Biomass hard carbon negative electrode material and preparation method thereof, negative electrode plate and sodium ion battery

Abstract

The invention provides a biomass hard carbon negative electrode material and a preparation method thereof, a negative electrode plate and a sodium ion battery. The preparation method comprises the following steps that a biomass precursor and a solution containing a deacidification chelating agent react in a supercritical fluid medium to obtain an intermediate material, and the deacidification chelating agent contains amino; and carrying out carbonization treatment on the intermediate material to obtain the biomass hard carbon negative electrode material. The invention aims to provide a technical scheme of combining supercritical fluid extraction before carbonization with a deacidification chelating agent, so that the damage to the pore structure of the material in the traditional acid / alkali washing process is avoided while the impurity removal is efficiently and deeply completed, and the specific capacity and the first-week coulombic efficiency of the biomass hard carbon negative electrode material are effectively improved. In addition, the supercritical fluid in the preparation method can be recycled, fluorine-containing wastewater is avoided, and the preparation method has the advantages of being environmentally friendly and low in cost.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of negative electrode materials, and particularly relates to a biomass hard carbon negative electrode material, a preparation method thereof, a negative electrode sheet and a sodium ion battery. BACKGROUND

[0002] With the increasing demand for clean energy worldwide, sodium ion batteries have shown broad application prospects in the energy storage field due to their advantages of abundant raw materials, low cost and high safety. As the preferred negative electrode material of sodium ion batteries, hard carbon materials have the advantages of stable structure and good cycle performance, but their commercial application still faces many challenges. Among them, the impurity problem in hard carbon negative electrode materials is particularly prominent. Biomass hard carbon negative electrode materials mostly contain various impurities such as organic, inorganic and microorganisms during the preparation process, especially inorganic salts and inorganic oxides remaining after high-temperature carbonization. The existence of these impurities not only reduces the electrical conductivity of the hard carbon negative electrode material, but also hinders the rapid transmission of sodium ions, resulting in a decrease in the reversible specific capacity of the biomass hard carbon negative electrode material and a shortening of the service life of the sodium ion battery.

[0003] The existing technology discloses a method for removing impurities from hard carbon negative electrode materials, which usually adopts a method of carbonization followed by strong acid (such as hydrofluoric acid) or strong base treatment. However, the above method has obvious limitations: (1) incomplete impurity removal: metal ions in biomass precursors and acid impurities (especially those present in micropores and mesopores) are difficult to be effectively removed by conventional acid / alkali washing, resulting in insufficient purity of the prepared hard carbon negative electrode material, which further affects its electrochemical performance; (2) high risk of structure damage: the use of strong acid or strong base treatment may damage the pore structure and surface functional groups of the material, resulting in a decrease in the specific surface area and pore collapse, which further affects the sodium storage performance of the hard carbon negative electrode material; (3) environmental pollution: traditional acid or alkali washing treatment requires multiple washing (such as acid washing-water washing-alkali washing-water washing), which is complex, time-consuming and water-consuming. In addition, the residual acid substances (such as hydrofluoric acid) may corrode the equipment and produce fluorine-containing wastewater, increasing the environmental protection treatment cost.

[0004] In addition, the pore structure, carbon layer spacing and surface chemical properties of the hard carbon negative electrode material are crucial to its sodium storage performance. Although traditional preparation methods can partially adjust the pore structure of the material, it is difficult to achieve precise control, resulting in poor specific capacity, first-cycle coulombic efficiency and rate performance of the prepared hard carbon negative electrode material, which further hinders the industrial application of hard carbon negative electrode materials.

[0005] Therefore, there is an urgent need to develop a preparation method for hard carbon negative electrode materials that is efficient, environmentally friendly and can simultaneously control the structure of the material, is simple to operate and has low cost, so as to meet the use requirements of high-performance sodium ion batteries. SUMMARY

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a biomass hard carbon anode material, its preparation method, anode sheet, and sodium-ion battery. This invention aims to provide a technical solution combining supercritical fluid extraction before carbonization with a deacidifying chelating agent. This method efficiently and deeply removes impurities while avoiding the damage to the material's pore structure caused by traditional acid / alkali washing processes. This results in a biomass hard carbon material with a porous and loose structure and a suitable specific surface area, effectively improving the specific capacity and first-cycle coulombic efficiency of the anode material. Furthermore, the supercritical fluid used in this preparation method is recyclable, eliminating the generation of fluoride-containing wastewater, and offering advantages of being environmentally friendly and cost-effective.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a biomass hard carbon anode material, the method comprising the following steps:

[0009] S1. A biomass precursor and a solution containing a deacidifying chelating agent are reacted in a supercritical fluid medium to obtain an intermediate material, wherein the deacidifying chelating agent contains an amino group;

[0010] S2. The intermediate material is subjected to carbonization treatment to obtain the biomass hard carbon anode material.

[0011] Supercritical fluid technology, due to its unique physicochemical properties (high diffusivity, strong solubility, and mild reaction conditions), has shown great potential in material purification and structure control. This invention employs a one-step impurity removal method combining supercritical fluid extraction with a deacidifying chelating agent before high-temperature carbonization. This method effectively removes various impurities from the micropores and mesopores of the material, avoiding the damage to the microporous structure caused by traditional acid / alkali washing processes after carbonization. This not only preserves the material's high specific surface area and porous structure, improving the electrochemical performance of biomass hard carbon anode materials, but also simplifies the process, reduces production costs, and minimizes environmental pollution. Specifically:

[0012] (1) The amino group in the deacidifying chelating agent is basic and can be protonated (-NH3) + This neutralizes residual acidic substances, thereby improving deacidification efficiency and preventing residual acid from corroding the carbon skeleton, which would increase surface defects and oxygen-containing functional groups, thus preventing a significant decline in the electrochemical performance of biomass hard carbon anode materials. Simultaneously, the deacidification chelating agent can chelate metal ions adsorbed in the biomass precursor to form stable complexes, thereby achieving metal ion removal and effectively preventing these metal ions from transforming into structurally stable oxides (such as Fe2O3) during subsequent high-temperature processes, which are difficult to remove, further improving the removal efficiency.

[0013] (2) Supercritical fluids have low viscosity and high diffusivity, which can carry deacidifying chelating agents to penetrate into micropores and mesopores that are difficult to reach by traditional acid washing or alkaline washing processes, deeply remove impurities in hard carbon anode materials, avoid pore collapse caused by strong acid (such as hydrofluoric acid) corrosion, retain the high specific surface area and pore structure of the material, and significantly improve the sodium storage performance of biomass hard carbon anode materials.

[0014] (3) The supercritical fluid and deacidifying chelating agent provided by the present invention can be recycled and reused, which reduces production costs and eliminates the need to treat hazardous chemical waste liquids (such as fluoride-containing wastewater after HF pickling), thereby reducing environmental pollution.

[0015] In summary, this invention effectively removes many impurities from hard carbon anode materials through the synergistic effect between supercritical fluid and amino-containing deacidifying chelating agents, while retaining the material's high specific surface area and porous structure, thereby improving the specific capacity and first-cycle coulombic efficiency of biomass hard carbon anode materials.

[0016] Preferably, in step S1, the biomass precursor is further subjected to pretreatment before the reaction.

[0017] Preferably, the pretreatment process includes sequentially crushing and acid washing the biomass precursor.

[0018] Preferably, before crushing, the particle size of the biomass precursor is 0.5mm to 2mm, for example, it can be 0.5mm, 0.8mm, 1mm, 1.2mm, 1.5mm, 1.8mm or 2mm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] This invention improves the yield of biomass hard carbon anode materials by controlling the particle size of the biomass precursor within a reasonable range, thereby facilitating thorough impurity removal and increasing the yield of biomass hard carbon anode materials. If the particle size of the biomass precursor is too small, a large amount of energy will be consumed during carbonization, and the loss on ignition will increase, resulting in a decrease in the yield of biomass hard carbon anode materials. Conversely, if the particle size of the biomass precursor is too large, insufficient impurity removal may occur later.

[0020] Preferably, the concentration of the acid solution in the pickling process is 0.2 mol / L to 5 mol / L, for example, it can be 0.2 mol / L, 0.5 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L or 5 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0021] In this invention, the acid solution exemplary includes at least one of hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, phosphoric acid, citric acid, or acetic acid.

[0022] Preferably, the pickling temperature is 20℃~100℃, and the pickling time is 0.5h~6h. The purpose of pickling is to remove metallic impurities such as sodium, potassium, calcium, and iron from the biomass precursor.

[0023] Specifically, the pickling temperature can be, for example, 20℃, 30℃, 50℃, 80℃ or 100℃; the pickling time can be, for example, 0.5h, 1h, 2h, 5h or 6h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] Preferably, in step S1, the volume ratio of the solution containing the deacidifying chelating agent to the supercritical fluid medium is (1~5):100, for example, it can be 1:100, 1.5:100, 2:100, 2.5:100, 3:100, 3.5:100, 4:100, 4.5:100 or 5:100, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the supercritical fluid medium comprises carbon dioxide and / or sulfur hexafluoride. Further, the supercritical fluid medium is carbon dioxide. This invention optimizes the surface structure of the prepared biomass hard carbon anode material by selecting a supercritical fluid medium containing functional groups such as oxygen, fluorine, or sulfur, thereby enabling it to interact with oxygen-containing active groups (e.g., hydroxyl, carboxyl groups) on the surface of the hard carbon anode material. For example, oxygen atoms can increase active sites, fluorine atoms can enhance interfacial compatibility, and sulfur atoms can expand the carbon interlayer spacing.

[0026] Preferably, in step S1, the solution containing the deacidifying chelating agent includes the deacidifying chelating agent and a solvent.

[0027] Preferably, the deacidifying chelating agent includes at least one of ethylenediamine, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraamine, or diethylenetriamine.

[0028] Preferably, based on the total mass of the solution containing the deacidifying chelating agent as 100%, the mass percentage of the deacidifying chelating agent is 0.1% to 5%, for example, it can be 0.1%, 0.5%, 1%, 2%, 3%, 4% or 5%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0029] This invention improves impurity removal efficiency by controlling the mass percentage of the deacidifying chelating agent within a reasonable range. If hard carbon anode materials are not thoroughly cleaned after acid washing, residual acid will corrode the carbon skeleton, increasing surface defects and oxygen-containing functional groups, leading to a significant decrease in electrochemical performance. The amino groups in the deacidifying chelating agent are basic and can be protonated (-NH3). + This neutralizes residual acidic substances, further improving deacidification efficiency without introducing additional acidic substances. Simultaneously, the deacidification chelating agent effectively complexes metal ions in the biomass precursor. Utilizing the low viscosity and high diffusivity of the supercritical fluid medium, the deacidification chelating agent can penetrate deep into the micropores and mesopores that are difficult to reach with traditional acid or alkali washing, effectively preventing pore collapse caused by strong acid or alkali corrosion. This ensures that the resulting biomass hard carbon anode material possesses both a high specific surface area and a suitable porous structure.

[0030] Preferably, the solvent includes ethanol and optionally water. The solvent ethanol in the deacidifying chelating agent has hydroxyl groups, which can form hydrogen bonds with polar small molecules, significantly improving the solubility of small molecule organic matter in biomass precursors in supercritical fluid media, and effectively removing residual small molecule organic matter in biomass precursors.

[0031] Preferably, based on the total mass of the solution containing the deacidifying chelating agent as 100%, the mass percentage of ethanol is 20% to 100%, for example, it can be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] In this invention, ethanol and water can form a composite solvent system with a supercritical fluid medium under supercritical conditions, further enhancing the solubility of small molecule organic matter. Simultaneously, the high permeability and low viscosity of the supercritical fluid medium allow it to rapidly penetrate the matrix. Through the formation of hydrogen bonds between the hydroxyl groups in ethanol and polar small molecules, the solubility of small molecule organic matter in biomass precursors in the supercritical fluid medium is significantly improved, thereby effectively removing residual small molecule organic matter from biomass precursors.

[0033] Preferably, in step S1, the gas pressure of the reaction is 3.8 MPa to 55 MPa, for example, it can be 3.8 MPa, 5 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, 50 MPa or 55 MPa, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] This invention improves the supercritical reaction process by adjusting the reaction pressure to a reasonable range and appropriately increasing the density of the supercritical fluid medium. This enhances the solubility of the supercritical fluid medium in small organic molecules and metal ions. If the reaction pressure is too low, the supercritical fluid medium cannot reach a supercritical state; if the reaction pressure is too high, it will lead to increased equipment load, higher energy consumption, and may trigger other side reactions.

[0035] Preferably, in step S1, the flow rate of the supercritical fluid medium in the reaction is 0.02 L / min to 1 L / min, for example, it can be 0.02 L / min, 0.05 L / min, 0.1 L / min, 0.2 L / min, 0.5 L / min, 0.8 L / min or 1 L / min, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] This invention improves the impurity removal efficiency by controlling the flow rate of the supercritical fluid medium in the reaction to a reasonable range. If the flow rate is too slow, the residence time of the supercritical fluid will be prolonged, the processing throughput will be reduced, and energy consumption will be increased; if the flow rate is too fast, the contact between the supercritical fluid and the biomass precursor will be insufficient, ultimately reducing the impurity removal efficiency.

[0037] Preferably, in step S1, the reaction temperature is 35℃ to 55℃, for example, it can be 35℃, 40℃, 45℃, 50℃ or 55℃, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0038] This invention further improves the impurity removal efficiency by controlling the reaction temperature within a reasonable range. If the reaction temperature is too low, the mass transfer rate and reaction kinetics of the supercritical fluid will be significantly slowed down, and phase separation is likely to occur near the critical point; if the reaction temperature is too high, the density of the supercritical fluid medium will be reduced, thereby weakening its ability to dissolve small organic molecules and metal ions.

[0039] Preferably, in step S1, the reaction time is 1h to 10h, more preferably 3h to 7h, for example, it can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0040] This invention further improves the impurity removal efficiency by controlling the reaction time within a reasonable range. If the reaction time is insufficient, impurities inside the hard carbon anode material cannot be completely removed, and the pore structure cannot be fully adjusted; if the reaction time is too long, it may damage the internal structure of the hard carbon anode material.

[0041] Specifically, when the supercritical fluid medium is carbon dioxide, its supercritical temperature is 31.1℃ and the reaction pressure is not lower than the critical pressure of 7.38MPa; when the supercritical fluid medium is sulfur hexafluoride, its supercritical temperature is 45.6℃ and the reaction pressure is not lower than the critical pressure of 3.76MPa.

[0042] Preferably, in step S1, after the reaction is completed, the process further includes depressurization, water washing, and drying.

[0043] In this invention, the pressure reduction to atmospheric pressure has the following technical effects: the micro-explosion effect generated by the rapid vaporization of the supercritical fluid medium during pressure reduction can pre-form a microporous structure inside the material. This not only preserves the porous and loose characteristics of the biomass hard carbon anode material, but also significantly increases its specific surface area, thus exhibiting excellent sodium storage capacity.

[0044] In this invention, the purpose of water washing is to remove small amounts of additives and deacidifying chelating agents remaining on the surface of the biomass precursor.

[0045] In this invention, the drying process exemplary includes at least one of freeze drying, vacuum drying, or forced-air drying.

[0046] Preferably, in step S2, the carbonization process includes a first carbonization process and a second carbonization process performed sequentially.

[0047] In this invention, both the first carbonization treatment and the second carbonization treatment are carried out in an inert atmosphere, which exemplarily includes at least one of argon or nitrogen.

[0048] Preferably, the temperature of the first carbonization treatment is 300℃~600℃, and the time of the first carbonization treatment is 3h~6h.

[0049] Specifically, the temperature of the first carbonization treatment can be, for example, 300℃, 400℃, 500℃ or 600℃; the time of the first carbonization treatment can be, for example, 3h, 4h, 5h or 6h, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0050] On the one hand, if the temperature of the first carbonization process is too high, it will cause a violent release of volatiles, destroying the natural framework structure of the biomass precursor, causing pore collapse, a decrease in specific surface area, and possible excessive cross-linking, thereby reducing the active sites for sodium storage. On the other hand, if the temperature of the first carbonization process is too low, it will be difficult to effectively remove volatiles and oxygen-containing groups. The residual impurities will cause structural disorder in subsequent high-temperature carbonization, thereby reducing the yield and conductivity of hard carbon anode materials.

[0051] On the other hand, if the first carbonization process takes too long, it will waste energy and may lead to over-carbonization, causing the pores of the hard carbon anode material to shrink or close unreasonably. If the first carbonization process takes too short, the reaction will be insufficient, and the residual volatiles will cause the material structure to be damaged during high-temperature carbonization, thereby reducing the electrochemical performance of the hard carbon anode material.

[0052] Preferably, the temperature of the second carbonization treatment is 1000℃~1600℃, and the time of the second carbonization treatment is 2h~10h.

[0053] Specifically, the temperature of the second carbonization process can be, for example, 1000℃, 1200℃, 1400℃ or 1600℃; the time of the second carbonization process can be, for example, 2h, 5h, 8h or 10h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] The temperature of the second carbonization process has a significant impact on the structure and performance of hard carbon anode materials. Specifically, if the temperature of the second carbonization process is too low, it is difficult to form a stable carbon framework structure, which cannot provide sufficient interlayer sodium storage sites; if the carbonization temperature of the second carbonization process is too high, it results in a narrower carbon interlayer spacing in the hard carbon anode material, reducing the active sites for sodium storage and porosity, ultimately affecting the electrochemical performance of the hard carbon anode material.

[0055] Secondly, the present invention provides a biomass hard carbon anode material, which is prepared by the method for preparing biomass hard carbon anode material as described in the first aspect. The biomass hard carbon anode material prepared by the present invention exhibits excellent sodium storage capacity.

[0056] Thirdly, the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector and a negative electrode active material layer disposed on at least one side of the current collector, the negative electrode active material layer comprising the biomass hard carbon negative electrode material as described in the second aspect.

[0057] Fourthly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode comprises the negative electrode as described in the third aspect.

[0058] In this invention, the electrolyte exemplarily includes liquid electrolyte, gel electrolyte or solid electrolyte, and this invention does not limit it.

[0059] The sodium-ion battery assembled from the biomass hard carbon anode material prepared in this invention has both high specific capacity and high first-cycle coulombic efficiency.

[0060] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] This invention provides a method for preparing biomass hard carbon anode materials, which can produce biomass hard carbon anode materials with a porous and loose structure and a suitable specific surface area, thereby significantly improving their electrochemical performance. It has the following advantages:

[0063] (1) The preparation method provided by the present invention can efficiently remove many impurity elements before high-temperature carbonization, avoid the damage to the pore structure of the material by subsequent acid washing or alkali washing processes, retain the porous and loose characteristics and high specific surface area of ​​the material, so that the prepared biomass hard carbon anode material has excellent sodium storage performance.

[0064] (2) The deacidifying chelating agent used in this invention can neutralize acidic substances and chelate metal ions without introducing other acidic substances. At the same time, the low viscosity and high diffusivity of supercritical fluids allow the deacidifying chelating agent to penetrate into micropores and mesopores that are difficult to reach by traditional acid or alkali washing processes, thereby increasing the solubility of small molecule organic matter and metal ions in biomass precursors in supercritical fluid media and further improving the impurity removal efficiency.

[0065] (3) No strong corrosive chemicals such as hydrofluoric acid are used in the entire preparation process, which reduces environmental pollution and significantly reduces production costs. Detailed Implementation

[0066] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0067] Example 1

[0068] This embodiment provides a biomass hard carbon anode material and its preparation method, the preparation method including the following steps:

[0069] S1. After crushing the coconut shell precursor with a particle size of 1 mm, it was placed in a 1 mol / L hydrochloric acid solution and acid-washed for 3 h under constant temperature water bath at 80℃. After cleaning and filtration, the filter residue was dried and placed in a supercritical reactor.

[0070] 500 mL of supercritical carbon dioxide and 10 mL of an ethanol-water solution containing ethylenediamine (based on a total mass of 100% ethylenediamine-containing ethanol-water solution, with ethylenediamine comprising 1% and ethanol comprising 70%) were added to the aforementioned supercritical reactor, and the reaction was carried out. The temperature inside the supercritical reactor was 40 °C, the reaction pressure was 8 MPa, the carbon dioxide flow rate was 0.04 L / min, and the reaction was allowed to proceed naturally for 3 hours. The gas valve was then quickly opened to obtain the intermediate material.

[0071] S2. The above intermediate material is repeatedly washed with deionized water, then dried at 100°C, and then subjected to a first carbonization treatment and a second carbonization treatment in a nitrogen atmosphere. After the treatment, it is naturally cooled to room temperature to obtain the biomass hard carbon anode material. The temperature of the first carbonization treatment is 500°C and the time is 3 hours, and the temperature of the second carbonization treatment is 1300°C and the time is 5 hours.

[0072] Example 2

[0073] The difference between this embodiment and Embodiment 1 is that, in step S1, with the total mass of the ethanol aqueous solution containing ethylenediamine as 100%, the mass percentage of ethylenediamine is 0.4%, while all other aspects are the same as in Embodiment 1.

[0074] Example 3

[0075] The difference between this embodiment and Embodiment 1 is that, in step S1, the total mass of the ethanol aqueous solution containing ethylenediamine is 100%, and the mass percentage of ethylenediamine is 5%. All other aspects are the same as in Embodiment 1.

[0076] Example 4

[0077] The difference between this embodiment and Embodiment 1 is that in step S1, the total mass of the ethanol aqueous solution containing ethylenediamine is 100%, and the mass percentage of ethanol is 40%. Everything else is the same as in Embodiment 1.

[0078] Example 5

[0079] The difference between this embodiment and Embodiment 1 is that in step S1, the total mass of the ethanol aqueous solution containing ethylenediamine is 100%, and the mass percentage of ethanol is 20%. Everything else is the same as in Embodiment 1.

[0080] Example 6

[0081] The difference between this embodiment and Embodiment 1 is that in step S1, 100 mL of supercritical carbon dioxide and 1 mL of an ethanol-water solution containing ethylenediamine are added to the supercritical reactor (based on the total mass of the ethanol-water solution containing ethylenediamine being 100%, the mass percentage of ethylenediamine being 1%, and the mass percentage of ethanol being 70%). Everything else is the same as in Embodiment 1.

[0082] Example 7

[0083] The difference between this embodiment and Embodiment 1 is that in step S1, 100 mL of supercritical carbon dioxide and 5 mL of an ethanol-water solution containing ethylenediamine are added to the supercritical reactor (based on the total mass of the ethanol-water solution containing ethylenediamine being 100%, the mass percentage of ethylenediamine being 1%, and the mass percentage of ethanol being 70%). Everything else is the same as in Embodiment 1.

[0084] Example 8

[0085] The difference between this embodiment and Embodiment 1 is that in step S1, the flow rate of supercritical carbon dioxide is 1 L / min, while all other aspects are the same as in Embodiment 1.

[0086] Example 9

[0087] The difference between this embodiment and Embodiment 1 is that in step S1, the reaction time is 5 hours, while all other steps are the same as in Embodiment 1.

[0088] Example 10

[0089] The difference between this embodiment and Embodiment 1 is that in step S1, ethylenediamine is replaced with triethylenetetramine of equal mass percentage, while all other aspects are the same as in Embodiment 1.

[0090] Example 11

[0091] The difference between this embodiment and Embodiment 1 is that in step S1, supercritical carbon dioxide is replaced with an equal volume of supercritical sulfur hexafluoride, and the reaction temperature is adjusted to 52°C. All other aspects are the same as in Embodiment 1.

[0092] Example 12

[0093] The difference between this embodiment and Embodiment 1 is that in step S1, the coconut shell precursor is replaced with an equal amount of bellflower precursor, while everything else is the same as in Embodiment 1.

[0094] Example 13

[0095] The difference between this embodiment and Embodiment 1 is that, in step S1, with the total mass of the ethanol aqueous solution containing ethylenediamine as 100%, the mass percentage of ethylenediamine is 0.05%, and the mass percentage of ethanol is 10%, while all other aspects are the same as in Embodiment 1.

[0096] Example 14

[0097] The difference between this embodiment and Embodiment 1 is that, in step S1, with the total mass of the ethanol aqueous solution containing ethylenediamine as 100%, the mass percentage of ethylenediamine is 10%, and the mass percentage of ethanol is 70%, while all other aspects are the same as in Embodiment 1.

[0098] Example 15

[0099] The difference between this embodiment and Embodiment 1 is that in step S1, 100 mL of supercritical carbon dioxide and 0.5 mL of an ethanol-water solution containing ethylenediamine are added to the supercritical reactor (based on the total mass of the ethanol-water solution containing ethylenediamine being 100%, the mass percentage of ethylenediamine being 1%, and the mass percentage of ethanol being 70%). Everything else is the same as in Embodiment 1.

[0100] Example 16

[0101] The difference between this embodiment and Embodiment 1 is that in step S1, 100 mL of supercritical carbon dioxide and 10 mL of an ethanol-water solution containing ethylenediamine are added to the supercritical reactor (based on the total mass of the ethanol-water solution containing ethylenediamine being 100%, the mass percentage of ethylenediamine being 1%, and the mass percentage of ethanol being 70%). Everything else is the same as in Embodiment 1.

[0102] Example 17

[0103] The difference between this embodiment and Embodiment 1 is that in step S1, the flow rate of carbon dioxide is 5 L / min, while all other aspects are the same as in Embodiment 1.

[0104] Comparative Example 1

[0105] The difference between this comparative example and Example 1 is that step S1 is adjusted as follows: Coconut shell precursors with a particle size of 1 mm are crushed and placed in a 1 mol / L hydrochloric acid solution. The mixture is then acid-washed for 3 hours under a constant temperature water bath at 80°C. After washing and filtration, the filter residue is dried and placed in a supercritical reactor. The temperature inside the supercritical reactor is adjusted to 40°C, the reaction pressure to 8 MPa, and the carbon dioxide flow rate to 0.04 L / min. After reacting for 3 hours, the reactor is allowed to cool naturally, and the gas valve is quickly opened to obtain the intermediate material. All other steps are the same as in Example 1.

[0106] Comparative Example 2

[0107] This comparative example provides a hard carbon anode material and its preparation method, the preparation method comprising the following steps:

[0108] S1. After crushing the coconut shell precursor with a particle size of 1 mm, it was placed in a 1 mol / L hydrochloric acid solution and acid-washed for 3 h in a constant temperature water bath at 80 °C. After cooling, it was washed with deionized water until the pH value was 7.5 and then dried at 100 °C.

[0109] S2. The purified coconut shell precursor obtained in step S1 is carbonized at low temperature of 500℃ for 3 hours in a nitrogen atmosphere; then transferred to a 1 mol / L hydrochloric acid solution and acid-washed at 60℃ for 10 hours, and washed with deionized water until neutral; then carbonized at high temperature of 1300℃ for 5 hours, and naturally cooled to obtain the hard carbon anode material.

[0110] Comparative Example 3

[0111] This comparative example provides a hard carbon anode material and its preparation method, the preparation method comprising the following steps:

[0112] S1. After crushing the coconut shell precursor with a particle size of 1 mm, it was placed in a 1 mol / L hydrochloric acid solution and acid-washed for 3 h in a constant temperature water bath at 80 °C. After cooling, it was washed with deionized water until the pH value was 7.5 and then dried at 100 °C.

[0113] S2. The purified coconut shell precursor obtained in step S1 is carbonized at low temperature of 500℃ for 3 hours in a nitrogen atmosphere; then transferred to a sodium hydroxide solution with a concentration of 1 mol / L, and alkali washed at 60℃ for 10 hours, and washed with deionized water until neutral; then carbonized at high temperature of 1300℃ for 5 hours, and naturally cooled to obtain the hard carbon anode material.

[0114] Comparative Example 4

[0115] This comparative example provides a hard carbon anode material and its preparation method, the preparation method comprising the following steps:

[0116] S1. After crushing the coconut shell precursor with a particle size of 1 mm, it was placed in a 1 mol / L hydrochloric acid solution and acid-washed for 3 h in a constant temperature water bath at 80 °C. After cooling, it was washed with deionized water until the pH value was 7.5 and then dried at 100 °C.

[0117] S2. The purified coconut shell precursor obtained in step S1 is carbonized at low temperature of 500℃ for 3 hours in a nitrogen atmosphere; then transferred to a 1 mol / L hydrochloric acid solution and acid-washed at 60℃ for 10 hours; then transferred to a 1 mol / L sodium hydroxide solution and alkaline-washed at 50℃ for 12 hours; washed with deionized water until neutral; then carbonized at high temperature of 1300℃ for 5 hours; and naturally cooled to obtain the hard carbon anode material.

[0118] Preparation of sodium-ion batteries

[0119] The hard carbon anode material, Super P, CMC, and SBR provided in the above examples and comparative examples were mixed into a homogenate at a mass ratio of 94:1.5:2:2.5 to obtain an anode slurry. The anode slurry was coated onto copper foil using a four-sided coating tool with a thickness of 120 μm, and then the membrane was dried in a vacuum drying oven at 100 °C for 2 h. The electrode membrane was punched into a disc with a radius of 0.6 mm using a punching machine. Sodium metal was used as the counter electrode, and NaClO4 with a concentration of 1 mol / L and EC+DEC (1:1 vol%) were used as the electrolyte. The separator was a PP / PE / PP three-layer separator. The membrane was assembled into a CR2016 button cell in a glove box.

[0120] Test conditions

[0121] The sodium-ion batteries prepared in the above examples and comparative examples were subjected to constant current charge-discharge tests with a current density of 0.1C (1C=300mAh / g) and a voltage range of 2V~0.005V.

[0122] The test results are shown in Table 1:

[0123] Table 1

[0124]

[0125] As can be seen from Table 1, the following conclusions can be drawn:

[0126] (1) Comparing the test data of Examples 1 to 11 in Table 1, it can be seen that the biomass hard carbon material prepared by the pre-carbonization supercritical fluid extraction combined with deacidification chelating agent purification process provided by the present invention exhibits excellent electrochemical performance within a wide range of supercritical process parameters. Specifically, the first-week charge specific capacity is stable in the range of 339.1 mAh / g to 359.1 mAh / g, and the first-week coulombic efficiency reaches 89.8% to 93.9%, fully verifying that the preparation method has wide process adaptability. Among them, Example 11 shows the best performance indicators, with the highest first-week charge specific capacity.

[0127] The above experimental results demonstrate that, under optimized supercritical reaction conditions, the deacidifying chelating agent can effectively remove metal ion impurities from biomass precursors through its chelating effect. Combined with the high diffusivity and solvent selectivity unique to supercritical fluids, it can fully dissolve and remove small molecule organic matter, inorganic salts, and other impurities remaining in the material pores from the matrix, avoiding pore collapse caused by strong acid corrosion. While retaining the high specific surface area and pore structure of hard carbon, it achieves efficient purification of the material, effectively improving the sodium storage capacity and first-cycle coulombic efficiency of biomass hard carbon materials.

[0128] (2) Based on the test results of Examples 1 and 11 in Table 1, it can be seen that deep impurity removal of biomass hard carbon materials can be achieved using different supercritical fluid media. Among them, the biomass hard carbon material prepared using sulfur hexafluoride as the supercritical fluid medium has a slightly higher first-cycle charging specific capacity. This is because sulfur hexafluoride contains functional groups such as fluorine and sulfur, which can interact with active hydroxyl and carboxyl groups on the surface of hard carbon, further optimizing the surface structure of hard carbon, increasing sodium storage sites and enhancing interfacial compatibility, thereby improving the comprehensive electrochemical performance of biomass hard carbon materials.

[0129] (3) Based on the test data of Example 1 and Example 12 in Table 1, the first-week charging specific capacity of Example 1 and Example 12 is 356.5mAh / g and 355.9mAh / g, respectively, and the first-week coulombic efficiency is 93.9% and 93.6%, respectively. Both have high first-week charging specific capacity and first-week coulombic efficiency, indicating that the impurity removal process using supercritical fluid extraction combined with deacidification chelating agent is suitable for different types of biomass precursor raw materials.

[0130] (4) Based on the test data of Examples 1, 2, 3 and 4 in Table 1, it can be seen that compared with the bio-hard carbon materials prepared by acid washing, alkali washing or combined acid-alkali cleaning, the bio-hard carbon materials prepared by supercritical extraction combined with deacidifying chelating agent purification process have superior electrochemical performance. Specifically, they exhibit higher first-week charging specific capacity and first-week coulombic efficiency. This indicates that acid washing or alkali washing can remove some impurities in biomass hard carbon materials, but cannot completely remove them. However, the supercritical extraction combined with deacidifying chelating agent purification process can maintain the porous structure of the material, effectively avoiding microporous structure damage and element residue caused by traditional acid-alkali washing, thereby significantly improving the electrochemical performance of biomass hard carbon materials. In addition, the supercritical extraction step is set before carbonization treatment, thus avoiding the damage to the microporous structure of the material caused by traditional acid washing and / or alkali washing after carbonization. Furthermore, the supercritical fluid medium and deacidifying chelating agent in this process route can be reused, further reducing costs and waste liquid treatment steps.

[0131] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a biomass hard carbon anode material, characterized in that, The preparation method includes the following steps: S1. A biomass precursor and a solution containing a deacidifying chelating agent are reacted in a supercritical fluid medium to obtain an intermediate material, wherein the deacidifying chelating agent contains an amino group; S2. The intermediate material is subjected to carbonization treatment to obtain the biomass hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, In step S1, the biomass precursor is further subjected to pretreatment before the reaction. And / or, the pretreatment process includes sequentially crushing and acid washing the biomass precursor; And / or, before crushing, the particle size of the biomass precursor is 0.5 mm to 2 mm; And / or, the concentration of the acid solution in the pickling treatment is 0.2 mol / L to 5 mol / L; And / or, the pickling temperature is 20℃~100℃, and the pickling time is 0.5h~6h.

3. The preparation method according to claim 1 or 2, characterized in that, In step S1, the volume ratio of the solution containing the deacidifying chelating agent to the supercritical fluid medium is (1~5):100; And / or, the supercritical fluid medium includes carbon dioxide and / or sulfur hexafluoride.

4. The preparation method according to claim 1, characterized in that, In step S1, the solution containing the deacidifying chelating agent includes the deacidifying chelating agent and a solvent; And / or, the deacidifying chelating agent includes at least one of ethylenediamine, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraamine, or diethylenetriamine; And / or, based on the total mass of the solution containing the deacidifying chelating agent as 100%, the mass percentage of the deacidifying chelating agent is 0.1% to 5%; And / or, the solvent includes ethanol and optionally water; And / or, based on the total mass of the solution containing the deacidifying chelating agent as 100%, the mass percentage of ethanol is 20% to 100%.

5. The preparation method according to claim 1, characterized in that, In step S1, the gas pressure of the reaction is 3.8 MPa to 55 MPa; And / or, in step S1, the flow rate of the supercritical fluid medium in the reaction is 0.02 L / min to 1 L / min; And / or, in step S1, the reaction temperature is 35°C to 55°C; And / or, in step S1, the reaction time is 1 h to 10 h.

6. The preparation method according to claim 1, characterized in that, In step S1, after the reaction is completed, the process also includes depressurization, water washing and drying.

7. The preparation method according to claim 1, characterized in that, In step S2, the carbonization process includes a first carbonization process and a second carbonization process performed sequentially. And / or, the temperature of the first carbonization treatment is 300℃~600℃, and the time of the first carbonization treatment is 3h~6h; And / or, the temperature of the second carbonization treatment is 1000℃~1600℃, and the time of the second carbonization treatment is 2h~10h.

8. A biomass hard carbon anode material, characterized in that, The biomass hard carbon anode material is prepared by the method described in any one of claims 1-7.

9. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one side of the current collector, the negative electrode active material layer including the biomass hard carbon negative electrode material as described in claim 8.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes the negative electrode as described in claim 9.

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