Negative plate and secondary battery

Abstract

The invention belongs to the technical field of battery preparation, and particularly relates to a negative plate and a secondary battery. A film-shaped structure is formed by a first polymer binder, so that the negative plate can better adapt to expansion and contraction of silicon-carbon composite particles, and volume change is effectively buffered; the problem that a traditional binder such as SBR is prone to breakage due to the fact that a point-like contact mode is adopted is solved, the structural stability of the silicon-carbon composite particles and the negative electrode piece is greatly improved, the problem that a silicon-based negative electrode is prone to expansion in the charging and discharging process is obviously solved, and by combining control over the structure and the average sphericity degree of the silicon-carbon composite particles, the service life of the silicon-carbon composite particles is prolonged. The negative electrode active material layer is conductive to reducing local stress concentration, constructing a stable conductive network, improving the conductivity of the negative electrode active material layer and reducing the internal resistance of a system, so that the rate capability and the cycle performance of the secondary battery are improved.

Description

Technical Field

[0001] This application belongs to the field of battery manufacturing technology, specifically relating to a negative electrode and a secondary battery. Background Technology

[0002] Silicon-carbon composite materials are considered an important candidate for next-generation lithium-ion battery anode materials due to their high specific capacity. However, when used as anode materials for lithium-ion batteries, silicon-carbon composite materials face the problem of severe volume expansion (approximately 300%) of silicon materials during charge and discharge. Furthermore, traditional anode binders (such as styrene-butadiene rubber SBR) fix the anode material through point contact (SBR point contact), resulting in fewer contact sites. This is especially true when using spherical silicon-carbon composite materials, which further reduces the contact sites between the conductive agent and the silicon-carbon composite material. Moreover, these binders struggle to adapt to the expansion and contraction of silicon-carbon particles, easily leading to adhesion failure, electrode structure damage, conductive network breakage, and decreased adhesion strength, ultimately affecting the battery's cycle performance, conductivity, and rate performance. Therefore, there is an urgent need to develop an anode sheet and secondary battery that combines high adhesion, high conductivity, and structural stability. Summary of the Invention

[0003] Therefore, the technical problem to be solved by this application is to overcome the problem that the existing technology uses traditional negative electrode binders with few point contact sites and is difficult to adapt to the expansion and contraction of negative electrode materials, thus affecting the cycle performance, conductivity and rate performance of the battery, thereby providing a negative electrode active material, negative electrode sheet and secondary battery.

[0004] In a first aspect, this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising silicon-carbon composite particles, a conductive agent, and a first polymer binder coating the surface of the silicon-carbon composite particles; the silicon-carbon composite particles comprising core particles and a carbon coating layer distributed on the surface of the core particles, the core particles comprising porous carbon material and silicon distributed within the cavities of the porous carbon material; the average sphericity of the silicon-carbon composite particles ≥ 0.85; the first polymer binder comprising one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethyl methacrylate-polyvinylidene fluoride copolymer, polymethyl methacrylate-polypropylene copolymer, and polypropylene-polymethyl methacrylate-polyvinylidene fluoride copolymer.

[0005] In one optional embodiment, the silicon content is 30%-70% by mass of the total silicon-carbon composite particles; and / or, the total pore volume of the silicon-carbon composite particles is 0.6 cm³. 3 / g~1.5cm 3 / g, the average pore size of the silicon-carbon composite particles is 1.5nm-20nm; the pore volume of micropores in the silicon-carbon composite particles accounts for 20%-99% of the total pore volume.

[0006] In one optional embodiment, the ID / IG value of the silicon-carbon composite particles is 0.7-1.5; and / or, the median particle size of the silicon-carbon composite particles is 50nm-500nm.

[0007] In one alternative embodiment, the thickness of the carbon coating layer is 5 nm to 20 nm.

[0008] In one optional embodiment, the conductive agent comprises carbon nanotubes having a length of 1 μm-10 μm and an aspect ratio of 100-1000.

[0009] In one optional embodiment, the mass ratio of the silicon-carbon composite particles, the conductive agent, and the first polymer binder is 80–99:0.2–8:0.2–10.

[0010] In one alternative embodiment, the negative electrode active material layer further includes a second polymer binder.

[0011] In one optional embodiment, the silicon-carbon composite particles further include graphite particles, and the mass content of the silicon-carbon composite particles is 7.5%-90% based on the total mass of the silicon-carbon composite particles and the graphite particles.

[0012] In one alternative embodiment, the repeating groups of the first polymeric adhesive include one or more of -CN groups and -CO- groups.

[0013] In one optional embodiment, the first polymeric adhesive comprises one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethacrylate-polyvinylidene fluoride copolymer, polymethacrylate-polypropylene copolymer, and polypropylene-polymethacrylate-polyvinylidene fluoride copolymer; and / or, the weight-average molecular weight of the first polymeric adhesive is 50,000 to 800,000.

[0014] In one alternative embodiment, the number of silicon-carbon composite particles in different regions of a 100µm×100µm unit area of ​​the negative electrode active material layer varies by ≤100 under a scanning electron microscope.

[0015] In one optional embodiment, the peel force between the negative electrode active material layer and the negative electrode current collector is 10 N / m-30 N / m.

[0016] In one alternative embodiment, the silicon content is 4%-50% by mass of the total mass of the negative electrode active material layer.

[0017] Secondly, this application provides a secondary battery, including the negative electrode sheet described in the first aspect.

[0018] In one optional embodiment, the secondary battery further includes a non-aqueous electrolyte; the non-aqueous electrolyte includes a carbonate solvent, a lithium salt, and additives; the carbonate solvent includes linear carbonate solvents and / or linear carboxylic acid ester solvents.

[0019] In one optional embodiment, the sum of the mass contents of linear carbonate solvents and linear carboxylic acid ester solvents is ≤60% based on the total mass of the non-aqueous electrolyte.

[0020] The technical solution of this application has the following advantages: The negative electrode sheet provided in this application includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer includes silicon-carbon composite particles, a conductive agent, and a first polymer binder coating the surface of the silicon-carbon composite particles. The silicon-carbon composite particles include core particles and a carbon coating layer distributed on the surface of the core particles. The core particles include porous carbon material and silicon distributed inside the cavities of the porous carbon material. The average sphericity of the silicon-carbon composite particles is ≥0.85. The first polymer binder comprises one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethyl methacrylate-polyvinylidene fluoride copolymer, polymethyl methacrylate-polypropylene copolymer, and polypropylene-polymethyl methacrylate-polyvinylidene fluoride copolymer. This application utilizes these first polymer binders to form a film structure, which better adapts to the expansion and contraction of silicon-carbon composite particles, effectively buffering volume changes and avoiding the problem of easy breakage caused by the point contact method of traditional SBR binders. This greatly improves the structural stability of silicon-carbon composite particles and negative electrode sheets, significantly improves the problem of easy expansion of silicon-based negative electrodes during charging and discharging, and, combined with the control of the structure and average sphericity of silicon-carbon composite particles, helps to reduce local stress concentration, construct a stable conductive network, improve the conductivity of the negative electrode active material layer, reduce the internal resistance of the system, and thus improve the rate performance and cycle performance of the secondary battery. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 These are scanning electron microscope (SEM) images of the negative electrode active material layer on the surface of the negative electrode sheet obtained in Example 1 of this application (the top two images are planar views, with the left one before rolling and the right one after rolling; the bottom two images are cross-sectional views, with the left one before rolling and the right one after rolling, both at a magnification of 500x). Figure 2 This is a scanning electron microscope image (3000x magnification) of the negative electrode active material layer of the negative electrode sheet obtained in Example 1 of this application. Detailed Implementation

[0023] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0024] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0025] Traditional negative electrode binders (such as styrene-butadiene rubber (SBR), PVDF, PPO, etc.) fix the negative electrode material through point contact, resulting in fewer contact sites between the negative electrode material and the conductive agent. This is especially true when using spherical silicon-carbon composite materials as the negative electrode material, which further reduces the contact sites between the conductive agent and the silicon-carbon composite material. Moreover, such negative electrode binders are difficult to adapt to the expansion and contraction of the negative electrode material, leading to a decrease in adhesion and conductivity, electrode structure damage, and consequently affecting battery cycle performance. To address the problems of traditional negative electrode binders having few point contact sites and being unable to adapt to the expansion and contraction of the negative electrode material, thus affecting battery conductivity and cycle performance, this application provides the following technical solution.

[0026] In a first aspect, this application provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising silicon-carbon composite particles, a conductive agent, and a first polymer binder coating the surface of the silicon-carbon composite particles; the silicon-carbon composite particles comprising core particles and a carbon coating layer distributed on the surface of the core particles, the core particles comprising porous carbon material and silicon distributed within the cavities of the porous carbon material; the average sphericity of the silicon-carbon composite particles ≥ 0.85; the first polymer binder comprising one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethyl methacrylate-polyvinylidene fluoride copolymer, polymethyl methacrylate-polypropylene copolymer, and polypropylene-polymethyl methacrylate-polyvinylidene fluoride copolymer.

[0027] This application utilizes first polymer binders to form a film-like structure coating the surface of the silicon-carbon composite particles. The use of silicon-carbon composite particles with high sphericity (≥0.85) significantly increases the contact sites between the silicon-carbon composite particles and the conductive agent, thereby improving the battery's conductivity. Furthermore, the film-like structure better accommodates the expansion and contraction of the silicon-carbon composite particles, effectively buffering volume changes and avoiding the breakage problems associated with point-contact binders like those used in traditional SBRs. This greatly improves the structural stability of the silicon-carbon composite particles and the negative electrode, significantly mitigating the problem of the negative electrode easily expanding during charging and discharging. Combined with the control of the silicon-carbon composite particle structure and average sphericity, it helps reduce local stress concentration, constructing a stable conductive network, improving the conductivity of the negative electrode active material layer, and reducing the system's internal resistance, thereby improving the rate performance and cycle performance of the secondary battery. For example, the average sphericity of the silicon-carbon composite particles can be 0.85, 0.9, 0.92, 0.93, 0.94, 0.96, 0.97, 0.99, 1, or within any two of these values. For example, the first polymeric binder comprises one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethacrylate-polyvinylidene fluoride copolymer, polymethacrylate-polypropylene copolymer, and polypropylene-polymethacrylate-polyvinylidene fluoride copolymer. The polyacrylate may be one or more of polymethyl methacrylate, polyethyl methacrylate, polyisobutyl methacrylate, polybutyl methacrylate, polyn-butyl methacrylate, polypropylene methacrylate, polymethyl methacrylate, methacrylic acid, polymethyl methacrylate, polyethyl methacrylate, polyisobutyl methacrylate, polybutyl methacrylate, polyn-butyl methacrylate, polyisooctyl methacrylate, polybutylene acrylate, and polypropylene methacrylate. The polyacrylate may be one or more of lithium polyacrylate and sodium polyacrylate. In polymethyl methacrylate-polyvinylidene fluoride copolymer, polymethyl methacrylate-polypropylene copolymer, and polypropylene-polymethyl methacrylate-polyvinylidene fluoride copolymer, the polymethyl methacrylate can be one or more of polymethyl methacrylate, polyethyl methacrylate, polyisobutyl methacrylate, polybutyl methacrylate, and poly(n-butyl methacrylate).

[0028] In one optional embodiment, the silicon content, based on the total mass of the silicon-carbon composite particles, is 30%-70%. Limiting the silicon content of the silicon-carbon composite particles to this range is beneficial for further improving the cycle performance of the negative electrode during charge-discharge while maintaining high battery conductivity. For example, the silicon content, based on the total mass of the silicon-carbon composite particles, is 30%, 40%, 50%, 60%, 70%, or within any two of the above values.

[0029] In one optional embodiment, the pore structure of the porous carbon material doped with carbon nanotubes includes micropores, mesopores, and macropores. The International Union of Pure and Applied Chemistry (IUPAC) classifies pores in powder materials according to size as micropores (pore size < 2 nm), mesopores (pore size 2–50 nm), and macropores (pore size > 50 nm).

[0030] In one optional embodiment, the total pore volume of the silicon-carbon composite particles is 0.6 cm³. 3 / g~1.5cm 3 The silicon-carbon composite particles have an average pore size of 1.5 nm to 20 nm; the micropore volume in the silicon-carbon composite particles accounts for 20% to 99% of the total pore volume. This application controls the total pore volume, pore size, and micropore ratio of the silicon-carbon composite particles within the above ranges, enabling better electrolyte wetting and further buffering of volume expansion during charge and discharge, thereby further improving the battery's cycle performance. For example, the total pore volume of the silicon-carbon composite particles is 0.6 cm³. 3 / g, 0.8cm 3 / g, 1.0cm 3 / g, 1.2cm 3 / g, 1.5cm 3 / g or within the range of any two of the above values. The average pore size of the silicon-carbon composite particles is 1.5nm, 5nm, 10nm, 20nm or within the range of any two of the above values. The pore volume of the micropores in the silicon-carbon composite particles accounts for 20%, 30%, 40%, 50%, 70%, 80%, 90%, 99% of the total pore volume or within the range of any two of the above values.

[0031] In one optional embodiment, the ID / IG value of the silicon-carbon composite particles is 0.7-1.5. This application controls the ID / IG value of the silicon-carbon composite particles within this range, effectively balancing the conductivity and structural stability of the material. An ID / IG value of 0.7-1.5 indicates the presence of moderate structural defects or disordered carbon in the material, which facilitates rapid lithium-ion insertion / extraction, improving rate performance; it also retains the good conductive network of the graphite domain, ensuring efficient electron transport. Furthermore, an ID / IG value within this range optimizes the volume expansion buffering capacity of the silicon-carbon composite particles during charging and discharging, alleviates the volume change stress of the negative electrode silicon, and reduces electrode cracking during charging and discharging. For example, the ID / IG value of the silicon-carbon composite particles is 0.7, 0.8, 0.9, 1.0, 1.2, 1.5, or falls within any two of the above values.

[0032] In one optional embodiment, the median particle size of the silicon-carbon composite particles is 50nm-500nm. For example, the median particle size of the silicon-carbon composite particles can be 50nm, 60nm, 70nm, 80nm, 100nm, 120nm, 150nm, 180nm, 200nm, 250nm, 280nm, 300nm, 320nm, 350nm, 400nm, 500nm, or within any two of the above values.

[0033] In one optional embodiment, the thickness of the carbon coating layer is 5 nm to 20 nm. For example, the thickness of the carbon coating layer can be 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 20 nm, or within any two of the above values.

[0034] In one optional embodiment, the conductive agent comprises carbon nanotubes with a length of 1 μm-10 μm and an aspect ratio of 100-1000. This application controls the length and aspect ratio of the carbon nanotubes within these ranges to achieve superior conductivity. When the aspect ratio or length of the carbon nanotubes is too small, the specific surface area of ​​the carbon nanotubes is too large, thus affecting battery cycle performance. When the aspect ratio or length of the carbon nanotubes is too large, the electron conduction path becomes complex, resulting in poor conductivity. For example, the length of the carbon nanotubes is 1 μm, 2 μm, 3 μm, 5 μm, 6 μm, 8 μm, 10 μm, or any two of these values. The aspect ratio is 100, 150, 200, 300, 500, 600, 800, 1000, or any two of these values.

[0035] In one optional embodiment, the mass ratio of the silicon-carbon composite particles, the conductive agent, and the first polymer binder is 80–99:0.2–8:0.2–10. For example, the mass ratio of the silicon-carbon composite particles, the conductive agent, and the first polymer binder is 80:8:10, 85:5:10, 90:5:5, 99:0.2:0.2, or falls within any two of these ranges.

[0036] In one optional embodiment, the negative electrode active material layer further includes graphite particles, and the mass content of the silicon-carbon composite particles is 7.5%-90% based on the total mass of the silicon-carbon composite particles and graphite particles. For example, the mass content of the silicon-carbon composite particles, based on the total mass of the silicon-carbon composite particles and graphite particles, can be 7.5%, 8%, 9%, 10%, 11%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 48%, 55%, 60%, 65%, 70%, 80%, 90%, or within any two of the above values.

[0037] In an optional embodiment, the negative electrode active material layer further includes a second polymer binder. The second polymer binder is a conventional polymer binder in the art, such as one or more of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyphenylene oxide (PPO), and polypropylene (PP). The mass content of the second polymer binder in the negative electrode active material layer is 1-10 wt%. For example, it can be 1 wt%, 2 wt%, 3 wt%, 5 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, or within any two of the above values.

[0038] In one optional embodiment, the repeating groups of the first polymer binder include one or more of -CN groups (cyano groups) and -CO- groups (carbonyl groups). These groups have a strong adsorption capacity for silicon, which can provide better adhesion, promote the contact between the conductive agent and the silicon-carbon composite particles, and thus give the battery higher conductivity.

[0039] In one optional embodiment, the first polymer binder comprises one or more of the following: acrylonitrile, a polymer copolymerized from one or more monomers selected from acrylate monomers, a polymer copolymerized from polyisocyanate and polyol, a polymer copolymerized from diacid and diamine, and a polymer copolymerized from diahydratomethane and diamine.

[0040] In one optional embodiment, the weight-average molecular weight of the first polymer binder is 50,000 to 800,000. By controlling the weight-average molecular weight of the first polymer binder within the above range, the bond strength (peel force) and processing performance can be effectively balanced: a suitable molecular weight (50,000 to 800,000) ensures that the polymer chains have sufficient cohesive strength and entanglement ability to provide good adhesion, while avoiding problems such as excessive melt viscosity or dispersion difficulties caused by excessively high molecular weight. This improves the film uniformity, electrode flexibility, and process adaptability of the negative electrode material, especially reducing defects such as cracking and peeling during electrode slurry coating, and optimizing the slurry dispersion with active materials and conductive agents. For example, the weight-average molecular weight of the first polymer binder is 50,000, 80,000, 110,000, 140,000, 170,000, 190,000, 210,000, 300,000, 400,000, 500,000, 600,000, 700,000, 800,000, or within any two of the above values.

[0041] In one optional embodiment, the powder conductivity of the silicon-carbon composite particles is 0.001 S / cm to 100 S / cm. By controlling the powder conductivity of the silicon-carbon composite particles within the above range, conductivity and dispersibility can be effectively balanced, ① ensuring that the conductive agent provides an efficient electron transport path and reducing the overall impedance of the electrode material; ② avoiding excessive agglomeration of the conductive agent, improving its uniform dispersion in the active material, thereby optimizing the microstructure of the electrode and enhancing the rate performance and cycle stability of the electrode. Furthermore, it reduces polarization at high current densities, improving the battery's fast-charging capability; and suppresses volume changes in the active material during charging and discharging through a uniform conductive network, extending battery life. For example, the powder conductivity of the silicon-carbon composite particles is 0.001 S / cm, 0.1 S / cm, 1 S / cm, 0.5 S / cm, 1 S / cm, 10 S / cm, 50 S / cm, 100 S / cm, or within any two of the above values. Method for determining powder conductivity: The powder conductivity of silicon-carbon composite particles is tested using a powder resistivity tester (two-electrode method). Before testing, the powder is loaded into an insulating mold (such as polytetrafluoroethylene) and mechanically pressed into a dense block to reduce the influence of contact resistance. Conductive silver paste is applied to the upper and lower surfaces of the powder or metal electrodes are placed on them.

[0042] In one optional embodiment, the range of the number of silicon-carbon composite particles in different regions of a 100µm × 100µm unit area under a scanning electron microscope (SEM) at 1000x magnification is ≤100. By controlling the range of the number of silicon-carbon composite particles in the negative electrode active material layer under SEM to be within the above range, the conductivity of the battery can be further improved. For example, the range of the number of silicon-carbon composite particles in different regions of a 100µm × 100µm unit area under a scanning electron microscope (SEM) at 1000x magnification is 5, 20, 30, 50, 60, 80, 100 or within any two of the above values.

[0043] In one optional embodiment, the peel force between the negative electrode active material layer and the negative electrode current collector is 10 N / m-30 N / m. By controlling the peel force within this range, it is possible to ensure that the active material layer and the current collector maintain close and uniform contact during cycling, reducing electron transport resistance at the interface. Controlling the peel force to 10-30 N / m is an effective means of achieving low internal resistance by optimizing the interface microstructure and mechanical stability. Within this range, the peel force can suppress microcracks or pores generated during electrode fabrication (such as coating and rolling), avoiding the formation of high-resistance points due to poor contact. Furthermore, the volume change of the active material is significantly reduced during lithium-ion insertion / extraction in the battery system. A moderate peel force can buffer stress, prevent interlayer separation, and maintain a long-term stable conductive network, thereby further reducing the battery's internal resistance. For example, the peel force between the negative electrode active material layer and the negative electrode current collector can be 10 N / m, 12 N / m, 15 N / m, 18 N / m, 20 N / m, 22 N / m, 25 N / m, 30 N / m or within any two of the above values.

[0044] In one optional embodiment, the silicon content is 4.5%-50% by mass of the total mass of the negative electrode active material layer. Limiting the silicon content of the negative electrode active material layer to this range is beneficial for further improving the problem of easy expansion of the negative electrode sheet during charging and discharging while maintaining high battery conductivity. If the silicon content is too low (<4.5%), it will lead to: ① a significant decrease in specific capacity: The theoretical specific capacity of silicon (~4200 mAh / g) is much higher than that of carbon materials (~372 mAh / g), and insufficient silicon content will weaken the energy density advantage of the negative electrode material. ② Limited improvement in expansion: The buffering effect of the carbon matrix on the volume expansion of silicon depends on sufficient silicon content. When the silicon content is too low, the carbon-dominated composite particles may still fail locally due to insufficient silicon phase dispersion. If the silicon content is too high (>50%), it will lead to: ① deterioration of the conductive network: Insufficient carbon phase ratio will reduce the overall conductivity of the particles, increase the internal resistance of the electrode, and affect rate performance and cycle stability. ② Increased volume expansion: When there is too much silicon phase, stress concentration occurs inside the particles during charging and discharging, which can easily lead to cracks or structural collapse, causing the active material to separate from the current collector. ③ Increased interfacial side reactions: An unstable SEI film is easily formed on a high-silicon surface, continuously consuming electrolyte and accelerating capacity decay. For example, based on the total mass of the negative electrode active material layer, the mass content of silicon element is 4.5%, 7%, 20%, 35%, 50%, or within any two of the above values.

[0045] In one optional embodiment, the negative electrode current collector may be one or more of copper foil, pure copper foil, nickel-plated copper foil, and carbon-coated copper foil.

[0046] In one alternative embodiment, the thickness of the negative electrode current collector is 3 μm to 18 μm; for example, the thickness of the negative electrode current collector is 3 μm, 5 μm, 10 μm, 15 μm, 18 μm or within any two of the above values.

[0047] In one alternative embodiment, the conductive agent may be selected from at least one of conductive carbon black, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0048] In one optional embodiment, the thickness of the active layer of the negative electrode material is 10 μm to 100 μm. For example, the thickness of the active layer of the negative electrode material can be 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, or within any two of the above values.

[0049] In one optional embodiment, the areal density of the active layer of the negative electrode material is 2.0 mg / cm³. 2 ~35mg / cm2 For example, the areal density of the active layer of the negative electrode material can be 2.0 mg / cm³. 2 5mg / cm 2 8mg / cm 2 10mg / cm 2 15mg / cm 2 20mg / cm 2 25mg / cm 2 30mg / cm 2 35mg / cm 2 Or it falls within the range formed by any two of the above values.

[0050] Secondly, this application provides a secondary battery, including the negative electrode sheet described in the first aspect.

[0051] In one optional embodiment, the secondary battery further includes a non-aqueous electrolyte; the non-aqueous electrolyte includes a carbonate solvent, a lithium salt, and additives; the carbonate solvent includes linear carbonate solvents and / or linear carboxylic acid ester solvents. For example, the linear carbonate solvent includes one or more of ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate. The linear carboxylic acid ester solvent includes one or more of methyl acetate, ethyl acetate, n-propyl acetate, methyl propionate, and ethyl propionate.

[0052] In one optional embodiment, the sum of the mass content of the linear carbonate solvent and the linear carboxylic acid ester solvent, based on the total mass of the non-aqueous electrolyte, is ≤60%. Studies have found that when the sum of the mass content of the linear carbonate solvent and the linear carboxylic acid ester solvent is ≤60%, it not only reduces the penetration of solvent molecules into the binder molecular chains, maintaining the stability of its cross-linked structure, but also prevents excessive softening of the binder, ensuring it maintains sufficient mechanical strength during charge and discharge to resist stress caused by volume changes in the negative electrode material, preventing the active layer from peeling off from the current collector, thereby improving battery cycle performance. When the mass content of the linear carbonate solvent and the linear carboxylic acid ester solvent exceeds 60%, the swelling effect intensifies non-linearly, leading to a sharp deterioration in electrode mechanical properties and affecting battery cycle performance. For example, the sum of the mass content of the linear carbonate solvent and the linear carboxylic acid ester solvent, based on the total mass of the non-aqueous electrolyte, can be 12%, 20%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or within any two of the above values.

[0053] In one alternative embodiment, the lithium salt comprises at least one selected from LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. The lithium salt is used in amounts commonly found in the art.

[0054] In an alternative embodiment, the secondary battery further includes a positive electrode plate, which is a lithium plate or includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector; the positive electrode active material layer includes a positive electrode active material; The specific type of the positive electrode active material is not particularly limited and can be selected according to requirements. As an example, the positive electrode active material may include, but is not limited to, lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), lithium iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, etc.). These materials can be used alone or in combination of two or more.

[0055] The positive electrode active material layer further includes at least one of a conductive agent and a binder. The conductive agent can be selected from at least one of conductive carbon black, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyvinyl alcohol (PVA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). It should be noted that the positive electrode active material, the conductive agent, and the binder are formulated according to the conventional ratio in the art. The positive electrode current collector uses materials well-known to those skilled in the art. For example, the positive electrode current collector is aluminum foil, etc.

[0056] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0057] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0058] The testing methods for the parameters involved in this invention are as follows: (1) Test method for average sphericity: After discharging the lithium-ion secondary battery to 0% SOC, the negative electrode sheet is disassembled and taken out. It is soaked in dimethyl carbonate (DMC) solvent for 12 hours and then rinsed with DMC to remove the lithium salt attached to the electrode sheet. The negative electrode active material layer is then rinsed off the electrode sheet with deionized water. After ultrasonication, the filtrate is removed by centrifugation and then dried. The obtained powder is observed by scanning electron microscope backscatter mode. In this mode, the contrast of silicon carbon particles (i.e. silicon carbon composite particles) is brighter, which can distinguish them from graphite and conductive carbon. The images of bright particles in the SEM image at a certain magnification (e.g., 2500 times) are analyzed by image processing software (e.g., Image Pro Plus) to obtain the perimeter and area of ​​each particle. The perimeter equivalent radius r1 and area equivalent radius r2 of each particle are calculated respectively. Then the average sphericity of each particle is S=r2 / r1. The average sphericity of each particle is then weighted by the number of particles to obtain the average sphericity of the matrix particles in the negative electrode sheet. (2) Method for determining the pore parameters of silicon-carbon composite particles: The silicon-carbon composite particle sample is fully degassed to remove water and impurities. The specific surface area is calculated by combining the adsorption-desorption isotherm of gas (such as nitrogen, argon or carbon dioxide) at low temperature with the BET model. The pore size distribution is analyzed by the BJH model to obtain the total pore volume, average pore size and the percentage of micropore volume in the total pore volume of silicon-carbon composite particles. (3) Test method for ID / IG of silicon-carbon composite particles: The ID / IG value of silicon-carbon composite particles is obtained by Raman spectroscopy. A confocal micro Raman spectrometer (such as Horiba LabRAM, Renishaw, etc.) is used, with a commonly used 532 nm laser wavelength and a spectral range of 1000-2000 cm⁻¹. -1(Covering both D and G peaks), exposure time 10-60 seconds, multiple accumulations (e.g., 3-5 times) to improve signal-to-noise ratio. Test multiple regions of the sample (at least 3-5 points) to avoid local inhomogeneities affecting the results. ID / IG calculation: ID / IG = D peak fitted area or intensity / G peak fitted area or intensity; (4) Method for determining the median particle size of silicon-carbon composite particles: The median particle size of silicon-carbon composite particles was determined using laser diffraction (LD). The specific steps are as follows: The silicon-carbon composite particles were dispersed in water or an organic solvent (such as ethanol), a dispersant (such as Tween 80) was added, and the mixture was sonicated (5-10 minutes) to prevent aggregation, resulting in a suspension. Instrument calibration: The instrument (such as a Malvern Mastersizer) was calibrated using a standard sample (such as polystyrene microspheres). The suspension was pumped into the sample cell, and the data was analyzed after laser scattering. The software automatically generated the D50.

[0059] (5) Method for determining the carbon coating layer of silicon-carbon composite particles: High-resolution transmission electron microscopy (HRTEM) can be used to directly observe the thickness, uniformity and interface bonding of the carbon coating layer with the silicon core. Combined with EDS (energy dispersive spectroscopy), the elemental distribution (such as C and Si) can be located, and the integrity of the coating can be verified.

[0060] (6) Test method for tube diameter and length-to-diameter ratio: The sample is observed by SEM to characterize the location and morphology of the sample. At high magnification, the length and diameter of carbon nanotubes can be observed more intuitively. The ratio of length to diameter can be indirectly estimated by comparing with a scale of known size, so as to obtain the approximate range of length-to-diameter ratio.

[0061] (7) Method for determining the weight-average molecular weight of the first polymer binder: The weight-average molecular weight of the first polymer binder was determined by gel permeation chromatography (GPC).

[0062] (8) Test method for Si element content: After discharging the lithium-ion secondary battery to 0% SOC, disassemble and remove the negative electrode sheet. Soak it in dimethyl carbonate (DMC) solvent for 12 hours, then rinse it with DMC to remove the lithium salt attached to the electrode sheet. After drying, treat the electrode sheet at 400℃ in an inert atmosphere for 2 hours (such as in a tube furnace under nitrogen or argon atmosphere). The negative electrode active material layer can then be peeled off from the current collector, and the negative electrode active material can be collected. In the silicon content test, a thermogravimetric analyzer is used. The sample amount used for the test is 5~15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature to 900℃ at a rate of 10℃ / min, and held at 900℃ for 40min. This allows the non-silicon components in the active layer of the negative electrode material to volatilize while the silicon is fully oxidized to silicon dioxide. The weight percentage at the end of the entire test process is the ash value of the negative electrode active material layer. The ash value is then divided by the molar mass of silicon dioxide (60) and multiplied by the molar mass of silicon (28) to obtain the percentage of silicon in the negative electrode active material layer. (9) Test method for the range of silicon-carbon composite particle quantity in different regions of the negative electrode active material layer: Principle: Backscattered electron (BSE) imaging is used to distinguish different materials. The intensity of the backscattered electron signal is related to the atomic number (Z) of the element; the higher the atomic number, the stronger the signal and the brighter it appears in the image. Silicon (Si, Z=14) > Carbon (C, Z=6) > Binder (mostly C, H, O). Therefore, in BSE mode, silicon-carbon composite particles are the brightest, pure graphite particles are gray, and conductive agents and binders are the darkest. This allows for very clear differentiation of target particles from other components.

[0063] Test process: Sample Preparation: Surface Observation: Take a small piece of electrode (e.g., 1cm x 1cm) and attach it to the sample stage with conductive adhesive. Since the electrode itself is conductive, gold plating is usually unnecessary (though a light gold plating may be applied if necessary to prevent charging). Cross-sectional Observation: Prepare a clean cross-section using ion polishing (CP). Cross-sectional observation provides a more accurate reflection of the distribution deep within the layer and is more reliable than surface observation.

[0064] Selection of Regions: On the cross-section of the electrode, select 10 non-overlapping fields of view for imaging, from the active layer side (surface) to the current collector side (bottom) and from left to right. Each field of view should encompass the entire electrode thickness. Under SEM, first locate and define the different regions to be compared (e.g., electrode center, edge, first, middle, and last sections along the coating direction) at low magnification (500X). Within each selected region, randomly select multiple fields of view (FOVs), for example, 10 points per region (each point measuring 100µm × 100µm). At high magnification (gradually 5,000x, 10,000x, 20,000x, depending on particle size), capture clear secondary electron (SE) or backscattered electron (BSE) images.

[0065] Image Analysis: a. Scale Calibration: Based on the scale of the SEM image, set the correspondence between pixels in the software and actual sizes (e.g., μm). b. Image Preprocessing: Adjust contrast and thresholding to separate particles from the background. For BSE images, a grayscale threshold can be set to specifically identify bright silicon-carbon particles. c. Recognition and Counting: The software automatically identifies and labels each individual particle. A minimum particle size can be set to eliminate noise. d. Data Extraction: The software outputs the total number of particles identified in each field of view, the number of silicon-carbon particles, and the area / diameter distribution of the particles. Calculate the average of the statistical results for all fields of view in each region (e.g., average number of particles). Calculate the range of the number of silicon-carbon composite particles using the following formula: Range of the number of silicon-carbon composite particles = Maximum average number of silicon-carbon composite particles - Minimum average number of silicon-carbon composite particles.

[0066] See Figure 1 As shown, the number of silicon-carbon composite particles in different regions of the negative electrode active material layer obtained in Example 1 of this application varies little, with a statistically significant difference in particle count of less than 100. See also Figure 2 As shown, the first polymer binder coats the surface of the silicon-carbon composite particles in a film-like form.

[0067] Example 1 This embodiment provides a method for preparing a negative electrode, including: (1) Deposited silicon nanoparticles (core particles) ① Preparation of porous carbon spheres: MOF powder (specifically ZIF-8 powder, Chinese name 2-methylimidazolium zinc salt) was mixed with ethanol at a mass ratio of 1:5 to obtain ZIF-8 slurry. The ZIF-8 slurry was spray granulated. The inlet temperature of the spray granulation was 180℃, the outlet temperature was 100℃, and the feed rate was 10mL / min to prepare spherical particles. High-temperature carbonization was carried out in an inert atmosphere (Ar2) (carbonization temperature was 800℃, time was 2h) to decompose MOFs and form porous carbon spheres.

[0068] ② Chemical Vapor Deposition (CVD) for Silicon Filling: Porous carbon spheres are heated to 600℃ in a CVD reaction chamber, and then a mixture of SiH4 and H2 with a volume ratio of 1:15 is introduced. Deposition is carried out at a temperature of 600℃, a gas pressure of 200 Pa, and a SiH4 gas flow rate of 50 sccm for 60 minutes to obtain deposited silicon nanoparticles (core particles).

[0069] (2) Preparation of the coating layer ① Pretreatment: The deposited silicon nanoparticles are soaked in dilute hydrofluoric acid (5% HF) to remove the natural oxide layer (SiO2) and metal impurities on the surface.

[0070] ② Carbon Coating: Pretreated silicon nanoparticles are placed in a quartz boat within a fluidized bed CVD reactor. An inert gas (such as high-purity argon, Ar2) is introduced to purge the air from the reactor. The reactor is heated to the set carbon source decomposition temperature (typically 900℃). Then, a mixture of methane (CH4) and Ar2 at a volume ratio of 1:9 is introduced. Deposition is carried out at 900℃, 100 Pa, and a CH4 flow rate of 50 sccm for 60 minutes, forming particles with a carbon coating on their surface. After deposition, the carbon source gas is stopped, and the reactor is cooled to room temperature under an inert atmosphere. The particles are then removed under an inert atmosphere (such as in a glove box). The resulting particles are then subjected to a short-time annealing treatment (60 min) at a higher temperature of 1200℃ in an inert atmosphere (such as high-purity argon, Ar2) to obtain silicon-carbon composite particles.

[0071] (3) Preparation of negative electrode sheet Silicon-carbon composite particles and graphite particles were mixed to obtain a negative electrode active material, in which the content of silicon-carbon composite particles was 16.1 wt% and the content of graphite particles was 83.9 wt%. The above negative electrode active material, conductive agent (CNT carbon nanotubes, tube length 5 μm, aspect ratio 500), binder (polyacrylonitrile, PAN, weight average molecular weight 152,000), and polyvinylidene fluoride (PVDF) were mixed and stirred at a mass ratio of 93:0.5:4:2.5. The solvent NMP was added and stirred to adjust the viscosity of the slurry, resulting in a negative electrode slurry with a viscosity of 6800 mPa·s at 25°C.

[0072] The negative electrode slurry was prepared with an areal density of 5.8 mg / cm³. 2 The carbon-coated copper foil (carbon layer thickness 2µm) with a thickness of 6μm was uniformly coated on both sides, and then dried in a coating oven at 105℃ to obtain a negative electrode sheet with a negative electrode active material layer. The compaction density of the negative electrode active material layer after rolling was 1.75 g / cm³. 3 Then, the electrode sheets are cut, laser wires are applied, and electrode tabs are welded to obtain the negative electrode sheet.

[0073] The secondary battery provided in this embodiment is prepared using the following method: (1) Preparation of positive electrode The positive electrode active material (lithium cobalt oxide, LCO), conductive agent (30wt% CNT carbon nanotubes and 70wt% carbon black), and binder (polyvinylidene fluoride) are mixed in a mass ratio of 98:1:1 to form a positive electrode slurry. This slurry is then coated onto a 9μm aluminum foil, dried, rolled, slit, and cut into sheets. Finally, tabs are welded on to form the positive electrode sheet.

[0074] (2) Electrolyte preparation An electrolyte was prepared by mixing ethylene carbonate (EC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and lithium hexafluorophosphate (LiPF6) in a mass ratio of 20:30:50:13.3.

[0075] (3) Battery assembly Bare cells are obtained by winding the negative electrode, polyethylene separator, and positive electrode in sequence. The bare cells are then placed in a battery soft pack aluminum-plastic film, top-sealed, side-sealed, dried to remove moisture, and injected with electrolyte. After top-sealing, formation, and sorting, a secondary battery is obtained.

[0076] Example 2-3 The preparation process is basically the same as in Example 1. The only difference is that in the preparation process of porous carbon balls in step (1), the mass ratio of MOF powder to ethanol, feed rate, inlet temperature and outlet temperature in the ZIF-8 slurry of spray granulation are adjusted, or in the carbon coating process in step (2), one or more of the parameters such as gas pressure, gas flow rate, deposition temperature and deposition time of fluidized bed chemical vapor deposition are adjusted, which makes the average sphericity of silicon-carbon composite particles different, as shown in Table 1.

[0077] Specifically, in Example 2, during the preparation of porous carbon spheres, during the carbon coating process, the reaction gas pressure was increased to 500 Pa in fluidized bed chemical vapor deposition. In Example 3, during the preparation of porous carbon spheres, the inlet temperature of the spray granulation was reduced to 150°C and the outlet temperature was reduced to 80°C. In the carbon coating process, the flow rate of methane (CH4) gas was reduced to 20 sccm in fluidized bed chemical vapor deposition.

[0078] Examples 4-7 The preparation process is basically the same as in Example 1. The only difference is that in the preparation of porous carbon spheres in step (1), one or more of the following parameters are adjusted: the composition of ZIF-8 slurry, carbonization temperature, composition of carbonization atmosphere, and whether etching is performed. This makes the pore parameters of silicon-carbon composite particles (e.g., total pore volume, average pore diameter, and / or percentage of micropore volume to total pore volume) different, as shown in Table 1.

[0079] Specifically, in Example 4: the carbonization temperature was lowered to 700°C, and an activation atmosphere was introduced simultaneously, with CO2 being introduced, so that the particles were carbonized in a mixed atmosphere of CO2 and Ar2 with a volume ratio of 3:97.

[0080] Example 5: When preparing ZIF-8 slurry, 50 wt% of surfactant poloxamer 407 (F127) was added to ZIF-8 powder, and the carbonization temperature was increased to 1000°C. The surfactant decomposed during the carbonization process, leaving mesopores.

[0081] Example 6: When preparing ZIF-8 slurry, nano-SiO was added at a mass of 75 wt% of ZIF-8 powder. Simultaneously, after high-temperature carbonization, porous carbon spheres were etched with a 5% HF solution to remove the SiO2 template. The etching time was 1 hour. After etching, macropores and / or mesopores formed by the SiO2 template occupying the space were left in the porous carbon spheres.

[0082] Example 7: The carbonization temperature was reduced to 600°C.

[0083] Examples 8-11 The preparation process is basically the same as in Example 1, except that the temperature of fluidized bed chemical vapor deposition (i.e., coating temperature) is adjusted during the carbon coating process in step (2), resulting in different ID / IG ratios of the silicon-carbon composite particles, as shown in Table 1. Specifically, in Example 8, the carbon coating temperature was increased to 1000℃; in Example 9, the carbon coating temperature was decreased to 800℃; in Example 10, the carbon coating temperature was decreased to 700℃; and in Example 11, the carbon coating temperature was increased to 1100℃.

[0084] Examples 12-13 The preparation process is basically the same as in Example 1. The only difference is that in the preparation process of porous carbon balls in step (1), the mass ratio of MOF powder to ethanol, the inlet temperature, the feed rate and other parameters in the ZIF-8 slurry of spray granulation are adjusted to make the median particle size of silicon-carbon composite particles different, as shown in Table 1.

[0085] Specifically, in Example 12: the mass ratio of ZIF-8 to ethanol in the slurry was adjusted to 1:10, the spray inlet temperature was adjusted to 200℃, and the spray feed rate was adjusted to 4mL / min.

[0086] Example 13: The mass ratio of ZIF-8 to ethanol in the slurry was adjusted to 1:3, the spray inlet temperature was adjusted to 150℃, and the spray feed rate was adjusted to 18mL / min.

[0087] Examples 14-15 The preparation process is basically the same as in Example 1, except that the deposition time of the fluidized bed chemical vapor deposition process in step (2) ② during carbon coating is adjusted, resulting in different carbon coating thicknesses. Specifically, in Example 14, the deposition time is adjusted to 20 minutes, and the carbon coating thickness in this example is 5.2 nm. In Example 15, the deposition time is adjusted to 80 minutes, and the carbon coating thickness in this example is 20 nm.

[0088] Examples 16-19 The preparation process is basically the same as in Example 1, except that carbon nanotubes with different lengths and aspect ratios are used. Specifically, Example 16 uses carbon nanotubes with a length of 1 μm and an aspect ratio of 100, Example 17 uses carbon nanotubes with a length of 10 μm and an aspect ratio of 1000, Example 18 uses carbon nanotubes with a length of 0.5 μm and an aspect ratio of 50, and Example 19 uses carbon nanotubes with a length of 12 μm and an aspect ratio of 1200.

[0089] Examples 20-21 The preparation process is basically the same as in Example 1, except that the type of the first polymer binder is different. In Example 20, polyacrylic acid with a weight average molecular weight of 700,000 was used instead of polyacrylonitrile, and in Example 21, polymethacrylate with a weight average molecular weight of 720,000 was used instead of polyacrylonitrile.

[0090] Examples 22-25 The preparation process was basically the same as in Example 1, except that PAN of different molecular weights was used as the first polymer binder instead of the PAN in Example 1. Example 22 used PAN with a weight-average molecular weight of 704,000, Example 23 used PAN with a weight-average molecular weight of 51,000, Example 24 used PAN with a weight-average molecular weight of 855,000, and Example 25 used PAN with a weight-average molecular weight of 48,900.

[0091] Example 26 The preparation process is basically the same as in Example 1, except that the different discharge viscosity of the negative electrode slurry causes different ranges in the number of silicon-carbon composite particles in different regions of a 100µm×100µm unit area under a scanning electron microscope at 1000x magnification, as shown in Table 1. Specifically, in Example 26, "adding solvent NMP and stirring to adjust the slurry viscosity to obtain a negative electrode slurry with a viscosity of 6800 mPa·s at 25°C" is replaced with "adding solvent NMP and stirring to adjust the slurry viscosity to obtain a negative electrode slurry with a viscosity of 10000 mPa·s at 25°C," which increases the range of the number of silicon-carbon composite particles in different regions to 120.

[0092] Examples 27-30 The preparation process is basically the same as in Example 1, except that the mass content of silicon-carbon composite particles and graphite in the negative electrode active material in the slurry is adjusted to make the silicon content in the negative electrode active material layer different, as shown in Table 2.

[0093] Examples 31-32 The preparation process is basically the same as in Example 1, except that the content of linear carbonate + linear carboxylic acid ester in the electrolyte is adjusted so that the sum of the mass contents of linear carbonate solvent and linear carboxylic acid ester solvent is different. Specifically, the content of linear carbonate + linear carboxylic acid ester in the electrolyte of Example 31 is adjusted to 60%, and the content of linear carbonate + linear carboxylic acid ester in the electrolyte of Example 32 is adjusted to 20%.

[0094] Example 31: Ethyl carbonate (EC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and lithium hexafluorophosphate (LiPF6) were mixed evenly in a mass ratio of 25:20:40:15 to prepare an electrolyte.

[0095] Example 32: Ethyl carbonate (EC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), PP, and lithium hexafluorophosphate (LiPF6) were mixed evenly in a mass ratio of 67:8:8:4:13 to prepare an electrolyte.

[0096] Comparative Example 1 The preparation process is basically the same as in Example 1. The only difference is that in the preparation process of porous carbon balls in step (1), the feed rate, inlet temperature, and outlet temperature of spray granulation are adjusted, or in the carbon coating process in step (2), one or more of the parameters such as gas pressure and gas flow rate of fluidized bed chemical vapor deposition are adjusted, which makes the average sphericity of silicon-carbon composite particles different, as shown in Table 1. Specifically, in the carbon coating process of this comparative example, the flow rate of methane (CH4) gas is reduced to 20 sccm, the deposition temperature is reduced to 700℃, and the deposition pressure is increased to 500Pa.

[0097] Comparative Example 2 The preparation process is basically the same as in Example 1, except that water is used instead of NMP solvent in the preparation of the negative electrode.

[0098] Comparative Examples 3-4 The preparation process was basically the same as in Example 1, except that the type of the first polymer binder was adjusted. In Comparative Example 3, PPO with a weight-average molecular weight of 50,000 was used instead of polyacrylonitrile, and in Comparative Example 4, PVDF with a weight-average molecular weight of 800,000 was used instead of polyacrylonitrile.

[0099] Table 1 Physical parameters of silicon-carbon composite particles

[0100] Table 2. Mass content of silicon-carbon composite particles and mass content of silicon element.

[0101] Experimental Example 1 1. Test of the peeling force of the negative electrode sheet (1) Sample preparation: a. Negative electrode sample: length > 300mm, width 24mm, sample quantity: no less than 5 samples per batch.

[0102] b. Apply PF adhesive to the steel plate, then apply double-sided tape (length same as the steel plate, width > 24mm), attach the electrode sheet to the double-sided tape, and roll it three times to remove air bubbles and wrinkles.

[0103] (2) Test: Fix the steel plate to one clamp of the electric tensile testing machine, while the other clamp holds the free end of the electrode. At a 180° angle to the steel plate, pull the electrode apart from the copper foil by 5 mm at the standard test rate (100 mm / min) to ensure the coating remains continuous and unbroken during the peel force measurement. Measure the peel force by the force required to continuously peel the coating from the copper foil to the steel plate, and convert this force to peel strength (unit: kgf). Note that the peel line is perpendicular to the direction of the force.

[0104] (3) Data processing: Record the maximum and average values, analyze the peel strength curve, remove outliers, and calculate the peel force using the following formula: σ180°=(F×g) / B σ180°—Peel strength, kN / m F—peeling force, kgf g—acceleration due to gravity, m / s² B—Sample width, mm.

[0105] 2. Lithium-ion battery EIS test at 25℃ and 50% SOC ①25℃ 50% SOC EIS Test: Discharge at 0.2C to the lower limit voltage at 25℃±5℃; let stand for 10 minutes; ② Adjust SOC: Charge at a constant current of 0.7C to 3.95V, then charge at a constant voltage of 0.05C for 60 minutes, followed by a 10-minute rest period. Then perform an AC impedance test with the following parameters: highest frequency: 50kHz~5kHz, starting frequency: consistent with the highest frequency value, lowest frequency: approximately 100mHz, scanning method: scanning from high frequency to low frequency, test mode: POTENTIOSTAT mode, AMPLITUDE: 5mV.

[0106] ③ Data preprocessing: Check the integrity of the raw data, eliminate outliers, and perform smoothing if necessary to reduce noise. Confirm the test frequency range (usually 10mHz-100kHz) and the number of test points.

[0107] ④ Nyquist plot: Plot the relationship between the real part (Z') and the imaginary part (-Z") of the impedance. At 50% SOC, it usually presents a semicircle (high frequency region) and a diagonal line (low frequency region).

[0108] ⑤ Equivalent circuit model fitting: Select a suitable equivalent circuit model (commonly Randle or modified Randle model), use nonlinear least squares method for fitting, and evaluate the goodness of fit (χ²). 2 value).

[0109] ⑥ Parameter extraction: Ohm resistance (R S): The intersection point of the high-frequency impedance curve and the left side of the real axis (Z' axis), the SEI film resistance (R SEI The diameter of the first (or overlapping with the second) semicircle appearing in the high-frequency region; charge transfer resistance (Rct): the diameter of the second semicircle in the Nyquist plot. Characteristics of 50% SOC data: Compared to extreme SOCs (such as 10% or 90%), 50% SOC typically exhibits: moderate charge transfer resistance, a more symmetrical Nyquist plot, better fit, and can serve as a benchmark for battery state of health (SOH) assessment. Rs, RSEI, Rct, and Rtotal are simulated based on its Nyquist plot, where Rtotal = Rs + RSEI + Rct.

[0110] 3. Test of rebound rate 48 hours after rolling: (1) Sampling: Take the negative electrode sheet after rolling. In this test example, take 5 square samples of the same size (the length of the negative electrode sheet in this test example is 75mm×1.7m).

[0111] (2) Testing: Initial thickness measurement (T0): The first thickness measurement is performed immediately after the electrode rolling is completed. The average value is calculated as the initial thickness T0 of the sample.

[0112] Place the tray in a constant temperature and humidity environment (25℃, relative humidity <1%) and let it stand for 48 hours. Ensure that the sample is not subjected to any external pressure or interference during this period.

[0113] Final thickness measurement (T48): After standing for 48 hours, immediately remove the samples. Using the same micrometer, measure the thickness of each sample again at the exact same location as when measuring T0. Calculate the average value as the final thickness T48 of the sample.

[0114] (3) Data processing and analysis: For each sample, calculate its thickness rebound value and rebound rate.

[0115] Thickness rebound value (μm) = T48 - T0; Thickness rebound rate (%) = [(T48-T0) / T0] × 100%; The average rebound value and average rebound rate of all samples were calculated to assess consistency and stability.

[0116] 4. Electrode peeling force at 0% SOC of the lower battery: (1) Preparation of negative electrode sample for testing peel strength: A. Discharge the battery to 3.0V empty charge (0soc), disassemble it and take samples of the negative electrode. Prepare a negative electrode with a length of 79mm and a width of 24mm. Sample quantity: 5 samples per batch.

[0117] B. Apply pressure-sensitive adhesive to the steel plate, then apply double-sided tape (length same as the steel plate, width > 24mm). Place the electrode sheet on the double-sided tape and roll it three times to remove air bubbles and wrinkles.

[0118] (2) Peel force test: Fix the steel plate to one clamp of the electric tensile testing machine, while the other clamp holds the free end of the electrode. At a 180° angle to the steel plate, pull the electrode apart from the copper foil by 5 mm at the standard test rate (100 mm / min) to ensure the coating remains continuous and unbroken during the peel force measurement. Measure the peel force by the force required to continuously peel the coating from the copper foil to the steel plate, and convert this force to peel strength (unit: kgf). Note that the peel line is perpendicular to the direction of the force.

[0119] (3) Data processing: Record the maximum and average values, analyze the peel strength curve, remove outliers, calculate the peel force according to the formula, and take the average value of the test results. The formula for calculating the peel force is as follows: σ180° = (F × g) / B; σ180°—Peel strength, KN / m; F—peeling force, kgf; g—acceleration due to gravity, m / s² 2 ; B—Sample width, mm.

[0120] 4. Electrode Resistors The surface resistance of the negative electrode was measured using the four-probe method.

[0121] Table 3 Performance Test Results

[0122] Experiment Example 2: Electrical Performance Testing 1. Loop testing The test temperature was 25±5℃. The battery was discharged at 0.5C to 3V and left to stand for 10 minutes; then it was charged at a constant current of 0.5C to 3.95V, and then charged at a constant voltage of 0.05C. After standing for 10 minutes, the battery thickness was measured and used as the initial thickness.

[0123] Charge at a constant current of 2.0C to 4.25V, charge at a constant voltage of 1.5C, switch to constant current of 1.5C to 4.55V, charge at a constant voltage of 0.05C, let stand for 10 minutes, discharge at 1.5C to 3.5V, then discharge at 0.7C to 3.0V, record the first discharge capacity, let stand for 10 minutes, and perform cycle tests using this charge and discharge procedure.

[0124] A full-charge thickness test is performed every 100 cycles. After 500 cycles, the capacity retention rate and thickness expansion rate are calculated. Capacity retention rate after 500 cycles = Discharge capacity of the 500th cycle / Discharge capacity of the first cycle; Thickness expansion rate after 500 cycles = (Thickness at full charge after 500 cycles - Initial thickness) / Initial thickness.

[0125] 2. Amplification performance test (1) Test preparation Equipment: Charge / discharge tester (e.g., Neware, Arbin), constant temperature chamber (optional), data recording system. Battery samples: New batteries require initial formation (initial charge / discharge activation). Environmental conditions: Typically conducted at a constant temperature of 25°C.

[0126] (2) Testing process Rate discharge test procedure: The test is conducted at 25±5℃: the lithium-ion battery is charged to 4.55V at a constant current of 0.2C, then charged to 0.05C at a constant voltage, left to stand for 10 minutes, and then discharged to 3V at a constant current of 0.2C. The discharge capacity at this time is Q0.

[0127] Then, charge at 0.2C to 4.55V, then charge at constant voltage to 0.05C, let stand for 10 minutes, and discharge at 0.5C to 3V; at this point, the discharge capacity is Q1, then the 0.5C rate performance = Q1 / Q0 × 100%; Then, charge at 0.2C to 4.55V, then charge at constant voltage to 0.05C, let stand for 10 minutes, and then discharge at 1C to 3V; at this time, the discharge capacity is Q2, and the 1C rate performance is Q2 / Q0×100%.

[0128] Then, charge at 0.2C to 4.55V, then charge at constant voltage to 0.05C, let stand for 10 minutes, and discharge at 2C to 3V; at this time, the discharge capacity is Q3, and the 2C rate performance is Q3 / Q0×100%.

[0129] Then, charge at 0.2C to 4.55V, then charge at constant voltage to 0.05C, let stand for 10 minutes, and then discharge at 3C to 3V; at this time, the discharge capacity is Q3, and the 3C rate performance is Q4 / Q0×100%.

[0130] Table 4 Electrical performance test results

[0131] As can be seen from Tables 3 and 4, compared with Comparative Examples 1-4, the structural stability and conductivity of the negative electrode active material layer on the negative electrode sheet prepared in each embodiment are significantly improved, greatly improving the problem of battery expansion and significantly improving the cycle performance, conductivity and rate performance of the battery.

[0132] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector. The negative active material layer includes silicon-carbon composite particles, a conductive agent, and a first polymer binder coating the surface of the silicon-carbon composite particles. The silicon-carbon composite particles include core particles and a carbon coating layer distributed on the surface of the core particles. The core particles include porous carbon material and silicon distributed inside the cavities of the porous carbon material. The average sphericity of the silicon-carbon composite particles is ≥0.

85. The first polymer binder includes one or more of polyacrylonitrile, polyacrylate, polyurethane, polyamide, polyimide, polyacrylic acid, polyacrylate, polymethyl methacrylate-polyvinylidene fluoride copolymer, polymethyl methacrylate-polypropylene copolymer, and polypropylene-polymethyl methacrylate-polyvinylidene fluoride copolymer.

2. The negative electrode sheet according to claim 1, characterized in that, The silicon content of the silicon-carbon composite particles is 30%-70% by mass; and / or the total pore volume of the silicon-carbon composite particles is 0.6 cm³. 3 / g~1.5cm 3 / g, the average pore size of the silicon-carbon composite particles is 1.5nm-20nm; the pore volume of micropores in the silicon-carbon composite particles accounts for 20%-99% of the total pore volume; and / or, the ID / IG value of the silicon-carbon composite particles is 0.7-1.5; and / or, the median particle size of the silicon-carbon composite particles is 50nm-500nm; and / or, the thickness of the carbon coating layer is 5nm-20nm.

3. The negative electrode sheet according to claim 1, characterized in that, The conductive agent includes carbon nanotubes with a length of 1μm-10μm and an aspect ratio of 100-1000; and / or, the mass ratio of the silicon-carbon composite particles, the conductive agent, and the first polymer binder is 80-99:0.2-8:0.2-10; and / or, the negative electrode active material layer further includes a second polymer binder.

4. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material layer also includes graphite particles, and the mass content of the silicon-carbon composite particles is 7.5%-90% based on the total mass of the silicon-carbon composite particles and the graphite particles.

5. The negative electrode sheet according to claim 1, characterized in that, The repeating groups of the first polymer binder include one or more of the -CN groups and -CO- groups.

6. The negative electrode sheet according to claim 1, characterized in that, The first polymer binder is coated on the surface of the silicon-carbon composite particles in the form of a film; and / or, the weight-average molecular weight of the first polymer binder is 50,000 to 800,000.

7. The negative electrode sheet according to claim 1, characterized in that, The number of silicon-carbon composite particles in different regions of a 100µm×100µm unit area of ​​the negative electrode active material layer, as observed under a scanning electron microscope, varies by a range of ≤100.

8. The negative electrode sheet according to claim 1, characterized in that, The peel force between the negative electrode active material layer and the negative electrode current collector is 10 N / m-30 N / m; and / or, based on the total mass of the negative electrode active material layer, the mass content of silicon element is 4.5%-50%.

9. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-8. Optionally, the secondary battery further includes a non-aqueous electrolyte; the non-aqueous electrolyte includes a carbonate solvent, a lithium salt, and additives; the carbonate solvent includes linear carbonate solvents and / or linear carboxylic acid ester solvents.

10. The secondary battery according to claim 9, characterized in that, Based on the total mass of the non-aqueous electrolyte, the sum of the mass contents of linear carbonate solvents and linear carboxylic acid ester solvents is ≤60%.

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