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WO2023045369A1 - 正极极片、二次电池、电池模块、电池包和用电装置 - Google Patents

正极极片、二次电池、电池模块、电池包和用电装置 Download PDF

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WO2023045369A1
WO2023045369A1 PCT/CN2022/094239 CN2022094239W WO2023045369A1 WO 2023045369 A1 WO2023045369 A1 WO 2023045369A1 CN 2022094239 W CN2022094239 W CN 2022094239W WO 2023045369 A1 WO2023045369 A1 WO 2023045369A1
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Prior art keywords
coating
positive electrode
electrode sheet
battery
secondary battery
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PCT/CN2022/094239
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English (en)
French (fr)
Inventor
吴燕英
王星会
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宁德时代新能源科技股份有限公司
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Publication of WO2023045369A1 publication Critical patent/WO2023045369A1/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
    • 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
    • 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
    • H01M4/04—Processes of manufacture in general
    • 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
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • 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 relates to the technical field of lithium batteries, in particular to a positive pole piece, a secondary battery, a battery module, a battery pack and an electrical device.
  • lithium-ion secondary batteries have been widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric Automotive, military equipment, aerospace and other fields. Due to the great development of lithium-ion secondary batteries, higher requirements have been put forward for their energy density, cycle performance and safety performance. In addition, due to the bulk phase doping and surface coating of positive active materials, the energy density modification effect of lithium ion secondary batteries is limited. Therefore, the industry is currently focusing on optimizing the structural design of lithium-ion secondary batteries. For example, in the case of a fixed cell size and material chemical system, if the positive electrode sheet of the lithium-ion secondary battery has a higher coating weight, the proportion of active materials can be significantly increased, thereby increasing the energy density of the cell.
  • the present application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode sheet, which can improve the rate performance, high and low temperature performance, and safety performance of a secondary battery using the positive electrode sheet, while improving its kinetic performance .
  • the present application provides a positive pole piece, a secondary battery, a battery module, a battery pack and an electrical device.
  • the first aspect of the present application provides a positive electrode sheet, including a current collector and a coating coated on at least one surface of the current collector, the coating includes a first coating and a second coating, and the first coating is coated On the surface of the current collector, the second coating is coated on the surface of the first coating, wherein the tortuosity of the first coating is 3.01-4.93, and the tortuosity of the second coating is 2.11-2.87.
  • this application comprehensively considers the processing convenience of the positive electrode sheet, the electrochemical performance and energy density of the secondary battery, and realizes the positive electrode sheet by adjusting the tortuosity of the first coating and the second coating of the positive electrode sheet. Gradient regulation of porosity. Although the mechanism is still unclear, it is speculated that the tortuosity of the positive electrode sheet is distributed in a gradient, which improves the wettability of the positive electrode sheet to the electrolyte, shortens the drying time of the positive electrode sheet, and improves the battery life of the lithium-ion secondary battery. Chemical properties and energy density.
  • the coating amount CW of the positive electrode sheet is greater than or equal to 400 mg/1540.25 cm 2 . Therefore, the coating amount CW of the positive electrode sheet can effectively increase the loading capacity of the positive electrode active material and ensure the energy density of the lithium ion secondary battery.
  • the mixing ratio of the primary particles is 10-50%; the ratio of Dv50 of the secondary particles to Dv50 of the primary particles is 1.5-10. Therefore, when the primary particles are within the above blending ratio range, the tortuosity of the positive electrode sheet can be effectively controlled, and the porosity distribution state of the positive electrode sheet can be improved.
  • the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is within the above range, the breakage of the secondary particles during the cold pressing process can be effectively improved, thereby improving the difficulty in cold pressing of the positive pole piece.
  • the thickness of the first coating is 20-140 ⁇ m
  • the thickness of the second coating is 20-140 ⁇ m. Therefore, when the thicknesses of the first coating layer and the second coating layer are within the above range, the positive electrode sheet has better processability, which is convenient for subsequent winding into a battery cell and assembling into a secondary battery.
  • the compacted density of the first coating is greater than the compacted density of the second coating.
  • the compaction of the first coating is 2.5-2.8 g/cc
  • the compaction of the second coating is 2.1-2.5 g/cc. Therefore, when the compacted density of the first coating is within the above-mentioned range, the first coating can support more positive electrode active materials, and the positive electrode active material particles can be fully contacted.
  • the compaction density of the second coating is within the above range, the electrolyte can be fully immersed in it, which improves the wettability of the positive electrode piece to the electrolyte.
  • the porosity of the first coating is 16-20%; the porosity of the second coating is 26-30%.
  • the positive electrode active material is LiNi x Co y Mnz Fe a Al b P c O 2 (wherein, 0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0 ⁇ z ⁇ 1, 0 ⁇ a ⁇ 1 , 0 ⁇ b ⁇ 0.8, 0 ⁇ c ⁇ 4). Therefore, by selecting a positive electrode active material with high gram capacity and good cycle performance, the lithium ion secondary battery can have higher energy density and better electrochemical performance.
  • a second aspect of the present application provides a secondary battery, including the positive electrode sheet of the first aspect of the present application.
  • a third aspect of the present application provides a battery module including the secondary battery of the second aspect of the present application.
  • a fourth aspect of the present application provides a battery pack, including the battery module of the third aspect of the present application.
  • the fifth aspect of the present application provides an electric device, including at least one selected from the secondary battery of the second aspect of the present application, the battery module of the third aspect of the present application, or the battery pack of the fourth aspect of the present application. kind.
  • the application provides a positive electrode sheet, a secondary battery, a battery module, a battery pack, and an electrical device.
  • the tortuosity of the first coating and the second coating of the positive pole piece By adjusting the tortuosity of the first coating and the second coating of the positive pole piece, so that the tortuosity of the first coating is greater than that of the second coating, the tortuosity of the positive pole piece can be distributed in a gradient, and then the tortuosity of the positive pole piece can be guaranteed.
  • the positive pole piece has a higher coating amount, the wettability of the positive pole piece to the electrolyte is improved, the drying time of the positive pole piece is shortened, and the electrochemical performance and power of the lithium-ion secondary battery are improved. academic performance.
  • Fig. 1 is a schematic diagram of the tortuosity of the positive electrode sheet of the present application.
  • Fig. 2 is a scanning electron microscope image of Example 2 of the present application.
  • FIG. 3 is a schematic diagram of a secondary battery according to an embodiment of the present application.
  • FIG. 4 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG. 3 .
  • FIG. 5 is a schematic diagram of a battery module according to an embodiment of the present application.
  • FIG. 6 is a schematic diagram of a battery pack according to an embodiment of the present application.
  • FIG. 7 is an exploded view of the battery pack according to one embodiment of the present application shown in FIG. 6 .
  • FIG. 8 is a schematic diagram of an electrical device in which a secondary battery is used as a power source according to an embodiment of the present application.
  • ranges disclosed herein are defined in terms of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit that define the boundaries of the particular range. Ranges defined in this manner may be inclusive or exclusive and may be combined arbitrarily, ie any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are contemplated. Additionally, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, the following ranges are all expected: 1-3, 1-4, 1-5, 2- 3, 2-4 and 2-5.
  • the numerical range "a-b” represents an abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers.
  • the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article, and "0-5" is only an abbreviated representation of the combination of these values.
  • a certain parameter is an integer ⁇ 2
  • a method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed in sequence, or may include steps (b) and (a) performed in sequence.
  • the method can also include step (c), means that step (c) can be added to the method in any order, for example, the method can include steps (a), (b) and (c), and can also include the step (a), (c) and (b), may also include steps (c), (a) and (b) and the like.
  • the term "or” is inclusive unless otherwise stated.
  • the phrase "A or B” means “A, B, or both A and B.” More specifically, the condition "A or B” is satisfied by either of the following: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists) ; or both A and B are true (or exist).
  • the present application proposes a positive electrode sheet, including a current collector and a coating coated on at least one surface of the current collector, the coating includes a first coating and a second coating, the first The coating is coated on the surface of the current collector, and the second coating is coated on the surface of the first coating, wherein the tortuosity of the first coating is 3.01-4.93, and the tortuosity of the second coating is 2.11-2.87.
  • the tortuosity is the ratio of the actual passage length L t of the substance in the porous medium to the distance (thickness) L 0 of the medium.
  • L t is the actual path length of lithium ions passing through the positive electrode active material particles
  • L is the thickness of the coating layer of the positive electrode sheet.
  • the positive pole piece will reduce the energy density of the lithium-ion secondary battery to a certain extent.
  • the present application arranges the first coating and the second coating on the surface of the current collector, and adopts corresponding control means (such as the mixing of primary particles and secondary particles, different cold compaction densities) to make the tortuousness of the first coating
  • the degree of tortuosity is greater than that of the second coating, so that the positive electrode sheet of the present application has both high active material loading capacity and excellent kinetic performance: the first coating has a larger tortuosity, which ensures that the positive electrode sheet has a higher
  • the loading capacity of the positive electrode active material significantly improves the energy density of the lithium-ion secondary battery; at the same time, the second coating has a small tortuosity, so that the electrolyte can fully infiltrate the positive electrode sheet, which is conducive to the diffusion of lithium ions in the positive electrode sheet , ensuring that the lithium-ion secondary battery has better kinetic performance, thereby improving its cycle performance and other electrochemical performance.
  • the coating amount CW of the positive electrode sheet is greater than or equal to 400 mg/1540.25 cm 2 .
  • FIG. 2 is a scanning electron microscope image (SEM) of the cross section of the positive electrode of the present application.
  • the primary particle size of the positive active material in the first coating is obviously smaller than the secondary particle size of the positive active material in the second coating.
  • the average particle size of primary particles in secondary particles refers to the average size of all primary particle sizes in the scanning electron microscope image at 10K magnification. Therefore, when the primary particles and the secondary particles are mixed, and the Dv50 and Dv90 of the primary particles and the secondary particles are within the above range, it can not only prevent the secondary particles from being broken during the cold pressing process, but also solve the problem of cold pressing of the positive pole piece.
  • the tortuosity of the coating can be adjusted, so that the tortuosity of the first coating is greater than that of the second coating, so that the positive electrode sheet has both high positive active material loading and excellent wettability.
  • the mixing ratio of the primary particles is 10-50%, and the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is 1.5-10. Therefore, when the mixing ratio of the primary particles is within the above range, the cold pressing performance of the positive electrode sheet can be significantly improved. When the ratio of the Dv50 of the secondary particles to the Dv50 of the primary particles is within the above range, the breakage of the secondary particles during the cold pressing process can be effectively improved, thereby improving the difficulty in cold pressing of the positive pole piece.
  • the thickness of the first coating is 20-140 ⁇ m
  • the thickness of the second coating is 20-140 ⁇ m. Therefore, when the thicknesses of the first coating layer and the second coating layer are within the above-mentioned range, the positive electrode sheet has good processability, which is convenient for subsequent winding into a battery cell and assembling into a secondary battery.
  • the compacted density of the first coating is greater than the compacted density of the second coating.
  • the compaction density of the first coating greater than that of the second coating, the regulation of the porosity and tortuosity of the positive pole piece is realized, thereby improving the impact of the positive pole piece on the electrolyte. At the same time, it ensures that the positive electrode sheet is loaded with high positive electrode active material.
  • the compaction of the first coating is 2.5-2.8 g/cc
  • the compaction of the second coating is 2.1-2.5 g/cc.
  • the compaction density of the first coating and the second coating is within the above range, the tortuosity of the first coating is high, the positive electrode active material content is high, and the particles are in close contact, and the tortuosity of the second coating is small,
  • the electrolyte can fully infiltrate the positive electrode sheet, thereby significantly improving the dynamic performance of the positive electrode sheet.
  • the porosity of the first coating is 16-20%; the porosity of the second coating is 26-30%.
  • the performance of the positive electrode sheet has both high active material coating amount and excellent electrolyte wettability. Therefore, not only the energy density of the lithium-ion secondary battery is significantly improved, but also the diffusion ability of lithium ions in the positive electrode sheet is improved, thereby improving the kinetic performance of the lithium-ion secondary battery.
  • the positive electrode active material is LiNi x Co y Mnz Fe a Al b P c O 2 (wherein, 0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, 0 ⁇ z ⁇ 1, 0 ⁇ a ⁇ 1 , 0 ⁇ b ⁇ 0.8, 0 ⁇ c ⁇ 4).
  • the performance of the positive electrode sheet can better fit the characteristics of the electrochemical system, thereby meeting the performance requirements of the lithium-ion secondary battery.
  • the average volume distribution particle diameter Dv50 refers to the particle diameter corresponding to when the cumulative volume distribution percentage of the positive electrode active material reaches 50%.
  • the average volume distribution particle diameter Dv90 refers to the particle diameter corresponding to when the cumulative volume distribution percentage of the positive electrode active material reaches 90%.
  • the volume average particle diameter Dv50 of the positive electrode active material can be measured by laser diffraction particle size analysis. For example, with reference to the standard GB/T 19077-2016, use a laser particle size analyzer (such as Malvern Master Size 3000) to measure.
  • Liquid absorption rate (2mm-residual height) * capillary cross-sectional area * 1.0g/cm3 (electrolyte density) / time
  • the tortuosity of the positive pole piece can be tested by mercury porosimetry, for example, referring to the standard GB/T 21650.1-2008, using a pore size analyzer (such as poremaster60GT).
  • the tortuosity is calculated according to the following formula:
  • V Mercury volume
  • ⁇ Hg Mercury density
  • S Total area of positive pole piece
  • ⁇ V i Pore volume change
  • D i Average pore diameter
  • E Pore index.
  • a secondary battery is provided.
  • a secondary battery typically includes a positive pole piece, a negative pole piece, an electrolyte, and a separator.
  • active ions are intercalated and extracted back and forth between the positive electrode and the negative electrode.
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the separator is arranged between the positive pole piece and the negative pole piece, which mainly plays a role in preventing the short circuit of the positive and negative poles, and at the same time allows ions to pass through.
  • the positive pole piece is the positive pole piece according to the first aspect of the present application.
  • the negative electrode sheet includes a negative electrode collector and a negative electrode film layer arranged on at least one surface of the negative electrode collector, and the negative electrode film layer includes a negative electrode active material.
  • the negative electrode current collector has two opposing surfaces in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposing surfaces of the negative electrode current collector.
  • a metal foil or a composite current collector can be used as the negative electrode current collector.
  • copper foil can be used as the metal foil.
  • the composite current collector may include a base layer of polymer material and a metal layer formed on at least one surface of the base material of polymer material.
  • Composite current collectors can be formed by metal materials (copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on polymer material substrates (such as polypropylene (PP), polyethylene terephthalic acid It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • negative electrode active materials known in the art for batteries can be used as the negative electrode active material.
  • the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material, lithium titanate, and the like.
  • the silicon-based material may be selected from at least one of elemental silicon, silicon-oxygen compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys.
  • the tin-based material can be selected from at least one of simple tin, tin oxide and tin alloy.
  • the present application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials of batteries can also be used. These negative electrode active materials may be used alone or in combination of two or more.
  • the negative electrode film layer may also optionally include a binder.
  • the binder can be selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethyl At least one of acrylic acid (PMAA) and carboxymethyl chitosan (CMCS).
  • the negative electrode film layer may also optionally include a conductive agent.
  • the conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.
  • the negative electrode film layer may optionally include other additives, such as thickeners (such as sodium carboxymethylcellulose (CMC-Na)) and the like.
  • thickeners such as sodium carboxymethylcellulose (CMC-Na)
  • the negative electrode sheet can be prepared in the following manner: the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binding agent and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.
  • a solvent such as deionized water
  • the electrolyte plays the role of conducting ions between the positive pole piece and the negative pole piece.
  • the present application has no specific limitation on the type of electrolyte, which can be selected according to requirements.
  • electrolytes can be liquid, gel or all solid.
  • an electrolytic solution is used as the electrolyte.
  • the electrolytic solution includes electrolyte salts and solvents.
  • the electrolyte salt can be selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonyl imide, At least one of lithium fluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium difluorooxalate borate, lithium difluorodifluorooxalatephosphate and lithium tetrafluorooxalatephosphate.
  • the solvent can be selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene propylene carbonate ester, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, butyl At least one of ethyl acetate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.
  • the electrolyte may optionally include additives.
  • additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain performances of the battery, such as additives that improve battery overcharge performance, additives that improve high-temperature or low-temperature performance of batteries, and the like.
  • the secondary battery further includes a separator.
  • the present application has no particular limitation on the type of the isolation membrane, and any known porous structure isolation membrane with good chemical stability and mechanical stability can be selected.
  • the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.
  • the separator can be a single-layer film or a multi-layer composite film, without any particular limitation.
  • the materials of each layer may be the same or different, and there is no particular limitation.
  • the positive pole piece, the negative pole piece and the separator can be made into an electrode assembly through a winding process or a lamination process.
  • the secondary battery may include an outer package.
  • the outer package can be used to package the above-mentioned electrode assembly and electrolyte.
  • the outer package of the secondary battery may be a hard case, such as a hard plastic case, an aluminum case, a steel case, and the like.
  • the outer packaging of the secondary battery may also be a soft bag, such as a bag-type soft bag.
  • the material of the soft case may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.
  • FIG. 3 shows a square-shaped secondary battery 5 as an example.
  • the outer package may include a housing 51 and a cover 53 .
  • the housing 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plates enclose to form an accommodating cavity.
  • the housing 51 has an opening communicating with the accommodating cavity, and the cover plate 53 can cover the opening to close the accommodating cavity.
  • the positive pole piece, the negative pole piece and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process.
  • the electrode assembly 52 is packaged in the containing cavity. Electrolyte is infiltrated in the electrode assembly 52 .
  • the number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.
  • the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.
  • FIG. 5 is a battery module 4 as an example.
  • a plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4 .
  • the plurality of secondary batteries 5 may be fixed by fasteners.
  • the battery module 4 may also include a case having a housing space in which a plurality of secondary batteries 5 are accommodated.
  • the above-mentioned battery modules can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.
  • the battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box.
  • the battery box includes an upper box body 2 and a lower box body 3.
  • the upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery module 4.
  • Multiple battery modules 4 can be arranged in the battery box in any manner.
  • the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application.
  • a secondary battery, a battery module, or a battery pack can be used as a power source of a power consumption device, and can also be used as an energy storage unit of the power consumption device.
  • Electric devices can include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but not limited thereto.
  • secondary batteries, battery modules or battery packs can be selected according to their usage requirements.
  • FIG. 8 is an example of an electrical device.
  • the electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.
  • a battery pack or a battery module may be used.
  • a device may be a cell phone, tablet, laptop, or the like.
  • the device is generally required to be light and thin, and a secondary battery can be used as a power source.
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, it is coated on an Al foil, dried, and cold-pressed to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, apply extrusion coating or transfer coating on Al foil, dry and cold press to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After being fully stirred and mixed evenly, it is coated on the first coating layer, dried and cold-pressed to obtain a positive electrode sheet.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, it is coated on an Al foil, dried, and cold-pressed to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After being fully stirred and mixed evenly, it is coated on the first coating layer, dried and cold-pressed to obtain a positive electrode sheet.
  • NMP N-methylpyrrolidone
  • the mixing ratio of the primary particle is 20%
  • the compacted density is: 2.2g/cc
  • the thickness of the coating layer is 91.5 ⁇ m
  • the coating weight CW 310mg/1540.25cm 2
  • the cold pressure is 30T.
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, apply extrusion coating or transfer coating on Al foil, dry and cold press to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, apply extrusion coating or transfer coating on Al foil, dry and cold press to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After being fully stirred and mixed evenly, it is coated on the first coating layer, dried and cold-pressed to obtain a positive electrode sheet.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, it is coated on an Al foil, dried, and cold-pressed to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the first coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully mixed in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1. After stirring and mixing evenly, apply extrusion coating or transfer coating on Al foil, dry and cold press to obtain the first coating.
  • NMP N-methylpyrrolidone
  • the positive electrode active material LiFePO 4 of the second coating, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are mixed in a N-methylpyrrolidone (NMP) solvent system in a weight ratio of 97:2:1.
  • NMP N-methylpyrrolidone
  • the mixing ratio of the primary particle is 60%
  • the compacted density is: 2.2g/cc
  • the thickness of the coating layer is 91.5 ⁇ m
  • the coating weight CW 310mg/1540.25cm 2
  • the cold pressure is 60T.
  • the positive pole piece is the positive pole piece in the above-mentioned embodiments and comparative examples.
  • a porous polymer film made of polyethylene (PE) is used as the separator.
  • PE polyethylene
  • the positive electrode sheet, the separator and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrodes to play the role of isolation, and the bare cell is obtained by winding. Put the bare cell in the outer package, inject the electrolyte and package it to obtain the secondary battery using the positive electrode sheet in each embodiment and comparative example.
  • Each of the above-prepared secondary batteries was left to stand for 30 minutes in a constant temperature environment of 25° C. respectively.
  • the 1C discharge capacity retention rate is (C 1 /C 0 )*100%.
  • 2C discharge to 2.5V 2C discharge to 2.0V, record the discharge capacity C 2 , and then stand still for 30 minutes.
  • the 2C discharge capacity retention rate is (C 2 /C 0 )*100%.
  • Performance test at 25°C Put each secondary battery prepared above in a high and low temperature box (model: SM-012PF, manufacturer: Guangdong Sanmu Technology Co., Ltd.) at 25°C, discharge at 1C to 2.5V, and discharge at 1C to 2.0V, then stand still for 5 minutes, charge to 3.65V according to 1C constant current, charge at constant voltage, the cut-off current is 0.05C, record the discharge capacity C 0 , then stand still for 30 minutes, discharge to 2.5V according to 1C, discharge at 1C to 2.0V, then stand still for 30 minutes, charge to 3.65V at a constant current of 1C, charge at a constant voltage with a cut-off current of 0.05C, and then stand still for 5 minutes.
  • a high and low temperature box model: SM-012PF, manufacturer: Guangdong Sanmu Technology Co., Ltd.
  • the discharge capacity retention rate at 25°C is (C 0 /C 0 )*100%.
  • Performance test at -25°C Adjust the temperature of the high and low temperature box to -25°C, let the secondary batteries prepared above stand for 120 minutes, discharge at 1C to 2.5V, and 1C to 2.0V, record the discharge capacity C 1 , Then let it sit for 5 minutes.
  • 60°C performance test adjust the high and low temperature box to 25°C, let the secondary batteries prepared above stand for 120 minutes, charge at a constant current of 1C to 3.65V, charge at a constant voltage with a cut-off current of 0.05C, and then stand for 5 minute. Adjust the temperature of the high and low temperature box to 60°C, let the secondary batteries prepared above stand for 120 minutes, discharge to 2.5V at 1C, and discharge to 2.0V at 1C, record the discharge capacity C 2 , and then let stand for 5 minutes.
  • the discharge capacity retention rate at 60°C is (C 2 /C 0 )*100%.
  • Each secondary battery prepared above was placed in a constant temperature environment of 25°C for 30 minutes, discharged at a constant current of 0.33C to 2.5V, left for 30 minutes, charged at a constant current of 0.33C to 3.65V, and then constant voltage Charging, cut-off current 0.05C, and then stand for 30 minutes.
  • Discharge at 0.33C to 2.5V record the discharge capacity C 0 , let it stand for 30 minutes, charge at a constant current of 0.33C to 3.65 V, then charge at a constant voltage with a cut-off current of 0.05C, and then let it stand for 5 minutes.
  • DCR resistance (U 1 ⁇ U 2 )/I.
  • the first coating adopts small particles of positive electrode active material, and the high cold compaction density makes the tortuosity of the first coating higher than that of the second coating.
  • the density makes the porosity of the positive electrode sheet distributed in a gradient, so it has achieved good results in the rate performance, high and low temperature performance, and the wettability of the electrolyte of the secondary battery.
  • the kinetic performance of the lithium ion secondary battery is also improved.
  • Example 2 and Example 3 have smaller diffusion resistance R f , their kinetic performance, high and low temperature performance, and rate performance are all improved.
  • R f diffusion resistance
  • the present application is not limited to the above-mentioned embodiments.
  • the above-mentioned embodiments are merely examples, and within the scope of the technical solutions of the present application, embodiments that have substantially the same configuration as the technical idea and exert the same effects are included in the technical scope of the present application.
  • various modifications conceivable by those skilled in the art are added to the embodiments, and other forms constructed by combining some components in the embodiments are also included in the scope of the present application. .

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Abstract

提供了一种正极极片，包括涂覆层和集流体，涂覆层位于集流体的至少一个表面，涂覆层包括第一涂层和第二涂层，第一涂层涂覆于集流体表面上，第二涂层涂覆于第一涂层表面，其中，第一涂层迂曲度为3.01～4.93，第二涂层迂曲度为2.11～2.87。

Description

正极极片、二次电池、电池模块、电池包和用电装置
相关申请的交叉引用
本申请要求享有于2021年09月23日提交的名称为“正极极片、二次电池、电池模块、电池包和用电装置”的中国专利申请202111117391.1的优先权，该申请的全部内容通过引用并入本文中。
技术领域
本申请涉及锂电池技术领域，尤其涉及一种正极极片、二次电池、电池模块、电池包和用电装置。
背景技术
近年来，随着锂离子电池的应用范围越来越广泛，锂离子二次电池广泛应用于水力、火力、风力和太阳能电站等储能电源系统，以及电动工具、电动自行车、电动摩托车、电动汽车、军事装备、航空航天等多个领域。由于锂离子二次电池取得了极大的发展，因此对其能量密度、循环性能和安全性能等也提出了更高的要求。另外，由于对正极活性物质的体相掺杂和表面包覆对于锂离子二次电池的能量密度改性效果有限。因此，业界目前着眼于优化锂离子二次电池的结构设计。例如，在固定电芯尺寸与材料化学体系的情况下，若锂离子二次电池的正极极片具有较高的涂敷重量，可以显著提高活性物质占比，进而提高电芯的能量密度。
但是随着正极极片的涂布量不断上升，锂离子二次电池的极片烘干困难，因此，生产锂离子二次电池时，烘烤时间和浸润时间也会相应增 长。同时电解液也难以充分浸润极片，锂离子二次电池的动力学性能也相应变差，无法满足对锂离子二次电池在性能上的要求。因此，现有的针对正极极片的设计仍有待改进。
发明内容
本申请是鉴于上述课题而进行的，其目的在于，提供一种正极极片，可以提高使用该正极极片的二次电池的倍率性能、高低温性能、安全性能的同时，改善其动力学性能。
为了达到上述目的，本申请提供了一种正极极片、二次电池、电池模块、电池包和用电装置。
本申请的第一方面提供了一种正极极片，包括集流体以及涂覆于集流体的至少一个面的涂层，涂层包括第一涂层和第二涂层，第一涂层涂覆于集流体的表面，第二涂层涂覆于第一涂层的表面，其中，第一涂层迂曲度为3.01～4.93，第二涂层迂曲度为2.11～2.87。
由此，本申请综合考量正极极片的加工便捷性、二次电池的电化学性能和能量密度，通过调控正极极片的第一涂层和第二涂层的迂曲度，实现了对极片的孔隙率的梯度调节。虽然机理尚不清楚，但推测正极极片的迂曲度呈梯度分布，提高了正极极片对电解液的浸润性、缩短了正极极片的烘干时间，进而改善了锂离子二次电池的电化学性能和能量密度。
在任意实施方式中，正极极片的涂布量CW大于等于400mg/1540.25cm 2。由此，正极极片的涂布量CW能有效提高正极活性物质的负载量，保证了锂离子二次电池的能量密度。
在任意实施方式中，第一涂层包括Dv50＝2～4μm、Dv90＝4～8μm的一次颗粒，第二涂层包括Dv50＝2～7μm、Dv90＝7～10μm的一次颗粒以 及Dv50＝4～12μm、Dv90＝13～30μm的二次颗粒。
由此，一次颗粒和二次颗粒混合使用且一次颗粒和二次颗粒的Dv50、Dv90在上述范围内时，能有效防止一次颗粒在冷压过程中破碎，进而改善正极极片冷压困难的问题。在任意实施方式中，相对于正极活性物质的总质量，第二涂层中，一次颗粒掺混比例为10～50％；二次颗粒的Dv50与一次颗粒的Dv50的比值为1.5～10。由此，一次颗粒在上述掺混比例范围内时，能有效调控正极极片的迂曲度，改善正极极片的孔隙率的分布状态。二次颗粒的Dv50与一次颗粒的Dv50的比值在上述范围内时，能有效改善二次颗粒在冷压过程中破碎的情况，进而改善了正极极片冷压困难的问题。
在任意实施方式中，第一涂层的厚度为20～140μm，第二涂层的厚度为20～140μm。由此，第一涂层和第二涂层的厚度在上述范围内时，正极极片有更好的加工性能，便于后续卷绕成电芯，装配至二次电池中。
在任意实施方式中，第一涂层的压实密度大于第二涂层的压实密度。由此，通过让第一涂层的压实密度大于第二涂层的压实密度，以此来实现对正极极片的孔隙率和迂曲度的调控，进而在改善了正极极片对电解液的浸润性的同时，保证了正极极片具有高正极活性物质负载量。
在任意实施方式中，第一涂层的压密2.5～2.8g/cc,第二涂层压密为2.1～2.5g/cc。由此，第一涂层的压实密度在上述范围内时，第一涂层能负载更多的正极活性物质，正极活性物质颗粒能充分接触。第二涂层的压实密度在上述范围内时，电解液能充分浸入其中，改善了正极极片对电解液的浸润性。
在任意实施方式中，第一涂层的孔隙率为16～20％；第二涂层的孔隙率为26～30％。由此，第一涂层和第二涂层在上述范围内时，能有效改 善正极极片对电解液的浸润性、缩短正极极片的烘干时间。
在任意实施方式中，正极活性物质为LiNi xCo yMn zFe aAl bP cO 2(其中，0≤x≤1，0≤y≤1，0≤z≤1，0≤a≤1，0≤b≤0.8，0≤c≤4)。由此，通过选择克容量高、循环性能好的正极活性物质能使锂离子二次电池具有较高的能量密度、较佳的电化学性能。
本申请的第二方面提供一种二次电池，包括本申请第一方面的正极极片。
本申请的第三方面提供一种电池模块，包括本申请的第二方面的二次电池。
本申请的第四方面提供一种电池包，包括本申请的第三方面的电池模块。
本申请的第五方面提供一种用电装置，包括选自本申请的第二方面的二次电池、本申请的第三方面的电池模块或本申请的第四方面的电池包中的至少一种。
本申请提供了正极极片、二次电池、电池模块、电池包、用电装置。通过调控正极极片的第一涂层和第二涂层的迂曲度，使得第一涂层的迂曲度大于第二涂层的迂曲度，正极极片的迂曲度可呈梯度分布，进而在保证正极极片有较高的涂布量的前提下，提高了正极极片对电解液的浸润性、缩短了正极极片的烘干时间，进而改善了锂离子二次电池的电化学性能和动力学性能。
附图说明
图1是本申请的正极极片迂曲度的示意图。
图2是本申请实施例2的扫描电镜图。
图3是本申请一实施方式的二次电池的示意图。
图4是图3所示的本申请一实施方式的二次电池的分解图。
图5是本申请一实施方式的电池模块的示意图。
图6是本申请一实施方式的电池包的示意图。
图7是图6所示的本申请一实施方式的电池包的分解图。
图8是本申请一实施方式的二次电池用作电源的用电装置的示意图。
附图标记说明：L t物质在孔介质中的实际通过路径长度；L 0介质距离；1电池包；2上箱体；3下箱体；4电池模块；5二次电池；51壳体；52电极组件；53顶盖组件。
具体实施方式
以下，适当地参照附图详细说明具体公开了本申请的正极极片、二次电池、电池模块、电池包和电学装置的实施方式。但是会有省略不必要的详细说明的情况。例如，有省略对已众所周知的事项的详细说明、实际相同结构的重复说明的情况。这是为了避免以下的说明不必要地变得冗长，便于本领域技术人员的理解。此外，附图及以下说明是为了本领域技术人员充分理解本申请而提供的，并不旨在限定权利要求书所记载的主题。
本申请所公开的“范围”以下限和上限的形式来限定，给定范围是通过选定一个下限和一个上限进行限定的，选定的下限和上限限定了特别范围的边界。这种方式进行限定的范围可以是包括端值或不包括端值的，并且可以进行任意地组合，即任何下限可以与任何上限组合形成一个范围。例如，如果针对特定参数列出了60-120和80-110的范围，理解为60- 110和80-120的范围也是预料到的。此外，如果列出的最小范围值1和2，和如果列出了最大范围值3，4和5，则下面的范围可全部预料到：1-3、1-4、1-5、2-3、2-4和2-5。在本申请中，除非有其他说明，数值范围“a-b”表示a到b之间的任意实数组合的缩略表示，其中a和b都是实数。例如数值范围“0-5”表示本文中已经全部列出了“0-5”之间的全部实数，“0-5”只是这些数值组合的缩略表示。另外，当表述某个参数为≥2的整数，则相当于公开了该参数为例如整数2、3、4、5、6、7、8、9、10、11、12等。
如果没有特别的说明，本申请的所有实施方式以及可选实施方式可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有技术特征以及可选技术特征可以相互组合形成新的技术方案。
如果没有特别的说明，本申请的所有步骤可以顺序进行，也可以随机进行，优选是顺序进行的。例如，某方法包括步骤(a)和(b)，表示该方法可包括顺序进行的步骤(a)和(b)，也可以包括顺序进行的步骤(b)和(a)。例如，提到该方法还可包括步骤(c)，表示步骤(c)可以任意顺序加入到方法中，例如，该方法可以包括步骤(a)、(b)和(c)，也可包括步骤(a)、(c)和(b)，也可以包括步骤(c)、(a)和(b)等。
如果没有特别的说明，本申请所提到的“包括”和“包含”表示开放式，也可以是封闭式。例如，“包括”和“包含”可以表示还可以包括或包含没有列出的其他组分，也可以仅包括或包含列出的组分。
如果没有特别的说明，在本申请中，术语“或”是包括性的。举例来说，短语“A或B”表示“A，B，或A和B两者”。更具体地，以下任一条件均满足条件“A或B”：A为真(或存在)并且B为假(或不存 在)；A为假(或不存在)而B为真(或存在)；或A和B都为真(或存在)。
正极极片
本申请的一个实施方式中，本申请提出了一种正极极片，包括集流体以及涂覆于集流体的至少一个面的涂层，涂层包括第一涂层和第二涂层，第一涂层涂覆于集流体的表面，第二涂层涂覆于第一涂层的表面，其中，第一涂层迂曲度为3.01～4.93，第二涂层迂曲度为2.11～2.87。
参见图1，迂曲度为物质在孔介质中的实际通过路径长度L t与介质距离(厚度)L 0的比值。在正极极片中，L t为锂离子穿过正极活性物质颗粒的实际路径长度，L 0为正极极片的涂覆层的厚度。
虽然机理尚不明确，但本申请人意外地发现：若正极极片的迂曲度越高，电解液也难以浸润正极极片，锂离子在其中的扩散性能也相对较差。反之，正极极片的迂曲度越低，电解液可以充分浸润正极极片，锂离子在其中的扩散性能较好，但是正极极片中的正极活性物质含量较低，因此，使用迂曲度较低的正极极片会在一定程度上降低锂离子二次电池的能量密度。
本申请通过在集流体表面上设置第一涂层和第二涂层，采取相应的调控手段(如一次颗粒和二次颗粒的混配、不同的冷压实密度)使得第一涂层的迂曲度大于第二涂层的迂曲度，从而本申请正极极片的兼具高活性物质负载量和优良的动力学性能：第一涂层具有较大的迂曲度，确保了正极极片有较高的正极活性物质负载量，显著提高了锂离子二次电池的能量密度；同时，第二涂层具有较小的迂曲度，使得电解液可以充分浸润正极片，有利于锂离子在正极片中扩散，确保了锂离子二次电池有着较好的动力学性能，进而改善了其循环性能和其他的电化学性能。
在一些实施方式中，从提高正极极片的正极活性物质的涂布量，进而提高锂离子二次电池的能量密度的角度出发，正极极片的涂布量CW大于等于400mg/1540.25cm 2。
在一些实施方式中，第一涂层包括Dv50＝2～4μm、Dv90＝4～7μm的一次颗粒，第二涂层包括Dv50＝2～7μm、Dv90＝7～10μm的一次颗粒以及Dv50＝4～12μm、Dv90＝13～30μm的二次颗粒。
参见图2，图2为本申请的正极极片断面的扫描电镜图(SEM)。第一涂层的正极活性物质一次颗粒大小明显小于第二涂层的正极活性物质二次颗粒大小。需要说明的是，二次颗粒中一次颗粒的平均粒径是指10K倍的扫描电镜图中所有一次颗粒粒径大小的平均值。由此，将一次颗粒和二次颗粒混配，且一次颗粒和二次颗粒的Dv50、Dv90在上述范围内时，不仅可以防止二次颗粒在冷压过程中破碎，进而解决正极极片冷压困难的问题，同时还可以调控涂层的迂曲度，使得第一涂层的迂曲度大于第二涂层的迂曲度，使得正极极片兼具高正极活性物质负载量和优良的浸润性。
在一些实施方式中，第二涂层中，相对于正极活性物质的总质量，一次颗粒掺混比例为10～50％，二次颗粒Dv50与一次颗粒Dv50的比值为1.5～10。由此，一次颗粒掺混比例在上述范围内时，可显著改善正极极片的冷压性能。二次颗粒的Dv50与一次颗粒的Dv50的比值在上述范围内时，能有效改善二次颗粒在冷压过程中破碎的情况，进而改善了正极极片冷压困难的问题。
在一些实施方式中，第一涂层的厚度为20～140μm，第二涂层的厚度为20～140μm。由此，第一涂层、第二涂层的厚度在上述范围内时，正极极片有着良好的加工性能，方便后续卷绕成电芯装配至二次电池当中。
在一些实施方式中，从调控第一涂层、第二涂层的迂曲度与孔隙率 的角度出发，第一涂层的压实密度大于第二涂层的压实密度。
由此，通过让第一涂层的压实密度大于第二涂层的压实密度，以此来实现对正极极片的孔隙率和迂曲度的调控，进而在改善了正极极片对电解液的浸润性的同时，保证了正极极片由高正极活性物质负载量。
在一些实施方式中，第一涂层的压密2.5～2.8g/cc,第二涂层压密为2.1～2.5g/cc。
由此，第一涂层和第二涂层压实密度在上述范围内时，第一涂层的迂曲度高，正极活性物质含量高、颗粒间紧密接触，第二涂层的迂曲度小，电解液可以充分浸润正极极片，进而显著提高正极极片的动力学性能。
在一些实施方式中，第一涂层的孔隙率为16～20％；第二涂层的孔隙率为26～30％。
由此，第一涂层的孔隙率和第二涂层的孔隙率在上述范围内时，正极极片的性能既有高活性物质涂覆量又有极佳的电解液浸润性。因此，不仅显著改善了锂离子二次电池的能量密度，而且提高了锂离子在正极极片中的扩散能力，进而改善了锂离子二次电池的动力学性能。
在一些实施方式中，正极活性物质为LiNi xCo yMn zFe aAl bP cO 2(其中，0≤x≤1，0≤y≤1，0≤z≤1，0≤a≤1，0≤b≤0.8，0≤c≤4)。
由此，正极极片的性能能更好地贴合电化学体系的特性，进而满足锂离子二次电池性能的需求。
另外，平均体积分布粒径Dv50是指，正极活性材料累计体积分布百分数达到50％时所对应的粒径。平均体积分布粒径Dv90是指，正极活性材料累计体积分布百分数达到90％时所对应的粒径。在本申请中，正极 活性材料的体积平均粒径Dv50可采用激光衍射粒度分析法测定。例如参照标准GB/T 19077-2016，使用激光粒度分析仪(例如Malvern Master Size 3000)进行测定。
吸液速率测试方法：
采用毛细管吸取2mm电解液，垂直放在冷压后的极片上静止200秒，观察毛细管中电解液残留高度；
吸液速率＝(2mm-残留高度)*毛细管截面积*1.0g/cm3(电解液密度)/时间
迂曲度测试方法：
在本申请中，正极极片的迂曲度可以采取压汞法进行测试，例如参照标准GB/T 21650.1-2008，采用孔径分析仪(例如poremaster60GT)。
迂曲度按如下公式计算：
τ＝(2.23-1.13Vρ Hg)(0.92y) 1+E
其中，
Figure PCTCN2022094239-appb-000001
V：水银体积；ρ Hg:水银密度；S：正极极片总面积；ΔV i：孔体积变化；D i：平均孔径；E：孔隙指数。
另外，以下适当参照附图对本申请的二次电池、电池模块、电池包和用电装置进行说明。
本申请的一个实施方式中，提供一种二次电池。
通常情况下，二次电池包括正极极片、负极极片、电解质和隔离膜。在电池充放电过程中，活性离子在正极极片和负极极片之间往返嵌入和脱出。电解质在正极极片和负极极片之间起到传导离子的作用。隔离膜设置在正极极片和负极极片之间，主要起到防止正负极短路的作用，同时可以使离子通过。
[正极极片]
正极极片为本申请第一方面的正极极片。
[负极极片]
负极极片包括负极集流体以及设置在负极集流体至少一个表面上的负极膜层，负极膜层包括负极活性材料。
作为示例，负极集流体具有在其自身厚度方向相对的两个表面，负极膜层设置在负极集流体相对的两个表面中的任意一者或两者上。
本申请的二次电池中，负极集流体可采用金属箔片或复合集流体。例如，作为金属箔片，可以采用铜箔。复合集流体可包括高分子材料基层和形成于高分子材料基材至少一个表面上的金属层。复合集流体可通过将金属材料(铜、铜合金、镍、镍合金、钛、钛合金、银及银合金等)形成在高分子材料基材(如聚丙烯(PP)、聚对苯二甲酸乙二醇酯(PET)、聚对苯二甲酸丁二醇酯(PBT)、聚苯乙烯(PS)、聚乙烯(PE)等的基材)上而形成。
本申请的二次电池中，负极活性材料可采用本领域公知的用于电池的负极活性材料。作为示例，负极活性材料可包括以下材料中的至少一种：人造石墨、天然石墨、软炭、硬炭、硅基材料、锡基材料和钛酸锂等。硅基材料可选自单质硅、硅氧化合物、硅碳复合物、硅氮复合物以及硅合金中的至少一种。锡基材料可选自单质锡、锡氧化合物以及锡合金中的至少一种。但本申请并不限定于这些材料，还可以使用其他可被用作电池负极活性材料的传统材料。这些负极活性材料可以仅单独使用一种，也可以将两种以上组合使用。
本申请的二次电池中，负极膜层还可选地包括粘结剂。粘结剂可选自丁苯橡胶(SBR)、聚丙烯酸(PAA)、聚丙烯酸钠(PAAS)、聚丙 烯酰胺(PAM)、聚乙烯醇(PVA)、海藻酸钠(SA)、聚甲基丙烯酸(PMAA)及羧甲基壳聚糖(CMCS)中的至少一种。
本申请的二次电池中，负极膜层还可选地包括导电剂。导电剂可选自超导碳、乙炔黑、炭黑、科琴黑、碳点、碳纳米管、石墨烯及碳纳米纤维中的至少一种。
本申请的二次电池中，负极膜层还可选地包括其他助剂，例如增稠剂(如羧甲基纤维素钠(CMC-Na))等。
本申请的二次电池中，可以通过以下方式制备负极极片：将上述用于制备负极极片的组分，例如负极活性材料、导电剂、粘结剂和任意其他组分分散于溶剂(例如去离子水)中，形成负极浆料；将负极浆料涂覆在负极集流体上，经烘干、冷压等工序后，即可得到负极极片。
[电解质]
电解质在正极极片和负极极片之间起到传导离子的作用。本申请对电解质的种类没有具体的限制，可根据需求进行选择。例如，电解质可以是液态的、凝胶态的或全固态的。
本申请的二次电池中，电解质采用电解液。电解液包括电解质盐和溶剂。
本申请的二次电池中，电解质盐可选自六氟磷酸锂、四氟硼酸锂、高氯酸锂、六氟砷酸锂、双氟磺酰亚胺锂、双三氟甲磺酰亚胺锂、三氟甲磺酸锂、二氟磷酸锂、二氟草酸硼酸锂、二草酸硼酸锂、二氟二草酸磷酸锂及四氟草酸磷酸锂中的至少一种。
本申请的二次电池中，溶剂可选自碳酸亚乙酯、碳酸亚丙酯、碳酸甲乙酯、碳酸二乙酯、碳酸二甲酯、碳酸二丙酯、碳酸甲丙酯、碳酸乙丙 酯、碳酸亚丁酯、氟代碳酸亚乙酯、甲酸甲酯、乙酸甲酯、乙酸乙酯、乙酸丙酯、丙酸甲酯、丙酸乙酯、丙酸丙酯、丁酸甲酯、丁酸乙酯、1,4-丁内酯、环丁砜、二甲砜、甲乙砜及二乙砜中的至少一种。
本申请的二次电池中，电解液还可选地包括添加剂。例如添加剂可以包括负极成膜添加剂、正极成膜添加剂，还可以包括能够改善电池某些性能的添加剂，例如改善电池过充性能的添加剂、改善电池高温或低温性能的添加剂等。
[隔离膜]
本申请的二次电池中，二次电池中还包括隔离膜。本申请对隔离膜的种类没有特别的限制，可以选用任意公知的具有良好的化学稳定性和机械稳定性的多孔结构隔离膜。
本申请的二次电池中，隔离膜的材质可选自玻璃纤维、无纺布、聚乙烯、聚丙烯及聚偏二氟乙烯中的至少一种。隔离膜可以是单层薄膜，也可以是多层复合薄膜，没有特别限制。在隔离膜为多层复合薄膜时，各层的材料可以相同或不同，没有特别限制。
本申请的二次电池中，正极极片、负极极片和隔离膜可通过卷绕工艺或叠片工艺制成电极组件。
本申请的二次电池中，二次电池可包括外包装。该外包装可用于封装上述电极组件及电解质。
本申请的二次电池中，二次电池的外包装可以是硬壳，例如硬塑料壳、铝壳、钢壳等。二次电池的外包装也可以是软包，例如袋式软包。软包的材质可以是塑料，作为塑料，可列举出聚丙烯、聚对苯二甲酸丁二醇酯以及聚丁二酸丁二醇酯等。
本申请对二次电池的形状没有特别的限制，其可以是圆柱形、方形或其他任意的形状。例如，图3是作为一个示例的方形结构的二次电池5。
在一些实施方式中，参照图4，外包装可包括壳体51和盖板53。其中，壳体51可包括底板和连接于底板上的侧板，底板和侧板围合形成容纳腔。壳体51具有与容纳腔连通的开口，盖板53能够盖设于开口，以封闭容纳腔。正极极片、负极极片和隔离膜可经卷绕工艺或叠片工艺形成电极组件52。电极组件52封装于容纳腔内。电解液浸润于电极组件52中。二次电池5所含电极组件52的数量可以为一个或多个，本领域技术人员可根据具体实际需求进行选择。
在一些实施方式中，二次电池可以组装成电池模块，电池模块所含二次电池的数量可以为一个或多个，具体数量本领域技术人员可根据电池模块的应用和容量进行选择。
图5是作为一个示例的电池模块4。参照图5，在电池模块4中，多个二次电池5可以是沿电池模块4的长度方向依次排列设置。当然，也可以按照其他任意的方式进行排布。进一步可以通过紧固件将该多个二次电池5进行固定。
可选地，电池模块4还可以包括具有容纳空间的外壳，多个二次电池5容纳于该容纳空间。
在一些实施方式中，上述电池模块还可以组装成电池包，电池包所含电池模块的数量可以为一个或多个，具体数量本领域技术人员可根据电池包的应用和容量进行选择。
图6和图7是作为一个示例的电池包1。参照图6和图7，在电池包1中可以包括电池箱和设置于电池箱中的多个电池模块4。电池箱包括上 箱体2和下箱体3，上箱体2能够盖设于下箱体3，并形成用于容纳电池模块4的封闭空间。多个电池模块4可以按照任意的方式排布于电池箱中。
另外，本申请还提供一种用电装置，用电装置包括本申请提供的二次电池、电池模块、或电池包中的至少一种。二次电池、电池模块、或电池包可以用作用电装置的电源，也可以用作用电装置的能量存储单元。用电装置可以包括移动设备(例如手机、笔记本电脑等)、电动车辆(例如纯电动车、混合动力电动车、插电式混合动力电动车、电动自行车、电动踏板车、电动高尔夫球车、电动卡车等)、电气列车、船舶及卫星、储能系统等，但不限于此。
作为用电装置，可以根据其使用需求来选择二次电池、电池模块或电池包。
图8是作为一个示例的用电装置。该用电装置为纯电动车、混合动力电动车、或插电式混合动力电动车等。为了满足该用电装置对二次电池的高功率和高能量密度的需求，可以采用电池包或电池模块。
作为另一个示例的装置可以是手机、平板电脑、笔记本电脑等。该装置通常要求轻薄化，可以采用二次电池作为电源。
实施例
以下，说明本申请的实施例。下面描述的实施例是示例性的，仅用于解释本申请，而不能理解为对本申请的限制。实施例中未注明具体技术或条件的，按照本领域内的文献所描述的技术或条件或者按照产品说明书进行。所用试剂或仪器未注明生产厂商者，均为可以通过市购获得的常规产品。
实施例1
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于第一涂层上，烘干、冷压后得到正极极片。
第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实密度为：2.6g/cc，涂覆层厚度为22.5μm，涂布重量CW＝90mg/1540.25cm 2，冷压力为40T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，其中一次颗粒掺混比例为30％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝310mg/1540.25cm 2，冷压力为10T。
实施例2
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压 实密度为：2.6g/cc，涂覆层厚度为22.5μm，涂布重量CW＝90mg/1540.25cm 2，冷压力为40T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为40％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝310mg/1540.25cm2，冷压力为20T。
实施例3
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实密度为：2.6g/cc，涂覆层厚度为22.5μm，涂布重量CW＝90mg/1540.25cm 2，冷压力为40T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为20％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝310mg/1540.25cm 2，冷压力为30T。
实施例4
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚 偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实密度为：2.6g/cc，涂覆层厚度为70μm，涂布重量CW＝279mg/1540.25cm 2，冷压力为30T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为30％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝121mg/1540.25cm 2，冷压力为15T。
实施例5
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实 密度为：2.7g/cc，涂覆层厚度为62μm，涂布重量CW＝258mg/1540.25cm 2，冷压力为55T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为30％，压实密度为：2.2g/cc，涂覆层厚度为42μm，涂布重量CW＝142mg/1540.25cm 2，冷压力为50T。
对比例1
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实密度为：2.6g/cc，涂覆层厚度为22.5μm，涂布重量CW＝90mg/1540.25cm 2，冷压力为70T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为5％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝310mg/1540.25cm 2，冷压力为21T。
对比例2
将第一涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚 偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于Al箔上烘干、冷压，得到第一涂层。
然后将第二涂层的正极活性物质LiFePO 4、导电剂Super P、粘结剂聚偏二氟乙烯(PVDF)按重量比97:2:1，在N-甲基吡咯烷酮(NMP)溶剂体系中充分搅拌混合均匀后，使用挤压涂布或转移涂布涂覆于第一涂层上，烘干、冷压后得到正极极片。
其中，第一涂层中，一次颗粒的Dv50＝2.0μm、Dv90＝4.0μm，压实密度为：2.6g/cc，涂覆层厚度为22.5μm，涂布重量CW＝90mg/1540.25cm 2，冷压力为70T。
第二涂层中，一次颗粒的Dv50＝5.5μm、Dv90＝8.2μm，二次颗粒的Dv50＝7.3μm、Dv90＝25μm，一次颗粒掺混比例为60％，压实密度为：2.2g/cc，涂覆层厚度为91.5μm，涂布重量CW＝310mg/1540.25cm 2，冷压力为60T。
上述实施例1～5、对比例1、对比例2的正极极片的相关参数如表1所示
表1：实施例1～5与对比例1、对比例2的正极极片参数
Figure PCTCN2022094239-appb-000002
Figure PCTCN2022094239-appb-000003
Figure PCTCN2022094239-appb-000004
上述实施例1～5、对比例1、对比例2的正极极片分别对其吸液速率、迂曲度进行测试。测试结果如下表2所示。
表2：实施例1～5与对比例1、对比例2的吸液速率、迂曲度测试结果
另外，将上述实施例1～5、对比例1、对比例2的正极极片分别如下所示制备成二次电池，进行性能测试。测试结果如下表3所示。
(1)二次电池的制备
正极极片采用上述各实施例和对比例中的正极极片。
将作为负极活性物质的人造石墨、导电剂乙炔黑、粘结剂丁苯橡胶(SBR)以及增稠剂碳甲基纤维素钠(CMC)按照重量比90：5：2：2：1在去离子水溶剂体系中充分搅拌混合均匀后，涂覆于铜箔上烘干、冷压，得到负极极片。
以聚乙烯(PE)制多孔聚合薄膜作为隔离膜。将正极片、隔离膜以及负极片按顺序重叠，使隔离膜处于正负极之间起到隔离的作用，并卷绕得到裸电芯。将裸电芯置于外包装中，注入电解液并封装，得到使用各实施例和对比例中的正极极片的二次电池。
(2)二次电池交流阻抗测试
使用电化学工作站(型号：VMP3，制造商：法国Bio-logic)，设置正常全电池测试的频率范围为500kHz～30mHz，振幅设为5mV，测试上述制备的二次电池的电化学阻抗。
(3)二次电池放电倍率测试
将上述制备的各二次电池，分别在25℃恒温环境下，静置30分钟。按照0.33C恒流放电至2.5V，0.33C恒流放电至2.0V，然后静置1小时，0.33C恒流充电至3.65V，恒压充电，截止电流为0.05C，静置30分钟，按照0.33C恒流放电至2.5V，然后0.33C恒流放电至2.0V，记录放电容量C 0，静置1小时，0.33C恒流充电至3.65V，恒压充电，截止电流0.05C，静置30分钟。
按照1C放电至2.5V，1C放电至2.0V，记录放电容量C 1，然后静置1小时，0.33C恒流充电至3.65V，恒压充电，截止电流0.05C，静置30分钟。1C放电容量保持率为(C 1/C 0)*100％。
按照2C放电至2.5V，2C放电至2.0V，记录放电容量C 2，然后静置30分钟。2C放电容量保持率为(C 2/C 0)*100％。
(4)二次电池高低温性能测试
25℃性能测试：将上述制备的各二次电池，分别置于25℃的高低温箱(型号：SM-012PF，制造商：广东三木科技有限公司)中，按照1C放电至2.5V，1C放电至2.0V，然后静置5分钟，按照1C恒流充电至3.65V，恒压充电，截止电流为0.05C，记录放电容量C 0，然后静置30分钟，按照1C放电至2.5V，1C放电至2.0V，然后静置30分钟，按照1C恒流充电至3.65V，恒压充电，截止电流为0.05C，然后静置5分钟。
25℃放电容量保持率为(C 0/C 0)*100％。
-25℃性能测试：调节高低温箱的温度至-25℃，将上述制备的各二次电池，静置120分钟，按照1C放电至2.5V，1C放电至2.0V，记录放电容量C 1，然后静置5分钟。
-25℃放电容量保持率为(C 1/C 0)*100％。
60℃性能测试：调节高低温箱至25℃，将上述制备的各二次电池，静置120分钟，按照1C恒流充电至3.65V，恒压充电，截止电流为0.05C，然后静置5分钟。调节高低温箱的温度至60℃，将上述制备的各二次电池，静置120分钟，按照1C放电至2.5V，1C放电至2.0V，记录放电容量C 2，然后静置5分钟。
60℃放电容量保持率为(C 2/C 0)*100％。
(5)二次电池直流阻抗测试
将上述制备的各二次电池，分别在25℃恒温环境下，静置30分钟，按照0.33C恒流放电至2.5V，静置30分钟，按照0.33C恒流充电至3.65V，再恒压充电，截止电流0.05C，然后静置30分钟。按照0.33C放电至2.5V，记录放电容量C 0，静置30分钟，按照0.33C恒流充电至3.65 V，再恒压充电，截止电流0.05C，然后静置5分钟。按照0.33C放电，截止电流0.5C0(此步调节上述制备的各二次电池50％SOC)，静置1小时，记录此时电压U 1，按照电流I＝5C放电30秒，记录此时电压U 2，然后静置5分钟。DCR阻值＝(U 1-U 2)/I。
表3：实施例1～5与对比例1、对比例2的性能测试结果
Figure PCTCN2022094239-appb-000005
根据上述结果可知，实施例1～5中的正极极片中，第一涂层采用小颗粒的正极活性物质、高冷压实密度使得第一涂层的迂曲度高于第二涂层的迂曲度，使得正极极片的孔隙率呈梯度分布，因而在二次电池的倍率性能、高低温性能、电解液的浸润性，均取得了良好的效果。并且，还提高了锂离子二次电池的动力学性能。
而相对于此，对比例1中得到的正极极片中第二涂层的一次颗粒掺混比例仅为5％，对比例2中得到的正极极片中第二涂层的一次颗粒掺混比例高达60％，正极极片的迂曲度偏大。因此，对比例1和对比例2的扩散阻抗R f较大，在动力学性能方面未取得有效提高。
另外，实施例1与实施例2、实施例3相比可知，虽然实施例2、实施例3有着较小的扩散阻抗R f，其动力学性能、高低温性能、倍率性能均有所改善。但继续提高第二涂层中一次颗粒掺混比例对锂离子二次电池的动力学性能无明显的提升效果。
需要说明的是，本申请不限定于上述实施方式。上述实施方式仅为示例，在本申请的技术方案范围内具有与技术思想实质相同的构成、发挥相同作用效果的实施方式均包含在本申请的技术范围内。此外，在不脱离本申请主旨的范围内，对实施方式施加本领域技术人员能够想到的各种变形、将实施方式中的一部分构成要素加以组合而构筑的其它方式也包含在本申请的范围内。

Claims (13)

  1. 一种正极极片，包括集流体以及涂覆于所述集流体的至少一个面的涂层，所述涂层包括包含正极活性物质的第一涂层和包含正极活性物质的第二涂层，所述第一涂层涂覆于集流体的表面，所述第二涂层涂覆于所述第一涂层的表面，
    其中，所述第一涂层迂曲度为3.01～4.93，所述第二涂层迂曲度为2.11～2.87。
  2. 根据权利要求1所述的正极极片，其中，所述正极极片的涂布总量CW大于等于400mg/1540.25cm 2。
  3. 根据权利要求1或2中任一项所述的正极极片，其中，所述第一涂层包括Dv50＝2～4μm且Dv90＝4～8μm的正极活性物质一次颗粒，所述第二涂层包括Dv50＝2～7μm且Dv90＝7～10μm的正极活性物质一次颗粒以及Dv50＝4～12μm且Dv90＝13～30μm的二次颗粒。
  4. 根据权利要求1～3中任一项所述的正极极片，其中，
    所述第二涂层中，相对于正极活性物质的总质量，所述一次颗粒掺混比例为10～50％，
    所述二次颗粒Dv50与一次颗粒Dv50的比值为1.5～10。
  5. 根据权利要求1～4中任一项所述的正极极片，其中，
    所述第一涂层的厚度为20～140μm，所述第二涂层的厚度为20～140μm。
  6. 根据权利要求1～5中任一项所述的正极极片，其中，所述第一涂层的压实密度大于所述第二涂层的压实密度。
  7. 根据权利要求1～6中任一项所述的正极极片，其中，所述第一涂层的压密2.5～2.8g/cc,所述第二涂层压密为2.1～2.5g/cc。
  8. 根据权利要求1～7中任一项所述的正极极片，其中，所述第一涂层的孔隙率为16～20％；所述第二涂层的孔隙率为26～30％。
  9. 根据权利要求1～8中任一项所述的正极极片，其中，所述正极活性物质为LiNi xCo yMn zFe aAl bP cO 2，其中，0≤x≤1，0≤y≤1，0≤z≤1，0≤a≤1，0≤b≤0.8，0≤c≤4。
  10. 一种二次电池，包括权利要求1～9中任一项所述的正极极片。
  11. 一种电池模块，包括权利要求10所述的二次电池。
  12. 一种电池包，包括权利要求11所述的电池模块。
  13. 一种用电装置，包括选自权利要求10所述的二次电池、权利要求11所述的电池模块或权利要求12所述的电池包中的至少一种。
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