Preparation method of hard carbon material with high initial efficiency and low specific surface area and application of hard carbon material in sodium-ion battery

Abstract

The invention discloses a preparation method of a high-first-effect and low-specific-surface-area hard carbon material and application of the hard carbon material in a sodium-ion battery, and belongs to the technical field of sodium-ion battery negative electrode materials, hard-shell biomass is used as a raw material and is subjected to three-stage (400 DEG C trepanning stage, 800 DEG C carbonization treatment stage and 1400 DEG C graphite layer forming stage) high-temperature annealing treatment, and the high-first-effect and low-specific-surface-area hard carbon material is obtained. The high-temperature annealing treatment product is subjected to acid pickling, water washing and drying to obtain the hard carbon material with high initial efficiency and low specific surface area, the surface porosity of the hard carbon material is effectively reduced in the medium-temperature treatment process, and meanwhile, the closed pore volume and the average pore size of the material are remarkably increased. The hard carbon material prepared by the method shows the characteristic of extremely low specific surface area (the specific surface area is as low as 1.9 m < 2 > / g), so that the sodium-ion battery can realize efficient and stable circulation, and meanwhile, the sodium-ion battery shows high capacity, high first efficiency and excellent rate capability.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery anode material technology, and particularly relates to a method for preparing a hard carbon material with high initial efficiency and low specific surface area and its application in sodium-ion batteries. Background Technology

[0002] The large-scale consumption of fossil fuels has caused irreversible pollution and damage to the global environment. Therefore, developing sustainable and clean new energy sources to replace fossil fuels has become an urgent priority. With the rapid development of new energy technologies, significant progress has been made in power generation technologies for renewable energy sources such as wind, solar, hydro, and tidal power. However, these energy sources are characterized by randomness, intermittency, and volatility, making it impossible to directly meet the demand for on-demand power supply. Therefore, developing efficient and reliable energy storage systems is key to solving this problem.

[0003] Lithium-ion batteries, due to their excellent stability, high discharge capacity, and low self-discharge rate, have been widely used in production and daily life. However, the limited reserves of lithium resources cannot simultaneously support the needs of electric vehicles and large-scale energy storage, thus necessitating the development of new energy storage battery technologies to achieve sustainable development. Sodium-ion batteries, with their similar working principle and battery structure to lithium-ion batteries, coupled with abundant sodium resources, low cost, and excellent overall performance, are gradually becoming an important alternative to lithium-ion batteries. Sodium-ion batteries can meet the requirements of the new energy sector for low cost, long lifespan, and high safety performance, effectively alleviating the lithium resource shortage problem and playing an increasingly important role in new energy storage applications. Currently, hard carbon materials, due to their high specific capacity, excellent cycle stability, and rate performance, have become the most promising anode materials for sodium-ion batteries.

[0004] However, the preparation of hard carbon materials typically relies on petroleum-based or coal-based precursors, which is not only costly but also contradicts environmental sustainability goals. Therefore, finding renewable and low-cost hard carbon precursors for developing hard carbon materials for sodium-ion batteries has become a current research hotspot. Furthermore, the inherent porous structure of biomass-derived hard carbon materials often leads to extremely low initial coulombic efficiency in sodium-ion batteries. Improving the pore and interface structure of hard carbon materials and enhancing their reversibility in sodium-ion batteries remains a challenge for those skilled in the art. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a method for preparing hard carbon materials with high initial efficiency and low specific surface area, and its application in sodium-ion batteries. The hard carbon material prepared using this method exhibits an extremely low specific surface area (as low as 1.9 m²). 2 The properties of sodium-ion batteries ( / g) enable them to achieve efficient and stable cycling, while also exhibiting high capacity, high initial efficiency, and excellent rate performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing hard carbon materials with high initial efficiency and low specific surface area, comprising the following steps: Hard-shelled biomass was used as raw material and subjected to a three-stage high-temperature annealing process. The high-temperature annealing product was then acid-washed, water-washed, and dried to obtain the high-efficiency, low-specific-surface-area hard carbon material. The three-stage high-temperature annealing process is divided into a pore-opening stage, a carbonization stage, and a graphite layer formation stage. The temperature during the pore-opening stage is 400°C, the temperature during the carbonization stage is 800°C, and the temperature during the graphite layer formation stage is 1400°C.

[0007] Hard-shelled biomass, with its wide availability, low cost, and renewability, is an ideal precursor for preparing hard carbon materials. Through processes such as pyrolysis and carbonization, shelled precursors can be transformed into hard carbon materials with excellent sodium storage performance. This not only provides a new avenue for the high-value utilization of agricultural waste but also offers a new material option for the sustainable development of sodium-ion batteries. This invention uses a simplified process to prepare a high-capacity, high-efficiency, and extremely low specific surface area sodium-ion battery anode. The high-temperature annealing process of this invention is carried out in three stages: the pore-opening stage (low-temperature zone) ensures effective dehydration and pore opening of the biomass, initially regulating the pore structure inside the hard carbon; the carbonization stage (medium-temperature zone) further deoxidizes the initial carbonization, forming a large number of C=C / CC bonds; and the graphite layer formation stage (high-temperature zone) closes the pores generated during dehydration and deoxidation to further form a closed-pore structure. The medium-temperature treatment effectively reduces the surface porosity of the material (i.e., effectively reduces the specific surface area) while significantly increasing the closed-pore volume and average pore size. The final true density of the hard carbon material is between 1.96 and 2.16 g / cm³. This optimized pore structure helps provide more sodium ion storage active sites while maintaining a low specific surface area, thereby improving the battery's reversible capacity while maintaining a high initial coulombic efficiency (first-efficiency). The high-first-efficiency, low-specific-surface-area hard carbon material prepared by this invention has a specific surface area below 10.4 m² / g (as low as 1.9 m² / g). Currently, commonly used hard carbon materials have a large specific surface area. After activation with phosphoric acid according to this invention, under the condition of maintaining a consistent high temperature, the medium-temperature regulation will lead to a decrease in the specific surface area of ​​hard-shell biomass. Therefore, this invention prepares a hard carbon material with a low specific surface area and high initial efficiency. Similarly, at high temperatures of 1200℃ and 1300℃, the specific surface area decreases with the addition of temperature control in the mid-temperature zone. However, this invention demonstrates that the specific surface area of ​​coconut shell hard carbon reaches an extremely low level at 1400℃. This indicates that at 1400℃, the open pores in the hard carbon are essentially converted into closed pores, making it the most suitable high-temperature carbonization temperature for coconut shells. In this invention, phosphoric acid is introduced to provide more defect sites, effectively creating an open pore in the coconut shell. While the high-temperature zone is indeed a crucial temperature for the conversion of open pores into closed pores, this invention focuses on increasing the number of micropores and further controlling the conversion of open pores into closed pores. Therefore, this invention can prepare hard carbon materials with extremely low specific surface areas.

[0008] Furthermore, the heating rate to the opening stage is 5°C / min, the heating rate to the carbonization stage is 5°C / min, and the heating rate to the graphite layer formation stage is 2°C / min.

[0009] Furthermore, the processing time for the opening stage is 2 hours, the processing time for the carbonization stage is 2 hours, and the processing time for the graphite layer formation stage is 2 hours.

[0010] Furthermore, the hard-shelled biomass is selected from coconut shells, walnut shells, peach kernels, or apricot kernels.

[0011] Furthermore, before the hard-shelled biomass undergoes the three-stage high-temperature annealing treatment, it also includes a step of soaking in phosphoric acid; The concentration of phosphoric acid is 0.1-0.5 mol / L, and the soaking time is 6-18 h.

[0012] Phosphoric acid, acting as both a regulator and dopant, etches the tight structure of lignin and hemicellulose in hard-shelled biomass, thereby creating more and larger closed pore spaces. By controlling the phosphoric acid concentration and soaking time, the biomass precursor can be made to have more defects, providing more active sites for sodium ion storage in subsequent processes. This invention achieves phosphorus atom doping through thermodynamic control in the mid-temperature range and the introduction of a phosphorus source, synergistically inducing the expansion of carbon interlayer spacing.

[0013] After immersion in phosphoric acid, the immersed material is centrifuged once to separate the solid and liquid. The resulting solid is dried and then subjected to a three-stage high-temperature annealing process.

[0014] Furthermore, the acid used in pickling is hydrochloric acid, sulfuric acid, or nitric acid.

[0015] After the three-stage high-temperature annealing process is completed, the resulting product is soaked in an acid solution again (acid washing) to further remove the ash and obtain hard carbon material for sodium-ion batteries.

[0016] Furthermore, the concentration of acid used during pickling is 0.2-5 mol / L.

[0017] Furthermore, all three stages of high-temperature annealing are performed under a protective atmosphere.

[0018] The present invention also provides a hard carbon material with high efficiency and low specific surface area prepared by the above method.

[0019] This invention also provides an application of the aforementioned high-efficiency, low-specific-surface-area hard carbon material in sodium-ion batteries, wherein the high-efficiency, low-specific-surface-area hard carbon material serves as the negative electrode. The shell-derived hard carbon negative electrode for sodium-ion batteries provided by this invention exhibits excellent performance.

[0020] The present invention also provides a sodium-ion battery, wherein the negative electrode material is the aforementioned high-efficiency, low-specific-surface-area hard carbon material.

[0021] Compared with the prior art, the present invention has the following advantages and technical effects: This invention utilizes phosphoric acid to etch the tight structure of lignin and hemicellulose in hard-shell biomass, thereby developing more and larger closed pore spaces. By controlling the phosphoric acid concentration and soaking time, the biomass precursor is endowed with more defects, providing more active sites for sodium ion storage in subsequent processes. While conventional one-step high-temperature annealing can obtain hard carbon materials, it often exhibits extremely low initial coulombic efficiency and low plateau capacity because the pore structure of carbon cannot be effectively closed. This invention modifies the high-temperature annealing process into three stages (pore-opening stage, carbonization stage, and graphite layer formation stage), where the pore-opening stage temperature is 400℃, the carbonization stage temperature is 800℃, and the graphite layer formation stage temperature is 1400℃. The low-temperature process (pore-opening stage) ensures effective dehydration and pore opening of the biomass, initially regulating the pore structure inside the hard carbon. The medium-temperature process (carbonization stage) ensures deoxidation of the carbon material, forming a large number of C / C=C bonds and further increasing the number of micropores. The high-temperature process (graphite layer formation stage) causes partial graphitization of the carbon layer and closes the pores generated during dehydration and deoxidation to further form a closed-pore structure, thereby effectively improving the reversibility of hard carbon materials in the sodium storage process.

[0022] The high-efficiency, low-specific-surface-area hard carbon material (shell-derived hard carbon) provided by this invention exhibits a micron-sized structure, good electrical conductivity, and a closed-pore structure. The hard carbon material prepared using the method of this invention displays an extremely low specific surface area (as low as 1.9 μm). 2 The properties of ( / g) enable sodium-ion batteries to achieve efficient and stable cycling, while also exhibiting high capacity, high initial efficiency, and excellent rate performance. The shell-derived hard carbon prepared by this invention has the potential for large-scale industrial application. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a scanning electron microscope image of coconut shell-derived hard carbon from Example 1.

[0024] Figure 2 This is a transmission electron microscope (TEM) image of coconut shell-derived hard carbon from Example 1.

[0025] Figure 3 The images show the X-ray diffraction (XRD) patterns of coconut shell-derived hard carbon in Examples 1-2 and Comparative Examples 1-2.

[0026] Figure 4The nitrogen (N2) adsorption-desorption curves of coconut shell-derived hard carbon in Examples 1-2 and Comparative Examples 1-2 are shown.

[0027] Figure 5 Small-angle X-ray scattering (SAXS) curves of coconut shell-derived hard carbon in Examples 1-2 and Comparative Examples 1-2.

[0028] Figure 6 The image shows the Raman fitting curve of coconut shell-derived hard carbon in Example 1.

[0029] Figure 7 The cycling curve of coconut shell-derived hard carbon in Example 1 at a current density of 20 mA / g is shown.

[0030] Figure 8 The rate performance of coconut shell-derived hard carbon in Example 1 is shown at current densities of 20-1000 mA / g.

[0031] Figure 9 The long cycling curve of coconut shell-derived hard carbon in Example 1 at a current density of 300 mA / g is shown. Detailed Implementation

[0032] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0033] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0034] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0035] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0036] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0037] Embodiments of the present invention provide a method for preparing a hard carbon material with high initial efficiency and low specific surface area, comprising the following steps: Hard-shelled biomass is used as raw material and subjected to a three-stage high-temperature annealing process. The high-temperature annealing product is then acid-washed, water-washed, and dried to obtain a hard carbon material with high efficiency and low specific surface area. The three-stage high-temperature annealing process consists of an opening stage, a carbonization stage, and a graphite layer formation stage. The temperature for the pore-opening stage is 400℃, the temperature for the carbonization stage is 800℃, and the temperature for the graphite layer formation stage is 1400℃.

[0038] In a preferred embodiment of the present invention, the processing time for the pore-opening stage is 2 hours, the processing time for the carbonization stage is 2 hours, and the processing time for the graphite layer formation stage is 2 hours.

[0039] In a preferred embodiment of the present invention, the heating rate to the pore-opening stage is 5°C / min, the heating rate to the carbonization stage is 5°C / min, and the heating rate to the graphite layer formation stage is 2°C / min.

[0040] In a preferred embodiment of the present invention, the hard-shelled biomass is selected from coconut shells, walnut shells, peach kernels, or apricot kernels. In the following embodiments of the present invention, coconut shells are used as an example for illustration.

[0041] In a preferred embodiment of the present invention, before the hard-shelled biomass undergoes a three-stage high-temperature annealing treatment, it further includes a step of soaking in phosphoric acid; The concentration of phosphoric acid is 0.1-0.5 mol / L, and the soaking time is 6-18 hours.

[0042] In a preferred embodiment of the present invention, the acid used during pickling is hydrochloric acid, sulfuric acid, or nitric acid.

[0043] In a preferred embodiment of the present invention, the concentration of acid used during pickling is 0.2-5 mol / L.

[0044] After the three-stage high-temperature annealing treatment, the resulting product is soaked in an acid solution again (acid washing) to further remove ash and obtain hard carbon material for sodium-ion batteries. After the three-stage high-temperature annealing treatment, the product is cooled to room temperature and then acid washed at a cooling rate of 1-5°C / min. In the following embodiments of the present invention, hydrochloric acid with a concentration of 1 mol / L is used for acid washing.

[0045] In a preferred embodiment of the present invention, all three stages of high-temperature annealing are performed under a protective atmosphere.

[0046] For example, the protective atmosphere is selected from nitrogen, argon, a nitrogen / hydrogen mixture, or an argon / hydrogen mixture, the gas pressure is atmospheric pressure, the gas is a flowing gas, and the gas flow rate is 20-100 sccm. In the following embodiments of the present invention, argon with a flow rate of 50 sccm is used as the protective atmosphere for illustration.

[0047] The embodiments of the present invention also provide a hard carbon material with high efficiency and low specific surface area prepared by the above method.

[0048] Embodiments of the present invention also provide an application of the above-mentioned high-efficiency, low-specific-surface-area hard carbon material in a sodium-ion battery, wherein the high-efficiency, low-specific-surface-area hard carbon material is used as the negative electrode.

[0049] An embodiment of the present invention also provides a sodium-ion battery, wherein the negative electrode material is the aforementioned high-efficiency, low-specific-surface-area hard carbon material.

[0050] In a preferred embodiment of the present invention, the assembly method of the sodium-ion battery is as follows: a high-efficiency, low-specific-surface-area hard carbon material is mixed with a binder to obtain a slurry, the obtained slurry is coated onto a current collector by a scraping method, dried to obtain a sodium-ion battery negative electrode, and then the sodium-ion battery is assembled under an argon atmosphere using an ether electrolyte containing sodium salt.

[0051] In a preferred embodiment of the present invention, during the battery assembly process, the current collector material is selected from one of pure copper foil, carbon-coated copper foil, pure aluminum foil, and carbon-coated aluminum foil.

[0052] In a preferred embodiment of the present invention, during the battery assembly process, the mass ratio of high-efficiency, low-specific-surface-area hard carbon material to binder is 9:1.

[0053] In a preferred embodiment of the present invention, during the battery assembly process, the coating thickness of the slurry is 50-200 μm; the drying temperature is 50-100°C, and the drying time is 12-48 h.

[0054] In a preferred embodiment of the present invention, during the battery assembly process, the binder is selected from one of polyvinylidene fluoride, sodium carboxymethyl cellulose, and sodium alginate.

[0055] In a preferred embodiment of the present invention, during the battery assembly process, the ether electrolyte is selected from ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether.

[0056] In a preferred embodiment of the present invention, during the battery assembly process, the sodium salt is selected from sodium hexafluorophosphate, sodium perchlorate and sodium trifluoromethanesulfonate, and the concentration of the sodium salt in the ether electrolyte is 0.5-2 mol / L.

[0057] In a preferred embodiment of the present invention, during the battery assembly process, the oxygen content in the argon atmosphere is less than 1 ppm and the water content is less than 1 ppm.

[0058] Unless otherwise specified, the room temperature in this invention is 25±2℃.

[0059] All raw materials used in the embodiments of the present invention were obtained through commercial purchase.

[0060] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0061] The technical solution of the present invention will be further illustrated by the following embodiments.

[0062] Example 1 This embodiment provides a method for preparing high-efficiency, low-specific-surface-area hard carbon materials using coconut shells as raw materials, the steps of which are as follows: Coconut shell powder (600 mesh) was soaked in 0.5 mol / L phosphoric acid for 18 hours. The phosphoric acid etched the tight structure of lignin and hemicellulose, thereby developing more and larger closed pore spaces. The precursor material obtained after soaking was centrifuged once to separate the solid and liquid. The resulting solid product was dried for 24 hours. The dried solid product was then subjected to high-temperature annealing under argon protection (flow rate of 50 sccm). The high-temperature annealing process was carried out in three stages: first, the temperature was increased from room temperature to 400℃ at a rate of 5℃ / min, and annealed at 400℃ for 2 hours (this is the first stage of pore opening); then, the temperature was increased to 800℃ at a rate of 5℃ / min, and annealed at 800℃ for 2 hours (this is the second stage of carbonization); then, the temperature was increased to 1400℃ at a rate of 2℃ / min, and annealed at 1400℃ for 2 hours (this is the third stage of graphite layer formation); after annealing, a cooling process was carried out at a rate of 3℃ / min. The product subjected to high-temperature annealing was soaked in 1 mol / L hydrochloric acid for 4 hours. After soaking in hydrochloric acid, the soaked material was washed with deionized water and dried to obtain a high-efficiency, low-specific-surface-area hard carbon material (coconut shell-derived hard carbon).

[0063] Figure 1 This is a scanning electron microscope (SEM) image of coconut shell-derived hard carbon from Example 1. Figure 2 The transmission electron microscope (TEM) image of the coconut shell-derived hard carbon in Example 1 shows that the coconut shell hard carbon exhibits abundant layered stacking and honeycomb structure. In the TEM image, numerous short-range ordered and long-range disordered graphite-like microcrystals can be observed, with each microcrystal consisting of a few layers of graphene sheets stacked together. These sheets exhibit significant bending, interlacing, and disorder. The interlayer spacing (d) of this coconut shell hard carbon is... 002 The wavelength is 0.378 nm.

[0064] The X-ray diffraction (XRD) pattern of coconut shell-derived hard carbon in Example 1 is shown below. Figure 3 XRD analysis showed that the (002) diffraction peak of the coconut shell-derived hard carbon in Example 1 shifted to a lower angle direction near 23°. This phenomenon indicates that the coconut shell-derived hard carbon in Example 1, due to the thermodynamic regulation process in the mid-temperature region and the phosphorus (P) atom doping achieved by the introduction of phosphoric acid, jointly induced the expansion of carbon interlayers, reflecting the decrease in the orderliness of the graphite microcrystalline structure and the increase in the interlayer spacing.

[0065] The N2 adsorption-desorption curves of coconut shell-derived hard carbon in Example 1 are shown below. Figure 4 It can be seen that the specific surface area of ​​the coconut shell-derived hard carbon in Example 1 is as low as 1.9 m². 2 / g.

[0066] The SAXS curve of coconut shell-derived hard carbon in Example 1 is shown below. Figure 5 It can be seen that the closed-cell radius of the coconut shell-derived hard carbon in Example 1 is 13.82 Å, and the true density is 1.96 g / cm³. 3 .

[0067] Figure 6 The Raman fitting curve of coconut shell-derived hard carbon in Example 1 is shown. Raman spectroscopy analysis shows that the integral intensity ratio of the D peak to the G peak of the coconut shell-derived hard carbon in Example 1 (I D / I G The value of 1.31 indicates that it has the typical high-defect, low-graphitization structural characteristics of hard carbon.

[0068] The coconut shell-derived hard carbon from Example 1 was used as the negative electrode to assemble a sodium-ion battery. The specific process was as follows: the coconut shell-derived hard carbon was mixed with the binder sodium alginate (the binder accounted for 10 wt% of the total mass of the coconut shell-derived hard carbon and the binder), and then coated onto a carbon-coated copper foil current collector by a scraping method. The coating thickness was 100 μm. After drying (drying at 80°C for 48 h), the negative electrode of the sodium-ion battery was obtained. Then, the sodium-ion battery was assembled in a glove box with an argon atmosphere containing less than 1 ppm oxygen and less than 1 ppm water. The electrolyte was an ether electrolyte, which was ethylene glycol dimethyl ether with 1 mol / L sodium hexafluorophosphate dissolved in it.

[0069] At a current density of 20 mA / g, the sodium-ion battery synthesized from coconut shell-derived hard carbon in Example 1 exhibits an initial coulombic efficiency of 90.4%, an initial reversible specific capacity of 331 mAh / g, and a specific capacity of 318 mAh / g after 100 cycles. Figure 7 The capacity retention rate after 100 cycles was 96.1%. The rate performance of the coconut shell-derived hard carbon in Example 1 at current densities of 20-1000 mA / g is shown in [reference needed]. Figure 8 It can be seen that at a high current density of 1000 mA / g, its specific capacity is 234 mAh / g, exhibiting good rate performance. After 1000 cycles at a high current density of 300 mA / g, its specific capacity is 221 mAh / g. Figure 9 ).

[0070] Comparative Example 1 This comparative example provides a method for preparing coconut shell-derived hard carbon. The preparation method is as follows: Coconut shell powder (mesh size same as in Example 1) is subjected to high-temperature annealing under argon gas (flow rate of 50 sccm). The high-temperature annealing is carried out in two stages: first, the temperature is increased from room temperature to 400℃ at a rate of 5℃ / min, and annealed at 400℃ for 2 h; then, the temperature is increased to 1400℃ at a rate of 2℃ / min, and annealed at 1400℃ for 2 h; after annealing, a cooling treatment is performed at a rate of 3℃ / min. The high-temperature annealed product is soaked in 1 mol / L hydrochloric acid for 4 h. After hydrochloric acid soaking, the soaked material is washed with deionized water and dried to obtain coconut shell-derived hard carbon. The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example is 20.2 m². 2 / g ( Figure 4 The closed-cell radius of the coconut shell-derived hard carbon obtained in this comparative example is 11.48 Å. Figure 5 The true density of the coconut shell-derived hard carbon obtained in this comparative example is 2.32 g / cm³. 3 The integral intensity ratio (ID to GG) of the coconut shell-derived hard carbon obtained in this comparative example. D / I G The value is 1.15.

[0071] The XRD pattern of the coconut shell-derived hard carbon synthesized in this comparative example is shown below. Figure 3 .

[0072] The sodium-ion battery assembly process is the same as in Example 1.

[0073] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 83.1%, an initial reversible specific capacity of 264 mAh / g, a specific capacity of 249 mAh / g after 100 cycles, and a capacity retention of 94.3% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 153 mAh / g.

[0074] Comparative Example 2 This comparative example provides a method for preparing coconut shell-derived hard carbon, comprising the following steps: Coconut shell powder (mesh size same as in Example 1) is soaked in 0.5 mol / L phosphoric acid for 18 h, utilizing the phosphoric acid to etch the tight structure of lignin and hemicellulose, thereby developing more and larger closed pore spaces; the precursor material obtained after soaking is centrifuged once to separate the solid and liquid, and the resulting solid product is dried for 24 h; the dried solid product is subjected to high-temperature annealing under argon protection (flow rate of 50 sccm), in two stages: first, the temperature is increased from room temperature to 400℃ at a rate of 5℃ / min, and annealed at 400℃ for 2 h; then, the temperature is increased to 1400℃ at a rate of 2℃ / min, and annealed at 1400℃ for 2 h; after annealing, a cooling treatment is performed at a rate of 3℃ / min. The high-temperature annealed product is soaked in 1 mol / L hydrochloric acid for 4 h. After soaking in hydrochloric acid, the material was washed with deionized water and dried to obtain coconut shell-derived hard carbon. The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example was 56.3 m². 2 / g ( Figure 4 The closed-cell radius of the coconut shell-derived hard carbon obtained in this comparative example is 13.15 Å. Figure 5 The true density of the coconut shell-derived hard carbon obtained in this comparative example is 2.16 g / cm³. 3 The integral intensity ratio (ID to GG) of the coconut shell-derived hard carbon obtained in this comparative example. D / I G The value is 1.30.

[0075] The XRD pattern of the coconut shell-derived hard carbon synthesized in this comparative example is shown below. Figure 3 It can be seen that the (002) crystal plane diffraction peak of the coconut shell-derived hard carbon synthesized in this comparative example is shifted to a lower angle compared to the coconut shell-derived hard carbon of Comparative Example 1. This phenomenon indicates that the phosphorus (P) atom doping achieved by the introduction of phosphate in the material of Comparative Example 2 induces the expansion of carbon interlayer, reflecting the decrease in the order of the graphite microcrystalline structure and the increase in the interlayer spacing.

[0076] The N2 adsorption-desorption isotherms of the coconut shell-derived hard carbon synthesized in this comparative example are shown below. Figure 4It can be seen that Comparative Example 2, by introducing phosphoric acid and achieving phosphorus (P) atom doping during the preparation process, increased the specific surface area of ​​its hard carbon material from 20.2 m² in Comparative Example 1. 2 / g increased significantly to 56.3 m 2 / g. This result indicates that the introduction of phosphorus, and the decomposition of phosphorus-containing functional groups (such as -P=O, -PO-, etc.) and the escape of gaseous products (such as P2O5, PH3, etc.) during its pyrolysis, have a significant pore-forming and pore-expanding effect on the carbon framework, thereby generating new micropores or expanding existing pores within the material. Simultaneously, phosphorus atoms, as heteroatoms, embed into the carbon matrix, disrupting the topological regularity of the six-membered carbon rings, effectively inhibiting the growth and ordered stacking of graphite crystallites, and promoting the formation of a more disordered, open, and high-defect-density amorphous carbon structure. These structural evolutions collectively lead to a significant increase in the specific surface area of ​​the material.

[0077] The SAXS curve of coconut shell-derived hard carbon in this comparative example is shown below. Figure 5 It can be seen that, compared with Comparative Example 1, Comparative Example 2, by introducing phosphoric acid and achieving phosphorus (P) atom doping, increased the closed-pore radius of the hard carbon material from 11.48 Å to 13.15 Å, and the true density from 2.32 g / cm³. 3 Decreased to 2.16 g / cm³ 3 These data collectively confirm that phosphorus-containing functional groups (such as -P=O, -POC) undergo pyrolysis in the intermediate temperature range (~300-600℃), accompanied by the in-situ generation and release of small molecule gases such as P2O5 and PH3. This process can be viewed as a self-sacrificing template / in-situ foaming mechanism, which directly induces the formation of nanoscale closed-pore cavities during the solidification of the amorphous carbon matrix through vapor phase etching and the construction of escape pathways. This mechanism ultimately leads to a significant increase in the closed-pore volume of hard carbon materials.

[0078] from Figure 5 It can also be seen that, compared with Comparative Example 2, Example 1, through mid-temperature control, increased the closed-pore radius of the hard carbon material from 13.15 Å to 13.82 Å, and the true density from 2.16 g / cm³. 3 Decreased to 1.96 g / cm 3 These data collectively confirm that the mid-temperature process significantly increases the closed-pore volume and average pore size of the material. This optimized pore structure facilitates the provision of more sodium ion storage active sites while maintaining a low surface area, thereby improving reversible capacity while maintaining a high initial coulombic efficiency.

[0079] The sodium-ion battery assembly process is the same as in Example 1.

[0080] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 89.6%, an initial reversible specific capacity of 303 mAh / g, a specific capacity of 292 mAh / g after 100 cycles, and a capacity retention of 96.3% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 189 mAh / g.

[0081] As can be seen from the data of Example 1 and Comparative Example 1, the present invention can still produce sodium-ion batteries with good performance even with a significant reduction in the specific surface area of ​​the material.

[0082] Example 2 This embodiment provides a method for preparing a high-efficiency, low-specific-surface-area hard carbon material (coconut shell-derived hard carbon). The steps are as follows: Coconut shell powder (mesh size same as in Example 1) is subjected to high-temperature annealing under argon gas (flow rate of 50 sccm). The high-temperature annealing is carried out in three stages: first, the temperature is increased from room temperature to 400℃ at a rate of 5℃ / min, and annealed at 400℃ for 2 h; then, the temperature is increased to 800℃ at a rate of 5℃ / min, and annealed at 800℃ for 2 h; then, the temperature is increased to 1400℃ at a rate of 2℃ / min, and annealed at 1400℃ for 2 h; after annealing, a cooling treatment is performed at a rate of 3℃ / min. The high-temperature annealed product is soaked in 1 mol / L hydrochloric acid for 4 h. After hydrochloric acid soaking, the soaked material is washed with deionized water and dried to obtain coconut shell-derived hard carbon. The specific surface area of ​​the coconut shell-derived hard carbon obtained in this embodiment is as low as 3.4 m². 2 / g ( Figure 4 The closed-cell radius of the coconut shell-derived hard carbon obtained in this embodiment is 12.51 Å. Figure 5 The true density of the coconut shell-derived hard carbon obtained in this embodiment is 2.21 g / cm³. 3 The integral intensity ratio (ID to GG) of the coconut shell-derived hard carbon obtained in this embodiment is... D / I G The value is 1.20.

[0083] The XRD pattern of coconut shell-derived hard carbon in this embodiment is shown below. Figure 3 The diffraction peak of the (002) crystal plane near 23° in this embodiment of coconut shell-derived hard carbon shows a shift towards a lower angle compared to Comparative Example 1. This phenomenon indicates that the thermodynamic regulation process in the mid-temperature region induced the expansion of the carbon interlayer in this embodiment, reflecting a decrease in the orderliness of the graphite microcrystalline structure and an increase in the interlayer spacing. Moreover, from... Figure 3 It can be seen that the (002) crystal plane diffraction peak of coconut shell-derived hard carbon in Example 1 at around 23° is shifted to a lower angle compared to Comparative Example 2 and Example 2.

[0084] The N2 adsorption-desorption isotherm results for coconut shell-derived hard carbon in this embodiment are shown below. Figure 4 It can be seen that, compared with Comparative Example 1, Example 2 introduced a medium-temperature control process during preparation, which increased the specific surface area of ​​the prepared hard carbon from 20.2 m² to... 2 / g significantly decreased to 3.4 m 2 / g. This data indicates that the mid-temperature treatment effectively reduces the surface porosity of the material, thereby physically limiting the excessive penetration and irreversible consumption of the electrolyte into the electrode material during the initial cycle. This is beneficial for guiding the formation of a denser and more stable solid electrolyte interphase (SEI) film, enabling precise control over the SEI formation site and thickness.

[0085] The SAXS curve of coconut shell-derived hard carbon in this embodiment is shown below. Figure 5 As can be seen from Comparative Example 1, Example 2, through mid-temperature control, increased the closed-pore radius of the hard carbon material from 11.48 Å to 12.51 Å, and the true density from 2.32 g / cm³. 3 Decreased to 2.21 g / cm 3 These data collectively confirm that the mid-temperature process significantly increases the closed-pore volume and average pore size of the material. This optimized pore structure facilitates the provision of more sodium ion storage active sites while maintaining a low surface area, thereby improving reversible capacity while maintaining a high initial coulombic efficiency.

[0086] The sodium-ion battery assembly process is the same as in Example 1.

[0087] At a current density of 20 mA / g, the sodium-ion battery assembled in this embodiment exhibits an initial coulombic efficiency of 84.3%, an initial reversible specific capacity of 292 mAh / g, a specific capacity of 281 mAh / g after 100 cycles, and a capacity retention of 96.2% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 175 mAh / g.

[0088] Comparative Example 3 Same as Example 1, except that the temperature of the third stage high-temperature annealing treatment is 1300°C.

[0089] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example is 59.5 m². 2 / g. The closed-cell radius of the coconut shell-derived hard carbon obtained in this comparative example is 12.56 Å. The true density of the coconut shell-derived hard carbon obtained in this comparative example is 2.20 g / cm³. 3 .

[0090] The sodium-ion battery assembly process is the same as in Example 1.

[0091] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 86.3%, an initial reversible specific capacity of 309 mAh / g, a specific capacity of 295 mAh / g after 100 cycles, and a capacity retention of 95.4% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 184 mAh / g.

[0092] Compared to Example 1 (third-stage high-temperature annealing at 1400℃), adjusting the third-stage high-temperature annealing to 1300℃ resulted in an increase in the specific surface area of ​​coconut shell-derived hard carbon, which also led to a decrease in sodium-ion battery performance. The fundamental reason lies in the discontinuous evolution of the material structure. At 1300℃, the carbon material is in a critical transitional range of structural evolution: on the one hand, the closed-pore structure formed by the previous pyrolysis collapses and decreases significantly due to the shrinkage of the carbon skeleton; on the other hand, the graphitization and ordering process of the carbon layers has not been fully carried out due to insufficient thermodynamic driving force, failing to form a sufficiently large graphite-like microcrystalline region with suitable interlayer spacing. This results in the overall sodium storage active site density of the material being reduced to the lowest level, and the sodium ion diffusion path being disordered, leading to a deterioration in kinetic performance. In contrast, the carbonization products of the third-stage high-temperature annealing at 1400℃ obtain higher thermodynamic energy, driving significant rearrangement of carbon atoms and forming larger and more ordered graphite-like microcrystalline structures. Although their closed-pore volume is reduced, these microcrystalline structures have expanded interlayer spacing, providing stable and reversible insertion / extraction channels for sodium ions. The platform capacity contributed by this embedded reaction mechanism effectively compensates for and surpasses the capacity loss caused by the reduction of closed pores.

[0093] Example 3 Same as Example 1, except that the coconut shell powder is soaked in phosphoric acid for 6 hours.

[0094] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this embodiment is as low as 10.4 m². 2 / g, closed-cell radius of 12.67Å, true density of 2.07g / cm³ 3 .

[0095] The sodium-ion battery assembly process is the same as in Example 1.

[0096] At a current density of 20 mA / g, the sodium-ion battery assembled in this embodiment achieves an initial coulombic efficiency of 87.6%, an initial reversible specific capacity of 314 mAh / g, a specific capacity of 300 mAh / g after 100 cycles, and a capacity retention of 95.5% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 204 mAh / g.

[0097] Compared to Example 1 (immersion time of 18 hours), adjusting the immersion time to 6 hours resulted in a slight decrease in sodium-ion battery performance, but it still achieved the technical effects of low specific surface area and high initial efficiency. The fundamental reason lies in the uneven dispersion of phosphoric acid, forming localized high-concentration and low-phosphorus regions, leading to phase separation. A compositional gradient exists within the precursor. Uneven phosphorus doping occurs: high-phosphorus regions exhibit excessive pore formation / destruction of the carbon framework; low-phosphorus regions show weak doping effects.

[0098] Comparative Example 4 Same as Example 1, except that the coconut shell powder is soaked in phosphoric acid for 24 hours.

[0099] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example is 44.2 m². 2 / g, closed-cell radius is 12.39Å, true density is 2.11g / cm³ 3 .

[0100] The sodium-ion battery assembly process is the same as in Example 1.

[0101] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 85.4%, an initial reversible specific capacity of 303 mAh / g, a specific capacity of 291 mAh / g after 100 cycles, and a capacity retention of 96.0% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 176 mAh / g.

[0102] Compared with Example 1 (soaking time of 18h), adjusting the soaking time to 24h resulted in an increase in the specific surface area of ​​the material and a decrease in the performance of the sodium-ion battery. The fundamental reason is that excessive solvent evaporation and changes in system viscosity may lead to precursor prepolymerization or component segregation.

[0103] Example 4 Same as Example 1, except that the concentration of phosphoric acid is 0.1 mol / L.

[0104] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this embodiment is as low as 5.6 m². 2 / g, closed-cell radius of 11.53Å, true density of 2.14g / cm³ 3 .

[0105] The sodium-ion battery assembly process is the same as in Example 1.

[0106] At a current density of 20 mA / g, the sodium-ion battery assembled in this embodiment exhibits an initial coulombic efficiency of 83.2%, an initial reversible specific capacity of 284 mAh / g, a specific capacity of 271 mAh / g after 100 cycles, and a capacity retention of 95.4% after 100 cycles. After 1000 cycles at a high current density of 300 mA / g, its specific capacity is 162 mAh / g.

[0107] Compared to Example 1 (phosphoric acid concentration of 0.5 mol / L), adjusting the phosphoric acid concentration to 0.1 mol / L resulted in a slight decrease in sodium-ion battery performance, but it still achieved the technical effects of low specific surface area and high initial efficiency. The fundamental reason is insufficient phosphorus doping level: the doping reaction is a concentration-driven process, and at low concentrations, the number of effective doping sites is insufficient, resulting in low doping efficiency.

[0108] Comparative Example 5 Same as Example 1, except that the concentration of phosphoric acid is 2 mol / L.

[0109] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example is 258.4 m². 2 / g, closed-cell radius of 9.94Å, true density of 2.34g / cm³ 3 .

[0110] The sodium-ion battery assembly process is the same as in Example 1.

[0111] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 80.6%, an initial reversible specific capacity of 231 mAh / g, a specific capacity of 220 mAh / g after 100 cycles, and a capacity retention of 95.2% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 103 mAh / g.

[0112] Compared to Example 1 (with a phosphoric acid concentration of 0.5 mol / L), adjusting the phosphoric acid concentration to 2 mol / L resulted in a significant increase in the specific surface area of ​​the material, but also led to a decrease in the performance of the sodium-ion battery. The fundamental reason lies in the uncontrolled pyrolysis kinetics: excess phosphoric acid causes an explosive release of gaseous byproducts (P2O5, PH3) during pyrolysis, generating intense internal pressure. Simultaneously, the strongly acidic environment may excessively catalyze the decomposition of precursors.

[0113] Comparative Example 6 Same as Example 1, except that the temperature in the first stage of opening is 200°C.

[0114] The specific surface area of ​​the coconut shell-derived hard carbon obtained in this comparative example is 278.6 m². 2 / g, closed-cell radius of 9.1Å, true density of 2.39g / cm³3 .

[0115] The sodium-ion battery assembly process is the same as in Example 1.

[0116] At a current density of 20 mA / g, the sodium-ion battery assembled in this comparative example exhibits an initial coulombic efficiency of 73.5%, an initial reversible specific capacity of 197 mAh / g, a specific capacity of 187 mAh / g after 100 cycles, and a capacity retention of 94.9% after 100 cycles. At a high current density of 300 mA / g for 1000 cycles, its specific capacity is 62 mAh / g.

[0117] Compared to Example 1 (opening stage temperature of 400℃), lowering the opening stage temperature to 200℃ resulted in a larger specific surface area of ​​the material and a decline in sodium-ion battery performance. The fundamental mechanism lies in the fact that the effective sodium storage structure of hard carbon depends on the topological evolution within the main pyrolysis range of 250-400℃. Within this temperature range, the precursor undergoes intense aromatization condensation and dehydrogenation / deoxygenation processes, accompanied by the vigorous escape of volatile small molecules (such as H2, CH4, and CO), thereby etching rich open-pore structures, topological defects, and nanoscale micropores into the gradually rigidifying non-graphitized carbon framework. The processing temperature of 200℃ did not reach the activation energy barrier of this reaction; the precursor only underwent physical dehydration and mild cross-linking, failing to trigger the pyrolysis pore-forming kinetics. This resulted in a significantly insufficient micropore volume and incomplete development of closed-pore structures, severely limiting the capacity contribution of sodium ions at defect adsorption sites and micropore filling sites (especially the storage of quasi-metallic sodium clusters corresponding to low-potential plateaus).

[0118] Comparative Example 7 Same as Example 1, except that the temperature of the third stage graphite layer formation stage is 1200°C.

[0119] The specific surface area of ​​the obtained coconut shell-derived hard carbon is 351.8 m². 2 / g. The closed-cell radius of the obtained coconut shell-derived hard carbon is 9.3 Å. The true density of the obtained coconut shell-derived hard carbon is 2.43 g / cm³. 3 .

[0120] The sodium-ion battery assembly process is the same as in Example 1.

[0121] Based on the synthesized coconut shell-derived hard carbon, in sodium-ion batteries, the initial coulombic efficiency is 79.6% at a current density of 20 mA / g, the initial reversible specific capacity is 205 mAh / g, the specific capacity after 100 cycles is 193 mAh / g, and the capacity retention after 100 cycles is 94.1%. At a high current density of 300 mA / g for 1000 cycles, the specific capacity is 54 mAh / g.

[0122] Compared to Example 1 (carbonization at 1400°C), adjusting the high-temperature carbonization temperature to 1200°C significantly increased the specific surface area of ​​the material, leading to a decrease in sodium-ion battery performance. The fundamental reason is that at 1200°C, the viscoelasticity of the carbon matrix makes it unable to withstand the internal pyrolysis gas pressure, causing the carbon walls separating the closed pores to rupture. This allows the originally isolated nanocavities to interconnect, forming an open mesoporous network. The dramatic increase in BET specific surface area directly confirms the material's transformation from a low specific surface area closed-pore structure to a high specific surface area open-pore structure. The open pores expose a huge internal surface area. This structural degradation causes the sodium storage mechanism to degenerate from efficient closed-pore filling and graphite-like interlayer embedding to inefficient surface adsorption and defect storage. The large open surface area leads to irreversible decomposition of the electrolyte, forming an excessively thick and unstable solid electrolyte interphase (SEI) film, irreversibly consuming large amounts of sodium ions and active materials. This manifests as an increased specific surface area, extremely low initial coulombic efficiency, and poor cycle stability.

[0123] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a hard carbon material with high initial efficiency and low specific surface area, characterized in that, Includes the following steps: Hard-shelled biomass was used as raw material and subjected to a three-stage high-temperature annealing process. The high-temperature annealing product was then acid-washed, water-washed, and dried to obtain the high-efficiency, low-specific-surface-area hard carbon material. The three-stage high-temperature annealing process is divided into a pore-opening stage, a carbonization stage, and a graphite layer formation stage. The temperature during the pore-opening stage is 400°C, the temperature during the carbonization stage is 800°C, and the temperature during the graphite layer formation stage is 1400°C.

2. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 1, characterized in that, The heating rate to the opening stage is 5°C / min, the heating rate to the carbonization stage is 5°C / min, and the heating rate to the graphite layer formation stage is 2°C / min.

3. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 1, characterized in that, The hard-shelled biomass is selected from coconut shells, walnut shells, peach kernels, or apricot kernels.

4. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 1, characterized in that, Before the hard-shelled biomass undergoes the three-stage high-temperature annealing treatment, it also includes a step of soaking in phosphoric acid; The concentration of phosphoric acid is 0.1-0.5 mol / L, and the soaking time is 6-18 h.

5. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 1, characterized in that, The acid used in pickling is hydrochloric acid, sulfuric acid, or nitric acid.

6. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 5, characterized in that, The concentration of acid used during pickling is 0.2-5 mol / L.

7. The method for preparing high-efficiency, low-specific-surface-area hard carbon materials according to claim 1, characterized in that, All three stages of high-temperature annealing were performed under a protective atmosphere.

8. A hard carbon material with high initial efficiency and low specific surface area, characterized in that, It is prepared according to any one of claims 1-7.

9. The application of a high-efficiency, low-specific-surface-area hard carbon material as described in claim 8 in a sodium-ion battery, characterized in that, The high-efficiency, low-specific-surface-area hard carbon material is used as the negative electrode.

10. A sodium-ion battery, characterized in that, The negative electrode material is the high-efficiency, low-specific-surface-area hard carbon material as described in claim 8.

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