Sodium-ion battery negative electrode material and preparation method and application thereof

Abstract

The application belongs to the technical field of sodium ion battery negative electrode materials, and discloses a sodium ion battery negative electrode material with high reversible specific capacity, high initial coulomb efficiency and excellent cycle stability, and a preparation method and application thereof. The preparation method comprises the following steps: (1) mixing biomass raw materials with water for hydrothermal carbonization treatment, and collecting hydrothermal carbon and hydrothermal liquid after solid-liquid separation; (2) performing evaporation concentration treatment on the hydrothermal liquid to obtain a hydrothermal concentrated product; (3) performing high-temperature carbonization treatment on the hydrothermal carbon in an inert atmosphere to obtain a porous hard carbon material; (4) mixing the porous hard carbon material with the hydrothermal concentrated product, and performing secondary carbonization treatment in an inert atmosphere, so that the hydrothermal concentrated product is pyrolyzed and carbonized to form a carbon layer to block open pores of the porous hard carbon material, and a closed-pore hard carbon material is obtained; and (5) performing acid pickling treatment on the closed-pore hard carbon material to obtain the sodium ion battery negative electrode material.

Description

Technical Field

[0001] This invention relates to the technical field of sodium-ion battery anode materials, and more specifically, to sodium-ion battery anode materials, their preparation methods, and applications. Background Technology

[0002] Sodium-ion batteries, due to the abundant and widely distributed sodium resources, are considered one of the potential alternative technologies to lithium-ion batteries in the field of large-scale energy storage. The anode material is a key component determining the electrochemical performance of sodium-ion batteries, among which hard carbon materials have become a widely studied anode material system due to their high reversible sodium storage capacity and suitable operating potential. Biomass raw materials are widely available and renewable, making them important precursors for the preparation of hard carbon materials. Bamboo, rice husks, wheat straw, and sugarcane bagasse are abundant in nature, and using them to prepare hard carbon anode materials has the advantages of readily available raw materials and low cost. However, current technologies for preparing hard carbon anode materials using biomass still face the following technical challenges.

[0003] First, the pore structure of carbon materials significantly impacts their electrochemical performance. Closed-pore structures in hard carbon materials are considered the primary sites for low-potential sodium ion storage, and appropriate closed-pore structures are beneficial for improving reversible specific capacity. However, biomass typically forms numerous open pores after high-temperature carbonization. These open-pore structures result in an excessively large specific surface area, leading to significant electrolyte decomposition on the carbon material surface during the initial charge-discharge process, forming a solid electrolyte interface film and causing increased irreversible capacity loss and a low initial coulombic efficiency. Therefore, effectively sealing the open pores on and near the surface of hard carbon materials, transforming them into closed-pore structures favorable for sodium storage, while simultaneously reducing the specific surface area to minimize irreversible capacity loss, is crucial for improving the overall electrochemical performance of hard carbon anode materials.

[0004] In existing technologies, a common strategy for pore sealing in hard carbon materials is to introduce exogenous carbonaceous precursors, which form a carbon layer on the hard carbon surface through pyrolysis and carbonization to seal open pores. However, most of the exogenous carbonaceous precursors currently used are derived from fossil resources, have complex compositions, and often contain impurities such as sulfur and heavy metals. During high-temperature carbonization, non-target elements may be introduced, affecting the purity and consistency of electrochemical performance of the carbon material. Furthermore, the batch stability of these exogenous carbonaceous precursors is significantly affected by the origin of the raw materials and the processing technology, which is detrimental to the batch-to-batch stability control of product performance. More importantly, the additional introduction of exogenous sealing materials increases raw material costs and processing steps, contradicting the original design intent of biomass-based hard carbon as a "green, low-cost, and sustainable" material.

[0005] Secondly, biomass feedstocks typically contain alkali and alkaline earth metals such as potassium, calcium, sodium, and magnesium. These metals catalyze localized graphitization during high-temperature carbonization, leading to an uneven microstructure in the carbon material and reducing the number of sodium-storing active sites. Furthermore, residual metal impurities can trigger side reactions during electrochemical cycling, affecting cycle stability. Therefore, effective impurity removal from biomass before carbonization is a crucial prerequisite for obtaining high-performance hard carbon materials. Hydrothermal carbonization is an effective method that combines carbonization pretreatment and impurity removal. Under hydrothermal conditions, alkali and alkaline earth metals in biomass can dissolve into the liquid phase and be removed. However, during the hydrothermal process, some organic components in the biomass also dissolve or degrade into the liquid phase, forming a hydrothermal fluid rich in soluble organic matter such as furfural, hydroxymethylfurfural, oligosaccharides, and organic acids, which has a high carbon content. In existing processes, this hydrothermal fluid is usually directly discharged as wastewater or treated as wastewater, which not only results in a significant waste of biomass carbon resources but also increases the environmental burden of wastewater treatment. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a sodium-ion battery anode material with high reversible specific capacity, high initial coulombic efficiency and excellent cycle stability, as well as its preparation method and application. The technical solution is as follows: A method for preparing sodium-ion battery anode materials includes the following steps: (1) The biomass raw material is mixed with water for hydrothermal carbonization, and the hydrothermal carbon and hydrothermal liquid are collected separately after solid-liquid separation; (2) The hydrothermal liquid is evaporated and concentrated to obtain a hydrothermal concentrated product; (3) The hydrothermal carbon is subjected to high-temperature carbonization under an inert atmosphere to obtain a porous hard carbon material; (4) After mixing the porous hard carbon material with the hydrothermal concentrate, a secondary carbonization treatment is carried out under an inert atmosphere to make the hydrothermal concentrate pyrolyze and carbonize to form a carbon layer that blocks the open pores of the porous hard carbon material, thereby obtaining a closed-pore hard carbon material. (5) The closed-cell hard carbon material is acid-washed to obtain the sodium-ion battery anode material.

[0007] The advantages of the sodium-ion battery anode material preparation method of the present invention are as follows: Hydrothermal carbonization directionally separates a single biomass raw material into solid-phase hydrothermal carbon and a hydrothermal liquid rich in soluble organic matter. The hydrothermal carbon, after high-temperature carbonization, is transformed into a porous hard carbon material with abundant pores, providing ample storage space for sodium ions and forming the structural basis for achieving high reversible specific capacity. The hydrothermal liquid, after evaporation and concentration, is transformed into a hydrothermal concentrate product with an appropriate softening point, serving as a carbon layer precursor for pore sealing. During the secondary carbonization process, the hydrothermal concentrate product softens and melts, penetrating into the pores of the porous hard carbon material. After pyrolysis and carbonization, a dense carbon layer is formed at the pore inlet, transforming open pores into a closed state, significantly reducing the specific surface area, greatly inhibiting the excessive growth of the solid electrolyte interfacial film, and improving the initial coulombic efficiency. Since the hydrothermal concentration product and the porous hard carbon material originate from the same biomass feedstock, they have a natural affinity in terms of chemical composition and surface properties. The wettability and permeability of the molten carbon source to the pores of hard carbon are significantly better than those of exogenous commercial carbon sources, enabling more uniform and in-depth pore sealing. The final acid washing treatment removes residual metal impurities and ensures cycle stability. In summary, this invention innovatively transforms hydrothermal fluids, which are usually treated as waste liquid, into a pore-sealing carbon source, realizing the synergistic high-value utilization of the solid and liquid phase components of a single biomass feedstock. The process is green and economical, effectively solving the technical challenge of simultaneously improving the reversible specific capacity and initial coulombic efficiency of biomass-based hard carbon anode materials.

[0008] As a further improvement to the above preparation method: in step (1), the biomass raw material is selected from at least one of bamboo, rice husk, wheat straw, corn cob, and sugarcane bagasse; the hydrothermal carbonization treatment temperature is 180-260°C, the time is 4-24 hours, and the mass ratio of the biomass raw material to water is 1:(5-15). Therefore, the above biomass raw materials are widely available and contain abundant cellulose, hemicellulose, and lignin components. Under hydrothermal carbonization conditions, they can produce a high proportion of soluble organic carbon components, which is beneficial for obtaining hydrothermal carbon with high carbon content and hydrothermal fluid with high organic carbon concentration.

[0009] As a further improvement to the above preparation method: In step (2), the evaporation and concentration process is carried out under reduced pressure, with an evaporation temperature of 60–120°C and an absolute pressure of 0.01–0.09 MPa, until the softening point of the hydrothermal concentrate is 80–200°C. Thus, reduced pressure evaporation can efficiently remove moisture at lower temperatures, avoiding excessive decomposition or carbonization of organic matter caused by high-temperature atmospheric pressure evaporation; using the softening point as the control index for the concentration endpoint ensures that the obtained hydrothermal concentrate has appropriate thermoplasticity, allowing it to soften and flow during subsequent heating to penetrate pores, and also having a sufficiently high residual carbon rate to form a dense carbon layer after carbonization.

[0010] As a further improvement to the above preparation method: in step (3), the high-temperature carbonization treatment is carried out at a temperature of 1000–1500°C, for a holding time of 2–6 hours, with a heating rate of 2–10°C / min, and the inert atmosphere is argon or nitrogen. Thus, the above carbonization process enables the hydrothermal carbon to be fully pyrolyzed and transformed into a hard carbon material with a typical disordered carbon microcrystalline structure, while the release of volatiles forms abundant micropores and mesopores in the carbon matrix.

[0011] As a further improvement to the above preparation method: in step (4), the mass ratio of the porous hard carbon material to the hydrothermal concentration product is 1:(0.05~0.3); thus, the open pores on the surface of the porous hard carbon material are effectively blocked while avoiding the formation of an excessively thick carbon layer on the outer surface of the particles by excessive carbon source, which would affect the transport kinetics of sodium ions. The temperature of the secondary carbonization treatment is 800~1200°C, the holding time is 1~4 hours, the heating rate is 2~5°C / min, and the inert atmosphere is argon or nitrogen. Thus, it can ensure that the hydrothermal concentration product is fully pyrolyzed and carbonized to form a dense carbon layer, without causing irreversible changes to the already formed hard carbon skeleton structure.

[0012] As a further improvement to the above preparation method: In step (4), the mixing method is melt coating: the hydrothermal concentrated product is heated to 20-50°C above its softening point to melt it, the porous hard carbon material is added to the molten hydrothermal concentrated product and stirred evenly, and then the secondary carbonization treatment is carried out after cooling; thus, the melt coating method utilizes the thermoplasticity of the hydrothermal concentrated product to transform it into a low-viscosity melt at a temperature above the softening point, and the molten carbon source is fully contacted with the porous hard carbon material by stirring and penetrates into the pores by capillary action. The process is simple and does not introduce additional solvents.

[0013] Alternatively, the mixing method can be a solution impregnation method: the hydrothermal concentrated product is dissolved in an organic solvent to form an impregnation solution, the porous hard carbon material is immersed in the impregnation solution, and after evaporation to remove the organic solvent, the secondary carbonization treatment is performed. Thus, the solution impregnation method dissolves the hydrothermal concentrated product in an organic solvent to form a low-viscosity solution. The high fluidity and low surface tension of the solution promote the deep penetration of the carbon source precursor into the deep pores of the porous hard carbon material, making it suitable for cases with small pore sizes or deep pores.

[0014] The two mixing methods mentioned above can be flexibly selected based on the softening point of the hydrothermal concentration product and the pore structure characteristics of the porous hard carbon material.

[0015] As a further improvement to the above preparation method: when the mixing method is melt coating, the stirring rate is 100–500 r / min, the stirring time is 0.5–4 hours, and after stirring, the mixture is naturally cooled to room temperature before the secondary carbonization treatment. This ensures that the carbon source is uniformly distributed on the surface of the hard carbon particles and fully penetrates the pores, while avoiding excessive shear force that could cause the hard carbon particles to break. When the mixing method is solution impregnation, the organic solvent is selected from at least one of tetrahydrofuran, toluene, N-methylpyrrolidone, and quinoline; the mass concentration of the hydrothermal concentrated product in the impregnation solution is 5%–30%; the impregnation temperature is 20–60°C, and the impregnation time is 2–12 hours. Therefore, the selected organic solvent has good dispersibility for the hydrothermal concentrated product, and its boiling point is moderate, making it easy to evaporate and remove.

[0016] As a further improvement to the above preparation method: In step (5), the acid solution used for pickling is hydrochloric acid solution or sulfuric acid solution with a concentration of 0.5-2 mol / L, the pickling temperature is 20-80°C, and the pickling time is 2-6 hours. Thus, hydrochloric acid and sulfuric acid can effectively dissolve the oxides or carbonates of impurities such as alkali metals, alkaline earth metals, and transition metals remaining in carbon materials, but they do not have a corrosive effect on the carbon skeleton and the formed carbon layer.

[0017] The sodium-ion battery anode material is prepared by the above-described preparation method; the weight gain of the carbon layer is 2-20%; the specific surface area is 4-12 m² / g; the reversible specific capacity at a current density of 0.1 A / g is 270-340 mAh / g, the initial coulombic efficiency is 84-91%; and the capacity retention rate after 500 cycles at a current density of 1 A / g is ≥89%.

[0018] Sodium-ion batteries use the aforementioned sodium-ion battery negative electrode material for their negative electrode.

[0019] The embodiments of the invention provided in this specification will be further described below with reference to the accompanying drawings and specific embodiments. Additional aspects and advantages of the embodiments of the invention provided in this specification will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the embodiments of the invention provided in this specification. Attached Figure Description

[0020] The accompanying drawings, which form part of the embodiments of the invention provided in this specification, are used to aid in understanding the embodiments of the invention provided in this specification. The content provided in the drawings and the related descriptions in the embodiments of the invention provided in this specification can be used to explain the embodiments of the invention provided in this specification, but do not constitute an undue limitation on the embodiments of the invention provided in this specification. In the drawings: Figure 1The image shows a SEM image of the acid-washed porous hard carbon material of Comparative Example 1.

[0021] Figure 2 This is a SEM image of the sodium-ion battery anode material from Example 1. Detailed Implementation

[0022] The embodiments of the invention provided in this specification will now be clearly and completely described in conjunction with the accompanying drawings. Those skilled in the art will be able to implement the embodiments of the invention provided in this specification based on these descriptions. Before describing the embodiments of the invention provided in this specification in conjunction with the accompanying drawings, it should be particularly noted that: The technical solutions and features provided in the embodiments of the invention provided in this specification, including the following description, can be combined with each other without conflict.

[0023] Furthermore, the embodiments of the inventions provided in this specification mentioned below are generally only a part of the embodiments of the inventions provided in this specification, and not all of them. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the inventions provided in this specification without creative effort should fall within the scope of protection of the embodiments of the inventions provided in this specification.

[0024] Regarding the terminology and units in the embodiments of the invention provided in this specification: The terms "comprising," "including," "having," and any variations thereof in the description, claims, and related parts of the embodiments of the invention provided in this specification are intended to cover non-exclusive inclusion. Furthermore, other relevant terms and units in the embodiments of the invention provided in this specification can be reasonably interpreted based on the relevant content of the embodiments of the invention provided in this specification.

[0025] Example 1

[0026] The sodium-ion battery anode material and its preparation method in this embodiment are as follows: (1) After washing and drying rice husks (60-100 mesh), mix them with deionized water at a mass ratio of 1:10 and load them into a hydrothermal reactor (60% filling). Perform hydrothermal carbonization treatment at 220°C for 12 hours. After the reaction, allow the mixture to cool naturally to room temperature. After opening the reactor, separate the solid and liquid phases by vacuum filtration and collect the solid phase hydrothermal carbon and the liquid phase hydrothermal liquid. Wash the hydrothermal carbon three times with deionized water and then dry it at 80°C for 12 hours for later use.

[0027] (2) The hydrothermal liquid was transferred into a rotary evaporator and concentrated under reduced pressure at an evaporation temperature of 60°C and an absolute pressure of 0.02MPa until it became a viscous paste. Its softening point was measured to be 120°C, and the hydrothermal concentrated product was obtained.

[0028] (3) Place the dried hydrothermal carbon in a tube furnace and heat it from room temperature to 1200°C at a heating rate of 5°C / min under an argon atmosphere. Hold it at the temperature for 4 hours for high-temperature carbonization treatment. After naturally cooling to room temperature, take it out to obtain porous hard carbon material.

[0029] (4) Mixing using the melt coating method: The hydrothermal concentrate was placed in a stainless steel container and heated to 150°C (30°C above its softening point) to completely melt it. Then, porous hard carbon material was added to the molten hydrothermal concentrate at a mass ratio of 1:0.15. The mixture was stirred at a stirring rate of 300 r / min for 2 hours. After stirring, it was allowed to cool naturally to room temperature. The cooled mixture was placed in a tube furnace and heated to 1000°C at a heating rate of 3°C / min under an argon atmosphere. It was held at this temperature for 2 hours for a secondary carbonization treatment. After cooling naturally to room temperature, the mixture was removed to obtain closed-cell hard carbon material.

[0030] (5) Add the closed-cell hard carbon material to a 1 mol / L hydrochloric acid solution (solid-liquid mass-volume ratio of 1 g: 20 mL), stir and acid wash at 60°C for 4 hours, filter, wash with deionized water until the pH of the filtrate is neutral, and vacuum dry at 80°C for 12 hours to obtain the sodium-ion battery anode material.

[0031] Tests showed that the specific surface area of ​​the sodium-ion battery anode material was 5.8 m² / g, and the carbon layer weight gain was 11.8%. The reversible specific capacity at a current density of 0.1 A / g was 326 mAh / g, the initial coulombic efficiency was 89.2%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 93.5%.

[0032] Let m1 be the mass of the porous hard carbon material in step (3) and m2 be the mass of the closed-cell hard carbon material in step (3). The weight gain rate is (m2-m1) / m1*100%.

[0033] Example 2

[0034] Compared with Example 1, the sodium-ion battery anode material and its preparation method in this example are different in that: in step (2), the evaporation temperature is 120°C, the absolute pressure is 0.01MPa, and the softening point of the hydrothermal concentrated product is 180°C; in step (4), the hydrothermal concentrated product is heated to 210°C to melt it.

[0035] The sodium-ion battery anode material was tested and found to have a specific surface area of ​​4.6 m² / g and a carbon layer weight gain of 13.6%. The reversible specific capacity at a current density of 0.1 A / g was 332 mAh / g, the initial coulombic efficiency was 89.8%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 94.1%.

[0036] Example 3

[0037] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that the temperature of the high-temperature carbonization treatment in step (3) is 1000°C and the holding time is 6 hours.

[0038] Tests showed that the specific surface area of ​​the sodium-ion battery anode material was 8.7 m² / g, and the carbon layer weight gain rate was 10.8%. The reversible specific capacity at a current density of 0.1 A / g was 296 mAh / g, the initial coulombic efficiency was 86.2%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 91.2%.

[0039] Example 4

[0040] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that the temperature of the high-temperature carbonization treatment in step (3) is 1500°C and the holding time is 2 hours.

[0041] The sodium-ion battery anode material was tested and found to have a specific surface area of ​​5.2 m² / g and a carbon layer weight gain of 11.2%. The reversible specific capacity at a current density of 0.1 A / g was 278 mAh / g, the initial coulombic efficiency was 88.9%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 92.5%.

[0042] Example 5

[0043] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that the mass ratio of porous hard carbon material to hydrothermal concentration product in step (4) is 1:0.05; the temperature of the secondary carbonization treatment is 1200°C and the holding time is 1 hour.

[0044] The sodium-ion battery anode material was tested and found to have a specific surface area of ​​11.5 m² / g and a carbon layer weight gain of 3.5%. The reversible specific capacity at a current density of 0.1 A / g was 312 mAh / g, the initial coulombic efficiency was 84.5%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 90.5%.

[0045] Example 6

[0046] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that the mass ratio of porous hard carbon material to hydrothermal concentration product in step (4) is 1:0.3; the temperature of the secondary carbonization treatment is 800°C and the holding time is 4 hours.

[0047] Tests showed that the specific surface area of ​​the sodium-ion battery anode material was 4.2 m² / g, with a carbon layer weight gain of 18.5%. The reversible specific capacity at a current density of 0.1 A / g was 288 mAh / g, the initial coulombic efficiency was 90.5%, and the capacity retention after 500 cycles at a current density of 1 A / g was 93.8%. This indicates that when excessive amounts of hydrothermal concentrate are used, although pore sealing is more complete (lowest specific surface area), some carbon source excessively penetrates into the pores, reducing the effective closed-pore volume. Simultaneously, the excessively thick carbon layer increases the diffusion path of sodium ions through the carbon layer into the closed pores, both of which contribute to a decrease in reversible specific capacity.

[0048] Example 7

[0049] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that the solution impregnation method is used for mixing in step (4): the hydrothermal concentrated product is dissolved in tetrahydrofuran to form an impregnation solution with a mass concentration of 15%, the porous hard carbon material is immersed in the impregnation solution (maintaining the mass ratio of porous hard carbon material to hydrothermal concentrated product at 1:0.15), impregnated at 40°C for 6 hours, and then the tetrahydrofuran solvent is removed by rotary evaporation at 60°C, and then the same secondary carbonization treatment as in Example 1 is performed.

[0050] The sodium-ion battery anode material was tested and found to have a specific surface area of ​​6.8 m² / g and a carbon layer weight gain of 10.6%. The reversible specific capacity at a current density of 0.1 A / g was 318 mAh / g, the initial coulombic efficiency was 88.8%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 93.2%.

[0051] Example 8

[0052] Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this example is that bamboo powder (60-100 mesh) is used instead of rice husk in step (1), the mass ratio of biomass raw material to water is 1:15, and the conditions of the remaining steps are the same as in Example 1.

[0053] The sodium-ion battery anode material was tested and found to have a specific surface area of ​​7.6 m² / g and a carbon layer weight gain of 10.9%. The reversible specific capacity at a current density of 0.1 A / g was 308 mAh / g, the initial coulombic efficiency was 87.2%, and the capacity retention rate after 500 cycles at a current density of 1 A / g was 91.9%.

[0054] Compare with Example 1 Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this comparative example is that steps (2) and (4) are not performed. That is, the porous hard carbon material obtained in step (3) is directly used as the anode material after being acid-washed in step (5).

[0055] Testing revealed that the acid-washed porous hard carbon material had a specific surface area of ​​376 m² / g, a reversible specific capacity of 268 mAh / g at a current density of 0.1 A / g, an initial coulombic efficiency of 48.5%, and a capacity retention of 80.2% after 500 cycles at a current density of 1 A / g. This indicates that while the untreated porous hard carbon material possesses a certain reversible specific capacity, its extremely low initial coulombic efficiency (only 48.5%) and poor cycling stability are due to the excessive growth of the solid electrolyte interfacial film caused by numerous open pores exposed to the electrolyte.

[0056] Figure 1 The image shows a SEM image of the acid-washed porous hard carbon material of Comparative Example 1. Figure 2 This is a SEM image of the sodium-ion battery anode material from Example 1. (Comparison) Figure 1 and Figure 2 As can be seen, after carbon coating, the surface of the hard carbon particles and the inner walls of the visible pores exhibit obvious rough textures, confirming that the carbon layer has been successfully deposited on the surface of the hard carbon framework. In addition, N2 adsorption-desorption tests showed that the specific surface area decreased from 376 m² / g to 5.8 m² / g, indicating that a large number of micropores and mesopores have also been effectively filled and blocked by the carbon layer.

[0057] Compare with Example 2 Compared with Example 1, the difference between the sodium-ion battery anode material and its preparation method in this comparative example is that the acid washing treatment in step (5) is not performed, that is, the closed-cell hard carbon material obtained in step (4) is directly used as the anode material.

[0058] Testing revealed that the closed-cell hard carbon material had a specific surface area of ​​5.6 m² / g, a reversible specific capacity of 298 mAh / g at a current density of 0.1 A / g, an initial coulombic efficiency of 82.5%, and a capacity retention of 78.5% after 500 cycles at a current density of 1 A / g. This indicates that without acid washing, residual metallic impurities in the material catalyze electrolyte decomposition during electrochemical cycling, leading to the deactivation of active sites. Although the initial coulombic efficiency remains high due to pore closure, cycling stability deteriorates significantly, with a capacity retention of only 78.5%.

[0059] Compare with Example 3 Compared with Example 1, the sodium-ion battery anode material and its preparation method in this comparative example are different in that: in step (4), the hydrothermal concentrate product is not used, but commercial medium-temperature coal tar pitch (softening point 85°C) is used instead of the hydrothermal concentrate product as the carbon layer precursor, the mass ratio of porous hard carbon material to coal tar pitch is 1:0.15, and the remaining operating conditions (the heating temperature of the melt coating method is adjusted to 115°C to adapt to the softening point of coal tar pitch) and subsequent step conditions are the same as in Example 1.

[0060] Testing revealed that the sodium-ion battery anode material had a specific surface area of ​​14.8 m² / g and a carbon layer weight gain of 7.6%. Its reversible specific capacity at a current density of 0.1 A / g was 283 mAh / g, its initial coulombic efficiency was 83.2%, and its capacity retention after 500 cycles at a current density of 1 A / g was 88.5%. It is evident that when commercial coal tar pitch is used to replace the hydrothermal concentrate, the wettability and permeability of the molten coal tar pitch to the porous hard carbon material are inferior to those of the same source hydrothermal concentrate due to differences in surface chemistry between the exogenous carbon source and biomass-based hard carbon. This results in poor pore sealing (specific surface area of ​​14.8 m² / g, higher than the 5.8 m² / g in Example 1), ultimately leading to lower initial coulombic efficiency and reversible specific capacity compared to Example 1.

[0061] Compare with Example 4 Compared with Example 1, the sodium-ion battery anode material and its preparation method in this comparative example are different in that: the evaporation and concentration treatment in step (2) is not performed, and in step (4), the porous hard carbon material is directly immersed in the unconcentrated original hydrothermal liquid (solid-liquid mass-volume ratio of 1g:50mL), soaked at 80°C for 12 hours, and then dried and evaporated at 120°C to remove moisture, and then subjected to the same secondary carbonization treatment and acid washing treatment as in Example 1.

[0062] Testing revealed that the specific surface area of ​​the sodium-ion battery anode material was 182 m² / g, with a carbon layer weight gain of 1.2%. The reversible specific capacity at a current density of 0.1 A / g was 271 mAh / g, the initial coulombic efficiency was 55.8%, and the capacity retention after 500 cycles at a current density of 1 A / g was 82.8%. This indicates that the concentration of organic carbon components in the unevaporated and unconcentrated original hydrothermal fluid was extremely low. The amount of carbon source deposited on the surface of the porous hard carbon material after drying was minimal (weight gain of only 1.2%). The carbon layer formed after secondary carbonization was insufficient to effectively seal the pores, and the specific surface area remained as high as 182 m² / g, resulting in significantly lower initial coulombic efficiency and cycle stability compared to Example 1.

[0063] In the above embodiments and comparative examples, the testing methods for each performance data are as follows: Specific surface area and total pore volume testing: Nitrogen adsorption-desorption tests were conducted at 77 K using a fully automated specific surface area and porosity analyzer. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method, and the total pore volume was converted from the amount of nitrogen adsorbed at a relative pressure P / P0 = 0.99.

[0064] Softening point test: The softening point was determined using a ring and ball method softening point tester in accordance with GB / T 4507 standard.

[0065] Electrochemical performance testing: The negative electrode material, acetylene black conductive agent, and polyvinylidene fluoride binder were mixed in a mass ratio of 8:1:1, and N-methyl-2-pyrrolidone was used as a solvent to prepare a slurry. This slurry was uniformly coated onto a copper foil current collector, vacuum dried at 120°C for 12 hours, and then punched into circular electrodes with a diameter of 12 mm. The surface loading of the active material was approximately 1.0–1.5 mg / cm³. 2 CR2032 coin cells were assembled in an argon-filled glove box (water and oxygen content both below 0.1 ppm). A sodium metal sheet was used as the counter / reference electrode, a Whatman GF / D glass fiber membrane as the separator, and a solution of 1 mol / L NaClO4 dissolved in ethylene carbonate / propylene carbonate (volume ratio 1:1) with 5 vol% fluoroethylene carbonate added as the electrolyte. Constant current charge-discharge tests were conducted at 25°C using a battery testing system, with a voltage window of 0.01–2.0 V (vs. NaClO4). + / Na). The reversible specific capacity and initial coulombic efficiency were derived from the discharge specific capacity and charge-discharge efficiency at a current density of 0.1 A / g during the first cycle. The cycle stability test was performed by activating the device for 3 cycles at 0.1 A / g, followed by 500 cycles at a current density of 1 A / g. The capacity retention rate was calculated using the following formula: Capacity retention rate = (Discharge specific capacity of the 500th cycle / Discharge specific capacity of the 1st cycle) × 100%.

[0066] The embodiments of the invention provided in this specification have been described above. Those skilled in the art will be able to implement the embodiments of the invention provided in this specification based on these descriptions. All other preferred embodiments and implementations obtained by those skilled in the art based on the above description of the embodiments of the invention provided in this specification without inventive effort should fall within the protection scope of the embodiments of the invention provided in this specification.

Claims

1. A method for preparing a sodium-ion battery anode material, characterized in that: Includes the following steps: (1) The biomass raw material is mixed with water and subjected to hydrothermal carbonization treatment. After solid-liquid separation, the hydrothermal carbon and hydrothermal liquid are collected separately. The biomass raw material is selected from at least one of bamboo, rice husk, wheat straw, corn cob, and sugarcane bagasse. The temperature of the hydrothermal carbonization treatment is 180-260°C and the time is 4-24 hours. The mass ratio of the biomass raw material to water is 1:(5-15). (2) The hydrothermal liquid is evaporated and concentrated to obtain a hydrothermal concentrated product; (3) The hydrothermal carbon is subjected to high-temperature carbonization under an inert atmosphere to obtain a porous hard carbon material; (4) After mixing the porous hard carbon material with the hydrothermal concentrate, a secondary carbonization treatment is carried out under an inert atmosphere to make the hydrothermal concentrate pyrolyze and carbonize to form a carbon layer that blocks the open pores of the porous hard carbon material, thereby obtaining a closed-pore hard carbon material. The mixing method is a melt coating method: the hydrothermal concentrated product is heated to 20-50°C above its softening point to melt it, the porous hard carbon material is added to the molten hydrothermal concentrated product and stirred evenly, and then cooled before undergoing the secondary carbonization treatment; or... The mixing method is a solution impregnation method: the hydrothermal concentrated product is dissolved in an organic solvent to form an impregnation solution, the porous hard carbon material is impregnated in the impregnation solution, and the organic solvent is evaporated and removed before the secondary carbonization treatment is performed; (5) The closed-cell hard carbon material is acid-washed to obtain the sodium-ion battery anode material.

2. The preparation method according to claim 1, characterized in that: In step (2), the evaporation and concentration process is carried out under reduced pressure, with an evaporation temperature of 60-120°C and an absolute pressure of 0.01-0.09 MPa, until the softening point of the hydrothermal concentrate is 80-200°C.

3. The preparation method according to claim 1, characterized in that: In step (3), the high-temperature carbonization treatment is carried out at a temperature of 1000-1500°C, the holding time is 2-6 hours, the heating rate is 2-10°C / min, and the inert atmosphere is argon or nitrogen.

4. The preparation method according to claim 1, characterized in that: In step (4), the mass ratio of the porous hard carbon material to the hydrothermal concentration product is 1:(0.05~0.3); the temperature of the secondary carbonization treatment is 800~1200°C, the holding time is 1~4 hours, the heating rate is 2~5°C / min, and the inert atmosphere is argon atmosphere or nitrogen atmosphere.

5. The preparation method according to claim 1, characterized in that: When the mixing method is melt coating: the stirring rate is 100-500 r / min, the stirring time is 0.5-4 hours, and after stirring, the mixture is naturally cooled to room temperature before the secondary carbonization treatment is performed; When the mixing method is solution impregnation: the organic solvent is selected from at least one of tetrahydrofuran, toluene, N-methylpyrrolidone, and quinoline; the mass concentration of the hydrothermal concentrated product in the impregnation solution is 5% to 30%; the impregnation temperature is 20 to 60°C; and the impregnation time is 2 to 12 hours.

6. The preparation method according to claim 1, characterized in that: In step (5), the acid solution used for pickling is hydrochloric acid solution or sulfuric acid solution with a concentration of 0.5 to 2 mol / L, the pickling temperature is 20 to 80°C, and the pickling time is 2 to 6 hours.

7. The sodium-ion battery anode material prepared by the preparation method according to any one of claims 1-6, characterized in that: The weight gain rate of the carbon layer is 2-20%; the specific surface area is 4-12 m². 2 / g; the reversible specific capacity at a current density of 0.1A / g is 270–340 mAh / g, and the initial coulombic efficiency is 84–91%; the capacity retention after 500 cycles at a current density of 1A / g is ≥89%.

8. A sodium-ion battery, characterized in that: Its negative electrode uses the sodium-ion battery negative electrode material as described in claim 7, or the sodium-ion battery negative electrode material prepared by any one of the preparation methods described in claims 1-6.

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