Resin-based hard carbon microspheres for sodium-ion batteries and a preparation method thereof

Abstract

The application relates to the technical field of sodium ion battery negative electrode materials, and discloses resin-based hard carbon microspheres for sodium ion batteries and a preparation method thereof, which comprises the following steps: mixing styrene, a crosslinking agent and a catalyst to prepare an oil phase, dispersing the oil phase in a polyvinyl alcohol aqueous solution, and preparing resin microspheres through micro-suspension polymerization; and then performing air atmosphere pre-oxidation and inert atmosphere carbonization treatment to obtain a target product. Through regulation and control of a micro-suspension polymerization formula and a step-by-step heat treatment process, a dense spherical structure with rich closed pores and a suitable interlayer spacing is constructed, and the technical problems of low tap density and easy melting collapse of pyrolysis of traditional resin-based hard carbon are effectively solved. The obtained hard carbon microspheres have uniform particle size distribution, extremely high tap density and excellent powder fluidity, simultaneously exhibit high reversible specific capacity, high initial efficiency and excellent rate cycle stability, and are suitable for large-scale commercial application.

Description

Technical Field

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

[0002] With the rapid development of the new energy industry, sodium-ion batteries, with their advantages of abundant sodium resources, low cost, and excellent low-temperature performance, are considered an ideal choice for large-scale energy storage and low-speed electric vehicles. Among various anode materials, hard carbon (amorphous carbon) has become the closest to commercialization among sodium-ion battery anode materials due to its large interlayer spacing, stable structure, and high sodium storage capacity. Resin-based hard carbon, compared to biomass-based hard carbon, has advantages such as high chemical purity, strong structural designability, and good product consistency, thus attracting widespread attention from academia and industry.

[0003] However, resin-based hard carbon materials still face numerous technical bottlenecks in practical applications, limiting their commercialization. Firstly, traditional resin-based hard carbon preparation processes often employ a bulk polymerization-mechanical crushing-carbonization route, where bulk resin is first synthesized, and then a powder precursor is obtained through mechanical crushing and grinding. This top-down process results in irregular blocky or flaky morphologies of the final hard carbon particles, with a wide particle size distribution and sharp edges. This irregular particle morphology not only causes poor powder flowability, making it difficult to adapt to large-scale automated coating processes, but also leads to low tap density. Furthermore, during electrode fabrication, the particles are loosely packed, sacrificing the battery's volumetric energy density.

[0004] Secondly, most commonly used polymer precursors (such as polystyrene) are thermoplastic resins with poor thermal stability. During high-temperature carbonization, these resins are prone to depolymerization and melt rheology at lower temperatures. This not only causes precursor particles to agglomerate and stick together during heat treatment, destroying the original microstructure, but also results in low carbonization yield due to decomposition and volatilization, increasing production costs. Furthermore, this unstabilized melting process often leads to excessive collapse of the carbonized material structure, making it difficult to form abundant micropores and closed-pore structures conducive to sodium ion storage and transport.

[0005] Finally, due to limitations in existing synthesis and heat treatment processes, the microcrystalline structure of the resulting hard carbon materials is often difficult to precisely control. Insufficient carbonization results in an excessively large specific surface area, leading to low first-cycle coulombic efficiency. Conversely, excessive carbonization or an improperly designed cross-linking structure can easily increase the graphitization of the carbon layers and shrink the interlayer spacing, making it difficult for larger sodium ions to insert and extract, thus reducing the material's reversible specific capacity and causing poor rate performance under high-current charge-discharge conditions. Therefore, developing a resin-based hard carbon microsphere with high tap density, high carbonization yield, and excellent electrochemical sodium storage performance, along with its preparation method, is a key problem to be solved in this field. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a resin-based hard carbon microsphere for sodium-ion batteries and its preparation method, which solves the problems of irregular particle morphology, low tap density, and easy melting and rheological changes in the precursor during high-temperature carbonization, leading to structural collapse and low carbon yield in existing resin-based hard carbon material preparation processes.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides resin-based hard carbon microspheres for sodium-ion batteries, employing the following technical solution: A resin-based hard carbon microsphere for sodium-ion batteries, wherein the hard carbon microsphere is made by oxidation and carbonization of a resin microsphere precursor; the resin microsphere precursor is polymerized from raw materials comprising the following parts by weight: 1 part styrene, 0.25-5 parts crosslinking agent, and 0.01-0.1 parts catalyst.

[0008] By employing the above technical solution, this invention utilizes a specific ratio of styrene and a crosslinking agent to construct a dense and stable three-dimensional polymer network framework. In this formulation system, styrene serves as the primary carbon source precursor, providing the basic carbon framework; the amount of crosslinking agent added is controlled between 0.25 and 5 times the mass of styrene. This wide adjustment window allows for controllable crosslinking density within the polymer microspheres. The appropriate crosslinking density ensures that the microspheres maintain their spherical structure without collapsing during subsequent high-temperature heat treatment, while also suppressing excessive graphitization during carbonization, thereby inducing the formation of an amorphous carbon structure rich in micropores and closed pores.

[0009] The amount of catalyst was precisely controlled to ensure a moderate rate of polymerization, avoiding internal defects in the microspheres due to an excessively rapid reaction or incomplete solidification due to an excessively slow reaction. The resulting hard carbon microspheres possess a complete spherical morphology, which can improve the packing density and processability of the electrode material.

[0010] Preferably, the weight ratio of styrene, crosslinking agent and catalyst in the raw materials is 1:(0.4-0.5):(0.015-0.025).

[0011] By employing the above technical solution, the ratio of crosslinking agent to catalyst is locked within the aforementioned preferred range, forming a polymer crosslinking network. At this ratio, the binding forces between polymer molecular chain segments are balanced, preventing both the problems of excessive crosslinking leading to a brittle skeleton and severe pore closure after carbonization, and the defects of insufficient crosslinking resulting in a loose structure, low carbonization yield, and poor sphericity. The precursor prepared in this formulation, after carbonization, yields a hard carbon material with both high specific capacity and high first-cycle coulombic efficiency.

[0012] Preferably, the crosslinking agent is divinylbenzene; the catalyst is benzoyl peroxide.

[0013] By adopting the above technical solution, divinylbenzene and styrene have similar chemical structures, which can achieve uniform copolymerization at the molecular level, eliminate phase separation, and ensure the uniformity of internal density of microspheres; benzoyl peroxide, as an initiator, has a half-life temperature that matches the polymerization process, which is conducive to the stable reaction.

[0014] Preferably, the microstructure of the hard carbon microspheres is an amorphous carbon structure, and its XRD pattern shows that d 002 The carbon layer spacing is 0.42-0.43 nm; the tap density of the hard carbon microspheres is 0.92-1.07 g / cm³. 3 .

[0015] By adopting the above technical solution, a larger d 002 The carbon interlayer spacing (0.42-0.43 nm) is superior to that of graphite (0.335 nm), providing ample channels for the insertion and extraction of larger sodium ions, reducing the ion diffusion barrier, and thus improving the rate performance and sodium storage capacity of the battery; simultaneously, it reaches a high spacing of 0.92-1.07 g / cm³. 3 The tap density indicates that the material has sphericity and particle flowability, which can effectively improve the compaction density of the battery electrode and thus improve the volumetric energy density of the whole battery.

[0016] Secondly, the present invention provides a method for preparing resin-based hard carbon microspheres for sodium-ion batteries, employing the following technical solution: A method for preparing resin-based hard carbon microspheres for sodium-ion batteries includes the following steps: (1) Synthesis of resin microspheres: Styrene, crosslinking agent and catalyst are mixed to form an oil phase solution. The oil phase solution is added to a polyvinyl alcohol aqueous solution and homogenized to form an emulsion. Then, the emulsion is heated and polymerized, and dried to obtain resin microspheres. (2) Oxidation of resin microspheres: The resin microspheres are pre-oxidized by heating in an air atmosphere to obtain oxidized microspheres; (3) Carbonization of oxide microspheres: The oxide microspheres are heated under an inert atmosphere for carbonization treatment, and then cooled to obtain resin-based hard carbon microspheres.

[0017] By employing the above technical solution, this invention combines micro-suspension polymerization with a stepwise heat treatment strategy to achieve the controllable preparation of high-performance hard carbon microspheres. Its core reaction mechanism and innovative points are described below: First, in the resin microsphere synthesis stage, a micro-suspension polymerization strategy of homogenization followed by polymerization is adopted. Through strong mechanical shearing, hydrophobic monomers (oil phase) containing initiators and crosslinking agents are forcibly dispersed in an aqueous phase containing a dispersant, forming micron-sized droplets with a narrow particle size distribution. At this point, polyvinyl alcohol molecules adsorb at the oil-water interface, forming a protective film to prevent droplet aggregation. During subsequent heating, the monomers undergo independent free radical copolymerization reactions within each tiny droplet, directly solidifying into spheres. This method avoids the problems of irregular particle morphology, numerous surface defects, and low tap density caused by mechanical crushing after traditional block polymerization.

[0018] Secondly, in the oxidation stage, oxygen is used to chemically modify the polymer microspheres at a specific temperature. Oxygen molecules attack active sites on the polymer chains (such as tertiary carbon atoms or double bond positions), introducing oxygen-containing functional groups such as carbonyl groups, hydroxyl groups, or ether bonds, and forming oxygen-bridged cross-linked structures between the polymer chains. This process transforms the originally easily meltable thermoplastic polymer into a thermosetting three-dimensional network structure, improving the heat resistance of the microspheres and ensuring that they do not undergo melt rheology or adhesion during subsequent high-temperature carbonization, thus maintaining their spherical morphology.

[0019] Finally, during the carbonization stage, the pre-oxidized rigid framework undergoes thermal dehydrogenation and deoxygenation reactions in an inert atmosphere, releasing non-carbon elements as small molecule gases. As the temperature rises, carbon atoms rearrange to form a disordered graphite structure. Due to the presence of cross-linked networks in the precursor, the ordered stacking of carbon layers is restricted by steric hindrance, preventing the formation of long-range ordered graphite crystals. Ultimately, this results in a hard carbon structure with large interlayer spacing and abundant closed pores. This microstructure is key to achieving high sodium storage capacity.

[0020] Preferably, in step (1), the mass ratio of the oil phase solution to the polyvinyl alcohol aqueous solution is 1:(1-10); and the mass fraction of the polyvinyl alcohol aqueous solution is 4-10%.

[0021] By employing the above technical solutions, the oil-to-water ratio and dispersant concentration directly affect the viscosity and interfacial tension of the emulsion. Within these ranges, the rheological stability of the emulsion system can be maintained, droplet collision and aggregation can be prevented, and a high yield can be ensured, making it suitable for large-scale production.

[0022] Preferably, in step (1), the rotation speed of the homogeneous dispersion is 10000-30000 r / min; and the dispersion time is 60-120 seconds.

[0023] By employing the above technical solution, the oil-phase droplets are forcibly refined to the micrometer scale using the ultra-strong shear force and turbulence effect generated by high rotational speed. The coordinated control of rotational speed and time determines the final microsphere size (D50). Shear energy input within this range can control the microsphere size to be between several micrometers and tens of micrometers, which is the optimal size range for shortening the sodium ion transport path and improving rate performance.

[0024] Preferably, in step (1), the heating polymerization procedure is as follows: the emulsion is heated to 75-85°C at a heating rate of 0.5-2°C / min for 1-3 hours, and then the temperature is raised to 90-100°C to continue the reaction for 3-5 hours.

[0025] By adopting the above technical solution, a segmented heating polymerization process is used. The low-temperature stage is within the initiator half-life range, which allows the polymerization reaction to start smoothly and avoids microsphere rupture or porosity caused by rapid polymerization; the high-temperature stage ensures complete conversion of residual monomers, improves the crosslinking degree and mechanical strength of resin microspheres, and reduces volatile matter loss in subsequent processing.

[0026] Preferably, in step (2), the pre-oxidation treatment involves heating the resin microspheres to 240-340°C at a heating rate of 0.5-5°C / min and reacting at a constant temperature for 1-6 hours.

[0027] In employing the above technical solution, the coordination of oxidation temperature and time is crucial. Temperatures below 240℃ result in low oxidation efficiency, failing to form a sufficient thermosetting surface layer; temperatures above 340℃ lead to microsphere combustion or excessive ablation. Controlling the heating rate prevents excessive heat concentration in the oxidation reaction, which could damage the microsphere structure. This step ensures that oxygen atoms can penetrate from the surface inwards and participate in cross-linking, forming homogeneous oxidized microspheres.

[0028] Preferably, in step (3), the carbonization process involves heating the oxidized microspheres to 800-1500°C and reacting at a constant temperature for 1-3 hours.

[0029] By employing the above technical solution, 800-1500℃ is the sensitive range for the formation of hard carbon structures. Within this temperature range, as the temperature increases, defects in the carbon network gradually decrease, conductivity improves, and the interlayer spacing gradually decreases. By adjusting this final temperature, a balance can be achieved between high capacity / large interlayer spacing and high initial efficiency / high conductivity according to application requirements, resulting in a negative electrode material with excellent overall performance.

[0030] This invention provides resin-based hard carbon microspheres for sodium-ion batteries and their preparation method. It has the following beneficial effects: 1. This invention constructs an amorphous hard carbon material with abundant closed-pore structure and expanded microcrystalline interlayer spacing by precisely controlling the ratio of styrene to divinylbenzene crosslinking agent and subsequent stepwise heat treatment process. This specific microstructure not only provides sufficient storage sites for sodium ions with larger radii, effectively improving the reversible specific capacity of the material, but also reduces the diffusion energy barrier of sodium ions between carbon layers, thereby endowing the anode material with excellent rate charge-discharge performance and cycle stability.

[0031] 2. This invention employs a micro-suspension polymerization process combined with low-temperature pre-oxidation and shaping technology to prepare hard carbon microspheres with uniform particle size distribution, smooth surface, and extremely high sphericity. Compared with the irregular blocky hard carbon prepared by traditional mechanical pulverization methods, the microsphere structure of this invention has superior powder flowability and particle packing efficiency, which is beneficial for forming a dense coating during electrode preparation, thereby improving the compaction density of the battery electrode and the volumetric energy density of the entire battery.

[0032] 3. The preparation method provided by this invention introduces oxygen-containing functional groups through a low-temperature pre-oxidation step, and constructs a thermosetting cross-linked network inside the polymer. This effectively solves the problems of polystyrene resins being prone to melting and sticking at high temperatures, leading to structural collapse and low carbon yield. This process route has a wide adjustment window for raw material ratio and reaction temperature, the process is stable and controllable, and the raw materials used are widely available and inexpensive, with a high carbonization yield. It is very suitable for the industrial mass production of sodium-ion battery anode materials. Attached Figure Description

[0033] Figure 1 SEM image of polystyrene resin-based hard carbon microspheres prepared in an embodiment of the present invention.

[0034] Figure 2 The XRD pattern of the sample prepared in this embodiment of the invention.

[0035] Figure 3 The nitrogen adsorption-desorption isotherm of the sample prepared in the embodiment of the present invention.

[0036] Figure 4 This is a graph showing the first charge and discharge curves of the assembled battery according to an embodiment of the present invention.

[0037] Figure 5 This is a rate performance diagram of the assembled battery according to an embodiment of the present invention.

[0038] Figure 6 This is a diagram showing the long-cycle performance of the assembled battery according to an embodiment of the present invention. Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments, comparative examples, and test examples in this specification. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0040] Examples 1-3: Example 1: This example provides a resin-based hard carbon microsphere for sodium-ion batteries and its preparation method, specifically including the following steps: (1) Synthesis of resin microspheres: At room temperature, according to the mass ratio of styrene to crosslinking agent 1:0.25 and the mass ratio of styrene to catalyst 1:0.01, 40g of styrene, 10g of divinylbenzene (crosslinking agent) and 0.4g of benzoyl peroxide (catalyst) were weighed and added to an Erlenmeyer flask and stirred for 30 minutes to form a uniform oil phase solution.

[0041] The oil phase solution was poured into 504g of a 4% (w / w) polyvinyl alcohol aqueous solution (dispersion) at a mass ratio of 1:10 to the oil phase solution. The mixture was then stirred and dispersed using a homogenizer at 10,000 rpm for 90 seconds to form an emulsion.

[0042] The dispersed emulsion was poured into a three-necked flask, and the electric stirrer was set to 250 r / min for continuous stirring. The mixture was heated to 80°C at a heating rate of 1°C / min and polymerized for 2 hours. Then the temperature was raised to 95°C and the reaction was continued for 4 hours.

[0043] After the reaction was completed, the product was cooled to room temperature, washed three times with deionized water, and dried in an oven at 80°C for 12 hours to obtain polystyrene-divinylbenzene resin microspheres.

[0044] (2) Oxidation of resin microspheres: The dried resin microspheres were placed in a muffle furnace and slowly heated to 240°C at a heating rate of 0.5°C / min under an air atmosphere. The temperature was kept constant for 1 hour to carry out low-temperature pre-oxidation treatment and obtain oxidized microspheres.

[0045] (3) Carbonization of oxide microspheres: The oxide microspheres were placed in a tube furnace and heated to 800°C at a heating rate of 2°C / min under nitrogen atmosphere protection, and reacted at a constant temperature for 2 hours. After natural cooling, resin-based hard carbon microspheres were obtained.

[0046] The results of this embodiment show that even at lower crosslinking agent ratios and carbonization temperatures, hard carbon materials with complete spherical morphology and dense skeletons can still be prepared using the micro-suspension polymerization and stepwise heat treatment process of this invention, confirming that the preparation method has a wide process window and good spherical stability.

[0047] Example 2: This example provides a resin-based hard carbon microsphere for sodium-ion batteries and its preparation method, specifically including the following steps: (1) Synthesis of resin microspheres: At room temperature, according to the mass ratio of styrene to crosslinking agent of about 1:0.43 and the mass ratio of styrene to catalyst of about 1:0.02, 42g of styrene, 18g of divinylbenzene (crosslinking agent) and 1g of benzoyl peroxide (catalyst) were weighed and added to an Erlenmeyer flask and stirred for 30 minutes to form a uniform oil phase solution.

[0048] The oil phase solution was poured into 300 mL of a 5% (w / w) polyvinyl alcohol aqueous solution (dispersion) at a mass ratio of approximately 1:5 to the dispersion. The mixture was then stirred and dispersed using a homogenizer at 13000 rpm for 90 seconds to form an emulsion.

[0049] The dispersed emulsion was poured into a three-necked flask, and the electric stirrer was set to 250 r / min for continuous stirring. The mixture was heated to 80°C at a heating rate of 1°C / min and polymerized for 2 hours. Then the temperature was raised to 95°C and the reaction was continued for 4 hours.

[0050] After the reaction was completed, the product was cooled to room temperature, washed three times with deionized water, and dried in an oven at 80°C for 12 hours to obtain polystyrene-divinylbenzene resin microspheres.

[0051] (2) Oxidation of resin microspheres: The dried resin microspheres were placed in a muffle furnace and heated to 250°C at a heating rate of 1°C / min under air atmosphere, and reacted at a constant temperature for 2 hours for low-temperature pre-oxidation treatment. This step effectively introduced oxygen-containing functional groups and stabilized the microsphere skeleton, preventing subsequent high-temperature melting.

[0052] (3) Carbonization of oxide microspheres: The oxide microspheres were placed in a tube furnace and heated to 1000℃ at a heating rate of 2℃ / min under the protection of inert gases such as nitrogen, and the temperature was kept constant for 2 hours. After natural cooling, the final resin-based hard carbon microspheres were obtained.

[0053] (4) Product characterization: The polystyrene resin-based hard carbon microspheres prepared in this embodiment were selected for testing: such as Figure 1 As shown in the SEM image, the obtained product exhibits a morphology with uniform particle size distribution, intact spherical structure, and smooth surface.

[0054] like Figure 2 The XRD pattern shows that the product exhibits a typical amorphous carbon structure, and its d 002 The carbon interlayer spacing is 0.4291 nm, and the larger interlayer spacing is conducive to the insertion of sodium ions.

[0055] like Figure 3 The nitrogen adsorption-desorption isotherm shows that the specific surface area of ​​the product is 1.209 m². 2 / g indicates a lower specific surface area and fewer open pores, which helps reduce irreversible capacity loss in the first cycle.

[0056] Example 3: This example provides a resin-based hard carbon microsphere for sodium-ion batteries and its preparation method, specifically including the following steps: (1) Synthesis of resin microspheres: At room temperature, according to the mass ratio of styrene to crosslinking agent 1:5 and the mass ratio of styrene to catalyst 1:0.1, 10g of styrene, 50g of divinylbenzene (crosslinking agent) and 1g of benzoyl peroxide (catalyst) were weighed and added to an Erlenmeyer flask and stirred for 30 minutes to form a uniform oil phase solution.

[0057] The oil phase solution was poured into 61g of a 10% (w / w) polyvinyl alcohol aqueous solution at a mass ratio of 1:1 to that of the dispersion. The mixture was then dispersed using a homogenizer at 30,000 rpm for 90 seconds to form an emulsion.

[0058] The dispersed emulsion was poured into a three-necked flask, and the electric stirrer was set to 400 r / min for continuous stirring. The mixture was heated to 80°C at a heating rate of 1°C / min and polymerized for 2 hours. Then the temperature was raised to 95°C and the reaction was continued for 4 hours.

[0059] After the reaction was completed, the product was cooled to room temperature, washed three times with deionized water, and dried in an oven at 80°C for 12 hours to obtain polystyrene-divinylbenzene resin microspheres.

[0060] (2) Oxidation of resin microspheres: The dried resin microspheres were placed in a muffle furnace and heated to 340°C at a heating rate of 5°C / min under an air atmosphere. The temperature was kept constant for 6 hours to carry out full oxidation and cross-linking treatment, and oxidized microspheres with high oxygen content were obtained.

[0061] (3) Carbonization of oxide microspheres: The oxide microspheres were placed in a tube furnace and heated to 1500°C at a heating rate of 5°C / min under nitrogen atmosphere protection, and reacted at a constant temperature for 2 hours. After natural cooling, resin-based hard carbon microspheres were obtained.

[0062] The hard carbon microspheres prepared in this embodiment underwent a high degree of cross-linking, oxidation, and high-temperature heat treatment. The resulting product has a highly dense structure and excellent physicochemical stability. This demonstrates that the preparation process of this invention can maintain good structural integrity and controllability even under harsh reaction conditions, reflecting the potential of this technical solution to optimize material performance through a wide range of parameter adjustments.

[0063] Comparative Examples 1-4: Comparative Example 1: This comparative example aims to verify the importance of the low-temperature pre-oxidation step in maintaining microsphere morphology and yield.

[0064] Compared with Example 2, the difference is that step (2) of oxidation treatment of resin microspheres was omitted. Specifically, the resin microspheres prepared and dried in step (1) were directly placed into a tube furnace and subjected to high-temperature carbonization under a nitrogen atmosphere. The remaining preparation steps and parameters were the same as in Example 2.

[0065] Comparative Example 2: This comparative example aims to verify the effect of crosslinking agent content on the stability of the resin microsphere skeleton and the final hard carbon pore structure.

[0066] Compared with Example 2, the difference is that in step (1), the mass ratio of styrene to crosslinking agent (divinylbenzene) is adjusted to 1:0.05 (i.e., 42g of styrene corresponds to 2.1g of divinylbenzene) to investigate the effect of insufficient crosslinking density on the material structure. The rest of the preparation steps and parameters are the same as in Example 2.

[0067] Comparative Example 3: This comparative example aims to verify the effect of carbonization temperature on the microstructure of hard carbon (such as interlayer spacing and degree of graphitization) and sodium storage mechanism.

[0068] Compared with Example 2, the difference is that the high-temperature carbonization treatment temperature in step (3) is increased to 2000℃, which aims to investigate the evolution of the microstructure of the material (such as interlayer spacing and graphitization degree) under extremely high temperature heat treatment conditions and its specific impact on the electrochemical sodium storage performance. The remaining preparation steps and parameters are the same as in Example 2.

[0069] Comparative Example 4: This comparative example aims to verify the advantages of the micro-suspension polymerization spheroidization process used in this invention over the traditional block crushing process in terms of tap density and electrochemical performance.

[0070] Compared with Example 2, the difference is that step (1) uses bulk polymerization instead of micro-suspension polymerization, and the product is irregular particles. Specifically, styrene, divinylbenzene, and benzoyl peroxide are mixed evenly, and without adding polyvinyl alcohol aqueous dispersion, the mixture is directly heated and cured in a sealed container at the same temperature to obtain block resin. The block resin is then mechanically pulverized and sieved to micron-sized powder, followed by subsequent oxidation and carbonization treatments. All other parameters (such as oxidation and carbonization regimes) are the same as in Example 2.

[0071] Test Example 1-2: Test Example 1: Physical Property Test This test case mainly measures the carbon yield, particle size distribution and tap density of the resin-based hard carbon materials obtained in Examples 1-3 and Comparative Examples 1-4 to verify the influence of different preparation process parameters and methods on the macroscopic physical properties of the materials.

[0072] Test method: Carbonization yield determination: Record the mass change of the sample before and after carbonization. The calculation formula is: yield = (mass of hard carbon after carbonization / mass of resin fed) × 100%.

[0073] Particle size distribution determination: The sample to be tested was dispersed in ethanol solvent and ultrasonically treated for 5 minutes to eliminate agglomeration. The particle size distribution of the sample was determined using a Malvern laser particle size analyzer, and the median diameter D50 value was recorded.

[0074] Tap density determination: The FZS-4 tap density meter was used for testing. A certain mass of powder sample was weighed and placed into a graduated cylinder. The vibration frequency was set to 300 times / min and the vibration amplitude was 3mm. The vibration was continued until the volume of the powder no longer changed. The final volume was recorded and the density was calculated.

[0075] Test results: The physical property test data of each group of samples are summarized in Table 1.

[0076]

[0077] Results Analysis and Conclusions: Based on the data analysis in Table 1, we can conclude that: Examples 1 to 3 all employed micro-suspension polymerization combined with oxidation and carbonization processes to prepare spherical hard carbon materials with high tap density. In Example 2, under specific crosslinking agent ratios and suitable oxidation and carbonization processes, an ideal particle size (8.9 μm) and a high tap density (1.07 g / cm³) were obtained. 3 This indicates that the combination of process parameters is beneficial for forming materials with a dense structure and excellent packing performance. With the increase of crosslinking degree and oxidation degree (Example 3), the thermal stability of the material skeleton is enhanced, and the carbonization yield increases to 59.1%, but the particle size also increases accordingly.

[0078] Comparing the data from each group can reveal the impact of the process mechanism: The crucial role of the pre-oxidation step: In Comparative Example 1, omitting the pre-oxidation step resulted in depolymerization and melt rheology of the polystyrene microspheres at high temperatures, leading to severe product agglomeration and an extremely low carbonization yield (16.2%), making it impossible to maintain the spherical morphology. This confirms that the oxygen-containing functional groups introduced during the low-temperature pre-oxidation process not only play a role in cross-linking and curing but also improve the thermal stability of the resin skeleton, which is key to achieving high yield and maintaining spherical shape.

[0079] Effect of crosslinking agent content: In Comparative Example 2, reducing the amount of crosslinking agent resulted in a loose resin network structure, with more volatiles escaping during pyrolysis, a carbonization yield of only 29.8%, and lower structural strength causing some microspheres to break and a decrease in tap density.

[0080] Effect of microstructure on density: Comparative Example 4, with its irregular blocky hard carbon prepared by mechanical pulverization, had a D50 (9.1 μm) similar to that of Example 2, but its tap density was only 0.68 g / cm³. 3 It is much lower than the 1.07 g / cm³ of Example 2. 3 This directly proves that the microsphere structure prepared by micro-suspension polymerization has better flowability and particle packing efficiency, which is beneficial to improving the compaction density and volumetric energy density of battery electrodes.

[0081] In summary, this invention successfully prepared resin-based hard carbon microspheres with high yield and high tap density by controlling the micro-suspension polymerization formula and oxidative carbonization process, solving the problems of complex processes and low density of traditional resin-based carbon materials.

[0082] Test Example 2: Electrochemical Performance Test This test case demonstrates the assembly and testing of button half-cells using the hard carbon materials prepared in the above examples and comparative examples to evaluate their actual electrochemical performance in sodium-ion battery anode applications, including initial coulombic efficiency, reversible specific capacity, rate performance, and cycle stability.

[0083] Experimental methods: Electrode preparation: The prepared hard carbon powder (active material), conductive carbon black, and polyvinylidene fluoride binder were mixed at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone solvent was added, and the mixture was stirred at high speed to form a slurry. The slurry was uniformly coated onto the surface of a copper foil current collector, dried in a vacuum oven at 120℃ for 12 hours, rolled, and then cut into circular electrode sheets with a diameter of 12 mm. The active material loading of a single electrode was controlled at 0.8 mg / cm³. 2 Up to 1.2 mg / cm 2 between.

[0084] Battery Assembly: CR2025 coin cells were assembled in an argon atmosphere glove box with water and oxygen contents both below 0.1 ppm. Sodium metal sheets were used as the counter and reference electrodes, and glass fiber filter paper was used as the separator. The electrolyte was a mixed solution of ethylene carbonate and diethyl carbonate (volume ratio 1:1) containing 1.0 mol / L sodium hexafluorophosphate, with 5% fluoroethylene carbonate added as an additive.

[0085] Electrochemical testing: Constant current charge-discharge tests were performed using a battery testing system (such as the Blue Electric CT2001A). The voltage test range was 0.01V to 2.0V (vs. Na+ / Na). The initial charge-discharge current density was 30mA / g, and the subsequent rate tests were performed with current density gradients of 0.1A / g, 0.5A / g, 1A / g, 2A / g, and 5A / g. Cycle life testing was conducted at a current density of 0.2A / g.

[0086] Test results: The sample prepared in Example 2 was selected as a typical representative, and its electrochemical performance curve is shown below. Figures 4 to 6 As shown. Figure 4 The results show that the sample of Example 2 exhibits a significant low-voltage plateau region in the voltage range of 0.01-2.0V, which is the main contributing region for sodium storage in hard carbon. Its initial charge specific capacity and discharge specific capacity are 471.5mAh / g and 372.9mAh / g, respectively, with an initial coulombic efficiency of 79.08%. Figure 5 Rate performance tests show that the material maintains a high capacity release even at high current densities, and the reversible specific capacity remains at 70.0 mAh / g when the current density increases to 3 A / g. Figure 6 The long-cycle curves show that after 100 cycles at a current density of 300 mA / g, the capacity retention is excellent and no significant capacity decay is observed.

[0087] Detailed electrochemical performance test data for each embodiment and comparative example are summarized in Table 2.

[0088]

[0089] Results Analysis and Conclusions: Table 2 and Figures 4-6 The test data reveals the structure-property relationship between the material's microstructure and electrochemical performance: Example 2 constructed hard carbon microspheres with abundant closed-pore structure and suitable interlayer spacing by controlling the appropriate degree of crosslinking and oxidative carbonization process, achieving a balance between high specific capacity (372.9 mAh / g) and high initial efficiency (79.1%). In contrast, Example 1, due to its lower carbonization temperature (800℃), had insufficient carbon layer rearrangement, resulting in a larger specific surface area, increased side reactions, and slightly lower initial efficiency. Example 3, after high-temperature treatment (1500℃), improved the degree of graphitization, and although the initial efficiency increased to 84.5%, the reduced interlayer spacing led to a decrease in sodium storage sites, resulting in a capacity decrease to 288.7 mAh / g.

[0090] Comparative data further confirm the mechanism of action of the key preparation process of this invention: Necessity of oxidative crosslinking: Comparative Example 1 was not pre-oxidized. During carbonization, the resin balls underwent structural collapse and softening, which prevented the formation of effective micropores and closed channels, resulting in extremely low sodium storage capacity (134.2 mAh / g) and poor cycle stability.

[0091] Effect on structural stability: Due to insufficient crosslinking agent content, the precursor skeleton of Comparative Example 2 has low strength and underdeveloped pore structure after carbonization, resulting in significantly inferior capacity and rate performance compared to Example 2.

[0092] Advantages of microsphere morphology: Although the blocky hard carbon prepared by mechanical pulverization in Comparative Example 4 has a similar chemical composition to that of Example 2, its irregular particle morphology results in a larger contact resistance of particles in the electrode sheet, and the stress concentration caused by the anisotropic volume expansion during charging and discharging is more severe. As a result, its rate performance (only 48.9 mAh / g at 2A / g) and cycle retention rate (81.3%) are lower than those of the spherical hard carbon in Example 2.

[0093] In summary, the resin-based hard carbon microspheres prepared by the present invention using micro-suspension polymerization combined with stepwise oxidation carbonization process optimize ion transport kinetics and structural stability by utilizing their complete spherical structure, and optimize the internal pore structure by controlling process parameters, exhibiting excellent comprehensive sodium storage performance.

Claims

1. A resin-based hard carbon microsphere for sodium-ion batteries, characterized in that, The hard carbon microspheres are made from resin microsphere precursors through oxidation and carbonization. The resin microsphere precursor is polymerized from raw materials comprising the following parts by weight: Styrene: 1 part; crosslinking agent: 0.25-5 parts; catalyst: 0.01-0.1 parts.

2. The resin-based hard carbon microspheres for sodium-ion batteries according to claim 1, characterized in that, The weight ratio of styrene, crosslinking agent, and catalyst in the raw materials is as follows: 1：(0.4-0.5)：(0.015-0.025)。 3. The resin-based hard carbon microspheres for sodium-ion batteries according to claim 1, characterized in that, The crosslinking agent is divinylbenzene; the catalyst is benzoyl peroxide.

4. The resin-based hard carbon microspheres for sodium-ion batteries according to claim 1, characterized in that, The microstructure of the hard carbon microspheres is an amorphous carbon structure, and its XRD pattern shows that d 002 The carbon layer spacing is 0.42-0.43 nm; the tap density of the hard carbon microspheres is 0.92-1.07 g / cm³. 3 .

5. A method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Synthesis of resin microspheres: Styrene, crosslinking agent and catalyst are mixed to form an oil phase solution, the oil phase solution is added to a polyvinyl alcohol aqueous solution, and the emulsion is formed by homogenization and dispersion. Then, the emulsion is heated and polymerized, and dried to obtain resin microspheres; (2) Oxidation of resin microspheres: The resin microspheres are pre-oxidized by heating in an air atmosphere to obtain oxidized microspheres; (3) Carbonization of oxide microspheres: The oxide microspheres are heated under an inert atmosphere for carbonization treatment, and then cooled to obtain resin-based hard carbon microspheres.

6. The method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to claim 5, characterized in that, In step (1), the mass ratio of the oil phase solution to the polyvinyl alcohol aqueous solution is 1:(1-10); the mass fraction of the polyvinyl alcohol aqueous solution is 4-10%.

7. The method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to claim 5, characterized in that, In step (1), the rotation speed of the homogeneous dispersion is 10000-30000 r / min; the dispersion time is 60-120 seconds.

8. The method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to claim 5, characterized in that, In step (1), the heating polymerization procedure is as follows: the emulsion is heated to 75-85℃ at a heating rate of 0.5-2℃ / min for 1-3 hours, and then the temperature is raised to 90-100℃ to continue the reaction for 3-5 hours.

9. The method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to claim 5, characterized in that, In step (2), the pre-oxidation treatment involves heating the resin microspheres to 240-340°C at a heating rate of 0.5-5°C / min and reacting at a constant temperature for 1-6 hours.

10. The method for preparing resin-based hard carbon microspheres for sodium-ion batteries according to claim 5, characterized in that, In step (3), the carbonization process involves heating the oxide microspheres to 800-1500℃ and reacting at a constant temperature for 1-3 hours.

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