A negative electrode sheet, a method for manufacturing the negative electrode sheet, and a battery

Abstract

The application provides a negative electrode sheet, a negative electrode sheet preparation method and a lithium ion battery, comprising a negative electrode current collector and a functional coating, the functional coating comprising a first functional coating and a second functional coating, the first functional coating being arranged on the surface of the current collector, and the second functional coating being arranged on the side of the first functional coating away from the negative electrode current collector; the elastic modulus of the first functional coating is smaller than that of the second functional coating. The negative electrode sheet provided by the application has a gradient elastic modulus structure from the side close to the negative electrode current collector to the side away from the negative electrode current collector, which is formed by the cooperation of the first functional coating and the second functional coating, and a controllable release direction is preset for the expansion stress, so that the stress is preferentially guided to the first functional coating which has stronger toughness, and the vicious cycle caused by the structural damage and the rigid constraint of the steel shell is avoided, and then the cycle performance of the high-silicon negative electrode applied to the steel shell laminated battery is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a negative electrode, a method for preparing the negative electrode, and a battery. Background Technology

[0002] Currently, the conventionally prepared negative electrode sheets in the industry are mechanically homogeneous bodies with uniform compaction density and mechanical modulus in the thickness direction. However, silicon negative electrodes undergo significant volume expansion during charging and discharging. The resulting huge internal stress has no effective channeling path within the homogeneous electrode sheet and can only be concentrated and released within the electrode sheet and at the interface, leading to a series of problems: First, the active material is prone to microcracks or even pulverization due to stress concentration, resulting in irreversible consumption of active lithium and battery capacity decay; Second, the uniform modulus allows the expansion stress to directly impact the interface between the electrode sheet and the current collector, easily leading to coating peeling and interruption of the electron conduction path; Third, such structural damage is superimposed on the rigid constraint of the steel shell, and the lack of a pre-set buffer area and stress guidance path inside the cell creates a vicious cycle of performance deterioration, severely limiting the cycle life of high-silicon negative electrode steel shell stacked batteries. Summary of the Invention

[0003] To address the problem of battery cycle performance degradation caused by expansion stress generated during charging and discharging of silicon anodes in existing technologies, a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery are provided.

[0004] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: On one hand, the present invention provides a negative electrode sheet, including a negative electrode current collector and a functional coating, wherein the functional coating includes a first functional coating and a second functional coating, the first functional coating is disposed on the surface of the current collector, and the second functional coating is disposed on the side of the first functional coating opposite to the negative electrode current collector; The elastic modulus of the first functional coating is less than that of the second functional coating; The first functional coating includes a first active material, a first binder, and a first conductive agent; The mass percentage of the first active material is 80%~85%; The first adhesive has a mass percentage content of 8% to 12%; The first conductive agent has a mass percentage content of 3% to 5%; The second functional coating includes a second active material, a second binder, and a second conductive agent; The second active material has a mass percentage content of 83% to 88%; The second adhesive has a mass percentage of 6% to 8%; The second conductive agent has a mass percentage content of 4% to 6%; The first functional coating further includes a toughening agent, and the second functional coating further includes a reinforcing agent, wherein the toughening agent has a mass percentage content of 1% to 3%, and the reinforcing agent has a mass percentage content of 1% to 2%. The second functional coating further includes a crosslinking agent, wherein the crosslinking agent has a mass percentage of 0.5% to 1%. The first adhesive includes a primary adhesive and an auxiliary adhesive, wherein the primary adhesive has a mass percentage of 5% to 8% and the auxiliary adhesive has a mass percentage of 2% to 4%.

[0005] Optionally, the first adhesive includes a primary adhesive and an auxiliary adhesive, wherein the primary adhesive includes one or more of styrene-butadiene rubber and polyacrylate compounds; and / or, The auxiliary binder includes one or more of sodium alginate, carboxylated nitrile rubber, and polyurethane compounds.

[0006] Optionally, the first conductive agent includes one or more of conductive carbon black, Ketjen black, carbon nanotubes, and graphene.

[0007] Optionally, the second adhesive includes one or more of polyvinylidene fluoride, polyimide, cyanate compounds, and peroxides.

[0008] Optionally, the second conductive agent includes a composite of carbon fiber and graphene, wherein the diameter of the carbon fiber is 5-10 μm and the length of the carbon fiber is 10-50 μm.

[0009] Optionally, the toughening agent comprises elastic fibers with a diameter of 0.5~2μm and a length of 50~200μm; The reinforcing agent includes one or more of nano-silica, nano-alumina, and nano-silicon carbide.

[0010] Optionally, the porosity of the first functional coating is greater than that of the second functional coating, wherein the porosity of the first functional coating is 25% to 45% and the porosity of the second functional coating is 15% to 28%.

[0011] Optionally, the thickness of the first functional coating is 30~70μm; The thickness of the second functional coating is 10~30μm.

[0012] Optionally, the above-mentioned method for preparing the negative electrode includes the following operations: The first active material, the first binder, the first conductive agent, the toughening agent, the pore-forming agent, and the first solvent are used to make a first slurry; The second active material, the second binder, the second conductive agent, the reinforcing agent, and the second solvent are used to make a second slurry; The first slurry and the second slurry are sequentially coated on the surface of the negative electrode current collector. After drying, the first solvent, the second solvent and the pore-forming agent are removed to obtain the negative electrode sheet.

[0013] Optionally, the pore-forming agent includes one or more of expandable polymer microspheres, ammonium carbonate, ammonium bicarbonate, azodicarbonamide, and polyethylene glycol, and the mass percentage of the pore-forming agent is 0.5% to 1.5% based on the total mass of the first functional coating as 100%.

[0014] Optionally, in the above method for preparing the negative electrode sheet, the preparation of the first slurry includes the following operations: An auxiliary binder is added to the first solvent, and after mixing and dissolving, a gel solution is obtained. The first active material and the first conductive agent are mixed, and then the mixture is added to the gel solution to obtain the first premixed solution. The main binder and pore-forming agent are added to the first premixed liquid, stirred and mixed evenly, and then the toughening agent is added. After stirring and dispersing again, the first slurry is obtained.

[0015] Optionally, in the above method for preparing the negative electrode, the preparation of the second slurry includes the following steps: The second solvent and the second binder are mixed to obtain a mixture; A second conductive agent is added to the mixture, and after stirring and dispersing, a second active material and a reinforcing agent are added, and the mixture is dispersed again to obtain a second premix. A crosslinking agent is added to the second premix, and after stirring evenly, a second slurry is obtained.

[0016] On the other hand, the present invention provides a battery comprising the aforementioned negative electrode sheet, or a negative electrode sheet prepared by the method for preparing the aforementioned negative electrode sheet.

[0017] The beneficial effects of this application are as follows: In the negative electrode sheet provided in this application, structural synergy and performance improvement are achieved by setting a gradient modulus double-layer functional coating on the negative electrode current collector. The first functional coating is set on the surface of the current collector and consists of 80%~85% by mass of a first active material, 5%~8% by mass of a main binder and 2%~4% by mass of an auxiliary binder, 8%~12% by mass of a first binder, 3%~5% by mass of a first conductive agent, and 1%~3% by mass of a toughening agent. The second functional coating is covered on the side of the first functional coating away from the current collector and consists of 83%~88% by mass of a second active material, 6%~8% by mass of a second binder, 4%~6% by mass of a second conductive agent, 1%~2% by mass of a reinforcing agent, and 0.5%~1% by mass of a crosslinking agent. The elastic modulus of the first functional coating is smaller than that of the second functional coating. The two work together to form a gradient elastic modulus structure that increases from the side closer to the current collector to the side farther from the current collector. Furthermore, the first functional coating on the surface of the current collector is a low-modulus, high-toughness stress buffer layer. The first functional coating can absorb and disperse the internal stress generated by the expansion of the silicon anode through its own deformation, molecular chain slippage, and flexible network stretching, avoiding direct stress impact on the current collector interface, effectively preventing the coating from peeling off from the current collector and ensuring the electron conduction path. The second functional coating covering the first functional coating has a high elastic modulus, which can constrain the irregular expansion of silicon particles from the outside, maintain the integrity of the electrode surface structure, reduce microcracks and breakage caused by stress concentration in the active material, and avoid irreversible consumption of active lithium. At the same time, the first and second functional coatings work together to form a gradient elastic modulus structure from the side closer to the anode current collector to the side farther away from the anode current collector, which pre-sets a controllable release direction for the expansion stress, so that the stress is preferentially conducted to the more resilient first functional coating, avoiding the vicious cycle of structural damage caused by the stress of the homogeneous electrode having nowhere to be released and the superposition of rigid constraints of the steel shell, thereby achieving a significant improvement in the cycle performance of high silicon anodes in steel-shell stacked batteries. Attached Figure Description

[0018] Figure 1 These are elastic modulus test diagrams of Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation

[0019] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0020] The present invention provides a negative electrode sheet, including a negative electrode current collector and a functional coating. The functional coating includes a first functional coating and a second functional coating. The first functional coating is disposed on the surface of the current collector, and the second functional coating is disposed on the side of the first functional coating opposite to the negative electrode current collector. The elastic modulus of the first functional coating is less than that of the second functional coating; The first functional coating includes a first active material, a first binder, and a first conductive agent; The mass percentage of the first active material is 80%~85%; The first adhesive has a mass percentage content of 8% to 12%; The first conductive agent has a mass percentage content of 3% to 5%; The second functional coating includes a second active material, a second binder, and a second conductive agent; The second active material has a mass percentage content of 83% to 88%; The second adhesive has a mass percentage of 6% to 8%; The second conductive agent has a mass percentage content of 4% to 6%; The first functional coating further includes a toughening agent, and the second functional coating further includes a reinforcing agent, wherein the toughening agent has a mass percentage content of 1% to 3%, and the reinforcing agent has a mass percentage content of 1% to 2%. The second functional coating further includes a crosslinking agent, wherein the crosslinking agent has a mass percentage of 0.5% to 1%. The first adhesive includes a primary adhesive and an auxiliary adhesive, wherein the primary adhesive has a mass percentage of 5% to 8% and the auxiliary adhesive has a mass percentage of 2% to 4%.

[0021] Specifically, structural synergy and performance improvement are achieved by setting a gradient modulus double-layer functional coating on the negative electrode current collector. The first functional coating is set on the surface of the current collector and consists of 80%~85% by mass of a first active material, 5%~8% by mass of a main binder and 2%~4% by mass of an auxiliary binder, 8%~12% by mass of a first binder, 3%~5% by mass of a first conductive agent, and 1%~3% by mass of a toughening agent. The second functional coating is covered on the side of the first functional coating away from the current collector and consists of 83%~88% by mass of a second active material, 6%~8% by mass of a second binder, 4%~6% by mass of a second conductive agent, 1%~2% by mass of a reinforcing agent, and 0.5%~1% by mass of a crosslinking agent. The elastic modulus of the first functional coating is smaller than that of the second functional coating. The two work together to form a gradient elastic modulus structure that increases from the side closer to the current collector to the side farther from the current collector. Furthermore, in the negative electrode sheet provided in this application, the first functional coating on the surface of the current collector is a low-modulus, high-toughness stress buffer. The first functional coating layer can absorb and disperse the internal stress generated by the expansion of the silicon anode through its own deformation, molecular chain slippage, and flexible network stretching, avoiding direct stress impact on the current collector interface, effectively preventing the coating from peeling off from the current collector, and ensuring the electron conduction path. The second functional coating covering the first functional coating has a high elastic modulus, which can constrain the irregular expansion of silicon particles from the outside, maintain the integrity of the electrode surface structure, reduce microcracks and breakage caused by stress concentration in the active material, and avoid irreversible consumption of active lithium. At the same time, the first and second functional coatings work together to form a gradient elastic modulus structure from the side closer to the anode current collector to the side farther away from the anode current collector, which pre-sets a controllable release direction for the expansion stress, so that the stress is preferentially conducted to the more resilient first functional coating, avoiding the vicious cycle of structural damage caused by the stress of the homogeneous electrode having nowhere to be released and the superposition of rigid constraints of the steel shell, thereby achieving a significant improvement in the cycle performance of high silicon anodes in steel-shell stacked batteries.

[0022] Specifically, by adjusting the mass percentage of each component in the first functional coating, a stress buffering function with low modulus and high toughness can be achieved. Specifically, 80%–85% of the primary active material (silicon-carbon composite material) ensures a high capacity foundation, thereby achieving high energy density in the battery; 8%–12% of the primary binder (a composite system of primary binder, auxiliary binder, and toughening agent) imparts excellent high elasticity, viscoelasticity, and elongation at break to the coating by forming an elastomer network, strong hydrogen bonding, and an elastomer toughening phase, becoming the key to achieving low modulus, and can efficiently absorb stress through molecular chain extension and movement; 3%–5% of the primary conductive agent (conductive carbon black) constructs a stable electronic conductivity pathway while avoiding excessive enhancement of modulus, ensuring conductivity. The proportions of the above components ensure that the first functional coating has sufficient capacity and good conductivity, while also fully leveraging the stress buffering effect of high toughness and low modulus. This effectively protects the current collector interface, buffers the expansion stress of the silicon anode, lays the foundation for the stress guiding mechanism of the gradient modulus structure, and thus helps to improve the battery cycle life and structural stability.

[0023] Furthermore, the mass percentage of the first active material can be 80%, 81%, 82%, 83%, 84%, or 85%; The mass percentage of the first adhesive can be 8%, 9%, 10%, 11%, or 12%. The mass percentage of the first conductive agent can be 3%, 4%, or 5%; Furthermore, the mass percentage of the first active material, the mass percentage of the first binder, and the mass percentage of the first conductive agent can be any value within the scope defined in this application.

[0024] Similarly, by limiting the mass percentage of each component in the second functional coating, the functions of high elastic modulus and high strength are achieved; among them, 83% to 88% of the second active material provides stable strength support for the coating while ensuring high capacity supply. The 6%~8% second binder undergoes a covalent cross-linking reaction, transforming thermoplasticity into thermosetting, which greatly improves the rigidity and thermal stability of the coating and achieves a high elastic modulus. The 4%~6% second conductive agent (a composite of carbon fiber and graphene) not only significantly enhances the surface hardness and elastic modulus, but also forms a highly efficient electron transport channel. By optimizing the percentage content of the second active material, the second binder, and the second conductive agent in the second functional coating, the second functional coating can effectively constrain the irregular expansion of silicon particles from the outside, maintain the integrity of the electrode surface structure, reduce the breakage of active materials and irreversible consumption of active lithium, and ensure excellent conductivity and structural stability. Together with the first functional coating, it forms a gradient elastic modulus structure from the side closer to the negative electrode current collector to the side farther from the negative electrode current collector, providing a controllable release direction for expansion stress, thereby improving both the cycle performance and energy density of the silicon negative electrode steel shell stacked battery.

[0025] Furthermore, the mass percentage of the second active material can be 83%, 84%, 85%, 86%, 87%, or 88%. The second adhesive may have a mass percentage of 6%, 7%, or 8%; The mass percentage of the second conductive agent can be 4%, 5%, or 6%; Furthermore, the mass percentage of the second active material, the mass percentage of the second binder, and the mass percentage of the second conductive agent can be any value within the scope defined in this application.

[0026] Specifically, 1% to 3% of the toughening agent can fully construct a flexible network in the first functional coating, ensuring sufficient toughness and tear resistance of the coating. It can efficiently disperse the expansion stress of the silicon anode through its own deformation and network stretching, without causing the coating modulus to be too low or the structure to be loose due to excessive toughening agent content. The 1%~2% reinforcing agent can be fully dispersed in the binder network of the second functional coating, effectively hindering the movement of molecular chains, strengthening the surface elastic modulus and hardness, improving the ability to restrain the expansion of silicon particles, and avoiding the impact of excessive content on the compatibility of the coating with other components and electron transport efficiency.

[0027] Specifically, during the drying or subsequent heat treatment of the negative electrode sheet, the crosslinking agent can promote the formation of covalent crosslinks in the linear polymer chains of the second binder, transforming thermoplasticity into thermosetting, improving the rigidity, hardness, and thermal stability of the second functional coating, and further optimizing the second functional coating's ability to constrain the irregular expansion of silicon particles. At the same time, the content range of 0.5% to 1% ensures that the crosslinking reaction is fully carried out, giving the coating sufficient structural strength to withstand the rolling pressure and maintain the integrity of the electrode surface, while avoiding excessive crosslinking agent leading to excessive brittleness and insufficient toughness of the coating, or affecting the compatibility with other components and electron transport efficiency. It works synergistically with the main binder, reinforcing agent, and other components of the second functional coating to further improve the high elastic modulus of the second functional coating.

[0028] Specifically, the crosslinking agent described in this application can be a cyanate ester or a peroxide, which, during electrode drying or subsequent heat treatment, causes covalent bonds to be formed between linear polymer chains, transforming thermoplasticity into thermosetting, and greatly improving the rigidity and thermal stability of the coating.

[0029] In some embodiments, the first adhesive includes a primary adhesive and a secondary adhesive, wherein the primary adhesive includes one or more of styrene-butadiene rubber and polyacrylate compounds; and / or, The auxiliary binder includes one or more of sodium alginate, carboxylated nitrile rubber, and polyurethane compounds.

[0030] Specifically, the main binder is selected from styrene-butadiene rubber or polyacrylate compounds, which form an elastomer network in the coating at a mass ratio of 5% to 8%. With its excellent high elasticity and viscoelasticity, it efficiently absorbs the internal stress generated by the expansion of the silicon anode through the extension and slippage of molecular chains. The auxiliary binder is one or more of sodium alginate, carboxylated nitrile rubber, or polyurethane compounds, which work synergistically with the main binder at a mass ratio of 2% to 4%. Sodium alginate can form strong hydrogen bonds with the hydroxyl groups on the silicon surface through hydroxyl groups, thereby improving the initial adhesion between the coating and the current collector. The carboxylated nitrile rubber or polyurethane compounds act as an elastomer toughening phase to further disperse stress and improve the elongation at break of the coating. This application limits the ratio of the main binder and the auxiliary binder to achieve a better synergistic effect, ensuring that the first functional coating has sufficient toughness and deformation capacity to effectively buffer stress and protect the current collector interface, while also ensuring the structural stability and adhesion performance of the coating, avoiding the structural loosening problem caused by excessively low modulus, thereby improving the cycle life and structural integrity of the battery.

[0031] The main adhesive may have a mass percentage of 5%, 6%, 7% or 8%, and the auxiliary adhesive may have a mass percentage of 2%, 3% or 4%.

[0032] In some embodiments, the first conductive agent includes one or more of conductive carbon black, Ketjen black, carbon nanotubes, and graphene.

[0033] Specifically, the first conductive agent can be any one of the above-mentioned conductive agents to achieve the corresponding conductive properties; Among them, conductive carbon black (Super P) and other materials can form a stable electronic conductive path in the coating, ensuring electron transport efficiency, while avoiding the problem of excessive enhancement of elastic modulus that may be caused by high aspect ratio conductive materials; Ketjen black, carbon nanotubes, graphene and other materials are beneficial to optimizing conductivity, ensuring that the first functional coating does not lose conductivity in the process of absorbing and dispersing the expansion stress of the silicon anode, ensuring the continuity of the electron conduction path, and can also work synergistically with the binder, active material and other components of the first functional coating to maintain the stability of the coating structure, which is conducive to achieving a balance between stress buffering and conductivity.

[0034] In some embodiments, both the first active material and the second active material may be selected from silicon-carbon composite materials.

[0035] In some embodiments, the second adhesive comprises one or more of polyvinylidene fluoride, polyimide, cyanate compounds, and peroxides.

[0036] Among them, polyvinylidene fluoride or polyimide, as the binder of the second functional coating, can form a strong binding and binding effect on the active material, providing basic rigidity for the coating; Cyanate esters or peroxides, as cross-linking binders, can promote the formation of covalent cross-links in linear polymer chains during electrode drying or subsequent heat treatment, transforming thermoplasticity into thermosetting and improving the rigidity and thermal stability of the second coating. When the above-mentioned binder is applied to the second functional coating, it ensures that the second functional coating has sufficient elastic modulus and strength, which can effectively constrain the irregular expansion of silicon particles and maintain the integrity of the electrode surface structure. It can also enhance the structural stability and resistance to rolling pressure of the coating. In combination with the conductive agent, active material and other components of the second functional coating, it can further optimize the electron transport efficiency and the utilization rate of active material.

[0037] In some embodiments, the second conductive agent comprises a composite of carbon fiber and graphene, wherein the carbon fiber has a diameter of 5-10 μm and a length of 10-50 μm.

[0038] Specifically, the carbon fibers with a diameter of 5-10 μm and a length of 10-50 μm can not only significantly improve the hardness and elastic modulus of the surface coating, providing support for constraining the irregular expansion of silicon particles and maintaining the integrity of the electrode surface structure, but also synergize with graphene to form a dense and efficient electron transport network; graphene has excellent electrical conductivity, which can further optimize electron conduction efficiency and improve the utilization rate of active materials. In this application, a composite of carbon fiber and graphene is used as a second conductive agent, which can not only strengthen the structural rigidity of the second functional coating, enabling it to withstand rolling pressure and effectively suppress microcracks and breakage caused by stress concentration in the active material, but also ensure the low resistance characteristics of the coating. It works synergistically with the first functional coating to improve the capacity retention rate of the battery at different discharge rates (0.5C, 1C rate) while achieving gradient elastic modulus stress guidance.

[0039] In some embodiments, the toughening agent comprises elastic fibers having a diameter of 0.5 to 2 μm and a length of 50 to 200 μm; The reinforcing agent includes one or more of nano-silica, nano-alumina, and nano-silicon carbide.

[0040] Specifically, the first functional coating of this application incorporates elastic fibers as a toughening agent, and the second functional coating is combined with reinforcing agents such as nano-silica. Among them, the elastic fibers with a diameter of 0.5~2μm and a length of 50~200μm can form a flexible network inside the first functional coating, improve the coating's toughness and tear resistance, effectively prevent the generation of cracks during deformation, and further disperse the expansion stress of the silicon anode in conjunction with the binder system, thereby enhancing the coating's ability to absorb stress through its own deformation and better protecting the current collector interface. The nano-silica reinforcing agent is dispersed in the binder network of the second functional coating, which effectively hinders the movement of molecular chains, further improves the elastic modulus and hardness of the surface of the second functional coating, strengthens the constraint effect of the second functional coating on the irregular expansion of silicon particles, and maintains the integrity of the electrode surface structure. The two work together to ensure the stress buffering effect of the first functional coating and the rigid constraint performance of the second functional coating. They can also form a gradient elastic modulus structure from the side closer to the negative electrode current collector to the side farther away from the negative electrode current collector, balancing the stress dissipation and structural stability of the negative electrode sheet. This further reduces problems such as active material breakage and coating peeling of the negative electrode sheet, and significantly improves the cycle life and structural reliability of high-silicon negative electrode steel shell stacked batteries.

[0041] In some embodiments, the porosity of the first functional coating is greater than that of the second functional coating, wherein the porosity of the first functional coating is 25% to 45% and the porosity of the second functional coating is 15% to 28%.

[0042] Specifically, the first functional coating has a high porosity of 25% to 45%. A pore-forming agent is added when preparing the corresponding slurry. The high porosity of the first functional coating is due to the porous structure brought about by the pore-forming agent. On the one hand, the high porosity of the first functional coating can further reduce the elastic modulus after the coating is compacted, providing sufficient deformation space for the expansion of the silicon anode and enhancing the stress buffering capacity. On the other hand, it is conducive to rapid electrolyte wetting, ensuring lithium-ion transport efficiency, and better dispersing internal stress with the flexible network structure, avoiding current collector interface peeling. The second functional coating has a low porosity of 15% to 28%, which is conducive to the formation of a dense structural layer. This can not only strengthen the constraint on the irregular expansion of silicon particles and maintain the integrity of the electrode surface structure, but also improve the coating hardness and modulus through the dense structure, withstand the rolling pressure and reduce the breakage of active materials. The porosity difference and modulus gradient of the first and second functional coatings are matched to ensure both the stress buffering and ion transport efficiency of the first functional coating and the rigid constraint and structural stability of the second functional coating.

[0043] In some embodiments, the thickness of the first functional coating is 30~70μm; The thickness of the second functional coating is 10~30μm.

[0044] Specifically, the 30~70μm thickness of the first functional coating provides ample physical space for stress buffering. Combined with its low modulus and high porosity characteristics, it can more fully absorb and disperse the internal stress generated by the expansion of the silicon anode through deformation, molecular chain slippage and flexible network stretching, avoiding stress directly acting on the anode current collector interface and effectively protecting the electron conduction path. A secondary functional coating thickness of 10~30μm helps to ensure its structural strength, which can both constrain the irregular expansion of silicon particles and maintain the integrity of the electrode surface structure, and prevent brittle fracture caused by excessive thickness leading to extended ion transport paths or excessive modulus.

[0045] In some embodiments, the method for preparing the above-mentioned negative electrode sheet includes the following operations: The first active material, the first binder, the first conductive agent, the toughening agent, the pore-forming agent, and the first solvent are used to make a first slurry; The second active material, the second binder, the second conductive agent, the reinforcing agent, and the second solvent are used to make a second slurry; The first slurry and the second slurry are sequentially coated on the surface of the negative electrode current collector. After drying, the first solvent, the second solvent and the pore-forming agent are removed to obtain the negative electrode sheet.

[0046] First, a first slurry is prepared by mixing a first active material, a first binder, a first conductive agent, a toughening agent, a pore-forming agent, and a first solvent in a specific ratio to ensure that each component is uniformly dispersed. The incorporation of the pore-forming agent pre-determines the pore structure. Simultaneously, a second active material, a second binder, a second conductive agent, a reinforcing agent, and a second solvent are formulated into a second slurry. Through component optimization, the high elastic modulus, high strength, and excellent conductivity of the surface layer are ensured.

[0047] By applying a double-layer coating, the first slurry and the second slurry are successively coated onto the surface of the negative electrode current collector, which not only ensures the tight bonding of the two functional coatings, but also avoids the mutual interference of different functional components. During the drying process, not only are the first solvent and the second solvent efficiently removed, but the pore-forming agent is also decomposed or volatilized, forming a preset porous structure in the first functional coating, which further optimizes the distribution of gradient porosity and elastic modulus.

[0048] In this preparation process, the gradient elastic modulus and porosity characteristics of the first and second functional coatings ensure that the core functions of the negative electrode sheet, such as stress buffering, rigid constraint and conductive transport, are fully utilized, ultimately achieving a significant improvement in the cycle performance and energy density of the high-silicon negative electrode steel shell stacked battery.

[0049] In some embodiments, the pore-forming agent includes one or more of expandable polymer microspheres, ammonium carbonate, ammonium bicarbonate, azodicarbonamide, and polyethylene glycol, and the mass percentage of the pore-forming agent is 0.5% to 1.5% based on the total mass of the first functional coating as 100%.

[0050] Specifically, this application uses expandable thermally decomposable polymer microspheres (Expancel) as a pore-forming agent. The pore-forming agent decomposes or volatilizes during the drying process of the negative electrode sheet, forming uniformly distributed pores inside the first functional coating. This not only further reduces the elastic modulus of the coating after compaction, reserving sufficient deformation space for the volume expansion during the charging and discharging process of the silicon negative electrode, but also optimizes the electrolyte wetting channel and improves the lithium-ion transport efficiency. The content range of 0.5% to 1.5% ensures that the number and size of pores are matched with the stress buffering requirements, avoiding excessive modulus and buffering failure due to insufficient pores, while avoiding loose coating structure, reduced mechanical strength or impact on electronic conduction efficiency due to excessive content.

[0051] In some embodiments, the preparation of the first slurry includes the following operations: An auxiliary binder is added to the first solvent, and after mixing and dissolving, a gel solution is obtained. The first active material and the first conductive agent are mixed, and then the mixture is added to the gel solution to obtain the first premixed solution. The main binder and pore-forming agent are added to the first premixed liquid, stirred and mixed evenly, and then the toughening agent is added. After stirring and dispersing again, the first slurry is obtained.

[0052] Specifically, in the above operations: Add deionized water to a mixing tank, and slowly add an auxiliary binder (such as sodium alginate powder) at 400 rpm. Continue stirring for 60 minutes until completely dissolved to form a gel. The first active material and Super P conductive agent were premixed in a dry powder mixer for 30 minutes to ensure uniform adhesion of the conductive agent and obtain a premixed solid powder. Then, the powder was added to the gel liquid, the mixer speed was increased to 800 rpm, and the mixture was dispersed for 120 minutes until the slurry was uniform and free of particles. Then, the speed was reduced to 300 rpm, and the main binder (styrene-butadiene rubber SBR emulsion) and pore-forming agent were added in sequence and stirred for 60 minutes. Add toughening agent slowly at a low speed of 300 rpm and disperse for 60 minutes to ensure uniform fiber distribution without shear damage. Add an appropriate amount of deionized water to adjust the slurry viscosity to 4000±500 mPa·s (@25℃), stir, and degas at 500 rpm for 30 minutes under a vacuum of -0.1 MPa to obtain the first slurry.

[0053] Specifically, the first solvent may be selected from deionized water.

[0054] In some embodiments, the preparation of the second slurry includes the following steps: The second solvent and the second binder are mixed to obtain a mixture; A second conductive agent is added to the mixture, and after stirring and dispersing, a second active material and a reinforcing agent are added, and the mixture is dispersed again to obtain a second premix. A crosslinking agent is added to the second premix, and after stirring evenly, a second slurry is obtained.

[0055] Specifically, in the above operations: Add NMP solvent to the reactor, and slowly add the second binder, polyvinylidene fluoride (PVDF), at 500 rpm. After the addition is complete, increase the speed to 1500 rpm and stir for 120 minutes until a completely dissolved mixture is formed. Adjust the rotation speed to 2000 rpm, add the second conductive agent, disperse at high speed for 90 minutes until a uniform conductive colloid is formed, reduce the rotation speed to 800 rpm, slowly add the second active material and reinforcing agent, and disperse for 120 minutes. The rotation speed was further reduced to 400 rpm, and the crosslinking agent was slowly added. The mixture was stirred continuously for 60 minutes to ensure uniformity. Finally, an appropriate amount of NMP was added to adjust the viscosity of the slurry to 3500±500 mPa·s (@25℃). Then, the mixture was degassed at 800 rpm for 30 minutes under a vacuum of -0.1MPa to obtain the second slurry.

[0056] Specifically, the second solvent is an organic solvent, which may be an NMP solvent.

[0057] Another embodiment of the present invention provides a battery including the aforementioned negative electrode sheet, or a negative electrode sheet prepared by the method for preparing the aforementioned negative electrode sheet.

[0058] Specifically, the battery includes the negative electrode sheet provided in this application. In the negative electrode sheet provided in this application, the first functional coating on the surface of the current collector is a low-modulus, high-toughness stress buffer layer. It can absorb and disperse the internal stress generated by the expansion of the silicon negative electrode through its own deformation, molecular chain slippage, and flexible network stretching, avoiding direct stress impact on the current collector interface, effectively preventing the coating from peeling off from the current collector, and ensuring the electron conduction path. The second functional coating covering the first functional coating has a high elastic modulus, which can constrain the irregular expansion of silicon particles from the outside, maintain the structural integrity of the electrode sheet surface, reduce microcracks and breakage of active materials caused by stress concentration, and avoid irreversible consumption of active lithium. At the same time, the first functional coating and the second functional coating work together to form a gradient elastic modulus structure from the side closer to the negative electrode current collector to the side farther away from the negative electrode current collector, which pre-sets a controllable release direction for the expansion stress, so that the stress is preferentially conducted to the more tough first functional coating, avoiding the vicious cycle of structural damage caused by the stress of the homogeneous electrode sheet having nowhere to be released and the rigid constraint of the steel shell superimposed, thereby achieving a significant improvement in the cycle performance of high silicon negative electrode in steel shell stacked battery.

[0059] The present invention will be further illustrated by the following examples.

[0060] Example 1 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, and includes the following operational steps: Preparation of the first slurry Add deionized water to a mixing tank, and slowly add 1.5% auxiliary binder (sodium alginate) at 400 rpm. Continue stirring for 60 minutes until completely dissolved to form a gel. 85% of the first active material and 4% of the conductive agent (Super P) were premixed in a dry powder mixer for 30 minutes to ensure uniform adhesion of the conductive agent, resulting in a premixed solid powder. This powder was then added to the gel solution, and the mixer speed was increased to 800 rpm. The mixture was dispersed for 120 minutes until the slurry was uniform and free of particles. The speed was then reduced to 300 rpm, and 7% of the main binder (styrene-butadiene rubber SBR emulsion) and 0.5% of the pore-forming agent were added sequentially. The mixture was stirred for 60 minutes. At a low speed of 300 rpm, slowly add 2% toughening agent (polyurethane fiber) and disperse for 60 minutes to ensure that the fiber is evenly distributed and not damaged by shear. Add an appropriate amount of deionized water to adjust the viscosity of the slurry to 4000±500 mPa·s (@25℃). Then transfer it to a mixer and degas at 500 rpm for 30 minutes under a vacuum of -0.1 MPa to obtain the first slurry.

[0061] Preparation of the second slurry Add NMP solvent to the reactor, and slowly add 7% of the second binder (PVDF) at 500 rpm. After the addition is complete, increase the speed to 1500 rpm and stir for 120 minutes until a completely dissolved mixture is formed. Adjust the rotation speed to 2000 rpm, add 4% of the second conductive agent (a composite of carbon fiber and graphene, 2.5% carbon fiber and 1.5% graphene), disperse at high speed for 90 minutes until a uniform conductive liquid is formed, reduce the rotation speed to 800 rpm, slowly add 87% of the second active material and 1.2% of the reinforcing agent (nano silica), and disperse for 120 minutes. The rotation speed was further reduced to 400 rpm, and 0.8% crosslinking agent (toluene diisocyanate) was slowly added. The mixture was stirred continuously for 60 minutes to ensure uniformity. Finally, an appropriate amount of NMP was added to adjust the viscosity of the slurry to 3500±500 mPa·s (@25℃). Then, the mixture was degassed at 800 rpm for 30 minutes under a vacuum of -0.1MPa to obtain the second slurry.

[0062] Preparation of negative electrode The first slurry and the second slurry are coated sequentially on the negative electrode current collector in a 1:1 ratio using a double-layer coating method. The coating is then baked, dried, rolled, and cut to obtain the negative electrode sheet.

[0063] The positive electrode can be any type of positive electrode conventionally used in the field, and this application does not impose any special restrictions on the positive electrode.

[0064] The negative electrode, positive electrode, separator, steel shell, and electrolyte are assembled into a steel-shell stacked lithium-ion battery.

[0065] Example 2 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: In the preparation of the first slurry, the main binder SBR content is 6%, the auxiliary binder sodium alginate content is 2%, and the pore-forming agent content is 1%. In the preparation of the second slurry, the second binder has a PVDF content of 6%, the crosslinking agent content of 1%, and the reinforcing agent nano-silica content of 2%.

[0066] Example 3 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: No auxiliary binder is added during the preparation of the first slurry.

[0067] Example 4 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: No pore-forming agent is added during the preparation of the first slurry.

[0068] Example 5 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: No crosslinking agent is added during the preparation of the second slurry.

[0069] Example 6 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: No reinforcing agent is added in the preparation of the second slurry.

[0070] Comparative Example 1 The negative electrode sheet is prepared using the conventional CMC-SBR system and assembled into a lithium-ion battery. The specific preparation method is as follows: Add deionized water (100 times the amount of CMC) to a stirred tank, and slowly add 1.8% CMC powder at 500 rpm. After the addition is complete, increase the speed to 1200 rpm and stir continuously for 180 minutes until the CMC is completely dissolved, forming a transparent viscous solution. The viscosity of the solution should be controlled at 3000±500 mPa·s (@25℃). Then add 2.5% Super P conductive agent to the CMC solution and disperse at 1500 rpm for 60 minutes to form a conductive adhesive. Adjust the speed to 1000 rpm and slowly add 92% SiO2. x After adding the active ingredient (C), increase the speed to 2000 rpm and disperse at high speed for 120 minutes to ensure the slurry is uniform and free of particles. Reduce the speed to 500 rpm and slowly add 3.7% SBR emulsion. Continue stirring for 60 minutes to ensure the SBR is uniformly dispersed. Finally, add an appropriate amount of deionized water to adjust the slurry viscosity to 3500±500 mPa·s (@25℃). Transfer to a planetary mixer and degas at 600 rpm for 40 minutes under a vacuum of -0.1MPa before discharging.

[0071] Comparative Example 2 This comparative example is used to illustrate the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operational steps in Example 1, with the following differences: The coating order of the first slurry and the second slurry is reversed, that is, the second slurry is coated on the surface of the negative electrode current collector first, and then the first slurry is coated.

[0072] Comparative Example 3 This embodiment illustrates the negative electrode sheet, negative electrode sheet preparation method, and lithium-ion battery disclosed in this invention, including most of the operations in Example 1, with the following differences: In the preparation of the first slurry, the percentage of sodium alginate as an auxiliary binder is 1.5%, the percentage of pore-forming agent is 2%, and the percentage of toughening agent is 0.5%. In the preparation of the second slurry, the crosslinking agent content is 1.5% and the reinforcing agent nano-silica content is 0.5%.

[0073] Performance testing Performance tests were performed on the examples and comparative examples prepared above: 1. The elastic modulus of the negative electrode sheets of Examples 1-6 and Comparative Examples 1-3 were tested: Test method: Using a nanoindenter, indentation tests were performed on the cross-section of the negative electrode sheet starting from the current collector interface, with an interval of 5 μm, to obtain the elastic modulus value at that point.

[0074] like Figure 1As shown, the test curve of Example 1 clearly verifies that the negative electrode sheet of this application has a gradient elastic modulus structure. The increase from 2.1 GPa on the negative electrode current collector side to 9.2 GPa on the surface side verifies that the first functional coating and the second functional coating work together to form a gradient elastic modulus structure from the side closer to the negative electrode current collector to the side farther away from the negative electrode current collector, that is, the structure of "low elastic modulus buffer layer - high elastic modulus constraint layer". This avoids the structural damage caused by the stress of the homogeneous electrode sheet having nowhere to be released and the vicious cycle of rigid constraint superposition in steel-cased batteries, thereby achieving a significant improvement in the cycle performance of high silicon negative electrode applied in steel-cased stacked batteries. Figure 1 The comparative tests show that its elastic modulus remains basically unchanged (about 4.5 GPa) throughout the thickness of the negative electrode, thus failing to achieve stress buffering and guidance.

[0075] 2. The specific elastic modulus test results of Examples 1-6 and Comparative Examples 1-3 are recorded in Table 1.

[0076] Table 1 As can be seen from the elastic modulus test results of Examples 1-6 and Comparative Examples 1-3 in Table 1, Example 1 exhibits a better elastic modulus gradient structure, with the elastic modulus of the first functional coating being 2.1 GPa and the elastic modulus of the second functional coating being 9.2 GPa, and the modulus gradient span reaching 7.1 GPa. In Example 2, by adjusting the proportion of material components, the elastic modulus gradient was further optimized. The elastic modulus of the first functional coating decreased to 2.0 GPa, mainly due to the increased amount of sodium alginate added. The elastic modulus of the second functional coating increased to 9.5 GPa, due to the increased content of crosslinking agent and reinforcing agent. The overall gradient range expanded to 7.5 GPa. In Examples 3-6, Example 3, without the addition of an auxiliary binder, showed that the elastic modulus of the first functional coating increased to 2.8 GPa, indicating that sodium alginate can effectively reduce the modulus of the underlying layer; Example 4, without the addition of a pore-forming agent, showed that the elastic modulus of the first functional coating increased to 2.9 GPa, proving that the pore-forming agent can reduce the modulus of the underlying layer by increasing porosity; Example 5, without the addition of a crosslinking agent, showed that the modulus of the second functional coating decreased to 6.8 GPa, indicating that the crosslinking agent plays a key role in improving the surface modulus; Example 6, without the addition of a reinforcing agent, showed that the modulus of the second functional coating decreased to 7.5 GPa, further indicating that nano-silica can further improve the rigidity of the second functional coating. In Comparative Example 1, a conventional CMC-SBR system was used. The test results show that its elastic modulus is uniformly maintained at 4.5 GPa in the electrode thickness direction. The elastic modulus of the first functional coating and the second functional coating are not different, which cannot achieve the directional guidance of expansion stress and easily leads to disordered stress concentration. In Comparative Example 2, the coating order of the two layers was reversed, so that the elastic modulus of the first functional coating was greater than the reverse modulus gradient between the elastic moduli of the second and second functional coatings. Specifically, the elastic modulus of the first functional coating was 9.1 GPa, and the elastic modulus of the second functional coating was 2.3 GPa. The high modulus layer was close to the current collector and could not buffer the stress, causing the expansion stress to directly impact the interface. The low modulus layer was located on the surface and could not restrain the active material, which easily caused the electrode surface to pulverize.

[0077] The elastic modulus of the first functional coating of Comparative Example 3 is 2.2 GPa, and the elastic modulus of the second functional coating is 9.0 GPa. Compared with other comparative examples, Comparative Example 3 meets the requirement that the elastic modulus of the first functional coating is less than that of the second functional coating in this application.

[0078] 3. Tests were conducted on Examples 1-6 and Comparative Examples 1-3: Slurry viscosity testing method: A rotational viscometer is used at 20 rpm. The shear resistance experienced by the rotor as it rotates in the slurry is measured. The test time is 60 seconds. After the reading stabilizes, the viscosity value is recorded in mPa·s (millipascals per second). If the slurry viscosity is between 4000 and 6000 mPa·s, the condition is "moderate"; below 4000 mPa·s, the condition is "low"; and above 6000 mPa·s, the condition is "high".

[0079] Test method for static suspension stability of slurry: The static suspension stability of slurry is reflected by observing whether sedimentation or stratification occurs after the slurry has been left to stand for a certain period of time.

[0080] Peel strength test method: Cut a 25mm wide × 100mm long sample from the rolled electrode sheet, and peel the coating off the current collector at a constant angle and speed using the 180° peel method. Record the force value during the peeling process and convert it into peel strength per unit width.

[0081] The method for testing the resistivity of a diaphragm is as follows: A 50mm × 50mm square sample is cut from the rolled electrode. The four-probe method is used, with four equidistant probes contacting the coating surface. Current is passed through the two outer probes, and voltage is measured through the two inner probes. This eliminates the influence of contact resistance and wire resistance, and accurately measures the resistivity of the coating.

[0082] The test results are entered into Table 2.

[0083] Table 2 The test data in Table 2 show that, due to the combination of main and auxiliary binders and the reasonable addition of toughening / reinforcing agents, the slurry in Examples 1 and 2 only showed slight sedimentation after 48 hours, with a peel strength of 31.8-32.5 N / m and a resistivity as low as 18.5-19.2 Ω*cm, taking into account suspension stability, interfacial adhesion and conductivity. Example 3 shows that the lack of auxiliary binder leads to increased sedimentation and decreased peel strength; Examples 5 and 6 show that the absence of crosslinking agent / reinforcing agent results in increased resistivity and impaired interfacial bonding and conductivity. Comparative Example 1 uses a conventional CMC-SBR system, which has weak adhesion and obvious sedimentation. Comparative Example 2 has a reverse gradient, which leads to extremely poor adhesion between the coating and the current collector. The peel strength is only 18.5 N / m, and the resistivity increases significantly. The modulus structure and component integrity directly affect the stability of the electrode interface and the continuity of the conductive path. The slurry obtained in Comparative Example 3 had a moderate viscosity, but slight sedimentation occurred after 12 hours, significant sedimentation after 24 hours, and severe sedimentation after 48 hours. Its suspension stability was significantly worse than that of Examples 1-6. It is speculated that the reason is that the ratio of various parameters (auxiliary binder, crosslinking agent, toughening agent, and reinforcing agent) during the preparation process was unbalanced, resulting in a decrease in the dispersibility and stability of the slurry. The peel strength was only 22.6 N / m, which was much lower than the 28.5-32.5 N / m of Examples 1-6. The interfacial adhesion was poor. Due to insufficient bonding system and toughening agent, the bonding force between the coating and the current collector was weakened. The membrane resistivity was 38.2 Ω*cm, which was much higher than that of the examples. The integrity of the conductive path was damaged. This was mainly due to the excessive crosslinking agent, which led to increased coating brittleness, uneven dispersion of conductive agent, and reduced electron transport efficiency.

[0084] 4. Tests were conducted on Examples 1-6 and Comparative Examples 1-3: Rate discharge performance test method: The test was conducted at 25℃ using the Xinwei charge-discharge test equipment. After standing for 10 minutes, the battery was charged to 4.5V using a constant current and constant voltage of 0.2C, with a cutoff current of 0.05C. After standing for 10 minutes, the battery was discharged to 3.0V using a constant current of 0.2C / 0.5C / 1.0C respectively, and the discharge capacity C was recorded. 0.2 / C 0.5 / C 1.0 .

[0085] 0.2C capacity percentage = C 0.2 / C 0.2 ×100% 0.5C capacity percentage = C 0.5 / C 0.2 ×100% 1.0C capacity percentage = C 1.0 / C 0.2 ×100% 25℃ Cyclic Performance Test Method: The battery was tested at 25℃ using a Xinwei charge-discharge tester. After resting for 10 minutes, the battery was charged to 4.5V using a 0.5C constant current and constant voltage method, with a cutoff current of 0.05C. After resting for 10 minutes, the battery was discharged to 3.0V using a 0.5C constant current method, and the initial capacity C0 was recorded. The charge-discharge test was repeated 500 times, and the discharge capacity C on the 500th cycle was recorded. 500 .

[0086] 25℃-500-week capacity retention rate = C 500 / C0×100% 45℃ Cyclic Performance Test Method: The battery was tested at 45℃ using a Xinwei charge-discharge tester. After resting for 10 minutes, the battery was charged to 4.5V using a 0.5C constant current and constant voltage method, with a cutoff current of 0.05C. After resting for 10 minutes, the battery was discharged to 3.0V using a 0.5C constant current method, and the initial capacity C0 was recorded. The charge-discharge test was repeated 400 times, and the discharge capacity C0 of the 400th cycle was recorded. 400 .

[0087] 45℃-400 cycles capacity retention rate = C 400 / C0×100%.

[0088] The test results are entered into Table 3.

[0089] Table 3 As can be seen from the test results in Table 3, Examples 1-2, by forming a complete low-modulus buffer and a high-modulus constraint gradient, can effectively alleviate silicon expansion stress and protect the interface and active material. The capacity retention rates at 25℃-500 cycles are 88.7% and 91.0%, respectively, and the capacity retention rates at 45℃-400 cycles are 76.5% and 86.8%, respectively, demonstrating excellent rate performance. In Examples 3-6, the gradient effect of elastic modulus caused by the absence of components was relatively poor, and the cycle retention rate and rate performance gradually decreased. In Comparative Example 1, stress concentration in the homogeneous electrode caused breakage of the active material and interface peeling, resulting in performance inferior to that of the embodiments. Comparative Example 2 shows that the reverse gradient cannot buffer stress and the constraint fails, resulting in a capacity retention rate of 76.3% (25℃-500 cycles) and 65.8% (45℃-400 cycles), with the lowest rate discharge rate. The rate discharge performance and cycle performance of Comparative Example 3 were inferior to those of Examples 1-6, and only slightly better than Comparative Example 1. The 0.5C capacity ratio was 95.1% and the 1C capacity ratio was 89.2%, which was lower than 97.9% and 94.6% of Example 1, respectively. The capacity retention rate at 25℃-500 cycles was 82.4% and the capacity retention rate at 45℃-400 cycles was 76.9%, which was far lower than 91.0% and 86.8% of Example 1, respectively. The core reason is that the content of each component deviated from the range defined in this application during preparation, the gradient modulus and pore structure did not reach the ideal state, the stress conduction was insufficient, the active material was easily broken and the interface was easily peeled off, and the conductivity and adhesion properties decreased, which ultimately led to a significant decrease in rate performance and cycle stability.

[0090] Based on the above test results, the first functional coating on the current collector surface of the negative electrode provided in this application is a low-modulus, high-toughness stress buffer layer. It can absorb and disperse the internal stress generated by the expansion of the silicon negative electrode through its own deformation, molecular chain slippage, and flexible network stretching, avoiding direct stress impact on the current collector interface, effectively preventing the coating from peeling off from the current collector, and ensuring the electron conduction path. The second functional coating covering the first functional coating has a high elastic modulus, which can constrain the irregular expansion of silicon particles from the outside, maintain the structural integrity of the electrode surface, reduce microcracks and breakage of active materials caused by stress concentration, and avoid irreversible consumption of active lithium. At the same time, the first and second functional coatings work together to form a gradient elastic modulus structure from the side closer to the negative electrode current collector to the side farther away from the negative electrode current collector, which pre-sets a controllable release direction for the expansion stress, so that the stress is preferentially conducted to the more tough first functional coating, avoiding the vicious cycle of structural damage caused by the stress of the homogeneous electrode having nowhere to be released and the superposition of rigid constraints of the steel shell, thereby achieving a significant improvement in the cycle performance of high-silicon negative electrodes in steel-shell stacked batteries.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector and a functional coating. The functional coating includes a first functional coating and a second functional coating. The first functional coating is disposed on the surface of the current collector, and the second functional coating is disposed on the side of the first functional coating that is opposite to the negative electrode current collector. The elastic modulus of the first functional coating is less than that of the second functional coating; The first functional coating includes a first active material, a first binder, and a first conductive agent; The mass percentage of the first active material is 80%~85%; The first adhesive has a mass percentage content of 8% to 12%; The first conductive agent has a mass percentage content of 3% to 5%; The second functional coating includes a second active material, a second binder, and a second conductive agent; The second active material has a mass percentage content of 83% to 88%; The second adhesive has a mass percentage of 6% to 8%; The second conductive agent has a mass percentage content of 4% to 6%; The first functional coating further includes a toughening agent, and the second functional coating further includes a reinforcing agent, wherein the toughening agent has a mass percentage content of 1% to 3%, and the reinforcing agent has a mass percentage content of 1% to 2%. The second functional coating further includes a crosslinking agent, wherein the crosslinking agent has a mass percentage of 0.5% to 1%. The first adhesive includes a primary adhesive and an auxiliary adhesive, wherein the primary adhesive has a mass percentage of 5% to 8% and the auxiliary adhesive has a mass percentage of 2% to 4%.

2. The negative electrode sheet according to claim 1, characterized in that, The primary adhesive comprises one or more of styrene-butadiene rubber and polyacrylate compounds; and / or, The auxiliary binder includes one or more of sodium alginate, carboxylated nitrile rubber, and polyurethane compounds.

3. The negative electrode sheet according to claim 1, characterized in that, The first conductive agent includes one or more of conductive carbon black, Ketjen black, carbon nanotubes, and graphene.

4. The negative electrode sheet according to claim 1, characterized in that, The second adhesive includes one or more of polyvinylidene fluoride, polyimide, cyanate compounds, and peroxides.

5. The negative electrode sheet according to claim 1, characterized in that, The second conductive agent comprises a compound of carbon fiber and graphene, wherein the diameter of the carbon fiber is 5-10 μm and the length of the carbon fiber is 10-50 μm.

6. The negative electrode sheet according to claim 1, characterized in that, The toughening agent comprises elastic fibers with a diameter of 0.5~2μm and a length of 50~200μm; The reinforcing agent includes one or more of nano-silica, nano-alumina, and nano-silicon carbide.

7. The negative electrode sheet according to claim 1, characterized in that, The porosity of the first functional coating is greater than that of the second functional coating, wherein the porosity of the first functional coating is 25% to 45% and the porosity of the second functional coating is 15% to 28%.

8. The negative electrode sheet according to claim 1, characterized in that, The thickness of the first functional coating is 30~70μm; The thickness of the second functional coating is 10~30μm.

9. The negative electrode sheet according to claim 1, characterized in that, The elastic modulus of the first functional coating is 1.5~4.0 GPa, and the elastic modulus of the second functional coating is 6.0~12.0 GPa.

10. The method for preparing the negative electrode sheet according to any one of claims 1 to 9, characterized in that, Includes the following operations: The first active material, the first binder, the first conductive agent, the toughening agent, the pore-forming agent, and the first solvent are used to make a first slurry; The second active material, the second binder, the second conductive agent, the reinforcing agent, and the second solvent are used to make a second slurry; The first slurry and the second slurry are sequentially coated on the surface of the negative electrode current collector. The first solvent, the second solvent, and the pore-forming agent are removed by drying to obtain the negative electrode sheet.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The pore-forming agent includes one or more of expandable polymer microspheres, ammonium carbonate, ammonium bicarbonate, azodicarbonamide, and polyethylene glycol. The mass percentage of the pore-forming agent is 0.5% to 1.5% based on the total mass of the first functional coating as 100%.

12. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The preparation of the first slurry includes the following operations: An auxiliary binder is added to the first solvent, and after mixing and dissolving, a gel solution is obtained. The first active material and the first conductive agent are mixed, and then the mixture is added to the gel solution to obtain the first premixed solution. The main binder and pore-forming agent are added to the first premixed liquid, stirred and mixed evenly, and then the toughening agent is added. After stirring and dispersing again, the first slurry is obtained.

13. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The preparation of the second slurry includes the following steps: The second solvent and the second binder are mixed to obtain a mixture; A second conductive agent is added to the mixture, and after stirring and dispersing, a second active material and a reinforcing agent are added, and the mixture is dispersed again to obtain a second premixed solution. A cross-linking agent is added to the second premix, and after stirring evenly, a second slurry is obtained.

14. A battery, characterized in that, This includes the negative electrode sheet as described in any one of claims 1 to 9, or the negative electrode sheet prepared by the method described in any one of claims 10 to 13.

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