Binder for silicon negative electrode and preparation method thereof

Abstract

The invention relates to the technical field of batteries, in particular to a binder for a silicon negative electrode and a preparation method of the binder. The embodiment of the invention provides a preparation method of a binder for a silicon negative electrode, which comprises the following steps: preparing a premixed solution A: adding a boron-containing compound into an aqueous solution of a hydroxyl-containing compound, and stirring and mixing to form the premixed solution A; preparing a premixed solution B: dissolving a hard monomer and a soft monomer in water to form the premixed solution B; the premixed solution A and the premixed solution B are mixed to form a mixed solution C, an initiator is added to enable the mixed solution C to be subjected to a copolymerization reaction, and the binder is prepared through the copolymerization reaction of the mixed solution C. The binder prepared by the method has excellent elasticity, toughness, rigidity and mechanical properties.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a binder for silicon negative electrodes and a preparation method thereof. Background Art

[0002] Silicon is widely used as the negative electrode material for lithium-ion batteries due to its high theoretical capacity, good conductivity, and abundant resources. However, silicon undergoes drastic volume changes during lithium insertion, with a volume expansion rate of up to 300-400%.

[0003] In lithium-ion batteries, binders are a key component of electrode manufacturing, playing a vital role in bonding the active material (such as silicon) to the current collector to ensure electrical contact. Traditional binders, such as polyvinylidene fluoride (PVDF), struggle to provide sufficient adhesion to accommodate the cyclical volume changes of silicon due to their weak van der Waals interactions. Traditional linear binders, such as sodium alginate (SA) and styrene-butadiene rubber (SBR), are prone to molecular chain slippage and breakage due to mechanical stress concentration on the linear chains.

[0004] Based on this, providing a new binder for silicon negative electrode to solve the above defects is an urgent problem to be solved. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the purpose of the present invention is to provide a binder for silicon negative electrode and a preparation method thereof, wherein the binder has excellent mechanical properties and can significantly improve the cycle performance of secondary batteries using silicon material as negative electrode.

[0006] To achieve the above objectives, the first aspect of the present application provides a method for preparing a binder for a silicon negative electrode, comprising: Preparing a premix solution A: adding the boron-containing compound to the aqueous solution of the hydroxyl-containing compound, stirring and mixing, to form the premix solution A; Preparing a premix solution B: dissolving the hard monomer and the soft monomer in water to form the premix solution B; Copolymerization: The premix solution A and the premix solution B are mixed to form a mixed solution C, an initiator is added to cause the mixed solution C to undergo a copolymerization reaction, and the binder is prepared by the copolymerization reaction of the mixed solution C.

[0007] The binder obtained by the preparation method described in the present application is formed by random copolymerization of hydroxyl-containing compounds, boron-containing compounds, hard monomers and soft monomers, and has excellent elasticity, toughness, rigidity and mechanical properties. The binder has a three-dimensional cross-linked network structure, and the borate bond has a strong bond strength, which can maintain the stability of the network structure of the binder. In addition, hydroxyl-containing compounds, boron-containing compounds, hard monomers and soft monomers contain rich polar groups and various hydrogen bonds. The intermolecular forces formed between different hydrogen bonds are different. When the silicon negative electrode expands, the different intermolecular forces can gradedly consume the stress generated by the silicon expansion, thereby reducing the damage to the electrode interface caused by the silicon expansion, having good adhesion properties, and being able to effectively maintain the surface morphology of the electrode plate; thereby reducing the interfacial impedance of the battery and improving the cycle performance of the battery.

[0008] In some embodiments, the boron-containing compound is selected from one or more of sodium tetraborate and sodium metaborate.

[0009] In some embodiments, the hard monomer is selected from one or more of methyl methacrylate, styrene, acrylonitrile, acrylic acid, acrylamide, and N-vinyl pyrrolidone.

[0010] In some embodiments, the soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, butadiene, isoprene, ethylene, and siloxane monomers.

[0011] In some embodiments, the mass ratio of the hard monomer to the soft monomer satisfies: (10-15): (7:2).

[0012] In some embodiments, the initiator is selected from one or more of ammonium persulfate, dibenzoyl peroxide, and dicumyl peroxide.

[0013] In some embodiments, the hydroxyl-containing compound includes a first compound and a second compound, wherein the first compound is a natural polymer compound, and the second compound is a water-soluble polyphenol compound.

[0014] In some embodiments, the water-soluble polyphenol compound is selected from one or more of tannic acid, catechin, anthocyanin, dopamine, gallic acid, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, and 3,4-dihydroxyphenylpropionic acid.

[0015] In some embodiments, the natural polymer compound is selected from one or more of polysaccharide polymer compounds, protein polymer compounds, nucleic acids, lignin, and natural rubber.

[0016] In some embodiments, the polysaccharide polymer compound is selected from one or more of peach gum, cellulose, starch, chitosan, alginate, and hyaluronic acid.

[0017] In some embodiments, the proteinaceous polymer compound is selected from one or more of collagen, silk protein, and gelatin.

[0018] In some embodiments, the mass ratio of the first compound to the second compound satisfies: (1-10):1.

[0019] The second aspect of the present application provides a binder for a silicon negative electrode, which is prepared according to the preparation method of the first aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A flow chart of the preparation method for this application; Figure 2 Fourier transform infrared spectroscopy test of peach gum, tannic acid and binder 2; Figure 3 X-ray photoelectron spectroscopy test of binder 2; Figure 4 Differential scanning calorimetry test for binder 2; Figure 5 is the stress-strain test of adhesive 2; Figure 6 180° peel test of silicon electrode under the action of adhesive 2; Figure 7 Nanoindentation test of silicon electrode under the action of binder 2; Figure 8 Atomic force microscopy test of silicon electrode under the action of binder 2; Figure 9 This is a test of the cycling performance of the silicon electrode under the action of binder 2; Figure 10 The rate performance test of silicon electrode under the action of binder 2; Figure 11 Scanning electron microscopy test of silicon electrode before and after cycling under the action of binder 2. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0023] In the description of this application, the terms "(1)", "(2)", "first", and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, a feature specified as "(1)", "(2)", "first", and "second" may explicitly or implicitly include at least one of the features. In the description of this application, "plurality" means at least two, such as two, three, etc., unless otherwise specifically specified.

[0024] In the description of this application, unless otherwise indicated, the term "or" is inclusive. That is, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0025] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the weights described in the examples of this application may be mass units known in the chemical industry, such as μg, mg, g, and kg.

[0026] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0027] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0028] The following describes the implementation of the present application in detail.

[0029] First aspect A first aspect of the present application provides a method for preparing a binder for a silicon negative electrode, comprising: Preparing a premix solution A: adding the boron-containing compound to the aqueous solution of the hydroxyl-containing compound, stirring and mixing, to form the premix solution A; Preparing a premix solution B: dissolving the hard monomer and the soft monomer in water to form the premix solution B; Copolymerization: The premix solution A and the premix solution B are mixed to form a mixed solution C, an initiator is added to cause the mixed solution C to undergo a copolymerization reaction, and the binder is prepared by the copolymerization reaction of the mixed solution C.

[0030] The binder obtained by the preparation method described in the present application is formed by random copolymerization of hydroxyl-containing compounds, boron-containing compounds, hard monomers and soft monomers, and has excellent elasticity, toughness, rigidity and mechanical properties. The binder has a three-dimensional cross-linked network structure, and the borate bond has a strong bond strength, which can maintain the stability of the network structure of the binder. In addition, hydroxyl-containing compounds, boron-containing compounds, hard monomers and soft monomers contain rich polar groups and various hydrogen bonds. The intermolecular forces formed between different hydrogen bonds are different. When the silicon negative electrode expands, the different intermolecular forces can gradedly consume the stress generated by the silicon expansion, thereby reducing the damage to the electrode interface caused by the silicon expansion, having good adhesion properties, and being able to effectively maintain the surface morphology of the electrode plate; thereby reducing the interfacial impedance of the battery and improving the cycle performance of the battery.

[0031] In some embodiments, the boron-containing compound is selected from one or more of sodium tetraborate (borax) and sodium metaborate. Sodium tetraborate and sodium metaborate are readily soluble in water and can form an electron-deficient structure [B(OH)4]⁻ (tetrahydroxyborate ion) in aqueous solution, which can complex with hydroxyl groups to form a stable coordination.

[0032] Preferably, the boron-containing compound is selected from sodium tetraborate.

[0033] In some embodiments, the molar ratio of the hydroxyl groups in the boron-containing compound to the hydroxyl-containing compound satisfies the following: 1:(2-4). When the molar number of hydroxyl groups in the hydroxyl-containing compound cannot be accurately calculated, the amount of the boron-containing compound may be determined by mass ratio. For example, the mass ratio of the boron-containing compound to the hydroxyl-containing compound satisfies the following: 1:(6-10). Excessive amounts of boron-containing compounds can cause the copolymerized binder network to become brittle, excessively cross-linked, and lack flexibility. Excessive amounts of hydroxyl compounds can result in insufficient cross-linking points during the copolymerization process, resulting in an overly soft network and poor mechanical properties.

[0034] In some embodiments, the premix solution A is stirred at a temperature of 50-100° C. Controlling the stirring temperature of the premix solution within the above temperature range is beneficial to accelerating the reaction kinetics, promoting the dehydration reaction, and facilitating the migration of hydroxyl groups to combine with tetraborate to form crosslinking points.

[0035] In some embodiments, the hard monomer is selected from one or more of methyl methacrylate, styrene, acrylonitrile, acrylic acid, acrylamide, and N-vinyl pyrrolidone. The polymer formed from the hard monomer has a high glass transition temperature and strong rigidity, which helps to increase the rigidity of the adhesive and further improve the mechanical properties of the adhesive.

[0036] Preferably, the hard monomer is selected from acrylic acid and acrylamide. The interfacial wettability and ionic carboxyl and amide groups provided by acrylic acid (AA) and acrylamide (AM) contribute to the formation of a uniform solid electrolyte interface (SEI) film. Ethyl acrylate (EA) also provides flexible links and polar groups, which help form a reversible hydrogen bonding network and enhance energy dissipation after the electrode is broken and stretched.

[0037] Furthermore, the mass concentration of the hard monomer is in the range of 0.1-0.2 g / ml. If the mass concentration of the hard monomer is too high, the reaction rate per unit volume will increase dramatically, requiring a high stirring rate and cooling in the reaction environment. This may also trigger localized reaction implosion, resulting in a broadened molecular weight distribution of the final product and even the formation of a gelled crosslinker structure. If the mass concentration of the hard monomer is too low, the free radicals generated by the decomposition of the initiator may react with the solvent or impurities, resulting in reduced initiation efficiency and prolonged reaction time.

[0038] In some embodiments, the soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, butadiene, isoprene, ethylene, and siloxane monomers. The polymer formed from the soft monomer has a low glass transition temperature and high toughness, which helps improve the flexibility and elasticity of the adhesive and further enhances the mechanical properties of the adhesive.

[0039] Preferably, the soft monomer is selected from ethyl acrylate.

[0040] Furthermore, the mass concentration of the soft monomer is in the range of 0.01-0.1 g / ml.

[0041] In some embodiments, the premixed solution B includes a hard monomer and a soft monomer, wherein the mass ratio of the hard monomer to the soft monomer satisfies the following range: (10-15): (7:2). The applicant has experimentally discovered that when the mass ratio of the hard monomer to the soft monomer meets the above range, a system structure with both "strength and toughness" can be achieved in the cross-linked polymer network. The hard monomer provides a rigid skeleton, improving modulus and strength, while the soft monomer provides flexible segments, which generate segment slip and energy dissipation under stress, enhancing ductility and crack resistance.

[0042] In some embodiments, the initiator is selected from one or more of ammonium persulfate, dibenzoyl peroxide, and dicumyl peroxide. The hydroxyl-containing compound, the hard monomer, and the soft monomer can undergo copolymerization under the action of the initiator.

[0043] In some embodiments, the copolymerization step further includes adding a pH regulator to adjust the pH of the mixed solution C to a range of (4-6). Adjusting the pH of the solution C to a reasonable range facilitates the copolymerization reaction, avoids excessive chain dissociation during the polymerization process, helps maintain stable borate bonds, and facilitates the formation of a uniform ternary network structure of the binder.

[0044] In some embodiments, the reaction temperature of the copolymerization step is between 50°C and 80°C. Preferably, the reaction temperature is 70°C. As can be understood, initiators decompose more actively at temperatures between 50°C and 80°C, generating more effective free radicals and achieving higher conversion rates. This not only increases the participation of soft monomers (low Tg) in the copolymerization process and improves copolymerization uniformity, but also reduces system viscosity, enhancing polymer chain mobility and preventing the generation of side reactions at high temperatures (e.g., 100°C).

[0045] In some embodiments, the hydroxyl-containing compound includes a first compound and a second compound.

[0046] In some embodiments, the first compound is a natural polymer compound. In this application, a natural polymer compound refers to a polymer compound having a molecular weight greater than 10 4 The first compound is a natural polymer compound with a large molecular weight, which is easy to obtain and has a high marketability, which is conducive to reducing the cost of industrial production. In addition, the polymer compound has a long molecular chain and there are interactions between the chains. Its hydroxyl groups can provide abundant reaction sites. The product formed by the copolymerization reaction retains the long-chain skeleton of the natural polymer and also introduces flexibility, hardness, and borate ester through the copolymerization reaction. The synthesized copolymer has good flexibility, rigidity, viscosity and processability.

[0047] In some embodiments, the natural polymer compound is selected from one or more of polysaccharide polymer compounds, protein polymer compounds, nucleic acids, lignin, and natural rubber.

[0048] Furthermore, the polysaccharide polymer compound is selected from one or more of peach gum, cellulose, starch, chitosan, alginate, and hyaluronic acid, wherein peach gum, cellulose, starch, chitosan, alginate, and hyaluronic acid all contain hydroxyl groups; peach gum, alginate, and hyaluronic acid also contain carboxyl groups.

[0049] Preferably, the polysaccharide polymer compound is selected from one or more of peach gum, alginate, and hyaluronic acid.

[0050] It can be understood that peach gum, alginate, and hyaluronic acid contain not only hydroxyl groups but also carboxyl groups. Hydroxyl groups and carboxyl groups, as different polar groups, are very easy to form intermolecular hydrogen bond interactions with active substances (such as silicon, silicon oxide, silicon carbon, silicon carbon / graphite).

[0051] More preferably, the polysaccharide polymer compound is selected from peach gum. The hydroxyl groups in peach gum can not only serve as functional side group donors for copolymerization with hard monomers and soft monomers, but can also complex with boron-containing compounds to form stable ligands.

[0052] Furthermore, the proteinaceous polymer compound is selected from one or more of collagen, silk protein, and gelatin. It is understandable that collagen, silk protein, and gelatin contain hydroxyl groups and carboxyl groups.

[0053] As you can understand, nucleic acids primarily refer to ribose / deoxyribose (DNA / RNA). The C3 and C5 positions of ribose (RNA) or deoxyribose (DNA) are hydroxyl groups. This means nucleic acids are natural polymers containing hydroxyl groups. Lignin is a polymer of phenylpropane units, which contains hydroxyl groups at both phenolic and alcoholic hydroxyl groups.

[0054] Natural rubber often introduces trace amounts of hydroxyl or carboxyl groups during its preparation process, and is widely used in industry and inexpensive. Therefore, natural rubber can also be used as the main chain compound for the synthesis of adhesives.

[0055] In some embodiments, the second compound is a water-soluble polyphenol compound. Water-soluble polyphenol compounds are rich in hydroxyl groups, which can serve as one of the reactants of borate esters, providing available hydroxyl groups for the formation of borate esters. In addition, the hydroxyl groups in polyphenol compounds exist in a variety of forms, for example, they can exist in the form of phenol. These hydroxyl groups can be physically cross-linked with the first compound, hard monomer, and soft monomer, which is beneficial to enhance the interaction force between molecules and help maintain the surface morphology of the electrode; they can also form multiple weak hydrogen bond interactions with the polar groups in the first compound, hard monomer, and soft monomer, thereby reducing the glass transition temperature of the copolymer product and increasing the toughness of the binder system.

[0056] In some embodiments, the water-soluble polyphenol compound is selected from one or more of tannic acid, catechin, anthocyanidin, dopamine, gallic acid, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, and 3,4-dihydroxyphenylpropionic acid. The water-soluble polyphenol compounds include catechol and pyrogallol. The hydroxyl groups in catechol and pyrogallol can form multiple weak hydrogen bond interactions with polar groups (e.g., carboxyl, amino, and carboxylate) in the copolymer system, weakening the strong hydrogen bonds between the copolymer molecules and reducing the strong intermolecular interactions, thereby lowering the glass transition temperature of the adhesive and improving the fluidity, viscosity, and toughness of the bond.

[0057] The water-soluble polyphenol compound is selected from one or more of tannic acid, catechin, anthocyanin, dopamine, gallic acid, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, and 3,4-dihydroxyphenylpropionic acid. The water-soluble polyphenol compound can provide more polar groups and improve the adhesion of the binder network.

[0058] Preferably, the water-soluble polyphenol compound is selected from tannic acid.

[0059] In some embodiments, the mass ratio of the first compound to the second compound satisfies the following ratio: (1-10):1. Preferably, the mass ratio of the first compound to the second compound satisfies the following ratio: (5-10):1. Further preferably, the mass ratio of the first compound to the second compound is 5:1. It is understood that the first compound is a polymer compound, and the first compound is selected as the main chain of the copolymerization product, which has a multi-point branching structure. This structure facilitates multi-point cross-linking, thereby enhancing the dimensional stability and moldability of the three-dimensional network system of its components, thereby helping to improve the strength of the system.

[0060] The beneficial effects of the present application are further illustrated below with reference to the examples.

[0061] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application. Example 1

[0062] Prepare premix solution A: Take 2.5g of peach gum powder, dissolve it in 39g of deionized water, and stir until it is completely dissolved to prepare a peach gum aqueous solution with a mass fraction of 6wt%; 0.5 g of sodium tetraborate decahydrate was added to the 6 wt % peach gum aqueous solution and stirred at 70° C. for 30 minutes to form a premixed solution A.

[0063] Prepare premixed solution B: 12 g of acrylic acid monomer, 4 g of ethyl acrylate monomer, and 1 g of acrylamide monomer were dissolved in 20 g of deionized water and stirred at 20° C. for 30 minutes to form a premixed solution B.

[0064] Copolymerization: Premix solution B was added to premix solution A to form mixed solution C. 0.4 g of potassium persulfate was added to mixed solution C, and the pH of mixed solution C was adjusted to 5.5 with a 5 wt% lithium hydroxide solution. The mixture was then mechanically stirred at 70°C for 4 hours. Binder 1 was obtained by centrifugation, filtration, and drying.

[0065] Preparation of button cells The negative electrode slurry was prepared with a mass ratio of nano-silicon: binder B1: conductive agent (super P) of 6:2:2. The slurry was coated on copper foil, vacuum-dried at 70°C, and cut into circular electrodes with a diameter of 14 mm for use as the negative electrode; a metallic lithium sheet was used as the positive electrode, and polypropylene was used as the separator; 1M LiPF6 was dissolved in EC:DEC (50:50 vol / vol) and 10vol% FEC was used as an additive as the electrolyte, and a button cell (CR2032) was assembled under an inert atmosphere. This button cell is denoted as B1. Example 2

[0066] Prepare premix solution A: Take 2.5g of peach gum powder and 0.5g of tannic acid powder, dissolve them in 47g of deionized water, and stir until completely dissolved to prepare a peach gum-tannic acid aqueous solution with a mass fraction of 6wt%; 0.5 g of sodium tetraborate decahydrate was added to the 6 wt % peach gum-tannic acid aqueous solution, and stirred at 70° C. for 30 minutes to form a premixed solution A.

[0067] Prepare premixed solution B: 12 g of acrylic acid monomer, 4 g of ethyl acrylate monomer, and 1 g of acrylamide monomer were dissolved in 20 g of deionized water and stirred at 20° C. for 30 minutes to form a premixed solution B.

[0068] Copolymerization: Premixed solution B was added to premixed solution A to form mixed solution C. 0.4 g of potassium persulfate was added to mixed solution C, and the pH of mixed solution C was adjusted to 5.5 with a 5 wt% lithium hydroxide solution. The mixture was then mechanically stirred at 70°C for 4 hours. Binder 2 was obtained by centrifugation, filtration, and drying. Accordingly, this button cell was labeled B2. Example 3

[0069] The difference from Example 2 is that the first compound is alginate. Example 4

[0070] The difference from Example 2 is that the second compound is catechin. Example 5

[0071] The difference from Example 2 is that the hard monomer is acrylonitrile. Example 6

[0072] The difference from Example 2 is that the soft monomer is butyl acrylate. Example 7

[0073] The difference from Example 2 is that the amount of peach gum powder is 0.5 g and the amount of tannic acid powder is 0.5 g. Example 8

[0074] The difference from Example 2 is that the peach gum powder is 5.0 g and the tannic acid powder is 0.5 g. Comparative Example 1

[0075] Prepare premix solution A: Take 2.5g of peach gum powder, dissolve it in 39g of deionized water, and stir until it is completely dissolved to prepare a peach gum aqueous solution with a mass fraction of 6wt%; 0.5 g of sodium tetraborate decahydrate was added to the 6 wt % peach gum aqueous solution and stirred at 70° C. for 30 minutes to form a premixed solution A.

[0076] Copolymerization of premixed solution A: 0.4 g of potassium persulfate was added to the premixed solution A, and the pH of the premixed solution A was adjusted to 5.5 with a 5 wt% lithium hydroxide solution. The solution was then mechanically stirred at 70°C for 4 hours. Binder D1 was obtained by centrifugation, filtration, and drying. The secondary battery prepared in this manner is labeled D1. Comparative Example 2

[0077] The same method as in Comparative Example 1 was used to obtain a binder Da.

[0078] Copolymerization of Premix Solution B: Premix Solution B was prepared using the same method as in Example 1. 0.4 g of potassium persulfate was added to the premix Solution B, and the pH of the premix Solution B was adjusted to 5.5 using a 5 wt% lithium hydroxide solution. Mechanical stirring was then performed at 70°C for 4 hours. Binder Db was obtained by centrifugation, filtration, and drying.

[0079] The binder Da and the binder Db were mixed at a ratio of 1:1 to obtain a binder d2 of Comparative Example 2. Accordingly, the secondary battery was marked as D2. Comparative Example 3

[0080] Premix solution A and premix solution B were prepared in the same manner as in Example 1. 0.4 g of potassium persulfate was added to the premixed solution B, and mechanical stirring was performed at 70°C for 4 hours. Then, premixed solution A was added, and mechanical stirring was continued at 70°C for 4 hours. Binder D3 was obtained after centrifugation, filtration, and drying. Accordingly, the secondary battery was labeled D3.

[0081] Secondary batteries made with the binders prepared using the methods of Examples 1-8 and Comparative Examples 1-3 were labeled B1-8 and D1-3, respectively. Electrochemical performance tests were performed on the secondary batteries, and the test results are shown in Table 1.

[0082] The electrochemical performance test method is as follows: First cycle charge and discharge test: The first cycle charge and discharge test was performed at a current density of 0.2A / g and a test voltage range of 0.01~1.5V.

[0083] Constant current charge and discharge test: Long cycle performance test is carried out at a current density of 1A / g, with a test voltage range of 0.01~1.5V; 50 cycles.

[0084] EIS AC impedance test: Perform AC impedance test in the frequency range of 0.01Hz-100kHz.

[0085] Table 1 Electrochemical performance of Examples 1-8 and Comparative Examples 1-3 / First discharge capacity (mAh / g) Capacity retention rate after 50 cycles (%) <![CDATA[R ct First lap (Ω)]]> <![CDATA[R ct / after 50 cycle(Ω)]]> B1 3120 80.2 79 180.6 B2 3450 92.5 65 79.6 B3 3213 85.0 85 100.3 B4 3197 84.6 78 115.6 B5 3302 76.3 75 123.8 B6 3208 79.6 82 155.5 B7 2897 82.0 76 135.9 B8 2988 75.2 78 175.0 D1 2700 60.1 93 280.8 D2 3091 75.6 85 220.5 D3 2895 70.8 88 250.6 It can be seen from the above table that when the hydroxyl-containing compound contains two different types of compounds, the cycle performance of the prepared adhesion is better. Furthermore, when the ratio of the first compound to the second compound is 5:1, the first-cycle discharge capacity, cycle stability and interface transfer impedance performance are optimal.

[0086] The first-cycle discharge specific capacity of Comparative Example 1 is poor, which is mainly due to the lack of the polymer skeleton's coordinated stress distribution and volume expansion relief effect; the lack of interfacial wettability and ion affinity provided by the carboxyl groups and amides in acrylic acid (AA) and acrylamide (AM), resulting in uneven SEI interface formation; and the lack of chemical energy-consuming chain links, such as the flexible links and polar groups provided by ethyl acrylate (EA), which help to form a reversible hydrogen bond network and improve energy dissipation after fracture and stretching.

[0087] The first-cycle coulombic efficiency and interface transfer impedance of Comparative Example 2 are poor, which is mainly due to the lack of a molecular-level chemical cross-linking network. After separate preparation and physical mixing, the two components are only physically entangled or have limited compatibility, and lack "synergistic chemical cross-linking points"; phase separation leads to discontinuous transmission paths, and microscopic phase separation is prone to occur during the electrode drying process. The diffusion impedances in the two component regions are different, forming an uneven ion migration path and increasing the interface impedance; the mechanical support is dispersed and stress cannot be evenly transmitted. The two components that exist separately cannot form an interpenetrating network, and there is a lack of overall synergistic hierarchical energy dissipation.

[0088] The capacity retention rate and interface transfer impedance of Comparative Example 3 are poor, which is mainly due to the lack of in-situ copolymerization or uniform co-crosslinking. Random copolymerization of poly (acrylic acid-acrylamide-ethyl acrylate) is carried out first, and peach gum-boron-tannic acid is mostly filled in the existing polymer matrix. Unlike Example 2, it forms a dynamic hydrogen bond network or borate ester bond with peach gum-boron-tannic acid through carboxyl, hydroxyl, amide, etc., and the interface binding energy decreases; the network nodes are uneven, the ion-electron coupling transmission is limited, and peach gum-boron-tannic acid is easily aggregated in local areas, resulting in localized electron / ion transmission and increased interface transfer impedance; in the "brittle matrix + soft filling" mode, when subjected to stress, peach gum-boron-tannic acid is restricted to yield and the main chain is insufficient to extend, and fails during the cycle. The volume effect causes the SEI film to rupture frequently, and the capacity retention rate is low.

[0089] Furthermore, further performance tests were conducted on Example 2, and the test methods and results are as follows.

[0090] Infrared spectrum test: peach gum, tannic acid, and binder 2 powder were used as samples, and the transmittance of the samples was tested at 25 degrees Celsius in the range of 400-4000cm-1. Figure 2 The infrared spectrum test pattern of adhesive 2 is shown. Figure 2 It can be seen that the -OH peak position and -C=O peak position of the binder are blue-shifted, which indicates that there are different intermolecular hydrogen bonds in the binder; -1 A new characteristic peak appears at , indicating the presence of BOC, which indicates the presence of borate structure in the binder.

[0091] XPS test: Using the powder of binder 2 as the sample, the full spectrum and fine spectrum of C1s, O1s, and B1s were tested at 25 degrees Celsius. Figure 3 The XPS analysis structure of binder 2 is shown. Figure 3 From the fitting results, it can be seen that ~192.1eV is the peak of BOC, ~192.8eV is the peak of BOB, and ~193.6eV is the peak of B-OH; this can further illustrate the presence of borate structure in the binder.

[0092] DSC test: The powder of binder 2 was used as the sample, the test temperature range was 0-200 degrees Celsius, and the heating rate was 10 degrees Celsius / min. Figure 4 The differential scanning calorimetry test of binder 2 is shown. The test results show that the glass transition temperature of binder 2 is 31.21°C, indicating that the molecular chain of binder 2 has good flexibility.

[0093] Stress-strain test: Taking the film after drying of adhesive 2 as an example, the test was carried out at 25 degrees Celsius and a tensile speed of 30 mm / min. Figure 5 The stress-strain analysis of Adhesive 2 is shown. It can be seen that the maximum strain of Adhesive 2 is 775% and the maximum stress is 5.52 MPa, demonstrating its excellent mechanical properties.

[0094] 180° peel test: The electrode sample is coated with a mixture of nano-silicon, Super P, and adhesive 2 on copper foil and then vacuum dried. The peeling is performed at 25 degrees Celsius and a peeling speed of 10 mm / min. Figure 6 The 180° peel test of the silicon electrode under the action of Adhesive 2 is shown. It can be seen that the average peel force of Adhesive 2 is 7.23N, demonstrating its excellent adhesion ability.

[0095] Nanoindentation test: The load-displacement curve was tested at 25°C with a maximum load of 1.2mN using a vacuum-dried electrode coated with a mixture of nanosilicon, Super P, and binder 2 on copper foil. Figure 7 The nanoindentation test of the silicon electrode under the action of Binder 2 is shown. It can be seen that under a maximum load of 1.2mN, Binder 2 is indented to a maximum depth of 2742nm, and the elastic recovery rate is 77.68%, demonstrating its excellent toughness and elastic recovery capabilities.

[0096] Atomic force microscopy testing: Atomic force microscopy testing was performed on the silicon electrodes under the action of the binder before and after cycling. Figure 8The atomic force microscopy test of the silicon electrode under the action of binder 2 is shown. It can be seen that the roughness of the silicon electrode under the action of the binder before cycling is 177nm, and the roughness of the silicon electrode after cycling is 370nm, demonstrating its excellent ability to maintain the electrode surface morphology.

[0097] Cyclic performance test: At 25 degrees Celsius, the silicon electrode under the action of binder 2 was subjected to a cyclic performance test in the voltage range of 0.01~1.0V and a current density of 2A / g. Figure 9 The cycling performance test of the silicon electrode under the action of binder 2 was demonstrated. The test results showed that after 500 cycles at a current density of 2A / g, the discharge capacity was 1823mAh / g, the capacity retention rate was 90.2%, and the unit capacity decay rate was 0.0208%, demonstrating its excellent cycling stability.

[0098] Rate performance test: At 25 degrees Celsius, the silicon electrode under the action of binder 2 was tested for rate performance in the voltage range of 0.01~1.0V and current densities of 0.5A / g, 1A / g, 2A / g, 4A / g, and 8A / g. Figure 10 The rate performance test of the silicon electrode under the action of binder 2 is demonstrated. The test results show that the discharge capacity is 1357.1 mAh / g at a current density of 8 A / g, demonstrating its excellent rate performance.

[0099] Scanning electron microscope test: At 25 degrees Celsius, the surface morphology of the silicon electrode under the action of the binder was observed before and after 100 cycles. Figure 11 Scanning electron microscopy (SEM) tests of a silicon electrode before and after cycling with Binder 2 were performed. The test results show that after 100 cycles, the silicon electrode surface is virtually crack-free, demonstrating that its combination of rigidity and flexibility effectively maintains the electrode's surface morphology.

[0100] In summary, it can be seen that the binder obtained by the preparation method of the present application has excellent elasticity, toughness and rigidity, as well as excellent adhesion, and can effectively maintain the electrode surface morphology; the secondary battery prepared therefrom has excellent rate performance and cycle stability.

[0101] It should be noted that although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a binder for a silicon negative electrode, comprising: Preparing a premix solution A: adding the boron-containing compound to the aqueous solution of the hydroxyl-containing compound, stirring and mixing, to form the premix solution A; Preparing a premix solution B: dissolving the hard monomer and the soft monomer in water to form the premix solution B; Copolymerization: The premix solution A and the premix solution B are mixed to form a mixed solution C, an initiator is added to cause the mixed solution C to undergo a copolymerization reaction, and the binder is prepared by the copolymerization reaction of the mixed solution C.

2. The method for preparing the adhesive according to claim 1, wherein: The boron-containing compound is selected from one or more of sodium tetraborate and sodium metaborate.

3. The method for preparing the adhesive according to claim 1, wherein: The hard monomer is selected from one or more of methyl methacrylate, styrene, acrylonitrile, acrylic acid, acrylamide, and N-vinyl pyrrolidone; and / or The soft monomer is selected from one or more of butyl acrylate, ethyl acrylate, butadiene, isoprene, ethylene, and siloxane monomers.

4. The method for preparing the adhesive according to claim 3, wherein: The mass ratio of the hard monomer to the soft monomer satisfies: (10-15): (7:2).

5. The method for preparing the adhesive according to claim 1, wherein: The initiator is selected from one or more of ammonium persulfate, dibenzoyl peroxide, and dicumyl peroxide.

6. The method for preparing the adhesive according to claim 1, wherein: The hydroxyl-containing compound includes a first compound and a second compound, wherein the first compound is a natural polymer compound, and the second compound is a water-soluble polyphenol compound.

7. The method for preparing the adhesive according to claim 6, characterized in that: The water-soluble polyphenol compound is selected from one or more of tannic acid, catechin, anthocyanin, dopamine, gallic acid, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyphenylacetic acid, and 3,4-dihydroxyphenylpropionic acid.

8. The method for preparing the adhesive according to claim 6, wherein: The natural polymer compound is selected from one or more of polysaccharide polymer compounds, protein polymer compounds, nucleic acids, lignin, and natural rubber; and / or The polysaccharide polymer compound is selected from one or more of peach gum, cellulose, starch, chitosan, alginate, and hyaluronic acid; and / or The protein macromolecular compound is selected from one or more of collagen, silk protein and gelatin.

9. The method for preparing the adhesive according to claim 6, wherein: The mass ratio of the first compound to the second compound satisfies: (1-10):

1.

10. A binder for a silicon negative electrode, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 9.

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