High-capacity, high-rate hard carbon negative electrode material and preparation method and application thereof

Abstract

The application provides a high-capacity and high-rate hard carbon negative electrode material and a preparation method and application thereof, and belongs to the technical field of sodium ion battery negative electrode materials. Specifically, biomass shell raw materials are coarsely broken and sieved to obtain biomass coarse powder; the biomass coarse powder is mixed with an oxidizing agent and then pre-oxidized; mechanical pulverization is performed to obtain biomass coarse powder with a D50 of 8-15 microns; the biomass coarse powder is added into mixed acid and heated in a water bath, and then washed with water until neutral to obtain a purified precursor; and finally, the precursor is subjected to staged high-temperature carbonization in a vacuum carbonization furnace. Through specific process parameters, the prepared hard carbon material has a hierarchical porous structure and a suitable graphite microcrystalline layer spacing, and has a low total content of metal impurities; when applied to a sodium ion battery negative electrode, the hard carbon material exhibits a high reversible specific capacity, a high initial coulombic efficiency, and excellent long cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, and in particular to a high-capacity, high-rate hard carbon anode material, its preparation method, and its application. Background Technology

[0002] With the rapid development of large-scale energy storage and low-speed electric vehicles, sodium-ion batteries have become one of the most promising next-generation electrochemical energy storage systems due to their advantages such as abundant sodium resources and low cost. Among many anode materials, hard carbon materials, with their suitable interlayer spacing for sodium ion insertion / extraction, abundant defects and pore structures, exhibit excellent sodium storage capacity and cycle stability, and are considered to be the most promising sodium battery anode material for practical application.

[0003] Biomass, due to its wide availability, low cost, and abundant carbon content, has become an ideal precursor for preparing hard carbon anode materials. However, hard carbon obtained by directly carbonizing untreated biomass usually suffers from problems such as high impurity content, non-uniform structural disorder, low initial coulombic efficiency, and poor rate performance, which restricts its application in high-performance sodium-ion batteries.

[0004] Currently, pre-oxidation and acid treatment of biomass precursors are common improvement methods. For example: Chinese patent CN118993020A discloses a method for preparing sodium-ion battery anode materials, which involves pre-oxidation in air to stabilize the precursor structure, followed by immersion in a single inorganic acid solution to remove impurities. However, this method is simplistic, using only a single acid such as sulfuric acid, which is insufficient for removing certain metallic impurities, affecting the purity of the final product. Furthermore, relying solely on an air atmosphere for pre-oxidation results in insufficient oxidation intensity and poor controllability, making it impossible to form a stable carbon framework suitable for sodium storage.

[0005] Chinese patent CN120622464A discloses a method for preparing element-doped biomass hard carbon anode materials, which involves immersing pretreated biomass raw materials in an oxidant solution followed by low-temperature pre-oxidation. However, this method is complex, and improper control of the pre-oxidation process can easily lead to a significant reduction in carbon yield, affecting production efficiency and economics. Secondly, the process may introduce new impurities, affecting the quality of the final product. Moreover, in the liquid mixing process, the mixture needs to be dried first, then dispersed, and then pre-oxidized. During the drying process, as the surface moisture decreases, more solution is drawn from the lower layer and migrates upward through the capillaries between particles (capillary effect). The solute dissolved in the water is transported to the surface along with the moisture. This process is continuous, resulting in uneven mixing concentration between the upper and lower layers. Furthermore, during drying, the surface may be dried to a blackened state while the lower layer remains undried, affecting product consistency.

[0006] Therefore, there is an urgent need to develop a comprehensive method that is simple in process, highly controllable, and can simultaneously achieve deep purification and effective structural regulation of biomass precursors, so as to prepare high-performance biomass hard carbon anode materials. Summary of the Invention

[0007] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a method for preparing high-capacity, high-rate hard carbon anode materials, which solves the problems of single treatment of biomass precursors, complex process control, possible introduction of new impurities, and lack of comprehensive treatment methods in the prior art. At the same time, this invention will also provide high-capacity, high-rate hard carbon anode materials prepared by this method; in addition, this invention will also provide applications of this high-capacity, high-rate hard carbon anode material.

[0008] To achieve the above and other related objectives, the present invention provides the following technical solutions: In a first aspect, the present invention provides a method for preparing a high-capacity, high-rate hard carbon anode material, comprising the following steps: S1. Raw material pretreatment: After coarsely crushing the biomass husk raw material, it is sieved to obtain coarse biomass powder; S2, Pre-oxidation treatment: The biomass coarse powder obtained in step S1 is mixed evenly with the oxidant, and then the temperature is raised to 200-300℃ at a heating rate of 2-5℃ / min. Pre-oxidation is carried out at this temperature. After the pre-oxidation is completed, the mixture is cooled naturally. The pre-oxidized material is then broken up and sieved to obtain pre-oxidized powder. S3, Crushing: The pre-oxidized powder obtained in step S2 is mechanically pulverized to control the median particle size D50 of the final powder to be between 8 and 15 micrometers. S4. Compound acid purification and etching: The pulverized powder from step S3 is added to a mixed acid and heated in a water bath. Then it is washed with water until neutral, removed and dried to obtain the purified precursor. S5, carbonization: The purified precursor obtained in step S4 is placed in a vacuum carbonization furnace and carbonized in stages under continuous vacuum or inert gas protection. After carbonization, it is naturally cooled to room temperature under vacuum or inert gas protection and then sieved to obtain high-capacity, high-rate hard carbon anode material.

[0009] Furthermore, in step S1, the biomass shell raw material is selected from one or more combinations of coconut shells, apricot shells, and peanut shells.

[0010] The choice of different biomass shell raw materials significantly affects the microstructure and electrochemical performance of hard carbon materials. Coconut shells, due to their unique cellulose structure, can form a more ordered carbon layer arrangement during carbonization, which is beneficial for the insertion and extraction of sodium ions. Apricot shells, with their moderate porosity, provide good channels for sodium ion transport, while peanut shells, with their high lignin content, contribute to the formation of a stable carbon framework. By rationally selecting and combining different biomass shell raw materials, the structural characteristics of hard carbon materials can be further optimized, improving their performance in sodium-ion batteries.

[0011] Furthermore, in step S1, the biomass husk raw material is coarsely crushed in a coarse crusher, then passed through a 100-mesh sieve. The sieve-passed material is washed with deionized water and dried to obtain coarse biomass powder.

[0012] Furthermore, in step S2, the oxidant is one or more of potassium persulfate, ammonium persulfate, and sodium persulfate.

[0013] Furthermore, the biomass coarse powder and oxidant are mixed at a mass ratio of 1:0.05 to 1:0.5.

[0014] Furthermore, in step S2, the pre-oxidation time is 9 to 12 hours.

[0015] Furthermore, in step S2, the mixing time is 0.5 to 2 hours to ensure that the biomass powder and oxidant are mixed evenly; after being mixed evenly, the mixture is placed in a high-temperature oven for pre-oxidation treatment in an air atmosphere.

[0016] In the pre-oxidation process, compared with liquid mixing, the present invention uses solid mixing with fewer variables, which is simpler and more efficient, reducing time and cost losses; moreover, it can avoid capillary effect, completely solve the problem of concentration difference between upper and lower layers, ensure uniformity of mixing concentration, and achieve uniform oxidation inside and outside the particles; at the same time, it can eliminate the coexistence of local carbonization and undried phenomena, making the reaction conditions more controllable and ensuring the consistency of pre-oxidation reaction.

[0017] Furthermore, in step S3, mechanical pulverization is performed using an air jet mill or a ball mill.

[0018] Furthermore, in step S4, the mixed acid is composed of hydrochloric acid, sulfuric acid and nitric acid, with the following mass percentages: hydrochloric acid 10%~50%, sulfuric acid 5%~40%, and nitric acid 15%~40%. This mass ratio is the direct mixing ratio of hydrochloric acid, sulfuric acid and nitric acid, wherein the concentration of hydrochloric acid is 37%, the concentration of sulfuric acid is 98%, and the concentration of nitric acid is 65%.

[0019] Furthermore, in step S4, the water bath heating temperature is 60~90℃, and the water bath heating time is 2~6 hours.

[0020] Furthermore, in step S4, the drying temperature is 80~120℃ and the drying time is 6~12 hours.

[0021] Furthermore, in step S5, the inert gas is argon or nitrogen.

[0022] Furthermore, in step S5, the segmented high-temperature carbonization specifically includes: in the first stage, heating to 500-700°C at a heating rate of 3-8°C / min and holding for 0.5-2 hours; in the second stage, continuing to heat to 900-1200°C at the same or different heating rates and holding for 1-3 hours.

[0023] Furthermore, in step S5, when the carbonization temperature is in the range of 500°C to 900°C, the absolute pressure inside the furnace is controlled to be maintained at 200-500 Pa; when the carbonization temperature rises above 900°C until the end of carbonization (the final carbonization temperature is 900~1200°C, including the endpoint value), the absolute pressure inside the furnace is controlled to be maintained at a high vacuum state of -50 Pa to 0 Pa.

[0024] The segmented carbonization process is the key technical feature of this invention. The first stage of medium-temperature treatment is conducive to the slow decomposition and preliminary carbonization of organic matter, while the second stage of high-temperature treatment promotes further rearrangement of the carbon structure and the formation of graphite microcrystals. Different vacuum levels are used to control different temperature ranges, which ensures the timely removal of volatile products during carbonization and avoids damage to the carbon structure caused by excessive oxidation.

[0025] Furthermore, in step S5, after naturally cooling to room temperature, the sample is taken out and passed through a 325-mesh sieve.

[0026] Furthermore, in step S5, the temperature is lowered under an inert gas or continuous vacuum to prevent air from entering the furnace during the cooling process and causing secondary oxidation of the sample.

[0027] In a second aspect, the present invention provides a high-capacity, high-rate hard carbon anode material prepared by the above-described preparation method.

[0028] Furthermore, this hard carbon material exhibits a typical amorphous structure, with a graphite interlayer spacing (d002) of 0.38–0.42 nm and a specific surface area of ​​5–50 m². 2 / g; The hard carbon material has a hierarchical porous structure, with micropores below 2 nm and mesopores of 2~50 nm as the main pore size distribution; The total content of residual metal impurities on the surface of the hard carbon material is less than 500 ppm.

[0029] A third aspect of the present invention provides a sodium-ion battery anode comprising the high-capacity, high-rate hard carbon anode material as an active material.

[0030] In a fourth aspect, the present invention provides a sodium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the aforementioned sodium-ion battery negative electrode.

[0031] Furthermore, the sodium-ion battery exhibits a reversible specific capacity of ≥330 mAh / g at a current density of 33 mA / g, an initial efficiency of greater than 88%, and a capacity retention of ≥84.69% after 1000 cycles at 1C.

[0032] As described above, the high-capacity, high-rate hard carbon anode material, its preparation method, and its application of the present invention have the following beneficial effects: 1. Low raw material cost and green renewable: This invention uses widely available and inexpensive agricultural waste (coconut shells, apricot shells, peanut shells) as raw materials, realizing the high-value utilization of resources and conforming to the concept of sustainable development.

[0033] 2. High efficiency of pretreatment process: Pre-oxidation can stabilize the framework structure and form sodium storage active sites. The composite acid system can achieve deep removal of impurities and synergistic regulation of surface properties. It overcomes the defect of incomplete removal of certain metal impurities by single sulfuric acid treatment in the existing technology and avoids the problem of significant reduction in carbon yield caused by improper control of conventional pre-oxidation.

[0034] 3. Pre-oxidation: This process is carried out under specific oxidants (persulfate series) and temperature ranges (200~300℃). It can introduce abundant oxygen-containing functional groups into the biomass molecular chain and initiate cross-linking reactions, effectively stabilizing its skeletal structure in the subsequent high-temperature carbonization process, inhibiting the excessive growth of graphite microcrystals, and facilitating the formation of more sodium-storing active sites.

[0035] 4. Mixed Acid Purification: An innovative composite acid system using hydrochloric acid, sulfuric acid, and nitric acid is employed. Hydrochloric acid effectively removes alkali metals and some metallic impurities; sulfuric acid has a strong dehydrating and carbonizing effect, contributing to the formation of a stable carbon framework and initial porosity; nitric acid, as a strong oxidant, further oxidizes and etches the carbon surface, increasing defects and micropores, and enhancing the material's hydrophilicity. The synergistic effect of these three components achieves deep removal of complex impurities from biomass and synergistic regulation of material surface properties—effects that cannot be achieved with single-acid treatments.

[0036] 5. Precise and controllable carbonization process: The segmented temperature design and precise pressure control are adopted. The absolute pressure is controlled at 200~500Pa in the medium temperature range (500~900℃) and the high temperature range (>900℃) is maintained at -50~0Pa high vacuum. This can effectively remove impurity gases, control the spacing between graphite microcrystals, avoid structural inhomogeneity caused by carbonization at a single temperature or pressure, and ensure the stability and consistency of the material structure.

[0037] 6. Excellent Electrochemical Performance: The hard carbon material prepared by the above-mentioned synergistic process possesses an optimized pore structure (abundant micro / mesopores) and suitable interlayer spacing (0.38~0.42nm), providing favorable conditions for the rapid transport and storage of sodium ions. When applied to the anode of sodium-ion batteries, it exhibits high reversible specific capacity (exceeding 330 mAh / g at a current density of 33 mA / g), high initial coulombic efficiency (typically exceeding 88%), and excellent long-term cycle stability (capacity retention ≥84.69%) and rate performance. Attached Figure Description

[0038] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the hard carbon anode material prepared in Example 1 of this invention.

[0039] Figure 2 This is a scanning electron microscope (SEM) image of the hard carbon anode material prepared in Example 1 of the present invention.

[0040] Figure 3 The first charge-discharge curve of a sodium-ion half-cell assembled with the hard carbon anode material prepared in Example 1 of this invention at a rate of 0.1C. Detailed Implementation

[0041] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0042] Based on the method described in this invention, the technical effects of this invention are further illustrated below through specific embodiments and comparative examples. Examples 1-6 are the preparation methods of this invention, and comparative examples 7-15 are processes used for comparison.

[0043] Example 1 High-capacity, high-rate hard carbon anode materials prepared from coconut shells S1. Raw material pretreatment: Take 500g of dried coconut shells, crush them with a coarse crusher, pass them through a 100-mesh sieve, wash them three times with deionized water, and dry them at 110℃ to obtain coarse coconut shell powder.

[0044] S2. Chemical pre-oxidation: Weigh 100g of coarse coconut shell powder and mix it with 20g of ammonium persulfate (i.e., the mass ratio of raw material to oxidant is 1:0.2) in a mixer for 2 hours; put the mixture into an open crucible and place it in a high-temperature oven. Under static air atmosphere, raise the temperature to 250℃ at a heating rate of 5℃ / min and keep it at this temperature for 10 hours; after the reaction is completed, cool it and break up the pre-oxidized material and pass it through a 100-mesh sieve.

[0045] S3. Crushing: The pre-oxidized material is fed into an air jet mill and crushed until the median particle size D50 is 10 micrometers.

[0046] S4. Purification and etching of the composite acid: Prepare a mixed acid solution in which the mass ratio of concentrated hydrochloric acid (37%), concentrated sulfuric acid (98%), and concentrated nitric acid (65%) is 30:40:30; add 100g of the powder obtained in step S3 to the mixed acid and stir and cook in an 80℃ water bath for 3 hours; after the reaction is completed, filter and wash with deionized water until the filtrate is neutral, and then vacuum dry at 100℃ for 12 hours to obtain the purified precursor.

[0047] S5. High-Temperature Carbonization with Segmented Pressure Control: The dried, purified precursor is placed in a programmable pressure-controlled high-temperature vacuum tube furnace. After evacuating the furnace to a low vacuum, the temperature is increased at a rate of 5°C / min. When the temperature reaches 500°C, the furnace pressure is controlled at 400Pa (if the furnace pressure is too low, high-purity nitrogen is reintroduced to precisely control the absolute pressure at 400Pa). This pressure is maintained, and the temperature is increased to 600°C and held for 1 hour. Then, the temperature is increased to 900°C at 2°C / min under 400Pa pressure. When the temperature reaches 900°C, the high vacuum system is activated, and the absolute pressure inside the furnace is rapidly reduced to 0 Pa (high vacuum state). Under this high vacuum, the temperature is increased at 2°C / min to the final carbonization temperature of 1200°C and held for 2 hours. After the holding period, the temperature is naturally cooled to below 100°C under continuous high vacuum. Nitrogen is then introduced to atmospheric pressure, and the sample is removed and passed through a 325-mesh sieve to obtain hard carbon material, denoted as HC-1.

[0048] Example 2 High-capacity, high-rate hard carbon anode materials prepared using apricot shells as raw materials S1. Raw material pretreatment: Take 500g of dried apricot shells, crush them with a coarse crusher, pass them through a 100-mesh sieve, wash them three times with deionized water, and dry them at 110℃ to obtain coarse apricot shell powder.

[0049] S2. Chemical pre-oxidation: Weigh 100g of coarse apricot shell powder and mix it with 30g of sodium persulfate (i.e., the mass ratio of raw material to oxidant is 1:0.3) in a mixer for 2 hours; put the mixture into an open crucible and place it in a high-temperature oven. Under static air atmosphere, raise the temperature to 300℃ at a heating rate of 5℃ / min and keep it at this temperature for 12 hours; after the reaction is completed, cool it and break up the pre-oxidized material and pass it through a 100-mesh sieve.

[0050] S3. Crushing: The pre-oxidized material is fed into an air jet mill and crushed until the median particle size D50 is 15 micrometers.

[0051] S4. Purification and etching of the composite acid: Prepare a mixed acid solution in which the mass ratio of concentrated hydrochloric acid (37%), concentrated sulfuric acid (98%), and concentrated nitric acid (65%) is 45:30:25; add 100g of the powder obtained in step S3 to the mixed acid and stir and cook in a water bath at 90℃ for 2 hours; after the reaction is completed, filter and wash with deionized water until the filtrate is neutral, and then vacuum dry at 100℃ for 12 hours to obtain the purified precursor.

[0052] S5. High-Temperature Carbonization with Segmented Pressure Control: The dried, purified precursor was placed in a programmable pressure-controlled high-temperature vacuum tube furnace. After evacuating the furnace to a low vacuum, the temperature was increased at a rate of 3°C / min. When the temperature reached 500°C, the pressure inside the furnace was controlled at 500 Pa. This pressure was maintained, and the temperature was increased to 600°C and held for 2 hours. Then, the temperature was increased at 3°C / min under 500 Pa pressure to the final carbonization temperature of 900°C. When the temperature reached 900°C, the high vacuum system was activated, and the absolute pressure inside the furnace was rapidly reduced to -50 Pa (high vacuum state). The temperature was held under this high vacuum for 3 hours. After the holding period, the temperature was naturally cooled to below 100°C under continuous high vacuum. Nitrogen gas was then introduced to atmospheric pressure, and the sample was removed and passed through a 325-mesh sieve to obtain hard carbon material, denoted as HC-2.

[0053] Example 3 High-capacity, high-rate hard carbon anode materials prepared using peanut shells as raw materials S1. Raw material pretreatment: Take 500g of dried peanut shells, crush them with a coarse crusher, pass them through a 100-mesh sieve, wash them three times with deionized water, and dry them at 110℃ to obtain coarse peanut shell powder.

[0054] S2. Chemical pre-oxidation: Weigh 100g of coarse peanut shell powder and mix it with 15g of potassium persulfate (i.e., the mass ratio of raw material to oxidant is 1:0.15) in a mixer for 2 hours; put the mixture into an open crucible and place it in a high-temperature oven, raise the temperature to 280℃ at a rate of 5℃ / min under static air atmosphere, and keep the temperature constant for 9 hours; after the reaction is completed, cool it and break up the pre-oxidized material and pass it through a 100-mesh sieve.

[0055] S3. Crushing: The pre-oxidized material is fed into an air jet mill and crushed until the median particle size D50 is 8 micrometers.

[0056] S4. Purification and etching of the composite acid: Prepare a mixed acid solution in which the mass ratio of concentrated hydrochloric acid (37%), concentrated sulfuric acid (98%), and concentrated nitric acid (65%) is 35:40:25; add 100g of the powder obtained in step S3 to the mixed acid and stir and cook in a water bath at 70℃ for 5 hours; after the reaction is completed, filter and wash with deionized water until the filtrate is neutral, and then vacuum dry at 100℃ for 12 hours to obtain the purified precursor.

[0057] S5. High-Temperature Carbonization with Segmented Pressure Control: The dried, purified precursor was placed in a programmable pressure-controlled high-temperature vacuum tube furnace. After evacuating the furnace to a low vacuum, the temperature was increased at a rate of 8°C / min. When the temperature reached 500°C, the pressure inside the furnace was controlled at 300 Pa. This pressure was maintained, and the temperature was increased to 700°C at a rate of 8°C / min and held for 0.5 hours. Then, the temperature was increased to 900°C at 5°C / min under 300 Pa pressure. When the temperature reached 900°C, the high vacuum system was activated, and the absolute pressure inside the furnace was rapidly reduced to -50 Pa (high vacuum state). Under this high vacuum, the temperature was increased to the final carbonization temperature of 1150°C at a rate of 5°C / min and held for 1 hour. After the holding period, the temperature was allowed to cool naturally to below 100°C under continuous high vacuum. Nitrogen gas was then introduced to atmospheric pressure, and the sample was removed and passed through a 325-mesh sieve to obtain hard carbon material, denoted as HC-3.

[0058] Example 4 Hard carbon anode materials were prepared using apricot shells as raw material and with a low proportion of oxidant. The only difference between this embodiment and Example 2 is that in step S2, the mass ratio of apricot shell powder to sodium persulfate is 1:0.05 (using the lower limit of the ratio), while all other aspects are the same as in Example 2. The resulting hard carbon material is designated as HC-4.

[0059] Example 5 High-capacity, high-rate hard carbon anode materials prepared using apricot shells as raw materials S1. Raw material pretreatment: Take 500g of dried apricot shells, crush them with a coarse crusher, pass them through a 100-mesh sieve, wash them three times with deionized water, and dry them at 110℃ to obtain coarse apricot shell powder.

[0060] S2. Chemical pre-oxidation: Weigh 100g of coarse apricot shell powder and mix it with 17g of ammonium persulfate (i.e., the mass ratio of raw material to oxidant is 1:0.17) in a mixer for 2 hours; put the mixture into an open crucible and place it in a high-temperature oven. Under static air atmosphere, raise the temperature to 300℃ at a heating rate of 5℃ / min and keep it at this temperature for 12 hours; after the reaction is completed, cool it and break up the pre-oxidized material and pass it through a 100-mesh sieve.

[0061] S3. Crushing: The pre-oxidized material is fed into an air jet mill and crushed until the median particle size D50 is 15 micrometers.

[0062] S4. Purification and etching of the composite acid: Prepare a mixed acid solution in which the mass ratio of concentrated hydrochloric acid (37%), concentrated sulfuric acid (98%), and concentrated nitric acid (65%) is 40:25:35; add 100g of the powder obtained in step S3 to the mixed acid and stir and cook in a water bath at 65℃ for 2 hours; after the reaction is completed, filter and wash with deionized water until the filtrate is neutral, and then vacuum dry at 100℃ for 12 hours to obtain the purified precursor.

[0063] S5. High-Temperature Carbonization with Segmented Pressure Control: The dried, purified precursor was placed in a programmable pressure-controlled high-temperature vacuum tube furnace. After evacuating the furnace to a low vacuum, the temperature was increased at a rate of 3°C / min. When the temperature reached 500°C, the pressure inside the furnace was controlled at 350 Pa. This pressure was maintained, and the temperature was increased to 600°C and held for 2 hours. Then, the temperature was increased to 900°C at 3°C / min under 350 Pa pressure. When the temperature reached 900°C, the high vacuum system was activated, and the absolute pressure inside the furnace was rapidly reduced to -30 Pa (high vacuum state). Under this high vacuum, the temperature was increased at 3°C / min to the final carbonization temperature of 1200°C and held for 2 hours. After the holding period, the temperature was allowed to cool naturally to below 100°C under continuous high vacuum. Nitrogen gas was then introduced to atmospheric pressure, and the sample was removed and passed through a 325-mesh sieve to obtain hard carbon material, denoted as HC-5.

[0064] Example 6 High-capacity, high-rate hard carbon anode materials prepared by high-hydrochloric acid ratio acid treatment The only difference between this embodiment and Example 2 is that in step S4, the ratio of the mixed acids is hydrochloric acid: sulfuric acid: nitric acid = 50:25:25 (mass ratio, with the upper limit for hydrochloric acid). All other aspects are the same as in Example 2. The resulting hard carbon material is designated HC-6.

[0065] Comparative Example 7 The only difference between this comparative example and Example 3 (both using peanut shells as raw material) is the carbonization process: segmented pressure control was not used, and the final carbonization temperature was 1150℃. Specifically, under atmospheric pressure argon protection, the temperature was directly increased to 1150℃ at 5℃ / min and held for 2 hours. The resulting material is designated D-HC-7.

[0066] Comparative Example 8 The difference between this comparative example and Example 1 is the raw material: pine wood chips were used instead of coconut shells; all other steps and parameters were exactly the same. The resulting material is designated as D-HC-8.

[0067] Comparative Example 9 The difference between this comparative example and Example 1 is the oxidant: hydrogen peroxide (H2O2) of equal mass is used instead of ammonium persulfate; all other steps and parameters are exactly the same. The resulting material is designated as D-HC-9.

[0068] Comparative Example 10 The difference between this comparative example and Example 1 is that the chemical pre-oxidation step is omitted (i.e., no oxidant is added), and the coarse coconut shell powder is directly subjected to subsequent crushing, acid purification, and carbonization treatment, while the other parameters remain unchanged. The resulting material is designated as D-HC-10.

[0069] Comparative Example 11 The difference between this comparative example and Example 1 lies in the pre-oxidation temperature: the pre-oxidation temperature is set to 150°C, which is lower than the 200-300°C range required by this invention. All other steps and parameters are exactly the same. The resulting material is designated D-HC-11.

[0070] Comparative Example 12 The difference between this comparative example and Example 1 lies in the pre-oxidation time: the pre-oxidation time was extended to 15 hours, exceeding the 9-12 hour range required by this invention. All other steps and parameters remained identical. The resulting material was designated D-HC-12.

[0071] Comparative Example 13 The difference between this comparative example and Example 1 lies in the control of carbonization pressure: during the intermediate-temperature carbonization stage from 500°C to 900°C, a slight positive pressure was not maintained; instead, the pressure was controlled at a relatively low 1000 Pa (exceeding the 200-500 Pa range of this invention), and then reduced to 0 Pa after 900°C. The remaining steps and parameters were the same. The resulting material is designated D-HC-13.

[0072] Comparative Example 14 The difference between this comparative example and Example 1 lies in the control of carbonization pressure: during the intermediate-temperature carbonization stage (500°C to 900°C), the pressure was controlled at 400 Pa (consistent with Example 1, meeting the 200~500 Pa requirement). However, during the high-temperature carbonization stage after 900°C, a high vacuum was not maintained; instead, the pressure was controlled at 200 Pa (outside the -50~0 Pa range of this invention). The remaining steps and parameters were the same. The resulting material is designated D-HC-14.

[0073] Comparative Example 15 The difference between this comparative example and Example 1 is that the raw material and oxidant are mixed in a liquid phase (the raw material powder and oxidant solution are mixed, and the mass of oxidant in the oxidant solution is the same as in Example 1). After mixing, the mixture is dried, dispersed, and then pre-oxidized. The remaining steps and parameters are the same. The resulting material is designated as D-HC-15.

[0074] Electrochemical performance testing Preparation of sodium-ion batteries: For secondary batteries, a 6-7% (w / w) polyvinylidene fluoride (PVDF) solution was prepared using N-methylpyrrolidone as a solvent. Hard carbon anode material, PVDF, and conductive carbon black were mixed with the PVDF solution at a mass ratio of 90:5:5 and coated onto copper foil. The coated electrode was then vacuum-dried in a 110°C vacuum oven for 4 hours and cut into 14mm diameter discs. These discs were then transferred to an argon-filled German Micron glove box and assembled into 2430 coin cells. A 1 mol / L NaPF6 three-component mixed solvent (EC:DMC:EMC = 1:1:1, volume ratio) was used as the electrolyte. A sodium metal sheet served as the counter electrode, and a 16μm thick Ube membrane was used as the separator. Electrochemical performance tests were conducted on the assembled half-cells using an Arbin electrochemical testing system. The charge / discharge voltage range was 0V to 2.0V, with a rate of 0.1C. Cyclic testing was performed at a rate of 1C.

[0075] The hard carbon materials prepared in the above embodiments and comparative examples were assembled into sodium-ion half-cells for testing. The key performance data are summarized in Table 1 below:

[0076] Results Analysis The data in the table above shows that: Examples 1-6 (HC-1 to HC-6) employing the complete process of this invention exhibit excellent and stable performance in three core indicators: reversible capacity (>330 mAh / g), initial efficiency (>88%), and high-rate capacity retention (≥84.69%). This demonstrates that the process remains highly efficient and stable even when parameters such as raw materials (coconut shells, apricot shells, peanut shells, or combinations thereof), oxidant ratio (1:0.05 to 1:0.3), and acid ratio (10-50% hydrochloric acid) vary within the scope of this invention, proving the breadth and feasibility of the claims of this invention.

[0077] Comparative Example 7 (D-HC-7) did not employ segmented pressure control, which prevented the timely removal of volatile products during the carbonization process. Furthermore, the material's graphitization and conductive network development were insufficient, resulting in a significant decrease in various properties, especially rate performance.

[0078] Comparative Example 8 (D-HC-8) was replaced with pine wood, and its performance was lower than that of the example due to the difference in its natural structure and shell material, which proved the necessity of using specific biomass shells as raw materials.

[0079] Comparative Example 9 (D-HC-9) had a different oxidation mechanism and effect when the oxidant was replaced with hydrogen peroxide, and its performance decreased, which proved the key role of the persulfate series of oxidants.

[0080] Comparative Example 10 (D-HC-10) was not chemically pre-oxidized, and the precursor structure was not effectively stabilized and modified, resulting in a decline in performance.

[0081] Comparative Examples 11 (D-HC-11) and 12 (D-HC-12) show that both excessively low pre-oxidation temperature (150°C) and excessively long pre-oxidation time (15h) impair the final performance. In particular, the first-efficiency of Comparative Example 12 was severely reduced due to over-reaction, demonstrating the importance of the range of pre-oxidation parameters.

[0082] Comparative Examples 13 (D-HC-13) and 14 (D-HC-14) verified the necessity of the two pressure control conditions: a slight positive pressure (200-500 Pa) in the mid-temperature range and a high vacuum (-50~0 Pa) in the high-temperature range. Deviation from either condition would lead to a significant deterioration in the rate performance of the material.

[0083] Comparative Example 15 (D-HC-15) changed the mixing method of the oxidant and raw materials from solid mixing to liquid mixing. Regarding the mixing uniformity, it was shown that the liquid mixing uniformity of this material was insufficient, leading to a decrease in performance, demonstrating the importance of the solid mixing method. The pre-oxidation of this invention uses solid mixing instead of liquid mixing, simplifying the drying step, avoiding capillary effects, reducing costs, and improving product consistency.

[0084] It is evident that the complete process system provided by this invention, which uses specific shell biomass as raw material, combines persulfate chemical pre-oxidation, specific ratio mixed acid purification, and segmented pressure-controlled carbonization at 500-900℃ with micro-positive pressure (200-500 Pa) and high vacuum above 900℃ (-50~0 Pa), is a necessary and sufficient condition for obtaining high-performance hard carbon anode materials. Examples 1-6 demonstrate that different implementations within the parameter range of this invention all achieve excellent and consistent performance, while Comparative Examples 7-14, through counter-examples, clearly define the boundaries upon which the technical solution of this invention can be established, highlighting its inventiveness and significant progress.

[0085] In summary, this invention involves coarsely crushing and sieving biomass husks to obtain coarse biomass powder; pre-oxidizing the powder after mixing with an oxidant; mechanically pulverizing the powder to a D50 of 8-15 micrometers; adding the powder to a mixed acid and heating it in a water bath, followed by washing with water until neutral to obtain a purified precursor; and finally, performing segmented high-temperature carbonization in a vacuum carbonization furnace. Through the synergistic effect of specific process parameters, this invention produces a hard carbon material with a hierarchical porous structure and suitable graphite microcrystal spacing, resulting in a low total content of metallic impurities. When applied to the anode of sodium-ion batteries, it exhibits high reversible specific capacity, high initial coulombic efficiency, and excellent long-cycle stability and rate performance. Therefore, this invention effectively overcomes the various shortcomings of existing technologies and possesses high industrial application value.

[0086] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can 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 those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for preparing a high-capacity, high-rate hard carbon anode material, characterized in that, Includes the following steps: S1. After coarsely crushing the biomass husk raw material, sieve it to obtain coarse biomass powder; S2. Mix the biomass coarse powder obtained in step S1 with the oxidant evenly, then raise the temperature to 200~300℃ and perform pre-oxidation at this temperature. After the pre-oxidation is completed, cool naturally, break up the obtained pre-oxidized material and sieve it to obtain pre-oxidized powder. The oxidant is one or a combination of potassium persulfate, ammonium persulfate and sodium persulfate. S3. Mechanically pulverize the pre-oxidized powder obtained in step S2 to make its D50 between 8 and 15 micrometers; S4. Add the pulverized powder from step S3 to the mixed acid and heat it in a water bath. Then wash it with water until it is neutral, take it out and dry it to obtain the purified precursor. S5. Place the purified precursor obtained in step S4 in a vacuum carbonization furnace and perform segmented carbonization under continuous vacuum or inert gas protection. After carbonization, allow it to cool naturally to room temperature under vacuum or inert gas protection, and then sieve it to obtain high-capacity, high-rate hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, In step S1, the biomass shell raw material is selected from one or more combinations of coconut shells, apricot shells, and peanut shells.

3. The preparation method according to claim 1, characterized in that, In step S2, the biomass powder and oxidant are mixed at a mass ratio of 1:0.05 to 1:0.5; the pre-oxidation time is 9 to 12 hours.

4. The preparation method according to claim 1, characterized in that, In step S4, the mixed acid is composed of hydrochloric acid, sulfuric acid and nitric acid, with the following mass percentages: hydrochloric acid 10%~50%, sulfuric acid 5%~40%, and nitric acid 15%~40%.

5. The preparation method according to claim 1, characterized in that, In step S4, the water bath heating temperature is 60~90℃, and the water bath heating time is 2~6 hours.

6. The preparation method according to claim 1, characterized in that, In step S5, the segmented high-temperature carbonization specifically includes: in the first stage, heating to 500-700°C at a heating rate of 3-8°C / min and holding for 0.5-2 hours; in the second stage, continuing to heat to 900-1200°C at the same or different heating rates and holding for 1-3 hours.

7. The preparation method according to claim 1, characterized in that, In step S5, when the carbonization temperature is in the range of 500°C to 900°C, the absolute pressure inside the furnace is controlled to be maintained at 200-500 Pa; when the carbonization temperature rises above 900°C until the carbonization is completed, the absolute pressure inside the furnace is controlled to be maintained at a high vacuum state of -50 Pa to 0 Pa.

8. A high-capacity, high-rate hard carbon anode material prepared by the preparation method according to any one of claims 1 to 7, characterized in that, The hard carbon material has a typical amorphous structure, with a graphite microcrystal spacing d002 of 0.38–0.42 nm and a specific surface area of ​​5–50 m². 2 / g; the hard carbon material has a hierarchical porous structure, and the pore size distribution is mainly composed of micropores below 2 nm and mesopores of 2~50 nm; the total content of residual metal impurities on the surface of the hard carbon material is less than 500 ppm.

9. A sodium-ion battery negative electrode, characterized in that, The sodium-ion battery anode comprises the high-capacity, high-rate hard carbon anode material as described in claim 8 as the active material.

10. A sodium-ion battery, characterized in that, The sodium-ion battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the negative electrode of the sodium-ion battery according to claim 9.

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