. At a current density of 1 A / g, the initial specific capacity was 110 to 227 mAh / g. The specific values ​​are shown in Table 1.

[0069] Table 1 Electrochemical properties of nitrogen-sulfur doped hard carbon electrodes

[0070]

Claims

1. A nitrogen / sulfur doped hard carbon negative electrode material, characterized in that: The negative electrode material has a nitrogen content of 0.5-15 wt %, a sulfur content of 0.1-10 wt %, and a hard carbon (002) crystal plane spacing of 0.38-0.45 nm.

2. A method for preparing a nitrogen / sulfur doped hard carbon negative electrode material, characterized in that: The following steps are involved: (1) placing chitosan particle powder on a cellulose pulp board, and pyrolyzing and carbonizing the chitosan particle powder at a medium to low temperature under the protection of an inert gas to obtain a nitrogen-doped precursor carbon; (2) dispersing the nitrogen-doped precursor carbon obtained in step (1) in a strong oxidizing solution, stirring in an oil bath at an appropriate temperature to carry out oxidation and doping reactions, and obtaining nitrogen / sulfur-doped expanded carbon after washing and drying; (3) The nitrogen / sulfur-doped expanded carbon obtained in step (2) is carbonized at high temperature under the protection of an inert gas, and then ball-milled, washed, and dried to obtain a nitrogen / sulfur-doped hard carbon negative electrode material.

3. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (1), the chitosan is one or more of α-chitosan and β-chitosan; and the cellulose pulp board is one of a broadleaf pulp board and a coniferous pulp board.

4. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (1), the inert gas is one of nitrogen (N2) and argon (Ar); the medium-low temperature carbonization temperature is 350-900°C, and the carbonization time is 4-12 hours.

5. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (2), the strong oxidizing solution is a mixed solution of concentrated sulfuric acid (98 wt%) and hydrogen peroxide (30 wt%); the dosage ratio of the precursor, ammonium persulfate, concentrated sulfuric acid and hydrogen peroxide is 1 g: (0.5-3) g: (3-15) mL: (0.5-3) mL.

6. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (2), the oil bath heating temperature is 40 to 95° C., the stirring rate is 600 to 1200 rpm, and the stirring time is 2 to 10 hours.

7. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (2), the detergent is deionized water, the drying temperature is 60-100° C., and the drying time is ≥4 hours.

8. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (3), the inert gas is one of nitrogen (N2) and argon (Ar), the high-temperature carbonization temperature is 1000-1400°C, and the carbonization time is 4-12 hours.

9. The method for preparing the nitrogen / sulfur doped hard carbon negative electrode material according to claim 1, characterized in that: In the step (3), the ball milling speed is 800-1600 rpm, and the ball milling time is 1-3 hours; the detergent is deionized water and ethanol, the drying temperature is 60-100° C., and the drying time is 2-10 hours.

10. A sodium ion battery comprising the nitrogen / sulfur doped hard carbon negative electrode material according to any one of claims 1 to 2.

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