Biomass-based hard carbon negative electrode material and preparation method and application thereof

Abstract

The application discloses a biomass-based hard carbon negative material and a preparation method and application thereof, and belongs to the technical field of carbon materials. The preparation method comprises the following steps: adding at least one first metal ion and at least one second metal ion into ammonia water, sufficiently dissolving to form a metal-ammonia complex solution; dissolving a biomass material in the metal-ammonia complex solution to obtain a uniform solution; washing and drying the uniform solution, and pyrolyzing and carbonizing to obtain a target material; the first metal ion is formed by an easily volatile metal with a boiling point not higher than temperature T, and the second metal ion is formed by a difficultly volatile sodium-philic metal with a boiling point higher than temperature T, and the temperature T is a pyrolysis carbonization temperature. The method dissolves cellulose by using a metal-ammonia complex solution, utilizes two types of metals with different boiling points, realizes sodium-philic metal single-atom uniform doping and nano-pore construction in cooperation in one step, and has the advantages of simple process, low cost, and excellent electrical performance of the obtained biomass-based hard carbon.

Description

Technical Field

[0001] This application belongs to the field of carbon materials technology, specifically relating to a biomass-based hard carbon anode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, due to their abundant sodium resources, low cost, and similar operating principle to lithium-ion batteries, are considered a promising next-generation electrochemical energy storage system. Among the anode materials for sodium-ion batteries, hard carbon materials, with their disordered carbon layer structure and large interlayer spacing, can effectively accommodate large-radius sodium ions for reversible insertion and extraction, making them one of the mainstream anode material options. In particular, biomass-based hard carbon derived from biomass precursors not only has widely available, renewable, and inexpensive raw material sources, but its natural microstructure, after carbonization, often inherits abundant pore structures, exhibiting high reversible specific capacity and good cycle stability. Therefore, developing high-performance, low-cost biomass-based hard carbon anode materials is of significant practical importance for promoting the industrialization of sodium-ion batteries.

[0003] Currently, the commonly used technical route for preparing biomass-based hard carbon anode materials is to clean and dry the biomass raw materials, followed by high-temperature pyrolysis and carbonization under an inert atmosphere to obtain hard carbon products with an amorphous carbon structure. To further improve the sodium storage capacity and rate performance of hard carbon materials, current research focuses mainly on two aspects: pore structure control and heteroatom doping modification. For example, existing technologies use metal salt solutions to impregnate biomass precursors, followed by high-temperature pyrolysis to obtain metal ion-modified hard carbon materials; alternatively, biomass carbonization is further combined with metal salts, organic carbon sources, and various dopant sources in multiple steps to prepare composite hard carbon anode materials with multi-element doping characteristics. These methods can achieve performance optimization by constructing suitable nanopores to increase sodium storage active sites and shorten ion transport paths, or by introducing metal and non-metal heteroatoms to change the electron cloud distribution of the carbon layer and reduce the sodium ion diffusion barrier.

[0004] However, current technologies for preparing metal-doped hard carbon anode materials based on biomass still have certain limitations. On the one hand, conventional liquid-phase impregnation or physical mixing processes are difficult to achieve sufficient and uniform penetration of metal ions into the microscopic cellular structure of biomass, resulting in uneven mixing of the precursor and metal source. This leads to problems such as dispersed distribution of active sites and poor consistency in electrochemical performance in the final carbonized product. On the other hand, in order to remove excess metal oxide byproducts generated during carbonization or to etch pores in the carbon framework, existing technologies often rely on cumbersome acid washing post-treatment processes. This not only increases the complexity of the process steps and water consumption, but the high-intensity acid treatment can also cause irreversible structural damage to the conductive network framework of hard carbon. In addition, although some hydrothermal pretreatment schemes can improve the mixing uniformity to a certain extent, they are highly dependent on high-temperature and high-pressure reaction equipment, and usually require the pore-forming process and the metal doping process to be carried out in separate steps.

[0005] In summary, existing technologies generally suffer from problems such as difficulty in coordinating pore formation and doping, difficulty in dispersing single atoms, uneven mixing of precursors, and complex and poor process controllability. Summary of the Invention

[0006] In view of this, the primary objective of this application is to provide a method for preparing biomass-based hard carbon anode materials. This method is simple, highly controllable, and can achieve a biomass-based hard carbon structure in one step by uniformly doping sodium-loving metal single atoms and constructing nanopores. It simultaneously reduces the sodium ion diffusion barrier, improves reversible capacity and rate performance, and realizes low-cost, high-performance, and scale-up preparation of biomass-based hard carbon anode materials.

[0007] To achieve the above objectives, this application adopts the following technical solution: One aspect of this application discloses a method for preparing a biomass-based hard carbon anode material, comprising the following steps: A metal-ammonia complex solution is provided, wherein the metal-ammonia complex solution uses ammonia water as a solvent and dissolves at least one first metal ion and at least one second metal ion; The biomass material is immersed in the metal-ammonia complex solution and stirred thoroughly to dissolve, thereby obtaining a homogeneous solution in which the metal ions coordinate with the cellulose in the biomass material. The homogeneous solution was washed with water and dried, followed by pyrolysis and carbonization to obtain a biomass-based hard carbon anode material. Wherein, the first metal ion is formed by a volatile metal with a boiling point not exceeding temperature T, and the second metal ion is formed by a non-volatile sodium-loving metal with a boiling point exceeding temperature T, wherein temperature T is the pyrolysis carbonization temperature.

[0008] Another aspect of this application discloses a biomass-based hard carbon anode material, prepared using the method described above, wherein the closed pore volume of the biomass-based hard carbon anode material is 0.05~0.15 cm³. 3 ·g -1 The closed-pore rate is 30-50%.

[0009] Another aspect of this application discloses a sodium-ion battery anode containing the biomass-based hard carbon anode material described in this application.

[0010] Another aspect of this application discloses a sodium-ion battery containing the sodium-ion battery negative electrode described in this application.

[0011] This application has at least the following beneficial effects: This application utilizes two types of metals with different boiling points at high temperatures to achieve a one-step synergistic effect of doping and pore-forming. Specifically, it combines a volatile metal at high temperatures and a non-volatile sodium-loving metal at high temperatures, using ammonia as a metal complexing agent to first form a metal-ammonia complex solution. This solution is then used as a solvent to dissolve cellulose in biomass materials, achieving uniform mixing of multiple metal ions at the molecular level through liquid-phase coordination, and anchoring them onto the cellulose template. The self-evaporation properties of the volatile metal at high temperatures are used for pore-forming, eliminating the need for subsequent acid washing and simplifying the process. Simultaneously, based on the atomic-level dispersion of metal ions in the precursor, they are successfully converted into single atoms after pyrolysis and carbonization, avoiding agglomeration. Through the synergistic effect of in-situ pore-forming and single-atom doping, single atoms lower the diffusion barrier, and multi-level pores increase capacity. Thus, a simple, low-cost, and environmentally friendly method is used to achieve a one-step synergistic hard carbon structure constructed by uniform single-atom doping of a sodium-loving metal and nanopores, enabling low-cost, high-performance biomass-based hard carbon anode materials that can be scaled up. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the preparation process of biomass-based hard carbon anode material in a preferred embodiment of this application.

[0013] Figure 2 Transmission electron microscope (TEM) images of the biomass-based hard carbon anode materials in Example 1 and Comparative Example 1.

[0014] Figure 3 The charge-discharge curves of the button battery prepared from the biomass-based hard carbon anode material in this embodiment are shown.

[0015] Figure 4 The charge-discharge curves of the button battery prepared with biomass-based hard carbon anode material are shown in the comparative example.

[0016] Figure 5 The rate performance diagram of the button cell using the biomass-based hard carbon anode material in this embodiment is shown.

[0017] Figure 6 The rate performance diagram of the coin cell using biomass-based hard carbon anode material is shown in the comparative example. Detailed Implementation

[0018] The embodiments of this application will be clearly and completely described below. The technical solutions in the embodiments described below are exemplary and only possible technical implementations of this application, not all possible implementations. Those skilled in the art can combine the embodiments of this application to obtain other embodiments without creative effort, and these embodiments are also within the protection scope of this application. It should be noted that in the description of this application, ordinal numbers or successive terms such as "step one", "step two", "first", "second", "then", "finally", "(1), (2)..." "S1, S2..." are used for the steps involved in the preparation process. Such expressions are only for the convenience of description and reference, and do not indicate or imply that the steps must be strictly performed in this specific order. Unless otherwise expressly limited in the text, those skilled in the art can reasonably change, combine or decompose the order of certain steps according to the process conditions, equipment layout or production efficiency considerations in actual application. Such adjustments do not depart from the substantive protection scope of the technical solution of this application.

[0019] The first aspect of this application discloses a method for preparing biomass-based hard carbon anode materials. This method constructs a special metal ion-cellulose coordination system, achieving uniform mixing and dispersion of multiple metal ions in a biomass precursor at the molecular scale through liquid-phase coordination, and anchoring them onto a cellulose template. Simultaneously, by utilizing the differentiated behaviors of different metal ions during pyrolysis and carbonization, it synergistically completes the in-situ construction of nanopores and atomic-level doping of sodium-loving active sites in one step, thereby obtaining a biomass-based hard carbon anode material with excellent sodium storage performance. This method features simple process steps, mild and controllable conditions, and avoids the cumbersome acid washing post-treatment steps in traditional processes, offering significant advantages in cost and environmental friendliness.

[0020] The preparation method of the biomass-based hard carbon anode material in this application mainly includes the following steps: S1. Prepare a metal-ammonia complex solution.

[0021] In this step, the metal-ammonia complex solution refers to a mixed solution of metal-ammonia complexes formed by the complexation of ions of two types of metals with different boiling points at high temperatures with ammonia water. Specifically, this metal-ammonia complex solution uses ammonia water as a solvent and dissolves at least one first metal ion and at least one second metal ion, ensuring uniform dispersion of metal ions at the solution level and exhibiting excellent dispersibility.

[0022] In this application, the subsequent pyrolysis carbonization temperature is defined as temperature T. The first metal ion is formed by a volatile metal with a boiling point not exceeding temperature T, and it mainly plays a role in high-temperature pore formation. Within the subsequent pyrolysis carbonization temperature range, its corresponding elemental metal or compound has a high saturated vapor pressure and is a type of metal ion that can escape or volatilize from the carbon matrix in gaseous form. During the pyrolysis carbonization process, the uniform escape behavior of these volatile metal ions can leave a large number of uniformly distributed in-situ nano-vacancy sites in the carbon skeleton, which evolve into closed pores that are conducive to sodium ion storage as the carbon layer structure rearranges. At the same time, its volatilization process avoids the residue of inactive metal impurities in the carbon product, thereby eliminating the need for acid washing and purification.

[0023] In some specific embodiments, considering the pyrolysis carbonization temperature range (1100-1500℃), the volatile metal is a metal with a boiling point not exceeding 1100℃, such as at least one of Mg and Zn, but not limited thereto. Specifically, magnesium has a boiling point of approximately 1090℃, and zinc has a boiling point of approximately 907℃, enabling effective volatilization and pore formation at conventional hard carbon pyrolysis temperatures. In practical applications, other volatile metal ions that can form metal-ammonia complexes with ammonia and exhibit similar volatilization behavior at pyrolysis temperatures can also be selected; this application does not exhaustively limit the range of such ions.

[0024] In this application, the second metal ion is formed from a non-volatile sodium-loving metal with a boiling point exceeding temperature T, primarily achieving metal species doping. Specifically, it is non-volatile at the corresponding pyrolysis carbonization temperature, can stably remain in the carbon framework, and exhibits good affinity or catalytic activity for sodium ions. During pyrolysis carbonization, these metal ions are reduced and anchored by surrounding carbon atoms, forming atomically dispersed metal single-atom or sub-nanometer cluster doping sites. These sodium-loving sites can significantly reduce the energy barrier required for sodium ions to insert and extract into hard carbon materials, improving the material's high-current charge-discharge capability, while also providing additional reversible capacity contribution.

[0025] In some specific embodiments, considering the pyrolysis carbonization temperature range (1100-1500℃), the non-volatile sodium-loving metal is a metal with a boiling point exceeding 1100℃, such as at least one of Cu, Mn, Ni, Fe, and Co, but is not limited thereto. The ions formed by the aforementioned transition metals all possess unpaired d-orbital electrons, enabling strong electronic interactions with the π-electron system in carbon materials, thereby optimizing the electronic structure of the carbon layer surface and enhancing the adsorption and catalytic conversion capabilities of sodium ions. In practical applications, other metal ions that are electrochemically active towards sodium, can form metal-ammonia complexes with ammonia, and can be stably doped under pyrolysis conditions can also be selected; this application does not exhaustively limit the range of such ions.

[0026] In this application, to ensure the uniform coexistence of the two types of metal ions in solution and their sufficient coordination with biomass materials, a preferred solution preparation method is provided. In some specific examples, the metal-ammonia complex solution is formed by thoroughly stirring an aqueous salt of a volatile metal and an aqueous salt of a non-volatile sodium-loving metal in ammonia water. Ammonia water plays a dual role in the system: (i) a metal complexing agent, as ammonia molecules can form stable ammonia complex ions with metal ions, allowing the metal ions to be uniformly dispersed in the form of complexed cations; (ii) the formed metal-ammonia complex solution subsequently serves as a cellulose solvent, as the metal complex coordinates with the hydroxyl groups (-OH) on cellulose molecules, thereby disrupting the strong hydrogen bond network between and within cellulose molecules, dissolving cellulose, and anchoring the metal to the cellulose template.

[0027] In this application, the water-soluble salt refers to a salt compound that can dissolve in water and form a homogeneous ionic solution. Specific examples include, but are not limited to, hydrochlorides, carbonates, and nitrates, which can be selected according to the type of metal. In some specific examples, the water-soluble salt is a carbonate. These water-soluble salts of metals are widely available, inexpensive, and have good compatibility with ammonia water systems.

[0028] It is understandable that the proportions of each component in the metal-ammonia complex solution have a significant impact on the uniformity of the final material structure. In some specific embodiments, the mass ratio of the water-soluble salt of the volatile metal to the water-soluble salt of the non-volatile sodium-loving metal is (10~30):(70~90), for example, any mass ratio or range between any two ratios such as 10:90, 15:85, 20:80, 25:75, and 30:70. Controlling the proportion of the volatile metal salt solution at a low level aims to ensure that a sufficient number of nanopores are generated during pyrolysis to contribute to sodium storage space, without causing a decrease in the strength of the carbon framework structure or failure of pore connectivity due to excessive volatile species. Conversely, controlling the proportion of the non-volatile sodium-loving metal salt solution at a high level ensures a sufficient density of sodium-loving active sites in the final hard carbon material, thereby effectively improving sodium ion transport kinetics and reversible capacity.

[0029] Furthermore, the mass ratio of the two salts to ammonia is 1:10. The concentration of ammonia is not specifically limited and can be a conventional concentration in the art, such as 15-25 wt%.

[0030] S2. Obtain a homogeneous solution in which metal ion ligands are coordinated with cellulose in biomass materials.

[0031] In this step, the biomass material is immersed in the metal-ammonia complex solution prepared in step S1 and stirred thoroughly to dissolve it. The coordination between the metal-ammonia complex ions and the functional groups on the cellulose molecular chains in the biomass forms a macroscopically uniform precursor solution with microscopically dispersed molecular-level dispersion. This is a uniform solution in which the metal ion ligands are coordinated with the cellulose in the biomass material. This is a key prerequisite for achieving the uniformity of the microstructure and the consistency of the electrochemical performance of the final biomass hard carbon material.

[0032] In this application, the homogeneous solution refers to a macroscopically clear, transparent, or translucent viscous liquid state that does not separate into layers or produce precipitation after standing, and where no obvious solid fiber particles can be observed under an optical microscope. The formation mechanism of this state lies in the fact that metal ions, complexed with ammonia, react with numerous hydroxyl groups (-OH) on the cellulose molecular chain through electrostatic attraction and coordination bonding, forming a stable cellulose-metal complex. This complex allows cellulose, which is originally insoluble in water, to dissolve in molecular form or swell and disperse in a highly dispersed aqueous medium, thereby achieving atomic-level uniform pre-dispersion of metal ions within the carbon source precursor.

[0033] In this application, there are no particular restrictions or requirements on the types of biomass materials used; any natural or man-made waste rich in cellulose can be used as the biomass raw material. Specific examples include, but are not limited to, at least one of the following: cellulose-containing agricultural waste, cellulose-containing chemical waste, and cellulose-containing industrial waste.

[0034] In some specific examples, the biomass material can be any one or a mixture of two or more of the following: waste textiles (such as cotton clothing scraps), waste packaging materials (such as corrugated boxes and waste cardboard), waste decorative materials (such as sawdust and wood chips), waste cotton, pine wood, poplar wood, bamboo, coconut shells, flax, ramie, bagasse, rice straw, wheat straw, α-cellulose, β-cellulose, microcrystalline cellulose, and carboxymethyl cellulose. It should be understood that any cellulose-containing biomass raw material capable of coordinating and dissolving with metal ammonia complex ions can be used in the technical solution of this application.

[0035] In this application, to ensure the full progress of the coordination reaction and obtain an ideal homogeneous solution, it is necessary to reasonably control process parameters such as the feed ratio, stirring time, and temperature. In some specific embodiments, the mass ratio of biomass material to metal-ammonia complex solution is 1:(4-15). For example, this mass ratio can be any ratio or a range between any two ratios, such as 1:4, 1:6, 1:8, 1:10, 1:12, and 1:15. If the amount of metal-ammonia complex solution used is too low (ratio less than 1:4), the biomass material may not be fully wetted and swollen, and some fibers may not be able to contact the metal ions, resulting in uneven coordination. If the amount used is too high (ratio greater than 1:15), it will lead to excessive waste of metal-ammonia complex solution and increased energy consumption in the subsequent drying process. Therefore, a suitable mass ratio range can ensure the dissolution effect while taking into account the economic efficiency of the process.

[0036] In some specific embodiments, the stirring and dissolving time is 0.5-12 hours. For example, it can be any value or a range between any two of 0.5 hours, 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, and 12 hours. A person skilled in the art can verify the stirring time experimentally. If the time is too short, the metal-ammonia complex ions will not coordinate completely with cellulose; if the time is too long, it will easily lead to cellulose degradation, causing a decrease in yield.

[0037] In some specific examples, the stirring and dissolution temperature is -20 to 30°C. For example, it can be any value from -20°C, -10°C, 0°C, 10°C, 20°C, 25°C, and 30°C, or any range between two temperatures, which can be selected according to needs. For instance, a low-temperature environment (such as -20 to 0°C) is beneficial for the metal-ammonia complex to disrupt the hydrogen bonds of cellulose. Operating at room temperature (20 to 30°C) has the advantages of simple process and low energy consumption. A suitable dissolution temperature can be selected according to specific research needs; the above temperature range provides a large adjustment window for process conditions.

[0038] S3. Wash and dry the resulting homogeneous solution to remove free uncoordinated components.

[0039] Specifically, after obtaining a homogeneous solution, a water washing process is required. The purpose of water washing is to remove free metal ions that have not effectively coordinated with cellulose, excess ammonia, and other soluble impurities from the solution, thereby obtaining biomass material with metal ion coordination. This step is not a traditional deep acid washing purification, but rather a simple purification and neutralization treatment of the precursor. Since most metal ions are already firmly bound to the cellulose molecular chain through coordination bonds, simple water washing will not lead to a significant loss of effective metal components. As a concrete example, the solution after dissolving the biomass can be directly added to water for washing.

[0040] The material is then dried to remove moisture, yielding a solid metal-cellulose coordination precursor. The drying method is not particularly limited; conventional methods in the art, such as forced-air drying, vacuum drying, or freeze drying, can be used. Specific conditions should be chosen to dry the material to a constant weight; this application does not impose strict limitations on these conditions. In some specific embodiments, the drying temperature is 50-90°C.

[0041] S4. The dried precursor is placed in a non-reactive atmosphere for pyrolysis and carbonization to obtain biomass-based hard carbon anode material.

[0042] In this step, on the one hand, the self-evaporation characteristics of low-boiling-point volatile metals at high temperatures are utilized to create pores in situ, thus eliminating the need for subsequent acid washing processes. Simultaneously, based on the atomic-level dispersion of metal ions in the precursor, they are successfully converted into single atoms after pyrolysis and carbonization, preventing aggregation. Specifically, single atoms lower the diffusion barrier and improve rate performance, while hierarchical pores increase capacity.

[0043] In this application, pyrolysis carbonization is carried out in a non-reactive atmosphere, which refers to a protective gaseous environment that does not participate in the oxidation or chemical reaction of the carbon material. Specifically, it can be one or a mixture of two or more of nitrogen, rare gases (such as argon, helium, etc.). There are no particular limitations or requirements on the specific pyrolysis carbonization temperature; it can be reasonably set according to the selected metal ions and the carbonization temperature of the biomass precursor. As a preferred example, the pyrolysis carbonization temperature is 1100-1500℃. For example, any temperature value or any range between two temperatures can be selected from 1100℃, 1150℃, 1200℃, 1250℃, 1300℃, 1350℃, 1400℃, 1450℃, and 1500℃. In this application, a high temperature range of 1100-1500℃ is selected to be compatible with the metal ions in the metal-ammonia complex solution. Specifically, this temperature range is much higher than the boiling point of volatile metals such as magnesium and zinc, which can ensure that they are fully volatilized and escaped, thereby efficiently constructing closed pores between carbon layers. At the same time, this temperature can promote sufficient graphitization and microcrystalline rearrangement and structural ordering of the carbon layers, improving the electronic conductivity of hard carbon materials. Furthermore, at this high temperature, the non-volatile transition metal ions are carbothermally reduced and form thermodynamically stable metal single atoms or sub-nanometer clusters, which are firmly anchored in the carbon defect sites, ensuring the long-term cycling stability of the active sites.

[0044] The specific holding time can be adjusted as needed. In some specific examples, the holding time is 0.5-4 hours. For example, it can be any value among 0.5 hours, 1 hour, 2 hours, 3 hours, and 4 hours, or a range between any two values. Too short a holding time may lead to incomplete carbonization and insufficient volatilization and pore formation; too long a holding time may lead to excessive densification of the carbon layer, and some closed pores may collapse due to carbon layer shrinkage. Controlling the holding time within a suitable range can achieve a better balance between the integrity of the carbon skeleton structure and the degree of pore development. Those skilled in the art can adjust it according to the actual situation.

[0045] Furthermore, the heating rate for pyrolysis carbonization can be a conventional carbonization heating rate in the art, such as 0.5-10 °C / min, without any particular limitations or requirements.

[0046] The second aspect of this application discloses a biomass-based hard carbon anode material, which is prepared using the method described above. This hard carbon anode material exhibits significant microstructural characteristics that distinguish it from products prepared by conventional methods. Specifically, the closed-pore volume of this biomass-based hard carbon anode material is 0.05~0.15 cm³. 3 ·g -1The closed pores mentioned here refer to isolated internal pores that are not directly connected to the outer surface of the material. These closed pores are formed by coating the nanocavities left by the in-situ escape of volatile metals with a carbon layer. They can provide effective space for the filling and storage of sodium ions, and are not prone to forming an unstable solid electrolyte interface film due to electrolyte side reactions during charging and discharging.

[0047] The closed-cell ratio of this biomass-based hard carbon anode material is 30-50%, where closed-cell ratio refers to the percentage of closed-cell volume to the total pore volume of the material. A high closed-cell ratio means that sodium ions are primarily stored in the closed pores within the carbon material, resulting in a storage mechanism with high reversibility and excellent cycle stability. Compared to hard carbon materials with a higher open-cell ratio, the hard carbon anode material prepared in this application, due to its high closed-cell ratio, can significantly reduce initial coulombic efficiency loss and improve the material's conductivity and rate performance.

[0048] This application further discloses a sodium-ion battery negative electrode containing a biomass-based hard carbon negative electrode material prepared by the method described above. Specifically, the biomass-based hard carbon negative electrode material is mixed with a conductive agent (such as acetylene black, Super P, etc.) and a binder (such as polyvinylidene fluoride PVDF, sodium carboxymethyl cellulose CMC, styrene-butadiene rubber SBR, etc.) in a certain proportion to form a slurry, which is then coated onto the surface of a current collector (such as copper foil, aluminum foil). After drying, rolling, and cutting, a sodium-ion battery negative electrode sheet is obtained. The conductive agent, binder, etc., can all be materials known in the art or independently developed without particular limitation.

[0049] This application also provides a sodium-ion battery comprising the aforementioned negative electrode. The sodium-ion battery typically also includes a positive electrode, a separator, and an electrolyte. The positive electrode active material, separator, and electrolyte can all be of types known in the art or independently developed without particular limitation. For example, the positive electrode active material can be a layered transition metal oxide, a polyanionic compound, or a Prussian blue analogue commonly used in the art; the separator can be glass fiber filter paper or a porous polyolefin film; the electrolyte is typically a carbonate or ether organic solution containing sodium salts (such as sodium perchlorate NaClO4 or sodium hexafluorophosphate NaPF6). Because the biomass-based hard carbon negative electrode material provided in this application has abundant closed pore volume and uniformly dispersed sodium-loving active sites, the assembled sodium-ion battery exhibits excellent rate performance and battery capacity in testing.

[0050] The present application will be further illustrated below with reference to specific embodiments. It should be noted that the specific embodiments below are for illustrative purposes only and do not limit the scope of the present application in any way.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0052] In addition, unless otherwise specified, methods without detailed conditions or steps are conventional methods, and the reagents and materials used are commercially available.

[0053] Example 1: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic copper carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic copper carbonate and basic zinc carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0054] Step (2) Preparation of biomass precursor: At 20°C, cotton (1 kg) and the metal-ammonia complex solution (10 kg) prepared in step (1) were mixed and stirred for 8 h to obtain a homogeneous solution of metal ion-coordinated biomass cellulose; the solution was directly added to deionized water for washing to obtain the biomass precursor; then, it was placed in a 60°C oven and dried to constant weight.

[0055] Step (3) Preparation of biomass-based hard carbon anode material: The dried biomass precursor in step (2) is placed in a high-temperature furnace and heated to 1300°C at a heating rate of 5°C / min. It is then kept at this temperature for 2 hours to pyrolyze and carbonize, thus obtaining the biomass-based hard carbon anode material.

[0056] Example 2: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0057] Step (2) Biomass precursor preparation: Refer to the implementation method of Example 1, except that cotton is replaced with an equal mass of bamboo.

[0058] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0059] Example 3: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0060] Step (2) Preparation of biomass precursor: Refer to the implementation method of Example 1, except that the stirring and dissolution time of cotton and metal-ammonia complex solution is 0.5h.

[0061] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0062] Example 4: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0063] Step (2) Preparation of biomass precursor: Refer to the implementation method of Example 1, except that the stirring and dissolution time of cotton and metal-ammonia complex solution is 12h.

[0064] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0065] Example 5: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic copper carbonate and basic magnesium carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic copper carbonate and basic magnesium carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0066] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0067] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0068] Example 6: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic nickel carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic nickel carbonate and basic zinc carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0069] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0070] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0071] Example 7: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic copper carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic copper carbonate and basic zinc carbonate is 90:10, and the mass ratio of the two salts to ammonia water is 1:10.

[0072] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0073] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0074] Example 8: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic copper carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic copper carbonate and basic zinc carbonate is 70:30, and the mass ratio of the two salts to ammonia water is 1:10.

[0075] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0076] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0077] Example 9: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0078] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0079] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1, the only difference is that: heat to 1100℃ at a heating rate of 3℃ / min, and keep at this temperature for 3 hours to pyrolyze and carbonize, and obtain biomass-based hard carbon anode material.

[0080] Example 10: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0081] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0082] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1, the only difference is that: heat to 1500℃ at a heating rate of 3℃ / min, and keep at this temperature for 3 hours to pyrolyze and carbonize, and obtain biomass-based hard carbon anode material.

[0083] Example 11: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic manganese carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic manganese carbonate and basic zinc carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0084] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0085] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0086] Example 12: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic ferric carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic ferric carbonate and basic zinc carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0087] Example 13: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Add basic cobalt carbonate and basic zinc carbonate to 20wt% ammonia water and stir thoroughly to prepare metal-ammonia complex solution, wherein the mass ratio of basic cobalt carbonate and basic zinc carbonate is 85:15, and the mass ratio of the two salts to ammonia water is 1:10.

[0088] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0089] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0090] Example 14: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0091] Step (2) Preparation of biomass precursor: Refer to the implementation method of Example 1, except that: cotton (1 kg) and the metal-ammonia complex solution (4 kg) prepared in step (1) are mixed and stirred.

[0092] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0093] Example 15: Biomass-based hard carbon anode material and its preparation Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0094] Step (2) Preparation of biomass precursor: Refer to the implementation method of Example 1, except that: cotton (1 kg) and the metal-ammonia complex solution (15 kg) prepared in step (1) are mixed and stirred.

[0095] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0096] Comparative Example 1 This comparative example uses the same implementation method as Example 1, except that the cotton is not treated.

[0097] Specifically, cotton is placed directly in a high-temperature furnace and heated to 1300°C at a heating rate of 5°C / min, held at that temperature for 2 hours, and then pyrolyzed and carbonized to obtain biomass-based hard carbon anode material.

[0098] Comparative Example 2 This comparative example uses the same implementation method as Example 1, except that the stirring and dissolution time of the cotton and metal-ammonia complex solution is 0.1 h.

[0099] The specific steps are as follows: Step (1) Preparation of metal-ammonia complex solution: Refer to Example 1.

[0100] Step (2) Preparation of biomass precursor: Refer to the implementation method of Example 1, except that the stirring and dissolution time of cotton and metal-ammonia complex solution is 0.1h.

[0101] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0102] Comparative Example 3 This comparative example uses the same implementation method as Example 1, except that the ratio of metal salts in the metal-ammonia complex solution is different.

[0103] The specific steps are as follows: Step (1) Preparation of metal-ammonia complex solution: The mass ratio of basic copper carbonate and basic zinc carbonate is 60:40.

[0104] Step (2) Preparation of biomass precursor: Refer to Example 1.

[0105] Step (3) Preparation of biomass-based hard carbon anode material: Refer to Example 1.

[0106] Material characterization and performance testing a: Porosity test The porosity of the biomass-based hard carbon anode materials prepared in the examples and comparative examples was tested, and the results are shown in Table 1: Table 1 Porosity test results of biomass-based hard carbon anode materials

[0107] As shown in Table 1, this application achieves uniform mixing of multiple metal ions at the molecular level through liquid-phase coordination and anchors them onto the cellulose template. Through in-situ self-sacrificing templates, low-boiling-point volatile metals are uniformly distributed on the cellulose template, and their self-evaporation properties at high temperatures create pores in situ, resulting in a uniform pore size distribution. Compared to Comparative Examples 1-3, the biomass-based hard carbon anode material in this embodiment possesses a rich closed-pore structure. This is mainly due to the addition of appropriate volatile metals that self-evaporate at high temperatures to form a porous structure. High-temperature graphite domains bend and shrink into closed pores, thus exhibiting high sodium storage capacity.

[0108] b: TEM characterization Figure 2 TEM characterization results of the biomass-based hard carbon anode materials in Example 1 and Comparative Example 1 are presented respectively. It can be seen that Example 1 exhibits abundant nanopores, with rich, long graphite-like layers and numerous closed pores, which stack to form a turbine layer of closed void domains. The abundant closed pores are related to the addition of pore-forming metal ions; during pyrolysis and carbonization, low-boiling-point metals volatilize, forming pores surrounded by long graphite-like layers to enclose and shrink the sites, forming closed sodium storage sites. In contrast, the microstructure of Comparative Example 1 is highly disordered, and it is difficult to identify obvious closed pore regions and long graphite-like layers; it has only a few nanopores.

[0109] c: Electrochemical performance testing Battery preparation: Biomass-based hard carbon anode material was mixed uniformly with conductive agent SuperP, binder sodium carboxymethyl cellulose, and styrene-butadiene rubber at a mass ratio of 8:1:0.5:0.5. A suitable amount of water was added and stirred to form a slurry. The slurry was then uniformly coated onto copper foil as a current collector. After drying, it was cut into circular electrode sheets with a diameter of 14 mm. Under vacuum conditions, the electrode sheets were dried at 80°C for approximately 3 hours with forced air drying, followed by vacuum drying at 80°C for 6 hours. They were then transferred to a glove box for later use. Battery assembly was carried out in an Ar atmosphere glove box, using a sodium metal sheet as the counter electrode, 1 mol / L NaPF6 dissolved in DME solution as the electrolyte, and GF / D as the separator, to assemble a CR2032 button cell. The battery was then heated in a 30°C oven at 0.025 A·g⁻¹. -1 The current density cycling was performed, and the rate testing was conducted at 0.025, 0.05, 0.1, 0.3, 0.5, and 1 A·g. -1 The test was conducted at a current density of [insert current density here]. The test results are shown in Table 2.

[0110] Table 2 Electrochemical performance of CR2032 button cells

[0111] As shown in Table 2, the biomass-based hard carbon anode material in this application significantly improves the electrochemical performance of the battery. Furthermore, combined with... Figure 3 and Figure 4 Compared to Comparative Examples 1-3, the batteries prepared with biomass-based hard carbon anode materials in these examples show significantly improved charge specific capacity and initial charge-discharge efficiency. Meanwhile, as... Figure 5 and 6 As shown, compared to Comparative Examples 1-3, the rate performance of the batteries prepared with biomass-based hard carbon anode materials in the examples is significantly improved.

[0112] Based on the above results, it can be concluded that the biomass-based derived hard carbon treated with the metal-ammonia complex solution formed by the two types of metals in this application can significantly improve the electrochemical performance of the material. This is mainly due to the atomic-level dispersion of the non-volatile sodium-loving metal ions in the precursor, which are successfully converted into single atoms after pyrolysis and carbonization, avoiding agglomeration. The single atoms reduce the diffusion barrier and improve the conductivity and rate performance of the hard carbon material. At the same time, the volatile metals form pores in situ, and the resulting hierarchical pores improve the battery capacity.

[0113] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing a biomass-based hard carbon anode material, characterized in that, Includes the following steps: Add at least one first metal ion and at least one second metal ion to ammonia water, and allow them to dissolve completely to form a metal-ammonia complex solution; The biomass material is immersed in the metal-ammonia complex solution and stirred thoroughly to dissolve, thereby obtaining a homogeneous solution in which the metal ions coordinate with the cellulose in the biomass material. The homogeneous solution was washed with water and dried, followed by pyrolysis and carbonization to obtain a biomass-based hard carbon anode material. Wherein, the first metal ion is formed from a volatile metal with a boiling point not exceeding temperature T, and the second metal ion is formed from a non-volatile sodium-loving metal with a boiling point exceeding temperature T, wherein temperature T is the pyrolysis carbonization temperature.

2. The preparation method according to claim 1, characterized in that, The volatile metal is at least one of Mg and Zn; And / or, the non-volatile sodium-loving metal is at least one of Cu, Mn, Ni, Fe, and Co.

3. The preparation method according to claim 1, characterized in that, The metal-ammonia complex solution is formed by adding water-soluble salts of volatile metals and water-soluble salts of non-volatile sodium-loving metals to ammonia water and stirring thoroughly. The mass ratio of the water-soluble salt of the volatile metal to the water-soluble salt of the non-volatile sodium-loving metal is (10~30):(70~90), and the mass ratio of the two water-soluble salts to ammonia is 1:

10.

4. The preparation method according to claim 1, characterized in that, The biomass material is at least one of cellulose-containing agricultural waste, cellulose-containing chemical waste, and cellulose-containing industrial waste.

5. The preparation method according to claim 1, characterized in that, The biomass material is one or a mixture of two or more of the following: waste textiles, waste packaging materials, waste decorative materials, waste cotton, pine wood, poplar wood, bamboo, coconut shell, flax, ramie, sugarcane bagasse, rice straw, wheat straw, α-cellulose, β-cellulose, microcrystalline cellulose, and carboxymethyl cellulose.

6. The preparation method according to claim 1, characterized in that, The mass ratio of the biomass material to the metal-ammonia complex solution is 1:(4-15), the stirring and dissolving time is 0.5-12h, and the stirring and dissolving temperature is -20~30℃.

7. The preparation method according to claim 1, characterized in that, The pyrolysis carbonization is carried out in a non-reactive atmosphere at a temperature of 1100-1500℃ for a holding time of 0.5-4h.

8. A biomass-based hard carbon anode material, characterized in that, The biomass-based hard carbon anode material prepared by the preparation method according to any one of claims 1-7 has a closed pore volume of 0.05~0.15 cm³. 3 ·g -1 The closed-pore rate is 30-50%.

9. A sodium-ion battery negative electrode, characterized in that, It contains the biomass-based hard carbon anode material as described in claim 8.

10. A sodium-ion battery, characterized in that, It contains the sodium-ion battery negative electrode as described in claim 9.

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