Negative electrode sheet and secondary battery

Abstract

The application belongs to the field of batteries, and discloses a negative electrode sheet and a secondary battery. The negative electrode sheet comprises a negative electrode coating, the negative electrode coating comprises a negative electrode active layer and a primer layer which are arranged in a stack, the negative electrode active layer comprises a negative electrode active material and an oil-based binder, the negative electrode active material comprises a silicon-based material, and the oil-based binder contains a cyano group; the primer layer is located between the negative electrode current collector and the negative electrode active layer; and the primer layer comprises a water-based binder, and the water-based binder contains a carboxyl group. By optimizing the binder structure in the negative electrode active layer and the primer layer, the conductivity and the anchoring force of the negative electrode active layer can be improved, the buffer capacity of the primer layer to the volume change of silicon can be enhanced, and the interface bonding between the negative electrode active layer and the primer layer can be improved, so that the negative electrode sheet has the advantages of good structural stability and good conductivity, thereby providing reliable structural support for the structural stability and the electrochemical performance of the high-silicon battery system under long-cycle working conditions.

Description

Technical Field

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

[0002] Silicon-carbon composite materials are considered the most promising next-generation lithium-ion battery anode active material due to their high theoretical specific capacity (the theoretical specific capacity of silicon is approximately 4200 mAh / g). However, the silicon particles in these materials undergo drastic volume expansion and contraction during battery charging and discharging (the volume change rate can reach over 300%). The resulting enormous mechanical stress can lead to problems such as bonding failure, conductive network breakage, and separation of the bonding interface between the active layer and the current collector. Ultimately, this results in a sharp decline in the battery's cycle performance and a deterioration in its rate performance. Summary of the Invention

[0003] In view of this, the technical problem to be solved by this application is to overcome the defects of existing silicon-based anode systems, such as active layer adhesion failure and insufficient interfacial bonding between the active layer and the current collector, and to comprehensively improve the cycle retention rate of the battery, reduce battery impedance and cycle expansion.

[0004] To achieve the above objectives, this application adopts the following technical solution.

[0005] According to an embodiment of this application, in a first aspect, a negative electrode sheet is provided, comprising a negative electrode current collector and a negative electrode coating disposed on at least one side surface of the negative electrode current collector in the thickness direction; the negative electrode coating comprises a negative electrode active layer and a base layer stacked thereon; the negative electrode active layer comprises a negative electrode active material and an oil-based binder, the negative electrode active material comprising a silicon-based material, and the oil-based binder comprising cyano groups; the base layer is located between the negative electrode current collector and the negative electrode active layer; the base layer comprises an aqueous binder, and the aqueous binder comprises carboxyl groups.

[0006] In some optional embodiments, the mass content of the cyano groups is A%, 0.05≤A≤2.5%, based on the mass of the negative electrode active layer.

[0007] In some alternative embodiments, the oil-based binder includes at least one of polyacrylonitrile, nitrile rubber, cyano-modified oil-based polyurethane, cyano-modified polyamide, cyano-modified polyimide, and cyano-modified cellulose.

[0008] In some alternative embodiments, the acid value of the primer coating is B mg KOH / g, with 40 ≤ B ≤ 80, based on the mass of the primer coating.

[0009] In some alternative embodiments, the water-based adhesive includes at least one of carboxyl-modified styrene-butadiene rubber, carboxyl-modified water-based polyurethane, and polyacrylic acid.

[0010] In some alternative embodiments, the thickness of the base coating is H1 μm, where 0.2 ≤ H1 ≤ 2.

[0011] In some alternative embodiments, the peel force between the negative electrode current collector and the negative electrode coating is FN / m, where F≥20.

[0012] In some optional embodiments, the number average molecular weight of the oil-based binder is Mn, where Mn ≤ 200,000.

[0013] In some alternative embodiments, the silicon-based material comprises silicon-carbon particles, any two 100 μm particles in the surface SEM image of the negative electrode. Within a 100 μm region, the difference in the number of silicon-carbon particles is C, where C ≤ 100.

[0014] In some optional embodiments, the silicon-carbon particles comprise porous carbon and silicon particles located within the porous carbon channels, and a carbon layer coating the surface of the porous carbon; satisfying at least one of the following conditions: (1) The sphericity of the silicon-carbon particles is φ, where φ ≥ 0.85; (2) The particle size Dv50 of the silicon carbide particles is 6μm-12μm; (3) The total pore volume of the porous carbon is 0.6 cm³. 3 / g-1.5cm 3 / g, with an average pore size of 1.5nm-50nm and a micropore volume ratio of 20%-99%; (4) The powder conductivity of the silicon carbide particles at 25℃ and 5MPa is 0.001S / cm-100S / cm; (5) In the Raman spectrum of the silicon-carbon particles, the Raman shift is 1350±10 cm⁻¹. -1 The peak at this location is designated as peak D, with a Raman shift of 1580 ± 10 cm. -1 The peak at the location is denoted as peak G, and the ratio of the peak intensity of peak D to peak G is ID / IG, where 0.7 ≤ ID / IG ≤ 1.5.

[0015] In some alternative embodiments, the mass percentage of silicon in the negative electrode active layer is 7%-50%, based on the mass of the negative electrode active layer.

[0016] In some optional embodiments, the negative electrode active material further includes graphite material, wherein the mass ratio of the silicon carbon particles to the graphite material is (20-60):(40-80) based on the negative electrode active material.

[0017] In some optional embodiments, the negative electrode active layer further includes a conductive agent and a dispersant, wherein the mass ratio of the negative electrode active material to the oil-based binder, the conductive agent, and the dispersant is (80-99):(0.2-10):(0.2-8):(0.2-5).

[0018] In some alternative embodiments, the dispersant includes at least one of polyethylene, polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride, polyvinylpyrrolidone, polytetrafluoroethylene, and epoxy resin.

[0019] In some optional embodiments, the thickness of the negative electrode active layer on one side is H2 μm, where 30≤H2≤260.

[0020] According to an embodiment of this application, in a second aspect, a secondary battery is also provided, the secondary battery including the negative electrode sheet described in the first aspect of this application.

[0021] In some optional embodiments, the secondary battery further includes an electrolyte comprising a linear ester solvent; the mass percentage of the linear ester solvent is E, ≤ 60%, based on the mass of the electrolyte.

[0022] In some optional embodiments, the linear ester solvent includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and isomethyl butyrate.

[0023] 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 coating disposed on at least one side surface of the negative electrode current collector in the thickness direction; the negative electrode coating includes a negative electrode active layer and a base layer stacked thereon; the negative electrode active layer includes a negative electrode active material and an oil-based binder, the negative electrode active material includes a silicon-based material, and the oil-based binder contains cyano groups; the base layer is located between the negative electrode current collector and the negative electrode active layer; the base layer includes an aqueous binder, and the aqueous binder contains carboxyl groups.

[0024] This application optimizes the binder structure in the negative electrode active layer and the base coating layer, thereby improving the conductivity and bonding anchoring force of the negative electrode active layer, enhancing the buffering capacity of the base coating layer to cope with silicon volume changes, and improving the interfacial bonding between the negative electrode active layer and the base coating layer. This results in a negative electrode sheet with advantages of good structural stability and excellent conductivity, thus providing a reliable structural guarantee for the structural stability and electrochemical performance of high-silicon battery systems under long-cycle conditions.

[0025] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Detailed Implementation

[0026] 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.

[0027] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0028] Silicon-based anodes typically use a water-based binder that is a blend of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC). CMC molecules contain a large number of carboxyl groups, which can form strong hydrogen bonds with the Si-OH naturally present on the surface of silicon particles, ensuring the interfacial bonding between silicon-based materials. SBR can provide high resilience and flexibility to buffer the expansion and contraction of silicon particles and maintain the integrity of the electrode structure.

[0029] However, this study found that, since SBR itself consists of discrete latex particles, after drying and forming a film, the particles only fuse at the contact points, forming a localized point-like bonding structure. The effective bonding area is small, and the anchoring force between the negative electrode material particles and between the active layer and the current collector is insufficient. This makes it difficult to cope with the repeated and drastic volume changes of the silicon-based material during cycling, especially for negative electrodes with high silicon content (>30%). As cycling progresses, the SBR bonding points are prone to fatigue failure, leading to negative electrode powder shedding and conductive network breakage. Furthermore, the point-like bonding of SBR cannot form a continuous constraint network. After the electrode is rolled, the elastic rebound is large, making it difficult to achieve the target compaction density. Moreover, SBR swells significantly after contact with the electrolyte, further deteriorating its bonding effect. In addition, the surface affinity between the SBR molecular chains and silicon particles is poor, making it difficult to effectively maintain the close contact between the conductive agent (such as carbon nanotubes) and the silicon particles, affecting the conductivity of the negative electrode.

[0030] To improve the adhesion between the negative electrode active layer and the current collector, related technologies often pre-coat a base layer on the current collector surface. This base layer can be a carbon coating mainly composed of conductive carbon black and a binder. Considering factors such as efficiency, environmental friendliness, and cost, aqueous binders like SBR are often chosen for the base layer. This study found that in existing silicon-based negative electrodes, the adhesion between the negative electrode active layer and the base layer is poor. The reason for this is that both the negative electrode active layer and the base layer use binders containing SBR. The dotted bonding characteristics of SBR result in weak interfacial adhesion between the negative electrode active layer and the base layer. Especially in high-silicon systems, the significant volume change of silicon during cycling generates significant shear stress at the interface between the negative electrode active layer and the base layer, easily causing the negative electrode active layer to peel off completely from the base layer surface (i.e., "delamination"), leading to a sharp increase in battery internal resistance and rapid capacity decay.

[0031] Based on this, in order to solve the defects of silicon-based anodes in related technologies, such as active layer adhesion failure and insufficient interfacial bonding between the active layer and the base layer, this application proposes the following solution.

[0032] According to a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode coating disposed on at least one side surface of the negative electrode current collector in the thickness direction; the negative electrode coating includes a negative electrode active layer and a base layer stacked thereon; the negative electrode active layer includes a negative electrode active material and an oil-based binder, the negative electrode active material includes a silicon-based material, and the oil-based binder contains cyano groups; the base layer is located between the negative electrode current collector and the negative electrode active layer; the base layer includes an aqueous binder, and the aqueous binder contains carboxyl groups.

[0033] In this application, the term "oil-based binder" refers to a binder that can be dissolved in an organic solvent (such as N-methylpyrrolidone, abbreviated as NMP; or dimethylacetamide, abbreviated as DMAC). This type of binder completely dissolves in the organic solvent to form a gel, which, after drying, can form a continuous film on the surface of the powder to achieve surface bonding. The term "water-based binder" refers to a binder that can be dissolved or dispersed in water or ethanol. This type of binder exists in water in the form of latex dispersion or water-soluble form, and the dried adhesive particles achieve a bonding effect through point contact.

[0034] This application introduces an oil-based binder containing cyano (-CN) groups into the negative electrode active layer. First, after drying, the oil-based binder forms a continuous film structure that coats the surface of the negative electrode material. This undoubtedly increases the anchoring force between the negative electrode material particles and between the active layer and the current collector. This not only effectively adapts to the volume changes of the silicon-based material and helps maintain the integrity of the negative electrode structure, but also improves the dispersibility of the negative electrode slurry and enhances the conductivity of the electrode sheet. Furthermore, the cyano groups in oil-based binders contain lone pairs of electrons. On the one hand, these lone pairs of electrons can interact with the π-electron system on the surface of conductive agents (such as carbon nanotubes), which is beneficial for building a stable conductive network inside the negative electrode active layer and reducing interfacial resistance. On the other hand, these lone pairs of electrons can also form hydrogen bonds, coordination interactions, and dipole-dipole electrostatic attraction with hydroxyl groups or silicon-oxygen bonds on the surface of silicon-based materials, thereby enhancing the adhesion between the oil-based binder and the silicon-based material and preventing adhesion failure. Thirdly, the cyano groups in the oil-based binder in the negative electrode active layer can also form strong hydrogen bonds with the carboxyl groups in the water-based binder in the base layer, enhancing the interfacial bonding force between the active layer and the base layer and suppressing the problem of negative electrode delamination.

[0035] Furthermore, this application incorporates a base coating layer between the negative electrode current collector and the negative electrode active layer. This transforms the point contact between the active layer and the current collector into a surface contact, further reducing interfacial impedance. The base coating layer also prevents direct contact between the electrolyte and the current collector, reducing side reactions. Additionally, it increases the surface roughness of the current collector, enhancing the adhesion of the active layer, and passivates the current collector surface, preventing copper leaching and contamination of the active material at high temperatures or high potentials. More importantly, the base coating layer uses a carboxyl-containing aqueous binder. Its flexible segments provide the base coating with a certain elastic buffering capacity, absorbing the stress generated by the volume expansion of silicon particles, alleviating stress accumulation within the active layer, and preventing pulverization of the active layer.

[0036] In summary, by optimizing the binder structure in the negative electrode active layer and the base coating, this application can improve the conductivity and bonding anchoring force of the negative electrode active layer, enhance the buffering capacity of the base coating to cope with silicon volume changes, and improve the interfacial bonding between the negative electrode active layer and the base coating. This results in a negative electrode sheet with advantages of good structural stability and excellent conductivity, thereby providing a reliable structural guarantee for the structural stability and electrochemical performance of high-silicon battery systems under long-cycle conditions.

[0037] In some embodiments, based on the mass of the negative electrode active layer, by controlling the mass content A of cyano groups between 0.05% and 2.5%, it is possible to ensure that the cyano groups provide sufficient lone pair electrons to adsorb and build a conductive network with the π-electron system of the conductive agent, and form hydrogen bonds / coordination with the silicon-based material surface to enhance cohesive adhesion. At the same time, it is also possible to maintain strong hydrogen bonding with the carboxyl groups of the undercoating layer, thereby improving the conductivity, structural stability, and cycle performance of the negative electrode sheet. If A is too small, the number of cyano groups in the oil-based binder will be insufficient, making it difficult to form a sufficient conductive network and interfacial hydrogen bonds, resulting in increased interfacial resistance and decreased peel strength. If A is too large, it means that the content of oil-based binder is too high, which will dilute the proportion of active material, reduce the energy density of the negative electrode sheet, and may increase electrode polarization.

[0038] It should be noted that the mass content of cyano groups can be obtained by conventional methods in the art, such as thermogravimetric analysis. For example, the mass content of cyano groups can be 0.05%, 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.5%, or values ​​within any range of two of the above values.

[0039] In some embodiments, the oil-based binder includes at least one of polyacrylonitrile (PAN), nitrile rubber, cyano-modified oil-based polyurethane, cyano-modified polyamide, cyano-modified polyimide, and cyano-modified cellulose. All of the above oil-based binders contain abundant cyano groups, which can form a continuous film structure after drying, increasing the anchoring force between negative electrode material particles and between the active layer and the current collector, effectively adapting to volume changes in silicon-carbon particles. Simultaneously, the nitrogen atoms in the cyano groups possess lone pairs of electrons. Firstly, these lone pairs of electrons can adsorb onto the π-electron system on the surface of conductive agents (such as carbon nanotubes), constructing a stable conductive network within the active layer. Secondly, these lone pairs of electrons can also form hydrogen bonds or coordination interactions with hydroxyl groups or siloxane bonds on the surface of silicon-carbon materials, enhancing the adhesion between the oil-based binder and the silicon-based material. Thirdly, the cyano group can form strong hydrogen bonds with the carboxyl groups of the aqueous binder in the undercoat, further improving the interfacial bonding force between the active layer and the undercoat. In addition, the above-mentioned binder has good solubility or dispersion properties in organic solvents (such as N-methylpyrrolidone), which is beneficial to the formation of a uniform negative electrode slurry, thereby improving the dispersion uniformity of silicon carbon particles in the electrode.

[0040] In this application, cyano-modified oil-based polyurethane, cyano-modified polyamide, cyano-modified polyimide, and cyano-modified cellulose can be prepared by conventional methods in the art. In one embodiment, as an example, cyano-modified polyamide can be obtained by surface grafting modification, bulk copolymerization modification, and Mannich reaction modification specifically for polyacrylamide. For example, cyano-modified polyamide can be prepared by the following method: dissolving mercaptosuccinic acid, acrylonitrile, and an initiator in an organic solvent, carrying out an addition reaction under a nitrogen atmosphere, removing part of the solvent by vacuum evaporation after the reaction is completed, precipitating in a precipitant, and drying under vacuum to obtain a cyano-containing diacid monomer, dissolving the above diacid monomer, diamine monomer, aromatic dianhydride, and two diisocyanates in an organic solvent, adding a catalyst in a reaction vessel, reacting first at a low temperature, and then gradually raising the temperature to a higher temperature and holding it to carry out a polycondensation reaction to obtain cyano-modified polyamide.

[0041] For example, the preparation method of carboxyl-modified water-based polyurethane includes: mixing isocyanate monomers and polyols in an acetate solvent (such as propylene glycol methyl ether acetate) and heating to carry out a prepolymerization reaction; then adding a hydrophilic chain extender containing carboxyl groups to carry out a chain extension reaction to obtain a carboxyl-modified polyurethane prepolymer; adding a neutralizing agent to the prepolymer to neutralize and form a salt, and emulsifying and dispersing it in deionized water to obtain a carboxyl-modified water-based polyurethane emulsion.

[0042] Furthermore, in some other embodiments of this application, the oil-based binder includes polyacrylonitrile, and the dispersant includes polyvinylidene fluoride (PVDF). The polyacrylonitrile molecular chain contains abundant cyano groups, which can form a strong hydrogen bond network with the carboxyl groups in the undercoating layer. Simultaneously, the lone pair electrons of its cyano groups can adsorb with the π-electron system of the conductive agent to construct a conductive network and form hydrogen bonds / coordination with the surface of the silicon-based material to enhance cohesive adhesion. PVDF, as a dispersant, has good flexibility, which can promote the uniform dispersion of silicon-carbon particles in the slurry and improve the deformation resistance of the active layer, better adapting to the volume expansion and contraction of silicon-carbon particles. The combined use of polyacrylonitrile ensures the conductive network and interfacial hydrogen bond anchoring, while PVDF improves dispersibility and deformation resistance, thereby maintaining the integrity of the electrode structure during cycling, further improving the cycle stability of the negative electrode and reducing battery expansion.

[0043] In some embodiments, based on the mass of the base coating, by controlling the acid value B (mgKOH / g) of the base coating to be between 40 mg KOH / g and 80 mg KOH / g, it can be ensured that the base coating contains an appropriate amount of carboxyl groups (-COOH), thereby forming a sufficient strong hydrogen bond cross-linking network with the cyano groups of the oil-based binder in the negative electrode active layer. This anchors the two coatings at the molecular level, significantly improving the interfacial adhesion between the base coating and the active layer, effectively resisting the shear stress generated by the volume expansion of silicon particles, inhibiting the "delamination" of the active layer, and maintaining good mechanical strength and chemical stability of the base coating. If B is too small, the carboxyl content is insufficient, the number of hydrogen bond binding sites is too small, the improvement in interfacial adhesion is limited, and the peeling force decreases. If B is too large, the excessive carboxyl groups may lead to excessive hydrophilicity of the base coating, deterioration of mechanical properties, and possible side reactions with metal ions dissolved from the electrolyte or positive electrode, thereby affecting the long-term cycle stability of the battery.

[0044] It should be noted that the acid value B refers to the number of milligrams of potassium hydroxide required to neutralize the carboxyl groups in 1g of dry adhesive. It can directly reflect the content of carboxyl groups in the base coating. The specific test method includes: using non-aqueous potentiometric titration, calculated according to the formula B = (V×C×56.11) / m, where V is the net volume consumed (mL, excluding blank), C is the concentration of KOH standard solution (mol / L), and m is the mass of dry adhesive (g). For example, the acid value B can be 40 mg KOH / g, 43 mg KOH / g, 46 mg KOH / g, 49 mg KOH / g, 52 mg KOH / g, 55 mg KOH / g, 58 mg KOH / g, 61 mg KOH / g, 64 mg KOH / g, 67 mg KOH / g, 70 mg KOH / g, 73 mg KOH / g, 76 mg KOH / g, 79 mg KOH / g, 80 mg KOH / g, or a value within the range of any two of the above values.

[0045] In some embodiments, the aqueous binder includes at least one of carboxyl-modified styrene-butadiene rubber, carboxyl-modified water-based polyurethane, and polyacrylic acid. The molecular chains of these aqueous binders all contain carboxyl (-COOH) groups, enabling them to disperse or dissolve in water to form an environmentally friendly aqueous primer slurry. More importantly, the carboxyl group, as a strong hydrogen bond donor, can form a strong hydrogen bond network with the cyano groups of the oil-based binder in the negative electrode active layer, thereby establishing a strong molecular-level anchor between the primer layer and the active layer, significantly improving interfacial adhesion, and effectively suppressing active layer delamination during cycling in the high-silicon negative electrode system. Furthermore, the aforementioned aqueous binder also possesses good film-forming properties and flexibility, capable of adapting to volume changes in silicon-carbon particles, further ensuring the structural stability of the negative electrode sheet.

[0046] It should be noted that this application uses emulsion polymerization to prepare water-based adhesives. The following explanation uses carboxyl-modified styrene-butadiene rubber as an example: First, styrene and butadiene monomers are added to a pure water system containing an emulsifier. The mixture is heated under nitrogen protection to initiate an emulsion polymerization reaction. Subsequently, during the polymerization process, acrylic monomers containing carboxyl groups (-COOH) are added for graft copolymerization, forming copolymer segments with carboxyl side groups. The reaction is terminated after reaching the target conversion rate, unreacted monomers are removed, the pH is adjusted, and the mixture is purified by filtration and the solid content is adjusted to finally obtain carboxyl-modified styrene-butadiene rubber latex. Furthermore, in the above polymerization process, the distribution density and grafting rate of carboxyl groups on the polymer chain can be adjusted by controlling the amount and method of adding acrylic acid, thereby optimizing the number of hydrogen bond sites between the base layer and the active layer and further improving interfacial adhesion.

[0047] In some implementations, by controlling the thickness H1 μm of the undercoat between 0.2 μm and 2 μm, it is possible to ensure that the undercoat provides sufficient carboxyl sites to form a strong hydrogen bond network with the cyano groups in the active layer, achieving molecular-level anchoring at the interface. Simultaneously, it maintains good continuous coverage and flexibility, transforming the point contact between the active layer and the current collector into a surface contact. Furthermore, the flexible segments can absorb the volume expansion stress of the silicon particles, effectively buffering the volume expansion of the silicon particles and preventing "delamination." If H1 is too thin, there are insufficient effective carboxyl sites, resulting in low hydrogen bond density and limited anchoring effect. If H1 is too thick, the coating resistance increases and it is prone to cracking, while also reducing the energy density of the negative electrode.

[0048] For example, the thickness of the base coating may be 0.20 μm, 0.32 μm, 0.44 μm, 0.56 μm, 0.68 μm, 0.80 μm, 0.92 μm, 1.04 μm, 1.16 μm, 1.28 μm, 1.40 μm, 1.52 μm, 1.64 μm, 1.76 μm, 1.88 μm, 2.00 μm, or a value within the range of any two of the above values.

[0049] Because this application utilizes a strong hydrogen bond network formed between the cyano groups in the oil-based binder and the carboxyl groups in the undercoat, it significantly enhances the interfacial bonding and structural stability between the undercoat and the negative electrode active layer. This allows them to work together as a whole to resist volume change stress during cycling, effectively transferring and dispersing interfacial loads, thereby improving the bonding force between the negative electrode coating and the negative electrode current collector. Therefore, in some embodiments, the peel force FN / m between the negative electrode current collector and the negative electrode coating can reach at least 20 N / m, thus providing a reliable guarantee for the structural integrity of the high-silicon negative electrode system during long-term cycling.

[0050] It should be noted that the test method for peel force F includes: taking a negative electrode sample, cutting it into specimens with a length greater than 300 mm and a width of 24 mm, taking no less than 5 specimens from each batch, attaching the specimens to the steel plate with double-sided tape, and rolling them three times with a roller to remove air bubbles and wrinkles. At one end of the specimen, using a tool, peeling the negative electrode coating from the negative electrode current collector in a 180° direction for about 5 mm to ensure that the negative electrode coating is continuous and does not break during the subsequent peel force test. Fixing the steel plate on one clamp of the tensile testing machine, clamping the free end of the negative electrode current collector separated from the specimen in another clamp, keeping the peel angle at 180°, starting the tensile testing machine, and peeling the negative electrode coating from the surface of the negative electrode current collector at a constant rate specified by the standard (e.g., 50 mm / min), recording the force value during the peeling process. The unit of peel force F is N / m.

[0051] In some embodiments, by controlling the number-average molecular weight (Mn) of the oil-based binder to not exceed 200,000, the binder can maintain good solubility or dispersibility in organic solvents, allowing the molecular chains to extend appropriately. After drying, it forms a continuous and flexible film structure, effectively coating silicon-carbon particles and adapting to their volume changes, maintaining the integrity of the electrode structure and the cohesive bonding within the active layer. Simultaneously, a suitable molecular weight helps the slurry maintain appropriate viscosity, ensuring coating uniformity and the dispersibility of silicon-carbon particles. If Mn is too high, it will lead to severe entanglement of the binder molecular chains and excessively high slurry viscosity, not only affecting the coating process but also potentially increasing the brittleness and reducing the flexibility of the film, making it difficult to effectively buffer the volume expansion of silicon particles, and easily causing film rupture and damage to the conductive network.

[0052] It should be noted that the test method for the number average molecular weight (Mn) of oil-based adhesives includes: using gel permeation chromatography (GPC / SEC) to separate components of different molecular weights based on molecular size exclusion effects. During testing, N,N-dimethylformamide containing lithium bromide is used as the solvent, and narrowly distributed polystyrene or polymethyl methacrylate is used as a standard for calibration. The number average molecular weight (Mn) is output through the gel permeation chromatography system. For example, the number average molecular weight (Mn) of oil-based adhesives can be 1000, 5000, 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, 90000, 100000, 120000, 150000, 200000, or a value within any two of the above ranges.

[0053] In some embodiments, the silicon-based material comprises silicon-carbon particles, and in the surface SEM image of the negative electrode, any two 100 μm particles... Within a 100 μm region, the difference in the number of silicon-carbon particles is C, where C ≤ 100. Since this application uses a cyano-containing oil-based binder, its excellent dispersion properties ensure uniform distribution of silicon-carbon particles in the active layer. Therefore, C ≤ 100 indicates that the silicon-carbon particles have excellent dispersion uniformity in the negative electrode active layer. Uniform particle distribution facilitates stress distribution during charging and discharging, synergistically working with the continuous film structure formed by the oil-based binder to reduce electrode structure damage caused by excessive local volume expansion. Simultaneously, it also helps to cooperate with cyano groups and conductive agents to construct a conductive network, forming a continuous and complete electron transport path, reducing local internal resistance differences, and improving the battery's cycle stability and rate performance. If C is too large (i.e., a significant difference in the number of particles in different regions), it indicates uneven dispersion. Agglomerated areas are prone to concentrated stress leading to electrode cracking and damage to the film coating of the oil-based binder, while sparse areas suffer from insufficient active material, affecting capacity utilization and thus degrading the battery's electrochemical performance.

[0054] It should be noted that the value of C can be obtained through conventional methods in the art, such as scanning electron microscopy (SEM). The specific testing method includes: cutting the negative electrode sample to a suitable size; since silicon-carbon particles have a certain degree of conductivity, the sample can be directly fixed on the sample stage and placed in a vacuum chamber; setting the accelerating voltage (e.g., 0.1kV to 30kV) and working distance; selecting the secondary electron (SE) or backscattered electron (BSE) signal mode; acquiring a clear surface morphology image at an appropriate magnification; randomly selecting at least two 100μm × 100μm rectangular regions on the surface of the negative electrode; counting the number of silicon-carbon particles in each region; and calculating the absolute value of the difference between the number of silicon-carbon particles in any two regions, which is the C value. For example, the value of C can be 1, 8, 15, 22, 29, 36, 43, 50, 57, 64, 71, 78, 85, 92, 100, or a value within any range of two of the above values.

[0055] In some embodiments, the silicon-based material further includes silicon-oxygen particles, and the silicon-oxygen bonds on the surface of the silicon-oxygen particles can also form hydrogen bonds or coordination interactions with the cyano groups in the oil-based binder, thereby enhancing cohesive bonding and maintaining the integrity of the electrode structure.

[0056] To further improve the overall electrochemical performance and structural stability of the silicon-carbon anode, this application optimizes the structural parameters of the silicon-carbon particles. Specifically, the silicon-carbon particles include porous carbon, silicon particles located within the porous carbon channels, and a carbon layer coating the surface of the porous carbon. In some embodiments, By controlling the sphericity φ of the silicon-carbon particles to be no less than 0.85, local stress concentration during charging and discharging can be reduced, which is beneficial for the oil-based binder to form a continuous and uniform film coating on its surface, reducing the risk of particle breakage and electrode structure damage. If the sphericity is too low, the particle shape will be irregular, and stress concentration is likely to occur at protrusions or edges, leading to particle cracking or pulverization, which in turn will destroy the integrity of the film coating of the oil-based binder and accelerate the failure of the electrode structure.

[0057] It should be noted that the test method for sphericity φ includes: analyzing scanning electron microscope (SEM) images of silicon carbide particles at a certain magnification (e.g., 1000x) using image processing software (e.g., Image ProPlus). Specifically, at least 10 silicon carbide particles are selected from the SEM image, and the perimeter and area of ​​each particle are measured. The equivalent radius r1 of the perimeter and the equivalent radius r2 of the area of ​​each particle are calculated respectively, and then the sphericity φ is determined according to the formula. i =r2 / r1 calculates the sphericity of each particle, and takes the average of the sphericity of all particles as the sphericity φ of the silicon-carbon particle.

[0058] By controlling the particle size (Dv50) of silicon-carbon particles to be between 6 μm and 12 μm, it is beneficial for silicon particles to be uniformly loaded within the carbon channels, while also ensuring good electron conduction and ion diffusion performance. If the particle size is too small, the specific surface area will be too large, leading to more side reactions and a decrease in first-efficiency; if the particle size is too large, the absolute value of the volume expansion of the silicon particles themselves will increase, which can easily lead to carbon shell cracking and electrode structure damage.

[0059] It should be noted that the particle size Dv50 of silicon carbide particles refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, that is, the volume content of particles smaller than or equal to this size accounts for 50% of the total particle volume. The Dv50 test method can refer to standard GB / T19077-2016 / ISO 13320:2009, and is tested using a laser particle size analyzer (Malvern Master Size 3000). For example, the particle size Dv50 of silicon carbide particles can be 6.0μm, 6.4μm, 6.8μm, 7.2μm, 7.6μm, 8.0μm, 8.4μm, 8.8μm, 9.2μm, 9.6μm, 10.0μm, 10.4μm, 10.8μm, 11.2μm, 11.6μm, 12.0μm, or values ​​within any two of the above ranges.

[0060] By controlling the total pore volume of porous carbon to 0.6 cm³ 3 / g-1.5cm 3With an average pore size between 1.5 nm and 50 nm and a micropore volume fraction between 20% and 99%, silicon-carbon particles possess a suitable pore structure. This provides a buffer space for silicon expansion while ensuring sufficient electrolyte wetting channels, which is beneficial for maintaining electrode cycle stability. If the total pore volume is too small, the buffer space is insufficient, and silicon expansion can easily lead to particle breakage; if the total pore volume is too large, the particle mechanical strength decreases, the tap density decreases, and the energy density is affected. Furthermore, a smaller average pore size results in a higher micropore fraction, which is beneficial for increasing specific surface area and buffer space. However, an excessively small average pore size or an excessively high micropore fraction may affect the full wetting of the electrolyte; conversely, an excessively large average pore size will relatively reduce the micropore fraction, which will weaken the buffering effect on silicon expansion.

[0061] It should be noted that the average pore size of porous carbon refers to the statistical average of the diameters of all pores in the porous carbon material. The total pore volume, average pore size, and micropore volume ratio of porous carbon can be obtained by conventional methods in the art, such as the BET test. For example, the total pore volume of porous carbon can be, for instance, 0.60 cm³. 3 / g, 0.65cm 3 / g, 0.70cm 3 / g, 0.80cm 3 / g, 0.90cm 3 / g, 1.00cm 3 / g, 1.10cm 3 / g, 1.15cm 3 / g, 1.20cm 3 / g, 1.25cm 3 / g, 1.30cm 3 / g, 1.35cm 3 / g, 1.40cm 3 / g, 1.45cm 3 / g, 1.50cm 3 / g or values ​​within the range of any two of the above values; the average pore size of porous carbon can be, for example, 1.5nm, 2nm, 3nm, 4nm, 5nm, 7nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, etc., or values ​​within the range of any two of the above values; the volume percentage of micropores in porous carbon can be, for example, 20%, 30%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, etc., or values ​​within the range of any two of the above values.

[0062] By controlling the powder conductivity of silicon-carbon particles at 25℃ and 5MPa within the range of 0.001S / cm-100S / cm, it is possible to ensure that the silicon-carbon particles themselves have good conductivity, which is beneficial for reducing electrode internal resistance and improving rate performance. If the conductivity is too low, the active material itself has poor conductivity, and even with the assistance of conductive agents, it is easy to cause local electron transport obstruction; if the conductivity is too high, it usually means that the degree of graphitization of the carbon layer is too high, which may lead to the material becoming brittle and reducing its ability to buffer volume expansion.

[0063] It should be noted that the powder conductivity can be obtained by conventional methods in the art, such as by using a powder resistivity meter. For example, the powder conductivity may be 0.001 S / cm, 0.005 S / cm, 0.01 S / cm, 0.02 S / cm, 0.05 S / cm, 0.1 S / cm, 0.2 S / cm, 0.5 S / cm, 1 S / cm, 2 S / cm, 5 S / cm, 10 S / cm, 20 S / cm, 50 S / cm, 100 S / cm, or a value within any two of the above ranges.

[0064] The Raman shift of the silicon-carbon particles is 1350 ± 10 cm⁻¹. -1 The peak at the specified location is denoted as the D peak. The D peak typically corresponds to a disordered structure in carbon materials (such as sp). 3 (Hybrid carbon, edge defects, etc.), its intensity reflects the defect density of the carbon layer; Raman shift is 1580±10 cm. -1 The peak at the specified location is denoted as the G peak, which corresponds to the sp peak in the graphite lattice. 2 The in-plane stretching vibrations of hybrid carbon reflect the content of ordered graphite structure. The ratio of the D peak intensity to the G peak intensity is ID / IG. ID / IG comprehensively characterizes the graphitization degree and defect density of the carbon layer. By controlling the ID / IG value between 0.7 and 1.5, the carbon layer can have a moderate degree of graphitization and defect density, ensuring good electronic conductivity while providing sufficient mechanical strength to suppress silicon expansion. If ID / IG is too small, it means that the degree of graphitization of the carbon layer is relatively high, the material is brittle, and it is prone to cracking during cycling. If ID / IG is too large, it means that there are relatively too many defects in the carbon layer, the conductivity decreases, and it may increase interfacial side reactions.

[0065] For example, the value of ID / IG can be 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, etc., or a value within the range of any two of the above values.

[0066] In some implementations, by controlling the mass percentage of silicon in the negative electrode active layer to be between 7% and 50%, based on the mass of the negative electrode active layer, both high energy density and cycle stability can be achieved. If the mass percentage of silicon is too low, the energy density improvement is limited, making it difficult to fully utilize the high capacity advantage of silicon-carbon materials; if the mass percentage of silicon is too high, the volume expansion effect is too strong, which may lead to electrode structure damage and cycle life degradation.

[0067] It should be noted that the test method for the mass percentage of silicon includes: discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, soaking it in dimethyl carbonate for 12 hours, rinsing to remove lithium salt, drying it, immersing the negative electrode sheet in water to peel the negative electrode active layer from the current collector, collecting the active material and drying it. Using a thermogravimetric analyzer, the temperature is raised from room temperature to 900℃ at a rate of 10℃ / min in an air or oxygen atmosphere and held for 40 minutes to allow non-silicon components to volatilize and silicon to be fully oxidized to silicon dioxide. The mass percentage of silicon is calculated based on the ash mass using the following formula: Mass percentage of silicon in the negative electrode active layer = [7 × ash mass / (15 × sample mass)] × 100%. For example, the mass percentage of silicon can be 7%, 11%, 15%, 19%, 23%, 27%, 31%, 35%, 39%, 43%, 47%, 50%, etc., or values ​​within any two of the above ranges.

[0068] In some embodiments, the negative electrode active material further includes graphite material. Based on the negative electrode active material, by controlling the mass ratio of silicon-carbon particles to graphite material to be between (20-60):(40-80), the high specific capacity of silicon-carbon material and the good cycle stability of graphite material can be balanced. That is, silicon-carbon particles provide high capacity, while graphite material buffers volume expansion and maintains the stability of the conductive network. Their synergistic effect can improve the electrochemical performance of the negative electrode. If the mass ratio is too small, i.e., the proportion of silicon-carbon particles is relatively low, the capacity improvement of the negative electrode is limited, making it difficult to leverage the high energy density advantage of silicon-carbon material. If the mass ratio is too large, i.e., the proportion of silicon-carbon particles is relatively high, the proportion of graphite is too small. This can easily cause structural damage to the electrode during cycling due to excessive volume expansion of silicon, resulting in rapid capacity decay.

[0069] For example, the mass ratio of silicon carbon particles to graphite material can be 20:80, 23:77, 26:74, 29:71, 32:68, 35:65, 38:62, 41:59, 44:56, 47:53, 50:50, 53:47, 56:44, 59:41, 60:40, or a value within any two of the above ranges.

[0070] In some embodiments, the negative electrode active layer further includes a conductive agent and a dispersant. The mass ratio of the negative electrode active material to the oil-based binder, the conductive agent, and the dispersant is (80-99):(0.2-10):(0.2-8):(0.2-5), which ensures that the components in the negative electrode active layer work synergistically, i.e., the active material provides capacity as the main component. The oil-based binder forms a continuous film structure, and its cyano lone pair electrons adsorb with the π-electron system of the conductive agent to build a conductive network on the one hand, and form hydrogen bonds / coordination with the surface of the silicon-based material to enhance cohesive adhesion on the other hand. The conductive agent builds an efficient conductive path, while the dispersant promotes the uniform dispersion of silicon-carbon particles. In this way, the structural stability, conductivity, and cycle performance of the negative electrode sheet can be improved, providing a guarantee for the long-life application of the high-silicon negative electrode system.

[0071] For example, the mass ratio of the negative electrode active material, oil-based binder, conductive agent, and dispersant can be, for example, 80:10:8:5, 81:8:6:4, 82:6:5:3, 83:5:4:2.5, 84:4:3:2, 85:3:2.5:1.5, 86:2.5:2:1.2, 87:2:1.5:1, 88:1.5:1.2:0.8, 89:1.2:1:0.6, 90:1:0.8:0.5, 92:0.8:0.6:0.4, 94:0.6:0.5:0.3, 96:0.4:0.3:0.25, 99:0.2:0.2:0.2, etc., or values ​​within the range of any two of the above values.

[0072] In some embodiments, the conductive agent includes at least one selected from conductive carbon fiber, single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, carbon black, conductive graphite, and graphene. The resistivity of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes is 10⁻⁶. -7 Ω·cm-10 -3 The electrode has a length of Ω·cm, which ensures excellent intrinsic conductivity; the length is 1μm-10μm, which facilitates the interconnection of conductive agents in the electrode to form a three-dimensional conductive network, thereby further improving the electron transport efficiency inside the electrode.

[0073] In some embodiments, the dispersant includes at least one selected from polyethylene, polyvinyl chloride, polyvinylidene fluoride, polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), polytetrafluoroethylene, and epoxy resin (EP). These dispersants exhibit good wetting and steric hindrance effects, enabling them to effectively adsorb onto the surface of silicon carbon particles, preventing particle agglomeration, thereby significantly improving the dispersion uniformity of silicon carbon particles in the negative electrode slurry, and thus ensuring the consistency of the active material distribution and the uniformity of the electrode structure in the coated electrode.

[0074] In some embodiments, by controlling the thickness H2 μm of the negative electrode active layer on one side to be between 30 μm and 260 μm, sufficient active material can be accommodated to improve energy density, while ensuring that the electrolyte fully wets the entire active layer, enabling rapid transport of lithium ions in the thickness direction. Furthermore, the continuous film structure formed by the oil-based binder can coat silicon carbon particles and buffer their volume expansion. At the same time, the carboxyl groups in the undercoat and the cyano groups in the active layer form a strong hydrogen bond network, providing a firm interface anchor. The two work together to prevent the electrode from cracking or "detaching".

[0075] It should be noted that the thickness of the negative electrode active layer on one side can be measured using conventional methods in the art, such as by using a micrometer or a cross-sectional scanning electron microscope. For example, the thickness of the negative electrode active layer on one side can be 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 120 μm, 140 μm, 160 μm, 180 μm, 200 μm, 230 μm, 260 μm, or a value within any two of the above ranges.

[0076] In some embodiments, the negative current collector includes at least one of copper foil, pure copper foil, nickel-plated copper foil, and carbon-coated copper foil.

[0077] According to a second aspect of this application, a secondary battery is provided, the secondary battery comprising the negative electrode sheet described in the first aspect of this application.

[0078] In some embodiments, the secondary battery further includes an electrolyte comprising a linear ester solvent. By controlling the mass percentage E of the linear ester solvent to not exceed 60% based on the mass of the electrolyte, the swelling effect of the electrolyte on the oil-based binder can be effectively reduced, maintaining the stability of the continuous film structure formed by the oil-based binder and the cyano-carboxyl hydrogen bond network between it and the undercoat layer. This ensures the structural integrity and interfacial adhesion of the negative electrode sheet during long-term cycling. If E is too large, the proportion of linear ester solvent is too high, which may correspondingly increase the risk of swelling of the oil-based binder. This can lead to a loose binder film structure and damage to the hydrogen bond network, thereby weakening the structural stability and interfacial adhesion of the negative electrode sheet. Simultaneously, an excessively high proportion of linear ester will reduce the dielectric constant of the electrolyte, affecting the complete dissociation of lithium salt and ionic conductivity, thus degrading the rate performance and cycle life of the battery.

[0079] It should be noted that the mass percentage E of the linear ester solvent can be obtained by conventional methods in the art, such as gas chromatography-mass spectrometry (GC-MS). For example, the mass percentage E of the linear ester solvent can be 4%, 8%, 12%, 16%, 20%, 24%, 28%, 32%, 36%, 40%, 44%, 48%, 52%, 56%, 60%, or a value within any two of the above ranges.

[0080] In some embodiments, the linear ester solvent includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and isobutyrate.

[0081] The present application will be further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products.

[0082] Example 1 This embodiment provides a method for preparing a secondary battery, including the following steps: Step 1: Preparation of the negative electrode 1) Preparation of the base coating 3.5 wt% conductive carbon (carbon black), 6 wt% carboxyl-modified styrene-butadiene rubber (SBR), 0.5 wt% sodium carboxymethyl cellulose (CMC), and 90 wt% deionized water were mixed and stirred until homogeneous. The pH was adjusted to no more than 8 and the viscosity to below 500 mPa·s to obtain a primer slurry. Then, this primer slurry was prepared at a concentration of 0.5 mg / cm³. 2 The areal density is uniformly coated on both sides of a 6μm thick copper foil, and then baked in two steps (coating oven temperature 95℃) to obtain a copper foil sheet containing a base coating.

[0083] 2) Preparation of negative electrode slurry The silicon-based anode material (content 25wt%, using spherical silicon-carbon particles with a sphericity of 0.97 and a total pore volume of 0.91 cm³) was used. 3A mixture of polyacrylonitrile (g) and flake artificial graphite (75wt%), conductive material (60wt% carbon nanotubes and 40wt% carbon black), binder (polyacrylonitrile, degree of polymerization 500-2000, number average molecular weight not exceeding 200,000), and dispersant (polyvinylidene fluoride) is mixed at a mass ratio of 96.1:1.68:1.5:0.72. N-methylpyrrolidone solvent is added and stirred. The pH of the slurry is adjusted to not exceed 8, the actual temperature in the mixing tank is controlled to not exceed 60℃, and the viscosity is adjusted to 6800 mPa·s to obtain the negative electrode slurry.

[0084] 3) Preparation of negative electrode sheet The negative electrode slurry prepared in step 2) was used at 5.8 mg / cm³. 2 The areal density is uniformly coated onto the surface of the copper foil sheet containing the base coating prepared in step 1). After two-step baking (coating oven temperature 105℃) and drying, it is placed in an 80℃ oven for 5 hours to obtain the negative electrode sheet. The negative electrode sheet is then rolled until the compaction density of the negative electrode coating is 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.

[0085] Step 2: Preparation of the positive electrode sheet A positive electrode slurry was prepared by mixing lithium cobalt oxide (positive active material), conductive agent (30wt% carbon nanotubes and 70wt% carbon black), and polyvinylidene fluoride (PVDF) binder in a mass ratio of 98:1:1. The slurry was coated onto a 9μm aluminum foil, dried, rolled, slit, cut into sheets, and then the tabs were welded to obtain the positive electrode sheet.

[0086] Step 3: Preparation of electrolyte In an argon-filled glove box (moisture content below 0.1 ppm, oxygen content below 0.1 ppm), ethylene carbonate and propylene carbonate (volume ratio 1:1) are added to 40% of the total electrolyte mass of a linear ester solvent (dimethyl carbonate: methyl ethyl carbonate: propyl propionate: ethyl propionate = 4:4:2:1), and stirred until homogeneous. Then, 11% of the total electrolyte mass of thoroughly dried lithium hexafluorophosphate and 4% of lithium bis(trifluoromethanesulfonyl)imide are added sequentially, followed by 2% of the total electrolyte mass of 1,3,6-hexanetrionitrile and 1% of adiponitrile, and stirring is continued until homogeneous. After the moisture and free acid content are tested and found to be within acceptable limits, the desired electrolyte is obtained.

[0087] Step 4: Preparation of secondary batteries The negative electrode sheet prepared in the first step, the polyethylene separator, and the positive electrode sheet prepared in the second step are sequentially wound to obtain a bare cell. The bare cell is placed in a battery aluminum shell or soft-pack aluminum-plastic film, and top-sealed, side-sealed, and dried to remove moisture. The electrolyte prepared in the third step is injected, and then top-sealed, formed, and sorted to obtain a secondary battery.

[0088] The preparation methods of Examples 2-18 and Comparative Examples 1-3 are basically the same as those of Example 1, with differences shown in Tables 1-2. In Examples 15 and 16, the mass ratio of lithium salt and nitrile additives remained unchanged; the mass ratio of linear ester solvent was altered by adjusting the mass ratios of ethylene carbonate and propylene carbonate in the electrolyte. "This means that the parameter values ​​are the same as in Example 1, but it does not exclude the possibility of reasonably foreseeable errors such as test errors."

[0089] It is understandable that the thickness of the base coating can be adjusted by controlling the amount of base coating slurry applied during the preparation of the base coating.

[0090] Table 1

[0091] Table 2

[0092] Test case 1. AC impedance test The test temperature was 25℃±5℃, and the battery state of charge (SOC) was 50%. The specific steps are as follows: (1) Discharge pretreatment: Discharge at a constant current of 0.2C to the lower limit voltage of the battery and let stand for 10 minutes.

[0093] (2) SOC adjustment: Charge at a constant current of 0.7C to 3.95V, then charge at a constant voltage until the current drops to 0.05C. The constant voltage time is 60min, and then let stand for 10min.

[0094] (3) AC impedance test: Using the constant potential mode, apply a sinusoidal AC signal with an amplitude of 5mV, and scan the frequency range from 50kHz to 100mHz, scanning from high frequency to low frequency (starting frequency from 50kHz to 5kHz, and the lowest frequency is about 100mHz). Record the real part (Z') and imaginary part (-Z'') of the battery impedance at different frequencies.

[0095] (4) Data preprocessing: Check the integrity of the raw data, remove outliers, and perform smoothing if necessary to reduce noise. Confirm the test frequency range (usually 10mHz-100kHz) and number of points.

[0096] (5) Nyquist plot: Plot the Nyquist plot with the real part of the impedance (Z') as the horizontal axis and the imaginary part of the impedance (-Z'') as the vertical axis. At 50% SOC, the plot usually appears as a semicircle in the high-frequency region and a diagonal line in the low-frequency region.

[0097] (6) Equivalent circuit fitting: Select a suitable equivalent circuit model (e.g., Randle model or improved Randle model), use nonlinear least squares method for fitting, and evaluate the goodness of fit (χ²). 2 value).

[0098] (7) Parameter extraction: Extract the following parameters from the fitting results: Ohmic resistance (abbreviated R) s ), corresponding to the intersection of the high-frequency region and the real axis; charge transfer resistance (abbreviated R) ct ), corresponding to the diameter of the semicircle; double-layer capacitor (abbreviated C) dl The characteristic frequency of the semicircle is calculated; the diffusion impedance (Warburg impedance, abbreviated Z) is obtained from the characteristic frequency of the semicircle. w This reflects the diffusion process of lithium ions inside the electrode.

[0099] (8) Substitute into the following formula to calculate the battery impedance Z 总 :

[0100] Where ω = 2 × π × f, f is the test frequency in Hz; j is the unit of the imaginary number, which is the symbolic representation of the action of "rotating 90 degrees".

[0101] Note: EIS data at 50% SOC has the characteristics of moderate charge transfer resistance, good symmetry of Nyquist plot, high fit and good reproducibility, making it suitable as a benchmark for evaluating battery state of health (SOH).

[0102] 2. Cyclic performance test Under a test temperature of 45℃, the capacitor was discharged at a constant current of 0.5C to 3.0V and allowed to stand for 10 minutes; then charged at a constant current of 0.5C to 3.95V, followed by constant voltage charging until the current dropped to 0.05C, and allowed to stand for 10 minutes; then charged at a constant current of 2.0C to 4.25V, followed by constant voltage charging until the current dropped to 1.5C; then charged at a constant current of 1.5C to 4.55V, followed by constant voltage charging until the current dropped to 0.05C, and allowed to stand for 10 minutes; finally, it was discharged at a constant current of 1.5C to 3.5V, and then discharged at a constant current of 0.7C to 3.0V, and allowed to stand for 10 minutes. This process constituted one cycle, and the cyclic test was repeated. The capacity retention rate after the 500th cycle was calculated by dividing the discharge capacity of the 500th cycle by the discharge capacity of the 1st cycle.

[0103] 3. Thickness expansion rate test Under a test temperature of 45℃, the battery was discharged at a constant current of 0.5C to 3.0V and allowed to stand for 10 minutes; then charged at a constant current of 0.5C to 3.95V, followed by constant voltage charging until the current dropped to 0.05C, and allowed to stand for 10 minutes; finally, it was charged at a constant current of 2.0C to 4.25V, followed by constant voltage charging until the current dropped to 1.5C; and then charged at a constant current of 1.5C to 4.55V, followed by constant voltage charging until the current dropped to 0.05C, and allowed to stand for 10 minutes. At this point, the battery was fully charged, and its thickness was measured and recorded as the initial thickness. The charge-discharge cycle was then performed according to the aforementioned test procedure. Every 100 cycles, the full charge process was repeated, and the thickness was measured. The thickness after the 500th full charge cycle was recorded, and the growth rate of this thickness relative to the initial thickness was taken as the thickness expansion rate of the battery after 500 cycles.

[0104] The test results are shown in Table 3.

[0105] Table 3

[0106] As can be seen from Tables 1-3, by optimizing the binder structure in the negative electrode active layer and the base layer, this application can improve the conductivity and bonding anchoring force of the negative electrode active layer, enhance the buffering capacity of the base layer to cope with silicon volume changes, and improve the interfacial bonding between the negative electrode active layer and the base layer. This results in the negative electrode sheet having the advantages of good structural stability and excellent conductivity, thereby providing a reliable structural guarantee for the structural stability and electrochemical performance of the high-silicon battery system under long-cycle conditions.

[0107] 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 invention.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating disposed on at least one surface of the negative electrode current collector in the thickness direction; characterized in that: The negative electrode coating includes a negative electrode active layer and a base coating layer stacked together; the negative electrode active layer includes a negative electrode active material and an oil-based binder, the negative electrode active material includes a silicon-based material, and the oil-based binder contains cyano groups; The base coating is located between the negative electrode current collector and the negative electrode active layer, and the base coating includes an aqueous binder containing carboxyl groups.

2. The negative electrode sheet according to claim 1, characterized in that, Based on the mass of the negative electrode active layer, the mass content of the cyano groups is A%, 0.05≤A≤2.5%; And / or, the oil-based adhesive includes at least one of polyacrylonitrile, nitrile rubber, cyano-modified oil-based polyurethane, cyano-modified polyamide, cyano-modified polyimide, and cyano-modified cellulose.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, Based on the mass of the base coating, the acid value of the base coating is B mg KOH / g, 40≤B≤80; And / or, the water-based adhesive includes at least one of carboxyl-modified styrene-butadiene rubber, carboxyl-modified water-based polyurethane, and polyacrylic acid.

4. The negative electrode sheet according to claim 3, characterized in that, The thickness of the base coating is H1 μm, 0.2≤H1≤2; And / or, the peel force between the negative electrode current collector and the negative electrode coating is FN / m, F≥20.

5. The negative electrode sheet according to claim 1 or 2, characterized in that, The number average molecular weight of the oil-based binder is Mn, where Mn≤200000; And / or, the silicon-based material comprises silicon-carbon particles, any two 100 μm particles in the surface SEM image of the negative electrode. Within a 100 μm region, the difference in the number of silicon-carbon particles is C, where C ≤ 100.

6. The negative electrode sheet according to claim 5, characterized in that, The silicon-carbon particles comprise porous carbon and silicon particles located within the porous carbon channels, and a carbon layer coating the surface of the porous carbon; satisfying at least one of the following conditions: (1) The sphericity of the silicon-carbon particles is φ, where φ ≥ 0.85; (2) The particle size Dv50 of the silicon carbide particles is 6μm-12μm; (3) The total pore volume of the porous carbon is 0.6 cm³. 3 / g-1.5cm 3 / g, with an average pore size of 1.5nm-50nm and a micropore volume ratio of 20%-99%; (4) The powder conductivity of the silicon carbide particles at 25℃ and 5MPa is 0.001S / cm-100S / cm; (5) In the Raman spectrum of the silicon-carbon particles, the Raman shift is 1350±10 cm⁻¹. -1 The peak at this location is designated as peak D, with a Raman shift of 1580 ± 10 cm. -1 The peak at the location is denoted as peak G, and the ratio of the peak intensity of peak D to peak G is ID / IG, where 0.7 ≤ ID / IG ≤ 1.

5.

7. The negative electrode sheet according to claim 5, characterized in that, Based on the mass of the negative electrode active layer, the mass percentage of silicon in the negative electrode active layer is 7%-50%; And / or, the negative electrode active material further includes graphite material, and the mass ratio of the silicon carbon particles to the graphite material is (20-60):(40-80) based on the negative electrode active material. And / or, the negative electrode active layer further includes a conductive agent and a dispersant, and the mass ratio of the negative electrode active material to the oil-based binder, the conductive agent and the dispersant is (80-99):(0.2-10):(0.2-8):(0.2-5).

8. The negative electrode sheet according to claim 7, characterized in that, The dispersant includes at least one of polyethylene, polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride, polyvinylpyrrolidone, polytetrafluoroethylene, and epoxy resin; And / or, the thickness of the negative electrode active layer on one side is H2 μm, 30≤H2≤260.

9. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-8.

10. The secondary battery according to claim 9, characterized in that, The secondary battery also includes an electrolyte, which comprises a linear ester solvent; Based on the mass of the electrolyte, the mass percentage of the linear ester solvent is E, where E ≤ 60%; And / or, the linear ester solvent includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and isobutyrate.

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