Positive pole piece, preparation method thereof and secondary battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a positive pole piece, a preparation method thereof and a secondary battery. The positive pole piece comprises a positive current collector and a positive active layer which is at least laminated on one surface of the positive current collector, and the positive active layer at least comprises a first active layer, a second active layer and a third active layer which are laminated and attached along the direction far away from the positive current collector; the first active layer, the second active layer and the third active layer comprise ternary active materials, and the elasticity modulus of the ternary active materials in the first active layer, the second active layer and the third active layer is gradually decreased in a gradient mode. The active layer in the positive pole piece adopts a microstructure gradient design, the characteristics of each layer of material are fully considered, the advantages of each layer of material are fully played, the utilization rate of the material performance is maximized, and the positive pole piece has the characteristics of high capacity, high magnification and long circulation.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and particularly relates to a positive electrode sheet and a preparation method thereof, as well as a secondary battery. Background Art

[0002] Lithium-ion batteries have the advantages of low cost, high safety, and high energy density, and have developed rapidly. Due to its unique layered crystal structure, ternary materials can provide more lithium active storage sites and have a high-capacity advantage, making them one of the popular cathode materials for lithium-ion batteries. By increasing the nickel content of ternary materials, the specific capacity can be effectively improved. However, generally speaking, the higher the nickel content, the worse the structural cycle stability of the ternary material lattice. A single high-nickel ternary material is difficult to simultaneously achieve high capacity, high cycle stability, and high rate performance. Especially in the design of thick electrodes with high energy density, the comprehensive performance of the structural stability, capacity, and rate performance of electrode materials is more important. This severely limits the application of ternary materials in battery systems with high energy density, high rate performance, and long cycle life. Summary of the Invention

[0003] The purpose of this application is to provide a positive electrode sheet and a preparation method thereof, as well as a secondary battery, aiming to solve to a certain extent the problem that existing ternary materials are difficult to simultaneously achieve the advantages of high capacity, high rate, and long cycle, which limits their application in batteries.

[0004] To achieve the above application purpose, the technical solutions adopted in this application are as follows:

[0005] In the first aspect, this application provides a positive electrode sheet, which includes a positive electrode current collector and at least a positive electrode active layer stacked on one surface of the positive electrode current collector. Along the direction away from the positive electrode current collector, the positive electrode active layer at least includes a first active layer, a second active layer, and a third active layer stacked and adhered together; the first active layer, the second active layer, and the third active layer include ternary active materials, and the elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer decrease in a gradient.

[0006] In some embodiments, the first active layer includes a medium-nickel single-crystal ternary active material with a molar ratio of nickel in transition metals of 50% - 60%, the second active layer includes a medium-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals of 50% - 60%, and the third active layer includes a high-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals higher than 60%.

[0007] In some embodiments, in the first active layer, the elastic modulus of the medium-nickel single-crystal ternary active material is 200 GPa - 300 GPa.

[0008] In some embodiments, in the second active layer, the elastic modulus of the medium-nickel polycrystalline ternary active material is 180 GPa to 250 GPa.

[0009] In some embodiments, in the third active layer, the elastic modulus of the high-nickel polycrystalline ternary active material is 150 GPa to 200 GPa.

[0010] In some embodiments, in the first active layer, the Dv50 of the medium-nickel single-crystalline ternary active material is 1 μm to 5 μm.

[0011] In some embodiments, in the second active layer, the Dv50 of the medium-nickel polycrystalline ternary active material is 8 μm to 15 μm.

[0012] In some embodiments, in the third active layer, the Dv50 of the high-nickel polycrystalline ternary active material is 8 μm to 15 μm.

[0013] In some embodiments, the porosity of the first active layer is 5% to 15%.

[0014] In some embodiments, the porosity of the second active layer is 20% to 35%.

[0015] In some embodiments, the porosity of the third active layer is 30% to 45%.

[0016] In some embodiments, the thickness ratio of the first active layer, the second active layer, and the third active layer is (10 - 30):(40 - 60):(20 - 40).

[0017] In some embodiments, the total thickness of the positive electrode active layer is 70 μm to 300 μm.

[0018] In some embodiments, in the first active layer, the chemical general formula of the medium-nickel single-crystalline ternary active material is LiNi x1 Co y1 Mn z1 O2, where x1 + y1 + z1 = 1, 0.5 ≤ x1 ≤ 0.6, 0.2 ≤ y1 ≤ 0.3, 0.2 ≤ z1 ≤ 0.3.

[0019] In some embodiments, in the second active layer, the chemical general formula of the medium-nickel polycrystalline ternary active material is LiNi x2 Co y2 Mn z2 O2, where x2 + y2 + z2 = 1, 0.5 ≤ x2 ≤ 0.6, 0.2 ≤ y2 ≤ 0.3, 0.2 ≤ z2 ≤ 0.3.

[0020] In some embodiments, in the third active layer, the chemical general formula of the high-nickel polycrystalline ternary active material is LiNi x3 Co y3 Mn z3 O2, where x3 + y3 + z3 = 1, 0.8 ≤ x3 ≤ 1, 0.05 ≤ y3 ≤ 0.15, 0.05 ≤ z3 ≤ 0.15.

[0021] In a second aspect, the present application provides a method for preparing a positive electrode plate, comprising the following steps:

[0022] Provide a positive electrode current collector, and prepare a first active layer including a ternary active material on at least one surface of the positive electrode current collector;

[0023] Prepare a second active layer including a ternary active material on the surface of the first active layer facing away from the positive electrode current collector;

[0024] Prepare a third active layer including a ternary active material on the surface of the second active layer facing away from the first active layer; to obtain a positive electrode plate; wherein, the elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer decrease in a gradient manner.

[0025] In some embodiments, the first active layer includes a medium-nickel single-crystalline ternary active material with a molar ratio of nickel in the transition metal of 50% - 60%; the second active layer includes a medium-nickel polycrystalline ternary active material with a molar ratio of nickel in the transition metal of 50% - 60%; the third active layer includes a high-nickel polycrystalline ternary active material with a molar ratio of nickel in the transition metal higher than 60%.

[0026] In a third aspect, the present application provides a secondary battery, which contains the above positive electrode plate and / or the positive electrode plate prepared by the above method.

[0027] The positive electrode tab provided in the first aspect of the present application, in which the positive electrode active layer at least includes a first active layer, a second active layer, and a third active layer that are laminated and adhered. Each active layer contains a ternary active material. The elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer decrease in a gradient manner. That is, along the thickness direction of the positive electrode tab, from the separator end to the current collector end, the elastic moduli of the ternary active materials increase in sequence. Through the gradient design of the elastic moduli of the three active layers (the first, second, and third active layers), among which the high-elasticity-modulus layer (the first active layer) can inhibit the volume expansion of particles during charge and discharge, and the low-elasticity-modulus layer (the third active layer) can buffer stress, reduce structural damage, and improve the cycle stability, enabling the positive electrode tab to have long cycle performance. In addition, the elastic modulus of the second active layer (polycrystalline material) is moderate, taking into account both lithium diffusion kinetics and structural stability, supporting fast charge and discharge, and enabling the positive electrode tab to have high-rate performance. Moreover, the ternary active materials included in the first active layer, the second active layer, and the third active layer have high capacity. Therefore, through the elastic modulus gradient design of the positive electrode active layer in the positive electrode tab of the present application, it is compatible with the expansion and contraction of the charge and discharge particles of the ternary active material, enhances the bonding force at the interface between the active layer and the current collector, reduces the formation of cracks inside the active layer and the cracking of the tab, and improves the long cycle stability. It comprehensively solves the problems of poor cycle stability of high-nickel ternary materials and easy structural damage of polycrystalline ternary materials, and achieves the balance between long cycle and high capacity. Moreover, the active layer in the positive electrode tab of the present application adopts a microstructure gradient design, fully considering the characteristics of each layer of material, giving full play to the advantages of each layer of material, maximizing the utilization rate of material performance, and realizing a positive electrode tab with high rate and long cycle.

[0028] The preparation method of the positive electrode tab provided in the second aspect of the present application is to prepare the first active layer, the second active layer, and the third active layer including the ternary active material in layers, ensuring that the material and structural characteristics of each layer are independently controllable, avoiding the interference of mixed layers, and the process controllability provides a realization basis for material design. It enables the elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer to decrease in a gradient manner, ensuring the designed performance. In the positive electrode tab prepared by the method of the present application, the elastic modulus gradient design of the positive electrode active layer is compatible with the expansion and contraction of the charge and discharge particles of the ternary active material, enhances the bonding force at the interface between the active layer and the current collector, reduces the formation of cracks inside the active layer and the cracking of the tab, and improves the long cycle stability. It comprehensively solves the problems of poor cycle stability of high-nickel ternary materials and easy structural damage of polycrystalline ternary materials, and achieves the balance between long cycle and high capacity.

[0029] In the secondary battery provided in the third aspect of the present application, due to the inclusion of a positive electrode tab with high capacity, high rate, and long cycle characteristics, it can improve the energy density, rate performance, and cycle life of the secondary battery. Description of the Drawings

[0030] To more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 is a schematic structural diagram of a positive electrode plate provided by an embodiment of the present application;

[0032] Figure 2 is a schematic flowchart of a preparation method of a positive electrode plate provided by an embodiment of the present application;

[0033] Among them, each reference numeral in the figure:

[0034] 1 - positive electrode current collector 2 - positive electrode active layer 21 - first active layer

[0035] 22 - second active layer 23 - third active layer. Detailed implementation manners

[0036] In order to make the technical problems to be solved, technical solutions and beneficial effects of the present application more clearly understood, the following further details the present application in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0037] In the present application, the term "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the front and back associated objects.

[0038] In the present application, "at least one" means one or more, and "a plurality" means two or more. "At least one (item)" or its similar expression below refers to any combination of these items, including any combination of single item (item) or plural items (items). For example, "at least one (item) of a, b or c", or, "at least one (item) of a, b and c" can both represent: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, c can be single or multiple respectively.

[0039] It should be understood that in various embodiments of the present application, the magnitudes of the serial numbers of the above processes do not mean the order of execution. Some or all steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0040] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0041] The weight of the relevant components mentioned in the embodiments of the specification of the present application not only can refer to the specific content of each component, but also can represent the proportional relationship of the weights between the components. Therefore, as long as the content of the relevant components in the embodiments of the specification of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the embodiments of the specification of the present application. Specifically, the mass described in the embodiments of the specification of the present application can be mass units well-known in the chemical industry such as μg, mg, g, kg, etc.

[0042] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be referred to as the second XX, and similarly, the second XX can also be referred to as the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0043] The term "single crystal" means that the entire material is composed of a continuous grain, and the atoms are completely ordered in three-dimensional space, having anisotropy (different physical properties in different directions).

[0044] The term "polycrystalline" means that the material is composed of a large number of small grains (crystals). Inside each grain, the atoms are ordered, but the orientations between the grains are random, and there are grain boundaries. The characteristics of polycrystalline are different from single crystal. It is composed of numerous tiny single crystal particles, has grain boundaries, poor structural stability, and is prone to cracking from the grain boundaries during cycling. In addition, polycrystalline is composed of a large number of randomly oriented small grains. Although each small grain itself is anisotropic, the randomly oriented small grains macroscopically cancel out the directional differences, so it shows isotropy as a whole statistically.

[0045] In the first aspect of the embodiments of the present application, a positive electrode plate is provided. As shown in the attached Figure 1 figure, it includes a positive electrode current collector 1 and at least a positive electrode active layer 2 stacked on one surface of the positive electrode current collector 1. Along the direction away from the positive electrode current collector 1, the positive electrode active layer 2 at least includes a first active layer 21, a second active layer 22, and a third active layer 23 stacked and attached; the first active layer 21, the second active layer 22, and the third active layer 23 include a ternary active material, and the elastic modulus of the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 decreases in a gradient.

[0046] The positive electrode sheet provided in the first aspect of the embodiment of the present application, in which the positive electrode active layer 2 at least includes a first active layer 21, a second active layer 22, and a third active layer 23 that are stacked and adhered. Each active layer contains a ternary active material. The elastic modulus of the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 decreases in a gradient manner. That is, along the thickness direction of the positive electrode sheet, from the separator end to the current collector end, the elastic modulus of the ternary active material increases in sequence. Through the design of the gradient decrease of the elastic modulus of the three active layers (the first, second, and third active layers), among which, the high-elasticity-modulus layer (the first active layer 21) can inhibit the volume expansion of particles during charge and discharge, and the low-elasticity-modulus layer (the third active layer 23) can buffer stress, reduce structural damage, and improve the cycle stability, so that the positive electrode sheet has long cycle performance. In addition, the elastic modulus of the second active layer 22 (polycrystalline material) is moderate, taking into account both lithium diffusion kinetics and structural stability, supporting fast charge and discharge, so that the positive electrode sheet has high rate performance. And, the ternary active materials included in the first active layer 21, the second active layer 22, and the third active layer 23 have high capacity. Therefore, through the elastic modulus gradient design of the positive electrode active layer 2 in the embodiment of the present application, it is compatible with the expansion and contraction of the charge and discharge particles of the ternary active material, enhances the bonding force at the interface between the active layer and the current collector, reduces the formation of cracks inside the active layer and the cracking of the electrode sheet, and improves the long cycle stability. It comprehensively solves the problems of poor cycle stability of high-nickel ternary materials and easy structural damage of polycrystalline ternary materials, and achieves the balance between long cycle and high capacity. And, the active layer in the positive electrode sheet of the embodiment of the present application adopts a microstructure gradient design, fully considering the characteristics of each layer of material, giving full play to the advantages of each layer of material, maximizing the utilization rate of material performance, and realizing a positive electrode sheet with high rate and long cycle.

[0047] In the embodiment of the present application, it is a relatively preferred consideration to adopt three layers for the positive electrode active layer 2. If only two active layers are included, the difference in mechanical strength such as the elastic modulus between the two layers of materials is relatively large, and the cyclic expansion and contraction of the active material are more intense, which is not conducive to the stability of the structure. Therefore, a structure with three active layers is adopted, introducing an intermediate transition layer, and the transition between each layer in terms of mechanical strength is more gentle. And having more than three layers has no greater significance and will increase the manufacturing complexity of the electrode sheet.

[0048] In some possible implementations, the first active layer 21 includes a medium-nickel single-crystal ternary active material with a molar proportion of nickel in transition metals of 50% to 60%. The second active layer 22 includes a medium-nickel polycrystalline ternary active material with a molar proportion of nickel in transition metals of 50% to 60%. The third active layer 23 includes a high-nickel polycrystalline ternary active material with a molar proportion of nickel in transition metals higher than 60%. It should be noted that in the high-nickel polycrystalline ternary active material with a molar proportion of nickel in transition metals higher than 60% included in the third active layer 23 of the embodiments of the present application, there is an ultra-high nickel with a molar proportion of nickel in transition metals higher than 90%. In the embodiments of the present application, the higher the nickel content in the ternary active material, the higher the corresponding specific capacity, but on the contrary, the worse the cycle structure stability. The polycrystalline ternary active material has good ion diffusion kinetics, better specific capacity and rate performance, but poor cycle structure stability; the single-crystal ternary active material is the opposite, the particles are not easy to crack during cycling, the structure stability is good but the rate performance is slightly poor. Based on this, the medium-nickel single-crystal material has the best structure stability. Through experimental observation and mechanism analysis, it is found that the microstructure cracking of the electrode sheet generally occurs along the interface between the current collector and the coating. Therefore, the medium-nickel single-crystal material is arranged in the first active layer 21 on the side attached to the current collector. The specific capacity of the medium-nickel single-crystal material used is relatively lower than that of the polycrystalline high-nickel material, but the structure stability is high and the long-cycle performance is good. Setting the medium-nickel single-crystal material at this interface is more stable and can better inhibit the cracking of the electrode sheet. The middle second active layer 22, as an intermediate transition layer, needs to provide mechanical strength between the upper layer and the lower layer. The medium-nickel polycrystalline material used has a lower strength than the medium-nickel single-crystal material but higher than the high-nickel polycrystalline material. Its long-cycle stability is better than that of the high-nickel polycrystalline material, and the specific capacity is higher than that of the medium-nickel single-crystal material. The high-nickel polycrystalline ternary active material with a molar proportion of nickel in transition metals higher than 60% has the best electrochemical performance, but the structure stability is low. Therefore, it is arranged on the outermost layer close to the separator side, which can be compatible with the flexible separator and minimize the risk of electrode cracking. Thus, in the embodiments of the present application, the first active layer 21 (medium-nickel single-crystal material), the single-crystal structure provides excellent cycle stability, and the medium-nickel component avoids drastic volume changes. The second active layer 22 (medium-nickel polycrystalline material), the polycrystalline structure improves the lithium diffusion rate and the rate performance; the medium-nickel component balances the capacity and stability. The third active layer 23 (high-nickel polycrystalline material), the high-nickel component significantly increases the specific capacity, and the polycrystalline structure enhances the lithium diffusion kinetics to make up for the rate deficiency of the single-crystal material. Through the synergistic effect of the three active layers, the problems of poor rate performance of the single-crystal material and poor cycle stability of the polycrystalline material are solved at the same time, and the unity of high capacity, high rate and long cycle is achieved.

[0049] In some possible implementation manners, the elastic modulus of the medium-nickel single-crystal ternary active material is 200 GPa to 300 GPa; exemplarily, it can be any typical but non-limiting point value such as 200 GPa, 210 GPa, 220 GPa, 230 GPa, 240 GPa, 250 GPa, 260 GPa, 270 GPa, 280 GPa, 290 GPa, 300 GPa or an interval value between any two point values. In the case of this elastic modulus, the medium-nickel single-crystal ternary active material of the first active layer 21 can provide better structural support and inhibit particle cracking.

[0050] In some possible implementation manners, the elastic modulus of the medium-nickel polycrystalline ternary active material is 180 GPa to 250 GPa; exemplarily, it can be any typical but non-limiting point value such as 180 GPa, 190 GPa, 200 GPa, 210 GPa, 220 GPa, 230 GPa, 240 GPa, 250 GPa or an interval value between any two point values. In this case, as an intermediate transition layer, the second active layer 22 can provide the mechanical strength between the first active layer 21 and the third active layer 23, and balance the diffusion kinetics and structural stability between different active layers.

[0051] In some possible implementation manners, the elastic modulus of the high-nickel polycrystalline ternary active material is 150 GPa to 200 GPa; exemplarily, it can be any typical but non-limiting point value such as 150 GPa, 160 GPa, 170 GPa, 180 GPa, 190 GPa, 200 GPa or an interval value between any two point values. In this case, it adapts to the volume expansion of the high-nickel polycrystalline ternary active material and reduces stress concentration.

[0052] In some possible implementation manners, in the first active layer 21, the elastic modulus of the medium-nickel single-crystal ternary active material is 200 GPa to 300 GPa; in the second active layer 22, the elastic modulus of the medium-nickel polycrystalline ternary active material is 180 GPa to 250 GPa; in the third active layer 23, the elastic modulus of the high-nickel polycrystalline ternary active material is 150 GPa to 200 GPa. In this case, through the modulus gradient optimization, the structural instability of the high-nickel material is alleviated and the cycle life is prolonged.

[0053] In some possible implementation manners, in the first active layer 21, the Dv50 of the medium-nickel single-crystal ternary active material is 1 μm to 5 μm; exemplarily, it can be any typical but non-limiting point value such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or an interval value between any two point values. In this case, the first active layer 21 formed by the medium-nickel single-crystal ternary active material with a small particle size is denser, which can improve the compaction density of the electrode sheet and enhance the energy density.

[0054] In some possible implementation manners, in the second active layer 22, the Dv50 of the medium-nickel polycrystalline ternary active material is 8 μm to 15 μm; exemplarily, it can be typical but non-limiting arbitrary point values such as 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm or interval values between any two point values.

[0055] In some possible implementation manners, in the third active layer 23, the Dv50 of the high-nickel polycrystalline ternary active material is 8 μm to 15 μm; exemplarily, it can be typical but non-limiting arbitrary point values such as 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm or interval values between any two point values.

[0056] In the above embodiments of the present application, the ternary active materials used in the second active layer 22 and the third active layer 23 have relatively large particle sizes, and the formed second active layer 22 and third active layer 23 have larger pore structures relative to the first active layer 21, which can improve the electrolyte wettability and ion diffusion performance of the electrode sheet and enhance the rate performance.

[0057] In some possible implementation manners, in the first active layer 21, the Dv50 of the medium-nickel single-crystalline ternary active material is 1 μm to 5 μm; in the second active layer 22, the Dv50 of the medium-nickel polycrystalline ternary active material is 8 μm to 15 μm; in the third active layer 23, the Dv50 of the high-nickel polycrystalline ternary active material is 8 μm to 15 μm. In this case, by combining the particle size gradient with the material characteristics, the high rate requirement and the high capacity target are simultaneously met.

[0058] In some possible implementation manners, the porosity of the first active layer 21 is 5% to 15%; exemplarily, it can be typical but non-limiting arbitrary point values such as 5%, 8%, 10%, 12%, 14%, 15% or interval values between any two point values. In this case, the porosity of the first active layer 21 is relatively low, and the active layer with a dense structure is beneficial to improving the energy density and cycle stability.

[0059] In some possible implementation manners, the porosity of the second active layer 22 is 20% to 35%; exemplarily, it can be typical but non-limiting arbitrary point values such as 20%, 25%, 28%, 30%, 32%, 35% or interval values between any two point values. In this case, the porosity of the intermediate layer is relatively large, which can promote the penetration of the electrolyte, improve the rate performance, and reserve space for volume expansion.

[0060] In some possible implementations, the porosity of the third active layer 23 is 30% to 45%; exemplarily, it can be any typical but non-limiting point value such as 30%, 33%, 35%, 38%, 40%, 42%, 45%, or an interval value between any two point values. In this case, the pores of the third active layer 23 are larger, and the high porosity alleviates the volume expansion of the high-nickel material and reduces structural damage.

[0061] In the positive electrode active layer 2 of the embodiments of the present application, the porosity of each active layer is crucial in the design principle of the electrode sheet. Especially in the thick electrode system, when the porosity increases, the strength of the electrode sheet decreases, and the cycle capacity retention rate drops. When the porosity decreases, it will affect the rate performance by restricting lithium ion conduction. Therefore, it is necessary to find a balance between capacity retention and rate performance. Under the condition of meeting the application requirements, the electrode sheet should be thinned as much as possible.

[0062] In some possible implementations, the porosity of the first active layer 21 is 5% to 15%; the porosity of the second active layer 22 is 20% to 35%; the porosity of the third active layer 23 is 30% to 45%. In this case, along the thickness direction of the electrode sheet, from the separator end to the current collector end, the porosity (tortuosity) of the ternary active material decreases in sequence; the porosity gradient design collaboratively solves the problems of volume expansion of the high-nickel material and electrolyte infiltration, and supports long cycle life and high rate performance.

[0063] It should be noted that the porosity refers to the percentage of the pore volume in the total volume of the material, reflecting the "looseness" of the electrode. The tortuosity is a dimensionless parameter describing the complexity of the pore channels. The higher the tortuosity, the more tortuous the pore path. It is generally considered that the relationship between porosity and tortuosity follows the Bruggeman correction formula, and porosity and tortuosity are usually negatively correlated. Materials with high porosity tend to have low tortuosity, while materials with low porosity have high tortuosity. High porosity can increase the electrolyte infiltration area and promote lithium ion transport, but may reduce the volume energy density. Low porosity can reduce electrolyte contact, increase ion transport resistance, but improve the energy density. High tortuosity can extend the lithium ion diffusion path, reduce the effective diffusion coefficient, and is not conducive to high rate performance. Low tortuosity can shorten the diffusion path, improve ion transport efficiency, and support fast charge and discharge. High porosity and high tortuosity may lead to "false infiltration" (the electrolyte is filled but the diffusion efficiency is low); low porosity and low tortuosity need to balance infiltration and transport.

[0064] In some possible implementation manners, in the positive electrode active layer 2, the thickness ratio of the first active layer 21, the second active layer 22, and the third active layer 23 is (10 - 30):(40 - 60):(20 - 40). In this case, the main function of the bottom layer (the first active layer 21) is the structural strength stability. Since the capacity of the medium nickel single crystal material is relatively low, its thickness ratio is relatively thin to avoid the influence of insufficient rate performance of the single crystal material. The middle layer (the second active layer 22) has the thickest thickness because it can balance the mechanical strength and the electrochemical performance. Therefore, by making full use of the high rate characteristics of the polycrystalline material, the maximum utilization of the performance can be satisfied. The thickness of the surface layer (the third active layer 23) is between that of the first active layer 21 and the second active layer 22. It has the highest electrochemical performance but the worst cycle stability. Therefore, it cannot be too thick to balance the capacity of the high nickel material and the structural stability.

[0065] Exemplarily, in the positive electrode active layer 2, the thickness ratio of the first active layer 21, the second active layer 22, and the third active layer 23 can be any typical but non - restrictive point value or interval value between any two point values, such as 10:60:30, 10:50:40, 20:50:30, 20:40:40, 20:60:20, 30:40:30, 30:50:20, etc.

[0066] In some possible implementation manners, the thickness of the positive electrode active layer 2 is 70μm - 300μm. Exemplarily, it can be any typical but non - restrictive point value or interval value between any two point values, such as 70μm, 100μm, 150μm, 200μm, 250μm, 300μm, etc. In this case, to adapt to the battery energy density requirement, the advantages of each layer of materials are maximized through thickness distribution to achieve overall performance balance.

[0067] In some possible implementation manners, the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 can be nickel - cobalt - manganese ternary material, nickel - cobalt - aluminum ternary material, doped nickel - cobalt - manganese ternary material, doped nickel - cobalt - aluminum ternary material, etc. The doping elements include elements such as Mg, Ti, Sb, Nb, Zr, etc. to improve the structural stability, or elements such as F, S, B, etc. are incorporated to improve the surface characteristics. Among them, the nickel - cobalt - aluminum ternary material LiNi 0.8 Co 0.15 Al 0.05 O2 has a high energy density (close to NCM811), excellent rate performance, and is suitable for fast charging scenarios.

[0068] In some possible implementation manners, in the first active layer 21, the chemical general formula of the medium nickel single crystal ternary active material is LiNi x1 Co y1 Mn z1O2, where x1 + y1 + z1 = 1, 0.5 ≤ x1 ≤ 0.6, 0.2 ≤ y1 ≤ 0.3, 0.2 ≤ z1 ≤ 0.3.

[0069] In some possible implementation manners, the medium nickel single crystal ternary active material includes single crystal LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622), single crystal LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523), or at least one of them. These medium nickel single crystal ternary active materials have a stable crystal structure and can improve the cycle performance of the positive electrode sheet.

[0070] In some possible implementation manners, in the second active layer 22, the chemical general formula of the medium nickel polycrystalline ternary active material is LiNi x2 Co y2 Mn z2 O2, where x2 + y2 + z2 = 1, 0.5 ≤ x2 ≤ 0.6, 0.2 ≤ y2 ≤ 0.3, 0.2 ≤ z2 ≤ 0.3.

[0071] In some possible implementation manners, the medium nickel polycrystalline ternary active material includes polycrystalline LiNi 0.6 Co 0.2 Mn 0.2 O2, polycrystalline LiNi 0.5 Co 0.2 Mn 0.3 O2, or at least one of them. These medium nickel polycrystalline ternary active materials have moderate specific capacity and cycle stability.

[0072] In some possible implementation manners, in the third active layer 23, the chemical general formula of the high nickel polycrystalline ternary active material is LiNi x3 Co y3 Mn z3 O2, where x3 + y3 + z3 = 1, 0.8 ≤ x3 ≤ 1, 0.05 ≤ y3 ≤ 0.15, 0.05 ≤ z3 ≤ 0.15.

[0073] In some possible implementation manners, the high nickel polycrystalline ternary active material includes LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811), LiNi 0.8 Co 0.15 Mn 0.05 O2 (NCM81505), LiNi 0.7 Co 0.15 Mn 0.15 O2 (NCM71515), LiNi0.9 Co 0.05 Mn 0.05 At least one of O2 (NCM90505). These high-nickel polycrystalline ternary active materials have a high specific capacity.

[0074] In the positive electrode plate of the embodiment of the present application, based on the fact that when ions are inserted and extracted in the active material particles, the particles will expand and contract in volume. According to the principle of elasticity, the smaller the elastic modulus under the same strain, the smaller the stress. Therefore, the internal stress generated by the ternary active material particles and the electrode during cycling is small, and the particles and the electrode plate are not easily damaged. A gradient design of the microstructure of the electrode plate is carried out for the ternary active material, and key factors such as the particle morphology, specific capacity, elastic modulus, porosity (tortuosity), etc. of the ternary active material with different nickel contents and crystal structures are considered simultaneously to achieve a positive electrode plate with high rate and long cycle life. Through the mechanical gradient design of the elastic modulus, the expansion and contraction of the charging and discharging particles of the ternary active material are compatible, the bonding force between the active layer and the current collector interface is enhanced, the crack formation inside the active layer and the cracking of the electrode plate are reduced, and the long cycle stability is improved. Moreover, by utilizing the different morphological characteristics and mechanical characteristics of the ternary active material, the tortuosity in the thickness direction of the electrode plate is regulated, the lithium ion diffusion at high rate is better utilized, and the concentration polarization is reduced.

[0075] The positive electrode plate of the above embodiment of the present application can be obtained by the following example method.

[0076] In the second aspect, the embodiment of the present application provides a method for preparing a positive electrode plate, as shown in the appendix Figure 1 shown, including the following steps:

[0077] S10. Provide a positive electrode current collector 1, and prepare a first active layer 21 including a ternary active material on at least one surface of the positive electrode current collector 1;

[0078] S20. Prepare a second active layer 22 including a ternary active material on the surface of the first active layer 21 facing away from the positive electrode current collector 1;

[0079] S30. Prepare a third active layer 23 including a ternary active material on the surface of the second active layer 22 facing away from the first active layer 21; to obtain a positive electrode plate; wherein, the elastic modulus of the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 decreases in a gradient.

[0080] The preparation method of the positive electrode plate in the embodiment of the present application prepares the first active layer 21, the second active layer 22, and the third active layer 23 including ternary active materials in layers, ensuring that the material and structural characteristics of each layer are independently controllable, avoiding mixed layer interference, and the process controllability provides a basis for material design. The elastic modulus of the ternary active materials in the first active layer 21, the second active layer 22, and the third active layer 23 decreases in a gradient manner, ensuring the designed performance. In the positive electrode plate prepared by the method of the embodiment of the present application, the elastic modulus gradient design of the positive electrode active layer 2 is compatible with the expansion and contraction of the ternary active material charge and discharge particles, enhances the bonding force at the interface between the active layer and the current collector, reduces the formation of cracks inside the active layer and the cracking of the electrode plate, and improves the long-cycle stability. It comprehensively solves the problems of poor cycle stability of high-nickel ternary materials and easy damage to the structure of polycrystalline ternary materials, and realizes the balance between long cycle and high capacity.

[0081] In some possible implementation manners, the first active layer 21 includes a medium-nickel single-crystal ternary active material with a molar ratio of nickel in the transition metal of 50% to 60%; the second active layer 22 includes a medium-nickel polycrystalline ternary active material with a molar ratio of nickel in the transition metal of 50% to 60%; the third active layer 23 includes a high-nickel polycrystalline ternary active material with a molar ratio of nickel in the transition metal higher than 60%. Through the synergistic effect of the three active layers, the problems of poor single-crystal rate performance and poor polycrystalline cycle stability are solved simultaneously, and the synergistic complementarity of high capacity, high rate, and long cycle is realized. Among them, the first active layer 21 (medium-nickel single-crystal material), the single-crystal structure provides excellent cycle stability, and the medium-nickel component avoids drastic volume changes. The second active layer 22 (medium-nickel polycrystalline material), the polycrystalline structure improves the lithium diffusion rate and the rate performance; the medium-nickel component balances the capacity and stability. The third active layer 23 (high-nickel polycrystalline material), the high-nickel component significantly increases the specific capacity, and the polycrystalline structure enhances the lithium diffusion kinetics to make up for the single-crystal rate deficiency.

[0082] In the above step S10:

[0083] In some possible implementation manners, the step of preparing the first active layer 21 includes: after making a mixed slurry of the medium-nickel single-crystal ternary active material, the first conductive agent, the first binder, and the first organic solvent, depositing it on the surface of the positive electrode current collector 1 and drying to form the first active layer 21.

[0084] In some possible implementation manners, in the first active layer 21, the Dv50 of the medium-nickel single-crystal ternary material is 1 μm to 5 μm; the elastic modulus of the medium-nickel single-crystal ternary material is 200 GPa to 300 GPa; the porosity of the first active layer 21 is 5% to 15%.

[0085] In some possible implementation manners, in the first active layer 21, the chemical general formula of the medium-nickel single-crystal ternary material is LiNi x1 Co y1Mn z1 O₂, where x₁ + y₁ + z₁ = 1, 0.5 ≤ x₁ ≤ 0.6, 0.2 ≤ y₁ ≤ 0.3, 0.2 ≤ z₁ ≤ 0.3.

[0086] In some possible implementation manners, the medium-nickel single-crystal ternary active material includes single-crystal LiNi 0.6 Co 0.2 Mn 0.2 O₂, single-crystal LiNi 0.5 Co 0.2 Mn 0.3 O₂ or at least one of them. These medium-nickel single-crystal ternary active materials have a stable crystal structure and can improve the cycle performance of the positive electrode sheet.

[0087] In some possible implementation manners, the positive current collector 1 includes but is not limited to any one of copper foil and aluminum foil.

[0088] In the above step S20:

[0089] In some possible implementation manners, the step of preparing the second active layer 22 includes: after making a mixed slurry from the medium-nickel polycrystalline ternary active material, the second conductive agent, the second binder, and the second organic solvent, depositing it on the surface of the first active layer 21 and drying to form the second active layer 22.

[0090] In some possible implementation manners, in the second active layer 22, the Dv50 of the medium-nickel polycrystalline ternary material is 8 μm to 15 μm; the elastic modulus of the medium-nickel polycrystalline ternary material is 180 GPa to 250 GPa; the porosity of the second active layer 22 is 20% to 35%.

[0091] In some possible implementation manners, in the second active layer 22, the chemical general formula of the medium-nickel polycrystalline ternary material is LiNi x2 Co y2 Mn z2 O₂, where x₂ + y₂ + z₂ = 1, 0.5 ≤ x₂ ≤ 0.6, 0.2 ≤ y₂ ≤ 0.3, 0.2 ≤ z₂ ≤ 0.3.

[0092] In some possible implementation manners, the medium-nickel polycrystalline ternary active material includes polycrystalline LiNi 0.6 Co 0.2 Mn 0.2 O₂, polycrystalline LiNi 0.5 Co 0.2 Mn 0.3 O₂ or at least one of them. These medium-nickel polycrystalline ternary active materials have moderate specific capacity and cycle stability.

[0093] In the above step S30:

[0094] In some possible implementation manners, the step of preparing the third active layer 23 includes: after making a mixed slurry from a high-nickel polycrystalline ternary active material, a third conductive agent, a third binder, and a third organic solvent, depositing the slurry on the surface of the second active layer 22 and drying to form the third active layer 23.

[0095] In some possible implementation manners, in the third active layer 23, the Dv50 of the high-nickel polycrystalline ternary material is 8 μm to 15 μm; the elastic modulus of the high-nickel polycrystalline ternary material is 150 GPa to 200 GPa; the porosity of the third active layer 23 is 30% to 45%.

[0096] In some possible implementation manners, in the third active layer 23, the chemical general formula of the high-nickel polycrystalline ternary material is LiNi x3 Co y3 Mn z3 O2, where x3 + y3 + z3 = 1, 0.8 ≤ x3 ≤ 1, 0.05 ≤ y3 ≤ 0.15, 0.05 ≤ z3 ≤ 0.15.

[0097] In some possible implementation manners, the high-nickel polycrystalline ternary active material includes at least one of LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.8 Co 0.15 Mn 0.05 O2, LiNi 0.7 Co 0.15 Mn 0.15 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2. These high-nickel polycrystalline ternary active materials have a high specific capacity.

[0098] In some possible implementation manners, the first conductive agent in the first active layer 21, the second conductive agent in the second active layer 22, and the third conductive agent in the third active layer 23 independently include: acetylene black (Super P), conductive graphite (such as KS-6, KS-15), carbon nanotubes (CNT), graphene, Ketjen black (ECP), carbon nanofibers (CNF), etc.

[0099] In some possible implementation manners, the contents of the first conductive agent in the first active layer 21, the second conductive agent in the second active layer 22, and the third conductive agent in the third active layer 23 are independently 1 wt% to 5 wt%. In specific embodiments, the content of the conductive agent can be typical but non-limiting contents such as 3 wt%, 4 wt%, 5 wt%, etc.

[0100] In some possible implementation manners, the first binder in the first active layer 21, the second binder in the second active layer 22, and the third binder in the third active layer 23 independently include: oil-based systems such as PVDF (polyvinylidene fluoride) and PTFE (polytetrafluoroethylene), and water-based systems such as LA132 / LA133 (polyacrylic acid), CMC (sodium carboxymethyl cellulose), and SBR (styrene-butadiene rubber latex).

[0101] In some possible implementation manners, the contents of the first binder in the first active layer 21, the second binder in the second active layer 22, and the third binder in the third active layer 23 are independently 2 wt% to 5 wt%. In specific embodiments, the contents of the binder can be typical but non-limiting contents such as 2 wt%, 3 wt%, 4 wt%, and 5 wt%.

[0102] In some possible implementation manners, the first organic solvent in the slurry for preparing the first active layer 21, the second organic solvent in the slurry for preparing the second active layer 22, and the third organic solvent in the slurry for preparing the third active layer 23 independently include: N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAC), tetrahydrofuran (THF), cyclopentanone, etc.

[0103] In some possible implementation manners, the mass percentages of the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 are independently 90% to 95%. Specifically, they can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0104] In some possible implementation manners, in the positive electrode active layer 2, the thickness ratio of the first active layer 21, the second active layer 22, and the third active layer 23 is (10 - 30):(40 - 60):(20 - 40);

[0105] In some possible implementation manners, the thickness of the positive electrode active layer 2 is 70 μm to 300 μm.

[0106] The above embodiments of the present application have been described in detail above and will not be elaborated here.

[0107] In a third aspect, an embodiment of the present application provides a secondary battery, which includes the above positive electrode sheet and / or the positive electrode sheet prepared by the above method.

[0108] In the secondary battery of the embodiment of the present application, since it includes a positive electrode sheet with high capacity, high rate, and long cycle characteristics, the energy density, rate performance, and cycle life of the secondary battery can be improved.

[0109] This application does not specifically limit the negative electrode sheet, electrolyte, separator, etc. in the secondary battery, and it can be applied to any battery system.

[0110] In some possible implementation manners, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (such as coke, etc.), hard carbon, or nitrides, tin-based oxides, tin-based oxides, tin alloys, and nano-negative electrode materials, etc. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0111] In some possible implementation manners, the steps of manufacturing the negative electrode sheet include: mixing the negative electrode active material with a conductive agent such as conductive carbon black, a binder such as carboxymethyl cellulose and styrene-butadiene rubber, and a solvent such as water in a mass ratio of (80-99):(1-5):(2-10):100 to form a positive electrode mixed slurry, then performing vacuum degassing, discharging, coating on a coater, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0112] In some possible implementation manners, the separator can block the passage of electrons and allow the passage of ions. Exemplarily, the separator includes, but is not limited to, at least one material among polypropylene fiber, polyacrylonitrile fiber, polyvinyl formal fiber, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fiber, and poly(p-phenylenediamine terephthalate).

[0113] In some possible implementation manners, the electrolyte includes at least one soluble metal salt. In some specific embodiments, the metal salt includes at least one of LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 SO2)N], where m and n are natural numbers. These electrolytic salts can all ensure high ionic conductivity of the electrolyte, and do not undergo harmful side reactions with electrode materials, electrolytes, separators, etc., and have good chemical stability.

[0114] In some possible implementation manners, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0115] In some possible implementation manners, the type of the battery cell includes a lithium-ion battery.

[0116] In some possible implementation manners, the battery cells of the present application can be assembled into a battery module. The number of battery cells included in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. The battery module can also include a housing having an accommodation space, and a plurality of battery cells are accommodated in the accommodation space.

[0117] In a possible implementation, the battery cells and / or battery modules can also be assembled into a battery pack, and the number of battery cells or battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0118] To enable those skilled in the art to clearly understand the above implementation details and operations of this application, and to significantly demonstrate the progress of the positive electrode sheet and its preparation method, as well as the secondary battery in the embodiments of this application, the above technical solutions are illustrated by way of multiple embodiments below.

[0119] Example 1

[0120] A positive electrode sheet has a total thickness of 85 μm. Among them, the thickness of the aluminum foil positive electrode current collector 1 is 15 μm, and the thickness of the positive electrode active layer 2 is 70 μm. Along the direction away from the positive electrode current collector 1, the positive electrode active layer 2 includes: a first active layer 21 with an elastic modulus of 260 GPa, including the active material single crystal NCM523, with a thickness of 7 μm (accounting for 10%), and a porosity of 8%; a second active layer 22 with an elastic modulus of 220 GPa, including the active material polycrystalline NCM622, with a thickness of 42 μm (accounting for 60%), and a porosity of 25%; a third active layer 23 with an elastic modulus of 180 GPa, including polycrystalline NCM811, with a thickness of 21 μm (accounting for 29%), and a porosity of 38%.

[0121] Its preparation includes the following steps:

[0122] (1) Slurry preparation: Mix the single crystal NCM523 powder, carbon nanotubes (CNTs), and PVDF in a mass ratio of 92:4:4 to make the first layer of slurry, with the solvent being NMP; mix the polycrystalline NCM622 powder, conductive graphite (KS-6), and PVDF in a mass ratio of 91:5:4 to make the second layer of slurry, with the solvent being NMP; mix the polycrystalline NCM811 with acetylene black (super P) and CMC in a mass ratio of 90:6:4 to make the third layer of slurry, with the solvent being a water / ethanol mixture (volume ratio 7:3);

[0123] (2) Preparation of the first active layer 21: Coating the first layer of slurry evenly on the aluminum foil current collector by slit extrusion coating, drying it with hot air at 80 °C, and controlling the solvent evaporation rate at 70% to obtain the incompletely dried first active layer 21;

[0124] (3) Preparation of the second active layer 22: Coating the second layer of slurry on the incompletely dried first active layer 21, using the residual solvent in the lower layer to promote interlayer bonding, drying it with hot air at 80 °C, and controlling the solvent evaporation rate at 80% to obtain the incompletely dried second active layer 22;

[0125] (4) Preparation of the third active layer 23: Coat the third layer of slurry on the incompletely dried second active layer 22, utilize the residual solvent in the lower layer to promote interlayer bonding, and perform hot air drying at 120 °C to completely remove the solvent. The first active layer 21, the second active layer 22, and the third active layer 23 are sequentially formed and laminated on the surface of the positive electrode current collector 1 to obtain the positive electrode active layer 2.

[0126] (5) Rolling process: Roll the positive electrode sheet under a pressure of 80 M to complete the final sheet forming and obtain the positive electrode sheet.

[0127] Example 2

[0128] A positive electrode sheet has a total thickness of 280 μm. Among them, the thickness of the aluminum foil positive electrode current collector 1 is 20 μm, and the thickness of the positive electrode active layer 2 is 260 μm. Along the direction away from the positive electrode current collector 1, the positive electrode active layer 2 includes: the first active layer 21 with an elastic modulus of 250 GPa, including the active material single crystal NCM622, with a thickness of 78 μm (accounting for 30%), and a porosity of 12%; the second active layer 22 with an elastic modulus of 200 GPa, including the active material polycrystalline NCM71515, with a thickness of 130 μm (accounting for 50%), and a porosity of 30%; the third active layer 23 with an elastic modulus of 160 GPa, including polycrystalline NCM90505, with a thickness of 52 μm (accounting for 20%), and a porosity of 42%.

[0129] Its preparation includes the following steps:

[0130] (1) Slurry preparation: Mix the single crystal NCM622 powder, carbon nanotubes (CNTs), and PVDF in a mass ratio of 92:4:4 to make the first layer of slurry, with the solvent being NMP; mix the polycrystalline NCM71515 powder, conductive graphite (KS-6), and PVDF in a mass ratio of 91:5:4 to make the second layer of slurry, with the solvent being NMP; mix the polycrystalline NCM90505, acetylene black (super P), and CMC in a mass ratio of 90:6:4 to make the third layer of slurry, with the solvent being a water / ethanol mixture (volume ratio 7:3);

[0131] (2) Preparation of the first active layer 21: Coat the first layer of slurry evenly on the aluminum foil current collector by slit extrusion coating, and perform hot air drying at 80 °C, controlling the solvent evaporation rate at 70% to obtain the incompletely dried first active layer 21;

[0132] (3) Preparation of the second active layer 22: Coat the second layer of slurry on the incompletely dried first active layer 21, utilize the residual solvent in the lower layer to promote interlayer bonding, and perform hot air drying at 80 °C, controlling the solvent evaporation rate at 80% to obtain the incompletely dried second active layer 22;

[0133] (4) Preparation of the third active layer 23: Coat the third layer of slurry on the incompletely dried second active layer 22, and utilize the residual solvent in the lower layer to promote interlayer bonding. Then, perform hot air drying at 120 °C to completely remove the solvent, and sequentially form the first active layer 21, the second active layer 22, and the third active layer 23 which are laminated and adhered on the surface of the positive electrode current collector 1 to obtain the positive electrode active layer 2.

[0134] (5) Rolling process: Roll the positive electrode sheet under a pressure of 80 M to complete the final sheet forming and obtain the positive electrode sheet.

[0135] Examples 3 - 4

[0136] Examples 3 - 4 respectively provide a positive electrode sheet, which is different from Example 1 in that: the conductive agent of the first layer of slurry is graphene, the binder is polyacrylic acid (PAA), the solvent is a water / ethanol mixture (volume ratio 9:1), the conductive agents of the second and third layers are Ketjenblack, the binder is polyimide (PI), the solvent is NMP, and the mass ratios of the active material, conductive agent, and binder in each layer remain unchanged. The corresponding ternary materials, thicknesses, and particle size D50 of each active layer are different, and the elastic modulus values of the prepared first active layer 21, second active layer 22, and third active layer 23 are different. See Table 1 below for details.

[0137] Examples 5 - 6

[0138] Examples 5 - 6 respectively provide a positive electrode sheet, which is different from Example 1 in that: the conductive agent of the first layer of slurry is carbon nanotubes (CNTs), the binder is polyacrylic acid (PAA), the solvent is a water / ethanol mixture (volume ratio 8:2), the conductive agents of the second and third layers of slurry are graphene, the binder is PVDF, the solvent is NMP, and the mass ratios of the active material, conductive agent, and binder in each layer remain unchanged. The corresponding ternary materials, thicknesses, and particle size D50 are different, and the porosities of the first active layer 21, second active layer 22, and third active layer 23 are different. See Table 1 below for details.

[0139] Example 7

[0140] Example 7 provides a positive electrode sheet, which is different from Example 1 in that: the thickness values of the first active layer 21, second active layer 22, and third active layer 23 are different, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0141] Comparative Example 1

[0142] Comparative Example 1 provides a positive electrode sheet, which is different from Example 1 in that: the ternary materials used in the first active layer 21, second active layer 22, and third active layer 23 are the same and the elastic modulus values are the same, both being 300 GPa, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0143] Comparative Example 2

[0144] Comparative Example 2 provides a positive electrode sheet, which is different from that of Example 1 in that: the ternary materials used in the first active layer 21, the second active layer 22, and the third active layer 23 are the same and have the same elastic modulus value of 150 GPa, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0145] Comparative Example 3

[0146] Comparative Example 3 provides a positive electrode sheet, which is different from that of Example 1 in that: the elastic modulus values of the first active layer 21, the second active layer 22, and the third active layer 23 increase in sequence, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0147] Comparative Example 4

[0148] Comparative Example 4 provides a positive electrode sheet, which is different from that of Example 1 in that: the elastic modulus value of the first active layer 21 is lower than that of the second active layer 22, and the elastic modulus value of the second active layer 22 is higher than that of the third active layer 23, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0149] Comparative Example 5

[0150] Comparative Example 5 provides a positive electrode sheet, which is different from that of Example 1 in that: it only includes the first active layer 21 with an elastic modulus value of 300 GPa and the second active layer 22 with an elastic modulus value of 250 GPa, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0151] Comparative Example 6

[0152] Comparative Example 6 provides a positive electrode sheet, which is different from that of Example 1 in that: it only includes the first active layer 21 with an elastic modulus value of 300 GPa and the third active layer 23 with an elastic modulus value of 200 GPa, and the corresponding ternary materials, thicknesses, and particle size D50 are different. See Table 1 below for details.

[0153] The specific information of the positive electrode sheets provided in the above examples and comparative examples is shown in Table 1 below;

[0154] Table 1

[0155]

[0156]

[0157] In order to verify the progressiveness of the embodiments of the present application, the positive electrode sheets prepared in the above embodiments and comparative examples were applied to lithium-ion batteries for the following electrical performance tests:

[0158] 1. Preparation steps of the test battery: Cut the electrode sheet into small round pieces with a diameter of 14 mm, assemble a CR2032 type button battery. The prepared electrode sheet is the positive electrode, the separator is of Celgard 2400 type, the counter electrode is a lithium metal sheet, and the electrolyte is 1 mol / L LiPF6 solution (optional), and the solvent is EC / DEC (vol% 1:1) (optional). Charge and discharge the assembled battery at a constant current of 0.05C or 0.1C for 3 cycles for full formation.

[0159] 2. Test conditions / methods for the battery rate performance: Charge and discharge in a constant current mode, with a voltage range of 2.5 - 4.3V, and the rate options are 0.1C, 1C, 3C (optional).

[0160] 3. Test conditions / methods for the battery cycle performance: Charge and discharge in a constant current mode for cycling, the cycling current is 1C, test the capacity retention rate after 100 cycles, and the voltage range is 2.5 - 4.3V.

[0161] 4. Test conditions / methods for the battery energy density: Based on the discharge curve obtained by testing at 0.1C in step 2, calculate according to the energy density formula: Unit mass energy density (Wh / kg) = Discharge capacity (mAh) × Average voltage (V) / Material mass (kg) (The test result of this method is the energy density of the active material body, without considering the mass of other components inside the battery).

[0162] The above test results are shown in Table 2 below:

[0163] Table 2

[0164]

[0165] It can be seen from the above test results that in the positive electrode sheet of the embodiment of the present application, along the direction away from the positive electrode current collector 1, the elastic modulus of the ternary active material in the first active layer 21, the second active layer 22, and the third active layer 23 shows a gradient design of decreasing gradient, which slows down the stress difference between layers and effectively relieves stress concentration, and is beneficial to reducing the tendency of the positive electrode sheet to crack, thereby improving the cycle capacity retention rate. The gradient design of the microstructure of the active layer in the positive electrode sheet fully considers the characteristics of each layer of material, gives play to the advantages of each layer of material, maximizes the utilization rate of material performance, and realizes a positive electrode sheet with high energy density, high rate and long cycle life.

[0166] In Comparative Example 1, ternary materials with high elastic modulus were used for all three active layers, reducing the rate performance and energy density of the positive electrode sheet. In Comparative Example 2, ternary materials with low elastic modulus were used for all three active layers, reducing the cycle performance of the positive electrode sheet. In Comparative Example 3, a reverse gradient design was adopted, which deteriorated the cycle capacity retention rate. In Comparative Example 4, a design method with high elastic modulus in the middle and low elastic modulus on both sides was adopted, also significantly reducing the cycle performance of the positive electrode sheet. In Comparative Examples 5 and 6, a two-layer design was adopted for the active layer, and the cycle stability was significantly reduced.

[0167] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode plate, characterized in that, It includes a positive current collector and at least a positive active layer stacked on one surface of the positive current collector. Along the direction away from the positive current collector, the positive active layer at least includes a first active layer, a second active layer, and a third active layer stacked and adhered; the first active layer, the second active layer, and the third active layer include ternary active materials, and the elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer decrease in a gradient.

2. The positive electrode sheet according to claim 1, wherein The first active layer includes a medium-nickel single-crystal ternary active material with a molar ratio of nickel in transition metals of 50% to 60%, the second active layer includes a medium-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals of 50% to 60%, and the third active layer includes a high-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals higher than 60%.

3. The positive electrode sheet according to claim 2, wherein, In the first active layer, the elastic modulus of the medium-nickel single-crystal ternary active material is 200 GPa to 300 GPa; and / or, in the second active layer, the elastic modulus of the medium-nickel polycrystalline ternary active material is 180 GPa to 250 GPa; and / or, in the third active layer, the elastic modulus of the high-nickel polycrystalline ternary active material is 150 GPa to 200 GPa.

4. The positive electrode sheet according to claim 3, characterized in that, In the first active layer, the Dv50 of the medium-nickel single-crystal ternary active material is 1 μm to 5 μm; and / or, in the second active layer, the Dv50 of the medium-nickel polycrystalline ternary active material is 8 μm to 15 μm; and / or, in the third active layer, the Dv50 of the high-nickel polycrystalline ternary active material is 8 μm to 15 μm.

5. The positive electrode sheet according to claim 4, wherein, The porosity of the first active layer is 5% to 15%; and / or, the porosity of the second active layer is 20% to 35%; and / or, the porosity of the third active layer is 30% to 45%.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized in that, In the positive active layer, the thickness ratio of the first active layer, the second active layer, and the third active layer is (10 - 30):(40 - 60):(20 - 40); and / or, the total thickness of the positive active layer is 70 μm to 300 μm.

7. The positive electrode sheet according to any one of claims 2 to 5, characterized in that In the first active layer, the chemical general formula of the medium-nickel single-crystal ternary active material is LiNi x1 Co y1 Mn z1 O2, where x1 + y1 + z1 = 1, 0.5 ≤ x1 ≤ 0.6, 0.2 ≤ y1 ≤ 0.3, 0.2 ≤ z1 ≤ 0.3; And / or, in the second active layer, the chemical general formula of the medium nickel polycrystalline ternary active material is LiNi x2 Co y2 Mn z2 O2, where x2 + y2 + z2 = 1, 0.5 ≤ x2 ≤ 0.6, 0.2 ≤ y2 ≤ 0.3, 0.2 ≤ z2 ≤ 0.3; And / or, in the third active layer, the chemical general formula of the high-nickel polycrystalline ternary active material is LiNi x3 Co y3 Mn z3 O2, where x3 + y3 + z3 = 1, 0.8 ≤ x3 ≤ 1, 0.05 ≤ y3 ≤ 0.15, 0.05 ≤ z3 ≤ 0.

15.

8. A method for preparing a positive electrode plate, characterized in that, It includes the following steps: Provide a positive current collector, and prepare a first active layer including ternary active materials on at least one surface of the positive current collector; Prepare a second active layer including ternary active materials on the surface of the first active layer facing away from the positive current collector; Prepare a third active layer including ternary active materials on the surface of the second active layer facing away from the first active layer; obtain a positive electrode plate; wherein, the elastic moduli of the ternary active materials in the first active layer, the second active layer, and the third active layer decrease in a gradient.

9. The method for preparing a positive electrode sheet according to claim 8, characterized in that, The first active layer includes a medium-nickel single-crystal ternary active material with a molar ratio of nickel in transition metals of 50% to 60%; the second active layer includes a medium-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals of 50% to 60%; the third active layer includes a high-nickel polycrystalline ternary active material with a molar ratio of nickel in transition metals higher than 60%.

10. A secondary battery, characterized in that, The secondary battery includes a positive electrode sheet as described in any one of claims 1 to 7 and / or a positive electrode sheet prepared by the method as described in any one of claims 8 to 9.

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