Small-size carbon microcrystal high-multiplying lignin-based hard carbon, and preparation method and application thereof

Abstract

The present application provides a kind of small size carbon crystallite high magnification lignin-based hard carbon and its preparation method and application, the method of the present application utilizes carboxylated lignin as carbon source, with ether oxygen bond-containing polymer as crosslinking agent, by mechanical hot-pressing rapid oxidation crosslinking and high temperature carbonization to prepare lignin-based hard carbon material with small size carbon crystallite.The preparation process of the present application method, mechanical hot-pressing avoids solvent use, and promotes the crosslinking of lignin and ether high molecule.The carboxyl group of carboxylated lignin and the end hydroxyl group of ether high molecule form ester bond, and at the same time, non-covalent crosslinking is carried out through hydrogen bond interaction between oxygen groups, which significantly improves the oxygen crosslinking degree.In the high temperature carbonization process, the highly crosslinked structure hinders the growth of carbon layer, can inhibit the growth and rearrangement of carbon crystallite, and form hard carbon with small size carbon crystallite, large interlayer spacing and high disorder.As sodium storage negative electrode, it can simultaneously improve initial coulombic efficiency, sodium storage capacity and rate, and show excellent electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of hard carbon materials technology, specifically relating to a high-ratio lignin-based hard carbon with small-sized carbon microcrystals, its preparation method, and its application. Background Technology

[0002] Global demand for advanced energy storage and conversion technologies such as batteries is growing rapidly. Lithium-ion batteries (LIBs) currently dominate the market due to their high energy density, good cycle performance, and long lifespan, but they also suffer from the limitations of limited and unevenly distributed lithium resources. Sodium, with its low cost and widespread and abundant resource distribution, has come into focus. Sodium-ion batteries (SIBs) not only possess energy storage mechanisms comparable to LIBs but also offer advantages such as high safety, low cost, and good low-temperature stability. They are expected to become an important supplement to lithium batteries in energy storage and specific power applications, contributing to energy transition and reducing global dependence on lithium resources.

[0003] Hard carbon, as a novel carbon-based energy storage material, has attracted considerable attention in the field of sodium-ion battery anodes due to its unique microstructure and performance advantages. Its advantages are mainly reflected in its low operating voltage and excellent cycle stability. However, hard carbon materials still suffer from problems such as low initial coulombic efficiency, low sodium storage capacity, and poor rate performance.

[0004] Hard carbon precursors mainly include three categories: biomass (such as wood, coconut shell, starch, cellulose, lignin), synthetic polymers (phenolic resin, polyaniline), and asphalt-based materials (coal tar pitch, petroleum coke). Among them, lignin, as a natural renewable precursor, has the following significant advantages: (1) its three-dimensional cross-linked aromatic structure gives it a high carbon yield (40-50%), and oxygen-containing functional groups (hydroxyl, methoxy) can induce disorder in the carbon layer, forming abundant nanopores; (2) it is widely available and low in cost, and as a by-product of the papermaking industry, it can be utilized as a resource, which meets the needs of sustainable development. However, the high aromaticity of lignin leads to a dense hard carbon structure, and the π-π interaction of the benzene ring causes the carbon layer to shrink, resulting in a small interlayer spacing and the formation of long carbon layers, which inhibits the disordered stacking of graphite-like microcrystals, resulting in poor sodium storage capacity and rate performance of hard carbon.

[0005] Regulating the molecular structure of precursors is an effective means to improve the sodium storage performance of hard carbon. Pre-oxidation can introduce oxygen-containing groups into the precursor, connect unsaturated aliphatic hydrocarbons or aromatic side chains, increase the degree of cross-linking of the molecular structure, inhibit melting and rearrangement during carbonization, and regulate the growth of carbon microcrystals. Chinese patent CN202411052551.2 describes the preparation of a precursor by reacting sodium lignin sulfonate with hexamethylenetetramine after desulfonation and activation. The precursor is then pre-oxidized and segmented carbonized to prepare a hard carbon anode material. This process is cumbersome and increases the preparation cost. Chinese patent CN118405685B describes the preparation of a lignin-based hard carbon anode material by cross-linking liquid lignin with cross-linking agents such as oxalic acid and formic acid, followed by pre-oxidation and carbonization. The material achieved a coulombic efficiency of over 90% for the first time, but the reversible capacity was only 300 mAh / g and the rate performance was poor. Du et al. (Carbon 178(2021)243-255) prepared a spherical precursor by spray drying a sodium lignin sulfonate solution and then pre-oxidizing it with air, followed by high-temperature carbonization to prepare a lignin-based hard carbon material. The hard carbon prepared by this process is highly graphitized with a small interlayer spacing, making it unsuitable as a sodium electrode anode. Chinese patent CN202310919956.0 provides a method to improve the graphitization ability of lignin, namely hot pressing. This method requires pre-oxidation of lignin before hot pressing, and the resulting carbon material has a small interlayer spacing, making it unsuitable for sodium storage in hard carbon anodes. Qu et al. (Green Chem, 2023, 25, 9873–9883) pre-oxidized lignin and performed hot pressing pretreatment to prepare highly graphitized lignin-based carbon materials through carbonization. These materials had large carbon crystallites and an interlayer spacing of only 0.34 nm, which is insufficient to meet the requirements for sodium storage in hard carbon anodes. Chen et al. (Green Chem, 2025, 10.1039 / D5GC00409H) mixed lignocellulose with water and then hot-pressed it to prepare hard carbon, but hot pressing made it difficult to control the content of the three components, resulting in difficulties in regulating the performance of the hard carbon. Chinese patent CN202510320978.4 describes a method for preparing hard carbon sodium storage with high reversible capacity by hot-pressing lignocellulose biomass raw materials to remove unstable hemicellulose components, but with limited improvement in rate performance.

[0006] In summary, lignin-based hard carbon materials have the following problems when used as a negative electrode in sodium-ion batteries:

[0007] (1) In terms of precursor preparation, air pre-oxidation and chemical oxidation cross-linking are time-consuming and complicated, and the repeated use of solvents reduces production efficiency, which is not conducive to large-scale production.

[0008] (2) In terms of structural control of hard carbon materials, the oxygen crosslinking degree of the air pre-oxidation method is low, while the chemical oxidation crosslinking method destroys the original structure. The hard carbon materials prepared have small carbon layer spacing, long carbon layers, and low disorder.

[0009] (3) In terms of sodium storage performance, lignin-based hard carbon has poor overall performance and it is difficult to balance the initial coulombic efficiency, specific capacity and rate performance. Summary of the Invention

[0010] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a method for preparing a high-ratio lignin-based hard carbon material with small-sized carbon microcrystals.

[0011] This invention utilizes carboxylated lignin as a carbon source and a polymer containing ether oxygen bonds as a crosslinking agent to prepare lignin-based hard carbon materials with small-sized carbon microcrystals through rapid oxidative crosslinking via mechanical hot pressing and high-temperature carbonization. First, alkali lignin is carboxylated to obtain carboxylated lignin, which is then hot-pressed with a polymer containing ether oxygen bonds to obtain a highly crosslinked precursor. Then, the crosslinked precursor is placed in an inert gas atmosphere and carbonized at high temperature to obtain the lignin-based hard carbon material with small-sized carbon microcrystals.

[0012] In the preparation process of this invention, mechanical hot pressing avoids the use of solvents and promotes the cross-linking of lignin and ether polymers. The carboxyl groups of carboxylated lignin form ester bonds with the terminal hydroxyl groups of the ether polymers, while simultaneously undergoing non-covalent cross-linking through hydrogen bonding between oxygen groups, significantly improving the degree of oxygen cross-linking. During high-temperature carbonization, the highly cross-linked structure hinders the growth of carbon layers, inhibiting the growth and rearrangement of carbon crystallites, forming hard carbon with small-sized carbon crystallites, large interlayer spacing, and high disorder. As a sodium storage anode, it can simultaneously improve initial coulombic efficiency, sodium storage capacity, and rate capability, exhibiting excellent electrochemical performance.

[0013] Another object of the present invention is to provide a lignin-based hard carbon material with small-sized carbon microcrystals prepared by the above method.

[0014] Another object of the present invention is to provide the application of the above-mentioned lignin-based hard carbon material with small carbon microcrystals in the anode of sodium-ion batteries.

[0015] In this invention, compared with the prior art, the method does not use any solvent and the preparation process is efficient and simple; the prepared hard carbon has small-sized carbon microcrystals and large interlayer spacing in structure; the hard carbon has high reversible capacity, high first coulombic efficiency and excellent rate performance in sodium-ion battery energy storage.

[0016] The objective of this invention is achieved through the following solution:

[0017] A method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals, characterized by comprising the following steps:

[0018] (1) Dissolve lignin in sodium hydroxide solution to prepare a lignin solution of 5-20 g / L, control the pH of the solution to 12, add a carboxylating reagent of 2-10 g / L, react at 90-100℃ for 1-6 h, and obtain carboxylated lignin by dialysis and drying.

[0019] (2) Carboxylated lignin is uniformly mixed with ether-containing oxygen bond polymers and then hot-pressed to obtain oxygen-crosslinked lignin precursors.

[0020] (3) The oxygen-crosslinked lignin precursor obtained in step (2) is carbonized at high temperature to obtain the lignin-based hard carbon;

[0021] The weight ratio of lignin: carboxylating agent: ether-oxygen bond-containing polymer is 100:(20-60):(25-250).

[0022] Preferably, the weight ratio of lignin: carboxylating agent: ether-oxygen bond-containing polymer is 100:(30-50):(80-150).

[0023] Preferably, in step (1), the lignin includes at least one of corn cob alkali lignin, sugarcane alkali lignin, bamboo alkali lignin, wheat straw alkali lignin, and enzymatically hydrolyzed lignin from the biorefining process.

[0024] The carboxylating agent is at least one of the following: sodium chloroacetate aqueous solution, ammonium chloroacetate aqueous solution, sodium difluorochloroacetate aqueous solution, and ammonium trichloroacetate aqueous solution.

[0025] Preferably, in step (1), the dialysis is performed in deionized water, using a 3000Da dialysis bag, and the dialysis time is 12-24 hours.

[0026] The drying process is at least one of vacuum drying, infrared drying, and freeze drying.

[0027] Preferably, in step (2), the ether-oxygen bond-containing polymer is one or more of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer, polyethylene glycol, and ethylene-vinyl acetate copolymer.

[0028] Preferably, in step (2), the molecular weight of the ether-containing oxygen bond polymer is 1000-15000 and the melting point is 50-100℃.

[0029] Preferably, in step (2), the method for uniform mixing is grinding, and the grinding conditions are 100-1000 rpm for 2-4 hours;

[0030] The hot pressing is performed by mechanically pressing the mixed sample using a manual flat vulcanizing machine. The hot pressing temperature is 130-260℃, the hot pressing pressure is 2-20MPa, and the hot pressing time is 0.5-4h.

[0031] Preferably, in step (3), the carbonization temperature is 1100-1600℃, the carbonization time is 2-4h, and the heating rate is 2-5℃ / min;

[0032] The carbonization process is as follows: the temperature is increased to 800°C at 5°C / min, then increased to 1300°C at 2°C / min, and held for 2 hours; the inert atmosphere is nitrogen or argon.

[0033] The method described in this invention produces a small-sized carbon microcrystal high-ratio lignin-based hard carbon.

[0034] The present invention relates to the application of a small-sized carbon microcrystalline high-rate lignin-based hard carbon in sodium-ion battery anode materials.

[0035] The invention will now be described in more detail:

[0036] (1) Dissolve alkali lignin in sodium hydroxide solution to prepare a lignin solution of 5-20 g / L, control the pH of the solution to 12, add 2-10 g / L carboxylating reagent, react at 90-100℃ for 1-6 h, and obtain carboxylated lignin by dialysis and drying.

[0037] This step uses sodium hydroxide solution as a solvent to fully dissolve alkali lignin, preparing a lignin solution of 5-20 g / L, and controlling the pH of the solution to 12. If the concentration of the lignin solution is too high, it will inhibit the carboxylation reaction; if the concentration of lignin is too low, it will affect the yield of carboxylated lignin. The concentration of the carboxylating reagent needs to be controlled at 2-10 g / L, the reaction temperature at 90-100℃, and the reaction time at 1-6 h. If the concentration of the carboxylating reagent is too low, the reaction temperature is too low, or the reaction time is too short, the lignin carboxylation modification will be incomplete, affecting the crosslinking process in step (2); if the concentration of the carboxylating reagent is too high, the reaction temperature is too high, or the reaction time is too long, it will increase the preparation cost. The reaction solution is dialyzed using a 3000 Da dialysis bag, and the dialysis time is 12-24 h. If the molecular weight cutoff of the dialysis bag is too high, the unreacted carboxylating reagent and lignin cannot be removed; if the molecular weight cutoff of the dialysis bag is too low, some reaction products will be lost.

[0038] (2) The carboxylated lignin is uniformly mixed with the ether-containing oxygen bond polymer and hot-pressed at 2-20 MPa and 130-260℃ for 0.5-4 h to obtain the oxygen crosslinked lignin precursor;

[0039] This step first involves grinding the carboxylated lignin obtained in (1) with the ether-containing oxygen-bonded polymer to obtain a mixed sample. Grinding is performed using a planetary ball mill for 2-4 hours. If the grinding time is too short, the material will not mix evenly. In this step, the ratio of carboxylated lignin to ether-containing oxygen-bonded polymer needs to be controlled, preferably 100:(80-150). If the amount of polymer is too low, the lignin crosslinking will be incomplete; if the amount of polymer is too high, it will affect the yield of hard carbon material in step (3).

[0040] The hot pressing step is carried out using a flat vulcanizing machine, where the material is evenly spread on a polyacetylimide film and pressure and temperature are applied. If the material is not spread evenly, it will result in uneven pressure on the material and uneven cross-linking of lignin and polymer. The hot pressing temperature for this step is 130-280℃, the time is 0.5-4h, and the pressure is 2-20MPa, preferably 5-10MPa. If the hot pressing temperature is too low or the time is too short, the cross-linking reaction will not occur; if the hot pressing temperature is too high or the time is too long, the lignin will decompose, reducing the yield; if the hot pressing pressure is too low, the cross-linking of lignin and polymer will be incomplete; if the hot pressing pressure is too high, it will aggravate the Π-Π aggregation of lignin, affecting the hard carbon performance of step (3). The melting point of the ether-oxygen bond-containing polymer selected for this step is 50-100℃. If the melting point is too high, the contact area between lignin and polymer will be small, resulting in poor cross-linking effect; if the melting point is too low, the polymer will liquefy severely during the hot pressing process, resulting in low cross-linking efficiency.

[0041] (3) The oxygen-crosslinked lignin precursor obtained in step (2) is placed in a graphite boat and carbonized at high temperature under the protection of inert gas to obtain lignin-based hard carbon material with small carbon microcrystals.

[0042] The inert atmosphere in this step can be nitrogen, argon, or other inert gases. Carbonization is a one-step process with a carbonization temperature of 1100–1600℃, a heating rate of 2–5℃ / min, and a carbonization time of 2–4 h. If the carbonization temperature is too low or the carbonization time is too short, the hard carbon will have a large specific surface area and few closed-cell structures, affecting the sodium storage capacity. If the carbonization temperature is too high or the time is too short, it will lead to excessive growth of hard carbon graphite microcrystals, shrinkage of interlayer spacing, and a decrease in sodium storage performance. If the heating rate is too slow, it will lead to shrinkage of the hard carbon layers and small interlayer spacing. If the heating rate is too fast, it will lead to incomplete carbonization, which is not conducive to the formation of disordered microcrystalline structures.

[0043] The lignin-based hard carbon material with small-sized carbon microcrystals prepared by the method of the present invention has a graphite crystallite size of 3-4 nm, a carbon interlayer spacing of 0.385-0.390 nm, and a closed pore size of 0.9-1.1 nm.

[0044] The present invention also provides the application of the above-mentioned lignin-based hard carbon material with small carbon microcrystals in the anode of sodium-ion batteries.

[0045] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0046] (1) This invention utilizes hot pressing to promote cross-linking between lignin and polymers, thereby increasing the degree of cross-linking in the lignin molecular structure, suppressing the problem of carbon layer shrinkage during lignin carbonization, and effectively regulating the microcrystalline structure characteristics of lignin-based hard carbon. This technology avoids the use of liquid solvents and improves molecular cross-linking efficiency, which is beneficial for industrial production.

[0047] (2) The lignin-based hard carbon material prepared by this invention has small carbon microcrystals, large carbon interlayer spacing and high degree of disorder, and has excellent sodium storage rate and cycle performance as a sodium-ion battery anode material. Attached Figure Description

[0048] Figure 1 These are infrared FTIR test images of the thermo-pressed oxygen crosslinking precursors obtained in Example 1 and Comparative Example 1 of the present invention.

[0049] Figure 2 These are XRD comparison images of lignin-based hard carbon with small-sized carbon microcrystals obtained in Example 1 and Comparative Example 1 of the present invention.

[0050] Figure 3 This is a comparison chart of SAXS test results for lignin-based hard carbon with small-sized carbon microcrystals obtained in Example 1 and Comparative Example 1 of the present invention.

[0051] Figure 4 These are the charge-discharge curves of lignin-based hard carbon with small-sized carbon microcrystals obtained in Example 1 and Comparative Examples 1, 2, 3, and 4 of this invention.

[0052] Figure 5 These are the rate performance curves of lignin-based hard carbon with small-sized carbon microcrystals obtained in Example 1 and Comparative Examples 1, 2, 3, and 4 of the present invention under different current densities. Detailed Implementation

[0053] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0054] The materials used in the following examples are all commercially available.

[0055] Example 1

[0056] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0057] 10g of the obtained carboxylated lignin and 10g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F127, molecular weight: 12000, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0058] The oxygen crosslinking precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0059] Example 2

[0060] 10g of purified sucralose lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 3g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0061] 10g of the obtained carboxylated lignin and 8g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (P123, molecular weight: 5800, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 180℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 8 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0062] The oxygen crosslinking precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0063] Example 3

[0064] 10g of purified wheat straw lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10g / L lignin solution. The pH of the solution was controlled at 12. 500mL of 8g / L ammonium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0065] 10g of the obtained carboxylated lignin and 12g of ethylene-vinyl acetate copolymer (molecular weight: 2000, melting point: 75℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 200℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 10 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0066] The oxygen-crosslinked precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1200℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0067] Example 4

[0068] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 500mL of 6g / L sodium difluorochloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0069] 10g of the obtained carboxylated lignin and 15g of polyethylene glycol (molecular weight: 10000, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 180℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 5MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0070] The oxygen-crosslinked precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1400℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled for 5 minutes in a high-speed oscillating ball mill to obtain lignin-based hard carbon.

[0071] Example 5

[0072] 10g of purified bamboo alkaloid lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0073] 10g of the obtained carboxylated lignin and 12g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F68, molecular weight: 8350, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 200℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6 MPa and maintained in a hot-pressed state for 2 hours. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0074] The oxygen-crosslinked precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1500℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled for 5 minutes in a high-speed oscillating ball mill to obtain lignin-based hard carbon.

[0075] Example 6

[0076] 10g of purified enzymatically hydrolyzed lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 3g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0077] 10g of the obtained carboxylated lignin and 10g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F127, molecular weight: 12000, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 170℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 8 MPa, maintaining the hot-pressed state for 1.5 hours. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0078] The oxygen crosslinking precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0079] Comparative Example 1 (Lignin and polymer without hot-pressing treatment compared to Example 1)

[0080] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0081] The obtained 10g carboxylated lignin and 10g polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F127, molecular weight: 12000, melting point: 60℃) were added to an agate ball mill jar and ball milled at 300rpm for 2h in a planetary ball mill to obtain a mixed sample.

[0082] The mixture obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0083] Comparative Example 2 (compared to Example 1, alkali lignin was not carboxylated)

[0084] 10g of purified corn cob alkali lignin and 10g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F127, molecular weight: 12000, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature, and the raw material was removed to obtain the oxygen crosslinking precursor.

[0085] The oxygen crosslinking precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0086] Comparative Example 3 (compared to Example 1, the hot pressing process only involves heating without pressurization)

[0087] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0088] 10g of the obtained carboxylated lignin and 10g of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer (F127, molecular weight: 12000, melting point: 60℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film, and the temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed on a hot press and kept under heating for 1 hour. After the material cooled to room temperature, the raw material was removed to obtain the precursor.

[0089] The precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0090] Comparative Example 4 (Compared to Example 1, no polymer was added; carboxylated lignin was directly hot-pressed and oxidized)

[0091] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0092] The obtained 10g of carboxylated lignin was evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature before being removed to obtain the precursor.

[0093] The precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0094] Comparative Example 5 (compared to Example 1, the low-melting-point polymer was replaced with a high-melting-point polymer)

[0095] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0096] 10g of the obtained carboxylated lignin and 10g of polyvinyl alcohol (molecular weight: 12000, melting point: 230℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature before being removed to obtain the precursor.

[0097] The precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0098] Comparative Example 6 (compared to Example 1, hot pressing was performed using an oxygen-free polymer)

[0099] 10g of purified corn cob alkali lignin was dissolved in 30wt.% sodium hydroxide solution to prepare 1000mL of 10 g / L lignin solution. The pH of the solution was controlled at 12. 1000mL of 5g / L sodium chloroacetate was added, and the reaction was carried out at 90℃ for 2h. The reaction solution was dialyzed against deionized water for 24h using a 3000Da molecular weight dialysis bag to obtain a carboxylated lignin solution. The solution was concentrated at 90℃ until the mass fraction of the concentrate was 50wt.%, and then the concentrated carboxylated lignin solution was freeze-dried to obtain carboxylated lignin powder.

[0100] 10g of the obtained carboxylated lignin and 10g of linear low-density polyethylene (molecular weight: 100,000, melting point: 92℃) were added to an agate ball mill jar and ball-milled at 300 rpm for 2 hours in a planetary ball mill to obtain a mixed sample. The mixed sample was then evenly spread on a polyacetylimide film. The temperature of the flat vulcanizing machine was set to 160℃. After the temperature stabilized, the sample was placed in the flat vulcanizing machine, and the pressure was increased to 6 MPa, maintaining the hot-pressed state for 1 hour. During the hot-pressing process, the pressure was monitored for changes and adjusted in a timely manner to maintain the initial pressure. After the hot-pressing was completed, the material was allowed to cool to room temperature before being removed to obtain the precursor.

[0101] The precursor obtained above was placed in a high-temperature tube furnace and carbonized at high temperature under a nitrogen atmosphere. The carbonization temperature was increased from room temperature to 800℃ at a rate of 5℃ / min, and then increased to 1300℃ at a rate of 2℃ / min, and held for 2 hours. After naturally cooling to room temperature, the carbonized product was ball-milled in a high-speed oscillating ball mill for 5 minutes to obtain lignin-based hard carbon.

[0102] Example Effect Description

[0103] The lignin-based hard carbon materials obtained in the examples and comparative examples were structurally characterized, and their performance in sodium-ion battery anode materials was tested. The results are shown in Table 1 and 2. Figures 1-4 .

[0104] The defect structure of the sample was characterized by microconfocal Raman spectroscopy. The length of the carbon crystallites and the spacing between carbon layers were measured using X-ray diffraction.

[0105] In sodium-ion battery performance testing, the lignin-based hard carbon material obtained in this invention was used to prepare the sodium-ion battery negative electrode with polyacrylic acid binder and conductive carbon black at a mass ratio of 8:1:1. A coin cell half-cell was assembled using a copper sheet as the positive electrode and 1M NaPF6 as the electrolyte for electrochemical testing. The testing window was 0–2.5V, and the charge / discharge current density was 0.05–5A / g.

[0106] Table 1. Carbon crystallite size and sodium storage capacity of lignin-based hard carbon prepared in different embodiments and comparative examples.

[0107]

[0108] Table 1 Explanation:

[0109] The carbon crystallite size La of the lignin-based hard carbon materials prepared in Examples 1-6 is less than 3.5 nm, Lc is about 1.0 nm, and the carbon interlayer spacing d is... 002 All are greater than 0.385 nm. In Example 1, the carbon crystallite size La is only 2.98 nm, Lc is 1.11 nm, and the carbon interlayer spacing d... 002 The carbon crystal size is 0.389 nm, and it has a relatively small closed-pore diameter of 0.96 nm. Its carbon crystallite size and closed-pore diameter are much smaller than those of the lignin-based hard carbon material obtained by hot-pressing cross-linking lignin and polymer in Comparative Example 1, the lignin-based hard carbon material obtained by uncarboxylated treatment in Comparative Example 2, the lignin-based hard carbon material obtained by heating without pressure pretreatment in Comparative Example 3, the lignin-based hard carbon material obtained by direct hot-pressing carboxylated lignin in Comparative Example 4, the hard carbon obtained by hot-pressing with high-melting-point polymer polyvinyl alcohol in Comparative Example 5, and the hard carbon obtained by hot-pressing with oxygen-free polymer linear low-density polyethylene in Comparative Example 6.

[0110] Regarding sodium storage performance, the initial reversible specific capacity of all examples was greater than 300 mAh / g (0.05 A / g), and the specific capacity at a high current of 5 A / g was greater than 200 mAh / g, with high initial coulombic efficiency (ICE > 80%). The hard carbon in Example 1, as a sodium-ion battery anode material, had an initial reversible specific capacity of 323 mAh / g and a high initial coulombic efficiency of 86.5%, and could still maintain a capacity of 228 mAh / g at a high current of 5 A / g, which was far superior to the various hard carbon materials in the comparative examples.

[0111] Compared to Example 1, Comparative Example 1 did not use hot-pressed crosslinking of lignin and ether-containing oxygen-bonded polymers, resulting in the failure of oxygen crosslinking reaction between the two. The precursor had a low content of oxygen-containing functional groups, and excessive carbon layer growth during carbonization led to longer carbon crystallite sizes. The resulting lignin-based hard carbon had higher carbon crystallite sizes (La, 4.31 nm) and closed-pore diameters (1.39 nm) than that of Example 1, while the interlayer spacing (d) was also higher. 002 (0.374nm) is smaller than that of Example 1, resulting in extremely poor rate performance and initial coulomb efficiency.

[0112] Compared to Example 1, Comparative Example 2 did not undergo lignin carboxylation treatment, resulting in fewer lignin crosslinking sites during thermal crosslinking and a lower content of oxygen-containing functional groups in the precursor. This led to a higher degree of graphitization and fewer defects during carbonization. Consequently, the carbon crystallite size La (4.68 nm) and closed-pore diameter (1.13 nm) of the resulting lignin-based hard carbon were significantly higher than those of Example 1, resulting in extremely poor rate performance and initial coulombic efficiency.

[0113] Compared to Example 1, Comparative Example 3 only involved heating without pressurization during the hot-pressing process. The lack of pressure resulted in reduced reactivity between carboxylated lignin and ether-containing oxygen-bonded polymers, leading to a lower degree of cross-linking and fewer oxygen-containing functional groups generated. Although the resulting hard carbon material had a lower carbon interlayer spacing d... 002 Similar to the example, but its carbon crystallite size La (4.39 nm) and closed pore diameter (1.16 nm) are both larger than those of Example 1, resulting in poorer rate performance and first coulomb efficiency.

[0114] Compared to Example 1, Comparative Example 4 did not add polymers and directly hot-pressed carboxylated lignin, resulting in poor pre-oxidation effect. The resulting hard carbon material had a large carbon crystallite size La (5.01 nm) and a large closed pore size (1.28 nm), leading to extremely poor rate performance and initial coulombic efficiency. Furthermore, the initial reversible specific capacity was also much lower than that of Example 1.

[0115] Compared to Example 1, Comparative Example 5 used a high-melting-point polymer as a crosslinking agent. During the hot-pressing process, both carboxylated lignin and polyvinyl alcohol were in a solid powder state, resulting in a small contact area and a much lower degree of crosslinking compared to the molten state. The resulting lignin-based hard carbon had a higher carbon crystallite size La (4.43 nm) and closed-pore diameter (1.21 nm) than the sample obtained in Example 1, and the carbon interlayer spacing d was also higher. 002 This is significantly lower than the sample obtained in Example 1, resulting in extremely poor rate performance, initial coulombic efficiency, and poor initial reversible specific capacity.

[0116] Compared to Example 1, Comparative Example 6 used oxygen-free linear low-density polyethylene as a crosslinking agent, resulting in poor crosslinking effect. This led to a low content of oxygen-containing functional groups in the hard carbon precursor, and the carbon crystallite size La (4.91 nm) and closed-pore diameter (1.29 nm) were both higher than those of the sample obtained in Example 1. The carbon interlayer spacing d 002 The small size (0.376nm) results in extremely poor rate performance and initial coulomb efficiency.

[0117] In summary, the mechanical thermo-pressure oxygen crosslinking method described in this invention can prepare lignin-based hard carbon materials with large interlayer spacing, small closed-pore diameter, and small-sized carbon microcrystals. As a negative electrode material for sodium-ion batteries, it exhibits significantly better performance than lignin-based hard carbon materials prepared by traditional methods.

[0118] Figure 1 This is an infrared test image of the hard carbon precursor obtained in Example 1 of this invention, after hot pressing at 1750 cm⁻¹. -1 The significant red shift of the peak corresponding to the non-conjugated ketone or carboxylic acid functional group at 1500 cm⁻¹ is likely due to hydrogen bond crosslinking, and the peak at 1500 cm⁻¹ is also likely due to the crosslinking of hydrogen bonds. -1 The significantly weakened peak corresponding to the phenolic hydroxyl group indicates that mechanical hot pressing helps promote the crosslinking of carboxylated lignin with ether-containing oxygen-bonded polymers. Meanwhile, at 1270 cm⁻¹... -1The peak corresponding to the C=O group on the benzene ring appears after hot pressing, which further illustrates that hot pressing can increase the content of C=O / OC=O bonds, which helps to suppress carbon layer shrinkage during subsequent carbonization and form lignin-based hard carbon with small carbon microcrystals and small-pore closed pores.

[0119] Figure 2 These are the XRD test curves of the lignin-based hard carbon materials prepared in Example 1 and Comparative Example 1 of this invention. From... Figure 2 As can be seen, the diffraction peak of the (100) crystal plane becomes significantly larger after hot pressing, indicating that hot pressing helps to reduce the crystallite size of hard carbon. The fitting results also show that hot pressing can effectively reduce the carbon crystallite size, which decreases from 4.31 nm to 3.25 nm.

[0120] Figure 3 These are comparison images of small-angle scattering tests of lignin-based hard carbon with small-sized carbon microcrystals obtained in Example 1 and Comparative Example 1 of this invention; from Figure 3 As can be seen from the SAXS region, Example 1 has a steeper shoulder peak and a higher peak intensity than Comparative Example 1, indicating that Example 1 has a richer number of small-volume closed pores.

[0121] Figure 4 The figures show the first charge-discharge curves of the lignin-based hard carbon materials prepared in Example 1 and Comparative Examples 1, 2, 3, and 4 of this invention as anode materials for sodium-ion batteries. As can be seen from the figures, the hard carbon prepared by hot pressing exhibits a high initial reversible capacity of 323 mAh / g at a current of 0.05 A / g, achieving a first-cycle efficiency of up to 86.5%. The initial reversible capacity of Comparative Example 1 (without hot pressing) is 289 mAh / g, lower than Example 1, and its first-cycle coulombic efficiency is 78.9%, slightly lower than Example 1. Furthermore, the capacities and first-cycle coulombic efficiencies of Comparative Examples 2, 3, and 4, prepared without carboxylation treatment, only by heating without pressurization, and without the addition of polymers, are also lower than those of Example 1.

[0122] Figure 5 This diagram shows the rate performance curves of lignin-based hard carbon materials prepared in Example 1 and Comparative Examples 1, 2, 3, and 4 of this invention as anode materials for sodium-ion batteries. Mechanical hot pressing significantly improves the rate performance of hard carbon. The hot-pressed hard carbon maintains a high reversible capacity of over 200 mAh / g at a high current of 5 A / g, reaching 228 mAh / g under the conditions of Example 1. In contrast, the rate performance of Comparative Examples 1, 2, 3, and 4 is poor, with Comparative Example 4 exhibiting the worst rate performance, maintaining only 55 mAh / g at 5 A / g, significantly inferior to the performance of Example 1. This demonstrates that the mechanical hot pressing oxygen crosslinking technology proposed in this invention represents a significant improvement over conventional pre-oxidation techniques.

Claims

1. A method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals, characterized in that, Includes the following steps: (1) Dissolve lignin in sodium hydroxide solution to prepare a lignin solution of 5~20 g / L, control the pH of the solution to 12, add carboxylating reagent of 2~10 g / L, react at 90-100 ℃ for 1~6 h, and obtain carboxylated lignin by dialysis and drying. (2) Carboxylated lignin is uniformly mixed with ether-containing oxygen-bonded polymers and then subjected to hot pressing to obtain an oxygen-crosslinked lignin precursor; (3) The oxygen-crosslinked lignin precursor obtained in step (2) is carbonized at high temperature to obtain the lignin-based hard carbon; The weight ratio of lignin: carboxylating agent solute: ether-oxygen bond-containing polymer is 100:(20~60):(25~250). In step (2), the ether-containing oxygen bond polymer is one or more of polyethylene glycol-polypropylene glycol-polyethylene glycol block copolymer, polyethylene glycol, and ethylene-vinyl acetate copolymer.

2. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 1, characterized in that, The weight ratio of lignin: carboxylating agent: ether-oxygen bond-containing polymer is 100: (30~50): (80~150).

3. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 2, characterized in that, In step (1), the lignin includes at least one of corn cob alkali lignin, sugarcane alkali lignin, bamboo alkali lignin, wheat straw alkali lignin, and enzymatically hydrolyzed lignin from the biorefining process. The carboxylating agent is at least one of sodium chloroacetate aqueous solution, ammonium chloroacetate aqueous solution, sodium difluorochloroacetate aqueous solution, and ammonium trichloroacetate aqueous solution.

4. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 3, characterized in that, In step (1), the dialysis is performed in deionized water using a 3000 Da dialysis bag, and the dialysis time is 12-24 h. The drying process is at least one of vacuum drying, infrared drying, and freeze drying.

5. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 1, characterized in that, In step (2), the molecular weight of the ether-containing oxygen bond polymer is 1000~15000 and the melting point is 50~100℃.

6. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 5, characterized in that, In step (2), the method for uniform mixing is grinding, and the grinding conditions are 100~1000 rpm for 2~4 hours; The hot pressing is performed by mechanically pressing the mixed sample using a manual flat vulcanizing machine. The hot pressing temperature is 130~260℃, the hot pressing pressure is 2~20 MPa, and the hot pressing time is 0.5~4 h.

7. The method for preparing high-ratio lignin-based hard carbon with small-sized carbon microcrystals according to claim 1, characterized in that, In step (3), the carbonization temperature is 1100~1600 ℃, the carbonization time is 2~4 h, and the heating rate is 2~5℃ / min; The carbonization process is as follows: heat up to 800 ℃ at 5 ℃ / min, then heat up to 1300 ℃ at 2 ℃ / min, and hold for 2 hours.

8. A small-sized carbon microcrystal high-ratio lignin-based hard carbon prepared by the method according to any one of claims 1 to 7.

9. The application of the small-sized carbon microcrystalline high-rate lignin-based hard carbon as described in claim 8 in sodium-ion battery anode materials.

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