Positive plate, battery, battery pack and electric equipment

Abstract

The invention provides a positive plate, a battery, a battery pack and electric equipment. The positive plate comprises a positive current collector, a first positive active layer and a second positive active layer, wherein the first positive active layer and the second positive active layer are stacked on at least one side of the positive current collector; the first positive electrode active layer comprises a first positive electrode active material, and the second positive electrode active material comprises a first positive electrode active material and a second positive electrode active material; the first positive electrode active material comprises at least one of a manganese-based positive electrode active material, an iron-based positive electrode active material and a manganese-iron mixed-based positive electrode active material, and the second positive electrode active material comprises at least one of a ternary positive electrode active material, lithium cobalt oxide, lithium nickelate and lithium titanate; the second positive electrode active layer further comprises a second conductive agent, and the second conductive agent comprises at least two linear conductive agents and at least one of a sheet-shaped conductive agent and a point-shaped conductive agent. The positive plate has a special structure and composition, and can effectively improve the impedance increase of the battery and improve the cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries and relates to a positive electrode sheet, and in particular to a positive electrode sheet, a battery, a battery pack and an electrical device. Background Art

[0002] Lithium-ion batteries have been widely used in electric vehicles (such as electric vehicles, hybrid vehicles, electric bicycles, etc.), energy storage systems (such as home energy storage systems, grid energy storage systems, renewable energy storage, etc.), medical equipment, household appliances, toys and entertainment equipment due to their advantages such as high energy density, high voltage platform, low self-discharge rate, lightweight, no memory effect, environmental protection, wide operating temperature range, and flexible design.

[0003] Currently, the cathode active materials used in lithium-ion batteries primarily include manganese-based, iron-based, manganese-iron mixed-based, ternary, and lithium cobalt oxide. While the first three materials offer advantages over other cathode active materials, such as a high voltage platform, high rate capability, high safety, low cost, and environmental friendliness, they suffer from varying degrees of manganese and iron ion dissolution during charge and discharge. This leads to increased battery impedance, damage to the negative electrode SEI film, and loss of active lithium, all of which negatively impact battery cycle performance. To prevent metal ion dissolution, a common technical solution is to coat the cathode material with a coating, such as a chelating agent coating, CEI film, or carbon coating. However, these coatings typically do not contribute to capacity, require additional processing, and increase costs.

[0004] Therefore, how to inhibit the dissolution of metal ions in manganese-based positive electrode materials, iron-based positive electrode materials and manganese-iron mixed-based positive electrode materials, improve battery impedance and enhance battery cycle performance is a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0005] In response to the above defects, the present invention provides a positive electrode sheet with a special structure and composition, which can effectively inhibit the dissolution of metal ions in manganese-based positive electrode materials, iron-based positive electrode materials and manganese-iron mixed-based positive electrode materials, improve the impedance growth of the battery and enhance the battery cycle performance.

[0006] The present invention also provides a battery comprising the positive electrode sheet, so the battery has a lower impedance growth rate and a longer cycle life.

[0007] The present invention also provides a battery pack comprising the positive electrode sheet or the battery, so that the battery pack has a lower impedance growth rate and a longer cycle life.

[0008] The present invention also provides an electrical device comprising the above-mentioned battery or the above-mentioned battery pack, so that the electrical device has higher endurance and stability.

[0009] A first aspect of the present invention provides a positive electrode sheet, the positive electrode comprising a positive electrode current collector, a first positive electrode active layer, and a second positive electrode active layer, wherein the first positive electrode active layer is disposed on at least one side of the positive electrode current collector, and the second positive electrode active layer is disposed on a side of the first positive electrode active layer away from the positive electrode current collector;

[0010] The first positive electrode active layer includes a first positive electrode active material, and the second positive electrode active layer includes the first positive electrode active material and a second positive electrode active material;

[0011] The first positive electrode active material includes at least one of a manganese-based positive electrode active material, an iron-based positive electrode active material, and a manganese-iron mixed-based positive electrode active material, and the second positive electrode active material includes at least one of a ternary positive electrode active material, lithium cobaltate, lithium nickelate, and lithium titanate;

[0012] The second positive active layer further includes a second conductive agent, and the second conductive agent includes at least two linear conductive agents, and at least one of a sheet-like conductive agent and a dot-like conductive agent.

[0013] The positive electrode sheet as described above, wherein the manganese-based positive electrode active material includes at least one of lithium manganese phosphate, lithium-rich manganese-based, lithium manganate, and lithium nickel manganate; the iron-based positive electrode active material includes lithium iron phosphate; and the manganese-iron mixed-based positive electrode active material includes lithium iron manganese phosphate.

[0014] The positive electrode sheet as described above, wherein the linear conductive agent comprises at least one of carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, arrayed carbon nanotubes, metal nanowires, and conductive polymer nanowires;

[0015] And / or, the flake-shaped conductive agent includes graphene and / or conductive graphite;

[0016] And / or, the dot-shaped conductive agent includes conductive carbon black and / or acetylene black.

[0017] In the positive electrode sheet as described above, the linear conductive agent includes a first linear conductive agent and a second linear conductive agent; the diameter of the first linear conductive agent is larger than the diameter of the second linear conductive agent.

[0018] In the positive electrode sheet as described above, the ratio of the diameters of the first linear conductive agent to the second linear conductive agent is 2.5-20.

[0019] The positive electrode sheet as described above, wherein the diameter of the linear conductive agent is 0.4 nm to 500 nm, and the aspect ratio is 50 to 1000; preferably, the diameter of the linear conductive agent is 5 nm to 50 nm, and the aspect ratio is 50 to 300;

[0020] And / or, the sheet diameter of the flake conductive agent is 0.5 μm to 50 μm, and the thickness is 0.3 nm to 200 nm;

[0021] And / or, the particle size of the dot-shaped conductive agent is 15 nm to 80 nm.

[0022] In the positive electrode sheet as described above, based on the total mass of the second conductive agent in the second positive electrode active layer, the total mass content of the linear conductive agent is 25% to 75%, and the total mass content of the remaining types of second conductive agents is 25% to 75%.

[0023] The positive electrode sheet as described above, wherein, based on the total mass of the first positive electrode active material and the second positive electrode active material in the second positive electrode active layer, the mass content of the second positive electrode active material is 5% to 95%, and the mass content of the first positive electrode active material is 5% to 95%.

[0024] The positive electrode sheet as described above, wherein, based on the total mass of the second positive electrode active layer and the first positive electrode active layer, the mass content of the second positive electrode active layer is 10% to 90%, and the mass content of the first positive electrode active layer is 10% to 90%.

[0025] The positive electrode sheet as described above, wherein the thickness of a single side of the first positive electrode active layer is 5 μm to 104 μm; and / or the thickness of a single side of the second positive electrode active layer is 5 μm to 104 μm.

[0026] The positive electrode sheet as described above, wherein the positive electrode further includes a conductive layer, which is arranged between the first positive electrode active layer and the second positive electrode active layer; the conductive layer includes a conductive material, and the conductive material includes at least one of carbon black, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, array tubes, graphene, and conductive graphite; preferably, the thickness of the conductive layer is 1μm to 3μm.

[0027] The positive electrode sheet as described above, wherein the electrode further comprises a polymer coating, and the polymer coating is provided on at least one side of the second positive electrode active layer away from the first positive electrode active layer; preferably, the thickness of the polymer coating is 0.5 μm to 5 μm.

[0028] The positive electrode sheet as described above, wherein the polymer in the polymer coating layer includes at least one of polyimide, polyetherimide, and polyacrylonitrile.

[0029] The positive electrode sheet as described above, wherein the median particle size D50 of the first positive electrode active material is 0.02 μm to 3 μm; and / or the median particle size D50 of the mixed material of the first positive electrode active material and the second positive electrode active material is 0.1 μm to 10 μm.

[0030] The positive electrode sheet as described above, wherein the first positive electrode active layer further comprises a first conductive agent, a first binder, and a first dispersant; based on the total mass of the first positive electrode active layer, the mass content of the first conductive agent is 0.5% to 10%, the mass content of the first binder is 0.5% to 10%, the mass content of the first dispersant is 0 to 1%, and the mass content of the first positive electrode active material is 89.5% to 99%;

[0031] And / or, the second positive electrode active layer also includes a second binder and a second dispersant; based on the total mass of the second positive electrode active layer, the mass content of the second conductive agent is 0.5% to 10%, the mass content of the second binder is 0.5% to 10%, the mass content of the second dispersant is 0 to 1%, and the total mass content of the first positive electrode active material and the second positive electrode active material is 89.5% to 99%.

[0032] The positive electrode sheet as described above, wherein the single-sided compaction density of the first positive electrode active layer is 1.8 g / cm 3 ~3g / cm 3 ; and / or, the single-sided compaction density of the second positive electrode active layer is 2g / cm 3 ~4.5g / cm 3 .

[0033] A second aspect of the present invention provides a battery, comprising the positive electrode sheet described in the first aspect.

[0034] A third aspect of the present invention provides a battery pack, comprising at least two positive electrode sheets according to the first aspect, or the batteries according to the second aspect.

[0035] A fourth aspect of the present invention provides an electrical device, which includes the battery described in the second aspect or the battery pack described in the third aspect.

[0036] The positive electrode sheet in the present invention includes a special structure and composition, which can not only intercept the manganese ions and iron ions dissolved from the first positive electrode active material (including at least one of manganese-based positive electrode active material, iron-based positive electrode active material, and manganese-iron mixed-based positive electrode active material) during the cyclic charge and discharge process, thereby improving the cycle performance of the battery, but also can reduce the content of the second positive electrode active material (including at least one of ternary positive electrode active material, lithium cobalt oxide, lithium nickel oxide, and lithium titanate) to a certain extent, reduce the risk of thermal runaway, and improve the safety of the battery; at the same time, through the combination of special conductive agents, different conductive agents can cooperate with each other to reduce the contact resistance between particles of different materials (i.e., the first positive electrode active material and the second positive electrode active material), thereby reducing the DC resistance growth rate of the battery and making the battery have higher cycle performance; in addition, the pore structure of the active layer can also be optimized, the infiltration and diffusion of the electrolyte can be promoted, and the structural stability of the electrode can be ensured. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the cross-section of a positive electrode sheet according to one embodiment of the present invention;

[0038] Figure 2 is a schematic structural diagram of a cross section of a positive electrode sheet according to another embodiment of the present invention;

[0039] Figure 3 Schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode sheet of the present invention in one embodiment;

[0040] Figure 4 Schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode sheet of the present invention in another embodiment;

[0041] Figure 5 Schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode sheet of the present invention in another embodiment;

[0042] Figure 6 Schematic diagram of the cross-sectional structure of a polymer coating in a positive electrode sheet of the present invention in one embodiment;

[0043] Figure 7 Schematic diagram of the cross-sectional structure of the polymer coating in the positive electrode sheet of the present invention in another embodiment;

[0044] Figure 8 Schematic diagram of the cross-sectional structure of the polymer coating in the positive electrode sheet of the present invention in another embodiment;

[0045] Figure 9 This is a cross-sectional SEM image of the positive electrode sheet in Example 1 of the present invention;

[0046] Figure 10 This is an EDS image of the first positive electrode active material in the positive electrode sheet in Example 1 of the present invention;

[0047] Figure 11 This is an EDS image of the second positive electrode active material in the positive electrode sheet in Example 1 of the present invention;

[0048] Figure 12 This is a surface SEM image of the second positive electrode active layer in the positive electrode sheet in Example 1 of the present invention;

[0049] Figure 13 1 is a cycle capacity retention curve of the battery corresponding to the positive electrode sheets in Example 1 of the present invention and Comparative Example 1;

[0050] Figure 14 : is the charge and discharge curve of the battery corresponding to the positive electrode sheet in Example 1 of the present invention;

[0051] Figure 15 : This is the charge and discharge curve of the battery corresponding to the positive electrode sheet in Comparative Example 1 of the present invention;

[0052] Figure 16 The discharge DC internal resistance curves of the lithium batteries corresponding to the electrodes in Example 1 and Comparative Example 2 of the present invention before cycling at 25°C are shown;

[0053] Figure 17 The discharge DC internal resistance curves of the lithium batteries corresponding to the electrodes in Example 1 and Comparative Example 2 of the present invention after 300 cycles of charge and discharge at 25°C;

[0054] Figure 18 This is an EDS line scan of manganese element in lithium nickel cobalt aluminum oxide of the positive electrode sheet in Example 1 of the present invention before cycling;

[0055] Figure 19 This is an EDS line scan of the manganese element in the lithium nickel cobalt aluminum oxide of the positive electrode sheet in Example 1 of the present invention after cycling.

[0056] Description of reference numerals:

[0057] 1-first positive electrode active layer; 2-second positive electrode active layer; 11-first positive electrode active material; 21-second positive electrode active material; 3-positive electrode current collector; 4-conductive layer; 5-polymer coating. DETAILED DESCRIPTION

[0058] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0059] Among the many positive electrode active materials, manganese-based positive electrode materials, iron-based positive electrode materials, and manganese-iron mixed-based positive electrode materials have been widely used in multiple technical fields due to their advantages such as high voltage platform, high rate performance, high safety, and low cost. However, during the cyclic charge and discharge process, these materials suffer from the dissolution of manganese and iron ions, leading to a series of problems such as increased battery impedance, damage to the negative electrode SEI film, and loss of active lithium, which affects the battery's cycle performance. Currently, coating measures are generally used to alleviate the dissolution problem of manganese and iron ions in the above materials, but this will cause capacity loss and increase costs.

[0060] In order to solve the above problems, the inventors tried to set a layer of positive electrode active material (including manganese-based positive electrode material, iron-based positive electrode material and manganese-iron mixed positive electrode material, hereinafter referred to as the first positive electrode active material) on the side away from the positive electrode current collector, which includes at least one of ternary positive electrode active material, lithium cobalt oxide, lithium nickel oxide and lithium titanate (hereinafter referred to as the second positive electrode active material) to intercept the manganese ions and iron ions dissolved in the first positive electrode active material, reduce the deposition of manganese ions and iron ions on the negative electrode surface, avoid destroying the SEI film and reduce the loss of active ions, thereby improving the cycle life of the battery. However, the crystal structure stability of the second positive electrode active material is poor, and it is easy to undergo phase change during the charge and discharge process, accompanied by the release of heat, which is easy to cause thermal runaway. At the same time, the particle size of the second positive electrode active material is generally large, which will lead to larger pores in the positive electrode active layer, which will not only affect the wetting and penetration of the electrolyte, but also have a negative impact on the stability of the electrode structure, thereby affecting the discharge performance, cycle performance and safety performance of the battery.

[0061] Therefore, the inventors blended the second positive electrode active material with the first positive electrode active material. On the one hand, this can optimize the pore structure of the positive electrode active layer, improve the wettability of the electrolyte, and enhance the stability of the electrode structure, so that the battery has better discharge performance, cycle performance and safety performance; on the other hand, it can also reduce the mass content of the second positive electrode active material, which can not only ensure its interception effect on manganese ions and iron ions, but also reduce the risk of thermal runaway and improve the safety of the battery.

[0062] However, after blending, the first and second cathode active materials are different materials, and the conductivity, lithium ion diffusion capacity, and charge-discharge volume change of different materials are inconsistent. This leads to a large contact resistance between the particles of different materials, which in turn causes an increase in the internal resistance growth rate of the electrode during the cycle. Therefore, it is necessary to improve the positive electrode active layer after blending to reduce the contact resistance between the different materials and thus reduce the DC resistance growth rate of the battery during the cycle.

[0063] After extensive research, the inventors found that linear conductive agents have better connectivity and conductivity than other conductive agents. By using two or more linear conductive agents for blending, and blending at least one of point-shaped conductive agents and sheet-shaped conductive agents, the problem of electrical connection failure at the direct contact interface of different materials can be significantly improved, the contact resistance between particles of different materials can be reduced, and the battery can have a lower DC resistance growth rate.

[0064] Among them, linear conductive agent refers to a conductive material with a significant extended structure in one dimension (length), while the scales in the other two dimensions (width and height) are within the nanoscale (≤500nm); point conductive agent refers to a conductive material with scales in the three dimensions of space within the nanoscale (≤100nm), usually in the form of particles or spheres; sheet conductive agent refers to a conductive material in the form of thin sheets, with the scale in one dimension (thickness) within the nanoscale (≤200nm).

[0065] Based on the above analysis, the first aspect of the present invention provides a positive electrode sheet, which includes a positive electrode collector, a first positive electrode active layer and a second positive electrode active layer, the first positive electrode active layer is arranged on at least one side of the positive electrode collector, and the second positive electrode active layer is arranged on the side of the first positive electrode active layer away from the positive electrode collector; the first positive electrode active layer includes a first positive electrode active material, and the second positive electrode active material includes a first positive electrode active material and a second positive electrode active material; the first positive electrode active material includes at least one of a manganese-based positive electrode active material, an iron-based positive electrode active material, and a manganese-iron mixed-based positive electrode active material, and the second positive electrode active material includes at least one of a ternary positive electrode active material, lithium cobaltate, lithium nickelate, and lithium titanate; the second positive electrode active layer also includes a second conductive agent, and the second conductive agent includes at least two linear conductive agents, and at least one of a sheet conductive agent and a point conductive agent.

[0066] Preferably, the second positive electrode active material includes a ternary positive electrode active material. The ternary positive electrode active material has the advantages of a wide charge and discharge voltage range and high capacity, which can well match the charge and discharge voltage window of the first positive electrode active material and improve the energy density of the battery cell.

[0067] Figure 1 Schematic diagram of the cross-section of the positive electrode sheet of the present invention in one embodiment. Figure 2 FIG1 is a schematic structural diagram of a cross section of a positive electrode sheet according to another embodiment of the present invention. Figure 1 and Figure 2 In the embodiment, the positive electrode sheet includes a positive electrode current collector 3 and a first positive electrode active layer 1 and a second positive electrode active layer 2; in one embodiment, the first positive electrode active layer 1 is arranged on one side of the positive electrode current collector 3, and the second positive electrode active layer 2 is arranged on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3 (such as Figure 1In another embodiment, the first positive electrode active layer 1 is disposed on both sides of the positive electrode current collector 3, and the second positive electrode active layer 2 is disposed on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3 (as shown in FIG. Figure 2 As shown); wherein, the first positive electrode active layer 1 includes a first positive electrode active material 11, and the second positive electrode active layer 2 includes a first positive electrode active material 11 and a second positive electrode active material 21.

[0068] When the first positive electrode active layer 1 and the second positive electrode active layer 2 are provided on both sides of the positive electrode current collector 3, the first positive electrode active layer 1 and the second positive electrode active layer 2 on both sides of the positive electrode current collector 3 are symmetrically arranged, that is, the thickness and composition of the first positive electrode active layer 1 on both sides are completely consistent, and the thickness and composition of the second positive electrode active layer 2 on both sides are completely consistent, thereby avoiding inconsistent capacity on both sides of the positive electrode current collector 3 and reducing the overall performance of the battery, and at the same time simplifying the electrode preparation process.

[0069] Specifically, the second conductive agent in the second positive electrode active layer 2 includes at least two types of linear conductive agents and a sheet-shaped conductive agent, or the second conductive agent includes at least two types of linear conductive agents and a dot-shaped conductive agent, or the second conductive agent includes at least two types of linear conductive agents, dot-shaped conductive agents, and a sheet-shaped conductive agent. The at least two types of linear conductive agents refer to at least two different types of linear conductive agents, or at least two types of the same type of linear conductive agent with different specifications (e.g., diameter, length, tube diameter (including inner and outer diameters), etc.) of the linear conductive agent.

[0070] It should be noted that the types of the first positive electrode active material 11 in the second positive electrode active layer 2 and the first positive electrode active material 11 in the first positive electrode active layer 1 may be the same or different.

[0071] The present invention does not impose any specific limitation on the material of the positive electrode current collector 3 , and it can be any conventional choice in the art, such as aluminum foil or nickel foil.

[0072] Preferably, the manganese-based positive electrode active material in the present invention comprises lithium manganese phosphate (LMP, LiMn b PO4, 0<b≤1), lithium-rich cathode, lithium manganese oxide (LMO, molecular formula is LiMn2O4), lithium nickel manganese oxide (LNMO, molecular formula is LiNi c Mn 1-c O4, 0.5≤c<1); iron-based positive electrode active materials include lithium iron phosphate (LFP, molecular formula LiFePO4); manganese-iron mixed-based positive electrode active materials include lithium manganese iron phosphate (LMFP); ternary positive electrode active materials include at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

[0073] Preferably, the molecular formula of lithium manganese iron phosphate is LiMnx Fe y M 1-x-y PO4, 0.2≤x≤0.9, 0.1≤y≤0.5, 0≤1-xy≤0.2, M comprises at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge. The lithium-rich manganese-based compound has a molecular formula of aLi2MnO3·(1-a)LiEO2, E is a 3d transition metal and / or a 4d transition metal, and 0<a<1.

[0074] Preferably, lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide each independently comprise the chemical composition shown in Formula 2,

[0075] LiNi r Co s E t G 1-r-s-t O2 formula 2,

[0076] In Formula 2, 0<r<1, 0<s<1, 0<t<1, and 0≤1-rst≤0.2, E is selected from Al or Mn, and G includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb and Ge.

[0077] The present invention does not specifically limit the types and specifications of the linear conductive agent, the sheet conductive agent, and the dot conductive agent, and conventional materials in the art can be used.

[0078] The present invention does not specifically limit the sources of the above-mentioned manganese-based positive electrode active materials, iron-based positive electrode active materials, manganese-iron mixed-based positive electrode active materials, ternary positive electrode active materials, lithium cobaltate, lithium nickelate, lithium titanate, and second conductive agents (linear conductive agents, sheet conductive agents, and point conductive agents). Commercially available products or products prepared by conventional preparation methods familiar to those skilled in the art can be used.

[0079] It is understood that the first positive electrode active layer 1 and the second positive electrode active layer 2 further include a binder and a dispersant. The binders in the first positive electrode active layer 1 and the second positive electrode active layer 2 can be the same or different, and the dispersants can be the same or different. The present invention does not specifically limit the types of the binder and dispersant, and they can be binders and dispersants commonly used in lithium-ion batteries. For example, the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, and polyimide; the dispersant can be selected from at least one of polyvinyl pyrrolidone (PVP), polyacrylic acid (PAA), and polyethylene glycol (PEG).

[0080] The first positive electrode active layer 1 also includes a conductive agent (hereinafter referred to as the first conductive agent). The present invention does not specifically limit the type of the conductive agent in the first positive electrode active layer 1. The conductive agent can be the same as the second conductive agent in the second positive electrode active layer 2, or it can be a conductive agent commonly used in lithium-ion batteries. For example, the conductive agent can be selected from at least one of carbon black (CB), acetylene black, conductive graphite, carbon fiber (VGCF), carbon nanotubes (CNT), and graphene (GN). The carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0081] The present invention does not impose any specific restrictions on the ratio of the components in the first positive electrode active layer 1 and the second positive electrode active layer 2 , and a suitable ratio can be selected according to actual conditions.

[0082] The present invention does not specifically limit the preparation method of the positive electrode. In one embodiment, it can be prepared by the following method:

[0083] The raw materials including the first positive electrode active material 11, the first conductive agent, the first binder, and the first dispersant are dispersed in N-methylpyrrolidone (NMP) solvent, and are fully stirred and mixed to form a uniform first positive electrode slurry; the raw materials including the first positive electrode active material 11, the second positive electrode active material 21, the second conductive agent, the second binder, and the second dispersant are dispersed in N-methylpyrrolidone (NMP) solvent, and are fully stirred and mixed to form a uniform second positive electrode slurry; the first positive electrode slurry is coated on at least one side of the positive electrode collector 3, and then the second positive electrode slurry is coated on the surface of the coating formed by the first positive electrode slurry. After drying, rolling, and slitting, the desired positive electrode sheet is obtained.

[0084] The positive electrode sheet in the present invention has a special electrode sheet structure and composition. Specifically, a second positive electrode active layer 2 is provided on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3. The first positive electrode active layer 1 includes a first positive electrode active material 11 (including at least one of a manganese-based positive electrode active material, an iron-based positive electrode active material, and a manganese-iron mixed-based positive electrode active material), and the second positive electrode active layer 2 includes the first positive electrode active material 11 and a second positive electrode active material 21 (including at least one of a ternary positive electrode active material, lithium cobaltate, lithium nickelate, and lithium titanate). At the same time, the second positive electrode active layer 2 also includes a second conductive agent (including at least two linear conductive agents, and at least one of a sheet conductive agent and a point conductive agent). Among them, the second positive electrode active layer 2 can, on the one hand, intercept the manganese ions and iron ions dissolved from the first positive electrode active material 11 during the cyclic charge and discharge process, thereby effectively reducing the metal deposition on the surface of the negative electrode, avoiding damage to the SEI film, and reducing the loss of active ions, thereby improving the cycle performance of the battery; on the other hand, it can also reduce the content of the second positive electrode active material 21 in the second positive electrode active layer 2 to a certain extent, reduce the risk of thermal runaway, improve the safety of the battery, and optimize the pore structure of the second positive electrode active layer 2, improve the electrolyte infiltration ability and the structural stability of the electrode, and further improve the cycle performance of the battery. The second conductive agent can significantly improve the problem of electrical connection failure between the interfaces where particles of different materials are in direct contact, reduce the contact resistance between particles of different materials, and make the battery have a lower DC resistance growth rate, thereby improving the cycle performance of the battery.

[0085] Furthermore, the specific types of the linear conductive agent, the sheet conductive agent and the dot conductive agent in the second conductive agent can be selected to reduce the contact resistance between particles of different materials, so that the battery has higher cycle performance.

[0086] In a specific embodiment, the linear conductive agent includes at least one of carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, array tube, metal nanowire, conductive polymer nanowire; the flake conductive agent includes graphene and / or conductive graphite; the point conductive agent includes conductive carbon black and / or acetylene black.

[0087] It should be noted that carbon nanotubes include single-walled carbon nanotubes, thin multi-walled carbon nanotubes and thick multi-walled carbon nanotubes, among which the diameter A of single-walled carbon nanotubes satisfies: 0.4nm≤A≤3nm, the diameter B of thin multi-walled carbon nanotubes satisfies: 5nm≤B≤20nm, and the diameter C of thick multi-walled carbon nanotubes satisfies: 20nm<C≤150nm.

[0088] In a specific embodiment, the linear conductive agent includes a first linear conductive agent and a second linear conductive agent; the diameter of the first linear conductive agent is larger than the diameter of the second linear conductive agent.

[0089] Specifically, the linear conductive agent in the second conductive agent includes two types of linear conductive agents having different diameters, namely a first linear conductive agent and a second linear conductive agent.

[0090] When the linear conductive agent includes two linear conductive agents with different diameters, the contact resistance between different positive electrode active material particles can be further reduced, the DC resistance growth rate of the battery can be reduced, and the battery can have higher cycle performance.

[0091] In a specific embodiment, the ratio of the diameters of the first linear conductive agent to the second linear conductive agent is 2.5 to 20. When the ratio of the diameters of the first linear conductive agent to the second linear conductive agent is within the aforementioned range, the cycle performance of the battery can be further improved.

[0092] Illustratively, the diameter ratio may be 2.5, 3, 4, 6, 8, 10, 12, 14, 16, 18, 20, or a range consisting of any two values ​​therein.

[0093] In one specific embodiment, the diameter of the linear conductive agent is 0.4 nm to 500 nm, and the aspect ratio is 50 to 1000. Preferably, the diameter of the linear conductive agent is 5 nm to 50 nm, and the aspect ratio is 50 to 300. When the linear conductive agent meets the aforementioned size range, the contact resistance between different particles can be further reduced, resulting in a lower DC resistance growth rate for the battery and improved battery cycle performance.

[0094] Exemplarily, the diameter of the linear conductive agent can be 0.4nm, 1nm, 5nm, 10nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm or a range consisting of any two of them; the aspect ratio can be 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 or a range consisting of any two of them.

[0095] In one embodiment, the flake conductive agent has a diameter of 0.5 μm to 50 μm and a thickness of 0.3 nm to 200 nm. When the flake conductive agent meets the aforementioned size range, the contact resistance between different particles can be further reduced, resulting in a lower DC resistance growth rate for the battery and improved battery cycle performance.

[0096] Exemplarily, the sheet diameter of the flake conductive agent can be 0.5μm, 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm or a range consisting of any two of these values; the thickness can be 0.3nm, 0.5nm, 1nm, 5nm, 10nm, 50nm, 100nm, 150nm, 200nm or a range consisting of any two of these values.

[0097] In one embodiment, the median particle size of the dot-shaped conductive agent is 15 nm to 80 nm. When the dot-shaped conductive agent meets the aforementioned size range, the contact resistance between different particles can be further reduced, resulting in a lower DC resistance growth rate for the battery and improved battery cycle performance.

[0098] Illustratively, the median particle size of the dot-shaped conductive agent may be 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, or a range consisting of any two of these values.

[0099] Furthermore, the mass content of the first positive electrode active material 11 and the second positive electrode active material 21 in the second positive electrode active layer 2, the mass ratio of the first positive electrode active layer 1 and the second positive electrode active layer 2, and the content of various conductive agents in the second conductive agent can be adjusted to make the battery have better overall performance.

[0100] In one embodiment, based on the total mass of the second conductive agent in the second positive electrode active layer 2, the total mass content of the linear conductive agent is 25% to 75%, and the total mass content of the remaining types of second conductive agents is 25% to 75%. Within this range, the various conductive agents in the second conductive agent can better cooperate, combining the advantages of different conductive agents to further reduce the contact resistance between different positive electrode active material particles, reduce the growth rate of DC resistance during battery cycling, and improve battery performance.

[0101] Specifically, when the second conductive agent includes two or more linear conductive agents of different specifications (e.g., carbon fibers of different diameters and / or lengths), or two or more different types of linear conductive agents (e.g., carbon fibers and single-walled carbon nanotubes), the total weight content of all linear conductive agents of all specifications and all types of linear conductive agents in the second conductive agent is between 25% and 75%. The remaining types of second conductive agents are point-shaped conductive agents and / or sheet-shaped conductive agents. When the second conductive agent includes point-shaped conductive agents, the weight content of the point-shaped conductive agents in the second conductive agent is between 25% and 75%. When the second conductive agent includes sheet-shaped conductive agents, the weight content of the sheet-shaped conductive agents in the second conductive agent is between 25% and 75%. When the second conductive agent includes both point-shaped conductive agents and sheet-shaped conductive agents, the total weight content of the sheet-shaped conductive agents and point-shaped conductive agents in the second conductive agent is between 25% and 75%.

[0102] Illustratively, the total mass content of the linear conductive agent in the second conductive agent can be 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or 75%, or a range consisting of any two of these values.

[0103] Illustratively, the total mass content of the remaining types of second conductive agents in the second conductive agent can be 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% or 75%, or a range consisting of any two of these values.

[0104] In the present invention, the surface and cross-section of the positive electrode sheet can be photographed by a scanning electron microscope (SEM) to obtain an SEM morphology image. Based on the morphology and size characteristics of each conductive agent (such as diameter, aspect ratio, sheet diameter, thickness, particle size, etc.), the type of conductive agent in the first positive electrode active layer 1 and the second positive electrode active layer 2 can be determined; even when the size of the conductive agents is similar, the morphology of the conductive agents (such as the cross-section, tortuosity, roughness, etc. of the conductive agents) can be used for judgment.

[0105] After determining the types of conductive agents contained in the first positive electrode active layer 1 and the second positive electrode active layer 2, a big data AI identification method can be used. That is, a library of SEM images of electrode sheet cross-sections containing different proportions of the conductive agents can be established. Through AI identification and comparison, information such as the density range and volume range of the different conductive agents can be calculated, thereby calculating the mass ratio of the various conductive agents in the second positive electrode active layer 2. During the identification process, a large amount of electrode sheet cross-sectional morphological feature information containing different proportions and types of conductive agents is required to ensure the reliability of the statistical results. Therefore, it is necessary to set up positive electrode sheets containing different types and different proportions of conductive agents in order to collect information.

[0106] In a specific embodiment, based on the total mass of the first positive electrode active material 11 and the second positive electrode active material 21 in the second positive electrode active layer 2, the mass content of the second positive electrode active material 21 is 5% to 95%, and the mass content of the first positive electrode active material 11 is 5% to 95%. Within this range, the first positive electrode active material 11 and the second positive electrode active material 21 can better cooperate with each other, not only ensuring the interception effect of manganese ions and iron ions in the bottom layer of the positive electrode sheet (i.e., the first positive electrode active layer 1), but also making the second positive electrode active layer 2 have a better pore structure, which is conducive to the wetting and penetration of the electrolyte, and improves the stability of the electrode sheet, further improving the discharge performance, cycle performance, and safety performance of the battery.

[0107] Illustratively, based on the total mass of the first positive electrode active material 11 and the second positive electrode active material 21 in the second positive electrode active layer 2, the mass content of the second positive electrode active material 21 is 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95%, or a range consisting of any two of these values.

[0108] Illustratively, based on the total mass of the first positive electrode active material 11 and the second positive electrode active material 21 in the second positive electrode active layer 2, the mass content of the first positive electrode active material 11 is 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 95%, or a range consisting of any two of these values.

[0109] In the present invention, the “mass content of the first positive electrode active material 11 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21” and the “mass content of the second positive electrode active material 21 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21” can be obtained by testing using the following method:

[0110] The electrode sheet and dressing (i.e., first positive electrode active layer 1 and second positive electrode active layer 2) were peeled off using adhesive tape to obtain the first positive electrode active layer 1 dressing and the second positive electrode active layer 2 dressing, respectively. Energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), and inductively coupled plasma mass spectrometry (ICP-MS) were used to analyze the main components of the first and second positive electrode active layers 1 and 2.

[0111] For example, when the first positive electrode active material 11 in the first positive electrode active layer 1 is lithium iron manganese phosphate, and the second positive electrode active material 21 in the second positive electrode active layer 2 is lithium iron manganese phosphate and lithium nickel cobalt aluminum oxide, EDS is used to test the stripped first positive electrode active layer 1 dressing and the second positive electrode active layer 2 dressing, respectively, to obtain the weight percentage of each element in the first positive electrode active layer 1 and the weight percentage of each element in the second positive electrode active layer 2. The content of each element obtained by the test can be analyzed to show that the first positive electrode active layer 1 contains Mn and Fe elements, which are consistent with the characteristic elements of lithium iron manganese phosphate; the second positive electrode active layer 2 contains not only Mn and Fe elements, but also Ni, Co, and Al elements, which proves that the active main material in the second positive electrode active layer 2 is a mixture of lithium iron manganese phosphate and lithium nickel cobalt aluminum oxide.

[0112] Similarly, XRD testing can be used to test the phase structures of the peeled first positive electrode active layer 1 dressing and the second positive electrode active layer 2 dressing, respectively. It can be determined that the active main material type of the first positive electrode active layer is lithium iron manganese phosphate, and the active main material in the second positive electrode active layer 2 is a mixture of lithium iron manganese phosphate and lithium nickel cobalt aluminum oxide. Through XRD refinement, the mass ratio of lithium nickel cobalt aluminum oxide to lithium iron manganese phosphate in the second positive electrode active layer 2 can be calculated, and the mass content of the first positive electrode active material 11 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21 can be further converted to obtain the total mass of the second positive electrode active material 21 in the first positive electrode active material 11 and the second positive electrode active material 21.

[0113] Similarly, the composition of the stripped second positive electrode active layer 2 dressing can also be tested by ICP to obtain the mass percentage of each element in the dressing (based on the total mass). Using the mass percentage of the characteristic elements Co and Al in lithium nickel cobalt aluminum oxide, the weight percentage of lithium nickel cobalt aluminum oxide can be calculated, and then the mass ratio of lithium nickel cobalt aluminum oxide to lithium manganese iron phosphate in the second positive electrode active layer 2 can be obtained, and the mass content of the first positive electrode active material 11 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21, as well as the total mass of the second positive electrode active material 21, can be obtained.

[0114] In one embodiment, based on the total mass of the second positive electrode active layer 2 and the first positive electrode active layer 1, the mass content of the second positive electrode active layer 2 is 10% to 90%, and the mass content of the first positive electrode active layer 1 is 10% to 90%. Within this range, not only can the second positive electrode active layer 2 retain manganese and iron ions, but it can also optimize the pore structure of the outer layer of the positive electrode sheet to improve lithium ion transmission.

[0115] Illustratively, based on the total mass of the second positive electrode active layer 2 and the first positive electrode active layer 1, the mass content of the second positive electrode active layer 2 is 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%, or a range consisting of any two of these values.

[0116] Illustratively, based on the total mass of the second positive electrode active layer 2 and the first positive electrode active layer 1, the mass content of the first positive electrode active layer 1 is 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%, or a range consisting of any two of these values.

[0117] In the present invention, the “mass content of the first positive electrode active material layer 1 in the total mass of the first positive electrode active material layer 1 and the second positive electrode active material layer 2” and the “mass content of the second positive electrode active material layer 2 in the total mass of the first positive electrode active material layer 1 and the second positive electrode active material layer 2” can be obtained by the following test method:

[0118] The thickness of the first positive electrode active layer 1 and the second positive electrode active layer 2 in the electrode sheet was determined by scanning electron microscopy (SEM), with the first positive electrode active layer 1 being d1 and the second positive electrode active layer 2 being d2. The electrode sheet and the dressing (i.e., the first positive electrode active layer 1 and the second positive electrode active layer 2) were peeled off using adhesive tape to obtain the first positive electrode active layer 1 and the second positive electrode active layer 2, respectively. Use a circular sampler to cut the first positive electrode active layer 1 and the second positive electrode active layer 2 obtained by peeling off, respectively, to obtain circular pieces of a certain area. The area, thickness and mass of the circular piece obtained by cutting off the first positive electrode active layer 1 are measured, and the area is recorded as S1, the thickness is t1 (t1 < d1), and the mass is m1. The area, thickness and mass of the circular piece obtained by cutting off the second positive electrode active layer 2 are measured, and the area is recorded as S2, the thickness is t2 (t2 < d2), and the mass is m2. Then, the compaction density of the first positive electrode active layer is ρ1 = m1 / (S1×t1), and the compaction density of the second positive electrode active layer is ρ2 = m2 / (S2×t2). The weight ratio of the first positive electrode active layer 1 to the second positive electrode active layer 2 is (d1×ρ1):(d2×ρ2), which is further converted to obtain the mass content of the first positive electrode active material 11 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21, and the mass content of the second positive electrode active material 21 in the total mass of the first positive electrode active material 11 and the second positive electrode active material 21.

[0119] Furthermore, the thickness of the first positive electrode active layer 1 and the thickness of the second positive electrode active layer 2 will also affect the performance of the electrode sheet, thereby affecting the overall performance of the battery.

[0120] In one specific embodiment, the thickness of the first positive electrode active layer 1 on a single side is 5 μm to 104 μm, and the thickness of the second positive electrode active layer 2 on a single side is 5 μm to 104 μm. Within this range, the lithium ion transmission path can be shortened to a certain extent, the electrode sheet surface resistance can be reduced, and the dynamic performance of the electrode sheet can be improved; at the same time, a high energy density can be maintained.

[0121] Illustratively, the thickness of a single side of the first positive electrode active layer 1 is 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 104 μm, or a range consisting of any two of these values; the thickness of a single side of the second positive electrode active layer 2 is 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 104 μm, or a range consisting of any two of these values.

[0122] The “thickness of the first positive electrode active layer 1 ” and the “thickness of the second positive electrode active layer 2 ” in the present invention can be obtained by testing the cross section of the electrode piece through SEM testing.

[0123] Furthermore, the structure of the electrode can be improved to enhance the battery's cycle performance, charge and discharge performance, and safety performance.

[0124] In one embodiment, the positive electrode sheet further includes a conductive layer disposed between the first positive electrode active layer 1 and the second positive electrode active layer 2. The conductive layer comprises a conductive material, including at least one of carbon black, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, arrayed carbon nanotubes, graphene, and conductive graphite. Preferably, the conductive layer has a thickness of 1 μm to 3 μm. The addition of the conductive layer improves the impedance between the first positive electrode active layer 1 and the second positive electrode active layer 2, enhances the electrode sheet's liquid retention and electrolyte wetting ability, facilitates electrolyte penetration and diffusion, and further improves the battery's cycling performance and charge-discharge capabilities.

[0125] Illustratively, the thickness of the conductive layer may be 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or a range consisting of any two values ​​thereof.

[0126] The “thickness of the conductive layer” in the present invention may be consistent with the test method of the “thickness of the first positive electrode active layer 1 ” described above, and will not be elaborated herein.

[0127] It should be noted that when the first positive electrode active layer 1 and the second positive electrode active layer 2 are provided on both sides of the positive electrode current collector 3 , the conductive layer can be provided on one side of the positive electrode current collector 3 or on both sides of the positive electrode current collector 3 .

[0128] In detail, Figure 3 FIG1 is a schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode of the present invention in one embodiment. Figure 4 FIG1 is a schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode of the present invention in another embodiment. Figure 5 FIG1 is a schematic diagram of the cross-sectional structure of the conductive layer in the positive electrode of the present invention in another embodiment. Figure 3-Figure 5 The middle positive electrode includes a positive electrode current collector 3 , a first positive electrode active layer 1 , a second positive electrode active layer 2 , and a conductive layer 4 .

[0129] In one embodiment, the first positive electrode active layer 1 is disposed on one side of the positive electrode current collector 3, the conductive layer 4 is disposed on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3, and the second positive electrode active layer 2 is disposed on the side of the conductive layer 4 away from the first positive electrode active layer 1 (e.g. Figure 3 shown).

[0130] In another embodiment, the first positive electrode active layer 1 is provided on one side of the positive electrode current collector 3, the conductive layer 4 is provided on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3, and the second positive electrode active layer 2 is provided on the side of the conductive layer 4 away from the first positive electrode active layer 1; the first positive electrode active layer 1 is provided on the other side of the positive electrode current collector 3, and the second positive electrode active layer 2 is provided on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3 (such as Figure 4 shown).

[0131] In another embodiment, the first positive electrode active layer 1, the conductive layer 4, and the second positive electrode active layer 2 are symmetrically arranged on both sides of the positive electrode current collector 3, wherein the first positive electrode active layer 1 is arranged on both sides of the positive electrode current collector 3, the conductive layer 4 is arranged on both sides of the first positive electrode active layer 1 away from the positive electrode current collector, and the second positive electrode active layer 2 is arranged on both sides of the conductive layer 4 away from the first positive electrode active layer 1 (such as Figure 5 shown).

[0132] It is understood that the conductive layer 4 also includes a binder. The present invention does not specifically limit the type of the binder, as long as it can provide a bonding effect. For example, the binder in the conductive layer 4 includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, and polyimide.

[0133] The present invention does not impose any specific restrictions on the mass ratio of the conductive material to the binder in the conductive layer, and can be adjusted according to actual conditions. Preferably, the mass ratio of the conductive material to the binder is 1:(0.5-4).

[0134] Illustratively, the mass ratio of the conductive material to the binder may be 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, or a range consisting of any two ratios therein.

[0135] In one embodiment, the electrode further comprises a polymer coating disposed on at least one side of the second positive electrode active layer 2 away from the first positive electrode active layer 1. Preferably, the polymer coating has a thickness of 0.5 μm to 5 μm. The addition of the polymer coating can reduce side reactions between the active material and the electrolyte, further improving the battery's cycle life and safety.

[0136] Illustratively, the thickness of the polymer coating may be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, or a range consisting of any two of these values.

[0137] The “thickness of the polymer coating” in the present invention may be consistent with the test method of the “thickness of the first positive electrode active layer 1 ” described above, and will not be described in detail here.

[0138] In a specific embodiment, the polymer in the polymer coating includes at least one of polyimide, polyetherimide, and polyacrylonitrile.

[0139] It should be noted that when the first positive electrode active layer 1 and the second positive electrode active layer 2 are provided on both sides of the positive electrode current collector 3 , the polymer coating can be provided on one side of the positive electrode current collector or on both sides of the current collector.

[0140] In detail, Figure 6 FIG1 is a schematic diagram of the cross-sectional structure of the polymer coating in the positive electrode of the present invention in one embodiment. Figure 7 FIG1 is a schematic diagram of the cross-sectional structure of the polymer coating in the positive electrode of the present invention in another embodiment. Figure 8 FIG1 is a schematic diagram of the cross-sectional structure of the polymer coating in the positive electrode of the present invention in another embodiment. Figure 6-Figure 8 The middle positive electrode includes a positive electrode current collector 3 , a first positive electrode active layer 1 , a second positive electrode active layer 2 , and a polymer coating 5 .

[0141] In one embodiment, the first positive electrode active layer 1 is disposed on one side of the positive electrode current collector 3, the second positive electrode active layer 2 is disposed on the side of the first positive electrode active layer 1 away from the positive electrode current collector 3, and the polymer coating 5 is disposed on the side of the second positive electrode active layer 2 away from the first positive electrode active layer 1 (e.g. Figure 6 shown).

[0142] In another embodiment, the first positive electrode active layer 1 is disposed on both sides of the positive electrode current collector 3, the second positive electrode active layer 2 is disposed on both sides of the first positive electrode active layer 1 away from the positive electrode current collector 3, and the polymer coating 5 is disposed on either side of the second positive electrode active layer 2 away from the first positive electrode active layer 1 (e.g. Figure 7 shown).

[0143] In another embodiment, the first positive electrode active layer 1, the second positive electrode active layer 2 and the polymer coating 5 are symmetrically arranged on both sides of the positive electrode current collector 3, wherein the first positive electrode active layer 1 is arranged on both sides of the positive electrode current collector 3, the second positive electrode active layer 2 is arranged on both sides of the first positive electrode active layer 1 away from the positive electrode current collector 3, and the polymer coating 5 is arranged on both sides of the second positive electrode active layer 2 away from the first positive electrode active layer 1 (such as Figure 8 shown).

[0144] Furthermore, the particle size distribution of the first positive electrode active material 11 and the particle size distribution of the mixed material of the first positive electrode active material 11 and the second positive electrode active material 21 can be adjusted to further optimize the performance of the electrode sheet.

[0145] In one specific embodiment, the median particle size D50 of the first positive electrode active material 11 is 0.02 μm to 3 μm. Within this range, the particle size of the first positive electrode active material 11 is relatively moderate. On the one hand, this allows the material particles to have a shorter lithium ion transmission path, reducing the lithium ion transmission resistance. At the same time, it also allows the material particles to have higher mechanical strength, avoiding material breakage caused by stress concentration during the charge and discharge process. On the other hand, this helps to improve the conductivity of the particles, thereby improving the material's rate performance and safety performance.

[0146] Illustratively, the median particle size D50 of the first positive active material 11 may be 0.02 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or a range consisting of any two of these values.

[0147] The "median particle size D50" in the present invention refers to the particle size corresponding to when the cumulative volume percentage of the particles reaches 50%, which can be obtained by testing with a laser particle size analyzer.

[0148] Specifically, the positive electrode needs to be pretreated before testing, including the following steps: using tape to peel off the electrode sheet and the dressing (i.e., the first positive electrode active layer 1 and the second positive electrode active layer 2) to obtain the first positive electrode active layer 1 dressing and the second positive electrode active layer 2 dressing. The peeled dressings are placed in hydrochloric acid and stirred or ultrasonicated, and the solid is dried after solid-liquid separation. The dried solid is ground and then dispersed in a solvent (e.g., ethanol, acetone, etc.), ultrasonicated to obtain a suspension, and the suspension is centrifuged to obtain the first positive electrode active material 11 and the mixed material of the first positive electrode active material 11 and the second positive electrode active material 21, respectively.

[0149] In one embodiment, the median particle size D50 of the mixture of the first positive electrode active material 11 and the second positive electrode active material 21 is 0.1 μm to 10 μm. Within this range, the pore structure of the second positive electrode active layer 2 can be further optimized, the wettability of the electrolyte can be improved, and the structural stability of the electrode can be ensured, thereby further improving the dynamic performance and safety performance of the battery.

[0150] Illustratively, the median particle size D50 of the mixed material may be 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, or a range consisting of any two of these values.

[0151] Furthermore, the mass content of each component in the first positive electrode active layer 1 and the second positive electrode active layer 2 can be controlled to improve the overall performance of the electrode.

[0152] In a specific embodiment, the first positive electrode active layer 1 further includes a first conductive agent, a first binder and a first dispersant; based on the total mass of the first positive electrode active layer 1, the mass content of the first conductive agent is 0.5% to 10%, the mass content of the first binder is 0.5% to 10%, the mass content of the first dispersant is 0 to 1%, and the mass content of the first positive electrode active material 11 is 89.5% to 99%.

[0153] Illustratively, based on the total mass of the first positive electrode active layer 1, the mass content of the first conductive agent is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, or a range consisting of any two of the values; the mass content of the first binder is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, or a range consisting of any two of the values; the mass content of the first dispersant is 0%, 0.2%, 0.4%, 0.6%, 0.8% or 1%, or a range consisting of any two of the values; the mass content of the first positive electrode active material 11 is 89.5%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%, or a range consisting of any two of the values.

[0154] In a specific embodiment, the second positive electrode active layer 2 also includes a second binder and a second dispersant; based on the total mass of the second positive electrode active layer 2, the mass content of the second conductive agent is 0.5% to 10%, the mass content of the second binder is 0.5% to 10%, the mass content of the second dispersant is 0 to 1%, and the total mass content of the first positive electrode active material 11 and the second positive electrode active material 21 is 89.5% to 99%.

[0155] Illustratively, based on the total mass of the second positive electrode active layer 2, the mass content of the second conductive agent is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, or a range consisting of any two of the values; the mass content of the second binder is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, or a range consisting of any two of the values; the mass content of the second dispersant is 0%, 0.2%, 0.4%, 0.6%, 0.8% or 1%, or a range consisting of any two of the values; the total mass content of the first positive electrode active material 11 and the second positive electrode active material 21 is 89.5%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%, or a range consisting of any two of the values.

[0156] In a specific embodiment, the single-sided compaction density of the first positive electrode active layer 1 is 1.8 g / cm 3 ~3g / cm 3 Within this range, the single-sided compaction density of the first positive electrode active layer 1 is relatively moderate, which not only helps to improve the capacity of the battery, but also does not affect the electrolyte infiltration and the liquid retention capacity of the electrode, and helps to improve the energy density and cycle performance of the battery.

[0157] For example, the single-sided compaction density of the first positive electrode active layer 1 can be 1.8 g / cm 3, 2g / cm 3 , 2.2g / cm 3 , 2.4g / cm 3 , 2.6g / cm 3 , 2.8g / cm 3 or 3g / cm 3 , or a range consisting of any two values.

[0158] The test method for the "compaction density of the first positive electrode active layer 1" in the present invention may be consistent with the above-mentioned test method, and will not be described in detail here.

[0159] In a specific embodiment, the single-sided compaction density of the second positive electrode active layer 2 is 2 g / cm 3 ~4.5g / cm 3 Within this range, the single-sided compaction density of the second positive electrode active layer 2 is relatively moderate, which not only improves the battery capacity, but also prevents the pores of the second positive electrode active layer 2 from being too small, thereby affecting the electrolyte infiltration ability and its own liquid retention ability, reducing polarization during the battery cycle and helping to improve the battery's cycle performance.

[0160] For example, the single-sided compaction density of the second positive electrode active layer 2 can be 2 g / cm 3 , 2.2g / cm 3 , 2.4g / cm 3 , 2.6g / cm 3 , 2.8g / cm 3 , 3g / cm 3 , 4.2g / cm 3 , 4.4g / cm 3 or 4.5g / cm 3 , or a range consisting of any two values.

[0161] A second aspect of the present invention provides a battery comprising the positive electrode sheet according to the first aspect. Therefore, the battery has a high cycle performance.

[0162] Generally, a battery consists of an electrolyte, a cell, and an enclosure that encapsulates the cell. The electrolyte is injected into the cell within the enclosure, and the cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrodes. The cell can be a laminated cell, where the positive electrode, separator, and negative electrode are alternately stacked, or a wound cell, where the positive electrode, separator, and negative electrode are stacked in sequence and then wound to form a wound cell.

[0163] The present invention does not specifically limit the structure of the negative electrode. In one embodiment, the negative electrode includes a negative electrode current collector and a negative electrode active layer disposed on at least a portion of the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, and a conductive agent.

[0164] The present invention does not specifically limit the material of the negative electrode current collector, and it can be any conventional material in the art. For example, the negative electrode current collector can be any one of copper foil, nickel foam, and copper foam.

[0165] The present invention does not specifically limit the type of negative electrode active material, and can be any negative electrode active material commonly used in batteries. For example, the negative electrode active material can be selected from at least one of graphite, hard carbon, soft carbon, mesocarbon microbeads, silicon-based negative electrode materials (primarily including silicon monoxide and silicon-carbon negative electrodes), and tin-based negative electrode materials (primarily including tin and tin alloys).

[0166] The present invention does not specifically limit the type of binder in the negative electrode, and can be any binder commonly used in current battery negative electrodes. For example, the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide, polyimide, polyvinyl alcohol, and sodium polyacrylate.

[0167] The present invention does not specifically limit the type of conductive agent in the negative electrode, and can be any conductive agent commonly used in current battery negative electrodes. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0168] The present invention does not specifically limit the preparation method of the negative electrode. In one embodiment, the negative electrode can be prepared by a method comprising the following steps:

[0169] The negative electrode active material, conductive agent and binder are dispersed in deionized water and fully stirred to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode collector, and the negative electrode sheet is obtained after drying, rolling and cutting.

[0170] The present invention does not impose any specific restrictions on the amounts of the negative electrode active material, the conductive agent, and the binder, which can be adjusted according to actual conditions.

[0171] The present invention does not specifically limit the composition of the electrolyte, and may include one or more solvents commonly used in current battery electrolytes, as well as electrolyte lithium salts commonly used in current battery electrolytes. For example, the solvent may include at least one of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, and γ-butyrolactone; and the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0172] The present invention does not impose any specific restrictions on the material of the separator, and can be any separator material commonly used in current batteries. For example, the separator can be selected from any of polypropylene separators (PP), polyethylene separators (PE), polypropylene / polyethylene two-layer composite films (PP / PE), polyimide electrospun separators (PI), polypropylene / polyethylene / polypropylene three-layer composite films (PP / PE / PP), cellulose non-woven separators, and separators with ceramic coatings.

[0173] A third aspect of the present invention provides a battery pack, comprising the positive electrode sheet of the first aspect or the battery of the second aspect. The battery pack has the same advantages as the positive electrode sheet, which will not be described in detail.

[0174] Generally, a battery pack includes multiple batteries as described above, which are connected as single cells to form a battery pack. These batteries can be electrically connected using conventional methods in the art, such as series connection, parallel connection, or a combination of these connection methods, without particular limitation.

[0175] A fourth aspect of the present invention provides an electrical device comprising the battery of the second aspect or the battery pack of the third aspect. The electrical device has the same advantages as the positive electrode described above, which will not be described in detail here.

[0176] The electrical equipment in the present invention can be conventional electrical equipment in the field, such as power equipment (such as electric vehicles, electric cars), electronic equipment (such as mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (such as watches, bracelets, VR glasses, etc.), and there is no special limitation on this.

[0177] Hereinafter, the positive electrode provided by the present invention will be described in detail through specific examples.

[0178] Example 1

[0179] Lithium manganese iron phosphate (molecular formula LiMn 0.6 Fe 0.4PO4, D50 is 0.06 μm), multi-walled carbon nanotube thick tubes (diameter is 50 nm, aspect ratio is 50), polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), N-methylpyrrolidone (NMP) are mixed in a mass ratio of 100:2:2.5:0.2:60, and the first positive electrode active slurry is obtained after uniform mixing;

[0180] Lithium nickel cobalt aluminum oxide (molecular formula LiNi 0.7 Co 0.2 Al 0.1 O2, D50 is 3.77μm), lithium manganese iron phosphate (molecular formula LiMn 0.6 Fe 0.4 PO4, D50 is 0.06 μm), a first linear conductive agent multi-walled carbon nanotube thick tube (diameter 50 nm, aspect ratio 50), a second linear conductive agent multi-walled carbon nanotube thin tube (diameter 10 nm, aspect ratio 300), carbon black (median particle size 40 nm), polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), and N-methylpyrrolidone (NMP) are mixed in a mass ratio of 60:40:0.75:0.5:0.75:2.5:0.2:55, and the mixture is evenly mixed to obtain a second positive electrode active slurry;

[0181] The first positive electrode active slurry prepared above is coated on one side of the positive electrode current collector aluminum foil, and then the second positive electrode active slurry is coated on the surface of the coating formed by the first positive electrode coating, and after drying, a single-side coated electrode sheet is obtained; the single-side coated electrode sheet is double-coated, and the first positive electrode active slurry and the second positive electrode active slurry are respectively coated on the other side of the aluminum foil according to the same method as above, with the coating surface density of each layer on both sides being consistent, and then drying is carried out; after drying, it is rolled and cut to obtain the positive electrode sheet of this embodiment;

[0182] The single-surface density of the first positive electrode active layer and the single-surface density of the second positive electrode active layer in the positive electrode sheet are both 125 g / m 2 The single-sided compaction density of the first positive electrode active layer is 2.4g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.78g / cm 3 ; The single-side thickness of the first positive electrode active layer is 52.4 μm, and the single-side thickness of the second positive electrode active layer is 44.9 μm; wherein, based on the total mass of the second positive electrode active layer and the first positive electrode active layer, the mass content of the second positive electrode active layer is 50%, and the mass content of the first positive electrode active layer is 50%; the D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 2.3 μm.

[0183] Example 2

[0184] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0185] In the second positive electrode active slurry, lithium nickel cobalt aluminum oxide is replaced with lithium nickel cobalt manganese oxide (molecular formula LiNi 0.7 Co 0.1 Mn 0.2 O2, D50 is 3.68 μm), and the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide is adjusted to 5:95, that is, based on the total mass of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the mass content of lithium manganese iron phosphate is 5%, and the mass content of lithium nickel cobalt manganese oxide is 95%;

[0186] In the second positive electrode active slurry, the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube, and carbon black is adjusted to 1:0.5:0.5, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 50%, the mass content of the multi-walled carbon nanotube thin tube is 25%, and the mass content of the carbon black is 25%;

[0187] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.4g / cm 3 ; The single-side thickness of the first positive electrode active layer is 93.75 μm, and the single-side thickness of the second positive electrode active layer is 7.35 μm; based on the total mass of the first positive electrode active layer and the second positive electrode active layer, the mass content of the first positive electrode active layer is 90%, and the mass content of the second positive electrode active layer is 10%; the D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 3.46 μm, and the others remain unchanged.

[0188] Example 3

[0189] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0190] In the second positive electrode active slurry, lithium nickel cobalt aluminum oxide is replaced with lithium nickel cobalt manganese oxide (molecular formula LiNi 0.7 Co 0.1 Mn 0.2 O2, D50 is 3.68 μm), and the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide is adjusted to 50:50, that is, based on the total mass of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the mass content of lithium manganese iron phosphate is 50%, and the mass content of lithium nickel cobalt manganese oxide is 50%;

[0191] In the second positive electrode active slurry, the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube, and carbon black is adjusted to 0.5:1:0.5, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 25%, the mass content of the multi-walled carbon nanotube thin tube is 50%, and the mass content of the carbon black is 25%;

[0192] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.1g / cm 3 The thickness of the first positive electrode active layer on one side is 52.1 μm, and the thickness of the second positive electrode active layer on one side is 40.3 μm; the D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 1.12 μm, and the others remain unchanged.

[0193] Example 4

[0194] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0195] In the second positive electrode active slurry, lithium nickel cobalt aluminum oxide is replaced with lithium nickel cobalt manganese oxide (molecular formula LiNi 0.7 Co 0.1 Mn 0.2 O2, D50 is 3.68 μm), and the mass ratio of lithium manganese iron phosphate to lithium nickel cobalt manganese oxide is adjusted to 95:5, that is, based on the total mass of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the mass content of lithium manganese iron phosphate is 95%, and the mass content of lithium nickel cobalt manganese oxide is 5%;

[0196] In the second positive electrode active slurry, the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube, and carbon black is adjusted to 0.75:0.75:0.5, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 37.5%, the mass content of the multi-walled carbon nanotube thin tube is 37.5%, and the mass content of the carbon black is 25%;

[0197] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.45g / cm 3 ; The single-side thickness of the first positive electrode active layer is 10.5 μm, and the single-side thickness of the second positive electrode active layer is 91.84 μm; based on the total mass of the first positive electrode active layer and the second positive electrode active layer, the mass content of the first positive electrode active layer is 10%, and the mass content of the second positive electrode active layer is 90%; the D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 0.14 μm, and the others remain unchanged.

[0198] Example 5

[0199] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 2, except that:

[0200] In both the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate was replaced with a lithium-rich manganese-based material (chemical formula: 0.8Li2MnO3·0.2LiMnO2, D50: 0.6 μm);

[0201] At the same time, the thick multi-walled carbon nanotubes were replaced with array tubes (diameter 25 nm, aspect ratio 500), and the mass ratio of the first linear conductive agent array tube, the second linear conductive agent multi-walled carbon nanotube thin tube, and the carbon black was adjusted to 0.5:0.5:1, that is, based on the total mass of the conductive agent, the mass content of the array tube was 25%, the mass content of the multi-walled carbon nanotube thin tube was 25%, and the mass content of the carbon black was 50%;

[0202] The single-sided compaction density of the first positive electrode active layer is 2.53 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.4g / cm 3 The thickness of the first positive electrode active layer on one side is 88.93 μm, and the thickness of the second positive electrode active layer on one side is 7.35 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 3.48 μm, and the others remain unchanged.

[0203] Example 6

[0204] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 3, except that:

[0205] In both the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate was replaced with a lithium-rich manganese-based material (chemical formula: 0.8Li2MnO3·0.2LiMnO2, D50: 0.6 μm);

[0206] The thick multi-walled carbon nanotubes were replaced with carbon fibers (diameter 200 nm, aspect ratio 200), and the mass ratio of the first linear conductive agent carbon fibers, the second linear conductive agent multi-walled carbon nanotube thin tubes, and the carbon black was adjusted to 0.5:0.5:1, that is, based on the total mass of the conductive agent, the mass content of the carbon fibers was 25%, the mass content of the multi-walled carbon nanotube thin tubes was 25%, and the mass content of the carbon black was 50%;

[0207] The single-sided compaction density of the first positive electrode active layer is 2.53 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.01g / cm 3The thickness of the first positive electrode active layer on one side is 49.41 μm, and the thickness of the second positive electrode active layer on one side is 41.53 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 1.56 μm, and the others remain unchanged.

[0208] Example 7

[0209] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 4, except that:

[0210] In both the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate was replaced with a lithium-rich manganese-based material (chemical formula: 0.8Li2MnO3·0.2LiMnO2, D50: 0.6 μm);

[0211] The thick multi-walled carbon nanotubes were replaced with single-walled carbon nanotubes (diameter 1 nm, aspect ratio 1000), and the mass ratio of the second linear conductive agent single-walled carbon nanotubes, the first linear conductive agent multi-walled carbon nanotube thin tubes, and carbon black was adjusted to 0.5:0.5:1, that is, based on the total mass of the conductive agent, the mass content of the single-walled carbon nanotubes was 25%, the mass content of the multi-walled carbon nanotube thin tubes was 25%, and the mass content of the carbon black was 50%;

[0212] The single-sided compaction density of the first positive electrode active layer is 2.53 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.57g / cm 3 The thickness of the first positive electrode active layer on one side is 9.88 μm, and the thickness of the second positive electrode active layer on one side is 87.55 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 0.67 μm, and the others remain unchanged.

[0213] Example 8

[0214] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 2, except that:

[0215] In the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate is replaced with lithium manganese phosphate material (chemical formula is LiMnPO4, D50 is 1.3μm), and lithium nickel cobalt manganese oxide is replaced with lithium nickel cobalt aluminum oxide (chemical formula is LiNi 0.7 Co 0.2 Al 0.1 O2, D50 is 3.77 μm);

[0216] The mass ratio of the first linear conductive agent, the multi-walled carbon nanotube thick tube, the second linear conductive agent, the multi-walled carbon nanotube thin tube, and the carbon black is adjusted to 0.7:0.7:0.6, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 35%, the mass content of the multi-walled carbon nanotube thin tube is 35%, and the mass content of the carbon black is 30%;

[0217] The single-sided compaction density of the first positive electrode active layer was 2.61 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.4g / cm 3 The thickness of the first positive electrode active layer on one side is 86.2 μm, and the thickness of the second positive electrode active layer on one side is 7.36 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 3.64 μm, and the others remain unchanged.

[0218] Example 9

[0219] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 3, except that:

[0220] In the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate is replaced with lithium manganese phosphate material (chemical formula is LiMnPO4, D50 is 1.3μm), and lithium nickel cobalt manganese oxide is replaced with lithium nickel cobalt aluminum oxide (chemical formula is LiNi 0.7 Co 0.2 Al 0.1 O2, D50 is 3.77 μm);

[0221] At the same time, the thick multi-walled carbon nanotubes were replaced with array tubes (diameter 25 nm, aspect ratio 500), and the mass ratio of the first linear conductive agent array tube, the second linear conductive agent multi-walled carbon nanotube thin tube, and carbon black was adjusted to 0.7:0.7:0.6, that is, the mass content of the array tube was 35%, the mass content of the multi-walled carbon nanotube thin tube was 35%, and the mass content of the carbon black was 30%;

[0222] The single-sided compaction density of the first positive electrode active layer is 2.6 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.98g / cm 3 The thickness of the first positive electrode active layer on one side is 48.07 μm, and the thickness of the second positive electrode active layer on one side is 41.95 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 2.52 μm, and the others remain unchanged.

[0223] Example 10

[0224] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 4, except that:

[0225] In the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate is replaced with lithium manganese phosphate material (chemical formula is LiMnPO4, D50 is 1.3μm), and lithium nickel cobalt manganese oxide is replaced with lithium nickel cobalt aluminum oxide (chemical formula is LiNi 0.7 Co 0.2 Al 0.1 O2, D50 is 3.77 μm);

[0226] The thick multi-walled carbon nanotubes were replaced with carbon fibers (diameter 200 nm, aspect ratio 200), and the mass ratio of the first linear conductive agent carbon fibers, the second linear conductive agent multi-walled carbon nanotube thin tubes, and the carbon black was adjusted to 0.7:0.7:0.6, that is, the mass content of the carbon fibers was 35%, the mass content of the multi-walled carbon nanotube thin tubes was 35%, and the mass content of the carbon black was 30%;

[0227] The single-sided compaction density of the first positive electrode active layer was 2.61 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.67g / cm 3 The thickness of the first positive electrode active layer on one side is 9.6 μm, and the thickness of the second positive electrode active layer on one side is 84.27 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 1.36 μm, and the others remain unchanged.

[0228] Example 11

[0229] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 3, except that:

[0230] In the first positive electrode active material and the second positive electrode active slurry, lithium manganese iron phosphate is replaced with lithium manganese oxide (chemical formula: LiMn2O4, D50: 1.3 μm), and the second positive electrode active slurry also includes lithium cobalt oxide (chemical formula: LiCoO2, D50: 1.1 μm). Then, the mass ratio of lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium cobalt oxide in the second positive electrode active slurry is 25:50:25;

[0231] The thick carbon nanotubes were replaced with single-walled carbon nanotubes (with a diameter of 1 nm and an aspect ratio of 1000), and the mass ratio of single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon black was adjusted to 0.5:0.5:1, that is, based on the total mass of the conductive agent, the mass content of the second linear conductive agent single-walled carbon nanotubes was 25%, the mass content of the first linear conductive agent multi-walled carbon nanotubes was 25%, and the mass content of the carbon black was 50%;

[0232] The single-sided compaction density of the first positive electrode active layer was 2.57 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.99g / cm 3The thickness of the first positive electrode active layer on one side is 48.64 μm, and the thickness of the second positive electrode active layer on one side is 41.8 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 2.5 μm, and the others remain unchanged.

[0233] Example 12

[0234] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that the positive electrode sheet in this embodiment further includes a conductive layer. The specific preparation method is as follows:

[0235] The conductive material (carbon black), binder (PVDF), and solvent (NMP) are uniformly mixed in a mass ratio of 100:0.1:70 to obtain a conductive layer slurry;

[0236] The first positive electrode active slurry in Example 1 is coated on one side of the positive electrode current collector aluminum foil, and then the conductive layer slurry prepared above is coated on the surface of the coating formed by the first positive electrode coating, and then the second positive electrode active slurry in Example 1 is coated on the surface of the coating formed by the conductive layer slurry. After drying, a single-sided coated electrode sheet is obtained; the single-sided coated electrode sheet is reversed, and the first positive electrode active slurry, the conductive layer slurry, and the second positive electrode active slurry are respectively coated on the other side of the aluminum foil according to the same method as above, with the coating surface density of each layer on both sides being consistent, and then dried; after drying, it is rolled and cut to obtain the positive electrode sheet of this embodiment; wherein the thickness of the conductive layer is 2 μm, and the others remain unchanged.

[0237] Example 13

[0238] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that the positive electrode sheet in this embodiment further includes a polymer coating. The specific preparation method is as follows:

[0239] The first positive electrode active slurry in Example 1 is coated on one side of the positive electrode current collector aluminum foil, and then the second positive electrode active slurry is coated on the surface of the coating formed by the first positive electrode coating, and then the polymer material polyimide is coated on the surface of the coating formed by the second positive electrode active slurry. After drying, a single-sided coated electrode sheet is obtained; the single-sided coated electrode sheet is reversed, and the first positive electrode active slurry, the second positive electrode active slurry and the polymer material are respectively coated on the other side of the aluminum foil according to the same method as mentioned above. The coating surface density of each layer on both sides is consistent, and then drying is performed; after drying, roller pressing and slitting are performed to obtain the positive electrode sheet of this embodiment; wherein, the thickness of the polymer coating is 1 μm, and the others remain unchanged.

[0240] Example 14

[0241] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0242] In the second positive electrode active slurry, lithium nickel cobalt aluminum oxide was replaced with lithium nickel oxide (chemical formula: NiCoO2, D50: 3.3 μm);

[0243] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.9g / cm 3 The thickness of the first positive electrode active layer on one side is 52.08 μm, and the thickness of the second positive electrode active layer on one side is 43.1 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 1.35 μm, and the others remain unchanged.

[0244] Example 15

[0245] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that:

[0246] In the second positive electrode active slurry, lithium nickel cobalt aluminum oxide is replaced by lithium titanate (chemical formula is Li4Ti5O 12 , D50 is 0.98 μm);

[0247] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.6g / cm 3 The thickness of the first positive electrode active layer on one side is 53 μm, and the thickness of the second positive electrode active layer on one side is 48.08 μm. The D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 0.65 μm, and the others remain unchanged.

[0248] Example 16

[0249] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube and carbon black in the second positive electrode active slurry is adjusted to 0.4:0.4:1.2, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 20%, the mass content of the multi-walled carbon nanotube thin tube is 20%, and the mass content of the carbon black is 60%.

[0250] Example 17

[0251] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube and carbon black in the second positive electrode active slurry is adjusted to 1.6:0.2:0.2, that is, based on the total mass of the conductive agent, the mass content of the multi-walled carbon nanotube thick tube is 80%, the mass content of the multi-walled carbon nanotube thin tube is 10%, and the mass content of carbon black is 10%.

[0252] Example 18

[0253] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that in the second positive electrode active slurry, the mass ratio of lithium manganese iron phosphate and lithium nickel cobalt aluminum oxide is adjusted to 3:97, that is, based on the total mass of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, the mass content of lithium manganese iron phosphate is 3%, and the mass content of lithium nickel cobalt manganese oxide is 97%;

[0254] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.4g / cm 3 The thickness of the first positive electrode active layer on one side is 52.4 μm, and the thickness of the second positive electrode active layer on one side is 36.76 μm; the D50 of the mixed material of lithium nickel cobalt aluminum oxide and lithium manganese iron phosphate in the second positive electrode active layer is 3.55 μm, and the others remain unchanged.

[0255] Example 19

[0256] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that, based on the total mass of the first positive electrode active layer and the second positive electrode active layer, the mass content of the first positive electrode active layer is 95%, and the mass content of the second positive electrode active layer is 5%;

[0257] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 2.78g / cm 3 ; The single-side thickness of the first positive electrode active layer is 98.9 μm, and the single-side thickness of the second positive electrode active layer is 4.5 μm; the others remain unchanged.

[0258] Example 20

[0259] The preparation method of the positive electrode sheet in this embodiment is basically the same as that in Example 1, except that, based on the total mass of the first positive electrode active layer and the second positive electrode active layer, the mass content of the first positive electrode active layer is 5%, and the mass content of the second positive electrode active layer is 95%;

[0260] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3The single-sided compaction density of the second positive electrode active layer is 2.78g / cm 3 ; The single-side thickness of the first positive electrode active layer is 5.2μm, and the single-side thickness of the second positive electrode active layer is 85.4μm; the others remain unchanged.

[0261] Comparative Example 1

[0262] Lithium manganese iron phosphate (molecular formula LiMn 0.6 Fe 0.4 PO4, D50 is 0.06 μm), multi-walled carbon nanotube thick tube (diameter 50 nm, aspect ratio 50), polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), and N-methyl pyrrolidone (NMP) are mixed in a mass ratio of 100:2:2.5:0.2:60, and the positive electrode active slurry is obtained after uniform mixing;

[0263] The positive electrode active slurry prepared above is coated on one side of the positive electrode current collector aluminum foil, and after drying, a single-sided coated electrode sheet is obtained; the single-sided coated electrode sheet is double-faced, and the positive electrode active slurry is coated on the other side of the aluminum foil in the same manner as above, with the coating density of each layer on both sides being uniform, and then dried; after drying, the positive electrode sheet of this embodiment is obtained by roller pressing and slitting;

[0264] The single-surface density of the positive electrode active layer in the positive electrode sheet is 250 g / m 2 The single-sided compaction density of the positive electrode active layer is 2.4g / cm 3 ; and the single-side thickness of the positive electrode active layer is 105μm.

[0265] Comparative Example 2

[0266] The preparation method of the electrode in this comparative example is basically the same as that in Example 1, except that:

[0267] The second positive electrode active slurry does not include the second linear conductive agent carbon nanotube thin tube, and the mass ratio of the first linear conductive agent multi-walled carbon nanotube thick tube to carbon black is 0.75:1.25, and the other remains unchanged.

[0268] Comparative Example 3

[0269] The preparation method of the electrode in this comparative example is basically the same as that in Example 1, except that:

[0270] The second positive electrode active slurry does not include lithium manganese iron phosphate, and the mass ratio of lithium nickel cobalt aluminum oxide, the first linear conductive agent multi-walled carbon nanotube thick tube, the second linear conductive agent multi-walled carbon nanotube thin tube, carbon black, polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), and N-methyl pyrrolidone (NMP) is 100:0.75:0.5:0.75:2.5:0.2:50;

[0271] The single-sided compaction density of the first positive electrode active layer is 2.4 g / cm 3 The single-sided compaction density of the second positive electrode active layer is 3.45g / cm 3 The thickness of the first positive electrode active layer on one side is 52.08 μm, and the thickness of the second positive electrode active layer on one side is 36.23 μm; the others remain unchanged.

[0272] Comparative Example 4

[0273] The preparation method of the electrode in this comparative example is basically the same as that in Example 1, except that:

[0274] The second positive electrode active slurry does not include the second linear conductive agent multi-walled carbon nanotube thin tubes and the first linear conductive agent multi-walled carbon nanotube thick tubes, and the mass ratio of lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, carbon black, polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), and N-methylpyrrolidone (NMP) is 60:40:2:2.5:0.2:55.

[0275] Comparative Example 5

[0276] The preparation method of the electrode in this comparative example is basically the same as that in Example 1, except that:

[0277] The second positive electrode active slurry does not include the first linear conductive agent multi-walled carbon nanotube thick tube, so the mass ratio of lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, the second linear conductive agent multi-walled carbon nanotube thin tube, carbon black, polyvinylidene fluoride (PVDF), polyvinyl pyrrolidone (PVP), and N-methylpyrrolidone (NMP) is 60:40:0.75:1.25:2.5:0.2:55.

[0278] Test example

[0279] 1. The electrode prepared in Example 1 was subjected to SEM test. The test results are shown in Figure 9 .

[0280] Figure 9 This is a cross-sectional SEM image of the positive electrode sheet in Example 1 of the present invention. Figure 9 In the image, the upper and lower electrode layers can be clearly distinguished. The upper, darker black layer is the second positive active layer, while the lower, lighter black layer is the first positive active layer. This is due to the different conductivity of the two electrodes. The conductive agent density in the upper active layer is more concentrated, resulting in better conductivity, while the conductive agent density in the lower active layer is more dispersed, resulting in weaker conductivity, resulting in the difference in brightness. Based on this, the thickness of the second positive active layer on one side can be measured to be approximately 44.9μm, while the thickness of the first positive active layer on one side is approximately 52.4μm.

[0281] 2. Use tape to peel off the electrode prepared in Example 1 to obtain the first positive electrode active layer dressing and the second positive electrode active layer dressing, respectively. EDS test is performed on each dressing. The test results are shown in Tables 1 and Figure 10 and Figure 11 .

[0282] Table 1

[0283]

[0284]

[0285] From Table 1 and Figure 10-11 It can be seen that:

[0286] The second positive electrode active layer contains Ni, Co, and Al (wt% > 0.2), characteristic elements of lithium nickel cobalt aluminum oxide (NCA), indicating the presence of NCA. The first positive electrode active layer contains Mn and Fe (wt% > 1%), consistent with the characteristic elements of lithium manganese iron phosphate (LMFP). Furthermore, the second positive electrode active layer also contains Mn and Fe, indicating that the active material of the second positive electrode active layer is a mixture of lithium manganese iron phosphate and lithium nickel cobalt aluminum oxide. Figure 10 This is the EDS image of the first positive electrode active layer in the positive electrode sheet in Example 1 of the present invention. Figure 11 This is an EDS graph of the second positive electrode active layer in the positive electrode sheet in Example 1 of the present invention.

[0287] 3. The surface of the positive electrode sheet prepared in Example 1 was subjected to SEM testing to obtain a surface SEM image of the second positive electrode active layer. The test results are shown in FIG. Figure 12 .

[0288] Figure 12 This is a surface SEM image of the second positive electrode active layer in the positive electrode sheet in Example 1 of the present invention. Figure 12 It can be seen that two linear conductive agents (multi-walled carbon nanotube thick tubes and multi-walled carbon nanotube thin tubes) and a dot-shaped conductive agent (carbon black) are clearly present in the second positive electrode active layer.

[0289] 4. The positive electrodes prepared in the above examples and comparative examples were made into batteries, and the electrochemical performance of the prepared batteries was tested, which specifically included the following steps:

[0290] The electrode sheets prepared above were cut into rectangular shapes measuring 3.2 cm (length) by 2.6 cm (width) and the tabs were extended. A 3.4 cm (length) by 2.8 cm (width) graphite anode was used. A dry cell was fabricated by stacking the cathode, separator, and anode in the order of cathode. The dry cell was placed in a 4 cm by 9 cm aluminum-plastic case, injected with electrolyte, and vacuum-sealed. The case was then subjected to formation, aging, and capacity separation to produce a lithium battery. The electrolyte consisted of lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), and dimethyl carbonate (PC) in a mass ratio of 11.3:23.05:24:0.5:1.

[0291] 1) Cyclic performance test

[0292] The lithium battery prepared above was charged to 4.35V at a constant current rate of 1 / 3C at room temperature of 25°C, and then charged at a constant voltage with a cut-off current rate of 0.02C and allowed to stand for 10 minutes; then discharged to 2V at a constant current rate of 1 / 3C, and the initial discharge capacity was recorded as C0; the battery was cycled 300 times according to the above charge and discharge mechanism, and the discharge capacity at the 300th cycle was recorded as C1. The cycle capacity retention rate (%) = (C1 / C0) × 100%. The test results are shown in Tables 2 and Figure 13-15 .

[0293] Figure 13 The cycle capacity retention rate curve of the battery corresponding to the electrode in Example 1 of the present invention and Comparative Example 1 is shown in FIG. Figure 13 It can be seen that compared with the positive electrode in Comparative Example 1 (i.e., LMFP single-layer positive electrode), the lithium battery prepared using the positive electrode in Example 1 (i.e., LMFP|&NCA double-layer positive electrode) has a higher capacity retention rate within 300 cycles at room temperature, and the difference becomes further larger with the increase in the number of cycles.

[0294] Figure 14 is the charge and discharge curve of the battery corresponding to the electrode in Example 1 of the present invention, Figure 15 is the charge and discharge curve of the battery corresponding to the electrode in Comparative Example 1 of the present invention, Figure 14 and 15 It can be seen that compared with the positive electrode in Comparative Example 1 (i.e., LMFP single-layer positive electrode), the lithium battery prepared using the positive electrode in Example 1 (i.e., LMFP|&NCA double-layer positive electrode) has a higher average voltage and less capacity attenuation during the cycle.

[0295] 2) Internal resistance test

[0296] The lithium battery before and after the above cycle was subjected to a DC internal resistance test, and the test method was as follows:

[0297] The battery was placed in a room temperature environment of 25°C and charged to 4.35V at a rate of 1 / 3C0. After that, constant voltage charging was performed with a cut-off current rate of 0.05C0 to obtain a battery with 100% SOC. The battery was left to stand for 10 minutes (as the start of the cycle), and a constant current discharge of 0.1C0 at a rate of 1 / 3C0 was performed. The battery was left to stand for 60 minutes, and the initial voltage at the last 1 second of the standstill was recorded as V0. The battery was then discharged at a constant current rate of 1.5C0 for 30 seconds, and the end voltage at the last 1 second of the discharge was recorded as V1. The battery was left to stand for 10 minutes, and a constant current charge of 0.0125C0 at a rate of 1 / 3C0 was performed (as the end of the cycle). The DC internal resistance is (V0-V1) / (1.5C0). The cycle test was repeated 9 times from the start of the above cycle to the end of the cycle. From the 1st cycle to the 9th cycle, the DCIR values ​​obtained correspond to 90% SOC, 80% SOC, 70% SOC, 60% SOC, 50% SOC, 40% SOC, 30% SOC, 20% SOC, and 10% SOC of the battery, respectively.

[0298] The test results are shown in Table 2 and Figure 16-17 .

[0299] Figure 16 The discharge DC internal resistance curves of the lithium batteries corresponding to the electrodes in Example 1 and Comparative Example 2 of the present invention before cycling at 25°C are shown; Figure 17 The DC internal resistance curves of lithium batteries corresponding to the electrodes in Example 1 and Comparative Example 2 after 300 cycles of charge and discharge at 25°C are shown. The comparison shows that the DC internal resistance of the lithium battery in Example 1 increases less during cycling, especially between 30% and 70% SOC. This demonstrates that the use of a special conductive agent can effectively suppress the rapid increase in the electrode's ohmic resistance.

[0300] 3) Security

[0301] The battery was charged at a constant current rate of 1 / 3C to 4.35V at room temperature (25°C), followed by constant voltage charging at a cutoff current rate of 0.02C. After full charge, a Φ5mm high-temperature resistant steel needle was used to penetrate the battery perpendicular to the narrow side of the soft-pack battery at a speed of 1mm / s (the steel needle remained in the battery). The battery was considered to have passed if it did not catch fire or explode. For each example and comparative example, 10 batteries were tested as a group, and the number of batteries that passed in a group was recorded. If the number of batteries that passed in a group was 6, it was recorded as 6 / 10 PASS. The test results are shown in Table 2.

[0302] Table 2

[0303]

[0304] From Table 2 we can see that:

[0305] Compared with Comparative Examples 1 to 5, the lithium batteries in Examples 1 to 20 can achieve higher cycle performance, lower DC internal resistance change rate and higher safety, and have higher comprehensive performance. Among them, the cycle capacity retention rate of Example 3 at room temperature is 78%, the 40% SOC DC internal resistance change rate after 300 cycles at room temperature is 3.03%, and the number of needle punctures is 9 / 10PASS. In contrast, the cycle capacity retention rate of Comparative Example 3 is only 59%, the 40% SOC DC internal resistance change rate is 13.79%, the safety is relatively low, and the number of needle punctures is 4 / 10PASS; although the lithium batteries in Comparative Examples 1, 2, 4, and 5 have higher safety, the room temperature cycle performance and DC internal resistance change rate are poor. It can be seen that the positive electrode sheet in the present invention can effectively improve the impedance growth of the battery and improve the battery cycle performance, while also improving the battery's needle puncture safety.

[0306] 5. The lithium batteries in Example 1 and Comparative Example 1 were cycled 300 times at room temperature (25°C) according to the charge and discharge mechanism in the cycle performance test. The lithium batteries after the cycle were disassembled to obtain the corresponding negative electrode sheets. The manganese concentration in the negative electrode sheets was detected using an inductively coupled plasma spectrometer (ICP). The test results are shown in Table 3.

[0307] Table 3

[0308]

[0309] From Table 3 we can see that:

[0310] Compared with the lithium battery in comparative example 1, the lithium battery in example 1 has a smaller mass content of manganese in the negative electrode after 300 cycles, which indicates that the positive electrode in example 1 can effectively reduce the dissolution of manganese in the positive electrode.

[0311] 6. The lithium battery in Example 1 was cycled for 300 cycles at room temperature (25°C) according to the aforementioned charge and discharge mechanism. The lithium battery before and after the cycle was disassembled to obtain the corresponding positive electrode sheet. The cross section of the positive electrode sheet was photographed using electron scanning microscopy-energy dispersive spectroscopy (SEM-EDS), and the lithium nickel cobalt aluminum oxide particles were scanned for manganese elements (it is known that NCA particles contain almost no Mn element). The test results are shown in FIG. Figure 18 and Figure 19 .

[0312] Figure 18 This is the EDS line scan of manganese element in lithium nickel cobalt aluminum oxide before cycling in the positive electrode of Example 1 of the present invention. Figure 19 This is the EDS line scan of manganese element in lithium nickel cobalt aluminum oxide after cycling in the positive electrode of Example 1 of the present invention. Figure 18 and Figure 19As can be seen, the Mn signal of the lithium nickel cobalt aluminum oxide material in the positive electrode of Example 1 increases during the charge-discharge cycle. This indicates that the manganese dissolved from the lithium manganese iron phosphate is captured by the lithium nickel cobalt aluminum oxide, thereby reducing the manganese dissolution from the positive electrode. Similarly, the ternary material also has the effect of capturing and intercepting the iron ions dissolved from LMFP.

[0313] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode comprises a positive electrode current collector, a first positive electrode active layer and a second positive electrode active layer, wherein the first positive electrode active layer is arranged on at least one side of the positive electrode current collector, and the second positive electrode active layer is arranged on a side of the first positive electrode active layer away from the positive electrode current collector; The first positive electrode active layer includes a first positive electrode active material, and the second positive electrode active layer includes the first positive electrode active material and a second positive electrode active material; The first positive electrode active material includes at least one of a manganese-based positive electrode active material, an iron-based positive electrode active material, and a manganese-iron mixed-based positive electrode active material, and the second positive electrode active material includes at least one of a ternary positive electrode active material, lithium cobaltate, lithium nickelate, and lithium titanate; The second positive electrode active layer further includes a second conductive agent, and the second conductive agent includes at least two linear conductive agents, and at least one of a sheet-like conductive agent and a dot-like conductive agent.

2. The positive electrode sheet according to claim 1, characterized in that The manganese-based positive electrode active material includes at least one of lithium manganese phosphate, lithium-rich manganese-based, lithium manganate, and lithium nickel manganate; the iron-based positive electrode active material includes lithium iron phosphate; and the manganese-iron mixed-based positive electrode active material includes lithium iron manganese phosphate.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The linear conductive agent comprises at least one of carbon fiber, single-walled carbon nanotube, multi-walled carbon nanotube, array tube, metal nanowire, and conductive polymer nanowire; And / or, the flake-shaped conductive agent includes graphene and / or conductive graphite; And / or, the dot-shaped conductive agent includes conductive carbon black and / or acetylene black.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The linear conductive agent includes a first linear conductive agent and a second linear conductive agent; the diameter of the first linear conductive agent is larger than the diameter of the second linear conductive agent.

5. The positive electrode sheet according to claim 4, characterized in that: The ratio of the diameters of the first linear conductive agent to the second linear conductive agent is 2.5-20.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized in that: The diameter of the linear conductive agent is 0.4 nm to 500 nm, and the aspect ratio is 50 to 1000; preferably, the diameter of the linear conductive agent is 5 nm to 50 nm, and the aspect ratio is 50 to 300; And / or, the sheet diameter of the flake conductive agent is 0.5 μm to 50 μm, and the thickness is 0.3 nm to 200 nm; And / or, the median particle size of the dot-shaped conductive agent is 15 nm to 80 nm.

7. The positive electrode sheet according to any one of claims 1 to 6, characterized in that: Based on the total mass of the second conductive agent in the second positive electrode active layer, the total mass content of the linear conductive agent is 25% to 75%, and the total mass content of other types of second conductive agents is 25% to 75%.

8. The positive electrode sheet according to any one of claims 1 to 7, characterized in that: Based on the total mass of the first positive electrode active material and the second positive electrode active material in the second positive electrode active layer, the mass content of the second positive electrode active material is 5% to 95%, and the mass content of the first positive electrode active material is 5% to 95%.

9. The positive electrode sheet according to any one of claims 1 to 8, characterized in that: Based on the total mass of the second positive electrode active layer and the first positive electrode active layer, the mass content of the second positive electrode active layer is 10% to 90%, and the mass content of the first positive electrode active layer is 10% to 90%.

10. The positive electrode sheet according to any one of claims 1 to 9, characterized in that: The thickness of the first positive electrode active layer on one side is 5 μm to 104 μm; and / or the thickness of the second positive electrode active layer on one side is 5 μm to 104 μm.

11. The positive electrode sheet according to any one of claims 1 to 10, characterized in that: The positive electrode also includes a conductive layer, which is arranged between the first positive electrode active layer and the second positive electrode active layer; the conductive layer includes a conductive material, and the conductive material includes at least one of carbon black, carbon fiber, single-walled carbon nanotubes, multi-walled carbon nanotubes, array tubes, graphene, and conductive graphite; preferably, the thickness of the conductive layer is 1μm to 3μm.

12. The positive electrode sheet according to any one of claims 1 to 11, characterized in that: The positive electrode sheet further includes a polymer coating, which is disposed on at least one side of the second positive electrode active layer away from the first positive electrode active layer. Preferably, the polymer coating has a thickness of 0.5 μm to 5 μm.

13. The positive electrode sheet according to claim 12, characterized in that: The polymer in the polymer coating includes at least one of polyimide, polyetherimide, and polyacrylonitrile.

14. The positive electrode sheet according to any one of claims 1 to 13, characterized in that: The median particle size D50 of the first positive electrode active material is 0.02 μm to 3 μm; and / or the median particle size D50 of the mixed material of the first positive electrode active material and the second positive electrode active material is 0.1 μm to 10 μm.

15. The positive electrode sheet according to any one of claims 1 to 14, characterized in that: The first positive electrode active layer further comprises a first conductive agent, a first binder, and a first dispersant; based on the total mass of the first positive electrode active layer, the mass content of the first conductive agent is 0.5% to 10%, the mass content of the first binder is 0.5% to 10%, the mass content of the first dispersant is 0 to 1%, and the mass content of the first positive electrode active material is 89.5% to 99%; And / or, the second positive electrode active layer also includes a second binder and a second dispersant; based on the total mass of the second positive electrode active layer, the mass content of the second conductive agent is 0.5% to 10%, the mass content of the second binder is 0.5% to 10%, the mass content of the second dispersant is 0 to 1%, and the total mass content of the first positive electrode active material and the second positive electrode active material is 89.5% to 99%.

16. The positive electrode sheet according to any one of claims 1 to 15, characterized in that: The single-sided compaction density of the first positive electrode active layer is 1.8 g / cm 3 ~3g / cm 3 ; and / or, the single-sided compaction density of the second positive electrode active layer is 2g / cm 3 ~4.5g / cm 3 .

17. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 16.

18. A battery pack, characterized in that: The battery pack comprises at least two positive electrode sheets according to any one of claims 1 to 16, or the battery according to claim 17.

19. An electrical device, characterized in that: The electrical device comprises the battery according to claim 17 or the battery pack according to claim 18.

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