Two-dimensional multi-level micro-nano bismuth carbon composite material as well as preparation method and application thereof

Abstract

The invention provides a two-dimensional multi-level micro-nano bismuth carbon composite material and a preparation method and application thereof.The material is of a multi-layer stacked micron sheet structure, and the stacked micron sheet structure is of a secondary multi-sheet-layer structure composed of a plurality of primary sheet layer units which are assembled with one another and have interlayer intervals; the primary lamellar unit comprises a carbon shell and bismuth nanoparticles wrapped by a carbon wall, and the bismuth nanoparticles wrapped by the carbon wall are embedded in the carbon shell to form dual-carbon constraining force on the bismuth nanoparticles. The preparation method comprises the following steps: preparing a two-dimensional multi-level bismuth oxybromide precursor; and converting the two-dimensional multi-level bismuth oxybromide precursor into the two-dimensional multi-level micro-nano bismuth carbon composite material by using a carbon thermal reduction method. The material is simple and safe in preparation operation and short in period. The two-dimensional multi-level structure enables the bismuth nano-particles to be densely embedded, high tap density and bismuth content are endowed, high sodium storage capacity is ensured when the two-dimensional multi-level structure is used for the sodium ion battery, and the rate capability is improved; the large interlayer spacing of the multi-layer carbon plate and the double-carbon barrier relieve the volume change, reduce the side reaction and improve the cycling stability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of sodium ion battery electrode materials, and particularly relates to a two-dimensional multi-level micro-nano bismuth-carbon composite material, a preparation method and application thereof. BACKGROUND

[0002] Currently, the key problem restricting the improvement of the energy density of sodium ion batteries mainly focuses on the negative electrode side of the battery. The traditional carbon-based negative electrode materials, such as graphite and hard carbon, have relatively limited sodium storage capacity, and the sodium storage mechanism has not been fully understood and still needs to be further studied. In contrast, the nitrogen group elements (such as phosphorus, antimony, bismuth, etc.) that store sodium in the form of alloying have high theoretical mass specific capacity and volume specific capacity, and if they can be effectively utilized, they are expected to significantly improve the energy density of sodium ion batteries. Among these elements, metallic bismuth has become one of the most promising negative electrode material candidates due to its good air stability, non-toxic and harmless characteristics, abundant raw material sources, and high specific capacity (mass specific capacity of 385 mAh / g and volume specific capacity of 3773 mAh / cm 3 ).

[0003] However, metallic bismuth as a sodium storage negative electrode material still faces many challenges in practical applications. During the charging and discharging cycle, metallic bismuth will undergo a huge volume change (about 252%), which is extremely easy to cause the collapse of the structure of the electrode material and thus lose activity. At the same time, the excessive expansion of the electrode material will expose a large amount of surface, accelerate the consumption of the electrolyte and trigger side reactions, and also increase the internal resistance of the electrode, which has a serious impact on the sodium storage capacity, cycle life and rate performance of the battery. In addition, the corrosion problem of the electrode material cannot be ignored. During the battery cycle, the direct contact of the bimetallic electrode with the acidic molecules in the electrolyte will cause the negative electrode to dissolve, especially at a large current density, the irreversible loss of the electrode material will further reduce the performance of the battery.

[0004] To solve the above problems, researchers have tried to construct bismuth nanomaterials through nanotechnology and coat a carbon layer on the surface of bismuth nanoparticles. This method can to some extent alleviate the volume expansion problem of bismuth and prolong the cycle life of the battery. However, existing technologies usually achieve this purpose by reducing the particle size and increasing the carbon content, which often leads to a decrease in sodium storage capacity. This optimization strategy at the expense of volume capacity is difficult to meet the needs of the commercial application of bismuth-based sodium storage negative electrode materials. In addition, the carbon coating method also affects the density of the material. For example, coating bismuth particles with substances such as polydopamine and tannic acid usually results in primary particles with a carbon-coated surface. Such particles have a large specific surface area and low tap density, which increases the ion migration path between particles and reduces the ion transport rate, which is not entirely beneficial to the cycle life, sodium storage performance and rate performance of the material. Therefore, selecting a reasonable carbon composite method is crucial to improving the performance of the material.

[0005] In related research, various preparation methods of bismuth-carbon composite materials have been reported. For example, patent CN114918426A discloses a bulk bismuth-carbon composite and its preparation method and application. The method uses bismuth citrate and carbon-containing substances as bismuth and carbon sources, and uses ammonia and other complexes, and prepares bulk bismuth-carbon composite through a two-step method of dissolution-carbonization. However, the product prepared by this method has uneven morphology, and bismuth nanoparticles are exposed on the surface of the bulk, which cannot be effectively protected by the carbon shell. After 2000 cycles at a current density of 5A / g, there is a significant capacity decay. In addition, the ammonia used in the preparation process is corrosive and volatile, and there is an explosion risk at high temperatures.

[0006] Patent CN114122407A discloses a preparation method and application of a bismuth nanoparticle composite material with double carbon layer protection. The method includes four steps of preparing a reaction solution, hydrothermal reaction, product dopamine coating, and heating carbonization. Although an organic ligand and dopamine coating are used to form a double carbon coating layer, the preparation steps are complicated. Moreover, the prepared bismuth-carbon composite still has the problem of bismuth nanoparticle exposure, and it is difficult to effectively control the distribution of bismuth nanoparticles. At the same time, the use of organic ligand and dopamine increases the carbon content while reducing the mass fraction of bismuth, resulting in an effective sodium storage capacity of the composite of only about 350 mAh / g, and a cycle number of less than 200 cycles.

[0007] The article "Facile Galvanic Replacement Construction of Bi@C Nanosheets Array as Binder-Free Anodes for Superior Sodium-Ion Batteries" reports a method for preparing vertically grown bismuth / carbon nanosheet arrays on a copper foam substrate through galvanic replacement, in-situ polymerization of dopamine, and annealing procedures. The specific process is as follows: after galvanic replacement of the reactants on a copper foil substrate for 48 h, washing and drying, then coating with dopamine to obtain a carbon layer precursor, and finally carbonizing at 500°C for 2 h in an Ar atmosphere. This unique carbon-coated nanosheet array structure can adapt to volume changes during the cycling process, maintain electrode stability, and promote the rapid transfer of electrons / ions. However, this method uses a copper foil as a substrate, which has limitations in synthesis. Moreover, the use of dopamine coating and carbonization does not effectively confine Bi nanoparticles within the carbon sheet layer, and the exposed Bi particles will affect the battery performance in subsequent cycles. The material has a reversible capacity of only 315.7 mAh / g after 1500 cycles at a current density of 1 A / g. Although two-dimensional materials have unique performance advantages, the synthesis of bismuth-based materials is mostly limited to nanowire (rod), spherical, and amorphous block structures, and the preparation of two-dimensional sheet materials still faces many challenges. In addition, the carbon coating method has a crucial impact on the morphology, Bi content, and overall performance of the material. Although the Bi / C composite strategy has been proven to improve the rate performance and cycle stability of the material, existing specific implementations have many problems in product control, such as difficulty in accurately controlling the morphology of the composite, the size of the Bi particles, the relative content of Bi / C, and the complex and unclear combination of Bi / C. Under high rate and long cycle (especially super-long cycle of tens of thousands of cycles) conditions, simple Bi / C composite products are difficult to provide stable sodium storage performance.

[0008] In summary, there is an urgent need to develop an effective structure design and a reasonable preparation method to alleviate the volume expansion problem of bismuth during sodium insertion and extraction, stabilize the electrode structure, fully utilize the high theoretical capacity of metallic bismuth, and obtain a metallic bismuth anode material with high tolerance (long cycle life), ultra-fast charging characteristics (high rate), and high energy density (high specific capacity), achieving all-around optimization of material performance and promoting commercialization. SUMMARY

[0009] To solve the problems mentioned in the background art, the present application provides a two-dimensional multi-level micro-nano bismuth / carbon composite material, a preparation method thereof, and applications thereof.

[0010] In a first aspect, the present application provides a two-dimensional multilayer micro-nano Bi-C composite material, which has a multilayer stacked microparticle structure composed of a secondary multi-layer structure of a plurality of primary layer units assembled with an interlayer spacing. The primary layer unit includes a carbon shell and Bi nanoparticles wrapped by a carbon wall, and the Bi nanoparticles wrapped by the carbon wall are embedded in the carbon shell, forming a double carbon constraint force on the Bi nanoparticles. Due to the unique multilayer structure and double carbon constraint force design, the Bi nanoparticles are effectively protected, the agglomeration of the Bi nanoparticles is limited, and the stability of the material is enhanced. The double carbon structure also improves the electrical conductivity of the material and provides a good channel for ion transmission, which can improve the electrochemical performance in energy storage applications.

[0011] In some specific embodiments, the material is a multilayer longitudinally ordered stacked square microparticle structure. The multilayer longitudinally ordered stacked square microparticle structure is beneficial for the directional transmission of electrons and ions, improves the electrical conductivity and ion migration rate of the material, and thus improves the charge and discharge performance of the material in the field of batteries.

[0012] In some specific embodiments, the average particle size of the Bi nanoparticles is in the range of 10 nm to 25 nm, and the mass fraction is 90.5% to 95%. The appropriate particle size of the Bi nanoparticles and the high content ensure that the material has a high mass specific capacity. Smaller particle size increases the contact area of the Bi nanoparticles with the electrolyte, improving the reaction activity; high content directly improves the overall power storage capacity of the material, and the material exhibits excellent performance in energy storage.

[0013] In some specific embodiments, the average thickness of the material is in the range of 280 nm to 320 nm, and the edge length is in the range of 1.5 μm to 2 μm. The appropriate thickness and edge length are beneficial for the penetration of the electrolyte and the diffusion of the ions, shorten the ion transmission path, and improve the charge and discharge efficiency and rate performance of the material.

[0014] In some specific embodiments, the stacked microparticle structure is assembled by 10 to 15 primary layer units with an interlayer spacing having an interlayer interaction force, and the thickness of the primary layer unit is about 20 nm to 32 nm. The interlayer spacing and the interaction force work together to ensure the relative sliding property between the layers, alleviate the volume change of the material during the charge and discharge process, maintain the stability of the structure, ensure the smooth transmission of ions between the layers, and improve the cycle stability and service life of the material.

[0015] In some specific embodiments, the tap density of the material is 2.80 g / cm 3 -3.16 g / cm 3 , and the specific surface area is 30 m 2 / g to 50 m 2The higher tap density allows the material to accommodate more active substances in a limited volume, thereby improving the energy density; the moderate specific surface area ensures sufficient contact between the material and the electrolyte while avoiding excessive consumption of the electrolyte due to an excessively large specific surface area, thereby improving the charge-discharge efficiency and cycle stability of the material and enhancing the comprehensive performance of the material in practical applications.

[0016] The bismuth nanoparticles of the two-dimensional multilayer micro-nano bismuth-carbon composite material are uniformly embedded in the primary sheet unit, and are not distributed on the surface of the primary sheet unit, so that the metal particles are completely protected. In addition, the nitrogen family elements other than phosphorus (P) generally exhibit stronger interlayer interaction forces than van der Waals forces, effectively avoiding the irreversible stacking and aggregation of sheet layers due to the weak van der Waals forces between the layers of the two-dimensional material. Unlike ordinary two-dimensional materials, the moderate specific surface area of the multilayer microsheet structure prevents excessive consumption of the electrolyte. At the same time, the inherent strong mechanical structure, enhanced electrical conductivity, high ion transport capacity, and high-density active sites of the two-dimensional material make it have unique advantages in electrochemical stability and activity that ordinary materials do not have, and have more potential for commercial application.

[0017] In a second aspect, the present application provides a preparation method of a two-dimensional multilayer micro-nano bismuth-carbon composite material, comprising:

[0018] S1, preparing a two-dimensional multilayer bismuth-oxygen-bromine precursor;

[0019] S2, converting the two-dimensional multilayer bismuth-oxygen-bromine precursor into a two-dimensional multilayer micro-nano bismuth-carbon composite material by using a carbon thermal reduction method.

[0020] In some specific embodiments, the step S1 comprises the following:

[0021] S11, uniformly mixing ethylene glycol and a mass fraction of 4%-6% nitric acid solution to obtain a mixed solution one;

[0022] S12, adding sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate to the mixed solution one in the order of (1-1.5):(1-1.5):(4-6):(4.8-7.2) by mass ratio, adjusting the pH value to 1.2-1.5, stirring at room temperature for 20-50 min, to obtain a mixed solution two;

[0023] S13, placing the mixed solution two in a sealed polytetrafluoroethylene reaction kettle, and performing hydrothermal reaction at 120-150 DEG C and keeping for 8-12 h, and then naturally cooling to room temperature, and centrifugally washing the obtained reaction product with water and anhydrous ethanol, and drying to obtain a two-dimensional multilayer bismuth-oxygen-bromine precursor.

[0024] In some specific embodiments, the step S2 comprises the following:

[0025] S21, place the two-dimensional multilevel bismuth oxybromide precursor in a quartz tube furnace, heat to 400-600 DEG C at a heating rate of 2-5 DEG C / min under a protective atmosphere, and keep for 1-2 h, wherein the protective atmosphere is one or a mixture of nitrogen, argon or hydrogen-argon;

[0026] S22, naturally cool to room temperature to obtain a two-dimensional multilevel micro-nano bismuth-carbon composite material.

[0027] In the above technical solution, the macromolecular carbon-containing organic matter polyvinylpyrrolidone and glucose are selected as carbon-coated raw materials, the high-viscosity molecular chains and oxygen-containing functional groups of the two are combined with Bi 3 + ordered hinge entanglement, and a tight wrapping and three-dimensional interconnected double-carbon layered structure is formed by subsequent carbothermal reduction, which enhances the tap density of the material and improves the electrical conductivity. The method breaks through the limitations of low material density and long ion migration path caused by conventional carbon coating only for primary particle coating, the raw materials used are cheap and easy to obtain, the preparation method is simple and short, the obtained material is uniform and dense without by-products, the raw material utilization rate is high, and the method has the conditions for large-scale production.

[0028] In a third aspect, the application provides an application of a two-dimensional multilevel micro-nano bismuth-carbon composite material, which is the application of the two-dimensional multilevel micro-nano bismuth-carbon composite material described in the first aspect or the two-dimensional multilevel micro-nano bismuth-carbon composite material prepared by the method described in the second aspect as a negative electrode of a sodium ion battery.

[0029] Compared with the prior art, the beneficial results of the application are:

[0030] (1) The application starts from the reaction mechanism at the atomic scale, uses carbothermal reduction method to accurately remove bromine and oxygen atoms in the multilevel bismuth oxybromide precursor, and then obtains a multilevel micro-nano bismuth-carbon composite material. Compared with traditional nanomaterial preparation methods, the preparation process discards the complex and tedious process and has innovation. The raw materials used are easy to obtain in the market and have low cost, which not only reduces the production cost, but also has significant advantages in process amplification and feasibility for large-scale industrial production.

[0031] (2), the internal structure of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared by the application is unique, and bismuth nanoparticles are densely embedded therein. This microstructure layout has multiple advantages: on the one hand, it gives the material a higher tap density and bismuth content, which is of great significance to improve the energy storage density of the material; on the other hand, it forms a short-range ion path, greatly promoting the ion transmission efficiency. In addition, the carbon-bismuth interface structure on the surface of the bismuth nanoparticles can significantly enhance the electrical conductivity of the material, while effectively inhibiting the agglomeration of nanoparticles, thereby significantly improving the cycle stability of the material during the charging and discharging process.

[0032] (3), the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared by the application is a multi-level double-carbon constrained layer with obvious interlayer spacing. This structure plays a key role when the material is applied in the energy storage field: on the one hand, it serves as a high-speed channel for sodium ion migration, significantly improving the ion migration rate and thus improving the charging and discharging performance of the material; on the other hand, during the charging and discharging process of the material, it can effectively buffer the volume change caused by ion intercalation and deintercalation, and at the same time protect the internal metal bismuth particles from corrosion by the electrolyte, thereby effectively enhancing the cycle resistance of the material and prolonging the service life of the material. BRIEF DESCRIPTION OF DRAWINGS

[0033] The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and, together with the description, serve to explain the principles of the application. Other embodiments and many of the intended advantages of the embodiments will be readily appreciated as the same becomes better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

[0034] Figure 1 is a flow chart of a preparation method of a two-dimensional multi-level micro-nano bismuth-carbon composite material according to an embodiment of the application;

[0035] Figure 2 is a conceptual diagram of a two-dimensional multi-level micro-nano bismuth-carbon composite material based on carbon thermal reduction method at atomic level and a structure diagram of the composite material according to an embodiment of the application;

[0036] Figure 3 is a scanning electron microscope (SEM) image and an X-ray diffraction (XRD) spectrum of a two-dimensional multi-level bismuth-oxygen bromide precursor prepared according to embodiment 1 of the application;

[0037] Figure 4 a, 4b is a scanning electron microscope (SEM) image of a two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to embodiment 1 of the application;

[0038] Figure 4 c is a transmission electron microscope (TEM) image of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0039] Figure 4 d is a diffraction pattern of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0040] Figure 4 e is a high-resolution image of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0041] Figure 4 f is an element mapping pattern of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0042] Figure 4 g-4i are spherical aberration electron microscope images of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0043] Figure 5 is a transmission electron microscope (TEM) image and particle size statistical diagram of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0044] Figure 6 is an X-ray diffraction (XRD) pattern and thermogravimetric analysis (TGA) diagram of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0045] Figure 7 is an X-ray photoelectron spectroscopy (XPS) full spectrum and XPS-C1s, XPS-N1s, XPS-O1s, XPS-Bi4f spectrum of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0046] Figure 8 is a tap density test calculation diagram of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0047] Figure 9 is a nitrogen isothermal adsorption-desorption curve (BET) and initial coulombic efficiency statistical line diagram of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application;

[0048] Figure 10 is a rate performance diagram of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to Embodiment 1 of the present application as a negative material of sodium ion battery;

[0049] Figure 11is a cycle performance chart of the two-dimensional multi-level micro-nano bismuth carbon composite material prepared according to the embodiment 1 of the present application as a sodium ion battery negative electrode material at a current density of 5 A / g;

[0050] Figure 12 a-12d is a scanning electron microscope (SEM) image and particle size distribution histogram of the spherical bismuth carbon composite particles prepared according to the comparative preparation of the present application. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments will be described clearly and completely below with reference to the drawings in the present application, and similar components are denoted by similar reference numerals in the drawings. Obviously, the following described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without any creative effort fall within the protection scope of the present application.

[0052] It should be understood that, when used in the specification and the appended claims, the terms “comprise” and “include” indicate the presence of described features, integers, steps, operations, elements, and / or components, but do not exclude one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0053] It should also be understood that the terms used in the present application specification are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the present application specification and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms, unless the context clearly indicates otherwise.

[0054] The present application embodiment proposes a preparation method flow chart of a two-dimensional multi-level micro-nano bismuth carbon composite material, as shown in Figure 1 The method comprises,

[0055] S100, preparing a two-dimensional multi-level bismuth oxybromide precursor.

[0056] S200, converting the two-dimensional multi-level bismuth oxybromide precursor into a two-dimensional multi-level micro-nano bismuth carbon composite material by using a carbon thermal reduction method.

[0057] In some specific embodiments, the step S100 comprises the following:

[0058] S101, uniformly mixing ethylene glycol and a mass fraction of 4%-6% nitric acid solution to obtain a mixed solution one;

[0059] S102, according to (1-1.5):(1-1.5):(4-6):(4.8-7.2) mass ratio, sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate are sequentially added to the mixed solution, the pH value is adjusted to 1.2-1.5, stirring at room temperature for 20-50 min, to obtain a second mixed solution;

[0060] S103, the second mixed solution is placed in a sealed polytetrafluoroethylene reactor, and a hydrothermal reaction is carried out at 120-150°C for 8-12 h, and then naturally cooled to room temperature, and the obtained reaction product is washed by centrifugation with water and anhydrous ethanol, and dried to obtain a two-dimensional multi-level bromine-oxygen bismuth precursor.

[0061] Specifically, sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate with a mass ratio of (1-1.5):(1-1.5):(4-6):(4.8-7.2) are dissolved in ethylene glycol to obtain a mixed solution, and the pH value of the solution is adjusted to 1.2-1.5 by a 4-6% nitric acid solution; the mixed solution after pH adjustment is placed in an environment of 25°C, and treated by continuous magnetic stirring for 30 min; then transferred to a sealed 100ml polytetrafluoroethylene reactor for hydrothermal reaction. After the reaction is completed, it is naturally cooled to room temperature; after the reactor is cooled, the reaction product is taken out, washed by centrifugation with water and anhydrous ethanol in sequence, and then dried to obtain a two-dimensional multi-level bromine-oxygen bismuth precursor.

[0062] Preferably, the hydrothermal reaction conditions are: a temperature range of 120-150°C, and a holding time of 8-12 h.

[0063] In some specific embodiments, the S200 step includes the following:

[0064] S201, the two-dimensional multi-level bromine-oxygen bismuth precursor is placed in a quartz tube furnace, heated to 400-600°C at a heating rate of 2-5°C / min under a protective atmosphere, and held for 1-2 h, wherein the protective atmosphere is one or a mixture of nitrogen, argon or hydrogen-argon;

[0065] S202, naturally cooled to room temperature to obtain a two-dimensional multi-level micro-nano bismuth-carbon composite material.

[0066] Specifically, the two-dimensional multi-level bromine-oxygen bismuth precursor obtained in S100 is placed in a quartz tube furnace, heated to 400-600°C at a heating rate of 2-5°C / min under one or a mixture of nitrogen, argon or hydrogen-argon, and held for 1-2 h, and then naturally cooled to room temperature to obtain a two-dimensional multi-level micro-nano bismuth-carbon composite material.

[0067] This invention also proposes a two-dimensional multi-level micro / nano bismuth-carbon composite material, with reference to... Figure 2 , Figure 2 A schematic diagram of a two-dimensional multi-level micro / nano bismuth-carbon composite material based on a carbothermal reduction method at the atomic level, and a schematic diagram of the composite material structure, are shown below, according to an embodiment of the present invention. Figure 2 As shown in the upper part, the two-dimensional multi-layered micro / nano bismuth-carbon composite material exhibits a multi-layered stacked micron-sheet structure. This stacked micron-sheet structure is a secondary multi-layered structure composed of several primary layered units assembled with interlayer spacing. Each primary layered unit includes a carbon shell and bismuth nanoparticles encased in carbon walls. The bismuth nanoparticles encased in carbon walls are embedded within the carbon shell, forming a dual-carbon constraint on the bismuth nanoparticles. Within each primary layered unit, the bismuth nanoparticles are encased in carbon walls, forming a core-shell-like structure, with the entire core-shell structure located within the outer carbon shell. Figure 2 The lower half shows the crystal structure changes of the two-dimensional multi-layered bismuth oxybromine precursor during the thermal carbon reduction process. During carbothermic reduction, the bismuth oxybromine precursor and the carbon-containing organic matter coated on its unit layers undergo the following reaction processes: 2BiOBr→Bi2O3+Br2↑, the carbon-containing organic matter decomposes into carbon, Bi2O3+3C→2Bi+3CO↑ (small amount), Bi2O3+1.5C→2Bi+1.5CO2↑ (dominant). After the complete reaction, the chemical bonds of bromine and oxygen atoms originally connected to Bi in the precursor are broken and escape as gas, while Bi retains its original atomic arrangement and forms a new two-dimensional multi-layered micro-nano bismuth carbon composite material with the remaining carbon layers and carbon shell.

[0068] In some specific embodiments, the two-dimensional multi-layered micro / nano bismuth-carbon composite material exhibits a multi-layered, longitudinally ordered, stacked square microsheet structure. This stacked square microsheet structure is assembled from 10-15 primary lamellar units via interlayer spacing with interlayer interactions. The average thickness is in the range of 280 nm-320 nm, the side length is 1.5 μm-2 μm, and the tap density is 2.80 g / cm³. 3 -3.16g / cm 3 Specific surface area is 30m² 2 / g-50m 2 / g. Wherein, the thickness of the primary sheet unit is about 20nm-32nm, the average particle size of the bismuth nanoparticles is in the range of 10nm-25nm, and the mass fraction is 90.5%-95%. The disordered stacking presents a spherical shape similar to a sea urchin, and the surface primary sheet unit is distributed randomly and oriented randomly. In contrast, the ordered stacking has the following advantages: first, it can construct specific horizontal or vertical channels for sodium ion migration; second, it can reduce the transmission of sodium ions between the unit sheet interfaces, reduce the interface impedance, and accelerate the transmission rate; third, it can vectorize the swelling stress and release it along the vertical direction, reducing the sodium resistance. The disordered stacking cannot have the above advantages, and its sodium ion transmission path is disordered and longer, requiring to cross multiple interfaces, with significantly increased impedance; the swelling process is disordered, which can easily cause stress concentration, hinder the sodium reaction, and even cause particle rupture, ultimately resulting in poor material performance.

[0069] The two-dimensional multi-level micro-nano bismuth carbon composite material of the present application has a bismuth nanoparticle and a multi-level micron primary sheet unit structure. Wherein, the bismuth nanoparticles are uniformly and densely embedded inside the multi-level micron primary sheet unit, and there is a thin layer of carbon wall near the surface of the bismuth nanoparticles, which together with the outer carbon shell forms a double carbon constraint layer. There is a clear interlayer spacing between the stacked square micron sheet structures, and the primary sheet unit is connected by covalent bond, ionic bond and metal bond, and the primary sheet unit layer is connected by van der Waals force. The densely embedded bismuth nanoparticles give the composite material high tap density and high bismuth content. When applied to sodium ion batteries, this feature can ensure high sodium storage capacity, effectively shorten the sodium ion migration path, accelerate the sodium ion charging and discharging rate, and improve the rate performance. In addition, the interlayer spacing of the stacked square micron sheet structure can be used as a high-speed channel for ion migration, effectively relieving the volume expansion of the material caused by sodium insertion; the double carbon constraint layer can effectively prevent the aggregation of internal bismuth nanoparticles, reduce their contact with the electrolyte, reduce the occurrence of side reactions, and prolong the cycle life.

[0070] Example 1

[0071] A preparation method of a two-dimensional multi-level micro-nano bismuth carbon composite material, specifically comprising the following steps:

[0072] Step one: 50 ml of ethylene glycol and 10 ml of 6% by mass nitric acid solution were measured and mixed uniformly. Then 0.1 g of sodium bromide, 0.1 g of glucose, 0.4 g of polyvinylpyrrolidone and 0.485 g of bismuth nitrate pentahydrate were sequentially added into the mixed solution. It was determined that the pH value of the mixed solution was 1.2 at this time. The mixed solution was placed in an environment of 25°C and continuously magnetically stirred for 30 min, and then the mixed solution was transferred to a 100 mL sealed polytetrafluoroethylene-lined stainless steel autoclave. The autoclave was placed in an oven and kept at 150°C for 12 h. After the reaction was completed, the autoclave was naturally cooled to room temperature, and the reaction product was taken out and washed with water and ethanol for three times respectively. After the washing was completed, the collected sample was placed in an electric heating air drying oven and dried at 65°C for 8 h. After cooling, a two-dimensional multi-level bromine-oxygen bismuth precursor was obtained.

[0073] Further reference Figure 3 , Figure 3 A scanning electron microscope (SEM) image and an X-ray diffraction pattern of the two-dimensional multi-level bromine-oxygen bismuth precursor prepared according to Embodiment 1 of the present application are shown. It can be directly found from the image that the two-dimensional multi-level bromine-oxygen bismuth precursor presents a morphology of multi-layer square micrometer sheet with a secondary multi-sheet structure assembled by primary sheet units. In the formation process of the two-dimensional multi-level bromine-oxygen bismuth precursor, the glucose molecule as a carbon source has stable molecular structure. The stability is derived from the fact that the influence of the acid-base degree on the hydrogen bond is relatively weak, and the hydrogen bond force is far smaller than the intermolecular cohesive force. Therefore, the glucose is hardly affected by the change of pH value. The viscosity of polyvinylpyrrolidone (PVP) is sensitive to the pH value. When the pH value of the system decreases, the interaction between PVP molecules increases, which is manifested as an increase in viscosity and an increase in mutual entanglement between molecules. Therefore, in the synthesis process of the material, the hydroxyl group in the glucose molecule is chemically bonded with Bi 3 +. In the thermal carbonization reaction, this combined part forms an inner carbon wall wrapping the bismuth nanoparticles. The long molecular chain of PVP is chelated with the glucose-Bi 3+ the combination of interconnection, which contains a long-chain carbon part further forms an outer carbon shell in the carbonization process. In addition, the two-dimensional multi-level BiOBr precursor unit presents a lamellar structure, which is mainly based on the crystal structure characteristics of BiOBr. The arrangement of atoms inside the BiOBr crystal determines its tendency to form a lamellar structure during growth. By using this characteristic, the secondary multi-lamellar structure assembled by the primary lamellar unit is constructed by introducing PVP, glucose and other hinges.

[0074] Step two: place the two-dimensional multi-level BiOBr precursor powder sample obtained in step one in a quartz tube furnace, and perform calcination treatment under Ar / H2(5%) mixed protective atmosphere. During calcination, the temperature in the furnace is raised to 450°C at a rate of 2°C / min, and the temperature is maintained at this temperature for 2h. After calcination, the furnace is naturally cooled to room temperature, and the two-dimensional multi-level micro-nano Bi / C composite material is finally obtained.

[0075] With reference to Figure 4 a-4i, Figure 4 a-4i respectively shows the scanning electron microscope (SEM) image, transmission electron microscope (TEM) image, diffraction pattern, high-resolution image, element mapping pattern and spherical aberration electron microscope image of the two-dimensional multi-level micro-nano Bi / C composite material prepared according to Example 1 of the present application. From Figure 4 a and Figure 4 The SEM images of a and b can clearly observe that the two-dimensional multi-level micro-nano Bi / C composite material presents a multi-layer longitudinal ordered laminated square microparticle structure as a whole. In-depth analysis of the local micro-morphology of the image edge shows that there is a large interlayer spacing between the multi-level carbon sheets. This interlayer spacing not only can serve as a high-speed channel for ion migration, significantly promoting the transmission of ions inside the material, but also can effectively alleviate the volume expansion phenomenon caused by the sodium insertion process. Through Figure 4 The TEM image of c can intuitively and clearly observe that the bismuth nanoparticles in the two-dimensional multi-level micro-nano Bi / C composite material present a uniform distribution state, which ensures that the material exhibits stable and uniform performance in various applications. Figure 4 The diffraction pattern of d and Figure 4 The high-resolution image of e, both of which show that the internal bismuth nanoparticles have good crystallinity, which means that the atomic arrangement inside the bismuth nanoparticles presents a highly ordered state. Combined with Figure 4Elemental mapping analysis of f shows that carbon (C), nitrogen (N), and bismuth (Bi) are uniformly distributed in the material. (Spherical aberration electron microscopy image) Figure 5 g-4i) further provides structural information at atomic resolution. The interplanar spacings marked in the figure, such as the (101) interplanar spacing d = 0.373 nm and the (003) interplanar spacing d = 0.393 nm, are also shown. At the same time, through precise measurement, statistics and fitting, the number of primary lamellar units of this composite material is determined to be 13 layers.

[0076] Further reference Figure 5 , Figure 6 Transmission electron microscopy (TEM) images and particle size distributions of the two-dimensional multilayer micro / nano bismuth-carbon composite material prepared according to Example 1 of the present invention are shown. The TEM images and particle size distributions reveal that the two-dimensional composite material has an average side length of 1.6 μm, an average thickness of 293.1 nm, an average thickness of approximately 24.3 nm for the primary lamellar units, and an average particle size of 21.7 nm for the bismuth nanoparticles.

[0077] Figure 6 The X-ray diffraction (XRD) pattern and thermogravimetric analysis (TGA) pattern of the two-dimensional multi-level micro / nano bismuth carbon composite material prepared according to Example 1 of the present invention are shown. Figure 6 Several distinct main peaks can be observed on the left, corresponding to different crystal planes of metallic bismuth. The main peaks near 27.2°, 38.0°, and 39.6° correspond to the (012), (104), and (110) crystal planes, respectively. By comparing with the standard card Bi NO.85-1329, it can be confirmed that a metallic bismuth phase exists in the material, and the presence and position of these characteristic peaks indicate that the bismuth nanoparticles in the material have good crystallinity and orderly atomic arrangement. Figure 7 The graph on the right shows two curves: the orange curve represents the change in material weight with temperature, and the blue dashed line represents the rate of weight change. Analysis of the entire thermogravimetric curve reveals a significant weight change peak around 291.2℃, with a rate of 0.74% / min, indicating a relatively rapid weight gain process near this temperature, specifically the reaction 4Bi + 3O₂ → 2Bi₂O₃. There is also a weight loss change around 390.8℃, specifically the reaction C + O₂ → CO₂↑, at a rate of -0.71% / min. The final weight of the remaining solid material, Bi₂O₃, is 102.1% of the initial weight. Calculating the mass percentage of Bi in Bi₂O₃ yields a final bismuth content of 91.6 wt%.

[0078] Next, refer to Figure 7 , Figure 7The full X-ray photoelectron spectroscopy (XPS) spectrum and XPS-C1s, XPS-N1s, XPS-O1s, and XPS-Bi4f spectra of the two-dimensional multi-layered micro / nano bismuth-carbon composite material prepared according to Example 1 of the present invention are shown. The fine spectra clearly show the bonding modes of each element and the type of nitrogen doping. (Bi4f fine spectrum...) Figure 7 The top right corner of the graph shows two main peaks, corresponding to Bi. 3 The presence of + and Bi indicates that bismuth exists in both trivalent and zero-valent states in the material. (C1s fine spectrum) Figure 7 (Bottom left) Through peak fitting, several different peaks can be seen, corresponding to bonding modes such as CC, CO, and CN. The CC peak represents a carbon-carbon covalent bond, the CO peak represents a carbon-oxygen covalent bond, and the CN peak represents a carbon-nitrogen covalent bond. (N1s fine spectrum) Figure 7 After peak segmentation (lower middle), three different types of nitrogen can be observed: graphitic nitrogen (N), pyridinic nitrogen (N), and pyrrolic nitrogen (N); O1s fine spectrum ( Figure 8 (Bottom right corner) After peak separation, you can see peaks corresponding to CO, Bi-O, etc. CO bonds, as mentioned above, reflect the connection between carbon and oxygen, while Bi-O bonds indicate that bismuth and oxygen form ionic bonds.

[0079] Figure 9 The diagram shows the calculated tap density of the two-dimensional multi-layered micro / nano bismuth-carbon composite material prepared according to Example 1 of the present invention. The test results show that its tap density is 3.03 g / cm³. 3 High tap density can fully utilize the high theoretical volumetric capacity of bismuth materials and improve the overall energy density of the battery. It is an important indicator for measuring the performance of materials in battery applications.

[0080] Final Reference Figure 9 , Figure 9 The nitrogen isothermal adsorption-desorption curve (BET) and initial coulombic efficiency statistical line graph of the two-dimensional multi-layered micro / nano bismuth-carbon composite material prepared according to Example 1 of the present invention are shown. Figure 9 As shown in the BET plot on the left, pink represents the adsorption process and blue represents the desorption process. At lower relative pressures, the adsorption amount increases slowly with increasing relative pressure, indicating that nitrogen molecules are mainly adsorbed within the micropores and pores of the material. Once the relative pressure reaches a certain value, the adsorption amount increases rapidly due to capillary condensation of nitrogen in the mesopores and macropores of the material. The desorption curve does not completely overlap with the adsorption curve, forming a hysteresis loop. This indicates the presence of a certain mesoporous structure in the material, with a pore size of 3.8 nm. Analysis of the adsorption-desorption curves reveals the pore size distribution of the material. The specific surface area of ​​this material is 48.8 m².2 / g, which is calculated by the BET theory. The specific surface area reflects the size of the material surface. A larger specific surface area means that the material has more active sites, which is beneficial for the electrochemical reaction. Figure 10 The blue curve in the right graph shows the initial coulombic efficiency of the two-dimensional multi-level micro-nano bismuth-carbon composite material under different samples. The initial coulombic efficiency is relatively stable, maintaining at about 85%. The final statistics show that the initial coulombic efficiency of the material is 85.7%. The initial coulombic efficiency is an important indicator of the electrochemical performance of electrode materials. A higher initial coulombic efficiency means that the material can store and release electrical energy more effectively during the first use, reducing energy loss.

[0081] Application Example

[0082] The two-dimensional multi-level micro-nano bismuth-carbon composite material prepared in Example 1 was used as the negative electrode material of sodium ion battery, and a button cell was assembled. After assembly, the battery was tested for related characterization: the rate performance of the battery was tested at different current densities of 1A / g to 250A / g; at the same time, the battery was tested for 12000 cycles at a current density of 5A / g.

[0083] Reference Figure 10 , Figure 11The figure shows the rate performance of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to the embodiment 1 of the present application as a negative material of sodium ion battery. As shown in the figure, the abscissa is the cycle number (n), from 0 to 280, which is used to record the number of charge and discharge cycles of the battery. The left ordinate is the specific capacity (mAh / g), which represents the amount of electricity that can be stored and released per gram of negative material during the charge and discharge process, reflecting the electricity storage capacity of the material. The right ordinate is the coulombic efficiency (C.E., %), which represents the ratio of the discharge capacity to the charge capacity of the battery, presented in percentage form, and is used to measure the energy utilization efficiency during the charge and discharge process of the battery. The current density is marked below the figure with different current density values from 1 A / g to 250 A / g, which represent the current density applied in different cycle stages, for studying the performance of the material at different charge and discharge rates. When the current density gradually increases from 1 A / g to 100 A / g, the discharge specific capacity decreases from 385.2 mAh / g to 337.8 mAh / g, with a capacity retention rate of 87.7%, indicating that as the current density increases, the discharge specific capacity of the material decreases, but still maintains a high proportion. At 200 A / g and 250 A / g ultra-high current density, the discharge capacity is 265.6 mAh / g and 220.9 mAh / g, respectively, with a capacity retention rate of 69.0% and 57.3%, respectively. Even at ultra-high current density, the material still maintains a certain discharge capacity. When the current density returns to 1 A / g, the material can be stably cycled, and the discharge specific capacity returns to a relatively high level, indicating that the material has excellent rate performance and can adapt to different charge and discharge rates. The coulombic efficiency curve shows that the coulombic efficiency is maintained at a relatively high level, close to 100%, indicating that the material has a small energy loss during the charge and discharge process and good reversibility of charge and discharge.

[0084] Continuing to refer to Figure 11 , Figure 12 The figure shows the cycle performance of the two-dimensional multi-level micro-nano bismuth-carbon composite material prepared according to the embodiment 1 of the present application as a negative material of sodium ion battery at a current density of 5 A / g. As shown in the figure, the two-dimensional multi-level micro-nano bismuth-carbon composite material can be stably cycled for 12000 cycles while maintaining a high specific capacity, and the capacity decay is very slight, with a specific capacity of 385.8 mAh / g and a capacity retention rate of 98.1% (393.4 mAh / g in the first cycle after activation), showing excellent cycle stability and ultra-long cycle life.

[0085] Embodiment 2

[0086] The difference from example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1:1:4:4.8, and the mass fraction of nitric acid is adjusted to 4%, and the pH value of the mixed solution is 1.5 at this time, and the subsequent steps are the same as those of example 1. The two-dimensional multi-level micro-nano bismuth carbon composite material is finally prepared. The average edge length of the composite material is 1.7pm, the average thickness of the material is 280nm, the number of primary sheet units is 12 layers, the thickness is about 23nm, the average diameter of bismuth nanoparticles is 10nm, the mass fraction of bismuth is 92%, and the tap density is 3.07g / cm 3 , and the specific surface area is 33m 2 / g.

[0087] Example 3

[0088] The difference from example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1:1:4:4.8, and the mass fraction of nitric acid is adjusted to 5%, and the pH value of the mixed solution is 1.4 at this time, and the subsequent steps are the same as those of example 1. The two-dimensional multi-level micro-nano bismuth carbon composite material is finally prepared. The average edge length of the composite material is 1.6pm, the average thickness of the material is 290nm, the number of primary sheet units is 13 layers, the thickness is about 22nm, the average diameter of bismuth nanoparticles is 12nm, the mass fraction of bismuth is 91.5%, and the tap density is 3.06g / cm 3 , and the specific surface area is 35m 2 / g.

[0089] Example 4

[0090] The difference from example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1.5:1.5:6:6, and the subsequent steps are the same as those of example 1. The two-dimensional multi-level micro-nano bismuth carbon composite material is finally prepared. The average edge length of the composite material is 1.7pm, the average thickness of the material is 310nm, the number of primary sheet units is 11 layers, the thickness is about 28nm, the average diameter of bismuth nanoparticles is 18nm, the mass fraction of bismuth is 90.5%, and the tap density is 2.80g / cm 3 , and the specific surface area is 38m 2 / g.

[0091] Example 5

[0092] The difference from Example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1.5:1.5:6:7.2, and the same subsequent steps as in Example 1 are followed to finally prepare the two-dimensional multi-level micro-nano bismuth carbon composite material. The composite material has an average edge length of 2.0 μm, an average thickness of 320 nm, a number of primary sheet layer units of 10 layers, a thickness of about 32 nm, an average diameter of bismuth nanoparticles of 25 nm, a mass fraction of bismuth of 93%, and a tap density of 3.06 g / cm 3 , a specific surface area of 42 m 2 / g.

[0093] Example 6

[0094] The difference from Example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1:1:4:7.2, and the mass fraction of nitric acid is adjusted to 5%, and the pH value of the mixed solution is 1.3 at this time, and the same subsequent steps as in Example 1 are followed to finally prepare the two-dimensional multi-level micro-nano bismuth carbon composite material. The composite material has an average edge length of 1.9 μm, an average thickness of 315 nm, a number of primary sheet layer units of 15 layers, a thickness of about 20 nm, an average diameter of bismuth nanoparticles of 17 nm, a mass fraction of bismuth of 93.5%, and a tap density of 3.11 g / cm 3 , a specific surface area of 50 m 2 / g.

[0095] Example 7

[0096] The difference from Example 1 is that the mass ratio of sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate in step one is 1:1:6:7.2, and the mass fraction of nitric acid is adjusted to 5%, and the pH value of the mixed solution is 1.4 at this time, and the same subsequent steps as in Example 1 are followed to finally prepare the two-dimensional multi-level micro-nano bismuth carbon composite material. The composite material has an average edge length of 1.5 μm, an average thickness of 320 nm, a number of primary sheet layer units of 14 layers, a thickness of about 23 nm, an average diameter of bismuth nanoparticles of 20 nm, a mass fraction of bismuth of 95%, and a tap density of 3.16 g / cm 3 , a specific surface area of 41 m 2 / g.

[0097] Example 8

[0098] The difference from Example 1 is that in Step 2, the temperature rising rate of the tube furnace is set to 5℃ / min, the reduction temperature is controlled at 600℃, and the holding time is set to 1h. Other steps are the same as Example 1. Finally, a two-dimensional multi-level micro-nano bismuth carbon composite material is prepared. The average edge length of the composite material is 1.6μm, the average thickness of the material is 290nm, the number of primary sheet units is 13 layers, the thickness is about 22nm, the average diameter of bismuth nanoparticles is 13nm, the mass fraction of bismuth is 93%, and the tap density is 3.10g / cm 3 , and the specific surface area is 48m 2 / g.

[0099] Example 9

[0100] The difference from Example 1 is that in Step 2, the protective atmosphere in the tube furnace is nitrogen. Other steps are the same as Example 1. Finally, a two-dimensional multi-level micro-nano bismuth carbon composite material is prepared. The average edge length of the composite material is 1.7μm, the average thickness of the material is 310nm, the number of primary sheet units is 11 layers, the thickness is about 28nm, the average diameter of bismuth nanoparticles is 25nm, the mass fraction of bismuth is 94%, and the tap density is 3.13g / cm 3 , and the specific surface area is 44m 2 / g.

[0101] Comparative Example 1

[0102] The difference from Example 1 is that in Step 1, the mixed solution is ethylene glycol and nitric acid solution with a mass fraction of 30%. The actual pH value of the mixed solution is -0.2. Other steps are the same as Example 1. The product prepared is spherical bismuth carbon composite particles instead of multi-layer sheet assembly, and the average particle size is 2.3μm.

[0103] Figure 12 Figures a-12d respectively show the scanning electron microscope (SEM) images and particle size distribution statistics of the spherical bismuth carbon composite particles prepared according to the comparative example of the present application. As shown in the figures, Figure 12 The magnification of a is 2.20k, and there are a large number of spherical bismuth carbon composite particles in the figure, which are densely distributed. The number of particles in the overall field of view is large, and the sizes of different particles have certain differences. Some particles are close to each other or even have slight agglomeration. Figure 12 The magnification of b is 10.0k, and it can be clearly observed that the spherical particles are relatively round and have high surface smoothness. Figure 12 Figure c is also an SEM image of spherical bismuth carbon composite particles. The blue line segment and red mark in the figure are used to measure the particle size, and Figure 12 Similarly, a presents a dense distribution of spherical particles. ​d is a particle size distribution statistical graph drawn by using Nano Measurer 1.2.5 software, the abscissa is the particle size (μm), and the ordinate is the percentage of particles in the particle size range. It can be seen from the graph that the particle size of the spherical bismuth / carbon composite particles is mainly concentrated in the range of 1 μm-4 μm, and the average particle size is 2.3 μm. The particles in a certain specific interval (about 1.5 μm-2 μm) have the highest proportion.

[0104] In addition, it is worth noting that patent CN115986090A discloses a nitrogen-doped bismuth / carbon composite microsphere material and its preparation method and application. Although both the present application and the patent product belong to bismuth / carbon composite materials, there are significant differences in design ideas and invention results. First of all, the present application takes two-dimensional material design as the starting point, fully utilizes the natural sheet structure of BiOBr, and guides Bi 3 + to connect with each other to build a multi-sheet system. The patent adopts a spherical design idea, although there is some commonality in the synthesis method, but the roles of the constituent materials are different, and the final synthesis results also have great differences. Secondly, the two-dimensional multi-level micro-nano bismuth / carbon composite prepared by the present application is a sheet material, in which the ultra-fine bismuth nanoparticles are completely and tightly embedded in the primary sheet unit by double carbon barriers, and the outer carbon shell reaches microns, with high tap density. The product of the patent is a secondary spherical microparticle assembled by nanosheet units, which contains bismuth nanoparticles in the nanosheet unit, and its specific surface area is significantly higher than that of the material of the present application. Finally, the two-dimensional material of the present application has higher bismuth content and tap density. The double carbon barriers can effectively prevent particle aggregation, resist electrolyte corrosion and alleviate particle swelling. The space between the layers not only serves as a high-speed ion transmission channel, but also effectively buffers volume changes. In addition, the two-dimensional material itself has the advantages of enhanced electrical conductivity and high ion transmission capacity. Therefore, the two-dimensional material of the present application is significantly superior to the product of the patent in terms of rate performance and cycle performance.

[0105] Obviously, those skilled in the art can make various modifications and changes to the embodiments of the present application without departing from the spirit and scope of the present application. In this way, the present application also aims to cover these modifications and changes if they are within the scope of the claims of the present application and their equivalents. The word "comprising" does not exclude the presence of other elements or steps not listed in the claims. The simple fact that certain measures are described in mutually different dependent claims does not mean that combinations of these measures cannot be used to advantage. Any reference signs in the claims should not be considered as limiting the scope.

Claims

1. A two-dimensional multi-level micro / nano bismuth-carbon composite material, characterized in that, The material exhibits a multi-layered stacked micron sheet structure, which is a secondary multi-layered structure composed of several primary sheet units assembled with interlayer spacing. Each primary sheet unit includes a carbon shell and bismuth nanoparticles wrapped by carbon walls. The bismuth nanoparticles wrapped by the carbon walls are embedded inside the carbon shell, forming a dual carbon constraint on the bismuth nanoparticles.

2. The two-dimensional multi-level micro / nano bismuth-carbon composite material according to claim 1, characterized in that, The material is a multi-layered, longitudinally ordered, stacked square microsheet structure.

3. The two-dimensional multi-level micro / nano bismuth-carbon composite material according to claim 1, characterized in that, The bismuth nanoparticles have an average particle size in the range of 10 nm to 25 nm and a mass fraction of 90.5% to 95%.

4. The two-dimensional multi-level micro / nano bismuth-carbon composite material according to claim 1, characterized in that, The material has an average thickness in the range of 280nm-320nm and a side length of 1.5μm-2μm.

5. The two-dimensional multi-level micro / nano bismuth-carbon composite material according to claim 1, characterized in that, The stacked microsheet structure is assembled from 10-15 layers of primary sheet units with interlayer spacing having interlayer interaction forces. The thickness of the primary sheet unit is about 20nm-32nm, and the interlayer interaction forces are van der Waals forces.

6. The two-dimensional multi-level micro / nano bismuth-carbon composite material according to claim 1, characterized in that, The tap density of the material is 2.80 g / cm³. 3 -3.16g / cm 3 Specific surface area is 30m² 2 / g-50m 2 / g.

7. A method for preparing a two-dimensional multi-level micro / nano bismuth-carbon composite material, characterized in that, The preparation method includes: S1, preparation of two-dimensional multi-level bismuth oxybromide precursor; S2, using the carbothermal reduction method, the two-dimensional multi-level bismuth oxybromine precursor is converted into a two-dimensional multi-level micro / nano bismuth carbon composite material.

8. The method for preparing two-dimensional multi-level micro / nano bismuth-carbon composite materials according to claim 7, characterized in that, The S1 step includes the following: S11, Ethylene glycol is mixed with a 4%-6% nitric acid solution to obtain mixed solution one; S12, according to the mass ratio of (1-1.5):(1-1.5):(4-6):(4.8-7.2), sodium bromide, glucose, polyvinylpyrrolidone and bismuth nitrate pentahydrate are added to the first mixed solution in sequence, the pH value is adjusted to 1.2-1.5, and the mixture is stirred at room temperature for 20-50 minutes to obtain the second mixed solution; S13, the mixed solution 2 is placed in a sealed polytetrafluoroethylene reactor and subjected to hydrothermal reaction at 120℃-150℃ for 8h-12h. After natural cooling to room temperature, the reactants are washed by centrifugation with water and anhydrous ethanol, and then dried to obtain the two-dimensional multilayer bismuth oxybromide precursor.

9. The method for preparing two-dimensional multi-level micro / nano bismuth-carbon composite materials according to claim 7, characterized in that, Step S2 includes the following: S21, the two-dimensional multi-level bismuth oxybromotriene precursor is placed in a quartz tube furnace and calcined at 400℃-600℃ at a heating rate of 2℃ / min-5℃ / min under a protective atmosphere, and held at that temperature for 1h-2h. The protective atmosphere is one or a mixture of nitrogen, argon, or a hydrogen-argon mixture. S22, naturally cooled to room temperature, to obtain the two-dimensional multi-level micro / nano bismuth carbon composite material.

10. The application of the two-dimensional multi-level micro / nano bismuth carbon composite material according to any one of claims 1-6, or the two-dimensional multi-level micro / nano bismuth carbon composite material prepared by the preparation method of any one of claims 7-9, as a negative electrode material for sodium-ion batteries.

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