Lfp electrode with composite active coating, method of making and battery

Abstract

The application relates to the technical field of battery materials, and discloses an LFP electrode with a composite active coating, a preparation method of the LFP electrode and a battery. The LFP electrode with the composite active coating comprises a current collector and a composite active coating coated on the surface of the current collector; the composite active coating comprises a bottom layer, a connecting layer and a surface layer which are sequentially arranged in layers; the active material of the bottom layer is high-pressure-density LFP material; the active material of the surface layer is high-power LFP material; the connecting layer comprises LFP secondary balls; at least part of the LFP secondary balls are distributed at opposite interfaces of the connecting layer; the LFP secondary balls distributed on one interface are embedded in the bottom layer and the connecting layer at the same time; the LFP secondary balls distributed on the other interface are embedded in the surface layer and the connecting layer at the same time; and the particle size of the LFP secondary balls is 2-10 mu m. The LFP electrode with the composite active coating provided by the application has high energy density, better fast charging performance and long service life.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and more specifically, to an LFP electrode with a composite active coating, a method for preparing the same, and a battery thereof. Background Technology

[0002] With the rapid development of electric vehicles and various energy storage systems, lithium iron phosphate (LFP), as a high-safety, long-life, and low-cost cathode material, is finding increasingly wider applications and its market demand is growing rapidly. In 2024 alone, domestic LFP consumption exceeded 2.4 million tons, and this is expected to continue to grow rapidly in the future. Especially in the energy storage field, it holds an absolute dominant position, and in the power battery field, its market share is also increasing. Furthermore, with improvements in LFP materials themselves and corresponding optimizations at the electrode, cell, and system levels, the energy density and fast-charging performance of LFP batteries are improving, leading to a year-on-year increase in their market share in the power battery sector. In the small-capacity and commercial vehicle sectors, LFP batteries have already achieved dominance, and in the passenger vehicle sector, their market share is steadily rising, gradually approaching and even surpassing ternary lithium batteries.

[0003] Current LFP batteries, while maintaining their advantages of long lifespan, high safety, and low cost, are seeing a new development trend and direction towards achieving both high energy density and fast charging performance. The demand for high energy density and fast charging, besides improving the anode and electrolyte, also places higher demands on the LFP cathode. We need higher compaction density LFP materials to improve battery energy density and provide sufficient redundancy for fast charging and long cycle life of the anode. However, high compaction cathode materials often result in lower specific capacity, higher internal resistance, and can even affect battery cycle life and safety. These characteristics often severely impact battery energy density and fast charging capabilities. Solving the energy density, fast charging, and lifespan issues entirely through lithium iron phosphate materials themselves is extremely difficult, and even if some can be achieved, the cost will be very high. Balancing energy density, fast charging, and lifespan is the impossible triangle in lithium iron phosphate battery development. Usually, a balance and trade-offs are required, and the results are often unsatisfactory.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide an LFP electrode with a composite active coating, a method for preparing the same, and a battery, in order to improve at least one of the problems mentioned in the background art.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides an LFP electrode with a composite active coating, comprising a current collector and a composite active coating coated on the surface of the current collector. The composite active coating comprises a base layer, a connecting layer, and a surface layer stacked sequentially. The bottom active material is high-pressure compacted LFP material, with a compaction density ≥ 2.6 g / cm³. 3 The lithium iron phosphate material has a bottom layer thickness of 100~200μm; The active material on the surface is a high-power LFP material with a D50 of 0.5~5μm and a specific surface area of ​​10~30m² / g. The particle surface is coated with a continuous carbon layer, which accounts for 1~3% of the mass of the high-power LFP material, and the thickness of the surface layer is 50~200μm. The connecting layer includes LFP secondary spheres, at least a portion of which are distributed at opposite interfaces of the connecting layer. The LFP secondary spheres distributed at one interface are simultaneously embedded in the bottom layer and the connecting layer; the LFP secondary spheres distributed at the other interface are simultaneously embedded in the top layer and the connecting layer; the D50 of the LFP secondary spheres is 2~10μm; the thickness of the connecting layer is 50~100μm.

[0007] In an optional embodiment, the compaction density of the high-pressure compacted LFP material is 2.6~2.85 g / cm³. 3 .

[0008] In an optional embodiment, the D50 of the high-density LFP material is 3~10μm, preferably 2~8μm, and more preferably 3~5μm.

[0009] In an optional implementation, the high-power LFP is a secondary sphere formed by primary particle sintering, and the primary particle size is 50~150nm.

[0010] In an optional embodiment, the high-pressure dense LFP material accounts for 10-50% by mass in the composite active coating; And / or, the high-power LFP material accounts for 30-80% of the mass of the composite active coating; And / or, the mass ratio of LFP secondary spheres in the composite active coating is 10-20%.

[0011] In an optional embodiment, the surface layer further includes 1-2% by mass of an adhesive and 0.2-1.5% by mass of a conductive agent; And / or, the bonding layer also includes 1-2% adhesive and 0.2-1.5% conductive agent by mass; And / or, the underlying layer also includes 1-2% adhesive and 0.2-2% conductive agent by mass.

[0012] In an optional embodiment, the adhesive is PVDF; Optionally, the conductive agent is selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene, and carbon nanofibers.

[0013] Secondly, embodiments of the present invention provide a method for preparing the aforementioned LFP electrode with a composite active coating, comprising: The first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are provided. The first positive electrode slurry includes high-pressure dense LFP material, the second positive electrode slurry includes LFP secondary spheres, and the third positive electrode slurry includes high-power LFP material. The first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are coated on the surface of the current collector. After all the slurries are dried, a three-layer structure is formed by stacking them in sequence. Then, the slurry is rolled to obtain an LFP electrode with a composite active coating. During the rolling process, at least a portion of the LFP secondary spheres that should be located in the connecting layer are embedded in the surface layer, and at least a portion of the LFP secondary spheres are embedded in the bottom layer.

[0014] In an optional embodiment, the solid content of the first positive electrode slurry is 60-70%, and / or the solid content of the second positive electrode slurry is 60-70%; and / or the solid content of the second positive electrode slurry is 60-70%.

[0015] Thirdly, the present invention provides a battery comprising an LFP electrode having a composite active coating as described in any of the foregoing embodiments.

[0016] The present invention has the following beneficial effects: The LFP electrode with a composite active coating provided in this invention uses high-power lithium iron phosphate (LFP) material for its surface layer. High-power LFP material has the characteristics of small particle size, large specific surface area, and excellent electronic conductivity, which can significantly improve the fast-charging performance of the electrode. When the surface layer thickness is within the range defined in this invention, it can ensure fast-charging performance while also taking into account the interface compatibility between the surface and the bottom layer, avoiding the risk of interlayer delamination caused by a single surface material. The bottom layer uses high-density lithium iron phosphate material. The high packing density of high-density lithium iron phosphate material can effectively improve the volumetric energy density of the electrode. When the bottom layer thickness is within the range defined in this invention, it can fully utilize its high-density advantage, providing core support for the overall energy density of the battery. Lithium iron phosphate secondary spheres (LFP secondary spheres) distributed within the connecting layer and embedded between the surface and the bottom layer not only act as conductors connecting the upper and lower layers, improving interlayer conductivity, but also serve as mechanical anchoring, improving the connection between the bottom and surface layers and increasing the peeling force. In addition, the secondary lithium iron phosphate itself has good capacity, rate capability, and cycle performance, which can further improve the battery performance. Therefore, the design of lithium iron phosphate secondary spheres in the bottom layer, surface layer and connecting layer of this application enables the electrode to have high energy density, excellent fast charging performance and good structural stability, thereby enabling the battery to have high energy density, better fast charging performance and long life. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the structure of an LFP electrode with a composite active coating provided in an embodiment of the present invention; Figure 2 SEM image of high-density LFP material; Figure 3 SEM image of high-power LFP material; Figure 4 It is a secondary spherical LFP material; Figure 5 This is a comparison chart of capacity tests for the embodiment and the comparative example; Figure 6 The comparative example is a cycle performance test of the embodiment and the comparative example. Detailed Implementation

[0019] 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.

[0020] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0021] This invention provides an LFP electrode with a composite active coating, comprising a current collector and a composite active coating coated on the surface of the current collector; The composite active coating comprises a base layer, a connecting layer, and a surface layer stacked sequentially. The bottom active material is high-pressure compacted LFP material, with a compaction density ≥ 2.6 g / cm³. 3 The lithium iron phosphate material has a bottom layer thickness of 100~200μm (e.g., 100μm, 150μm or 200μm). The active material on the surface is a high-power LFP material with a D50 of 0.5~5μm (e.g., 0.5μm, 1μm or 2μm) and a specific surface area of ​​10~30m² / g (e.g., 10 m² / g, 20 m² / g or 30 m² / g). The particle surface is coated with a continuous carbon layer, which accounts for 1~3% (e.g., 1%, 2% or 3%) of the mass of the high-power LFP material. The thickness of the surface layer is 50~200μm (e.g., 50μm, 100μm or 200μm). The connecting layer includes LFP secondary spheres, at least a portion of which are distributed at opposite interfaces of the connecting layer. The LFP secondary spheres distributed at one interface are simultaneously embedded in the bottom layer and the connecting layer; the LFP secondary spheres distributed at the other interface are simultaneously embedded in the top layer and the connecting layer. The D50 of the LFP secondary spheres is 2~10μm (e.g., 2μm, 3μm, 4μm, 5μm, 8μm or 10μm, preferably 3~5μm), and the thickness of the connecting layer is 50~100μm (e.g., 50μm, 80μm or 100μm).

[0022] High-power LFP materials refer to lithium iron phosphate (LiFePO4) cathode materials that have significantly improved rate performance and power density through modification methods such as nano-sizing, carbon coating, element doping, and particle gradation. They can achieve 4–10C fast charging, low internal resistance, and high cycle life, while retaining the inherent advantages of LFP such as high safety and low cost.

[0023] The LFP electrode with a composite active coating provided in this invention uses high-power lithium iron phosphate (LFP) material for its surface layer. High-power LFP material has small particle size, large specific surface area, and excellent electronic conductivity, which can significantly improve the fast-charging performance of the electrode. When the surface layer thickness is within the range defined in this invention, it can ensure fast-charging performance while also considering the interface compatibility between the surface and the bottom layer, avoiding the risk of interlayer delamination caused by a single surface material. The bottom layer uses high-density lithium iron phosphate material. High-density lithium iron phosphate material has a high packing density, which can effectively improve the volumetric energy density of the electrode. When the bottom layer thickness is within the range defined in this invention, it can fully utilize its high-density advantage, providing core support for the overall energy density of the battery. Lithium iron phosphate secondary spheres (LFP secondary spheres) distributed within the connecting layer and embedded between the surface and bottom layers can not only act as conductors connecting the upper and lower layers, improving interlayer conductivity, but also play a mechanical anchoring role, improving the connection between the bottom and surface layers and increasing the peeling force. In addition, the secondary lithium iron phosphate itself has good capacity, rate capability, and cycle performance, which can further improve the battery performance. Therefore, the design of lithium iron phosphate secondary spheres in the bottom layer, surface layer, and connecting layer of this application enables the electrode to possess both high energy density, excellent fast-charging performance, and good structural stability, thereby giving the battery high energy density, good fast-charging performance, and long lifespan. When the thickness of the bottom layer is within the aforementioned range, it ensures electrode energy density while avoiding problems such as excessively long ion diffusion paths and decreased charge / discharge efficiency due to excessive thickness, or insufficient structural strength and susceptibility to breakage due to insufficient thickness. When the thickness of the connecting layer is within the aforementioned range, it ensures that the secondary spheres are fully distributed and play a role in interlayer connection and mechanical anchoring, avoiding insufficient secondary sphere distribution and poor interlayer conductivity due to excessive thickness, or increased overall electrode volume and decreased energy density due to excessive thickness. When the thickness of the surface layer is within the aforementioned range, it fully utilizes the fast-charging advantages of high-power LFP materials, avoiding insufficient fast-charging performance due to excessive thinness, or increased ion diffusion resistance and impact on battery cycle performance due to excessive thickness, thereby giving the electrode better electrochemical performance.

[0024] It should be noted that when secondary spheres are distributed in the coating, the particle size of the secondary spheres needs to be within the range required by this invention. They should not be too large or too small. If they are too small, the interlayer connection will be weakened. If they are too large, the integrity of the interface between the surface layer and the bottom layer will be damaged, resulting in an increase in the interlayer gap. This will not only reduce the interlayer conductivity and prevent the mechanical anchoring effect, but also affect the overall compaction effect of the electrode. Consequently, stress concentration will occur during the charging and discharging process of the battery, reducing the cycle stability and energy density of the battery.

[0025] Optionally, the mass ratio of high-pressure dense LFP material in the composite active coating is 10-50% (e.g., 10%, 20%, 30%, 40%, or 50%); and / or, the mass ratio of high-power LFP material in the composite active coating is 30-80% (e.g., 30%, 40%, 50%, 60%, 70%, or 80%); and / or, the mass ratio of LFP secondary spheres in the composite active coating is 10-20%.

[0026] Optionally, to achieve better electrochemical performance of the electrode, the compaction density of the high-density LFP material is 2.6~2.85 g / cm³. 3 (e.g., 2.6 g / cm) 3 2.7 g / cm 3 2.8 g / cm 3 Or 2.85 g / cm 3 ).

[0027] Optionally, the D50 of the high-density LFP material is 3~10 μm (e.g., 3 μm, 5 μm, 8 μm or 10 μm), preferably 2~8 μm, and more preferably 3~5 μm. When the D50 of the high-density LFP material is within this range, the particle size uniformity is good. This ensures that the material has high packing density and tap density, fully utilizing the advantages of high density to improve the electrode volumetric energy density. It also avoids problems such as excessive specific surface area and severe agglomeration due to excessively small particle size, or poor material dispersibility and loose interlayer bonding due to excessively large particle size, ensuring a stable underlying structure and excellent electrochemical performance.

[0028] Optionally, to improve the electrochemical performance of the electrode, the high-power LFP is made of ultrafine LFP particles coated with a conductive layer, with a D50 controlled at 0.5~5μm, a specific surface area of ​​10~30m² / g, and an electronic conductivity ≥10. - With a particle size of ³S / cm, this type of high-power LFP particle has a small and uniform particle size. The conductive layer on the surface can effectively reduce the electron transport resistance and respond quickly to charge and discharge signals. At the same time, its large specific surface area can increase the contact area with the electrolyte, accelerate ion diffusion, further enhance the fast charging performance of the surface layer, and form a good particle size gradient match with the underlying high-density LFP material, improve the interlayer interface compatibility, and prevent the surface layer from falling off.

[0029] Optionally, the surface layer further includes 1-2% (e.g., 1%, 1.5%, or 2%) of adhesive and 0.2-1.5% (e.g., 0.2%, 0.5%, 0.8%, 1%, or 1.5%) of conductive agent by mass, with the remainder being high-power LFP material; and / or, the bonding layer further includes 1-2% (e.g., 1%, 1.5%, or 2%) of adhesive and 0.2-1.5% (e.g., 0.2%, 0.5%, 0.8%, 1%, or 1.5%) of conductive agent by mass, with the remainder being LFP secondary spheres; and / or, the bottom layer further includes 1-2% (e.g., 1%, 1.5%, or 2%) of adhesive and 0.2-2% (e.g., 0.2%, 0.5%, 1%, or 2%) of conductive agent by mass, with the remainder being high-density LFP material.

[0030] Optionally, the adhesive is PVDF.

[0031] Optionally, the conductive agent is selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene, and carbon nanofibers.

[0032] The method for preparing an LFP electrode with a composite active coating provided in this embodiment of the invention includes: The first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are provided. The first positive electrode slurry includes high-pressure dense LFP material, the second positive electrode slurry includes LFP secondary spheres, and the third positive electrode slurry includes high-power LFP material. The first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are coated on the surface of the current collector. After all the slurries are dried, a three-layer structure is formed by stacking them in sequence. Then, the slurry is rolled to obtain an LFP electrode with a composite active coating. During the rolling process, at least a portion of the LFP secondary spheres that should be located in the connecting layer are embedded in the surface layer, and at least a portion of the LFP secondary spheres are embedded in the bottom layer.

[0033] The preparation method provided in the embodiments of the present invention can produce the electrode sheet provided in the embodiments of the present invention.

[0034] Specifically, the preparation method is as follows: S1. Preparation of slurry (1) Preparation of the first positive electrode slurry The high-density LFP material, binder, conductive agent and first solvent are mixed evenly to form a first positive electrode slurry with a solid content of 60-70% (e.g. 60%, 65% or 70%).

[0035] Optionally, the first solvent is NMP.

[0036] Optionally, the first positive electrode slurry also includes a dispersant, which accounts for about 0.01 to 0.1% (e.g., 0.01%, 0.03%, 0.04%, 0.05% or 0.1%) by mass in the slurry, preferably 0.03 to 0.05%. The dispersant may be polyvinylpyrrolidone (PVP).

[0037] (2) Preparation of the second positive electrode slurry High-power LFP material, binder, conductive agent, LFP secondary spheres and second solvent are mixed evenly to form a second positive electrode slurry with a solid content of 60-70% (e.g., 60%, 65% or 70%).

[0038] Optionally, the second solvent is NMP.

[0039] Optionally, the second positive electrode slurry also includes a dispersant, which accounts for about 0.01 to 0.1% of the slurry by mass, preferably 0.03 to 0.05%. The dispersant can be polyvinylpyrrolidone (PVP).

[0040] (3) Preparation of the third cathode slurry High-power LFP material, binder, conductive agent and second solvent are mixed evenly to form a third positive electrode slurry with a solid content of 60-70% (e.g. 60%, 65% or 70%).

[0041] Optionally, the third solvent is NMP.

[0042] Optionally, the third cathode slurry also includes a dispersant, which accounts for about 0.01 to 0.1% of the slurry by mass, preferably 0.03 to 0.05%. The dispersant can be polyvinylpyrrolidone (PVP).

[0043] In the aforementioned first cathode slurry, second cathode slurry and third cathode material, the content of active materials (high-pressure dense LFP, high-power LFP and secondary spheres) is the same as that of existing conventional LFP cathode slurries, for example 95~98wt% (e.g. 95wt%, 96wt%, 97wt% or 98wt%).

[0044] S2, Coating The first positive electrode slurry is coated on the positive electrode current collector and then dried at 50~80°C (e.g., 50°C, 60°C, 70°C or 80°C) to form the bottom layer; Then, the second positive electrode slurry is output through the lower die of the dual-die coating machine and directly coated onto the bottom layer. The third positive electrode slurry is simultaneously coated through the upper die of the dual-die coating machine and completely covers the coating formed by the second positive electrode slurry. After drying at 50~80℃, it is rolled. The rolling action squeezes the LFP secondary spheres that were originally located in the middle layer into the bottom and surface layers, forming an interlocking structure between the three layers and improving the connection stability between the layers. Finally, the metal is cut and punched to form the positive electrode sheet.

[0045] Optionally, the positive current collector is an Al foil.

[0046] An embodiment of the present invention provides a battery including an LFP electrode with a composite active coating provided in the embodiment of the present invention.

[0047] Example 1 97.75% high-density LFP material (compacted density of 2.7 g / cm³) 3 The first positive electrode slurry with a solid content of 69% is prepared by uniformly mixing 1.8% binder (PVDF), 0.4% conductive agent (conductive carbon black), 0.05% dispersant (PVP), and first solvent (NMP) with a D50 of 5μm.

[0048] 98.0% LFP secondary spheres (D50 of 3μm), 1.65% binder (PVDF), 0.3% conductive agent (conductive carbon black), 0.05% dispersant (PVP), and a second solvent (NMP) were mixed evenly to prepare a second positive electrode slurry with a solid content of 68%.

[0049] A third cathode slurry with a solid content of 68% was prepared by uniformly mixing 98.2% high-power LFP material (a continuous carbon layer on the surface, the carbon layer accounting for 2.1% of the mass of the high-power LFP material, D50 of 0.8μm, and specific surface area of ​​18 m² / g), 1.3% binder (PVDF), 0.5% conductive agent (conductive carbon black), 0.03% dispersant (PVP), and a third solvent (NMP).

[0050] The first positive electrode slurry is coated onto the positive electrode current collector (aluminum foil) and then dried at 80°C to form the bottom layer; Then, the second positive electrode slurry is output through the lower die of the dual-die coating machine and directly coated onto the bottom layer. The third positive electrode slurry is simultaneously coated through the upper die of the dual-die coating machine and completely covers the coating formed by the second positive electrode slurry. It is then dried at 65°C, rolled, and finally slit and punched to form the positive electrode sheet.

[0051] The composite active coating of the prepared positive electrode sheet has a bottom layer thickness of 150 μm, a connecting layer thickness of 100 μm, and a surface layer thickness of 80 μm.

[0052] Example 2 97.75% high-density LFP material (compacted density of 2.7 g / cm³) 3 The first positive electrode slurry with a solid content of 69% is prepared by uniformly mixing 1.8% binder (PVDF), 0.4% conductive agent (conductive graphite), 0.05% dispersant (PVP), and first solvent (NMP) with a D50 of 3μm.

[0053] 98.0% LFP secondary spheres (D50 of 5μm), 1.65% binder (PVDF), 0.3% conductive agent (conductive carbon black), 0.05% dispersant, and a second solvent (NMP) were mixed evenly to prepare a second positive electrode slurry with a solid content of 68%.

[0054] A third cathode slurry with a solid content of 68% was prepared by uniformly mixing 98.2% high-power LFP material (a continuous carbon layer on the surface, the carbon layer accounting for 2.1% of the mass of the high-power LFP material, with a D50 of 1.3μm and a specific surface area of ​​14 m² / g), 1.3% binder (PVDF), 0.5% conductive agent (conductive carbon black), 0.03% dispersant (PVP) and a third solvent (NMP).

[0055] The first positive electrode slurry is coated onto the positive electrode current collector (aluminum foil) and then dried at 80°C to form the bottom layer; Then, the second positive electrode slurry is output through the lower die of the dual-die coating machine and directly coated onto the bottom layer. The third positive electrode slurry is simultaneously coated through the upper die of the dual-die coating machine and completely covers the coating formed by the second positive electrode slurry. It is then dried at 65°C, rolled, and finally slit and punched to form the positive electrode sheet.

[0056] The composite active coating of the prepared positive electrode sheet has a bottom layer thickness of 100 μm, a connecting layer thickness of 50 μm, and a surface layer thickness of 50 μm.

[0057] Example 3 97.75% high-density LFP material (compacted density of 2.7 g / cm³) 3 The first positive electrode slurry with a solid content of 69% is prepared by uniformly mixing 1.8% binder (PVDF), 0.4% conductive agent (conductive graphite), 0.05% dispersant (PVP), and first solvent (NMP) with a D50 of 10μm.

[0058] 98.0% LFP secondary spheres (D50 of 3μm), 1.65% binder (PVDF), 0.3% conductive agent (conductive carbon black), 0.05% dispersant (PVP), and a second solvent (NMP) were mixed evenly to prepare a second positive electrode slurry with a solid content of 68%.

[0059] A third cathode slurry with a solid content of 68% was prepared by uniformly mixing 98.2% high-power LFP material (a continuous carbon layer on the surface, the carbon layer accounting for 2.1% of the mass of the high-power LFP material, D50 of 2μm, and specific surface area of ​​10 m² / g), 1.3% binder (PVDF), 0.5% conductive agent (conductive carbon black), 0.03% dispersant (PVP) and a third solvent (NMP).

[0060] The first positive electrode slurry is coated onto the positive electrode current collector (aluminum foil) and then dried at 80°C to form the bottom layer; Then, the second positive electrode slurry is output through the lower die of the dual-die coating machine and directly coated onto the bottom layer. The third positive electrode slurry is simultaneously coated through the upper die of the dual-die coating machine and completely covers the coating formed by the second positive electrode slurry. It is then dried at 65°C, rolled, and finally slit and punched to form the positive electrode sheet.

[0061] The composite active coating of the prepared positive electrode sheet has a bottom layer thickness of 200 μm, a connecting layer thickness of 200 μm, and a surface layer thickness of 100 μm.

[0062] Comparative Example 1 In this comparative example, the same first positive electrode slurry as in Example 1 was coated onto the positive electrode current collector, and after drying and rolling, a coating with a thickness of 330 μm was formed.

[0063] Comparative Example 2 In this comparative example, the same third positive electrode slurry as in Example 1 was coated onto the positive electrode current collector, and after drying and rolling, a coating with a thickness of 330 μm was formed.

[0064] Comparative Example 3 In this comparative example, the same first positive electrode slurry as in Example 1 was coated onto the positive electrode current collector and dried. Then, a third positive electrode slurry was coated onto the dried coating surface, dried, and finally rolled to obtain a current collector with a two-layer coating structure. After rolling, a coating with a thickness of 330 μm was obtained, of which the bottom layer thickness was 215 μm and the surface layer thickness was 115 μm.

[0065] Comparative Example 4 This comparative example is basically the same as Example 1, except that the D50 of the secondary sphere is 1 μm.

[0066] Comparative Example 5 This comparative example is basically the same as Example 1, except that the D50 of the secondary sphere is 12 μm.

[0067] Experimental Example The positive electrode material, conductive carbon and binder obtained in the above embodiments and comparative examples are mixed and prepared in a ratio of 97.5:1.5:1. The slurry is then coated on aluminum foil, dried, rolled, slit and die-cut, and finally assembled with graphite negative electrode to form a 5AH soft pack battery. After preparation, conventional formation is performed, and the formed battery is then subjected to 2.5°C at 45°C. Within a voltage range of 3.65V, charge and discharge at a 1C rate, and record the cycle capacity retention. Record the test results in Table 1 and... Figure 5 and 6 As shown.

[0068] Table 1. Electrochemical performance of the positive electrode sheets prepared in each embodiment and comparative example after being assembled into batteries.

[0069] As can be seen from Table 1, the positive electrode sheets prepared in each embodiment of the present invention all have good electrochemical performance.

[0070] Comparing Comparative Example 1 with Example 1, the electrical performance of Comparative Example 1 is significantly worse. This indicates that the positive electrode sheet prepared entirely using high-density lithium iron phosphate (LFP) material as the active material, compared to the positive electrode sheet prepared with a high-density active material layer and a connecting layer containing secondary spheres, has significantly better electrical performance. The secondary spheres (LFP spheres) are distributed within the connecting layer and embedded between the surface and bottom layers. They not only act as conductors connecting the upper and lower layers, improving interlayer conductivity, but also provide mechanical anchoring, improving the connection between the surface and bottom layers and increasing peel strength. Furthermore, the secondary lithium iron phosphate spheres themselves have excellent capacity, rate capability, and cycle performance, further enhancing battery performance. This results in a battery with high energy density, better fast charging, and longer lifespan. Comparing Comparative Example 2 with Example 1, the cycle performance of Comparative Example 2 is significantly worse, indicating that the positive electrode sheet prepared entirely with high-power lithium iron phosphate material as the active material cannot improve its cycle performance compared to the positive electrode sheet prepared with high-pressure dense active material layer and high-power active material layer combined with a connecting layer containing secondary spheres, due to the absence of high-pressure dense active material. Comparing Comparative Example 3 with Example 1, the cycling performance of Comparative Example 3 is significantly worse, indicating that if there is only a high-power lithium iron phosphate layer and a high-density active material layer, without secondary ball bonding, the connection stability between the two layers will be poor, resulting in poor cycling performance. Comparing Comparative Example 4 with Example 1, Comparative Example 4 showed poorer cycle life and capacity, indicating that if the particle size of the secondary spheres is too large, it will damage the integrity of the interface between the surface and the bottom layer, leading to an increase in the interlayer gap. This not only reduces the interlayer conductivity and prevents the mechanical anchoring function, but also affects the overall compaction effect of the electrode, resulting in stress concentration during battery charging and discharging, and reducing the cycle stability and energy density of the battery.

[0071] Comparing Comparative Example 5 with Example 1, Comparative Example 5 showed poorer cycling performance, indicating that if the particle size of the secondary spheres is too small, the bonding effect will be weakened.

[0072] In summary, the LFP electrode with a composite active coating provided by the embodiments of the present invention has better electrochemical performance because the surface active material is high-power lithium iron phosphate material (LFP material) and the bottom active material is high-density lithium iron phosphate material, and the surface and bottom layers are stably bonded together by lithium iron phosphate secondary spheres.

[0073] 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. An LFP electrode with a composite active coating, characterized in that, Includes a current collector and a composite active coating applied to the surface of the current collector; The composite active coating comprises a bottom layer, a connecting layer, and a top layer stacked sequentially. The underlying active material is high-pressure dense LFP material, and the high-pressure dense LFP material has a compaction density ≥ 2.6 g / cm³. 3 The lithium iron phosphate material has a bottom layer thickness of 100~200μm; The active material of the surface layer is a high-power LFP material, the D50 of which is 0.5~5μm, the specific surface area is 10~30m² / g, the particle surface is coated with a continuous carbon layer, the carbon layer accounts for 1~3% of the mass of the high-power LFP material, and the thickness of the surface layer is 50~200μm. The connecting layer includes LFP quadratic spheres, at least a portion of which are distributed at opposite interfaces of the connecting layer. The LFP quadratic spheres distributed at one interface are simultaneously embedded in the bottom layer and the connecting layer; the LFP quadratic spheres distributed at the other interface are simultaneously embedded in the top layer and the connecting layer; the D50 of the LFP quadratic spheres is 2~10μm; and the thickness of the connecting layer is 50~100μm.

2. The LFP electrode with a composite active coating according to claim 1, characterized in that, The compaction density of the high-density LFP material is 2.6~2.85 g / cm³. 3 .

3. The LFP electrode with a composite active coating according to claim 1, characterized in that, The D50 of the high-pressure dense LFP material is 3~10μm, preferably 2~8μm, and more preferably 3~5μm.

4. The LFP electrode with a composite active coating according to claim 1, characterized in that, The high-power LFP is a secondary sphere formed by primary particle sintering, with the primary particle size being 50~150nm.

5. The LFP electrode with a composite active coating according to claim 1, characterized in that, The high-pressure dense LFP material accounts for 10-50% of the mass ratio in the composite active coating; And / or, the high-power LFP material accounts for 30-80% by mass in the composite active coating; And / or, the LFP secondary spheres account for 10-20% of the mass ratio in the composite active coating.

6. The LFP electrode with a composite active coating according to claim 5, characterized in that, The surface layer also includes an adhesive at a mass ratio of 1-2% and a conductive agent at a mass ratio of 0.2-1.5%. And / or, the connecting layer further includes 1-2% adhesive and 0.2-1.5% conductive agent by mass; And / or, the underlying layer further includes 1-2% adhesive and 0.2-2% conductive agent by mass.

7. The LFP electrode with a composite active coating according to claim 6, characterized in that, The adhesive is PVDF; Optionally, the conductive agent is selected from at least one of conductive carbon black, conductive graphite, carbon nanotubes, graphene, and carbon nanofibers.

8. The method for preparing an LFP electrode with a composite active coating as described in any one of claims 1 to 7, characterized in that, include: A first positive electrode slurry, a second positive electrode slurry, and a third positive electrode slurry are provided. The first positive electrode slurry includes the high-pressure dense LFP material, the second positive electrode slurry includes LFP secondary spheres, and the third positive electrode slurry includes the high-power LFP material. The first positive electrode slurry, the second positive electrode slurry, and the third positive electrode slurry are coated on the surface of the current collector. After all the slurries are dried, a three-layer structure is formed by stacking them in sequence. Then, the slurry is rolled to obtain the LFP electrode with the composite active coating. During the rolling process, at least a portion of the LFP secondary spheres that should be located in the connecting layer are embedded in the surface layer, and at least a portion of the LFP secondary spheres are embedded in the bottom layer.

9. The preparation method according to claim 8, characterized in that, The first positive electrode slurry has a solid content of 60-70%, and / or the second positive electrode slurry has a solid content of 60-70%; and / or the second positive electrode slurry has a solid content of 60-70%.

10. A battery, characterized in that, Including the LFP electrode with a composite active coating as described in any one of claims 1 to 7.

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