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

Abstract

This application provides a negative electrode sheet and its preparation method, as well as a secondary battery. The negative electrode sheet includes a negative electrode current collector, a first negative electrode active material layer, and a second negative electrode active material layer. The first negative electrode active material layer is disposed on one side surface of the negative electrode current collector, and the second negative electrode active material layer is disposed on the side surface of the first negative electrode active material layer opposite to the negative electrode current collector. The first negative electrode active material layer includes a first negative electrode active material, a first PI resin, an elastic polymer, and a first dispersant, and does not contain conductive material. The second negative electrode active material layer includes a second negative electrode active material, a second PI resin, a conductive material, and a second dispersant, and does not contain elastic polymer; the elastic polymer includes rubber-based polymers. The negative electrode sheet provided by this application enables the negative electrode sheet to possess both good cycle stability and conductivity, effectively suppressing expansion and improving the cycle performance and rate performance of the battery.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, specifically relating to a negative electrode sheet and its preparation method, and a secondary battery. Background Technology

[0002] Silicon-based anode materials are considered core anode materials for improving the energy density of lithium-ion batteries due to their extremely high theoretical specific capacity (far exceeding that of traditional graphite anodes). However, they experience volume expansion of up to 300% during battery charging and discharging, leading to pulverization of the anode sheet structure and shedding of the active material layer. In addition, silicon has poor conductivity, which seriously affects the cycle performance and rate performance of the battery, limiting its large-scale application.

[0003] To address the aforementioned issues, existing technologies commonly employ carbon coating, elemental doping modification, and binder modification. Among these, binder modification, by regulating the adhesion and structural stability within the electrode, adapts to the volume deformation characteristics of silicon-based materials, offering advantages such as strong process adaptability, low modification costs, and significant effects. Polyimide (PI), due to its rigid imide five-membered ring structure in its molecular backbone, possesses excellent high-temperature resistance, mechanical strength, and chemical stability, and is widely used for the binder modification of silicon-carbon anodes. However, existing polyimide (PI) applications in silicon-carbon anodes often employ a single cross-linked network, which cannot simultaneously address volume expansion buffering and electronic conduction efficiency, resulting in poor cycle performance and rate capability of the battery. Summary of the Invention

[0004] To address the problem that existing anode sheets containing silicon-based anodes and PI cannot simultaneously achieve volume expansion buffering and electronic conduction efficiency, resulting in poor battery cycle performance and rate performance, this application provides an anode sheet, its preparation method, and a secondary battery.

[0005] To solve the above-mentioned technical problems, in a first aspect, this application provides a negative electrode sheet, including a negative electrode current collector, a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is disposed on one side surface of the negative electrode current collector, and the second negative electrode active material layer is disposed on the side surface of the first negative electrode active material layer opposite to the negative electrode current collector. The first negative electrode active material layer includes a first negative electrode active material, a first PI resin, an elastic polymer, and a first dispersant, and does not contain conductive materials. The second negative electrode active material layer includes a second negative electrode active material, a second PI resin, a conductive material, and a second dispersant, and does not contain an elastic polymer in the second negative electrode active material layer; The elastic polymer includes rubber-based polymers.

[0006] Preferably, a gradient transition layer is formed between the first negative electrode active material layer and the second negative electrode active material layer, the gradient transition layer being formed by the interpenetration and cross-linking of molecular chains in the first PI resin and molecular chains in the second PI resin.

[0007] Preferably, the thickness of the first negative electrode active material layer is H1, the thickness of the second negative electrode active material layer is H2, and the ratio of H1 to H2 is 1:(1.2~1.5).

[0008] Preferably, the total thickness of the first negative electrode active material layer and the second negative electrode active material layer is 50μm~100μm.

[0009] Preferably, in the first negative electrode active material layer, the mass content of the elastic polymer is 8%~12%, the mass content of the first negative electrode active material is 63%~79%, the mass content of the first PI resin is 12%~22%, and the mass content of the first dispersant is 1%~3%. And / or, in the second negative electrode active material layer, the mass content of the conductive material is 12%~18%, the mass content of the second negative electrode active material is 57%~75%, the mass content of the second PI resin is 12%~22%, and the mass content of the second dispersant is 1%~3%.

[0010] Preferably, both the first negative electrode active material and the second negative electrode active material include at least one of silicon-carbon composite material and carbon material; The silicon-carbon composite material includes silicon particles and a carbon coating layer. The carbon coating layer covers the outer surface of the silicon particles. The thickness of the carbon coating layer is 10 nm to 50 nm, and the D50 particle size of the silicon particles is 50 nm to 200 nm. The specific surface area of ​​the silicon-carbon composite material is 10 m². 2 / g~30m 2 / g.

[0011] Preferably, the rubber polymer includes at least one of heat-resistant modified styrene-butadiene rubber and carboxylated nitrile rubber; The heat-resistant modified styrene-butadiene rubber is obtained by modifying styrene-butadiene rubber with a silane coupling agent. The heat-resistant modified styrene-butadiene rubber retains ≥90% of its elasticity after being kept at 150℃-180℃ for 20min-30min. And / or, both the first PI resin and the second PI resin include oil-soluble polyimide.

[0012] Secondly, this application provides a method for preparing the above-mentioned negative electrode sheet, comprising the following steps: The first negative electrode active material, the first PI resin, the elastic polymer, the first dispersant and the first solvent are mixed evenly to obtain the first negative electrode slurry; The second negative electrode active material, the second PI resin, the conductive material, the second dispersant, and the second solvent are mixed evenly to obtain the second negative electrode slurry; The first negative electrode slurry is coated on at least one side of the negative electrode current collector and dried to obtain a first negative electrode active material layer; the second negative electrode slurry is coated on the surface of the first negative electrode active material layer away from the negative electrode current collector and dried to obtain a second negative electrode active material layer, thus obtaining an unrolled negative electrode sheet. The unrolled negative electrode sheet is rolled and cured to obtain the negative electrode sheet.

[0013] Preferably, the curing temperature is 150℃~180℃, and the curing time is 20min~30min.

[0014] Thirdly, this application provides a secondary battery, including the negative electrode sheet described above, or a negative electrode sheet prepared by the method described above.

[0015] The negative electrode sheet provided in this application has the following effects: 1) The first negative electrode active material layer contains a first PI resin and an elastic polymer to form a PI-elastic polymer composite network, which can effectively buffer the volume expansion of the silicon-carbon composite material during charging and discharging, and prevent the negative electrode sheet from pulverizing and falling off; the second negative electrode active material layer contains a second PI resin and a conductive material to form a PI-conductive carbon composite network, which can significantly improve the electronic conduction efficiency of the electrode sheet and improve the rate performance of the battery. The two work together to enable the negative electrode sheet to have both good cycle stability and conductivity, effectively suppress expansion, improve the cycle performance and rate performance of the battery, and solve the problems of volume expansion and poor conductivity of silicon-carbon negative electrodes. 2) The negative electrode sheet provided in this application can be directly used in high-energy-density lithium-ion batteries and is suitable for various lithium-ion battery products such as mobile phones, laptops, and electric vehicles. It has a wide range of applications and good prospects for industrial application. Detailed Implementation

[0016] 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 embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0017] This application provides a negative electrode sheet, including a negative electrode current collector, a first negative electrode active material layer and a second negative electrode active material layer, wherein the first negative electrode active material layer is disposed on one side surface of the negative electrode current collector, and the second negative electrode active material layer is disposed on the side surface of the first negative electrode active material layer opposite to the negative electrode current collector. The first negative electrode active material layer includes a first negative electrode active material, a first PI resin, an elastic polymer, and a first dispersant, and does not contain conductive materials. The second negative electrode active material layer includes a second negative electrode active material, a second PI resin, a conductive material, and a second dispersant, and does not contain an elastic polymer in the second negative electrode active material layer; The elastic polymer includes rubber-based polymers.

[0018] Specifically, the first negative electrode active material layer contains no conductive material; electron conduction is achieved solely through the conductivity of the first negative electrode active material itself. The first PI resin and elastic polymer contained in the first negative electrode active material layer form a PI-elastic polymer composite network, effectively suppressing volume expansion during the charging and discharging of the silicon-carbon composite material, preventing electrode cracking and active material shedding, reducing interfacial impedance fluctuations, improving the cycle stability of the electrode, and extending battery life. Simultaneously, it maintains good bonding between the first negative electrode active material layer and the negative electrode current collector, and between the first and second negative electrode active material layers, avoiding interfacial peeling problems caused by volume expansion; ensuring the elastic buffer and interfacial adhesion of the first negative electrode active material layer. The second negative electrode active material layer contains no elastic polymer, increasing the proportion of conductive material. The second PI resin and conductive material form a PI-conductive carbon composite network, effectively improving the electron conduction efficiency of the second negative electrode active material layer, enhancing kinetics, and improving battery rate performance.

[0019] The negative electrode sheet provided in this application has the following effects: 1) The first negative electrode active material layer contains a first PI resin and an elastic polymer to form a PI-elastic polymer composite network, which can effectively buffer the volume expansion of the silicon-carbon composite material during charging and discharging, and prevent the negative electrode sheet from pulverizing and falling off; the second negative electrode active material layer contains a second PI resin and a conductive material to form a PI-conductive carbon composite network, which can significantly improve the electronic conduction efficiency of the electrode sheet and improve the rate performance of the battery. The two work together to enable the negative electrode sheet to have both good cycle stability and conductivity, effectively suppress expansion, improve the cycle performance and rate performance of the battery, and solve the problems of volume expansion and poor conductivity of silicon-carbon negative electrodes. 2) The negative electrode sheet provided in this application can be directly used in high-energy-density lithium-ion batteries and is suitable for various lithium-ion battery products such as mobile phones, laptops, and electric vehicles. It has a wide range of applications and good prospects for industrial application.

[0020] In some embodiments, a gradient transition layer is formed between the first negative electrode active material layer and the second negative electrode active material layer, the gradient transition layer being formed by the interpenetration and cross-linking of molecular chains in the first PI resin and molecular chains in the second PI resin.

[0021] Specifically, the gradient transition layer is formed by the interpenetrating and cross-linking of molecular chains in the first PI resin and the second PI resin. This enables electrons from the PI-conductive carbon composite network in the second negative electrode active material layer to be conducted to the silicon-carbon composite material of the first negative electrode active material layer through the interpenetrating and cross-linked PI molecular chains at the interface of the two layers, forming a complete electron transport pathway and ensuring the smooth progress of the charging and discharging process.

[0022] No additional conductive carbon material is added to the first negative electrode active material layer, but conductivity and battery charging and discharging are achieved through the following two aspects: First, the first negative electrode active material is selected from silicon-carbon composite material, which itself contains an amorphous carbon coating layer and has a certain conductivity, which can conduct electrons; Second, the first negative electrode active material layer and the second negative electrode active material layer are tightly bonded, and the electrons of the PI-conductive carbon composite network in the second negative electrode active material layer can be conducted to the silicon-carbon composite material of the first negative electrode active material layer through the PI molecular chains that interpenetrate and cross-link at the interface of the two layers, forming a complete electron transport pathway and ensuring the smooth progress of the charging and discharging process.

[0023] In some embodiments, the thickness of the first negative electrode active material layer is H1, the thickness of the second negative electrode active material layer is H2, and the ratio of H1 to H2 is 1:(1.2~1.5).

[0024] Specifically, when the H1:H2 ratio is in the range of 1:(1.2~1.5), the first negative electrode material layer is thinner, while the second negative electrode material layer is thicker. Because the second negative electrode material layer contains more conductive material, it ensures electron conduction throughout the negative electrode, improves electron conduction efficiency, reduces impedance, and enhances the battery's rate performance. A slightly thinner first negative electrode material layer, while increasing impedance, effectively suppresses the expansion of the silicon-carbon composite material, increases adhesion between the first and second negative electrode material layers, and ensures peel strength. If the H1:H2 ratio is less than 1:(1.2~1.5), the first negative electrode material layer is too thin, reducing adhesion between the current collector and the first negative electrode material layer, and reducing adhesion between the second and first negative electrode material layers. This reduces peel strength, making the first and second negative electrode material layers prone to detachment during cycling, thus reducing battery cycle performance. If the ratio of H1:H2 is greater than 1:(1.2~1.5), the thickness of the second negative electrode material layer is too low, the impedance of the negative electrode increases, the electron conduction efficiency decreases, the rate performance of the battery decreases, the cycle performance decreases, and the rate performance decreases.

[0025] In specific embodiments, the ratio of H1:H2 can be 1:1.2, 1:1.25, 1:1.28, 1:1.3, 1:1.2, 1:1.35, 1:1.4, 1:1.45, 1:1.5 or any two of the above.

[0026] In some embodiments, the total thickness of the first negative electrode active material layer and the second negative electrode active material layer is 50 μm to 100 μm.

[0027] Specifically, the total thickness of the first negative electrode active material layer and the second negative electrode active material layer is in the range of 50μm to 100μm. The appropriate thickness range can ensure that there is sufficient active material load per unit area to support the basic energy density of the cell; at the same time, it can ensure that the electrolyte is fully wetted, the lithium ion transport rate is high, avoid excessive polarization, and improve the rate performance and cycle performance of the battery.

[0028] If the total thickness of the first and second negative electrode active material layers is too low, the active material loading will be insufficient, the energy density of the battery will decrease, the electrode sheets will be prone to detachment and powder shedding, and the cycle performance of the battery will decline. If the total thickness of the first and second negative electrode active material layers is too high, the lithium-ion transport distance will increase, the kinetic performance will decrease, and the rate performance and cycle performance of the battery will decline.

[0029] In a specific embodiment, the total thickness of the first negative electrode active material layer and the second negative electrode active material layer can be 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, 85μm, 90μm, 95μm, 100μm or any two of the above.

[0030] In some embodiments, in the first negative electrode active material layer, the mass content of the elastic polymer is 8%~12%, the mass content of the first negative electrode active material is 63%~79%, the mass content of the first PI resin is 12%~22%, and the mass content of the first dispersant is 1%~3%. And / or, in the second negative electrode active material layer, the mass content of the conductive material is 12%~18%, the mass content of the second negative electrode active material is 57%~75%, the mass content of the second PI resin is 12%~22%, and the mass content of the second dispersant is 1%~3%.

[0031] Specifically, in the first negative electrode active material layer, the mass content of the first PI resin is 12%~22%, and the mass content of the elastic polymer is in the range of 8%~12%. The first negative electrode active material layer contains a relatively large amount of elastic polymer. The PI-elastic polymer composite network formed by the elastic polymer and the first PI resin can effectively absorb the volume expansion stress during the charging and discharging process of the silicon-carbon composite material, prevent electrode cracking and active material layer shedding, reduce interface impedance fluctuations, improve the cycle stability of the electrode, and extend the battery life. At the same time, it maintains a good bond between the first negative electrode active material layer and the negative electrode current collector, as well as a good bond between the first negative electrode active material layer and the second negative electrode active material layer, avoiding interface peeling problems caused by volume expansion.

[0032] If the mass content of the elastic polymer in the first negative electrode active material layer is less than 8%, the adhesion between the first negative electrode active material layer and the negative electrode current collector decreases, and the adhesion between the first negative electrode active material layer and the second negative electrode active material layer also decreases. This makes the negative electrode sheet prone to powdering and detachment, leading to a decrease in the battery's cycle performance and rate performance. If the mass content of the elastic polymer is greater than 12%, the excessively high elastic polymer content reduces the content of the first negative electrode active material, resulting in a decrease in energy density. Simultaneously, the electronic impedance increases, the electronic conduction of the first negative electrode active material layer decreases, and the battery's rate performance and cycle performance deteriorate.

[0033] In a specific embodiment, the mass content of the elastic polymer in the first negative electrode active material layer can be 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, or any two of the above.

[0034] In the second negative electrode active material layer, the mass content of conductive material is 12%~18%, and the mass content of the second PI resin is 12%~22%. The high content of conductive material in the second negative electrode active material layer, along with the PI-conductive carbon composite network formed by the conductive material and the second PI resin, effectively improves the electronic conduction efficiency of the second negative electrode active material layer, enhances kinetics, and improves battery rate performance. If the mass content of conductive material is less than 12%, the electronic impedance of the second negative electrode active material layer increases, the electronic conduction rate decreases, and the battery's rate performance and cycle performance deteriorate. If the mass content of conductive material is greater than 18%, the content of the second negative electrode active material decreases, the energy density decreases, and excessive conductive material fills the pores inside the electrode, blocking electrolyte wetting, increasing the lithium-ion transport path, increasing polarization, and further deteriorating rate performance and cycle performance.

[0035] In a specific embodiment, the mass content of conductive material in the second negative electrode active material layer can be 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, or any two of the above.

[0036] In specific embodiments, the mass content of the first dispersant can be 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 1.9%, 2%, 2.3%, 2.5%, 2.7%, 2.8%, 3%, or any two of the above. The mass content of the first negative electrode active material can be 63%, 65%, 68%, 70%, 72%, 75%, 77%, 79%, or any two of the above. The mass content of the first PI resin can be 12%, 13%, 14%, 15%, 16%, 17%, 19%, 20%, 21%, 22%, or any two of the above.

[0037] In specific embodiments, the mass content of the second dispersant can be 1%, 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, 1.9%, 2%, 2.3%, 2.5%, 2.7%, 2.8%, 3%, or any two of the above. The mass content of the second negative electrode active material can be 57%, 58%, 59%, 60%, 62%, 64%, 65%, 67%, 69%, 70%, 72%, 73%, 75%, or any two of the above. The mass content of the second PI resin can be 12%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, or any two of the above.

[0038] If the mass content of the second negative electrode active material and the mass content of the first negative electrode active material meet the above range, it can ensure that there is sufficient active material load per unit area to support the basic energy density of the battery cell.

[0039] The mass content of the first dispersant and the mass content of the second dispersant meet the above range, which can play a role in uniform dispersion, avoid agglomeration, ensure that the thickness of the first negative electrode active material layer and the second coating negative electrode active material layer are uniform and the structure is stable, and improve the batch consistency of the negative electrode sheet.

[0040] In some embodiments, both the first negative electrode active material and the second negative electrode active material include at least one of silicon-carbon composite material and carbon material; The silicon-carbon composite material includes silicon particles and a carbon coating layer. The carbon coating layer covers the outer surface of the silicon particles. The thickness of the carbon coating layer is 10 nm to 50 nm, and the D50 particle size of the silicon particles is 50 nm to 200 nm. The specific surface area of ​​the silicon-carbon composite material is 10 m². 2 / g~30m 2 / g.

[0041] Specifically, the silicon-carbon composite material includes a carbon coating layer with a thickness ranging from 10 nm to 50 nm. This carbon coating layer buffers the volume deformation of silicon particles and also provides conductivity, ensuring the electronic conduction of the first negative electrode active material layer. If the thickness of the carbon coating layer is too thin, the silicon particles expand and crack, making the silicon-carbon composite material prone to fragmentation and pulverization. Simultaneously, conductivity decreases, the impedance of the negative electrode increases, and battery cycle performance and rate performance deteriorate. If the thickness of the carbon coating layer is too thick, lithium-ion transport resistance increases, rate performance deteriorates, compaction density decreases, battery energy density decreases, and cycle life is reduced.

[0042] In specific embodiments, the thickness of the carbon coating layer can be 10nm, 12nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, 50nm, or any two of the above.

[0043] The D50 particle size of silicon particles ranges from 50nm to 200nm, reducing the risk of particle cracking and pulverization, maintaining the integrity of the particle structure and conductive network, lowering impedance, and improving battery cycle performance and rate performance. If the silicon particle size is too large, the volume deformation during battery charging and discharging is large, resulting in severe particle pulverization, decreased battery cycle performance, and increased ion diffusion paths, leading to poor rate performance. If the silicon particle size is too small, the specific surface area is too large, increasing side reactions in the electrolyte, further reducing battery cycle performance; the particles are also prone to agglomeration, resulting in uneven slurry dispersion, poor electrode consistency, and decreased battery cycle life.

[0044] In specific embodiments, the D50 particle size of the silicon particles can be 50nm, 70nm, 80nm, 100nm, 75nm, 120nm, 60nm, 140nm, 90nm, 150nm, 85nm, 200nm, or any two of the above.

[0045] The specific surface area of ​​silicon-carbon composite material is 10 m². 2 / g~30m 2A specific surface area within the specified range (g) helps the electrolyte to fully wet the material, reduces polarization, provides sufficient electrochemical reaction activity interfaces, lowers the interfacial impedance for lithium-ion intercalation / deintercalation, improves reaction kinetics, and enhances the battery's cycle performance and rate performance. If the specific surface area of ​​the silicon-carbon composite material is too low, the internal porosity is small, electrolyte wetting is difficult, particles are prone to cracking and pulverization, impedance increases, and battery cycle performance and rate performance deteriorate. If the specific surface area of ​​the silicon-carbon composite material is too high, the slurry viscosity increases, dispersion is uneven, electrode consistency is poor, irreversible capacity increases during the first charge, electrolyte side reactions increase, and battery cycle performance and rate performance decrease.

[0046] In a specific embodiment, the specific surface area of ​​the silicon-carbon composite material can be 10 m². 2 / g、12m 2 / g, 15m 2 / g、17m 2 / g、20m 2 / g、23m 2 / g、25m 2 / g、27m 2 / g、29m 2 / g、30m 2 / g or either of the above.

[0047] In some embodiments, the carbon material includes graphite.

[0048] In some embodiments, the rubber polymer includes at least one of temperature-resistant modified styrene-butadiene rubber and carboxylated nitrile rubber; The heat-resistant modified styrene-butadiene rubber is obtained by modifying styrene-butadiene rubber with a silane coupling agent. The heat-resistant modified styrene-butadiene rubber retains ≥90% of its elasticity after being kept at 150℃-180℃ for 20min-30min. And / or, both the first PI resin and the second PI resin include oil-soluble polyimide.

[0049] Specifically, the elastic polymer is selected from the above types, which can realize the elastic buffer of the first negative electrode active material layer, effectively alleviate the volume expansion of up to 300% during the charging and discharging of silicon-carbon composite material, avoid the pulverization of the electrode structure and the shedding of the active material layer, and at the same time enhance the adhesion between the first negative electrode active material layer and the negative electrode current collector, enhance the interfacial adhesion between the first negative electrode active material layer and the second negative electrode active material layer, improve the adhesion of the negative electrode sheet, and ensure the structural integrity of the negative electrode sheet.

[0050] The first PI resin and the second PI resin are selected from the above types and have good adhesion and chemical stability. The second PI resin is the core bonding component of the second negative electrode active material layer, which ensures the adhesion of the second negative electrode active material layer.

[0051] In some preferred embodiments, the oil-soluble polyimide includes at least one of ethynyl-terminated polyimide and polyamic acid-type oil-soluble PI.

[0052] In some preferred embodiments, the conductive carbon material includes one or more of conductive carbon black, graphene, carbon nanotubes, Ketjen black, and VGCF.

[0053] Specifically, the conductive carbon material is selected from the above types and can improve the conductivity of the negative electrode.

[0054] In some embodiments, both the first dispersant and the second dispersant comprise at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polyacrylate, polyurethane dispersant, and polyethylene oxide.

[0055] Specifically, the first and second dispersants are selected from the above-mentioned types, which can play a role in uniform dispersion, avoid agglomeration, ensure uniform dispersion of slurry, and thus ensure uniform thickness and structural stability of the first negative electrode active material layer and the second coating negative electrode active material layer, thereby improving the batch consistency of negative electrode sheets.

[0056] Secondly, this application provides a method for preparing the above-mentioned negative electrode sheet, comprising the following steps: The first negative electrode active material, the first PI resin, the elastic polymer, the first dispersant and the first solvent are mixed evenly to obtain the first negative electrode slurry; The second negative electrode active material, the second PI resin, the conductive material, the second dispersant, and the second solvent are mixed evenly to obtain the second negative electrode slurry; The first negative electrode slurry is coated on at least one side of the negative electrode current collector and dried to obtain a first negative electrode active material layer; the second negative electrode slurry is coated on the surface of the first negative electrode active material layer away from the negative electrode current collector and dried to obtain a second negative electrode active material layer, thus obtaining an unrolled negative electrode sheet. The unrolled negative electrode sheet is rolled and cured to obtain the negative electrode sheet.

[0057] The method for preparing the negative electrode sheet provided in this application does not require complex equipment and cumbersome steps. It can achieve industrial production using conventional stirring, coating, and rolling equipment. The process is simple, reduces production costs, and avoids the operational difficulties and quality fluctuations caused by complex processes.

[0058] In some embodiments, the curing temperature is 150℃~180℃, and the curing time is 20min~30min.

[0059] Specifically, within the aforementioned curing temperature and time range, the first and second negative electrode active material layers undergo curing treatment. The PI molecular chains at the interface of the two layers interpenetrate and cross-link, forming a seamless gradient transition layer, thus tightly bonding the first and second negative electrode active material layers. If the curing temperature is too low or the curing time is too short, the gradient transition layer cannot be formed. If the curing temperature is too high, the molecular chains break and decompose, destroying the integrity of the already formed cross-linked network. This increases the rigidity of the gradient transition layer, making it prone to cracking and breakage, reducing electrode adhesion, decreasing electronic conductivity, and lowering cycle performance and other properties.

[0060] In specific embodiments, the curing temperature can be 150℃, 155℃, 158℃, 160℃, 163℃, 165℃, 167℃, 170℃, 172℃, 175℃, 177℃, 180℃, or any two of the above. The curing time can be 20min, 21min, 22min, 24min, 25min, 26min, 27min, 28min, 29min, 30min, or any two of the above.

[0061] In some embodiments, a first negative electrode active material, a first PI resin, an elastic polymer, a first dispersant, and a first solvent are mixed uniformly to obtain a first negative electrode slurry, comprising the following steps: drying the first negative electrode active material, the first PI resin, and the elastic polymer at 100-120°C for 1-2 hours until the moisture content is ≤0.5%; mixing the dried first PI resin with a portion of the first solvent uniformly to obtain a first PI resin solution with a mass concentration of 15-25%; adding the dried first negative electrode active material, the dried elastic polymer, the first PI resin solution, the first dispersant, and another portion of the first solvent into a stirring device, stirring at a speed of 300 r / min to 800 r / min for 30 min to 60 min, and then grinding with a sand mill until the slurry particle size D50 is 1-5 μm to obtain a uniform and fine first negative electrode slurry.

[0062] In some embodiments, the second negative electrode active material, the second PI resin, the conductive material, the second dispersant, and the second solvent are mixed uniformly to obtain a second negative electrode slurry, comprising the following steps: drying the second negative electrode active material, the second PI resin, and the conductive material at 100-120°C for 1-2 hours until the moisture content is ≤0.5%; mixing the dried second PI resin with a portion of the second solvent to obtain a second PI resin solution with a mass concentration of 15-25%; adding the dried second negative electrode active material, the dried conductive material, the second PI resin solution, the second dispersant, and another portion of the second solvent into a stirring device, stirring at a speed of 300 r / min to 800 r / min for 40 min to 70 min, and then grinding with a sand mill until the slurry particle size D50 is 1-3 μm to obtain a uniform and fine second negative electrode slurry.

[0063] In some embodiments, coating the first negative electrode slurry onto at least one side surface of the negative electrode current collector and drying it to obtain a first negative electrode active material layer includes the following steps: coating the first negative electrode slurry onto at least one side surface of the negative electrode current collector and drying it at 110°C to 130°C for 10 min to 20 min, and obtaining a first negative electrode active material layer after drying.

[0064] In some embodiments, coating the second negative electrode slurry onto the surface of the first negative electrode active material layer away from the negative electrode current collector and drying it to obtain the second negative electrode active material layer includes the following steps: coating the second negative electrode slurry onto the surface of the first negative electrode active material layer away from the negative electrode current collector and drying it at 110°C to 130°C for 15 min to 25 min to obtain the second negative electrode active material layer.

[0065] In some embodiments, rolling the unrolled negative electrode sheet includes the following steps: rolling the unrolled negative electrode sheet under a pressure of 5-10 MPa.

[0066] In some embodiments, the areal density of the negative electrode sheet after roll forming is 1.2 g / cm³. 3 -1.5g / cm 3 .

[0067] In some embodiments, the negative current collector includes at least one of copper foil, composite copper foil, and composite current collector.

[0068] In some embodiments, the thickness of the negative electrode current collector is 8 μm to 12 μm.

[0069] The adhesion between the first and second negative electrode active material layers is achieved through two synergistic aspects: First, the first negative electrode slurry contains a first PI resin, and the second negative electrode slurry also contains a second PI resin. After the two layers are coated, during subsequent curing at 150-180℃, the PI molecular chains at the interface of the two layers will interpenetrate and cross-link to form a seamless gradient transition layer, making the first and second negative electrode active material layers tightly bonded. Second, after the first negative electrode slurry is coated, it is first dried at 110-130℃ to remove the solvent, forming a stable first negative electrode active material layer. Then, the second negative electrode slurry is coated. The second PI resin in the second negative electrode slurry can form physical adsorption and chemical bonding with the first PI resin in the first negative electrode active material layer, further enhancing the adhesion between the two layers and preventing the two layers from peeling off due to volume expansion during charging and discharging.

[0070] Thirdly, this application provides a secondary battery, including the negative electrode sheet described above, or a negative electrode sheet prepared by the method described above.

[0071] The secondary battery provided in this application includes the aforementioned negative electrode. A first PI resin and an elastic polymer form a PI-elastic polymer composite network, which can effectively buffer the volume expansion of the silicon-carbon composite material during charging and discharging, and prevent the negative electrode from pulverizing and falling off. A second PI resin and a conductive material form a PI-conductive carbon composite network, which can significantly improve the electronic conduction efficiency of the electrode and improve the rate performance of the battery. The two work together to enable the negative electrode to have both good cycle stability and conductivity, effectively suppress expansion, improve the cycle performance and rate performance of the battery, and solve the problems of volume expansion and poor conductivity of silicon-carbon negative electrodes.

[0072] In some embodiments, the secondary battery includes lithium-ion batteries, sodium-ion batteries, solid-state batteries, etc.

[0073] In some embodiments, the secondary battery includes a positive electrode sheet, which includes a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent.

[0074] The positive electrode active material includes at least one of lithium-containing active materials and sodium-containing active materials.

[0075] Lithium-containing active materials include one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate doped with metal or non-metal, and lithium nickel cobalt manganese oxide doped with metal or non-metal.

[0076] The doped metal includes at least one of Al, Mn, Mg, Ti, Zr, Nb, V, Co, Zn, W, and Cr.

[0077] Doped nonmetals include at least one of C, S, halogens, P, and B.

[0078] The positive electrode binder includes one or more of polyvinylidene fluoride, polyacrylonitrile, polytetrafluoroethylene, polyvinyl alcohol, and polyurethane.

[0079] Positive conductive agents include one or more of acetylene black, conductive carbon black, carbon fiber, carbon nanotubes, and Ketjen black.

[0080] The specific embodiments of the present invention will be further explained and illustrated below through examples, but this does not mean that the scope of protection of the present invention is limited to the scope described in the examples.

[0081] Example 1 S1: Negative electrode preparation S11 raw material pretreatment: Both the first and second negative electrode active materials are selected from silicon-carbon composite materials. The silicon-carbon composite material comprises silicon particles and a carbon coating layer. The silicon particles have a D50 particle size of 100 nm, and the carbon coating layer coats the outer surface of the silicon particles with a thickness of 20 nm. The specific surface area of ​​the silicon-carbon composite material is 20 m². 2 / g.

[0082] The first PI resin is selected from acetylene-terminated polyimide. The second PI resin is selected from acetylene-terminated polyimide.

[0083] The first dispersant is selected from polyvinylpyrrolidone. The second dispersant is selected from polyethylene glycol.

[0084] The elastic polymer is selected from heat-resistant modified styrene-butadiene rubber. The conductive carbon material is selected from conductive carbon black.

[0085] The first solvent is selected from NMP (N-methylpyrrolidone), and the second solvent is selected from NMP.

[0086] The first negative electrode active material, the second negative electrode active material, the first PI resin, the second PI resin, the elastic polymer, and the conductive carbon material were dried at 110°C for 1.5 hours until the moisture content was ≤0.5%.

[0087] The first PI resin was partially dissolved in the first solvent to prepare a first PI resin solution with a mass concentration of 20%, and stirred until there were no particles.

[0088] The second PI resin was partially dissolved in the first solvent to prepare a first PI resin solution with a mass concentration of 20%, and stirred until there were no particles.

[0089] S12: Preparation of the first negative electrode slurry: Take the first negative electrode active material, the first PI resin, the elastic polymer, the first dispersant, and the first solvent in a mass ratio of 45:12:6:1.5:7, put them into a double planetary mixer, stir at a speed of 500 r / min for 45 min, and then grind them with a sand mill until the slurry particle size D50 is 3 μm to obtain a uniform and fine first negative electrode slurry.

[0090] S13: Preparation of the second negative electrode slurry: Take the second negative electrode active material, the second PI resin, the conductive carbon material, the second dispersant, and the second solvent in a mass ratio of 45:12:8:1.5:8, put them into a double planetary mixer, stir at a speed of 600 r / min for 55 min, and then grind them with a sand mill until the slurry particle size D50 is 2 μm to obtain a uniform and fine second negative electrode slurry.

[0091] S14: Coating The first negative electrode slurry was uniformly coated on both sides of a 10 μm thick copper foil and dried at 120°C for 15 min to form the first negative electrode active material layer. Then, the second negative electrode slurry was coated on the side of the first negative electrode active material layer facing away from the copper foil to obtain the second negative electrode active material layer. The thickness of the first negative electrode active material layer was controlled to be 25 μm and the thickness of the second negative electrode active material layer to be 32.5 μm. The layers were dried at 120°C for 20 min to obtain an unrolled negative electrode sheet. The ratio of H1 to H2 was 1:1.3.

[0092] S15: Roller Curing The unrolled negative electrode sheet is placed in a rolling mill and rolled under a pressure of 8 MPa to achieve an areal density of 1.3 g / cm³. 3 Then it was placed in an oven and cured at 170°C for 25 minutes. During the curing process, the PI molecular chains at the interface of the two layers interpenetrate and crosslink to form a seamless gradient transition layer. After cooling to room temperature, the negative electrode sheet was obtained.

[0093] Tests showed that after curing at 170℃, the heat-resistant modified styrene-butadiene rubber retained 92% of its elasticity, with no signs of aging or brittleness.

[0094] S2: Preparation of positive electrode sheet LCO, SP, conductive paste, and PVDF were mixed evenly in a mass ratio of 97%:1%:0.5%:1.5%, and NMP was added and mixed evenly to obtain the positive electrode paste.

[0095] The positive electrode slurry described above is coated on both sides of the aluminum foil, and then dried and rolled to obtain the positive electrode sheet.

[0096] S3: Fabrication of lithium-ion batteries The separator is a PP / PE composite membrane with a thickness of 20μm. The positive electrode sheet prepared in S2, the negative electrode sheet prepared in S1, and the separator are wound together to form a core. The core and the aluminum-plastic film are then used to make a lithium-ion battery. Then, processes such as electrolyte injection and formation are performed. Finally, the battery's electrical performance is tested.

[0097] Examples 2-15 Examples 2-15 are largely the same as Example 1, with the following differences: In the first negative electrode active material layer, the mass content of the first negative electrode active material, the mass content of the first PI resin, the mass content of the elastic polymer, and the mass content of the first dispersant are different, as detailed in Table 1. In the second negative electrode active material layer, the mass content of the second negative electrode active material, the mass content of the second PI resin, the mass content of the conductive carbon material, and the mass content of the second dispersant are different, as detailed in Table 1.

[0098] Table 1 Continued from Table 1 Example 16 This embodiment is the same as embodiment 1 in most steps, except that in step S14 the thickness of the first negative electrode active material layer is controlled to be 30 μm, the thickness of the second negative electrode active material layer is controlled to be 45 μm, and the ratio of H1 to H2 is 1:1.5. The rest is the same as in embodiment 1.

[0099] Example 17 This embodiment is the same as embodiment 1 in most steps, except that in step S14 the thickness of the first negative electrode active material layer is controlled to be 30 μm, the thickness of the second negative electrode active material layer is controlled to be 37 μm, and the ratio of H1 to H2 is 1:1.23. The rest is the same as in embodiment 1.

[0100] Example 18 This embodiment is the same as most of the steps in Embodiment 1. The difference is that in step S14, the thickness of the first negative electrode active material layer is controlled to be 30 μm, the thickness of the second negative electrode active material layer is controlled to be 30 μm, and the ratio of H1 to H2 is 1:1. The rest is the same as in Embodiment 1.

[0101] Example 19 This embodiment is the same as most of the steps in Embodiment 1. The difference is that in step S14, the thickness of the first negative electrode active material layer is controlled to be 30 μm, the thickness of the second negative electrode active material layer is controlled to be 48 μm, and the ratio of H1 to H2 is 1:1.6. The rest is the same as in Embodiment 1.

[0102] Example 20 This embodiment is the same as embodiment 1 in most steps, except that step S11 uses a silicon-carbon composite material, wherein the silicon particles have a D50 particle size of 50 nm and the carbon coating layer has a thickness of 10 nm; the specific surface area of ​​the silicon-carbon composite material is 30 m². 2 / g. The rest is the same as in Example 1.

[0103] Example 21 This embodiment is the same as embodiment 1 in most steps, except that step S11 uses a silicon-carbon composite material, wherein the D50 particle size of the silicon particles is 200 nm, the thickness of the carbon coating layer is 30 nm, and the specific surface area of ​​the silicon-carbon composite material is 10 m². 2 / g. The rest is the same as in Example 1.

[0104] Example 22 This embodiment is largely the same as Embodiment 1, except that step S11 uses a silicon-carbon composite material, wherein the D50 particle size of the silicon particles is 30 nm, the thickness of the carbon coating layer is 8 nm, and the specific surface area of ​​the silicon-carbon composite material is 35 m². 2 / g. The rest is the same as in Example 1.

[0105] Example 23 This embodiment is the same as embodiment 1 in most steps, except that step S11 uses a silicon-carbon composite material, wherein the D50 particle size of the silicon particles is 210 nm, the thickness of the carbon coating layer is 38 nm, and the specific surface area of ​​the silicon-carbon composite material is 8 m². 2 / g. The rest is the same as in Example 1.

[0106] Example 24 This embodiment is largely the same as Example 1, except that the elastic polymer is selected from carboxylated nitrile rubber, the first PI resin is selected from polyamic acid type oil-soluble PI, and the second PI resin is selected from polyamic acid type oil-soluble PI. The rest is the same as in Example 1.

[0107] Example 25 This embodiment is the same as Example 1 in most steps, except that the elastic polymer is selected from unmodified styrene-butadiene rubber. The rest is the same as in Example 1.

[0108] Comparative Example 1 This comparative example is the same as most of the steps in Example 1, except that step S1 is different. Specifically, silicon-carbon composite material, conductive carbon black, styrene-butadiene rubber, and CMC are mixed evenly in a mass ratio of 88:5:5:2, and deionized water is added and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of a copper foil and dried to obtain a negative electrode sheet. The thickness of the copper foil and the thickness of the active coating on one side of the negative electrode sheet are the same as in Example 1, and the silicon-carbon composite material is the same as in Example 1.

[0109] Comparative Example 2 This comparative example is the same as most of the steps in Example 1, except that there is no first PI resin in step S1, and the rest is the same as in Example 1.

[0110] Comparative Example 3 This comparative example is the same as Example 1 in most steps, except that there is no second PI resin in step S1, and the binder used in step S12 is styrene-butadiene rubber. The rest is the same as in Example 1.

[0111] Battery performance test: The batteries prepared in the above embodiments and comparative examples were subjected to the following tests. (1) Mass energy density: At room temperature, the battery prepared above was charged at a constant current of 1C to 4.2V, then charged at a constant voltage of 4.3V with a cutoff current of 0.05C, and then discharged at a constant current of 1C to 3.0V. The discharge capacity C1 of the battery was tested, and the mass energy density of the battery was calculated.

[0112] (2) 25℃ cycle test: The battery prepared above was charged to 4.2V at a constant current of 1C under an ambient temperature of 25°C, then charged at a constant voltage of 4.2V with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 1C. This cycle was repeated for 500 cycles. Calculate the 500-cycle capacity retention rate = discharge capacity at week 500 / average discharge capacity of weeks 1-3 × 100%.

[0113] (3) 45℃ high temperature cycling test: The battery prepared above was placed in an environment of 45°C and charged to 4.2V with a constant current of 1C, then charged at 4.2V with a constant voltage, with a cutoff current of 0.05C, and then discharged to 3.0V with a constant current of 1C. This cycle was repeated for 500 cycles. Calculate the 500-cycle capacity retention rate = discharge capacity at week 500 / average discharge capacity of weeks 1-3 × 100%.

[0114] The thickness H1 of the fully charged battery in the first week and the thickness H2 of the fully charged battery in the 500th week are tested. The thickness expansion rate of the battery after 500 cycles is calculated as (H2-H1) / H1*100%.

[0115] (4) Ratio performance At room temperature, charge at a constant current of 1C to 4.2V, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C, and then discharge at a constant current of 0.2C to 3.0V. Record the discharge capacity C1. Then charge at a constant current of 1C to 4.2V, then charge at a constant voltage of 4.2V with a cutoff current of 0.05C, and then discharge at a constant current of 3C to 3.0V. Record the discharge capacity C2.

[0116] 3C discharge capacity ratio = C2 / C1 * 100%.

[0117] (5) Peel strength of negative electrode Using a universal electronic tensile testing machine, take a 25mm*25mm negative electrode sample, clamp it on the tensile testing machine, and perform a peel test at a speed of 100mm / min.

[0118] The test results are shown in Table 2.

[0119] Table 2 Tables 1 and 2 show that, compared with Comparative Examples 1-3, the negative electrode in Comparative Example 1 lacks the first PI resin, second PI resin, and elastic polymer, resulting in low electrode peel strength, low volumetric energy density, low capacity retention at room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. Similarly, the first negative electrode active material layer in Comparative Example 2 does not contain the first PI resin, and the second negative electrode active material layer in Comparative Example 3 does not contain the second PI resin. Both Comparative Examples 2 and 3 exhibit low electrode peel strength, low volumetric energy density, low capacity retention at room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. The difference in performance is illustrated by comparison. The first negative electrode active material layer contains a first PI resin and an elastic polymer, which can form a PI-elastic polymer composite network. This network can effectively buffer the volume expansion of the silicon-carbon composite material during charging and discharging, preventing the negative electrode sheet from pulverizing and falling off. The second negative electrode active material layer contains a second PI resin and a conductive material, which can form a PI-conductive carbon composite network. This network can significantly improve the electronic conductivity of the electrode sheet and improve the rate performance of the battery. The synergy between the two enables the negative electrode sheet to have both good cycle stability and conductivity, effectively suppressing expansion and improving the cycle performance and rate performance of the battery, thus solving the problems of volume expansion and poor conductivity of silicon-carbon negative electrodes.

[0120] Comparing Examples 1-4 and Examples 5-6, in Example 5, the mass content of the elastic polymer in the first negative electrode material layer is less than 8%, resulting in low electrode peel strength, low volumetric energy density, low capacity retention at both room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. This is presumably because the low mass content of the elastic polymer reduces the adhesion between the first negative electrode active material layer and the negative electrode current collector, and also reduces the adhesion between the first and second negative electrode active material layers. This makes the negative electrode prone to powdering and detachment, leading to decreased cycle performance and rate performance. In Example 6, the mass content of the elastic polymer in the first negative electrode material layer is higher than 12%, resulting in low volumetric energy density, low capacity retention at both room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. This is presumably because the high elastic polymer content reduces the content of the first negative electrode active material, lowering the energy density. Simultaneously, the increased electronic impedance reduces the electronic conduction of the first negative electrode active material layer, leading to decreased rate performance and cycle performance. Comparing Examples 1-4, 7-9 with Examples 12 and 13, in Example 12, the mass content of the first PI resin in the first negative electrode material layer is less than 12%, resulting in low electrode peel strength, low volumetric energy density of the battery, low capacity retention rate at room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. In Example 13, the mass content of the first PI resin in the first negative electrode material layer is higher than 22%, resulting in low volumetric energy density of the battery, low capacity retention rate at room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. It is speculated that this is because the mass content of the first PI resin in the first negative electrode material layer is not within the range of 12% to 22%, making it unable to form a PI-elastic polymer composite network with the elastic polymer. This network cannot effectively suppress silicon expansion, the active material layer is prone to detachment, and the cycle performance and rate performance decrease. By comparison, it can be seen that in the first negative electrode active material layer, the mass content of the elastic polymer is 8%~12% and the mass content of the first PI resin is 12%~22%. The PI-elastic polymer composite network formed by the elastic polymer and the first PI resin can effectively absorb the volume expansion stress of the silicon-carbon composite material during the charging and discharging process, thereby improving the cycle stability and rate performance of the electrode.

[0121] Comparing Examples 1-4, 7-9 and Examples 10-11, 14-15, in Examples 10-11, the mass content of the conductive carbon material in Example 10 was less than 12%, and in Example 14, the mass content of the second PI resin was less than 12%. Both examples exhibited low volumetric energy density, low capacity retention at both room temperature and high temperature, high high-temperature cycle expansion rate, and poor rate performance. It is speculated that when the mass content of the conductive material is less than 12%, the electronic impedance of the second negative electrode active material layer increases, the electron conduction rate decreases, and the rate performance and cycle performance of the battery decrease. When the mass content of the second PI resin is less than 12%, an effective PI-conductive carbon composite network cannot be formed, resulting in a decrease in electron conduction rate and a reduction in both rate performance and cycle performance. In Example 11, the mass content of the conductive carbon material was higher than 18%, and in Example 15, the mass content of the second PI resin was higher than 22%. Both resulted in low volumetric energy density, low capacity retention at both room and high temperatures, high high-temperature cycle expansion, and poor rate performance. The higher mass content of the conductive carbon material (above 18%) suggests a decrease in the content of the second negative electrode active material, leading to lower energy density. Excessive conductive material fills the pores inside the electrode, blocking electrolyte wetting, increasing lithium-ion transport paths, increasing polarization, and thus reducing rate and cycle performance. Similarly, the higher mass content of the second PI resin (above 22%) suggests excessive resin, which blocks the pores inside the electrode, increasing electrolyte wetting, increasing lithium-ion transport paths, increasing polarization, and thus reducing rate and cycle performance. This comparison demonstrates that when the mass content of the conductive material in the second negative electrode active material layer is 12%–18%, and the mass content of the second PI resin is 12%–22%, the PI-conductive carbon composite network formed by the conductive material and the second PI resin effectively improves the electronic conduction efficiency of the second negative electrode active material layer, enhances kinetics, and improves battery rate performance.

[0122] Comparing Examples 1, 16-17, and Examples 18-19, in Example 18, H1:H2 is greater than 1:(1.2~1.5), resulting in low volumetric energy density, low capacity retention at both room and high temperatures, high high-temperature cycle expansion rate, and poor rate performance. It is speculated that the thickness of the second negative electrode material layer is too thin, increasing the impedance of the negative electrode sheet, reducing electron conduction efficiency, and thus decreasing the rate and cycle performance of the battery. In Example 19, H1:H2 is less than the range of 1:(1.2~1.5), resulting in low volumetric energy density, low capacity retention at both room and high temperatures, high high-temperature cycle expansion rate, and poor rate performance. It is speculated that the thickness of the first negative electrode material layer is too thin, reducing the adhesion between the negative electrode current collector and the first negative electrode material layer, reducing the adhesion between the second negative electrode material layer and the first negative electrode material layer, and reducing the peel strength of the negative electrode sheet. During cycling, the first and second negative electrode material layers are prone to detachment, leading to decreased cycle performance and rate performance of the battery. The ratio of H1 to H2 is in the range of 1:(1.2~1.5). The first negative electrode material layer has a smaller thickness, while the second negative electrode material layer has a larger thickness. Because the second negative electrode material layer contains more conductive material, it can ensure the electron conduction of the entire negative electrode sheet, improve the electron conduction efficiency of the negative electrode sheet, reduce the impedance of the negative electrode sheet, and improve the rate performance of the battery. The first negative electrode material layer has a slightly smaller thickness. While increasing the impedance of the negative electrode sheet, it can also effectively suppress the expansion of the silicon-carbon composite material, increase the adhesion between the first negative electrode material layer and the negative electrode current collector, and ensure the peel strength of the negative electrode sheet.

[0123] Comparing Examples 1, 20-21, and Examples 22-23, the D50 particle size of the silicon particles in Example 22 is less than 50 nm, while the D50 particle size of the silicon particles in Example 23 is greater than 210 nm. The resulting batteries have low volumetric energy density, low capacity retention at room temperature and high temperature, high high temperature cycle expansion rate, and poor rate performance. This indicates that a D50 particle size of silicon particles in the range of 50 nm to 200 nm reduces the risk of particle cracking and pulverization, maintains the integrity of the particle structure and conductive network, reduces impedance, and improves the battery's cycle performance and rate performance.

[0124] Examples 1 and 24 illustrate that oil-soluble polyimides, including at least one of ethynyl-terminated polyimides and polyamic acid-type oil-soluble PI, have good adhesion and chemical stability, and all have the same improvement effect. Comparing Examples 1 and 25, the elastic polymer in Example 25 is selected from unmodified styrene-butadiene rubber. The battery exhibits low volumetric energy density, low capacity retention at both room and high temperatures, high high-temperature cycle expansion rate, and poor rate performance. This indicates that the rubber polymer selected from the materials provided in this application includes at least one of temperature-resistant modified styrene-butadiene rubber and carboxylated nitrile rubber. The temperature-resistant modified styrene-butadiene rubber is obtained by modifying styrene-butadiene rubber with a silane coupling agent. After being kept at 150℃-180℃ for 20-30 minutes, the elasticity retention rate of the temperature-resistant modified styrene-butadiene rubber is ≥90%. This enables elastic buffering of the first negative electrode active material layer, effectively mitigating the up to 300% volume expansion of the silicon-carbon composite material during charging and discharging, preventing electrode structure pulverization and active material layer detachment. Simultaneously, it enhances the adhesion between the first negative electrode active material layer and the negative electrode current collector, strengthens the interfacial adhesion between the first and second negative electrode active material layers, improves the adhesion of the negative electrode sheet, and ensures the structural integrity of the negative electrode sheet.

[0125] 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, a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer is disposed on one side surface of the negative electrode current collector, and the second negative electrode active material layer is disposed on the side surface of the first negative electrode active material layer opposite to the negative electrode current collector. The first negative electrode active material layer includes a first negative electrode active material, a first PI resin, an elastic polymer, and a first dispersant, and does not contain conductive materials. The second negative electrode active material layer includes a second negative electrode active material, a second PI resin, a conductive material, and a second dispersant, and does not contain an elastic polymer in the second negative electrode active material layer; The elastic polymer includes rubber-based polymers.

2. The negative electrode sheet according to claim 1, characterized in that, A gradient transition layer is formed between the first negative electrode active material layer and the second negative electrode active material layer. The gradient transition layer is formed by the interpenetration and cross-linking of molecular chains in the first PI resin and molecular chains in the second PI resin.

3. The negative electrode sheet according to claim 1, characterized in that, The thickness of the first negative electrode active material layer is H1, and the thickness of the second negative electrode active material layer is H2, with H1:H2 being 1:(1.2~1.5).

4. The negative electrode sheet according to claim 1 or 3, characterized in that, The total thickness of the first negative electrode active material layer and the second negative electrode active material layer is 50μm~100μm.

5. The negative electrode sheet according to claim 1, characterized in that, In the first negative electrode active material layer, the mass content of the elastic polymer is 8%~12%, the mass content of the first negative electrode active material is 63%~79%, the mass content of the first PI resin is 12%~22%, and the mass content of the first dispersant is 1%~3%. And / or, in the second negative electrode active material layer, the mass content of the conductive material is 12%~18%, the mass content of the second negative electrode active material is 57%~75%, the mass content of the second PI resin is 12%~22%, and the mass content of the second dispersant is 1%~3%.

6. The negative electrode sheet according to claim 1, characterized in that, Both the first negative electrode active material and the second negative electrode active material include at least one of silicon-carbon composite materials and carbon materials; The silicon-carbon composite material includes silicon particles and a carbon coating layer. The carbon coating layer covers the outer surface of the silicon particles. The thickness of the carbon coating layer is 10 nm to 50 nm, and the D50 particle size of the silicon particles is 50 nm to 200 nm. The specific surface area of ​​the silicon-carbon composite material is 10 m². 2 / g~30m 2 / g.

7. The negative electrode sheet according to claim 1, characterized in that, The rubber polymers include at least one of temperature-resistant modified styrene-butadiene rubber and carboxylated nitrile rubber; The heat-resistant modified styrene-butadiene rubber is obtained by modifying styrene-butadiene rubber with a silane coupling agent. The heat-resistant modified styrene-butadiene rubber retains ≥90% of its elasticity after being kept at 150℃-180℃ for 20min-30min. And / or, both the first PI resin and the second PI resin include oil-soluble polyimide.

8. A method for preparing a negative electrode sheet according to any one of claims 1-7, characterized in that, Includes the following steps: The first negative electrode active material, the first PI resin, the elastic polymer, the first dispersant and the first solvent are mixed evenly to obtain the first negative electrode slurry; The second negative electrode active material, the second PI resin, the conductive material, the second dispersant, and the second solvent are mixed evenly to obtain the second negative electrode slurry; The first negative electrode slurry is coated on at least one side of the negative electrode current collector and dried to obtain a first negative electrode active material layer; the second negative electrode slurry is coated on the surface of the first negative electrode active material layer away from the negative electrode current collector and dried to obtain a second negative electrode active material layer, thus obtaining an unrolled negative electrode sheet. The unrolled negative electrode sheet is rolled and cured to obtain the negative electrode sheet.

9. The method for preparing the negative electrode sheet according to claim 8, characterized in that, The curing temperature is 150℃~180℃, and the curing time is 20min~30min.

10. A secondary battery, characterized in that, Includes the negative electrode sheet according to any one of claims 1-7, or the negative electrode sheet prepared by the method for preparing the negative electrode sheet according to any one of claims 8-9.

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