WO2023133844A1 - 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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WO2023133844A1
WO2023133844A1 PCT/CN2022/072148 CN2022072148W WO2023133844A1 WO 2023133844 A1 WO2023133844 A1 WO 2023133844A1 CN 2022072148 W CN2022072148 W CN 2022072148W WO 2023133844 A1 WO2023133844 A1 WO 2023133844A1
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active material
material layer
positive electrode
composite particles
particles
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PCT/CN2022/072148
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English (en)
French (fr)
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蒲晶晶
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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Priority to CN202280023692.4A priority Critical patent/CN117083729A/zh
Priority to EP22896848.3A priority patent/EP4239717A1/en
Priority to PCT/CN2022/072148 priority patent/WO2023133844A1/zh
Priority to US18/331,246 priority patent/US20230335722A1/en
Publication of WO2023133844A1 publication Critical patent/WO2023133844A1/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
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • 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
    • 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
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/20—Mountings; Secondary casings or frames; Racks, modules or packs; Suspension devices; Shock absorbers; Transport or carrying devices; Holders
    • H01M50/204—Racks, modules or packs for multiple batteries or multiple cells
    • H01M50/207—Racks, modules or packs for multiple batteries or multiple cells characterised by their shape
    • H01M50/209—Racks, modules or packs for multiple batteries or multiple cells characterised by their shape adapted for prismatic or rectangular cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/20—Mountings; Secondary casings or frames; Racks, modules or packs; Suspension devices; Shock absorbers; Transport or carrying devices; Holders
    • H01M50/249—Mountings; Secondary casings or frames; Racks, modules or packs; Suspension devices; Shock absorbers; Transport or carrying devices; Holders specially adapted for aircraft or vehicles, e.g. cars or trains
    • 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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2220/00—Batteries for particular applications
    • H01M2220/20—Batteries in motive systems, e.g. vehicle, ship, plane
    • 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 batteries, and specifically relates to a positive pole piece, a secondary battery, a battery module, a battery pack and an electrical device.
  • Lithium-ion secondary batteries are widely used in electric vehicles and consumer electronics products due to their advantages such as high energy density, high output power, long cycle life and low environmental pollution.
  • the positive pole piece is one of the core components of the lithium-ion secondary battery, and its performance has an important impact on the electrochemical performance of the lithium-ion secondary battery.
  • lithium-ion secondary batteries With the wide application of lithium-ion secondary batteries, higher and higher requirements are put forward for the electrochemical performance of lithium-ion secondary batteries, and the performance requirements for positive electrode sheets are also gradually increased.
  • the present application provides a positive pole piece, a secondary battery, a battery module, a battery pack, and an electrical device.
  • the secondary battery can take into account high rate performance and energy density.
  • the first aspect of the present application provides a positive electrode sheet, including: a positive electrode current collector; a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, and the positive electrode active material layer is sequentially stacked in a direction away from the surface
  • the first active material layer and the second active material layer are provided.
  • the first active material layer includes first composite particles, and the first composite particles include first lithium iron phosphate particles and first lithium iron phosphate particles coated on the surface of the first lithium iron phosphate particles.
  • the second active material layer includes second composite particles, the second composite particles include second lithium iron phosphate particles and a second carbon layer coated on the surface of the second lithium iron phosphate particles, wherein the graphite of the first composite particles The degree of graphitization is greater than the degree of graphitization of the second composite particles.
  • the positive electrode sheet of the embodiment of the present application includes two layers of active material layers, each layer of active material layer includes lithium iron phosphate particles coated with a carbon layer, and the conductivity of the carbon layer is relatively high.
  • the carbon layer can significantly improve the conductivity of the active material layer, and can significantly improve the contact resistance between the positive active material layer and the positive current collector, thereby increasing the electrical conductivity of the positive electrode sheet, thereby increasing the use of the positive electrode sheet.
  • the rate performance of the battery is relatively high.
  • the two active material layers are the first active material layer and the second active material layer, the first active material layer is arranged closer to the positive electrode current collector, the graphitization of the first active material layer is relatively higher, and the preparation of the first
  • the solid content of the slurry of the active material layer is relatively low
  • the second active material layer is set away from the positive current collector, the graphitization of the second active material layer is relatively low, and the solid content of the slurry for preparing the second active material layer is relatively high High; there is a difference in the solid content of the slurry for preparing the first active material layer and the second active material layer, resulting in a gradient difference in solvent concentration.
  • the combination of the two is conducive to the thick coating of the positive active material layer, thereby improving the use of the positive electrode.
  • the energy density of the secondary battery of the pole piece is conducive to the thick coating of the positive active material layer, thereby improving the use of the positive electrode.
  • the energy density of the secondary battery of the pole piece is conducive to the thick coating of the
  • the degree of graphitization of the first composite particles is 0.3-0.6, and optionally, the degree of graphitization of the first composite particles is 0.3-0.5.
  • the degree of graphitization of the first composite particles is relatively high, which can further improve the conductivity of the first active material layer, and the solid content of the slurry containing the first composite particles is relatively low.
  • the degree of graphitization of the second composite particles is 0.05-0.3, and optionally, the degree of graphitization of the second composite particles is 0.1-0.2.
  • the degree of graphitization of the second composite particles is relatively low, and on the basis of improving the conductivity of the second active material layer, the solid content in the slurry for preparing the second active material layer can be increased.
  • the combination of the slurry with low solid content and the slurry with high solid content can further reduce the difficulty of thick coating and increase the thickness of the positive electrode active material layer.
  • the first composite particles include primary particles
  • the second composite particles include secondary particles formed by aggregation of a plurality of primary particles
  • the average particle diameter D50 of the first composite particles is smaller than the average particle diameter D50 of the second composite particles.
  • the average particle diameter D50 of the first composite particles is 1 ⁇ m ⁇ 4 ⁇ m; optionally, it is 1 ⁇ m ⁇ 3 ⁇ m.
  • the use of the first composite particles in the above particle size range can increase the contact area with the electrolyte and improve the rate performance of the secondary battery; and can increase the compaction density when forming the first active material layer, thereby increasing the energy of the secondary battery density.
  • the average particle diameter D50 of the second composite particles is 6 ⁇ m ⁇ 12 ⁇ m, optionally 8 ⁇ m ⁇ 10 ⁇ m.
  • the secondary particles are spherical or spherical in shape.
  • the specific surface area of this kind of morphology is relatively large, which can increase the contact area between the second composite particle and the electrolyte, which is beneficial to the infiltration performance of the electrolyte on the second composite particle, thereby improving the transmission performance of lithium ions, and further improving the second composite particle. rate performance of the secondary battery.
  • the specific surface area of the first composite particle is 10m 2 /g-15m 2 /g; optionally, it is 12m 2 /g-14m 2 /g.
  • the specific surface area of the first composite particles is relatively large, the dispersion is uniform, and the wettability between the first composite particles and the electrolyte can be improved.
  • the specific surface area of the second composite particle is 5m 2 /g-10m 2 /g; optionally, it is 6m 2 /g-8m 2 /g.
  • the specific surface area of the second composite particle is relatively small, and its corresponding particle size is relatively large, which can form more pores with the first composite particle, which is beneficial to the transmission of lithium ions.
  • the ratio of the coating weight CW1 of the first active material layer to the coating weight CW2 of the second active material layer is 0.8 to 1.2;
  • the ratio of the coating weights CW2 of the two active material layers was 1.
  • the first active material layer with a higher degree of graphitization and the second active material layer with a lower degree of graphitization cooperate with each other to improve the coating characteristics of the positive electrode active material layer, especially for thick coating processing, thereby improving the positive electrode activity The overall thickness of the material layer.
  • the coating weight CW1 of the first active material layer is 0.115 mg/cm 2 ⁇ CW1 ⁇ 0.195 mg/cm 2 .
  • the coating weight CW2 of the second active material layer is 0.115 mg/cm 2 ⁇ CW2 ⁇ 0.195 mg/cm 2 .
  • the first active material layer includes a first conductive agent, and the mass content of the first conductive agent is A if the total weight of the first active material layer is 100%; the second active material layer includes a second conductive agent. agent, based on the total weight of the second active material layer as 100%, the mass content of the second conductive agent is B; wherein, A ⁇ B; alternatively, 1wt% ⁇ B-A ⁇ 3wt%. Compared with the degree of graphitization of the second active material layer, the degree of graphitization of the first active material layer is relatively high. On the basis of ensuring the conductivity of the first active material layer, the conductive agent can be reduced in the first active material layer. The dosage can increase the mass ratio of the first composite particles, thereby improving the energy density of the secondary battery.
  • the powder compacted density of the first active material layer under a pressure of 600 MPa is 2.4g/cc ⁇ 2.65g/cc, optionally 2.5g/cc ⁇ 2.6g/cc.
  • the energy density of the secondary battery can be significantly improved by adopting the above range of compacted density of the powder.
  • the powder compacted density of the second active material layer under a pressure of 600 MPa is 2.2 g/cc ⁇ 2.45 g/cc, optionally 2.35 g/cc ⁇ 2.45 g/cc.
  • the particles in the slurry for preparing the second active material layer have excellent fluidity, dispersibility and process performance, and the particles in the second active material layer have excellent fluidity and are uniformly dispersed; compared with the preparation of the first active material layer
  • the mutual cooperation of the slurries is beneficial to making the positive electrode active material layer and improving the quality of the positive electrode sheet.
  • the second aspect of the present application provides a secondary battery, including a positive pole piece, a separator, and a negative pole piece, wherein the positive pole piece is the positive pole piece according to any embodiment of the first aspect of the present application.
  • the third aspect of the present application provides a battery module, including the secondary battery according to the second aspect of the present application.
  • the fourth aspect of the present application provides a battery pack, including the secondary battery of the second aspect of the present application or the battery module of the third aspect of the present application.
  • the fifth aspect of the present application provides an electric device, including 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.
  • Fig. 1 is a schematic structural view of the positive pole piece provided by some embodiments of the present application.
  • Fig. 2 is a cross-sectional view of the positive pole piece shown in Fig. 1 along the A-A line;
  • FIG. 3 is a schematic structural view of an electrode assembly of a secondary battery provided in some embodiments of the present application.
  • Fig. 4 is an exploded schematic diagram of a secondary battery provided by some embodiments of the present application.
  • Fig. 5 is a schematic structural diagram of a battery module provided by some embodiments of the present application.
  • Fig. 6 is a schematic structural diagram of a battery pack provided by some embodiments of the present application.
  • Fig. 7 is a schematic structural diagram of an electrical device provided by some embodiments of the present application.
  • Fig. 8 is a TEM image of the first active material layer of the positive electrode sheet provided by some embodiments of the present application.
  • Fig. 9 is a TEM image of the second active material layer of the positive electrode sheet provided by some embodiments of the present application.
  • Fig. 10 is a partial SEM image of the positive electrode sheet provided by some embodiments of the present application.
  • Y thickness direction; 1, electrical device; 10, battery pack; 11, lower box; 12 upper box; 20, battery module;
  • Positive electrode active material layer 521. First active material layer; 522. Second active material layer;
  • 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 comprising steps (a) and (b) means that the method may comprise steps (a) and (b) performed sequentially, and may also comprise steps (b) and (a) performed sequentially.
  • the method may also include step (c) means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), and may also include 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 embodiments of the present application provide a positive electrode sheet.
  • Fig. 1 is a schematic structural view of the positive electrode sheet provided by some embodiments of the present application
  • Fig. 2 is a cross-sectional view of the positive electrode sheet shown in Fig. 1 taken along line A-A.
  • the positive electrode sheet 50 of the embodiment of the present application includes a positive electrode collector 51 and a positive electrode active material layer 52 disposed on at least one surface 511 of the positive electrode collector 51 .
  • the positive electrode current collector 51 has two surfaces 511 opposite to each other in its own thickness direction X, and the positive electrode active material layer 52 is disposed on any one of the two surfaces 511 of the positive electrode current collector 51, or may be disposed on both surfaces respectively. 511.
  • the X direction shown in FIG. 1 represents the thickness direction of the positive electrode current collector 51 .
  • the positive current collector 51 can be a metal or composite current collector.
  • the metal for example, a metal foil material or a porous metal plate, such as aluminum or an aluminum alloy, can be used.
  • the positive electrode current collector 51 uses aluminum foil.
  • a composite current collector for example, it may include a polymer material base layer and a metal layer formed on at least one surface 511 of the polymer material base layer.
  • the composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver or silver alloy, etc.) on a polymer material base (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.).
  • a metal material such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver or silver alloy, etc.
  • a polymer material base 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.
  • PET ethylene glycol ester
  • PBT polybutylene terephthalate
  • PS polystyrene
  • PE polyethylene
  • the positive electrode active material layer 52 includes a positive electrode active material.
  • the positive electrode active material is a lithium iron phosphate active material, which can perform reversible deintercalation/intercalation of lithium ions and migration of electrons during charge and discharge cycles.
  • the positive electrode current collector 51 collects the current and output.
  • the ferrous ion in lithium iron phosphate particles is at the 4c position of the oxygen octahedron, and the lithium ion is at the 4a position of the oxygen octahedron.
  • the oxygen atoms in the tetrahedral structure phosphate ions are separated and fail to form a continuous FeO 6 network, resulting in poor electronic conductivity of lithium iron phosphate particles; and the lithium ion diffusion coefficient in lithium iron phosphate particles is low.
  • the low electrical conductivity and low lithium ion diffusion coefficient of lithium iron phosphate particles lead to poor rate performance using such lithium iron phosphate particles.
  • the carbon layer has the advantages of good conductivity and large specific surface area.
  • the inventor considers coating the carbon layer on the surface of the lithium iron phosphate particles to form graphitized lithium iron phosphate particles, which can build a good conductive network on the surface of the lithium iron phosphate particles, thereby Improve the conductivity of the positive electrode sheet 50 .
  • the inventors have found that in order to improve the conductivity of the positive electrode sheet, the degree of graphitization of lithium iron phosphate particles is relatively high, and lithium iron phosphate particles with a higher degree of graphitization will increase the difficulty of coating to form a positive electrode active material layer, which is not conducive to Thick coating processing.
  • the positive electrode active material layer 52 is divided into a two-layer structure.
  • the positive electrode active material layer 52 includes the first active material layer 521 stacked in sequence in the direction away from the surface 511 of the positive electrode current collector 51 and the second active material layer 522 , both active material layers use graphitized lithium iron phosphate particles.
  • the first active material layer 521 is disposed close to the surface 511 of the positive electrode collector 51
  • the second active material layer 522 is disposed away from the surface 511 of the positive electrode current collector 51.
  • the first active material layer 521 includes first composite particles, the first composite particles include first lithium iron phosphate particles and the first carbon layer coated on the surface 511 of the first lithium iron phosphate particles, and the second active material layer 522 includes the second Composite particles, the second composite particles include second lithium iron phosphate particles and a second carbon layer coated on the surface 511 of the second lithium iron phosphate particles.
  • Each layer can improve conductivity, and can significantly improve the contact resistance between the positive electrode active material layer 52 as a whole and the positive electrode current collector 51, thereby increasing the electrical conductivity, thus increasing the rate of the secondary battery using the positive electrode active material layer 52 performance.
  • the degree of graphitization refers to the degree to which carbon atoms form a close-packed hexagonal graphite crystal structure. The closer the lattice size is to the lattice parameters of ideal graphite, the higher the degree of graphitization is. It is used here for lithium iron phosphate Characterization of the coated carbon layer.
  • the degrees of graphitization of the two active material layers are different, and the degree of graphitization of the first composite particles in the first active material layer 521 is greater than that of the second composite particles in the second active material layer 522, in other words, the first composite particles
  • the degree of graphitization of the particles is relatively higher, the poorer their compatibility with the solvent in the slurry such as N-methylpyrrolidone NMP, and it is more difficult to disperse, and the solid content in the slurry for preparing the first active material layer 521 is relatively high. lower.
  • the graphitization degree of the lithium iron phosphate particles is high, the particles are easy to slip, which can enhance the flexibility of the positive electrode sheet 50 and reduce the risk of corner cracking of the electrode assembly during winding.
  • the degree of graphitization of the second composite particles is relatively low, so the second composite particles have better compatibility with solvents such as N-methylpyrrolidone NMP, and are easy to disperse.
  • the solid content in the slurry for preparing the second active material layer 522 Relatively high. Due to the difference in solid content, a solvent concentration gradient difference can be formed between the slurry for preparing the first active material layer 521 and the preparation of the second active material layer 522, which is conducive to the thick coating of the positive electrode active material layer 52, thereby improving the secondary The energy density of the battery.
  • the positive electrode sheet 50 includes two layers of active material layers, each layer of active material layers includes lithium iron phosphate particles coated with a carbon layer, and the conductivity of the carbon layer is relatively high.
  • the carbon layer can significantly improve the conductivity of the active material layer, and can significantly improve the contact resistance between the positive electrode active material layer 52 and the positive electrode current collector 51, thereby increasing the conductivity of the positive electrode sheet 50, thus improving the use of the positive electrode sheet.
  • the rate performance of the secondary battery of 50 includes two layers of active material layers, each layer of active material layers includes lithium iron phosphate particles coated with a carbon layer, and the conductivity of the carbon layer is relatively high.
  • the carbon layer can significantly improve the conductivity of the active material layer, and can significantly improve the contact resistance between the positive electrode active material layer 52 and the positive electrode current collector 51, thereby increasing the conductivity of the positive electrode sheet 50, thus improving the use of the positive electrode sheet.
  • the rate performance of the secondary battery of 50 includes two layers of active material layers, each layer of active material layers includes lithium iron phosphate particles
  • the two active material layers are respectively the first active material layer 521 and the second active material layer 522, the first active material layer 521 is arranged closer to the positive electrode current collector 51, and the graphitization of the first active material layer 521 is relatively more High, the solid content of the slurry for preparing the first active material layer 521 is relatively low; the second active material layer 522 is set away from the positive electrode current collector 51, the graphitization of the second active material layer 522 is relatively low, and the preparation of the second active material
  • the solid content of the slurry of the layer 522 is relatively high; the difference in the solid content of the slurry for preparing the first active material layer 521 and the second active material layer 522 leads to a gradient difference in the solvent concentration, and the combination of the two is beneficial to the positive electrode activity.
  • the thick coating of the material layer 52 can improve the energy density of the secondary battery using the positive electrode sheet 50 .
  • the use of the positive electrode sheet 50 of the embodiment of the present application enables the secondary battery to take into account both high rate performance and energy density as well as good comprehensive electrochemical performance.
  • the degree of graphitization of the first composite particles is 0.3-0.6.
  • the lower limit of the degree of graphitization of the first composite particles may be 0.3, 0.4 or 0.5
  • the upper limit of the degree of graphitization of the first composite particles may be 0.6, 0.5 or 0.4.
  • the range of the degree of graphitization of the first composite particles may be a reasonable combination of any upper limit and any lower limit above, and optionally, the degree of graphitization of the first composite particles is 0.3-0.5.
  • the degree of graphitization of the first composite particles is relatively high, which can further improve the conductivity of the first active material layer 521 , and the solid content of the slurry containing the first composite particles is relatively low.
  • the degree of graphitization of the second composite particles is 0.05-0.3, and the lower limit of the degree of graphitization of the second composite particles may be 0.05, 0.1, 0.15, 0.2, or 0.25.
  • the upper limit of the degree of graphitization of the second composite particles may be 0.15, 0.20, 0.25, 0.30 or the like.
  • the range of the graphitization degree of the second composite particles may be a reasonable combination of any upper limit and any lower limit above, and optionally, the graphitization degree of the second composite particles is 0.1-0.2.
  • the degree of graphitization of the second composite particles is relatively low, and the solid content in the slurry for preparing the second active material layer 522 can be increased on the basis of improving the conductivity of the second active material layer 522 .
  • the combination of the slurry with low solid content and the slurry with high solid content can further reduce the difficulty of thick coating and increase the thickness of the positive electrode active material layer 52 .
  • the average particle size of the composite particles is too small or the specific surface area is too high, the contact area between the composite particles and the electrolyte is too large, and under the action of high voltage or strong oxidation, the electrolyte is prone to side reactions on the surface of the composite particles , worsen the gas production problem, increase the heat production, and deteriorate the safety performance and cycle performance of the secondary battery; if the average particle size of the composite particles is too large or the specific surface area is too low, lithium ions will be embedded in the composite particles during charging and discharging. If the detachment path is too long, the kinetic performance of the secondary battery will be affected.
  • the average particle diameter D50 and/or the specific surface area of the composite particles are adjusted to achieve the purpose of improving the performance of the secondary battery.
  • D50 refers to the particle size corresponding to when the cumulative volume percentage of composite particles reaches 50%, that is, the median particle size in volume distribution.
  • D50 can be measured, for example, using a laser diffraction particle size distribution measuring instrument (eg, Malvern Mastersizer 3000).
  • the first composite particles include primary particles
  • the second composite particles include secondary particles formed by aggregation of a plurality of primary particles
  • the average particle diameter D50 of the first composite particles is smaller than the average particle diameter D50 of the second composite particles.
  • the average particle diameter D50 of the first composite particles is 1 ⁇ m ⁇ 4 ⁇ m.
  • the lower limit of the average particle diameter D50 of the first composite particles may be 1 ⁇ m, 2 ⁇ m, 3 ⁇ m, or the like.
  • the upper limit of the average particle diameter D50 of the first composite particles may be 2 ⁇ m, 3 ⁇ m, 4 ⁇ m, or the like.
  • the range of the average particle diameter D50 of the first composite particles may be a reasonable combination of any upper limit and any lower limit above, and may be 1 ⁇ m ⁇ 3 ⁇ m.
  • the use of the first composite particles in the above particle size range can increase the contact area with the electrolyte and improve the rate performance of the secondary battery; and when the first active material layer 521 is formed, the compaction density can be increased, thereby improving the secondary battery. Energy Density.
  • the average particle diameter D50 of the second composite particles is 6 ⁇ m ⁇ 12 ⁇ m.
  • the lower limit of the average particle diameter D50 of the second composite particles may be 6 ⁇ m, 7 ⁇ m, 8 ⁇ m, 9 ⁇ m, 10 ⁇ m or 11 ⁇ m
  • the upper limit of the average particle diameter D50 of the second composite particles may be 7 ⁇ m, 8 ⁇ m, 9 ⁇ m, 10 ⁇ m, 11 ⁇ m or 12 ⁇ m.
  • the range of the average D50 of the second composite particle may be a reasonable combination of any upper limit and any lower limit above, and may be 8 ⁇ m ⁇ 10 ⁇ m.
  • the slurry when the slurry forms the second active material layer 522, the slurry is not easy to agglomerate and the like; and the relatively large average particle size D50 of the second composite particles can significantly improve the secondary The rate performance of the battery.
  • the second composite particle has a spherical or spherical shape, and the specific surface area of this shape is relatively large, which can increase the contact area between the second composite particle and the electrolyte, and is beneficial to the electrolyte for the second composite.
  • the wetting properties of the particles can improve the transport performance of lithium ions, thereby improving the rate performance of the secondary battery.
  • the specific surface area of the first composite particle is 10 m 2 /g to 15 m 2 /g.
  • the lower limit of the specific surface area of the first composite particles is 10m 2 /g, 11m 2 /g, 12m 2 /g or 13m 2 /g
  • the upper limit of the specific surface area of the first composite particles is 11m 2 /g, 12m 2 /g, 13m 2 /g, 14m 2 /g or 15m 2 /g, etc.
  • the range of the specific surface area of the first composite particle can be a reasonable combination of any upper limit and any lower limit above, and can be selected from 12m 2 /g to 14m 2 /g g.
  • the specific surface area of the first composite particles is relatively large, the dispersion is uniform, and the wettability between the first composite particles and the electrolyte can be improved.
  • the specific surface area of the second composite particle is 5 m 2 /g to 10 m 2 /g.
  • the lower limit of the specific surface area of the second composite particles is 5 m 2 /g, 6 m 2 /g or 7 m 2 /g
  • the upper limit of the specific surface area of the second composite particles is 7 m 2 /g, 8 m 2 /g, 9 m 2 /g or 10m 2 /g, etc.
  • the range of the specific surface area of the second composite particle may be a reasonable combination of any upper limit and any lower limit above, and may be 6m 2 /g-8m 2 /g.
  • the specific surface area of the second composite particles is relatively small, the particle diameter is relatively large, and many voids are formed by stacking.
  • a gap gradient difference can be formed between the first active material layer 521 and the second active material layer 522, and there is It is beneficial to the transmission of lithium ions and improves the kinetic performance.
  • the ratio of the coating weight CW1 of the first active material layer 521 to the coating weight CW2 of the second active material layer 522 is 0.8 ⁇ 1.2.
  • the lower limit of the ratio may be 0.8, 0.9, 1.0 or 1.1; the upper limit of the ratio may be 0.9, 1.0, 1.1 or 1.2.
  • the range of the ratio may be a reasonable combination of any upper limit and any lower limit above.
  • the ratio of the coating weight CW1 of the first active material layer 521 to the coating weight CW2 of the second active material layer 522 is 1.
  • the first active material layer 521 with a higher degree of graphitization and the second active material layer 522 with a lower degree of graphitization cooperate with each other to improve the coating characteristics of the positive electrode active material layer 52, which is especially beneficial for thick coating processing, thereby The overall thickness of the positive electrode active material layer 52 is increased.
  • the coating weight CW1 of the first active material layer 521 is 0.115mg/cm 2 cm 2 , 0.130mg/cm 2 , 0.135mg/cm 2 , 0.140mg/cm 2 , 0.145mg/cm 2 , 0.150mg/cm 2 , 0.155mg/cm 2 , 0.160mg/cm 2 , 0.165mg/cm 2 , 0.170mg/cm 2 , 0.175mg/cm 2 or 0.180mg/cm 2 ; the upper limit of CW1 can be 0.125mg/cm 2 , 0.130mg/cm 2 , 0.135mg/cm 2 , 0.140mg/cm 2 , 0.145mg /cm 2 , 0.150mg/cm 2 , 0.155mg/cm 2 , 0.160mg/cm 2 , 0.165mg/cm 2 , 0.170mg/cm 2 , 0.115m
  • the coating weight CW2 of the second active material layer 522 is 0.115 mg/cm 2 ⁇ CW2 ⁇ 0.195 mg/cm 2 ;
  • the lower limit of CW2 can be 0.115 mg/cm 2 , 0.120 mg/cm 2 , 0.125 mg /cm 2 , 0.130mg/cm 2 , 0.135mg/cm 2 , 0.140mg/cm 2 , 0.145mg/cm 2 , 0.150mg/cm 2 , 0.155mg/cm 2 , 0.160mg/cm 2 , 0.165mg/cm 2 2 , 0.170mg/cm 2 , 0.175mg/cm 2 or 0.180mg/cm 2 ;
  • the upper limit of CW2 can be 0.125mg/cm 2 , 0.130mg/cm 2 , 0.135mg/cm 2 , 0.140mg/cm 2 , 0.145 mg/cm 2 , 0.150m
  • the cooperation between the first active material layer 521 and the second active material layer 522 can increase the overall coating weight of the positive electrode active material.
  • the powder compacted density of the first active material layer 521 under a pressure of 600 MPa is 2.4 g/cc to 2.65 g/cc
  • the powder compacted density of the first active material layer 521 under a pressure of 600 MPa is The lower limit can be 2.4g/cc, 2.45g/cc, 2.50g/cc or 2.55g/cc
  • the upper limit of the powder compacted density of the first active material layer 521 under a pressure of 600MPa can be 2.45g/cc, 2.50g /cc, 2.55g/cc, 2.60g/cc or 2.65g/cc.
  • the first active material layer 521 adopting the above powder compaction density range has a relatively high compaction density, which helps to increase the energy density of the secondary battery.
  • the powder compacted density of the second active material layer 522 under a pressure of 600 MPa is 2.2 g/cc to 2.45 g/cc
  • the powder compacted density of the second active material layer 522 under a pressure of 600 MPa is The lower limit may be 2.2 g/cc, 2.25 g/cc, 2.30 g/cc or 2.35 g/cc.
  • the upper limit of the compacted powder density of the second active material layer 522 under a pressure of 600 MPa may be 2.30 g/cc, 2.35 g/cc, 2.40 g/cc or 2.45 g/cc.
  • the second active material layer 522 in the above powder compacted density range, the compacted density of the second active material layer 522 is relatively low, and the particles in the slurry for preparing the second active material layer 522 have excellent fluidity, dispersion
  • the fluidity of the particles in the second active material layer 522 is relatively excellent, and its dispersion is uniform; it cooperates with the slurry for preparing the first active material layer 521, which is conducive to making the positive electrode active material layer 52 and improving the positive electrode. Pole quality.
  • the powder compaction density of the first active material layer 521 and the second active material layer 522 cooperate with each other to increase the compaction density of the positive electrode active material layer 52 as a whole, thereby increasing the energy density of the secondary battery.
  • the positive electrode active material layer 52 may also optionally include a conductive agent.
  • the conductive agent is not particularly limited, and may be a conductive agent known in the art.
  • the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
  • the conductive agent can reduce the internal resistance of the positive electrode active material layer 52 and can increase the conductivity between the positive electrode active material layer 52 and the positive electrode current collector 51 .
  • the use of carbon-coated lithium iron phosphate particles can improve the conductivity of the positive electrode active material layer 52 to a certain extent, thereby reducing the amount of conductive agent to a certain extent, thereby relatively improving the conductivity of lithium iron phosphate.
  • the dosage of the particles improves the utilization rate of the positive electrode active material, is beneficial to obtain low electrode sheet resistance, and can increase the energy density of the secondary battery.
  • the first active material layer 521 includes a first conductive agent, and the mass content of the first conductive agent is A when the total weight of the first active material layer 521 is 100%.
  • the second active material layer 522 includes a second conductive agent, based on the total weight of the second active material layer 522 as 100%, the mass content of the second conductive agent is B; wherein, A ⁇ B; alternatively, 1wt% ⁇ B-A ⁇ 3wt%.
  • the degree of graphitization of the first active material layer 521 is relatively higher.
  • the first active material layer 521 The amount of the conductive agent can be reduced, and the mass proportion of the first composite particles can be increased, so that the energy density of the secondary battery can be increased.
  • the positive active material layer 52 may also optionally include a binder.
  • the binder is not particularly limited, and may be known in the art.
  • the binder may include styrene-butadiene rubber (SBR), water-based acrylic resin (water-based acrylic resin), carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE ), one or more of ethylene-vinyl acetate copolymer (EVA) and polyvinyl alcohol (PVA).
  • SBR styrene-butadiene rubber
  • water-based acrylic resin water-based acrylic resin
  • CMC carboxymethyl cellulose
  • PVDF polyvinylidene fluoride
  • PTFE polytetrafluoroethylene
  • EVA ethylene-vinyl acetate copolymer
  • PVA polyvinyl alcohol
  • the binder is used for the bonding of the composite particles and the conductive agent to ensure that the internal conductive network of the positive electrode sheet 50 has good structural stability. Since the conductivity of the binder itself is relatively poor, a relatively small amount of the binder can be used. for low sheet resistance.
  • the embodiment of the present application also provides a method for preparing a positive electrode sheet.
  • the positive electrode sheet of the embodiment of the present application can be prepared by coating.
  • the first positive electrode slurry is coated on at least one surface of the positive electrode current collector to obtain the first active material layer;
  • the second positive electrode slurry is coated on the surface of the first active material layer away from the positive electrode current collector, The second active material layer is obtained; then, the first active material layer and the second active material layer are dried, cold-pressed, etc. to form a positive electrode active material layer on the positive electrode current collector to obtain a positive electrode sheet.
  • the preparation method of the positive electrode sheet includes the following steps:
  • the first composite particles, the first conductive agent, the binding agent and any other components are dispersed in a solvent (such as N-methylpyrrolidone NMP) to form the first positive electrode slurry; the first positive electrode slurry is coated on On the positive current collector;
  • a solvent such as N-methylpyrrolidone NMP
  • the second composite particles, the second conductive agent, the binding agent and any other components are dispersed in a solvent (such as N-methylpyrrolidone NMP) to form a second positive electrode slurry; the second positive electrode slurry is coated on On the first positive electrode slurry;
  • a solvent such as N-methylpyrrolidone NMP
  • the first active material layer and the second active material layer can be obtained respectively, thereby obtaining a positive electrode sheet.
  • the embodiment of the present application further provides a secondary battery.
  • Fig. 3 is a schematic structural view of an electrode assembly of a secondary battery provided by some embodiments of the present application
  • Fig. 4 is a schematic exploded view of a secondary battery provided by some embodiments of the present application.
  • the secondary battery of the embodiment of the present application includes a positive pole piece 50 , a negative pole piece 60 , a separator 70 and an electrolyte.
  • the positive pole piece 50 is the positive pole piece of the embodiment of the first aspect of the present application, or the positive pole piece obtained by the preparation method of the embodiment of the second aspect of the present application. Due to the use of the positive pole piece of the embodiment of the first aspect of the present application or the positive pole piece obtained by the preparation method of the embodiment of the second aspect of the present application, the secondary battery of the present application has both high rate performance and energy density.
  • the negative electrode sheet 60 can be a metal lithium sheet, or a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector.
  • the negative electrode active material layer includes a negative electrode active material.
  • the negative electrode current collector is copper foil.
  • the negative electrode active material in the negative electrode active material layer may be a negative electrode active material known in the art for secondary batteries.
  • 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 active material layer may optionally further 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 active material layer may optionally further 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 active material 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 60 can be prepared in the following manner: the above-mentioned components used to prepare the negative electrode sheet 60, such as negative electrode active material, conductive agent, binder and any other components are dispersed in a solvent (such as deionized water) to form 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 60 can be obtained.
  • a solvent such as deionized water
  • the present application has no particular limitation on the type of the isolation membrane 70 , and any well-known isolation membrane 70 with a porous structure having good chemical stability and mechanical stability can be selected.
  • the material of the isolation film 70 can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.
  • the isolation film 70 may be a single-layer film, or a multi-layer composite film, and is not particularly limited. When the isolation film 70 is a multilayer composite film, the materials of each layer may be the same or different, and there is no particular limitation.
  • the positive pole piece 50 , the negative pole piece 60 and the separator 70 can be fabricated into the electrode assembly 40 by a winding process or a lamination process.
  • the electrode assembly 40 shown in FIG. 3 is a wound-type electrode assembly.
  • the electrolyte plays the role of conducting ions between the positive pole piece 50 and the negative pole piece 60 .
  • 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.
  • the electrolyte is an electrolytic solution.
  • the electrolytic solution includes electrolyte salts and solvents.
  • the electrolyte salt may be selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, trifluoromethane At least one of lithium sulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium difluorooxalate borate, lithium difluorodifluorooxalatephosphate and lithium tetrafluorooxalatephosphate.
  • the solvent may be selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, Butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate At least one of ester, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.
  • the electrolyte also optionally includes 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 30 includes a top cover assembly 31 and a casing 32 , as well as an electrode assembly 40 and an electrolyte accommodated in the casing 32 .
  • the electrode assembly 40 includes a positive pole piece 50 , a negative pole piece 60 and a separator 70 .
  • the positive pole piece 50 or the negative pole piece 60 includes tabs.
  • active ions are intercalated and extracted back and forth between the positive electrode sheet 50 and the negative electrode sheet 60 .
  • the electrolyte plays the role of conducting ions between the positive pole piece 50 and the negative pole piece 60 .
  • the separator 70 is arranged between the positive pole piece 50 and the negative pole piece 60, and mainly plays a role of preventing short circuit of the positive and negative poles, and at the same time allows ions to pass through.
  • the secondary battery 30 may be a wound type or laminated type battery, such as one of lithium ion secondary battery, lithium primary battery, sodium ion battery, and magnesium ion battery, but is not limited thereto.
  • the case 32 of the secondary battery may be a hard shell, such as a hard plastic shell, aluminum shell, steel shell, or the like.
  • the casing 32 of the secondary battery may also be a soft case, such as a pouch-type soft case.
  • the material of the soft case may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.
  • secondary batteries may be assembled into a battery module.
  • 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 schematic structural diagram of a battery module provided by some embodiments of the present application. As shown in FIG. 5 , in the battery module 20 , multiple secondary batteries 30 may be arranged in sequence along the length direction of the battery module 20 . Of course, it can also be arranged in any other manner. Further, the plurality of secondary batteries 30 may be fixed by fasteners.
  • the battery module 20 may further include a case having an accommodation space in which a plurality of secondary batteries 30 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.
  • FIG. 6 is a schematic structural diagram of a battery pack provided by some embodiments of the present application.
  • the battery pack 10 may include a battery box and a plurality of battery modules 20 disposed in the battery box.
  • the battery box includes an upper box body 12 and a lower box body 11 , the upper box body 12 can cover the lower box body 11 and form a closed space for accommodating the battery module 20 .
  • Multiple battery modules 20 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. 7 is a schematic structural diagram of an electrical device provided by some embodiments of the present application; the electrical device 1 includes a secondary battery 30 .
  • the electrical device 1 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 secondary batteries of Examples 1 to 35 and Comparative Examples 1-4 were prepared according to the following methods.
  • Aluminum foil with a thickness of 13 ⁇ m was used.
  • the first composite particles, conductive carbon black, binder polyvinylidene fluoride, and dispersant shown in Table 1 are fully stirred and mixed according to a preset mass ratio, and then N-methylpyrrolidone is added to stir and disperse to form the first positive electrode
  • the slurry has a viscosity of 8000mPa ⁇ s to 20000mPa ⁇ s.
  • the mass ratio of the first composite particle to the total mass of the conductive carbon black, the binder, and the dispersant is 97:2.5:0.5.
  • the first composite particles are graphitized lithium iron phosphate with an olivine structure.
  • the second composite particles, conductive carbon black, binder polyvinylidene fluoride, and dispersant shown in Table 2 are fully stirred and mixed according to a preset mass ratio, and then N-methylpyrrolidone is added to stir and disperse to form a second positive electrode
  • the slurry has a viscosity of 8000mPa ⁇ s to 20000mPa ⁇ s.
  • the mass ratio of the second composite particle to the total mass of the conductive carbon black, the binder, and the dispersant is 97:2.5:0.5.
  • the first composite particles are graphitized lithium iron phosphate with an olivine structure.
  • the first positive electrode slurry and the second positive electrode slurry are coated on the aluminum foil by double-sided double-chamber coating equipment, dried, cold-pressed, slit, and prepared to obtain the positive electrode sheet, wherein, when coating, the first The positive electrode slurry is located between the second positive electrode slurry and the aluminum foil.
  • Copper foil with a thickness of 6 ⁇ m was used.
  • Negative electrode active material graphite conductive carbon black, binder styrene-butadiene rubber emulsion (SBR), thickener sodium carboxymethylcellulose (CMC), in an appropriate amount of deionized water in a weight ratio of 95:1.0:2.0:2 Fully stir and mix to form a uniform negative electrode slurry; apply the negative electrode slurry on the negative electrode current collector, and obtain the negative electrode sheet after drying and other processes.
  • SBR binder styrene-butadiene rubber emulsion
  • CMC thickener sodium carboxymethylcellulose
  • Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a volume ratio of 3:7 were uniformly mixed to obtain an organic solvent, and then 1 mol/L LiPF 6 was uniformly dissolved in the above organic solvent.
  • the positive pole piece, the separator, and the negative pole piece are stacked in sequence, then wound into a battery core and packed into the packaging shell, the above electrolyte is injected into the battery core, and then sealed, left standing, hot and cold pressed, formed and other processes to obtain a lithium-ion secondary battery.
  • the Malvern 2000 (MasterSizer 2000) laser particle size analyzer is used for testing according to the GB/T19077-2016/ISO 13320: 2009 standard.
  • the sample After the sample is poured into the sampling tower, it circulates to the test optical system with the solution.
  • the particles are irradiated by the laser beam, and the particle size distribution characteristics of the particles can be obtained by receiving and measuring the energy distribution of the scattered light (shading degree: 8- 12%) Specifically, the sample can be measured according to the GB/T19077-2016/ISO 13320:2009 standard.
  • the ZEISS sigma 300 scanning electron microscope was used to test the sample, and then the test was carried out according to the standard JY/T010-1996 to observe the shape of the sample.
  • Sample preparation Use scissors to cut a 5mm*5mm sample (including the abnormal area) and paste it on the sample stage with conductive adhesive, and use tweezers to make holes around the abnormal area to make a mark.
  • Test process move the sample about 50 times to confirm the abnormal position, the abnormal position focuses on 30K, 10K, 5K, 3K, 1K, 500, 200, 50 (the minimum magnification is appropriate to include the entire abnormal area), and the normal area focuses on 30K, 10K , 5K, 3K, 1K, 500 shooting.
  • Lithium-ion secondary battery 1C rate discharge capacity retention rate (%) 1C rate discharge capacity/0.33C rate discharge capacity ⁇ 100%.
  • the lithium-ion secondary battery At 25°C, put the lithium-ion secondary battery on hold for 30 minutes, charge it to 3.65V with a constant current of 0.33C, the constant voltage charging cut-off current is 0.05C, put it aside for 5 minutes, then discharge it at 0.33C, the cut-off current is 0.5C, put it aside 60 minutes; then discharge at 5C for 30 seconds, the voltage V1 at the end of the recording, rest for 40 seconds, then charge with a constant current of 3.75C for 30 seconds, rest for 60 minutes, then charge with a constant current of 0.33C to 3.65V, constant voltage charging cut-off current 0.05C, rest for 5 minutes, then discharge at 0.33C, cut-off current 0.9C, rest for 60 minutes, then discharge at 5C for 30 seconds, record the voltage V2 at the end, rest for 40 seconds; then charge with a constant current of 3.75C for 30 seconds, Set aside for 5min.
  • Comparative Example 1 uses a layer of graphitized positive electrode active material layer, and its rate performance is relatively better. Compared with Comparative Example 1, the energy densities of Examples 1 to 27 have been obtained Significantly improved. Therefore, the secondary batteries of Example 1 to Example 27 can simultaneously achieve both rate performance and energy density.
  • the degree of graphitization of the first active material layer and the second active material layer in Examples 1 to 6 is higher, the rate performance of the secondary battery is better, the DC resistance is lower, and as the degree of graphitization increases, the rate The better the performance. And the rate capability and energy density of Example 3 are relatively high.
  • Example 7 to Example 14 the content of the graphitized first active material layer contained in the secondary battery is relatively high, its rate performance is relatively good, and the DC resistance is low; and due to the compaction density of the first active material layer Relatively large, the energy density of the secondary battery has been improved.
  • Example 12 to Example 15 on the basis of using the first active material layer with a relatively high degree of graphitization, the mass content of the conductive agent in the first active material layer is reduced, and the rate performance of the secondary battery does not deteriorate significantly , and the energy density has been improved.
  • Example 16 to Example 21 within a reasonable compacted density range, as the compacted density of the first active material layer increases, the energy density of the secondary battery increases, and the rate performance also increases accordingly.
  • Example 22 to Example 27 within a reasonable compacted density range, as the compacted density of the second active material layer increases, the energy density of the secondary battery increases, and the rate performance also increases accordingly.
  • Fig. 8 is the TEM (Transmission Electron Microscope) figure of the first composite particle of embodiment 1
  • Fig. 9 is the TEM figure of the second composite particle of embodiment 1
  • Fig. 10 is the local SEM (Scanning of the positive pole piece of embodiment 1) Electron Microscope) diagram.
  • the average particle diameter D50 of the first active material layer in Example 1 is relatively small, and the particle distribution is tight; the average particle diameter D50 of the second active material layer is relatively large, and the first active material layer A porous structure can be formed between the second active material layer and the second active material layer.

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Abstract

本申请涉及一种正极极片、二次电池、电池模块、电池包和用电装置。该正极极片包括正极集流体；设置于正极集流体的至少一个表面上的正极活性物质层，正极活性物质层包括离开表面的方向依次层叠设置的第一活性物质层和第二活性物质层，第一活性物质层包括第一复合颗粒，第一复合颗粒包括第一磷酸铁锂颗粒和包覆于第一磷酸铁锂颗粒表面的第一碳层，第二活性物质层包括第二复合颗粒，第二复合颗粒包括第二磷酸铁锂颗粒和包覆于第二磷酸铁锂颗粒表面的第二碳层，其中，第一复合颗粒的石墨化度大于第二复合颗粒的石墨化度。本申请采用该种正极极片的二次电池能兼顾较高的倍率性能和能量密度。

Description

正极极片、二次电池、电池模块、电池包和用电装置 技术领域
本申请属于电池技术领域，具体涉及一种正极极片、二次电池、电池模块、电池包和用电装置。
背景技术
锂离子二次电池由于具备能量密度大、输出功率高、循环寿命长和环境污染小等优点而被广泛应用于电动汽车以及消费类电子产品中。正极极片是锂离子二次电池的核心部件之一，其性能对锂离子二次电池的电化学性能具有重要影响。
随着锂离子二次电池的广泛应用，对锂离子二次电池的电化学性能提出了越来越高的要求，对正极极片的性能要求也逐步提高。
发明内容
本申请提供一种正极极片、二次电池、电池模块、电池包和用电装置，二次电池能兼顾较高的倍率性能和能量密度。
为了实现上述目的，本申请第一方面提供一种正极极片，包括：正极集流体；设置于正极集流体的至少一个表面上的正极活性物质层，正极活性物质层包括离开表面的方向依次层叠设置的第一活性物质层和第二活性物质层，第一活性物质层包括第一复合颗粒，第一复合颗粒包括第一磷酸铁锂颗粒和包覆于第一磷酸铁锂颗粒表面的第一碳层，第二活性物质层包括第二复合颗粒，第二复合颗粒包括第二磷酸铁锂颗粒和包覆于第二磷酸铁锂颗粒表面的第二碳层，其中，第一复合颗粒的石墨化度大于第二复合颗粒的石墨化度。
由此，本申请实施例的正极极片，一方面，正极极片包括两层活性物质层，每层活性物质层均包括碳层包覆的磷酸铁锂颗粒，碳层的导电性较高，碳层能够显著提高活性物质层的导电性，且能够显著改善正极活性物质层和正极集流体之间的接触阻抗，从而提高正极极片的电导率，由此提高采用该正极极片的二次电池的倍率性能。另一方面，两层活性物质层分别为第一活性物质层和第二活性物质层，第一活性物质层更靠近正极集流体设置，第一活性物质层的石墨化相对更高，制备第一活性物质层的浆料的固含量相对较低；第二活性物质层背离正极集流体设置，第二活性物质层的石墨化相对较低，制备第二活性物质层的浆料的固含量相对较高；制备第一活性物质层和第二活性物质层的浆料的固含量存在差异导致溶剂浓度存在梯度差，二者配合使用，有利于正极活性物质层的厚涂布，从而提升采用该正极极片的二次电池的能量密度。
在任意实施方式中，第一复合颗粒的石墨化度为0.3～0.6，可选地，第一复合颗粒的石墨化度为0.3～0.5。第一复合颗粒的石墨化度相对较高，能够进一步提高第一活性物质层的导电性，且包含第一复合颗粒的浆料的固含量相对较低。
在任意实施方式中，第二复合颗粒的石墨化度为0.05～0.3，可选地，第二复合颗粒的石墨化度为0.1～0.2。第二复合颗粒的石墨化度相对较低，在提高第二活性物质层的导电性的基础上，能够提高制备第二活性物质层的浆料中的固含量。低固含量的浆料和高固含量的浆料相互配合，能够进一步降低厚涂布的难度，并提高正极活性物质层的厚度。
在任意实施方式中，第一复合颗粒包括一次颗粒，第二复合颗粒包括多个一次颗粒聚集形成的二次颗粒，第一复合颗粒的平均粒径D50小于第二复合颗粒的平均粒径D50。第二复合颗粒和第一复合颗粒存在粒径差异，能够形成多孔隙结构，从而改善二次电池的动力学性能。
在任意实施方式中，第一复合颗粒的平均粒径D50为1μm～4μm；可选为1μm～3μm。采用上述粒径范围的第一复合颗粒，能够提高与电解液的接触面积，提高二次电池的倍率性能；并且在形成第一活性物质层时能够提高压实密度，进而提升二次电池的能量密度。
在任意实施方式中，第二复合颗粒的平均粒径D50为6μm～12μm，可选为8μm～10μm。采用上述粒径范围的第二复合颗粒，在浆料形成第二活性物质层时，浆料中不容易发生团聚等现象；并且第二复合颗粒的相对较大平均粒径D50能够显著提升二次电池的倍率性能。
在任意实施方式中，二次颗粒为球型或类球型形貌。该种形貌的比表面积相对较大，能够提高第二复合颗粒和电解液之间的接触面积，有利于电解液对第二复合颗粒的浸润性能，从而改善锂离子的传输性能，进而提升二次电池的倍率性能。
在任意实施方式中，第一复合颗粒的比表面积为10m 2/g～15m 2/g；可选为12m 2/g～14m 2/g。第一复合颗粒的比表面积相对较大，分散均匀，能够提高第一复合颗粒和电解液之间的浸润性。
在任意实施方式中，第二复合颗粒的比表面积为5m 2/g～10m 2/g；可选为6m 2/g～8m 2/g。第二复合颗粒的比表面积相对较小，其对应的粒径相对较大，能够和第一复合颗粒形成较多孔隙，有利于锂离子的传输。
在任意实施方式中，第一活性物质层的涂布重量CW1与第二活性物质层的涂布重量CW2的比值为0.8～1.2；可选地，第一活性物质层的涂布重量CW1与第二活性物质层的涂布重量CW2的比值为1。较高石墨化度的第一活性物质层和较低石墨化度的第二活性物质层相互协同，能够改善正极活性物质层的涂布特性，尤其有利于进行厚涂布加工，从而提高正极活性物质层整体的厚度。
在任意实施方式中，第一活性物质层的涂布重量CW1为0.115mg/cm 2≤CW1≤0.195mg/cm 2。
在任意实施方式中，第二活性物质层的涂布重量CW2为0.115mg/cm 2≤CW2≤0.195mg/cm 2。
在任意实施方式中，第一活性物质层包括第一导电剂，以第一活性物质层的总重量为100％计，第一导电剂的质量含量为A；第二活性物质层包括第二导电剂，以第二活性物质层的总重量为100％计，第二导电剂的质量含量为B；其中，A＜B；可选地，1wt％≤B-A≤3wt％。相较于第二活性物质层的石墨化度，第一活性物质层的石墨化度相对较高，在确保第一活性物质层的导电性的基础上，第一活性物质层中可减少导电剂的用量，并能够提高第一复合颗粒的质量占比，从而能够提高二次电池的能量密度。
在任意实施方式中，第一活性物质层在600MPa压强下的粉体压实密度为2.4g/cc～2.65g/cc，可选为2.5g/cc～2.6g/cc。采用上述粉体压实密度范围能够显著提高二次电池的能量密度。
在任意实施方式中，第二活性物质层在600MPa压强下的粉体压实密度为2.2g/cc～2.45g/cc，可选为2.35g/cc～2.45g/cc。制备第二活性物质层的浆料中的颗粒具有优异的流动性、分散性和工艺性能，第二活性物质层中的颗粒的流动性较为优异，其分散均匀；与制备第一活性物质层的浆料相互配合，有利于制作正极活性物质层，并提高正极极片的品质。
本申请第二方面提供一种二次电池，包括正极极片、隔离膜和负极极片，正极极片为如本申请第一方面任一实施例的正极极片。
本申请第三方面提供一种电池模块，包括如本申请第二方面的二次电池。
本申请第四方面提供一种电池包，包括如本申请第二方面的二次电池或本申请第三方面的电池模块。
本申请第五方面提供一种用电装置，包括如本申请第二方面的二次电池、本申请第三方面的电池模块或本申请第四方面的电池包。
附图说明
为了更清楚地说明本申请实施例的技术方案，下面将对本申请实施例中所需要使用的附图作简单地介绍，显而易见地，下面所描述的附图仅仅是本申请的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据附图获得其他的附图。
图1是本申请一些实施例提供的正极极片的结构示意图；
图2是图1所示的正极极片沿A-A线作出的剖视图；
图3是本申请一些实施例提供的二次电池的电极组件的结构示意图；
图4是本申请一些实施例提供的二次电池的分解示意图；
图5是本申请一些实施例提供的电池模块的结构示意图；
图6是本申请一些实施例提供的电池包的结构示意图；
图7是本申请一些实施例提供的用电装置的结构示意图；
图8是本申请一些实施例提供的正极极片的第一活性物质层的TEM图；
图9是本申请一些实施例提供的正极极片的第二活性物质层的TEM图；
图10是本申请一些实施例提供的正极极片的局部SEM图。
其中，附图标记说明如下：
Y、厚度方向；1、用电装置；10、电池包；11、下箱体；12上箱体；20、电池模块；
30、二次电池；31、顶盖组件；32、壳体；
40、电极组件；
50、正极极片；51、正极集流体；511、表面；
52、正极活性物质层；521、第一活性物质层；522、第二活性物质层；
60、负极极片；70、隔离膜。
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活性材料及其制造方法、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，方法包括步骤(a)和(b)，表示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，例如，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]
第一方面，本申请实施例提出了一种正极极片。
图1是本申请一些实施例提供的正极极片的结构示意图，图2是图1所示的正极极片沿A-A线作出的剖视图。如图1和图2所示，本申请实施例的正极极片50包括正极集流体51和设置于正极集流体51的至少一个表面511上的正极活性物质层52。
正极集流体51具有在其自身厚度方向X彼此相对的两个表面511，正极活性物质层52设置在正极集流体51的两个表面511的其中任意一者上，也可以分别设置于两个表面511。图1中所示的X方向表示正极集流体51的厚度方向。
正极集流体51可采用金属或复合集流体。作为金属，示例性地，可以采用金属箔材或多孔金属板，例如铝或铝合金，具体地，正极集流体51采用铝箔。作为复合集流体，示例性地，可包括高分子材料基层和形成于高分子材料基层至少一个表面511上的金属层。复合集流体可通过将金属材料(如铝、铝合金、镍、镍合金、钛、钛合金、银或银合金等)形成在高分子材料基层(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。
正极活性物质层52包括正极活性物质，正极活性物质为磷酸铁锂活性材料，在充放电循环过程中能够进行锂离子的可逆脱嵌/入嵌以及电子的迁移，正极集流体51将电流汇集并输出。
一般而言，磷酸铁锂颗粒中的亚铁离子处于氧八面体的4c位，锂离子处于氧八面体的4a位，聚阴离子磷酸离子结构热稳定性较好，但是八面体结构的FeO 6被四面体结构磷酸离子中的氧原子分开而未能形成连续的FeO 6网络，导致磷酸铁锂颗粒的电子电导率较差；并且磷酸铁锂颗粒中的锂离子扩散系数较低。磷酸铁锂颗粒的低电导率以及低锂离子扩散系数导致采用此种磷酸铁锂颗粒的倍率性能较差。
碳层具有导电性好和比表面积大等优点，发明人考虑在磷酸铁锂颗粒的表面包覆碳层形成石墨化的磷酸铁锂颗粒，能够在磷酸铁锂颗粒表面构建良好的导电网络，从而提高正极极片50的导电性。但是发明人发现为了提高正极极片的导电性，磷酸铁锂颗粒的石墨化程度相对较高，而较高石墨化度的磷酸铁锂颗粒会增加涂布形成正极活性物质层的难度，不利于厚涂布加工。
基于上述问题，在本申请实施例中将正极活性物质层52分为两层结构，换言之，正极活性物质层52包括离开正极集流体51的表面511的方向依次层叠设置的第一活性物质层521和第二活性物质层522，两层活性物质层均采用石墨化的磷酸铁锂颗粒。具体地，第一活性物质层521靠近正极集流体51的表面511设置，第二活性物质 层522背离正极集流体51的表面511设置。第一活性物质层521包括第一复合颗粒，第一复合颗粒包括第一磷酸铁锂颗粒和包覆于第一磷酸铁锂颗粒表面511的第一碳层，第二活性物质层522包括第二复合颗粒，第二复合颗粒包括第二磷酸铁锂颗粒和包覆于第二磷酸铁锂颗粒表面511的第二碳层。各层均能够改善导电性，且能够显著改善正极活性物质层52整体和正极集流体51之间的接触阻抗，从而提高电导率，由此提高采用该正极活性物质层52的二次电池的倍率性能。
在本文中，石墨化度是指碳原子形成密排六方石墨晶体结构的程度，其晶格尺寸越能接近理想石墨的点阵参数，石墨化度就也越高，这里用来对磷酸铁锂包覆碳层的表征。
两层活性物质层的石墨化度的程度不同，第一活性物质层521的第一复合颗粒的石墨化度大于第二活性物质层522的第二复合颗粒的石墨化度，换言之，第一复合颗粒的石墨化度相对更高，其与浆料中的溶剂例如N-甲基吡咯烷酮NMP的相容性越差，较难分散，制备第一活性物质层521的浆料中的的固含量相对较低。并且，磷酸铁锂颗粒的石墨化度较高时，颗粒之间容易滑移，能够增强正极极片50的柔韧性，降低电极组件在卷绕时出现拐角开裂的风险。第二复合颗粒的石墨化度相对较低，则第二复合颗粒与溶剂例如N-甲基吡咯烷酮NMP的相容性较好，易于分散，制备第二活性物质层522的浆料中的固含量相对较高。由于固含量的差异，制备第一活性物质层521的浆料和制备第二活性物质层522之间能够形成溶剂浓度梯度差，有利于正极活性物质层52的厚涂布，从而能够提高二次电池的能量密度。
根据本申请实施例的正极极片50，一方面，正极极片50包括两层活性物质层，每层活性物质层均包括碳层包覆的磷酸铁锂颗粒，碳层的导电性较高，碳层能够显著提高活性物质层的导电性，且能够显著改善正极活性物质层52和正极集流体51之间的接触阻抗，从而提高正极极片50的电导率，由此提高采用该正极极片50的二次电池的倍率性能。另一方面，两层活性物质层分别为第一活性物质层521和第二活性物质层522，第一活性物质层521更靠近正极集流体51设置，第一活性物质层521的石墨化相对更高，制备第一活性物质层521的浆料的固含量相对较低；第二活性物质层522背离正极集流体51设置，第二活性物质层522的石墨化相对较低，制备第二活性物质层522的浆料的固含量相对较高；制备第一活性物质层521和第二活性物质层522的浆料的固含量存在差异导致溶剂浓度存在梯度差，二者配合使用，有利于正极活性物质层52的厚涂布，从而提升采用该正极极片50的二次电池的能量密度。
因此，采用本申请实施例的正极极片50，使得二次电池能够同时兼顾较高的倍率性能和能量密度以及良好的综合电化学性能。
在一些实施方式中，第一复合颗粒的石墨化度为0.3～0.6。第一复合颗粒的石墨化度的下限可以为0.3、0.4或0.5，第一复合颗粒的石墨化度的上限可以为0.6、0.5或0.4。第一复合颗粒的石墨化度的范围可以是上述任一上限和任一下限的合理组合，可选地，第一复合颗粒的石墨化度为0.3～0.5。第一复合颗粒的石墨化度相对较高，能够进一步提高第一活性物质层521的导电性，且包含第一复合颗粒的浆料的固含量相对较低。
在一些实施方式中，第二复合颗粒的石墨化度为0.05～0.3，第二复合颗粒的石墨化度的下限可以为0.05、0.1、0.15、0.2或0.25等。第二复合颗粒的石墨化度的上限可以为0.15、0.20、0.25或0.30等。第二复合颗粒的石墨化度的范围可以是上述任一上限和任一下限的合理组合，可选地，第二复合颗粒的石墨化度为0.1～0.2。第二复合颗粒的石墨化度相对较低，在提高第二活性物质层522的导电性的基础上，能够提高制备第二活性物质层522的浆料中的固含量。低固含量的浆料和高固含量的浆料相互配合，能够进一步降低厚涂布的难度，并提高正极活性物质层52的厚度。
如果复合颗粒的平均粒径过小或者比表面积过高，则复合颗粒与电解液的接触面积过大，在高电压或强氧化性的作用下，电解液易于在复合颗粒的表面511发生副反应，恶化产气问题，产热量增加，二次电池的安全性能及循环性能变差；如果复合颗粒的平均粒径过大或者比表面积过低，则充放电过程中锂离子在复合颗粒内嵌入与脱出的路径过长，二次电池的动力学性能受到影响。
为了提高二次电池的性能，在本申请中通过对复合颗粒的平均粒径D50和/或比表面积进行调控，以此实现提高二次电池的性能的目的。D50指复合颗粒累计体积百分数达到50％时所对应的粒径，即体积分布中位粒径。D50例如可以使用激光衍射粒度分布测量仪(例如Malvern Mastersizer 3000)进行测量。
在一些实施方式中，第一复合颗粒包括一次颗粒，第二复合颗粒包括多个一次颗粒聚集形成的二次颗粒，第一复合颗粒的平均粒径D50小于第二复合颗粒的平均粒径D50。第二复合颗粒和第一复合颗粒存在粒径差异，能够形成多孔隙结构，从而改善二次电池的动力学性能。
作为一些示例，第一复合颗粒的平均粒径D50为1μm～4μm。第一复合颗粒的平均粒径D50的下限可以为1μm、2μm或3μm等。第一复合颗粒的平均粒径D50的上限可以为2μm、3μm或4μm等。第一复合颗粒的平均粒径D50的范围可以是上述任一上限和任一下限的合理组合，可选为1μm～3μm。采用上述粒径范围的第一复合颗粒，能够提高与电解液的接触面积，提高二次电池的倍率性能；并且在形成第一活性物质层521时能够提高压实密度，进而提升二次电池的能量密度。
作为一些示例，第二复合颗粒的平均粒径D50为6μm～12μm。第二复合颗粒的平均粒径D50的下限可以为6μm、7μm、8μm、9μm、10μm或11μm，第二复合颗粒的平均粒径D50的上限可以为7μm、8μm、9μm、10μm、11μm或12μm。第二复合颗粒的平均D50的范围可以是上述任一上限和任一下限的合理组合，可选为8μm～10μm。采用上述粒径范围的第二复合颗粒，在浆料形成第二活性物质层522时，浆料不容易发生团聚等现象；并且第二复合颗粒的相对较大平均粒径D50能够显著提升二次电池的倍率性能。
可选地，第二复合颗粒为球形或类球形形貌，该种形貌的比表面积相对较大，能够提高第二复合颗粒和电解液之间的接触面积，有利于电解液对第二复合颗粒的浸润性能，从而改善锂离子的传输性能，进而提升二次电池的倍率性能。
在一些实施方式中，第一复合颗粒的比表面积为10m 2/g～15m 2/g。第一复合颗粒的比表面积的下限为10m 2/g、11m 2/g、12m 2/g或13m 2/g，第一复合颗粒的比表面积 的上限为11m 2/g、12m 2/g、13m 2/g、14m 2/g或15m 2/g等，第一复合颗粒的比表面积的范围可以为上述任一上限和任一下限的合理组合，可选为12m 2/g～14m 2/g。第一复合颗粒的比表面积相对较大，分散均匀，能够提高第一复合颗粒和电解液之间的浸润性。
在一些实施方式中，第二复合颗粒的比表面积为5m 2/g～10m 2/g。第二复合颗粒的比表面积的下限为5m 2/g、6m 2/g或7m 2/g，第二复合颗粒的比表面积的上限为7m 2/g、8m 2/g、9m 2/g或10m 2/g等，第二复合颗粒的比表面积的范围可以为上述任一上限和任一下限的合理组合，可选为6m 2/g～8m 2/g。第二复合颗粒的比表面积相对较小，其粒径相对较大，堆积形成较多空隙。将包含第二复合颗粒的浆料涂敷于第一活性物质层521上形成第二活性物质层522时，第一活性物质层521和第二活性物质层522之间能够形成空隙梯度差，有利于锂离子的传输，改善动力学性能。
在一些实施方式中，第一活性物质层521的涂布重量CW1与第二活性物质层522的涂布重量CW2的比值为0.8～1.2。该比值的下限可以为0.8、0.9、1.0或1.1；该比值的上限可以为0.9、1.0、1.1或1.2。该比值的范围可以是上述任一上限和任一下限的合理组合，可选地，第一活性物质层521的涂布重量CW1与第二活性物质层522的涂布重量CW2的比值为1。较高石墨化度的第一活性物质层521和较低石墨化度的第二活性物质层522相互协同，能够改善正极活性物质层52的涂布特性，尤其有利于进行厚涂布加工，从而提高正极活性物质层52整体的厚度。
作为一些示例，第一活性物质层521的涂布重量CW1为0.115mg/cm 2≤CW1≤0.195mg/cm 2；CW1的下限可以为0.115mg/cm 2、0.120mg/cm 2、0.125mg/cm 2、0.130mg/cm 2、0.135mg/cm 2、0.140mg/cm 2、0.145mg/cm 2、0.150mg/cm 2、0.155mg/cm 2、0.160mg/cm 2、0.165mg/cm 2、0.170mg/cm 2、0.175mg/cm 2或0.180mg/cm 2；CW1的上限可以为0.125mg/cm 2、0.130mg/cm 2、0.135mg/cm 2、0.140mg/cm 2、0.145mg/cm 2、0.150mg/cm 2、0.155mg/cm 2、0.160mg/cm 2、0.165mg/cm 2、0.170mg/cm 2、0.175mg/cm 2、0.180mg/cm 2、0.185mg/cm 2、0.190mg/cm 2或0.195mg/cm 2。CW1的范围可以是上述任一上限和任一下限的合理组合。
在一些示例中，第二活性物质层522的涂布重量CW2为0.115mg/cm 2≤CW2≤0.195mg/cm 2；CW2的下限可以为0.115mg/cm 2、0.120mg/cm 2、0.125mg/cm 2、0.130mg/cm 2、0.135mg/cm 2、0.140mg/cm 2、0.145mg/cm 2、0.150mg/cm 2、0.155mg/cm 2、0.160mg/cm 2、0.165mg/cm 2、0.170mg/cm 2、0.175mg/cm 2或0.180mg/cm 2；CW2的上限可以为0.125mg/cm 2、0.130mg/cm 2、0.135mg/cm 2、0.140mg/cm 2、0.145mg/cm 2、0.150mg/cm 2、0.155mg/cm 2、0.160mg/cm 2、0.165mg/cm 2、0.170mg/cm 2、0.175mg/cm 2、0.180mg/cm 2、0.185mg/cm 2、0.190mg/cm 2或0.195mg/cm 2。CW2的范围可以是上述任一上限和任一下限的合理组合。
第一活性物质层521和第二活性物质层522相互配合能够提高正极活性物质的整体涂布重量。
在一些实施方式中，第一活性物质层521在600MPa压强下的粉体压实密度为2.4g/cc～2.65g/cc，第一活性物质层521在600MPa压强下的粉体压实密度的下限可以为2.4g/cc、2.45g/cc、2.50g/cc或2.55g/cc、第一活性物质层521在600MPa压强下的粉体 压实密度的上限可以为2.45g/cc、2.50g/cc、2.55g/cc、2.60g/cc或2.65g/cc。其范围可以为上述任一上限和任一下限的合理组合，可选为2.5g/cc～2.6g/cc。采用上述粉体压实密度范围的第一活性物质层521，其压实密度相对较高，有助于提升二次电池的能量密度。
在一些实施方式中，第二活性物质层522在600MPa压强下的粉体压实密度为2.2g/cc～2.45g/cc，第二活性物质层522在600MPa压强下的粉体压实密度的下限可以为2.2g/cc、2.25g/cc、2.30g/cc或2.35g/cc。第二活性物质层522在600MPa压强下的粉体压实密度的上限可以为2.30g/cc、2.35g/cc、2.40g/cc或2.45g/cc。其范围可以为上述任一上限和任一下限的合理组合，可选为2.35g/cc～2.45g/cc。采用上述粉体压实密度范围的第二活性物质层522，第二活性物质层522的压实密度相对较低，制备第二活性物质层522的浆料中的颗粒具有优异的流动性、分散性和工艺性能，第二活性物质层522中的颗粒的流动性较为优异，其分散均匀；与制备第一活性物质层521的浆料相互配合，有利于制作正极活性物质层52，并提高正极极片的品质。
第一活性物质层521和第二活性物质层522的粉体压实密度相互配合，能够提高正极活性物质层52整体的压实密度，从而提高二次电池的能量密度。
正极活性物质层52还可选地包括导电剂。对导电剂没有特别的限制，可以是本领域已知的导电剂。作为示例，导电剂可以包括超导碳、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。导电剂能够降低正极活性物质层52的内阻，并能够增加正极活性物质层52和正极集流体51之间的导电性。在本申请实施例中，采用碳包覆的磷酸铁锂颗粒能够在一定程度上提高正极活性物质层52的导电性，由此能够在一定程度上降低导电剂的用量，从而相对提高磷酸铁锂颗粒的用量，提高正极活性物质的利用率，有利于获得低极片电阻，并能够提高二次电池的能量密度。
在一些实施方式中，第一活性物质层521包括第一导电剂，以第一活性物质层521的总重量为100％计，第一导电剂的质量含量为A。第二活性物质层522包括第二导电剂，以第二活性物质层522的总重量为100％计，第二导电剂的质量含量为B；其中，A＜B；可选地，1wt％≤B-A≤3wt％。相较于第二活性物质层522的石墨化度，第一活性物质层521的石墨化度相对较高，在确保第一活性物质层521的导电性的基础上，第一活性物质层521中可减少导电剂的用量，并能够提高第一复合颗粒的质量占比，从而能够提高二次电池的能量密度。
正极活性物质层52还可选地包括粘结剂。对粘结剂没有特别的限制，可以是本领域已知的粘结剂。作为示例，粘结剂可以包括丁苯橡胶(SBR)、水性丙烯酸树脂(water-based acrylic resin)、羧甲基纤维素(CMC)、聚偏二氟乙烯(PVDF)、聚四氟乙烯(PTFE)、乙烯-醋酸乙烯酯共聚物(EVA)及聚乙烯醇(PVA)中的一种或多种。
粘结剂用于复合颗粒与导电剂的粘接，保证正极极片50内部导电网络具有良好的结构稳定性，由于粘结剂本身的导电性较差，因此可采用相对少量的粘结剂，以获得低极片电阻。
[正极极片的制备方法]
第二方面，本申请实施例还提供了一种正极极片的制备方法。
本申请实施例的正极极片可以采用涂布方式制备。例如，将第一正极浆料涂布于正极集流体的至少一个表面上，获得第一活性物质层；将第二正极浆料涂布于第一活性物质层的背离正极集流体的表面上，获得第二活性物质层；之后将第一活性物质层和第二活性物质层经过烘干、冷压等工序，即在正极集流体上形成正极活性物质层，获得正极极片。
作为一些示例，正极极片的制备方法包括以下步骤：
将第一复合颗粒、第一导电剂、粘结剂和任意其他的组分分散于溶剂(例如N-甲基吡咯烷酮NMP)中，形成第一正极浆料；将第一正极浆料涂布在正极集流体上；
将第二复合颗粒、第二导电剂、粘结剂和任意其他的组分分散于溶剂(例如N-甲基吡咯烷酮NMP)中，形成第二正极浆料；将第二正极浆料涂覆在第一正极浆料上；
将第一正极浆料和第二正极浆料经烘干、冷压等工序后，即可分别得到第一活性物质层和第二活性物质层，从而得到正极极片。
[二次电池]
第三方面，本申请实施例还提供了一种二次电池。
图3是本申请一些实施例提供的二次电池的电极组件的结构示意图；图4是本申请一些实施例提供的二次电池的分解示意图。
如图3和图4所示，本申请实施例的二次电池包括正极极片50、负极极片60、隔离膜70和电解液。
正极极片50采用本申请第一方面实施例的正极极片，或者采用本申请第二方面实施例的制备方法所获得的正极极片。由于使用了本申请第一方面实施例的正极极片或本申请第二方面实施例的制备方法所获得的正极极片，使得本申请的二次电池同时兼顾较高的倍率性能和能量密度。
负极极片60可以采用金属锂片，也可以采用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活性物质层，负极活性物质层包括负极活性材料。其中，负极集流体为铜箔。
在一些实施方式中，负极活性材料层中的负极活性材料可采用本领域公知的用于二次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。
在一些实施方式中，负极活性材料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。
在一些实施方式中，负极活性材料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。
在一些实施方式中，负极活性材料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。
在一些实施方式中，可以通过以下方式制备负极极片60：将上述用于制备负极极片60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60。
本申请对隔离膜70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70。在一些实施方式中，隔离膜70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70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70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。
在一些实施方式中，正极极片50、负极极片60和隔离膜70可通过卷绕工艺或叠片工艺制成电极组件40。图3中示出的电极组件40为卷绕式电极组件。
电解质在正极极片50和负极极片60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。电解质可以是液态的、凝胶态的或全固态的。
在一些实施方式中，电解质采用电解液。电解液包括电解质盐和溶剂。
在一些实施方式中，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。
在一些实施方式中，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。
在一些实施方式中，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。
请继续参阅图3和图4，二次电池30包括顶盖组件31和壳体32、以及容纳于壳体32内的电极组件40和电解质。电极组件40包括正极极片50、负极极片60和隔离膜70。正极极片50或负极极片60包含极耳。在二次电池30充放电过程中，活性离子在正极极片50和负极极片60之间往返嵌入和脱出。电解质在正极极片50和负极极片60之间起到传导离子的作用。隔离膜70设置在正极极片50和负极极片60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。具体地，该二次电池30可以为卷绕式或叠片式的电池，如锂离子二次电池、锂一次电池、钠离子电池、镁离子电 池中的一种，但并不局限于此。
在一些实施方式中，二次电池的壳体32可以是硬壳，例如硬塑料壳、铝壳、钢壳等。二次电池的壳体32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。
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是本申请一些实施例提供的电池模块的结构示意图，如图5所示，在电池模块20中，多个二次电池30可以是沿电池模块20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30进行固定。
可选地，电池模块20还可以包括具有容纳空间的外壳，多个二次电池30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是本申请一些实施例提供的电池包的结构示意图，如图6所示，在电池包10中可以包括电池箱和设置于电池箱中的多个电池模块20。电池箱包括上箱体12和下箱体11，上箱体12能够盖设于下箱体11，并形成用于容纳电池模块20的封闭空间。多个电池模块20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7是本申请一些实施例提供的用电装置的结构示意图；该用电装置1包括二次电池30。该用电装置1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1对二次电池30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～实施例35以及对比例1-4的二次电池均按照下述方法进行制备。
1、正极集流体的制备
采用厚度为13μm的铝箔。
2、正极极片的制备
2-1、第一正极浆料的制备
将表1所示的第一复合颗粒、导电炭黑、粘结剂聚偏氟乙烯、分散剂按预设质量比充分搅拌混合，然后加入N-甲基吡咯烷酮搅拌，分散，制成第一正极浆料，其粘度为8000mPa·s～20000mPa·s。其中，第一复合颗粒和导电炭黑总质量、粘结剂、分散剂的质量比为97∶2.5∶0.5。第一复合颗粒为石墨化的橄榄石型结构的磷酸铁锂。
2-2、第二正极浆料的制备
将表2所示的第二复合颗粒、导电炭黑、粘结剂聚偏氟乙烯、分散剂按预设质量比充分搅拌混合，然后加入N-甲基吡咯烷酮搅拌，分散，制成第二正极浆料，其粘度为8000mPa·s～20000mPa·s。其中，第二复合颗粒和导电炭黑总质量、粘结剂、分散剂的质量比为97∶2.5∶0.5。第一复合颗粒为石墨化的橄榄石型结构的磷酸铁锂。
2-3、正极极片的制备
采用双面双腔涂布设备将第一正极浆料和第二正极浆料涂布至铝箔上，烘干、冷压、分切、制备得到正极极片，其中，在涂布时，第一正极浆料位于第二正极浆料和铝箔之间。
3、负极集流体的制备
采用厚度为6μm的铜箔。
4、常规负极极片的制备
将负极活性材料石墨、导电炭黑、粘结剂丁苯橡胶乳液(SBR)、增稠剂羧甲基纤维素钠(CMC)、按95∶1.0∶2.0∶2重量比在适量的去离子水中充分搅拌混合，使其形成均匀的负极浆料；将负极浆料涂覆于负极集流体上，经烘干等工序后，得到负极极片。
5、隔离膜
采用PP薄膜。
6、电解液的制备
将体积比为3∶7的碳酸乙烯酯(EC)和碳酸甲乙酯(EMC)混合均匀，得到有机溶剂，然后将1mol/L的LiPF 6均匀溶解在上述有机溶剂中。
7、锂离子二次电池的制备
将正极极片、隔离膜、负极极片依次层叠设置，然后卷绕成电芯并装入包装外壳中，将上述电解液注入到电芯中，之后经过密封、静置、热冷压、化成等工序，得到锂离子二次电池。
测试部分
复合颗粒的测试
(1)复合颗粒的平均粒径D50测试
采用马尔文2000(MasterSizer 2000)激光粒度仪按照GB/T19077-2016/ISO  13320：2009标准进行测试，测试过程：前处理：取一洁净烧杯，加入适量待测样品，添加分散剂，超声120W/5min确保样品在分散剂中完全分散。
测试：样品倒入进样塔后随溶液循环到测试光路系统，颗粒在激光束的照射下，通过接受和测量散向光的能量分布就可以得出颗粒的粒度分布特征(遮光度：8-12％)具体可按照GB/T19077-2016/ISO 13320：2009标准对样品进行测定。
(2)复合颗粒的石墨化度测试
将复合颗粒平铺在玻璃片上，置于激光拉曼仪下，观察其表面的拉曼峰，采用拉曼光谱图中两峰强度之比值I1360/I1580，测定3-5个点取平均值，以此测试复合颗粒的石墨化度。
(3)复合颗粒的形貌测试
采用ZEISS sigma 300扫描电子显微镜进行样品测试，然后参照标准JY/T010-1996进行测试，对样品形貌进行观测。
步骤如下：
1.制样：用剪刀剪取5mm*5mm大小(包含异常区域)样品粘贴在粘有导电胶的样品台上，并用镊子在异常区域周边扎孔做好标识。
2.参数设置：模式：In-lens，电压：10KV，光阑：30um，工作距离：4.5mm。
3.测试流程：50倍左右移动样品确认异常位置，异常位置聚焦30K、10K、5K、3K、1K、500、200、50(最小倍以包含整个异常区域为宜)，正常区域聚焦30K、10K、5K、3K、1K、500拍摄。
二次电池的性能测试
(1)锂离子二次电池的倍率性能测试
在25℃下，将新鲜锂离子二次电池搁置30分钟，以0.33C的倍率恒流放电至2.5V，再以0.33C恒流放电至2.0V，之后搁置60分钟，再以0.33C恒流充电至3.65V，恒压充电截止电流为0.05C。
在25℃下，搁置30分钟，以0.33C的倍率恒流放电至2.5V，再以0.33C恒流放电至2.0V，之后搁置30分钟，测试得到锂离子二次电池0.33倍率放电容量
在25℃下，搁置30分钟，以1C的倍率恒流放电至2.5V，再以1C恒流放电至2.0V，之后搁置30分钟，测试得到锂离子二次电池1C倍率放电容量。
锂离子二次电池1C倍率放电容量保持率(％)＝1C倍率放电容量/0.33C倍率放电容量×100％。
(2)直流内阻(DCR)测试
在25℃下，将锂离子二次电池搁置30分钟，以0.33C恒流充电至3.65V，恒压充电截止电流为0.05C，搁置5分钟，再以0.33C放电，截止电流0.5C，搁置60分钟；然后以5C放电30秒，记录结束时电压V1，搁置40秒，再以3.75C恒流充电30秒，搁置60分钟，再以0.33C恒流充电至3.65V，恒压充电截止电流为0.05C，搁置5分钟，再以0.33C放电，截止电流0.9C，搁置60分钟，然后以5C放电30秒，记录结束时电压V2，搁置40秒；再以3.75C恒流充电30秒，搁置5min。
DCR计算公式：R＝(V2-V1)/I，单位为mΩ。
(3)二次电池的重量能量密度测试的示例性方法如下：
用电子秤称量电池重量W(单位：kg)；在25℃下，用1/3C恒流充电至电池最高额定电压，然后恒压充电至电流降至0.05C，静置15min，再用1/3C恒流放电至电池最低额定电压，静置5min；获得电池的放电能量；重复测试3次取平均值即为电池的平均放电能量E(单位：Wh)；电池的重量能量密度＝E/W。
实施例1～27和对比例1～3的参数如表1和表2所示。
表1
Figure PCTCN2022072148-appb-000001
Figure PCTCN2022072148-appb-000002
表2
Figure PCTCN2022072148-appb-000003
Figure PCTCN2022072148-appb-000004
实施例1～27和对比例1～4的测试结果如表3所示。
表3
Figure PCTCN2022072148-appb-000005
Figure PCTCN2022072148-appb-000006
由表1至表3可知，对比例1采用了一层石墨化度的正极活性物质层，其倍率性能相对较优，相较于对比例1，实施例1至实施例27的能量密度得到了显著提升。故，实施例1至实施例27的二次电池能够同时兼顾倍率性能和能量密度。
实施例1至实施例6的第一活性物质层和第二活性物质层的石墨化度较高，二次电池的倍率性能较好，直流阻抗较低，且随着石墨化度的增高，倍率性能越好。且实施例3的倍率性能和能量密度相对较高。
实施例7至实施例14，二次电池中包含的石墨化的第一活性物质层的含量较高，其倍率性能相对较好，直流阻抗较低；且由于第一活性物质层的压实密度相对较大，二次电池的能量密度得到了提升。
实施例12至实施例15，在采用相对较高的石墨化度的第一活性物质层的基础上，减少第一活性物质层中的导电剂的质量含量，二次电池的倍率性能无明显恶化，且能量密度得到了提升。
实施例16至实施例21，在合理的压实密度范围内，随着第一活性物质层的压实密度的增大，二次电池的能量密度增大，倍率性能也随之提升。
实施例22至实施例27，在合理的压实密度范围内，随着第二活性物质层的压实密度的增大，二次电池的能量密度增大，倍率性能也随之提升。
图8为实施例1的第一复合颗粒的TEM(Transmission Electron Microscope)图，图9为实施例1的第二复合颗粒的TEM图，图10为实施例1的正极极片的局部SEM(Scanning Electron Microscope)图。如图8至图10所示，实施例1的第一活性物 质层的平均粒径D50相对较小，颗粒分布紧密；第二活性物质层的平均粒径D50相对较大，第一活性物质层和第二活性物质层之间能够形成多孔隙结构。
虽然已经参考优选实施例对本申请进行了描述，但在不脱离本申请的范围的情况下，可以对其进行各种改进并且可以用等效物替换其中的部件。尤其是，只要不存在结构冲突，各个实施例中所提到的各项技术特征均可以任意方式组合起来。

Claims (14)

  1. 一种正极极片，包括：
    正极集流体；
    设置于所述正极集流体的至少一个表面上的正极活性物质层，所述正极活性物质层包括离开所述表面的方向依次层叠设置的第一活性物质层和第二活性物质层，所述第一活性物质层包括第一复合颗粒，所述第一复合颗粒包括第一磷酸铁锂颗粒和包覆于所述第一磷酸铁锂颗粒表面的第一碳层，所述第二活性物质层包括第二复合颗粒，所述第二复合颗粒包括第二磷酸铁锂颗粒和包覆于所述第二磷酸铁锂颗粒表面的第二碳层，其中，所述第一复合颗粒的石墨化度大于所述第二复合颗粒的石墨化度。
  2. 根据权利要求1所述的正极极片，其中，
    所述第一复合颗粒的石墨化度为0.3～0.6，可选地，所述第一复合颗粒的石墨化度为0.3～0.5；和/或
    所述第二复合颗粒的石墨化度为0.05～0.3，可选地，所述第二复合颗粒的石墨化度为0.1～0.2。
  3. 根据权利要求1或2所述的正极极片，其中，
    所述第一复合颗粒包括一次颗粒，所述第二复合颗粒包括多个一次颗粒聚集形成的二次颗粒，所述第一复合颗粒的平均粒径D50小于所述第二复合颗粒的平均粒径D50。
  4. 根据权利要求3所述的正极极片，其中，
    所述第一复合颗粒的平均粒径D50为1μm～4μm；可选为1μm～3μm，和/或
    所述第二复合颗粒的平均粒径D50为6μm～12μm，可选为8μm～10μm。
  5. 根据权利要求3或4所述的正极极片，其中，
    所述二次颗粒为球型或类球型形貌。
  6. 根据权利要求1至5任一项所述的正极极片，其中，
    所述第一复合颗粒的比表面积为10m 2/g～15m 2/g；可选为12m 2/g～14m 2/g，和/或
    所述第二复合颗粒的比表面积为5m 2/g～10m 2/g；可选为6m 2/g～8m 2/g，
  7. 根据权利要求1至6任一项所述的正极极片，其中，
    所述第一活性物质层的涂布重量CW1与所述第二活性物质层的涂布重量CW2的比值为0.8～1.2；
    可选地，所述第一活性物质层的涂布重量CW1与所述第二活性物质层的涂布重量 CW2的比值为1。
  8. 根据权利要求7所述的正极极片，其中，
    所述第一活性物质层的涂布重量CW1为0.115mg/cm 2≤CW1≤0.195mg/cm 2；和/或
    所述第二活性物质层的涂布重量CW2为0.115mg/cm 2≤CW2≤0.195mg/cm 2。
  9. 根据权利要求1至8任一项所述的正极极片，其中，
    所述第一活性物质层包括第一导电剂，以所述第一活性物质层的总重量为100％计，所述第一导电剂的质量含量为A；
    所述第二活性物质层包括第二导电剂，以所述第二活性物质层的总重量为100％计，所述第二导电剂的质量含量为B；
    其中，A＜B；可选地，1wt％≤B-A≤3wt％。
  10. 根据权利要求1至9任一项所述的正极极片，其中，
    所述第一活性物质层在600MPa压强下的粉体压实密度为2.4g/cc～2.65g/cc，可选为2.5g/cc～2.6g/cc；和/或
    所述第二活性物质层在600MPa压强下的粉体压实密度为2.2g/cc～2.45g/cc，可选为2.35g/cc～2.45g/cc。
  11. 一种二次电池，包括正极极片、隔离膜和负极极片，所述正极极片为如权利要求1至10任一项所述的正极极片。
  12. 一种电池模块，包括如权利要求11所述的二次电池。
  13. 一种电池包，包括如权利要求11所述的二次电池或如权利要求12所述的电池模块。
  14. 一种用电装置，包括如权利要求11所述的二次电池、如权利要求12所述的电池模块或如权利要求13所述的电池包。
PCT/CN2022/072148 2022-01-14 2022-01-14 正极极片、二次电池、电池模块、电池包和用电装置 WO2023133844A1 (zh)

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010267540A (ja) * 2009-05-15 2010-11-25 Panasonic Corp 非水電解質二次電池
JP2015125832A (ja) * 2013-12-25 2015-07-06 トヨタ自動車株式会社 二次電池用電極の製造方法
CN107742709A (zh) * 2017-10-17 2018-02-27 中国科学院青岛生物能源与过程研究所 一种磷酸铁锂电池正极活性材料及其制备和应用
CN110071292A (zh) * 2019-04-04 2019-07-30 桑顿新能源科技有限公司 一种锂离子电池正极极片的制备方法及其正极极片
CN110581256A (zh) * 2019-10-17 2019-12-17 朱虎 一种磷酸铁锂正极的制备方法
CN111640940A (zh) * 2019-03-01 2020-09-08 宁德时代新能源科技股份有限公司 负极片及二次电池

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20210044117A (ko) * 2019-10-14 2021-04-22 주식회사 엘지화학 리튬 이차전지용 양극 활물질의 제조 방법 및 상기 방법에 의해 제조된 리튬 이차전지용 양극 활물질
CN113330601A (zh) * 2020-08-31 2021-08-31 宁德新能源科技有限公司 极片、电化学装置和电子装置
US20230111642A1 (en) * 2021-10-12 2023-04-13 Xiamen Hithium Energy Storage Technology Co., Ltd. Electrode sheet and electrode sheet assembly
CN216213552U (zh) * 2021-10-12 2022-04-05 厦门海辰新能源科技有限公司 一种极片和极片组件

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2010267540A (ja) * 2009-05-15 2010-11-25 Panasonic Corp 非水電解質二次電池
JP2015125832A (ja) * 2013-12-25 2015-07-06 トヨタ自動車株式会社 二次電池用電極の製造方法
CN107742709A (zh) * 2017-10-17 2018-02-27 中国科学院青岛生物能源与过程研究所 一种磷酸铁锂电池正极活性材料及其制备和应用
CN111640940A (zh) * 2019-03-01 2020-09-08 宁德时代新能源科技股份有限公司 负极片及二次电池
CN110071292A (zh) * 2019-04-04 2019-07-30 桑顿新能源科技有限公司 一种锂离子电池正极极片的制备方法及其正极极片
CN110581256A (zh) * 2019-10-17 2019-12-17 朱虎 一种磷酸铁锂正极的制备方法

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