A hard carbon material and preparation method thereof and sodium ion battery

Abstract

The present invention provides a hard carbon material, a preparation method thereof, and a sodium ion battery, and specifically relates to the field of sodium ion battery materials. The preparation method of the hard carbon material comprises the following steps: providing lignin, and pre-treating the lignin to obtain a purified raw material; mixing the purified raw material with a phosphorus source to obtain a precursor; and carbonizing the precursor to obtain the hard carbon material. In the present invention, a mixture of a phosphorus source and lignin is carbonized under an inert atmosphere to obtain a phosphorus-doped hard carbon material. The introduction of phosphorus can change the microstructure of the hard carbon, making it have more defects and a larger carbon layer spacing, providing more active sites for the embedding / de-embedding of sodium atoms, thereby increasing the energy storage capacity and improving the electrochemical performance. The preparation method is simple and the cost is low.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion battery materials, and in particular to a hard carbon material, a preparation method thereof, and a sodium ion battery. Background Art

[0002] Since the commercialization of lithium-ion batteries in the 1990s, they have been widely used in fields such as 3C, energy storage, and power. However, the uneven global distribution and scarcity of lithium resources have hampered the healthy development of lithium-ion batteries. Sodium is widely distributed around the world, has abundant reserves, is widely available, and is inexpensive. Furthermore, sodium-ion batteries operate on similar principles to lithium-ion batteries, both belonging to the "rocking chair" category. With the rapid development of positive and negative electrode materials for sodium-ion batteries in recent years, sodium-ion batteries are expected to replace or complement lithium-ion batteries in areas such as energy storage and low-speed electric vehicles. Graphite, as a negative electrode material for lithium-ion batteries, has excellent electrochemical properties. However, due to thermodynamic reasons, sodium ions have difficulty forming stable intercalation compounds with graphite, resulting in poor application in sodium-ion battery systems.

[0003] Hard carbon, a material suitable for sodium-ion battery anodes, offers advantages such as low cost, ease of preparation, low sodium storage potential, and high capacity, making it the most widely studied anode material for sodium-ion batteries. Lignin is a widely available, abundant, and renewable biomass resource. Currently, lignin is primarily burned to provide heat energy, resulting in relatively low economic returns. Lignin has a relatively high carbon content, and its phenolic hydroxyl groups can undergo chemical modification to form benzoquinone groups. The reversible conversion between the two provides pseudocapacitance, while also promoting electron conduction and ion transfer to a certain extent. Carbon materials function by providing surface adsorption sites for charge storage. By reducing the cost of hard carbon and increasing its energy density, lignin-based carbon materials can be effectively used as sodium-ion battery electrode materials. However, hard carbon materials prepared from lignin have shortcomings in electrochemical properties such as cycling stability and first coulombic efficiency, requiring manipulation of their microstructure. Summary of the Invention

[0004] In view of the above shortcomings of the prior art, the present invention provides a hard carbon material, a preparation method thereof, and a sodium ion battery to improve the problems of insufficient electrochemical performance of hard carbon materials such as cycle stability and first coulombic efficiency.

[0005] To achieve the above-mentioned purpose and other related purposes, the present invention provides a method for preparing a hard carbon material, comprising the following steps: providing lignin and pretreating the lignin to obtain a purified raw material; mixing the purified raw material with a phosphorus source to obtain a precursor; and carbonizing the precursor to obtain the hard carbon material.

[0006] In one example of the present invention, the steps of pretreating the lignin are as follows: dispersing the lignin in water to obtain a lignin suspension; adding an alkaline solution to the lignin suspension, adjusting the pH of the lignin suspension until the lignin is dissolved, and filtering to obtain a lignin filtrate; heating the lignin filtrate in a water bath, and adding an acidic solution to adjust the pH of the lignin filtrate until the lignin precipitates; filtering, washing, drying, grinding, and sieving the precipitated lignin to obtain a purified raw material.

[0007] In one example of the present invention, the lignin includes any one or more of enzymatic lignin, alkali lignin, ethanol lignin, sulfate lignin, and organic lignin.

[0008] In one example of the present invention, the purified raw material is mixed with a phosphorus source to prepare a precursor, which includes: placing the purified raw material in a solvent, stirring evenly to prepare a first stock solution; adding a phosphorus source to the first stock solution, ultrasonically treating it for 1 to 4 hours, and freeze-drying it to obtain the precursor.

[0009] In an example of the present invention, the solvent includes an organic solvent or water, and the organic solvent includes any one or more of ethanol, methanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dioxane.

[0010] In one example of the present invention, the phosphorus source includes any one or more of phosphoric acid, elemental phosphorus, phosphates or phosphate ester compounds; and the mass ratio of the phosphorus source to the purified raw material is 0.5:1 to 2.5:1.

[0011] In one example of the present invention, a pre-oxidation treatment is also included before the precursor is carbonized. The pre-oxidation treatment includes: placing the precursor in a tube furnace in an air atmosphere, raising the temperature in the tube furnace to 150-250°C at a heating rate of 1-10°C / min, keeping it warm for 1-4 hours, and then cooling it to room temperature.

[0012] In one example of the present invention, the precursor is carbonized to obtain a hard carbon material, which includes: placing the pre-oxidized precursor in a tubular furnace in an inert atmosphere, and performing a first-stage carbonization and a second-stage carbonization, wherein the first-stage carbonization includes: raising the temperature to 200-250°C at a heating rate of 3-10°C / min, and keeping warm for 1-4 hours; the second-stage carbonization includes: raising the temperature to 1000-1500°C at a heating rate of 3-10°C / min, and keeping warm for 1-4 hours; after the insulation is completed, cooling to room temperature, and sieving to obtain the hard carbon material.

[0013] The present invention also provides a hard carbon material, which is prepared according to the above preparation method.

[0014] The present invention also provides a sodium ion battery, comprising a positive electrode plate, a negative electrode plate, a separator and an electrolyte, wherein the negative electrode plate comprises a negative electrode current collector and the above-mentioned hard carbon material arranged on the negative electrode current collector.

[0015] The present invention's hard carbon material is produced by carbonizing phosphorus-doped lignin under an inert atmosphere. The introduction of phosphorus modifies the hard carbon's microstructure, resulting in more defects and a larger interlayer spacing. This provides more active sites for the insertion and extraction of sodium atoms, thereby increasing energy storage capacity and improving electrochemical performance. This addresses the issues of low capacity and coulombic efficiency in hard carbon anode materials. Furthermore, the use of lignin, a biomass papermaking waste product, as a carbon source provides an effective approach for the high-value utilization of lignin. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 : A flow chart for preparing the hard carbon material of the present invention;

[0018] Figure 2 : A flow chart of lignin pretreatment in the present invention;

[0019] Figure 3 : Flow chart of the preparation of the precursor in the present invention. DETAILED DESCRIPTION

[0020] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0021] It should be noted that "%" in this specification represents mass percentage.

[0022] See also Figure 1The present invention provides a method for preparing a hard carbon material, which specifically comprises the following steps:

[0023] S1. providing lignin and pretreating the lignin to obtain a purified raw material;

[0024] S2, mixing the purified raw material with a phosphorus source to prepare a precursor;

[0025] S3, carbonizing the precursor to obtain the hard carbon material

[0026] In step S1, the lignin includes any one or more of enzymatic lignin, alkali lignin, ethanol lignin, sulfate lignin, or organic lignin, for example, enzymatic lignin, alkali lignin, organic lignin, or a combination of enzymatic lignin and ethanol lignin, which are not listed here. It should be noted that when the lignin is a combination of two or more, there is no restriction on the ratio between the components in the combination, and they can be mixed in any ratio.

[0027] See also Figure 2 , the steps of pre-treating lignin in step S1 are as follows:

[0028] S11, dispersing the lignin in water to obtain a lignin suspension;

[0029] S12, adding an alkaline solution to the lignin suspension, adjusting the pH of the lignin suspension until the lignin is dissolved, and filtering to obtain a lignin filtrate;

[0030] S13, heating the lignin filtrate in a water bath, and adding an acidic solution to adjust the pH of the lignin filtrate until lignin precipitates;

[0031] S14, filtering, washing, drying, and grinding the precipitated lignin to obtain a purified raw material.

[0032] In step S12, an alkaline solution is added to adjust the lignin suspension to alkaline to dissolve the lignin. Preferably, the alkaline solution is an aqueous solution of sodium hydroxide or potassium hydroxide, and the pH of the suspension is 12.

[0033] In step S13, an acidic solution is added to adjust the lignin filtrate to acidity to precipitate the purified lignin. Preferably, the water bath heating temperature is 70°C; the acidic solution is a sulfuric acid aqueous solution or a hydrochloric acid aqueous solution, and the pH of the lignin filtrate is 2.

[0034] In step S14, the precipitated lignin is filtered and washed to neutrality, followed by drying, grinding, and sieving. The filtration, washing, drying, grinding, and sieving steps can be performed according to conventional procedures in the art. For example, the filtrate can be washed multiple times with deionized water until the filtrate is neutral, and then dried in a drying oven at 60°C for 24 hours. Preferably, the particle size of the purified raw material is ≥200 mesh.

[0035] Pretreatment of lignin can reduce the ash content in lignin and improve its purity.

[0036] See also Figure 3 In step S2, the purified raw material is mixed with a phosphorus source to obtain a precursor, which includes:

[0037] S21, placing the purified raw material in a solvent and stirring evenly to obtain a first stock solution;

[0038] S22. Add a phosphorus source to the first stock solution, perform ultrasonic treatment for 1 to 4 hours, and freeze-dry to obtain a precursor.

[0039] In step S21, the solvent includes an organic solvent or water, and the organic solvent includes any one or more of ethanol, methanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dioxane, that is, the organic solvent can be any one of the organic solvents listed above, such as ethanol, or acetone, or N,N-dimethylformamide, etc. The organic solvent can also be any two or more combinations of the organic solvents listed above, such as a composition of ethanol and methanol, or a composition of methanol and acetone, or a composition of ethanol, N,N-dimethylformamide and N,N-dimethylacetamide, which are not listed here. It should be noted that when the organic solvent is a composition of two or more, there is no restriction on the ratio between the components in the composition, and they can be mixed in any proportion. In other embodiments, the organic solvent can also be selected from the organic solvents commonly used in the art that are not listed above. Preferably, the organic solvent is ethanol.

[0040] In step S22, the phosphorus source is any one or more of phosphoric acid, elemental phosphorus, phosphate or phosphate ester compounds. For example, phosphoric acid or elemental phosphorus or a combination of elemental phosphorus and phosphate ester compounds, etc., are not listed here one by one. It should be noted that when the phosphorus source is a combination of two or more, there is no restriction on the ratio between the components in the combination, and they can be mixed in any proportion. The mass ratio of the phosphorus source to the lignin is 0.5:1 to 2.5:1. For example, the mass ratio of the phosphorus source to the lignin can be any value within the range of 0.5:1 to 2.5:1, such as 0.5:1, 1:1, 2:1, 2.5:1, etc.; preferably, the phosphorus source is phosphoric acid. The time of ultrasonic treatment is 1 to 4 hours. For example, the time of ultrasonic treatment can be any value within the range of 1 to 4 hours, such as 1 hour, 2 hours, 3 hours or 4 hours. Ultrasonic treatment makes the mixing of the phosphorus source and lignin more uniform, and the precursor obtained by freeze drying can retain the original structure of the material to the greatest extent, increase the reaction area, and improve the consistency of the product. The introduction of phosphorus can change the microstructure of hard carbon, making it have more defects and larger carbon layer spacing, providing more active sites for the insertion / extraction of sodium atoms, thereby increasing the energy storage capacity and improving the electrochemical performance.

[0041] Among them, in step S22, if the phosphorus source is one or more of phosphoric acid, elemental phosphorus or phosphate compounds, the solvent in step S21 is an organic solvent, that is, lignin is dissolved in the organic solvent to obtain the first stock solution; if the phosphorus source is phosphate, since phosphate is insoluble in organic solvents and easily agglomerates in organic solvents, the solvent in step S21 is water, that is, lignin is dispersed in water to obtain the first stock solution.

[0042] In step S3, a pre-oxidation treatment is also included before the precursor is carbonized, including: placing the precursor in a tube furnace, under an air atmosphere, raising the temperature in the tube furnace to 150-250°C at a heating rate of 1-10°C / min, keeping warm for 1-4 hours, and then cooling to room temperature. For example, the heating rate of pre-oxidation can be any value within the range of 1-10°C / min, such as 1°C / min, 3°C / min, 5°C / min, 8°C / min or 10°C / min, the temperature of pre-oxidation can be any value within the range of 150-250°C, such as 150°C, 200°C, 220°C, 240°C or 250°C, and the time of pre-oxidation can be any value within the range of 1-4 hours, such as 1h, 2h, 3h or 4h. Preferably, the temperature of pre-oxidation is 220-240°C. Through pre-oxidation treatment, oxygen-containing functional groups, such as carbonyl groups, are introduced on the surface of the precursor, and pre-oxidation can achieve preliminary carbonization to introduce more defects during the carbonization process, thereby inhibiting the growth and orientation of the carbon layer, increasing the interlayer spacing of the graphite microcrystals, and facilitating the insertion / extraction of sodium ions.

[0043] After the pre-oxidation is completed, the precursor is carbonized, including: placing the pre-oxidation-treated precursor in a tubular furnace in an inert atmosphere, and performing a first-stage carbonization process and a second-stage carbonization process, wherein the first-stage carbonization includes: raising the temperature to 200-250°C at a heating rate of 3-10°C / min, and keeping it warm for 1-4 hours; the second-stage carbonization includes: raising the temperature to 1000-1500°C at a heating rate of 3-10°C / min, and keeping it warm for 1-4 hours; after the insulation is completed, cooling to room temperature is performed, and sieving to obtain the hard carbon material. For example, the heating rate can be any value within the range of 3 to 10°C / min, such as 3°C / min, 5°C / min, 8°C / min, or 10°C / min; the carbonization temperature in the first stage can be any value within the range of 200 to 250°C, such as 200°C, 220°C, 240°C, or 250°C; and the carbonization temperature in the second stage can be any value within the range of 1000 to 1500°C, such as 1000°C, 1100°C, 1300°C, or 1500°C. If the heating rate during carbonization is too fast, the carbon yield will be reduced, while if the heating rate is too slow, the time and energy costs will be too high. Therefore, the heating rate for carbonization of lignin in the present invention is preferably 4 to 6°C / min. After the second stage of insulation, the temperature is lowered to room temperature under an inert atmosphere and the sample is removed to prevent the sample from oxidizing and deteriorating in an air atmosphere due to excessive temperature. Preferably, the temperature of the first stage carbonization is 220-240° C., the temperature of the second stage carbonization is 1100-1300° C., the inert atmosphere is argon, and the particle size of the hard carbon material is ≥200 mesh.

[0044] The present invention also provides a hard carbon material, which is prepared using the preparation method described above. The hard carbon material of the present invention introduces phosphorus into the hard carbon structure, changes the microstructure of the hard carbon, and makes it have more defects and a larger carbon layer spacing, providing more active sites for the insertion / extraction of sodium atoms, thereby increasing the energy storage capacity and improving the electrochemical performance.

[0045] The present invention also provides a sodium-ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer comprises the hard carbon material of the present invention. It should be noted that the positive electrode sheet, negative electrode sheet, separator, and electrolyte of the sodium-ion battery are all prepared using conventional methods in the art.

[0046] The present invention is further illustrated below by means of several examples. The chemical reagents used in the following examples can all be obtained through commercial means.

[0047] Example 1

[0048] Weigh 5g of purified enzymatic lignin and dissolve it in 20mL of ethanol, add 10g of phosphoric acid, ultrasonically treat for 2h, and freeze-dry to obtain a precursor. Place the precursor in a tubular furnace, and in an air atmosphere, raise the temperature in the tubular furnace to 230℃ at a heating rate of 6℃ / min and keep it warm for 2h for pre-oxidation. After cooling to room temperature, take it out and grind and sieve; Place the pre-oxidized precursor in a tubular furnace, and in an inert atmosphere, raise the temperature in the tubular furnace to 230℃ at a heating rate of 5℃ / min and keep it warm for 2h, then raise the temperature in the tubular furnace to 1200℃ at a heating rate of 5℃ / min, keep it warm for 2h, and cool to room temperature. Sieve to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 6.5%.

[0049] Example 2

[0050] Take 5g of purified alkali lignin and dissolve it in 20mL of ethanol, add 10g of phosphoric acid, ultrasonically treat for 2h, and freeze-dry to obtain a precursor. Place the precursor in a tubular furnace, and in an air atmosphere, raise the temperature in the tubular furnace to 250℃ at a heating rate of 3℃ / min and keep it warm for 2h for pre-oxidation. After cooling to room temperature, take it out and grind and sieve; Place the pre-oxidized precursor in a tubular furnace, and in an inert atmosphere, raise the temperature in the tubular furnace to 250℃ at a heating rate of 3℃ / min and keep it warm for 2h. Then, raise the temperature in the tubular furnace to 1500℃ at a heating rate of 4℃ / min, keep it warm for 4h, cool to room temperature, and sieve to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 4.3%.

[0051] Example 3

[0052] 5 g of purified ethanol lignin was weighed and dissolved in 20 mL of ethanol. 8.5 g of phosphoric acid was added, and the mixture was ultrasonically treated for 2 h. The precursor was freeze-dried to obtain a precursor. The precursor was placed in a tube furnace, and under an air atmosphere, the temperature in the tube furnace was raised to 250 ° C at a heating rate of 7 ° C / min and kept warm for 2 h for pre-oxidation. After cooling to room temperature, the precursor was taken out and ground and sieved. The pre-oxidized precursor was placed in a tube furnace, and under an inert atmosphere, the temperature in the tube furnace was raised to 250 ° C at a heating rate of 7 ° C / min, kept warm for 2 h, and then raised to 1500 ° C at a heating rate of 7 ° C / min. After keeping warm for 4 h, it was cooled to room temperature and sieved to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 5.1%.

[0053] Example 4

[0054] 5 g of purified enzymatic lignin was weighed and dissolved in 20 mL of ethanol. 2.5 g of red phosphorus was added, and the product was ultrasonically treated for 4 h. The product was freeze-dried to obtain a precursor. The precursor was placed in a tube furnace, and in an air atmosphere, the temperature in the tube furnace was raised to 200 ° C at a heating rate of 10 ° C / min and kept warm for 4 h for pre-oxidation. After cooling to room temperature, the product was taken out and ground and sieved. The pre-oxidized precursor was placed in a tube furnace, and in an inert atmosphere, the temperature in the tube furnace was raised to 200 ° C at a heating rate of 10 ° C / min, kept warm for 4 h, and then raised to 1300 ° C at 10 ° C / min. After keeping warm for 4 h, the product was cooled to room temperature and sieved to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 5.8%.

[0055] Example 5

[0056] 5 g of purified enzymatic lignin was weighed and dispersed in 20 mL of deionized water. 10.8 g of ammonium dihydrogen phosphate was added, and the mixture was ultrasonically treated for 1 hour. The precursor was freeze-dried to obtain a precursor. The precursor was placed in a tubular furnace, and under an air atmosphere, the temperature in the tubular furnace was raised to 150°C at a heating rate of 1°C / min and kept warm for 1 hour for pre-oxidation. After cooling to room temperature, the mixture was taken out and ground and sieved. The pre-oxidized precursor was placed in a tubular furnace, and under an inert atmosphere, the temperature in the tubular furnace was raised to 220°C at a heating rate of 4°C / min, kept warm for 2 hours, and then raised to 1000°C at a heating rate of 3°C / min. After keeping warm for 1 hour, the mixture was cooled to room temperature and sieved to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 4.7%.

[0057] Example 6

[0058] Weigh 5g of purified enzymatic lignin and disperse it in 20mL of deionized water. Add 12.5g of diammonium hydrogen phosphate, ultrasonically treat for 1h, and freeze-dry to obtain a precursor. Place the precursor in a tubular furnace, and in an air atmosphere, raise the temperature in the tubular furnace to 150°C at a heating rate of 1°C / min and keep it warm for 1h. After cooling to room temperature, take it out and grind and sieve; place the pre-oxidized precursor in a tubular furnace, and in an inert atmosphere, raise the temperature in the tubular furnace to 150°C at a heating rate of 3°C / min and keep it warm for 1h. Then, raise the temperature in the tubular furnace to 1100°C at a heating rate of 6°C / min, keep it warm for 3h, cool to room temperature, and sieve to obtain a hard carbon material. According to elemental analysis, the phosphorus content of the hard carbon material in this embodiment is 5.5%.

[0059] Comparative Example

[0060] 5 g of purified enzymatically hydrolyzed lignin was dissolved in 20 mL of ethanol, sonicated for 4 h, and freeze-dried to obtain the precursor. The precursor was heated to 250°C in air at 3°C / min and held for 4 h. After cooling to room temperature, it was ground, sieved, and placed in a tube furnace. Under an inert atmosphere, the temperature was increased to 250°C at a rate of 3°C / min, held for 4 h, and then increased to 1000°C at a rate of 3°C / min. The temperature was held for 2 h before cooling to room temperature.

[0061] Table 1: Contents of C, H, N, and S in the lignin raw materials of Examples 1 to 6 and the comparative example

[0062] sample C(％) H(％) N(％) S(％) Enzymatic lignin 58.02 5.68 0.46 0.00 Alkali lignin 58.23 4.36 0.15 2.95 Ethanol lignin 54.18 5.63 0.22 0.08

[0063] The hard carbon materials prepared in Examples 1 to 6 and the comparative example were respectively assembled into sodium ion batteries, and the process was as follows: 0.16 g of hard carbon material was weighed, 0.02 g of conductive agent Super P and 0.02 g of binder PVDF were added, and after thorough grinding, 2 mL of NMP was added and mixed. After slurrying was uniformly mixed, the slurry was drawn onto 15 μm thick aluminum foil to form a negative electrode sheet; then, in an argon glove box, a metal sodium sheet was used as the counter electrode, a glass fiber was used as the diaphragm, and 1 mol / L NaPF6 / PC was used as the electrolyte to assemble a CR2032 button battery.

[0064] The battery was tested for its initial charge and discharge capacity at a rate of 0.3C in the voltage range of 2.0 to 4.0 V, and the initial coulombic efficiency was calculated. The test results are shown in Table 2.

[0065] Table 2: Phosphorus content of hard carbon materials prepared in Examples 1 to 6 and Comparative Examples and performance test results of assembled batteries

[0066]

[0067] It can be clearly seen from the test results in Table 2 that the first discharge capacity and efficiency of Examples 1 to 6 are significantly improved compared with the comparative example. This is because the doped phosphorus element in Examples 1 to 6 changes the microstructure of the hard carbon, causing it to have more defects and a larger carbon layer spacing, providing more active sites for the insertion / extraction of sodium atoms, thereby improving the energy storage capacity.

[0068] The hard carbon material of the present invention is prepared by carbonizing phosphorus-doped lignin under an inert atmosphere. The introduction of phosphorus can change the microstructure of the hard carbon, making it have more defects and a larger carbon layer spacing, providing more active sites for the insertion / extraction of sodium atoms, thereby increasing the energy storage capacity and improving the electrochemical performance, thus solving the problem of low capacity and coulombic efficiency of hard carbon negative electrode materials. At the same time, the use of biomass papermaking waste lignin as a carbon source provides an effective way to high-value utilization of lignin. Therefore, the present invention effectively overcomes some practical problems in the prior art and has high utilization value and significance.

[0069] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A method for preparing a hard carbon material, characterized in that: The steps include: Providing lignin and pretreating the lignin to obtain a purified raw material; mixing the purified raw material with a phosphorus source to prepare a precursor; performing carbonization treatment on the precursor to obtain the hard carbon material; Among them, before carbonizing the precursor, a pre-oxidation treatment is also included, and the pre-oxidation treatment includes: placing the precursor in a tube furnace in an air atmosphere, raising the temperature in the tube furnace to 150-250°C at a heating rate of 1-10°C / min, keeping the temperature for 1-4 hours, and then cooling to room temperature.

2. The method for preparing a hard carbon material according to claim 1, wherein: The steps of pretreating the lignin are as follows: dispersing the lignin in water to obtain a lignin suspension; adding an alkaline solution to the lignin suspension, adjusting the pH of the lignin suspension until the lignin is dissolved, and filtering to obtain a lignin filtrate; heating the lignin filtrate in a water bath, and adding an acidic solution to adjust the pH of the lignin filtrate until lignin precipitates; The precipitated lignin is filtered, washed with water, dried, ground, and sieved to obtain a purified raw material.

3. The method for preparing a hard carbon material according to claim 1, wherein: The lignin includes any one or more of enzymatic lignin, alkali lignin, ethanol lignin, sulfate lignin, and organic lignin.

4. The method for preparing a hard carbon material according to claim 1, wherein: The steps of mixing the purified raw material with a phosphorus source to prepare a precursor are as follows: placing the purified raw material in a solvent and stirring uniformly to obtain a first stock solution; A phosphorus source is added to the first stock solution, and the solution is ultrasonically treated for 1 to 4 hours, followed by freeze-drying to obtain the precursor.

5. The method for preparing a hard carbon material according to claim 4, wherein: The solvent includes an organic solvent or water, and the organic solvent includes any one or more of ethanol, methanol, acetone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone or dioxane.

6. The method for preparing a hard carbon material according to claim 4, wherein: The phosphorus source includes any one or more of phosphoric acid, elemental phosphorus, phosphate or phosphate ester compounds; the mass ratio of the phosphorus source to the purified raw material is 0.5:1 to 2.5:

1.

7. The method for preparing a hard carbon material according to claim 1, wherein: Carbonizing the precursor to obtain a hard carbon material includes: placing the pre-oxidized precursor in a tubular furnace in an inert atmosphere to perform a first-stage carbonization and a second-stage carbonization, wherein the first-stage carbonization includes: raising the temperature to 200-250°C at a heating rate of 3-10°C / min and keeping it warm for 1-4 hours; the second-stage carbonization includes: raising the temperature to 1000-1500°C at a heating rate of 3-10°C / min and keeping it warm for 1-4 hours; cooling to room temperature after the insulation is completed, and sieving to obtain the hard carbon material.

8. A hard carbon material, characterized in that The hard carbon material is prepared according to the method for preparing the hard carbon material according to any one of claims 1 to 7.

9. A sodium ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that: The negative electrode sheet includes a negative electrode current collector and the hard carbon material according to claim 8 disposed on the negative electrode current collector.

Images