Hard carbon materials with closed-cell structures, preparation methods and applications

Abstract

This invention discloses a hard carbon material with a closed-pore structure, its preparation method, and its applications. The hard carbon material, as determined by gas adsorption, has an average pore size of 5 nm-6 nm and a total closed-pore volume of 0.12 cm³. 3 / g-0.30cm 3 / g; XRD characterization revealed that the crystallite size of the hard carbon material was 4.0 nm–5.0 nm in the basal plane direction (La) and 1.00 nm–1.30 nm in the direction perpendicular to the basal plane (Lc). In this invention, the large-pore size and large closed-pore volume of the hard carbon material can provide more active sodium storage sites, which is beneficial for improving the initial coulombic efficiency of the hard carbon material. The narrower and longer graphite crystallite structure in the hard carbon material is also conducive to achieving large pore size and large closed-pore volume.

Description

Technical Field

[0001] This invention relates to the field of secondary battery anode materials technology, and more specifically, to hard carbon materials with closed-pore structures, their preparation methods, and applications. Background Technology

[0002] With the development of the new energy industry, lithium-ion batteries have been widely used in electric vehicles and other industries. However, their further development is constrained by a series of key issues, such as lithium resource reserves and the cost of lithium-ion batteries. Sodium-ion batteries have the advantages of low cost and abundant and widely distributed sodium reserves, making them a promising alternative to lithium-ion batteries.

[0003] Hard carbon, as a negative electrode material for sodium-ion batteries, possesses a disordered amorphous structure that provides more active sodium storage sites due to its numerous defects and micropores, thus enhancing the electrical performance of sodium-ion batteries. Existing technologies utilize biomass as a raw material to prepare hard carbon materials. Plant biomass, rich in cellulose, hemicellulose, lignin, and other substances, is the most abundant renewable resource on Earth. However, the electrochemical performance of hard carbon prepared by directly carbonizing plant biomass is not ideal. One major reason is the inherently low initial coulombic efficiency of hard carbon materials, which limits their application range. Therefore, improving the initial coulombic efficiency of hard carbon materials is an urgent problem to be solved.

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

[0005] The purpose of this invention is to provide hard carbon materials with closed-pore structures, their preparation methods, and their applications, thereby improving the problem of low initial coulombic efficiency of hard carbon materials.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a hard carbon material, wherein the average pore size of the hard carbon material is 5 nm-6 nm and the total pore volume of closed pores is 0.12 cm³, as determined by gas adsorption. 3 / g-0.30cm 3 / g;

[0008] XRD characterization revealed that the crystallite size of the hard carbon material was 4.0 nm-5.0 nm in the basal plane direction La and 1.00 nm-1.30 nm in the direction perpendicular to the basal plane.

[0009] In an optional embodiment, the hard carbon material satisfies at least one of the following (1)-(7):

[0010] (1) The interlayer spacing d(002) of the hard carbon crystals in the hard carbon material is 0.390 nm-0.400 nm;

[0011] (2) The specific surface area of ​​the hard carbon material is 4 cm². 2 / g-8cm 2 / g;

[0012] (3) The hard carbon material I D / I G It is 1.7-1.8;

[0013] (4) The hard carbon material is doped with at least one of phosphorus, nitrogen or sulfur, and the content of the doped element is ≥3wt%, preferably 4wt%-10wt%;

[0014] (5) The hard carbon material includes biomass-based hard carbon;

[0015] (6) The reversible discharge specific capacity of the hard carbon material is >340mAh / g;

[0016] (7) The first-efficiency of the hard carbon material is >87%.

[0017] Secondly, the present invention provides a method for preparing the hard carbon material described in the foregoing embodiments, comprising the following steps:

[0018] Biomass raw materials are oxidized to obtain oxidation products;

[0019] The oxidation product was washed with acid to obtain a purified product.

[0020] The purified product was subjected to pre-carbonization and carbonization sequentially to obtain the hard carbon material.

[0021] In an optional embodiment, the preparation method further satisfies at least one of the following ac:

[0022] a. The biomass raw material has a cellulose content ≥35wt% and a lignin content ≥15wt%; preferably, the biomass raw material has a cellulose content of 35wt%-45wt% and a lignin content of 17wt%-40wt%; more preferably, the biomass raw material has a cellulose content of 35wt%-45wt% and a lignin content of 30wt%-40wt%.

[0023] b. The biomass raw materials include plant-based biomass raw materials; more preferably, the plant-based biomass raw materials are selected from at least one of sugarcane bagasse, straw, corn cobs, paper pulp, and wood;

[0024] c. The oxygen content in the biomass raw material is W1, the oxygen content in the oxidation product is W2, and (W1-W2) / W1 ≥ 65%; preferably, (W1-W2) / W1 is 90%-200%.

[0025] In an optional embodiment, the oxidation process includes gas-phase oxidation and / or liquid-phase oxidation.

[0026] In an optional embodiment, the oxidation reaction further satisfies at least one of the following (1)-(8):

[0027] (1) The gas phase oxidation is specifically a heating treatment in an oxygen-containing atmosphere, with a gas flow rate of 1L / min-3L / min;

[0028] (2) The oxygen-containing gas used in the gas-phase oxidation includes at least one of air, oxygen or carbon dioxide;

[0029] (3) The heating temperature for the gas phase oxidation is 200℃-400℃, and the time is 6h-12h;

[0030] (4) The liquid phase oxidation specifically includes treating the biomass raw material in a solution containing an oxidant;

[0031] (5) The oxidant used in the liquid phase oxidation includes at least one of hydrogen peroxide, peracetic acid, sodium dichromate, chromic acid, nitric acid, potassium permanganate or ammonium persulfate;

[0032] (6) An oxidizing agent is added during the liquid phase oxidation process; more preferably, the oxidizing agent includes ferrous sulfate; more preferably, the mass ratio of ferrous sulfate to biomass is 1:(2-10);

[0033] (7) The liquid phase oxidation process is performed at a temperature of 30℃-50℃ for 10 min-60 min.

[0034] (8) The ratio of biomass raw material used in the liquid phase oxidation to the oxidant is 1g:(0.003-0.13)mol.

[0035] In an optional embodiment, the acid solution includes a group containing a doping element, wherein the doping element group is selected from at least one of a phosphate group, a carboxylic acid group, and a sulfate group;

[0036] Preferably, the acid solution includes phosphoric acid and / or phytic acid; more preferably, the concentration of phytic acid in the acid solution is 20wt%-30wt%, and the concentration of phosphoric acid is 15wt%-20wt%.

[0037] Preferably, the pickling time in the pickling step is 4-8 hours;

[0038] Preferably, the oxidation product is washed with water before and after the acid pickling.

[0039] In an optional embodiment, the pre-carbonization temperature is 700℃-1000℃, and the pre-carbonization time is 2h-4h;

[0040] And / or, the carbonization temperature is 1200℃-1400℃, and the carbonization time is 1h-4h;

[0041] And / or, the pre-carbonized product is further pulverized before the carbonization step; preferably, the pulverization method is ball milling, with a ball milling speed of 100 r / min-400 r / min and a ball milling time of 4 h-10 h.

[0042] Thirdly, the present invention provides a negative electrode sheet, comprising the hard carbon material described in the foregoing embodiments or the hard carbon material prepared by any one of the methods described in the foregoing embodiments.

[0043] Fourthly, the present invention provides a secondary battery, including the negative electrode sheet described in the foregoing embodiments.

[0044] Fifthly, the present invention provides an electrical device including the secondary battery described in the foregoing embodiments.

[0045] The present invention has the following beneficial effects:

[0046] In this invention, the large-pore size and large closed-pore volume of the hard carbon material can provide more active sodium storage sites, which is beneficial to improving the initial coulombic efficiency of the hard carbon material. The narrower and longer graphite microcrystal structure in the hard carbon material is also conducive to achieving large-pore size and large closed-pore volume. The hard carbon composite material prepared by this invention has an excellent closed-pore structure, low specific surface area, and better reversible discharge specific capacity and initial coulombic efficiency. Simultaneously, by controlling the oxygen content in the precursor after oxidation treatment, the pore structure of the hard carbon material can be effectively improved, thereby enhancing the electrochemical performance of the product. Attached Figure Description

[0047] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 The image shown is a SEM image of the hard carbon material prepared in Example 1 of this invention, with a magnification of 5000×.

[0049] Figure 2 yes Figure 1 The magnified view of a part is 50000×. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products, and all biomass used has been dried with a moisture content of less than 1 wt%.

[0051] This invention provides a hard carbon material, the average pore size of which is 5nm-6nm and the total pore volume of closed pores is 0.12cm³, as determined by gas adsorption. 3 / g-0.30cm 3 / g;

[0052] XRD characterization revealed that the crystallite size of the hard carbon material was 4.0 nm-5.0 nm in the basal plane direction La and 1.00 nm-1.30 nm in the direction perpendicular to the basal plane.

[0053] In this embodiment of the invention, the average pore size of the hard carbon material is determined to be 5nm-6nm by gas (e.g., nitrogen) adsorption method. Specifically, it can be any value between 5nm, 5.2nm, 5.4nm, 5.6nm, 5.8nm, 6nm, or 5nm-6nm; the total pore volume of the closed pores is 0.12cm³. 3 / g-0.30cm 3 / g, specifically 0.12cm 3 / g, 0.15cm 3 / g, 0.18cm 3 / g, 0.21cm 3 / g, 0.24cm 3 / g, 0.27cm 3 / g, 0.30cm 3 / g or 0.12cm 3 / g-0.30cm 3 Any value between / g. Hard carbon materials with suitable mesopore size and high closed-pore volume can provide more active sodium storage sites, which is beneficial to improving the initial coulombic efficiency of hard carbon materials.

[0054] In this embodiment of the invention, the crystallite size La of the hard carbon material has a basal plane orientation of 4.0 nm-5.0 nm, specifically any value between 4.0 nm, 4.1 nm, 4.2 nm, 4.3 nm, 4.4 nm, 4.5 nm, 4.6 nm, 4.7 nm, 4.8 nm, 4.9 nm, 5.0 nm, or 4.0 nm-5.0 nm; the perpendicular orientation Lc has a basal plane orientation of 1.00 nm-1.30 nm, specifically any value between 1.00 nm, 1.05 nm, 1.10 nm, 1.15 nm, 1.2 nm, 1.25 nm, 1.30 nm, or 1.00 nm-1.30 nm. The narrower and longer graphite crystallite structure in the hard carbon material is beneficial for achieving large pore size and large closed-pore volume.

[0055] In an optional embodiment, the hard carbon material satisfies at least one of the following (1)-(7):

[0056] (1) The interlayer spacing d(002) of the hard carbon crystal in the hard carbon material is 0.390nm-0.400nm, specifically it can be any value between 0.390nm, 0.395nm, 0.400nm or 0.390nm-0.400nm. A larger interlayer spacing is beneficial to the diffusion of sodium ions and to improving the stability of the hard carbon material in the sodiumization / desodiumization process.

[0057] (2) The specific surface area of ​​the hard carbon material is 4 cm². 2 / g-8cm 2 / g, specifically 4cm 2 / g, 5cm 2 / g、6cm 2 / g、7cm 2 / g、8cm 2 / g or 4cm 2 / g-8cm 2 Any value between / g; a certain specific surface area is beneficial to improve the provision of active sodium storage sites, but too large a specific surface area will reduce defects and closed pores, thereby leading to a decrease in capacity and first-efficiency.

[0058] (3) The hard carbon material I D / I G It is 1.7-1.8, specifically it can be 1.7, 1.72, 1.74, 1.76, 1.78, 1.8 or any value between 1.7 and 1.8;

[0059] (4) The hard carbon material is doped with at least one of phosphorus, nitrogen or sulfur, and the content of the doping element is ≥3wt%, specifically it can be any value of 3wt%, 5wt%, 7wt%, 9wt%, 11wt%, 13wt%, or ≥3wt%, preferably 4wt%-10wt%; the presence of the doping element is beneficial to improving the capacity and first coulombic efficiency of the hard carbon material.

[0060] (5) The hard carbon material includes biomass-based hard carbon.

[0061] (6) The reversible discharge specific capacity of the hard carbon material is >340mAh / g; more preferably, the reversible discharge specific capacity is >360mAh / g.

[0062] (7) The first-efficiency of the hard carbon material is >87%; more preferably, the first-efficiency is >91%.

[0063] The present invention also provides a method for preparing the hard carbon material described in the foregoing embodiments, comprising the following steps:

[0064] Biomass raw materials are oxidized to obtain oxidation products;

[0065] The oxidation product was washed with acid to obtain a purified product.

[0066] The purified product was subjected to pre-carbonization and carbonization sequentially to obtain the hard carbon material.

[0067] During the oxidation process, cellulose glycosidic bonds undergo oxidative cleavage, inducing molecular and chain cross-linking. The oxidative cross-linking process of cellulose generates more C=O and retains more CO, which is beneficial for subsequent pre-carbonization and carbonization pyrolysis processes. This results in the formation of narrower and longer graphite microcrystal structures, thereby creating larger and more closed pores.

[0068] The acid washing step removes ash from the oxidation products. Furthermore, the reaction of some of the acid with the oxidation products yields a wood precursor containing doped elements. Pre-carbonization and subsequent carbonization produce doped hard carbon materials, which are more conducive to improving the capacity and initial coulombic efficiency of the hard carbon materials. Pre-carbonization helps reduce the oxygen content in the material. Without pre-carbonization, the oxygen content in the purified products is too high, resulting in poor structural stability and difficulty in pore shrinkage during carbonization, leading to an excessively large specific surface area and reduced defects.

[0069] This invention utilizes biomass as a raw material to prepare hard carbon materials. The raw materials are widely available and inexpensive, reducing environmental pollution and effectively controlling production costs. Furthermore, biomass raw materials are rich in cellulose and lignin, which, through oxidative cross-linking, increase the C=O structure. Subsequent acid washing not only reduces impurities but also allows for elemental doping. The carbonized material exhibits good electrical conductivity, which is beneficial for improving the performance of the final battery. In optional embodiments, the preparation method further satisfies at least one of the following criteria:

[0070] a. The biomass raw material contains ≥35wt% cellulose and 15wt% lignin; preferably, the biomass raw material contains 35wt%-45wt% cellulose and 17wt%-40wt% lignin; more preferably, the biomass raw material contains 35wt%-45wt% cellulose and 30wt%-40wt% lignin. It is difficult to achieve doping by acid washing after cellulose oxidation. Therefore, if the cellulose content is too high and the lignin content is low, the effect of oxidation acid washing on improving the electrical properties of hard carbon materials is limited.

[0071] b. The biomass raw materials include plant-based biomass raw materials; more preferably, the plant-based biomass raw materials are selected from at least one of sugarcane bagasse, straw, corn cobs, paper pulp, and wood; the raw materials are widely available and inexpensive, reducing environmental pollution and effectively controlling the preparation cost.

[0072] c. The oxygen content in the biomass raw material is W1, and the oxygen content in the oxidation product is W2, with (W1-W2) / W1 ≥ 65%, specifically any value of 65%, 100%, 150%, 200%, 250%, or ≥ 65%; preferably, (W1-W2) / W1 is 90%-200%. Increasing the degree of oxidation is beneficial for producing narrower and longer graphite microcrystal structures, thereby forming larger and more numerous closed pores.

[0073] In an optional embodiment, the oxidation process includes gas-phase oxidation and / or liquid-phase oxidation, preferably liquid-phase oxidation.

[0074] In an optional embodiment, the gas-phase oxidation further satisfies at least one of the following (1)-(3):

[0075] (1) The gas phase oxidation is specifically a heating treatment in an oxygen-containing atmosphere, with a gas flow rate of 1L / min-3L / min;

[0076] (2) The oxygen-containing gas used in the gas-phase oxidation includes at least one of air, oxygen or carbon dioxide;

[0077] (3) The heating temperature for the gas phase oxidation is 200℃-400℃ and the time is 6h-12h.

[0078] In an optional embodiment, the liquid-phase oxidation further satisfies at least one of the following (1)-(5):

[0079] (1) The liquid phase oxidation specifically includes treating the biomass raw material in a solution containing an oxidant, wherein the concentration of the oxidant in the solution is 20wt%-50wt%;

[0080] (2) The oxidant used in the liquid phase oxidation includes at least one of hydrogen peroxide, peracetic acid, sodium dichromate, chromic acid, nitric acid, potassium permanganate or ammonium persulfate;

[0081] (3) An oxidizing agent is added during the liquid phase oxidation process; more preferably, the oxidizing agent includes reducing metal ions such as ferrous sulfate; more preferably, the mass ratio of ferrous sulfate to biomass is 1:(2-10);

[0082] (4) The liquid phase oxidation process is performed at a temperature of 30℃-50℃ for 10 min-60 min.

[0083] (5) The ratio of biomass raw material used in the liquid phase oxidation to the oxidant is 1g:(0.003-0.13)mol.

[0084] The specific steps of liquid-phase oxidation include: mixing a reducing metal salt solution, such as ferrous sulfate solution, with biomass raw material for 0.5-2 hours, followed by solid-liquid separation to obtain a solid phase, which is the biomass raw material adsorbed with reducing metal ions; preferably, the mass ratio of ferrous ions in the ferrous sulfate solution to the biomass raw material is 1:(14-50); then, the biomass raw material adsorbed with reducing metal ions is mixed with an oxidant in the liquid phase to react and obtain the oxidation product.

[0085] In the liquid-phase oxidation step, ferrous ions are mainly used for embedding into the fibers of biomass. The subsequently added oxidant reacts with these ions to generate hydroxyl radicals, inducing bond breaking and cross-linking of cellulose. In gas-phase oxidation, the different weight loss ranges of lignin and cellulose make it difficult to control the oxidation temperature and degree. Hydrothermal pre-oxidation is cumbersome and difficult to industrialize. Compared to gas-phase and hydrothermal oxidation, liquid-phase oxidation is simpler to operate, has stronger oxidizing power, and is easier to control.

[0086] In an optional embodiment, the acid solution includes a group containing a dopant element, wherein the dopant element group is selected from at least one of a phosphate group, a carboxylic acid group, and a sulfate group; during the pickling process, the hydroxyl groups of lignin undergo an esterification reaction with the dopant element group to form a bridge, thereby forming a wood precursor rich in dopant element.

[0087] Preferably, the acid solution includes phosphoric acid and / or phytic acid; more preferably, the concentration of phytic acid in the acid solution is 20wt%-30wt%, and the concentration of phosphoric acid is 15wt%-20wt%. Using mixed acids for pickling can increase the types of impurities removed, effectively improve the ash removal effect, and thus contribute to the improvement of the first-stage efficiency.

[0088] Preferably, in the pickling step, the pickling time is 4-8 hours, and the pickling solution is pickled until it is neutral or close to neutral.

[0089] Preferably, the oxidation product is washed with water before and after the acid pickling.

[0090] In an optional embodiment, the pre-carbonization temperature is 700℃-1000℃, specifically any value between 700℃, 800℃, 900℃, 1000℃ or 700℃-1000℃, and the pre-carbonization time is 2h-4h, specifically any value between 2h, 3h, 4h or 2h-4h. Pre-carbonization is carried out at a lower temperature, which is beneficial to reduce the oxygen content in the purified product and to maintain the stability of the material structure and pore structure shrinkage during the carbonization process.

[0091] In an optional embodiment, the carbonization temperature is 1200℃-1400℃, specifically any value between 1200℃, 1250℃, 1300℃, 1350℃, 1400℃ or 1200℃-1400℃, and the carbonization time is 1h-4h, specifically any value between 1h, 2h, 3h, 4h or 1h-4h. The carbonization temperature affects the specific surface area and interlayer spacing of the material. If the carbonization temperature is too low, the specific surface area will be too large, and defects will be reduced.

[0092] In an optional embodiment, the pre-carbonized product is further pulverized before the carbonization step; preferably, the pulverization method is ball milling, with a ball milling speed of 100 r / min-400 r / min and a ball milling time of 4 h-10 h. Pulverization can effectively destroy the original structure of the material, which is beneficial to the shrinkage of the pore structure during the subsequent carbonization process and improves its surface defects.

[0093] The present invention also provides a negative electrode sheet, comprising the hard carbon material described in the foregoing embodiments or the hard carbon material prepared by any one of the methods described in the foregoing embodiments.

[0094] The present invention also provides a secondary battery, including the negative electrode sheet described in the foregoing embodiments.

[0095] The present invention also provides an electrical device, including the secondary battery described in the foregoing embodiments.

[0096] The features and performance of the present invention will be further described in detail below with reference to the embodiments.

[0097] Example 1

[0098] This embodiment provides a method for preparing hard carbon materials, specifically including the following steps:

[0099] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 8g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add 30% hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 40min, then discard the solution to obtain the oxidized precursor.

[0100] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a 1:1 volume ratio. After washing with water and drying, it was pre-carbonized at 800℃ for 3 hours to obtain a phosphorus-rich biomass precursor. This precursor was then ball-milled at 300 r / min for 10 hours, followed by carbonization and cooling at 1300℃ to obtain hard carbon material. SEM images are shown below. Figure 1 and Figure 2 As shown.

[0101] Example 2

[0102] This embodiment provides a method for preparing hard carbon materials, specifically including the following steps:

[0103] Take 30g of sugarcane bagasse (cellulose content 43.1wt%, lignin content 25wt%) and 10g of ferrous sulfate, add them to 300mL of deionized water, stir for 1.5h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:1.5), stir at 45℃ for 30min, then discard the solution to obtain the oxidized precursor.

[0104] The precursor was acid-washed for 4.5 h with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1, then washed with water and dried. After pre-carbonization at 800℃ for 3 h, a phosphorus-rich biomass precursor was obtained. After ball milling at 300 r / min for 10 h, the precursor was carbonized and cooled at 1300℃ to obtain hard carbon material.

[0105] Example 3

[0106] This embodiment provides a method for preparing hard carbon materials, specifically including the following steps:

[0107] Take 30g of corn cob (cellulose content 39.7wt%, lignin content 17.8wt%), 5g of ferrous sulfate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:1), stir at 45℃ for 35min, discard the supernatant, place the bottom product in a vacuum drying oven at 85℃ to dry, and obtain the oxidized precursor.

[0108] The precursor was acid-washed for 7 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ for 3 hours to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1300℃ to obtain hard carbon material.

[0109] Example 4

[0110] This embodiment provides a method for preparing hard carbon material, which differs from Example 1 mainly in the use of a different mixed acid for pickling. The method specifically includes the following steps:

[0111] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 8g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 30min, then discard the solution to obtain the oxidized precursor.

[0112] The precursor was acid-washed with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a 1:1 volume ratio for 6 hours, and then pre-carbonized at 800℃ for 3 hours to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, the precursor was carbonized and cooled at 1300℃ to obtain hard carbon material.

[0113] Example 5

[0114] This embodiment provides a method for preparing hard carbon material, the main difference from Embodiment 1 being a lower pre-carbonization temperature, specifically including the following steps:

[0115] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 8g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 30min, then discard the solution to obtain the oxidized precursor.

[0116] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 600℃ for 3 hours to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1300℃ to obtain hard carbon material.

[0117] Example 6

[0118] This embodiment provides a method for preparing hard carbon material, the main difference from Embodiment 1 being a reduction in ball milling speed, specifically including the following steps:

[0119] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 8g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 40min, then discard the solution to obtain the oxidized precursor.

[0120] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ for 3 hours to obtain a phosphorus-rich biomass precursor. After ball milling at 100 r / min for 6 hours, it was carbonized and cooled at 1300℃ to obtain hard carbon material.

[0121] Example 7

[0122] This embodiment provides a method for preparing hard carbon material, the main difference from Example 1 being a lower carbonization temperature, specifically including the following steps:

[0123] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 8g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 40min, then discard the solution to obtain the oxidized precursor.

[0124] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1200℃ to obtain hard carbon material.

[0125] Example 8

[0126] This embodiment provides a method for preparing hard carbon material, the main difference from Example 7 being a reduction in the amount of ferrous sulfate used, specifically including the following steps:

[0127] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 5g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 40min, then discard the solution to obtain the oxidized precursor.

[0128] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1200℃ to obtain hard carbon material.

[0129] Example 9

[0130] This embodiment provides a method for preparing hard carbon material, the main difference from Example 7 being the increased amount of ferrous sulfate, specifically including the following steps:

[0131] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%), 15g of ferrous sulfate heptahydrate, add to 300mL of deionized water, stir for 1h to mix, then add hydrogen peroxide solution (molar ratio of H2O2 to ferrous sulfate is 1:2), stir at 45℃ for 40min, then discard the solution to obtain the oxidized precursor.

[0132] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1200℃ to obtain hard carbon material.

[0133] Example 10

[0134] This embodiment provides a method for preparing hard carbon material, which differs from Example 7 mainly in the oxidation method, and specifically includes the following steps:

[0135] Take 30g of paper pulp (paper pulp from cork kraft paper mill, with cellulose content of 41.2wt% and lignin content of 35.65wt%) and place it in a nitrogen atmosphere, introduce air, and heat to 300℃ for 9 hours with an air flow rate of 2L / min to obtain the oxidized precursor.

[0136] The precursor was acid-washed for 6 hours with a mixture of phytic acid (50% by mass) and phosphoric acid (35% by mass) in a volume ratio of 1:1. After washing with water and drying, it was pre-carbonized at 800℃ to obtain a phosphorus-rich biomass precursor. After ball milling at 300 r / min for 10 hours, it was carbonized and cooled at 1200℃ to obtain hard carbon material.

[0137] Comparative Example 1

[0138] This embodiment provides a method for preparing hard carbon material. The remaining operations are the same as in Example 1, except that the raw material used is microcrystalline cellulose.

[0139] Comparative Example 2

[0140] This embodiment provides a method for preparing hard carbon material. The remaining operations are the same as in Example 1, except that ferrous sulfate heptahydrate is not added.

[0141] Comparative Example 3

[0142] This embodiment provides a method for preparing hard carbon materials. The remaining operations are the same as in Example 1, except that no mixed acid is added for treatment.

[0143] Comparative Example 4

[0144] This embodiment provides a method for preparing hard carbon material. The remaining operations are the same as in Example 1, except that pre-carbonization is not performed, and the material is directly carbonized after ball milling.

[0145] Comparative Example 5

[0146] This embodiment provides a method for preparing hard carbon material. The remaining operations are the same as in Example 1, except that ball milling is not performed after pre-carbonization, and carbonization is performed directly.

[0147] Comparative Example 6

[0148] This embodiment provides a method for preparing hard carbon material. The remaining operations are the same as in Example 1, except that high-temperature carbonization at 1000°C is used.

[0149] Application Examples

[0150] The specific surface area of ​​the hard carbon materials in each embodiment and comparative example was measured by N2 adsorption-desorption. The specific surface area and pore structure were determined using gas adsorption, and the interlayer spacing was calculated using XRD. The incident light wavelength was [wavelength value missing]. And the intensity of typical D and G peaks was determined using Raman spectroscopy for I D / I G The calculation results are shown in Table 1:

[0151] Table 1

[0152]

[0153]

[0154] Analysis of Table 1 shows that in this invention, the oxidation effect of hydroxyl radicals generated by the reaction of ferrous sulfate and hydrogen peroxide can effectively oxidize cellulose in the raw material, oxidize and cleave cellulose glycosidic bonds, and induce molecular cross-linking and chain cross-linking. The oxidative cross-linking process of cellulose produces more C=O and retains more CO, increasing the precursor O content. During carbonization and pyrolysis, this results in narrower and longer graphite microcrystal structures, thus forming larger closed pores. Raw materials that do not undergo hydroxyl radical oxidation treatment have smaller pore volumes and average pore sizes.

[0155] Phosphoric acid and phytic acid treatment can effectively remove ash. The esterification reaction between the hydroxyl groups of lignin and the phosphate groups in the mixed acid of phytic acid and phosphoric acid forms a bridge, thereby forming a phosphorus-rich wood precursor, which can effectively improve the interlayer spacing, reversible discharge specific capacity, and first coulombic efficiency of the hard carbon prepared subsequently.

[0156] The oxygen content before carbonization is significantly lower than that of the sample after pre-oxidation treatment, which is beneficial to structural stability and pore shrinkage. The oxygen content of the sample without pre-carbonization is too high. Subsequent ball milling and carbonization is not conducive to structural stability and pore shrinkage, resulting in a large specific surface area and fewer defects.

[0157] During the preparation process, ball milling effectively disrupts the original structure, facilitating pore shrinkage and improving surface defects during subsequent carbonization. Precursors not treated with ball milling exhibit significantly lower specific surface area, reversible discharge specific capacity, and initial coulombic efficiency after carbonization.

[0158] During the carbonization process, the carbonization temperature is compared with the control of the interlayer spacing. A lower carbonization temperature can maintain the interlayer spacing, but it results in a larger specific surface area of ​​the carbonized product, poor reversible discharge specific capacity, and poor first coulombic efficiency.

[0159] Sodium-ion batteries were prepared using the negative electrode materials obtained in each embodiment and comparative example according to the following methods.

[0160] A method for preparing a button-type sodium-ion battery includes the following steps:

[0161] According to the ratio of active material:SP:CMC:SBR=94:2:1.5:2.5, weigh out the negative electrode material, SP, CMC and SBR respectively and mix them evenly in deionized water to prepare a slurry; coat the evenly mixed slurry onto the aluminum foil current collector, bake it in an oven at 80℃ for 1 hour, and then take it out and cool it to room temperature.

[0162] Adjust the roller spacing and roll the electrode sheets. Cut the rolled electrode sheets into small round pieces with a diameter of 14mm and weigh them as m1. Similarly, cut the aluminum foil current collector into aluminum foil round pieces with a diameter of 14mm and weigh them as m2. Where (m1-m2)*0.94 is the mass of the active material, denoted as m3. Place the weighed round pieces into an 80℃ oven and vacuum dry for 12 hours.

[0163] The vacuum-dried small discs were transferred to a glove box, and sodium discs were used as the counter electrode and auxiliary electrode. The electrolyte was 1M NaPF6 / EC:DMC:DEC = 2:2:1, and a glass fiber diaphragm was used as the separator. Sodium-ion button cells were assembled in a glove box where the oxygen and water content were both less than 0.01ppm.

[0164] The assembled button-type sodium-ion batteries were left to stand for 12 hours. The electrochemical performance of the settled button-type sodium-ion batteries was then tested under constant current using the Wuhan Landian Battery Testing System. The test results are shown in Table 2.

[0165] Table 2

[0166]

[0167] As can be seen from the examples and comparative examples 1-6 in Table 2, the hard carbon prepared using raw materials rich in cellulose and lignin exhibits the best reversible discharge specific capacity and initial coulombic efficiency. The pretreatment uses ferrous sulfate Fe... 2+ Embedded in cellulose, the reaction with added hydrogen peroxide generates hydroxyl radicals that strongly oxidize the cellulose, promoting the formation of more C=O structures. Subsequent treatment with a mixed acid of phosphoric acid and phytic acid not only removes impurities but also crosslinks with lignin. During subsequent pre-carbonization, the in-situ formation of CO and PO bonds on the surface effectively increases specific capacity and initial coulombic efficiency. Ball milling further reduces surface defects, facilitating pore shrinkage during subsequent carbonization. The resulting hard carbon exhibits a low specific surface area and excellent reversible discharge specific capacity and initial coulombic efficiency. Furthermore, by controlling the oxygen content in the precursor after oxidation treatment, the pore structure of the hard carbon material can be effectively improved, enhancing the product's electrochemical performance.

[0168] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0169] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

[0170] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing a hard carbon material, comprising the following steps: The biomass raw material is oxidized to obtain the oxidation product; the oxidation treatment includes liquid phase oxidation, and an oxidation aid, namely ferrous sulfate, is added during the liquid phase oxidation process; The oxidation product is eluted with an acid solution to obtain a purified product. The acid solution contains a group containing a dopant element, and the dopant element group is selected from at least one of a phosphate group, a carboxylic acid group, and a sulfate group. The purified product was subjected to pre-carbonization and carbonization sequentially to obtain the hard carbon material. The pre-carbonization temperature is 700℃-1000℃, and the pre-carbonization time is 2h-4h. And / or, the carbonization temperature is 1200℃-1400℃, and the carbonization time is 1h-4h; And / or, the pre-carbonized product is further pulverized before the carbonization step.

2. The method for preparing hard carbon material according to claim 1, characterized in that, The preparation method also satisfies at least one of the following ac: a. The biomass raw material contains ≥35wt% cellulose and ≥15wt% lignin; b. The biomass raw materials include plant-based biomass raw materials; c. The oxygen content in the biomass raw material is W1, the oxygen content in the oxidation product is W2, and (W1-W2) / W1≥65%.

3. The method for preparing hard carbon material according to claim 2, characterized in that, The biomass raw material contains 35wt%-45wt% cellulose and 17wt%-40wt% lignin.

4. The method for preparing hard carbon material according to claim 2, characterized in that, The biomass raw material contains 35wt%-45wt% cellulose and 30wt%-40wt% lignin.

5. The method for preparing hard carbon material according to claim 2, characterized in that, The plant-based biomass raw materials are selected from at least one of sugarcane bagasse, straw, corn cobs, paper pulp, and wood.

6. The method for preparing hard carbon material according to claim 2, characterized in that, (W1-W2) / W1 is 90%-200%.

7. The method for preparing hard carbon material according to claim 1, characterized in that, The oxidation treatment also satisfies at least one of the following (1)-(5): (1) The liquid phase oxidation specifically includes treating biomass raw materials in a solution containing an oxidant; (2) The oxidant used in the liquid phase oxidation includes at least one of hydrogen peroxide, peracetic acid, sodium dichromate, chromic acid, nitric acid, potassium permanganate or ammonium persulfate; (3) The mass ratio of ferrous sulfate to biomass is 1:(2-10); (4) The liquid phase oxidation process is performed at a temperature of 30℃-50℃ for 10 min-60 min. (5) The ratio of biomass raw material used in the liquid phase oxidation to the oxidant is 1g: (0.003-0.13)mol.

8. The method for preparing hard carbon material according to claim 1, characterized in that, The acid solution includes phosphoric acid and / or phytic acid.

9. The method for preparing hard carbon material according to claim 1, characterized in that, The concentration of phytic acid in the acid solution is 20wt%-30wt%, and the concentration of phosphoric acid is 15wt%-20wt%.

10. The method for preparing hard carbon material according to claim 1, characterized in that, In the pickling step, the pickling time is 4-8 hours.

11. The method for preparing hard carbon material according to claim 1, characterized in that, The oxidation products are washed with water before and after the pickling process.

12. The method for preparing hard carbon material according to claim 1, characterized in that, The pulverization method is ball milling, with a ball milling speed of 100 r / min-400 r / min and a ball milling time of 4 h-10 h.

13. The hard carbon material obtained by the preparation method according to any one of claims 1-12, characterized in that, The hard carbon material, as determined by gas adsorption, has an average pore size of 5 nm-6 nm and a total closed-pore volume of 0.12 cm³. 3 / g-0.30cm 3 / g; XRD characterization revealed that the crystallite size of the hard carbon material was 4.0 nm-5.0 nm in the basal plane direction La and 1.00 nm-1.30 nm in the direction perpendicular to the basal plane. The specific surface area of ​​the hard carbon material is 4 cm². 2 / g-6.82cm 2 / g, the reversible discharge specific capacity of the hard carbon material is >340mAh / g, and the first-efficiency of the hard carbon material is >87%.

14. The hard carbon material according to claim 13, characterized in that, The hard carbon material satisfies at least one of the following (1)-(4): (1) The interlayer spacing d(002) of the hard carbon crystals in the hard carbon material is 0.390 nm-0.400 nm; (2) The hard carbon material I D / I G It is 1.7-1.8; (3) The hard carbon material is doped with at least one of phosphorus, nitrogen or sulfur, and the content of the doped element is ≥3wt%; (4) The hard carbon material includes biomass-based hard carbon.

15. The hard carbon material according to claim 13, characterized in that, The doping element content in the hard carbon material is 4wt%-10wt%.

16. A negative electrode sheet, characterized in that, Including hard carbon materials prepared by any one of claims 1-12.

17. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 16.

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