WO2024045504A1 - 粘结剂、制备方法、正极极片、二次电池及用电装置 - Google Patents

粘结剂、制备方法、正极极片、二次电池及用电装置 Download PDF

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Publication number
WO2024045504A1
WO2024045504A1 PCT/CN2023/076199 CN2023076199W WO2024045504A1 WO 2024045504 A1 WO2024045504 A1 WO 2024045504A1 CN 2023076199 W CN2023076199 W CN 2023076199W WO 2024045504 A1 WO2024045504 A1 WO 2024045504A1
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Prior art keywords
binder
preparation
polyvinylidene fluoride
positive electrode
battery
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PCT/CN2023/076199
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English (en)
French (fr)
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段连威
刘会会
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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 WO2024045504A1 publication Critical patent/WO2024045504A1/zh

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Classifications

    • 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
    • H01M4/621—Binders
    • H01M4/622—Binders being polymers
    • H01M4/623—Binders being polymers fluorinated polymers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F114/00—Homopolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen
    • C08F114/18—Monomers containing fluorine
    • C08F114/20—Vinyl fluoride
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F2/00—Processes of polymerisation
    • C08F2/04—Polymerisation in solution
    • C08F2/10—Aqueous solvent
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
    • C08F2/00—Processes of polymerisation
    • C08F2/34—Polymerisation in gaseous state
    • 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
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • 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
    • H01M10/058—Construction or manufacture
    • 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
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present application relates to the technical field of secondary batteries, and in particular to an adhesive, a preparation method, a positive electrode sheet, a secondary battery, a battery module, a battery pack and an electrical device.
  • secondary batteries have been widely used in energy storage power systems such as hydraulic, thermal, wind and solar power stations, as well as in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. With the popularization of secondary battery applications, higher requirements have been placed on its cycle performance and service life.
  • Binders are commonly used materials in secondary batteries and are in great demand for battery pole pieces, separators, packaging, etc.
  • the existing binders have poor adhesion and often require a large amount of addition to meet the adhesive strength requirements of the pole pieces, which will limit the improvement of battery energy density. Therefore, existing adhesives still need to be improved.
  • This application was made in view of the above-mentioned problems, and its purpose is to provide a binder that can exert excellent bonding force at a low addition amount, so that the pole piece has sufficient bonding strength, and Can improve battery cycle performance.
  • this application provides a binder, which includes polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.
  • This binder can ensure sufficient adhesion of the pole pieces at a low addition amount and improve High battery cycle performance.
  • the polydispersity coefficient of polyvinylidene fluoride is 1.8-2.5, optionally 1.9-2.3.
  • the polydispersity coefficient of ultra-high molecular weight polyvinylidene fluoride is within an appropriate range.
  • the weight-average molecular weight distribution of ultra-high molecular weight polyvinylidene fluoride is uniform and the performance is balanced, which can ensure that the binder has the properties of a pole piece at a low addition amount. With sufficient adhesion, the capacity retention rate of the battery during cycling is further improved.
  • the Dv50 particle size of polyvinylidene fluoride is 100 ⁇ m to 200 ⁇ m, optionally 120 ⁇ m to 200 ⁇ m.
  • ultra-high molecular weight polyvinylidene fluoride still has good processing properties and can ensure the production efficiency of pole pieces and batteries.
  • the polyvinylidene fluoride has a crystallinity of 40% to 46%, optionally 41% to 46%.
  • the crystallinity of polyvinylidene fluoride is controlled within an appropriate range, and the binder is added in a low amount to meet the electrode plate adhesion and battery cycle performance. It will not have an excessive impact on the battery flexibility and can still meet the electrode plate requirements. production needs.
  • the viscosity of the glue prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone is 2000mPa ⁇ s ⁇ 5000mPa ⁇ s, optionally 2100mPa ⁇ s ⁇ 4300mPa ⁇ s, wherein the viscosity of polyvinylidene fluoride is The mass content is 2%, based on the total mass of the glue.
  • Controlling the viscosity of the polyvinylidene fluoride glue within a suitable range and adding a low amount of binder can ensure excellent adhesion of the pole piece.
  • a second aspect of the application also provides a method for preparing a binder, which includes the following steps: providing vinylidene fluoride monomer and a solvent, performing a first-stage polymerization reaction to obtain a first product; The second stage polymerization reaction is carried out under a gas atmosphere; a chain transfer agent is added to carry out the third stage polymerization reaction to obtain polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.
  • the preparation method of the binder can prepare an ultra-high molecular weight polyvinylidene fluoride binder through segmented polymerization.
  • the binder can meet the bonding strength requirements of the electrode piece at a low addition amount, helps increase the loading capacity of the positive active material in the electrode piece, and improves the capacity retention rate of the battery during cycling.
  • the reaction temperature of the first-stage polymerization reaction is 45°C to 60°C
  • the reaction time is 4 hours to 10 hours
  • the initial polymerization pressure is 4MPa to 6MPa.
  • the reaction temperature of the second stage polymerization reaction is 60°C to 80°C
  • the reaction time is 2 hours to 4 hours
  • the reaction pressure is 6MPa to 8MPa.
  • reaction time of the third stage polymerization reaction is 1 hour to 2 hours.
  • the uniformity of the weight average molecular weight of the polymerized product can be controlled to ensure that the product has a lower
  • the polydispersity coefficient improves the consistency of the performance of polyvinylidene fluoride, allowing the pole piece to have excellent adhesion with a low amount of binder added, and the cycle capacity retention rate of the battery can be further improved.
  • the chain transfer agent includes one or more of cyclohexane, isopropanol, methanol, and acetone.
  • the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.
  • the amount of chain transfer agent used is 1.5% to 3% of the mass of vinylidene fluoride monomer.
  • the first stage reaction includes the following steps: adding a solvent and a dispersant to the container to remove oxygen from the reaction system; adding an initiator and a pH adjuster to the container to adjust the pH value to 6.5-7 , then add vinylidene fluoride monomer to bring the pressure in the container to 4MPa ⁇ 6MPa; after stirring for 30 to 60 minutes, raise the temperature to 45°C to 60°C to perform the first stage of polymerization.
  • the amount of solvent used is 2 to 8 times the mass of vinylidene fluoride monomer.
  • the dispersant includes one or more of cellulose ethers and polyvinyl alcohol.
  • the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.
  • the amount of dispersant is 0.1% to 0.3% based on the mass of vinylidene fluoride monomer.
  • the initiator is an organic peroxide.
  • the organic peroxide includes t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethylperoxydicarbonate, diisopropylperoxydicarbonate, and One or more types of tert-butyl peroxypivalate.
  • the amount of initiator used is 0.15% to 1% based on the mass of vinylidene fluoride monomer.
  • the pH adjusting agent includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and ammonia.
  • the amount of pH adjuster is 0.05% to 0.2% of the total mass of vinylidene fluoride monomer.
  • a third aspect of the present application provides a positive electrode sheet, including a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector.
  • the positive electrode film layer includes a positive electrode active material, a conductive agent, and a binder in any embodiment.
  • the adhesive prepared by the preparation method in any embodiment.
  • the mass fraction of the binder is 0.4% to 0.6%, based on the total mass of the positive electrode film layer.
  • Controlling the mass fraction of the binder within an appropriate range while ensuring effective bonding force can increase the loading of active materials in the battery pole pieces, helping to further improve the power performance of the battery.
  • a secondary battery including an electrode assembly and an electrolyte.
  • the electrode assembly includes a separator, a negative electrode sheet and the positive electrode sheet of the third aspect of the present application.
  • the secondary battery is either lithium-ion or sodium-ion.
  • a battery module including the secondary battery of the fourth aspect of the present application.
  • a battery pack is provided, including the battery module of the fifth aspect of the present application.
  • an electrical device including at least one of the secondary battery of the fourth aspect of the present application, the battery module of the fifth aspect, or the battery pack of the sixth aspect of the present application.
  • Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.
  • Figure 2 is an exploded view of the secondary battery according to an embodiment of the present application shown in Figure 1;
  • FIG. 3 is a schematic diagram of a battery module according to an embodiment of the present application.
  • Figure 4 is a schematic diagram of a battery pack according to an embodiment of the present application.
  • FIG 5 is an exploded view of the battery pack according to an embodiment of the present application shown in Figure 4;
  • Figure 6 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application
  • Figure 7 is a bonding force-displacement diagram of Example 10 and Comparative Example 1;
  • Figure 8 is a graph showing the battery capacity retention rate and the number of cycles in Example 10 and Comparative Example 1.
  • Ranges disclosed herein are defined in terms of lower and upper limits. 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 of the endpoints, and may be arbitrarily combined, that is, 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, understand that ranges of 60-110 and 80-120 are also expected. Additionally, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, Then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.
  • the numerical range “ab” 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" means that all real numbers between "0-5" have been listed in this article, and "0-5" is just an abbreviation of these numerical combinations.
  • a certain parameter is an integer ⁇ 2
  • the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially.
  • step (c) means that step (c) may be added to the method in any order.
  • the method may include steps (a), (b) and (c). , may also include steps (a), (c) and (b), may also include steps (c), (a) and (b), etc.
  • condition "A or B” is satisfied by any of the following conditions: 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).
  • Polyvinylidene fluoride is currently one of the most widely used binder types in secondary batteries.
  • the viscosity of traditional polyvinylidene fluoride is low, and a large amount of addition is often required to ensure effective bonding of active materials, thereby enabling the pole pieces to achieve effective bonding force.
  • increasing the dosage of traditional polyvinylidene fluoride will reduce the load of active materials in the pole pieces, affecting battery performance. The improvement of efficiency performance is difficult to meet the requirements for battery cycle performance.
  • this application proposes a binder, which includes polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.
  • binder refers to a chemical compound, polymer or mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium.
  • polyvinylidene fluoride refers to polymers with vinylidene fluoride as the main synthetic monomer.
  • Polymers include on the one hand chemically homogeneous polymers prepared by polymerization reactions, but with varying degrees of polymerization, molar mass and chain A collection of macromolecules that differ in length.
  • the term on the other hand also includes derivatives of aggregates of macromolecules formed by polymerization reactions which are obtainable by reaction, for example addition or substitution, of functional groups in said macromolecules and which may be chemically homogeneous or chemically non-uniform compounds.
  • Polyvinylidene fluoride herein includes both homopolymers and copolymers.
  • weight average molecular weight refers to the sum of the weight fractions of molecules of different molecular weights in the polymer multiplied by their corresponding molecular weights.
  • the dispersion medium of the adhesive is an oily solvent.
  • the oily solvent include but are not limited to dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dicarbonate Methyl ester, ethyl cellulose, polycarbonate. That is, the binder is dissolved in the oily solvent.
  • a binder is used to fix the electrode active material and/or conductive agent in place and adhere them to the conductive metal component to form an electrode.
  • the binder serves as a positive electrode binder and is used to bind the positive electrode active material and/or conductive agent to form an electrode.
  • the binder serves as a negative electrode binder and is used to bind the negative electrode active material and/or conductive agent to form an electrode.
  • the weight average molecular weight of polyvinylidene fluoride is 5 million to 6 million, 6 million to 7 million, 7 million to 8 million, 8 million to 9 million, 6 million to 9 million, 7 million to 9 million any of them.
  • the fluorine element in polyvinylidene fluoride forms hydrogen bonds with the hydroxyl groups or/and carboxyl groups on the surface of the active material and the current collector surface, which can improve the adhesion of the pole piece.
  • the weight average molecular weight is 500 Polyvinylidene fluoride of 10,000 to 9 million can improve the adhesion of the pole pieces at a low level of addition and improve the capacity retention rate of the battery during the cycle.
  • the above-mentioned binder can ensure sufficient adhesion of the electrode piece at a low addition amount, which is beneficial to improving the energy density of the battery and the cycle performance of the battery.
  • the weight average molecular weight of polyvinylidene fluoride can be tested using methods known in the art, such as gel chromatography, such as using Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detection). 2141) for testing.
  • the test method is to use a polystyrene solution sample with a mass fraction of 3.0% as a reference and select a matching chromatographic column (oil: Styragel HT5DMF7.8*300mm+Styragel HT4).
  • NMP N-methylpyrrolidone
  • polyvinylidene fluoride has a polydispersity coefficient of 1.8 to 2.5.
  • the polydispersity coefficient of polyvinylidene fluoride can be selected from 1.8 to 1.9, 1.9 to 2.0, 2.0 to 2.1, 2.1 to 2.2, 2.2 to 2.3, 2.3 to 2.4, 2.4 to 2.5, 1.8 to 2.0, 2.0 Any one of ⁇ 2.2, 2.2 ⁇ 2.4, 1.9 ⁇ 2.3, 2 ⁇ 2.3.
  • polydispersity coefficient refers to the ratio of the weight average molecular weight of the polymer to the number average molecular weight of the polymer.
  • number average molecular weight refers to the sum of the mole fractions of molecules of different molecular weights in the polymer multiplied by their corresponding molecular weights.
  • the polydispersity coefficient of polyvinylidene fluoride is too large, the degree of polymerization of polyvinylidene fluoride will be dispersed, reducing the uniformity of the binder, causing the binder to be unable to uniformly adhere the positive active material to the current collector, and thus It affects the cycle performance of the battery and also reduces the solid content of the slurry, making it impossible to further improve the energy density of the battery. If the polydispersity coefficient of polyvinylidene fluoride is too small, the preparation process will be more difficult and the yield rate will be low, resulting in production higher cost.
  • ultra-high molecular weight poly Vinylidene fluoride has a uniform weight average molecular weight distribution and stable performance, which can ensure that the binder has sufficient adhesion to the pole piece at a low addition amount, further improving the capacity retention rate of the battery during cycling.
  • polyvinylidene fluoride has a suitable polydispersity coefficient, which can effectively increase the solid content of the slurry and reduce production costs.
  • the polydispersity coefficient can be tested using methods known in the art, such as gel chromatography, such as Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141).
  • gel chromatography such as Waters 2695 Isocratic HPLC gel chromatograph (differential refractive index detector 2141).
  • a polystyrene solution sample with a mass fraction of 3.0% is used as a reference to select a matching chromatographic column (oil: Styragel HT5DMF7.8*300mm+Styragel HT4).
  • NMP N-methylpyrrolidone
  • polyvinylidene fluoride has a Dv50 particle size of 100 ⁇ m to 200 ⁇ m.
  • the Dv50 particle size of polyvinylidene fluoride can be selected from 120 ⁇ m to 140 ⁇ m, 140 ⁇ m to 160 ⁇ m, 160 ⁇ m to 180 ⁇ m, 180 ⁇ m to 200 ⁇ m, 120 ⁇ m to 150 ⁇ m, 150 ⁇ m to 180 ⁇ m, 120 ⁇ m to 200 ⁇ m, and 140 ⁇ m to 200 ⁇ m. Any kind.
  • Dv50 particle size refers to the particle size corresponding to when the cumulative particle size distribution number of particles reaches 50% in the particle size distribution curve. Its physical meaning is that particles with a particle size smaller (or larger) than it account for 50%. %.
  • the Dv50 particle size of polyvinylidene fluoride is too large, it will be relatively difficult to dissolve the polyvinylidene fluoride, which will reduce the dispersion of the binder and affect the uniform distribution of the positive active material on the current collector and the cycle performance of the battery. At the same time, the polyvinylidene fluoride will The dissolution is difficult, which reduces the speed of the pulping process; if the Dv50 particle size of polyvinylidene fluoride is too small, the adhesive force of the pole piece decreases.
  • ultra-high molecular weight polyvinylidene fluoride still has good processing properties and can ensure the production efficiency of pole pieces and batteries.
  • a suitable range of Dv50 particle size of polyvinylidene fluoride can also enable the amount of binder to be controlled at a low level without excessive negative impact on the bonding performance. This effectively improves the damage to pole pieces and battery performance caused by high amounts of binders in traditional technologies.
  • the polyvinylidene fluoride has a crystallinity of 40% to 46%.
  • the crystallinity of polyvinylidene fluoride can be selected from 41% to 42%, 42% to 43%, 43% to 44%, 41% to 42%, 42% to 43%, 43% to 44% Any one of %, 44% to 45%, 45% to 46%, 41% to 43%, 43% to 45%, and 41% to 46%.
  • the crystallinity of polyvinylidene fluoride is usually required to be below 50%.
  • the crystallinity of the polyvinylidene fluoride in this application is within a suitable range, and the binder is added in a low amount to meet the adhesion force of the electrode piece and battery cycle performance, and will not have an excessive impact on the flexibility of the electrode piece. It can meet the production needs of pole pieces.
  • the crystallinity can be tested using methods known in the art, such as differential scanning thermal analysis.
  • 0.5 g of polyvinylidene fluoride is placed in an aluminum crucible, shaken flat, and the crucible lid is covered, under a nitrogen atmosphere, with a purge gas of 50 ml/min and a protection of 70 ml/min. Air, the heating rate is 10°C per minute, the test temperature range is -100°C ⁇ 400°C, and the differential scanning calorimeter (DSC) of the American TA Instrument model Discovery 250 is used to test and eliminate thermal history.
  • DSC differential scanning calorimeter
  • the viscosity of the glue prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone is 2000mPa ⁇ s ⁇ 5000mPa ⁇ s, in which the mass content of polyvinylidene fluoride is 2%, based on the total content of the glue Mass meter.
  • the viscosity of the glue prepared by dissolving polyvinylidene fluoride in N-methylpyrrolidone can be selected from 2100mPa ⁇ s to 2700mPa ⁇ s, 2700mPa ⁇ s to 3400mPa ⁇ s, or 3400mPa ⁇ s to 3800mPa ⁇ s.
  • the viscosity of the polyvinylidene fluoride glue is too high, the viscosity of the prepared binder solution will be too high and difficult to stir, which will reduce the dispersibility of the binder and make it difficult for the binder to evenly adhere the positive active material to the assembly. In terms of fluid, it affects the cycle performance of the battery. At the same time, the viscosity of the binder solution is too large, which reduces the speed of the pulping process; if the viscosity of the binder solution is too small, it is difficult for the pole piece to have sufficient adhesion at low addition amounts. .
  • the binder solution needs to have a certain viscosity to prevent the positive electrode active materials and conductive agents and other additives from settling, so that the slurry can be stored more stably.
  • a binder with a mass fraction of at least 7% is required to achieve it.
  • the polyvinylidene fluoride of the present application is The expected viscosity of the glue can be achieved at a dosage of 2%, which provides a basis for reducing the content of the binder in the positive electrode film layer.
  • Controlling the viscosity of the polyvinylidene fluoride glue within a suitable range and adding a low amount of binder can ensure excellent adhesion of the pole pieces.
  • the viscosity of the binder solution can be tested using methods known in the art, such as the rotational viscometer test method.
  • the Lichen Technology NDJ-5S rotational viscometer for testing.
  • a method for preparing an adhesive including The following steps: provide vinylidene fluoride monomer and solvent, perform the first stage polymerization reaction, and obtain the first product; perform the second stage polymerization reaction of the first product in a water-insoluble gas atmosphere; add a chain transfer agent, and perform the third stage polymerization reaction.
  • Stage polymerization reaction produces polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million.
  • the first product can be the reaction liquid formed after the first stage polymerization reaction, or the product after processing and purification of the above reaction liquid.
  • multiple parts of the first product are mixed, and the second stage polymerization reaction is performed under a water-insoluble gas atmosphere. It can be understood that multiple portions of the first product can be simultaneously prepared through multiple reaction kettles, or can be prepared multiple times through one reaction kettle. The uniformity of the polyproduct can be improved through multiple, segmented synthesis methods.
  • ultra-high molecular weight polyvinylidene fluoride can be prepared, so that the binder can meet the needs of the electrode plate bonding force at a low addition amount, which helps to improve the positive electrode active material in the electrode plate.
  • the load capacity is beneficial to improving the capacity retention rate of the battery during the cycle.
  • the first product with a certain molecular weight is first formed in the first stage of polymerization reaction, and then the molecular chain segment with the target molecular weight is formed in the second stage of polymerization reaction, and then the molecular weight of the polymer is controlled through the third stage polymerization reaction to avoid excessive molecular weight.
  • segmented polymerization can improve the utilization rate of the reactor during the preparation of polyvinylidene fluoride, save time, and reduce the amount of polyvinylidene fluoride in the reactor. Dwell time.
  • the cooperation of the first-stage polymerization reaction, the second-stage polymerization reaction, and the third-stage polymerization reaction is conducive to further improving the production efficiency of polyvinylidene fluoride.
  • the reaction temperature of the first stage polymerization reaction is 45°C to 60°C. In some embodiments, the reaction temperature of the first-stage polymerization reaction can be selected from any one of 45°C to 50°C, 50°C to 55°C, 55°C to 60°C, and 45°C to 55°C.
  • the reaction time of the first stage polymerization reaction is 4 hours to 10 hours. In some embodiments, the reaction time of the first stage polymerization reaction can be selected from 4 hours to 5 hours, 5 hours to 6 hours, 6 hours to 7 hours, 7 hours to 8 hours, 8 hours to 9 hours, 9 hours to Any one of 10 hours, 4 hours to 6 hours, 6 hours to 8 hours, 8 hours to 10 hours, 5 hours to 10 hours.
  • the initial polymerization pressure is 4MPa to 6MPa. In some implementations, the initial polymerization pressure can be selected from 4MPa to 5MPa or 5MPa to 6MPa. In some embodiments, the initial polymerization pressure is higher than the critical pressure of vinylidene fluoride.
  • the reaction temperature of the second stage polymerization reaction is 60°C to 80°C. In some embodiments, the reaction temperature of the second stage polymerization reaction can be selected from 60°C to 70°C, or 70°C to 80°C.
  • reaction time of the second stage polymerization reaction is 2 hours to 4 hours. In some embodiments, the reaction time of the second stage polymerization reaction can be selected from any one of 2 hours to 3 hours, or 3 hours to 4 hours.
  • the reaction pressure of the second stage polymerization reaction is 6MPa ⁇ 8MPa. In some embodiments, the reaction pressure of the second stage polymerization reaction can be selected from any one of 6MPa to 7MPa and 7MPa to 8MPa.
  • the reaction time of the third stage polymerization reaction is 1 hour to 2 hours.
  • the uniformity of the weight average molecular weight of the polymerized product can be controlled to ensure that the product has a lower
  • the polydispersity coefficient improves the balance of polyvinylidene fluoride performance, allowing the pole piece to have excellent adhesion with a low amount of binder added, and the cycle capacity retention rate of the battery can be further improved.
  • the chain transfer agent includes one or more of cyclohexane, isopropyl alcohol, methanol, and acetone.
  • Water-insoluble gas refers to a gas with a gas solubility less than 0.1L.
  • Gas solubility refers to the volume of gas when it is dissolved in 1L of water and reaches saturation when the pressure of the gas is 1.013 ⁇ 10 5 Pa at 20°C.
  • the water-insoluble gas is selected from one or more of nitrogen, oxygen, hydrogen, and methane.
  • the amount of chain transfer agent is 1.5% to 3% of the total mass of vinylidene fluoride monomer.
  • the amount of chain transfer agent can also be, for example, 2% or 2.5%.
  • Controlling the amount of chain transfer agent within an appropriate range can control the polymer chain length, thereby obtaining a polymer with a suitable molecular weight range and uniform distribution.
  • the first stage polymerization reaction includes the following steps:
  • the amount of solvent used is 2 to 8 times the total mass of vinylidene fluoride monomer.
  • the amount of solvent used may also be, for example, 3, 4, 5, 6 or 7 times the total mass of vinylidene fluoride monomer.
  • the solvent is deionized water.
  • the dispersant includes one or more of cellulose ethers and polyvinyl alcohol.
  • the cellulose ether includes one or more of methyl cellulose ether and carboxyethyl cellulose ether.
  • the amount of dispersant is 0.1% to 0.3% of the total mass of vinylidene fluoride monomer.
  • the amount of dispersant used may also be, for example, 0.2% of the total mass of vinylidene fluoride monomer.
  • the initiator is an organic peroxide.
  • organic peroxides include t-amyl peroxypivalate, t-amyl peroxypivalate, 2-ethylperoxydicarbonate, diisopropylperoxydicarbonate, and One or more types of tert-butyl peroxypivalate.
  • the amount of initiator is 0.15% to 1% of the total mass of vinylidene fluoride monomer.
  • the amount of initiator used may also be, for example, 0.2%, 0.4%, 0.6% or 0.8% of the mass of the vinylidene fluoride monomer.
  • the pH adjusting agent includes one or more of potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, and ammonia.
  • the amount of pH adjuster is 0.05% to 0.2% of the total mass of vinylidene fluoride monomer.
  • the amount of pH adjuster can be, for example, 0.1% or 0.15% of the total mass of vinylidene fluoride monomer.
  • the positive electrode sheet includes a positive electrode current collector and is disposed on at least one surface of the positive electrode current collector.
  • the positive electrode film layer includes a positive electrode active material, a conductive agent and a binder in some embodiments or a binder prepared by a preparation method in some embodiments.
  • the positive electrode sheet has excellent bonding force with a low additive amount of binder.
  • the mass fraction of the binder is 0.4% to 0.6%, based on the total mass of the positive electrode film layer. In some embodiments, the mass fraction of the binder is one of 0.4% to 0.5% and 0.5% to 0.6%.
  • Controlling the mass fraction of the binder within an appropriate range can not only ensure that the electrode pieces have sufficient adhesive force, but also increase the loading of active materials in the battery electrode pieces, helping to further improve the power performance of the battery.
  • the positive electrode current collector has two surfaces facing each other in its own thickness direction, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector.
  • the positive electrode current collector may be a metal foil or a composite current collector.
  • the metal foil aluminum foil can be used.
  • the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer.
  • the composite current collector can be formed by forming metal materials (aluminum, aluminum alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc.) on polymer material substrates (such as polypropylene (PP), polyterephthalate It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • PP polypropylene
  • PBT polybutylene terephthalate
  • PS polystyrene
  • PE polyethylene
  • the cathode active material may be a cathode active material known in the art for batteries.
  • the cathode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds.
  • this application is not limited to these materials, and other possible materials can also be used.
  • lithium transition metal oxides may include, but are not limited to, lithium cobalt oxides (such as LiCoO 2 ), lithium nickel oxides (such as LiNiO 2 ), lithium manganese oxides (such as LiMnO 2 , LiMn 2 O 4 ), lithium Nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1/3 Co 1/3 Mn 1/3 O 2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O 2 (can also be abbreviated to NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O 2 (can also be abbreviated to NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O 2 (can also be abbreviated to NCM 622 ), LiNi At least one of 0.8 Co 0.1 Mn 0.1 O 2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as Li Li
  • the olivine structure contains Examples of lithium phosphates may include, but are not limited to, lithium iron phosphate (such as LiFePO 4 (also referred to as LFP)), composites of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO 4 ), lithium manganese phosphate and carbon. At least one of composite materials, lithium iron manganese phosphate, and composite materials of lithium iron manganese phosphate and carbon.
  • lithium iron phosphate such as LiFePO 4 (also referred to as LFP)
  • composites of lithium iron phosphate and carbon such as LiMnPO 4
  • LiMnPO 4 lithium manganese phosphate and carbon.
  • At least one of composite materials, lithium iron manganese phosphate, and composite materials of lithium iron manganese phosphate and carbon At least one of composite materials, lithium iron manganese phosphate, and composite materials of lithium iron manganese phosphate and carbon.
  • the positive electrode film layer optionally further includes a conductive agent.
  • 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 positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components in a solvent (such as N -methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode piece can be obtained.
  • a solvent such as N -methylpyrrolidone
  • the negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, where the negative electrode film layer includes a negative electrode active material.
  • the negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.
  • the negative electrode current collector may be a metal foil or a composite current collector.
  • the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base material.
  • the composite current collector can be formed by forming metal materials (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyterephthalate It is formed on substrates such as ethylene glycol ester (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).
  • PP polypropylene
  • PBT polybutylene terephthalate
  • PS polystyrene
  • PE polyethylene
  • the negative active material may be a negative active material known in the art for batteries.
  • the negative active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, and the like.
  • the silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon carbon composites, silicon nitrogen composites and silicon alloys.
  • the tin-based material may be selected from at least one of elemental tin, tin oxide compounds and tin alloys.
  • the present application is not limited to these materials, and other traditional materials that can be used as battery negative electrode active materials can also be used. Only one type of these negative electrode active materials may be used alone, or two or more types may be used in combination.
  • the negative electrode film layer optionally further includes a binder.
  • the binder can be selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polysodium acrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), poly At least one of methacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).
  • the negative electrode film layer optionally further includes a conductive agent.
  • the conductive agent may 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 optionally includes other auxiliaries, 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 by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative active materials, conductive agents, binders and any other components 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 piece can be obtained.
  • a solvent such as deionized water
  • the electrolyte plays a role in conducting ions between the positive and negative electrodes. This application There are no specific restrictions on the type of electrolyte, you can choose according to your needs.
  • the electrolyte can be liquid, gel, or completely solid.
  • the electrolyte is an electrolyte solution.
  • the electrolyte solution includes electrolyte salts and solvents.
  • the electrolyte salt may be selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bistrifluoromethanesulfonimide, trifluoromethane At least one of lithium sulfonate, lithium difluorophosphate, lithium difluoroborate, lithium dioxaloborate, lithium difluorodioxalate phosphate and lithium tetrafluoroxalate phosphate.
  • the solvent may be selected from the group consisting of ethylene carbonate, propylene carbonate, methylethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl 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 optionally further includes additives.
  • additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain properties of the battery, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.
  • the secondary battery further includes a separator film.
  • a separator film There is no particular restriction on the type of isolation membrane in this application. Any well-known porous structure isolation membrane with good chemical stability and mechanical stability can be used.
  • the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride.
  • the isolation film can be a single-layer film or a multi-layer composite film, with no special restrictions. When the isolation film is a multi-layer composite film, the materials of each layer can be the same or different, and there is no particular limitation.
  • the positive electrode piece, the negative electrode piece and the separator film can be made into an electrode assembly through a winding process or a lamination process.
  • the secondary battery may include an outer packaging.
  • the outer packaging can be used to package the above-mentioned electrode assembly and electrolyte.
  • the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.
  • the outer packaging of the secondary battery may also be a soft bag, such as a bag-type soft bag.
  • the soft bag may be made of plastic. Examples of plastic include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like.
  • FIG. 1 shows a square-structured 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 side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity.
  • the housing 51 has an opening communicating with the accommodation cavity, and the cover plate 53 can cover the opening to close the accommodation cavity.
  • the positive electrode piece, the negative electrode piece and the isolation film can be formed into the electrode assembly 52 through a winding process or a lamination process.
  • the electrode assembly 52 is packaged in the containing cavity.
  • the electrolyte soaks into 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.
  • secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in the battery module can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery module.
  • FIG. 3 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 can be fixed by fasteners.
  • the battery module 4 may further include a housing having a receiving space in which a plurality of secondary batteries 5 are received.
  • the above-mentioned battery modules can also be assembled into a battery pack.
  • the number of battery modules contained in the battery pack can be one or more. Those skilled in the art can select the specific number 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 2 and a lower box 3.
  • the upper box 2 can be covered with the lower box 3 and form a An enclosed 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 electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided by the present application.
  • the secondary battery, battery module, or battery pack may be used as a power source for the electrical device, or may be used as an energy storage unit for the electrical device.
  • the electric device may 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, and electric golf carts). , electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited to these.
  • a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.
  • FIG. 6 is an electrical device as an example.
  • the electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc.
  • a battery pack or battery module can be used.
  • the device may be a mobile phone, a tablet, a laptop, etc.
  • the device is usually required to be thin and light, and a secondary battery can be used as a power source.
  • the first stage of polymerization reaction Add 4kg of deionized water and 2g of methylcellulose ether to the 10L autoclave of No.1 and No.2, vacuum and replace O2 with N2 three times, and add 5g of tert-butyl peroxide again Pivalate and 2g of sodium bicarbonate, and filled with 1kg of vinylidene fluoride
  • the pressure of the monomer reaches 5MPa, mix and stir for 30 minutes, raise the temperature to 45°C, and react for 4 hours;
  • the second stage of polymerization reaction transfer the reaction liquid in No. 1 and No. 2 reactors to No. 3 reactor, fill with nitrogen to a pressure of 7MPa, raise the temperature to 70°C, and stir for 3 hours;
  • the third stage of polymerization reaction add 40g of cyclohexane and continue the reaction for 1 hour, then stop the reaction.
  • the reaction system is centrifuged and the solid phase is collected, washed and dried to obtain the polyvinylidene fluoride binder.
  • NMP N-methylpyrrolidone
  • NMP N-methylpyrrolidone
  • Metal lithium sheets are used as negative electrode sheets.
  • Example 1 The positive electrode sheet, negative electrode sheet, separator and electrolyte in Example 1 were charged Assembled into a button cell in the box.
  • Example 1 Basically the same as Example 1, the difference is that the reaction times in the first stage polymerization reaction are adjusted to 5h, 6h, 7h, and 8h respectively, and the cyclohexane in the third stage polymerization reaction is adjusted to 35g, 30g, and 25g respectively. , 20g, the specific parameters are shown in Table 1.
  • Example 2 It is basically the same as Example 2, except that the mass fraction of the polyvinylidene fluoride binder is adjusted, based on the total mass of the positive electrode film layer.
  • the specific parameters are as shown in Table 1.
  • Example 2 It is basically the same as Example 1, except that the mass fraction of the polyvinylidene fluoride binder is adjusted, based on the total mass of the positive electrode film layer.
  • the specific parameters are as shown in Table 1.
  • Example 3 It is basically the same as Example 3, except that the mass fraction of the polyvinylidene fluoride binder is adjusted, based on the total mass of the positive electrode film layer.
  • the specific parameters are as shown in Table 1.
  • Example 4 It is basically the same as Example 4, except that the mass fraction of the polyvinylidene fluoride binder is adjusted, based on the total mass of the positive electrode film layer.
  • the specific parameters are as shown in Table 1.
  • Example 5 It is basically the same as Example 5, except that the mass fraction of the polyvinylidene fluoride binder is adjusted, based on the total mass of the positive electrode film layer.
  • the specific parameters are as shown in Table 1.
  • the binder is polyvinylidene fluoride with a weight average molecular weight of 800,000, purchased from Dongyangguang Company, model number 701A, and the mass fraction of the binder is adjusted to 2%, based on the positive electrode film The total mass of the layer is measured, and the specific parameters are shown in Table 1.
  • the binder is polyvinylidene fluoride with a weight average molecular weight of 1 million, and the preparation method is:
  • the binder is polyvinylidene fluoride with a weight average molecular weight of 2 million, and the preparation method is:
  • the binder is polyvinylidene fluoride with a weight average molecular weight of 3 million, and the preparation method is:
  • the binder is polyvinylidene fluoride with a weight average molecular weight of 4 million, and the preparation method is:
  • the battery capacity retention rate test process is as follows: At 25°C, charge the button battery to 3.65V at a constant current of 1/3C, then charge to a constant voltage of 3.65V until the current is 0.05C, leave it aside for 5 minutes, and then discharge at 1/3C to 2.5V, and the resulting capacity is recorded as the initial capacity C0. Repeat the above steps for the same battery, and record the discharge capacity Cn of the battery after the nth cycle.
  • the battery capacity retention rate data corresponding to Examples 1 to 17 or Comparative Examples 1 to 6 in Table 1 is the data measured after 500 cycles under the above test conditions, that is, the value of P500.
  • Figure 7 is a bonding force-displacement diagram between Example 10 and Comparative Example 1. It can be seen from the figure that at the same displacement, the bonding force of Example 10 is significantly higher than that of Comparative Example 1, indicating that under When the amount of binder added is low, the polyvinylidene fluoride binder provided in this application enables the pole piece to have excellent bonding force.
  • Figure 8 is a graph showing the battery capacity retention rate and the number of cycles in Example 10 and Comparative Example 1. It can be seen from the figure that after 500 battery cycles After several times, the cycle capacity retention rate of Example 10 is significantly higher than that of Comparative Example 1, indicating that the polyvinylidene fluoride binder provided by the present application can improve the performance of the battery during the cycle when the amount of binder added is low. The cycle capacity retention rate effectively improves the damage to pole pieces and battery performance caused by the high dosage of binders in traditional technology.
  • the binders in Examples 1 to 17 all include polyvinylidene fluoride with a weight average molecular weight of 5 million to 9 million. From the comparison between Examples 1 to 17 and Comparative Example 1, it can be seen that a polyvinylidene fluoride binder with a weight average molecular weight of 5 million to 9 million can make the pole piece have excellent bonding force and improve the cycle performance of the battery at a low addition amount. Capacity retention during the process.
  • a polyvinylidene fluoride binder with a polydispersity coefficient of 1.8 to 2.5 and a weight average molecular weight of 5 million to 9 million can achieve excellent bonding of the pole pieces at a low addition amount. power, the battery has a high capacity retention rate during cycling.
  • the polyvinylidene fluoride provided by the present application with a weight average molecular weight of 5 million to 9 million has a crystallinity of 40% to 46%.
  • the weight average molecular weight of the binder is compared with traditional The weight average molecular weight of the binder has increased significantly, which can effectively reduce the amount of binder used.
  • the crystallinity of polyvinylidene fluoride has not increased significantly and is still below 50%, which is within the allowable value range and can meet the extreme requirements. Application requirements for sheet adhesives.
  • Example 2 From the comparison of Example 2, Examples 7-8 and Example 6, it can be seen that when the mass fraction of the binder is 0.4%-0.6%, based on the total mass of the positive electrode film layer, the binder can ensure that the electrode piece has sufficient The adhesion force of the battery is improved, and the capacity retention rate of the battery during cycling is further improved. Improve step by step. From the comparison of Example 2, Examples 7-8 and Example 9, it can be seen that when the binder mass fraction is 0.7%, excessively high binder content will not significantly improve the adhesive force of the pole piece and the cycle performance of the battery. , on the contrary, it is not conducive to the improvement of active material loading and limits the further improvement of battery energy density.

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Abstract

本申请提供了一种粘结剂、制备方法、正极极片、二次电池及用电装置。粘结剂包括重均分子量为500万～900万的聚偏氟乙烯。该粘结剂在低添加量下就能够保证极片具有足够的粘结力，电池具有提高的循环性能，该粘结剂有助于减少电池中粘结剂的用量，利于极片中活性材料负载量的进一步提高。

Description

粘结剂、制备方法、正极极片、二次电池及用电装置
交叉引用
本申请引用于2022年8月30日递交的名称为“粘结剂、制备方法、正极极片、二次电池及用电装置”的第202211044756.7号中国专利申请，其通过引用被全部并入本申请。
技术领域
本申请涉及二次电池技术领域，尤其涉及一种粘结剂、制备方法、正极极片、二次电池、电池模块、电池包及用电装置。
背景技术
近年来，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。随着二次电池应用的普及，对其循环性能、使用寿命等也提出了更高的要求。
粘结剂是二次电池中的常用材料，在电池的极片、隔离膜、封装处等均有很大需求。但是现有的粘结剂粘结性差，往往需要大量添加才能满足极片粘结力的要求，这会限制电池能量密度的提升。因此，现有的粘结剂仍有待改进。
发明内容
本申请是鉴于上述课题而进行的，其目的在于，提供一种粘结剂，该粘结剂在低添加量下即可以发挥优异的粘结力，使得极片具有足够的粘结强度，并且能够提高电池的循环性能。
为了达到上述目的，本申请提供了一种粘结剂，粘结剂包括重均分子量为500万～900万的聚偏氟乙烯。
该粘结剂在低添加量下就能够保证极片具有足够的粘结力，提 高电池的循环性能。
在任意实施方式中，聚偏氟乙烯的多分散系数为1.8～2.5，可选为1.9～2.3。
超高分子量的聚偏氟乙烯的多分散系数在合适范围内，超高分子量的聚偏氟乙烯的重均分子量分布均匀，性能均衡，能够保证粘结剂在低添加量下就使得极片具有足够的粘结力，电池在循环过程中的容量保持率进一步提高。
在任意实施方式中，聚偏氟乙烯的Dv50粒径为100μm～200μm，可选为120μm～200μm。
控制聚偏氟乙烯的Dv50粒径在合适范围内，超高分子量的聚偏氟乙烯依然具有良好的加工性能，能够保证极片和电池的生产效率。
在任意实施方式中，聚偏氟乙烯的结晶度为40％～46％，可选为41％～46％。
控制聚偏氟乙烯的结晶度在合适范围内，粘结剂在低添加量满足极片粘结力和电池循环性能的基础上，不会对电池柔性带来过大影响，依然能够满足极片的生产需要。
在任意实施方式中，聚偏氟乙烯溶于N-甲基吡咯烷酮制得的胶液的粘度为2000mPa·s～5000mPa·s，可选为2100mPa·s～4300mPa·s，其中聚偏氟乙烯的质量含量为2％，基于胶液的总质量计。
控制聚偏氟乙烯的胶液的粘度在合适范围内，低添加量的粘结剂就能够保证极片具有优异的粘结力。
本申请的第二方面还提供一种粘结剂的制备方法，包括以下步骤：提供偏氟乙烯单体和溶剂，进行第一段聚合反应，得到第一产物；将第一产物在非水溶性气体氛围下进行第二段聚合反应；加入链转移剂，进行第三段聚合反应，得到重均分子量为500万～900万的聚偏氟乙烯。
该粘结剂的制备方法通过分段聚合，能够制备出超高分子量的聚偏氟乙烯粘结剂。该粘结剂在低添加量下即可以满足极片粘结力的需求，有助于提高极片中正极活性材料的负载量，利于提高电池在循环过程中的容量保持率。
在任意实施方式中，第一段聚合反应的反应温度为45℃～60℃，反应时间为4小时～10小时，初始聚合压力为4MPa～6MPa。
在任意实施方式中，第二段聚合反应的反应温度为60℃～80℃，反应时间为2小时～4小时，反应压力为6MPa～8MPa。
在任意实施方式中，第三段聚合反应的反应时间为1小时～2小时。
控制各个阶段聚合反应的反应压力、反应时间、反应温度在合适的范围内，在实现聚偏氟乙烯重均分子量提高的同时，可以控制聚合产物重均分子量的均匀性，保证产物具有较低的多分散系数，提高聚偏氟乙烯性能的一致性，使得极片在粘结剂低添加量下即具有优异的粘结力，且电池的循环容量保持率能够进一步提高。
在任意实施方式中，链转移剂包括环己烷、异丙醇、甲醇以及丙酮中的一种或多种。
在任意实施方式中，非水溶性气体选自氮气、氧气、氢气、甲烷中的一种或多种。
在任意实施方式中，链转移剂的用量为偏氟乙烯单体质量的1.5％～3％。
在任意实施方式中，第一阶段反应包括以下步骤：向容器中加入溶剂和分散剂，去除反应体系中的氧气；向所述容器中加入引发剂和pH调节剂，调节pH值至6.5～7，然后加入偏氟乙烯单体，使容器中的压力达到4MPa～6MPa；搅拌30分钟～60分钟后，升温至45℃～60℃，进行第一段聚合反应。
在任意实施方式中，溶剂的用量为偏氟乙烯单体质量的2～8倍。
在任意实施方式中，分散剂包括纤维素醚和聚乙烯醇中的一种或多种。
在任意实施方式中，纤维素醚包括甲基纤维素醚和羧乙基纤维素醚中的一种或多种。
在任意实施方式中，分散剂的用量为偏氟乙烯单体质量的0.1％～0.3％。
在任意实施方式中，引发剂为有机过氧化物。
在任意实施方式中，有机过氧化物包括过氧化新戊酸叔戊酯、过氧化叔戊基新戊酸酯、2-乙基过氧化二碳酸酯、二异丙基过氧化二碳酸酯以及叔丁基过氧化新戊酸酯中的一种或多种。
在任意实施方式中，引发剂的用量为偏氟乙烯单体质量的0.15％～1％。
在任意实施方式中，pH调节剂包括碳酸钾、碳酸氢钾、碳酸钠、碳酸氢钠以及氨水中的一种或多种。
在任意实施方式中，pH调节剂的用量为偏氟乙烯单体总质量的0.05％～0.2％。
本申请的第三方面提供一种正极极片，包括正极集流体以及设置在正极集流体至少一个表面的正极膜层，正极膜层包括正极活性材料、导电剂和任意实施方式中的粘结剂或任意实施方式中的制备方法制备的粘结剂。
在任意实施方式中，粘结剂的质量分数为0.4％～0.6％，基于正极膜层的总质量计。
控制粘结剂的质量分数在合适范围内在确保有效粘结力的同时能够提高电池极片中活性物质的负载量，有助于进一步提高电池的功率性能。
在本申请的第四方面，提供一种二次电池，包括电极组件和电解液，所述电极组件包括隔离膜、负极极片和本申请第三方面的正极极片，可选地，二次电池为锂离子电池或钠离子电池。
在本申请的第五方面，提供一种电池模块，包括本申请第四方面的二次电池。
在本申请的第六方面，提供一种电池包，包括本申请第五方面的电池模块。
在本申请的第七方面，提供一种用电装置，包括本申请第四方面的二次电池、第五方面的电池模块或第六方面的电池包中的至少一种。
附图说明
图1是本申请一实施方式的二次电池的示意图；
图2是图1所示的本申请一实施方式的二次电池的分解图；
图3是本申请一实施方式的电池模块的示意图；
图4是本申请一实施方式的电池包的示意图；
图5是图4所示的本申请一实施方式的电池包的分解图；
图6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；
图7是实施例10和对比例1的粘结力-位移图；
图8是实施例10和对比例1的电池容量保持率与循环次数的曲线图。
附图标记说明：
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盖板。
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，所述方法包括步骤(a)和(b)，表示所述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，所述提到所述方法还可包括步骤(c)，表示步骤(c)可以任意顺序加入到所述方法，例如，所述方法可以包括步骤(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，所述“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)。
聚偏氟乙烯是目前二次电池中使用最为广泛的粘结剂种类之一。然而，传统聚偏氟乙烯的粘度低，往往需要大量添加才能保证活性物质的有效粘结，从而使得极片达到有效的粘结力。然而传统聚偏氟乙烯用量的提高会降低活性材料在极片中的负载量，影响电池功 率性能的提升，难以满足对于电池循环性能的要求。
[粘结剂]
基于此，本申请提出了一种粘结剂，粘结剂包括重均分子量为500万～900万的聚偏氟乙烯。
在本文中，术语“粘结剂”是指在分散介质中形成胶体溶液或胶体分散液的化学化合物、聚合物或混合物。
在本文中，术语“聚偏氟乙烯”是指以偏氟乙烯为主要合成单体的聚合物，聚合物一方面包括通过聚合反应制备的化学上均一的、但在聚合度、摩尔质量和链长方面不同的大分子的集合体。该术语另一方面也包括由聚合反应形成的这样的大分子集合体的衍生物，即可以通过上述大分子中的官能团的反应，例如加成或取代获得的并且可以是化学上均一的或化学上不均一的化合物。本文中的聚偏氟乙烯既包括均聚物，也包括共聚物。
在本文中，术语“重均分子量”是指聚合物中用不同分子量的分子所占的重量分数与其对应的分子量乘积的总和。
在一些实施方式中，粘接剂的分散介质是油性溶剂，油性溶剂的示例包括但不限于二甲基乙酰胺、N，N-二甲基甲酰胺、N-甲基吡咯烷酮、丙酮、碳酸二甲酯、乙基纤维素、聚碳酸酯。即，粘结剂溶解于油性溶剂中。
在一些实施方式中，粘结剂用于将电极活性物质及/或导电剂固定在合适位置并将它们粘附在导电金属部件以形成电极。
在一些实施方式中，粘结剂作为正极粘结剂，用于粘结正极活性材料及/或导电剂以形成电极。
在一些实施方式中，粘结剂作为负极粘结剂，用于粘结负极活性材料及/或导电剂以形成电极。
在一些实施方式中，聚偏氟乙烯的重均分子量为500万～600万、600万～700万、700万～800万、800万～900万、600万～900万、700万～900万中的任意一种。
聚偏氟乙烯中的氟元素与活性材料表面及集流体表面的羟基或/和羧基形成氢键作用，能够提高极片的粘结力。重均分子量为500 万～900万的聚偏氟乙烯，能够在低水平添加量下提高极片的粘结力，提高电池在循环过程中的容量保持率。
上述粘结剂在低添加量下就能够保证极片具有足够的粘结力，有利于提高电池的能量密度以及电池的循环性能。
在本申请中，聚偏氟乙烯的重均分子量的测试可以选用本领域已知的方法进行测试，例如采用凝胶色谱法进行测试，如采用Waters 2695 Isocratic HPLC型凝胶色谱仪(示差折光检测器2141)进行测试。在一些实施方式中，测试方法为以质量分数为3.0％的聚苯乙烯溶液试样做参比，选择匹配的色谱柱(油性：Styragel HT5DMF7.8*300mm+Styragel HT4)。用纯化后的N-甲基吡咯烷酮(NMP)溶剂配置3.0％的聚偏氟乙烯胶液，配置好的溶液静置一天，备用。测试时，先用注射器吸取四氢呋喃，进行冲洗，重复几次。然后吸取5ml实验溶液，排除注射器中的空气，将针尖擦干。最后将试样溶液缓缓注入进样口。待示数稳定后获取数据，读取重均分子量。
在一些实施方式中，聚偏氟乙烯的多分散系数为1.8～2.5。在一些实施方式中，聚偏氟乙烯的多分散系数可选为1.8～1.9、1.9～2.0、2.0～2.1、2.1～2.2、2.2～2.3、2.3～2.4、2.4～2.5、1.8～2.0、2.0～2.2、2.2～2.4、1.9～2.3、2～2.3中的任意一种。
在本文书，术语“多分散系数”是指聚合物的重均分子量与聚合物的数均分子量的比值。
在本文中，术语“数均分子量”是指聚合物中用不同分子量的分子所占的摩尔分数与其对应的分子量乘积的总和。
若聚偏氟乙烯的多分散系数过大，则聚偏氟乙烯的聚合度较为分散，降低粘结剂的均匀性，导致粘结剂无法将正极活性材料均匀的粘附在集流体上，进而影响电池的循环性能，同时也使得浆料的固含量下降，无法进一步提高电池的能量密度；若聚偏氟乙烯的多分散系数过小，制备工艺难度较大，且优率较低，导致生产成本较高。
控制聚偏氟乙烯的多分散系数在合适范围内，超高分子量的聚 偏氟乙烯的重均分子量分布均匀，性能稳定，能够保证粘结剂在低添加量下就使得极片具有足够的粘结力，电池在循环过程中的容量保持率进一步提高。另外聚偏氟乙烯具有合适的多分散系数，能有效提升浆料固含量，降低生产成本。
本申请中，多分散系数的测试可以选用本领域已知的方法进行测试，例如采用凝胶色谱法进行测试，如采用Waters 2695 Isocratic HPLC型凝胶色谱仪(示差折光检测器2141)进行测试。在一些实施方式中，以质量分数为3.0％的聚苯乙烯溶液试样做参比，选择匹配的色谱柱(油性：Styragel HT5DMF7.8*300mm+Styragel HT4)。用纯化后的N-甲基吡咯烷酮(NMP)溶剂配置3.0％的聚偏氟乙烯胶液，配置好的溶液静置一天，备用。测试时，先用注射器吸取四氢呋喃，进行冲洗，重复几次。然后吸取5ml实验溶液，排除注射器中的空气，将针尖擦干。最后将试样溶液缓缓注入进样口。待示数稳定后获取数据。分别读取重均分子量a和数均分子量b。多分散系数＝a/b。
在一些实施方式中，聚偏氟乙烯的Dv50粒径为100μm～200μm。在一些实施方式中，聚偏氟乙烯的Dv50粒径可选为120μm～140μm、140μm～160μm、160μm～180μm、180μm～200μm、120μm～150μm、150μm～180μm、120μm～200μm、140μm～200μm中的任意一种。
在本文中，术语“Dv50粒径”指在粒度分布曲线中，颗粒的累计粒度分布数达到50％时所对应的粒径，它的物理意义是粒径小于(或大于)它的颗粒占50％。
若聚偏氟乙烯的Dv50粒径过大，聚偏氟乙烯溶解相对困难，降低粘结剂的分散性，影响正极活性材料在集流体上的均匀分布和电池的循环性能，同时聚偏氟乙烯的溶解困难，降低制浆过程的速度；若聚偏氟乙烯的Dv50粒径过小，极片的粘结力下降。
控制聚偏氟乙烯的Dv50粒径在合适范围内，超高分子量的聚偏氟乙烯依然具有良好的加工性能，能够保证极片和电池的生产效率。同时合适范围的聚偏氟乙烯的Dv50粒径，还能使得粘结剂的用量可以被控制在较低的水平，且不会对粘结性能造成过大的负面影响， 从而有效改善了传统技术中高用量粘结剂带来的极片和电池性能受损的情况。
参照GB/T 19077-2016粒度分布激光衍射法，用50ml烧杯称量0.1g～0.13g的聚偏氟乙烯粉料，再称取5g无水乙醇，加入到装有聚偏氟乙烯粉料的烧杯中，放入长度约2.5mm的搅拌子，并用保鲜膜密封。将样品放入超声机超声5min，转移到磁力搅拌机用500转/分钟的速度搅拌20分钟以上，每批次产品抽取2个样品测试取平均值。采用激光粒度分析仪进行测定，如英国马尔文仪器有限公司的Mastersizer 2000E型激光粒度分析仪进行测试。
在一些实施方式中，聚偏氟乙烯的结晶度为40％～46％。在一些实施方式中，聚偏氟乙烯的结晶度可选为41％～42％、42％～43％、43％～44％、41％～42％、42％～43％、43％～44％、44％～45％、45％～46％、41％～43％、43％～45％、41％～46％中的任意一种。
若聚偏氟乙烯的结晶度过小，聚合物分子链的规整密堆积程度下降，影响粘结剂的化学稳定性和热稳定性。但是若聚偏氟乙烯的结晶度过大，聚偏氟乙烯链段的可移动性降低，影响极片的柔韧性，同时聚偏氟乙烯的溶解困难，降低制浆过程的速度。在极片生产过程中，通常要求聚偏氟乙烯的结晶度在50％以下。
本申请的聚偏氟乙烯的结晶度在合适范围内，粘结剂在低添加量满足极片粘结力和电池循环性能的基础上，不会对极片的柔性带来过大影响，依然能够满足极片的生产需要。
本申请中，结晶度的测试可以选用本领域已知的方法进行测试，如采用差式扫描热分析法进行测试。在一些实施例中，将0.5g聚偏氟乙烯置于铝制坩埚中，抖平，盖上坩埚盖子，在氮气气氛下，以50毫升/分钟的吹扫气，以70毫升/分钟的保护气，升温速率为每分钟10℃，测试温度范围-100℃～400℃，利用美国TA仪器型号为Discovery 250的差示扫描量热仪(DSC)进行测试并消除热历史。
此测试将会得到聚偏氟乙烯的DSC曲线，并对曲线进行积分，峰面积即为聚偏氟乙烯的熔融焓ΔH(J/g)，粘结剂结晶度＝(ΔH/ΔHm)×100％，其中ΔHm为聚偏氟乙烯的标准熔融焓(晶态熔 化热)，ΔHm＝104.7J/g。
在一些实施方式中，聚偏氟乙烯溶于N-甲基吡咯烷酮制得的胶液的粘度为2000mPa·s～5000mPa·s，其中聚偏氟乙烯的质量含量为2％，基于胶液的总质量计。在一些实施方式中，聚偏氟乙烯溶于N-甲基吡咯烷酮制得的胶液的粘度可选为2100mPa·s～2700mPa·s、2700mPa·s～3400mPa·s、3400mPa·s～3800mPa·s、3800mPa·s～4300mPa·s、2100mPa·s～4300mPa·s、2700mPa·s～4300mPa·s、3400mPa·s～4300mPa·s中的任意一种。
若聚偏氟乙烯的胶液的粘度过大，制备的粘结剂溶液的粘度过大，难以搅动，降低粘结剂的分散性，使得粘结剂难以将正极活性材料均匀的粘附在集流体上，影响电池的循环性能，同时粘结剂溶液的粘度过大，降低制浆过程的速度；若粘结剂溶液的粘度过小，极片在低添加量下难以具有足够的粘结力。
另外制备正极浆料时，粘结剂溶液需要具有一定的粘度，才能防止正极活性材料以及导电剂等助剂的沉降，使浆料能较稳定地存储。现有技术中，要达到2000mPa·s～5000mPa·s的胶液粘度，至少需要质量分数为7％的粘结剂才能实现，基于正极膜层的质量计，而本申请的聚偏氟乙烯在2％的用量下就可以实现胶液的预期粘度，为降低粘结剂在正极膜层中的含量提供了基础。
控制聚偏氟乙烯的胶液的粘度在合适范围内，低添加量的粘结剂就能够保证极片具有优异的粘结力。
本申请中，粘结剂溶液的粘度可以采用本领域已知的方法进行测试，如旋转粘度计测试法。作为示例，用500ml烧杯分别称取7g聚偏氟乙烯和343g N-甲基吡咯烷酮(NMP)，配置成质量分数2％的胶液，使用力辰高速研磨机搅拌分散，转速800转/分钟，搅拌时间120分钟后超声震荡30分钟去除气泡。在室温下，使用力辰科技NDJ-5S旋转粘度计进行测试，选用3号转子插入胶液，保证转子液面标志和胶液液面相平，以12转/分钟的转子转速测试粘度，6分钟后读取粘度数据即可。
本申请的一个实施方式中，提供一种粘结剂的制备方法，包括 以下步骤：提供偏氟乙烯单体和溶剂，进行第一段聚合反应，得到第一产物；将第一产物在非水溶性气体氛围下进行第二段聚合反应；加入链转移剂，进行第三段聚合反应，得到重均分子量为500万～900万的聚偏氟乙烯。
可以理解，第一产物既可以为第一段聚合反应后形成的反应液，也可以为上述反应液加工提纯后的产物。
在一些实施方式中，将多份第一产物混合，在非水溶性气体氛围下进行第二段聚合反应。可以理解，多份第一产物可以通过多个反应釜同步制备，也可以通过一个反应釜多次制备。通过多次、分段合成的方法可以提高聚产物的均匀度。
采用分段法进行聚合反应，能制备得到超高分子量的聚偏氟乙烯，使得粘结剂在低添加量下即可以满足极片粘结力的需求，有助于提高极片中正极活性材料的负载量，利于提高电池在循环过程中的容量保持率。同时先在第一段聚合反应中形成具有一定分子量的第一产物，再在第二段聚合反应形成目标分子量的分子链段，再通过第三段聚合反应调控聚合物的分子量能够避免分子量过高降低聚偏氟乙烯重均分子量的均匀性，提高产物的均一性；而且分段聚合能够提升聚偏氟乙烯制备过程中反应器的利用率，节约时间，减少聚偏氟乙烯在反应器中的停留时间。通过第一段聚合反应、第二段聚合反应、第三段聚合反应相互配合，有利于进一步提升聚偏氟乙烯的生产效率。
在一些实施方式中，第一段聚合反应的反应温度为45℃～60℃。在一些实施方式中，第一段聚合反应的反应温度可选为45℃～50℃、50℃～55℃、55℃～60℃、45℃～55℃中的任意一种。
在一些实施方式中，第一段聚合反应的反应时间为4小时～10小时。在一些实施方式中，第一段聚合反应的反应时间可选为4小时～5小时、5小时～6小时、6小时～7小时、7小时～8小时、8小时～9小时、9小时～10小时、4小时～6小时、6小时～8小时、8小时～10小时、5小时～10小时中的任意一种。
在一些实施方式中，初始聚合压力为4MPa～6MPa。在一些实施 方式中，初始聚合压力可选为4MPa～5MPa或5MPa～6MPa。在一些实施方式中，初始聚合压力高于偏氟乙烯的临界压力。
在一些实施方式中，第二段聚合反应的反应温度为60℃～80℃。在一些实施方式中，第二段聚合反应的反应温度可选为60℃～70℃、70℃～80℃中的任意一种。
在一些实施方式中，第二段聚合反应的反应时间为2小时～4小时。在一些实施方式中，第二段聚合反应的反应时间可选为2小时～3小时、3小时～4小时中的任意一种。
在一些实施方式中，第二段聚合反应的反应压力为6MPa～8MPa。在一些实施方式中，第二段聚合反应的反应压力可选为6MPa～7MPa、7MPa～8MPa中的任意一种。
在一些实施方式中，第三段聚合反应的反应时间为1小时～2小时。
控制各个阶段聚合反应的反应压力、反应时间、反应温度在合适的范围内，在实现提高聚偏氟乙烯重均分子量的同时，可以控制聚合产物重均分子量的均匀性，保证产物具有较低的多分散系数，提高聚偏氟乙烯性能的均衡度，使得极片在粘结剂低添加量下即具有优异的粘结力，且电池的循环容量保持率能够进一步提高。
在一些实施方式中，链转移剂包括环己烷、异丙醇、甲醇以及丙酮中的一种或多种。
非水溶性气体是指气体溶解度小于0.1L的气体。气体溶解度是指在20℃时，气体的压强为1.013×105Pa，在1L水里溶解达到饱和状态时气体的体积。在一些实施方式中，非水溶性气体选自氮气、氧气、氢气、甲烷中的一种或多种。
在一些实施方式中，链转移剂的用量为偏氟乙烯单体总质量的1.5％～3％。链转移剂的用量例如还可以是2％或2.5％。
链转移剂的用量控制在合适范围内，能使得聚合物链长可控，从而获得分子量范围合适、分布均一的聚合物。
在一些实施方式中，第一段聚合反应包括以下步骤：
向容器中加入溶剂和分散剂，去除反应体系中的氧气；
向所述容器中加入引发剂和pH调节剂，调节pH值至6.5～7，然后加入偏氟乙烯单体，使容器中的压力达到4MPa～6MPa；
搅拌30分钟～60分钟后，升温至45℃～60℃，进行第一段聚合反应。
升温进行聚合反应前，先将物料混合均匀，能使反应进行得更彻底，制备的聚合物的重均分子量、结晶度以及粒径更均匀。
在一些实施方式中，溶剂的用量为偏氟乙烯单体总质量的2～8倍。溶剂的用量例如还可以是偏氟乙烯单体总质量的3、4、5、6或7倍。在一些实施方式中，溶剂为去离子水。
在一些实施方式中，分散剂包括纤维素醚和聚乙烯醇中的一种或多种。
在一些实施方式中，纤维素醚包括甲基纤维素醚和羧乙基纤维素醚中的一种或多种。
在一些实施方式中，分散剂的用量为偏氟乙烯单体总质量的0.1％～0.3％。分散剂的用量例如还可以是偏氟乙烯单体总质量的0.2％。
在一些实施方式中，引发剂为有机过氧化物。
在一些实施方式中，有机过氧化物包括过氧化新戊酸叔戊酯、过氧化叔戊基新戊酸酯、2-乙基过氧化二碳酸酯、二异丙基过氧化二碳酸酯以及叔丁基过氧化新戊酸酯中的一种或多种。
在一些实施方式中，引发剂的用量为偏氟乙烯单体总质量的0.15％～1％。引发剂的用量例如还可以是偏氟乙烯单体质量的0.2％、0.4％、0.6％或0.8％。
在一些实施方式中，pH调节剂包括碳酸钾、碳酸氢钾、碳酸钠、碳酸氢钠以及氨水中的一种或多种。
在一些实施方式中，pH调节剂的用量为偏氟乙烯单体总质量的0.05％～0.2％。pH调节剂的用量例如还可以是偏氟乙烯单体总质量的0.1％或0.15％。
[正极极片]
正极极片包括正极集流体以及设置在正极集流体至少一个表面 的正极膜层，正极膜层包括正极活性材料、导电剂和一些实施方式中的粘结剂或一些实施方式中的制备方法制备的粘结剂。
该正极极片在粘结剂低添加量下具有优异的粘结力。
在一些实施方式中，粘结剂的质量分数为0.4％～0.6％，基于正极膜层的总质量计。在一些实施方式中，粘结剂的质量分数为0.4％～0.5％、0.5％～0.6％中的一种。
若粘结剂的质量分数过高，过多的粘结剂会造成正极活性材料在极片中的负载量下降，导致电池的能量密度降低，限制电池容量的发挥。
若粘结剂的质量分数过低，达不到足够的粘结效果，一方面无法将足量的导电剂和正极活性材料粘结到一起，极片的粘结力小；另一方面粘结剂无法紧密结合于活性物质表面，导致极片表面容易脱粉，造成电池的循环性能下降。
控制粘结剂的质量分数在合适范围内，在确保极片具有足够粘结力的同时，能够提高电池极片中活性物质的负载量，有助于进一步提高电池的功率性能。
作为示例，正极集流体具有在其自身厚度方向相对的两个表面，正极膜层设置在正极集流体相对的两个表面的其中任意一者或两者上。
在一些实施方式中，所述正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，可采用铝箔。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层至少一个表面上的金属层。复合集流体可通过将金属材料(铝、铝合金、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。
在一些实施方式中，正极活性材料可采用本领域公知的用于电池的正极活性材料。作为示例，正极活性材料可包括以下材料中的至少一种：橄榄石结构的含锂磷酸盐、锂过渡金属氧化物及其各自的改性化合物。但本申请并不限定于这些材料，还可以使用其他可 被用作电池正极活性材料的传统材料。这些正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。其中，锂过渡金属氧化物的示例可包括但不限于锂钴氧化物(如LiCoO2)、锂镍氧化物(如LiNiO2)、锂锰氧化物(如LiMnO2、LiMn2O4)、锂镍钴氧化物、锂锰钴氧化物、锂镍锰氧化物、锂镍钴锰氧化物(如LiNi1/3Co1/3Mn1/3O2(也可以简称为NCM333)、LiNi0.5Co0.2Mn0.3O2(也可以简称为NCM523)、LiNi0.5Co0.25Mn0.25O2(也可以简称为NCM211)、LiNi0.6Co0.2Mn0.2O2(也可以简称为NCM622)、LiNi0.8Co0.1Mn0.1O2(也可以简称为NCM811)、锂镍钴铝氧化物(如LiNi0.85Co0.15Al0.05O2)及其改性化合物等中的至少一种。橄榄石结构的含锂磷酸盐的示例可包括但不限于磷酸铁锂(如LiFePO4(也可以简称为LFP))、磷酸铁锂与碳的复合材料、磷酸锰锂(如LiMnPO4)、磷酸锰锂与碳的复合材料、磷酸锰铁锂、磷酸锰铁锂与碳的复合材料中的至少一种。
在一些实施方式中，正极膜层还可选地包括导电剂。作为示例，所述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。
在一些实施方式中，可以通过以下方式制备正极极片：将上述用于制备正极极片的组分，例如正极活性材料、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)中，形成正极浆料；将正极浆料涂覆在正极集流体上，经烘干、冷压等工序后，即可得到正极极片。
[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，所述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。
在一些实施方式中，所述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。
在一些实施方式中，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。所述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。所述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，负极膜层还可选地包括粘结剂。所述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。
在一些实施方式中，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。
在一些实施方式中，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。
在一些实施方式中，所述电解质采用电解液。所述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，所述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]
在一些实施方式中，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。
在一些实施方式中，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。
在一些实施方式中，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。
在一些实施方式中，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。
在一些实施方式中，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1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2，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能够盖设于所述开口，以封闭所述容纳腔。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封装于所述容纳腔内。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3是作为一个示例的电池模块4。参照图3，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4和图5是作为一个示例的电池包1。参照图4和图5，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，所述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。所述二次电池、电池模块、或电池包可以用作所述用电装置的电源，也可以用作所述用电装置的能量存储单元。所述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。
作为所述用电装置，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6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。
一、制备方法
实施例1
1)粘结剂的制备
第一段聚合反应：在1号、2号10L的高压釜中加入4kg的去离子水和2g的甲基纤维素醚，抽真空并用N2置换O2三次，再次加入5g叔丁基过氧化新戊酸酯和2g的碳酸氢钠，并充入1kg的偏氟乙烯 单体使压力达到5MPa，混合搅拌30min，升温到45℃，反应4h；
第二段聚合反应：将1号、2号反应釜中的反应液转移到3号反应釜当中，充入氮气至压力7MPa，升温到70℃，搅拌反应3h；
第三段聚合反应：加入40g环己烷后继续反应1h，停止反应。将反应体系离心后收集固相，洗涤、干燥即得到聚偏氟乙烯粘结剂。
2)正极极片的制备
将3961.8g磷酸铁锂，24.6g的聚偏氟乙烯粘结剂，57.4g的乙炔黑在行星式搅拌罐中，公转转速25r/min，搅拌25min，其中粘结剂的质量分数为0.6％，基于正极膜层的总质量计；
在搅拌罐中加入2.4kg的N-甲基吡咯烷酮(NMP)溶液，公转速度25r/min，自转速度950r/min，搅拌75min；
在搅拌罐中加入12.3g分散剂，以公转速度25r/min，自转速度1350r/min，搅拌65min；
搅拌结束，测试浆料粘度，粘度控制在8000～15000mPa·s。
如粘度偏高，加入N-甲基吡咯烷酮(NMP)溶液使得粘度降低到上述范围，加入NMP溶液后按照公转速度25r/min，自转速度1250r/min，搅拌30min，得到正极浆料。将制得的正极浆料刮涂到涂碳铝箔上面，110℃烘烤15min，冷压后裁剪成直径15mm的圆片，即得到正极极片。
3)负极极片
以金属锂片作为负极极片。
4)隔离膜
以聚丙烯膜作为隔离膜。
5)电解液的制备
在氩气气氛手套箱中(H2O<0.1ppm，O2<0.1ppm)，将有机溶剂碳酸乙烯酯(EC)/碳酸甲乙酯(EMC)按照体积比3/7混合均匀，加入LiPF6锂盐溶解于有机溶剂中，搅拌均匀，配置1M LiPF6EC/EMC溶液得到电解液。
6)电池的制备
将实施例1中的正极极片、负极极片、隔离膜和电解液在扣电 箱中组装成扣式电池。
实施例2～5
与实施例1基本相同，区别在于，将第一段聚合反应中的反应时间分别调整为5h、6h、7h、8h，将第三段聚合反应中的环己烷分别调整为35g、30g、25g、20g，具体参数如表1所示。
实施例6～9
与实施例2基本相同，区别在于，调整了聚偏氟乙烯粘结剂的质量分数，基于正极膜层的总质量计，具体参数如表1所示。
实施例10～11
与实施例1基本相同，区别在于，调整了聚偏氟乙烯粘结剂的质量分数，基于正极膜层的总质量计，具体参数如表1所示。
实施例12～13
与实施例3基本相同，区别在于，调整了聚偏氟乙烯粘结剂的质量分数，基于正极膜层的总质量计，具体参数如表1所示。
实施例14～15
与实施例4基本相同，区别在于，调整了聚偏氟乙烯粘结剂的质量分数，基于正极膜层的总质量计，具体参数如表1所示。
实施例16～17
与实施例5基本相同，区别在于，调整了聚偏氟乙烯粘结剂的质量分数，基于正极膜层的总质量计，具体参数如表1所示。
对比例1
与实施例1基本相同，该粘结剂为重均分子量为80万的聚偏氟乙烯，购买于东阳光公司，型号为701A牌号，并调整粘结剂的质量分数为2％，基于正极膜层的总质量计，具体参数如表1所示。
对比例2
与实施例1基本相同，该粘结剂为重均分子量为100万的聚偏氟乙烯，制备方法为：
在10L的高压釜中加入4kg的去离子水和2g的甲基纤维素醚，抽真空并用N2置换O2三次，再次加入5g叔丁基过氧化新戊酸酯和2g的碳酸氢钠，并充入1kg的偏氟乙烯，使体系压力达到7MPa，混 合搅拌30min，升温到45℃，反应4h后加入40g的环己烷继续反应，当反应釜内压力降到2MPa时停止反应。将反应体系离心后收集固相，洗涤、干燥即得到重均分子量为100万的聚偏氟乙烯。
对比例3
与实施例1基本相同，该粘结剂为重均分子量为200万的聚偏氟乙烯，制备方法为：
在10L的高压釜中加入4kg的去离子水和2g的甲基纤维素醚，抽真空并用N2置换O2三次，再次加入5g叔丁基过氧化新戊酸酯和2g的碳酸氢钠，并充入1kg的偏氟乙烯，使体系压力达到7MPa，混合搅拌30min，升温到45℃，反应6h后加入28g的环己烷继续反应，当反应釜内压力降到2MPa时停止反应。将反应体系离心后收集固相，洗涤、干燥即得到重均分子量为200万的聚偏氟乙烯。
对比例4
与实施例1基本相同，该粘结剂为重均分子量为300万的聚偏氟乙烯，制备方法为：
在10L的高压釜中加入4kg的去离子水和2g的甲基纤维素醚，抽真空并用N2置换O2三次，再次加入5g叔丁基过氧化新戊酸酯和2g的碳酸氢钠，并充入1kg的偏氟乙烯，使体系压力达到7MPa，混合搅拌30min，升温到45℃，反应7h后加入26g的环己烷继续反应，当反应釜内压力降到2MPa时停止反应。将反应体系离心后收集固相，洗涤、干燥即得到重均分子量为300万的聚偏氟乙烯。
对比例5
与实施例1基本相同，该粘结剂为重均分子量为400万的聚偏氟乙烯，制备方法为：
在10L的高压釜中加入4kg的去离子水和2g的甲基纤维素醚，抽真空并用N2置换O2三次，再次加入5g叔丁基过氧化新戊酸酯和2g的碳酸氢钠，并充入1kg的偏氟乙烯，使体系压力达到7MPa，混合搅拌30min，升温到45℃，反应8h后加入22g的环己烷继续反应，当反应釜内压力降到2MPa时停止反应。将反应体系离心后收集固相，洗涤、干燥即得到重均分子量400万的聚偏氟乙烯。
二、电池性能测试
1、粘结剂性质测试
1)重均分子量测试
采用Waters 2695 Isocratic HPLC型凝胶色谱仪(示差折光检测器2141)。以质量分数为3.0％的聚苯乙烯溶液试样做参比，选择匹配的色谱柱(油性：Styragel HT5DMF7.8*300mm+Styragel HT4)。用纯化后的N-甲基吡咯烷酮(NMP)溶剂配置3.0％的聚偏氟乙烯胶液，配置好的溶液静置一天，备用。测试时，先用注射器吸取四氢呋喃，进行冲洗，重复几次。然后吸取5ml实验溶液，排除注射器中的空气，将针尖擦干。最后将试样溶液缓缓注入进样口。待示数稳定后获取数据，读取重均分子量。
2)多分散系数测试
采用Waters 2695 Isocratic HPLC型凝胶色谱仪(示差折光检测器2141)。以质量分数为3.0％的聚苯乙烯溶液试样做参比，选择匹配的色谱柱(油性：Styragel HT5DMF7.8*300mm+Styragel HT4)。用纯化后的N-甲基吡咯烷酮(NMP)溶剂配置3.0％的聚偏氟乙烯胶液，配置好的溶液静置一天，备用。测试时，先用注射器吸取四氢呋喃，进行冲洗，重复几次。然后吸取5ml实验溶液，排除注射器中的空气，将针尖擦干。最后将试样溶液缓缓注入进样口。待示数稳定后获取数据。分别读取重均分子量a和数均分子量b。多分散系数＝a/b。
3)Dv50测试
参照GB/T 19077-2016粒度分布激光衍射法，用50ml烧杯称量0.1g～0.13g的聚偏氟乙烯粉料，再称取5g无水乙醇，加入到装有聚偏氟乙烯粉料的烧杯中，放入长约2.5mm的搅拌子，并用保鲜膜密封。将样品放入超声机超声5min，转移到磁力搅拌机用500r/min的搅拌20min以上，每批次产品抽取2个样品测试取平均值。采用激光粒度分析仪进行测定，如英国马尔文仪器有限公司的Mastersizer2000E型激光粒度分析仪进行测试。
4)结晶度测试
将0.5g聚偏氟乙烯置于铝制坩埚中，抖平，盖上坩埚盖子，在 氮气气氛下，以50ml/min的吹扫气，以70ml/min的保护气，升温速率为10℃/min，测试温度范围-100℃～400℃，利用美国TA仪器型号为Discovery 250的差示扫描量热仪(DSC)进行测试并消除热历史。
此测试将会得到聚偏氟乙烯的DSC曲线，并对曲线进行积分，峰面积即为聚偏氟乙烯的熔融焓ΔH(J/g)，聚偏氟乙烯结晶度＝(ΔH/ΔHm)×100％，其中ΔHm为聚偏氟乙烯的标准熔融焓(晶态熔化热)，ΔHm＝104.7J/g。
5)胶液粘度测试
用500ml烧杯分别称取7g聚偏氟乙烯和343g N-甲基吡咯烷酮(NMP)，配置成质量分数2％的胶液，使用力辰高速研磨机搅拌分散，转速800r/min，搅拌时间120min后超声震荡30min去除气泡。在室温下，使用力辰科技NDJ-5S旋转粘度计进行测试，选用3号转子插入胶液，保证转子液面标志和胶液液面相平，以12r/min的转子转速测试粘度，6min后读取粘度数据即可。
2、极片性能测试
1)粘结力测试
参考GB-T2790-1995国标《胶粘剂180°剥离强度实验方法》，本申请实施例和对比例的粘结力测试过程如下：
用刀片截取宽度为30mm，长度为100-160mm的试样，将专用双面胶贴于钢板上，胶带宽度20mm，长度90-150mm。将前面截取的极片试样的正极膜层面贴在双面胶上，后用2kg压辊沿同一个方向滚压三次。将宽度与极片等宽，长度为250mm的纸带固定于极片集流体下方，并且用皱纹胶固定。打开三思拉力机电源(灵敏度为1N)，指示灯亮，调整限位块到合适位置，将钢板未贴极片的一端用下夹具固定。将纸带向上翻折，用上夹具固定，利用拉力机附带的手动控制器上的“上行”和“下行”按钮调整上夹具的位置。然后进行测试并读取数值。将极片受力平衡时的力除以胶带的宽度作为单位长度的极片的粘结力，以表征正极膜层与集流体之间的粘结强度，得到如图7所示的实施例10和对比例1的粘结力-位移图。
3、电池性能测试
1)电池容量保持率测试
电池容量保持率测试过程如下：在25℃下，将扣式电池以1/3C恒流充电至3.65V，再以3.65V恒定电压充电至电流为0.05C，搁置5min，再以1/3C放电至2.5V，所得容量记为初始容量C0。对上述同一个电池重复以上步骤，并同时记录循环第n次后电池的放电容量Cn，则每次循环后电池容量保持率Pn＝(Cn/C0)×100％，以P1、P2……P500这500个点值为纵坐标，以对应的循环次数为横坐标，得到如图8所示的实施例10和对比例1的电池容量保持率与循环次数的曲线图。
该测试过程中，第一次循环对应n＝1、第二次循环对应n＝2、……第500次循环对应n＝500。表1中实施例1～17或对比例1～6对应的电池容量保持率数据是在上述测试条件下循环500次之后测得的数据，即P500的值。
三、各实施例、对比例测试结果分析
按照上述方法分别制备各实施例和对比例的电池，并测量各项性能参数，结果见下表1。
表1实施例和对比例参数以及性能测试表

图7为实施例10与对比例1的粘结力-位移图，从图中可以看出，在相同位移时，实施例10的粘结力明显高于对比例1的粘结力，表明在粘结剂添加量较低的情况下，本申请提供的聚偏氟乙烯粘结剂使得极片具有优异的粘结力。图8为实施例10与对比例1的电池容量保持率与循环次数的曲线图，从图中可以看出，在电池循环500 次后，实施例10的循环容量保持率明显高于对比例1，表明在粘结剂添加量较低的情况下，本申请提供的聚偏氟乙烯粘结剂能够提高电池在循环过程中的循环容量保持率，有效改善传统技术中高用量粘结剂带来的极片和电池性能受损的情况。
从实施例1～5和对比例2～5对比可知，重均分子量500万～900万的聚偏氟乙烯粘结剂在低添加量的情况下，能够进一步提高极片的粘结力和电池的容量保持率，有助于进一步降低极片中的粘结剂含量以及提高电池的活性材料负载量。
实施例1～17中的粘结剂均包括重均分子量为500万～900万的聚偏氟乙烯。从实施例1～17与对比例1对比可知，重均分子量500万～900万的聚偏氟乙烯粘结剂在低添加量下即使得极片具有优异的粘结力且能够提高电池在循环过程中的容量保持率。
从实施例1～17中可知，多分散系数为1.8～2.5的重均分子量为500万～900万的聚偏氟乙烯粘结剂，在低添加量下就能够使得极片具有优异的粘结力，电池在循环过程中具有高的容量保持率。
从实施例1～17中得知，Dv50粒径为100μm～200μm的重均分子量为500万～900万的聚偏氟乙烯粘结剂，在低添加量的情况下就能够使得极片具有优异的粘结力，电池在循环过程中具有高的容量保持率。
从实施例1～17中得知，本申请提供的重均分子量为500万～900万的聚偏氟乙烯的结晶度为40％～46％，该粘结剂的重均分子量相比于传统粘结剂的重均分子量显著提高，可以有效降低粘结剂的使用量，但是聚偏氟乙烯的结晶度并没有大幅度增长，依然在50％以下，处于许用值范围内，能够满足极片粘结剂的应用需求。
从实施例1～17中得知，聚偏氟乙烯溶于N-甲基吡咯烷酮制得的胶液的质量分数为2％时，胶液的粘度为2000mPa·s～5000mPa·s。这使得粘结剂在低添加量下就能够保证极片具有足够的粘结力。
从实施例2、实施例7～8与实施例6对比可知，当粘结剂的质量分数为0.4％～0.6％，基于正极膜层的总质量计时，粘结剂就能够保证极片具有足够的粘结力，且电池在循环过程中的容量保持率进一 步提高。从实施例2、实施例7～8和实施例9对比可知，当粘结剂质量分数为0.7％时，过高的粘结剂含量不会显著提升极片的粘结力和电池的循环性能，反而不利于活性物质负载量的提升，限制了电池能量密度的进一步提高。
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 (29)

  1. 一种用于二次电池极片的粘结剂，其特征在于，所述粘结剂包括重均分子量为500万～900万的聚偏氟乙烯。
  2. 根据权利要求1所述的粘结剂，其特征在于，所述聚偏氟乙烯的多分散系数为1.8～2.5。
  3. 根据权利要求1所述的粘结剂，其特征在于，所述聚偏氟乙烯的Dv50粒径为100μm～200μm。
  4. 根据权利要求1至3中任一项所述的粘结剂，其特征在于，所述聚偏氟乙烯的结晶度为40％～46％。
  5. 根据权利要求1至3中任一项所述的粘结剂，其特征在于，所述聚偏氟乙烯溶于N-甲基吡咯烷酮制得的质量含量为2％的所述聚偏氟乙烯的胶液的粘度为2000mPa·s～5000mPa·s。
  6. 一种用于二次电池极片的粘结剂的制备方法，其特征在于，包括以下步骤：
    提供偏氟乙烯单体和溶剂，进行第一段聚合反应，得到第一产物；
    将所述第一产物在非水溶性气体氛围下进行第二段聚合反应；
    加入链转移剂，进行第三段聚合反应，得到重均分子量为500万～900万的聚偏氟乙烯。
  7. 根据权利要求6所述的制备方法，其特征在于，所述第一段聚合反应的反应温度为45℃～60℃，反应时间为4小时～10小时，初始聚合压力为4MPa～6MPa。
  8. 根据权利要求6所述的制备方法，其特征在于，所述第二段 聚合反应的反应温度为60℃～80℃，反应时间为2小时～4小时，反应压力为6MPa～8MPa。
  9. 根据权利要求6至8中任一项所述的制备方法，其特征在于，所述第三段聚合反应的反应时间为1小时～2小时。
  10. 根据权利要求6至8中任一项所述的制备方法，其特征在于，所述链转移剂选自环己烷、异丙醇、甲醇、丙酮中的一种或多种。
  11. 根据权利要求6至8中任一项所述的制备方法，其特征在于，所述非水溶性气体选自氮气、氧气、氢气、甲烷中的一种或多种。
  12. 根据权利要求6至8中任一项所述的制备方法，其特征在于，所述链转移剂的用量为所述偏氟乙烯单体总质量的1.5％～3％。
  13. 根据权利要求6至8中任一项所述的制备方法，其特征在于，所述第一段聚合反应包括以下步骤：
    向容器中加入溶剂和分散剂，去除反应体系中的氧气；
    向所述容器中加入引发剂和pH调节剂，调节pH值至6.5～7，然后加入偏氟乙烯单体，使所述容器中的压力达到4MPa～6MPa；
    搅拌30分钟～60分钟后，升温至45℃～60℃，进行第一段聚合反应。
  14. 根据权利要求13所述的制备方法，其特征在于，所述溶剂的用量为所述偏氟乙烯单体总质量的2～8倍。
  15. 根据权利要求13所述的制备方法，其特征在于，所述分散剂包括纤维素醚和聚乙烯醇中的一种或多种。
  16. 根据权利要求15所述的制备方法，其特征在于，所述纤维素醚包括甲基纤维素醚和羧乙基纤维素醚中的一种或多种。
  17. 根据权利要求13所述的制备方法，其特征在于，所述分散剂的用量为所述偏氟乙烯单体总质量的0.1％～0.3％。
  18. 根据权利要求13所述的制备方法，其特征在于，所述引发剂为有机过氧化物。
  19. 根据权利要求18所述的制备方法，其特征在于，所述有机过氧化物包括过氧化新戊酸叔戊酯、过氧化叔戊基新戊酸酯、2-乙基过氧化二碳酸酯、二异丙基过氧化二碳酸酯以及叔丁基过氧化新戊酸酯中的一种或多种。
  20. 根据权利要求13所述的制备方法，其特征在于，所述引发剂的用量为所述偏氟乙烯单体总质量的0.15％～1％。
  21. 根据权利要求13所述的制备方法，其特征在于，所述pH调节剂包括碳酸钾、碳酸氢钾、碳酸钠、碳酸氢钠以及氨水中的一种或多种。
  22. 根据权利要求13所述的制备方法，其特征在于，所述pH调节剂的用量为所述偏氟乙烯单体总质量的0.05％～0.2％。
  23. 一种正极极片，包括正极集流体以及设置在正极集流体至少一个表面的正极膜层，所述正极膜层包括正极活性材料、导电剂和权利要求1至5中任一项所述的粘结剂或权利要求6至22中任一项所述的制备方法制备的粘结剂。
  24. 根据权利要求23所述的正极极片，其特征在于，所述粘结剂的质量分数为0.4％～0.6％，基于所述正极膜层的总质量计。
  25. 一种二次电池，其特征在于，包括电极组件和电解液，所述电极组件包括隔离膜、负极极片和权利要求23或24所述的正极极片。
  26. 根据权利要求25所述的二次电池，其特征在于，所述二次电池为锂离子电池或钠离子电池。
  27. 一种电池模块，其特征在于，包括权利要求25或26所述的二次电池。
  28. 一种电池包，其特征在于，包括权利要求27所述的电池模块。
  29. 一种用电装置，其特征在于，包括选自权利要求25或26所述的二次电池、权利要求27所述的电池模块或权利要求28所述的电池包中的至少一种。
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