Core-shell structure binder for silicon negative electrode of lithium battery as well as preparation method and application of core-shell structure binder

Abstract

The invention relates to a core-shell structure binder for a silicon negative electrode of a lithium battery and a preparation method and application of the core-shell structure binder, emulsion particles of the binder have a core-shell structure and comprise a core layer and a shell layer coating the core layer, a core layer monomer of the core layer comprises methyl methacrylate, butyl acrylate and methacrylic acid, and a core layer monomer of the shell layer comprises methyl methacrylate, butyl acrylate and methacrylic acid. Shell layer monomers of the shell layer comprise methyl methacrylate, butyl acrylate, methacrylic acid and vinyl versatate. The preparation method comprises the following steps: step 1, preparing a core layer seed emulsion; step 2, shell layer polymerization; and step 3, post-processing. The application refers to application of the binder in preparation of a silicon negative electrode of a lithium battery or preparation of the lithium battery. The binder disclosed by the invention can effectively buffer the volume change of silicon by virtue of synergistic mechanical properties and excellent interface characteristics, improve the electrochemical performance, cycling stability and rate capability of the battery, maintain the integrity of an electrode and solve the problem of electrode failure caused by volume expansion of a silicon-based negative electrode in a cycling process.

Description

Technical Field

[0001] This invention relates to a core-shell structure binder for silicon anodes in lithium batteries, its preparation method, and its application, belonging to the field of lithium-ion battery technology. Background Technology

[0002] Lithium-ion batteries, as highly efficient energy storage devices, are widely used in electric vehicles, portable electronic products, and energy storage systems. Currently, commercially available anode materials are mainly graphite, which has a limited theoretical specific capacity (372 mAh g / g). - ¹), making it difficult to meet the ever-increasing demand for high energy density. Silicon-based materials (such as silicon-carbon composites and silicon-oxygen materials) have extremely high theoretical specific capacities (up to 4200 mAh g⁻¹). - ¹) Silicon has become a research hotspot for next-generation anodes. However, silicon undergoes volume changes of up to 300% during charging and discharging, leading to electrode structure rupture, active material shedding, and continuous reconstruction of the solid electrolyte interphase (SEI) film, resulting in rapid capacity decay and shortened cycle life. Binders, as key auxiliary materials in electrodes, play a crucial role in maintaining electrode structural integrity and interfacial stability. Currently widely used traditional water-based binders such as styrene-butadiene rubber (SBR) emulsions and carboxymethyl cellulose (CMC) offer advantages such as environmental friendliness and low cost, but their insufficient mechanical strength and toughness make them unable to effectively buffer the volume expansion of silicon materials. Furthermore, small-molecule emulsifier residues in SBR are prone to migration or demulsification during electrode processing, affecting electrode uniformity and interfacial stability. While water-soluble binders such as sodium alginate (SA) have strong adhesive power, their high molecular chain rigidity and brittleness make them unable to adapt to the repeated expansion and contraction behavior of silicon particles. Summary of the Invention

[0003] To overcome the above-mentioned defects of the prior art, the present invention provides a core-shell structure binder for silicon anodes of lithium batteries, its preparation method and application. The synergistic mechanical properties and excellent interfacial characteristics of the binder can effectively buffer the volume change of silicon and improve the cycle life and rate performance of the battery.

[0004] The technical solution adopted in this invention is: a core-shell structure binder for a silicon anode of a lithium battery, wherein the latex particles have a core-shell structure, including a core layer and a shell layer covering the core layer. The core layer monomers, by mass parts, include 80-90 parts of methyl methacrylate (MMA), 9-19 parts of butyl acrylate (BA), and 0.5-2 parts of methacrylic acid (MAA). The shell layer monomers, by mass parts, include 20-30 parts of methyl methacrylate (MMA), 68-78 parts of butyl acrylate (BA), 0.5-2 parts of methacrylic acid (MAA), and 0.5-2 parts of ethylene tert-carbonate (VV-10).

[0005] Preferably, the core layer monomers comprise, by mass parts, 85 parts methyl methacrylate, 14 parts butyl acrylate, and 1 part methacrylic acid.

[0006] Preferably, the shell monomers of the shell layer include 25 parts methyl methacrylate, 73 parts butyl acrylate, 1 part methacrylic acid, and 1 part ethylene tert-carbonate by mass.

[0007] Preferably, the glass transition temperature of the shell is lower than that of the core.

[0008] Preferably, the glass transition temperature of the core layer is 40℃~65℃, and the glass transition temperature of the shell layer is -35℃~-15℃.

[0009] Furthermore, the glass transition temperature of the core layer is 45°C, and the glass transition temperature of the shell layer is -24°C.

[0010] The preparation method of any of the core-shell structure binders for lithium battery silicon anodes disclosed in this invention includes the following steps: Step 1: Preparation of core layer seed emulsion: The premixed core layer monomers are pre-emulsified to obtain a core layer pre-emulsion. The core layer pre-emulsion is heated to the first polymerization temperature under nitrogen protection, and an initiator is added to react until a blue phase is observed, thus obtaining the core layer seed emulsion. Step 2: Shell polymerization: The premixed shell monomers are pre-emulsified to obtain a shell pre-emulsion. The shell pre-emulsion is then semi-continuously added to the core seed emulsion. The initiator is added in stages, and the temperature is raised to the second polymerization temperature for isothermal polymerization. Step 3: Post-processing: Cool and heat the polymerized emulsion, adjust the pH value, and filter to obtain the core-shell structure binder for the silicon anode of lithium battery.

[0011] Preferably, the first polymerization temperature is 75℃~80℃.

[0012] Preferably, the second polymerization temperature is 78°C.

[0013] Preferably, the initiator is ammonium persulfate (APS) with a purity ≥99.99%.

[0014] Preferably, the pre-emulsification treatment in step one and step two uses the same emulsifier, which is a compound of anionic and nonionic emulsifiers.

[0015] Preferably, the anionic emulsifier is SR-10 or DNS-86, and the nonionic emulsifier is TW-80 or RHODASURF6530.

[0016] Preferably, the emulsifier further includes a buffer, wherein the buffer is sodium bicarbonate (NaHCO3).

[0017] Preferably, in the process of semi-continuously adding the shell pre-emulsion to the core seed emulsion in step two, the mass of the core seed emulsion is 5% of the mass of the shell pre-emulsion.

[0018] Preferably, the dropwise addition time of the shell pre-emulsion is 2h to 4h.

[0019] Preferably, in step three, the pH value is adjusted to 8.0~8.5.

[0020] The application of any of the core-shell structure binders for lithium-ion battery silicon anodes disclosed in this invention, or the core-shell structure binders for lithium-ion battery silicon anodes prepared by any of the preparation methods of the core-shell structure binders for lithium-ion battery silicon anodes disclosed in this invention, in the preparation of lithium-ion battery silicon anodes or lithium-ion batteries.

[0021] The beneficial effects of this invention are: (1) The binder of the present invention achieves a gradient mechanical response with a soft shell and a hard core by controlling the mechanical properties and functional group distribution of the shell and core layers during the preparation process. The core layer is the rigid support skeleton of the binder, and the shell layer is the elastic buffer layer. Its synergistic mechanical properties and excellent interface characteristics can effectively buffer the volume change of silicon, improve the electrochemical performance, cycle stability and rate performance of the battery, maintain the integrity of the electrode, and solve the problem of electrode failure caused by volume expansion of silicon-based anodes during cycling. It is especially suitable for use in silicon-carbon anodes or pure silicon anodes.

[0022] (2) The binder of the present invention is prepared by semi-continuous seed emulsion polymerization, which has no small molecule emulsifier residue, reduces impurity ions, has excellent interface stability, and can effectively improve battery life.

[0023] (3) The adhesive of the present invention is a water-based adhesive, which is environmentally friendly and economical, has low production cost, and has significant industrial application value. Attached Figure Description

[0024] Figure 1 This is a flowchart of one embodiment of the preparation method of the core-shell structure binder for the silicon anode of the lithium battery of the present invention; Figure 2 This is a particle size distribution diagram of the adhesive in Embodiment 1 of the present invention. Detailed Implementation

[0025] This invention discloses a core-shell structure binder for silicon anodes in lithium batteries, specifically a soap-free emulsion binder (aqueous binder) for silicon-carbon anodes or pure silicon anodes. The latex particles of the binder have a core-shell structure, including a core layer and a shell layer covering the core layer. The core layer serves as a rigid support framework, and the shell layer serves as an elastic buffer layer, forming a gradient structure with a hard core and a soft shell (hard core, soft shell). The core layer is mainly copolymerized from core layer monomers (or hard monomers). The core layer monomers, by mass parts, include 80 to 90 parts of methyl methacrylate (e.g., 80, 85, or 90 parts), 9 to 19 parts of butyl acrylate (e.g., 9, 15, or 19 parts), and 0.5 to 2 parts of methacrylic acid (e.g., 0.5, 1, or 2 parts). The shell layer is mainly copolymerized from shell layer monomers (soft monomers and functional monomers). The shell layer monomers, by mass parts, include 20 to 30 parts of methyl methacrylate (e.g., 20, 25, or 30 parts), 68 to 78 parts of butyl acrylate (e.g., 68, 75, or 78 parts), 0.5 to 2 parts of methacrylic acid (e.g., 0.5, 1, or 2 parts), and 0.5 to 2 parts of ethylene tert-carbonate (e.g., 0.5, 1, or 2 parts, to enhance resistance to electrolyte swelling).

[0026] A preferred embodiment of the core layer comprises, by mass parts, 85 parts methyl methacrylate, 14 parts butyl acrylate, and 1 part methacrylic acid.

[0027] A preferred embodiment of the shell layer comprises, by mass parts, 25 parts methyl methacrylate, 73 parts butyl acrylate, 1 part methacrylic acid, and 1 part ethylene tert-carbonate.

[0028] The glass transition temperature of the shell layer is preferably lower than that of the core layer. The glass transition temperature of the core layer is typically 40°C to 65°C, for example, 40°C, 45°C, 50°C, 60°C or 65°C, preferably 45°C. The glass transition temperature of the shell layer is typically -35°C to -15°C, for example, -35°C, -30°C, -25°C, -20°C or -15°C, preferably -24°C.

[0029] See Figure 1 The present invention also discloses a method for preparing the core-shell structure binder of any of the lithium battery silicon anodes disclosed herein, which employs a semi-continuous seed emulsion polymerization method and includes the following steps: Step 1: Preparation of core layer seed emulsion: The premixed core layer monomers are pre-emulsified to obtain a core layer pre-emulsion. The core layer pre-emulsion is heated to the first polymerization temperature under nitrogen protection, and an initiator is added to react until a blue phase is observed, thus obtaining the core layer seed emulsion. Step 2: Shell polymerization: The premixed shell monomers are pre-emulsified to obtain a shell pre-emulsion. The shell pre-emulsion is then semi-continuously added to the core seed emulsion. The initiator is added in stages, and the temperature is raised to the second polymerization temperature for isothermal polymerization. Step 3: Post-processing: Cool and heat-preserve (age) the polymerized emulsion, adjust the pH value and filter to obtain the core-shell structure binder for the silicon anode of lithium battery.

[0030] The first polymerization temperature is preferably 75°C to 80°C, for example, 75°C, 78°C, or 80°C. The second polymerization temperature is preferably 78°C.

[0031] The initiator is preferably ammonium persulfate.

[0032] Preferably, the pre-emulsification treatment in step one and step two uses the same emulsifier, which is a compound of anionic and nonionic emulsifiers, which can improve the mechanical and chemical stability of the adhesive.

[0033] The anionic emulsifier is preferably SR-10 or DNS-86, and the nonionic emulsifier is preferably TW-80 or RHODASURF6530. The emulsifier may also include a buffer, preferably sodium bicarbonate.

[0034] In step two, during the semi-continuous addition of the shell pre-emulsion to the core seed emulsion, the mass of the core seed emulsion is preferably 5% of the mass of the shell pre-emulsion.

[0035] The preferred dripping time for the shell pre-emulsion is 2h to 4h, for example, 2h, 3h or 4h.

[0036] In step three, the pH value is preferably adjusted to 8.0~8.5, for example 8.0, 8.3 or 8.5.

[0037] In step three, a filter screen can be used to filter the emulsion after the pH value has been adjusted. The filter screen is preferably a 200-mesh filter screen.

[0038] This invention also discloses the application of any of the above-mentioned core-shell structure binders or the products obtained by their preparation methods in battery manufacturing, specifically including: For preparing a silicon anode for lithium batteries, the silicon anode comprises a silicon-based active material (e.g., nano-silicon or silicon-carbon composite material), a conductive agent (e.g., conductive carbon black or carbon nanotubes), the core-shell structure binder, a negative electrode current collector (e.g., copper foil), and a corresponding solvent (e.g., water or N-methylpyrrolidone). The method for preparing lithium batteries includes assembling a silicon anode containing the core-shell structure binder together with a positive electrode, a separator, and an electrolyte to form a lithium-ion battery.

[0039] Example 1: S1: Preparation of emulsifier solution and initiator solution: S101: Accurately weigh 0.3g NaHCO3, 1g SR-10, 0.017g RHODASURF6530 and mix with 49ml deionized water. Stir at room temperature until completely dissolved to obtain an emulsifier solution. S102: Accurately weigh 0.6g of ammonium persulfate (APS) and dissolve it in 10ml of deionized water to obtain an initiator solution.

[0040] Two portions of each emulsifier solution and initiator solution were prepared, one portion for preparing the core seed emulsion and the other portion for shell polymerization.

[0041] S2: Preparation of nuclear layer seed emulsion: S201: Premix 85g MMA, 14g BA and 1g MAA core layer monomers; S202: The premixed core layer monomer is added dropwise to the emulsifier solution through a constant pressure separatory funnel, maintaining a constant temperature of 45°C and stirring at 300 rpm, controlling the addition time to 3.5 h, and continuing to stir for 30 min after the addition is complete to obtain the core layer pre-emulsion. S203: Transfer the pre-emulsion of the core layer to a four-necked flask equipped with a constant pressure separatory funnel, a reflux condenser, a thermometer, and an electric stirrer. Under nitrogen protection, heat the core layer in the emulsion to 78°C, add the initiator solution in stages, and react until the emulsion turns blue. Cool in an ice-water bath to obtain the core layer seed emulsion.

[0042] S3: Shell aggregation: S301: Premix 25g MMA, 73g BA, 1g MAA and 1g VV-10 shell monomers; S302: The premixed shell monomer is added dropwise to the emulsifier solution through a constant pressure separatory funnel, maintaining a constant temperature of 45°C and stirring at 300 rpm, controlling the addition time to 30 min, and continuing to stir for 30 min after the addition is complete to obtain the shell pre-emulsion. S303: Under nitrogen protection, heat the kernel layer seed emulsion to 77°C; S304: Take the kernel seed emulsion (5% of the mass of the shell pre-emulsion) and place it in a flask. Add the shell pre-emulsion to the kernel seed emulsion dropwise at a constant rate using a peristaltic pump. The total dropwise addition time is 3 hours. A semi-continuous reaction method is adopted: pause for 15 minutes after every 1 hour of addition, and the total reaction time is 3.5 hours. The initiator solution addition strategy is as follows: add the initiator solution for the first time 10 minutes after the start of addition, and then add it every 10 minutes thereafter. All initiator solutions are added within 3.5 hours. After the dropwise addition is completed, raise the temperature to 78°C and keep it at that temperature for 1 hour.

[0043] S4: Post-processing: The reaction system was slowly cooled to 40°C, and the pH value was adjusted to 8.0 with ammonia while continuously stirring. The mixture was then filtered through a 200-mesh filter to obtain the core-shell structure binder for the silicon anode of lithium batteries, denoted as B1.

[0044] Example 2: The difference between this implementation and Example 1 is as follows: In step S101, SR-10 is replaced with DNS-86, and RHODASURF6530 is replaced with TW-80, while the dosage remains unchanged.

[0045] Apart from the differences mentioned above, the other preparation steps and methods are the same as in Example 1.

[0046] The core-shell structure binder for the prepared lithium battery silicon anode is denoted as B2.

[0047] Example 3: The difference between this embodiment and Embodiment 1 is that: In step S101, SR-10 is replaced with DNS-86, while the dosage remains unchanged.

[0048] Apart from the differences mentioned above, the other preparation steps and methods are the same as in Example 1.

[0049] The core-shell structure binder for the prepared lithium battery silicon anode is designated as B3.

[0050] Example 4: The difference between this embodiment and Embodiment 1 is that: In step S101, RHODASURF6530 is replaced with TW-80, while the dosage remains unchanged.

[0051] Apart from the differences mentioned above, the other preparation steps and methods are the same as in Example 1.

[0052] The core-shell structure binder for the prepared lithium battery silicon anode is designated as B4.

[0053] Example 5: The difference between this embodiment and Embodiment 1 is that: In step S301, the amount of BA is changed to 72.5g and the amount of VV-10 is changed to 1.5g (the mass percentage of VV-10 is 1.5%).

[0054] Apart from the differences mentioned above, the other preparation steps and methods are the same as in Example 1.

[0055] The core-shell structure binder for the prepared lithium battery silicon anode is designated as B5.

[0056] Example 6: The difference between this embodiment and Embodiment 1 is that: In step S301, the amount of BA is changed to 72g and the amount of VV-10 is changed to 2g (the mass percentage of VV-10 is 2%).

[0057] Apart from the differences mentioned above, the other preparation steps and methods are the same as in Example 1.

[0058] The core-shell structure binder for the prepared lithium battery silicon anode is designated as B6.

[0059] The performance of the adhesive emulsions prepared in each embodiment was tested, and the test results are detailed in Table 1: Table 1. Comparison of the properties of the adhesive emulsions prepared in each example.

[0060] As shown in the comparative data in Table 1, B1 (the binder prepared in Example 1) exhibits a viscosity that meets the requirements for use as a binder when the solid content remains relatively stable, while also having the lowest gelation rate. The particle size distribution of B1 was measured as follows: Figure 2 As shown, the specific particle size distribution values ​​are detailed in Table 2: Table 2. Particle size distribution of B1

[0061] As can be seen from Table 2, the particle size distribution of the emulsion is relatively concentrated, indicating that the functional groups of B1 (the binder prepared in the example) are evenly distributed and the interfacial interaction is consistent. At the same time, the SEI film is more uniform, which can effectively improve the cycle life.

[0062] Unless otherwise specified or further limited to one preferred or optional technical means being another, the preferred and optional technical means disclosed in this invention can be arbitrarily combined to form several different technical solutions.

Claims

1. A core-shell structure binder for a silicon anode in a lithium-ion battery, characterized in that... Its latex particles have a core-shell structure, including a core layer and a shell layer covering the core layer. The core layer monomers, by mass parts, include 80 to 90 parts of methyl methacrylate, 9 to 19 parts of butyl acrylate, and 0.5 to 2 parts of methacrylic acid. The shell layer monomers, by mass parts, include 20 to 30 parts of methyl methacrylate, 68 to 78 parts of butyl acrylate, 0.5 to 2 parts of methacrylic acid, and 0.5 to 2 parts of ethylene tert-carbonate.

2. The core-shell structure binder for the silicon anode of a lithium battery according to claim 1, characterized in that... The glass transition temperature of the shell is lower than that of the core.

3. The core-shell structure binder for the silicon anode of a lithium battery according to claim 2, characterized in that... The glass transition temperature of the core layer is 40℃~65℃, and the glass transition temperature of the shell layer is -35℃~-15℃.

4. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to any one of claims 1-3, characterized in that... Includes the following steps: Step 1: Preparation of core layer seed emulsion: The premixed core layer monomers are pre-emulsified to obtain a core layer pre-emulsion. The core layer pre-emulsion is heated to the first polymerization temperature under nitrogen protection, and an initiator is added to react until a blue phase is observed, thus obtaining the core layer seed emulsion. Step 2: Shell polymerization: The premixed shell monomers are pre-emulsified to obtain a shell pre-emulsion. The shell pre-emulsion is then semi-continuously added to the core seed emulsion. The initiator is added in stages, and the temperature is raised to the second polymerization temperature for isothermal polymerization. Step 3: Post-processing: Cool and heat the polymerized emulsion, adjust the pH value, and filter to obtain the core-shell structure binder for the silicon anode of lithium battery.

5. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to claim 4, characterized in that... The first polymerization temperature is 75℃~80℃.

6. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to claim 4, characterized in that... The second polymerization temperature is 78°C.

7. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to claim 4, characterized in that... The initiator is ammonium persulfate.

8. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to claim 4, characterized in that... The pre-emulsification treatment in both Step 1 and Step 2 uses the same emulsifier, which is a compound of anionic and nonionic emulsifiers.

9. The method for preparing the core-shell structure binder for the silicon anode of a lithium battery according to claim 4, characterized in that... In step two, during the semi-continuous addition of the shell pre-emulsion to the core seed emulsion, the mass of the core seed emulsion is 5% of the mass of the shell pre-emulsion.

10. The application of the core-shell structure binder of the lithium battery silicon anode according to any one of claims 1-3, or the core-shell structure binder of the lithium battery silicon anode prepared by the preparation method of the core-shell structure binder of the lithium battery silicon anode according to any one of claims 4-9, in the preparation of lithium battery silicon anodes or lithium batteries.

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