Positive plate, preparation method thereof and secondary battery

Abstract

The invention discloses a positive plate, a preparation method of the positive plate and a secondary battery. The positive plate comprises a positive current collector and a negative current collector, the first positive electrode active material layer is arranged on at least one surface of the current collector; the second positive electrode active material layer is arranged on the outer surface of the first positive electrode active material layer; wherein the first positive electrode active material layer comprises a first active material, the first active material comprises lithium-rich manganese-based positive electrode material particles, and the second positive electrode active material layer comprises a second active material. Layered coating is adopted for the positive plate, the problem of uneven dispersion of the positive electrode material during preparation of coating slurry is avoided, the formula of each layer of slurry is not affected, the lithium-rich manganese-based positive electrode material in the first active material layer can supplement lithium for the second active material layer, and the positive electrode materials in the two layers have a synergistic effect, so that the lithium-rich manganese-based positive electrode material can be used for lithium supplement for the second active material layer. The voltage window of the first positive electrode active material layer is wide, the compatibility with the working voltage of the second positive electrode active material layer is good, and the first efficiency of the battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a positive electrode sheet, a preparation method thereof, and a secondary battery. Background Art

[0002] With the continuous development of electronic devices, higher requirements are put forward for the energy density and cycle performance of lithium-ion batteries. For example, lithium cobaltate, as a commonly used positive electrode material for lithium-ion batteries, has a relatively high specific capacity and working voltage. However, at high voltages, the cycle performance of lithium cobaltate will significantly decline, restricting its application in high-energy-density lithium-ion batteries.

[0003] Currently, the methods for improving the cycle performance of lithium cobaltate at high voltages mainly include surface coating, bulk doping, etc. Surface coating can form a stable protective layer on the surface of lithium cobaltate particles to prevent the erosion of the electrolyte on lithium cobaltate, thereby improving the cycle performance. Bulk doping can change the crystal structure of lithium cobaltate and improve its structural stability, thereby improving the cycle performance. However, these methods often have problems such as complex processes, high costs, and limited effects. For another example, ternary materials have a high energy density, but face great challenges in high-temperature cycling; lithium iron phosphate has good safety and cycle performance, but in the long cycle of lithium iron phosphate, generally a lithium supplement agent is used to supplement lithium to increase the long-cycle performance. The lithium iron manganese phosphate material with an olivine structure has the same structure as the lithium iron phosphate material, a similar specific capacity, and a higher plateau voltage, so it can greatly improve the energy density. However, this material has poorer conductivity and poor low-temperature performance, so it is difficult to be used alone.

[0004] As a positive electrode material with a discharge specific capacity exceeding 250 mAh·g -1 , the lithium-rich manganese-based material has great commercial prospects. At the same time, it has high safety and obvious cost advantages, and is expected to become the next-generation lithium-ion positive electrode material that can be industrialized. Currently, its application is mainly in high-voltage systems (upper limit voltage 4.2 - 4.8 V) to ensure high capacity utilization. However, this also brings a series of disadvantages that hinder its industrialization process: 1) The irreversible capacity is large during the first charge and discharge process; 2) The capacity and voltage decay seriously during the cycle process; 3) The ion diffusion coefficient of the material is low, the rate performance is poor, and the low-temperature performance is poor; 4) During the cycle process, due to the continuous change of the structure, the capacity will continuously increase beyond the rated capacity, making it difficult to design the battery management system. Whether a lithium battery positive electrode composite material system can be found that can not only exert the advantages of the lithium-rich manganese-based material but also make up for its deficiencies is crucial.

[0005] Patent CN115911609A discloses a lithium-rich manganese-based lithium-ion battery for improving the initial efficiency. In this patent, it is added together with the positive active material, conductive agent, binder, etc. during the slurry mixing stage in the process of making the battery cell to form a positive electrode slurry. However, mixing several materials with different particle sizes into a slurry will invisibly increase the difficulty of slurry mixing. On the one hand, it is impossible to ensure the dispersion stability of the slurry, and on the other hand, it may change the original mature slurry mixing process and produce more adverse results. Summary of the Invention

[0006] The object of the present invention is to provide a positive electrode sheet that can improve the problems of low specific capacity, low initial efficiency, and cycle decay of traditional positive electrode materials in a high-voltage system.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A positive electrode sheet, comprising:

[0009] A positive electrode current collector;

[0010] A first positive electrode active material layer disposed on at least one surface of the current collector;

[0011] A second positive electrode active material layer disposed on the outer surface of the first positive electrode active material layer;

[0012] Wherein, the first positive electrode active material layer includes a first active material, the first active material includes a lithium-rich manganese-based positive electrode material, the second positive electrode active material layer includes a second active material, the second active material includes at least one of lithium cobaltate, lithium iron phosphate, lithium manganese iron phosphate, and ternary positive electrode materials, and the ternary positive electrode materials include at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium nickel manganese oxide.

[0013] Preferably, the single-sided areal density of the first positive electrode active material layer is less than the single-sided areal density of the second positive electrode active material layer.

[0014] Preferably, the single-sided areal density of the first positive electrode active material layer is 2-10 mg / cm 2 , and the single-sided areal density of the second positive electrode active material layer is 16-40 mg / cm 2 .

[0015] Preferably, the thickness of the first active material layer is less than the thickness of the second active material layer.

[0016] Preferably, the D50 particle size of the lithium-rich manganese-based positive electrode material is 2-8 μm, the D50 particle size of the lithium cobaltate or ternary positive electrode material is 2-18 μm, and the D50 particle size of the lithium iron phosphate or lithium manganese iron phosphate is 0.2-2 μm.

[0017] Another object of the present invention is to provide a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0018] S1. Mix the first active material, conductive agent and binder in a solvent, and stir evenly to obtain the slurry of the first positive electrode active material layer;

[0019] S2. Mix the second active material, super composite conductive agent and binder in a solvent, and stir evenly to obtain the slurry of the second positive electrode active material layer;

[0020] S3. Coating the slurry of the first positive electrode active material layer and the slurry of the second positive electrode active material layer on the positive electrode current collector in sequence, drying and rolling, thus obtaining the positive electrode sheet.

[0021] Preferably, in steps S1 and S2, the stirring is carried out in a segmented manner, and the segmented stirring sequentially includes pre-stirring, high-speed stirring and defoaming stirring.

[0022] Preferably, the rotation speed of the pre-stirring is 1800-2200 r / min, and the stirring time is 50-70 min; the rotation speed of the high-speed stirring is 2900-3300 r / min, and the stirring time is 300-360 min; the rotation speed of the defoaming stirring is 1000-1400 r / min, and the stirring time is 50-70 min.

[0023] Preferably, the viscosities of the slurry of the first positive electrode active material layer and the slurry of the second positive electrode active material layer are 4000-6000 mPa·s respectively, and the fineness is ≥8.

[0024] Another object of the present invention is to provide a secondary battery, comprising the above-mentioned positive electrode sheet.

[0025] Compared with the prior art, the present invention has at least the following beneficial effects: The positive electrode sheet of the present invention adopts layered coating, which avoids the problem of uneven dispersion of the positive electrode material during the preparation of the coating slurry, and ensures that the formula of each layer of slurry is not affected. The lithium-rich manganese-based positive electrode material in the first active material layer can supply lithium for the second active material layer, and the positive electrode materials in the two layers have a synergistic effect. The voltage window of the first positive electrode active material layer is relatively wide, and it has good compatibility with the working voltage of the second positive electrode active material layer, improving the initial efficiency of the battery. Detailed Embodiments

[0026] In order to make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0027] In the first aspect according to the present invention, the present invention provides a positive electrode sheet, comprising:

[0028] A positive electrode current collector;

[0029] A first positive electrode active material layer disposed on at least one surface of the current collector;

[0030] A second positive electrode active material layer disposed on the outer surface of the first positive electrode active material layer;

[0031] Wherein, the first positive electrode active material layer comprises a first active material, the first active material comprises a lithium-rich manganese-based positive electrode material, the second positive electrode active material layer comprises a second active material, the second active material comprises at least one of lithium cobaltate, lithium iron phosphate, lithium manganese iron phosphate, and ternary positive electrode materials, and the ternary positive electrode materials comprise at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, and lithium nickel manganate.

[0032] The positive electrode sheet in the present invention has two positive electrode active material layers. The function of the layered arrangement is that on the one hand, it can avoid the problem of difficult pulp dispersion of different positive electrode materials, and on the other hand, it can effectively maintain the original formulations of the first positive electrode material and the second positive electrode material respectively, so that the homogenization process is not affected.

[0033] The lithium-rich manganese-based positive electrode material in the present invention is specifically xLi2MnO3·(1-x)LiMO2, where 0 < x < 1. The lithium-rich manganese-based can serve as both a positive electrode material and a slow-release lithium supplement agent. In addition to releasing a large amount of active lithium in the first cycle, active lithium continues to participate in the reaction during its cycling process.

[0034] In the positive electrode sheet of the present invention, with the second active material as the main component and the first active material as an auxiliary material, when they are used in combination, a synergistic effect will occur between them, and the first active material slowly releases lithium to supplement the second active material.

[0035] In addition, the first active material in the present invention is a high-voltage positive electrode material with a relatively wide voltage window, usually 2.0 - 4.8V. When used together with the second active material, it is activated at a voltage of 4.6V in the first cycle, and the other electrical performance test voltages should be consistent with the operating voltage of the second active material. For example, when the second active material is lithium cobaltate with a voltage of 4.53V, the full cell is activated at a voltage of 4.6V in the first cycle to improve the first efficiency of the lithium cobaltate battery, and the other electrical performance test voltages are 4.53V, meeting the voltage requirements of the high-voltage lithium cobaltate system; for example, when the second active material is lithium iron phosphate, the full cell is activated at a voltage of 4.6V in the first cycle to improve the first efficiency of the lithium iron phosphate battery, and the other electrical performance test voltages are 3.65V, meeting the voltage requirements of the lithium iron phosphate system. That is, the voltage window range of the first active material adapts to all the operating voltages of the second active material provided by the present invention.

[0036] In some embodiments, the single-sided surface density of the first positive electrode active material layer is less than that of the second positive electrode active material layer.

[0037] In some embodiments, the single-sided surface density of the first positive electrode active material layer is 2-10 mg / cm 2 , specifically, it can be 2 mg / cm 2 , 3 mg / cm 2 , 4 mg / cm 2 , 5 mg / cm 2 , 6 mg / cm 2 , 7 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , and may include but are not limited to the above surface densities; the single-sided surface density of the second positive electrode active material layer is 16-40 mg / cm 2 , specifically, it can be 16 mg / cm 2 , 17 mg / cm 2 , 18 mg / cm 2 , 19 mg / cm 2 , 20 mg / cm 2 , 21 mg / cm 2 , 22 mg / cm 2 , 23 mg / cm 2 , 24 mg / cm 2 , 25 mg / cm 2 , 26 mg / cm 2 , 27 mg / cm 2 , 28 mg / cm 2 , 29 mg / cm 2 , 30 mg / cm 2 , 31 mg / cm 2 , 32 mg / cm 2 , 33 mg / cm 2 , 34 mg / cm 2 , 35 mg / cm 2 , 36 mg / cm 2 , 37 mg / cm 2 , 38 mg / cm 2 , 39 mg / cm 2 , 40 mg / cm 2 , and may include but are not limited to the above surface densities.

[0038] As described above, the main function of the first positive electrode active material layer is to supplement lithium for the second positive electrode active material layer. There is a difference in areal density between the two layers, constructing an "ion and electron high-speed channel", reducing the diffusion resistance of lithium ions, reducing electrode polarization, improving the rate performance of the battery, enabling the battery to still maintain good performance during high-current charge and discharge, slowing down capacity decay, and extending the service life of the battery.

[0039] In some embodiments, the thickness of the first active material layer is less than the thickness of the second active material layer.

[0040] In some embodiments, the particle size D50 of the lithium-rich manganese-based positive electrode material is 2 - 8 μm, specifically it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, and can include but not be limited to the values listed above; the particle size D50 of lithium cobaltate or ternary positive electrode material is 2 - 18 μm, specifically it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, and can include but not be limited to the values listed above; the particle size D50 of lithium iron phosphate or lithium manganese iron phosphate is 0.2 - 2 μm, specifically it can be 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, and can include but not be limited to the values listed above.

[0041] In a second aspect of the present invention, the present invention also provides a method for preparing the above positive electrode sheet, comprising the following steps:

[0042] S1. Mix the first active material, conductive agent, and binder in a solvent, and stir evenly to obtain a slurry for the first positive electrode active material layer;

[0043] S2. Mix the second active material, super composite conductive agent, and binder in a solvent, and stir evenly to obtain a slurry for the second positive electrode active material layer;

[0044] S3. Coating the slurry for the first positive electrode active material layer and the slurry for the second positive electrode active material layer on the positive electrode current collector in sequence, drying and roll pressing to obtain the positive electrode sheet.

[0045] In some embodiments, in step S1 and step S2, the stirring adopts segmented stirring, and the segmented stirring sequentially includes pre-stirring, high-speed stirring, and defoaming stirring.

[0046] Among them, the super composite conductive agent is a composite slurry of carbon black and carbon nanotubes.

[0047] In some embodiments, the rotational speed of the pre-stirring is 1800 - 2200 r / min, preferably 2100 r / min; the stirring time is 50 - 70 min, preferably 60 min; the rotational speed of the high-speed stirring is 2900 - 3300 r / min, preferably 3100 r / min; the stirring time is 300 - 360 min, preferably 330 min; the rotational speed of the defoaming stirring is 1000 - 1400 r / min, preferably 1400 r / min; the stirring time is 50 - 70 min, preferably 60 min.

[0048] In some embodiments, the viscosities of the slurry of the first positive electrode active material layer and the slurry of the second positive electrode active material layer are 4000 - 6000 mPa·s respectively, and the fineness is ≥8.

[0049] A third object of the present invention further provides a secondary battery, which includes a battery cell formed by the above-mentioned positive electrode sheet, separator and negative electrode sheet, an electrolyte, and a housing for encapsulating the battery cell and the electrolyte.

[0050] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer may be one or more of, but not limited to, graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium. Among them, the graphite may be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based materials may be selected from one or more of elemental silicon, silicon oxides, silicon-carbon composites, and silicon alloys; the tin-based materials may be selected from one or more of elemental tin, tin oxides, and tin alloys. The negative electrode current collector is usually a structure or part for collecting current. The negative electrode current collector may be various materials suitable for being used as the negative electrode current collector of a lithium-ion battery in the art. For example, the negative electrode current collector may be, but not limited to, a metal foil, and more specifically, may be, but not limited to, a copper foil.

[0051] Among them, the separator may be various materials suitable for being used as the separator of a lithium-ion battery in the art. For example, it may be a combination of one or more of, but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0052] Among them, the electrolyte includes an organic solvent, an electrolyte lithium salt, and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-proof electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include at least one of, but not limited to, film-forming additives, conductive additives, flame retardant additives, overcharge prevention additives, additives for controlling the content of H2O and HF in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0053] To make the technical solutions and advantages of the present invention clearer, the present invention and its beneficial effects will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0054] Example 1

[0055] Preparation of the positive electrode sheet:

[0056] S1. Mix 0.5Li2MnO3·0.5LiMO2, a lithium-rich manganese-based cathode material with a particle size D50 of 5 μm, a conductive agent (CNT), a conductive agent (SP), and a binder (PVDF) in an N-methylpyrrolidone solvent according to a mass ratio of 0.5Li2MnO3·0.5LiMO2:CNT:SP:PVDF = 95.5:0.5:2:2. First, stir at a speed of 2100 r / min for 60 min, then stir at a speed of 3100 r / min for 330 min, and finally stir at a speed of 1400 r / min for 60 min to obtain the slurry of the first positive electrode active material layer.

[0057] S2. Mix LCO, a lithium cobaltate cathode material with a particle size D50 of 12 μm, a super composite conductive agent, and a binder PVDF in an N-methylpyrrolidone solvent according to a mass ratio of LCO:super composite conductive agent:PVDF = 98.3:0.7:1. First, stir at a speed of 2100 r / min for 60 min, then stir at a speed of 3100 r / min for 330 min, and finally stir at a speed of 1400 r / min for 60 min to obtain the slurry of the second positive electrode active material layer.

[0058] Among them, the super composite conductive agent is a composite slurry of carbon black and carbon nanotubes, and carbon black:carbon nanotubes ≈ 5:5.

[0059] S3. Coating the above-prepared slurry on the positive current collector successively to form a first positive active material layer and a second positive active material layer, and obtaining a positive electrode sheet after drying and rolling; wherein, the coating surface density parameters of the first positive active material layer and the second positive active material layer are shown in Table 1.

[0060] Preparation of negative electrode sheet:

[0061] Prepare a slurry for the negative active material layer by mixing 97.5 wt% graphite, 0.9 wt% CMC, and 1.6 wt% styrene-butadiene rubber (SBR), with a solid content of 45 wt%. Coat the slurry on the negative current collector using a coater, and obtain a negative electrode sheet after drying and rolling.

[0062] Preparation of lithium-ion battery:

[0063] Wind the above-prepared negative electrode sheet, positive electrode sheet, and separator together to form a wound core. The width of the battery core is 62.5 mm. Package it with an aluminum-plastic film, bake to remove moisture, then inject the electrolyte, and perform hot pressing and forming to obtain a lithium-ion battery.

[0064] Examples 2 - 4

[0065] The difference from Example 1 is the coating surface density of the slurry for the first active material layer and the slurry for the second active material layer in step S3. The specific parameters are shown in Table 1.

[0066] The rest is the same as Example 1 and will not be elaborated here.

[0067] Example 5

[0068] The difference from Example 1 is that the lithium cobaltate positive electrode material in step S2 is replaced with a lithium iron phosphate positive electrode material. The formulation of the lithium iron phosphate slurry is as follows: Mix lithium iron phosphate particles with a D50 particle size of 1 μm, a conductive agent (CNT), a conductive agent (SP), and a binder (PVDF) in a mass ratio of 96.5:0.5:1:2.

[0069] The rest is the same as Example 1 and will not be elaborated here.

[0070] Example 6

[0071] The difference from Example 1 is that the lithium cobaltate positive electrode material in step S2 is replaced with a ternary positive electrode material lithium nickel cobalt manganese oxide. The formulation of the lithium nickel cobalt manganese oxide slurry is as follows: Mix lithium nickel cobalt manganese oxide 6-series ternary positive electrode particles with a D50 particle size of 3 μm, a conductive agent (CNT), a conductive agent (SP), and a binder (PVDF) in a mass ratio of 97:1:0.3:1.7.

[0072] The rest is the same as Example 1 and will not be elaborated here.

[0073] Comparative Example 1

[0074] Different from Example 1, the first active material layer is not prepared, and only the lithium cobaltate cathode material slurry is coated.

[0075] The rest is the same as in Example 1 and will not be elaborated here.

[0076] Comparative Example 2

[0077] Different from Example 5, the first active material layer is not prepared, and only the lithium iron phosphate cathode material slurry is coated.

[0078] The rest is the same as in Example 1 and will not be elaborated here.

[0079] Comparative Example 3

[0080] Different from Example 6, the first active material layer is not prepared, and only the lithium nickel cobalt manganese oxide cathode material slurry is coated.

[0081] The rest is the same as in Example 1 and will not be elaborated here.

[0082] Comparative Example 4

[0083] Different from Example 1, the lithium-rich manganese-based cathode material 0.5Li2MnO3·0.5LiMO2, lithium cobaltate material, super composite conductive agent, and PVDF are mixed in a mass ratio of 19.4:77.6:1.3:1.7 to prepare a cathode coating slurry, which is coated on the cathode current collector in a single layer.

[0084] The rest is the same as in Example 1 and will not be elaborated here.

[0085] Comparative Example 5

[0086] Different from Example 1, the surface density of the coated slurry in step S3, and the specific parameters are shown in Table 1.

[0087] The rest is the same as in Example 1 and will not be elaborated here.

[0088] The batteries obtained from the above Examples 1 to 6 and Comparative Examples 1 to 5 were respectively subjected to the following performance tests:

[0089] Cycling test: At 45 °C, charge at a constant current of 0.5C to the charge cut-off voltage, then charge at a constant voltage to 0.05C, and then discharge at 0.5C to the cut-off current. Repeat the above charge and discharge process until the battery capacity drops to 80% of the initial capacity.

[0090] Initial efficiency: Charge at a constant current of 0.5C to the charge cut-off voltage of 4.6V, then charge at a constant voltage to 0.05C; then discharge at 0.5C to the cut-off current.

[0091] The test results were also tabulated in Table 1.

[0092] Among them, the parameters and experimental results of the above Examples 1-6 and Comparative Examples 1-5 are shown in Table 1.

[0093] Table 1

[0094]

[0095] Analysis of test results: As can be seen from the above table, compared with the comparative examples, the initial efficiency and cycling performance of the battery in the examples of the present invention are better. This shows that: during the first cycle activation of the battery, the lithium-rich manganese-based material can play a role in lithium compensation to compensate for the irreversible lithium loss caused by the formation of the solid electrolyte interface (SEI) film, thereby improving the initial efficiency of the battery; at the same time, the lithium-rich manganese-based material has a function of slow-release lithium compensation during the cycling process, which can extend the cycling performance of the battery.

[0096] Different coating materials can improve the initial efficiency and cycling performance of the battery on the basis of the original material while maintaining the characteristics of the original material.

[0097] Compared with Comparative Example 4, Example 1 uses layered coating, and the test performance of the battery is better than that of single-layer coating, indicating that the use of a suitable double-layer coating can better give full play to the lithium compensation characteristics of the lithium-rich manganese-based material. In the present invention, the first thin-coated lithium-rich manganese-based material is in the inner layer, and the second thick-coated main material is in the outer layer. During discharge, the lithium ions returning from the negative electrode will preferentially return to the second thick-coated main material, and the improvement of the initial efficiency will be better; during the cycling process, the thin-coated lithium-rich manganese-based material in the inner layer can reduce the interference of side reactions on the material, and the slow-release lithium compensation will be relatively more persistent; in addition, the surface density of the layered coating is also adjusted according to the characteristics of the material. Compared with Comparative Example 5, the surface density of the first lithium-rich manganese-based single-sided coating in Example 1 is lower, which is more conducive to the battery to maintain the characteristics of the second material, give full play to the characteristics of the lithium-rich manganese-based material to assist in lithium compensation of the second material, improve the initial efficiency and cycling life of the battery on the basis of maintaining the characteristics of the second material, and reduce the adverse effects such as capacity and voltage attenuation of the lithium-rich manganese-based material during the cycling process.

[0098] In summary, a positive electrode sheet provided by the present invention uses a layered coating active material layer, which avoids the problem of uneven dispersion of the positive electrode material during the preparation of the coating slurry, and ensures that the formula of each layer of slurry is not affected. The lithium-rich manganese-based positive electrode material in the first active material layer can perform lithium compensation for the second active material layer, and the positive electrode materials in the two layers have a synergistic effect. The voltage window of the first positive electrode active material layer is wide, and it has good compatibility with the working voltage of the second positive electrode active material layer.

[0099] Based on the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains are also able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions, or variations made by those skilled in the art based on the present invention fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A positive electrode sheet, characterized in that: include: Positive electrode current collector; A first positive electrode active material layer is disposed on at least one surface of the current collector; a second positive electrode active material layer, disposed on the outer surface of the first positive electrode active material layer; The first positive electrode active material layer includes a first active material, and the first active material includes a lithium-rich manganese-based positive electrode material; The second positive electrode active material layer includes a second active material, and the second active material includes at least one of lithium cobalt oxide, lithium iron phosphate, lithium iron manganese phosphate, and a ternary positive electrode material.

2. The positive electrode sheet according to claim 1, characterized in that: The single-surface areal density of the first positive electrode active material layer is smaller than the single-surface areal density of the second positive electrode active material layer.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The single surface density of the first positive electrode active material layer is 2-10 mg / cm 2 The single surface density of the second positive electrode active material layer is 16-40 mg / cm 2 .

4. The positive electrode sheet according to claim 1, characterized in that: The thickness of the first active material layer is smaller than the thickness of the second active material layer.

5. The positive electrode sheet according to claim 1, characterized in that: The particle size D50 of the lithium-rich manganese-based positive electrode material is 2-8 μm, the particle size D50 of the lithium cobalt oxide or ternary positive electrode material is 2-18 μm, and the particle size D50 of the lithium iron phosphate or lithium manganese iron phosphate is 0.2-2 μm.

6. A method for preparing a positive electrode sheet according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1, mixing a first active material, a conductive agent and a binder in a solvent, stirring evenly, to obtain a first positive electrode active material layer slurry; S2, mixing the second active material, the conductive agent and the binder in a solvent, stirring evenly, to obtain a second positive electrode active material layer slurry; S3, sequentially coating the first positive electrode active material layer slurry and the second positive electrode active material layer slurry on the positive electrode current collector, drying and rolling, so as to obtain the positive electrode sheet.

7. The method for preparing a positive electrode sheet according to claim 6, characterized in that: In step S1 and step S2, the stirring is performed by segmented stirring, and the segmented stirring includes pre-stirring, high-speed stirring and defoaming stirring in sequence.

8. The method for preparing a positive electrode sheet according to claim 7, characterized in that: The speed of the pre-stirring is 1800-2200 r / min, and the stirring time is 50-70 min; the speed of the high-speed stirring is 2900-3300 r / min, and the stirring time is 300-360 min; the speed of the defoaming stirring is 1000-1400 r / min, and the stirring time is 50-70 min.

9. The method for preparing a positive electrode sheet according to claim 6, characterized in that: The viscosity of the first positive electrode active material layer slurry and the second positive electrode active material layer slurry are respectively 4000-6000 mPa·s, and the fineness is ≥8 μm.

10. A secondary battery, characterized in that: The positive electrode sheet comprises the positive electrode sheet as claimed in any one of claims 1 to 5.

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