Silicon-carbon negative electrode with double-layer gradient structure and preparation method of silicon-carbon negative electrode

Abstract

The invention discloses a silicon-carbon negative electrode with a double-layer gradient structure and a preparation method of the silicon-carbon negative electrode, and relates to the technical field of lithium ion battery electrode materials. The silicon-carbon negative electrode structure sequentially comprises an upper electrode layer, a lower electrode layer and a current collector layer from top to bottom. The current collector layer is a copper foil, the upper electrode layer comprises a large-particle active substance, a composite conductive agent and a binder, and the lower electrode layer comprises a small-particle active substance, a composite conductive agent and a binder. The prepared silicon-carbon negative electrode with the double-layer gradient structure has the advantages of high capacity retention rate and low expansion rate.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery electrode materials technology, specifically to a silicon-carbon anode with a double-layer gradient structure and its preparation method. Background Technology

[0002] The rapid development of new energy materials places increasingly higher demands on battery performance and capacity. The specific capacity of traditional graphene anodes is only 372 mAh / g; in contrast, the theoretical mass specific capacity and theoretical volumetric specific capacity of silicon anodes are as high as 4200 mAh / g and 9786 mAh / cm³, respectively. 3 Furthermore, silicon anodes have an ultra-negative potential of -3.04V. In addition, silicon has high natural reserves and low price, making it one of the most promising anode materials.

[0003] In traditional single-layer silicon-carbon anodes, large particle materials can reduce specific surface area and side reactions, but the ion transport path is long, making lithium plating more likely during fast charging. Small particle materials shorten the ion diffusion distance, but the large specific surface area leads to low initial efficiency and rapid electrolyte consumption. Meanwhile, the lithium storage mechanism of silicon-carbon materials is an alloying reaction, resulting in significant volume expansion during charge and discharge, reaching up to 300%. An unreasonable particle layering design can accelerate electrode structure pulverization, causing capacity decay. While existing bilayer electrodes increase density through layering, the large differences in particle size of active materials within the same layer, such as a D50 difference greater than 2 μm between graphite and silicon-carbon, leads to uneven interfacial stress and exacerbates crack formation.

[0004] For high-nickel ternary cathode systems, oxygen evolution intensifies during fast charging, necessitating a cathode with higher reaction uniformity. Current technologies employ sulfur-loaded / coating layer structures, but fail to address the synergistic optimization of expansion constraint and ion transport in silicon-carbon anodes. Therefore, developing a silicon-carbon anode material that can simultaneously improve ion transport rate and interfacial stability is urgently needed. Summary of the Invention

[0005] The purpose of this invention is to provide a silicon-carbon anode with a double-layer gradient structure and its preparation method, thereby solving the following technical problems: Silicon-carbon materials exhibit significant volume expansion during charging and discharging. Although existing bilayer electrodes increase density through layering, the large differences in particle size of active materials within the same layer lead to uneven interfacial stress and exacerbate crack formation.

[0006] The objective of this invention can be achieved through the following technical solutions: This invention provides a method for preparing a silicon-carbon anode with a dual-layer gradient structure. The silicon-carbon anode comprises, from top to bottom, an upper electrode layer, a lower electrode layer, and a current collector layer. The current collector layer is a copper foil. The upper electrode layer comprises large-particle active material, a composite conductive agent, and a binder. The lower electrode layer comprises small-particle active material, a composite conductive agent, and a binder. The method for preparing the silicon-carbon anode with the double-layer gradient structure includes the following steps: S1. Slurry preparation: The upper slurry is obtained by mixing large-particle active material, composite conductive agent and binder, and the lower slurry is obtained by mixing small-particle active material, composite conductive agent and binder. S2. Gradient coating: Simultaneous coating is performed using an extrusion double-die coating machine. The lower layer slurry is first coated onto the current collector, and the upper layer is wet-pressed to composite the upper layer slurry, resulting in a silicon-carbon anode semi-finished product with a double-layer gradient structure. S3. Drying and rolling: The silicon-carbon anode semi-finished product with a double-layer gradient structure is first dried. Solvent replacement technology is used during the drying process. Then it is rolled into shape to obtain the silicon-carbon anode with a double-layer gradient structure.

[0007] As a further embodiment of the present invention: the upper slurry comprises the following raw materials in parts by weight: 94-97 parts of large particulate active material, 0.5-1.5 parts of composite conductive agent, and 1.5-5.5 parts of binder; the lower slurry comprises the following raw materials in parts by weight: 94-97 parts of small particulate active material, 0.5-1.5 parts of composite conductive agent, and 1.5-5.5 parts of binder.

[0008] As a further aspect of the present invention: the large-particle active material is a graphite-silicon-carbon composite with a composite ratio of 75-85:15-25 and D50=10-14μm; the small-particle active material is a graphite-silicon-carbon composite with a composite ratio of 75-85:15-25 and D50=6-8μm.

[0009] As a further aspect of the present invention: the composite conductive agent is a compound obtained by a mass ratio of conductive carbon black SP and conductive carbon nanotubes CNT-2001 of 4:1; the binder is any one of styrene-butadiene rubber and carboxymethyl cellulose.

[0010] As a further aspect of the present invention: the density of the lower layer accounts for 60%, and the density of the upper layer accounts for 40%.

[0011] As a further aspect of the present invention: the temperature in the S3 drying process is 55 to 110°C, the heating rate is 5°C / min, and the time is 10-60min.

[0012] As a further aspect of the present invention: the solvent used in the solvent replacement technology of the S3 drying process is a mixture of isopropanol and water.

[0013] As a further aspect of the present invention: the compaction density in the S3 compaction process is 1.5-1.8 g / cc.

[0014] The beneficial effects of this invention are: (1) This invention uses an extrusion-type dual-die coating machine to simultaneously coat the upper and lower slurries, controlling the consistency of particle size within the same layer and the particle size gradient between layers, thereby simultaneously improving ion transport rate and interface stability. In the dual-layer gradient structure, the upper slurry uses large-particle active material, whose low specific surface area reduces side reactions, is suitable for high-nickel ternary cathode systems, and can also serve as a buffer layer to alleviate silicon expansion. The lower slurry uses small-particle active material, which shortens the ion diffusion path, accelerates lithium-ion insertion, and improves fast charging capability. Compared with a single-layer electrode, the capacity retention rate is increased by 28% to 85% after 500 cycles of 2C fast charging. At the same time, the higher density of the upper layer provides mechanical support, the moderate porosity of the lower layer reserves expansion space, and the interlayer is formed by a flexible transition zone by a binder network to dissipate interfacial stress.

[0015] (2) The present invention uses a composite conductive agent composed of conductive carbon black SP and conductive carbon nanotube CNT-2001. CNT-2001 has a small size and a large specific surface area, which can form a more developed conductive network with carbon black, exhibiting good conductivity and low interfacial impedance, and ensuring that the electrode has good charge and discharge performance.

[0016] (3) The present invention employs solvent replacement technology in the drying process to prevent interlayer mixing. The difference in D50 between graphite and silicon carbide is less than 1 μm, and the uniformity of particle size in the same layer ensures homogenization of the same layer, matching the expansion coefficients of the same layer materials and inhibiting crack initiation. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1: A method for preparing a silicon-carbon anode with a double-layer gradient structure, comprising the following steps: S1. Large-particle active material is obtained by compounding graphite with a D50 of 10-14μm and silicon-carbon composite at a ratio of 85:15; small-particle active material is obtained by compounding graphite with a D50 of 6-8μm and silicon-carbon composite at a ratio of 85:15; composite conductive agent is obtained by compounding conductive carbon black SP and conductive carbon nanotubes CNT-2001 at a mass ratio of 4:1; upper slurry is obtained by mixing 94g of large-particle active material, 0.5g of composite conductive agent and 1.5g of styrene-butadiene rubber; lower slurry is obtained by mixing 94g of small-particle active material, 0.5g of composite conductive agent and 1.5g of styrene-butadiene rubber. S2. Simultaneous coating is performed using an extrusion-type dual-die coating machine. The lower layer is first coated onto the copper foil with a lower layer slurry density of 60%. The upper layer is then wet-pressed onto the upper layer slurry with a higher layer density of 40%, resulting in a silicon-carbon anode semi-finished product with a double-layer gradient structure. S3. The silicon-carbon anode semi-finished product with a double-layer gradient structure is first dried. During the drying process, the heating rate is 5°C / min, the time is 10min, and the temperature range is 55 to 110°C. At the same time, a mixed solvent of isopropanol and water is used for solvent replacement. Then, it is rolled into shape with a rolling density of 1.5g / cc to obtain the silicon-carbon anode with a double-layer gradient structure.

[0019] Example 2: A method for preparing a silicon-carbon anode with a double-layer gradient structure, comprising the following steps: S1. Large-particle active material is obtained by compounding graphite with a D50 of 10-14μm and silicon-carbon composite at a ratio of 80:20; small-particle active material is obtained by compounding graphite with a D50 of 6-8μm and silicon-carbon composite at a ratio of 80:20; composite conductive agent is obtained by compounding conductive carbon black SP and conductive carbon nanotubes CNT-2001 at a mass ratio of 4:1; upper slurry is obtained by mixing 95g of large-particle active material, 1g of composite conductive agent and 3.5g of styrene-butadiene rubber; lower slurry is obtained by mixing 95g of small-particle active material, 1g of composite conductive agent and 3.5g of styrene-butadiene rubber. S2. Simultaneous coating is performed using an extrusion-type dual-die coating machine. The lower layer is first coated onto the copper foil with a lower layer slurry density of 60%. The upper layer is then wet-pressed onto the upper layer slurry with a higher layer density of 40%, resulting in a silicon-carbon anode semi-finished product with a double-layer gradient structure. S3. The silicon-carbon anode semi-finished product with a double-layer gradient structure is first dried. During the drying process, the heating rate is 5°C / min, the time is 40min, and the temperature range is 55 to 110°C. At the same time, a mixed solvent of isopropanol and water is used for solvent replacement. Then, it is rolled into shape with a rolling density of 1.6g / cc to obtain the silicon-carbon anode with a double-layer gradient structure.

[0020] Example 3: A method for preparing a silicon-carbon anode with a double-layer gradient structure, comprising the following steps: S1. Large-particle active material is obtained by compounding graphite with a D50 of 10-14μm and silicon-carbon composite in a ratio of 75:25; small-particle active material is obtained by compounding graphite with a D50 of 6-8μm and silicon-carbon composite in a ratio of 75:25; composite conductive agent is obtained by compounding conductive carbon black SP and conductive carbon nanotubes CNT-2001 in a mass ratio of 4:1; upper slurry is obtained by mixing 97g of large-particle active material, 1.5g of composite conductive agent and 5.5g of styrene-butadiene rubber; lower slurry is obtained by mixing 97g of small-particle active material, 1.5g of composite conductive agent and 5.5g of styrene-butadiene rubber. S2. Simultaneous coating is performed using an extrusion-type dual-die coating machine. The lower layer is first coated onto the copper foil with a lower layer slurry density of 60%. The upper layer is then wet-pressed onto the upper layer slurry with a higher layer density of 40%, resulting in a silicon-carbon anode semi-finished product with a double-layer gradient structure. S3. The silicon-carbon anode semi-finished product with a double-layer gradient structure is first dried. During the drying process, the heating rate is 5°C / min, the time is 60min, and the temperature range is 55 to 110°C. At the same time, a mixed solvent of isopropanol and water is used for solvent replacement. Then, it is rolled into shape with a rolling density of 1.8g / cc to obtain the silicon-carbon anode with a double-layer gradient structure.

[0021] Comparative Example 1: A method for preparing a silicon-carbon anode, comprising the following steps: S1. An active material is obtained by compounding graphite with a D50 of 6-14μm and silicon-carbon composite in a ratio of 75:25; a composite conductive agent is obtained by compounding conductive carbon black SP and conductive carbon nanotubes CNT-2001 in a mass ratio of 4:1; and a slurry is obtained by mixing 97g of active material, 1.5g of composite conductive agent and 5.5g of styrene-butadiene rubber. S2. Coating is performed using an extrusion-type dual-die coating machine to obtain silicon-carbon anode semi-finished product; S3. The silicon-carbon anode semi-finished product is first dried. During the drying process, the heating rate is 5°C / min, the time is 60min, and the temperature range is 55 to 110°C. At the same time, a mixed solvent of isopropanol and water is used for solvent replacement. Then, it is rolled into shape with a rolling density of 1.8g / cc to obtain the silicon-carbon anode.

[0022] Comparative Example 2: A method for preparing a silicon-carbon anode, comprising the following steps: S1. Large-particle active material is obtained by compounding graphite with a D50 of 10-14μm and silicon-carbon composite in a ratio of 75:25; small-particle active material is obtained by compounding graphite with a D50 of 6-8μm and silicon-carbon composite in a ratio of 75:25; 97g of large-particle active material, 1.5g of conductive carbon black SP, and 5.5g of styrene-butadiene rubber are mixed to obtain the upper slurry; 97g of small-particle active material, 1.5g of conductive carbon black SP, and 5.5g of styrene-butadiene rubber are mixed to obtain the lower slurry. S2. Simultaneous coating is performed using an extrusion-type dual-die coating machine. The lower layer is first coated onto the copper foil with a lower layer slurry density of 60%. The upper layer is then wet-pressed onto the upper layer slurry with a higher layer density of 40%, resulting in a silicon-carbon anode semi-finished product. S3. The silicon-carbon anode semi-finished product is first dried. During the drying process, the heating rate is 5°C / min, the time is 60min, and the temperature range is 55 to 110°C. At the same time, a mixed solvent of isopropanol and water is used for solvent replacement. Then, it is rolled into shape with a rolling density of 1.8g / cc to obtain the silicon-carbon anode.

[0023] Comparative Example 3: A method for preparing a silicon-carbon anode, comprising the following steps: S1. Large-particle active material is obtained by compounding graphite with a D50 of 10-14μm and silicon-carbon composite in a ratio of 75:25; small-particle active material is obtained by compounding graphite with a D50 of 6-8μm and silicon-carbon composite in a ratio of 75:25; composite conductive agent is obtained by compounding conductive carbon black SP and conductive carbon nanotubes CNT-2001 in a mass ratio of 4:1; upper slurry is obtained by mixing 97g of large-particle active material, 1.5g of composite conductive agent and 5.5g of styrene-butadiene rubber; lower slurry is obtained by mixing 97g of small-particle active material, 1.5g of composite conductive agent and 5.5g of styrene-butadiene rubber. S2. Simultaneous coating is performed using an extrusion-type dual-die coating machine. The lower layer is first coated onto the copper foil with a lower layer slurry density of 60%. The upper layer is then wet-pressed onto the upper layer slurry with a higher layer density of 40%, resulting in a silicon-carbon anode semi-finished product. S3. First, dry the silicon-carbon anode semi-finished product. During the drying process, the heating rate is 5°C / min, the time is 60min, and the temperature range is 55 to 110°C. Then, roll it into shape with a rolling density of 1.8g / cc to obtain the silicon-carbon anode.

[0024] Performance testing: Examples 1-3 and Comparative Examples 1-3 were assembled with lithium iron phosphate coated aluminum foil and ceramic separators, respectively, welded, covered with aluminum-plastic film, and initially encapsulated with a heat sealer; electrolyte (a solution of lithium hexafluorophosphate dissolved in ethylene carbonate) was injected, aged, vacuumed, heat-sealed a second time, and shaped to obtain battery samples for performance testing.

[0025] 1. Specific Capacity: According to GB 31241-2022 "Safety Technical Specifications for Lithium-ion Batteries and Battery Packs for Portable Electronic Products", the sample was first charged at 0.2A at 25℃. When the terminal voltage of the battery or battery pack reached the charging limit voltage U... cl When the current is less than or equal to 0.02A, switch to constant voltage charging until the charging current is less than or equal to 0.02A. Stop charging, let it rest for 10 minutes, and then discharge at the recommended current I. cr Perform constant current discharge until discharge termination voltage U de The capacity provided during discharge is the actual capacity of the sample; the specific capacity is calculated, and the test results are shown in Table 1. 2. Capacity Retention Rate: According to GB / T 44027.1-2024 "Determination Methods for Carbon Materials - Part 1: Determination of Initial Discharge Specific Capacity, Initial Coulombic Efficiency, and Capacity Retention Rate at Different Discharge Rates", samples were prepared at 25℃ and 50% relative humidity, and the capacity retention rate was calculated. ; ; In the formula: C 2(dis) : Initial charge specific capacity when charged and discharged at a 2C rate, expressed in milliampere-hours per gram (mAh / g); Q 2(dis) The initial charge capacity when charged and discharged at a 2C rate, expressed in milliampere-hours (mAh). m: The mass of the active material in the battery, in grams (g); R: Capacity retention rate at 2C / 0.2C, expressed as a percentage (%); C r(dis) The first discharge specific capacity when charged and discharged at a rate of 0.2C is expressed in milliampere-hours per gram (mAh / g); the test results are shown in Table 1. 3. Volumetric expansion rate: According to GB / T 44027.2-2024 "Determination of Carbon Materials - Part 2: Determination of Expansion Rate", the sample was prepared at 25℃ and 50% relative humidity, and the expansion rate was calculated. ; In the formula: D1: The thickness of the electrode material coating on the electrode sheet after vacuum drying, in micrometers (μm); D2: The thickness of the electrode material coating on the electrode sheet after cycling, in micrometers (μm); R expansion Expansion rate; test results are shown in Table 1.

[0026] Table 1: Performance Test Data Statistics of Examples 1-3 and Comparative Examples 1-3

[0027] As can be seen from Table 1, the inconsistent specific capacity and expansion rate of Examples 1-3 were due to the inconsistent proportion of added silicon-carbon composites; Comparative Example 1, as a single-layer electrode, performed far worse than the double-layer electrode in both capacity retention and expansion rate; Comparative Example 2, because it only used conductive carbon black SP as the conductive agent, had lower conductivity and higher interfacial impedance compared to composite conductive agents, resulting in poor capacity retention and expansion rate; Comparative Example 3, because it did not use solvent replacement technology, had a higher expansion rate than the Examples.

[0028] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A method for preparing a silicon-carbon anode with a double-layer gradient structure, characterized in that, The silicon-carbon anode comprises, from top to bottom, an upper electrode layer, a lower electrode layer, and a current collector layer; the current collector layer is a copper foil; the upper electrode layer comprises large-particle active material, a composite conductive agent, and a binder; and the lower electrode layer comprises small-particle active material, a composite conductive agent, and a binder. The method for preparing the silicon-carbon anode with the double-layer gradient structure includes the following steps: S1. Slurry preparation: The upper slurry is obtained by mixing large-particle active material, composite conductive agent and binder, and the lower slurry is obtained by mixing small-particle active material, composite conductive agent and binder. S2. Gradient coating: The upper and lower layers are coated simultaneously using an extrusion double-die coating machine. The lower layer slurry is coated onto the current collector, and the upper layer is coated onto the lower slurry simultaneously. After low-temperature drying in an oven, a silicon-carbon anode semi-finished product with a double-layer gradient structure is obtained. S3. Drying and rolling: The silicon-carbon anode semi-finished product with a double-layer gradient structure is first dried. Solvent replacement technology is used during the drying process. Then it is rolled into shape to obtain the silicon-carbon anode with a double-layer gradient structure.

2. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 1, characterized in that, The upper slurry comprises the following raw materials in parts by weight: 94-97 parts of large-particle active material, 0.5-1.5 parts of composite conductive agent, and 1.5-5.5 parts of binder; the lower slurry comprises the following raw materials in parts by weight: 94-97 parts of small-particle active material, 0.5-1.5 parts of composite conductive agent, and 1.5-5.5 parts of binder.

3. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 1, characterized in that, The large-particle active material is a graphite-silicon-carbon composite with a composite ratio of 75-85:15-25 and D50=10-14μm; the small-particle active material is a graphite-silicon-carbon composite with a composite ratio of 75-85:15-25 and D50=6-8μm.

4. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 2, characterized in that, The composite conductive agent is a compound obtained by mixing conductive carbon black SP and conductive carbon nanotubes CNT in a mass ratio of 4:1; the binder is any one or more of styrene-butadiene rubber and carboxymethyl cellulose.

5. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 1, characterized in that, The density of the lower layer accounts for 60%, and the density of the upper layer accounts for 40%.

6. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 1, characterized in that, The temperature for S3 drying is 55 to 110°C, the heating rate is 5°C / min, and the time is 10 to 60 min.

7. The method for preparing a silicon-carbon anode with a double-layer gradient structure according to claim 1, characterized in that, The solvent used in the S3 drying process for solvent replacement technology is a mixture of isopropanol and water.

8. The method for preparing a high-fast-charging performance double-layer gradient silicon-carbon anode according to claim 1, characterized in that, The compaction density during S3 compaction is 1.5-1.8 g / cc.

9. A silicon-carbon anode with a double-layer gradient structure, characterized in that, It is prepared by the preparation method described in any one of claims 1-7.

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