Composite positive electrode sheet and preparation method and application thereof

Abstract

This invention relates to the field of lithium battery technology, and more particularly to a composite positive electrode sheet, its preparation method, and its application. The composite positive electrode sheet includes a positive current collector and a double-layer structure sequentially stacked on the current collector; the double-layer structure consists, from top to bottom, a lithium iron phosphate-doped ternary positive electrode material layer and a lithium iron phosphate layer. The composite positive electrode sheet provided by this invention improves the battery voltage platform and energy density while ensuring battery safety performance, and also effectively improves battery rate performance and cycle performance.

Description

Technical Field

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

[0002] Currently, the main cathode materials for lithium-ion power batteries are lithium iron phosphate (LFP) and ternary materials (NCM). However, LFP cathode materials suffer from low energy density and poor kinetic and low-temperature performance; NCM has shortcomings in terms of safety, cycle life, and cost.

[0003] The physical and electrochemical properties of LFP and NCM differ significantly. The limited functional diversity of materials exacerbates the difficulty of performance optimization, and a single material cannot simultaneously meet the comprehensive requirements of high energy density, high safety, and long lifetime. Simple physical mixing cannot effectively control the electrode microstructure, limit lithium-ion transport efficiency, and cause problems such as interfacial instability and poor rate performance, thus failing to realize synergistic effects. Summary of the Invention

[0004] In view of this, the present invention provides a composite positive electrode sheet, its preparation method, and its application. The composite positive electrode sheet provided by the present invention, while ensuring battery safety performance, improves the battery voltage platform and energy density, and effectively enhances battery rate performance and cycle performance.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a composite positive electrode sheet, the composite positive electrode sheet comprising a positive current collector and a double-layer structure sequentially stacked on at least one side surface of the positive current collector, wherein the direction closer to the aluminum foil of the current collector is the lower layer and the direction farther from the aluminum foil of the current collector is the upper layer, the lower layer of the double-layer structure is a lithium iron phosphate layer and the upper layer is a lithium iron phosphate mixed ternary positive electrode material layer.

[0006] Furthermore, the upper lithium iron phosphate-doped ternary cathode material layer of the composite cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 δ takes values ​​of 3.2-6.3.

[0007] Where α represents the percentage of lithium iron phosphate content; β represents the percentage of ternary cathode material content; γ represents the percentage of conductive agent content; θ represents the percentage of binder content; B1 represents the specific surface area of ​​lithium iron phosphate cathode material; and B2 represents the specific surface area of ​​ternary cathode material, all in m². 2 / g; C1 is the compacted density of lithium iron phosphate cathode material, and C2 is the compacted density of ternary cathode material, in g / cm³.3 Q1 represents the specific capacity of the lithium iron phosphate cathode material, and Q2 represents the specific capacity of the ternary cathode material, both in mAh / g.

[0008] Furthermore, α+β+γ+θ=1; in the upper lithium iron phosphate doped ternary cathode material layer, the content of ternary cathode material β is 0-50%; preferably, the content of ternary cathode material β is 10-40%.

[0009] Furthermore, the conductive agent content γ is 0-2%; the binder content θ is 0.5-2.5%, and preferably the binder content θ is 1-1.8%.

[0010] Furthermore, the lithium iron phosphate has an olivine-type structure with the chemical formula LiFePO4, and the ternary cathode material is lithium nickel cobalt manganese oxide with a layered rock salt structure and the chemical formula LiNi. x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, x + y < 1.

[0011] Furthermore, the specific surface area B1 of the lithium iron phosphate cathode material is 10-15 m². 2 / g; the specific surface area B2 of the ternary cathode material is 0.3-1.3m². 2 / g.

[0012] Furthermore, the compaction density C1 of the lithium iron phosphate cathode material is 2.2-3.0 g / cm³. 3 The compaction density C2 of the ternary cathode material is 2.8-3.6 g / cm³. 3 .

[0013] Furthermore, the specific capacity Q1 of the lithium iron phosphate cathode material is 135-148 mAh / g; the specific capacity Q2 of the ternary cathode material is 190-230 mAh / g.

[0014] Furthermore, the mixing ratio of the upper lithium iron phosphate cathode material to the ternary cathode material is 50:50-99:1.

[0015] Furthermore, the coating mass ratio of the upper lithium iron phosphate cathode material to the ternary cathode material layer and the lower lithium iron phosphate layer is 1~9:9~1.

[0016] In a second aspect, the present invention provides the composite positive electrode sheet described in the first aspect, comprising the following steps: (1) Preparation of upper lithium iron phosphate doped ternary cathode material slurry and lower lithium iron phosphate layer slurry; (2) The upper layer of lithium iron phosphate mixed ternary cathode material slurry and the lower layer of lithium iron phosphate slurry are coated onto the cathode current collector in a double layer, and then dried, rolled and die-cut to obtain the cathode sheet.

[0017] Furthermore, the coating is a double-layer coating performed by extrusion coating.

[0018] Thirdly, the present invention provides the application of the composite positive electrode sheet described in the first aspect in lithium batteries.

[0019] Fourthly, the present invention provides a lithium battery, the lithium battery comprising a positive electrode, a negative electrode, a separator and an electrolyte; the positive electrode is the composite positive electrode as described in the first aspect or a composite positive electrode prepared by the preparation method described in the second aspect.

[0020] Furthermore, the negative electrode active composite material includes one or more of graphite negative electrode materials, silicon-oxygen negative electrode materials, and modified silicon-oxygen negative electrode materials.

[0021] Compared with the prior art, the present invention has achieved the following beneficial effects: The composite cathode sheet provided by this invention employs a double-layer coating method. The upper layer uses a ternary cathode material doped with lithium iron phosphate, and the lower layer uses a lithium iron phosphate cathode. This method ensures battery safety while improving the battery voltage platform and energy density, and effectively enhances the battery's rate performance and cycle performance. The upper layer, using a ternary cathode material doped with lithium iron phosphate, satisfies the need for improved specific capacity, voltage platform, and energy density. The lower layer, in contact with the current collector, uses a lithium iron phosphate cathode, ensuring the high safety performance of the battery cell. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0023] Figure 1 This is the cell rate discharge curve of Embodiment 1 of the present invention. Detailed Implementation

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] The present invention provides a composite positive electrode sheet, the composite positive electrode sheet comprising a positive current collector and a double-layer structure sequentially stacked on at least one side surface of the positive current collector, wherein the direction closer to the aluminum foil of the current collector is the lower layer and the direction farther away from the aluminum foil of the current collector is the upper layer, the lower layer of the double-layer structure is a lithium iron phosphate layer and the upper layer is a lithium iron phosphate mixed ternary positive electrode material layer.

[0026] Furthermore, the upper lithium iron phosphate-doped ternary cathode material layer of the composite cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 δ takes values ​​of 3.2-6.3.

[0027] Where α represents the percentage of lithium iron phosphate content; β represents the percentage of ternary cathode material content; γ represents the percentage of conductive agent content; θ represents the percentage of binder content; B1 represents the specific surface area of ​​lithium iron phosphate cathode material; and B2 represents the specific surface area of ​​ternary cathode material, all in m². 2 / g; C1 is the compacted density of lithium iron phosphate cathode material, and C2 is the compacted density of ternary cathode material, in g / cm³. 3 Q1 represents the specific capacity of the lithium iron phosphate cathode material, and Q2 represents the specific capacity of the ternary cathode material, both in mAh / g.

[0028] If the value of δ is too large, the proportion of ternary components may be too high, which will lead to poor safety and increased costs; or the ternary surface compaction may be too large, making the material easy to crush and affecting cycle performance. If the value is too small, it indicates that the proportion of ternary components is too low, the energy density is low, and the rate performance is poor.

[0029] Further, α+β+γ+θ=1; in the upper lithium iron phosphate doped ternary cathode material layer, the content of ternary cathode material β is 0-50%; the β can be any value between 0-50%, such as 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, etc.; preferably, the content of ternary cathode material β is 10-40%.

[0030] Further, the conductive agent content γ is 0-2%; γ can be any value between 0-2%, for example, 0.1%, ..., 0.5%, ..., 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, preferably 1-2%; the binder content θ is 0.5-2.5%, θ can be any value between 0.5-2.5%, for example, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%; preferably, the binder content θ is 1-1.8%.

[0031] Furthermore, the lithium iron phosphate has an olivine-type structure with the chemical formula LiFePO4, and the ternary cathode material is lithium nickel cobalt manganese oxide with a layered rock salt structure and the chemical formula LiNi. x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, x + y < 1.

[0032] Furthermore, the specific surface area B1 of the lithium iron phosphate cathode material is 10-15 m². 2 / g; the specific surface area B2 of the ternary cathode material is 0.3-1.3m². 2 / g. The B1 can be 10-15m. 2 Any value between / g, for example, 10.5m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, 12.5m 2 / g、13m 2 / g, 13.5m 2 / g、14m 2 / g, 14.5m 2 / g, etc.; B1 can be 0.3-1.3m 2 Any value between / g, for example, 0.4m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g etc.

[0033] Furthermore, the compaction density C1 of the lithium iron phosphate cathode material is 2.2-3.0 g / cm³. 3 The compaction density C2 of the ternary cathode material is 2.8-3.6 g / cm³. 3 The C1 can be 2.2-3.0 g / cm³. 3 Any value between these two values, for example, 2.3 g / cm³. 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 2.9g / cm 3 etc.; C2 can be 2.8-3.6 g / cm³. 3 Any value between these two values, for example, 2.9 g / cm³. 3 3.0g / cm 3 3.1g / cm 3 3.2g / cm 3 3.3g / cm 3 3.4g / cm 3 3.5g / cm 3 wait.

[0034] Furthermore, the specific capacity Q1 of the lithium iron phosphate cathode material is 135-148 mAh / g; the specific capacity Q2 of the ternary cathode material is 190-230 mAh / g. Q1 can be any value between 135-148 mAh / g, such as 136 mAh / g, 137 mAh / g, 138 mAh / g, 139 mAh / g, 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, etc.; Q2 can be any value between 190-230 mAh / g, such as 195 mAh / g, 200 mAh / g, 205 mAh / g, 210 mAh / g, 215 mAh / g, 220 mAh / g, 225 mAh / g, etc.

[0035] It should be noted that the thickness of the double-layer structure set on the positive electrode current collector is determined based on the cell capacity and energy density. It should not be too thick or too thin. The total thickness of the positive electrode multilayer structure and the current collector is generally between 80mm and 200mm.

[0036] Furthermore, the mixing ratio of the upper lithium iron phosphate cathode material to the ternary cathode material is 50:50-99:1; preferably 60:40-90:10. If the ternary mixing ratio is too low, the energy density will be low and the rate performance will be poor; if the ternary mixing ratio is too high, the safety will be deteriorated and the cost will increase.

[0037] Because ternary cathode materials have a layered rock salt structure, they exhibit excellent rate performance and a small specific surface area. Since the upper layer of the electrode has a high current, the upper layer uses lithium iron phosphate (LFP) blended with ternary cathode material, which can quickly accept the insertion and extraction of lithium ions during high-rate charge and discharge, significantly improving the battery's rate performance. Simultaneously, the low specific surface area of ​​the ternary cathode material reduces side reactions with the electrolyte, improving the battery's processability and high-temperature storage stability. The lower layer, in contact with the current collector, uses an olivine-type LFP layer, ensuring battery safety. Furthermore, the lower LFP layer and the upper LFP blended ternary cathode material layer ensure good contact and interface adhesion between the upper and lower layers, better meeting processing performance requirements. The double-layer coating method, with the upper layer being LFP blended with ternary cathode material and the lower layer being LFP, significantly improves the voltage platform and battery energy density, while also enhancing rate performance and cycle performance, and ensuring cell safety.

[0038] Furthermore, the coating mass ratio of the upper lithium iron phosphate cathode material to the ternary cathode material layer to the lower lithium iron phosphate layer is 1~9:9~1; preferably 3~7:7~3.

[0039] Furthermore, the current collector includes, but is not limited to, aluminum foil, copper foil, and titanium mesh. Because aluminum foil has advantages such as high strength, good conductivity, easy surface treatment, and low cost, the current collector is preferably aluminum foil.

[0040] The present invention provides the composite positive electrode sheet as described in the first aspect, comprising the following steps: (1) Preparation of upper lithium iron phosphate doped ternary cathode material slurry and lower lithium iron phosphate layer slurry; (2) The upper layer of lithium iron phosphate mixed ternary cathode material slurry and the lower layer of lithium iron phosphate slurry are coated onto the cathode current collector in a double layer, and then dried, rolled and die-cut to obtain the cathode sheet.

[0041] Furthermore, the coating is a double-layer coating performed by extrusion coating.

[0042] The present invention provides the application of the composite positive electrode sheet described in the first aspect in lithium batteries.

[0043] The present invention provides a lithium battery, the lithium battery comprising a positive electrode, a negative electrode, a separator and an electrolyte; the positive electrode is the composite positive electrode as described in the first aspect or a composite positive electrode prepared by the preparation method described in the second aspect.

[0044] Furthermore, the negative electrode active composite material includes one or more of graphite negative electrode materials, silicon-oxygen negative electrode materials, and modified silicon-oxygen negative electrode materials.

[0045] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0046] Example 1 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of upper-layer lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode main material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 90:10, the content of conductive agent γ is 1.8%, and the content of binder θ is 1.7%; The lower lithium iron phosphate cathode slurry ②, wherein the upper lithium iron phosphate blended ternary system layer of the double-coated cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 ; Among them, the specific surface area B1 of the lithium iron phosphate cathode material is 12.25, and the specific surface area B2 of the ternary cathode material is 0.507, with units of m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.6, and the compaction density C2 of ternary cathode material is 3.45, both in g / cm³. 3 ; The specific capacity Q1 of lithium iron phosphate cathode material is 145, and the specific capacity Q2 of ternary cathode material is 220, both in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0047] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0048] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0049] Example 2: A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of upper-layer lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode main material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 80:20, the content of conductive agent γ is 1.8%, and the content of binder θ is 1.7%; The lower lithium iron phosphate cathode slurry ②, wherein the upper lithium iron phosphate blended ternary system layer of the double-coated cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 ; Among them, the specific surface area B1 of the lithium iron phosphate cathode material is 12.25, and the specific surface area B2 of the ternary cathode material is 0.507, with units of m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.6, and the compaction density C2 of ternary cathode material is 3.45, both in g / cm³. 3 ; The specific capacity Q1 of lithium iron phosphate cathode material is 145, and the specific capacity Q2 of ternary cathode material is 220, both in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0050] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0051] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0052] Example 3: A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode main material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 70:30, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; the lower lithium iron phosphate cathode slurry ②, wherein the upper lithium iron phosphate blended ternary system layer of the double-coated cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 Among them, the specific surface area B1 of the lithium iron phosphate cathode material is 12.25, and the specific surface area B2 of the ternary cathode material is 0.507, with units of m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.6, and the compaction density C2 of ternary cathode material is 3.45, both in g / cm³. 3 ; The specific capacity Q1 of lithium iron phosphate cathode material is 145, and the specific capacity Q2 of ternary cathode material is 220, both in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0053] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0054] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0055] Example 4: A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode main material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 80:20, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; the lower lithium iron phosphate cathode slurry ②, wherein the upper lithium iron phosphate blended ternary system layer of the double-coated cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 ; Among them, the specific surface area B1 of the lithium iron phosphate cathode material is 10.46, and the specific surface area B2 of the ternary cathode material is 0.71, with units of m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.5, and the compaction density C2 of ternary cathode material is 3.5, with units of g / cm³. 3 ; The specific capacity Q1 of lithium iron phosphate cathode material is 144, and the specific capacity Q2 of ternary cathode material is 215, both in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0056] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0057] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0058] Example 5: A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode main material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 80:20, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; the lower lithium iron phosphate cathode slurry ②, wherein the upper lithium iron phosphate blended ternary system layer of the double-coated cathode sheet satisfies the following relationship: δ=(α+β)÷α× × × ×(γ+θ)×10 3 ; Among them, the specific surface area B1 of the lithium iron phosphate cathode material is 13.9, and the specific surface area B2 of the ternary cathode material is 0.96, with units of m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.3, and the compaction density C2 of ternary cathode material is 3.4, with units of g / cm³. 3 ; The specific capacity Q1 of lithium iron phosphate cathode material is 146, and the specific capacity Q2 of ternary cathode material is 205, both in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0059] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0060] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0061] Comparative Example 1 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Lithium iron phosphate material α and ternary cathode material β were blended at a mass ratio of 9.5:0.5 to form a cathode composite active material. Cathode active material α+β, conductive agent γ, and binder θ were mixed at a mass ratio of 96.5:1.8:1.7 and added to solvent NMP via a multi-step process. After thorough stirring, a cathode slurry was obtained. The specific surface area B1 of the lithium iron phosphate cathode material was 12.25, and the specific surface area B2 of the ternary cathode material was 0.507 (units: m²). 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.6, and the compaction density C2 of ternary cathode material is 3.45, with units of g / cm³. 3 The specific capacity Q1 of the lithium iron phosphate cathode material is 145, and the specific capacity Q2 of the ternary cathode material is 220, with units of mAh / g. Then, the cathode mixture slurry is directly coated onto a 12μm carbon-coated aluminum foil, and after drying, rolling, and die-cutting, the cathode sheet is obtained. (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0062] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0063] Comparative Example 2 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 40:60, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; Lower lithium iron phosphate cathode slurry ②: The specific surface area B1 of the lithium iron phosphate cathode material is 12.25, and the specific surface area B2 of the ternary cathode material is 0.507, in m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.6, and the compaction density C2 of ternary cathode material is 3.45, with units of g / cm³. 3The specific capacity Q1 of the lithium iron phosphate cathode material is 145, and the specific capacity Q2 of the ternary cathode material is 220, with units of mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, the cathode sheet is obtained.

[0064] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0065] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0066] Comparative Example 3 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Lithium iron phosphate material α was used as the positive electrode active material. The positive electrode material α, conductive agent γ, and binder θ were mixed in a mass ratio of 96.5:1.8:1.7 and added to the solvent NMP via a multi-step process. After thorough stirring, a positive electrode slurry was obtained. Other conditions were the same as in Example 1 and will not be repeated here. The positive electrode slurry was then directly coated onto a 12μm carbon-coated aluminum foil, and after drying, rolling, and die-cutting, a positive electrode sheet was obtained. (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0067] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0068] Comparative Example 4 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded at a mass ratio of 90:10, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; the lower lithium iron phosphate cathode slurry ② has a specific surface area B1 of 14 for lithium iron phosphate cathode material and a specific surface area B2 of 0.35 for ternary cathode material, in m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.9, and the compaction density C2 of ternary cathode material is 2.8, with units of g / cm³. 3 The specific capacity Q1 of the lithium iron phosphate cathode material is 148, and the specific capacity Q2 of the ternary cathode material is 190, with units of mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, the cathode sheet is obtained.

[0069] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0070] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0071] Comparative Example 5 A method for preparing a lithium-ion battery includes the following steps: (a) Preparation of lithium-ion battery cathode Preparation of the upper lithium iron phosphate blended ternary cathode slurry ①: The overall proportion of the cathode material is 96.5%, of which lithium iron phosphate cathode material α and ternary cathode material β are compounded in a mass ratio of 70:30, the conductive agent content γ accounts for 1.8%, and the binder content θ accounts for 1.7%; the lower lithium iron phosphate cathode slurry ② has a specific surface area B1 of 10 for lithium iron phosphate cathode material and a specific surface area B2 of 1.1 for ternary cathode material, in m². 2 / g; The compaction density C1 of lithium iron phosphate cathode material is 2.2, and the compaction density C2 of ternary cathode material is 3.6, with units of g / cm³. 3The specific capacity Q1 of the lithium iron phosphate cathode material is 140, and the specific capacity Q2 of the ternary cathode material is 225, in mAh / g. Slurry ① and slurry ② are coated onto a 12μm carbon-coated aluminum foil by extrusion coating, with the weight ratio of the lower layer to the upper layer being 1:1. After drying, rolling, and die cutting, the cathode sheet is obtained.

[0072] (b) Preparation of lithium-ion battery anode The graphite anode material was mixed with carbon black Super-P, binder CMC and SBR in a mass ratio of 96:1:1.3:1.7, and then added to pure water solvent to dissolve and stir fully to obtain anode slurry. The anode slurry was coated onto 6μm copper foil, and after drying, rolling and cutting, anode sheets were obtained.

[0073] (c) Preparation of lithium-ion batteries The positive and negative electrode sheets prepared in (a) and (b) are assembled together with a PE separator coated with ceramic layers on both sides, encapsulated and injected with carbonate electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and perform electrochemical testing.

[0074] The parameters of Examples 1-5 and Comparative Examples 1-5 are summarized in Table 1.

[0075] Table 1. Parameters of Examples 1-5 and Comparative Examples 1-5

[0076] Lithium-ion battery safety testing: Capacity test: The battery was charged and discharged at 0.33C and 1C respectively to obtain the battery capacity data at 0.33C and 1C. Internal resistance test: Use a DC internal resistance tester to test the battery's internal resistance; Hot box test: Place the fully charged battery in an explosion-proof oven, start at 25°C, increase the temperature to 130°C at a rate of 5°C / min, maintain the temperature for 60 minutes, and record the temperature rise and voltage change of the battery.

[0077] Needle penetration test: At 25°C, a 5mm steel needle is used to penetrate the geometric center of a fully charged battery at a speed of 25mm / s and held for 30 minutes. The temperature rise and voltage change of the battery are recorded.

[0078] Table 2. Battery internal resistance, battery capacity, and maximum battery temperature at 130℃ in the hot box.

[0079] Lithium-ion battery electrical performance testing: I. Testing Methods 1. Rate discharge capacity retention test At room temperature (25℃), it is charged to 4.2V using a constant current and constant voltage method at 1C, and discharged to 2.0V at different rates (0.33C, 1C, 2C, 3C, 4C).

[0080] 2. High and low temperature discharge capacity retention test It can be charged to 3.65V by 1C constant current and constant voltage, discharged to 2.0V by 1C at different temperatures (25℃, 0℃), and discharged to 1.6V by 1C at different temperatures (-10℃, -20℃, -30℃).

[0081] II. Test Results 1. Rate discharge capacity retention test results The rate discharge capacity retention test results of lithium-ion batteries in all embodiments and comparative examples are shown in Table 3. The rate discharge curves of the cells in the embodiments are shown in the figure. Figure 1 As shown.

[0082] Table 3. Battery Discharge Capacity Retention Rate Test Results

[0083] 2. High and low temperature discharge capacity retention test results The high and low temperature discharge capacity retention test results of all embodiments and comparative examples of lithium-ion batteries are shown in Table 4.

[0084] Table 4. Battery High and Low Temperature Discharge Capacity Retention Test Results

[0085] The data above shows that the more ternary cathode material is mixed, the higher the energy density and the better the rate performance. However, the thermal stability and safety deteriorate accordingly. The data from the examples show that, within a certain range, mixing different proportions of ternary cathode material generally results in better energy density, safety performance, rate performance, and high and low temperature performance. A comparison between Example 1 and Comparative Example 1 shows that, under the premise of the same ternary material mixing ratio, direct overall mixing followed by single-layer coating does not result in significant differences in energy density and rate performance, but safety performance is significantly reduced. Comparative Example 2 shows that when the upper layer contains more than 50% ternary material... The battery's thermal stability decreased significantly, and the needle penetration temperature rise increased accordingly. Comparative Example 3 shows that while the pure lithium iron phosphate cathode has a relatively low needle penetration temperature rise and excellent safety performance, it suffers from high internal resistance, low energy density, and poor discharge capacity retention and high / low temperature performance, offering no advantages in electrical performance. Comparative Examples 4 and 5 show that ternary cathodes have low specific capacity and low compaction, resulting in poor material contact and high polarization. Conversely, excessively high specific capacity and large specific surface area, coupled with excessive compaction, pose a risk of material crushing. The rolled material is also relatively fragile, further affecting the cell's rate performance and high / low temperature performance. Therefore, it can be seen that to simultaneously meet the comprehensive requirements of high energy density, high safety, and rate performance, a double-layer coating technology with a δ value between 3.2 and 6.3 can better satisfy the electrochemical performance requirements of energy density, safety, and rate performance.

[0086] 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 composite positive electrode sheet, characterized in that, The composite positive electrode sheet includes a positive current collector and a double-layer structure stacked sequentially on at least one side of the surface of the positive current collector, wherein the direction closer to the aluminum foil of the current collector is the lower layer and the direction farther away from the aluminum foil of the current collector is the upper layer. The lower layer of the double-layer structure is a lithium iron phosphate layer and the upper layer is a lithium iron phosphate mixed ternary positive electrode material layer.

2. The composite positive electrode sheet as described in claim 1, characterized in that, The upper lithium iron phosphate-doped ternary cathode material layer of the composite cathode sheet satisfies the following relationship: δ = (α + β) ÷ α × × × × (γ + θ) × 10 3 , where δ ranges from 3.2 to 6.3; Where α represents the percentage of lithium iron phosphate content; β represents the percentage of ternary cathode material content; γ represents the percentage of conductive agent content; θ represents the percentage of binder content; B1 represents the specific surface area of ​​lithium iron phosphate cathode material; and B2 represents the specific surface area of ​​ternary cathode material, all in m². 2 / g; C1 is the compacted density of lithium iron phosphate cathode material, and C2 is the compacted density of ternary cathode material, in g / cm³. 3 Q1 represents the specific capacity of the lithium iron phosphate cathode material, and Q2 represents the specific capacity of the ternary cathode material, both in mAh / g.

3. The composite positive electrode sheet as described in claim 2, characterized in that, α+β+γ+θ=1; or, in the upper lithium iron phosphate doped ternary cathode material layer, the content of ternary cathode material β is 0-50%; preferably, the content of ternary cathode material β is 10-40%. Alternatively, the conductive agent content γ is 0-2%; the binder content θ is 0.5-2.5%; preferably, the binder content θ is 1-1.8%.

4. The composite positive electrode sheet as described in claim 1, characterized in that, Lithium iron phosphate has an olivine-type structure and the chemical formula LiFePO4. The ternary cathode material is lithium nickel cobalt manganese oxide, which has a layered rock salt structure and the chemical formula LiNi. x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, x + y < 1.

5. The composite positive electrode sheet as described in claim 2, characterized in that, The specific surface area B1 of the lithium iron phosphate cathode material is 10-15 m². 2 / g; the specific surface area B2 of the ternary cathode material is 0.3-1.3m². 2 / g; Alternatively, the compaction density C1 of the lithium iron phosphate cathode material is 2.2-3.0 g / cm³. 3 The compaction density C2 of the ternary cathode material is 2.8-3.6 g / cm³. 3 ; Alternatively, the specific capacity Q1 of the lithium iron phosphate cathode material is 135-148 mAh / g; and the specific capacity Q2 of the ternary cathode material is 190-230 mAh / g.

6. The composite positive electrode sheet as described in claim 1, characterized in that, The mixing ratio of the upper lithium iron phosphate cathode material to the ternary cathode material is 50:50-99:1; preferably 60:40-90:

10.

7. The composite positive electrode sheet as described in claim 1, characterized in that, The coating mass ratio of the upper lithium iron phosphate cathode material layer to the ternary cathode material layer and the lower lithium iron phosphate layer is 1-9:9-1; preferably 3-7:7-3.

8. The method for preparing the composite positive electrode sheet according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Preparation of upper lithium iron phosphate doped ternary cathode material slurry and lower lithium iron phosphate layer slurry; (2) The upper layer of lithium iron phosphate mixed ternary cathode material slurry and the lower layer of lithium iron phosphate slurry are coated onto the cathode current collector in a double layer, and then dried, rolled and die-cut to obtain the cathode sheet.

9. The application of the composite positive electrode sheet as described in any one of claims 1-7 in lithium batteries.

10. A lithium battery, the lithium battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode is the composite positive electrode as described in any one of claims 1-7.

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