A cross-linked lignin-derived hard carbon material, its preparation method and application

Abstract

This invention relates to the field of sodium-ion battery materials, and discloses a cross-linked lignin-derived hard carbon material, its preparation method, and its applications. Lignin is mixed with an aldehyde-containing reagent in a solvent and then subjected to a cross-linking reaction to obtain cross-linked lignin. The cross-linked lignin is then directly heated to 1100-1400℃ under an inert atmosphere for high-temperature carbonization to obtain the cross-linked lignin-derived hard carbon material. The cross-linked lignin-based hard carbon material obtained by this invention has a large interlayer spacing and abundant closed-cell structure. The sodium-ion battery prepared with this material has a discharge capacity as high as 417 mAh / g, a plateau capacity ratio of 74.5%, and excellent rate performance, significantly simplifying the production process and reducing energy consumption.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials, and more specifically, to a cross-linked lignin-derived hard carbon material, its preparation method, and its application. Background Technology

[0002] Hard carbon materials are composed of randomly oriented and twisted graphene nanosheets, possessing large interlayer spacing and abundant closed-pore structures, enabling them to efficiently store sodium ions. Furthermore, the raw materials for hard carbon synthesis are abundant (mainly biomass, bitumen, polymers, etc.) and inexpensive, making it considered the most commercially viable anode material for sodium-ion batteries.

[0003] Pre-oxidation is a common strategy in the synthesis of hard carbon materials. It introduces oxygen-containing groups into the precursor, promoting molecular cross-linking during pyrolysis and thus regulating the growth of carbon crystallites. Patent CN121097076A discloses a hard carbon anode material for sodium-ion batteries and its preparation method. Phenolic resin and asphalt are mixed and sequentially subjected to pre-oxidation, pre-carbonization, and carbonization treatments. Pre-oxidation promotes cross-linking between the asphalt and phenolic resin, forming more closed pores and improving sodium storage performance. Patent CN121005392A regulates the formation of lignin molecules into macromolecules through pre-oxidation or cross-linking, followed by a first calcination to form open pores, and a second high-temperature calcination to form closed pores. Therefore, existing technologies generally consider pre-oxidation to regulate the cross-linking between precursors, forming closed pores during further calcination.

[0004] However, the aforementioned existing technologies use a large amount of acid and alkali reagents, lacking environmental friendliness, and require pre-oxidation and pre-carbonization processes, making the preparation process complex. More importantly, relying on traditional pre-oxidation methods to modify lignin often makes it difficult to precisely control the crosslinking density. If the crosslinking density is insufficient, the aliphatic side chains in the lignin cannot undergo sufficient aromatization, making it difficult to form large-sized ordered aromatic lamellae. This structural defect not only leads to lattice distortion and reduced interlayer spacing, significantly increasing the resistance to sodium ion transport, but also severely limits the plateau capacity performance of the material due to incomplete closed-cell development. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, which requires complex heat treatment processes such as pre-oxidation or staged calcination to maintain the stability of lignin microstructure and is difficult to accurately lock large interlayer spacing and high proportion of plateau capacity, this invention provides a lignin-derived hard carbon material with strong covalent cross-linking to regulate structure and pore structure, as well as its preparation method and application.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: A method for preparing a cross-linked lignin-derived hard carbon material includes the following steps: S1, mixing lignin with an aldehyde-containing reagent in a solvent and then carrying out a cross-linking reaction to obtain cross-linked lignin; S2, directly heating the cross-linked lignin to 1100~1400℃ under an inert atmosphere for high-temperature carbonization to obtain the cross-linked lignin-derived hard carbon material.

[0007] Preferably, the cross-linked lignin is directly heated to 1200~1400℃ under an inert atmosphere for high-temperature carbonization.

[0008] Preferably, the lignin is at least one of alkali lignin, dealkali lignin, enzymatically hydrolyzed lignin, ground wood lignin, and organic lignin.

[0009] Furthermore, the mass ratio of lignin to aldehyde-containing reagent is 4:1 to 1:4.

[0010] Further, the solvent is at least one selected from alkaline aqueous solution, dimethyl sulfoxide, N-methylpyrrolidone, N,N-dimethylformamide, and tetrahydrofuran.

[0011] Preferably, the alkaline aqueous solution is at least one of NaOH and KOH, with a concentration of 1~5 mol / L.

[0012] Preferably, the solvent is an alkaline aqueous solution.

[0013] Furthermore, the aldehyde-containing reagent is at least one of formaldehyde, acetaldehyde, glyoxal, propionaldehyde, malondialdehyde, n-butyraldehyde, succinaldehyde, n-pentanaldehyde, and glutaraldehyde.

[0014] Preferably, the aldehyde-containing reagent is at least one of formaldehyde and glyoxal.

[0015] Furthermore, the mass concentration of the aldehyde-containing reagent is 10% to 40%.

[0016] Preferably, when the aldehyde-containing reagent is a monoaldehyde, the mass concentration of the aldehyde-containing reagent is 30% to 40%.

[0017] Furthermore, the cross-linking reaction is carried out at a temperature of 40~95℃ for 2~8 hours.

[0018] Preferably, the heating rate during high-temperature carbonization is 1~10℃ / min.

[0019] Preferably, the heating rate during high-temperature carbonization is 1~5℃ / min.

[0020] A cross-linked lignin-derived hard carbon material is prepared by the method for preparing the cross-linked lignin-derived hard carbon material.

[0021] Furthermore, the interlayer spacing of the cross-linked lignin-derived hard carbon material is 0.36~0.38 nm.

[0022] An application of the cross-linked lignin-derived hard carbon material for the preparation of sodium-ion batteries.

[0023] Preferably, it is used to prepare anode materials for sodium-ion batteries.

[0024] A sodium-ion battery using the cross-linked lignin-derived hard carbon material.

[0025] Existing technologies rely on weak intermolecular forces or low-concentration incomplete cross-linking to control the planar structure of benzene rings. This invention utilizes a homogeneous liquid system (strong alkaline aqueous solution or polar organic solvent) to fully expand and highly activate lignin molecules, thereby enabling deep polycondensation with a high proportion of highly active aldehyde-containing reagents, constructing a strongly covalently cross-linked network with high thermal stability. This invention achieves substantial control over the pore structure of hard carbon by precisely controlling the degree of cross-linking, rather than merely limiting it to carbon layer arrangement. During subsequent high-temperature pyrolysis, chemical bond forces directly counteract the micro-stress generated by lignin during rapid volatilization, thus preventing disordered stacking and collapse shrinkage of carbon layers. This allows for the production of hard carbon materials with stable, larger interlayer spacing and abundant closed-pore structures without the need for pre-oxidation or pre-carbonization.

[0026] Compared with the prior art, the beneficial effects of the technical solution of the present invention are: 1. Achieved precise control over the microstructure of hard carbon This invention constructs a stable chemical scaffold between lignin molecules through liquid-phase crosslinking, effectively resisting structural shrinkage and collapse during high-temperature pyrolysis. This allows the prepared hard carbon material to maintain a large interlayer spacing of approximately 0.38 nm and spontaneously form a rich closed-pore structure. The material obtained by this invention has a more regular microcrystalline arrangement and fewer structural defects, providing an ideal microscopic environment for the storage and diffusion of sodium ions.

[0027] 2. Significantly improved sodium storage capacity and platform share. The hard carbon material prepared by this invention exhibits significant advantages in electrochemical performance, achieving a discharge capacity of 417 mAh / g with a plateau capacity ratio as high as 74.5%. This not only improves the overall energy density of the battery but also demonstrates excellent rate performance. At a high current of 100 mA / g, the capacity retention rate reaches 76.4%, effectively solving the problems of low plateau ratio and poor kinetic performance commonly found in lignin-based hard carbon materials. Attached Figure Description

[0028] Figure 1 The images show the X-ray diffraction patterns of the hard carbon materials prepared in Example 1 and Comparative Example 1.

[0029] Figure 2 The SAXS diagrams are of the hard carbon materials prepared in Example 1 and Comparative Example 1.

[0030] Figure 3 The hard carbon materials prepared in Example 1 and Comparative Example 1 are used as the negative electrode of a sodium-ion battery, and the charge-discharge diagrams are shown for the second cycle at a current density of 20 mA / g.

[0031] Figure 4 The capacity contribution diagram for the hard carbon materials prepared in Example 1 and Comparative Example 1 as a negative electrode of a sodium-ion battery during the second cycle at a current density of 20 mA / g is shown.

[0032] Figure 5 Rate curves of the hard carbon materials prepared in Example 1 and Comparative Example 1 as negative electrodes of sodium-ion batteries at different current densities.

[0033] Figure 6 The capacity contribution of the hard carbon materials prepared in Examples 2 and 3 as anodes of sodium-ion batteries during the second cycle at a current density of 20 mA / g is shown in the figure.

[0034] Figure 7 The X-ray diffraction patterns of the hard carbon materials prepared in Comparative Examples 2 and 3 are shown.

[0035] Figure 8 The X-ray diffraction patterns are of the hard carbon materials prepared in Examples 4 and 5.

[0036] Figure 9 The charge-discharge curves of the hard carbon materials prepared in Comparative Examples 2 and 3 as anodes of sodium-ion batteries during the second cycle at a current density of 20 mA / g.

[0037] Figure 10 The hard carbon materials prepared in Examples 4 and 5 are used as the negative electrode of a sodium-ion battery, and the charge-discharge curves are shown in the second cycle at a current density of 20 mA / g.

[0038] Figure 11 The capacity contribution of the hard carbon materials prepared in Comparative Examples 2 and 3 as anodes in sodium-ion batteries during the second cycle at a current density of 20 mA / g.

[0039] Figure 12 The capacity contribution of the hard carbon materials prepared in Examples 4 and 5 as anodes of sodium-ion batteries during the second cycle at a current density of 20 mA / g is shown in the figure.

[0040] Figure 13 The image shows a comparison of the Raman spectra of the hard carbon materials prepared in Example 1 and Comparative Example 1.

[0041] Figure 14The image shows a comparison of the Raman spectra of the hard carbon materials prepared in Example 2 and Example 3.

[0042] Figure 15 The image shows a comparison of the Raman spectra of the hard carbon materials prepared in Comparative Example 2 and Comparative Example 3.

[0043] Figure 16 The image shows a comparison of the Raman spectra of the hard carbon materials prepared in Example 4 and Example 5. Detailed Implementation

[0044] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in this technical field.

[0045] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0046] Example 1 Alkali-reduced lignin and formaldehyde (37% by mass) solution were dispersed in 1 M NaOH solution at a ratio of 1:2, and the cross-linking reaction was carried out at 65°C for 3 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the cross-linked lignin product.

[0047] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised at a rate of 5 °C / min, the temperature was raised to 1400 °C, held for 2 h, and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0048] Example 2 Enzymatically hydrolyzed lignin and a 20% glyoxal solution were dispersed in a tetrahydrofuran solution at a ratio of 2:1 (mass ratio), and the cross-linking reaction was carried out at 90°C for 2 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the cross-linked lignin product.

[0049] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1300℃ at a rate of 10 ℃ / min, held for 4 h, and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0050] Example 3 A mixture of organic lignin and formaldehyde and glyoxal (mass fractions of 30% and 10%, respectively) was dispersed in an N-methylpyrrolidone solution at a 1:1 mass ratio, and the crosslinking reaction was carried out at 40°C for 8 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the crosslinked lignin product.

[0051] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1200℃ at a rate of 1℃ / min, held at that temperature for 6 h, and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0052] Example 4 Alkali-reduced lignin was dispersed in a tetrahydrofuran solution at a 1:1 mass ratio with glyoxal (20% by mass) and glutaraldehyde (10% by mass) solutions, and the cross-linking reaction was carried out at 90°C for 2 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the cross-linked lignin product.

[0053] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1400℃ at a rate of 5℃ / min, held at that temperature for 4 h, and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0054] Example 5 Alkali lignin was dispersed in a tetrahydrofuran solution at a mass ratio of 1.5:2 with solutions of n-butyraldehyde (15% by mass) and n-pentanaldehyde (15% by mass). The cross-linking reaction was carried out at 95°C for 2 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the cross-linked lignin product.

[0055] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1300℃ at a rate of 10 ℃ / min, held for 4 h, and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0056] Comparative Example 1 Dealkalized lignin and deionized lignin were dispersed in a 1 M NaOH solution at a mass ratio of 1:2 and reacted at 65 °C for 3 h. The pH was adjusted to 5 with acid to induce precipitation. The precipitate was washed, filtered, and dried at 60 °C overnight to obtain the lignin product.

[0057] The lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised at a rate of 5 °C / min, the temperature was raised to 1400 °C, held for 2 hours and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0058] Comparative Example 2 Groundwood lignin was dispersed in N,N-dimethylformamide (DMF) solution at a mass ratio of 2:0.75 with glyoxal (20%) and propionaldehyde (20%) solutions, and the crosslinking reaction was carried out at 110°C for 2 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and dried at 60°C overnight to obtain the crosslinked lignin product.

[0059] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1100℃ at a rate of 10 ℃ / min, held for 4 h and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0060] Comparative Example 3 Organic lignin was dispersed in a dimethyl sulfoxide (DMSO) solution at a mass ratio of 2:1.25 with formaldehyde (30%) and glutaraldehyde (10%) solutions, and the mixture was crosslinked at 80°C for 2 h. The pH was adjusted to approximately neutral, the precipitate was washed and filtered, and then dried at 60°C overnight to obtain the crosslinked lignin product.

[0061] The cross-linked lignin product was placed in a tube furnace, argon gas was introduced, the temperature was raised to 1100℃ at a rate of 10 ℃ / min, held for 4 h and then cooled to room temperature. After washing with water and drying, hard carbon material was obtained.

[0062] Test methods The electrode sheet of the negative electrode material and the assembly method of the sodium-ion battery include the following steps: The obtained hard carbon material, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 8:1:1, stirred thoroughly, and then coated onto copper foil. The mixture was then dried in a vacuum drying oven at 80℃ for 12 h, and cut into electrode sheets of a specific size using a die-cutting machine. Battery assembly was performed in an argon-protected glove box. The electrolyte was a 1 mol / L NaPF6 solution in ethylene glycol dimethyl ether, and a sodium metal sheet was used as the counter electrode.

[0063] (1) Electrochemical performance characterization Constant current charge-discharge test (GCD): Charge-discharge cycle test is performed at a current density of 20 mA / g to examine the specific capacity and charge-discharge curve characteristics of the material.

[0064] Capacity contribution analysis: The capacity of the ramp area and the plateau area and their proportion are calculated by the charge and discharge curves.

[0065] Rate performance testing: The capacity retention of the material is tested at different current densities.

[0066] (2) Microstructure characterization Phase analysis: X-ray diffraction (XRD) was used to measure the (002) crystal plane diffraction peaks of the material and to calculate the interlayer spacing.

[0067] Pore ​​structure analysis: The scattering characteristics of the material are observed by small-angle X-ray scattering (SAXS) to assess the abundance of closed-pore structures.

[0068] Carbon structural order: obtained through Raman spectroscopy. I D / IG It is used to reflect the degree of microscopic disorder and defects in materials.

[0069] Analysis and Explanation 1. Microstructure evolution and phase analysis like Figure 1 , Figure 7 and Figure 8 As shown in the XRD patterns, the hard carbon prepared in Example 1 has the optimal interlayer spacing (d = 0.38 nm), while the interlayer spacing of the uncrosslinked Comparative Example 1 is only 0.358 nm. The shrinkage of the interlayer structure increases the resistance to sodium ion transport, which is detrimental to ion storage. Furthermore, with the decrease in crosslinking density and deviation from process parameters, lattice distortion in the samples leads to a significant reduction in interlayer spacing. The interlayer spacing of Comparative Example 2 shrinks to 0.361 nm, and that of Comparative Example 3 shrinks to 0.358 nm; Examples 4 and 5 also shrink to 0.369 nm and 0.372 nm, respectively. These results demonstrate that only under the specific crosslinking conditions defined in this invention can the tight packing of carbon layers be broken, resulting in a significant increase in interlayer spacing and thus reducing the resistance to sodium ion transport.

[0070] Examples 1-3 I D / I G The values ​​were 1.47, 1.54, and 1.61, respectively, significantly lower than the 1.76 of Comparative Example 1. Figures 13-14 This demonstrates that chemical pre-crosslinking can effectively induce the carbon layer to evolve towards low defects and high order. Comparative Examples 2 and 3, due to their lower calcination temperatures, could not induce sufficient aromatization of the residual aliphatic side chains, resulting in the materials failing to form large-sized ordered aromatic lamellae and instead leaving numerous microscopic defects that hinder ion storage. I D / I G The values ​​reached 1.74 and 1.71 respectively. Figure 15 The ratios of Examples 4 and 7 were 1.56 and 1.51, respectively, indicating an improvement in orderliness. However, due to deviations in crosslinking density and heating kinetics, they were not the optimal sodium storage state. Figure 16 ).

[0071] Regarding pore structure characterization, Example 1 showed a higher peak position (q=0.11Å) compared to Comparative Example 1. -1 It has a more pronounced hump ( Figure 2 This proves that it has formed a rich closed pore structure inside, which is a prerequisite for achieving high capacity.

[0072] 2. Analysis of Electrochemical Performance Results Example 1 achieved a discharge capacity of up to 417 mAh / g by utilizing optimal interlayer spacing and closed-cell structure. Figure 3Furthermore, its discharge capacity is mainly contributed by the low-voltage plateau region, with the plateau accounting for as much as 74.5%. Figure 4 Even at a high current density of 100 mA / g, it can still retain 76.4% of its initial capacity. Figure 5 ).Depend on Figure 6 As can be seen, Examples 2 and 3, which use organic solvent systems, also maintained a high platform capacity ratio through precise pre-crosslinking control. Figure 6 In contrast, in Comparative Example 1, the lignin native structure could not independently resist thermal stress in the absence of a strong covalent cross-linking scaffold, resulting in severe microstructural collapse and loss of a large number of sodium storage active sites, with a discharge capacity of only about 220 mAh / g. Figure 3 In Comparative Examples 2 and 3, the excessive lignin content led to a decrease in aldehyde content and a sparse pre-crosslinked network. Furthermore, low-temperature carbonization prevented sufficient aromatization of the aliphatic side chains, resulting in a decrease in their total capacity to 170 mAh / g and 213 mAh / g, respectively. Figure 9 The platform's share dropped to 61.6% and 54.5% respectively. Figure 11 ).

[0073] Depend on Figure 12 As can be seen from the comparison, the ramp capacity of Example 4 is slightly higher ( Figure 10 Although the carbonization temperature of Example 5 was increased to 1300℃, the use of long-chain monoaldehyde reagents and the rapid heating rate resulted in the side chains not being fully converted into large-size ordered aromatic structures, resulting in a platform ratio of 62.45% and a total capacity performance that were inferior to Example 1.

[0074] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a cross-linked lignin-derived hard carbon material, characterized in that, Includes the following steps: S1. After mixing lignin with an aldehyde-containing reagent in a solvent, a cross-linking reaction is carried out to obtain cross-linked lignin; S2. Crosslinked lignin is directly heated to 1100~1400℃ under an inert atmosphere for high-temperature carbonization to obtain crosslinked lignin-derived hard carbon materials.

2. The method for preparing cross-linked lignin-derived hard carbon material according to claim 1, characterized in that, The mass ratio of lignin to aldehyde-containing reagent is 4:1 to 1:

4.

3. The method for preparing cross-linked lignin-derived hard carbon material according to claim 1, characterized in that, The solvent is at least one of the following: alkaline aqueous solution, dimethyl sulfoxide, N-methylpyrrolidone, N,N-dimethylformamide, and tetrahydrofuran.

4. The method for preparing cross-linked lignin-derived hard carbon material according to claim 1, characterized in that, The aldehyde-containing reagent is at least one of formaldehyde, acetaldehyde, glyoxal, propionaldehyde, malondialdehyde, n-butyraldehyde, succinaldehyde, n-pentanaldehyde, and glutaraldehyde.

5. The method for preparing cross-linked lignin-derived hard carbon material according to claim 1, characterized in that, The mass concentration of the aldehyde-containing reagent is 10% to 40%.

6. The method for preparing cross-linked lignin-derived hard carbon material according to claim 1, characterized in that, The cross-linking reaction is carried out at a temperature of 40-95°C for 2-8 hours.

7. A cross-linked lignin-derived hard carbon material, characterized in that, It is prepared by the method for preparing cross-linked lignin-derived hard carbon materials according to any one of claims 1 to 7.

8. The cross-linked lignin-derived hard carbon material according to claim 8, characterized in that, The interlayer spacing of the cross-linked lignin-derived hard carbon material is 0.36~0.38 nm.

9. An application of the cross-linked lignin-derived hard carbon material according to claim 8, characterized in that, Used in the manufacture of sodium-ion batteries.

10. A sodium-ion battery, characterized in that, Use the cross-linked lignin-derived hard carbon material as described in claim 8.

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