Starch-derived hard carbon negative electrode material and preparation method and application thereof

Abstract

The application discloses a starch-derived hard carbon negative electrode material and a preparation method and application thereof, and comprises the following steps: after starch aqueous solution is heated and stirred to a gelatinized state, the starch aqueous solution is mixed with glutaraldehyde aqueous solution, and the pH value of the mixed solution is adjusted to be acidic to perform a cross-linking reaction, so that a cross-linked starch suspension is obtained; the cross-linked starch suspension is pretreated to remove residual glutaraldehyde, so that a cross-linked starch precursor is obtained; and after dehydration, the cross-linked starch precursor is carbonized at high temperature under the protection of an inert atmosphere, and the starch-derived hard carbon negative electrode material is obtained. According to the application, the starch is cross-linked by glutaraldehyde, a stable three-dimensional interconnected network structure is constructed, and dehydration treatment is combined, so that the removal of hydroxyl groups in the precursor is promoted, the foaming phenomenon of the cross-linked starch precursor in the high-temperature carbonization process is inhibited, the starch precursor is more likely to form a hard carbon material with a uniform structure and rich closed pores after high-temperature carbonization, and it is more favorable to provide active sites and storage space for sodium ion storage, and the reversible specific capacity and rate performance of the material are improved.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery / capacitor anode materials, and particularly relates to a starch-derived hard carbon anode material, its preparation method, and its application. Background Technology

[0002] With the continuous growth of global energy demand, the large-scale consumption of fossil fuels has led to increasingly prominent problems such as climate change and environmental pollution, which have become important factors restricting the sustainable development of the economy and society. Electrochemical energy storage, with its advantages of high energy density, excellent conversion efficiency, and fast response time, has attracted widespread attention in large-scale energy storage and portable electronic devices. Lithium-ion batteries, due to their high energy density (150-500 Wh / kg), have been widely used in electric vehicles, consumer electronics, and other fields. However, lithium resources are relatively limited, and there are problems such as high cost and uneven geographical distribution, which to some extent restricts the further promotion and application of lithium-ion batteries in the field of large-scale energy storage. In contrast, sodium resources are abundant, widely distributed, and have low cost, making sodium-ion batteries a promising complementary technology route in the field of large-scale grid energy storage. The development of high-quality anode materials is the key to improving the performance of sodium battery systems. Among them, hard carbon materials are considered to be one of the anode materials with the greatest commercial application potential due to their advantages of high safety, low cost, and wide availability of raw materials.

[0003] Starch, as a widely available and low-cost biomass-based hard carbon precursor, has promising application prospects. However, starch molecules contain a large number of glycosidic bonds, which are prone to breakage during pyrolysis, generating volatile products such as CO2, H2, and CH4, resulting in poor thermal stability of the precursor. Starch is also prone to significant foaming during direct carbonization, which damages the starch structure, increases surface defects, and reduces carbon yield, ultimately leading to low compaction density of the resulting hard carbon material and affecting the energy density of sodium-ion batteries. To address the foaming problem of starch precursors during carbonization, Chinese invention patent application CN118754092A discloses a method for preparing corn starch-based hard carbon sodium-ion battery anode material. This method uses soluble starch as raw material, employs diammonium hydrogen phosphate, sodium hypophosphite, citric acid, and phthalic anhydride as crosslinking agents, and can be combined with zinc acetate as a pore-forming agent. The hard carbon anode material is prepared through steps such as crosslinking, freeze-drying, segmented calcination, alcohol washing, and drying. This method can suppress the foaming phenomenon during starch carbonization and maintain the spherical structure to a certain extent. However, its preparation process is relatively complex and requires multiple freeze-drying and segmented calcination treatments, resulting in a long process cycle. At the same time, the capacity retention of the obtained hard carbon material at higher current densities still has room for further improvement. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, the present invention aims to provide a starch-derived hard carbon anode material, its preparation method, and its applications. Glutaraldehyde is used as a crosslinking agent, and starch is crosslinked via a liquid-phase crosslinking method to obtain a crosslinked starch precursor. Crosslinking creates a stable three-dimensional interconnected network structure between starch molecular chains, which helps suppress the thermal decomposition and volatile release of the crosslinked starch precursor during subsequent high-temperature carbonization. Subsequently, dehydration treatment promotes the removal of some hydroxyl groups (-OH) in the precursor, forming stable C=O and C=C structures, thereby reducing the degree of foaming during subsequent high-temperature carbonization and maintaining stable particle morphology. High-temperature carbonization of the treated precursor yields a hard carbon anode material rich in closed-cell structures, thus improving the sodium storage performance of the material. The hard carbon anode material prepared by this invention can be used in sodium-ion batteries and sodium-ion capacitor systems.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a starch-derived hard carbon anode material includes the following steps: Step 1: Heat and stir the starch aqueous solution until it gelatinizes, then mix it with glutaraldehyde and adjust the pH of the mixed solution to acidic to carry out the cross-linking reaction, thus obtaining a cross-linked starch suspension; Step 2: Pre-treat the cross-linked starch suspension to remove residual glutaraldehyde and obtain the cross-linked starch precursor; Step 3: Dehydrate the cross-linked starch precursor; Step 4: The dehydrated cross-linked starch precursor is subjected to high-temperature carbonization under an inert atmosphere to obtain starch-derived hard carbon anode material.

[0006] In step 1, the starch:water:glutaraldehyde ratio is (5~10)g:50mL:(2~5)mL by mass-volume ratio; the starch is one or more of potato starch and corn starch mixed in any ratio.

[0007] In step 1, the heating and stirring process is as follows: the starch aqueous solution is heated and stirred at 50~95℃ for 40~60 minutes, and then stirred at 40~60℃ for 1~2 hours.

[0008] In step 1, the mass concentration of the glutaraldehyde aqueous solution is 25~50 wt%.

[0009] In step 1, the pH value of the mixed solution is adjusted to 2-4 to carry out the cross-linking reaction, and the cross-linking reaction time is 2-3 hours.

[0010] In step 2, the pretreatment process is as follows: solid-liquid separation, washing and filtration, and drying of the cross-linked starch suspension.

[0011] In step 3, the dehydration temperature is at least 220°C, and the dehydration time is 4~12h.

[0012] In step 4, the inert atmosphere is argon, the carbonization temperature is 1000~1600℃, the heating rate is 5~10℃ / min, and the holding time is 1~3h.

[0013] The present invention also provides a hard carbon anode material prepared by the above-mentioned method for preparing starch-derived hard carbon anode material.

[0014] The present invention also provides an application of the above-mentioned hard carbon anode material in the anode active material of sodium-ion batteries or sodium-ion capacitors.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses glutaraldehyde as a crosslinking agent. The glutaraldehyde molecule contains two active aldehyde groups, has high reactivity, and is easy to crosslink with the hydroxyl groups on the starch molecular chain, thereby forming a stable three-dimensional interconnected network structure between the molecular chains. It has the advantages of relatively mild reaction conditions, simple process and significant crosslinking effect.

[0016] 2. This invention constructs a stable three-dimensional interconnected network structure by cross-linking starch with glutaraldehyde. Combined with dehydration treatment, it promotes the removal of some hydroxyl groups (-OH) in the precursor, which can effectively inhibit the foaming phenomenon of cross-linked starch precursor during high-temperature carbonization, maintain the morphology and structural integrity of particles, improve the thermal stability of the precursor, and provide favorable conditions for the subsequent construction of uniform and controllable hard carbon microstructures.

[0017] 3. This invention, through the synergistic regulation of crosslinking and dehydration, makes it easier for starch precursors to form hard carbon materials with uniform structure, abundant closed pores, and better pore structure and interlayer spacing after high-temperature carbonization. This provides more active sites and storage space that are conducive to sodium ion storage, thereby improving the reversible specific capacity and rate performance of the material.

[0018] In summary, this invention utilizes glutaraldehyde to crosslink starch precursors, combined with dehydration treatment for synergistic regulation. This method offers advantages such as mild reaction conditions, simple process, and significant crosslinking effect, enabling the construction of a stable three-dimensional bridging network structure between starch molecular chains. It effectively suppresses foaming of the precursor during high-temperature carbonization, maintaining particle morphology and structural integrity. Furthermore, it facilitates the formation of uniformly structured, highly closed-cell hard carbon materials after high-temperature carbonization, thereby enhancing the material's reversible specific capacity, sodium storage performance, and application value. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for preparing starch-derived hard carbon anode materials.

[0020] Figure 2 These are scanning electron microscope (SEM) images of the hard carbon anode materials prepared in Examples 1-2 and Comparative Examples 1-2; wherein, Figure 2 (a) is Comparative Example 1. Figure 2 (b) is Comparative Example 2. Figure 2 (c) is Example 1. Figure 2 (d) is Example 2.

[0021] Figure 3 The graph shows a comparison of the rate performance of the hard carbon anode half-cells prepared in Examples 1-3 and Comparative Examples 1-3.

[0022] Figure 4 The chart shows a comparison of the charge-discharge curves of the hard carbon anode half-cells prepared in Examples 1-3 and Comparative Examples 1-3 at 1C and 10C rates. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the present invention is not limited to the following embodiments. It should be noted that those skilled in the art can make several modifications, equivalent substitutions or alterations to the present invention without departing from the spirit and essence of the present invention. All technical solutions falling within the protection scope of the claims of the present invention should be protected by the present invention.

[0024] like Figure 1 As shown, a method for preparing a starch-derived hard carbon anode material includes the following steps: Step 1: Preparation of cross-linked starch; Biomass starch is added to water to prepare a starch aqueous solution. The starch aqueous solution is heated and stirred at 50-95°C for 40-60 minutes to induce preliminary gelatinization of the starch. Then, stirring is continued at 40-60°C for 1-2 hours to obtain a stable and homogeneous gelatinized starch precursor system. Next, 2-5 mL of a 25-50 wt% glutaraldehyde aqueous solution is added to the system, and the pH of the solution is adjusted to 2-4. The reaction is allowed to proceed for 2-3 hours, during which the starch and glutaraldehyde undergo a cross-linking reaction, forming a stable three-dimensional interconnected network structure between the starch molecular chains. This helps to inhibit the thermal decomposition and volatile release of the precursor during subsequent high-temperature carbonization. The biomass starch can be selected from one or more of potato starch and corn starch, mixed in any ratio. The mass-to-volume ratio of starch to water in the starch aqueous solution is 5:50-10:50 g / mL.

[0025] Step 2: Washing and Separation; The cross-linked starch suspension was centrifuged to separate the solid and liquid phases, and the resulting solid product was collected. During the process, the solid product was repeatedly washed with ethanol to remove residual glutaraldehyde from the reaction system. The washed solid product was then dried to obtain the cross-linked starch precursor. The drying temperature was 40–80 °C, and the drying time was 8–12 h.

[0026] Step 3: Dehydration treatment of cross-linked starch; The cross-linked starch precursor was placed in a muffle furnace for dehydration. The dehydration temperature was at least 220°C, and the dehydration time was 8-12 hours.

[0027] Step 4: High-temperature carbonization; The cross-linked starch precursor was subjected to high-temperature carbonization in a tube furnace under an inert atmosphere to obtain a starch-derived hard carbon anode material. The inert atmosphere was argon, the carbonization temperature was 1000~1600℃, the heating rate was 5~10℃ / min, and the holding time was 1~3h.

[0028] This invention provides the application of the hard carbon anode material obtained by the above preparation method in the anode active material of sodium-ion batteries or sodium-ion capacitors. Specifically, the hard carbon material can be mixed with a conductive agent and a binder to form a slurry, and the slurry can be coated on the surface of a current collector for use in the assembly of battery devices.

[0029] Example 1 A method for preparing a starch-derived hard carbon anode material includes the following steps: Step 1: Add 10g of corn starch to 50mL of water to prepare a starch aqueous solution. Heat and stir the starch aqueous solution at 90℃ for 50min to gelatinize the starch. Then continue stirring at 50℃ for 1h. Add 3mL of 50wt% glutaraldehyde aqueous solution to the gelatinized starch system, adjust the pH of the system solution to 3, and react for 2h to allow the starch and glutaraldehyde to undergo a cross-linking reaction.

[0030] Step 2: The system after the cross-linking reaction was centrifuged to separate the solid and liquid components, and the resulting solid product was collected and washed multiple times with ethanol to remove residual glutaraldehyde in the system; then the washed solid product was placed in an 80℃ forced-air oven and dried for 8 hours to obtain the cross-linked starch precursor.

[0031] Step 3: Place the cross-linked starch precursor in a muffle furnace and dehydrate it at 220°C for 8 hours.

[0032] Step 4: Under argon protection, the dehydrated precursor is placed in a tube furnace for high-temperature carbonization at a temperature of 1400℃ and a heating rate of 5℃ / min. The carbonization is then maintained at the target temperature for 2 hours to obtain starch-derived hard carbon anode material.

[0033] Example 2 The raw materials and preparation process used in this embodiment are basically the same as those in Example 1. The difference is that in step 3, the cross-linked starch precursor is placed in a muffle furnace and dehydrated at 220°C for 12 hours.

[0034] Example 3 The raw materials and preparation process used in this embodiment are basically the same as those in Example 1. The difference is that in step 1, 5 mL of 25 wt% glutaraldehyde aqueous solution is added to the gelatinized starch system and the reaction is carried out for 3 hours.

[0035] Comparative Example 1 The raw materials and preparation process used in this comparative example are basically the same as those in Example 1, except that the dehydration treatment in step 3 was not performed.

[0036] Comparative Example 2 The raw materials and preparation process used in this comparative example are basically the same as those in Example 1. The difference is that in step 3, the cross-linked starch precursor is placed in a muffle furnace and dehydrated at 220°C for 4 hours.

[0037] Comparative Example 3 Step 1: Place corn starch in a muffle furnace and dehydrate it at 220°C for 8 hours to obtain the dehydrated precursor.

[0038] Step 2: Under argon protection, the dehydrated precursor is placed in a tube furnace for high-temperature carbonization treatment at a carbonization temperature of 1400℃ and a heating rate of 5℃ / min. The carbonization temperature is then maintained at the target temperature for 2 hours to obtain hard carbon anode material.

[0039] Preparation of the negative electrode sheet for sodium-ion batteries: The hard carbon materials obtained in Examples 1-3 and Comparative Examples 1-3 were used as active materials and mixed with a conductive agent (conductive carbon black, Carbot) and a binder (polyvinylidene fluoride, PVDF) at a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) was added as a dispersion medium, and the mixture was stirred evenly to form an electrode slurry. The electrode slurry was coated onto the surface of a carbon-coated aluminum foil current collector and pre-dried at 80°C. Then, it was transferred to a vacuum drying oven and dried at 80°C and a vacuum degree less than -0.09 MPa for 12 h. After drying, the electrode sheets were cut into round sheets with a diameter of 12 mm using a punching machine to serve as the negative electrode sheets.

[0040] Electrochemical performance testing: CR2032 half-cells were assembled in an inert atmosphere glove box. The negative electrode sheets prepared in Examples 1-3 and Comparative Examples 1-2 were used as working electrodes, metallic sodium as the counter electrode and reference electrode, a glass fiber separator was used, and the electrolyte was a 1 M NaPF6 solution prepared with diethylene glycol dimethyl ether (DIGLYME) as the solvent. After assembly, the cells were allowed to stand for 6 hours, and then constant current charge-discharge tests were performed at room temperature within a voltage window of 0.01-2 V. Rate performance tests were conducted sequentially at 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C. Figure 3 As shown in the rate performance test results, the discharge specific capacity of each sample gradually decreased with increasing current density. However, the sample from the example maintained a high capacity at all rates, significantly better than the comparative sample. The hard carbon material prepared with a dehydration time of 8 hours exhibited the best rate performance, indicating that the crosslinking and dehydration steps have a significant impact on the structural stability and electrochemical performance of the hard carbon material. Crosslinking treatment can construct a stable starch molecular network, improve the structural strength of the precursor, and suppress foaming during subsequent carbonization. Dehydration treatment can remove some bound water and small molecule volatile components from the precursor before high-temperature carbonization, achieving slow gas release, reducing structural stress and morphological collapse risk during high-temperature carbonization, thereby improving the rate performance and cycle stability of the material. The performance test results obtained at 10 C rate are shown in Table 1.

[0041] Table 1 Performance test results at 10 C ratio Example 1 yes 2h 8h 231.2 Example 2 yes 2h 12h 177.9 Example 3 yes 3h 8h 193.7 Comparative Example 1 yes 2h 0h 185.6 Comparative Example 2 yes 2h 4h 153.7 Comparative Example 3 no 0h 8h 169.4 Figure 2 Scanning electron microscope (SEM) images of the hard carbon anode materials prepared in Examples 1-2 and Comparative Examples 1-2. Figure 2As shown in (c) and (d), dehydration treatment helps maintain the original morphology and structural integrity of starch granules. This is because the dehydration treatment promotes the removal of some -OH groups in the precursor, forming more stable C=O and C=C structures, thereby enhancing the thermal stability of the precursor and reducing the rapid decomposition and volatile gas release caused by glycosidic bond breakage during subsequent pyrolysis. This, in turn, reduces the softening, swelling, and foaming phenomena of starch granules during high-temperature carbonization. When the dehydration temperature is 220℃ and the dehydration time is 8h, the degree of precursor dehydration is suitable, which can better maintain the spherical morphology of starch granules and is also conducive to the formation of a relatively uniform hard carbon skeleton during subsequent carbonization. As shown in Table 1, the electrochemical performance test results show that the hard carbon material prepared under the above dehydration conditions exhibits the best rate performance when assembled into a sodium-ion battery, especially when the dehydration time is 8h, the discharge specific capacity at a 10C rate reaches 231.1mAh / g. This is mainly because appropriate dehydration treatment, while maintaining stable particle morphology, further optimizes the pore structure and closed-pore distribution of the material, giving the resulting hard carbon material a microstructure more conducive to the rapid transport and intercalation / deintercalation of sodium ions. This reduces electrode polarization and improves sodium storage reaction kinetics, ultimately resulting in superior rate performance. It should be noted that the material prepared with a dehydration time of 0 h also exhibits a certain capacity advantage at high rates, but this phenomenon mainly stems from the foaming and expansion of the undehydrated precursor during carbonization, leading to a decrease in material bulk density. At the same coating thickness, its electrode areal loading is lower than other samples, resulting in a higher apparent capacity in high-current tests. However, this material has a low capacity at low rates and suffers from low density, severe foaming, and poor electrode processing consistency, which are unfavorable for practical large-scale applications.

[0042] Meanwhile, the hard carbon material prepared using cross-linked starch as a precursor exhibits superior rate performance compared to materials prepared using uncross-linked ordinary starch as a precursor. This is because cross-linking constructs a relatively stable three-dimensional interconnected network structure between starch molecular chains, effectively suppressing foaming and structural collapse during subsequent high-temperature carbonization. It also facilitates the formation of a uniform, highly closed-cell hard carbon material, thereby improving sodium ion transport kinetics and resulting in superior rate performance compared to materials obtained using uncross-linked ordinary starch precursors.

Claims

1. A method for preparing a starch-derived hard carbon anode material, characterized in that, Includes the following steps: Step 1: Heat and stir the starch aqueous solution until it gelatinizes, then mix it with glutaraldehyde aqueous solution and adjust the pH of the mixed solution to acidic to carry out the cross-linking reaction, thus obtaining a cross-linked starch suspension; Step 2: Pre-treat the cross-linked starch suspension to remove residual glutaraldehyde and obtain the cross-linked starch precursor; Step 3: Dehydrate the cross-linked starch precursor; Step 4: The dehydrated cross-linked starch precursor is subjected to high-temperature carbonization under an inert atmosphere to obtain starch-derived hard carbon anode material.

2. The method for preparing starch-derived hard carbon anode material according to claim 1, characterized in that, In step 1, the starch:water:glutaraldehyde aqueous solution ratio is (5~10)g:50mL:(2~5)mL by mass-volume ratio; the starch is one or more of potato starch and corn starch mixed in any ratio.

3. The method for preparing the starch-derived hard carbon anode material according to claim 1, characterized in that, In step 1, the heating and stirring process is as follows: the starch aqueous solution is heated and stirred at 50~95℃ for 40~60 minutes, and then stirred at 40~60℃ for 1~2 hours.

4. The method for preparing starch-derived hard carbon anode material according to claim 1, characterized in that, In step 1, the mass concentration of the glutaraldehyde aqueous solution is 25~50 wt%.

5. The method for preparing the starch-derived hard carbon anode material according to claim 1, characterized in that, In step 1, the pH of the mixed solution is adjusted to 2-4, and the cross-linking reaction is carried out for 2-3 hours.

6. The method for preparing starch-derived hard carbon anode material according to claim 1, characterized in that, In step 2, the pretreatment process is as follows: solid-liquid separation, washing and filtration, and drying of the cross-linked starch suspension.

7. The method for preparing the starch-derived hard carbon anode material according to claim 1, characterized in that, In step 3, the dehydration temperature is at least 220°C, and the dehydration time is 4~12h.

8. The method for preparing starch-derived hard carbon anode material according to claim 1, characterized in that, In step 4, the inert atmosphere is argon, the carbonization temperature is 1000~1600℃, the heating rate is 5~10℃ / min, and the holding time is 1~3h.

9. A hard carbon anode material prepared by the method for preparing starch-derived hard carbon anode material as described in any one of claims 1-7.

10. The application of the hard carbon anode material as described in claim 8 in the anode active material of sodium-ion batteries or sodium-ion capacitors.

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