Preparation method of phosphorus-doped and closed-pore synergistically regulated biomass hard carbon and application of phosphorus-doped and closed-pore synergistically regulated biomass hard carbon in negative electrode of sodium-ion battery

Abstract

The invention discloses a preparation method of phosphorus-doped and closed-pore synergistically regulated biomass hard carbon and application of the phosphorus-doped and closed-pore synergistically regulated biomass hard carbon in a negative electrode of a sodium-ion battery, and belongs to the technical field of electrode materials. The preparation method comprises the following steps: preparing a water-based starch suspension with a certain mass fraction by taking starch as a biomass precursor, carrying out a cross-linking reaction through sodium trimetaphosphate to form a phosphate ester bond bridged stable network structure, pre-oxidizing the cross-linked precursor, and then carbonizing in a high-temperature inert atmosphere to obtain the water-based starch composite material. And the hard carbon material with expanded interlayer spacing, phosphorus doping and high closed pore volume is prepared. The material shows high reversible capacity, high initial coulombic efficiency, excellent rate capability and cycling stability when being used as a sodium-ion battery negative electrode active material. According to the preparation method disclosed by the invention, through a simple cross-linking-carbonizing process, cooperative regulation and control of heteroatom doping and a closed-pore structure are realized, and a new strategy is provided for large-scale preparation of the high-performance biomass hard carbon negative electrode.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, and in particular relates to a method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed pores and its application in the anode of sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries (SIBs), with their significant advantages of abundant sodium resources and low manufacturing costs, and their high similarity to lithium-ion batteries in their working mechanism, have become a highly competitive and ideal technology choice for large-scale energy storage. Hard carbon materials, due to their wide availability of carbon sources, strong designability of microstructures, and excellent cycle stability, are widely recognized as the most promising anode materials for sodium-ion batteries. However, hard carbon materials still face bottlenecks in practical applications: their initial coulombic efficiency is relatively low, their reversible capacity is insufficient, and their rate performance is difficult to meet the requirements of high-power scenarios. The core issue lies in the slow transport kinetics of sodium ions in hard carbon materials, coupled with the lack of sufficient closed-pore structures within the material, which fails to provide efficient storage sites for sodium ions.

[0003] Currently, the core strategies for improving the electrochemical performance of hard carbon materials mainly focus on two directions: heteroatom doping and pore-closure engineering. Heteroatom doping (such as phosphorus, nitrogen, and sulfur) can effectively expand the carbon interlayer spacing and improve the electronic conductivity of materials. Among them, phosphorus doping has become a research hotspot in this field due to its significant regulatory effect. On the other hand, the construction of pore structures can directly increase sodium ion storage sites, especially playing an irreplaceable role in improving the low-potential plateau capacity of materials. Unfortunately, existing technical solutions often require complex multi-step processes to achieve heteroatom doping and pore structure regulation. This is not only cumbersome and costly, but also difficult to simultaneously achieve both preparation efficiency and precise structural regulation, thus restricting the large-scale application of hard carbon materials.

[0004] Therefore, developing a biomass hard carbon preparation technology that is simple to process, efficient to prepare, and can synergistically achieve precise control of phosphorus doping and closed-pore structure has become the key to breaking through current technical bottlenecks and promoting the industrialization of sodium-ion batteries. It is also an important technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for preparing biomass hard carbon through synergistic regulation of phosphorus doping and closed-pore structure, and its application in the anode of sodium-ion batteries. By crosslinking starch with sodium trimetaphosphate (STMP), phosphorus doping and closed-pore structure regulation are achieved in one step, thus solving the problems existing in the prior art. This results in a significant improvement in the first-time coulombic efficiency, reversible capacity, rate performance, and cycle stability of hard carbon materials for sodium-ion batteries, meeting commercialization requirements.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure includes the following steps: Aqueous starch suspensions were prepared using starch as a raw material. Sodium trimetaphosphate and sodium sulfate (as buffer reagents) were added to the aqueous starch suspension. The pH of the system was adjusted to 10-12 using an alkaline solution, and the cross-linking reaction was carried out by stirring. After the reaction was completed, the pH of the system was adjusted to 5-7 using an acidic solution. After washing and drying, cross-linked starch was obtained. The cross-linked starch was subjected to pre-oxidation and carbonization treatments in sequence to obtain biomass hard carbon with synergistic regulation of phosphorus doping and closed pores.

[0007] In this invention, an aqueous starch suspension is cross-linked with sodium trimetaphosphate (STPP) under alkaline conditions to obtain stable cross-linked starch with phosphate ester bonds. The obtained cross-linked starch intermediate is then pre-oxidized in air to obtain a pre-oxidized intermediate. The pre-oxidized intermediate is then carbonized at high temperature under an inert atmosphere to obtain biomass hard carbon with phosphorus doping and closed-cell synergistic regulation. In the alkaline aqueous solution system, STPP first undergoes hydrolysis to generate a highly reactive sodium tripolyphosphate (STPP) intermediate. This intermediate then acts as a bridge, with the phosphorus atoms at both ends undergoing nucleophilic substitution reactions with the hydroxyl groups from two independent starch molecules to form phosphate diester bonds. Therefore, a stable covalent cross-linked network is established between starch molecules. This structural change endows the starch with enhanced functional properties, laying the foundation for the subsequent high-temperature preparation of high-performance hard carbon.

[0008] Precise pH control is crucial throughout the preparation process. During the cross-linking reaction, excessively high pH leads to a highly alkaline environment that triggers the hydrolysis and degradation of starch molecular chains, resulting in an uneven cross-linking network and excessive side reactions. This increases structural defects and degrades the electrochemical performance of the final hard carbon material. Conversely, excessively low pH fails to effectively drive the cross-linking reaction, leading to insufficient cross-linking and preventing the precursor from forming the ideal hard carbon structure with large interlayer spacing and abundant nanopores during subsequent carbonization, ultimately resulting in extremely low sodium storage capacity. During the neutralization phase after the reaction, excessively high pH after adjustment causes the residual alkalinity to continue the cross-linking reaction, leading to batch-to-batch inconsistencies. Residual sodium ions are also introduced into the final product, becoming active centers for electrochemical side reactions, reducing coulombic efficiency and posing safety hazards. Conversely, excessively low pH after adjustment causes the acidic environment to hydrolyze and break the valuable cross-linking bonds, destroying the constructed three-dimensional network and causing the final hard carbon structure to collapse and its capacity to plummet. Therefore, only by maintaining the optimal alkaline pH during the reaction to ensure efficient and uniform crosslinking, and by precisely neutralizing to neutral after the reaction to stabilize the structure and remove impurities, can a high-performance hard carbon anode for sodium-ion batteries be successfully prepared.

[0009] Furthermore, the crosslinking reaction is carried out at a temperature of 40-60°C for 1-4 hours and at a stirring rate of 200-600 rpm.

[0010] Furthermore, the amount of sodium trimetaphosphate added is 10-30% of the mass of the starch; the amount of sodium sulfate added is 10% of the mass of the starch.

[0011] Furthermore, the specific operation steps of the pre-oxidation are as follows: under an air atmosphere, the temperature is increased to 200-300℃ at a heating rate of 1-3℃ / min, and held at this temperature for 2-10 hours.

[0012] Furthermore, the specific operation steps of the carbonization treatment are as follows: under a nitrogen atmosphere, the temperature is increased to 1100-1500℃ at a heating rate of 5-10℃ / min, and held at this temperature for 2-4 hours.

[0013] Further, the alkaline solution is a 0.5-2M NaOH solution; the acidic solution is a 1M HCl solution.

[0014] The present invention also provides a biomass hard carbon prepared by the above preparation method, which exhibits synergistic regulation of phosphorus doping and closed-pore structure.

[0015] The present invention also provides an application of the biomass hard carbon with synergistic regulation of phosphorus doping and pore closure in sodium-ion batteries.

[0016] The present invention also provides a sodium-ion battery electrode, wherein the phosphorus doping and closed-pore synergistically regulated biomass hard carbon is used as the active ingredient.

[0017] The present invention also provides a sodium-ion battery, wherein the electrode of the sodium-ion battery is a negative electrode.

[0018] Compared with the prior art, the present invention has the following advantages and technical effects: This invention crosslinks starch using STMP to form a stable network structure bridged by phosphate ester bonds. The crosslinking precursor is then pre-oxidized and subsequently carbonized under a high-temperature inert atmosphere, simultaneously achieving phosphorus doping and closed-pore structure control. Phosphorus doping significantly expands the carbon interlayer spacing, promoting sodium ion diffusion; closed pores provide abundant sodium storage sites, significantly improving the low-potential plateau capacity. The resulting hard carbon material exhibits high reversible capacity, high initial coulombic efficiency, excellent rate performance, and cycling stability, overcoming the technical bottlenecks of low initial efficiency and poor rate performance in traditional hard carbon anodes.

[0019] The raw materials for preparing biomass-based hard carbon materials in this invention have the advantages of readily available renewable resources and economic efficiency in large-scale production, providing a feasible path for the development of low-cost, high-performance energy storage devices. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 SEM image of SOHC prepared in Example 1; Figure 2 The TEM and EDS spectra of SOHC prepared in Example 1 are shown. Figure 3 The image shows the HRTEM image of the SOHC prepared in Example 1. Figure 4 True density and closed-cell volume diagrams of SOHC in Example 1 and OHC in Comparative Example 1; Figure 5 The XRD patterns are of SOHC in Example 1 and OHC in Comparative Example 1. Figure 6 The Raman spectra of SOHC in Example 1 and OHC in Comparative Example 1 are shown. Figure 7 The first charge-discharge curves of SOHC prepared in Example 1 and OHC prepared in Comparative Example 1 are shown. Figure 8 The rate performance graphs show the SOHC prepared in Example 1 and the OHC prepared in Comparative Example 1. Figure 9 The graph shows the cycling performance of SOHC prepared in Example 1 and OHC prepared in Comparative Example 1. Detailed Implementation

[0021] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0022] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0023] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0024] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0025] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0026] This invention provides a method for preparing biomass hard carbon through synergistic regulation of phosphorus doping and pore closure, comprising the following steps: 1) Raw material preparation and suspension preparation: Starch is selected as the biomass raw material and a certain mass fraction of aqueous starch suspension is prepared; 2) Crosslinking reaction: Add sodium sulfate (10% of starch mass) as a buffer to the aqueous starch suspension, and then add sodium trimetaphosphate (10-30% of starch mass, for example, 10%, 20% or 30%); adjust the pH of the system to 10-12 (for example, pH=11) using 0.5-2M (for example, 1M) NaOH solution, and stir continuously at 200-600 rpm (for example, 300 rpm) for 1-4 hours (for example, 3 hours) at a temperature of 40-60℃ (for example, 45℃). 3) Post-treatment of cross-linked products: After the reaction is completed, the pH of the system is adjusted to 5-7 (for example, pH=6) using 1M HCl solution. The product is washed with deionized water and dried to obtain cross-linked starch. 4) Pre-oxidation treatment: Place the dried cross-linked starch in an air atmosphere and heat it to 200-300℃ (exemplary, temperature is 250℃) at a heating rate of 1-3℃ / min (exemplary, heating rate is 2℃ / min), and keep it at this temperature for 2-10h (exemplary, holding time is 6h) to complete the pre-oxidation process; 5) Carbonization treatment: The pre-oxidized product is transferred to a nitrogen atmosphere and heated to 1100-1500℃ (exemplary, temperature is 1300℃) at a heating rate of 5-10℃ / min (exemplary, heating rate is 5℃ / min), and held at this temperature for 2-4h (exemplary, holding time is 2h). After cooling, the phosphorus-doped and closed-pore synergistically regulated biomass hard carbon is obtained.

[0027] This method can be used to prepare biomass hard carbon with synergistic regulation of phosphorus doping and closed pores.

[0028] This phosphorus-doped and pore-closed biomass hard carbon can be used in sodium-ion batteries, or as an active component in sodium-ion battery electrodes, serving as the negative electrode of the battery.

[0029] Unless otherwise specified, "room temperature" in this invention refers to 25±2℃.

[0030] All raw materials used in this invention were purchased from the market.

[0031] In this invention, the biomass precursor can be any biomass material such as corn starch, potato starch, cassava starch, wheat starch, and sweet potato starch. The specific implementation scheme below uses corn starch as an example for illustration, which is not intended to limit the biomass material.

[0032] The technical solution of the present invention will be further illustrated by the following embodiments.

[0033] Example 1 A method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure, comprising the following steps: 1) Add water to corn starch to obtain an aqueous corn starch suspension with a mass fraction of 30%; 2) Add sodium sulfate (10% by weight of corn starch) and sodium trimetaphosphate (10% by weight of corn starch) to the obtained aqueous corn starch suspension. Adjust the pH of the system to 11 with 1M NaOH solution. Stir the reaction for 3 hours at 45℃ water bath with a stirring rate of 300 rpm. 3) After the reaction was completed, the pH of the system was adjusted to 6 with 1M HCl solution, the product was washed with deionized water and dried in an oven at 40℃ for 16h to obtain cross-linked starch; 4) Place the dried cross-linked starch in an air atmosphere and heat it to 250°C at a heating rate of 2°C / min, and keep it at this temperature for 6 hours to complete the pre-oxidation process; 5) The pre-oxidized product was transferred to a nitrogen atmosphere and heated to 1300℃ at a heating rate of 5℃ / min. It was then carbonized at this temperature for 2 hours. After cooling, the biomass hard carbon with phosphorus doping and closed-pore synergistic regulation was obtained, denoted as 10%-SOHC.

[0034] Figure 1 This is a SEM image of the SOHC prepared in Example 1. Figure 1 It can be seen that the morphology of SOHC consists of irregular bulk and nanosheets.

[0035] Figure 2 The images show the TEM and EDS spectra of the SOHC prepared in Example 1. Figure 2 It can be seen that SOHC has successfully achieved uniform phosphorus doping, which is beneficial to expanding the carbon interlayer spacing, promoting ion transport, and enhancing its rate performance.

[0036] Figure 3 The image shows the HRTEM image of the SOHC prepared in Example 1. Figure 3 It is known that SOHC contains a large number of closed-pore structures and many edge defect sites, which can enhance the Na content within the closed pores. + Its reversible embedding / de-embedding capability.

[0037] Example 2 Same as Example 1, except that the amount of sodium trimetaphosphate added is 4% of the mass of corn starch, and the resulting phosphorus-doped and closed-pore synergistically regulated biomass hard carbon is denoted as 4%-SOHC.

[0038] Example 3 Same as Example 1, except that the amount of sodium trimetaphosphate added is 20% of the mass of corn starch, and the resulting phosphorus-doped and closed-pore synergistically regulated biomass hard carbon is denoted as 20%-SOHC.

[0039] Example 4 Same as Example 1, except that the amount of sodium trimetaphosphate added is 30% of the mass of corn starch, and the resulting phosphorus-doped and closed-pore synergistically regulated biomass hard carbon is denoted as 30%-SOHC.

[0040] Comparative Example 1 A method for preparing biomass hard carbon, comprising the following steps: 1) Place the corn starch in an air atmosphere and heat it to 250°C at a heating rate of 2°C / min, and keep it at this temperature for 6 hours to complete the pre-oxidation process; 2) The pre-oxidized product is transferred to a nitrogen atmosphere and heated to 1300℃ at a heating rate of 5℃ / min. It is then carbonized at this temperature for 2 hours. After cooling, biomass hard carbon is obtained, denoted as OHC.

[0041] Figure 4 The true density and closed-cell volume diagrams for SOHC in Example 1 and OHC in Comparative Example 1 are provided by [the relevant authority / organization]. Figure 4It can be seen that, compared to the hard carbon obtained by directly pre-oxidizing and carbonizing corn starch, the true density of hard carbon obtained by cross-linking starch is 1.2249 g / cm³. 3 It dropped to 1.1529 g / cm³ 3 The corresponding closed-pore volume is 0.3759 g / cm³. 3 It has increased to 0.425cm. 3 / g, this large closed-pore volume is beneficial for Na + Filling improves the reversible capacity at low voltage.

[0042] Figure 5 The images show the XRD patterns of SOHC in Example 1 and OHC in Comparative Example 1. Figure 5 It is known that STMP crosslinking treatment can increase the interlayer spacing (0.373 nm to 0.384 nm) of its derived hard carbon, which is beneficial to Na + Insertion and de-embedding improve rate performance.

[0043] Figure 6 The Raman spectra of SOHC in Example 1 and OHC in Comparative Example 1 are obtained from... Figure 6 It can be seen that the hard carbon obtained by STMP crosslinking treatment has I d / I g The decrease from 1.146 to 1.012 indicates that it can reduce the defect concentration of its derived hard carbon, which is beneficial to reducing irreversible sodium loss and improving the first coulombic efficiency.

[0044] comprehensive Figures 4-6 It can be seen that appropriate STMP crosslinking treatment optimizes the closed-cell structure, interlayer spacing, surface functional group and defect concentration of hard carbon, thereby improving sodium storage performance.

[0045] Application Example 1 The electrochemical performance of SOHC prepared in Example 1 and OHC prepared in Comparative Example 1 was tested in a half-cell. The specific method was as follows: S1. Dissolve 80mg of SOHC or OHC, 10mg of super P and 10mg of sodium alginate in deionized water and stir continuously to form a uniform electrode slurry. Using a four-sided coating tool with a thickness of 150um, evenly coat the obtained electrode slurry onto the aluminum foil current collector. After drying, press it on a tablet press to obtain a sodium-ion battery hard carbon negative electrode sheet with a diameter of 11mm.

[0046] S2. The obtained sodium-ion battery hard carbon negative electrode sheet was used as the working electrode for the preparation of CR2032 coin cell. A commercially available metallic sodium sheet was used as the counter electrode. A commercially available sodium-ion electrolyte (1M NaPF6in DIGLYME=100Vol%) was used. The separator was a GF / D glass fiber membrane. The cells were assembled in an argon-protected glove box.

[0047] Figure 7 The first-cycle charge-discharge curves are shown for the SOHC prepared in Example 1 and the OHC prepared in Comparative Example 1. Figure 7 It can be seen that at a current density of 0.1C (1C=300mA / g), compared with the capacity of OHC of 287.4mAh / g and the initial coulombic efficiency of 80.02%, SOHC has an initial reversible specific capacity of 341.3mAh / g and a high initial coulombic efficiency of 86.89%, which is much higher than that of hard carbon obtained by direct pre-oxidation carbonization of corn starch. It also has both high initial coulombic efficiency and high reversible specific capacity. This comprehensive performance is better than most of the biomass hard carbon anodes reported in the current technology.

[0048] Figure 8 The graph shows the rate performance of SOHC prepared in Example 1 and OHC prepared in Comparative Example 1. Figure 8 It is known that SOHC also has excellent rate performance, with a specific capacity of 177.3 mAh / g at a high current density of 10C (10C=3000mA / g), which is higher than that of OHC (44.2 mAh / g).

[0049] Figure 9 The graphs show the cycling performance of SOHC prepared in Example 1 and OHC prepared in Comparative Example 1. Figure 9 It can be seen that SOHC also has excellent cycling stability. At 1C, after 600 cycles, the capacity retention rate is still above 78.6%, which is much higher than that of OHC (58.9%).

[0050] The materials prepared in Examples 1-4 and Comparative Example 1 were used as negative electrode materials for electrochemical performance testing, and the results are shown in Table 1.

[0051] Table 1 Performance test results of different anode materials As can be seen from Table 1, the biomass hard carbon material prepared by the present invention with phosphorus doping and closed-pore synergistic regulation can achieve high reversible specific capacity, high initial coulombic efficiency, excellent rate performance and cycle stability when used as the anode of sodium-ion batteries.

[0052] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing biomass hard carbon through synergistic regulation of phosphorus doping and closed-pore structure, characterized in that, Includes the following steps: Aqueous starch suspensions were prepared using starch as a raw material. Sodium trimetaphosphate and sodium sulfate were added to the aqueous starch suspension, and the pH of the system was adjusted to 10-12 using an alkaline solution. The mixture was stirred to carry out the cross-linking reaction. After the reaction was completed, the pH of the system was adjusted to 5-7 using an acidic solution. After washing and drying, cross-linked starch was obtained. The cross-linked starch was subjected to pre-oxidation and carbonization treatments in sequence to obtain biomass hard carbon with synergistic regulation of phosphorus doping and closed pores.

2. The method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure according to claim 1, characterized in that, The crosslinking reaction is carried out at a temperature of 40-60℃ for 1-4 hours and at a stirring rate of 200-600 rpm.

3. The method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure according to claim 1, characterized in that, The amount of sodium trimetaphosphate added is 10-30% of the mass of the starch; The amount of sodium sulfate added is 10% of the mass of the starch.

4. The method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure according to claim 1, characterized in that, The specific steps of the pre-oxidation are as follows: under an air atmosphere, the temperature is increased to 200-300℃ at a heating rate of 1-3℃ / min, and held at this temperature for 2-10 hours.

5. The method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure according to claim 1, characterized in that, The specific steps of the carbonization process are as follows: under a nitrogen atmosphere, the temperature is increased to 1100-1500℃ at a heating rate of 5-10℃ / min, and held at this temperature for 2-4 hours.

6. The method for preparing biomass hard carbon by synergistic regulation of phosphorus doping and closed-pore structure according to claim 1, characterized in that, The alkaline solution is a 1M sodium hydroxide solution; the acidic solution is a 1M hydrochloric acid solution.

7. A biomass hard carbon with synergistic regulation of phosphorus doping and closed pores, prepared by the preparation method according to any one of claims 1-6.

8. The application of biomass hard carbon with phosphorus doping and closed-pore synergistic regulation as described in claim 7 in sodium-ion batteries.

9. A sodium-ion battery electrode, characterized in that, The active ingredient is biomass hard carbon with phosphorus doping and closed-pore synergistic regulation as described in claim 7.

10. A sodium-ion battery, characterized in that, The sodium-ion battery electrode as described in claim 9 is used as the negative electrode.

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