Bilayer composite positive electrode sheet, preparation method and lithium-ion battery

Abstract

This invention discloses a bilayer composite positive electrode sheet, its preparation method, and a lithium-ion battery. The bilayer composite positive electrode sheet includes: a current collector and a first positive electrode active material layer and a second positive electrode active material layer sequentially stacked on at least one surface of the current collector. The first positive electrode active material layer includes a polycrystalline ternary lithium metal oxide doped with a first element, which has a hollow structure. The first element is selected from at least three of W, Nb, Ti, Mg, Al, and Zr. The second positive electrode active material layer includes phosphorus-doped lithium iron phosphate. The highly conductive network of the first positive electrode active material layer in this application greatly reduces the ohmic polarization from the current collector to the active material, providing a fast electron transport channel for the entire electrode. At the same time, the thermal stability and interfacial stability of the second positive electrode active material layer provide a safety guarantee for the high energy density material in the first positive electrode active material layer.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a double-layer composite positive electrode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and lack of memory effect, have been widely used in consumer electronics, electric vehicles, and large-scale energy storage systems. With the continuous expansion of application scenarios, the market is placing increasingly higher comprehensive demands on the energy density, power characteristics, safety, and cycle life of lithium-ion batteries, and single cathode material systems are gradually reaching their performance bottlenecks. Currently, commercially available lithium-ion battery cathode materials are mainly divided into two categories: olivine-structured materials represented by lithium iron phosphate (LiFePO4), and materials based on lithium nickel cobalt manganese oxide (LiNiO2). x Co y Mn z O2) or lithium nickel cobalt aluminum oxide (LiNi x Co y Al z Ternary materials with layered structures, represented by O2.

[0003] Lithium iron phosphate (LFP) boasts excellent safety and ultra-long cycle life due to the intrinsic thermal stability of its olivine structure, while also offering lower raw material costs. However, its low intrinsic electronic conductivity and small lithium-ion diffusion coefficient result in relatively poor rate performance and low-temperature performance. Ternary materials, especially high-nickel systems, can achieve higher energy densities due to their higher specific capacity and operating voltage. However, their layered structure is prone to oxygen release, phase transitions, and cation mixing under deep delithiation, high temperatures, or overcharge conditions, leading to the risk of thermal runaway. Consequently, their cycle stability and safety are significantly inferior to those of LFP.

[0004] In summary, current single cathode material systems are insufficient to simultaneously meet market demands for high safety and long lifespan.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a double-layer composite positive electrode sheet, a preparation method, and a lithium-ion battery that balances safety and cycle performance.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a double-layer composite positive electrode sheet, comprising: a current collector and a first positive electrode active material layer and a second positive electrode active material layer sequentially stacked on at least one surface of the current collector; The first positive electrode active material layer includes a polycrystalline ternary lithium metal oxide doped with a first element, the polycrystalline ternary lithium metal oxide having a hollow structure; the first element is selected from at least three of W, Nb, Ti, Mg, Al, and Zr; The second positive electrode active material layer includes phosphorus-doped lithium iron phosphate.

[0008] In an optional embodiment, the polycrystalline ternary lithium metal oxide is spherical; And / or, the general chemical formula of the polycrystalline ternary lithium metal oxide is LiNi. x Co y Mn z M w O2, where x+y+z+w=1 and 0.5 ≤ x<1, and M is the first element.

[0009] In an optional embodiment, the total molar amount of the first element in the polycrystalline ternary lithium metal oxide accounts for 0.1%-2.0% of the total molar amount of all metals except lithium in the polycrystalline ternary lithium metal oxide; And / or, the hollowness HD of the secondary particles of the hollow spherical polycrystalline ternary lithium metal oxide is 0.2-0.6, where HD = D1 / D2, D1 is the equivalent sphere diameter of the hollow region, and D2 is the outer diameter of the particle.

[0010] In an optional embodiment, the P / Fe molar ratio in the phosphorus-doped lithium iron phosphate is 1.01-1.10; And / or, the lithium iron phosphate is provided with a carbon coating layer on its surface, and the carbon content in the lithium iron phosphate is 1.0 wt% - 3.0 wt%.

[0011] In an optional embodiment, the mass ratio of the polycrystalline ternary lithium metal oxide to the lithium iron phosphate is 1-3:1; And / or, the compaction density of the bilayer composite positive electrode sheet is 2.6 g / cm³. 3 - 3.4 g / cm 3 .

[0012] Secondly, the present invention provides a method for preparing the double-layer composite positive electrode sheet according to any one of the foregoing embodiments, comprising: A first positive electrode active slurry containing polycrystalline ternary lithium metal oxide is coated on the surface of the current collector, followed by initial drying to form a first positive electrode active material layer on the current collector. A second positive electrode active slurry containing phosphorus-doped lithium iron phosphate is coated on the surface of the first positive electrode active material layer, then dried again, and subsequently rolled and slit to obtain the double-layer composite positive electrode sheet.

[0013] In an optional embodiment, the D50 of the secondary particles of the hollow spherical polycrystalline ternary lithium metal oxide is 8.0 μm-15.0 μm; And / or, the primary particle D50 of the phosphorus-doped lithium iron phosphate is 50 nm-200 nm.

[0014] In an optional embodiment, the first positive electrode active slurry includes the polycrystalline ternary lithium metal oxide, conductive agent, binder and solvent, and the mass ratio of the polycrystalline ternary lithium metal oxide, conductive agent and binder in the first positive electrode active slurry is (90-97):(1.5-5):(1.5-5). And / or, the second positive electrode active slurry includes the lithium iron phosphate, conductive agent, binder and solvent, and the mass ratio of the lithium iron phosphate, conductive agent and binder in the second positive electrode active slurry is (92-96):(2-6):(2-6).

[0015] In an optional embodiment, the conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, graphene, and Ketjen black. And / or, the adhesive is polyvinylidene fluoride (PVDF). And / or, the solvent is N-methylpyrrolidone (NMP); And / or, the initial and / or re-drying temperature is 80℃-130℃, and the drying time is 1 min-5 min.

[0016] Thirdly, the present invention provides a lithium-ion battery comprising the double-layer composite positive electrode sheet described in any of the foregoing embodiments.

[0017] The present invention has the following beneficial effects: This application achieves complementary advantages by combining a first cathode active material layer with a multi-element doped hollow ternary lithium metal oxide, which possesses high conductivity, fast charging capability, and structural buffering capacity, with a second cathode active material layer with phosphorus-doped lithium iron phosphate, which has high stability, safety, and interface protection functions. The high conductivity network of the first cathode active material layer greatly reduces ohmic polarization from the current collector to the active material, providing a fast electron transport channel for the entire electrode, thereby effectively avoiding the disadvantage of the relatively low intrinsic electronic conductivity of lithium iron phosphate material in the second cathode active material layer; at the same time, the thermal stability and interface stability of the second cathode active material layer provide a safety guarantee for the high energy density material in the first cathode active material layer, and together they construct a stable electrode-electrolyte interface. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0019] The applicant's research found that, in order to achieve a balance between high safety and long lifespan and improve overall performance, a double-layer or multi-layer coating technique can be used to progressively composite active materials with different properties onto the same current collector. However, if the focus is limited to interlayer composite of active materials without specifically addressing the microscopic scale of the active particles themselves to synergistically optimize the ion / electron transport capacity of the bottom layer and the structural strain tolerance of the surface layer, then significant interfacial impedance and stress mismatch issues will arise, hindering further improvements in overall performance.

[0020] Therefore, embodiments of the present invention provide a double-layer composite positive electrode sheet, comprising: a current collector and a first positive electrode active material layer and a second positive electrode active material layer sequentially stacked on at least one surface of the current collector; The first positive electrode active material layer includes a polycrystalline ternary lithium metal oxide doped with a first element, the polycrystalline ternary lithium metal oxide having a hollow structure; the first element is selected from at least three of W, Nb, Ti, Mg, Al, and Zr; The second positive electrode active material layer includes phosphorus-doped lithium iron phosphate.

[0021] In this application's bilayer composite cathode electrode, the active material in the first cathode active material layer is a polycrystalline ternary lithium metal oxide with a hollow structure. This unique hollow structure provides a three-dimensional short path for electrolyte wetting and rapid lithium-ion diffusion. Simultaneously, the cavities in the hollow structure act as buffer cells, effectively absorbing the volume expansion stress of the active material during charging and discharging, maintaining the integrity of the electrode structure. Furthermore, the active material in the first cathode active material layer is doped with a first element. This first element effectively expands the crystal lattice, increases electronic conductivity, and pins the lattice to suppress phase transitions, thus optimizing electronic conductivity and structural stability. In summary, by using a polycrystalline ternary lithium metal oxide with a hollow structure and doped with a first element as the active material in the first cathode active material layer, it is beneficial to reduce the interfacial resistance and ohmic polarization of electrons transported from the current collector to the active material.

[0022] In this application's bilayer composite cathode electrode, the active material in the second cathode active material layer is phosphorus-doped lithium iron phosphate. The inherently excellent thermal stability of lithium iron phosphate, combined with its location on the outermost side of the electrode, effectively blocks or delays the transfer of heat and oxygen to the inner high-energy-density ternary material under thermal abuse conditions, significantly improving the battery's thermal safety margin. Furthermore, phosphorus doping stabilizes the olivine structure of lithium iron phosphate and forms a denser and more stable solid electrolyte interphase (CEI) film on its surface, greatly suppressing the continuous decomposition of the electrolyte and the dissolution of transition metal ions.

[0023] This application achieves complementary advantages by combining a first cathode active material layer with a multi-element doped hollow ternary lithium metal oxide, which possesses high conductivity, fast charging capability, and structural buffering capacity, with a second cathode active material layer with phosphorus-doped lithium iron phosphate, which has high stability, safety, and interface protection functions. The high conductivity network of the first cathode active material layer greatly reduces ohmic polarization from the current collector to the active material, providing a fast electron transport channel for the entire electrode, thereby effectively avoiding the disadvantage of the relatively low intrinsic electronic conductivity of lithium iron phosphate material in the second cathode active material layer; at the same time, the thermal stability and interface stability of the second cathode active material layer provide a safety guarantee for the high energy density material in the first cathode active material layer, and together they construct a stable electrode-electrolyte interface.

[0024] In an optional embodiment, the polycrystalline ternary lithium metal oxide is spherical; spherical particles are beneficial to the rheology and coating uniformity of the slurry, improve the surface flatness of the electrode and the consistency of interlayer contact, reduce interface defects, and enhance the continuity of electron / ion transport paths.

[0025] In an optional embodiment, the polycrystalline ternary lithium metal oxide has the general chemical formula LiNi. x Co y Mn z M w O2, where x+y+z+w=1 and 0.5 ≤ x<1, and M is the first element. High-nickel ternary materials are beneficial for ensuring high specific capacity and voltage plateau, while the introduction of the first element M enables lattice control, balancing energy density and structural stability.

[0026] In an optional embodiment, the total molar amount of the first element in the polycrystalline ternary lithium metal oxide accounts for 0.1%-2.0% of the total molar amount of all metals except lithium in the polycrystalline ternary lithium metal oxide, for example 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, and 2.0%. Micro-doping can effectively pin the crystal lattice and optimize the carrier concentration, while excessive doping can easily induce impurities or destroy the layered order.

[0027] In an optional embodiment, the hollowness HD of the secondary particles of the hollow spherical polycrystalline ternary lithium metal oxide is 0.2-0.6, for example, 0.20, 0.24, 0.29, 0.33, 0.38, 0.42, 0.47, 0.51, 0.56, 0.60; where HD = D1 / D2, D1 is the equivalent sphere diameter of the hollow region, and D2 is the outer diameter of the particle. Appropriate hollowness provides a short path for the three-dimensional diffusion of lithium ions and buffers for volume expansion, while maintaining the mechanical strength and tap density of the particles, avoiding excessive cavitation that could lead to breakage of the conductive network.

[0028] It should be noted that the secondary particles of hollow spherical polycrystalline ternary lithium metal oxide in this application are produced by spray granulation. When the inlet air temperature is 180-240℃ and the feed rate is 15-35 mL / min during the spray drying process, the hollowness HD can meet the requirements.

[0029] In an optional embodiment, the P / Fe molar ratio in the phosphorus-doped lithium iron phosphate is 1.01-1.10, for example, 1.010, 1.020, 1.030, 1.040, 1.050, 1.060, 1.070, 1.080, 1.090, or 1.100. The slight excess of phosphorus can stabilize the olivine framework and promote the formation of a dense CEI, enhancing the interface passivation capability without significantly sacrificing electronic conductivity.

[0030] In an optional embodiment, the lithium iron phosphate (LFP) surface is provided with a carbon coating layer, wherein the carbon content of the LFP is 1.0 wt% - 3.0 wt%, for example 1.0 wt%, 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.4 wt%, 2.6 wt%, 2.8 wt%, or 3.0 wt%. This constructs a continuous conductive network to compensate for the intrinsically low conductivity of LFP, while physically blocking electrolyte erosion and synergistically improving interfacial kinetics and chemical stability through phosphorus doping.

[0031] In an optional embodiment, the mass ratio of the polycrystalline ternary lithium metal oxide to the lithium iron phosphate is 1-3:1, for example 1.0:1, 1.2:1, 1.4:1, 1.6:1, 1.8:1, 2.0:1, 2.2:1, 2.4:1, 2.6:1, 2.8:1, 3.0:1; while ensuring the safety barrier performance of the second positive electrode active material layer, maximizing the contribution of high-energy materials in the first positive electrode active material layer is more conducive to improving the overall performance of the electrode.

[0032] In an optional embodiment, the compaction density of the bilayer composite positive electrode sheet is 2.6 g / cm³. 3 - 3.4 g / cm 3Examples of values ​​include 2.60 g / cm³, 2.69 g / cm³, 2.78 g / cm³, 2.87 g / cm³, 2.96 g / cm³, 3.05 g / cm³, 3.14 g / cm³, 3.23 g / cm³, 3.32 g / cm³, and 3.40 g / cm³. These values ​​balance porosity and electrode density, ensuring sufficient electrolyte wetting and ion transport channels while maintaining good electronic pathways and mechanical integrity.

[0033] The present invention also provides a method for preparing the double-layer composite positive electrode sheet according to any one of the foregoing embodiments, comprising: A first positive electrode active slurry containing polycrystalline ternary lithium metal oxide is coated on the surface of the current collector, followed by initial drying to form a first positive electrode active material layer on the current collector. A second positive electrode active slurry containing phosphorus-doped lithium iron phosphate is coated on the surface of the first positive electrode active material layer, then dried again, and subsequently rolled and slit to obtain the double-layer composite positive electrode sheet.

[0034] In the preparation process of the double-layer composite positive electrode sheet, the first positive electrode active material layer is formed first and then the second positive electrode active material layer is coated to avoid interlayer missolution and component migration; the staged drying controls the solvent gradient evaporation to ensure clear interlayer interfaces and strong bonding, and suppress stress delamination.

[0035] In an optional embodiment, the D50 of the secondary particles of the hollow spherical polycrystalline ternary lithium metal oxide is 8.0 μm-15.0 μm, for example, 8.0 μm, 8.8 μm, 9.6 μm, 10.4 μm, 11.2 μm, 12.0 μm, 12.8 μm, 13.6 μm, 14.4 μm, and 15.0 μm. This particle size range is beneficial for balancing the slurry dispersibility, coating uniformity, and ion diffusion path length, avoiding agglomeration due to excessive fineness and scratches or interface roughness due to excessive coarseness.

[0036] In an optional embodiment, the primary particle D50 of the phosphorus-doped lithium iron phosphate is 50 nm-200 nm, for example 50 nm, 67 nm, 83 nm, 100 nm, 117 nm, 133 nm, 150 nm, 167 nm, 183 nm, 200 nm; nanoscale primary particles can shorten the lithium ion solid-phase diffusion distance and improve the reaction activity; combined with carbon coating, high interfacial reaction efficiency can be achieved within a limited thickness.

[0037] In an optional embodiment, the first positive electrode active slurry includes the polycrystalline ternary lithium metal oxide, conductive agent, binder, and solvent. The mass ratio of the polycrystalline ternary lithium metal oxide, conductive agent, and binder in the first positive electrode active slurry is (90-97):(1.5-5):(1.5-5), for example, 90.0:5.0:5.0, 91.0:4.5:4.5, 92.0:4.0:4.0, 93.0:3.5:3.5, 94.0:3.0:3.0, 95.0:2.5:2.5, 96.0:2.0:2.0, and 97.0:1.5:1.5. The high proportion of active material ensures energy density, and the appropriate amount of conductive agent and binder work together to construct a conductive network and mechanical framework, taking into account conductivity, adhesion, etc.

[0038] In an optional embodiment, the second positive electrode active slurry includes lithium iron phosphate, a conductive agent, a binder, and a solvent. The mass ratio of lithium iron phosphate, conductive agent, and binder in the second positive electrode active slurry is (92-96):(2-6):(2-6), for example, 92.0:6.0:2.0, 92.8:5.6:1.6, 93.6:5.2:1.2, 94.4:4.8:0.8, 95.2:4.4:0.4, and 96.0:4.0:0. The high proportion of active material ensures energy density, and an appropriate amount of conductive agent and binder work together to construct a conductive network and mechanical framework, taking into account conductivity, adhesion, etc.

[0039] In an optional embodiment, the conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, graphene, and Ketjen black. In an optional embodiment, the adhesive is polyvinylidene fluoride (PVDF). In an optional embodiment, the solvent is N-methylpyrrolidone (NMP). In an optional embodiment, the initial and / or re-drying temperature is 80℃-130℃, for example 80℃, 86℃, 92℃, 98℃, 104℃, 110℃, 116℃, 122℃, 128℃, 130℃; the drying time is 1 min-5 min, for example 1.0 min, 1.4 min, 1.8 min, 2.2 min, 2.6 min, 3.0 min, 3.4 min, 3.8 min, 4.2 min, 4.6 min, 5.0 min; to remove the solvent from the first positive electrode active slurry.

[0040] The present invention also provides a lithium-ion battery, including the double-layer composite positive electrode sheet described in any one of the foregoing embodiments.

[0041] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0042] Example 1 A method for preparing a double-layer composite positive electrode sheet includes the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. Sintering was carried out at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (Al) with an HD of 0.35 and a D50 of 13.5 μm.

[0043] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B1), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0044] (3) Preparation and coating of the first positive electrode active material layer slurry: Al, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0045] (4) Preparation and coating of the second positive electrode active material layer slurry: B1, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0046] (5) Electrode post-treatment: The electrode coated with the double-layer slurry is thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A1 to B1 being 2:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0047] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0048] Example 2 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (1), the inlet air temperature of the spray dryer is adjusted to 200℃ and the feed rate is 30 mL / min, to prepare a W / Nb / Mg co-doped hollow spherical NCM material with HD of 0.25 and D50 of 11.0 μm, specifically including the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 200℃ and a feed rate of 30 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A2) with an HD of 0.25 and a D50 of 11.0 μm.

[0049] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B2), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0050] (3) Preparation and coating of the first positive electrode active material layer slurry: A2, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0051] (4) Preparation and coating of the second positive electrode active material layer slurry: B2, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0052] (5) Electrode post-treatment: The electrode coated with the double-layer slurry is thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A2 to B2 being 2:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0053] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0054] Example 3 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (1), the inlet air temperature of the spray dryer is adjusted to 240℃ and the feed rate is 20 mL / min, to prepare a W / Nb / Mg co-doped hollow spherical NCM material with HD of 0.50 and D50 of 15.0 μm, specifically including the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 240℃ and a feed rate of 20 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A3) with an HD of 0.50 and a D50 of 15.0 μm.

[0055] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B3), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0056] (3) Preparation and coating of the first positive electrode active material layer slurry: A3, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0057] (4) Preparation and coating of the second positive electrode active material layer slurry: B3, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0058] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A3 to B3 being 2:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0059] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0060] Example 4 A method for preparing a bilayer composite positive electrode sheet differs from Example 1 in that: in step (1), the doping element is changed to Ti / Al / Zr, and a Ti / Al / Zr co-doped hollow spherical NCM material with HD of 0.35 and D50 of 13.5 μm is prepared. The method specifically includes the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Tetrabutyl titanate, aluminum nitrate nonahydrate, and zirconium nitrate pentahydrate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain Ti / Al / Zr co-doped hollow spherical NCM material (A4) with HD of 0.35 and D50 of 13.5 μm.

[0061] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B4), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0062] (3) Preparation and coating of the first positive electrode active material layer slurry: A4, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0063] (4) Preparation and coating of the second positive electrode active material layer slurry: B4, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0064] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A4 to B4 being 2:1, and rolled to a compaction density of 3.15 g / cm³. 3Cut.

[0065] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0066] Example 5 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (2), the amount of ammonium dihydrogen phosphate is adjusted to prepare carbon-coated phosphorus-doped lithium iron phosphate with a phosphorus-iron molar ratio of P / Fe of 1.01. The specific steps include: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A5) with an HD of 0.35 and a D50 of 13.5 μm.

[0067] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.01) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B5), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0068] (3) Preparation and coating of the first positive electrode active material layer slurry: A5, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0069] (4) Preparation and coating of the second positive electrode active material layer slurry: B5, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0070] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A5 to B5 being 2:1, and rolled to a compaction density of 3.15 g / cm³.3 Cut.

[0071] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0072] Example 6 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (2), the amount of ammonium dihydrogen phosphate is adjusted to prepare carbon-coated phosphorus-doped lithium iron phosphate with a phosphorus-iron molar ratio of P / Fe of 1.10. The specific steps include: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A6) with an HD of 0.35 and a D50 of 13.5 μm.

[0073] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.10) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B6), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0074] (3) Preparation and coating of the first positive electrode active material layer slurry: A6, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0075] (4) Preparation and coating of the second positive electrode active material layer slurry: B6, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0076] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A6 to B6 being 2:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0077] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0078] Example 7 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (5), the coating mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is adjusted to 1:1, specifically including the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. The mixture was sintered at 780℃ for 12 hours in an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A7) with an HD of 0.35 and a D50 of 13.5 μm.

[0079] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B7), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0080] (3) Preparation and coating of the first positive electrode active material layer slurry: A7, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0081] (4) Preparation and coating of the second positive electrode active material layer slurry: B7, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0082] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A7 to B7 being 1:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0083] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0084] Example 8 A method for preparing a double-layer composite positive electrode sheet differs from Example 1 in that: in step (5), the coating mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is adjusted to 3:1, specifically including the following steps: (1) Preparation of the first positive electrode active material: Nickel, cobalt and manganese acetates were prepared into a solution with a Ni:Co:Mn molar ratio of 82:12:5. Ammonium metatungstate, 0.3% niobium oxalate and 0.3% magnesium acetate, equivalent to 0.4% of the total molar amount of each metal except lithium, were added and stirred evenly to obtain a precursor solution. The precursor solution was spray-dried at an inlet air temperature of 220℃ and a feed rate of 25 mL / min to obtain precursor powder. The precursor powder was mixed with lithium carbonate, wherein the amount of lithium carbonate added was 1.05 times the total molar amount of each metal except lithium. Sintering was carried out at 780℃ for 12 hours under an oxygen atmosphere to obtain a W / Nb / Mg co-doped hollow spherical NCM material (A8) with an HD of 0.35 and a D50 of 13.5 μm.

[0085] (2) Preparation of the second positive electrode active material: Ferrous oxalate, ammonium dihydrogen phosphate (P / Fe=1.05) and glucose were mixed and ball-milled, and calcined at 650°C for 10 hours in argon to obtain carbon-coated phosphorus-doped lithium iron phosphate (B8), wherein the mass fraction of carbon was 2.0 wt% and the primary particle D50 was 100 nm.

[0086] (3) Preparation and coating of the first positive electrode active material layer slurry: A8, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 96:2:2, coated on aluminum foil, and dried at 120°C for 2 minutes to form the first positive electrode active material layer.

[0087] (4) Preparation and coating of the second positive electrode active material layer slurry: B8, conductive carbon black and PVDF are dispersed evenly in NMP at a mass ratio of 94:4:2 and continuously coated onto the first positive electrode active material layer.

[0088] (5) Electrode post-treatment: The electrodes coated with the double-layer slurry are thoroughly dried, i.e., dried at 120℃ for 2 minutes, with the mass ratio of A8 to B8 being 3:1, and rolled to a compaction density of 3.15 g / cm³. 3 Cut.

[0089] (6) Battery assembly: Using the above-mentioned electrode as the positive electrode, artificial graphite as the negative electrode, Celgard 2325 as the separator, and 1M LiPF6 in EC / DMC / EMC (1:1:1 vol%) with 2% VC as the electrolyte, a 3.2 Ah soft-pack full cell was assembled and electrochemical tests were performed. The test voltage range was 2.0-4.2 V.

[0090] Comparative Example 1 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (1), the spray drying inlet temperature is adjusted to 160℃ and the feed rate is 10 mL / min, while the remaining steps and parameters are the same as in Example 1, and a W / Nb / Mg co-doped NCM material with HD of 0 and D50 of 18 μm is prepared.

[0091] Comparative Example 2 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (1), the spray drying inlet temperature is adjusted to 260℃ and the feed rate is 40 mL / min, while the remaining steps and parameters are the same as in Example 1, and a W / Nb / Mg co-doped hollow spherical NCM material with HD of 0.8 and D50 of 6 μm is prepared.

[0092] Comparative Example 3 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (1), no doping elements are added, and an undoped hollow spherical NCM material is prepared.

[0093] Comparative Example 4 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (1), only 1.0% ammonium metatungstate is added for single-element doping to prepare W single-doped hollow spherical NCM material.

[0094] Comparative Example 5 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (1), only 0.5% ammonium metatungstate and 0.5% magnesium acetate are added for dual-element doping to prepare W / Mg dual-doped hollow spherical NCM material.

[0095] Comparative Example 6 A method for preparing a positive electrode sheet differs from Example 1 in that: in step (2), undoped ordinary carbon-coated lithium iron phosphate (P / Fe=1.00) is used as the second active material.

[0096] Comparative Example 7 A method for preparing a positive electrode sheet differs from Example 1 in that only a single-layer positive electrode is prepared. Step (4) is omitted, and the first positive electrode active material layer slurry in step (3) is used for coating only once. The total coating quality is the same as that of the double-layer electrode sheet in Example 1, thus preparing a single-layer ternary positive electrode sheet.

[0097] Comparative Example 8 A method for preparing a positive electrode sheet differs from Example 1 in that only a single-layer positive electrode is prepared. Step (3) is omitted, and the second positive electrode active material layer slurry in step (4) is used for coating only once. The total coating mass is the same as that of the double-layer electrode sheet in Example 1, thus preparing a single-layer lithium iron phosphate positive electrode sheet.

[0098] The parameters of each embodiment and comparative example are shown in Table 1. The method for testing the hollowness includes: taking a cross-sectional sample of the prepared cathode material, calculating the equivalent sphere diameter D1 of the internal hollow region, the outer diameter of the particle D2, and the hollowness HD = D1 / D2.

[0099] Table 1 Material parameters and structural designs of Examples 1-8 and Comparative Examples 1-8

[0100] Test Result Analysis The electrical performance of the pouch cells prepared in the above embodiments and comparative examples was tested, and the key results are shown in Table 2.

[0101] Table 2 Electrochemical performance of pouch cells in Examples 1-8 and Comparative Examples 1-8

[0102] According to Tables 1 and 2: (1) Effectiveness of the double-layer composite structure: Compared with the pure ternary cathode, the double-layer composite cathode of this application has basically the same internal resistance and rate performance, but significantly better cycle life and lower discharge temperature rise; compared with the pure lithium iron phosphate cathode, the advantages in internal resistance and rate performance are more significant. This proves that the double-layer composite cathode of this application successfully combines the power advantages of ternary cathode with the life and temperature rise advantages of lithium iron phosphate.

[0103] (2) Influence of hollowness in the first positive electrode active material layer: When the hollowness of the polycrystalline ternary lithium metal oxide in the first positive electrode active material layer of this application is too low, the internal resistance and rate capability deteriorate significantly; when the hollowness is too high, the cycle life is even worse than that of pure ternary materials. The hollowness value of Example 1 shows the best overall performance, indicating that there is a suitable range for hollowness, and both excessively high and excessively low hollowness lead to performance imbalance.

[0104] (3) Ternary multi-element co-doped first cathode active material layer: Compared with undoped, single-doped, and double-doped, the internal resistance of multi-element co-doped material decreases and the cycle life increases in turn. This proves that multi-element synergistic doping has a progressive gain, and single or double-element doping cannot achieve the same effect.

[0105] (4) Phosphorus-doped lithium iron phosphate in the second positive electrode active material layer: The phosphorus-doped sample has a significantly better cycle life than the undoped sample, and the rate performance and internal resistance are basically the same, indicating that phosphorus doping has a clear effect on improving interface stability and there is an optimal doping range.

[0106] (5) Coating quality ratio: If the areal density ratio is too low, the rate performance will decrease, and if it is too high, the cycle life will be sacrificed. The value of Example 1 achieves the best balance between rate and cycle, and can be adjusted according to application requirements.

[0107] In summary, compared with the prior art, the technical solution of the present invention has the following beneficial effects: (1) The first positive electrode active material layer of this application adopts a hollow spherical ternary material doped with multiple elements, which takes into account both high capacity and structural stability; the second positive electrode active material layer adopts phosphorus-doped lithium iron phosphate, which simultaneously improves safety and conductivity. The two are combined in a specific ratio to achieve breakthroughs in energy density, safety and lifespan.

[0108] (2) The hollow spherical structure of the first positive electrode active material layer provides a short path for ion diffusion, and multi-element doping (especially high-valence W, Nb, and Ti) enhances electronic conductivity, jointly constructing an efficient three-dimensional charge transport network. Phosphorus doping in the second positive electrode active material layer also improves the intrinsic conductivity of lithium iron phosphate. At the same time, the combination of the buffering effect of the hollow structure of the first positive electrode active material layer and the low-strain characteristics of the second positive electrode active material layer, as well as the strictly controlled particle size distribution, effectively matches the volume changes of different materials during cycling, greatly alleviates internal stress, and ensures the structural integrity of the electrode during long-term cycling.

[0109] (3) The double-layer coating process involved in this invention is highly compatible with existing lithium-ion battery electrode manufacturing lines, requiring no complex equipment modifications. Stable large-scale production can be achieved by precisely controlling the formulation, viscosity, and coating parameters of the two slurries. This solution provides a clear and feasible technical path for developing next-generation power and energy storage batteries that combine high performance and high safety.

[0110] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A double-layer composite positive electrode sheet, characterized in that, include: A current collector and a first positive electrode active material layer and a second positive electrode active material layer are sequentially stacked on at least one surface of the current collector; The first positive electrode active material layer includes a polycrystalline ternary lithium metal oxide doped with a first element, the polycrystalline ternary lithium metal oxide having a hollow structure; the first element is selected from at least three of W, Nb, Ti, Mg, Al, and Zr; The second positive electrode active material layer includes phosphorus-doped lithium iron phosphate.

2. The double-layer composite positive electrode sheet according to claim 1, characterized in that, The polycrystalline ternary lithium metal oxide is spherical; And / or, the general chemical formula of the polycrystalline ternary lithium metal oxide is LiNi. x Co y Mn z M w O2, where x+y+z+w=1 and 0.5 ≤ x<1, and M is the first element.

3. The double-layer composite positive electrode sheet according to claim 1, characterized in that, The total molar amount of the first element in the polycrystalline ternary lithium metal oxide accounts for 0.1%-2.0% of the total molar amount of all metals except lithium in the polycrystalline ternary lithium metal oxide; And / or, the hollowness HD of the secondary particles of the polycrystalline ternary lithium metal oxide is 0.2-0.6, where HD = D1 / D2, D1 is the equivalent sphere diameter of the hollow region, and D2 is the outer diameter of the particle.

4. The double-layer composite positive electrode sheet according to claim 1, characterized in that, The P / Fe molar ratio in the phosphorus-doped lithium iron phosphate is 1.01-1.10; And / or, the lithium iron phosphate is provided with a carbon coating layer on its surface, and the carbon content in the lithium iron phosphate is 1.0 wt% - 3.0 wt%.

5. The double-layer composite positive electrode sheet according to claim 1, characterized in that, The mass ratio of the polycrystalline ternary lithium metal oxide to the lithium iron phosphate is 1-3:1; And / or, the compaction density of the bilayer composite positive electrode sheet is 2.6 g / cm³. 3 - 3.4 g / cm 3 .

6. A method for preparing a double-layer composite positive electrode sheet according to any one of claims 1-5, characterized in that, include: A first positive electrode active slurry containing polycrystalline ternary lithium metal oxide is coated on the surface of the current collector, followed by initial drying to form a first positive electrode active material layer on the current collector. A second positive electrode active slurry containing phosphorus-doped lithium iron phosphate is coated on the surface of the first positive electrode active material layer, then dried again, and subsequently rolled and slit to obtain the double-layer composite positive electrode sheet.

7. The method for preparing the double-layer composite positive electrode sheet according to claim 6, characterized in that, The secondary particles of the polycrystalline ternary lithium metal oxide have a D50 of 8.0 μm-15.0 μm; And / or, the primary particle D50 of the phosphorus-doped lithium iron phosphate is 50 nm-200 nm.

8. The method for preparing the double-layer composite positive electrode sheet according to claim 6, characterized in that, The first positive electrode active slurry includes the polycrystalline ternary lithium metal oxide, conductive agent, binder and solvent, and the mass ratio of polycrystalline ternary lithium metal oxide, conductive agent and binder in the first positive electrode active slurry is (90-97):(1.5-5):(1.5-5). And / or, the second positive electrode active slurry includes the lithium iron phosphate, conductive agent, binder and solvent, and the mass ratio of the lithium iron phosphate, conductive agent and binder in the second positive electrode active slurry is (92-96):(2-6):(2-6).

9. The method for preparing the double-layer composite positive electrode sheet according to claim 8, characterized in that, The conductive agent is selected from at least one of conductive carbon black, carbon nanotubes, graphene, and Ketjen black. And / or, the adhesive is polyvinylidene fluoride; And / or, the solvent is N-methylpyrrolidone; And / or, the initial and / or re-drying temperature is 80℃-130℃, and the drying time is 1 min-5 min.

10. A lithium-ion battery, characterized in that, Includes the double-layer composite positive electrode sheet as described in any one of claims 1-5.

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