Biomass hard carbon negative electrode material based on controllable pre-oxidation crosslinking as well as preparation method and application of biomass hard carbon negative electrode material

Abstract

The invention relates to a biomass hard carbon negative electrode material based on controllable pre-oxidation crosslinking as well as a preparation method and application of the biomass hard carbon negative electrode material. The method comprises: pretreating biomass with a metal salt structure directing agent; carrying out pre-oxidation crosslinking on the pretreated biomass precursor in an oxygen-enriched atmosphere; and carrying out stepped carbonization on the pre-oxidized cross-linked intermediate under the protection of an inert atmosphere to obtain the biomass hard carbon negative electrode material. The hard carbon material prepared by the invention has moderate specific surface area (5-50 m < 2 > / g), high closed porosity (gt, 40%) and expanded graphite interlamellar spacing (0.38-0.40 nm). Due to the unique structure, the high reversible capacity (gt, 330 mAh / g) and excellent cycling stability are realized when the biomass hard carbon material is used for the negative electrode of the sodium ion battery, and the problems of low reversible capacity and poor rate capability of the traditional biomass hard carbon are solved.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage materials technology, specifically relating to a biomass hard carbon anode material based on controllable pre-oxidative crosslinking, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, due to their abundant resources and low cost, show great promise for large-scale energy storage. Developing high-performance, low-cost anode materials is one of the keys to the commercialization of sodium-ion batteries. Hard carbon materials, with their abundant carbon source, disordered microstructure, and suitable sodium-ion intercalation channels, are considered highly promising anode materials for sodium-ion batteries.

[0003] Biomass (such as wood, straw, and coconut shells) is an ideal precursor for preparing hard carbon materials due to its wide availability, renewability, and low cost. However, hard carbon materials obtained by direct carbonization of biomass often suffer from problems such as low first-cycle coulombic efficiency and poor rate performance. This is mainly due to the uncontrollable pore structure, excessively high specific surface area, and excessive oxygen-containing functional groups formed during the direct carbonization process of biomass, resulting in significant irreversible capacity loss.

[0004] To improve the performance of biomass hard carbon, researchers typically employ post-treatment methods such as pre-oxidation, activation, and doping. However, these methods often have limitations: conventional pre-oxidation has limited precision in controlling the microstructure, and improper control can lead to a significant reduction in carbon yield; while post-treatment methods that introduce activators such as strong acids and bases can increase capacity, they are often accompanied by problems such as environmental pollution, equipment corrosion, and increased energy consumption. In particular, acid-based structure-directing agents used in existing technologies (such as oxalic acid and citric acid disclosed in CN 118206101 A) may remain in the material, introducing unintended heteroatom doping, thereby affecting the material's electronic conductivity, initial efficiency, and long-term cycling stability.

[0005] It is particularly important to note that, from a life-cycle perspective, traditional hard carbon production processes, especially those with low yields or involving highly polluting chemicals, have significant environmental footprints (such as global warming potential), which contradicts the development concept of green energy storage. Therefore, developing a low-carbon production method that can precisely control the microstructure of biomass carbon precursors, ensuring high electrochemical performance while also achieving high carbon yield and low environmental impact, has significant scientific and practical value. Summary of the Invention

[0006] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing biomass hard carbon anode materials based on controllable pre-oxidative crosslinking. This method, through a unique combination of processes, not only enhances the electrochemical performance of the material but also reduces environmental impact at the source by increasing carbon yield.

[0007] Another object of the present invention is to provide a biomass hard carbon anode material prepared by the above preparation method.

[0008] Another object of the present invention is to provide the application of the above-mentioned biomass hard carbon anode material.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A low-carbon preparation method for biomass hard carbon anode materials based on controllable pre-oxidative crosslinking includes the following steps: (I) Precursor pretreatment and structure orientation: The biomass raw material is cleaned, crushed into blocks or granules, and then immersed in a volatile metal salt structure orientation agent solution. After being fully immersed, it is dried to obtain the pretreated biomass precursor. (II) Controllable pre-oxidative crosslinking: The pretreated biomass precursor obtained in step (I) is placed in an atmosphere furnace and heated to 200-600°C at a heating rate of 0.5-5°C / min under an oxygen-rich atmosphere and held for 1-10 hours to obtain a pre-oxidative crosslinking intermediate. (III) Step-by-step high-temperature carbonization: The pre-oxidized crosslinking intermediate obtained in step (II) is transferred to a high-temperature tube furnace. Under the protection of an inert atmosphere, the temperature is first raised to 600~900℃ and held for 0.5~3 hours. Then, the temperature is raised to 1200~1500℃ at the same heating rate and held for 1~5 hours. After natural cooling, the material is crushed, ground, cleaned and dried to obtain the biomass hard carbon anode material.

[0010] This invention discovers that using volatile metal salts (such as zinc chloride) as structure-directing agents, in synergy with a pre-oxidation crosslinking process under specific conditions, can produce unexpected technical effects: In the pre-oxidation stage, the metal salt can catalyze the crosslinking and aromatization of biomass components, forming stable oxygen bridges and framework structures; and in the subsequent step-by-step carbonization process, the metal salt can completely volatilize at high temperatures, leaving no residue in the final hard carbon product, thus avoiding the potential negative impact of heteroatom doping on initial efficiency and stability. This "catalysis-volatilization" mechanism is fundamentally different from acid-based structure-directing agents. The synergistic effect described in this invention is specifically manifested in: Structural synergy: Metal salts guide the formation of a stable and well-developed cross-linked network during the pre-oxidation stage. This network can effectively suppress the disordered collapse of the structure and the formation of a large number of open pores during high-temperature carbonization, thereby guiding the generation of an ideal hard carbon structure with moderate specific surface area, high closed pore rate and expanded interlayer spacing.

[0011] Yield synergy: The robust cross-linked network significantly reduces the escape of small molecule volatiles during carbonization, thereby greatly improving the final yield (carbon fixation rate) of hard carbon, which is the fundamental reason for achieving low-carbon manufacturing.

[0012] Preferably, in step (I), the biomass raw material is one or more of the following: redwood, bamboo, straw, coconut shell, walnut shell, peanut shell, cotton shell, pine, eucalyptus, and balsa wood; the size of the crushed biomass raw material is small pieces of 1-20 mm square or particles of 2-10 mesh.

[0013] Preferably, in step (I), the metal salt structure directing agent is one or more of zinc chloride, ferric chloride, zinc nitrate, and ferric nitrate.

[0014] Preferably, in step (I), the mass concentration of the metal salt structure guiding agent solution is 0.1% to 5%; the impregnation time is 2 to 24 hours; and the drying temperature is 60 to 120°C.

[0015] Preferably, in step (II), the oxygen-enriched atmosphere is air, or a mixture of oxygen and an inert gas with an oxygen volume fraction of 21% to 50%; the preferred heating rate of the programmed heating is 1 to 2 °C / min; the holding temperature is 250 to 300 °C; and the holding time is 2 to 6 hours.

[0016] More preferably, in step (II), the temperature of the heat preservation is 250°C, and the heat preservation time is 2 hours.

[0017] Preferably, in step (III), the inert atmosphere is nitrogen or argon; the preferred procedure for the stepped high-temperature carbonization is: first, heat to 800-900℃ at a rate of 2-10℃ / min and hold for 1-2 hours, then continue to heat to 1300-1400℃ and hold for 2-4 hours.

[0018] More preferably, the stepped high-temperature carbonization process is as follows: first, the temperature is raised to 800~900℃ and held for 1~2 hours, and then the temperature is raised to 1300℃ and held for 2 hours.

[0019] This invention also protects the biomass hard carbon anode material prepared by the above-described method, which has a rich nanoporous structure, a graphitized interlayer spacing d002 ranging from 0.38 to 0.40 nm, and a BET specific surface area of ​​5 to 50 m². 2 / g, closed-cell ratio greater than 40%.

[0020] This invention also protects a sodium-ion battery whose negative electrode active material comprises the above-mentioned biomass hard carbon negative electrode material.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. An innovative synergistic strategy of "controllable pre-oxidative crosslinking assisted by volatile metal salt structure-directing agents" is proposed. This strategy introduces a volatile metal salt catalyst during the pre-oxidation stage to guide the formation of an ideal precursor structure, which is then completely removed after carbonization, achieving "pure" control over the microstructure of hard carbon materials. This process combination and the specific type of structure-directing agent used are not disclosed in existing technologies, and the resulting "catalysis-volatilization" mechanism and subsequent effects are not readily apparent.

[0022] 2. Significant improvement in overall material performance: Through the above-mentioned synergistic process, the prepared hard carbon material has a suitable specific surface area (5-50 m²). 2 With its high closed-pore ratio (>40%) and expanded graphite interlayer spacing (0.38-0.40 nm), this unique structure enables it to achieve both high reversible capacity (>330 mAh / g) and excellent cycle stability when used as a sodium-ion battery anode, solving the problems of low reversible capacity and poor rate performance of traditional biomass hard carbon.

[0023] 3. Unexpectedly low carbon emission effects were achieved: This invention, through a synergistic process, increases the hard carbon yield by more than 15% compared to direct carbonization. According to life cycle assessment results, taking the production of 1 kg of hard carbon material as a functional unit, the global warming potential of this invention's process is reduced by approximately 20.75% compared to traditional direct carbonization. This significant environmental advantage can only be achieved through the synergistic effect of the two-step process of "structure-directing agent impregnation" and "controlled pre-oxidative crosslinking," which cannot be achieved by implementing either step alone or by using other types of structure-directing agents. Attached Figure Description

[0024] To more clearly illustrate the technical solutions adopted in the embodiments of the present invention or the prior art, the accompanying drawings involved in the description of the embodiments or the prior art will be briefly introduced below. It should be noted that these drawings only represent some embodiments of the present invention, and those skilled in the art can derive other related drawings based on these drawings without creative effort.

[0025] Figure 1 These are microscopic CT and morphological comparison images of the hard carbon materials prepared in Example 1 (second row of images) and Comparative Example 1 (first row of images) of the present invention (SEM morphology images (a, d), CT structure images (b, e), and HRTEM microstructure images (c, f)).

[0026] Figure 2 These are the initial charge-discharge curves of the hard carbon materials prepared in Example 1 and Comparative Example 1 of this invention.

[0027] Figure 3 This is a rate performance diagram of the hard carbon materials prepared in Example 1 and Comparative Example 1 of the present invention.

[0028] Figure 4 This is a full life cycle assessment diagram (global warming potential value) of the preparation routes corresponding to the hard carbon materials prepared in the embodiments and comparative examples of this invention.

[0029] Figure 5 This invention compares the application of its technical route in biomass raw materials such as walnuts, peanuts, beech wood, cotton, pine wood, coconuts, and bamboo. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. All raw materials involved in the present invention can be purchased directly from the market. For process parameters not specifically specified, conventional techniques can be referred to.

[0031] To clearly demonstrate the objectives, technical solutions, and advantages of the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be comprehensively and accurately described below. It should be understood that the following embodiments are only for illustrating the present invention and are not intended to limit it. Unless otherwise expressly stated, all technical and scientific terms used herein follow the general definitions within the technical field, and all reagents mentioned meet industrial or analytical purity standards. Furthermore, the accompanying drawings and descriptions are intended to help those skilled in the art to deeply understand this application, and are not intended to limit the subject matter covered by the claims.

[0032] Regarding the "range" mentioned in this application, it is defined by setting a lower limit and an upper limit, which define the boundaries of a specific range. Such a range may or may not include its endpoints, and can be freely combined; that is, any lower limit can be combined with any upper limit to form a new range.

[0033] Unless otherwise specified, all embodiments and their optional solutions in this application can be combined with each other to create new technical solutions. Similarly, unless otherwise specified, all technical features and their optional features in this application can also be combined with each other to form new technical solutions.

[0034] Example 1 (I) After cleaning the mahogany blocks, break them into small pieces of about 5 mm square. Prepare a 2% zinc chloride aqueous solution, immerse the wood blocks in the solution for 12 hours, remove them and dry them in an oven at 105°C to obtain the pretreated biomass precursor.

[0035] (II) The above pretreated precursor is placed in a muffle furnace and heated to 250°C at a heating rate of 1°C / min under an air atmosphere. The temperature is then maintained at this temperature for 6 hours and naturally cooled to obtain a pre-oxidized crosslinking intermediate.

[0036] (III) The pre-oxidized crosslinking intermediate was placed in a tube furnace and heated to 900°C at a rate of 5°C / min and held for 1 hour under a nitrogen atmosphere. Then, the temperature was increased to 1300°C at a rate of 5°C / min and held for 2 hours. After naturally cooling to room temperature, the resulting carbon block was crushed and ground, washed with deionized water until neutral, and vacuum dried at 120°C for 12 hours to obtain the biomass hard carbon anode material, denoted as HC-1. This preparation route was defined as PHC, the indirect carbonization route.

[0037] Example 2 It is basically the same as Example 1, except that in step (II), the heat preservation temperature for pre-oxidative crosslinking is 200°C.

[0038] Example 3 It is basically the same as Example 1, except that in step (II), the heat preservation temperature for pre-oxidative crosslinking is 400°C.

[0039] Example 4 It is basically the same as Example 1, except that in step (II), the heat preservation temperature for pre-oxidative crosslinking is 600°C.

[0040] Example 5 (Influence of Structure Directing Agent Concentration) It is basically the same as Example 1, except that in step (I), the mass concentration of the zinc chloride aqueous solution used is 0.5%.

[0041] Example 6 (Types of Structure-Directing Agents) It is basically the same as Example 1, except that in step (I), the structure directing agent used is a 1% ferric chloride aqueous solution.

[0042] Comparative Example 1 The "structure-directing agent impregnation" step (I) and the "controlled pre-oxidative crosslinking" step (II) were skipped. Instead, the crushed and cleaned poplar blocks were directly carbonized under nitrogen atmosphere, heated to 1300℃ at a rate of 5℃ / min and held for 2 hours. The resulting material was designated HC-D1. This preparation route was defined as DHC, the direct carbonization route.

[0043] Comparative Example 2 Only step (II) of "controlled pre-oxidative crosslinking" was performed, but step (I) of "structure-directing agent impregnation" was not performed. That is, the mahogany block was directly pre-oxidized in air by heating to 250°C at 1°C / min and holding for 6 hours, and then the same step (III) as in Example 1 was performed. The resulting material was designated HC-D2.

[0044] Comparative Example 3 (using structure-directing agent alone) The precursor was impregnated with a structure-directing agent (2% zinc chloride) in step (I), but the pre-oxidative crosslinking in step (II) was not performed. Instead, the impregnated and dried precursor was directly carbonized in step (III). The resulting material is designated as HC-D3.

[0045] Comparative Example 4 (using acid-based structure-directing agents) The process is essentially the same as in Example 1, except that in step (I), a 2% aqueous solution of citric acid is used as the structure directing agent. The resulting material is designated as HC-D4.

[0046] Performance testing and characterization Electrochemical performance testing: The biomass hard carbon materials prepared in the preparation examples and comparative examples were used to prepare negative electrode slurry according to the mass ratio of hard carbon: conductive agent: binder = 8:1:1, and coated on the current collector (copper foil). The slurry was then dried and sliced, and C2032 coin cells were assembled in a glove box. After standing for 12 hours, the cells were placed in the Xinwei Battery test channel for relevant sodium storage performance testing.

[0047] Physical property characterization: The specific surface area of ​​the material was tested using a specific surface area and pore size analyzer (BET); the crystal structure of the material was tested using an X-ray diffractometer (XRD) and the interlayer spacing (d002) was calculated; the carbon structural disorder (ID / IG) of the material was tested using a Raman spectrometer; and the carbon yield was calculated by weighing.

[0048] Life Cycle Assessment: A full life cycle assessment was conducted on the PHC process of Example 1 and Comparative Example 1 (conventional direct carbonization process DHC) of this invention, with the production of 1 kg of hard carbon material as the functional unit. The test results of the examples and comparative examples are summarized in the following table: Table 1 Results analysis: Structure and electrochemical properties: The pre-carbonized structure was characterized using scanning electron microscopy (SEM) and three-dimensional micro-CT. For example... Figure 1 As shown in a and d, after applying an additional pre-carbonization process to the rosewood precursor, significant organic aggregates were added to the surface morphology distribution of the rosewood block. These organic aggregates filled most of the original cell wall pores of the wood, further affecting the heat transfer during the carbonization process and altering its carbonization process. Figure 1The CT structural views of b and e further illustrate that during the pre-carbonization process, the internal structural channels of the wood underwent further condensation, aggregation, and structural collapse. Simultaneously, the high-resolution transmission electron microscopy (HRTEM) image of the PHC hard carbon material prepared by the pre-carbonization process also shows a more pronounced graphite-like microcrystalline region than that of the DHC material. The diffraction ring intensity in the attached figure further demonstrates that the PHC material has a more obvious distribution of graphite microcrystalline regions. Figure 1 (c, f in the text)

[0049] As can be seen from the data in Table 1: 1. Synergistic effect is crucial: Example 1 (synergistic process) demonstrates superior reversible capacity (337 mAh / g), first-efficiency (85%), and specific surface area control (15 m²). 2 The optimal balance was achieved in terms of g / g, interlayer spacing (0.39 nm), and carbon yield (28%). The performance of Comparative Example 2 (pre-oxidation only) and Comparative Example 3 (structure directing agent only) was significantly lower than that of Example 1, demonstrating that the synergistic effect of the present invention cannot be achieved by performing either step alone.

[0050] 2. Advantages of volatile metal salts: The first-efficiency (85%) of Example 1 (ZnCl2) was significantly higher than that of Comparative Example 4 (citric acid, 79%), and it also had a lower specific surface area and a higher carbon yield. This demonstrates that using volatile metal salts avoids the negative impact of heteroatom residues on the first-efficiency and more effectively guides the formation of the desired structure, resulting in unexpected performance improvements.

[0051] 3. The criticality of process parameters: The pre-oxidation temperature (Examples 3, 1, 4, 5) and the concentration of the structure-directing agent (Example 6, 1) both have a significant impact on performance, proving that the optimality of the parameter range of the present invention is not obvious.

[0052] Environmental benefits: from Figure 4 According to the life cycle assessment data, the global warming potential (GWP) of the process in Example 1 of this invention is only 3.38 kg CO2eq / kg PHC, which is about 20.75% lower than that of the conventional process in Comparative Example 1 (4.26 CO2eq / kg DHC). The GWP values ​​of Comparative Examples 2, 3, 4, and 5 are all higher than those of Example 1, which strongly demonstrates that only a complete and synergistic process of "impregnation with volatile metal salt structure-directing agents", "controlled pre-oxidative crosslinking", and "step-by-step high-temperature carbonization" can maximize carbon yield and environmental benefits.

[0053] Route universality verification: like Figure 5As shown, the direct carbonization process DHC and the indirect carbonization process PHC of this patent application were applied to different biomass raw materials. It was found that the application of this process to walnut, peanut, beech, cotton, pine, coconut and bamboo raw materials can quickly improve the reversible capacity under different current density ranges, which greatly verifies the universality of the technology of this invention.

[0054] in conclusion: This invention, through a unique synergistic process design of "volatile metal salt structure-directing agent-assisted controlled pre-oxidative crosslinking," not only successfully prepared high-performance biomass hard carbon materials with high capacity, high initial efficiency, moderate specific surface area, large interlayer spacing, and high closed-porosity, but also reduced carbon emissions at the source by improving yield. LCA analysis confirmed its significant low-carbon advantages. This effect is unprecedented and unpredictable in existing technologies, providing a high-performance and green sustainable anode material solution for sodium-ion batteries.

[0055] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing biomass hard carbon anode material based on controllable pre-oxidative crosslinking, characterized in that, Includes the following steps: (I) The biomass raw material is cleaned and crushed, then immersed in a metal salt structure directing agent solution, fully wetted and dried to obtain a pretreated biomass precursor; (II) The pretreated biomass precursor obtained in step (I) is heated to 200-600°C at a heating rate of 0.5-5°C / min under an oxygen-enriched atmosphere and held for 1-10 hours to obtain a pre-oxidized crosslinking intermediate. (III) The pre-oxidized crosslinking intermediate obtained in step (II) is heated to 600~900℃ and kept at that temperature for 0.5~3 hours under an inert atmosphere, and then heated to 1200~1500℃ and kept at that temperature for 1~5 hours. After natural cooling, it is crushed, ground, cleaned and dried to obtain the biomass hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, In step (I), the volatile metal salt structure directing agent is one or more of zinc chloride, ferric chloride, zinc nitrate, and ferric nitrate; the mass concentration of the metal salt structure directing agent solution is 0.1% to 5%; the impregnation time is 2 to 24 hours; and the drying temperature is 60 to 120°C.

3. The preparation method according to claim 1 or 2, characterized in that, In step (I), the biomass raw material is one or more of redwood, bamboo, straw, coconut shell, walnut shell, and peanut shell; the size of the crushed biomass raw material is small pieces of 1-20 mm square or particles of 2-10 mesh.

4. The preparation method according to claim 1, characterized in that, In step (II), the oxygen-enriched atmosphere is air, or a mixture of oxygen and an inert gas with an oxygen volume fraction of 21% to 50%; the temperature rise rate of the programmed temperature rise is 1 to 2 °C / min.

5. The preparation method according to claim 1, characterized in that, In step (II), the temperature for heat preservation is 250~300℃; the heat preservation time is 2~6 hours.

6. The preparation method according to claim 1, characterized in that, In step (III), the temperature is first raised to 800-900℃ at a rate of 2-10℃ / min and held for 1-2 hours, and then raised to 1300-1400℃ and held for 2-4 hours; the inert atmosphere is nitrogen or argon.

7. The preparation method according to claim 6, characterized in that, First, raise the temperature to 800~900℃ and hold for 1~2 hours, then continue to raise the temperature to 1300℃ and hold for 2 hours.

8. A biomass hard carbon anode material prepared by any one of claims 1 to 7.

9. The biomass hard carbon anode material according to claim 8, characterized in that, Its BET specific surface area is 5~50 m² 2 / g, closed-pore ratio greater than 40%; graphitized interlayer spacing d002 ranges from 0.38 to 0.40 nm.

10. A sodium-ion battery, characterized in that, Its negative electrode active material comprises the biomass hard carbon negative electrode material as described in claim 8 or 9.

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