A pine-based hard carbon anode material, its preparation method and application

Abstract

This invention belongs to the field of sodium-ion battery technology and discloses a pine-based hard carbon anode material and its preparation method and application. (1) Pretreatment: Pine raw materials from the cold region of Inner Mongolia are crushed and cleaned to obtain pretreated pine raw materials; (2) High-temperature carbonization is used to construct a carbon skeleton to obtain a hard carbon precursor; (3) Room-temperature KOH alkalization is used to regulate the interlayer spacing and closed-pore structure to obtain a hard carbon material; (4) Electrode preparation: The hard carbon material is mixed with a conductive agent and a binder, and N-methyl-2-pyrrolidone is used as a solvent to make a slurry. The slurry is coated on a copper foil current collector and dried to obtain a pine-based hard carbon anode material. This invention develops a new process for high-temperature carbonization to construct a stable skeleton in conjunction with room-temperature KOH alkalization for precise control. The room-temperature alkalization process reduces the overall energy consumption of the preparation by more than 65%. The raw materials are agricultural and forestry waste from cold regions, which are cheap, readily available, green and sustainable. The process is simple and controllable and has excellent prospects for industrial scale-up.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and more specifically to a pine-based hard carbon anode material for sodium-ion batteries and its preparation method. Background Technology

[0002] Under the "dual carbon" strategy, large-scale energy storage technology has become a key support for the development of renewable energy. Sodium-ion batteries are an ideal candidate for large-scale energy storage systems due to the abundance and wide distribution of sodium resources, low cost, and similar electrochemical working principle to lithium-ion batteries. They have broad application prospects in energy storage power stations, low-speed electric vehicles, and other fields.

[0003] Hard carbon materials, with their amorphous carbon layer structure, well-developed pore defects, and good chemical stability, have become the preferred anode material for the commercial application of sodium-ion batteries. They primarily achieve sodium ion storage through multiple mechanisms including intercalation, filling, and adsorption. The carbon interlayer spacing and closed-pore structure of hard carbon materials are the core factors determining their sodium storage performance. An ideal hard carbon anode material needs to possess both a large carbon interlayer spacing and a high proportion of closed-pore structures: a large carbon interlayer spacing can lower the barrier to sodium ion intercalation and diffusion, improving ion transport kinetics; a high proportion of closed-pore structures can serve as effective filling sites for sodium ions, significantly increasing the specific capacity of sodium storage, while reducing electrolyte over-wetting and improving the initial coulombic efficiency.

[0004] Existing hard carbon anode materials are mostly prepared using a synergistic process of "high-temperature carbonization + high-temperature alkalization / etching". Although this can achieve a certain structural control effect, this type of process has obvious drawbacks: First, high-temperature alkalization has high energy consumption, which significantly increases the cost of industrial production and has stringent requirements for the equipment's high-temperature resistance and corrosion resistance. Second, high-temperature alkalization is prone to excessive etching of the carbon skeleton, causing the collapse of the closed-cell structure and making it impossible to form a stable high proportion of closed cells, which affects the material's cycle stability. Third, the high-temperature alkalization reaction rate is fast, and the etching uniformity is poor, making it difficult to accurately control the carbon interlayer spacing. This can easily lead to problems such as excessively large or small interlayer spacing, resulting in an imbalance between ion transport and sodium storage performance. Fourth, existing biomass-based hard carbon materials mostly use ordinary temperate / subtropical biomass raw materials without considering the influence of the growth environment on the raw material composition and structure. The lignin content and fiber density of the raw materials are insufficient, making them unsuitable for mild structural control processes, which further limits the development of room-temperature alkalization processes.

[0005] Biomass-based hard carbon uses agricultural and forestry waste as raw materials, offering advantages such as low cost and green sustainability. Pine trees, as a common agricultural and forestry waste, possess natural tubular channels and lignin fiber networks, making them an excellent precursor for preparing hierarchical porous hard carbon. However, current technologies lack research on hard carbon preparation from pine trees in the cold regions of Inner Mongolia. Due to their long growth cycle and low-temperature growth stress, pine trees in this region have higher lignin content and denser fiber structures, resulting in a more stable graphitized carbon skeleton after carbonization. Furthermore, current technologies have not achieved synergistic control of "high-temperature carbonization + room-temperature alkalization," making it difficult to obtain hard carbon materials with large interlayer spacing and high stability closed pores under conditions of low energy consumption and low equipment requirements. This makes it impossible to simultaneously achieve the high capacity, high rate, and long cycle performance of sodium-ion batteries.

[0006] Therefore, developing a method for preparing hard carbon anode materials by combining high-temperature carbonization and room-temperature alkalization techniques using pine wood from the cold regions of Inner Mongolia is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] In view of this, the present invention provides a pine-based hard carbon anode material, its preparation method, and its application. The purpose of this invention is to overcome the defects of existing hard carbon anode material preparation processes and provide a pine-based hard carbon anode material for sodium-ion batteries and its preparation method. Through the synergistic effect of high-temperature carbonization and room-temperature KOH alkalization, combined with the unique raw material advantages of pine trees in the cold regions of Inner Mongolia, precise and mild control of the interlayer spacing and closed-pore structure of the hard carbon material is achieved. This reduces energy consumption and equipment requirements while obtaining hard carbon materials with electrochemical performance significantly superior to traditional high-temperature alkalization processes.

[0008] One objective of this invention is to provide a method for preparing pine-based hard carbon anode material, the specific steps of which include:

[0009] (1) Pretreatment: Pine raw materials from the cold region of Inner Mongolia are crushed into small pieces, washed repeatedly with deionized water 3 times to remove impurities and soluble ash, and dried at 60℃ for 12 hours to constant weight to obtain pretreated pine raw materials. The beneficial effects of the above-mentioned technical means are: removing impurities from the raw materials and ensuring the uniformity of the subsequent carbonization and alkalization processes; and the raw materials of pine trees in the cold regions of Inner Mongolia do not require complicated deashing treatment, and the ash content can be controlled at a low level simply by washing with water, thus simplifying the process steps.

[0010] (2) High-temperature carbonization to construct carbon skeleton: The pretreated pine raw material is placed in an alumina crucible and placed in a tube furnace for high-temperature carbonization. It is then naturally cooled to room temperature to obtain a hard carbon precursor with a graphitized dense carbon skeleton structure and a basic closed-pore structure. The beneficial effects of the above-mentioned technical methods are as follows: High-temperature carbonization at 900-1500℃ is the core step in constructing a stable carbon framework. Slow heating and high-temperature holding allow the precursors from cold-region pine trees to fully remove volatiles, promoting deep rearrangement of the carbon framework and forming a dense carbon framework of graphitized microcrystals. This avoids the problem of excessive defects in low-temperature carbonization and provides a stable supporting structure for subsequent room-temperature alkali etching, which is the basis for effective etching in room-temperature alkali etching. At the same time, the rearrangement of the carbon framework during high-temperature carbonization can naturally form a basic closed-pore structure of micropore-mesopore composite, providing initial sites for sodium ion storage. The carbon interlayer spacing of this hard carbon precursor is 0.36~0.38 nm, and the specific surface area is 200~600 m². 2 / g.

[0011] (3) Room temperature KOH alkalization to regulate interlayer spacing and closed-pore structure: The hard carbon precursor was placed in a planetary ball mill and ball milled at 1100 r / min for 8 h to increase the specific surface area, so that the subsequent KOH etching solution could penetrate into the particle evenly and ensure the uniformity of etching. After ball milling, the hard carbon particles were dried at 60°C, immersed in KOH aqueous solution, and etched by stirring at room temperature for 12 h. After etching, the particles were filtered and dried at 60°C. Then, the particles were immersed in HCl aqueous solution to acid wash, neutralize the residual KOH, and remove the inorganic salt impurities generated during the etching process. After acid washing, the particles were filtered and dried at 60°C. The acid-washed hard carbon material was mixed with ultrapure water and washed four times by centrifugation at 8000 r / min for 10 min each time, for a total of four washes, to thoroughly remove residual acid radicals and salt impurities. After filtration and drying, a hard carbon material with large interlayer spacing and high closed pores was obtained.

[0012] The beneficial effects of the above-mentioned technical methods are as follows: Room-temperature KOH alkalization, based on the dense carbon framework constructed by high-temperature carbonization, achieves precise and gentle control of the hard carbon structure. On the one hand, KOH, as a strong activator, preferentially etches amorphous carbon defects at room temperature, stripping away local carbon layers and further expanding the carbon layer spacing from 0.36~0.38 nm to 0.37~0.40 nm, providing ample intercalation and transport channels for sodium ions. On the other hand, the room-temperature alkalization reaction rate is mild, allowing for precise modification and enrichment of the basic closed-pore structure, increasing the closed-pore ratio to 30%~60%, while avoiding the collapse of the unsupported framework or etching at high temperatures, forming a stable microporous-mesoporous composite closed-pore structure. Furthermore, room-temperature KOH alkalization can introduce abundant CO oxygen-containing functional groups onto the surface of hard carbon materials, providing additional chemisorption sites for sodium ions.

[0013] (4) Electrode preparation: The hard carbon material, Super P conductive agent and PVDF binder are mixed in mass ratio and slurry is prepared with N-methyl-2-pyrrolidone (NMP) as solvent. The slurry is coated on copper foil current collector with a thickness of 150 μm and vacuum dried at 120℃ for 12 h to obtain pine-based hard carbon negative electrode with large interlayer spacing and high closed pores. The coated copper foil can be cut into circular negative electrode sheets with a diameter of 12 mm according to the requirements for battery assembly.

[0014] Preferably, in step (1), the lignin content of the pine raw material from the cold region of Inner Mongolia is 32%~38%, and the cellulose content is 35%~39%.

[0015] Preferably, in step (1), the material is crushed to 1~2 cm.

[0016] Preferably, in step (2), the specific operation of high-temperature carbonization is as follows: the pretreated pine raw material is placed in a tube furnace, argon gas is introduced for protection, the gas flow rate is 100 mL / min, and the temperature is heated to 900-1500℃ at a heating rate of 2℃ / min, and kept at the temperature for 2h.

[0017] A further preferred method is to purge the furnace with argon gas for 1 hour before introducing argon gas to remove oxygen and prevent oxidation of the pine raw material during carbonization.

[0018] Further optimization reveals that the optimal temperature for high-temperature carbonization is 1500℃. At this temperature, the carbon skeleton exhibits the best compactness and graphitization degree, resulting in the best structural regulation effect during subsequent room-temperature alkalization.

[0019] Preferably, in step (3), the concentration of the KOH aqueous solution is 1-5 mol / L, the optimal concentration is 3 mol / L, and the optimal room temperature etching time is 12 h. Under these conditions, the etching effect and structural stability of the alkali are optimal, and the electrochemical performance of the hard carbon material is the best.

[0020] Preferably, the stirring rate for room temperature etching is 200 r / min; the liquid-to-solid ratio of the KOH aqueous solution to the hard carbon precursor is 30 mL / g.

[0021] Preferably, in step (3), the concentration of the HCl aqueous solution is 2 mol / L and the acid washing time is 3 h.

[0022] Preferably, in step (3), the particle size D of the hard carbon precursor after ball milling is... 50 It is 60 μm.

[0023] Preferably, in step (4), the mass ratio is 8:1:1 and the solid content of the pulp is 27%.

[0024] The second objective of this invention is to provide a pine-based hard carbon anode material, which is the result of synergistic regulation of high-temperature carbonization and room-temperature KOH alkalization, exhibiting a unique microstructure and excellent electrochemical performance. Its core features are: Structural characteristics: The hard carbon anode material has a carbon interlayer spacing of 0.37~0.40 nm, a closed-cell ratio of 30%~60%, and a total pore volume of 0.2~0.6 cm³. 3 / g; The material surface is rich in CO oxygen-containing functional groups, and the interior is a graphitized carbon layer structure; The large interlayer spacing forms a continuous and spacious sodium ion transport channel, reducing ion diffusion resistance. The high proportion of stable closed pores constructs sufficient sodium ion filling and storage sites, and the CO oxygen-containing functional groups provide additional chemisorption sites.

[0025] Performance characteristics: At a current density of 0.1 A / g, the initial reversible specific capacity is ≥396 mAh / g, the plateau capacity is ≥254 mAh / g, the capacity retention rate after 100 cycles is ≥99%, and the cycle decay rate is ≤0.01% / cycle; at a current density of 1 A / g, the capacity retention rate after 500 cycles is ≥85%, and the specific capacity is still above 90 mAh / g at a high current density of 5 A / g, combining high specific capacity, high cycle stability and excellent rate performance.

[0026] Process-derived characteristics: The preparation process does not require high-temperature alkalization, and the energy consumption is reduced by more than 65% compared with the traditional high-temperature alkalization process. The raw material is agricultural and forestry waste from the cold region of Inner Mongolia, which has low ash content and excellent structure, and does not require complex pretreatment.

[0027] The third objective of this invention is to provide an application of pine-based hard carbon anode material in sodium-ion batteries. The sodium-ion battery includes pine-based hard carbon anode material, as well as a sodium-ion battery cathode, electrolyte, and separator. The components are assembled according to conventional sodium-ion battery assembly processes to form a sodium-ion half-cell or full-cell.

[0028] Preferably, the positive electrode is sodium iron pyrophosphate (NFPP), the electrolyte is diethylene glycol dimethyl ether + NaPF6 electrolyte, and the separator is a glass fiber separator. The components have good compatibility and the full cell has excellent electrochemical performance: the capacity retention rate is ≥99% after 100 cycles at a current density of 0.1 A / g, and the cycle decay rate is ≤0.01% / cycle, making it suitable for large-scale energy storage applications.

[0029] As can be seen from the above technical solution, compared with the prior art, the beneficial effects achieved by the present invention include at least the following: Technological innovation, low energy consumption and low equipment requirements: For the first time, the synergistic control of "high temperature carbonization + room temperature KOH alkalization" is achieved, replacing the traditional 400~900℃ high temperature alkalization process. The overall energy consumption is reduced by more than 65%, and there is no need for high temperature resistant and high corrosion alkalization equipment, which significantly reduces the cost of industrial production. Moreover, the room temperature alkalization reaction rate is mild, easy to control and easy to scale up, solving the problem of the difficulty of industrialization of existing processes. Innovative raw materials with superior structure and strong adaptability: For the first time, pine trees from the cold regions of Inner Mongolia are selected as biomass precursors. Due to the low-temperature growth environment, the lignin content of this raw material is 32%~38%, and the fiber structure is dense, providing a stable supporting structure for room temperature alkalization, which is the key to achieving effective etching at room temperature. In addition, the raw material has low ash content and does not require complicated deashing treatment, further simplifying the process. The raw material is agricultural and forestry waste, which is inexpensive, readily available, green and sustainable. Precise structural control and significantly improved performance: Room-temperature KOH alkalization, based on a stable carbon framework, enables precise control of carbon interlayer spacing (0.37~0.40 nm) and closed-pore structure (30%~60%), avoiding excessive etching and structural collapse caused by high-temperature alkalization. The prepared hard carbon material achieves the synergistic effect of intercalation, filling, and adsorption triple sodium storage mechanisms, and its electrochemical performance is significantly better than that of traditional high-temperature alkalization products, with an initial reversible specific capacity of ≥396 mAh / g at 0.1 A / g and a capacity retention of ≥85% after 500 cycles at 1 A / g. Balancing performance and cost, with promising industrialization prospects: This invention features a simple and controllable process with precise optimization of parameters at each step. It requires no complex equipment, uses inexpensive raw materials and reagents, and produces a hard carbon anode material with good compatibility with sodium iron pyrophosphate cathodes and conventional electrolytes. The assembled full cell exhibits excellent cycle stability, providing a high-quality anode material option for the large-scale commercial application of sodium-ion batteries and possessing significant industrial application value. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0031] Figure 1 This is a SEM image of the hard carbon anode material prepared in Example 1 of the present invention.

[0032] Figure 2 Figure 1 shows the HRTEM image of the hard carbon anode material prepared in Example 1 of the present invention. Figure 2a is a high-resolution transmission electron microscope image of Comparative Example 1, and Figure 3b is the analysis of the carbon layer lattice fringe spacing calculated from the HRTEM image.

[0033] Figure 3 The images show the EIS diagrams of the hard carbon anode materials prepared in Examples 1, 2, and 3 of this invention. Figure 4 Figure 1 shows the CV curves of the hard carbon anode material prepared in Example 1 of the present invention at 0.1 mV / s and multiple scan rates. Figure 2a shows the CV curves of the first three scans (1st, 2nd, 3rd) of Example 1 at a scan rate of 0.1 mV / s. Figure 3b shows the CV curves of Example 1 at different scan rates (0.1, 0.2, 0.4, 0.8, 1 mV / s). Figure 5 The diagram shows the ion diffusion coefficients of the hard carbon anode materials prepared in Examples 1, 2, and 3 of this invention. Figure 6 Figure a shows the first charge-discharge curves of the hard carbon anode materials prepared in Examples 1, 2, and 3 of this invention at a current density of 0.1 A / g, and the corresponding ramp capacity and plateau capacity. Figure a is the first constant current charge-discharge curve of Examples 1, 2, and 3 at a current density of 0.1 A / g, and Figure b is a bar chart of the ramp capacity and plateau capacity distribution of the corresponding charge-discharge curves. Figure 7 The graph shows the cycling performance of the hard carbon anode materials prepared in Examples 1, 2, and 3 of this invention at a current density of 0.1 A / g. Figure 8 The rate performance diagrams of the hard carbon anode materials prepared in Examples 1, 2 and 3 of the present invention at different current densities are shown. Figure 9 The graph shows the cycling performance of the hard carbon anode material prepared in Example 1 of this invention at a current density of 1 A / g.

[0034] Figure 10 The graph shows the cycling performance of the hard carbon anode material prepared in Example 2 of this invention at a current density of 1 A / g.

[0035] Figure 11 The graph shows the cycling performance of the hard carbon anode material prepared in Example 3 of this invention at a current density of 1 A / g.

[0036] Figure 12 The graph shows the cycle performance of the hard carbon anode materials prepared in Examples 1, 2 and 3 of this invention in a full cell assembled with NFPP.

[0037] Figure 13 This is a SEM image of the hard carbon anode material prepared in Comparative Example 1 of the present invention.

[0038] Figure 14Figure 1 shows the HRTEM image of the hard carbon anode material prepared in Comparative Example 1 of this invention. Figure 1a is a high-resolution transmission electron microscope image of Comparative Example 1, and Figure 1b is the analysis of the carbon layer lattice fringe spacing calculated from the HRTEM image.

[0039] Figure 15 The graph shows the cycling performance of the hard carbon anode material prepared in Comparative Example 1 of this invention at a current density of 1 A / g.

[0040] Figure 16 The graph shows the cycling performance of the hard carbon anode material prepared in Comparative Example 2 of this invention at a current density of 1 A / g.

[0041] Figure 17 The graph shows the cycling performance of the hard carbon anode material prepared in Comparative Example 3 of this invention at a current density of 1 A / g.

[0042] Figure 18 This is a SEM image of the hard carbon anode material prepared in Comparative Example 2 of the present invention.

[0043] Figure 19 Figure 1 shows the HRTEM image of the hard carbon anode material prepared in Comparative Example 2 of the present invention. Figure 2a is a high-resolution transmission electron microscope image of Comparative Example 2, and Figure 2b is the analysis of the carbon layer lattice fringe spacing calculated from the HRTEM image.

[0044] Figure 20 This is a SEM image of the hard carbon anode material prepared in Comparative Example 3 of the present invention.

[0045] Figure 21 Figure 1 shows the HRTEM image of the hard carbon anode material prepared in Comparative Example 3 of the present invention. Figure 2a is a high-resolution transmission electron microscope image of Comparative Example 3, and Figure 3b is the analysis of the carbon layer lattice fringe spacing calculated from the HRTEM image.

[0046] Figure 22 This is a SEM image of the hard carbon anode material prepared in Example 3 of the present invention.

[0047] Figure 23 EIS diagrams of the hard carbon anode materials prepared in Example 1 of the present invention and Comparative Examples 1, 2, and 3; Figure 24 Figure a shows the first charge-discharge curves of the hard carbon anode materials prepared in Example 1 and Comparative Examples 1, 2, and 3 of this invention at a current density of 0.1 A / g, and the corresponding ramp capacity and plateau capacity. Figure a is the first constant current charge-discharge curve of Example 1 and Comparative Examples 1, 2, and 3 at a current density of 0.1 A / g. Figure b is a bar chart of the ramp capacity and plateau capacity distribution of the corresponding charge-discharge curves. Figure 25The graph shows the cycling performance of the hard carbon anode materials prepared in Example 1 and Comparative Examples 1, 2, and 3 at a current density of 0.1 A / g. Figure 26 The graphs show the rate performance of the hard carbon anode materials prepared in Example 1 and Comparative Examples 1, 2, and 3 of this invention at different current densities.

[0048] Figure 27 Figure 1 shows the BET test results of the hard carbon anode material prepared in Example 1 of this invention under N2 adsorption-desorption and CO2 adsorption-desorption tests; Figure 2a is the N2 adsorption-desorption curve of Example 1, Figure 2b is the pore size distribution of N2 adsorption-desorption of Example 1, Figure 2c is the CO2 adsorption-desorption curve of Example 1, and Figure 2d is the pore size distribution of CO2 adsorption-desorption of Example 1. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] To address the shortcomings of existing technologies, this invention utilizes pine trees from the cold regions of Inner Mongolia as a unique precursor and develops a new process of "high-temperature carbonization to construct a stable framework + room-temperature KOH alkalization for precise control," replacing the traditional high-temperature alkalization process. This solves the problems of high energy consumption, easy collapse of the carbon framework, and difficulty in performance control in existing processes. At the same time, by leveraging the raw material advantages of cold-region pine trees, the electrochemical performance of hard carbon materials is significantly improved, providing a new solution for the industrial production of low-cost, high-performance hard carbon anodes for sodium-ion batteries.

[0051] This invention uses pine trees from the cold regions of Inner Mongolia as a biomass precursor. It employs a synergistic process—high-temperature carbonization to construct a stable carbon framework and room-temperature KOH alkalization for precise structural control—to replace the traditional high-temperature alkalization process. This achieves efficient control over the interlayer spacing and closed-pore structure of hard carbon materials. Large interlayer spacing and highly stable closed-pore hard carbon materials can be obtained without high-temperature heating, with interlayer spacing reaching 0.37–0.40 nm and closed-pore content ranging from 30% to 60%. The prepared pine-based hard carbon anode material possesses both excellent sodium ion transport channels and sufficient sodium storage active sites, achieving high performance at 0.1 A g. -1 The first reversible specific capacity at the current density is ≥396 mAh g. -1 1 Ag -1The capacity retention rate is ≥85% after 500 cycles at current density, and the electrochemical performance is significantly better than that of hard carbon materials prepared by traditional high-temperature alkalization process. At the same time, the room temperature alkalization process reduces the overall energy consumption of preparation by more than 65%. The raw materials are agricultural and forestry waste from cold regions, which are cheap, readily available, green and sustainable. The process is simple and controllable, and has excellent prospects for industrial scale-up.

[0052] The present invention conducts a series of tests on the prepared pine-based hard carbon anode material, and the test methods are as follows: 1. Electrochemical kinetic characterization: Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to analyze the reversibility of electrochemical reactions, storage ion kinetics and interfacial impedance characteristics of the materials.

[0053] 2. Electrochemical performance testing: The initial charge-discharge specific capacity, cycle stability, and rate performance of the material are evaluated through constant current charge-discharge testing (GCD). 3. Microstructure characterization: Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the microstructure, carbon layer structure and interlayer spacing of the material.

[0054] Material source: The pine trees planted on the campus of Inner Mongolia University of Technology are used as raw materials in this invention. According to preliminary research, the lignin content is 32%~38% and the cellulose content is 35%~39%. Specific values ​​within this range can be selected, and will not be elaborated here.

[0055] Example 1 A method for preparing a pine-based hard carbon anode material for sodium-ion batteries includes the following steps: (1) Pretreatment: Pine raw materials from the cold region of Inner Mongolia were crushed into small pieces of 1-2 cm, washed three times with deionized water to remove surface impurities, and dried in a forced-air drying oven at 60℃ for 12 h until constant weight was obtained to obtain pretreated pine raw materials with a lignin content of 32%; (2) High-temperature carbonization to construct the carbon framework: The pretreated pine raw material was placed in an alumina crucible and then placed in a tube furnace. Argon gas was first introduced for 1 h to remove oxygen from the furnace, followed by argon gas protection at a flow rate of 100 mL / min. The temperature was increased to 1500℃ at a rate of 2℃ / min, held for 2 h, and then naturally cooled to room temperature to obtain a hard carbon precursor. The carbon interlayer spacing of the hard carbon precursor was 0.36 nm, and the specific surface area was 350 m². 2 / g; (3) Room temperature KOH alkalization to regulate interlayer spacing and closed-pore structure: The hard carbon precursor was placed in a planetary ball mill and milled at 1100 r / min for 8 h. After drying at 60℃, hard carbon particles with a particle size D50 of 60 μm were obtained. The dried hard carbon particles were immersed in 3 mol / L KOH aqueous solution at a liquid-to-solid ratio of 30 mL / g and etched at room temperature with stirring for 12 h. After filtration, they were dried at 60℃. Then, they were immersed in 2 mol / L HCl aqueous solution and acid-washed at room temperature with stirring for 2 h. After filtration, they were dried at 60℃. The acid-washed hard carbon material was mixed with ultrapure water, centrifuged at 8000 r / min and washed 4 times for 10 min each time. After filtration, they were dried at 60℃ to obtain a hard carbon material with a large interlayer spacing and high closed-pore structure. The carbon interlayer spacing of this material was 0.3919 nm, the closed-pore ratio was 45%, and the total pore volume was 0.4 cm. 3 / g; (4) Electrode preparation: The above-mentioned hard carbon material, Super P conductive agent and PVDF binder are mixed in a mass ratio of 8:1:1 and NMP is used as solvent to prepare a uniform slurry with a solid content of 27%. The slurry is coated on copper foil with a coating thickness of 150 μm and vacuum dried at 120℃ for 12 h to obtain pine-based hard carbon negative electrode.

[0056] The aforementioned hard carbon anode was assembled with a sodium metal sheet to form a sodium-ion half-cell, and its electrochemical performance was tested. Simultaneously, it was assembled with a sodium iron pyrophosphate cathode, diethylene glycol dimethyl ether + NaPF6 electrolyte, and a glass fiber membrane to form a sodium-ion full-cell, and its full-cell performance was tested. The test results are as follows: SEM image ( Figure 1 The results show that the material is etched to a moderate degree, achieving an optimal balance between structural integrity and pore development; at the same time, the overall framework of the particles is not damaged, electronic conductivity is maintained well, and the specific surface area is within the optimal range.

[0057] TEM image ( Figure 2 The data shows that the material is a high-stability closed-cell hard carbon material with a large interlayer spacing of up to 0.3919 nm and a closed-cell ratio of 45%.

[0058] EIS chart ( Figure 3 This indicates that the material has the lowest charge transfer impedance and the optimal ion diffusion kinetics. CV curve ( Figure 4 The display material exhibits good electrochemical reversibility and minimal polarization; Ion diffusion coefficient diagram ( Figure 5 The sodium ion diffusion rate of the material is significantly higher than that of traditional hard carbon materials; First charge-discharge curve ( Figure 6 The results show that at a current density of 0.1 A / g, the initial reversible specific capacity reaches 396.62 mAh / g, and the plateau capacity reaches 254.72 mAh / g; Cyclic performance ( Figure 7 The results show that after 100 cycles at a current density of 0.1 A / g, the capacity retention is 99% and the coulombic efficiency remains stable at over 99%. Ratio performance ( Figure 8 The data shows that even at a high current density of 5 A / g, the specific capacity of the material remains above 90 mAh / g; High current cycling performance ( Figure 9 The results show that after 500 cycles at a current density of 1 A / g, the capacity retention exceeds 85%. All-battery ( Figure 12 The results showed that the full cell assembled with sodium iron pyrophosphate had the smallest decay value after 100 cycles, demonstrating excellent cycle stability.

[0059] Example 2 Using the exact same preparation process as in Example 1, except that the KOH etching concentration was changed to 1 mol / L, a hard carbon anode material was obtained.

[0060] The material was assembled into a sodium-ion half-cell and its electrochemical performance was tested. The results showed that, compared with the 3 mol / L KOH alkalization treatment in Example 1, the low-concentration KOH etching used in Example 2 had insufficient modification effect on the material, and the material had a low degree of graphitization and too many defect sites. Compared with the sample of Example 1, the initial coulombic efficiency and sodium storage specific capacity of this material were significantly lower. After 500 cycles at a high current density of 1 A / g, the capacity retention was lower than that of the sample of Example 1 (see Example 1). Figure 10 ).

[0061] Example 3 Using the exact same preparation process as in Example 1, except that the KOH etching concentration was changed to 5 mol / L, a hard carbon anode material was obtained.

[0062] The material was assembled into a sodium-ion half-cell and tested. The results showed that, compared with the 3 mol / L KOH alkalization treatment in Example 1, the high-concentration KOH etching used in Example 3 caused the collapse of the dense carbon framework constructed by high-temperature carbonization. Figure 22 ), compared with the SEM image of Comparative Example 3 ( Figure 20 In contrast, this material suffers from the worst electrochemical performance due to the severe non-selective etching caused by excessive KOH, which not only destroys the macroscopic framework of the particles but also excessively removes oxygen-containing functional groups and physical support structures between the hard carbon layers, resulting in rapid capacity decay. Furthermore, due to the large number of open closed-pore structures, although the carbon interlayer spacing has increased, the structural stability is poor, and the ion diffusion resistance is increased. The rate performance and cycling stability of this material are far worse than those of Example 1. The specific capacity at a high current of 5 A / g is significantly reduced, and the material collapses after 327 cycles at 1 A / g, making continuous cycling difficult and exhibiting significant capacity decay. The high-current cycling performance is significantly worse (see Example 1). Figure 11 ).

[0063] Comparative Example 1 This comparative example only uses a high-temperature carbonization process, omitting the subsequent room-temperature KOH alkaline etching step, and adjusts the carbonization temperature to 900℃ to finally obtain a hard carbon anode material. It only explores the effect of the reduced carbonization temperature, and its preparation process differs from that of Example 1.

[0064] The material was assembled into a sodium-ion half-cell and tested. The results showed that: Compared to Example 1, Comparative Example 1 has a larger charge transfer impedance value, greater resistance to electron / ion transport on the electrode surface, and a smaller slope of the inclined line than Example 1, indicating poorer ion diffusion kinetics.

[0065] Lower temperature modification is insufficient; particles are mainly prismatic with a slightly rough surface, a small amount of fine particles agglomerate, insufficient pore development, and limited ion transport channels. Figure 13 ); The carbon interlayer spacing only increased to 0.3365 nm ( Figure 14 The material has a closed-cell content of only 20%, low graphitization degree, too many defects, and low sodium storage capacity. Furthermore, its capacity retention after 200 cycles at a current density of 1 A / g is far lower than that of the hard carbon material obtained in Comparative Example 3 at a carbonization temperature of 1500℃ (see Comparative Example 3). Figure 15 ).

[0066] Comparative Example 2 This comparative example only uses a high-temperature carbonization process, omitting the subsequent room-temperature KOH alkaline etching step, and adjusts the carbonization temperature to 1300℃ to finally obtain a hard carbon anode material. It only explores the effect of the reduced carbonization temperature, and its preparation process differs from that of Example 1.

[0067] The material was assembled into a sodium-ion half-cell and tested. The results showed that: Lower temperature modification is insufficient, resulting in a wide particle size distribution, with large particles exhibiting surface depressions and micropore development, while fine particles agglomerate and fill the pores, reducing ion transport efficiency. Figure 18 The carbon interlayer spacing only increased to 0.3678 nm. Figure 19 The sodium storage capacity is relatively low, and the capacity retention rate after 200 cycles at a current density of 1 A / g is much lower than that of the hard carbon material obtained at a carbonization temperature of 1500℃ in Comparative Example 3 (see...). Figure 16 ).

[0068] Comparative Example 3 This comparative example only uses a high-temperature carbonization process, omitting the subsequent room-temperature KOH alkaline etching step, and adjusts the carbonization temperature to 1500℃ to finally obtain a hard carbon anode material. Only the effect of carbonization temperature is investigated, and its preparation process differs from that of Example 1.

[0069] The material was assembled into a sodium-ion half-cell and tested. The results showed that: The hard carbon material obtained at this carbonization temperature is mainly composed of layered particles with layered cracks on the surface, resulting in reduced agglomeration and improved dispersibility. Figure 20 The degree of graphitization was significantly improved, the proportion of interlayered sodium storage increased, the layered fractures formed continuous sodium ion transport channels, the degree of ordering of the carbon layers was greatly improved, exhibiting graphite-like layered arrangement characteristics, the number of closed pores reached its peak, the pore size and distribution were more regular, and the carbon layer spacing increased to 0.3789 nm. Figure 21 The sodium storage capacity is relatively high, and the capacity retention after 200 cycles at a current density of 1 A / g is higher than that of the hard carbon materials obtained at the carbonization temperatures used in Comparative Examples 1 and 2 (see...). Figure 17 ).

[0070] In summary, Example 1, obtained by etching with 3 mol / L KOH, is the optimal carbon material. Figure 23 It was found that, compared with Comparative Examples 1, 2, and 3, Example 1 had the lowest charge transfer impedance, the lowest resistance to electron / ion transport on the electrode surface, and the steepest slope line, indicating that it had the best ion diffusion kinetics.

[0071] The electrode material provided by this invention (Example 1) exhibits a significantly superior total discharge specific capacity compared to the comparative material at a current density of 0.1 A / g. Figure 24 Furthermore, the capacity of both the platform region and the slope region are simultaneously improved, indicating that the present invention, through a specific modification scheme, simultaneously optimizes the bulk reaction kinetics and surface / interface activity of the material, effectively activates the energy storage active sites, significantly improves the specific capacity and reaction kinetics performance of the material, and solves the technical problems of low capacity and limited reaction kinetics of electrode materials in the prior art.

[0072] The electrode material of Example 1 provided by this invention has a significantly higher initial specific capacity and a much higher specific capacity after cycling stabilization than all comparative materials. Figure 25 The energy storage capacity of the material was 2.5 times that of Comparative Example 1, 1.8 times that of Comparative Example 2, and 1.4 times that of Comparative Example 3, demonstrating that the modified scheme of this invention significantly improves the energy storage capacity of the material. During long-term cycling, the coulombic efficiency remained stable at around 100%, with no irreversible side reactions occurring, indicating that the material exhibits good structural reversibility during charge and discharge and possesses excellent cycle reliability.

[0073] The electrode material of Example 1 provided by this invention exhibits a significantly higher discharge specific capacity than the comparative material at all rates from 0.1 to 5.0 A / g. Figure 26Even at a high current of 5.0 A / g, it maintains considerable energy storage capacity. When the current density recovers to 0.1, 1.0, and 2.0 A / g, the specific capacity can quickly recover to the initial level without significant irreversible capacity loss. This indicates that the material has excellent structural stability during repeated rate switching and can withstand the stress changes caused by rapid charging and discharging, avoiding structural collapse and deactivation of active sites. Furthermore, its high capacity at low current and stable performance at high current prove that it can be adapted to various application scenarios from conventional charging and discharging to rapid charging and discharging, and has a wider range of practical application value.

[0074] Example 1 of the electrode material provided by the present invention was tested in BET ( Figure 27 N2 adsorption-desorption test (ab) and CO2 adsorption-desorption test (cd) were performed respectively to further prove the closed-pore ratio. The closed-pore ratio of Example 1 was calculated to be 44.73%≈45% by mercury porosimetry combined with nitrogen adsorption-desorption and carbon dioxide adsorption-desorption.

[0075] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a pine-based hard carbon anode material, characterized in that, The specific steps include: (1) Pretreatment: The raw materials of Inner Mongolia pine are crushed, washed and dried to obtain pretreated pine raw materials; (2) High-temperature carbonization to construct carbon skeleton: The pretreated pine raw material is carbonized at high temperature and then naturally cooled to room temperature to obtain hard carbon precursor; (3) Room temperature KOH alkalization to regulate interlayer spacing and closed-cell structure: After ball milling and drying, the hard carbon precursor is immersed in KOH aqueous solution and etched by stirring at room temperature. After etching, it is filtered and dried. Then it is immersed in HCl aqueous solution for acid washing, filtered and dried. After washing, filtration and drying, hard carbon material is obtained. (4) Electrode preparation: The hard carbon material, Super P conductive agent and PVDF binder are mixed in mass ratio, and N-methyl-2-pyrrolidone is used as solvent to make slurry. The slurry is coated on copper foil current collector and dried to obtain pine-based hard carbon anode material.

2. The preparation method according to claim 1, characterized in that, In step (1), the lignin content of the Inner Mongolia pine raw material is 32%~38%, and the cellulose content is 35%~39%.

3. The preparation method according to claim 1, characterized in that, In step (1), the material is crushed to 1-2 cm.

4. The preparation method according to claim 1, characterized in that, In step (2), the specific operation of high-temperature carbonization is as follows: the pretreated pine raw material is placed in a tube furnace, argon gas is introduced for protection, the gas flow rate is 100 mL / min, and the temperature is heated to 900-1500℃ at a heating rate of 2℃ / min, and held for 2 h.

5. The preparation method according to claim 1, characterized in that, In step (3), the concentration of the KOH aqueous solution is 1-3 mol / L, the liquid-solid ratio of the KOH aqueous solution to the hard carbon precursor is 30 mL / g, and the etching time is 12 h.

6. The preparation method according to claim 1, characterized in that, In step (3), the concentration of the HCl aqueous solution is 2 mol / L, and the acid washing time is 3 h.

7. The preparation method according to claim 1, characterized in that, In step (3), the particle size D of the hard carbon precursor after ball milling 50 It is 60 μm.

8. The preparation method according to claim 1, characterized in that, In step (4), the mass ratio of the hard carbon material, Super P conductive agent, and PVDF binder is 8:1:1, and the solid content of the pulp is 27%.

9. The pine-based hard carbon anode material obtained by any one of the preparation methods according to claims 1-8, characterized in that, The hard carbon anode material has a carbon interlayer spacing of 0.37~0.40 nm, a closed-cell ratio of 30%~60%, and a total pore volume of 0.2~0.6 cm³. 3 / g.

10. The application of the pine-based hard carbon anode material according to claim 9 in sodium-ion batteries.

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