Positive electrode sheet, method for manufacturing the same, and battery

Abstract

The application relates to a positive electrode sheet and a preparation method and a battery thereof. The positive electrode sheet comprises a current collector, and a first active material layer and a second active material layer which are sequentially stacked on at least one side surface of the current collector in the thickness direction; the first active material layer is located between the current collector and the second active material layer; wherein the first active material layer comprises a first active material, the second active material layer comprises a second active material, the bulk modulus of the second active material is greater than that of the first active material, and the Young's modulus of the second active material is greater than that of the first active material; the thickness ratio of the first active material layer to the second active material layer is 1:4-4:1. In this way, the high-voltage-resistant second active material layer and the low-voltage-resistant first active material layer can realize good functional cooperation and produce significant synergistic effect, so that the cycle performance and storage performance of the battery can be effectively improved while the energy density and rate performance are considered.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode sheet, its preparation method, and a battery. Background Technology

[0002] To improve the energy density and rate performance of lithium-ion batteries, the art typically employs techniques to increase the compaction density of the cathode. On one hand, increasing the compaction density increases the content of active material per unit volume, thereby improving the battery's energy density. On the other hand, increasing the compaction density increases the contact area between active material particles and between active material particles and the current collector, which helps to reduce electron transport resistance, promote lithium-ion diffusion, reduce polarization, and improve the battery's rate performance.

[0003] However, when increasing the compaction density of the positive electrode, the active material particles are subjected to greater rolling pressure, which significantly increases the risk of breakage. In particular, the active material particles on the surface of the positive electrode are directly impacted by the rolling pressure and are more prone to breakage. This can lead to an aggravation of the side reactions between the active material and the electrolyte, and deteriorate the cycle performance of the battery. Summary of the Invention

[0004] In view of this, the present application provides a positive electrode sheet, a method for preparing the same, and a battery to solve at least one problem existing in the prior art.

[0005] In a first aspect, embodiments of this application provide a positive electrode sheet, including a current collector, and a first active material layer and a second active material layer sequentially stacked on at least one surface of the current collector along its thickness direction; the first active material layer is located between the current collector and the second active material layer; wherein...

[0006] The first active material layer includes a first active material, the second active material layer includes a second active material, the bulk modulus of the second active material is greater than the bulk modulus of the first active material, and the Young's modulus of the second active material is greater than the Young's modulus of the first active material.

[0007] The thickness ratio of the first active material layer to the second active material layer is 1:4 to 4:1.

[0008] In conjunction with the first aspect of this application, in an optional embodiment, the bulk modulus of the first active substance is 100 GPa to 115 GPa, and the bulk modulus of the second active substance is 115 GPa to 135 GPa; and / or, the Young's modulus of the first active substance is 55 GPa to 65 GPa, and the Young's modulus of the second active substance is 65 GPa to 80 GPa.

[0009] In conjunction with the first aspect of this application, in an optional embodiment, the sum of the thicknesses of the first active material layer and the second active material layer is 50 μm to 150 μm.

[0010] In conjunction with the first aspect of this application, in an optional embodiment, the first active material layer further includes a first adhesive, and the second active material layer further includes a second adhesive; the first adhesive and the second adhesive satisfy at least one of the following conditions: (a) the elongation at break of the second adhesive is greater than the elongation at break of the first adhesive; (b) the elongation at break of the first adhesive is 150% to 350%, and the elongation at break of the second adhesive is 300% to 400%.

[0011] In conjunction with the first aspect of this application, in an optional embodiment, the first active material comprises a first ternary material; the second active material comprises a second ternary material and / or lithium iron phosphate; the first ternary material and the second ternary material satisfy at least one of the following conditions: (a) the first ternary material and / or the second ternary material are single crystal particles; (b) the particle size D50 of the first ternary material and / or the second ternary material is 1.5 μm to 4.0 μm; (c) the surface of the first ternary material and / or the second ternary material has an alumina coating layer.

[0012] In conjunction with the first aspect of this application, in an optional embodiment, the first active material comprises a first lithium nickel cobalt manganese oxide ternary material, and the second active material comprises a second lithium nickel cobalt manganese oxide ternary material; the first lithium nickel cobalt manganese oxide ternary material and the second lithium nickel cobalt manganese oxide ternary material satisfy at least one of the following conditions: (a) the molar percentage of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material is higher than the molar percentage of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material; (b) the molar percentage of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material is 5% to 20%, and the molar percentage of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material is 20% to 40%.

[0013] In conjunction with the first aspect of this application, in an optional embodiment, the first active material comprises a first lithium nickel cobalt manganese oxide ternary material, and the second active material comprises a second lithium nickel cobalt manganese oxide ternary material;

[0014] The second lithium nickel cobalt manganese oxide ternary material includes a doping element, wherein the doping element has a valence state of tetravalent or higher; the doping element satisfies at least one of the following conditions: (a) the doping element includes at least one of Mo, Ta, Nb, and Ti; (b) the content of the doping element is 500ppm to 5000ppm.

[0015] Secondly, embodiments of this application provide a method for preparing a positive electrode sheet as described in any of the first aspects, characterized in that the method includes the following steps:

[0016] The first active material, the first binder and the first conductive agent are added to the first solvent and mixed evenly to obtain the first slurry;

[0017] The second active material, the second binder, and the second conductive agent are added to the second solvent and mixed evenly to obtain the second slurry.

[0018] The first slurry and the second slurry are laminated onto at least one side surface of the current collector along the thickness direction, with the first slurry being closer to the current collector than the second slurry. After drying, the positive electrode sheet is obtained.

[0019] In conjunction with the second aspect of this application, in an alternative embodiment, the method satisfies at least one of the following features:

[0020] (1) The mass ratio of the first active material, the first binder and the first conductive agent is (94~98):(0.5~3):(0.5~3);

[0021] (2) The mass ratio of the second active material, the second binder and the second conductive agent is (94~98):(0.5~3):(0.5~3);

[0022] (3) The first adhesive and the second adhesive each independently comprise at least one of polytetrafluoroethylene, polyvinylidene fluoride, and carboxymethyl cellulose;

[0023] (4) The first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers;

[0024] (5) The first solvent and the second solvent each independently comprise N-methylpyrrolidone;

[0025] (6) The solid content of the first slurry and / or the second slurry is 68%~75%;

[0026] (7) The viscosity of the first slurry and / or the second slurry is 6000 mPa·s to 10000 mPa·s;

[0027] (8) The areal density of the first slurry and / or the second slurry coating is 100 g / m². 2 ~500g / m 2 .

[0028] Thirdly, embodiments of this application provide a battery comprising a positive electrode sheet prepared by any of the methods described in the first aspect or the second aspect.

[0029] Compared with the prior art, the embodiments of this application have the following beneficial effects:

[0030] The positive electrode sheet, its preparation method, and battery provided in this application embodiment include a current collector and a first active material layer and a second active material layer sequentially stacked on at least one side surface of the current collector along its thickness direction. The first active material layer is located between the current collector and the second active material layer. The first active material layer includes a first active material, and the second active material layer includes a second active material. The bulk modulus of the second active material is greater than that of the first active material, and the Young's modulus of the second active material is greater than that of the first active material. That is, the pressure resistance of the second active material is higher than that of the first active material. This results in the mechanical strength and pressure resistance of the second active material layer located on the surface of the positive electrode sheet being significantly higher than that of the first active material layer. This allows for increased compaction density through higher rolling pressure while effectively reducing the breakage of the second active material during the rolling process. Since the first active material layer is buffered and protected by the second active material layer, a material with relatively lower pressure resistance can be selected, further ensuring compaction density while reducing costs. When the first active material layer and... When the thickness ratio of the second active material layer is too large, the thickness of the second active material layer in the entire active material layer is relatively small, making it difficult to effectively reduce the breakage of the second active material during the rolling process. This results in the battery's cycle performance not being effectively improved. When the thickness ratio of the first active material layer to the second active material layer is too small, although the thickness of the second active material layer in the entire active material layer is relatively large, it can reduce the breakage of the second active material during the rolling process, reduce side reactions, and thus improve the battery's cycle performance and storage performance. However, because the thickness ratio of the first active material layer in the entire active material layer is relatively small, it is not conducive to ensuring the overall compaction density of the positive electrode sheet, thus making it difficult to balance the battery's energy density. Therefore, in this embodiment, the thickness ratio of the first active material layer to the second active material layer is controlled at 1:4 to 4:1. In this way, the high-voltage second active material layer and the low-voltage first active material layer can achieve better functional coordination and produce a significant synergistic effect, thereby effectively improving the battery's cycle performance and storage performance while also balancing energy density and rate performance.

[0031] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0032] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0033] Figure 1 A schematic flowchart illustrating a method for preparing a positive electrode sheet according to an embodiment of this application;

[0034] Figure 2 This is a scanning electron microscope image of the ternary material on the surface of the positive electrode of the battery corresponding to Example 1 after a 45°C cycle performance test;

[0035] Figure 3 The image shows a scanning electron microscope (SEM) image of the ternary material on the surface of the positive electrode of the battery corresponding to Comparative Example 1 after a 45°C cycle performance test.

[0036] Figure 4 The image shows a scanning electron microscope (SEM) image of the ternary material on the surface of the positive electrode of the battery corresponding to Comparative Example 6 after a 45°C cycle performance test. Detailed Implementation

[0037] To make the technical solution and beneficial effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional experimental conditions. Unless otherwise specified, all reagents and raw materials used in this invention are commercially available.

[0038] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of this application. However, it will be apparent to those skilled in the art that this application can be practiced without one or more of these details. In other instances, to avoid confusion with this application, some technical features well-known in the art have not been described; that is, not all features of actual embodiments are described herein, nor are well-known functions and steps described in detail.

[0039] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. When used herein, the singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” and / or “including,” when used in this specification, identify the presence of the stated features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups. When used herein, the term “and / or” includes any and all combinations of the associated listed items.

[0040] To fully understand this application, detailed steps and structures will be presented in the following description to illustrate the technical solution of this application. Preferred embodiments of this application are described in detail below; however, in addition to these detailed descriptions, this application may have other implementation methods.

[0041] Unless otherwise defined, the technical and scientific terms used in this application have the same meanings as those in the technical and scientific field to which this application pertains.

[0042] Unless otherwise specified, the techniques or conditions described in the following embodiments are generally performed in accordance with conventional techniques or conditions described in the literature in this field, or in accordance with the product manual and the manufacturer's recommendations. All numerical ranges in the following embodiments include endpoint values.

[0043] This application provides a positive electrode sheet, which includes a current collector and a first active material layer and a second active material layer sequentially stacked on at least one side surface of the current collector along its thickness direction; the first active material layer is located between the current collector and the second active material layer; wherein the first active material layer includes a first active material, the second active material layer includes a second active material, the bulk modulus of the second active material is greater than the bulk modulus of the first active material, the Young's modulus of the second active material is greater than the Young's modulus of the first active material, and the thickness ratio of the first active material layer to the second active material layer is 1:4 to 4:1.

[0044] Bulk modulus represents a material's resistance to compression, while Young's modulus represents its stiffness. Higher bulk modulus and Young's modulus indicate better mechanical properties and higher pressure resistance. In this embodiment, the pressure resistance of the second active material is higher than that of the first active material. This results in the mechanical strength and pressure resistance of the second active material layer located on the surface of the positive electrode being significantly higher than that of the first active material layer. This allows for increased compaction density through higher rolling pressure while effectively reducing the breakage of the second active material during the rolling process. Since the first active material layer is buffered and protected by the second active material layer, a material with relatively lower pressure resistance can be selected, further ensuring compaction density while reducing costs. When the thickness ratio of the first and second active material layers is too large, the second active material layer accounts for a small proportion of the total active material layer thickness, making it difficult to effectively reduce the breakage of the second active material during the rolling process. This results in ineffective improvement in the battery's cycle performance. When the thickness ratio of the first active material layer to the second active material layer is too small, although the second active material layer accounts for a larger proportion of the total active material layer thickness, which can reduce the breakage of the second active material during the rolling process and reduce side reactions, thereby improving the cycle performance and storage performance of the battery, the first active material layer accounts for a smaller proportion of the total active material layer thickness, which is not conducive to ensuring the overall compaction density of the positive electrode sheet, thus making it difficult to balance the energy density of the battery. Therefore, in the embodiments of this application, the thickness ratio of the first active material layer to the second active material layer is controlled at 1:4 to 4:1. In this way, the high-voltage second active material layer and the low-voltage first active material layer can achieve better functional coordination and produce a significant synergistic effect, thereby effectively improving the cycle performance and storage performance of the battery while balancing energy density and rate performance.

[0045] The current collector is not particularly limited in this embodiment; any current collector well-known to those skilled in the art and suitable for positive electrode sheets can be used. For example, the current collector can be at least one of aluminum foil, porous aluminum foil, or carbon-coated aluminum foil. The first active material layer and the second active material layer can be sequentially stacked on one surface of the current collector along the thickness direction, or they can be sequentially stacked on both surfaces of the current collector along the thickness direction.

[0046] For example, the thickness ratio of the first active material layer and the second active material layer can be 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1 or any value between any two of the above ranges.

[0047] In some embodiments, the sum of the thicknesses of the first active material layer and the second active material layer can be 50 μm to 150 μm, for example, 50 μm, 75 μm, 100 μm, 125 μm, 150 μm, or any value between any two of the above ranges. This helps to ensure the energy density and cycle performance of the battery.

[0048] In some embodiments, the bulk modulus of the first active substance can be 100 GPa to 115 GPa, for example, 100 GPa, 105 GPa, 110 GPa, 115 GPa or any value between any two of the above ranges, and the bulk modulus of the second active substance can be 115 GPa to 135 GPa, for example, 115 GPa, 120 GPa, 125 GPa, 130 GPa, 135 GPa or any value between any two of the above ranges.

[0049] Controlling the bulk modulus of the first and second active materials within the aforementioned range helps ensure a suitable transition in the withstand voltage of the first and second active materials, thereby improving the overall compaction uniformity of the positive electrode sheet and ultimately enhancing its comprehensive performance.

[0050] In some embodiments, the Young's modulus of the first active substance can be 55 GPa to 65 GPa, for example, 55 GPa, 60 GPa, 65 GPa or any value between any two of the above ranges, and the Young's modulus of the second active substance can be 65 GPa to 80 GPa, for example, 65 GPa, 70 GPa, 75 GPa, 80 GPa or any value between any two of the above ranges.

[0051] Controlling the Young's modulus of the first and second active materials within the aforementioned range helps ensure a suitable transition in the withstand voltage of the first and second active materials, thereby improving the overall compaction uniformity of the cathode sheet and ultimately enhancing its comprehensive performance.

[0052] In some embodiments, the first active material may include a first ternary material; the second active material may include a second ternary material and / or lithium iron phosphate.

[0053] Ternary materials possess high theoretical specific capacity and a high voltage plateau, as well as high compaction density. Therefore, including ternary materials in both the first and second active materials helps to better ensure the battery's energy density. Lithium iron phosphate generally has a small particle size (D50 approximately 0.5μm~2.0μm), a bulk modulus of approximately 120GPa, and a Young's modulus of approximately 72GPa, exhibiting good voltage resistance. Therefore, including lithium iron phosphate in the second active material helps to ensure the voltage resistance level of the second active material layer. Furthermore, the surface of lithium iron phosphate can be coated with a carbon coating layer, resulting in better conductivity and greater resistance to high currents, which further enhances the battery's rate performance.

[0054] For example, the first active material may be a first ternary material; the second active material may be a second ternary material and / or lithium iron phosphate.

[0055] In some specific embodiments, the first active material is a first ternary material; the second active material is a second ternary material.

[0056] The mechanical properties of ternary materials are mainly affected by their composition and are also affected by differences in unit cell orientation (specifically, for example, by controlling the orientation of high-orientation precursors). Therefore, when both the first and second active materials are ternary materials, the first and second active materials can have different mechanical properties by adjusting the composition of the ternary materials and controlling the unit cell orientation, thereby satisfying that the bulk modulus of the second active material is greater than that of the first active material, and the Young's modulus of the second active material is greater than that of the first active material.

[0057] Optionally, the first ternary material and / or the second ternary material are single-crystal particles. Single-crystal particles have a more stable structure, higher compaction density, and better cycle performance. Therefore, having one or both of the first and second ternary materials as single-crystal particles can better balance the energy density and cycle performance of the battery.

[0058] In a specific example, both the first ternary material and the second ternary material are single-crystal particles.

[0059] Optionally, the particle size D50 of the first ternary material and / or the second ternary material can be 1.5 μm to 4.0 μm, for example, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, or any value between any two of the above ranges. This helps to ensure the material's pressure resistance and compaction density, thereby improving the battery's cycle performance and energy density.

[0060] In a specific example, the particle size D50 of both the first ternary material and the second ternary material is 1.5 μm to 4.0 μm.

[0061] Optionally, the surfaces of the first ternary material and / or the second ternary material have an alumina coating. The alumina coating can block the contact between the ternary material and the electrolyte and the outside air, thereby reducing side reactions, ensuring the capacity of the ternary material, and improving the energy density and cycle performance of the battery.

[0062] In a specific example, both the first ternary material and the second ternary material have an aluminum oxide coating on their surfaces.

[0063] In some specific embodiments, the content of the alumina coating layer on the surface of the first ternary material and / or the second ternary material can be 500 ppm to 50,000 ppm. That is, the mass percentage of the alumina coating layer in the first ternary material or the second ternary material is 0.05 wt% to 5 wt%, for example, it can be 0.05 wt%, 0.15 wt%, 0.25 wt%, 0.35 wt%, 0.45 wt%, 5 wt%, or any value between any two of the above ranges.

[0064] In some embodiments, the first active material may include a first lithium nickel cobalt manganese oxide ternary material, and the second active material may include a second lithium nickel cobalt manganese oxide ternary material; the molar ratio of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material is higher than the molar ratio of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material.

[0065] The molar percentage of Mn in Ni, Co, and Mn mentioned above is the normalized molar amount of Mn, which is (molecular amount of Mn / (molecular amount of Ni + mole amount of Co + mole amount of Mn)) × 100%.

[0066] In lithium nickel cobalt manganese oxide ternary materials, the higher the valence state of the transition metal, the stronger the chemical bond between it and oxygen. That is, the chemical bond strength between Mn and O, the chemical bond strength between Co and O, and the chemical bond strength between Ni and O increase in that order. Thus, by setting the molar ratio of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material to be higher than that in the first lithium nickel cobalt manganese oxide ternary material, the first and second active materials can have different mechanical properties. It is easy to achieve that the bulk modulus of the second active material is greater than that of the first active material, and the Young's modulus of the second active material is greater than that of the first active material.

[0067] Optionally, the molar percentage of Mn in the first lithium nickel cobalt manganese oxide ternary material in Ni, Co, and Mn can be 5% to 20%, for example, 5%, 10%, 15%, 20%, or any value between any two of the above ranges. The molar percentage of Mn in the second lithium nickel cobalt manganese oxide ternary material in Ni, Co, and Mn can be 20% to 40%, for example, 20%, 25%, 30%, 35%, 40%, or any value between any two of the above ranges.

[0068] By controlling the molar ratio of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material and the molar ratio of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material within the aforementioned range, the bulk modulus and Young's modulus of the first and second active materials can be controlled within a suitable range. This is beneficial for ensuring that the withstand voltage of the second active material is higher than that of the first active material, while also allowing for a suitable transition in the withstand voltage of the first and second active materials. This is beneficial for improving the overall compaction uniformity of the cathode sheet, and thus for improving the comprehensive performance of the cathode sheet.

[0069] In some embodiments, the first active material includes a first lithium nickel cobalt manganese oxide ternary material, and the second active material includes a second lithium nickel cobalt manganese oxide ternary material; the second lithium nickel cobalt manganese oxide ternary material includes a dopant element, and the dopant element has a valence state of tetravalent or higher.

[0070] In some specific embodiments, the first active material can be a first lithium nickel cobalt manganese oxide ternary material, and the second active material can be a second lithium nickel cobalt manganese oxide ternary material.

[0071] Optionally, the doping element includes at least one of Mo, Ta, Nb, and Ti. For example, the doping element is at least one of Mo, Ta, Nb, and Ti.

[0072] By doping the second nickel-cobalt-manganese ternary material with high-valence ions (e.g., Mo) with valence states of four or higher (e.g., Mo) 6+ Ta 5+ 、Nb 5+ Ti 4+ (etc.) can introduce oxygen vacancies, suppress lattice slip, thereby increasing the Young's modulus of the material, which can improve the pressure resistance of the second active material.

[0073] When the content of the dopant element is too low, its effect on improving the bulk modulus and elastic modulus of the material is limited; when the content of the dopant element is too high, it will reduce the compaction density of the material, thereby affecting the energy density of the battery. Therefore, in some embodiments, the content of the dopant element can be 500ppm to 5000ppm, for example, 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm or any value between any two of the above ranges.

[0074] In some embodiments, the first active material layer may further include a first adhesive, and the second active material layer may further include a second adhesive, wherein the elongation at break of the second adhesive is greater than that of the first adhesive.

[0075] The second active material in the second active material layer has high pressure resistance. Combined with the second binder with higher elongation at break, it can better improve the mechanical strength and pressure resistance of the second active material layer, enhance the buffering capacity against rolling pressure, thereby further reducing the breakage of the second active material during the rolling process, and thus further improving the cycle performance of the battery.

[0076] In some specific embodiments, the elongation at break of the first binder can be 150% to 350%, for example, 150%, 200%, 250%, 300%, 350%, or any value between any two of the above ranges; the elongation at break of the second binder can be 300% to 400%, for example, 300%, 350%, 400%, or any value between any two of the above ranges. This helps to control the withstand voltage levels of the first and second active material layers at appropriate levels, while ensuring the compaction density of the positive electrode sheet, thereby better balancing the cycle performance and energy density of the battery.

[0077] This application also provides a method for preparing a positive electrode sheet as described in any of the foregoing embodiments. Please refer to... Figure 1 The method for preparing the positive electrode sheet provided in this application includes the following steps:

[0078] S1: Add the first active material, the first binder and the first conductive agent to the first solvent and mix them evenly to obtain the first slurry;

[0079] S2: Add the second active material, the second binder, and the second conductive agent to the second solvent and mix them evenly to obtain the second slurry;

[0080] S3: The first slurry and the second slurry are laminated on at least one side of the current collector along the thickness direction, with the first slurry being closer to the current collector than the second slurry. After drying, a positive electrode sheet is obtained.

[0081] In this embodiment, a first slurry comprising a first active material and a second slurry comprising a second active material are first prepared. Then, the first and second slurries are coated onto at least one side surface of the current collector along its thickness direction at predetermined positions, thereby forming a first active material layer and a second active material layer sequentially stacked on at least one side surface of the current collector along its thickness direction. The preparation method of this embodiment is simple and easy to implement, and the resulting positive electrode sheet has the beneficial effects of the positive electrode sheet described in any of the foregoing embodiments.

[0082] It should be noted that although the steps in the flowchart above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Moreover, at least some of the steps in the flowchart above may include multiple steps or stages, and these steps or stages are not necessarily completed at the same time, nor are they necessarily performed sequentially.

[0083] In some embodiments, in step S1, the mass ratio of the first active material, the first binder, and the first conductive agent can be (94~98):(0.5~3):(0.5~3), for example, 94:0.5:0.5, 95:2:3, 96:1.5:2.5, 97:1:2, 98:3:3, or any other ratio within the above range. This helps to ensure the structural stability and electrochemical performance of the first active material layer subsequently formed by coating the first slurry.

[0084] For example, the first conductive agent may include at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers. Specifically, the first conductive agent may be at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers.

[0085] For example, the first solvent may include N-methylpyrrolidone. Specifically, the first solvent may be N-methylpyrrolidone.

[0086] In some embodiments, the solid content of the first slurry can be 68% to 75%, for example, it can be 68%, 70%, 72%, 74%, 75% or any value between any two of the above ranges.

[0087] In some embodiments, the viscosity of the first slurry can be 6000 mPa·s to 10000 mPa·s, for example, it can be 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s or any value between any two of the above ranges.

[0088] Controlling at least one of the solid content and viscosity of the first slurry within the above-mentioned range facilitates the execution of subsequent coating processes and helps to improve the quality and performance of the formed first active material layer.

[0089] In some embodiments, the areal density of the first slurry coating can be 100 g / m². 2 ~500g / m 2 For example, it can be 100g / m 2 200g / m 2 300g / m 2 400g / m 2 500g / m 2 Or any value between any two of the above ranges. This helps to control the areal density of the first active material layer within a suitable range, thereby ensuring the compaction density of the final positive electrode sheet.

[0090] In some embodiments, in step S2, the mass ratio of the second active material, the second binder, and the second conductive agent is (94~98):(0.5~3):(0.5~3), for example, it can be 94:0.5:0.5, 95:2:3, 96:1.5:2.5, 97:1:2, 98:3:3, or any other ratio within the above range. This helps to ensure the structural stability and electrochemical performance of the second active material layer subsequently formed by coating the second slurry.

[0091] For example, the second conductive agent may include at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers. Specifically, the second conductive agent may be at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers.

[0092] In some embodiments, the second solvent may include N-methylpyrrolidone. Exemplarily, the second solvent may be N-methylpyrrolidone.

[0093] In some embodiments, the solid content of the second slurry can be 68% to 75%, for example, 68%, 70%, 72%, 74%, 75% or any value between any two of the above ranges.

[0094] In some embodiments, the viscosity of the second slurry can be 6000 mPa·s to 10000 mPa·s, for example, it can be 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s or any value between any two of the above ranges.

[0095] Controlling at least one of the solid content and viscosity of the second slurry within the above-mentioned range facilitates the execution of subsequent coating processes and helps to improve the quality and performance of the formed second active material layer.

[0096] In some embodiments, the areal density of the second slurry coating can be 100 g / m². 2 ~500g / m 2 For example, it can be 100g / m 2 200g / m 2 300g / m 2 400g / m 2 500g / m 2 Or any value between any two of the above ranges. This helps to control the areal density of the second active material layer within a suitable range, thereby ensuring the compaction density of the final positive electrode sheet.

[0097] In step S3 above, the first slurry and the second slurry can be coated separately. For example, the first slurry can be coated on at least one side surface of the current collector along the thickness direction, and after drying, a first active material layer can be formed; then the second slurry can be coated on the first active material layer, and after drying, a second active material layer can be formed. The first slurry and the second slurry can also be coated simultaneously. For example, a double-layer die can be used to simultaneously coat the first slurry and the second slurry at a set position, and after drying, a first active material layer and a second active material layer can be sequentially stacked on the current collector.

[0098] In step S3, the drying temperature can be between 90℃ and 150℃, for example, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, or any value within any two of the above ranges. The drying time can be between 30s and 300s, for example, 30s, 50s, 100s, 150s, 200s, 250s, 300s, or any value within any two of the above ranges. This helps ensure the efficiency of the drying process and avoids affecting the materials in the active material layer.

[0099] In some embodiments, step S3 may include rolling, die-cutting and punching steps after drying to obtain a positive electrode sheet that meets the requirements.

[0100] In some embodiments, the first active material in step S1 may include a first lithium nickel cobalt manganese oxide ternary material, and the second active material in step S2 may include a second lithium nickel cobalt manganese oxide ternary material; the preparation method of the first lithium nickel cobalt manganese oxide ternary material and the second lithium nickel cobalt manganese oxide ternary material may include the following steps:

[0101] Step S11: Under a protective gas atmosphere, nickel salt, cobalt salt, and manganese salt are added to deionized water to obtain a mixed solution. A precipitant and a complexing agent are then added to the mixed solution to carry out the reaction.

[0102] For example, the protective gas can be an inert gas (specifically, nitrogen, helium, argon, etc.).

[0103] For example, the nickel salt can be at least one of nickel sulfate (NiSO4·6H2O), nickel nitrate (Ni(NO3)2·6H2O), nickel chloride (NiCl2·6H2O), nickel acetate (Ni(CH3COO)2·4H2O), and nickel hydroxide (Ni(OH)2).

[0104] For example, the cobalt salt can be at least one of cobalt sulfate (CoSO4·7H2O), cobalt nitrate (Co(NO3)2·6H2O), cobalt chloride (CoCl2·6H2O), and cobalt hydroxide (Co(OH)2).

[0105] For example, the manganese salt can be at least one of manganese sulfate (MnSO4·H2O), manganese nitrate (Mn(NO3)2·4H2O), manganese chloride (MnCl2·4H2O), and manganese hydroxide (Mn(OH)2).

[0106] For example, the precipitant can be at least one of sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH3·H2O), sodium carbonate (Na2CO3), and ammonium bicarbonate (NH4HCO3).

[0107] For example, the complexing agent may be at least one of ammonia (NH3·H2O), citrate (specifically, sodium citrate), oxalate (specifically, ammonium oxalate), and ethylenediaminetetraacetic acid (EDTA).

[0108] Step S12: The reaction product of step S11 is subjected to aging, settling, filtration, washing and drying in sequence to obtain the precursor.

[0109] Step S13: Mix the precursor obtained in step S12 with lithium salt and place it in an oxygen-containing atmosphere for the first calcination. After the calcination is completed, perform a cooling treatment to obtain a pre-sintered material.

[0110] For example, the lithium salt can be at least one of lithium hydroxide (LiOH·H2O), lithium carbonate (Li2CO3), and lithium nitrate (LiNO3).

[0111] For example, the temperature of the first calcination can be 600℃~1200℃, such as 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, 1200℃ or any value between any two of the above ranges.

[0112] In some embodiments, in step S13, after the precursor is mixed with the lithium salt, a metal doping source (the doping element in the metal doping source has a valence state of tetravalent or higher) can be added to dop the final nickel cobalt manganese oxide ternary material with an element having a valence state of tetravalent or higher, thereby increasing the Young's modulus of the material.

[0113] Step S14: The pre-sintered material obtained in step S13 is placed in an oxygen-containing atmosphere for a second calcination to obtain a ternary material containing Ni, Co and Mn (i.e., lithium nickel cobalt manganese oxide ternary material).

[0114] For example, the temperature of the second calcination can be 300℃~500℃, such as 300℃, 400℃, 500℃ or any value between any two of the above ranges.

[0115] In some embodiments, in step S14, an alumina coating layer may also be formed on the surface of the lithium nickel cobalt manganese oxide ternary material. The alumina coating layer can block the contact between the ternary material and the electrolyte and the outside air, thereby reducing side reactions, ensuring the capacity of the ternary material, and improving the energy density and cycle performance of the battery.

[0116] In the above embodiments, a nickel-cobalt-manganese ternary material (NCM ternary material) with elemental content conforming to a preset stoichiometric ratio and a stable layered structure can be prepared by using a co-precipitation and two-step calcination method.

[0117] This application also provides a battery comprising a positive electrode sheet as described in any of the foregoing embodiments or a positive electrode sheet prepared by the method described in any of the foregoing embodiments.

[0118] The battery in this application is a secondary battery. Furthermore, the secondary battery can be a lithium-ion battery.

[0119] It should be understood that the beneficial effects of the positive electrode in the above embodiments are all applicable to the battery.

[0120] The technical solution of this application will be further described below with reference to several embodiments and comparative examples.

[0121] First, prepare ternary materials A and F, which are to be used in the examples and comparative examples.

[0122] The preparation steps of ternary material A are as follows: First, under an argon gas atmosphere, Ni is... 2+ Co 2+ and Mn 2+ A mixed solution (i.e., a mixed aqueous solution of nickel salt NiSO4·6H2O, cobalt salt CoSO4·7H2O, and manganese salt MnSO4·H2O), sodium hydroxide as a precipitant, and ammonia as a complexing agent were mixed and reacted. The reaction product was then subjected to aging, settling, filtration, washing, and drying to obtain a precursor. Next, the precursor was mixed with lithium hydroxide (the molar ratio of the precursor to lithium in the lithium salt was 1:1.08), and MoO3, a metal dopant source, was added (the amount added made the Mo doping content 3000ppm). The mixture was then placed in an oxygen-containing atmosphere and calcined at 850℃ for the first time. After calcination, the mixture was cooled to obtain a pre-sintered material. Next, the pre-sintered material was placed in an oxygen-containing atmosphere and calcined at 400℃ for the second time to prepare a ternary material A containing Ni, Co, and Mn. 2+ Co 2+ Mn 2+ The molar ratio is 33:33:33, and the surface coating mass percentage is 800 ppm of alumina coating.

[0123] The preparation steps of ternary material B are basically the same as those of ternary material A. The main difference lies in the adjustment of the amount of nickel salt, cobalt salt, and manganese salt added, so that the Ni content in the obtained ternary material B is... 2+ Co 2+ Mn 2+ The molar ratio is 40:20:40.

[0124] The preparation steps of ternary material C are basically the same as those of ternary material A. The main difference lies in the adjustment of the amount of nickel salt, cobalt salt, and manganese salt added, so that the Ni content in the obtained ternary material C is... 2+ Co 2+ Mn 2+ The molar ratio is 51.5:24:24.5.

[0125] The preparation steps of ternary material D are basically the same as those of ternary material A. The main difference lies in the adjustment of the amount of nickel salt, cobalt salt, and manganese salt added, so that the Ni content in the obtained ternary material D is... 2+ Co 2+ Mn 2+ The molar ratio is 58:20:22.

[0126] The preparation steps of ternary material E are basically the same as those of ternary material A, with the main difference being: firstly, the amount of nickel salt, cobalt salt, and manganese salt added is adjusted to increase the amount of Ni in the prepared ternary material E. 2+ Co 2+ Mn 2+ The molar ratio is 70:15:15; secondly, the ternary material was not doped with Mo.

[0127] The preparation steps of ternary material F are basically the same as those of ternary material A, with the main difference being: firstly, the amount of nickel salt, cobalt salt, and manganese salt added is adjusted to increase the amount of Ni in the prepared ternary material F. 2+ Co 2+ Mn 2+ The molar ratio is 83:7:10; secondly, the ternary material was not doped with Mo.

[0128] The particle size, bulk modulus, Young's modulus, and energy density of all the ternary materials prepared above were tested using the following methods:

[0129] (1) Bulk modulus B and Young's modulus E: An indirect method was used to test the bulk modulus B and Young's modulus E after the ternary material was prepared into a compact. Details are as follows:

[0130] Weigh out the ternary material powder, place it in a mold, apply a specific pressure of 500 MPa on a tablet press, hold the pressure for a period of time to form a cylindrical compact. Test the prepared compact using the ultrasonic pulse method to measure its density (ρ), measure its volume V using the displacement method, and weigh its mass m, where ρ = m / V. Use an ultrasonic probe to emit and receive longitudinal waves (P-wave) and transverse waves (S-wave), measure their propagation time in the compact, and calculate the wave velocities (Vp and Vs). Calculate according to the following formula:

[0131] Equivalent bulk modulus Beff: Beff = ρ(Vp) 2 -3 / 4Vs 2 );

[0132] Equivalent shear modulus Geff: Geff = ρVs 2 ;

[0133] Equivalent Young's modulus Eeff: Calculated using the formula E=9BG / (3B+G);

[0134] A cylindrical compact was subjected to uniaxial compression on a universal testing machine, and the load and displacement were measured simultaneously to obtain the stress-strain curve. In the initial linear stage, the slope is the equivalent Young's modulus Eeff.

[0135] The intrinsic modulus of particles is derived from the compact modulus: The measured Beff and Geff are the equivalent moduli of the compact including pores. The intrinsic bulk modulus B, shear modulus G, and Young's modulus E are obtained by back-calculation using the Mori-Tanaka model.

[0136] (2) Particle size test: Malvern laser diffraction particle size analyzer was used. 0.06g~0.10g of sample was taken with a sample spoon and added to a 50mL beaker. Sodium hexametaphosphate was added and water was added. The mixture was stirred with a glass rod and water was added to the sample injection system. The mixture was sonicated (60% ultrasonic intensity, 60s) and tested according to the laser diffraction method for particle size analysis in GB-T 19077-2016.

[0137] (3) Element content test: Inductively coupled plasma atomic emission spectrometry (ICP-OES) was used to test the content of lithium, nickel, cobalt, manganese, sodium, magnesium, aluminum, potassium, copper, calcium, iron, zinc and silicon in part 2 of the chemical analysis method for lithium nickel cobalt manganese oxide YS-T1006.2-2014.

[0138] The test results are shown in Table 1.

[0139] Example 1

[0140] The preparation of the positive electrode in this embodiment includes the following steps:

[0141] Step S101: The ternary material F (first active material), conductive carbon black (Super P), conductive carbon nanotubes (CNT), and the first binder polyvinylidene fluoride (molecular weight approximately 800,000 Da, elongation at break 319%) prepared above are added to the NMP solvent in a mass ratio of 97:1:1:1, and stirred evenly to obtain the first slurry;

[0142] Step S102: The ternary material A (second active material), conductive carbon black (Super P), conductive carbon nanotubes (CNT), and the second binder polyvinylidene fluoride (molecular weight approximately 1,100,000 Da, elongation at break 392%) prepared above are added to NMP solvent in a mass ratio of 97:1:1:1 and stirred evenly to obtain the second slurry;

[0143] Step S103: Using a double-layer die, the first slurry obtained in step S101 and the second slurry obtained in step S102 are layered and coated onto one side of an aluminum foil with a thickness of 12 μm, with the first slurry closer to the current collector than the second slurry. After drying, a first active material layer and a second active material layer are formed sequentially layered on the aluminum foil. The weight of the coating per unit area (first active material layer plus second active material layer) is 18.5 mg / cm². 2Then, the same process is used to coat the first slurry and the second slurry on the other side of the aluminum foil along the thickness direction, and the positive electrode is obtained after drying; wherein, the thickness ratio of the first active material layer and the second active material layer in the positive electrode is 1:1.

[0144] Example 2

[0145] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0146] Replace the first active material in step S101 with ternary material E, and replace the second active material in step S102 with ternary material B.

[0147] Example 3

[0148] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0149] In the prepared positive electrode sheet, the thickness ratio of the first active material layer and the second active material layer is adjusted to 1:4. The sum of the thicknesses of the first active material layer and the second active material layer remains unchanged.

[0150] Example 4

[0151] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0152] In the prepared positive electrode sheet, the thickness ratio of the first active material layer and the second active material layer is adjusted to 3:2. The sum of the thicknesses of the first active material layer and the second active material layer remains unchanged.

[0153] Example 5

[0154] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0155] In the prepared positive electrode sheet, the thickness ratio of the first active material layer to the second active material layer is adjusted to 4:1. The sum of the thicknesses of the first and second active material layers remains unchanged.

[0156] Example 6

[0157] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0158] The types of the first and second adhesives in steps S101 and S102 are interchanged. That is, the first adhesive is polyvinylidene fluoride with a molecular weight of approximately 1,100,000 Da and an elongation at break of 392%, and the second adhesive is polyvinylidene fluoride with a molecular weight of approximately 800,000 Da and an elongation at break of 319%.

[0159] Comparative Example 1

[0160] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0161] In step S101, the first active material is replaced with ternary material A, and in step S102, the second active material is replaced with ternary material F. In other words, the types of the first and second active materials are interchanged.

[0162] Comparative Example 2

[0163] The preparation method of the positive electrode in this embodiment is basically the same as that in Example 1, with the main difference being:

[0164] Replace the first active material in step S101 with ternary material B, and replace the second active material in step S102 with ternary material E.

[0165] Comparative Example 3

[0166] The preparation method of the positive electrode in this comparative example is basically the same as that in Example 1, with the main difference being:

[0167] In the prepared positive electrode sheet, the thickness ratio of the first active material layer and the second active material layer is adjusted to 1:9. The sum of the thicknesses of the first active material layer and the second active material layer remains unchanged.

[0168] Comparative Example 4

[0169] The preparation method of the positive electrode in this comparative example is basically the same as that in Example 1, with the main difference being:

[0170] In the prepared positive electrode sheet, the thickness ratio of the first active material layer to the second active material layer is adjusted to 9:1. The sum of the thicknesses of the first and second active material layers remains unchanged.

[0171] Comparative Example 5

[0172] The preparation method of the positive electrode in this comparative example is basically the same as that in Example 1, with the main difference being:

[0173] The first active material in step S101 is replaced with ternary material C, and step S102 is omitted. Accordingly, in step S103, the first slurry is coated only on both sides of the aluminum foil along the thickness direction to form a first active material layer. The thickness of the first active material layer is the same as the sum of the thicknesses of the first active material layer and the second active material layer in Example 2.

[0174] Comparative Example 6

[0175] The main difference between the preparation method of the positive electrode in this comparative example and that in Example 1 is that the first active material in step S101 is replaced with ternary material D, and step S102 is omitted. Accordingly, in step S103, only the first slurry is coated on both sides of the aluminum foil along the thickness direction to form the first active material layer. The thickness of the first active material layer is the same as the sum of the thicknesses of the first active material layer and the second active material layer in Example 1.

[0176] The test method for the elongation at break of the first and second adhesives used in the above embodiments and comparative examples is as follows: Using NMP as a solvent, the adhesive is prepared into a 7% (w / w) solution, poured into an 8mm deep, 100mm diameter aluminum disc mold, filled to the brim, smoothed, and left to stand for 12 hours to obtain cured adhesive. Then, strips of 15mm × 50mm are cut out. A universal testing machine is used, equipped with clamps to ensure secure clamping of the adhesive strip sample without damage. Simultaneously, an extensometer is installed to measure the deformation of the sample during tensile testing. The tensile speed is 50mm / min. The original gauge length (L0) is determined. After the sample breaks, the gauge length (L1) is measured. The elongation at break is calculated as ((L1-L0) / L0) × 100%.

[0177] The positive electrode sheets obtained in the above embodiments and comparative examples were used to prepare batteries, and the performance of the batteries was tested.

[0178] The battery preparation steps are as follows: The positive electrode sheets obtained in the above embodiments and comparative examples are rolled to a compaction density of 3.4 g / cc, serving as the positive electrode sheets; artificial graphite powder, conductive carbon, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:2:1:1, and then deionized water is added in a high-speed mixer and mixed uniformly to form a negative electrode slurry with a solid content of 48%. The negative electrode slurry is coated onto one side of an 8 μm thick copper foil using a transfer coating machine and dried, maintaining a coating weight of 10.4 mg / cm² per unit area after drying. 2 Then, the same process is used to coat and dry the other side of the copper foil to obtain a negative electrode semi-finished product. The negative electrode semi-finished product is rolled to a compaction density of 1.5 g / cc to serve as the negative electrode. The above positive electrode, negative electrode and separator are wound to form a core. Then, the exposed metal foil parts of the positive and negative electrode are processed and welded into tabs. The core is wrapped with aluminum-plastic film to form a semi-finished cell. After drying, electrolyte is injected. After vacuum sealing, standing, formation and shaping processes, a lithium-ion battery is obtained.

[0179] The prepared lithium-ion battery was subjected to the following tests:

[0180] (1) 45℃ Cyclic Performance Test: First, in an environment of 45℃, the first charge and discharge were performed. The voltage was charged to 4.4V under a constant current of 1C charging current. After that, the charging cutoff current was 0.05C under a constant voltage of 4.4V. After resting for 15 minutes, the voltage was discharged to 2.8V under a constant current of 1C discharging current. The discharge capacity of the first cycle was recorded as C1. Then, the voltage was charged to 4.4V under a constant current of 1C charging current. After resting for 15 minutes, the voltage was discharged to 2.8V under a 1C current. The discharge capacity of the first cycle was recorded as C2. Then, the charge and discharge cycles were performed until the capacity decayed to 80% of C2. The number of cycles corresponding to the 80% discharge capacity retention rate was recorded.

[0181] (2) 60℃ storage reversible capacity retention test: At 60℃, the lithium-ion battery was left to stand for 30 minutes and then discharged at a constant current of 1C to a voltage of 2.8V. The lithium-ion battery was then charged at a constant current of 1C to a voltage of 4.4V, and then charged at a constant voltage until the current ≤0.05C. Then it was discharged at 1C to 2.8V, and the discharge capacity was recorded as C1. After the fully charged battery was stored at 60℃ for 180 days, it was discharged at a 1C1 rate to 2.8V, and the remaining capacity Cm was recorded. It was then charged at a constant current of 1C1 to a voltage of 4.4V, and then charged at a constant voltage until the current ≤0.05C. Finally, it was discharged at a 1C1 rate to 2.8V, and the discharge capacity was recorded as Cn. The storage reversible capacity retention rate = (Cn / C1) × 100%.

[0182] (3) Gas generation during storage at 70℃: At 25℃, the battery volume was measured by water displacement and the volume was V1. After being fully charged, the battery was placed in a 70℃ constant temperature chamber for 30 days. The battery volume was measured by water displacement and the volume was V2. The growth rate of gas generation during battery storage = ((V2-V1) / V1)×100%.

[0183] (4) 1C energy density: First, before winding, record the weight m1 of the positive electrode. Calculate the foil area V1 by measuring the length and width of the electrode. The aluminum foil mass m2 = ρ 铝箔 V1 was charged and discharged in an environment of 25℃. It was charged at a constant current of 1C to a voltage of 4.4V, and then charged at a constant voltage of 4.4V with a cutoff current of 0.05C. After resting for 15 minutes, it was discharged at a constant current of 1C to a voltage of 2.8V. The discharge capacity of the first cycle was recorded as C1, the discharge energy as E1, and the energy density as E1 / ((m1-m2)×97%).

[0184] The test results are shown in Table 2.

[0185] Table 1

[0186]

[0187] Table 2

[0188]

[0189] In the positive electrode sheet prepared in Example 1, the second active material in the second active material layer is a high-voltage-resistant ternary material A (high bulk modulus B, high Young's modulus E), and the first active material in the first active material layer is a low-voltage-resistant material F (low bulk modulus B, low Young's modulus E). In the negative electrode sheet prepared in Comparative Example 6, the active material in the single-layer active material layer is a ternary material D, and the theoretical Ni, Co, and Mn contents in its active material layer are comparable to those in Example 1. The results in Table 1 show that, through the synergistic effect of the first and second active material layers, the cycle performance of the battery corresponding to Example 1 is significantly improved compared to Comparative Example 6, the storage performance is improved, high-temperature storage gas production is significantly reduced, and it also has a higher energy density. This confirms the synergistic effect of the first and second active material layers.

[0190] In the positive electrode sheet prepared in Example 2, the second active material in the second active material layer is a high-voltage-resistant ternary material B (high bulk modulus, high Young's modulus), and the first active material in the first active material layer is a low-voltage-resistant material E (low bulk modulus, low Young's modulus). In the negative electrode sheet prepared in Comparative Example 5, the active material in the single-layer active material layer is a ternary material C, and the theoretical Ni, Co, and Mn contents in its active material layer are comparable to those in Example 2. As can be seen from the results in Table 1, the overall performance of the battery corresponding to Example 2 is significantly better than that of Comparative Example 5. This is consistent with the comparison results of Example 1 and Comparative Example 6 above, further confirming the synergistic effect of the first and second active material layers.

[0191] In the positive electrode sheet prepared in Example 1, the second active material in the second active material layer is a high-voltage-resistant ternary material A (high bulk modulus B, high Young's modulus E), and the first active material in the first active material layer is a low-voltage-resistant material F (low bulk modulus B, low Young's modulus E). In contrast, in the positive electrode sheet prepared in Comparative Example 1, the second active material in the second active material layer is a low-voltage-resistant material F, and the first active material in the first active material layer is a high-voltage-resistant ternary material A. That is, compared to Example 1, the types of active materials in the two active material layers are interchanged. As can be seen from the results in Table 1, the battery corresponding to Example 1 has significantly better cycle performance and storage performance, lower gas production during high-temperature storage, and higher energy density. The main issue is that in the positive electrode sheet prepared in Comparative Example 1, the second active material in the second active material layer on the surface is made of low-pressure-resistant material F. Due to its low pressure resistance, the surface layer is subjected to pressure impacts without buffering during the rolling process. This results in a large pressure impact and frequent breakage (including cracking) of the active material. The exposed new interface after the active material breaks is not protected by an aluminum oxide coating layer, making it prone to side reactions with the electrolyte. This leads to increased impedance, increased gas production, and consequently, a drop in battery cycle life and a deterioration in storage performance.

[0192] In the positive electrode sheet prepared in Example 2, the second active material in the second active material layer is a high-voltage-resistant ternary material B (high bulk modulus, high Young's modulus), and the first active material in the first active material layer is a low-voltage-resistant material E (low bulk modulus, low Young's modulus). In contrast, in the negative electrode sheet prepared in Comparative Example 2, the second active material in the second active material layer is a low-voltage-resistant material E, and the first active material in the first active material layer is a high-voltage-resistant ternary material B. That is, compared to Example 2, the types of active materials in the two active material layers are interchanged. As can be seen from the results in Table 1, the performance of the battery corresponding to Example 2 is significantly better than that of the battery corresponding to Comparative Example 2. This is consistent with the comparison results of Example 1 and Comparative Example 1.

[0193] As can be seen from the data in Table 1 for Examples 1, 3 to 5, and Comparative Examples 3 and 4, when the second active material in the second active material layer is all made of high-pressure-resistant ternary material A, and the first active material in the first active material layer is all made of low-pressure-resistant material F, as the proportion of the second active material layer thickness in the total thickness of the two active material layers increases, the particle breakage of the active material after rolling decreases, side reactions decrease, and the cycle performance and storage performance of the battery improve accordingly. At the same time, the gas production during high-temperature storage decreases. However, the energy density of the battery decreases accordingly. As can also be seen from the data in Table 1, as the pressure resistance of the ternary material increases, its energy density decreases accordingly. Therefore, in order to effectively improve the cycle performance and storage performance of the battery while taking into account the energy density, the thickness ratio of the first active material layer and the second active material layer needs to be controlled at 1:4 to 4:1, so that the high-pressure-resistant second active material layer and the low-pressure-resistant first active material layer can achieve better functional coordination and produce a significant synergistic effect.

[0194] In the positive electrode sheet prepared in Example 1, the elongation at break of the second binder in the second active material layer is higher than that of the first binder in the first active material layer. In Example 6, the binders in the first and second active material layers are the same, and their elongation at break is low. As can be seen from the results in Table 1, the overall performance of the battery corresponding to Example 1 is better than that of the battery corresponding to Example 6. This indicates that the combination of a second active material layer with higher pressure resistance and a second binder with higher elongation at break can further improve the mechanical strength and pressure resistance of the second active material layer, enhance its buffering capacity against rolling pressure, and thus further reduce the breakage of the active material when directly exposed to rolling pressure, thereby further improving the cycle performance and storage performance of the battery. Therefore, having a higher elongation at break of the second binder in the second active material layer than that of the first binder in the first active material layer is a superior technical solution.

[0195] Figures 2 to 4 The images show scanning electron microscope (SEM) images of the ternary material on the surface of the positive electrode layer after cycling performance testing at 45°C for the batteries corresponding to Example 1, Comparative Example 1, and Comparative Example 6, respectively. Figure 2 It can be seen that in Example 1, there is virtually no particle breakage in the ternary material on the surface of the positive electrode. Figure 3 It can be seen that in Comparative Example 1, the ternary material on the surface of the positive electrode contains significantly more broken particles. (From...) Figure 4 As can be seen, in Comparative Example 6, some particles in the ternary material on the surface of the positive electrode were broken. This is consistent with the performance test results of the corresponding battery mentioned above.

[0196] It should be noted that the positive electrode sheet embodiments, positive electrode sheet preparation method embodiments, and battery embodiments provided in this application belong to the same concept; the technical features in the technical solutions described in each embodiment can be arbitrarily combined without conflict.

[0197] It should be understood that the above embodiments are exemplary and not intended to encompass all possible implementations of this application. Various modifications and changes can be made to the above embodiments without departing from the scope of this disclosure. Similarly, the various technical features of the above embodiments can be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only illustrate several implementations of the present invention and do not limit the scope of protection of this patent.

Claims

1. A positive electrode plate, characterized in that, The system includes a current collector, and a first active material layer and a second active material layer sequentially stacked on at least one surface of the current collector along its thickness direction; the first active material layer is located between the current collector and the second active material layer; wherein, The first active material layer includes a first active material, and the second active material layer includes a second active material. The bulk modulus of the second active material is greater than that of the first active material, and the Young's modulus of the second active material is greater than that of the first active material. The bulk modulus of the first active material is 100 GPa to 115 GPa, and the bulk modulus of the second active material is 115 GPa to 135 GPa. The Young's modulus of the first active material is 55 GPa to 65 GPa, and the Young's modulus of the second active material is 65 GPa to 80 GPa. The thickness ratio of the first active material layer to the second active material layer is 1:4 to 4:

1.

2. The positive electrode sheet according to claim 1, characterized in that, The sum of the thicknesses of the first active material layer and the second active material layer is 50 μm to 150 μm.

3. The positive electrode sheet according to claim 1, characterized in that, The first active material layer further includes a first adhesive, and the second active material layer further includes a second adhesive; the first adhesive and the second adhesive satisfy at least one of the following conditions: (a) the elongation at break of the second adhesive is greater than the elongation at break of the first adhesive; (b) the elongation at break of the first adhesive is 150% to 350%, and the elongation at break of the second adhesive is 300% to 400%.

4. The positive electrode sheet according to claim 1, characterized in that, The first active material includes a first ternary material; the second active material includes a second ternary material and / or lithium iron phosphate; the first ternary material and the second ternary material satisfy at least one of the following conditions: (a) the first ternary material and / or the second ternary material are single crystal particles; (b) the particle size D50 of the first ternary material and / or the second ternary material is 1.5 μm to 4.0 μm; (c) the surface of the first ternary material and / or the second ternary material has an alumina coating layer.

5. The positive electrode sheet according to claim 1, characterized in that, The first active material includes a first lithium nickel cobalt manganese oxide ternary material, and the second active material includes a second lithium nickel cobalt manganese oxide ternary material; the first lithium nickel cobalt manganese oxide ternary material and the second lithium nickel cobalt manganese oxide ternary material satisfy at least one of the following conditions: (a) the molar percentage of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material is higher than the molar percentage of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material; (b) the molar percentage of Mn in Ni, Co, and Mn in the first lithium nickel cobalt manganese oxide ternary material is 5% to 20%, and the molar percentage of Mn in Ni, Co, and Mn in the second lithium nickel cobalt manganese oxide ternary material is 20% to 40%.

6. The positive electrode sheet according to claim 1, characterized in that, The first active material includes a first ternary lithium nickel cobalt manganese oxide material, and the second active material includes a second ternary lithium nickel cobalt manganese oxide material. The second lithium nickel cobalt manganese oxide ternary material includes a doping element, wherein the doping element has a valence state of tetravalent or higher; the doping element satisfies at least one of the following conditions: (a) the doping element includes at least one of Mo, Ta, Nb, and Ti; (b) the content of the doping element is 500ppm to 5000ppm.

7. A method for preparing a positive electrode sheet as described in any one of claims 1 to 6, characterized in that, The method includes the following steps: The first active material, the first binder and the first conductive agent are added to the first solvent and mixed evenly to obtain the first slurry; The second active material, the second binder, and the second conductive agent are added to the second solvent and mixed evenly to obtain the second slurry. The first slurry and the second slurry are laminated onto at least one side surface of the current collector along the thickness direction, with the first slurry being closer to the current collector than the second slurry. After drying, the positive electrode sheet is obtained.

8. The method for preparing the positive electrode sheet according to claim 7, characterized in that, The method satisfies at least one of the following characteristics: (1) The mass ratio of the first active material, the first binder and the first conductive agent is (94~98):(0.5~3):(0.5~3); (2) The mass ratio of the second active material, the second binder and the second conductive agent is (94~98):(0.5~3):(0.5~3); (3) The first adhesive and the second adhesive each independently comprise at least one of polytetrafluoroethylene, polyvinylidene fluoride, and carboxymethyl cellulose; (4) The first conductive agent and the second conductive agent each independently include at least one of conductive carbon black, conductive carbon nanotubes, conductive graphite, carbon nanotubes, graphene, and conductive carbon fibers; (5) The first solvent and the second solvent each independently comprise N-methylpyrrolidone; (6) The solid content of the first slurry and / or the second slurry is 68%~75%; (7) The viscosity of the first slurry and / or the second slurry is 6000 mPa·s to 10000 mPa·s; (8) The areal density of the first slurry and / or the second slurry coating is 100 g / m². 2 ~500g / m 2 .

9. A battery, characterized in that, The positive electrode sheet includes the positive electrode sheet prepared by any one of claims 1 to 6 or the positive electrode sheet prepared by the method of claim 7 or 8.

Images