A corn flour / glucaric acid cross-linked derivative hard carbon negative electrode material, a preparation method and application thereof

Abstract

The application belongs to the technical field of sodium ion battery negative electrode materials, and discloses a corn powder / glucose acid cross-linking derived hard carbon negative electrode material and a preparation method and application thereof; the preparation method is as follows: corn powder and glucose acid are stirred at a ratio of 10:7-20, the modified corn powder after drying is prepared for carbonization at a low temperature of 500-700 DEG C in an inert gas for 1-3 hours, and then high-temperature carbonization is carried out at 1100-1600 DEG C for 1-3 hours to prepare hard carbon. The preparation method of the green and environment-friendly, low-cost and high-cycle-stability sodium ion battery biomass hard carbon negative electrode material has the characteristics of mass production, controllable hard carbon surface morphology and excellent electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery anode materials, and relates to a method for preparing a sodium-ion battery biomass hard carbon anode material. Specifically, it relates to a method for preparing a sodium-ion battery biomass hard carbon anode material using corn flour and gluconic acid as raw materials through modification, pre-carbonization and high-temperature carbonization. Background Technology

[0002] High-energy-density, long-life rechargeable batteries are crucial for supporting modern electric transportation and distributed energy storage. Sodium-ion batteries, due to their abundant sodium resources, low cost, and high safety, are considered one of the most promising technologies for large-scale energy storage and can effectively complement lithium-ion batteries. The anode material is a key component determining the energy density and cycle life of sodium-ion batteries. However, because the radius of sodium ions (1.02 Å) is significantly larger than that of lithium ions (0.76 Å), and the interlayer spacing of traditional graphite anodes is small (approximately 0.335 nm), it is difficult to achieve effective insertion and extraction of sodium ions, resulting in extremely low sodium storage capacity.

[0003] Hard carbon is a highly disordered carbon material with large interlayer spacing (0.37-0.42 nm), abundant defects, and difficulty in graphitization at high temperatures. Its unique "house of cards" structure is formed by randomly stacked and cross-linked curved graphene sheets, which provides ample space for the insertion / extraction and adsorption of sodium ions. It is considered to be the most commercially promising anode material for sodium-ion batteries.

[0004] Currently, the precursors for hard carbon anodes mainly include three categories: resin-based (such as phenolic resin), carbohydrate-based (such as sucrose), and biomass-based. Among them, biomass-based precursors (such as coconut shells, bamboo, fruit shells, rice husks, etc.) have significant cost reduction potential due to their wide availability, renewability, and lowest cost. Typically, biomass raw materials are directly pyrolyzed in an inert atmosphere (such as N2, Ar) at a high temperature of 1000-1600℃ to carbonize them and form a hard carbon structure. However, existing biomass hard carbon preparation technologies have the following bottlenecks: (1) Difficulty in structural control: Natural biomass has complex components, and the microstructure uniformity after carbonization is poor, making it difficult to accurately control the interlayer spacing and closed-pore structure; (2) Insufficient heteroatom doping: Most biomass has a limited heteroatom content, requiring additional dopants to be added, increasing the complexity of the process; (3) Unstable raw material supply: High-quality precursors such as coconut shells rely on imports, posing supply chain risks. To address these issues, researchers in the field are currently attempting to optimize biomass precursors using strategies such as acid treatment, organic acid esterification modification, and template methods. However, the complexity and cost of these processes still need to be reduced. The primary challenge in the biomass hard carbon route is the screening and determination of precursors. Currently, commonly used biomass raw materials mainly consist of cellulose, hemicellulose, and lignin, which are significantly affected by factors such as origin and season, leading to inconsistent batch performance of hard carbon products. Corn flour, a byproduct of corn processing, has a huge annual output and low price. It is rich in starch (approximately 70-75%), protein (approximately 8-10%), and small amounts of lipids and cellulose. Its molecular structure is rich in hydroxyl (-OH), amide (-CONH-), and COC bonds, which can serve as a carbon source while providing natural nitrogen and oxygen doping. However, directly carbonizing corn flour at high temperatures presents significant technical obstacles: during pyrolysis, the glycosidic bonds in the starch-rich corn flour undergo severe breakage, releasing a large amount of volatile components, resulting in severe melt foaming of the precursor. This process causes a surge in the material's specific surface area and collapses its pore structure, ultimately leading to a sharp decline in the initial coulombic efficiency (ICE) and reversible specific capacity of the prepared hard carbon, severely limiting its electrochemical performance. Gluconic acid (C6H) 12 O7 is a product of glucose oxidation. Its molecule contains a carboxyl group (-COOH) and multiple hydroxyl groups (-OH). It has strong chelating ability and reactivity. In the current technology, it is usually only used for complexing metal ions or pH adjustment. There are no reports on its use as a covalent cross-linking agent to stabilize and regulate the network structure of biomass precursors rich in starch, protein and other components.

[0005] Based on this, this invention proposes a novel method for the directional construction of cross-linked structures using corn flour and gluconic acid as precursors and by adjusting their mass ratio. This patent proposes a new strategy for directionally regulating the hard carbon structure by adding gluconic acid to the biomass precursor corn flour, enhancing the designability of the precursor structure. High-performance hard carbon anode materials for sodium-ion batteries are prepared through reasonable modification and carbonization processes. Summary of the Invention

[0006] This invention aims to improve existing processes by providing a low-cost, industrially feasible method for preparing a high-performance sodium-ion battery biomass hard carbon anode material with a unique hierarchical structure. This method involves controlling the ratio of corn flour to gluconic acid in combination with specific modification, pre-carbonization, and high-temperature carbonization processes. Addressing the shortcomings of existing technologies, this invention innovatively utilizes the directional esterification and cross-linking of gluconic acid in a pH=3±0.5 environment. Through covalent bonding between the hydroxyl groups of corn flour and the carboxyl groups of gluconic acid (molecular needle-and-thread effect), a stable precursor network is constructed. Furthermore, through ratio optimization and synergistic two-step carbonization, a unique hierarchical structure of "large voids-dense carbon walls" is formed, significantly improving sodium ion diffusion efficiency, suppressing side reactions, and overcoming the bottleneck of poor cycling performance in traditional biomass hard carbon anodes.

[0007] The technical solution of the present invention will be described in detail below.

[0008] A method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material, wherein the method involves controlling the ratio of corn flour to gluconic acid, and preparing the sodium-ion battery biomass hard carbon anode material through hydrothermal stirring, pre-carbonization, and high-temperature carbonization. The specific preparation steps include the following: S1. Dissolve corn flour and gluconic acid in deionized water and adjust the pH value to 3±0.5; then stir the mixture to carry out the cross-linking reaction. After the reaction is completed, dry the mixture to obtain modified corn flour. S2. The collected modified corn flour is pre-carbonized at low temperature under an inert gas. S3. The carbonized product is then carbonized at high temperature under an inert gas to finally obtain hard carbon material.

[0009] This invention utilizes gluconic acid as a crosslinking agent to achieve a multi-component synergistic effect with corn flour, forming a uniform hydrogel. Glucoic acid molecules contain one carboxyl group and five hydroxyl groups. Under pH 3 ± 0.5 conditions, its carboxyl group undergoes directional esterification / amidation reactions with the hydroxyl groups of starch and the amino groups of protein in corn flour, forming covalent ester and amide bonds, constructing a starch-protein-gluconic acid ternary crosslinking network. These covalent bonds act like "molecular needles and thread," covalently binding gluconic acid molecules to the carbohydrate and protein backbone of corn flour, forming a uniform and stable three-dimensional precursor network. Its five hydroxyl groups do not participate in the crosslinking process, giving the gluconic acid molecules a "star-shaped" multi-hydroxyl structure, creating a steric hindrance effect that inhibits excessive local crosslinking and ensures a uniform molecular-level distribution of crosslinking points. If crosslinking agents containing only carboxyl groups without multiple hydroxyl groups (such as citric acid and acetic acid) are used, the crosslinking reaction becomes uncontrollable, easily leading to local aggregation or phase separation, making uniform locking impossible. This cross-linked network plays multiple roles during pyrolysis: First, it "locks" the components in a uniform molecular distribution state through covalent bonds, inhibiting phase separation of starch, protein, and lipids in corn flour during pyrolysis, fundamentally solving the problem of uncontrollable structure caused by fluctuations in biomass raw material components; Second, in the pre-carbonization stage, the pyrolysis of gluconic acid releases volatile gases such as CO2 and H2O, forming nanoscale bubbles in the softened precursor matrix, while the breaking of cross-linked bonds releases local stress, inducing the formation of micron-sized voids; In the high-temperature carbonization stage, the micron-sized voids evolve into "large voids," and the dense areas of the cross-linked network form "dense carbon walls," ultimately constructing a unique hierarchical structure of "large voids-dense carbon walls"; Third, nitrogen atoms in corn flour proteins are retained in situ during carbonization, forming nitrogen doping, while oxygen-containing functional groups released by the pyrolysis of gluconic acid form oxygen doping, achieving synergistic enhancement of electronic conductivity and sodium ion adsorption capacity through nitrogen-oxygen co-doping.

[0010] Preferably, the corn flour in step S1 has a particle size of 300-700 μm. This particle size range ensures that gluconic acid can fully penetrate the corn flour particles and form a gradient cross-linked structure, avoiding surface densification that hinders diffusion due to excessively small particle size, and uneven cross-linking due to excessively large particle size, ultimately ensuring the stable formation of the "large void-dense carbon wall" hierarchical structure.

[0011] More preferably, the total starch content of the corn flour is 70.8-74.1% by mass, and the proportion of amylopectin in the total starch is 71-92.5% by mass; the protein content of the corn flour is 8.1-9.2% by mass.

[0012] Preferably, the mass ratio of corn flour to gluconic acid in step S1 is 10:7~20. More preferably, the mass ratio of corn flour to gluconic acid is 10:15.

[0013] Preferably, in step S1, the mass ratio of the total mass of corn flour and gluconic acid to the mass ratio of the deionized water is 1:1 to 7. More preferably, the mass ratio of the total mass of corn flour and gluconic acid to the mass ratio of the deionized water is 1:3. At this dosage, the corn flour and gluconic acid can be completely dissolved, thereby achieving thorough and uniform mixing and cross-linking, while facilitating subsequent drying and reducing energy consumption.

[0014] Preferably, in step S1, gluconic acid is added to deionized water beforehand, followed by corn flour, and the pH is adjusted to 3 ± 0.5. More preferably, the pH is adjusted using NaOH solution, and more preferably, the NaOH solution concentration is 1-7 mol / L, even more preferably 3 mol / L. When pH = 3 ± 0.5, the -COOH and -COO groups in gluconic acid... - The molar ratio is approximately 1:4 to 1:9. At this point, the degree of carboxylation is moderate, which can ensure esterification and cross-linking with the hydroxyl groups of corn flour, while avoiding excessive etching and uncontrolled cross-linking due to excessively high acidity (such as pH=2), or insufficient cross-linking due to excessively low acidity (such as pH=4).

[0015] Preferably, the stirring speed in step S1 is 250-700 r / min, the stirring temperature is 50-100°C, and the stirring time is 3-7 hours. More preferably, the stirring speed in step S1 is 400 r / min, and the stirring temperature is 80°C. ℃, stirring time is 5 hours.

[0016] Preferably, the drying conditions in step S1 are vacuum drying at a temperature of 70~90°C. The temperature is ℃, more preferably 80℃, and the drying is carried out until the moisture is completely removed. The moisture content is preferably not higher than 10%, and the drying time is preferably 4 hours.

[0017] Preferably, in step S2, the pre-carbonization temperature is 500-700℃, the heating rate is 1-10℃ / min, and the holding time is 1-3 hours. More preferably, in step S2, the pre-carbonization temperature is 600℃, the holding time is 2 hours, and the heating rate is 5℃ / min. Pre-carbonization mainly completes the pyrolysis of the precursor, the curing of the cross-linked network, and the formation of the initial porous structure through volatile gases.

[0018] Preferably, the inert gas in steps S2 and S3 is at least one of nitrogen, helium, and argon, and more preferably argon or high-purity argon.

[0019] Preferably, in step S3, the carbonization temperature is 1100-1600℃, the holding time is 1-3 hours, and the heating rate is 1-10℃ / min. More preferably, in step S3, the carbonization temperature is 1500℃, the holding time is 2 hours, and the heating rate is 5℃ / min. High-temperature carbonization is a key step in achieving high graphitization of the carbon skeleton and final shaping of the pore structure.

[0020] Preferably, the interlayer spacing of the hard carbon material obtained in step S3 is 0.376-0.380 nm, more preferably 0.378 nm. The specific surface area is 10.833-18.223 m². 2 / g, preferably 10.833m 2 / g. Pore volume is 0.019-0.028cm³. 3 / g, preferably 0.019cm 3 / g; at a current density of 50mA / g, the initial coulombic efficiency is 86.88%-90.69%, preferably 90.69%.

[0021] The present invention also provides a biomass negative electrode hard carbon material prepared by any of the above preparation methods.

[0022] The present invention also provides an application of the biomass negative electrode hard carbon material prepared by any of the above preparation methods in the field of sodium-ion batteries.

[0023] This invention combines corn flour and gluconic acid for the preparation of hard carbon, achieving the following: (1) Low production cost: Corn flour (a byproduct of corn processing) and gluconic acid (a product of glucose oxidation) are used as raw materials, which are widely available and inexpensive, and do not require metal additives or complex solvent systems, significantly reducing the cost of raw materials. (2) Simple and controllable process: The preparation method involves only three steps: hydrothermal crosslinking, pre-carbonization, and high-temperature carbonization. The operating conditions are mild (pH=3±0.5, atmospheric pressure), and no high-pressure reactor or special equipment is required, making it easy to scale up for industrial production. (3) Unique microstructure: The carboxyl groups of gluconic acid can crosslink / chelate with the hydroxyl groups or protein amino groups in corn flour to construct a uniform and stable three-dimensional precursor network; after two carbonization steps, a unique hierarchical structure of "large voids-dense carbon walls" is formed. (4) Excellent electrochemical performance: In-situ nitrogen-oxygen co-doping synergistically enhances electronic conductivity and sodium ion adsorption capacity, while the hierarchical structure ensures high specific capacity, high first-efficiency and long-cycle stability.

[0024] Compared with the prior art, the beneficial effects of this invention are mainly reflected in: This invention provides a method for producing biomass hard carbon materials for sodium-ion batteries that are widely available and inexpensive. The method uses corn flour and gluconic acid as raw materials, resulting in extremely low costs and achieving high-value utilization of biomass. Furthermore, by controlling the ratio of corn flour to gluconic acid and employing specific modification processes, a unique hierarchical structure of "large voids-dense carbon walls" is created. This morphology originates from the introduction of gluconic acid, increasing surface roughness and providing more active sites, thus enabling the customization of materials to meet different performance requirements. The sodium-ion battery biomass hard carbon anode material produced by this method is simple to manufacture, environmentally friendly, low-cost, and uses renewable raw materials. When used as a sodium-ion battery anode, it exhibits controllable surface morphology and excellent cycle stability, showing promising prospects for industrial application. Attached Figure Description

[0025] Figure 1 The X-ray diffraction (XRD) patterns of the hard carbon materials prepared in Example 2 and Comparative Example 1 are shown.

[0026] Figure 2 Scanning electron microscope (SEM) image of the hard carbon material prepared for Comparative Example 1.

[0027] Figure 3 Scanning electron microscope (SEM) image of the hard carbon material prepared for Comparative Example 4.

[0028] Figure 4 Scanning electron microscope (SEM) image of the hard carbon material prepared in Example 2.

[0029] Figure 5 The graph shows the cycling performance of the hard carbon materials prepared in Example 2 and Comparative Example 1 at a current density of 50 mA / g.

[0030] Figure 6 The graph shows the cycling performance of the hard carbon materials prepared in Example 2 and Comparative Example 1 at a current density of 1000 mA / g. Detailed Implementation

[0031] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, and instruments used in the embodiments, unless otherwise specified, can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention can all achieve the reactions and obtain the desired products. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0033] Example 1: Corn flour source: Northeast yellow corn flour, variety: Xianyu 335, harvested in autumn 2024, batch: 20241015; total starch content: 72.5%, of which amylopectin accounts for 78.3% of the total starch, and protein content: 9.2%.

[0034] Source of gluconic acid: 50% gluconic acid was purchased from Sanmu Chemical Reagent Co., Ltd.

[0035] Corn flour and gluconic acid were mixed at a mass ratio of 10:7, and three times the total mass of deionized water was added. The pH was then adjusted to 3 ± 0.5 (preferably using a 3 mol / L NaOH solution). The mixture was stirred at 80°C and 400 r / min for 5 hours to obtain a mixed solution. The mixed solution was first filtered, and then vacuum dried at 80°C until all moisture was removed to obtain modified corn flour. The modified corn flour was pre-carbonized at 600°C under argon at 5°C / min for 2 hours. After carbonization, the product was collected, and then carbonized at 1500°C under argon at 5°C / min for 2 hours to obtain a hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0036] Example 2: Using the raw materials described in Example 1, corn flour and gluconic acid were mixed at a mass ratio of 10:15, and three times the total mass of deionized water was added. The pH was then adjusted to 3 ± 0.5. The mixture was stirred at 80°C and 400 r / min for 5 hours to obtain a mixed solution. The mixed solution was first filtered, and then vacuum dried at 80°C until all moisture was removed to obtain modified corn flour. The modified corn flour was pre-carbonized at 600°C under argon at 5°C / min for 2 hours. After carbonization, the product was collected, and then carbonized at 1500°C under argon at 5°C / min for 2 hours to obtain a hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0037] Appendix Figure 1The XRD patterns of the hard carbon anode materials prepared in Example 2 and Comparative Example 1 are shown. The diffraction peak positions of the sample (002) were calculated using the Bragg equation. It was found that the interlayer spacing of Example 2 was larger than that of Comparative Example 1. The larger interlayer spacing is beneficial to the rapid insertion / extraction of sodium ions.

[0038] Appendix Figure 2 The SEM image for Comparative Example 1 shows a smooth material surface, exhibiting the intrinsic morphology of direct carbonization of pure corn flour. This is because without the addition of gluconic acid, the corn flour only undergoes self-pyrolysis during carbonization, resulting in structural collapse and the inability to form a three-dimensional cross-linked network.

[0039] Appendix Figure 3 The SEM image for Comparative Example 4 shows a macroporous structure on the material surface. This is because sulfuric acid lacks the carboxyl functional groups found in gluconic acid, thus preventing effective esterification / amidation cross-linking reactions. Consequently, during pyrolysis, components such as starch and protein are prone to phase separation, failing to "lock" the components in a uniformly distributed molecular state as in Example 2, ultimately resulting in a macroporous structure with lower structural strength.

[0040] Appendix Figure 4 The SEM image for Example 2 shows a uniform and dense "bubble-like" protrusion structure on the material surface. This morphology originates from the introduction of gluconic acid, which increases surface roughness and provides more active sites. Compared with the comparative examples, especially compared with the direct carbonization of corn flour in Comparative Example 1, the micron-sized cavities in this structure act as "ion buffers," shortening the sodium ion diffusion distance and improving rate performance; the dense carbon walls provide mechanical strength and buffer volume expansion, significantly improving cycle stability; the surface protrusions and spherical structures increase the electrolyte contact area and reduce interfacial impedance; and the dense carbon walls reduce irreversible side reaction sites, improving the first coulombic efficiency.

[0041] Appendix Figure 5 The graphs show the cycle performance of simulated sodium-ion batteries from Example 2 and Comparative Example 1 within a voltage range of 0.01–2.5 V at 50 mAh / g. Figure 6 The graph shows the cycle performance of simulated sodium-ion batteries from Example 2 and Comparative Example 1 in a voltage range of 0.01~2.5V at 1000mA / g. (See attached figure.) Figure 5 and attached Figure 6 As can be seen, the hard carbon anode material prepared in the embodiments of the present invention exhibits excellent cycling performance. The initial discharge capacity under 50 mA / g conditions is as high as 328.83 mAh / g. At a current density of 1000 mA / g, the reversible specific capacity of the prepared hard carbon remains at 246.36 mAh / g after 650 cycles, and the cycling performance is stable with a cycling coulombic efficiency close to 100%.

[0042] Example 3: Using the raw materials described in Example 1, corn flour and gluconic acid were mixed at a mass ratio of 10:20, and three times the total mass of deionized water was added. The pH was then adjusted to 3 ± 0.5. The mixture was stirred at 80°C and 400 r / min for 5 hours to obtain a mixed solution. The mixed solution was first filtered, and then vacuum dried at 80°C until all moisture was removed to obtain modified corn flour. The modified corn flour was pre-carbonized at 600°C under argon at 5°C / min for 2 hours. After carbonization, the product was collected, and then carbonized at 1500°C under argon at 5°C / min for 2 hours to obtain a hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0043] Example 4: Corn flour source: North China white corn flour, variety: Zhengdan 958, harvested in summer 2024, batch: 20240820; total starch content 70.8%, of which amylopectin accounts for 71.2% of the total starch, and protein content 8.7%.

[0044] The source of gluconic acid is the same as in Example 1.

[0045] Using the corn flour in this example, and with other steps and conditions consistent with Example 2, a hard carbon material was finally obtained. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0046] Example 5: Corn flour source: Southwest glutinous corn flour, variety: Chuannuo No. 1, harvested in autumn 2024, batch: 20241008; total starch content: 74.1%, of which amylopectin accounts for 92.5% of the total starch, and protein content: 8.1%.

[0047] The source of gluconic acid is the same as in Example 1.

[0048] Using the corn flour in this example, and with other steps and conditions consistent with Example 2, a hard carbon material was finally obtained. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0049] Comparative Example 1: The corn flour from Example 1 was used, and the process was carried out in accordance with that of Example 2, except that gluconic acid was not added.

[0050] The specific preparation method includes: corn flour and 3 times the mass of deionized water are stirred at 80℃ and a stirring speed of 400 r / min for 5 hours to obtain a mixed solution; the mixed solution is first filtered, and then vacuum dried at 80℃ until the water is completely removed to obtain modified corn flour; the modified corn flour is pre-carbonized at 600℃ under argon at 5℃ / min for 2 hours; after carbonization, the product is collected, and then carbonized at 1500℃ under argon at 5℃ / min for 2 hours to finally obtain hard carbon material. The interlayer spacing, specific surface area and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0051] Comparative Example 2: The raw materials described in Example 1 were used, and the procedure was carried out in accordance with that in Example 2, except that no heating was performed during the water bath.

[0052] The specific preparation method includes: mixing corn flour and gluconic acid at a mass ratio of 10:15, adding three times the total mass of deionized water (3 times the mass of corn flour and gluconic acid), and then adjusting the pH to 3 ± 0.5. The mixture is stirred at 400 r / min for 5 hours at room temperature (preferably 10-40℃, 25℃ in this example) to obtain a mixed solution; the mixed solution is first filtered, and then vacuum dried at 80℃ until all moisture is removed to obtain modified corn flour; the modified corn flour is pre-carbonized at 600℃ under argon at 5℃ / min for 2 hours; after carbonization, the product is collected, and then carbonized again at 1500℃ under argon at 5℃ / min for 2 hours to finally obtain a hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0053] Comparative Example 3: Potato source: Northeast potato flour, variety: Kexin No. 1, harvested in autumn 2024, batch: 20241020; total starch content: 78.5%, of which amylopectin accounts for 82.3% of the total starch, and protein content: 2.8%.

[0054] The source of gluconic acid is the same as in Example 1.

[0055] The procedure was carried out in accordance with Example 2, except that potato flour was used instead of corn flour.

[0056] The specific preparation method includes: mixing potato flour and gluconic acid at a mass ratio of 10:15, adding three times the total mass of deionized water (potato flour and gluconic acid), and then adjusting the pH to 3 ± 0.5. The mixture is stirred at 80°C and 400 r / min for 5 hours to obtain a mixed solution; the mixed solution is first filtered, then vacuum dried at 80°C until all moisture is removed to obtain modified potato flour; the modified potato flour is pre-carbonized at 600°C under argon at 5°C / min for 2 hours; after carbonization, the product is collected, and then carbonized again at 1500°C under argon at 5°C / min for 2 hours to finally obtain hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0057] Comparative Example 4: The corn flour source is the same as in Example 1.

[0058] Sulfuric acid source: Sulfuric acid (concentration 98%) was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0059] The procedure was carried out in accordance with Example 2, except that sulfuric acid was used instead of gluconic acid.

[0060] The specific preparation method includes: mixing corn flour and 50% sulfuric acid at a mass ratio of 10:15, adding deionized water at a mass ratio of 3 times the total mass of corn flour and sulfuric acid, and then adjusting the pH to 3±0.5. The mixture is stirred at 80℃ and 400 r / min for 5 hours to obtain a mixed solution; the mixed solution is first filtered, and then vacuum dried at 80℃ until all moisture is removed to obtain modified corn flour; the modified corn flour is pre-carbonized at 600℃ under argon at 5℃ / min for 2 hours; after carbonization, the product is collected, and then carbonized at 1500℃ under argon at 5℃ / min for 2 hours to finally obtain hard carbon material. The interlayer spacing, specific surface area, and pore volume of the material are shown in Table 1, and the electrochemical performance is shown in Table 2.

[0061] Performance testing The materials prepared in the above embodiments and comparative examples were assembled into coin half-cells for electrochemical testing. Specific steps included: using a 1 mol / L NaPF6 solution as the electrolyte, dimethyl ethylene glycol (DME) as the solvent, and Whatman GF / F glass fiber as the separator. The batteries were assembled in the following order: positive electrode shell, hard carbon negative electrode sheet, electrolyte, separator, sodium sheet, and negative electrode shell, and then sealed using a sealing machine. The hard carbon negative electrode sheet was prepared by mixing the prepared hard carbon negative electrode material, conductive carbon black, and sodium carboxymethyl cellulose in a mass ratio of 8:1:1 with an appropriate amount of deionized water, and homogenized for 30 min to form a slurry. The slurry was then uniformly coated onto bright aluminum foil and vacuum dried at 80°C for 12 hours. Finally, the electrode sheet was cut into circular electrodes with a diameter of 12 mm. The active material loading for each electrode was 0.9-1.5 mg. After the batteries were allowed to stand for 24 hours, their electrochemical performance was evaluated using a Newwell testing system and a Chenhua electrochemical workstation.

[0062] Electrochemical tests were conducted under a constant temperature condition of 30℃, primarily consisting of constant current charge-discharge tests. These tests mainly assessed reversible capacity, cycle life, and coulombic efficiency. Battery cycle performance was tested at a current density of 50 mA / g. The constant current charge-discharge test procedure was: 5 min rest - constant current discharge - 5 min rest - constant current charge, for 50 cycles. Battery cycle performance was also tested at a current density of 1000 mA / g. The constant current charge-discharge test procedure was: 5 min rest - constant current discharge - 5 min rest - constant current charge, for 650 cycles.

[0063] The interlayer spacing, specific surface area, and pore volume of the materials obtained in each embodiment and comparative example were tested. Specific parameters are shown in Table 1.

[0064] As can be seen from the table above, the hard carbon prepared by modifying corn flour with gluconic acid as a modifier in each embodiment and comparative example has a low specific surface area. In Examples 1-3, the optimal gluconic acid ratio is 10:15, which has the smallest specific surface area and is the optimal amount of gluconic acid added.

[0065] The initial coulombic efficiency, initial discharge capacity, and capacity under ultra-high current of the batteries obtained in each embodiment and comparative example were tested, further demonstrating that the sodium-ion battery using the hard carbon negative electrode of this application still maintains a high charge-discharge capacity after cycling, as detailed in Table 2:

[0066] As can be seen from the table above, the modified hard carbon material prepared by this invention exhibits significantly better performance than the comparative example in terms of cycle stability and initial coulombic efficiency. Implementing the technical solution of this invention can effectively improve the overall performance of sodium-ion battery anode materials. Furthermore, Examples 1 and 4-5 show that the method of this invention is not affected by the variety of raw material corn flour, and its electrochemical performance is comparable and significantly better than the comparative example.

[0067] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications are possible without departing from the technical solutions described in the claims.

Claims

1. A method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material, comprising the following steps: S1. Dissolve corn flour and gluconic acid in deionized water and adjust the pH to 3±0.5; then stir the mixture to carry out a cross-linking reaction. After the reaction is completed, dry the mixture to obtain modified corn flour. S2. The collected modified corn flour is pre-carbonized at low temperature under an inert gas. S3. The carbonized product is then carbonized at high temperature under an inert gas to finally obtain hard carbon material.

2. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, In step S1, the particle size of the corn flour is 300~700 μm.

3. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1 is characterized in that, The mass ratio of corn flour to gluconic acid in step S1 is 10:7~20; And / or, in step S1, the total mass ratio of corn flour and gluconic acid to the mass ratio of deionized water is 1:1~7.

4. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, In step S1, the stirring speed is 250-700 r / min, the stirring temperature is 50-100℃, and the stirring time is 3-7 hours.

5. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, In step S2, the temperature for low-temperature pre-carbonization under inert gas is 500-700℃, the holding time is 1-3 hours, and the heating rate is 1-10℃ / min.

6. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, In step S3, the high-temperature carbonization temperature is 1100-1600℃, the holding time is 1-3 hours, and the heating rate is 1-10℃ / min.

7. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, The mass ratio of corn flour to gluconic acid in step S1 is 10:

15. The total mass ratio of corn flour and gluconic acid to deionized water is 1:3, the stirring speed is 400 r / min, the stirring temperature is 80℃, and the stirring time is 5 hours. And / or, in step S2, the pre-carbonization temperature is 600℃, the holding time is 2 hours, and the heating rate is 5℃ / min; And / or, in step S3, the carbonization temperature is 1500℃, the holding time is 2 hours, and the heating rate is 5℃ / min.

8. The method for preparing a corn flour / gluconic acid crosslinked derivative hard carbon anode material according to claim 1, characterized in that, The interlayer spacing of the hard carbon material obtained in step S3 is 0.376-0.380 nm, and the specific surface area is 10.833-18.223 m². 2 / g, pore volume is 0.019-0.028cm³ 3 / g, and / or, the initial coulombic efficiency at a current density of 50mA / g is 86.88%-90.69%.

9. A corn flour / gluconic acid crosslinked derivative hard carbon anode material prepared by the preparation method according to any one of claims 1-8.

10. The application of a corn flour / gluconic acid crosslinked derivative hard carbon anode material prepared by the preparation method according to any one of claims 1-8 in the field of sodium-ion batteries.

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