An electrode assembly and a battery

Abstract

The application provides an electrode assembly and a battery. The electrode assembly of the application comprises a positive electrode sheet, a separator and a negative electrode sheet arranged in a stack, wherein the positive electrode sheet comprises a single-sided positive electrode sheet located at the outermost side and a double-sided positive electrode sheet located at the inner side in the thickness direction of the electrode assembly, the single-sided positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on one side surface of the positive electrode current collector close to the center of the electrode assembly, and the double-sided positive electrode sheet comprises a positive electrode current collector and positive electrode active layers located on both side surfaces of the positive electrode current collector; wherein the first negative electrode active layer corresponding to the single-sided positive electrode sheet comprises at least two layers of negative electrode coating layers arranged in a stack in the thickness direction of the negative electrode sheet, and the porosity of each layer of negative electrode coating layers increases layer by layer from the surface of the negative electrode current collector to the direction away from the negative electrode current collector. The battery of the application comprises the above electrode assembly. The electrode assembly of the application can improve the problem of lithium precipitation in the negative electrode area corresponding to the single-sided positive electrode sheet, and improve the cycle capacity retention rate and fast charging performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to an electrode assembly and a battery including the electrode assembly. Background Technology

[0002] Lithium-ion batteries are important electrochemical energy storage devices, and their energy density, cycle life, and safety are considered core performance indicators. Laminated lithium-ion batteries are widely used in consumer and power battery fields due to their advantages such as low internal resistance, good rate performance, and high space utilization.

[0003] However, during battery cycling, a significant and detrimental phenomenon is that the negative electrode region directly adjacent to the outermost single-sided positive electrode of a stacked battery has a much higher risk of lithium plating than the negative electrode region adjacent to the inner double-sided positive electrode. Lithium plating irreversibly consumes active lithium, leading to a decrease in cycle retention. Summary of the Invention

[0004] Research has revealed that during the cycling process of stacked batteries, the outermost single-sided positive electrode, with only one positive active layer, has a smaller reaction area than the inner double-sided positive electrode. Therefore, with the same total current flowing through a smaller reaction area, the current density of the single-sided positive electrode is greater, leading to increased polarization (overpotential). To maintain circuit voltage balance, the negative overpotential of the corresponding negative electrode region increases, causing the negative electrode potential to drop below the 0V lithium plating critical point, thus triggering lithium plating. This results in irreversible consumption of active lithium and a decrease in cycle retention rate. The lithium plating situation is particularly severe during high-current charging (such as fast charging), significantly deteriorating the battery's fast-charging performance.

[0005] To address the lithium plating problem in the negative electrode region corresponding to a single-sided positive electrode in existing stacked batteries, this invention provides an electrode assembly and a battery including the electrode assembly. The electrode assembly of this invention improves the lithium-ion transport performance of the negative electrode active layer corresponding to the single-sided positive electrode, reduces polarization in the negative electrode region corresponding to the single-sided positive electrode, and lowers the total overpotential of the negative electrode, thereby improving the lithium plating problem in the negative electrode region corresponding to the single-sided positive electrode and enhancing the battery's cycle capacity retention and fast-charging performance.

[0006] To achieve the above objectives, a first aspect of the present invention provides an electrode assembly, the electrode assembly comprising a positive electrode sheet, a separator, and a negative electrode sheet stacked together. In the thickness direction of the electrode assembly, the positive electrode sheet comprises two single-sided positive electrode sheets located on the outermost side of the electrode assembly and at least one double-sided positive electrode sheet located inside the electrode assembly. The single-sided positive electrode sheet comprises a positive current collector and a positive active layer located on the positive current collector near the inner surface of the electrode assembly. The double-sided positive electrode sheet comprises a positive current collector and positive active layers located on both sides of the positive current collector. The negative electrode sheet includes a negative electrode current collector and negative electrode active layers located on both sides of the negative electrode current collector. The negative electrode active layer corresponding to the single-sided positive electrode sheet is the first negative electrode active layer, and the negative electrode active layer corresponding to the double-sided positive electrode sheet is the second negative electrode active layer. The first negative electrode active layer includes at least two negative electrode coating layers stacked along the thickness direction of the negative electrode sheet. The porosity of each negative electrode coating layer increases progressively from the surface of the negative electrode current collector toward the direction away from the negative electrode current collector.

[0007] A second aspect of the present invention provides a battery including an electrolyte and the electrode assembly provided in the first aspect.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: By comprising at least two negative electrode coating layers stacked along the thickness direction of the negative electrode sheet in the first negative electrode active layer corresponding to the single-sided positive electrode sheet, and with the porosity of each negative electrode coating layer increasing progressively from the surface of the negative electrode current collector towards the direction away from the negative electrode current collector, the porosity of the surface negative electrode coating layer near the separator or positive electrode sheet is the highest. Lithium ions from the single-sided positive electrode sheet first reach the surface of the high-porosity negative electrode coating layer and can be rapidly and uniformly absorbed and transported, reducing the accumulation of lithium ions at the negative electrode sheet / electrolyte interface and the resulting concentration polarization. Meanwhile, the porosity of the bottom negative electrode coating layer near the negative electrode current collector is the lowest, ensuring good electronic contact and low resistance between the negative electrode active material and the negative electrode current collector. When lithium ions diffuse to the first negative electrode... When the active layer is formed, electrons can be rapidly reduced and embedded into the negative electrode active material, further suppressing the polarization inside the first negative electrode active layer. At the same time, the porosity increases layer by layer (or the porosity gradually decreases from the surface negative electrode coating to the bottom negative electrode coating), which can also avoid abrupt changes in porosity between negative electrode coatings, reduce or even prevent lithium-ion transport from being suddenly blocked at the interface of abrupt changes in porosity, thus reducing the total overpotential of the first negative electrode active layer corresponding to the single-sided positive electrode sheet during charging, raising its actual operating potential and moving it away from the 0V lithium plating critical point, effectively suppressing lithium plating, improving the cycle capacity retention rate of the battery, and in particular, reducing the polarization of the first negative electrode active layer during high-rate charging, reducing lithium plating, and improving the fast-charging performance of the battery.

[0009] Other features and advantages of the present invention will be described in detail in the following detailed description section.

[0010] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0011] Figure 1 The figure shown is a cross-sectional schematic diagram of the electrode assembly of the present invention.

[0012] Figure 2 The image shown is a SEM image of the negative electrode sheet of this invention. Detailed Implementation

[0013] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the invention. Unless otherwise specified herein, data ranges include endpoints.

[0014] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0015] In a first aspect, an electrode assembly is provided, comprising a positive electrode sheet, a separator, and a negative electrode sheet stacked together. In the thickness direction of the electrode assembly, the positive electrode sheet comprises two single-sided positive electrode sheets located on the outermost side of the electrode assembly and at least one (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40) double-sided positive electrode sheet located inside the electrode assembly. The single-sided positive electrode sheet comprises a positive current collector and a positive active layer located on one side surface of the positive current collector near the center of the electrode assembly. The double-sided positive electrode sheet comprises a positive current collector and positive active layers located on both sides of the positive current collector. The negative electrode sheet includes a negative electrode current collector and negative electrode active layers located on both sides of the negative electrode current collector. The negative electrode active layer corresponding to the single-sided positive electrode sheet is the first negative electrode active layer, and the negative electrode active layer corresponding to the double-sided positive electrode sheet is the second negative electrode active layer. The first negative electrode active layer includes at least two negative electrode coating layers stacked along the thickness direction of the negative electrode sheet. The porosity of each negative electrode coating layer increases progressively from the surface of the negative electrode current collector toward the direction away from the negative electrode current collector.

[0016] According to a specific implementation method, such as Figure 1 As shown, the electrode assembly 1 includes a positive electrode 2, a separator 3, and a negative electrode 4 stacked together. In the thickness direction Y of the electrode assembly, the positive electrode 2 includes two single-sided positive electrode 21 located on the outermost side of the electrode assembly and two double-sided positive electrode 22 located inside the electrode assembly. The single-sided positive electrode includes a positive current collector and a positive active layer located on the side surface of the positive current collector near the center of the electrode assembly. The double-sided positive electrode includes a positive current collector and positive active layers located on both sides of the positive current collector. The negative electrode 4 includes a negative current collector 41 and negative active layers located on both sides of the negative current collector 41. The negative active layer corresponding to the single-sided positive electrode 21 is the first negative active layer 421, and the negative active layer corresponding to the double-sided positive electrode is the second negative active layer 422.

[0017] In this invention, the porosity of the negative electrode coating refers to the percentage of pore volume in the negative electrode coating to the total volume of the coating. It can be understood that the porosity of the negative electrode coating refers to the porosity of a single layer of the negative electrode coating in the first negative electrode active layer. The porosity of the negative electrode coating can be measured by methods such as cross-sectional scanning electron microscopy (SEM). This involves polishing the cross-section of the first negative electrode active layer along its thickness direction using an ion milling machine (rather than directly shearing to avoid shear stress damaging the structure), performing high-resolution imaging of the first negative electrode active layer under SEM, arbitrarily selecting a 20μm × 20μm area as the analysis region in each negative electrode coating layer, and using image analysis software (e.g., ImageJ, NanoMeasurer, Matlab, ParticleMetric, etc.) to calculate the area ratio of pores within this 20μm × 20μm area. Finally, arbitrarily selecting five 20μm × 20μm areas in each negative electrode coating layer to measure the area ratio of pores, and calculating the arithmetic mean as the porosity of that negative electrode coating layer.

[0018] In this invention, the porosity of the negative electrode coating is closely related to the packing state of the particles inside the coating, and is affected by the combined influence of the particle size and morphology of the negative electrode active material.

[0019] In stacked lithium-ion batteries, the electrode assembly typically consists of stacked positive electrode sheets, separators, and negative electrode sheets. During charging, the outermost single-sided positive electrode sheet only has an active layer on its side closest to the center of the electrode assembly. Therefore, the local current density experienced by the single-sided positive electrode sheet during charging is significantly higher than that of the inner double-sided positive electrode sheet region. This non-uniformity in current density distribution leads to an increase in the overpotential of the single-sided positive electrode sheet, which in turn forces a voltage balancing mechanism to lower the potential of the adjacent negative electrode region. When the negative electrode potential drops below 0V, lithium ions will deposit on the negative electrode surface in the form of metallic lithium, i.e., lithium plating occurs. Lithium plating irreversibly consumes active lithium and also leads to a rapid decline in cycle performance, especially during high-current charging, where lithium plating becomes more severe and significantly degrades the battery's fast-charging performance.

[0020] Analysis revealed that the negative electrode region corresponding to the single-sided positive electrode needs to withstand a much higher lithium-ion flux than the conventional region during charging. However, its single homogeneous negative electrode active layer structure is unable to quickly and uniformly accommodate and conduct a large influx of lithium ions in a short period of time, thus causing lithium-ion accumulation and overpotential increase at the interface.

[0021] Based on this, this application proposes an electrode assembly in which the porosity of the first negative electrode active layer corresponding to the single-sided positive electrode increases layer by layer from the surface of the negative electrode current collector toward the direction away from the negative electrode current collector. This structure means that the surface negative electrode coating with the highest porosity is near the separator and the single-sided positive electrode. During charging, a large number of lithium ions from the single-sided positive electrode first reach the surface negative electrode coating with higher porosity. The porous channels inside facilitate the wetting and distribution of the electrolyte, enabling rapid absorption and transport of lithium ions, effectively reducing the degree of lithium ion accumulation at the negative electrode / electrolyte interface, thereby reducing the concentration polarization of the first negative electrode active layer corresponding to the single-sided positive electrode. The bottom negative electrode coating with the lowest porosity is near the negative electrode current collector. The negative electrode active material has a dense packing density, ensuring good electrical conductivity between the negative electrode active material in the negative electrode active layer and the negative electrode current collector. The contact between the lithium ions and the active layer reduces the resistance of the first negative electrode layer. When lithium ions diffuse into the first negative electrode active layer, they can quickly gain electrons, be reduced, and embed into the active material, further suppressing the polarization inside the first negative electrode active layer. At the same time, the porosity increases layer by layer from the bottom negative electrode coating to the top negative electrode coating, that is, the porosity gradually decreases from the top negative electrode coating to the bottom negative electrode coating. This avoids the sudden obstruction of lithium ion diffusion at the interface between negative electrode coatings with abrupt changes in porosity, which would lead to local accumulation and cause lithium plating. Therefore, by setting the first negative electrode active layer corresponding to the single-sided positive electrode sheet to have a structure in which the porosity increases layer by layer from the surface of the negative electrode current collector towards the direction away from the current collector, the total overpotential required by the first negative electrode active layer during charging is reduced, thereby raising its actual operating potential and moving it away from the lithium plating critical point. This effectively suppresses the occurrence of lithium plating and significantly improves the cycle retention rate and fast charging performance of the battery.

[0022] In this invention, by configuring the first negative electrode active layer corresponding to the single-sided positive electrode in the stacked electrode assembly as having a structure in which the porosity increases layer by layer from the surface of the negative electrode current collector toward the direction away from the current collector, the problem of lithium plating in the negative electrode region corresponding to the single-sided positive electrode can be improved compared with the prior art, thereby enhancing the battery's cycle capacity retention and fast charging performance. To further improve the effect, one or more of the technical features can be further optimized.

[0023] In some embodiments, the first negative electrode active layer includes 2 to 4 layers (e.g., 2, 3 or 4 layers) of negative electrode coating layered along the thickness direction of the negative electrode sheet.

[0024] In some embodiments, the thickness of each negative electrode coating layer may be the same or different.

[0025] In some embodiments, such as Figure 1 As shown, the negative electrode 4 adjacent to the single-sided positive electrode 21 has a first negative electrode active layer 421 on one side corresponding to the single-sided positive electrode, and a second negative electrode active layer 422 on the other side corresponding to the double-sided positive electrode.

[0026] In some embodiments, such as Figure 1 As shown, the negative electrode 4, which is not adjacent to the single-sided positive electrode 21, has a second negative electrode active layer 422 on both sides, which corresponds to the double-sided positive electrode.

[0027] In some embodiments, the porosity difference between two adjacent negative electrode coating layers is 5%-15%, for example, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0028] In some embodiments, the porosity of the second negative electrode active layer is 25%-40%, for example, 25%, 27%, 29%, 30%, 31%, 33%, 35%, 37%, 39% or 40%.

[0029] In this invention, the porosity of the second negative electrode active layer refers to the porosity of the second negative electrode active layer on one side. When the second negative electrode active layer is located on the surface of the negative electrode adjacent to the single-sided positive electrode, it refers to the porosity of the second negative electrode active layer on that side. When the second negative electrode active layer is located on both surfaces of the negative electrode that are not adjacent to the single-sided positive electrode, it refers to the porosity of the second negative electrode active layer on either side. The porosity of the second negative electrode active layer in the electrode assembly can be the same or different, but all of them satisfy the range of 25%-40%.

[0030] In some embodiments, the first negative electrode active layer includes a first negative electrode coating and a second negative electrode coating stacked along the thickness direction of the negative electrode sheet. The second negative electrode coating is located between the first negative electrode coating and the negative electrode current collector, and the porosity of the first negative electrode coating is greater than that of the second negative electrode coating. It can be understood that "the porosity of the first negative electrode coating is greater than that of the second negative electrode coating" means that the porosity of the first negative electrode coating located on the same side as the negative electrode current collector is greater than that of the second negative electrode coating.

[0031] The side of this bilayer structure closer to the electrolyte has higher porosity, which facilitates the rapid entry of ions; the side closer to the negative electrode current collector has lower porosity and higher density, which is beneficial for electron conduction and provides structural support. Therefore, it can further optimize electron and lithium-ion transport in the first negative electrode active layer, improve lithium plating, and enhance the battery structural integrity, thereby further improving the battery's cycle performance and fast charging performance.

[0032] In some embodiments, the porosity of the first negative electrode coating is 35%-50%, for example, 35%, 37%, 39%, 40%, 41%, 43%, 45%, 47%, 49% or 50%.

[0033] In some embodiments, the porosity of the second negative electrode coating is 25%-40%, for example, 25%, 27%, 29%, 30%, 31%, 33%, 35%, 37%, 39% or 40%.

[0034] In this invention, the porosity of the first negative electrode coating and the porosity of the second negative electrode coating refer to the porosity of a single-layer negative electrode coating.

[0035] According to a specific implementation method, such as Figure 2 As shown, the first negative electrode active layer 421 includes a first negative electrode coating 4211 and a second negative electrode coating 4212 stacked along the thickness direction Y of the negative electrode sheet. The second negative electrode coating 4212 is located between the first negative electrode coating 4211 and the negative electrode current collector. The porosity of the first negative electrode coating is 35%-50%, the porosity of the second negative electrode coating is 25%-40%, and the difference in porosity between two adjacent negative electrode coating layers is 5%-15%.

[0036] In some embodiments, the negative electrode active material of the first negative electrode coating includes first silicon-based particles, and the negative electrode active material of the second negative electrode coating includes second silicon-based particles. The average particle size of the first silicon-based particles is larger than that of the second silicon-based particles. The larger average particle size of the first silicon-based particles results in fewer contact points between them and a larger gap size, thereby forming or influencing the high porosity of the first negative electrode coating. This is beneficial for further improving the electrolyte wetting and lithium-ion transport of the surface layer of the first negative electrode coating. Conversely, the smaller average particle size of the first silicon-based particles results in more contact points between them, leading to their close packing in the second negative electrode active layer. This forms and influences the low porosity of the second negative electrode coating, further improving its conductivity. Therefore, it can further reduce the polarization of the first negative electrode active layer, reduce lithium plating, and improve battery cycle performance and fast charging performance.

[0037] In some embodiments, the average particle size of the first silicon-based particles is 3μm-15μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm or 15μm.

[0038] In some embodiments, the average particle size of the second silicon-based particles is 0.5 μm-3 μm, for example, 0.5 μm, 0.7 μm, 0.9 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm or 3 μm.

[0039] According to one specific embodiment, the average particle size of the first silicon-based particle is 3μm-15μm, the average particle size of the second silicon-based particle is 0.5μm-3μm, and the average particle size of the first silicon-based particle is greater than the average particle size of the second silicon-based particle.

[0040] In this invention, the average particle size of the first silicon-based particles can be measured by the following methods: For example, the battery is discharged to 0% SOC, the negative electrode is disassembled, and lithium salt on the surface of the negative electrode is removed by rinsing with DMC. An argon ion cut is used to cut a cross-section of the negative electrode in the thickness direction. From the SEM image of the first negative electrode coating cross-section, arbitrarily select 100 μm × 100 μm, identify and randomly select 100 silicon-based particles, and draw the smallest rectangle or square that completely surrounds each particle (i.e., a rectangle or square tangent to the four sides of the particle's edge). The length of the longer side of the rectangle or the length of any side of the square is the particle size of that single silicon-based particle. If the number of silicon-based particles in a single image is insufficient, multiple images can be taken until the observed number of silicon-based particles reaches 100. The arithmetic mean of the particle sizes of these 100 silicon-based particles is calculated, which is the average particle size of the first silicon-based particle. Alternatively, before preparing the first negative electrode coating, the average particle size of the first silicon-based particles can be obtained by measuring it using a laser particle size analyzer after thorough stirring. The average particle size of the second silicon-based particle can be obtained by referring to the test method for the average particle size of the first silicon-based particle described above.

[0041] In some embodiments, the first negative electrode coating includes a first silicon-based particle, the second negative electrode coating includes a second silicon-based particle, the first silicon-based particle includes spherical silicon-based particles, and the second silicon-based particle includes blocky silicon-based particles and optionally ("optionally" means that they may or may not be present) spherical silicon-based particles.

[0042] In some embodiments, the first negative electrode coating includes first silicon-based particles, the second negative electrode coating includes second silicon-based particles, the first silicon-based particles include spherical silicon-based particles, and the second silicon-based particles include blocky silicon-based particles.

[0043] In some embodiments, the morphology of the bulk silicon-based particles includes at least one of regular and irregular shapes, wherein the irregular shape includes at least one of cuboid and cube.

[0044] In some embodiments, the blocky silicon-based particles have an average of 4 to 10 edges.

[0045] In this invention, the average number of edges of the bulk silicon-based particles refers to the arithmetic mean of the number of edges on the surface of any 50 bulk silicon-based particles in the negative electrode active layer. The number of edges of a single bulk silicon-based particle in the negative electrode active layer is observed by SEM, and twice the observed number is taken as the number of edges of a single bulk silicon-based particle. The arithmetic mean of the number of edges on the surface of 50 bulk silicon-based particles is taken as the average number of edges of the bulk silicon-based particles.

[0046] In the first negative electrode coating, spherical silicon-based particles support each other in a point-contact manner, loosely stacking to form pores. These pores provide channels for the electrolyte, further ensuring that the first negative electrode coating on the surface has excellent lithium-ion transport kinetics. In the second negative electrode coating, the blocky silicon-based particles have irregular shapes and larger contact areas, which can form a more stable framework, ensuring the long-term cycle stability of the electron conduction network and electrode structure. This further improves the lithium intercalation kinetics of the first negative electrode active layer corresponding to the single-sided positive electrode sheet, improves its easy lithium deposition problem, and enhances the battery cycle performance and fast charging performance.

[0047] In some embodiments, the first negative electrode coating includes first silicon-based particles, and the second negative electrode coating includes second silicon-based particles. The first silicon-based particles include spherical silicon-based particles, and the second silicon-based particles include both bulk silicon-based particles and spherical silicon-based particles. While the first negative electrode coating includes spherical silicon-based particles, improving the lithium-ion transport performance of the battery, the second negative electrode coating includes both bulk and spherical silicon-based particles. The bulk silicon-based particles form a framework, and the spherical silicon-based particles, with their smaller size and multi-point contact characteristics, fill the spaces between the bulk silicon-based particles, enhancing the contact between them. This further enhances the conductivity and structural stability of the second negative electrode coating, thereby further reducing the polarization of the first negative electrode active layer, reducing lithium plating, and improving battery cycle performance and fast-charging performance.

[0048] In some embodiments, the sphericity of the spherical silicon-based particles is 0.8-0.99, for example, 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 0.99.

[0049] In this invention, the sphericity of the spherical silicon-based particles can be tested using the following methods: for example, a flat, deformation-free cross-section along the thickness direction of the negative electrode sheet is prepared using ion beam cutting; a cross-sectional image of the negative electrode active layer is captured using a scanning electron microscope (SEM); and within an arbitrarily selected 100μm × 100μm area in the image, silicon-based particles are identified using image analysis software (e.g., ImageJ, NanoMeasurer, Matlab, ParticleMetric, etc.), and the radius r of the equivalent circle representing the projected circumference of a single silicon-based particle is calculated. 1 The radius r of the equivalent circle of the projected area of ​​silicon-based materials 2 The sphericity of a single silicon-based material = r 2 / r 1 The sphericity of any 100 silicon-based particles is statistically analyzed and the arithmetic mean is calculated. This arithmetic mean is the sphericity of the silicon-based particles. If the number of silicon-based particles in a single image is less than 100, multiple images are taken until the total number reaches 100.

[0050] In some embodiments, in the second negative electrode coating, the weight ratio of the spherical silicon-based particles to the bulk silicon-based particles is (1-2.5):1, for example, 1:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2:1, 2.2:1, 2.4:1, or 2.5:1. The second negative electrode coating contains a mixture of spherical and bulk silicon, with the spherical silicon surrounding or filling the bulk silicon. The elastic deformation capability of the spherical silicon can absorb the localized stress generated by the expansion of the bulk silicon, preventing rapid pulverization of the bulk silicon; simultaneously, the small size of the spherical silicon better maintains the electrical contact between particles, further reducing polarization and suppressing lithium plating. A weight ratio of (1-2.5):1 for spherical silicon-based particles to bulk silicon-based particles means that spherical silicon dominates, ensuring that buffering and SEI stability are the primary functions. If the ratio is lower than the above range (e.g., less than 1), there is too much bulk silicon and insufficient buffering, resulting in a sharp decrease in cycle life. If the ratio is higher than the above range (e.g., greater than 2.5), there is too much spherical silicon. An excessively high proportion of spherical silicon will lower the overall compaction density of the negative electrode, resulting in a reduction in the storage capacity per unit volume, which is not conducive to improving energy density.

[0051] In some embodiments, the first negative electrode coating includes first silicon-based particles, the second negative electrode coating includes second silicon-based particles, and the weight content of the first silicon-based particles in the first negative electrode coating is greater than the weight content of the second silicon-based particles in the second negative electrode coating.

[0052] Lithium ions are first embedded in the first negative electrode coating. When the weight content of silicon-based particles in the first negative electrode coating is greater than that in the second negative electrode coating, the advantages of the higher silicon-based particle content and larger specific capacity of the first negative electrode coating can be fully utilized to directly alleviate the lithium ion accumulation pressure on the surface of the first negative electrode active layer and reduce the concentration polarization of the first negative electrode active layer. Meanwhile, the second negative electrode coating has a lower silicon-based particle content, lower volume expansion change, and higher compaction density, which can provide mechanical support and stable electron collection for the entire electrode, further reducing the concentration polarization of the first negative electrode active layer. As a result, the risk of lithium plating in the first negative electrode active layer is further reduced, and the cycle stability and fast charging performance of the battery are further improved.

[0053] In some embodiments, the weight content of the first silicon-based particles in the first negative electrode coating is 10%-50%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%.

[0054] In some embodiments, the weight content of the second silicon-based particles in the second negative electrode coating is 5%-25%, for example, 5%, 7%, 9%, 11%, 13%, 15%, 17%, 19%, 21%, 23%, or 25%.

[0055] According to one specific embodiment, the weight content of the first silicon-based particles in the first negative electrode coating is 10%-50%, the weight content of the second silicon-based particles in the second negative electrode coating is 5%-25%, and the weight content of the first silicon-based particles in the first negative electrode coating is greater than the weight content of the second silicon-based particles in the second negative electrode coating.

[0056] In some embodiments, the negative electrode active material of the first negative electrode coating includes first graphite particles, and the negative electrode active material of the second negative electrode coating includes second graphite particles, wherein the average particle size of the first graphite particles is larger than the average particle size of the second graphite particles. The accumulation of the larger average particle size of the first graphite particles on the surface of the first negative electrode coating forms larger-scale pore channels, which is beneficial for the rapid penetration of electrolyte into the bottom coating; the accumulation of the smaller average particle size of the second graphite particles forms a second negative electrode coating with lower porosity, which helps to improve the interfacial contact between the second negative electrode coating and the negative electrode current collector, reduce resistance, and thereby further reduce the polarization of the first negative electrode active layer corresponding to the single-sided positive electrode sheet, reduce lithium plating, and improve battery cycle performance, especially reducing polarization during high-rate charging (e.g., fast charging) and improving the fast charging performance of the battery.

[0057] In some embodiments, the average particle size of the first graphite particles is 15μm-30μm, for example, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm or 30μm.

[0058] In some embodiments, the average particle size of the second graphite particles is 10μm-25μm, for example, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm or 25μm.

[0059] According to one specific embodiment, the average particle size of the first graphite particle is 15μm-30μm, the average particle size of the second graphite particle is 10μm-25μm, and the average particle size of the first graphite particle is greater than the average particle size of the second graphite particle.

[0060] In some embodiments, the first negative electrode coating includes first silicon-based particles and first graphite particles, and the second negative electrode coating includes second graphite particles (excluding the second silicon-based particles). The weight content of the first silicon-based particles in the first negative electrode coating is greater than the weight content of the second silicon-based particles in the second negative electrode coating. The underlying second negative electrode coating contains only graphite particles, which can further reduce the expansion of the underlying second negative electrode coating, enhance the stability of the electrode structure, and further improve the cycle stability of the battery.

[0061] In some embodiments, the first negative electrode active layer includes a third silicon-based particle, the second negative electrode active layer includes a fourth silicon-based particle, and the electrode assembly satisfies the following relationship: S 1 2 , among which, S 1 =(D 19 -D 11 ) / D 15 S 2 =(D 29 -D 21 ) / D 25 D 19 D 15 D 11 These are the Dv90, Dv50, and Dv10 of the third silicon-based particle, respectively, in μm. 29 D 25 D 21 These are Dv90, Dv50, and Dv10, respectively, of the fourth silicon-based particle, in μm.

[0062] In this invention, it is understood that the third silicon-based particle is composed of silicon-based particles in the negative electrode coating of the first negative electrode active layer. For example, when the first negative electrode active layer includes two negative electrode coatings, the third silicon-based particle is composed of the first silicon-based particles in the first negative electrode coating and the second silicon-based particles in the second negative electrode coating.

[0063] By controlling the particle size distribution width S of silicon-based particles in different negative electrode active layers, the electrode assembly satisfies the following relationship: S 1 2 ​​The first negative electrode active layer of the single-sided positive electrode uses small-diameter silicon-based particles with a more uniform particle size distribution. On the one hand, the smaller particle size of the silicon-based particles can shorten the diffusion path of lithium ions in the first negative electrode active layer, reducing the driving force (i.e., overpotential) required for lithium ion diffusion inside the particles during high-rate charging, which can directly alleviate electrochemical polarization. On the other hand, the small-diameter silicon-based particles have less deformation when they expand, and the resulting internal stress is easier to release and buffer. The integrity of the silicon-based particles is better maintained, which helps to maintain the structural integrity and electrical contact of the silicon-based particles in the first negative electrode active layer during cycling, thereby forming a dense and stable SEI film. This slows down the impedance growth caused by silicon-based particle breakage and repeated SEI regeneration, and stabilizes the overpotential during long-term cycling. Therefore, it can further improve the lithium plating problem in the negative electrode region corresponding to the single-sided positive electrode. The second negative electrode active layer of the double-sided positive electrode uses silicon-based particles with a larger particle size and wider distribution. While ensuring high capacity, the use of small particles to fill the gaps between large particles improves the electrode density and conductive network connectivity of the second negative electrode active layer, maintaining good energy density.

[0064] In some embodiments, the first negative electrode active layer comprises a third silicon-based particle, and the electrode assembly satisfies the relationship: 1.2 ≤ S 1 ≤9.3 (e.g., 1.2, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, or 9.3), where S 1 =(D 19 -D 11 ) / D 15 D 19 D 15 D 11 These are Dv90, Dv50, and Dv10, respectively, of the third silicon-based particle, in μm.

[0065] In some embodiments, the second negative electrode active layer comprises a fourth silicon-based particle, and the electrode assembly satisfies the relationship: 1.4 ≤ S 2 ≤10.6 (e.g., 1.4, 1.8, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or 10.6), where S 2 =(D 29 -D 21 ) / D 25 D 29 D 25 D 21 These are Dv90, Dv50, and Dv10, respectively, of the fourth silicon-based particle, in μm.

[0066] In this invention, Dv10 represents the particle size corresponding to 10% of the cumulative particle size distribution of silicon-based particles arranged from smallest to largest; Dv50 represents the particle size corresponding to 50% of the cumulative particle size distribution of silicon-based particles arranged from smallest to largest; and Dv90 represents the particle size corresponding to 90% of the cumulative particle size distribution of silicon-based particles arranged from smallest to largest. In this invention, the volumetric particle size distribution of the third silicon-based particle can be obtained by measuring and statistically processing the Dv10, Dv50, and Dv90 of the silicon-based particles within an arbitrarily selected 100μm × 100μm area in an SEM image of the surface of the first negative electrode active layer, using image analysis software (e.g., ImageJ, NanoMeasurer, Matlab, ParticleMetric, etc.). In this invention, the volumetric particle size distribution of the fourth silicon-based particle can be obtained by measuring the volumetric particle size distribution of the third silicon-based particle in an SEM image of the surface of the second negative electrode active layer, referring to the testing method used for the volumetric particle size distribution of the third silicon-based particle. In this invention, the Dv10, Dv50, and Dv90 of the third or fourth silicon-based particles can also be obtained by testing with a laser particle size analyzer. For example, before preparing the negative electrode sheet, the silicon-based particles are thoroughly stirred and then the Dv10, Dv50, and Dv90 of the silicon-based particles are measured by a laser particle size analyzer.

[0067] In some embodiments, the first negative electrode active layer includes third silicon-based particles, wherein the third silicon-based particles have a Dv50 of D. 15 D 15 The range is 4μm-13μm, for example, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm or 13μm.

[0068] In some embodiments, the Dv90 of the third silicon-based particle is 18μm-38μm, for example, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm, 30μm, 32μm, 34μm, 36μm or 38μm.

[0069] In some embodiments, the Dv10 of the third silicon-based particle is 0.8μm-3μm, for example, 0.8μm, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm or 3μm.

[0070] According to one specific embodiment, the Dv50 of the third silicon-based particle is 4μm-13μm, the Dv90 of the third silicon-based particle is 18μm-38μm, the Dv10 of the third silicon-based particle is 0.8μm-3μm, and the electrode assembly satisfies the relationship: 1.2≤S 1 ≤9.3.

[0071] In some embodiments, the second negative electrode active layer includes a fourth silicon-based particle, wherein the Dv50 of the fourth silicon-based particle is D. 25 D 25 The range is 3μm-10μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm.

[0072] In some embodiments, the Dv90 of the fourth silicon-based particle is 15μm-32μm, for example, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm, 26μm, 27μm, 28μm, 29μm, 30μm, 31μm or 32μm.

[0073] In some embodiments, the Dv10 of the fourth silicon-based particle is 0.2μm-1μm, for example, 0.2μm, 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm or 1μm.

[0074] According to one specific embodiment, the Dv50 of the fourth silicon-based particle is 3μm-10μm, the Dv90 of the fourth silicon-based particle is 15μm-32μm, the Dv10 of the fourth silicon-based particle is 0.2μm-1μm, and the electrode assembly satisfies the relationship: 1.4≤S 2 ≤10.6.

[0075] In some embodiments, the first negative electrode active layer includes a first negative electrode conductive agent, and the second negative electrode active layer includes a second negative electrode conductive agent. The first negative electrode conductive agent and the second negative electrode conductive agent each independently include at least one of carbon nanotubes, graphene, and conductive carbon black.

[0076] In some embodiments, the first negative electrode conductive agent comprises carbon nanotubes and conductive carbon black, wherein the carbon nanotubes account for 10%-50% by weight in the first negative electrode conductive agent, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%. The conductive carbon black provides point contacts, and the carbon nanotubes provide line connections, forming a composite conductive network in the first negative electrode active layer. This synergistically enhances the conductivity of the first negative electrode active layer, reduces impedance and polarization, further reduces lithium plating, and improves the cycle stability and fast-charging performance of the battery.

[0077] In some embodiments, the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is greater than the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer. This strengthens the electronic conduction network of the first negative electrode active layer, compensates for the disadvantage of reduced electronic conduction performance that may be caused by its high porosity, ensures that the first negative electrode active layer of the corresponding single-sided positive electrode sheet can quickly transfer charge, improves the overall reaction kinetics uniformity of the battery, further reduces the overpotential of the first negative electrode active layer, suppresses lithium plating, and improves the cycle performance and fast charging performance of the battery.

[0078] In some embodiments, the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is 1.2%-3%, for example, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8% or 3%.

[0079] In some embodiments, the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer is 1%-2%, for example, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%.

[0080] According to one specific embodiment, the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is 1.2%-3%, and the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer is 1%-2%, wherein the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is greater than the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer.

[0081] In some embodiments, the first negative electrode active layer includes a first negative electrode binder, and the second negative electrode active layer includes a second negative electrode binder. The first negative electrode binder and the second negative electrode binder each independently include at least one of polyacrylic acid, sodium alginate, polyimide, styrene-butadiene rubber, and sodium carboxymethyl cellulose.

[0082] In some embodiments, the first negative electrode binder includes at least one of polyacrylic acid, sodium alginate, and styrene-butadiene rubber. Using polyacrylic acid, sodium alginate, or styrene-butadiene rubber—which have higher elastic modulus, stronger adhesion, and better compatibility with silicon-based particles—as the first negative electrode binder can enhance the adhesion and contact between silicon-based particles and conductive agents within the negative electrode active layer, improve the lithium-ion diffusion rate, reduce sheet resistance, further reduce the polarization of the first negative electrode active layer of the corresponding single-sided positive electrode sheet, further reduce lithium plating, and improve the battery's cycle and fast-charging performance.

[0083] In some embodiments, the first negative electrode binder comprises polyacrylic acid and styrene-butadiene rubber. Polyacrylic acid has strong adhesion, providing strong anchoring for silicon-based particles, while styrene-butadiene rubber has flexibility. Thus, the polyacrylic acid and styrene-butadiene rubber together suppress the drastic volume change of the first negative electrode active layer under high current, maintain the stability of the SEI film of the first negative electrode active layer, further reduce the polarization caused by SEI film thickening, further reduce lithium plating, and improve the cycle and fast-charging performance of the battery.

[0084] In some embodiments, the first negative electrode binder comprises polyacrylic acid and styrene-butadiene rubber, wherein the polyacrylic acid accounts for 30%-70% by weight in the first negative electrode binder, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65% or 70%.

[0085] In some embodiments, the second negative electrode binder includes at least one of styrene-butadiene rubber and sodium carboxymethyl cellulose.

[0086] In some embodiments, the weight percentage of the first negative electrode binder in the first negative electrode active layer is greater than the weight percentage of the second negative electrode binder in the second negative electrode active layer. On the one hand, the more first binder in the first negative electrode active layer, the better the adhesion, thereby enhancing the cohesion of the first negative electrode active layer, improving the contact between silicon-based particles and conductive agents within the negative electrode active layer, thereby increasing the lithium-ion diffusion rate, reducing the sheet resistance, further reducing the polarization of the first negative electrode active layer corresponding to the single-sided positive electrode sheet, reducing lithium plating, and improving the cycle performance and fast charging performance of the battery. On the other hand, the more first binder in the first negative electrode active layer, the more effectively it can alleviate the volume change of silicon-based particles during cycling, reduce silicon-based particle breakage and continuous side reactions, reduce the impedance caused by the thickening of the SEI film, further improve the lithium plating problem, and improve the cycle performance and fast charging performance of the battery.

[0087] In some embodiments, the weight percentage of the first negative electrode binder in the first negative electrode active layer is 2.5%-4.5%, for example, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2% or 4.5%.

[0088] In some embodiments, the weight percentage of the second negative electrode binder in the second negative electrode active layer is 1.5%-3%, for example, 1.5%, 1.7%, 2%, 2.2%, 2.5%, 2.7%, 3%.

[0089] According to one specific embodiment, the weight percentage of the first negative electrode binder in the first negative electrode active layer is 2.5%-4.5%, and the weight percentage of the second negative electrode binder in the second negative electrode active layer is 1.5%-3%, wherein the weight percentage of the first negative electrode binder in the first negative electrode active layer is greater than the weight percentage of the second negative electrode binder in the second negative electrode active layer.

[0090] In some embodiments, the sheet resistance of the first negative electrode active layer is less than that of the second negative electrode active layer. To accommodate the relatively high local current density of the single-sided positive electrode, the sheet resistance of the first negative electrode active layer corresponding to the single-sided positive electrode is reduced, thereby enabling rapid electron transport within the first negative electrode active layer. The lithium-ion insertion reaction is uniformly distributed in the thickness direction and in-plane of the first negative electrode active layer, further reducing the polarization of the first negative electrode active layer, improving lithium plating and the capacity decay caused by lithium plating, and enhancing battery cycle stability and fast charging performance.

[0091] In some embodiments, the sheet resistance of the first negative electrode active layer is 10 Ω·cm. 2 -18Ω·cm 2 For example, 10Ω·cm 2 11Ω·cm 2 12Ω·cm 2 13Ω·cm 2 14Ω·cm 2 15Ω·cm 2 16Ω·cm 2 17Ω·cm 2 Or 18Ω·cm 2 .

[0092] In some embodiments, the sheet resistance of the second negative electrode active layer is 18 Ω·cm. 2 -30Ω·cm 2 For example, 18Ω·cm 2 20Ω·cm 2 22Ω·cm 2 24Ω·cm 2 25Ω·cm 2 26Ω·cm 2 28Ω·cm 2 or 30Ω·cm 2 .

[0093] In this invention, the sheet resistance of the negative electrode active layer can be measured by the following methods: for example, disassembling the battery, removing the negative electrode sheet, removing the lithium salt on the surface of the negative electrode active layer, or using the original negative electrode sheet of the unassembled battery as a test sample, taking out the negative electrode sheet in a dry room with a dew point below -40°C, briefly immersing and cleaning the negative electrode sheet in an anhydrous solvent (such as DMC), and then thoroughly drying it in a vacuum oven. Using an electrode resistance meter, the cut negative electrode sheet is placed between the two electrodes, a controllable pressure (5 MPa, holding pressure for 15-30 seconds) is applied, and the total resistance through the electrode sheet is measured.

[0094] According to one specific embodiment, the sheet resistance of the first negative electrode active layer is 10 Ω·cm. 2 -18Ω·cm 2 The sheet resistance of the second negative electrode active layer is 18 Ω·cm. 2 -30Ω·cm 2 The surface resistance of the first negative electrode active layer is less than that of the second negative electrode active layer.

[0095] In some embodiments, the compaction density of the first negative electrode active layer is less than the compaction density of the second negative electrode active layer.

[0096] In this invention, the compaction density of the first negative electrode active layer is lower than that of the second negative electrode active layer. On the one hand, the low compaction density of the first negative electrode active layer, combined with the progressively increasing porosity of the negative electrode coating in the first negative electrode active layer, ensures the ion transport channels of the first negative electrode active layer, optimizes the transport paths of ions and electrons in the first negative electrode active layer, further reduces the polarization of the first negative electrode active layer, and reduces the occurrence of lithium plating in the first negative electrode active layer. On the other hand, the second negative electrode active layer has a higher compaction density, which can also improve the volumetric energy density of the battery and enhance the contact between the negative electrode active material and the negative electrode current collector in the second negative electrode active layer. Thus, while improving lithium plating, enhancing the battery's cycle performance and fast charging performance, the battery also achieves high power and high energy output, thereby improving the battery's energy density.

[0097] In this invention, the compaction density of the negative electrode active layer can be measured by, for example, by disassembling the battery, removing the negative electrode sheet, and removing the lithium salt on the surface of the negative electrode active layer; or by using the original negative electrode sheet of the unassembled battery as a test sample, selecting at least 20 sites on the negative electrode sheet, measuring the thickness of the negative electrode sheet at each site using a micrometer, and taking the average value h. 1 The negative electrode sheet is punched into a shape with an area of ​​S using a stamping die. 1 Take 10 discs, weigh each disc, and take the average mass m. 1The above-mentioned discs are immersed in a solvent that can dissolve the negative electrode active layer but does not corrode the negative electrode current collector. The negative electrode active layer is removed by ultrasonication or wiping. After the negative electrode current collector is removed, dried, and weighed, the average mass m is taken. 2 Calculate the average thickness h of 10 circular negative electrode wafers. 1 ; Calculate the average thickness h of the 10 circular negative electrode current collectors. 2 The areal density M of the single-sided negative electrode active layer 1 =[(m 1 -m 2 ) / 2] / S 1 The compaction density of a single-sided negative electrode active layer = M 1 / (h 1 -h 2 ).

[0098] In some embodiments, the compaction density of the first negative electrode active layer is 1.35 g / cm³. 3 -1.7g / cm 3 For example, 1.35 g / cm³ 3 1.38g / cm 3 1.4g / cm 3 1.42g / cm 3 1.45g / cm 3 1.48g / cm 3 1.5g / cm 3 1.52g / cm 3 1.55g / cm 3 1.58g / cm 3 1.6g / cm 3 1.62g / cm 3 1.65g / cm 3 1.68g / cm 3 Or 1.7g / cm 3 .

[0099] In some embodiments, the compaction density of the second negative electrode active layer is 1.5 g / cm³. 3 -1.75g / cm 3 For example, 1.5g / cm 3 1.52g / cm 3 1.55g / cm 3 1.58g / cm 3 1.6g / cm 3 1.62g / cm 3 1.65g / cm 3 1.68g / cm 3 1.7g / cm 3 1.72g / cm3 Or 1.75g / cm 3 .

[0100] According to one specific embodiment, the compaction density of the first negative electrode active layer is 1.35 g / cm³. 3 -1.7g / cm 3 The compaction density of the second negative electrode active layer is 1.5 g / cm³. 3 -1.75g / cm 3 The compaction density of the first negative electrode active layer is less than that of the second negative electrode active layer.

[0101] In some embodiments, such as Figure 1 As shown, the negative electrode current collector 41 includes a first negative electrode current collector 411 and a second negative electrode current collector 412. One side surface of the first negative electrode current collector 411 is the first negative electrode active layer 421 and the other side surface is the second negative electrode active layer 422. Both sides of the second negative electrode current collector 412 are the second negative electrode active layer 422. The thickness of the first negative electrode current collector is greater than the thickness of the second negative electrode current collector.

[0102] The negative electrode current collector is the main channel for electron transport. The thicker it is, the larger its cross-sectional area, and the greater the current it can carry. This makes the first negative electrode current collector, which carries the first negative electrode active layer with higher porosity, thicker, with a stronger current load and lower transmission resistance. This ensures that electrons can be transported quickly and evenly to all parts of the electrode with a lower voltage drop (i.e., less ohmic polarization), thereby supporting the lithium intercalation reaction at high rates, avoiding the aggravation of local polarization caused by insufficient electron supply, further reducing lithium plating, and improving the cycle performance and fast charging performance of the battery.

[0103] In some embodiments, the thickness of the first negative electrode current collector is 7μm-12μm, for example, 7μm, 8μm, 9μm, 10μm, 11μm or 12μm.

[0104] In some embodiments, the thickness of the second negative electrode current collector is 5μm-8μm, for example, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm or 8μm.

[0105] According to one specific embodiment, the thickness of the first negative electrode current collector is 7μm-12μm, the thickness of the second negative electrode current collector is 5μm-8μm, and the thickness of the first negative electrode current collector is greater than the thickness of the second negative electrode current collector.

[0106] In some embodiments, the first negative current collector and the second negative current collector are copper foils.

[0107] In some embodiments, such as Figure 1As shown, the separator 3 includes a first separator 31 and a second separator 32. The first separator 31 corresponds to the single-sided positive electrode 21, and the second separator 32 corresponds to the double-sided positive electrode 22. The porosity of the first separator is greater than that of the second separator. Using a high-porosity separator on the side of the high-porosity first negative electrode active layer synergistically promotes the high-speed transport of lithium ions between the single-sided positive electrode and the corresponding first negative electrode active layer, ensuring lithium ion supply, making the potential of the first negative electrode active layer more stable, and further suppressing lithium plating caused by polarization of the first negative electrode active layer. On the side of the low-porosity second negative electrode active layer, using a low-porosity separator can provide a more uniform ion flow and enhance safety, further optimizing the ion transport environment inside the battery and improving the cycle consistency of the battery.

[0108] In some embodiments, the porosity of the first diaphragm is 45%-55%, for example, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54% or 55%.

[0109] In some embodiments, the porosity of the second diaphragm is 37%-43%, for example, 37%, 38%, 39%, 40%, 41%, 42% or 43%.

[0110] According to one specific embodiment, the porosity of the first diaphragm is 45%-55%, and the porosity of the second diaphragm is 37%-43%, wherein the porosity of the first diaphragm is greater than that of the second diaphragm.

[0111] In this invention, the porosity of the separator can be measured by the following method: the battery is disassembled, and the separator is removed, or the original separator not used for battery assembly is used as a test sample. The test sample is tested according to the standard "GB / T 36363-2018 Polyolefin Separator for Lithium-ion Batteries" to obtain the porosity of the separator. For example, the gas replacement method is used, with helium gas, and the test is conducted in an environment with a temperature of 23±2℃ and a relative humidity of 50±5%. Porosity% = (1-ρ a / ρ t )×100%, where ρ a Apparent density of the substrate layer, in g / cm³ 3 ;ρ t The true density of the substrate layer, in g / cm³. 3 Five parallel samples were tested for each of the first and second diaphragms, and the arithmetic mean was taken as the final result.

[0112] A second aspect of the present invention provides a battery including an electrolyte and an electrode assembly provided in the first aspect of the present invention.

[0113] In some embodiments, the electrolyte can be a conventional electrolyte in the art. For example, the electrolyte may include an organic solvent, a lithium salt, and additives. The organic solvent includes carbonate solvents and carboxylic acid ester solvents. The carbonate solvent includes at least one selected from ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The carboxylic acid ester solvent includes at least one selected from ethyl propionate (EP), propyl propionate (PP), ethyl acetate (EA), and ethyl butyrate (EB). The lithium salt includes lithium hexafluorophosphate (LiPF6). The additive comprises at least one of lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP); the additive comprises at least one of nitrile compounds, fluorocarbonates, fluorocarboxylic acid esters, and sulfur-containing compounds.

[0114] In some instances, the battery is a lithium-ion rechargeable battery.

[0115] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0116] Example 1 (1) Preparation of single-sided and double-sided positive electrode plates The positive electrode active material (lithium cobalt oxide), positive electrode binder (polyvinylidene fluoride PVDF500), and positive electrode conductive agent (conductive carbon black: carbon nanotubes (weight ratio) = 1:1) were mixed in N-methylpyrrolidone (NMP) solvent at a weight ratio of 96:2:2 and continuously stirred under the action of a stirrer to form a uniform and flowing positive electrode slurry. Subsequently, the positive electrode slurry was coated on one side of the positive electrode current collector (aluminum foil) and dried in a vacuum oven at 120°C for 6 hours. Then, it was rolled and slit to obtain the desired single-sided positive electrode sheet. The positive electrode slurry was coated on both sides of the positive electrode current collector (aluminum foil) and dried in a vacuum oven at 120°C for 6 hours. Then, it was rolled and slit to obtain the desired double-sided positive electrode sheet.

[0117] (2) Preparation of negative electrode sheet The second graphite particles (artificial graphite), the second silicon-based particles (silicon-carbon composite particles), the first negative electrode conductive agent (carbon nanotubes: conductive carbon black (weight ratio) = 1:1), and the first negative electrode binder (styrene-butadiene rubber: polyacrylic acid (weight ratio) = 6:4) were mixed in an aqueous solvent at a weight ratio of 80.9:13.8:2.1:3.2, and continuously stirred under the action of a stirrer to form a uniform and fluid first negative electrode slurry. The first graphite particles (artificial graphite), the first silicon-based particles (silicon-carbon composite particles), and the first negative electrode were then mixed at a weight ratio of 62:32.7:2.1:3.2. Conductive agent (carbon nanotubes: conductive carbon black (weight ratio) = 1:1) and first negative electrode binder (styrene-butadiene rubber: polyacrylic acid (weight ratio) = 6:4) are mixed in an aqueous solvent and continuously stirred under the action of a stirrer to form a uniform and flowing second negative electrode slurry. The first negative electrode slurry is coated on one side of the first negative electrode current collector (copper foil) and dried in a vacuum oven at 120°C for 6 hours to obtain the second negative electrode coating. The second negative electrode slurry is then coated on the surface of the second negative electrode coating and dried in a vacuum oven at 120°C for 6 hours to obtain the first negative electrode coating, thereby obtaining the first negative electrode active layer. The second graphite particles (artificial graphite), the second silicon-based particles (silicon-carbon composite particles), the second negative electrode conductive agent (carbon nanotubes: conductive carbon black (weight ratio) = 1:1), and the second negative electrode binder (styrene-butadiene rubber: polyacrylic acid (weight ratio) = 6:4) were mixed in an aqueous solvent at a weight ratio of 49.2:46.5:1.8:2.5. The mixture was continuously stirred under the action of a stirrer to form a uniform and flowing third negative electrode slurry. The third negative electrode slurry was coated on the other side of the first negative electrode current collector (copper foil) and dried in a vacuum oven at 120°C for 6 hours to obtain the second negative electrode active layer. Then, after rolling and wire bonding, a negative electrode sheet corresponding to the single-sided positive electrode sheet was obtained. The first negative electrode active layer corresponds to the single-sided positive electrode sheet, and the second negative electrode active layer corresponds to the double-sided positive electrode sheet. The third negative electrode slurry is coated on both sides of the second negative electrode current collector (copper foil), and then dried in a vacuum oven at 120°C for 6 hours to obtain the second negative electrode active layer. After rolling and wire bonding, a negative electrode sheet corresponding to the double-sided positive electrode sheet is obtained. The second negative electrode active layers on both sides correspond to the double-sided positive electrode sheet. The first negative electrode active layer comprises two negative electrode coatings stacked along the thickness direction of the negative electrode sheet, namely the first negative electrode coating and the second negative electrode coating. The coatings are arranged from the surface of the negative electrode current collector towards the direction away from it. The porosity of each negative electrode coating layer increases progressively. The porosity of the first negative electrode coating is 42.5%, and the porosity of the second negative electrode coating is 32.6%. The difference in porosity between adjacent negative electrode coating layers is 42.5% - 32.6% = 9.9%. The porosity of the first negative electrode coating is greater than that of the second negative electrode coating. The porosity of the second negative electrode active layer is 32.5%. The average particle size of the first silicon-based particles is greater than that of the second silicon-based particles. The average particle size of the first silicon-based particles is 9.7 μm, and the average particle size of the second silicon-based particles is 7.78 μm. The first silicon-based particles are spherical silicon-based particles, and the second silicon-based particles are... The base particles are a mixture of spherical and bulk silicon-based particles. The sphericity of the spherical silicon-based particles is 0.92. In the second negative electrode coating, the weight ratio of spherical silicon-based particles to bulk silicon-based particles is 1.5:1. The average particle size of the first graphite particles is larger than that of the second graphite particles. The average particle size of the first graphite particles is 22.7 μm, and the average particle size of the second graphite particles is 17.2 μm. The weight content of the first silicon-based particles in the first negative electrode coating is greater than that of the second silicon-based particles in the second negative electrode coating. The weight content of the first silicon-based particles in the first negative electrode coating is 32.7%, and the weight content of the second silicon-based particles in the second negative electrode coating is 13.8%. The sheet resistance of the first negative electrode active layer is less than that of the second negative electrode active layer. The sheet resistance of the first negative electrode active layer is 15.8 Ω·cm. 2 The sheet resistance of the second negative electrode active layer is 20.3 Ω·cm. 2 The compaction density of the first negative electrode active layer is less than that of the second negative electrode active layer; the compaction density of the first negative electrode active layer is 1.5 g / cm³. 3 The compaction density of the second negative electrode active layer is 1.7 g / cm³. 3 The third silicon-based particle has a Dv90 of 28.5 μm (D 19 =28.5), the Dv50 of the third silicon-based particle is 8.7μm (D 15 =8.7), the Dv10 of the third silicon-based particle is 1.9μm (D 11 =1.9), the Dv90 of the fourth silicon-based particle is 23.5μm (D 29 =23.5), the Dv50 of the fourth silicon-based particle is 6.8μm (D 25 =6.8), the Dv10 of the fourth silicon-based particle is 0.6μm (D 21 =0.6), S 1 =(D 19 -D 11 ) / D 15=(28.5-1.9) / 8.7=3.06, S 2 =(D 29 -D 21 ) / D 25 =(23.5-0.6) / 6.8=3.37, S 1 2 The weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is greater than that of the second negative electrode conductive agent in the second negative electrode active layer. The weight percentage of the first negative electrode binder in the first negative electrode active layer is greater than that of the second negative electrode binder in the second negative electrode active layer. The thickness of the first negative electrode current collector is greater than that of the second negative electrode current collector. The thickness of the first negative electrode current collector is 8.5 μm, and the thickness of the second negative electrode current collector is 6.9 μm.

[0118] (3) Electrolyte preparation In an argon-filled glove box (moisture content <1 ppm, oxygen content <1 ppm), the following non-fluorinated solvents, propyl propionate, ethyl propionate, propylene carbonate, and ethylene carbonate, were mixed in a volume ratio of 3:4:2:1 to form a homogeneous solvent. Then, 21.2% of fluoroethylene carbonate, 16% of LiPF6, and 2.4% of 1,3,6-hexanetricarbonate based on the total weight of the electrolyte were slowly added and stirred until homogeneous to obtain the desired lithium-ion battery electrolyte.

[0119] (4) Preparation of diaphragm Ceramic particles (boehmite) and binder (polyacrylic acid) are added to deionized water at a weight ratio of 95:5. After thorough stirring, a first slurry with a solid content of 25% is obtained. The first slurry is coated onto one side of the substrate layer (polyethylene) using a gravure roller. After drying in a multi-section oven at 60°C, a carrier layer with ceramic coating located on one side of the substrate layer is formed. Polyvinylidene fluoride (in granular form) is added to deionized water to obtain a second slurry with a solid content of 8%. The second slurry is coated onto both sides of the carrier layer using a gravure roller and then dried in a multi-section oven at 60°C to form a coating layer. This process prepares a first separator corresponding to a single-sided positive electrode and a second separator corresponding to a double-sided positive electrode. By adjusting the coating process (e.g., the coverage of polyvinylidene fluoride particles) or the diameter of the substrate fiber, the porosity of the first separator is made greater than that of the second separator. The porosity of the first separator is 48.7%, and the porosity of the second separator is 40.3%.

[0120] (5) Lithium-ion battery preparation The positive electrode, separator, and negative electrode obtained above are stacked to form an electrode assembly, which is then subjected to processes such as electrolyte injection, vacuum packaging, room temperature standing, and high temperature formation to obtain the desired lithium-ion battery.

[0121] ​Example 2 group This set of examples illustrates the effects of changes in the porosity difference between two adjacent negative electrode coating layers.

[0122] This embodiment is based on Embodiment 1, except that the porosity of the first negative electrode coating and / or the porosity of the second negative electrode coating are changed, so that the difference in porosity between the two adjacent negative electrode coatings is changed, as detailed in Table 1.

[0123] Table 1 “ "Indicates the same as in Example 1" Example 3 This embodiment is based on Embodiment 1, except that the average particle size of the first silicon-based particle is 5.2 μm and the average particle size of the second silicon-based particle is 8.6 μm. The average particle size of the first silicon-based particle is smaller than that of the second silicon-based particle.

[0124] Example 4 group This embodiment group is based on Embodiment 1, except that the morphology of the first silicon-based particles is changed, as detailed in Table 2.

[0125] Table 2 Example 5 This embodiment is based on Embodiment 1, except that the porosity of the first negative electrode coating is 38.4%, the porosity of the second negative electrode coating is 32.6%, the porosity difference between the two adjacent negative electrode coatings is 38.4%-32.6%=5.8%, the average particle size of the first graphite particles is smaller than the average particle size of the second graphite particles, the average particle size of the first graphite particles is 15.5μm, and the average particle size of the second graphite particles is 17.2μm.

[0126] Example 6 group This set of embodiments is used to illustrate the effects of changes in the weight content of the first silicon-based particles in the first negative electrode coating and / or the weight content of the second silicon-based particles in the second negative electrode coating.

[0127] This set of embodiments is based on Embodiment 1, except that the weight content of the first silicon-based particles in the first negative electrode coating and / or the weight content of the second silicon-based particles in the second negative electrode coating are changed, as detailed in Table 3.

[0128] Table 3 Example 7 group This set of embodiments is used to illustrate the effects that occur when the sheet resistance of the first negative electrode active layer and / or the sheet resistance of the second negative electrode active layer changes.

[0129] This set of embodiments is based on Embodiment 1, except that the sheet resistance of the first negative electrode active layer and / or the sheet resistance of the second negative electrode active layer are changed by changing the weight content of the conductive agent, as detailed in Table 4.

[0130] Table 4 Example 8 group This set of examples is used to illustrate the effects when the compaction density of the first negative electrode active layer and / or the compaction density of the second negative electrode active layer changes.

[0131] This set of embodiments is based on Embodiment 1, except that the compaction density of the first negative electrode active layer and / or the compaction density of the second negative electrode active layer are changed, as detailed in Table 5.

[0132] Table 5 Example 9 group This set of examples is used to illustrate when S 1 2 The impact of changes.

[0133] This set of embodiments is based on Embodiment 1, except that S is changed. 1 and / or S 2 This makes S 1 2 Changes have occurred; see Table 6 for details.

[0134] Table 6 Example 10 group This set of embodiments is used to illustrate the effects that occur when the thickness of the first negative electrode current collector and / or the thickness of the second negative electrode current collector changes.

[0135] This set of embodiments is based on Embodiment 1, except that the thickness of the first negative electrode current collector and / or the thickness of the second negative electrode current collector are changed, as detailed in Table 7.

[0136] Table 7 Example 11 group This set of embodiments is used to illustrate the effects that occur when the porosity of the first diaphragm and / or the porosity of the second diaphragm changes.

[0137] ​​This set of embodiments is based on Embodiment 1, except that the porosity of the first diaphragm and / or the porosity of the second diaphragm are changed, as detailed in Table 8.

[0138] Table 8 Example 12 group This set of examples illustrates the effects of changes in the porosity of the second negative electrode active layer.

[0139] Example 12-1 This embodiment is based on Embodiment 1, except that the porosity of the second negative electrode active layer is 23%.

[0140] Example 12-2 This embodiment is based on Embodiment 1, except that the porosity of the second negative electrode active layer is 42%.

[0141] Example 13 This embodiment is based on Embodiment 1, except that the second negative electrode active layer also includes two negative electrode coatings stacked along the thickness direction of the negative electrode sheet.

[0142] Example 14 group This set of examples illustrates the effects of changes in the porosity of the first negative electrode coating.

[0143] This set of embodiments is based on Embodiment 1, except that the porosity of the first negative electrode coating is changed, as detailed in Table 9.

[0144] Table 9 Comparative Example 1 This embodiment is based on Embodiment 1, except that the first negative electrode active layer is not layered, the porosity of the first negative electrode active layer is 42.5%, and the weight content of silicon-based particles in the first negative electrode active layer is 46.5%.

[0145] Comparative Example 2 This embodiment is based on Embodiment 1, except that the porosity of the first negative electrode coating is less than that of the second negative electrode coating. The porosity of the first negative electrode coating is 42.5%, and the porosity of the second negative electrode coating is 44.6%. The average particle size of the first silicon-based particles is 9.7 μm, and the average particle size of the second silicon-based particles is 12 μm. The average particle size of the first silicon-based particles is less than that of the second silicon-based particles.

[0146] Test case The batteries prepared in the examples and comparative examples were subjected to the following performance tests, and the test results are shown in Table 10: (1) Capacity retention rate during 25℃ cycling: At 25℃±2℃, the battery was charged at a constant current and constant voltage of 0.7C to the upper limit voltage of 4.53V, then cut off at 0.05C, and then discharged at a constant current of 0.2C to the lower limit voltage of 3V. The initial discharge capacity is denoted as C. 1 After 10 minutes of rest, the cycle is as follows: 3C constant current / constant voltage charging to 4.25V, cut off at 2C, then 2C constant current / constant voltage charging to 4.48V, cut off at 1.5C, then 1.5C constant current / constant voltage charging to the upper limit voltage of 4.53V, cut off at 0.18C, rest for 5 minutes, and then discharge at 0.7C to the lower limit voltage of 3V. After 500 cycles, 0.7C constant current / constant voltage charging is performed to the upper limit voltage of 4.53V, cut off at 0.05C, and then 0.2C constant current discharge is performed to the lower limit voltage of 3V. The final discharge capacity is recorded as C. 2 Capacity retention rate = (C 2 / C 1 )×100%, (2) Lithium plating after 3C fast charging: In an environment of 25±2℃, charge at 3C DC until 0.05C is cut off, let stand for 5 minutes, discharge at 0.7C to 3V, repeat the above steps 20 times, then disassemble the battery and remove the negative electrode. The evaluation criteria for the lithium plating of the negative electrode are as follows: no lithium plating is grade 0, lithium plating area on the negative electrode surface is less than or equal to 2% is grade 1, lithium plating area on the negative electrode surface is greater than 2% and less than or equal to 5% is grade 2, lithium plating area on the negative electrode surface is greater than 5% and less than or equal to 10% is grade 3, lithium plating area on the negative electrode surface is greater than 10% and less than or equal to 20% is grade 4, and lithium plating area on the negative electrode surface is greater than 20% is grade 5.

[0147] (3) Volumetric energy density: The battery was left to stand for 1 hour at (25±2)℃. It was then charged at a constant current of 0.5C to 4.53V, and then charged at a constant voltage of 4.53V to a current of 0.05C, and left to stand for 10 minutes. Next, it was discharged at a constant current of 0.2C to 3V and left to stand for 10 minutes. The discharge capacity was recorded as Q, the average discharge plateau voltage as G, the lithium-ion battery thickness as H, the lithium-ion battery length as Y, the lithium-ion battery width as Z, and the volumetric energy density as (Q×G) / (H×Y×Z), in Wh / L.

[0148] Table 10 As can be seen from Table 10, by comparing the comparative example and the embodiment, the room temperature cycle capacity retention rate of the embodiment is improved, the lithium plating situation is significantly improved, and a high volumetric energy density is guaranteed. This indicates that by setting the first negative electrode active layer corresponding to the single-sided positive electrode in the stacked electrode assembly to a structure in which the porosity increases layer by layer from the surface of the negative electrode current collector towards the direction away from the current collector, the problem of lithium plating in the negative electrode region corresponding to the single-sided positive electrode can be improved, thereby improving the cycle capacity retention rate and fast charging performance of the battery, and the volumetric energy density is effectively maintained.

[0149] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. An electrode assembly, characterized in that, The electrode assembly includes a positive electrode sheet, a separator, and a negative electrode sheet stacked together. In the thickness direction of the electrode assembly, the positive electrode sheet includes two single-sided positive electrode sheets located on the outermost side of the electrode assembly and at least one double-sided positive electrode sheet located inside the electrode assembly. The single-sided positive electrode sheet includes a positive current collector and a positive active layer located on the side surface of the positive current collector near the center of the electrode assembly. The double-sided positive electrode sheet includes a positive current collector and positive active layers located on both sides of the positive current collector. The negative electrode sheet includes a negative electrode current collector and negative electrode active layers located on both sides of the negative electrode current collector. The negative electrode active layer corresponding to the single-sided positive electrode sheet is the first negative electrode active layer, and the negative electrode active layer corresponding to the double-sided positive electrode sheet is the second negative electrode active layer. The first negative electrode active layer includes at least two negative electrode coating layers stacked along the thickness direction of the negative electrode sheet. The porosity of each negative electrode coating layer increases progressively from the surface of the negative electrode current collector toward the direction away from the negative electrode current collector.

2. The electrode assembly according to claim 1, wherein, The first negative electrode active layer includes 2 to 4 negative electrode coating layers stacked along the thickness direction of the negative electrode sheet; And / or, the difference in porosity between two adjacent negative electrode coating layers is 5%-15%; And / or, the porosity of the second negative electrode active layer is 25%-40%.

3. The electrode assembly according to claim 1, wherein, The first negative electrode active layer includes a first negative electrode coating and a second negative electrode coating stacked along the thickness direction of the negative electrode sheet. The second negative electrode coating is located between the first negative electrode coating and the negative electrode current collector. The porosity of the first negative electrode coating is greater than that of the second negative electrode coating. Preferably, the porosity of the first negative electrode coating is 35%-50%, and / or the porosity of the second negative electrode coating is 25%-40%.

4. The electrode assembly according to claim 3, wherein, The first negative electrode coating includes first silicon-based particles, and the second negative electrode coating includes second silicon-based particles. The average particle size of the first silicon-based particles is larger than the average particle size of the second silicon-based particles. Preferably, the average particle size of the first silicon-based particles is 5 μm-15 μm, and / or the average particle size of the second silicon-based particles is 3 μm-10 μm. And / or, the first negative electrode coating comprises first silicon-based particles, the second negative electrode coating comprises second silicon-based particles, the first silicon-based particles comprise spherical silicon-based particles, the second silicon-based particles comprise bulk silicon-based particles and optionally earth-shaped silicon-based particles; preferably, the sphericity of the spherical silicon-based particles is 0.8-0.99; preferably, in the second negative electrode coating, the weight ratio of the spherical silicon-based particles to the weight of the bulk silicon-based particles is (1-2.5):1; And / or, the first negative electrode coating includes first graphite particles, the second negative electrode coating includes second graphite particles, and the average particle size of the first graphite particles is greater than the average particle size of the second graphite particles; preferably, the average particle size of the first graphite particles is 15μm-30μm, and / or, the average particle size of the second graphite particles is 10μm-25μm. And / or, the first negative electrode coating includes first silicon-based particles, the second negative electrode coating includes second silicon-based particles, and the weight content of the first silicon-based particles in the first negative electrode coating is greater than the weight content of the second silicon-based particles in the second negative electrode coating; preferably, the weight content of the first silicon-based particles in the first negative electrode coating is 10%-50%, and / or, the weight content of the second silicon-based particles in the second negative electrode coating is 5%-25%.

5. The electrode assembly according to claim 1, wherein, The surface resistance of the first negative electrode active layer is less than that of the second negative electrode active layer; And / or, the sheet resistance of the first negative electrode active layer is 10 Ω·cm 2 -18Ω·cm 2 ; And / or, the sheet resistance of the second negative electrode active layer is 18 Ω·cm 2 -30Ω·cm 2 ; And / or, the compaction density of the first negative electrode active layer is less than the compaction density of the second negative electrode active layer; And / or, the compaction density of the first negative electrode active layer is 1.35 g / cm³. 3 -1.7g / cm 3 ; And / or, the compaction density of the second negative electrode active layer is 1.5 g / cm³. 3 -1.75g / cm 3 .

6. The electrode assembly according to claim 1, wherein, The first negative electrode active layer includes a third silicon-based particle, the second negative electrode active layer includes a fourth silicon-based particle, and the electrode assembly satisfies the following relationship: S 1 2 , of which S 1 =(D 19 -D 11 ) / D 15 S 2 =(D 29 -D 21 ) / D 25 D 19 D 15 D 11 These are the Dv90, Dv50, and Dv10 of the third silicon-based particle, respectively, in μm. 29 D 25 D 21 These are Dv90, Dv50, and Dv10 of the fourth silicon-based particle, respectively, in μm;​ And / or, the first negative electrode active layer includes a third silicon-based particle, and the electrode assembly satisfies the relationship: 1.2 ≤ S 1 ≤9.3, where S 1 =(D 19 -D 11 ) / D 15 D 19 D 15 D 11 These are Dv90, Dv50, and Dv10 of the third silicon-based particle, respectively, in μm; And / or, the second negative electrode active layer includes a fourth silicon-based particle, and the electrode assembly satisfies the relationship: 1.4 ≤ S 2 ≤10.6, where S 2 =(D 29 -D 21 ) / D 25 D 29 D 25 D 21 These are Dv90, Dv50, and Dv10 of the fourth silicon-based particle, respectively, in μm; And / or, the first negative electrode active layer includes a third silicon-based particle, wherein the Dv50 of the third silicon-based particle is D 15 D 15 The thickness ranges from 4μm to 13μm. And / or, the second negative electrode active layer includes a fourth silicon-based particle, wherein the Dv50 of the fourth silicon-based particle is D 25 D 25 The range is 3μm-10μm.

7. The electrode assembly according to claim 1, wherein, The first negative electrode active layer includes a first negative electrode conductive agent, and the second negative electrode active layer includes a second negative electrode conductive agent. The first negative electrode conductive agent and the second negative electrode conductive agent each independently include at least one of carbon nanotubes, graphene, and conductive carbon black. Preferably, the first negative electrode conductive agent comprises carbon nanotubes and conductive carbon black, wherein the carbon nanotubes account for 10%-50% of the weight of the first negative electrode conductive agent; Preferably, the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is greater than the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer; more preferably, the weight percentage of the first negative electrode conductive agent in the first negative electrode active layer is 1.2%-3%, and / or, the weight percentage of the second negative electrode conductive agent in the second negative electrode active layer is 1%-2%.

8. The electrode assembly according to claim 1, wherein, The first negative electrode active layer includes a first negative electrode binder, and the second negative electrode active layer includes a second negative electrode binder. The first negative electrode binder and the second negative electrode binder each independently include at least one of polyacrylic acid, sodium alginate, polyimide, styrene-butadiene rubber, and sodium carboxymethyl cellulose. Preferably, the first negative electrode binder includes at least one of polyacrylic acid, sodium alginate, and styrene-butadiene rubber; more preferably, the first negative electrode binder includes polyacrylic acid and styrene-butadiene rubber, wherein the polyacrylic acid accounts for 30%-70% of the weight of the first negative electrode binder; Preferably, the second negative electrode binder includes at least one of styrene-butadiene rubber and sodium carboxymethyl cellulose; Preferably, the weight percentage of the first negative electrode binder in the first negative electrode active layer is greater than the weight percentage of the second negative electrode binder in the second negative electrode active layer; more preferably, the weight percentage of the first negative electrode binder in the first negative electrode active layer is 2.5%-4.5%; and / or, the weight percentage of the second negative electrode binder in the second negative electrode active layer is 1.5%-3%.

9. The electrode assembly according to any one of claims 1-8, wherein, The negative electrode current collector includes a first negative electrode current collector and a second negative electrode current collector. One side surface of the first negative electrode current collector is the first negative electrode active layer and the other side surface is the second negative electrode active layer. Both sides of the second negative electrode current collector are the second negative electrode active layer. The thickness of the first negative electrode current collector is greater than the thickness of the second negative electrode current collector. Preferably, the thickness of the first negative electrode current collector is 7μm-12μm, and / or the thickness of the second negative electrode current collector is 5μm-8μm. Preferably, the first negative electrode current collector and the second negative electrode current collector are copper foils. And / or, the separator includes a first separator and a second separator, the first separator corresponding to a single-sided positive electrode and the second separator corresponding to a double-sided positive electrode, the porosity of the first separator being greater than that of the second separator; preferably, the porosity of the first separator is 45%-55%, and / or, the porosity of the second separator is 37%-43%.

10. A battery, wherein, The battery includes an electrolyte and an electrode assembly according to any one of claims 1-9.

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