Lignin polyaniline composite binder, preparation thereof and application of lignin polyaniline composite binder in sodium ion battery hard carbon negative electrode

Abstract

The invention discloses a lignin polyaniline composite binder, preparation thereof and application of the lignin polyaniline composite binder in a hard carbon negative electrode of a sodium-ion battery. The method comprises the following steps: firstly, performing carboxymethylation modification on lignin to obtain carboxymethyl lignin; then, polyhydric alcohol serves as a cross-linking agent, a covalent bond is formed through dehydration condensation of hydroxyl on a molecular chain of the polyhydric alcohol and carboxyl of the carboxymethyl lignin, meanwhile, the polyhydric alcohol, the conductive polyaniline and the carboxyl of the carboxymethyl lignin are subjected to cross-linking compounding through the hydrogen-bond interaction between the hydroxyl of the polyhydric alcohol and amino on a molecular chain of the conductive polyaniline, and the lignin polyaniline composite binder is constructed. According to the composite binder, the compatibility of a hard carbon negative electrode and an ester or ether electrolyte is improved, and the structural integrity of the electrode and a stable SEI film can be effectively maintained in the circulation process, so that the first effect, the circulation stability and the rate capability are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the technical field of hard carbon anode binders, specifically relating to a lignin-polyaniline composite binder, its preparation, and its application in the hard carbon anode of sodium-ion batteries. Background Technology

[0002] Sodium-ion batteries, due to their strong adaptability to high and low temperatures, low cost, safety, and environmental friendliness, show broad application prospects in the energy storage field. Their overall performance mainly depends on the electrode materials, electrolyte characteristics, and the interfacial synergistic effect between them. Hard carbon batteries, due to their low cost and high reversible capacity (>300 mAh·g), are also promising. -1 ) and low operating potential (approximately 0.1 V vs Na) + With advantages such as α-Na, it is considered the most promising anode material for sodium-ion batteries. However, defects on the hard carbon surface (oxygen-containing functional groups, open pores, and heteroatoms) can catalyze the rapid decomposition of the electrolyte, leading to a decrease in its first-cycle coulombic efficiency, rate performance, and cycle performance.

[0003] Binders are key components of battery electrodes. In addition to providing basic bonding, they can also passivate defects on the hard carbon surface, thereby regulating the interfacial stability between the electrode and the electrolyte. Traditional binders such as sodium carboxymethyl cellulose (CMC) and sodium polyacrylate (PAA) have good adhesion and thermal stability due to the functional groups such as carboxyl and hydroxyl groups on their molecular chains. However, when these binders are applied to hard carbon anodes, there are three problems: (1) The hydroxyl and carboxyl functional groups enriched on their surface are prone to significant side reactions with the electrolyte, resulting in the formation of an unstable solid electrolyte interfacial film on the electrode surface; (2) Their one-dimensional linear molecular structure lacks sufficient elasticity and is difficult to effectively buffer the continuous changes in electrode volume caused by sodium ion insertion and extraction; (3) Poor ionic / electronic conductivity easily increases electrode resistance. These factors collectively result in poor electrochemical performance of hard carbon anodes using CMC or PAA in ester and ether electrolytes: the first-cycle coulombic efficiency is below 91%, the reversible specific capacity is below 340 mAh / g, and the cycle life does not exceed 1000 cycles; in terms of rate performance, in ester electrolytes, the reversible specific capacity at a current density of 1.0 A / g does not exceed 120 mAh / g; in ether electrolytes, the reversible specific capacity at a current density of 2.0 A / g does not exceed 250 mAh / g.

[0004] Lignin is the most abundant aromatic polymer with a three-dimensional network structure in plants. It is low in cost, renewable, and contains functional groups such as hydroxyl, carboxyl, methoxy, and carbonyl groups, making it an ideal raw material for developing adhesives.

[0005] Fan et al. (Small (2025) 21(18):2412003.) prepared hard carbon anodes using alkali lignin directly as a binder. When the alkali lignin content was 10 wt%, the prepared anodes exhibited a peel strength of 3.5 N / mm, achieved a first-cycle coulombic efficiency of 91% and a reversible specific capacity of approximately 330 mAh / g in an ether electrolyte system; at a high current density of 2 A / g, it still maintained a reversible specific capacity of 262 mAh / g, and after 700 cycles, it still maintained a reversible specific capacity of 244 mAh / g. Jiao et al. (Green Chem. (2024) 26: 6643-6655.) used sodium lignosulfonate (LS) and CMC in a 1:1 mass ratio to achieve a peel strength of 4.6 N / mm on the hard carbon anode. In an ether electrolyte, they achieved an initial coulombic efficiency of 87% and a reversible specific capacity of 348 mAh / g. The reversible specific capacity was maintained at 2 A / g and 276 mAh / g after 1000 cycles. In ester electrolyte systems, Gong et al. (ACS Sustain. Chem. Eng. (2021)9:12708-12717.) found that the hard carbon anode using LS as a binder had a coulombic efficiency of 91% in the first cycle and a reversible specific capacity of 222~257 mAh / g (increasing to 251~284 mAh / g after 10 cycles), both of which were superior to CMC.

[0006] However, existing technologies still have the following limitations: lignin itself has limited bonding properties, and existing composite binders mainly rely on physical hydrogen bonding, which results in weak forces and easy molecular chain slippage, leading to low bond strength (peel strength < 5 N / mm) and a cycle life generally not exceeding 1000 cycles. Secondly, when lignin is used alone, its hydroxyl and carboxyl functional groups remain exposed to the electrolyte, and existing composite binders based on physical hydrogen bonding cannot effectively shield these active groups, thus continuously catalyzing electrolyte decomposition. Furthermore, due to the poor ionic conductivity of lignin and its composites used in existing solutions, it is difficult to improve rate performance.

[0007] To balance high conductivity and adhesion performance in electrodes, polyaniline, possessing both intrinsic conductivity and multiple bonding capabilities, has become an ideal material. For example, He et al. (Ind. Eng. Chem. Res. (2020) 59:2680−2688.) reported a series of polyaniline conductive polymer binders for silicon-based anodes, which maintained a reversible specific capacity of 1776 mAh / g after 100 cycles at a current density of 0.5 A / g, significantly outperforming the 473 mAh / g of commercial CMC binders. However, compared to silicon-based anodes, hard carbon surfaces are mainly composed of sp²-hybridized graphite-like microcrystals and sp³-hybridized disordered carbon structures. The interaction between these surfaces and the nitrogen-containing groups of polyaniline relies primarily on weak van der Waals forces and limited hydrogen bonds, lacking the strong chemical bonding found between polyaniline and silicon. For example, Li et al. (J. Energy Storage, (2025) 141:119252.) prepared a polyaniline-hard carbon composite material by coating polyaniline onto the surface of hard carbon using a two-step synthesis method. Although this material has better conductivity than uncoated hard carbon, its first-cycle coulombic efficiency in ester electrolytes is only 57%, and its long-term cycling stability is poor (cycle life less than 1000 cycles). Therefore, the role of polyaniline as a binder alone is still limited, and it needs to be used in combination with other organic polymers to further improve its bonding and electrochemical performance in hard carbon anodes.

[0008] Currently, lignin-polyaniline composite systems have been extensively studied in fields such as supercapacitors, flexible electronic devices, and lithium-ion batteries. For example, Sun et al. (J. Colloid Interface Sci. (2025) 678:40-49.) prepared a polyaniline / alkali lignin electrode material, where polyaniline provides the conductive pathway and alkali lignin provides the redox active sites, synergistically improving the charge storage capacity and rate performance of supercapacitors. Chinese patent CN115714181A discloses a low-temperature resistant lignin binder system for lithium-ion batteries, which combines lignin, polyaniline, graphite, silica, polyacrylate, and water. The prepared graphite anode exhibits conductivity and demonstrates excellent low-temperature storage and rate performance even at extremely low temperatures of -57℃. However, existing lignin / polyaniline composite materials are mostly used as active electrode materials or conductive additives, and their preparation processes are usually complex, easily leading to polyaniline agglomeration. The dense conductive network formed by this aggregation blocks the micropores of hard carbon, hindering the insertion and extraction of Na⁺, making it difficult to use directly as a binder for hard carbon anodes.

[0009] In summary, existing hard carbon anode binders have some performance shortcomings: insufficient adhesion and conductivity, and difficulty in effectively passivating their own oxygen-containing groups and defects on the hard carbon surface, resulting in the inability to construct a stable electrode / electrolyte interface. Consequently, it is difficult to improve the first-cycle coulombic efficiency, cycle performance, and rate performance of hard carbon anodes, thus limiting their large-scale application. Summary of the Invention

[0010] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing a lignin-polyaniline composite adhesive.

[0011] This invention utilizes carboxymethyl lignin, polyaniline, and polyol crosslinking to prepare a conductive lignin-polyaniline composite adhesive with a multi-layered crosslinked network of covalent and hydrogen bonds. Specifically, lignin is first modified by carboxymethylation to obtain carboxymethyl lignin. Then, using a polyol as a crosslinking agent, the hydroxyl groups on the polyol's molecular chain undergo dehydration condensation with the carboxyl groups of carboxymethyl lignin to form covalent bonds. Simultaneously, the hydrogen bonds between the hydroxyl groups of the polyol and the amino groups on the conductive polyaniline molecular chain crosslink the three components to construct the lignin-polyaniline composite adhesive.

[0012] Another objective of this invention is to provide a lignin-polyaniline composite binder prepared by the above method. In this binder, the three-dimensional network structure of lignin and the rigid-flexible segmental structure of polyaniline interpenetrate and produce a synergistic effect. Combined with the cross-linking effect of the polyol, this significantly enhances the peel strength of the hard carbon anode, thereby effectively buffering the volume change stress of the hard carbon anode during cycling. Secondly, the composite of lignin and polyaniline not only reduces the exposure of carboxyl and hydroxyl functional groups in the binder but also promotes the uniform dispersion of the electrode material. Furthermore, the hydroxyl, carboxyl, amino, and imine groups contained in the molecular chains of polyaniline, carboxymethyl lignin, and polyol can form strong hydrogen bonds with the oxygen-containing functional groups on the hard carbon surface, effectively shielding defects on the hard carbon surface and significantly suppressing interfacial side reactions. Polyaniline, with its excellent electronic conductivity, can construct a three-dimensional conductive network in the hard carbon anode, significantly improving charge transport kinetics. Therefore, the hard carbon anode using this composite binder exhibits good interfacial compatibility in both ester and ether electrolytes, and can effectively maintain the integrity and stability of the SEI film structure during cycling, thereby significantly improving the first-efficiency, cycling and rate performance of the hard carbon anode.

[0013] The total content of polar functional groups such as hydroxyl and carboxyl groups in the lignin-polyaniline composite binder provided by the present invention is low, not exceeding 2.7 mmol / g.

[0014] Another object of the present invention is to provide the application of the above-mentioned lignin-polyaniline composite binder in the hard carbon anode of sodium-ion batteries.

[0015] The lignin-polyaniline composite binder prepared using this invention results in a hard carbon anode with a peel strength ≥6.0 N / mm. In ester-based electrolytes: at a current density of 0.02 A / g, its charge transfer resistance ≤50 Ω, initial reversible specific capacity ≥370 mAh / g, and initial coulombic efficiency ≥92%; at a high current density of 1 A / g, the specific capacity ≥120 mAh / g, and after 1500 cycles, the specific capacity ≤110 mAh / g. In ether-based electrolytes: at a current density of 0.05 A / g, the charge transfer resistance ≤2 Ω, initial reversible specific capacity ≥370 mAh / g, and initial coulombic efficiency ≥95%; at a high current density of 2 A / g, the reversible specific capacity ≥290 mAh / g, and after 1500 cycles, the capacity remains above 250 mAh / g.

[0016] To achieve this objective, the present invention adopts the following technical solution:

[0017] In a first aspect, the present invention provides a method for preparing a lignin-polyaniline composite adhesive, comprising the following steps:

[0018] (1) Dissolve lignin in an alkaline aqueous solution, then add a carboxylating reagent and heat to react, to obtain a carboxymethyl lignin solution;

[0019] (2) Adjust the carboxymethyl lignin solution from step (1) to acidity, add polyol, and heat to crosslink the reaction to obtain a carboxymethyl lignin polyol composite solution;

[0020] (3) Cool the carboxymethyl lignin polyol composite solution from step (2) to 0-5°C, add aniline monomer, and carry out oxidative polymerization under the action of an initiator, and dry to obtain lignin polyaniline composite binder.

[0021] The amounts of each reactant, by weight, are as follows:

[0022] 100 parts of lignin;

[0023] 50-100 parts of carboxylating reagent;

[0024] 40-80 parts of polyol;

[0025] 10-30 parts of aniline monomer;

[0026] Preferably, the amounts of each reactant are as follows, by weight:

[0027] 100 parts of lignin;

[0028] 80-100 parts of carboxylating reagent;

[0029] 50-70 parts of polyol;

[0030] 10-20 parts of aniline monomer.

[0031] Preferably, the lignin in step (1) includes at least one of alkali lignin extracted by acid pulping, enzymatically hydrolyzed lignin, and organic solvent lignin; more preferably, it includes at least one of alkali lignin extracted by acid pulping and enzymatically hydrolyzed lignin.

[0032] Preferably, the phenolic hydroxyl content of the lignin in step (1) is 2.0 to 4.0 mmol / g (e.g., 2.0 mmol / g, 2.5 mmol / g, 3.0 mmol / g, 3.5 mmol / g, 4.0 mmol / g, etc.). Any other specific value within this range can be selected, and will not be elaborated here.

[0033] Preferably, the alkaline aqueous solution in step (1) includes at least one of sodium hydroxide solution and potassium hydroxide solution.

[0034] Preferably, the pH value of the alkaline aqueous solution in step (1) is 10 to 12.

[0035] Preferably, the mass fraction of lignin in the alkaline aqueous solution in step (1) is 20-40% (e.g., 20%, 25%, 30%, 35%, 40%, etc.). Any other specific value within this range can be selected, and will not be described in detail here.

[0036] Preferably, the carboxylating agent in step (1) includes at least one of sodium cyanoacetate, sodium monochloroacetate, sodium α-bromoacetate, and sodium trifluoroacetate.

[0037] Preferably, the heating reaction temperature in step (1) is 70 to 90 ℃. The heating temperature can be 70 ℃, 75 ℃, 80 ℃, 85 ℃, 90 ℃, etc. Any other specific point value within this range can be selected, and will not be described in detail here.

[0038] Preferably, the heating reaction time in step (1) is 2 to 8 hours (e.g., 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, etc.); any other specific point value within the above range can be selected, and will not be described in detail here.

[0039] Preferably, the acid regulator used in step (2) to adjust to acidity includes at least one of sulfuric acid solution, p-toluenesulfonic acid and hydrochloric acid solution, with a mass fraction of 4 to 8% (e.g., 4%, 5%, 6%, 7%, 8%, etc.). Any other specific point value within this range can be selected, and will not be described in detail here.

[0040] Preferably, the pH value adjusted to acidity in step (2) is 1 to 3 (e.g., 1, 2, 3, etc.); any other specific value within this range can be selected, and will not be described in detail here.

[0041] Preferably, the polyol in step (2) is a polyol with a carbon chain length of 2 to 6, including at least one of glycerol, butylene glycol, ethylene glycol and sorbitol.

[0042] Preferably, the temperature of the heating crosslinking reaction in step (2) is 60 to 90°C (e.g., 60, 70, 80 and 90°C); any other specific point value within this range can be selected, and will not be described in detail here.

[0043] Preferably, the heating crosslinking reaction time in step (2) is 2 to 4 h (e.g., 2.0 h, 2.5 h, 3.0 h, 3.5 h, 4.0 h, etc.); any other specific point value within this range can be selected, and will not be described in detail here.

[0044] Preferably, the initiator in step (3) includes one of ammonium persulfate and potassium persulfate.

[0045] Preferably, the mass ratio of aniline monomer to initiator in step (3) is 10:1 to 20:1.

[0046] Preferably, the oxidative polymerization time in step (3) is 12 to 24 h (e.g., 12, 14, 16, 18, 20, 22 and 24 h); any other specific point value within this range can be selected, and will not be described in detail here.

[0047] Preferably, the drying in step (3) is spray drying, and the drying temperature is 180 to 220°C (e.g., 180°C, 190°C, 200°C, 220°C, etc.). Any other specific point value within this range can be selected, and will not be described in detail here.

[0048] Preferably, the preparation method of the lignin-polyaniline composite adhesive includes the following steps:

[0049] (1) Add 5-10% alkaline regulator to water to adjust the pH to 10-12; then, dissolve lignin in the alkaline aqueous solution to prepare a lignin solution with a mass fraction of 20-40%, and react it with a carboxylating agent at 70-90℃ for 2-8 h to obtain a carboxymethyl lignin solution.

[0050] (2) Add 4-8% acidic regulator to carboxymethyl lignin solution to adjust the pH value to 1-3, add polyol, and heat at 60-90℃ for 2-4 h to crosslink the reaction to obtain carboxymethyl lignin polyol composite solution;

[0051] (3) Cool the carboxymethyl lignin polyol composite solution to 0-5℃, then add aniline monomer, and oxidize and polymerize it with an initiator for 12-24 h. Spray dry the reaction solution at 180-220℃ to obtain lignin polyaniline composite binder.

[0052] Secondly, the present invention provides a lignin-polyaniline composite adhesive prepared by the above preparation method.

[0053] Thirdly, the present invention provides the application of the above-mentioned lignin-polyaniline composite binder in the hard carbon anode of sodium-ion batteries.

[0054] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0055] (1) The binder can simultaneously improve the bonding strength and conductivity, ensuring that the hard carbon anode can maintain excellent structural integrity during processing, long-term storage and cycling.

[0056] (2) The binder exhibits good compatibility with both ester and ether electrolytes, promoting the formation of a uniform and dense thin SEI film on the surface and significantly improving the long-term cycling stability of the hard carbon anode. Its unique conductive network structure can greatly enhance the rate performance of the hard carbon anode by reducing interfacial impedance. The negatively charged carboxyl groups in the binder have a strong adsorption effect on sodium ions, which can significantly improve the sodium storage capacity of the hard carbon anode. This is of great value for constructing high-capacity hard carbon anodes.

[0057] (3) The lignin-polyaniline composite binder developed in this invention has advantages such as simple preparation process, wide availability of raw materials and low cost, and has the potential for large-scale industrial application, thus providing a technical solution for developing low-cost, high-energy-density and long-cycle-life sodium-ion batteries. Attached Figure Description

[0058] Figure 1 The graphs show the rate performance test results of the hard carbon anodes corresponding to Example 3 and Comparative Example 1 in ester and ether electrolytes.

[0059] Figure 2 The graphs show the cycling performance of the hard carbon anodes corresponding to Example 3 and Comparative Example 1 in ester and ether electrolytes.

[0060] Figure 3 The images show the SEM morphology of the hard carbon anodes corresponding to Example 3 and Comparative Example 1 after cycling in an ester electrolyte for 1500 cycles. Detailed Implementation

[0061] 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.

[0062] 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.

[0063] Example 1

[0064] Prepare an alkaline aqueous solution with a pH of 10 by adding a 5% sodium hydroxide solution to a certain amount of water. Weigh 100 g of organic solvent lignin (phenolic hydroxyl content 2.0 mmol / g) and dissolve it in 400 g of alkaline aqueous solution to prepare a 20% lignin solution. Add 50 g of sodium cyanoacetate at 70℃ and react for 2 h to obtain a carboxymethyl lignin solution.

[0065] Add 4% sulfuric acid (by mass) to the above carboxymethyl lignin solution to adjust the pH of the system to 1, add 40 g of ethylene glycol, and carry out a cross-linking reaction at 60°C for 2 h to obtain a carboxymethyl lignin-ethylene glycol composite solution.

[0066] The carboxymethyl lignin glycol composite solution was cooled to 0°C, and then 10 g of aniline monomer and 1 g of ammonium persulfate were added sequentially. An oxidative polymerization reaction was carried out at this temperature for 12 hours. The product mixture was then spray-dried at 180°C to obtain the lignin-polyaniline composite binder.

[0067] Example 2

[0068] Prepare an alkaline aqueous solution with a pH of 12 by adding a 10% potassium hydroxide solution to a certain amount of water. Weigh 100 g of enzymatically hydrolyzed lignin (phenolic hydroxyl content 4.0 mmol / g) and dissolve it in 150 g of alkaline aqueous solution to prepare a 40% lignin solution. Add 100 g of sodium α-bromoacetate at 90℃ and react for 8 h to obtain a carboxymethyl lignin solution.

[0069] Add 8% p-toluenesulfonic acid as a regulator to the above carboxymethyl lignin solution to adjust the pH of the system to 3, add 80 g of glycerol, and carry out a cross-linking reaction at 90℃ for 2 h to obtain a carboxymethyl lignin-glycerol composite solution.

[0070] The carboxymethyl lignin-glycerol composite solution was cooled to 5°C, and then 30 g of aniline monomer and 1.5 g of potassium persulfate were added sequentially. An oxidative polymerization reaction was carried out at this temperature for 24 hours. The product mixture was then spray-dried at 220°C to obtain the lignin-polyaniline composite binder.

[0071] Example 3

[0072] Prepare an alkaline aqueous solution with a pH of 10 by adding a 5% sodium hydroxide solution to a certain amount of water. Weigh 100 g of alkali lignin (phenolic hydroxyl content 2.0 mmol / g) and dissolve it in 400 g of alkaline aqueous solution to prepare a 20% lignin solution. Add 50 g of sodium monochloroacetate at 70℃ and react for 2 h to obtain a carboxymethyl lignin solution.

[0073] Add 4% hydrochloric acid as a pH adjuster to the above carboxymethyl lignin solution to adjust the pH of the system to 1, add 40 g of glycerol, and carry out a cross-linking reaction at 60℃ for 2 h to obtain a carboxymethyl lignin-glycerol composite solution.

[0074] The carboxymethyl lignin-glycerol composite solution was cooled to 0°C, and then 10 g of aniline monomer and 1 g of ammonium persulfate were added sequentially. An oxidative polymerization reaction was carried out at this temperature for 12 hours. The product mixture was then spray-dried at 200°C to obtain the lignin-polyaniline composite binder.

[0075] Example 4

[0076] Prepare an alkaline aqueous solution with a pH of 12 by adding a 10% potassium hydroxide solution to a certain amount of water. Weigh 50 g of alkali lignin (phenolic hydroxyl content 4.0 mmol / g) and 50 g of enzymatically hydrolyzed lignin (phenolic hydroxyl content 4.0 mmol / g) and dissolve them in 150 g of alkaline aqueous solution to prepare a 40% lignin solution. Add 100 g of sodium trifluoroacetate at 90℃ and react for 8 h to obtain a carboxymethyl lignin solution.

[0077] Add 8% p-toluenesulfonic acid as a regulator to the above carboxymethyl lignin solution to adjust the pH of the system to 3, add 80 g of sorbitol, and carry out a cross-linking reaction at 90℃ for 2 h to obtain a carboxymethyl lignin-sorbitol composite solution.

[0078] The carboxymethyl lignin-sorbitol composite solution was cooled to 5°C, and then 30 g of aniline monomer and 1 g of potassium persulfate were added sequentially. An oxidative polymerization reaction was carried out at this temperature for 24 hours. The product mixture was then spray-dried at 200°C to obtain the lignin-polyaniline composite binder.

[0079] Example 5

[0080] An alkaline aqueous solution with a pH of 11 was prepared by adding an 8% sodium hydroxide solution to a certain amount of water. 40 g of alkali lignin (phenolic hydroxyl content 3.0 mmol / g), 40 g of enzymatically hydrolyzed lignin (phenolic hydroxyl content 3.0 mmol / g), and 20 g of organic solvent lignin (phenolic hydroxyl content 3.0 mmol / g) were weighed and dissolved in 300 g of alkaline aqueous solution to prepare a 25% lignin solution. 90 g of sodium monochloroacetate was added at 80℃, and the reaction was allowed to proceed for 6 h to obtain a carboxymethyl lignin solution.

[0081] Add 6% hydrochloric acid as a pH adjuster to the above carboxymethyl lignin solution to adjust the pH of the system to 2, add 60 g of glycerol, and carry out a cross-linking reaction at 75°C for 3 h to obtain a carboxymethyl lignin-glycerol composite solution.

[0082] The carboxymethyl lignin-glycerol composite solution was cooled to 3°C, and then 15 g of aniline monomer and 1 g of ammonium persulfate were added sequentially. An oxidative polymerization reaction was carried out at this temperature for 16 hours. The resulting reaction solution was filtered and infrared dried at 80°C for 12 hours to obtain the lignin-polyaniline composite binder. The product mixture was then spray-dried at 210°C to obtain the lignin-polyaniline composite binder.

[0083] Comparative Example 1 (commercial CMC was used directly as the adhesive, compared to Example 3)

[0084] Commercial CMC (average molecular weight 150,000, degree of carboxyl substitution 0.7) was directly used as a binder for hard carbon anodes.

[0085] Comparative Example 2 (compared to Example 3, commercial PAA was used directly as the adhesive)

[0086] Commercial PAA (average molecular weight 100,000, degree of carboxyl substitution 1.0) was directly used as a binder for hard carbon anodes.

[0087] Comparative Example 3 (compared to Example 3, carboxymethyl lignin was used directly as a binder)

[0088] A certain amount of water was mixed with a 5% sodium hydroxide solution to prepare alkaline water with a pH of 10. 100 g of alkali lignin (phenolic hydroxyl content 2.0 mmol / g) was weighed and dissolved in 400 g of alkaline water to prepare a 20% lignin solution. 50 g of sodium monochloroacetate was added at 70°C, and the reaction was allowed to proceed for 2 h to obtain a carboxymethyl lignin solution. The resulting carboxymethyl lignin solution was spray-dried at 200°C to obtain a carboxymethyl lignin composite binder.

[0089] Comparative Example 4 (compared to Example 3, carboxymethyl lignin and glycerol were used directly as composite binders)

[0090] Prepare alkaline water with a pH of 10 by adding a 5% sodium hydroxide solution to a certain amount of water. Weigh 100 g of alkali lignin (phenolic hydroxyl content 2.0 mmol / g) and dissolve it in 400 g of alkaline water to prepare a 20% lignin solution. Add 50 g of sodium monochloroacetate at 70℃ and react for 2 h to obtain a carboxymethyl lignin solution.

[0091] Add 4% hydrochloric acid as a mass adjuster to the above carboxymethyl lignin solution to adjust the pH value of the system to 1, add 40 g of glycerol, and carry out a cross-linking reaction at 60℃ for 2 h to obtain a carboxymethyl lignin-glycerol composite solution. Spray dry the carboxymethyl lignin-glycerol composite solution at 200℃ to obtain a lignin-glycerol composite binder.

[0092] Comparative Example 5 (compared to Example 3, carboxymethyl lignin and polyaniline were used directly as composite binders)

[0093] Prepare alkaline water with a pH of 10 by adding a 5% sodium hydroxide solution to a certain amount of water. Weigh 100 g of alkali lignin (phenolic hydroxyl content 2.0 mmol / g) and dissolve it in 400 g of alkaline water to prepare a 20% lignin solution. Add 50 g of sodium monochloroacetate at 70℃ and react for 2 h to obtain a carboxymethyl lignin solution.

[0094] The carboxymethyl lignin solution was cooled to 0°C, and 4% hydrochloric acid was added to adjust the pH of the system to 1. Then, 10 g of aniline monomer and 1 g of ammonium persulfate were added sequentially, and an oxidative polymerization reaction was carried out at this temperature for 12 hours. The product mixture was spray-dried at 200°C to obtain the carboxymethyl lignin polyaniline composite binder.

[0095] Comparative Example 6 (compared to Example 3, glycerol and polyaniline were used directly as composite binders)

[0096] A 5% sodium hydroxide solution was added to a certain amount of water to prepare an alkaline aqueous solution with a pH of 10. 40 g of glycerol was dissolved in 400 g of the alkaline aqueous solution to prepare a 20% glycerol solution. The glycerol solution was cooled to 0°C, and 4% hydrochloric acid was added as a pH adjuster to adjust the pH to 1. Subsequently, 10 g of aniline monomer and 1 g of ammonium persulfate were added sequentially, and an oxidative polymerization reaction was carried out at this temperature for 12 hours. The product mixture was spray-dried at 200°C to obtain a glycerol-polyaniline composite binder.

[0097] Comparative Example 7 (compared to Example 3, polyaniline was used directly as a binder)

[0098] Commercial polyaniline (average molecular weight 5000) was used directly as an aqueous binder for sodium-ion hard carbon anodes.

[0099] Results and Analysis:

[0100] The lignin-polyaniline composite binder obtained in the examples was structurally characterized, and the electrochemical behavior of the hard carbon anode of a sodium-ion battery assembled with this binder was studied. The results are shown in Table 1 and 2. Figures 1-3 .

[0101] Preparation of hard carbon anode for sodium-ion batteries: The binder, conductive carbon black, and hard carbon active particles from the examples and comparative examples were mixed in a mass ratio of 10:10:80 and uniformly coated onto a copper current collector to serve as the hard carbon anode for sodium-ion batteries. A 1 mol / L vinyl carbonate / diethyl carbonate (EC / DEC) electrolyte and Whatman glass fiber as the separator were assembled together into a CR2032 battery casing to obtain a sodium-ion symmetrical half-cell.

[0102] Structural characterization of the adhesive: The content of carboxyl and hydroxyl groups in the adhesive was determined using an automatic potentiometric titrator. 0.1 g of sample was accurately weighed and acidified with 0.2 mL of 2 mol / L hydrochloric acid. Subsequently, 20 mL of dimethylformyl aqueous solution (containing 0.03 g of p-hydroxybenzoic acid as an internal standard) was accurately added. After the sample was completely dissolved by electromagnetic stirring, a saturated calomel reference electrode and a glass indicator electrode were inserted, and titration was performed using tetrabutylammonium hydroxide standard solution. The titration endpoint was determined using a first-order derivative curve. This curve would show three characteristic peaks, corresponding to the titration endpoints of hydrochloric acid, carboxylic acid, and hydroxyl groups, respectively. Based on the titration volume corresponding to each characteristic peak, the titration volume consumed by the internal standard (p-hydroxybenzoic acid) (blank value) was subtracted to calculate the content of carboxyl and hydroxyl groups in the sample.

[0103] The peel strength of the hard carbon anode was tested using a computer-controlled electronic universal testing machine (TSKL-10D) at a temperature of 25°C and a peel rate of 6 cm / min. The average mechanical properties were taken as the final data.

[0104] Electrochemical performance testing of the hard carbon anode of sodium-ion batteries: Impedance performance testing was conducted using an electrochemical workstation (Bio-Logic VMP-3e), with a frequency range of 0.01 Hz to 100 kHz and a voltage amplitude of 5 mV. First-cycle charge-discharge testing, cycle performance testing, and rate performance testing were performed using a Xinwei battery testing system (CT-4008-5V50mA-164). In the ester electrolyte, the constant current charging and discharging high and low current densities set for the first-cycle charge-discharge test were 1 A / g and 0.02 A / g, respectively. The charge-discharge current densities set for the rate performance test were 0.02 A / g, 0.05 A / g, 0.1 A / g, 0.2 A / g, 0.4 A / g, 0.6 A / g, 0.8 A / g, and 1 A / g, respectively. Five cycles were performed at each current density, and the average value was taken as the test result. The constant current charge / discharge current density set for the cycle performance test was 1 A / g. For the ether electrolyte, the high and low constant current charges / discharge current densities set for the first week of the charge / discharge test were 2 A / g and 0.05 A / g, respectively. The rate performance test was set with charge / discharge current densities of 0.05 A / g, 0.1 A / g, 0.25 A / g, 0.5 A / g, 1 A / g, 2 A / g, and 5 A / g, with 6 cycles at each current density, and the average value was taken as the test result. The constant current charge / discharge current density set for the cycle performance test was 2 A / g.

[0105] Table 1. Total hydroxyl and carboxyl content of different binders, corresponding peel strength of hard carbon anodes, and their electrochemical performance in ester electrolytes.

[0106]

[0107] Table 2. Electrochemical performance of hard carbon anodes with different binders in ether electrolytes.

[0108]

[0109] The data in Tables 1 and 2 show that the binders prepared in Examples 1-5 have lower total carboxyl and hydroxyl content (all below 2.70 mmol / g), significantly lower than those in Comparative Examples 1-5 (all above 3.12 mmol / g). This structural feature helps to significantly improve the interfacial compatibility between the hard carbon anode and the electrolyte. Meanwhile, the average peel strength of the hard carbon anodes corresponding to Examples 1-5 is all above 6.0 N / mm, far exceeding the highest value of the comparative examples (5.5 N / mm), indicating excellent bonding performance. Furthermore, in ether / ester electrolytes, the charge transfer impedance of the hard carbon anodes corresponding to Examples 1-5 is lower than the highest value of Comparative Examples 1-6. Based on these characteristics, the hard carbon anodes using the binders of Examples 1-5 exhibit higher first-cycle reversible specific capacity and coulombic efficiency, as well as more stable cycling performance, in both ester and ether electrolytes. Among them, Example 3 shows the most outstanding overall performance, with the lowest total carboxyl and hydroxyl content (2.53 mmol / g) and an average peel strength as high as 6.5 N / mm. In ester electrolytes, the charge transfer impedance at 0.02 A / g rate is only 40 Ω, with an initial reversible specific capacity of 381 mAh / g and a coulombic efficiency of 94%. At a high rate of 1 A / g, the reversible specific capacity reaches 124 mAh / g, and it retains 115 mAh / g after 1500 cycles. In ether electrolytes, the charge transfer impedance at 0.05 A / g rate is as low as 1.4 Ω, with an initial reversible specific capacity of 378 mAh / g and a coulombic efficiency of 97%. At a high rate of 2 A / g, the reversible specific capacity reaches 305 mAh / g, and it retains 262 mAh / g after 1500 cycles, significantly outperforming all comparative examples. These advantages can be attributed to the following:

[0110] Compared to Example 3, Comparative Example 1 used only CMC as a binder. Its molecules are rich in negatively charged carboxyl and hydroxyl functional groups (total content reaches 5.40 mmol / g). These groups cannot be effectively shielded or neutralized, thus catalyzing the decomposition of the ester electrolyte and disrupting the stability of the SEI film. Simultaneously, relying solely on CMC makes it difficult to construct a stable three-dimensional cross-linked network structure, resulting in an average peel strength of only 4.5 N / mm for the hard carbon anode, making it prone to structural instability during long-term charge-discharge cycles. These defects collectively lead to a significant decrease in the electrochemical performance of the hard carbon anode. In the ester electrolyte, its charge transfer resistance reached 129 Ω, and the first-cycle reversible specific capacity and first-cycle coulombic efficiency were only 319 mAh / g and 81%, respectively. Rate performance also deteriorated significantly, with the reversible specific capacity dropping to 65 mAh / g at a current density of 1 A / g. Cycling stability was also poor, with the reversible specific capacity remaining at only 32 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode with CMC binder still performed poorly: the charge transfer resistance reached 6.2 Ω, the first-cycle reversible specific capacity and the first-cycle coulombic efficiency were 315 mAh / g and 86%, respectively; at a current density of 2 A / g, the reversible specific capacity was 247 mAh / g, which further decreased to 176 mAh / g after 1500 cycles.

[0111] Compared to Example 3, Comparative Example 2 used only PAA as a binder. PAA molecules are rich in negatively charged carboxyl functional groups (total content reaches 6.34 mmol / g). These groups cannot be effectively shielded or neutralized, thus catalyzing decomposition reactions and disrupting the stability of the SEI film in ester electrolytes. Furthermore, relying solely on PAA makes it difficult to construct a stable three-dimensional cross-linked network structure, resulting in an average peel strength of only 5.2 N / mm for the hard carbon anode, making it prone to structural instability during long-term charge-discharge cycles. These defects collectively lead to a significant decrease in the electrochemical performance of the hard carbon anode. In ester electrolytes, its charge transfer resistance reaches 150 Ω, and the first-cycle reversible specific capacity and first-cycle coulombic efficiency are only 311 mAh / g and 88%, respectively. Rate performance also deteriorates significantly, with the reversible specific capacity dropping to 80 mAh / g at a current density of 1 A / g. Cyclic stability is also poor, with the reversible specific capacity remaining at only 63 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode with PAA binder still performed poorly: the charge transfer resistance reached 7.0 Ω, the first-cycle reversible specific capacity and the first-cycle coulombic efficiency were 320 mAh / g and 92%, respectively; at a current density of 2 A / g, the reversible specific capacity was 264 mAh / g, which further decreased to 200 mAh / g after 1500 cycles.

[0112] Compared to Example 3, Comparative Example 3 used only carboxymethyl lignin as a binder. This system still retained a significant amount of hydroxyl and carboxyl functional groups (total content as high as 3.41 mmol / g), which catalyzed the decomposition of the ester electrolyte and disrupted the stability of the SEI film. Furthermore, due to insufficient crosslinking strength, the binder's adhesive properties were significantly reduced (the average mechanical peel strength of the hard carbon anode was only 3.1 N / mm), making the electrode material prone to structural instability during long-term charge-discharge cycles. These defects collectively led to a significant decrease in the electrochemical performance of the hard carbon anode. In the ester electrolyte, its charge transfer resistance reached 135 Ω, and the first-cycle reversible specific capacity and first-cycle coulombic efficiency were only 340 mAh / g and 75%, respectively; the rate performance also deteriorated significantly, with the reversible specific capacity dropping to 98 mAh / g at a current density of 1 A / g; the cycle stability was also poor, with the reversible specific capacity remaining at only 84 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode corresponding to this binder still performed poorly: the charge transfer resistance reached 6.3 Ω, the first-cycle reversible specific capacity and the first-cycle coulombic efficiency were 326 mAh / g and 89%, respectively; at a current density of 2 A / g, the reversible specific capacity was 250 mAh / g, which further decreased to 191 mAh / g after 1500 cycles.

[0113] Compared to Example 3, Comparative Example 4 used only carboxymethyl lignin and glycerol to construct a crosslinked composite system. Due to insufficient crosslinking strength, the composite system still contained a large number of hydroxyl and carboxyl functional groups (3.27 mmol / g), which would still catalyze the decomposition of ester electrolytes and damage the stability of the SEI film. Furthermore, the intrinsic electronic insulation properties of both significantly increased the electrode internal resistance. In terms of mechanical properties, due to insufficient crosslinking strength, the average peel strength of the hard carbon anode corresponding to the obtained composite binder was only 5.5 N / mm, making it difficult to maintain the stability of the electrode structure during long-term cycling. These defects collectively led to a significant decrease in the electrochemical performance of the hard carbon anode. In ester-based electrolytes, the charge transfer resistance reached 122 Ω, with a first-cycle reversible specific capacity and first-cycle coulombic efficiency of only 349 mAh / g and 79%, respectively. Rate performance also deteriorated significantly, with the reversible specific capacity dropping to 101 mAh / g at a current density of 1 A / g. Cycling stability was also poor, with the reversible specific capacity remaining at only 50 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode corresponding to this binder still performed poorly: the charge transfer resistance reached 5.7 Ω, with a first-cycle reversible specific capacity and first-cycle coulombic efficiency of 300 mAh / g and 85%, respectively. At a current density of 2 A / g, the reversible specific capacity was 201 mAh / g, further decreasing to 175 mAh / g after 1500 cycles.

[0114] Compared to Example 3, Comparative Example 5 directly used carboxymethyl lignin and polyaniline as composite binders. Due to insufficient crosslinking strength, the composite system still contained a large number of hydroxyl and carboxyl functional groups (3.12 mmol / g), which would still catalyze the decomposition of ester electrolytes and damage the stability of the SEI film. In terms of mechanical properties, due to insufficient crosslinking strength and lack of plasticizer, the mechanical peel strength of the hard carbon anode corresponding to the obtained composite binder was only 1.8 N / mm, making it difficult to maintain the stability of the electrode structure during long-term cycling. The above defects together led to a significant decrease in the electrochemical performance of the hard carbon anode. In ester electrolytes, its charge transfer resistance reached 114 Ω, and the first-cycle reversible specific capacity and first-cycle coulombic efficiency were only 334 mAh / g and 74%, respectively; the rate performance also deteriorated significantly, with the reversible specific capacity dropping to 84 mAh / g at a current density of 1 A / g; the cycling stability was also poor, with the reversible specific capacity remaining at only 43 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode corresponding to this binder still performed poorly: the charge transfer resistance reached 5.2 Ω, the first-cycle reversible specific capacity and the first-cycle coulombic efficiency were 337 mAh / g and 81%, respectively; at a current density of 2 A / g, the reversible specific capacity was 224 mAh / g, which further decreased to 163 mAh / g after 1500 cycles.

[0115] Compared to Example 3, Comparative Example 6 directly used glycerol and polyaniline as composite binders. Glycerol has a simple molecular structure, containing only three hydroxyl groups, and mainly binds to polyaniline through hydrogen bonds, resulting in a reduced hydroxyl content after composite formation. However, this interaction is weak, and the flexible segments of glycerol are unable to effectively resist interfacial stress, causing the binder layer to easily slip or crack under mechanical loads (mechanical peel strength is only 3.8 N / mm), making it difficult to maintain the structural stability of the electrode during long-term cycling. These defects collectively lead to a significant decrease in the electrochemical performance of the hard carbon anode. In ester electrolytes, its charge transfer resistance reaches 120 Ω, and the first-cycle reversible specific capacity and first-cycle coulombic efficiency are only 320 mAh / g and 82%, respectively; the rate performance also deteriorates significantly, with the reversible specific capacity dropping to 70 mAh / g at a current density of 1 A / g; the cycling stability is also poor, with the reversible specific capacity remaining at only 45 mAh / g after 1500 cycles at a current density of 1 A / g. In ether-based electrolytes, the hard carbon anode corresponding to this binder still performed poorly: the charge transfer resistance reached 5.5 Ω, the first-cycle reversible specific capacity and the first-cycle coulombic efficiency were 298 mAh / g and 87%, respectively; at a current density of 2 A / g, the reversible specific capacity was 217 mAh / g, which further decreased to 158 mAh / g after 1500 cycles.

[0116] Compared to Example 3, Comparative Example 7 directly used polyaniline as a binder. Due to its strong conductivity, polyaniline exhibits a charge transfer impedance of only 45 Ω in ester-based electrolytes and only 1.7 Ω in ether-based electrolytes, even lower than in Examples 2 and 5. Furthermore, since the polyaniline molecular chain does not contain hydroxyl or carboxyl groups, its corresponding functional group content is 0 mmol / g. Although this material has good compatibility with ester-based electrolytes, the bonding strength of single polyaniline is low, with a mechanical peel strength of only 2.3 N / mm, making it difficult to maintain the structural stability of the electrode during long-term cycling. Simultaneously, polyaniline as a binder is prone to agglomeration, failing to achieve uniform coating of hard carbon, resulting in numerous defects on the hard carbon surface being exposed to the electrolyte environment, thus causing a significant decrease in the electrochemical performance of the negative electrode. In ester-based electrolytes, the hard carbon anode using this binder exhibited a reversible specific capacity of only 314 mAh / g in the first cycle, with a coulombic efficiency of 69%. Rate performance also deteriorated significantly, with the reversible specific capacity dropping to 59 mAh / g in the first cycle at 1 A / g; after 1500 cycles at 1 A / g, the reversible specific capacity remained at only 37 mAh / g. In ether-based electrolytes, the system also performed poorly: the reversible specific capacity and coulombic efficiency in the first cycle were 246 mAh / g and 75%, respectively; the reversible specific capacity was 236 mAh / g at 2 A / g, further decreasing to 114 mAh / g after 1500 cycles.

[0117] Figure 1 The rate performance of the hard carbon anodes of Example 3 and Comparative Example 1 in ester and ether electrolytes was compared. In ester electrolytes, when the current density varied from 0.02 to 1 A / g, the reversible specific capacities of the hard carbon anode of Example 3 were 381, 366, 342, 273, 175, 147, 133, and 124 mAh / g, respectively, all significantly higher than those of Comparative Example 1. In ether electrolytes, within the current density range of 0.05 to 5 A / g, the specific capacities of the hard carbon anode of Example 3 were 378, 372, 360, 347, 329, 302, and 202 mAh / g, respectively, also significantly better than those of Comparative Example 1. This performance advantage is mainly attributed to two aspects: First, the hard carbon anode of Example 3 has a lower charge transfer resistance (see Tables 1 and 2), which is conducive to the rapid migration of sodium ions; second, its excellent interfacial compatibility with ester electrolytes promotes the formation of a solid electrolyte interfacial film that is more conducive to the conduction and diffusion of sodium ions, thereby jointly improving the rate performance of the material.

[0118] Figure 2The cycling performance test results of the hard carbon anodes in Example 3 and Comparative Example 1 are presented. In ester electrolytes, after 1500 cycles at a current density of 1 A / g, the reversible specific capacity of the anode in Example 3 remained at 112 mAh / g, significantly higher than the 32 mAh / g of Comparative Example 1. In ether electrolytes, after 1500 cycles at a current density of 2 A / g, its capacity remained at 262 mAh / g, also significantly better than the 176 mAh / g of Comparative Example 1. This is mainly due to the high bonding strength of the composite binder, which ensures the structural stability of the anode during cycling, while the low content of hydroxyl and carboxyl functional groups effectively improves the interfacial compatibility between the electrode and the electrolyte, thus jointly achieving excellent cycling performance.

[0119] To more intuitively compare the morphological changes of the hard carbon anodes used in Comparative Example 1 and Example 3 after long cycling, an ester electrolyte system was selected, and the surface morphology of the electrode after 1500 cycles was observed using a scanning electron microscope (SEM). Figure 3 As shown in Figure a, due to significant side reactions between the binder used in Comparative Example 1 and the ester electrolyte, it was almost completely decomposed after 1500 cycles, resulting in the loss of its bonding function. This led to extensive exposure of the hard carbon active material and significant cracking of the electrode. Meanwhile... Figure 3 In example b, the binder corresponding to Example 3 showed weak side reactions with the electrolyte and could still adhere firmly to the hard carbon surface after cycling, maintaining effective coating of the active material, further confirming that its corresponding hard carbon anode has superior cycling performance.

[0120] 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 lignin-polyaniline composite adhesive, characterized in that, Includes the following steps: (1) Dissolve lignin in an alkaline aqueous solution, then add a carboxylating reagent and heat to react, to obtain a carboxymethyl lignin solution; (2) Adjust the carboxymethyl lignin solution from step (1) to acidity, add polyol, and heat to crosslink the reaction to obtain a carboxymethyl lignin polyol composite solution; (3) Cool the carboxymethyl lignin polyol composite solution from step (2) to 0-5°C, add aniline monomer, and carry out oxidative polymerization under the action of an initiator, and dry to obtain lignin polyaniline composite binder.

2. The preparation method according to claim 1, characterized in that, The amounts of each reactant, by weight, are as follows: 100 parts of lignin; 50-100 parts of carboxylating reagent; 40-80 parts of polyol; 10-30 parts of aniline monomer; Preferably, the amounts of each reactant are as follows, by weight: 100 parts of lignin; 80-100 parts of carboxylating reagent; 50-70 parts of polyol; 10-20 parts of aniline monomer.

3. The preparation method according to claim 1 or 2, characterized in that, The carboxylating agent in step (1) includes at least one of sodium monochloroacetate, sodium α-bromoacetate, sodium cyanoacetate, and sodium trifluoroacetate; And / or, the temperature of the heating reaction in step (1) is 70–90 °C; And / or, the heating reaction time in step (1) is 2 to 8 h.

4. The preparation method according to claim 1 or 2, characterized in that, The acid adjuster used in step (2) to adjust to acidity includes at least one of sulfuric acid solution, p-toluenesulfonic acid and hydrochloric acid solution, with a mass fraction of 4-8%; And / or, the polyol in step (2) is a polyol with a carbon chain length of 2 to 6, including at least one of glycerol, butylene glycol, ethylene glycol and sorbitol; And / or, the pH value adjusted to acidity in step (2) is 1 to 3; And / or, the temperature of the heating crosslinking reaction in step (2) is 60–90 °C; And / or, the heating crosslinking reaction in step (2) takes 2 to 4 hours.

5. The preparation method according to claim 1 or 2, characterized in that, The initiator in step (3) includes one of ammonium persulfate and potassium persulfate; And / or, the mass ratio of aniline monomer to initiator in step (3) is 10:1 to 20:1; And / or, the oxidative polymerization time in step (3) is 12 to 24 h.

6. The preparation method according to claim 1 or 2, characterized in that, The lignin in step (1) includes at least one of alkali lignin, enzymatically hydrolyzed lignin, and organic solvent lignin; And / or, the phenolic hydroxyl content of the lignin in step (1) is 2.0 to 4.0 mmol / g; And / or, the pH value of the alkaline aqueous solution in step (1) is 10 to 12.

7. The preparation method according to claim 1 or 2, characterized in that, The alkaline aqueous solution in step (1) includes at least one of sodium hydroxide solution and potassium hydroxide solution; And / or, the lignin in step (1) has a mass fraction of 20-40% in an alkaline aqueous solution; And / or, the drying in step (3) is spray drying, and the drying temperature is 180 to 220°C.

8. The preparation method according to claim 1 or 2, characterized in that, Includes the following steps: (1) Add 5-10% alkaline regulator to water to adjust the pH to 10-12; then, dissolve lignin in the alkaline aqueous solution to prepare a lignin solution with a mass fraction of 20-40%, and react it with a carboxylating agent at 70-90℃ for 2-8 h to obtain a carboxymethyl lignin solution. (2) Add 4-8% acidic regulator to carboxymethyl lignin solution to adjust pH to 1-3, add polyol, and heat at 60-90℃ for 2-4 h to crosslink the reaction to obtain carboxymethyl lignin polyol composite solution; (3) Cool the carboxymethyl lignin polyol composite solution to 0-5℃, then add aniline monomer, and oxidize and polymerize it with an initiator for 12-24 h. Spray dry the reaction solution at 180-220℃ to obtain lignin polyaniline composite binder.

9. A lignin-polyaniline composite adhesive obtained by the preparation method according to any one of claims 1 to 8.

10. The application of the lignin-polyaniline composite binder of claim 9 in the hard carbon anode of sodium-ion batteries.

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