Hard carbon negative electrode material and preparation method and application thereof

Abstract

The invention discloses a hard carbon negative electrode material and a preparation method and application thereof. The hard carbon negative electrode material has a hierarchical porous structure, wherein micropores of 0.35-0.85 nm account for 40-50% of the total pore volume, and mesopores of 2-25 nm account for 30-50% of the total pore volume; the average closed pore diameter of the material is 1.7 to 2.2 nm, and the closed pore volume is 0.091 to 0.17cm < 3 >. G <-1 >; the hard carbon negative electrode material has a short-range pseudo graphite microcrystalline structure, the average transverse size of the hard carbon negative electrode material is 3.81-4.18 nm, and the average longitudinal size of the hard carbon negative electrode material is 0.91-1.11 nm. According to the method, a high-concentration acid etching process is adopted, nanoscale impurity phases in a primary carbonization product of industrial lignin are accurately removed, and then the target hard carbon negative electrode material is prepared through secondary carbonization. The hard carbon negative electrode material has high reversible sodium storage capacity, excellent first coulombic efficiency and long cycle stability, the preparation process is simple and easy to operate, and the production cost is low.

Description

Technical Field

[0001] This invention relates to the field of secondary battery anode materials, and more particularly to a hard carbon anode material, its preparation method, and its application. Background Technology

[0002] Sodium resources are abundant, evenly distributed globally, and inexpensive. Sodium-ion batteries, with their similar working principle and superior performance to lithium-ion batteries, show broad application prospects in large-scale energy storage systems, low-speed electric vehicles, and 5G base stations. Therefore, the development of sodium-ion batteries is of great significance. Hard carbon, due to its low cost, wide availability, and advantages such as high theoretical specific capacity and low operating voltage, has become the preferred anode material for sodium-ion batteries, attracting widespread attention and research enthusiasm from the global academic and industrial communities, and possessing enormous commercial potential. However, hard carbon anodes still face problems such as low initial coulombic efficiency, insufficient sodium storage capacity, and poor cycle and rate performance, which seriously hinder the commercialization process of sodium-ion batteries.

[0003] Hard carbon (HCC) is composed of curved and folded graphene microregions, defects, and pores, exhibiting a complex structure and abundant sodium storage sites. Its typical constant-current charge-discharge curve for sodium storage can be divided into a high-potential ramp region (> 0.1 V) and a low-potential plateau region (< 0.1 V). Numerous studies have shown that the plateau capacity is mainly related to sodium ion deposition in closed pores and ultramicropores (< 1 nm). Therefore, improving the sodium storage capacity in the plateau region and reducing irreversible sodium ion loss in the ramp region are effective strategies for increasing the energy density of the full cell. Thus, designing and constructing appropriately sized closed nanopores and ultramicropores in the HCC bulk phase is key to improving the sodium storage capacity in the low-potential plateau region. Simultaneously, the rational construction of ultramicropores and the reduction of defect content help improve the initial coulombic efficiency (OCE) of HCC. In summary, there is an urgent need to develop simple, efficient, and highly controllable methods to simultaneously regulate the nanopores, ultramicropores, pseudo-graphite structure, and surface defects of HCC, thereby improving its OCE, plateau capacity, rate performance, and cycle stability.

[0004] Lignin is the second most abundant renewable biomass resource on Earth after cellulose, mainly derived from a byproduct of the papermaking industry. This material boasts high carbon content (approximately 45%), low cost, wide availability, and good consistency, making it a preferred precursor for preparing hard carbon anode materials. However, due to the high aromaticity of lignin, its direct pyrolysis products are typically dense hard carbon, lacking a nanoporous structure and suitable carbon framework. This results in poor sodium storage capacity and rate performance, with a low initial coulombic efficiency. Tests have shown that hard carbon anode materials prepared by direct pyrolysis of lignin have a sodium storage capacity approaching 320 mAh·g⁻¹ and an initial coulombic efficiency approaching 85%.

[0005] To further improve the sodium storage capacity, rate performance, and initial coulombic efficiency of lignin-based hard carbon, this invention targets the cooking waste liquor (black liquor) generated by alkaline pulping in the papermaking industry. First, it undergoes evaporation and concentration treatment, then deliberately adds an impurity phase followed by acidification treatment, ultimately obtaining a lignin raw material containing a uniformly dispersed high content of nanoscale impurity phases. This impurity phase can reserve space for the pore growth of hard carbon during subsequent carbonization, which is beneficial for synthesizing hard carbon anode materials with well-developed pores. Summary of the Invention

[0006] The purpose of this invention is to provide a high-efficiency, high-capacity industrial lignin-based hard carbon anode material with excellent rate performance and cycle stability, which can avoid or improve the related defects mentioned in the prior art.

[0007] In a first aspect, the present invention provides a hard carbon anode material having a hierarchical porous structure, wherein the pore size of the hierarchical porous structure is micropores of 0.35-0.85 nm and mesopores of 2-25 nm, wherein the micropore volume accounts for 40-50% of the total pore volume and the mesopore volume accounts for 30-50% of the total pore volume; The hard carbon anode material has an average closed-pore diameter of 1.7–2.2 nm and a closed-pore volume of 0.091–0.117 cm³. 3 ·g -1 ; The hard carbon anode material has a short-range pseudo-graphite microcrystalline structure with an average lateral size of 3.81–4.18 nm, an average longitudinal size of 0.91–1.11 nm, and an interlayer spacing of 0.373–0.380 nm.

[0008] According to the present invention, a hard carbon anode material has at least the following beneficial effects: (1) The hard carbon anode material of the present invention has more suitable average interlayer spacing of 0.373 to 0.380 nm, which is beneficial to improving the sodium storage capacity of the slope region and the plateau region.

[0009] (2) The hard carbon anode material of the present invention has a hierarchical porous structure and a short-range ordered pseudo-graphite structure, which allows sodium ions to be transported, diffused and stored rapidly, which is beneficial to improving the sodium storage capacity and rate performance of the hard carbon material.

[0010] (3) The hard carbon anode material of the present invention, as a sodium-ion battery anode material, has a first-charge specific capacity higher than 401 mAh·g at a current density of 0.1C. -1 The initial Coulomb efficiency is ≥90%.

[0011] (4) The hard carbon anode material of the present invention, as a sodium-ion battery anode material, still has a current density greater than 290 mAh·g at 2C. -1 Specific capacity.

[0012] (5) The hard carbon anode material of the present invention, as a sodium-ion battery anode material, still has a capacity retention rate of more than 85% after 150 cycles at 2C.

[0013] Secondly, the present invention provides a method for preparing the above-mentioned hard carbon anode material, comprising the following steps: S1. Add Na2SO4 to the black liquor obtained in the papermaking process, mix evenly, adjust the pH to <3 with sulfuric acid to precipitate lignin, filter and dry, and further mechanically pulverize to obtain industrial lignin powder. S2. The industrial lignin powder obtained in step S1 is subjected to low-temperature carbonization under a protective atmosphere to obtain a carbonized material. S3. The carbonized material obtained in step S2 is mechanically crushed to obtain carbon powder. The carbon powder is added to an acid solution and heated to react, resulting in reacted carbon powder. S4. Wash, filter, and dry the carbon powder obtained after the reaction in step S3 to obtain dry carbon powder. S5. The dry carbon powder obtained in step S4 is subjected to high-temperature carbonization under a protective atmosphere to obtain a hard carbon anode material.

[0014] This invention introduces uniformly dispersed nanoscale impurity phases into industrial lignin. These impurity phases occupy reserved spaces during the low-temperature carbonization stage, which not only facilitates the formation of the target hierarchical porous structure but also inhibits the growth of long-range pseudo-graphite structures while promoting the formation of short-range ordered pseudo-graphite structures. The nanoscale dispersed impurity phases are mainly Na2SO4 and iron- and potassium-containing metal compounds.

[0015] Furthermore, this invention utilizes acid solution heating etching to remove impurity phases from the carbon powder obtained from the primary carbonization process; subsequent high-temperature treatment promotes the growth of abundant closed-pore, ultra-micropore, and mesoporous structures within the carbon material. In addition, the removal of impurity phases optimizes the defect structure and oxygen-containing functional group structure of hard carbon during the secondary high-temperature carbonization process.

[0016] Among them, the short-range ordered pseudo-graphite structure can promote the storage and transport of sodium ions; abundant closed pores and ultra-micropores are conducive to the formation of sodium clusters, thereby increasing the sodium storage capacity in the plateau region; at the same time, the ultra-microporous structure and the optimized defect and oxygen-containing functional group structure can reduce the irreversible loss of sodium ions and improve the first coulombic efficiency; while the hierarchical mesoporous structure can shorten the diffusion path of sodium ions, promote the diffusion and transport of sodium ions, and ultimately improve the rate performance of the material.

[0017] In some embodiments of the present invention, the content of Na2SO4 in step S1 is 5-10% of the black liquor, the drying temperature is 50-80°C, and the diameter of the mechanically pulverized powder is 50-70 μm.

[0018] In some embodiments of the present invention, the initial low-temperature carbonization temperature in step S2 is 300-1000°C, and the heat preservation time is 1-10 hours.

[0019] In some embodiments of the present invention, the particle size of the powder particles in step S3 is 10 to 100 μm.

[0020] A suitable particle size is conducive to the acid solution etching fully, creating more reserved space, which is beneficial for the subsequent secondary carbonization growth of more porous structures.

[0021] In some embodiments of the present invention, the acid solution in step S3 is one or more of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, and the concentration of the acid solution is 4 to 10 M. If it is a mixed acid, the concentration refers to the sum of the concentrations of each component. The liquid-solid ratio of the acid solution to the carbon powder is 20 to 40:1 (mL / g).

[0022] Using high-concentration acid and heating facilitates the etching of numerous nanoscale pre-reserved pore spaces, providing favorable conditions for subsequent secondary high-temperature carbonization growth of hierarchical porous, microporous, mesoporous, and closed-pore structures. A liquid-to-solid ratio below 20:1 results in insufficient acid and incomplete etching of impurity phases; a ratio above 40:1 increases costs and impacts profitability. The total concentration of the mixed acid should be between 4 and 10 M to ensure sufficient etching intensity to remove nanoscale impurity phases.

[0023] In some embodiments of the present invention, the heating temperature in step S3 is 40–95°C, and the reaction time is 1–10 h.

[0024] In some embodiments of the present invention, the drying temperature in step S4 is 50-70°C and the drying time is 24-48 hours.

[0025] In some embodiments of the present invention, the carbonization temperature in step S5 is 1100℃~1600℃, the carbonization time is 1~10h, and the carbonization heating rate is 2~10℃ / min.

[0026] In some embodiments of the present invention, the protective atmosphere described in steps S2 and S5 is an inert gas, specifically argon or nitrogen, and the flow rate of the protective atmosphere is 100 to 400 sccm.

[0027] The present invention provides a method for preparing a hard carbon anode material, which has at least the following beneficial effects: (1) This invention uses black liquor from the papermaking industry as raw material, introduces Na2SO4 impurity phase and precipitates it with acid to obtain lignin. Its bulk phase contains a large number of nano-sized uniformly dispersed Na2SO4, iron- and potassium-containing metal compounds and other impurity phases. A primary carbonization is obtained through primary carbonization, and the primary carbonization is treated with a high-concentration acid solution. During the reaction process, the uniformly dispersed impurity phase in the primary carbonization is etched away. Further high-temperature carbonization is carried out to obtain a hard carbon material with a large number of micropores, mesopores and nano-closed pores. This hard carbon material has a large sodium storage capacity, high initial coulombic efficiency, excellent rate capability and cycling performance. This invention etches the impurity phase of low-temperature carbon by adjusting the acid concentration, reaction heating and time, and further optimizes the internal pore structure and surface oxygen-containing functional groups and defect structure of the material by high-temperature carbonization.

[0028] (2) The hard carbon anode material prepared by the present invention not only has good performance, but also high efficiency and low cost, and has good industrialization prospects.

[0029] Thirdly, the hard carbon anode material provided by this invention is used in secondary batteries.

[0030] In some embodiments of the present invention, the secondary battery includes one of a sodium-ion battery and a lithium-ion battery.

[0031] In some embodiments of the present invention, the above-mentioned hard carbon negative electrode material is provided as the active material of the electrode, and is mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry is then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet is cut into a circular piece of a certain size, with a sodium sheet as the counter electrode. The electrolyte is selected from sodium salts of NaBF4, NaPF6, NaClO4, NaFSI, and NaTFSI, and organic solvents selected from dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. A coin cell is then assembled.

[0032] In some embodiments of the present invention, the above-mentioned hard carbon negative electrode material is provided as the active material of the electrode, and is mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry, which is then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet is cut into a circular piece of a certain size, with a lithium sheet as the counter electrode. The electrolyte is selected from lithium salts of LiBF4, LiPF6, LiFSI, and LiTFSI, and organic solvents selected from dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. A coin cell is then assembled. Attached Figure Description

[0033] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 The first constant current charge-discharge curve of the hard carbon anode material prepared according to Example 1 is shown.

[0035] Figure 2 The first constant current charge-discharge curve of the hard carbon anode material prepared according to Example 2 is shown.

[0036] Figure 3 The image shows a transmission electron microscope (TEM) image of the hard carbon anode material prepared according to Example 1.

[0037] Figure 4 This is a high-resolution image of the hard carbon anode material prepared according to Example 1.

[0038] Figure 5 The X-ray diffraction pattern is shown for the primary carbonized material prepared according to Example 1.

[0039] Figure 6 The image shows the micropore size distribution curve of the hard carbon anode material prepared according to Example 1.

[0040] Figure 7 The image shows the mesopore size distribution curve of the hard carbon anode material prepared according to Example 1.

[0041] Figure 8 The small-angle scattering curve is shown for the hard carbon anode material prepared according to Example 1. Detailed Implementation

[0042] The present invention will now be described in detail with reference to the accompanying drawings and specific examples.

[0043] Example 1

[0044] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) Add 5% Na2SO4 to the lignin-containing black liquor produced by papermaking process, stir evenly and add sulfuric acid solution to adjust pH < 3 to precipitate lignin, filter, dry at 70℃ for 24h and mechanically crush to obtain lignin powder with a particle size of 60μm, take 20g of lignin powder and carbonize in an argon atmosphere furnace, the heating rate is 2℃ / min, the holding temperature is 700℃, the holding time is 1h, and the material is naturally cooled to obtain primary carbonized material. The primary carbonized material is mechanically crushed to a particle size of 50~55μm. (2) Add 5g of the obtained primary carbonized material to 150mL of 6M H2SO4 solution, heat to 85℃ and stir for 5h. (3) Filter and wash with deionized water until neutral, then put it in an oven at 70°C for 24 hours to obtain dry powder; (4) The dried powder obtained above is carbonized in an argon atmosphere. The heating rate is 2℃ / min, the holding temperature is 1500℃, the holding time is 2h, and it is naturally cooled. Then it is pulverized by airflow to a particle size of ~20μm to obtain hard carbon anode material. (5) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into circular pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1. The capacity was tested every 10 cycles after 150 cycles at a current density of 2C. The final capacity retention rate was 86%.

[0045] Example 2

[0046] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) Add 10% Na2SO4 to the lignin-containing black liquor in step (1) of Example 1, replace the argon gas in steps (1) and (4) of Example 1 with nitrogen gas, and keep the other operation steps the same as in Example 1 to obtain hard carbon anode material; (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0047] Example 3

[0048] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) Set the heat preservation temperature in step (1) of Example 1 to 300℃ and the heat preservation time to 5h. Replace the sulfuric acid in step (2) of Example 1 with hydrochloric acid. The concentration of hydrochloric acid is 4M. Other operation steps are consistent with Example 1 to obtain hard carbon anode material. (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0049] Example 4

[0050] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) Set the heat preservation temperature in step (1) of Example 1 to 1000℃ and the heat preservation time to 10h. Replace the sulfuric acid in step (2) of Example 1 with nitric acid. The concentration of nitric acid is 10M. Other operation steps are consistent with Example 1 to obtain hard carbon anode material. (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0051] Example 5

[0052] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) The heat preservation temperature of step (4) in Example 1 is 1600℃ and the heat preservation time is 5h. The sulfuric acid in step (2) of Example 1 is replaced with phosphoric acid. The carbonized material in step (1) of Example 1 is mechanically crushed to a particle size of 5-10μm. Other operation steps are consistent with Example 1 to obtain hard carbon anode material. (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0053] Example 6

[0054] This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) The carbonized material in step (1) of Example 1 is mechanically crushed to a particle size of 80-100 μm. The heat preservation temperature in step (4) of Example 1 is 1100℃ and the heat preservation time is 10h. The 6M sulfuric acid in step (2) of Example 1 is replaced with a mixed acid solution of 3M sulfuric acid and 3M hydrochloric acid. Other operation steps are consistent with Example 1 to obtain hard carbon anode material. (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0055] Example 7

[0056] This embodiment provides a method for preparing a lignin-based hard carbon anode material. The specific steps are the same as in Embodiment 1, except that its performance in lithium-ion batteries is tested according to the following steps.

[0057] The hard carbon anode material obtained in Example 1 was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry, which was then coated onto a copper foil current collector. The slurry was dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into circular pieces of a certain size. A coin cell was assembled using a lithium sheet as the counter electrode and 1M LiPF6 ester as the electrolyte. The electrochemical lithium storage performance is shown in Table 1.

[0058] Comparative Example 1 This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) In step (1) of Example 1, Na2SO4 was not added to the black liquor of papermaking, and other operation steps were kept the same as in Example 1 to obtain hard carbon anode material; (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0059] Comparative Example 2 This embodiment provides a method for preparing a lignin-based hard carbon anode material, the specific steps of which are as follows: (1) Replace the 6M sulfuric acid in step (2) of Example 1 with 2M sulfuric acid, and keep the other operation steps the same as in Example 1 to obtain hard carbon anode material; (2) The hard carbon negative electrode material obtained above was used as the active material of the electrode. It was mixed with conductive carbon black and PVDF binder in a ratio of 95:2.5:2.5 to form a slurry. The slurry was then coated onto an aluminum foil current collector and dried in a vacuum drying oven at 80°C for 24 hours to obtain an electrode sheet. The electrode sheet was cut into round pieces of a certain size. A coin cell was assembled using a sodium sheet as the counter electrode and 1M NaPF6 ether as the electrolyte. The electrochemical sodium storage performance is shown in Table 1.

[0060] Test Example 1 In this test, the charge-discharge performance of Examples 1-7 and Comparative Examples 1-2 was tested using button cells at room temperature (25℃). The test results are shown in Table 1.

[0061] Table 1. Electrochemical performance of Examples 1-7 and Comparative Examples 1-2

[0062] Table 1 shows the battery performance test results, indicating that the sodium-ion anode half-cells assembled using lignin-based hard carbon obtained through a high-concentration acid etching process all achieved initial coulombic efficiencies exceeding 90% and reversible capacities greater than 400 mAh·g⁻¹. -1 Maintaining high rate performance, the reversible capacity at 2C current density is greater than 290 mAh·g. -1 .

[0063] Test Example 2 This test case examined the micropores, mesopores, closed pores, and closed pore volumes of Examples 1-7 and Comparative Examples 1-2. The test results are shown in Table 2. The closed pore volume was calculated using the true density of the tested hard carbon anode material, and the average closed pore diameter was calculated by fitting the small-angle scattering curve of the tested hard carbon anode material. Using a Micromeritics 3Flex physical adsorption analyzer, the N2 and CO2 adsorption-desorption isotherms of the hard carbon anode material were tested at 77 K and 273 K to characterize the pore structure. Subsequently, based on the adsorption branches of the isotherms, nonlocal density functional theory (NLDFT) was used as the primary analytical method, combined with the specific surface area calculated by the multi-point Brunauer-Emmett-Teller (BET) method for auxiliary verification. Finally, the pore size distribution of the hard carbon anode material was obtained, and the test data are shown in Table 2.

[0064] Table 2. Distribution and proportion of micropores and mesopores in Examples 1-7 and Comparative Examples 1-2, and volume of closed pores and closed pores.

[0065] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A hard carbon anode material, characterized in that, The hard carbon anode material has a hierarchical porous structure, wherein the pore size of the hierarchical porous structure consists of micropores with a diameter of 0.35–0.85 nm and mesopores with a diameter of 2–25 nm. The micropore volume accounts for 40–50% of the total pore volume, and the mesopore volume accounts for 30–50% of the total pore volume. The hard carbon anode material has an average closed-pore diameter of 1.7–2.2 nm and a closed-pore volume of 0.091–0.117 cm³. 3 ·g -1 ; The hard carbon anode material has a short-range pseudo-graphite microcrystalline structure with an average lateral size of 3.81–4.18 nm, an average longitudinal size of 0.91–1.11 nm, and an interlayer spacing of 0.373–0.380 nm.

2. The hard carbon anode material according to claim 1, characterized in that, The preparation method of the hard carbon anode material includes the following steps: S1. Add Na2SO4 to the black liquor obtained from the papermaking process, mix evenly, adjust the pH to <3 with sulfuric acid to precipitate lignin, filter and dry, and further mechanically pulverize to obtain industrial lignin powder. S2. The industrial lignin powder obtained in step S1 is subjected to low-temperature carbonization under a protective atmosphere to obtain a carbonized material. S3. The carbonized material obtained in step S2 is mechanically crushed to obtain carbon powder. The carbon powder is added to an acid solution and heated to react, resulting in reacted carbon powder. S4. Wash, filter, and dry the carbon powder obtained after the reaction in step S3 to obtain dry carbon powder. S5. The dry carbon powder obtained in step S4 is subjected to high-temperature carbonization under a protective atmosphere to obtain a hard carbon anode material.

3. The method for preparing the hard carbon anode material according to claim 2, characterized in that, In step S1, the content of Na2SO4 is 5-10% of the black liquor, the drying temperature is 50-80℃, and the diameter of the industrial lignin powder is 50-70μm.

4. The method for preparing the hard carbon anode material according to claim 2, characterized in that, The carbonization temperature in step S2 is 300–1000℃, and the carbonization time is 1–10h.

5. The method for preparing the hard carbon anode material according to claim 2, characterized in that, The acid solution mentioned in step S3 is one or more of sulfuric acid, hydrochloric acid, phosphoric acid, and nitric acid. The concentration of the acid solution is 4 to 10 M. If it is a mixed acid, the concentration refers to the sum of the concentrations of each component. The liquid-solid ratio of the acid solution to the carbon powder is 20 to 40:1 (mL / g).

6. The method for preparing the hard carbon anode material according to claim 2, characterized in that, The heating reaction temperature in step S3 is 40–95°C, and the heating reaction time is 1–10 h.

7. The method for preparing the hard carbon anode material according to claim 2, characterized in that, The drying temperature in step S4 is 50–70°C, and the drying time is 24–48 hours.

8. The method for preparing the hard carbon anode material according to claim 2, characterized in that, The carbonization temperature in step S5 is 1100℃~1600℃, the carbonization time is 1~10 h, and the carbonization heating rate is 2~10℃ / min.

9. The application of the hard carbon anode material as described in claim 1 in sodium-ion secondary batteries or lithium-ion secondary batteries.

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