A negative electrode sheet, a method for manufacturing the same, and an application thereof

Abstract

This invention discloses a negative electrode sheet, its preparation method, and its application. The negative electrode sheet includes a current collector and a negative electrode active material layer disposed on at least one surface of the current collector. The negative electrode active material layer includes a silicon-based active material, a carboxyl-containing binder, and an oxide filler with hydroxyl groups on its surface. The hydroxyl groups on the surface of the oxide filler form chemical bonds with the carboxyl groups of the carboxyl-containing binder. Compared to existing technologies, this invention significantly enhances the adhesion of the negative electrode sheet through a chemical bonding mechanism, effectively suppressing electrode pulverization and particle shedding caused by volume expansion of high-silicon negative electrodes during cycling. Furthermore, it optimizes the electrode structure and electrochemical performance by limiting particle size matching and synergistic component ratio. This electrode sheet is particularly suitable for high-silicon negative electrode systems and can significantly improve the cycle stability of lithium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a negative electrode sheet, its preparation method and application. Background Technology

[0002] Silicon-based anode materials are known for their extremely high theoretical specific capacity (Li). 4.4 Silicon (4200 mAh / g) has become a research hotspot for lithium-ion battery anode materials. However, silicon undergoes approximately 100% volume expansion during lithium insertion and volume contraction after delithiation, causing the electrode to repeatedly experience extreme volume strain during cycling. This repeated expansion-contraction behavior triggers a series of problems: the binder is repeatedly stretched, resulting in plastic deformation or even breakage; the adhesion between active material particles and current collectors decreases; active particles detach from the electrode and lose electrical contact; and the electrode pulverizes and cracks, ultimately leading to rapid capacity decay. When the silicon content is high (silicon mass fraction ≥ 15%), these problems are particularly severe, and insufficient adhesion has become a key bottleneck restricting the cycle life of high-silicon anodes.

[0003] To improve the adhesion of the negative electrode sheet, existing technologies mainly employ the following methods: (1) Use high-adhesion adhesives. For example, carboxyl-containing adhesives such as polyacrylic acid, polyacrylonitrile-polyacrylic acid copolymer, and sodium carboxymethyl cellulose. The carboxyl groups in these adhesives can form Si-OC chemical bonds with the natural oxide layer on the silicon surface, thus providing stronger adhesion than traditional PVDF. However, in high-silicon systems, relying solely on the chemical bonding of the adhesive itself is still insufficient to meet the requirements for cycle stability.

[0004] (2) Increase the amount of binder. Increasing the binder content can improve the adhesion to a certain extent, but it will reduce the content of active materials and sacrifice the energy density of the battery. More importantly, when the amount of binder exceeds a certain threshold, the improvement of adhesion tends to saturate. Continuing to increase the amount of binder will lead to a decrease in cohesive strength due to the excessive thickness of the binder layer, and the adhesion will decrease instead of increase.

[0005] (3) Introducing inorganic oxides as fillers. Existing studies have added inorganic oxides such as Al2O3, SiO2, and ZrO2 to the negative electrode sheet. However, existing technologies usually regard these oxides as "inert fillers" and only focus on their mechanical support or auxiliary function as electrode additives, without paying attention to the contribution of the chemical interaction between oxides and binders to the adhesion.

[0006] Therefore, how to effectively improve the adhesion of the negative electrode sheet, suppress electrode pulverization and particle shedding under high silicon content conditions, and at the same time avoid the initial coulombic efficiency decline caused by the introduction of additives is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to provide a negative electrode sheet, its preparation method, and its application, to solve the problems of insufficient adhesion, electrode pulverization, and rapid capacity decay caused by volume expansion during cycling of existing high-silicon content negative electrode sheets, so as to achieve the effects of enhanced adhesion, improved initial coulombic efficiency, and improved cycling stability.

[0008] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a negative electrode sheet, comprising a current collector and a negative electrode active material layer disposed on at least one surface of the current collector; the negative electrode active material layer comprises a silicon-based active material, a carboxyl-containing binder, and an oxide filler with hydroxyl groups on its surface. In this process, the hydroxyl groups on the surface of the oxide filler form chemical bonds with the carboxyl groups of the carboxyl-containing binder.

[0009] Preferably, the hydroxyl groups on the surface of the oxide filler undergo a condensation reaction with the carboxyl groups of the carboxyl-containing binder to generate a characteristic absorption peak with a carboxyl group, the characteristic absorption peak being located at 1550~1620 cm⁻¹. -1 Interval.

[0010] Preferably, the oxide filler is selected from at least one of alumina, zirconium oxide, silicon dioxide, titanium dioxide, magnesium oxide, and cerium oxide.

[0011] Preferably, the specific surface area of ​​the oxide filler is 20~200m². 2 / g.

[0012] Preferably, the ratio λ of the particle size D50 of the silicon-based active material to the particle size D50 of the oxide filler satisfies: 0.003≤λ≤0.1.

[0013] Preferably, at least one of the following conditions is met: (a) The oxide filler content in the negative electrode active material layer is 0.1% to 5% by mass; (b) The mass ratio of the oxide filler to the carboxyl-containing binder is 1:2 to 1:20; (c) The silicon content in the silicon-based active material is ≥30% by mass; (d) The carboxyl-containing binder has a mass content of 2-12% in the negative electrode active material layer; (e) The mass content of carboxyl groups in the carboxyl-containing binder is ≥2 mmol / g.

[0014] Preferably, the carboxyl-containing binder is selected from one or more of polyacrylic acid, polyacrylonitrile grafted carboxyl copolymer, sodium carboxymethyl cellulose, sodium alginate, and carboxymethyl chitosan.

[0015] Preferably, the negative electrode active material layer further includes a conductive agent, which includes at least one of carbon nanotubes, carbon nanofibers, and sheet-like conductive graphite; the content of the conductive agent is 0.1% to 3% of the total mass of the negative electrode active material layer.

[0016] Secondly, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1, spreading oxide powder evenly in a drying tray with a layer thickness ≤1cm, drying at 120~200℃ for 2~6h, and sealing immediately after drying for later use or for direct use; S2. Dissolve the carboxyl-containing binder in a solvent and stir until completely dissolved to obtain a binder solution; S3. Dry mix the silicon-based active material, oxide and conductive agent, then add part of the binder solution and conductive agent dispersion, knead and disperse to make the multi-components uniformly dispersed and form a product with multi-element particle size distribution; S4. Add the remaining binder solution and conductive agent dispersion to the product of step S3 to fully disperse all components and obtain a slurry; S5. Add solvent to the slurry of step S4 to dilute it to a viscosity of 2000~8000 mPa·s, then degas under vacuum to obtain a homogeneous slurry; S6. Coat the homogeneous slurry of step S5 onto the current collector and dry it; roll-press the dried electrode to a compaction density of 0.9~1.6 g / cm³. 3 That is, the negative electrode sheet is obtained.

[0017] Thirdly, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the aforementioned negative electrode.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: This invention effectively suppresses electrode pulverization and particle shedding caused by volume expansion of high-silicon anodes during cycling by chemically bonding between hydroxyl groups on the oxide surface and carboxyl groups in the binder.

[0019] This invention improves the first coulombic efficiency by limiting the crystal form and specific surface area of ​​the oxide, thereby reducing the consumption of lithium ions in the electrolyte by the active sites on the oxide surface.

[0020] This invention limits the particle size ratio of silicon-based active material to oxide, allowing oxide particles to fill the gaps between silicon particles, thus avoiding particle bridging and improving electrode compaction density and structural uniformity. Furthermore, by limiting the mass ratio of oxide to binder, it constructs an efficient chemical bonding network while ensuring energy density, avoiding performance degradation caused by saturation of adhesive strength or excessive binder thickness.

[0021] This invention is particularly applicable to high-silicon anode systems with a silicon-based active material content of ≥15%, and can effectively solve the technical problems of large volume expansion and high adhesion requirements in high-silicon systems. Detailed Implementation

[0022] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0023] In this invention, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0024] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0025] It should be understood that in various embodiments of the present invention, the order of the above-mentioned processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0026] According to a first aspect of the present invention, the present invention provides a negative electrode sheet, comprising a current collector and a negative electrode active material layer disposed on at least one surface of the current collector; the negative electrode active material layer comprises a silicon-based active material, a carboxyl-containing binder, and an oxide filler with hydroxyl groups on its surface; In this process, the hydroxyl groups on the surface of the oxide filler form chemical bonds with the carboxyl groups of the carboxyl-containing binder.

[0027] The core of this invention lies in the condensation reaction between the hydroxyl groups (M-OH) on the surface of the oxide filler and the carboxyl groups (-COOH) of the carboxyl-containing binder, forming a chemical bond containing a carboxyl salt: -COO-M-, i.e., M-OH + -COOH → -COO-M- + H2O. The formation of this chemical bond can be verified by Fourier transform infrared spectroscopy; the characteristic peak of -COOH before the reaction is located at 1710 cm⁻¹. -1Corresponding to the C=O stretching vibration peak, the characteristic peak of -COO-M- after the reaction is located at 1550-1620 cm⁻¹. -1 The peak corresponds to the antisymmetric stretching vibration of carboxylate; simultaneously, the peak of the OH stretching vibration of hydroxyl groups on the oxide surface is at 3200-3600 cm⁻¹. -1 The strength is weakened; the formation of this chemical bond means that the oxide particles are not simply "mixed" in the binder, but are connected to the binder network through chemical bonds, becoming "chemical cross-linking points" of the binder network.

[0028] In some embodiments of the present invention, the hydroxyl groups on the surface of the oxide filler undergo a condensation reaction with the carboxyl groups of the carboxyl-containing binder to generate a characteristic absorption peak with a carboxyl group, the characteristic absorption peak being located at 1550~1620 cm⁻¹. -1 Interval.

[0029] When the oxide filler and the carboxyl-containing binder come into contact in the electrode, Fourier transform infrared spectroscopy can detect the characteristic absorption peak of the condensation of hydroxyl groups on the oxide surface with the carboxyl groups of the binder to form carboxylate groups. This characteristic absorption peak is located in the range of 1550~1620 cm⁻¹. -1 The range, distinct from the characteristic absorption peak of free carboxyl groups at 1710 cm⁻¹ -1 Specific testing methods: Take the negative electrode sheet of a battery discharged to 0% SOC (without grinding) and perform FTIR testing using the KBr pressing method or the ATR method; calculate the 1550~1620 cm⁻¹ peak value by peak fitting of the FTIR spectrum. -1 Peak area and 1710 cm -1 The ratio of peak areas reflects the degree of chemical bonding; when the ratio is greater than 0.1, it indicates the presence of significant chemical bonding.

[0030] In some embodiments of the present invention, the oxide filler is selected from at least one of alumina, zirconium oxide, silicon dioxide, titanium dioxide, magnesium oxide, and cerium oxide.

[0031] Among them, the applicable oxides include, but are not limited to: Al2O3 (aluminum oxide): Al-OH +-COOH→-COO-Al-+H2O; ZrO2 (zirconia): Zr-OH +-COOH→-COO-Zr-+H2O; SiO2 (silicon dioxide): Si-OH +-COOH→-COO-Si-+H2O; TiO2 (titanium dioxide): Ti-OH +-COOH→-COO-Ti-+H2O; MgO (magnesium oxide): Mg-OH +-COOH→-COO-Mg-+H2O; CeO2 (cerium oxide): Ce-OH + -COOH → -COO-Ce- +H2O; All oxides containing hydroxyl groups on their surface can react with carboxyl-containing binders to form the chemical bonds described in this invention. The positions of the -COO-M- characteristic peaks of different oxides are shown in Table 1 below.

[0032] Table 1 When the oxide filler is SiO2, a unique double bonding mechanism exists: on the one hand, the -OH on the surface of SiO2 undergoes a condensation reaction with the -COOH of the carboxyl-containing binder (Si-OH + -COOH → -COO-Si- + H2O), forming the first chemical bond; on the other hand, the -OH on the surface of SiO2 can also react with the Si-OH on the surface of the silicon-based active material (from SiO2). x The reaction (layer) generates Si-O-Si bonds (Si-OH + HO-Si → Si-O-Si + H2O), forming a second chemical bond. This second chemical bond can be confirmed by FTIR: the antisymmetric stretching vibration peak of Si-O-Si appears at 1050-1080 cm⁻¹. -1 In contrast, this peak is absent or significantly weaker in the control electrode that does not contain SiO2. This indicates that SiO2 simultaneously forms a "chemical bridge" structure with both the binder and the silicon-based active material (binder-COO-Si-O-Si-silicon-based active material). This unique mechanism is not present in other metal oxides (such as Al2O3 and ZrO2), which can only chemically bond with the binder and cannot directly bond with the silicon-based active material.

[0033] In some embodiments of the present invention, the specific surface area of ​​the oxide filler is 20~200m². 2 / g.

[0034] In this invention, the crystal form and specific surface area of ​​the oxide filler are optimized: when Al2O3 is used, the γ-Al2O3 crystal form is preferred, and the specific surface area is controlled between 20 and 100 m². 2 / g; When ZrO2 is used, the monoclinic crystal form is preferred, with a specific surface area of ​​20~100m². 2 / g; When using SiO2, amorphous state is preferred, with a specific surface area of ​​20~100m². 2 / g; When using TiO2, rutile type is preferred, with a specific surface area of ​​20~100m². 2 / g. The reason is that if the specific surface area is too high, the oxide will consume a large amount of lithium to form the SEI film during the first charge, resulting in a significant decrease in the first efficiency, and will continue to consume lithium during cycling, accelerating capacity decay; while using an oxide with a low specific surface area introduces a very small SEI area, so the first efficiency is not affected, and it also has better chemical and thermal stability.

[0035] In some embodiments of the present invention, the ratio λ of the particle size D50 of the silicon-based active material to the particle size D50 of the oxide filler satisfies: 0.003≤λ≤0.1.

[0036] λ is determined by scanning electron microscopy (SEM) to capture cross-sectional or surface images of the electrode, and image analysis software is used to statistically analyze the particle size of at least 200 particles to calculate D50. The range of 0.003 ≤ λ ≤ 0.1 is set because: when λ < 0.003, excessively high specific surface area leads to an increase in SEI film area during the first charge, consuming more lithium and reducing initial efficiency. Simultaneously, the high density of hydroxyl groups on the surface requires more binder coating, potentially exceeding its coating capacity. When λ > 0.1, metal oxide particles cannot enter the interstices of active material particles, instead bridging above the particles and forming voids below them, reducing the packing ratio. This also causes chemical bonding failure and hinders the filling of interstices by small-diameter active materials, disrupting particle size distribution optimization. The preferred range is 0.005 ≤ λ ≤ 0.05. Oxide particles within this range can enter the interstices of small-diameter active materials, maintaining a suitable specific surface area to avoid excessive lithium consumption and providing sufficient but not excessive chemical bonding sites. When the D50 of the silicon-based active material is 7.6μm, 4μm and 12μm, the corresponding D50 ranges of the oxide filler are 22.8nm~760nm, 12nm~400nm and 36nm~1200nm, respectively; the preferred D50 ranges of the oxide filler are 38nm~380nm, 20nm~200nm and 60nm~600nm, respectively.

[0037] In some embodiments of the present invention, at least one of the following conditions is satisfied: (a) The mass content of oxide filler in the negative electrode active material layer is 0.1%~5%; (b) The mass ratio of oxide filler to carboxyl-containing binder is 1:2 to 1:20; (c) The silicon content in the silicon-based active material is ≥30% by mass; (d) The mass content of carboxyl-containing binder in the negative electrode active material layer is 2~12%; (e) The mass content of carboxyl groups in carboxyl-containing binders is ≥2 mmol / g.

[0038] The condition in (a) is met because: when the oxide filler content is less than 0.1%, the insufficient number of particles leads to insufficient chemical cross-linking points, resulting in insignificant improvement in adhesion (where <5% is due to testing error), thus it is not worthwhile to add more; while when the content exceeds 5%, the content of active material decreases accordingly, the energy density loss is greater than 5.8%, and too many oxide particles will exceed the coating capacity of the binder, with uncoated particles becoming stress concentration points, resulting in a decreasing improvement in marginal adhesion, and the porosity of the electrode may increase due to excessive particles. The preferred content range is 0.5-3%, at which point the adhesion improvement is significant and the energy density loss is controllable; 1% is recommended as a starting point to achieve the best balance between adhesion and energy density.

[0039] Meeting condition (b) ensures that the hydroxyl groups on the oxide surface are fully coated by the binder, and each oxide particle is covered by at least one layer of binder molecules, so that the chemical bonding sites are fully exposed and not obscured by other oxide particles. When the mass ratio of oxide filler to binder is less than 1:2, the oxide surface area is too large, exceeding the coating capacity of the binder. A large number of particle surfaces are not covered by the binder, resulting in a lack of chemical bond connections between particles, becoming stress concentration points, thereby reducing the overall strength of the electrode. When the ratio is greater than 1:20, the number of metal oxide particles is insufficient, the chemical crosslinking points are insufficient, the improvement in adhesion is not significant, and the benefits of adding metal oxides cannot compensate for the loss caused by the reduction of active material. The preferred range is 1:5 to 1:15. Within this range, the hydroxyl groups on the surface of the metal oxide can be fully coated by the binder, the chemical bonding sites are fully utilized, sufficient chemical crosslinking points are formed in the binder network, and sufficient free segments are still maintained to provide elasticity.

[0040] Satisfying (e) means that in carboxyl-containing adhesives, the content of -COOH is ≥2 mmol / g, preferably ≥5 mmol / g; for polyacrylonitrile grafted carboxyl copolymer adhesives, the content of -COOH is also ≥2 mmol / g, preferably ≥5 mmol / g.

[0041] In some embodiments of the present invention, the carboxyl-containing binder is selected from one or more of polyacrylic acid, polyacrylonitrile grafted carboxyl copolymer, sodium carboxymethyl cellulose, sodium alginate, and carboxymethyl chitosan.

[0042] In some embodiments of the present invention, the negative electrode active material layer further includes a conductive agent, which includes at least one of carbon nanotubes, carbon nanofibers, and sheet-like conductive graphite; the content of the conductive agent is 0.1% to 3% of the total mass of the negative electrode active material layer.

[0043] The conductive agent is preferably carbon nanotubes, which can be wound around the surface of oxide particles, allowing the oxide particles to act as anchor points in the conductive network. Since the oxide particles themselves do not undergo volume expansion, they can provide stable support points, effectively preventing the conductive network from slipping or breaking when the silicon-based material undergoes volume expansion.

[0044] According to a second aspect of the present invention, the present invention provides a method for preparing a negative electrode sheet, comprising the following steps: S1, spreading oxide powder evenly in a drying tray with a layer thickness ≤1cm, drying at 120~200℃ for 2~6h, and sealing immediately after drying for later use or for direct use; S2. Dissolve the carboxyl-containing binder in a solvent and stir until completely dissolved to obtain a binder solution; S3. Dry mix the silicon-based active material, oxide and conductive agent, then add part of the binder solution and conductive agent dispersion, knead and disperse to make the multi-components uniformly dispersed and form a product with multi-element particle size distribution; S4. Add the remaining binder solution and conductive agent dispersion to the product of step S3 to fully disperse all components and obtain a slurry; S5. Add solvent to the slurry of step S4 to dilute it to a viscosity of 2000~8000 mPa·s, then degas under vacuum to obtain a homogeneous slurry; S6. Coat the homogeneous slurry of step S5 onto the current collector and dry it; roll-press the dried electrode to a compaction density of 0.9~1.6 g / cm³. 3 This yields the negative electrode sheet.

[0045] According to a third aspect of the present invention, the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the aforementioned negative electrode.

[0046] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0047] Example 1

[0048] Preparation of negative electrode sheet S1. γ-Al2O3 powder (with an average particle size of 30 nm and a specific surface area of ​​180 nm) is used. 2 The γ-Al₂O₃ powder (99.9% purity) was spread evenly in a drying tray with a layer thickness ≤1cm and dried at 120℃ for 2 hours. After drying, it was immediately sealed for later use. The γ-Al₂O₃ powder constituted 0.8% of the negative electrode active material layer. S2. Dissolve the polyacrylonitrile grafted carboxyl copolymer in N-methylpyrrolidone and stir until completely dissolved to obtain a binder solution with a solid content of 48%; wherein, the mass content of the polyacrylonitrile grafted carboxyl copolymer in the negative electrode active material layer is 8%; S3. Dry mix CVD silicon carbide material (with a D50 of 8μm and a silicon mass content of 48%), γ-Al2O3 powder, conductive carbon black and single-walled carbon nanotubes in a mass ratio of 90:0.8:0.5:0.5, add a portion of the binder solution and conductive agent dispersion, knead and disperse for 60 min to make the multi-components uniformly dispersed and form a product with a multi-element particle size distribution; S4. Add the remaining binder solution and conductive agent dispersion to the product of step S3 to fully disperse all components and obtain a slurry; S5. Add solvent to the slurry of step S4 to dilute to a viscosity of 2000 mPa·s, then degas under vacuum to obtain a homogeneous slurry; S6. Coat the homogeneous slurry of step S5 onto the current collector and dry; roll-press the dried electrode to a compaction density of 1.1 g / cm³. 3 This yields the negative electrode sheet.

[0049] Example 2

[0050] The difference from Example 1 is that in step S1, the γ-Al2O3 powder is replaced with anatase TiO2 (with an average particle size of 15 nm and a specific surface area of ​​180 m²). 2 / g), with a mass content of 1.2%, and the remaining steps are the same as in Example 1.

[0051] Example 3

[0052] The difference from Example 1 is that in step S1, the γ-Al2O3 powder is replaced with cubic MgO (with an average particle size of 45 nm and a specific surface area of ​​80 m²). 2 / g), with a mass content of 1.2%, and the remaining steps are the same as in Example 1.

[0053] Example 4

[0054] Unlike Example 1, in step S1, the γ-Al2O3 powder was replaced with cubic ZrO2 (with an average particle size of 15 nm and a specific surface area of ​​180 m²). 2 / g), with a mass content of 1.2%, and the remaining steps are the same as in Example 1.

[0055] Example 5

[0056] The difference from Example 1 is that in step S1, the γ-Al2O3 powder is replaced with amorphous SiO2 (with an average particle size of 150 nm and a specific surface area of ​​50 m²). 2 / g), with a mass content of 1.2%, and the remaining steps are the same as in Example 1.

[0057] Example 6

[0058] The difference from Example 1 is that in step S1, the γ-Al2O3 powder is replaced with cubic fluorite CeO2 (with an average particle size of 15 nm and a specific surface area of ​​180 m²). 2 / g), with a mass content of 1.2%, and the remaining steps are the same as in Example 1.

[0059] Example 7

[0060] The difference from Example 1 is that in step S1, the γ-Al2O3 powder is replaced with a mixed powder of Al2O3 and TiO2, with a mass content of 0.5% for each. The remaining steps are the same as in Example 1.

[0061] Comparative Example 1 Unlike Example 1, γ-Al2O3 was not added; the remaining steps were the same as in Example 1.

[0062] Comparative Example 2 Unlike Example 1, the γ-Al₂O₃ powder has an average particle size of 10 nm and a specific surface area of ​​280 m². 2 / g, the remaining steps are the same as in Example 1.

[0063] Comparative Example 3 Unlike Example 1, the γ-Al₂O₃ powder has an average particle size of 1 μm and a specific surface area of ​​9 m². 2 / g, the remaining steps are the same as in Example 1.

[0064] Comparative Example 4 The difference from Example 1 is that the mass content of γ-Al2O3 powder is 5.5%, while the remaining steps are the same as in Example 1.

[0065] Comparative Example 5 Unlike Example 1, the mass content of γ-Al2O3 powder is 0.3%, and its mass ratio with the binder is less than 1:20. The remaining steps are the same as in Example 1.

[0066] Comparative Example 6 The difference from Example 1 is that in step S2, the polyacrylonitrile grafted carboxyl copolymer is replaced with polyacrylonitrile, while the remaining steps are the same as in Example 1.

[0067] The negative electrode sheets obtained from the above embodiments and comparative examples were subjected to the following performance tests. The test results are shown in Tables 2 and 3.

[0068] First Coulomb Efficiency Test: The electrode sheets were assembled into a CR2032 coin cell, and a lithium metal sheet was used as the counter electrode. The first charge-discharge test was conducted at a rate of 0.05C, and the ratio of the first discharge capacity to the first charge capacity was calculated.

[0069] 180° Peel Strength Test: Cut electrode samples with a width of 25 mm and a length of 100 mm, and use a universal testing machine to perform a 180° peel test at a speed of 50 mm / min. Record the peel force and take the average value of 5 tests.

[0070] 3C rate discharge retention test: The electrode sheets are assembled into a soft-pack full cell. After activation at 0.1C for 3 weeks, it is charged at 1C constant current to the cutoff voltage. After standing for 5 minutes, it is discharged at 3C constant current to the cutoff voltage. The ratio of 3C discharge capacity to 1C discharge capacity is calculated.

[0071] Peel strength retention test after 100 cycles: The electrode sheets were assembled into a soft-pack full cell. After 100 charge-discharge cycles at 25°C with 1C / 1C, the battery was disassembled, the negative electrode sheet was removed, dried, and the peel strength after the cycles was tested according to the above-mentioned 180° peel strength test method. The ratio of the peel strength to the initial peel strength was calculated.

[0072] 500-cycle capacity retention test: The electrodes are assembled into a full cell and charged and discharged at 25°C for 500 cycles at 1C / 1C. The ratio of the discharge capacity of the 500th cycle to the discharge capacity of the 3rd cycle is calculated.

[0073] Table 2 Table 3 As can be seen from the test results of Examples 1-6 in Table 2, using a low specific surface area (20-100m²) 2 The oxide filler ( / g) can significantly reduce the initial coulombic efficiency loss: (1) Al2O3 (γ phase): using 50m 2 When using a low specific surface area scheme of / g, the estimated overall first-effect loss is <0.5%, while using a 200m² scheme... 2 When using a high specific surface area scheme of / g, the overall first-effect loss reaches 1-3%; (2) TiO2 (anatase): using 20m 2 With a low specific surface area of ​​ / g, the additional bulk phase loss is approximately 0.1%, and the estimated overall first-effect loss is <0.8%, while using a 220m² solution results in a loss of approximately 0.1% in bulk phase and <0.8% in first-effect loss. 2 When using a high specific surface area scheme of / g, the overall first-effect loss reaches 1.5-3.5%; (3) MgO (cubic phase): using 40m 2 When using a low specific surface area scheme of / g, the estimated overall first-effect loss is <0.5%, while using a 220m² scheme... 2 When using a high specific surface area scheme of / g, the overall first-effect loss reaches 2-4%; (4) ZrO2 (cubic phase): using 100m 2When using a low specific surface area scheme of / g, the estimated overall first-effect loss is <0.5%, while using a 200m² scheme... 2 When using a high specific surface area scheme of / g, the overall first-effect loss reaches 1.5-2.5%; (5) SiO2 (amorphous): using 20m 2 With a low specific surface area of ​​ / g, the additional bulk phase loss is approximately 0.7%, and the estimated overall first-effect loss is <1.0%, while using a 200m² solution results in a lower specific surface area of ​​ / g. 2 When using a high specific surface area scheme with / g (gas phase method), the overall first-effect loss reaches 2-5%; (6) CeO2 (cubic fluorite): using 100m 2 When using a low specific surface area scheme of / g, the estimated overall first-effect loss is <0.5%, while using a 200m² scheme... 2 When using a high specific surface area scheme ( / g), the overall first-effect loss reaches 1.5-2.5%. The above data demonstrates that by limiting the specific surface area of ​​the oxide filler to 20-200 m² / g, the overall first-effect loss can be achieved. 2 / g can control the initial effect loss to within 1%, preferably to within 0.5%, which is significantly better than the high specific surface area scheme.

[0074] As can be seen from the test results of Examples 1-7 in Table 3, the present invention significantly improves the adhesion and cycle stability of the negative electrode sheet through the chemical bonding between the hydroxyl groups on the oxide surface and the carboxyl groups of the carboxyl-containing binder: (1) Significantly improved adhesion: The 180° peel strength of Examples 1-7 is 5.9-6.3 N / m, which is 127%-142% higher than that of Comparative Example 1 (without oxide filler) of 2.6 N / m, proving that the chemical bonding mechanism effectively enhances the adhesion of the electrode sheet.

[0075] Improved rate performance: The 3C rate discharge retention rate of Examples 1-7 was 71%-75%, which is 6-10 percentage points higher than that of Comparative Example 1 (65%). This indicates that the chemical bonding network helps maintain the integrity of the electrode structure and improves lithium-ion transport.

[0076] Significantly improved cycling stability: After 100 cycles, the peel strength retention rate of Examples 1-7 was 72%-75%, which is 17-20 percentage points higher than that of Comparative Example 1 (55%); the capacity retention rate after 500 cycles was 75%-82%, which is 5-12 percentage points higher than that of Comparative Example 1 (70%), demonstrating that chemical bonding effectively inhibits electrode pulverization and particle shedding during cycling.

[0077] Synergistic effect of mixed oxides: Example 7 (Al2O3 + TiO2 mixed addition) achieved a 180° peel strength of 6.3 N / m and a capacity retention of 83% after 500 cycles, which is slightly better than Examples 1-6 with single oxide addition, indicating that the mixed addition of different oxides may produce a synergistic effect.

[0078] Compared with Example 1, Comparative Example 1 (without oxide filler) has a 180° peel strength of only 2.6 N / m, a 3C rate discharge retention rate of 65%, a peel strength retention rate of 55% after 100 cycles, and a capacity retention rate of 70% after 500 cycles. All performance characteristics are significantly lower than those of Example 1, demonstrating that adding oxides and using chemical bonding mechanisms is the key to improving performance.

[0079] Compared to Example 1, Comparative Example 2 (oxide particle size too small, λ<0.003) achieved a 180° peel strength of 5.3 N / m, a 3C rate discharge retention rate of 76%, and a peel strength retention rate of 77% after 100 cycles, but its capacity retention rate after 500 cycles was only 50%, far lower than the 82% of Example 1. This is because the excessively small particle size resulted in an excessively large specific surface area (280 μm²). 2 / g), continuously consumes lithium during the cycle, accelerating capacity decay.

[0080] Compared to Example 1, Comparative Example 3 (with excessively large oxide particle size, λ>0.1) exhibited a 180° peel strength of only 2.0 N / m, a 3C rate discharge retention rate of 63%, a peel strength retention rate of 32% after 100 cycles, and a capacity retention rate of 57% after 500 cycles, showing a significant decrease in all performance characteristics. This is because the excessively large particle size prevents oxide particles from entering the interparticle spaces of the active material, resulting in a bridging effect and the failure of chemical bonds.

[0081] Compared to Example 1, Comparative Example 4 (with excessively high oxide content and a mass ratio to binder > 1:2) exhibited a 180° peel strength of only 1.6 N / m, a 3C rate discharge retention rate of 60%, a peel strength retention rate of 35% after 100 cycles, and a capacity retention rate of 60% after 500 cycles, with no significant improvement in adhesion. This is because the excessively high oxide content significantly reduces the interfacial compatibility between the material and the current collector, exceeding the binder's encapsulation capacity, resulting in a decrease in the proportion of active materials. Simultaneously, the oxide particles tend to agglomerate and disperse unevenly, leading to an increase in microcracks within the coating.

[0082] Compared to Example 1, Comparative Example 5 (with excessively low oxide content, mass ratio to binder <1:20) exhibited a 180° peel strength of only 2.7 N / m, a 3C rate discharge retention rate of 66%, a peel strength retention rate of 55% after 100 cycles, and a capacity retention rate of 71% after 500 cycles, showing little improvement in adhesive strength. This is because the excessively low oxide content resulted in insufficient chemical crosslinking points, making it impossible to construct an effective chemical bonding network. Compared to Example 1, Comparative Example 6 (without -COOH groups in the binder) showed a 180° peel strength of only 1.1 N / m, a 3C rate discharge retention rate of 50%, a peel strength retention rate of 20% after 100 cycles, and a capacity retention rate of only 4% after 500 cycles, indicating a sharp deterioration in all performance aspects. This demonstrates that the chemical bonding between -COOH groups and -OH groups on the oxide surface is the core mechanism for improving performance; without chemical bonding, simple physical mixing cannot provide sufficient adhesive strength.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector and a negative electrode active material layer disposed on at least one surface of the current collector; the negative electrode active material layer includes a silicon-based active material, a carboxyl-containing binder, and an oxide filler with hydroxyl groups on its surface; In this process, the hydroxyl groups on the surface of the oxide filler form chemical bonds with the carboxyl groups of the carboxyl-containing binder.

2. The negative electrode sheet according to claim 1, characterized in that, The hydroxyl groups on the surface of the oxide filler undergo a condensation reaction with the carboxyl groups of the carboxyl-containing binder, generating a characteristic absorption peak with carboxyl groups located at 1550~1620 cm⁻¹. -1 Interval.

3. The negative electrode sheet according to claim 1, characterized in that, The oxide filler is selected from at least one of alumina, zirconium oxide, silicon dioxide, titanium dioxide, magnesium oxide, and cerium oxide.

4. The negative electrode sheet according to claim 1 or 3, characterized in that, The specific surface area of ​​the oxide filler is 20~200m². 2 / g.

5. The negative electrode sheet according to claim 1, characterized in that, The ratio λ of the particle size D50 of the silicon-based active material to the particle size D50 of the oxide filler satisfies: 0.003≤λ≤0.

1.

6. The negative electrode sheet according to claim 1, characterized in that, At least one of the following conditions must be met: (a) The oxide filler content in the negative electrode active material layer is 0.1% to 5% by mass; (b) The mass ratio of the oxide filler to the carboxyl-containing binder is 1:2 to 1:20; (c) The silicon content in the silicon-based active material is ≥30% by mass; (d) The carboxyl-containing binder has a mass content of 2-12% in the negative electrode active material layer; (e) The mass content of carboxyl groups in the carboxyl-containing binder is ≥2 mmol / g.

7. The negative electrode sheet according to claim 1, characterized in that, The carboxyl-containing binder is selected from one or more of polyacrylic acid, polyacrylonitrile grafted carboxyl copolymer, sodium carboxymethyl cellulose, sodium alginate, and carboxymethyl chitosan.

8. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material layer further includes a conductive agent, which includes at least one of carbon nanotubes, carbon nanofibers, and sheet-like conductive graphite; the content of the conductive agent is 0.1% to 3% of the total mass of the negative electrode active material layer.

9. A method for preparing a negative electrode sheet according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Spread the oxide powder evenly in a drying tray, with a layer thickness of ≤1cm, and dry at 120~200℃ for 2~6h. After drying, seal immediately for later use or use directly. S2. Dissolve the carboxyl-containing binder in a solvent and stir until completely dissolved to obtain a binder solution; S3. Dry mix the silicon-based active material, oxide and conductive agent, then add part of the binder solution and conductive agent dispersion, knead and disperse to make the multi-components uniformly dispersed and form a product with multi-element particle size distribution; S4. Add the remaining binder solution and conductive agent dispersion to the product of step S3 to fully disperse all components and obtain a slurry; S5. Add solvent to the slurry of step S4 to dilute it to a viscosity of 2000~8000 mPa·s, then degas under vacuum to obtain a homogeneous slurry; S6. Coat the homogeneous slurry of step S5 onto the current collector and dry it; roll-press the dried electrode to a compaction density of 0.9~1.6 g / cm³. 3 That is, the negative electrode sheet is obtained.

10. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode is the negative electrode as described in any one of claims 1-8.

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