Preparation method and application of additive induced coordination compound derived Bi nitrogen-doped carbon composite material

Abstract

The invention relates to the field of sodium ion batteries, and discloses a preparation method and application of an additive induced coordination compound derived Bi nitrogen-doped carbon composite material. The preparation method comprises the following steps: chelating an organic ligand with Bi ions in bismuth salt through a mechanochemical method to obtain a precursor Bi-HMT; subsequently, mixing and grinding the precursor and an additive to obtain an intermediate product, and performing high-temperature pyrolysis and carbon thermal reduction on the intermediate product to obtain a final product Bi nitrogen-doped carbon composite material. When the material is used as a sodium ion battery negative electrode, a nitrogen-doped carbon skeleton generated based on in-situ conversion of a coordination compound and an additive can be tightly combined with Bi particles, agglomeration among the Bi particles is avoided, volume change caused by intercalation and deintercalation of sodium ions is effectively relieved, heteroatom nitrogen is introduced through an organic ligand to modify the carbon skeleton, and the specific surface area of the material is increased. The chemical affinity of the material is improved, the overall conductivity of the composite material is improved, and the cycling stability and the rate capability of the material are enhanced.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, and more particularly to a method for preparing Bi nitrogen-doped carbon composite materials derived from additive-induced coordination compounds and their applications. Background Technology

[0002] In recent years, sodium-ion batteries have been regarded as an ideal alternative to lithium-ion batteries due to their lower cost, higher safety, and abundant resource reserves, and have become a research hotspot. However, their low energy density and poor first-coulombic efficiency limit their practical commercial application. Therefore, developing high-performance sodium-ion battery materials to meet the needs of large-scale applications is a key problem that urgently needs to be solved.

[0003] Anode materials are a crucial component of sodium-ion batteries, directly impacting their overall performance. Hard carbon has become the mainstream choice due to its structural stability and excellent electrochemical performance. However, the high manufacturing cost and limited sodium storage capacity of hard carbon severely restrict the commercial application of sodium-ion batteries. Bismuth (Bi), with its multi-step alloying reaction, has a high theoretical specific capacity (386 mAh g⁻¹). -1 Bi metal exhibits a competitive advantage in the field of sodium-ion battery anode materials due to its high volume change (up to 251%) during cycling. This volume change leads to electrode fragmentation, causing active material to detach from the current collector, interrupting electron transport and accelerating battery capacity decay. Furthermore, the large volume change causes repeated damage and repair of the solid electrolyte interphase (SEI), resulting in irreversible electrolyte consumption and high charge transfer resistance. These problems lead to a continuous deterioration in the capacity of Bi electrode materials, making their cycle stability and rate performance significantly lower than practical application requirements, ultimately hindering their commercial application.

[0004] To address the aforementioned issues, various strategies have been adopted, including designing micro / nanostructures, metal composites, and carbon coating, to further improve the performance of Bi electrodes. For example, patent CN 117766731 A discloses a carbon-coated Bi nanocomposite material and its preparation method, and introduces its application in the anode of high-rate lithium-ion / sodium-ion batteries. This patent proposes to prepare Bi-C / G nanomaterials with porous structures by reacting Bi particles with graphene through a CO-Bi interface, thereby achieving excellent capacity retention and stable cycling performance at high current densities. First, polyvinylpyrrolidone, bismuth nitrate, and 2-methylimidazole are added to an ethylene glycol solution in which graphene is uniformly dispersed, and after freeze-drying, a bismuth metal organic framework material Bi-MOF / G is obtained; subsequently, the precursor is calcined and held at a reducing atmosphere to finally obtain a Bi-C / G composite material with a uniform structure. However, this technical solution has a relatively complex process and lengthy steps, which may lead to high energy consumption and reduce the controllability and reproducibility of product preparation.

[0005] Patent CN 118237592 A discloses a method for preparing and applying a fast-charging porous Bi-Sn binary alloy material. This patent addresses the problems of severe volume expansion and poor structural stability associated with single-metal Bi by combining Bi and Sn to construct an alloy and introducing a porous structure, thereby improving its specific capacity. The process involves mixing bismuth oxide, stannous oxide, and magnesium powder to obtain a mixed precursor; then placing the mixed precursor in an alloy reaction vessel and calcining it under a protective atmosphere to obtain an intermediate product; finally, the intermediate product is acid-washed in tartaric acid and dried at low temperature to obtain a porous Bi-Sn binary alloy anode material. However, this process involves steps such as alloy reaction vessel calcination, acid washing, and freeze-drying, resulting in high production costs and safety hazards, making large-scale industrial production difficult.

[0006] Patent CN 118231611 A discloses an alloy-type anode material for sodium-ion batteries and its preparation method. The patent first dissolves bismuth nitrate pentahydrate and polyvinylpyrrolidone in a mixed solution of ethylene glycol and nitric acid to obtain Bi nanoparticles via a hydrothermal method. Subsequently, the Bi nanoparticles, hexadecyltrimethylammonium bromide, and tetraethyl orthosilicate are dissolved in ethanol and deionized water, and after isothermal reaction and centrifugation washing, Bi@mSiO2 is obtained. Finally, Bi@mSiO2 and polyvinylpyrrolidone are dissolved in deionized water, and after freeze-drying, carbonization, and washing, a Bi@mC composite material is obtained. While this method can prepare Bi-based composite materials with good performance, its complex preparation process and the use of large amounts of organic solvents may pose risks to the environment and human health.

[0007] In summary, although the above methods can alleviate the volume expansion of Bi-based materials and improve their structural stability to some extent, the problem of Bi particle agglomeration and growth during pyrolysis remains unresolved. The prepared materials cannot simultaneously possess excellent electrochemical performance and simple process feasibility, exhibiting significant limitations. Furthermore, the hydrothermal synthesis and the use of various organic solvents significantly increase the energy consumption, time, and cost of the preparation process, and impose additional environmental burdens. Therefore, this method has poor universality and is difficult to apply to the commercial large-scale preparation of battery electrode materials. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for preparing Bi nitrogen-doped carbon composite materials derived from additive-induced coordination compounds and their applications. This method utilizes a carbon framework generated through in-situ conversion of additives and organic ligands to restrict the growth of Bi particles, effectively controlling the size of Bi particles during pyrolysis and ensuring their tight embedding on the carbon framework surface, thereby mitigating volume changes during charge and discharge. Simultaneously, the in-situ generated carbon framework exhibits better bonding with Bi particles compared to externally introduced carbon materials. Introducing nitrogen heteroatoms into the carbon framework further enhances the material's chemical affinity and conductivity. As a sodium-ion battery anode material, this composite material exhibits high specific capacity, excellent cycle stability, and good rate performance. Furthermore, the preparation method of this invention has advantages such as simple operation, short reaction time, and low organic solvent consumption.

[0009] The specific technical solution of this invention is as follows:

[0010] First, this invention provides a method for preparing Bi nitrogen-doped carbon composite materials derived from additive-induced coordination compounds, comprising the following steps:

[0011] (1) Bismuth salt and organic ligand hexamethylenetetramine were placed in a ball mill jar and the precursor Bi-HMT was obtained by mechanical chemical reaction and washing.

[0012] (2) Grind and mix the precursor Bi-HMT with the additive to obtain the intermediate product.

[0013] (3) The intermediate product is pyrolyzed under an inert protective atmosphere to obtain Bi nitrogen-doped carbon composite material derived from additive-induced coordination compound.

[0014] The additive-induced coordination compound-derived Bi nitrogen-doped carbon composite material prepared by the method of the present invention has the following advantages: (1) The additive restricts the precursor Bi-HMT and can prevent Bi particles from agglomerating during high-temperature calcination; (2) Compared with carbon introduced by other means, the nitrogen-doped carbon generated in situ during the calcination of the additive and coordination compound is more tightly bound to the Bi particles, which can effectively slow down the volume change caused by sodium ion insertion / extraction and enhance the electrochemical stability of the material; (3) The incorporation of nitrogen element in the carbon skeleton can improve the chemical affinity of the material, provide more active sites, and improve the conductivity of the composite material; (4) When the composite material is used as the negative electrode of sodium ion battery, its composite structure can effectively promote electrolyte wetting and rapid migration of sodium ions, and has excellent cycle stability and rate performance.

[0015] Furthermore, this invention employs a mechanochemical method to prepare the precursor Bi-HMT, and obtains Bi nitrogen-doped carbon composite materials through additive grinding and high-temperature calcination. As a precursor synthesis method, the mechanochemical method avoids the drawbacks of traditional hydrothermal, high-temperature solid-state, and vapor deposition methods, reducing solvent consumption, energy waste, and excessively long reaction times. This method features a simple preparation process, low reaction temperature, environmental friendliness, and high yield, making it suitable for large-scale industrial production.

[0016] Preferably, in step (2), the additive is polyethylene glycol, citric acid or ascorbic acid, with polyethylene glycol being the most preferred.

[0017] This invention reveals that the type of additive has different effects on the precursor, thus affecting the structure of the pyrolysis product; therefore, the selection of additives is crucial. Polyethylene glycol (PEG), when ground and mixed with the precursor, effectively adheres to the precursor surface, ensuring complete coating and dispersing large precursor particles into fine powder. This prevents material agglomeration and particle growth during pyrolysis, maintaining the nanoscale size of Bi particles. Citric acid, as a weak reducing agent, promotes the reduction of Bi particles during carbonization after uniform mixing with the precursor, allowing grain growth to dominate and preventing particle agglomeration into micrometer-sized particles. However, because citric acid cannot adequately protect the material during subsequent carbonization, its effect is not as good as PEG. Ascorbic acid, as a strong reducing agent, significantly accelerates the reduction process of Bi particles and promotes their formation; however, its strong reducing effect causes Bi metal particles to be directly reduced into large spherical particles, resulting in the worst effect.

[0018] Preferably, in the Bi nitrogen-doped carbon composite material, Bi particles are confined and distributed within the nitrogen-doped carbon framework, with a particle size of 30–50 nm.

[0019] In this invention, the ideal particle size range of the final product Bi particles is between 30 nm and 50 nm. If the size of the Bi particles is too large (especially at the micrometer level), severe volume expansion will occur during cycling, leading to a decrease in the cycling stability of the material and a sharp shortening of the cycle life. Conversely, if the size of the Bi particles is too small, the particles are prone to agglomeration due to electrostatic adsorption, resulting in an increase in side reactions and a decrease in the first-cycle coulombic efficiency.

[0020] Preferably, in step (2), the mass ratio of the precursor Bi-HMT to the additive is (0.5~1.5):(1.5~3).

[0021] This invention reveals that the ratio of the precursor Bi-HMT to the additive significantly affects the morphology of the final product. Specifically, if the additive content is too low, it cannot adequately coat the precursor surface, resulting in insufficient reduction and protection of Bi particles during pyrolysis, leading to abnormal particle agglomeration and growth. If the additive content is excessive, it will completely coat the precursor and cause uneven additive distribution, disrupting the uniformity of the Bi particle structure during subsequent pyrolysis.

[0022] Preferably, in step (1), the molar ratio of the bismuth salt to hexamethylenetetramine is (1~2):(0.5~4).

[0023] Preferably, in step (1), the bismuth salt is bismuth chloride, bismuth nitrate, or bismuth sulfate; most preferably, it is bismuth nitrate.

[0024] This invention discovers that the type of bismuth salt affects the properties of the final material during ball milling. During ball milling, anions assist in coordination to form precursors, which decompose during pyrolysis, forming carbon skeletons with different structures. Ultimately, through comparison, this invention found that nitrate ions significantly improve material performance, chloride ions have a relatively smaller impact on overall material performance, while sulfate ions have the least effect.

[0025] Preferably, in step (1), the mechanochemical reaction (ball milling) of the present invention includes two schemes:

[0026] Option A: One-step ball milling method is adopted: the total mass ratio of the bismuth salt and the organic ligand hexamethylenetetramine to the mass of the ball milling beads is 1:(10~40); the mechanochemical reaction time is 1~8 h, and the rotation speed is 300~600 rpm.

[0027] In the ball milling process of Scheme A above: First, the material is crushed into particles or powder under the strong mechanical force of the ball mill. Under the high temperature generated by the ball mill, the organic ligand molecule HMT absorbs energy and causes the molecular bond to break. The broken molecular bond then coordinates and chelates with Bi metal ions to form Bi-HMT complex.

[0028] Option B: Two-step ball milling method, including the following stages:

[0029] The first stage mainly involves the crushing of bismuth salt and the organic ligand hexamethylenetetramine; the total mass ratio of bismuth salt and organic ligand hexamethylenetetramine to the mass of the milling beads is 1:(10~20), the milling time is 2~3 hours (forward rotation for 20~40 minutes, pause for 10~20 minutes, reverse rotation for 20~40 minutes, and so on), and the rotation speed is 200~400 rpm;

[0030] The second stage involves further refining the crushed bismuth salt and hexamethylenetetramine to promote the reaction. The total mass ratio of bismuth salt and the organic ligand hexamethylenetetramine to the ball milling beads is 1:(15~30). The ball milling time is 3~5 h (50~70 min forward rotation, 3~7 min pause, 50~70 min reverse rotation, and so on), and the rotation speed is 400~600 rpm. The precursor Bi-HMT is finally obtained.

[0031] In the ball milling process of Scheme B above, the material is first broken into smaller particles by mechanical force at medium to high speed (200~400 rpm) in the first stage. Meanwhile, a longer pause time (10-20 min) can appropriately lower the temperature of the material in the ball mill jar, preventing side reactions or incomplete reactions. When the material size is broken down to approximately the same size, the ball-to-material ratio is appropriately increased in the second stage, and high-speed ball milling (400-600 rpm) is performed again to further refine and mix the broken particles evenly. During the ball milling process, the reaction mainly occurs on the surface of the ball milling beads and the contact surface of the ball mill jar. When the ball milling beads collide with each other, the contact surface generates high temperature. The organic ligand molecule HMT absorbs energy, causing the valence bond to break. The broken valence bond then coordinates and chelates with Bi metal ions to form Bi-HMT complexes. In the second stage, appropriately increasing the ball-to-material ratio (1:(15-30)) and the ball milling time (3-5 h) and shortening the pause time (3-7 min) can better maintain the high temperature state, making it easier for the materials to react and achieve uniform synthesis between the reactants. Compared with Scheme A, the precursor Bi-HMT particles obtained by Scheme B are smaller in size and do not exhibit agglomeration, which is more conducive to obtaining uniform nanoparticles in the subsequent carbonization process.

[0032] Preferably, in step (1), the detergent used for washing is anhydrous ethanol, ethylene glycol, isopropanol or deionized water.

[0033] Preferably, in step (3), the pyrolysis temperature is 400~700 ℃, the heating rate is 1~8 ℃ / min, and the holding time is 1~5 h.

[0034] During pyrolysis, the aforementioned pyrolysis process parameters significantly affect the final Bi particles. For example, if the pyrolysis temperature is too low, the reaction rate will decrease, the reaction time will increase, and the material may not be completely pyrolyzed within the specified time, resulting in a mixture of reactants and intermediates. Conversely, if the pyrolysis temperature is too high, the surface of the material may be over-carbonized, leading to overburning and deformation, as well as excessive energy consumption. If the holding time is too long, the material particles will recrystallize and grow, resulting in a significant increase in particle size. If the holding time is too short, the carbonization process will be insufficient, leading to inadequate product purity. If the heating rate is too fast, the temperature difference between the inside and outside of the Bi-HMT precursor will be too large, resulting in pyrolysis of the outer part but not the inner part, which will damage the material's structure and morphology, leading to low product uniformity. If the heating rate is too slow, the reaction time at the same temperature needs to be extended, affecting overall production efficiency. Ultimately, this invention controls the pyrolysis process parameters within the aforementioned range, achieving the best overall effect.

[0035] Preferably, in step (3), the inert protective atmosphere is argon or nitrogen.

[0036] Secondly, this invention also provides the application of the additive-induced coordination compound-derived confined Bi nitrogen-doped carbon composite material obtained by the above preparation method as a negative electrode material for sodium-ion batteries.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] (1) The present invention obtains Bi-HMT precursors through a relatively simple mechanical ball milling and washing method, which effectively avoids the problems of large energy and long time required for the synthesis of materials by traditional hydrothermal method, high temperature solid phase method, vapor phase deposition method, etc. The preparation process is simple, the reaction temperature is low, and the yield is high, which can be used for large-scale industrial production.

[0039] (2) In this invention, the additive is ground and mixed with the precursor Bi-HMT to promote the dispersion of Bi-HMT and avoid the agglomeration of the precursor material during pyrolysis. At the same time, the additive can act as a reducing agent during pyrolysis, so that the nucleation rate of Bi particles exceeds the grain growth rate, thereby realizing the generation of nanoscale Bi particles.

[0040] (3) Compared with carbon introduced through other means, the carbon generated by the in-situ conversion of coordination compounds and additives in this invention can bind tightly to Bi particles. This carbon framework acts as a confinement mechanism, effectively inhibiting the aggregation and growth of Bi particles during electrochemical cycling; on the other hand, it can buffer the volume changes caused by sodium ion insertion / extraction. The two work together to avoid material breakage and decomposition, significantly enhancing the cycling stability of the material.

[0041] (4) The method of the present invention introduces heteroatom nitrogen to dope carbon materials, which improves the chemical affinity of the materials and provides more active sites, thereby improving the overall conductivity of the composite material.

[0042] (5) This invention improves the conductivity of materials by generating conductive carbon substrates through in-situ conversion, alleviates volume changes during charge and discharge, and reduces the agglomeration of active materials while nano-sizing the materials. When used as the negative electrode material for sodium-ion batteries, this composite material exhibits high actual capacity, good cycle stability, and good rate performance. Among the two ball milling processes provided, Scheme B, compared to Scheme A, promotes the uniform formation of the precursor by changing the ball milling parameters and performing secondary ball milling. At the same time, through additive grinding and subsequent pyrolysis, the obtained Bi nitrogen-doped carbon composite material has a better morphology and smaller particle size.

[0043] (6) The Bi nitrogen-doped carbon composite material prepared by the method of the present invention exhibits good electrochemical performance when used as a negative electrode for sodium-ion batteries over a wide potential range of 0.01-1.5 V, and the actual capacity can be maintained at 371 mAh g. -1 The rate capability returns to the initial capacity after cycling. Bi nitrogen-doped carbon composite materials prepared by high-energy ball milling yield high-purity products with simple preparation and good stability, making them suitable for large-scale use in the commercial fabrication of electrode materials for sodium-ion batteries. Attached Figure Description

[0044] Figure 1 The X-ray powder diffraction pattern of the precursor Bi-HMT prepared in Example 1 of this invention;

[0045] Figure 2 This is a scanning electron microscope image of the precursor Bi-HMT prepared in Example 1 of the present invention;

[0046] Figure 3 This is a scanning electron microscope image of the final product prepared in Example 1 of the present invention;

[0047] Figure 4 This is a scanning electron microscope image of the final product prepared in Comparative Example 2 of the present invention;

[0048] Figure 5 This is a scanning electron microscope image of the final product prepared in Comparative Example 3 of the present invention;

[0049] Figure 6 The X-ray powder diffraction pattern of the final product prepared in Example 1 of this invention;

[0050] Figure 7 The final product prepared in Example 1 of this invention is 0.5 A g. -1 Under normal cycling performance;

[0051] Figure 8 The rate performance of the final product prepared in Example 1 of this invention at different current densities is shown. Detailed Implementation

[0052] The present invention will be further described in detail below through embodiments, but the present invention is not limited to the following examples.

[0053] Example 1

[0054] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0055] (2) Add 30 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0056] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0057] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with ethylene glycol and anhydrous ethanol to obtain the precursor Bi-HMT;

[0058] (5) The precursor Bi-HMT obtained in step (4) is mixed and ground with polyethylene glycol at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0059] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0060] Example 2

[0061] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0062] (2) Add 40 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0063] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0064] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with isopropanol and deionized water to obtain the precursor Bi-HMT;

[0065] (5) The precursor Bi-HMT obtained in step (4) is mixed and ground with polyethylene glycol at a mass ratio of 1:1 to obtain a uniform intermediate product;

[0066] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0067] Example 3 (The difference from Example 1 is that citric acid is used as an additive)

[0068] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0069] (2) Add 30 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0070] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0071] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with ethylene glycol and anhydrous ethanol to obtain the precursor Bi-HMT;

[0072] (5) The precursor obtained in step (4) is mixed and ground with citric acid at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0073] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0074] Example 4 (The difference from Example 2 is that bismuth chloride is used as the bismuth salt)

[0075] (1) Add 1 mmol of bismuth chloride and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0076] (2) Add 40 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0077] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 8 hours at a speed of 300 rpm. After the ball milling is completed, obtain the mixed product.

[0078] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with isopropanol and deionized water to obtain the precursor Bi-HMT;

[0079] (5) The precursor obtained in step (4) is mixed and ground with citric acid at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0080] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 600 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0081] Example 5

[0082] (1) Add 1 mmol of bismuth chloride and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0083] (2) Add 40 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0084] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 8 hours at a speed of 300 rpm. After the ball milling is completed, obtain the mixed product.

[0085] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with isopropanol and deionized water to obtain the precursor Bi-HMT.

[0086] (5) The precursor obtained in step (4) is mixed and ground with citric acid at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0087] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 700 °C for 3 hours at a heating rate of 5 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0088] Example 6

[0089] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0090] (2) Add 20 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0091] (3) Place the ball milling jar described in (2) into the planetary ball mill and assemble it properly. Perform ball milling for 2 hours (30 min forward rotation, 15 min rest, 30 min reverse rotation, and repeat this process). The rotation speed during ball milling is 200 rpm.

[0092] (4) After the ball milling in step (3) is completed, add 10 g of agate ball milling beads with a diameter of 10 mm into the ball milling jar, and then perform ball milling for 4 hours (60 min forward, 5 min rest, 60 min reverse, and so on) at a speed of 500 rpm.

[0093] (5) Take out the mixed product obtained in step (4) and wash and centrifuge it multiple times with ethylene glycol and anhydrous ethanol to obtain the precursor Bi-HMT.

[0094] (6) The precursor obtained in step (5) is mixed and ground with polyethylene glycol at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0095] (7) The intermediate product obtained in step (6) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0096] Comparative Example 1 (the difference from Example 2 is that bismuth sulfate is used as the bismuth salt)

[0097] (1) Add 1 mmol of bismuth sulfate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0098] (2) Add 40 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0099] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0100] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with isopropanol and deionized water to obtain the precursor Bi-HMT.

[0101] (5) The precursor obtained in step (4) is mixed and ground with polyethylene glycol at a mass ratio of 1:1 to obtain a uniform intermediate product;

[0102] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0103] Comparative Example 2 (the difference from Example 1 is that ascorbic acid was used as an additive)

[0104] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0105] (2) Add 30 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0106] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0107] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with ethylene glycol and anhydrous ethanol to obtain the precursor Bi-HMT.

[0108] (5) The precursor obtained in step (4) is mixed and ground with ascorbic acid at a mass ratio of 1:2 to obtain a uniform intermediate product;

[0109] (6) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0110] Comparative Example 3 (the difference from Example 1 is that no additives were used)

[0111] (1) Add 1 mmol of bismuth nitrate and 3 mmol of hexamethylenetetramine to the ball mill jar;

[0112] (2) Add 30 g of agate grinding beads with a diameter of 10 mm into the grinding jar;

[0113] (3) Place the ball milling jar described in (2) into a planetary ball mill and assemble it properly. Perform ball milling for 6 hours at a speed of 400 rpm. After the ball milling is completed, obtain the mixed product.

[0114] (4) Take out the mixed product obtained in step (3) and wash and centrifuge it multiple times with ethylene glycol and anhydrous ethanol to obtain the precursor Bi-HMT.

[0115] (5) The intermediate product obtained in step (5) is pyrolyzed under an argon atmosphere and heated to 500 °C for 2 h at a heating rate of 2 °C / min to obtain Bi nitrogen-doped carbon composite material.

[0116] Performance testing

[0117] The final products obtained from each embodiment and comparative example were mixed with a conductive agent (Super P) and a binder (PVDF) in a mass ratio of 7:2:1. First, the product and conductive agent were mixed in a fixed ratio, and after multiple grindings and uniform mixing, a fixed proportion of PVDF was added, along with an appropriate amount of solvent N-methylpyrrolidone (NMP) to form a homogeneous slurry. This slurry was then smoothly coated onto a current collector (copper foil), dried at 80 °C for 12 hours, and then cut into 12 mm electrode sheets using a cutting machine. A glass fiber membrane was used as the separator, a sodium metal sheet as the counter electrode, and 1.0 mol·L⁻¹... -1 A half-cell was assembled using NaPF6 as the solute and diethylene glycol dimethyl ether as the solvent. Assembly was performed in an argon-filled glove box, proceeding from bottom to top in the following order: positive electrode shell, prepared electrode, separator, electrolyte, sodium metal sheet, nickel foam, and negative electrode shell. The assembled sodium-ion half-cell was allowed to stand for 24 hours before electrochemical testing was conducted under constant temperature conditions.

[0118] Figure 1 The XRD pattern of the precursor Bi-HMT obtained after ball milling and washing in Example 1 shows that the precursor has good crystallinity.

[0119] Figure 2 The image shows the SEM image of the Bi-HMT precursor obtained after ball milling and washing in Example 1. It can be seen that the precursor exhibits a sea urchin-like morphology, and its size is between 200 and 300 nm.

[0120] Figure 3 The image shows a SEM image of the material after pyrolysis treatment in Example 1. Polyethylene glycol was uniformly distributed on the surface of the Bi-HMT precursor by grinding, and the aggregation between Bi metal particles was effectively restricted during the pyrolysis process. In the image, Bi spherical particles can be seen uniformly distributed on the surface of the carbon skeleton, with a size between 30 and 50 nm.

[0121] Figure 4 The image shows the SEM image of the material after pyrolysis treatment in Comparative Example 2. Without any additives, the Bi-HMT precursor particles are not constrained during pyrolysis, resulting in uneven Bi particle size and wide particle size distribution. In the image, Bi particles with obvious size differences can be seen accumulating on the surface of the carbon skeleton, with sizes ranging from 50 nm to 1 μm.

[0122] Figure 5The image shows the SEM image of the material after pyrolysis treatment in Comparative Example 3. Although the reduction process was enhanced by uniformly mixing ascorbic acid and Bi-HMT precursor through grinding, the excessive reducing power of ascorbic acid caused partial reduction of Bi-HMT before pyrolysis, resulting in significant agglomeration and growth of Bi metal particles. As can be seen in the image, there are isolated large Bi metal particles with a size between 5 and 10 μm.

[0123] Figure 6 The image shows the XRD pattern of the pyrolysis product of Example 1. The results in the image indicate that the final product is a cubic Bi (PDF#85-1329), with a not very obvious broad diffraction peak appearing near 25°. This peak belongs to amorphous carbon, which proves the successful preparation of Bi nitrogen-doped carbon composite material.

[0124] Figure 7 Example 1 was performed at a current density of 0.5 A g. -1 The cycle performance graph shows that after 250 cycles at this current density, the battery capacity remains at 331 mAh g. -1 This indicates that the material has good cycle stability.

[0125] Figure 8 The graph shows the rate performance of Example 1 at different current densities, at 0.2 A g. -1 0.5 A g -1 1 A g -1 2 A g -1 5 A g -1 10A g -1 20 A g -1 30 A g -1 The specific capacity at the current density is 371 mAh g. -1 360mAh g -1 349 mAh g -1 338 mAh g -1 321 mAh g -1 302 mAh g -1 287 mAh g -1 and 252 mAh g -1 The material has a high capacity, and it can still return to the corresponding capacity when switching from a high current to a low current, indicating that the material has high stability and excellent rate performance.

[0126] Table 1 shows the capacity and capacity retention of the batteries in Examples 1-6 and Comparative Examples 1-3 at the 250th cycle.

[0127] Table 1

[0128]

[0129] Note: Current density is 0.5 A g -1 (Voltage window is 0.01~1.5 V) Test

[0130] Table 1 summarizes the results of each embodiment and comparative example at 0.5 μg. -1 The capacity and capacity retention of the half-cell at 250 cycles under the given current density. Where:

[0131] (1) The difference between Examples 1 and 3, and Comparative Examples 2 and 3 is the type of additives used (Comparative Example 3 did not use any additives): Compared with Example 3, Comparative Examples 2 and 3 had significantly higher capacity and capacity retention at 250 cycles. This is because the type of additive has different effects on the protection of the material. Polyethylene glycol (Example 1) can better disperse and adhere to the surface of the precursor, ensuring that the precursor is completely coated, so that smaller Bi metal particles are formed in the subsequent carbonization process. Citric acid (Example 3) is a weak reducing agent and cannot completely protect the material, so the subsequent carbonization process cannot fully protect the material. Ascorbic acid (Comparative Example 2) is a strong reducing agent. When mixed with the precursor, the strong reducing effect causes the Bi metal particles to be directly reduced to form large spherical particles. Without any additives (Comparative Example 3), the in-situ carbon derived from ligands can only limit the growth and aggregation of Bi metal particles to a certain extent. At this time, the size range of Bi metal particles obtained by pyrolysis is large, and uniformity is difficult to guarantee.

[0132] (2) Comparing Example 6 with Example 1, it can be seen that different ball milling processes will have a certain impact on the performance of the product. In Example 6 using scheme B, the agglomeration between precursor materials is reduced by two-step ball milling and adjusting the interval time when ball milling. After grinding and carbonization with additives, the Bi particles of the final sample are smaller and more uniformly dispersed.

[0133] (3) Comparing Examples 2 and 4 with Comparative Example 1, it can be found that the type of bismuth salt affects the performance of the final material during the ball milling process. During the ball milling process, anions assist in coordination to form precursors and decompose during pyrolysis to form carbon skeletons with different structures. Nitrate (Example 2) can better improve the performance of the material, while chloride (Example 4) and sulfate (Comparative Example 1) have a smaller effect on improving the overall performance of the material.

[0134] (4) Comparing Example 4 and Example 5, it can be found that the pyrolysis conditions will affect the final structure of the material. If the heating rate is too slow and the temperature is too high, the grain growth process of the material will exceed the grain nucleation process, resulting in the material agglomeration and growth. If the heating rate is too fast, the internal and external temperatures of the material will be too high, the pyrolysis process will be incomplete, and the ligands and additives will not be completely converted into carbon skeletons.

Claims

1. A method for preparing a Bi nitrogen-doped carbon composite material derived from an additive-induced coordination compound, characterized in that... include: (1) Mix bismuth salt and hexamethylenetetramine and ball mill to produce a mechanochemical reaction, wash, and obtain the precursor Bi-HMT, wherein the bismuth salt is bismuth chloride, bismuth nitrate or bismuth sulfate; (2) The precursor Bi-HMT and the additive are mixed by grinding to obtain an intermediate product; the molar ratio of the precursor Bi-HMT to the additive is (0.5~1.5):(1.5~3); the additive is polyethylene glycol, citric acid or ascorbic acid; (3) The intermediate product is pyrolyzed under an inert protective atmosphere to obtain a Bi nitrogen-doped carbon composite material; in the Bi nitrogen-doped carbon composite material, Bi particles are confined and distributed on the surface of the nitrogen-doped carbon skeleton, and the particle size of the Bi particles is 30~50 nm.

2. The preparation method according to claim 1, characterized in that: In step (1), the bismuth salt is bismuth nitrate.

3. The preparation method according to claim 1 or 2, characterized in that: In step (1), the molar ratio of the bismuth salt to hexamethylenetetramine is (1~2):(0.5~4).

4. The preparation method according to claim 1 or 2, characterized in that: In step (1), The total mass ratio of the bismuth salt and hexamethylenetetramine to the mass of the milling beads is 1:(10~40). The diameter of the grinding beads is 8~12 mm; The mechanochemical reaction takes 1 to 8 hours and rotates at 300 to 600 rpm.

5. The preparation method according to claim 1 or 2, characterized in that: In step (1), the mechanochemical reaction includes the following stages: First stage: The total mass ratio of bismuth salt and hexamethylenetetramine to the mass of the milling beads is 1:(10~20). Rotate clockwise for 20~40 minutes, pause for 10~20 minutes, and rotate counterclockwise for 20~40 minutes. Repeat this cycle for 2~3 hours and 200~400 rpm. Second stage: The total mass ratio of bismuth salt and hexamethylenetetramine to the mass of the milling beads is 1:(15~30). Rotate forward for 50~70 minutes, pause for 3~7 minutes, and then rotate backward for 50~70 minutes. Repeat this cycle for 3~5 hours and 400~600 rpm.

6. The preparation method according to claim 1, characterized in that: In step (1), the detergent used for washing is anhydrous ethanol, ethylene glycol, isopropanol or deionized water.

7. The preparation method according to claim 1, characterized in that: In step (2), the additive is polyethylene glycol.

8. The preparation method according to claim 1, characterized in that: In step (3), the inert protective atmosphere is argon or nitrogen.

9. The preparation method according to claim 1, characterized in that: In step (3), the pyrolysis temperature is 400~700 ℃, the heating rate is 1~8 ℃ / min, and the holding time is 1~5 h.

10. The application of the additive-induced coordination compound-derived Bi nitrogen-doped carbon composite material obtained by the preparation method according to any one of claims 1 to 9 as a negative electrode material for sodium-ion batteries.

Images