Sucrose-polymer composite hard carbon and a preparation method thereof

Abstract

This invention discloses a sucrose-polypropylene composite hard carbon and its preparation method, belonging to the technical field of sodium-ion battery anode materials. The preparation method of the composite hard carbon includes using sucrose as a raw material, first preparing a precursor, mixing it with polypropylene, ball milling it, pre-oxidizing it at low temperature, and then carbonizing it at high temperature. The closed-pore volume of the material of this invention increases, the interlayer spacing expands, and the packing density of the material increases, thereby improving the energy density of the material. The reversible specific capacity of the hard carbon material of this invention can be increased from the initial 271 mAh g. ‑1 Increased to 387mAh.g ‑1 The capacity ratio in the low-voltage plateau region can be increased from 54% to 64%. Under optimal process conditions (M-SC:PP = 2:1, pre-oxidation at 330℃), the sucrose-polypropylene composite hard carbon prepared at 100 mA g... ‑1 It still retains 240mAh after 300 cycles. ‑1 Its capacity is far higher than that of directly calcined sucrose-based hard carbon materials (100 mAh / g). ‑1 The material exhibits good cycle stability and rate performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, and in particular to a sucrose-polymer composite hard carbon and its preparation method. Background Technology

[0002] With the rapid growth of global demand for renewable energy storage and electric vehicles, developing low-cost, safe, and high-performance rechargeable battery systems has become one of the core issues in the field of energy materials. While lithium-ion batteries dominate the market, the limited availability and uneven geographical distribution of lithium resources, leading to rising costs and supply chain risks, has prompted the research and industrial communities to actively seek alternative technologies. Among numerous candidates, sodium-ion batteries are considered one of the most promising next-generation large-scale energy storage technologies due to their abundant sodium resources, low cost, and similar working principle to lithium-ion batteries.

[0003] The commercial application of sodium-ion batteries heavily relies on breakthroughs in high-performance electrode materials, with anode materials being one of the key bottlenecks. Because the radius of sodium ions (approximately 1.02 Å) is larger than that of lithium ions (approximately 0.76 Å), the interlayer spacing of traditional graphite anode materials used in lithium-ion batteries is limited, resulting in slow sodium ion insertion / extraction kinetics and extremely low reversible capacity. Therefore, developing novel anode materials suitable for efficient sodium-ion storage is imperative.

[0004] Among numerous candidate materials, hard carbon has emerged as the most promising choice for sodium-ion battery anode materials. Hard carbon is a non-graphitized carbon that maintains a highly disordered structure even under high-temperature carbonization (typically >1000℃). Its structural features include abundant defects, expanded graphite-like microcrystalline interlayer spacing, and a complex pore system composed of open pores, closed pores, and micropores. This unique structure provides multiple mechanisms for sodium ion storage: in the higher potential slope region, sodium ions can be adsorbed onto defects, surfaces, and pore surfaces; in the low potential plateau region, sodium ions can be embedded in the expanded carbon interlayers or fill the nanopores, thus contributing high specific capacity and suitable operating potential.

[0005] Despite its promising prospects, ideal hard carbon anode materials must meet several stringent requirements: First, they must possess high reversible specific capacity and first-cycle coulombic efficiency to improve the overall energy density of the battery; second, they need excellent long-term cycle stability; third, raw materials must be widely available, inexpensive, and have simple preparation processes to align with the low-cost positioning of sodium-ion batteries for large-scale energy storage; and fourth, they need suitable tap density to achieve high volumetric energy density. Therefore, current research on hard carbon materials focuses on optimizing their microstructure through precursor selection and structural control, balancing the ratio of closed pores (contributing to plateau capacity) to defects / open pores (affecting first-cycle efficiency and ramp capacity), ultimately achieving a perfect balance between comprehensive electrochemical performance and manufacturing cost.

[0006] Among numerous hard carbon precursors, biomass or sugar derivatives have attracted widespread attention due to their renewability, high carbon content, and diverse molecular structures. Sucrose (C 12 H 22 O 11 Sucrose, as a widely available and inexpensive natural disaccharide, is one of the classic precursors for the preparation of hard carbon. Its molecular structure contains abundant hydroxyl groups, which readily cross-link during pyrolysis, forming a rigid structure that inhibits graphitization and yields typical hard carbon materials. Sucrose-based hard carbon typically exhibits high disorder and abundant nanopores, generally providing a high ramp capacity when used as a negative electrode in sodium-ion batteries. However, using pure sucrose as a precursor for hard carbon preparation presents several significant challenges, limiting its performance ceiling and industrialization potential:

[0007] (1) Low carbon yield: During the high-temperature pyrolysis of sucrose, the carbon skeleton shrinks and loses weight severely due to the violent release of a large number of small molecule gases (such as H2O, CO, CO2, hydrocarbons), resulting in a low carbon yield (generally less than 30%). This not only increases the cost of raw material consumption, but also means that the yield of effective carbon materials obtained under the same process is limited, which is not conducive to economic benefits.

[0008] (2) Insufficient plateau capacity, total capacity needs improvement: The sodium storage capacity of pure sucrose-derived hard carbon mainly comes from the high-potential slope region, corresponding to surface / defect adsorption and micropore filling. However, in the critical low-potential plateau region that determines energy density (typically below 0.1V vs. Na), the sodium storage capacity is significantly reduced. + The capacity contribution of sodium ions (Na) is relatively limited. This is usually related to the insufficient development of appropriately sized "closed-pore" structures in the material suitable for sodium ion insertion or quasi-metallic sodium filling. The pore structure formed by the pyrolysis of pure sucrose may be more inclined towards open pores or unsuitable-sized closed pores, resulting in a low plateau capacity and making it difficult for the overall reversible specific capacity to break through the 300 mmAh / g threshold, thus limiting the energy density of the battery.

[0009] (3) Limited structural control methods: The final hard carbon structure of sucrose is controlled by only changing the carbonization temperature or heating rate, which has limited effect and is not precise enough. It is difficult to achieve independent and precise control of key parameters such as closed-pore volume, pore size distribution and carbon layer spacing.

[0010] (4) First-cycle coulombic efficiency problem: Excessive specific surface area and too many openings / defects will lead to a large amount of irreversible electrolyte decomposition and solid electrolyte interphase (SEI) formation, thereby reducing the first-cycle coulombic efficiency and consuming the limited sodium source of the positive electrode.

[0011] To address the aforementioned problems, much effort has been made in this field, such as:

[0012] Chinese patent application CN120674479A, entitled "A Composite Hard Carbon Material for Sodium-ion Batteries and Its Preparation Method," discloses a method for preparing composite hard carbon by liquid coating of sucrose hydrothermal carbon spheres with petroleum asphalt followed by a synergistic carbonization reaction. This method improves the first-cycle coulombic efficiency to some extent. Its core mechanism is to use an external soft carbon layer to induce graphitization of the internal hard carbon, reducing defects and thus effectively suppressing side reactions. However, its strategy has obvious limitations: it sacrifices reversible capacity for high efficiency. Compared with the uncoated sample, its first-cycle discharge specific capacity decreased from 413 mAh / g to 362 mAh / g, indicating that the plateau capacity may be compressed. This patent focuses on modifying surface defects to improve first-cycle efficiency but does not address the fundamental issue of improving the internal closed-pore volume and plateau capacity of the hard carbon.

[0013] Chinese patent application CN120607240A, entitled "A Method for Preparing Porous Hard Carbon," uses zinc salt as a "self-sacrificing template" to simultaneously create open and closed pores in sucrose hard carbon. Its advantages are: the process is simpler and more environmentally friendly than traditional template methods (no acid washing required), and by optimizing the amount of zinc salt, a high reversible specific capacity (497 mAh / g) is achieved under specific conditions (such as Example 6), while maintaining a high first-week coulombic efficiency (84%). This indicates that it can effectively increase sodium storage sites. This patent focuses on "sculpting" pores through the physicochemical action of an external template agent (zinc salt), a complex process whose results are difficult to control stably. Its high capacity may overly rely on the adsorption capacity (slope region) contributed by the large number of micropores / open pores generated by zinc salt pore creation, while the controllable formation mechanism of a uniformly sized closed-pore structure that stably contributes to the plateau capacity is not clearly explained or proven in the patent.

[0014] Chinese patent application CN118754098A, entitled "A Method for Preparing Anthracite / Sucrose-Based Composite Carbon Materials through Two-Stage Carbonization," describes a method for preparing composite carbon materials using "ball milling and mixing of anthracite and sucrose + two-stage carbonization (pretreatment at 400℃ and final carbonization at 1200℃) and strong alkali / acid washing for impurity removal." This method combines the high conductivity and excellent rate performance of anthracite (soft carbon precursor) with the high capacity potential of sucrose (hard carbon precursor), achieving a balanced performance. The rate performance is particularly outstanding, with a reversible capacity of 271.8 mAh / g at 0.1C, but the rate performance is also excellent (maintaining 229 mAh / g at 10C). However, the process is complex and highly polluting: the core impurity removal step requires prolonged (2 hours each) heating and washing with high-concentration strong alkali (5M KOH) and strong acid (5% HCl). This process generates a large amount of chemical wastewater, resulting in poor environmental performance, contradicting the trend of green manufacturing, and increasing subsequent treatment costs. Limited capacity improvement and no optimization of platform capacity: Although the capacity of the composite material (271.8mAh / g) is better than that of pure anthracite (178.2mAh / g), it is significantly lower than that of pure sucrose hard carbon (279.7mAh / g), indicating that its composite strategy has little effect on improving the total capacity, especially the platform capacity.

[0015] Chinese patent application CN120483096A, entitled "A method for preparing a high-performance hard carbon anode material for sodium-ion batteries," describes a method that involves thoroughly mixing a biomass precursor and a nonionic surfactant. The surfactant's high wettability is used to uniformly coat the precursor. During high-temperature activation, PEG melts and penetrates deep into the coconut shell precursor. During carbonization, PEG decomposes into hydrocarbon radicals and hydroxyl radicals, promoting the development of the carbon layer within the hard carbon. This results in a highly closed-pore structure and a short-range ordered carbon layer structure conducive to sodium storage, improving the kinetic performance and specific capacity during charge and discharge. The hard carbon material of this invention increases the specific capacity of biomass hard carbon from 280 mAh / g to 389 mAh / g, and the first-cycle coulombic efficiency from 82% to over 89%. However, this method has at least the following drawbacks: (1) It mainly relies on the physical coating and surface pyrolysis modification of pre-made biomass carbon by polymers, and its ability to create closed-cell structures inside hard carbon is limited; (2) The natural biomass (such as coconut shells) has complex composition and uneven structure, which easily leads to poor batch consistency of products and makes it difficult to industrialize; (3) This method does not pay attention to the capacity and proportion of low voltage platforms, and the performance improvement dimension needs to be improved; (4) This method does not mention carbon yield, and the PEG pyrolysis carbon residue rate used is low, which may lead to higher raw material costs.

[0016] Therefore, the key to promoting its practical application lies in how to significantly improve its carbon yield and low potential plateau capacity while retaining the cost advantages and slope capacity of sucrose-based hard carbon through effective material design strategies. Summary of the Invention

[0017] One of the objectives of this invention is to provide a method for preparing sucrose-polymer composite hard carbon to solve the above-mentioned problems.

[0018] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for preparing sucrose-polymer composite hard carbon, comprising the following steps:

[0019] (1) The sucrose solution was subjected to hydrothermal carbonization, and after hydrothermal treatment, it was washed and dried to obtain the sucrose-based hard carbon precursor M-SC;

[0020] (2) After mixing the precursor M-SC obtained in step (1) with the polymer and ball milling it into powder, it is directly carbonized at high temperature;

[0021] (3) After high-temperature carbonization, the black material is removed after the material cools down to obtain sucrose-polymer composite hard carbon anode material.

[0022] During the formation of sucrose-based hard carbon, a large number of active small molecules overflow, resulting in numerous open pores and an excessively large specific surface area, thus causing low initial coulombic efficiency. Direct high-temperature calcination of polypropylene decomposes it into small molecule overflow matrix, making it difficult to convert into functional carbon materials. This invention, by combining sucrose and polymers and utilizing the cross-linking effect between sucrose and polypropylene, reduces the specific surface area of ​​sucrose-based hard carbon and constructs a closed-pore structure in situ within it, thereby improving the specific capacity and initial coulombic efficiency of sucrose-based hard carbon. It also provides a sustainable approach, reusing polyolefin waste into value-added energy storage products and increasing the carbon yield of biomass-based hard carbon to over 50%.

[0023] As a preferred technical solution

[0024] In step (2), before high-temperature carbonization, the mixed ball milled material is first subjected to low-temperature pre-oxidation, and then the high-temperature carbonization is carried out.

[0025] As a further preferred technical solution, the low-temperature pre-carbonization is carried out in a tube furnace, the low-temperature pre-oxidation time is 1 to 5 hours, the low-temperature pre-carbonization temperature is 200℃ to 400℃, the heating rate is 1 to 7℃ / min, and oxygen / air gas is introduced during the heating and holding process for pre-oxidation.

[0026] As a further preferred technical solution

[0027] The low-temperature pre-oxidation time is 1 hour; the low-temperature pre-oxidation temperature is 330°C; and the low-temperature pre-oxidation heating rate is 2°C / min.

[0028] As a preferred technical solution, in step (1), the hydrothermal carbonization is carried out in a hydrothermal kettle at a hydrothermal temperature of 160-200°C; the hydrothermal heat preservation time is 8-24 hours; and the washing solvent is distilled water or ethanol.

[0029] As a further preferred technical solution

[0030] The hydrothermal temperature is 180℃, and the hydrothermal insulation time is 8 hours.

[0031] As a preferred technical solution

[0032] In step (2), the polymer is selected from PP (polypropylene), PE (polyethylene), and PC (polycarbonate), and the composite ratio of M-SC and polymer is 1.5 to 3:1.

[0033] As a further preferred technical solution

[0034] The polymer is PP, and the composite ratio of M-SC to the polymer is 2:1.

[0035] As a preferred technical solution

[0036] In step (2), the high-temperature carbonization is carried out at a heating rate of 1 to 7°C / min to 1000°C to 1300°C and held for 8 to 12 hours. Inert gas is introduced for protection during the heating and holding process.

[0037] The second objective of this invention is to provide a composite hard carbon prepared by the above-described preparation method.

[0038] This invention optimizes the microstructure of hard carbon, especially the closed-pore structure, by appropriately adjusting the sucrose / polypropylene composite ratio and preferably performing pre-oxidation before high-temperature carbonization and optimizing the pre-oxidation process. This creates more reversible sodium storage conditions, thereby improving the electrochemical performance of sucrose-polypropylene composite hard carbon in sodium-ion batteries. Specifically, through extensive experiments, we discovered that when sucrose hydrothermal intermediate (M-SC) is compounded with polymers (such as PP / PC) and then calcined at high temperatures, polypropylene can adhere to the carbon matrix under the catalysis of M-SC. This significantly reduces the specific surface area of ​​the material and the number of open pores, but the number of closed pores does not increase. Therefore, a pre-oxidation step was added before high-temperature carbonization. Our research showed that pre-oxidation leads to more complete cross-linking of M-SC and PP. During pre-oxidation, the presence of oxygen accelerates the decomposition of both M-SC and PP, generating various aromatic compounds. The carbon matrix also becomes more active in the oxygen atmosphere, resulting in a high concentration of free radicals. This can lead to bonding through free radical-surface reactions. The two components construct a stable cross-linked structure during pre-oxidation, enabling it to resist the collapse of the pore structure caused by high temperatures during high-temperature carbonization. During subsequent carbonization, a certain number of micropores and ultramicropores are maintained, and the number of closed pores increases significantly, which greatly promotes the storage of sodium ions in the plateau region. With the assistance of the polymer, the open pores that should have formed in the sucrose hydrothermal carbon spheres (M-SC) during high-temperature carbonization are transformed into closed pores. This transformation occurs through crosslinking of the polymer and M-SC. During pre-oxidation, the polymer releases light aromatic compounds that crosslink with M-SC. These compounds deposit at the open pore entrances of the M-SC, forming a polymer-based carbon layer, thereby modulating the pore structure of the M-SC. Pre-oxidation opens the interlayer spacing of the sucrose-polypropylene composite hard carbon, resulting in smaller graphite crystallites, increased defect content, and a significant increase in the number of micropores and the volume of closed pores. When configured as the anode of a sodium-ion battery, hard carbon (SP) exhibits optimal closed pores. 2-1 It has a high reversible capacity of 387 mAh / g (OC-330℃), an initial coulombic efficiency of 83%, and excellent cycling and rate performance.

[0039] Compared with existing technologies, such as the method disclosed in CN120483096A that uses PEG melt-coating of natural biomass such as coconut shells, the present invention has the following significant differences and advantages:

[0040] (1) Different structural regulation mechanisms: This invention achieves endogenous and targeted closed-pore structure construction from the carbon skeleton formation source through molecular-level composite and pre-oxidation cross-linking of sucrose precursor and polypropylene, resulting in a more significant increase in platform capacity; (2) Advantages of raw material system: This invention uses sucrose with uniform composition as the main carbon source, solving the problem of difficulty in industrialization; (3) More critical performance improvement dimensions: This invention not only improves the reversible specific capacity to a similar high level (387mAh / g), but also increases the capacity ratio of the platform region from 54% to 64%, which is of decisive significance for improving the energy density of the whole battery. At the same time, on the basis of improving capacity, the first efficiency is significantly increased from 63% (sucrose-based benchmark) to 83%, solving the common problem of low first efficiency of high-capacity hard carbon; (4) Better industrialization economy: This invention clarifies that after introducing polymers such as polypropylene and undergoing pre-oxidation process, the carbon yield of the material is increased from 42% to more than 50%, which significantly improves the raw material utilization efficiency and reduces production costs.

[0041] Compared with existing technologies, the advantages of this invention are: the reversible specific capacity of the invented hard carbon material can be increased from the initial 270 mAh / g to 387 mAh / g, and the capacity ratio of the low voltage plateau region can be increased from 54% to 64%, 100 mA / g. -1 After 300 cycles, it still retains a capacity of 240 mAh / g, which is much higher than that of directly calcined sucrose-based hard carbon materials (100 mAh / g). The material has good cycling stability and rate performance.

[0042] Furthermore, this invention provides a method for processing waste polyolefin materials, successfully converting them into a functional material for use in sodium-ion batteries. Moreover, since the preparation process omits processes such as acid washing and alkali washing, no large amount of wastewater is generated, thus solving the problem of environmental pollution and having good social and economic benefits. Attached Figure Description

[0043] Figure 1 This is a comparison graph showing the cycle performance of negative electrode sheets made of sucrose-based hard carbon in different embodiments and comparative examples of the present invention.

[0044] Figure 2 This is a TEM image of sucrose-based hard carbon from Comparative Example 1 of the present invention.

[0045] Figure 3 This is a TEM image of sucrose-based hard carbon from Example 3 of the present invention;

[0046] Figure 4 This is a TEM image of the sucrose-based hard carbon of Example 5 of the present invention. Detailed Implementation

[0047] To explain the technical content, objectives, and effects of the present invention in detail, the following specific embodiments are provided to further illustrate the content of the present invention. However, the content of the present invention is far more than the following examples.

[0048] Unless otherwise specified, all materials used in the following examples and comparative examples are commercially available.

[0049] In the following examples, the polypropylene used was manufactured by McLean, with a molecular weight of 6000±500 and a softening point of 155~165℃; the polyethylene used was manufactured by Kramar, with a molecular weight of 1800. Those skilled in the art will understand that other types of PP and PC can also be used to complete this invention.

[0050] Example 1:

[0051] A high specific capacity sucrose-polypropylene composite hard carbon, the preparation method of which is as follows:

[0052] (1) Prepare a 0.2 mol / L sucrose solution, measure 70 mL of the above sucrose solution into a 100 mL polytetrafluoroethylene hydrothermal reactor, place it in a vacuum oven at 180 °C and keep it warm for 8 h. After the hydrothermal treatment is completed, wash with water, filter and dry to obtain reddish-brown blocky sucrose-based hard carbon precursor M-SC.

[0053] (2) The precursor M-SC obtained in step (1) and polypropylene (PP) are ground into powder in a ball mill jar at a mass ratio of 1:1. Then, high-temperature carbonization is carried out in a tube furnace. The specific method is: heating to 1300℃ in an argon atmosphere at a heating rate of 5℃ / min and holding for 8h.

[0054] (3) After the material cools to room temperature, the black material is taken out of the tube furnace to obtain sucrose-polypropylene composite hard carbon material.

[0055] Example 2:

[0056] The difference between this embodiment and embodiment 1 is that the mass ratio of M-SC to PP in step (2) is 1.5:1, while the rest is the same as in embodiment 1.

[0057] Example 3:

[0058] The difference between this embodiment and embodiment 1 is that the mass ratio of M-SC to PP in step (2) is 2:1, while the rest is the same as in embodiment 1.

[0059] Example 4:

[0060] The difference between this embodiment and embodiment 1 is that the mass ratio of M-SC to PP in step (2) is 3:1, while the rest is the same as in embodiment 1.

[0061] Example 5:

[0062] (1) Prepare a 0.2 mol / L sucrose solution, measure 70 mL of the above sucrose solution into a 100 mL polytetrafluoroethylene hydrothermal reactor, place it in a vacuum oven at 180 °C and keep it warm for 8 h. After the hydrothermal treatment is completed, wash with water, filter and dry to obtain reddish-brown blocky sucrose-based hard carbon precursor M-SC.

[0063] (2) The precursor M-SC obtained in step (1) and polypropylene (PP) are ground into powder in a ball mill jar at a mass ratio of 2:1. Then, low-temperature pre-oxidation is carried out in a tube furnace. The specific method is: heating to 330°C in an oxygen atmosphere at a heating rate of 2°C / min and holding for 1 hour.

[0064] (3) After the material cools down to room temperature, take out the black material from the tube furnace and transfer it to a high-temperature tube furnace for high-temperature calcination. The specific method is to heat it to 1300℃ in an argon atmosphere at a heating rate of 5℃ / min and keep it at that temperature for 8 hours.

[0065] (4) After the material cools to room temperature, the black material is taken out of the tube furnace to obtain sucrose-polypropylene composite hard carbon material.

[0066] Example 6:

[0067] The difference between this embodiment and embodiment 5 is that the atmosphere in the tubular furnace during calcination in step (2) is air, while the rest is the same as in embodiment 5.

[0068] Example 7:

[0069] The difference between this embodiment and embodiment 5 is that the heat preservation temperature in step (2) is 260°C, while the rest is the same as in embodiment 5.

[0070] Example 8:

[0071] The difference between this embodiment and embodiment 5 is that the heat preservation temperature in step (2) is 400℃, while the rest is the same as in embodiment 5.

[0072] Example 9:

[0073] The difference between this embodiment and embodiment 5 is that in step (2), polypropylene is replaced with polyethylene, while the rest is the same as in embodiment 5.

[0074] Comparative Example 1:

[0075] The difference between this comparative example and Example 1 is that PP is not added. The precursor M-SC is ground into powder and then sintered at 1300°C for 8 hours in an argon atmosphere at a heating rate of 5°C / min. That is, no composite with polypropylene or pre-oxidation treatment is performed. The rest is the same as in Example 4.

[0076] Comparative Example 2:

[0077] The difference between this comparative example and Example 5 is that PP is not added in step (2), but the rest is the same as Example 5.

[0078] Performance test example:

[0079] The hard carbon anode material prepared in the above examples and comparative examples was mixed with conductive agent carbon black (SP) and binder (PVDF) at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added. The mixture was prepared into a slurry in a planetary ball mill. Then, it was coated onto copper foil with a 125 μm doctor blade, dried in a forced-air oven at 60°C for 2 hours, and then dried in a vacuum at 105°C for 12 hours to prepare a negative electrode sheet for button cells.

[0080] Assemble the aforementioned negative electrode for button cells inside a glove box, allow it to stand at room temperature for 12 hours, and then apply it at 20 mA / g under a voltage range of 0.01–2.5V. -1 After three cycles at a current density of 100 mA.g -1 Current density was used for charge-discharge cycle testing, and the results are shown in Table 1.

[0081] Table 1: Specific capacity of sodium-ion batteries during the first charge / discharge cycle (20 mA g) -1 (current density)

[0082]

[0083] By comparing the first-cycle charge-discharge specific capacity of various embodiments and comparative examples, the introduction of polymers such as PP and PC significantly improved the first-cycle coulombic efficiency (ICE). Analysis of the capacity distribution in the sloping and plateau regions of the first-cycle discharge curve showed that the introduction of PP increased the capacity proportion in the plateau region. Furthermore, pre-oxidation treatment further improved the reversible specific capacity, especially in Example 5, where the reversible specific capacity increased to 387 mAh / g, and the sodium storage proportion in the plateau region of the first-cycle discharge specific capacity reached 64%, an improvement of 12 percentage points compared to Comparative Example 1. Hard carbon anode materials with abundant sodium storage behavior in the plateau region are key to achieving low-voltage, high-energy-density sodium-ion batteries. Figure 1 Cyclic performance graphs for Comparative Example 1, Example 3, Example 5, and Example 6 are shown.

[0084] Table 2. Material pore size information for different embodiments and comparative examples.

[0085]

[0086] Table 2 lists the pore size information of some examples and comparative examples. The specific surface area of ​​the material decreased by 30 times after the introduction of polypropylene. This is due to the sealing effect caused by the deposition of aromatic compounds from polypropylene decomposition at the sucrose openings. The decrease in specific surface area increases the first-cycle coulombic efficiency of hard carbon. Furthermore, pre-oxidation creates a large number of micropores and ultramicropores in the carbon matrix. In Example 5, the micro-to-mesopore ratio reached 6.19. In the carbon dioxide isothermal adsorption-desorption experiment, S... BET It reached 395.8m 2 / g, while nitrogen isothermal adsorption-desorption S BET Only 16.9m 2 The / g result demonstrates the presence of numerous micropores in the material. These micropores exhibit a molecular sieve effect, selectively allowing desolvated sodium ions to enter while preventing electrolyte solvent molecules from penetrating. This reduces electrolyte decomposition on the electrode surface and significantly improves the first-cycle coulombic efficiency, which explains why Example 5 achieved a first-cycle coulombic efficiency of 83%. Furthermore, true density testing revealed that the pre-oxidation process significantly increased the closed-cell volume of the sucrose-polypropylene composite hard carbon, and closed-cell filling has been shown to contribute to the sodium storage capacity of the plateau region (see Table 2).

[0087] Table 3. Graphite crystallite size and defect content in different embodiments and comparative examples.

[0088]

[0089] Table 3 compares the changes in the internal microstructure of each comparative example and embodiment. X-ray diffraction was used to measure the interlayer spacing (d) of the materials. 002 ), and L, representing the size of graphite crystallites, was calculated according to the Sher formula. a L c Raman spectroscopy was used to characterize the carbon layer, obtaining the ratio A of the D peak to the G peak, which represents the degree of disorder. D / A G .

[0090] Table 3 shows that directly introducing PP has little effect on the spacing of the carbon layers and the size of the graphite domains. The introduction of PP increases the defect content, while the pre-oxidation treatment opens up the carbon layers. 002 As the number of particles increases, the size of the graphite crystallites decreases, and the disorder increases. The transmission electron microscope image of Comparative Example 1 is shown below. Figure 2 As shown, the transmission electron microscope image of the material in Example 3 is as follows. Figure 3 As shown, the transmission electron microscope image of the material in Example 5 is as follows. Figure 4 As shown, comparison Figure 2 , Figure 3 and Figure 4The TEM image clearly shows an increase in closed pores inside the hard carbon of Example 5 due to the random orientation of the carbon layers, and a further increase in the disorder of the carbon layers. This is consistent with the A shown in Table 2. D / A G Increase, L a L c Reduced information consistency and the fragmentation and disorder of the graphite microcrystalline structure inside hard carbon after pre-oxidation are the reasons for the increase in closed pores. These changes in microstructure affect the electrochemical performance of sucrose-polypropylene composite hard carbon, specifically manifested as larger interlayer spacing, more closed-pore structures, lower specific surface area, and a higher proportion of micro- and mesopores, which will drive hard carbon towards characteristics such as a long low-voltage plateau, high initial efficiency, and high specific capacity.

[0091] In addition, the mass of the matrix material before and after calcination and the prepared hard carbon material were weighed using a balance, and the carbonization yield of the material in the high-temperature carbonization stage was calculated. The results are shown in Table 4.

[0092] Table 4. Comparison of carbon yield in the high-temperature carbonization stage between different embodiments and comparative examples.

[0093]

[0094] Table 4 shows that the carbon yield significantly improved after the introduction of PP, exceeding 50%, which is higher than that of sucrose-based hard carbon composed of polypropylene. This indicates that some PP can be converted into functional carbon materials under the action of M-SC. Further pre-oxidation reduced the carbon yield somewhat, possibly due to the loss of some carbon caused by oxygen-assisted combustion. However, compared to the 37.67% carbon yield after sintering following M-SC pre-oxidation, the introduction of PP still improved the carbon yield.

[0095] 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 method for preparing sucrose-polymer composite hard carbon, characterized in that, Includes the following steps: (1) The sucrose solution was subjected to hydrothermal carbonization, and after hydrothermal treatment, it was washed and dried to obtain the sucrose-based hard carbon precursor M-SC; (2) After mixing the precursor M-SC obtained in step (1) with the polymer and ball milling it into powder, it is directly carbonized at high temperature; (3) After high-temperature carbonization, the black material is removed after the material cools down to obtain sucrose-polymer composite hard carbon anode material.

2. The method of claim 1, wherein, In step (2), before high-temperature carbonization, the mixed ball milled material is first subjected to low-temperature pre-oxidation, and then the high-temperature carbonization is carried out.

3. The method of claim 2, wherein, The low-temperature pre-carbonization is carried out in a tube furnace. The low-temperature pre-oxidation time is 1 to 5 hours, the low-temperature pre-carbonization temperature is 200℃ to 400℃, the heating rate is 1 to 7℃ / min, and oxygen / air gas is introduced during the heating and holding process for pre-oxidation.

4. The method of claim 3, wherein, The low-temperature pre-oxidation time is 1 hour; the low-temperature pre-oxidation temperature is 330°C; and the low-temperature pre-oxidation heating rate is 2°C / min.

5. The method according to claim 1, characterized in that, In step (1), the hydrothermal carbonization is carried out in a hydrothermal reactor at a temperature of 160–200°C; the hydrothermal holding time is 8–24 h; and the washing solvent is distilled water or ethanol.

6. The method of claim 5, wherein, The hydrothermal temperature is 180℃, and the hydrothermal insulation time is 8 hours.

7. The method of claim 1, wherein, In step (2), the polymer is selected from PP, PE, and PC; the composite ratio of M-SC and polymer is 1.5 to 3:

1.

8. The method of claim 7, wherein, The polymer is PP, and the composite ratio of M-SC to the polymer is 2:

1.

9. The method according to claim 1, characterized in that, In step (2), the high-temperature carbonization is carried out at a heating rate of 1 to 7°C / min to 1000°C to 1300°C and held for 8 to 12 hours. Inert gas is introduced for protection during the heating and holding process.

10. The composite hard carbon prepared by any one of claims 1 to 9.

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