A high tap-density hard carbon material, a preparation method and application thereof

Abstract

The present application relates to the technical field of electrochemical energy storage materials, in particular to a high tap density hard carbon material and a preparation method and application thereof. The method comprises the following steps: starch and nano-carbon particles are subjected to mechanical-chemical treatment at a mass ratio of 100: (1-5), so that the starch is refined and plastic deformation occurs, and at the same time, the nano-carbon particles are coated on the surface of the starch to obtain a composite precursor; the composite precursor is subjected to stepwise programmed temperature pre-oxidation treatment in an oxygen-containing atmosphere to obtain a treated material; and the treated material is subjected to carbonization treatment in an inert atmosphere to obtain a high tap density hard carbon material. Through the synergistic effect of mechanical-chemical treatment and stepwise programmed temperature pre-oxidation, the present application realizes precise control of the morphology of starch particles and uniform coating of nano-carbon, and the obtained hard carbon material has high tap density, high reversible specific capacity, excellent initial coulombic efficiency, good cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage materials technology, and in particular to a high tap density hard carbon material, its preparation method, and its application. Background Technology

[0002] Hard carbon materials have become an important choice for anode materials in sodium-ion and lithium-ion batteries due to their excellent cycle stability and safety characteristics. However, the current mainstream preparation processes mostly use direct carbonization of biomass raw materials or simple pre-oxidation followed by carbonization. Although the processes are simple, they face several technical bottlenecks in practical applications.

[0003] From a material performance perspective, the hard carbon particles obtained from the direct carbonization of traditional biomass (such as starch and sucrose) are mostly irregularly shaped flakes or spheres with large gaps between particles, resulting in a tap density generally lower than 0.75 g / cm³. 3 This fundamentally restricts further improvements in battery volumetric energy density. The core reason for this bottleneck is that existing processes lack the ability to control particle morphology and microstructure. Conventional physical mixing methods cannot achieve precise engineering design of the particle morphology of biomass raw materials such as starch, making it difficult to form geometric configurations conducive to high-density packing, thus hindering effective breakthroughs in packing density.

[0004] Meanwhile, to improve the interfacial stability and electrochemical performance of materials, researchers have attempted to introduce coating modification methods, such as chemical vapor deposition (CVD) or sol-gel methods. However, these methods themselves have process defects such as cumbersome steps, high equipment requirements, and difficulty in scaling up. Furthermore, the coating layer is prone to detachment during high-temperature carbonization, making it difficult to guarantee the interfacial strengthening effect and limiting the improvement of the overall performance of the material.

[0005] In the pre-oxidation stage, traditional processes often use a single temperature platform or a relatively fast heating rate. This method can easily lead to excessive oxidation of the particle surface and insufficient internal cross-linking, or even damage to the precursor structure due to local overheating.

[0006] Therefore, developing a novel green preparation process that can achieve synergistic control over particle morphology engineering, interface structure strengthening, and high-density packing is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to address the problems existing in the prior art by providing a high tap density hard carbon material, its preparation method, and its application. Through the synergistic effect of mechanochemical treatment and step-programmed temperature pre-oxidation, precise control of starch particle morphology and uniform coating of nano-carbon are achieved. The resulting hard carbon material has high tap density, high reversible specific capacity, excellent first coulombic efficiency, good cycling stability, and rate performance.

[0008] To achieve the above objectives, the present invention provides a method for preparing a high tap density hard carbon material, comprising the following steps: S1. Starch and nano-carbon particles are subjected to mechanochemical treatment at a mass ratio of 100:(1-5) to refine the starch and cause plastic deformation, while the nano-carbon particles are coated on the surface of the starch to obtain a composite precursor. S2. The composite precursor is subjected to step-by-step temperature-programmed pre-oxidation treatment in an oxygen-containing atmosphere to obtain the treated material. S3. The treated material is carbonized under an inert atmosphere to obtain a high tap density hard carbon material.

[0009] In an optional embodiment, in S1, the starch is selected from corn starch; the nano-carbon particles are selected from at least one of carbon black, carbon nanotubes, graphene nanosheets and nano-graphite.

[0010] In an optional embodiment, in S1, the rotation speed of the mechanochemical treatment is 2000-3000 r / min, and the time is 1-20 min.

[0011] In an optional embodiment, in S1, the mechanochemical treatment is carried out in a high-speed pulverizer; the high-speed pulverizer is selected from an air jet mill or a mechanical impact mill.

[0012] In an optional implementation, in S1, the particle size distribution D of the composite precursor is... 50 It is 2-15μm.

[0013] In this invention, during the mechanochemical treatment process in S1, mechanical collisions and shear forces cause the starch to undergo size refinement and plastic deformation, forming starch particles with polyhedral geometric features. The polyhedron includes at least one of tetrahedron, hexahedron, and octahedron. At the same time, nano-carbon particles are embedded and coated on the surface of the starch particles by mechanical force, forming a core-shell structured composite precursor.

[0014] In an optional embodiment, in S2, the oxygen-containing atmosphere is selected from air or a nitrogen-oxygen mixture, the volume fraction of oxygen in the oxygen-containing atmosphere is 10-30%, and the flow rate of the oxygen-containing atmosphere is 0.5-2 L / min.

[0015] In an optional implementation, in S2, the stepped temperature-programmed pre-oxidation treatment is carried out in a temperature-controlled furnace; the stepped temperature-programmed pre-oxidation treatment includes six successively increasing temperature stages, each with a holding time.

[0016] In one optional embodiment, the six sequentially increasing temperature stages are 197-203°C (first temperature), 207-213°C (second temperature), 217-223°C (third temperature), 227-233°C (fourth temperature), 237-243°C (fifth temperature), and 247-253°C (sixth temperature), preferably 200°C, 210°C, 220°C, 230°C, 240°C, and 250°C.

[0017] In one optional embodiment, the heating rate from room temperature to the first temperature is 2-5°C / min, the heating rate from the first temperature to the second temperature is 1-2°C / min, the heating rate from the second temperature to the third temperature is 1-2°C / min, the heating rate from the third temperature to the fourth temperature is 1-2°C / min, the heating rate from the fourth temperature to the fifth temperature is 1-2°C / min, and the heating rate from the fifth temperature to the sixth temperature is 1-2°C / min.

[0018] In one optional embodiment, the holding time for each temperature stage is 15-360 min; preferably, the holding time for the first temperature, second temperature, third temperature, fourth temperature, and fifth temperature is 15-30 min, and the holding time for the sixth temperature is 30-360 min.

[0019] In an optional embodiment, in S3, the carbonization process adopts a two-stage heating process: first, the temperature is raised to 880-920℃ at 5-10℃ / min and held for 0.8-1.2h, and then the temperature is raised to 1000-1500℃ at 1-3℃ / min and held for 1-3h.

[0020] The present invention also provides a high tap density hard carbon material, which is prepared according to the preparation method of the high tap density hard carbon material.

[0021] In one optional embodiment, the tap density of the high-tap-density hard carbon material is 0.8-1.15 g / cm³. 3 .

[0022] The present invention also provides the application of the high tap density hard carbon material in sodium-ion batteries or lithium-ion batteries.

[0023] The beneficial effects of this invention are as follows: 1. This invention significantly improves the tap density of hard carbon materials through a synergistic process of mechanochemical treatment and stepped temperature-programmed pre-oxidation. On the one hand, during the mechanochemical treatment, high-intensity mechanical collisions and shear forces cause starch to undergo plastic deformation and size refinement, forming particles with polyhedral geometric features (such as tetrahedrons, hexahedrons, and octahedrons). This regular particle morphology is conducive to achieving close packing between particles. On the other hand, the stepped temperature-programmed pre-oxidation treatment causes starch molecular chains to gradually break, recombine, and cross-link within a wide temperature range, forming a stable and dense precursor skeleton. This effectively avoids the problems of excessive surface oxidation and insufficient internal cross-linking that are easily caused by traditional single-temperature pre-oxidation, laying a structural foundation for the high-density packing of subsequent carbonization products.

[0024] 2. The hard carbon material prepared by this invention possesses both high reversible specific capacity and excellent initial coulombic efficiency. Mechanochemical treatment causes nano-carbon particles to uniformly coat the surface of starch particles, forming a core-shell structured composite precursor. This coating layer effectively inhibits particle aggregation during subsequent carbonization, improves the conductivity of the material, and provides more active sites for lithium / sodium ion storage. Simultaneously, the stepped temperature-programmed pre-oxidation process ensures the uniformity and sufficiency of the pre-oxidation process, avoiding structural defects caused by localized overheating or uneven oxidation, thereby guaranteeing the electrochemical activity of the carbonized product.

[0025] 3. The hard carbon material prepared by this invention exhibits excellent cycling stability and rate performance. Thanks to the stable interfacial structure formed by the uniform coating of nano-carbon particles and the uniform cross-linked framework formed by step-by-step pre-oxidation, the resulting hard carbon material can maintain structural integrity during long-term cycling, effectively suppressing capacity decay.

[0026] 4. The method of this invention is simple, reproducible, and suitable for large-scale production. Compared with the complex coating processes in existing technologies (such as CVD and sol-gel methods), this invention can simultaneously achieve morphology control of starch granules and uniform coating of nano-carbon through a single mechanochemical treatment. The process flow is short, the equipment requirements are low, and it is easy to scale up industrially. Furthermore, the stepped temperature-programmed pre-oxidation process, through six progressively increasing temperature stages and precise holding time control, allows the material to undergo sufficient physicochemical changes in different temperature ranges, which is beneficial for achieving batch-to-batch product performance consistency.

[0027] In summary, this invention achieves precise control of starch particle morphology and uniform coating of nano-carbon through the synergistic effect of mechanochemical treatment and step-by-step temperature-programmed pre-oxidation. The resulting hard carbon material possesses high tap density, high reversible specific capacity, excellent first coulombic efficiency, good cycle stability, and rate performance, which can meet the application requirements of high-energy-density sodium-ion batteries or lithium-ion battery anode materials and has significant application value. Attached Figure Description

[0028] Figure 1 This is an appearance characterization diagram of the material after processing in Embodiment 1 of the present invention; Figure 2 This is an appearance characterization diagram of the high tap density hard carbon material in Example 1 of the present invention; Figure 3 This is a SEM characterization image of the high tap density hard carbon material in Example 1 of this invention; Figure 4 This is a SEM characterization image of the hard carbon material in Comparative Example 1 of this invention; Figure 5 This is a SEM characterization image of the hard carbon material in Comparative Example 2 of this invention; Figure 6 This is a charge-discharge curve of the half-cell prepared using the hard carbon material in Example 1 of this invention at a current density of 20 mA / g; Figure 7 This is a cycling performance curve of the half-cell prepared using the hard carbon material of Example 1 at a current density of 100 mA / g. Figure 8 This is a rate performance diagram of the half-cell prepared using the hard carbon material in Example 1 of this invention at different current densities. Detailed Implementation

[0029] The following embodiments are provided to better understand the present invention and are not limited to the described embodiments. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0030] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0031] Example 1 This embodiment provides a method for preparing a high tap density hard carbon material, including the following steps: 1000g of corn starch and 20g of nano-graphite were added to a mechanical impact pulverizer for mechanochemical treatment. The speed was set to 3000 r / min and the time to be 5 min, to obtain a composite precursor, D. 50 It is 8μm.

[0032] The composite precursor was placed in a temperature-controlled furnace and subjected to a step-by-step temperature-programmed pre-oxidation treatment under an air atmosphere (oxygen volume fraction of 21%, flow rate of 1 L / min) according to the following procedure: heating from room temperature to 200℃ at 5℃ / min and holding for 30 min; heating to 210℃ at 2℃ / min and holding for 30 min; heating to 220℃ at 2℃ / min and holding for 30 min; heating to 230℃ at 2℃ / min and holding for 30 min; heating to 240℃ at 1℃ / min and holding for 30 min; heating to 250℃ at 1℃ / min and holding for 360 min; thus obtaining the treated material.

[0033] The processed material was carbonized under argon protection: first, the temperature was increased to 900℃ at 10℃ / min and held for 1 hour, then increased to 1300℃ at 2℃ / min and held for 2 hours; finally, it was cooled in the furnace to obtain a high tap density hard carbon material.

[0034] Example 2 This embodiment provides a method for preparing a high tap density hard carbon material, which differs from Embodiment 1 in that the rotation speed of the mechanical chemical treatment is modified to 2000 r / min.

[0035] Example 3 This embodiment provides a method for preparing a high tap density hard carbon material, which differs from Embodiment 1 in that the amount of "nanographite" is modified to 10g.

[0036] Example 4 This embodiment provides a method for preparing a high tap density hard carbon material, which differs from Embodiment 1 in that "nanographite" is replaced with an equal amount of "carbon nanotubes".

[0037] Example 5 This embodiment provides a method for preparing a high tap density hard carbon material. The difference from Embodiment 1 is that the carbonization conditions are modified to "first heat to 900℃ at 10℃ / min and hold for 1h, then heat to 1500℃ at 2℃ / min and hold for 2h".

[0038] Comparative Example 1 This comparative example provides a method for preparing hard carbon material, which differs from Example 1 in that the amount of "nanographite" is modified to 100g.

[0039] Comparative Example 2 This comparative example provides a method for preparing hard carbon material, which differs from Example 1 in that the addition of "nanographite" is omitted.

[0040] Comparative Example 3 This comparative example provides a method for preparing hard carbon material, which differs from Example 1 in that the "mechanical-chemical treatment" step is omitted. Instead, 1000g of corn starch and 20g of nano-graphite are simply mixed and then directly subjected to a step-by-step temperature-programmed pre-oxidation treatment and carbonization treatment.

[0041] Comparative Example 4 This comparative example provides a method for preparing hard carbon material, which differs from Example 1 in that the "step-by-step temperature-controlled pre-oxidation treatment" is modified to a "one-step pre-oxidation treatment". Specifically, the composite precursor is placed in a temperature-controlled furnace and heated from room temperature to 250°C at a rate of 5°C / min under an air atmosphere (oxygen volume fraction of 21%, flow rate of 1L / min) and held for 3 hours to obtain the treated material.

[0042] Experimental Example 1: Material Morphology Characterization The appearance characterization diagram of the processed material in Example 1 is as follows: Figure 1 As shown. From Figure 1 As can be seen, the treated material is macroscopically a black powder. The appearance characterization diagram of the high-tap-density hard carbon material in Example 1 is shown below. Figure 2 As shown. From Figure 2 As can be seen, high tap density hard carbon materials also appear as black powder on a macroscopic scale.

[0043] The hard carbon materials in Example 1, Comparative Example 1, and Comparative Example 2 were characterized using scanning electron microscopy (SEM). The SEM image of the high tap density hard carbon material in Example 1 was obtained, as shown below. Figure 3 As shown. From Figure 3 As can be seen, the product particles have regular polyhedral morphologies (including tetrahedrons, hexahedrons, octahedrons, etc.). This morphology is conducive to close packing between particles and is an important structural basis for obtaining high tap density. The SEM characterization image of the hard carbon material in Comparative Example 1 is shown below. Figure 4 As shown. From Figure 4 As can be seen, the surface of the starch granules is completely coated with excessive nano-carbon, forming a relatively thick coating layer. This excessive coating may hinder direct contact between particles, which is not conducive to improving the packing density. The SEM characterization image of the hard carbon material in Comparative Example 2 is shown below. Figure 5 As shown. From Figure 5 As can be seen, the product particles exhibit severe agglomeration and irregular morphology, further confirming that the appropriate introduction of nano-carbon has a positive effect on particle dispersion and morphology control.

[0044] Experiment Example 2: Electrochemical Performance Testing The hard carbon materials obtained in Examples 1-5 and Comparative Examples 1-4 were used as active materials. They were mixed uniformly at a mass ratio of active material: conductive carbon black (SP): sodium carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 93:2:1.5:3.5, and coated onto copper foil to form the working electrode. A sodium metal sheet was used as the counter electrode, glass fiber as the separator, and a 1 mol / L sodium hexafluorophosphate (NaPF6) solution was used as the electrolyte. The solvent was a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. Battery assembly was carried out in a glove box filled with high-purity argon gas, where the water and oxygen content were both below 0.1 ppm.

[0045] The assembled half-cell was subjected to constant current charge-discharge testing using a battery testing system, with a voltage window of 0.002-3.0V (vs. Na). + / Na). The reversible specific capacity and initial coulombic efficiency of the materials were tested at a current density of 20 mA / g; the cycling performance was tested at a current density of 100 mA / g (200 cycles); and the rate performance was tested at different current densities (20-2000 mA / g). The tap density of each product powder was also determined using a tap density meter.

[0046] The test results of reversible specific capacity, initial coulombic efficiency, and tap density are shown in Table 1.

[0047] Table 1. Test results of reversible specific capacity, initial coulombic efficiency, and tap density.

[0048] As can be seen from the data in Table 1, the hard carbon materials obtained in Examples 1-5 using the method of this invention generally have high tap densities (0.80-1.10 g / cm³). 3 All of them exhibited good reversible specific capacity (309.9-335.6 mAh / g) and initial coulombic efficiency (84.6-93.2%). In contrast, the tap density, specific capacity, and initial coulombic efficiency of the products obtained in Comparative Examples 1-4 all decreased to varying degrees. Specific analysis of each comparative example shows that the tap density of Comparative Example 1 (excess nano-carbon) was only 0.60 g / cm³. 3 The specific capacity and coulombic efficiency also decreased significantly, indicating that the amount of nano-carbon added needs to be controlled within an appropriate range. Excessive coating is detrimental to particle aggregation and electrochemical performance. Comparative Example 2 (without nano-carbon) showed severe agglomeration, with a tap density of only 0.60 g / cm³. 3This demonstrates that the introduction of nano-carbon is crucial for particle dispersion and morphology control. Comparative Example 3 (omitting mechanochemical treatment) showed lower tap density and electrochemical performance than Example 1, indicating that mechanochemical treatment is a key step in achieving particle refinement, plastic deformation, and uniform nano-carbon coating. Comparative Example 4 (using one-step pre-oxidation instead of stepped heating) also showed lower tap density and electrochemical performance than Example 1, indicating that stepped temperature-programmed pre-oxidation helps form a stable and uniform precursor structure, avoiding structural defects caused by localized overheating or uneven oxidation.

[0049] Furthermore, the charge-discharge curves of the half-cell prepared using the hard carbon material of Example 1 at a current density of 20 mA / g are shown below. Figure 6 As shown. From Figure 6 As can be seen, at a current density of 20 mA / g, its reversible specific capacity is 330.5 mAh / g, and its initial coulombic efficiency is 92.1%. The cycling performance curves of the half-cell prepared using the hard carbon material in Example 1 at a current density of 100 mA / g are shown below. Figure 7 As shown. From Figure 7 As can be seen, at a current density of 100 mA / g, after 200 cycles, the capacity retention rate is 93.3%, demonstrating excellent cycle stability. The rate performance of the half-cell prepared using the hard carbon material of Example 1 at different current densities is shown in the figure. Figure 8 As shown. From Figure 8 As can be seen, it exhibits good rate performance in the current density range of 20-2000 mA / g.

[0050] In summary, this invention achieves precise control of starch particle morphology and uniform coating of nano-carbon through a mechanochemical treatment combined with a stepped temperature-programmed pre-oxidation process. The resulting hard carbon material has high tap density, high reversible specific capacity, and excellent cycle stability, which can meet the application requirements of high-energy-density sodium-ion battery anode materials.

[0051] Finally, it should be noted that the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a high tap density hard carbon material, characterized in that, Includes the following steps: S1. Starch and nano-carbon particles are subjected to mechanochemical treatment at a mass ratio of 100:(1-5) to refine the starch and cause plastic deformation, while the nano-carbon particles are coated on the surface of the starch to obtain a composite precursor. S2. The composite precursor is subjected to step-by-step temperature-programmed pre-oxidation treatment in an oxygen-containing atmosphere to obtain the treated material; S3. The treated material is carbonized under an inert atmosphere to obtain a high tap density hard carbon material.

2. The method for preparing high tap density hard carbon material according to claim 1, characterized in that, In S1, the starch is selected from corn starch; the nano-carbon particles are selected from at least one of carbon black, carbon nanotubes, graphene nanosheets and nano-graphite.

3. The method for preparing high tap density hard carbon material according to claim 1, characterized in that, In S1, the rotation speed of the mechanochemical treatment is 2000-3000 r / min, and the time is 1-20 min.

4. The method for preparing high tap density hard carbon material according to claim 1, characterized in that, In S2, the stepped temperature-increasing pre-oxidation treatment includes six progressively increasing temperature stages, each with a set holding time.

5. The method for preparing high tap density hard carbon material according to claim 4, characterized in that, The six temperature ranges that increase sequentially are 197-203℃, 207-213℃, 217-223℃, 227-233℃, 237-243℃, and 247-253℃.

6. The method for preparing high tap density hard carbon material according to claim 4, characterized in that, The holding time for each temperature stage is 15-360 minutes.

7. The method for preparing high tap density hard carbon material according to claim 1, characterized in that, In S3, the carbonization process adopts a two-stage heating process: first, the temperature is raised to 880-920℃ at 5-10℃ / min and held for 0.8-1.2h, and then the temperature is raised to 1000-1500℃ at 1-3℃ / min and held for 1-3h.

8. A high tap density hard carbon material, characterized in that, The high tap density hard carbon material is prepared according to any one of claims 1-7.

9. The high tap density hard carbon material according to claim 8, characterized in that, The tap density of high-tap-density hard carbon materials is 0.8-1.15 g / cm³. 3 .

10. The application of the high tap density hard carbon material according to claim 8 or 9 in sodium-ion batteries or lithium-ion batteries.

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