WO2024067231A1 - 一种正极片及锂离子电池 - Google Patents

一种正极片及锂离子电池 Download PDF

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WO2024067231A1
WO2024067231A1 PCT/CN2023/119599 CN2023119599W WO2024067231A1 WO 2024067231 A1 WO2024067231 A1 WO 2024067231A1 CN 2023119599 W CN2023119599 W CN 2023119599W WO 2024067231 A1 WO2024067231 A1 WO 2024067231A1
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Prior art keywords
positive electrode
conductive agent
material layer
carbon black
sheet according
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PCT/CN2023/119599
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English (en)
French (fr)
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黄腾
胡瑜磊
许占
袁晓涛
何科峰
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比亚迪股份有限公司
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Publication of WO2024067231A1 publication Critical patent/WO2024067231A1/zh

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application belongs to the technical field of secondary batteries, and specifically relates to a positive electrode sheet and a lithium-ion battery.
  • conductive agents are added to lithium iron phosphate electrodes.
  • the primary function of conductive agents is to improve electronic conductivity.
  • Conductive agents collect microcurrents between active materials and between active materials and current collectors to reduce the contact resistance of electrodes, increase the migration rate of electrons in lithium batteries, and reduce battery polarization.
  • conductive agents can also improve the processability of pole pieces, promote the infiltration of electrolytes into pole pieces, and correspond to changes in the diffusion impedance of the electrode liquid phase.
  • the present application provides a positive electrode sheet and a lithium-ion battery.
  • the present application provides a positive electrode sheet, comprising:
  • a positive electrode active material selected from lithium iron phosphate materials
  • a conductive agent wherein the conductive agent comprises carbon black
  • the positive electrode material layer meets the following conditions: 3.2% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 30.0%
  • x is the weight percentage of the carbon black in the conductive agent
  • a is the weight percentage of the conductive agent based on 100% of the positive electrode active material
  • ⁇ is the electrode tortuosity of the positive electrode material layer
  • ⁇ is the porosity of the positive electrode material layer.
  • the positive electrode material layer meets the following conditions: 3.5% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 15%.
  • the weight percentage x of the carbon black in the conductive agent is 30% to 100%.
  • the weight percentage x of the carbon black in the conductive agent is 40% to 70%.
  • the weight percentage a of the conductive agent is 0.5% to 2.5% based on 100% of the positive electrode active material.
  • the weight percentage a of the conductive agent is 0.75% to 1.5% based on 100% of the positive electrode active material.
  • the electrode tortuosity ⁇ of the positive electrode material layer is 1.5 to 3.5.
  • the electrode tortuosity ⁇ of the positive electrode material layer is 2.5 to 3.2.
  • the porosity ⁇ of the positive electrode material layer is 10% to 40%.
  • the porosity ⁇ of the positive electrode material layer is 15% to 30%.
  • the average particle size of the carbon black is 20-100 nm.
  • the conductive agent further includes one or more of carbon nanotubes and graphene.
  • the carbon nanotubes have an average diameter of 2-60 nm and an average length of 1-15 ⁇ m; the graphene has an average thickness of 1-100 nm, an average length of 0.2-20 ⁇ m, and an average width of 0.2-20 ⁇ m.
  • the positive electrode material layer is a single-layer structure.
  • the positive electrode material layer includes multiple layers of positive electrode coatings, and the weight percentage x of the carbon black in the conductive agent is the percentage of the total mass of the carbon black in the multiple layers of positive electrode coatings to the total mass of the conductive agent.
  • the positive electrode sheet also includes a current collector, the positive electrode material layer is arranged on the current collector, and along the direction away from the current collector, the carbon black content of the multiple layers of the positive electrode coating increases layer by layer, and the content of other additives in the multiple layers of the positive electrode coating decreases layer by layer.
  • the present application provides a lithium-ion battery, comprising the positive electrode sheet as described above.
  • the inventors conducted a large number of experiments on the use rules of the conductive agent, that is, the interaction between the addition ratio of the conductive agent and the electrode structure based on the positive electrode system of the lithium iron phosphate battery, and found that: controlling the weight percentage x of the carbon black in the conductive agent, with the positive electrode of ...
  • the weight percentage a of the conductive agent calculated as 100% of the active material, the electrode tortuosity ⁇ of the positive electrode material layer and the porosity ⁇ of the positive electrode material layer satisfy the relationship 3.2% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 30.0%, which can associate the physical properties of the positive electrode material layer itself with the construction of the conductive network, effectively optimize the liquid phase diffusion impedance and electronic conduction performance of the positive electrode material layer, and improve the kinetic performance of the positive electrode sheet.
  • FIG. 1 is an equivalent circuit diagram provided by the present application.
  • lithium iron phosphate-based material refers to a positive electrode active material containing lithium iron phosphate.
  • the surface of the lithium iron phosphate further includes a carbon coating layer to form the positive electrode active material.
  • the “average particle size of the carbon black” is obtained by measuring 300 carbon blacks using a scanning electron microscope image and averaging the particle sizes of the 300 carbon blacks.
  • the average length of the carbon nanotubes and “the average diameter of the carbon nanotubes” are obtained by taking the average value after measuring 300 carbon nanotubes through scanning electron microscope images.
  • the average thickness of the graphene”, “the average length of the graphene” and “the average thickness of the graphene” are obtained by measuring 300 graphenes using scanning electron microscope images. “The average thickness of the graphene” is obtained by taking the average value of the thickness of the graphene, and “the average length of the graphene” is obtained by taking the average value of the longest diameter of the graphene, which is the distance between the two farthest points on the surface of the graphene. “The average width of the graphene” is obtained by taking the average value of the maximum width of the graphene in the direction perpendicular to the longest diameter, which refers to the maximum value of the width of the graphene in the direction perpendicular to the longest diameter.
  • the embodiment of the present application provides a positive electrode sheet, including a positive electrode material layer, wherein the positive electrode material layer includes a positive electrode active material and a conductive agent, wherein the positive electrode active material is selected from lithium iron phosphate materials, and the conductive agent includes carbon black, and the positive electrode material layer meets the following conditions: 3.2% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 30.0%
  • x is the weight percentage of the carbon black in the conductive agent
  • a is the weight percentage of the conductive agent based on 100% of the positive electrode active material
  • ⁇ is the electrode tortuosity of the positive electrode material layer
  • ⁇ is the porosity of the positive electrode material layer.
  • the inventors conducted a large number of experiments on the usage rules of mixed conductive agents, that is, the interaction between the addition ratio of the conductive agent and the electrode structure, and found that: controlling the weight percentage x of the carbon black in the conductive agent, the weight percentage a of the conductive agent based on 100% of the positive electrode active material, the electrode tortuosity ⁇ of the positive electrode material layer and the porosity ⁇ of the positive electrode material layer to satisfy the relationship 3.2% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 30.0%, can associate the physical properties of the positive electrode material layer itself with the construction of the conductive network, effectively optimize the liquid phase diffusion impedance and electronic conduction performance of the positive electrode material layer, and improve the kinetic performance of the positive electrode sheet.
  • the positive electrode material layer satisfies the following conditions: 3.5% ⁇ 100*x*a/( ⁇ / ⁇ ) ⁇ 15%.
  • the conductive network construction rules in the lithium iron phosphate positive electrode are defined, and this rule interacts with the material electrode structure selection.
  • the optimal value of the liquid phase diffusion impedance can be achieved within a certain range, that is, the optimal improvement of the kinetics is achieved, ensuring that an effective conductive network can be constructed by optimizing the proportion of the conductive agent under different lithium iron phosphate positive electrode structural parameters.
  • the weight percentage x of the carbon black in the conductive agent is 30% to 100%.
  • the weight percentage x of the carbon black in the conductive agent can be 30%, 33%, 36%, 39%, 40%, 43%, 46%, 49%, 50%, 53%, 56%, 59%, 60%, 63%, 66%, 69%, 70%, 73%, 76%, 79%, 80%, 83%, 86%, 89%, 90%, 93%, 96%, 99% or 100%.
  • the weight percentage x of the carbon black in the conductive agent is 40% to 70%.
  • Carbon black has a high liquid retention capacity, which is beneficial to the diffusion process of lithium ions in the electrode.
  • the weight percentage x of the carbon black in the conductive agent is beneficial to ensure the liquid retention of the positive electrode material layer, thereby improving the diffusion efficiency of lithium ions between the positive electrode material layer and the electrolyte.
  • other conductive agents besides carbon black can be added to the conductive agent.
  • an efficient conductive network is constructed from multiple dimensions of points, lines, and surfaces to improve its electronic conduction efficiency.
  • the weight percentage a of the conductive agent is 0.5% to 2.5% based on 100% of the positive electrode active material.
  • the weight percentage a of the conductive agent based on 100% of the positive electrode active material can be 0.5%, 0.6%, 0.8%, 0.9%, 1.0%, 1.2%, 1.4%, 1.7%, 1.9%, 2%, 2.1%, 2.3%, 2.4% or 2.5%.
  • the weight percentage a of the conductive agent is 0.75% to 1.5% based on 100% of the positive electrode active material.
  • the contact area between the conductive agents can be increased while ensuring a lower amount of conductive agent added, thereby reducing the impedance of the positive electrode and avoiding the problem of reduced battery energy density caused by adding too much conductive agent.
  • the electrode tortuosity ⁇ of the positive electrode material layer is 1.5 to 3.5.
  • the electrode tortuosity ⁇ of the positive electrode material layer may be 1.5, 1.7, 1.9, 2, 2.1, 2.3, 2.4, 2.5, 2.7, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4 or 3.5.
  • the electrode tortuosity ⁇ of the positive electrode material layer is 2.5 to 3.2.
  • the electrode tortuosity ⁇ represents the degree of curvature of the porous electrode transmission path.
  • the electrode tortuosity ⁇ can be obtained through the three-dimensional reconstruction simulation fitting test of the electrode FIB-SEM.
  • the electrode tortuosity ⁇ of the positive electrode material layer is within the above range, it is beneficial to improve the embedding and extraction efficiency of lithium ions in the positive electrode material layer, as well as the infiltration and diffusion of the electrolyte in the positive electrode material layer, while ensuring that there is a suitable distance between the positive electrode active materials, improving the diffusion efficiency of the electrolyte and reducing the impact on electronic conduction.
  • the porosity ⁇ of the positive electrode material layer is 10% to 40%.
  • the porosity ⁇ of the positive electrode material layer can be 10%, 12%, 13%, 16%, 19%, 20%, 23%, 26%, 29%, 30%, 33%, 36%, 39% or 40%.
  • the porosity ⁇ of the positive electrode material layer is 15% to 30%.
  • the positive electrode material layer can have a higher liquid retention capacity, thereby ensuring sufficient penetration of the electrolyte into the positive electrode material layer and reducing ion conduction impedance.
  • the porosity ⁇ of the positive electrode material layer and the electrode tortuosity ⁇ of the positive electrode material layer jointly affect the diffusion impedance of the electrolyte in the positive electrode material layer.
  • ⁇ / ⁇ is the main influencing factor of electrolyte diffusion.
  • the larger the ⁇ / ⁇ value the greater the liquid phase diffusion impedance.
  • More conductive agent carbon black needs to be added to increase the liquid retention in the positive electrode material layer to offset the influence of liquid phase diffusion impedance.
  • the smaller the ⁇ / ⁇ value the proportion of carbon black added can be reduced accordingly. Therefore, the carbon black conductive agent can be adjusted according to the structure of the electrode material.
  • the addition ratio is regulated to optimize the liquid phase diffusion impedance of the positive electrode material layer and improve the kinetic performance of the electrode.
  • the average particle size of the carbon black is 20-100 nm.
  • the average particle size of the carbon black can be 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm.
  • the conductive agent further comprises one or more of carbon nanotubes and graphene.
  • the carbon nanotubes have an average diameter of 2-60 nm and an average length of 1-15 ⁇ m; the graphene has an average thickness of 1-100 nm, an average length of 0.2-20 ⁇ m, and an average width of 0.2-20 ⁇ m.
  • the average diameter of the carbon nanotubes may be 2 nm, 5 nm, 9 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm or 60 nm.
  • the average length of the carbon nanotubes may be 1 ⁇ m, 2 ⁇ m, 2.5 ⁇ m, 3.5 ⁇ m, 4.5 ⁇ m, 5.0 ⁇ m, 6.5 ⁇ m, 7.0 ⁇ m, 8.5 ⁇ m, 9.0 ⁇ m, 10.5 ⁇ m, 12 ⁇ m, 14.5 ⁇ m or 15 ⁇ m.
  • the average thickness of the graphene may be 1 nm, 2 nm, 5 nm, 9 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm or 100 nm, and the average length of the graphene may be 0.2 ⁇ m, 1 ⁇ m, 2 ⁇ m, 2.5 ⁇ m, 3.5 ⁇ m, 4.5 ⁇ m, 5.
  • the average width of the graphene can be 0.2 ⁇ m, 1 ⁇ m, 2 ⁇ m, 2.5 ⁇ m, 3.5 ⁇ m, 4.5 ⁇ m, 5.0 ⁇ m, 6.5 ⁇ m, 7.0 ⁇ m, 8.5 ⁇ m, 9.0 ⁇ m, 10.5 ⁇ m, 12 ⁇ m, 14.5 ⁇ m, 15 ⁇ m or 20 ⁇ m.
  • the morphology of the conductive agent is related to the overlapping area between the positive electrode material layers and the contact area between the conductive agent and the positive electrode active material.
  • the positive electrode material layer is a single-layer structure.
  • the positive electrode material layer includes multiple layers of positive electrode coatings
  • the weight percentage x of the carbon black in the conductive agent is the percentage of the total mass of the carbon black in the multiple layers of positive electrode coatings to the total mass of the conductive agent.
  • the positive electrode sheet also includes a current collector, the positive electrode material layer is disposed on the current collector, and along the direction away from the current collector, the carbon black content of the multiple layers of the positive electrode coating increases layer by layer, and the content of other additives in the multiple layers of the positive electrode coating decreases layer by layer.
  • the conductive agent in each layer is a conductive agent system composed of carbon black and other conductive agents, wherein carbon black has a high liquid retention capacity, which is beneficial to the diffusion process of lithium ions in the electrode.
  • carbon black has a high liquid retention capacity, which is beneficial to the diffusion process of lithium ions in the electrode.
  • the wetting effect of the electrolyte on the positive electrode material layer can be improved.
  • Carbon nanotubes and graphene, as other conductive agents have high electronic conductivity and need to be arranged more on the positive electrode material layer close to the current collector side to enhance the bonding with the current collector.
  • the layered arrangement of carbon black and other conductive agents is beneficial to building a better conductive network in the multi-layer coating system.
  • the current collector is selected from a metal material that can conduct electrons.
  • the current collector includes one or more of Al, Ni, tin, copper, and stainless steel.
  • the current collector is selected from aluminum foil, carbon-coated aluminum foil, or perforated aluminum foil.
  • the positive electrode material layer further includes a positive electrode binder, and the positive electrode active material, the positive electrode binder and the conductive agent are blended to obtain the positive electrode material layer.
  • the positive electrode binder includes polyvinylidene fluoride, a copolymer of vinylidene fluoride, polytetrafluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene, a copolymer of tetrafluoroethylene-hexafluoropropylene, a copolymer of tetrafluoroethylene-perfluoroalkyl vinyl ether, a copolymer of ethylene-tetrafluoroethylene, a copolymer of vinylidene fluoride-tetrafluoroethylene, a copolymer of vinylidene fluoride-trifluoroethylene, a copolymer of vinylidene fluoride-trichloroethylene, a copolymer of vinylidene fluoride-fluoroethylene, a copolymer of vinylidene fluoride-hexafluoropropylene-tetraflu
  • the content of the positive electrode active material is 90% to 99%, and the content of the positive electrode binder is 0.5% to 5%.
  • Another embodiment of the present application provides a lithium-ion battery, comprising the positive electrode sheet as described above.
  • the lithium-ion battery further includes a negative electrode sheet, a separator, and a non-aqueous electrolyte.
  • the separator includes single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP/PE, double-layer PP/PP and triple-layer PP/PE/PP separators.
  • the nonaqueous electrolyte includes a solvent and a lithium salt.
  • the solvent includes one or more of an ether solvent, a nitrile solvent, a carbonate solvent and a carboxylate solvent.
  • the lithium salt includes one or more of LiPF6 , LiPO2F2 , LiBF4 , LiSbF6 , LiAsF6 , LiN( SO2CF3 ) 2 , LiN ( SO2C2F5 ) 2 , LiC( SO2CF3 ) 3 , and LiN ( SO2F ) 2 .
  • This embodiment is used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, including follow these steps:
  • Lithium iron phosphate, a binder, nitrogen methyl pyrrolidone and a conductive agent are mixed to prepare a slurry, and the slurry is coated on an aluminum foil in a single-layer coating manner, wherein the amount of lithium iron phosphate added is 100 parts, the total amount of the conductive agent added is 1.5 parts, the amount of carbon black added is 0.65 parts, and in addition to carbon black, carbon nanotubes are added in an amount of 0.85 parts, wherein the content of thick tube CNT5% is 0.65 parts, the content of thin tube CNT4.3% is 0.2 parts, the weight percentage x of carbon black in the conductive agent is 43%, and the weight percentage a of the conductive agent is 1.5% based on the positive electrode active material as 100%.
  • a positive electrode sheet is prepared, and the positive electrode sheet and the corresponding negative electrode sheet are assembled into a battery, and the battery design capacity is 1.8Ah.
  • the porosity measured by mercury injection method is 22.1%, and the electrode tortuosity measured by FIB-SEM simulation fitting is 2.8.
  • test conditions of mercury intrusion method are: pressure range: 0.10 ⁇ 61000psia; contact angle: 130°; test temperature: 14 ⁇ 15°C;
  • the test process of FIB-SEM simulation fitting includes original image optimization, material segmentation, reconstruction (input segmentation parameters: pores 0-100; other solid materials: 100-140; lithium iron phosphate: 140-254), and software simulation results.
  • Example 2 is used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is double-layer coating, the upper layer is a small-particle lithium iron phosphate layer, wherein the lithium iron phosphate addition amount is 100 parts, the upper layer conductive agent addition amount is 0.8 parts, wherein the carbon black addition amount is 0.5 parts, in addition to carbon black, carbon nanotubes are also added, and the addition amount is 0.3 parts, and the carbon nanotubes are thin tubes CNT4.3%;
  • the lower layer is a lithium iron phosphate mixed electrode layer with large and small particles, wherein the lithium iron phosphate addition amount is 100 parts, the lower layer conductive agent addition amount is 0.8 parts, the carbon black addition amount is 0.5 parts, in addition to carbon black, carbon nanotubes are also added, and the addition amount is 0.3 parts, and the carbon nanotubes are thin tubes CNT4.3%, the weight percentage x of carbon black in the conductive agent is 62%, and the weight percentage a of the conductive agent is 0.8% based on the positive electrode active material as 100%, the porosity measured by mercury injection method is 19.6%
  • Example 3 is used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is double-layer coating, the upper layer is a large-particle lithium iron phosphate layer, wherein the lithium iron phosphate addition amount is 100 parts, the upper layer conductive agent addition amount is 0.95 parts, the carbon black addition amount is 0.7 parts, in addition to carbon black, carbon nanotubes are also added, and the addition amount is 0.25 parts, and the carbon nanotubes are thin tubes CNT4.3%;
  • the lower layer is a lithium iron phosphate mixed electrode layer with large and small particles, wherein the lithium iron phosphate addition amount is 100 parts, the lower layer conductive agent addition amount is 0.87 parts, the carbon black addition amount is 0.57 parts, in addition to carbon black, carbon nanotubes are also added, and the addition amount is 0.3 parts, and the carbon nanotubes are thin tubes CNT4.3%
  • the weight percentage x of carbon black in the conductive agent is 70%, and the weight percentage a of the conductive agent is 0.91% based on the positive electrode active material as 100%, the porosity measured by mercury injection method is 23.1%
  • Example 4 is used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is double-layer coating, the upper layer is a lithium iron phosphate mixed electrode layer with large and small particles, wherein the lithium iron phosphate addition amount is 100 parts, the upper layer conductive agent addition amount is 0.77 parts, the carbon black addition amount is 0.5 parts, in addition to carbon black, carbon nanotubes are also added, and the addition amount is 0.27 parts, and the carbon nanotubes are a mixture of thick tube CNT5% and thin tube CNT4.3%, the thick tube CNT5% addition amount is 0.1 parts, and the thin tube CNT4.3% addition amount is 0.17 parts; the lower layer is a lithium iron phosphate mixed electrode layer with large and small particles, wherein the lithium iron phosphate addition amount is 100 parts, and the lower layer conductive agent is The added amount is 0.81 parts, the added amount of carbon black is 0.5 parts, in addition to carbon black, carbon nanotubes are also added, and the added amount is 6.98 parts.
  • the carbon nanotubes are a mixture of thick tube CNT5% and thin tube CNT4.3%.
  • the thick tube CNT5% added amount is 0.1 parts, and the thin tube CNT4.3% added amount is 0.21 parts.
  • the weight percentage x of carbon black in the conductive agent is 63%, and the weight percentage a of the conductive agent is 0.79% based on the positive electrode active material as 100%.
  • the porosity measured by mercury injection method is 20.4%, and the electrode tortuosity measured by FIB-SEM simulation fitting is 2.88.
  • Examples 5 to 30 are used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and include most of the operation steps in Example 1, except that:
  • the weight percentage x of carbon black in the conductive agent, the weight percentage a of the conductive agent based on 100% lithium iron phosphate, the electrode tortuosity ⁇ of the positive electrode material layer and the porosity ⁇ of the positive electrode material layer are shown in Table 1 for Examples 5 to 30.
  • Comparative Example 1 is used to compare and illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is single-layer coating, the added amount of lithium iron phosphate is 100 parts, the total added amount of conductive agent is 1.7 parts, and carbon nanotubes are added in addition to carbon black, and the added amount is 1.275 parts, of which the content of thick tube CNT5% is 0.2 parts, the content of thin tube CNT4.3% is 1.075 parts, and the added amount of carbon black is 0.425 parts.
  • the weight percentage x of carbon black in the conductive agent is 25%, and the weight percentage a of the conductive agent is 1.7% based on the positive electrode active material as 100%.
  • the porosity measured by mercury injection method is 19.81%, and the electrode tortuosity measured by FIB-SEM simulation fitting is 3.1.
  • Comparative Example 2 is used to compare and illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is double-layer coating, the upper layer is a small-particle lithium iron phosphate layer, wherein the lithium iron phosphate addition amount is 100 parts, the conductive agent addition amount is 1.6 parts, and carbon nanotubes are added in addition to carbon black, and the addition amount is 1.3 parts.
  • the carbon nanotubes are thin tubes CNT4.3%, and the carbon black addition amount is 0.3 parts;
  • the lower layer is a lithium iron phosphate mixed electrode layer with large and small particles, wherein the lithium iron phosphate addition amount is 100 parts, the conductive agent addition amount is 1.6 parts, and carbon nanotubes are added in addition to carbon black, and the addition amount of carbon nanotubes is 1.3 parts.
  • the carbon tubes are thin tubes CNT4.3%, and the carbon black addition amount is 0.3 parts.
  • the weight percentage x of carbon black in the conductive agent is 19%, and the weight percentage a of the conductive agent is 1.6% based on the positive electrode active material as 100%.
  • the porosity measured by mercury injection method is 21.0%, and the electrode tortuosity measured by FIB-SEM simulation fitting is 2.98.
  • Comparative Example 3 is used to compare and illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and includes most of the operation steps in Example 1, except that:
  • the positive electrode coating method is double-layer coating, the upper layer is a large-particle lithium iron phosphate layer, in which the lithium iron phosphate addition amount is 100 parts, the conductive agent addition amount is 4.25 parts, and carbon nanotubes are added in addition to carbon black, and the addition amount is 1.5 parts.
  • the carbon nanotubes are thin tubes CNT4.3%, and the carbon black addition amount is 2.75 parts;
  • the lower layer is a lithium iron phosphate mixed electrode layer with large and small particles, in which the lithium iron phosphate addition amount is 100 parts, the conductive agent addition amount is 4.0 parts, and carbon nanotubes are added in addition to carbon black, and the carbon nanotubes are added in an amount of 0.95 parts.
  • the carbon tubes are thin tubes CNT4.3%, the amount of carbon black added is 3.05 parts, the weight percentage x of carbon black in the conductive agent is 70%, the weight percentage a of the conductive agent is 4.13% based on the positive electrode active material as 100%, the porosity measured by mercury injection method is 26.7%, and the electrode tortuosity measured by FIB-SEM simulation fitting is 2.5.
  • Comparative Examples 4 to 6 are used to illustrate the positive electrode sheet, lithium ion battery and preparation method thereof disclosed in the present application, and include most of the operation steps in Example 1, except that:
  • the weight percentage x of carbon black in the conductive agent, the weight percentage a of the conductive agent based on 100% lithium iron phosphate, the electrode tortuosity ⁇ of the positive electrode material layer and the porosity ⁇ of the positive electrode material layer are shown in Comparative Examples 4 to 6 in Table 1.
  • the positive electrode sheet prepared above was subjected to the following performance tests:
  • Standard charging At room temperature 25 ⁇ 5°C, charge to 3.8V at 0.33C constant current and constant voltage, cut-off current 0.05C; leave for 10 minutes.
  • the main steps include:
  • test parameters frequency range: 100000-0.05Hz; amplitude: 5mV; temperature: 25°C; 10 points are taken for every 10 times frequency range
  • the liquid phase diffusion resistance R ion is calculated.
  • the prepared positive electrode sheets were tested for longitudinal resistivity, DC internal resistance at 50% SOC, and liquid phase diffusion impedance. From the test results, it can be seen that the positive electrode sheets prepared according to the conductive network design rules provided in the present application have relatively low resistivity, DC internal resistance, and liquid phase diffusion impedance, indicating that by matching the addition ratio of carbon black in the multi-element conductive agent with the structural parameters of the positive electrode material layer, the electronic conductivity and ion conductivity of the positive electrode sheet can be taken into account at the same time, and the optimal value of the liquid phase diffusion impedance can be achieved, which is conducive to building an effective conductive network under different lithium iron phosphate positive electrode structural parameter conditions.

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Abstract

一种正极片，包括正极材料层，所述正极材料层包括正极活性材料和导电剂，所述正极活性材料选自磷酸铁锂类材料，所述导电剂包括有炭黑，所述正极材料层满足以下条件：3.2％≤100*x*a/(τ/ε)≤30.0％。同时，还公开了包括上述正极片的锂离子电池。

Description

一种正极片及锂离子电池
本申请要求于2022年9月29日提交中国专利局、申请号为202211195629.7、申请名称为“一种正极片及锂离子电池”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本申请属于二次电池技术领域，具体涉及一种正极片及锂离子电池。
背景技术
受结构限制，磷酸铁锂材料自身的电子导电率很低，限制了其在锂离子电池中的发挥。目前在磷酸铁锂电极中都会选择加入导电剂，导电剂的首要作用是提高电子电导率，导电剂在活性物质之间、活性物质与集流体之间起到收集微电流的作用以减小电极的接触电阻，提高锂电池中电子的迁移速率，降低电池极化。此外，导电剂也可以提高极片加工性，促进电解液对极片的浸润，对应电极液相扩散阻抗的变化。但是对于需要加入何种导电剂以及导电剂的加入量，并没有普适的规则。传统的电极，导电剂的使用过于单一，导电剂与导电剂之间搭配难以形成比较高效的导电网络。同一种导电剂配方往往会应用在不同的电极中，对于不同电极材料来说，采用同种导电剂配方往往得到不同的作用，因此，现有导电剂添加配比不能充分发挥电极性能。
发明内容
针对现有磷酸铁锂电池存在由导电剂构建的导电网络性能不足的问题，本申请提供了一种正极片及锂离子电池。
本申请解决上述技术问题所采用的技术方案如下：
一方面，本申请提供了一种正极片，包括：
正极材料层，所述正极材料层包括：
正极活性材料，选自磷酸铁锂类材料；和
导电剂，所述导电剂包括炭黑；
所述正极材料层满足以下条件：
3.2％≤100*x*a/(τ/ε)≤30.0％
其中，x为所述炭黑在所述导电剂中的重量百分比；
a为以所述正极活性材料为100％计所述导电剂的重量百分比；
τ为所述正极材料层的电极曲折度；
ε为所述正极材料层的孔隙率。
可选的，所述正极材料层满足以下条件：
3.5％≤100*x*a/(τ/ε)≤15％。
可选的，所述炭黑在所述导电剂中的重量百分比x为30％～100％。
可选的，所述炭黑在所述导电剂中的重量百分比x为40％～70％。
可选的，以所述正极活性材料为100％计导电剂的重量百分比a为0.5％～2.5％。
可选的，以所述正极活性材料为100％计导电剂的重量百分比a为0.75％～1.5％。
可选的，所述正极材料层的电极曲折度τ为1.5～3.5。
可选的，所述正极材料层的电极曲折度τ为2.5～3.2。
可选的，所述正极材料层的孔隙率ε为10％～40％。
可选的，所述正极材料层的孔隙率ε为15％～30％。
可选的，所述炭黑的平均粒径为20-100nm。
可选的，所述导电剂还包括碳纳米管和石墨烯中的一种或多种。
可选的，所述碳纳米管的平均直径为2-60nm，平均长度为1-15μm；所述石墨烯的平均厚度为1-100nm，平均长度为0.2-20μm，平均宽度为0.2-20μm。
可选的，所述正极材料层为单层结构。
可选的，所述正极材料层包括多层正极涂层，所述炭黑在所述导电剂中的重量百分比x为多层正极涂层中炭黑质量总和与导电剂质量总和的百分比。
可选的，所述正极片还包括有集流体，所述正极材料层设置于所述集流体上，沿远离所述集流体的方向，多层所述正极涂层的炭黑含量逐层增大，多层所述正极涂层的其他添加剂含量逐层降低。
另一方面，本申请提供了一种锂离子电池，包括如上所述的正极片。
根据本申请提供的正极片，发明人基于磷酸铁锂电池的正极体系，通过大量试验对导电剂的使用规则，即对导电剂的添加比例与电极结构之间的交互关系进行了试验，发现：控制所述炭黑在所述导电剂中的重量百分比x、以所述正 极活性材料为100％计所述导电剂的重量百分比a、所述正极材料层的电极曲折度τ和所述正极材料层的孔隙率ε满足关系式3.2％≤100*x*a/(τ/ε)≤30.0％的限定，能够使正极材料层本身的物理性质与导电网络的构建相关联，有效优化正极材料层的液相扩散阻抗以及电子传导性能，提升正极片的动力学性能。
附图说明
图1是本申请提供的等效电路图。
具体实施方式
为了使本申请所解决的技术问题、技术方案及有益效果更加清楚明白，以下结合附图及实施例，对本申请进行进一步详细说明。应当理解，此处所描述的具体实施例仅仅用以解释本申请，并不用于限定本申请。
在本申请的描述中，“磷酸铁锂类材料”指代含有磷酸铁锂的正极活性材料，在一些情况下，所述磷酸铁锂的表面还包括有碳包覆层以形成所述正极活性材料。
在本申请的描述中，“所述炭黑的平均粒径”扫描电镜图片测量300个炭黑，由300个炭黑的粒径取平均值得出。
在本申请的描述中，“所述碳纳米管的平均长度”、“所述碳纳米管的平均直径”均由扫描电镜图片测量300个碳纳米管后取平均值得出。
“所述石墨烯的平均厚度”、“所述石墨烯的平均长度”和“所述石墨烯的平均厚度”由扫描电镜图片测量300个石墨烯，“所述石墨烯的平均厚度”由石墨烯的厚度取平均值得出，“所述石墨烯的平均长度”由石墨烯的最长径取平均值得出，所述石墨烯的最长径即所述石墨烯表面相距最远的两点之间的距离，“所述石墨烯的平均宽度”由石墨烯与最长径垂直的方向的最大宽度取平均值得出，所述石墨烯与最长径垂直的方向的最大宽度指的是所述石墨烯在与所述最长径垂直方向上的宽度的最大值。
本申请实施例提供了一种正极片，包括正极材料层，所述正极材料层包括正极活性材料和导电剂，所述正极活性材料选自磷酸铁锂类材料，所述导电剂包括有炭黑，所述正极材料层满足以下条件：
3.2％≤100*x*a/(τ/ε)≤30.0％
其中，x为所述炭黑在所述导电剂中的重量百分比；
a为以所述正极活性材料为100％计所述导电剂的重量百分比；
τ为所述正极材料层的电极曲折度；
ε为所述正极材料层的孔隙率。
发明人基于磷酸铁锂电池的正极体系，通过大量试验对混合导电剂的使用规则，即对导电剂的添加比例与电极结构之间的交互关系进行了试验，发现：控制所述炭黑在所述导电剂中的重量百分比x、以所述正极活性材料为100％计所述导电剂的重量百分比a、所述正极材料层的电极曲折度τ和所述正极材料层的孔隙率ε满足关系式3.2％≤100*x*a/(τ/ε)≤30.0％的限定，能够使正极材料层本身的物理性质与导电网络的构建相关联，有效优化正极材料层的液相扩散阻抗以及电子传导性能，提升正极片的动力学性能。
在一些实施例中，所述正极材料层满足以下条件：
3.5％≤100*x*a/(τ/ε)≤15％。
通过对上述关系式范围的进一步限制，定义了磷酸铁锂正极中的导电网络构建规则，并且这种规则是和材料电极结构选择有交互作用的，通过多元导电剂中炭黑的添加比例与正极材料的结构参数相匹配，在某一范围内能够实现液相扩散阻抗的最优值，即实现动力学的最优提升，保证在不同磷酸铁锂正极结构参数条件下也能通过导电剂的比例优选构建有效的导电网络。
在一些实施例中，所述炭黑在所述导电剂中的重量百分比x为30％～100％。
具体的，所述炭黑在所述导电剂中的重量百分比x可以为30％、33％、36％、39％、40％、43％、46％、49％、50％、53％、56％、59％、60％、63％、66％、69％、70％、73％、76％、79％、80％、83％、86％、89％、90％、93％、96％、99％或100％。
在一些实施例中，所述炭黑在所述导电剂中的重量百分比x为40％～70％。
炭黑具有高的保液能力，有利于锂离子在电极中的扩散过程，通过调节所述炭黑在所述导电剂中的重量百分比x处于上述范围时，有利于保证所述正极材料层的保液量，进而提高锂离子在正极材料层与电解液之间的扩散效率，进一步的，为提高正极材料层的电子传导性能，可在所述导电剂中加入除炭黑外的其他导电剂，通过复合导电剂的使用从点、线、面多维度构建了高效的导电网络，以提高其电子传导效率。
在一些实施例中，以所述正极活性材料为100％计所述导电剂的重量百分比a为0.5％～2.5％。
具体的，以所述正极活性材料为100％计所述导电剂的重量百分比a可以为0.5％、0.6％、0.8％、0.9％、1.0％、1.2％、1.4％、1.7％、1.9％、2％、2.1％、2.3％、2.4％或2.5％。
在一些实施例中，以所述正极活性材料为100％计所述导电剂的重量百分比a为0.75％～1.5％。
随着导电剂的含量的提高，有利于提高正极片的电子导通能力，但是过高的导电剂添加对于正极片的电子导通能力提高的影响较小，同时会导致能量密度的下降，通过对于导电剂的添加量限制，综合上述关系式的限定，能够在保证较低的导电剂添加量的前提下提高导电剂之间的接触面积，降低正极片的阻抗，同时避免添加过多导电剂导致的电池能量密度减低的问题。
在一些实施例中，所述正极材料层的电极曲折度τ为1.5～3.5。
在具体的实施例中，所述正极材料层的电极曲折度τ可以为1.5、1.7、1.9、2、2.1、2.3、2.4、2.5、2.7、2.9、3.0、3.1、3.2、3.3、3.4或3.5。
在一些实施例中，所述正极材料层的电极曲折度τ为2.5～3.2。
电极曲折度τ代表了多孔电极传输路径的弯曲程度。具体的，电极曲折度τ可通过极片FIB-SEM的三维重构仿真拟合测试得到，当所述正极材料层的电极曲折度τ处于上述范围时，有利于提高锂离子在正极材料层中的嵌入和脱出效率，以及所述正极材料层中电解液的浸润和扩散，同时保证正极活性材料之间具有合适的距离，提高电解液的扩散效率同时减少对于电子传导的影响。
在一些实施例中，所述正极材料层的孔隙率ε为10％～40％。
在具体的实施例中，所述正极材料层的孔隙率ε可以为10％、12％、13％、16％、19％、20％、23％、26％、29％、30％、33％、36％、39％或40％。
在一些实施例中，所述正极材料层的孔隙率ε为15％～30％。
当所述正极材料层的孔隙率ε处于上述范围中时，能够使所述正极材料层具有较高的保液量，保证电解液对于正极材料层的充分渗透，减小离子传导阻抗。
同时，通过试验可知，所述正极材料层的孔隙率ε与所述正极材料层的电极曲折度τ共同影响电解液在正极材料层中的扩散阻抗，τ/ε是电解液扩散的主要影响因素，τ/ε值越大代表液相扩散阻抗越大，需要添加更多的导电剂炭黑，以提高正极材料层中的保液量，以抵消液相扩散阻抗的影响；τ/ε值越小，炭黑添加的比例可以相应减少。因此，可根据电极材料的结构对炭黑导电剂的 添加比例进行调控，从而能够优化正极材料层的液相扩散阻抗，提高电极的动力学性能。
在一些实施例中，所述炭黑的平均粒径为20-100nm。
在具体的实施例中，所述炭黑的平均粒径可以为20nm、30nm、40nm、50nm、60nm、70nm、80nm、90nm或100nm。
在一些实施例中，所述导电剂还包括碳纳米管和石墨烯中的一种或多种。
在一些实施例中，所述碳纳米管的平均直径为2-60nm，平均长度为1-15μm；所述石墨烯的平均厚度为1-100nm，平均长度为0.2-20μm，平均宽度为0.2-20μm。
具体的，所述碳纳米管的平均直径可以为2nm、5nm、9nm、10nm、15nm、20nm、25nm、30nm、35nm、40nm、45nm、50nm、55nm或60nm。所述碳纳米管的平均长度可以为1μm、2μm、2.5μm、3.5μm、4.5μm、5.0μm、6.5μm、7.0μm、8.5μm、9.0μm、10.5μm、12μm、14.5μm或15μm。
所述石墨烯的平均厚度可以为1nm、2nm、5nm、9nm、10nm、15nm、20nm、25nm、30nm、35nm、40nm、45nm、50nm、55nm、60nm、70nm、75nm、80nm、85nm、90nm或100nm，所述石墨烯的平均长度可以为0.2μm、1μm、2μm、2.5μm、3.5μm、4.5μm、5.0μm、6.5μm、7.0μm、8.5μm、9.0μm、10.5μm、12μm、14.5μm、15μm或20μm，所述石墨烯的平均宽度为可以为0.2μm、1μm、2μm、2.5μm、3.5μm、4.5μm、5.0μm、6.5μm、7.0μm、8.5μm、9.0μm、10.5μm、12μm、14.5μm、15μm或20μm。
所述导电剂的形态与其在正极材料层之间相互搭接的面积以及导电剂与正极活性材料之间的接触面积相关，通过限制所述碳纳米管、所述炭黑和所述导电石墨的形态参数，有利于在导电剂之间，以及导电剂和正极活性材料之间构建合理的导电网络，减少正极片的内部接触电阻。
在一些实施例中，所述正极材料层为单层结构。
在另一些实施例中，所述正极材料层包括多层正极涂层，所述炭黑在所述导电剂中的重量百分比x为多层正极涂层中炭黑质量总和与导电剂质量总和的百分比。
在一些实施例中，所述正极片还包括有集流体，所述正极材料层设置于所述集流体上，沿远离所述集流体的方向，多层所述正极涂层的炭黑含量逐层增大，多层所述正极涂层的其他添加剂含量逐层降低。
对于多次正极涂层的结构，各层中的导电剂选用炭黑与其他导电剂复合的导电剂体系，其中炭黑具有高的保液能力，有利于锂离子在电极中的扩散过程，通过在沿远离所述集流体的方向逐层增大炭黑的含量，可以提高电解液对于正极材料层的浸润效果，而作为其他导电剂的碳纳米管与石墨烯具有高的电子导电率，需要更多地设置于正极材料层靠近集流体侧，增强与集流体的结合，炭黑与其他导电剂的分层布置有利于在多层涂布体系中构建较好的导电网络。
在一些实施例中，所述集流体选自可传导电子的金属材料，在一些实施例中，所述集流体包括Al、Ni、锡、铜、不锈钢的一种或多种；在一些实施例中，所述集流体选自铝箔、涂炭铝箔或打孔铝箔。
在一些实施例中，所述正极材料层还包括有正极粘结剂，所述正极活性材料、所述正极粘结剂和所述导电剂共混得到所述正极材料层。
在一些实施例中，所述正极粘结剂包括聚偏氟乙烯、偏氟乙烯的共聚物、聚四氟乙烯、偏氟乙烯-六氟丙烯的共聚物、四氟乙烯-六氟丙烯的共聚物、四氟乙烯-全氟烷基乙烯基醚的共聚物、乙烯-四氟乙烯的共聚物、偏氟乙烯-四氟乙烯的共聚物、偏氟乙烯-三氟乙烯的共聚物、偏氟乙烯-三氯乙烯的共聚物、偏氟乙烯-氟代乙烯的共聚物、偏氟乙烯-六氟丙烯-四氟乙烯的共聚物、热塑性聚酰亚胺、聚乙烯及聚丙烯等热塑性树脂；丙烯酸类树脂；以及苯乙烯丁二烯橡胶中的一种或多种。
在一些实施例中，以所述正极材料层的总质量为100％计，所述正极活性材料的含量为90％～99％，所述正极粘结剂的含量为0.5％～5％。
本申请的另一实施例提供了一种锂离子电池，包括如上所述的正极片。
在一些实施例中，所述锂离子电池还包括负极片、隔膜和非水电解液。
在一些实施例中，所述隔膜包括单层PP(聚丙烯)、单层PE(聚乙烯)、双层PP/PE、双层PP/PP和三层PP/PE/PP隔膜。
在一些实施例中，所述非水电解液包括溶剂和锂盐。
所述溶剂包括包括醚类溶剂、腈类溶剂、碳酸酯类溶剂和羧酸酯类溶剂中的一种或多种。
所述锂盐包括LiPF6、LiPO2F2、LiBF4、LiSbF6、LiAsF6、LiN(SO2CF3)2、LiN(SO2C2F5)2、LiC(SO2CF3)3、LiN(SO2F)2中的一种或多种。
以下通过实施例对本申请进行进一步的说明。
表1
实施例1
本实施例用于说明本申请公开的正极片、锂离子电池及其制备方法，包括 以下操作步骤：
将磷酸铁锂，粘结剂，氮甲基吡咯烷酮和导电剂混合制备成浆料，将上述浆料涂覆在铝箔上，涂布方式为单层涂布，其中磷酸铁锂添加量为100份，导电剂的总添加量为1.5份，其中炭黑添加量为0.65份，除炭黑以外还添加有碳纳米管，添加量为0.85份，其中粗管CNT5％含量为0.65份，细管CNT4.3％含量为0.2份，炭黑在所述导电剂中的重量百分比x为43％，以所述正极活性材料为100％计所述导电剂的重量百分比a为1.5％，经过烘烤、辊压制备得到正极片，将正极片与对应的负极片组装成电池，电池设计容量为1.8Ah。压汞法实测孔隙率为22.1％，通过FIB-SEM仿真拟合实测电极曲折度为2.8。
压汞法的测试条件为：压力范围：0.10～61000psia；接触角：130°；测试温度：14～15℃；
FIB-SEM仿真拟合的测试过程包括原始图片优化、材料分割、重构(输入分割参数：孔隙0-100；其他固体材料：100-140；磷酸铁锂：140-254)、软件模拟结果。切割范围：10μm×10μm×10μm；切割厚度：10nm。
实施例2
实施例2用于说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为双层涂布，上层为小颗粒磷酸铁锂层，其中磷酸铁锂添加量为100份，上层导电剂的添加量为0.8份，其中炭黑添加量为0.5份，除炭黑以外还添加有碳纳米管，其添加量为0.3份，该碳纳米管为细管CNT4.3％；下层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂的添加量为100份，下层导电剂的添加量为0.8份，炭黑添加量为0.5份，除炭黑以外还添加有碳纳米管，其添加量为0.3份，该碳纳米管为细管CNT4.3％，炭黑在所述导电剂中的重量百分比x为62％，以所述正极活性材料为100％计所述导电剂的重量百分比a为0.8％，通过压汞法实测孔隙率为19.6％，FIB-SEM仿真拟合实测电极曲折度为2.86。
实施例3
实施例3用于说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为双层涂布，上层为大颗粒磷酸铁锂层，其中磷酸铁锂添加量为100份，上层导电剂的添加量为0.95份，炭黑添加量为0.7份，除炭黑以外还添加有碳纳米管，其添加量为0.25份，该碳纳米管为细管CNT4.3％；下层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂的添加量为100份，下层导电剂的添加量为0.87份，炭黑添加量为0.57份，除炭黑以外还添加有碳纳米管，其添加量为0.3份，该碳纳米管为细管CNT4.3％，炭黑在所述导电剂中的重量百分比x为70％，以所述正极活性材料为100％计所述导电剂的重量百分比a为0.91％，通过压汞法实测孔隙率为23.1％，FIB-SEM仿真拟合实测电极曲折度为2.75。
实施例4
实施例4用于说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为双层涂布，上层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂添加量为100份，上层导电剂的添加量为0.77份，炭黑添加量为0.5份，除炭黑以外还添加有碳纳米管，其添加量0.27份，该碳纳米管为粗管CNT5％与细管CNT4.3％混合搭配，粗管CNT5％添加量为0.1份，细管CNT4.3％添加量为0.17份；下层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂的添加量为100份，下层导电剂的添加量为0.81份，炭黑添加量为0.5份，除炭黑以外还添加有碳纳米管，其添加量为6.98份，该碳纳米管为粗管CNT5％与细管CNT4.3％混合搭配，粗管CNT5％添加量为0.1份，细管CNT4.3％添加量为0.21份，炭黑在所述导电剂中的重量百分比x为63％，以所述正极活性材料为100％计所述导电剂的重量百分比a为0.79％，通过压汞法实测孔隙率为20.4％，FIB-SEM仿真拟合实测电极曲折度为2.88。
实施例5～30
实施例5～30用于说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
导电剂中炭黑的重量百分比x、以磷酸铁锂为100％计导电剂的重量百分比a、正极材料层的电极曲折度τ和正极材料层的孔隙率ε如表1中实施例5～30所示。
对比例1
对比例1用于对比说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为单层涂布，磷酸铁锂添加量为100份，导电剂总添加量为1.7份，除炭黑以外还添加有碳纳米管，其添加量为1.275份，其中粗管CNT5％含量为0.2份，细管CNT4.3％含量为1.075份，炭黑添加量为0.425份，炭黑在所述导电剂中的重量百分比x为25％，以所述正极活性材料为100％计所述导电剂的重量百分比a为1.7％，压汞法实测孔隙率为19.81％，通过FIB-SEM仿真拟合实测电极曲折度为3.1。
对比例2
对比例2用于对比说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为双层涂布，上层为小颗粒磷酸铁锂层，其中磷酸铁锂添加量为100份，导电剂添加量为1.6份，除炭黑以外还添加有碳纳米管，其添加量1.3份，该碳纳米管为细管CNT4.3％，炭黑添加量为0.3份；下层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂的添加量为100份，导电剂添加量为1.6份，除炭黑以外还添加有碳纳米管，碳纳米管添加量为1.3份，该碳管为细管CNT4.3％，炭黑添加量为0.3份，炭黑在所述导电剂中的重量百分比x为19％，以所述正极活性材料为100％计所述导电剂的重量百分比a为1.6％，通过压汞法实测孔隙率为21.0％，FIB-SEM仿真拟合实测电极曲折度为2.98。
对比例3
对比例3用于对比说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
正极涂布方式为双层涂布，上层为大颗粒磷酸铁锂层，其中磷酸铁锂添加量为100份，导电剂添加量为4.25份，除炭黑以外还添加有碳纳米管，其添加量1.5份，该碳纳米管为细管CNT4.3％，炭黑添加量为2.75份；下层为大、小颗粒搭配的磷酸铁锂混合电极层，其中磷酸铁锂的添加量为100份，导电剂添加量为4.0份，除炭黑以外还添加有碳纳米管，碳纳米管添加量为0.95份，该 碳管为细管CNT4.3％，炭黑添加量为3.05份，炭黑在所述导电剂中的重量百分比x为70％，以所述正极活性材料为100％计所述导电剂的重量百分比a为4.13％，通过压汞法实测孔隙率为26.7％，FIB-SEM仿真拟合实测电极曲折度为2.5。
对比例4～6
对比例4～6用于说明本申请公开的正极片、锂离子电池及其制备方法，包括实施例1中大部分操作步骤，其不同之处在于：
导电剂中炭黑的重量百分比x、以磷酸铁锂为100％计导电剂的重量百分比a、正极材料层的电极曲折度τ和正极材料层的孔隙率ε如表1中对比例4～6所示。
性能测试
对上述制备得到的正极片进行如下性能测试：
1、极片纵向电阻率测试：
采用BER1300极片电阻仪，实验面积：153.94mm2；采样数目：10；保压时间：30s；压强：25MPa
1)测试前对极片进行110℃真空干燥1h
2)擦探针，设备进行校准复位
3)将极片置于载样台上，盖上上盖板，将载样台置于测试腔中，移动载样台使测试孔卡位至下端子，关上防护门
4)每张待测极片取10个点，点击开始试验，输入箔材厚度，保存路径，开始测试。
2、直流内阻测试：
1)先进行RPTs测试：
RPTs测试方法(常温进行测试)
标准充电：常温25±5℃下，0.33C恒流恒压充到3.8V，截止电流0.05C；搁置10min。
标准放电：常温25±5℃下，0.33C恒流放到2.0V；搁置10min。
循环3次，取第3次放电容量为电池放电容量C0；
2)将全电池按C0容量调50％SOC
3)室温下静置2h，取静置结束后电压E1，1.5C恒流放电30s，取放电结束后电压E2。其中，直流内阻DCIR*C＝(E1-E2)/1.5。
3、液相扩散阻抗测试：
主要包括以下步骤：
1)组装对称电池
2)EIS测试，测试参数：频率范围：100000-0.05Hz；振幅：5mV；温度：25℃；每10倍频率范围取点10个
3)通过软件拟合出扩散部分的斜率和截距，通过Zview软件拟合得到Rs值，等效电路图如图1所示：
代入公式(1)(2)

计算得出液相扩散阻抗Rion。
得到的测试结果填入表2。
表2

表2中对制备的正极片进行了纵向电阻率、50％SOC下直流内阻以及液相扩散阻抗的测试，从测试结果可以看出，根据本申请提供的导电网络设计规则制备出来的正极片都具有相对较低的电阻率、直流内阻以及液相扩散阻抗，说明通过多元导电剂中炭黑的添加比例与正极材料层的结构参数相匹配，能够同时兼顾正极片的电子传导性能和离子传导性能，实现液相扩散阻抗的最优值，有利于在不同磷酸铁锂正极结构参数条件下构建有效的导电网络。
以上所述仅为本申请的较佳实施例而已，并不用以限制本申请，凡在本申请的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本申请的保护范围之内。

Claims (17)

  1. 一种正极片，其特征在于，包括：
    正极材料层，所述正极材料层包括：
    正极活性材料，选自磷酸铁锂类材料；和
    导电剂，所述导电剂包括炭黑；
    所述正极材料层满足以下条件：
    3.2％≤100*x*a/(τ/ε)≤30.0％
    其中，x为所述炭黑在所述导电剂中的重量百分比；
    a为以所述正极活性材料为100％计所述导电剂的重量百分比；
    τ为所述正极材料层的电极曲折度；
    ε为所述正极材料层的孔隙率。
  2. 根据权利要求1所述的正极片，其特征在于，所述正极材料层满足以下条件：
    3.5％≤100*x*a/(τ/ε)≤15％。
  3. 根据权利要求1或2所述的正极片，其特征在于，所述炭黑在所述导电剂中的重量百分比x为30％～100％。
  4. 根据权利要求3所述的正极片，其特征在于，所述炭黑在所述导电剂中的重量百分比x为40％～70％。
  5. 根据权利要求1～4任意一项所述的正极片，其特征在于，以所述正极活性材料为100％计导电剂的重量百分比a为0.5％～2.5％。
  6. 根据权利要求5所述的正极片，其特征在于，以所述正极活性材料为100％计导电剂的重量百分比a为0.75％～1.5％。
  7. 根据权利要求1～6任意一项所述的正极片，其特征在于，所述正极材料层的电极曲折度τ为1.5～3.5。
  8. 根据权利要求7所述的正极片，其特征在于，所述正极材料层的电极曲折度τ为2.5～3.2。
  9. 根据权利要求1～8任意一项所述的正极片，其特征在于，所述正极材料层的孔隙率ε为10％～40％。
  10. 根据权利要求1～9任意一项所述的正极片，其特征在于，所述正极材料层的孔隙率ε为15％～30％。
  11. 根据权利要求1～10任意一项所述的正极片，其特征在于，所述炭黑的平均粒径为20-100nm。
  12. 根据权利要求1～11任意一项所述的正极片，其特征在于，所述导电剂还包括碳纳米管和石墨烯中的一种或多种。
  13. 根据权利要求12所述的正极片，其特征在于，所述碳纳米管的平均直径为2-60nm，平均长度为1-15μm；所述石墨烯的平均厚度为1-100nm，平均长度为0.2-20μm，平均宽度为0.2-20μm。
  14. 根据权利要求1～13任意一项所述的正极片，其特征在于，所述正极材料层为单层结构。
  15. 根据权利要求1～13任意一项所述的正极片，其特征在于，所述正极材料层包括多层正极涂层，所述炭黑在所述导电剂中的重量百分比x为多层正极涂层中炭黑质量总和与导电剂质量总和的百分比。
  16. 根据权利要求15所述的正极片，其特征在于，所述正极片还包括有集流体，所述正极材料层设置于所述集流体上，沿远离所述集流体的方向，多层所述正极涂层的炭黑含量逐层增大，多层所述正极涂层的其他添加剂含量逐层降低。
  17. 一种锂离子电池，其特征在于，包括如权利要求1～16任意一项所述的 正极片。
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