A biomass-based hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery

Abstract

The present invention discloses a biomass-based hard carbon negative electrode material, a preparation method thereof, and a sodium ion battery, relating to the technical field of battery energy storage. The preparation method comprises the following steps: preparing a first mixed solution, the first mixed solution comprising a low melting point metal chloride, a high melting point metal nitrate, an acid solution and an aqueous solvent, and the pH of the first mixed solution being any value in the range of 1.0 - 5.0; adding a biomass raw material to the first mixed solution, and preparing a second mixed solution after mixing evenly; performing suction filtration and drying on the second mixed solution in sequence to prepare a biomass precursor; placing the biomass precursor in an inert protective gas and performing segmented calcination on it, and successively pulverizing and sieving the calcined product to prepare a hard carbon negative electrode material. The structural defects and open pore structure of the hard carbon negative electrode material prepared by the present invention are optimized, thereby improving the sodium ion energy storage performance and the first-cycle Coulombic efficiency of the hard carbon negative electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of battery energy storage, and particularly relates to a biomass-based hard carbon negative electrode material, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] High-performance negative electrode materials are one of the key factors promoting the industrialization process of sodium-ion batteries. Among many negative electrode materials, hard carbon has become the most widely used negative electrode material for sodium-ion batteries due to its advantages such as rich carbon source, high sodium storage capacity, and low cost. The precursors of hard carbon have a wide range of sources, which can be mainly divided into two categories: polymer materials and biomass. Compared with polymer materials, biomass precursors not only have lower costs but also wider sources, such as carbohydrates, starches, proteins, cellulose, hemicellulose, and lignin, and can retain their unique morphological structures during the carbonization process, providing more possibilities for the design and optimization of negative electrode materials for sodium-ion batteries.

[0003] In the prior art, the prepared hard carbon has a high sodium storage capacity, a low working potential, and excellent cycle stability. Its microstructure shows highly disordered characteristics, consisting of randomly stacked anisotropic graphite-like microcrystals with low crystallinity. This unique structure enables hard carbon to have a larger interlayer spacing, and at the same time, there are more structural defects and a larger specific surface area. These characteristics not only facilitate the insertion and extraction of sodium ions but also have an important impact on the formation process of the solid electrolyte interface film. However, a large number of defects and open-pore structures in the hard carbon structure also bring some challenges. For example, during the first charge and discharge process, these defects and pores are prone to consuming too much electrolyte, forming "dead sodium" sites, resulting in additional irreversible capacity loss, thereby reducing the first Coulomb efficiency of hard carbon. Summary of the Invention

[0004] An object of the first aspect of the present invention is to provide a preparation method of a biomass-based hard carbon negative electrode material, so as to solve the technical problem that the large number of structural defects and open-pore structures in the biomass-based hard carbon negative electrode material in the prior art lead to low energy storage.

[0005] Another object of the first aspect of the present invention is to further improve the dispersion stability of metal ions.

[0006] An object of the second aspect of the present invention is to provide a hard carbon negative electrode material, which is prepared according to the above preparation method.

[0007] An object of the third aspect of the present invention is to provide a sodium-ion battery including the hard carbon negative electrode material prepared by the above preparation method.

[0008] According to the object of the first aspect of the present invention, the present invention provides a preparation method of a biomass-based hard carbon negative electrode material, including the following steps:

[0009] Prepare a first mixed solution, which includes a low-melting-point metal chloride, a high-melting-point metal nitrate, an acid solution and a water solvent, and the pH of the first mixed solution is any value in the range of 1.0 - 5.0;

[0010] Add the biomass raw material to the first mixed solution, and after mixing evenly, prepare a second mixed solution;

[0011] Perform suction filtration and drying on the second mixed solution in sequence to prepare a biomass precursor;

[0012] Place the biomass precursor in an inert protective gas and perform staged calcination on it, and successively pulverize and screen the calcination product to prepare the hard carbon anode material; where:

[0013] The low-melting-point metal chloride and the acid solution form a eutectic solvent system, the melting point of the metal ions in the low-melting-point metal chloride is lower than 650 °C, the melting point of the metal ions in the high-melting-point metal nitrate is higher than 1100 °C, and the calcination temperature of the staged calcination includes a first temperature and a second temperature, the first temperature is any value in the range of 400 °C - 800 °C, and the second temperature is any value in the range of 1100 °C - 1600 °C.

[0014] Optionally, the low-melting-point metal chloride is any one or more of tin chloride, zinc chloride, gallium chloride, aluminum chloride, sodium chloride, potassium chloride, lead chloride;

[0015] The high-melting-point metal nitrate is any one or more of titanium nitrate, zirconium nitrate, molybdenum nitrate, vanadium nitrate, chromium nitrate, niobium nitrate, iron nitrate.

[0016] Optionally, the molar ratio of the low-melting-point metal chloride, the high-melting-point metal nitrate, the acid solution and the water solvent in the first mixed solution is any value in the range of (0.01 - 5):(0.001 - 10):(0.1 - 10):1.

[0017] Optionally, the acid solution is any one or several of hypochlorous acid, hydrochloric acid, sulfuric acid, sulfurous acid, acetic acid, formic acid, citric acid, phosphoric acid.

[0018] Optionally, the pH of the first mixed solution is any value in the range of 1.0 - 3.0.

[0019] Optionally, before the step of performing suction filtration and drying on the second mixed solution in sequence to prepare a biomass precursor, the following steps are further included:

[0020] The second mixed solution is mechanically activated by high-energy ball milling for any value within the range of 0.1 h to 6 h.

[0021] Optionally, the biomass raw material is any one or more of bamboo, wood, reed, coffee residue, sugarcane bagasse, cotton stalk, hemp stalk, wheat straw, rice straw, corn straw, corn cob, coconut shell, durian shell, rice husk, walnut shell, peanut shell, hazelnut shell, starch, and lignin.

[0022] Optionally, the mass ratio of the biomass raw material to the first mixed solution in the second mixed solution is any value within the range of 0.01 - 5:1.

[0023] According to the object of the second aspect of the present invention, the present invention also provides a biomass-based hard carbon anode material prepared by the preparation method of the hard carbon anode material according to any one of the above.

[0024] According to the object of the third aspect of the present invention, the present invention also provides a sodium ion battery, including a hard carbon anode material prepared by the preparation method of the hard carbon anode material according to any one of the above. The initial discharge specific capacity of the sodium ion battery is higher than 389 mAh / g, the initial Coulombic efficiency is greater than 94.5%, and the capacity retention rate after 2000 cycles is higher than 92.25%.

[0025] The present invention forms a eutectic solvent system by a low-melting-point metal chloride and an acid solution, and combines the introduction of a high-melting-point metal nitrate. Under the condition that the pH is any value within the range of 1.0 - 5.0, the biomass raw material is pretreated, which can accurately regulate the chemical composition and microstructure of the biomass precursor, promote the dissolution of non-carbon components in the biomass raw material and the optimization of the pore structure, and through staged calcination in an inert atmosphere, ensure the ordering of the interlayer structure, defect regulation, generation of closed pores and improvement of conductivity of the hard carbon anode material. After pulverization and sieving, a hard carbon anode material with uniform particle size and optimized structure is prepared, effectively improving its specific capacity, rate performance and cycle stability, reducing irreversible capacity loss, and improving the initial Coulombic efficiency, making it more suitable for high-performance sodium ion batteries.

[0026] Furthermore, by setting the pH to any value within the range of 1.0 - 3.0, the present invention enables the acid solution in the eutectic solvent to provide a stronger hydrogen bond effect, that is, the low-melting-point metal chloride is more stable as a hydrogen bond acceptor, and the high-melting-point metal nitrate can maintain higher solubility, avoiding the formation of hydroxide precipitates, reducing the viscosity, enhancing the fluidity and improving the dissolution ability of the eutectic solvent, so as to more effectively penetrate and act on the biomass raw material, improve its shearing and dissolution ability, further weaken the hydrogen bond network of cellulose, reduce the crystallinity, thereby reducing the collapse of amorphous carbon, making the hard carbon structure more uniform and improving the cycle stability.

[0027] The above description is only an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and implement it according to the content of the specification, the following describes in detail with reference to the preferred embodiments of the present invention and the accompanying drawings as follows. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Some specific embodiments of the present invention will be described in detail hereinafter with reference to the accompanying drawings in an exemplary but not limiting manner. The same reference numerals in the drawings denote the same or similar components or parts. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings:

[0029] Figure 1 is a schematic flowchart of a method for preparing a hard carbon negative electrode material according to an embodiment of the present invention;

[0030] Figure 2 is an XRD pattern of the hard carbon negative electrode material prepared according to Example 1 of the present invention;

[0031] Figure 3 is an SEM image of the hard carbon negative electrode material prepared according to Example 1 of the present invention;

[0032] Figure 4 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon negative electrode material prepared according to Example 1 of the present invention;

[0033] Figure 5 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon negative electrode material prepared according to Comparative Example 1 of the present invention;

[0034] Figure 6 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon negative electrode material prepared according to Comparative Example 2 of the present invention;

[0035] Figure 7 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon negative electrode material prepared according to Comparative Example 4 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The following further describes in detail the specific embodiments of the present invention in conjunction with the accompanying drawings and embodiments. The following embodiments are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0037] To make the above objects, features, and advantages of the present application more obvious and understandable, the following will describe the specific embodiments of the present application in detail with reference to the accompanying drawings. It can be understood that the specific embodiments described herein are only used to explain the present application, rather than limiting the present application. Additionally, it should be noted that for the convenience of description, only the parts related to the present application rather than all the structures are shown in the drawings. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0038] The terms "comprise" and "have" in this application and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but optionally further includes steps or units not listed, or optionally further includes other steps or units inherent to these processes, methods, products, or devices.

[0039] Referring to "embodiments" herein means that the specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0040] Figure 1 is a schematic flowchart of a method for preparing a hard carbon negative electrode material according to an embodiment of the present invention.

[0041] As Figure 1 shown, the present invention provides a method for preparing a biomass-based hard carbon negative electrode material, comprising the following steps:

[0042] Step S100: Prepare a first mixed solution, which includes a low-melting-point metal chloride, a high-melting-point metal nitrate, an acid solution, and a water solvent, and the pH of the first mixed solution is any value in the range of 1.0 - 5.0;

[0043] Step S200: Add the biomass raw material to the first mixed solution, and after mixing evenly, prepare a second mixed solution;

[0044] Step S300: Perform suction filtration and drying on the second mixed solution in sequence to prepare a biomass precursor;

[0045] Step S400: Place the biomass precursor in an inert protective gas and perform staged calcination on it, and successively crush and screen the calcination product to prepare a hard carbon negative electrode material. Among them, the low-melting-point metal chloride and the acid solution form a eutectic solvent system. The melting point of the metal ions in the low-melting-point metal chloride is lower than 650 °C, and the melting point of the metal ions in the high-melting-point metal nitrate is higher than 1100 °C. The calcination temperature of the staged calcination includes a first temperature and a second temperature. The first temperature is any value between 400 °C and 800 °C, and the second temperature is any value between 1100 °C and 1600 °C.

[0046] In this embodiment, in the preparation method of the hard carbon negative electrode material, first prepare a first mixed solution including a low-melting-point metal chloride, a high-melting-point metal nitrate, an acid solution and an aqueous solvent. Then add the biomass raw material to the first mixed solution, and after mixing evenly, prepare a second mixed solution. Successively perform suction filtration and drying on the second mixed solution to prepare a biomass precursor. After that, place the biomass precursor in an inert protective gas and perform staged calcination on it, and successively crush and screen the calcination product to prepare a hard carbon negative electrode material. Among them, the low-melting-point metal chloride and the acid solution form a eutectic solvent system. The calcination temperature of the staged calcination includes a first temperature and a second temperature. The first temperature is any value between 400 °C and 800 °C, that is, the first temperature can be 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C or 800 °C, or any value between 400 °C and 800 °C. The second temperature is any value between 1100 °C and 1600 °C, that is, the second temperature can be 1100 °C, 1200 °C, 1300 °C, 1400 °C, 1500 °C or 1600 °C, or any value between 1100 °C and 1600 °C. The pH of the first mixed solution is any value between 1.0 and 5.0, that is, the pH of the first mixed solution can be 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 or 5.0, or any value between 1.0 and 5.0.

[0047] In this embodiment, by forming a eutectic solvent system with a low-melting-point metal chloride and an acid solution, combined with the introduction of a high-melting-point metal nitrate, under the condition that the pH is any value between 1.0 and 5.0, the chemical composition and microstructure of the biomass precursor can be precisely regulated, the dissolution of non-carbon components in the biomass raw material and the optimization of the pore structure can be promoted, and through staged calcination in an inert atmosphere, the interlayer structure of the hard carbon negative electrode material can be ensured to be ordered, defect regulation, closed pore generation and conductivity improvement can be achieved. After crushing and screening, a hard carbon negative electrode material with uniform particle size and optimized structure is prepared, effectively improving its specific capacity, rate performance and cycle stability, reducing irreversible capacity loss, and improving the first Coulomb efficiency, making it more suitable for high-performance sodium ion batteries.

[0048] In this embodiment, under the first temperature condition, the low-melting-point metal melts to fill the pores of cellulose formed after the hydrolysis of the biomass raw material, and the high-melting-point metal forms metal oxides for stabilizing the carbon structure. The molten low-melting-point metal can act as a dispersant to promote the uniform distribution of the high-melting-point metal oxides, so as to improve the etching uniformity of the high-melting-point metal oxides in the carbon material under the second temperature condition. Under the second temperature condition, the molten low-melting-point metal further vaporizes and forms uniformly distributed nanopores in the carbon material, increasing the specific surface area of the hard carbon negative electrode material. The high-melting-point metal oxides are subjected to carbothermal reduction to form nanopores, further increasing the sodium storage sites of the hard carbon negative electrode material. At the same time, the vaporization etching of the low-melting-point metal can improve the reduction etching effect of the high-melting-point metal oxides, making the pore structure distribution in the carbon material more uniform, so that the sodium-ion battery based on the hard carbon negative electrode material has high capacity, low specific surface area, high rate performance and long cycle life.

[0049] In this embodiment, the eutectic metal chloride acts as a hydrogen bond acceptor, and the acid solution acts as a hydrogen bond donor. The two are mixed to form a eutectic solvent system. The eutectic solvent system can precisely cut the cell structure of the biomass raw material, thereby realizing closed pores in the derived hard carbon, that is, the eutectic solvent effectively changes the pore structure by simultaneously shearing and dissolving the amorphous components to form closed pores with adjustable sizes. At the same time, the highly active chloride ions in the eutectic solvent system have a strong charge effect and can form stable hydrogen bonds with the hydroxyl groups in the cellulose molecules, thereby effectively shearing the originally tight hydrogen bond network structure in the cellulose crystalline region and regulating the cellulose crystallinity. In addition, cellulose molecules can exhibit certain basic characteristics or exist in a protonated form in the solvent system. The lone pair electrons in its molecules can coordinate with Lewis acids, and metal ions, as typical Lewis acid cations, can interact with the electron-donating sites in the cellulose molecules, further weakening the hydrogen bond forces inside the cellulose macromolecules. This dual action mechanism not only significantly reduces the crystallinity of cellulose but also realizes the precise regulation of cellulose, thus contributing to the formation of a closed pore structure, reducing irreversible capacity loss, promoting the formation of micropores and interlayer defects, improving sodium ion storage capacity, and enhancing cycle stability.

[0050] In this embodiment, the acid component in the deep eutectic solvent system can not only purify the biomass precursor, removing the inorganic impurities in the biomass precursor, but also synchronously remove part of the lignin and hemicellulose, increasing the cellulose content in the biomass precursor. It also introduces oxygen-containing functional groups such as carbonyl and hydroxyl groups into the derived hard carbon as an oxidant, significantly improving the sodium storage performance of the hard carbon material. At the same time, the nitrate in the high-melting-point metal nitrate reacts with cellulose to obtain nitrocellulose, thereby introducing groups such as nitro groups. These groups escape during subsequent carbonization, increasing the overall etching of the graphite layer, thus playing a role in pore formation. The cellulose is converted into a rich short-range ordered carbon layer, effectively closing the open pores generated during the pyrolysis process. By chemically treating and regulating the components of the biomass precursor, the control of the microstructure of the derived hard carbon is achieved. The obtained hard carbon anode material will have a higher degree of graphitization, a lower specific surface area and oxygen content. That is, the hard carbon anode material with the above structural characteristics has a large number of sodium ion intercalation sites and fewer irreversible sodium storage sites, and the reversible sodium storage capacity of the hard carbon anode material will be improved accordingly. The mechanical activation method can significantly reduce the particle size of the material, increase the specific surface area, thereby providing more active sites, enhancing the reaction activity of the material, increasing the energy state of the material, and making it easier to participate in the electrochemical reaction.

[0051] In this embodiment, the pH of the first mixed solution is any value in the range of 1.0 - 5.0. By setting the acidic condition of the first mixed solution within the above range, not only can a stable deep eutectic solvent system be formed, improving the shear force of the deep eutectic solvent, promoting the hydrolysis of the biomass raw material into cellulose, but also the solubility of metal ions in the low-melting-point metal chloride and high-melting-point metal nitrate can be increased, preventing the metal ions from forming insoluble hydroxides or precipitates, and at the same time preventing the metal ions from forming complexes with chloride ions, further improving the dispersion uniformity of the metal ions.

[0052] In this embodiment, the first temperature is any value in the range of 400°C - 800°C, and the second temperature is any value in the range of 1100°C - 1600°C. By setting the first temperature and the second temperature within the above corresponding ranges, the low-melting-point metal in the low-melting-point metal chloride can be completely melted under the condition of the first temperature, so that the molten low-melting-point metal can uniformly adhere to the surface of the carbon material, and at the same time act as a dispersant to uniformly disperse the high-melting-point metal oxide formed under the condition of the first temperature. Under the condition of the second temperature, the low-melting-point metal is gasified and etched to form nanopores in the carbon material, and at the same time the high-melting-point metal oxide is subjected to carbothermal reduction to form uniformly distributed nanopores in the carbon material, further increasing the specific surface area of the hard carbon anode material.

[0053] In this embodiment, by using the first mixed solution to pretreat the biomass raw material, the preparation of high-performance hard carbon anode materials by one-step pyrolysis is realized. Compared with the traditional two-step carbonization method, it has significant process advantages and economy, simplifies the preparation process, avoids the separation of intermediate products and secondary heat treatment, reduces the energy consumption by about 30%-40%, and shortens the production cycle by more than 50%. In addition, the one-step pyrolysis method can better control the microstructure of the hard carbon anode material, making it have a higher specific surface area, a more uniform pore distribution and more excellent electrochemical performance, providing a more efficient and environmentally friendly preparation method for the application of hard carbon anode materials in the fields of sodium-ion batteries, supercapacitors, etc.

[0054] In a further embodiment, the low-melting-point metal chloride is any one or more of tin chloride, zinc chloride, gallium chloride, aluminum chloride, sodium chloride, potassium chloride, lead chloride, and the high-melting-point metal nitrate is any one or more of titanium nitrate, zirconium nitrate, molybdenum nitrate, vanadium nitrate, chromium nitrate, niobium nitrate, iron nitrate. In this embodiment, the melting point of the low-melting-point metal is lower than 650 °C, and the melting point of the high-melting-point metal is higher than 1100 °C. By mixing the low-melting-point metal chloride, the high-melting-point metal nitrate and the acid solution to prepare the first mixed solution, the low-melting-point metal and the high-melting-point metal in the first mixed solution are simultaneously attached to the cellulose formed by the hydrolysis of the biomass raw material. When calcined at the first temperature, the low-melting-point metal is heated and melted and attached to the surface of the carbon material, and the high-melting-point metal reacts to form a high-melting-point metal oxide and is evenly attached to the surface of the carbon material through the dispersion action of the molten low-melting-point metal, so that when calcined at the second temperature, the low-melting-point metal is gasified and etched to form nanopores, and the high-melting-point metal oxide is carbon thermally reduced to form nanopores, thereby increasing the specific surface area of the hard carbon anode material, optimizing the pore structure of the hard carbon anode material, increasing the sodium storage performance of the hard carbon anode material, and thus increasing the charge-discharge specific capacity of the sodium-ion battery based on the hard carbon anode material.

[0055] In a further embodiment, the molar ratio of the low-melting-point metal chloride, the high-melting-point metal nitrate, the acid solution and the water solvent in the first mixed solution is any value in (0.01-5):(0.001-10):(0.1-10):1. In this embodiment, by setting the molar ratio of the low-melting-point metal chloride, the high-melting-point metal nitrate, the acid solution and the water solvent within the above range, it is convenient for the eutectic metal chloride and the acid solution to form a stable eutectic solvent system, and the acid solution and the water solvent to form an acidic condition with a pH of 1.0-5.0, so that the eutectic solvent system can fully hydrolyze the biomass raw material to form cellulose, destroy the hydrogen bond network structure of the cellulose, reduce the crystallinity of the cellulose, and at the same time ensure the uniform dispersion of various metal ions in the first mixed solution, preventing precipitation or the formation of complexes.

[0056] In this embodiment, by controllably adjusting the type and concentration of metal ions, the interlayer spacing of graphite microcrystals and the micropore size can be regulated, thereby comprehensively regulating the interfacial / bulk electrochemistry of hard carbon. Metal ions within the above concentration range can catalyze the growth of graphite flakes into long-range ordered graphene sheets. At high temperatures, the metal ions break the sp3 carbon bridges between the graphene sheets, enabling the graphene sheets to rearrange freely, forming nano-graphite domains and carbon micropores, and promoting the removal of irreversible oxygen-containing defects through the coordination between metal catalytic ions and oxygen-containing defects. The optimized hard carbon has expanded graphite regions and highly developed nanopores, reducing the defect content and facilitating the rapid storage of sodium ions. At the same time, the M-N4-C (M is a metal atom) structure can catalyze the rapid decomposition of NaPF6, endowing it with a thin and inorganic-rich solid electrolyte interface phase and fast interfacial sodium ion storage kinetics, and triggering a local electric field within the graphite domains to provide a Coulomb force to reduce the diffusion barrier and accelerate the bulk sodium ion storage kinetics.

[0057] In a further embodiment, the acid solution is any one or more of hypochlorous acid, hydrochloric acid, sulfuric acid, sulfurous acid, acetic acid, formic acid, citric acid, and phosphoric acid. In this embodiment, when the acid solution is a strong acid, that is, hydrochloric acid or sulfuric acid, it helps to increase the solubility of high-melting-point metal nitrates and prevent the formation of hydroxide precipitates in the solution. When the acid solution is a weak acid, that is, acetic acid or citric acid, it can form stable metal-organic complexes with metal ions, improving the dispersion uniformity of metal ions in the solvent and avoiding agglomeration. When the acid solution is an oxidizing acid, that is, hypochlorous acid or sulfuric acid, it can partially oxidize cellulose, increasing its hydrolysis rate and promoting the shearing of the biomass cell wall. When the acid solution is an organic acid, that is, acetic acid or citric acid, it can form complexes on the surface of cellulose, improving the uniformity of the microporous structure after carbonization. When the acid solution is sulfuric acid or phosphoric acid, it can promote the degradation of lignin, reduce the carbon skeleton collapse during high-temperature carbonization, and form an appropriate amount of micropores. When the acid solution is acetic acid or citric acid, it can regulate the growth of the carbon skeleton, inhibit the excessive specific surface area, and improve the cycle stability. The acid solution in this embodiment can be selected according to the needs from the corresponding acidic solutions, and no limitation is imposed thereon.

[0058] In a further embodiment, the pH of the first mixed solution is any value in the range of 1.0 - 3.0, that is, the pH of the first mixed solution can be 1.0, 1.5, 2.0, 2.5 or 3.0, or any value in the range of 1.0 - 3.0. In a strongly acidic environment with a pH of 1.0 - 3.0, the acid solution in the deep eutectic solvent provides stronger hydrogen bonding, making the low-melting metal chloride more stable as a hydrogen bond acceptor. The high-melting metal nitrate can maintain higher solubility, avoiding the formation of hydroxide precipitates, reducing the viscosity of the deep eutectic solvent, enhancing its fluidity and dissolution ability, thus more effectively penetrating and acting on the biomass raw material, improving its shearing and dissolution ability, further weakening the hydrogen bond network of cellulose, reducing the crystallinity, thereby reducing the collapse of amorphous carbon and making the hard carbon structure more uniform, and improving the cycle stability.

[0059] In a further embodiment, before step S300, the following steps are further included:

[0060] Mechanically activate the second mixed solution. The mechanical activation method is high-energy ball milling, and the mechanical activation time is any value in the range of 0.1 h - 6 h.

[0061] In this embodiment, before filtering and drying the second mixed solution, it is necessary to mechanically activate the second mixed solution. Treating the second mixed solution by mechanical activation can significantly reduce the particle size of the biomass raw material in the second mixed solution, increase the specific surface area, thus providing more active sites, enhancing the reaction activity of the biomass raw material, reducing the activation energy, making the reaction more uniform, and the product performance more stable, which can greatly improve the reaction rate and shorten the process flow.

[0062] In this embodiment, through mechanical activation, the densely packed metal nanoparticles in the first mixed solution can be embedded in the hydrogen bond network structure of highly conductive cellulose, which can significantly improve the electrochemical performance of the hard carbon negative electrode material. The densely packed structure of metal nanoparticles not only provides abundant active sites but also forms a continuous electron conduction network, thus greatly promoting the rapid transport of electrons and ions. This structural design effectively reduces the charge transfer resistance and maximizes the utilization rate of metal active substances, making them show higher efficiency in electrochemical reactions. In addition, the nanoporous structure of the hard carbon negative electrode material plays an important role during the cycling process. It not only provides sufficient channels for the penetration of the electrolyte and ion diffusion but also can effectively relieve the stress concentration problem caused by volume changes of metal nanoparticles during charge and discharge. Through this buffering effect, the structural integrity of the hard carbon negative electrode material is maintained, thus significantly enhancing its rate performance and cycle stability.

[0063] In this embodiment, the ball-to-material ratio of the high-energy ball milling method is any value in the range of 0.1 - 10:1, and the ball milling speed is any value in the range of 100 rpm - 500 rpm. That is, the ball-to-material ratio can be 0.1:1, 0.5:1, 1:1, 2:1, 3:1, 5:1, 7:1, or 10:1, or any value in the range of 0.1 - 10:1. The ball milling speed can be 100 rpm, 150 rpm, 200 rpm, 300 rpm, 400 rpm, or 500 rpm, or any value in the range of 100 rpm - 500 rpm. By setting the ball-to-material ratio and the ball milling speed within the above ranges respectively, the particle distribution uniformity in the second mixed solution is improved, the specific surface area is increased, and the sodium ion storage performance of the hard carbon anode material is optimized.

[0064] In a further embodiment, the biomass raw material is any one or more of bamboo, wood, reed, coffee grounds, bagasse, cotton stalks, hemp stalks, wheat straw, rice straw, corn straw, corn cobs, coconut shells, durian shells, rice husks, walnut shells, peanut shells, hazelnut shells, starch, lignin. The biomass raw materials involved in this embodiment have a wide range of sources, covering different types such as lignocellulose, herbaceous cellulose, fruit shells, agricultural waste, and single-component organic substances (starch, lignin). Due to the different chemical compositions, microstructures, and ash contents of different biomass raw materials, their effects on the electrochemical properties such as specific capacity, pore structure, conductivity, and cycle stability are different during the preparation process of the hard carbon anode material. By reasonably selecting and optimizing the biomass raw materials, comprehensive technical effects such as high specific capacity, high initial Coulomb efficiency, long cycle life, and excellent rate performance can be achieved in the sodium storage hard carbon material.

[0065] In a further embodiment, the mass ratio of the biomass raw material to the first mixed solution in the second mixed solution is any value in the range of 0.01 - 5:1. That is, the mass ratio of the biomass raw material to the first mixed solution can be 0.01:1, 0.1:1, 0.5:1, 1:1, 1.5:1, 2:1, 3:1, 4:1, or 5:1, or any value in the range of 0.01 - 5:1. In this embodiment, by setting the mass ratio of the biomass raw material to the first mixed solution in the second mixed solution within the above ratio range, it can ensure that the biomass is completely infiltrated, and maintain the uniformity and dissolution ability of the eutectic solvent system, ensure that the biomass reacts fully, avoid excessive degradation or insufficient modification, and improve the microstructure uniformity of the hard carbon material. At the same time, metal chlorides and high-melting-point metal nitrates can be evenly dispersed, and metal coordination complexes are formed inside the biomass structure, ensuring the uniform etching effect of metals during the subsequent carbonization process, improving the controllability of the pore structure of the carbon material, increasing the specific capacity, reducing metal residues, improving the electrochemical stability of the electrode, and increasing the cycle life.

[0066] The present invention also provides a biomass-based hard carbon negative electrode material, which is prepared according to the preparation method of the above hard carbon negative electrode material.

[0067] The present invention also provides a sodium-ion battery, which includes the hard carbon negative electrode material prepared according to the preparation method of the above hard carbon negative electrode material. The initial discharge specific capacity of the sodium-ion battery is higher than 389 mAh / g, the initial Coulombic efficiency is greater than 94.5%, and the capacity retention rate after 2000 cycles is higher than 92.25%. Regarding the preparation method of the hard carbon negative electrode material, it will not be elaborated here one by one. In this embodiment, by using the hard carbon negative electrode material prepared by the above preparation method as the negative electrode material of the sodium-ion battery, the discharge specific capacity, the initial Coulombic efficiency, and the rate performance of the sodium-ion battery can be significantly improved.

[0068] In this embodiment, in the preparation method of the sodium-ion battery, using the hard carbon negative electrode material prepared in the above embodiment as the negative electrode active material, after weighing the negative electrode active material, the conductive agent, the first binder, and the second binder according to a mass ratio of 95:2:2:1, deionized water is added, and stirred for 6 h to obtain a uniform black slurry. The black slurry is in a viscous paste state. The above black slurry is evenly coated on the copper foil, and then vacuum dried at 80 °C for 12 h to obtain a pole piece with a uniform thickness. Then, the pole piece is punched into a circular piece with a diameter of 14 mm as the negative electrode. Using a metal circular piece with a diameter of 14 mm as the negative electrode, a glass fiber as the separator, and 1 mol / L of NaPF6 as the electrolyte, a CR2025 type button battery is assembled in a glove box filled with argon. After the battery stands for 24 h, the charge-discharge performance test is carried out. Here, the conductive agent is acetylene black, the first binder is sodium carboxymethyl cellulose, and the second binder is styrene-butadiene rubber.

[0069] The present application will be further described in detail below with reference to specific embodiments.

[0070] Example 1

[0071] In the preparation method of the hard carbon negative electrode material in this embodiment, first, tin chloride, molybdenum nitrate, and acetic acid are dissolved in water, and their molar ratio is 0.5:0.1:5:1. After stirring, a first mixed solution is obtained. Then, bamboo powder is added to the first mixed solution, wherein the mass ratio of the bamboo powder to the first mixed solution is 0.1:1. Subsequently, the second mixed solution is mechanically activated at a rotation speed of 300 rpm for 30 min, and the ball-to-material ratio of the mechanical activation is 3:1. The product after mechanical activation is filtered by suction and dried at 80 °C to obtain a pretreated biomass precursor. The obtained biomass precursor is heated to 800 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 0.5 h, and then heated to 1300 °C at a rate of 3 °C / min and held for 3 h, and then cooled with the furnace. Finally, the calcined product is taken out and pulverized and sieved to obtain a hard carbon negative electrode material for sodium-ion batteries.

[0072] Example 2

[0073] In the preparation method of the hard carbon negative electrode material in this example, first, zinc chloride, molybdenum nitrate, and phosphoric acid are dissolved in water, and their molar ratio is 5:10:5:1. After stirring, a first mixed solution is obtained. Then, bamboo powder is added to the first mixed solution, where the mass ratio of bamboo powder to the first mixed solution is 0.5:1. Subsequently, mechanical activation is carried out at a rotation speed of 500 rpm for 10 min, and the ball-to-material ratio of mechanical activation is 3:1. The product after mechanical activation is filtered by suction and dried at 80 °C to obtain a pretreated biomass precursor. The obtained biomass precursor is heated to 600 °C at a rate of 5 °C / min in a tube furnace under a nitrogen atmosphere and held for 2 h, then heated to 1600 °C at a rate of 3 °C / min and held for 1 h, and then cooled with the furnace. Finally, the calcined product is taken out and pulverized and sieved to obtain the hard carbon negative electrode material for sodium-ion batteries.

[0074] Example 3

[0075] In the preparation method of the hard carbon negative electrode material in this example, first, aluminum chloride, iron nitrate, and acetic acid are dissolved in water, and their molar ratio is 0.01:0.05:0.1:1. After stirring, a first mixed solution is obtained. Then, bamboo powder is added to the first mixed solution, where the mass ratio of bamboo powder to the first mixed solution is 0.01:1. Subsequently, mechanical activation is carried out at a rotation speed of 100 rpm for 6 h, and the ball-to-material ratio of mechanical activation is 5:1. The product after mechanical activation is filtered by suction and dried at 80 °C to obtain a pretreated biomass precursor. The obtained biomass precursor is heated to 400 °C at a rate of 3 °C / min in a tube furnace under a nitrogen atmosphere and held for 2 h, then heated to 1100 °C at a rate of 2 °C / min and held for 12 h, and then cooled with the furnace. Finally, the calcined product is taken out and pulverized and sieved to obtain the hard carbon negative electrode material for sodium-ion batteries.

[0076] Example 4

[0077] In the preparation method of the hard carbon negative electrode material in this embodiment, first, zinc chloride, molybdenum nitrate, and formic acid are dissolved in water, and their molar ratio is 0.5:0.001:10:1. After stirring, a first mixed solution is obtained. Then, coconut shell powder is added to the first mixed solution, where the mass ratio of the coconut shell powder to the first mixed solution is 5:1. Subsequently, mechanical activation is carried out at a rotation speed of 300 rpm for 30 min, and the ball-to-material ratio of mechanical activation is 10:1. The product after mechanical activation is subjected to suction filtration and dried at 80 °C to obtain a pretreated biomass precursor. The obtained biomass precursor is heated to 800 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 0.5 h, then heated to 1500 °C at a rate of 3 °C / min and held for 3 h, and then cooled with the furnace. Finally, the calcined product is taken out and pulverized and sieved to obtain the hard carbon negative electrode material for sodium-ion batteries.

[0078] Example 5

[0079] In the preparation method of the hard carbon negative electrode material in this embodiment, first, tin chloride, cobalt nitrate, and acetic acid are dissolved in water, and their molar ratio is 0.5:0.05:2:1. After stirring, a first mixed solution is obtained. Then, coconut shell powder is added to the first mixed solution, where the mass ratio of the coconut shell powder to the first mixed solution is 0.05:1. Subsequently, mechanical activation is carried out at a rotation speed of 300 rpm for 60 min, and the ball-to-material ratio of mechanical activation is 0.1:1. The product after mechanical activation is subjected to suction filtration and dried at 80 °C to obtain a pretreated biomass precursor. The obtained biomass precursor is heated to 500 °C at a rate of 5 °C / min in a tubular furnace under a nitrogen atmosphere and held for 6 h, then heated to 1300 °C at a rate of 3 °C / min and held for 2 h, and then cooled with the furnace. Finally, the calcined product is taken out and pulverized and sieved to obtain the hard carbon negative electrode material for sodium-ion batteries.

[0080] Comparative Example 1

[0081] The difference between Comparative Example 1 and Example 1 is only that the first mixed solution is a composition of acetic acid and water, and the molar ratio of acetic acid to water is 5:1. The pretreated biomass precursor is directly heated to 1300 °C and held for 3 h to prepare the hard carbon negative electrode material for sodium-ion batteries.

[0082] Comparative Example 2

[0083] The difference between Comparative Example 2 and Example 1 is only that the first mixed solution does not include tin chloride.

[0084] Comparative Example 3

[0085] The difference between Comparative Example 3 and Example 1 is only that the first mixed solution does not include molybdenum nitrate.

[0086] Comparative Example 4

[0087] The difference between Comparative Example 4 and Example 1 is only that the second mixed solution is not mechanically activated and only stirred.

[0088] The hard carbon anode materials prepared in Examples 1-5 and Comparative Examples 1-4 were used as the anode active materials of the corresponding sodium-ion batteries respectively, and the electrochemical performances of the sodium-ion batteries prepared based on the hard carbon anode materials prepared in Examples 1-5 and Comparative Examples 1-4 were tested, and the test result graphs of the electrochemical performances shown in Table 1 were obtained. Here, the test temperature in the electrochemical performance test was 25 °C, and the charge-discharge cut-off voltage was 0.01-2.5 V.

[0089]

[0090] As shown in Table 1, when the hard carbon anode materials prepared in Examples 1-5 were used as the anode active materials of the sodium-ion batteries, the first charge specific capacity of the corresponding sodium-ion batteries was higher than 389 mAh / g, the first Coulombic efficiency was higher than 94%, the discharge specific capacity in the first cycle at a current density of 1 A / g was higher than 349 mAh / g, and the capacity retention rate after 2000 cycles at a current density of 1 A / g was higher than 92%. Compared with the hard carbon anode materials prepared in Comparative Example 1 where only the biomass raw material was pretreated with acid solution, Comparative Example 2 where the first mixed solution did not include low-melting-point metal chloride, Comparative Example 3 where the first mixed solution did not include high-melting-point metal nitrate, and Comparative Example 4 where the second mixed solution was not mechanically activated and used as the anode active materials of the sodium-ion batteries, the sodium-ion batteries prepared from the hard carbon anode materials prepared in Examples 1-5 of the present invention all had significantly improved discharge specific capacity, first Coulombic efficiency and rate performance. That is, the hard carbon anode materials prepared by the one-step pyrolysis method of the present invention have the advantages of high first efficiency, low cost, high capacity and long life.

[0091] Figure 2 is the XRD pattern of the hard carbon anode material prepared according to Example 1 of the present invention, Figure 3 is the SEM image of the hard carbon anode material prepared according to Example 1 of the present invention, Figure 4 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon anode material prepared according to Example 1 of the present invention, Figure 5 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon anode material prepared according to Comparative Example 1 of the present invention, Figure 6 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon anode material prepared according to Comparative Example 2 of the present invention, Figure 7 is the first-cycle discharge curve of the sodium-ion battery with the hard carbon anode material prepared according to Comparative Example 4 of the present invention.

[0092] As Figure 2As shown, the XRD pattern of the hard carbon anode material prepared in Example 1 includes two peak positions, (002) and (100). Among them, the position of the (002) peak appears at 2θ = 26°, and the corresponding interlayer spacing is about 3.4 Å. The position of the (100) peak appears at 2θ = 45°, indicating that the material of the product prepared in Example 1 is hard carbon. And the peak intensity ratio of the (002) peak to the (100) peak is 2, that is, the intensity of the (002) peak is higher and the intensity of the (100) peak is weaker, indicating that the crystal layer structure of the sample is more layered and neatly stacked, with a stronger diffraction signal in the direction perpendicular to the layer plane, indicating that the atomic arrangement in the plane is relatively irregular or disordered. The peak intensity ratio of 2 further indicates the orderliness of the interlayer arrangement of the hard carbon anode material prepared in Example 1, that is, the product prepared in Example 1 has a certain degree of graphitization, and the hard carbon anode material is successfully prepared.

[0093] As Figure 3 shown, the hard carbon anode material prepared in Example 1 has a porous honeycomb structure, that is, the biomass precursor obtained by pretreatment changes from a submicron cluster structure composed of neatly arranged fibers to a unique porous honeycomb structure, so that during the high-temperature carbonization process, the porous honeycomb-structured biological precursor can form richer closed pores, thereby increasing the sodium storage sites of the hard carbon anode material and improving the plateau capacity.

[0094] As Figure 4 shown, Figure 4 is the charge-discharge curve of Example 1. The charge curve and discharge curve of the hard carbon anode material prepared in Example 1 both show a high-voltage ramp region and a low-voltage quasi-plateau region, and both the charge specific capacity and the discharge specific capacity can reach 400 mAh / g. This shows that the pretreatment using the deep eutectic solvent system dissolves more lignocellulose, and combined with the high-melting-point nitrate to form more optimized nanoporous structures in the carbon material, while increasing the specific surface area and reducing structural defects, thus playing a further etching role on the hard carbon anode material. More nanoporous structures are more conducive to the storage of sodium ions.

[0095] As Figure 5 shown, the charge-discharge curves of the products prepared in Comparative Example 1 do not have a low-voltage quasi-plateau region, indicating that the pretreatment of the biomass raw material only with acid solution in Comparative Example 1 cannot completely hydrolyze the biomass raw material, resulting in the product after subsequent calcination not being graphitized hard carbon.

[0096] As Figure 6 and Figure 7As shown, in the charge-discharge curves of the products prepared in Comparative Example 2 and Comparative Example 4, although there are plateau-like regions in the low-voltage range, the discharge specific capacities in Comparative Example 2 and Comparative Example 4 are both lower than 362 mAh / g. That is, when the low-melting-point metal chloride is absent in the first mixed solution or the second mixed solution is not mechanically activated, the biomass raw material cannot be completely hydrolyzed to form cellulose.

[0097] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0098] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent should be subject to the appended claims.

Claims

1. A preparation method of a biomass-based hard carbon anode material, characterized in that, It includes the following steps: Prepare a first mixed solution, the first mixed solution includes a low-melting-point metal chloride, a high-melting-point metal nitrate, an acid solution and a water solvent, and the pH of the first mixed solution is any value in the range of 1.0 - 5.0; Add the biomass raw material to the first mixed solution, and after mixing evenly, prepare a second mixed solution; Filter the second mixed solution by suction and dry it in sequence to prepare a biomass precursor; Place the biomass precursor in an inert protective gas and carry out staged calcination on it, and successively crush and screen the calcination product to prepare the hard carbon negative electrode material; where: The low-melting-point metal chloride and the acid solution form a eutectic solvent system, the melting point of the metal ions in the low-melting-point metal chloride is lower than 650 °C, the melting point of the metal ions in the high-melting-point metal nitrate is higher than 1100 °C, the calcination temperature of the staged calcination includes a first temperature and a second temperature, the first temperature is any value in the range of 400 °C - 800 °C, and the second temperature is any value in the range of 1100 °C - 1600 °C; The low-melting-point metal chloride is any one or more of tin chloride, zinc chloride, gallium chloride, aluminum chloride, sodium chloride, potassium chloride, lead chloride; The high-melting-point metal nitrate is any one or more of titanium nitrate, zirconium nitrate, molybdenum nitrate, vanadium nitrate, chromium nitrate, niobium nitrate, iron nitrate; The molar ratio of the low-melting-point metal chloride, the high-melting-point metal nitrate, the acid solution and the water solvent in the first mixed solution is any value in the range of (0.01 - 5):(0.001 - 10):(0.1 - 10):

1.

2. The method for preparing the hard carbon negative electrode material according to claim 1, characterized in that, The acid solution is any one or several of hypochlorous acid, hydrochloric acid, sulfuric acid, sulfurous acid, acetic acid, formic acid, citric acid, phosphoric acid.

3. The method for preparing the hard carbon negative electrode material according to claim 2, characterized in that, The pH of the first mixed solution is any value in the range of 1.0 - 3.

0.

4. The preparation method of the hard carbon negative electrode material according to claim 3, characterized in that, Before the step of filtering the second mixed solution by suction and drying it in sequence to prepare a biomass precursor, the following steps are further included: Mechanically activate the second mixed solution, the way of mechanical activation is the high-energy ball milling method, and the time of mechanical activation is any value in the range of 0.1 h - 6 h.

5. The method for preparing the hard carbon negative electrode material according to any one of claims 1 - 4, characterized in that, The biomass raw material is any one or several of bamboo, wood, reed, coffee residue, sugarcane bagasse, cotton stalk, hemp stalk, wheat straw, rice straw, corn straw, corn cob, coconut shell, durian shell, rice husk, walnut shell, peanut shell, hazelnut shell, starch, lignin.

6. The method for preparing the hard carbon negative electrode material according to claim 5, characterized in that, The mass ratio of the biomass raw material and the first mixed solution in the second mixed solution is any value in the range of 0.01 - 5:

1.

7. A biomass-based hard carbon negative electrode material, characterized in that Prepared by the method for preparing the hard carbon negative electrode material according to any one of claims 1 - 6.

8. A sodium-ion battery, characterized in that, The hard carbon negative electrode material obtained by the preparation method of the hard carbon negative electrode material according to any one of claims 1-6, the first discharge specific capacity of the sodium ion battery is higher than 389 mAh / g, the first Coulomb efficiency is greater than 94.5%, and the capacity retention rate after 2000 cycles is higher than 92.25%.

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