Hard carbon material, preparation method thereof, negative electrode and battery

Abstract

This invention relates to the field of battery materials technology, and discloses hard carbon materials, their preparation methods, anodes, and batteries. The hard carbon material provided in this invention comprises multiple particles containing closed pores. The closed-pore rate of the hard carbon material is 80-88%, the total closed-pore volume is 0.2-0.28 cm³ / g, and the proportion of closed pores with a pore size in the range of 1-2 nm is 75-80%. The preparation method of this hard carbon material includes a gradient-heating pre-oxidation treatment of the resin powder before heat treatment. The hard carbon material provided by this invention can provide sufficient reversible storage sites for sodium ions, reduce electrolyte intrusion and side reactions, synergistically improve the reversible capacity, initial coulombic efficiency, and cycle stability of the hard carbon anode, achieving optimal matching between pore structure and electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically, to hard carbon materials, their preparation methods, negative electrodes, and batteries. Background Technology

[0002] With the increasing prevalence of portable electronic devices and electric vehicles, the demand for lithium-ion batteries with high energy density and long cycle life is constantly growing. Hard carbon anode materials, due to their unique advantages such as wide availability, low cost, good structural stability, and high specific capacity, have attracted much attention in sodium-ion batteries, lithium-ion batteries, and other fields, becoming one of the research hotspots in recent years and showing promising application prospects. Hard carbon possesses a low sodium / lithium intercalation plateau and high capacity. In sodium-ion batteries, its sodium storage mechanism exhibits a plateau region below 0.1V, which can improve the average operating voltage and energy density of the battery.

[0003] However, current hard carbon anode materials cannot effectively utilize the sodium storage capacity in the plateau region below 0.1V.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide hard carbon materials and their preparation methods, negative electrodes and batteries, in order to improve at least one of the problems mentioned in the background art.

[0006] This invention is implemented as follows: In a first aspect, embodiments of the present invention provide a hard carbon material comprising a plurality of particles containing closed pores, wherein the closed pore rate of the hard carbon material is 80-88%, the total volume of closed pores is 0.2-0.28 cm³ / g, and the proportion of closed pores with a pore size in the range of 1-2 nm is 75-80%.

[0007] In an optional embodiment, the oxygen content in the hard carbon material is 0.8 to 1.2 wt%, preferably, the oxygen content in the hard carbon material is 1.06 to 1.12 wt%. In an optional implementation, the oxygen content on the particle surface is greater than the oxygen content inside the particle.

[0008] In an optional embodiment, the hard carbon material has a Dmax of 25~30μm and a Dv50 of 5~8μm.

[0009] Secondly, embodiments of the present invention provide a method for preparing a hard carbon material, comprising: Pre-oxidation: The resin powder is treated at 200~220℃ for 40~80min in an oxygen-containing atmosphere, then heated to 300~320℃ for 40~80min, and finally heated to 380~400℃ for 40~80min to obtain the pre-oxidized product. The molecular structure of the resin powder is a three-dimensional network structure. Heat treatment: Switch the oxygen-containing atmosphere to an inert atmosphere, continue to heat the pre-oxidized product, and perform heat treatment at a temperature of 1200~1500℃ for 2~4 hours.

[0010] In an optional implementation, the oxygen-containing atmosphere is an air atmosphere; Optionally, the heating rate of the pre-oxidation process is 1~2℃ / min; Optionally, the heating rate of the heat treatment process is 1~5℃ / min.

[0011] In an optional embodiment, the method for preparing the resin powder includes: Small molecule short-chain molecular structure resin raw materials are cured to form a three-dimensional network molecular structure resin, and then crushed; Optionally, the oxygen content in the resin raw material is 10-15% by mass; Optionally, the resin raw material is selected from at least one of phenolic resin, epoxy resin, polyurethane resin, unsaturated polyester resin and acrylic resin; Optionally, the curing method involves holding the small-molecule, short-chain molecular structure resin raw material at 95~105℃ for 120-240 minutes.

[0012] In an optional embodiment, the resin powder has a Dmax of 25-30 μm and a Dv50 of 5-8 μm.

[0013] Thirdly, embodiments of the present invention provide a negative electrode, wherein the active layer comprises the hard carbon material provided in the embodiments of the present invention or the hard carbon material prepared by the preparation method provided in the embodiments of the present invention.

[0014] Fourthly, embodiments of the present invention provide a battery, including a negative electrode provided in embodiments of the present invention.

[0015] The present invention has the following beneficial effects: The closed-pore volume and closed-pore ratio of hard carbon materials correspond to sodium storage in the plateau region. Hard carbon closed pores (especially <3nm ultramicropores) are the main source of low-voltage plateau capacity of sodium electrodes, while 1~2 nm closed pores have the highest filling efficiency and contribute the most to the plateau capacity. The 1~2 nm closed-pore structure of hard carbon materials provided by this invention, with its high proportion, high ratio, and moderate pore volume, can not only provide sufficient reversible storage sites for sodium ions, but also reduce electrolyte intrusion and side reactions, synergistically improving the reversible capacity, first coulombic efficiency, and cycle stability of hard carbon anodes, achieving the optimal match between pore structure and electrochemical performance.

[0016] The method for preparing hard carbon materials provided by the present invention involves pre-oxidizing the resin powder under a gradient temperature rise in an oxygen atmosphere before heat treatment, which can produce hard carbon materials with closed-pore parameters within the requirements of the present invention and have excellent electrochemical performance. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0018] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0019] In response to the problems existing in the current technology, the inventor conducted research and made the following considerations: The structure of hard carbon materials is complex and diverse. Its internal pore structure, microstructure, and surface functional groups all affect the platform capacity.

[0020] On the one hand, during the carbonization process of hard carbon, the irregular arrangement of carbon atoms creates various defects and active sites. While these defects can facilitate the adsorption and diffusion of sodium or lithium ions to some extent, excessive defects can lead to unstable and uneven plateau capacity. On the other hand, the closed-pore and open-pore structures in hard carbon materials contribute differently to the plateau capacity. Open-pore structures facilitate electrolyte penetration and ion transport, while closed-pore structures reduce the direct contact area between the electrolyte and the hard carbon material, reducing side reactions and thus maintaining the stability of the electrode structure. However, current hard carbon preparation processes struggle to precisely control the ratio and distribution of these two pore structures, thereby affecting the performance of the plateau capacity.

[0021] Furthermore, the inventors discovered that the selection and processing method of the precursor also have a significant impact on the platform capacity of the hard carbon anode material. Different precursors have different chemical compositions, molecular structures, and thermal stability, resulting in significant differences in the microstructure and properties of the hard carbon materials generated after carbonization.

[0022] Therefore, the present invention proposes the following solution: This invention provides a hard carbon material comprising multiple particles. The closed-cell rate of the hard carbon material is 80-88% (e.g., 80%, 82%, 84%, 86%, or 88%, or any value within the range of any two of the aforementioned values), the total closed-cell volume is 0.2-0.28 cm³ / g (e.g., 0.2³ / g, 0.22³ / g, 0.24³ / g, 0.26³ / g, or 0.28³ / g, or any value within the range of any two of the aforementioned values), and the proportion of closed-cells with a pore size in the range of 1-2 nm is 75-80% (e.g., 75%, 78%, or 80%, or any value within the range of any two of the aforementioned values).

[0023] The closed-pore volume and closed-pore ratio of hard carbon materials correspond to sodium storage in the plateau region. Hard carbon closed pores (especially <3nm ultramicropores) are the main source of low-voltage plateau capacity of sodium electrodes, while 1~2 nm closed pores have the highest filling efficiency and contribute the most to the plateau capacity. The 1~2 nm closed-pore structure of hard carbon materials provided by this invention, with its high proportion, high ratio, and moderate pore volume, can provide sufficient reversible storage sites for sodium ions, reduce electrolyte intrusion and side reactions, and synergistically improve the reversible capacity, first coulombic efficiency, and cycle stability of hard carbon anodes, achieving a good match between pore structure and electrochemical performance. Understandably, without theoretical constraints, if the proportion of closed pores in the 1-2nm range is too small, there will be insufficient effective sodium storage sites, leading to a low low-pressure plateau capacity and difficulty in improving the overall specific capacity. If the proportion of closed pores in the 1-2nm range is too large, it will easily cause excessively loose internal particle structure, which will aggravate volume expansion during charge and discharge, reduce structural stability, and worsen cycle life. If the total volume of closed pores is too small, there will be insufficient sodium storage space, which will directly manifest as a low specific capacity and insufficient plateau capacity. If it is too large, it will lead to insufficient material skeleton strength, which will cause problems such as a decrease in initial coulombic efficiency, difficulty in electrode compaction, and severe electrode expansion. If the closed pore ratio is too small, the sodium storage capacity will be limited, and the energy density will be difficult to meet the application requirements. In addition, if it is too large, it will lead to a significant decrease in particle density, which will not only make it difficult to improve the compaction density, but also lead to a deterioration in rate performance and a decrease in cycle stability due to an increase in structural defects.

[0024] In hard carbon (non-graphitized carbon), the irregular stacking of 2 to 4 layers of twisted graphite microcrystals with short-range order forms nanoscale cavities (mostly <3nm in diameter, mainly ultramicropores / micropores), which are closed pores.

[0025] In this invention, the method for determining various characteristics of hard carbon materials is as follows: 1. Test method for the percentage of closed pores in the 1~2nm range: Small-angle X-ray scattering (SAXS) works by using the electron density difference (carbon vs. air / vacuum inside the pore) to generate scattering, and simultaneously detects both open and closed pores, without being limited by the diffusion of probe molecules.

[0026] The 1~2 nm range corresponds to a scattering vector q≈3.14~6.28 nm. - ¹(2θ≈0.5°~1°)

[0027] This method can be used to determine the percentage of closed pores in the 1-2 nm range. The percentage of closed pores in the 1-2 nm range is defined by the determined scattering vector range. ,conduct Weighted integration yields the scattering integral intensity corresponding to an aperture size of 1-2 nm. The integral formula is Simultaneously, the scattering vector range corresponding to all effective closed pores in the sample. conduct Weighted integration yields the total integrated intensity of scattering across the entire aperture. The integral formula is The scattering integral intensity I corresponds to its volume distribution V(d), and the obtained volume distribution Through formula Convert to quantity distribution Where d is the pore size. Integrating the number distribution over the pore size range of 1~2nm yields... For all effective closed pores, the corresponding pore diameter range Integrating the quantity distribution yields... .

[0028] 2. Test methods for total closed-cell volume and closed-cell ratio: The total closed-cell volume in a unit mass of hard carbon material was determined using a combination of helium specific gravity and mercury intrusion method, and calculated by the difference between the true density of helium (ρg) and the true density of liquid (ρL). Total closed-cell volume V closed = Volume of mercury intrusion method (1 / ρL) - Volume of helium specific gravity method (1 / ρg); Closed-pore ratio = V closed (Closed-cell volume) / Helium density method volume × 100%.

[0029] Principle: Based on Boyle's Law, helium (with extremely small molecules, approximately 0.26 nm) is used as the displacement medium, allowing it to penetrate almost all pores of hard carbon. Mercury does not wet carbon and requires pressure to enter the pores, typically covering an area of ​​≈3 nm to 1000 μm (3 nm corresponds to 400 MPa). Core logic: Helium method: eliminates both open and closed pores → yields the smallest volume (pure framework). Mercury intrusion porosimetry: only eliminates open pores, including closed pores → yields a larger volume (framework + closed pores).

[0030] In some embodiments, to achieve better electrochemical performance of the hard carbon material, the particle size Dmax of the hard carbon material is 25~30μm (e.g., 25μm, 28μm or 30μm, or any value within the range between any two of the aforementioned values); and Dv50 is 5~8μm (e.g., 5μm, 6μm, 7μm or 8μm, or any value within the range between any two of the aforementioned values).

[0031] In some embodiments, the oxygen content in the hard carbon material is 0.8–1.2 wt% (e.g., 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, or 1.2 wt%, or any value within the range of any two of the aforementioned values). The oxygen content in the hard carbon material is 1.06–1.12 wt% (e.g., 1.06 wt%, 1.08 wt%, 1.10 wt%, or 1.12 wt%, or any value within the range of any two of the aforementioned values). An oxygen content within this range indicates a higher number of active sites in the hard carbon material, which contributes to better electrochemical performance. It is understood that, without theoretical constraints, the oxygen content in the hard carbon material should not be too high. Excessive oxygen content can lead to a significant decrease in initial coulombic efficiency, an increase in side reactions, and accelerated electrolyte decomposition. It can also cause electrode expansion, poor cycle stability, and deterioration in rate performance.

[0032] In some embodiments, the oxygen content on the surface of the particles is greater than the oxygen content inside the particles. That is, the oxygen content is slightly higher on the surface and slightly lower inside. For example, the oxygen content of the material can be tested using an oxygen content analyzer (e.g., a Thermo Fisher Scientific FlashSmart elemental analyzer); the oxygen distribution can be detected by cutting the material particles using a focused ion beam (FIB) and mapping the cross-section using SEM-EDS. It is understood that the above oxygen content distribution results in: high oxygen on the surface: improving electrolyte wettability, inducing the formation of a uniform and stable SEI film, reducing interfacial impedance, improving rate performance, and reducing initial irreversible capacity; low oxygen inside: improving electronic conductivity and structural stability, reducing bulk side reactions and volume expansion, enhancing closed-pore sodium storage capacity, and improving reversible capacity and cycle life; gradient distribution avoids abrupt changes in interfacial impedance, achieving a balance between rapid sodium ion transport and stable storage.

[0033] The method for preparing hard carbon material provided in this embodiment of the invention includes: S1, Pre-oxidation: (1) The resin raw material is cured and crushed to obtain resin powder.

[0034] The resin raw material is a commercially available small-molecule short-chain molecular structure (weight average molecular weight 1000~5000, for example 1000, 2000, 3000, 4000 or 5000, or any value within the range of any two of the aforementioned values). It cannot be used directly, as direct application will cause foaming and expansion during subsequent heat treatment, making it impossible to obtain the product required by this invention. After curing, a three-dimensional network structure is formed, resulting in better molecular stability.

[0035] Optionally, the curing method involves holding the small-molecule short-chain molecular structure resin at 95~105℃ (e.g., 95℃, 100℃ or 105℃, or any value within the range of any two of the aforementioned values) for 120~240 min (e.g., 120 min, 150 min, 180 min, 210 min or 240 min, or any value within the range of any two of the aforementioned values).

[0036] In some embodiments, the resin powder is further subjected to vibratory sieving after crushing, which can improve the flowability of the resin powder, reduce agglomeration, and improve the subsequent processability of the resin powder. It should be understood that this application does not limit the frequency and time of vibratory sieving, etc. Those skilled in the art can select appropriate vibratory sieving frequency and time according to actual experimental conditions (such as vibratory sieving instruments, etc.), all of which are within the protection scope of this application.

[0037] Optionally, the oxygen content in the resin raw material is 10-15% by mass (e.g., 10%, 12%, 13%, or 15%, or any value within the range of any two of the aforementioned values). By selecting a resin with a suitable oxygen content as the raw material, the product will also contain a certain amount of oxygen after heat treatment, which can introduce more active sites and thus improve the electrochemical performance of the material. After the resin powder with an oxygen content of 10-15% by mass is treated by the pre-oxidation method described in this invention, the oxygen content can be increased to 18-20%, thereby increasing the oxygen content of the final product.

[0038] Optionally, the resin raw material is selected from at least one of phenolic resin, epoxy resin, polyurethane resin, unsaturated polyester resin and acrylic resin.

[0039] Optionally, to ensure that the prepared hard carbon material has a better particle size distribution and further improve the electrochemical performance of the material, the single particle size of the resin powder is 25~30μm (e.g., 25μm, 28μm or 30μm, or any value within the range of any two of the aforementioned values), and the median particle size is 5~8μm (e.g., 5μm, 6μm, 7μm or 8μm, or any value within the range of any two of the aforementioned values).

[0040] (2) The resin powder is placed in an oxygen-containing atmosphere at 200~220℃ (e.g., 200℃, 210℃ or 220℃, or any value within the range of any two of the aforementioned values) for 40~80 min (e.g., 40 min, 60 min or 80 min, or any value within the range of any two of the aforementioned values), then heated to 300~320℃ (e.g., 300℃, 310℃ or 320℃, or any value within the range of any two of the aforementioned values) for 40~80 min (e.g., 40 min, 60 min or 80 min, or any value within the range of any two of the aforementioned values), and finally heated to 380~400℃ (e.g., 380℃, 390℃ or 400℃, or any value within the range of any two of the aforementioned values) for 40~80 min (e.g., 40 min, 60 min or 80 min, or any value within the range of any two of the aforementioned values) to obtain the pre-oxidized product.

[0041] The process involves mild oxidation at 200-220℃, deep pore formation at 300-320℃, and structural stabilization at 380-400℃, during which the resin undergoes further cross-linking. This gradient heating process allows for precise control of the internal pore structure of the resin precursor, preferentially generating uniformly sized and regularly distributed nanoscale closed pores. Compared to traditional single-temperature pre-oxidation, gradient heat treatment effectively suppresses macropore formation and micropore collapse, resulting in 75-80% of the particles having closed pores with a diameter of 1-2 nm, while controlling the total closed pore volume at 0.2-0.28 cm³ / g and maintaining a stable closed-pore rate of 80%-88%. This high-proportion, high-density, and moderately sized 1-2 nm closed-pore structure provides ample reversible storage sites for sodium ions and reduces electrolyte intrusion and side reactions, synergistically improving the reversible capacity, initial coulombic efficiency, and cycle stability of the hard carbon anode, achieving a good match between pore structure and electrochemical performance.

[0042] The pre-oxidation process can remove volatiles (such as moisture and small molecule organic matter) from the resin powder, avoid the collapse of the carbon skeleton due to violent gas generation during subsequent high-temperature carbonization, reduce the material volume shrinkage rate, and improve the structural integrity of the final hard carbon. The gradient heating treatment of this invention can increase the proportion of closed pores in the 1~2nm range in the obtained hard carbon, so that the closed pores can reach 75~80%.

[0043] The pre-oxidation process should be carried out at a temperature not lower than 200°C to avoid insufficient cross-linking affecting the electrochemical performance of the material, and not higher than 400°C to avoid excessive carbonization and agglomeration.

[0044] Optionally, the heating rate of the pre-oxidation process is 1~2℃ / min (e.g., 1℃ / min, 1.5℃ / min, or 2℃ / min, or any value within the range of any two of the aforementioned values). Heating at this rate avoids sudden temperature increases that could lead to resin melting or structural collapse.

[0045] S2, Heat Treatment: Switch from an oxygen-containing atmosphere to an inert atmosphere, continue heating, and perform heat treatment on the pre-oxidized product. The heat treatment temperature is 1200~1500℃ (e.g., 1200℃, 1300℃, 1400℃ or 1500℃, or any value within the range of any two of the aforementioned values), and the heat treatment time is 2~4h (2h, 3h or 4h, or any value within the range of any two of the aforementioned values).

[0046] Alternatively, the inert atmosphere may be a nitrogen atmosphere or a rare gas (such as argon).

[0047] During the heat treatment process, excessively low temperatures and short times will reduce the number of closed pores in the hard carbon matrix, increase the specific surface area, and lead to a decrease in capacity. On the other hand, excessively high temperatures or long times will result in over-sintering, which will cause severe graphitization of the hard carbon material and affect its capacity.

[0048] Optionally, the heating rate during the heat treatment process is 1~5℃ / min (e.g., 1℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min, or any value within the range of any two of the aforementioned values). Heating at this rate is beneficial for the slow dehydrogenation, condensation, and framework rearrangement of the precursor, enabling the controllable formation of effective closed pores of 1~2nm with uniform size and stable structure. At the same time, it reduces defects and disordered carbon formation, balancing high specific capacity and good structural stability. However, if the heat treatment cycle is too long, the production efficiency will decrease significantly, and excessive carbonization shrinkage is likely to occur, resulting in insufficient closed pore volume and low sodium storage capacity. If the heating rate is too high, exceeding 5℃ / min, the internal thermal stress will be too great, which may cause severe collapse of the pore structure or local excessive expansion, resulting in uneven distribution of closed pores and an increase in macropores. This will also exacerbate the initial irreversible capacity loss and worsen cycle stability and electrode compaction performance.

[0049] Optionally, the type of oxygen-containing atmosphere is not limited; it can be a mixture of oxygen and other inert gases. However, considering its ease of acquisition, an air atmosphere is selected in the embodiments of the present invention.

[0050] In this invention, the heating method is not limited. The above-mentioned pretreatment and heat treatment are usually carried out by one or more of the following: box furnace, pusher furnace, roller furnace, tunnel furnace, rotary furnace, etc. The heat treatment heating method is generally one or more of the following: resistance wire heating, silicon carbide rod heating, and silicon molybdenum rod heating.

[0051] The negative electrode provided in this embodiment of the invention comprises a hard carbon material provided by this invention or a hard carbon material prepared by the preparation method provided by this invention in its active layer. Because this negative electrode comprises the hard carbon material provided by this embodiment of the invention, it exhibits superior electrochemical performance.

[0052] The battery provided in this embodiment of the invention includes a negative electrode provided in this embodiment. Because it includes the negative electrode provided in this embodiment, the battery exhibits superior electrochemical performance.

[0053] Optionally, the battery is a lithium-ion battery or a sodium-ion battery, preferably a sodium-ion battery.

[0054] Example 1 (1) Phenolic resin (thermosetting type, weight average molecular weight of about 3000) was cured (100℃, held for 240 min) and then crushed. The particle size of the phenolic resin powder was determined by laser diffraction of particle size distribution according to GB / T 19077-2016 using a Malvern Mastersizer2000E laser particle size analyzer. The particle size of the phenolic resin powder was found to be Dv50=7.53μm and Dmax=26.74μm.

[0055] (2) The resin powder processed in the previous step is passed through a vibrating screen to improve its fluidity. The vibrating screen frequency is 30Hz and the screen mesh size is 325 mesh.

[0056] (3) The resin powder after the previous step is pre-oxidized in an air atmosphere in a rotary kiln. It is treated at 200℃ for 60 min, then heated to 320℃ for 60 min, and finally heated to 400℃ for 60 min. The heating rate of the whole process is 2℃ / min.

[0057] (4) Switch the air atmosphere to nitrogen atmosphere and continue to heat the temperature to 1400℃ for 4 hours at a rate of 2℃ / min to obtain resin-based hard carbon anode material.

[0058] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0059] Example 2 This embodiment is basically the same as Embodiment 1, except that the resin raw material is replaced with epoxy resin (thermosetting type, weight average molecular weight of about 3300).

[0060] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0061] Example 3 This embodiment is basically the same as Embodiment 1, except that: The pre-oxidation process in step (3) is as follows: treat at 210℃ for 50 min, then heat up to 310℃ for 50 min, and finally heat up to 390℃ for 50 min. The heating rate of the whole process is 1℃ / min.

[0062] The heat treatment process in step (4) is as follows: switch the air atmosphere to a nitrogen atmosphere, and continue to heat the temperature to 1300℃ for 3 hours at a rate of 2℃ / min to obtain the resin-based hard carbon anode material.

[0063] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0064] Example 4 This embodiment is basically the same as Embodiment 1, except that: The pre-oxidation process in step (3) is as follows: the temperature is raised to 220℃ for 40 min, then raised to 300℃ for 40 min, and finally raised to 380℃ for 40 min. The heating rate of the whole process is 1.5℃ / min.

[0065] The heat treatment process in step (4) is as follows: switch the air atmosphere to a nitrogen atmosphere, and continue to heat the temperature to 1500℃ for 2 hours at a rate of 5℃ / min to obtain the resin-based hard carbon anode material.

[0066] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0067] Example 5 This embodiment is basically the same as Embodiment 1, except that: The pre-oxidation process in step (3) is as follows: treat at 220℃ for 50 min, then heat up to 300℃ for 50 min, and finally heat up to 380℃ for 50 min. The heating rate of the whole process is 2℃ / min.

[0068] The heat treatment process in step (4) is as follows: switch the air atmosphere to a nitrogen atmosphere, and continue to heat the temperature to 1200℃ for 2 hours at a rate of 5℃ / min to obtain the resin-based hard carbon anode material.

[0069] Example 6 This embodiment is basically the same as Embodiment 1, except that: The heat treatment process in step (4) is as follows: switch the air atmosphere to a nitrogen atmosphere, and continue to heat the gas at a rate of 2℃ / min to 1300℃ for 2 hours to obtain the resin-based hard carbon anode material.

[0070] Example 7 This embodiment is basically the same as Embodiment 1, except that: The pre-oxidation process in step (3) is as follows: the temperature is raised to 220℃ for 40 min, then raised to 300℃ for 40 min, and finally raised to 380℃ for 40 min. The heating rate of the whole process is 1.5℃ / min.

[0071] The heat treatment process in step (4) is as follows: switch the air atmosphere to a nitrogen atmosphere, and continue to heat the temperature to 1200℃ for 4 hours at a rate of 2℃ / min to obtain the resin-based hard carbon anode material.

[0072] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0073] Comparative Example 1 This comparative example is basically the same as Example 1, except that in step (3), it is treated at 200°C for 3 hours.

[0074] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0075] Comparative Example 2 This comparative example is basically the same as Example 1, except that in step (3), it is treated at 320°C for 3 hours.

[0076] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0077] Comparative Example 3 This comparative example is basically the same as Example 1, except that in step (3), it is treated at 400°C for 3 hours.

[0078] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0079] Comparative Example 4 This comparative example is basically the same as Example 1, except that in step (3), it is treated at 100°C for 3 hours.

[0080] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0081] Comparative Example 5 This comparative example is basically the same as Example 1, except that in step (3), it is treated at 600°C for 3 hours.

[0082] The material was damaged due to excessively high temperature during the pretreatment process.

[0083] Comparative Example 6 This comparative example is basically the same as Example 1, except that step (3) is omitted.

[0084] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0085] Comparative Example 7 This comparative example is basically the same as Example 1, except that the heat treatment temperature in step (4) is 1100℃.

[0086] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0087] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0088] Comparative Example 8 This comparative example is basically the same as Example 1, except that the phenolic resin was not cured, that is, the resin that was pre-oxidized was a small molecule short-chain molecular structure resin.

[0089] Expansion occurred during the heat treatment process, resulting in the failure of hard carbon material manufacturing.

[0090] The product characteristics of the resin-based hard carbon anode material were measured and recorded in Table 1 below.

[0091] Table 1. Hard carbon oxygen content, closed-cell volume, and closed-cell ratio of the examples and comparative examples.

[0092] As can be seen from Table 1, the closed-cell parameters of the hard carbon materials prepared by the methods provided in the various embodiments of the present invention all meet the requirements of the present invention. Comparative Examples 1-8 did not prepare hard carbon materials according to the method provided in this invention, and the closed-cell parameters obtained were not within the range required by this invention. Among them, Comparative Example 5 suffered material damage due to excessively high pre-oxidation temperature.

[0093] Experimental Example The hard carbon materials prepared in each embodiment and comparative example were used to make batteries, and their electrochemical performance was measured.

[0094] The hard carbon material prepared in the above examples and comparative examples was mixed with conductive carbon black and a binder to form a slurry, which was then uniformly coated onto aluminum foil and dried to form an electrode. A sodium sheet was used as the counter electrode, and a glass fiber membrane was selected as the separator. A mixed solution of 1 mol / L NaPF6 and ethylene carbonate (EC): dimethyl carbonate (DEC) = 1:1 vol.% was used as the electrolyte to form a coin cell. The testing conditions were as follows: the initial discharge test was performed by discharging to 0.00V at 30mA / g, followed by recharging to 2.0V, and the test was conducted under constant temperature conditions of 25℃. The plateau capacity was below 0.1V.

[0095] Record the test results in Table 2.

[0096] Table 2 shows the electrochemical performance of the batteries prepared in each example and comparative example.

[0097] By comparing Comparative Examples 1-3 with Example 1 using the data in Tables 1 and 2, it was found that the pre-oxidation process did not follow the gradient heating treatment required by the present invention, and the closed-pore parameters and oxygen content of the hard carbon material obtained were not within the range required by the present invention, resulting in relatively poor electrochemical performance. Comparing Comparative Example 4 with Example 1, the closed-cell parameters and oxygen content of Comparative Example 4 are not within the range required by the present invention, and the electrochemical sodium storage performance is poor. This may be because the pre-oxidation temperature is low, resulting in a low degree of resin pre-oxidation cross-linking, insufficient closed-cell formation, small effective pore volume and uneven pore size distribution, which affects the capacity of hard carbon materials applied to batteries. Comparing Comparative Example 6 with Example 1, the closed-pore parameters and oxygen content of Comparative Example 6 are not within the range required by the present invention, and the electrochemical sodium storage performance is poor. This may be because there is no pre-oxidation process, the closed pores cannot develop in an orderly manner, the pore size distribution is disordered and the effective closed pore ratio is extremely low, and the pore structure is prone to collapse, thereby affecting the reversible specific capacity, first coulombic efficiency and cycle stability of hard carbon materials applied to batteries. Comparing Comparative Example 7 with Example 1, the pore-closure parameters and oxygen content of Comparative Example 7 are not within the range required by the present invention, and the electrochemical sodium storage performance is poor. This may be due to insufficient pore formation, poor structural regularity, and a low proportion of effective sodium storage channels, which affects the reversible capacity and cycle stability of the sodium-ion battery.

[0098] In summary, the 1-2 nm closed-pore structure with a high proportion and moderate pore volume in the hard carbon material provided by this invention not only provides sufficient reversible storage sites for sodium ions but also reduces electrolyte intrusion and side reactions, synergistically improving the reversible capacity, initial coulombic efficiency, and cycle stability of the hard carbon anode, achieving optimal matching between pore structure and electrochemical performance. In a preferred embodiment, the hard carbon material also contains a suitable amount of oxygen, which imparts an appropriate number of active sites to the hard carbon material, further improving the electrochemical performance of the material.

[0099] The method for preparing hard carbon materials provided by the present invention involves pre-oxidizing the resin powder under a gradient temperature rise in an oxygen atmosphere before heat treatment, which can produce hard carbon materials with closed-pore parameters within the requirements of the present invention and have excellent electrochemical performance.

[0100] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A hard carbon material, characterized in that, The hard carbon material comprises multiple particles containing closed pores. The closed pore rate of the hard carbon material is 80-88%, the total volume of closed pores is 0.2-0.28 cm³ / g, and the proportion of closed pores with a pore size in the range of 1-2 nm is 75-80%.

2. The hard carbon material according to claim 1, characterized in that, The oxygen content in the hard carbon material is 0.8~1.2 wt%, preferably 1.06~1.12 wt%.

3. The hard carbon material according to claim 2, characterized in that, The oxygen content on the surface of the particle is greater than the oxygen content inside the particle.

4. The hard carbon material according to any one of claims 1 to 3, characterized in that, The hard carbon material has a Dmax of 25~30μm and a Dv50 of 5~8μm.

5. A method for preparing a hard carbon material, characterized in that, include: Pre-oxidation: The resin powder is treated at 200~220℃ for 40~80min in an oxygen-containing atmosphere, then heated to 300~320℃ for 40~80min, and finally heated to 380~400℃ for 40~80min to obtain the pre-oxidized product. The molecular structure of the resin powder is a three-dimensional network structure. Heat treatment: Switch the oxygen-containing atmosphere to an inert atmosphere, continue to heat the pre-oxidized product, and perform heat treatment at a temperature of 1200~1500℃ for 2~4 hours.

6. The preparation method according to claim 5, characterized in that, The oxygen-containing atmosphere is an air atmosphere; Optionally, the heating rate of the pre-oxidation process is 1~2℃ / min; Optionally, the heating rate of the heat treatment process is 1~5℃ / min.

7. The preparation method according to claim 5, characterized in that, The method for preparing the resin powder includes: Small molecule short-chain molecular structure resin raw materials are cured to form a three-dimensional network molecular structure resin, and then crushed; Optionally, the oxygen content in the resin raw material is 10-15% by mass; Optionally, the resin raw material is selected from at least one of phenolic resin, epoxy resin, polyurethane resin, unsaturated polyester resin and acrylic resin; Optionally, the curing method involves holding the small-molecule, short-chain resin raw material at 95~105℃ for 120~240 minutes.

8. The preparation method according to claim 6, characterized in that, The resin powder has a Dmax of 25-30 μm and a Dv50 of 5-8 μm.

9. A negative electrode, characterized in that, Its active layer includes the hard carbon material as described in any one of claims 1 to 4 or the hard carbon material prepared by the preparation method as described in any one of claims 5 to 8.

10. A battery, characterized in that, Includes the negative electrode as described in claim 9.

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