Lithium ion battery positive pole piece and lithium ion battery

Abstract

The invention discloses a lithium ion battery positive pole piece and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The lithium ion battery positive pole piece provided by the invention comprises a current collector, a first active material layer arranged on at least one surface of the current collector and a second active material layer arranged on the surface of the first active material layer, the first active material layer comprises lithium iron phosphate as a first positive electrode active material, and the second active material layer comprises nickel cobalt lithium manganate as a second positive electrode active material; the lithium iron phosphate meets the following relational expression: 0.5 < = (SSA1 * W1) / D50-1 < = 4; and 3% < = W1 < = 10%. According to the invention, collaborative optimization of the inner-layer lithium iron phosphate and the outer-layer nickel cobalt lithium manganate in morphology, content and function is realized, and the rate capability, cycle life and thermal safety of the battery are synergistically improved while high energy density is ensured.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a positive electrode sheet for a lithium-ion battery and a lithium-ion battery. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] The development of lithium-ion battery cathode material technology has always faced the challenge of synergistically optimizing multiple performance indicators such as energy density, safety, and cycle life. Layered transition metal oxides with high nickel content (such as lithium nickel cobalt manganese oxide) possess advantages in high specific capacity and high operating voltage, making them key materials for achieving high energy density in batteries. However, with increasing nickel content, the structural stability of the material decreases, making it prone to severe exothermic side reactions under thermal or electrical abuse conditions, posing a risk of thermal runaway. Its insufficient intrinsic safety has become a bottleneck restricting its widespread application in high-performance power batteries.

[0004] To improve the safety of high-nickel cathode systems, the industry typically considers incorporating olivine-type phosphate materials (such as lithium iron phosphate) with better thermal stability. Common composite strategies include bulk doping, surface coating, and physical mixing or bilayer structure design in the electrode layers. These methods improve the thermal safety of the system to some extent, but also introduce new technical problems: simple physical mixing may lead to the coupling of interfacial side reactions between the two materials in the electrolyte, which may exacerbate gas generation; while using methods such as bilayer coating to partition different materials, although theoretically functional separation can be achieved, improper matching can make it difficult to coordinate the electrochemical characteristics (such as operating voltage range, ionic / electronic conductivity) and kinetic behavior of the inner and outer layers, which may lead to obstruction of lithium-ion transport at the interlayer interface; or at high rates, the polarization difference may cause severe uneven reaction progress between the two layers; and additional side reactions may also be triggered, which may impair cycle life and interfacial stability.

[0005] Therefore, how to improve battery safety, rate performance and cycle life in a synergistic way by optimizing the structural design of the double-layer composite cathode while maintaining high energy density is an urgent problem to be solved. Summary of the Invention

[0006] In view of this, the present invention provides a positive electrode sheet for a lithium-ion battery and a lithium-ion battery. The present invention achieves synergistic optimization of the morphology, content and function of the inner lithium iron phosphate layer and the outer lithium nickel cobalt manganese oxide layer, thereby synergistically improving the rate performance, cycle life and thermal safety of the battery while ensuring high energy density.

[0007] In a first aspect, the present invention provides a positive electrode sheet for a lithium-ion battery, comprising a current collector, a first active material layer disposed on at least one surface of the current collector, and a second active material layer disposed on the surface of the first active material layer. The first active material layer includes lithium iron phosphate as the first positive electrode active material, and the second active material layer includes lithium nickel cobalt manganese oxide as the second positive electrode active material; The lithium iron phosphate satisfies the following relationship: 0.5 ≤ (SSA1 × W1) / D 50-1 ≤4; Where SSA1 is the specific surface area of ​​lithium iron phosphate, in m². 2 / g; W1 is the mass fraction of the first active substance layer in the total mass of the first and second active substance layers, which satisfies 3%≤W1≤10%; D 50-1 The median particle size of lithium iron phosphate is given in μm.

[0008] Preferably, W1 satisfies 5% ≤ W1 ≤ 10%.

[0009] Preferably, the specific surface area of ​​the lithium iron phosphate is 10~22 m². 2 / g.

[0010] Preferably, the median particle size of the lithium iron phosphate is 0.7~2 μm.

[0011] Preferably, the general chemical formula of the lithium nickel cobalt manganese oxide is LiNi. x Co y Mn 1-x-y O2, where 0.6 ≤ x ≤ 0.92, 0 <y<0.4。

[0012] Preferably, the lithium nickel cobalt manganese oxide has a median particle size of 8-18 μm and a specific surface area of ​​0.3-1.0 m². 2 / g.

[0013] Preferably, the first active material layer further includes a first conductive agent and a first binder, and the mass ratio of the first positive electrode active material, the first conductive agent and the first binder is (94~97): (1~3): (2~4); the second active material layer further includes a second conductive agent and a second binder, and the mass ratio of the second positive electrode active material, the second conductive agent and the second binder is (96~98): (1~3): (1~2).

[0014] Preferably, the coating amount of the first active material layer is 5~20 g / m². 2 The coating amount of the second active material layer is 145~165 g / m². 2 .

[0015] Secondly, the present invention provides a lithium-ion battery, including the above-mentioned lithium-ion battery positive electrode sheet.

[0016] Thirdly, the present invention provides an electrical device including the aforementioned lithium-ion battery.

[0017] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention correlates and controls the specific surface area (SSA1), its mass fraction in the total active material (W1), and median particle size (D) of the inner lithium iron phosphate. 50-1 The relationship between these three factors provides a precise quantitative basis for the design of bilayer composite cathode plates. This parameter effectively coordinates the functions of the inner lithium iron phosphate layer in constructing a conductive network, providing structural buffering, and stabilizing the electrochemical interface, solving the problem of the difficulty in systematically optimizing the material parameters and ratios of the functional layers in a bilayer structure.

[0018] (2) Through the above-mentioned quantitative design, the present invention synergistically improves the overall performance of lithium-ion batteries while ensuring high energy density dominated by high proportion of nickel cobalt manganese oxide. Specifically, the battery has excellent high-rate discharge performance and long cycle life at the same time, which significantly alleviates the contradiction between the two in traditional design; at the same time, the thermal stability and interface stability of the battery are enhanced, and the safety performance is fundamentally improved.

[0019] (3) The parameter relationship established by the present invention has clear process guidance, which is conducive to improving production consistency and yield, ensuring the controllability and consistency of battery product performance degradation throughout the entire life cycle, and meeting the stringent requirements of high-end power batteries for high safety, long life and reliability. Detailed Implementation

[0020] 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.

[0021] This invention provides a positive electrode sheet for a lithium-ion battery, comprising a current collector, a first active material layer disposed on at least one surface of the current collector, and a second active material layer disposed on the surface of the first active material layer; The first active material layer includes lithium iron phosphate as the first positive electrode active material, and the second active material layer includes lithium nickel cobalt manganese oxide as the second positive electrode active material; The lithium iron phosphate satisfies the following relationship: 0.5 ≤ (SSA1 × W1) / D 50-1 ≤4; Where SSA1 is the specific surface area of ​​lithium iron phosphate, in m². 2 / g; W1 is the mass fraction of the first active substance layer in the total mass of the first and second active substance layers, which satisfies 3%≤W1≤10%; D 50-1 The median particle size of lithium iron phosphate is given in μm.

[0022] The numerator term "SSA1×W1" in the above-mentioned technical solution of this invention comprehensively characterizes the total effective active interface contribution provided by the lithium iron phosphate layer. The specific surface area SSA1 determines the size of the reactive interface per unit mass of material, while the mass fraction W1 determines the relative content of the material in the overall electrode. Their product reflects the overall ability of this functional layer to participate in charge transfer and interfacial reactions at the electrode scale. The denominator term D... 50-1 This is related to the average diffusion path length of lithium ions within the solid phase of lithium iron phosphate particles. The larger the particle size, the greater the kinetic resistance to the migration of ions from the inside of the particle to the surface.

[0023] While existing technologies have addressed the introduction of stabilizing materials such as lithium iron phosphate into bilayer composite cathodes to improve safety, their improvement approaches are typically limited to isolated adjustments or simple combinations of parameters for single or multiple materials. For example, optimizing the particle size or specific surface area of ​​lithium iron phosphate alone to control its processability or intrinsic reactivity. However, this invention simultaneously constrains the aforementioned three key parameters, solving the performance synergy problem inherent in high-nickel systems.

[0024] When the value of the above relationship is below 0.5, it means that either the mass ratio of the lithium iron phosphate layer is too low, or its particles are too coarse or its specific surface area is too small, preventing it from forming a continuous and efficient electron / ion hybrid conductive network between the current collector and the highly active layer. Its value as a functional layer cannot be realized, and its contribution to improving the overall rate performance of the battery is negligible. When the value of the above relationship is above 4, it usually corresponds to the use of excessively fine nanoparticles or an excessively high specific surface area. This drastically increases the active sites on the material surface and the contact area with the electrolyte, not only deteriorating the slurry processing performance but also triggering serious interfacial side reactions, leading to irreversible consumption of active lithium and electrolyte, thus damaging the battery's cycle life and long-term interfacial stability. When the value of the above relationship is within a specific range, the addition of a small amount of lithium iron phosphate can achieve three functions: first, constructing a robust conductive pathway to ensure excellent rate performance; second, forming a stable initial interface to suppress side reactions; and third, acting as a mechanical and chemical buffer layer to improve overall safety. More preferably, 0.5 ≤ (SSA1×W1) / D 50-1 ≤2.5, further, 0.5≤(SSA1×W1) / D 50-1 ≤2. It should be noted that, in the calculation, SSA1 and D... 50-1 All calculations are performed using unitless values, with the values ​​in meters.2 The values ​​when / g and the values ​​when the unit is μm.

[0025] Through the aforementioned quantitative design, this invention synergistically improves the overall performance of lithium-ion batteries while ensuring high energy density dominated by lithium nickel cobalt manganese oxide with a high proportion of lithium nickel cobalt manganese oxide. Specifically, the battery simultaneously possesses excellent high-rate discharge performance and long cycle life, significantly alleviating the contradiction between the two in traditional designs; at the same time, the battery's thermal stability and interface stability are enhanced, and safety performance is fundamentally improved.

[0026] In an optional embodiment of the present invention, W1 satisfies 5%≤W1≤10%. If W1 is too small, the lithium iron phosphate layer will be too thin, making it impossible to construct a continuous and effective three-dimensional conductive network and mechanical buffer layer between the current collector and the high-nickel layer; if W1 is too large, it will excessively crowd out the proportion of high-capacity nickel cobalt manganese oxide, significantly sacrificing the overall energy density of the battery.

[0027] In an optional embodiment of the present invention, the specific surface area of ​​the lithium iron phosphate is 10~22 m². 2 / g, more preferably 12~20 m 2 / g. Lithium iron phosphate with a suitable specific surface area provides sufficient active surface, facilitating close contact with the conductive agent and constructing an excellent electronic conductivity network. Simultaneously, the moderate specific surface area avoids the problems of severe agglomeration of nanomaterials, uncontrolled slurry viscosity, and exacerbated side reactions with the electrolyte caused by excessively high surface energy. Furthermore, it is beneficial to obtain a suitable pore structure after rolling, balancing ion transport and electrode strength. The specific surface area value mentioned in this invention is obtained by measuring using the BET nitrogen adsorption method.

[0028] The lithium iron phosphate used in this invention can be an unmodified conventional material, or it can be carbon-coated and doped with metal ions (such as Mg). 2+ Ti 4+ Zr 4+ Materials modified with (etc.) to further improve their intrinsic electronic conductivity or structural stability. The morphology of lithium iron phosphate can be approximately spherical, plate-like, or other irregular polyhedra, and its primary particles can be single crystals or polycrystalline aggregates.

[0029] In an optional embodiment of the present invention, the median particle size of the lithium iron phosphate is 0.7~2 μm, more preferably 0.8~1.5 μm. While a smaller particle size is beneficial for shortening the lithium-ion diffusion path, it drastically increases the specific surface area and deteriorates the slurry rheology and electrode processability; a larger particle size significantly increases ion migration resistance, especially during fast charging and discharging, potentially leading to severe concentration polarization and limiting rate performance. The median particle size is measured using a laser particle size analyzer.

[0030] In an optional embodiment of the present invention, the chemical general formula of the lithium nickel cobalt manganate is LiNi x Co y Mn 1-x-y O2, where 0.6 ≤ x ≤ 0.92 and 0 < y < 0.4. For example, medium-nickel systems such as NCM622 and NCM712 or high-nickel systems such as NCM811 and NCM9055 can be selected.

[0031] In an optional embodiment of the present invention, the median particle size of the lithium nickel cobalt manganate is 8 - 18 μm. Larger particle sizes are beneficial for obtaining high tap density and electrode compaction density, thereby improving the volumetric energy density while reducing the total surface area of the particles. The specific surface area is 0.3 - 1.0 m 2 / g; a lower specific surface area can effectively limit the contact area between the material and the electrolyte, reduce interfacial side reactions and transition metal dissolution at high voltages, and is beneficial for improving long cycle life and high-temperature stability. The lithium nickel cobalt manganate can be single-crystal particles or polycrystalline secondary agglomerates, preferably polycrystalline secondary agglomerates.

[0032] In an optional embodiment of the present invention, the first active material layer further includes a first conductive agent and a first binder, and the mass ratio of the first cathode active material, the first conductive agent, and the first binder is (94 - 97) : (1 - 3) : (2 - 4); the second active material layer further includes a second conductive agent and a second binder, and the mass ratio of the second cathode active material, the second conductive agent, and the second binder is (96 - 98) : (1 - 3) : (1 - 2).

[0033] In an optional embodiment of the present invention, the first conductive agent and the second conductive agent can each independently be selected from one or several of conductive carbon black (such as Super P, Ketjenblack), carbon nanotubes, graphene, and carbon fibers. Preferably, it is a composite system of conductive carbon black and carbon nanotubes to construct a three-dimensional conductive network of point-line combination.

[0034] In an optional embodiment of the present invention, the first binder and the second binder can each independently be selected from one or several of polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylic acid and its salts. Preferably, it is polyvinylidene fluoride because it has excellent chemical stability and bonding strength at high voltages.

[0035] In an optional embodiment of the present invention, the coating amount of the first active material layer is 5 - 20 g / m 2 , more preferably 8 - 15 g / m 2 ; the coating amount of the second active material layer is 145 - 165 g / m 2 , more preferably 150 - 160 g / m 2 .

[0036] The present invention does not impose any special limitations on the preparation method of the above-mentioned lithium-ion battery positive electrode sheet, and preferably includes the following steps: Slurry preparation: The first and second active substances are mixed with the corresponding conductive agents and binders in a solvent at high speed according to the specified proportions until a uniform and stable slurry is formed. The solid content of the slurry can be 40-80%. Double-layer coating: The slurry is uniformly coated onto the current collector (such as aluminum foil or carbon-coated aluminum foil with a thickness of 8-12μm) using a double-layer coating machine to form a double-layer structure; Drying and Rolling: The solvent is thoroughly dried in an oven at 80℃~130℃, followed by rolling at room temperature. The rolled electrodes can be cut to the required size as needed.

[0037] The present invention also provides a lithium-ion battery, including the above-described positive electrode sheet. The lithium-ion battery further includes a negative electrode sheet, a separator, an electrolyte, and outer packaging.

[0038] The negative electrode sheet may include graphite, silicon-based materials, soft / hard carbon, and other negative electrode active materials, coated on a copper foil current collector. The separator may be a polyethylene, polypropylene microporous membrane, or a ceramic-coated composite membrane thereof. The electrolyte contains lithium salts (such as LiPF6) and organic solvents (such as a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate), and may contain film-forming additives, flame-retardant additives, etc.

[0039] The present invention also provides an electrical device comprising the aforementioned lithium-ion battery.

[0040] The electrical device provided by this invention refers to any equipment, system or vehicle that uses the aforementioned lithium-ion battery as its power supply source or energy storage core unit.

[0041] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not have any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used. In the following embodiments, the specific surface area value was obtained by BET nitrogen adsorption method, and the particle size value was obtained by Malvern 2000 laser particle size analyzer.

[0042] Example 1 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 Its particle size is 0.8 μm and its specific surface area (SSA1) is 20 m². 2Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0043] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0044] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight per unit area of ​​13.3 g / m² is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon have a dry weight of 152.9 g / m² per unit area. 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved. 3 The material is rolled to obtain a positive electrode sheet. The first active material layer accounts for 8% of the total mass of the two active material layers, W1.

[0045] Example 2 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 The micrometer diameter (μm) is 1.0 μm, and the specific surface area (SSA1) is 14 m². 2 Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0046] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0047] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight per unit area of ​​13.3 g / m² is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon have a dry weight of 152.9 g / m² per unit area. 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved. 3 The material is rolled to obtain a positive electrode sheet. The first active material layer accounts for 8% of the total mass of the two active material layers, W1.

[0048] Example 3 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 The micrometer diameter (μm) is 1.2 μm, and the specific surface area (SSA1) is 14 m². 2 Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0049] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0050] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight per unit area of ​​13.3 g / m² is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon have a dry weight of 152.9 g / m² per unit area. 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved. 3 The material is rolled to obtain a positive electrode sheet. The first active material layer accounts for 8% of the total mass of the two active material layers, W1.

[0051] Example 4 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 Its particle size is 0.8 μm and its specific surface area (SSA1) is 20 m². 2 Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0052] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0053] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight of 8.3 g / m² per unit area is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon, with a dry weight per unit area of ​​157.9 g / m². 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved. 3 The material is rolled to obtain a positive electrode sheet. The first active material layer has a mass fraction W1 of 5% of the total mass of the two active material layers.

[0054] Example 5 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 The micrometer diameter (μm) is 1.0 μm, and the specific surface area (SSA1) is 14 m². 2 Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0055] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0056] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight of 8.3 g / m² per unit area is achieved.2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon, with a dry weight per unit area of ​​157.9 g / m². 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved. 3 The material is rolled to obtain a positive electrode sheet. The first active material layer has a mass fraction W1 of 5% of the total mass of the two active material layers.

[0057] Example 6 This embodiment provides a positive electrode sheet for a lithium-ion battery, the preparation method of which is as follows: (1) Preparation of the first active material layer slurry: The median particle size (D) was selected. 50-1 The micrometer diameter (μm) is 1.2 μm, and the specific surface area (SSA1) is 14 m². 2 Lithium iron phosphate (LFP) was used as the first positive electrode active material. Lithium iron phosphate, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 900) were mixed at a mass ratio of 95:1.5:0.5:3. N-methylpyrrolidone (NMP) solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a first active material slurry with a solid content of 65%.

[0058] (2) Preparation of the second active material layer slurry: A median particle size of 15 μm and a specific surface area of ​​0.5 m² were selected. 2 / g single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was used as the second positive electrode active material. NCM811, conductive carbon black (Super P), carbon nanotubes, and polyvinylidene fluoride (PVDF, model 5130) were mixed at a mass ratio of 96.8:1.5:0.5:1.2. NMP solvent was added to a high-speed mixer, and the mixture was homogeneously mixed to form a second active material slurry with a solid content of 70%.

[0059] (3) Double-layer coating and electrode preparation: Using precision double-layer coating technology, the slurry prepared in steps (1) and (2) is coated onto one surface of a carbon-coated aluminum foil current collector with a thickness of 12 μm and 2 μm coatings on both sides. By controlling the coating process parameters, a dry weight of 8.3 g / m² per unit area is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon, with a dry weight per unit area of ​​157.9 g / m². 2 The second active material layer (lithium nickel cobalt manganese oxide layer) is then applied. After drying, the same double-layer coating process is performed on the other side of the carbon-coated aluminum foil current collector, followed by drying. Then, a compaction density of 3.45 g / cm³ is achieved.3 The material is rolled to obtain a positive electrode sheet. The first active material layer has a mass fraction W1 of 5% of the total mass of the two active material layers.

[0060] Comparative Example 1 The difference between this comparative example and Example 4 is that the median particle size (D) of lithium iron phosphate in this comparative example is different. 50-1 The surface area is 3 μm, and the specific surface area (SSA1) is 8 m². 2 / g.

[0061] Comparative Example 2 The difference between this comparative example and Example 4 is that the median particle size (D) of lithium iron phosphate in this comparative example is different. 50-1 The micrometer diameter (μm) is 0.58 μm, and the specific surface area (SSA1) is 25 m². 2 / g, and the mass fraction W1 of the first active material layer in this comparative example is 10% of the total mass of the two active material layers. That is, by controlling the coating process parameters, a unit area dry weight of 16.6 g / m² is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon, with a dry weight per unit area of ​​149.6 g / m². 2 The second active material layer (lithium nickel cobalt manganese oxide layer).

[0062] Comparative Example 3 The difference between this comparative example and Example 5 is that the mass fraction W1 of the first active material layer in this comparative example is 2% of the total mass of the two active material layers. That is, by controlling the coating process parameters, a dry weight of 3.3 g / m² is achieved. 2 The first active material layer (lithium iron phosphate layer) and the layer directly stacked thereon, with a dry weight per unit area of ​​162.9 g / m². 2 The second active material layer (lithium nickel cobalt manganese oxide layer).

[0063] Comparative Example 4 The difference between this comparative example and Example 4 is that this comparative example does not contain a lithium iron phosphate layer, but only a single layer of lithium nickel cobalt manganese oxide is coated. The dry weight per unit area of ​​the lithium nickel cobalt manganese oxide layer is 166.2 g / m². 2 .

[0064] Parameter values ​​and relational expression (SSA1×W1) / D for Examples 1-6 and Comparative Examples 1-4 50-1 The calculated values ​​are summarized in Table 1.

[0065] Table 1. Parameter values ​​and calculation results for the examples and comparative examples. serial number <![CDATA[D 50-1 (μm)]]> <![CDATA[SSA1(m 2 / g)]]> <![CDATA[W1]]> <![CDATA[(SSA1× W1) / D 50-1 ]]> Example 1 0.8 20 8% (20 × 0.08) / 0.8 = 2.00 Example 2 1.0 14 8% (14 × 0.08) / 1.0 = 1.12 Example 3 1.2 12 8% (12 × 0.08) / 1.2 = 0.80 Example 4 0.8 20 5% (20 × 0.05) / 0.8 = 1.25 Example 5 1.0 14 5% (14 × 0.05) / 1.0 = 0.70 Example 6 1.2 12 5% (12 × 0.05) / 1.2 = 0.50 Comparative Example 1 3.0 8 5% (8 × 0.05) / 3.0 ≈ 0.13 Comparative Example 2 0.58 25 10% (25 × 0.10) / 0.58 ≈ 4.31 Comparative Example 3 1.0 14 2% (14 × 0.02) / 1.0 = 0.28 Comparative Example 4 - - 0% not applicable Test case Preparation of lithium-ion batteries: Artificial graphite, conductive carbon black, carbon nanotubes, sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 93:2:2:1.5:1.5. Deionized water was added and stirred to prepare a slurry with a solid content of 48%. This slurry was uniformly coated on both sides of an 8 μm thick copper foil. After drying and rolling, a coating weight of 104.29 g / m² was obtained. 2 The negative electrode sheet was prepared. The positive electrode sheets from Examples 1-6 and Comparative Examples 1-4, along with the aforementioned negative electrode sheets, were cut and welded with tabs, and then wound together with a polyethylene-based ceramic-coated separator to form a battery cell. The battery cell was placed in an aluminum-plastic film packaging bag, and an EC / EMC / DMC (volume ratio 3:5:2) electrolyte containing 1.0M LiPF6 was injected. After vacuum sealing, settling, formation (0.1C constant current charging to 3.6V), shaping, and capacity testing, a soft-pack lithium-ion battery with a rated capacity of 5Ah was obtained.

[0066] The testing method is as follows: 1. Energy density test: At 25°C, the battery was charged at a constant current of 0.2C to 4.2V, then switched to constant voltage charging until the current dropped to 0.05C. After resting for 30 minutes, it was discharged at a constant current of 0.2C to 2.8V, and the discharge capacity was recorded. Based on the cell's mass and volume, its gravimetric energy density (Wh / kg) and volumetric energy density (Wh / L) were calculated.

[0067] 2. High-rate long-cycle test: At 25°C, the battery was charged at a constant current of 1C to 4.2V, then switched to constant voltage charging until the current dropped to 0.05C, and immediately discharged at a constant current of 3C to 2.8V. This constitutes one cycle. The discharge capacity (C0) of the first cycle was recorded. This "1C charge / 3C discharge" cycle was repeated, and the discharge capacity (C0) of the 500th cycle was recorded. 500 ), and according to the formula "Capacity retention rate = (C 500 The capacity retention rate after high-rate cycling is calculated using the formula: (C0) × 100%.

[0068] 3. DC internal resistance (DCR) growth rate test: At 25°C, charge the battery to 50% state of charge (SOC). Test the initial DC internal resistance (DCR0): Apply a 10-second 3C discharge pulse and record the voltage drop (ΔV) at the start of the pulse. Calculate the DC internal resistance using ΔV / ΔI. After completing the 300th "1C charge / 3C discharge" cycle (refer to "2. High-rate long-cycle test" in the test method), adjust the battery to 50% SOC again and test the DC internal resistance (DCR0) after the cycle using the same method. 300 The DCR growth rate is calculated using the formula "[(DCR) 300Calculate using "- DCR0) / DCR0] × 100%".

[0069] 4. Thermal stability test: Disassemble the batteries that have not undergone cycling, as well as those that have completed 500 high-rate long-cycle tests (refer to "2. High-rate long-cycle test" in the test method). Remove the positive electrode, wash it with dimethyl carbonate (DMC), and dry it. Take an appropriate amount of the positive electrode material after these cycles and test it using a differential scanning calorimeter (DSC). The test conditions are: under an inert atmosphere (such as argon), heat from 50°C to 350°C at a heating rate of 5°C / min, and record the peak temperature of the exothermic reaction. The higher the peak temperature, the better the thermal stability of the material after long-term cycling.

[0070] The test results are summarized in Tables 2 and 3.

[0071] Table 2 Electrochemical test results of lithium-ion batteries in the examples and comparative examples serial number Weight energy density (Wh / kg) Volumetric energy density (Wh / L) High-rate cycling capacity retention (500 cycles, %) Initial DC internal resistance (mΩ) DC internal resistance growth rate (300 cycles, %) Example 1 265 725 88.5 15.2 28 Example 2 265 725 86.1 15.8 35 Example 3 265 725 84.3 16.5 41 Example 4 270 735 85.0 15.5 32 Example 5 270 735 82.5 16.2 39 Example 6 270 735 80.1 17.0 46 Comparative Example 1 270 735 71.2 18.5 68 Comparative Example 2 260 710 65.8 14.0 105 Comparative Example 3 272 740 73.5 19.1 75 Comparative Example 4 275 750 76.4 14.5 52 Table 3. Thermal stability test results of lithium-ion batteries in the examples and comparative examples. serial number Peak DSC temperature before cycling (°C) Peak DSC temperature (°C) after 500 cycles Peak temperature decay (°C) Example 1 230.1 226.8 3.3 Example 2 229.4 225.9 3.5 Example 3 228.1 224.5 3.6 Example 4 225.1 221.7 3.4 Example 5 224.0 220.2 3.8 Example 6 222.1 218.0 4.1 Comparative Example 1 227.5 218.5 9.0 Comparative Example 2 231.1 215.3 15.8 Comparative Example 3 223.0 214.1 8.9 Comparative Example 4 221.9 210.5 11.4 Note: The "DSC peak temperature before cycling" data in Table 3 are the average test values ​​of brand-new, uncycled batteries prepared in the same batch as each example / comparative example; the "DSC peak temperature after 500 cycles" data are the average test values ​​of batteries in the same batch after completing 500 high-rate cycles. The two sets of data are from different battery samples to ensure the consistency of the initial state of the batteries.

[0072] As can be seen from the experimental results in Tables 2 and 3, using Examples 1-6 of the present invention (where the parameters satisfy 0.5 ≤ (SSA1 × W1) / D) 50-1 The lithium-ion batteries prepared by ≤ 4) achieve excellent synergy and balance in terms of energy density, high-rate cycle life, power retention rate and long-term thermal stability.

[0073] All embodiments exhibited a capacity retention of over 80% and a DC internal resistance growth rate of less than 50% after 500 cycles of a demanding 1C charge / 3C discharge cycle. This demonstrates that, within the parameter range of this invention, the inner lithium iron phosphate layer effectively constructs a stable conductive network and mechanical buffer layer, mitigating the degradation of the high-nickel layer structure and interfacial side reactions during repeated fast charge and discharge cycles, thereby simultaneously resolving the contradiction between high rate capability and long cycle life.

[0074] Comparative Examples 1 and 3, due to insufficient contribution from the lithium iron phosphate layer (overly coarse particles or too low proportion), failed to form an effective buffer, resulting in significantly worse high-rate cycling performance and internal resistance growth compared to the examples. Comparative Example 2, due to the use of excessively fine, high-specific-surface-area lithium iron phosphate with an excessively high proportion, although initially exhibiting low internal resistance, triggered severe interfacial side reactions during cycling, leading to a sharp capacity decay and a surge in internal resistance. Comparative Example 4 (pure ternary system), lacking functional layer protection, showed the most severe degradation in high-rate cycling performance and interfacial deterioration.

[0075] As can be seen, this invention successfully designed a bilayer composite cathode by precisely controlling the quantitative relationship between the specific surface area, mass fraction, and median particle size of the inner lithium iron phosphate layer. This design, while preserving the high energy density of the high-nickel system to the greatest extent, endows the battery with excellent high-rate cycle performance, a slow power decay rate, and outstanding long-cycle thermal stability, achieving a synergistic improvement in high safety, long lifespan, and fast charge / discharge performance.

[0076] 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 positive electrode sheet for a lithium-ion battery, comprising a current collector, a first active material layer disposed on at least one surface of the current collector, and a second active material layer disposed on the surface of the first active material layer, characterized in that, The first active material layer includes lithium iron phosphate as the first positive electrode active material, and the second active material layer includes lithium nickel cobalt manganese oxide as the second positive electrode active material; The lithium iron phosphate satisfies the following relationship: 0.5 ≤ (SSA1 × W1) / D 50-1 ≤4; Where SSA1 is the specific surface area of ​​lithium iron phosphate, in m². 2 / g; W1 is the mass fraction of the first active substance layer in the total mass of the first and second active substance layers, which satisfies 3%≤W1≤10%; D 50-1 The median particle size of lithium iron phosphate is given in μm.

2. The lithium-ion battery positive electrode sheet as described in claim 1, characterized in that, The condition W1 satisfies 5% ≤ W1 ≤ 10%.

3. The lithium-ion battery positive electrode sheet as described in claim 1, characterized in that, The specific surface area of ​​the lithium iron phosphate is 10~22 m². 2 / g.

4. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The median particle size of the lithium iron phosphate is 0.7~2 μm.

5. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The general chemical formula of the lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O2, where 0.6 ≤ x ≤ 0.92, 0 <y<0.4。 6. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The median particle size of the lithium nickel cobalt manganese oxide is 8–18 μm, and the specific surface area is 0.3–1.0 m². 2 / g.

7. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The first active material layer further includes a first conductive agent and a first binder, and the mass ratio of the first positive electrode active material, the first conductive agent and the first binder is (94~97): (1~3): (2~4); the second active material layer further includes a second conductive agent and a second binder, and the mass ratio of the second positive electrode active material, the second conductive agent and the second binder is (96~98): (1~3): (1~2).

8. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The coating amount of the first active material layer is 5~20 g / m 2 The coating amount of the second active material layer is 145~165 g / m². 2 .

9. A lithium-ion battery, characterized in that, Including the positive electrode sheet of a lithium-ion battery as described in any one of claims 1 to 8.

10. An electrical appliance, characterized in that, Including the lithium-ion battery as described in claim 9.