High specific capacity nutshell / lignin composite hard carbon material and preparation and application thereof

Abstract

This invention discloses a high-specific-capacity nutshell / lignin composite hard carbon material and its preparation and application. The method uses nutshells and industrial lignin as raw materials, employing oxidation and cross-linking strategies to efficiently composite the two materials, followed by high-temperature carbonization to prepare high-specific-capacity hard carbon. First, lignin is oxidized and degraded to increase its oxygen-containing functional groups and active sites. Further, cross-linking promotes efficient composite formation of lignin with components in the nutshell, constructing a stable carbon framework. Finally, high-temperature carbonization yields the nutshell / lignin composite hard carbon. This hard carbon uses the nutshell as a rigid structure, utilizing the binding properties and oxygen-containing functional groups of lignin to increase structural stability and sodium storage sites, significantly improving its specific capacity and rate performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, specifically relating to a high-specific-capacity nutshell / lignin composite hard carbon material and its preparation and application. Background Technology

[0002] Sodium-ion batteries possess abundant reserves, a wide operating temperature range, high thermal stability and safety, fast charging and long cycle life, and strong compatibility with lithium battery production lines, thus showing broad prospects for industrial application. Hard carbon, with its high sodium storage capacity, high plateau capacity, and low operating voltage, is considered the most promising anode material for sodium-ion batteries. In particular, biomass hard carbon has attracted much attention due to its wide distribution, abundant resources, and environmental friendliness. However, it faces several challenges in electrochemical performance and production, hindering its large-scale application. For example, its first-cycle coulombic efficiency is low; its surface contains unstable oxygen functional groups such as -OH and -COOH, which easily react with the electrolyte, causing electrolyte decomposition and irreversible sodium ion intercalation. It also generates a thick and unstable SEI film, severely affecting battery performance improvement.

[0003] However, biomass raw materials have a complex composition, containing not only major components such as cellulose but also a large number of impurities such as inorganic minerals. Furthermore, the morphology and composition of different batches of biomass are greatly affected by the growth environment, resulting in batch-to-batch variations of over 15% in the prepared hard carbon products after carbonization, making it impossible to guarantee stable and consistent battery performance. Currently, precise structural control is difficult to achieve in the preparation of biomass hard carbon, and it is challenging to stably control key structural parameters such as porosity and active sites in large-scale production. Simultaneously, how to screen low-cost carbon sources that are not limited by geography and season, and how to design green and efficient preparation processes to achieve target cost control, are challenges that need to be overcome in the industrialization process.

[0004] Fruit shells (such as walnut shells and coconut shells) are ideal hard carbon precursors due to their high carbon content, rigid structure, low ash content, and natural porosity. For example, Nita et al. (Journal of Energy Chemistry, 2021, 58: 207-218) used waste walnut shells and coconut shells to obtain low specific surface area (<10 m²) carbonization materials through a one-step carbonization method. 2Walnut shell hard carbon and coconut shell hard carbon (approximately 1000 g / g) exhibit reversible capacities of 314 mAh / g and 294 mAh / g, respectively, in sodium-ion batteries (with a current density of 50 mA / g). Verma et al. (Journal of Energy Storage, 2024, 103: 114336) obtained pistachio shell hard carbon via a one-step carbonization method, achieving a reversible capacity of 302 mAh / g (with a current density of 50 mA / g) in sodium-ion batteries. By pretreating the fruit shells (acid washing, pre-carbonization, catalytic oxidation, etc.), the interlayer spacing and pore distribution of the resulting hard carbon can be controlled, giving the material excellent conductivity, a layered structure suitable for sodium storage, and high stability. This significantly improves the energy density, rate capability, and cycle performance of the sodium-ion battery anode. For example, Wang et al. (Sustainable Materials and Technologies, 2022, 33: e00446) used a method combining hydrochloric acid treatment and high-temperature carbonization. Hazelnut shells were used as a precursor, soaked in 36-38% hydrochloric acid for 36 hours to remove inorganic impurities. After high-temperature carbonization, hazelnut shell hard carbon was obtained. The hard carbon prepared by this method exhibited a disordered graphite structure with an interlayer spacing increased to 0.383 nm (compared to 0.375 nm in the untreated sample). D / I G The ratio increased from 1.62 to 1.74, increasing its defects and active sites; it achieved 91% first-time efficiency in sodium-ion batteries, and the reversible capacity increased to 342 mA hg. -1 (Untreated sample: 299 mA hg) -1 ), at 20 mA g -1After 100 cycles under current, the capacity retention rate is 91%. However, the use of concentrated hydrochloric acid is detrimental to environmental protection and experimental safety, and may excessively corrode the material's structure, reducing its stability and affecting its service life. Chinese patent CN120774403A uses waste areca nut shells as raw materials, pre-carbonizing them in air (200-350℃). The resulting pre-carbonized material is then pretreated by hydrothermal, ball milling, or molten salt-assisted methods, followed by high-temperature carbonization in an inert atmosphere to obtain hard carbon. This produces hard carbon with suitable interlayer spacing and optimized pore structure, providing more sodium ion active sites and improving the initial coulombic efficiency (66.01%) and reversible capacity (324.9 mAh / g). However, the process is cumbersome and energy-intensive (especially hydrothermal and molten salt methods), and may introduce impurities (such as molten salt residue), increasing costs and post-processing difficulty. Furthermore, residual oxygen-containing compounds on the surface may also contribute to the problem. The consumption of sodium and irreversible sodium is still relatively high, which limits the further improvement of the first effect. Chinese patent CN118145622A uses high temperature and high pressure treatment of sulfite to regulate the composition and structure of the shell, and then performs high temperature carbonization. The hard carbon obtained has a capacity of 303mAh / g, but the capacity improvement is limited and the residual impurities are difficult to handle. Environmental issues also hinder its further industrial application. At present, hard carbon from the shell has the following problems: (1) The structure of natural biomass (lignin, cellulose, hemicellulose content) is not uniform, and the microstructure of the hard carbon is difficult to control precisely, resulting in batches being uneven; (2) The surface of the shell has few functional groups, requiring cumbersome pretreatment for activation. Excessive pretreatment processes (such as hydrothermal carbonization and molten salt method) will increase energy consumption, which is not conducive to large-scale production; (3) The carbon layer spacing of untreated shells is usually narrow (about 0.375 nm), which is not conducive to the efficient insertion and extraction of sodium ions, limiting the reversible capacity.

[0005] Lignin-based hard carbon is derived from byproducts of the papermaking industry. Its sources are concentrated, low-cost, and high in carbon (approximately 60%), requiring no complex pretreatment and can be obtained through simple carbonization. It possesses numerous surface functional groups and a unique microstructure: the pyrolysis products rich in phenylpropane units form short-range ordered, long-range disordered graphite-like microcrystals with large carbon interlayer spacing, facilitating sodium ion insertion. The three-dimensional cross-linked network inhibits carbon layer stacking, constructing a multi-level pore structure (open / closed pores) to increase the density of sodium storage sites, making it an excellent precursor for hard carbon. Meng et al. (Journal of Power Sources, 2023, 581: 233475) prepared lignin hard carbon by carbonizing lignin extracted from corn cobs at high temperature, which had a specific capacity of 311 mAh / g (30 mA / g); Chen et al. (Journal of Electroanalytical Chemistry, 2022, 919: 116526) prepared lignin hard carbon with high defects (I0.05) by enzymatically hydrolyzing lignin as raw material and carbonizing it at high temperature. D / I GThe lignin-based hard carbon (d002) exhibits an initial efficiency of 81.2% and a reversible capacity of 303 mAh / g (50 mA / g). Furthermore, lignin-based hard carbon combines the advantages of sustainable raw materials (waste utilization) and structural design. Its sodium storage performance can be further optimized through simple pretreatment and material composite design, demonstrating significant competitiveness in sodium-ion battery anodes. For example, Lin et al. (Carbon, 2020, 157: 316-323.) prepared lignin-based hard carbon using a synergistic strategy of low-temperature pre-oxidation (200℃ air) and high-temperature carbonization. The carbonyl groups (C=O) introduced by pre-oxidation significantly enhanced molecular cross-linking, effectively inhibiting the ordered growth of graphite crystallites during carbonization, while simultaneously expanding the interlayer spacing (d002 increased from 3.86 Å to 4.01 Å) and providing abundant nanopores (specific surface area increased significantly from 3.8 Å to 30.9 m²). 2 / g), exhibiting a reversible capacity of 336 mAh / g (25 mA / g), good rate performance (235.6 mAh / g, 2000 mA / g), and stable long cycling (94% capacity retention after 250 cycles). However, its pretreatment time is too long and it is relatively sensitive to temperature (only showing good performance at 200℃), which is not conducive to its industrial application. Chinese patent CN121005392A first uses oxidation or cross-linking to treat lignin powder, and then uses a two-step carbonization method to make the carbon layers cross-bend to form a closed-pore structure, finally obtaining a closed-pore hard carbon material. Through cross-linking-calcination synergistic regulation, the carbon layer spacing is shrunk to 0.37-0.39 nm, the XRD (002) peak is enhanced and the full width at half maximum is reduced, and the Raman I D / I G The value decreases, which increases the graphitization degree of lignin and forms uniform closed pores rather than open interconnected pores; it has a reversible capacity of 340 mAh / g. However, the two high-temperature calcination processes, especially the second calcination temperature of up to 1700℃, have high energy consumption and complex processes, which may affect the cost and efficiency of large-scale production; at present, lignin-based hard carbon has the following problems: (1) The molecular structure of natural lignin is complex and non-uniform, which makes it difficult to effectively control the defect density of hard carbon after carbonization. Too many defects are not conducive to structural stability; (2) The three-dimensional cross-linking network of lignin is difficult to precisely control during carbonization, which affects the orderly stacking of carbon layers and the formation of closed-pore structures; (3) The ash content of lignin is high, which affects the further improvement of the first effect.

[0006] Therefore, to address the aforementioned problems, researchers have combined the characteristics of different biomass materials to prepare hard carbon by combining two types of biomass, achieving the goal of leveraging their respective strengths and compensating for their weaknesses. Yin et al. (Journal of Energy Storage, 2025, 113:115649) used walnut shells as raw material, removing impurities through hydrothermal pretreatment, and then mixed them with asphalt in a certain ratio (e.g., 10:1). After high-temperature carbonization, they prepared a biomass hard carbon composite material. This strategy utilizes a soft carbon asphalt coating to seal the open pores in the hard carbon, significantly reducing the material's specific surface area (from 859.52 m²). 2 / g decreased to 5.41 m 2 The optimized material, by reducing electrolyte side reactions, improves sodium ion storage performance. The optimized material achieves an initial charge capacity of 291.9 mAh / g at 50 mA / g, with an initial coulombic efficiency improved to 74.95%, and retains a reversible capacity of 257 mAh / g after 1000 cycles at 1 A / g. Fang et al. (ACS Applied Energy Materials, 2025) prepared a structurally optimized hard carbon material by chemically crosslinking lignin with aromatic-ring-rich phenolic resin, followed by high-temperature carbonization. This expanded the interlayer spacing to approximately 0.40 nm, which is beneficial for sodium ion storage. + Diffusion is achieved, while reducing surface defect density and suppressing side reactions. At a lignin / phenolic resin mass ratio of 7:3, the reversible capacity reaches 321 mAh / g, with an initial efficiency of 92.8%, and the capacity retention rate reaches 89.6% after 600 cycles at 1000 mA / g. Chinese patent CN117163941B pre-oxidizes fruit shells (such as coconut shells, walnut shells, etc.) in a low-oxygen atmosphere (oxygen partial pressure 1-15v%, temperature 300-600℃) to obtain heat-treated material, which is then compounded with starch (such as wheat starch, corn starch, etc.) and calcined in stages to finally obtain biomass-starch composite hard carbon material. By physically blocking starch with biochar powder, foaming and coalescence of starch during pyrolysis are inhibited. At the same time, more disordered graphite microstructures and closed pores are constructed, improving the carbon interface compatibility, thereby synergistically improving the sodium storage performance of the material. The reversible specific capacity can reach 340 mAh / g, the initial coulombic efficiency is 88%, and the capacity retention rate is as high as 97% after 200 cycles at 2C rate.

[0007] In summary, although researchers have optimized the structure and improved electrochemical performance by effectively combining two different biomass components, the physical properties of the different biomass components are significantly different. It is difficult to achieve uniform dispersion of multiple biomass components at the molecular or nanoscale through simple mechanical mixing. The weak interface or discontinuous phase results in a small interlayer spacing of the prepared hard carbon, few reversible adsorption sites and a limited number of sodium storage pores, which limits the further improvement of specific capacity, especially plateau capacity. Summary of the Invention

[0008] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing high-specific-capacity nutshell / lignin composite hard carbon materials.

[0009] This invention uses coconut shells and industrial lignin as raw materials, employing oxidation and cross-linking strategies to efficiently composite the two, followed by high-temperature carbonization to prepare high-specific-capacity hard carbon. First, lignin is oxidized and degraded to increase its oxygen-containing functional groups and active sites. Then, cross-linking promotes efficient composite formation of lignin with components in the coconut shell, constructing a stable carbon framework. Finally, high-temperature carbonization yields coconut shell / lignin composite hard carbon. This hard carbon uses the coconut shell as a rigid structure, utilizing the binding properties and oxygen-containing functional groups of lignin to increase structural stability and sodium storage sites, significantly improving its specific capacity and rate performance.

[0010] Another object of the present invention is to provide a high specific capacity nutshell / lignin composite hard carbon material prepared by the above method. This hard carbon has an interlayer spacing of 0.390–0.400 nm, an active functional group C=O content of 10–14 at%, and a closed-cell content of 0.10–0.16 cm⁻¹. 3 / g; as a sodium ion anode, the reversible specific capacity at a current density of 50mA / g is not less than 370 mAh / g, the rate capacity at a current density of 2A / g is not less than 260 mAh / g, and the capacity retention rate after 1000 cycles is not less than 75%.

[0011] Another object of the present invention is to provide the application of the above-mentioned high specific capacity nutshell / lignin composite hard carbon material in the negative electrode of sodium-ion batteries.

[0012] To achieve this objective, the present invention adopts the following technical solution:

[0013] In a first aspect, the present invention provides a method for preparing a high-specific-capacity nutshell / lignin composite hard carbon material, comprising the following steps:

[0014] (1) Add an oxidizing agent solution to a lignin solution with a pH of 11-13 to carry out an oxidation reaction, and then dry after cooling to obtain oxidized lignin;

[0015] (2) The nut shell particles, oxidized lignin and crosslinking agent are ball-milled and mixed, dried and then crosslinked. The mixture is then acid-washed and dried to obtain the nut shell / lignin crosslinked complex.

[0016] (3) Carbonize the shell / lignin crosslinked complex to obtain shell / lignin composite hard carbon.

[0017] Preferably, the shell in step (2) includes at least one of macadamia nut shells, walnut shells, camellia shells, coconut shells, pine cone shells, and pistachio shells, more preferably at least one of macadamia nut shells and walnut shells.

[0018] Preferably, the fruit shell particles in step (2) are obtained by crushing the fruit shells and sieving them through a 20-200 mesh sieve, and the particle size is 850-150 μm.

[0019] Preferably, the lignin in step (1) is at least one of the following: enzymatically hydrolyzed lignin extracted from biorefining residue, organic solvent lignin obtained from solvent pulping, and alkali lignin extracted from black liquor from alkaline pulping.

[0020] Preferably, the lignin solution with a pH of 11 to 13 in step (1) is obtained by the following method: preparing lignin into an aqueous solution with a mass concentration of 10 to 40%, and then adding an alkaline solution to adjust the pH to 11 to 13.

[0021] More preferably, the alkali in the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, and ammonia water; the mass concentration of the alkaline solution is 10-20%.

[0022] Preferably, the mass ratio of lignin to oxidant in step (1) is 10:(1-2).

[0023] Preferably, the oxidant in step (1) includes at least one of hydrogen peroxide, ozone, sodium persulfate, potassium permanganate, and sodium hypochlorite.

[0024] Preferably, the mass concentration of the oxidant solution in step (1) is 20±5%.

[0025] Preferably, the oxidation reaction in step (1) is carried out at a temperature of 70 to 110°C and for a reaction time of 0.5 to 4 hours.

[0026] Preferably, the drying in steps (1) and (2) is at least one of blower drying and infrared drying; more preferably, it is carried out at 100-120°C for 4-8 hours.

[0027] Preferably, the mass ratio of oxidized lignin to fruit shell particles in step (2) is 10:(30-90).

[0028] Preferably, the mass ratio of oxidized lignin to crosslinking agent in step (2) is 10:(2-10).

[0029] Preferably, the crosslinking agent in step (2) is an aldehyde compound, including at least one of formaldehyde, acetaldehyde and glutaraldehyde.

[0030] Preferably, the crosslinking agent in step (2) is mixed with the shell particles and oxidized lignin in the form of an aqueous solution; the concentration of the crosslinking agent in the aqueous solution is 20-30 wt%.

[0031] Preferably, the ball milling time in step (2) is 10 to 30 minutes; the ball milling speed is 800 to 1400 rpm.

[0032] Preferably, the temperature of the crosslinking reaction in step (2) is 250-400°C and the reaction time is 0.5-3h.

[0033] Preferably, the crosslinking reaction in step (2) is carried out in a tube furnace.

[0034] Preferably, the crosslinking reaction in step (2) is carried out in an air atmosphere.

[0035] Preferably, the acid used for pickling in step (2) is an acid solution with a mass concentration of 10-20%, and the acid in the acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid.

[0036] Preferably, the carbonization in step (3) refers to carbonizing at 250-400°C for 5-20 minutes and then carbonizing at 1200-1500°C for 1-3 hours. More preferably, it refers to carbonizing at 300-400°C for 5-15 minutes and then carbonizing at 1300-1500°C for 1-2 hours.

[0037] Preferably, the carbonization heating rate in step (3) is 3 to 5 °C / min, more preferably 3 to 4 °C / min.

[0038] Preferably, the carbonization in step (3) is carried out in an inert gas atmosphere.

[0039] More preferably, the inert gas includes at least one of argon, helium, and nitrogen.

[0040] Secondly, the present invention provides a high specific capacity nutshell / lignin composite hard carbon material prepared by the above preparation method.

[0041] Thirdly, the present invention provides the application of the above-mentioned high specific capacity nutshell / lignin composite hard carbon material in the negative electrode of sodium-ion batteries.

[0042] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0043] (1) The shell / lignin composite hard carbon prepared by the present invention has a large carbon layer spacing, abundant surface oxygen-containing functional groups and a stable cross-linked framework, and has high specific capacity and rate performance when used in sodium-ion batteries.

[0044] (2) This invention increases the number of oxygen-containing groups by oxidative modification of industrial lignin, thereby improving the reactivity and increasing the chemical activity of the fruit shell surface. At the same time, it increases the proportion of lignin, increases the crosslinking density, provides more active sites to achieve high reversible specific capacity, and shows significant commercial application potential.

[0045] (3) This invention utilizes the rigid framework of the fruit shell to suppress carbon layer collapse through one-step oxidative crosslinking (250-400℃), while the active functional groups of lignin directionally modify the pores and expand the interlayer spacing to form a stable carbon framework. Combined with acid washing to remove impurities, this process replaces multiple processes and high-energy-consuming treatments. Using fruit shells and industrial lignin as raw materials, this process not only solves the problem of performance fluctuations in natural fruit shells, but also provides a high-performance, low-cost, and green manufacturing solution for sodium electrode anodes through standardized oxidation temperature control. Attached Figure Description

[0046] Figure 1 This is the Raman spectrum of the lignocellulosic composite hard carbon prepared in Example 1 of this invention.

[0047] Figure 2 This is the X-ray photoelectron spectrum of the lignocellulosic composite hard carbon prepared in Example 1 of this invention.

[0048] Figure 3 This is a rate performance test chart of the nutshell lignin composite hard carbon prepared in Example 1 of the present invention.

[0049] Figure 4 This is a test chart of the cycling performance of the fruit shell lignin composite hard carbon prepared in Example 1 of the present invention. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0051] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0052] Example 1

[0053] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0054] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0055] Take 9g of nutshell particles and 1g of oxidized lignin powder, add 2g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place the mixture in a tube furnace, oxidize and crosslink at 350℃ in air atmosphere for 2h, then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then place in a 120℃ forced-air drying oven for 8h to obtain nutshell / lignin crosslinked composite;

[0056] The obtained composite was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain a coconut shell / lignin composite hard carbon.

[0057] Example 2

[0058] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0059] Dissolve 100g of alkali lignin in 300ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 100ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry at 120℃ for 8h to obtain oxidized lignin.

[0060] Take 6g of nutshell granules and 2g of oxidized lignin powder, add 4g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place the mixture in a tube furnace, oxidize and crosslink at 350℃ in air atmosphere for 2h, then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and dry in a 120℃ forced-air drying oven for 8h to obtain nutshell / lignin crosslinked composite;

[0061] The obtained composite was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain a coconut shell / lignin composite hard carbon.

[0062] Example 3

[0063] The walnut shells were crushed and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0064] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry at 120℃ for 8h to obtain oxidized lignin.

[0065] Take 8g of nutshell granules and 1g of oxidized lignin powder, add 2g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place the mixture in a tube furnace, oxidize and crosslink at 350℃ in air atmosphere for 2h, then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then place in a 120℃ forced-air drying oven for 8h to obtain nutshell / lignin crosslinked composite;

[0066] The obtained composite was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain a coconut shell / lignin composite hard carbon.

[0067] Example 4

[0068] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0069] Dissolve 100g of organic solvent lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry at 120℃ for 8h to obtain oxidized lignin.

[0070] Take 7g of nutshell granules and 2g of oxidized lignin powder, add 4g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place the mixture in a tube furnace, oxidize and crosslink at 350℃ in air atmosphere for 2h, then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then place in a 120℃ forced-air drying oven for 8h to obtain nutshell / lignin crosslinked composite;

[0071] The obtained composite was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain a coconut shell / lignin composite hard carbon.

[0072] Comparative Example 1 (Compared to Example 1, the fruit shells were directly used to make hard carbon)

[0073] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0074] Take 9g of macadamia nut shell particles and ball mill them for 20 minutes at a speed of 1200 rpm. Then place them in a tube furnace and carbonize them at 350℃ in an Ar atmosphere for 5 minutes. Then raise the temperature to 1300℃ and hold for 2 hours. Cool down to room temperature to obtain macadamia nut shell hard carbon.

[0075] Comparative Example 2 (Compared to Example 1, no lignin was added, and the fruit shell was oxidized and cross-linked alone)

[0076] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0077] Take 9g of nutshell particles, add 2g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place in a tube furnace, and perform oxidative crosslinking at 350℃ in air atmosphere for 2h, cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then place in a 120℃ forced-air drying oven for 8h to obtain the nutshell crosslinked product;

[0078] The obtained cross-linked shell product was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain cross-linked hard carbon from the shell.

[0079] Comparative Example 3 (Compared to Example 1, the nutshell was only compounded with oxidized lignin, without oxidative cross-linking)

[0080] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0081] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0082] Take 9g of fruit shell granules and 1g of oxidized lignin powder and ball mill them in a ball mill for 20 minutes at a speed of 1200 rpm.

[0083] The ball-milled mixture was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain lignocarbon from the fruit shell.

[0084] Comparative Example 4 (compared to Example 1, alkali oxidized lignin was cross-linked alone and then carbonized to produce hard carbon)

[0085] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0086] Take 1g of oxidized lignin, add 2g of 25wt% glutaraldehyde aqueous solution, and ball mill in a ball mill for 20min at a speed of 1200 rpm. Dry in a 120℃ forced-air drying oven for 4h. After drying, put the powder into a tube furnace and oxidize and crosslink at 350℃ in air atmosphere for 2h. Then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin crosslinked product.

[0087] The oxidized lignin crosslinked material was placed in a tube furnace and heated at a rate of 3℃ / min. It was carbonized at 350℃ in an Ar atmosphere for 5 minutes, then heated to 1300℃ and held for 2 hours. Finally, it was cooled to room temperature to obtain oxidized lignin crosslinked hard carbon.

[0088] Comparative Example 5 (compared to Example 1, alkali lignin was directly converted into hard carbon)

[0089] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry at 120℃ for 8h to obtain oxidized lignin.

[0090] Take 5g of oxidized lignin powder and ball mill it for 20min at a speed of 1200 rpm. Then place the ball-milled powder in a tube furnace and heat it at a rate of 3℃ / min. Carbonize it at 350℃ in an Ar atmosphere for 5min, then raise the temperature to 1300℃ and hold it for 2h. Cool it down to room temperature to obtain oxidized lignin hard carbon.

[0091] Comparative Example 6 (compared to Example 1, untreated lignin was directly produced into hard carbon after ball milling)

[0092] Take 5g of alkali lignin powder and ball mill it for 20min at a speed of 1200 rpm. Then place the ball-milled powder in a tube furnace and heat it at a rate of 3℃ / min. Carbonize it at 350℃ in an Ar atmosphere for 5min, then raise it to 1300℃ and hold it for 2h. Finally, cool it down to room temperature to obtain industrial lignin hard carbon.

[0093] Comparative Example 7 (compared to Example 1, untreated lignin was combined with fruit shell)

[0094] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0095] 9g of coconut shell granules and 1g of alkali lignin powder were mixed, and 2g of 25wt% glutaraldehyde aqueous solution was added. The mixture was ball-milled for 20min at a speed of 1200 rpm and dried in a 120℃ forced-air drying oven for 4h. After drying, the mixture was placed in a tube furnace and subjected to oxidative crosslinking at 350℃ in an air atmosphere for 2h. Then, it was cooled to room temperature, washed with 10wt% hydrochloric acid solution and deionized water, and then dried in a 120℃ forced-air drying oven for 8h to obtain the coconut shell / industrial lignin crosslinked composite.

[0096] The obtained composite was placed in a tube furnace and carbonized at 350°C for 5 minutes in an Ar atmosphere at a heating rate of 3°C / min. Then, the temperature was increased to 1300°C and held for 2 hours. Finally, the temperature was cooled to room temperature to obtain a fruit shell / industrial lignin composite hard carbon.

[0097] Comparative Example 8 (compared to Example 1, using a ball milling followed by liquid phase crosslinking method)

[0098] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0099] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0100] Take 9g of coconut shell particles and 1g of oxidized lignin powder, add 2g of 25wt% glutaraldehyde aqueous solution, and ball mill in a ball mill for 20min at a speed of 1200 rpm. Place the ball-milled mixture in a three-necked flask, add 50ml of water, and react at room temperature to 80℃ for 4h. After cooling to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then dry in a forced-air oven at 120℃ for 8h to obtain the coconut shell / lignin liquid-phase crosslinked composite.

[0101] The obtained composite was placed in a tube furnace and carbonized at 350°C for 5 minutes in an Ar atmosphere at a heating rate of 3°C / min. Then, the temperature was increased to 1300°C and held for 2 hours. Finally, the temperature was cooled to room temperature to obtain nutshell / lignin liquid-phase crosslinked hard carbon.

[0102] Comparative Example 9 (compared to Example 1, the crosslinking temperature was increased to 450°C)

[0103] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0104] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0105] Take 9g of nutshell granules and 1g of oxidized lignin powder, add 2g of 25wt% glutaraldehyde aqueous solution, ball mill in a ball mill for 20min at a speed of 1200 rpm, dry in a 120℃ forced-air drying oven for 4h, after drying, place the mixture in a tube furnace, oxidize and crosslink at 450℃ in air atmosphere for 2h, then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then place in a 120℃ forced-air drying oven for 8h to obtain nutshell / lignin crosslinked composite;

[0106] The obtained composite was placed in a tube furnace and carbonized at 350°C for 5 minutes in an Ar atmosphere at a heating rate of 3°C / min. Then, the temperature was increased to 1300°C and held for 2 hours. Finally, the temperature was cooled to room temperature to obtain a high-temperature cross-linked hard carbon composite of nutshell / lignin.

[0107] Comparative Example 10 (Compared to Example 1, the shell and lignin were mixed and pre-carbonized directly, then carbonized at high temperature, without oxidative cross-linking)

[0108] Macadamia nut shells were crushed, and the powder was passed through a 30-mesh sieve to obtain shell particles with a particle size of less than 600 μm.

[0109] Dissolve 100g of alkali lignin in 200ml of water, adjust the pH to 12 with 10wt% NaOH solution, add 50ml of 20wt% hydrogen peroxide solution, react at 80℃ for 2h, cool down and dry in a 120℃ forced-air drying oven for 8h to obtain oxidized lignin.

[0110] Take 9g of nutshell particles and 1g of oxidized lignin powder and mix them in a ball mill for 20min at a speed of 1200 rpm. Dry in a 120℃ forced-air drying oven for 4h. After drying, place the mixture in a tube furnace and pre-carbonize at 450℃ for 2h under an Ar atmosphere. Then cool to room temperature, wash with 10wt% hydrochloric acid solution and deionized water, and then dry in a 120℃ forced-air drying oven for 8h to obtain the nutshell / lignin pre-carbonized composite.

[0111] The obtained composite was placed in a tube furnace and heated at a rate of 3°C / min. It was carbonized at 350°C in an Ar atmosphere for 5 minutes, then heated to 1300°C and held for 2 hours. Finally, it was cooled to room temperature to obtain a pre-carbonized hard carbonized nutshell / lignin composite.

[0112] Results and analysis:

[0113] The nutshell lignin composite hard carbon prepared in Example 1 was applied to the anode of a sodium-ion battery, and material characterization and electrochemical testing were performed. The results are shown in Tables 1 and 2. Figures 1-4 .

[0114] The interlayer spacing of the samples in this invention was characterized using X-ray diffraction.

[0115] The closed-pore volume of the sample of this invention was measured using a fully automated density analyzer and X-ray small-angle scattering.

[0116] The C=O content on the surface of the sample of the present invention was characterized by X-ray photoelectron spectroscopy.

[0117] I of the present invention D / I G Raman spectroscopy (HJY LabRAM Odyssey) was used for testing, and the D peak (approximately 1350 cm⁻¹) in the Raman spectrum was analyzed. -1 ) and G peak (approximately 1580cm) -1 Perform peak fitting and take the area ratio to obtain I. D / I G The higher the ratio, the more disordered the structure and the more defect sites there are.

[0118] The specific procedure for electrochemical performance testing is as follows: The prepared nutshell / lignin composite hard carbon, conductive carbon black, and sodium polyacrylate (PAA-Na 4wt%) were mixed uniformly in a ball mill flask at a mass ratio of 8:1:1. The mixture was then ball-milled at 200 r / min for 40 minutes using a planetary ball mill. The viscosity of the slurry could be adjusted using a binder solution until a uniform suspension with good coating properties was obtained. The slurry was then uniformly coated onto the surface of a copper foil current collector, and subsequently placed in a vacuum drying oven and heat-treated at 110 ℃ for 10 hours to completely remove the solvent. The dried electrode material was then compacted using a precision roller press and finally punched into circular electrode sheets with a diameter of 12 mm. Each electrode sheet was precisely weighed and stored in an inert atmosphere glove box for later use.

[0119] The assembly of the sodium-ion half-cell was carried out in a strictly humidity- and oxygen-controlled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm). The battery adopted a standard CR2032 button cell structure, and the specific assembly process was as follows: First, a metallic sodium counter electrode and a glass fiber separator (Whatman GF / B type) were placed in sequence in the negative electrode shell, and an appropriate amount of electrolyte (1 mol / L NaPF6 / DME solution) was added; then, the prepared hard carbon working electrode was stacked in sequence with a stainless steel gasket and a spring sheet; finally, the positive electrode shell was used to seal the battery to complete the assembly.

[0120] The electrochemical performance of the electrode materials was evaluated using the Neware battery testing system. Constant current charge-discharge tests were conducted within a voltage window of 0.01–3.0 V, with an initial current density set at 50 mA g. -1 Rate performance testing employed a stepped current density variation mode, at 50, 100, 200, 500, 1000, 2000, and 50 mA g. -1 Five charge-discharge cycles were performed at each current density.

[0121] Table 1. Structural characteristics of nutshell / lignin composite hard carbon and comparative examples 1-10

[0122]

[0123] Table 2 Electrochemical properties of nutshell / lignin composite hard carbon and comparative examples 1–10

[0124]

[0125] Explanation of Tables 1 and 2:

[0126] As shown in Table 1, the nutshell / lignin composite hard carbon prepared in Examples 1-4 all have a high closed-cell volume (>0.1 cm). 3 The composite of lignin with a large interlayer spacing (>0.390 nm) and a functional group C=O of not less than 10.0 at% demonstrates that the composite with lignin expands the interlayer spacing of hard carbon and provides abundant closed pores, which is conducive to the insertion of sodium ions and provides more active sites. At the same time, the reversible active adsorption functional group C=O introduced by lignin is beneficial to improving the adsorption strength and dynamic performance of sodium ions.

[0127] As shown in Table 2, Example 1 exhibits a high reversible specific capacity of 393 mAh / g at a current density of 50 mA / g, with a retention rate of 93.7% after 200 cycles, demonstrating good cycle stability. Simultaneously, it exhibits a specific capacity of 282 mAh / g at a current density of 2 A / g, with a retention rate of 75.6% after 1000 cycles, demonstrating excellent rate performance, significantly superior to similar materials. This is mainly attributed to the material's large interlayer spacing, numerous closed pores, and reversible active functional groups, which enable the hard carbon structure to achieve efficient sodium storage performance and cycle stability in sodium-ion batteries.

[0128] Compared to Example 1, Comparative Example 1 directly carbonized hard carbon using fruit shells as a single raw material. Because the fruit shells themselves have a dense structure, few active functional groups, and lack the binding and functional group modification of oxidized lignin, resulting in a simpler structure, the hard carbon formed has fewer sodium storage sites and fewer closed pores (0.045 cm²). 3 The capacity is relatively low (320mAh / g).

[0129] Compared to Example 1, Comparative Example 2 used fruit shells as raw materials for oxidative cross-linking before producing hard carbon. Although this increased the closed-cell content (0.085 cm⁻¹), it still yielded better results. 3 It improved the reversible active functional groups (C=O content increased by only 4.3% compared to Comparative Example 1), but lacked lignin cross-linking network and functional group supplementation, and did not provide more active sites, resulting in less capacity improvement (only 15mAh / g compared to Comparative Example 1).

[0130] Compared to Example 1, Comparative Example 3 uses nutshells and oxidized lignin as raw materials to directly mix and carbonize hard carbon. Although the closed-cell content, interlayer spacing and reversible active functional groups are improved, the composite is not strong and the structure is not stable. The improvement in capacity (only 20 mAh / g compared with Comparative Example 1) and stability is small.

[0131] Compared to Example 1, Comparative Examples 4, 5, and 6 are hard carbons made from oxidized lignin or industrial lignin. Although the interlayer spacing is increased, which is beneficial for the insertion of sodium ions, they lack the rigid framework support of the nutshell, are prone to collapse, have insufficient structural strength, and the lignin itself has a high ash content, resulting in low initial efficiency and poor reversible capacity (less than 321 mAh / g).

[0132] Compared to Example 1, Comparative Example 7 used industrial lignin and fruit shells as raw materials, which were first pre-oxidized and cross-linked before being made into hard carbon; although the closed-cell content (0.095 cm⁻¹) was lower, the results showed that the carbon content was lower. 3 The interlayer spacing (0.389nm) is acceptable, but the lignin is not oxidized, the functional group activity is low, the composite effect is insufficient, and the improvement on rate performance (253mAh / g) and reversible capacity (358mAh / g) is limited.

[0133] Compared to Example 1, Comparative Example 8, which involved liquid-phase crosslinking, only showed slight surface bonding, failing to form a continuous three-dimensional crosslinked framework. The weak interface between the nutshell and lignin made the carbon layers prone to stacking during carbonization, leading to closed-cell collapse (the closed-cell content decreased to 0.098 cm). 3 The specific capacity was significantly lower than that of Example 1 (346 mAh / g) due to the lack of 350℃ solid-phase oxidation crosslinking, low retention of C=O functional groups (C=O content was only 7.2 at%), and inability to stably expand the interlayer spacing.

[0134] Compared to Example 1, the crosslinking temperature of Comparative Example 9 was too high, and excessive pyrolysis led to the decomposition of a large number of oxygen-containing functional groups in lignin, resulting in a significant loss of C=O (only 5.8 at%). At the same time, the crosslinking skeleton was excessively shrunk, the closed-cell volume was drastically reduced, and the sodium storage sites were reduced, resulting in a specific capacity that was far lower than that of Example 1 (only 331 mAh / g).

[0135] Compared to Example 1, the rapid carbonization of lignin in Comparative Example 10 resulted in disordered stacking of carbon layers, making it difficult to form closed pores and easy to interconnect them. C=O was not stably retained, resulting in fewer sodium storage sites and lower utilization, thus reducing the specific capacity (337 mAh / g). At the same time, without crosslinking agents and oxidative crosslinking, the two components were only physically mixed, and there was no chemical bonding at the interface, so a crosslinked framework could not be formed. The cycle stability (67.5%) was also not as good as that of Example 1.

[0136] Figure 1 This is the Raman spectrum of the nutshell / lignin composite hard carbon prepared in Example 1, from which the D peak (approximately 1350 cm⁻¹) is observed. -1 ) and G peak (approximately 1580cm) -1 The composite hard carbon I was obtained by fitting the data. D / I G For version 2.02, I in other embodiments D / I G All > 2, while I of all comparative examples D / I G The values ​​are all <2, indicating that the shell / lignin composite hard carbon has a higher degree of disorder and defect density, with abundant defects, providing more adsorption active sites for sodium ions.

[0137] Figure 2 This is the X-ray photon energy spectrum of the nutshell / lignin composite hard carbon prepared in Example 1. As can be seen from the figure, the nutshell / lignin composite hard carbon has a high oxygen content and more reversible active functional groups, which can improve the surface properties of hard carbon and provide pseudocapacitance, thereby enhancing electrochemical performance.

[0138] Figure 3 This is a rate performance test chart of the nutshell / lignin composite hard carbon prepared in Example 1. The nutshell / lignin composite hard carbon has an initial charge specific capacity of 393 mAh / g at a current density of 50 mA / g, and a reversible capacity of 368 mAh / g after 50 cycles; the charge specific capacity is 282 mAh / g at a current density of 2 A / g. This is due to the high closed-pore content of the composite hard carbon, which provides more platform capacity and large interlayer spacing, which facilitates the intercalation and reversible adsorption of sodium ions by the reversible adsorption active functional groups.

[0139] Figure 4 This is a cycling performance test chart of the nutshell / lignin composite hard carbon prepared in Example 1. After 1000 cycles at a current density of 2 A / g, the nutshell / lignin composite hard carbon maintained a reversible charge specific capacity of 215 mAh / g, with a capacity retention rate of 75.6%. This is mainly attributed to the increased content of the reversible active functional group C=O, which is beneficial to the rapid and reversible adsorption of sodium ions and improves its rate performance; and the rigid framework provided by the nutshell, which ensures the cycling stability of the material.

[0140] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a high specific capacity nutshell / lignin composite hard carbon material, characterized in that, Includes the following steps: (1) Add an oxidizing agent solution to a lignin solution with a pH of 11-13 to carry out an oxidation reaction, and then dry after cooling to obtain oxidized lignin; (2) The nut shell particles, oxidized lignin and crosslinking agent are ball-milled and mixed, dried and then crosslinked at 250-400℃. After acid washing and drying, the nut shell / lignin crosslinked complex is obtained. (3) Carbonize the shell / lignin crosslinked complex to obtain shell / lignin composite hard carbon.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of lignin to oxidant is 10:(1-2); And / or, the oxidant in step (1) includes at least one of hydrogen peroxide, ozone, sodium persulfate, potassium permanganate, and sodium hypochlorite; And / or, the oxidation reaction in step (1) is carried out at a temperature of 70–110°C for a reaction time of 0.5–4 h; And / or, the mass concentration of the oxidant solution in step (1) is 20 ± 5%.

3. The preparation method according to claim 1, characterized in that, In step (2), the mass ratio of oxidized lignin to fruit shell particles is 10:(30-90). And / or, in step (2), the mass ratio of oxidized lignin to crosslinking agent is 10:(2-10); And / or, the crosslinking agent in step (2) is an aldehyde compound, including at least one of formaldehyde, acetaldehyde and glutaraldehyde; And / or, the temperature of the crosslinking reaction in step (2) is 250–400°C, and the reaction time is 0.5–3 h; And / or, the crosslinking agent in step (2) is mixed with the shell particles and oxidized lignin in the form of an aqueous solution; the concentration of the crosslinking agent in the aqueous solution is 20-30 wt%.

4. The preparation method according to claim 1, 2, or 3, characterized in that, The ball milling time in step (2) is 10 to 30 minutes; the ball milling speed is 800 to 1400 rpm. And / or, the crosslinking reaction in step (2) is carried out in an air atmosphere.

5. The preparation method according to claim 1, 2, or 3, characterized in that, The shells mentioned in step (2) include at least one of macadamia nut shells, walnut shells, camellia shells, coconut shells, pine cone shells, and pistachio shells, more preferably at least one of macadamia nut shells and walnut shells; And / or, the particle size of the shell particles in step (2) is 850–150 μm; And / or, the lignin in step (1) is at least one of the following: enzymatically hydrolyzed lignin extracted from biorefining residue, organic solvent lignin obtained from solvent pulping, and alkali lignin extracted from black liquor from alkaline pulping.

6. The preparation method according to claim 1, 2, or 3, characterized in that, The carbonization in step (3) refers to carbonizing at 250-400℃ for 5-20 minutes, and then carbonizing at 1200-1500℃ for 1-3 hours. More preferably, carbonizing at 300-400℃ for 5-15 minutes, and then carbonizing at 1300-1500℃ for 1-2 hours. And / or, the carbonization heating rate in step (3) is 3 to 5 °C / min, more preferably 3 to 4 °C / min.

7. The preparation method according to claim 1, 2, or 3, characterized in that, The lignin solution with a pH of 11-13 mentioned in step (1) is obtained by the following method: lignin is prepared into an aqueous solution with a mass concentration of 10-40%, and then an alkaline solution is added to adjust the pH to 11-13; And / or, the acid used for pickling in step (2) is an acid solution with a mass concentration of 10-20%; And / or, the carbonization in step (3) is carried out in an inert gas atmosphere.

8. The preparation method according to claim 7, characterized in that, The inert gas includes at least one of argon, helium, and nitrogen; And / or, the acid in the acid solution is at least one of hydrochloric acid, acetic acid, nitric acid and sulfuric acid; And / or, the alkali in the alkaline solution is at least one of sodium hydroxide, potassium hydroxide, and ammonia water; the mass concentration of the alkaline solution is 10-20%.

9. A high specific capacity nutshell / lignin composite hard carbon material prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the high specific capacity nutshell / lignin composite hard carbon material as described in claim 9 in the negative electrode of a sodium-ion battery.

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