Hard carbon nanospheres, preparation method and application thereof, and sodium ion battery anode

Abstract

The application provides a hard carbon nanosphere, a preparation method and application thereof, and a sodium ion battery anode, and belongs to the technical field of sodium ion battery anode materials. 2,6-diacetylpyridine, 1,8-diaminonaphthalene and a copper salt are reacted in a solvent to obtain a copper coordination polymer precursor; the copper coordination polymer precursor is subjected to first-step pyrolysis under an inert atmosphere to obtain an intermediate product; the intermediate product is subjected to acid pickling treatment; and the product after the acid pickling treatment is subjected to second-step pyrolysis under an inert atmosphere to obtain the hard carbon nanosphere. The hard carbon nanosphere with highly disordered and twisted graphite lattices is prepared through copper ion-mediated polymer precursor pyrolysis, the interlayer spacing is expanded, and the number of stacked layers is limited, so that the sodium ion storage performance is improved.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery anode materials, and particularly relates to a hard carbon nanosphere, its preparation method and application, and a sodium-ion battery anode. Background Technology

[0002] Sodium-ion batteries are considered an ideal alternative for large-scale energy storage due to the abundance and low cost of sodium resources. However, the large radius of sodium ions (0.102 nm) makes their embedding in traditional graphite anodes difficult, thus limiting their practical application. Hard carbon materials, with their disordered graphite lattice and tunable structure, have become strong candidates for anode materials in sodium-ion batteries. Currently, hard carbon materials are mainly prepared by the pyrolysis of biomass (such as resins and starch) or synthetic polymers (such as phenolic resins). However, biomass precursors suffer from inconsistencies in structure and complex composition, while synthetic polymer precursors have the advantages of high purity and controllable structure. Nevertheless, existing technologies struggle to precisely control the graphite lattice topology of hard carbon (such as interlayer spacing and number of stacked layers), resulting in low sodium-ion storage capacity, poor rate performance, and insufficient cycle stability.

[0003] For example, a study published in *Energy & Environmental Science* by a team from Wenzhou University points out that clarifying the relationship between the sodium storage performance of hard carbon and parameters such as interlayer spacing is a current challenge. Calculations demonstrate that when the interlayer spacing is ≥0.38 nm, the sodium ion diffusion barrier is significantly reduced. This indirectly confirms that interlayer spacing smaller than this value (e.g., 0.34-0.37 nm) is a key reason for the limited performance of existing materials. A review article from Central South University, "A Comprehensive Understanding of Closed Pores in Hard Carbon Anodes for High-Energy Sodium-Ion Batteries," describes the ideal local microstructure of hard carbon as short-range ordered microcrystals formed by stacking graphene nanosheets in several layers (typically 2-4 layers). This indirectly suggests that stacking layers exceeding this range (e.g., 5-6 layers) may represent structures with high order, which are unfavorable for sodium storage. A paper published in *Joule* by teams from Huazhong University of Science and Technology and Wuhan University points out that traditional carbonization techniques, due to the coupling of pyrolysis, crystallization, and graphitization processes, make it difficult to precisely control the structure of carbon-based materials. The progress summary from the Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, also mentions that the active structure of hard carbon for sodium storage remains unclear, severely restricting its structural design. It is evident that existing hard carbon materials typically possess locally ordered graphite structures with small interlayer spacing (approximately 0.34-0.37 nm) and a large number of stacked layers (5-6 layers), which hinders the rapid insertion and extraction of sodium ions. Furthermore, the lack of uniform morphology and defect regulation further limits their electrochemical performance. Therefore, developing a hard carbon material with a highly disordered graphite lattice, expanded interlayer spacing, and a limited number of stacked layers is crucial for improving the performance of sodium-ion batteries. Summary of the Invention

[0004] To address the issues of low capacity, poor rate performance, and insufficient cycle stability of existing hard carbon materials in sodium-ion storage, this invention proposes a hard carbon nanosphere, its preparation method, and its application as a sodium-ion battery anode. This invention prepares hard carbon nanospheres with highly disordered and twisted graphite lattices through copper ion-mediated pyrolysis of polymer precursors, thereby achieving expanded interlayer spacing and a limited number of stacked layers, thus improving sodium-ion storage performance.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention proposes a method for preparing hard carbon nanospheres, comprising the following steps:

[0007] 2,6-Diacetylpyridine (DAP), 1,8-diaminonaphthalene (DAN) and copper salt were reacted in a solvent to obtain a copper coordination polymer precursor via Schiff base condensation and coordination polymerization.

[0008] The copper coordination polymer precursor was subjected to a first-step pyrolysis under an inert atmosphere to obtain an intermediate product;

[0009] The intermediate product is subjected to acid washing, and the acid-washed product is subjected to a second pyrolysis under an inert atmosphere to obtain the hard carbon nanospheres.

[0010] Furthermore, the ratio of 2,6-diacetylpyridine, 1,8-diaminonaphthalene, copper salt, and solvent is 1 mmol: 1 mmol: 1 mmol: 5 mL.

[0011] Furthermore, the copper salt includes copper chloride (CuCl2).

[0012] Furthermore, the temperature of the first pyrolysis step is 900℃, the heating rate is 5℃ / min, and the holding time is 1 hour.

[0013] Furthermore, the pickling process includes immersion in an inorganic acid solution, wherein the inorganic acid is at least one of sulfuric acid, hydrochloric acid, or nitric acid.

[0014] Furthermore, the temperature of the second pyrolysis step is 1200-1500℃, the heating rate is 5℃ / min, and the holding time is 1 hour.

[0015] This invention also proposes a hard carbon nanosphere prepared according to the above preparation method, wherein the hard carbon nanosphere has a graphite interlayer spacing of 0.38-0.40 nm, a graphite stacking layer number of 2-4 layers, and a specific surface area of ​​27-41 m². 2 / g.

[0016] The present invention also proposes an application of the above-mentioned hard carbon nanospheres in sodium-ion batteries.

[0017] Furthermore, the hard carbon nanospheres serve as the anode active material in sodium-ion batteries.

[0018] The present invention also proposes a sodium-ion battery anode comprising the above-mentioned hard carbon nanospheres as the active material of the sodium-ion battery anode.

[0019] Compared with the prior art, the present invention has the following advantages and technical effects:

[0020] (1) This invention utilizes the coordination polymerization reaction of 2,6-diacetylpyridine, 1,8-diaminonaphthalene, and copper salt, combined with a two-step pyrolysis and acid washing process, to achieve precise control over the microstructure of hard carbon. The raw material ratio, pyrolysis temperature, and acid washing conditions can all be systematically adjusted, resulting in high process repeatability and suitability for large-scale preparation. During pyrolysis, copper ions not only promote the cross-linking and graphitization of the precursor but also expand the interlayer spacing after acid washing following the second pyrolysis step. Comparative experiments show that the interlayer spacing and electrochemical performance of the copper-free sample are significantly lower than those of the copper-containing sample, proving that the introduction of copper ions is the key to achieving excellent performance.

[0021] (2) This invention successfully prepared hard carbon nanospheres with highly disordered and twisted graphite lattices through a copper ion-mediated polymer precursor pyrolysis process. This structure has an expanded interlayer spacing and a limited number of graphite stacked layers, which is conducive to the rapid insertion and extraction of sodium ions, and significantly improves the specific capacity, first efficiency and cycle stability of the material. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0023] Figure 1 This is a schematic diagram of the preparation process of hard carbon nanospheres in an embodiment of the present invention;

[0024] Figure 2 The microstructure characterization of Cu-HCs900-1300 prepared in Example 1 of this invention is shown in the following figures: (a) is a scanning electron microscope image, (b) is a low-resolution transmission electron microscope (TEM) image, (c) is a high-resolution TEM image, (d) is the interlayer spacing test result, (e) is the low-resolution TEM image selected corresponding to the energy dispersive X-ray spectrum, and (f)-(i) are the energy dispersive X-ray spectra of C, N, O and Cu elements, respectively.

[0025] Figure 3 X-ray diffraction patterns of the hard carbon nanospheres prepared in Examples 1-4 and Comparative Example 1;

[0026] Figure 4 (a) shows the nitrogen adsorption-desorption curves of the hard carbon nanospheres prepared in Examples 1-4 and Comparative Example 1, and (b) shows the pore size distribution.

[0027] Figure 5 The following are the electrochemical performance test results for the examples and comparative samples, where (a) is the electrochemical performance test result of the Cu-HCs900-1300 sample at 0.05 A·g. -1 (a) shows the performance test results of 1-3 cycles at current density, and (b) shows the rate performance test results of the example and comparative samples. (c) shows the performance test results of the example and comparative samples at 100 mA·g. -1 Cyclic performance under the given conditions, (d) for Cu-HCs900-1300 sample at 5000 mA·g -1 Cyclic performance under certain conditions;

[0028] Figure 6 To reveal the efficient sodium storage mechanism of Cu-HCs 900-1300 based on DFT (density functional theory) theoretical calculations, (a) is the sodium intercalation model of bilayer graphite, (b) is the change in binding energy caused by sodium ion intercalation under different interlayer spacing, (c) is the sodium intercalation model with different number of layers, and (d) is the change in binding energy caused by sodium ion intercalation under different number of layers.

[0029] Figure 7 This is a comparison chart of the electrochemical performance tests of the samples from Example 1 (Cu-HCs900-1300 (two-step pyrolysis)) and Comparative Example 2 (Cu-HCs900-1300 (one-step pyrolysis)). Detailed Implementation

[0030] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0031] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0032] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0033] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0034] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0035] This invention provides a method for preparing hard carbon nanospheres, comprising the following steps:

[0036] 2,6-Diacetylpyridine (DAP), 1,8-diaminonaphthalene (DAN) and copper salt were reacted in a solvent to obtain a copper coordination polymer precursor via Schiff base condensation and coordination polymerization.

[0037] The copper coordination polymer precursor was subjected to a first-step pyrolysis under an inert atmosphere to obtain an intermediate product.

[0038] The intermediate product was acid-washed, and the acid-washed product was then subjected to a second pyrolysis under an inert atmosphere to obtain hard carbon nanospheres.

[0039] In this invention, 2,6-diacetylpyridine and 1,8-diaminonaphthalene undergo Schiff base condensation and coordination polymerization in the presence of copper salt to form a copper coordination polymer precursor. Subsequently, a two-step pyrolysis and acid washing process is used to achieve controlled transformation of the carbon structure and removal of copper species, ultimately forming hard carbon nanospheres with a twisted graphite lattice. This method, through the coordination and catalytic effect of copper ions, guides the formation of a highly disordered hard carbon structure with expanded interlayer spacing and a limited number of stacked layers, providing an ideal microstructural basis for efficient sodium storage.

[0040] In a preferred embodiment of the present invention, the molar ratio of 2,6-diacetylpyridine, 1,8-diaminonaphthalene, copper salt, and solvent is 1 mmol:1 mmol:1 mmol:5 mL. The reaction of 2,6-diacetylpyridine, 1,8-diaminonaphthalene, and copper salt in a 1:1:1 molar ratio ensures complete Schiff base formation and copper coordination, avoiding waste of raw materials or generation of byproducts. This improves the uniformity and reproducibility of the precursor structure, which is beneficial for the controllable evolution of the carbon structure during subsequent pyrolysis.

[0041] In a preferred embodiment of the present invention, the preparation of the copper coordination polymer precursor is carried out under an acidic environment; more preferably, oxalic acid is added during the preparation process. Oxalic acid provides mild acidic conditions during the Schiff base condensation of 2,6-diacetylpyridine and 1,8-diaminonaphthalene, promoting the formation of imine bonds (-C=N-) while avoiding side reactions and improving reaction efficiency and product purity. Furthermore, oxalic acid can form a soluble complex with copper ions, promoting uniform dispersion of the copper salt in ethanol in the early stages of the reaction, which is beneficial for the subsequent formation of a structurally uniform copper coordination polymer precursor with the Schiff base ligand.

[0042] In a preferred embodiment of the present invention, the copper salt comprises copper chloride (CuCl2). Copper chloride is highly soluble in ethanol, readily coordinates with organic ligands, and can effectively catalyze carbonization and participate in structure orientation during pyrolysis. The use of copper chloride is beneficial for forming uniform copper coordination polymers, promoting the subsequent expansion of graphite interlayer spacing and the formation of defect structures.

[0043] In a preferred embodiment of the present invention, the temperature of the first pyrolysis step is 900°C, the heating rate is 5°C / min, and the holding time is 1 hour. The first pyrolysis step can initially carbonize the copper coordination polymer to form a carbon-based intermediate containing copper nanoparticles. This step realizes the initial graphitization and structural fixation of the precursor, laying the foundation for subsequent acid washing to remove unstable copper species and the second step of high-temperature structural adjustment.

[0044] In a preferred embodiment of the present invention, the pickling treatment includes immersion in an inorganic acid solution, wherein the inorganic acid is at least one of sulfuric acid, hydrochloric acid, or nitric acid. Immersion in an inorganic acid (such as sulfuric acid, hydrochloric acid, or nitric acid) can dissolve and remove residual metallic copper and unstable copper-containing species after pyrolysis, remove copper particles leaving vacancies and structural defects, further expand the interlayer spacing, and enhance the adsorption and sodium intercalation active sites of the carbon framework.

[0045] In a preferred embodiment of the invention, the pickling process further includes a step of adding FeCl3 solution for further treatment. The FeCl3 solution is used as an oxidizing etching agent. After the first pyrolysis step, the intermediate product may contain copper or copper oxide particles that are not completely coated with carbon. FeCl3, as a mild oxidizing agent, can undergo a redox reaction with copper to convert the sparingly soluble copper into soluble Cu. 2+It is then further removed by acid washing.

[0046] In a preferred embodiment of the present invention, the temperature of the second pyrolysis step is 1200-1500℃, the heating rate is 5℃ / min, and the holding time is 1 hour. The second pyrolysis step can further graphitize the carbon skeleton, stabilize the interlayer structure, and simultaneously regulate the number of stacked layers and specific surface area. The high-temperature treatment makes the carbon structure more stable, while maintaining a large interlayer spacing and a limited number of stacked layers, which is conducive to the rapid diffusion and storage of sodium ions.

[0047] For example, in an embodiment of the present invention, a method for preparing hard carbon nanospheres specifically includes the following steps (see schematic diagram of the preparation process). Figure 1 ):

[0048] (1) Weigh 10 mmol of 2,6-diacetylpyridine (DAP), 10 mmol of 1,8-diaminonaphthalene (DAN), and 100.0 mg of oxalic acid, dissolve them in 50 mL of anhydrous ethanol, and stir at room temperature until completely dissolved and mixed to obtain a light purple-red mixed solution. Place the mixed solution in a 60 °C oil bath and stir continuously for 8 hours. Observe that the solution turns into a clear deep purple-red (DAP-DAN). Then, add 10 mmol of CuCl2·2H2O to this solution and stir continuously at room temperature for 12 hours. After the reaction is completed, centrifuge to obtain a brown-black precipitate, wash it with ethanol, and finally dry it in an oven to obtain a brown-black copper coordination polymer powder (DAP-DAN-Cu).

[0049] (2) First step pyrolysis: The above-mentioned dried copper coordination polymer powder was evenly placed in a graphite boat, placed in a tube furnace, and heated from room temperature to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere. The temperature was then held at this temperature for 1 hour and then naturally cooled to room temperature to obtain the intermediate product after the first step pyrolysis.

[0050] (3) Acid washing treatment: Immerse the intermediate product in 0.5M H2SO4 solution for 12 hours to remove some unstable impurities. Then, add FeCl3 solution for further treatment to completely remove metallic copper or copper oxide particles. Finally, wash repeatedly with ultrapure water until the supernatant is neutral to completely remove by-products and impurities. Dry the washed sample.

[0051] (4) Second step pyrolysis: The acid-washed and dried sample is placed again in the graphite boat in the tube furnace. Under nitrogen atmosphere, the temperature is raised to 1200-1500℃ at a rate of 5℃ / min and held for 1 hour for the second step pyrolysis. Then it is naturally cooled to room temperature to obtain hard carbon nanospheres (Cu-HCs).

[0052] This invention also proposes a hard carbon nanosphere prepared according to the above preparation method. The hard carbon nanosphere has an average particle size of 50-200 nm, a graphite interlayer spacing of 0.38-0.40 nm, 2-4 graphite stacking layers, and a specific surface area of ​​27-41 m². 2 / g.

[0053] This invention also proposes an application of the above-mentioned hard carbon nanospheres in sodium-ion batteries.

[0054] All raw materials used in the embodiments of this invention were purchased commercially.

[0055] In the embodiments of this invention, room temperature refers to "25±3℃".

[0056] The technical solution of the present invention will be further illustrated by the following embodiments.

[0057] Example 1

[0058] A method for preparing hard carbon nanospheres specifically includes the following steps:

[0059] (1) Weigh 10 mmol of 2,6-diacetylpyridine (DAP), 10 mmol of 1,8-diaminonaphthalene (DAN), and 100.0 mg of oxalic acid, add them to 50 mL of anhydrous ethanol, and stir at room temperature until completely dissolved and mixed to obtain a light purple-red mixed solution. Place the mixed solution in an oil bath at 60 °C and stir continuously for 8 hours. Observe that the solution turns into a clear dark purple-red color. Then, add 10 mmol of CuCl2·2H2O to this solution and stir continuously at room temperature for 12 hours. After the reaction is completed, centrifuge to obtain a brown-black precipitate, wash it with ethanol, and finally dry it in an oven to obtain a brown-black copper coordination polymer powder.

[0060] (2) First step pyrolysis: The above-mentioned dried copper coordination polymer powder was evenly placed in a graphite boat, placed in a tube furnace, and heated from room temperature to 900°C at a heating rate of 5°C / min under a nitrogen atmosphere. The temperature was then held at this temperature for 1 hour and then naturally cooled to room temperature to obtain the intermediate product after the first step pyrolysis.

[0061] (3) Acid washing treatment: The intermediate product was immersed in 0.5M H2SO4 solution for 12 hours to remove some unstable impurities. Then, 50mL of FeCl3 solution was added for further treatment. Finally, the sample was washed repeatedly with ultrapure water until the supernatant was neutral. The washed sample was then dried.

[0062] (4) Second step pyrolysis: The acid-washed and dried sample is placed again in the graphite boat in the tube furnace. Under nitrogen atmosphere, the temperature is raised to 1300℃ at a rate of 5℃ / min and held for 1 hour for the second step pyrolysis. Then it is naturally cooled to room temperature to obtain hard carbon nanospheres (denoted as Cu-HCs900-1300).

[0063] Example 2

[0064] Same as Example 1, except that the second pyrolysis step is to raise the temperature to 1200℃, and the resulting hard carbon nanospheres are denoted as Cu-HCs900-1200.

[0065] Example 3

[0066] Same as Example 1, except that the second pyrolysis step is to raise the temperature to 1400℃, and the resulting hard carbon nanospheres are denoted as Cu-HCs900-1400.

[0067] Example 4

[0068] Same as Example 1, except that the second pyrolysis step is to raise the temperature to 1500℃, and the resulting hard carbon nanospheres are denoted as Cu-HCs900-1500.

[0069] Comparative Example 1

[0070] A hard carbon nanosphere without added copper ions was prepared using the same method as in Example 1, except that CuCl2·2H2O was not added in step (1). Specifically:

[0071] (1) Weigh 10 mmol of 2,6-diacetylpyridine (DAP), 10 mmol of 1,8-diaminonaphthalene (DAN) and 100.0 mg of oxalic acid, dissolve them in 50 mL of anhydrous ethanol, stir at room temperature until completely dissolved and mixed to obtain a light purple-red mixed solution. Place the mixed solution in a 60 °C oil bath and stir continuously for 8 hours. Observe that the solution turns into a clear dark purple-red color. Stir continuously at room temperature for 12 hours to obtain polymer powder.

[0072] (2)-(4) Same as in Example 1, the hard carbon nanospheres without added copper ions are denoted as HCs900-1300.

[0073] Performance testing

[0074] 1. Microscopic morphological characterization

[0075] The microstructure characterization of Cu-HCs900-1300 prepared in Example 1 of this invention is shown in the figure. Figure 2In the figures, (a) is a scanning electron microscope (SEM) image, (b) is a low-resolution transmission electron microscope (TEM) image, (c) is a high-resolution TEM image, (d) is the interlayer spacing test result, (e) is the low-resolution TEM image selected corresponding to the energy-dispersive X-ray spectroscopy (EDX-X) spectrum, and (f)-(i) are the EDX spectra of C, N, O, and Cu elements, respectively. Based on (a) and (b), the prepared hard carbon material exhibits a uniform spherical shape (diameter -100 nm). (c) shows that the hard carbon has a distorted graphite lattice with 2-3 stacked graphite layers. The interlayer spacing measured in the yellow box area of ​​(c), as shown in (d), is 0.381 nm, which is much larger than the interlayer spacing of graphite (0.34 nm), which is beneficial for sodium ion storage. The EDX spectra in (e)-(i) show that the hard carbon mainly contains C, N, O, and Cu elements.

[0076] 2. X-ray diffraction pattern

[0077] The X-ray diffraction (XRD) patterns of the hard carbon nanospheres prepared in Examples 1-4 and Comparative Example 1 are shown below. Figure 3 As can be seen, all samples exhibit broad (002) and (100) diffraction peaks at approximately 23° and 44°, indicating that they possess an amorphous structure. Compared to the HCs900-1300 sample in Comparative Example 1, the (002) diffraction peaks of all Cu-HCs samples show broadening and a significant leftward shift, indicating an increased interlayer spacing and enhanced structural disorder. Copper particle peaks were also detected, and the Cu-HCs900-1300 sample exhibited the strongest copper particle diffraction peaks with increasing pyrolysis temperature. Furthermore, based on Bragg's law, the interlayer spacing d of Cu-HCs900-1300 was calculated. 002 =0.386nm is significantly greater than HCs900-1300 (d 002 =0.35nm) and graphite (d 002 =0.34nm), which is consistent with Figure 2 The results in (d) are consistent with those in the middle, indicating that the introduction of copper expands the interlayer spacing of hard carbon, which is beneficial for the insertion and extraction of sodium ions.

[0078] 3. Nitrogen adsorption-desorption test

[0079] The nitrogen adsorption-desorption test results of the hard carbon nanospheres prepared in Examples 1-4 and Comparative Example 1 are shown in the figure. Figure 4 In Figure (a), the nitrogen adsorption / desorption isotherms of the examples and comparative samples are shown. It can be seen that the nitrogen adsorption / desorption isotherms of all samples exhibit Type IV curves, indicating the presence of a mesoporous structure. Based on the Brunauer-Emmett-Teller formula, the specific surface area of ​​all Cu-HCs samples in the examples is significantly higher than that of the HCs900-1300 samples (from 3.41 m²). 2 ·g-1 Increased to 41 m 2 ·g -1 (b) shows the pore size distribution, confirming that all samples possess a hierarchical porous structure ranging from micropores to macropores; however, the HCs900-1300 sample exhibits only limited porosity. These results indicate that introducing Cu into the precursor... 2+ It can significantly regulate the pore structure and increase the specific surface area during pyrolysis, thereby promoting the efficient storage of sodium ions.

[0080] 4. Electrochemical performance testing

[0081] Electrode preparation and battery assembly: 120.0 mg of the hard carbon nanospheres prepared in Examples 1-4 and Comparative Example 1 were weighed and mixed with 15.0 mg of conductive additive (Super P) and 15.0 mg of binder (polyvinylidene fluoride, PVDF), respectively, until homogeneous. The mixture was added to N-methylpyrrolidone (NMP) solvent and degassed for 15 minutes using a degassing mixer to form a homogeneous slurry. The slurry was then uniformly coated onto copper foil using a coating machine and subsequently vacuum dried at 120 °C for 12 hours. The loading of active material on the copper foil was 1 mg / cm³. 2 After stamping, the working electrode is obtained. In an argon-filled glove box, the working electrode prepared above, a sodium sheet with a diameter of 15.6 mm and a thickness of 0.5 mm as the counter electrode, a separator (GF / D, Whatman) and an electrolyte (1.0 M NaPF6 diethylene glycol dimethyl ether solution) are sequentially assembled into a CR2032 button cell.

[0082] Performance Testing: The assembled batteries underwent electrochemical performance testing on a battery testing system (Newway), with a voltage window of 0.01 to 3.0 V (vs. Na). + / Na).

[0083] The electrochemical performance test results of the examples and comparative samples are shown in the figure. Figure 5 Wherein (a) is the Cu-HCs900-1300 sample at 0.05 A·g -1 (a) shows the performance test results of 1-3 cycles at current density, and (b) shows the rate performance test results of the example and comparative samples. (c) shows the performance test results of the example and comparative samples at 100 mA·g. -1 Cyclic performance under the given conditions, (d) for Cu-HCs900-1300 sample at 5000 mA·g -1 Cyclic performance under the given conditions. (a) Figure shows the Cu-HCs900-1300 electrode at 0.05 A·g. -1 It exhibited 500.87 mAh·g in the first cycle at the specified current density. -1The discharge specific capacity is high, the first charge-discharge efficiency (ICE) reaches 72.73%, and the plateau capacity is 208.96 mAh·g. -1 (<0.1 V), accounting for 57% of the total capacity. Figure (b) shows the rate performance of all samples. It can be seen that Cu-HCs900-1300 exhibits the best reversible specific capacity at different current densities, namely: 365.64, 342.48, 329.66, 311.48, 294.72, 273.62, 254.07 and 215.58 mAh·g. -1 Moreover, when the current density recovers to 0.05 A·g -1 At that time, the reversible capacity of the Cu-HCs900-1300 electrode can still be restored to 348 mAh·g. -1 The capacity loss was only 4.66%. In contrast, the copper-free sample HCs900-1300 at 0.05 A·g... -1 and 5 A·g -1 The reversible capacity under these conditions is only 199.19 mAh·g. -1 (It is 54% of Cu-HCs900-1300) and 106.11 mAh·g -1 (This is 49.2% of Cu-HCs900-1300). Furthermore, the cycling performance of the material was investigated; Figure (c) shows the sample at 0.1 A·g. -1 Under the given cycling conditions, Cu-HCs900-1300 exhibited the best performance (high capacity retention) after 100 cycles, which may be closely related to its unique structure (nanospheres, large interlayer spacing, and appropriate graphite stacking layers). Furthermore, the cycling performance of Cu-HCs900-1300 at high current densities was investigated, as shown in Figure (d), at a current density of 5 A·g⁻¹. -1 The Cu-HCs900-1300 electrode exhibited excellent cycling performance, maintaining a capacity retention of 78.55% and a coulombic efficiency close to 100% after 5000 cycles. These results demonstrate that the unique structure of the hard carbon nanospheres prepared in the examples, especially Cu-HCs900-1300, endows them with superior electrochemical performance.

[0084] Figure 6 To reveal the efficient sodium storage mechanism of Cu-HCs 900-1300 based on DFT (Density Functional Theory) theoretical calculations, (a) presents a sodium intercalation model in bilayer graphite, (b) shows the binding energy variation caused by sodium ion intercalation under different interlayer spacings, (c) presents a sodium intercalation model with different numbers of layers, and (d) shows the binding energy variation caused by sodium ion intercalation under different numbers of layers. It can be seen that when the graphite interlayer spacing increases from 3.4 Å to 4.0 Å, the sodium... + The binding energy gradually decreases, proving that increasing the interlayer spacing is beneficial to Na.+ Storage. Meanwhile, with a fixed interlayer spacing (3.85 Å), as the number of stacked layers (1-4 layers) increases, Na... + The binding energy initially decreased and then increased, indicating that an appropriate number of layers (approximately 3 layers) can promote efficient sodium storage. This further demonstrates the reason why the target material Cu-HCs900-1300 possesses excellent electrochemical performance.

[0085] Comparative Example 2

[0086] Same as Example 1, except that the two-step pyrolysis is eliminated, and the copper coordination polymer precursor is directly pyrolyzed to 1300°C in one step, specifically including the following steps:

[0087] (1) Weigh 10 mmol of 2,6-diacetylpyridine (DAP), 10 mmol of 1,8-diaminonaphthalene (DAN), and 100.0 mg of oxalic acid, add them to 50 mL of anhydrous ethanol, and stir at room temperature until completely dissolved and mixed to obtain a light purple-red mixed solution. Place the mixed solution in an oil bath at 60 °C and stir continuously for 8 hours. Observe that the solution turns into a clear dark purple-red color. Then, add 10 mmol of CuCl2·2H2O to this solution and stir continuously at room temperature for 12 hours. After the reaction is completed, centrifuge to obtain a brown-black precipitate, wash it with ethanol, and finally dry it in an oven to obtain a brown-black copper coordination polymer powder.

[0088] (2) One-step pyrolysis: The above-mentioned dried copper coordination polymer powder was uniformly placed in a graphite boat, placed in a tube furnace, and heated from room temperature to 1300°C at a heating rate of 5°C / min under a nitrogen atmosphere. The temperature was then maintained at this temperature for 1 hour, and then naturally cooled to room temperature to obtain the intermediate product after the first step of pyrolysis.

[0089] (3) Acid washing treatment: The intermediate product was immersed in 0.5M H2SO4 solution for 12 hours to remove some unstable impurities. Then, 50mL of FeCl3 solution was added for further treatment. Finally, the product was washed repeatedly with ultrapure water until the supernatant was neutral. The washed sample was dried to obtain hard carbon nanospheres.

[0090] Figure 7 A comparison of the electrochemical performance tests of the samples from Example 1 (Cu-HCs 900-1300 (two-step pyrolysis)) and Comparative Example 2 (Cu-HCs 900-1300 (one-step pyrolysis)) shows that at 100 mA·g -1 The specific capacity of the sample obtained by two-step pyrolysis under the given conditions was much higher than that obtained by one-step pyrolysis, indicating that two-step pyrolysis can provide more active sites for sodium storage.

[0091] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. Use of hard carbon nanospheres in sodium-ion batteries, characterized in that, The hard carbon nanospheres have a graphite interlayer spacing of 0.38-0.40 nm, a graphite stacking layer number of 2-4 layers, and a specific surface area of 27-41 m 2 / g. The preparation method of the hard carbon nanospheres comprises the following steps: reacting 2,6-diacetylpyridine, 1,8-diaminonaphthalene and a copper salt in a solvent to obtain a copper coordination polymer precursor; carrying out first pyrolysis on the copper coordination polymer precursor under an inert atmosphere to obtain an intermediate product; carrying out acid pickling treatment on the intermediate product and carrying out second pyrolysis on the product after acid pickling under an inert atmosphere to obtain the hard carbon nanospheres; the usage ratio of the 2,6-diacetylpyridine, 1,8-diaminonaphthalene, copper salt and solvent is 1 mmol:1 mmol:1 mmol:5 mL; the copper salt comprises copper chloride; the temperature of the second pyrolysis is 1200-1500 DEG C, the heating rate is 5 DEG C / min, and the holding time is 1 hour; the temperature of the first pyrolysis is 900 DEG C, the heating rate is 5 DEG C / min, and the holding time is 1 hour. 2.The application of the hard carbon nanospheres in sodium ion batteries according to claim 1, characterized in that, the acid pickling treatment comprises soaking treatment using an inorganic acid solution, and the inorganic acid is at least one of sulfuric acid, hydrochloric acid or nitric acid.

3. A sodium-ion battery anode, characterized in that, The hard carbon nanospheres prepared in the application of any one of claims 1-2.

Images