Composite positive electrode sheet, method for manufacturing the same, secondary battery, and electric device

Abstract

The application provides a composite positive electrode sheet and a preparation method thereof, a secondary battery and an electric device, and relates to the technical field of secondary batteries. The composite positive electrode sheet comprises a current collector, a first functional layer arranged on at least one surface of the current collector, and a second functional layer arranged on the surface of the first functional layer away from the current collector. The first functional layer comprises a first positive electrode active material; the first positive electrode active material is a single-crystal flaky particle; the second functional layer comprises a second positive electrode active material; and the second positive electrode active material is a polycrystalline spherical particle. The composite positive electrode sheet of the application solves the problem of electronic and ionic transmission mismatch. The bottom single-crystal flaky particle forms surface contact with the current collector to greatly reduce the interface electronic impedance; and the surface polycrystalline spherical particle constructs connected pores to significantly reduce the bulk phase ionic transmission impedance. The structure realizes kinetic matching, and effectively improves the rate performance and cycle stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and more specifically, to a composite positive electrode sheet and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] With the rapid development of energy storage technology and related end-use applications, higher requirements are being placed on the comprehensive performance of secondary batteries, including energy density, power density, and cycle life. The electrochemical performance of a battery largely depends on the structure and design of its electrodes, especially the microstructure of the positive electrode, which has a decisive influence on the battery's kinetic performance.

[0003] Traditional positive electrode plates typically employ a homogeneous structural design, where a single morphology or type of positive electrode active material is mixed with a conductive agent and binder, and then uniformly coated onto a smooth current collector surface. During the charging and discharging process of the electrode plate, the interface region near the current collector primarily handles electron collection and transport, while the bulk interior of the electrode plate, especially the side near the separator, primarily handles ion diffusion and transport.

[0004] However, this traditional homogeneous structure design has inherent physical contradictions in practical applications, making it difficult to simultaneously meet the requirements of electron and ion transport. On the one hand, it is usually difficult to form good surface contact between conventional homogeneous particulate materials and flat current collectors, often exhibiting point or line contact, resulting in a small actual contact area and high contact electronic impedance, thus limiting the high-rate charge and discharge capability of the electrode. On the other hand, if the particle shape and stacking method are changed to increase the interface contact area, it often easily leads to narrow and tortuous stacking pores deep inside the electrode, greatly increasing the long-distance diffusion impedance of ions deep inside the electrode, resulting in greater battery polarization.

[0005] In summary, existing electrodes constructed from a single homogeneous material suffer from a severe "electron-ion transport mismatch." The high electronic impedance at the current collector interface and the high ion diffusion impedance in the electrode bulk phase mutually restrict each other, preventing existing electrode structures from simultaneously meeting the synergistic requirements of low interfacial contact resistance and high ion transport rate. This, in turn, severely impacts the overall rate performance and cycle stability of the secondary battery. An innovative electrode structure design is urgently needed to overcome this bottleneck.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a composite positive electrode sheet and its preparation method, a secondary battery, and an electrical device. The composite positive electrode sheet reduces the interfacial electronic impedance by using bottom monocrystalline flake particles and reduces the bulk ion transport impedance by using top polycrystalline spherical particles, thus synergistically solving the problem of electron and ion transport mismatch and significantly improving the rate and cycle performance of the battery.

[0008] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a composite positive electrode sheet, comprising a current collector, a first functional layer disposed on at least one surface of the current collector, and a second functional layer disposed on the first functional layer away from the surface of the current collector; The first functional layer includes a first positive electrode active material; the first positive electrode active material is a single crystal-like particle; The second functional layer contains a second positive electrode active material; the second positive electrode active material is a polycrystalline spherical particle.

[0009] In an optional implementation, the mass ratio of the first functional layer to the second functional layer is 1:(3~9); and / or, The dry film thickness ratio of the first functional layer to the second functional layer is 1:(1.5~10); and / or, The total compaction density of the composite cathode sheet is 2.5 g / cm³. 3 ~3.5g / cm 3 ; and / or, The aspect ratio of the single-crystal crystalline particles is (5~20):1; and / or, The primary particle size of the single-crystal crystalline particles is 0.5 μm to 8 μm; and / or, The thickness of the first functional layer is 5 μm to 60 μm; and / or, The polycrystalline spherical particles are secondary aggregated spheres with a particle size of 5 μm to 14 μm; and / or, The primary grain size of the polycrystalline spherical particles is 50 nm to 900 nm; and / or, The thickness of the second functional layer is 20 μm to 120 μm; and / or, In the first functional layer, the inactive crystal planes of the monocrystalline particles are parallel to and attached to the current collector surface; and / or, The current collector is aluminum foil; and / or, The thickness of the current collector is 6μm~20μm; In an optional embodiment, the first positive electrode active material and the second positive electrode active material are each independently selected from at least one of layered oxides and polyanionic compounds.

[0010] In an optional embodiment, the layered oxide comprises Na x The sodium-electric layered oxide M1M2O2, wherein M1 is at least one of Ni, Co, Mn, Fe, and Cu; M2 is at least one of W, Zr, Ca, Zn, Al, Mg, Ti, Nb, Y, La, Sr, and Mo; and x ranges from 0.4 to 1.05; and / or, The polyanionic compound is selected from at least one of Na3V2(PO4)3 and NaFePO4.

[0011] In an optional embodiment, the first functional layer further includes a first binder and a first conductive agent; in the first functional layer, the mass ratio of the first positive electrode active material, the first binder, and the first conductive agent is (85~97):(2~8):(1~7); and / or, The second functional layer also includes a second binder and a second conductive agent. In the second functional layer, the mass ratio of the second positive electrode active material, the second binder and the second conductive agent is (83~96):(2~9):(2~8).

[0012] In an optional embodiment, the first adhesive and the second adhesive are each independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose; and / or, The first conductive agent and the second conductive agent are each independently selected from at least one of superconducting carbon black, acetylene black, Ketjen black, carbon fiber, and graphene; and / or, The surface roughness Ra of the current collector in contact with the first functional layer is 0.1 μm to 0.5 μm.

[0013] Secondly, the present invention provides a method for preparing a composite positive electrode sheet as described in any of the foregoing embodiments, comprising: Provide a current collector; and coat at least one surface of the current collector with a first slurry containing a first positive electrode active material to obtain a first functional layer, wherein the first positive electrode active material is a single crystal particle. A second slurry containing a second positive electrode active material is coated onto the side of the first functional layer away from the current collector to obtain a second functional layer. The second positive electrode active material is a polycrystalline spherical particle. The coated electrode sheet is subjected to roll pressing to obtain the composite positive electrode sheet.

[0014] In an optional embodiment, in the step of rolling the coated electrode, the total compaction density of the electrode is controlled to be 2.5 g / cm³. 3 ~3.5g / cm 3 ; and / or, The coated electrode is subjected to a roll forming process, which causes the inactive crystal planes of the monocrystalline particles in the first functional layer to be parallel to and adhere to the surface of the current collector; and / or, The preparation steps of the first slurry and the second slurry each independently include: dissolving the binder in N-methylpyrrolidone solvent to obtain a glue solution, and then uniformly mixing the positive electrode active material and the conductive agent in the glue solution; and / or, Before coating at least one surface of the current collector with a first slurry containing a first positive electrode active material, the method further includes: subjecting the surface of the current collector to electrochemical etching or plasma treatment, such that the surface roughness Ra of the current collector in contact with the first functional layer is 0.1 μm to 0.5 μm; and / or, Before the coating electrode is rolled, the method further includes: drying the coated electrode at 105°C; and / or, After the coated electrode sheet is rolled, the method further includes: cutting the rolled electrode sheet to a preset size; and / or A double-layer co-extrusion coating device is used to simultaneously coat the first slurry and the second slurry onto the surface of the current collector.

[0015] Thirdly, the present invention provides a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte; The positive electrode sheet is a composite positive electrode sheet as described in any of the foregoing embodiments.

[0016] In an optional embodiment, the secondary battery is a sodium-ion battery; the negative electrode sheet includes a hard carbon negative electrode active material; The electrolyte is an electrolyte containing sodium hexafluorophosphate.

[0017] Fourthly, the present invention provides an electrical device comprising the secondary battery described in the foregoing embodiments.

[0018] Compared with existing technologies, this invention provides a composite positive electrode sheet that synergistically solves the severe electron and ion transport mismatch problem in traditional electrodes by setting a double-layer structure with active materials of specific morphologies on the surface and outer side of the current collector. The first functional layer on the surface of the current collector uses single-crystal-shaped particles as the active material. Due to the geometric characteristics of the plate-shaped particles, they easily slide during the electrode forming process, allowing the flat crystal faces to be parallel and adhered to the current collector surface. This large-area "surface-to-surface contact" method completely breaks through the limitations of the traditional point or line contact between particles and flat current collectors, significantly increasing the actual contact area of ​​the interface, thereby greatly reducing the interfacial contact electronic impedance between the current collector and the active material, and providing a high-quality channel for efficient electron conduction.

[0019] Meanwhile, the second functional layer, located on the side of the first functional layer opposite to the current collector, uses polycrystalline spherical particles as the active material. The accumulation of these spherical particles can construct a well-organized and highly interconnected porous network deep within the electrode. This highly developed porous structure significantly shortens the ion diffusion distance and reduces the tortuosity of the ion transport path, allowing the electrolyte to fully wet the electrode. This significantly reduces the long-distance diffusion resistance of ions within the electrode, ensuring an extremely high ion transport rate.

[0020] By combining the underlying monocrystalline crystalline structure with the surface polycrystalline spherical structure, this electrode creates a high-speed electron transport network near the current collector and a smooth ion diffusion channel near the separator. These complementary advantages achieve a high degree of matching between electron and ion dynamics within the electrode, effectively reducing battery polarization and significantly improving the high-rate charge / discharge capability and cycle stability of the rechargeable battery. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a SEM (scanning electron microscope) image of the single-crystal positive electrode active material particles used in Example 1 of this application; Figure 2 This is a SEM image of the polycrystalline spherical positive electrode active material particles used in Example 1 of this application; Figure 3 This is a cross-sectional SEM image of the positive electrode sheet prepared in Comparative Example 2 (single crystal coating) of this application; Figure 4 The XRD (X-ray diffraction) comparison spectrum of the positive electrode prepared in Comparative Example 2 of this application before rolling is shown. Figure 5 The XRD (X-ray diffraction) comparison spectra of the positive electrode prepared in Comparative Example 2 of this application after rolling are shown. Figure 6 This is a cross-sectional SEM image of the positive electrode sheet prepared in Comparative Example 3 (single polycrystalline coating) of this application; Figure 7 This is a comparison of the EIS (electrochemical impedance spectroscopy) of sodium-ion batteries prepared in Example 1 and Comparative Example 1 at 50% SOC (state of charge). Detailed Implementation

[0023] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0024] This application provides a composite cathode, which aims to synergistically solve the inherent electron transport and ion transport mismatch problem in secondary battery electrodes by differentiating the spatial arrangement and microstructure of the materials inside the electrode.

[0025] The composite positive electrode sheet includes: (1) a current collector, (2) a first functional layer disposed on at least one surface of the current collector, and (3) a second functional layer disposed on the first functional layer away from the surface of the current collector.

[0026] The first functional layer contains a first positive electrode active material; the first positive electrode active material is a single crystal-like particle; the second functional layer contains a second positive electrode active material; the second positive electrode active material is a polycrystalline spherical particle.

[0027] The first functional layer, serving as an electron transport optimization layer near the current collector, contains a first positive electrode active material, which is a monocrystalline particle. The principle behind using monocrystalline particles is that these particles have a regular two-dimensional planar structure. During electrode forming, their flat, inactive crystal planes can adaptively spread parallel to each other and closely adhere to the surface of the current collector. This microstructure arrangement transforms the contact mode between the active material and the current collector from traditional point or line contact to large-area surface contact, thereby significantly increasing the actual effective contact area of ​​the interface and substantially reducing the interfacial contact electronic impedance between the current collector and the active material layer, providing excellent interfacial conditions for efficient electron conduction.

[0028] Meanwhile, the second functional layer, serving as an ion transport optimization layer away from the current collector side, contains a second positive electrode active material, which is composed of polycrystalline spherical particles. The principle behind using polycrystalline spherical particles is that when these particles are stacked within the electrode, a rich, well-organized, and highly interconnected pore network naturally forms between them. This three-dimensional porous structure not only facilitates thorough wetting of the electrolyte but also provides wider and shorter channels for ion migration, thereby significantly reducing the long-distance diffusion resistance of ions within the bulk phase of the electrode and ensuring a high ion transport rate.

[0029] In summary, the composite cathode provided in this embodiment achieves a high degree of complementarity and matching between electron dynamics and ion dynamics in physical space by setting monocrystalline flake particles in the first functional layer to reduce electron transport impedance and setting polycrystalline spherical particles in the second functional layer to reduce ion transport impedance. This bilayer composite structure effectively eliminates the performance shortcomings of a single homogeneous electrode, thereby significantly improving the rate performance of secondary batteries containing this composite cathode, reducing internal resistance polarization, and extending cycle life.

[0030] In some embodiments of this application, the material parameters, proportions of each functional layer in the composite cathode and the overall parameters of the cathode have been specifically optimized to further improve the synergistic matching of electron and ion transport.

[0031] In some implementations, the mass ratio of the first functional layer to the second functional layer is 1:(3~9). For example, it can be 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:8.5, 1:9, etc.

[0032] In some embodiments, the dry film thickness ratio of the first functional layer to the second functional layer is 1:(1.5~10). For example, it can be 1:1.5, 1:2, 1:3, 1:4, 1:5, 1:7, 1:8, 1:9, 1:10, etc.

[0033] The mass ratio of the first functional layer to the second functional layer is set to 1:(3~9), or the dry film thickness ratio is set to 1:(1.5~10). Controlling the ratio within these ranges ensures that the bottom layer has sufficient single-crystal particles to construct a dense electronic conductive network, while allowing the surface layer to occupy sufficient bulk space to provide abundant porosity and capacity. If the bottom layer accounts for too high a proportion, it will lead to excessively thick layered accumulation of single-crystal particles, blocking the ion transport channels inside the electrode; if the surface layer accounts for too high a proportion, it may weaken the bottom layer's effect on improving interfacial electronic contact.

[0034] In some embodiments, the total compaction density of the composite cathode is 2.5 g / cm³. 3 ~3.5g / cm 3 For example, it could be 2.5 g / cm³. 3 2.6g / cm 3 2.8g / cm 3 2.9g / cm 3 3.0g / cm 3 3.1g / cm 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 etc.

[0035] An appropriate compaction density can balance the microstructural stability and conductivity of the electrode. This pressure range is sufficient to cause the monocrystalline particles in the first functional layer to slide, allowing their flat, inactive crystal planes to spread parallel and adhere tightly to the current collector surface, thereby forming a large-area surface contact and significantly reducing interfacial electronic impedance. At the same time, this pressure will not cause severe brittle fracture of the polycrystalline spherical particles in the second functional layer, effectively preserving the interconnecting pores for electrolyte wetting and ion transport.

[0036] In some embodiments, the aspect ratio of the monocrystalline flake particles is (5~20):1. For example, it can be 5:1, 6:1, 8:1, 10:1, 12:1, 14:1, 16:1, 18:1, 20:1, etc.

[0037] In some embodiments, the primary particle size of the single-crystal particles is 0.5 μm to 8 μm. For example, it can be 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc.

[0038] In some implementations, the thickness of the first functional layer is 5 μm to 60 μm. For example, it can be 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, 45 μm, 50 μm, 60 μm, etc.

[0039] In some embodiments, the polycrystalline spherical particles are secondary aggregated spheres with a particle size of 5 μm to 14 μm. For example, they can be 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 13 μm, 14 μm, etc.

[0040] In some embodiments, the primary grain size of the polycrystalline spherical particles is 50 nm to 900 nm. For example, it can be 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 800 nm, 900 nm, etc.

[0041] In some embodiments, the thickness of the second functional layer is 20 μm to 120 μm. For example, it can be 20 μm, 30 μm, 40 μm, 60 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, etc.

[0042] In some embodiments, in the first functional layer, the inactive crystal facets of the monocrystalline particles are attached parallel to the current collector surface.

[0043] In some embodiments, the current collector is aluminum foil.

[0044] In some embodiments, the thickness of the current collector is 6μm to 20μm; for example, it can be 6μm, 8μm, 10μm, 12μm, 14μm, 15μm, 16μm, 18μm, 20μm, etc.

[0045] Regarding microstructure parameters, the aspect ratio of the monocrystalline flake particles in the first functional layer is (5~20):1, the primary particle size is 0.5μm~8μm, and the thickness of the first functional layer is 5μm~60μm. Monocrystalline particles with aspect ratios within this range possess excellent two-dimensional spreading characteristics, and with a specific layer thickness range, they can optimally achieve parallel orientation of inactive crystal planes. Meanwhile, the polycrystalline spherical particles in the second functional layer are secondary aggregated spheres, with secondary sphere diameters of 5μm~14μm and primary grain diameters of 50nm~900nm, and the thickness of the second functional layer is 20μm~120μm. Nanoscale primary grains shorten the solid-phase ion diffusion distance, while micron-scale secondary sphere stacking constructs a smooth liquid-phase ion transport pore network. Furthermore, this embodiment uses aluminum foil with a thickness of 6μm~20μm as the current collector to balance the mechanical strength and high energy density requirements of the battery.

[0046] In some embodiments of this application, the chemical composition of the positive electrode active material used in the composite positive electrode sheet is specifically defined.

[0047] In some embodiments, the first positive electrode active material and the second positive electrode active material are each independently selected from at least one of layered oxides and polyanionic compounds.

[0048] Both of these types of materials exhibit excellent sodium / lithium storage activity. By independently selecting the two layers within this range, the overall performance of the electrode can be flexibly controlled. For example, the first and second functional layers can be made of layered oxide materials with identical chemical compositions, maintaining the distinction only in microscopic particle morphology between monocrystalline flakes and polycrystalline spherical shapes. This simplifies the material system and avoids side reactions between different material interfaces.

[0049] In some embodiments, the layered oxide comprises Na x M1M2O2 is a sodium-electric layered oxide, wherein M1 is at least one of Ni, Co, Mn, Fe and Cu; M2 is at least one of W, Zr, Ca, Zn, Al, Mg, Ti, Nb, Y, La, Sr and Mo; and x ranges from 0.4 to 1.05.

[0050] The introduction of M2 element can support the crystal lattice structure and increase the interlayer spacing, which is beneficial to maintaining the integrity of the crystal structure and further reducing the resistance to sodium ion insertion and extraction within the crystal lattice. The stoichiometric coefficient x of sodium ranges from 0.4 to 1.05, and an appropriate sodium content can ensure that the material has good initial phase stability and high reversible capacity.

[0051] In some embodiments, the polyanionic compound is selected from at least one of Na3V2(PO4)3 and NaFePO4.

[0052] Polyanionic compounds possess extremely high structural and thermal stability. When applied to the composite electrode structure described in this embodiment, combined with the extremely low interfacial electronic impedance resulting from the single-crystal planarization of the first functional layer, the inherent low electronic conductivity of polyanionic compounds can be effectively overcome. This significantly improves their rate performance and energy conversion efficiency while ensuring high safety.

[0053] In some embodiments, the first functional layer further includes a first binder and a first conductive agent; in the first functional layer, the mass ratio of the first positive electrode active material, the first binder and the first conductive agent is (85~97):(2~8):(1~7).

[0054] In the first functional layer, the mass ratio of the first positive electrode active material is 85-97 parts by mass; for example, it can be 85 parts by mass, 87 parts by mass, 89 parts by mass, 90 parts by mass, 92 parts by mass, 94 parts by mass, 95 parts by mass, 96 parts by mass, 97 parts by mass, etc. In the first functional layer, the mass ratio of the first binder is 2-8 parts by mass; for example, it can be 2 parts by mass, 3 parts by mass, 4 parts by mass, 4.5 parts by mass, 5 parts by mass, 6 parts by mass, 7 parts by mass, 7.5 parts by mass, 8 parts by mass, etc. In the first functional layer, the mass ratio of the first conductive agent is 1-7 parts by mass; for example, it can be 1 part by mass, 2 parts by mass, 3 parts by mass, 3.5 parts by mass, 4 parts by mass, 5 parts by mass, 6 parts by mass, 6.5 parts by mass, 7 parts by mass, etc.

[0055] In some embodiments, the second functional layer further includes a second binder and a second conductive agent, wherein the mass ratio of the second positive electrode active material, the second binder and the second conductive agent in the second functional layer is (83~96):(2~9):(2~8).

[0056] In the second functional layer, the mass ratio of the second positive electrode active material is 83-96 parts by mass; for example, it can be 83 parts by mass, 85 parts by mass, 87 parts by mass, 89 parts by mass, 90 parts by mass, 92 parts by mass, 94 parts by mass, 95 parts by mass, 96 parts by mass, etc. In the second functional layer, the mass ratio of the second binder is 2-9 parts by mass; for example, it can be 2 parts by mass, 3 parts by mass, 4 parts by mass, 5 parts by mass, 6 parts by mass, 7 parts by mass, 8 parts by mass, 8.5 parts by mass, 9 parts by mass, etc. In the second functional layer, the mass ratio of the second conductive agent is 2-8 parts by mass; for example, it can be 2 parts by mass, 3 parts by mass, 4 parts by mass, 4.5 parts by mass, 5 parts by mass, 6 parts by mass, 7 parts by mass, 7.5 parts by mass, 8 parts by mass, etc.

[0057] Specifically, the first functional layer includes not only a monocrystalline first positive electrode active material, but also a first binder and a first conductive agent. In the first functional layer, the mass ratio of the first positive electrode active material, the first binder, and the first conductive agent is configured as (85~97):(2~8):(1~7). This ratio ensures sufficient adhesion for the monocrystalline particles during the rolling process, while simultaneously establishing a basic electron permeation network in the particle gaps with an appropriate amount of conductive agent. Furthermore, the second functional layer includes a second binder and a second conductive agent. In the second functional layer, the mass ratio of the second positive electrode active material, the second binder, and the second conductive agent is configured as (83~96):(2~9):(2~8). Because the polycrystalline spherical particles in the second functional layer create abundant pores, appropriately adjusting the ratio of the conductive agent and the binder can effectively enhance the mechanical strength and electronic conductivity of the porous structure without blocking ion transport channels.

[0058] To further leverage the resistance-reducing advantages of the monocrystalline particles in the first functional layer, this embodiment specifically defines the surface morphology of the current collector, ensuring that the surface roughness Ra of the current collector in contact with the first functional layer is between 0.1 μm and 0.5 μm. This specific micro-roughness range has a significant synergistic effect: on the one hand, it provides moderate surface unevenness, greatly enhancing the bonding force between the first functional layer and the current collector through a mechanical anchoring effect, preventing the electrode from detaching during long-term cycling; on the other hand, the degree of roughness fluctuation does not interfere with the directional spreading of the monocrystalline particles, ensuring that the flat, inactive crystal facets of the monocrystalline particles can still form a tight and large-area surface contact with the current collector, thereby achieving extremely low interfacial electronic impedance while ensuring the stability of the electrode structure.

[0059] In some embodiments, the first adhesive and the second adhesive are each independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

[0060] In some embodiments, the first conductive agent and the second conductive agent are each independently selected from at least one of superconducting carbon black, acetylene black, Ketjen black, carbon fiber and graphene.

[0061] In some embodiments, the surface roughness Ra of the current collector in contact with the first functional layer is 0.1 μm to 0.5 μm. For example, it can be 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, etc.

[0062] In terms of material selection, by using carbon-based conductive agents of different dimensions, a three-dimensional conductive network can be constructed inside and outside the composite electrode.

[0063] This application provides a method for preparing a composite positive electrode as described in any of the foregoing embodiments. This method utilizes a specific slurry layering coating and rolling process to construct a bilayer structure with differentiated microstructures within the electrode, thereby synergistically optimizing the electron and ion transport performance of the electrode. Specifically, the preparation method includes: Step S1: Provide a current collector; and coat at least one surface of the current collector with a first slurry containing a first positive electrode active material to obtain a first functional layer, wherein the first positive electrode active material is a single crystal particle.

[0064] In this step, the current collector, serving as the physical support substrate and electron collecting network for the electrode, can be made of conventional metal foil such as aluminum foil. The first slurry is typically prepared by thoroughly mixing monocrystalline flake particles, a first binder, a first conductive agent, and an appropriate amount of solvent (such as N-methylpyrrolidone, NMP). Because the selected first positive electrode active material has a significantly anisotropic monocrystalline flake structure (i.e., a large aspect ratio and a flat microstructure), when the first slurry is coated on the surface of the current collector, these flake particles initially form a base coating rich in two-dimensional extensibility at the current collector interface, laying the material foundation for the subsequent construction of a large-area surface contact interface.

[0065] Step S2: A second slurry containing a second positive electrode active material is coated onto the side of the first functional layer away from the current collector to obtain a second functional layer. The second positive electrode active material is a polycrystalline spherical particle.

[0066] In this step, the second slurry is typically composed of polycrystalline spherical particles, a second binder, a second conductive agent, and a solvent. Because the polycrystalline spherical particles (usually micron-sized secondary spheres formed by the agglomeration of nanoscale primary grains) naturally accumulate, abundant gaps are created between the particles. Coating these particles onto the surface of the electrode layer above the first functional layer allows for the construction of a regular and highly interconnected three-dimensional porous network in this region. This porous network greatly facilitates the deep wetting of the subsequent electrolyte and provides short, unobstructed diffusion channels for long-distance ion migration. It should be noted that steps S1 and S2 can be achieved in actual production through multiple sequential coatings (i.e., coating the underlayer, drying, and then coating the surface layer), or by using a double-layer co-extrusion coating device to simultaneously coat the first and second slurries onto the current collector surface. The latter method is more conducive to forming superior interlayer fusion and physical bonding.

[0067] Step S3: Roll pressing is performed on the coated electrode to obtain the composite positive electrode.

[0068] After coating (and the necessary drying and solvent removal processes), the electrode needs to undergo roll pressing. This roll pressing step not only improves the overall compaction density and volumetric energy density of the electrode, but is also a core key process that induces the synergistic evolution of the internal microstructure of the electrode. The specific mechanism is as follows: Under the strong mechanical pressure and shear force applied by the rolls, the flat monocrystalline particles in the first functional layer easily undergo interlayer sliding and spatial rotation, ultimately resulting in a significant preferred orientation. This causes the flat, inactive crystal surfaces of the monocrystalline particles to spread parallel under force and closely adhere to the flat current collector surface. This morphological evolution transforms the contact mode between the active layer and the current collector from point contact to large-area surface contact, greatly reducing the interfacial contact electronic impedance. At the same time, the applied mechanical pressure further compacts and consolidates the polycrystalline spherical particles in the second functional layer. However, thanks to the structural characteristics of polycrystalline spheres, they can still retain their original interconnected pore network intact without being completely destroyed under appropriate pressure.

[0069] In some preferred embodiments, the specific process parameters, processing steps and equipment selection in the preparation method of the composite positive electrode have been further optimized to ensure that the first functional layer and the second functional layer can perfectly achieve synergy in their physical structures.

[0070] In some embodiments, during the step of rolling the coated electrode, the total compaction density of the electrode is controlled to be 2.5 g / cm³. 3 ~3.5g / cm 3 For example, it could be 2.5 g / cm³. 3 2.6g / cm 3 2.8g / cm 3 2.9g / cm 33.0g / cm 3 3.1g / cm 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 etc.

[0071] Controlling the compaction density plays a decisive role in the formation of the electrode's bilayer microstructure. When the applied compaction density is within a certain range, the resulting mechanical pressure and shear force are precisely able to overcome the internal friction between the monocrystalline particles in the bottom first functional layer, causing the monocrystalline particles to slide and rotate in space. During this stress process, the monocrystalline particles, due to their two-dimensional anisotropic morphology, spread their flat, inactive crystal surfaces parallel to the force and eventually adhere tightly to the current collector surface, thereby reshaping the traditional point contact mode into a large-area surface contact mode, greatly reducing the interfacial electronic contact impedance. At the same time, controlling the upper limit of the compaction density effectively avoids excessive mechanical destructive force being transmitted to the outer second functional layer, ensuring that the polycrystalline spherical particles do not undergo severe brittle fracture, thus perfectly preserving the interconnected pore network formed by the accumulation of spherical particles, providing structural protection for electrolyte wetting and rapid ion transport.

[0072] In some embodiments, the coated electrode is rolled so that the inactive crystal faces of the monocrystalline particles in the first functional layer are parallel to the current collector surface.

[0073] In some embodiments, the preparation steps of the first slurry and the second slurry each independently include: dissolving the binder in an N-methylpyrrolidone solvent to obtain a glue solution, and then uniformly mixing the positive electrode active material and the conductive agent in the glue solution.

[0074] Regarding the slurry preparation process, the preparation steps of the first and second slurries each independently adopt a "sol-gel first, then mix" process path. Specifically, a predetermined proportion of binder is first completely dissolved in a highly polar organic solvent such as N-methylpyrrolidone (NMP) and stirred to obtain a uniform, transparent slurry. Subsequently, the corresponding positive electrode active material and conductive agent powder are gradually added and uniformly mixed into the slurry. This preparation logic ensures that the polymer binder chains are fully extended and form a uniform coating layer on the surface of the active material and conductive agent particles, effectively avoiding agglomeration caused by direct mixing of powders. This imparts excellent rheological stability to the slurry, laying the foundation for subsequent uniform coating.

[0075] In some embodiments, before coating at least one surface of the current collector with a first slurry containing a first positive electrode active material, the method further includes: electrochemically etching or plasma treating the surface of the current collector to achieve a surface roughness Ra of 0.1 μm to 0.5 μm between the current collector and the first functional layer. For example, it can be 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, etc.

[0076] In some embodiments, before the coated electrode is rolled, the coated electrode is dried at 105°C.

[0077] To further enhance the mechanical stability of the electrode, a pretreatment step is included before coating the current collector with the first slurry. Specifically, electrochemical etching or plasma treatment techniques can be used to micro-texturize the current collector, such as smooth aluminum foil, so that the surface roughness Ra of the current collector in contact with the first functional layer reaches 0.1 μm to 0.5 μm. This specific submicron roughness can form dense physical anchor points on the current collector surface, significantly improving the mechanical bonding force between the adhesive and the substrate (i.e., electrode peel strength). Simultaneously, since the micro-ripples of this roughness are much smaller than the geometric size of single-crystal particles, it does not hinder the parallel bonding of the inactive crystal planes of the single-crystal particles, achieving a perfect balance between high adhesion and low contact resistance.

[0078] In some embodiments, after the coated electrode sheet is rolled, the method further includes: cutting the rolled electrode sheet to a preset size.

[0079] In some embodiments, a two-layer co-extrusion coating device is used to simultaneously coat the first slurry and the second slurry onto the surface of the current collector.

[0080] In the coating and molding process, this "wet-on-wet" simultaneous coating technology not only significantly simplifies the process and reduces baking energy consumption, but more importantly, it allows the first and second slurries to undergo microscopic-scale macromolecular chain diffusion and particle interpenetration at the interface in a wet state. This results in extremely strong interlayer physical bonding after drying and curing, effectively preventing interlayer delamination of the electrode during charge-discharge cycles. After completing the double-layer coating, the electrode is dried at 105°C, a temperature that safely and efficiently evaporates the NMP solvent without causing thermal degradation of the binder. Finally, the rolled electrode is cut to a preset size according to specific specifications to obtain the composite positive electrode for secondary battery assembly.

[0081] This application also provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte; the positive electrode is a composite positive electrode as described in any of the foregoing embodiments.

[0082] In some embodiments, the secondary battery is a sodium-ion battery; the negative electrode includes a hard carbon negative electrode active material; and the electrolyte is an electrolyte containing sodium hexafluorophosphate.

[0083] This application provides a secondary battery. The secondary battery mainly comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode uses a composite positive electrode with a single-crystal bottom layer and a polycrystalline surface layer. The separator is located between the positive and negative electrodes, preventing direct contact and short circuits while allowing ion shuttle. The electrolyte serves as a medium for ion transport between the positive and negative electrodes. Furthermore, the secondary battery typically includes a battery casing (such as an aluminum casing, steel casing, or aluminum-plastic film) for encapsulating the above components and tabs for connecting to external circuits. Depending on the internal charge carriers and chemical system, the secondary battery can include, but is not limited to, sodium-ion batteries, lithium-ion batteries, or potassium-ion batteries; depending on the physical form of the electrolyte, it can also encompass traditional liquid batteries, semi-solid-state batteries, and all-solid-state batteries. In particular, in practical applications of sodium-ion batteries, the negative electrode can use hard carbon or other materials as the negative electrode active material, and the electrolyte can be an electrolyte containing sodium hexafluorophosphate. These components, when matched with the composite positive electrode optimized in this application, can work together to achieve excellent high-rate charge and discharge capabilities and long cycle life.

[0084] This application also provides an electrical device, including the secondary battery described in the foregoing embodiments.

[0085] This application also provides an electrical device. The device is internally equipped with the aforementioned secondary battery, which serves as the core power source or basic energy storage unit for providing operating power. Due to the inclusion of a secondary battery with extremely low internal resistance polarization and excellent cycle stability, the device achieves significant improvements in overall battery life, high-power instantaneous output, and long service life. In specific end-application scenarios, the types of this electrical device are very broad, including but not limited to: consumer portable electronic devices (such as smartphones, tablets, laptops, digital cameras, smart wearable devices, etc.), transportation vehicles (such as pure electric new energy vehicles, hybrid vehicles, electric bicycles, electric motorcycles, electric balance scooters, drones, electric ships, etc.), large-scale energy storage systems (such as wind and solar grid-connected energy storage power stations, household energy storage cabinets, communication base station backup power supplies, etc.), as well as various handheld power tools (such as electric drills, electric wrenches), intelligent robots, and medical electronic instruments, etc.

[0086] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0087] Example 1 This embodiment provides a composite cathode sheet and the sodium-ion battery prepared therefrom, which can be used as a basic performance evaluation benchmark for a "bottom monocrystalline + surface polycrystalline" bilayer structure.

[0088] (1) Preparation of inner layer (first functional layer): The single-crystal sodium-ion ternary cathode material NFM333 (NaNi) was prepared. 1 / 3 Fe 1 / 3Mn 1 / 3 O2), conductive carbon black SP, conductive carbon nanotubes CNT, and binder PVDF are prepared in a mass ratio of 96wt%:1wt%:1wt%:2wt%.

[0089] PVDF was dissolved in NMP, and other components were then uniformly mixed into the PVDF adhesive solution. The solid content of the slurry was controlled at 60% to obtain a single-crystal ternary material layer slurry. The single-crystal primary particles had a particle size of 6.2 μm and an aspect ratio of 8:1.

[0090] (2) Preparation of outer layer (second functional layer): The polycrystalline particulate sodium ternary cathode material NFM333 (NaNi) is prepared. 1 / 3 Fe 1 / 3Mn 1 / 3 O2), conductive carbon black SP, conductive carbon nanotubes CNTs, and binder PVDF are prepared in a mass ratio of 95wt%:1wt%:2wt%:2wt%. They are then dissolved and mixed in the same manner, controlling the solid content to 60%, to obtain a polycrystalline ternary material slurry. The primary polycrystalline particles have a particle size of 400nm, and the secondary spherical particles have a particle size of 9.5μm.

[0091] (3) Coating and forming: The monocrystalline paste is sprayed onto one side of the aluminum foil, and then the polycrystalline paste is sprayed onto the monocrystalline layer. The surface density of the electrode on one side is 117 g / m². 2 (20g / m monocrystalline layer) 2 Thickness 7μm; polycrystalline layer 97g / m 2 The coating thickness is 31 μm, and the ratio of monocrystalline to polycrystalline coating thickness is 1:4.42. After drying at 105℃, the unrolled electrode sheet is placed in a roller press, and the total compaction density is controlled to be 3.1 g / cm³. The composite positive electrode sheet is then cut.

[0092] (4) Battery assembly: The above positive electrode, separator and negative electrode are wound or stacked to obtain a core pack, and sodium-ion battery is obtained through assembly, baking, liquid injection and formation processes.

[0093] The microstructures of the aforementioned monocrystalline and polycrystalline cathodes can be found in the reference section. Figure 1 (SEM image of a single-crystal cathode) and Figure 2 (SEM image of polycrystalline particle cathode).

[0094] Example 2 This embodiment provides a composite cathode and the sodium-ion battery prepared therefrom, mainly used to investigate the influence of changes in the morphology parameters of the outer polycrystalline spherical particles on the performance of the cathode.

[0095] The only difference between this example and Example 1 is that the primary particle size of the polycrystalline material is 700 nm and the secondary spherical particle size is 8.2 μm. All other steps and parameters are the same as in Example 1.

[0096] Example 3 This embodiment provides a composite cathode and the sodium-ion battery prepared therefrom, mainly used to investigate the effect of changes in the ratio of the outer polycrystalline layer slurry on the performance of the cathode.

[0097] The only difference between this and Example 1 is that in the outer polycrystalline slurry, the mass ratio of polycrystalline cathode material, conductive carbon black SP, conductive carbon nanotubes CNT, and binder PVDF is adjusted to 94.5wt%:1.5wt%:2wt%:2wt%, while the other steps remain unchanged.

[0098] Example 4 This embodiment provides a composite cathode and the sodium-ion battery prepared therefrom, mainly used to investigate the influence of changes in the morphology parameters of the inner single-crystal particles on the electrode performance. The only difference from Example 1 is that the primary particle size of the single-crystal particles is adjusted to 4.8 μm, and the aspect ratio is 8:1; the remaining steps remain unchanged.

[0099] Example 5 This embodiment provides a composite cathode sheet and the sodium-ion battery prepared therefrom. Based on Example 4, the influence of changes in the slurry ratio of the inner monocrystalline layer is further investigated. The difference from Example 4 is that the mass ratio of the monocrystalline layer formulation is adjusted to monocrystalline cathode material: SP:CNT:PVDF = 95wt%:1.5wt%:1.5wt%:2wt%.

[0100] Example 6 This embodiment provides a composite positive electrode and the sodium-ion battery prepared therefrom, mainly used to investigate the influence of changes in the ratio of the surface density and thickness of the inner and outer coatings on the electrode performance.

[0101] The only difference between it and Example 1 is that the surface density of the single-crystal coating is adjusted to 40 g / m². 2 (Thickness 13μm), polycrystalline coating surface density adjusted to 67g / m² 2(Thickness 21.5μm), at this time the thickness ratio of single crystal to polycrystalline is 1:1.654.

[0102] Example 7 This embodiment provides a composite cathode sheet and the sodium-ion battery prepared therefrom, mainly used to investigate the effect of reducing the content of conductive agent and binder in the inner monocrystalline layer slurry.

[0103] The only difference between this and Example 1 is that the mass ratio of the single crystal layer formulation is adjusted to single crystal cathode material: SP:CNT:PVDF = 96.6wt%:1.2wt%:1.2wt%:1wt%, while the other steps remain unchanged.

[0104] Comparative Example 1 This comparative example provides a composite cathode sheet and the sodium-ion battery prepared therefrom, mainly used to examine the comparative effect when the thickness of the single-crystal coating is too large (i.e., the ratio of the inner and outer layer thicknesses exceeds the preferred range). The difference from Example 1 is that the single-crystal surface density is increased to 60 g / m². 2 (Thickness 19.5μm), polycrystalline facet density reduced to 57g / m 2 (Thickness 18.5μm), at this time the thickness ratio is 1:0.95.

[0105] Comparative Example 2 This comparative example provides a positive electrode and the sodium-ion battery prepared therefrom, mainly used to examine the comparison of using only a single homogeneous "all-monocrystalline" coating structure. The difference from Example 1 is that the electrode coating consists of only one monocrystalline layer. The monocrystalline surface density is 117 g / m². 2 (Thickness 38μm), polycrystalline surface density is 0, that is, the thickness ratio is 1:0.

[0106] The internal structure and crystal orientation characteristics of the electrode in Comparative Example 2 can be referred to respectively. Figure 3 (SEM image of electrode cross-section in Comparative Example 2) Figure 4 (Comparative Example 2: XRD comparison of electrode plates before rolling) and Figure 5 (Comparative Example 2: XRD comparison of electrode plates after rolling).

[0107] Comparative Example 3 This comparative example provides a positive electrode sheet and the sodium-ion battery prepared therefrom, mainly used to examine the comparison of using only a single homogeneous "fully polycrystalline" coating structure. The difference from Example 1 is that the coating is only a single polycrystalline layer. The areal density is 117 g / m² (thickness 38 μm), the single-crystal areal density is 0, i.e., the thickness ratio is 0:1.

[0108] The internal pore morphology of Comparative Example 3 is referenced. Figure 6 (SEM image of the electrode cross-section in Comparative Example 3).

[0109] Comparative Example 4 This comparative example provides a composite positive electrode sheet and the sodium-ion battery prepared therefrom, mainly used to examine the comparison when a coating sequence opposite to that of the present application is adopted (i.e., conventional understanding in the prior art).

[0110] The difference from Example 1 is that reverse coating is used, that is, the inner layer near the current collector is coated with polycrystalline material and the outer layer is coated with monocrystalline material, while the other parameters remain unchanged.

[0111] Performance testing experiment 1. Test Method Description: Before detailing the embodiments and comparative examples, the battery cell-related performance testing methods involved in this application are as follows: (1) DCR (DC internal resistance) growth rate test method: At 25°C, the state of charge (SOC) of the secondary battery was adjusted to 50% using a 0.5C current, and the voltage V1 was recorded. Then, the battery was discharged at a 1C current for 30 seconds, and the voltage V2 was recorded. The initial DCR was calculated as (V1-V2) / 1C. Subsequently, the battery was subjected to 200 constant power charge-discharge cycles of 1P / 1P, and the DCR of the 200th cycle was recorded. The DCR growth rate of the 200th cycle was obtained by dividing the DCR of the 1st cycle by the DCR of the 200th cycle and subtracting 1.

[0112] (2) Cycle life (capacity retention) test method: Charge-discharge tests were conducted at 25°C. The battery was first charged to 3.8V at 1P power, then discharged to 1.5V at 1P power, and the discharge capacity of the first cycle was recorded. The battery was then subjected to 2000 1P / 1P charge-discharge cycles, and the discharge capacity of the battery on the 2000th cycle was recorded. Dividing the discharge capacity of the 2000th cycle by the discharge capacity of the first cycle yielded the capacity retention rate after 2000 cycles.

[0113] 2. Experimental Results and Test Analysis: The performance of the electrodes and batteries prepared in Examples 1-7 and Comparative Examples 1-4 was tested, and the specific test data are shown in Table 1.

[0114] Table 1. Performance test results of samples in each experimental group

[0115] analyze: (1) As shown in Table 1 above, the composite positive electrode sheets of Examples 1-7 exhibit excellent comprehensive performance in terms of positive electrode resistivity (all below 20 Ω·cm), high-rate capacity retention (excellent high-rate performance, some reaching over 95%), and long-cycle performance (capacity retention of over 82% after 2000 cycles). This strongly demonstrates that the structure of "bottom single crystal + surface polycrystalline" can achieve perfect decoupling and synergy of electron-ion transport.

[0116] Combination Figure 7 (Comparing the EIS plots of Example 1 and Comparative Example 1) it can be seen that Example 1 has both extremely low high-frequency impedance (very small charge transfer resistance) and low mid-to-low frequency diffusion impedance.

[0117] (2) Regarding the underlying single-crystal arrangement mechanism, refer to Figure 3 and Figure 4 After roll forming, the diffraction peak intensity of the 003 crystal plane of the monocrystalline electrode increases significantly, and the peak area ratio of the 003 / 104 crystal plane increases dramatically from 1.95 before roll forming to 3.21.

[0118] This provides irrefutable evidence from the perspective of microscopic crystal planes. After being subjected to rolling pressure, the inactive crystal planes of the single crystal particles do indeed undergo a high degree of orientation and parallel laying. This is the fundamental mechanism by which the bottom layer of this application can achieve "face-to-face" contact and significantly reduce electronic contact impedance.

[0119] (3) Regarding the negative impact of thickness ratio imbalance (Comparative Example 1), when the thickness of the single crystal coating is excessively increased, although the electrode resistivity decreases further, the narrowing of the ion channel caused by the stacking of single crystal particles is amplified, resulting in obstructed ion transport, uneven electrochemical reaction, and accelerated decay of active material near the current collector side. Ultimately, the capacity retention rate after 2000 cycles decreases significantly to 77%, and the DCR growth rate surges to 52.2%.

[0120] (4) Although Comparative Example 2 (single crystal) has extremely low resistivity (14.26 Ω·cm) and high peel strength, its ion dynamics are extremely poor due to the lack of a porous network provided by the polycrystalline surface, resulting in a significantly lower rate discharge performance than the example. Although Comparative Example 3 (single polycrystalline) has abundant pores and good rate performance, its initial resistivity is extremely high (25.02 Ω·cm) due to point contact, and the grain boundary cracking in the later stage of the cycle easily causes the active material to fall off from the current collector (peel strength is only 18.47 N / mm), resulting in extremely severe DCR deterioration (58.4%).

[0121] (5) The design of placing polycrystalline material at the bottom and monocrystalline material on the surface completely violates the laws of electron and ion transport. Its positive electrode film resistivity is as high as 24.32 Ω·cm, and its rate performance, cycle retention rate, and DCR growth rate are all inferior to those of Example 1. This data directly breaks the inherent bias of the prior art in using polycrystalline material as the bottom layer, highlighting the irreplaceable nature and significant progress of the layered logic of this application.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions 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 composite positive electrode, characterized in that, It includes a current collector, a first functional layer disposed on at least one surface of the current collector, and a second functional layer disposed on the first functional layer away from the surface of the current collector; The first functional layer includes a first positive electrode active material; the first positive electrode active material is a single crystal-like particle; The second functional layer contains a second positive electrode active material; the second positive electrode active material is a polycrystalline spherical particle.

2. The composite positive electrode sheet as described in claim 1, characterized in that, The mass ratio of the first functional layer to the second functional layer is 1:(3~9); and / or, The dry film thickness ratio of the first functional layer to the second functional layer is 1:(1.5~10); and / or, The total compaction density of the composite cathode sheet is 2.5 g / cm³. 3 ~3.5g / cm 3 ; and / or, The aspect ratio of the single-crystal crystalline particles is (5~20):1; and / or, The primary particle size of the single-crystal crystalline particles is 0.5 μm to 8 μm; and / or, The thickness of the first functional layer is 5 μm to 60 μm; and / or, The polycrystalline spherical particles are secondary aggregated spheres with a particle size of 5 μm to 14 μm; and / or, The primary grain size of the polycrystalline spherical particles is 50 nm to 900 nm; and / or, The thickness of the second functional layer is 20 μm to 120 μm; and / or, In the first functional layer, the inactive crystal planes of the monocrystalline particles are parallel to and attached to the current collector surface; and / or, The current collector is aluminum foil; and / or, The thickness of the current collector is 6μm to 20μm.

3. The composite positive electrode sheet as described in claim 1, characterized in that, The first positive electrode active material and the second positive electrode active material are each independently selected from at least one of layered oxides and polyanionic compounds.

4. The composite positive electrode sheet as described in claim 3, characterized in that, The layered oxide includes the chemical formula Na. x The sodium-electric layered oxide M1M2O2, wherein M1 is at least one of Ni, Co, Mn, Fe, and Cu; M2 is at least one of W, Zr, Ca, Zn, Al, Mg, Ti, Nb, Y, La, Sr, and Mo; and x ranges from 0.4 to 1.05; and / or, The polyanionic compound is selected from at least one of Na3V2(PO4)3 and NaFePO4.

5. The composite positive electrode sheet as described in claim 1, characterized in that, The first functional layer further includes a first binder and a first conductive agent; in the first functional layer, the mass ratio of the first positive electrode active material, the first binder, and the first conductive agent is (85~97):(2~8):(1~7); and / or, The second functional layer also includes a second binder and a second conductive agent. In the second functional layer, the mass ratio of the second positive electrode active material, the second binder and the second conductive agent is (83~96):(2~9):(2~8).

6. The composite positive electrode sheet as described in claim 5, characterized in that, The first adhesive and the second adhesive are each independently selected from at least one of polyvinylidene fluoride, polytetrafluoroethylene, and sodium carboxymethyl cellulose; and / or, The first conductive agent and the second conductive agent are each independently selected from at least one of superconducting carbon black, acetylene black, Ketjen black, carbon fiber, and graphene; and / or, The surface roughness Ra of the current collector in contact with the first functional layer is 0.1 μm to 0.5 μm.

7. A method for preparing a composite positive electrode sheet as described in any one of claims 1-6, characterized in that, include: Provide current collectors; In addition, a first slurry containing a first positive electrode active material is coated on at least one surface of the current collector to obtain a first functional layer, wherein the first positive electrode active material is a single crystal-like particle. A second slurry containing a second positive electrode active material is coated onto the side of the first functional layer away from the current collector to obtain a second functional layer. The second positive electrode active material is a polycrystalline spherical particle. The coated electrode sheet is subjected to roll pressing to obtain the composite positive electrode sheet.

8. The method for preparing the composite positive electrode sheet as described in claim 7, characterized in that, In the step of rolling the coated electrode, the total compaction density of the electrode is controlled to be 2.5 g / cm³. 3 ~3.5g / cm 3 ; and / or, The coated electrode is rolled to make the inactive crystal planes of the single crystal particles in the first functional layer parallel to the surface of the current collector. And / or, The preparation steps of the first slurry and the second slurry each independently include: dissolving the binder in N-methylpyrrolidone solvent to obtain a glue solution, and then uniformly mixing the positive electrode active material and the conductive agent in the glue solution; and / or, Before coating at least one surface of the current collector with a first slurry containing a first positive electrode active material, the method further includes: subjecting the surface of the current collector to electrochemical etching or plasma treatment, such that the surface roughness Ra of the current collector in contact with the first functional layer is 0.1 μm to 0.5 μm; and / or, Before the coating electrode is rolled, the method further includes: drying the coated electrode at 105°C; and / or, After the coating electrode sheet is rolled, the method further includes: cutting the rolled electrode sheet to a preset size; and / or, A double-layer co-extrusion coating device is used to simultaneously coat the first slurry and the second slurry onto the surface of the current collector.

9. A secondary battery, characterized in that, It includes positive electrode, negative electrode, and electrolyte; The positive electrode sheet is the composite positive electrode sheet according to any one of claims 1-6; Preferably, the secondary battery is a sodium-ion battery; the negative electrode sheet includes a hard carbon negative electrode active material; and the electrolyte is an electrolyte containing sodium hexafluorophosphate.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 9.

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