A fluorine-free adhesive of silk fibroin cross-linked polyacrylic acid and a preparation method and application thereof

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a silk fibroin cross-linked polyacrylic acid fluorine-free binder as well as a preparation method and application thereof, the method comprising the following steps: dissolving polyacrylic acid in deionized water to prepare a polyacrylic acid colloidal solution with a mass fraction of 3-10%; dissolving silk fibroin in deionized water to prepare a silk fibroin solution with a mass fraction of 3-10%; mixing the polyacrylic acid colloidal solution and the silk fibroin solution uniformly according to the proportion that the silk fibroin accounts for 10-50% of the total mass of the polyacrylic acid and the silk fibroin to obtain a mixed colloidal solution; adding an ethanol solution into the mixed colloidal solution to convert the silk fibroin in a disordered state into an ordered state, and then performing heating and drying to obtain the silk fibroin cross-linked polyacrylic acid fluorine-free binder. The fluorine-free binder can effectively enhance the adhesion and mechanical properties of the binder, and can solve the problems of poor electrochemical performance and short cycle life of the silicon negative electrode caused by the poor adhesion and mechanical properties of the binder.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a fluorine-free binder of silk fibroin cross-linked polyacrylic acid, and a preparation method and application thereof. Background Art

[0002] The graphite anode material currently used is close to 372mAh g -1 The specific capacity of silicon anode materials is far from meeting the demand for high-performance lithium-ion batteries in 5G high-power electronic products. Silicon anode materials have a theoretical specific capacity that is more than 10 times that of graphite anode materials and have become the most promising alternative material to meet this demand. However, when silicon is used as anode material in lithium-ion batteries, it still faces significant volume expansion changes (>300%) during cycling, leading to structural fragmentation and pulverization, poor long-term cycle stability, and poor conductivity that restricts ion diffusion, which in turn limits the excellent performance of lithium-ion batteries. The use of traditional binders cannot solve this large volume expansion problem, making it difficult to fully utilize the high capacity advantages of silicon anode materials.

[0003] Binders are essential components of electrodes, primarily adhering active materials and conductive carbon black to the current collector. The forces acting between them generally include van der Waals forces, hydrogen bonds, electrostatic interactions, and covalent bonds. Silk fibroin can transform from conformation I (amorphous and α-helical structure) to conformation II (β-pleated structure) under different external environmental conditions (such as pH, temperature, and solvent type). The formation of the β-pleated structure significantly enhances cohesion due to the increased density of internal hydrogen bonds and the strengthening of intermolecular interactions. The rational use of silk fibroin to prepare binders with good adhesion and cohesion is crucial for improving and enhancing the electrochemical performance of batteries and is currently a research focus. Summary of the Invention

[0004] The purpose of the present invention is to address the shortcomings of the existing technology, such as the inability of traditional binders to adapt to the large volume expansion of silicon negative electrodes during cycling, which leads to pulverization and shedding of the electrodes and poor electrochemical performance of the battery. The present invention provides a binder with enhanced cohesion, which can alleviate phenomena such as electrode structure rupture and thus improve the electrochemical performance of the battery.

[0005] To achieve the above object, the present invention provides a fluorine-free binder of silk fibroin cross-linked polyacrylic acid, which comprises the following steps:

[0006] S1. dissolving polyacrylic acid in deionized water to prepare a polyacrylic acid colloidal solution with a mass fraction of 3-10%;

[0007] S2. dissolving silk fibroin in deionized water to prepare a silk fibroin solution with a mass fraction of 3-10%;

[0008] S3, mixing the polyacrylic acid colloidal solution of step S1 and the silk fibroin solution of step S2 uniformly according to 10-50% of silk fibroin in total mass of polyacrylic acid and silk fibroin, to obtain a mixed colloidal solution;

[0009] S4, adding an ethanol solution to the mixed colloidal solution of step S3 for transformation of silk fibroin from a disordered state to an ordered state, and then drying by heating to obtain a fluorine-free adhesive of silk fibroin cross-linked polyacrylic acid.

[0010] As a further preferred technical solution of the present application, the resistivity of the deionized water is 18.2 MΩ.m.

[0011] As a further preferred technical solution of the present application, the mass fraction of the ethanol solution is 1-5%.

[0012] As a further preferred technical solution of the present application, the temperature of the heating drying is 100-150°C.

[0013] As a further preferred technical solution of the present application, in step S3, the mixing is uniform by stirring, and the stirring time is at least 30 minutes.

[0014] As a further preferred technical solution of the present application, the molecular weight (Mn) of the polyacrylic acid is 420-460 kDa, and the molecular weight (Mn) of the silk fibroin is 6-10 kDa.

[0015] As a further preferred technical solution of the present application, in step S3, the silk fibroin accounts for 20-40% of the total mass of polyacrylic acid and silk fibroin.

[0016] According to another aspect of the present application, the present application also provides a fluorine-free adhesive prepared by the above method.

[0017] According to another aspect of the present application, the present application also provides a fluorine-free adhesive as an adhesive for preparing a silicon negative electrode. In the application, silicon nanoparticles are used as active materials, Super P is used as a conductive additive, the fluorine-free adhesive and the active materials and the conductive additive are mixed in water to form a slurry, which is coated on a conductive foil, and then dried and cut to obtain a silicon negative electrode. The silicon negative electrode can be applied to a lithium ion battery.

[0018] The present application uses silk fibroin as a cross-linking agent, utilizes the rich polar groups contained in silk fibroin to interact with the carboxyl groups of polyacrylic acid through non-covalent hydrogen bonds, and cooperates with the in-situ conformational transition characteristics of silk fibroin to construct a binder network structure with enhanced cohesion. The fluorine-free adhesive of the present application can well maintain the structural stability of the electrode by enhancing the cohesion of the binder, thereby fully exerting the characteristics of high capacity of the silicon negative electrode.

[0019] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0020] 1) The preparation method of the fluorine-free binder of the present invention is simple, the materials are readily available, and it is convenient for large-scale industrial production.

[0021] 2) The fluorine-free binder of the present invention is applied to the silicon negative electrode of lithium ion batteries to effectively enhance the adhesive properties and mechanical properties of the binder, which leads to poor electrochemical performance and short cycle life of the silicon negative electrode. The button half-cell prepared by the binder of the present invention has a good performance at 0.1C (1C = 4200mAh g -1 ) for 2 cycles and then cycled at 0.2C for 200 cycles, the specific capacity was 1900 mAh g -1 , with excellent cycle stability, while the capacity of Si / PAA prepared by traditional technology using polyacrylic acid (PAA) as silicon negative electrode binder is only maintained at 700mAh / g after 190 cycles at 0.84A / g, which shows that the improvement effect of the present invention is significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Figure 1 Infrared spectra of untreated silk fibroin (pre-SF) and pure silk fibroin treated at different temperatures.

[0024] Figure 2 This is the infrared spectrum of the combination of silk fibroin and polyacrylic acid, which verifies the existence of a large number of hydrogen bond interactions between the two.

[0025] Figure 3 Peel test of electrodes prepared with fluorine-free adhesives containing different ratios of silk fibroin and polyacrylic acid;

[0026] Figure 4 This is a cycling performance diagram of a silicon negative electrode made with a fluorine-free adhesive (prepared in Example 1) in which silk fibroin is combined with polyacrylic acid.

[0027] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0028] The following describes the specific embodiments of the present invention in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0029] Unless otherwise defined, the technical terms used in the following examples have the same meanings as commonly understood by those skilled in the art to which this invention belongs. The experimental reagents used in the following examples, unless otherwise specified, are conventional biochemical reagents; the experimental methods described, unless otherwise specified, are conventional methods.

[0030] Example 1

[0031] (1) 0.7 g of polyacrylic acid (PAA, Mn≈450 kDa) and 0.3 g of silk fibroin (SF, Mn≈6-10 kDa) were dissolved in 13.3 g and 9.7 g of deionized water (the resistivity of deionized water is 18.2 megohm.m), respectively.

[0032] (2) The two solutions in step (1) are mixed and stirred thoroughly to obtain a mixed colloidal solution.

[0033] (3) 0.6 g of 5% ethanol solution was added dropwise to the mixed colloidal solution, and then the mixture was kept at 140° C. for 12 h in a vacuum drying oven to obtain a fluorine-free adhesive with enhanced cohesion.

[0034] Example 2

[0035] (1) 0.9 g of polyacrylic acid and 0.1 g of silk fibroin were dissolved in 17.1 g and 3.23 g of deionized water (the resistivity of deionized water is 18.2 megohm.m), respectively.

[0036] (2) The two solutions in step (1) are mixed and stirred thoroughly to obtain a mixed colloidal solution.

[0037] (3) 0.45 g of ethanol solution (5%) was added dropwise to the mixed colloidal solution, and then kept in a vacuum drying oven at 140° C. for 12 h to obtain an adhesive with enhanced cohesion for characterization testing.

[0038] Example 3

[0039] (1) 0.3 g of polyacrylic acid and 0.7 g of silk fibroin were dissolved in 5.7 g and 22.63 g of deionized water (the resistivity of deionized water is 18.2 megohm.m), respectively.

[0040] (2) The two solutions in step (1) are mixed and stirred thoroughly to obtain a mixed colloidal solution.

[0041] (3) 0.85 g of ethanol solution (5%) was added dropwise to the mixed colloidal solution, and then kept in a vacuum drying oven at 140° C. for 12 h to obtain an adhesive with enhanced cohesion for characterization testing.

[0042] Comparative Example 1

[0043] As a control experiment for Example 1, a conventional adhesive was prepared. Specifically, 1 g of polyacrylic acid was dissolved in 19 g of deionized water (the resistivity of deionized water was 18.2 megohm·m) to prepare a fluorine-free adhesive.

[0044] Comparative Example 2

[0045] As a control experiment of Example 1, the only difference is that polyacrylic acid and silk fibroin are dissolved in deionized water at the same time to prepare a mixed colloidal solution. The remaining steps and material amounts are consistent with Example 1.

[0046] In Comparative Example 2, polyacrylic acid and silk fibroin were dissolved in deionized water at the same time, resulting in agglomeration. The bonding performance of the resulting adhesive was significantly worse than that of Example 1.

[0047] The products of the above embodiments and comparative examples were tested.

[0048] The conformational transition of silk fibroin under temperature and the cross-linking of silk fibroin with polyacrylic acid were studied by infrared spectroscopy. Figure 1 As shown in the infrared spectra of untreated silk fibroin (pre-SF) and silk fibroin treated at different temperatures, the C=O peak increases from 1645 cm -1 (1640-1650cm -1 corresponding to disordered structure) continuously redshifted to 1625 cm -1 (1620-1640cm -1 corresponding to the β-sheet structure).

[0049] like Figure 2 As shown in the figure, after cross-linking silk fibroin with polyacrylic acid, the C=O peak belonging to the polyacrylic acid domain shifted from 1700 cm -1 Offset to 1697cm -1 OH peak (~3200cm -1 ) and -NH peak (~3280cm -1 ) becomes wider and flatter (e.g. Figure 2 Example 1, the weight ratio of silk fibroin is 30%). These phenomena indicate that there are a large number of hydrogen bonds between silk fibroin and polyacrylic acid. It is worth noting that when the silk fibroin content is too low (Example 2, the weight ratio of silk fibroin is 10%) or too high (Example 3, the weight ratio of silk fibroin is 70%), only the 1697 cm -1 (polyacrylic acid domain) or 1633cm -1(Silk fibroin domain) at the main C = O peak. After a large number of experiments, the optimal range is 20-60% of the total mass of polyacrylic acid and silk fibroin, and the best effect is achieved when the silk fibroin content is 30%. Among them, when the silk fibroin content is moderate (30%), the infrared spectrum of Example 1 prepared at 1697cm -1 (polyacrylic acid domain) and 1634cm -1 Preliminary observations show that Example 1 achieves a balance between the adhesive strength of polyacrylic acid and the cohesive strength of silk fibroin.

[0050] In order to further demonstrate the beneficial technical effects of the present invention, the fluorine-free binders of Examples 1-3 above were used as binders for silicon negative electrodes of lithium-ion batteries, and Comparative Example 1 was used as a control group. The preparation method of the silicon negative electrode is as follows:

[0051] Silicon nanoparticles were used as the active material, Super P as the conductive additive, and the fluorine-free binder from Examples 1-3 as the binder. The mass ratio of active material, conductive additive, and binder was 8:1:1. Water was used as the solvent to form a uniform slurry, which was then applied to copper foil. The electrode was then dried in a vacuum drying oven at 140°C for 12 hours and cut into small discs with a diameter of 10 mm to serve as the silicon negative electrode.

[0052] A button-type lithium-ion battery was assembled using 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 10% fluoroethylene carbonate (FEC) and 1% vinylene carbonate (VC) as the electrolyte, lithium sheets as symmetrical electrodes, porous polypropylene (Celgard 2500) as the separator, and CR 2032 stainless steel as the battery case.

[0053] Because SF undergoes a conformational transition (from a disordered structure to a β-structure) upon heating, the dense structure within the β-structure forms strong hydrogen bonds with the carboxyl groups in PAA, enhancing cohesion. Optimal adhesion and mechanical strength cannot be achieved with either too little or too much SF (SF content of 10% or 70%). Too little SF fails to provide improved mechanical strength, while too much SF significantly reduces adhesion. However, the right amount of SF combined with PAA achieves optimal adhesion and mechanical strength. Figure 3 The peel test can prove the above point.

[0054] Cyclic performance test such as Figure 4 The Si / PAA electrode prepared with PAA as binder in Comparative Example 1 exhibited a capacity of 700 mAh / g after 190 cycles at 0.84 A / g, and the electrode prepared with PAA-SF-30 in Example 1 exhibited a capacity of 700 mAh / g after 190 cycles at 0.84 A / g. -1After 190 cycles, the specific capacity was 1900 mAh / g. The cycling performance of Examples 2 and 3 was better than that of Comparative Example 1, but worse than that of Example 1, indicating that optimizing the silk fibroin / polyacrylic acid ratio (the weight ratio of silk fibroin was 30%) can achieve optimal cycling performance. Therefore, the present invention preferably comprises 20-40% of the total weight of polyacrylic acid and silk fibroin, and more preferably 30%.

[0055] To explore the specific effect of the SF / PAA ratio on performance, based on the preparation method of Example 1, only the SF / PAA ratio was changed to prepare the binder. Table 1 then lists the electrochemical properties of electrodes prepared with different binder ratios. The comparison further illustrates the importance of the SF / PAA ratio.

[0056] Table 1

[0057]

[0058] Although specific embodiments of the present invention are described above, those skilled in the art should understand that these are merely examples and that various changes or modifications may be made to the embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is limited only by the appended claims.

Claims

1. A method for preparing a fluorine-free binder of silk fibroin cross-linked polyacrylic acid, characterized in that: The following steps are involved: S1. dissolving polyacrylic acid in deionized water to prepare a polyacrylic acid colloidal solution with a mass fraction of 3-10%; S2. dissolving silk fibroin in deionized water to prepare a silk fibroin solution with a mass fraction of 3-10%; S3, mixing the polyacrylic acid colloidal solution of step S1 and the silk fibroin solution of step S2 uniformly so as to obtain a mixed colloidal solution, with the silk fibroin accounting for 10-50% of the total mass of the polyacrylic acid and the silk fibroin; S4. Adding ethanol solution to the mixed colloidal solution of step S3 to transform the disordered silk fibroin into an ordered state, and then heating and drying to obtain a fluorine-free binder of silk fibroin cross-linked polyacrylic acid.

2. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: The resistivity of the deionized water is 18.2 MΩ·m.

3. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: The mass fraction of the ethanol solution is 1-5%.

4. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: The temperature of the heating and drying is 100-150°C.

5. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: In step S3, the mixture is mixed by stirring for at least 30 minutes.

6. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: The molecular weight of the polyacrylic acid is 420-460 kDa, and the molecular weight of the silk fibroin is 6-10 kDa.

7. The method for preparing the fluorine-free binder of silk fibroin cross-linked polyacrylic acid according to claim 1, characterized in that: In step S3, the silk fibroin accounts for 20-40% of the total mass of the polyacrylic acid and the silk fibroin.

8. A fluorine-free adhesive, characterized in that The method according to any one of claims 1 to 7 is used for preparation.

9. Use of the fluorine-free binder according to claim 8 as a binder in preparing a silicon negative electrode.

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