Lignin-based three-dimensional crosslinking binder as well as preparation method and application thereof

Abstract

The invention provides a lignin-based three-dimensional crosslinking binder as well as a preparation method and application thereof, and belongs to the field of lithium ion batteries. The lignin-based three-dimensional cross-linking binder is obtained after polyacrylic acid and polycaprolactone graft modified lignin are subjected to an in-situ thermal esterification reaction, and the weight of the polycaprolactone graft modified lignin accounts for 10%-30% of the total weight of the polyacrylic acid and the polycaprolactone graft modified lignin. According to the invention, lignin is taken as a core node of a net structure, polyacrylic acid is taken as an adhesion chain segment, and polycaprolactone is taken as a functional module, so that a novel rigid network structure is constructed. According to the lignin-based three-dimensional cross-linking binder disclosed by the invention, the ionic conductivity of a silicon-based negative electrode and the rate capability of a battery under high current density are improved, and the electrochemical impedance is also reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically, to a lignin-based three-dimensional crosslinking binder, its preparation method, and its application. Background Technology

[0002] With the rapid development of the global new energy vehicle industry, the market's performance requirements for power batteries are constantly increasing, especially the demand for high-energy-density lithium-ion batteries. Although lithium-ion batteries have made significant breakthroughs in energy storage, the theoretical specific capacity of traditional graphite anodes (approximately 372 mAh / g) is approaching its physical limit, making it difficult to meet the needs of next-generation high-energy-density batteries. Therefore, developing new high-performance electrode materials has become a key research direction. Silicon-based anodes have attracted much attention due to their extremely high theoretical specific capacity (approximately 4200 mAh / g, more than 10 times that of graphite) and are considered an ideal choice for next-generation high-energy-density lithium-ion batteries.

[0003] However, silicon-based materials exhibit significant volume expansion (up to 300%) during charge and discharge, leading to the pulverization of active materials, repeated rupture and regeneration of the solid electrolyte interface film, and consequently, rapid capacity decay and reduced cycle life. These technical bottlenecks severely restrict the commercial application of silicon-based anodes.

[0004] In electrode material systems, binders not only fix active materials and maintain electrode structural stability, but also affect the integrity of the conductive network and ion transport efficiency. Therefore, developing high-performance binders has become a crucial breakthrough in mitigating the volume expansion of silicon-based materials and improving battery cycle stability. Traditional binders (such as PVDF and CMC) are typically based on linear molecular structures, and their mechanical strength and adhesion are insufficient to adapt to the drastic volume changes of silicon-based materials during charge and discharge, leading to electrode structure damage and performance degradation. To overcome this challenge, researchers are dedicated to developing novel multifunctional binders, mainly including the following categories: 1. Three-dimensional cross-linked binders: enhancing mechanical strength and effectively buffering volume expansion by constructing cross-linked network structures; 2. Self-healing binders: achieving self-repair of damaged interfaces using dynamic chemical bonds (such as hydrogen bonds and reversible covalent bonds); 3. Conductive binders: combining adhesion and conductivity, reducing the amount of conductive agent used and optimizing charge transport efficiency. Through the synergistic effect of multifunctional binders, it is expected that while maintaining high energy density, the cycle stability and rate performance of silicon-based anodes can be significantly improved, thereby promoting their practical application in high-performance lithium-ion batteries.

[0005] The aromatic ring structure of lignin interacts with conductive agents, and its semi-rigid three-dimensional framework effectively suppresses the sulfur shuttle effect and volume expansion, greatly improving the cycle stability of lithium-sulfur batteries and maintaining good performance even with ultra-low binder dosages. Lignin, with its multifunctional properties and natural three-dimensional framework structure, not only shows potential in traditional materials fields but also demonstrates broad application prospects in energy storage and novel battery technologies. Therefore, lignin holds promise for the more comprehensive development of materials suitable for binders of various electrodes in lithium-ion batteries.

[0006] Existing technology discloses a lignin derivative composite binder and its preparation method and application in silicon-based anode sheets. Specifically, it involves reacting lignin grafted with polyacrylic acid and polyethylene glycol in a solvent under heat to obtain the lignin derivative composite binder. This invention addresses the problem of low functional group reactivity in lignin due to cohesive forces by functionalizing lignin through a cross-linked network. Simultaneously, the continuous ether bonds on the polyethylene glycol also facilitate the directional transport of lithium ions, positively impacting the rate performance of SiO electrodes and overcoming lithium ion transport barriers caused by thickness issues at high loading rates. Therefore, it improves the cycle performance and rate performance of the electrode material. However, the binder offers limited improvement in rate performance at high current densities, and the electrochemical impedance remains relatively high. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the defects of existing silicon-based anode binders for lithium-ion batteries, such as high electrochemical impedance and limited improvement in rate performance at high current density. The invention provides a lignin-based three-dimensional crosslinking binder that not only improves the ionic conductivity of silicon-based anodes and significantly improves the rate performance of batteries, but also reduces electrochemical impedance.

[0008] Another objective of this invention is to provide a lithium-ion battery negative electrode sheet.

[0009] Another object of the present invention is to provide a lithium-ion battery.

[0010] The above-mentioned objective of the present invention is achieved through the following technical solution: This invention provides a lignin-based three-dimensional crosslinking adhesive, which is obtained by thermal esterification of lignin grafted with polyacrylic acid and polycaprolactone; wherein the weight of the polycaprolactone grafted with lignin accounts for 10% to 30% of the total weight of the polyacrylic acid and polycaprolactone grafted with lignin.

[0011] This invention uses polycaprolactone-grafted modified lignin to composite with polyacrylic acid, and after thermal esterification, a lignin-based three-dimensional crosslinked binder with a three-dimensional crosslinked network is obtained.

[0012] The lignin-based three-dimensional crosslinking adhesive of this invention uses lignin as the core node of the network structure, polyacrylic acid as the adhesive segment, and polycaprolactone as the functional module to construct a novel network structure with rigidity. The introduction of polycaprolactone segments not only improves the reactivity of lignin but also significantly enhances the cationic diffusion performance of the adhesive. The thermal esterification reaction effectively enhances the crosslinking strength of the entire network, thereby improving the mechanical stability of the lignin-based three-dimensional crosslinking adhesive.

[0013] Preferably, the weight of the polycaprolactone-grafted modified lignin accounts for 10%, 20%, or 30% of the total weight of the polyacrylic acid and polycaprolactone-grafted modified lignin. The weight percentage of polycaprolactone-grafted modified lignin affects the performance of the binder.

[0014] Specifically, the grafting rate of polycaprolactone in the polycaprolactone-grafted modified lignin is 70-80%, and the total grafting amount is approximately 26560-30354 g / mol.

[0015] Specifically, the method for producing polyacrylic acid includes the following steps: adding an initiator to an aqueous acrylic acid solution; and stirring at high speed for 3-5 hours at 60-80°C under an inert atmosphere to obtain high-viscosity polyacrylic acid.

[0016] Specifically, the molecular weight distribution range of the polyacrylic acid is 3000~5000.

[0017] Specifically, the initiator is one of ammonium persulfate or potassium persulfate.

[0018] Specifically, the acrylic acid aqueous solution contains 25-40 wt% acrylic acid, preferably 33.3 wt%.

[0019] Specifically, the inert gas is selected from nitrogen or argon.

[0020] Specifically, the polycaprolactone-grafted modified lignin is obtained by ring-opening polymerization of cyclic ester monomers and lignin, the reaction temperature is 80~150℃, the reaction time is 6~24 hours, and the mass ratio of cyclic ester monomers to lignin is 0.5~5:1.

[0021] Specifically, the preparation method of the polycaprolactone-grafted modified lignin includes the following steps: Cyclic ester monomers and catalysts are activated at 90°C for 20-40 minutes, followed by the addition of lignin; under an inert gas atmosphere, the reaction is carried out at 80-150°C for 6-24 hours to obtain polycaprolactone-grafted modified lignin.

[0022] Preferably, the cyclic ester monomer is one of ε-caprolactone, γ-butyrolactone, δ-valerolactone, β-propiolactone, and maleolactone.

[0023] Preferably, the catalyst is one of 2,6-diphenylhydroxybenzoic acid, trifluoromethanesulfonic acid, diphenyl phosphate, citric acid, methanesulfonic acid, stannous octoate, and dibutyltin dilaurate.

[0024] Preferably, the selected lignin is alkali lignin or enzymatically hydrolyzed lignin.

[0025] Preferably, the mass ratio of the cyclic ester monomer to lignin is 1 to 3:1; specifically, the mass ratio of the cyclic ester monomer to lignin is 1:1, 1.7:1, 2:1, or 3:1.

[0026] Specifically, the inert gas is nitrogen or argon.

[0027] Preferably, the thermal esterification reaction is an in-situ thermal esterification reaction. Specifically, the in-situ thermal esterification includes: uniformly coating a mixture of polyacrylic acid, polycaprolactone-grafted lignin, silicon-based negative electrode active material, and conductive agent onto the negative electrode current collector, followed by a thermal esterification reaction.

[0028] Mixing binder components such as polyacrylic acid and polycaprolactone grafted lignin with the negative electrode slurry to be bonded, followed by a thermal esterification reaction, can better coat the negative electrode material and enhance the structural strength of the adhesive network.

[0029] Specifically, the temperature of the thermal esterification reaction is 100~200℃.

[0030] Specifically, the thermal esterification reaction takes 1 to 3 hours.

[0031] This invention also protects the application of the above-mentioned lignin-based three-dimensional crosslinking binder in the preparation of silicon-based anode sheets.

[0032] This invention also protects a lithium-ion battery negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer on the surface of the negative electrode current collector; The preparation of the negative electrode active material layer includes the following steps: uniformly mixing polyacrylic acid, polycaprolactone-grafted modified lignin, silicon-based negative electrode active material and conductive agent, coating it on the surface of the negative electrode current collector, and carrying out a thermal esterification reaction.

[0033] Specifically, the silicon-based anode active material is at least one of SiO, Si, or Si / C composite.

[0034] Specifically, the conductive agent is selected from conductive carbon black or carbon nanotubes.

[0035] Specifically, the mass ratio of the silicon-based negative electrode active material, conductive agent, and binder is 6~8:1~3:0.5~2.

[0036] The present invention also provides a lithium-ion battery, comprising a positive electrode, a battery separator, an electrolyte, and a negative electrode, wherein the negative electrode is the aforementioned lithium-ion battery negative electrode.

[0037] Specifically, the positive electrode includes at least one of elemental lithium, lithium iron phosphate, and NCM.

[0038] Specifically, the electrolyte includes lithium salts and carbonate organic solvents, wherein the lithium salts include, but are not limited to, LiPF6, and the carbonate solvents include, but are not limited to, at least one of fluoroethylene carbonate, vinylene carbonate, ethylene carbonate, and diethyl carbonate.

[0039] Specifically, the battery separator includes at least one of glass fiber separator and PP separator.

[0040] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a lignin-based three-dimensional crosslinking binder, obtained by thermal esterification of lignin grafted with polyacrylic acid and polycaprolactone. The introduction of polycaprolactone segments improves the ionic conductivity of silicon-based anode materials, significantly enhancing rate performance at high current densities and reducing electrochemical impedance. In cycle testing, electrodes using this lignin-based three-dimensional crosslinking binder exhibit longer cycle life, maintaining good capacity retention even at high current densities. Attached Figure Description

[0041] Figure 1 0.5A g -1 Cyclic diagrams of battery electrodes prepared with different binders at different current densities.

[0042] Figure 2 Rate capability diagram of batteries prepared with different binders (1C=1000mAh g) -1 ).

[0043] Figure 3 Electrochemical impedance spectroscopy (EIS) diagrams of batteries prepared with different binders after 5 cycles. Detailed Implementation

[0044] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0045] Example 1 A lignin-based three-dimensional crosslinking adhesive is prepared by the following method: Preparation of polyacrylic acid (PAA): The acrylic acid monomer was purified by column chromatography to remove polymerization inhibitors. 50 g of purified acrylic acid and 100 g of deionized water were placed in a three-necked flask, and 0.25 g of persulfate (APS) was added as a thermal initiator. The reaction was carried out under a nitrogen atmosphere at 60 °C with high-speed stirring for 3 h to obtain high-viscosity polyacrylic acid. The product was cut into small pieces, washed repeatedly with water, and freeze-dried to obtain the polyacrylic acid polymer. The molecular weight distribution of the obtained polyacrylic acid polymer ranged from 3000 to 5000.

[0046] Preparation of polycaprolactone-modified alkali lignin (PCL-g-AL): In a three-necked flask, ε-caprolactone and the catalyst dibutyltin dilaurate were activated at 90 °C for 30 min, with the catalyst amount being 0.06–0.12 g. Subsequently, lignin was added to the three-necked flask, with 5.14 g of ε-caprolactone and 3 g of lignin. The reaction was carried out at 100 °C for 24 hours under a nitrogen atmosphere, and then the reaction was terminated by adding triethylamine (0.1 g). The reaction mixture was then transferred to methanol at -20 °C, with the volume of methanol being 10 times that of ε-caprolactone to promote precipitation of the product. Subsequently, the product was centrifuged at 5000 rpm for 10 min to separate the polycaprolactone-modified alkali lignin. The product was washed with ice-cold methanol to remove residual ε-caprolactone and polycaprolactone. Finally, the product was vacuum dried at 40 °C for 24 hours to obtain polycaprolactone-modified alkali lignin. According to the gel permeation chromatography (GPC) data, the total grafting amount was 28415 g / mol and the grafting rate was 74.89%.

[0047] Preparation of lithium-ion batteries: Polyacrylic acid and polycaprolactone-grafted modified alkali lignin were dissolved in 1,4-dioxane, and a binder solution was prepared at a ratio of 9:1. SiO and conductive carbon black were mixed at a mass ratio of 7:2, and a suitable mass was weighed into an agate ball mill jar, with the material volume not exceeding 1 / 2 of the jar. The SiO and conductive carbon black mixture was ball milled at 400 r / min for 10 h. A slurry was prepared by mixing active material, conductive agent, and binder at a mass ratio of 7:2:1. After the slurry was thoroughly mixed for 5-6 h, it was coated with a doctor blade to obtain an active material loading of 1.0 mg / cm³. -2The wetted electrode was placed in a 60 °C forced-air drying oven for 1-2 h to remove most of the solvent, followed by heat treatment in a 150 °C vacuum drying oven for 2 h to complete the thermal esterification step. The dried electrode sheet was then cut into 12 mm diameter electrode sheets using a slicer. Polypropylene was used as the battery separator, and a mixed electrolyte of ethylene carbonate:diethyl carbonate containing 1 mol / L LiPF6, 10 wt% fluoroethylene carbonate, and 1 wt% vinylene carbonate was used as the battery electrolyte, wherein the volume ratio of ethylene carbonate to diethyl carbonate was 1:1. Coin half-cells were assembled in an argon-filled glove box.

[0048] Example 2 The preparation of polycaprolactone-grafted modified alkali lignin is the same as in Example 1, except that: polyacrylic acid and polycaprolactone-grafted modified alkali lignin are dissolved in 1,4-dioxane and a binder solution is prepared in a ratio of 8:2.

[0049] Example 3 The preparation of polycaprolactone-grafted modified alkali lignin is the same as in Example 1, except that polyacrylic acid and polycaprolactone-grafted modified alkali lignin are dissolved in 1,4-dioxane and a binder solution is prepared in a 7:3 ratio.

[0050] Example 4 The preparation of polycaprolactone-grafted modified alkali lignin is the same as in Example 1, except that the mass ratio of cyclic ester monomers to lignin is 4:1.

[0051] Comparative Example 1 The preparation method of polyacrylic acid is the same as that of polyacrylic acid in Example 1. Each 0.2 g of polyacrylic acid is dissolved in 5 mL of 1,4-dioxane to prepare an adhesive solution.

[0052] Result detection Testing methods: The lithium-ion batteries prepared in the above examples and comparative examples were subjected to the following performance tests: (1) Constant current charge-discharge test: The constant current charge-discharge test records the battery's cycle performance and rate performance data under a constant current density. The test uses the Xinwei charge-discharge test system to test the battery's cycle performance and rate performance. The battery test conditions are a constant temperature of 25 ℃. The battery needs to be left to stand for 10 h before cycling and activated once with a small current before the constant current charge-discharge test is performed.

[0053] The lithium-ion batteries prepared in the above examples and comparative examples were subjected to a temperature of 0.5 A g. -1 Constant current charge-discharge tests were performed at current densities, and the test results are as follows: Figure 1 As shown. From Figure 1It can be seen that Example 1 has the best cycling performance, retaining 936.37 mAh g after 300 cycles. -1 The discharge specific capacity of the battery is typically reduced after multiple cycles due to material aging, expansion, or loss. However, the capacity retention rate of Example 1 reaches approximately 89.4% (initial capacity of 1049.98 mAhg). -1 This is a remarkably good performance for silicon-based anode batteries. Furthermore, due to... Figure 1 It can be seen that although the battery capacity decreases to some extent with the increase of cycle number, the rate of capacity decay is relatively slow, indicating a high cycle life. Generally speaking, silicon-based anodes are prone to structural damage and capacity decay during cycling due to volume expansion issues, but this battery exhibits relatively good stability. With the increase of polycaprolactone-grafted modified lignin, the cycle performance also shows a significant decrease, attributed to the decline in the mechanical properties of the lignin-based three-dimensional crosslinking binder. The low-rigidity lignin-based three-dimensional crosslinking binder adapts to the expansion of the active material and changes accordingly, resulting in the inability to suppress the volume effect of the silicon-based material, and also lengthening the lithium-ion transport channels, which has a significant adverse effect on the maintenance of capacity and life.

[0054] The rate performance test results of the examples and Comparative Example 1 are as follows: Figure 2 As shown. Figure 2 Example 1, at a current density of 2C, still has 973.31 mAh g⁻¹. -1 Its capacity is far higher than that of Comparative Example 1, which has a capacity of 439.53 mAh g. -1 The capacity. When the current density returned to 0.1C, Example 1 still had 1453.56 mAh g. -1 The capacity indicates that this electrode possesses excellent rate performance. The battery maintains good capacity even at high current densities, meaning it is stable not only during normal charge and discharge but also exhibits strong rate performance at higher rates. The battery capacity did not show drastic decay at high current densities, indicating that the conductive network and structural design effectively mitigate the effects of high-rate charge and discharge. In contrast, corresponding to previous results, the rate performance of the electrode decreased significantly with increasing amounts of polycaprolactone-grafted modified alkali lignin. The SiO electrode prepared using a binder without crosslinked lignin, lacking the three-dimensional framework support of lignin, struggled to maintain electrode integrity, leading to contact failure of the conductive network during rapidly changing rate tests. It is evident that lignin plays a role in improving rate performance; the addition of PCL segments not only enhances the reactivity of lignin but also improves the cationic diffusion performance of the lignin-based three-dimensional crosslinked binder, thus achieving higher and more stable rate performance.

[0055] The rate performance of the lignin-based three-dimensional crosslinked adhesive obtained in Example 4 was also significantly better than that of Comparative Example 1.

[0056] (2) Electrochemical impedance spectroscopy: Electrochemical impedance spectroscopy involves applying a sinusoidally varying cross-linking perturbation voltage to the electrode, causing the electrode voltage to change according to a sinusoidal wave pattern, thereby obtaining an impedance spectrum, and then studying the electrode kinetics and ion diffusion mechanism. The test was conducted using an electrochemical workstation, at room temperature, applying a sinusoidal AC voltage with an amplitude of 5 mV to the battery in the frequency range of 0.01 Hz to 100 kHz.

[0057] Electrochemical impedance spectroscopy was performed on the half-cells prepared with different binders in Example 1 and Comparative Example 1. The test results are as follows: Figure 3 As shown. By Figure 3 It can be seen that after 5 cycles, Example 1 has the lowest impedance, at 25.85Ω, compared to 63.47Ω in Comparative Example 1, indicating that the lignin-based three-dimensional crosslinking binder has a good wetting effect on the electrolyte. The slope of the low-to-mid-frequency region of Example 1 is higher than that of Comparative Example 1, indicating that the electrode can maintain its stability and the integrity of the conductive network during cycling, and a stable SEI is generated, resulting in better charge transfer. Example 1 exhibits a lower impedance value in the electrochemical impedance spectroscopy test, especially after cycling, the impedance remains at a low level, indicating that its electrode material has high ion transport efficiency during charge and discharge. This advantage is mainly attributed to the three-dimensional framework structure of lignin and the introduction of polycaprolactone segments, the latter not only improving cation diffusion performance but also promoting rapid lithium-ion conduction. The electrochemical impedance of the lignin-based three-dimensional crosslinking binders obtained in Examples 2-4 is also significantly better than that of Comparative Example 1.

[0058] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A lignin-based three-dimensional crosslinking adhesive, characterized in that, The lignin-based three-dimensional crosslinking adhesive is obtained by thermal esterification of polyacrylic acid and polycaprolactone grafted modified lignin, wherein the weight of the polycaprolactone grafted modified lignin accounts for 10% to 30% of the total weight of the polyacrylic acid and polycaprolactone grafted modified lignin. The polycaprolactone-grafted modified lignin is obtained by ring-opening polymerization of cyclic ester monomers and lignin. The ring-opening polymerization reaction is carried out at a temperature of 80~150℃ and for a time of 6~24 hours. The mass ratio of the cyclic ester monomers to lignin is 0.5~5:

1. The molecular weight distribution range of the polyacrylic acid is 3000~5000.

2. The lignin-based three-dimensional crosslinking adhesive as described in claim 1, characterized in that, The method for producing polyacrylic acid includes the following steps: adding an initiator to an aqueous acrylic acid solution; stirring and reacting at 60-80°C for 3-5 hours under an inert atmosphere to obtain polyacrylic acid, wherein the mass percentage of acrylic acid in the aqueous acrylic acid solution is 25-40 wt%.

3. The lignin-based three-dimensional crosslinking adhesive as described in claim 1, characterized in that, The thermal esterification reaction is an in-situ thermal esterification reaction.

4. The lignin-based three-dimensional crosslinking adhesive as described in claim 1, characterized in that, The temperature of the thermal esterification reaction is 100~200℃, and the time of the thermal esterification reaction is 1~3h.

5. The lignin-based three-dimensional crosslinking adhesive as described in claim 1, characterized in that, The preparation method of the polycaprolactone-grafted modified lignin includes the following steps: activating cyclic ester monomers and catalysts at 90°C for 20-40 minutes, followed by adding lignin; reacting at 80-150°C for 6-24 hours under an inert gas atmosphere to obtain polycaprolactone-modified lignin.

6. The lignin-based three-dimensional crosslinking adhesive as described in claim 5, characterized in that, The lignin is alkali lignin or enzymatically hydrolyzed lignin.

7. The lignin-based three-dimensional crosslinking adhesive as described in claim 5, characterized in that, The mass ratio of the cyclic ester monomer to lignin is 1~3:

1.

8. The application of the lignin-based three-dimensional crosslinking binder according to any one of claims 1 to 7 in silicon-based negative electrode sheets.

9. A lithium-ion battery negative electrode sheet, characterized in that, The negative electrode includes a negative electrode current collector and a negative electrode active material layer on the surface of the negative electrode current collector. The preparation of the negative electrode active material layer includes the following steps: The polyacrylic acid, polycaprolactone-grafted modified lignin, silicon-based negative electrode active material and conductive agent described in claim 1 are uniformly mixed and coated on the surface of the negative electrode current collector, and then subjected to a thermal esterification reaction.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the lithium-ion battery negative electrode as described in claim 9.

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