Positive pole piece, preparation method thereof and secondary battery

Abstract

The positive pole piece comprises a current collector and a positive pole material layer, the positive pole material layer is arranged on the current collector, the porosity of the position, away from the current collector, of the positive pole material layer is larger than that of the position, close to the current collector, of the positive pole material layer, and the positive pole material layer comprises a tungsten-containing additive; and the tungsten-containing additive content of the positive electrode material layer far away from the current collector is greater than the tungsten-containing additive content of the positive electrode material layer close to the current collector. The positive pole piece has high electronic conductivity and high ionic conductivity.

Description

Technical Field

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

[0002] In recent years, with the rapid development of the global electric vehicle market, higher requirements and standards have been placed on the performance of secondary ion batteries, especially the increasing demand for secondary ion batteries that combine high energy density, high power, and long cycle life. However, the energy density of existing power secondary batteries is insufficient, limiting the driving range of electric vehicles.

[0003] Electrodes are a key factor determining the performance of secondary batteries. Currently, the common approach is to improve the electrode structure, i.e., to prepare high-load thick electrodes and implement rolling, in order to increase the proportion of active material. However, increasing the electrode thickness will increase the charge migration distance and the resistance of the secondary battery, which will restrict the transport dynamics of electrons and ions, reduce the electronic conductivity and ionic conductivity, and ultimately lead to the deterioration of lithium battery performance such as power cycle and hinder its energy density improvement. Summary of the Invention

[0004] The purpose of this application is to provide a positive electrode sheet and its preparation method, as well as a secondary battery, to solve the problem of low electronic conductivity and ionic conductivity of the positive electrode sheet.

[0005] To achieve the objectives of this application, the following technical solution is provided: In a first aspect, the present invention provides a positive electrode sheet, comprising a current collector and a positive electrode material layer, wherein the positive electrode material layer is disposed on the current collector, the porosity of the positive electrode material layer away from the current collector is greater than the porosity of the positive electrode material layer near the current collector, the positive electrode material layer comprises a tungsten-containing additive, and the content of the tungsten-containing additive in the positive electrode material layer away from the current collector is greater than the content of the tungsten-containing additive in the positive electrode material layer near the current collector.

[0006] In some embodiments, along the thickness direction of the positive electrode material layer, the porosity of the positive electrode material layer increases in the direction away from the current collector.

[0007] In some embodiments, the content of the tungsten-containing additive increases in the direction away from the current collector along the thickness direction of the positive electrode material layer.

[0008] In some embodiments, the positive electrode material layer includes a first positive electrode material layer and a second positive electrode material layer, the first positive electrode material layer is disposed on the current collector, the second positive electrode material layer is disposed on the first positive electrode material layer, and the second positive electrode material layer includes the tungsten-containing additive; the porosity of the second positive electrode material layer is greater than the porosity of the first positive electrode material layer.

[0009] In some embodiments, the tungsten-containing additive accounts for 0.8% to 9% of the mass of the second cathode material layer.

[0010] In some embodiments, the chemical formula of the tungsten-containing additive is Li. a M a W 1-a O3, M includes at least one of V, Nb or Ta, and a is 0.5 to 0.9.

[0011] In some embodiments, the porosity of the first positive electrode material layer is 10% to 20%.

[0012] In some embodiments, the porosity of the second positive electrode material layer is 30% to 40%.

[0013] In some embodiments, the thickness of the first positive electrode material layer is 100 μm to 200 μm.

[0014] In some embodiments, the thickness of the second positive electrode material layer is 40 μm to 160 μm.

[0015] In some embodiments, the first positive electrode material layer includes a first positive electrode material, the second positive electrode material layer includes a second positive electrode material, the particle size of the first positive electrode material is 3μm~16μm, and the particle size of the second positive electrode material is 10μm~16μm.

[0016] In some embodiments, the first cathode material includes a first sub-cathode material and a second sub-cathode material, wherein the particle size of the first sub-cathode material is larger than the particle size of the second sub-cathode material.

[0017] In some embodiments, the first cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide.

[0018] In some embodiments, the second cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide.

[0019] In some embodiments, the particle size of the first sub-cathode material is 10 μm to 16 μm.

[0020] In some embodiments, the particle size of the second sub-cathode material is 3μm to 5μm.

[0021] In some embodiments, the mass ratio of the first sub-cathode material to the second sub-cathode material is 1:9 to 9:1.

[0022] In a second aspect, the present invention provides a method for preparing a positive electrode sheet, the method being used to produce a positive electrode sheet as described in the embodiments of the first aspect, the method comprising: preparing a positive electrode slurry, the positive electrode slurry comprising a tungsten-containing additive; coating the positive electrode slurry onto a current collector, and drying it to obtain the positive electrode sheet.

[0023] Thirdly, the present invention provides a secondary battery comprising a negative electrode, a separator, an electrolyte, and a positive electrode as described in the embodiments of the first aspect.

[0024] The positive electrode sheet provided by this invention controls the porosity of the positive electrode material layer away from the current collector to be greater than that near the current collector. This results in a higher compaction density of the positive electrode material layer near the current collector, leading to tighter connections between particles and the formation of more conductive pathways. This, in turn, improves the electronic conductivity of the positive electrode material layer near the current collector. Meanwhile, the greater porosity of the positive electrode material layer away from the current collector not only improves the ionic conductivity of the outer side of the positive electrode material layer but also increases the wetting degree of the electrolyte on the positive electrode sheet and improves the overall liquid retention rate of the positive electrode material layer, thereby further improving the energy density of the positive electrode sheet. Meanwhile, the present invention also adds tungsten-containing additives to the positive electrode material layer, and controls the content of tungsten-containing additives to be higher on the outer side of the positive electrode material layer. As a fast ion conductor, tungsten-containing additives have high ionic conductivity and can provide a fast ion migration channel, thereby further improving the ionic conductivity on the outer side of the positive electrode material layer, accelerating the transport of lithium ions inside the positive electrode sheet, reducing the degree of polarization, improving the battery power performance, and improving the battery dynamic performance, reducing battery impedance or slowing down the growth of battery impedance, improving cycle performance, and enabling the battery to have good long-term stability. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 This is a cross-sectional schematic diagram of a positive electrode sheet according to one implementation method; Figure 2 This is a cross-sectional schematic diagram of the positive electrode sheet in another implementation method; Figure 3This is a flowchart of a method for preparing a positive electrode sheet according to one implementation. Detailed Implementation

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0029] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terminology used in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used in this application includes any and all combinations of one or more of the associated listed items.

[0030] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0031] This invention provides a positive electrode sheet 100, please refer to... Figure 1 The cathode material includes a current collector 10 and a positive electrode material layer 20, wherein the positive electrode material layer 20 is disposed on the current collector 10, the porosity of the positive electrode material layer 20 away from the current collector 10 is greater than the porosity of the positive electrode material layer 20 near the current collector 10, and the positive electrode material layer 20 includes a tungsten-containing additive, the content of the tungsten-containing additive in the positive electrode material layer 20 away from the current collector 10 is greater than the content of the tungsten-containing additive in the positive electrode material layer 20 near the current collector 10.

[0032] In a specific embodiment, the positive electrode 100 can be a lithium-ion positive electrode 100, which is a key carrier for lithium-ion insertion / deintercalation and electron conduction; of course, the positive electrode 100 can also be a sodium-ion positive electrode 100. The current collector 10 is used to support the positive electrode material layer 20 and provide it with mechanical support. The current collector 10 is also used to conduct electrons; optionally, the current collector 10 can be an aluminum foil.

[0033] In a specific embodiment, the positive electrode material layer 20 serves to store and release lithium ions, and electron and ion conduction channels need to be formed on the positive electrode material layer 20 to allow for the flow of electrons and ions. The positive electrode material layer 20 may include a positive electrode active material (hereinafter referred to as the positive electrode material), a conductive agent, a binder, etc.

[0034] In a specific embodiment, please refer to Figure 1 The positive electrode material layer 20 may include a first region 20A close to the current collector 10 and a second region 20B away from the current collector 10, wherein the porosity of the second region 20B is greater than that of the first region 20A. Alternatively, the compaction density of the first region 20A is greater than that of the second region 20B. It should be noted that the porosity and compaction density of the positive electrode material layer 20 are negatively correlated; the higher the compaction density, the lower the porosity.

[0035] In a specific embodiment, the positive electrode 100 further includes a tungsten-containing additive, which is a fast-ion conductor containing tungsten (W) and lithium (Li), allowing lithium ions to migrate rapidly within the solid lattice of the tungsten-containing additive. The tungsten-containing additive content in the second region 20B is greater than that in the first region 20A. Optionally, the tungsten-containing additive has the chemical formula Li. a M a W 1-a O3, M includes at least one of V, Nb or Ta, and a is 0.5 to 0.9. Optionally, a can be 0.5, 0.6, 0.7, 0.8 or 0.9.

[0036] The positive electrode 100 provided by this invention controls the porosity of the positive electrode material layer 20 away from the current collector 10 to be greater than that of the positive electrode material layer 20 near the current collector 10. This results in a higher compaction density of the positive electrode material layer 20 near the current collector 10, leading to a tighter connection between particles and the formation of more conductive pathways. This, in turn, improves the electronic conductivity of the positive electrode material layer 20 near the current collector 10. Meanwhile, the greater porosity of the positive electrode material layer 20 away from the current collector 10 not only improves the ionic conductivity of the positive electrode material layer 20 on its outer side but also increases the wettability of the electrolyte on the positive electrode 100 and improves the overall liquid retention rate of the positive electrode material layer 20, thereby further improving the energy density of the positive electrode 100. Meanwhile, the present invention also adds a tungsten-containing additive to the positive electrode material layer 20, and controls the content of the tungsten-containing additive to be higher on the outer side of the positive electrode material layer 20. As a fast ion conductor, the tungsten-containing additive has high ionic conductivity and can provide a fast ion migration channel, thereby further improving the ionic conductivity on the outer side of the positive electrode material layer 20, accelerating the transport of lithium ions inside the positive electrode 100, reducing the degree of polarization, improving the battery power performance, and improving the battery dynamic performance, reducing battery impedance or slowing down the growth of battery impedance, improving cycle performance, and enabling the battery to have good long-term stability.

[0037] In some embodiments, along the thickness direction of the positive electrode material layer 20, the porosity of the positive electrode material layer 20 increases in the direction away from the current collector 10; and / or, the content of tungsten-containing additives increases in the direction away from the current collector 10. Specifically, the direction perpendicular to the current collector 10 and the positive electrode material layer 20 is the thickness direction of the positive electrode material layer 20; the porosity and / or the content of tungsten-containing additives of the positive electrode material layer 20 are both positively correlated with the increase in the thickness of the positive electrode material layer 20.

[0038] In a specific embodiment, the porosity of the positive electrode material layer 20 can exhibit a gradient variation along the thickness direction of the positive electrode material layer 20. This gradient variation means that the porosity content varies at different locations along the thickness direction of the positive electrode material layer 20. The gradient variation can be linear or step-like. Linear variations include both linear and curvilinear variations.

[0039] In a specific embodiment, the positive electrode material layer 20 may include multiple sub-material layers stacked along the thickness direction, and the porosity difference between adjacent sub-material layers may be the same. For example, the multiple sub-material layers include the (n-1)th sub-material layer (porosity a1), the nth sub-material layer (porosity a2), and the (n+1)th sub-material layer (porosity a3); wherein a2-a1=a3-a2.

[0040] In a specific embodiment, the tungsten-containing additive content of the positive electrode material layer 20 can vary in a gradient along the thickness direction of the positive electrode material layer 20. For example, the tungsten-containing additive content of the (n-1)th sub-material layer is b1, the tungsten-containing additive content of the nth sub-material layer is b2, and the tungsten-containing additive content of the (n+1)th sub-material layer is b3; wherein, b2-b1=b3-b2.

[0041] This invention controls the porosity and tungsten-containing additive content of the positive electrode material layer 20 to increase incrementally, resulting in a gradient porosity distribution structure within the positive electrode material layer 20. The ionic conductivity of the positive electrode material layer 20 increases from the inside out (away from the current collector 10), while the electronic conductivity increases from the outside in. The changes in electronic and ionic conductivity conform to the transport dynamics of electrons and ions, which not only reduces the polarization phenomenon of the positive electrode 100 but also improves the material utilization rate of the positive electrode 100, thus contributing to the improvement of the energy density and cycle performance of the secondary battery. The tungsten-containing additive further increases the change in ionic conductivity, further accelerating the transport of lithium ions inside the electrode, reducing the degree of polarization, and improving the battery power performance.

[0042] For some implementation methods, please refer to Figure 2 The positive electrode material layer 20 includes a first positive electrode material layer 21 and a second positive electrode material layer 22. The first positive electrode material layer 21 is disposed on the current collector 10, and the second positive electrode material layer 22 is disposed on the first positive electrode material layer 21. The second positive electrode material layer 22 includes a tungsten-containing additive. The porosity of the second positive electrode material layer 22 is greater than that of the first positive electrode material layer 21.

[0043] In a specific embodiment, the positive electrode material layer 20 may include at least two layers, namely a first positive electrode material layer 21 and a second positive electrode material layer 22 stacked together. The first positive electrode material layer 21 is disposed between the second positive electrode material layer 22 and the current collector 10. The first positive electrode material layer 21 and the second positive electrode material layer 22 have different porosities and different tungsten-containing additive contents.

[0044] In a specific embodiment, the porosity of the second positive electrode material layer 22 is greater than that of the first positive electrode material layer 21, and the second positive electrode material layer 22 includes a tungsten-containing additive, while the first positive electrode material layer 21 does not contain a tungsten-containing additive. In other embodiments, the porosity of the second positive electrode material layer 22 is greater than that of the first positive electrode material layer 21, and both the first and second positive electrode material layers 21 and 22 include a tungsten-containing additive, with the tungsten-containing additive content of the second positive electrode material layer 22 being greater than that of the first positive electrode material layer 21.

[0045] The present invention provides a positive electrode material layer 20 comprising a first positive electrode material layer 21 and a second positive electrode material layer 22, thereby forming a double-layer structure for the positive electrode sheet 100, which can effectively improve the energy density and power cycle performance of the battery. The first positive electrode material layer 21 is connected to the current collector 10, and improves electron extraction efficiency and energy density through high electronic conductivity and high proportion of active materials. The second positive electrode material layer 22 is located on the outer layer and has greater porosity and fast ion conductor, which can provide a fast ion migration channel to facilitate lithium ion insertion and extraction.

[0046] In some embodiments, the tungsten-containing additive accounts for 0.8% to 9% of the mass of the second cathode material layer 22. Optionally, the tungsten-containing additive accounts for 0.8%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or 9% of the mass of the second cathode material layer 22.

[0047] By ensuring that the mass percentage of tungsten-containing additives in the second cathode material layer 22 is within the range mentioned above, the ionic conductivity of the second cathode material layer 22 can be guaranteed, significantly improving the lithium-ion conduction effect. It can also avoid the situation where the ion channels are too diffuse due to excessive tungsten-containing additives, thus preventing an increase in the internal resistance of the secondary battery.

[0048] In some embodiments, the porosity of the first positive electrode material layer 21 is 10% to 20%. Optionally, the porosity of the first positive electrode material layer 21 can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, or 20%.

[0049] In some embodiments, the porosity of the second positive electrode material layer 22 is 30% to 40%. Optionally, the porosity of the second positive electrode material layer 22 can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, or 40%.

[0050] By satisfying the porosity of the first positive electrode material layer 21 and the porosity of the second positive electrode material layer 22 within the above-mentioned range, the porosity of the first positive electrode material can be less than that of the second positive electrode material layer 22. The first positive electrode material obtains a high compaction density, and its electronic conductivity increases. The second positive electrode material layer 22 forms a highly efficient ion transport network channel, and its ion conductivity increases.

[0051] In some embodiments, the thickness of the first positive electrode material layer 21 is 100 μm to 200 μm. Optionally, the thickness of the first positive electrode material layer 21 can be 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm.

[0052] In some embodiments, the thickness of the second positive electrode material layer 22 is 40 μm to 160 μm. Optionally, the thickness of the second positive electrode material layer 22 can be 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, or 160 μm.

[0053] By satisfying the above-mentioned ranges for the thickness of the first positive electrode material layer 21 and the thickness of the second positive electrode material layer 22, a high-efficiency ion transport network channel can be generated in the positive electrode material layer 20, thereby improving the structural stability and ion transport performance of the positive electrode 100 during the secondary battery cycle and improving the cycle performance of the secondary battery using the positive electrode 100.

[0054] In some embodiments, the first positive electrode material layer 21 includes a first positive electrode material, and the second positive electrode material layer 22 includes a second positive electrode material. The particle size of the first positive electrode material is 3 μm to 16 μm, and the particle size of the second positive electrode material is 10 μm to 16 μm. Specifically, the particle size distribution range of the first positive electrode material is larger than that of the second positive electrode material.

[0055] In a specific embodiment, the first cathode material can be polycrystalline and / or monocrystalline particles; the second cathode material can be polycrystalline particles. The particle size of the smallest particle (first cathode material) in the first cathode material layer 21 can be smaller than the particle size of the smallest particle (second cathode material) in the second cathode material layer 22. In this way, by controlling the particle morphology and particle size distribution, the packing voids can be reduced, and the compaction density of the first cathode material layer 21 can be increased.

[0056] In a specific embodiment, the particle size of the first positive electrode material can be 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or 16μm. The particle size of the second positive electrode material can be 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or 16μm.

[0057] In a specific embodiment, the first cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide; the second cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide. Optionally, the first cathode material and the second cathode material can be the same or different; for example, both the first cathode material and the second cathode material can be lithium nickel cobalt manganese oxide, lithium iron phosphate, or lithium cobalt oxide; or, the first cathode material includes lithium iron phosphate, and the second cathode material includes lithium cobalt oxide.

[0058] The present invention controls the porosity distribution of the first positive electrode material layer 21 and the second positive electrode material layer 22 by setting different distribution ranges of the first positive electrode material and the second positive electrode material material layer 22, so that the first positive electrode material layer 21 and the second positive electrode material layer 22 form a pore structure with a suitable gradient distribution, reducing the resistance to ion transport and facilitating the transfer of ions between the positive electrode plates 100.

[0059] In some embodiments, the first cathode material includes a first sub-cathode material and a second sub-cathode material, wherein the particle size of the first sub-cathode material is larger than that of the second sub-cathode material. Specifically, the first cathode material layer 21 includes two sub-cathode materials with different particle size ranges, namely a first sub-cathode material and a second sub-cathode material, wherein the minimum particle size of the first sub-cathode material is larger than the maximum particle size of the second sub-cathode material. The first sub-cathode material can be polycrystalline; the second cathode material can be polycrystalline or single-crystal particles.

[0060] In a specific embodiment, the first sub-cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide; the second sub-cathode material includes one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, and lithium cobalt oxide. Optionally, the first sub-cathode material and the second sub-cathode material can be the same or different; for example, both the first sub-cathode material and the second sub-cathode material can be lithium nickel cobalt manganese oxide, lithium iron phosphate, or lithium cobalt oxide; or, the first sub-cathode material includes lithium iron phosphate, and the second sub-cathode material includes lithium cobalt oxide.

[0061] This invention reduces the porosity of the first positive electrode material layer 21 by setting two types of positive electrode particles with different particle sizes in the first positive electrode material layer 21 and filling the pores between the first positive electrode materials with a smaller particle size second positive electrode material. This increases its compaction density, allowing the first positive electrode material layer 21 to form a sufficient number of conductive paths, thereby significantly improving the electronic conductivity of the first positive electrode material layer 21. It also prevents the accumulation of lithium ions on the surface of the positive electrode 100, enabling the positive electrode 100 to better cooperate with the negative electrode and improve the cycle performance of the secondary battery.

[0062] In some embodiments, the particle size of the first sub-cathode material is 10 μm to 16 μm. Optionally, the particle size of the first sub-cathode material can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or 16 μm.

[0063] In some embodiments, the particle size of the second sub-cathode material is 3 μm to 5 μm. Optionally, the particle size of the second sub-cathode material can be 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, 4.2 μm, 4.4 μm, 4.6 μm, 4.8 μm, or 5 μm.

[0064] In some embodiments, the mass ratio of the first sub-cathode material to the second sub-cathode material is 1:9 to 9:1. Optionally, the mass ratio of the first sub-cathode material to the second sub-cathode material can be 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, or 9:1.

[0065] This invention also provides a method for preparing a positive electrode sheet; please refer to [the relevant documentation]. Figure 3 The method for preparing the positive electrode sheet provided in the above embodiments includes the following steps: Step S100: Prepare a positive electrode slurry, which includes a tungsten-containing additive. Step S200: The positive electrode slurry is coated onto the current collector and dried to obtain the positive electrode sheet.

[0066] In a specific embodiment, the positive electrode slurry includes a solid phase component and a liquid phase component (solvent). The solid phase component includes a positive electrode material, a conductive agent, and a binder. The positive electrode material includes one or more of lithium nickel manganese cobalt oxide, lithium iron phosphate, and lithium cobalt oxide. The mass percentage of the positive electrode material in the solid phase component is 80% to 98%; optionally, the mass percentage of the positive electrode material is 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, or 98%.

[0067] In a specific embodiment, the conductive agent includes at least one selected from superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene oxide, and carbon nanofibers. Optionally, the conductive agent accounts for 1% to 10% of the mass of the solid phase component; optionally, the mass percentage of the conductive agent is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%.

[0068] In a specific embodiment, the binder includes at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polyimide (PI), propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. Optionally, the binder accounts for 1% to 10% of the solid phase component by mass; optionally, the binder accounts for 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% by mass.

[0069] In specific embodiments, the current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymeric material substrate and a metal material layer formed on at least one surface of the polymeric material substrate. For example, the metal material may include at least one of aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys. For example, the polymeric material substrate may include at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0070] In a specific embodiment, the positive electrode sheet is typically formed by coating a positive electrode slurry onto a current collector, followed by drying and cold pressing. The positive electrode slurry is typically formed by dispersing the positive electrode material, optional conductive agent, optional binder, and any other components in a solvent and stirring until homogeneous. The solvent can be N-methylpyrrolidone (NMP), but is not limited to it. The solid component accounts for 40% to 70% of the slurry by mass. Optionally, the solid component accounts for 40%, 50%, 60%, or 70% by mass.

[0071] In some embodiments, in step S100, a positive electrode slurry is prepared, which includes a tungsten-containing additive. Specifically, this includes preparing a first positive electrode slurry and a second positive electrode slurry, wherein the second positive electrode slurry includes a tungsten-containing additive. The chemical formula of the tungsten-containing additive is Li. a M a W 1-a O3, M includes at least one of V, Nb or Ta, and a is 0.5 to 0.9.

[0072] In a specific embodiment, the first positive electrode slurry includes a first sub-positive electrode material and a second sub-positive electrode material; wherein, the particle size of the first sub-positive electrode material is 10μm~16μm, the particle size of the second sub-positive electrode material is 3μm~5μm, and the mass ratio of the first sub-positive electrode material to the second sub-positive electrode material is 1:9~9:1.

[0073] In a specific embodiment, the proportions of the second positive electrode material, tungsten-containing additive, conductive agent and binder in the second positive electrode slurry are 80%~98%, 0.8%~9%, 1%~10% and 1%~10%, respectively.

[0074] In some embodiments, in step S200, the positive electrode slurry is coated onto the current collector and dried to obtain the positive electrode sheet. Specifically, this includes coating the first positive electrode slurry onto the current collector, coating the second positive electrode slurry onto the first positive electrode slurry, drying, and cold pressing to obtain the positive electrode sheet.

[0075] In a specific embodiment, the thickness of the first positive electrode material layer is 100μm~200μm, and the thickness of the second positive electrode material layer is 40μm~160μm.

[0076] The present invention also provides a secondary battery, which includes a negative electrode, a separator, an electrolyte, and a positive electrode provided in the above embodiments; or the secondary battery includes a positive electrode obtained by the above preparation method. The negative electrode may include a negative electrode material, a conductive agent, and a binder, wherein the negative electrode material includes artificial graphite, natural graphite, etc.

[0077] The technical solution of the present invention will be described in detail below through specific embodiments.

[0078] Example 1 This embodiment provides a positive electrode sheet, including a current collector (aluminum foil), a first positive electrode material layer, and a second positive electrode material layer stacked together. The first positive electrode material layer includes a first positive electrode material (lithium nickel cobalt manganese oxide), and the second positive electrode material layer includes a second positive electrode material (lithium nickel cobalt manganese oxide) and a tungsten-containing additive (Li). 0.7 V 0.7 W 0.3 O3). The first cathode material includes the first sub-cathode material and the second sub-cathode material.

[0079] The first cathode material layer has a thickness of 150 μm and a porosity of 16%; the second cathode material layer has a thickness of 100 μm and a porosity of 35%. The particle size of the first sub-cathode material is 12 μm to 16 μm, and the particle size of the second sub-cathode material is 4 μm to 5 μm; the mass ratio of the first sub-cathode material to the second sub-cathode material is 3:7. The particle size of the second cathode material is 12 μm to 16 μm.

[0080] The method for preparing the positive electrode sheet provided in this embodiment includes the following steps: 1) The conductive agent (acetylene black), binder (PVDF), and first cathode material are weighed at a mass ratio of 6:4:90 and added to a solvent (NMP). After stirring, a first cathode slurry with a solid content of 45% is obtained. The conductive agent (acetylene black), binder (PVDF), second cathode material, and tungsten-containing additive are weighed at a mass ratio of 6:4:90:4 and added to a solvent (NMP). After stirring, a second cathode slurry with a solid content of 45% is obtained.

[0081] 2) A dual-head coating machine is used for coating. First, the first positive electrode slurry is coated on the current collector, and then the second positive electrode slurry is coated on the first positive electrode slurry. After drying and cold pressing, the positive electrode sheet is obtained.

[0082] Example 2 This embodiment provides a positive electrode sheet. The difference between this embodiment and Embodiment 1 is that the tungsten-containing additive is Li. 0.5 Nb 0.5 W 0.5 O3. The thickness of the first cathode material layer is 160 μm and the porosity is 15%; the thickness of the second cathode material layer is 120 μm and the porosity is 35%. The particle size of the first sub-cathode material is 10 μm to 14 μm, and the particle size of the second sub-cathode material is 3 μm to 4 μm; the mass ratio of the first sub-cathode material to the second sub-cathode material is 5:5.

[0083] The difference between the preparation method of this embodiment and that of Embodiment 1 is as follows: In step 1), the conductive agent (conductive carbon black), binder (polyimide PI), and first cathode material are weighed in a mass ratio of 6:3:91 and added to the solvent (NMP). After stirring, a first cathode slurry with a solid content of 50% is obtained. The conductive agent (conductive carbon black), binder (polyimide PI), second cathode material, and tungsten-containing additive are weighed in a mass ratio of 6:3:91:4 and added to the solvent (NMP). After stirring, a second cathode slurry with a solid content of 50% is obtained.

[0084] All other steps and parameter settings are consistent with those in Example 1.

[0085] Example 3 This embodiment provides a positive electrode sheet. The difference between this embodiment and Embodiment 1 is that the tungsten-containing additive is Li. 0.9 Ta 0.9 W 0. 1O3. The thickness of the first cathode material layer is 140 μm and the porosity is 12%; the thickness of the second cathode material layer is 90 μm and the porosity is 33%. The particle size of the first sub-cathode material is 10 μm to 16 μm, and the particle size of the second sub-cathode material is 3 μm to 5 μm; the mass ratio of the first sub-cathode material to the second sub-cathode material is 7:3.

[0086] The difference between the preparation method of this embodiment and that of Embodiment 1 is as follows: In step 1), the conductive agent, binder, and first positive electrode material are weighed and mixed in a mass ratio of 5:4:91, and after stirring, a first positive electrode slurry with a solid content of 60% is obtained. The conductive agent, binder, second positive electrode material, and tungsten-containing additive are weighed and mixed in a mass ratio of 5:4:91:4, and after stirring, a second positive electrode slurry with a solid content of 60% is obtained.

[0087] All other steps and parameter settings are consistent with those in Example 1.

[0088] Comparative Example 1 This comparative example provides a positive electrode sheet. The difference between this comparative example and Example 1 is that the positive electrode sheet only includes a first positive electrode material layer, and the thickness of the first positive electrode material layer is 250 μm; other steps and parameter settings are consistent with Example 1.

[0089] Comparative Example 2 This comparative example provides a positive electrode sheet. The difference between this comparative example and Example 1 is that the positive electrode sheet only includes a second positive electrode material layer, and the thickness of the second positive electrode material layer is 250 μm; other steps and parameter settings are consistent with Example 1.

[0090] Comparative Example 3 This comparative example provides a positive electrode sheet. The difference between this comparative example and Example 1 is that it includes a current collector (aluminum foil), a second positive electrode material layer and a first positive electrode material layer stacked together. The second positive electrode material layer is disposed on the current collector, and the first positive electrode material layer is disposed on the side of the second positive electrode material layer facing away from the current collector. Other steps and parameter settings are consistent with Example 1.

[0091] Comparative Example 4 This comparative example provides a positive electrode sheet. The difference between this comparative example and Example 1 is that the second positive electrode material layer does not include tungsten-containing additives; other steps and parameter settings are consistent with Example 1.

[0092] Comparative Example 5 This comparative example provides a positive electrode sheet. The difference between this comparative example and Example 1 is that the mass ratio of tungsten additive in the second positive electrode material layer is 15%; other steps and parameter settings are consistent with Example 1.

[0093] The positive electrode plates provided in Examples 1-3 and Comparative Examples 1-5 were subjected to the following tests: (a) Porosity testing, the testing method is as follows: (1) Sample preparation: The cross-section of the positive electrode was prepared by argon ion polishing (CP); (2) Image acquisition: The cross-section of the prepared positive electrode sheet was observed using a scanning electron microscope (SEM) to obtain clear microscopic images of the upper and lower layers (i.e., the second positive electrode material layer and the first positive electrode material layer); (3) Use the image processing software IMAGEJ to process and analyze the SEM photos, calculate the porosity of different regions, and record it in Table 1.

[0094] Test methods (ii) Energy density test, the test method is as follows: Weigh the battery and record the weight as m. Place the battery in the fixture, apply a force of 3000N, charge it to 4.2V with a constant current of 1C, let it rest for 30 minutes, discharge it to 2.75V with a constant current of 1C, let it rest for 30 minutes, and repeat this cycle 3 times. Calculate the discharge capacity (in Ah) and energy E (average of three cycles). Discharge energy density = E / m (in Wh / kg). Record the calculation results in Table 1.

[0095] (III) Ratio performance test, the test method is as follows: At 25°C, the battery was charged to 4.2V at a 1C rate, and then discharged to 2.75V at 1C, 2C, and 4C rates respectively. The discharge capacity was recorded, the capacity retention rate was calculated, and the results are recorded in Table 1.

[0096] (iv) Cyclic performance test, the test method is as follows: At 25°C, the battery was charged to 4.2V at a 1C rate and discharged to 2.75V at a 1C rate. A full charge and discharge cycle test was performed for 1000 cycles. The discharge capacity was recorded, and the capacity retention rate was calculated and recorded in Table 1.

[0097] (v) DCR test, the test method is as follows: At 25°C, the battery was charged to 4.2V at a 1C rate, and then discharged at a 1C rate to adjust the charge to 50% SOC. After resting for 1 hour, the battery was discharged at 1C for 10 seconds. The voltage at the end of the resting period and the end of the discharge period were recorded, and the DCR was calculated and recorded in Table 1.

[0098] Table 1

[0099] Based on Examples 1-3, Comparative Examples 1-3, and Table 1, it can be seen that this application, by controlling the layering of positive electrode material layers on the surface of the current collector and controlling the porosity of the first positive electrode material layer to be less than that of the second positive electrode material layer, forms a gradient porosity distribution structure in the positive electrode material layer. At this time, the ionic conductivity and electronic conductivity of the second positive electrode material layer and the first positive electrode material layer are well matched, which can improve the uniformity of current and lithium ion concentration distribution along the electrode thickness direction. At this time, the lithium battery has a small DCR value, excellent 4C rate performance, and the capacity retention rate after 1000 cycles at 1C reaches up to 89%. At the same time, the pore structure formed by the second positive electrode material layer and the first positive electrode material layer can increase the degree of electrolyte wetting of the electrode sheet, improve the liquid retention rate of the positive electrode material layer, and help improve the energy density of the electrode sheet.

[0100] In contrast, in the comparison documents 1-3, the electron and lithium-ion transport dynamics in the cathode material layer are more restricted, the uniformity of current and lithium-ion concentration distribution along the thickness direction is weakened, the 1C@1000 cycle performance of the lithium battery is significantly reduced, and the liquid retention rate of the cathode material layer is reduced, which also leads to a significant decrease in the 1C energy density of the lithium battery.

[0101] Based on Example 1, Comparative Examples 4-5, and Table 1, it can be seen that by adding a tungsten-containing additive to the second cathode material layer and adjusting the mass ratio of the tungsten-containing additive in the second cathode material layer, this application can significantly improve the internal resistance of the lithium battery and enhance its cycle performance. On the one hand, the tungsten-containing additive can form a sufficient number of fast ion conductors in the cathode material layer, thereby improving the lithium-ion conduction effect. On the other hand, as a fast ion conductor, the tungsten-containing additive can provide a fast ion migration channel, balancing the ion migration rates between the second cathode material layer and the first cathode material layer to be consistent, enabling lithium ions to be extracted from the cathode material layer in a timely and efficient manner. This improves the battery's power performance, enhances its kinetic performance, reduces its impedance, slows down its impedance growth, and significantly improves the battery's cycle performance.

[0102] In the description of the embodiments of this application, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0103] The above-disclosed embodiments are merely preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art will understand that all or part of the processes for implementing the above embodiments and equivalent variations made in accordance with the claims of this application are still within the scope of this application.

Claims

1. A positive electrode plate, characterized in that, include: current collector; A positive electrode material layer is disposed on the current collector. The porosity of the positive electrode material layer away from the current collector is greater than the porosity of the positive electrode material layer near the current collector. The positive electrode material layer includes a tungsten-containing additive. The content of the tungsten-containing additive in the positive electrode material layer away from the current collector is greater than the content of the tungsten-containing additive in the positive electrode material layer near the current collector.

2. The positive electrode sheet according to claim 1, characterized in that, Along the thickness direction of the positive electrode material layer, the porosity of the positive electrode material layer increases in the direction away from the current collector; and / or, the content of the tungsten-containing additive increases in the direction away from the current collector.

3. The positive electrode sheet according to claim 1, characterized in that, The positive electrode material layer includes a first positive electrode material layer and a second positive electrode material layer. The first positive electrode material layer is disposed on the current collector, and the second positive electrode material layer is disposed on the first positive electrode material layer. The second positive electrode material layer includes the tungsten-containing additive. The porosity of the second positive electrode material layer is greater than that of the first positive electrode material layer.

4. The positive electrode sheet according to claim 3, characterized in that, The tungsten-containing additive accounts for 0.8% to 9% of the mass of the second cathode material layer; and / or The chemical formula of the tungsten-containing additive is Li a M a W 1-a O3, M includes at least one of V, Nb or Ta, and a is 0.5 to 0.

9.

5. The positive electrode sheet according to claim 3, characterized in that, The porosity of the first positive electrode material layer is 10%~20%; and / or The porosity of the second positive electrode material layer is 30%~40%; and / or The thickness of the first positive electrode material layer is 100 μm to 200 μm; and / or The thickness of the second positive electrode material layer is 40μm~160μm.

6. The positive electrode sheet according to claim 3, characterized in that, The first positive electrode material layer includes a first positive electrode material, and the second positive electrode material layer includes a second positive electrode material. The particle size of the first positive electrode material is 3μm to 16μm, and the particle size of the second positive electrode material is 10μm to 16μm.

7. The positive electrode sheet according to claim 6, characterized in that, The first positive electrode material includes a first sub-positive electrode material and a second sub-positive electrode material, wherein the particle size of the first sub-positive electrode material is larger than the particle size of the second sub-positive electrode material.

8. The positive electrode sheet according to claim 7, characterized in that, The first cathode material includes one or more of lithium nickel manganese cobalt oxide, lithium iron phosphate, and lithium cobalt oxide; and / or The second cathode material includes one or more of lithium nickel manganese cobalt oxide, lithium iron phosphate, and lithium cobalt oxide; and / or The particle size of the first sub-cathode material is 10 μm to 16 μm; and / or The particle size of the second sub-cathode material is 3μm~5μm; and / or The mass ratio of the first sub-cathode material to the second sub-cathode material is 1:9 to 9:

1.

9. A method for preparing a positive electrode sheet, characterized in that, The preparation method is used to manufacture the positive electrode sheet as described in any one of claims 1-8, and the preparation method includes: A positive electrode slurry is prepared, wherein the positive electrode slurry includes a tungsten-containing additive; The positive electrode slurry is coated onto the current collector and dried to obtain the positive electrode sheet.

10. A secondary battery, characterized in that, It includes a negative electrode, a separator, an electrolyte, and a positive electrode as described in any one of claims 1-8.

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