A lignin-based hard carbon material, its preparation method and application

Abstract

This invention relates to a lignin-based hard carbon material, its preparation method, and its application. The method utilizes the dissolution and re-precipitation of lignin to control the preparation of a high-closed-pore content hard carbon material. Specifically, dissolved lignin is first obtained through acid treatment. Small molecules are added to the dissolved lignin to modify its structure. The solution is then diluted to below the minimum aqueous phase concentration and centrifuged to allow the modified nano-lignin to re-aggregate and precipitate. The modified lignin is then carbonized twice under an inert atmosphere, at low and high temperatures respectively. After cooling, the hard carbon material is obtained. This preparation method utilizes the water-soluble properties of p-toluenesulfonic acid to dissolve lignin. During the lignin dissolution process, the composition of the lignin is controlled at the molecular level. By controlling the lignin structure during the dissolution and re-precipitation process, a high-closed-pore hard carbon material is prepared, providing more active sites for sodium ion storage, increasing the plateau capacity, reducing side reactions during cycling, and thus effectively improving the electrochemical performance of the material.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery anode material technology, specifically a lignin-based hard carbon material and its preparation method and application. Background Technology

[0002] With the rapid development of renewable energy, high-performance energy storage technology has become a key support for energy transition. Lithium-ion batteries dominate the portable electronic devices and electric vehicles due to their high energy density and long cycle life. However, the limited global reserves of lithium resources (only 0.0065% of the Earth's crust), uneven geographical distribution, and volatile prices severely restrict their application in grid-scale energy storage. Sodium-ion batteries, with their abundant sodium resources (2.74% of the Earth's crust, 420 times that of lithium) and environmental friendliness, show significant application prospects in large-scale energy storage. Their working principle is similar to that of lithium-ion batteries, and they can be rapidly industrialized using mature production processes. However, the larger radius of sodium ions compared to lithium ions makes it difficult to achieve effective insertion / extraction in traditional graphite anodes. Therefore, developing high-performance anode materials has become a key breakthrough.

[0003] Given the economic and sustainable characteristics of biomass, its potential as a precursor for hard carbon synthesis has attracted widespread attention. However, the heterogeneity of raw materials poses a challenge to its large-scale production. Lignin, extracted from biomass, is the second most abundant organic material globally and can provide a large amount of valuable hard carbon products. As an aromatic macromolecule, lignin-based hard carbon generally exhibits low capacity and initial efficiency. This is because its high specific surface area triggers numerous side reactions, excessively consuming electrolyte. Furthermore, due to the complex composition of lignin, the prepared hard carbon has small interlayer spacing and low porosity. The plateau capacity of sodium-ion batteries is mainly provided by sodium ions intercalating into the interlayer and filling the "pores." While electrolyte can enter mesopores, it cannot enter micropores. Therefore, a high micropore content can reduce the occurrence of side reactions and improve cycle stability. Simultaneously, the reversibility of sodium ion insertion / extraction in closed pores is relatively good. Therefore, the prepared hard carbon with abundant closed pores (micropore size) helps reduce the formation of dead sodium, improving the plateau capacity and cycle stability of sodium-ion batteries. Currently, there are few methods for preparing hard carbon with high closed-pore content by controlling the lignin structure at the molecular level, and the operation process is complicated and time-consuming.

[0004] Based on this, the present invention uses water-soluble organic acids to dissolve lignin in peony shells, and adds small molecules to modify the lignin structure during the lignin dissolution process. The solution is then diluted to below the minimum aqueous phase concentration, so that the modified lignin is re-precipitated. By changing the type and content of small molecules, hard carbon materials with high closed-pore content can be prepared, thereby improving the sodium storage performance of biomass hard carbon. Summary of the Invention

[0005] The purpose of this invention is to provide a lignin-based hard carbon material, its preparation method, and its application. The preparation method utilizes the dissolution and re-precipitation of lignin to regulate the preparation of hard carbon materials with high closed-pore content, thereby improving the electrochemical performance of biomass hard carbon anode materials, including the charge-discharge specific capacity, cycle stability, and rate performance of sodium-ion batteries.

[0006] This invention utilizes a water-soluble organic acid to regulate the lignin structure at the molecular level to prepare hard carbon materials. This organic acid has water-soluble properties, and the hydrophilicity of lignin can be altered by using the water-soluble properties of p-toluenesulfonic acid, allowing lignin to dissolve in the p-toluenesulfonic acid solution. Small molecules are added to modify the lignin structure, and the lignin is re-precipitated by diluting the p-toluenesulfonic acid solution to below the minimum aqueous phase concentration. By utilizing the dissolution and re-precipitation process of lignin, the lignin structure is regulated at the molecular level, significantly improving the closed-pore content and electrochemical performance of the prepared hard carbon.

[0007] The present invention provides a method for preparing a lignin-based hard carbon material, which specifically includes the following steps: (1) Dissolve p-toluenesulfonic acid powder in water to prepare solution A; then heat solution A in a water bath, add peony shell to solution A, heat in a water bath and stir, take it out immediately after stirring and add deionized water to solution A, filter to obtain filtrate B, at which time lignin is dissolved in filtrate B; (2) Add small molecules to the filtrate B obtained in step (1) and stir. Then add deionized water to dilute filtrate B, then centrifuge, discard the supernatant, and the precipitate obtained is the modified lignin. Wash the modified lignin with deionized water until the washing solution is neutral, and then dry it. (3) The modified lignin obtained in step (2) is first calcined at low temperature under an inert atmosphere. After the low temperature calcination is completed, it is naturally cooled to room temperature. Then it is calcined at high temperature under an inert atmosphere. After the high temperature calcination is completed, it is first cooled down by a program and then naturally cooled to room temperature. The resulting material is ground and sieved to obtain lignin-based hard carbon material.

[0008] Furthermore, in step (1), the water used is deionized water, the mass fraction of p-toluenesulfonic acid in solution A is 50-95%, the stirring speed is 100-300 rpm, the stirring time is 10-60 min, the water bath temperature is 50-95℃, and after stirring, the solution is immediately removed and deionized water is added to dilute the solution A until the mass fraction of p-toluenesulfonic acid in solution A is 15-25%; Furthermore, in step (2), the small molecule is an inorganic salt, and the amount of the small molecule is 1-15% of the amount of lignin extracted (a blank experiment without adding small molecules is used to determine the amount of lignin extracted from filtrate B; the method for determining the amount of lignin extracted from filtrate B is existing technology and will not be elaborated here). The stirring speed is 100-300 rpm, the stirring time is 1-5 h, deionized water is added to dilute filtrate B to a mass fraction of 2-12 wt% for p-toluenesulfonic acid, the centrifugation speed is 6000-10000 rpm, and the drying temperature is 60℃-95℃. Furthermore, the small molecule in step (2) is calcium carbonate or zinc oxide.

[0009] Furthermore, in step (3), the low-temperature calcination temperature is 300℃-600℃ and the holding time is 1-3h; the high-temperature calcination temperature is 1200-1500℃ and the holding time is 2-4h; the heating rate is 2-10℃ / min; the cooling rate is 5-15℃ / min; and the sieve selected for sieving is 250 mesh or 300 mesh. The present invention also aims to provide a lignin-based hard carbon material prepared by the above method and its application as a negative electrode material for sodium-ion batteries.

[0010] The proposed method utilizes the dissolution and re-precipitation of lignin in p-toluenesulfonic acid solution. During the dissolution process, small molecules are added to modify the lignin structure, thereby achieving molecular-level control over the lignin structure to prepare high-closed-pore content hard carbon. The closed pores provide more storage sites for sodium ions, reducing side reactions during cycling. This method produces hard carbon with good stability and high reproducibility, offering a new option for exploring the large-scale preparation of high-performance, high-capacity sodium-ion battery anode materials. This invention, by controlling the lignin structure at the molecular level, allows for the regulation of the closed-pore content of hard carbon. The p-toluenesulfonic acid is reusable, meeting green chemistry requirements. It also features a short production cycle, low equipment requirements, and significant application potential.

[0011] Compared with the prior art, the present invention has the following beneficial effects: (1) The preparation method proposed in this invention utilizes the dissolution and re-precipitation of lignin to regulate lignin at the molecular level. The target product is obtained through three steps: dissolution, precipitation and carbonization. The material obtained has a stable structure, high yield, strong reproducibility and excellent performance, providing a choice for exploring the large-scale synthesis of sodium-ion battery anode materials with high rate characteristics and long cycle life. (2) The present invention utilizes the dissolution and reprecipitation process, using only simple water bath stirring, filtration and centrifugation. The process is short, simple and easy to implement, with low equipment requirements and short production cycle, and has great application potential. (3) The lignin-based hard carbon material prepared by the method of the present invention has more micropore-level closed pores, providing more sites for sodium ion storage, thereby effectively improving the sodium storage capacity and capacity retention rate of the material; (4) When the hard carbon material prepared by the present invention is used as the negative electrode of sodium-ion battery, it is tested at a current density of 0.1C. The first discharge specific capacity is as high as 364 mAh / g, the first cycle ICE is 80.49%, the reversible capacity is 293 mAh / g, and the plateau capacity is as high as about 200 mAh / g. The above shows that this lignin-based hard carbon material not only has high sodium storage performance but also has abundant closed pores, making it a potential application material for high energy density and high power density sodium-ion batteries. Attached Figure Description

[0012] Figure 1 The images show the XRD patterns of HCS-CaCO3 prepared in Example 1, HCS-ZnO prepared in Example 2, and HCS hard carbon prepared in Comparative Example 1. Figure 2 The charge-discharge curve of a sodium-ion battery using the HCS-CaCO3 hard carbon material prepared in Example 1 as the negative electrode at a current density of 0.1C is shown. Figure 3 The charge-discharge curve of a sodium-ion battery using the HCS-ZnO hard carbon material prepared in Example 2 as the negative electrode at a current density of 0.1C is shown. Figure 4 The cycling performance curve of a sodium-ion battery using HCS hard carbon material prepared as the negative electrode in Comparative Example 1 at a current density of 0.1C. Figure 5 Cyclic performance curve of sodium-ion battery with HCS-CaCO3 hard carbon material prepared in Example 1 as negative electrode at 0.1C current density; Figure 6 Cyclic performance curve of sodium-ion battery with HCS-ZnO hard carbon material prepared in Example 2 as negative electrode at 0.1C current density; Figure 7 Cycling performance curves of sodium-ion batteries using HCS hard carbon material prepared in Comparative Example 1 as the negative electrode at a current density of 0.1C. Figure 8 The rate charge-discharge curve of a sodium-ion battery using the HCS-CaCO3 hard carbon material prepared in Example 1 as the negative electrode. Figure 9 The CV curve of a sodium-ion battery using the HCS-CaCO3 hard carbon material prepared in Example 1 as the negative electrode is shown. Figure 10The CV curve of a sodium-ion battery using the HCS-ZnO hard carbon material prepared in Example 2 as the negative electrode is shown. Figure 11 The CV curve of a sodium-ion battery using HCS hard carbon material prepared in Comparative Example 1 as the negative electrode. Detailed Implementation

[0013] To better understand the content of this invention, it will be further described below with reference to specific embodiments and accompanying drawings. The following embodiments are based on the technology of this invention and provide detailed implementation methods and operating steps, but the scope of protection of this invention is not limited to the following embodiments.

[0014] Example 1: (1) Dissolve 40g of p-toluenesulfonic acid powder in 10g of deionized water to prepare solution A. Heat solution A in a water bath to 80°C. Weigh 5g of peony shell and add it to solution A. Heat in a water bath at 80°C and stir at 200rpm for 20min. After stirring, take it out immediately, add 150mL of deionized water, and filter to obtain filtrate B. (2) According to the blank experiment, the amount of lignin extracted from filtrate B in this example is about 1g. Add 0.15g of calcium carbonate to filtrate B, stir at 200rpm for 3h, then add 600mL of deionized water, centrifuge at 8000rpm for 6min, discard the supernatant, and the precipitate obtained is the modified lignin. Wash the modified lignin with deionized water until the washing liquid is neutral, and then dry it in a forced-air drying at 80℃. (3) Grind the modified lignin obtained in step (2) into a mortar and place it into a corundum ceramic boat. Place the corundum ceramic boat into a tube furnace and heat it to 400°C at a heating rate of 5°C / min. Then, keep it in flowing N2 for 2 hours for low-temperature calcination. After the low-temperature calcination, let it cool down to room temperature. Then, heat it to 1300°C at a heating rate of 5°C / min and keep it in flowing Ar for 3 hours for high-temperature calcination. After the high-temperature calcination, let it cool down to 300°C at a cooling rate of 10°C / min and then let it cool down to room temperature. Grind the black powder obtained after cooling and sieve it through a 250-mesh or 300-mesh sieve to obtain lignin-based hard carbon material, denoted as HCS-CaCO3.

[0015] Example 2: (1) Dissolve 40g of p-toluenesulfonic acid powder in 10g of deionized water to prepare solution A. Heat solution A in a water bath to 80°C. Weigh 5g of peony shell and add it to solution A. Heat in a water bath at 80°C and stir at 300rpm for 20min. After stirring, take it out immediately, add 150mL of deionized water, and filter to obtain filtrate B. (2) According to the blank experiment, the amount of lignin extracted from filtrate B in this example is about 1g. Add 0.15g of zinc oxide to filtrate B, stir at 300rpm for 3h, then add 600mL of deionized water, centrifuge at 8000rpm for 6min, discard the supernatant, and the precipitate obtained is the modified lignin. Wash the modified lignin with deionized water until the washing liquid is neutral, and then dry it in a forced-air drying at 80℃. (3) Grind the modified lignin obtained in step (2) into a mortar and place it into a corundum ceramic boat. Place the corundum ceramic boat into a tube furnace and heat it to 400°C at a heating rate of 5°C / min. Then, keep it in flowing N2 for 2 hours for low-temperature calcination. After the low-temperature calcination, let it cool down to room temperature. Then, heat it to 1300°C at a heating rate of 5°C / min and keep it in flowing Ar for 3 hours for high-temperature calcination. After the high-temperature calcination, let it cool down to 300°C at a cooling rate of 10°C / min and then let it cool down to room temperature. Grind the black powder obtained after cooling and sieve it through a 250-mesh or 300-mesh sieve to obtain lignin-based hard carbon material, denoted as HCS-ZnO.

[0016] Comparative Example 1: Commercially available lignin purchased from Aladdin was ground in a mortar and placed in a corundum ceramic boat. The corundum ceramic boat was then placed in a tube furnace and heated to 400°C at a heating rate of 5°C / min. It was then calcined at a low temperature in flowing N2 for 2 hours. After the low temperature calcination, the temperature was allowed to cool naturally to room temperature. The temperature was then increased to 1300°C at a heating rate of 5°C / min and calcined at a high temperature in flowing Ar for 3 hours. After the high temperature calcination, the temperature was reduced to 300°C at a cooling rate of 10°C / min and then allowed to cool naturally to room temperature. The resulting black powder was ground and sieved through a 250-mesh or 300-mesh sieve to obtain hard carbon material, denoted as HCS.

[0017] XRD tests were performed using a D / max-γβ type X-ray diffractometer manufactured by Rigaku Electric Co., Ltd. of Japan to analyze the degree of graphitization and interlayer spacing of hard carbon materials.

[0018] Please see Figure 1 XRD analysis showed that Examples 1, 2, and Comparative Example 1 all exhibited broad peaks at the (002) crystal plane around 23°, which is a typical hard carbon structure. The (002) peak of hard carbon corresponds to the degree of disorder and the interlayer spacing of hard carbon. It can be seen that the (002) peak of HCS-CaCO3 prepared in Example 1 is shifted to the left compared with the (002) peak of HCS-ZnO prepared in Example 2 and HCS prepared in Comparative Example 1, indicating that the HCS-CaCO3 material has a larger degree of disorder and interlayer spacing, which is beneficial to the storage of sodium ions.

[0019] The prepared hard carbon material was used as the negative electrode material for sodium-ion batteries, and the battery performance of the sodium-ion battery negative electrode was measured using CR2025 coin-shaped battery test material. First, a slurry was prepared by mixing active material (HCS-CaCO3 prepared in Example 1, HCS-ZnO prepared in Example 2, or HCS prepared in Comparative Example 1), polymer binder (prepared as a 1.5% CMC solution using water as a solvent), binder SBR, and conductive material (super-P-Li) to prepare the working electrode. The active material accounted for 95 wt% of the slurry, the polymer binder for 1.5 wt%, the binder for 1.5 wt%, and the conductive material for 1 wt%. This slurry was then coated onto copper foil and dried at 60°C for 24 hours. The battery was assembled in a glove box, using a Whatman glass fiber separator and a Na sheet as the counter electrode, and DIGLYME containing 1M NaCF3SO3 as the electrolyte. The battery was tested on a LANHE battery testing system (CT2001A, Wuhan, China) within a voltage window of 0.01-3V (relative to Na). + / Na) to perform battery charge / discharge tests.

[0020] Figure 2 The results show that the HCS-CaCO3 hard carbon material prepared in Example 1, when used as a negative electrode material for sodium-ion batteries, exhibits an initial discharge specific capacity of 364 mAh / g, a charge-discharge efficiency of 80.49%, and a plateau capacity of nearly 210 mAh / g, demonstrating excellent sodium storage performance.

[0021] Figure 3 The results show that when the HCS-ZnO hard carbon material prepared in Example 2 is used as a negative electrode material for sodium-ion batteries, the initial discharge specific capacity is 297.83 mAh / g, the charge specific capacity is 210.9 mAh / g, the charge-discharge efficiency is 70.81%, and the plateau capacity is about 140 mAh / g.

[0022] Figure 4 The results show that the HCS hard carbon material prepared in Comparative Example 1, when used as a negative electrode material for sodium-ion batteries, exhibits an initial discharge specific capacity of 171.6 mAh / g, a charge specific capacity of 107.3 mAh / g, a charge-discharge efficiency of 62.53%, and a plateau capacity of only about 45 mAh / g when tested at a current density of 0.1C. This indicates that the reversible capacity of the HCS-CaCO3 hard carbon material prepared in Example 1 is approximately 185 mAh / g higher than that of the HCS prepared in Comparative Example 1, demonstrating that the present invention significantly improves the sodium storage performance of lignin-based hard carbon. The plateau capacity increase of approximately 165 mAh / g indicates that the increase in reversible capacity is mainly due to the portion of sodium ions stored in the closed pores.

[0023] Figure 5 The HCS-CaCO3 hard carbon prepared in Example 1 was used as the negative electrode of a sodium-ion battery. After constant current discharge test at a current density of 0.1C, the capacity retention rate was more than 90% after 70 cycles, which shows excellent cycle stability. Figure 6 The HCS-ZnO hard carbon prepared in Example 2 was used as the negative electrode of a sodium-ion battery. After constant current discharge test at a current density of 0.1C, the capacity retention rate was more than 90% after 70 cycles, which shows excellent cycle stability. Figure 7 The HCS hard carbon prepared for Comparative Example 1 was used as the anode in a sodium-ion battery. A constant current discharge test was conducted at a current density of 0.1C. After 70 cycles, the capacity retention was only 32.36%, indicating poor cycle stability. These data show that the present invention significantly improves the cycle stability of lignin-based hard carbon, which is related to the increased plateau capacity. Sodium ion adsorption in hard carbon defects easily leads to dead sodium, resulting in a decrease in reversible capacity. The capacity retention rates of Examples 1 and 2 are similar; the only difference is that Example 1 has a higher capacity.

[0024] Figure 8 The results of the rate charge-discharge test of the HCS-CaCO3 hard carbon material prepared in Example 1 as the negative electrode material of sodium-ion battery show that when the current density recovers to 0.05C, the capacity of the sodium-ion battery recovers to 302mAh / g, indicating that the sodium-ion desodiuming and sodium-intercalation prepared in this invention has good reversibility.

[0025] Electrochemical tests were performed on the materials using the Chenhua CHI760E. Figure 9 , Figure 10 and Figure 11 The cyclic voltammetry test results for the hard carbon materials prepared in Examples 1, 2, and 1 (Comparative Example 1) show two peaks at 0.01 V and 0.1 V, corresponding to the redox reactions during charge and discharge. The results show that the intensity of the redox peaks is: Example 1 > Example 2 > Comparative Example 1, which corresponds to the charge and discharge capacity trends of the three materials. These results indicate that the hard carbon materials prepared in Examples 1 and 2, through lignin dissolution and re-precipitation followed by the addition of small molecules to modify the lignin structure during dissolution, possess excellent high reversible capacity and high plateau capacity characteristics, making them potential application materials for high-energy-density and high-power-density sodium-ion batteries.

[0026] The above description is merely an embodiment of the present invention and is not intended to limit the present invention in any way. The present invention can also have other embodiments based on the above structure and function, which will not be listed hereafter. Therefore, any simple modifications, equivalent changes, and alterations made by those skilled in the art to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a lignin-based hard carbon material, characterized in that, Specifically, the following steps are included: (1) Dissolve p-toluenesulfonic acid powder in water to prepare solution A; then heat solution A in a water bath, add peony shell to solution A, heat in a water bath and stir, remove immediately after stirring and add deionized water to solution A, filter to obtain filtrate B; (2) Add small molecules to the filtrate B obtained in step (1) and stir. Then add deionized water to dilute filtrate B, then centrifuge, discard the supernatant, and the precipitate obtained is the modified lignin. Wash the modified lignin with deionized water until the washing solution is neutral, and then dry it. (3) The modified lignin obtained in step (2) is first calcined at low temperature under an inert atmosphere. After the low temperature calcination is completed, it is naturally cooled to room temperature. Then it is calcined at high temperature under an inert atmosphere. After the high temperature calcination is completed, it is first cooled down by a program and then naturally cooled to room temperature. The resulting material is ground and sieved to obtain lignin-based hard carbon material.

2. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, The water used in step (1) is deionized water. The mass fraction of p-toluenesulfonic acid in solution A is 50-95%, and the water bath temperature is 50-95℃. After stirring, the solution is immediately taken out and deionized water is added to dilute it to the mass fraction of p-toluenesulfonic acid in solution A to 15-25%.

3. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, In step (1), the stirring speed is 100-300 rpm and the stirring time is 10-60 min; in step (2), the stirring speed is 100-300 rpm and the stirring time is 1-5 h.

4. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, The small molecules in step (2) are inorganic salts, and the amount of small molecules is 1-15% of the amount of lignin extracted. Deionized water is added to dilute filtrate B to a mass fraction of 2-12 wt% of p-toluenesulfonic acid. The centrifugation speed is 6000-10000 rpm, and the drying temperature is 60℃-95℃.

5. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, The small molecules in step (2) are calcium carbonate or zinc oxide.

6. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, In step (3), the low-temperature calcination temperature is 300℃-600℃ and the holding time is 1-3h; the high-temperature calcination temperature is 1200-1500℃ and the holding time is 2-4h; the heating rate is 2-10℃ / min and the cooling rate is 5-15℃ / min.

7. The method for preparing lignin-based hard carbon material as described in claim 1, characterized in that, In step (3), the sieve selected for sieving is 250 mesh or 300 mesh.

8. The lignin-based hard carbon material obtained by any of the preparation methods described in claims 1 to 7.

9. The application of the lignin-based hard carbon material as described in claim 8 as a negative electrode material for sodium-ion batteries.

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