Sodium-ion battery hard carbon negative electrode material derived based on waste fruit shells as well as preparation method and application of sodium-ion battery hard carbon negative electrode material

Abstract

The invention discloses a sodium-ion battery hard carbon negative electrode material derived based on waste fruit shells and a preparation method and application thereof. The method comprises the following steps: washing, drying and crushing waste shells from betel nut shells, transferring the crushed shells into a muffle furnace, carbonizing the shells in an air atmosphere, and naturally cooling the shells to obtain a pre-carbonized material; the pre-carbonized material is pretreated through a hydrothermal method, a ball milling method or a molten salt auxiliary method; transferring the pretreated product into an atmosphere furnace, performing high-temperature carbonization under the protection of inert gas, and cooling after carbonization is finished; and cooling, soaking in an acid solution, washing with deionized water and ethanol until the solution is neutral, and drying, grinding and sieving the obtained product. The waste betel nut shell precursor is adopted, the prepared hard carbon material has proper interlayer spacing and pore structure through proper regulation and control of pretreatment, more sodium ion active sites are provided, and the first coulombic efficiency, rate capability and cycling stability of the material are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of sodium ion battery electrode material preparation, and particularly relates to a waste fruit shell derived sodium ion battery hard carbon negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries (LIBs) have been widely concerned and applied due to their high energy density, long cycle life, low self-discharge rate and good safety. However, the limited reserves and uneven distribution of lithium resources make it difficult to meet the low-cost requirements of large-scale energy storage in the future. Sodium ion batteries (SIBs) have become a research hotspot due to the abundance and potential cost advantage of sodium element, and are considered as one of the most important alternatives to lithium ion batteries.

[0003] Electrode materials are an important part of sodium ion batteries, which directly affect the energy density, cycle stability and overall efficiency of the battery. Carbon-based materials are the first choice for sodium ion battery negative electrode materials due to their excellent electrical conductivity, flexible preparation method, low cost and environmental friendliness. Graphite is a commonly used negative electrode material for lithium ion batteries, but it is not suitable for sodium ion batteries. This is because the radius of sodium ion (0.106 nm) is about 55% larger than that of lithium ion (0.076 nm), making it difficult for sodium ion to normally intercalate and deintercalate between graphite layers. Hard carbon refers to carbon that is difficult to graphitize under high temperature conditions above 2500℃, and is composed of disordered regions and graphite-like regions, with a structure called the "card house" model. Hard carbon has a larger interlayer spacing, which is beneficial for sodium ion storage, and is considered one of the most promising negative electrode materials for sodium ion batteries.

[0004] The precursors of hard carbon include biomass, resin and polymer, among which biomass precursors are the most common. Biomass has a rich microstructure and is rich in carbon elements. In addition, biomass resources are abundant and easy to obtain, relatively inexpensive, sustainable and environmentally friendly, making it a stable carbon source for sodium ion battery hard carbon materials. Converting biomass into high-value electrode materials not only promotes the development of sodium ion battery energy storage technology, but also promotes the efficient use of biomass, contributing to the development of sustainable green energy. Hard carbon is usually prepared by carbonization under inert atmosphere (such as argon) at high temperature (usually 800-1600℃). Although the preparation process is simple, the hard carbon obtained by direct carbonization has low initial coulombic efficiency (ICE), poor rate performance and limited long-term cycle stability. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a waste fruit shell derived sodium ion battery hard carbon negative electrode material and a preparation method and application thereof, in order to improve the electrochemical performance of the material.

[0006] To solve the above technical problems, according to one aspect of the present application, a preparation method of a hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells is provided, comprising: Step one, washing and drying the waste fruit shells derived from betel nut shells; Step two, crushing the dried waste fruit shells and transferring them to a muffle furnace, carbonizing in an air atmosphere at a pyrolysis temperature of 250-350 DEG C, and obtaining a pre-carbonized material after natural cooling; Step three, pre-treating the pre-carbonized material by a hydrothermal method, a ball milling method or a molten salt assisted method; Step four, transferring the pre-treated product to a gas furnace, high-temperature carbonizing under the protection of an inert gas at a pyrolysis temperature of 1200-1400 DEG C, and cooling after carbonization; Step five, placing the cooled product in an acidic solution, then washing it with deionized water and ethanol until it is neutral, drying, grinding and sieving the obtained product to obtain the hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells.

[0007] Further, in step two, the heating rate of the muffle furnace is 5 DEG C / min, and the pyrolysis temperature is 300 DEG C.

[0008] Further, in step three, the solvent used in the hydrothermal method is one of deionized water, 1 mol / L sodium hydroxide solution and 1 mol / L potassium hydroxide solution, the ratio of pre-carbonized material to solvent is 1:10 (g / mL), the hydrothermal temperature is 180 DEG C, and the time is 18 h.

[0009] Further, in step three, the planetary ball mill used in the ball milling method has a rotation speed of 500 rpm, the ball milling time is 12 h, and the mass ratio of grinding balls to pre-carbonized material is 1-7:1.

[0010] Further, in step three, the salt used in the molten salt assisted method is sodium chloride, the mass ratio of sodium chloride to pre-carbonized material is 1-2:1, and the time is 8 h.

[0011] Further, in step four, the inert gas for high-temperature carbonization is argon, the heating rate is 5 DEG C / min, and the pyrolysis temperature is 1300 DEG C.

[0012] Further, in step five, the acidic solution is a 1 mol / L dilute hydrochloric acid solution, and the soaking time is 12 h.

[0013] According to another aspect of the present application, a hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells obtained by any one of the above preparation methods is provided.

[0014] According to another aspect of the present application, there is provided the use of the above-mentioned biomass-derived hard carbon anode material for sodium-ion batteries in the preparation of a carbon-based anode material for sodium-ion batteries.

[0015] According to another aspect of the present application, there is provided a sodium-ion battery comprising a carbon-based anode material for sodium-ion batteries prepared from the above-mentioned biomass-derived hard carbon anode material for sodium-ion batteries.

[0016] The biomass precursor is pre-treated to prepare the hard carbon material with suitable interlayer spacing and pore structure, which provides more sodium ion active sites, and improves the first coulombic efficiency, rate performance and cycle stability of the material. The biomass precursor used in the present application is abundant and easy to obtain, and the pre-treatment method used is simple and environmentally friendly. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is the SEM image of the biomass-derived hard carbon material prepared in Comparative Example 1; Figure 2 is the SEM image of the biomass-derived hard carbon material prepared in Example 2; Figure 3 is the XRD image of the biomass-derived hard carbon material prepared in Comparative Example 1 and Examples 1 and 2; Figure 4 is the first cycle charge-discharge curve of the biomass-derived hard carbon material prepared in Comparative Example 1 and Example 2; Figure 5 is the rate performance curve of the biomass-derived hard carbon material prepared in Comparative Example 1 and Example 2; Figure 6 is the cycle performance curve of the biomass-derived hard carbon material prepared in Comparative Example 1 and Example 2; Figure 7 is the cyclic voltammogram of the biomass-derived hard carbon material prepared in Comparative Example 1; Figure 8 is the cyclic voltammogram of the biomass-derived hard carbon material prepared in Example 2. DETAILED DESCRIPTION

[0018] The biomass precursor has a rich microstructure, and by pre-treatment and proper regulation, the electrochemical performance of the prepared hard carbon can be effectively improved, which is of great significance for the development of high-performance biomass hard carbon materials. Based on this, a typical embodiment of the present application provides a preparation method of a biomass-derived hard carbon anode material for sodium-ion batteries, which is prepared by treating the pre-carbonized product by different pre-treatment methods.

[0019] Step one, the waste fruit shell derived from betel nut shell is washed and dried.

[0020] Specifically, the discarded fruit shells are placed in deionized water for ultrasonic washing to remove surface dust and impurities, and the washed discarded fruit shells are placed in a blast drying oven for drying. The temperature of the blast drying oven is preferably 80° C., and the drying time is 12 hours.

[0021] Step 2: The dried waste shells are crushed and transferred to a muffle furnace. In an air atmosphere, the pyrolysis temperature is controlled at 250-350° C. for carbonization, and the pre-carbonized material is obtained after natural cooling.

[0022] In this step, in some preferred embodiments, the pyrolysis temperature can be selected from 250°C, 280°C, 300°C, 320°C, and 350°C.

[0023] In a further preferred embodiment, the heating rate of the muffle furnace is 5°C / min, the pyrolysis temperature is 300°C, and the holding time is 2 h.

[0024] Step three: pre-treating the pre-carbonized material by a hydrothermal method, a ball milling method or a molten salt assisted method.

[0025] When the hydrothermal method is used, the solvent used is one of deionized water, 1 mol / L sodium hydroxide solution and 1 mol / L potassium hydroxide solution. The ratio of pre-carbonized material to solvent is preferably 1:10 (g / mL), the hydrothermal temperature is 180°C, and the time is 18 h.

[0026] When the ball milling method is used, preferably, the rotation speed of the planetary ball mill is 500 rpm, the ball milling time is 12 h, and the mass ratio of the grinding balls to the pre-carbonized material is 1-7:1.

[0027] When the molten salt assisted method is used, the salt used is sodium chloride, the mass ratio of sodium chloride to pre-carbonized material is preferably 1-2:1, and the time is 8 hours.

[0028] Step 4: Transfer the pretreated product to an atmosphere furnace and perform high-temperature carbonization under the protection of inert gas at a pyrolysis temperature of 1200-1400°C. After carbonization, cool the product down. The inert gas used for high-temperature carbonization is preferably argon.

[0029] In this step, in some preferred embodiments, the pyrolysis temperature can be selected from 1200°C, 1250°C, 1300°C, 1350°C, and 1400°C.

[0030] In a further preferred embodiment, the heating rate is 5°C / min, the pyrolysis temperature is 1300°C, the holding time is 2h, and the cooling rate is 5°C / min.

[0031] Step 5: After cooling, soak in an acidic solution, then wash with deionized water and ethanol until neutral, dry, grind, and sieve the obtained product to obtain the said hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells.

[0032] The product is placed in a forced air drying oven and dried at 80° C. for 12 h. The acidic solution is preferably a 1 mol / L dilute hydrochloric acid solution, and the soaking time is 12 h.

[0033] The hard carbon anode material for sodium-ion batteries derived from waste nutshells obtained in the above embodiment can be used to prepare hard carbon material electrode sheets. The hard carbon anode material for sodium-ion batteries derived from waste nutshells is ground and mixed with acetylene black and polyvinylidene fluoride in a mass ratio of 8:1:1. N-methylpyrrolidone is added and stirred to obtain a uniformly mixed electrode slurry. The slurry is evenly coated on copper foil using a coating machine, dried in a vacuum drying oven for 12 hours, and then punched into disc electrodes using a sheet punch to obtain hard carbon material electrode sheets. The prepared hard carbon material electrode sheets are further used to prepare sodium-ion batteries.

[0034] The technical solutions claimed in the present invention are further illustrated below by means of some examples. However, the examples and comparative examples are intended to illustrate embodiments of the present invention and do not exceed the scope of the subject matter of the present invention. The scope of protection of the present invention is not limited by the examples. Unless otherwise specified, the materials and reagents used in the present invention can be obtained from commercial products in the art. Example 1

[0035] The betel nut shell was ultrasonically washed with deionized water for 6 h to remove surface dust and impurities, and then dried in a forced air drying oven at 80 °C for 12 h.

[0036] The dried betel nut shell was placed in a grinder and crushed. After crushing, it was transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere and kept at that temperature for 2 h. After natural cooling, the pre-carbonized material was obtained.

[0037] The pre-carbonized material and deionized water were placed in a hydrothermal autoclave at a ratio of 1:10 (g / mL), kept at 180°C for 18 h, and a pretreated product was obtained.

[0038] The pretreated product was placed in a tube furnace, and under the protection of inert gas argon, the temperature was increased from room temperature to 1300°C at a heating rate of 5°C / min, kept at that temperature for 2 h, and cooled to room temperature at a cooling rate of 5°C / min.

[0039] After cooling, it is soaked in 1 mol / L dilute hydrochloric acid solution for 12 h, then washed with deionized water and ethanol until neutral, and the obtained product is placed in a blast drying oven for drying at 80°C for 12 h, and then ground and sieved to obtain a hard carbon material. Example 2

[0040] The betel nut shell is washed with deionized water for 6 h to remove surface dust impurities, and then dried at 80°C in a blast drying oven for 12 h after washing.

[0041] The dried betel nut shell is crushed in a crusher, and then transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere for 2 h, and then naturally cooled to obtain a pre-carbonized material.

[0042] The pre-carbonized material is placed in a hydrothermal kettle with 1 mol / L sodium hydroxide solution at a ratio of 1:10 (g / mL) at a temperature of 180°C for 18 h to obtain a pretreated product.

[0043] The pretreated product is placed in a tube furnace and heated to 1300°C at a heating rate of 5°C / min from room temperature under the protection of inert gas argon, and then cooled to room temperature at a cooling rate of 5°C / min.

[0044] After cooling, it is soaked in 1 mol / L dilute hydrochloric acid solution for 12 h, then washed with deionized water and ethanol until neutral, and the obtained product is placed in a blast drying oven for drying at 80°C for 12 h, and then ground and sieved to obtain a hard carbon material. Example 3

[0045] The betel nut shell is washed with deionized water for 6 h to remove surface dust impurities, and then dried at 80°C in a blast drying oven for 12 h after washing.

[0046] The dried betel nut shell is crushed in a crusher, and then transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere for 2 h, and then naturally cooled to obtain a pre-carbonized material.

[0047] The pre-carbonized material is placed in a hydrothermal kettle with 1 mol / L potassium hydroxide solution at a ratio of 1:10 (g / mL) at a temperature of 180°C for 18 h to obtain a pretreated product.

[0048] The pretreated product is placed in a tube furnace and heated to 1300°C at a heating rate of 5°C / min from room temperature under the protection of inert gas argon, and then cooled to room temperature at a cooling rate of 5°C / min.

[0049] After cooling, the mixture was immersed in a 1 mol / L dilute hydrochloric acid solution for 12 h, and then washed with deionized water and ethanol until neutral. The obtained product was placed in a forced air drying oven and dried at 80°C for 12 h. After drying, it was ground and sieved to obtain a hard carbon material. Example 4

[0050] The betel nut shell was ultrasonically washed with deionized water for 6 h to remove surface dust and impurities, and then dried in a forced air drying oven at 80 °C for 12 h.

[0051] The dried betel nut shell was placed in a grinder and crushed. After crushing, it was transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere and kept at that temperature for 2 h. After natural cooling, the pre-carbonized material was obtained.

[0052] The grinding balls and the pre-carbonized material were placed in a planetary ball mill at a mass ratio of 1:1 and ball milled at a speed of 500 rpm for 12 h to obtain a pretreated product.

[0053] The pretreated product was placed in a tube furnace, and under the protection of inert gas argon, the temperature was increased from room temperature to 1300°C at a heating rate of 5°C / min, kept at that temperature for 2 h, and cooled to room temperature at a cooling rate of 5°C / min.

[0054] After cooling, the product was immersed in a 1 mol / L dilute hydrochloric acid solution for 12 h, and then washed with deionized water and ethanol until neutral. The obtained product was placed in a forced air drying oven and dried at 80°C for 12 h. After drying, it was ground and sieved to obtain a hard carbon material. Example 5

[0055] The betel nut shell was ultrasonically washed with deionized water for 6 h to remove surface dust and impurities, and then dried in a forced air drying oven at 80 °C for 12 h.

[0056] The dried betel nut shell was placed in a grinder and crushed. After crushing, it was transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere and kept at that temperature for 2 h. After natural cooling, the pre-carbonized material was obtained.

[0057] The grinding balls and the pre-carbonized material were placed in a planetary ball mill at a mass ratio of 3:1 and ball milled at a speed of 500 rpm for 12 h to obtain a pretreated product.

[0058] The pretreated product was placed in a tubular furnace, and under the protection of inert gas argon, the temperature was increased from room temperature to 1300°C at a heating rate of 5°C / min, kept at that temperature for 2 h, and cooled to room temperature at a cooling rate of 5°C / min.

[0059] After cooling, it is soaked in 1 mol / L dilute hydrochloric acid solution for 12 h, then washed with deionized water and ethanol until neutral, and the obtained product is placed in a blast drying oven for drying at 80°C for 12 h, and then ground and sieved to obtain a hard carbon material. Example 6

[0060] The betel nut shell is washed with deionized water for 6 h to remove surface dust impurities, and then dried at 80°C in a blast drying oven for 12 h.

[0061] The dried betel nut shell is crushed in a crusher, and then transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere for 2 h, and then naturally cooled to obtain a pre-carbonized material.

[0062] The grinding balls and the pre-carbonized material are placed in a planetary ball mill at a mass ratio of 7:1, and ball milled at a speed of 500 rpm for 12 h to obtain a pretreated product.

[0063] The pretreated product is placed in a tube furnace and heated to 1300°C at a heating rate of 5°C / min from room temperature under the protection of inert gas argon, and then cooled to room temperature at a cooling rate of 5°C / min.

[0064] After cooling, it is soaked in 1 mol / L dilute hydrochloric acid solution for 12 h, then washed with deionized water and ethanol until neutral, and the obtained product is placed in a blast drying oven for drying at 80°C for 12 h, and then ground and sieved to obtain a hard carbon material. Example 7

[0065] The betel nut shell is washed with deionized water for 6 h to remove surface dust impurities, and then dried at 80°C in a blast drying oven for 12 h.

[0066] The dried betel nut shell is crushed in a crusher, and then transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere for 2 h, and then naturally cooled to obtain a pre-carbonized material.

[0067] The pre-carbonized material is placed in 100 mL of a sodium chloride solution and soaked at room temperature for 8 h, and the mass ratio of sodium chloride to pre-carbonized material is 1:1. After soaking, it is filtered and placed in a blast drying oven for drying at 80°C for 12 h to obtain a pretreated product.

[0068] The pretreated product is placed in a tube furnace and heated to 1300°C at a heating rate of 5°C / min from room temperature under the protection of inert gas argon, and then cooled to room temperature at a cooling rate of 5°C / min.

[0069] After cooling, the mixture was immersed in a 1 mol / L dilute hydrochloric acid solution for 12 h, and then washed with deionized water and ethanol until neutral. The obtained product was placed in a forced air drying oven and dried at 80°C for 12 h. After drying, it was ground and sieved to obtain a hard carbon material. Example 8

[0070] The betel nut shells were ultrasonically washed with deionized water for 6 h to remove surface dust and impurities, and then dried in a forced air drying oven at 80 °C for 12 h.

[0071] The dried betel nut shell was placed in a grinder and crushed. After crushing, it was transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere and kept at that temperature for 2 h. After natural cooling, the pre-carbonized material was obtained.

[0072] The pre-carbonized material was placed in 100 mL of sodium chloride solution and soaked at room temperature for 8 h. The mass ratio of sodium chloride to pre-carbonized material was 2:1. After soaking, it was filtered and placed in a forced air drying oven and dried at 80°C for 12 h to obtain a pretreated product.

[0073] The pretreated product was placed in a tube furnace, and under the protection of inert gas argon, the temperature was increased from room temperature to 1300°C at a heating rate of 5°C / min, kept at that temperature for 2 h, and cooled to room temperature at a cooling rate of 5°C / min.

[0074] After cooling, the product was immersed in a 1 mol / L dilute hydrochloric acid solution for 12 h, and then washed with deionized water and ethanol until neutral. The obtained product was placed in a forced air drying oven and dried at 80°C for 12 h. After drying, it was ground and sieved to obtain a hard carbon material.

[0075] Comparative Example 1 The betel nut shell was ultrasonically washed with deionized water for 6 h to remove surface dust and impurities, and then dried in a forced air drying oven at 80 °C for 12 h.

[0076] The dried betel nut shell was placed in a grinder and crushed. After crushing, it was transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min in an air atmosphere and kept at that temperature for 2 h. After natural cooling, the pre-carbonized material was obtained.

[0077] The pre-carbonized material was placed in a tubular furnace and, under the protection of inert gas argon, heated from room temperature to 1300°C at a heating rate of 5°C / min, kept at that temperature for 2 h, and cooled to room temperature at a cooling rate of 5°C / min.

[0078] After cooling, the product was immersed in a 1 mol / L dilute hydrochloric acid solution for 12 h, and then washed with deionized water and ethanol until neutral. The obtained product was placed in a forced air drying oven and dried at 80°C for 12 h. After drying, it was ground and sieved to obtain a hard carbon material.

[0079] The areca shell-derived hard carbon negative electrode material obtained in each example and comparative example was mixed with acetylene black and polyvinylidene fluoride at a mass ratio of 8:1:1, N-methylpyrrolidone was added and stirred to obtain a uniformly mixed electrode slurry, the battery slurry was uniformly coated on a copper foil using a coating machine, and then placed in a vacuum drying oven for vacuum drying for 12 h, and then punched into a circular electrode sheet with a diameter of 12 mm to obtain a hard carbon material electrode sheet.

[0080] The electrode sheet obtained above was used as a negative electrode, a glass fiber (Whitman, GF / D) circular sheet with a diameter of 19 mm was used as a separator, a sodium metal sheet with a diameter of 12 mm and a thickness of 0.2 mm was used as a counter electrode and a reference electrode, and an electrolyte was a 1 mol / L sodium perchlorate / ethylene carbonate / dimethyl carbonate solution. A sodium ion battery was assembled in a glove box filled with high-purity argon according to the structure of a CR2016 standard button cell, and the battery was tested on a battery test platform at a current density of 20 mA / g (0.1C) for charge and discharge testing.

[0081] Table 1. Main parameters and electrochemical performance of examples 1-8 and comparative example 1

[0082] Figure 1 、 2 The SEM images of comparative example 1 and example 2 are shown in FIG. 2, it can be seen that the hard carbon material of example 2 shows a more abundant pore structure after sodium hydroxide etching, providing abundant sodium ion active sites, which is beneficial to the storage of sodium ions and can effectively improve the capacity; Figure 3 The XRD patterns of comparative example 1 and examples 1 and 2 are shown in FIG. 3, and all show characteristic peaks near 23° and 43°, corresponding to the diffraction of (002) and (100) crystal planes, respectively, indicating that the materials all belong to amorphous carbon materials; Figure 4 The charge and discharge curves of comparative example 1 and example 2 at a current density of 20 mA / g are shown in FIG. 4, and example 2 shows a first coulombic efficiency of 66.01% and an initial reversible specific capacity of 324.9 mAh / g; Figure 5 The rate performance graph shown in FIG. 5 shows that the material of example 2 shows better rate performance; Figure 6 The cycle performance graph of comparative example 1 and example 2 at 200 mA / g is shown in FIG. 6, and after 50 cycles, the charge capacity of comparative example 1 is 130.4 mAh / g, and the capacity retention rate is 49.99%, and the charge capacity of example 2 is 193.3 mAh / g, and the capacity retention rate is 58.02%; Figure 7 、 8The cyclic voltammograms of Comparative Example 1 and Example 2 at a scan rate of 0.1 mV / s are shown. Compared with Comparative Example 1, Example 2 shows better coincidence in the second and third cycles, indicating that it has higher sodium ion intercalation / deintercalation reversibility.

[0083] The above embodiments are only the preferred embodiments of the present application, which describe the basic principles and characteristics of the present application. The present application is not limited to the above specific embodiments. Any improvement and optimization within the scope of the disclosed method is within the protection scope of the present application.

Claims

1. A method for preparing a hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells, characterized in that: include: Step 1: washing and drying the discarded shells from betel nut shells; Step 2: The dried waste shells are crushed and transferred to a muffle furnace. In an air atmosphere, the pyrolysis temperature is controlled at 250-350°C for carbonization, and the pre-carbonized material is obtained after natural cooling; Step 3, pre-treating the pre-carbonized material by a hydrothermal method, a ball milling method or a molten salt assisted method; Step 4: Transfer the pretreated product to an atmosphere furnace, and under the protection of inert gas, control the pyrolysis temperature to 1200-1400°C for high-temperature carbonization, and cool it down after the carbonization is completed; Step 5: After cooling, soak in an acidic solution, then wash with deionized water and ethanol until neutral, dry, grind, and sieve the obtained product to obtain the said hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells.

2. The preparation method according to claim 1, wherein: In step 2, the heating rate of the muffle furnace is 5°C / min, and the pyrolysis temperature is 300°C.

3. The preparation method according to claim 1 or 2, characterized in that: In step 3, the solvent used in the hydrothermal method is one of deionized water, 1 mol / L sodium hydroxide solution and 1 mol / L potassium hydroxide solution, the ratio of pre-carbonized material to solvent is 1:10 (g / mL), the hydrothermal temperature is 180°C, and the time is 18 h.

4. The preparation method according to claim 1 or 2, characterized in that: In step 3, the planetary ball mill used in the ball milling method has a rotation speed of 500 rpm, a ball milling time of 12 h, and a mass ratio of grinding balls to pre-carbonized material of 1-7:

1.

5. The preparation method according to claim 1 or 2, characterized in that: In step three, the salt used in the molten salt-assisted method is sodium chloride, the mass ratio of sodium chloride to pre-carbonized material is 1-2:1, and the time is 8 hours.

6. The preparation method according to claim 1, wherein: In step 4, the inert gas for high-temperature carbonization is argon, the heating rate is 5°C / min, and the pyrolysis temperature is 1300°C.

7. The preparation method according to claim 1 or 6, characterized in that: In step five, the acidic solution is a dilute hydrochloric acid solution with a concentration of 1 mol / L, and the soaking time is 12 h.

8. A hard carbon negative electrode material for sodium ion batteries derived from discarded fruit shells obtained according to the preparation method of any one of claims 1 to 7.

9. The use of the hard carbon negative electrode material for sodium ion batteries derived from waste fruit shells as claimed in claim 8 in the preparation of carbon-based negative electrode materials for sodium ion batteries.

10. A sodium ion battery, characterized in that: It includes the sodium ion battery carbon-based negative electrode material prepared from the sodium ion battery hard carbon negative electrode material derived from waste fruit shells as described in claim 8.

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