Starch-based hard carbon negative electrode material and preparation method thereof

Abstract

The present application relates to the technical field of negative electrode material, and more particularly to a starch-based hard carbon negative electrode material and a preparation method thereof, which comprises the following steps: taking starch as raw material, introducing 2-phosphonate butane-1,2,4-tricarboxylic acid and guanidino acetic acid for phosphorus and nitrogen co-doping modification after gelatinization, preparing a uniform composite hydrosol, drying, crushing and screening to obtain a blending precursor powder, and finally preparing the starch-based hard carbon negative electrode material through three-stage programmed temperature carbonization in an inert atmosphere and subsequent post-processing of crushing, washing, drying and screening. Through cross-linking modification and segmented temperature control carbonization, the starch-based hard carbon negative electrode material effectively improves the defect of easy melting and collapse of starch pyrolysis, synergistically improves the electronic conductivity and sodium ion diffusion kinetics of the material, and significantly improves the sodium storage capacity, the first coulombic efficiency and the cycle and rate stability.

Description

Technical Field

[0001] This invention relates to the field of anode material technology, and in particular to a starch-based hard carbon anode material and its preparation method. Background Technology

[0002] Anode materials are the core component determining the overall performance and cost of sodium-ion batteries. Traditional graphite anodes, due to the large radius of sodium ions, cannot form stable intercalation compounds, making them unsuitable for sodium-ion battery systems. Alloy-based and titanium-based non-carbon anodes suffer from defects such as severe volume expansion and low specific capacity, failing to meet practical application requirements. Hard carbon materials, with their unique microcrystalline structure characterized by long-range disorder and short-range order, possess adjustable carbon interlayer spacing, abundant sodium storage sites, and a stable framework structure, making them currently recognized as the most commercially promising anode material for sodium-ion batteries.

[0003] Based on precursor sources, raw materials for hard carbon preparation mainly fall into three categories: petrochemical byproducts, synthetic polymers, and biomass. While chemical-based precursors such as petroleum asphalt and coke have a stable supply, they generally suffer from high impurity content and insufficient environmental friendliness, resulting in low initial coulombic efficiency of the obtained hard carbon. Resin-based synthetic polymer precursors allow for precise control of molecular structure, leading to materials with excellent performance, but their high raw material costs and high carbonization energy consumption make them unsuitable for low-cost energy storage needs. Biomass-based hard carbon, due to its wide availability, renewability, environmental friendliness, and considerable carbon yield, has become a research hotspot in the field of hard carbon materials. Among various biomass precursors, starch, as a typical polysaccharide natural polymer, possesses characteristics such as high purity, uniform composition, easily modifiable molecular structure, and regular natural particle morphology. Furthermore, its abundant sources and low cost make it an ideal precursor for preparing high-performance hard carbon anodes.

[0004] However, current starch-based hard carbon preparation technology still has defects: starch itself has poor thermal stability. During direct high-temperature carbonization, the hydroxyl groups and glycosidic bonds on the molecular chain are easily broken and a large number of volatile components are released, causing severe melting foaming and particle collapse. In the end, carbon materials with excessive specific surface area and disordered pore structure are formed, which not only leads to a large amount of irreversible capacity loss and low initial coulombic efficiency, but also significantly reduces the tap density of the material, which restricts the volumetric energy density of the battery. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a starch-based hard carbon anode material and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a method for preparing a starch-based hard carbon anode material, comprising the following steps: S1. Mix starch with water to prepare starch milk, heat and gelatinize to obtain starch colloid, then add an aqueous solution of phosphorus compound and a nitrogen compound to the starch colloid to react and obtain a composite hydrosol; S2. Perform a first post-treatment on the composite hydrosol to obtain the blended precursor powder; S3. Under an inert atmosphere, the blending precursor powder is subjected to a three-stage programmed heating treatment, and after furnace cooling, a crude carbonized product is obtained. S4. The crude carbonization product is subjected to a second post-treatment to obtain starch-based hard carbon anode material.

[0007] Further, the preparation process of starch colloid in step S1 specifically includes: mixing starch with deionized water to prepare a starch slurry with a mass fraction of 10-20%; transferring the starch slurry to a constant temperature water bath stirring reaction device; heating to 70-85℃ and stirring at a constant temperature for 20-40 minutes. The starch is any one or a mixture of two or more of corn starch, tapioca starch, potato starch, and wheat starch; the stirring speed is controlled at 200-500 rpm.

[0008] Furthermore, in step S1, the phosphorus-containing compound is 2-phosphonobutane-1,2,4-tricarboxylic acid, and the nitrogen-containing compound is guanidinoacetic acid.

[0009] Further, in step S1, the feed ratio of starch, phosphorus-containing compound, and nitrogen-containing compound, based on dry weight, is as follows: 100 parts starch, 10-18 parts aqueous solution of phosphorus-containing compound with a mass fraction of 40-60%, and 5-10 parts nitrogen-containing compound. In some embodiments, the feed ratio of starch, phosphorus-containing compound, and nitrogen-containing compound, based on dry weight, is as follows: 100 parts starch, 12-16 parts aqueous solution of phosphorus-containing compound with a mass fraction of 45-55%, and 6-9 parts nitrogen-containing compound.

[0010] Furthermore, the reaction conditions in step S1 include: under continuous mechanical stirring, controlling the stirring rate at 300-600 rpm; adding the phosphorus-containing aqueous solution and the nitrogen-containing compound by dropwise addition or batch addition, with the addition time controlled at 15-30 min; after the addition is completed, continuing to stir at a constant temperature of 40-50℃ for 20-40 min. The order of addition is as follows: first, the phosphorus-containing aqueous solution is added dropwise, and after its dropwise addition is complete, the nitrogen-containing compound is added in batches; the pH value of the system is controlled at 5.0-7.0 during the reaction.

[0011] Further, the first post-processing in step S2 includes: first drying, first pulverization, and first sieving; wherein, the temperature of the first drying is 70-90℃, and the drying time is 8-15h; the first pulverization is carried out using a planetary ball mill, with the milling speed controlled at 200-400rpm and the milling time at 10-30min; the first sieving uses a 150-250 mesh standard sieve. The first drying is either forced-air drying or vacuum drying; the ball-to-material mass ratio for the first pulverization is (5-10):1, and the grinding balls are made of zirconium oxide or agate; after the first sieving, the undersize component is collected as a blending precursor powder.

[0012] Furthermore, the three-stage temperature ramp-up process in step S3 specifically includes: Phase 1: Increase the temperature from room temperature to 250-280℃ at a rate of 1-3℃ / min, and continue to maintain the temperature for 1-3 hours after reaching the target temperature; Phase 2: Increase the temperature from 250-280℃ to 550-650℃ at a rate of 0.5-2℃ / min, and continue to maintain the temperature for 1-2 hours after reaching the target temperature; Phase 3: Increase the temperature from 550-650℃ to 1000-1200℃ at a rate of 2-5℃ / min, and then maintain the temperature for 2-4 hours after reaching the target temperature.

[0013] The inert atmosphere is high-purity nitrogen, and the gas flow rate is controlled at 50-200 mL / min; the room temperature is 20-30℃.

[0014] Further, the second post-processing in step S4 includes a second crushing, washing, a second drying, and a second sieving. The second crushing is performed using a planetary ball mill, with the milling speed controlled at 200-400 rpm and the milling time at 10-30 min. Washing involves using hot deionized water at 80-90℃ to perform 3-5 vacuum filtrations on the powder. The second drying temperature is 70-90℃, and the drying time is 8-15 h. The second sieving uses a 100-200 mesh standard inspection sieve. The ball-to-material mass ratio in the second crushing is (3-8):1; the vacuum degree of the vacuum filtration is 0.06-0.09 MPa; the second drying is vacuum drying; and the undersize component after the second sieving is collected as the final starch-based hard carbon anode material.

[0015] On the other hand, the present invention provides a starch-based hard carbon anode material prepared by the above preparation method.

[0016] The beneficial effects of this invention are: 1. This invention first prepares starch into a gelatinized colloid, then introduces phosphorus- and nitrogen-containing compounds into a liquid-phase composite. The phosphorus-containing compound is 2-phosphonobutane-1,2,4-tricarboxylic acid, and the nitrogen-containing compound is guanidinoacetic acid. These two components can form hydrogen bonds and covalent bonds with the hydroxyl groups of the starch molecular chains, constructing a molecular-level cross-linked network to anchor the starch molecular chains. During the low-temperature pre-carbonization stage, the cross-linking sites first form carbonaceous framework nodes, restricting the melting and flow of starch molecules, buffering the impact of pyrolysis gas generation, and mitigating particle foaming and structural collapse. This results in carbonaceous particles with regular morphology and dense structure, which helps improve the material's tap density and provides a structural basis for increasing the volumetric energy density of the battery.

[0017] 2. This invention employs a three-stage programmed temperature rise to control the pyrolysis process in stages, constructing a hierarchical porous system while maintaining the specific surface area within a reasonable range. A lower specific surface area reduces the contact area between the material and the electrolyte, minimizing ion consumption during the initial charge-discharge process due to the formation of the solid electrolyte interfacial film, reducing irreversible capacity loss, and improving the initial coulombic efficiency. The hierarchical pores create continuous ion transport channels, accelerating electrolyte wetting and sodium ion diffusion, ensuring the material's high-current charge-discharge capability.

[0018] 3. This invention optimizes the carbon microcrystalline structure through in-situ co-doping of phosphorus and nitrogen. Benefiting from the uniform dispersion of the liquid-phase precursor, phosphorus and nitrogen atoms can be embedded relatively uniformly into the carbon lattice, reducing local agglomeration. Nitrogen doping enhances the intrinsic electronic conductivity of the material and introduces active sites for sodium ion adsorption; phosphorus doping induces lattice distortion, expands the carbon interlayer spacing, and lowers the sodium ion insertion / extraction energy barrier. The synergistic effect of these two dopants improves the material's reversible specific capacity, rate performance, and cycle stability.

[0019] 4. This invention uses natural starch as a carbon source, which has a wide range of raw material sources and low cost. The process parameters are controllable, the batch stability of the materials is good, and no complicated preparation equipment is required. It has both performance and cost advantages and is suitable for the large-scale development of sodium-ion batteries. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the following examples and comparative examples, 2-phosphonobutane-1,2,4-tricarboxylic acid (CAS No.: 37971-36-1), guanidinoacetic acid (CAS No.: 352-97-6), phytic acid (CAS No.: 83-86-1), phosphoric acid (CAS No.: 7664-38-2), and urea (CAS No.: 57-13-6) were all analytical grade with a purity ≥98.0%; sodium carboxymethyl cellulose, styrene-butadiene rubber latex, and acetylene black were all battery-specific reagents; and sodium metal sheets, glass fiber separators, and electrolytic copper foils were all commercial battery-grade auxiliary materials.

[0022] Example 1 S1. Weigh 100g of corn starch, add 567mL of deionized water to mix and prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 78℃, and stir at 350rpm for 30min to obtain starch colloid; weigh 7g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 7mL of deionized water to dissolve and prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, after the dropwise addition is complete, add 7.5g of guanidinoacetic acid in batches, the addition time is 20min, and the stirring is maintained at 450rpm during the addition process; after the addition is complete, stir at 45℃ for 30min to obtain composite hydrosol.

[0023] S2. The composite hydrosol was dried at 80℃ for 12 hours, then taken out and ball-milled at 300 rpm for 20 minutes. The mixture was then passed through a 200-mesh standard sieve, and the sieved component was taken to obtain the blended precursor powder.

[0024] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen at a flow rate of 120 mL / min, and implement a three-stage programmed temperature increase: Stage 1 increases from 25℃ to 265℃ at a rate of 2℃ / min and holds for 2 hours; Stage 2 increases to 600℃ at a rate of 1℃ / min and holds for 1.5 hours; Stage 3 increases to 1100℃ at a rate of 3℃ / min and holds for 3 hours; cool the furnace to room temperature to obtain the crude carbonized product.

[0025] S4. The crude carbonization product is ball-milled at 300 rpm for 20 min, washed 4 times with 85℃ hot deionized water under reduced pressure, and the filter cake is vacuum dried at 80℃ for 12 h. The cake is then passed through a 150-mesh standard inspection sieve, and the sieved component is taken to obtain the starch-based hard carbon anode material.

[0026] Example 2 S1. Weigh 100g of cassava starch, add 900mL of deionized water to mix and prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 70℃, and stir at 200rpm for 40min to obtain starch colloid; weigh 4g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 6mL of deionized water to dissolve and prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, after the dropwise addition is complete, add 5g of guanidinoacetic acid in batches, the addition time is 15min, and the stirring is maintained at 300rpm during the addition process; after the addition is complete, stir at 40℃ for 40min to obtain composite hydrosol.

[0027] S2. The composite hydrosol was vacuum dried at 70℃ for 15h, then taken out and ball-milled at 200rpm for 30min. The mixture was then passed through a 150-mesh standard sieve, and the sieved component was taken to obtain the blended precursor powder.

[0028] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen at a flow rate of 50 mL / min, and implement a three-stage programmed temperature increase: Stage 1: increase from 20℃ to 250℃ at a rate of 1℃ / min and hold for 3 hours; Stage 2: increase to 550℃ at a rate of 0.5℃ / min and hold for 2 hours; Stage 3: increase to 1100℃ at a rate of 2℃ / min and hold for 3 hours; cool to room temperature with the furnace to obtain the crude carbonized product.

[0029] S4. The crude carbonization product is ball-milled at 200 rpm for 30 min, washed three times with 80℃ hot deionized water under reduced pressure, and the filter cake is vacuum dried at 70℃ for 15 h. The filter cake is then passed through a 100-mesh standard inspection sieve, and the sieved component is taken to obtain the starch-based hard carbon anode material.

[0030] Example 3 S1. Weigh 100g of potato starch, add 400mL of deionized water to mix and prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 85℃, and stir at 500rpm for 20min to obtain starch colloid; weigh 10.8g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 7.2mL of deionized water to dissolve and prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, and after the dropwise addition is complete, add 10g of guanidinoacetic acid in batches, with the addition time being 30min, and keep stirring at 600rpm during the addition process; after the addition is complete, stir at 50℃ for 20min to obtain composite hydrosol.

[0031] S2. The composite hydrosol was dried at 90℃ for 8 hours, then removed and ball-milled at 400rpm for 10 minutes. The mixture was then passed through a 250-mesh standard sieve, and the sieved component was collected to obtain the blended precursor powder.

[0032] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen at a flow rate of 200 mL / min, and implement a three-stage programmed temperature increase: Stage 1: increase from 30℃ to 280℃ at 3℃ / min and hold for 1 h; Stage 2: increase to 650℃ at 2℃ / min and hold for 1 h; Stage 3: increase to 1100℃ at 5℃ / min and hold for 3 h; cool to room temperature with the furnace to obtain the crude carbonized product.

[0033] S4. The crude carbonization product is ball-milled at 400 rpm for 10 min, washed 5 times with 90℃ hot deionized water under reduced pressure, the filter cake is dried under vacuum at 90℃ for 8 h, and passed through a 200-mesh standard test sieve. The component that passes through the sieve is taken to obtain the starch-based hard carbon anode material.

[0034] Example 4 S1. Weigh 100g of wheat starch, add 733mL of deionized water to prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 75℃, and stir at 300rpm for 35min to obtain starch colloid; weigh 5.4g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 6.6mL of deionized water to prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, and after the dropwise addition is complete, add 6g of guanidinoacetic acid in batches, with the addition time being 25min, and keep stirring at 400rpm during the addition process; after the addition is complete, stir at 42℃ for 35min to obtain composite hydrosol.

[0035] S2. The composite hydrosol was dried at 75°C for 14 hours, then removed and ball-milled at 250 rpm for 25 minutes. The mixture was then passed through a 180-mesh standard sieve, and the sieved component was collected to obtain the blended precursor powder.

[0036] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen at a flow rate of 80 mL / min, and implement a three-stage programmed temperature increase: Stage 1 increases from 25℃ to 260℃ at a rate of 1.5℃ / min and holds for 2.5 h; Stage 2 increases to 580℃ at a rate of 1℃ / min and holds for 2 h; Stage 3 increases to 1000℃ at a rate of 2.5℃ / min and holds for 4 h; cool to room temperature with the furnace to obtain the crude carbonized product.

[0037] S4. The crude carbonization product is ball-milled at 250 rpm for 25 min, washed 4 times with 82℃ hot deionized water under reduced pressure, the filter cake is vacuum dried at 75℃ for 14 h, and passed through a 120 mesh standard test sieve. The component that passes through the sieve is taken to obtain the starch-based hard carbon anode material.

[0038] Example 5 S1. Weigh 100g of corn starch, add 456mL of deionized water to mix and prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 82℃, and stir at 450rpm for 25min to obtain starch colloid; weigh 8.8g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 7.2mL of deionized water to dissolve and prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, and after the dropwise addition is complete, add 9g of guanidinoacetic acid in batches, with the addition time being 20min, and keep stirring at 550rpm during the addition process; after the addition is complete, stir at 48℃ for 25min to obtain composite hydrosol.

[0039] S2. The composite hydrosol was vacuum dried at 85℃ for 10h, then taken out and ball-milled at 350rpm for 15min. The mixture was then passed through a 220-mesh standard sieve, and the sieved component was taken to obtain the blended precursor powder.

[0040] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen gas at a flow rate of 150 mL / min, and implement a three-stage programmed temperature increase: Stage 1: increase from 25℃ to 275℃ at 2.5℃ / min and hold for 1.5 h; Stage 2: increase to 620℃ at 1.5℃ / min and hold for 1 h; Stage 3: increase to 1200℃ at 4℃ / min and hold for 2 h; cool to room temperature with the furnace to obtain the crude carbonized product.

[0041] S4. The crude carbonization product is ball-milled at 350 rpm for 15 min, washed three times with 88℃ hot deionized water under reduced pressure, and the filter cake is vacuum dried at 85℃ for 10 h. The cake is then passed through a 180-mesh standard inspection sieve, and the sieved component is taken to obtain the starch-based hard carbon anode material.

[0042] Example 6 S1. Weigh 50g of corn starch and 50g of tapioca starch, add 567mL of deionized water to mix and prepare starch slurry, transfer to a constant temperature water bath stirring device, heat to 80℃, and stir at 400rpm for 30min to obtain starch colloid; weigh 7.5g of 2-phosphonobutane-1,2,4-tricarboxylic acid, add 7.5mL of deionized water to dissolve and prepare an aqueous solution, first add the aqueous solution dropwise to the starch colloid, after the dropwise addition is complete, add 8g of guanidinoacetic acid in batches, the addition time is 20min, and the stirring is maintained at 500rpm during the addition process; after the addition is complete, stir at 45℃ for 30min to obtain composite hydrosol.

[0043] S2. The composite hydrosol was vacuum dried at 80℃ for 12h, then taken out and ball-milled at 300rpm for 20min. The mixture was then passed through a 200-mesh standard sieve, and the sieved component was taken to obtain the blended precursor powder.

[0044] S3. Place the blended precursor powder in a tube furnace, introduce high-purity nitrogen gas at a flow rate of 100 mL / min, and implement a three-stage programmed temperature increase: Stage 1: increase from 25℃ to 270℃ at 2℃ / min and hold for 2 hours; Stage 2: increase to 600℃ at 1.2℃ / min and hold for 1.5 hours; Stage 3: increase to 1150℃ at 3.5℃ / min and hold for 2.5 hours; cool to room temperature with the furnace to obtain the crude carbonized product.

[0045] S4. The crude carbonization product is ball-milled at 300 rpm for 20 min, washed 4 times with 85℃ hot deionized water under reduced pressure, and the filter cake is vacuum dried at 80℃ for 12 h. The cake is then passed through a 150-mesh standard inspection sieve, and the sieved component is taken to obtain the starch-based hard carbon anode material.

[0046] Comparative Example 1 The difference between this comparative example and Example 1 is that 2-phosphonobutane-1,2,4-tricarboxylic acid is not added; the rest of the preparation steps and parameters are the same as in Example 1.

[0047] Comparative Example 2 The difference between this comparative example and Example 1 is that guanidinoacetic acid is not added, while the rest of the preparation steps and parameters are the same as in Example 1.

[0048] Comparative Example 3 The difference between this comparative example and Example 1 is that 2-phosphonobutane-1,2,4-tricarboxylic acid and guanidinoacetic acid are not added; the rest of the preparation steps and parameters are the same as in Example 1.

[0049] Comparative Example 4 The difference between this comparative example and Example 2 is that 2-phosphonobutane-1,2,4-tricarboxylic acid is replaced with an equal mass of phosphoric acid, while the rest of the preparation steps and parameters are the same as in Example 2.

[0050] Comparative Example 5 The difference between this comparative example and Example 3 is that guanidinoacetic acid is replaced with an equal mass of urea, while the rest of the preparation steps and parameters are the same as in Example 3.

[0051] Comparative Example 6 The difference between this comparative example and Example 4 is that 2-phosphonobutane-1,2,4-tricarboxylic acid is replaced with an equal mass of phytic acid, while the rest of the preparation steps and parameters are the same as in Example 4.

[0052] Accurately weigh 800 mg of hard carbon anode material, 100 mg of acetylene black conductive agent, 50 mg of sodium carboxymethyl cellulose (CMC), and 125 mg of styrene-butadiene rubber latex (SBR, solid content 40%) prepared in Examples 1-6 and Comparative Examples 1-6 at room temperature, ensuring that the dry basis mass ratio of active material: conductive agent: binder is 8:1:1, and the dry basis mass ratio of CMC to SBR is 1:1.

[0053] The weighed hard carbon anode material, acetylene black, and CMC were placed in a 50 mL ball mill jar, and 1.8 mL of deionized water and 8 g of zirconium oxide milling balls were added. The ball-to-material mass ratio was 10:1. After sealing, the mixture was ball-milled at 300 rpm for 60 min. After the ball milling was completed, SBR emulsion was added, and the mixture was ball-milled at 200 rpm for another 15 min to obtain a uniform electrode slurry without obvious particles.

[0054] An automatic coating machine was used to uniformly coat the slurry onto the smooth surface of a 10μm thick electrolytic copper foil. The coating gap was set to 200μm and the coating speed was 5mm / s. After coating, the copper foil was transferred to a forced-air drying oven and dried at 80℃ for 30min to remove most of the solvent.

[0055] The dried electrode sheets were rolled using a roller press to a total thickness of 85±5μm, with the compaction density controlled at 0.85-0.95g / cm³. 3 The electrodes were punched into circular sheets with a diameter of 12 mm using a punching machine, weighed using a 0.01 g analytical balance, and screened for active substance loading of 1.0-1.2 mg / cm³. 2 The electrode sheets were selected, with the loading amount calculated as (total electrode mass - mass of blank copper foil of the same size) / electrode area. The selected electrodes were placed in a vacuum drying oven and dried at 100℃ and a vacuum of -0.09MPa for 12 hours. After drying, they were transferred to a glove box under argon protection and sealed for later use.

[0056] All assembly operations were completed in an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm. A φ14mm, 0.5mm thick sodium metal electrode and a φ16mm glass fiber diaphragm were prepared. The electrolyte was 1mol / L NaClO4 with an EC / DEC volume ratio of 1:1 and containing 5% fluoroethylene carbonate.

[0057] Assemble the battery in the following order: positive electrode shell, working electrode (carbon side up), add 40 μL of electrolyte, cover with separator, add 20 μL of electrolyte, place sodium plate, place φ15 mm, 0.5 mm thick stainless steel gasket, place spring clip, and snap on negative electrode shell; the total electrolyte volume is 60 μL, ensuring complete wetting of the separator and electrode. Place the assembled battery in a sealing machine at 800 kg / cm². 2 Pressure sealing was performed; after removal, residual electrolyte was wiped off the surface, and the samples were allowed to stand at room temperature (25±2℃) for 12 hours. Three batteries were assembled in parallel for each sample group. The following tests were conducted: The battery was tested using a battery testing system in a constant temperature environment of 25±2℃, with a voltage range of 0.01-2.5V (vsNa / Na). +The specific capacity is calculated based on a baseline of 300 mAh / g, with 0.1C corresponding to a current density of 30 mA / g, and so on. Discharge at a constant current of 0.1C to 0.01V, allow to stand for 5 minutes, then charge at a constant current to 2.5V; record the initial discharge specific capacity and the initial charge specific capacity, and calculate the initial coulombic efficiency using the following formula:

[0058] In the formula: η ICE For the first coulomb efficiency, %; C charge,1st The specific capacity for the first charge is expressed in mAh / g; C discharge,1st The first discharge specific capacity is expressed in mAh / g.

[0059] After two cycles of activation at 0.1C, the circuit was sequentially cyclicated for 5 cycles at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C, with a resting period of 2 minutes after each charge-discharge cycle. The discharge specific capacity of the 5th cycle at each rate was recorded as the corresponding rate performance data. Finally, the circuit was returned to 0.1C for 5 cycles, and the rate capacity recovery rate was calculated. After the rate test, the circuit was continuously charged and discharged for 100 cycles at a current density of 1C. The discharge specific capacity of each cycle was recorded, and the cycle capacity retention rate was calculated using the following formula:

[0060]

[0061] In the formula: R recovery This represents the capacity recovery rate, expressed in %; C recovery The discharge specific capacity after high-rate testing back to 0.1C is expressed in mAh / g; C initial The initial 0.1C discharge specific capacity is expressed in mAh / g; R capacity Cyclic capacity retention, in %; C discharge ,n is the discharge specific capacity of the nth cycle, in mAh / g (n=100 in this test); C discharge ,1 represents the discharge specific capacity in the first cycle of the long cycle, in mAh / g.

[0062] The experimental results are shown in Table 1: Table 1. Experimental results of different embodiments and comparative examples

[0063] As shown in Table 1, the starch-based hard carbon anode materials prepared in Examples 1-6 of this invention exhibit stable initial coulombic efficiencies of 87.0%-88.0% at 0.1C, reversible charge specific capacities of 346.5-374.7 mAh / g at 0.1C, reversible specific capacities maintained at 208.7-223.5 mAh / g at 5C high rates, rate capacity recovery rates all exceeding 95.7%, and capacity retention rates after 100 cycles at 1C not less than 92.8%. All electrochemical performance characteristics are significantly superior to those of Comparative Examples 1-6. This demonstrates that the phosphorus-nitrogen co-doping modification combined with a three-stage programmed temperature-increasing carbonization process of this invention can simultaneously improve the reversible sodium storage capacity, initial coulombic efficiency, rate performance, and cycle stability of starch-based hard carbon, effectively solving the common industry problems of easy structural collapse, high irreversible capacity, and poor kinetic performance of starch-based hard carbon materials.

[0064] The core mechanism of this invention's performance enhancement lies in first constructing a molecular-level cross-linking network between starch molecular chains using 2-phosphonobutane-1,2,4-tricarboxylic acid and guanidinoacetic acid. This anchors and fixes the starch molecular chains, forming stable carbonaceous skeleton nodes during the low-temperature pre-carbonization stage. This restricts melt flow during starch pyrolysis, preventing particle foaming and structural collapse, and precisely controlling the material's specific surface area within a reasonable range. This significantly reduces irreversible sodium ion consumption during the first charge-discharge process, ensuring high initial coulombic efficiency. Simultaneously, phosphorus and nitrogen atoms are uniformly embedded in the carbon lattice via covalent bonds. Phosphorus atoms expand the carbon layers through lattice distortion, increasing the interlayer spacing to a range suitable for sodium ion insertion and extraction, reducing the steric hindrance and energy barrier for sodium ion insertion and extraction. Nitrogen atoms enhance the intrinsic electronic conductivity of the carbon skeleton and introduce numerous sodium ion adsorption active sites at the edges of carbon microcrystals. The synergistic effect of these two factors simultaneously improves the material's reversible sodium storage capacity and electron and ion transport kinetics. In addition, the three-stage programmed temperature carbonization process achieves precise pore formation through stepwise pyrolysis, constructing a hierarchical pore structure with "micropores as the main component and mesopores as the auxiliary component". This provides ample storage sites for sodium ions, provides a three-dimensional channel for electrolyte wetting and rapid ion diffusion, and ensures the structural stability of the carbon skeleton, ultimately achieving excellent rate performance and cycle stability.

[0065] Comparative Example 3, without any phosphorus or nitrogen doping modification, exhibited the worst performance among all samples. Its initial coulombic efficiency at 0.1C was only 79.0%, its reversible charge specific capacity at 0.1C was only 283.4 mAh / g, its high-rate reversible specific capacity at 5C was only 142.6 mAh / g, and its capacity retention after 100 cycles at 1C was only 82.4%. This is because when unmodified starch is directly carbonized, the molecular chains lack cross-linking and anchoring during pyrolysis, making it prone to melting, foaming, and particle collapse. This results in a carbon material with an excessively large specific surface area and disordered pore structure. During the initial charge and discharge, the formation of the SEI film consumes a large amount of sodium ions, leading to extremely high irreversible capacity. Simultaneously, the carbon layers tend to stack in an ordered manner, resulting in a high degree of graphitization. The spacing between carbon layers cannot accommodate the reversible insertion and extraction of large-radius sodium ions, and the material has poor intrinsic electronic conductivity, leading to high resistance to sodium ion solid-phase diffusion. Ultimately, this results in comprehensive performance degradation.

[0066] Comparative Examples 1 and 2, with only phosphorus and nitrogen sources added respectively, showed significantly lower performance than Examples 1-6. Specifically, Comparative Example 1, without phosphorus doping, had an initial coulombic efficiency of only 83.0% at 0.1C and a high-rate reversible specific capacity of only 168.5 mAh / g at 5C. Comparative Example 2, without nitrogen doping, had an initial coulombic efficiency of only 84.0% at 0.1C and a high-rate reversible specific capacity of only 179.2 mAh / g at 5C. This is because adding a single component cannot form a complete three-dimensional cross-linked network, resulting in limited inhibition of starch pyrolysis and melting. The specific surface area of ​​the material remains high after carbonization, leading to greater irreversible consumption of the SEI film. Furthermore, single doping cannot achieve synergistic performance optimization. Single phosphorus doping cannot introduce sufficient surface active sites, resulting in insufficient pseudocapacitive contribution and ineffective improvement of the material's electronic conductivity. Single nitrogen doping cannot effectively expand the carbon layer, leading to large steric hindrance for sodium ion intercalation and deintercalation, limiting the bulk sodium storage capacity. Therefore, the performance is far lower than that of the co-doped examples.

[0067] Comparative Examples 4-6 replaced the special dopant of the present invention with conventional reagents such as phosphoric acid, urea, and phytic acid. All performance characteristics were lower than those of Examples 1-6. The initial coulombic efficiency at 0.1C was only 83.0%-85.0%, the reversible specific capacity at 5C high rate was only 173.9-191.7 mAh / g, and the capacity retention rate after 100 cycles at 1C was only 87.5%-90.2%. This is because the binding ability of conventional dopants to starch molecules is significantly reduced, and they cannot achieve the same modification effect as the dopants of this invention: Phosphoric acid is a small-molecule inorganic acid that cannot form a long-chain cross-linked structure with starch. It is easily volatilized and lost during pyrolysis, resulting in low effective phosphorus doping content and poor interlayer spacing control; Urea contains only a single amino group, which has weak binding force with starch hydroxyl groups. It has a low pyrolysis temperature, and most of the nitrogen element is decomposed and lost during carbonization, resulting in insufficient number of nitrogen doping active sites; Although phytic acid contains multiple phosphonic acid groups, its molecular chain is too rigid and cannot form a uniform cross-linked network in starch colloid. The cross-linking with starch is uneven, and the effect of improving the carbon interlayer spacing after carbonization is limited. Ultimately, all the performance is inferior to the embodiments of this invention.

[0068] In summary, this invention modifies starch by co-doping with phosphorus and nitrogen using 2-phosphonobutane-1,2,4-tricarboxylic acid and guanidinoacetic acid, combined with a three-stage programmed temperature carbonization process, thereby constructing a stable cross-linked network in the starch precursor and synergistically improving the electronic conductivity and sodium ion diffusion kinetics of the material.

[0069] In the description of this specification, the reference to terms such as "example," "various examples," etc., means that a specific feature, structure, material, or characteristic described in connection with that example or preparation is included in at least one example or preparation of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same example or preparation. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more examples or preparations.

[0070] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a starch-based hard carbon anode material, characterized in that, Includes the following steps: S1. Mix starch with water to prepare starch milk, heat and gelatinize to obtain starch colloid, then add an aqueous solution of phosphorus compound and a nitrogen compound to the starch colloid to react and obtain a composite hydrosol; S2. Perform a first post-treatment on the composite hydrosol to obtain the blended precursor powder; S3. Under an inert atmosphere, the blending precursor powder is subjected to a three-stage programmed heating treatment, and after furnace cooling, a crude carbonized product is obtained. S4. The crude carbonization product is subjected to a second post-treatment to obtain starch-based hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, The preparation process of starch colloid in step S1 specifically includes: mixing starch with deionized water to prepare starch milk with a mass fraction of 10-20%, transferring the starch milk to a constant temperature water bath stirring reaction device, heating to 70-85℃ and stirring at a constant temperature for 20-40 minutes.

3. The preparation method according to claim 1, characterized in that, In step S1, the phosphorus-containing compound is 2-phosphonobutane-1,2,4-tricarboxylic acid, and the nitrogen-containing compound is guanidinoacetic acid.

4. The preparation method according to claim 1, characterized in that, In step S1, the feeding ratio of starch, phosphorus-containing compound and nitrogen-containing compound, based on dry basis mass parts, is as follows: 100 parts starch, 10-18 parts of a phosphorus-containing compound aqueous solution with a mass fraction of 40-60%, and 5-10 parts nitrogen-containing compound.

5. The preparation method according to claim 1, characterized in that, The reaction conditions in step S1 include: Under continuous mechanical stirring conditions, the stirring rate is controlled at 300-600 rpm; The process of adding the aqueous solution of phosphorus-containing compounds and nitrogen-containing compounds is by dripping or adding in batches, and the addition time is controlled within 15-30 minutes. After adding the ingredients, continue stirring at a constant temperature of 40-50℃ for 20-40 minutes.

6. The preparation method according to claim 1, characterized in that, The first post-processing in step S2 includes: first drying, first pulverization, and first sieving; The first drying process involves a temperature of 70-90℃ and a drying time of 8-15 hours. The first crushing process is carried out using a planetary ball mill, with the ball mill speed controlled at 200-400 rpm and the ball milling time at 10-30 min; The first sieving uses a 150-250 mesh standard sieve.

7. The preparation method according to claim 1, characterized in that, The three-stage temperature ramp-up process in step S3 specifically includes: Phase 1: Increase the temperature from room temperature to 250-280℃ at a rate of 1-3℃ / min, and continue to maintain the temperature for 1-3 hours after reaching the target temperature; Phase 2: Increase the temperature from 250-280℃ to 550-650℃ at a rate of 0.5-2℃ / min, and continue to maintain the temperature for 1-2 hours after reaching the target temperature; Phase 3: Increase the temperature from 550-650℃ to 1000-1200℃ at a rate of 2-5℃ / min, and then maintain the temperature for 2-4 hours after reaching the target temperature.

8. The preparation method according to claim 1, characterized in that, The second post-processing in step S4 includes a second crushing, washing, a second drying, and a second sieving; The second grinding is carried out using a planetary ball mill, with the ball mill speed controlled at 200-400 rpm and the ball milling time at 10-30 min; The powder is washed with hot deionized water at 80-90℃ for 3-5 times under reduced pressure filtration. The second drying temperature is 70-90℃, and the drying time is 8-15 hours; The second sieving uses a 100-200 mesh standard inspection sieve.

9. A starch-based hard carbon anode material prepared by the preparation method according to any one of claims 1-8.

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